Page 388 - Chemical process engineering design and economics

368 Chapter 7

To guide the reactor selection process, Walas [7] has classified reactions
according to the operating mode (batch or continuous), reactor type (tank, tank
battery, tubular), flow type (back mixed, multistage back mixed), and the phases in
contact. This reactor classification in Table 7.2 indicates if a particular reactor
arrangement is commonly used, rarely used, or not feasible.
Economics determines whether to use a continuous flow or a batch reactor.
Generally, if the residence time is large and the production rate small, select a
batch reactor. This relationship is shown in Figure 7.1, which can be used to ob-
tain a preliminary selection of a reactor. When the application is located in over-
lapping areas or near a boundary, make a careful analysis to determine the most
economic choice.
There are two ideal models for developing reactor-sizing relationships: the
plug flow and the perfectly stirred-tank models. In the plug-flow model, the reac-
tants flowing through the reactor are continuously converted into products. Dur-
ing reaction there is no radial variation of concentration, backmixing or forward
mixing. In a perfect STR, the reactants are thoroughly mixed so that the concen-
tration of all species and temperature are uniform throughout the reactor and equal
to that leaving the reactor.

Figure 7.1 Application areas for several reactor types. From Ref. 8.

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