Global kinetics

Lasaga, A. C. (1981). Dynamic treatment of geochemical cycles global kinetics. In "Kinetics of Geochemical Processes" (A. C. Lasaga and R. J. Kirkpatrick, eds), pp. 69-110. Mineral. Soc. Amer., Washington, DC. 

With the use of isothermal calorimetry, very accurate heat generation rates can be acquired as a function of time. By measurement at several temperatures, global kinetic parameters can be determined, assuming that the reaction mechanism remains the same within the temperature interval investigated. The heat production of the substance under test can be expressed as ... 

Data from ARC experiments can be used to determine the global kinetics [123,126] of a highly energetic reaction. When the onset is first observed in an ARC experiment, the concentrations have not changed significantly, and therefore ... 

Although in determining the onset temperature and the global kinetics in the initial stage of the reaction the, corrections are of lesser importance, these corrections have a dramatic effect on the maximum self-heat rate and the maximum temperature. Care must be taken in the interpretation of such data from an experiment with a phi-factor greater than 1. For direct simulations of plant situations, a phi-factor of 1.0 to 1.05 is used [89],... 

If agitation fails during a semi-batch operation, the transfer of heat will essentially stop. The resulting increase in temperature depends on the concentration of the reactants at that moment, the global kinetics, and the mass transfer rate. The effect of the temperature increase is easily simulated in a reaction calorimeter both with and without addition of reactants. 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

E. B. Ledesma, N. D. Marsh, A. K.Sandrowitz, and M. J. Womat, Global kinetic rate parameters for the formation of polycycUc aromatic hydrocarbons [PAH] from die pyrolysis of catechol A model compound representative of soUd fuel moieties. Energy Fuels 16(6), 1331—1336... 

They focus on the ID simulation of an urea SCR system. The system includes a model for N02 production on a DOC, a model for urea injection, urea decomposition and hydrolysis catalyst, a model for a vanadium-type SCR catalyst and a model for NH3 decomposition on a clean-up catalyst. The catalyst models consist of a ID monolith model with global kinetic reactions on the washcoat surface, kinetic parameters have been taken from literature or adjusted to experimental data from literature. The complete model was implemented in AVL BOOST (2006). AVL BOOST is an engine cycle and gas exchange simulation software tool, which allows for the building of a model of the entire engine. 

In this section we shall discuss the development of a global kinetic model for DOC. The basic model reactions considered in the DOC model are summarized in Table II. Here the real HC mixture is modeled by two characteristic hydrocarbons—propene and decane. Propene represents more reactive, light hydrocarbons, which practically do not adsorb during cold start, while decane is a representative of heavier hydrocarbons with significant adsorption on... 

When a simple, fast and robust model with global kinetics is the aim, the reaction kinetics able to predict correctly the rate of CO, H2 and hydrocarbons oxidation under most conditions met in the DOC consist of semi-empirical, pseudo-steady state kinetic expressions based on Langmuir-Hinshelwood surface reaction mechanism (cf., e.g., Froment and Bischoff, 1990). Such rate laws were proposed for CO and C3H6 oxidation in Pt/y-Al203 catalytic mufflers in the presence of NO already by Voltz et al. (1973) and since then this type of kinetics has been successfully employed in many models of oxidation and three-way catalytic monolith converters... 

Several different models were proposed for the slow NOx storage process, while only few details and approximate models are available for the highly transient NOx reduction within the rich phase, lasting only several seconds. The models can be divided into two groups, depending on whether the internal diffusion in the particles of the NOx storage material is considered explicitly, or this effect is included implicitly into the evaluated kinetic parameters. The models can be further differentiated by the level of complexity for the reaction kinetics description, i.e., either (simplified) microkinetic scheme or the global kinetics. 

For a comprehensive kinetic description of the NH3 + N0/N02 reacting system in a wide range of temperatures and N02/N0X feed ratios a global kinetic model was developed, based on the whole set of reactions in Table V. 

Adsorption on the interior site. For aqueous solutions such adsorption is usually classified by either ionic type chemical steps, or by a relatively loose adsorption with low heat of adsorption, that is by physical adsorption. In either case the process has a high rate constant so that this step usually does not influence the global kinetics. 

The last term in Equation 1 represents the global kinetics, expressed as the rate of adsorption per unit volume of bed. It cannot be written in terms of concentrations in the fluid phase—the equations must be solved to do this—but it may be expressed quantitatively in terms of Ci by writing an expression for the diffusion rate ... 

Global Kinetic Models for Supercritical Water Oxidation of Organic Substances... 

Tester, J.W. et al., Revised global kinetic measurements of methanol oxidation in supercritical water, Indust. Eng. Chem. Res., 32(1), 236-239, 1993a. 

Although not very commonly used (with the exception of the initial rate procedure for slow reactions), the differential method has the advantage that it makes no assumption about what the reaction order might be (note the contrast with the method of integration, Section 3.3.2), and it allows a clear distinction between the order with respect to concentration and order with respect to time. However, the rate constant is obtained from an intercept by this method and will, therefore, have a relatively high associated error. The initial rates method also has the drawback that it may miss the effect of products on the global kinetics of the process. 

Carbon Monoxide Oxidation. Analysis of the carbon monoxide oxidation in the boundary layer of a char particle shows the possibility for the existence of multiple steady states (54-58). The importance of these at AFBC conditions is uncertain. From the theory one can also calculate that CO will bum near the surface of a particle for large particles but will react outside the boundary layer for small particles, in qualitative agreement with experimental observations. Quantitative agreement with theory would not be expected, since the theoretical calculations, are based on the use of global kinetics for CO oxidation. Hydroxyl radicals are the principal oxidant for carbon monoxide and it can be shown (73) that their concentration is lowered by radical recombination on surfaces within a fluidized bed. It is therefore expected that the CO oxidation rates in the dense phase of fluidized beds will be suppressed to levels considerably below those in the bubble phase. This expectation is supported by studies of combustion of propane in fluidized beds, where it was observed that ignition and combustion took place primarily in the bubble phase (74). More attention needs to be given to the effect of bed solids on gas phase reactions occuring in fluidized reactors. 

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