Page 313 - Introduction to chemical reaction engineering and kinetics

Chapter 12

Batch Reactors (BR)

The general characteristics of a batch reactor (BR) are introduced in Chapter 2, in
connection with its use in measuring rate of reaction. The essential picture (Figure 2.1)
in a BR is that of a well-stirred, closed system that may undergo heat transfer, and be
of constant or variable density. The operation is inherently unsteady-state, but at any
given instant, the system is uniform in all its properties.
In this chapter, we first consider uses of batch reactors, and their advantages and
disadvantages compared with continuous-flow reactors. After considering what the es-
sential features of process design are, we then develop design or performance equa-
tions for both isothermal and nonisothermal operation. The latter requires the energy
balance, in addition to the material balance. We continue with an example of optimal
performance of a batch reactor, and conclude with a discussion of semibatch and semi-
continuous operation. We restrict attention to simple systems, deferring treatment of
complex systems to Chapter 18.

12.1 USES OF BATCH REACTORS

Batch reactors are used both in the laboratory for obtaining design and operating data
and in industrial processes for production of chemicals.
The use of batch reactors in the laboratory is described in Section 2.2.2 for the inter-
pretation of rate of reaction, in Section 3.4.1.1 for experimental methodology, and in
Chapter 4 and subsequent chapters for numerical treatment of kinetics experimental
data for various types of reacting systems.
The use of batch reactors in commercial processes is usually most suitable for small-
volume production, particularly for situations in which switching from one process or
product to another is required, as in the manufacture of pharmaceuticals. Typically, in
such processes, the value of the products is relatively large compared with the cost of
production. However, batch reactors may also be used for large-volume production
(England, 1982). Examples are the production of “vinyl” (polyvinyl chloride or PVC)
involving suspension polymerization, and of emulsion-polymerized latex. In some
cases, it may be desirable to deviate from a strictly batch process. For example, it may
be necessary to replenish periodically a limiting reactant which has been consumed, or,
if products of different purity are required, portions of the batch may be removed at
different times. In this type of operation, referred to as a semibatch process (Section
12.4) the system is no longer closed, since some mass is added to, or removed from,
the system during the process. If addition of reactants and removal of products occur
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