Reactor design, practical consideration

The choice of reactor will be very dependent on the requirements of the chemical reaction scheme, the relative importance of mixing and heat transfer, and practical considerations (e.g., the effect of solids in the process materials of construction flexibility). A comparison of the typical performance of different designs is given in Table 5. HEX Reactors are discussed in more depth in Chapter 4. 

In industrial practice, the laboratory equipment used in chemical synthesis can influence reaction selection. As issues relating to kinetics, mass transfer, heat transfer, and thermodynamics are addressed, reactor design evolves to commercially viable equipment. Often, more than one type of reactor may be suitable for a given reaction. For example, in the partial oxidation of butane to maleic anhydride over a vanadium pyrophosphate catalyst, heat-transfer considerations dictate reactor selection and choices may include fluidized beds or multitubular reactors. Both types of reactors have been commercialized. Often, experience with a particular type of reactor within the organization can play an important part in selection. 

Figure 1 shows a computational framework, representing many years of Braun s research and development efforts in pyrolysis technology. Input to the system is a data base including pilot, commercial and literature sources. The data form the basis of a pyrolysis reactor model consistent with both theoretical and practical considerations. Modern computational techniques are used in the identification of model parameters. The model is then incorporated into a computer system capable of handling a wide range of industrial problems. Some of the applications are reactor design, economic and flexibility studies and process optimization and control. 

In practical reactor design, however, the engineer has been faced with critical problems occurring at conversions and temperatures considerably beyond the range where published data and theory can be directly applied. He has been therefore obliged to rely largely on empirical and semitheoretical methods. 

Industrial practice often confronts the development engineer with networks that are considerably more complicated than that of cyclohexene hydroformylation in the example above. Additional simplifications may then be desirable or necessary in order to arrive at a model that remains manageable in the highly iterative applications called for in reactor design and optimization and possibly on-line process control. A useful and usually successful way of achieving such streamlining is to place all network nodes at end members or non-trace intermediates, ignoring the fact that some of them may be at trace-level intermediates [10]. 

The data form the basis of pyrolysis models consistent with both theoretical and practical considerations. The resulting models are integrated into a complete reactor simulation, which is then applied in design and optimization work. The simulation includes detailed models describing process-side heat and momentum transfer, thermophysical properties, and fired-side radiative heat transfer. 

It is important to match mixing equipment capabilities with process requirements. While it is desirable to have an optimum design and operating conditions for every step in the process sequence, it is seldom practical to do so. For example, specialty and pharmaceutical processes require the use of multipurpose reactors. An important consideration is to understand how less-than-ideal equipment wiU function in aU stages of operation. 

The case studies presented in this chapter illustrate reactor design procedures for a carefully selected set of reacting systems wherein the physical dimensions of the reactor (diameter, height) and fixed and operating parameters (catalyst loading, superficial velocity, impeller speed, and other) were calculated. As a postscript to these studies, we would like to consolidate and emphasize certain fundamental and practical considerations in reactor selection and design. 

The manner in which the rates of reactant consumption and of formation of each individual product varies with reactant concentrations and temperature affords information that is useful in two ways first, as providing the basis for the reactor design if the reaction is to be operated on a significant scale, and second, and more to the immediate point, to give a framework within which a reaction mechanism can be formulated. Whatever the practical difficulties of obtaining this information, and they can be considerable with microporous catalysts and those undergoing rapid deactivation, it is essential for mechanistic analysis. It cannot be stressed to strongly that no formulation of a reaction mechanism can be accepted as plausible until shown to be consistent with experimentally determined kinetics. The corollary is however equally important the mechanism cannot be deduced from kinetic measurements alone, because many different mechanisms can lead to the same kinetic expression. 

The understanding of chemical equilibrium and optimum problems is extremely important from an academic and practical point of view, particularly in reactor design and control. However, the relationship between these two problems is not well understood. Historically, equilibrium-optimum considerations have been proclaimed in the famous Le Chatelier s principle. In chemistry, this principle is used to influence reversible chemical reactions. For example, the equilibrium conversion of an exothermic reaction, that is, a reaction liberating heat, is more favorable at lower temperature, so cooling of the reaction mixture shifts the equilibrium to the product side. Le Chatelier s principle is part of the curriculum of university students in chemistry and chemical engineering. Unfortunately, the relation between this principle and the analysis of equilibria and optima often is not presented clearly. In particular, there is no explicit explanation of how to apply Le Chatelier s principle, which has been formulated for closed systems at equilibrium (so at zero value of the net overall reaction rate), to continuous-flow reactors, in which the reaction rate certainly is not zero. This section is based on an article by Yablonsky and Ray (2008), which aims at bridging this gap between the concepts of equHihrium and optimum. 

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