Changes in the permeability of membranes due to their decompression during permeation

Abstract

Release of pressure during permeation experiments leads to a large transient increase in the apparent diffusivity of gases. In some systems changes in the diffusivity are coupled with increases in the selectivity. The apparent diffusion coefficients of O 2 and of CO 2 increase by a factor of 35 in natural rubber (NR) membranes and by a factor of 50 (for oxygen) in internally plasticized PVC. This phenomenon is ascribed to transient changes that take place in the membrane as result of a sudden build up of the internal pressure of the dissolved gases. The relaxation constants for the return of the system to equilibrium were calculated to be in the range of 30-50 sec. Improvement of the performance of permeation systems by a cyclic mode of operation, which permits the exploitation of the high flow rates coupled with high selectivities that momentarily may be induced at low pressures is also discussed.