Feed

Semibatch

Semiplug flow

Batch

Plug-

flow

Source: Smith and Petela, 1991.

time in the reactor. This requirement implies that to minimize waste from multiple reactions in series, an ideal-batch or plug-flow reactor performs better than a continuous well-mixed reactor.

Reactor Conversion

For the series reaction of Equations 3.7(3) and 3.7(4), a low concentration of PRODUCT in the reactor minimizes the formation of WASTE BY-PRODUCT. This reduction can be achieved by low conversion in the reactor.

If the reaction involves more than one feed, operating with the same low conversion on all feeds is not necessary. By using an excess of one feed, the chemical engineer can operate with relatively high conversion of other feed material and still inhibit series waste by-product formation.

Unfortunately, low conversion in the reactor (whether on one or all of the feeds) can increase both energy use and the cost of separation and recycling. However, if an existing separation system has spare capacity, reducing conversion to reduce waste by-product formation via series reaction is still worth considering.

Whereas increasing conversion always increases the formation of waste by-products via series reactions, the same is not always true of parallel reactions. Whether waste by-product formation via parallel reactions increases or decreases with increasing conversion depends on the order of the primary and secondary reactions.

Reactor Concentration

One or more of the following actions improves selectivity:

For a given reactor design, use an excess of one of the feeds when more than one feed is involved. If the by-product reaction is reversible and involves a decrease in the number of moles, increase the concentration of inerts.

If the by-product reaction is reversible and involves an increase in the number of moles, decrease the concentration of inerts.

Separate the product during the reaction before continuing further reaction and separation. Recycle waste by-products to the reactor. Where this recycling is possible, the waste by-products should be recovered and recycled to extinction.

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