High-flux CHA zeolite membranes for H2 separations

Abstract

Two types of CHA zeolite membranes (SAPO-34 and SSZ-13) were used for the separations of CO2/H2 and H2/CH4 binary mixtures at pressures up to 2.8 MPa. Separation performances of both membranes were improved after synthesis modifications. The SAPO-34 membranes had higher permeances than the SSZ-13 membranes because less SAPO-34 crystals were grown into the support pores. The best SAPO-34 membrane showed CO2/H2 separation selectivity of 17.6 and CO2 permeance of 40.3 × 10−7 mol/(m2 s Pa) (CO2 permeability of 72220 barrers); this membrane also had H2/CH4 separation selectivity of 42.2 and H2 permeance of 14.5 × 10−7 mol/(m2 s Pa) (H2 permeability of 25980 barrers) for equimolar ratio feed at pressure drop of 0.2 MPa and 298 K. These data were far beyond the upper bounds of the current polymeric and zeolite membranes in CO2/H2 and H2/CH4 plots. The effects of pressure, temperature, feed flow rate, feed concentration and water vapor on membrane performances through SAPO-34 and SSZ-13 membranes were comparably studied in CO2/H2 or H2/CH4 mixtures. Both membranes displayed similar trends in permeances and selectivities with the changes of the parameters, except for temperature and pressure dependences of selectivities and CH4 permeance in H2/CH4 mixtures, respectively. The different behaviors were attributed to the differences in the compositions and defect distributions of the two membranes. For both membranes, CO2/H2 selectivities increased but H2/CH4 selectivities decreased with the increase of pressure. Preferential adsorption dominated the CO2/H2 separations and molecular sieving did the H2/CH4 separations through both membranes. Single-gas permeances dependence of pressure drop was predicted by the Maxwell–Stefan diffusion model.