Reactors

The reactor is the center of a process and can be a primary source of waste. The quality of mixing in a reactor is crucial. Unexpected flow patterns and mixing limitations cause problems in commercial reactors designed from bench-scale research data.

The three classes of mixing are defined as follows:

Macro scale mixing refers to blending the feed so that every gallon in a reactor has the same average composition. Mixing on the macro scale is controlled through the use of agitators, educators, and chargers. Micro scale mixing refers to interdispersing the feed to give uniform composition to the 10-100 mp scale. Mixing on the micro scale is controlled by eddies and is not affected much by agitation. Molecular scale mixing is complete when every molecule in the reactor is surrounded by exactly the same molecules at least on a time-averaged basis. This mixing is almost totally driven by diffusion.

A completely well-stirred-tank reactor implies good mixing in all these levels. Many agitated vessels are well mixed on the macro scale, however, no commercial reactor is perfectly well mixed or thoroughly plug-flow. This incomplete mixing may be unimportant if the reaction is dominant or all reactions are slow. In many applications, the reaction and mixing times are often similar and must be carefully studied.

Sometimes one level of mixing controls the reaction rates in a bench reactor, and another level controls them in the plant. For example, in the following commercial reaction:

where A, B, and D are reactants; P is the product; and Q and X are by-products; a major discrepancy in yields between laboratory and plant reactors was found. The scale of mixing regimes was determined to be the source of the difference. The reaction in Equation 3.10(1) was fast, the reaction in Equation 3.10(2) was the slow-rate determining step and the reaction in Equation 3.10(3) was intermediate in speed. In the laboratory reactor, A reacted with B before it could form X. In the plant, it took a few seconds to get A and B together. During this mixing time, enough A was lost to X to seriously reduce the yield. A 1-mm-diameter stream disperses much faster than a 50mm one.

The vessel flow patterns, feed introduction methods, and mixing at all levels can create waste and operational problems. Companies should consider the following options to prevent pollution generated in reactors.

(a) (b)
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