Methanation kinetic parameters

Controlled elimination of mass and heat transport resistances is an important prerequisite for obtaining intrinsic kinetic parameters of the fast exothermic reaction of partial oxidation of methane to synthesis gas. It has been demonstrated that under conditions of strong transport limitations erroneous conclusions concerning the reaction scheme can be derived [7-9]. It was determined in this laboratory that transport limitations are practically absent over a wide range of operating conditions if one portion of the catalyst (< 40 pm) is diluted with -5 portions of an... 

The other state variables are the fugacity of dissolved methane in the bulk of the liquid water phase (fb) and the zero, first and second moment of the particle size distribution (p0, Pi, l )- The initial value for the fugacity, fb° is equal to the three phase equilibrium fugacity feq. The initial number of particles, p , or nuclei initially formed was calculated from a mass balance of the amount of gas consumed at the turbidity point. The explanation of the other variables and parameters as well as the initial conditions are described in detail in the reference. The equations are given to illustrate the nature of this parameter estimation problem with five ODEs, one kinetic parameter (K ) and only one measured state variable. 

Kinetic expressions for the three step pathway given above for the partial oxidation of methane to formaldehyde over a vanadium oxide-silica catalyst were determined by Spencer and Periera [17]. The kinetic parameters... 

Kinetic parameters of methane hydroxidation with hydrogen peroxide... 

Table 2 Kinetic Parameters for the Phenol-Methanal or Lignin-Methanal Reactions...

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. 

A kinetic model [15], with adapted kinetic parameters [25], was used, which accounts for oxidation by oxygen of carbon monoxide, propene, methane, and hydrogen, and also includes inhibition effects caused by nitrogen oxide. The following net production rates were applied in Eq. (26) ... 

Other steps used in the model assume that the heterogeneous conversion of methane is limited to the gas-phase availability of oxygen, O2 adsorption is fast relative to the rate of methane conversion, and heat and mass transports are fast relative to the reaction rates. Calculations for the above model were conducted for a batch reactor using some kinetic parameters available for the oxidative coupling of methane over sodium-promoted CaO. The results of the computer simulation performed for methane dimerization at 800 °C can be found in Figure 7. It is seen that the major products of the reaction are ethane, ethylene, and CO. The formation of methanol and formaldehyde decreases as the contact time increases. 

An infrared study of CO adsorption on Ru-Au supported on magnesia suggested that this bimetallic behaves differently from Ru-Cu, with no evidence of Au segregation at the cluster surface, (nor separate Au clusters although ruthenium and gold are practically immiscible in the bulk). At temperatures below 383 K where the reaction between cyclopropane and hydrogen adopted routes to propane or methane + ethane, no interaction between Au and Ru containing up to 36% Au was evident from the kinetic parameters.However, a more complete examination (unpublished) of these catalysts by XPS, EXAFS, SAXS, and other techniques has been made and it is believed that the surface contained Ru atoms only. 

Table 2.5. Kinetic Parameters for the Steam Methane Reforming over Ni-, Pt-, and Ir-Based Supported Catalysts54-57...

The starting-point and basic-level principles of models, which can be defined as comprehensive or inclusive, must be the following the combinatorial approach for the compilation of a kinetic scheme and the use of independent values of kinetic parameters. The latter means the flat refusal to use any parameter optimization algorithms based on adjustment of the whole model or its blocks to some selected experimental data. These two basic principles are already discussed above in Sections II.A and II.B with reference to the GRI-Mech (combinatorial approach) and the methane-to-methanol oxidation model developed by Vedeneev and co-authors (use of independent kinetic parameters). However, we could not find any example of consistent employment of both principles in conjunction. 

For many cases Eq. (3.1) serves as a good approximation. However, one must keep in mind that the more correct form of mass action equation must contain activity values, which in the general case differ from concentrations (or, in other words, activity coefficients are not equal to 1). Deviations of activities from concentrations are most pronounced at relatively low temperatures and high pressures, i.e., when properties of the reaction system display a pronounced difference from those of an ideal gas. Uncertainty of kinetic simulations can therefore increase if values of kinetic parameters obtained at low pressures are used to model high-pressure processes in the framework of Eq. (3.1). Among processes of interest announced in this work, at least one—oxidation of methane-to-methanol—severely needs high pressures, at which the non-ideality of the reaction system can in principle manifest itself. 

The above thermochemical values were used to fill the heterogeneous module of the kinetic scheme for the OCM reaction over a model Li/MgO catalyst with corresponding kinetic parameters (see Table III). In combination with a scheme of homogeneous methane oxidation, this set of reactions forms the desired micro-kinetic description. It allowed us to re-consider specific features of the OCM process and to obtain some unexpected results. 

Even though the authors could not avoid some adjustment of selected kinetic parameters, what is explicable taking into account the extraordinary complexity of the system. As a result, they succeeded in reproducing in their simulations some important features of the real system and validated their micro-kinetic model against high-pressure spatially resolved experimental data for catalytic partial oxidation of methane. 

Simulations demonstrate, however, that variations in kinetic parameters of reactions under consideration lead to substantial consequences. Figure 15 shows how relatively small variations in the rate constant for reaction (30) influence the SID in methane-ethane mixtures. In such a reaction system (which models real compositions of natural gas) competition of different channels of ethyl-oxygen reaction overlaps (and very probably interferes) with methyl-oxygen chemistry. The latter is even somewhat qualitatively different there are no variations in mono-molecular reactions of methylperoxy radicals at temperatures below 900 K (only dissociation to methyl and 02) and all their bi-molecular reactions lead to branching as a nearest consequence. As to the ethyl-oxygen chemistry, it is much more rich and much less definite at the same time. So in this particular case, small variations in kinetic parameters lead to very substantial consequences. 

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

In Methylosinus trichosporium OB3b that expresses a particulate monooxygenase, the concentration of copper plays a significant role in the kinetic parameters for the consumption of both methane and trichloroethene (Sontoh and Semrau 1998). For methane, Vmax decreased from 300 to 82 when the concentration of Cu increased from 2.5 to 20 pm, and Ks decreased from 62 to 8.3 under these conditions. For trichloroethene, Vmax and Ks were unmeasurable at Cu concentrations of 2.5 juM even in the presence of formate, but for concentrations of 20 pM in the presence of formate were 4.1 and 7.9. 

Arai et al. (1986) determined the kinetic parameters for methane combustion over Lao.6Sro 4MnC>3 prepared by decomposition of metal acetates and/or nitrates calcined... 

Kinetic parameters for methane combustion over La Sr MnO calcined at different temperatures ... 

Some indication of active-site acidic functionality in P450 dioxygen scission can be revealed by the comparative evaluation of kinetic parameters in protonated and deuterated waters. The use of kinetic solvent isotope effects (KSIEs) has been valuable in the determination of potential proton-linked dioxygen activating enzymes such as P450 -, methane monooxygenase (MMO), ... 

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... 

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