The influence of polarity on flux and rejection behaviour in solvent resistant nanofiltration—Experimental observations

Abstract

The separation characteristics of a dense polydimethylsiloxane (PDMS) membrane were studied\nusing mixtures comprising xylene, cyclohexane or n-heptane with oxygenate components at\nconcentrations up to 75%. The effects of polarity on flux and rejection performance were\ndetermined through a test matrix of solvent type, concentration, filtration pressure, crossflow rate\nand the degree of membrane crosslinking.\nIn all cases involving alcohols, the more polar compound in the feed mixture was partially rejected\nby the membrane and the extent of rejection was dependent on the polarity as quantified by\nsolubility parameter. The rejection-concentration profiles for several alcohol/solvent mixtures\nexhibited a maximum, with the highest rejection around 30%. Mixtures containing MTBE did not\nseparate, i.e. no rejection was observed.\nRejection increased with increasing pressure and crossflow rate but was largely unaffected by the\ndegree of membrane crosslinking. Component flux was affected by the oxygenate concentration in\nthe mixture, which was attributed in part to changes in the degree of membrane swelling with\ncomposition. Experimental findings suggest that the separation is primarily governed by\nmulticomponent solvent/oxygenate/membrane swelling equilibria, and results compare favourably\nwith swelling isotherms available in the open literature.