The Role of Chemical Reactions

The successful, practical implementation of a chemical reaction is not a trivial exercise. The creative application of matraial from a number of technical areas is almost always required. Operating conditions must be chosen so that the reaction proceeds at an acceptable rate and to an acceptable extent The maximum extent to which a reaction can proceed is determined by stoichiometry and by the branch of thermodynanucs known as chemical equilibrium. This book begins with a short discussion of the principles of stoichiometry that are most applicable to chemical reactions. A working knowledge of chenucal equilibrium is presumed, based on prior chenustry and/or chenucal engineering coursework. Howevra, the book contains problems and examples that will help to reinforce this material. 

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Alternatively, the conservation of atomic species is commonly expressed in the form of chemical equations, corresponding to chemical reactions. We refer to the stoichiometric constraints expressed this way as chemical reaction stoichiometry. A simple system is represented by one chemical equation, and a complex system by a set of chemical equations. Determining the number and a proper set of chemical equations for a specified list of species (reactants and products) is the role of chemical reaction stoichiometry. 

Certain crude approaches are available to predict overall results, that is, nonequilibrium compositions. More refined techniques are available for the analysis of simplified models. Solution of the reaction kinetics of homogeneous gas phase combustion is possible through numerical solution of the rate equations. With the exception of the role of an overall highly exothermic reaction, the procedures are similar to those described in the preceding section on nozzle processes. The solution of the droplet burning problem including the role of chemical reaction rates, while not particularly tractable, is feasible. 

A more detailed description of the role of chemical reaction engineering is available in the paper by Tunca et al.,12 along with some examples and recent trends. Another useful source is the EPA Green Chemistry Web site (www.epa.gov/oppt/greenchemistry). 

In some cases the kinetic currents are small (sometimes 10% or even less) even at their maximum height when compared with waves of the equimolar solutions that are diffusion-controlled. This type of behaviour has been observed in particular in systems in which the waves of are obscured by the supporting electrolyte. However, not all kinetic currents are so small and whilst abnormally small currents may indicate kinetic currents, currents of the normal height do not allow us to exclude the role of chemical reactions. (Some catalytic currents are abnormally high.)... 

Feldmann, H. F., "The Role of Chemical Reaction Engineering in Coal Gasification," Chemical Reaction Engineering Reviews, Advances in Chemistry, No. 148, pp. 132-155 (1975). 

For a better comprehension of the chemistry of a groundwater system the redox status needs to be well-defined. Until recently, most efforts have relied solely upon Ej.j or pE, intensity factors, as the master variable. However, it is apparent that these intensity factors do not truly represent the redox status of a system because some pertinent redox couples are not electroactive and redox reactions are generally slow and are not at equilibrium. In this paper, the oxidative capacity, a capacity factor, is operationally defined and shown to be a better descriptive parameter of the redox status. Determination of the OXC of an aqueous system allows investigators to cla.ssify the system in terms of well-defined geochemical and microbial parameters. This classification combined with other predictive tools, such as a redox titration, allows one to predict the identity and assess the role of chemical reactions and microbial populations within a specific groundwater system. As such, the capacity factor OXC should be determined in water quality assessment. 

Sundmacher and Qi (Chapter 5) discuss the role of chemical reaction kinetics on steady-state process behavior. First, they illustrate the importance of reaction kinetics for RD design considering ideal binary reactive mixtures. Then the feasible products of kinetically controlled catalytic distillation processes are analyzed based on residue curve maps. Ideal ternary as well as non-ideal systems are investigated including recent results on reaction systems that exhibit liquid-phase splitting. Recent results on the role of interfadal mass-transfer resistances on the attainable top and bottom products of RD processes are discussed. The third section of this contribution is dedicated to the determination and analysis of chemical reaction rates obtained with heterogeneous catalysts used in RD processes. The use of activity-based rate expressions is recommended for adequate and consistent description of reaction microkinetics. Since particles on the millimeter scale are used as catalysts, internal mass-transport resistances can play an important role in catalytic distillation processes. This is illustrated using the syntheses of the fuel ethers MTBE, TAME, and ETBE as important industrial examples. 

The role of chemical reaction engineering in catalyst development has often been minor. The primary problem [1] is that macroscopic chemical kinetic equations do not allow the d uction of a unique mechanism. In 1987, Cleaves et aL [2] introduced a reactor to acquire kinetic data at the elementary step level (in contrast to macroscopic kinetics used in conventional chemical reaction engineering). The netwoik of elementary steps and the kinetic parameters of these elementary steps most accurately represent tte chemical reaction(s), and such data can be directly used in catalyst devebpment. This reactor is now popularly known as the TAP (temporal analysis ofproducts) reactor. The type of kinetic data possible with a TAP reactor, viz. the reaction mechanism and the kinetic parameters of the elementary steps, is also useful in chemical reaction engineering where non-steady state operation is considered and where changes in reaction mechanisms can occur within the reactor. In the 1990 s, a second-generation TAP reactor [3] appeared, with inq)roved signal-to-noise ratios. 

An exceptionally productive area of in vivo voltammetric applications is represented by neurobiology [160-162]. Voltammetric microelectrodes can be successfully applied for the detection of those substances that possess distinguished redox properties. As an example of the role of chemical reaction kinetics coupled with the heterogeneous electron transfer the detection of neurotransmitter dopamine in the extracellular brain liquid of living rats has been chosen. 

The meeting concentrated also on the role of chemical reactions coupled with electrode reactions, i.e. more on chemistry studied electrochemically. Electrochemistry is an excellent method for studying chemistry of short living intermediates and the newly developed — or developing — procedures make it possible to study these processes in more detail. The use of ultramicroelectrodes combined with very high sweep rates and the possibility of using these electrodes in... 

Both experimental and numerical investigations of the role of chemical reaction kinetics in turbulent combustion are complicated by the strong coupling of hydrodynamics with thermochemistry and by resolution requirements hydrodynamic and fliermodynamical spatial and temporal scales span many orders of magnitude in flames with high Reynolds and Damkohler numbers (the latter being the ratio of characteristic flow time scales to chemical time scales). Thus DNS of practical tur-... 

Driving Forces for Shrinkage explains the role of chemical reactions and the pressure exerted by osmotic, disjoining, and capillary phenomena. The concept of moisture stress, widely used in soil science, is a phenomenological measure of all of these factors. 

The relation between the extent of separation and the extent of reaction is briefly considered in Section 5.1. How chemical reactions alter the separation equilibria in gcts-liquid, vapor-liquid, liquid-liquid, solid-liquid, surface adsorption equilibria, etc., is described in Section 5.2. The role of chemical reactions in aitering the separation in... 

In Section 5.2, we considered the change in separation equiUhrium due to chemical reactions. Many such separation processes in practice do not have phases in equUih-rium. This may have come about, for example, due to inadequate contact time between the phases in the device being used. The extent of separation achieved wiU be controlled by the extent of transfer of the species between the phases. Other conditions remaining constant, the higher the rate of transfer, the larger the extent of separation achieved. The species transport rate from one phase to another then controls the actual separation achieved. Chemical reactions can influence this interphase species transport rate. The role of chemical reactions in the separation achieved in rate-controlled equilibrium separation processes is, therefore, the subject of this section. Mass transfer and separation in gas-Uquid systems are covered first, followed by liquid-liquid systems. 

These physical separation methods are often reinforced by chemical reactions, which are usually reversible. An elementary treatment of the role of chemical reactions in enhancing separation across a broad spectrum of phase equilibrium driven processes and membrane based processes has been included. The level of treatment in this book assumes familiarity with elementary principles of chemical engineering thermodynamics and traditional... 

The role of chemical reactions is still sigrrificant, because they bring into play energies much higher than the interfacial ones and frequently the reachons between hquid and solid result in the formation of new phases. We can thus dishnguish three... 

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