REACTORS FOR HOMOGENEOUS REACTIONS

On an industrial scale, BRs are primarily intended for homogeneous liquid-phase reactions and less frequently for gas-phase reactions. On a laboratory scale, however, BRs with a constant volume are often used for the determination of the kinetics of homogeneous gas-phase reactions. BRs are typically used industrially for the production of fine chemicals via organic liquid-phase reactions, such as drug synthesis, and the manufacture of paints, pesticides, and herbicides. 

A BR has several advantages in the realization of industrial reactions. It is flexible, allowing the same reactor to be used for multiple, chemically different reactions. This is a clear advantage for the manufacture of an assortment of fine chemicals the production can be adjusted and rearranged according to market demand. Different reactions often require very different reaction times (batch time), since reaction velocities vary considerably. For a BR, this is, however, a minor problem the reaction time can easily be altered, as required, by allowing a longer or a shorter reaction time until the desired product distribution is obtained. 

For certain types of reactions, it is desirable to change the operation conditions during the course of the reaction. For instance, in the case of reversible exothermic reactions, it is favorable to have a higher temperature at the beginning of the reaction to enhance the reaction kinetics, whereas toward the end of the reaction the temperature should be reduced. This procedure (decreasing temperature ramp) is favorable, since the equilibrium composition is more favorable at lower temperatures. The desired conditions in a BR can be established by a computer-controlled temperature trajectory the optimal temperature-time scheme can be theoretically determined in advance and realized by cooling control. 

The scaleup of kinetic data obtained in the laboratory to a BR operating on an industrial scale is fairly simple, as the reaction time in the laboratory corresponds directly to the reaction time on the factory scale, provided that the reactors in general operate under similar conditions. This advantage has diminished in importance, however, since the tremendous increase in the theoretical knowledge of chemical reaction engineering. 

Source Data from Trambouze, R, van Langenhem, H., and Wauquier, J.R, Chemical Reactors—Design/Engineering Operation, Editions Technip, Paris, 1988. DE = double jacket, SE = welded external coil. 

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However, before extrapolating the arguments from the gross patterns through the reactor for homogeneous reactions to solid-catalyzed reactions, it must be recognized that in catalytic reactions the fluid in the interior of catalyst pellets may diSer from the main body of fluid. The local inhomogeneities caused by lowered reactant concentration within the catalyst pellets result in a product distribution different from that which would otherwise be observed. 

Runaway criteria developed for plug-flow tubular reactors, which are mathematically isomorphic with batch reactors with a constant coolant temperature, are also included in the tables. They can be considered conservative criteria for batch reactors, which can be operated safer due to manipulation of the coolant temperature. Balakotaiah et al. (1995) showed that in practice safe and runaway regions overlap for the three types of reactors for homogeneous reactions (1) batch reactor (BR), and, equivalently, plug-flow reactor (PFR), (2) CSTR, and (3) continuously operated bubble column reactor (BCR). 

The above equations are the same as those of the plug-flow tubular reactor for homogeneous reactions (Smith, 1981 Levenspiel, 1972). The differential form of equation (3.122) can be written in several equivalent forms (Levenspiel, 1972) ... 

The present chapter applies the fundamental principles of CRE to specialized examples of practical interest. Many standard texts on CRE start with a detailed presentation of reactors for homogeneous reactions and then extend the treatment to multiphase reactors. In the present chapter, homogeneous reactors constitute only a small part of the treatment and are covered under one of several case studies included. The bulk of the treatment is devoted to multiphase reactions and reactor characteristics. A broad outline of the scope of the chapter is presented in Eigure 11.1. 

The simple batch reactor for homogeneous reactions is the most common type. From this kind of reactor, many kinetic data have been obtained. AU the reactants are charged in at the beginning of the reaction, with no mass transfer occurring until the reaction is complete. Examples of batch reactions are the ammonolysis of nitrochlorobenzenes, hydrolysis of esters, and polymerization of butadiene and styrene in aqueous suspension. These will be treated in more detail in later chapters. 

It is a good idea to run the laboratory reactor without catalyst to check for homogeneous reactions. However, this method does not work when the homogeneous reaction involves reactants that do not occur in the feed but are created by a heterogeneous reaction. It then becomes important to maintain the same ratio of free volume to catalyst volume in the laboratory reactor used for intrinsic kinetic studies as in the pilot or production reactors. 

In practice, it is often possible with stirred-tank reactors to come close to the idealized mixed-flow model, providing the fluid phase is not too viscous. For homogenous reactions, such reactors should be avoided for some types of parallel reaction systems (see Figure 5.6) and for all systems in which byproduct formation is via series reactions. 

Many chemical reactions are performed on a batch basis, in which a reactor is filled with solvents, substrates, catalysts and anything else required to make the reaction proceed, the reaction is then performed and finally the reactor is emptied and the resultant mixture separated (Figure 11.2). Conceptually, a batch reactor is similar to a scaled up version of a reaction in a round-bottomed flask, although obviously the engineering required to realize a large scale reaction is much more complicated. Batch reactors are suitable for homogeneous reactions, and also for multiphasic reactions provided that efficient mixing between the phases may be achieved so that the reaction occurs at a useful rate. 

Most real reactors are not homogeneous but use catalysts (1) to make reaction occur at temperatures lower than would be required for homogeneous reaction and (2) to attain a higher selectivity to a particular product than would be attained homogeneously. One may then ask whether any of the previous material on homogeneous reactions has any relevance to these situations. The answer fortunately is yes, because the same equations are used. However, catalytic reaction rate expressions have a quite different meaning than rate expressions for homogeneous reactions. 

Chemat, E, Poux, M., Di Martino, J.L. andBerlan, J., A new continuous-flow recycle microwave reactor for homogeneous and heterogeneous chemical reactions, Chem. Eng. Tech., 1996,19(5), 420. 

S. P. Chitra and R. Govind. Synthesis of optimal serial reactor structures for homogeneous reactions. AIChEJ., 31 185, 1985. 

For reactions where high-pressure requirements do not allow large diameter tanks for homogeneous reaction kinetics, a loop reactor can be used. The loop is a recycle reactor made of small diameter tubes. Feed can be supplied continuously at one location in the loop and product withdrawal at another. 

In this form, the two-mode model is identical to the classical steady-state two-phase model of a tubular catalytic reactor with negligible axial dispersion. There is also a striking structural similarity between the two-mode models for homogeneous reactions and two-phase models for catalytic reactions in the practical limit of Per 1. This could be seen more clearly when Eqs. (137) and (138) are rewritten as... 

Batch reactors are used primarily to determine rate law parameters for homogeneous reactions. This determination is usually achieved by measuring concentration as a function of time and then using either the differential, integral, or least squares method of data analysis to determine the reaction order, a, and specific reaction rate, k. If some reaction parameter other than concentration is monitored, such as pressure, the mole balance must be rewritten in terms of the measured variable (e.g., pressure). 

Chitra, S. P.. and Govind. R. Synthesis of Optimal Serial Reactor Structures for Homogeneous Reactions. Part I Isothermal Reactors." AlChE J. 31, 177 (1985a). 

One type of reactor which can be useful for kinetic measurements is the continuous stirred tank reactor (CSTR). The kinetic model is identical with that for the recirculation reactor and the designs are based on the reactors used for homogeneous reactions. Carberry et have described a CSTR which... 

We may first divide tubular reactors into those designed for homogeneous reactions, and therefore basically just an empty tube, and those designed for a heterogeneously catalyzed reaction, and hence to be packed with a catalyst. Both types can of course be operated adiabatically, and it was the simplest model of these that we discussed in the last chapter. If the temperature of the reactor is to be controlled this is through the wall, and the associated problems of heat transfer now arise. These include transfer at the wall and subsequent radial diffusion across the flowing reactants. In the empty tubular reactor there may be considerable variations in flow rate across the tube. For example, in the slow laminar flow the fluid... 

The kinetics and reactor designs we have considered thus far are for homogeneous reactions. The remainder of the book is devoted to heterogeneous reactions. Many systems are heterogeneous because a catalyst is necessary, and this substance is commonly (but not always) in a phase different from that of the reactants and products. Accordingly, our first objective will be a study (Chaps. 8 and 9) of heterogeneous catalysis and kinetics of heterogeneous catalytic reactions. 

Data for homogeneous reactions is most often obtained in a batch reactor. After postulating a rate law and combining it with a mole balance, we next use any or all of the methods in Step S to process the data and arrive at the reaction orders and specific reaction rate constants. 

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