Influence of capillary condensation effects on mass transport through porous membranes

Abstract

The importance of capillary condensation effects in connection with properly quantifying the rate of mass transport through porous media has been already described (J. Coll. Interf. Sci. 110 (1986) 544; J. Membr. Sci. 66 (1992) 259; J. Membr. Sci. 182 (2001) 91). However, there is still not enough work done in order to understand, quantify and possibly exploit this phenomenon. In this work permeation and binary diffusion experiments have been performed at ambient temperature in combination with measurements of the adsorption isotherms using porous Vycor glass possessing a mean pore radius of approximately 4 nm. Main focus was set on a comparison of the transport of (a) permanent gases (helium, nitrogen, methane and hydrogen) and (b) condensable gases (butane, propane and freon 112) as a function of the pressure conditions in the membrane. The adsorption isotherms of the condensable gases were measured using a volumetric method. The results indicate the occurrence of capillary condensation at a reduced pressure P/P0 of approximately 0.8 for all three gases. There is a good agreement between the volume of condensate at saturation and the pore volume determined in independent porosimetric studies. Results of a large set of experiments with butane revealed that the permeation is only a function of the mean gas pressure in the membrane. This contradicts the available models suggested in (J. Coll. Interf. Sci. 110 (1986) 544; J. Membr. Sci. 66 (1992) 259) predicting that the transport mechanism in pores changes with different pressure conditions. Finally, in binary isobaric diffusion experiments with pure helium on one side of the membrane and pure butane on the other, a remarkable increase in selectivity of the transport process with increasing pressure in the membrane was observed.