A Composite Membrane Based on Polydecylmethylsiloxane for the Separation of Hydrocarbons Mixtures at Reduced Temperatures

The permeability of n-butane and methane as well as their mixture through a PDecMS/MFFK composite membrane at reduced temperatures up to 0°C is for the first time studied in this work. According to the data of SEM, the thickness of the selective layer of PDecMS is 5 μm. It is shown that both the permeability coefficient of butane and ideal butane/methane selectivity \({{\alpha }_{{{{{{{\text{C}}}_{{\text{4}}}}{{{\text{H}}}_{{{\text{10}}}}}} \mathord{\left/ {\vphantom {{{{{\text{C}}}_{{\text{4}}}}{{{\text{H}}}_{{{\text{10}}}}}} {{\text{C}}{{{\text{H}}}_{{\text{4}}}}}}} \right. \kern-0em} {{\text{C}}{{{\text{H}}}_{{\text{4}}}}}}}}}\) increase as the temperature decreases from 60 down to 0°C. Thus, the permeability coefficient of butane is 11 400 Barrer at 0°C. It is important to emphasize that the ideal butane/methane selectivity of PDecMS/MFFK of 60 at 0°C is twofold higher than similar values for MDK and PDMS membranes (27 and 32, respectively). This is first of all associated with the difference in the values of the sorption selectivity α^S of these polymers. Thus, the values of \(\alpha _{{{{{{{\text{C}}}_{{\text{4}}}}{{{\text{H}}}_{{{\text{10}}}}}} \mathord{\left/ {\vphantom {{{{{\text{C}}}_{{\text{4}}}}{{{\text{H}}}_{{{\text{10}}}}}} {{\text{C}}{{{\text{H}}}_{{\text{4}}}}}}} \right. \kern-0em} {{\text{C}}{{{\text{H}}}_{{\text{4}}}}}}}}^{{\text{S}}}\) for PDecMS and PDMS at 0°C estimated based on the enthalpy of sorption are 170 and 95, respectively. In addition, the difference in the activation energies of diffusion of methane in PDecMS, PDMS, and MDK provides a sharper increase in the butane/methane permselectivity for PDecMS when compared to PDMS and MDK in the case of decreasing the measurement temperature. In the case of a C₄H₁₀ (35 vol %)/CH₄ mixture, the butane/methane permselectivity of a PDecMS/MFFK membrane decreases down to 34, which is typical for all the membranes based on polysiloxanes.