Ion transport membranes applications

Oxygen-Ion Transport Membrane Applications in Selective Oxidation... 

FI CU RE 4.29 A schematic illustration of the ion transport membrane (ITM) device developed and patented by Air Products and Chemicals, Inc. The supported membrane wafers are separated by spacer rings and attached to a common product withdrawal tube. (From Armstrong P. A., Stein V.E.E., Bennet D.I., Foster E.P., Ceramic Membrane Development for Oxygen Sypply to Gasification Applications, Air Products and Chemicals, Inc., Allentown, PA, 2002. With permission.)... 

This article provides a brief survey of oxygen-ion transport membrane materials and their applications as membrane reactors in the selective oxidation of light alkanes. 

Application in catalyst and inorganic membranes preparation. J. Mater. Chem. 1999 9 55-65 Guizard C., Julbe A. Nanophase ceramic ion transport membranes for oxygen separation and gas stream enrichment. In Recent Advances in Gas Separation by Microporous Ceramic Membranes, Kanellopoulos N.K., ed., Amsterdam Elsevier, 2000, pp. 435-471 Guizard C., Barboiu M., Bac A., Hovnanian N. Hybrid organic-inorganic membranes with specific transport properties. Applications in separation and sensors technology. Separ. Purif. Technol. 2001 25 167-180... 

Solid mixed ionic-electronic conductors (MIECs) exhibit both ionic and electronic (electron-hole) conductivity. Naturally, in any material there are in principle nonzero electronic and ionic conductivities (a i, a,). It is customary to limit the use of the term MIEC to those materials in which a, and 0, 1 do not differ by more than two orders of magnitude. It is also customary to use the term MIEC if a, and Ogi are not too low (o, a i 10 S/cm). Obviously, there are no strict rules. There are processes where the minority carriers play an important role despite the fact that 0,70 1 exceeds those limits and a, aj,i< 10 S/cm. In MIECs, ion transport normally occurs via interstitial sites or by hopping into a vacant site or a more complex combination based on interstitial and vacant sites, and electronic (electron/hole) conductivity occurs via delocalized states in the conduction/valence band or via localized states by a thermally assisted hopping mechanism. With respect to their properties, MIECs have found wide applications in solid oxide fuel cells, batteries, smart windows, selective membranes, sensors, catalysis, and so on. 

In connection with the content of this section, dynamic features of ion transports through polyvinyl chloride membranes [27,28], ion-exchange resin membranes [29,30], or BLMs [31-36] have been discussed in the light of VCTTMs. For wide and pertinent applications of the VCTTM, however, further investigations have been required on the experimental and theoretical methods to analyze VCTTM quantitatively. 

Extensive studies have been carried out concerning ion transfers, electron transfers and combinations of ion and electron transfers at liquid-liquid interfaces. Po-larography and voltammetry at liquid-liquid interfaces are of analytical importance, because they are applicable to ionic species that are neither reducible nor oxidizable at conventional electrodes. They are also usefid in studying charge-transfer processes at liquid-liquid interfaces or at membranes solvent extractions, phase transfer catalyses, ion transport at biological membranes, etc. are included among such processes. 

An outstanding example of the application of the theories and methods of electrochemical kinetics to an apparently different field of high interest in biological science is found in the fundamental investigation of ion transport through biological membranes. Two concise, but very clear reviews on this subject have been written by de Levie [108, 109] references to other reviews and further bibliography can be found therein. 

We summarize what is special with these prototype fast ion conductors with respect to transport and application. With their quasi-molten, partially filled cation sublattice, they can function similar to ion membranes in that they filter the mobile component ions in an applied electric field. In combination with an electron source (electrode), they can serve as component reservoirs. Considering the accuracy with which one can determine the electrical charge (10 s-10 6 A = 10 7 C 10-12mol (Zj = 1)), fast ionic conductors (solid electrolytes) can serve as very precise analytical tools. Solid state electrochemistry can be performed near room temperature, which is a great experimental advantage (e.g., for the study of the Hall-effect [J. Sohege, K. Funke (1984)] or the electrochemical Knudsen cell [N. Birks, H. Rickert (1963)]). The early volumes of the journal Solid State Ionics offer many pertinent applications. 

Membranes and Osmosis. Membranes based on PEI can be used for the dehydration of oiganic solvents such as 2-propanol, methyl ethyl ketone, and toluene (451), and for concentrating seawater (452—454). On exposure to ultrasound waves, aqueous PEI salt solutions and brominated poly(2,6-dimethylphenylene oxide) form stable emulsions from which it is possible to cast membranes in which submicrometer capsules of the salt solution are embedded (455). The rate of release of the salt solution can be altered by surface—active substances. In membranes, PEI can act as a proton source in the generation of a photocurrent (456). The formation of a PEI coating on ion-exchange membranes modifies the transport properties and results in permanent selectivity of the membrane (457). The electrochemical testing of salts (458) is another possible application of PEI. 

Commercially available membranes are usually reinforced with woven, synthetic fabrics to improve the mechanical properties. Several hundred thousand square meters of IX membranes are now produced annually, and the mechanical and electrochemical properties are varied by the manufacturers to suit the proposed applications. The electrochemical properties of most importance for ED are (/) the electrical resistance per unit area of membrane (2) the ion transport number, related to current efficiency (3) the electrical water transport, related to process efficiency and (4) the back-diffusion, also related to process efficiency. 

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