Silicon carbide reactor

In addition to flow systems that use conventional heating, microwave-assisted flow chemistry has also been reported [78,86-88]. Organ reported the synthesis of benzimidazole through microwave-assisted flow chemistry (Scheme 1.16) [86]. One of the critical components of the flow system was a silicon carbide reactor tube. As mentioned previously, these materials have a very high absorption of microwave radiation and heat very rapidly upon irradiation. The ability of the silicon carbide to transfer heat is also very high thus, this material is ideally suited for the constraction of a reactor tube that wiU be used... 

The fuel for the Peach Bottom reactor consisted of a uranium-thorium dicarbide kernel, overcoated with pyrolytic carbon and silicon carbide which were dispersed in carbon compacts (see Section 5), and encased in graphite sleeves [37]. There were 804 fuel elements oriented vertically in the reactor core. Helium coolant flowed upward through the tricusp-shaped coolant channels between the fuel elements. A small helium purge stream was diverted through the top of each element and flowed downward through the element to purge any fission products leaking from the fuel compacts to the helium purification system. The Peach... 

Fluidized-bed CVD was developed in the late 1950s for a specific application the coating of nuclear-fuel particles for high temperature gas-cooled reactors. PI The particles are uranium-thorium carbide coated with pyrolytic carbon and silicon carbide for the purpose of containing the products of nuclear fission. The carbon is obtained from the decomposition of propane (C3H8) or propylene... 

The literature proposes a relatively large number of HEX reactors that have been designed and built of different materials such as glass, stainless steel, polyether ether ketone (PEEK), and silicon carbide (SiC). A presentation can be found in Amdoimaz et al. [13]. 

Gas-phase process, of ethylene-propylene polymer manufacture, 10 711 Gas-phase reactions flow mixing for, 14 613 pressure and, 14 623 Gas-phase reactor (GPR), 20 533 Gas-phase sedimentation, 18 142 Gas-phase synthesis, in silicon carbide manufacture and processing, 22 533 Gas pipeline systems, 12 366 Gas pretreatment, 13 841 Gas processing, in petroleum refining, 18 663... 

Zinc oxide in solid or fine particle form is kept in a reactor cavity that is subjected to irradiation from solar concentrators [92], The dissociation products are zinc (vapor) and oxygen for this first reaction AG=0 at about 2235K [91], The reactor is made of materials like inconel steel, zirconia, silicon carbide or graphite [68,89,92], The graphite is used in special designs to avoid direct contact with chemical species [68], The dissociation products are then cooled rapidly to separate zinc and oxygen, transporting the... 

A review article on the CVD processes used to form SiC and Si3N4 by one of the pioneers in this area, Erich Fitzer [Fitzer, E., and D. Hegen, Chemical vapor deposition of silicon carbide and silicon nitride—Chemistry s contribution to modem silicon ceramics, Angew. Chem. Int. Ed. Engl, 18, 295 (1979)], describes the reaction kinetics of the gas-phase formation of these two technical ceramics in various reactor arrangements (hot wall, cold... 

Silicon carbide s relatively low neutron cross section and good resistance to radiation damage make it useful in some of its new forms in nuclear reactors (qv). Silicon carbide temperature-sensing devices and structural shapes fabricated from the new dense types are expected to have increased stability. Silicon carbide coatings (qv) may be applied to nuclear fuel elements, especially those of pebble-bed reactors, or silicon carbide may be incorporated as a matrix in these elements (153,154). 

Figure 2 is a schematic diagram of the two-section reactor body and accessories. The fluid bed section was made of self-bonded silicon carbide, 16 inches high by 6% inches o.d. with a recessed flange. The recess accommodated a 120-mesh porosity silicon carbide gas distribution plate. The nickel manifold assembly was topped by a heavy support flange. This manifold sup-... 

Fig. 5. Radioactivity after shutdown per watt of thermal power for A, a liquid-metal fast breeder reactor, and for a D—T fusion reactor made of various structural materials B, HT-9 ferritic steel C, V-15Cr-5Ti vanadium—chromium—titanium alloy and D, silicon carbide, SiC, showing the million-fold advantage of SiC over steel a day after shutdown. The radioactivity level after shutdown is also given for E, a SiC fusion reactor using the neutron reduced...

The reactor wells are pre-loaded with catalyst (diluted with silicon carbide if desired) and inserted into the module. Two graphite seals are used per well, one to pressure seal each vial and one to prevent bypass flow around the vial. A single central bolt in tension supplies the sealing force onto these seals (Fig. 3.12b). 

Pebble-Bed Modular Reactor (PBMR) A nuclear reactor technology that utilizes tiny silicon carbide-coated uranium oxide granules sealed in pebbles about the size of oranges, made of graphite. Helium is used as the coolant and energy transfer medium. This containment of the radioactive material in small quantities has the potential to achieve an unprecedented level of safety. This technology may become popular in the development of new nuclear power plants. 

The research at MIT has been done in the cold-wall vertical tube reactor shown in Figure 14. The wafer is aligned almost parallel to the flow on a vertical silicon carbide-coated susceptor. The wafer is heated by optical radiation from high-intensity lamps to a temperature of 775°C. Silane was introduced... 

Just as in the new Applied Materials reactor, the problem being addressed is the reduced throughput with conventional epi reactors as wafers get larger. In this reactor configuration, 50 wafers can be placed on the 25 tapered cavities placed radially within the bell jar. There are resistance heaters above and below the silicon carbide-coated graphite wafer holders, and heat loss at the outer periphery is compensated for with external heat lamps. 

A recuperative bayonet sulphuric acid decomposition reactor has been designed by researchers at Sandia National Laboratories (SNL) that features all-silicon carbide (SiC) construction for the heated parts, can be made from readily available SiC shapes, makes the most use of heat recuperation, and has all of its fluid connections at sufficiently low temperatures that conventional seal materials can be used. Bench-scale experiments using electric resistance heaters as the energy source have verified that the design functions as intended. 

In terms of safety, two issues are regularly debated. First, the issue of nuclear waste and, second, concerns over potential terrorist attacks on nuclear power plants. The first objection may be overcome through the introduction of new types of power plants, such as the pebble-bed modular reactor.This type of reactor uses graphite balls flecked with tiny amounts of uranium, rather than conventional fuel rods. With the fuel encased in graphite and impermeable silicon carbide, the theory is that the waste should be relatively easy to dispose of.The terrorism fears are less easily addressed and may ultimately stall the construction of new plants in countries such as the U.S., where these worries are greatest. 

Figure 14. Corrosion behavior of reduction reactor materials samples ( 7), Inconel 625 (O), silicon (Q), silicon nitride (%), alonized Inconel 62 (M silicon carbide and ( f), Inconel 657. Furnace temperature, 482°C S03, 25 see/min steam, 58 see/min argon, 78 see/min.

Reactor filling glass beads, d = 1.5 mm.Diluent Silicon carbide, d = 0.2 mm. 

An effective way to improve the isothermicity of reactors is to dilute the catalyst with inert particles, preferably of a material with a high heat conductivity, such as silicon carbide (heat conductivity in the solid state about 40 times that of porous alumina). In the diluted bed, the heat generated per unit volume of bed will be lowered, and together with an increased effective heat conductivity this will result in a more even radial temperature distribution. 

Figure 13B shows the calculated temperature differences for the same cases as considered before, but with catalyst beds diluted with silicon carbide to one third of the original catalyst concentration. It can be seen that the temperature differences are appreciably smaller than in the undiluted case (note the differences in temperature scale between Figures 13A and 13B). The dilution with good thermally conducting material is particularly effective at the low velocities in short beds because the convective contribution to the effective heat conductivity is then relatively small. It can be inferred that in microflow reactors (D = 1 cm L = 10 cm) and in bench-scale reactors (D = 2 cm L = 1 m) with diluted beds radial temperature differences are less than 1-2 °C for the considered cases, which is quite acceptable. 

Figure 20 presents some results of comparative tests on hydrodesulfurization of a heavy gasoil in a bench-scale and a microflow reactor over two catalysts, both diluted with small particles of silicon carbide. It can be inferred that results are the same irrespective of the reactor scale, the same difference in relative performance of the two catalysts being observed in both reactor types. 

Diluent in microflow test Silicon carbide, d = 0.05 mm. Feed Middle East heavy gasoil, 1.64 %w S.Operating conditions WHSV, WABT, hydrogen/oil ratio, partial pressures of hydrogen and hydrogen sulfide same in commercial and microflow reactor. 

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... 

Figure 15 Silicon carbide heat exchange/reactor design for steam/naphtha cracking process. (From Ref. 28, reprinted with permission of the American Institute of Chemical Engineers.)...

Silicon carbide, widely employed as an abrasive (carborundum), is finding increasing use as a refractory. It has a better thermal conductivity at high temperatures than any other ceramic and is very resistant to abrasion and corrosion especially when bonded with silicon nitride. Hot-pressed, self-bonded SiC may be suitable as a container for the fuel elements in high-temperature gas-cooled reactors and also for the structural parts of the reactors. Boron carbide, which is even harder than silicon carbide, is now readily available commercially because of its value as a radiation shield, and is being increasingly used as an abrasive. 

Advanced waste form work is also being carried out in the Ceramics and Graphite Section at PNL, where high temperature gas-cooled reactor fuel technology is applied to waste solidification. Waste particles are coated with pyrolytic carbon followed by a cover coat of silicon carbide. These coated particles would then be placed in a matrix of inert material contained in a canister of yet another material. 

Coated spherical Th02- or U02-particles are increasingly utilized in the fuel of gas-cooled high temperature reactors. Their 50 to 1500 pm core of uranium(IV) oxide is manufactured using conventional sintering techniques. This is then pyrolytically coated with many layers of carbon and silicon carbide (see Section 5.7.5.1). 

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