CENTRIFUGATION J. 0. MALONEY University o f Kansas, Lawrence, Kan.
P
ROBABLY the most outstanding event in centrifugation during the last year was a 1-day symposium on the subject which occurred a t the Louisville, Ky., meeting of the American Institute of Chemical Engineers in March. This wm the first time a program on this unit operation has been held by a national society, and the interest in the subject was denionstrated by the attendance of more than 300 persons. Nine papers were given and covered the areas of theory, selection, evaluation and scale-up of equipment, and the application of centrifugals to three specific separation problems. The first paper by Storrow ( 4 7 A ) summarizes the results of 8 years of work on the flow relations of a liquid passing through a cake which had been deposited in a perforated baskrt centrifuge. In the experiments, he reports the cake was always covered n i t h liquid and never allowed to dry out. Some of his conclusions are interesting, significant, and worth summarizing.
For five materials studied the effect of compressive stress on the ermeability of the cake was minor. T i e basic flow equation was substantiated by experimental verification of the effect of each variable appearing in it. The flow of liquid through the cakes was streamline. Determination of the permeability of materials using a filter cell and a centrifugc gave results which agreed within 20%, and with certain materials a n agreement of 1% existed. Experimental evidence showed that the cakes formed during a centrifuging operation do not always develop in a cylindrical configuration, but by using a method for averaging the cake thickness, a reasonable prediction of the flow rate can be made. He terminates his treatment by pointing out that the problems associated with liquid retention in the cake, with washing of the cake, and with the formation of a cake in the basket when the cake is not submerged; all these remain to be attacked. The second paper (&A) by Smith gives a general treatment of the centrifugals available for chemical processing. Schneider (49A) discusses the advantages of the centrifugal for liquid-solid separations, and lists a number of cautions and illustrations for the conduct of centrifugal filtrations. An extensive treatment of the factors to be considered by the process engineer in making a preliminary selection of Centrifugal equipment was presented by Bingeman ( 7 A ) . He classifies centrifugal equipment on the basis of dcsign, and gives the centrifugal force for the various machines developed, as well as the power requirements, typical handling capacities, dimensions, and suppliers. His costs are expressed in relative terms, but precise values are givrn by Smith ( 4 4 A ) and Flood ( % l A ) . Bingeman concludes with an exampk showing how to make a preliminary selection. The authors of the next two papers concerning the manner in which sperific machines are scaled up recommend that tests always be conducted. Irving (98A)discusses the performance of the push-type centrifugal machine from the standpoint of particle size, slurry concentration, liquid viscosity, crystal degredation, washing, and residual moisture content. For the residual moisture in a cake of fixed thickness he gives
482
where ,%I = residual moisture content K = a constant A = crystal surface area per unit mass of crystals I.( = viscosity of liquid C = centrifugal force t = drying spin time This equation has some similarities t o one of Bartkus and Simons (sa) in which they show that the residual moisture content is a function of the centrifugal force to the -0.5 power. However, the definitive work on residual liquid contcnt in a centrifugal cake is yet to be published. I n order to predict the performance of plant-size equipment Irving ends by describing how the smallscale tests are conducted on a unit 12 inches in diameter. The technique used by Merco to develop recommendations for plant installations was presented next. The authors (%!A) state that preliminary studies are made with a test tube centrifuge, followed by ones using a small continuous unit called the L\VA-205, and finally with a pilot-size Merco. Apparently some modification of Ambler’s ( S A ) sigma function is used in making the final scale-up. Two examples are included to show how well the capacity of larger units could be predicted. The Hermetic separator and its application to industrial liquid-liquid separations is described by Sullivan (48A). A special doublc-bowl vacuum centrifuge was the suliject of the next paper (35A). This unit is designed to remove both floating and sinking particles, as well as dissolved air from viscous gelatin solutions which exhibits severe foaming characteristics and is subject to bacterid attack. The final talk was by Ambler ( I A ) who treated two related topics. The first is a description of the application of centrifuges to cornstarch refining. The second is an item-by-item consideration of the terms found in the “sigma” value as it applies to a disk centrifuge separating gluten. The sigma function is an assembly of the variables relating to the characteristics of the centrifuge. He considers, among other items, the effect on the separation of gluten of the angular speed, the distance between disks, the disk angle, the angle that the upper and loner portions of the b o d makes with the vertical, and the distance from the axis of the centrifuge to the disk stack. In
JAMES 0. MALONEY is head of the department of chemical engineering and director of the Research Foundation at the University of Kansas. H e was born in St. Joseph, Mo., in 1915, and received his B.S.Ch.E. from the University of Illinois in 1936 and his M.S. and Ph.D. from The Pennsylvania State College in 1939 and 1941, respectively. Maloney is a member of the ACS and the AIChE.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 48, No. 3
CENTRIFUGATION
The centrifuge
...key equipment in making Diesel and gas turbine fuels
addition he discusscs the interrelation of these variables and some of the practical considerations which modify the theory. .4nibler’s valuable contributions continue to be in the development of theory and in showing its limitation in practice. The results of this symposium are encouraging and plans are being made for an additional one of a more fundamental nature. Several review articles have appeared. Broadwell ( I b A ) in a two-article series continues his reviews ( 11 A ) of the work done in Europe for the period 1953-55. He organizes his material principally around the specific material being processed and includes the application of centrifugal equipment to the refining of vegt:table and fish oils, animal fats, soap, sugar, petroleum, chemicals, and fermented liquors. This particular arrangement can be particularly useful to anyone interested in specific applications; the 274 references cover the field. iis one of a series of articles on solids-liquid separation Flood ( 2 1 A ) has written on centrifugals. His short section on “basic thinking” can be criticized on two points, He recommends the treatment in Perry ( 1 5 A )on centrifugals, but apparently overlooks the fact that this material i3 over 20 years old. His statement that the theory of cent’rifugnl filtration has bocn studird extensively needs more than minor correction. Essentially only two workers, Storrow and Grace, have reported on the theory of centrifugal filtration, and the relative importance of the contributions of these two men to centrifugal filtration seems to have escaped him. The rest of his article gives a satisfactory description of the equipment available together with approximate prices. In some respects Smith’s article (44A) of 1952 remains more useful. Hulsen (%A) presents an illustrated description of centrifugal equipment. A 35-page report containing instructions for conducting pilot plant tests using the Merco centrifuge is available (38A). It includes procedures for conducting concentration, washing, clarification, soluble recovery, and classification studies. I n addition there are many suggestions made on ways to overcome difficulties that arise during a test-a contribution to the practical aspects of the subject. A translation (89A) of a German book ( 6 A ) on the application of the gas centrifuge to isotope separation has been made. Such a translation could make available to a wider group information leading to a better understanding of this technique for separating isotopcs.
viewer’s attention. Performance data, costs, and design modifications are presented for a Podbielniak centrifugal extraction used in the extraction on a commercial scale of Chloromycctin and penicillin (4A). A method for sealing the basket of a centrifuge to prevent the leakage of sugar crystals consists of fitting adjustable metal hoops tightly against the side of the basket a t its top and bottom (3OA). A new procedure for cleaning Westfalia centrifuges, which have been used on the purification of heavy oil, is said to reduce the down time 60% (39A). Goodwin (23A) has presented some genera! information on the selection of electrical drives for automatic batch centrifugals. Other new developments are found in Table I.
Applications During this review period the major descriptions of the application of centrifugals have been the refining of vegetable oils and the preparation of heavy petroleum oils for use as fuels in Diesel engines and gas turbines. In the refining of vegetable oils the objects have been to increase the recovery of marketable products, to reduce waste disposal problems, and to minimize operating costs. Centrifuges are becoming increasingly important in the realization of all these objectives. Because of t,he dificulty in making accurate predictions on the performance of plants from pilot scale data, one company has installed a full-scale, multipurpose vegetable-oil processing plant (10A ), which they believe t o be the best way to sell refining processes employing their centrifugal equipment. Such an installation is an interesting com-
New Equipment Those interested in a detailed description of the manner in which perforated basket centrifugals are fabricated by the Rroadbent Co. are referred t o a recent article ( 4 . M ) which contains 19 illustrations and is more extensive and detailed than any one which has come t o this reMarch 1956
centrifuges separate potato starch-Nippon Japan
Shiryo Co., Tokushima, Shikoku,
INDUSTRIAL AND ENGINEERING CHEMISTRY
483
UNIT OPERATIONS REVIEW
Centrifuges separate gluten and starch-LuDo
& LuYm Corp., Cebu, Philippines
These two applications and others of a miscellaneous iintuie mentary on the state of our knowledge of certain wit operations are tabulated in Table 11. and unit processes. The vegetable-oil refining procedures in the country are not always suitable for operation elsewhere. A process particularly adaptable to foreign iustallatiom for carryRECENT PATENTS ing out this operation continuously and one in which disk centriThe patents on centilfugal equipment granted in the Unltcd fuges are used has been described (9A). I n addition to the deStates during the last year include: scription a list of installations and operating data is presented. Application of centrifugal equipment to specific separations .4n interest in the use of heavy fuel oil in Diesel engine, and i n (6B, 11B, I d R , d l B , 26B, 28B, SOB) gas turbines has existed for a number of years. The declining use of steam locomotive which burned these heavy oils, the development of large heavy oil surpluses, the increasTable 1. New Equipment Developments in Centrifugals ing cost of light Diesel fuels, and Itrm ADDliCatiOn >\lanufactures Rcfcrrnce _. their possible nsp as jet fuel for mili(824) Bird-Humholt centrifngal drier Dewatering of granular materials Bird Stainlrss basket centrifuge Corrosive niaterials Brondbent ( 5 4 '4 ) tary aircraft all have contributed to a Cream separator De Laval (16A) De Laval Air-Tight Dooble-ho~\-lvacuum centrifuge Cleaning and deaeration of gpla- Eastman Kodak (56.4) growing interest in using the relatively tine solut,ions inexpensive but heavy residues from Titan T.i
