Steam-carbon reaction rate equations

This paper surveys the field of methanation from fundamentals through commercial application. Thermodynamic data are used to predict the effects of temperature, pressure, number of equilibrium reaction stages, and feed composition on methane yield. Mechanisms and proposed kinetic equations are reviewed. These equations cannot prove any one mechanism however, they give insight on relative catalyst activity and rate-controlling steps. Derivation of kinetic equations from the temperature profile in an adiabatic flow system is illustrated. Various catalysts and their preparation are discussed. Nickel seems best nickel catalysts apparently have active sites with AF 3 kcal which accounts for observed poisoning by sulfur and steam. Carbon laydown is thermodynamically possible in a methanator, but it can be avoided kinetically by proper catalyst selection. Proposed commercial methanation systems are reviewed. 

Fischer-Tropsch synthesis can be regarded as a surface polymerization reaction since monomer units are produced from the reagents hydrogen and carbon monoxide in situ on the surface of the catalyst. Hence, a variety of hydrocarbons (mainly n-paraffines) are formed from hydrogen and carbon monoxide by successive addition of C, units to hydrocarbon chains on the catalyst surface (Equation 12.1). Additionally, carbon dioxide (Equation 12.3) and steam (Equations 12.1 and 12.2) are produced C02 affects the reaction just a little, whereas H20 shows a strong inhibiting effect on the reaction rate when iron catalysts are used. 

Relative viscosities are calculated from viscosities for the individual components at 0° (II7), weighting them on a mole fraction basis. The change in diffusivities and viscosities with temperature and pressure is assumed to be independent of gas mixture. If desired, more accurate calculations of diffusivities and viscosities of gas mixtures can be made using the approaches of Wilke (IIS) and Bromley and Wilke (II0), respectively. Table V presents relative values for Dfree, m, and p across the stagnant film for the gas-carbon reactions. Substituting these values in Equation (42), the relative reaction rates in Zone III for the gas-carbon reactions are calculated and also presented in Table V. Qualitatively, the rates of the carbon-oxygen and carbon-steam reactions are predicted to be about twice the rate... 

The effect of the addition of a potassium promoter to a nickel steam reforming catalyst has been probed in terms of the propensity of the catalyst to resist carbon formation. It has been found that potassium facilitates a reduced accumulation of carbon by decreasing the rate of hydrocarbon decomposition on the catalyst and by increasing the rate of steam gasification of filamentary carbon from the catalyst. The effect of the promoter on the carbon removal reaction is evident in an enhancement of the pre-exponential factor in the rate equation by promotion of water adsorption on the catalyst surface. 

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 ... 

The apparent increase in rate may arise from the increase in surface as carbon was removed from the original particle. Although the conventional experimental rate equation for the carbon-steam reaction (6) states that steam is a part of the rate equation, it further states that steam also inhibits the rate. The test procedure used here maintains inlet steam at a fixed velocity and in large excess thus any effect of the level of steam conversion should be negligible on the integral rates calculated here. In the runs shown in Figure 2, the maximum use of steam by carbon in a 5-min. period varied from 3.6 to 8.5% of the total steam available. Of... 

Rate Equations for the Carbon-Carbon dioxide and Steam Reactions... 

These rate equations describe the kinetics and mechanisms of the gasification reactions of carbons by carbon dioxide and steam. As such, they describe the processes of activation by carbon dioxide and steam giving insights into mechanisms of carbon removal (the activation process), together with differences between activations by different agents and the effects of inhibition by product gases. 

The gas-phase reaction of carbon monoxide and steam to produce carbon dioxide and hydrogen has been studied in the presence of a Siemens ozonizer discharge. A factorial design was used to determine the effect of input electrical power, pressure, space velocity, and temperature on the conversion of carbon monoxide. With the aid of an empirical equation, derived from the factorial design data, the region of maximum conversion of carbon monoxide within the limits of the factors was determined. The rate of approach to thermodynamic equilibrium was investigated for one set of experimental conditions and was compared with previous work. The effect of changing the surface-to-volume ratio of the reactor upon carbon monoxide conversion was also determined. 

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... 

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