Mixed gas sorption and transport study in solubility selective polymers

Membrane separation technology has recently emerged as a potential alternative technique for removing higher hydrocarbons (C₃₊) from natural gas. For economic reasons, membranes for this application should be organic vapor selective materials such as poly(dimethylsiloxane) (PDMS) or poly(1-trimethylsilyl-1-propyne) (PTMSP). These polymers, often called solubility selective polymers, sieve penetrant molecules based largely on relative penetrant solubility in the polymer. The sorption and transport properties in such polymers have been reported previously. However, most studies present only pure gas sorption and transport properties. Mixture properties, which are important for estimating membrane separation performance, are less often reported. In addition, mixed gas sorption and diffusion data in such polymers, to the best of our knowledge, have never been investigated before. This research work provides a fundamental database of mixture sorption, diffusion, and permeation data in solubility selective polymers. Two solubility selective polymers were studied: poly(dimethylsiloxane) (PDMS) and poly(1-trimethylsilyl-1-propyne) (PTMSP). The vapor/gas mixture was n-C4H10/CH4. CH4 partial pressures ranged from 1.1 to 16 atm, and [subscript n-]C₄H₁₀ partial pressures ranged from 0.02 to 1.7 atm. Temperatures studied ranged from -20 to 50 oC. The pure and mixed gas [subscript n-]C₄H₁₀ and CH₄ permeability and solubility coefficients in PDMS and PTMSP were determined experimentally using devices constructed specifically for these measurements. The pure and mixed gas diffusion coefficients were calculated from permeability and solubility data. In rubbery PDMS, the presence of [subscript n-]C₄H₁₀ increases CH₄ permeability, solubility, and diffusivity. On the other hand, the presence of CH₄ does not measurably influence [subscript n-]C₄H₁₀ sorption and transport properties. The [subscript n-]C₄H₁₀/CH₄ mixed gas permeability selectivities are lower than those estimated from pure gas measurements. This difference is due to both lower solubility and diffusivity selectivities in mixtures relative to those in pure gas. Plasticization of PDMS by [subscript n-]C₄H₁₀ does little to n-C4H10/CH₄ mixed gas diffusivity selectivity. Increases in mixed gas permeability selectivity with increasing [subscript n-]C₄H₁₀ activity and decreasing temperature were mainly due to increases in solubility selectivity. Unlike PDMS, the presence of [subscript n-]C₄H₁₀ decreases CH₄ permeability, solubility, and diffusivity in PTMSP. The competitive sorption and the blocking effects significantly reduce CH₄ solubility and diffusion coefficients in the polymer, respectively. However, similar to PDMS, the presence of CH₄ has no measurable influence on [subscript n-]C₄H₁₀ sorption and transport properties. [subscript n-]C₄H₁₀ /CH₄ mixed gas permeability selectivities in PTMSP are higher than those determined from the pure gas measurements. This deviation is a result of higher solubility and diffusivity selectivities in mixtures relative to the pure gas values. Mixed gas permeability, solubility, and diffusivity selectivities in PTMSP increased with increasing [subscript n-]C₄H₁₀ activity and decreasing temperature. The partial molar volumes of [subscript n-]C₄H₁₀ and CH₄ in the polymers were determined from sorption and dilation data. The dilation isotherms of PDMS and PTMSP in mixtures agree with estimates based on pure gas sorption and dilation measurements. The partial molar volumes of n-C4H10 and CH4 in PDMS are similar to those in liquids. In contrast, the partial molar volumes of [subscript n-]C₄H₁₀ and CH₄ in glassy PTMSP are substantially lower than those in liquids. Several models were used to fit the experimental data. For instance, the FFV model, the activated diffusion model, and the Maxwell-Stefan model were employed to describe the mixture permeability data in PDMS. Based on the Maxwell-Stefan analysis, the influence of coupling effects on permeation properties in PDMS were negligible. The dual mode sorption and permeation models were used to describe the mixed gas data in PTMSP. The dual mode permeability model must be modified to account for [subscript n-]C₄H₁₀ -induced reductions in CH₄ diffusion coefficients (i.e., the blocking effect). The FFV model provides poor correlations in PTMSP. There seems to be other factors, besides FFV per se, contributing to the temperature and concentration dependence of diffusion coefficients in PTMSP.