Natural gas permeation in polyimide membranes

摘要

Polyimide membranes derived from 6FDA-DAM:DABA and 6FDA-6FpDA:DABA copolymers have been used to separate 50/50 CO_2/CH_4 mixtures and multicomponent synthetic natural gas mixtures at 35℃ and feed pressures up to 55 atm. For 6FDA-DAM:DABA 2:1 membranes the effects of thermal annealing and covalent crosslinking are decoupled with respect to effects on permeabilities and selectivity. Crosslinking at 295℃ with 1,4-butylene glycol and 1,4-cyclohexanedimethanol increases CO_2 permeabilities by factors of 4.1 and 2.4, respectively, at 20 atm feed pressure, without a loss in selectivity, relative to crosslinking at 220℃. Thermal annealing and crosslinking also reduce CO_2 plasticization effects. Crosslinking of DABA-containing copolymers, therefore, can produce membranes with tunable transport properties that offer significantly higher performance with better plasticization-resistance than that reported in the literature for the commercial polymers Matrimid~(~R) and cellulose acetate for CO_2 removal from natural gas mixtures. Separation of complex mixtures containing CO_2, CH_4, C_2H_6, C_3H_8, and C_4H_(10) or toluene results in a significant decrease of the CO_2 permeability, but only a moderate decrease in the CO_2/CH_4 selectivity.