Methane cracking reaction

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. 

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. 

The reaction between CH4 and CO2 can proceed above 640 °C [2]. At such temperatures, carbon deposition occurs via Boudouard reaction (Eq. (22.3)) and methane cracking reaction (Eq. (22.4)). 

The exothermic CO disproportionation is fevored at lower temperature than the endothermic methane cracking reaction. However at 700 °C, temperature often used to perform the reaction, carbon can be formed via the two reactions. 

Although olefins are intermediates in this reaction, the final product contains a very low olefin concentration. The overall reaction is endothermic due to the predominance of dehydrogenation and cracking. Methane and ethane are by-products from the cracking reaction. Table 6-1 shows the product yields obtained from the Cyclar process developed jointly by British Petroleum and UOP. ° A simplified flow scheme for the Cyclar process is shown in Figure 6-6. 

Hydrogenolysis of 2-methylpentane, hexane, and methylcyclopentane has been also studied on tungsten carbide, WC, a highly absorptive catalyst, at 150-350 °C in a flow reactor [80], These reforming reactions were mainly cracking reactions leading to lower molar mass hydrocarbons. At the highest temperature (350 °C) all the carbon-carbon bonds were broken, and only methane was formed. At lower temperatures (150-200 °C) product molecules contained several carbon atoms. 

Sometime in the early twentieth century it was found that if the steel tubes in the furnace had certain kinds of dirt in them, the cracking reactions were faster and they produced less methane and coke. These clays were acting as catalysts, and they were soon made synthetically by precipitating silica and alumina solutions into aluminosilicate cracking catalysts. The tube fumace also evolved into a more efficient reactor, which performs catalytic cracking (FCC), which is now the workhorse reactor in petroleum... 

Recently, Takenaka et studied a series of base metal catalysts supported on various ceramic oxides for catalytic cracking of kerosene fuel. Yields of H2 and methane from a model kerosene fuel (52 wt% n-Ci2, 27 wt% diethylbenzene and 21 wt% t-butylcyclohexane) over various base metals at 600°C are shown in Figure 33. Ni/Ti02 showed the highest catalytic activity for the cracking reaction of kerosene fuel, and also maintained a better performance for the kerosene feed that contained benzothiophene. However, the catalytic performance of the... 

We conclude, therefore, that the mechanisms of catalytic cracking reactions on nickel metal and nickel carbide are closely comparable, but that the latter process is subject to an additional constraint, since a mechanism is required for the removal of deposited carbon from the active surfaces of the catalyst. Two phases are present during reactions on the carbide, the relative proportions of which may be influenced by the composition of the gaseous reactant present, but it is not known whether the contribution from reactions on the carbide phase is appreciable. Since reactions involving nickel carbide yielded products other than methane, surface processes involved intermediates other than those mentioned in Scheme I, although there is also the possibility that if cracking reactions were confined to the metal present, entirely different chemical changes may proceed on the surface of nickel carbide. 

Parameters for the compensation line found for cracking reactions (74,151, 212, 213,218), methane exchange (21), and also including data for the oxidation of ethylene and propylene (209a) on this metal are given in Table II, J. 

Arrhenius parameters for the methanation reaction on alumina-supported Group VIII metals (227b) were close to the line for cracking reactions on several metals (Table III, A). Activity was based on the numbers of surface metal atoms and a compensation relation was described from these data we calculate c = 0.1185 0.0117, B = 15.216 1.068, and oL = 0.491. 

The analysis of the product gas, calculated to a hydrogen-free basis, is shown in Table II. The high percentage of methane and the predominance of n-butane and n-pentane over the branched compounds suggest that they were produced by a thermal cracking reaction with hydrogenation of the olefins. 

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. 

Ethane, propane, and butane, usually present in smaller concentrations in addition to methane in most natural gases, react in the steam reforming in similar way, with the overall reaction corresponding to Equation (35). With higher hydrocarbons, as contained in naphtha, the reaction is more complex. Higher paraffins in naphtha feed will be first completely cracked down in a methane-forming reaction, which proceeds between 400 and 600 °C and could be described, for example, as follows (Eq. 56) ... 

Steam reforming was the primary reaction over these nickel catalysts. The presence of hydrocarbons (G2 to G5) which would indicate cracking reactions occurred to the extent of less than 10% in the reaction products. The presence of methane, which would indicate partial reforming, did not exceed 5% in the reaction products. There does not appear to be any significant difference in product selectivity for the n-hexane steam reforming reaction over nickel on the 2 quite different supports—zeolite vs. alumina. Carbonaceous residues accumulated in the case of all the nickel catalysts where reforming activity was sustained and the carbon deposition on the zeolite catalysts compared favorably with G56. 

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