High Temperature and High Pressure Gas Separation Properties of Polybenzimidazole Membranes Dried at Different Temperatures

Polybenzimidazole (PBI) membranes are mechanically and chemically resilient making them suitable for the separation of gases at high pressure and high temperature.  Hydrogen/carbon dioxide gas selectivities ranging from 5 to 24 have been reported for PBI membranes measured at temperatures up to 250 °C, illustrating significant temperature-dependent permselectivity properties.  We investigated the effect of drying temperature and testing conditions on the gas permeation properties of PBI membranes.  Four sets of PBI membranes were cast and then subjected to a final drying temperature of 250, 300, 350, or 400 °C.  The gas separation properties of the membranes dried at 250 °C and at 300 °C were measured at 35, 100, 200, and 300 °C with upstream pressures of 5, 15, 30, and 45 atm.  The PBI membranes measured at 35 °C exhibited H₂/CO₂ selectivities that ranged from 18 to 50 when tested with pure gases and up to 72 when tested with gas mixtures over the pressure range studied.  PBI membranes dried at 250 °C and tested at 35 °C demonstrated H₂/CO₂ ideal selectivities that ranged from 18 at 5 atm to 33 at 30 atm. When the same membranes were tested at 300 °C, the H₂/CO₂ ideal selectivity remained almost unchanged (28 at 5 atm and 30 at 30 atm).  Membranes dried at 300 °C exhibited improved separation properties when tested at 35 °C, showing a H₂/CO₂ ideal selectivity of 49 at 5 atm, 53 at 30 atm, and 47 at 45 atm.  Using a gas mixture, the H₂/CO₂ selectivity at 35 °C and 30 atm was even higher at 72.  These results suggest that PBI membranes need to be dried at high temperatures to fully maximize their separation properties.