Methane, pyrolytic

Biomedical. Heart-valve parts are fabricated from pyrolytic carbon, which is compatible with living tissue. Such parts are produced by high temperature pyrolysis of gases such as methane. Other potential biomedical apphcations are dental implants and other prostheses where a seal between the implant and the living biological surface is essential. Plasma and arc-wire sprayed coatings are used on prosthetic devices, eg, hip implants, to achieve better bone/tissue attachments (see Prosthetic and BiOLffiDiCALdevices). 

Of the many forms of carbon and graphite produced commercially, only pyrolytic graphite (8,9) is produced from the gas phase via the pyrolysis of hydrocarbons. The process for making pyrolytic graphite is referred to as the chemical vapor deposition (CVD) process. Deposition occurs on some suitable substrate, usually graphite, that is heated at high temperatures, usually in excess of 1000°C, in the presence of a hydrocarbon, eg, methane, propane, acetjiene, or benzene. 

Chlorination of Hydrocarbons or Chlorinated Hydrocarbons. Chlorination at pyrolytic temperatures is often referred to as chlorinolysis because it involves a simultaneous breakdown of the organics and chlorination of the molecular fragments. A number of processes have been described for the production of carbon tetrachloride by the chlorinolysis of various hydrocarbon or chlorinated hydrocarbon waste streams (22—24), but most hterature reports the use of methane as the primary feed. The quantity of carbon tetrachloride produced depends somewhat on the nature of the hydrocarbon starting material but more on the conditions of chlorination. The principal by-product is perchloroethylene with small amounts of hexachloroethane, hexachlorobutadiene, and hexachloroben2ene. In the Hbls process, a 5 1 mixture by volume of chlorine and methane reacts at 650°C the temperature is maintained by control of the gas flow rate. A heat exchanger cools the exit gas to 450°C, and more methane is added to the gas stream in a second reactor. The use of a fluidi2ed-bed-type reactor is known (25,26). Carbon can be chlorinated to carbon tetrachloride in a fluidi2ed bed (27). 

Methane is unique among hydrocarbons in being thermodynamically stable with respect to its elements. It follows that pyrolytic reactions to convert it to other hydrocarbons are energetically unfavourable and will be strongly equilibrium-limited. This is in marked contrast to the boranes where mild thermolysis of B2H6 or B4H10, for example, readily yields mixtures of the higher boranes (p. 164). Vast natural reserves of CH4 gas exist but much is wasted... 

The deposition of pyrolytic graphite in a fluidized bed is used in the production of biomedical components such as heart valves, ] and in the coating of uranium- and thorium-carbides nuclear-fuel particles for high temperature gas-cooled reactors, for the purpose of containing the products of nuclear fission.fl" The carbon is obtained from the decomposition of propane (CgHg) or propylene (CgHg) at 1350°C, or of methane (CH4) at 1800°C. Its structure is usually isotropic (see Ch. 4). 

Lucas, P., and Marchand, A., Pyrolytic Carbon Deposition from Methane, Carbon, 28(1) 207-219 (1990)... 

The pyrolytic reforming reactor was a packed bed in a quartz tube reactor. Quartz was selected to reduce the effect of the reactor construction material on the hydrocarbon decomposition rate. ° The reactor was packed with 5.0 0.1 g of AC (Darco KB-B) or CB (BP2000) carbon-based catalyst. The reactor was heated electrically and operated at 850—950 °C, and the reactants had a residence time of 20—50 s, depending on the fuel. The reactor was tested with propane, natural gas, and gasoline as the fuels. Experiments showed that a flow of 80% hydrogen, with the remainder being methane, was produced for over 180 min of continuous operation.The carbon produced was fine particles that could be blown out... 

Carbonaceous compounds can also form in the absence of a catalyst by free-radical, gas-phase condensation reactions. The formation of this pyrolytic carbon is known in steam-reforming reactors where it can be controlled to some extent by minimizing the free volume within the reactor chamber. This type of carbon does not form readily with methane but can be severe with larger hydrocarbons. The compounds formed by free-radical reactions tend to be quite different from the graphitic carbon formed by metal catalysts. For example, Lee et al. showed that the compounds formed by passing pure, undi-... 

To illustrate the utility of the bimolecular QRRK theory, consider the recombination of CHjCl and CHjCl radicals at temperatures in the range 800-l,5(X) C. This recombination process is important in the chlorine-catalyzed oxidative pyrolytic (CCOP) conversion of methane into more valuable C2 products, and it has been studied recently by Karra and Senkan (1988a). The following composite reaction mechanism represents the complex process ... 

The molten material attacks quartz. Therefore, quartz boats coated with carbon by pyrolytic decomposition of methane should be used in refining the compound to obtain high purity material. 

Under pyrolytic conditions at temperatures above 300°C, generally within 500-800°C, the pyrolysis reaction forms alkenes by carbon-hydrogen bond scissions. An early experiment, where propane was heated to 575°C for 4 min in a silica flask, yielded propylene by dehydrogenation [Eqs. (2.19)-(2.21)] at a somewhat slower rate than it yielded methane and ethylene by cracking 54... 

In the production of carbon tetrachloride, chlorination is carried out in excess chlorine. The lower-boiling, partially chlorinated products then enter into a series of reactors where they react with added chlorine to achieve almost full chlorination of methane. In another process called chlorinolysis, higher aliphatic hydrocarbons undergo exhaustive chlorination at pyrolytic temperature (>600°C).177 182 183 Under such conditions carbon-carbon bond fission and simultaneous chlorination occur. Aliphatic hydrocarbon wastes are the preferred feedstock, as they react with about 20% excess chlorine. 

The presence of a considerable proportion of methylated bodies in low temperature tar and its origin must be explained. The fact that the yield of methane remains largely the same even when tar formation is completely inhibited would indicate that the methyl groups of coal possibly do not participate in forming the methylated bodies in tar. It is not unlikely, therefore, that such methylated bodies in tar are synthesized during pyrolytic reaction of the hydroaromatic structure (via methylenes). 

Carbon films and graphite films have been prepared by vacuum evaporation [28,42,76,77], pyrolysis [29,36,78-83], screen printing [46,62,63,65,66,84], and laser photoactivation of sites on a graphite or glassy carbon substrate [85]. Various pyrolytic processes have been successful, most based on the deposition of volatile precursor compounds. For example, methane can be pyrolyzed while... 

Carbon deposition occurs on the surface of a substrate inserted into the carbonization system using hydrocarbon gases, such as methane and propane [45], This process is a kind of chemical vapor deposition (CVD) and the products are called pyrolytic carbons. In order to control the structure, the deposition conditions have to be controlled. The deposition can occur on either static or dynamic substrates. In the former, the substrate is placed in a furnace, which is heated either by direct passing of electric currents or from the surroundings. In the latter, small substrate particles are fluidized... 

Methane, or rather natural gas (which may contain carbon oxides, higher hydrocarbons, and inert gases), is of great interest as a source of pyrolytically grown fibers because of its relatively low cost. 

At the same time there is an increase of carbon dioxide, carbon monoxide, methane, propene, and other gaseous components. The low-boiling components are carbon oxides and hydrocarbon while the higher-boiling components are esters. Mechanistically, the pyrolytic conversion of PMMA to its monomer is a radical process [24]. Two radicals are formed by the action of heat of the polymer chain (Scheme 24.1). 

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