Reverse Separation in Fractionation Pumps

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Abstract
Measurements by Trevoy and the writer, to be published shortly elsewhere, have shown that the relative volatilities of mixtures of liquids increase surprisingly at low rates of distillations. A consequence is that organic liquids release volatiles into high vacuum apparatus more readily than would be deduced from measurements made at usual pressures of, say, 1–100 microns. The composition of condensate in an oil‐filled diffusion pump is found to be a function of position in the pump, the lightest constituents straying preferentially to the high vacuum end. As an example, Octoil contaminated with less than 1 percent of butyl phthalate in the last boiler of a 3‐stage pump was found to pass vapor through the high vacuum entrance as though the boiler had contained 27 percent butyl phthalate. To epitomize the situation, a vapor pump is a specialized form of still with two exits. The intention of the designer, expressed in the geometry of the pump, is to force the lighter molecules to one exit but molecular perversity causes them to wander to the other. The effect is probably unimportant except where the highest vacuums are required, though it does point to the desirability of thorough prepurification of pump oils; the task of fractionation, before and during use, is harder than we had supposed.

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