MOLECULAR DISTILLATION r
BENCHFOR TESTING VITAMINA CONCENTRATES Left, apparatus for making testing solutions, purification and reoovery of solvents, etc.: rzght, apparatus for determining acid values; center, Hilger vitameter in a light-tight booth into which the operator retires, pulling a curtain behind him.
‘1USEFUL
VACUITY
D.H.KILLEFFER 60 East 42hd Street, New York, N. Y.
S
TRANGE things happen in vacua. By merely moving molecules farther apart and reducing their collisions with one another, various impossibilities are moved over into the realm of industrial utility. One of these, and it is decidedly the most enticing at present, is the actual distillation of so delicate and so refractory a substance as vitamin A to remove it in pure form from fish liver oil. Already this distillation on a pilot-plant scale can produce enough pure vitamin A to supply one-tenth of the United States’ demand. But that comes later. First we must orient ourselves in the realms of nothingness and with that as a background follow out the devious paths which led to that useful result. If the story apparently goes afield, remember that that is the habit of research and not simply an ingenious device to maintain suspense. To get this story straight we must not only look into vacuity, we must consider mercury’s isotopes, failures of dielectrics] petroleum lubricants] the mildewing of photographic films in the tropics, and vitamins. All are part of this vacuous story. The beginning lies in the vacant spaces of matter. In our atmosphere there exist some lox5molecules in each cubic centimeter. Each of these myriads of molecules moves with great rapidity in a straight line until it meets another molecule. The frequency of these collisions, from which both meeting molecules are presumed to rebound in new straight lines, is so great that on the average no such straight path can be longer than a ten-thousandth of a millimeter. The intense activities of swarming bees are tame compared with those of molecules even a t ordinary temperatures However, let us suppose that our cubic centimeter of gas is ex-
panded a million fold and subsequently another million fold to a pressure of about 1 0 - 9 mm. Then the number of molecules in each cubic centimeter will have been reduced to 107, and the chances of collision very much lessened. At that point, where the average uninterrupted path of each niolecule is nearly a mile long we will have passed the limit of experimental vacua and will have reached nearly to the degree of vacancy of the tails of comets. Far beyond that region lies interstellar vacuum where perhaps a single molecule occupies a cubic centimeter. This look beyond present human possibility is taken to show that the range of emptiness is only beginning to be explored. What has now been found in the relatively dense matter a t to 10-7 millimeter of mercury absolute pressure .suggests the wealth of opportunity that lies beyond. In this region of practical vacua, from 10l2to lo8 molecules pack themselves into each cubic centimeter. In spite of that they are able to flash about the comparative vacancy of space in paths from several centimeters t o even a large fraction of a kilometer in length before colliding with one another. Obviously conditions in such surroundings are very different from those a t atniospheric pressures. This idea of giving molecules room to travel by removing others in their way appealed to Brqinsted and Hevesy as a way to separate the isotopes of mercury. By allowing molecules shot out from liquid mercury to strike cold targets placed with due regard to the lengths of their free paths as determined by their relative weights, this separation was one of the first applications of the idea which has since developed into the practical art of molecular distillation. 966
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INDUSTRIAL AND ENGINEERISG CIIEMISTRY
Later an Englishman, C. It. Burch, became interested in vscua to help in impregnating condenser dielectrics. Ordinary soaking, he found, left tiny voids in the paper or hoard which interfered with perfect performance. This led him to exhaust the air from the paper and the fluid before impregnation and thus get rid of the voids. His interest extended to the study of higher and higher vnoua until he was able to evaporate materials which were previously considered undistillahle. Ultimately he developed an apparatus in which the mast refraotory luhrieatigoilcouldbe refined by “niolecular distillation.” Because the method involved individual molecules and their uninterrupted fliKhht across short gaps, tire mutual effects of different kinds of molecules were of no consequence, and azentropic mixtures apparently did not exist. The commercial feasibility of molecular distillation was sonn rlernonstrated, and a patent wa8 issued for the process. Burch’s excursions into high Y&CUILled him to devise new piimps cnperzted hy petroleum vapors snd to design continuously evucuated transmitting equipment which ii now in daily use sending radio mezsqes from Rugby, England, to India and Australia. Our trail now leads to the laboratory of the Eastman Kodak Company where Hickmsn was studying the drying of motion picture film. In the same way that Bureh was led to vscuum in the treatment of paper, IIiekman began usuing v m u m for the dryina of film and by an independent, hut almost parellel, path continued to the evolution of vaouum pumps, pump fluids, and moleculrir stills.
It v m aoon apparent that. the niolecular still could be used to evaporate the natural fats and oils which, as a class, had always heen eonsidered entirely nonvolatile. At last nn apparatus w&s available which transferred these refraotory suhstnnces into the realm of volatile liquids and opened up a vast ficld for research and industrial development. The natural oils are more difficult to distill than petroleum residikes, hut they offergreacer revards. Their main bulk consiSts of glycerides evaporating within a narrow temperature range, together with & smaller qrlantity oi accessory substances, sterols, odors, vitamins, ete., evaporating more readily. Molecular d i p tillat,ion strips off these valuahle materials, leaving a residue of bland, nearly odorless oil. It is thus no longer necessary to saponify the entire bulk of the oil to separate the small quantity ofmcessory substances. Long before these possibilities were fully realized, vzsious oils were distilled experirnentdly. Cod liver oil gave beautiful giddcn fractions which deposited a yellow wax. Did the wax contain the vitamin? On the chance that it did a surnple was successfully assayed for vitamin D, and a patent, application u‘as lodged for the molecular distillation of vitamins. Then, onring to the press of matters photographic, the investigation was t,emporarily discontinued. The issuance of the patent for distilling vitamins brought a request for a license which revived the xvhuhole matter again. After %me negotiatinns an arrangement wa6 made, and subsequently further lots of cod liver oil were pmessed for their vitamin content in a newly built molecular still. The results were far better than anyone could have anticipated, and samples of concentrated vitamins showed exceedingly high potencis. Naturally the whole prospect changed. What had been a mere idea quickly became the center of renewed activity. New agreements were negotiated between the parties, ttnd immediat,e steps were taken to secure rights under the Burch p a t e n t which covered the ’general (Top) FEEDnxn W I T B D , , ~ ~ ~ ~ , principles o f m o l e c u l a r SYEITEXS ON PIWTM o m c m m distillation. Research was QTILlr prosecuted on a wider scale (RoUom) DISTILLING Coiums in the laboratory to iron Wmm THE I3Ic-n CONCENTRAout details. A pilot plant OF VlTAMlN ESTW?S 1.5 RECURPD was erected which proved able to treat a hundred
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INDUSTRIAL AND ENGINEERING CHEMISTRY
gallons of oil per day. Indeed, the fortunate chance which directed this effort to fatty oils instead of petroleum oils has now worked out into a thriving, growing, and useful young industry. All of this has dealt with how molecular distillation came to be. Nothing has been said about what it is. Actually it is so simple as scarcely to need description. The material t o be distilled, the distilland, is caused t o flow evenly over a heated surface. Directly opposite this is a relatively cooler surface annular in shape. Between the two is as nearly nothing as it is possible to producea very high vacuum. The distance between the heated liquid film and the condensing surface is adjusted to be less than the mean free path of the molecules to be distilled. Across that gap, molecules jump. That is all there is to it except for an arrangement to catch the distillate. The key to the matter is in the production and maintenance of a sufficiently high vacuum, which must necessarily be greater with heavier molecules whose mean free paths are shortened by their greater weight. Research on this phase of the problem is complete to the point of commercial utility but not to the satisfaction of those responsible. They still think of interstellar space and real vacuity, even though present practice is less than a mere millionth of an atmosphere. Some of the aspects of this type of distillation are interesting. Utilizing molecular projectiles as is here done involves no boiling
VOL. 29, NO. 9
point as a control, no vapor pressure, no aeeotropic mixtures. Control of the process has been accomplished by adding to the distilland a dye, already found to volatilize simultaneously with the desired material, whose color indicates its presence in the distillate. The results of applying this technic t o fish liver oils have been to remove much of their odor and to separate from them vitamin A in the natural unsaponified condition in potencies up t o 500,000 units per gram., These ester concentrates can be saponified and redistilled t o produce pure vitamin A alcohol with a potency greater than 3,000,000 U. S. P. units per gram. The technic has also shown that there are several separate vitamins in the complex we know as natural vitamin D. Already on the market as products of this operation are cholesterol, vitamin A (esters and alcohol), and odorless vitaminless oil. To these, others soon will be added. The usual question we might ask in such a situation is: What further commercial development can be seen? The present attitude is to learn more about the process before deciding that. Experimental equipment will be made available for research institutions but will not at present be licensed for commercial application. The minutiae of this high-vacuum technic and of the results obtained with it are contained in the following papers by Hickman and his associates and in others soon t o be published.
APPARATUS AND METHODS K. C. D. HICKMAN Eastman Kodak Company, Rochester, N. Y.
M
OLECULAR distillation means the transfer of vapor
from the warmer surface of a liquid to the cooler surface of a nearby condenser, the space between the two being evacuated sufficiently to prevent any obstruction of the vapor. The free path of the majority of the molecules emerging is then less than the distance between the surfaces, and distillation occurs a t the lowest possible temperatures. The method was devised by Br6nsted and Hevesy (2) for separating the isotopes of mercury, and was adopted by Burch (S), Washburn (IS), and Waterman (14)for distilling other substances. T o Burch we owe a complete new technology (4) for the purification of organic substances. The early molecular stills were of the simplest construction, consisting of two concentric vessels (2) or a vessel with a reentrant top (7). The bottom served as the boiler and the top as the condenser, and the space between the two was evacuated to about mm. The performance of such apparatus fell far short of the ideal. Under ideal conditions the separation of a mixed liquid by molecular distillation should be superior to t h a t made a t higher pressures (3, 13) I n practice this is prevented ih various ways. The distilla is usually viscous and remains unstirred because the ebullition accompanying ordinary boiling is absent. The distilling surface becomes impoverished of its more volatile constituents and ceases t o be representative of the bulk of liquid. The distillate fractions contain less of the light and more of the heavy constituents than they should. Also, the fractions alter during withdrawal from the still because part of the later condensate becomes mixed with part of the earlier as it flows slowly from the large condensing surface. These tendencies reduce the separation to less than the ideal. On the contrary, distillations a t higher pressures benefit from their imperfections because part of the distillate falls back into the boiler and is reevaporated. This secondary evaporation and the washing of the vapors which
it entails cause a better than theoretical separation and are, indeed, the bases of the a r t of fractionation. With the simple molecular stills many hours of vacuum treatment are needed t o degas the material, and, although only the surface a t any moment is involved in distillatioa, the whole bulk of distilland is maintained a t or above the distilling temperature. Thus, the process which has been atlopted to save the material from thermal decomposition may actually involve a prolonged exposure t o heat. These inherent drawbacks to the molecular pot still can be eliminated by making a still in two parts, one t o store the liquid a t low temperature, the other t o effect distillation . a t high temperature. By keeping L the mass of oil in the distilling region small compared with t h a t in the storage space, the total exposure t o heat can be diminished greatly. Furthermore, by bringing the substance a little at a time from one part of the storage space t o the distilling region a n a returning it t o another part of the storage sphce, some assurance can be gained t h a t all portions of the substance have had a n equally favorable opportunity for degassing and distillation. This form of construction is, in effect, the one used commercially (4, FIGURE 1. SIMPLB 10) in continuous molecular stills FORMOF MOLECULAR where the substance is conveyed a little a t a time over a succesSTILL
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