Experiment No.11 SEDIMENTATION 1. Objective: To determine the effect of initial concentration and initial height on sedimentation characteristics. 2. Intended Learning Outcomes (ILOs): The students shall be able to: 2.1 understand the different types of sedimentation. 2.2 determine the settling velocity and the solid concentration in a batch settling test. 3. Discussion: When a dilute slurry is settled by gravity into a clear fluid and a slurry of higher solids concentration, the process is called sedimentation or sometimes thickening. To illustrate the method for determining the settling velocities and the mechanisms of settling, a batch settling test is carried out by placing a uniform concentration of the slurry in a graduated cylinder. At the start, as shown in Figure 11.1-a, all the particles settle by free settling in suspension zone B. The particles in zone B settle at a uniform rate at the start, and a clear liquid zone A appears in Fig.11.1-b. The height z drops at a constant rate. Also, zone D begins to appear, which contains the settled particles at the bottom. Zone C is a transition layer whose solids content varies from that in zone B to that in zone D. After further settling, zones B and C disappear as shown in Fig. 11.1-c. Then compression first appears, this moment is called the critical point. During compression, liquid is expelled upward from zone D and the thickness of zone D decreases. Fig. 11.1 Batch sedimentation results: (a) original uniform suspension, (b) zones of settling after a given time, (c) compression of zone D after zones B and C disappears, (d) clear liquid interface height z versus time of settling. 124 the average concentration of the suspension if zi is the height of the slurry. and 60 min. As shown. which is at slope of the line. and zi is the intercept of the tangent of the curve. This is repeated for other times. Place 100 ml of water separately in each four 250 ml graduated cylinder. Repeat procedure 2 to procedure 4 from Procedure I. This is calculated by c1zi = cozo or c1 = (zo / zi) co where co is the original slurry concentration in kg/m3 at zo height and t = 0. II. with slope –dz/dt = v1. At this point the height is z1. Then. and plot of settling velocity versus concentration is made. measure and record what happen to the rise of sludge interface at the base of the cylinder at these convinient time intervals. 4. 11. experimental rates of each sludge are necessary. 5g. 30 min. Determine the settling velocity. stir the solution. Place the solution into four separate graduated cylinders with different initial heights. Allow the mixture to attain ultimate height and composition readings. Procedure: I. The settling velocity v is determined by drawing a tangent to the curve in Fig. 8g. Prepare a 10% concentration by weight of calcium carbonate of about 200-ml solution. 11. therefore. 3.zi) / t The concentration c1 is. Effect of Initial Concentration on Sedimentation 1.1-4d the height z of the clear-liquid interface is plotted versus time. 125 . Effect of Initial Height on Sedimentation Characteristics 1. The crtitical point is shown at point C. Add 3g.In Fig.250 ml Garduated Cylinder Stopwatch Analytical Balance Spatula Watch Glass Stirring Rod Ruler Materials: Calcium Carbonate 5. and compute for the solid concentration. Observe the interface developed after 15 min. the velocity of settling. Stir the resulting mixture 2. Resources: Apparatus: 4 . v1 = (z1 . Tabulate the attained data and plot the height of suspended solids against time. and 10g calcium carbonate simultaneously in each graduated cylinder. 45 min. 4. 2. Since sludges vary greatly in their settling rate. 3.1-d at a a given time t1. is constant at first. cm 3g CaCO3 Time.6. cm 8g CaCO3 Time. min 0 15 30 45 60 Settling velocity: Solid concentration: ____________________________________ ____________________________________ Height of interface. cm 5g CaCO3 Time. min 0 15 30 45 60 Settling velocity: Solid concentration: ____________________________________ ____________________________________ 126 . min 0 15 30 45 60 Settling velocity: Solid concentration: ____________________________________ ____________________________________ Height of interface. Data and Results: Course: Group No: Group Members: Experiment No: Section: Date Performed: Date Submitted: Instructor: Effect of Initial Concentration on Sedimentation Height of interface. cm 0 15 30 45 60 Settling velocity: Solid concentration: ____________________________________ ____________________________________ 127 . cm 0 15 30 45 60 Settling velocity: Solid concentration: ____________________________________ ____________________________________ Time.Height of interface. min z. cm 10g CaCO3 Time. min 0 15 30 45 60 Settling velocity: Solid concentration: ____________________________________ ____________________________________ Effect of Initial Height on Sedimentation Characteristics Time. min z. min z. cm 0 15 30 45 60 Settling velocity: Solid concentration: ____________________________________ ____________________________________ Time. Calculations: 128 .Time. min z. cm 0 15 30 45 60 Settling velocity: Solid concentration: ____________________________________ ____________________________________ 7. 129 . Conclusion: 130 .8. 01828 x10-3 Pa-s and ρ = 1. calculate the length of the settling tank. A gravity settling tank is to be used to clean waste water from an oil refinery.202 kg/m3.5 in.10 in. The waste water contains 1% oil volume as small droplets ranging in size from 100 to 1000 microns which will be removed from the water before the latter is to be discharged into the river./min Height of sludge at the end of free-settling period 6. 2. Height of sludge when settled completely 1.32 kPa. The dust particles can be considered spherical with a density of 1280 kg/m 3.5 in. Answers: 131 . Determine the terminal seetling velocity of dust particles having a diameter of 60 microns at 294. What is free settling? 3. 5. the diameter of which is equal to the depth of the liquid suspension in it. Questions/Problems: 1. A laboratory test on a suspension of a solid in a liquid gave the following information: Original height of sludge before settling 10 in.3 K and 101. The tank is rectangular section 2 ft wide by 4 ft deepwith provisions for smooth continuous discharge of clean water and skimming off of oil at the surface of the discharge end. What is the time it would take for the solid to settle to a height of 20% of the original height of the sludge? 10. If 10000 gal/min of waste water is to be cleaned of oil droplets. Free settling rate 0.87. One thousand cubic feet of similar sludge is to be settled in a vertical cylindrical tank.9. For air: μ = 0. Height of the sludge at the end of 120 minutes 4 in. 6. specific gravity of oil is 0. What is hindered settling? 4. Differentiate batch and continuous sediementation. . United Kingdom: New Academic Science. H.11. (2010). H. R. Germany: Wiley-VCH Verlag & Co. Boca Raton.A. and Ghajar. C. rating and thermal design. Further Readings: Cao. Boston: McGraw-Hill Professional.J. Kothandaraman. Introduction to membrane science and technology.S. Heat transfer in process engineering. A. E. Kakac. (2011). and Rudramoorthy.).). (2011). Strathmann. M. Florida: CRC Press. Massachusetts: Butterworth-Heinemann.Y. A. Fluid mechanics and machinery (3rd ed. New York: McGraw-Hill. The safety relief valve handbook: design and use of process safety valves to ASME. Liu. Cengel. and Pramuanjaroenkij. (2009). 132 . Hellemans.(2011). Heat exchangers: selection.P. (2012). Heat and mass transfer: fundamentals and applications (4th ed.. Process Skills Members do not demonstrate targeted process skills. Group conflicts are cooperatively managed at all times. Members do not need to be supervised by the teacher. Clean and orderly workplace with Messy workplace during occasional mess during and after the experiment. and Teamwork Group conflicts have to be settled by the teacher.12. Members occasionally demonstrate targeted process skills. Members always demonstrate targeted process skills. Members occasionally demonstrate needed skills. Safety Precautions Members do not follow safety precautions. Clean and orderly workplace at all times during and after the experiment. Members have defined responsibilities most of the time. Members are able to set-up the materials with supervision. Neatness and Orderliness Ability to do independent work Members finish on time with incomplete data. Group conflicts are cooperatively managed most of the time. and after the experiment. Members finish ahead of time with complete data and time to revise data. II. Laboratory Skills Manipulative Skills Members do not demonstrate needed skills. Assessment (Rubric for Laboratory Performance): BEGINNER 1 CRITERIA ACCEPTABLE 2 PROFICIENT 3 I. Experimental Set-up Members are unable to set-up the materials. Members are on tasks and have defined responsibilities at all times. Members follow safety precautions at all times. by the teacher. Members are able to set-up the material with minimum supervision. Experiment Members do not know their tasks and have no Cooperative defined responsibilities. Members require Members require supervision by the occasional supervision teacher. Members follow safety precautions most of the time. Work Habits Time Members do not finish Management / on time with incomplete Conduct of data. Members always demonstrate needed skills. Other Comments / Observations: TOTAL SCORE RATING 100% 133 = ( TotalScore ) x 24 SCORE .