HIGH VACUUM m$
KENNETH HICKMAN
136 PELHAM ROAD, ROCHESTER I O ,
N. Y.
During the 2 years since the last appearance of this review (33), little strikingly new has emerged, though this is no indication that significant developments have not occurred in classified projects. References to vacuum distillation are fewer but vacuum metallurgy and coating of metals and salts b y evaporation and spluttering are on the increase. M u c h attention has been ,given to electrical vacuum gages.
open-path still is represented by a multiplate fractionating assembly,
apparently intended for the fine separation of heavy petroleum residues, where the distilland falls down a heated inclined plane while the distillate is caused to climb upward over a plurality of cooled rollers which transfer intermediate fractions to the distilland a t appropriate positions (67). S7ariousimprovement patents (56, 37) have been issued for the centrifugal short-path still, of which perhaps the most significant covers the use of magnetic braking (77) to heat the rotor, This reduces loss of radiant heat to the casing but transfers the relatively large thermal load to an oversize driving motor, An important paper of monograph proportions by Booy ( 9 ) , emanating from H. I. Waterman’s laboratory in Delft, traces the development of repetitive multiplate distillation and then describes an improved horizontal fractionator which employs an internal tubular condenser fitted with backward-slanting delivery rods to return distillates one or more places upstream. Performance data and theoretical treatment are included. The struggle to achieve an equilibrium type fractionating column of reasonable throughput, low pressure drop and, hence low thermal hazard, continues unabated, and worth-while improvements are noted, though perfection is not yet in distant view. The wetted wall (56) and spinning band (78) column with narrow clearance, which have shown such phenomenal S U C C ~ S Sa t normal pressures, lose both power and throughput under high vacuum. An open-path column with sufficient clearance for tenuous vapors has previously been described (IS) in which a heated jacket continuously revaporized and returned the charge to a central, cooled condenser which rotated and again flung the distillate to the hot walls. Since reflux was continuously traveling downwards in the form of unvaporized residues and vapor was continually passing upward to regions of lower pressure, some fractional distillation was secured. This arrangement, devised independently by two experimenters, spent 8 years in the patent office before a patent was finally awarded (38)! A missing feature was added by a third team (66) who proposed keeping the hot walls continuously wetted and agitated by a brush (91) fastened to the cold rotor. The conundrum-how to equilibrate with vapor without the use of vapor pressure-still invites the ingenious answer. A major factor contributing to the difficulties of this type of research is the lack of a selection of reliable test fluids, grouped chiefly in pairs, which will obey Raoult’s law or a t least exhibit a constant relative volatility over the usable range of compositions. It is audacious, to say the least, to attempt research in multiple redistillation without precise information about a single distillation. A team in the Kodak laboratories set out in 1949 on a general exploration of this subject with the pair of fluids, 2-ethyl hexyl phthalate and %ethyl hexyl sebacate, and the results have recently appeared (78, 7 4 ) . A basic question is whether the t v o liquids mentioned and other common fluids exhibit an ac-
P
ERHAPS the most noteworthy events in the past 2 years were the launching of the magazine Vacuum and the sponsoring, for the first time, by the American Institute of Chemical Engineers, of a convention devoted to high vacuum processes and methods. That the usage of vacuum m-as increasing, in spite of lack of front page news, was evidenced by the comprehensive advertisements (66) of one leading company (Figure 1) and the immense size of pumping equipment displayed by another (18) The diversity of pumps, coaters, and freezedrying equipment offered a t the Physical Society’s Exhibition (28) (London) was testimony to the interest and competence alive in other countries. I n January 1951, SV. Edwards & Co., London, manufacturers of vacuum equipment, issued the first number of a quarterly journal devoted to, and entitled, Vacuum. Similar in format to the French journal, Le V i d e , but with less orientation toward electronics, the new magazine is important because of the broadly planned series of abstracts that it inaugurates. Four main classes of subject: 1. General science and engineering 2. Vacuum apparatus and auxiliaries 3. Vacuum processing techniques 4. Special subsidiary subjects
are subdivided into 39 sections, of which 25 were occupied in the first issue. The abstract pages are perforated for loose leaf storage. The introductory article in the main text by Andrade ( 4 ) was follon-ed by other reviews on freeze drying and film coating, all of high literary caliber. Subsequent issues have maintained the standard, particularly with general surveys, but the journal has not yet attracted many original papers and publication has been delayed. The meeting of the American Institute of Chemical Engineers at French Lick, Ind., May 11 to 14, 1952, had been well planned and it attracted upward of 300 chemical engineers and physicists. The scope of subject was considerably wider than the sponsoring society’s title but created a much-needed awareness of the ramifications of low pressure research and industry. Three papers converging on supersonic flow ( 6 0 ) , high altitude wind tunnel testing (63), and flow visualization ($6) of aircraft models, were particularly noteworthy. The present resort to steam ejectors energized by boilers of multithousand horsepower for creating the supersonic draft in tunnels with diameters measured in inches suggests that the invention of a better method would be welcomed. An equally useful convention which took place 22 months earlier but was not covered in our last review, was the symposium on vacuum physics (46) in Birmingham, England. Pumps (as),design of large vacuum systems (62), vacuum desiccation ( 7 ) , vacuum film coating (&), and vacuum metallurgy (59) were described by well-known authorities. Passing now to individual elements of technology, the truly 44
January 1953
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INDUSTRIAL AND ENGINEERING CHEMISTRY
commodation or evaporation coefficient of unity in the high vacuum still. Relatively unimportant a t ordinary pressures, where net evaporation is necessarily a small fraction of the gross traffic a t the liquid vapor interface, the evaporation coefficient may become the limiting factor in the open-path still, not only of rate of distillation but of composition of distillate. Some results of these researches were clear cut. The clean purified liquids, with surfaces less than 0.1 second old, showed coefficients of unity when rates of evaporation were below 2 grams per meter of liquid surface per second (40). Stagnant liquids, such as ordinarily exposed in pot stills, often showed coe5cients far less than unity and the surfaces would separate into two zones designated “torpid” and “working” by the author (34). The repression of evaporation was associated chiefly with the torpid area. While ordinarily the torpidity persisted indefinitely, and was shown to beset most crude liquid mixtures, it could be eliminated by the simple act of overflowing the surface during distillation. The low coefficients associated ivith the torpid habit suggested that a relatively large area (500 to 1) should be exposed for evaporation when generating vapor for measurements of equilibrium pressure (41). The authors described a rotating barrel tensimeter for this purpose. They also found that the composition of emergent vapor varied according to whether the liquid surfaces were converging or diverging; a was higher from an expanding surface. The data gathered in this work (74) have been assembled in charts (41) which allegedly display the value of one theoretical molecular (projective or short-path) plate over an extended range of temperatures from the mixture ethyl hexyl phthalate and ethyl hexyl sebacate named above. The likely performances of various stills are marked on appropriate areas of the charts. Valuable chiefly as a preliminary approach, the work stems back theoretically to the Langmuir-Knudsen rate equation and to the high vacuum tensimeter in the measurements of p in that equation. These tensimeters (39, 76) have generally utilized a small stagnant pool as the source of vapor and are thus themselves now suspects as standard instruments until redesigned. Vacuum metallurgy has been the subject of many patents and papers but the review or how-to-do-it article predominates. Important among these is the summary by Kroll (49) of true metallurgical processes (excluding analysis by vacuum fusion, and vacuum coating). The 88 references show that all the common metals, alloys, and metallic salts have been surveyed for reduction or purification in vacuo, Tables give the temperature a t which a commercially acceptable distilling pressure of 1.8 mm. is reached. Titanium is conspicuously absent from the discussion. High purity iron (44)has been produced in 25-pound batches by melting in vacuo and holding a t 1 micron for some hours, flushing with hydrogen and pouring a t 5-om. pressure of hydrogen. The properties of high chromium steel are reported as greatly improved by vacuum melting (24). The production of zirconium by the Kroll and Van Arkel processes are reviewed and properties of the metal are given in foil and sheet form (63). The vapor pressures and rates of evaporation of titanium have been measured over an extended range (14) and data for tantalum (92) and zirconium are also available (71). Fusion of metals on the laboratory scale is described for a furnace utilizing 50 to 725 watts over a temperature range of 850’ to 2000’ C. (8). Zinc has been separated from silver and lead in the pilot plant distillation of Parkes process crusts a t pressures of 10 to 30microns (70). On a much smaller scale sodium has been distilled from aluminum to provide an analytical method (55). The rates (69) of evaporation of zinc and the pressure (76)of vapor over zinc-silver alloys have been determined. The reduction of barium oxide in vacuo by carbon has been explored over a considerable range of temperatures and equilibria (11). A vacuum furnace has been described for preparing single crystals of metals (6%’).Another small high temperature vacuum furnace is noted (8).
45
The chief news concerning vacuum coating of metals, nonreflecting salts, and protective layers is the increased scale of operations (8, 30, 31). General reviews are less in evidence (16,43, 67, 68) than original papers. Thus the reflectivity (50) of thin films has been examined; also the electrical resistivity of antimony films (54). Aluminum films deposited by evaporation were oxidized anodically and the dielectric strength was determined (58). The effect of thickness on gettering properties of barium has been examined for evaporated films ($7). Graticules have been produced by deposition of geometric film patterns (1). The evaporative coating of rolled sheet material
COURTESY NATIONAL RESEARCH CORP
Figure 1.
Vacuum Fusion Gas Analyzer
has been improved by provision of multiple chambers for continuous degassing (79). Constructions for the removal of moisture from the sheet preliminary to coating have also been patented ( 3 ) . During 1952 the very large vacuum pump emerged for inspection, operated by oil for metallurgical and nuclear applications (18) and by steam for evacuating and driving supersonic wind tunnels. A curious effect (35) has been discovered which applies to all oil-operated diffusion pumps. Oils are necessarily mixtures or become so during use, and the constituents have a range of volatilities. The range increases enormously when the rate of evaporation declines, as it does toward the entrance of the pump, and this allows the contaminants to stray into the high vacuum manifold. There is thus a contest between the ability of the stages in a fractionating pump to move the volatiles forward and the propensity of the contaminants to wander backward toward the low temperature end. In this general connection an important study(51) has beenmade of various fractionating pumps and pump oils. Minor alterations of construction, alteration of cooling pattern and wattage input profoundly affected ultimate performance. Silicone oils retained their preeminent position and tri-o-cresyl phosphate, contrary to previous reports, operated excellently. An entirely new principle of fractionation pumping has just been announced ($3) by the same group. A still for preparing oil for diffusion pumps is said to produce a filling which will not show separation in a three-stage pump (6). Designs for two high speed diffusion pumps are offered in an Atomic Energy Commission publication (64). The molecular drag pump has
INDUSTRIAL AND ENGINEERING CHEMISTRY
46
received generalized mathematical treatment ( 4 7 ) , and a method of computing mechanical pump speeds has been offered ( 3 2 ) . Gages to measure low pressures now include diaphragm instruments. Where the sensitivity must remain high (17), electrical inductance is used in place of mechanical coupling. For slightly higher pressures the diaphragm actuating a dial needle (15, 46, 61) is available. A novel variation uses pulsating sylphon bellows (48). For the many detailed improvements in electrical gages, reference is made to the abstract section S o . 20 in Vacuum. We mention here only the ionization gage for very low pressures, which has a wire collector (6) instead of the usual plate to minimize the effects of x-ray. In the field of water removal, which includes freeze drying of plasma, tissue, and other biological specimens, and the low temperature concentration of fruit juices, an outstanding article-contained in two issues of Le V i d e (19, 20)-has been contributed by Dunnoyer, who adorns a theory of desiccation with an extended mathematical treatment. rl review of freeze drying is given by Harris in Vucuum (29). The use of dielectric heating in the vacuum cabinet is described in a United States patent (10). Continuous removal of ice from the condenser is secured by making the latter a stream of chilled oil ( 1 2 ) . NOTE. Owing to circumstances beyond the control of editor or reviewer, the assignment for this review was not made until 5 weeks before the due date. Complete coverage has not been accorded except in certain areas of vacuum distillation. LITERATURE CITED
(1) Aitchison, P. M., and Aitchison, R. E.,Australian J . A p p l . Phys.,
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1,75-9 (1950). Alberman, K. B., J . Scz. Instr., 27, 280-2 (1950). Alexander, P., Brit. Patent 639,099 (1950). *4ndrade, E. N. da C., V a c u z m , 1, 3-10 (1951). Aeam and Ortel, Le V i d e , 6, 1063 (1951). Bayard, R. T., and Alpert, D., Rev. Sc%.Instr., 21, 571 (1950). Beckett, L. G., J . Sei. Instr., Suppl. 1, 66-8 (1951). Benson, N. C., Hass, G., and Scott, K . W., J . Opt. Soc. Amer., 40,687-90 (1950). Booy, H., “De Moleculaire Destillatie AIS Hulpmiddel Bij Het Ondereoek Van Aardolieresiduen,” July 9, 1952. Bradbury, S., U. S. Patent 2,512,991 (1950). Brown, C., J . Chem. P h y s . , 18, 1311-13 (1950). Brown, G. H., and Hayler, C. N., U. S. Patent Application 536,491 (1949). Byron, E. S., Bowman, J. R . , and Coull, J., IND.ENG.CHEM., 43,1002-10 (1951). Carpenter, L. G., Proc. Phys. Soc., 64, 57-66 (1951). Clarke Instrument Co., C h e m . Eng. N e w s , 28, 2463 (July 17, 1950). Defour, C., L e V i d e , 5,837-41 (1950). Dibeler, V. H., and Cordero, T . , J . Research X a l l . Bur. Standards, 46,l-4 (1951). Distillation Products Industrics, Advt., Business Week (March 29,1952). Dunnoyer, L., Le V i d e , 6,1025-40 (1951). Ibid., pp. 1077-90. D’Yarmett, E. C., U. S.Patent 2,040,837 (May 19, 1936). Edwards, J. W., Johnston, H. L., and Blackburn, P. E., J . Am. Chem. Soc., 73,172-4 (1951). Edwards Co., W., Advt., Nature, 170, LXXXVIII (Aug. 9, 1952). _.__
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Handbook of Scientific Instruments and Apparatus, 1 Lowther Gardens, Prince Consort Road, London, S.W.7, England, The Physical Society, 1952. Harris, R. J. C., V a c u u m , 1, 11-25 (1951). Hartman, A. H., Plastics Industry (April 1952). Hartman, A. H., and Schneider, M., Metal Finishing (September, 1951). Henry, R., Le V i d e , 5, 859-65 (1950). Hickman, K., IND. EIW. CHEM.,43, 68-9 (1951). Hickman, K., Ibid., 44, 1892-1902 (1952). Hickman, K., Rev.Sci. Instr., 22, 141-6 (1951). Hickman, K . C. D., U. S. Patent 2,538,967 (Jan. 23, 1951). Ibid., 2,554,703 (May 29, 1951). Ibid., 2,609,335 (Sept. 2 , 1952). Hickman, X., Hecker, J., and Embree, N., IND.ENG.CHEM., 9.264 11937). Hickman; K., and Trevoy, D. J., Ibid., 44, 1882-8 (1952). Ibid., pp, 1903-11. Holland, L., J . Sci. Instr., Suppl. No. 1, 59-62 (1951). Holland, L., V a c u u m . 1,23-36 (1951). Hopkins, B. E., Jenkins, G. C. H.. and Stone. J. E. N.. J . I r o n Steel I n s t . (London), 168, 377-83 (1951). Inst. of Physics of Birmingham, Midland Branch, Symposium, June 27-8, 1950. Instruments, 23,1136 (1950). Jacobs, R. B., J . A p p l . Phys., 22, 217 (1951). Koll, R., Havens, R . , and La Gow, H., Rev. Sci. Instr., 21, 596 (1950). Kroll, W. J., V a c u u m , 1, 163-84 (1951). Kuhn, H., and Wilson, B. A., Proc. Phys. Soc., 63B, 745-55 (1950). Latham, D., Power, B. D., and Dennis, N. T. M., Vacuum, 1, 97 (1951). Lazarus, D . , and Chipman, D. R., Rev. Sci. Instr., 22, 211-2 (1951). Leonard, B. R., and Vivien, J. N., t o be published in Chem. Eng.
Progr. Leverton, W. F., and Dekker, A. J., P h y s . Rev., 80, 732-6 (1950). Light Metals, 14,149-50 (1951). Lindsey, E. E., Kiefer, J. M., and Huffine, C. L., IND.ENG. CHEM., 44,225-30 (1952). Lomas, J., M a c h . Lloyd, 22, 87-92 (1950). Lomer, 1’. D., Proc. Phys. Soc., 63B, 818-20 (1950). Malcolm, E. D., J . Sei. Instr., Suppl. No. 1, 63-6 (1951). Maslach, G. J., to be published in Chem. Eng. Progr. Matheson, H., and Eden, M., Rev.Sei. Instr., 19, 502 (1948). Millen, T. S., J . Sci. Instr., Suppl. KO.1, 7-10 (1951). Miller, G. L., Murez Ltd., Rm., 1, 184-96 (1951). Natl. Nuclear Energy Ser., Div. V, 3, “Miscellaneous Physical and Chemical Techniques,” (Los Alamos Project), 252-82, 1952. Nat!. Research Corp., Chem. Eng. News, 30, 1418-9 (1952). Perry, E. S., and Mansing, F. J., U. S.Patent 2,539,699 (Jan. 20,1951). Robinson, T., Ibid., 2,586,717, Rosenblatt, D., Mach. Design, 23, 141-4 (1951). St. Claire, €1. W.,and Spenlove, M. J., J . Metals, 3, 1192-7 (1951 ) Schlechtcn, A. W., and Doeling, R. F., Ibid., 3, 327--30 (1951). Skinner, G. B., Edwards, J. IT., and Johnston, H . L., J . Am. Chem. Sac., 73,174-6 (1951). Stoll, C., U. S.Patent 2,384,500. Trevoy, D. J . , A n a l . Chem., 24, 1382-3 (1952). EYG.CHEM.,44, 1888-92 (1952). Trevoy, D. J., IND. Underwood, E. E., and Averbach, B. L., J . .Metals, 3, 1198-1202
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fl9)Al).
Veihoek, F. H., and Marshall, A. L., J . Am. Chem. Soc.. 61, 2737 (1939). White, A. B.. and Jacobs, R. B., Distillation Products Industries, Brit. Patent 635,436 (1951). Williamson, L. J., J . A p p l . Chem., 19, No. 1, 33-40 (1951).
