Disentangling the gap between pure and mixed-gas performance of thin film composite membranes through improved cell design and testing methods

Abstract

Testing thin film composite (TFC) membrane coupons at low stage-cuts (≤5 %) in a sweep-gas permeation system is a common practice to obtain mixed-gas separation properties for benchmarking performance and making scale-up decisions. However, even under these idealized conditions, mixed-gas permeance and selectivity can be more than 30 % lower than their pure-gas values, partially due to concentration polarization, an effect that typically intensifies with increased membrane permeance. This study investigates the effect of cell design on mixed-gas testing using PolyActive™ TFC membranes with pure-gas CO₂ permeance of 1700–3100 gas permeance unit (GPU), covering the permeance range of most state-of-the-art CO₂/N₂ separation membranes. We designed and 3D-printed a counter-current permeation cell with enhanced feed and sweep flow efficiency, resulting in a 33–41 % increase in mixed-gas CO₂ permeance compared to traditional permeation cells. Furthermore, we compared sweep-gas and vacuum permeation methods using traditional permeation cells, revealing that the latter delivers 41 % higher mixed-gas CO₂ permeance, because vacuuming effectively minimizes the downstream concentration polarization. These findings highlight the importance of cell design and permeation apparatus selection in lab-scale mixed-gas testing, with strong implications for module design and process optimization at the industrial scale.