Fluid—solid reactions

From the point of view of chemical metallurgy, the class of heterogeneous reactions in which a fluid (gas or liquid) contacts a solid, reacts with it and transforms it into the product is very important and involves many of the principles pertaining to heterogeneous reactions. Fluid-solid reactions may essentially be represented in the following three ways  

A (fluid) + B (solid) — Fluid products A (fluid) + B (solid) — Solid products A (fluid) + B (solid) — Fluid and solid products 

Let a fluid-solid reaction of the following type be considered. 

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L. K. Doraiswany and M. M. Shamia, Heterogeneous Reactions, Hnaijses, Examples and Reactor Design, Vol. 2, Fluid—Fluid Solid Reactions, ]ohxi Wiley Sons, Inc., New York, 1984, pp. 299—300. 

Product layer diffusion Many fluid-solid reactions generate ash or oxide layers that impede further reaction. 

The analysis of fluid-solid reactions is easier when the particle geometry is independent of the extent of reaction. Table 11.6 lists some situations where this assumption is reasonable. However, even when the reaction geometry is fixed, moving boundary problems and sharp reaction fronts are the general rule for fluid-solid reactions. The next few examples explore this point. 

Stable and radioactive tracers have been used extensively in catalysis to validate reaction networks, test for intermediates, confirm reaction orders, distinguish between intra- and inter-molecular mechanisms, establish rate limiting steps, docviment direct participation of surface atoms in fluid-solid reactions, etc. A unique feature of tracer studies is that Individual reaction steps can be followed in a complicated set of reactions without perturbing the chemical composition of the... 

In some heterogeneous reactions, for instance, in noncatalytic fluid-solid reactions, the resistances to the reaction may be taken to occur in series. However, in some other reactions, such as catalytic solid-solid reactions, more complicated series-parallel relationships among the resistances must be considered. 

In some fluid-solid reactions, nucleation of the product constitutes an important step. With increasing size of the solid or with increasing reaction temperature, the time within which nucleation is important becomes small compared to the total reaction time, thereby making nucleation relatively less important. 

Strictly speaking, the validity of the shrinking unreacted core model is limited to those fluid-solid reactions where the reactant solid is nonporous and the reaction occurs at a well-defined, sharp reaction interface. Because of the simplicity of the model it is tempting to attempt to apply it to reactions involving porous solids also, but this can lead to incorrect analyses of experimental data. In a porous solid the chemical reaction occurs over a diffuse zone rather than at a sharp interface, and the model can be made use of only in the case of diffusion-controlled reactions. 

The dissolution of porous minerals, the combustion of porous carbon, the reaction between porous carbon and carbon dioxide, and the formation of nickel carbonyl from pure nickel are some examples of fluid-solid reactions where the reactant solid is porous and where no solid reaction product is formed. A reaction of this type can be represented as... 

Reactions among solids occurring through gaseous intermediates can be viewed as coupled gas-solid reactions, and thus can be analyzed in the light of the discussions presented in the preceding sections regarding fluid-solid reactions. 

Volume 1 Gas-solid and solid-solid reactions Volume 2 Fluid-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 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). 

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). 

Continuous reactors for fluid-solid reactions involve continuous flow for both fluid and solid phases. With the assumptions made in Section 22.2.1 about the fluid, we focus only... 

In this chapter, we consider reactors for fluid-solid reactions in which the solid particles are in motion (relative to the wall of the vessel) in an arbitrary pattern brought about by upward flow of the fluid. Thus, the solid particles are neither in ideal flow, as in the treatment in Chapter 22, nor fixed in position, as in Chapter 21. We focus mainly on the fluidized-bed reactor as an important type of moving-particle reactor. Books dealing with fluidization and fluidized-bed reactors include those by Kunii and Levenspiel (1991), Yates (1983), and Davidson and Harrison (1963). 

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