Page 252 - Modeling of Chemical Kinetics and Reactor Design

222 Modeling of Chemical Kinetics and Reactor Design

reactors are used in the pharmaceutical, biochemical, or multi-product
plants as in the dye industry. These reactors or autoclaves require
suitable access for inserting agitators, coils, or other internal devices
and for cleaning. Figure 4-2 shows a steam-jacketed autoclave and
Figure 4-3 is a 120-gal, steam-jacketed autoclave for processing
organic chemicals at 2,000 psi and 300°F. The advantages of a batch
reactor are:

• Simple in construction.
• Small instrumentation and cost.
• Flexibility of operation.

The principal disadvantage of a batch reactor is in the labor cost.
Labor cost includes the time it takes to fill the reactor, heat it to
reaction temperature, cool it after completion of the reaction, discharge
the reactor contents, and clean the reactor for the next batch. These
procedures increase the overall labor costs per unit of production.
Another disadvantage involves the difficulty to control heat transfer
and product quality. Chemical reaction rates usually increase with
temperature and with more intimate contact between reactants. Mechanical
agitation promotes the flow of heat by forcing convection of the mass
and by reducing the film resistance at the vessel wall. Additionally,
agitation breaks up agglomerated solids thereby increasing the contact
surface and the rate at which reacting species come into close proximity.
An important purpose of agitation or mixing is to bring a number
of materials together in a physically homogeneous mixture. Two or
more fluids are either blended or dispersed as emulsions; fluids and
finely divided solids are dispersed as suspensions, gases dispersed as
fluids, or soluble substances dissolved. Mixing of process fluids is
reviewed in Chapter 7.

SEMI-BATCH REACTORS

Figure 4-4 shows a semi-batch reactor with outside circulation and
the addition of one reactant through the pump. Semi-batch reactors
have some reactants that are charged into the reactor at time zero,
while other reactants are added during the reaction. The reactor has
no outlet stream. Some reactions are unsuited to either batch or
continuous operation in a stirred vessel because the heat liberated
during the reaction may cause dangerous conditions. Under these