Sedimentation PDF

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Practical report on sedimentation...

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Batch sedimentation Batch it is where the reactants are initially fed on a process and left for some time and collected later. In sedimentation the feed is a mixture of solids and liquids. The mixture is contained on the vertical tubes and left for some time for solid particles to settle down[ CITATION Mik16 \l 7177 ], the settling down of solid particles is due to the gravitational force. The particles will settle in a fluid if its density is greater than that of a fluid in which it is suspended. The particle size affects the settling velocity of the particles within a fluid. The larger particles will have a smaller settling velocity as compared to the smaller particles. In addition to particle size the density and concentration also affects the sedimentation rate. These includes the particle shape and orientation in this experiment batch sedimentation was assessed by a jar test, where the mixture of solids and liquids is allowed to settle and the height of clear liquid is measured as a function of the settling time. Usually the batch sedimentation is called the gravity sedimentation. A further factor influencing the sedimentation rate is the degree of agitation of the suspension. Gentle stirring may produce accelerated settling if the suspension behaves as a non-Newtonian fluid in which the apparent viscosity is a function of the rate of shear. The change in apparent viscosity can probably be attributed to the re-orientation of the particles. The effect of stirring is, however, most marked on the consolidation of the final sediment, in which the particles can be prevented by gentle stirring. The height of suspension does not generally affect either the rate of sedimentation or the consistency of the sediment ultimately obtained. If the ratio of the diameter of the vessel to the diameter of the particle is greater than about 100, the walls of the container appear to have no effect on the rate of sedimentation. For smaller values, the sedimentation rate may be reduced because of the retarding influence of the walls. Table 1 shows the time in seconds and the height within the three different cylinders which contained a mixture of solids and liquid in millimetres. Table 1: Time and the height of batch sedimentation

Time (s) 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

Height in first cylinder(c m) 84.8 79.5 73.3 68.0 61.0 55.0 49.5 43.5 37.0 30.0 23.0 17.6 11.6 11.0 10.5 10.3

Height in second cylinder(m m) 819 784 744 705 662 624 580 535 495 460 420 375 335 329 250 215

Height in third cylinder(m m) 750 720 680 620 600 540 490 470 400 320 280 240 180 160 130 65

32 34 36 38 40 42 44 46 48 50 52 54 56

10.2 10.1 10.0 10.0 10.0

165 135 109 104 99 96 92 90 90 89 89 89 89

65,5 64 64 63 63 63 63

Figure 1 is the graph of the second cylinder with a mixture of solids and liquids used for the practical. The graph shows that the height of solids decreases with an increase in time, the trend means the solid particles were allowed to settle down under the influence of gravitational force. At some time the particles reaches an equilibrium where the height remains constant, although at that time the upper part which is highly concentrated of liquids is not pure but the smaller solid particles has no effect on the height.

References Scholz, M., 2016. Wetlands for water pollution control(second edition). science direct, pp. 1-5.

Batch settling time and height in the second cylinders 900

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Height (mm)

600

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100

0

0

4 6

2

8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56

Time (s)

Figure 1: height and settling time in batch sedimentation

Calculating the settling velocity of the particles on the second cylinder. Settling velocity=

=

∆h t 819 ×10 −3 (¿¿−3)−(335 ×10 ) 24 ¿

= 0.011 m/s...