On the driving force of methanol pervaporation through a microporous methylated silica membrane

The pervaporation transport of methanol through an amorphous microporous methylated silica membrane was studied experimentally and analyzed through modeling within a Maxwell-Stefan (MS) framework. The experimental conditions cover a temperature range of 60-155 degrees C and absolute pressures up to 16 bar. Exerting higher absolute pressures on the liquid feed-side of the membrane did not lead to enhanced fluxes, confirming that (i) the selective layer of the 56.8 cm(2) membrane was a closed layer and defect-free, and (ii) the chemical potential gradient of the permeating component is an appropriate driving force to describe the transport through the microporous membrane. Both the adsorption isotherm and the heat of adsorption (-Delta H(i,)ads) were determined, and a two-site Langmuir isotherm adequately correlated the heterogeneous adsorption behavior of methanol in amorphous silica. Different levels of detail were adopted for modeling the diffusion transport through the selective layer, after having accounted for the resistance of the different support layers. Two models based on the MS approach described best the data: one model had a constant diffusivity and the other model had a loading-dependent diffusivity. The latter is equivalent to the classical pervaporation model, where the flux is proportional to the fugacity difference over the membrane and an exponential temperature dependency of the permeance. The presented derivation eliminates the inconsistencies of earlier interpretations given in the literature. Although there is a slight preference for the latter, easy-to-use model, no further statistical discrimination could be made based on the data.