Well-stirred batch reactors

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The stirred tank batch reactor is still the most widely used reactor type both in the laboratory and industry. A batch reactor is one in which a feed material is treated as a whole for a fixed period of time. Batch reactors may be preferred for small-scale production of high priced products, particularly if many sequential operations are carried out to obtain high product yields. Batch reactors may also be justified when multiple, low volume products are produced in the same equipment or when continuous flow is difficult, as it is with highly viscous or sticky solids-laden liquids. Because residence time can be more uniform in batch reactors, better yields and higher selectivity may be obtained than with continuous reactors.

Batch reactors are often used because of their convenience mainly in laboratory experimentation. Industrial practice generally favors processing continuously rather than in single batches, because overall investment and operating costs usually are less onerous. Data obtained in batch reactors can be well defined and used to predict performance of larger scale, continuous-flow reactors. Almost all batch reactors are well stirred; thus, ideally, compositions are uniform throughout and residence times of all contained reactants are constant.

Although various types of agitators can be used with different shear patterns imposed, the major problem with stirred tank reactors is abrasion of the matrix particles. This abrasion is not only due to the mixer blades or generated shear but also to some extent to the air injection. By packing the gel beads, porous particles or other packing in a draft tube, abrasion can be limited. In fact, a submerged packed bed with external recycle results, and the mixer acts as a pump and the rest of the vessel as a recirculation reservoir. Impellers such as helical ribbon, screw or anchor are preferred over turbines or propellers for their gentler stirring. Smaller particles are less subject to shear and these also have fewer risks of diffusion limit of the conversion rates. Their separation at the exit of the reactor might, however, be more problematic.

Batch reactors are rarely used as stand-alone reactor configurations in municipal or industrial wastewater treatment units, except for certain cases such as pH-adjustment of a stream (i.e. by addition of alkali to neutralize a batch) or for chemical precipitation of dissolved metals (i.e. removal of metal ions by alkaline precipitation).

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