Methane formation kinetics

Kinetics. Extensive studies of the kinetics of methane synthesis were reported by White and co-workers (10,11, 12, 13, 14, 15). They studied the reaction between CO and hydrogen over a reduced nickel catalyst on kieselguhr at 1 atm and 300°-350°C (10). They correlated the rate of methane formation by the equation ... 

The kinetic expression was derived by Akers and White (10) who assumed that the rate-controlling factor in methane formation was the reaction between the adsorbed reactants to form adsorbed products. However, the observed temperature-dependence of the rate was small, which indicates a low activation energy, and diffusion was probably rate-controlling for the catalyst used. 

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

It should be noted at this point that primary and secondary reaction products can be distinguished not only by kinetic data (13) but also by suppression of the secondary reactions. E.g substitution of 2,2,2-trifluoroethanol for p-dioxane as solvent for HCoCCO) suppresses homologation and methane formation addition of a phosphine to give the less acidic catalyst HCo(CO)3PR3 has the same effect, as has the substitution of the less acidic catalyst HMn(CO)5. 

In the first set of measurements the rate of carbon build-up on a Ni(lOO) surface was measured at various temperatures as follows (1) surface cleanliness was established by AES (2) the sample was retracted into the reaction chamber and exposed to several torr of CO for various times at a given temperature (3) after evacuation the sample was transferred to the analysis chamber and (4) the AES spectra of C and Ni were measured. Two features of this study are noteworthy. First, two kinds of carbon forms are evident - a carbidic type which occurs at temperatures < 650 K and a graphite type at temperatures > 650 K. The carbide form saturates at 0.5 monolayers. Second, the carbon formation data from CO disproportionation indicates a rate equivalent to that observed for methane formation in a H2/CO mixture. Therefore, the surface carbon route to product is sufficiently rapid to account for methane production with the assumption that kinetic limitations are not imposed by the hydrogenation of this surface carbon. 

Englezos, P., A Model for the Formation Kinetics of Gas Hydrates from Methane, Ethane, and Their Mixtures, M.S. Thesis, University of Calgary, Alberta (1986). 

Also in Bishnoi s laboratory, Dholabhai et al. (1993) studied the effect of electrolytes on methane hydrate formation kinetics. They found that after the equilibrium fugacity (or driving force) is adjusted for the presence of salt, hydrate growth kinetics are quantitatively described by the pure water kinetics model of Englezos. 

By using the steady state kinetic equations, it is then possible to express k and k3 as a function of the overall turnover frequency for CO-conversion to hydrocarbons (Nco), the overall turnover frequency for methane formation (NCH ), the probability for chain growth (a), the steady state coverage of the precursor A and the value of the equilibrium constant K. In table I the expressions for the kj and k are given. 

Not much information is published on the kinetics of partial oxidation. The methane concentration is about 8 times higher than indicated by reaction (71), but as expected increasing pressure promotes methane formation. From a mere thermodynamical point of view no soot should be present at 1300 °C and O/C > 1. However, the raw sythesis gas contains more or less soot, depending on feedstock. Gasification of heavy oils fractions yields about 1-2% soot, but with methane the soot content is close to zero. 

Since there is no reason to believe that methane is formed in any other reaction, the rate of methane production is a measure of the rate of the initiation reaction (1). Lin and Back (c/. also Davis and Williamson , Quinn , Gordon ) have made a careful study of the kinetics of methane formation over a range of pressure and temperature. At higher pressures the process is first order in ethane, but there is a falling-off of the first-order rate coefficient at lower pressures. The first-order rate coefficient can be expressed as... 

The yields of individual volatile hydrocarbons obtained on HZSM5 in the kinetic range of autocatalysis are recorded in Table 1- The first compound to observe is methane, the second is ethene and thereafter the typical compounds to be formed in carbenium ion reactions (propane, i-butane and i-pentane) are the major products. Initial methane formation (on the fresh very active catalyst at... 

After the devolatilization and rapid-rate methane formation stages are completed, char gasification occurs at a relatively slow rate various models to describe the gasification kinetics of this material for various limited ranges of conditions have been proposed. The differential rates of reaction of devolatilized coal chars are a function of temperature, pressure, gas composition, carbon conversion, and prior history. 

PH2 = hydrogen partial pressure, atm /r = relative reactivity factor for rapid-rate methane formation dependent on the particular carbonaceous solid (defined as unity for air-pretreated Ireland mine coal char) a = kinetic parameter dependent on gas composition and pressure... 

Later studies show that there is a kinetic isotope effect for CO hydrogenation. Kobori et al. obtained an inverse isotope for CO hydrogenation with a 4.5 wt. percent Ru/Si02 catalyst with kjj/kQ = 0.57 + 0.12 for methane formation and kjj/k = 0.43 + 0.11 for C2 formation in the hydrogenation of surface carbon, predeposited by disproportionation of CO on a clean metal surface. 

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