The influence of polarity on flux and rejection behaviour in solvent resistant nanofiltration - experimental observations

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