Preparation of carbon molecular sieve membranes on low-cost alumina hollow fibers for use in C3H6/C3H8 separation

Abstract

We herein report the preparation of thin carbon molecular sieve (CMS) membranes on 10 cm-long alumina hollow fiber supports for use in the separation of C3H6/C3H8 gas mixtures. Considering the manufacturing costs and scalability by modulation, low grade alumina powder ( < 20 US$/kg) was selected as the inorganic material for the porous support following its extrusion into a hollow fiber structure. An intermediate layer of alumina was initially introduced onto the hollow fiber via a sol-gel method to reduce the effective pore size, and a commercial polyimide (Matrimid 5218 (R)) film was then coated onto this alumina layer. Following subsequent pyrolysis, the resulting CMS membrane exhibited a C3H6 permeance of 45 GPU and a C3H6/C3H8 selectivity of 16.5. The separation performance for this gas mixture was lower than that of single-component permeance (i.e., 80 GPU C3H6 and 30 C3H6/C3H8 ideal gas selectivity) due to guest-guest interactions between the permeating C3H6 and C3H8 molecules. Finally, long-term stability tests for the CMS membrane at 2 bar over 500 h indicated a tendency to reach stabilization, whereas the test carried out at 7 bar exhibited a continuous decrease in gas permeance, which remains an issue for the use of our CMS membrane system in industrial applications.