Overall Reaction Engineering

In this section, we still restrict ourselves to the consideration of systems where only the overall behavior is of interest, but we extend the analysis to actual chemical reactors. Indeed, the discussion in the previous section was limited to the overall kinetics of multicomponent mixtures seen from the viewpoint of chemical reaction engineering, the discussion was in essence limited to the behavior in isothermal batch reactors, or, equivalently, in isothermal plug flow reactors. In this section, we present a discussion of reactors other than these two equivalent basic ones. The fundamental problem in this area is concisely discussed next for a very simple example. 

Suppose one has performed experiments with the mixture under consideration in a batch reactor, and one has obtained experimentally the overall kinetics—the R ) function such that dCldt = —R(C). For instance, one could obtain R C) = if the intrinsic kinetics are in fact first order and the initial concentration distribution is l (l,x) = exp(—x). If one were to regard R(C) as a true (rather than an apparent) kinetic law, one would eonclude that in a CSTR with dimensionless residence time T the exit overall concentration is delivered by the (positive) solution of TC +C = 1. The correct value is in fact C = exp(—y)/(l + Ty) , and the difference is not a minor one. (To see that easily, consider the long time asymp- 

one needs to deconvolute R(C) into some appropriate intrinsic kinetic law. What form the latter might take be suggested by experiments performed with single components. 

one needs to use the intrinsic kinetics as the basis for the analysis of a reactor other than a batch or plug flow reactor. 

In this section, we restrict our analysis to the second step of this procedure. The nomenclature we use is as follows Cp(x) is the concentration distribution in the feed to any given reactor, normalized so that cp(y) = ycp(y) = 1, and Ce(x) is the corresponding distribution in the product stream. Let C = CE(y) is the overall residue. Finally, let Tbe the dimensionless residence time in the reactor, that is, the actual residence time times the average value of the frequency factor in the feed mixture. 

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C. Overall Reaction Engineering Appendix A Orthogonal Complement Appendix B... 

Reaction Engineering. Electrochemical reaction engineering considers the performance of the overall cell design ia carrying out a reaction. The joining of electrode kinetics with the physical environment of the reaction provides a description of the reaction system. Both the electrode configuration and the reactant flow patterns are taken iato account. More ia-depth treatments of this topic are available (8,9,10,12). 

Safety in Chemical Reaction Engineering 991 The overall heat transfer eoeffieient U is defined by... 

The immobilization procedure may alter the behavior of the enzyme (compared to its behavior in homogeneous solution). For example, the apparent parameters of an enzyme-catalyzed reaction (optimum temperature or pH, maximum velocity, etc.) may all be changed when an enzyme is immobilized. Improved stability may also accrue from the minimization of enzyme unfolding associated with the immobilization step. Overall, careful engineering of the enzyme microenvironment (on the surface) can be used to greatly enhance the sensor performance. More information on enzyme immobilization schemes can be found in several reviews (7,8). 

Elementary reactions have integral orders. However, for overall reactions the rate often cannot be written as a simple power law. In this case orders will generally assume non-integral values that are only valid within a narrow range of conditions. This is often satisfactory for the description of an industrial process in terms of a power-rate law. The chemical engineer in industry uses it to predict how the reactor behaves within a limited range of temperatures and pressures. 

Since most synthetic applications require enzymes catalyzing nonnatural substrates, their properties often have to be improved. One way to achieve this is to optimize reaction conditions such as pH, temperature, solvents, additives, etc. [6-9]. Another way is to modulate the substrates without compromising the synthetic efficiency of the overall reaction [10]. In most cases for commercial manufacturing, however, the protein sequences have to be altered to enhance reactivity, stereoselectivity and stability. It was estimated that over 30 commercial enzymes worldwide have been engineered for industrial applications [11]. Precise prediction of which amino acids to mutate is difficult to achieve. Since the mid 1990s, directed evolution... 

A new type of power supply for electric cars eliminates the need for recharging. A fuel cell is a battery that produces electricity while reactants are supplied continuously from an external source. Because reactants continuously flow into the cell, a fuel cell is also known as a flow battery. Unlike the fuel supply of a more conventional battery, the fuel supply in a fuel cell is unlimited. As in the combustion of gasoline in a conventional engine, the overall reaction in a fuel cell is the oxidation of a fuel by oxygen. 

The overall reactions given in Table 2-2 can be used to produce both electrical energy and heat. The maximum work available from a fuel source is related to the free energy of reaction in the case of a fuel cell, whereas the enthalpy (heat) of reaction is the pertinent quantity for a heat engine, i.e.. 

In the above example e is equal to 0.830. The AG of the overall reaction can be expressed in terms Of the corresponding battery voltage and for the hydrogen-oxygen reaction at 25 C, its value is 1.229 V. As the temperature increases this thermodynamic equilibrium value will decrease by a factor of 0.84 mV per C. If the water produced remains in the gas phase, the ratio of AG /AH increases to 0.911. So as we see, these values are much higher than what can be obtained by a heat engine where the efficiency is defined by the ratio of the temperature difference of the hot... 

These complications show wli we emphasize simple and qualitative problems in this course. In reactor engineering the third decimal place is almost always meaningless, and even the second decimal place is fiequently suspect. Our answers may be in error by several orders of magnitude through no fault of our own, as in our example of the temperature dependence of reaction rates. We must be suspicious of our calculations and make estimates with several approximations to place bounds on what may happen. Whenever a chemical process goes badly wrong, we are blamed. This is why chemical reaction engineers must be clever people. The chemical reactor is the least understood and the most complex unif of any chemical process, and its operation usually dominates the overall operation and controls the economics of most chemical processes. 

Thus hydrocarbons and CO are products of incomplete combustion, which occurs primarily when the engine (and the catalyst) are cold. NO, is produced primarily by free-radical reactions with atmospheric N2 in the high temperatures (2500°C) within the engine cylinders. The overall reaction can be written as... 

Most standard chemical engineering tests on kinetics [see those of Car-berry (50), Smith (57), Froment and Bischoff (19), and Hill (52)], omitting such considerations, proceed directly to comprehensive treatment of the subject of parameter estimation in heterogeneous catalysis in terms of rate equations based on LHHW models for simple overall reactions, as discussed earlier. The data used consist of overall reaction velocities obtained under varying conditions of temperature, pressure, and concentrations of reacting species. There seems to be no presentation of a systematic method for initial consideration of the possible mechanisms to be modeled. Details of the methodology for discrimination and parameter estimation among models chosen have been discussed by Bart (55) from a mathematical standpoint. 

This chapter gives an introduction to the subject of chemical reaction engineering. The first part introduces basic definitions and concepts of chemical reaction engineering and chemical kinetics and the importance of mass and heat transfer to the overall chemical reaction rate. In the second part, the basic concepts of chemical reactor design are covered, including steady-state models and their use in the development... 

Another classification of chemical reactors is according to the phases being present, either single phase or multiphase reactors. Examples of multiphase reactors are gas liquid, liquid-liquid, gas solid or liquid solid catalytic reactors. In the last category, all reactants and products are in the same phase, but the reaction is catalysed by a solid catalyst. Another group is gas liquid solid reactors, where one reactant is in the gas phase, another in the liquid phase and the reaction is catalysed by a solid catalyst. In multiphase reactors, in order for the reaction to occur, components have to diffuse from one phase to another. These mass transfer processes influence and determine, in combination with the chemical kinetics, the overall reaction rate, i.e. how fast the chemical reaction takes place. This interaction between mass transfer and chemical kinetics is very important in chemical reaction engineering. Since chemical reactions either produce or consume heat, heat removal is also very important. Heat transfer processes determine the reaction temperature and, hence, influence the reaction rate. 

The preceding discussion has concentrated on the selection of catalytically active components. Although this is an essential task, this is just one aspect of the whole catalytic process, which also includes selection of catalyst support and the design of the overall catalyst in relation to reaction engineering requirements, so that not only activity and selectivity but also mechanical and chemical stability are ensured. For catalyst supports, the design variables are the degree and the form of the dispersion of the catalytic active components, and the porosity of the support. 

While reactor temperature often plays a dominant role on reaction rate, it is not the only contribution to the rate expression. We also have the influence of the concentrations of the reacting species as symbolized by / (.. in Eq. (4.5). This expression can range from simple to very complex. In the simplest form the rate of reaction is proportional to the reactant concentrations raised to their stoichiometric coefficients. This is true for an elementary step where it is assumed that the molecules have to collide to react and the frequency of collisions depends upon the number of molecules in a unit volume. In reality, matters are far more complicated. Several elementary steps with unstable intermediates are usually involved, even for the simplest overall reactions. When the intermediates are free radicals, there can be a hundred or more elementary steps. From an engineering viewpoint it is impractical to deal with scores of elementary steps and intermediates and we usually seek an overall rate expression in terms of the stable, measurable (in principle) components in the reactor. In theory we can derive... 

Pseudo- and overall reaction orders. Kinetic textbooks describe other, more complicated methods applicable to other forms of proposed rate equations, mostly for evaluation of results from batch reactors. However, if the development chemist or engineer can commission experiments—as opposed to having to evaluate existing data—he can often save himself much effort by determination of pseudo- and overall reaction orders. For example, for a reaction A + B — product(s) and power-law rate equation —rk = kmCfCB, three series of experiments suggest themselves ... 

For the particle sizes used in industrial reactors (> 1.5 mm), intraparticle transport of the reactants and ammonia to and from the active inner catalyst surface may be slower than the intrinsic reaction rate and therefore cannot be neglected. The overall reaction can in this way be considerably limited by ammonia diffusion through the pores within the catalysts [211]. The ratio of the actual reaction rate to the intrinsic reaction rate (absence of mass transport restriction) has been termed as pore effectiveness factor E. This is often used as a correction factor for the rate equation constants in the engineering design of ammonia converters. 

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