Page 102 - The engineering of chemical reactions

Chapter 3

SINGLE REACTIONS IN
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7 CONTINUOUS ISOTHERMAL

REACTORS .

CONTINUOUS REACTORS

n this chapter we consider the fundamentals of reaction in continuous isothermal
reactors. Most industrial reactors are operated in a continuous mode instead of batch
I because continuous reactors produce more product with smaller equipment, require less
labor and maintenance, and frequently produce better quality control. Continuous processes
are more difficult to start and stop than batch reactors, but they make product without
stopping to change batches and they require minimum labor.
Batch processes can be tailored to produce small amounts of product when needed.
Batch processes are also ideal to measure rates and kinetics in order to design continuous
processes: Here one only wants to obtain information rapidly without generating too much
product that must be disposed of. In pharmaceuticals batch processes are sometimes desired
to assure quality control: Each batch can be analyzed and certified (or discarded), while
contamination in a continuous processes will invariably lead to a lot of worthless product
before certifiable purity is restored. Food and beverages are still made in batch processes
in many situations because biological reactions are never exactly reproducible, and a batch
process is easier to “tune” slightly to optimize each batch. Besides, it is more romantic to
produce beer by “beechwood aging,” wine by stamping on grapes with bare feet, steaks by
charcoal grilling, and similar batch processes.
We will develop mass balances in terms of mixing in the reactor. In one limit the
reactor is stirred sufficiently to mix the fluid completely, and in the other limit the fluid
is completely unmixed. In any other situation the fluid is partially mixed, and one cannot
specify the composition without a detailed description of the fluid mechanics. We will
consider these “nonideal” reactors in Chapter 8, but until then all reactors will be assumed
to be either completely mixed or completely unmixed.

THE CONTINUOUS STIRRED TANK REACTOR

Here we consider the situation where mixing of fluids is sufficiently rapid that the com-
position does not vary with position in the reactor. This is a “stirred-tank” or “backmix
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