17CF022387 Usoro Sedimentation PDF

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iCOVENANT UNIVERSITY, OTA.COLLEGE OF ENGINEERINGDEPARTMENT OF CHEMICAL ENGINEERINGEXPERIMENT 3SEDIMENTATIONBYUSORO UBONG EMMANUEL17CFSUBMITTED TOTHE DEPARTMENT OF CHEMICAL ENGINEERINGIN PARTIAL FULFILMENT OF REQUIREMENTSFOR CHEMICAL ENGINEERING LABORATORY II(CHE 322)AUGUST 4, 2020.TITLE PAGEiiABSTRA...

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COVENANT UNIVERSITY, OTA. COLLEGE OF ENGINEERING DEPARTMENT OF CHEMICAL ENGINEERING EXPERIMENT 3 SEDIMENTATION BY

USORO UBONG EMMANUEL 17CF022387 SUBMITTED TO THE DEPARTMENT OF CHEMICAL ENGINEERING

IN PARTIAL FULFILMENT OF REQUIREMENTS FOR CHEMICAL ENGINEERING LABORATORY II (CHE 322) AUGUST 4, 2020. TITLE PAGE

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ABSTRACT This experiment was performed to determine the effect of initial mass concentration on the settling characteristics of sand slurries. To that end, four samples of sand slurries (200g, 250g, 300g and 350g) were each mixed with 1 litre of water and subjected to batch sedimentation, wherein interface heights of the supernatant and the fluid with solids were recorded and plotted against time. Graphical demonstration was used in the experiment in order to analyze the relationship between the height of the interface and time. 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. That is, height of the interface varied inversely with time. Furthermore, graphs were plotted in order determine the rate constant b, critical sedimentation point HC and settling rate of each of the slurries. It was observed that mass concentration indeed has an effect on settling characteristics; in particular, the mass concentration varies inversely with settling rate. That is to say that at lower concentration, sand will allow faster initial settling rates, with the 200g sample having the fastest settling rate of 0.85mm/s.

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TABLE OF CONTENTS

TITLE PAGE ..................................................................................................................... i ABSTRACT ................................................................................................................... ii TABLE OF CONTENTS .............................................................................................. iii CHAPTER ONE: INTRODUCTION ............................................................................ 1 1.1 Definition and Scope ............................................................................................. 1 1.2 Objectives .............................................................................................................. 1 1.3 Significance ........................................................................................................... 1 1.4 Limitations ............................................................................................................ 1 CHAPTER TWO: THEORY ......................................................................................... 2 2.1 Batch Settling Process ........................................................................................... 2 2.2 Batch Settling Curve ............................................................................................. 2 2.3 Method .................................................................................................................. 3 CHAPTER THREE: METHODOLOGY ....................................................................... 4 3.1 Materials and Apparatus ....................................................................................... 4 3.2 Procedure ............................................................................................................... 4 CHAPTER FOUR: RESULTS ....................................................................................... 5 CHAPTER FIVE: DISCUSSION OF RESULTS .......................................................... 6 5.1 Plots of Height of Interface vs. Time for the Sand Samples ................................. 6 5.2 Plot of ln(H-H∞) vs. Time for the Sand Samples .................................................. 8 5.1 Possible errors which occurred ...........................................................................10 CHAPTER SIX: CONCLUSION ................................................................................. 11 REFERENCES ............................................................................................................. 12 APPENDIX .................................................................................................................. 13

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CHAPTER ONE: INTRODUCTION 1.1 Definition and Scope Sedimentation is the physical process of allowing particles in suspension in water to settle out of the suspension and become sediment, coming to rest against a barrier (Gregory and Edward, 2010). This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism. In other words, Sedimentation is the separation of a dilute slurry or suspension by gravity settling into a clear fluid and slurry of higher solids content (Geankoplis , 2012). The world today is one where materials need to be extracted from time to time in order that they become stand-alone. More often than not, process industries would always be saddled with the task of solid-liquid separation. It is therefore essential to understand the Batch sedimentation phenomenon as it is a widely used method in solid-liquid separation This laboratory report discusses an experiment on batch sedimentation performed on given weights of a sand samples. This work aims to investigate this sedimentation process by determining the effect of initial concentration and to plot the relationship of it with the settling rate (settling time).

1.2 Objectives The objectives of the sedimentation experiment are as follows: 1. 2. 3. 4. 5.

To study the effect of initial concentration on sedimentation rates To Construct settling rate curves from a single batch test To determine the rate of sedimentation of a sample of particles in a liquid To study the effects of particle weights on the rate of sedimentation The experiment aimed to observe the relationship of settling time with slurry concentration.

1.3 Significance Sedimentation has a wide range of industrial applications. It is used in the measurement of the viscosity of high viscous fluids and in the treatment (clarification) of water and wastewater. It is also used in creating suspended material (floc) used in coagulation or, in other treatment processes, such as lime softening. It is therefore essential to understand the principles of sedimentation and how it is affected by concentration and particle size.

1.4 Limitations The sedimentation rate of particles in fluids are affected by some factors, such as: the ratio of the diameter of the vessel to the diameter of the particle, concentration of the particles, turbulence, position of the vessel, nature of the fluids, particle shapes, sizes and weights. This large number of affecting factors means that the results and experiment as a whole is vulnerable to several errors that may arise from any one of these factors. Therefore, great caution must be taken to ensure accurate results.

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CHAPTER TWO: THEORY Sedimentation is the separation of a suspension into a supernatant clear fluid and rather dense slurry containing a higher concentration of solid (Brown, 1950). The distinction between these two zones can be observed in this experiment by performing a batch settling process where the suspension is allowed to settle into a clear liquid zone and sediment.

2.1 Batch Settling Process Sedimentation is the process where the water has little or no movement and the suspended solids settle to the bottom under the force of gravity and form sediments. As sedimentation continuous, zones are formed (Harker et al, 2013).

Figure 2.1. Sedimentation of concentrated suspensions (a) Type 1 settling (b) Type 2 settling

In Figure 2.1a, after an initial brief acceleration period, the interface between the clear liquid and the suspension moves downwards at a constant rate which forms a layer of sediment at the bottom of the container. When this interface approaches the layer of sediment, its rate of fall decreases until the “critical settling point” is reached when a direct interface is formed between the sediment and the clear liquid. Further sedimentation results solely from a consolidation of the sediment, with liquid being forced upwards around the solids which are then forming a loose bed with the particles in contact with one another. As the flow area gradually reduces, the rate progressively diminishes. In Figure 2.1a, a stage in the sedimentation process is illustrated. A is clear liquid, B is suspension of the original concentration, C is a layer through which the concentration gradually increases, and D is sediment. The sedimentation rate remains constant until the upper interface corresponds with the top of zone C and it then falls until the critical settling point is reached when both zones B and C will have disappeared. A second and rather less common mode of sedimentation as shown in Figure 2.1b is obtained when the range of particle size is very great. The sedimentation rate progressively decreases throughout the whole operation because there is no zone of constant composition, and zone C extends from the top interface to the layer of sediment

2.2 Batch Settling Curve As the sedimentation proceeds, the accumulation of solids at the bottom causes flocs to compress, and liquid is expelled to the upper zones. When the weight of the solid is equal to the compressive strength of the flocs, the settling process ends. Figure 2.2 shows a typical batch settling curve of a slurry.

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Figure 2.2 Batch Settling Curve (McCabe et. al., 1993)

Figure 2.2 shows that during the early phases of settling, the velocity (slope of the height vs. time plot) is fairly constant. When Zone B disappears, the rate of settling starts to decrease until the final height is reached. Settling rates greatly depend on feed concentration, and in the latter stages, on the initial height, Zo. In general, the higher the initial concentration, the smaller the rate of settling. This is because the upward velocity of the displaced fluid is great, and there would be steeper velocity gradients in the fluid.

2.3 Method In this experiment, the slurries are made up of water and sand. In the analysis of sedimentation data, it is useful to create settling curves. With these plots, the effects of slurry concentration on the settling behaviour of sand can be determined; •

The interface height is calculated using the formula: 𝐼𝑛𝑡𝑒𝑟𝑝ℎ𝑎𝑠𝑒 ℎ𝑒𝑖𝑔ℎ𝑡 (𝑐𝑚) =

•

𝜋𝐷2 4

The settling velocity is then calculated as the difference of the interface height over the time elapsed: 𝑆𝑒𝑡𝑡𝑙𝑖𝑛𝑔 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 =

•

𝐼𝑛𝑡𝑒𝑟𝑝ℎ𝑎𝑠𝑒 ℎ𝑒𝑖𝑔ℎ𝑡 (𝑚𝐿 )

(𝐼𝑛𝑡𝑒𝑟𝑝ℎ𝑎𝑠𝑒 ℎ𝑒𝑖𝑔ℎ𝑡1−𝐼𝑛𝑡𝑒𝑟𝑝ℎ𝑎𝑠𝑒 ℎ𝑒𝑖𝑔ℎ𝑡2) 𝑇𝑖𝑚𝑒1−𝑇𝑖𝑚𝑒2

The sedimentation curve (for each concentration and height variation) is then plotted as the graph of the interface height vs. time.

The rate of sedimentation during this period is given approximately by: −

dH dT

= b(H − H∞)

( 2.3.1)

The time taken for the sludge line to fall from a height Hc, corresponding to the critical settling point, to a height H is given by: −bt = ln(H − H∞ ) − ln(HC − H∞)

( 2.3.2)

*H=height of the sludge line at time t, H∞ = the final height of the sediment, b = constant for a given suspension, Hc=Critical settling point. Thus, if ln(H −H∞) is plotted against t, a straight line of slope −b is obtained. The values of H∞ are determined largely by the surface film of liquid adhering to the particles.

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CHAPTER THREE: METHODOLOGY 3.1 Materials and Apparatus 4 Graduated Cylinders (1000 mL) Stopwatch, Digital Balance Stirring rod Mortar and pestle Spatula Beaker • Sand and water

• • • • • • •

3.2 Procedure 1. Prepare four of one litre (1000mL) graduated measuring cylinders. 2. The sand was pounded into fine particles and Weighed (100g, 150, 200g and 250g) using the weighing scale. 3. The samples were placed in beakers and with 1000mL of tap water. The suspensions were then mixed vigorously to ensure uniform distribution of particles. 4. The suspensions were transferred to the prepared graduated cylinder with careful agitation. 5. Immediately after the suspension was agitated, timing was started. 6. In each case, the rate of sedimentation was studied by measuring the changes in height of the various solid/liquid interfaces with respect to time until there was no more significant changes in the data recorded for the height of the clear zone. 7. The graph of the interface height versus time was plotted and results discussed in Chapter Five.

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CHAPTER FOUR: RESULTS Table 4.1: Results for 200g, 250g, 300g and 350g Sand Samples H - H∞ ln(H - H∞) -dH/dT= b(H- H∞) Height H (mm) t (secs) 200g 250g 300g 350g 200g 250g 300g 350g 200g 250g 300g 350g 200g 250g 300g 350g 0 340 345 335 341 303 291 247 256 5.7137 5.6733 5.5094 5.5452 0.85 0.76 0.67 0.77 60 327 331 318 322 290 277 230 237 5.6699 5.6240 5.4381 5.4681 0.81 0.72 0.62 0.71 120 310 315 298 304 273 261 210 219 5.6095 5.5645 5.3471 5.3891 0.76 0.68 0.57 0.66 180 291 295 279 287 254 241 191 202 5.5373 5.4848 5.2523 5.3083 0.71 0.63 0.52 0.61 240 275 277 263 263 238 223 175 178 5.4723 5.4072 5.1648 5.1818 0.67 0.58 0.47 0.53 300 255 260 245 245 218 206 157 160 5.3845 5.3279 5.0562 5.0752 0.61 0.54 0.42 0.48 360 234 240 233 222 197 186 145 137 5.2832 5.2257 4.9767 4.9200 0.55 0.48 0.39 0.41 420 214 222 218 206 177 168 130 121 5.1761 5.1240 4.8675 4.7958 0.5 0.44 0.35 0.36 480 191 205 199 183 154 151 111 98 5.0370 5.0173 4.7095 4.5850 0.43 0.39 0.3 0.29 540 175 190 181 162 138 136 93 77 4.9273 4.9127 4.5326 4.3438 0.39 0.35 0.25 0.23 600 158 175 172 143 121 121 84 58 4.7958 4.7958 4.4308 4.0604 0.34 0.31 0.23 0.17 660 138 155 157 127 101 101 69 42 4.6151 4.6151 4.2341 3.7377 0.28 0.26 0.19 0.13 720 115 135 130 107 78 81 42 22 4.3567 4.3944 3.7377 3.0910 0.22 0.21 0.11 0.07 780 102 126 105 85 65 72 17 0 4.1744 4.2767 2.8332 0.18 0.19 0.05 0 840 88 108 88 85 51 54 0 0 3.9318 3.9890 0.14 0.14 0 0 900 66 90 88 85 29 36 0 0 3.3673 3.5835 0.08 0.09 0 0 960 63 88 88 85 26 34 0 0 3.2581 3.5264 0.07 0.09 0 0 1020 62 85 88 85 25 31 0 0 3.2189 3.4340 0.07 0.08 0 0 1080 59 83 88 85 22 29 0 0 3.0910 3.3673 0.06 0.08 0 0 1140 56 81 88 85 19 27 0 0 2.9444 3.2958 0.05 0.07 0 0 1200 53 78 88 85 16 24 0 0 2.7726 3.1781 0.04 0.06 0 0 1320 50 74 88 85 13 20 0 0 2.5649 2.9957 0.04 0.05 0 0 1380 49 71 88 85 12 17 0 0 2.4849 2.8332 0.03 0.04 0 0 1440 47 69 88 85 10 15 0 0 2.3026 2.7081 0.03 0.04 0 0 1500 46 67 88 85 9 13 0 0 2.1972 2.5649 0.03 0.03 0 0 1560 45 65 88 85 8 11 0 0 2.0794 2.3979 0.02 0.03 0 0 1620 44 63 88 85 7 9 0 0 1.9459 2.1972 0.02 0.02 0 0 1680 43 61 88 85 6 7 0 0 1.7918 1.9459 0.02 0.02 0 0 1740 41 59 88 85 4 5 0 0 1.3863 1.6094 0.01 0.01 0 0 1800 40 57 88 85 3 3 0 0 1.0986 1.0986 0.01 0.01 0 0 1860 39 56 88 85 2 2 0 0 0.6931 0.6931 0.01 0.01 0 0 1920 38 55 88 85 1 1 0 0 0.0000 0.0000 0 0 0 0 1980 37 54 88 85 0 0 0 00 0 0 0 2040 37 54 88 85 0 0 0 00 0 0 0 2100 37 54 88 85 0 0 0 00 0 0 0

From Table 4.1 above, plots of ln(H − H∞ ) against time t were obtained for the different sand samples. The equations of the curves obtained are listed respectively as follows; a) b) c) d)

y = -0.0028x + 6.1645 y = -0.0026x + 6.1238 y = -0.0027x + 5.7718 y = -0.0030x + 5.8207

Thus, if these equations are compared with equation (2.3.2) in Chapter Two: Theory, the rate constant for a suspension b as well as the critical sedimentation point HC can be obtained; Table 4.2: Sand Samples and their respective H c and b values

Hc b Mass of Sample(g) 200 512.563 250 510.596 300 409.115 350 422.208

0.0028 0.0026 0.0027 0.003

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CHAPTER FIVE: DISCUSSION OF RESULTS 5.1 Plots of Height of Interface vs. Time for the Sand Samples The plots obtained all show a similar trend. The graphs are initially linear, indicating a constant and high settling rate. The graph then begins to curve; indicating that the settling rate has began to decrease due to the solid’s resistance to flow of the liquid as interface height decreases.

H (mm)

Plot of Height (mm) vs. Time (secs) for 200g 400 350 300 250 200 150 100 50 0 0

500

1000

1500

2000

2500

t (s) Figure 5.1.1: Plot of Height (mm) vs. Time (secs) for 200g

Figure 5..1.1 above shows a downward sloping graph indicating interface height varies inversely with time. The initial linear section indicates a constant and high settling rate, the curved section shows that the rate has begun to decrease and the final linear section indicates the compression zone and thus final height

Plot of Height (mm) vs. Time (secs) for 250g H (mm)

400 300 200 100 0 0

500

1000

1500

2000

2500

t (s) Figure 5.1.2: Plot of Height (mm) vs. Time (secs) for 25 0g

Figure 5.1.2 above shows a downward sloping graph indicating interface height varies inversely with time. The initial linear section indicates a constant and high settling rate, the curved section shows that the rate has begun to decrease and the final linear section indicates the compression zone and thus final height . 6

Plot of Height (mm) vs. Time (secs) for 300g

H (mm)

400 350 300 250 200 150 100 50 0 0

500

1000

1500

2000

2500

t (s) Figure 5.1.3: Plot of Height (mm) vs. Time (secs) for 3 00g

Figure 5.1.3 above shows a downward sloping graph indicating interface height varies inversely with time. The initial linear section indicates a constant and high settling rate, the curved section shows that the rate has begun to decrease and the final linear section indicates the compression zone and thus final height

Plot of Height (mm) vs. Time (secs) for 350g 400

H (mm)

300 200 100 0 0

500

1000

1500

2000

2500

t (s) Figure 5.1.4: Plot of Height (mm) vs. Time (secs) for 35 0g

Figure 5.1.4 above shows a downward sloping graph indicating interface height varies inversely with time. The initial linear section indicates a constant and high settling rate, the curved section shows that the rate has begun to decrease and the final linear section indicates the compression zone and thus final height.

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5.2 Plot of ln(H-H∞) vs. Time for the Sand Samples

Plot of ln(H-H∞) vs. Time (secs) for 200g 7.0000

ln(H - H∞)

6.0000 5.0000

y = -0.0028x + 6.1645

4.0000 3.0000 2.0000 1.0000 0.0000 0

500

1000

1500

2000

2500

t (s) Figure 5.2.1: Plot of ln(H-H∞) vs. Time (secs) for 200g

Observation of figure 5.5 above shows that ln(H −H∞) varies inversely with time t. The curve continues to slope downwards indicating that the interface height H reduces further until it is equal to the final height of sediment, H∞. The point at which H= H∞ is represented by the end of the curve and this is the point where sediment has fully settled. Furthermore, the equation of the curve, y = -0.0028x + 6.1645, can be compared with equation (2.3.2). From this it was determined that the settling rate constant, b = 0.0028 and the critical sedimentation point Hc =512.563mm.

Plot of ln(H-H∞) vs. Time (secs) for 250g

ln(H - H∞)

7.0000

6.0000 5.0000

y = -0.0026x + 6.1238

4.0000 3.0000 2.0000 1.0000 0.0000 0

500

1000

1500

2000

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t (s) Fi...