Complex methanation process

Carbon in CH4 can be derived from C02 or fermentation of organic matter. The process of methane generation is a complex multistep process, by several species of bacteria, which results in the following general reaction ... 

From the chemically simple process of methane or gas oil to carbon black to the first more complex petrochemical process to isopropanol the scope of this industry has expanded to the production of a truly vast range of large-scale chemicals. Here we will cover outlines of representative samples of the more important of these chemicals. For more detailed information consult the items listed in the Further Reading section at the end of this chapter. 

The basic elements for life, such as carbon, nitrogen, and sulfur, can exist in the biosphere in several oxidation states, ranging from C(-l-4) in carbon dioxide to C(—4) in methane, N(5-l-) in nitrate to N(—3) in ammonia, or S(- -6) in sulfate to S(—2) in hydrogen sulfide. Interconversions of these various species constitute their global biogeochemical cycles which are sustained by complex biological processes, with bacteria playing a prominent role [3,6-10]. 

The oxidation of hydrocarbons follows very complex ramified processes. In the case of methane, all the mechanisms proposed make use of the radicals CH3 and OH. The most probsble process (Semenov, 1960) involves a branching chain with the formation of formaldehyde, and takes account of latest kinetic data and of modern concepts of the free energy of free radical-stable molecule reactions. Its validity is confirmed by the accord between values calculated for each elementary step and the experimental results. 

Historically, formaldehyde has been and continues to be manufactured from methanol. EoUowing World War II, however, as much as 20% of the formaldehyde produced in the United States was made by the vapor-phase, noncatalytic oxidation of propane and butanes (72). This nonselective oxidation process produces a broad spectmm of coproducts (73) which requites a complex cosdy separation system (74). Hence, the methanol process is preferred. The methanol raw material is normally produced from synthesis gas that is produced from methane. 

Methane, chlorine, and recycled chloromethanes are fed to a tubular reactor at a reactor temperature of 490—530°C to yield all four chlorinated methane derivatives (14). Similarly, chlorination of ethane produces ethyl chloride and higher chlorinated ethanes. The process is employed commercially to produce l,l,l-trichloroethane. l,l,l-Trichloroethane is also produced via chlorination of 1,1-dichloroethane with l,l,2-trichloroethane as a coproduct (15). Hexachlorocyclopentadiene is formed by a complex series of chlorination, cyclization, and dechlorination reactions. First, substitutive chlorination of pentanes is carried out by either photochemical or thermal methods to give a product with 6—7 atoms of chlorine per mole of pentane. The polychloropentane product mixed with excess chlorine is then passed through a porous bed of Fuller s earth or silica at 350—500°C to give hexachlorocyclopentadiene. Cyclopentadiene is another possible feedstock for the production of hexachlorocyclopentadiene. 

The mixed refrigerant cwcle was developed to meet the need for hq-uefying large quantities of natural gas to minimize transportation costs of this fuel. This cycle resembles the classic cascade cycle in principle and may best be understood by referring to that cycle. In the latter, the natural gas stream after purification is cooled successively by vaporization of propane, ethylene, and methane. Each refrigerant may be vaporized at two or three pressure levels to increase the natural gas coohng efficiency, but at a cost of considerable increased process complexity. 

Similar results are obtained for dre deposition of the carbides of these metals using methane as a source of carbon, atrd silicon tetrahalides for the preparation of silicides. These reactions are more complex than dre preparation of the diborides because of the number of carbides atrd silicides that the tratrsition metals form, some of which have wide ranges of non-stoichiometry. The control of the ratio of the partial pressures of dre ingoing gases is therefore important as a process variable. 

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