FIXED-BED CATALYTIC REACTORS FOR FLUID-SOLID REACTIONS

Fixed-Bed Catalytic Reactors for Fluid-Solid Reactions 

This chapter is devoted to fixed-bed catalytic reactors (FBCR), and is the first of four chapters on reactors for multiphase reactions. The importance of catalytic reactors in general stems from the fact that, in the chemical industry, catalysis is the rule rather than the exception. Subsequent chapters deal with reactors for noncatalytic fluid-solid reactions, fluidized- and other moving-particle reactors (both catalytic and noncatalytic), and reactors for fluid-fluid reactions. 

In this chapter, we first cite examples of catalyzed two-phase reactions. We then consider types of reactors from the point of view of modes of operation and general design considerations. Following introduction of general aspects of reactor models, we focus on the simplest of these for pseudohomogeneous and heterogeneous reactor models, and conclude with a brief discussion of one-dimensional and two-dimensional models. 

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Chapter 21 Fixed-Bed Catalytic Reactors for Fluid-Solid Reactions... 

In a fixed-bed catalytic reactor for a fluid-solid reaction, the solid catalyst is present as a bed of relatively small individual particles, randomly oriented and fixed in position. The fluid moves by convective flow through the spaces between the particles. There may also be diffusive flow or transport within the particles, as described in Chapter 8. The relevant kinetics of such reactions are treated in Section 8.5. The fluid may be either a gas or liquid, but we concentrate primarily on catalyzed gas-phase reactions, more common in this situation. We also focus on steady-state operation, thus ignoring any implications of catalyst deactivation with time (Section 8.6). The importance of fixed-bed catalytic reactors can be appreciated from their use in the manufacture of such large-tonnage products as sulfuric acid, ammonia, and methanol (see Figures 1.4,11.5, and 11.6, respectively). 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

A semicontinuous reactor for a fluid-solid reaction involves the axial flow of fluid downward through a fixed bed of solid particles, the same arrangement as for a fixed-bed catalytic reactor (see Figure 15.1(b)). The process is thus continuous with respect to the fluid and batch with respect to the solid (Section 12.4). 

In this chapter rate equations are set up for fluid-solid reactions. In Chapter 11 an example of such a reaction carried out in a fixed bed of particles is worked out and illustrates the diflererice in behavior of the reactor as compared with a fixed bed catalytic reactor. 

Gas-solid reactors are the best-known two-phase catalytic reactor types operated continuously. The major reactors for solid-catalyzed gas phase reactions are the fixed-bed, the fluid-bed and the entrained-flow reactor. 

The high turbulence created in the fluid-solid mixture leads to much higher heat transfer coefficients than those which can be obtained in fixed beds. The resulting uniformity of the fluidized bed makes it applicable for effecting catalytic reactions, especially highly exothermic and temperature sensitive reactions. However, the fluidization technology is much more complicated than that associated with fixed bed reactors. 

One of the most common catalytic reactors is the fixed-bed type, in which the reaction mixture flows continuously through a tube filled with a stationary bed of catalyst pellets. Because of its importance, and because considerable information is available on its performance, most attention will be given to this reactor type. Fluidized-bed and slurry reactors are also considered later in the chapter. Some of the design methods given are applicable also to fluid-solid noncatalytic reactions. The global rate and integrated conversion-time relationships for noncatalytic gas-solid reactions will be considered in Chap. 14. 

Esterification of organic compounds often involves multiphase catalytic reactions in which contact of liquid (organic substrate) and solid (catalyst) phases are involved. The most common esterification processes fall into the category of two phase (liquid-solid) reactions. Both slurry and fixed bed reactors can be used for ion exchange resin catalyzed esterification reactions. The overall performance of these reactors depends on the inter phase mass transfer, intrinsic kinetics of reaction, physicochemical properties and mixing of the fluid phases. For a continuous process, fixed bed reactors should be preferred, however, in fixed bed reactors small catalyst particles cause higher pressure drop. Special type of support trays may also be required to support small catalyst particles in fixed bed reactors. 

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