High Surface Area Membrane Reactors

Conventional membrane surfaces arc cither flat (as a foil, plate or square channel) or regularly curved (as in a tube or circular channel). Potentially there may be stagnation zones in which carbonaceous materials arc deposited. Moreover, not every site on the membrane surface participates in the catalytic reaction. 

Membranes with non-traditional surfaces have been suggested [Gryaznov et al., 1975]. Pd-based membranes in the form of cellular foils were fabricated and used to carry out a hydrogen-consuming and a hydrogen-generating reaction on the opposite sides of the membrane. The foils have oppositely directed, alternate projections of hemispherical or half-ellipsoidal shapes. This type of configuration, in principle, provides uniform 

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It is important to attain as high an area as possible for a membrane reactor. Configurations with multilayer planar membranes, coiled membranes, or as multiple tubes also can be used for similar processes with potentially very high surface areas, as sketched in Figure 12-6. 

The next step for application is the development of handsome modules that can be combined in larger units for separation or catalytic membrane reactors. In separation one strives to high surface-area-to-volume ratios in catalytic membrane reactors this ratio will depend on the volumetric catalytic rates compared to the required permeation fluxes. 

Bicontinuous morphologies, such as the gyroid phase, formed by block copolymers are particularly interesting for generating nanoporous high surface area materials for potential applications as membrane reactors. Hashimoto et ak, used... 

In order to avoid carbon deposition, steam usually needs to be intentionally co-fed with the hydrocarbon vapor introduced into the reactor. Following initial fuel steam reforming, hydrogen, and to a lesser extent CO, are adsorbed on the high surface area fuel-side catalyst and react with oxygen anions to form H2O and CO2. The exothermic oxidation reactions produce heat which drives endothermic steam reforming reactions in a packed bed of catalyst adjacent to the membrane. Steam formed over the oxidation catalysts diffuses to the packed bed of steam reforming catalysts and reacts with hydrocarbon fuel by the reaction ... 

Besides, gas-liquid reactions can be performed within a membrane microreactor where membrane serves for product separation and thereby limits product inhibition [100]. In this version, the fabrication and operation of new hybrid membrane microreactors for gas-liquid-solid reactions is described. The reactors consist of porous stainless steel tubes onto which carbon nanofibers (CNFs) are grown as catalyst support (Figure 9.26). CNFs have high surface area, so they can be efficiently used as a catalyst support. 

Tubular carbon-coated cathodes. MFCs require high surface areas and porosities typical of wastewater reactors. One new approach to wastewater treatment has been to use tubular ultrafiltration membranes, providing high surface areas for filtering the treated water (180 to 6800 m /m ). Based on that idea of high surface areas provided by these membranes, Zuo et al. (2007) developed a tubular cathode by applying a... 

If the membrane surface area in the reactor is sufficiently high, conversion depends only on the selectivity of the membrane. In this case, all the natural gas that is not lost by transfer through the membrane will be converted in the reactor. Because of the large costs of the high... 

Non-oxide ceramic materials such as silicon carbide has been used commercially as a membrane support material and studied as a potential membrane material. Silicon nitride has also the potential of being a ceramic membrane material. In fact, both materials have been used in other high-temperature structural ceramic applications. Oxidation resistance of these non-oxide ceramics as membrane materials for membrane reactor applications is obviously very important. The oxidation rate is related to the reactive surface area thus oxidation of porous non-oxide ceramics depends on their open porosity. The generally accepted oxidation mechanism of porous silicon nitride materials consists of two... 

A fixed-bed reactor often suffers from a substantially small effectiveness factor (e.g., 10 to 10 for a fixed-bed steam reformer according to Soliman et al. [1988]) due to severe diffusional limitations unless very small particles are used. The associated high pressure drop with the use of small particles can be prohibitive. A feasible alternative is to employ a fluidized bed of catalyst powders. The effectiveness factor in the fluidized bed configuration approaches unity. The fluidization system also provides a thermally stable operation without localized hot spots. The large solid (catalyst) surface area for gas contact promotes effective catalytic reactions. For certain reactions such as ethylbenzene dehydrogenation, however, a fluidized bed operation may not be superior to a fixed bed operation. To further improve the efficiency and compactness of a fluidized-bed reactor, a permselective membrane has been introduced by Adris et al. [1991] for steam reforming of methane and Abdalla and Elnashaie [1995] for catalytic dehydrogenation of ethylbenzene to styrene. 

Membranes that arc catalytically active or impregnated with catalyst do not suffer from any potential catalyst loss or attrition as much as other membrane reactor configurations. This and the above advantage have the implication that the former requires a lower catalyst concentration per unit volume than the latter. It should be mentioned that the catalyst concentration per unit volume can be further increased by selecting a high "packing density" (surface area per unit volume) membrane element such as a honeycomb monolith or hollow fiber shape. 

The requirement for catalytic surface area may determine in what form the catalyst should be incorporated in the membrane reactor. If the underlying reaction calls for a very high catalytic surface area, the catalyst may need to be packed as pellets and contained inside the membrane tubes or channels rather than impregnated on the membrane surface or inside the membrane pores due to the limited available area or volume in the lauer case. 

Finally, some authors [31,130,131] employed 7-AI2O3 thin supported layers (pore size 4 nm) for ethylene partial oxidation in membrane reactors with separate feed of reactants. In such cases the membrane material had a specific surface area high enough to guarantee a direct catalyst support. 

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