PREDICTION OF FLUX AND SELECTIVITY IN PERVAPORATION THROUGH A MEMBRANE

Flux and selectivity in pervaporation through a membrane can be predicted via the solution-diffusion mechanism by the use of solubility and diffusivity models. The solubility model developed takes into account the free energy contributions in the mixture system of multicomponent penetrants and a polymer from combinatorial-entropy, free-volume, interactional-enthalpy, and elastic factors. The diffusivity model developed is the hybrid model that combines the key features of the foe-volume and molecular models. In the hybrid model, penetrant molecular thickness and polymer molecular parameters are used to determine the term equivalent to the preexponential factor of the free-volume model, and the free-volume expression is used to relate penetrant diffusivity to penetrant size and the free volume of the mixture system including concentration contributions from the multicomponent penetrants. Both the solubility and diffusivity models have been verified with the sorption of single-component aromatic vapors in a semicrystalline polyethylene film. Based on the single-component solubility and diffusivity model parameters determined from the sorption, the prediction of flux and selectivity is in line with experimental data obtained from the pervaporation of a mixture of toluene, p-xylene, and mesitylene through the polyethylene him at various temperatures and different permeate pressures. The effects of temperature on flux and selectivity are investigated via both the model prediction and experiments. Also predicted are the effects of permeate vapor pressure on flux and selectivity, and the concentration and diffusivity profiles across the membrane.