Catalytic reactions and reactors

Doraiswamy, L.K. Catalytic reactions and reactors - a surface science approach. Prog. Surf. Sci. 1991, 37, 1-277. 

E. E. Petersen, Chemical Reaction Analysis, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1965. A more advanced text emphasizing fluid-solid catalytic reactions and reactor design. 

Marin, G. B., Kapteijn, F., van Diepen, A. E., and Moulijn, J. A., Catalytic reaction and reactor engineering, in. Combinatorial Catalysis and High Throughput Catalyst Design and Testing (E. G. Derouane Ed.), pp. 239-281. Kluwer Academic Publishers (2000). 

Process. As soHd acid catalysts have replaced Hquid acid catalysts, they have typically been placed in conventional fixed-bed reactors. An extension of fixed-bed reactor technology is the concept of catalytic distillation being offered by CR L (48). In catalytic distillation, the catalytic reaction and separation of products occur in the same vessel. The concept has been appHed commercially for the production of MTBE and is also being offered for the production of ethylbenzene and cumene. 

Heterogeneous catalytic systems offer the advantage that separation of the products from the catalyst is usually not a problem. The reacting fluid passes through a catalyst-filled reactor m the steady state, and the reaction products can be separated by standard methods. A recent innovation called catalytic distillation combines both the catalytic reaction and the separation process in the same vessel. This combination decreases the number of unit operations involved in a chemical process and has been used to make gasoline additives such as MTBE (methyl tertiai-y butyl ether). 

After reactivity and selectivity, the next complication we encounter with all catalytic reactions is that there are essential transport steps of reactants and products to and from the catalyst. Therefore, in practice catalytic reaction rates can be thoroughly disguised by mass transfer rates. In fact, in many industrial reactors the kinetics of individual reactions are quite unknown, and some engineers would regard knowledge of their rates as unimportant compared to the need to prepare active, selective, and stable catalysts. The role of mass transfer in reactions is therefore essential in describing most reaction and reactor systems, and this will be a dominant subject in this chapter. 

In parallel with this development, we discuss the chemical and petroleum industries and the major processes by which most of the classical products and feedstocks are made. We begin in Chapter 2 with a section on The Real World, in which we describe the reactors and reactions in a petroleum refinery and then the reactions and reactors in making polyester. These are all catalytic multiphase reactors of almost unbelievable size and complexity. By Chapter 12 the principles of operation of these reactors will have been developed. 

We regard the essential aspects of chemical reaction engineering to include multiple reactions, energy management, and catalytic processes so we regard the first seven chapters as the core material in a course. Then the final five chapters consider topics such as environmental, polymer, sohds, biological, and combustion reactions and reactors, subjects that may be considered optional in an introductory course. We recommend that an instmctor attempt to complete the first seven chapters within perhaps 3/4 of a term to allow time to select from these topics and chapters. The final chapter on multiphase reactors is of course very important, but our intent is only to introduce some of the ideas that are important in its design. 

The general principle of two-phase catalysis in polar solvents, for example, in water, is shown in the simplified diagram of Fig. 1. The metal complex catalyst, which can be solubilized by hydrophilic ligands, converts the reactants A + B into the product C. The product is more soluble in the second than in the first phase and can be separated from the catalyst medium by simple phase separation. Excellent mixing and contacting of the two phases are necessary for efficient catalytic reaction, and thus the reactor is normally well stirred. 

CFBs as reactors, the solids residence time is an important parameter. Previous studies mostly assess operations at moderate values of the solids circulation rates (<100kg/m2s), whereas gas-catalytic reactions and, e.g., biomass pyrolysis require completely different operating conditions. 

The field of chemical kinetics and reaction engineering has grown over the years. New experimental techniques have been developed to follow the progress of chemical reactions and these have aided study of the fundamentals and mechanisms of chemical reactions. The availability of personal computers has enhanced the simulation of complex chemical reactions and reactor stability analysis. These activities have resulted in improved designs of industrial reactors. An increased number of industrial patents now relate to new catalysts and catalytic processes, synthetic polymers, and novel reactor designs. Lin [1] has given a comprehensive review of chemical reactions involving kinetics and mechanisms. 

The various volumetric mass-transfer coefficients are defined in a manner similar to that discussed for gas-liquid and fluid-solid mass transfer in previous sections. There are a large number of correlations obtained from different gas-liquid-solid systems. For more details see Shah (Gas-Liquid-Solid Reactor Design, McGraw-Hill, 1979), Ramachandran and Chaudhari (Three-Phase Catalytic Reactors, Gordon and Breach, 1983), and Shah and Sharma [Gas-Liquid-Solid Reactors in Carberry and Varma (eds.), Chemical Reaction and Reactor Engineering, Marcel Dekker, 1987],... 

Combining the kinetics of the catalytic reaction and its interplay with transport phenomena as well as with the hydrodynamics of the chemical reactor to be used as the basis of shaping and operating the final catalyst. 

Knowledge of the rate of a catalytic reaction and its selectivity is essential for its application in practice. More precisely, the kinetics of the reaction is required in the form of a rate expression, i.e. the function that tells how the reaction rate varies with temperature, pressure and composition of the reacting system. The kinetics determine the size of the catalytic reactor for a given overall production rate and without it, design would be highly speculative. 

Several reactors are presently used for studying gas-solid reactions. These reactors should, in principle, be useful for studying gas-liquid-solid catalytic reactions. The reactors are the ball-mill reactor (Fig. 5-10), a fluidized-bed reactor with an agitator (Fig. 5-11), a stirred reactor with catalyst impregnated on the reactor walls or placed in an annular basket (Fig. 5-12), a reactor with catalyst placed in a stationary cylindrical basket (Fig. 5-13), an internal recirculation reactor (Fig. 5-14), microreactors (Fig. 5-16), a single-pellet pulse reactor (Fig. 5-17), and a chromatographic-column pulse reactor (Fig. 5-18). The key features of these reactors are listed in Tables 5-3 through 5-9. The pertinent references for these reactors are listed at the end of the chapter. 

The influence of activity changes on the dynamic behavior of nonisothermal pseudohomogeneoiis CSTR and axial dispersion tubular reactor (ADTR) with first order catalytic reaction and reversible deactivation due to adsorption and desorption of a poison or inert compound is considered. The mathematical models of these systems are described by systems of differential equations with a small time parameter. Thereforej the singular perturbation methods is used to study several features of their behavior. Its limitations are discussed and other, more general methods are developed. 

The balance equations of pseudohomogeneous reactors with catalytic reaction and activity changes were developed under the simplifying assvuoptions [3] The nononolecular catalytic reaction... 

If we adopt this global view, another question arises. Catalysts, as they work in catalytic reactors, are just the intermediary product of a solid state reaction chain which starts with preparation and activation, before the catalysts is contacted with the feed, and is followed by deactivation and, ultimately, "death". The question is what are the relationships among solid-state reactions occurring during the initial activation, catalytic reaction and deactivation ... 

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