PERVAPORATION MEMBRANES FOR ETHANOL-WATER MIXTURE PREPARED BY PLASMA POLYMERIZATION .3. PERFLUOROCARBON MEMBRANES

Pervaporation membranes for the ethanol-water mixture were prepared by plasma polymerization of tetrafluoroethylene, perfluoropropane, and perfluoropropylene onto porous substrates. The influence of the monomers on the elemental ratio (F/C) of the polymer depositions by X-ray photoelectron spectroscopy was rather small compared with that of the W/FM parameter (W = wattage for plasma excitation, FM = mass flow rate of a monomer). The optical emission spectroscopy indicated the similarity of gaseous species formed in the plasmas. The membranes were found ethanol-permselective, showing separation coefficients (alpha(EtOH)) around 4-7 and a wide range of permeation rates (J), 10-10(-2) kg/m2 h, for the 4.8 wt % ethanol solution at 40-degrees-C. The alpha(EtOH) of the membranes with thicker depositions could be correlated to the F/C ratios as a measure of membrane hydrophobicity. It was thought that, by making a plot alpha(EtOH) against J values for the perfluorocarbon membranes, they could be classified into three groups on thickness of deposition. The ethanol-separation mechanisms for each group, which may contain four kinds of mass transfer schemes, i.e., distillation through larger pores, flow of sorption layer at the liquid-membrane interface, and diffusions through deposition or substrate, were also discussed.