Silicon reaction zone

The first and second pair of tubes were connected to a U-shaped reactor by the silicon reaction zone. In the center of the tubes, silicon slabs served as a recuperation zone for the fluids entering and leaving the system. Their impressive efficiency was demonstrated by CFD simulations revealing the highest heat transfer resistance in the fluid itself and thus temperature profiles across the width of the ducts (see Figure 2.39). 

Figure 7.44 Schematic of the suspended tube reactor developed by Arana et al. [558] the silicon slabs served for lateral heat transfer homogeneous combustion took place in the silicon reaction zone.

A typical 20-MW, a-c furnace is fitted with three 45-in. (114.3-cm) prebaked amorphous carbon electrodes equdateraHy spaced, operating on a three-phase delta connection. The spacing of the electrodes is designed to provide a single reaction zone between the three electrodes. The furnace is rotated to give one revolution in two to four days or it may be oscillated only. Rotation of the furnace relative to the electrodes minimizes silicon carbide buildup in the furnace. 

Internal heat exchange is realized by heat conduction from the microstructured reaction zone to a mini channel heat exchanger, positioned in the rear of the reaction zone [1,3,4], The falling film micro reactor can be equipped, additionally, with an inspection window. This allows a visually check of the quality of film formation and identification of flow misdistribution. Furthermore, photochemical gas/liquid contacting can be carried out, given transparency of the window material for the band range of interest [6], In some cases an inspection window made of silicon was used to allow observation of temperature changes caused by chemical reactions or physical interactions by an IR camera [4, 5]. 

The copyrolysis of 1 wt% dibromotetrafluoro-p-xylylene with commercially available hexafluoro-p-xylene (Aldrich) with metals was examined and it was found that it was indeed possible to prepare films that were spectroscopically indistinguishable from those deposited from dimer. The PA-F films obtained are of excellent quality, having dielectric constants of2.2-2.3 at 1 MHz and dissociation temperatures up to 530°C in N2. A uniformity of better than 10% can be routinely achieved with a 0.5-gm-thick film on a 5-in. silicon wafer with no measurable impurities as determined by XPS. During a typical deposition, the precursor was maintained at 50°C, the reaction zone (a ceramic tube packed with Cu or Ni) was kept at 375-550°C, and the substrate was cooled to -10 to -20°C. The deposited film had an atomic composition, C F 0 = 66 33 1 3 as determined by XPS. Except for 0, no impurities were detected. Within instrumental error, the film is stoichiometric. Poly(tetrafluoro-p-xylylene) has a theoretical composition ofC F = 2 1. Figure 18.2 illustrates the XPS ofthe binding energy... 

The main requirement to methylchloride, ethylchloride and chlorobenzene is the absence of impurities, by-products and especially moisture. With even the slightest amount of liquid entering the reaction zone, the products start to hydrolyse and condense, the activity of the contact mass or copper-silicon alloy decreases, and the process subsides. That is why the technology of direct synthesis usually provides for a device to dehydrate alkyl- and arylchlorides. For this purpose one can pass methyl- or ethylchloride through the tower sprayed with sulfuric acid, or use other dehydrating substances (burnt CaCF, AI203 and zeolites, e.g. burnt klinoptilo-lite). 

The particle size of metallic silicon also has a noticeable effect on the chemical activity of contact mass in case of coarse grinding, the reactivity of powder is lower than required in case of fine grinding the reactivity rises sharply, and the heat has to be intensively diverted from the reaction zone. 

The dependence of combustion temperature and velocity for the Si-N2 system as a function of dilution with /3-Si3N4 powder is shown in Fig. 38. In this case, at constant gas pressure, remains constant and is equal to the dissociation temperature of silicon nitride, for dilutions up to 60 wt %. However, with increasing dilution, the combustion front velocity increases. Also, increasing the overall heat evolution, smaller dilution by nonmelted nitride powder promotes coalescence of liquid Si particles, leading to an increase in the average reactant particle size, as well as to formation of thin liquid Si films, which blocks nitrogen infiltration to the reaction zone (Mukasyan et al, 1986). The same effects were also observed for the AI-N2 and B-N2 systems (Mukasyan, 1994), which are characterized by dissociation of the final product in the combustion wave. 

To overcome the objectionable reoxidation of formaldehyde and decomposition at the temperature of the reaction zone in the oxidation of methane, it has been proposed to react the formaldehyde as fast as formed with some substance to give a compound more stable under the conditions of the reaction and thus to increase the yields obtainable. It is claimed 101 that a reaction between the newly formed formaldehyde and annnonia to form a more stable compound, hexamethylene-tetramine, is possible under certain conditions, so that the formaldehyde is saved from destruction and can be obtained in a technically satisfactory yield. The hexamethylenetetramine is prepared by oxidizing methane with air in the presence of ammonia gas. A mixture consisting of six volumes of methane, twelve volumes of oxygen, and four volumes of ammonia gas is passed through a constricted metal tube which is heated at the constriction. The tube is made of such a metal as copper, silver, nickel, steel, iron, or alloys of iron with tin, zinc, aluminum, or silicon or of iron coated with one of these metals. Contact material to act as a catalyst when non-catalytic tubes are used in the form of wire or sheets of silver, copper, tin, or alloys may be introduced in the tube. At atmospheric pressure a tube temperature... 

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