(348i) Mixed-Gas Sorption of Ternary Mixtures of CO2, CH4 and C2H6 in PIM-1: Experimental Measurement and Thermodynamic Modelling

Gas separation with dense polymeric membranes follows the solution-diffusion mechanism. Often only pure-gas solubility is measured and the very few measurements of solubility of gas mixtures in polymers reported are for the case of binary mixtures. In real applications, however, the presence of third components and contaminants can significantly alter the performance of the membrane, due to plasticization and competitive exclusion.

To characterize these effects, we measured the sorption isotherms of C₂H₆, CO₂, and CH₄ in PIM-1, in binary and ternary conditions, using a pressure decay apparatus, working at constant composition and variable pressure of the gas phase. A new measurement protocol was designed to measure sorption of ternary gas mixtures for the first time.

The results show strong competitive sorption effects in all binary and ternary cases analyzed. In the binary cases, both CO₂ and C₂H₆ have a strong exclusion power over CH₄, whereas the effect of CH₄ on the two gases is limited. Surprisingly, C₂H₆ has a strong exclusion power over CO₂ as well, even though these gases have very similar condensability and pure-gas sorption in PIM-1. In the ternary case, the superposition of the effects highlighted in binary tests is observed: both CO₂ and CH₄ are excluded due to the presence of C₂H₆, while C₂H₆ solubility is barely altered compared to pure-gas conditions. Overall, the effect of C₂H₆ on CO₂/CH₄ solubility-selectivity is modest, compared to the corresponding binary case.

The Non-Equilibrium Lattice Fluid (NELF)^[1] model predictions for binary and ternary sorption, performed using only pure-gas parameters as input, are in very good agreement with the data, indicating that the model is a reliable tool to study mixed-gas sorption, even in complex scenarios.

References

^[1] F. Doghieri, G.C. Sarti, Macromolecules, 24 (1996) 7885–7896

^[2] O. Vopička, M.G. De Angelis, N. Du, N. Li, M.D. Guiver, G.C. Sarti, J. Membr. Sci., 459 (2014) 264–276

Figure. Third component (C₂H₆) effects on (a) CO₂ and (b) CH₄ solubility in PIM-1 at 35 °C. Empty symbols: pure gases. Striped symbols 19-81 mol% CO₂/CH₄ binary mixture.^[2] Filled symbols 15-15-70 mol% CO₂/C₂H₆/CH₄ ternary mixture. Solid lines represent NELF model predictions of pure- and mixed gas sorption isotherms.