A generalized solution—diffusion model of the pervaporation process through composite membranes Part II. Concentration polarization, coupled diffusion and the influence of the porous support layer

Abstract

The central part of the model is based on the solution—diffusion process which describes fluxes of low molecular weight components in the dense active layer. using the Maxwell-Stefan theory, coupled equations for the diffusive fluxes through the dense layer are derived. The concentrations on the phase boundaries inside the membrane are determined by solubility equilibria with the adjacent phases of the feed and permeate phase, respectively. They are calculated using the UNIQUAC model as described in Part I of this series. Fluxes through the porous sublayer are calculated by the general kinetic gas flow model which includes Knudsen flow and laminar flow as special cases. The solution—diffusion model and the pore flow model are combined resulting in a resistance-in-series model for fluxes through the dense layer and the support layer. The generalized model is applied to the pervaporation of six aqueous/organic mixtures through the standard poly(vinyl alcohol)/poly(acrylonitrile) (PVA/PAN) composite membrane. It turns out that the strong non-ideal solubility behavior of the liquid mixture components in PVA and diffusive coupling effects play the dominant roles in the pervaporation process. More simplified models which do not account for these physico-chemical effects fail to describe fluxes and separation diagrams in PVA/PAN membranes. Introducing the influence of the porous support layer leads to almost complete agreement with experimental data even at elevated permeate pressures. The influence of concentration polarization has also been studied in some detail and is found to be of minor importance compared to the other contributions.