Types of equipment and contacting patterns

There is a wide choice of contacting methods and equipment for gas-solid reactions. As with other solids-handling problems, the solution finally adopted may depend very much on the physical condition of the reactants and products, i.e. particle size, any tendency of the particles to fuse together, ease of gas-solid separation, etc. One type of equipment, the rotary kiln, has already been mentioned (Chapter 2, Fig. 2.4) and some further types of equipment suitable for continuous operation are shown in Fig. 3.37. The concepts of macromixing in the solid phase and dispersion in the gas phase as discussed in the previous section will be involved in the quantitative treatment of such equipment. 

The principle of the moving bed of Fig. 3.37a in cross-flow to the air supply is used for roasting zinc blende and for the combustion of large coal on chain grate stokers. Another kind of moving bed is found in hopper-type reactors in which particles are fed at the top and continuously move downwards to be discharged at 

Rotary kilns (Chapter 2, Fig. 2.4) have advantages where the solid particles tend to stick together as in cement manufacture, and in the reduction and carbonylation steps in the purification of nickel. In rotary kilns, the flow of gas may be co-current or countercurrent to the solids. 

If the solid particles can be maintained in the fluidised state without problems of agglomeration or attrition, the fluidised bed reactor Fig. 1.45c is likely to be preferred. For short contact times at high temperatures the dilute phase transfer line reactor (Fig. 3.37 /) has advantages. 

Raked hearth reactors were once extensively used in the metals extraction industries but are now being superseded. 

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