Methane pyrolysis

Dean, A., Detailed kinetic modeling of autocatalysis in methane pyrolysis, J. Phys. Chem., 94, 1432, 1990. 

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,... 

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. 

At the conditions when the rate Hmiting step is crystallization of the target carbon phase (diamond) from the energy saturated primary carbon atoms C generated by the methane pyrolysis, the stepwise process can be written as a series of steps ... 

The recent use of the transmission electron microscopy of high resolu tion at the in situ condition at large enough pressure of methane resulted in the direct observation of the metal nanoparticle liquefaction at the cata lytic methane pyrolysis. Thus, the formation of carbon fibers and nano tubes often results from fluidization of the catalyticaUy active phase via its oversaturation with carbon at the catalyst operation. This may happen to a variety of processes when the deposition of graphitized carbon is pre ceded by the primary atomic or another energy saturated carbon species formed on the surface of the catalyticaUy active metals (see Figure 5.2). Supposedly, the formation of the very specific structures of the carbon fil ament, like the so caUed fishbone structure (see Figure 5.3B), may be... 

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. 

Primary, secondary and tertiary mechanisms for methane pyrolysis determined by the Leeds Expert System... 

When wood is heated an enormous variety of chemicals are produced water vapour, non-eondensable gases (carbon monoxide, carbon dioxide, hydrogen and methane), pyrolysis produets (methanol, acetone, acetic acid, and complex hydrocarbons and volatile tars). A carbon-rich char (charcoal) remains. The relative proportions of these produets vary depending on the amount of oxygen admitted, on the temperature and on the physieal configuration of the furnace. 

Serrano DP, Betas JA, Guil-Lopez R (2009) H2 production from methane pyrolysis over commercial carbon catalysts kinetic and deactivation study. Int J Hydrogen Energ 34 4488-4494... 

Fig. 3. Argon plasma jet with water-cooled annulus for methane pyrolysis. (Redrawn from Leutner, H. W., Stokes, C S. Ind. Eng. Chem. 53, 341 (1961), by permission of the authors and the publishers, the American Chemical Society)...

Fig. 4. Schematic of plasmatron system used for methane pyrolysis. (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)...

Rokstad, O. A., Olsvik, O., Jenssen, B. and Holmer, A., Ethylene, acetylene, and benzene from methane pyrolysis, in Novel Production Methods for Ethylene, Light Hydrocarbons and Aromatics (L. F. Albright, B. L. Cryness, and S. Nowak, Eds.), Marcel Dekker, New York, 1992, pp. 256-272. 

Figure 9-2. Kinetics of the double-step pyrolysis of hydrocarbons in plasma-chemical jet reactor. Step 1, methane pyrolysis inplasmajet of hydrogen Step 2, propane pyrolysis, injection of propane is delayed with respect to start of the process (1) 0.17 ms, (2) 0.37 ms, (3) 0.73 ms. Propane injection temperature 293 K propane-to-methane flow rate ratio 1 2.

Process characteristics Pyrolysis of methane Pyrolysis of propane Pyrolysis of benzene... 

Keywords compression chemical reactor, methane pyrolysis, acetylene, natural gas conversion, hydraulic ram. 

According to the calculations very high temperamres can be achieved in bubbles in a short time. To investigate the possibilities of methane pyrolysis by compression in bubbles the simplified Kassel reactions scheme was used. 

Autocatalysis, carbon formation and surface effects The third or autocatalytic stage In the methane pyrolysis. In which the yield of ethane begins to rise sharply again after the steady-state plateau (fig. 1) Is not predicted or explained by the reaction mechanism postulated above. The autocatalysis Is most evident In the yield of ethane, but almost certainly affects the other products as well, although It Is less obvious because their yields are already rising sharply. Autocatalysis has frequently been reported in the decomposition of methane, and under various conditions of pressure, temperature, conversion or surface, may have a variety of causes. It is most commonly associated with the formation of carbon, and attributed to reactions occuring at a carbon surface. 

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