Basis of Permeability/Selectivity Tradeoff Relations in Polymeric Gas Separation Membranes

TLDR

Polymer membrane gas separation follows a trade‑off where higher permeability reduces selectivity, a relationship empirically captured by Robeson’s limits expressed as α = β (PA/PB) with parameters linked to gas size and condensability. The study aims to develop a fundamental theory explaining this permeability/selectivity trade‑off in polymer membranes. The theory models λ as a function of gas size and β as a function of λ, gas condensability, and a single adjustable parameter.

Abstract

Gas separation properties of polymer membrane materials follow distinct tradeoff relations: more permeable polymers are generally less selective and vice versa. Robeson1 identified the best combinations of permeability and selectivity for important binary gas pairs (O2/N2, CO2/CH4, H2/N2, etc.) and represented these permeability/selectivity combinations empirically as αA/B = βA/B , where PA and PB are the permeability coefficients of the more permeable and less permeable gases, respectively, αA/B is selectivity (=PA/PB), and λA/B and βA/B are empirical parameters. This report provides a fundamental theory for this observation. In the theory, λA/B depends only on gas size. βA/B depends on λA/B, gas condensability, and one adjustable parameter.