Mapua Institute of TechnologySchool of Chemical Engineering, Chemistry, Biological

Engineering and Material Science and Engineering

CHEMICAL ENGINEERING LABORATORY 1

EXPERIMENT 10: SETTLING VELOCITY

Bismonte, Ma. Elizabetha, Gara, John Patrick N.a, Surnit, Rebecca T.a, Tabaquero, Renz James T.a

aBachelor of Science in Chemical Engineering, Mapua Institute of Technology, School of Chemistry and Chemical Engineering

ABSTRACT

The objectives of this experiment were to analyze a settling velocity curve of different suspensions from a single batch test, to determine the effect of initial concentration of the suspensions of sedimentation rate, to determine the effect of initial suspension height of the suspensions on sedimentation rate and to determine the effect of

the addition of coagulant in the sedimentation rate. The students were able to graph the data points gathered from the experiment. The students’ analysis of the settling velocity of suspensions with different concentration but with the same amount of solvent is that as time progresses, the settling velocity decreases. Meanwhile, settling velocity of chalk suspensions with same concentrations also yields the same observations to that of those with different concentrations. Therefore, the students concluded that the settling velocity of the three suspensions decreases with time. The initial concentration of suspensions is inversely proportional to

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ARTICLE INFO

Keywords:SedimentationSuspensionCoagulantSettling Velocity

Date PerformedDecember 2, 2014

Date SubmittedDecember 9, 2014

the sedimentation rate since there are many particles in the suspension; the velocity of the particle is slow, taking some time before the particles reach the bottom part of the vessel.

1. INTRODUCTION Sedimentation is the accumulation

of sediments in a specified mixture (Vanoni, 1975). It is a phenomenon where the particles in a suspension settle out of the fluid and rest at the bottom of the container (Julien, 2010).

Several factors affect the sedimentation process of a mixture. The density is the most important factor in this process. The material that is denser will tend to settle at the bottom leaving the less dense component at the top layer (Bustos, 1999). These are also affected by the forces acting on them such as centrifugal force, electromagnetism, and gravity. It is commonly applied to processes which involved purifying of a mixture to separate the solid materials from the liquid mixture. Example of this is in water treatment plants where the treated water is made to be still in order for the solid particles or sludge to resolve at the bottom of the tank.

The process starts with a fast settling of the solid particles creating a layer of clear liquid and a transition layer. In the transition layer, the concentration of the solid particles is less of the original concentration. The settling process will continue to progress until only two layers are seen in the column (Garcia, 2008).

In this experiment, chalk will be dissolved in the water and it will put into the Sedimentation apparatus to observe the settling of the chalk particles at the bottom. It aims to determine the following: to analyze the settling velocity curves of different suspensions, and to determine the effect of initial concentration and initial suspension

height of suspensions on sedimentation rate (Gutierrez et al., 2005).

2. METHODOLOGY

The experiment had four objectives. The first one was to analyze settling velocity curves of different suspensions from a single batch test. The second was to determine the effect of initial concentration of the suspensions on sedimentation rate. Third was to determine the effect of initial suspension and the height of the suspensions on sedimentation rate. The fourth one was to determine the effect of adding coagulant in the sedimentation rate.

The equipment and materials used in this experiment were the sedimentation apparatus, digital balance, 1-L plastic graduated cylinder, stop watch, pail, funnel, powdered chalk, distilled water and stirrer.

Figure 2.1: The Sedimentation Apparatus

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Figure 2.2: Parts of Sedimentation Apparatus

Figure 2.3: Monitoring and Recording of the Height of Interface

To begin the experiment, three types of suspension were prepared. Suspension A (10% chalk) was prepared by mixing 150 grams of powdered chalk and 1350 ml of distilled water. Suspension B (5% chalk) was prepared by mixing 70 grams of powdered chalk and 1350 ml of distilled water. Suspension C (5% chalk) was prepared by mixing 35 grams of powdered chalk with 675 ml of distilled water. Then, the suspensions were poured separately into the glass cylinders of the sedimentation apparatus as the initial height of each was noted and recorded. Next, for every 1 minute, the height of the interface between the clear supernatant liquid and slurry for each suspension was monitored and recorded. This was continuously done until at least five similar values of height were obtained. Lastly, a graph was constructed by plotting the interface height (y-axis) versus settling time (x-axis) for each suspension.

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3. RESULTS AND DISCUSSION

The table below shows the data obtained by observing the interface height of three chalk suspensions:

Table 3.2: Data Gathered

Table 3.3: Data Gathered

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Table 3.1: Data Gathered

TimeInterface Height

A B C

0 71.72 70 34.041 69.34 66 29.85

2 67.95 58 25.91

3 66.8 57 21.9

4 65.02 54.2 18.4

5 62.87 51.3 16.5

6 60.96 48.1 15.24

7 59.93 45.2 14.3

8 56.64 42.1 13.7

9 54.61 39.4 13.4

10 52.32 36.7 12.9

11 49.53 34.6 12.7

12 48.01 32.9 12.3

13 47.24 32.1 12.15

14 46.48 31.1 11.85

15 46.63 30.5 11.7

16 45.87 30 11.5

17 45.47 29.5 11.3

18 45.16 29.3 11.2

19 44.56 28.9 11.1

20 44.25 28.5 10.9

21 44.07 28.3 10.7

22 43.82 28 10.6

23 42.98 27.8 10.45

24 42.9 27.5 10.325 42.52 27.2 10.2

26 42.37 26.9 10.1

27 42.27 26.8 10

TimeInterface Height

A B C28 41.94 26.5 9.929 41.81 26.4 9.830 41.68 26.1 9.731 41.48 26 9.632 41.15 25.8 9.533 41.02 25.6 9.434 40.84 25.5 9.3535 40.64 25.4 9.336 40.39 25.1 9.237 40.18 25 9.138 40.07 24.9 9.0539 39.52 24.8 940 39.44 24.7 8.9541 39.37 24.5 8.942 39.07 24.4 8.8543 39.07 24.3 8.844 38.89 24.1 8.745 38.74 24 8.6546 38.58 23.9 8.647 38.43 23.8 8.5548 38.33 23.5 8.549 38.25 23.4 8.4550 38.07 23.3 8.451 37.95 23.2 8.3552 37.95 23.2 8.353 37.82 23 8.354 37.62 22.9 8.25

TimeInterface Height

A B C55 37.47 22.8 8.256 37.29 22.7 8.257 37.16 22.6 8.1558 37.16 22.5 8.159 36.98 22.4 8.160 36.91 22.35 8.161 36.83 22.3 8.0562 36.75 22.2 863 36.53 22.1 864 36.53 22.05 7.9565 36.35 21.9 7.966 36.35 21.9 7.967 36.27 21.8 7.968 36.2 21.7 7.969 36.04 21.6 7.970 35.86 21.6 7.8571 35.86 21.5 7.872 35.71 21.4 7.873 35.71 21.3 7.874 35.64 21.3 7.875 35.56 21.2 7.7576 35.56 21.1 7.777 35.41 20 7.778 35.26 19 7.779 35.26 19 7.780 35.26 19 7.781 35.26 19 7.782 35.26 19 7.7

Settling Velocity Curves

The settling velocity of three chalk suspensions with different concentrations was observed in the glass cylinders of the sedimentation apparatus. Suspension A and B have the same amount of solvent but with concentrations of 11.11 and 0.512 g/mL respectively while Suspension B and C have the same concentration but with different amount of solvents used.

Settling Velocity of Suspensions with different concentration but with the same amount of solvent.

0 10 20 30 40 50 60 70 80 900

10

20

30

40

50

60

70

80

Different Concentrations

Suspension A Suspension B

Settling Time, min

Inte

rfac

e H

eigh

t, m

Figure 3.1: Comparison of chalk suspensions with different concentrations

In Figure 3.1, the settling rate for both suspension were increasing for 13 seconds but at t = 2 seconds, we observed that the particles in suspension B settled at the bottom of the vessel faster than suspension A. This is because as the concentration is increased, the settling velocity decreases (Mohammed, 2013). There are a lot of factors to consider like gravity, viscous forces,

particle path, size and shape and interactions of the particles to each other and to the wall. Both suspensions have different particle sizes and shapes. Spherical particles follows a vertical path when settling and have increasing settling velocity with increasing diameter due to the uniform viscous forces and impact of water striking the particle while irregular shaped particles follows unstable path and decreases the settling velocity with increasing the volume (Mohammed, 2013). The interactions of the particles with each other and with the wall affects the settling velocity since the particles are bumping each other and the wall which causes the distraction in the path of the moving particle and slows down the settling velocity.

Settling velocity of chalk suspensions with same concentrations

0 10 20 30 40 50 60 70 80 900

2

4

6

8

10

12

Same Concentrations

Suspension B Suspension C

Settling Time, min

Inte

rfac

e H

eigh

t, m

Figure 3.2: Comparison of chalk suspensions with same concentrations

Figure 3.2 shows the graph of chalk suspensions B and C with the same concentration of 0.0512 g/mL but with different amount of solvent used. We observed that the settling rate of suspension C has a fast increasing rate for only 5 seconds compared to suspension B with 13 seconds. This is because there are more particles in

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suspension B than in C thus, more solids are still settling down the vessel, making suspension C achieve the ultimate height of the settled solids faster than suspension B. Also, the suspension height of B is higher than C which takes longer the particle to settle down the vessel.

Settling velocity profile of the three suspensions

0 10 20 30 40 50 60 70 80 900

10

20

30

40

50

60

70

80

Settling Velocity Curves

Suspension A Suspension BSuspension C

Settling Time, min

Inte

rfac

e H

eigh

t, m

Figure 3.3: Settling velocity curve of the three suspensions

Figure 3.3 shows the settling velocity curves of suspension A, B and C. We observed that the settling velocity of the three suspensions decreases as time increases. Also, suspension C achieved the ultimate height of the settled solids fastest among the three suspensions. This is because the amount of the particles is smaller wherein few particles only are settling thus, the particle quickly reaches a constant velocity (Zhang).

4. CONCLUSION

The objectives of this experiment were achieved. We were able to analyze settling velocity curves of different suspensions from a single batch test and determine the effect of initial concentration and initial suspension height of the suspensions on sedimentation rate.

The settling velocity of the three suspensions decreases with time.

The initial concentration of suspensions is inversely proportional to the sedimentation rate since there are many particles in the suspension; the velocity of the particle is slow, taking some time before the particles reach the bottom part of the vessel. The initial suspension height of the suspensions is inversely proportional to the sedimentation rate since the particles in the lower suspension height is nearer the bottom of the vessel and achieved the ultimate height of the settled solids faster compared with the higher one.

5. REFERENCES

Vanoni, V. A. 1975. Sedimentation Engineering: Classic Edition. 1.

Julien, P. Y. 2010. Erosion and Sedimentation. 24-25.

Bustos, M. C. 1999. Sedimentation and Thickening: Phenomenological Foundation and Mathematical Theory. 52-53.

Garcia, M. H. 2008. Sedimentation Engineering: Processes, Measurements, Modeling, and Practice.33-34.

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Gutierrez, C. L., Ngo, R. L, 2005, Chemical Engineering Laboratory Manual Part 1, 85-86.

Mohammed, Muhaned A.R., (2013). Studying the Factors Afecting the Setling Velocity of Solid Particles in Non- Newtonian Fluids, Journal (NUCEJ) Vol.16No.1, 2013, p.41 – 50. NUCEJ Vol.16 No1 Mohammed 41

http://www.che.iitb.ac.in/online/system/files/92/course_details/FM+304.pdf

http://lorien.ncl.ac.uk/ming/particle/cpe124p2.html

6. ACKNOWLEDGEMENT

The group would like to thank God for the talents and resources He bestowed. The group would like to thank Dr. Allan Soriano and the laboratory assistant for their knowledge and guidance in the experiment’s theory and procedures.

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