The Combined Effect of Diffusion and Evaporation on the Molecular Distillation of Ideal Binary Liquid Mixtures

Abstract

Abstract A treatment which takes into account the effects of both diffusion in the liquid film and evaporation on the rate of molecular distillation of binary liquid mixtures in the falling film and rotary cone stills has been developed. The conditions under which the rate of molecular distillation in these stills is determined by either evaporation alone or evaporation together with diffusion have been identified. The interface temperature, film thickness, and concentration profiles were obtained by a numerical solution of the system equations for both cases. For the falling film stills, the limiting case of an evaporation rate controlled process was approached when, at the inlet of the still, the ratio EI = k 1 δ 0/D was less than 0.1 for all the ratios KI = k 2/k 1 greater than 0.086, where k 1 and k 2 are the evaporation rate coefficients of the more and less volatile components of the mixture, respectively, D is the diffusion coefficient of the more volatile component, δ0 is the initial film thickness, and the subscript I refers to the conditions at the liquid film surface. For k 2/k 1 > 0.4, the above approximation was still satisfactory for all values of EI < 1.5. The rotary cone stills were found not to be affected by diffusion in the liquid film and, therefore, well represented by the evaporation rate controlling mechanism over the full range of distillation parameters investigated.