Kinetics macrokinetics

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... 

Oxygen reduction can be accelerated by an application of electrodes with high surface area, e.g. the porous electrodes [9, 13]. The porous electrodes usually consist of catalysts, hydrophobic agent (polytetrafluoroethylene-PTFE) and conductive additive. Electrode kinetics on the porous electrodes is complicated by the mass and charge transfer in the pores and is called the macrokinetics of electrode processes . 

In order to estimate the region of this approximation applicability, it is necessary to examine macrokinetics of a polymeranalogous reaction with explicit allowance for the diffusion of a reagent Z into a globule. In this case, the profile of its constituent monomeric units will be fuzzy rather than stepwise (see Fig. 1). This brings up two questions. The first one is how this profile depends on kinetic and diffusion parameters of a reaction system. The second question is concerned with the effect of the profile shape on the statistical characteristics of the chemical structure of the products of a polymeranalogous reaction. A rigorous theory has been developed [22,23] which enables us to answer these questions. The main concepts of this theory are outlined in the subsequent Sections. 

When considering the macrokinetics of PAR described by equations (Eq. 17), it is reasonable to focus on two limiting regimes. The first of these, the kinetically-controlled regime, takes place provided the rate of diffusion of molecules Z appreciably exceeds that of the chemical reaction. In this case, a uniform concentration Z = Ze should be established all over the globule after time interval t R2/D. Subsequently, during the interval t 1 /kZe, which is considerably larger than f[Pg.152]

Quite special is the consideration of the kinetically-controlled regime of the reaction in a globule. For this limiting macrokinetic regime probability... 

Integration with the (micro) kinetics, in other words the kinetics of the pertinent free biocatalysts or of the immobilized biocatalysts including mass transfer, yields the overall reactor description or macrokinetics in later sections. In order to come up with these descriptions, a mass balance over the bioreactor should be drawn up (Figure 11.11). 

In the washcoat, reaction rates are modeled via global reaction mechanisms. In such a global or macrokinetic reaction mechanism, several microkinetic adsorption, reaction and desorption steps are lumped together, reducing the overall number of kinetic parameters considerably. For some catalysts,... 

The main difference between metals and polymers is related to the fact that transitions from one state to another in polymers occur (as a result of changing of environmental conditions, primarily temperature) not as jumps but continuously. This leads to the absence of a clearly defined line or transition front. Additionally, because of die low heat and temperature conductivity of polymeric materials, a change in material properties may take place over a large volume,or even simultaneously throughout the whole mass of an article, although the local transition rates and degrees of conversion may be different. Thus it is necessary to develop a macrokinetic model of the transition. This model must describe the combined effects of non-stationary heat transfer and reaction kinetics and is used to determine the temperature and conversion fields. 

The macrokinetics of zinc extraction is discussed in detail in Ref. 8. It is a combination of a reaction kinetics term (55) with the Maxwell-Stefan (54) or eddy diffusion (56). The rate law is as follows ... 

A. Ya. Rozovsky, Heterogeneous Chemical Reactions Kinetics and Macrokinetics, Nauka, Moscow, 1980. 

The net kinetics of the overall reaction (steps 1-7) are normally called effective kinetics or macrokinetics, in contrast to the kinetics of the chemical transformation (steps 3-5) which are termed intrinsic kinetics or microkinetics. 

Limitations The interaction of the kinetics of the chemical reaction and transport phenomena of the reactions are described as macrokinetics, in which transport phenomena, such as mass and heat transfer, adsorption, and desorption have a substantial impact on the time-dependent development of the reaction. The macro-kinetic is important for reactor engineering, construction, operation, and safety. However, for development and comparison of heterogeneous catalysts, the macroki-netic is not suitable, as the comparison of different catalytic materials is often hampered by the transport phenomena. 

The kinetics of the process is important in view of economic viability, because it is the crucial factor for productivity. For an assessment the complete macrokinetics of both reaction steps have to be considered (i.e. heat and mass transfer as well as the rates of the gas-solid reactions). Thermal reduction of an oxide and its reoxidation by CO2 are reversible gas-solid reactions. The reaction rate of a solid reacting with a gas to form another solid material is described by Equation (8) ... 

In addition to the formation of active centres and participation in elementary processes, the discussion of which forms the main topic of this volume, monomers very often react with some component(s) of the polymerizing medium under complex formation. This reaction is very important. Complex formation lowers the effective monomer concentration, and changes in the polymerization rate usually occur. When the complex is much more active than the monomer, it may react preferentially with the active centre. This, of course, changes the addition mechanism and kinetics. When the monomer and complex also compete, the macrokinetics need not necessarily change. Usually, however, the mechanism of the whole process is greatly complicated, and a kind of copolymerization occurs. 

One of the most important issues of macrokinetic studies is distinguishing the kinetic region in which the observed rate is governed by kinetic dependences. Obviously, intrinsic kinetic data are necessary for catalyst characterization and reactor design. Traditionally, in the kinetic studies of heterogeneous catalysis, to distinguish the kinetic regime from fhe... 

Present research efforts aim mainly at obtaining the important parameters from a study of the macrokinetics of combustion. It is important to estimate the effects of flame retardants, chemical structure of the polymer and polymer composition on variations of the solid- and gas-phase reaction kinetics. 

This volume is including information about thermal and thermooxidative degradation of polyolefine nanocomposites, modeling of catalytic complexes in the oxidation reactions, modeling the kinetics of moisture adsorption by natural and synthetic polymers, new trends, achievements and developments on the effects of beam radiation, structural behaviour of composite materials, comparative evaluation of antioxidants properties, synthesis, properties and application of polymeric composites and nanocomposites, photodegradation and light stabilization of polymers, wear resistant composite polymeric materials, some macrokinetic phenomena, transport phenomena in polymer matrix, liquid crystals, flammability of polymeric materials and new flame retardants. 

Macrokinetic reaction rates of vibrationaiiy excited molecules are self-consistent with the influence of the reactions on vibrational distribution functions / E), which can be taken into account by introducing into the Fokker-Planck kinetic equation (3-130) an additional flux related to the reaction ... 

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