Every year, the world uses close to 100 trillion scf (standard cubic feet) of natural gas. All of this gas requires treatment before it enters the pipeline, making natural gas processing by far the largest market for industrial gas separation processes and equipment. Of this huge market, membranes have less than a 5% share, but this is changing; membrane-based removal of natural gas contaminants is growing faster than any other segment of the membrane gas separation business. This paper gives an overview of the membrane technology in current use for natural gas treatment and outlines the future prospects.

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Permeability, diffusion, and solubility coefficients for H2O vapor in four different 6FDA‐based polyimides were determined at temperatures between 25 and 45°C and over a wide range of relative humidities. The solubility of H2O vapor in some of the polyimides studied can be described by the “dual‐mode sorption” model whereas in other polyimides it is represented by the Flory‐Huggins equation, which suggests that the latter polymers are plasticized by H2O. The solubility of H2O vapor in the polyimides decreases as the temperature is raised and increases with increasing polarity of the polymer. The diffusion coefficients for H2O in the polyimides studied either increase or pass through a weak maximum with increasing H2O activity, or concentration in the polymers. The latter behavior is probably due to a clustering of H2O molecules in the polyimides at higher H2O activities or concentrations. The diffusion coefficients for H2O decrease as the chain‐packing density of the polyimides increases. The permeability coefficients for H2O vapor in 6FDA‐based polyimide membranes either increase slightly or are constant as the H2O activity is increased. The experimental values of the permeability coefficients are consistent with the values determined from diffusion and solubility coefficients. The permeability of the polyimides to H2O vapor appears to be controlled by the solubility of H2O in the polymers. The polyimides studied exhibit a very high selectivity for H2O vapor relative to CH4, and therefore are potentially useful membrane materials for the dehydration of natural gas. ©1995 John Wiley & Sons, Inc.

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