Composite membranes support layer, importance

One of the most important degradation mechanisms of SLM is an emulsification ofthe membrane phase due to lateral shear forces. Therefore, formation of barrier layers on the membrane surface by physical deposition [98] or by interfacial polymerization could prevent instability [99, 100]. A polysulfone support with N-methylpyiTolidone as a solvent was coated by a poly(ether ketone) layer as the outside layer and gave a specific composite membrane support. Such composite hoUow-fiber membranes showed significant improvement in stabUity in copper ions permeation. 

An excellent review of composite RO and nanofiltration (NE) membranes is available (8). These thin-fHm, composite membranes consist of a thin polymer barrier layer formed on one or more porous support layers, which is almost always a different polymer from the surface layer. The surface layer determines the flux and separation characteristics of the membrane. The porous backing serves only as a support for the barrier layer and so has almost no effect on membrane transport properties. The barrier layer is extremely thin, thus allowing high water fluxes. The most important thin-fHm composite membranes are made by interfacial polymerization, a process in which a highly porous membrane, usually polysulfone, is coated with an aqueous solution of a polymer or monomer and then reacts with a cross-linking agent in a water-kniniscible solvent. 

Another important group of anisotropic composite membranes is formed by solution-coating a thin (0.5-2.0 xm) selective layer on a suitable microporous support. Membranes of this type were first prepared by Ward, Browall, and others at General Electric [52] and by Forester and Francis at North Star Research [17,53] using a type of Langmuir trough system. In this system, a dilute polymer solution in a volatile water-insoluble solvent is spread over the surface of a water-filled trough. 

If a membrane has a graded pore structure but is made in one processing step, frequently from the same material across its thickness, it is called an asymmetric membrane. If, on the other hand, the membrane has two or more distinctively different layers made at different steps, the resulting structure is called a composite membrane. Almost invariably in the case of a composite membrane, a predominantly thick layer provides the necessary mechanical strength to other layers and the flow paths for the permeate and is called the support layer or bulk support. Composite membranes have the advantage that the separating layer and the support layer(s) can be tailored made with different materials. Permselective and permeation properties of the membrane material are critically important while the material for the support layer(s) is chosen for mechanical strength and other consideration such as chemical inertness. The composite membranes can have... 

In porous composite membranes, the support layer(s) can play an important role in the reactor performance. This, for example, is the case with consecutive reactions such as partial oxidations where intermediate products are desirable. Harold et al. [1992] presented a concept in which two reactants are introduced to a two-layer membrane system from opposite sides ethylene on the membrane side while oxygen on the support side. The mass transfer resistance of the support layer lowers the oxygen concentration in the catalytic zone and directs the preferred intermediate product, acetaldehyde, toward the membrane side. Thus the support layer structure enhances the yield of acetaldehyde. 

An ideal pervaporation membrane should consist of an ultra thin defect free skin layer (dense layer) supported by a porous support. The skin layer is perm-selective and hence responsible for the selectivity of the membrane. However, the porous support also plays an important role in overall performance of the membrane. The effect of the porous support, of a composite membrane, on the permeation properties of the membrane is discussed in details in the composite membranes... 

To ensure high permeabilities, it is important to work with low membrane thickness without compromising membrane integrity. For this purpose, several techniques for the production of composite membranes, in which thin palladium alloy layers are deposited onto porous supports have been developed (Fig. 9.8) and are summarized by Drioli et al. [11]. The main problems related to composite membranes concern the achievement of defect-free deposited layers which maintain performance both with time, and also with thermal cycling. Usually, the dif-... 

Numerous kinds of polymers in combination with solvents and other additives are used for preparation of membranes for specific purposes. In reverse osmosis, for example, composite membranes have become very important and have been studied intensively. These membranes are composed of two layers, each with a specific function, such as mechanical support or selective separation. Table 2 shows some examples of polymers used for preparation of reverse osmosis membranes. 

Controlled removal of the template is especially important when zeolite based membranes are involved consisting of a continuous MFI layer on a ceramic or sintered metal support (ref. 14). In these novel composite ceramic membranes the formation of cracks during template removal would be detrimental. The unique properties (ref. 14) of metal-supported MFl-layer membranes prove that indeed crack formation can be essentially prevented. 

The difference in coefficients of thermal expansion between the substrate and the deposited film is another important factor to consider. Table 13.2 lists the coefficients of thermal expansion for some materials relevant to this discussion. It is clear that these materials cover a wide range of values of the coefficients of thermal expansion. Since process applications of the composite Pd and Pd/alloy membranes involve high temperatures and temperature cycling in certain cases, the matching of the values between the support and the membrane layer is one of the critical factors for obtaining a leak-stable membrane. 

Interfacial polymerization has become a very important and useful technique for the synthesis of thin-film composite RO and NF membranes [5, 13]. Polymerization occurs at the interface between two immiscible solvents that contain the reactants (Fig. 3.6-8). For instance, a UF membrane is immersed in an aqueous diamine solution. The excess of water is removed, and the saturated support is put in contact with an organic phase that contains an acyl chloride. As a consequence, the two monomers react to form a thin layer (1 to 0.1 pm) of PA on top of the U F membrane. 

Types of membranes for reverse osmosis. One of the more important membranes for reverse-osmosis desalination and many other reverse-osmosis processes is the cellulose acetate membrane. The asymmetric membrane is made as a composite film in which a thin dense layer about 0.1 to 10 pm thick of extremely fine pores supported upon a much thicker (50 to 125 pm) layer of microporous sponge with little resistance to permeation. The thin, dense layer has the ability to block the passage of quite small solute molecules. In desalination the membrane rejects the salt solute and allows the solvent water to pass through. Solutes which are most effectively excluded by the cellulose acetate membrane are the salts NaCl, NaBr, CaClj, and NajSO sucrose and tetralkyl ammonium salts. The main limitations of the cellulose acetate membrane are that it can only be used mainly in aqueous solutions and that it must be used below about 60°C. 

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