Ramin Daniels

The role of boundary layers in the removal of volatile organic compounds from water by pervaporation

Removal of volatile organic compounds (VOCs) from water by pervaporation is dominated by boundary layer effects (concentration polarization). A simple analysis shows these effects to be much more severe in pervaporation than in ultrafiltration and reverse osmosis because of the high VOC enrichment that can be obtained by pervaporation. In pervaporation, the concentration of solute at the membrane surface is often one-tenth or less of the concentration in the bulk solution because of the huge concentration polarization effect. In this paper, we present a rigorous treatment of concentration polarization using the resistances-in-series model and include the contribution of convective flow to transport in the boundary layer. The resulting general expression is valid for compounds that are enriched in the permeate as well as for compounds that are depleted in the permeate. The effects of operating conditions on pervaporation performance are discussed, and compared to data obtained with spiral-wound modules. Experimental data demonstrate that increasing the permeate pressure in pervaporation does not necessarily reduce the VOC flux although it reduces the driving force for permeation.

The effect of concentration polarization on the separation of volatile organic compounds from water by pervaporation

Concentration polarization dominates the separation of dissolved volatile organic compounds from water by pervaporation. This is particularly true with hydrophobic organics, such as toluene and trichloroethylene, for which concentration polarization is severe even in highly turbulent membrane modules. With these compounds, measured separation factors can be 10 to 20% of the intrinsic separation factors in the absence of concentration polarization. As a result of concentration polarization, unexpected permeation properties are observed. For example, the organic flux is independent of membrane thickness over a wide range, whereas the water flux decreases with membrane thickness. Consequently, thicker membranes are preferred over thinner ones. Also, the organic flux is relatively independent of permeate pressure over a wide range, whereas the water flux decreases as the permeate pressure increases. This means that the separation performance improves as the driving force across the membrane decreases, contrary to normal membrane behavior. These and other consequences of concentration polarization are described in this paper.

Olefin Recovery from Chemical Industry Waste Streams

The objective of this project was to develop a membrane process to separate olefins from paraffins in waste gas streams as an alternative to flaring or distillation. Flaring these streams wastes their chemical feedstock value; distillation is energy and capital cost intensive, particularly for small waste streams.