Problem E1 Batch Sedimentation53 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 and 10% CaCO3 slurries. An increase in the viscosity of the medium will lower the settling velocities of the particles.1 Start-Up 7. four zones develop: zone A which is the region of supernatant liquid.1 7. zone C which is the region of variable concentration. the region of settled solutes. zones B and C gradually disappear while zones A and D increase. a layer of homogeneously dispersed solutes. the level of the sludge shrinks to a final level known as the ultimate height. Record the initial heights of the slurries in the tubes. It settles. time b.5% and 10% by weight CaCO3). In the former case.1. . NOTE: Ask the lab instructor/technician to show you the proper way of shaking the tubes. and zone D. More concentrated slurry behaves differently.3 Bulk density Bulk viscosity Prepare a table for the following data: a. Fill the other tubes with the least concentrated slurry (5%) to different levels of 700 mm and 500 mm. Clear liquid-slurry interface vs. 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.2. This is called hindered settling. the particles settle individually and the settling characteristics depend on the viscosity of the medium. At the start there is only one zone. or more correctly. the particles settle as a single body with a very much lowered velocity due to mutual interference among them. and the particle size.1. zone B which is the remains of the homogeneous zone developed at the start.1. also known as the transition zone. while an increase in the particle size and density will favor faster settling. The plot of height of demarcation line vs. relative density of the particles with respect to that of the medium. Concentrated sludge levels vs. in this case. the other involves more concentrated slurry. 7. The derivative of the curve with respect to time is the settling velocity. Allow the slurries to stand undisturbed. The demarcation line becomes more and more distinct as the mixture approaches the end of the settling period. Each tube should be filled to the same level (900 mm) for the 5%.5%. time 7. After some time. This is a function of the initial concentration and is independent of the initial height of the slurry. 7.2. OPERATING PROCEDURE AND CONDITIONS 7.1 7. settling by gravity proceeds such that after some time there will be a clear demarcation line between the clear liquid and the slurry.2 With constant stirring. 7. Also. pour each slurry in separate sedimentation tubes (label the tubes). As the operation continues.2 Prepare the appropriate amounts of CaCO3 slurries (5%.2 Experimental Procedure 7. time will give the settling characteristics of the slurry.Problem E1 Batch Sedimentation 54 slurry. At the middle of the separation. Obtain the following slurry characteristics and operating conditions: Concentration Temperature 7. Use calibrated pails in preparing the slurries. Each sedimentation tube has an approximate capacity of 20 liters. Where: 8. Call this graph 3.3 Plot the sedimentation curves for all the sedimentation tubes on a single graph (height of interface. Plot on one graph log(z – z∞) vs.3. f). f z0 z z0 z difference of the height of the interface at an indicated time divided by the distance ultimately settles.3.2 8.3 Clean Up 7. Do this until there are no observable changes in the heights being measured. . Discard the clear liquid in the sink. The wet CaCO3 may be sun-dried for reuse. DID YOU LEAVE EVERYTHING SAFE AND TIDY? 8. Allow the solids to settle and then decant the mixture. v vs. Allow the slurries to stand for 24 hours to obtain the final compaction reading. Call this graph 2. 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. Call this graph 4.2. z∞.2. prepare the sedimentation rate curves (settling velocity. note also the rise of the sludge at the base of the tubes at each corresponding time interval.3 7. Prepare the rate of settling vs.4 8. 7. 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).1 8.1 Graphical Treatment 8.1.4 55 Note the height of the interface (between the clear liquid and the subsiding dispersion) at a convenient time interval (2 or 3 minutes).1. time.Problem E1 Batch Sedimentation 7. mass rate of sedimentation. If observable. time for all the sedimentation tubes. g/cm2-hr).2 Obtain the final compaction reading in each tube.1 7. fraction settled.1. TREATMENT OF RESULTS 8. Fraction settled is the fraction settled. ) = (zi – z)/ Call this graph 5. Be careful not to disturb the dispersions.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. ).1. z vs. Call this graph 1. From the settling data. describe the operation of any two commercial sedimentation equipment.2.2 Discuss the usefulness and the applicability of the analyses performed above in the design of commercial sedimentation equipment. 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. 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.1 8. 10. QUESTIONS 10. 9. 10. Determine the ratio of sedimentation volume/actual volume to the final volume/actual volume for each sedimentation tube.2.4 Calculate the settling velocities for all the sedimentation tubes using appropriate formulas. Compare the calculated settling velocities with the observed settling velocities.1 Interpret each of the graphs in the Treatment of Results. ANALYSIS AND INTERPRETATION OF DATA 9.3 With the aid of diagrams.2.2 8.2 Calculations 8.3 8.2 For the CaCO3 slurry. Calculate the average size of the particles using the minimum settling velocity obtained from the sedimentation rate curve of the least concentrated slurry.1 Discuss the significance of the critical point and the limiting concentration in sedimentation operations.Problem E1 Batch Sedimentation 56 8. 9.2.3 Compare graphs 4 and 5. 10. .