Upper bound of polymeric membranes for mixed-gas CO₂/CH₄ separations

Membrane polymers with high permeability and high selectivity are preferred for gas separations. However, there exists a trade-off or upper bound, i.e., polymers with higher permeability often exhibit lower selectivity, and vice versa. The upper bound for separation of various gas pairs has been empirically drawn and theoretically rationalized using pure-gas data. However, for CO₂/CH₄ separation, the high pressure CO₂ and non-methane hydrocarbons can plasticize polymers, increasing mixed-gas CO₂ permeability and decreasing mixed-gas CO₂/CH₄ selectivity. This study aims to apply a modified free volume theory to interpret CO₂/CH₄ separation performance in polymeric membranes. The model satisfactorily describes the pure-gas upper bounds for various gas pairs including CO₂/CH₄, the effect of high pressure CO₂ on mixed-gas CO₂/CH₄ separation properties, and the practical mixed-gas upper bound for CO₂/CH₄ separations. The CO₂ is found to have an estimated glass transition temperature of 108K. The assumptions of this model are discussed, and future work to improve this model is proposed.