Removal of trace organics from aqueous solutions: Effect of membrane thickness

A resistance-in-series model is used to describe the pervaporation performance of elastomeric membranes in the removal of volatile organic components from water. Equations have been derived to describe the organic component flux as a function of feed concentration, permeability of the organic component in the membrane, membrane thickness and liquid boundary layer mass transfer coefficient. The model has been verified using both homogeneous and composite membranes of polydimethylsiloxane, ethylene propylene rubber and polyoctenamer. Membranes with a wide range of thicknesses have been prepared and the pervaporation behaviour for the removal of toluene and trichloroethylene from aqueous solutions has been studied. The experiments show that the hydrodynamic boundary layer resistance is of great importance. For highly permeable polymers such as polydimethylsiloxane mass transfer in the boundary layer is rate determining and should be considered carefully in further development of the process. For less permeable polymers such as ethylene propylene rubber this effect becomes more dominant with decreasing membrane thickness. The water fluxes are inversely proportional to the thickness of the actual separating layer and they depend strongly on the type of elastomer used. A proper choice of the elastomeric material and the thickness of the separating layer will determine the selectivity of the process.