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Reactor Design 377

Table 7.7 Approximate Mixer Power for Stirred-Tank Reactors

Application Power"
hp/lOOOgal

Blending" 0.2-0.5
Homogeneous Reaction 3 0.5-1.5
Reaction with Heat Transfer 3 1.5-5.0
Liquid-Liquid Mixtures" 5.0
Liquid-Gas Mixtures 3 5.0-10.0
Slurries" 10.0
Fermentation 15 3.0-10.0
Emulsion Polymerization 11 6.0-7.0
Suspension Polymerization b 3.0-10.0
Solution Polymerization 11 15.0-40.0

a) Source: Reference 7
b) Source: Reference 15
3
c) To convert to W/m multiply by 197.0.

Constant density implies that the volumetric flow rate from reactor to reac-
tor is constant. The relationships listed in Table 7.4 apply to any number of
CSTRs in series. The subscript, n, refers to the reactor number and also to the
number of the entering stream. The subscript, n + 1, refers to the number of the
leaving stream. Equations 7.4.1 to 7.4.3, are the mole balance for reactant A,
the energy equation, and the rate equation.
STRs are usually never completely filled unless top withdrawal of the liq-
uid is required. At the top of the reactor, we will allow some empty volume,
called head space. Blaasel [15] recommends allowing 15% head space for reac-
tors less than 1.9 m 3 (500 gal) and 10% head space for reactors greater than 1.9
m 3 (500 gal). After calculating the reaction volume, then add the headspace ac-
cording to these rules to obtain the reactor volume. After calculating the reactor
volume, select a standard reactor from a manufacturer. A standard reactor is less
expensive than a reactor made-to-order. Table 7.3 lists standard-size reactors,
which will vary somewhat from manufacturer to manufacturer. In Table 7.3, the
rated capacity is the reaction volume, and the actual volume includes the head-
space. Because the manufacturer has allowed for headspace in this case, we need
not allow headspace according to the above rules.
To transfer heat, size either a STR with a jacket or one with internal coils.
Try the jacketed reactor first because it is the least costly. The available heat-

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