THE UNIVERSITY OF THE WEST INDIES

FACULTY OF ENGINEERING

DEPARTMENT OF CHEMICAL & PROCESS ENGINEERING

CHEMICAL ENGINEERING LABORATORY 2 (CHNG 2009/2010)

2008 – 2009

SED2 – Sedimentation Studies II

Date conducted: 23rd October 2008

Date submitted: 06th November 2008

Name: MEERA RAMPERSAD

Student ID No: 807001964

Group No: P

TABLE OF CONTENTS

ABSTRACT....................................................................................................................................1

OBJECTIVES..................................................................................................................................1

APPARATUS & PROCEDURE.....................................................................................................2

Apparatus:....................................................................................................................................2

Procedure:....................................................................................................................................2

THEORY.........................................................................................................................................3

RESULTS & CALCULATIONS....................................................................................................5

Sample Calculations:...................................................................................................................8

DISCUSSION..................................................................................................................................9

Precautions that were taken during the experiment...................................................................11

Possible errors which occurred..................................................................................................11

Safety Consideration to be made when conducting this experiment.........................................11

CONCLUSION & RECOMMENDATIONS................................................................................12

Conclusion:................................................................................................................................12

Recommendations:....................................................................................................................12

REFERENCES..............................................................................................................................14

APPENDIX A..................................................................................................................................1

LIST OF TABLES

Table 1: Settling Velocities of the Suspension................................................................................5

Table 2: Height of Suspension and Final Compaction Height........................................................6

LIST OF FIGURES

Figure 1: Suspension Interface Height vs. Time Plot......................................................................6

Figure 2: Settling Velocities Vs Height for the Four Initial Heights Plot.......................................7

Figure 3: Plot of Suspension Interface Height vs time for the 750mm Suspension........................7

ABSTRACT

The objectives of this experiment were to investigate the effect of the initial height for the

550mm, 650mm, 750mm and 850mm suspensions with a constant mass concentration of 4% on

the sedimentation, to analyse the rate of settling velocity against the initial height from a single

batch test, to identify the concentration at which the mass rate of solids is a minimum (C L) at an

initial height of 75cm and to determine the settling velocity at this height. In order to achieve the

objectives, the vessels were individually agitated and allowed to settle and the height of the

interface was recorded ever thirty seconds. From the results, it was found that there existed a

decreasing linear variation initially followed by a decreasing non-linear region which levelled off

to a decreasing linear region for the height of the interface against time for all the suspensions.

Since the concentration of the suspensions was constant the initial heights varied by an almost

constant factor of 1.2 which could have been used to plot the graphs for the 750mm and 850 mm

suspensions.

Secondly it was deduced that the settling velocity varied in a non-linear decreasing manner with

the height where as the settling velocity decreased as the height of the interface increased and

finally the limiting concentration CL and settling velocity vL at this concentration for the 750mm

suspension were identified to be 4.69% and -0.0002239ms-1 respectively. The results for the

850mm suspension were not as expected due to errors which may have occurred during the

experiment. Generally the results obtained during this experiment were adequate to make a good

analysis of the objectives.

OBJECTIVES

Sedimentation is the partial separation or concentration of suspended solid particles from a liquid

by gravity. The equipment used for settling by gravity is either thickeners or clarifiers. In the

thickening process the concentration of the suspended solids in the feed stream is increased

whereas in clarification the suspended particles are removed to make a clear effluent. The types

of sedimentation processes used are affected by factors such as particle size, liquid viscosity,

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solution densities, as well as the characteristics of the slurry within the solution. This process is

important industrially because it can be used to treat wastewater by removing suspending solids

from it or it can be used to recover valuable solids such as ores from washing solutions. In this

experiment the settling process is a batch clarification process where the suspension is allowed to

settle into a clear liquid zone and sediment.

The objectives of the experiment were to:

1. Investigate the effect of the initial height with a constant mass concentration of 4% on

sedimentation characteristics.

2. Analyse the rate of settling velocity versus initial height curve from a single batch test.

3. Identify the concentration at which the mass rate of the solids is a minimum (C L) at an

initial height of 75cm.

4. Determine the settling velocity (νL) at this height.

APPARATUS & PROCEDURE

Apparatus:

Refer to Appendix A for the sketch of the Armfield W2 Sedimentation apparatus that was used in

the experiment.

The suspension used was green coloured chalk and water. This was placed in cylindrical glass

vessels that were vertically oriented in the Armfield W2 sedimentation apparatus. A back light

on the apparatus provided the lighting needed. On the front of the vessels were measuring tape

ranging from 0-940 cm. A stopwatch was used to measure the rate of settling.

Procedure:

The following procedure was carried out during the experiment [CHNG2009/2010 – Chemical

Engineering Laboratory]:

1. The back lighting on the apparatus was turned on by pressing the switch located at the

side of the apparatus.

2

2. The tube with the height of 55cm was carefully removed and was shook until a uniform

suspension of the chalk was obtained.

3. The tube was then quickly replaced onto the apparatus and the stopwatch was started.

The initial height of the suspension was measured.

4. The height of the suspension interface at 30second intervals was measured until the

process reached the final compaction stages where the rate of change of height suddenly

decreased.

5. The final compaction height was recorded after 24 hours.

6. Steps 2-5 were repeated for the other three initial heights (65cm, 75cm, and 85cm).

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THEORY

There are two basic types of settling that takes place, hindered settling and free settling.

Hindered settling occurs when the concentration of the solution is high enough for there to be

significant interaction between the particles. The frictional force exerted at a given velocity of

the particles relative to the fluid may be greatly increased as a result of modifications to the flow

pattern. [Particle Technology & Separation Processes]. Free or particulate settling occurs when

the concentration of the solution is low and the particles are sufficiently far apart to freely settle

regardless of their nature. Faster settling particles may collide with slower moving ones where

they may cohere or continue downward at their own settling velocity. The cohered particles

settle at a rate greater than that of the individual particles and there is a gradual transition from

particulate settling into zone settling regime where the particles are forced to settle as a mass.

The settling rate of the mass is a function of the solids concentration. [Perry’s Chemical

Engineers Handbook]. The sedimentation velocities of the particles tend to decrease steadily as

the concentration of the suspension is increased. However for very dilute solutions the settling

velocities may be up to 1.5 times the normal terminal velocities due to the formation of clusters

of particles which settle in well defined streams.

For concentrated suspensions there are two main types of settling Type 1 and Type 2. In Type 1

settling, the suspension settles into four zones, a clear liquid zone, a constant composition zone, a

variable composition zone and sediment. In Type 2 settling, there are only three zones with the

constant composition zone absent. When the suspension is first allowed to settle, there is a brief

period of acceleration, followed by the constant downward movement of the interface between

the clear liquid and the suspension. The sediment accumulates at the bottom of the vessel. When

the interface approaches the layer of sediment, its rate of fall decreases until the critical settling

point is reached i.e. when the direct interface is formed between the sediment and the clear

liquid. Further sedimentation results in the agglomeration of the sediment and the increased

upward movement of the liquid. [Particle Technology & Separation Processes]

For batch settling carried out in vertical tubes the main factors which affect the process are the

height of the suspension, the diameter of the vessel, and the volumetric concentration. Generally 4

the height of the suspension does not affect the rate of sedimentation or the consistency of the

sediment obtained. However if the position of the interface is plotted as a function of time for

two or more different height, then it is seen that they vary by a constant factor. If the ratio of the

diameter of the vessel to the diameter of the particle is greater than 100 then the walls of the

container have no effect on the rate of sedimentation.[Particle Technology & Separation

Processes]

The concentration of the suspension however affects the rate of sedimentation such that at higher

concentrations the rate of fall of sludge is lower because the upward velocity of the displaced

fluid is greater hence the velocity gradients are steeper. The movement of the particles is retarded

and the settling rate is less than that for free settling. When the concentration of a suspension is

low the settling velocity is uniform and the particles move at an almost constant rate. For a

continuous process if the settling velocity is high then some of the particles may be withdrawn

with the clear liquid.

The shape and orientation of the vessel affects the rate of sedimentation such that if the walls of

the vessel are vertical and the cross-sectional area does not vary with depth then the shape of the

vessels can be considered to be is independent of the sedimentation rate. If the vessel is inclined

at an angle then its shape cannot be ignored. [Particle Technology & Separation Processes].

Initially the rate of settling is linear in the settling process and the rate of sedimentation can be

calculated by the following equation [CHNG2009/2010 – Chemical Engineering Laboratory]:

Gs=−S×ms

V where Gs –Mass settling rateS – Slope of the initial line

ms – Mass of solids added

V – Volume of the test vessel

For a continuous sedimentation process, the rate of settling in each zone depends on it being at

least fast as the rate at which solids are being fed to the system. The concentration at which the

mass transfer rate of solids is a minimum CL, the settling velocity at this concentration vL, and

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the rate of upward flow of water, thus need to be considered when designing settling tanks and

thickeners. CL may be found by graphically by the following method: [CHNG2009/2010–

Chemical Engineering Laboratory].

1. Plotting a graph of suspension height interface against time for the periods of constant rate,

hindered settling and final compaction.

2. Drawing tangents to the two linear points on the curve i.e. the constant rate and final

compaction zones.

3. Drawing a bisector at the point of intersection of these two tangents. The point at which the

bisector intersects the sedimentation curve is the critical point and it corresponds to a height

Hi on the y-axis of the graph. This height represents that at which the slurry would occupy if

all the solids were present at limiting concentration.

CL may then be found empirically by the following equation:

CL H i¿=Co Ho where Co is the original slurry concentration

Ho is the original height

H i¿

is the y-intercept of the tangent at the critical point Hi

vL is the gradient of the tangent to sedimentation curve

RESULTS & CALCULATIONS

The raw results collected during the experiment can be found in Appendix A.

Table 1: Settling Velocities of the Suspension

Original Height of Suspension Ho / m 0.50 0.65 0.75 0.85Height the settling velocity is measured /m Settling Velocity / x10-4 ms-1

0.30 -4.55 -2.94 -1.92 -2.000.35 -6.86 -3.53 -2.67 -2.640.40 -11.11 -6.06 -3.70 -3.510.45 -18.75 -8.57 -5.17 -4.500.50 -41.67 -12.82 -7.57 -5.88

6

Table 2: Height of Suspension and Final Compaction Height

Original Height of Suspension / m Final compaction height / m

0.55 0.086

0.65 0.102

0.75 0.115

0.85 0.128

Figure 1: Suspension Interface Height vs. Time Plot

7

Figure 2:

Settling Velocities Vs Height for the Four Initial Heights Plot

Figure 3: Plot of Suspension Interface Height vs time for the 750mm Suspension

8

Sample Calculations:

1. Settling velocity, v L

From the graph of suspension height vs time, the settling velocity is the gradient of the

tangent to the curve at any point.

Gradient = vL=

ΔSuspension heightΔ Time

For the 550mm suspension, at 0.30m

vL=Δ−0 . 3Δ660

=−4 .54×10−4 ms−1

2. For the 750mm suspension:

a. Limiting concentration, C L:

Co = 4% Ho = 0.75m

H i¿

= 0.640m (found from the graph of suspension interface height vs time).

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CL=Co H o

H i¿

CL=0 .04×0 .750

0 . 640=0 .0469

CL = 4.69%

b. Settling velocity, v L:

Gradient of the tangent at the critical point =

0 .34−0 . 641340

=−0. 0002239 ms−1

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DISCUSSION

Sedimentation can either be a batch or continuous process. When the process is continuous the

concentration at which the mass rate of the solids is especially important for the correct

functioning of the system and design of the equipment used. Sedimentation is widely utilised in

the process industry. It is the major process employed by sewage water treatment companies to

purify the water so that it can be released back into the environment without causing an

imbalance of the ecosystem. Sedimentation processes are also employed by companies that use

minerals or solids as the raw materials or final products for their processes where sedimentation

is used to recover these minerals or solids from suspensions. In this experiment the

sedimentation process carried out was a batch process and the analysis carried out here is

comparable to any industrial settling process.

In this experiment, the effect of concentration of the suspension was not investigated since the

concentrations of the suspensions were constant at 4%. Since the settling vessels were long

cylindrical tubes that were vertically oriented, the vessel had little or no effect on the rate of

sedimentation. On analysing the graph of suspension interface height against time it was deduced

that the relationship was a decreasing one. Initially the graphs were linear as shown in Figure 1,

which indicated that the rate of settling was the highest. The rate of settling then fell off when

the consolidation started to form since the resistance to the flow of liquid increased and the

height of the interface decreased. This is represented by the curved part of the graph. In the final

consolidation region which is linear, the sediment settled the slowest since the displaced fluid

had to flow through the small spaces between the particles.

When an x=y line was drawn from the origin, it was seen the ratio of the distances of this line

from the 550mm curve to the origin and to the 650mm curve to the origin varied by a constant

factor of approximately 1.2. The distance between the 650mm and 750mm curves varied by

approximately the same factor of 1.2. It was expected that the distance between the 750mm and

850mm graphs would vary by this same factor. However this was not obtained due to errors

which may have occurred during the experiment. The height of the suspension interface varied

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with time by a constant factor of 1.2 and if this factor was known for two sets of heights then the

curves for the 750mm, 850mm etc could be reproduced with needing experimental data.

The settling velocity was found for the different suspensions over the region 300 – 500mm. It

calculated from the gradient of the tangents to these points on the height of the suspension

interface vs time graph. From Figure 2, the general trend was that the velocity decreased non-

linearly for the different heights of suspension examined. This was in agreement with the theory

since as the settling progresses the upward movement of the liquid is reduced due to the fact that

the some of the sediment begins to coagulate. This caused the area over which the particles

moved to decrease thus the settling velocity decreased. The order of decreasing settling velocity

was 550mm, 650mm, 750mm and 850mm respectively with the 550mm suspension settling the

fastest and the 850mm suspension settling the slowest. Also the settling velocity was a function

of height such that as the settling velocity decreased the height of the suspension increased. Thus

it can be concluded that greater volumes of suspension have lower settling velocities and hence

take a longer time to settle.

The plot of suspension interface height vs time for the initial suspension height of 750mm,

Figure 3, was plotted to find the limiting concentration, CL and settling velocity, vL at the critical

point. This was found by drawing tangents from the initial and final linear parts of the

suspension height curve. A bisector was then drawn at the point where the tangents intersected to

touch the suspension height curve at the critical point which corresponded to a suspension

interface height Hi. The tangent at the critical point was extended to cut the y-axis at the point

H i¿

. An empirical relationship between Hi , H i

¿

and Co and CL was used to calculate the value of

CL which was calculated to be 0.0469 or approximately 4.69%. The settling velocity at the

critical point was obtained by calculating the gradient of the tangent to the critical point. The

corresponding value of vL at this point was -0.0002239ms-1. This value of CL is important for

continuous sedimentation processes and it usually lies between Co and Cmax which this is the

minimum and maximum concentrations of the suspension respectively.

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Precautions that were taken during the experiment:

1. The chalk and water suspension was gently agitated for approximately 2minutes by holding

the vessel at either end (stopper at the top to the base of the vessel) and turning it through a

45° angle. This was to ensure that the suspension was thoroughly mixed.

2. Immediately after the suspension was agitated the suspension was quickly replaced in the

sedimentation apparatus and timing was started.

3. The coloured chalk was used as the sediment and the back light on the apparatus made it

easier to detect the height of the interface.

4. One person measured the time for the duration of the experiment and another person agitated

the suspension and measured the height of the interface so as to reduce errors due to human

reaction.

5. Shaking of the table where the sedimentation apparatus was placed on was avoided.

Possible errors which occurred:

1. Errors occurred when measuring the height of the interface since there were suspended

particles in this zone which made it difficult to actual determine the boundaries of the

interface.

2. Settling started as soon as the agitation was stopped. Thus there was a time delay when the

vessel had to be fitted onto the apparatus.

3. For the 850mm, the time delay was greater since this vessel was the heaviest and took a

longer time to replace in the apparatus.

4. The degree of agitation was different for each of the suspensions since it was manually done.

Safety Consideration to be made when conducting this experiment:

1. The apparatus had grooves which the vessels fit into to hold the vessels securely while the

rate of sedimentation was observed.

2. The equipment had a back light which made it easier to see the zones of settling.

3. Care was exercised when agitating the mixture since the vessel was made of glass. Also the

vessel was held securely at the top (placing hand over the stopper) and bottom during the

agitation.

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4. Personal protective equipment such as safety glasses, laboratory coats and closed footwear

were worn which were adequate for the experiment. Gloves were not needed for handling

the suspension.

CONCLUSION & RECOMMENDATIONS

Conclusion:

The following conclusions can be drawn from the analysis of the experiment:

1. The effect of the initial height of the suspension with a constant mass concentration of 4%

on the sedimentation characteristics was such that for each height the settling characteristics

varied by a constant factor of approximately 1.2. The suspension interface height varied with

the time linearly for the first an last periods but non-linearly during the middle period. This

trend was exhibited for the four suspensions used.

2. The results for the 850mm suspension were not as expected because settling which not

measured started before the vessel was replaced onto the apparatus.

3. The settling velocity varied in a non-linear decreasing manner with the height of the

interface where it was seen that the settling velocity decreased as the height of the interface

increased. Also, settling occurred the fastest for the 550mm, 650mm, 750mm and 850mm

suspensions respectively.

4. The limiting concentration at which the mass rate of the solids is a minimum (C L) at an

initial height of 750mm was found to be 4.69% and the settling velocity (vL) at this height

was found to be -0.0002239ms-1.

5. The final settling heights for the 550mm, 650mm, 750mm and 850mm were 86mm, 102mm,

115mm and 128mm respectively.

Recommendations:

1. The apparatus should be equipped with an electric stirrer to eliminate the time delay when

replacing the cylinders onto the apparatus.

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2. The time taken to measure the interface should be longer (about 1 minute) so that there

would be fewer particles in the interface making it easier to measure.

3. The suspension should be gently agitated so that the final consolidation of the sediment

would be more tightly packed.

4. The results should be trended while the experiment is being carried out so that any major

errors which occurred can be reduced by repeating the experiment as in the case of the

850mm suspension.

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REFERENCES

The University of the West Indies, Laboratory Manual, CHNG2009/2010 – Chemical

Engineering Laboratory, 2008 – 2009 Department of Chemical Engineering

J.M. Coulson & J.F. Richardson. Chemical Engineering Volume 2 (5 th Edition) Chapter 5 .

Particle Technology & Separation Processes. J.R. Backhurst, J.H. Harker

Perry’s Chemical Engineers Handbook. Chapter 18 “Liquid Solid Operation & Equipment”

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APPENDIX A

1. Raw results collected from the experiment

2. Diagram of Apparatus used in the experiment

1