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Encyclopedia of Physical Science and Technology EN014A-654 July 28, 2001 16:35

Reactors in Process Engineering 27

3. Design Parameters previously mentioned assumptions can be met even in a
long tube if the mixing characteristics indicate high dis-
The major design parameters for a semibatch reactor are
persion levels in the reactor. This is particularly true of
similar to a batch reactor with the addition of flow into or
gassed liquids where the bubbling in the column mixes
out of the tank.
the liquid.
4. Applications
2. Classification
The advantage of this reactor, with feed only, is for the
The continuous-stirred tank reactor is one of the two pri-
control of heat of extremely exothermic reactions. By
mary types of ideal flow reactors. It is also referred to as
inputting the feed gradually during the course of the reac-
a mixed-flow reactor, back-mix reactor, or constant-flow
tion, the concentration of feed in the reactor can be kept
stirred-tank reactor.
lower than in normal batch operation. Also, the tempera-
ture of the feed stream, when cooler than the reaction mix-
ture, has a quenching effect. Some of the heat released dur- 3. Design Parameters
ing the reaction is used to heat the feed material, thereby
The CSTR is not an integral reactor. Since the same con-
reducing the required capacity of the heating coils.
centration exists everywhere, and the reactor is operating
The semibatch can also be used to control the kinetics
at steady state, there is only one reaction rate at the aver-
in multiple reaction sequences. The selectivity may be
age concentration in the tank. Since this concentration is
shifted to one reaction by adding a reactant slowly. This
low because of the conversion in the tank, the value for
keeps one reactant concentration high with respect to the
the reaction rate is also low. This is particularly significant
other.
for higher-order reactions compared with integral reactor
The semibatch can also be used for continuous product
systems.
removal, such as vaporization of the primary product. This
Time is still an important variable for continuous sys-
can increase yield in equilibrium limited reactions.
tems, but it is modified to relate to the steady-state condi-
tions that exist in the reactor. This time variable is referred
5. Advantages–Disadvantages to as space time. Space time is the reactor volume divided
by the inlet volumetric flow rate. In other words, it is the
The temperature-controlling features of this reaction sch-
time required to process one reactor volume of feed mate-
eme dominate selection and use of the reactor. However,
rial. Since concentration versus real time remains constant
the semibatch reactor does have some of the advantages of
during the course of a CSTR reaction, rate-data acquisi-
batch reactors: temperature programming with time and
tion requires dividing the difference in concentration from
variable reaction time control.
the inlet to the outlet by the space time for the particular
The temperature conditions and the batch nature of
reactor operating conditions.
this reactor are the primary operational difficulties and
make the reactor impractical for most reactions, even for
computer-controlled systems. The majority of reactions 4. Applications
considered for semibatch are highly exothermic and, as
The CSTR is particularly useful for reaction schemes that
such, are dangerous and require special attention.
require low concentration, such as selectivity between
multiple reactions or substrate inhibition in a chemostat
C. Continuous-Stirred Tank (see Section IV). The reactor also has applications for het-
erogeneous systems where high mixing gives high contact
1. Description
time between phases. Liquid–liquid CSTRs are used for
The continuous-stirred tank reactor (CSTR) has continu- the saponification of fats and for suspension and emul-
ous input and output of material. The CSTR is well mixed sion polymerizations. Gas–liquid mixers are used for the
with no dead zones or bypasses in ideal operation. It may oxidation of cyclohexane. Gas homogeneous CSTRs are
or may not include baffling. The assumptions made for the extremely rare.
ideal CSTR are (1) composition and temperature are uni-
form everywhere in the tank, (2) the effluent composition
5. Advantages–Disadvantages
is the same as that in the tank, and (3) the tank operates at
steady state. The advantages for CSTRs include (1) steady-state op-
We traditionally think of the CSTR as having the ap- eration; (2) back mixing of heat generated by exothermic
pearance of a mixing tank. This need not be the case. The reactions,whichincreasesthereactionrateandsubsequent