Methane cracking

A series of kinetic studies on the carbon filament formation by methane decomposition over Ni catalysts was reported by Snoeck et al. [116]. The authors derived a rigorous kinetic model for the formation of the filamentous carbon and hydrogen by methane cracking. The model includes the following steps ... 

Snoeck, J., Froment, G., and Fowles, M., Kinetic study of the carbon filament formation by methane cracking on a nickel catalyst, /. Catal., 169, 250,1997. 

Figure 8, based on methane cracking into graphite, illustrates that this is effectively so, confirming earliers results by Rostrup-Nielsen (20). 

The reason why the minimum steam ratio goes down with temperature is not known with certainty. One possibility is that the competing reactions of carbon production and consumption have such kinetics that the rate of coke consumption increases faster with temperature than the rate of coke generation, which suggests that the carbon-steam reaction has a higher activation energy than the methane cracking and carbon monoxide disproportionation reaction. 

The elevated temperatures used in the reactor may damage the catalysts. Of particular importance is the possible carbon formation by methane cracking ... 

Through an analogous procedure Snoeck and Froment [1997] obtained the following equation for the coke production by methane cracking ... 

Coking reactions occur in parallel with the reforming reactions and are undesirable as they cause poisoning of the surface of the catalyst pellets. This leads to lower catalyst activity and the need for more frequently catalyst reloading. The coking reactions are the CO-reduction, methane cracking and Boudouard reaction, given by the respective equilibrium reactions [4] ... 

Eremin41 reported that the amount of methane cracked by a high frequency discharge is proportional to the current consumed and to the amount of excess hydrocarbons. The reaction was found to be of the first order and the rate was directly proportional to the discharge energy and inversely proportional to the original amount... 

Side reactions are the reverse water gas shift reaction, the Boudouard reaction and methane cracking, where the former two are suppressed at temperatures > 820 °C and the latter two form carbon in the temperature range 550 - 700 °C [68]. 

Since coal is abundandy available in the world and thus should about maintain its cost level in the future, R D efforts should concentrate on coal-based processes. The advanced processes of methane cracking and coal cracking (HYDROCARB) have a huge economic potential [55]. 

The analysis by STEM of Ni and Ni-Cr catalysts after the carbon deposition by the methane cracking reaction shows the formation of graphitic filaments in large amounts of whisker and octopus types. 

Extremely high rates of temperature change (10 K/s) show that it is possible to intermpt the methane cracking chain before solid carbon is produced from acetylene. Thus the problem of coke formation can be eliminated. In any case possible formation of coke in liquids is not so detrimental problem as in the conventional hydrocarbon crackers - in view of low wettability in many liquids coke could be relatively easier separated. 

Inside the catalyst pores, the composition of the gas is at the equilibrium composition shown above. The values of PcmlPco and Pm IPcm of the bulk fluid and the equilibrium composition are greater than the values fliased on Ni catalyst) measured for (Boudouard reaction) and Kg (methane cracking reaction) ... 

This comparison shows that there is a potential to form carbon from the methane-cracking reaction in the inside of the reformer wall at this location in the reformer tube. Detailed, proprietary kinetic expressions for the reactions 8-10 indicate that while reaction 9 will form carbon, the coke will be gasified by steam and CO2 (reactions 8 and 10), so there is no net accumulation of carbon in this example. However, as the steam-to-hydrocarbon feed ratio is reduced further there will be a point where there is an accumulation of carbon because the coking rate of reaction 9 will be greater than the combined gasification rates of reactions 8 and 10. 

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