Reactors for Heterogeneous Systems

Catalyst selection involves two features productivity and selectivity. The process rate is a subtle combination of four limiting steps adsorption/desorption of reac-tants/product, surface reaction between species, diffusion through pores and diffusion through external film. Pore structure, surface area, nature and distribution of active sites play a crucial role in forming the process rate at the level of catalyst 

Contact of reactants involves design decisions regarding the following aspects  

The reversible reactions deserve particular attention. The in-situ removal of a product by reactive distillation, reactive extraction or by using selective membrane diffusion should be investigated. 

Hydraulic design aims at the realization of an intensive heat and mass transfer. For two-phase gas-liquid or gas-solid systems, the choice is between different regimes, such as dispersed bubbly flow, slug flow, churn-turbulent flow, dense-phase transport, dilute-phase transport, etc. 

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There are many ways that two phases can be contacted, and for each the design equation will be unique. Design equations for these ideal flow patterns may be developed without too much difficulty. However, when real flow deviates considerably from these, we can do one of two things we may develop models to mirror actual flow closely, or we may calculate performance with ideal patterns which bracket actual flow. Fortunately, most real reactors for heterogeneous systems can be satisfactorily approximated by one of the five ideal flow patterns of Fig. 17.1. Notable exceptions are the reactions which take place in fluidized beds. There special models must be developed. 

Mass and heat balance equations for typical gas-liquid reactors in heterogeneous systems at steady state... 

Markowz G, Schirrmeister S, Albrecht J, Becker F, Schiltte R, Caspary KJ, Klemm E (2005) Microstructured reactors for heterogeneously catalysed gas-phase reactions on an industrial scale. Chem Eng Technol 28 459 -64 Muller A, Cominos V, Hessel V, Horn B, Schiirer J, Ziogas A, Jahnisch K, Hillmann V, GroBer V, Jamc KA, Bazzanella A, Rinke G, Kraut M (2005) Fluidic bus system for chemical process engineering in the laboratory and for small-scale production. Chem Eng J 107 205-214 Pennemann H, Watts P, Haswell S, Hessel V, Lowe H (2004) Org Proc Res Dev 8 422... 

In many of these operations the engineer is concerned primarily with prediction of pressure losses. However, the heat transfer rate through the tube wall into the gas or the liquid phase is also of major concern in heat-exchange equipment. In the design of chemical reactors for heterogeneous gas-liquid systems, it is necessary to be able to predict not only pressure drops and rates of heat transfer into or out of the channel, but also the rates of mass transfer from the gas into the liquid phase. 

Power-law kinetic rate expressions can frequently be used to quantify homogeneous reactions. However, many reactions occur among species in different phases (gas, liquid, and solid). Reaction rate equations in such heterogeneous systems often become more complicated to account for the movement of material from one phase to another. An additional complication arises from the different ways in which the phases can be contacted with each other. Many important industrial reactors involve heterogeneous systems. One of the more common heterogeneous systems involves gas-phase reactions promoted with porous solid catalyst particles. 

The CSTR is particularly useful for reaction schemes that require low concentration, such as selectivity between multiple reactions or substrate inhibition in a chemostat (see Section IV). The reactor also has applications for heterogeneous systems where high mixing gives high contact time between phases. Liquid-liquid CSTRs are used for the saponification of fats and for suspension and emulsion polymerizations. Gas-liquid mixers are used for the oxidation of cyclohexane. Gas homogeneous CSTRs are extremely rare. 

To analyze reaction mechanisms in complex catalytic systems, the application of micropulse techniques in small catalytic packed beds has been used. Christoffel [33] has given an introduction to these techniques in a comprehensive review of laboratory reactors for heterogeneous catalytic processes. Mtlller and Hofmann [59,61] have tested the dynamic method in the packed bed reactor to investigate complex heterogeneous reactions. Kinetic parameters have been evaluated by a method, which employs concentration step changes and the time derivatives of concentration transients at the reactor outlet as caused by a concentration step change at the reactor inlet. 

Equation (2) (as an ordinary differential equation) and Eq. (3) apply now with Eq. (4). As already implied, a laboratory well-mixed reactor for heterogeneous catalysis is more difficult to realize than a PFR. Many versions have been used 12), and Froment and Bischoff 13) illustrate reactors with external recycle, with internal recycle 1,14), and with an internal spinning basket 15). When using these reactors for experiments in the transient regime, it is important to keep to a minimum the volume outside the bed of catalyst. Internal recycle reactors involve bearings exposed to hot reactive gases and require a magnetic drive system for leak-proof operation. Exter-... 

FIGURE 2.4. Schematic representation of the optical-fiber bundled array photocatalytic reactor system (Reprinted with permission from Environ. Sci. Tech., 32(3), N.J. Peill and M.R. Hoffmann, Mathematical model of a phtoocatlaytic piber-optic cable reactor for heterogeneous photocatalysis, 398-404. Copyright 1998 American Chemical Society ). 

Polymerization reactors are a specific kind of chemical reactors in which polymerization reactions take place therefore, in principle, they can be analyzed following the same general rules applicable to any other chemical reactor. The basic components of a mathematical model for a chemical reactor are a reactor model and rate expressions for the chemical species that participate in the reactions. If the system is homogeneous (only one phase), these two basic components are pretty much what is needed on the other hand, for heterogeneous systems formed by several phases (emulsion or suspension polymerizations, systems with gaseous monomers, slurry reactors or fluidized bed reactors with solid catalysts, etc.), additional transport and/or thermodynamic models may be necessary to build a realistic mathematical representation of the system. In this section, to illustrate the basic principles and components needed, we restrict ourselves to the simplest case, that of homogeneous reactors in other sections, additional components and more complex cases are discussed. 

The term space velocity has somewhat different connotations when dealing with heterogeneous catalytic reactors. For such systems, the space velocity is the ratio of the mass flow rate of feed to the mass of catalyst used (W) ... 

Heterogeneous reactions involving more than one phase are a common feature of organic synthesis. Among them are gas-liquid reactions, liquid-liquid reactions, solid-liquid reactions, and gas-liquid-solid reactions (e.g., the slurry reaction). The main features of various two-phase reactions are covered in Chapters 14 and 15, and Chapter 16 discusses reactors for such systems. Chapter 17 covers the analysis of three-phase reactions and reactors. 

For heterogeneous systems and nonlinear homogeneous systems more detailed models of the reacting environment during the fluid s passage through the reactor are necessary. 

The differential reactor is used to evaluate the reaction rate as a function of concentration for a heterogeneous system. It consists of a tube that contains a small amount of catalyst as shown schematically in Figure 4-17. The conversion of the reactants in the bed is extremely small due to the small amount of catalyst used, as is the change in reactant concentration through the bed. The result is that the reactant concentration through the reactor is constant and nearly equal to the... 

BRADLEY A. SAVILLE is an Associate Professor of Chemical Engineering at i the University of Toronto. He received his B.Sc. and Ph.D. in chemical engi-neering at the University of Alberta. He is the author or co-author of over 25 research articles on enzyme kinetics, pharmacokinetics, heterogeneous reactions in biological systems, and reactors for immobilized enzymes. He is a member, of the Chemical Institute of Canada, the Canadian Society of Chemical Engineering, and Professional Engineers Ontario. 

Chapter 1 reviews the concepts necessary for treating the problems associated with the design of industrial reactions. These include the essentials of kinetics, thermodynamics, and basic mass, heat and momentum transfer. Ideal reactor types are treated in Chapter 2 and the most important of these are the batch reactor, the tubular reactor and the continuous stirred tank. Reactor stability is considered. Chapter 3 describes the effect of complex homogeneous kinetics on reactor performance. The special case of gas—solid reactions is discussed in Chapter 4 and Chapter 5 deals with other heterogeneous systems namely those involving gas—liquid, liquid—solid and liquid—liquid interfaces. Finally, Chapter 6 considers how real reactors may differ from the ideal reactors considered in earlier chapters. 

So far we have concentrated on homogeneous reactions in ideal reactors. The reason is two-fold because this is the simplest of systems to analyze and is the easiest to understand and master also because the rules for good reactor behavior for homogeneous systems can often be applied directly to heterogeneous systems. 

For practical purposes it is often beneficial to use a heterogeneous system with the enzyme as a solid preparation which easily can be separated from the product in the liquid phase. Solid enzyme preparatiorrs can conveniently be used in packed bed and stirred tank reactors. As in other cases with heterogeneous catalysis, mass trarrsfer limitations can reduce the overall reaction rate, but usually this is no major problem. 

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