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
