Methane steam reforming rate equations

Rostrup-Nielsen found that the intrinsic reaction rate, rj, for methane steam reforming is correlated with the sulphur coverage by equation 6 (2). In the adiabatic prereformer, the sulphur acts as a pore mouth poison and as the reactions are restricted by pore diffusion 2,8), the effective activity of the sulphur poisoned catalyst pellet can be described by an empirical relation, equation 7, between the effective pellet reaction rate, rp, and the average sulphur coverage, 0av (7/... 

Bodrov et al. (1964) found in the carbon dioxide reforming of methane that for CO2ICH4 1, the reaction rate can be described by equation (3.59) developed for steam reforming of methane. The study concluded that methane does not react with CO2 but that it does react with steam. The steam is produced when hydrogen (formed from the adsorption and dissociation of methane) reacts with CO2 in the water-gas shift reaction ... 

Analysis of this rate equation shows that it takes into consideration the reversible reaction step in the steam reforming. The rate equation implies that the forward reaction is first order with respect to methane, zero order with respect to steam (possibly due to the high steam /C ratio in the experiments) and is inhibited by hydrogen. The reverse reaction is independent of the methane concentration, second order in hydrogen, first order in CO and is inhibited by H2O. 

All these factors are functions of the concentration of the chemical species, temperature and pressure of the system. At constant diffu-sionai resistance, the increase in the rate of chemical reaction decreases the effectiveness factor while al a constant intrinsic rate of reaction, the increase of the diffusional resistances decreases the effectiveness factor. Elnashaie et al. (1989a) showed that the effect of the diffusional resistances and the intrinsic rate of reactions are not sufficient to explain the behaviour of the effectiveness factor for reversible reactions and that the effect of the equilibrium constant should be introduced. They found that the effectiveness factor increases with the increase of the equilibrium constants and hence the behaviour of the effectiveness factor should be explained by the interaction of the effective diffusivities, intrinsic rates of reaction as well as the equilibrium constants. The equations of the dusty gas model for the steam reforming of methane in the porous catalyst pellet, are solved accurately using the global orthogonal collocation technique given in Appendix B. Kinetics and other physico-chemical parameters for the steam reforming case are summarized in Appendix A. 

Xu and Froment (1989a) have developed new, more general intrinsic rate equations for the steam reforming of methane using an integral... 

The mechanism of steam reforming of methane over a nickel catalyst (equation 168) or over different nickel alumina catalysts has been investigated at 873K k The rate of reaction on a commercial co-precipitated catalyst A (poorly crystallized y-Al203 containing 75% Ni) is given by equation 169 ... 

A suggestion has been made that the rate-determining steps in the steam reforming of methane carried out over two different catalysts are not the same. The absence of H-D exchange in methane when catalyst A was used implies that step 1 is irreversible and ratedetermining (see the scheme in equation 171). 

A low effectiveness factor — see Figure 3.23 — implies that the effectiveness factor and thus the effective rate for the steam reforming of methane is inversely proportional to the Thiele modulus [199] and hence the equivalent particle diameter assuming that the particle is isotherm. For a first-order equilibrium rate expression, a general effectiveness factor can be evaluated as shown in [199] [389]. For a large equilibrium constant, this equation can be simplified to ... 

Steam methane reforming on nickel-based catalysts is the main process for industrial production of hydrogen or synthesis gas. Numerous studies have been reported on the kinetics of these reactions [1, 3, 59, 167]. Xu and Froment [167] investigated a large number of detailed mechanisms and proposed the intrinsic rate equations for the steam reforming of methane on the nickel-alumina catalyst, i.e., (4.193), (4.194) and (4.195) which have been widely used. These rate equations were thus used by... 

Equation (2.95) infers that for a tenfold increase in pressure, an increase of 46 mV in reversible cell potential is observed. As the pressure is increased, the mass transport rates and gas solubilities also increase. Anyhow, the increased pressure may promote some reactions decomposition of methane to carbon/hydrogen, methanation (methane formation) and carbon deposition and methane formation are favoured. The carbon deposition may lead to the plugging of gas passages in the anode. The steam reformation also gets inhibited due to higher pressure, which is a... 

Here k denotes reaction rate constant, superscripts / and b stand for forward and backward reactions respectively, subscripts SMR and WGS stand for steam methane reforming and water gas shift reactions respectively, and the square brackets represent the mole fraction of the specie. The heat sources due to these reactions, to be added to the energy conservation equation are given by ... 

Ideally, future work on the model would investigate the connection between capital cost and hydrogen production rate. For example, the capital cost of steam methane reforming may be linked to the production rate as in Equation (2), which will provide a more complicated trade-off. This type of analysis is possible with H2Sim, but requires the user to adjust capital costs to reflect the unit size. 

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