Kinetics of Steam Reforming

All steam reforming catalysts in the activated form contain metallic nickel as active component, but the composition and structure of the support and the nickel content differ considerably in the various commercial brands. Thus the theoretical picture is less uniform than for the ammonia synthesis reaction, and the number of scientific publications is much smaller. The literature on steam reforming kinetics published before 1993 is summarized by Rostrup - Nielsen [362], and a more recent review is given by K. Kochloefl [422]. There is a general agreement that the steam reforming reaction is first order with respect to methane, but for the other kinetic parameters the results from experimental investigations differ considerably for various catalysts and reaction conditions studied by a number of researchers. 

As an example for an analytical expression, the formula given by Moe and Gerhard [423] is presented  

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  

In the extrinsic reaction rate, mass transfer plays a dominant role. The combined effect of the molecular diflusion of the reactants from the bulk gas through the gas film around a catalyst particle to the geometrical surface of the particle, and to some extent the Knudsen diflusion within the catalyst pores, are the limiting factors. As the intrinsic reaction is fast, the reactants will have reacted before they travel down the lenght of the pore. The effectivity of a steam reforming catalyst, that is, how much of the catalyst particle is utilized, varies with the reaction conditions and is only about 1 % at the exit. Because of this, the apparent activity increases with decreasing particle size, and the geometrical shape of the catalyst particle also has a distinct influence (Section 4.1.1.3). 

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)  

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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. 

The kinetics of steam reforming have been studied extensively [1-3]. However a general rate expression which fits the various experimental findings and ties together all the rate equations reported in literature has not been found yet. The approach used by Froment et al.[4] seems to be the most promising. Obviously a critical evaluation of the kinetic expression obtained by Froment et al.[4] is beyond the scope of this short paper. 

The anode is usually a thin layer of Ni-YSZ cermet. At temperatures over 1173K, the kinetics of steam reforming reactions are faster than the electrochemical oxidation of hydrogen or carbon monoxide. The other principal component is the bipolar plate, which is commonly fabricated from LaCrOa (Mg-doped) which is predominantly an electronic conductor and is compatible with other cell components at 1273 K. It is however relatively brittle and expensive, and there is a continuous effort to replace it with other materials preferably with a metallic system. 

The kinetics of steam reforming has been widely studied, and there are a number of rate equations available in the literature (6).To design a steam reformer, equations expressing the intrinsic reaction rates in the following form are required ... 

Various approaches have been applied to establish intrinsic kinetics of steam reforming of hydrocarbons [415],... 

Berman, A., Karn, R.K. and Epstein, M. (2005) Kinetics of steam reforming of methane on Ru/Al20j catalyst promoted... 

Experimental results concerning the development of a small-scale 1 kW autothermal reformer of propane were reported by Rampe et al. [76]. In the proposed reactor, two reactions occur on a metal honeycomb structure coated with platinum. Air and water are mixed before they are fed to the reactor in counterflow to the product gas outside the reactor wall, where the water is vaporized and the steam and air are heated up. Then, they are mixed with propane at the bottom of the reactor. It was verified that the preheating operation mode led to about a 4% higher efficiency, since the higher inlet air temperature causes a higher temperature level in the reaction zone, resulting in improved kinetics of the reforming reaction. 

Blaylock, D.W., Ogura, T., Green, W.H., Beran, G.J.O. Computational investigation of thermochemistry and kinetics of steam methane reforming on Ni(lll) under realistic conditions. J. Phys. Chem. C 2009, 113, 4898-908. 

Kinetic Parameters of Steam Reforming of Methane in this Study (T = 450-550°C, 1 atm)... 

The dependence of the molecular diffusion coefficient on the temperature and the total pressure can also be used to unravel the influence of external mass transfer. However, the temperature influences both the diffusivity and the intrinsic kinetics, whereas a variation of the total pressure at constant partial pressures of the reactants affects only the diffusivity. Kolbl et al. [93] applied this method in their investigation of steam reforming in microchannel reactors. 

Parameters for the Kinetic Model of Steam Reforming of Methanol on BASF K3-110, A Cu/Zn0/A12Q3 Catalyst. 

Although attempts have been made to establish kinetics for steam reforming on the basis of a micro-kinetic approach [23] and lately on first principles [264], most work is based on empirical kinetics, which has been sufficient to develop highly sophistieated models for tubular reformers. 

K. K, (2007) Kinetic modelling of steam reforming of ethanol for the production of hydrogen over C0/AI2O3 catalyst. Chem. Eng J., 125, 139-147. 

This type of cell operates at the highest temperature (1000 °C) of all current fuel cell types. This high operating temperature has many advantages including producing steam suitable for co-generation, the ability to reform many fuels efficiently and the overall fast kinetics of the... 

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