VOCs (Volatile Organic Compounds)-contaminated water in chemical processes such as painting, solvent degreasing, printing and dry-cleaning causes many environmental problems and affects the health of plants and animals. Pervaporation of many separation technologies offers economical advantages in the removal of trace chlorinated hydrocarbon from aqueous solution.
Pervaporation is a membrane process in which a liquid mixture is in contact with the membrane on the feed side at atmospheric pressure and where permeate is removed as a vapor because of a low vapor pressure existing on the permeate side. Among various polymeric membranes, PDMS (polydimethylsiloxane) was used in pervaporation process because this polymer has shown good permeability together with reasonable selectivity.
From the pervaporation experiments, the flux of trichloroethylene (TCE) increased linearly with feed composition but the flux of water was slightly increased because of TCE-water coupling. And the separation efficiency of TCE was better than that of PCE. Higher flux was obtained at the lower downstream pressure and higher operating temperature. And flux of TCE was increased rapidly with feed flow rates because of the removal of concentration polarization. The selectivities for each case were shown inversely with flux.
In polymeric membrane separation performance depends on glass transition temperature and degree of crystallinity. The effect of crosslinking is similar to that of crystallinity and important in the crosslinked polymer such as PDMS. Therefore, permeation flux and selectivity depend strongly on the crosslinking density. Crosslinking densities of each membrane were measured by sorption experiment of cyclohexane.
High crosslinking density means the reduction of solubility and diffusivity for target component. The reduction of diffusivity for each membrane was calculated by sorption experiments of single component and Fujita's free volume theory. Flux and selectivity of trichloroethylene have a tendency to decrease with crosslinking density. Chain immobilization and reduction of diffusivity could explain these results.