The critical time of transition to the convective mode of the unsteady-state evaporation of a horizontal layer of a binary solution into a stagnant gas is theoretically and experimentally determined. It is shown that the slow diffusion mode of evaporation can be destabilized and replaced by the convective mode if the molecular weight of one of the liquid components is higher than the molecular weight of the gas and the molecular weight of the other liquid components is lower than the molecular weight of the gas. The neutral stability curve of the diffusion mode is determined using the Navier-Stokes equation in the Boussinesq approximation, the continuity equation, and the convective diffusion equations of the evaporating components. The transition between the modes is faster than the molecular diffusion; therefore, the depth of penetration of the vapor of the light component into the gas in the diffusion mode can be taken as the linear scale of the phenomenon considered. By the Galerkin method, the neutral stability curve of the diffusion mode of evaporation is determined and the critical time of bifurcation of the mode is found as a function of the composition of an aqueous butanol solution. The theoretical and experimental data are in good agreement.

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An integrated mass-exchange process combining liquid-membrane separation with crystallization from solution is experimentally and theoretically investigated. Experiments on separation of binary mixtures of paraffins that have close properties and form mutual solid solutions are carried out. The maximal degree of separation is attained using an aqueous ethanol membrane containing 65 wt % ethanol. From hexadecane-octadecane and hexadecane-tetracosane mixtures containing 86 mol % hexadecane, crystals containing 96.3 and 99.8 mol % hexadecane, respectively, are obtained. The solubility of hexadecane, octadecane, and tetracosane in aqueous ethanol solutions of various concentrations is experimentally studied over a wide temperature range. Using the solubility data, thermodynamic calculations of the liquid-liquid and liquid-crystals equilibria are carried out for the upper and lower boundaries of the aqueous alcohol membrane. A mathematical model of the integrated process is obtained by compiling a set of equations for the diffusion fluxes of the components in the liquid membrane and expressions describing the phase equilibria at the membrane boundaries. A comparison of the experimental data on the separation efficiency with the results of the calculations shows that the diffusion model adequately describes the main features of the integrated process.