Experiment 2 Solid-Liquid Separation: Batch Sedimentation
I.
Introduction
Solid-liquid separation involves the separation of two phases namely, solid and liquid, from a suspension. An example of solid-liquid separation process is sedimentation. The process of sedimentation involves the separation of a dilute slurry or a suspension by gravity settling into a clear fluid and a slurry of higher solids concentration. The materials to be settled are usually solid particles such as silts or clay present in water systems (Geankoplis, 2003). Sedimentation can be accomplished by decreasing the medium fluid velocity to almost zero so the particles would no longer remain in suspension due to fluid dynamics, thereby promoting settling (Soediono, 1989). During batch sedimentation, suspension of particles are allowed to stand in a settling tank or column (Latsa et al., 2005). Figure 1 below illustrates the basic stages in the sedimentation process.
' Figure 1. Progress in Batch Sedimentation (left) and settling zones(right) (McCabe et al., 1993) At time equal to zero, the total height of the suspension is labeled ZO where the concentration of the suspended particles are the same at any point. As time progresses, there is a formation of zones. Zone A is called the clear layer where particle concentration is almost zero, Zone B is called the sedimentation zone and the concentration is uniform and equal to the original suspension, the interface at which zone A and B coincides is called the sedimentation interface. The interface height is the measurement from the bottom of the settling chamber to the sedimentation interface. The settled solids are located in Zone D (sediment layer) and above Zone D is called the transition layer or a densely-packed Zone C (McCabe et al., 1993). As the sedimentation process continues, the depths of zones A and D increase (Figures 1b to e) while the depth of zone B decreases. The depth of zone C almost remains constant (Figures 1b to c). Eventually, zone B disappears and the solids are deposited at zone D and C (Figures 1d to e) (McCabe et al., 1993). The settling of particles is generally affected by the container size and its interaction with other particles. If its distance from the wall is sufficient enough or if the ratio of the particle diameter to the diameter of the container is less than 1:200 or if the particle concentration is less than 0.2 volume percent, the interference will be less than 1%. In this case 1
the particle is said to be free settling, otherwise the term hindered settling is applied since the particles become too crowded and settling rate proceeds at a lower rate (Geankoplis, 2003). The reasons for modification of the settling rate of particles in a concentrated suspension include: (a) when a wide range of particle sizes are present in the feed, differential settling rates between large and small particles lead to modification of the effective density of the suspension, (b) the upward velocity of the fluid is greater at higher concentrations, (c) the velocity gradients in the fluid surrounding the particles are greater due to the closer proximity of the particles and (d) the ability of particles to aggregate is enhanced at higher concentrations (Latsa et al., 2005). Other less apparent factors affect the sedimentation rate aside from the particle size, density and concentration, and fluid viscosity. These include particle shape and orientation, convection currents in the surrounding fluid, and chemical pretreatment of the feed suspension (Foust et al., 2008). Sedimentation of a suspension is generally assessed by a jar test, usually using a graduated cylinder, during which a suspension is allowed to settle and the height of the clear liquid suspension interface is measured as a function of the settling time. The expected raw data to be gathered are height of interface zones in terms of volume markings and the corresponding time. The cross-sectional area of the graduated cylinder will be determined by measuring its inside diameter. To obtain the actual interface height, the recorded volumes are divided by the crosssectional area of the cylinder. Figure 2 below shows how the interface height can be determined in a jar or cylinder test.
Figure 2. Zones of settling after a given time and determination of the interface height (Tarleton & Wakeman, 2007)
The recorded volume markings are divided by the cross-sectional area of the cylinder to obtain the actual interface height (z) (Tarleton & Wakeman, 2007): 𝑣𝑜𝑣𝑙𝑢𝑚𝑒𝑙𝑒𝑣𝑒𝑙
z = 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟𝑎𝑟𝑒𝑎(1) 2
The interface height (z) is plotted against time in Figure 2. It can be observed that the slope of the plot, which is the velocity of settling, is constant before reaching the critical point C. With the limitations to the number of data which can be gathered, the velocity can be estimated by finding the slopes between adjacent points using equation 1 (Geankoplis, 2003).
Figure 3. Height of Interface vs Time curve (Foust, 1980) The settling rate or settling velocity can be obtained from the plot shown above. The instantaneous settling velocity is the slope of the line tangent to the curve -dz/dt = v or given by the equation:
(2) where v is the settling rate and z is the interface heights with respect to time interval t. One of the simplest methods to determine the critical point of a settling curve is to draw two tangent lines, one in the free settling portion, the other at the final compression at the near end of settling in which interface height becomes almost constant as seen in Figure 2. In the same figure, an angle bisector of the two tangent lines, which vertex is the intersection point, is drawn. The intersection of the angle bisector and the settling curve is an estimate of the critical point (Foust et al., 1980). Solid-liquid separation is applied in industrial processes which include recovery and processing of solids or purification of liquids (Svarovsky, 2000). Sedimentation is often utilized in the food industry such as “separating dirt and debris from the raw material, crystals from their mother liquor and dust or product particles from air streams” (Earle, 2004). It is also one of the most widely used unit operations in wastewater treatment systems in removing grit and particulate matter, sludge from the bioreactor, chemical flocs in a chemical process (Carlsson, 1998). This experiment highlighted the effects of varying slurry concentration to the sedimentation characteristics of flour and the construction of the settling curve of the flour slurry.
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II.
Objectives
This experiment aimed to determine the effect of varying slurry concentration to the sedimentation process. Specifically, it aimed to: a. Determine the relationship between time and interface height at different slurry concentrations; and b. Determine the relationship between time and settling rate at different the slurry concentrations.
III.
Scope and Limitations
The experiment was focused on determining the effect of varying solids concentration to the settling rate of the flour particles. Four varying flour concentrations namely 20 g/L, 40 g/L and 60 g/L were used. The effect of varying fluid height, temperature, and varying particle sizes were beyond the scope of the experiment and were assumed constant all throughout. The experiment was only applicable to batch still fluid processes and the effects of fluid dynamics on the settling of the particle was not considered. Gravity was the sole force acting on the particles. These limitations implied that any irrelevant results to the experiment were accounted to those factors that weren't considered. Moreover, the investigation of the effect of different flour concentrations implied that only the rate at which the solids settled were expected in the results. The experiment was conducted at the School of Technology’s Unit Operations Laboratory at standard temperature and pressure. The experiment approximately lasted for 16 hours until all the solids have completely settled. Necessary data were gathered after the experiment which were used to plot the relationship between time and settling rate at varying slurry concentrations as well as between time and interface height at varying slurry concentrations IV.
Methodology A. Materials Vernier caliper 1000-mL graduated cylinder Stirring rod Baking soda sample Distilled water Aluminum foil
Ruler Stopwatch 500 ml Beaker Analytical balance PPE (e.g. mask, gloves)
A. Methods The Material Safety Data Sheet of flour was reviewed prior to the experiment proper and proper personal protective equipment attire was observed. Furthermore, 4
laboratory materials such as 1000-mL graduated cylinders and 500-mL beakers were borrowed from the lab technician. The apparatuses were further clean and freed from contaminants. The inner diameter of the 1000-mL graduated cylinder was measured using the Vernier caliper. Simultaneously, 20 g, 40 g, and 60 g of flour were weighed on a 500-mL beaker. A significant amount of distilled water was poured into the 500-mL beaker and was slowly mixed with the previously weighed 20 g of flour using stirring rod. Subsequently, the mixed solution was poured into the 1-L graduated cylinder and was diluted into the 1000 mL mark. The contents were then stirred again using stirring rod for 30 seconds to ensure uniform particle concentration distribution. After stirring, the height of the mixture was recorded. The mixture was allowed to settle and the volume reading of the interface height was then noted for every 2 minutes interval. The particles were found to fully settled the next day of the experiment. The same steps were repeated for the two other concentrations namely 40 g/L and 60 g/L. After the experiment, the workplace was clean as well as the laboratory apparatus and were returned to the lab technician. The gathered data were then subjected for further analysis.
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V.
Results and Discussion
Table 1. Calculated Height of Interface and Settling Rate Time Interval Height of Interface (cm) (mins) 20 g/L 40 g/L 60 g/L 36.45 35.90 36.45 0 36.38 35.55 36.27 2 36.31 35.19 36.27 4 35.72 35.01 36.09 6 35.72 35.01 36.09 8 35.54 34.83 36.09 10 35.36 34.65 36.09 12 35.36 34.65 36.09 14 35.36 34.65 36.09 16 35.18 34.65 36.09 18 34.99 34.65 36.09 20 34.63 34.65 36.09 22 34.27 34.65 36.09 24 34.27 34.65 36.09 26 34.27 34.47 36.09 28 34.27 34.47 36.09 30 34.27 34.47 35.91 40 34.27 34.29 35.72 50 34.08 34.11 35.72 60 34.08 33.93 35.54 70 34.08 33.93 35.54 80 33.90 33.93 35.36 90 33.90 33.75 35.36 100 33.90 33.75 35.18 110 33.35 33.75 35.18 120 2.19 3.41 4.92 1170
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Settling rate (cm/min) 20 g/L 40 g/L 60 g/L 0.0000 0.0000 0.0000 0.03645 0.00036 0.00018 0.01823 0.00036 0.00000 0.09721 0.00018 0.00019 0.00000 0.00000 0.00000 0.01823 0.00018 0.00000 0.01519 0.00019 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.01013 0.00000 0.00000 0.00911 0.00000 0.00000 0.01657 0.00000 0.00000 0.01519 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00019 0.00000 0.00000 0.00000 0.00000 0.00000 0.00000 0.00019 0.00000 0.00019 0.00019 0.00304 0.00019 0.00000 0.00000 0.00019 0.00019 0.00000 0.00000 0.00000 0.00203 0.00000 0.00019 0.00000 0.00019 0.00000 0.00000 0.00000 0.00019 0.00456 0.00000 0.00000 0.02664 0.50565 0.31849
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36.5
interface heihgt (cm)
36 35.5
20g/ml
y = -0.0095x + 36.272 R² = 0.959
35
40g/ml 60g/ml
y = -0.0133x + 35.051 R² = 0.7862
34.5
Linear (20g/ml)
34
Linear (40g/ml) 33.5
Linear (60g/ml)
y = -0.0201x + 35.567 R² = 0.6836
33 32.5 0
20
40
60
80
100
120
140
time (min)
Figure 2. Plot of Interface Height vs. time for the three flour concentrations
The interface height was determined by dividing the measured volume from the graduated cylinder by the area of the cylinder. Figure 1 shows the relative slopes of the three concentrations and it can be noted that the 20g/ml flour concentration has the largest slope (-0.0201) for the observation time of 120 minutes. This observation is supported by the fact that lower concentrations of suspended solid, the farther it is from hindered settling. As time progressed, the difference in interface height readings between successive time intervals slowly decreased due to the resistance caused by other solid particles coming in contact with the settling particle. The plot shown in Figure 1 did not account for the total time it took for the solids to fully settle. Figure 2 includes this data. It is noted that no data were obtained from 120 minutes to the time it took to fully settle, therefore, no inference regarding the critical point of the settling point can be acknowledged. The critical point is supposedly the point in time where the settling rate decreases appreciably. However, the relative settling rate for each concentration can be illustrated. Shown in Figure 3, it is observed that the settling rates provided a fluctuating value. This irregularity can be accounted by the experimental errors that occurred during measurement for the interface line is not inherently apparent. As time progressed, the settling rate can be observed to decrease. This phenomenon was due to the crowding of solid particles causing resistance to their downward motion. Another perspective can be taken that as solids accumulate in region C, it becomes more difficult for the solids to push out the water below the interface height since the path in which water can pass through becomes narrower.
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interface heihgt (cm)
40 35 30
20g/ml
25
40g/ml
y (60 g/ml) = -0.0269x + 36.921 R² = 0.9892 y (40g/ml) = -0.0272x + 35.567 15 R² = 0.9917 y (20 g/ml) = -0.0286x + 35.885 10 R² = 0.9921 5
20
60g/ml Linear (20g/ml) Linear (40g/ml) Linear (60g/ml)
0 0
500
1000
1500
time (min)
Figure 3. Plot of Interface Height vs. time (fully settled)
0.35
settling rate (cm/min)
0.3 0.25 0.2
20g/ml 40g/ml 60g/ml
0.15 0.1 0.05 0 -0.05
0
20
40
60
80
100
120
140
time (min) Figure 4. Settling Rate vs. Time
VI.
Conclusion
The concentration of the solid greatly affects the settling of solids particles in suspension in water or any fluid. The interface height decreases slower in higher solids concentration (40 and 60 g/ml) than for the lower solids concentration (20 g/ml). The settling rate of solid particles decreases as time progresses and as the interface height decreases due to the crowding of solid particles, thus, causing hindered settling. 8
VII.
Recommendation
Better relationship between settling rate and time ca be observed if data was gathered at constant time intervals from first settling to full settling in order to establish a reliable curve that closely resembles that of the theoretical settling curve. Furthermore, the choice of sample should be carefully chosen so as to establish an apparent interface height for ease of measurement reading
VIII. Appendices A. Raw Data Time Interval (mins) 0
Volume Readings (mL) 20 g/L 40 g/L 60 g/L 1000 1000 1000
2
998
990
995
4
996
980
995
6
980
975
990
8
980
975
990
10
975
970
990
12
970
965
990
14
970
965
990
16
970
965
990
18
965
965
990
20
960
965
990
22
950
965
990
24
940
965
990
26
940
965
990
28
940
960
990
30
940
960
990
40
940
960
985
50
940
955
980
60
935
950
980
70
935
945
975
80
935
945
975
9
90
930
945
970
100
930
940
970
110
930
940
965
120
915
940
965
1170
60
95
135
B. Sample Calculation All data calculated are based on 20 g/L concentration For the area,
𝜋 2 𝐷 ; 𝐷 = 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 4 𝜋 = (5.88 𝑐𝑚)2 4
𝐴𝑟𝑒𝑎 =
𝐴𝑟𝑒𝑎 = 27.43 𝑐𝑚2 For the interface height, 𝐼𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒 ℎ𝑒𝑖𝑔ℎ𝑡 =
𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑐𝑦𝑙𝑖𝑛𝑑𝑒𝑟 𝐴𝑟𝑒𝑎 =
1000 𝑐𝑚3 27.43𝑐𝑚2
𝐼𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒 ℎ𝑒𝑖𝑔ℎ𝑡 = 36.45 𝑐𝑚 For the settling rate, 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦 = =
ℎ𝑒𝑖𝑔ℎ𝑡 1 − ℎ𝑒𝑖𝑔ℎ𝑡 2 𝑡𝑖𝑚𝑒 𝑒𝑙𝑎𝑝𝑠𝑒𝑑
36.45 𝑐𝑚 − 36.38 𝑐𝑚 2 𝑚𝑖𝑛 = 0.03645 𝑐𝑚/𝑚𝑖𝑛
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C. Photos
Figure 5. Measuring the inside diameter of cylinder Figure 6. Covering the cylinder with aluminum
Figure 7. Taking the volume reading
Figure 8. 20 g/L, 40 g/L, and 60 g/L samples of flour
XI. References Carlsson, B. (1998). An introduction to sedimentation theory in wastewater. Systems and Control, 1-7. Retrieved February 11, 2017 from http://www.it.uu.se/research/project/jass/material/sett98.pdf Earle, R. L. (2004). Unit Operations in Food Processing. The New Zealand Institute of Food Science & Technology (Inc.). Retrieved February 11, 2017 from http://www.nzifst.org.nz/unitoperations/mechseparation3.htm Foust, A. (1980). Principles of unit operations. John Wiley & Sons (Asia) Pte Ltd. Foust, A. S., Wenzel, L. A., Clump, C. W., Maus, L., & Andersen, L. B. (2008). Principles of Unit Operations, 2nd Edition. John Wiley & Sons, Singapore. 11
Geankoplis, J. C. (2003). Transport processes and separation process principles (4th Ed). New Jersey: Pearson Education, Inc. Latsa, M., Assimacopoulos, D., Stamou, A., & Markatos, N. (2005). Batch Sedimentation. Applied Mathematical Modeling. Elsevier B.V. McCabe, Warren L., Smith, J., & Harriot, P. (1993). Unit operations of chemical engineering. Chemical Engineering Science (5th ed., Vol. 6). New York: McGraw Hill, Inc. http://doi.org/10.1016/0009-2509(57)85034-9 Soediono, B. (1989). Sedimentation. Journal of Chemical Information and Modeling, 53, 160. http://doi.org/10.1017/CBO9781107415324.004 Svarovsky, L. (200). Solid-liquid Separation (4th Ed) . Butterworth-Heinemann. http://dx.doi.org/10.1016/B978-075064568-3/50023 Tarleton, E. S. & Wakeman, R. J. (2007). “Solid/Liquid Separation: Equipment Selection and Process Desig”. Elsevier Ltd.
Team Members: Camarote, Bryle Kristiann C. Romelo, Nimrod B. Valdon, Sarah Jane I. Date Performed: February 13, 2017 Date Submitted: February 27, 2017
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