ON THE MECHANISM OF SEPARATION OF ETHANOL/WATER MIXTURES BY PERVAPORATION

A solution--diffusion model for the permeation of liquid mixtures through polymeric membranes taking into account coupling of fluxes has been developed. The model is applied to the separation by pervaporation of ethanol-water mixtures through cellulose acetate. In order to determine the activities of the permeating components in the poly-meric membrane, values of polymer-liquid and liquid---liquid interaction parameters are needed; polymer-liquid interaction parameters have been determined from swelling ex-periments and liquid-liquid interaction parameters have been calculated from excess free energy of mixing data taken from the literature. Concentration profiles of water and ethanol in cellulose acetate membranes have been calculated using (a) apparent concentration independent diffusion coefficients, and (b) diffusion coefficients with exponential concentration dependence and two adjustable parameters. It is discussed that the transport of ethanol-water mixtures by pervaporation cannot be explained by using concentration independent diffusion coefficients