(814b) Membrane Gas Separation Processes: Improved Performances Through Unsteady Cyclic Operation

Membrane processes based on dense polymeric materials offer attractive performances for gas separation applications at industrial scale such as air separation, hydrogen purification, natural gas treatment or volatile organic compounds recovery [1].

Steady state operation is systematically performed for industrial installations and the separation performances are primarily governed by the membrane selectivity (a), which corresponds to the ratio of the compounds permeability through the polymer. In several cases however, the separation specifications cannot be reached due to too low membrane selectivity. In that event, the unique separation performances which can be obtained in some cases when a transient regime is applied can be of interest, especially when the differences between the diffusion coefficients of the permeating species largely exceed the differences in permeability. Paul explored this issue in a pioneering study, based on a cyclic process with synchronous valve operation [2]. We recently extended the analysis thanks to numerical simulation methods and optimization techniques which offer the possibility to explore asynchronous operation modes [3]. The concept proposed by Paul shows however a major limitation. Because the upstream composition has to remain unchanged, an infinitely low stage cut has to be applied (i.e. a very low fraction of the feed permeates through the membrane); as a consequence, the recovery ratio of the module is infinitely small.

In this study, a different approach, based on unsteady cyclic operation over long time scale (i.e. much larger than the time-lag of the system), a completely unexplored research area, will be detailed and described. The evolution of the upstream and downstream compartments pressure of a membrane module under cyclic unsteady operation have been first simulated through a tailor made computer program. The predictions obtained by simulation have been experimentally validated on a dedicated installation equipped with a hollow fiber gas separation module for air separation application (PPO fibers, Piccolo module from Parker). In a second step, the set of operating conditions which maximize the separation performances of the unsteady process have been identified through optimization. It will be shown for the first time that unsteady operation offers the opportunity to improve separation selectivity and overall recovery compared to steady operation [4].

The results of this exploratory study suggest to more systematically investigate the possibilities and limitations of gas separation modules under transient operation. A series of potential target applications (gas pairs) of industrial interest will be detailed. The possibilities of improvement of the concept and future challenges will finally be addressed.

References 

[1] Baker, R, Membrane Technology and Applications, 2^nd ed., John Wiley & Sons, New York, 2004

[2] Paul, D.R. "Membrane separation of gases using steady cyclic operation." Ind. Eng. Chem. Process Des. Dev, 1971: 375-379.

[3] Corriou, J.P., C. Fonteix, et E. Favre. «Optimization of a pulsed operation of gas separation by membrane.» AIChE Journal 54, n° 5 (2008): 1224-1234.

[4] Favre, E., Castel, C., Corriou, J.P., Wang, L. " Procédé de séparation membranaire en régime discontinu" French patent  # FR 11 60587.