Abstract
The nonideal behavior of polymeric membranes during separation of gas mixtures can be quantified via the solution-diffusion theory from experimental mixed-gas solubility and permeability coefficients. In this study, CO2-CH4 mixtures were sorbed at 35 °C in 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA)-m-phenylenediamine (mPDA)—a polyimide of remarkable performance. The existence of a linear trend for all data of mixed-gas CO2 versus CH4 solubility coefficients—regardless of mixture concentration—was observed for 6FDA-mPDA and other polymeric films; the slope of this trend was identified as the ratio of gas solubilities at infinite dilution. The CO2/CH4 mixed-gas solubility selectivity of 6FDA-mPDA and previously reported polymers was higher than the equimolar pure-gas value and increased with pressure from the infinite dilution value. The analysis of CO2-CH4 mixed-gas concentration-averaged effective diffusion coefficients of equimolar feeds showed that CO2 diffusivity was not affected by CH4. Our data indicate that the decrease of CO2/CH4 mixed-gas diffusion, and permeability selectivity from the pure-gas values, resulted from an increase in the methane diffusion coefficient in mixtures. This effect was the result of an alteration of the size sieving properties of 6FDA-mPDA as a consequence of CO2 presence in the 6FDA-mPDA film matrix.
Highlights
Conventional distillation or absorption and adsorption systems are reliable and, for existing plants, economically feasible technologies
The CO2 /CH4 mixed-gas solubility selectivity of 6FDA-mPDA and previously reported polymers was higher than the equimolar pure-gas value and increased with pressure from the infinite dilution value
Most experimental studies reported in the literature are based on pure-gas permeation properties of membrane materials (e.g., cellulose acetate, polysulfone, polyphenylene oxide, polyimides, polymers of intrinsic microporosity (PIM), and others) [2,3,4,5,6]
Summary
Conventional distillation or absorption and adsorption systems are reliable and, for existing plants, economically feasible technologies. Membrane units could potentially replace these traditional unit operations, ensuring better economics and lower environmental impact [1]. To fully exploit the potential of membrane-based gas-separations, polymeric materials of high permeability and permeability selectivity are required. These materials must be mechanically strong for formation of integral asymmetric and thin-film composite membranes and should ensure stable separation performances over time. Most experimental studies reported in the literature are based on pure-gas permeation properties of membrane materials (e.g., cellulose acetate, polysulfone, polyphenylene oxide, polyimides, polymers of intrinsic microporosity (PIM), and others) [2,3,4,5,6]. The pure-gas CO2 /CH4 permeability selectivity of PIM-1 is ~15
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