Other Multiphase Reactors

Mass transfer steps are essential in any multiphase reactor because reactants must be transferred from one phase to another. When we consider other multiphase reactors in later chapters, we will see that mass transfer rates fiequently control these processes. In this chapter we consider a simpler example in the catalytic reactor. This is the first example of a multiphase reactor because the reactor contains both a fluid phase and a catalyst phase. However, this reactor is a very simple multiphase reactor because the catalyst does not enter or leave the reactor, and reaction occurs only by the fluid reacting at the catalyst surface. 

This situation describes an emulsion reactor in which reacting drops (such as oil drops in water or water drops in oil) flow through the CSTR with stirring to make the residence time of each drop obey the CSTR equation. A spray tower (liquid drops in vapor) or bubble column or sparger (vapor bubbles in a continuous liquid phase) are also segregated-flow situations, but these are not always mixed. We wiU consider these and other multiphase reactors in Chapter 12. 

The quantity couples the two equations. For a membrane reactor this is simply the area of the membrane, but for other multiphase reactors the interfacial area may vary with conditions. 

In the new edition, the material on Chemical Reactor Design has been re-arranged into four chapters. The first covers General Principles (as in the earlier editions) and the second deals with Flow Characteristics and Modelling in Reactors. Chapter 3 now includes material on Catalytic Reactions (from the former Chapter 2) together with non-catalytic gas-solids reactions, and Chapter 4 covers other multiphase reactor systems. Dr J. C. Lee has contributed the material in Chapters 1, 2 and 4 and that on non-catalytic reactions in Chapter 3, and Professor W. J. Thomas has covered catalytic reactions in that Chapter. 

In spite of this enticing come-on, we will not solve this problem for the moment, being content with its illustration of a typical two-phase reactor balance formulation using the PFR model. We hasten to add, however, that the solution to the set of equations (7-140) and (7-141) with the initial and boundary conditions given is identical to that for the much simpler set of (7-54) and (7-139). In the following sections we shall pursue in detail the developments using the by-now familiar dispersion model for tubular reactors, and in Chapter 8 will treat a number of other multiphase reactor models. 

As in the case of other multiphase reactors discussed in this chapter, topical material divides itself rather naturally into three major aspects hydrodynamics, transport, and reaction processes. We will stay with fairly simple approaches, particularly in the area of hydrodynamics and correlations. An extensive amount of research continues to this day on trickle beds, so we cannot attempt to present the latest word. 

As one might expect, these are conveniently subdivided into correlations for gas-liquid coefficients and for liquid-solid coefficients. The overall structure of these correlations is not much different from those we have seen for other multiphase reactors, but the correlation coefficients, of course, are very-specific to the application. 

Equations 12.S.b-l, 2 reduce to a large variety of special cases. As written, they are the standard equations for interfacial mass transfer used in Chapters 6 and 14 (also see Pavlica and Olson [60]. They are also the basis of the cross-flow models for fluidized beds (see Chapter 13) or other multiphase reactors, and have been used for heat transfer studies. 

This applies to other multiphase reactors, not just trickle-beds. When the reactions in trickle-beds involve nonvolatile liquid reactants which limit the reaction rate, then the interpretation of liquid-phase tracer data in a TBR can be approached based on Case 2 discussed in Section 6.1.1. Clearly, scale-up is possible under conditions discussed in that section. In case of volatile liquid reactants or rates governed by both gas and liquid reactants the trickle-bed must be considered as a system with two flowing phases discussed in Section 6.1.2 and scale-up is difficult. 

Fig. 1 shows various types of BCR. Compared to other multiphase reactors some of the main advantages of BCR are ... 

Multiphase Reactors. The overwhelming majority of industrial reactors are multiphase reactors. Some important reactor configurations are illustrated in Figures 3 and 4. The names presented are often employed, but are not the only ones used. The presence of more than one phase, whether or not it is flowing, confounds analyses of reactors and increases the multiplicity of reactor configurations. Gases, Hquids, and soHds each flow in characteristic fashions, either dispersed in other phases or separately. Flow patterns in these reactors are complex and phases rarely exhibit idealized plug-flow or weU-stirred flow behavior. 

New reactor technologies are currently under development, and these include meso- and micro-structured reactors or the use of membranes. Among meso-structured reactors, monolithic catalysts play a pre-eminent role in environmental applications, initially in the cleaning of automotive exhaust gases. Beside this gas-solid application, other meso-structures such as membranes [57, 58], corrugated plate or other arranged catalysts and, of course, monoliths can be used as multiphase reactors [59, 60]. These reactors also offer a real potential for process intensification, which has already been demonstrated in commercial applications such as the production of hydrogen peroxide. 

In most multiphase reactors the reactions occur only in one phase, with the other phase... 

This shows how catalytic reactions compare with other interfacial reactions. In a fixed bed reactor the catalyst (in phase ) has an infinite residence time, which can be ignored in the expressions we derived in previous chapters. For a moving bed reactor in which catalyst moves through the reactor, we have a true multiphase reactor because the residence time of the catalyst phase is not infinite. 

Conclusions. In tubular multiphase reactors with an exothermic reaction where one phase with a high throughput serves to carry the heat of reaction out of the reactor, a sudden flow reduction in this phase (whether accompanied by a similar reduction in the other phases or not) can lead to a considerable transient temperature rise, well above the new steady state temperature. The maximum excess temperature depends in a complex way upon the rate of the flow reduction, the flow rates in the different phases, the heat capacities and the reaction rates of the system. 

Apart from the flow regimes, several other issues control the performance of these multiphase reactors. For example, in a gas-liquid reactor, the rate of mass... 

Multiphase Reactors Reactions between gas-liquid, liquid-liquid, and gas-liquid-solid phases are often tested in CSTRs. Other laboratory types are suggested by the commercial units depicted in appropriate sketches in Sec. 19 and in Fig. 7-17 [Charpentier, Mass Transfer Rates in Gas-Liquid Absorbers and Reactors, in Drew et al. (eds.), Advances in Chemical Engineering, vol. 11, Academic Press, 1981]. Liquids can be reacted with gases of low solubilities in stirred vessels, with the liquid charged first and the gas fed continuously at the rate of reaction or dissolution. Some of these reactors are designed to have known interfacial areas. Most equipment for gas absorption without reaction is adaptable to absorption with reaction. The many types of equipment for liquid-liquid extraction also are adaptable to reactions of immiscible liquid phases. 

An application of microfluidic reactors is the development of a membraneless fuel cell. Two streams, one containing a fuel such as methanol, the other an oxygen-saturated acid or alkaline stream, are merged without mixing. The laminar flow pattern in the narrow channel helps to maintain separate streams without the use of membrane separators. Opposite walls function as the electrodes and are doped with catalyst. Ion exchange, protons for the add system, takes place through the liquid-liquid interface. This is an example of a solid-liquid-liquid-solid multiphase reactor. ... 

However, to the author s knowledge, this model has not been applied for the prediction of multiphase reactors mostly due to the complexity of the suggested closure relations. On the other hand, this paper serves as a useful reference as the exact derivation of the k — e model equations are given and discussed. Parts of this modeling work have been adopted in many papers. 

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