Aromatic block co-polyimide membranes for sour gas feed separations

Permeation properties of pure and gas mixture consisting of CO2, H2S, CH4, N2 and C2H6 through dense films of the block co-polyimides (6FDA-mPDA)-(6FDA-durene) are studied at 35-55 °C and pressures of 100-500 psi for combined separation of CO2, H2S and N2 from acid gas streams. The H2S composition in the gas mixture is up to 20 vol%. This rigid block copolymer exhibits relatively constant or slightly decrease in pure gas permeability with increasing feed pressure for most of the penetrants. The pure gas CO2/CH4 and N2/CH4 selectivities are up to 61 and 2.6 respectively. The mixed gas studies also show relatively constant or slightly decrease in permeability coefficients with increase in pressure for most of the penetrants. One significant fact is that, at feed pressure of 500 psi and 20 vol% H2S in feed gas mixture (consisting of CO2, N2, CH4 and H2S), H2S/CH4 and CO2/CH4 separation factors are 23 and 27 respectively. These values are within the same order of magnitude or even better than those obtained in other state-of-the-art polymeric membranes that have been reported. Another unique feature of this co-polyimide is that, in addition to being selective to acid gas, it is also more selective to N2 as compare to CH4. Moreover, the CO2/CH4 separation factor does not suffer significant loss to the same degree as in the case of cellulose acetate (CA), even under these much more aggressive environments. This stability at moderate pressures and high H2S concentration is quite remarkable, as most reported studies are focused on low H2S concentrations and low pressures.