Molecular distillation studies of several fatty acids
Mote, Julian Francis
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The theory involved in actual distillation phenomena encountered in molecular distillation has, to the present date, been only lightly explored. Scientific research has been replaced mainly with the need for improving the practical applications ot the process. As a result, although few theoretical correlations have been evolved, the present day high-speed centrifugal molecular stills are capable of effecting separations and purifications economically impossible by any other means. Molecular distillation has found wide commercial application in the separation and purification of such normally non-distillable, high molecular weight, thermally unstable substances as vitamins A and E, industrial synthetics, plasticizers, fatty acid dimers, and the like. In investigating the theory underlying molecular distillation, only a few classes of compounds have been used; the most important of these being pilot dyes, and to a lesser extent, fatty acids. The use of the latter class of compounds has been slight, despite their ease of analysis, ready, solubility in various carrier oils, and the availability of a large number of the series for investigative purposes. It was the purpose of this investigation to molecular distill a number of these fatty acids and, from a study of the elimination curves obtained, to determine the effects of molecular weight, molecular structure, and degree of unsaturation on these maxima. The five-inch magnetically-driven centrifugal molecular still employed for the investigation was modified slightly from a previous design. Two metal diffusion pumps which were a part of the vacuum pumping system were replaced by a third megavac forepump, which was in turn connected through a manifold arrangement with the original two that were in the system. A gravity-feed system was also installed; the modification consisted of elevating the feed tank above the still head assembly, and installing a needle valve in the feed line for accurate feed rate control. Preliminary vacuum tests were roads on the modified still, and the development of a synthetic constant-yield oil for use in the investigation was undertaken. From a number of distillation tests made on individual and blended mixtures of various petroleum oils, it was found that a blend of 225 milliliters of Voltesso No 36, 175 milliliters of light mineral, and 425 milliliters of heavy mineral oil distilled in approximately constant volumetric quantities over the temperature range of 86 to 146 degrees Centigrade. The development of a constant-yield oil led to the standardization of an operating procedure which was to be used for all the tests. The operating variables that were held constant for the distillation studies included: feed rate 60 to 65 milliliters per minute; operating pressure, 12 ± 2 microns of mercury; feed-residue temperature difference, 24 ± 1/2 degrees Centigrade; rotor speed, 1000 ± 50 revolutions per minute; condenser water temperature, 45 ± 1 degrees Centigrade; and one pass of the feed mixture across the rotor per fraction. Six saturated fatty acids, pentadecylic, palmitic, margaric, stearic, nonadecylic, and arachidic; one monoethenoid, oleic; and one diethenoid, linoleic, were then individually distilled using the standardized operating procedure devised in the investigation. The elimination curves were plotted and the maxima were found to be 100, 102, 110, 112, 127, 122, 110, 108 degrees Centigrade, respectively. These maxima were then compared to determine what effect molecular weight, molecular structure, and degree of unsaturation had on the maxima. It was concluded from this investigation that the addition of two CH₂ groups in the molecule of a homologous series of long-chain even numbered carbon atom aliphatic fatty acids raises the elimination maximum 10 degrees Centigrade. It was found that no valid prediction as to the effect of the addition of one CH₂ group to a homologous series of long-chain aliphatic fatty acids can be made. The study indicated that molecular structure; that is, chain length, has a definite, but unpredictable influence on the elimination maximum. It was also found that the addition of each unconjugated double bond in a molecule of a homologous series of fatty acids lowers the elimination maximum two degrees Centigrade.
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