Mass transport of CO2 over CH4 controlled by the selective surface barrier in ultra-thin CHA membranes

The adsorption and mass transport of CO2 and CH4 in CHA zeolite were studied experimentally. First, large and well-defined CHA crystals with varying Si/Al ratios and morphologies ideal for adsorption studies were prepared. Then, adsorption isotherms were measured, and adsorption parameters were estimated from the data. In the next step, permeation experiments for pure components and mixtures were conducted for a defect-free CHA membrane with a Si/Al ratio of 80 and a thickness of 600 nm over a wide temperature range. A maximum selectivity of 243 in combination with a CO2 permeance of 70 x 10(-7) mol/(m(2) s Pa) was observed for a feed of an equimolar CO2/CH4 mixture at 273 K and 5.5 bar. Finally, a simple model accounting for adsorption and diffusion through the surface barriers and the interior of the pores of the membrane was fitted to the permeation data. The fitted model indicated that the surface barrier was a surface diffusion process at the pore mouth with higher activation energy than the diffusion process within the pores. The model also showed that the highly selective mass transport in the membrane was mostly a result of a selective surface barrier and, to a lesser extent, a result of adsorption selectivity.

Automated summary

This study experimentally investigated the adsorption and mass transport of CO2 and CH4 in CHA zeolite. Large and well-defined CHA crystals were prepared for adsorption studies, and adsorption isotherms were measured to estimate adsorption parameters. Permeation experiments with a defect-free CHA membrane also showed high selectivity for CO2 and CH4 under specific conditions.