Chemical kinetics heterogenous

Figure 4.9. Schematic representation of enzyme regulation principles as a basis for biokinetic model building. While ordinary kinetics are analogous to chemical kinetics (heterogeneous catalysis), enzyme induction and repression are typical biological phenomena creating transients. (From Moser, 1984.)...

Several factors can influence reaction rates. Concentration and reaction rate are related. niiaui Key Terms reaction rate chemical kinetics heterogeneous reaction lull nai catalysis homogeneous catalyst heterogeneous catalyst LC order rate-determining step... 

Chemical reactions obey the rules of chemical kinetics (see Chapter 2) and chemical thermodynamics, if they occur slowly and do not exhibit a significant heat of reaction in the homogeneous system (microkinetics). Thermodynamics, as reviewed in Chapter 3, has an essential role in the scale-up of reactors. It shows the form that rate equations must take in the limiting case where a reaction has attained equilibrium. Consistency is required thermodynamically before a rate equation achieves success over tlie entire range of conversion. Generally, chemical reactions do not depend on the theory of similarity rules. However, most industrial reactions occur under heterogeneous systems (e.g., liquid/solid, gas/solid, liquid/gas, and liquid/liquid), thereby generating enormous heat of reaction. Therefore, mass and heat transfer processes (macrokinetics) that are scale-dependent often accompany the chemical reaction. The path of such chemical reactions will be... 

Mass Transfer. The reaction rate of heterogeneous reactions may be controlled by the rates of diffusion of the reacting species, rather than the chemical kinetics. 

The first of these factors pertains to the complications introduced in the rate equation. Since more than one phase is involved, the movement of material from phase to phase must be considered in the rate equation. Thus the rate expression, in general, will incorporate mass transfer terms in addition to the usual chemical kinetics terms. These mass transfer terms are different in type and number in different kinds of heterogeneous systems. This implies that no single rate expression has a general applicability. 

Mechanistic rate laws assume that only chemical kinetics is operational and transport phenomena are not occurring. Consequently, it is difficult to determine mechanistic rate laws for most solid phase systems due to the heterogeneity of the solid phase system caused by different particle sizes, porosities, and types of retention sites. 

In the case of heterogeneous surface burning of a particle, consideration must be given to the question of whether diffusion rates or surface kinetic reaction rates are controlling the overall burning rate of the material. In many cases, it cannot be assumed that the surface oxidation kinetic rate is fast compared to the rate of diffusion of oxygen to the surface. The surface temperature determines the rate of oxidation and this temperature is not always known a priori. Thus, in surface combustion the assumption that chemical kinetic rates are much faster than diffusion rates cannot be made. 

Mukahy, M. F. R., and B. C. Young. Heterogeneous reactions of OH radicals. Int. J. Chem. Kinet. Symp. 1 (Chemical Kinetics Data for the Lower and Upper Atmosphete) 595-609, 1975. 

At low temperatures (T<1320 °C) and small particles, combustion regime (I) prevails [11,74,75]. Regime (I) is controlled by chemical kinetics intraparticle (reaction control), see Figure 55. The oxygen content is constant at any radius inside the particle since the rate of diffusion is fast compared to the rate of heterogeneous reaction. The particle then burns with reducing density and a constant diameter, see Figure 55. 

Although the major objective of this paper has been to illustrate the concepts involved in the development of detailed chemical kinetic mechanisms for reactions taking place in the gas phase, a very short introduction is provided here to illustrate the application of the foregoing concepts to heterogeneous systems. 

The philosophy used to develop detailed chemical kinetic mechanisms for gas-phase reactions can, in principle, be extended to treat heterogeneous reactions, provided diffusion is also considered in the final analysis. Clearly, the problem in heterogeneous catalysis is considerably more complex because of the close proximity of a large number of atoms and their collective effect on reaction kinetics and mechanisms, and the inevitable variation of catalyst structure with time—for example, as a result of sintering and poisoning. 

In siunmary, although the application of detailed chemical kinetic modeling to heterogeneous reactions is possible, the effort needed is considerably more involved than in the gas-phase reactions. The thermochemistry of surfaces, clusters, and adsorbed species can be determined in a manner analogous to those associated with the gas-phase species. Similarly, rate parameters of heterogeneous elementary reactions can be estimated, via the application of the transition state theory, by determining the thermochemistry of saddle points on potential energy surfaces. 

Billingham, N. C. and A. D. Jenkins, Free Radical Polymerization in Heterogeneous Systems, Chap. 6 in Comprehensive Chemical Kinetics, Vol. 14A, C. H. Bamford and C. F. H. Tipper, eds., American Elsevier, New York, 1976. 

While many techniques have evolved to evaluate surface intermediates, as will be discussed below, it is equally important to also obtain information on gas phase intermediates, as well. While the surface reactions are interesting because they demonstrate heterogeneous kinetic mechanisms, it is the overall product yield that is finally obtained. As presented in a text by Dumesic et al. one must approach heterogeneous catalysis in the way it has been done for gas phase systems, which means using elementary reaction expressions to develop a detailed chemical kinetic mechanism (DCKM). DCKMs develop mechanisms in which only one bond is broken or formed at each step in the reaction scheme. The DCKM concept was promoted and used by numerous researchers to make great advances in the field of gas phase model predictions. 

Gerecke, A., A. Thielmann, L. Gutzwiller, and M. J. Rossi, The Chemical Kinetics of HONO Formation Resulting from Heterogeneous Interaction of N02 with Flame Soot, Geophys. Res. Lett., 25, 2453-2456 (1998). 

There is nothing unique about the determination of the mechanism of electrochemical reactions. Electrochemical kinetics is a parallel field to that of heterogeneous chemical kinetics and basically the mechanism tasks in the two related fields are the same. There are three goals that must be reached consecutively. 

Electrochemical reactions are heterogeneous chemical reactions accompanied by electrical charge transfer across the interface. Besides ordinary variables of chemical kinetics, such as concentration, temperature, etc., electrochemical kinetics is characterized by an additional independent variable, electrode potential. The rate of electrochemical processes may vary quite significantly (exponentially) with the electrode potential. 

Although such studies are in their early stages, this example clearly demonstrates that we have the measurement tools to investigate the complex interaction of hydrodynamics and chemical kinetics in the complex porous medium represented by a fixed bed. Looking to the future, we may expect experiments of this nature to demonstrate how a catalyst with intrinsic high selectivity can produce a far wider product distribution when operated in a fixed-bed environment as a result of the spatial heterogeneity in hydrodynamics and hence, for example, mass transfer characteristics between the inter-pellet space within the bed and the internal pore space of the catalyst. 

In this chapter we provide the fundamental concepts of chemical and biochemical kinetics that are important for understanding the mechanisms of bioreactions and also for the design and operation of bioreactors. First, we shall discuss general chemical kinetics in a homogeneous phase and then apply its principles to enzymatic reactions in homogeneous and heterogeneous systems. 

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