Page 389 - Chemical process engineering design and economics

Reactor Design 369

STIRRED-TANK REACTOR SELECTION

The operating mode of a stirred-tank reactor may be either continuous or batch.
A STR consists of a vessel to contain the reactants, a heat exchanger, a mixer,
and baffles to prevent vortex formation and to increase turbulence, enhancing
mixing.
To evaluate and select a STR, consider the following factors:

1. mixing
2. heat transfer
3. jacket pressure drop
4. cleaning

Sufficient power must be supplied to the liquid to approach the ideal
model of a thoroughly-mixed reacting system. Inadequate mixing results in a
longer average residence time and thus a larger reactor volume than for the ideal
model. Designing a mixing system requires selecting and sizing the impeller,
baffles, and electric motor. For a preliminary design, all that is necessary is to
estimate the mixer power.
An important consideration when sizing a STR is heating or'cooling the
reactor contents. There are several heat exchangers, which are classified as ei-
ther an internal or external heat exchanger. The internal heat exchangers are
immersed directly into the reacting liquid and consist of spiral coils, harp coils,
and hollow or plate baffles. We will only consider spiral coils when designing
an STR.
The external heat exchanger may either be a jacket or a she 11-and-tube heat
exchanger. For the latter, the reactor contents circulate through an external flow
loop containing the heat exchanger. The jacket types, as illustrated in Figure
7.2, consist of the simple jacket - with or without a spiral baffle or nozzles for
promoting turbulence - the partial pipe coil, and the dimple jacket. The simple
jacket consists of an outer cylinder enclosing part of the reactor. A heat-
transfer fluid flows in the annular area surrounding the reactor, as shown in Fig-
ure 7.2. If the heat-transfer rate is limited by the jacket heat-transfer coefficient,
then increase the turbulence in the jacket by using a spiral baffle or nozzles. The
spiral baffle is wound around and welded to the reactor. The baffle channels the
fluid from the jacket entrance to the jacket exit. Channeling the fluid increases
its velocity and turbulence, resulting in a higher heat transfer coefficient. The
partial pipe coil is formed by cutting a pipe along its longitudinal axis. Then,
the coil is wrapped around the reactor in a helix and welded onto the reactor
shell. The dimple jacket consists of hemispherical dimples pressed into a thin
plate, which is then wrapped around and welded onto the reactor. The jacket
area covers about 80% of the reactor surface, consisting of a bottom elliptical
head and a cylindrical shell.

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