UNIT O P E R A T I O N S
Centrifugation
T H E theory of centrifugal separation has always lagged behind practice. Manufacturers still use test performance as a basis of recommendation, and have used theory only as a means of explaining test results or as a basis of evaluating the possible advantages or disadvantages of changing rotational speed, and ring dam adjustment. I t is very encouraging to find two excellent articles on centrifugal theory-Storrow's work on centrifugal filtration and Jury and Locke's work on centrifugal sedimentation. As in the past, the majority of the application-type articles appear in publications not readily available to the chemical cnginecr. Such articles contain interesting and useful performance and maintenance data that will add to the process engineer's background and better enable him to select the best equipment. Hydrocyclones continue to be a popular item in the literature, and a symposium on this topic was included in the 1957 AIME annual meeting in S e w Orleans. Mining Engineering contains several articles that will quickly acquaint the process engineer with the background and application potential of the liquidsolids cyclone. A centrifugation symposium was included in the AIChE meeting a t Salt Lake City. However, the papers presented were not published during 1958. Hammond includes a section on "centrifuzal separation" in his new book (77).
JAMES E. FLOOD, JR., received his 6.S.Ch.E. from Villanova in 1941. Flood began specializing in the solids-liquid separation field with the Sharples Corp. developing centrifugal equipment. He came to Du Pont's Engineering Service Division in 1950 and i s now consulting on centrifugation and filtration problems. He is a registered professional engineer and a member of AIChE.
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Centrifugation articles have increased in number and in the breadth of fields covered. A number of new applications were reported, including several on hydro-
cyclones.
Trends Centrifugal manufacturers did not introduce major new equipment last year. Apparently, application of the various new machines introduced about two years ago has kept the manufacturers occupied. Pressurized units by Bird, Merco, and Sharples have been sold and some are in operation. The new: center-slung: batch-automatic centrifugals of Fletcher and Tolhurst are also being placed in operation. .A modification of the Sharples S o z l jector (7) has been reported. X second ring-dam (covered by parent applications) has been added at the bottom of the clarifier bowl to control the amount of heavy phase recycle. Excess heavy phase spills over the recycle dam. while the correct amount needed to satisfy the nozzles and properly maintain the oil-ivater interface fiows into the boiyl. Advantages of the new bocvl are outlined in the reference along with a description of its operation when applied to purification of residual fuel for diesel engines. In recent years, some centrifugal manufacturers began to charge a fee for laboratory test work. Originally. a nominal laboratory fee was introduced to discourage small-scale process development work at the manufacturer's expense. Lately, the charges have been increased and some companies are considering rates which \vi11 reflect the total cost of the Ivork.
Theory Jury and Locke presented theoretical solutions (with mathematical details) to the problem of continuous centrifugation in a disk centrifuge (76). Analytical and numerical solutions for the case of
INDUSTRIAL AND ENGINEERING CHEMISTRY
free settling of a dilute slurry in a centrifugal field are developed and the results of a specific centrifuge problem are recorded. T h e solids in &e sample problem kcere 2 microns or larger and, according to the author's theoretical equations, were readily separated. Therefore, it was not surprising that recovery in excess of 99.9% \vas measured on the test centrifuge (Xierco). The most interesting point of this article is the introduction of a neu' term, b (not named by the author).
where q = flow rate through a single passage, cc. 'sec. 7 = viscosity, poises (g. 'cm.j(sec.j 0 = disk angle J = distance benveen disks, cm. Ap = density difference. g. cc. D = particle diameter, cin. w = angiilar velocity, radian sec. For values of b less than 21.-. essentially complete centrifugal separation results. For values greater than 21.-. partial separation or classification occurs until. finally. the removal of solids approaches zero as b approaches infinity. The b factor may prove to be a useful tool for evaluating centrifugal separation in various commercial centrifuges. In another article ( 4 ) Sharples reported its intention to use sigma values to rate their disk-type centrifuges. Developed by -4mbler ( 7 ) . the sigma method mathematically reduces a sedimentation-type centrifuge to an equivalent gravity settling tank so that various centrifuge designs can be compared. As Jury and Locke stated in their article, a direct comparison of their equations and .4mbler's sigma equations is not obvious. The proof of either or both theoretical methods should become apparent as more accurate process data are obtained on numerous centrifugal applications. Storrow (Z)!tackling the problem of centrifugal filtration (called hydroextraction in England), has reported his mathematical analysis and confirming laboratory work. Flow-rate equations for the case of centrifugal cakes wherein the pores are filled with liquid are proposed and, according to the author. confirmed.
Several new centrifugal applications have been announced. Increased use of hydrocyclones in nuclear and chemical processing is evident
Also. as in other centrifugal filtration research (Q), slow-filtering materials (starch, chalk, kieselguhr, etc.) are used as test materials. As most of the important industrial applications of centrifugal equipment are for dewatering freedraining solids, a large degree of extrapolation will be required from the slowfiltering materials studied to the fastfiltering materials handled in practice. Storrow discusses this point as well as other possible problems that remain to be studied. Jur) and Locke‘s work on centrifugal sedimentation and Storrow‘s Jvork on centrifugal filtration adds additional theoretical equations to those already developed bv Ambler and Grace. The theory now needs modification and. most of all, substantiation by firm process data obtained on plant installations of centrifugal equipment. Applications Lime Recovered from Well Water. Three articles discuss \vel]-water treatment facilities which remove calcium hardness by a process that converis the calcium salts to lime. A continuous, solid-bowl centrifuge is the key to this procedure because it dewaters the lime slurry sufficiently (6595 solids) to permit economical lime kiln operation. Herod reports on new facilities at Dayton, Ohio (73)?and the plant at Lansing, hlich. (74). Rice reports on the well-water treatment a t the St. Regis (Fla.) plant (22). Sewage Treatment. Tivo articles describe the use of the continuous, solidbowl centrifuge in the treatment of sewage sludges. Jenks reports (7,5) on the installations at north San Mateo County and a t San Leandro, Calif., presenting performance data. costs, and reasons for using the continuous centrifuge. Best performance was obtained with an 18 X 28 inch centrifuge at feed rate of 30 gal., min. Feed solids were 5yc and the cake averaged 30y0 solids. Effluent from the centrifuge (called centrate by the author) contains approximately 2 to 2.5% solids. A 40 X 60 inch centrifuge !vas installed to handle 280 ga1:’min. of sludge and to produce 90 cubic yards of centrifuge cake per day. Hamlin discusses (70) the San Leandro operation in considerable detail. Diesel Fuel Oil. Landis (78) describes the DeLaval AC-VO nozzle-type centrifuge installed at the Navy’s DieselElectric Power Plant at Subic Bay in the Philippines. The centrifugal installation washes various diesel fuels \vith
water solutions and separates clean oil from the crudes. The plant \vas designed to treat all possible fuel oils that may be available to the Philippine plant in the event of a national emergency. Tabulated performance data before and after centrifugal treatment are listed. Coal Preparation Plants. Three articles (77, 20, 25) presented in a symposium on coal preparation plants discuss coal dewatering and review data. limitations, and performances for coal handling equipment. The merits of vertical and horizontal centrifugals are discussed. These articles are somewhat generalized but they serve as background information for the chemical process engineer. Cutting Oils. Lipton (79) presents a case for centrifugal clarification of machine shop cutting oils and uses the experience of his company to prove his point. Analysis of oils before and aftei centrifugation are tabulated. The type of centrifugal equipment is listed as a high-speed centrifugal separator. Flo\v rates are not presented. Barium-140. Ayers and Legler (2) report on a new batch process to recover kilocurie quantities of barium-140 from short-cooled nuclear fuel elements. Liquid-solids separations are made in a special batch, suspended, so!id-basket centrifuge which serves as reactor as well as a decanter. The basket. 163/4 inches in inside diameter and 63/’4 inches deep, is almost hidden in the maze of skimmers, compartments, seals, and supports. Remotely operated, the centrifuge, with all its complex attachments, is reported to have been trouble-free. Hydrocyclones. The papers presented at the AIME symposium on cyclones at Yew Orleans are published in Mining EnEineering. Valuable data covering the development, application, and performance of liquid-solids c>-clones can be obtained from the reference. AS frequently happens, the chemical engineer must translate the data from another industry (in this case: the mining and ore beneficiation industry) and extrapolate the operation into chemical industry-type separations. Erickson ( 8 ) reviews the development of the liquid-solid cyclone from 1939 to the present data and illustrates the various designs such as the Humphrey, Dutch State Mines: Centriclone, Dorrclone, and others. Herkenhoff (72) presents a method of selecting a cyclone for classification applications. Morris (27) compiled operating data from 24 plants showing feed, overflow, and underflow particle sizes as well as flow rates for cyclones ranging from 3 to 48 inches in
diameter. De\’aney (6) discusses the use of cyclones in grinding [aconite. Salter (23) reports on the estimated 100 cyclones installed in Arizona mining operations since 1954. The plant installations of cyclones at Uranium Reduction Co. are reviewed by Curfman (5). Burch ( 3 )deals with a cyclone separation problem in an atomic reactor plant \\.here a hydrocyclone removes reactor products. Some data on particle size, flow rates, and others are presented for 0.2.5-. 0.40-, and 0.56-inch diameter cyclones. The article contains a table sho\ving dimensions of these three sizes of cyclones which appear to be Dorrclones (Dorr-Oliver). The hydrocyclone system was operated approximately 1800 hours to demonstrate operability and establish performance data. Simulated corrosion products averaging in the 0.5 micron range were used for tests. Flow rates for the three sizes of cyclones were 0.25>0.75, and 1.5 gal.,‘min., respectively. at a pressure drop of 80 feet. Literature Cited (1) .Ambler, C. M., Chem. Eng. Progr. 48. 150-6 (1952). ( 2 ) Ayers, .A. L , Legler, B. M., Ibid., 54, 83-6 (February 1958). (3) Burch, W. D., Ibzd., 79-82 (Februarv 1958). (4) Chern. Week 85, 87-8 (Oct. 11, 1958). (5) Curfman, R. L., Mining Eng. 10, 768-9 11958). (6) DeVaney, F. D., Ibid., 9, 880-2 (1957). ( 7 ) Diesel Pozeer 36, 22-4 (September 1958). (8) Erickson, S. E., Mining Eng. 9, 869-72 (1957). (9) Grace, H. P., Chem. Eng. Progr. 49, 303-67, 427-536 (1953). (10) Hamlin, G. H., W’eJtestern City 34, 32-7 (April 1958). (11) Hammond, K., “Separation and Purification of Materials,” Philosophical Library, New York (1958). (12) Herkenhoff, E:. C., Mining Eng. 9, 873-6 (1957). (13’) Herod, B. C., P i t and Quarry 50, 128-32 (Mav 1958). (14) Ibid., 51, 120-5 (October 1958). (15) Jenks! J. H., Wastes Eng. 29, 360-1 (July 1958). (16) Jury, H., Locke, \V. L., A.I.Ch.E. Journal 3,480-3 (2957). (17) ,Kennedy, G. H., Walker, J. L., Jr.: Mtnzng Eng.9,1329-30 (1957). (18) L.andis, D. M., Bahret, E. G., Poze’er 102,87-9 (March 1958). (19, Lipton, A. H., McKibben, K. I.‘.. Lubrication Eng. 14,252-5 (June 1958). (20) McMorris, M’. L., Mining Eng. 9, 1332-3 (1957). (21) Morris, T. M., Ibid., 877-9 (1957, (22) Rice, E. S., Pafer Trade J. 142, 36-7 (Aug. 11, 1958). (23) Salter, R., King, E. J., Mining En,g. 9, 883-9 (1957). (24) Storrow, J. A4.,A.I.Ch.E. Journal 3, 528-34 (December 1957). (25) Vonfled, J. M., M i n i n g Ens. 9, 1333-5 (1957).
VOL. 51, NO. 3, PART II
MARCH 1959
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