EFFECTS OF TEMPERATURE AND PRESSURE ON GAS PERMSELECTION PROPERTIES IN ASYMMETRIC MEMBRANES

The effects of operating temperature and pressure on the transport beh aviors of various gases through asymmetric membranes with surface defe cts were analysed based on the resistance model. The model takes into account simultaneous Knudsen and Poiseuille flows through the porous m edium and solution-diffusion how through the dense medium of the skin layer. Two polyethersulfone asymmetric hollow fiber membranes having d ifferent surface porosity were prepared from a spinning solution conta ining N-methyl-2-pyrrolidone as a solvent and water as a nonsolvent ad ditive. Water was also used as the internal and external coagulant dur ing spinning of these hollow fibers. The pressure-normalized fluxes of pure hydrogen, helium, oxygen and nitrogen through both the membranes were measured at various temperatures and pressures and ideal separat ion factors for He/N-2 and O-2/N-2 were calculated. The observed exper imental data agree well with model predictions. The transport behavior s of the fast gases (He and O-2) and slow gas (N-2) in the asymmetric membranes are dramatically different because of variations in relative contribution of the pore flow for different gases. For the membranes prepared in this study, the pressure-normalized fluxes of fast gases i ncreased faster with increasing temperature compared to that of the sl ow gas. As a result, the ideal separation factor increased with temper ature. The relative contribution of gas permeability in the pores decr eased with increasing temperature. in contrast, separation factors dec reased faster with increasing pressure as the slow gas permeability by Poiseuille flow became increasingly more significant.