Industrially relevant CHA membranes for CO2/CH4 separation

Abstract

In the present work, single channel tubular zeolite CHA membranes with a length of 50 cm and a membrane area of 100 cm2 were evaluated by SEM and permeation experiments with single components and industrially relevant humid mixtures. The membranes comprised well-intergrown and smooth CHA films with a thickness of <500 nm supported on the inside of a highly porous tube. For single component permeation, a very low SF6 permeance of 4.5 × 10−10 mol/(m2‧s‧Pa) was observed, which indicated nearly defect free membranes. On the contrary, the membranes displayed a very high CO2 permeance of 128 × 10−7 mol/(m2‧s‧Pa), which illustrated the very high permeability of the CHA pores. Finally, the membranes displayed excellent selectivity for separation of industrially relevant CO2/CH4/H2O mixtures, which was attributed to selective interaction between the CO2 molecules and the polar water molecules in the pores. The highest observed CO2/CH4 separation selectivity was as high as 198 in combination with a CO2 permeance of 14 × 10−7 mol/(m2‧s‧Pa) at a feed pressure of 600 kPa (including 2.2 kPa water) and 293K. The corresponding CO2 flux was 0.39 mol/(m2‧s) and the corresponding CO2/CH4 separation factor was 162. The observed membrane performance was reduced by concentration polarisation due to the limited feed flow in the experimental setup. The corresponding selectivity and CO2 permeance corrected for concentration polarisation were as high as 236 and 16 × 10−7 mol/(m2‧s‧Pa), and the corrected CO2 flux was 0.44 mol/(m2‧s) and corrected separation factor was 198. An estimate showed that even at a low feed pressure of 500 kPa, it would be sufficient with as few as 64 membranes for processing of a feed of 100 Nm3/h raw biogas, i.e. the capacity of a typical biogas plant at a large farm, to biomethane with high purity. These results illustrated that the membranes are promising candidates for industrial separation of CO2 from e.g. natural gas and biogas.