Effects of temperature and pressure on gas permselection properties in asymmetric membranes

Abstract

The effects of operating temperature and pressure on the transport behaviors of various gases through asymmetric membranes with surface defects were analysed based on the resistance model. The model takes into account simultaneous Knudsen and Poiseuille flows through the porous medium and solution-diffusion flow through the dense medium of the skin layer. Two polyethersulfone asymmetric hollow fiber membranes having different surface porosity were prepared from a spinning solution containing N-methyl-2-pyrrolidone as a solvent and water as a nonsolvent additive. Water was also used as the internal and external coagulant during 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 separation factors for He N2 and O2 N2 were calculated. The observed experimental data agree well with model predictions. The transport behaviors of the fast gases (He and O2) and slow gas (N2) 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 increased faster with increasing temperature compared to that to that of the slow gas. As a result, the ideal separation factor increased with temperature. The relative contribution of gas permeability in the pores decreased with increasing temperature. In contrast, separation factors decreased faster with increasing pressure as the slow gas permeability by Poiseuille flow became increasingly more significant. © 1995.