Fluidized Beds of Noncatalytic Solids

TABLE 17.10. Overall Heat Transfer Coefficients in Agitated Tanks IU Btu/(hr)(sqft)(°R] 

Fluid Inside Jacket Fluid In Vessel Wall Material Agitation u 

Steam water C.I. and loose lead lining agitated 4-9 

Cold water cold water enameled C.I. none 43 

TABLE 17.10. Overall Heat Transfer Coefficients in Agitated Tanks 

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Part III Beyond the Fundamentals presents material not commonly covered in textbooks, addressing aspects of reactors involving more than one phase. It discusses solid catalyzed fluid-phase reactions in fixed-bed and fluidized-bed reactors, gas-solid noncatalytic reactions, reactions involving at least one liquid phase (gas-liquid and liquid-liquid), and multiphase reactions. This section also describes membrane-assisted reactor engineering, combo reactors, homogeneous catalysis, and phase-transfer catalysis. The final chapter provides a perspective on future trends in reaction engineering. 

Two fixed-bed reactors can be used in parallel, one reacting and the other regenerating. However, there are many disadvantages in carrying out this type of reaction in a packed bed. The operation is not under steady state conditions, and this can present control problems. Eventually, the bed must be taken off line to replace the solid. Fluidized beds (to be discussed later) are usually preferred for gas-solid noncatalytic reactions. 

Homogeneous reactions Homogeneous noncatalytic reactions are normally carried out in a fluidized bed to achieve mixing of the gases and temperature control. The solids of the bed act as a heat sink or source and facilitate heat transfer from or to the gas or from or to heat-exchange surfaces. Reactions of this type include chlorination of hydrocarbons or oxidation of gaseous fuels. 

All of these roasting furnace reactors operate continuously. They are noncatalytic gas-solid heterogeneous reactors. The multiple hearth has characteristics similar to plug flow operation. The flash roaster approaches CSTR, and the third option is a fluidized bed configuration. 

The types of reactors used for catalytic and noncatalytic gas-solid reactions are also often similar. Moving-bed reactors are used in blast furnaces and cement kilns. Fluidized-bed reactors are used for the roasting of sulflde ores and regeneration of catalytic cracking catalyst, and fixed-bed reactors are used to remove sulfur compounds from ammonia synthesis feed gas. When regeneration of the solid reactant is desired, two or more reactors operating in parallel are required if continuous, steady-state operation is to be achieved. 

This section provides brief descriptions of industrial processes in which noncatalytic gas-solid reactions play a major role. Although by no means complete, the discussion includes both traditional processes, such as the blast furnace for the production of iron from ore and the regeneration of fluidized-bed catalytic cracking catalyst, and newer processes such as the dry capture of SO2 from flue gas and the production of silicon for semiconductor applications. Each of the three primary reactor types is represented in the processes described. 

Equipment for heterogeneous reactions is particularly flexible, since each phase can be processed more or less independently. In the fluidized-bed reactor (Fig. 1-4) the reactants flow continuously through and out of the reactor, but the solid-catalyst phase is withdrawn, regenerated, and returned. In the lime kiln (an example of a gas-solid noncatalytic reactor) the two phases pass continuously and countercurrently through the reactor. In heterogeneous liquid-solid polymerization systems the slurry of catalyst and reaction mixture flow together through the reactors. Walas, Brotz, and particularly van Krevelen have summarized the various types of... 

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. 

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. 

For gas-soM catalytic reactors also, fixed-bed, fluidized-bed, and moving-bed reactors are commonly employed. However, since all gas-solid noncatalytic reactions are inherently time dependent, time becomes an unavoidable parameter in the analysis. We briefly outline the procedures for the three reactor types mentioned and also touch upon a few other types. In view of the special importance of fluidized-bed reactors for these systems, two case studies involving the use of this reactor are presented in Section 11.5 (Case Studies 11.6 and 11.12). Dutta and Gualy (1999) give a comparative evaluation of fixed- and fluidized-bed reactors. 

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