Batch Sedimentation

Problem E1 Batch Sedimentation

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Problem E1

BATCH SEDIMENTATION 1. OBJECTIVES 1.1 To determine the effects of initial concentration on sedimentation characteristics. 1.2 To determine the effect of initial height on sedimentation characteristics. 1.3 To determine the average size particles using experimental settling data. 2. REFERENCES Foust, Alan., et. al. Principles of Unit Operations, 2nd Ed. John Wiley and Sons, In., 1980. Geankoplis, Christie J., Transport Processes and Unit Operations, 3rd Ed. Prentice-Hall International, Inc., 1993. McCabe, Warren L., Smith, Julian C., and Harriott, Peter. Unit Operations of Chemical Engineering, 5th Ed. New York: McGraw-Hill, Inc., 1993. Perry, Robert H. and Don Green (Editor), Perry’s Chemical Engineers’ Handbook, 6th Ed. New York: McGraw-Hill, 1984. 3. EQUIPMENT AND MATERIALS W2 Sedimentation Apparatus (220 Volts) Stopwatch Calcium Carbonate (CaCO3) Viscometer (220 V)

Top Loading Balance Calibrated Pails Beaker (1L) / Stirring Rod

4. SAFETY The sedimentation tubes are made of glass. There should be extra care when covering them with rubber stoppers or when shaking them (to achieve homogeneity of the slurry). Members of the group that are handling the tubes must be given enough room to avoid any untoward incident. Likewise, discarding the slurries must also be done properly. For this, follow the clean up procedure carefully. 5. PRELIMINARY To achieve the objectives of this experiment, it is important that one reviews well the principles of Batch Sedimentation (Foust and Geankoplis). In addition, one must be familiar with the difference between batch sedimentation and continuous settling operations, and the equations necessary in analyzing them. Check if the light bulbs of the sedimentation apparatus are working. If not, ask the technician to have the defective bulb replaced. Note also the dimensions of the tubes. It may be useful to perform a trial run for one of the slurries; this should provide a basis in deciding on the most convenient time interval. 6. THEORY Sedimentation is the method of separation of dilute slurry into a clear supernatant liquid and concentrated slurry called sludge. Gravity effects this separation. There are two general cases in sedimentation: one involves dilute

Problem E1 Batch Sedimentation

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slurry, the other involves more concentrated slurry. In the former case, the particles settle individually and the settling characteristics depend on the viscosity of the medium, relative density of the particles with respect to that of the medium, and the particle size. An increase in the viscosity of the medium will lower the settling velocities of the particles; while an increase in the particle size and density will favor faster settling. Also, in this case, settling by gravity proceeds such that after some time there will be a clear demarcation line between the clear liquid and the slurry. The demarcation line becomes more and more distinct as the mixture approaches the end of the settling period. More concentrated slurry behaves differently. It settles, or more correctly, the particles settle as a single body with a very much lowered velocity due to mutual interference among them. This is called hindered settling. At the start there is only one zone, a layer of homogeneously dispersed solutes. At the middle of the separation, four zones develop: zone A which is the region of supernatant liquid, zone B which is the remains of the homogeneous zone developed at the start, zone C which is the region of variable concentration, also known as the transition zone, and zone D, the region of settled solutes. As the operation continues, zones B and C gradually disappear while zones A and D increase. After some time, the level of the sludge shrinks to a final level known as the ultimate height. The plot of height of demarcation line vs. time will give the settling characteristics of the slurry. The derivative of the curve with respect to time is the settling velocity. This is a function of the initial concentration and is independent of the initial height of the slurry. 7. OPERATING PROCEDURE AND CONDITIONS 7.1 Start-Up 7.1.1

7.1.2

Prepare the appropriate amounts of CaCO3 slurries (5%, 7.5% and 10% by weight CaCO3). Each sedimentation tube has an approximate capacity of 20 liters. Use calibrated pails in preparing the slurries. Obtain the following slurry characteristics and operating conditions: Concentration Temperature

7.1.3

Bulk density Bulk viscosity

Prepare a table for the following data: a. Clear liquid-slurry interface vs. time b. Concentrated sludge levels vs. time

7.2 Experimental Procedure 7.2.1

7.2.2

With constant stirring, pour each slurry in separate sedimentation tubes (label the tubes). Each tube should be filled to the same level (900 mm) for the 5%, 7.5%, and 10% CaCO3 slurries. Fill the other tubes with the least concentrated slurry (5%) to different levels of 700 mm and 500 mm. Record the initial heights of the slurries in the tubes. Shake each of the tube thoroughly to assure uniform dispersion of the solids. The tubes are then mounted to the sedimentation stand all at the same time. Allow the slurries to stand undisturbed. NOTE: Ask the lab instructor/technician to show you the proper way of shaking the tubes.

Problem E1 Batch Sedimentation

7.2.3

7.2.4

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Note the height of the interface (between the clear liquid and the subsiding dispersion) at a convenient time interval (2 or 3 minutes). If observable, note also the rise of the sludge at the base of the tubes at each corresponding time interval. Do this until there are no observable changes in the heights being measured. Allow the slurries to stand for 24 hours to obtain the final compaction reading, z∞. Be careful not to disturb the dispersions.

7.3 Clean Up 7.3.1 7.3.2

Obtain the final compaction reading in each tube. Pour the content of each tube in a single container (may not be applicable in some situations if there is a class that will use the setup the next day). Allow the solids to settle and then decant the mixture. Discard the clear liquid in the sink. The wet CaCO3 may be sun-dried for reuse. DID YOU LEAVE EVERYTHING SAFE AND TIDY?

8. TREATMENT OF RESULTS 8.1 Graphical Treatment 8.1.1 8.1.2 8.1.3

Plot the sedimentation curves for all the sedimentation tubes on a single graph (height of interface, z vs. time, ). Call this graph 1. Plot on one graph log(z – z∞) vs. time for all the sedimentation tubes. Call this graph 2. From the settling data, prepare the sedimentation rate curves (settling velocity, v vs. fraction settled, f). Fraction settled is the

fraction settled, f 

z0  z z0  z

difference of the height of the interface at an indicated time divided by the distance ultimately settles. Call this graph 3. Where:

8.1.4

8.1.5

z0 = initial height of the interface z∞ = final height of the interface z = height of the interface at any time Prepare a concentration-mass rate curve for each sedimentation tube in a single graph (concentration of slurry vs. mass rate of sedimentation, g/cm2-hr). Call this graph 4. Prepare the rate of settling vs. concentration curves from the sedimentation curves obtained for the least concentrated slurry using (refer to Foust and Geankoplis):

zic = z0c0 Where: c0 = initial concentration of the slurry zi = intercept of the tangent drawn at a point in the sedimentation curve c = the corresponding concentration for zi obtained using the equation above v = slope of the tangent of the curves in graph 1 or the settling velocity at time, ) = (zi – z)/ Call this graph 5.

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8.2 Calculations 8.2.1 8.2.2 8.2.3

8.2.4

Calculate the settling velocities for all the sedimentation tubes using appropriate formulas. Compare the calculated settling velocities with the observed settling velocities. Calculate the average size of the particles using the minimum settling velocity obtained from the sedimentation rate curve of the least concentrated slurry. Determine the ratio of sedimentation volume/actual volume to the final volume/actual volume for each sedimentation tube.

sedimentat ion volume  final volume 

(height of interface at critical point ) (tube area) initial weight of solids

(final height of interface) (tube area) initial weight of solids

9. ANALYSIS AND INTERPRETATION OF DATA 9.1 Interpret each of the graphs in the Treatment of Results. 9.2 For the CaCO3 slurry, obtain a mathematical statement that describes the dependence of settling velocity on (a) the concentration of the slurry (b) the initial height of the slurry. 9.3 Compare graphs 4 and 5. 10. QUESTIONS 10.1 Discuss the significance of the critical point and the limiting concentration in sedimentation operations. 10.2 Discuss the usefulness and the applicability of the analyses performed above in the design of commercial sedimentation equipment. 10.3 With the aid of diagrams, describe the operation of any two commercial sedimentation equipment.