Johannes G. Wijmans

The separation of dissolved organics from water by pervaporation

The ability of thin composite membranes to separate dissolved organic solvents from water by pervaporation has been examined. The separation performance of a pervaporation process was considered as a combination of an evaporation step and a diffusion step. For the experimental work, poly (dimethylsiloxane) and polyolefin composite membranes were incorporated into 0.1–0.3 m2, 2-inch diameter laboratory spiral-wound test modules. Typical fluxes were in the range 0.2–1.5 L/m2-h, depending on the operating conditions and the feed solution. The membranes used in this work were non-selective for polar solvents such as ethanol (αmem< 1) so that the overall enrichment of polar solvents in the permeate was typically 5–10 fold. With non-polar solvents such as ethyl acetate, chloroform and 1,1,2-trichloroethane, where the membrane selectivity, αmem, is greater than 1, organic enrichment in the permeate of 50–400 fold was obtained. Pervaporation therefore appears to be an economically viable method of removing non-polar solvents from dilute aqueous streams. Preliminary calculations show plant capital costs in the range

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

2–5/gal of feed water per day and operating costs of

Gas permeation through composite membranes

2–10/1,000 gal of feed water treated.

The design of membrane vapor–gas separation systems

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

Abstract Composite gas separation membranes generally consist of a selective, ultrathin top layer backed by a nonselective porous support. The top layer performs the separation and the support provides mechanical strength. A composite membrane will possess a selectivity close to the intrinsic selectivity of the top layer only if most of the permeation resistance lies within this top layer. To make high flux composite membranes, it is necessary to minimize the thickness of the selective layer. This, in turn, means the porous support must be very permeable. A simple model shows that the minimum thickness of a defect-free selective layer in a composite membrane having the intrinsic selectivity of the permselective layer is limited by the resistance of the porous support membrane. The model is supported by experimental gas and vapor permeation data for polysulfone-silicone rubber composites.

A Vapor Permeation Processes for the Separation of Aromatic Compounds from Aliphatic Compounds

Abstract Membrane vapor–gas separation systems are beginning to be applied to a number of gas separation problems in the petrochemical and refinery areas. In this paper, some of the factors that affect the design of these systems are described using, as an application example, the separation of propylene from nitrogen in polyolefin resin degassing vents.

Olefin Recovery from Chemical Industry Waste Streams

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.

Permeability, permeance and selectivity: A preferred way of reporting pervaporation performance data

A number of rubbery and glassy membranes have been prepared and evaluated in vapor permeation experiments for separation of aromatic/aliphatic mixtures, using 5/95 (wt:wt) toluene/methylcyclohexane (MCH) as a model solution. Candidate membranes that met the required toluene/MCH selectivity of ≥ 10 were identified. The stability of the candidate membranes was tested by cycling the experiment between higher toluene concentrations and the original 5 wt% level. The best membrane produced has a toluene permeance of 280 gpu and a toluene/MCH selectivity of 13 when tested with a vapor feed of the model mixture at its boiling point and at atmospheric pressure. When a series of related membrane materials are compared, there is a sharp trade-off between membrane permeance and membrane selectivity. A process design study based on the experimental results was conducted. The best preliminary membrane design uses 45% of the energy of a conventional distillation process.

Membrane separation of nitrogen from natural gas: A case study from membrane synthesis to commercial deployment

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.