A critical review of cyclic transient membrane gas separation processes: State of the art, opportunities and limitations

Membrane gas separations under transient state conditions are rather unexplored compared to steady-state operations. In a first step, the various theoretical and experimental studies performed on membrane gas separation processes under cyclic operations are reviewed. With respect to the cycle duration, these operations are classified into short and long classes. In a second step, a systematic analysis of the potential interest of the short class of cyclic operation compared to steady-state performances, namely an improved selectivity or a reverse selectivity, has been performed based on trade-off correlations applied to 2145 gas pairs. A series of gas pairs of potential interest for transient operation has been identified thanks to this approach. Four case studies ({He,N2}, {He,Ar}, {H2,C3H8} and UF6 isotopes), based on experimental solubility and diffusion data, have been investigated and the optimized separation performances for cyclic operations are compared to steady-state operations in terms of selectivity and productivity. Generally speaking, the possibility to increase the separation selectivity by cyclic operation logically needs to be carefully counterbalanced by a decrease in productivity. Finally, further work, potential applications and unexplored issues of cyclic membrane gas separation processes are discussed.