CENTRIFUGATION mg UNIVERSIlY
JAMES 0. MALONEY OF KANSAS, LAWRENCE, KANS.
M
ORE detailed information on the basic features of scale-up, the cost of equipment, and the fundamentals of isotope separation by centrifugal action have become available during the past year. Continued publication of such information will materially assist the chemical engineer in making preliminary designs and estimates on centrifugal equipment. Dickey and Bryden (6) have given a general discussion of the theory and practice of centrifugal separation and have described the application of centrifuges to the chemical industry. This book, together with those of Perry (15) and Riegel (16), furnish a good general description of the field of centrifugal equipment. Smith (26) has presented information on the types of centrifuges, the principal manufacturers of such equipment for the process industries, and the semiempirical test methods for selecting the proper type of apparatus. Test equipment used for preliminary selection of centrifugals includes a bottle centrifuge, a 12-inch batch centrifuge with both perforate and imperforate bowls, and a high speed, small diameter laboratory centrifugal. Detailed instructions are given on the methods for conducting the tests. Smith’s principal contribution of a fundamental nature involves the development of a method for using data obtained on the draining rate as a function of cake thickness in a 12-inch-diameter perforate-bowl machine t o predict the capacity of a large diameter machine. He also shows how the washing time required to reduce the soluble material in the cake in a large diameter centrifuge t o a definite value can be estimated from small scale tests. He also shows that an exponential equation is useful for extrapolating the draining rate information to thick cakes. The information contained in this article, coupled with that of Eckhardt (‘7) on the cost of centrifugal separators, will permit preliminary economic analysis of a liquid-solid separation problem. Eckhardt presents the cost of both suspended and underdriven basket-type centrifugals as a function of construction, basket diameter, type of drive, and unloading mechanism. For suspended units the basic cost ranges from $1200 to $4600 for steel units of 1to 11cubic feet of basket capacity. The maximum cost of a unit with the most expensive special features--namely, a stainless steel unit with a mechanical unloader and an explosionproof motor-would range from $2100 t o $8150 for a basket capacity of 1 to 11 cubic feet. The underdriven basket centrifugals are less expensive than the suspended types in the larger capacity. Basic costs range from $1200 t o $2700 for steel units of 1 t o 10.5 cubic feet capacity. Unfortunately the centrifugal force developed by the various standard machines is not given in this article. Typical cycle times for a 40-inch-diameter basket operating a t an unstated speed on a variety of materials are from 2 t o 30 minutes. Prices for the ter hleer automatic and continuous centrifugal and the Bird continuous solid-bowl machine are given. High speed centrifugal units vary widely depending upon the job to be donc. Ranges of operating and initial costs are given. Gernigon (9) presents descriptive material on various types of clarifiers and separators. He shows how the location of the dams on a liquid-liquid centrifuge can be calculated and finally lists a number of applications. Lyons and Johnson (11) have an extensive article on the application of the Bird continuous centrifugal t o classification of cement slurry, closed-circuit grinding, and classification. They point out that the advantages of a continuous centrifugal
include the possibility of finer particle separations, more reproducible products, a minimum of labor costs, classification which is readily controlled, and concentrates which frequently issue drier than from a filter. The disadvantages include high initial investment, continuous power cost, intelligent mechanical supervision, and under certain conditions, wearing of the solids conveyer mechanism. Corrigon (6) discusses the application of centrifugals t o oil purification. He defines two types of capacity, throughput and effective, the former being the total capacity of the machine irrespective of the degree of purification required, and the effective capacity being the quantity of oil the unit will process at the required purity. He states that mixing water with the oil and “wet centrifuging” assists in washing out heavy impurities, in removing water-soluble materials, and in reducing the corrosive action of certain materials. There are disadvantages for such operation which include possibility of emulsion formation, loss of special additives either by extraction or concentration at the water-oil interface, and loss of oil if the water seal breaks. Andersson ( I ) describes the purging and washing of sugar magmas. He reports that purging increases with increased speed, but beyond ten minutes of wringing not much additional effect results. Smart (23) presents the results of centrifuging a molasses in an alpha-Lava1 type centrifuge. Boruff ( 4 ) gives a brief description of the by-products recovered from alcohol fermentation process using a solid bowl-suspended basket centrifuge for clarifying screen stillage. A centrifugal for the recovery of barite from rotary drilling mud has been reported ( I S ) . In this operation the mud is first treated to remove the large size sand particles by screening, then the mud and water mixture is sent t o the centrifuge in which the barite is separated from the lighter material and stored for further use. Recovery of barite is from 80-90%. Another type of centrifugal manufactured by the same company has been used for corn starch separations. Smith (95) has written a brief review on new centrifugal equipment. The countercurrent gas centrifuge has been considered by Benedict (3). It is reported that this type of unit waa successfully developed for the separation of uranium isotopes. Benedict points out that this method of separation differs from usual diffusional separations in that the enrichment factor is independent of the molecular attraction between the molecules or the properties of the diffusion barrier, and that it is proportional to the difference in molecular weight between the two isotopes. He develops equations for the minimum work t o circulate the process gas and the minimum downflow. The serious disadvantage of such a unit is the low capacity of a single centrifuge. About 19,000 centrifuges would be required to produce 100 grams of C13 per day a t 90% concentration. He states that the method may be useful in the separation of heavy gases of widely differing molecular weights. Design Patents. Tho11 (99) describes a batch centrifugal suitable for operation under vacuum or pressure or in an inert gas atmosphere. The unit is equipped with a sealed unloader for intermittent discharge of the solids. A sealed skimmer pipe is also described. Terhune (98) has patented a double-bowl separator in which the lighter material passes over the rim of the inner bowl and the heavier material over the rim of the outer bowl. 8
January 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
Strzynski (87) has arranged outlets for discharging solids through nozzles tangentially t o the bowl in such a manner that they jet backward relative to the direction of rotation. This results in a reduction in the power which is required to drive the unit. Sharples (88) has devised a scheme for separating solids from liquids in a perforate basket, conducting the solids away and evaporating the liquid with the solids by a stream of gas. Schutte and Mack (18) have a method for separating B light solid from a heavy liquid and continuously discharging the solid. Schutte (17) and McCurdy (18) have also obtained patents on centrifugal separators. Process Patents. The drowning of nitrocellulose with water and the separation of the waste nitration acid and the wash water are described by Peanon and Askcraft (14). An apparatus equipped to remove the gases from solid-liquid mixtures in order to hasten the settling of the solid particles in a centrifuge has been devised by Flowers (8). Kopplin (10) has patented a method of washing sugar in a centrifugal by wing intermittent spraying. There is a switch mechanism for achieving the intermittent action. A method for separating blast furnace slag from water has been patented by Bartholomew (8). Continuous soap manufacture employing centrifugal equipment has been patented by Sender (19, BO). Sender has also patented a device (81) for discharging a viscous effluent from a. centrifuge. Smely (84) has mtented a centrifugal apparatus adopted for sugar processing. It is known that fundamental investigations of the behavior of centrifugal equipment are under way in universities and in industry. It is hoped that the results of such studies will be made available in the near future.
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LITERATURE CITED (1) Andersson, G., Socker Handl., 2, 379-406 (1946). (2) Bartholomew, T., U. S. Patent 2,422,464 (June 17, 1947). (3) Benedict, M., Chem. Eng. Progrees, Trans. Am. I d . C h . Engrs., 43, 41-60 (1947). (4) Boruff, C. S., IND.ENQ. CHBM.,39, 602-7 (1947). (5) Corrigon, B., Iron Age, 159, 66-62 (1947). (6) Dickey, G. D.. and Bryden, C. L., “Theory and Practice of Filtration,” New York, Reinhold Pub. Corp., 1946. (7) Eckhardt, H., Chem. Eng., 54, 121-3 (1947). (8) Flowers, A. E., U.S. Patent 2,417,747 (Mar. 18, 1947). (9) Gernigon, E., Chimie & industrie, 54, 382-8 (1945). (IO) Kopplin, F. W., U. 8. Patent 2,418,776 (Apr. 8, 1947). (11) Lyons, 8. C., and Johnson, A. L., Am. Inst. Mining Met. Engrs., Mining Technol., 11, No. 4; Tech. Pub. 2195 (1947). (12) McCurdy, H., U. S. Patent 2,425,110 (Aug. 5, 1947). (13) Memo Centrifugal Co., “Recovery of Barite from Rotary Drilling Mud by Centrifugal Separation,” San Franafsco, Calif. (14) Pearson, J. D., and Askcraft, D. C., Brit. Patent 578,691 (July 9, 1946). (15) Perry, J. H., “Chemical Engineers’ Handbook,” New York, McGraw-Hill Book Co., 1941. (16) Riegel, E. R., “Chemical Machinery,” New York, Reinhold Pub. Corp., 1944. (17) Schutte, A. H., U.S. Patent 2,398,967 (Apr. 23, 1946). (18) Schutte, A. H., and Mack, A. W., Ibid., 2,394,015 (Feb. 6, 1946). (19) Sender, L., Ibad., 2,411,468 (Nov. 19, 1946). (20) I W . , 2,411,469 (Nov. 19, 1946). (21) I W . , 2,412,099 (Dec. 3, 1946). (22) Sharplea, L. P., IbM., 2,409,713 (Oct. 22, 1946). (23) Smart, G. S.,Intern. Sugar J., 48, 293-6 (1946). (24) Smely, V., U. 8.Patent 2,416,073 (Feb. 18, 1947). (25) Smith, J. C., C h .Id.,61, 417-18 (1947). (26) Smith, J. C., IND. ENQ. CHEM.,39, 474-9 (1947). (27) Strzynski, G. J., U. S. Patent 2,410,313 (Oct. 29, 1946). .(28) Terhune, C. F., Canadian Patent 435,913 (July 23, 1946). (29) Tholl, J. F., U. 5. Patent 2,396,622 (Mar. 12, 1946).
RECEIVED November 8, 1947.
CRUSHING AND GRINDIN6 LINCOLN T. WORK, METAL a THERMIT CORPORATION, RAHWAY, N. J,
T
HE report presented last year (98) referred to the relatively slow progress which takes place in this old unit operation. . The importance of the growing knowledge of measurement was also stressed, since, particularly in the subsieve range, this gives information needed for exact analysis of the operation. Progress during the past year is not great, but it is substantial. Methods of Measurement. Several survey papers have been published. McCabe (13) presented a classification of methods, giving a brief and illuminating discussion of each type. He showed the increasing importance of the electron microscope and of the x-ray scattering by fine grains. Vernon (86) discussed various sedimentation, centrifuging, and microscopic methods in a brief paper of somewhat lesser scope. The Road and Building Materials Group of the Society of Chemical Industry and the Institution of Chemical Engineers held a joint meeting on February 4, 1947, a t which was presented a symposium on particle sire measuremenbs. Heywood (8) presented a paper on scope of analysis and standardization, giving limits of application and a somewhat detailed description of various methods. Davies (8)presented an extensive study of the sedimentation of small suspended particles, showing a treatment for spheres, flat particles, and clouds. He discussed the effecta of Brownian movement and diffusion, and the problem of accurate sampling from air streams. Gregg (6)discussed adsorption and
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heat of wetting methods for measuring surface erea. He observed that adsorption of gases yields better than relative values. Although there are problems in adsorption from liquids and in heat of wetting measurements, there are also elements of promise. Lea and Nurse (11)dincussed permeability methods indicating the practicability of the method in spite of some difficulties in securing absolute values. Skinner and Boas-Traube (81) discussed light extinction methods, including the effects of particle concentration and of length of light path on light extinction. Walton (87) showed the application of electron microscopy, recognizing that present instrumentation is reaching its limit of resolution and that improved performance in present instruments will be made. These papers are a11quantitative in their approach, and a wealth of data is contained in them. A second session gave the industrial applications. This included papers by Stairmand (93) on practical aspects, Newman (18) on paint pigments, Schofield and Russell (19) on soils, Smith (2%) on the radio industry, and Whipp and Bernhardt (88) on bitumen emulsions. Other work on spec& aspects of measurement is briefly cited. Johnson and La Mer (10)discussed light scattering by sulfur sols. DeVore and Pfund (S), working with pigment materials in various media, presented some interesting data on the spectral transmission of Alma of dielectric powders having uniform particle size.
