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Centrifugation by James E. Flood, Jr., E. I . d u Pont de Nemours & Co., Inc., Wilmington, Del. The gas centrifuge is proposed a s the potential key to inexpensive atomic energy
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The influx of European developed centrifugals continues Sharples has acquired the Trenntechnik and Baker Perkins offers the Escher-Wyss is finding a place in more and more industries, centrifuge application information and the know-how of equipment selection have not been found in recent literature. Both the users and the manufacturers continue to maintain silence to gain an advantage over competition in a highly competitive market. Hence the three articles on centrifugal equipment on pages 429-44 of this issue are a welcome addition to the literature. New applications of centrifugal equipment included catalyst separation in polyolefin processes, liquid-liquid separation for removing water from crude oil at the well site, and centrifugal processing of edible fats in meat processing. Special types of centrifugal equipment received wide public attention during the past two years. For example, the gas centrifuge was discussed in the Wall Street Journal as well as in several technical reports. Liquid cyclone studies were reported by several investigators. Two major equipment developments were reporled in the 1959-1960 period. The pressurized Merco for the Phillips polyolefin process represents an important first for the Merco Division of DorrOliver, Inc. Several centrifugals have been sealed for 0 to 1 p.s.i.g. operation in the past, but Merco made the advance to 150 p.s.i.g. design (above, right). The other major equipment development was the Bird Machine Co.’s new pilot plant continuous centrifuge. This unit has a 6-inch diameter bowl and has been designed for test work as well as pilot plant service. The previous centrifugation review appeared in March 1959 (p. 344). This review includes articles and equipment developments since that report and therefore deals with the literature from January 1959 to December 1960.
A L T H O U G H THE CENTRIFUGE
View Of Merco pc-20 pressure centrifuge at Dominion Tar 8t Chemi-
cal
cO.t
Ontario,
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adding the relatively new underdriven batch automatic Fletcher Tornado, as well as the conventional suspended basket centrifugal, to Sharples’ already extensive line of centrifugal equipment. In addition, Sharples has purchased the Trenntechnik centrifuge developed in Germany. I t is a centrifugal screen, very similar to the Dorr-Oliver Mercone and the Hey1 and Patterson Centri-plane
centrifugals, and is called the Super Conejector (below). The development of a 6-inch-diameter solid-bowl continuous centrifuge has been completed by Bird Machine Co. (p. 490). I t represents a radical departule from the conventional design of the large Bird centrifuges. Traditionally, the centrifuge bowl is mounted between two bearings, which means the bearings
Trends and New Equipment
Competition remains keen in the centrifugal equipment field, and various manufacturers have offered mew centrifugal designs for the chemical and processing industries. The centrifugal division of the Fletcher Co. was acquired by Sharples, thereby
Sharples’ Super Conejector is the Trenntechnik centrifuge design developed in Germany VOL. 53, NO. 6
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pilot plant operation for centrifugal processing studies of a wide variety of processes.
Theory
Bird Machine Co.'s 6-inch solid bowl continuous centrifuge has overhung bowl that can be dismantled without disturbing bearings, drive belts, or gear box
must be removed when the bowl is disassembled. The new 6-inch centrifuge, however, has an overhung bowl that can be dismantled without disturbing the bearings, drive belts, or gear box. The Bird designers have developed a unit that can be dismantled, cleaned, and reassembled in less than 30 minutes. Also, the 6-inch centrifuge can be sealed for pressure operation up to 150 p.s.i.g. The conveying speed (differential speed between the bowl and the conveyor) can be adjusted readily while the machine is rotating. Cake samples resulting from different conveying speeds or various cake retention periods under centrifugal force can be obtained on a slurry sample as small as 5 gallons. The new Bird pilot plant centrifuge is similar to the Sharples P-600 Super-D-Canter described in 1958 centrifugation review. A new centrifugal is available from Baker-Perkins (below). The EscherWyss, a multistage pusher type unit, available for many years in Europe, is now offered in this country. With the multistage design, friction between the cake and the basket screen is reduced. Many materials that could not be
pushed across a 24-inch basket can be handled by the Escher-iVyss with four or more short pushes. Less power is required for the centrifugal dewatering of slurries on the Escher-Wyss because the bulk of the mother liquor is separated from the solids on the portion of the screen closest to the centerline of the centrifuge (the area of low centrifugal force). However, the final cake dewatering is accomplished on the screen a t the maximum diameter (the area of the highest centrifugal force). The DeLaval Laboratory at Poughkeepsie, N. Y . , has been described in detail ( 3 ) , with emphasis on the pilot plant potential of the facility. A complete plant having as many as four stages of centrifugation with the necessary auxiliaries, such as pumps, feed tanks, mix tanks, and instrumentation, can be made available. The equipment is sized for processing flows in the range of 20 to 180 gallons per hour. In the past, pilot plants have been installed by centrifugal manufacturers for specific applications such as vegetable oil refining. However, DeLaval is the first company to announce it5 intent to set up
stage
wyss mu'ti-
pusher-type unit, able for many years in is "Ow Offered by BakerPerkins
490
INDUSTRIAL A N D ENGINEERING CHEMISTRY
An interesting repor1 by Bingeman and Coates ( 7 ) described centrifugal filtration studies that confirmed Darcy's law for liquid flow through packed beds. The experimenter had difficulty reproducing results because the solids seemed to pack differently each lime they were placed in the centrifuge basket, even though the cake was reported 10 be incompressible. The article is of gineral interest, but ir would be especially helpful if the authors could determine the reasons for nonreproducible filter cake properties in the various runs. For those interested in the mechanical design of centrifuge bowls, Goldberg and Sadowsky (7) have prepared a mathematical analysis of the stresses in an elliptical roror. A new technique for the testing of paper-making pulp fibers was presented by Thode and others (77). Pulp pads approximately 1 inch in diameter are formed on a Mire screen placed at the end of a modified test tube. After the bulk of liquor is drawn through the pad, but before the surface of the liquid reaches the surface of the cake, filtration is stopped and the test tube inserted in a bottle centrifuge. After centrifugal dewatering, the pad is dried and retained moisture recorded. A standard International centrifuge was modified for this test: and all the required equipment is clearly illustrated and described. Excellent correlation of paper sheet strength properties with centrifugal water retention was obtained in the study (77). Also, the centrifugal test offers a convenient method of estimating the swollen volume of paper-making pulps. While the test procedure and much of the data are intended for use in the paper industry. the article provides excellent background on centrifugal dewarering that can be applied to chemicals normally handled in the process industry. One graph shows centrifugally retained water us. spin time. and the data clearly show that retention time has a minor influence whereas centrifugal force has a major effect. Moisture was reduced to 190 to 1507, (wet basis) at 1000 and 3000 g, respectively. The influence of surface area is apparent, in that retained moisture in creased as a given pulp was refined. Refining the pulp shreds thick fibers into thinner fibers, thereby exposing new surface, Unfortunately, the increase in surface area was not reported (77), but the unrefined or raw fiber test pad retained 190y0 moisture. and the maximum degree of refining resulted in a test pad that contained 300 yomoisture.
an Cost Data
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A compilation of centrifugal equipment costs has been prepared by Smith (76). These data are presented in the generally accepted graphical form that has limited usefulness for the average engineer. For example, one can only guess that the cirves stop where they do because the manufacturers do not make larger or smaller units. Further, there is no way of knowing how many standard centrifuges are available between the extremes of the cost curves. Applications
The results of field tests conducted by Shell Oil Co. and reported by Sergesketter (74) indicate that centrifugal separation of water from crude oil is more economical than the heater-treater system now in general use. The increasing demand for natural gas is raising its price. I t is becoming uneconomical to burn natural gas to heat the crude oil to break the oil-water emulsion. The savings in natural gas more than pay for the operation of the centrifugal equipment. Tubular centrifuges, disk centrifuges, and disk centrifuges with peripheral noizles may be used for this application. The centrifugal processing of packing house waste was described by Downing ( 5 ) . Rapid heating, followed by centrifugal separation with a minimum of retention time at high temperatures, produces a valuable food product from waste that formerly was discarded. A centrifugal process for upgrading inedible tallow was discussed by Little and Downing (9), with flow sheets and process data given. The inedible rendering industry lost the bulk of the domestic soap manufacture to detergents. Forced to refine the tallow for new end uses, the industry turned to centrifugal processing to improve quality and to reduce operating costs through continuous processing and automation, The dewatering of coal by the Bird Humbolt centrifugal was described by Watters (78). Final moisture content of large coal (+0.25 inch to -0.75 inch) is less than 3 %. Catalyst separation in the Phillip’ polyolefin process is discussed in the report by Weedman and others To meet a product specification of less than 8 p.p.m., catalyst solids ranging in size from 40 microns to less ihan 1 micron were removed from the polyolefin polymer solution in the initial test work by the Model-9 Merco pilot unit, and later in the plant by the PC-30 centrifuge. Scale-up from 6 to 175 gallons per minute was successfully achieved. This article merits praise for its discussion cf process information and the scale-up procedures without revealing confidential
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information, such as type of polymer and solvent used. A process that converts the sludge produced in water softeners to usable chemical lime was described by Cronan (4). The key operation is performed by a solid bowl continuous centrifuge that removes magnesium, iron, and aluminum compounds from the recovered lime sludge before it is calcined for return to the process water softeners. The undesired comprwnds are present as finely divided solids that remain in the centrifuge effluent while the heavier CaC08 solids are separated and dewatered prior to calcining.
Gas Centrifuge Recently the concept of separating gases by their differences in specific gravity has bezn receiving attention. The most comprehensive article on the gas centrifuge was presented by Groth ( 8 ) ,who discussed various designs as well as the economics of isotope separation. Much background information of a general nature has appeared (72). Gernot Zippe, a West German scientist, was awarded a German patent on gas centrifuges in June 1960, and he has prepared several reports ( 2 0 , 2 7 ) on isotope enrichment by gas centrifugation as compared with gas diffusion. The design of the gas centrifuge is radically different from that of liquid centrifuges, in that the stresses in the gas centrifuge are essentially all selfstress of rotation or “hoop stresses,” whereas the liquid centrifuge bowl design must allow for liquid loading which is greater than the rotational self-stress. Therefore, the gas centrifuge can turn a t much higher speeds and generate much higher centrifugal forces. The gas centrifuge will be of interest in atomic energy studies, but it is doubtful if it will find economical applications in the chemical and process industries in the immediate future. liquid Cyclone
An excellent article by Bradley and Pulling (2) discussed the floh pattern in a 3-inch liquid cyclone. Dye was injected into the cyclone at various points, and photographs recorded the accumulation or dispersion of the dye at various points inside a transparent cyclone. Unfortunately, the photographs lack detail, but a good concept of the flow pattern is obtained; a strong argument is made for the existence of the “mantle,” an annular region between the outer downward vortex and the inner upward vortex where there is essentially no radial velocity component. A new method of defining the conical area of centrifugal separation was reported to improve the
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correlation of data obtained on the cyclone, and data to support this argument were presented. A comprehensive study of cyclones in the size range of 5- to 50-mm. diameter was reported by Engel and Weisman (6). Illustrations, with dimensions, of seven cyclones used in the study are shown, and the results of tests with thoria-urania slurries (possible nuclear reactor fuels) are presented. The data correlation allows prediction of concentration efficiency of a given cyclone, provided that properties and particle size distribution of the slurry feed are known. The data also confirmed Matschke and Dahlstrom’s (70) empirical equation for head loss across the qclone with an average deviation of less than 5%. A novel liquid cyclone design was described by Sineath (75). In this unit, the feed is introduced at the top (not tangential), and rotation is achieved by passing the slurry through a series of fins attached to a stationary solid central core inside the cyclone. Both “underflow” and “overflow” are taken off the bottom of the unit. The “underflow” is through an annular opening at the periphery of the cyclone, and the “overflow” is from a central nozzle at the bottom of the cyclone. Low energy loss for equal capacity and efficiency are the reported advantages of the new design. Bibliography (1) Bingeman, J. B., Coates, J . , A.1.Ch.E. Journal 6, 58 (1960). (2) Bradley, D., Pulling, D. J., Trans. h t . Chem. Ezg. ( - o n d o n ) 37, 34 (1959). (3) Crauer. L. S., IND.ENG. CHEM.52, 52A (June 1960). (4) Cronan, C. S., Chern. Eng. 66, 47 (Ji.ne 29, 1959). (5) Downing. F. P.. J . Am. Oil Chemirts‘ SOC. 36, 319 (1759). ‘ (6) Engel, F. C., Weisman, J., A.7.Ch.E. Journal 6, 262 (1960). (7) Goldberg, M. A,, Sadowsky, M., J . A$$. Mechanics 26, 549 (1959). 8 ) Groth, W. E., Research 12, 467 (1959). 9) Little, T. H., Downing, F. P., J . Am. Oil Chemists’ SOC.37, 26 (1960). (10) Matschke, D. E., Dahlstrom, D. A., Chem. Eng. Progr. 54, 60 (December 1958). (11) Naylor, T. R., Mine &? Quarry Eng. 24, 510 (1958). (12) Nucleonics 18, 17-18 (1960). (13) Sager, F., Petrol. Refiner 38, 1 9 3 (March 1959). (ld) Sergesketter, M. J., 021 Gas J . 57, 162 (September 1959). (15) Sineath, H. H., Chem. Eng. Progr. 55, 59 (November 1959). (16) Smith, J. C., Chem. Eng. 67, 148 (Sept. 5, 1960). (17) Thode, E. F., Bergomi, J. G., Jr., Unson, R. E., Ta$$t 43, 502 (1960). (18) Watters, E. A , Mzntng Congr. J. 46, 39 (1960). (19) Weedman, J. A., Payne, W. E., Johnson, D. W., Chem. Eng. Progr. 55, 49 (February 1959). (20) Zippr. Gernot, U. S. At. Energy Comm. TPD-5753. (1960). (21) Ibid., ORO-315: \
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