FLUID-SOLID NONCATALYTIC REACTIONS

A glib generalization is that the design equations for noncatalytic fluid-solid reactors can be obtained by combining the intrinsic kinetics with the appropriate 

TABLE 11.6 Examples of Fluid-Solid Noncatalytic Reactions 

Particle geometry largely Particle geometry strongly 

Hydrogen storage in a metal lattice Production of acetylene from CaC2 

Film dijfusion With a fast surface reaction on a nonporous particle, mass transfer limitations can arise in the fluid phase. 

The spouting regime of fluidization is used for the fluid-solid noncatalytic reactions, especially drying and combustion. 

The solids and the fluid have similar densities in liquid fluidization. The consequence is that most liquidized beds operate in the particulate regime where there is a smooth transition from incipient fluidization to pneumatic transport without bubble formation or slugging. They typically operate at near isothermal conditions and have good mass transfer between the liquid and the suspended solids. As a first approximation, the solid phase is well mixed and the liquid phase is in piston flow. There may also be a gas phase. Typical applications are in cell culture, including wastewater treatment. The specialized literature gives details. 

Cases where a solid directly participates in an overall reaction include the burning of solid fuels, the decoking of cracking catalyst, the reduction of iron ore with hydrogen, and the purification of water in an ion exchange bed. A unifying aspect of all these examples is that the solid participates directly, appears in the reaction stoichiometry, and will ultimately be consumed or exhausted. Often the size and shape of the 

Particle Geometry Largely Unaffected by Reaction Particle Geometry Strongly Affected by Reaction 

Decoking of catalyst pellets Ion exchange reactions Hydrogen storage in metal lattice Semiconductor doping Combustion of coal Reduction of ore Production of acetylene from CaC2 Semiconductor etching 

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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. 

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. 

The mechanism of many of the noncatalytic fluid-solid reactions can be described by a model in between unreacted core and homogeneous reactions models. Ishida and Wen (9) formulated such a model using the zone reaction concept of Ausman and Watson (10). In this model the reaction is not restricted to the surface of the core as in the unreacted core model but occurs homogeneously within a retreating core of reactant. Wen and Ishida (11) combined the grain concept with the zone reaction model and analyzed the reaction of SO2 with CaO particles. In the study conducted by Mantri, Gokarn and Doraiswamy (12) the concept of finite reaction zone model was further developed. 

In catalytic systems morphological changes of the pore structure, brought upon by the reaction and sorption processes, typically result in a reduction of the available pore volume. In some instances the internal pore structure is eventually blocked and becomes completely inaccessible to transport and/or reaction. In the field of noncatalytic fluid-solid reactions and acid rock dissolution, on the other hand, the chemical reaction consumes the solid matrix of the porous medium leading eventually to fragmentation and... 

A heterogeneous catalytic reaction occurs at or very near the fluid-solid interface. The principles that govern heterogeneous catalytic reactions can be applied to both catalytic and noncatalytic fluid-solid reactions. These two other types of heterogeneous reactions involve gas-liquid and gas-Hquid-solid systems. Reactions between gases and liquids are usually mass-transfer limited. 

HtUrogateous Reaed n Systems The mathematical treatment is quite difleient if the exchanger is treated as a solid phase. The conceptual models are then similar to those developed for noncatalytic fluid-solid reactions. These kinetic models have been applied soccessliiUy to some km-exchtuige pro-... 

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