Fluid-Particle Reactors Design

Though some real industrial reactions may never yield to simple analysis, this should not deter us from studying idealized systems. These satisfactorily represent many real systems and in addition may be taken as the starting point for more involved analyses. Here we consider only the greatly simplified idealized systems in which the reaction kinetics, flow characteristics, and size distribution of solids are known. 

Referring to Fig. 26.1, let us discuss briefly the various types of contacting in gas-solid operations. 

Solids and Gas Both in Plug Flow, When solids and gas pass through the reactor in plug flow, their compositions change during passage. In addition, such operations are usually nonisothermal. 

The plug flow contacting of phases may be accomplished in many ways by countercurrent flow as in blast furnaces and cement kilns [Fig. 26. ( )], by cross-flow as in moving belt feeders for furnaces [Fig. 26.1(6)], or by cocurrent flow as in polymer driers [Fig. 26.1(c)]. 

Solids in Mixed Flow, The fluidized bed [Fig. 26.1(d)] is the best example of a reactor with mixed flow of solids. The gas flow in such reactors is difficult to characterize and often is worse than mixed flow. Because of the high heat capacity of the solids, isothermal conditions can frequently be assumed in such operations. 

---

Chapter 26 Fluid-Particle Reactors Design Iron ore Coke... 

Friis and Hamielec (48) offered some comments on the continuous reactor design problem suggesting that the dispersed particles have the same residence time distribution as the dispersing fluid and the system can be modeled as a segregated CSTR reactor. 

In addition to flow, thermal, and bed arrangements, an important design consideration is the amount of catalyst required (W), and its possible distribution over two or more stages. This is a measure of the size of the reactor. The depth (L) and diameter (D) of each stage must also be determined. In addition to the usual tools provided by kinetics, and material and energy balances, we must take into account matters peculiar to individual particles, collections of particles, and fluid-particle interactions, as well as any matters peculiar to the nature of the reaction, such as reversibility. Process design aspects of catalytic reactors are described by Lywood (1996). 

After introducing some types of moving-particle reactors, their advantages and disadvantages, and examples of reactions conducted in them, we consider particular design features. These relate to fluid-particle interactions (extension of the treatment in Chapter 21) and to the complex flow pattern of fluid and solid particles. The latter requires development of a hydrodynamic model as a precursor to a reactor model. We describe these in detail only for particular types of fluidized-bed reactors. 

The most important methods for the determination of kinetics of catalyzed reactions are described here. We emphasize the problems and pitfalls in obtaining reliable reaction rates. The many diagnostic tests are briefly discussed and some warnings are given to limitations of commonly used laboratory reactors. Finally, it is worth noting that reaction rates can be expressed per unit mass of catalyst, per unit catalytic surface, per unit external particle area or per unit volume of the reactor, fluid or catalyst. For chemical reactor design it is best to express reaction rates in terms of unit catalyst volume. 

The problem of the optimal particle shape and size is crucial for packed bed reactor design. Generally, the larger the particle diameter, the cheaper the catalyst. This is not usually a significant factor in process design - more important are the internal and external diffusion effects, the pressure drop, the heat transfer to the reactor walls and a uniform fluid flow. 

In the next section various reaction models are considered. Then global rate equations are developed in Sec. 14-3 for one model (shrinking core). In Sec. 14-4 integrated conversion-vs-time relationships (for single particles) are presented. Such relationships are suitable for use in design of reactors in which the fluid phase is completely mixed. In Secs. 14-5 and 14-6 all these results will be applied to reactor design. 

We have seen how problems of particle size distribution of reactant and solid products can be employed in the design of fluid-bed reactors. The conversion obtained at the reactor exit depends on these distributions plus various other factors. The equations presented so far were based on continuous solids feed. In calculating the conversions, it is easier to divide the solid reactants into discrete ranges (each with an average size) and express the conversion as the sum from all the ranges. Furthermore, size distribution is usually determined by screen analysis, which gives discrete measurements. 

In slurry reactors, the catalyst particles are freely dispersed in the fluid phase (water) and consequently, the photocatalyst is fully integrated in the liquid mobile phase. The immobilized catalyst reactor design features a catalyst anchored to a fixed support, dispersed on the stationary phase (the catalyst-support system). 

---