Chapter 1

RESEARCH PROBLEM & ITS SETTING

Introduction

Water has always been a vital material for man’s existence. Its uses for drinking, transport,

recreation, agriculture and industry show immediately the extent to which it is an integral

part of our life. But because water is a good solvent, it picks up impurities easily and the

substances dissolved and/or suspended in it affect water’s suitability to the purpose for

which it is required.

In industries requiring water for steam boilers, one of the most common problems

encountered is water hardness. Hardness in water is mainly caused by ions of calcium and

magnesium, which are present with anions such as HCO3- ; SO42-, Cl-, NO3- and SO4

2-. Hard

water, when heated, precipitates calcium carbonate, which then clogs boilers and pipes

resulting to a poor conductance of heat and a decrease in boiler efficiency.

Ion exchange is a process that can be used to reduce or remove the hardness of water. It is a

reversible process where ions of a given species are displaced from an insoluble solid

substance (exchange medium) by ions of another species dissolved in water. Most

companies use synthetic resins, which are expensive and not readily available, as exchange

mediums for ion exchangers. That is why previous researchers Abroguena, et al wanted to

“develop a substitute for these resins that is economical and is locally available for easy

access”. They have found that coconut husk fiber (CHF) “proved to be a potential alternative

for the expensive synthetic resins used in the ion-exchange of softening water…”

(Abroguena, et al, 2004).

Statement of the Problem

There have been a number of studies that were conducted to demonstrate the feasibility of

coconut husk fiber (CHF) as cation exchanger. Although they have showed that CHF could

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display adsorbent characteristics, most of the parameters that describe CHF’s performance

as cation exchanger, such as its isotherm behavior, were not thoroughly established. Thus,

this study principally aims to evaluate the usefulness of the CHF as a synthetic-resin-

alternative by characterizing its performance as cation exchanger in the softening process of

removing water hardness.

The following are the specific questions that the researchers are going to answer after

performing the study:

1. What are the characteristics of coconut husk fiber, its hardness-removal capacity and adsorption isotherm?

2. What is the kinetic behavior of coconut husk fiber in the softening process?

3. Is coconut husk fiber comparable to commercial ion exchange resins in terms of performance and operational cost?

Objectives of the Study

Because the feasibility of the application of the CHF as cation exchanger in the water

softening process has already been demonstrated (Abroguena, et al, 2004), the question thus,

lies in what extent and degree can a given mass of CHF reduce water hardness to some

specific level. This study, thus, aims to measure CHF’s cation exchange-related parameters

and compare the organic adsorbent to commercial ion exchangers.

Specific objectives of this study include:

1. To determine the moisture content, adsorption isotherm, and hardness-removal capacity of coconut husk fiber.

2. To determine the minimum contact time between the CHF and hard water for effective softening and to determine its breakthrough time.

3. To be able to describe the comparability of the CHF ion exchanger to commercial ion exchangers in terms of performance and over-all cost.

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Statement of Hypotheses

Based on the study problems, the researchers formulate the following hypotheses:

1. CHFIE is better than commercial ion exchangers in terms

a. Performance

b. Over-all cost

Theoretical Background

Ion exchange is a “process in which solid particles containing exchangeable cations or

anions are contacted with an electrolyte solution to change the composition of the solution”.

(McCabe et. al., 2006) Major applications are seen in water softening by exchanging

calcium ions for sodium ions. Ion-exchange capability exists in some natural material such

as clays and zeolites, but most processes use synthetic ion-exchange resins. These are

prepared from organic polymers such as cross-linked polystyrene to which ionizable groups

have been added.

Besides clays and zeolites, “naturally existing materials have been studied by various

researchers to determine their ion-exchange properties and they have shown that cellulosic

materials, such as the CHF, exhibit cation-exchange properties.” (Abrogueña et. al., 2004)

Cellulosic fibers have ordered (crystalline) and disordered (amorphous) regions. In addition,

they have inner voids or pore spaces of which volume and inner surface areas affect their

accessibility, reactivity, and adsorption properties. These “void fractions allow significant

volume of water to pass through, thus, these portions of the less ordered amorphous regions

where the adsorption process takes place must have a significant influence on the adsorption

properties of these fibers”. (Malej, 2003)

Coconut Husk Fibers

“The sponge-like structure of CHF has the ability to adsorb large quantities of water very

quickly but it will not be waterlogged as excess water drains away. The accessibility of

cellulose to water varies with the size and distribution of the crystalline and amorphous

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regions.” (Maini & Santos, 1992) CHF accessibility then is “exhibited in its interactions

with water because water is able to destroy its weaker hydrogen bonds but cannot penetrate

into the crystalline regions of the fiber and this explains why Na+ ions are attached or

adsorbed on the amorphous region of the CHF.” (Malej, 2003)

Table 1 and Table 2 show the physical properties and composition of CHF.

Table 1. Physical Properties of CHF

Physical Properties of CHF

Diameter / width (in micron) 16

Length (in inches) 6 - 8

Density (g/cc) 1.40

Breaking elongation % 30

Moisture regain at 65 % R. H (%) 10.5

Swelling in water (diameter) 5%

Source: “About Coir”, www.karan-carpets.com

Table 2. Composition of Coconut Husk Fiber

Chemical composition of Coir

Water solubles 5.25%

Pectin and related compounds 3.00%

Hemi - cellulose 0.25%

Lignin 45.84%

Cellulose 43.44%

Ash 2.22%----------------

100.00%Source: “About Coir”, www.karan-carpets.com

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Ion Exchange Equilibrium

Accordingly, “the design of the ion exchange units is based upon ion exchange equilibrium”

(Smith, 2005). The generalized reaction equation for the exchange of ions A and a cation

exchange resin can be expressed as

nR-A+ + Bn+ ↔ Rn -Bn+ + nA+

where R- = an anionic group attached to exchange resin

A+ Bn+ = ions in solution

The equilibrium expression fro this reaction is

K A→ B=¿¿¿

where K A→B = selectivity coeff. a function of ionic strength

[Rn- Bn+], [R-A+]n = mole fraction of A+ and B+ exchange resin

[A+]n, [ Bn+] = concentration of A+ and B+ in solution, mole/ L

qA, qB = conc. of A and B on resin sites, eq/L

CA, CB = conc. of A and B in the solution, mg/L

The selectivity constant depends upon the valence, nature and concentration of the ion in the

solution.

The rates of ion-exchange reactions are “limited by the rates of diffusion of the counter ions

in the external solution and in the pores of the resin particles. Equilibrium can be assumed to

exist at any point inside each particle, and predicting the concentration changes with time

would require solution of the unsteady state diffusion equations.” (McCabe et. al., 2006)

Adsorption Theory

A component “can be separated from a mixture if it selectively adsorbs onto a solid surface.

This is the basis of the adsorption unit process. Adsorbents are usually porous solids, and

adsorption occurs mainly on the pore walls "inside" particles.” (Price, 2003) Ion exchange is

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a similar process; however, in this case ions create complexes with the solid instead of

adsorbing.

Adsorption equilibrium data is typically plotted in the form of an adsorption isotherm (i.e. at

constant temperature) with the mass adsorbed on the y-axis and the mass in the fluid on the

x-axis. “The shape of the curve is significant and factors heavily into design. Isotherms that

are “favorable” permit higher solid loadings at lower solution concentrations. These tend to

start out steep and level out. Isotherms which start out flat are "unfavorable", since they only

work well at high concentrations of solute. “(Price, 2003)

Isotherm adsorption models are used in hard water treatment to predict the ability of CHF to

remove water hardness down to a specific value. Several fits have been proposed for

isotherms. “A linear isotherm seems to work for very dilute solutions, but not for many

others. The Freundlich isotherm describes physical adsorption from liquids and can also be

used for the adsorption of hydrocarbon gases on activated carbon.” (Brown, 2001) It is a two

parameter model, of the form:

q=k Cn

Where q is the adsorbance in terms of mg hardness/ g medium, C is the solution’s

concentration while k and n are constants. The equation can be linearized into the form:

log q=log k+n log C

Perhaps, the best-known isotherm is the Langmuir isotherm, given by

q=qoC

k+C

where qo is the maximum adsorbance. The equation can also be linearized in the form,

1q= k

qo

C+ 1qo

“Langmuir isotherm assumes that the number of adsorption sites is fixed and that adsorption

is reversible. The Langmuir and Freundlich isotherms are only applicable to batch adsorber

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systems where sufficient time is provided to allow equilibrium between the ions in solution

and the ions adsorbed on the media to occur ” (Brown, 2001).

Breakthrough Theory

Adsorption is a transient process. The amount of material adsorbed within a bed depends

both on position and time. “As fluid enters a bed of adsorbent, it comes in contact with the

first few layers of absorbent. Solute adsorbs, filling up some of the available sites. Soon, the

adsorbent near the entrance is saturated and the fluid penetrates farther into the bed before

all solute is removed. Thus the active region shifts down through the bed as time goes on”

(Price, 2003).

The fluid emerging from the bed will have little or no solute remaining -- at least until the

bulk of the bed becomes saturated. The break point occurs when the concentration of the

fluid leaving the bed spikes as unadsorbed solute begins to emerge. The bed has become

ineffective.

As the concentration wave moves through the bed, most of the mass transfer is occurring in

a fairly small region. This mass transfer zone moves down the bed until it "breaks through".

The shape of the mass transfer zone depends on the adsorption isotherm, flow rate, and the

diffusion characteristics.

Many applications of adsorbers to raw water use a fixed-bed approach where the water

stream flows through a stationary mass of adsorbent. During the flow through the adsorbent,

many of the ions are expected to come into contact with active surface sites and thus be

retained on the surface of the adsorbing media. The design of these fixed adsorbers requires

the determination of two significant design parameters: minimum contact time and life of

the bed.

The minimum contact time is “determined through a series of kinetics tests in which a

known mass of media is exposed to “hard water” of known concentration for specific

amounts of time. At the end of each time increment, the concentration of the pollutant in the

solution is measured and the ratio of the final concentration to the initial concentration is

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plotted against time. The time at which the curve begins to flatten is the minimum contact

time required for the development of pseudo-equilibrium. Although removal may continue

past this minimum time, longer contact times in general are not economically efficient

because the size of the adsorber will need to be much greater for minimal additional

removal.” (Brown, 2001)

The service life of the bed “can be determined using the bed depth-service time [BDST]

model, which is based on the Bohart and Adams “quasi-chemical” rate law. The assumption

behind the Bohart and Adams equation is that equilibrium is not instantaneous and therefore,

the rate of the sorption reaction is proportional to the fraction of sorption capacity still

remaining on the media.” (Cooney, 1999) The linearized BDST model equation is as

follows:

tb=¿

No D

εv Co

− 1k Co

ln(Co

Cb

−1)¿

where tb = time until breakthrough (min)

C0 = initial concentration of pollutant (mg/L)

Cb = breakthrough concentration of pollutant (mg/L)

v = fluid velocity or loading rate (m/min)

e = porosity of the filter

K = quasi-chemical rate constant from Bohart and Adams theory (L/mg-sec)

N0 = capacity of the media for each pollutant in a multi-component solution (mg pollutant per cubic meter of filter volume)

D = depth of the filter bed

Significance of Study

The water used by many industries is usually sourced out from rivers, lakes, springs and

deep wells. Because water dissolves almost everything, water from these sources is naturally

hard. Since most industries today employ water in their processes that involves heat transfer

operations, especially in steam boilers, and since carbonate hardness of water result to scale

formation, hence, loss of energy, the need to soften water becomes a necessity.

Although prices of synthetic resins used in the softening process of ion-exchange have

dropped significantly during the past few decades, still it cannot be considered cheap.

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Furthermore, these resins when purchased are only available for bulk orders in selected key

cities like Cebu and Manila because most of the manufacturers are based abroad. Thus, the

cost of the resin is not the only problem but the availability as well.

CHF, a waste generated in coconut processing, is cheap, biodegradable, abundant and

locally available. So, the use of CHF in ion exchange will greatly reduce the cost of water

softening while minimizing environmental problems by lessening non-biodegradable waste.

Scope and Delimitations

This study has concentrated on the evaluation and/or measurement of parameters that

characterize CHF’s adsorption behavior. It included water testing, in which water samples

were obtained or prepared from the deionized water available at the Chemical Engineering

Laboratory of Xavier University. All of the experiments were conducted inside the said

laboratory. For the comparison of the CHF ion exchanger with the commercial type, the

commercial ion exchanger available in the ChE laboratory was used.

This study gave emphasis on the determination of the values of the process design

parameters that will maximize the efficiency of CHF in water hardness removal at a reduced

cost. However, this study will not include the determination of the effect of temperature on

the performance of CHF ion exchanger due to the limitations brought about by the

unavailability of apparatuses.

Definition of Terms

Anion refers to a negatively charged ion.

Breakthrough time is the elapsed time between initial contact of the ions in water with the

surface of CHF and the time at which the concentration of effluent flowing out of the

column is greater than 4 ppm.

Capacity is the amount of hardness ions removed per given mass of medium. For this paper,

capacity is reported as mg Ca2+/ g media.

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Cation refers to positively charged ion.

Hard water is water that has a high mineral content which consists of high levels of metal

ions, mainly calcium and magnesium in the form of carbonates.

Ion exchange is a process in which ions are exchanged between a solution and an ion

exchanger, an insoluble solid or gel. Ion exchange is a reversible process and the ion

exchanger can be regenerated or loaded by washing with an excess of the ions to be

exchanged.

Ion exchangers are either cation exchangers for positively charged cations or anion

exchangers for negatively charged anions. Ion exchange is a reversible process and the ion

exchanger can be regenerated or loaded by washing with an excess of the ions to be

exchanged.

Leakage is defined as the appearance of a low concentration of undesirable ions in the

column effluent during the beginning of the exhaustion.

Regeneration refers to process of treating the calcium-loaded resin with sodium chloride

solution, bringing it back to sodium form and making it ready for another cycle of operation.

Resin is an insoluble and porous polymer. The polymer have a very high molecular weight

and carry a functional group with either a positive (+) or negative (-) charge, known as

exchange sites.

Selectivity Constant is the constant of the equilibrium expression for the reaction equation

of the ion-exchange process. It is a function of ionic strength and is not a true constant.

Softening is the process of removing sparingly soluble calcium and magnesium ions with

less objectionable sodium ions. For this paper, softened water refers to hard water which

hardness had been reduced to 4 ppm.