Other Methanation Processes

Methanation catalysts or, alternatively, pressure-swing absorption are used to purity synthesis gas and refineiy hydrogen throughout the world. The methanation reaction has also been applied in other interesting and useful processes although not yet on a veiy significant scale. 

Pure hydrogen can be recovered by a methanation procedure from olefin plant tail gas that contains methane, ethylene and carbon monoxide. Typical methanation catalysts will remove up to 0.3% carbon monoxide and 0.3% ethylene at 270°C, 30 atm pressure and about 6000 h space velocity. A catalyst which does not crack ethylene to form carbon but which can produce ethane or 

TABLE 9.16. Catalytic Rich Gas (CRG) Reforming and Substitute Natural Gas (SNG). 

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Steam Utilization. Less steam is used in the RMProcess than is required for conventional shift conversion even though in other methanation processes as little as one-half of the total syngas is processed through shift conversion in order to achieve a near-stoichiometric balance of hydrogen and carbon monoxide for methanation. 

Under certain conditions of temperature and pressure, and in the presence of free water, hydrocarbon gases can form hydrates, which are a solid formed by the combination of water molecules and the methane, ethane, propane or butane. Hydrates look like compacted snow, and can form blockages in pipelines and other vessels. Process engineers use correlation techniques and process simulation to predict the possibility of hydrate formation, and prevent its formation by either drying the gas or adding a chemical (such as tri-ethylene glycol), or a combination of both. This is further discussed in SectionlO.1. 

Catalytic reactions at somewhat lower temperatures also produce ethylene and other olefins. When coupled with a methane process to methyl chloride, this reaction results ia a new route to the light hydrocarbons that is of considerable interest. 

Oxychlorination of Hydrocarbons. Methane was oxychlorinated with HCl and oxygen over a 4 3 3 CuCl—CUCI2—KCl molten mixture to give a mixture of chlorinated methanes, 60 mol % of which was carbon tetrachloride (28). Aqueous 20% HCl was used in the multistep process as the source of the acid. Anhydrous HCl is more typically used. Other oxychlorination processes can be made to yield high percentages of carbon tetrachloride starting from any of several hydrocarbon feeds (29—31). The typical reaction temperature is 400—600°C (see Chlorocarbons and chlorohydrocarbons. Methyl cm oRiDE Methylene cphoride and Cphoroform). 

The chapter by Eisenlohr et al. deals with the results of large scale pilot operations using a newly developed high-nickel catalyst with hot-gas recycle for temperature control. This and other work, conducted by Lurgi Mineraloeltechnik GmbH, with South African Coal and Oil Limited (SASOL), are the bases of the methanation process which Lurgi is proposing for commercial plants. 

L. Seglin We have heard today two rather novel approaches to methanation the steam-moderated RMProcess and the slurry methanation process. These are the results of new or recent R D. What can we visualize beyond that Would it be some other exotic process ... 

HCN is produced commercially by the reaction of ammonia, methane, and air over a platinum catalyst or from the reaction of ammonia and methane. HCN is also obtained as a by-product in the manufacture of acrylonitrile and may be generated during many other manufacturing processes (Pesce 1994). In 1999, there were 34 companies operating 47 HCN production facilities in... 

In addition to photocleavage of water with photocatalysts, other photosynthetic processes such as photochemical C02 reduction, resulting in the formation of CO or methane, and the photochemical fixation of nitrogen are of great interest ... 

Steam, at high temperatures (975-1375 K) is mixed with methane gas in a reactor with a Ni-based catalyst at pressures of 3-25 bar to yield carbon monoxide (CO) and hydrogen (H ). Steam reforming is the process by which methane and other hydrocarbons in natural gas are converted into hydrogen and carbon monoxide by reaction with steam over a nickel catalyst on a ceramic support. The hydrogen and carbon monoxide are used as initial material for other industrial processes. 

Methylene Chloride tdichtaromethane). CAS 75-09-2. As with the other members of the methyl series of chlorinated hydrocarbons, methylene chloride can he produced hy direct chlorination of methane. The usual procedure involves a modification of the simple methane process. The product from Ihe first chlorination passes through aqueous zinc chloride, contacting methanol at about 100 C. Thus. HCl from chlorination is used to displace the alcohol group, producing additional methyl chloride. This is further chlorinated to methylene chloride. Methylene chloride reacts violently in the presence of alkali or alkaline earth metals and will hydrolyze to formaldehyde in the presence of an aqueous base. Alkvlalion reactions occur at both functions, thus di-suhstiiulioiis result. For example. 

With three or more stages for each refrigerant, the power consumption for the cascade cycle is found to compete quite favourable with other liquefaction processes, especially in arctic conditions. This is mainly because of the low flow rate of refrigerant. The cascade cycle is also more flexible in operation, since each circuit of refrigerant can be separately controlled. For the classical cascade, however, an overlap may occur of refrigerants like methane and ethylene. Thus, as methane condenses at an elevated pressure, this will inherently cause some throttling losses. 

The former is a volume-decreasing reaction, while the latter is not. Both reactions are exothermic. Methanation is a deep hydrogenation reaction for carbon monoxide and WGSR is a complete oxidation reaction in which carbon monoxide is oxidized into carbon dioxide and water is reduced with the formation of hydrogen. As in the preparation of methane, other hydrocarbons, low alcohols and particularly, carbon dioxide and water are formed. Because of the presence of water, WGSR always occurs in the methanation process, which reduces the selectivity and yield of the desired product. 

Fig. 5.28 The principle behind the ab initio calculation of heat of formation (enthalpy of formation) using an isodesmic reaction. Methanol and hydrogen are (conceptually) made from methane and water (other isodesmic reactions could be used) the 0 K enthalpy input for this is the ab initio energy difference between the products and reactants. Graphite, hydrogen and oxygen are converted into methane and water and into methanol and hydrogen, with input of the appropriate heats of formation. The heat of formation of methanol at 0 K follows from equating the heat of formation of methanol with the sum of the energy inputs for the other two processes. The diagram is not meant to imply that methanol necessarily lies above its elements in enthalpy...

A number of other commercial processes have been described which are similar to the CRG processes discussed above. For example, Skov12 has claimed a process in which half the reactant stream is fed to the first reactor. The product of this reactor is combined with the remainder of the reactant stream and fed to a second reactor in which a methane content of greater than 95% is achieved. Similarly, Nikki13 has claimed a process in which, after steam reforming at 350—550 °C, a CH4 content of 98% is achieved by methanation at a temperature of 220 °C. 

For more than 30 years, di-7i-methane processes were limited to the three different types of (3,y-unsaturated systems mentioned above. However, recent studies have shown that the rearrangement is applicable to 2-aza-1,4-dienes [13]. Thus, triplet-sensitized irradiation of compound 11 affords the cyclopropylimine 12 and the TV-vinylaziridine 13 (Sch. 6). The photoreaction of 11 represents the first example of a 2-aza-di-7i-methane rearrangement (2-ADPM) that brings about the formation of a heterocyclic product. The reaction has been extended to other 2-azadienes that yield the corresponding cyclopropylimines regioselectively [13b]. 

Among the unimolecular cyclizations, the synthesis yielding a single product with excellent yields was the cyclization of allyloxy alcohols brought about by a ruthenium complex. The other unimolecular processes exhibited very low yields and/or mixture of products. The most popular synthesis of 1,3-dioxocins involved the cyclization of 1,5-alcohols with the insertion of a carbon unit. Such acetalization of both acyclic dialcohols or hydroxyl groups bound to rings is particularly efficient and the cyclization of methane-diphenols or dihydroxybenzophenones with dihalomethanes was of wide applications. Palladium-promoted cyclization of chloromercurio compounds showed to be certainly less effective even if it presented some cases in reasonable yields. The sole example synthesis of 1,3-dioxocins by transformation of another ring has no preparative interest. 

There are good reasons to suppose that a similar phenomenon of fluidization of nanoparticles of the same catalytically active metals may happen to some other catalytic processes when they are accompanied by the formation of graphitized carbon. Examples of such processes are catalytic pyrolysis of methane... 

In this case, one of the two ir-bonds is a cyclopropane unit and a vinyl cyclobutane is formed (equation 17), the photorearrangement still resembling closely the di-i -methane process of equation (1). Such an unsymmetric substrate should in principle afford, besides the cyclobutane (path b), also dicyclopropane (path a) by opening of the other o-bond in the cyclopropyl diradical intermediate this pathway has not been observed so far. 

Beyond metabolism of carbohydrates, there are several other biological processes common in prokaryotes that consume oxygen. For example, methylotrophic organisms can metabolize Ci compounds such as methane, methanol, formaldehyde, and formate, as in... 

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