Reactor Design and Analysis

Actually, chain-growth polymerization involves other reactions. The reaction shown above is a propagation reaction, as defined in Chapter 5. This reaction is accompanied by an initiation reaction, which provides a source of free radicals, and by a termination reaction, which consumes free radicals. For polystyrene polymerization, the termination reaction is the combination of two live polymer radicals, such as those shown above, to form a molecule of dead polymer. 

Suppose that we are asked to design or analyze a reactor system in which multiple reactions are taking place. In general, our work would have two simultaneous objectives  

To produce the desired product at the specified production rate using the smallest reactor possible (or the smallest amount of catalyst possible), i.e., to maximize the reaction rate. 

The second objective makes the problem much more difficult than the single-reaction problems that we solved earlier, in Chapter 4. Usually, it is not possible to minimize the reactor volume and maximize the reaction selectivity simultaneously. Frequently, there is a trade-off between rate and selectivity. Ultimately, this trade-off requires an economic analysis. However, the final analysis often favors selectivity over rate because there is a large and continuing cost penalty associated with converting a valuable raw matraial into a low-value by-product There are several important questions that must be considered in designing/analyzing multiple-reactor systems  

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Y. S. Tang. Ph.D has more than 35 years of experience in the field of thermal and fluid flow. His research interests have covered aspects of thermal hydraulics that are related to conventional and nonconventional power generation systems, with an emphasis on nuclear reactor design and analysis that focuses on liquld-meta -cooled reactors. Dr. Tang is co-author of Radioactive Waste Management published by Taylor 8 Francis, and Thermal Analysis of Liquid Metal Fast Breeder Reactors, He received a B5. from National Central University In China and an MS. in mechanical engineering from the University of Wisconsin. He earned his Ph.D. 

The resin system selected to initiate these studies is a step-growth anhydride cured epoxy. The approach to the kinetic analysis is that which is prevalent in the chemical engineering literature on reactor design and analysis. Numerical simulations of oligomeric population density distributions approximate experimental data during the early stages of the cure. Future research will... 

Our interest in this chapter is with the mass and energy balances for chemical reactors, and in electrochemical cells. We consider first the mass and energy balances for tank and tubular reactors, and then for a general black-box chemical reactor, since these balance equations are an important application of the thermodynamic equations for reacting mixtures and the starting point for practical reactor design and analysis. Finally, we consider equilibrium and the energy balance for electrochemical systems such as batteries and fuel-cells, and the use of electrochemical cells for thermodynamic measurements. 

The mass and energy balance equations developed in Secs. 14.1 and 14.2 are the basic equations used in reactor design and analysis. In many cases, however, our needs are much more modest than in engineering design. In particular, we may not be interested in such details as the type of reactor used and the concentration and temperature profiles or time history in the reactor, but merely in the species mass and total energy balances for the reactor. In such situations one can use the general black-box equations of Table 8.4-1 ... 

Bischolf, Chemical Reactor Design and Analysis, John Wiley, New York, 1979) and in a review by Ray [W.H. Ray, J. Macromolec. ScL, Rev. Macromolec. 

Worldwide, not as many nuclear engineering professionals are involved in criticality safety analysis as are involved in reactor analysis. More importantly, perhaps, these limited number of professionals tend to be more widely dispersed into smaller groups than are usually found in reactor design and analysis... 

Apply different mixing models in chemical reactor design and analysis. 

In order to be useful in reactor design and analysis, the reaction rate must be an intensive variable, i.e., one that does not depend on the size of the system. Also, it is very convenient to define the reaction rate so that it refers explicitly to one of the chemical species that participates in the reaction. The reference species usually is shown as part of the symbol for the reaction rate, and the reference species should be specified in the units of the reaction rate. 

This chapter presents an overview of chemical kinetics and introduces some of the molecular phenomena that provide a foundation for the field. The relationship between kinetics and chemical thermodynamics is also treated. The information in this chapter is sufficient to allow us to solve some problems in reactor design and analysis, which is the subject of Chapters 3 and 4. In Chapter 5, we will return to the subject of chemical kinetics and treat it more fundamentally and in greater depth. 

Based on more than a century of experimental and theoretical study of the kinetics of many different chemical reactions, some rules of thumb concerning the form of the rate equation have evolved. There are important exceptions to each of these ndes-of-thumb. Nevertheless, the following five generalizations permit chemical engineers to attack many practical problems in reactor design and analysis. 

Despite its lack of a strong theoretical justification, Eqn. (2-1) is very useful in a practical sense. It frequently provides a reasonable starting point for the analysis of experimental kinetic data as well, as for reactor design and analysis. 

The key word in this sentence is accurate. It is possible to predict rates reasonably well for very simple gas-phase reactions via quantum-mechanically based molecular simulations and to make order-of-magnitude predictions for more complex reactions. However, at diis point in time, rate equations that are accurate enough for reactor design and analysis must be developed from experimental data. 

The design equation must be integrated in order to solve problems in reactor design and analysis. In order to actually perform the integration, the temperature must be known as a function of either time or composition. This is because the rate equation r,- contains one or more constants that depend on temperature. 

When the value of K Ca is small compared to 1, the reaction is nearly first order in A. On the other hand, when the value of KaCa is large compared to 1, the reaction is close to zero order in A. At concentrations between these extremes, the reaction might appear to have a fractional order, perhaps 0.5 or so. However, the use of fractional-order rate equations can lead to difficulty in reactor design and analysis. Therefore, let s test Eqn. (6-6) to determine whether it provides an adequate fit of the data in Table 6-3. 

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