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Experiment No. 11 SEDIMENTATION 1. Objective: To determine determine the effect of initial concentration and initial initial height height on sedimentation characteristics. 2. Intended Learning Otcome! "ILO!#: The students shall be able to: 2.1 understand the different different types of sedimentation. sedimentation. 2.2 determine the settling settling velocity and the solid solid concentration in a batch batch settling test. $. Di!c!!ion: When a dilute slurry is settled by gravity into a clear fluid and a slurry of higher solids concentration, the process is called sedimentation or sometimes thickening. To To illustrate the method for determining the settling velocities and the mechanisms of settling, a batch settling test is carried out by placing a uniform concentration of the slurry in a graduated cylinder. At the start, as shown in igure 11.1!a, all the particles settle by free settling in suspension "one #. The particles in "one # settle at a uniform rate at the start, and a clear li$uid "one A appears appears in ig.11.1!b. The height z height z drops drops at a constant rate. Also, "one % begins to appear, which contains the settled particles at the bottom. &one ' is a transition layer whose solids content varies from that in "one # to that in "one %. After further settling, "ones # and ' disappear as shown in ig. 11.1!c. Then compression first appears, this moment is called the critical point. %uring compression, li$uid is e(pelled upward from "one % and the thickness of "one % decreases.
ig. 11.1 #atch sedimentation results: )a* original uniform suspension, )b* "ones of settling after a given time, )c* compression of "one % after "ones # and ' disappears, )d* clear li$uid interface height z versus z versus time of settling.
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+n ig. 11.1!d the height z of the clear!li$uid interface is plotted versus time. As shown, the velocity of settling, which is at slope of the line, is constant at first. The crtitical point is shown at point '. -ince sludges vary greatly in their settling rate, e(perimental rates of each sludge are necessary. The settling velocity v is determined by drawing a tangent to the curve in ig. 11.1!d at a a given time t 1, with slope –dz/dt = v 1. At this point the height is z1, and zi is the intercept of the tangent of the curve. Then, v 1 = (z1 - z )i / t The concentration c1 is, therefore, the average concentration of the suspension if zi is the height of the slurry. This is calculated by c1 zi = co zo or c1 = (zo / z )i co where co is the original slurry concentration in kgm / at zo height and t 0 . This is repeated for other times, and plot of settling velocity versus concentration is made.
%. &e!orce!: Apparatus: ! 2 ml 3arduated 'ylinder -topwatch Analytical #alance -patula Watch 3lass -tirring 5od 5uler
4aterials: 'alcium 'arbonate
'. (rocedre: I.
E))ect o) Initia* +oncentration on Sedimentation 1. 6lace 1 ml of water separately in each four 2 ml graduated cylinder. Add /g, g, 7g, and 1g calcium carbonate simultaneously in each graduated cylinder. -tir the resulting mi(ture 2. 8bserve the interface developed after 1 min9 / min9 min9 and min. measure and record what happen to the rise of sludge interface at the base of the cylinder at these convinient time intervals. /. Allow the mi(ture to attain ultimate height and composition readings. 4. Tabulate the attained data and plot the height of suspended solids against time. %etermine the settling velocity, and compute for the solid concentration.
II.
E))ect o) Initia* ,eig-t on Sedimentation +-aracteri!tic! 1. 6repare a 1; concentration by weight of calcium carbonate of about 2!ml solution. stir the solution. 2. 6lace the solution into four separate graduated cylinders with different initial heights. /. 5epeat procedure 2 to procedure from 6rocedure +.
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. Data and &e!*t!: +or!e: /rop No: /rop Member!:
Experiment No: Section: Date (er)ormed: Date Sbmitted: In!trctor:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> =eight of interface, cm g 'a'8/
Time, min 1 / -ettling velocity: -olid concentration:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> =eight of interface, cm 7g 'a'8/
Time, min 1 / -ettling velocity: -olid concentration:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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=eight of interface, cm 1g 'a'8/
Time, min 1 / -ettling velocity: -olid concentration:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Time, min
", cm
1 / -ettling velocity: -olid concentration:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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Time, min
", cm
1 / -ettling velocity: -olid concentration:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
Time, min
", cm
1 / -ettling velocity: -olid concentration:
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
0. +a*c*ation!:
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. +onc*!ion:
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. 3e!tion!4(rob*em!: 1. %ifferentiate batch and continuous sediementation. 2. What is free settling? /. What is hindered settling? . %etermine the terminal seetling velocity of dust particles having a diameter of microns at 2@./ and 11./2 k6a. The dust particles can be considered spherical with a density of 127 kgm /. or air: μ 0 .1727 (1 !/ 6a!s and ρ 0 1.22 kgm/. . A gravity settling tank is to be used to clean waste water from an oil refinery. The waste water contains 1; oil volume as small droplets ranging in si"e from 1 to 1 microns which will be removed from the water before the latter is to be discharged into the river. The tank is rectangular section 2 ft wide by ft deepwith provisions for smooth continuous discharge of clean water and skimming off of oil at the surface of the discharge end. +f 1 galmin of waste water is to be cleaned of oil droplets, specific gravity of oil is .7B, calculate the length of the settling tank. . A laboratory test on a suspension of a solid in a li$uid gave the following information: 8riginal height of sludge before settling 1 in. ree settling rate .1 in.min =eight of sludge at the end of free!settling period . in. =eight of the sludge at the end of 12 minutes in. =eight of sludge when settled completely 1. in. 8ne thousand cubic feet of similar sludge is to be settled in a vertical cylindrical tank, the diameter of which is e$ual to the depth of the li$uid suspension in it. What is the time it would take for the solid to settle to a height of 2; of the original height of the sludge? 15. An!6er!:
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11. 7rt-er &eading!: 'ao, <. )21*. Heat transfer in process engineering. #oston: 4c3raw!=ill 6rofessional. 'engel,C.A. and 3haDar, A.E. )211*. Heat and mass transfer: fundamentals and applications (4th ed.). Few Cork: 4c3raw!=ill. =ellemans, 4. )2@*. he safet! relief valve hand"oo#: design and use of process safet! valves to $%&' . 4assachusetts: #utterworth!=einemann. akac,-., Giu, =., and 6ramuanDaroenkiD, A. )212*. Heat echangers: selection rating and thermal design. #oca 5aton, lorida: '5' 6ress. othandaraman, '.6. and 5udramoorthy, 5.)211*. *luid mechanics and machiner! (+ rd ed.). Hnited ingdom: Few Academic -cience. -trathmann, =. )211*. ,ntroduction to mem"rane science and technolog! . 3ermany: Wiley!I'= Ierlag J 'o.
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