Modelling of methane and n-butane sorption, diffusion and permeation in polydimethylsiloxane using PC-SAFT

Abstract

Published sorption, diffusion and permeation data for methane and n-butane in polydimethylsiloxane (PDMS) from −20 to 50 °C was simulated using a perturbed chain statistical association theory (PC-SAFT) based model. The use of a temperature-dependent interaction parameter within the PC-SAFT model allowed the pure gas sorption data to be very well represented. The mixed gas sorption results were fully predictable from these pure gas parameters, without the introduction of any additional parameters, and agreed well with the experimental data. The model was also able to model the dilation behavior of PDMS under various gas compositions, making it possible to analyse gas sorption properties using pure gas sorption data only. A diffusion model coupled with the PC-SAFT model was capable of fitting both pure and mixed gas permeation data well by applying an exponential expression to account for such dilation in the diffusivity term. Only two parameters (i.e. infinite dilution mobility coefficient L0 and plasticization factor β) were used and no coupling effect between the two penetrants was needed. The activation energies of L0 were 11.7 and 13.4 kJ mol−1 for methane and n-butane. Moreover, the model was also able to calculate the concentration profiles of the penetrants across the membrane thickness. For n-butane, the mass concentration profile changed from linear to non-linear when the feed pressure increased from 4 to 11 atm for 8 mol% n-butane at 25 °C. Conversely, methane showed a linear concentration profile under both conditions.