A study into the permeability and compressibility …improving the permeability and compressibility...

Queensland University of Technology

A study into the permeability and

compressibility properties of

Australian bagasse pulp

By

Thomas J. Rainey

B.Eng (Chem), Hons I

A THESIS SUBMITTED FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

School of Engineering Systems

Queensland University of Technology

2009

i

IMPORTANT NOTICE

The information in this thesis is confidential and should not be disclosed for

any reason nor relied on for a particular use or application. Any invention or

other intellectual property described in this document remains the property of

Queensland University of Technology.

Thomas J. Rainey, A study of bagasse pulp filtration

ii

© Copyright 2009

by Thomas J. Rainey

Thomas J. Rainey – A study of bagasse pulp filtration

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Keywords

Sugarcane; bagasse; pulp; paper; permeability; compressibility; filtration;

Kozeny-Carman; drainage; forming; steady-state; dynamic; chemical additives;

flocculants.


iv

Executive Summary

This is an experimental study into the permeability and compressibility

properties of bagasse pulp pads. Three experimental rigs were custom-built for

this project. The experimental work is complemented by modelling work.

Both the steady-state and dynamic behaviour of pulp pads are evaluated in the

experimental and modelling components of this project.

Bagasse, the fibrous residue that remains after sugar is extracted from

sugarcane, is normally burnt in Australia to generate steam and electricity for

the sugar factory. A study into bagasse pulp was motivated by the possibility of

making highly value-added pulp products from bagasse for the financial benefit

of sugarcane millers and growers. The bagasse pulp and paper industry is a

multibillion dollar industry (1). Bagasse pulp could replace eucalypt pulp

which is more widely used in the local production of paper products. An

opportunity exists for replacing the large quantity of mainly generic paper

products imported to Australia. This includes 949,000 tonnes of generic

photocopier papers (2). The use of bagasse pulp for paper manufacture is the

main application area of interest for this study.

Bagasse contains a large quantity of short parenchyma cells called ‘pith’.

Around 30% of the shortest fibres are removed from bagasse prior to pulping.

Despite the ‘depithing’ operations in conventional bagasse pulp mills, a large

amount of pith remains in the pulp. Amongst Australian paper producers there

is a perception that the high quantity of short fibres in bagasse pulp leads to

poor filtration behaviour at the wet-end of a paper machine. Bagasse pulp’s

poor filtration behaviour reduces paper production rates and consequently

revenue when compared to paper production using locally made eucalypt pulp.

Pulp filtration can be characterised by two interacting factors;

permeability and compressibility. Surprisingly, there has previously been very

little rigorous investigation into neither bagasse pulp permeability nor


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compressibility. Only freeness testing of bagasse pulp has been published in

the open literature. As a result, this study has focussed on a detailed

investigation of the filtration properties of bagasse pulp pads.

As part of this investigation, this study investigated three options for

improving the permeability and compressibility properties of Australian

bagasse pulp pads. Two options for further pre-treating depithed bagasse prior

to pulping were considered. Firstly, bagasse was fractionated based on size.

Two bagasse fractions were produced, ‘coarse’ and ‘medium’ bagasse fractions.

Secondly, bagasse was collected after being processed on two types of juice

extraction technology, i.e. from a sugar mill and from a sugar diffuser. Finally

one method of post-treating the bagasse pulp was investigated. The effects of

chemical additives, which are known to improve freeness, were also assessed

for their effect on pulp pad permeability and compressibility.

Pre-treated Australian bagasse pulp samples were compared with several

benchmark pulp samples. A sample of commonly used kraft Eucalyptus

globulus pulp was obtained. A sample of depithed Argentinean bagasse, which

is used for commercial paper production, was also obtained. A sample of

Australian bagasse which was depithed as per typical factory operations was

also produced for benchmarking purposes.

The steady-state pulp pad permeability and compressibility parameters

were determined experimentally using two purpose-built experimental rigs. In

reality, steady-state conditions do not exist on a paper machine. The

permeability changes as the sheet compresses over time. Hence, a dynamic

model was developed which uses the experimentally determined steady-state

permeability and compressibility parameters as inputs. The filtration model

was developed with a view to designing pulp processing equipment that is

suitable specifically for bagasse pulp. The predicted results of the dynamic

model were compared to experimental data.

The effectiveness of a polymeric and microparticle chemical additives for

improving the retention of short fibres and increasing the drainage rate of a

bagasse pulp slurry was determined in a third purpose-built rig; a modified


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Dynamic Drainage Jar (DDJ). These chemical additives were then used in the

making of a pulp pad, and their effect on the steady-state and dynamic

permeability and compressibility of bagasse pulp pads was determined.

The most important finding from this investigation was that Australian

bagasse pulp was produced with higher permeability than eucalypt pulp, despite

a higher overall content of short fibres. It is thought this research outcome

could enable Australian paper producers to switch from eucalypt pulp to

bagasse pulp without sacrificing paper machine productivity. It is thought that

two factors contributed to the high permeability of the bagasse pulp pad.

Firstly, thicker cell walls of the bagasse pulp fibres resulted in high fibre

stiffness. Secondly, the bagasse pulp had a large proportion of fibres longer

than 1.3 mm. These attributes helped to reinforce the pulp pad matrix.

The steady-state permeability and compressibility parameters for the

eucalypt pulp were consistent with those found by previous workers.

It was also found that Australian pulp derived from the ‘coarse’ bagasse

fraction had higher steady-state permeability than the ‘medium’ fraction.

However, there was no difference between bagasse pulp originating from a

diffuser or a mill.

The bagasse pre-treatment options investigated in this study were not

found to affect the steady-state compressibility parameters of a pulp pad.

The dynamic filtration model was found to give predictions that were in

good agreement with experimental data for pads made from samples of pre-

treated bagasse pulp, provided at least some pith was removed prior to pulping.

Applying vacuum to a pulp slurry in the modified DDJ dramatically

reduced the drainage time. At any level of vacuum, bagasse pulp benefitted

from chemical additives as quantified by reduced drainage time and increased

retention of short fibres. Using the modified DDJ, it was observed that under

specific conditions, a benchmark depithed bagasse pulp drained more rapidly

than the ‘coarse’ bagasse pulp.


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In steady-state permeability and compressibility experiments, the addition

of chemical additives improved the pad permeability and compressibility of a

benchmark bagasse pulp with a high quantity of short fibres. Importantly, this

effect was not observed for the ‘coarse’ bagasse pulp. However, dynamic

filtration experiments showed that there was also a small observable

improvement in filtration for the ‘medium’ bagasse pulp. The mechanism of

bagasse pulp pad consolidation appears to be by fibre realignment. Chemical

additives assist to lubricate the consolidation process.

This study was complemented by pulp physical and chemical property

testing and a microscopy study. In addition to its high pulp pad permeability,

‘coarse’ bagasse pulp often (but not always) had superior physical properties

than a benchmark depithed bagasse pulp.


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List of publications

Journal articles Rainey, T.J., Doherty, W.O.S., Brown, R.J., Martinez, D.M., and Kelson, N.A.

– Pressure Filtration of Australian bagasse pulp, Appita J. (2009) submitted to Transport Porous Med.

Rainey, T.J., Doherty, W.O.S., Brown, R.J., Martinez, D.M., and Kelson, N.A.

- An Experimental Study of Australian Sugarcane Bagasse Pulp Permeability, Appita J. (2009) accepted for publication.

Peer-reviewed conference papers Doherty, B. and Rainey, T. - Bagasse fractionation by the soda process, Proc.

Aust. Soc. Sugar Cane Technol., Mackay. (2006). 545-554. Conference papers Rainey, T.J., Doherty, W.O.S., Brown, R.J., Kelson, N.A. and Martinez, D.M. -

Determination of the Permeability Parameters of Bagasse Pulp from Two Different Sugar Extraction Methods. In Proceedings Tappi Engineering Pulping and Environmental Conference, Session 4.1, Portland, Oregon, USA. (2008).

Rainey, T., Brown, R., Martinez, D.M., and Doherty, B. - The use of CFD to

simulate the behaviour of bagasse pulp suspensions during the dewatering process, Appita conference, Melbourne. (2006). Available online at QUT e-prints.

Several of the above papers are available online at the following site: http://eprints.qut.edu.au/


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Acknowledgements

I would like to thank all of my supervisors, without whom this thesis

would not be possible. I would like to thank Bill Doherty for helping me cope

with much of the structure and direction of my work – your door was always

open to me; Richard Brown - you always helped me with resource issues and

smooth progression through the milestones; Mark Martinez at UBC for

assistance with developing the dynamic filtration model and the long

discussions interpreting my data; and Neil Kelson for helping me with the

coding and article writing. All of you helped me in many ways.

This work would not have been possible without the financial

contribution of the Federal Government’s Sugar Research and Development

Corporation PhD scholarship fund, and income support from QUT’s Sugar

Research and Innovation. I would like to thank the Queensland Government’s

financial contribution through the PhD Smart State Fund. I also acknowledge

the financial contribution of the Faculty of Built Environment and Engineering

and the Engineering Systems theme. Your significant financial contributions

are deeply appreciated.

I would like to thank my family, particularly Jenni for her unending

patience through all those sleepless nights. Thank you Mum and Dad for your

support. Thank you Anna for entertaining me through my final year.

I would like to acknowledge the generous in-kind contribution of the

following organisations: the Australian Pulp and Paper Institute, particularly

Loi Nguyen, for use of their facilities; CSR Sugar and Mr. Paul Turnbull for

assistance with collection of the bagasse; Covey Consulting (Geoff Covey);

Appita (Ralph Coghill); Scion (Alan Dickson); Visy (Darren Ralston); Amcor

(Karl Osswald); Central Pulp and Paper Research Institute (Dr Roy and Dr

Sood); and HurterConsult (Bob Hurter).

Thank you Neil McKenzie for helping build the experimental equipment.

Thank you also to the countless others that contributed to this thesis.


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Formal supervisory team

Dr. Richard J. Brown, QUT, Built Environment and Engineering

Dr. William O.S. Doherty, QUT, Sugar Research and Innovation

Dr. Neil A. Kelson, QUT, High Performance Computing

Informal supervisor

Prof. D. Mark Martinez, University of British Columbia, Department of

Chemical and Biological Engineering

The project participants wish to acknowledge receipt of project funding

from the Australian Government and the Australian Sugarcane Industry as

provided by the Sugar Research and Development Corporation

This research is proudly supported by the Queensland Government’s

Growing the Smart State PhD Funding Program and may be used to assist

public policy development. However, the opinions and information contained

in the research do not necessarily represent the opinions of the Queensland

Government or carry any endorsement by the Queensland Government. The

Queensland Government accepts no responsibility for decisions or actions

resulting from any opinions or information supplied.


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This thesis is dedicated to my Dad, thank you for enriching my life,

and to my daughter Anna, welcome to the world.

“Even though I walk through the valley of the shadow of death, I

will fear no evil, for you are with me; your rod and your staff, they

comfort me.” – Ps 23:4


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The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To

the best of my knowledge and belief, the thesis contains no materials previously

published or written by another person except where due reference is made.

Signature ___________________

Date __________________


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Contents

Executive Summary -------------------------------------------------- iv

Abbreviations and nomenclature --------------------------------xvii

Terminology----------------------------------------------------------- xx

Chapter 1 Introduction --------------------------------------- ------1

1.1. Background and motivation 2

1.1.1. The production of sugar and bagasse from sugarcane 3 1.1.2. Potential uses of bagasse 6 1.1.3. Paper manufacture 7 1.1.4. Issues with bagasse paper manufacture in the

Australian context 10 1.1.5. The benefits of flocculants to assist paper formation 12

1.2. Research aim 12

1.3. Statement of objectives 12

1.4. Statement of novelty 14

1.5. Summary of thesis chapters 16

Chapter 2 Theory and Literature Review -------------------- 19

2.1. Background 19

2.2. Bagasse pulp properties 21

2.2.1. Bagasse pulp yield 21 2.2.2. Bagasse pulp fibre morphology 22 2.2.3. Chemical character of bagasse pulp fibres 23 2.2.4. Bagasse pulp physical properties 24

2.3. Pulp permeability and compressibility parameters 26

2.3.1. Steady-state permeability theory 26 2.3.2. Steady-state compressibility theory 35 2.3.3. Dynamic filtration theory 36 2.3.4. Non-Darcy flow 39 2.3.5. Equipment used in filtration studies 40 2.3.6. Additional filtration theory of particular importance to

this study 48

2.4. Chemical additives 52

2.4.1. The mechanism of CPAM and microparticle dual polymer systems for pulp flocculation 52

2.4.2. Flocculant systems 53 2.4.3. Literature on flocculants used for bagasse pulp 55


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2.4.4. Using the Dynamic Drainage Jar as a tool for comparing flocculants 56

2.4.5. Summary of chemical additives literature and theory 57

2.5. Summary of theory and literature review 58

Chapter 3 Experimental procedure and modelling----------60

3.1. Overview of experimental and modelling methodology 61

3.1.1. Preparation of Australian bagasse pulp 62 3.1.2. Physical and chemical property testing 63 3.1.3. Steady-state permeability property testing 63 3.1.4. Steady-state compression testing 64 3.1.5. Dynamic filtration modelling and verification 64 3.1.6. Effect of chemical additives on the drainage and

retention properties 65 3.1.7. Flow diagram of the experimental and modelling

methodology 65

3.2. Bagasse pulp preparation 67

3.2.1. Collection of raw materials 67 3.2.2. Pulp sample preparation 71 3.2.3. Test for statistical significance between two

populations of pulp samples 76

3.3. Physical and chemical property testing procedure 78

3.3.1. Chemical characterisation of pulp and bagasse 78 3.3.2. Pulp physical property testing 79 3.3.3. Fibre length analysis 80 3.3.4. Microscopy investigation 81

3.4. Steady-state permeability testing equipment and experimental procedure 83

3.5. Quasi steady-state compressibility experimental procedure 88

3.6. Dynamic filtration modelling and experimental verification procedure 90

3.6.1. Dynamic filtration modelling procedure 91 3.6.2. Verification of the dynamic model 91

3.7. Equipment and procedure for testing the effect of chemical additives 92

3.7.1. Methodology – Effect of shear 94 3.7.2. Methodology – Effect of vacuum 95 3.7.3. Methodology – Effect of chemical additives on

permeability and compressibility. 97

3.8. Summary of the experimental investigation 97

Chapter 4 Results and discussion -------------------------------99


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4.1. Results of bagasse chemical pulping 100

4.1.1. Bagasse pulping kinetics 100 4.1.2. Effect of bagasse pre-treatment on Australian bagasse

pulp yield 101 4.1.3. Effect of bagasse pre-treatment on bagasse pulp kappa

number 103 4.1.4. Summary of bagasse pulping analyses 103

4.2. Results of physical and chemical property testing 104

4.2.1. Pulp chemical analysis results 104 4.2.2. Pulp physical property results 107 4.2.3. Fibre length distribution analysis 112 4.2.4. Microscopic analysis 115 4.2.5. Summary of pulp physical and chemical property

testing 116

4.3. Results of steady-state permeability testing 117

4.3.1. Data from steady-state permeability testing 117 4.3.2. Effect of bagasse pre-treatment on pulp permeability

properties 121 4.3.3. Review of bagasse pulp steady-state permeability

model 122 4.3.4. Comparison of steady-state permeability data with

previous work 123 4.3.5. Summary of steady-state permeability experiments 125

4.4. Results of quasi steady-state compressibility testing 126

4.4.1. Suitability of the power law steady-state compressibility model 127

4.4.2. Pulp steady-state compressibility data and comparison with the findings of previous workers 128

4.4.3. The effect of pre-treament on bagasse compressibility 129 4.4.4. Summary of steady-state compressibility testing 131

4.5. Results of dynamic filtration modelling and validation 131

4.5.1. Predictions of the dynamic model 131 4.5.2. Dynamic filtration experiments and comparison with

predicted values 136 4.5.3. Summary of dynamic filtration modelling 137

4.6. Results of chemical additives testing 139

4.6.1. The effect of shear and additives on pulp retention 140 4.6.2. The effect of chemical additives and vacuum 146 4.6.3. The effects of chemical additives on permeability and

compressibility parameters 151 4.6.4. The effect of chemical additives on bagasse pulp’s

dynamic filtration behaviour 158


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4.6.5. Summary of the effect of chemical additives on pulp permeability and compressibility 161

Chapter 5 Conclusions ------------------------------------------ 163

5.1. Findings of this thesis 164

5.2. Recommendations for future work 167

References------------------------------------------------------------ 169

Appendix A Supplementary material for dynamic filtration modeling -------------------------------- --------------- 183

A.1 Derivation of the dimensional governing equation for the dynamic filtration model 184

A.2 Non-dimensionalising of dynamic model for FORTRAN 188

A.3 FORTRAN 77 program for the dynamic filtration model 191

A.4 Graphs comparing predictions of dynamic filtration model with experimental data 197

Appendix B Summary of pulp samples ---------------------- 203

Appendix C Supplementary photographs of experimental work ----------------------------------------------- 207

Appendix D Table of Students t distribution---------------- 211

Appendix E Fibre length data of pulp sample -------------- 213

Appendix F Engineering drawings of compression cell--- 215


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Abbreviations and nomenclature

Abbreviations

APPI Australian Pulp and Paper Institute

CPAM Cationic polyacrylamide

CPPRI Central Pulp and Paper Research Institute

CSF Canadian Standard Freeness

DCM Dichloromethane

DDJ Dynamic Drainage Jar

FQA Fibre Quality Analyser

OD Oven dry

LSD Least Significant Difference used by Scion for comparing

populations of pulp fibres

PAM Polyacrylamide

PPJ Positive Pulse Jar

QUT Queensland University of Technology

SRI A QUT institute; Sugar Research and Innovation

UBC University of British Columbia

WRV Water retention value


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Nomenclature

A cm2 the cross sectional area of a porous bed for use with Darcy’s

Law

a - an experimental constant for use in the Davies Kozeny factor

correction

b - an experimental constant for use in the Davies Kozeny factor

correction

c g/cm3 pulp concentration

D the flexural term in for the filtration governing equation; used

in Appendix A.

D* - the dimensionless form of D used for coding the dynamic

filtration model in FORTRAN (Appendix A)

g cm/s2 acceleration due to gravity

h cm the height of the pulp mat in the compressibility cell

K cm2 Darcy’s permeability constant

k - the Kozeny factor

k’ cm2 a permeability constant used by El-Sharkawy and co-workers

for measuring an Indian bagasse pulp

∆L cm the height of a bed of porous material for use with Darcy’s Law

∆l cm the distance between the two manometers

L mm the length of a fibre or capillary

M kPa a compressibility constant, used in the expression Ps=McN

m kPa a compressibility constant, , used in the expression Ps=m Ф n

m* cm-1 the ratio of surface area to volume of a capillary

N - a compressibility constant, used in the expression Ps=McN

n - a compressibility constant, used in the expression Ps=m Ф n

ni - the number of fibres with length Li

∆P mPa the pressure drop across a bed of porous material for use with

Darcy’s Law,

∆p mPa the pressure drop between two manometers

Q cm3/s the flow rate through a porous material for use with Darcy’s

Law

Sv cm2/cm3 the specific surface area of pulp fibre

t min time


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T - the dimensionless time used for coding the dynamic filtration

model in FORTRAN

u m/min the compression rate of the platen during experiments with the

compressibility cell

vr

cm/s velocity

x cm the distance from the top platen of the compressibility cell

X - the dimensionless distance from the top platen of the

compressibility cell used for coding the dynamic filtration

model in FORTRAN

Greek letters

α cm3/g pulp swelling factor,

ε - pulp porosity (between 0 and 1)

µ mPa.s liquid viscosity

Φ - solidity (i.e. volume solids fraction)

ρ g/cm3 density

σ standard deviation

τ Student’s t- statistic

τs viscous stress tensor

Subscripts

e equivalent length, as distance through a capillary, Le, or velocity

through a capillary, ue.

f the fluid phase

i the number of a fibre or population, as in fibre length Li

PE pooled estimate

s the solid phase

w weighted basis, as in Lw.

0 initial


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Terminology

Chemical additives includes microparticle and polymeric additives that

increase fibre flocculation (see ‘fibre’).

‘Coarse’ bagasse bagasse that is retained on a 12.5 mm screen.

Compressibility the compression behaviour of a pulp mat or pad (see

‘pulp mat’ and ‘pulp pad’).

Digester a pulp and paper reactor (see Reactor) for the digestion

of lignocellulosic material by delignification to produce

pulp.

Dynamic varies with time. For some experiments in this study

the dynamic effect is the varying compressive load

which changes the filtration properties and shape of a

pulp pad over time.

Fibre normally refers to pulp fibre. Composed of mainly

liberated schlerenchyma cells and some parenchyma

material. This term is conventionally used in the pulp

and paper industry. Bagasse is often described as

‘material’ rather than ‘fibre’ in this thesis to assist with

clarity on whether the parent material or the pulp is

being discussed.

Filtration in this study includes the combined effects of

permeability and compressibility behaviour in a pulp

mat or pad. In this thesis, this term is reserved to

describe dynamic behaviour.


xxi

‘Fine’ bagasse very short fibre that passes through a 4 mm screen.

Contains mainly pith (see ‘pith’).

Fines very short material which includes fibres.

Freeness an experimental measure of the ability of pulp to drain

freely.

Forming the process of producing a pulp mat from a pulp slurry.

Hardwood a wood species containing short fibres, such as eucalypt.

Kappa number a measure of the residual lignin content.

‘Medium’ bagasse bagasse which passes a 12.5 mm screen but is retained

on a 4 mm screen.

Non-wood a fibre resource not derived from wood, such as

sugarcane bagasse, wheat straw, kenaf, sorghum,

bananas or hemp.

Permeability the ability of a fluid to permeate a porous material.

Pith very short parenchyma cells which exists in sugarcane

and which do not have the characteristics of fibres.

Porosity the void fraction of a porous material on a volume basis

(see also ‘solidity’).

Pre-treatment describes how the fibrous material is treated prior to

or preparation pulping.

Pulp mat used in this thesis to mean a thin sheet of fibres less

than a few millimetres produced from a dilute pulp

slurry, as at the wet end of a paper machine.

Pulp pad is used for very thick pulp mats, such as those used for

experiments in this thesis. Applies to mats more than a

few millimetres and up to 300 mm in depth.


xxii

Quasi steady-state an approximation of steady-state conditions (see

‘steady-state’).

Reactor equipment for carrying out a chemical reaction. In this

thesis it has the same meaning as a digester (see

digester).

Solidity the solids fraction of a porous material on a volume

basis (see ‘porosity).

Steady-state invariant with time. For some experiments in this study

the level of compression of a pulp pad is constant (or

almost constant, i.e. ‘quasi steady-state’) meaning the

filtration properties are not changing with time.

Chapter 1 - Introduction

1

Chapter 1

Introduction

The study presented in this thesis investigates the permeability and

compressibility properties of Australian bagasse pulp pads1. Three options are

assessed for improving these properties of pulp pads. Firstly, the permeability

and compressibility properties of pulp prepared from ‘coarse’ and ‘medium’

size fractions of bagasse are compared. Secondly, bagasse produced from two

different modes of cane juice extraction, i.e. a mill and a diffuser, are

considered. Finally, the effect of flocculating chemical additives, namely

cationic polyacrylamide and bentonite, are examined. The steady-state

permeability and compressibility parameters for bagasse pulp are determined

experimentally using purpose built experimental equipment and are used as

inputs for a dynamic filtration model. The permeability and compressibility

properties of Australian bagasse pulp are compared with numerous benchmarks

including eucalypt pulp and bagasse pulp from Argentina.

In this chapter, the background and motivation for the work is presented

in section 1.1. Also, the aims (section 1.2) and the objectives (section 1.3) are

provided. A statement on the novel aspects of this study is in section 1.4 and

the chapter concludes with a summary of the thesis structure (section 1.5).

1 The ‘Terminology’ section in the preamble to this thesis should be read in order to assist the reader’s understanding of key concepts.


2

1.1. Background and motivation

The Australian sugar industry is mainly a single commodity producer

with raw sugar as the primary product. It consists of 25 raw sugar factories

producing $1.5 billion worth of raw sugar. Australia exports 80-85% of its raw

sugar product and so the industry is exposed to fluctuations in the world sugar

price (e.g. 3). Recently the Australian sugar industry was faced with a near

financial collapse because of the low sugar price, a series of poor crops and a

strengthening Australian dollar. As such the sugar industry is seeking to

diversify its product stream to increase revenue and reduce its dependence on

sugar. To achieve this, the industry is investigating new products that can be

made from the fibrous sugarcane residue, that is, bagasse. Of particular interest

to the sugar industry is the use of bagasse in paper manufacture.

Figure 1.1 shows a sketch of a billet of cane. The ‘bast’ is the external

part of the plant and the ‘pith’ is the internal part of the plant. The bast and the

pith are particularly important in the context of this thesis. The good

papermaking fibre from bagasse is mainly derived from the bast portion of the

sugarcane plant. Bagasse pulp quality is believed to be detrimentally affected

by short ‘pith’ material (length < 0.3 mm). This material is liberated by the

sugar extraction process and constitutes 30% of the bagasse. The short material

can block the holes in the paper mat, preventing water from draining through it,

reducing the production rate and various quality characteristics of the final

paper product (4-6). It is thought that ‘depithing’ of the bagasse by removing

30% of the shortest bagasse material is essential to make pulp of acceptable

quality (e.g. (7), (8)).


3

Figure 1.1 Sketch of a billet of sugarcane showing the bast and pith

regions of the plant (adapted from 9).

Paper machines making fine papers (e.g. tissues, photocopier papers) use

short pulp fibres, such as bagasse and eucalypt, and are generally faster than

machines making paper products from longer fibres. Consequently these

machines require fibres with good filtration properties (i.e. better web drainage)

to generate a homogeneous sheet.

Industry experts have observed that replacing hardwood pulp with

bagasse pulp reduces paper machine production rates by 25-30% (10, 11). For

a small 70 t/d tissue machine, using bagasse would result in a loss of revenue of

$8 million per year. Improving the drainage properties of bagasse pulp would

allow higher production rates, reducing the competitive advantage of eucalypt

pulp over bagasse pulp in terms of production rate.

1.1.1. The production of sugar and bagasse from sugarcane

In Australia, sugarcane is typically harvested and brought to the sugar

factory as short lengths of cane called ‘billets’.

The billets are crushed in a sugar factory to extract the juice which

contains the sugar (13% - 15% of the plant). The juice is concentrated and

sugar crystals are produced. The fluid surrounding the sugar crystals, (i.e.


4

molasses), has a high sugar content (around 40%) which is sold for cattle feed

or converted into fuel ethanol. In Australia, revenue from molasses and its

products is very small compared to the revenue from sugar. The fibre left over

from the crushing process, i.e. bagasse, constitutes around 14% of the plant.

Australia produces 10 million tonnes of bagasse annually. The production of

sugar and bagasse is shown in Figure 1.2.

Figure 1.2 Schematic of sugarcane processing and bagasse utilisation

options.

There are two main methods of extracting juice from cane. Sugar ‘mills’

are used almost exclusively in Australian raw sugar factories. Sugar ‘diffusers’

are not common in Australia but are commonly used overseas.

Bagasse fibres are severely damaged by sugar milling. A typical sugar

milling roller unit is shown in Figure 1.3. Sucrose is extracted after opening up

the parenchyma cells (mainly in the pith). The opening up of the pith occurs

when the sugarcane is initially shredded in the hammermills, when it is

processed in the subsequent roller mills (typically, six roller mills) and also in

the final dewatering mill. However, some shear forces are also exerted in the

roller mills in between the hammermills and the dewatering mill in a sugar mill

that uses only a milling train.

Bagasse

Sugarcane Juice

Molasses

Sugar

Conventional ethanol

Crushing


5

Figure 1.3 Sketch of a six roller unit where juice is extracted. Several of

these units follow the shredder and precede the final

dewatering roller mill (12).

The other form of juice extraction technology is a sugar ‘diffuser’ (see

Figure 1.4). For a diffuser, after cane preparation in a hammermill, and possibly

a first roller mill, the cane passes over a perforated plate, and juice or press

water is sprayed onto the fibrous bed to extract more sucrose. Along the

diffuser the juice is heated and lime is added to maintain a pH of 7. Subsequent

to the diffuser, the cane is passed through one or two drying roller mills.

Figure 1.4 Sketch of a sugar diffuser (13).

Perforated plate

Rollers Feed

chute Pressure feeder rolls

Pressure feeder chute

Delivery

nip

Feed nip


6

The subtle differences between mill bagasse and diffuser bagasse may

affect pulp quality. Thus, this project examined the impact of the extraction

process on pulp fibres.

1.1.2. Potential uses of bagasse

In Australia, bagasse is normally burnt for its fuel value to produce steam

and renewable electricity. Despite its 50% moisture content, bagasse has a high

calorific value, 9.9 MJ/kg (14). Bagasse’s opportunity cost, based on its fuel

value, is low ($30-$60 per tonne) because of the availability of high quality

local coal.

Bagasse has properties suitable for a range of highly value-added

renewable products such as pulp and paper products, polymers, building

materials and renewable fuels (Figure 1.5).

Figure 1.5 Potential uses of bagasse.

One of the main uses of bagasse internationally, which is overlooked in

Australia, is pulp and paper manufacture, which is the topic of this thesis.

Dozens of countries (e.g. China, India, Argentina and Mexico) make bagasse

pulp and paper products estimated to be worth between $4 billion and $10

billion annually. This estimate is based on bagasse pulp accounting for around

3% of the pulp and paper industry (15). Imported bagasse paper products from

South Africa, India and China are available in Australian supermarkets and

could be displaced by local production, generating revenue for sugarcane

millers and growers.

Pulp and paper Steam & electricity

Cellulosic ethanol

Fibreboard and

building materials

Waxes and resinsDietary fibre

Hydrothermal liquefaction

biofuels Bagasse

Polymers

Animal feed


7

Bagasse fibre is also used in building products. It is often used as a cheap

raw material in fibreboard or used as a filler in other products including

composite materials. Sometimes it is combined with cement and sand to make

reinforced concrete (16). These building products are common in India.

Recently, Australian researchers have investigated bagasse lignin and

cellulose as resources for renewable polymeric materials (17, 18). Revenue

from bagasse based polymers may grow substantially in the medium-term

because consumer demand for plastics produced from renewable and

sustainable resources is likely to increase.

There has been considerable interest for using lignocellulosic material,

including bagasse, for the production of fuel ethanol (19). However, at this

point in time, there is very little fuel ethanol commercially produced from

lignocellulosics, although a significant research effort is being undertaken in

Australia and overseas to realise this long-term goal that would reduce society’s

dependence on fossil fuels.

1.1.3. Paper manufacture

Modern paper manufacture is a highly capital intensive industry with

many stages of value adding being required until the paper finally reaches the

consumer.

The raw fibrous material is sourced from the forestry or agricultural

sector and usually stored in a large stockpile. Some pre-treatment often occurs,

such as the removal of pith in the case of bagasse, or fungal treatment in the

case of woodchips. In the case of bagasse, two stage depithing usually occurs.

The first stage is a ‘moist depithing’ where the bagasse from the mill is

separated into two fractions using a hammermill and a screen. The second

stage is ‘wet depithing’ which brings the total amount of pith that is removed to

30%.

The fibrous material is then broken down by mechanical or chemical

means into individual pulp ‘fibres’. The ‘fibres’ are often the tracheids

(softwood) or schlerenchyma cells (for bagasse), which are typically 1.0-

3.0 mm in length. For bagasse chemical pulp, which is investigated in this


8

study, the bagasse is normally loaded into a twin-screw horizontal ‘digester’

and the fibres are exposed to chemicals, such as caustic soda, elevated

temperature and pressure. Soda and soda AQ pulping, as used in this study, is

commonly used for bagasse chemical pulp. Bagasse lignin is much more

reactive than wood lignin, so the pulping conditions are very mild. Industrially,

the chemical charge is 12%-16% of sodium hydroxide (as NaOH on dry fibre),

at 170 °C but the pulping time is only 10 min to 12 min (20).

The formation of a fibre mat is a filtration process whereby a suspension

of fibres are deposited one layer at a time and water drains through the formed

mat (21). The result is that the flexible fibres overlay one another forming a

sheet of paper (see Figure 1.6).

(i) (ii)

Figure 1.6 Fibre orientation during paper formation at the microscopic

level shown in: (i) plan view (22); and (ii) elevation view (21)

It is thought that for bagasse pulp, the high content of short pith fibres

impede drainage which consequently reduces paper production rate.

There are two types of equipment commonly used to form a pulp mat

from a pulp slurry. These are the traditional ‘Fourdrinier former’ and the

modern ‘Twin-wire former’ which are normally designed for processing wood

pulp. Pulp fibres are suspended in water at 0.01% to 1% and are pumped into a

headbox.

The Fourdrinier former consists of continuous moving fabric. The pulp is

distributed along the width of the fabric and passes over a series of hydrofoils

Forming

fabric


9

which creates a pulsating vacuum underneath, promoting the drainage of the

pulp (Figure 1.7). At high speeds, instability of the free surface of the pulp

suspension on the polymer fabric can occur because of the pulsating vacuum

(23). The desire to increase the manufacturing speed ultimately resulted in the

development of the Twin-wire former.

Figure 1.7 Elevation view of a Fourdrinier former (24)

Twin-wire forming involves pulp leaving a headbox slice and impinging

on a converging gap between two polymer fabrics (24). The dewatering rate is

higher in twin-wire forming than on a Fourdrinier former. The dewatering

occurs at both surfaces of the pulp/paper mat and a large mechanical force is

applied.

Figure 1.8 Elevation sketch of a roll former adapted from Parker (25)

B headbox C slice G hydrofoils

Fabric 1

Fabric 2


10

Following paper formation, the moist wet sheet is mechanically pressed,

dried and reeled. Once a continuous dry paper sheet is produced, it is slit, cut

and packaged ready for the public.

1.1.4. Issues with bagasse paper manufacture in the Australian context

Eucalypt (a hardwood) is the material of choice for most Australian pulp

and paper manufacturers making fine papers, such as tissue and photocopier

paper, due to its ready local availability and its good pulp properties, such as

high tensile strength and printability. The average fibre length of bagasse pulp

is short, around 1.2 mm, and so it is a potential alternative to eucalypt (which

has an average fibre length of 0.8 mm to 1.0 mm) for paper production in

Australia. Despite the cost advantage of using bagasse ($30-$60 per tonne)

when compared to eucalypt ($150 per tonne) and the large existing sugar

industry, no bagasse-based products are made in Australia. The reasons for the

preferred use of eucalypt pulp include (26):

� Processing issues due to the poor filtration properties of bagasse pulp;

� Bagasse pulp has inferior strength properties relative to eucalypt pulp;

� High capital cost, given the enormous economies of scale achieved by

many modern wood pulp mills; and

� The remoteness of cane farms relative to Australian wood pulp mills

(see Figure 1.9).

An important focus of the work described in this thesis is the

investigation of options to improve bagasse pulp filtration properties. As will

be shown, the filtration characteristics can be improved by carefully preparing

bagasse prior to pulping.


11

(i) (ii)

Figure 1.9 Sketch of Australia showing (i) the paper industry

concentrated in south-eastern Australia (only three relatively

modern mills in Brisbane) (15) and (ii) the sugar industry

concentrated in north-eastern Australia (coloured yellow 27).

Additionally, the question of whether bagasse pulp could be produced

with permeability and strength properties similar to eucalypt is also addressed

as part of this investigation. If comparable properties for bagasse pulp and

eucalypt pulp can be achieved then there would be numerous benefits of

switching to bagasse pulp. As well as the lower cost of the raw material,

current consumer attitudes are favourable towards products with a perceived

environmental benefit. ‘Tree-free’ paper that is made without wood fibre

reduces deforestation. This is a particularly sensitive topic in southern

Australia where the current pulp and paper industry in concentrated.

The sugar extraction process also reduces the quality of the bagasse

fibres, reducing the pulp strength characteristics, including tensile strength, tear

strength and burst strength (28).

Bagasse pulp is normally processed on equipment designed for wood

pulp. To this end, if the filtration properties of bagasse pulp could be

characterised, this would provide valuable information to assist the design of

specialised bagasse pulp processing equipment using Computational Fluid

Dynamics, such as pulp washers and paper machine sheet forming fabrics.


12

Hence, complementary modelling of the bagasse filtration properties is

investigated in this study. However, equipment design using the model is

outside the scope of this investigation.

1.1.5. The benefits of flocculants to assist paper formation

Apart from improving the preparation of bagasse prior to pulping,

chemical additives, i.e. flocculants, also improve the production rate of a

paper/board machine. Pietschker and co-workers (29) for example reported

30 m/min increase in machine speed when a good drainage aid program was

employed at a corrugated medium mill and the increased fibre retention

resulted in $5-$20 per tonne reduction in fibre requirements. If similar results

were achieved say in a tissue mill with a production capacity of 70 t/d using

Australian bagasse, the increase in revenue resulting from the increased

production rate would be around $2-3 million per year and fibre savings of up

to $500,000 per year. The use of effective flocculants improves the quality of

formed bagasse sheets. For example, wire-marks are common in bagasse

photocopier paper.

1.2. Research aim

The aim of this project is to experimentally investigate three options to

improve the permeability and compressibility parameters of Australian bagasse

pulp pads with a view to making bagasse pulp competitive with local eucalypt

pulp. The experimental investigation is complemented by modelling of the

filtration behaviour of bagasse pulp pads.

1.3. Statement of objectives

The objectives of this project are:

1. To investigate three options to improve permeability and

compressibility properties of Australian bagasse pulp pads. The options

considered were:

1a: To use pulp derived from two selected fractions of non-pith

bagasse material.


13

1b: To use bagasse obtained from two different sugar extraction

methods.

1c: To select and add flocculants to the pulp slurry prior to pad

formation.

The effect of each option on the steady-state permeability and

compressibility constants is quantified. The bagasse pulp from this

study is compared to bagasse pulp produced industrially and also

eucalypt pulp.

2. To develop a dynamic filtration model that is well suited for Australian

bagasse pulp using the steady-state permeability and compressibility

parameter data experimentally obtained in (1).

3. To validate the dynamic model obtained in (2) by performing dynamic

filtration experiments with the pulp samples derived in (1).

The outcomes of the experimental and modelling filtration study, along

with supplementary pulp physical and chemical property testing, provided

guidance on the suitability of Australian bagasse pulp for various paper

products.

The relationship between objectives 1, 2 and 3 are shown in Figure 1.10.


14

Figure 1.10 Flow diagram of the relationship between the project

objectives.

1.4. Statement of novelty

To the author’s knowledge, there is no information on bagasse pulp pad

compressibility and permeability in the open literature. The study reported in

this thesis is a first attempt at investigating the permeability and compressibility

properties of bagasse pulp.

A bagasse pulp filtration study was performed as this has not been

studied extensively as will be shown in the literature review. Experimental

work and complementary mathematical modelling were used.

In this section, the novel aspects of the experimental study and

complementary theoretical modelling work are reported in light of the existing

body of knowledge described shortly in the literature review (Chapter 2).

Objective 3: Dynamic model validation

Australian bagasse pulp

preparation

Pulp from fractioned bagasse from a mill or diffuser with and without

flocculants added

Objective 1: Steady-state experiments

Experimentally obtain the steady-state compressibility and permeability parameters for each pulp and statistically

compare the pulp samples.

Objective 2: Dynamic filtration modelling

Use steady-state permeability and compressibility data in a dynamic filtration

model to predict the dynamic filtration behaviour

Dynamic

filtration

experiments

Verification of

dynamic filtration

model

Commercial

bagasse pulp

from Argentina

Australian

eucalypt

pulp


15

The novelty of the experimental work

This study includes an investigation into whether three treatment options

have any influence on the steady-state permeability and compressibility of a

bagasse pulp pad. These options are the choice of mill or diffuser bagasse,

fractionation of the depithed bagasse stock prior to pulping and the addition of

flocculants to bagasse pulp slurry.

Several experimental rigs were specially designed and fabricated during

the course of this project in order to investigate the material properties of pulp

pads.

The first experimental rig, a ‘permeability cell’, tested the steady-state

permeability parameters of bagasse and eucalypt pulp pads. The collected data

were verified in dynamic filtration experiments using a second purpose built

experimental rig, namely a ‘compressibility cell’. This approach is different

from that used by previous authors. The approach in this study is believed to be

unique because the steady-state permeability data is measured using one rig and

verified using a dynamic filtration model in a second rig. In recent studies by

other workers, the steady-state permeability and compressibility parameters are

inferred directly from dynamic filtration experiments alone.

A third piece of experimental equipment, a ‘modified Dynamic Drainage

Jar’ was purpose built to optimise a flocculant system that is suitable for

bagasse pulp. The equipment was used to vary the flocculant addition rate,

shear and the level of vacuum. Their effect on the fines retention and the

drainage time of a pulp slurry was measured. This flocculant system that was

optimised for use with a pulp slurry was then used to make pulp pads.

Quantifying the effect of microparticle and polymeric additives (i.e.

flocculants) on bagasse and wood pulp pad steady-state compressibility and

permeability parameters has not been performed before. Normally, the

effectiveness of flocculants as drainage aids is almost always measured by the

‘freeness’ of a pulp slurry rather than by pad permeability. This is because

measuring freeness is a simple experimental procedure compared to measuring

pad permeability and compressibility. Pad permeability more accurately


16

represents paper mat filtration than the freeness test. The effect of these

additives on the dynamic filtration behaviour is also studied for the first time.

In addition to the novelty resulting from the filtration study, the type of

laboratory reactor used to produce the majority of the pulp samples (i.e. a

‘flow-through’ reactor) has not previously been used for processing bagasse.

The novelty of the modelling work

Regarding the mathematical modelling work, a review of the open

literature indicated that a dynamic filtration model has not previously been

developed and verified using a non-wood pulp, such as bagasse pulp. The

values for the various permeability and compressibility factors which are

required in the dynamic model presented in this thesis are determined, both for

bagasse pulp and wood pulp. It is believed that the model developed in this

study is valid over a wider range of compression rates than many existing

models. The range of compression rates for which other models are valid is a

potential contribution for a further study, but this is outside the scope of this

study.

1.5. Summary of thesis chapters

Chapter 2 presents theory and a literature review for a study into bagasse

pulp filtration, setting this project in the context of the wider body of

knowledge. Areas covered include bagasse pulping, chemical additives,

filtration theory and experimental equipment used by other workers for

studying paper formation. The gaps in the body of knowledge are identified.

Chapter 3 presents the experimental equipment and procedure for the

project. The chapter describes how the bagasse pulp samples were prepared,

the chemical and physical properties of the pulp samples and the equipment and

procedure for the steady-state permeability and compressibility testing. The

method used for coding and verifying the dynamic filtration model is explained.

Finally the approach for the chemical additives testing is described.

Chapter 4 gives detailed results and discussion from this experimental

and modelling investigation. The pulp characteristics are provided and


17

compared to previous workers. Data is collected to test for differences between

pulp samples and for use in verifying a dynamic filtration model. The findings

of a chemical additives study are presented.

Chapter 5 summarises the main conclusions resulting from the work, and

areas of further research are suggested.

Appendices are attached with supplementary material. The material

relating to the filtration modelling is in Appendix A. Information on the pulp

samples prepared in this study is in Appendix B. Numerous supplementary

photographs were taken during the experimental investigations to further

illustrate the experiments (Appendix C). Student’s t test is most commonly

used for statistically comparing populations of pulp samples, the table of t

values is provided in Appendix D. Data on the pulp fibre length distributions

are provided in Appendix E. Appendix F is the engineering drawings of the

equipment used for measuring pulp pad compression.


18

This page is deliberately blank.

Chapter 2 - Theory and Literature Review

19

Chapter 2

Theory and Literature Review

This chapter describes the theory and literature that is relevant to a study

on bagasse pulp filtration.

This chapter proceeds by describing the background literature related to

bagasse pulp (section 2.1), and bagasse pulp’s properties (section 2.2). Pulp

permeability and compressibility theory is discussed (section 2.3). The use of

chemical additives for improving flocculation and drainage of pulp suspensions

is reviewed (section 2.4). A summary of the theory and literature review is

presented (section 2.5).

2.1. Background

Bagasse can be used to manufacture various pulp types including

mechanical pulp (6), semichemical pulp (30), and chemical pulp (31). It is used

to manufacture various paper grades including newsprint (32-36), fine papers

(36, 37), tissue and packaging grades (38).

Due to the wide range of applications, the body of literature on bagasse

pulping is large (4-8, 26, 28, 30, 32-63). The consensus is that pith

detrimentally affects most properties of the pulp, particularly the filtration

properties and that good depithing is essential for high quality bagasse pulp.

The strength properties of paper sheets made from bagasse pulp are generally

weaker than for those made from equivalent hardwood fibres. The body of

literature pertaining to bagasse paper formation is much smaller (39-41, 50, 53,


20

64) and is focussed on the use of chemical additives that can be used to

improve the filtration properties. It is the authors view that despite the

consensus that pith adversely affects the filtration properties of bagasse pulp,

there is a significant gap in the literature when it comes to scientifically

quantifying its effect. This was one of the main objectives of this study.

A review of bagasse utilisation for the manufacture of paper, board and

composite materials reveals that the groundwork for bagasse paper manufacture

was laid between 1960 and 1980. Josef E. Atchison stands widely recognised as

the authority on bagasse paper manufacture having provided a significant

contribution to the literature over an extended period (see for example 8, 32, 33,

43, 44). His work is very well known amongst the non-wood pulp researcher

community.

Until recently, the focus of research into bagasse papermaking in

Australia has been on maximising pulp strength properties. The most

comprehensive work on Australian bagasse paper manufacture was published

in the early 1980s by Gartside and co-workers (28, 51, 65). The work by

Gartside and his group focussed on the relatively poor strength of bagasse pulp

caused by the milling process. These workers concluded that a change to the

milling process is required for Australian bagasse to be used as feedstock in

paper manufacture. This conclusion was misleading and incorrect. The

following important factors were not considered in the work of Gartside and his

group:

� Pulp strength, although usually important, is not critical to meet the

specific demands of many pulp and paper products. For example water

absorption is critical in fluff pulp products. High pulp strength is

detrimental to tissue softness and a balance between softness and strength

is required. Bagasse pulp is usually well suited to both of these

applications;

� The drainage and retention properties of the pulp. In the opinion of this

author, these properties are considered to be of more importance for the

economics of a bagasse paper mill than pulp strength;


21

� The mode of sugar/juice extraction from sugarcane;

� The pulping process employed; and

� The availability of specialised cane varieties, with stronger fibre strength.

In addition to the above list, some technical developments within the

sugar and pulp & paper industries have emerged since Gartside’s work, such as

the advent of anthraquinone as a soda pulping additive, used in this study, and

the development of cane varieties that are better suited to paper manufacture.

Following chronologically on from the work of Gartside and co-workers,

Edwards in 1990 (66), investigated alternative juice extraction technology in an

attempt to improve pulp strength properties. Although this approach improved

pulp strength, it resulted in poor juice extraction.

There is a gap in the literature of Australian bagasse paper research

between Edward’s work in 1990 until other works by the author of this thesis,

i.e. Rainey, and his colleagues, Doherty and Lavarack commencing 2003 and

continuing until today (55, 67-69). These researchers investigated multiple by-

products from the pulping of bagasse such as organosolv lignin and soda lignin

and ethanol in addition to paper pulp. These studies were mainly focussed on

making co-products and bagasse pulp rather than converting bagasse pulp into

paper.

2.2. Bagasse pulp properties

Apart from the work by Gartside and co-workers, pulp property data used

in this thesis is largely drawn from three excellent articles; Giertz and Varma

(4), Paul and Kasiviswanathan (70) and Triana and co-workers (71).

2.2.1. Bagasse pulp yield

The unbleached pulp yield for soda bagasse pulp after screening is

reported by numerous authors (4, 28, 70, 71) in Table 2.1. There is variability

in the reported pulp yield, from 50% (71) to 61.4% (28). The yield reported by

Gartside (28) is significantly higher than found by most other researchers. The


22

fibres used by Gartside have different morphology which is discussed later in

this chapter.

Table 2.1 Soda bagasse pulp yield reported by various workers.

Kappa

number, -

Screened

yield, %

Rejects, %

Paul (3.8:1 fibre to pith) (70) 14 54.4 0

Giertz and Varma (4)

(14% NaOH, soda pulp, 30 min) 20 51 7.6

(14% NaOH, 60 min) 18.5 56.9 2

Gartside (Cane variety NCo310)

(28)

21.7 61.4 Not reported

Triana (71) 29 50 Not reported

2.2.2. Bagasse pulp fibre morphology

Useful chemical characterisation and fibre morphology data are provided

by Triana and co-workers (71). The data is for pulp derived from both

conventional cane and ‘energy cane’ varieties. Energy cane is bred for its high

fibre content rather than its sugar content. Selected data is reproduced in Table

2.2 along with some fibre morphology data reported by Gartside and his group

for Australian bagasse pulp (28). The narrower width of the fibres reported by

Gartside and co-workers is due to a smaller lumen rather than due to a thinner

cell wall.

The bagasse pulp fibres measured in the detailed morphology studies by

Gartside and co-workers (28) and Triana and co-workers (71) were

substantially longer and narrower than reported in a number of other works,

such as (50, 64, 65). It is presumed that the average fibre lengths reported in

(28) and (71) were obtained from pulp containing mainly sclerenchyma

material and that the lengths of a limited number of fibres (typically only 1000

fibres) were measured manually with a projection microscope. However, other

authors reported the average fibre length as measured by an automated image

capture and analysis (e.g. Kajaani Fibrelab apparatus (50, 64)).


23

Table 2.2 Fibre morphology data of washed conventional cane and

energy cane by previous workers.

Fibre

length

(mm)

Fibre

diameter

(�m)

Lumen

diameter

(�m)

Wall

thickness

(�m)

Ja 60-5 and Ba. 43-26 (71) 1.1-1.3 22-23 10.8-13.1 4.9-5.6

CSR6-81 (71) 1.4 22 12 5

Triton (28) 1.43 17.6 8.1 4.8

NCo310 (28) 1.33 13.6 4.1 4.8

(65) 1.11

Co

nv

enti

on

al c

ane

var

ieti

es

(50, 64) 0.73

Energy

Cane

C90-176 and

C90-178 (71) 2.0-2.1 23 13 5

2.2.3. Chemical character of bagasse pulp fibres

The chemical composition of bagasse pulp and the parent bagasse has

been reported by several authors. The characterisation method used by Triana

and co-workers (71) is the most similar to that used in this thesis. Their results

are reproduced below (Table 2.3). The cellulose component of the bagasse is

around 47% for conventional cane and increases to 73% for bagasse pulp.

Giertz and Varma (4) noted that the ash content decreased with increased

depithing.

Table 2.3 Chemical characterisation of washed cane and unbleached

bagasse pulp (71).

Conventional cane Energy cane

Bagasse pulp Parent bagasse Bagasse pulp Parent bagasse

Cellulose (%) 73 47 72-73 45-46

Hemicellulose (%) 23 27 23-24 28

Lignin (%) 2.4 23 2.3 22

Ash (%) 1.7 1 1.5 1


24

2.2.4. Bagasse pulp physical properties

The physical properties of bagasse pulp are highly dependent on the level

of depithing. Both Paul and Kasiviswanathan (70) and Giertz and Varma (4)

studied the effect of pith level and pulping conditions on the strength of bagasse

pulp. Paul and Kasiviswanathan focussed specifically on the effect of pith

levels on soda bagasse pulp, which was similar to the chemical process used in

this study (i.e. soda AQ). The study by Giertz and Varma looked at the effect

of pith level incidentally to the focus of their research and had similar findings.

The physical properties of unbleached pulp reported by various authors are

presented in Table 2.4. The results in the table published by Paul and

Kasiviswanathan are for pulps with a fibre to pith ratio similar to that used in

this study (i.e. 3.43:1).


25

Table 2.4 Physical properties of unbleached bagasse pulp reported by

previous workers.

Gartside

et al. (28)

Paul

et al. (70)

Giertz

et al. (4)

Triana

et al. (71)

Freeness CSF (mL) 540 250 313 558 558 313

Tensile Index (Nm.g) 85 112 70 62.9 28 40

Tear Index (mN.m2/g) 6.2 5.55 5.5 6.1 5.2 5.1

Burst Index (kPa.m2/g) 6.1 3.6 1.4 2.4

Apparent Density (g/cm3) 0.664

Paul and Kasiviswanathan (70) varied the level of depithing and report

the response of the bagasse pulp physical properties. In their study, the ‘fibre to

pith’ ratio is reported. The fibre to pith ratio is 1.8:1 to 2.0:1 for ‘whole

bagasse’, 2.6:1 to 2.8:1 for moist depithed bagasse, 3.0:1 to 3.8:1 for wet

depithed bagasse. The pulp samples were refined to 40 Schopper-Riegler

freeness. Some results from their study are reproduced in Table 2.5. In their

study, as the level of depithing increased (i.e. higher fibre to pith ratio), the

initial pulp freeness increased and the strength properties increased.

Table 2.5 Effect of depithing on unbleached bagasse pulp properties

(70).

Fibre to pith ratio 0.86:1 1.84:1 2.25:1 2.79:1 3.43:1 3.8:1 5.2:1

Initial pulp freeness (SR) 53 40 35 32 29 26 24

Final pulp freeness (SR) 53 40 40 41 39 40 40

Burst ‘factor’ (-) 33 31 32 34 36 36 41

Tear ‘factor’ (-) 36 43 49 51 55 58 68

Breaking length (km) 6.2 6.5 6.7 6.8 7.0 7.2 7.7


26

2.3. Pulp permeability and compressibility parameters

There is a significant body of literature where the compression of bagasse

was investigated by Australian workers, such as by Kent (72-74). This work

was performed with the intent of increasing sugar extraction from the sugar

milling process. The compression properties of bagasse are similar to that of

soil (75). The mechanisms involved in dynamic pulp pad filtration, particularly

particle collapse and flexural stiffness, are quite different to those that occur

during soil consolidation. To this end, the literature review focussed on

permeability theory generally, and also on pulp pad filtration more specifically.

The discussion of pulp pad permeability and compressibility that follows

has been divided into general steady-state permeability theory (section 2.3.1)

and steady-state compressibility theory (section 2.3.2), dynamic filtration where

permeability and compressibility are interdependent and are a function of time

(section 2.3.3) and the special case of non-Darcy flow (section 2.3.4).

Equipment which has been used previously for measuring the permeability and

compressibility of pulp pads is outlined (2.3.5). Additional theory which is

relevant to this study is presented (section 2.3.6).

2.3.1. Steady-state permeability theory

2.3.1.1. Darcy’s Law

The theory of steady-state laminar flow through a homogeneous porous

media is based on Darcy’s Law.

In 1856, Henry Darcy, developed an empirical law for the design and

construction of water distribution systems in Dijon, France. In his simple

experiment sand was packed into a column (see Figure 2.1) and water was

pumped through it.


27

Figure 2.1 Sketch defining the parameters used in Darcy’s law for flow

through a homogeneous rigid porous media.

Darcy measured the flow rate through the packed bed and measured the

pressure drop between the top of the bed and the bottom of the bed, �P, with

manometers. The resulting equation, Darcy’s original correlation is

( )L

P'K

A

Q

∆

∆=

Equation 2.1

where Q is the volumetric flow rate through a bed of porous material with

cross-sectional area A, �P is the frictional pressure drop across the length (�L)

of the porous media bed, and K’ is a permeability constant which is dependent

on viscosity.

Although Darcy’s law is useful in this form for water, K’ is dependent on

the viscosity as well as geometric factors. In order to separate the dependence

of the permeability constant on viscosity, �, K’=K/� is substituted where K is a

permeability constant that is independent of water viscosity (76, 77). K is

dependent on the shape, arrangement and porosity of the rigid material. This

amendment to Darcy’s correlation (Equation 2.1) became known as Darcy’s

Law (Equation 2.2) (77).

L

PK

A

Q

∆µ

∆=

Equation 2.2

p2 (mPa)

p1 (mPa)

�L (cm)

� (mPa)

Q (cm3/s)

Q (cm3/s)

Rigid porous

material


28

2.3.1.2. Relating K to porosity, �; the Kozeny-Carman equation and its derivation from Poiseuille’s Law

In paper manufacture, the permeability of a pulp mat is affected by the

varying concentration and hence porosity of the developing pulp mat as well as

by the structural arrangement of the mat. There are a wide range of steady-state

permeability models relating Darcy’s permeability to the porosity. Scheidegger

(77) gives an exhaustive discourse on the various models that have been

derived. These correlations are broadly classified into (i) empirical

correlations; (ii) capillaric models; and (iii) hydraulic radius theories.

By far the most commonly used correlation between Darcy’s permeability

constant, K, and porosity for pulp and paper research is the Kozeny-Carman

equation (78-80).

The Kozeny-Carman equation is based on hydraulic radius theory.

Kozeny originally proposed the basic theory in 1927 which was significantly

modified by Carman in 1937 and 1956. The original form of the equation is

usually attributed to Kozeny due to his experimental work, although Blake (as

reported in 77) had previously derived a similar form of the equation from

Darcy’s law. The assumptions of all hydraulic theories include (77):

• Fluid motion occurs like motion through capillaries

• No tangential component of the fluid velocity

• No pores are sealed off

• The pores are randomly distributed

• The pores are uniform in size

• The porosity is not too high

• Diffusion phenomena are absent

The derivation of the Kozeny-Carmen equation starts by assuming that

flow through porous media can be approximated by assuming that the liquid


29

flows under laminar conditions through a series of parallel circular capillaries

(i.e. fixed parallel cylindrical tubes) as shown in Figure 2.2.

Figure 2.2 Sketch to illustrate the derivation of the Kozeny-Carman

equation.

Using Poiseuille’s law for a single capillary where ue is the actual

velocity through the capillary, Le is the length of the capillary, �P is the

pressure drop across the capillary and d is the diameter of the capillary

e

2

eL

P

32

du

∆

µ=

Equation 2.3

By definition, the ratio of surface area to volume (m*) of a capillary is

4

d

dL

Ld4*m

e

e

2

=π

π=

Equation 2.4

So d=4m* and substituting into Equation 2.3 gives

Solid material

Hollow capillaries


30

e0

2

eL

P

k

*mu

∆

µ=

Equation 2.5

Where k0 is a constant.

The average (i.e. superstitial) velocity across the bed of porous material,

u, is lower than the velocity in the capillary. Also the length of the capillary,

Le, is longer and more tortuous than the actual length of the pulp pad, L. i.e.

L

Luu e

eε

=

Equation 2.6

Substituting Equation 2.6 into Equation 2.5 yields

L

P

k

*m

L

Lu

0

22

e

∆

µ

ε��

��

�=

Equation 2.7

Defining a new constant, k

2

e0

L

Lkk �

�

��

�=

Equation 2.8

And now applying the definition of m* over the whole of the porous

material,

( ) vS1particlesof areasurface

volumepore*m

ε−

ε==

Equation 2.9

( ) L

P

1kS

1u

2

3

2

v µ

∆

ε−

ε=

Equation 2.10


31

This on comparison with Darcy’s Law implies the Kozeny-Carman

equation which is Equation 2.11.

( )2

3

2

v 1kS

1K

ε−

ε=

Equation 2.11

where the porosity (i.e. the void fraction) of the material is �, the specific

surface area of the material (i.e. surface area per unit volume of porous

material) is Sv, and k is known as the ‘Kozeny factor’. The Kozeny factor, k, is

often referred to as a shape factor because it depends on the orientation and

interconnectivity of the channels. Unlike K, Sv is independent of concentration.

For this reason, Sv is more useful than K to compare the permeability of pulp

samples over a wide concentration range. Sv is used extensively in this thesis to

compare the steady-state permeability properties of pulp samples.

Values for Sv are widely reported in the literature for wood pulp (81-86).

The earliest reported values were by Robertson and Mason (86) for a sulfite

wood pulp. Sv was reported to be 2300 cm-1 for pulp that has never been dried

and 4100 cm-1 for pulp that has previously been dried. Gren (84) investigated

Sv as a function of kappa number for a sulphate wood pulp. Values of Sv were

reported to be between 2000 cm-1 and 3000 cm-1. These previous findings are

discussed in more detail when they are compared to the bagasse pulp measured

in this study (Chapter 4).

The most common variants of the Kozeny-Carman equation as presented

in Equation 2.11 include: using the solidity of the porous material � instead of

the porosity (� = 1 - �); separating the generalised specific surface area Sv into

a specific surface area multiplied by a tortuosity factor �, Sv2�

2; and simplifying

the kSv2 expression into a single term. The Kozeny-Carmen factor, k is often

applied as a constant for pulp fibres, 5.55, which was measured by Brown (87).

Despite its age this permeability model is still used today because of its

simplicity and accuracy. The Kozeny-Carman model is still being tailored

today for applications in a wide range of industries, such as in the coal industry

(88) and for groundwater management (89).


32

A common criticism of the Kozeny-Carman model is that k is actually a

function of porosity, and hence is variable. Soon after its inception, it was

found by Davies (90) and experimentally verified by Ingmanson and co-

workers (81), that for fibrous materials it can be represented by:

( )( )[ ]3

2/1

3

1b11

ak ε−+

ε−

ε=

Equation 2.12

where a = 3.5 and b = 57 for fibrous materials. Other authors use a=4.0,

b=57 (81, 91) for rigid materials.

Notably, the value of Sv was found to vary depending on whether k was

fixed (at 5.55) or whether it varied according to Equation 2.12. Ingmanson (81)

found that for a wood pulp, for a fixed value for k, Sv was 4200 cm-1 but for a

variable k, Sv was 2900 cm-1.

Other criticisms of Kozeny’s approach include the use of arbitrary

correlations by many authors to reproduce results experimentally as well as

failure of the model under conditions of turbulence, high porosity or tangential

flow. Many modifications to the Kozeny model exist including those concisely

reported by Mauret and Renaud (92) which are all adaptations of the Kozeny-

Carman equation (Equation 2.11), mostly incorporating corrections for k (such

as Davies’s Equation 2.12).


33

Table 2.6 Reproduction of Kozeny constant correlations (92).

2.3.1.3. Alternatives to the Kozeny-Carman equation for steady-state permeability

Dullien (93) provides a good summary of the relationship between

permeability and porosity proposed by various investigators, part of which is

reproduced below. These correlations are all of a similar form to Kozeny’s

correlation. Kozeny compared his experimental data and his correlation with

that of Zunker and Terzaghi. Carman (1937) acknowledges Slichter as having

made a major contribution to the development of steady-state permeability

correlations.

Table 2.7 The form of the relation between Darcy’s permeability factor

and porosity developed by various workers (93)

Relation Author

�3.3 Slichter (1898)

�/(1-�)2 Zunker (1920)

[(�-0.13)/(1-�)1.3]2 Terzaghi (1925)

�3/(1-�)2 Blake (1922), Kozeny (1927), Carman (1937, 1948)

The only known work into measuring the permeability constant of

bagasse pulp was undertaken recently by El-Sharkawy and co-workers (50, 64)

for a commercial Indian pulp. In this work, an axial feed pressure screen was

used to process the pulp and the improvement in pad permeability was

measured using a pulsed ultrasound Doppler anemometer.


34

The form of the permeability model used by these authors is not

commonly used; the permeability constant is proportional to the first power of

the porosity in the numerator. This is the form first developed by Zunker viz

( )21

'kKε−

ε=

Equation 2.13

where k' is a permeability constant.

El-Sharkawy and co-workers found the permeability constant for their

commercial bagasse pulp was 2.36×10-9 cm2. The work of El-Sharkawy and

co-workers is discussed further when their data is compared with the data

generated in this study (i.e. in Chapter 4).

There are a number of equations relating porosity to Darcy’s permeability

constant that are quite different to those discussed so far. One such correlation

is the Happel equations. Happel derived his equations directly from the Navier

Stokes equations for flow over an array of parallel cylinders both parallel and

perpendicular to flow (94). There is a fundamental difference to the approach

of Happel and Kozeny-Carman in that Happel derived the equations for flow

over an array of cylinders whereas the Kozeny-Carman model is derived from

Poiseulle’s law for flow through capillaries.

For flow parallel to an array of cylinders, Happel’s equation is (95)

( )( ) ( ) ( )( )2

2

11431ln2132

DK ε−−ε−+−ε−−

ε−=

Equation 2.14

For flow perpendicular to an array of cylinders, Happel’s equation is

( )( )( )

��

�

�

+ε−

−ε−+��

��

�

ε−ε−=

11

11

1

1ln

132

DK

2

22

Equation 2.15


35

Happel’s approach was further developed by Jackson and James (96). In

reality, assuming all the pulp fibres are cylindrically shaped, most pulp fibres

are aligned roughly perpendicular to flow and there is some component of the

fibre being aligned parallel to flow. Jackson and James used Happel’s

equations to determine the permeability of cylinders arranged in a three

dimensional cubical lattice (96). If flow passes axially to a perfect cubical

lattice, exactly two thirds of the cylinders are perpendicular to flow and exactly

one third of the cylinders are parallel to flow (see Figure 2.3). Thus

( ) ( )( )φ+φφ

= 231

132

2

FF32

DK

Equation 2.16

where

( ) ( ) ( ) ( )2

1 43ln2F φ−φ+−φ−=φ from Equation 2.14

( ) ( )( )1

1lnF

2

2

2+φ

−φ+φ−=φ from Equation 2.15

Figure 2.3 Sketch illustrating Jackson and James (91) approach to

developing their steady-state permeability model.

2.3.2. Steady-state compressibility theory

Only one compressibility model for pulp pads was observed in the

literature. The various authors use variants on the following model


36

Ps = M c N

Equation 2.17

where c is the pulp concentration and M and N are experimental

constants.

This compressibility model is used universally for steady-state conditions

by the authorities in the field, such as Ingmanson in particular and also Gren

(81, 82, 84, 85). For comparative purposes, it is normally essential to rearrange

the compressibility factors used by each author into M and N.

2.3.3. Dynamic filtration theory

Dynamic filtration modelling was performed in this study so that pulp

processing equipment can be designed specifically for bagasse pulp, although

this is beyond the scope of this study. Also, for this study, steady-state

approximations are adequate for the initial phases of the investigation, but to

increase similitude with industrial paper making dynamic modelling is

considered. Dynamic filtration modelling is currently in vogue in the modern

pulp and paper literature.

2.3.3.1. The continuity and Navier-Stokes equations

A brief background on the origins of the fluid mechanics equations is

provided below. This thesis uses a filtration model built from these equations.

In summary both the equations describing the conservation of mass, the

continuity equations, and the conservation of momentum, the Navier-Stokes

equations, are required to fully describe the fluid kinematics and interaction

between pulp permeability and compressibility.

The equations of continuity are:

0vt

=•∇ρ+∂

ρ∂

Equation 2.18

recalling that divergence is defined as


37

z

v

y

v

x

vv

∂

∂+

∂

∂+

∂

∂=•∇

The form of the Navier Stokes equations for incompressible flow is

gVPDt

VD 2

s ρ+∇µ+−∇=ρ

Equation 2.19

The derivation of the equations of momentum start with Cauchy’s

equation which was improved first by Navier in 1822 (97) and further refined

by Stokes in 1845.

The left hand term in Equation 2.19 is the acceleration term (or inertial

term), the right hand side of Equation 2.19 consists of the pressure gradient

term ( P∇ ), the viscous force term ( V2∇µ ) and the gravity term ( gρ ).

Assuming incompressible laminar flow through a cylindrical pipe,

Poiseuille’s Law can be derived from the Navier Stokes equations. For this

derivation, see (98)

Equation 2.19 is for incompressible flow of a single phase media. Pulp

mats are compressible two phase porous media so a number of amendments are

made. The momentum balance on the fibres and the fluid must be considered

separately and take into account the porosity of the structure. There exists

momentum transfer between the phases and the generalised viscous stress

tensor is used (i.e. �s instead of �). In summary, for a porous media such as a

pulp pad, the momentum equations are applied to both the fibres and the fluid.

The general formula for momentum balance on the fibres is

( ) isssfs mgPPDt

uD+φρ+τφ•∇+∇−∇φ−=φρ

Equation 2.20


38

Where u is the velocity of the fibres, �s is the viscous stress tensor. The

subscript f refers to the fluid phase and the subscript s refers to the solid phase

(i.e. the fibre). mi is the interphase momentum transfer

The general formula for momentum balance on the fluid is

( ) issff mg)1(P)1(Dt

vD)1( −φρ+τφ−•∇+∇φ−−=φ−ρ

Equation 2.21

Where v is the velocity of the fluid.

These equations are used to develop the governing equation used in this

thesis (section 2.4).

2.3.3.2. Filtration through compressible pulp mats and pads

There are a number of papers that have derived equations to calculate the

mechanics of fluid flow through compressible pulp media including porosity,

permeability and compressibility for various conditions (81, 95, 99-102).

Landman and co-workers (100) provide the governing equations for the basic

fluid mechanics of 1-D compressible porous media that can be determined

experimentally in a 1-D flow cell (Figure 2.4). The governing equations are

derived from the continuity equations, Navier-Stokes equations, Darcy’s Law

and the compressibility relation above. Landman and co-workers consider the

cases where (i) the initial pulp slurry is initially networked, as in a pulp pad, or

unnetworked, as in a pulp slurry, and (ii) the equipment used is a constant

pressure device (i.e. the piston exerts a constant force at variable rate as the pad

forms) or constant rate device (i.e. the piston exerts a constant rate despite the

increasing resistance as the pad forms). The four practical cases for which she

derives the solidity are:

• initially unnetworked fibre suspension & constant pressure

apparatus;

• initially unnetworked fibre suspension & constant rate apparatus;


39

• initially networked fibre suspension & constant pressure

apparatus; and

• initially networked fibre suspension & constant rate apparatus.

Figure 2.4 Schematic of a 1-D flow cell where a piston is expressing

liquid through an initially un-networked suspension.

This thesis considers the permeability and compressibility of a pulp pad

using an initially networked model.

2.3.4. Non-Darcy flow

The above equations all assume laminar flow. For flow at a sufficiently

high velocity, turbulence occurs and the more complex Forcheimer equation

can be used instead of Darcy’s Law (103).

2

vv*K

P βρ+µ

=∇−

Equation 2.22

Where � is an experimental constant and K* is a permeability constant

that is analogous to that used in Darcy’s Law.


40

2.3.5. Equipment used in filtration studies

This thesis uses an initially networked model for the permeability and

compressibility study since the steady-state permeability and compressibility

parameters. These parameters can be measured using simple equipment (see

sections 2.3.5.1). However, optimisation of a good chemical additives system

must be performed using equipment where a slurry is used rather than a pulp

pad, i.e. the pulp is initially un-networked. The equipment used for these

studies is described in section 2.3.5.2.

2.3.5.1.Equipment used for permeability and compressibility testing of pulp pads

The equipment used by various workers into the permeability and

compressibility testing of pulp pads are fairly simple (81-86). Sometimes, the

focus of the investigation is on pad permeability only (86). Some

investigations attempt to measure the compressibility and permeability

properties simultaneously (81-85). The equipment in all of these investigations

involves loading a pulp slurry into a (most commonly) transparent vessel. For

permeability studies, the pressure gradient is measured by manometers as well

as the water flow rate. For compressibility studies, the response of a hydraulic

or mechanical load to the height of the pulp pad is measured.

Figure 2.5 shows the cell used by Robertson and Mason which is fairly

typical of permeability and compressibility studies. In this particular

arrangement, the pulp is loaded into a 40 mm cylinder which is bounded by a

plunger with a reinforced 100 mesh screen at the top of the pad. The flow rate

is measured by timing the drop in level of a measuring cylinder, and the

pressure head is measured by the difference in fluid height between the cylinder

and a side arm.


41

Figure 2.5 Sketch of a permeability cell used by Robertson and Mason

(86).

For the case of measuring compressibility simultaneously, the plunger

compresses the pulp pad in the experiment and there is continuous monitoring

of the flow rate and pressure.

In previous permeability and compressibility studies, the temperature of

the water was not reported, so it is assumed the experiments were carried out at

ambient temperature. The author acknowledges that industrial paper forming

occurs at elevated temperature which affects pulp pad permeability and

compressibility. Ambient temperature was used in this study in order to be

consistent with that used by previous workers for comparative purposes.

In reality, paper manufacture involves thin pulp mats rather than thick

pulp pads. Experimentally measuring the permeability and compressibility of

thin pulp mats is difficult, and beyond the scope of this study. For this reason,

previous authors, as well as this author, measured the permeability and

compressibility of pulp pads because it only requires very simple equipment. It

is common practice to use the results from experiments with pulp pads to

represent the behaviour of pulp mats.


42

2.3.5.2.Equipment for filtration studies of fibre suspensions

A review of equipment used previously for accurately simulating sheet

formation on a paper machine, from pulp slurry, was conducted. For this study,

equipment that was well suited to testing the effects of chemical additives under

shear conditions and vacuum was investigated.

A Dynamic Drainage Jar (DDJ) is a simple stirred vessel into which a

dilute pulp slurry and flocculants are added. The water and fine fibrous

material passes through a permeable screen with 75 �m holes and the fraction

of fines retained above the screen is measured. By increasing the stirrer speed

(i.e. shear), the effectiveness of flocculants under high shear conditions can be

determined. Flocculation effectiveness is measured by the retention of fine

fibrous material. A higher level of retained fines over a wide range of stirrer

speeds means that flocculation has improved. The DDJ is most commonly used

to test the effectiveness of various flocculants and so it is discussed in more

detail in section 2.4. A DDJ was used in this study.

Figure 2.6 Sketch of a Dynamic Drainage Jar.


43

Many modifications of the DDJ exist for simulating the industrial

forming process. Modifications of the DDJ are the subject of a number of

papers (for example 104, 105). Modifications of the DDJ usually permit pad

formation to look at the combined effect of mechanical entrapment and

colloidal interaction.

The DDJ was modified by Britt and Unbehend in 1980 (106) to measure

the dryness of a sheet after exposure to a controlled vacuum. The vacuum was

applied to simulate the suction created on the Fourdrinier former; however, it

was not a pulsed vacuum. Britt and Unbehend (106) describe a method for

testing a dynamic drainage rate. An observation was that over-flocculation

created channels in the fibre pad, improving the initial drainage rate but once

the water was removed from the interstices of the pad air was sucked through

the sheet when vacuum was applied, resulting in higher final sheet moisture

content. Pulp suspensions of lower initial drainage rate tended to form more

consolidated and uniform sheets which when subjected to vacuum resulted in a

sheet of lower moisture content.

In 1982, the application of vacuum to a modified DDJ was automated

(107). In trials, Britt applied vacuum for 5 s and measured the final consistency

of the pad. In 1985, Britt further illustrated that pulp which drained quickly had

poor final dryness when vacuum was applied and that a certain level of fines

can improve the final sheet dryness (108). The following mechanism was

suggested: when the shortest fibres are mobile with respect to the fibre pad, the

fines migrate to the interstices of the forming web, sealing or plugging some of

the openings and slowing drainage but when the fines are flocculated and

attached to fibres, they are no longer free to migrate to the interstices and

drainage is not impeded.

The modification by Forsberg, called the “Dynamic Drainage Analyser”

(DDA) in (104) involved a microprocessor which served two functions; to

control (i) the duration of chemical addition in order to investigate contact time

between retention chemicals and fibres and (ii) the duration of stirring in order

to investigate different shear conditions. In this arrangement, it was intended to

form a pad to investigate mechanical entrapment of fibres as well as the


44

colloidal interaction. See Figure 2.7, for an illustration of the arrangement.

The DDA provided information on retention, drainage, porosity and wet web

dryness. The DDA recorded the vacuum level as a function of time. Figure 2.8

shows that the graphical output provided information on the drainage rate under

vacuum (time from point a to point c) and the final porosity of the dry sheet

(magnitude of vacuum at point d). The modified DDJ used in this study most

closely resembles Forsberg’s DDA.

Figure 2.7 Diagram of Forsberg’s Dynamic Drainage Analyser (104).

Figure 2.8 Graphical output of the DDA(104).

Although laboratory equipment that more accurately simulates the

industrial forming process exists, they came with increasing cost. In order to


45

improve the behaviour of bagasse pulp, a reasonably accurate and affordable

method of simulating the pulp dewatering process must be achieved in the

laboratory.

In order to further improve the similarity between laboratory equipment

and a Fourdrinier paper former, it was necessary to introduce pressure pulses

into the equipment. Various modifications of the DDJ attempted to incorporate

pulsed vacuum such as that described by Hubbe (105).

The modification by Hubbe, the “Positive Pulse Jar” (PPJ) (105) as

shown in Figure 2.9, introduced pressure pulses by a Bellows pump, pumping

dilutant under the jar. Previous versions had introduced pulses by vacuum

pump. The advantage of this method was that it more accurately simulated the

refluidising of the fibre mat facing the fabric, resulting in reduced fines content

in this region. The PPJ also investigated the use of a specialised rotor to

simulate uniform shear, as opposed to the random turbulence obtained in a

standard DDJ. The pressure pulses reduced retention. Importantly, the use of

the specialised rotor also resulted in reduced retention compared to the standard

impeller used by Britt.

Figure 2.9 Diagram of the PPJ and specialised rotor (105).

The Australian Pulp and Paper Institute (APPI) have pilot laboratory

equipment that more accurately simulates a Fourdrinier style paper machine;

see Figure 2.10 taken from Xu and Parker (109). It contains a moving belt with

hydrofoils attached in order to simulate the pressure pulses of a Fourdrinier

former. The equipment does not take into account the velocity profile of the

stock leaving the headbox slice.


46

Figure 2.10 Moving Belt Drainage Former (109).

Melbourne University in conjunction with CSIRO Forestry and Forest

Products (now called Ensis) also developed laboratory forming equipment that

simulates the velocity profile of the stock leaving the slice of an industrial

paper machine (110-112). This configuration is shown in Figure 2.11.

Importantly, this equipment is capable of aligning the fibres, approximating

fibre alignment on a paper machine. It does not take into account pressure

pulses characteristic of a Fourdrinier former.

Figure 2.11 Setup of the laboratory former by Helmer (110-112).

Another sophisticated piece of laboratory equipment outlined by Kataja

and Hirsila (113) should be mentioned here. Although it does not simulate the

forming process as accurately as the laboratory formers, it can be used to obtain

very detailed data about pulp pad formation; more than any other laboratory

equipment encountered. The equipment used by Kataja and Hirsila can be used

to measure the velocity of fibres at various heights in a dewatering fluid flow,


47

see Figure 2.12. This equipment is particularly useful for developing numerical

models of pulp suspension behaviour. The unit consists of a sealed tank with a

riser tube. Inside the riser tube is a wire and support grid. The suspension of

fibre is allowed to drain through the wire. The fibre is retained on the wire and

forms a fibre mat. The water level inside the riser is measured with an

ultrasonic surface detector. The vertical velocity of the fibres is measured

through the wire by four pulsed ultrasound Doppler anemometers. The signal

sent from the surface detector and the anemometers are processed and the data

is captured. The probes can measure the vertical velocity of the fibres up to

70 mm above the wire. The water level in the riser tube is adjusted by valves

V1, V2 and V3. Assuming valves V1 and V3 have adequate accuracy and

response time, the programmable logic could be modified to allow a time-

varying pressure pulse, simulating the effect of foils in Fourdrinier forming.

El-Sharkawy (50) used the equipment outlined by Kataja and Hirsila to

control bagasse pulp quality through fractionation and refining (50). This work

is the only published work with data presented on the drainage properties of

bagasse pulp.

Figure 2.12 Ultrasound anemometry for measuring filtration of fibre

suspension (113).


48

To measure the efficacy of chemical additives under both vacuum and

shear, a modified DDJ that most closely resembles that developed by Forsberg

was used for this project. It was simple to construct and gives an indication of

how chemical additives would perform under the dual effects of shear and

vacuum. The main differences are that it will involve a laptop computer to log

the data, the vacuum will be controlled by actuating a bleed valve on the

vacuum vessel and flow rate will be measured with digital scales. The

modified DDJ more closely resembles a Fourdrinier former than a Twin-wire

former.

2.3.6. Additional filtration theory of particular importance to this study

2.3.6.1.Steady state permeability theory

In the case of pulp fibres in the swollen state, a considerable amount of

water occupies the pores of the fibres. Incorporating � into the Kozeny-Carman

model (Equation 2.11) allows this study to obtain information on potential

strength generation during refining as well as permeability data. If the swelling

factor of the fibres is � cm3/g, then the porosity is related to the concentration, c

g/cm3, by � = 1 – �c.

Inserting into (Equation 2.11) and rearranging obtains

( ) ( )c1Sk

1Kc

3/1

2

v

2

3/12 α−��

��

�

α=

Equation 2.23

Plotting (Kc2)1/3 against concentration, c, will give a linear relation.

Darcy’s permeability, K, is determined from permeability experiments using

equation (1) and c is calculated from the height and diameter of the pulp pad for

a known mass of pulp. The specific surface area, Sv, and the swelling factor, �,

are calculated from the slope and the intercept of the graph. This method was

first used by Robertson and Mason (86). Sv and � are then inserted back into

equation (2) to test the agreement of the experimental data with the Kozeny-

Carman model. The Kozeny factor, k, is frequently assumed to be constant.

For randomly packed fibrous beds, k was determined to be 5.55 (114).


49

Values for � are reported in the same literature in which Sv is reported

(81-86). Reported values of � vary more than the reported values of Sv.

Ingmanson and co-workers report values as low as 1.65 cm3/g for wood pulp

whilst Robertson and Mason report � as high as 4.5 cm3/g for a sample of never

dried wood pulp.

The advantage of this method for quantifying the steady-state

permeability of pulp samples is that it requires a very simple experimental

method and also the equipment required is extremely simple. A pulp pad is

created by draining pulp slurry into a transparent vessel which is reinforced at

the bottom by a mesh. The flow rate is measured using a collection vessel and

stopwatch. The pressure drop per unit length can be measured by manometers.

The height of the pulp pad is measured to determine the pulp concentration,

assuming that the pulp concentration is approximately uniform in the absence

of significant hydraulic pressure. Darcy’s permeability constant, K, can be

determined from this data.

2.3.6.2.Steady-state compressibility theory

The compressibility equipment was designed so that the pulp could be

loaded into a cell and compressed with a permeable top platen which expresses

water. This is shown diagrammatically in Figure 2.13, indicating the pressure

on the solid phase, Ps, at the platen. The distance, x, is defined from the top

platen.

The hydraulic pressure at the top surface of the pulp pad is negligible so

the force on the fibres equals the force exerted on the platen.

The compression model used is the power law model viz Ps = M c N


50

Figure 2.13 Sketch of the compressibility cell.

The pulp concentration can be related to the solidity (that is, the volume

solids fraction), which is used in the dynamic model, by � = �c. Values for �

are in the range of 3.2-3.8 cm3/g.

For this geometry,

Ps=m �n

Equation 2.24

where m and n are experimental constants analogous to, and calculated

from, M and N.

2.3.6.3.The dynamic filtration model

This study follows the analysis of Landman and co-workers (100) for a

one dimensional constant rate filtration (i.e. platen moves with constant speed)

using an initially networked suspension. Martinez has built on this work for

pulp and paper applications (115, 116). The modifications include the

incorporation of the Kozeny-Carman steady-state permeability model (Equation

2.11) and the power law steady-state compressibility model (Equation 2.24).

The derivation of the governing equations for the filtration of a pulp pad is

provided in Appendix A.

Using the definitions presented in Figure 2.13, the dimensional form of

the governing equation for constant rate filtration is

Expressed

water

x

Depth into the pulp

mat

Ps

Height, h

Applied pressure

Loaded

pulp

Permeable

top platen

Impermeable

base


51

( )dx

d

dt

dh

dx

dD

dx

d

dt

d φ−��

�

� φφ=

φ

Equation 2.25

Where

( ) ( ) ( )�

mnK 1D

1n−φφφ−φ=φ

Equation 2.26

K(�) is the permeability as predicted by the Kozeny Carman model

(Equation 2.11). This governing equation is subjected to the initial condition

�(x,0) = �0 as the solidity is uniform throughout the cell, as well as the

following boundary conditions:

Boundary condition at the top platen

dt

dhu 0,x −==

0dx

d=

φ

Equation 2.27

Boundary condition at the base

0u 0,x ==

( )( ) 1nmn1Kdt

dh

dx

d−φφ−φ

µ=

φ

Equation 2.28

Solution of the dynamic model requires the factors m and n calculated

from steady state compression experiments, Sv and � from permeability

experiments. These equations are non-dimensionalised before being solved

(see Appendix A for the non-dimensional equations). The solution of these

equations provides values for � over the ranges of x and t.

For comparative purposes, the model predictions for � are determined at

x=0 for all t and consequently Ps is calculated (Equation 2.24). In the

experimental setup, the Instron measures the load on the top platen which is

converted to pressure. Validation of the model occurs if the experimental

pressure data matches the model predictions for solids pressure at the surface of

the pulp pad.


52

Both a constant k (k = 5.55), and variable k (Equation 2.12) with the

relevant values of Sv and �, are investigated for use in the dynamic model.

The dynamic model assumes that 100% of the fibre is retained by the

platen and also neglects friction between the platen and the side wall.

The effect of applying vacuum at the bottom boundary, which occurs in a

Fourdrinier former has the same effect as increasing the pressure at the top

boundary.

2.4. Chemical additives

As mentioned in Chapter 1, the use of an effective flocculant system in

paper manufacture increases production rates, improves paper quality and

reduces raw material requirements. A reduction in the quantity of organic

material in the effluent also improves environmental performance. In this

thesis, cationic polyacrylamide (CPAM) is combined with microparticles.

2.4.1. The mechanism of CPAM and microparticle dual polymer

systems for pulp flocculation

Cationic polyacrylamide (CPAM) is used widely as a drainage aid for all

types of chemical pulp but has been shown to be suitable for applications with a

high amount of fine fibres, such as in mechanical pulp (117). Bagasse pulp

similarly has a very high quantity of fine fibre and so CPAM was the polymer

selected for this study. CPAM’s mode of action is straightforward. The

cationic polymer attaches to the negatively charged surface of the fibres

resulting in neutralisation and flocculation.

The addition of anionic microparticles, such as bentonite or colloidal

silica can further improve flocculation by bridging the cationic flocculant

chains (see Figure 2.14).


53

Figure 2.14 Mechanism of silica microparticles (118).

The important difference between conventional polymer flocculation and

microparticle systems is that under conditions of high shear, such as those in a

papermachine headbox, the bonds formed with polymers are destroyed, but

microparticle systems have the ability to reflocculate the fibres after being

subjected to high shear.

2.4.2. Flocculant systems

A large volume of work has been undertaken in developing and

comparing chemical additives for various types of pulp (mainly wood grades)

(e.g. 107, 119, 120-126). The progression of additive chemistry has been from

the single polymer systems (pre 1970s) to dual polymer systems (1970s, 1980s)

to polymer and microparticle systems (1990s to present). The following

articles on flocculant research are all for wood pulp grades, often with high

amounts of very short fibre.

Hubbe (124) and Rojas & Hubbe (127) define three forms of chemicals

widely used as drainage additives: coagulants; flocculants; and microparticles.

Hubbe defines coagulants as compounds of high positive charge density which

act to neutralise the negative charge on fibres and ‘ionic trash’. Ionic trash is

undesirable very small fibres that are generated in mechanical pulping and has

very high surface area; bagasse pith may be considered ionic trash. Examples

of coagulants include aluminium sulphate (or alum), polyamines and

polyethyleneimine (PEI). Flocculants are polymers that link fine particles

together. Flocculants are often very high molecular weight copolymers of

acrylamide (PAM). Microparticles are very small negatively charged particles,

such as colloidal silica and bentonite, that interact with cationic flocculants (e.g.


54

CPAM) or cationic starch and further improve flocculation. Brouillette and co-

workers (128), Sherman and Keiser (129) and Ledda et al. (130) all describe

various microparticle systems.

Flocculant systems often consist of various combinations of coagulants,

flocculants and microparticles. These studies tend to focus on fibre retention

rather than drainage. This point is noted by Allen and Yaraskavitch (119).

This study gives an excellent review of the dewatering potential of a large

number of systems. They make the following pertinent observations:

microparticle systems improve dewatering in alkaline systems; CPAM’s give a

small improvement in dewatering; and several other systems (e.g. PEI,

polyDADMAC and dual polymer) improve drainage at the expense of final

sheet moisture in vacuum dewatering. Britt and Unbehend (108) also observed

this effect.

Recently Carr (123, 131-133) has strongly advocated silica nanoparticles

rather than microparticles. Carr claims that the shear resistance of a particle

attached to a surface is inversely proportional to its size i.e. the smaller the

particle, the greater the shear resistance (131). Carr claims inventorship of

nanoparticles as a flocculant. However, Duffy (from Nalco Chemicals), in

1993, had previously noted that nanometer sized silica particles were extremely

efficient (118).

Miyanishi and Shigeru (134, 135) optimised flocculation and drainage by

comparing various microparticle systems and controlling the zeta potential (i.e.

the streaming potential which is an indication of the charge of the “white”

water). Miyanishi and Shigeru looked at the effect of adding various

chemicals, (alum, anionic polyacylamide, DADMAC, bentonite and CPAM), in

various sequences on both types of pulp containing and free from ionic trash. It

was found that alum, CPAM and then bentonite was the best sequence for acid

papermaking in the presence of ionic trash, with a 6% increase in flocculation

(as measured by improved turbidity) and 65% improvement in their defined

measure of “drainage”. DADMAC, anionic polyacrylamide and then bentonite

was found to be the best sequence for alkaline papermaking, with a 7%

improvement in flocculation and 50% improvement in “drainage”.


55

Kumar also used zeta potential to improve the retention of bagasse pulp

in a less thorough study (54). Using a DDJ, the bagasse pulp was fractionated.

The best order for retention aids was found to be rosin-starch-alum-filler. The

best zeta potential for retention was found to be -5 mV.

In contrast to the studies by Miyanishi and Shigeru (134, 135) and Kumar

(54), Britt (121) found that in dynamic systems, although zeta potential

provides additional information, flocculation can be improved without any

change in the zeta potential.

As can be observed from the variability in optimised flocculant systems,

the optimum chemical additives system is dependent on the pulp and needs to

be determined on a case-by-case basis. It appears from the literature that

CPAM and bentonite should give reasonable improvements in pulp drainage.

It is noted that most articles tend to focus on the fibre retention properties

of chemical additives rather than the drainage properties, which is the focus of

this thesis.

2.4.3. Literature on flocculants used for bagasse pulp

The literature on flocculants used for bagasse pulp is limited.

Abril’s work during the 1980s is the best reported literature with regards

to developing flocculant systems for improving the drainage behaviour of

bagasse pulp (39-41). Abril’s work was published in Spanish which was

translated into English because of its relevance to this study. Abril investigated

the effect of polymer drainage and retention aids on bagasse pulp (41). In this

laboratory study Abril used a DDJ to assess a range of drainage and retention

aids namely dextran, polyethyleneimine, anionic PAM and polyamideamine.

The polyethyleneimine and polyamideamine showed the biggest improvement

in retention and the best improvement in freeness.

In a further study (40), Abril tested modified polyamideamines, modified

polyethyleneimines (PEI’s) and CPAM in the laboratory. One of the modified

PEI’s gave the best drainage properties and was tested industrially. In the

industrial trial, the headbox freeness and retention (as measured by whitewater


56

consistency) both improved, permitting the machine speed to be increased from

245-270 m/min to 300 m/min.

Ibrahem and co-workers (53) looked at PAM as a filler retention aid for

bagasse paper. The fillers investigated were titanium dioxide, silica and kaolin.

Strength data is provided for pulp containing each filler over a range of PAM

addition. PAM can improve filler retention by between 63% and 86%. It does

not contain information about the effect of PAM addition on drainage.

There is no known literature on the use of microparticles as a drainage aid

for bagasse pulp.

2.4.4. Using the Dynamic Drainage Jar as a tool for comparing

flocculants

The DDJ has been described in section 2.3.5.2. It was developed by Britt

and Unbehend in the early 1970s for comparing the effectiveness of flocculants

under the high shear conditions that exist in a paper machine. Pulp flocculants

can be tested very quickly in the laboratory using this equipment. The DDJ has

become the standard test method used by the paper industry to test the

suitability of pulp flocculants under high shear. Several TAPPI test methods

have been written that use this device.

In 1977, Unbehend, Britt’s colleague and frequent co-author, describes

the equipments use for measuring fines and colloidal retention. This paper

forms the basis of Tappi (the Technical Association of the Pulp and Paper

Industry) Test method T261 “Fines fraction by weight of paper stock by wet

screening”, making the equipment part of a standard test procedure.

In the standard test method, turbulence of the stock is maintained to

prevent pad formation (122). As shown in Figure 2.15, a stock is processed

through the DDJ giving the fines retention as a function of stirrer speed (line

A). When the same stock is processed with a strong dispersant, 50 ppm of

TAMOL 850 and the pH adjusted to 10.5 with sodium carbonate in the

presence of ultrasonic dispersion forces, a minimum fines retention line (line B)

is obtained, which is a characteristic of the stock. Britt proposed that the


57

colloidal forces are measured by the difference between line A and line B.

Flocculants raise the A line and dispersants lower it.

Figure 2.15 Retention in Dynamic Drainage Jar as a function of stirrer

speed (122).

From the literature, the DDJ is a reasonable approach to optimising a

chemical additives system.

2.4.5. Summary of chemical additives literature and theory

There is a large body of literature for pulp chemical additives. The

literature on bagasse pulp chemical additives is small and not recent. Although

microparticle systems are not very new, the effectiveness of these flocculant

systems are not reported for bagasse pulp.

The literature on the effect of chemical additives to improve the drainage

properties is not large as many studies focus solely on fines retention.

Miyanishi and Shigeru (134, 135), and Abril are the best works in measuring

the improvements in drainage caused by chemical additives. In every case,

these workers measure freeness rather than permeability which is a more

vigorous measure of drainage. As will be discussed, this thesis investigates the

compressibility and permeability of bagasse pulp. Quantifying the effect of

microparticle systems on the compressibility and permeability parameters of a

bagasse pulp has not been published before.

Stirrer speed, rpm

Fin

es r

eten

tio

n,

%


58

No work exists quantifying the effects of flocculants on the permeability

and compressibility parameters required for modelling dynamic pulp

filtration/formation behaviour.

2.5. Summary of theory and literature review

The background literature for bagasse pulping has been discussed. The

work of Gartside and co-workers (28, 51, 65) stands out as the most thorough

work performed in Australia. However previous published work with bagasse

in Australia has traditionally focussed strongly on improving the pulp strength

properties and did not consider pulp filtration properties.

The properties of bagasse pulp have been reported by numerous workers

apart from Gartside and co-workers (e.g. 71). The fibre morphology and

chemical character have been described. The pulp physical properties depend

on the level of depithing.

The pulp permeability and compressibility theory has been described for

both steady-state filtration and dynamic filtration under compression. The

Kozeny-Carman equation is the most common steady-state permeability

correlation linking Darcy’s permeability factor, K, to porosity. The power-law

compression model is the most common steady-state model for pulp pads. The

dynamic filtration model developed in this thesis is based on an initially

networked filtration model (100). The equipment designs most commonly used

in pulp filtration studies have been discussed.

The method of testing the effectiveness of flocculants using a DDJ has

been presented, along with the mechanism of pulp fibre flocculation.

This study investigates the permeability and compressibility of bagasse

pulp which has not been performed extensively. The gaps in the literature have

been identified as foreshadowed in section 1.4. Particularly, this thesis adds to

the existing literature.

� The two options for treating bagasse prior to pulping

(fractionation and the mode of juice extraction) have not


59

previously been considered with a view to improving their

permeability and compressibility properties.

� Obtaining steady-state permeability data on steady-state

equipment and confirming the data with a second piece of

equipment is a unique approach.

� Quantifying the effect of flocculants on pulp pad steady-state and

dynamic permeability and compressibility has not been previously

studied.

� Finally, a dynamic filtration model has not been previously

investigated for a non-wood pulp such as bagasse pulp.

This study is the first time that the filtration of bagasse pulp has been

directly compared to wood pulp.


60

Chapter 3

Experimental procedure and

modelling

The experimental component of the research plan was substantial. For the

filtration study three pieces of experimental equipment were constructed

specifically for the study, namely the ‘permeability cell’, the ‘compressibility cell’

and the modified DDJ (all terms are defined later in this chapter). This was in

addition to the three styles of digestion equipment used to produce the pulp

samples.

The research plan was implemented in six stages of experimentation and

modelling. This chapter proceeds in the order described by the experimental and

modelling methodology (section 3.1). The bagasse was treated and pulped using

the three types of digestion equipment, a ‘flow-through’ reactor, a ‘Parr’ reactor

and an ‘air-bath’ reactor (section 3.2). The chemical and physical properties of

the pulp were analysed as well as the fibre morphology (section 3.3). The steady-

state permeability of a pulp pad was measured using a custom built ‘permeability

cell’ (section 3.4). The steady-state compressibility of a bagasse pulp pad was

measured using a custom built ‘compressibility cell’ (section 3.5). The steady-

state permeability and compressibility of bagasse pulp was compared to numerous

benchmark pulp samples and the findings of previous workers for wood pulp.

The steady-state permeability and compressibility parameters for bagasse pulp

pads were used in a dynamic model which was coded in FORTRAN and

compared to experimental data obtained under dynamic filtration conditions

Chapter 3- Experimental procedure and modelling

61

(section 3.6). A suitable chemical additive system was optimised with a modified

Dynamic Drainage Jar (DDJ) using a bagasse pulp slurry, and the effect of

vacuum on drainage time was examined. The modified DDJ was also used to

obtain complementary information about the drainage behaviour through thin pulp

mats rather than thick pulp pads. The effect of chemical additives on the steady-

state permeability and compressibility constants of a bagasse pulp pad was

quantified (section 3.7). Finally a summary of the experimental procedure is

presented (section 3.8).

3.1. Overview of experimental and modelling methodology

The aims and objectives were achieved in six phases using the following

program of work.

Phase 1 Fractionated bagasse pulp from milled and diffuser bagasse

was prepared. A benchmark Australian eucalypt pulp and a

commercial bagasse pulp were obtained (section 3.2);

Phase 2 The physical and chemical properties of the bagasse pulp

were determined (section 3.3);

Phase 3 The steady-state permeability parameters of a bagasse pulp

pad were determined using simple permeability

experimental equipment. The effect of bagasse fraction and

the mode of juice extraction on pulp pad permeability was

examined without the addition of flocculants (section 3.4);

Phase 4 The steady-state compressibility parameters of a bagasse

pulp pad were determined using simple compression

equipment. The effect of bagasse fraction and the mode of

juice extraction on pulp pad compressibility was examined,

also without flocculants added (section 3.5);

Phase 5 The values of the steady-state permeability and

compressibility parameters were used by a dynamic model

for predicting the solidity and consequently load pressure of


62

a pulp pad compressed under dynamic conditions. The

model values are compared to data from dynamic filtration

experiments (section 3.6);

Phase 6 A suitable chemical additives system is optimised using a

modified DDJ using a pulp slurry rather than a pulp pad as

used in phases 3-5. The effect of additives on fines

retention and drainage time is determined. The effect of

chemical additives on the steady-state parameters of a

bagasse pulp pad is determined by repeating phases 3, 4 and

5 above (section 3.7).

The above order of research is used in the experimental procedure section

(Chapter 3) and the results section (Chapter 4).

3.1.1. Preparation of Australian bagasse pulp

Bagasse was prepared in a manner to maximise its permeability properties

and permit its long term storage for this study.

For Objective 1a, bagasse was separated into three fractions prior to pulping

using two wire mesh sieves of different aperture sizes (12.5 mm and 4 mm). The

three bagasse sizes produced were nominated: ‘coarse’, ‘medium’ and ‘fine’ pith

material. The terms ‘coarse’ bagasse pulp and ‘medium’ bagasse pulp are used

extensively in this thesis and refer to pulp which originated from the coarse and

medium fractions of bagasse respectively. Pulp from the ‘fine’ material blocks

the pores of the paper mat as it forms, reducing the drainage rate and sheet quality

(e.g. poor formation and wire-marks). Removing as much ‘fine’ material as

possible prior to pulping improves the drainage properties of the mat.

For Objective 1b, samples of bagasse from the different modes of juice

extraction were collected (i.e. milled and diffuser bagasse) from the same factory.

Several other pulp samples were prepared or obtained for comparative

purposes. The most important comparative pulp samples are: Eucalyptus globulus

pulp; pulp produced from Argentinean depithed bagasse used at a commercial

pulp mill; and a conventionally depithed Australian bagasse pulp.


63

3.1.2. Physical and chemical property testing

The Australian bagasse pulp samples were evaluated for their chemical

properties. The chemical analyses of the pulp samples included carbohydrate

composition by High Performance Liquid Chromatography, Klasson and acid

soluble lignin, ash, extractives and pulp yield.

Bagasse is commonly used for the production of linerboard, writing paper

and tissues amongst other products. The pulp samples were evaluated for strength

properties (tensile, tear, burst and short-span compression) over a range of

refining levels, fibre length distribution and optical properties amongst other

properties. The suitability of the bagasse pulp produced in this study for these

grades were assessed.

The pulp fibres were thoroughly measured for their morphology including

the distributions of fibre length, using a Kajaani fibre length analyser, as well as

other parameters including wall thickness and collapse ratio using a confocal laser

microscope.

3.1.3. Steady-state permeability property testing

The effect of bagasse preparation on the steady-state permeability properties

of pulp pads was studied. Objective 1a and Objective 1b (section 1.3) were

investigated with respect to the steady-state permeability properties.

Pulp permeability was measured in a simple experimental apparatus referred

to as a ‘permeability cell’. A transparent Perspex tube filled with pulp and

attached to a constant head tank was used to achieve steady-state flow.

Using this simple equipment, the suitability of the Kozeny-Carman

permeability model could be quickly determined.

The variables measured in the steady-state testing are the pulp specific

surface area, Sv, and the swelling factor, �. These parameters were determined for

use in the Kozeny-Carman permeability model (78, 79). These steady-state

variables are required for the dynamic filtration model. The findings of this

permeability study are compared to that of previous workers for wood pulp, as


64

well as to the only known previous work on bagasse permeability which was

performed recently (64).

The optimum values of Sv and � depend on whether a constant or a variable

Kozeny factor, ‘k’ is used. For this study, both constant and variable k was used.

Ingmanson and co-workers (81) found that using a variable Kozeny factor

resulted in an increase in the prediction for � of around 25% and a decrease in the

prediction for Sv of around 7% for wood pulp. The variation in Sv and � is

measured for non-wood pulp.

Objective 1a and Objective 1b were investigated using Student’s t-test with

respect to their effect on the compressibility properties Sv and �.

The steady-state permeability of eucalypt pulp, pine pulp and Argentinean

bagasse pulp was also measured.

3.1.4. Steady-state compression testing

The steady-state compressibility behaviour of pulp pads was measured using

simple compression equipment, i.e. a ‘compressibility cell’, using a simple Power-

Law correlation between load pressure and pulp concentration, Ps = M C N (see

Chapter 2 for definitions).

The pulp pad was initially compressed over a very long time-period to

measure the quasi steady-state compressibility parameters. The steady-state

factors M and N are necessary for the dynamic filtration modelling. Objective 1a

and Objective 1b were investigated using Student’s t-test with respect to their

effect on the compressibility properties M and N.

The same samples tested for their steady-state permeability were also

measured for their steady-state compressibility and compared to the numerous

benchmark pulp samples used in this study, including eucalypt, as well as

previous workers.

3.1.5. Dynamic filtration modelling and verification

In dynamic filtration, the permeability properties change as the pulp pad

compresses. Once the steady state compressibility and permeability parameters


65

are determined, the dynamic filtration of bagasse pulp can be predicted using a

filtration model similar to that used for wood pulp (100). This model requires the

steady-state permeability parameters, Sv and �, and the compressibility

parameters, M and N, for bagasse pulp previously determined experimentally.

The model was non-dimensionalised and coded in FORTRAN. The model

predicts the dynamic filtration behaviour using the experimentally determined

steady-state permeability and compressibility parameters. The actual dynamic

filtration behaviour of the pulp is then measured experimentally in the

compressibility cell. The experimental data is compared with the predictions of

the dynamic model in order to verify the model.

3.1.6. Effect of chemical additives on the drainage and retention

properties

A chemical additives system is optimised using a Dynamic Drainage Jar.

This was performed using a pulp slurry. The equipment was modified to also

investigate the effect of vacuum on fine fibre retention and drainage time.

In previous phases of this study, thick pulp pads were investigated because

the permeability and compressibility can be determined with simple equipment.

The behaviour of pulp pads is frequently used by numerous workers to represent

the behaviour of thin pulp mats (e.g. 81, 82, 83, 84-86). In this phase, the

modified DDJ was also used to obtain additional information on the behaviour of

thin bagasse pulp mats, which more closely resembles a Fourdrinier former than a

Twin-wire former.

Finally, the effect of chemical additives on pulp pad steady-state and

dynamic permeability and compressibility is quantified.

3.1.7. Flow diagram of the experimental and modelling methodology

The relationship between sections of the experimental and modelling

methodology is shown in Figure 3.1. The numbers in the figure are the phases of

the methodology described at the start of section 3.1. This is a theme of this

thesis. Chapter 3, the experimental and modelling procedure, and Chapter 4, the

results and discussion, proceed in the same order as the methodology.


66

Figure 3.1 Flow diagram of the experimental and modelling methodology

for bagasse pulp.

Phase 1. Bagasse pulp preparation

Steady-state experiments with bagasse pulp pads

Phase 3. Steady-state

permeability

experiments

Obtain Sv and �

Phase 4. Steady-state

compressibility

experiments

Obtain M and N

Bagasse pulp

Phase 5. Dynamic filtration modeling and experiments

with bagasse pulp pads

Dynamic filtration model

Use steady-state

parameters, Sv, �, M and N

Dynamic

compression

experiments

Model

verification

Phase 6.

Development of a

chemical additives

system using a

pulp slurry

Phase 2. Physical

and chemical

property testing

Bagasse preparation

Fractionated bagasse ‘coarse’ vs ‘medium’ pulp from mill or diffuser

(Chemical characterisation only)

Bagasse pulp slurry

With and without flocculants

Bagasse pulp pad


67

3.2. Bagasse pulp preparation

The preparation and storage of bagasse and pulp created some challenges

since bagasse is extremely bulky with a specific mass of 150 kg/m3. Bagasse also

degrades quickly due to the presence of a residual sugar. It was necessary to wash

it and dry it as quickly as possible for long-term storage in a large walk in

refrigerator at 4 °C. The large number of pulp samples generated in this report are

summarised in Appendix B.

The treatment of the bagasse prior to pulping is presented in section 3.2.1.

The pulp samples were prepared in Melbourne and QUT using three types of

reaction equipment as described in section 3.2.2. The statistical methods used to

determine whether there is a difference between populations of pulp samples are

provided in section 3.2.3. The effect of bagasse pre-treatment on yield and kappa

number is provided in section 4.1. The physical and chemical properties of the

pulp are provided in section 4.2.

3.2.1. Collection of raw materials

3.2.1.1. Australian bagasse

As previously mentioned, bagasse was collected from both a sugar diffuser

and a sugar mill and fractionated into three fractions ‘coarse’, ‘medium’ and ‘fine’

fractions.

The pre-treatment procedure used in this study is intended to maximise the

permeability of Australian bagasse pulp pads and also to minimise degradation of

the bagasse for long term storage. The total amount of pith removed (around

43%) was higher than normally used by industry to achieve acceptable bagasse

pulp permeability (typically 30%).

Bagasse was collected from CSR Invicta sugar factory. The Invicta milling

train consisted of a shredder and five milling units including the final dewatering

mill. The Invicta diffuser consisted of a separate shredder, a preliminary milling

unit, the diffuser and a final dewatering mill.


68

On 28th September 2006 ten 75 L bins were lined with garbage bags. Six

bins were filled with bagasse from the final dewatering mill of the milling train.

Four were filled with bagasse from the final dewatering mill following the

diffuser. Each bin was filled with 10 kg of bagasse. The bagasse in these bins

were turned over several times in order to reduce the temperature and moisture

content and hence degradation during transport. The bins arrived at QUT,

Carseldine Campus on 4th October 2006.

The bagasse obtained from the sugar mill was from cane species Q208B (B

for burnt). The bagasse collected from the diffuser was TellB. It was not possible

to collect bagasse of the same variety of cane from both the mill and the diffuser

during the visit. As will be shown in Chapter 4, the difference in cane varieties

was inconsequential. No difference was found in the pulping kinetics (section

4.1.1) or the permeability and compressibility characteristics (sections 4.3 and

4.4).

The fibre content of the parent cane was measured by factory staff and

determined to be 15.6% (wet basis) for both varieties of cane. The fibre content

of Australian cane is typically 10% to 17%. The fibre content of the cane was

towards the higher end of this range.

The bagasse was washed in copious amounts of water to remove sugar using

a cement mixer. The bagasse mixture was drained through a 4 mm wire mesh.

The fines in the filtrate were recovered by refiltering the filtrate through the

bagasse bed several times. Only 3% of the fines were lost through this washing

process.

The bagasse was allowed to dry to 10% moisture, see Figure 3.2. The

bagasse piles were rotated with one another for even exposure to the sun.


69

Figure 3.2 Photograph of bagasse drying outside on tarpaulins.

The washed milled and diffuser bagasse was separated into three fractions

prior to pulping using two wire mesh sieves of different aperture sizes, 12.5 mm

and 4 mm respectively. Subsamples of around 50 g of bagasse were manually

sieved for approximately 3 min to achieve the separation. The three bagasse sizes

produced were nominated: ‘coarse’ which accounts for the 25 % of the bagasse

that is retained on the 12.5 mm sieve (i.e. +2 mesh); ‘medium’ (i.e. 4.0 mm to

12.5 mm) which accounts for the 35% of the bagasse that passes the 12.5 mm

sieve but is retained on the 4.0 mm sieve (i.e. +6 mesh); and ‘fine’ which accounts

for around 40 % of the bagasse and passes through the 4.0 mm sieve (i.e. -6

mesh).

The fractionated bagasse samples are shown in Figure 3.3 (a), (b) and (c)

together with samples of ‘whole’ (unfractionated) Australian bagasse (d). The

‘coarse’ bagasse (a) contains a much higher content of large chip-like material

compared to the ‘medium’ bagasse (b). These definitions of ‘coarse’, ‘medium’,

Bin 1 Bin 2

Bin 3 Bin 4

Bin 5 Bin 6

Bin 7 Bin 8

Bin 9 Bin 10


70

‘fine’ and ‘whole’ bagasse pulp are used throughout this thesis. The ‘fine’ fraction

is assumed to be mainly pith material so this terminology is used interchangeably.

Figure 3.3 Photographs of (a) ‘coarse’, (b) ‘medium’ and (c) ‘fine’

fractions of Australian bagasse, and (d) Australian ‘whole’

bagasse.

A sample of Australian bagasse was sieved in order to remove 30% of its

shortest material. This is typical of overseas industrial depithing operations. The

pulp produced from this Australian bagasse is herein referred to as ‘30% depithed’

bagasse pulp.

After washing and fractionating, the bagasse was then stored in a walk-in

fridge (4 °C) until it was ready to be pulped.

3.2.1.2. Argentinean bagasse

A depithed Argentinean bagasse sample that is used by the company

Ledesma Paper Mill to make writing papers was included in the evaluation. Their

depithing process removed 30% of the finest material (i.e. the pith). This sample

50 mm 50 mm

50 mm

(a) (b)

(c) (d)

50 mm


71

was stored in the fridge but not washed so as not to alter its preparation conditions

prior to pulping. The pulp produced from this sample is referred to as

‘Argentinean’ bagasse pulp.

3.2.1.3. Wood material

Samples of Eucalyptus globulus and Pinus radiata wood material were

supplied in pulp form by Ensis and the Australian Pulp and Paper Research

Institute (APPI) respectively. These organisations are two of Australia’s leading

pulp and paper research and development companies. No pre-treatment was

performed on these samples. The pulping conditions are provided in section

3.2.2.3.

3.2.2. Pulp sample preparation

A large number of pulp samples were required to (i) investigate

permeability and compressibility differences between pulp samples originating

from different size fractions of bagasse2 as well as differences in milled and

diffuser bagasse pulp, (ii) undertake the required physical property testing and (iii)

investigate the effect of chemical additives on the flocculation of pulp fibres.

The large number of pulp samples was particularly important to achieve

Objective 1. If the populations were compared with a small number of large

cooks, then subtle changes in cooking conditions between the cooks could

potentially affect the outcome. To test whether there is a difference between

fractionated milled and diffuser bagasse, samples were pulped in an APPI digester

containing six 1.5 L cells (discussed in more detail in section 3.2.2.1). This

reactor is called a ‘flow-through’ digester herein. Samples of bagasse were

cooked in a randomised order.

Several pulp samples were produced in a much larger batch 18.5 L reactor

for a number of reasons. The size of the samples produced in the APPI ‘flow-

through’ digester were not sufficient for destructive physical property testing

(section 3.3). Also, for experiments involving chemical additives (section 3.7),

2 For bagasse with a very high proportion of short fibres, it was not possible to pulp the material because it was difficult to circulate the liquor in the APPI ‘flow-through’ digester.


72

the pulp had to be disposed after each experiment. As such, a much larger batch

of pulp was prepared so that subtle differences that may affect pulping

experiments could be eliminated as a potential source of error.

Over 60 pulp samples were produced in the course of the project. Each

bagasse pulp sample produced was labelled with a unique number which is

referred to hereafter in this thesis. The pulping conditions, origin of the bagasse,

yield and kappa number for each pulp sample are provided in Appendix B.

Supplementary photographs of the pulping equipment are provided in

Appendix C.

3.2.2.1. Bagasse pulping in the ‘flow-through’ digester

The APPI ‘flow-through’ digester consists of six cells into which bagasse is

packed. Each cell is 1.5 L. Hot cooking liquor is pumped from a 50 L tank

through each of the cells and drains back into the tank. The flow diagram of the

equipment is reproduced in Figure 3.4 (136) and a photograph of the equipment is

provided in Figure 3.5.

Figure 3.4 Sketch of the 6 cell ‘flow-through’ digester at APPI (136).

Digestion cells

Liquor inlet lines


73

Figure 3.5 Photograph of the APPI ‘flow-through’ digester showing the 6

digestion cells.

Bagasse was soaked in warm water for 20 min to soften it so that each 180 g

sample of ‘medium’ bagasse and 210 g of ‘coarse’ bagasse could be packed into

the digester cells. The increase in flexibility is due to the plasticisation of the

lignin rather than bending of the sclerenchyma pulp fibres.

Fifty litres of cooking liquor was recirculated through six cells containing

100-200 g of fractionated bagasse (air dry basis). The pulping conditions were 0.4

M sodium hydroxide (approx. 13.8% Na2O on oven dry fibre) and 0.1%,

anthraquinone, AQ, (on oven dry fibre) at 145 °C.

In this reactor, the cells can be independently isolated from the cooking

liquor by manual valves. The liquor is heated indirectly by steam. When the

liquor reaches temperature, the liquor can be circulated immediately through the

material in the cell.

An initial kinetics study was performed to determine the cooking time

required to achieve a pulp with a kappa number (i.e. residual lignin content) of 20.

A pulp screen was not available during the trials with the APPI digester, so the

kappa number was measured on unscreened pulp. The cooking length was varied

between 5 min and 70 min. It was found that only 30 min of cooking time was

Cells

Inlet liquor lines from common header (valve on each)

Outlet liquor

lines


74

required. Normally several hours is required to produce wood pulp using this

equipment (136). Bagasse pulp is well known to delignify much more quickly

than wood chips due to the high reactivity of grass lignins.

The depithed bagasse obtained from Argentina’s Ledesma Mill was also

pulped under these conditions for 30 min using the ‘flow-through’ reactor.

At the end of a cook, the pulp was transferred to a standard disintegrator.

The pulp was disintegrated for 10,000 rev. The pulp was thoroughly washed with

water and dewatered using a very large steel Buchner funnel.

A total of 30 pulp samples were generated in the ‘flow-through’ digester.

3.2.2.2. Bagasse pulping in a batch ‘Parr reactor’

It was not possible to pulp whole (unfractionated) bagasse or the ‘fine’

fractionated material in the APPI ‘flow-through’ digester because the liquor

would pool on top of the bagasse and not permeate through the bed of bagasse.

Consequently, samples of whole and fine Australian bagasse were pulped to a

target kappa number of 20 in the 18.5 L batch reactor at 170 °C for 105 min at a

liquor to fibre ratio of 14:1 with a concentration of approximately 0.4 M sodium

hydroxide and 0.1% AQ. The Parr reactor is shown in Figure 3.6. The reactor is

electrically heated. The time to temperature for the majority of the experiments

was typically 45 min - 60 min. 1 kg of bagasse (10%-15% moisture) was loaded

into the Parr reactor in each cook. The ‘Parr reactor’ is cooled by ambient water

flowing through serpentine cooling coils and takes 60 min to reach a temperature

at which the vessel can be safely handled.


75

Figure 3.6 The QUT 18.5 L Parr reactor.

In addition to the ‘whole’ and ‘fine’ bagasse pulp samples, large quantities

of bagasse pulp originating from ‘coarse’ and ‘medium’ fractions of bagasse were

produced in this reactor for physical property testing and the tests involving

chemical additives.

Pulp produced from ‘30% depithed’ bagasse was also produced in this

reactor under these cooking conditions. This pulp is used for benchmarking

purposes as it has been depithed to a similar level to that used by overseas

commercial operations. This benchmark pulp was cooked in this reactor so as to

prevent any pith material from being washed into the liquor. It is acknowledged

that this benchmark pulp sample was cooked at a higher temperature than the

majority of the pulp samples cooked using the ‘flow-through’ reactor. It was

decided that preserving the pith in this benchmark bagasse pulp sample, as could

be achieved using the Parr reactor, was very important.

In order to determine whether bleaching had any effect on the permeability

and compressibility properties, one of the pulp samples produced using the ‘Parr

reactor’ (Sample 56) was bleached using calcium hypochlorite according to Tappi

test method UM-206 from 28 brightness to 54 brightness (137).

Crane for moving

vessel head

Heating

jacket

Temperature

controller

Vessel head


76

3.2.2.3. Benchmark wood pulp

The two wood pulp samples were supplied by Australian pulp and paper

research and development organisations.

Eucalypt pulp was prepared at Ensis, Melbourne, Australia, using an ‘air-

bath’ reactor. The wood is loaded into a sealed cell with 2 L volume and the cell

is loaded into a pressure vessel and heated with steam. The cooking conditions

used to produce the pulp were 11.75% Na2O on oven dry fibre, sulphidity of 25%,

cooking temperature of 165oC for 2 h. The eucalypt was pulped to a target of 20

kappa.

A sample of kraft pine pulp was obtained from APPI, also in Melbourne.

The sample was similarly prepared in an air-bath reactor and pulped to a kappa

number of 20 using kraft pulping chemicals. The concentration of the cooking

chemicals that were used is not known.

3.2.2.4. Pulp screening

Each pulp sample was screened through a 200 �m slotted Packer screen

with water recirculation. The pulp samples were not allowed to dry at any stage.

The pulp was placed in a dough mixer to break up the pulp in order to make the

moisture content homogeneous enabling consistent dry substance measurements

to be obtained. The pulp was stored at 25% consistency in a 4 °C refrigerator for

the duration of the project.

3.2.3. Test for statistical significance between two populations of pulp

samples

In some experiments, it was necessary to test whether a sample group was

from a single population or two independent populations. For these tests of

statistical significance, the mean value of a parameter is compared using Student’s

‘two-sample test’ (e.g. reference 138). Also known as Student’s t test, it is

particularly suitable for comparing populations when the sample size is small,

such as in this study (139). The test determines whether independently obtained

samples are consistent with the null hypothesis that they are from populations

with equal means.


77

Suppose we have two independent populations (xi , i = 1,2,…,nx) and (yj

j=1,2,…nj). The pooled estimate of standard deviation between the two samples

is

( ) ( )2nn

s1ns1ns

yx

2

yy

2

xx2

PE−+

−+−=

Equation 3.1

Where PEs is the pooled estimate of standard deviation

nx is the number of samples in the first population

ny is the number of samples in the second population

sx is the estimated standard deviation of the first population

sy is the estimated standard deviation of the second population

Equation 2.26 assumes that the values are obtained from one experiment.

When an experiment has a limited sample size, repeating the experiment for each

sample improves confidence of each datum. Determining the pooled estimate

from the average values for each sample over a number of tests, improves

confidence in the accuracy of the values for each sample, xi and yi. The estimate

of standard deviation for each population is now

y

y'

y

x

x'

xr

ss;

r

ss ==

Equation 3.2

where sx’ is the standard deviation of a population of averaged values

r is the number of times the experiment is performed. Substituting this value into

Equation 3.1 gives

( ) ( )

2nn

r

s1n

r

s1n

syx

y

2

y

y

x

2

xx

2

*−+

−+−

=

Equation 3.3


78

For determining whether the sample group is from a single or two

populations, Student’s Test Statistic, � is:

yx

*n

1

n

1s

yx

+

−=τ

Equation 3.4

Where x and y are the average values for the two hypothetical populations.

The observed value of � is compared to tables of Student’s Test Statistic to

determine statistical significance provided in Appendix D. The tables provide

threshold values of � for each degree of freedom and confidence interval. Values

of � greater than those in the table mean that the sample group is from two

populations for the number of degrees of freedom and desired confidence interval.

3.3. Physical and chemical property testing procedure

The physical and chemical properties of selected bagasse pulp samples were

characterised. The chemical composition of several bagasse and bagasse pulp

samples was determined as well as eucalypt pulp (section 3.3.1). The pulp

physical properties of a ‘coarse’ bagasse pulp (Sample 56) and a benchmark

Australian bagasse pulp (Sample 58) were studied at various levels of refining

(section 3.3.2) and the results were compared to the findings of other researchers.

The fibre length distribution of all pulp samples was determined by a Kajaani

Fibre Length Analyser and by a Fibre Quality Analysis unit (section 3.3.3). A

microscopy study was undertaken to determine the fibre dimensions including cell

wall thickness, lumen diameter and collapse of the fibres (section 3.3.4).

3.3.1. Chemical characterisation of pulp and bagasse

The following chemical analyses of bagasse and pulp samples were

undertaken using the methods of the National Renewable Energy Laboratory for

biomass characterisation (140).


79

o Total solids in biomass;

o Carbohydrates in biomass by High Performance Liquid

Chromatography;

o Acid-soluble lignin in biomass;

o Ash;

o Extractives;

o Kappa number; and

o Pulp yield.

The chemical composition of six bagasse pulp samples was analysed as well

as a sample of eucalypt pulp. The bagasse pulp samples analysed included

‘coarse’ and ‘medium’ fractions of milled and diffuser bagasse pulp (four

samples; 26, 27, 20 and 39), whole bagasse pulp (Sample 53) and pulp from

Argentinean depithed bagasse (Sample 32). The parent bagasse material of each

of the bagasse pulp samples was also analysed.

The cellulose content is related to the quantities of glucan and xylan in the

pulp/bagasse hydrolysate and the hemicellulose content is related to the quantity

of arabinan.

3.3.2. Pulp physical property testing

Two samples of Australian bagasse pulp were tested at the Central Pulp and

Paper Research Institute (CPPRI), in India. CPPRI have significant experience

with bagasse pulp and paper testing.

The samples analysed were a ‘coarse’ bagasse pulp (Sample 56) and the

‘30% depithed’ bagasse pulp (Sample 58). Sample 58 is the benchmark sample

and Sample 56 is a sample with high permeability (refer Chapter 5). Both

samples were produced from Australian bagasse.

The pulp samples were analysed for strength properties that are typically

required for photocopier paper, tissue and boxes. The strength properties

analysed were tensile, tear, short-span compression and burst strength properties.

For these tests, handsheets of 60 g/m2 were formed. The handsheets were tested

after conditioning at temperature 27 ± 1 ºC and a relative humidity of 65 ± 2%

according to ISO 5269/1. Test specimens were cut from the handsheets. The


80

tensile and tear tests involve measuring the force required in the direction of the

sheet and perpendicular to the sheet respectively before the specimen fails. The

compression test involves compressing the specimen until it buckles. The burst

test involves clamping a specimen and applying a hydrostatic force, allowing the

specimen to bulge, until the specimen fails.

The water retention value (WRV) was also measured. WRV is an important

parameter for non-wood pulp as it provides an indication of the ability to dry a

sheet of paper. The apparent density of the pulp samples was also determined.

The effect of refining was determined. Pulps were beaten in the laboratory

PFI mill to three freeness levels according to the ISO 5264/2 method. Again,

handsheets of 60 gsm were prepared according to the ISO 5269/1 method.

3.3.3. Fibre length analysis

The fibre length analysis for the majority of the pulp samples as performed

on a Kajaani Fibre Analyser FS100 at Petrie Mill, Brisbane, Australia. The results

for fibre length produced from this unit are a length weighted basis, i.e.

=ii

2

ii

wLn

LnL

Equation 3.5

Where Lw is the length weighted fibre length, Li is the length of the ith fibre,

ni is the number of fibres with length Li.

For curl and kink properties, three samples (Sample 56, Sample 58 and

Sample 60) were analysed on a Fibre Quality Analyser LDA 96, supplied by

Optest Equipment, at the University of British Columbia, UBC. 5000 fibres were

analysed for each sample.

The curl index is calculated using the formula

1l

LindexCurl −= Equation 3.6

l


81

where L is the true length of the fibre and l is the apparent length of the fibre

(see Figure 3.7).

Figure 3.7 Sketch of a longitudinal section of a pulp fibre.

The kink index is related to the number and magnitude of bends in the fibre.

Apart from the discussion in Chapter 4, some additional fibre length

distribution data is provided in Appendix E.

3.3.4. Microscopy investigation

The microscopy investigation on the morphology of the pulp fibres was

performed by Scion, New Zealand, using a confocal laser microscope. This

investigation was performed to gain insight into the shape of both bagasse pulp

fibres and eucalypt pulp fibres and how they might behave during pad formation.

Three samples were analysed, a milled ‘coarse’ Australian bagasse pulp

(Sample 26), a milled ‘medium’ bagasse pulp (Sample 27) and the eucalyptus

pulp. 500 fibres from each pulp sample were chemically dehydrated by solvent

exchange through a graded series of water/acetone solutions. Fibres were then

mounted in a Spurr’s resin and the resin was allowed to cure before the surface

was sectioned and polished for image analysis. The images were analysed using

Scion’s image analysis software. A small quantity of chipped and broken fibres

as well as contaminants is removed during the image analysis.

Figure 3.8 shows a typical fibre section with the fibre width and thickness

dimensions determined in the image analysis. The orientation of the fibre was

optimised so as to minimise the fibre area, according to the definitions listed

below.

l L


82

Figure 3.8 An image of a bagasse fibre cross section showing fibre width

and thickness for the microscopy investigation.

The following measurements were calculated for each fibre, amongst others:

• Fibre width. This is the longest cross-sectional dimension as shown

in Figure 3.8.

• Fibre thickness. This is the shortest cross-sectional dimension as

shown in Figure 3.8.

• Wall area. The area of the black portion in Figure 3.8.

• Centreline perimeter. The perimeter of a line drawn between the

inside and outside walls of the fibre as shown in Figure 3.8.

• Wall thickness. The wall area divided by the centreline perimeter.

• Fibre area. This is the area bounded by a rectangle around the fibre.

• Fibre perimeter. The perimeter of the outside wall of the fibre.

• Lumen area. The area of the lumen inside the inner fibre wall.

• Lumen perimeter. The perimeter of the inside wall of the fibre.

• Collapse ratio. The fibre width divided by the fibre thickness.

• Maximum and minimum wall thicknesses.

Fibre thickness

Fibre width

Fibre area (bounded

by rectangle) Fibre perimeter


83

These measurements were compiled for each sample of 500 fibre sections.

The distributions of these parameters were collected.

Elsewhere in this thesis, Student’s t test is used for statistical comparisons of

populations. For the microscopy investigation, a different statistical test was used.

For this analysis the variability data were supplied by an external organisation and

their statistical test was employed. The results of the image analysis provide the

mean of each parameter as well as a ‘Least Significant Difference’ (LSD) between

means. If the difference in means of two pulp samples is greater than the LSD,

then the samples are significantly different using a 95% confidence interval.

3.4. Steady-state permeability testing equipment and experimental

procedure

The primary objective of the steady-state permeability study was to examine

the effect of bagasse preparation and processing on pulp permeability properties

and to compare the permeability data with the findings of previous workers for

wood pulp (e.g. 81) and bagasse pulp (64).

Australian bagasse from a sugar mill and a diffuser was separated into three

size fractions (i.e. ‘coarse’, ‘medium’ and ‘fine’ fractions) prior to chemical

pulping. For comparative purposes, the permeability properties of kraft eucalypt

pulp, a hardwood pulp with short fibres typically around 0.8 mm in length, were

determined as the benchmark for this study. The permeability of several other

pulp samples was also measured including pulp made from milled bagasse that

has had 30% of the finest material removed (Sample 58), unfractionated milled

Australian bagasse (i.e. ‘whole’ bagasse) pulp (Sample 53), and depithed bagasse

pulp from Ledesma Sugar, Pulp and Paper Mill in Argentina (Sample 32).

Finally, the permeability of a kraft pine pulp, a long fibre pulp typically 3 mm in

length, was measured.

The pressure drop and flow rate data from the experiments were used to

determine Darcy’s permeability, K, and consequently the specific surface area, Sv,

and the swelling factor, �, an indicator of the strength generation potential during

refining, were determined for use in the Kozeny-Carman permeability model (78,

79).


84

The ‘Kozeny factor’, k, is actually a function of pulp porosity, although a

constant is often used. The optimum values of Sv and � depend on whether a

constant or a variable k is used. For this study, both constant and variable k was

used. Ingmanson and co-workers (81) found that using a variable Kozeny factor

resulted in an increase in the prediction for � of around 25% and a decrease in the

prediction for Sv of around 7% for wood pulp.

The secondary objective of the permeability experiments was to obtain data

for the dynamic filtration model examined.

Pulp permeability was measured in a simple, custom built, experimental

cell, hereafter referred to as the ‘permeability cell’.

Figure 3.9 shows the schematic diagram of the experimental equipment

assembly which was custom built to obtain the permeability data. A photograph

is also provided (Figure 3.10) with the permeability cell highlighted in the bottom

right hand corner of the figure. The main feature of the permeability apparatus is

a permeability cell made from a Perspex tube with an internal diameter of 41 mm

and height of 300 mm. The cell has an airtight seal at the top - a rubber bung that

is connected to a manual valve. The bottom is supported by reinforced screen of

100 mesh. The cell is connected to two manometers to measure the pressure drop

(�p) across two positions of the pulp pad (�l).

30 g of equivalent dry pulp and 3 L of distilled water were added to a

disintegrator to make a pulp slurry of 0.9 % consistency. Two litres of this slurry

was slowly poured into the permeability cell to form a saturated pad. The slurry

was vigorously agitated as it was added to the cell to ensure uniform layering of

the pulp fibres in the cell. Although the water supply was not de-aerated, it was

consistent in every experiment. The data is similar to that obtained by other

workers using de-aerated water (86).


85

Figure 3.9 Schematic diagram of the apparatus used for permeability

measurements.

Cell ID 41 mm

�l

Water layer

Manual

valve

Town water

supply

Water level

Constant

head tank

Overflow

�p

Q

Pulp mat

�L, �P


86

Figure 3.10 Photograph of permeability cell.

The town water supply valve to the constant head tank was opened until the

tank overflowed and a constant head was maintained above the manual water

valve to the cell. The manual valve was opened and water flowed from the

constant head tank through the cell. The manual valve was adjusted until the

height in the manometer was constant (typically 5-15 min). The pulp pad height

(�L) was recorded to determine the pulp concentration for a known pulp mass.

The flow rate of the water through the cell was measured (Q) with a measuring

cylinder and a stopwatch and the difference in water height between the two

Constant head tank

Manometer

level

Manual valve

Permeability

cell

Rubber bung

Manometer

offtakes

Mesh screen


87

manometers was recorded (�p). The pressure drop �p applies over the distance

between the two manometers, �l. The hydraulic compression is negligible so

�p/�l is extrapolated over the full height of the pulp pad to determine �P/�L that

is required for the calculation of K using Darcy’s Law (Equation 2.2).

The town water supply temperature was 23 °C. The temperature of the

water supply varied only 2 °C during the period of the experiments. The variation

of the town water supply temperature did not significantly affect the pulp pad

permeability.

Great care was taken to ensure that the pulp pad was constantly saturated

with water by maintaining a pool of water above the pulp pad at all times. If the

pulp pad dries out, the fibres contract and the pulp pad could become unevenly

distributed across the cross section of the cell and channelling of water could

occur.

After these measurements at the lowest flow rate were recorded, the flow

rate of water through the cell was increased incrementally and the values for �p,

�L and Q were recorded.

Once these measurements were completed the supply of water to the cell

was turned off and another 500 mL of pulp slurry was then added to the

permeability cell. Values for �L, Q, and �p were again recorded over a range of

flow rates. Finally the remaining pulp slurry was added.

When fully loaded with pulp, the pad was compressed to heights of

210 mm, 180 mm and 150 mm using compressed air. In order to compress the

pulp pad, the permeability cell, including the rubber bung was detached from the

constant head tank. A compressed air line was attached to the rubber bung (and

hence the permeability cell) by a barbed nipple. At this point, the pulp pad is very

compressed (>0.1 g/cm3) and a pool of water above the pulp pad was easily

maintained as the compressed air was applied. Obtaining pressure drop and flow

rate data over a wide concentration range was important for accurate calculation

of Sv and �.


88

Sv and � were calculated from the intercept and slope of a graph of

concentration against (Kc2)1/3 as per Equation 2.23.

It was observed during the permeability experiments that at high pulp

concentrations (i.e. >0.1 g/cm3) repeatable results were readily obtained since it

was easier to avoid channelling than at low concentrations. Obtaining repeatable

results was more challenging in the low concentration range between 0.06 g/cm3-

0.08 g/cm3. At these low concentrations, the following problems were

occasionally encountered: (i) channelling. This problem could often, but not

always, be observed by water ‘fast-tracking’ down the inner walls of the Perspex

cell. At lower concentrations, the calculated permeability data obtained was

continuously checked by plotting (Kc2)1/3 against concentration, c. When

channelling occurred, the datum was obviously far too high (typically an order of

magnitude higher) and the datum was subsequently rejected; and (ii) pulp slurry

entrained in the manometer lines. This was overcome by purging the lines.

For the Australian bagasse pulp samples, the permeability experiment was

performed at least twice and the average Sv and � are presented in the results

section. For the other pulp samples, the permeability experiments were performed

typically five times and the average for Sv and � are presented in the results

section.

3.5. Quasi steady-state compressibility experimental procedure

The primary objective of the steady-state compressibility study was to

examine the effect of bagasse preparation on pulp steady-state compressibility

properties and to compare the findings with those for wood pulp (21, 6,7, 8).

The secondary objective of the compressibility study is to verify the steady-

state compression model and obtain steady-state compressibility data required for

the dynamic filtration model in section 3.6.

A simple, custom made, ‘compression cell’ (this term is used hereafter) was

designed, fabricated and mounted to an Instron 5500R capable of a maximum

load of 100 kN although for this study the load applied did not exceed 5 kN.

Photographs of the cell assembly are shown in Figure 3.11. The engineering


89

drawings are available in Appendix F. The cell is 100 mm in height, the platen is

10 mm thick, resulting in a total possible working height of 90 mm. The platen is

fitted with a shamband and Teflon ring to prevent water flowing around the

platen, and the platen is drilled with thirty 6 mm holes for the water to evacuate

(see Figure 3.11).

Pulp samples were disintegrated to 0.9% consistency. The barrel of the

compressibility cell was removed from the base and suspended on a screen of

100 mesh. The disintegrated pulp was added to the barrel of the compressibility

cell and the bulk of the water was allowed to drain through the mesh. Once the

desired height of pulp in the barrel was reached, the barrel, the supporting screen

and the loaded pulp could be transferred to the base and bolted in. The pulp pad

remained saturated during the transfer; in practice, this was easy to achieve. The

platen of the compressibility cell is then connected to the Instron, ready to

commence the compression experiment (see Figure 3.11).

(a) (b)

Figure 3.11 Photographs of (a) the loaded compressibility cell with the

barrel fixed onto the base and (b) the top platen and the base of

the cell when the cell is dismantled.

‘Quasi steady-state’ behaviour is the experimental approximation to the

actual steady-state behaviour. This is reached by compressing the pulp pad over a

very long time period. During steady-state compression, the permeability effects

are insignificant. The hydraulic load (i.e. the head of water above the platen) is

��

��

��

��

��


90

negligible compared to the applied mechanical load, and so the concentration

distribution is uniform throughout the bed. The graph of log(Ps) against log(c) is

linear. Values of M and N are obtained from the slope and intercept of the graph.

For all of the quasi steady-state compressibility tests, the platen finished

compressing the pulp pad 15 mm above the base of the cell. The platen was

lowered very slowly over 300 min at a constant rate of 0.25 mm/min (that is,

75 mm over 300 min). The wood pulp samples and the Argentinean bagasse pulp

(Sample 32) were compressed several times to obtain average values of M and N.

The Instron load and time were logged. The load on the platen was recorded by

the Instron and the applied pressure was calculated. The measured load was

reduced by the frictional resistance between the Teflon seal and the barrel. This

was typically equivalent to 2.5 – 4.5 kPa. This was measured by compressing the

cell when loaded with water.

3.6. Dynamic filtration modelling and experimental verification

procedure

This development of a dynamic filtration model was performed to assist

with the future development of specialised equipment for the processing of

bagasse pulp and also because a dynamic model more closely resembles the

industrial paper manufacturing process.

Two pulp parameters affect the drainage properties of fibre beds;

compressibility and permeability under dynamic conditions (84).

The steady-state permeability and compressibility parameters required for

the dynamic filtration model were previously collected (sections 3.4 and 3.5).

The model was coded (section 3.6.1) and pulp pad experimental data was obtained

under dynamic conditions for comparison and verification of the bagasse pulp

dynamic filtration model (section 3.6.2). The verification of the dynamic model

followed the sequence,

1. Steady-state permeability tests of depithed bagasse pulp to calculate the

permeability constants (the specific surface area, Sv and swelling factor, �)

(section 3.4).


91

2. Compression tests of depithed bagasse pulp to calculate the steady state

compressibility constants, M and N (section 3.5).

3. Calculation of the fibre pressure at the top surface of the pulp pad at higher

compression rates using the dynamic model (section 3.6.1). The model uses

the physical constants obtained from section 3.4 and 3.5.

4. Dynamic filtration of depithed bagasse pulp for comparison with the

predictions of the dynamic model (section 3.6.2).

3.6.1. Dynamic filtration modelling procedure

The governing equations (Equation 2.25 to Equation 2.28) were non-

dimensionalised, programmed in the language FORTRAN 77 and compiled.

FORTRAN 77 was chosen for its compatibility with the library subroutines used

to solve the non-dimensionalised governing equation. The derivation of the non-

dimensional governing equation from the dimensional governing equation is

available in the supplementary modelling material in Appendix A, as is the

FORTRAN code. The resolution of the output was 100 increments in height, h,

and time, t. The experimental compressibility constants M and N for each pulp

sample were converted to m and n for use in the dynamic model. The four

physical constants, m, n, Sv and � are inputs for the program. Other inputs into

the model include the initial pulp concentration and the platen speed.

The program outputs the solidity throughout the height of the cell for many

discreet time intervals. For comparison with the experimental data, the predicted

solidity at the top platen is determined and converted to fibre pressure (using

Equation 2.24). The model predictions using both a constant k (k = 5.55) and a

variable k (Equation 2.12) can then be compared with experimental data.

3.6.2. Verification of the dynamic model

The same pulp samples which were tested for their steady-state permeability

and compressibility properties were also tested under dynamic conditions. The

model was verified with over 20 pulp samples.

The experimental procedure for the dynamic filtration experiments used the

equipment and procedure used for the steady-state compression experiments


92

outlined in section 3.5, but with a higher compression rate so that the dynamic

effects can be observed. In the dynamic filtration experimental phase, the

compression speed was increased by 100 times in most instances to 25 mm/min

(that is 75 mm over 3 min) and the load on the platen was recorded and converted

to average pressure over the platen.

For the dynamic filtration experiments, the compression cell was loaded to

75 mm in depth, leaving 15 mm clearance to the platen, and compressed to 15

mm. The lower initial height of the pad is required for the dynamic testing

because the calculated values of the compressibility constants are valid over the

limited range of pressures used in the quasi steady state testing.

The repeatability of the dynamic filtration experiments was investigated

using two samples of ‘medium’ diffuser bagasse pulp. The pulp samples were

made into pulp pads, compressed under dynamic conditions, slurried again and

the experiment was repeated.

A sample was also tested under dynamic conditions before and after

bleaching.

3.7. Equipment and procedure for testing the effect of chemical additives

The chemical additives experiments were conducted in three phases. A

suitable shear stable chemical additives system was optimised using a DDJ and

bagasse pulp slurry. Section 3.7.1 provides the methodology used for optimising

a flocculant system for a bagasse pulp slurry with a DDJ (DDJ equipment has

already been described in length in section 2.4.4). The DDJ was then modified to

investigate the performance of the chemical additive under vacuum. The modified

DDJ was used to obtain additional information on the behaviour of thin pulp mats,

without chemical additives (section 3.7.2). Finally, the chemical additives were

used to make pulp pads. The steady-state permeability and compressibility

experiments and dynamic filtration experiments were repeated (section 3.7.3).

Data obtained using the DDJ was collected in duplicate. The work program for

this section of work is outlined in Figure 3.12. Up until this point, all the


93

permeability and compressibility testing was performed without chemical

additives.

Figure 3.12 Flow diagram of the experimental program for chemical

additives.

Four types of bagasse pulp produced in the 18.5 L batch Parr reactor were

investigated; a ‘whole’ bagasse pulp (Sample 53); a depithed Australian bagasse

pulp (30% of shortest material removed as “pith” prior to pulping, Sample 58); a

‘coarse’ bagasse pulp (Sample 56); and a ‘medium’ bagasse pulp (Sample 60).

The pulping conditions were 90 min, 15% Na2O, 0.1% AQ, at 170 °C.

A range of additives used for pulp and paper manufacture were obtained

from Ciba Specialty Chemicals. Ciba recommended Percol 182, a high molecular

weight cationic polyacrylamide (CPAM) in conjunction with Hydracol ONZ, a

modified bentonite microparticle, as the most effective chemical additives based

on their own work. Hence, this study only optimised one chemical additives

system.

1. Develop a shear stable

flocculant system for a bagasse

pulp slurry

Use a DDJ looking at shear only on pulp slurry

(Section 3.7.1)

2. The effect of vacuum on bagasse

pulp slurry flocculation

with and without the flocculant system developed in (1)

using a modified DDJ (Section 3.7.2)

3. The effect of the flocculants on

pulp pad steady-state and dynamic

permeability and compressibility

Pulp pads made using the flocculant system developed in (1) and measured in the permeability and compressibility

cells (Section 3.7.3)


94

Up until this point in the study, the permeability and compressibility of thick

pulp pads was studied. In reality, industrial paper manufacture actually involves

water draining through thin pulp mats. Thin pulp mats were produced, without

using chemical additives, in the modified DDJ. The drainage of pulp slurry

through thin pulp mats provided complementary information to the pulp pad

filtration data collected in the previous phases.

3.7.1. Methodology – Effect of shear

The efficacy of the flocculants was tested using a DDJ (Figure 2.6) for their

suitability in bagasse paper manufacture. Their efficacy was quantified using

fines retention in this part of the study. CPAM was tested in the range 0.01% to

0.5% (on dry fibre) and the bentonite was tested from 0% to 1.6% (on dry fibre).

Tappi test method T 261 cm-00 was followed for measuring the retention of fines

(141). A standard 76 �m screen was used.

500 mL of pulp slurry was made up to 0.1% consistency. The pinch valve

at the base of the vessel, below the screen, was opened for 30 s and the fines in the

filtrate (typically 120 mL) were collected and measured by filtering the filtrate

through a GP-C glass microfiber filter paper. The experiments were conducted

over a range of stirrer speeds from 500 rpm to 1500 rpm as recommended by the

test method.

To measure the total quantity of fines, 500 mL of pulp slurry at 0.1%

consistency was washed with several quantities of wash water containing Tamol

182 dispersant until the filtrate was transparent. This is in accordance with the

test method. The determination of the total quantity of fines using this method is

required for calculating the percentage of fines retained during experiments with

CPAM and bentonite.

The addition rate of the chemical additives were optimised on a sample of

‘whole’ bagasse pulp. The optimum addition rate for the ‘whole’ bagasse pulp

was then used for the other pulp samples investigated, i.e. ‘coarse’ bagasse pulp,

‘medium’ bagasse pulp and the benchmark ‘30% depithed bagasse pulp.


95

3.7.2. Methodology – Effect of vacuum

Industrially, not only is shear applied during paper manufacture but vacuum

is also applied. There are no standard test methods although numerous attempts

have been made to simulate both shear and vacuum in the laboratory (Chapter 2).

The DDJ was modified to allow the pulp suspension to dewater under a

controlled vacuum. The DDJ was connected to a small filtrate vessel which is

under vacuum, supplied by a small vacuum pump. The vacuum level was

measured using a pressure transducer that sent a signal back to the PLC. The PLC

regulates the position of a bleed valve to control the vacuum level. The flow rate

was measured by a 2000 g set of scales with an analogue output. The data output

from the scales and the pressure transducer were logged. A diagram of the setup

is shown in Figure 3.13 and a photograph is provided in Figure 3.14. The

experimental setup closely resembles that described by Forsberg (104).

Figure 3.13 Diagram of modified Dynamic Drainage Jar to measure the

drainage properties of pulps.

PLC

Vacuum

Pump

Scales

Bleed line

Data logger

PT

DDJ

Receiver


96

Figure 3.14 Photograph of the modified dynamic drainage jar with vacuum

control and data logging.

The vacuum was varied between 0 kPa and 40 kPa. Applying even a small

vacuum to the DDJ resulted in the entire amount of water in the slurry (i.e. almost

all of the 500 mL) passing through the screen in well under the 30 s required by

the Tappi test method. Tappi test method T 261 cm-00 was modified so that the

experiment ended when there was no water level in the DDJ (i.e. it had all passed

through the screen to become filtrate). The time for the water to pass through the

screen was recorded and the fines retention was calculated.

The effect of vacuum on the bagasse pulp fines retention and drainage time

was studied both with and without CPAM and bentonite drainage aids.

The fines retention data obtained using this method are not comparable with

the fines retention data presented in the previous section. The formation of a pulp

pad using the modified method increases the fines retention when compared to the

standard Tappi method.

DDJ

Controllers and data logging

equipment

Flowmeter

Filtrate line

Stirrer

Filtrate receiver

vessel

Digital weigh scales

(logged)

Pressure

transducer

Vacuum line

Vacuum controller

solenoid

Vacuum bleed line


97

3.7.3. Methodology – Effect of chemical additives on permeability and

compressibility.

The procedure for measuring the steady-state and dynamic permeability and

compressibility in sections 3.4, 3.5 and 3.6 were repeated using the four types of

bagasse pulp used in the study of chemical additives (i.e. samples 53, 56, 58 and

60) but with the addition of the CPAM and bentonite dual additive system

optimised in section 3.7.1. The required concentrations of flocculants were

determined (see section 4.6).

For both the permeability and compressibility experiments, the chemical

additives were added to the pulp slurry immediately prior to loading into the

respective equipment. The CPAM was added first and the slurry was stirred with

a large spatula for 30 s, followed by the addition of bentonite.

3.8. Summary of the experimental investigation

The washing, drying and cold storage of bagasse were necessary to

minimise degradation of the bagasse. This was undertaken as quickly as possible

once the bagasse was delivered from the sugar factory. Turning the bagasse over

at the factory in order to cool it quickly from the processing temperature of the

milling train is also thought to reduce degradation. The large volume of bagasse

required for this thesis also presented logistical challenges.

A large number of bagasse pulp samples were produced in a ‘flow-through’

reactor in Melbourne. In order to pack the maximum amount of material into the

digester, it was necessary to impregnate bagasse with hot water to make the

bagasse more flexible. This made it easier to pack bagasse into the cells.

The chemical characterisation and the majority of the fibre length

distribution analysis was conducted in Brisbane. The response of the pulp to

beating was undertaken in India as the pulp refining equipment was not available

in Queensland. The microscopy work was conducted in New Zealand by Scion as

the expertise in pulp fibre analysis was not available here.


98

Three types of reactor were necessary to undertake the work program. The

‘flow-through’ digester was used mainly for comparing populations of pulp

samples in compressibility and permeability experiments. The 18.5 L ‘Parr

reactor’ was used for preparing large quantities of stock pulp. Pulp prepared in

this reactor was used for destructive testing such as physical property testing and

testing with chemical additives. The benchmark wood species supplied were

produced with an ‘air-bath reactor’.

Three types of experimental equipment were constructed for this thesis,

namely a simple ‘permeability cell’, a simple ‘compressibility cell’ and a

‘modified DDJ’.

The ‘permeability cell’ was used for measuring the steady-state permeability

parameters, Sv and �, of bagasse and wood pulp pads.

The compressibility cell was used initially to measure the steady-state

compressibility parameters M and N of a bagasse and wood pulp pads. Quasi

steady-state conditions were reached by compressing the pulp pad over a very

long time. The compressibility cell was then used to measure the dynamic

filtration properties of a pulp pad under a constant rate of compression. The load

on the pulp pad was compared to the predictions of the dynamic filtration model

generated in FORTRAN 77 using the steady-state permeability and

compressibility parameters.

Finally, the modified DDJ was used to optimise a suitable shear-stable

chemical additives system for a bagasse pulp slurry. The chemical additives were

added to the slurry used to make pulp pads in the permeability and compressibility

experiments and the effect of chemical additives on the steady-state permeability

and compressibility parameters were measured. The effect on the dynamic

filtration was also measured in the compressibility cell. The modified DDJ

allowed for complementary information to be collected using thin pulp mats.

Chapter 4- Results and discussion

99

Chapter 4

Results and discussion

This chapter presents the findings of the investigation into bagasse pulp

filtration, comparing the permeability and compressibility data of Australian

bagasse pulp with numerous benchmark pulp samples and the findings of previous

workers, such as El-Sharkawy and co-workers for bagasse pulp (64), and

Ingmanson for wood pulp (82). The effect of bagasse preparation and flocculants

affects the permeability and compressibility properties of a bagasse pulp pad. Of

particular importance are the steady-state permeability parameters, the specific

surface area (Sv) and the swelling factor (�) and the compressibility factors M and

N. These parameters are independent of pulp concentration.

The dynamic filtration model developed in Chapter 3 is validated.

Supplementary information is provided to assist with identifying potential bagasse

paper products.

Similarly to Chapter 3, this chapter proceeds in the same order as the

research methodology outlined in section 3.1. An analysis of the kappa number

and yield data obtained during the pulping experiments with the ‘flow-through’

reactor is provided (section 4.1). The chemical character of the bagasse pulp was

determined and compared to the parent bagasse. The physical properties of the


100

bagasse pulp are compared to the findings of previous workers. The fibre

morphology is also measured (section 4.2). The steady-state permeability (section

4.3) and compressibility (section 4.4) of bagasse pulp pads was investigated. This

information was used to determine whether the fraction of bagasse or the mode of

juice extraction has a measurable effect on permeability and/or compressibility.

The steady-state permeability and compressibility parameters for bagasse pulp

were used in the dynamic model developed in this study. The output of the model

is compared to experimental data obtained under dynamic filtration conditions

(section 4.5). A suitable flocculant system was optimised with a DDJ using a pulp

slurry. The effect of vacuum on the drainage time of a bagasse pulp slurry was

examined. The effect of chemical additives on the permeability and

compressibility constants of a pulp pad was quantified (section 4.6).

4.1. Results of bagasse chemical pulping

In order to achieve a pulp with a kappa number of 20, kinetics experiments

were undertaken using the ‘flow-through’ reactor to determine the required

cooking time (section 4.1.1). The effect of pre-treatment conditions on the yield

(section 4.1.2) and kappa number (section 4.1.3) were examined. A summary of

the findings of the bagasse chemical pulping study is provided in section 4.1.4.

4.1.1. Bagasse pulping kinetics

The cooking conditions for the kinetics experiments were 0.4 M caustic

soda and 0.1% AQ on dry fibre at 145 °C. The pulping time was varied between

6 min and 70 min. Screening of the pulp was not possible at APPI. Figure 4.1

shows the effect of cooking time on the unscreened kappa number of the

‘medium’ milled Q208B bagasse pulp produced in the flow-through reactor. The

kappa number decreased linearly with increasing cooking time. This chemical

concentration had been previously successful for cooking bagasse in a previous

study (68). Previous work had established that the cooking time for eucalypt

using the flow-through reactor is 3 h - 5 h (136) indicating that bagasse is far more

easy to pulp compared to eucalypt.


101

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 10 20 30 40 50 60 70 80

Pulping time, min

Kap

pa

num

ber,

-

Figure 4.1 Effect of cooking time on the kappa number of ‘medium’

milled Q208B unscreened bagasse pulp.

The milled and diffuser bagasse required similar cooking conditions to

achieve a kappa number of 20 on unscreened material3. From the kinetics study it

was established that a cooking time of 30 min was adequate to achieve a kappa

number of 20.

When the pulped samples were later screened with a Packer screen (200 �m

slots), the rejects was in the range 3% to 7% of total pulp weight.

Data on screen rejects are available in the summary of the pulp samples

produced for this study (i.e. Appendix B).

4.1.2. Effect of bagasse pre-treatment on Australian bagasse pulp yield

The effect of the two pre-treatment variables on the pulp yield, i.e. the

fraction of bagasse (coarse or medium fractions) and the method used to obtain

the bagasse (i.e. milled or diffuser extraction process) is presented in Table 4.1.

These pulp samples were prepared under identical conditions in the ‘flow-through

reactor’, 30 min at 145 °C. The average values presented are for Samples 20-43

(excluding the bagasse from Argentina, Samples 32 and 37). Each of these

3 The kappa number of the screened samples were 3-5 units higher than the unscreened samples.


102

samples is classified as being derived from four categories, ‘coarse’ bagasse from

a mill, ‘medium’ bagasse from a mill, ‘coarse’ bagasse from a diffuser and

‘medium’ bagasse from a diffuser. The variables were tested for statistical

significance using Student’s pooled t-test at a 95% confidence interval.

The fraction of bagasse used (‘coarse’ or ‘medium’) had a significant effect

on pulp yield for both the milled and diffuser bagasse, as shown by the high � (i.e.

Student’s test statistic) values. The pulp yield of ‘coarse’ bagasse was higher than

that for ‘medium’ bagasse. The value of �PE for pulp yield is very small (1.3% for

milled bagasse population and 1.8% for diffuser bagasse population).

Although ‘coarse’ diffuser bagasse had 1.5% higher pulp yield than ‘coarse’

milled bagasse, this was found not to be statistically significant implying that the

method of bagasse preparation was not a factor.

Table 4.1 Summary of average bagasse pulp yield fraction obtained in

the ‘flow-through’ reactor.

Milled, %

(number of samples)

Diffuser, % (number of

samples)

Coarse 54.04 (6) 55.71 (3)

Medium 50.15 (5) 50.66 (4)

�PE 1.31 1.83

� 4.909 3.614

The overall average pulp yield for Australian bagasse samples was 52.5%.

The screened rejects were 3% - 7%. However, the screened yield for the

Argentinean sample was much higher, 61.8%. The difference between the

Australian pulp and Argentinean pulp may be related to differences in cane

variety.

The average pulp yield for Australian bagasse obtained is slightly lower

than found by previous workers, perhaps due to the reasonably high level of

screened rejects or simply due to the type of digester used in pulping. Paul and


103

Kasiviswanathan (70) reported screened pulp yield of 54.4% with 0% screen

rejects for a highly depithed soda pulp. Giertz and Varma (4) reported 51% pulp

yield with screen rejects of 7.6%. This was increased to 56.9% when increased

cooking time to reduce the screened rejects to 2%.

4.1.3. Effect of bagasse pre-treatment on bagasse pulp kappa number

Student’s t test, as in the previous section, was used to analyse the kappa

number of pulp samples originating from different bagasse preparation methods.

The average kappa number of ‘coarse’ bagasse pulp was 2 units higher than that

for ‘medium’ bagasse pulp (Table 4.2). As �PE for the kappa number was very

low there was a statistically significant difference in kappa number at a 95%

confidence interval between the ‘coarse’ and ‘medium’ bagasse pulp. This is

supported by the high � value. The increase in residual lignin content for ‘coarse’

bagasse pulp accounted for a small part (0.3%) of the 4% increase in the pulp

yield between the bagasse fractions. No statistically significant difference in

kappa number was observed between the pulp obtained from the diffuser bagasse

and the pulp obtained from milled bagasse for either the ‘coarse’ or ‘medium’

fractions.

Table 4.2 Summary of average bagasse pulp kappa number in the ‘flow-

through’ reactor.

Milled, -

(number of samples)

Diffuser, -

(number of samples)

Coarse 25.2 (6) 25.8 (3)

Medium 23.3 (5) 23.5 (4)

�PE 1.01 1.00

� 2.97 3.00

4.1.4. Summary of bagasse pulping analyses

A brief kinetics study that only considered time as a variable was performed

on the delignification of Australian bagasse in the ‘flow-through’ reactor. The

chemical concentration was 0.4 M caustic and 0.1% AQ and the temperature was


104

145 °C. It was found that a cooking time of 30 min was required to make bagasse

pulp with an unscreened kappa number of 20.

Fractionation of bagasse had a significant effect on pulp yield and kappa

number.

The bagasse pulp yield and the pulp’s kappa number were not affected by

whether the pulp originated from bagasse was processed by a mill or a diffuser.

4.2. Results of physical and chemical property testing

In this section, the results of the pulp chemical and physical properties are

presented and discussed (section 4.2.1 and section 4.2.2). The fibre length

distribution data is provided in section 4.2.3. The findings of the microscopy

investigation are shown in section 4.2.4. The chemical and physical property

testing are summarised in section 4.2.5.

4.2.1. Pulp chemical analysis results

The results from the chemical analysis of the bagasse and pulp samples are

presented in Table 4.3 along with those of eucalypt pulp. All samples had

negligible quantities of Dichloromethane (DCM) extractives. The acid insoluble

lignin content was typically 21% in bagasse and less than 3% for the pulp. Both

the pulp and bagasse had around 1% acid soluble lignin. The lignin content is

consistent with the pulp kappa number (Appendix B).

The ash content of the whole bagasse (6.9%) was significantly higher than

for the ‘coarse’ and ‘medium’ fractions of the bagasse (1.8%-2.5%). Depithing

the bagasse has the added effect of reducing the ash content. Depithing removed

some fine dirt that was entrained in the whole bagasse. This effect has been noted

by previous workers, for example, Paul and Kasiviswanathan (70). For the

fractionated bagasse samples that were generated using the ‘flow-through’ reactor,

around 75% of the ash was removed during pulping. Less than 20% of the ash

from the whole bagasse produced in the ‘Parr’ reactor was removed during

pulping. This is believed to occur in the ‘flow-through’ reactor by liquor washing

the ash out of the pulp. The geometry of the ‘flow-through’ reactor is more

conducive to circulation of liquor through the bed of bagasse.


105

The glucan content of the bagasse pulp hydrolysate was increased slightly

by fractionating the bagasse prior to pulping from 65.7% for the whole bagasse

pulp up to 75% for ‘coarse’ diffuser pulp. The diffuser pulp had slightly higher

glucan content than the milled bagasse pulp (around 69%).

The arabinan content of all the bagasse pulp samples was similar (mainly 4-

5%).

The depithed Argentinean bagasse had a lower glucan content and higher

arabinan content than the fractionated Australian bagasse pulp. However this did

not translate to any difference in the pulp composition.

The chemical composition of the eucalypt pulp hydrolysate is unremarkable

except to note that it has a higher glucan content and lower arabinan content than

any of the bagasse pulp samples. This result was expected because eucalypt is

grown primarily for its fibre content, unlike Australian sugarcane.

106

Table 4.3 Chemical analysis of bagasse, bagasse pulp and eucalypt pulp.

Sample

%DCM

extractives

on air dry wt

%Acid

Insoluble

Residue

%Acid

soluble

lignin

%Ash on

OD

bagasse

%Glucan %Xylan %Arabinan

Milled coarse bagasse (parent of Sample 26) 0.42% 21.05 1.02 2.245 45.6 27.2 2.2 Milled medium bagasse (parent of Sample 27) 0.49% 21.45 1.03 2.588 40.5 24.3 3.4 Diffuser coarse bagasse (parent of Sample 20) 0.45% 21.44 1.01 1.784 42.4 24.2 3.8 Diffuser medium bagasse (parent of Sample 39) 0.34% 21.69 0.99 2.477 40.9 24.3 5.8 Argentinean depithed bagasse (parent of Sample 32) 0.79% 21.29 0.99 2.330 37.0 23.7 7.1 B

agas

se

Whole bagasse (parent of Sample 53) 0.48% 24.16 0.96 6.869 38.9 22.4 6.9

Milled coarse pulp (Sample 26) 0.17% 3.00 0.93 0.484 68.4 23.3 4.1 Milled medium pulp (Sample 27) 0.19% 2.42 0.94 0.659 69.2 23.6 2.7 Diffuser coarse pulp (Sample 20) 0.18% 2.32 0.97 0.378 73.3 25.3 4.6 Diffuser medium pulp (Sample 39) 0.32% 2.60 0.96 0.551 75.3 25.4 5.9 Argentinean bagasse pulp (Sample 32) 0.46% 3.42 0.90 0.862 71.8 26.9 4.2

Bag

asse

pulp

Whole bagasse pulp (Sample 53) 0.15% 1.43 0.95 5.704 65.7 23.9 4.1

Eucalypt pulp 0.24% 2.60 1.04 0.799 77.7 23.2 1.0

106

Thom

as J. Rain

ey –

A stu

dy o

f bag

asse pulp

filtration

Chapter 4 - Results and discussion

107

4.2.2. Pulp physical property results

For this study, only one bagasse pulp (‘coarse’ pulp, Sample 56) was

evaluated for its physical properties. The results presented in Table 4.4 are

compared to the benchmark “30% depithed” Australian bagasse pulp (Sample 58).

Additional comparisons were also conducted with the findings of previous

workers.

Table 4.4 Physical properties of a ‘coarse’ bagasse pulp with a

benchmark Australian bagasse pulp (142).

Property ‘Coarse’ bagasse pulp

(Sample 56)

Benchmark bagasse

pulp; ‘30% depithed’

pulp (Sample 58)

Amount of PFI refining

0

(rev)

1000

(rev)

2000

(rev)

0

(rev)

500

(rev

1000

(rev)

Freeness CSF (mL) 615 410 290 485 250 200

Tensile Index (Nm.g) 70.2 73.4 76.0 74.0 81.5 82.5

Tear Index (mN.m2/g) 7.05 6.20 5.60 5.60 4.35 4.30

Burst Index (kPa.m2/g) 3.50 3.80 4.25 3.60 4.50 5.35

Short Span Compression

Index (kNm/kg)

31.2 32.1 34.0 32.6 35.0 37.7

Water Retention Value (%) 255 262 281 274 287 292

Apparent Density (g/cm3) 0.67 0.72 0.75 0.60 0.72 0.81

Fibre Strength Index 10.5 - - 9.0 - -

The ‘coarse’ bagasse pulp (Sample 56) starts with a much higher freeness

than the benchmark pulp (Sample 58, Table 4.4) as would be expected from its

higher permeability (see section 4.3).


108

In comparison to the benchmark Australian bagasse pulp, the ‘coarse’

bagasse pulp has similar tensile strength for a given freeness. For both pulp

samples, refining, as shown in Figure 4.2, only made a modest improvement in

tensile strength (+10%). The Australian benchmark pulp had slightly higher

tensile strength than reported by most previous workers (4, 70). The pulp had

lower tensile strength than reported by Gartside and co-workers (28).

The ‘coarse’ bagasse pulp had much higher tear strength than the

benchmark pulp but for both samples, as shown in Figure 4.3, refining had a

significant detrimental effect on its tear index (-20%). The Australian bagasse

pulp had tear strength similar to those reported by previous workers (4, 70).

As shown in Figure 4.4, ‘coarse’ bagasse pulp had similar burst strength to

the benchmark pulp, but higher than the findings of other workers (4). Refining

was moderately effective for the burst index of ‘coarse’ bagasse pulp (+21%) but

was extremely effective for the benchmark pulp (+49%). The datum from

Gartside and co-workers (28) are unusually high and may be as a consequence of

the much longer fibre length of their pulp. It is noted that the bagasse pulp used

by Gartside and co-workers had a longer fibre length, typically 1.38 mm for most

species of cane (although one variety is reported to be as long as 1.55 mm) than

the bagasse pulp in this study (~1 mm).

In this study, tensile and burst strengths did not improve by changing the

treatment conditions which is a different finding to that reported by Paul and

Kasiviswanathan (70). In this study, tear improved dramatically by changing the

pre-treatment conditions.


109

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700

Canadian Standard Freeness (ml)

Te

nsile I

nde

x (

Nm

/g)

Sample 56 - 'Coarse' bagasse pulpSample 58 - 'benchmark bagasse pulpGartside and coworkersGiertz and VarmaPaul and KasiviswanathanLinear (Sample 56 - 'Coarse' bagasse pulp)

Figure 4.2 Tensile index as a function of freeness.

0

1

2

3

4

5

6

7

8

0 100 200 300 400 500 600 700


Te

ar

Inde

x (

mN

.m2/g

)

Sample 56 - 'Coarse' bagasse pulpSample 58 - benchmark bagasse pulpGartside and coworkersGiertz and VarmaPaul and KasiviswanathanLinear (Sample 56 - 'Coarse' bagasse pulp)

Figure 4.3 Tear index as a function of freeness.


110

0

1

2

3

4

5

6

7

0 100 200 300 400 500 600 700


Bu

rst In

dex (

kP

a.m

2/g

)

Sample 56 - 'Coarse' bagasse pulpSample 58 - benchmark bagasse pulpGartside and coworkersPaul and KasiviswanathanLinear (Sample 56 - 'Coarse' bagasse pulp)

Figure 4.4 Burst index as a function of freeness.

The benchmark pulp had slightly better compression properties than the

‘coarse’ bagasse pulp (Figure 4.5). Refining provided a minor improvement to

compressive strength. The ‘coarse’ bagasse pulp had better WRV than the

benchmark pulp (Figure 4.6). Refining was slightly detrimental to WRV. The

density of the benchmark pulp was initially better than the ‘coarse’ bagasse pulp

but deteriorated rapidly when refined (Figure 4.7).

The short-span compression, WRV and density of the bagasse pulp in this

study were not compared with previous workers as the associated pulp freeness is

not reported in other literature.


111

20

22

24

26

28

30

32

34

36

38

40

0 100 200 300 400 500 600 700


Sho

rt s

pan c

om

pre

ssio

n in

dex (

kN

.m/g

)

Sample 56 - 'Coarse' bagasse pulp

Sample 58 - benchmark bagasse pulp

Figure 4.5 Short span compression as a function of freeness.

200

220

240

260

280

300

0 100 200 300 400 500 600 700


Wate

r re

tentio

n v

alu

e, %

Sample 56 - 'Coarse' bagasse pulp


Figure 4.6 Water Retention Value as a function of freeness.


112

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 100 200 300 400 500 600 700


Ap

pare

nt

de

nsity (

g/c

m3)

Sample 56 - 'Coarse bagasse pulp


Figure 4.7 Density of bagasse pulp as a function of freeness.

4.2.3. Fibre length distribution analysis

Table 4.5 provides a summary of the average fibre length of the samples

analysed mainly using the FS100 unit. The order of decreasing average fibre

length is ‘coarse’ (1.09 mm) > ‘medium’ (0.95 mm) > ‘fine’ (0.52 mm). The

presence of short pith fibres reduced the average fibre length of the ‘whole’

bagasse pulp (Sample 53) and the ‘30% depithed’ bagasse pulp (Sample 58). The

Argentinean bagasse pulp (Sample 32) has the same average fibre length as the

‘30% depithed’ Australian bagasse pulp (Sample 58). All bagasse pulp samples

have a longer fibre length than the eucalypt pulp (0.77 mm), apart from the ‘fine’

bagasse pulp. Pine pulp has the longest fibre length (2.65 mm). There was no

difference between diffuser and mill bagasse pulp.

Table 4.5 also presents a summary of the fines content. The FS100 unit

reported this information as the length of the first decile of the population, i.e. the

more fines, the shorter the length of the smallest 10% of fibres. It is noted that

every bagasse pulp sample have a higher proportion of short fibres than the

eucalypt pulp.


113

Table 4.5 Summary of fibre length analyses using a Kajaani FS100 on a

length weighted basis.

Pulp sample

Average

fibre

length, mm

Maximum length of

the shortest 10% of

fibres, mm

'coarse' (average for all samples)

1.09 0.31

'medium' (average for all samples)

0.95 0.27

'fine' (Sample 51) 0.52 0.11 ‘whole' (Sample 53) 0.8 0.21 Bagasse pulp with 30% fibre removal (Sample 58) *

0.965 Not available

'Argentinean bagasse' (Sample 32)

0.96 0.23

Eucalypt 0.77 0.39 Pine pulp 2.65 0.65

*sample analysed on FQA at UBC. The maximum length of the smallest 10% of fibres is not

available on this unit.

Figure 4.8 shows the fibre length distribution of Australian pulps derived

from fractionated bagasse, with those of Argentinean and eucalypt pulps. The

figure shows that the fraction of fibres shorter than 0.3 mm in length for bagasse

pulp is far higher than that for the eucalypt pulp. However, the bagasse pulp

samples have more fibres greater than 1.3 mm in length. The ‘coarse’ and

‘medium’ bagasse pulp samples often have a bimodal distribution which is more

pronounced in the ‘coarse’ bagasse samples than the ‘medium’ bagasse samples; a

curious observation. More data is available in Appendix E.


114

0

5

10

15

20

0 0.5 1 1.5 2 2.5

Fibre length (mm)

Perc

ent le

ngth

fra

ctio

n (

%)

Eucalypt

Argentinean bagasse pulp (Sample 32)

Pulp from coarse bagasse material (Sample 20)

Pulp from medium bagasse material (Sample 30)

Pulp from fine material (Sample 51)

Figure 4.8 Fibre length distribution of pulp samples including ‘coarse’

‘medium’ and ‘fine’ bagasse pulp, bagasse pulp from

Argentina and eucalypt pulp.

The results for curl and kink index are presented in Table 4.6 as they were

produced on a FQA. The curl and kink index for the ‘medium’ bagasse pulp was

slightly lower than the ‘coarse’ bagasse pulp. It is presumed that the higher level

of mechanical damage may give appreciably higher curl and kink index for the

‘medium’ bagasse pulp fibres. Both the coarse and medium bagasse pulp fibres

were found to have a lower kink index than the 30% depithed bagasse pulp and

slightly lower curl.

Table 4.6 Curl and kink index measurements for bagasse pulp samples

Curl, length

weighted basis (-)

Kink index (mm-1)

Coarse bagasse pulp (Sample 56) 0.055 0.78

Medium bagasse pulp (Sample 60) 0.045 0.65

30% depithed bagasse pulp (Sample 58) 0.079 1.24


115

4.2.4. Microscopic analysis

Confocal laser microscope and image analyses were used to obtain

additional information about fibre morphology. Over 500 fibre sections were

analysed for each of three pulp samples, ‘coarse’ milled bagasse (Sample 26),

‘medium’ milled bagasse (Sample 27) and the eucalypt pulp. Some of the key

results of the microscopy investigation are reported in Table 4.7.

Table 4.7 Results of microscopy study using a confocal laser microscope

and image analysis (143).

Bagasse pulp

derived from

the coarse

fraction of

milled bagasse

Bagasse pulp

derived from

the medium

fraction of

milled bagasse

Eucalypt

pulp

Least Significant

Difference (LSD)

between means

Fibre width (�m) 20.2 20.7 18.6 0.820

Fibre thickness (�m) 13.9 12.7 11.6 0.365

Wall thickness (�m) 5.13 4.72 4.19 0.151

Fibre area (�m2) 214 200 159 12.4

Fibre perimeter (�m) 68.5 67.6 60.9 2.26

Wall area (�m2) 186 169 132 9.65

Lumen area (�m2) 31.0 27.9 26.3 4.41

Collapse ratio (-) 1.48 1.66 1.63 0.0591

Minimum wall thickness

(�m)

3.46 3.18 2.75 0.126

Maximum wall

thickness (�m)

7.03 6.70 6.09 0.243

The fibre widths of the ‘coarse’ and ‘medium’ pulp samples were around

20.5 �m. The difference between the means of the two samples was less than the

Least Significant Difference, so there is no statistically significant difference in

fibre width between the ‘coarse’ and ‘medium’ bagasse pulp samples. The

bagasse pulp width was slightly wider than the eucalypt pulp, 18.55 �m.

The ‘coarse’ bagasse pulp fibres were significantly thicker (13.86 �m) than

the ‘medium’ bagasse pulp fibres (12.67 �m). Fibres from both bagasse pulp


116

samples were thicker than those from the eucalypt pulp sample, 11.63 �m.

Similarly, the fibre walls of the ‘coarse’ bagasse pulp fibres (5.13 �m) were

significantly thicker than for the ‘medium’ bagasse pulp fibres (4.72 �m) and the

eucalypt pulp fibres (4.19 �m).

The ‘coarse’ bagasse pulp fibres had a higher wall area (186 �m2) than the

‘medium’ bagasse fibres (169 �m2). The fibres from both bagasse pulp samples

had much higher wall area than the eucalypt pulp sample (132 �m2). The lumen

area for the ‘coarse’ bagasse pulp res (31.0 �m2) was found to be slightly larger

than the eucalypt pulp fibres (26.3 �m2). The mean lumen area of the ‘medium’

bagasse pulp fibres could not be differentiated statistically from either the ‘coarse’

or eucalypt pulp fibres.

The collapse ratio for the ‘coarse’ bagasse pulp (1.48) was substantially

greater than for the ‘medium’ bagasse pulp and the eucalypt bagasse pulp. This is

potentially due to the higher mechanical damage to the ‘medium’ bagasse material

in the milling train.

The morphological data suggests that the bagasse pulp fibres have thicker

cell walls and are less likely to collapse than the eucalypt pulp fibres.

4.2.5. Summary of pulp physical and chemical property testing

The ash content of the bagasse is significantly reduced by the pre-treatment.

More ash is washed away by the ‘flow-through’ reactor than by the ‘Parr’ reactor.

However, there was little difference in composition between the fractionated

Australian bagasse and the Argentinean bagasse.

The ‘coarse’ fraction of bagasse significantly improved the initial freeness,

tear properties and WRV of the pulp compared to the benchmark Australian pulp.

The pre-treament did not improve tensile or burst properties and had a slightly

negative impact on apparent density and compressive strength. Refining did not

significantly improve any strength properties and reduced the tear strength.

Fractionating the bagasse makes the bagasse pulp more suitable for products

where tear strength is important. As shall be shown in more detail in the next


117

section, the increase in freeness with pre-treatment improves permeability and

improves the prospects for increasing production rates.

The fibre length of the ‘coarse’ bagasse pulp is longer than the ‘medium’

bagasse pulp. The Argentinean bagasse pulp has similar fibre length to Australian

bagasse pulp prepared under the same conditions. All bagasse pulps had a much

wider fibre length distribution than the eucalypt pulp. They also had far more

short fibres than the eucalypt pulp.

The bagasse pulp fibres have thicker walls than the eucalypt fibres. This

suggests that they may be more rigid.

4.3. Results of steady-state permeability testing

This section gives data for K (for two fixed concentrations), Sv and �. The

effect of using a constant and variable k on the optimum values for Sv and � is

reported (section 4.3.1). A statistical analysis to determine the effect of pre-

treatment conditions on pulp pad permeability is shown in section 4.3.2. A

discussion of the suitability of the Kozeny-Carman steady-state permeability

model for bagasse pulp is provided in section 4.3.3. The results are compared with

the findings of previous workers (section 4.3.4). The findings of the permeability

study given the high proportion of short material are surprising. Finally a

summary of the steady-state permeability results is given in section 4.3.5.

4.3.1. Data from steady-state permeability testing

For each pulp sample tested, K was determined from Darcy’s Law over a

wide concentration range. Table 4.8 shows K, at only two concentrations,

0.08 g/cm3 and 0.12 g/cm3. Sv and � values were determined from the intercept

and slope of graphs of (Kc2)1/3 against concentration, such as those shown in

Figure 4.9.

118

Table 4.8 Comparison of permeability parameters obtained from the Kozeny-Carman model with both a constant and variable Kozeny

factor.

Sample name

Fraction

Permeability K

(×108 cm2) at � =

0.08 g/cm3

Permeability K

(×108 cm2) at � =

0.12 g/cm3

Specific surface area Sv (cm-1)

constant k factor

Specific surface area Sv (cm-1),

variable k factor

Swelling factor �(cm3/g),

constant k factor

Swelling factor �(cm3/g),

variable k factor

43 Coarse Milled 37.3 9.31 1540 1160 3.44 4.03 26 Coarse Milled 51.7 13.3 1420 1010 3.27 3.96 38 Coarse Milled 26.9 5.19 1570 1180 3.84 4.50 20 Coarse Diffuser 37.5 8.56 1520 1120 3.52 4.19 21 Coarse Diffuser 40.9 6.15 1390 980 3.61 4.39

Crude average for Coarse fraction 38.8 8.51 1490 1090 3.54 4.21

18 Medium Milled 30.6 7.47 1820 1290 3.33 4.04 27 Medium Milled 26.6 6.95 1830 1290 3.38 4.10 42 Medium Milled 23.1 6.45 2260 1610 3.10 3.75 39 Medium Diffuser 24.1 6.15 2170 1630 3.20 3.76 35 Medium Diffuser 18.2 4.81 2760 1970 3.00 3.64

Crude average Medium fraction 24.5 6.37 2170 1560 3.20 3.94

58 30% depithed Milled 6.80 1.77 4640 4020 2.97 3.19

51 Fine 2.2 0.58 14100 9880 2.01 2.52

53 Whole 4.40 1.73 20200 11200 1.11 1.67

32; Argentinean depithed bagasse 17.8 4.16 2100 1510 3.58 4.29

Eucalypt 15.4 3.88 2480 1920 3.38 3.93

Pine 36.4 1.59 327 319 7.17 7.60

118

Thom

as J. Rain

ey –

A stu

dy o

f bag

asse pulp

filtration


119

0

0.0005

0.001

0.0015

0.002

0.00 0.05 0.10 0.15 0.20

c (g/cm3)

(Kc

2)1

/3,

g2/3cm

-4/3

Coarse bagasse pulp

Medium bagasse pulp

Unfractionated bagasse pulp

Fine bagasse pulp

Figure 4.9 Graphs of (Kc2)1/3

against concentration

A statistical analysis of K would only provide a comparison for a specific

pulp concentration. A statistical analysis was performed on Sv and � rather than K

as they are independent of concentration. Optimum values for Sv and � were

determined for both constant and variable k using a least squares regression

method. The following analysis between pulp samples compares the results for Sv

and � using a constant k.

Table 4.8 shows that fractionating the bagasse prior to pulping has a

significant effect on Sv. For Australian bagasse pulp samples, the samples

obtained from the ‘coarse’ bagasse fraction has the lowest Sv value whilst the pulp

obtained from ‘whole’ bagasse has the highest Sv value. The average Sv was

1490 cm-1 for Australian bagasse pulp obtained from the coarse bagasse fraction

and 2170 cm-1 for Australian bagasse pulp obtained from the medium bagasse

fraction.

The benchmark ‘30% depithed’ bagasse pulp also had higher Sv, 4640 cm-1

(Sample 58) than the pulp samples obtained from coarse and medium fractions of

bagasse. The very high values for Sv obtained for the fine bagasse pulp (Sample


120

51) and the whole bagasse pulp (Sample 53) was associated with the very high

proportion of pith material. Pith has a very high surface area to volume ratio and

consequently increases Sv.

The Sv of Argentinean bagasse pulp (Sample 32), 2100 cm-1, was higher

than the Australian pulps derived from the ‘coarse’ and ‘medium’ bagasse

fractions.

Although both the Australian ‘30% depithed’ bagasse pulp (Sample 58) and

the Argentinean bagasse pulp both had 30% of the shortest material removed, they

have quite different values for Sv. The difference between the Sv values was

possibly due to the difference between the APPI ‘flow-through’ reactor and the

batch Parr reactor. Alternatively the difference may have been related to the cane

variety. Argentinean sugarcane is bred specifically to produce fibre for paper

manufacture (144).

Significantly, the Australian bagasse pulp had a lower Sv than the eucalypt

pulp, 2480 cm-1, meaning that it has higher permeability. This is the most

important finding of this bagasse pulp study. The thicker fibre walls and the

higher proportion of longer fibres contributed to a more open pulp pad matrix,

despite the higher fraction of short fibres (section 4.2.3). A significant proportion

of pith was removed by the pre-treatment and possibly also by the cooking

process in the ‘flow-through’ digester, reducing the influence of pith on bagasse

pulp permeability.

The Sv for eucalypt pulp was much less than the benchmark ‘30% depithed’

bagasse pulp as expected.

The Sv for pine pulp was much lower than the pulp obtained from Australian

bagasse pulp because of its longer fibre length.

For the Australian bagasse pulp samples, when a variable k is used in the

calculation of Sv the value of Sv is about 400 cm-1 (~25%) on average lower than

when it is calculated with constant k. As mentioned previously, the factor k gives

an indication of the tortuosity for the capillaries through the pulp pad.


121

The � values of the pulps derived from coarse and medium Australian

bagasse fractions were not very different from Argentinean bagasse (Sample 32)

and eucalypt pulp (see Table 4.8). The benchmark ‘30% depithed’ bagasse pulp

(Sample 58), also had a slightly lower � value. � affects the permeability

properties as well as Sv. � values obtained with a variable k were about 25%

higher than the values obtained with a constant k. High � values should also

indicate the potential for strength generation during refining. However, pulp

samples with higher values of � were not observed to have better strength

generation properties during refining.

4.3.2. Effect of bagasse pre-treatment on pulp permeability properties

The breakdown of Sv and � values for pulp samples originating from

different bagasse fractions and different sugar extraction processes are shown in

Table 4.9.

Table 4.9 Average Sv and � values for pulp samples originating from

Australian bagasse.

Sv (cm-1) � (cm3/g)

Coarse Medium Coarse Medium

Milled 1510 1970 3.52 3.27

Diffuser 1460 2470 3.57 3.10

The Sv and � values for each type of bagasse pulp were compared using

Student’s pooled t-test with a 95% confidence interval. The test statistic, �, used

to compare ‘coarse’ and ‘medium’ bagasse pulp is shown in Table 4.10 The �

value indicates that there is a difference in Sv between coarse and medium bagasse

pulp for either milled and diffuser bagasse pulp. The test statistic for � is less

clear. There is very strong evidence that there is a difference in � between pulp

derived from coarse and medium bagasse from the diffuser (high � = 6.07) but not

from the mill (low � = 1.83).


122

Table 4.10 � values for Sv and � for pulp samples obtained from different

bagasse fractions.

� for Sv values

(-)

� for � values

(-)

� within milled bagasse population 4.24 1.83

� within diffuser bagasse population 4.69 6.07

The low values for the test statistic, �, show that there is no difference

between the pulp samples obtained from the two sugar extraction methods for

either Sv nor � (Table 4.11).

Table 4.11 � values for Sv and � test statistic for pulp samples obtained

from different sugar extraction methods.

� for Sv values

(-)

� for � values

(-)

� within coarse bagasse population 0.87 0.25

� within medium bagasse population 2.41 1.78

4.3.3. Review of bagasse pulp steady-state permeability model

The values of Sv and � presented in Table 4.8 obtained using a constant and

variable Kozeny factor were inserted back into the Kozeny-Carman model and

compared with the original experimental data (shown in Figure 4.10). The

Kozeny-Carman model with either a constant k (the solid line) or a variable k (the

dashed line) reasonably predicts the experimental permeability data over the

concentration range used in this study.

A dynamic permeability and compressibility model is developed and

verified later in this chapter. The dynamic model uses data extrapolated beyond

the concentration range considered here. Extrapolating the permeability model

slightly above the concentration range used in the cell shows that the model

predicts higher permeability with a variable Kozeny factor than with a constant

factor. This suggests that the tortuosity of the capillaries in the pulp pad decrease


123

as the pulp concentration increases. It will be shown later in this chapter that

extrapolating the Kozeny-Carman model with a variable Kozeny factor beyond

this concentration range allows the dynamic model to give good predictions.

1.00E-08

1.00E-07

1.00E-06

0.05 0.10 0.15 0.20

Concentration, c (g/cm3)

Pe

rmea

bility,

K (

cm

2)

Experimental results, coarse bagasse pulp

Experimental results, medium bagasse pulp

Experimental results, fine bagasse pulp

Kozeny prediction

Kozeny prediction with Davies Kozeny factor

correction

Kozeny prediction (medium)

Figure 4.10 Comparison of the Kozeny-Carman model with experiment

data with both a constant and variable Kozeny factor.

4.3.4. Comparison of steady-state permeability data with previous work

The only published bagasse pulp permeability data is by El-Sharkawy and

co-workers who examine a commercial bagasse pulp from India (50, 64).

Unfortunately, it was not possible to determine values of Sv from their study as

the variation of K with concentration was not provided. This work reported a

“Kozeny-Carman permeability constant”. The paper contained unusual units for

the constant (cm2) when k is normally unitless. Upon discussion with the authors,

this paper used an obscure permeability model and not the Kozeny-Carman

model, viz

( )21

'kKε−

ε= Equation 4.1

where k’ is a permeability constant


124

In their study, bagasse pulp was fractionated using an axial feed pressure

screen with 0.06 mm slots to reduce the fines content and improve steady-state

permeability. Their data is presented in Table 4.12. They were able to improve

permeability by 30% by pressure screening increasing k’ from 2.36×10-9 cm2 to

3.08×10-9 cm2.

Table 4.12 Values for bagasse pulp “Permeability constant” reported by

El-Sharkawy and co-workers (50, 64)

Permeability constant

of original pulp (cm2)

Canadian Standard

Freeness (ml)

Original pulp 2.36×10-9 290

Screened pulp 3.08×10-9 459

Rejected

material 5.61×10-10 200

The results of this work have been recalculated into the form of their

‘permeability constant’. It was found that the ‘permeability constant’ for ‘coarse’

and ‘medium’ Australian bagasse pulp was between 2×10-8 cm2 and 4×10-8 cm2.

This is compared to 2.36×10-9 cm2 for El-Sharkawy and co-worker’s original

Indian bagasse pulp, and 3.08×10-9 for their most permeable screened pulp, a ten-

fold improvement.

For data collected in this study, the form of the conventional Kozeny-

Carman model was found to have better agreement with the experimental data

than the permeability model used by El-Shakawy and co-workers.

Table 4.13 shows a comparison of the Sv and � values from this study with

those of previous workers that investigated wood pulp. Most of these workers did

not specify the wood species used. The specific surface area, Sv, for the bagasse

pulp samples was lower than that found by previous workers for wood pulp.

Previous workers have reported a wide range for �, from 1.65 cm3/g up to


125

4.5 cm3/g. The values of � for bagasse pulp were in this range. Sv and � for the

benchmark eucalypt pulp sample measured in this study was very similar to that

of Robertson and Mason for a kraft wood pulp (86).

Table 4.13 Comparison of Sv and � measured by various workers.

Pulp source Investigator Kozeny

factor

Other details Specific

surface area,

Sv (cm-1

)

Swelling

factor, �

(cm3/g)

Soda AQ ‘Coarse’ and ‘Medium’

Australian Bagasse

This study Constant, 5.55

Not dried 1490-2170 3.20-3.54

Soda AQ ‘Coarse’ and ‘Medium’

Australian Bagasse

This study Variable Not dried 1100-1600 3.94-4.21

Eucalypt pulp This study Constant Not dried 2480 3.38

Soda AQ bagasse with 30% of pith

removed

This study Constant, 5.55

Not dried 4644 2.97

Sulfite wood pulp Robertson and Mason (86)

Constant, 5.55

Previously dried ~4100 2.80-3.08

Not dried ~2300 4.4-4.5

Kraft wood pulp Robertson and Mason (86)

Constant, 5.55

Not dried ~2300 3.66-4.27

Wood pulp Ingmanson and co-

workers (81)

Constant 4200 1.65

Variable ~2900 ~2.07

Sulphate wood pulp Gren (84) Constant, 5.55

2000-3000 4.8

The relative changes in bagasse pulp Sv values (~25%) between a variable

and constant k were more than the 7% previously reported by Ingmanson and co-

workers (81) for wood pulp. The change in � values was the same as previously

reported for wood pulp, 25%.

4.3.5. Summary of steady-state permeability experiments

The steady-state permeability properties of Australian bagasse pulp have

been measured in a simple custom built cell and reported. The steady-state

permeability data are needed for the dynamic filtration model.


126

Pulp derived from fractionated Australian bagasse was produced in the

laboratory with permeability that compares favourably with eucalypt pulp, despite

a higher overall fine fibre content. It is thought the fibre stiffness and the high

proportion of fibres greater than 1.3 mm in length creates a highly permeable

bagasse pulp pad.

Australian pulp derived from the ‘coarse’ bagasse fraction has better steady-

state permeability than the ‘medium’ fraction, as measured by lower Sv. The

‘coarse’ bagasse pulp from a diffuser has a higher � value. This confirms that

there is a difference between the fractions of bagasse (Objective 1a) with respect

to pulp permeability.

There was not found to be a difference in bagasse pulp permeability

between bagasse pulp from a diffuser and a mill (Objective 1b).

For bagasse pulp, a variable k gives a 25% higher value for � and a 25%

lower value of Sv compared with a constant k. This is similar to the findings

obtained for wood pulps.

The findings from the steady-state permeability study are consistent with the

findings of previous workers. A standard Australian bagasse pulp (Sample 58)

had worse Sv than wood pulp but was improved dramatically by the pre-treatment

and pulping procedure. The values of � for bagasse pulp are within the range of

findings for other workers using wood pulp.

Good agreement was found to exist between the experimental data and the

theoretical predictions for the permeability properties of Australian bagasse pulp

using the Kozeny-Carman model with a constant or variable k.

4.4. Results of quasi steady-state compressibility testing

In this section, the steady-state compressibility parameters of pulp pads, M

and N, are determined from experiments using the compressibility cell. The

steady-state power law compressibility model was found to be suitable (section

4.4.1). The values of M and N are presented and compared to the findings of

previous workers (section 4.4.2). These values for M and N are used in the


127

dynamic model later in this chapter. The effect of pre-treatment conditions on the

compressibility parameters is compared statistically (section 4.4.3). A summary

of the quasi steady-state compressibility tests is presented in section 4.4.4.

4.4.1. Suitability of the power law steady-state compressibility model

The pressure of the platen was calculated from the load recorded by the

Instron. For each pulp sample, the platen pressure was plotted as a function of

concentration on a log-log scale. The compressibility factor, N was determined

from the slope of the linear approximation and the compressibility factor, M was

determined as the exponent of the abscissa intercept. Figure 4.11 is an example of

this graph for experimental data.

log(P) = 2.65*log(c) + 3.50

0

0.5

1

1.5

2

2.5

-1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5

log (concentration) (g/cm3)

log

(p

ressu

re)

(kP

a)

Figure 4.11 Plot of the log of the platen pressure against the log of the pulp

concentration for a sample of bagasse pulp compressed under

quasi steady-state conditions (Sample 39; experimental data is

shown as the thin solid line and the linear approximation is

shown as the thick dashed line).

The figure demonstrates that the power law steady-state compressibility

relationship, Ps = M � N is suitable.


128

4.4.2. Pulp steady-state compressibility data and comparison with the

findings of previous workers

Table 4.14 is a summary of the compressibility constants M and N obtained

from quasi steady state compressibility experiments using a range of pulp

samples. These values were used as inputs to the dynamic model presented later

in this chapter.

Table 4.14 Table of values for the compressibility factors N and M found

in this study.

This study

Sample number Fraction Source N M (kPa)

Sample 26 Coarse Mill 2.83 7650 Sample 38 Coarse Mill 2.60 3320 Sample 43 Coarse Mill 2.94 8040 Sample 20 Coarse Diffuser 2.66 4990 Sample 21 Coarse Diffuser 2.68 3960 Sample 18 Medium Mill 2.72 6090 Sample 27 Medium Mill 2.61 3730 Sample 42 Medium Mill 2.56 3780 Sample 35 Medium Diffuser 2.72 4490 Sample 39 Medium Diffuser 2.65 3190

Average 2.70 4920

Whole bagasse pulp Whole Mill 2.74 4960 Fine bagasse pulp Fine Mill 3.23 25900

Argentinean bagasse pulp 2.82 8450 Eucalypt pulp 2.43 4780

Pine pulp 2.47 6010

The compressibility parameters for wood pulp determined by previous

workers were recalculated in terms of M and N. The results for eucalypt and pine

from this study are similar to the findings by previous workers for wood pulp

(Table 4.15).

Gren and Hedstrom (85) report N over the full range of kappa numbers for

chemical pulps, from 2.22 for a fully bleached pulp to 2.37 for a 100 kappa pulp

whilst Ingmanson reports N to be between 2.66 and 3.17 (81-83). Gren and

Hedstrom (85) noted that the difference in their results compared to Ingmanson’s

(81) is due to the different compression range used in their experiments. Our

results for N were all within the range reported by these authors.


129

Table 4.15 Table of values for the compressibility factors N and M found

by other workers.

Previous studies

Sample description N M (kPa)

Unclassified wood pulp at moderate

levels of compression (85) 2.22-2.37

Not

provided

Wood pulp (81) 2.66 1450

Beaten wood pulp (82, 83) 3.12 2950

Beaten wood pulp (82, 83) 3.14 3160

Unbeaten wood pulp (82, 83) 3.17 3480

Unbeaten wood pulp (82, 83) 3.14 3190

M was found to vary significantly between samples, however N was less

variable. The values of M reported in this study (generally 3000 kPa to 8000 kPa)

were often slightly higher than previously reported (1450 kPa to 3480 kPa). This

is possibly due to the concentration range which was selected as being suitable for

the dynamic filtration study in the next phase of the project.

The bagasse pulp was not statistically different from the eucalypt pulp in

terms of M and N (determined using Student’s t-test with a 95% confidence

interval). This is despite the significant difference in the fibre morphology.

Bagasse pulp has a longer fibre length and thicker cell walls than the eucalypt

pulp.

4.4.3. The effect of pre-treament on bagasse compressibility

The pulp compressibility parameters were again statistically compared using

Student’s t test. The pulp samples analysed were subdivided into four categories.

These categories are pulp produced from ‘coarse’ and ‘medium’ fractions of

milled and diffuser bagasse.

The average values for N and M within each of the milled and diffuser

bagasse pulp populations are provided in Table 4.16


130

Table 4.16 Table of average N and M values for milled and diffuser,

coarse and medium bagasse.

N (-) M (kPa)

Coarse Medium Coarse Medium

Milled 2.79 2.63 5890 4410

Diffuser 2.67 2.57 4440 3030

Firstly the effect of the bagasse fraction on N and M will be determined

followed by the effect of the method of juice extraction

4.4.3.1. The effect of bagasse fraction on N and M

The values of � comparing the difference between coarse and medium

bagasse pulp is presented for each of the milled and diffuser bagasse pulp

samples. The low values of � in Table 4.17 indicates that the bagasse fraction did

not affect the value of N within either the milled or diffuser bagasse pulp

populations. Similarly, the bagasse fraction was not found to affect the value of

M.

Table 4.17 � values for N and M comparing ‘coarse’ and ‘medium’

bagasse pulp.

Value of � for

comparing N

values (-)

Value of � for

comparing M

values (-)

� within milled bagasse population 1.46 0.866

� within diffuser bagasse population 1.30 2.64

4.4.3.2. The effect of the mode of juice extraction on N and M

The effect of the mode of juice extraction on N and M is now determined

within each of the milled and diffuser bagasse pulp populations. Table 4.18

shows the test statistic, �, calculated to determine whether there is an effect of the


131

mode of juice extraction on M and N firstly within the ‘coarse’ bagasse population

and then the ‘medium’ bagasse population.

Table 4.18 � values for N and M comparing pulp from two methods of

juice extraction.

� values comparing

mean N values (-)

� values comparing

mean M values (-)

within coarse bagasse population 0.940 0.724

within medium bagasse population 0.709 1.36

4.4.4. Summary of steady-state compressibility testing

The power law compressibility model is suitable for bagasse pulp and wood

pulp samples. The values for M and N were measured for use in the dynamic

model. The values for N were consistent with the findings of previous workers

for wood pulp, but the values of M were slightly higher than previously reported.

Bagasse pre-treatment was not found to affect the compressibility

parameters, rejecting the notion that either fractionated bagasse or the mode of

juice extraction (i.e. Objective 1a and Objective 1b) affects the steady-state

compressibility.

4.5. Results of dynamic filtration modelling and validation

The output of the dynamic model was tested using a sensitivity analysis of

the key permeability and compressibility variables, Sv, �, M and N (section 4.5.1).

The output of the dynamic model was compared with experimental data obtained

under dynamic filtration conditions (section 4.5.2) and a summary is provided

(section 4.5.3).

4.5.1. Predictions of the dynamic model

The output from the dynamic model (Equation 2.25 to Equation 2.28)

calculated by the FORTRAN program provides the solidity as a function of both

the position within the cell as well as time. The dimensionless output of the

model is easily converted back to dimensional form. An example of the output for


132

Australian bagasse pulp is shown in Figure 4.12 and Figure 4.13 where the time

axis was scaled to 3 min. The data presented used typical values for the

permeability and compressibility parameters obtained in this study. The solidity,

� = �c is initially constant through the cell (in Figure 4.12 as t 0 min) but

during the experiment, the solidity at the top platen is higher than lower in the cell

(Figure 4.13). The concentration gradient throughout the cell is roughly uniform

and small.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 20 40 60 80

x (mm)

� (

-)

t = 0 min

t = 1 min

t = 2 min

t = 2.5 min

t = 3 min

Figure 4.12 Output from the dynamic model for Sample A; graph of � as a

function of depth below the platen.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 2 3

t (min)

� (

-)

at the top platen

at the base

Figure 4.13 Output from the dynamic model for Sample A; graph of � as a

function of time.


133

Sv and � were determined from the permeability experiments in the

concentration range 0.05 g/cm3 to 0.2 g/cm3. The dynamic filtration model

governing equation used these factors beyond this concentration range in the

dynamic filtration experiments, as mentioned in section 4.3.3. It will be shown in

section 4.5.2 that this is a valid assumption provided a variable Kozeny factor, k,

is used.

A sensitivity analysis of the effect of compression rate, M, N and Sv on the

calculated Ps was performed. Each of these parameters were varied one at a time

and compared to a ‘base-case’ where all other parameters were fixed. The solidity

predicted from the FORTRAN model was converted to platen pressure, Ps using

Equation 2.24.

The effect of � was not studied explicitly. � is assumed to be invariant with

concentration for a given pulp sample. Increasing � has the same effect as

increasing concentration viz

( )n

s cmP α=

Equation 4.2

recalling that m and n are calculated from M and N. Hence � has a

considerable effect on the dynamic model.

Firstly, it was necessary to determine the compression rate required to

observe dynamic effects for use in the ‘base-case’ conditions. Figure 4.14 shows

the dynamic filtration effects where the compressibility and permeability

parameters were held constant. There was only a small increase in solids pressure

when the compression (75 mm) was undertaken over 20 min compared to 5 hr.

From the modelling, steady-state compression behaviour appears to occur when

the pulp pad is compressed for around 20 minutes. Compressing the pulp pad

over 5 hr in the previous experimental phase appears to be a valid timescale that

achieves quasi steady-state conditions. The data in Figure 4.14 was obtained

using �0 = 0.04 and Sv =1740 cm-1 which is the crude average for the milled

bagasse samples in Table 4.9, N=2.71 and M=5100 are the crude averages for

milled bagasse samples in Table 4.16. A compression time of 3 min for the


134

experiments was selected based on the observation in the figure that this

compression rate is fast enough for the model to exhibit the effects of dynamic

filtration.

0

50

100

150

200

250

300

0 0.2 0.4 0.6 0.8 1

C/Cmax

Ps (

kP

a)

75 mm compression over 5 hr

75 mm compression over 20 min



Sv=1740; N=2.71

Figure 4.14 Effect of compression rate on the fibre pressure; dynamic

model predictions; �0 = 0.04, Sv = 1740 cm-1

; N = 2.71 kPa; M

= 5100.

The effect of M, N and Sv were compared by changing one variable at a

time and comparing it to the ‘base case’. The ‘base case’ uses the same values for

the permeability and compressibility parameters in Figure 4.14, at a compression

rate of 75 mm in 3 min, i.e. �0 = 0.04, Sv =1740 cm-1, N=2.71 and M=5100. The

results are shown in Figure 4.17 to Figure 4.19. Figure 4.17 shows the effect of

increasing Sv from 1740 cm-1 to 2000 cm-1, Figure 4.18 shows the effect of

increasing N from 2.71 to 3.2, and Figure 4.19 shows the effect of increasing M

from 5100 kPa to 7000 kPa. Sv and N have quite a significant affect on Ps but M

only has a minor effect.


135

0

50

100

150

200

250

300

0 0.25 0.5 0.75 1

C/Cmax

Ps (

kP

a)

Sv=1740 - Base case

Sv=2000

Figure 4.15 Effect of Sv on the dynamic model predictions compared to the

base case; �0 = 0.04, Sv = 1740 cm-1

; N = 2.71 kPa; M = 5100,

compression time = 3min.

0

50

100

150

200

250

300

0

C/Cmax

Ps (

kP

a)

N=3.2

N=2.71 - Base case

Figure 4.16 Effect of N on the dynamic model predictions compared to the

base case; �0 = 0.04, Sv = 1740 cm-1

; N = 2.71 kPa; M = 5100,



136

0

50

100

150

200

250

300

0 0.25 0.5 0.75 1

C/Cmax

Ps (

kP

a)

M=5100 - Base case M=7000

Figure 4.17 Effect of M on the dynamic model predictions compared to the

base case; �0 = 0.04, Sv = 1740 cm-1

; N = 2.71 kPa; M = 5100,


4.5.2. Dynamic filtration experiments and comparison with predicted

values

In order to verify the dynamic model, the fibre pressure Ps at the top platen

was calculated from � at the top platen using Equation 2.24 and compared to

experimentally derived data.

Figure 4.18 and Figure 4.19 show the experimentally determined pressure at

the top platen compared to Ps calculated from the dynamic model. It was found

that for all pulp samples, the best agreement with experimental data occurs when

using a variable Kozeny factor (Equation 2.12) rather than a constant Kozeny

factor (i.e. k = 5.55). This means that the tortuosity of the capillaries through the

pulp pad is changing as the pad compresses. The bagasse pulp samples and the

pine pulp sample had very good agreement between the experimental data and the

model predictions. The eucalypt pulp had only fair agreement. The reason for the

poorer performance in predicting the behaviour of eucalypt pulp is not known.

Two pulp samples had poor agreement between the dynamic model and

experimental data; the pulp made from whole bagasse (Sample 53) and fine

bagasse (Sample 51). These pulp samples have the highest fine fibre content and


137

deviation from the dynamic model is thought to have been caused by incomplete

retention of fine fibre by the platen. The comparison between the predictions of

the dynamic model and the experimental data for other pulp samples are presented

in the supplementary material (Appendix A).

The repeatability of the dynamic filtration experiments was studied using

two medium fractions of bagasse pulp. Each pulp pad was compressed twice

under identical conditions. The experiments were repeatable. The data from

these experiments is attached in Appendix A.

During the dynamic filtration testing, the compression of a sample of pulp

was measured both before and after bleaching. Bleaching the pulp did not affect

the dynamic filtration behaviour of the pulp.

4.5.3. Summary of dynamic filtration modelling

Two purpose built pieces of laboratory equipment were used to determine

the steady-state permeability and compressibility parameters of bagasse pulp pads.

These steady-state parameters were used in a dynamic filtration model to

accurately predict the compressive load during dynamic filtration of a bagasse

pulp pad. The sensitivity of the variables M, N and Sv on the platen load pressure

was determined.

The dynamic filtration model provided good prediction of the fibre pressure

of a compressed bagasse pulp pad at high compression rates when significant

dynamic effects occur. It is particularly accurate for bagasse pulp, provided some

pith is removed, and pine pulp. The model only provides qualitative predictions

for eucalypt pulp at high compression rates. The Kozeny-Carman permeability

model allows the dynamic model to give excellent predictions, particularly when a

variable Kozeny factor is used (Equation 2.12), rather than a constant Kozeny

factor (i.e. k = 5.55).


138

0

20

40

60

80

100

0 1 2 3

Time (min)

Pre

ss

ure

(k

Pa

)

Experimental

Theoretical; constant k

Theoretical; variable k

0

20

40

60

80

100

0 1 2 3

Time (min)

Pre

ss

ure

(k

Pa

)

Experimental



0

20

40

60

80

100

0 1 2 3

Time (min)

Pre

ssu

re (

kP

a)

Experimental



0

20

40

60

80

100

0 1 2 3

Time (min)

Pre

ss

ure

(k

Pa

) Experimental



(a) (b)

(c) (d)

Figure 4.18 Comparison of the dynamic model with experimental data for

bagasse pulp (constant and variable k) (a) Medium milled

bagasse pulp (Sample 20); (b) Coarse milled bagasse pulp

(Sample 26); (c) Medium diffuser bagasse pulp (Sample 18); (d)

Coarse diffuser bagasse pulp (Sample 39).


139

0

20

40

60

80

100

0 1 2 3

Time (min)

Pre

ssu

re (

kP

a)

Experimental



0

20

40

60

80

100

0 1 2 3

Time (min)

Pre

ss

ure

(kP

a)

Experimental



0

20

40

60

80

100

0 1 2 3

Time (min)

Pre

ssu

re (

kP

a)

Experimental



(a) (b)

(c)

Figure 4.19 Comparison of the dynamic model with experimental data for

(a) Argentinean bagasse pulp, (b) eucalypt pulp and (c) pine

pulp (constant and variable k).

4.6. Results of chemical additives testing

A chemical additive system which is stable under shear conditions was

optimised for use with a bagasse pulp slurry using a DDJ. Pulp retention was the

principal measure of efficacy for determining the addition rate for the chemical

additives (section 4.6.1). The DDJ was modified to investigate the effect of

vacuum on the drainage time and retention of bagasse pulp slurries both with and

without flocculants (section 4.6.2). The modified DDJ simulates the Fourdrinier

forming process. The effect of chemical additives on the steady-state and

dynamic permeability and compressibility behaviour of bagasse pulp pads was


140

determined (sections 4.6.3 and 4.6.4). A summary of the effect of chemical

additives is presented (section 4.6.5).

4.6.1. The effect of shear and additives on pulp retention

A high molecular weight cationic polyacrylamide (CPAM), a flocculant

commonly used for wood-based paper manufacture, was investigated at varying

concentrations. The supplier recommended applying CPAM in the range of

0.02%-0.08% (on dry fibre) which is typical for mechanical wood pulp with a

high quantity of short fibre. The product is sold commercially as Ciba Percol 182.

The effectiveness of various concentrations of CPAM was measured on

‘whole’ bagasse pulp (Sample 53) beyond the ranges suggested by the supplier

using a DDJ. The effectiveness of flocculants was quantified by measuring the

retention of short fibres (i.e. ‘fines retention’). The flocculation improved with

increasing levels of CPAM as shown in Figure 4.20. Even a very small quantity

of CPAM improved flocculation. At 0.5% CPAM, over-flocculation was

observed which would lead to a ‘patchy’ paper sheet appearance and poor sheet

strength properties. An addition rate of 0.05% CPAM, which is the mid-point

recommended by the supplier for a mechanical pulp heavily laden with fine fibre

was used as a basis for further experiments with flocculants. This concentration is

known to be economical for wood-based paper production, improving fibre

distribution, whilst also improving drainage and retention.


141

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400 1600

Shear (rpm)

Fin

es r

ete

ntio

n,

%

Whole bagasse pulp

Whole bagasse pulp + 0.01% CPAM

Whole bagasse + 0.05% CPAM


Figure 4.20 The effect of CPAM concentration and shear on fines retention

on a whole bagasse pulp (Sample 53).

The effect of bentonite addition to a pulp slurry containing 0.1% fibre and

0.05% CPAM (on dry fibre) was investigated. Ciba recommended that their

modified bentonite, Hydracol ONZ, should be added at a rate of 0.3% after the

CPAM is added to the pulp slurry. Bentonite was added 1 min after CPAM was

added to the pulp slurry in the DDJ.

It was found that the addition of CPAM and bentonite, at this rate, actually

reduced the fines retention. A high level of bentonite was observed in the filtrate.

Consequently, the effect of bentonite addition rate on a bagasse fibre-CPAM

system was explored. Figure 4.21 shows that for a pulp slurry containing 0.1%

fibre and 0.05% CPAM, the optimum addition rate of bentonite was around

0.06%. A high level of shear (1500 rpm) was used to compare the effect of the

level of bentonite. Above this addition level, bentonite is in excess and not

attached to the fibre flocs.


142

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0.0% 0.2% 0.4% 0.6% 0.8% 1.0% 1.2% 1.4% 1.6%

Bentonite addition (on dry fibre)

Fin

es R

ete

ntio

n

Whole bagasse pulp, 1500 rpm

0.05% Percol 182 (CPAM)

in all samples

Figure 4.21 The effect of bentonite addition rate on the fines retention of a

whole bagasse pulp (Sample 53) with 0.05% CPAM added.

The effect of both CPAM and bentonite on the flocculation of a whole

bagasse pulp is shown in Figure 4.22 over a wide range of shear conditions.

Using 0.05% CPAM, the best fines retention was achieved with 0.06% of

bentonite. This was the most effective rate for whole bagasse pulp over the full

range of shear conditions. These addition rates of CPAM and bentonite were then

applied to other pulp samples, including ‘coarse’, ‘medium’ and the benchmark

‘30% depithed’ bagasse pulp.


143

0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000 1200 1400 1600

Shear (rpm)

Fin

es r

ete

ntion

Whole bagasse pulp


Whole bagasse pulp + 0.05%CPAM +0.06% bentonite

Whole bagasse pulp + 0.05%CPAM + 0.3% bentonite

Figure 4.22 The effect of shear, CPAM and bentonite addition rate on the

fines retention of a whole bagasse pulp.

The addition of (i) 0.05% CPAM and (ii) 0.05% CPAM + 0.06% bentonite

was then tested on the benchmark ‘30% depithed’ bagasse pulp (Sample 58).

Depithed bagasse pulp has better drainage properties than whole bagasse pulp as

shown in (section 4.3). Figure 4.23 shows that depithed bagasse pulp had the

same response to chemical additives as the whole bagasse pulp. The addition of

0.05% CPAM improved fines retention and was enhanced by the addition of

0.06% of bentonite.


144

0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000 1200 1400 1600

Shear (rpm)

Fin

es r

ete

ntio

n

Depithed bagasse pulp

Depithed bagasse pulp + 0.05% CPAM

Depithed bagasse pulp + 0.05% CPAM + 0.06% bentonite


fines retention of a ‘30% depithed’ bagasse pulp (Sample 58).

The effect of CPAM and bentonite on bagasse pulp derived from the

‘coarse’ bagasse pulp (Sample 56), which has high permeability, was performed

(Figure 4.24). The addition of CPAM and CPAM & bentonite on the ‘coarse’

bagasse pulp mirrored the results for the ‘30% depithed’ bagasse pulp and the

whole bagasse pulp.


145

0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000 1200 1400 1600

Shear (rpm)

Fin

es r

ete

ntio

n

'Coarse' bagasse pulp

'Coarse' bagasse pulp + 0.05% CPAM

'Coarse' bagasse pulp + 0.05% CPAM +0.6kg/t bentonite

Figure 4.24 The effect of shear CPAM and bentonite addition rate on the

fines retention of a ‘coarse’ bagasse pulp.

Finally, the CPAM/bentonite system was tested on ‘medium’ bagasse pulp

(Sample 60, Figure 4.25), again with good performance.

0%

20%

40%

60%

80%

100%

0 200 400 600 800 1000 1200 1400 1600

Shear (rpm)

Fin

es r

ete

ntion

Medium bagasse pulp

Medium bagasse pulp + 0.05% CPAM

Medium bagasse pulp +0.05%CPAM + 0.06% bentonite


fines retention of a ‘medium’ bagasse pulp.


146

In summary, the combination of 0.05% CPAM + 0.06% bentonite chemicals

for improving the retention of all types of bagasse pulp was effective under all

shear rates. This two chemical system gives good fines retention, typically 80%-

100%, under a range of shear conditions for every bagasse pulp except ‘whole’

bagasse pulp.

4.6.2. The effect of chemical additives and vacuum

The effect of vacuum and shear on the fines retention of a whole bagasse

pulp, without any flocculants added, was studied to look into the effect of

vacuum. During this process, a pulp mat was formed through which the pulp

slurry was filtered. This did not occur in the previous experiments with the DDJ.

The data is shown in Figure 4.26. Vacuum clearly had a profound effect on fines

retention.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 500 1000 1500 2000

Shear (rpm)

Fin

es r

ete

ntio

n

0kPa vacuum

20kPa vacuum

40kPa vacuum

Whole bagasse pulp

Not Tappi method-

100% of water is

removed

Figure 4.26 The effect of vacuum and shear on the fines retention of a

whole bagasse pulp (Sample 53), no chemical additives.

The time taken for the DDJ to drain was measured as a function of vacuum.

The drainage time decreased quickly as the first 10 kPa of vacuum were applied

(Figure 4.27). The data in Figure 4.27 was collected at a single moderate level of

shear. The stirrer speed was set to 1000 rpm.


147

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50

Vacuum kPa

Fin

es r

ete

ntio

n

0

20

40

60

80

100

120

140

160

180

200

Dra

inage

tim

e (

s)

Fines retention Drainage time

Whole bagasse pulp

Stirrer speed 1000rpmRead from right hand axis

Read from left hand axis

Figure 4.27 The effect of vacuum on the fines retention and drainage time

of a whole bagasse pulp (Sample 53), 1000 rpm shear, no

chemical additives.

The experiment was repeated for ‘30% depithed’ bagasse pulp (Sample 58,

Figure 4.28), ‘coarse’ bagasse pulp (Sample 56, Figure 4.29) and ‘medium’

bagasse pulp (Sample 60, Figure 4.30). In these experiments, the effect of

chemical additives was also investigated as a function of vacuum. The chemicals

added were 0.05% CPAM and 0.06% bentonite.


148

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50

Vacuum kPa

Fin

es r

ete

ntion

0

10

20

30

40

50

60

70

80

90

100

Dra

ina

ge t

ime

(s)

Fines retention 'depithed' bagasse pulp no additives

Fines retention 'depithed' bagasse pulp 0.05% CPAM + 0.06% bentonite

Drainage time 'depithed' bagasse pulp no additives

Drainage time 'depithed' bagasse pulp 0.05% CPAM + 0.06% bentonite

Read from right hand axis


Figure 4.28 The effect of vacuum and chemical additives on the fines

retention and drainage time of a ‘30% depithed’ bagasse pulp

(Sample 58), 1000 rpm shear.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50 60 70

Vacuum kPa

Fin

es r

ete

ntion

0

10

20

30

40

50

60

70

80

90

100

Dra

ina

ge t

ime

(s)

Fines retention no additives Fines retention 0.05% CPAM + 0.06% bentonite

Drainage time no additives Drainage time 0.05% CPAM + 0.06% bentonite




retention and drainage time of a ‘coarse’ bagasse pulp (Sample

56), 1000 rpm shear.


149

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50

Vacuum kPa

Fin

es r

ete

ntion

0

10

20

30

40

50

60

70

80

90

100

Dra

ina

ge t

ime

(s)

Fines retention no additives Fines retention 0.05% CPAM + 0.06% bentonite

Drainage time no additives Drainage time 0.05% CPAM + 0.06% bentonite

Read from right hand

axis

Read left right hand axis


retention and drainage time of a ‘medium’ bagasse pulp

(Sample 60), 1000 rpm shear.

For each bagasse pulp examined, the CPAM/bentonite system improved the

retention of fines and the drainage time.

These data provided an unexpected result. When no chemical additives

were used, the ‘30% depithed’ bagasse pulp (Sample 58) initially had a longer

drainage time than the ‘coarse’ bagasse pulp (Sample 56) but as the vacuum

increased, the drainage time of the ‘depithed’ bagasse pulp improved and the

drainage time became quicker than the ‘coarse’ bagasse pulp. This effect is

shown in Figure 4.31. At a vacuum level greater than 10 kPa, the ‘30% depithed’

bagasse pulp drained more quickly than the ‘coarse’ bagasse pulp.


150

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0 10 20 30 40 50 60 70Vacuum kPa

Fin

es r

ete

ntion

0

10

20

30

40

50

60

70

80

90

100

Dra

ina

ge t

ime

(s)

Fines retention 'Coarse bagasse' Test 56 no additives Fines retention 'Depithed bagasse' Test 58 no additives

Drainage time 'Coarse bagasse' Test 56 no additives Drainage time 'Depithed bagasse' Test 58 no additives

No additives




of ‘coarse’ (Sample 56) and ‘30% depithed’ (Sample 58)

bagasse pulp, 1000 rpm shear, no flocculants added.

This effect was exacerbated when chemical additives were used (0.05%

CPAM and 0.06% bentonite). At 5 kPa, the ‘30% depithed’ bagasse pulp had

faster drainage than the ‘coarse’ bagasse pulp.


151

50%

55%

60%

65%

70%

75%

80%

85%

90%

95%

100%

0 10 20 30 40 50

Vacuum kPa

Fin

es r

ete

ntion

0

10

20

30

40

50

60

70

80

90

100

Dra

ina

ge t

ime

(s)

Fines retention 'Coarse bagasse' with additives Fines retention 'Depithed bagasse' with additives

Drainage time 'Coarse bagasse' with additives Drainage time 'Depithed bagasse' with additives

With additives




of ‘coarse’ (Sample 56) and ‘30% depithed’ (Sample 58)

bagasse pulp, 1000 rpm shear, with flocculants added.

It was anticipated that the drainage time of the ‘coarse’ bagasse pulp under

vacuum and shear conditions would be quicker than the ‘depithed’ bagasse pulp

based on its substantially lower Sv alone (~4600 cm-1 for ‘30% depithed’ bagasse

pulp compared to ~1500 cm-1 for ‘coarse’ bagasse pulp). This affect was also

observed for ‘medium’ bagasse pulp. Fibre to fibre interactions during

compression evidently plays an important role during bagasse pulp pad formation

in the DDJ. This is explored further in the next section.

4.6.3. The effects of chemical additives on permeability and

compressibility parameters

4.6.3.1. The effect of chemical additives on bagasse pulp permeability parameters

The study into the permeability properties of bagasse pulp was revisited

using the CPAM/bentonite additive system. The experimental procedure used in

section 3.4 was repeated with the exception that the chemical additives were

added to the pulp slurry prior to loading into the cell. Figure 4.33 and Figure 4.34

show the graph of (Kc2)1/3 against c for a ‘coarse’ and ‘medium’ bagasse pulp

respectively. As can be observed from the figures, there was not found to be a


152

statistically significant difference in the slope or intercept of these plots and

consequently no difference in Sv or �. This was confirmed using Student’s t-test

with a 95% confidence interval, using the pooled estimate of standard deviation

from section 4.3.

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.0018

0.00 0.05 0.10 0.15 0.20


(Kc

2)1

/3

Sample 43 no additives

Sample 43 with additives

Figure 4.33 The effect of chemical additives on the permeability of a

‘coarse’ bagasse pulp (Sample 43).


153

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.0016

0.00 0.05 0.10 0.15 0.20


(Kc

2)1

/3

Sample 18 no additives

Sample 18 with additives

Linear (Sample 18 no additives)

Figure 4.34 The effect of chemical additives on the permeability of a

‘medium’ bagasse pulp (Sample 18).

The effect of additives was significant for a ‘30% depithed’ bagasse pulp

(Sample 58), see Figure 4.35. ‘30% depithed’ bagasse pulp had higher

permeability when chemical additives were used, although the permeability was

still lower than that of eucalypt (without additives). This was confirmed at a 95%

confidence interval.


154

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

0.0014

0.000 0.050 0.100 0.150 0.200


(Kc

2)1

/3

Eucalypt pulp, no additivesWhole bagasse pulp, no additives'Depithed' bagasse pulp, no additives'Depithed' bagasse pulp, with additivesLinear (Eucalypt pulp, no additives)

Figure 4.35 The effect of chemical additives on the permeability of a ‘30%

depithed’ bagasse pulp (Sample 58).

The results for Sv and � are shown in Table 4.19. The chemical additives

had a strong affect on the ‘30% depithed’ bagasse pulp, greatly reducing its Sv but

not on the ‘coarse’ or ‘medium’ bagasse pulp. There was not found to be any

statistically significant difference in � for any bagasse pulp sample using a 95%

confidence interval.

Table 4.19 Effect of additives on the permeability parameters Sv and �.

Parameter Bagasse pulp type No additives With additives

Coarse (Sample 43) 3.44 3.45

Medium (Sample 18) 3.33 2.98

� (-)

�PESD, �=0.216

30% depithed (Sample 58) 2.97 3.26

Coarse (Sample 43) 1540 1580

Medium (Sample 18) 1820 2080

Sv (cm-1)

�PESD, Sv=211 cm-1

30% depithed (Sample 58) 4640 3060


155

4.6.3.2. Effect of chemical additives on bagasse pulp compressibility parameters

The investigation into the steady state compressibility of bagasse pulp in

section 4.4 was revisited using the CPAM/bentonite system. Figure 4.36 and

Figure 4.37 shows typical results for a ‘coarse’ and ‘medium’ bagasse pulp

respectively. In both figures, the data is shown prior to chemical additives and

after the addition of chemical additives. There was no difference in the steady-

state compression factors M and N.

Figure 4.36 The effect of chemical additives on the quasi steady-state

compression of ‘coarse’ bagasse pulp (Sample 20).


156

Figure 4.37 The effect of chemical additives on the steady-state

compression of ‘medium’ bagasse pulp (Sample 18).

The effect of chemical additives is more pronounced on the steady state

compressibility of a ‘30% depithed’ bagasse pulp (Figure 4.38). This mirrors the

observation that chemical additives only affect the permeability parameters of a

‘depithed’ bagasse pulp.


157

Figure 4.38 The effect of chemical additives on the steady-state

compression of ‘depithed’ bagasse pulp.

Typical results for the steady-state compressibility test are shown in Table

4.20. The results were duplicated using other pulp samples (Sample 26, a ‘coarse’

bagasse pulp and Sample 42, a ‘medium’ bagasse pulp). The only statistically

significant result is that the chemical additives system affected the ‘depithed’

bagasse pulp compressibility parameters by increasing both M and N.

Table 4.20 Typical effect of chemical additives on bagasse pulp

compressibility parameters.

Parameter Bagasse pulp type No additives With additives



Log M (kPa)

�PESD, m=0.15




N, -

�PESD, n=0.12



158

4.6.4. The effect of chemical additives on bagasse pulp’s dynamic

filtration behaviour

In section 4.6.3, it was found that chemical additives affected the Sv, M and

N of only the ‘30% depithed’ pulp. The ‘coarse’ and ‘medium’ bagasse pulp

could not be shown, statistically speaking, to be affected by chemical additives.

The dynamic filtration tests in section 3.6 were revisited to look at the effect

of chemical additives. The results for ‘coarse’ pulp (Sample 43), ‘medium’ pulp

(Sample 18) and ‘30% depithed’ pulp (Sample 58) are shown in Figure 4.39.

Similar results for the ‘coarse’ and ‘medium’ pulps were obtained using Sample

26 (a ‘coarse’ pulp) and Sample 42 (a ‘medium’ pulp).

Under dynamic conditions, the governing equation (Equation 2.25) is

dominated by the flexural term D(�), reproduced below. In the case of the ‘30%

depithed’ bagasse pulp, the reduced load has been caused by an increase in the

permeability term (K(�)) and compression term (mn �n-1).

( ) ( ) ( )�

mnK 1D

1n−φφφ−φ=φ

Although it could not be determined experimentally using a 95% confidence

interval that the chemical additives affect either the steady-state permeability or

compressibility properties of ‘coarse’ or ‘medium’ bagasse pulp, there was some

anecdotal evidence of their effect demonstrated in the dynamic experiment, Figure

4.39. The figure shows that there is a small reduction in load pressure when

chemicals are added to the ‘medium’ pulp.

159

0

10

20

30

40

50

60

70

80

90

100

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2

Concentration (g/cm3)

Pre

ssu

re (

kP

a)

'Coarse' bagasse pulp (Sample 43) no additives 'Coarse' bagasse pulp (Sample 43) with additives

'Medium' bagasse pulp (Sample 18) no additives 'Medium' bagasse pulp (Sample 18) with additives

'Depithed' bagasse pulp, (Sample 58) no additives 'Depithed' bagasse pulp (Sample 58) with additives

'30% Depithed' pulp

'Medium' pulp

'Coarse' pulp

Figure 4.39 The effect of chemical additives on the dynamic filtration of ‘depithed’, ‘coarse’ and ‘medium’ bagasse pulp.

159

Chap

ter 4 –

Resu

lts and d

iscussio

n


160

Let us now re-examine the finding in section 4.6.2 where the ‘30%

depithed’ bagasse pulp had faster drainage under vacuum in the modified DDJ

than the ‘coarse’ bagasse pulp, particularly when chemical additives were used.

The possible mechanisms for consolidation of compressible fibrous media are

(from 102):

1. Fibre collapse

2. Bending of fibres & fibre realignment

3. Breaking of fibres

The Australian bagasse fibres are very rigid and significant fibre collapse

was not observed in the microscopy study. Fibre breakage is not occurring due to

the high repeatability of the permeability and compressibility experiments. This

leaves the bending of fibres and fibre realignment.

Further insight is gained by considering the larger improvement in drainage

rate for the ‘depithed’ bagasse when chemicals are added compared to the ‘coarse’

and ‘medium’ pulp samples. It seems unlikely that the chemical additives cause

the Australian bagasse pulp fibres to become more flexible. If there was a

significant bending effect brought about by chemical additives, one would expect

the three different types of pulp (i.e. ‘coarse’, ‘medium’ and ‘depithed’ pulp) to

exhibit a similar change in M and N. This was not observed. Although the ‘30%

depithed’ bagasse pulp had a higher load during steady-state compression when

chemical additives were used, it seems unlikely that it is more rigid than the

‘coarse’ and ‘medium’ pulp samples.

By a process of elimination, it seems the most likely mechanism of

Australian bagasse pulp consolidation is fibre realignment. The use of chemical

additives with bagasse pulp improves fibre realignment by creating a lubricating

effect, binding the “pith” fibres to the larger fibres. When using chemical

additives, the higher the level of pith, the greater the improvement in dynamic

filtration properties (see Figure 4.39).

In the initial stages of pad formation, without chemical additives, the pith

fibres roam freely and block pores in the pulp pad. With chemical additives, the

pith is attached to the longer fibres and are held back in suspension slightly


161

allowing a more porous pad during pad formation. Under the dynamic conditions

in the DDJ, as vacuum increased, the ‘30% depithed’ bagasse pulp pad

consolidated better than the ‘coarse’ and ‘medium’ pulp samples. Under the

dynamic conditions in the compression cell, the ‘coarse’ and ‘medium’ pulp

samples filtered more easily.

For processing low consistency pulp suspensions in conditions similar to a

DDJ, the pulp drains fastest when ‘30% depithed’ bagasse is used in conjunction

with a chemical additives system, but significant vacuum must be applied. The

difficulty with this approach is that the ‘30% depithed’ had a lower water

retention value (255% for ‘coarse’ bagasse pulp and 274% for ‘30% depithed’

bagasse pulp) which makes the sheet harder to dry. Also, the modified DDJ more

closely resembles the Fourdrinier former. It is not known how the filtration of the

pulp samples would compare in a Twin-wire former.

For processing initially networked fibre pads under dynamic conditions,

‘coarse’ bagasse pulp was the most easily dewatered bagasse pulp which

improved when chemical additives are used. The ‘coarse’ bagasse pulp

performed better when either flocculants were not used or when the vacuum level

was low.

4.6.5. Summary of the effect of chemical additives on pulp permeability

and compressibility

It was found that addition of 0.05% CPAM (as Ciba Percol 182) and 0.06%

modified bentonite (as Ciba Hydracol ONZ) improved the retention of bagasse

pulp fines over a wide range of shear using a DDJ.

Applying vacuum to the DDJ had the effect of dramatically reducing the

drainage time. Every bagasse pulp benefitted from the addition of the flocculants

in the DDJ, as measured by reduced drainage time and increased fines retention, at

any level of vacuum.

In steady-state permeability and compressibility experiments, the addition of

flocculants could only be determined to improve Sv for one type of pulp; the ‘30%


162

depithed’ pulp. However, dynamic filtration experiments showed that there is

also a significant improvement for the ‘medium’ bagasse pulp.

The mechanism of bagasse pulp consolidation is by fibre realignment which

is assisted by chemical additives.

For initially unnetworked suspensions, and the conditions that occur in the

DDJ, the fastest drainage rate was achieved by a standard depithing regime

practiced by industry (i.e. removal of 30% of the shortest fibres) using high levels

of vacuum and chemical additives. These conditions most closely resemble those

of a Fourdrinier former rather than a Twin-wire former. The lower WRV of the

‘coarse’ bagasse pulp indicates that it would dry more quickly.

For initially networked fibre pads, as in the compression cell and

permeability cell, the ‘coarse’ bagasse pulp was the easiest to filter.

Chapter 5 - Conclusions

163

Chapter 5

Conclusions

A study of bagasse pulp was motivated by the possibility of making highly

value-added products from bagasse for the financial benefit of sugarcane millers

and growers. In Australia, there is a perception that bagasse pulp always has poor

filtration characteristics which results in slower paper production compared to

local eucalypt pulp. Surprisingly, there has previously been very little rigorous

investigation into bagasse pulp permeability and compressibility. Only freeness

testing of bagasse pulp has been published in the open literature. Consequently,

this study focussed on improving the filtration properties of bagasse pulp pads.

This study investigated three options for improving the permeability and

compressibility properties of Australian bagasse pulp pads. Firstly, the effect of

the bagasse size, whether ‘coarse’ or ‘medium’ fractions, was considered. The

effect of the mode of juice extraction, whether from a mill or a diffuser, was

determined. Finally the effectiveness of chemical additives, which are known to

improve freeness of pulp slurries, was assessed.

The pre-treated Australian bagasse pulp samples were compared with

samples of eucalypt pulp, depithed Argentinean bagasse pulp that is used

industrially, and a benchmark Australian bagasse pulp that also had 30% of its

shortest fibres removed.

The steady-state permeability and compressibility parameters of bagasse

pulp pads were determined experimentally using two purpose-built experimental


164

rigs. These parameters were used as inputs for a dynamic filtration model which

more accurately represents industrial paper manufacture. The filtration model was

developed with a view to assist with the development of specialised bagasse pulp

processing equipment. The predicted results of the dynamic model were

compared to experimental data.

The effectiveness of a CPAM and bentonite chemical additives for

improving the retention of fines and increasing the drainage rate of bagasse pulp

slurry was determined in a modified Dynamic Drainage Jar. These chemical

additives were then used to make a pulp pad and their effect on the steady-state

and dynamic permeability and compressibility were determined.

5.1. Findings of this thesis

The most important finding presented in this thesis is that Australian

bagasse pulp was produced with permeability higher than eucalypt pulp, despite a

higher overall fine fibre content. It is hoped that this higher permeability will

enable Australian paper producers to switch from using Australian eucalypt pulp

to bagasse pulp without sacrificing paper machine productivity. The high fibre

stiffness, resulting from thicker fibre walls, and the high proportion of fibres

greater than 1.3 mm in length created a highly permeable bagasse pulp pad. By

fractionating the bagasse and using the ‘flow-through’ reactor appears to have

mitigated the negative influence of the pith particles.

The specific surface area, Sv, for eucalypt pulp was consistent with the

findings of previous workers. The benchmark Australian bagasse pulp had worse

permeability than the eucalypt pulp which is in harmony with the conventional

wisdom which holds that bagasse pulp normally has poor permeability properties.

Australian pulp derived from the ‘coarse’ bagasse fraction had higher

steady-state permeability than the ‘medium’ fraction as measured by the specific

surface area, Sv. However, there was not found to be a difference in bagasse pulp

steady-state permeability between bagasse pulp from a diffuser or a mill.


165

The values for the swelling factor, �, were similar for the bagasse pulp

samples and the eucalypt pulp which were all within the ranges reported by

previous workers for wood pulp.

For bagasse pulp, a variable Kozeny factor, k, resulted in a higher value for

� and a lower value for Sv compared with a constant k. This was similar to the

findings obtained for wood pulps reported by Ingmanson (81).

The values for the steady-state compressibility constants M and N were

measured for a wide range of pulp samples. The values for N were generally

consistent with the findings of previous workers for wood pulp, although the

values of M were slightly higher. The bagasse pre-treatment options were not

found to affect the steady-state compressibility parameters of a pulp pad.

The steady-state permeability and compressibility parameters, Sv, �, M and

N, were used in a dynamic filtration model to accurately predict the compressive

load in dynamic filtration of a bagasse pulp pad. The model was particularly

sensitive to Sv, � and N but less sensitive to M.

The dynamic model was particularly accurate for bagasse pulp, provided at

least some pith was removed. The Kozeny-Carman permeability model allowed

the dynamic model to give excellent predictions when a variable Kozeny factor

was used (Equation 2.12), rather than a constant Kozeny factor.

A microparticle chemical additive system, 0.05% CPAM and 0.06%

modified bentonite, improved the retention of bagasse pulp fines over a wide

range of shear using a DDJ. Applying vacuum dramatically reduced the drainage

time. At any level of vacuum, bagasse pulp benefitted from the chemical

additives as measured by reduced drainage time and increased fines retention.

The DDJ was also used to obtain additional information about the behaviour of

thin bagasse pulp mats without flocculants being added.

In steady-state permeability and compressibility experiments involving pulp

pads, the addition of chemical additives could only be determined to improve Sv

for one type of pulp; the ‘30% depithed’ pulp. However, dynamic filtration


166

experiments showed that there was a small improvement in permeability for the

‘medium’ bagasse pulp.

The mechanism of bagasse pulp consolidation appears to be by fibre

realignment. Chemical additives assist by lubricating the fibres during the

consolidation process.

For initially unnetworked suspensions, and the conditions found in the DDJ

which is similar to Fourdrinier forming, the fastest drainage rate was achieved by

a standard depithing regime practiced by industry (i.e. removal of 30% of the

shortest fibres) using a significant level of vacuum and chemical additives.

However, the lower WRV of the ‘coarse’ bagasse pulp indicates that it would dry

more quickly.

For initially networked fibre pads, as in the compression cell and

permeability cell, the ‘coarse’ bagasse pulp was the easiest to filter.

The physical properties of the ‘coarse’ bagasse pulp were compared to the

benchmark Australian bagasse pulp. The ‘coarse’ bagasse pulp had significantly

improved initial freeness, tear properties and WRV of the pulp. However, the

‘coarse’ bagasse pulp did not have higher tensile strength or burst properties and

had slightly worse apparent density and compressive strength. Also, refining did

not significantly improve any strength property. The bagasse pulp had acceptable

physical properties for the production of generic versions of each paper grade

considered (i.e. photocopier papers, tissues and packaging), by comparison with

Indian bagasse pulp.

In summary, this study has shown that bagasse pulp can be produced with

pulp pad permeability properties that are superior to eucalypt pulp, contrary to

conventional wisdom. The high permeability arises from the stiff pulp fibres and

the high proportion of longer fibres creating an open matrix. Given its higher

pulp pad permeability, ‘coarse’ bagasse pulp could be used for a range of

applications where its properties are superior to conventional bagasse pulp.


167

5.2. Recommendations for future work

No optimisation of the cane varieties was performed in the study presented

in this thesis. High fibre energy canes should be developed in Australia and

evaluated for their permeability, compressibility and strength properties.

Increasing the fibre content has the benefits of improving the economy of scale for

a bagasse pulp and paper mill and it also increases the amount of renewable

energy available. Energy canes have the potential to significantly improve the

economics of a bagasse paper industry in Australia. The opinion of the author is

that should a bagasse pulp mill be built in Australia, development of energy canes

would be inevitable.

A dynamic filtration model was developed and verified for bagasse pulp at

ambient conditions. This model will be a valuable tool for assisting the

development of pulp processing equipment that is specially designed for

processing bagasse pulp. This work could be conducted as a further study.

This thesis has outlined methods to improve the filtration properties of

bagasse pulp. Using these methods, the sheet drying performance may now

become the processing step limiting machine production rate. Improving the

sheet drying performance was beyond the scope of this study. Only a few

measurements of WRV were taken. A further study on improving the sheet

drying properties of bagasse pulp would be interesting.

This study recommends careful treatment of bagasse prior to pulping and

the use of chemical additives to improve the filtration properties of bagasse pulp

pads. An investigation into the pulp properties of heavily depithed bagasse pulp

that has been post-processed by pressure screening (as recommended in 50, 64)

may reveal further improvements in bagasse pulp pad permeability.

The issues which are preventing the development of a bagasse pulp industry

in Australia (outlined in section 1.1.3) are (i) the poor filtration properties of

bagasse pulp, (ii) the poor physical strength properties, (iii) high capital cost and


168

(iv) the remoteness of cane farms to existing pulp mills. For the first issue, there

has recently been significant progress made to improve the filtration properties of

bagasse pulp as outlined in this thesis and also by El-Sharkawy and co-workers

(50, 64). For the second issue, hopefully pulp strength will be improved by

breeding more appropriate cane varieties and changing the juice extraction

method in alignment with the work by Gartside and coworkers (28, 51, 65). For

the third issue, technologies that reduce the capital cost of a bagasse pulp mill

should be explored.

Targeted research into replacing the expensive liquor chemical recovery

plant appears to have great potential for dramatically reducing the capital cost of a

bagasse pulp mill. The ease with which bagasse is pulped makes it a prime

candidate for researching alternative processes which don’t require conventional

chemical recovery technology. Three such alternative processes are: using the

liquor to make fertiliser (50, 64, 145, 146); using electrostatic membranes to

recover pulping chemicals (20); and using organic solvents, such as formic acid,

that can be recovered by distillation (147-150).

References

169

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Appendix A-Supplementary material for dynamic filtration modeling

183

Appendix A

Supplementary material for

dynamic filtration modeling


184

A.1 Derivation of the dimensional governing equation for the dynamic

filtration model

This is the derivation of the governing equations for a constant rate

filtration apparatus. The mass and momentum balance equations are provided

in Chapter 2.

1D assumption; u (velocity of fibres) = v (velocity of water) = dh

dt at the top

platen

Mass balance on the fibres dh

dt+∇⋅(φu)=0

dφdt

−δφu

δx=0 (C.1.1)

Similarly, mass balance for the water

d(1−φ)

dt+∇⋅(1−φ)v=0

−dφdt

−d(1−φ)v

dx=0 (C.1.2)

(C.1.1)+(C.1.2)

δ(φu)

δx+ d(1−φ)v

dx=0

δ[φu+(1−φ)v]

δx=0

→φu+(1−φ)v=c(t) where c(t) is a constant.

sub in Boundary Condition: at x=h(t),u=v=−dh

dt

→−φdh

dt−(1−φ)

dh

dt=c(t)

c(t)=−dh

dt


185

→φu+(1−φ)v=−dh

dt

Momentum balance

Fibres:

ρsφDu

Dt=−φ∇P

f−∇P

s+∇⋅(φτ

s)+ρ

sφg+m (C.1.3)

Du

Dt=0

as inertia not significant

g→0 as gravity effects insignificant

τs=0

as shear effects insignificant Fluid:

ρfφDv

Dt=−(1−φ)∇P

f+∇⋅[(1−φ)τ

f]+ρ

fφg−m (C.1.4)

(C.1.3)+(C.1.4)

0=−φ∇Pf+∇P

s

in 1-D

dPf

dx+ dP

s

dx=0

which is Terzaghi’s principle.

d(Pf+P

s)

dx=0

Pf+P

s=c(t)

At bottom Ps=σ(t) since P

f=0 at x=0 recall

m=(1−φ)α(φ)(u−v) back to equation (C.1.4)

−(1−φ)∇Pf−m=0

dPf

dx=α(φ)(u−v) (C.1.5)

sub (C.1.5) into (C.1.3)


186

0=−φα(φ)(u−v)−dP

s

dx+(1−φ)α(φ)(u−v)

0=−(1−φ) dP

s

dx+α(φ)[(1−φ)u−(1−φ)v]

recall

φu+(1−φ)v=−dh

dt

so

0=−(1−φ) dP

s

dx+α(φ)[(1−φ)u+

dh

dt+φu]

→(1−φ) dP

s

dx=α(φ)[u+

dh

dt]

(1−φ)

α(φ)

dPs

dx−dh

dt=u

recall (C.1.1) is

dφdt

−δφu

δx=0

eliminate u

dφdt

−

dφ[ 1−φ

α(φ)

dPs

dx−dh

dt]

dx=0

dφdt

= d

dx ��

��φ(1−φ)

α(φ)

dPs

dx+ dh

dt

dφdx

use fitting model α(φ)= µ

K(φ)

and recall

Ps=MφN

f'(φ)= dP

s

dφ

dPs

dx= dP

s

dφ

dφdx


187

→dP

s

dx=f'(φ)

dφdx

so

dφdt

= d

dx ��

��φ(1−φ)K(φ)f'(φ)

µ

dφdx

+ dh

dt

dφdx

let

D(φ)= φ(1−φ)K(φ)f'(φ)

µ

dφdt

= d

dx ��

��

D(φ) dφdx

−dh

dt

dφdt

(C.1.6)

(C.1.6) is the governing equation recall

u= (1−φ)

α(φ)

dPs

dx−dh

dt

= (1−φ)

α(φ)f'(φ)

dφdx

−dh

dt

Boundary condition 1

x=h(t),u=−dh

dt

−dh

dt=

(1−φ)

α(φ)f'(φ)

dφdx

−dh

dt

→dφdx

=0

Boundary condition 2 x=0, u=0

0= (1−φ)

α(φ)f'(φ)

dφdx

−dh

dt

→dφdx

= dh

dt

α(φ)

(1−φ)f'(φ)


188

A.2 Non-dimensionalising of dynamic model for FORTRAN

The dimensional governing equations need to be non-dimensionalised for use with the FORTRAN function libraries.

Dimension Dimensional variable Dimensionless variable

Distance from platen x (m) X (-) =

x

h0−u

0t

Time t (min) t* (-) =

u0

h0

t

Flexural term D(φ) D*(φ) (-) =

D(φ)

u0h

0

1

1 by definition, the reason will be demonstrated shortly

Where h0 is defined as the initial height of the platen above the base and u

0

is the speed of the platen (u0= dh

dt)

recall the governing equation in dimensional form

dφdt

= d

dx ��

��

D(φ) dφdx

−dh

dt

dφdx

we can immediately transform the spatial co-ordinates

dφdt

= d

dX��

��D(φ)

(h0−u

0t)2

dφdX

−u

0

h0−u

0t

dφdX

(C.2.1)

Firstly, we need to establish a couple of simple relations, by the chain rule

dφdx

= δφ

δX

δX

δx+

δφ

δt

δt

δx

but at a constant rate δt

δx=0 so

δφ

δx=

1

h0−u

0t

δφ

δX (C.2.2)

also

x=X(h0−u

0t)→

δx

δt=−u

0X (C.2.3)

By the chain rule, the LHS of (1) becomes


189

δφ

δt=

δφ

δt*δt*

dt+

δφ

δx

δx

δt

substitute (C.2.2) and (C.2.3) and rearranging, the LHS of (C.2.1) reduces to

δφ

δt= u

0

h0

δφ

δt*+ u

0

h0

X

1−t*δφ

δX (C.2.4)

substituting (4) into (3) and rearranging

u0

h0

δφ

δt*= d

dX��

��D(φ)

h2

0(1−t*)2

dφdX

−u

0

h0

1−X

1−t*δφ

δX

Multiplying all terms by (1−t*)2h

0

u0

we get

(1−t*)2 δφ

δt*=

δ

δX ��

��D(φ)

u0h

0

dφdX

−(1−X)(1−t*) δφ

δX

by definition D(φ)= φ(1−φ)K(φ)f'(φ)

µ substituting the definitions of

K(φ)= 1

kS2

v

(1−φ)3

φ2 and f'(φ)=MNφN−1 we get

D(φ)= NM

µkS2

vuoh0

(1−φ)4φN−2

applying the definition of D*(φ) we end up with the non-dimensional

form of the governing equation

(1−t*)2 δφ

δt*=

δ

δX ��

��

D*(φ) dφdX

−(1−X)(1−t*) δφ

δX (C.2.5)

This is the form of the Governing equation required by the FORTRAN

NAB library functions D03PCF and D03PZF

(C.2.5) is subject to the boundary conditions

Boundary condition (1) at X = 1

δφ

δX=0

Boundary condition (2) at X = 0

δφ

δx=

δh

δt

φ

D(φ)


190

substituting (C.2.2) and the definition of D(φ) we get

1

h0−u

0t

δφ

δX=

u0φ

u0h

0D*(φ)

substituting the definition of t* and rearranging

δφ

δX=

φ(1−t*)

D*(φ)


191

A.3 FORTRAN 77 program for the dynamic filtration model

**************************************************************** * * Tom Raineys pulp compression modelling program * adapted from * D03PCF Example Program Text * * VERSION 6: FINAL * * Assisted by Neil Kelson * * This version is used for bagasse pulp compression modelling * by adjusting the parameters below * * * To compare with experimental data need to specify PhiInit; Hinit; MPHI; * NPHI; Sv; and DHDT. * Generates two files: fort.21 (output for Ps) and fort.22 (output for phi) * **************************************************************** * .. Parameters ..

INTEGER NOUT PARAMETER (NOUT=21) INTEGER NPDE, NPTS, INTPTS, ITYPE, NEQN, NIW, NWK, NW * PARAMETER (NPDE=2,NPTS=20,INTPTS=6,ITYPE=1,NEQN=NPDE*NPTS, * + NIW=NEQN+24,NWK=(10+6*NPDE)*NEQN, * + NW=NWK+(21+3*NPDE)*NPDE+7*NPTS+54) PARAMETER (NPDE=1,NPTS=50,INTPTS=50,ITYPE=1,NEQN=NPDE*NPTS, + NIW=NEQN+24,NWK=(10+6*NPDE)*NEQN, + NW=NWK+(21+3*NPDE)*NPDE+7*NPTS+54)

* INTPTS changed from 6 to 50 to increase resolution of X in output * .. Scalars in Common .. * DOUBLE PRECISION DPHI DOUBLE PRECISION PHIINIT, MPHI, NPHI * .. Local Scalars .. DOUBLE PRECISION ACC, HX, PI, PIBY2, TOUT, TS INTEGER I, IFAIL, IND, IT, ITASK, ITRACE, M * .. Local Arrays .. DOUBLE PRECISION U(NPDE,NPTS), UOUT(NPDE,INTPTS,ITYPE), W(NW), + X(NPTS), XOUT(INTPTS) INTEGER IW(NIW) * .. External Functions ..


192

DOUBLE PRECISION X01AAF EXTERNAL X01AAF * .. External Subroutines .. EXTERNAL BNDARY, D03PCF, D03PZF, PDEDEF, UINIT * .. Intrinsic Functions .. INTRINSIC SIN * .. Common blocks .. COMMON /VBLE/PHIINIT,MPHI,NPHI

* .. Data statements .. XOUT(1) = 1.0D-6

DO 60 I = 2, 51 XOUT(I) = 0.02+XOUT(I-1)

60 CONTINUE

* .. Executable Statements .. WRITE (NOUT,*) 'Raineys compression testing results - Ps' WRITE (NOUT+1,*) 'Raineys compression testing results - PHI only' ACC = 1.0D-6 M = 0 ITRACE = 0 MPHI = 4774D+0 * MPHI unitless NPHI = 2.4333D+0 * NPHI in Pa PHIINIT = 0.025429D+0 * Typical value in compression experiments

IND = 0 ITASK = 1 * * Set spatial mesh points * PIBY2 = 0.5D0*X01AAF(PI) HX = PIBY2/(NPTS-1) X(1) = 0.0D0 X(NPTS) = 1.0D+0 DO 20 I = 2, NPTS - 1 X(I) = SIN(HX*(I-1))

20 CONTINUE * * Set initial conditions * TS = 0.0D0 TOUT = 0.1D-5 * for testing - reduce TOUT step


193

* TOUT = 0.05D0 * Tom:

WRITE (NOUT,99999) ACC, PHIINIT, MPHI, NPHI WRITE (NOUT,99998) (XOUT(I),I=1,50) * Tom: Change from I=1,6 to I=1,50 to accommodate new X columns * Set the initial values CALL UINIT(U,NPTS)

ILOOPS = 87 * Tom: Changed above line from 5 to 83 to get values of TOUT 0 to * 0.83(=75mm/90mm at constant rate)

DO 40 IT = 1, ILOOPS

IFAIL = -1 TOUT = 0.01D0+TOUT * Tom: Introduce a linear timestep * * Call the solver CALL D03PCF(NPDE,M,TS,TOUT,PDEDEF,BNDARY,U,NPTS,X,ACC,W,NW,IW, + NIW,ITASK,ITRACE,IND,IFAIL) * * Interpolate solution at required spatial points CALL D03PZF(NPDE,M,U,NPTS,X,XOUT,INTPTS,ITYPE,UOUT,IFAIL)

WRITE (NOUT+1,99996) TOUT, (UOUT(1,I,1),I=1,INTPTS) WRITE (NOUT,99995) TOUT, ((MPHI*(UOUT(1,I,1))**NPHI),I=1,INTPTS) * Tom: to alternate between PS output and PHI output make active the correct * line here and also make active the appropriate FORMAT line (i.e. * either 99996 or 99995).

40 CONTINUE

* * Print integration statistics * WRITE (NOUT,99997) IW(1), IW(2), IW(3), IW(5) STOP * 99999 FORMAT (//' Accuracy requirement = ',D12.5,/ + ' PHIINIT = ',D12.5,/


194

+ ' MPHI = ',D12.5,/ + ' NPHI = ',D12.5,/) 99998 FORMAT (' T/ X ',50F8.4,/) 99997 FORMAT (' Number of integration steps in time ', + I4,/' Number of residual evaluations of resulting ODE ' + 'sys', + 'tem',I4,/' Number of Jacobian evaluations ', + ' ',I4,/' Number of iterations of nonlinear solve', + 'r ',I4,/) 99996 FORMAT (1X,F8.4,' PHI',50F8.4) * Tom: Change above format from 6F8.4 to 50F8.4 to display new columns * Tom: for X in output as governed by INTPTS 99995 FORMAT (1X,F8.4,' PS',50F8.3) * Switch between 99996 and 99995 END **************************************************************************** SUBROUTINE UINIT(U,NPTS) * Routine for PDE initial conditon

* .. declarations .. INTEGER I, NPTS DOUBLE PRECISION PHIINIT, MPHI, NPHI DOUBLE PRECISION U(1,NPTS)

* .. common blocks.. COMMON /VBLE/PHIINIT,MPHI,NPHI

* .. Executable Statements .. DO 20 I = 1, NPTS U(1,I) = PHIINIT 20 CONTINUE

RETURN END **************************************************************************** SUBROUTINE PDEDEF(NPDE,T,X,U,DUDX,P,Q,R,IRES)* .. Scalar Arguments .. DOUBLE PRECISION T, X INTEGER IRES, NPDE * .. Array Arguments .. DOUBLE PRECISION DUDX(NPDE), P(NPDE,NPDE), Q(NPDE), R(NPDE), + U(NPDE) * .. Scalars in Common .. DOUBLE PRECISION DPHI DOUBLE PRECISION PHIINIT,MPHI, NPHI DOUBLE PRECISION MU, KOZ, SV, DHDT, HINITIAL* .. Common blocks ..


195

COMMON /VBLE/PHIINIT,MPHI,NPHI

* NPHI (-) and MPHI (Pa) are the exponent and pre-exponent for the * compression correlation * mu is the viscosity (Pa.s) * koz is the variable kozeny factor (or set to 5.55) * Sv is the specific surface area (m^-1) * DHDT is the rate of the piston (m/s) * HINITIAL is the initial height (m) * Average Milled bagasse pulp MPHI * Average Milled bagasse pulp NPHI

MU = 0.001D0 * Viscosity of water in Pa.s KOZ = 3.5*((1-3.5*U(1))**3)*(1+(57*((3.5*U(1))**3)))/ + ((3.5*U(1))**0.5) * Koz can be set as constant k=5.55 if desired SV = 191700D0 * SV as m-1; this value is for optimum for variable koz factor

DHDT = 4.1667D-4 * in m/s - 75 mm displacement over 3 MINS HINITIAL = 0.073D0 * Initial height of the platen is 90mm, height in metres

* .. Executable Statements ..

DPHI = MPHI*NPHI*((1-U(1))**4)*((U(1))**(NPHI-2)) + /(MU*KOZ*SV*SV*DHDT*HINITIAL) Q(1) = -(1-X)*(1-T)*DUDX(1) R(1) = DPHI*DUDX(1) P(1,1) = (1-T)*(1-T)

RETURN END **************************************************************************** SUBROUTINE BNDARY(NPDE,T,U,UX,IBND,BETA,GAMMA,IRES) * .. Scalar Arguments .. DOUBLE PRECISION T INTEGER IBND, IRES, NPDE * .. Array Arguments .. DOUBLE PRECISION BETA(NPDE), GAMMA(NPDE), U(NPDE), UX(NPDE)

* .. Executable Statements ..

IF (IBND.EQ.0) THEN BETA(1) = 1.0D+0 GAMMA(1) = -U(1)*(1-T)


196

ELSE BETA(1) = 1.0D+0 GAMMA(1) = 0.0D+0 END IF

RETURN END


197

A.4 Graphs comparing predictions of dynamic filtration model with

experimental data

Commencing next page

198


198

199

199

Appendix A - Supplementary material for dynamic filtration modeling

200


200

201

201

Appendix A - Supplementary material for dynamic filtration modeling

202


202

203

Appendix B

Summary of pulp samples

204

Table B.1 Summary of bagasse pulping conditions for all pulp samples presented in this thesis.

Sample name Cook date Origin Fraction Reactor type Cooking conditions Cooking

time Screened

yield Screened kappa

number % rejects

Bagasse pulps (min) (-) (-) (%)

Sample 8 14/11/2006 Diffuser Medium Flow-through digester 0.4M NaOH & 0.1%AQ 40 0.5086 5.58%

Sample 9 14/11/2006 Diffuser Medium Flow-through digester 0.4M NaOH & 0.1%AQ 55

Sample 10 14/11/2006 Diffuser Medium Flow-through digester 0.4M NaOH & 0.1%AQ 70 0.4965 19.4 4.40%

Sample 11 14/11/2006 Mill Medium Flow-through digester 0.4M NaOH & 0.1%AQ 40 0.4436 21.9 4.68%






Kin

etics s

tudy




Sample 20 17/11/2006 Diffuser Coarse Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 0.5678 26.7 2.98%

Sample 21 17/11/2006 Diffuser Coarse Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 0.5615 26 3.62%

Sample 23 17/11/2006 Diffuser Medium Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 0.5201 22.7 5.93%

Sample 24 17/11/2006 Diffuser Coarse Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 0.5419 24.6 5.38%

Sample 26 20/11/2006 Mill Coarse Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 0.5467 26.2 3.70%

Sample 27 20/11/2006 Mill Medium Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 0.5098 24.3 6.07%

Sample 29 20/11/2006 Mill Coarse Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 0.5354 26 5.57%



Sample 32 21/11/2006 Ledesma Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 0.6180* 22.5 4.47%





Sample 37 21/11/2006 Ledesma Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 *Combined with Sample 32



Sample 40 22/11/2006 Diffuser Coarse Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 Not recorded

Sample 41 22/11/2006 Mill Medium Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 0.4718 12.08%

Sample 42 22/11/2006 Mill Medium Flow-through digester 0.4M NaOH & 0.1%AQ, 145 deg 30 0.5082 22 3.98%


Thom

as J. Rain

ey, A

study o

f bag

asse pulp

filtration

204

205

Sample name Cook date Origin Fraction Reactor type Cooking conditions Cooking

time Screened

yield

Screened kappa

number %

rejects

(min) (-) (-) (%)

Sample 52 17/07/2007 Mill Whole bagasse Large Parr Reactor

12.5% Na2O, 14:1 liq fibre, 170 deg, 105 minutes, no

AQ 105 0.5260 40 0.24%

Sample 53 19/07/2007 Mill Whole bagasse Large Parr Reactor

15% Na2O, 0.1% AQ, 12:1 liq fibre, 105 mins, 170

deg 105 9.2

Sample 55 9/04/2008 Mill Medium Large Parr Reactor 15% Na2O, 0.1% AQ, 12:1 liq fibre, 90 mins, 170 deg 90 0.4066 9.2 1.09%

Sample 56 14/04/2008 Mill Coarse Large Parr Reactor 15% Na2O, 0.1% AQ, 12:1 liq fibre, 60 mins, 170 deg 60 0.4924 16.2 5.09%

Sample 57 16/04/2008 Mill Coarse Large Parr Reactor 15% Na2O, 0.1% AQ, 12:1 liq fibre, 90 mins, 170 deg 90 0.4910 10.3 2.80%

Sample 58 23/04/2008 Mill 30% depithed Large Parr Reactor 15% Na2O, 0.1% AQ, 12:1 liq fibre, 60 mins, 170 deg 60 0.4068 14.3 8.07%

Sample 59 28/04/2008 Mill Fine Large Parr Reactor 15% Na2O, 0.1% AQ, 12:1 liq fibre, 90 mins, 170 deg 90 0.2868 8.1 8.61%

Sample 60 21/07/2008 Mill Medium Large Parr Reactor 15% Na2O, 0.1% AQ, 12:1 liq fibre, 60 mins, 170 deg 60 0.4902 15.7 5.74%

Wood pulps

Eucalyptus Globulus Ensis Air-bath reactor 11.75% Na2O, 25% sulfidity, 165 deg 120 19

Pinus Radiata APPI Air-bath reactor 20

Appen

dix

B - S

um

mary

of p

ulp

samples

205

206

This page is deliberately blank.

Appendix C-Supplementary photographs of experimental work

207

Appendix C

Supplementary photographs of

experimental work


208

Figure C.1 Photograph of bagasse packed into a cage ready for insertion

into a ‘flow-through’ reactor cell.

Figure C.2 Photograph of a loaded ‘flow-through’ reactor cell.

Packed bagasse

Top flange

Inlet liquor line

Outlet liquor line

Appendix C-Supplementary photographs of experimental work

209

Figure C.3 Photograph of the inside of the unloaded 18.5 L Parr reactor.

Figure C.4 Photograph of the 18.5 L Parr reactor vessel head.

Serpentine

cooling coils

��

��

��

� ��

��

��


210

Figure C.5 Barrel of compressibility cell immediately prior to loading

with pulp slurry.

Figure C.6 Compressibility cell mid-way through a compressibility

experiment.

Figure C.7 Relaxed pulp pad after compression experiment showing

layering of pulp.

��

��

��

��

��

�

�

��

��

211

Appendix D

Table of Students t distribution


212

The point tabulated is t, where P(tv>t)=p and tv has Student’s t-distribution with

v degrees of freedom.

Table D.1 Table of Student’s t statistic

p 0.25 0.1 0.05 0.025 0.01 0.005 0.0025 0.001 0.0005

v

1 1.000 3.078 6.314 12.71 31.82 63.66 127.3 318.3 636.6

2 0.816 1.886 2.920 4.303 6.965 9.925 14.09 22.33 31.60

3 0.765 1.638 2.353 3.182 4.541 5.841 7.453 10.21 12.92

4 0.741 1.533 2.132 2.776 3.747 4.604 5.598 7.173 8.610

5 0.727 1.476 2.015 2.571 3.365 4.032 4.773 5.893 6.869

6 0.718 1.440 1.943 2.447 3.143 3.707 4.317 5.208 5.959

7 0.711 1.415 1.895 2.365 2.998 3.499 4.029 4.785 5.408

8 0.706 1.397 1.860 2.306 2.896 3.355 3.833 4.501 5.041

9 0.703 1.383 1.833 2.262 2.821 3.250 3.690 4.297 4.781

10 0.700 1.372 1.812 2.228 2.764 3.169 3.581 4.144 4.587

11 0.697 1.363 1.796 2.201 2.718 3.106 3.497 4.025 4.437

12 0.695 1.356 1.782 2.179 2.681 3.055 3.428 3.930 4.318

13 0.694 1.350 1.771 2.160 2.650 3.012 3.372 3.852 4.221

14 0.692 1.345 1.761 2.145 2.624 2.977 3.326 3.787 4.140

15 0.691 1.341 1.753 2.131 2.602 2.947 3.286 3.733 4.073

16 0.690 1.337 1.746 2.120 2.583 2.921 3.252 3.686 4.015

17 0.689 1.333 1.740 2.110 2.567 2.898 3.222 3.646 3.965

18 0.688 1.330 1.734 2.101 2.552 2.878 3.197 3.610 3.922

19 0.688 1.328 1.729 2.093 2.539 2.861 3.174 3.579 3.883

20 0.687 1.325 1.725 2.086 2.528 2.845 3.153 3.552 3.850

21 0.686 1.323 1.721 2.080 2.518 2.831 3.135 3.527 3.819

22 0.686 1.321 1.717 2.074 2.508 2.819 3.119 3.505 3.792

23 0.685 1.319 1.714 2.069 2.500 2.807 3.104 3.485 3.767

24 0.685 1.318 1.711 2.064 2.492 2.797 3.091 3.467 3.745

25 0.684 1.316 1.708 2.060 2.485 2.787 3.078 3.450 3.725

30 0.683 1.310 1.697 2.042 2.457 2.750 3.030 3.385 3.646

40 0.681 1.303 1.684 2.021 2.423 2.704 2.971 3.307 3.551

50 0.679 1.299 1.676 2.009 2.403 2.678 2.937 3.261 3.496

60 0.679 1.296 1.671 2.000 2.390 2.660 2.915 3.232 3.460

120 0.677 1.289 1.658 1.980 2.358 2.617 2.860 3.160 3.373

� 0.674 1.282 1.645 1.960 2.326 2.576 2.807 3.090 3.291

213

Appendix E

Fibre length data of pulp sample

214

Table E.1 Results of fibre length distribution analysis for all pulp samples measured in this thesis.

Instumentation

and

First

decile

First

quartile

Second

quartile

Third

quartile

Ninth

decile

location

Sample

name

Diffuser

or

Milled

Coarse or

Medium

Number

of fibres10% 25% 50% 75% 90%

Average% fines

(<0.2mm)

18 Milled Medium 4999 0.29 0.51 0.87 1.37 1.89 1.01

20 Diffuser Coarse 3013 0.29 0.51 0.91 1.39 1.93 1.03

21 Diffuser Coarse 5016 0.4 0.72 1.2 1.7 2.23 1.27

26 Milled Coarse 5035 0.32 0.55 0.99 1.51 2.08 1.11

27 Milled Medium 5016 0.3 0.53 0.9 1.41 2.02 1.04

30 Milled Medium 5048 0.24 0.44 0.78 1.24 1.71 0.9

32 N/A N/A 5034 0.23 0.42 0.75 1.28 1.94 0.96

34 Milled Coarse 5027 0.28 0.51 0.94 1.45 2.12 1.09

35 Diffuser Medium 5023 0.26 0.49 0.87 1.31 1.79 0.98

38 Milled Coarse 5024 0.31 0.54 0.91 1.47 2.08 1.07

39 Diffuser Medium 5031 0.27 0.47 0.8 1.23 1.76 0.92

42 Milled Medium 5004 0.24 0.42 0.72 1.14 1.6 0.84

43 Milled Coarse 5020 0.26 0.45 0.82 1.37 2 0.99

51 Milled Fine 5066 0.11 0.21 0.39 0.67 1.09 0.52

53 Milled Unfrac. 4980 0.21 0.38 0.66 1.07 1.56 0.8

Kaj

aani

FS

100 a

t A

mco

r, P

etri

e M

ill

Eucalypt 5036 0.39 0.57 0.77 0.98 1.15 0.77

56 Milled Coarse 8097 1.148 10.1

58 Milled 30%

depithed 5857 0.965 17.45

FQ

A a

t

UB

C

60 Milled Medium 4573 1.149 11.1

Thom

as J. Rain

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asse pulp

filtration

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215

Appendix F

Engineering drawings of compression

cell

216

Thomas J. Rainey - A study of bagasse pulp filtration

216

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Appendix F - Engineering drawings of compression cell

217

First

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A study into the permeability and compressibility …improving the permeability and compressibility...

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Transcript of A study into the permeability and compressibility …improving the permeability and compressibility...