Kinetic models, chemical steps involved

To summarize these short remarks on modelling of BWR contamination buildup, it can be stated that all the existing models suffer from the same problems as the corresponding PWR models (see Section 4.4.3.6.), i. e. primarily from the fact that some of the fundamental steps are still insufficiently known. To develop a causative model based on physico-chemical principles, further work is claimed to be necessary to elucidate the mechanisms and the kinetics of the steps involved in activity buildup (Alder et al., 1992). Finally, it is still an open question whether it is possible to reflect the very complex conditions in a real BWR plant satisfactorily by a mathematical model that can be applied in daily work. 

Experimental determination of Ay for a reaction requires the rate constant k to be determined at different pressures, k is obtained as a fit parameter by the reproduction of the experimental kinetic data with a suitable model. The data are the concentration of the reactants or of the products, or any other coordinate representing their concentration, as a function of time. The choice of a kinetic model for a solid-state chemical reaction is not trivial because many steps, having comparable rates, may be involved in making the kinetic law the superposition of the kinetics of all the different, and often unknown, processes. The evolution of the reaction should be analyzed considering all the fundamental aspects of condensed phase reactions and, in particular, beside the strictly chemical transformations, also the diffusion (transport of matter to and from the reaction center) and the nucleation processes. 

Fig. 14.10 Reaction path diagram [149] illustrating major steps in volatile-N conversion in flames for different nitrogen species hydrogen cyanide (HCN), ammonia (NH3), cya-nuric acid (HNCO), acetonitrile (CH3CN), and pyridine (C5H5N). The diagram is based on chemical kinetic modeling at moderate fuel-N concentrations. Solid lines denote elementary reaction pathways, while dashed arrows denote routes that involve intermediates and reactions not shown.

The development and application of a rigorous model for a chemically reactive system typically involves four steps (1) development of a thermodynamic model to describe the physical and chemical equilibrium (2) adoption and use of a modeling framework to describe the mass transfer and chemical reactions (3) parameterization of the mass-transfer and kinetic models based upon laboratory, pilot-plant, or commercial-plant data and (4) use of the integrated model to optimize the process and perform equipment design. 

Of course, this is a required result if the kinetic model has any pretense to validity, and it is important that the B V model attains it for the limit of / = 0, not only for the simple one-step, one-electron process, but also in the context of an arbitrary multistep mechanism. The derivation here was carried out for a mechanism in which the prereactions and postreactions involve net charge transfer however the same outcome can be obtained by a similar method for any reaction sequence, as long as it is chemically reversible and a true equilibrium can be established. 

Prior to applying the methods to simplify the kinetic models of complex reactions, taking into account kinetic participation of the involved steps, we emphasize once again the importance of such a procedure. Finding the base kinetic model of a reaction is the identification of the fundamental essence of a chemical transformation. In fact, finding the base mechanism of the reaction is to reveal the nature of chemical transformation. The base reaction mechanism is some kind of core , which enables the in-depth understanding of the observed phenomena and highlights the fundamental ones. 

Stoichiometric model, sloidiiometrtc reaction scheme in enzyme kinetics, a chemical reaction equation in which molecules involved in the reaction, including the enzyme, are represented as letters. These also indicate the molar concentrations. The numbers in front of the letters are the stoichiometric coefficients, which indicate how many moles or molecules reactant are involved in the corresponding reaction step (the coefficient is usually omitted). The simplest example of a S.M. in enzyme kinetics is E + S si ES - E + P. More complicated reactions are more conveniently expressed by enzyme graphs. 

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