Fluid—solid reactors

Fluid-solid systems cover a major class of chemical reactions and encompass both liquid-solid and gas-solid systems. In either case, the fluid phase is a single homogeneous fluid. The solid phase acts as a catalyst and its arrangement in the 

The catalytic wall reactor with channel diameters in the range 50-1000 pm and a length dependent on the reaction time required circumvents these shortcomings. However, in most cases, the catalytic surface area provided by the wall alone is insufficient for the chemical transformation and therefore the specific surface area has to be increased by chemical treatment of channel walls or by coating them with highly porous support layers. This can be done by using a variety of techniques such as sol-gel, electrophoretic and chemical or physical vapor deposition [8, 9]. 

The three fundamental operational parameters described bdow characterize the MSR pressure drop, residence time distribution and mass transfer rates. 

The pressure drop during the passage of a fluid through a reactor is an important parameter related to the optimization of the energy consumption. Pressure drop will be considered assuming non-compressible fluids and taking the standard assumption of continuum mechanics. Gas properties at temperatures up to 600 K and at a minimum pressure of 0.1 MPa will be used. Fluid velocities less than 10 m s will be considered in channels with hydraulic diameters less than 1 mm. Under these conditions, the fluid flow is laminar and compressibility effects can be neglected [10]. 

To avoid flow maldistribution in the bed, the particle diameter should not be larger than one-tenth of the tube diameter (dp dt/10) and the channel length should be higher than 50 particle diameters (Lbed 50dt). This may lead to a relatively high pressure drop in the MSR, which can be estimated with the modified Ergun equation [11]  

This section draws heavily from the book Chemical Reactor Engineering by Levenspiel [1]. 

FIGVRES.13 Various contacting patterns in fluid-solid reactors (a-c) countercurrent, crosscurrent, and cocurrent plug flow (d) intermediate gas flow, mixed solid flow (e) semi-batch operations. From Levenspiel [1], copyright 1972 by John Wil r Sons, Inc. Reprinted by permission of John Wiley Sons, Inc. 

Chapter 5 Ceramic Powder Synthesis with Solid Phase Reactant 

Equation (5.41) assumes that the gas is of a uniform composition throughout the reactor at all times. If the gas composition changes with the time or position within the reactor, a different equation must be used. To account for the effect of particle size distribution in addition to the residence time distribution is difficult because different size particles can remain in the reactor for different periods of time. To account for these effects completely a population balance must be performed, where the conversion is an internal variable (see Chapter 3). This type of treatment is beyond the scope of this chapter. A simplified method of accounting for the effects of a particle size distribution, mQt), on the mean conversion, is by 

This equation assumes that all the particles of different size have the ssune residence time within the reactor. This is not always a good assumption because fine particles follow the gas stream lines much 

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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... 

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). 

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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