Upgrading of raw natural gas using high-performance polymer membranes

The separation of CO{sub 2} and H{sub 2}S from raw natural gas is essential for enhancing the heating value of the gas and for providing environmental safety during nation and use. Membrane separation processes are particularly useful for this application because they are inherently energy-efficient. Membrane processes also offer other important advantages, such as ease of scale-up due to module design, good space and weight efficiency and great flexibility in handling variations in the flow rate, pressure and composition of the raw natural gas streams. In a coordinated effort, the Gas Research Institute and the Department of Energy, Morgantown Energy Technology Center are funding an extensive study at Syracuse University on the relationships between the chemical structure of various polymers and their permeability to different gases. Based on criteria derived from this study, novel fluorine-containing polyimides that exhibit very high C0{sub 2}/CH{sub 4}, selectivities as well as significant H{sub 2}S/CH{sub 4}, selectivities were synthesized. These ``dense`` (homogeneous) membranes separated various mixtures of C0{sub 2}/CH{sub 4}, and H{sub 2}S/CH{sub 4}/CO{sub 2} at 35{degree}C (95{degree}F) and at pressures up to 10 atm (147 psi). The results of these measurements agree well with computer simulations. The C0{sub 2} content of CH{sub 4}/CO{sub 2}more » mixtures containing as much as 40 mole percent C0{sub 2} can be reduced with the polyamide membranes to pipeline specifications ({le} 2 mole percent C0{sub 2}). To reduce the H{sub 2}S content of H{sub 2}S/CH{sub 4}/CO{sub 2} mixtures to pipeline specifications ({le} 4 ppM H{sub 2}S), the membranes are best combined with conventional gas-absorption processes, such as a selective amine. Such ``hybrid`` processes for the upgrading of natural gas can be more economical than conventional gas-absorption processes alone.« less