Silver salt facilitated transport membranes for olefin/paraffin separations: Carrier instability and a novel regeneration method

Abstract

Facilitated transport membranes have been the subject of continuing study in the membrane gas separation field for over 50 years. Despite laboratory performance that far exceeds the polymer upper bound for many gas pairs, facilitated transport membranes have failed to achieve commercial success. This failure is in part due to instability of the carrier species that are responsible for the facilitation effect. Here, we describe the impact of exposure to light, hydrogen, hydrogen sulfide, and acetylene on a silver salt facilitated olefin transport membrane. All of these treatments are shown to significantly reduce the ethylene/ethane selectivity of membranes composed of a Pebax® 2533/AgBF4 selective layer. For example, the mixed-gas ethylene/ethane selectivity of such a membrane decreases from an initial value of 40 to 1.1 after 34 days of exposure to ambient light, due to photo-reduction of the silver carrier ions. In an effort to stabilize membrane performance, a regeneration method was discovered that uses a peroxide/acid liquid or vapor phase treatment to oxidize reduced silver carriers within the membrane. This in-situ regeneration method is shown to reverse the performance degradation of membranes exposed to light, hydrogen, and acetylene. Such a technique used in a cyclic process is a possible means of extending the lifetime of silver salt facilitated olefin transport membranes, and although many challenges remain, could play a role in moving these membranes toward practical reality.