CH3510: MOMENTUM TRANSFER AND MO LAB

Department of Chemical Engineering

Submitted by-

Mohammed Rhazy – CH10B100

Shivani Patel – CH10B101

Saurabh Bhandari – CH10B102

SEDIMENTATION

Sedimentation Studies Apparatus

Objective: To study the batch sedimentation process.

Aim: To determine the effect of initial concentration and initial suspension height on sedimentation rates.

Introduction: Sedimentation is the process of letting suspended materials settle by gravity. Suspended material may be partials, such as clay or silts, originally present in the source water. More commonly suspended material or floc is created from material in the water and the chemical used in coagulation or in the other treatment process, such as lime softening. Sedimentation is accomplished by decreasing the velocity of the water being treated to a point below which the particle will no longer remain in suspension. When the velocity no longer supports the transport of the particles, gravity will remove them from the flow. Some of the more common types of factors to consider are: The size and the type of particles to be removed have a significant effect on the operation of the sedimentation tank. Because of their density, sand or silt can be removed very easily. In contrast colloidal material, small particles that stay in suspension and make the water seem cloudy, will not settle until the material is coagulated and flocculated by the addition of a chemical, such as an iron salt or aluminum sulphate. The shape of the particle also affects its settling characteristics.

Theory: For any batch sedimentation experiment, on slurry of known concentration, the height of a liquid-solid interface is obtained as a function of time. Slopes of this curve at any point of time represent settling velocities of the suspension at that time and are characteristics of specific solid concentration. At

the beginning of a batch sedimentation process, the solid is uniformly disturbed in the liquid. The total depth of suspension is maximum but after a short while the solid have settled to give a zone of clear liquid. After sometimes it is divided into three zone, clear liquid zone, partial dense zone and zone. Earlier dense zone increase then decreases.

Experimentation setup

In sedimentation, the particles experience a period of accelerated fall and a period of constant velocity fall. The constant velocity period is usually of more importance, as the accelerated fall period is very short relative to the constant velocity period.

Description: The set up consists of five cylinders made of borosilicate glass. The cylinders are mounted on vertical back-panel, which is illuminated from behind. Measuring scale are provided for each of the cylinders.

Materials required: Graduated cylinders (1.8L), stop watch, CaCO3, Distilled water, spatula, CaCO3 (ppt.), stirrer, weighing machine, funnel.

Experimental Procedure:

1) Prepare four different slurry solutions. Solution 1 - 55 gm CaCO3 in 1.8 ltr of water. Solution 2 - 75 gm CaCO3 in 1.8 ltr of water. Solution 3 - 95 gm CaCO3 in 1.8 ltr of water. Solution 4 - 75 gm CaCO3 in 1.8 ltr of water.

2) Fill the solution in cylinders. 3) For the first three cylinders, take (55,75,95) gm CaCO3 in 1.8 liters water

and note down the initial height, which is same. 4) For second two cylinders, take (75,75) gm CaCO3 in 1.8 liters water and

note down the initial height which is different. 5) Connect the electric supply. 6) Switch ON the light.

7) Stir the solution of all cylinders until a uniform solution is achieved. Record the initial time.

8) Start the stop watch and record the height of clear liquid interface at every 2 minute interval.

9) Also note down the time. 10) Record the final height and time for each cylinder. 11) The values were then tabulated and graphs were plotted to obtain various parameters.

Observation & Calculation:

S.No. Amount of CaCO3 (gm) in 1.8 ltr of water Initial height of interface

Cylinder 1 55 100

Cylinder 2 75 100

Cylinder 3 95 100

Cylinder 4 75 45

Cylinder 5 75 60

Observation Table:

Time( in min) Cylinder-1 Cylinder-2 Cylinder-3 Cylinder-4 Cylinder-5

0 100 100 100 45 60

2 96 96.4 97 38.5 56

4 92.5 93.1 93.8 34.5 53.5

6 89.4 90.3 91.3 31.4 51.2

8 86.2 87.3 88.7 27.5 48.7

10 82.7 84.6 86.3 24.5 46.3

12 79.6 81.4 83.5 21.2 43.4

14 76.8 79 81.4 18.8 41.2

16 73.8 76.4 79.2 16.6 39

18 70.4 73.7 76.8 14.3 36.5

20 67.3 71 74.8 12.9 34.5

22 64.3 68.3 72.5 11.8 32.3

24 60.9 65.5 70.3 11 30.2

26 57.8 63 68.3 10.4 28.2

28 54.7 60.4 66.2 9.9 26.4

30 51.7 58.1 64.3 9.6 24.7

32 48.5 55.3 62.3 9 23.4

34 45.7 53.1 60.5 8.8 22.3

36 42.7 50.5 58.6 8.4 21.3

38 39.3 47.9 56.9 8 20.5

40 36.7 45.9 55.4 7.7 19.7

42 33.9 43.6 53.7 7.4 19.2

44 30.9 41.3 52.1 7 18.8

46 28.2 39.3 50.6 6.8 18.3

48 25.3 37.2 49.1 6.5 17.8

50 23.1 35.3 47.8 6.2 17.3

52 22.4 34.4 46.6 6 17

54 21.8 33.6 45.4 5.7 16.6

56 21.3 32.8 44.2 5.5 16.2

58 20.9 32.1 43.2 5.3 15.8

60 20.5 31.4 42.3 5.1 15.5

62 20.1 30.7 41.4 5 15.2

64 19.7 30.1 40.6 4.9 14.9

66 19.4 29.5 39.8 4.8 14.6

68 19 28.9 39.1 4.8 14.4

70 18.7 28.5 38.5 4.7 14.1

72 18.4 28.1 37.9 4.7 13.9

74 18.2 27.7 37.4 4.6 13.6

76 18 27.3 36.9 4.6 13.3

78 17.8 27 36.4 4.5 13.1

80 17.6 26.7 36 4.5 12.9

82 17.4 26.4 35.6 4.5 12.6

84 17.3 26.1 35.3 4.4 12.4

86 17.2 25.9 35 4.4 12.2

88 17.2 25.7 34.8 4.4 12

90 17.2 25.6 34.6 4.4 11.8

Interface Height (cm)

Graphs

Calculations:

Source : Prof. Basavaraj’s lecture notes

According, to the above figure, at time ‘t’ the interface between clear liquid and suspension concentration C is at a height ‘h’ from the base of the vessel and velocity of the interface is the slope of the curve at this time. Therefore, velocity of the interface at time ‘t’ is(h1-h)/t. If we observe the graph, there exists a linear relationship between height and time for some time interval. In this interval terminal settling velocity of the interface remains same. And its value is equal to the slope of the linear part. Thus, using the above mentioned concept we the terminal settling velocities were found out as:

Cylinder 1 – 1.211 cm/min Cylinder 2 – 1.082 cm/min Cylinder 3 – 0.875 cm/min Cylinder 4 – 0.994 cm/min Cylinder 5 – 1.031 cm/min

The above results show that the terminal velocity decreases with increase in concentration of suspension. However, the terminal velocity does not vary much with the change in height.

Conclusion

1. From the terminal velocity of the first three cylinders we can say that the velocity decreases with increase in concentration. This causes the settling time to vary for different concentrations in hindered settling, i.e. more the concentration more is the hindrance and hence lesser is the terminal velocity.

Cylinder 1 – 1.211 cm/min (55 gm/1.8 l) – 100 cm Cylinder 2 – 1.082 cm/min (75 gm/1.8 l) – 100 cm Cylinder 3 – 0.875 cm/min (95 gm/1.8 l) – 100 cm

2. From the other section of the experiment we can say that terminal velocity is not affected much by the change in height, hence it is not dependent on initial height of the interface.

Cylinder 2 – 0.994 cm/min (75 gm/1.8 l) – 100 cm Cylinder 4 – 1.031 cm/min (75 gm/1.8 l) – 045 cm Cylinder 5 – 1.082 cm/min (75 gm/1.8 l) – 060 cm

Precautions and Maintenance instructions:

1) Keep apparatus clean and free from dust 2) Always drain and clean the cylinders after use 3) Solutions should be mixed properly before starting the experiment 4) The columns should be bound with screws sufficiently tight.

Sources of Error:

1) Parallax error due to difference in interface and eye level 2) Inaccuracies in the readings due to unclear interface at the beginning 3) Error from the least count of the scale on the cylinder 4) Time delay between in taking readings for different cylinders 5) Not stirring the CaCO3 in the solution which may lead to a concentration

that is slightly different from what we expect

References

1) Dr. Basavaraj M. Gurappa – Mechanical Operations Lecture notes on Sedimentation-4,5.

2) Warren L. McCabe, Julian C. Smith, Peter Harriott, Unit operations of Chemical Engineering,