Catalytic reactions fluid-solid

Many industrially important reactions are characterized by an interface across which heat/mass transfer occurs. They are called heterogeneous reactions. They include fluid-fluid reactions, gas-solid catalytic reactions, gas-solid noncatalytic reactions, and solid-solid reactions. 

However, before extrapolating the arguments from the gross patterns through the reactor for homogeneous reactions to solid-catalyzed reactions, it must be recognized that in catalytic reactions the fluid in the interior of catalyst pellets may diSer from the main body of fluid. The local inhomogeneities caused by lowered reactant concentration within the catalyst pellets result in a product distribution different from that which would otherwise be observed. 

For a catalytic reaction of a reactant from a single fluid phase (either gas or liquid) to take place on a solid catalyst, diffusion processes also play a role, so in the complete process the following steps can be distinguished ... 

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. 

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). 

Next we assume that the mass transfer rate of A to or Ifom the surface limits the rate of etching or growth, or that the concentration of A in the flowing or bulk fluid Ca]> is not identical to the concentration in the fluid near the solid surface Cas- (Recall that we used the same notation in the catalytic reactions of Chapter 7. Note also that we have a problem with subscript s designating either the gas concentration of A at the surface or the fact that we have a solid.)... 

In the case of two fluids, two films are developed, one for each fluid, and the corresponding mass-transfer coefficients are determined (Figure 3.2). In a fluid-solid system, there is only one film whereas the resistance within the solid phase is expressed by the solid-phase diffusion coefficient, however, in many cases an effective mass-transfer coefficient is used in the case of solids as well. Consider the irreversible catalytic reaction of the form... 

Reaction rate expressions of solid-fluid catalytic reactions... 

Figure 5.2 Concentration and temperature profiles for fluid —solid catalytic reactions.

In many catalytic reactions, solid, liquid, and gas phases are involved, and the phase behavior often has a strong influence on mixing and mass transfer and consequently on the catalytic performance. Supercritical fluids, especially supercritical CO2, have gained considerable attention as environmentally benign solvents (e.g., (94y). The combined use of in situ transmission and ATR-IR spectroscopy together with video monitoring is a promising approach for elucidation of the behavior of a... 

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