TRANSPORT THROUGH ZEOLITE FILLED POLYMERIC MEMBRANES

In this paper the effect of zeolite particles incorporated in rubbery polymers on the pervaporation properties of membranes made from these polymers is discussed. Pervaporation of methanol/toluene mixtures was carried out with membranes prepared from the toluene selective polymer EPDM and the methanol selective polymers Viton and Estane 5707. From the results of the pervaporation experiments it could be concluded tha t the addition of the hydrophilic zeolite NaX as well as the hydrophob ic zeolite silicalite-1 leads to an increase in methanol flux and a de crease in toluene flux through the membranes. Pervaporation experiment s with bi-layer membranes consisting of an unfilled polymer layer and a polymer layer filled with zeolite particles demonstrated that the ef fect of addition of particles depends on their position in the membran e. Furthermore, the component flux through the membranes as a function of the volume fraction of zeolite is modelled with existing theories describing the permeability of heterogeneous materials. The results sh ow that the apparent permeability of the dispersed phase is lower than the intrinsic permeability of the dispersed phase when the flux throu gh the particle is restricted by the polymer phase. This phenomenon wa s confirmed by numerical simulation of the transport in the membrane t hrough a plane parallel to the transport direction. The simulations ar e carried out for an unfilled membrane, a membrane filled with an impe rmeable particle, a rubber particle and with a particle which shows La ngmuir sorption behaviour. The reason for the discrepancy between the apparent permeability and the intrinsic permeability is that the appar ent permeability of the zeolite phase is calculated by dividing the fl ux with the driving force over the entire membrane which is larger tha n that over the particle. In case of numerical simulation the concentr ation in every position in the plane is known and therefore the intrin sic permeability of the filler can be calculated on basis of the actua l driving force. This treatment results in a permeability which is cor rect over several orders of magnitude, (C) 1998 Elsevier Science B.V.