Concentration polarization in SAPO-34 membranes at high pressures

Separation of CO2/CH4 mixtures through tubular SAPO-34 membranes with high permeances was strongly affected by concentration polarization at feed pressures above about 1 MPa. The faster-permeating CO2 was depleted near the membrane surface because it could not diffuse fast enough from the bulk phase. Concentration polarization increased as pressure increased at constant feed rates because fluxes through the membranes increased, gas diffusivities decreased, and the velocity across the membrane decreased because gas densities increased. The CO2/CH4 selectivity, at 298 K and a feed pressure of 5.8 MPa, increased by about 160% and the CO2 flux increased by about 80% when Teflon spacers were inserted inside the membrane tube and the feed flow rate was increased by a factor of three. The spacers and the increased feed rate increased the gas velocity and decreased the diffusion distance from the bulk phase to the membrane surface, and thus they decreased the polarization boundary layer. The spacers did not affect separations at low pressures, where gas velocities and diffusivities were higher and fluxes were lower.