Membranes and Modules

Membranes

With the much broader knowledge of membrane structure and membrane manufacture accumulated in the development of desalination membranes in the 1970s pervaporation processes gained new interest. The separation characteristic of a membrane process is determined by the difference in transport rates of the components through the membrane only, not by liquid-vapour equilibria, and azeotropic mixtures can easily be separated. Since only the heat of evaporation of the permeate vapour is lost a single step membrane process saves energy compared with, e.g., distillation.

Two different types of pervaporation membranes were developed at about the same time in the beginning of the 1980s:

• Hydrophilic membranes, with a preferential permeation for water, used mainly for the removal of water from organic solvents and solvent mixtures, with an emphasis on azeotropic mixtures.

• Organophilic membranes for the removal of volatile organic components from water and gas streams.

In both applications composite membranes are used, allowing for very thin separation layers but with sufficient chemical, mechanical and thermal stability. Because the composite structure flat sheet configurations are preferred. The substructure of both types of pervaporation membranes is very similar: a porous support membrane with an asymmetric pore structure is laid onto a carrier layer of a woven or non-woven textile fabric and a basic ultrafiltration membrane is formed. On the free side of this porous substructure the pores have diameters in the order of 20-50 nm which widen up to the fabric side to the micrometre range. Polyester, polypropylene and similar fibres are used for the textile carrier layer; structural polymers such as polyacrylonitrile, polyetherimide, polysulfone, polyethersulfone and polyvinyliden fluoride form the porous support.

On this substructure a thin dense layer (in the range of 0.5-5 |im thick) is coated, which effects the separation. Different coating techniques are in use, most commonly a solution of the respective polymer in an appropriate solvent is spread onto the porous substructure. The solvent is then evaporated, followed by further treatment to effect cross-linking of the polymer.

In hydrophilic membranes the separating layer is made from cross-linked polyvinyl alcohol (PVA), from polyimides, or natural polymers such as chitosan or cellulose acetate (CA), with PVA dominant. For organophilic membranes, the separation layer is formed mostly from siloxanes such as polydimethylsiloxane (PDMS), or polyoctylmethyl siloxane (POMS).

In recent years new efforts have been made in academia and industry to develop new membranes for organic-organic separation. Of specific interest are the separation of olefins from paraffins, e.g. propene from propane, aromatics such as benzene or toluene from aliphatic hydrocarbons or the separation of the xylene isomers. To date, no industrialization has been achieved. The only industrial processes in this area are the separation of the light alcohols methanol and ethanol from their mixtures with hydrocarbons, ethers and esters. The membranes in use are, however, more of the hydrophilic type, in which the more polar alcohols replace the water.

To date, only polymeric membranes have been applied in pervaporation and vapour permeation processes. Thermal, mechanical and chemical stability of the porous substructure are limiting the operation range of this type of membrane, more than the stability of the separating layer. Demand for higher operation temperatures and chemical resistance have stimulated the development of inorganic substructures, and porous ceramics in particular. These can be coated by cross-linked polymeric separating layers similar to those on polymeric substructures. In more recent developments organic separation layers are applied, either by coating the porous substructure with a layer of zeolites or by reducing the size of the pore to molecular dimensions. The separation mechanism of these membranes is even more complex than that of polymeric separating layers, as molecular sieving effects, caused by shape and size of molecules, and molecule-surface interaction decide whether a component can pass through the membrane or will be retained.

Modules

Design of modules for pervaporation and vapour permeation processes was based on the experience gained in water treatment by membranes, such as ultrafiltration and reverse osmosis processes. However, significant modifications had to be made because of the specific requirements of pervaporation and vapour permeation processes.

The partial vapour pressure at the permeate side has to be reduced in both processes to fairly low values, especially when low final concentrations have to be reached in the retentate. Therefore any pressure losses, even in the range of a few millibars have to be avoided at the permeate side. Since any feed mixture will contain organic components at high concentration, mostly at elevated temperatures, the chemical stability of all module components, such as spacer and potting material and glues is critical. To date, two types of modules are most widely applied:

• Plate modules, mainly used for dehydration applications, with permeate channels as open as applicable. Stainless steel is used as a construction material for support plates for the membranes and for spacers. The permeate channels are preferably open over the circumference of the modules which

Figure 1 Vapour-liquid equilibrium curves for common pharmaceutical solvents which azeotrope with water. All can be dehydrated using pervaporation.

are assembled inside a special vacuum vessel that also house the permeate condenser. Alternative designs are very similar to plate heat exchangers, in which the supported membrane replace the heat exchanger plates. These modules are closed to the outside, with internal ducts feed and retentate, and for permeate removal. • Spiral wound modules with stainless steel central tubes, but otherwise similar to those known from the conventional membrane processes, are mainly used for organophilic membranes. One or several of the spiral wound modules are housed inside a pressure tube and assembled in conventional skids. In a special design, the sandwich structures of membranes and permeate and feed spacer are welded together and not spirally wrapped around the central tube but arranged as Sat sheets on the central tube for the removal of the permeate.

Very rarely, hollow fibres are used, generally with the feed flow inside the bore of the fibre. For the more conventional arrangement - feed Sow at the shell side of the fibre - permeate pressure losses inside the bore may become detrimental for the process.

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Solar Panel Basics

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