Methane steam reforming kinetic expressions

All hydrocarbons in the feed higher than methane are assumed to be instantly cracked into CH4, CO2, 112, and CO. Consequently the reaction system inside a reformer tube is described by the rate expressions of the kinetics of steam reforming in the methane reactions I, II, and III. 

K3 is the equilibrium constant for reaction (42), which is the product of the equilibrium constants for reactions (41) and (37) K4] Kiy. For the ratio C02/C0 the authors assume only a slight deviation from the equilibrium and use an empirical relation without a kinetic term C02/C0=/(CH4 conversion, S/C ratio, K,7). Other kinetic expressions may be found in [362], [418], [422], For the reaction mechanism [422] of steam reforming of methane, the following scheme (Eqs. 51-55) was proposed ... 

Ross and Steel (1973) studied the mechanism of the steam reforming of methane over a coprecipitated nickel alumina catalyst (75% Ni) in a powder form (250-335 pm). The reaction was carried out at low total pressures (0-1 mm Hg), in the temperature range 500-600 C and at a low steam to methane ratio (2.0-0.2). The kinetics are summarized by the following expression ... 

P.3.2.3 Kinetics The reaction given by Equation 9.22a describes methane reforming with steam, whereas Equation 9.22b describes the water-gas shift reaction. The kinetic expression used for the CH4 reforming reaction shall be given by the following simple relation ... 

They reported similar findings to those of Bodrov et al (1967) including the attainment of a steady state after 40 hours and they represented the kinetic data by similar expressions. Xu and Froment (1989) studied the steam reforming of methane over a crushed nickel-magnesium-alumina catalyst containing 15% nickel in a reactor of 10.7mm diameter at temperatures between 500 and 575°C and pressures between 5 and 15 bar. Soliman et al (1992) studied steam reforming over a nickel-calcium aluminate-spinel catalyst in a commercial microreactor 6mm in diameter at temperatures from 475 to 550 C and pressures from 2 to 4 atmospheres. 

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. 

Early work on the kinetics of the steam reforming of methane [59] was based on the assumption that the methane adsorption was ratedetermining, in agreement with the general assumption of a first-order dependence on methane concentration. Later work by Khomenko et al. [271] avoided the discussion of a rate-determining step instead, the researchers inserted the quasi steady-state approximation in terms of the Temkin identity [75], and the following rate expression was obtained for the temperature range of 470-700°C ... 

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