Temperature Dependence of Light Hydrocarbons Sorption and Transport in Dense Membranes Based on Tetradecyl...

Solubility-selective polymer membranes are promising materials for C₃₊ hydrocarbons removal from methane and other permanent gas streams. To this end, a dense solubility-selective membrane based on crosslinked poly(tetradecyl methyl siloxane) was synthesized. Sorption of methane, ethane, and n-butane in the polymer was measured in the temperature range of 5-35 °C. An abnormal temperature dependence of sorption was detected, contradicting the generally accepted view of sorption as an exothermic process. In particular, methane shows minimal sorption at 5 °C. The abnormal temperature behavior was found to be related to crystallization of the alkyl side chains at temperatures below ~10 °C. Gas permeability determined by sorption and permeation methods are in reasonable agreement with each other and decrease in the order n-C₄H₁₀ > C₂H₆ > CH₄. The solubility of these alkanes changes in the same order indicating that poly(tetradecyl methyl siloxane) is indeed the sorption-selective membrane. The diffusivities and permeabilities of studied alkanes declined with decreasing temperature, whereas the n-C₄H₁₀/CH₄ permselectivity increases with decreasing temperature, reaching a value of 23 at 5 °C.