Types of Multiphase Reactors

The book starts with a review of kinetics and the batch reactor in Chapter 2, and the material becomes progressively more complex until Chapter 12, which describes all the types of multiphase reactors we can think of This is the standard, linear, boring progression followed in essentially all textbooks. 

Classification by Phase Despite the generic classification by operating mode, reactors are designed to accommodate the reactant phases and provide optimal conditions for reaction. Reactants may be fluid(s) or solid(s), and as such, several reactor types have been developed. Singlephase reactors are typically gas- (or plasma- ) or liquid-phase reactors. Two-phase reactors may be gas-liquid, liquid-liquid, gas-solid, or liquid-solid reactors. Multiphase reactors typically have more than two phases present. The most common type of multiphase reactor is a gas-liquid-solid reactor however, liquid-liquid-solid reactors are also used. The classification by phases will be used to develop the contents of this section. 

Monolith reactors have recently found applications in performing catalytic three-phase reactions (see Chapter 9). There is also growing interest in the chemical industries for this novel type of multiphase reactor. A proper modeling of the monolith reactor is a necessary step in order to estimate the overall performance of the reactor. 

Discussions on flow modeling so far have been more or less restricted to singlephase reactors. However, in a broad range of application areas, multiple phases are involved in chemical reactions (see examples cited by Ramachandran and Choudhari, 1983 Doraiswamy and Sharma, 1984 Kunii and Levenspiel, 1991 Shah, 1991 Dudukovic et al, 1999). Reactors carrying out such reactions are generically termed multiphase reactors. There are several types of multiphase reactors and several methods are available to classify these reactors. One of the simplest methods of... 

Because of energy, sustainability, and security issues, multiphase reactors will have an ever-increasing role in all the countries. Considerable effort is needed to better predict the very complex phenomena occurring in the many types of multiphase reactors. 

A major difference between the two types of multiphase reactors is that the amount of catalyst in the slurry reactor is only 0.01-1% of the total volume, whereas it is 50-60% of the volume of the packed bed. In this chapter the slurry reactor is considered first, because it is the more common type and because having the catalyst concentration as a variable makes it easier to evaluate the kinetic models. 

TodtjJ., Lucke.J., Schligerl.K. and A.Renken. "Gas hold-up and longitudinal dispersion on different types of multiphase reactors and their possible applications for microbial processes". Chem.Engng.Sci. 32 (1977) 369. 

In summary, the progress of the modem chemical industry relies heavily on catalysis for high selectivity/green chemistry and sustainable technology. However, the effectiveness of a catalyst system is pivotally dependent on the availability of appropriate multiphase contacting devices, which are commonly referred to as multiphase reactors. Chapter 3 gives a detailed discussion on the types of multiphase reactors available and the rationale for their selection for a specific reaction system. 

In direct contrast to intrinsic kinetics, the transport processes (mass/heat transfer coefficient) depend on the type of multiphase reactor, its size, and operating parameters. Thus, one can have an order or two of magnitude changes in the gas-Uquid mass transfer coefficient, k a, when shifting over from packed columns to stirred... 

As mentioned in Section 2.2, the main issue relating to scale-up of a multiphase reactor is the values of the transport coefficients, and Once these values are determined, comparison of the parameters in the parentheses of the right-hand side of Equation 2.19 allows the rate-controlling step for a given set of operating conditions in a given type of multiphase reactor. The worked examples in Chapters 7A, 7B, 8, and 9 illustrate this procedure. 

Multiphase reactions may involve gas-liquid, gas-liquid-solid (solid as catalyst or reactant), liquid-liquid, liquid-liquid-solid reactions, etc. The reactions may vary from very slow to very fast, endothermic to highly exothermic. Based on the reaction characteristics, different types of multiphase reactors are used in industrial practice. A number of texts deaUng with design of multiphase reactors are available (Satterfield 1970 Shah 1979 Ramachandran and Chaudhari 1983 Westerterp et al. 1988 Deckwer 1992). Considerable information on theoretical, hydrodynamics, and mass transfer aspects of different multiphase reactors has become available since the publication of the above texts. This recent information is likely to allow rational, simple and yet reliable designs of many industrially important multiphase reactors. In this book, different types of multiphase reactors falUng under two categories—(1) gas-liquid and (2) gas-liquid-solid— are considered. The basic aim is to provide user-friendly, simple, and reasonably accurate design procedure for each multiphase reactor. 

Three-phase catalytic reactions are the focus of discussion in the present chapter. Further, in most applications, the catalysts used are expensive. Therefore, effective utilization and snstenance of the catalyst activity are important aspects in the economics. This initial discnssion is based on the value of the Hatta number defined by Equation 2.4. The reactor ntilization is related to its ability to achieve the intrinsic enhancement of the reaction rate by the catalyst nsed. As discussed in Section 2.2, the intrinsic kinetics of the catalyzed reaction are nnaffected by the type of multiphase reactor. Therefore, the reactor utilization needs to be reinterpreted keeping in view the large changes in both and a that are possible with a venturi loop reactor. 

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