Methane pyrolysis reactions

Methane—the major constituent of natural gas—is not only an excellent fuel but an important chemical. This is illustrated by the various reactions in Fig. 6.4 where each of the products are important reactants themselves. The thermodynamics of methane pyrolysis reactions is shown in Fig. 6.5 for equilibrium conditions. If, however, the hydrogen produced is removed from the reaction system, then the product yields can be substantially higher. This has been demonstrated using thin palladium membranes which allow only H2 to pass through. Similarly, when the pyrolysis occurs on a hot wire, the hydrogen produced can diffuse out of the reaction zone faster than the other heavier product, and therefore, equilibrium conditions do not prevail. Thus, in Fig. 6.5, though reaction 1 is preferred thermodynamically, it is found that experimentally reaction 4 occurs on a hot wire forming aromatic oil. 

Decomposition. Acetaldehyde decomposes at temperatures above 400°C, forming principally methane and carbon monoxide [630-08-0]. The activation energy of the pyrolysis reaction is 97.7 kj/mol (408.8 kcal/mol) (27). There have been many investigations of the photolytic and radical-induced decomposition of acetaldehyde and deuterated acetaldehyde (28—30). 

Figure 4.2 presents a simplified flow diagram of the ENCOAL Liquid from Coal (LFC) process. The process upgrades low-rank coals to two fuels, Process-Derived Coal (PDF ) and Coal-Derived Liquid (CDL ). Coal is first crushed and screened to about 50 mm by 3 mm and conveyed to a rotary grate dryer, where it is heated and dried by a hot gas stream under controlled conditions. The gas temperature and solids residence time are controlled so that the moisture content of the coal is reduced but pyrolysis reactions are not initiated. Under the drier operating conditions most of the coal moisture content is released however, releases of methane, carbon dioxide, and monoxide are minimal. The dried coal is then transferred to a pyrolysis reactor, where hot recycled gas heats the coal to about 540°C. The solids residence time... 

Because H2 and CO are produced with such high selectivities and conversions at these short residence times, the primary mechanism of formation of these products must be methane pyrolysis. The surface reactions which produce H2 and CO occur in an oxygen-depleted environment, and the major surface species are probably adsorbed C or CHx and H. The C reacts with oxygen to produce CO, which desorbs before being f urther oxidized to CO2. Adsorbed H atoms may either combine to form H2 which desorbs or react with oxygen to make adsorbed OH species, which then combine with additional adsorbed H atoms to form H2O. Thus,... 

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... 

It involves the displacement of hydrogen, or an alkyl group, from a saturated carbon atom by a methyl radical. For a methane pyrolysis, its history goes back to Kassels attempt to write a chain reaction for the pyrolysis of methane. The evidence for it is quite unsubstantial. 

The industrially important direct methane conversion processes comprise oxidative coupling, reductive coupling including pyrolysis reactions, partial oxidation, halogenation and oxyhalogenation,26 and ammoxidation. Other direct conversions include alkylation, electrophilic substitution, and C-H bond activation over various complex and super acid catalysts. Several of these direct conversion technologies remain to be exploited to achieve their full commercial potentials. 

Building on the foundation of the hydrocarbon oxidation mechanisms developed earlier, it is possible to characterize the flame as consisting of three zones [1] a preheat zone, a reaction zone, and a recombination zone. The general stmeture of the reaction zone is made up of early pyrolysis reactions and a zone in which the intermediates, CO and H2, are consumed. For a very stable molecule like methane, little or no pyrolysis can occur during the short residence time within... 

Although methane is the simplest hydrocarbon, the elucidation of the mechanism of its pyrolysis has proved a matter of considerable difficulty. The reaction proceeds in a very different way from the other paraffin pyrolyses a carbon-carbon single bond is much weaker than a carbon-hydrogen bond so that C-C bond ruptures are important in all of the other pyrolyses. In the methane pyrolysis C-C bond rupture becomes gradually more important as product e.g. ethane) accumulates, so that the character of the process changes as reaction proceeds. 

The main products of the methane pyrolysis are ethylene, acetylene and hydrogen, with smaller amounts of ethane this was established in studies made by Gordon " and by Palmer et In addition, carbon is deposited on the walls of the reaction vessel. 

Palmer and Hirt followed the course of the methane pyrolysis by observing the formation of carbon films on the surface of the reaction vessel. They varied the methane concentration over a factor of 20, and found the kinetics to be first order. 

This conclusion that the activation energy is about 103 kcal.mole removes a difficulty that had been found with regard to the reaction mechanism. The only plausible Initiation reactions in the methane pyrolysis are... 

On the basis of these considerations it is possible to understand why a chain reaction is not important in the methane pyrolysis. Because the radicals are p radicals the order of the chain mechanism is bound to be greater than unity, and this factor, together with the high activation energy for reaction (2), leads to a low rate for the chain process. 

Eisenberg and Bliss and Palmer et al have studied the time-course of the methane pyrolysis there is an initial acceleration followed by retardation. Both studies show that ethane accelerates the reaction. The work of Palmer et indicates that there is a deposition of vitreous carbon on the walls of the vessel, that the reaction is inhibited by carbon, and that the rate is not appreciably affected by the surface volume ratio. They also find that the reaction is strongly accelerated by added naphthalene, which tends to produce carbon nuclei very rapidly. They conclude that the formation of nuclei has a strong effect on the rate of decomposition. The inhibition by hydrogen may then be due to its removal of nuclei. The accelerating effect of added ethane is attributed to its more rapid decomposition, with accompanying formation of nuclei aside from this, ethane is a good source of free radicals. 

As an example, we consider in outline the mechanism generated by Chinnick et al. [14] for methane pyrolysis. The reaction types are defined in Table 4.1 and a set of rules is associated with each of them. For example, decomposition corresponds to the rupture of every unique single bond in a molecule M, taking care to identify only unique reactions. Limitations can be placed on the reactions, essentially through a generalization of the associated rate constants. Radical isomerization, for example, is only permitted in the Chinnick system for 1-4, 1-5 and 1-6 H shifts, and since it does not occur for hydrocarbons with carbon chains less than C4, it is absent for methane. 

Similar pyrolysis reaction can also occur during reforming of higher hydrocarbons. In fact, higher hydrocarbons tend to decompose more easily than methane and therefore the risk of carbon formation is even higher with vaporized liquid petroleum fuels than with natural gas. Another source of carbon formation is from... 

These reactions increase the heating value of the gas product, since methane has a high heat of combustion. However, these reactions are very slow except under high pressure and in the presence of a catalyst. Another source of the methane in the syngas is the pyrolysis process. Reaction R-4.11 is the reverse steam methane reforming reaction. All reactions that produce methane are exothermic reactions. 

Fig. 5. Relations between the concentrations of methane pyrolysis products and the distance or time over which the reaction occurred. A temperature-time (distance) profile of the plasma jet is also shown. (Redrawn from Kinetika i termodinam. khim. reaktsii v nizkotemperaturnoi plazme, Polak, L. (ed.). Moscow Nauka 1965, by permission of Professor L. Polak, Institute of Petrochemical Synthesis of the Academy of Sciences of the U.S.S.R., Moscow)...

The K-butane pyrolysis is analysed here as an initial, simple example of a pyrolysis reaction mechanism. It is important to note that the pyrolysis reactions of small hydrocarbons are fundamental to the proper understanding of the whole process. In fact, the pyrolysis mechanism displays a typical hierarchical structure and the small hydrocarbons must be analysed first. Fig. 1 shows the main and minor products from K-butane decomposition, under isothermal conditions, at 1,093 K and 1 atm. Ethylene, propylene and methane are the main products, while only trace amount of butenes, ethane, benzene and cyclopenta-diene are observed. These model predictions have been confirmed and validated by several experimental measurements (Dente and Ranzi, 1983). 

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