On the subtle balance between competitive sorption and plasticization effects in asymmetric hollow fiber gas separation membranes

Abstract

The paper describes the influence of a varying feed composition of CO 2/CH 4 and CO 2/N 2 mixtures on the gas separation performance of integrally skinned asymmetric PES/PI hollow fibers with an effective skin thickness of 0.27 μm. Normally, thin membrane structures (<3 μm) show accelerated plasticization behavior induced by CO 2 in pure gas measurements. This study shows that introducing an inert gas to the CO 2 feed mixture apparently suppresses plasticization. This effect is more pronounced at higher concentrations of inert gas, supported by a continuous drop in CO 2 permeance as a function of CO 2 fugacity. At a concentration of 80% inert gas in the feed mixture, the CO 2 permeance reduces more than 35% from its initial value, whereas the reduction is 8–10% with 2% inert gas in the feed mixture. However, a mixed gas permeation model predicts for all experimentally used gas compositions similar decreases in CO 2 permeance. Plasticization effects seem to be counterbalanced by competitive sorption. This effect becomes larger with increasing inert gas concentration. At 80% inert gas plasticization effects appear to be completely counterbalanced by competitive sorption. Besides that, for all gas compositions, the separation factor decreases with increasing feed pressure, generally assumed as an indication of plasticization. However, such a selectivity decrease is also predicted by the dual mode sorption model, which neglects effects of plasticization. More pronounced indication of plasticization effects is observed when the N 2 permeance decay is followed in time after the membrane has been in contact with CO 2 at elevated CO 2 partial pressures. A significant enhanced N 2 permeance is observed due to polymer network dilation, which decreases very slowly in time. There seems to be a subtle balance between plasticization and competitive sorption during mixed gas experiments with integrally skinned asymmetric hollow fibers, which results in the observed phenomenon.