Pervaporation for the mixture of benzene and cyclohexane through PPOP membranes

Abstract

Pervaporation and sorption for the liquid mixture of benzene and cyclohexane by using poly[bis(phenoxy)phosphazene] (PPOP) membranes were characterized. Diffusivities, solubilities, interaction parameters, plasticization coefficients, and heats of sorption for benzene and cyclohexane in the membrane were determined from the vapor sorption kinetics and isotherms, which were measured gravimetrically by a continuous microbalance. Pervaporation operations offered a maximum selectivity of 3 at 35°C and the 35-50% feed concentration level of benzene. Normalized fluxes increased but the pervaporation selectivity decreased slightly with temperature. The binary transport in the membrane was modeled by the solution-diffusion model with an exponential form diffusivity to the concentration of each permeant. Coupling coefficients imbedded in the model were numerically determined by a proposed computation method. The model can illustrate not only concentration and diffusivity profiles for the permeants within the membrane, but their transport mechanism, which is consistent with the hypothesis of "two layers" in the membrane. The analysis of activation energy for transport indicates that the permeation barrier for pervaporation is reduced much more than that of the single-vapor permeation through an unswollen membrane.