(355a) Carbon Molecular Sieve Membranes for N2/CH4 Separation

Many natural gas reserves are contaminated by a high content of nitrogen which has to be removed to meet pipeline specifications. Currently available nitrogen rejection technologies include cryogenic distillation, pressure swing adsorption (PSA), and lean oil absorption. Polymer-based membrane separation for N₂/CH₄ has not been pursued widely because of the small size difference between N₂ and CH₄ and the competing effects of diffusion and sorption. These challenges lead to unattractive permselectivity using traditional polymeric materials due to the broad distribution of segmental motions of polymer chains. Carbon Molecular Sieve (CMS) membranes overcome this problem by providing a hyper-rigid structure and distinctive bimodal pore size distribution.

Different pyrolysis conditions for Matrimid^® polyimide and resultant separation performance of CMS membranes were studied in this work. It was shown that N₂/CH₄ selectivity was highly improved with increased pyrolysis temperature. Sorption tests were performed to deconvolute permeability into diffusion coefficient and sorption coefficient. The temperature effects on permeability, sorption coefficient and diffusion coefficient were also studied.