Understand the real upper bound for CO₂/CH₄ separation

Membrane materials with high permeability and high selectivity are preferred for gas separations. However, there exists a trade-off (or upper bound): polymers with higher permeability often exhibit lower selectivity, and vice versa. The upper bounds have been empirically drawn and theoretically predicted. While the upper bounds for light gas pairs such as O₂/N₂ can accurately describe the limit of membrane materials, the upper bound for CO₂/CH₄ separation does not depict state-of-the-art of material research. For example, the performance of commercial CO₂/CH₄ separation membrane (cellulose acetate, CA) is way below the upper bound, and there seem to be many materials showing much better separation properties. This paper presents experimental data of CA membranes tested with simulated CO₂/CH₄ mixtures. The effect of feed pressure and composition on CO₂/CH₄ separation properties is interpreted with a free volume model, which is then used to predict a more realistic upper bound for industrial CO₂/CH₄ separations.