SORPTION AND DIFFUSION OF CHLORINATED HYDROCARBONS IN SILICALITE-FILLED PDMS MEMBRANES

Pervaporation performance of a membrane is determined as well by the sorption as by the diffusion characteristics of the permeating components in the membrane. In order to enhance the selectivity of PDMS membranes, e.g. for the separation of VOC's from water, a filler (silicalite) may be incorporated in the membrane. Vapour-sorption experiments of CI-HC's in PDMS and silicalite showed that the sorption in PDMS is determined by the hydrophobicity of the organic component, whereas for the sorption in the zeolite the molecular sieving effect is the dominant factor. As a result of this, voluminous organics are prevented from entering into the pores of the zeolite. The sorption capacity of the (un)filled membrane strongly decreases by a temperature rise from 20 degrees to 70 degrees C, whereas the sorption in the zeolite is not significantly affected. In most cases, the sorption capacity of the zeolite exceeds the one of the PDMS. With respect to the diffusion, small and light molecules, as well as molecules with a double bond, are favoured in comparison with the heavy ones. This rule applies for the diffusion both in the zeolite and in the (un)filled PDMS. The diffusion coefficient in silicalite is however a factor of 1000 smaller than the one in PDMS so that the positive effect of the filler can mainly be attributed to a kind of reservoir function for the organics in the membrane.