Comparison of pervaporation and perstraction for the separation of p‑xylene/m‑xylene mixtures using PDMS and CTA membranes

The separation of the individual xylene isomers remains challenging and energy intensive. Since the utilization of membrane separations could bring an improvement, we compare the efficacy of two related membrane methods, pervaporation and perstraction, for the separation of p‑xylene/m‑xylene mixtures using membranes from polydimethylsiloxane (PDMS) and cellulose triacetate (CTA) at 40 °C. Methanol was used as the receiving liquid in perstraction and as a membrane entrainer; feeds containing 0–80 mol.% of methanol were studied. While PDMS showed almost no selectivity to p‑xylene over m‑xylene during pervaporation, the selectivity reached up to 1.40 when operated in the perstraction mode with the feed enriched with methanol. CTA performed better than PDMS for all studied mixtures, the highest selectivity of 1.93 was observed for the perstraction of the molten eutectic mixture with no methanol in the feed. The likely reason for the enhancement of the selectivity are the isomer-specific interactions with the polar moieties in the polymer plasticized by methanol. Overall, it was demonstrated that a simple change in the mode of operation from pervaporation to perstraction and the use of a membrane entrainer can appreciably enhance the p‑xylene to m‑xylene selectivity of the common and reliable membrane materials.