Membranes silicon carbide

The dip and spin coating methods are the most practiced techniques as indicated in the open literature. Many materials have been made into porous membranes by this route including the more commonly referenced materials such as metal oxides, silicon carbide, silicon nitride, silicon and aluminum oxynitrides and glasses. 

Partially pvrolvzed poivsilastyrene membranes. Molecular sieve membranes can also be made from porous solids other than carbon and zeolitic materials. One such potential candidate is the family of precursors of silicon carbide. Among the possible precursors, polysilastyrene (phenylmeihylsilane-dimethylsilane copolymer) is commercially available and soluble in many common solvents such as toluene and tetrahydrofuran and can be crosslinked by UV radiation. Polysilastyrene is comprised of long chains of silicon atoms ... 

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... 

Different supports are used, (see Section 10.6.4) with different geometry (discs or tubes), thickness, porosity, tortuosity, composition (alumina, stainless steel, silicon carbide, mullite, zirconia, titania, etc.), and symmetry or asymmetry in its stmcture. Tubular supports are preferable compared to flat supports because they are easier to scale-up (implemented as multichannel modules). However, in laboratory-scale synthesis, it is usually found that making good quality zeolite membranes on a tubular support is more difficult than on a porous plate. One obvious reason is the fact that the area is usually smaller in flat supports, which decreases the likelihood of defects. In Figure 10.1, two commercial tubular supports, one made of a-alumina (left side) and the other of stainless steel (right side) used in zeolite membrane synthesis, are shown. Both ends of the a-alumina support are glazed and both ends of the stainless steel support are welded with nonporous stainless steel to assure a correct sealing in the membrane module and prevent gas bypass. 

The synthesis of nanophase ceramics is one of these concepts, it allows micro-porous ceramic materials with ceramic grains in the nanometer range to be obtained. Research in the field of nanophase materials is very active. A number of results on the control of microstructure and temperature stability of metal oxide ceramics can be applied to membrane preparation. Works carried out on non-oxide ceramics such as silicon carbide, silicon oxinitride or aluminum nitride should be regarded in order to extend the domain of available membrane materials. 

The Crystal FT filters are made of 100% self-bonded, re crystallised silicon carbide wuth multi-layer SiC membranes. The filter carrier is characterised by high porosity and an open three-dimensional pore structure, Crystar says modules with 4CFT and 5CFT filters showed time and energy savings of 36%. 

The group of Nishimura at Tsukuba has employed a wide variety of deposition techniques in their membrane work [59-68]. In their early studies, palladium was electroplated onto both sides of the substrates [61]. Electroless deposition using PdCl2 with SnCl2 sensitizers was also used [59, 60]. The authors recommend abrasion using 0.05 pm (50 nm) diameter alumina particles to remove thick native layers of contaminants after alloy fabrication and rolling of foils [60]. It should be noted that workers skilled in the art avoid abrasives such as diamond paste or silicon carbide (SiC) to nunimize possible formation of refractory carbides or silicides with the reactive substrates. 

Zeolite monoliths have been found to be useful for such applications as rotatory adsorbers for use as dehumidifiers and desiccant cooling processes [360] or in VOC treatment systems [379]. Alumina-coated silicon carbide monoliths have also been employed as supports for B-ZSM-5 membranes [380] providing a larger surface area per unit volume, compared to traditional membrane supports. With these membranes, the same authors have reported n/t-butane and Hj/t-butane separation selectivities of 35 and 77, respectively [119]. Also, SIL-1 membranes supported on stainless steel grids (Figure 11.30) have shown a good performance in the separation of /t-butane mixtures, with separation factors as high as 53 at 63°C [362]. 

Wach et al. used a precursor polymer blend to develop a silicon carbide ceramic coating on porous alumina substrate as a gas separation membrane. A small amount (5 to 20 wt%) of polyvinylsilane was blended with... 

Wach, R. A., Sugimoto, M., Yoshikawa, M., Formation of silicone carbide membrane by radiation curing of polycarbosilane and polyvinylsilane and its gas separation up to 250°C. Journal of the American Ceramic Society 2007, 90(1), 275-278. 

The choice of the catalyst structure was optimized. Indeed, our previous activity was relevant to the optimization of the temperature profile along the catalytic bed, in order to ensure the lowest strain for the operation of the membrane [64, 65]. We found that a high thermal conductivity material, such as silicon carbide, along with a radial flow geometry may lead to a more isothermal profile along the catalytic bed. 

The reactor start-up followed the step of membrane thermal pre-treatment (Fig. 6.8) the electrical heating tapes were turned off and the reactor was fed with a mixture of methane, air and nitrogen in order to heat the combustion zone and the catalyst up to the operating conditions in two different steps. In the first step methane combustion occurred homogeneously on the silicon carbide foam. 

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