Moving-Solids Reactors

Fixed-bed reactors are ideal for many solid-catalyzed gas reactions. The contacting of the solid by the gas tends to be quite uniform, and long contact 

Many types of gas-solid reactors have been designed to allow motion of the solid relative to the fixed walls of the reactors. This motion is desired for one of the following reasons  

To enhance heat transfer between the particle and the environment 

To accommodate size changes of the particles concurrent with reaction 

As the gas flow is increased beyond the behavior of the bed depends on the density difference between the particles and the suspending fluid. If the 

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Other designs of kilns use static shells rather than rotating shells and rely on mechanical rakes to move solid material through the reactor. 

If a very high viscosity is required, die granulated polymer can be postcondensed in die solid state at 160-190° C for 4-24 h. The postcondensation step can be done batchwise in large revolving reactors or can be carried out in a continuous manner using tall moving-bed reactors. Surprisingly, die water concentration is not critical to the rate of postcondensation. The method employed for PA-6 is similar to tiiat for PA-6,6 (Example lc). As a result of a postcondensation step for 24 h at 190°C, the i/inh in 85% formic acid is increased from 0.8 to 1.35 (Afn = 18,000-35,000). 

In this study, a simulated countercurrent moving bed reactor (SCMBR) with four parts by switching the inlets and outlets of the parts cyclically is employed in order to avoid abrasion occurring from the movement of a solid catalyst. Based on the above concepts, we focused on the performance of a SCMBR for the oxidation of CO at low concentration in absence of H2 over Pt/AlaOs catalyst adsorbent. For the first stqj of the overall nractor draign, the performance of a SCMBR is experimentally investigated and compared with that of a PBR for the reaction. 

Reactors in which the solid catalyst particles remain in a fixed position relative to one another (fixed bed, trickle bed, and moving bed reactors). 

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. 

Fluidized-Bed and Other Moving-Particle Reactors for Fluid-Solid Reactions... 

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). 

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. 

When a chemical reaction occurs in the system, each of these types of behavior gives rise to a corresponding type of reactor. These range from a fixed-bed reactor (Chapter 21-not a moving-particle reactor), to a fluidized-bed reactor without significant carryover of solid particles, to a fast-fluidized-bed reactor with significant carryover of particles, and ultimately a pneumatic-transport or transport-riser reactor in which solid particles are completely entrained in the rising fluid. The reactors are usually operated commercially with continuous flow of both fluid and solid phases. Kunii and Levenspiel (1991, Chapter 2) illustrate many industrial applications of fluidized beds. 

If larger particles are used in a moving-bed reactor, there is some sacrifice over temperature control and fluid-solid exchange. However, the pressure drop is much less than in bubbling fluidized beds, and erosion by particles is largely avoided. Furthermore, the fluid-solid contacting is close to ideal, and so performance is enhanced. 

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