Pyrolytic hydrocarbon

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). 

With respect to the formation of unwanted polyaromatic hydrocarbons in the pyrolytic process, it has been shown that conditions can be maintained where such fonuation is negligible according to EPA and OSHA standards. As production rates are increased, it will be incumbent on any manufacturer to maintain a set of operating parameters which produce an environmentally-benign product however, current information regarding the process for fiber formation reveals no barriers to accomplishing this. 

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... 

Processing. The process requires a monofilament carbon-fiber core which is heated resistively in a tubular glass reactor shown schematically in Fig. 19.1. PI A carbon monofilament is pre-coated with a 1 pm layer of pyrolytic graphite to insure a smooth deposition surface and a constant resistivity. 1 1 SiC is then deposited by the reaction of silane and a hydrocarbon. Other precursors such as SiCl4, and CH3SiCl3 are also being investigated. A fiber cross-section is shown in Fig. 19.2.P1... 

The performance of a novel microwave-induced pyrolysis process was evaluated by studying the degradation of HDPE and aluminiutn/polymer laminates in a semibatch bench-scale apparatus. The relationship between temperature, residence time of the pyrolytic products in the reactor, and the chemical composition of the hydrocarbon fraction produced was investigated. 28 refs. 

SBA-Kellogg A pyrolytic process for making ethylene and acetylene from saturated hydrocarbon gases, similar to the Hoechst HTP process but with the addition of steam which increases the yield of C2 gases. 

Chantal, P.D., Kaliaguine, S., Grand-maissen, J.L., Mahay, A., Production of hydrocarbons from Aspen Poplar pyrolytic oils over HZMS-5, Appl. Catal., 1984, 10, 317. 

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-... 

In addition to the fact that carbon formed by gas-phase pyrolysis is chemically different from that which forms catalytically on Ni, it is important to recognize that carbon formed by pyrolysis forms on the surface rather than in the bulk of the material. Because of this, pyrolysis does not result in pitting of the surfaces to which the hydrocarbon is exposed. Furthermore, on porous Ni cermets, carbon fiber formation can lead to fracture of the electrode caused by the stresses induced by the carbon fibers. Such stresses do not occur upon deposition of pyrolytic carbon. 

As is well known, when hydrocarbons transported in an inert gas such as helium are heated, carbon or pyrolytic graphite will deposit on the walls. In the HTCVD, the ethylene helps make the particles stable and, in doing so, carbon is transported into the chamber together with the silicon. It is a sort of symbiosis in the transport between the silicon and carbon. 

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. 

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