Membrane permselective

In open fibers the fiber wall may be a permselective membrane, and uses include dialysis, ultrafiltration, reverse osmosis, Dorman exchange (dialysis), osmotic pumping, pervaporation, gaseous separation, and stream filtration. Alternatively, the fiber wall may act as a catalytic reactor and immobilization of catalyst and enzyme in the wall entity may occur. Loaded fibers are used as sorbents, and in ion exchange and controlled release. Special uses of hoUow fibers include tissue-culture growth, heat exchangers, and others. 

Gas Separation. During the 1980s, gas separation using membranes became a commercially important process the size of this appHcation is stiH increasing rapidly. In gas separation, one of the components of the feed permeates a permselective membrane at a much higher rate than the others. The driving force is the pressure difference between the pressurized feed gas and the lower pressure permeate. 

Catalytic A catalytic-membrane reactor is a combination heterogeneous catalyst and permselective membrane that promotes a reaction, allowing one component to permeate. Many of the reactions studied involve H9. Membranes are metal (Pd, Ag), nonporous metal oxides, and porous structures of ceran iic and glass. Falconer, Noble, and Speriy [in Noble and Stern (eds.), op. cit., pp. 669-709] review status and potential developments. 

Films of the copolymers are, as with Nafion, saponified and used for permselective membranes. They have a much higher tensile strength than the Du Pont material and are also claimed to have a higher ion exchange capacity. 

There are two major frontiers in membrane research, one technological and the other scientific. At the technological frontier, chemical engineers can make important contribntions to the development of new materials, the engineering of stractnre or morphology into membranes, and the identification of new ways of using permselective membranes. 

Figure 10.5 Principle of operation of a catalyst particle coated with a permselective membrane (a) selective addition of reactants, (b) selective removal of products.

One of the most studied applications of Catalytic Membrane Reactors (CMRs) is the dehydrogenation of alkanes. For this reaction, in conventional reactors and under classical conditions, the conversion is controlled by thermodynamics and high temperatures are required leading to a rapid catalyst deactivation and expensive operative costs In a CMR, the selective removal of hydrogen from the reaction zone through a permselective membrane will favour the conversion and then allow higher olefin yields when compared to conventional (nonmembrane) reactors [1-3]... 

A special case of interfaces between electrolytes are those involving membranes. A membrane is a thin, ion-conducting interlayer (most often solid but sometimes also a solution in an immiscible electrolyte) separating two similar liquid phases and exhibiting selectivity (Fig. 5.1). Nonselective interlayers, interlayers uniformly permeable for all components, are called diaphragms. Completely selective membranes (i.e., membranes that are permeable for some and impermeable for other substances) are called permselective membranes. 

When the original compositions of the outer phases are different, the permselective membrane will prevent the complete leveling of these compositions. Some equilibrium component distribution between phases (a) and (p) will be established, and between points A and B a potential difference called the membrane potential (or transmembrane potential) (p will develop. This potential difference is determined by... 

The high specificity required for the analysis of physiological fluids often necessitates the incorporation of permselective membranes between the sample and the sensor. A typical configuration is presented in Fig. 7, where the membrane system comprises three distinct layers. The outer membrane. A, which encounters the sample solution is indicated by the dashed lines. It most commonly serves to eliminate high molecular weight interferences, such as other enzymes and proteins. The substrate, S, and other small molecules are allowed to enter the enzyme layer, B, which typically consist of a gelatinous material or a porous solid support. The immobilized enzyme catalyzes the conversion of substrate, S, to product, P. The substrate, product or a cofactor may be the species detected electrochemically. In many cases the electrochemical sensor may be prone to interferences and a permselective membrane, C, is required. The response time and sensitivity of the enzyme electrode will depend on the rate of permeation through layers A, B and C the kinetics of enzymatic conversion as well as the charac-... 

If the electrochemical sensor does not require a permselective membrane, immobilization of the enzyme onto the surface of the electrode is possible. Glassy carbon graphite reticulated vitreous carbon and carbon paste electrodes... 

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

Basically, three kinds of membranes are being studied inorganic oxide membranes, polymer-based membranes, and metal and metal alloy membranes. Some combinations of these are also used, such as impregnating inorganic oxide membranes with catalytic materials. A key term in this held is permselective membrane, which is a thin material that can allow a certain component of a mixture, but not other components, to pass through (or permeate) from one side to the other. 

Quantitative analytical treatments of the effects of mass transfer and reaction within a porous structure were apparently first carried out by Thiele (20) in the United States, Dam-kohler (21) in Germany, and Zeldovitch (22) in Russia, all working independently and reporting their results between 1937 and 1939. Since these early publications, a number of different research groups have extended and further developed the analysis. Of particular note are the efforts of Wheeler (23-24), Weisz (25-28), Wicke (29-32), and Aris (33-36). In recent years, several individuals have also extended the treatment to include enzymes immobilized in porous media or within permselective membranes. The important consequence of these analyses is the development of a technique that can be used to analyze quantitatively the factors that determine the effectiveness with which the surface area of a porous catalyst is used. For this purpose we define an effectiveness factor rj for a catalyst particle as... 

Koresh, J.E. and A. Sofer, Molecular sieve carbon permselective membrane, Part I. Presentation of a new device for gas mixture separation, Sep. Sci. Technol., 18, 723, 1983. 

Grafting of functional monomers onto fluoropolymers produced a wide variety of permselective membranes. Grafting of styrene (with the following sulfonation), (meth)acrylic acids, 4-vinylpyridine, A-vinylpyrrolidone onto PTFE films gave membranes for reverse omosis,32-34 ion-exchange membrane,35-39 membranes for separating water from organic solvents by pervaporation,49-42 as well as other kinds of valuable membranes. 

Sloot, H. 1991. A non-permselective membrane reactor for catalytic gas phase reactions. Thesis, University of Twcntc, Enschede. 

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