A GENERALIZED SOLUTION DIFFUSION-MODEL OF THE PERVAPORATION PROCESS THROUGH COMPOSITE MEMBRANES .2. CONCENTRATION POLARIZATION, COUPLED DIFFUSION AND THE INFLUENCE OF THE POROUS SUPPORT LAYER

The central part of the model is based on the solution-diffusion proce ss which describes fluxes of low molecular weight components in the de nse active layer. Using the Maxwell-Stefan theory, coupled equations f or the diffusive fluxes through the dense layer are derived. The conce ntrations on the phase boundaries inside the membrane are determined b y solubility equilibria with the adjacent phases of the feed and perme ate phase, respectively. They are calculated using the UNIQUAC model a s described in Part I of this series. Fluxes through the porous sublay er are calculated by a general kinetic gas flow model which includes K nudsen flow and laminar flow as special cases. The solution-diffusion model and the pore flow model are combined resulting in a resistance-i n-series model for fluxes through the dense layer and the support laye r. The generalized model is applied to the pervaporation of six aqueou s/organic mixtures through the standard poly(vinyl alcohol)/poly(acryl onitrile) (PVA/PAN) composite membrane. It turns out that the strong n on-ideal solubility behavior of the liquid mixture components in PVA a nd diffusive coupling effects play the dominant roles in the pervapora tion process. More simplified models which do not account for these ph ysico-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 polariz ation has also been studied in some detail and is found to be of minor importance compared to the other contributions.