Solids fluid-solid reactors

Fig. 4. Multiphase fluid and fluid—solids reactors (a) bubble column, (b) spray column, (c) slurry reactor and auxiUaries, (d) fluidization unit, (e) gas—bquid—sobd fluidized reactor, (f) rotary kiln, and (g) traveling grate or belt drier.

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

Figure 26.1 Various contacting patterns in fluid-solid reactors a-d) countercurrent, crosscurrent, and cocurrent plug flow d) intermediate gas flow, mixed solid flow (e) semibatch operations.

Flow of Two Fluids. The major applications are in absorption, extraction, and distillation, with and without reaction. Other applications, also quite important, are for shell-and-tube or double-pipe heat exchangers, and noncatalytic fluid-solid reactors (blast furnace and ore-reduction processes). 

It is clear from this discussion that the design of mechanically agitated photochemical reactors requires a knowledge of eg, db, and aL. These parameters can be estimated from the correlations reported in Section II. For the multilamp reactor, the effects of internals on eg, db, and a L should be appropriately evaluated. No similar, reliable design procedures have yet been developed for solid-fluid photocatalytic reactors. 

The Unipol process employs a fluidized bed reactor (see Section 3.1.2) for the preparation of polyethylene and polypropylene. A gas-liquid fluid solid reactor, where both liquid and gas fluidize the solids, is used for Ziegler-Natta catalyzed ethylene polymerization. Hoechst, Mitsui, Montedison, Solvay et Cie, and a number of other producers use a Ziegler-type catalyst for the manufacture of LLDPE by slurry polymerization in hexane solvent (Fig. 6.11). The system consists of a series of continuous stirred tank reactors to achieve the desired residence time. 1-Butene is used a comonomer, and hydrogen is used for controlling molecular weight. The polymer beads are separated from the liquid by centrifugation followed by steam stripping. 

Reactors for solid-solid reactions are designed in the same way as that for the fluid-solid reactors (see Section 5.10) but with these reactions the mixing of the gas does not need to be considered. 

All the design equations for ideal catalytic or fluid-solid reactors can be obtained from their homogeneous reactor analogs merely by substituting the catalyst or solid weight, W, for the reactor volume, V. The reactor volume is merely the catalyst weight W divided by the bulk density of the catalyst pj,. In the catalytic or fluid-solid reactor design equations,, based on catalyst mass, must of course be used. 

A glib generalization is that the design equations for noncatalytic fluid-solid reactors can be obtained by combining the intrinsic kinetics with the appropriate transport equations. The experienced reader knows that this is not always possible even for the solid-catalyzed reactions considered in Chapter 10 and is much more difficult when the solid participates in the reaction. The solid surface is undergoing change. See Table 11.6. Measurements usually require transient experiments. As a practical matter, the measurements will normally include mass transfer effects and are often made in pilot-scale equipment intended to simulate a full-scale reactor. Consider a gas-solid reaction of the general form... 

Table 6.1 Different types of fluid-solid reactors, their advantages, and limitations.

Figure 3.35. Differing situations in a fluid/solid reactor as a function of the velocity of the fluid phase, along with fluidization diagrams for the various reactor types. The pressure drop, Ap, increases in solid bed operations until the minimum velocity for fluidization is reached, t f. Above this velocity, fluidized bed conditions exist. In case of particles with diameters about 0.5 cm, the spouted bed can be operated in fluidized conditions above the minimum velocity of spouting A further increase in velocity finally leads to the elutriation of the solid phase.

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