Methane oxidation rate constants

A typical oxidation is conducted at 700°C (113). Methyl radicals generated on the surface are effectively injected into the vapor space before further reaction occurs (114). Under these conditions, methyl radicals are not very reactive with oxygen and tend to dimerize. Ethane and its oxidation product ethylene can be produced in good efficiencies but maximum yield is limited to ca 20%. This limitation is imposed by the susceptibiUty of the intermediates to further oxidation (see Figs. 2 and 3). A conservative estimate of the lower limit of the oxidation rate constant ratio for ethane and ethylene with respect to methane is one, and the ratio for methanol may be at least 20 (115). 

Extensive research has been conducted on catalysts that promote the methane—sulfur reaction to carbon disulfide. Data are pubhshed for sihca gel (49), alurnina-based materials (50—59), magnesia (60,61), charcoal (62), various metal compounds (63,64), and metal salts, oxides, or sulfides (65—71). Eor a sihca gel catalyst the rate constant for temperatures of 500—700°C and various space velocities is (72)... 

Figure 3.44 Conversion rates and product selectivity of partial methane oxidation performed under constant heating power as a function of the methane/oxygen ratio [112. ...

In the M. trichosporium OB3b system, a third intermediate, T, with kmax at 325 nm (e = 6000 M-1cm 1) was observed in the presence of the substrate nitrobenzene (70). This species was assigned as the product, 4-nitrophenol, bound to the dinuclear iron site, and its absorption was attributed primarily to the 4-nitrophenol moiety. No analogous intermediate was found with the M. capsulatus (Bath) system in the presence of nitrobenzene. For both systems, addition of methane accelerated the rate of disappearance of the optical spectrum of Q (k > 0.065 s-1) without appreciatively affecting its formation rate constant (51, 70). In the absence of substrate, Q decayed slowly (k 0.065 s-1). This decay may be accompanied by oxidation of a protein side chain. 

Many extensive models of the high-temperature oxidation process of methane have been published [20, 20a, 20b, 21], Such models are quite complex and include hundreds of reactions. The availability of sophisticated computers and computer programs such as those described in Appendix I permits the development of these models, which can be used to predict flow-reactor results, flame speeds, emissions, etc., and to compare these predictions with appropriate experimental data. Differences between model and experiment are used to modify the mechanisms and rate constants that are not firmly established. The purpose here is to point out the dominant reaction steps in these complex... 

Table 16.3 Names, abbreviations, pseudo-first-order rate constants, and half-lives of polyhalo-genated alkanes in Fe(II)/goethite suspension. Experimental conditions 25 m L" goethite, pH 7.2, tgq>24 h. Fe(II) = 1 mM. b Standard deviation, c number of replicates, d t =5 h. Reprinted with permission from Pecher K, Haderline SB, Schwarzenbach RP (2002) Reduction of polyhalo-genated methanes by surface-bound Fe(II) in aqueous suspensions of iron oxides. Environ Sci Technol 36 1734-1741. Copyright 2002 American Chemical Society...

For the above applied oxidation of methane to carbon dioxide on some metal oxide catalysts, also a first-order reaction was assumed [10, pp. 182 and 193], However, in combinatorial catalysis it may be sufficient to have a first rough idea about the underlying kinetics. Without having prior information about the kinetics, the performance of a reactor is provided with a huge uncertainty. This is obvious if one considers the wide variation of reaction rates. Pre-exponential factors of reaction rate constants derived by the transition-state theory vary widely from approximately 10 to 1016 s-1 [10]. This first information might then be used to develop a pilot plant for the up-scaling and for further detailed kinetic examinations. 

Figure 2. Temperature dependence of the ratio of isotopic rate constants, < ki2/ki3 for methane oxidation by various oxidants.

The oxidation of methane was very slow under the experimental conditions employed The slowest rates are those with anhydrite as oxidant. Because the ratio of the rate constants, a, is dependent upon the oxidant, it is difficult to estimate the carbon isotope selectivity during sulfate reduction at temperatures relevant to TSR in sour gas occurrences. However, the effects are substantial with the cupric oxide-manganese dioxide and hematite-anhydrite trends in Figure 2 giving extrapolated a-values of about 1.02 and 1.04 respectively at 200°C. 

The presence of the intermediate methane in the ethane and propane oxidation experiments, coupled with failure to detect ethane or ethene intermediates in the propane experiments constitutes indirect evidence that the reaction rate constants are in the order k >k >k, This order is confirmed by comparing Tablls°5fnlo tVlan t s rate constants calculated on the assumption of first order kinetics, decrease with percent reaction. This is presumably the consequence of the closed reactor conditions and for this reason, rate constants are not given in the tables. However, on average, the assumption of first order kinetics with respect to... 

A simple model of the chemical processes governing the rate of heat release during methane oxidation will be presented below. There are simple models for the induction period of methane oxidation (1,2.>.3) and the partial equilibrium hypothesis (4) is applicable as the reaction approaches thermodynamic equilibrium. However, there are apparently no previous successful models for the portion of the reaction where fuel is consumed rapidly and heat is released. There are empirical rate constants which, due to experimental limitations, are generally determined in a range of pressures or concentrations which are far removed from those of practical combustion devices. To calculate a practical device these must be recalibrated to experiments at the appropriate conditions, so they have little predictive value and give little insight into the controlling physical and chemical processes. 

For evaluation of the role of ozone in oxidation of hydrocarbons, the comparison of the reaction rates of ozone and oxygen with a certain hydrocarbon may be of use. The rate constant of this reaction is by 103 times higher [8], at minimum, for ozone and methane as reactants than that for oxygen and methane at the temperature as high as 200 °C. On the other hand, the concentration of ozone in the atmosphere is rather low under normal conditions. 

The efficiency of the initiation effect of triplet oxygen depends on the hydrocarbon structure, i.e., on the strength of the attacked C —H bond. For instance, the ratio of the rate constants of the reaction of oxygen with formaldehyde and methane is 1.3 x 109 at 100 °C [8]. This indicates that intermediates of oxidation may be more sensitive towards oxidation than the original substrate which may contribute to the appearance of heterogeneous regions where the oxidation takes place preferably. 

The experimental results are presented in Figure 3. In this figure the amount of coke deposited on the catalyst has been plotted versus the volume of methane (at STP) fed into the reactor. Both absolute amounts of coke on a 5 g charge of catalyst, and percent coke by weight are reported. Since the feed flow rate of methane was maintained constant at 0.31 liters (STP)/min, the abscissa also represents time. Each point on Figure 3 represents an experimental run of approximately 12-15 hours duration including the reduction and subsequent oxidation of the catalyst. 

Heavier hydrocarbons behave similarly, although rate constants of their OH oxidation are the higher the more carbon atoms there are in the molecule (eg rate constant for pentane is of the order of 103 higher than that for methane) the oxidation is also much faster in the case of unsaturated or aromatic organic hydrocarbons and their derivatives. 

Derivations of equation (4) involve a microscopic viewpoint. The reasoning, in its simplest form, is that the reaction rate is proportional to the collision rate between appropriate molecules, and the collision rate is proportional to the product of the concentrations. Implicit in this picture is the idea that equation (4) will be valid only if equation (1) represents a process that actually occurs at the molecular level. Equation (1) must be an elementary reaction step, with v[ molecules of each molecular species i interacting in the microscopic process equation (4) will not be meaningful if equation (1) is the overall methane-oxidation reaction CH -1- 2O2 CO2 -1- 2H2O, for example. Thus, there are two basic problems in chemical kinetics the first is to determine the reaction mechanism, that is, to find the elementary steps by which the given reaction proceeds, and the second is to determine the specific rate constant k for each of these steps. These two problems are discussed in Sections B,2 and B.3, respectively. 

The second-order rate constant for oxidation of methane via OH attack is approximately 3 X ICR15 cm3/(molecule sec). 

Methane oxidation kinetics was assessed as follows. In these experiments, cells were inoculated into medium containing different initial amounts of copper sulfate and grown to an optical density at 600 nm of 0.5-0.7. Aliquots were then placed in closed vials at different initial dissolved methane concentrations. These aliquots were incubated under optimal conditions, and head-space samples were taken at four different time points (1-4 h) for determination of methane concentrations by gas chromatography. From these data, initial methane consumption rates determined for different methane concentrations were used to obtain the Michaelis-Menten parameters Ks (half-saturation constant) and Vmax (rate at substrate saturation). Under these conditions sMMO was not expressed, as described previously (9). 

Fig. 15. Effect of initial ethane concentration on the self-ignition delay values (7ign) in oxidation of methane-ethane mixtures modeling at different values of reaction (28) rate constant (T = 773 K, P = 70bar, [O2]0 = 15.4%). (1) k28 = 3 x 105s (2) k28 = 6 x 105s (3) k28 = 1.2 x lOV1.

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