Page 284 - Modeling of Chemical Kinetics and Reactor Design

254 Modeling of Chemical Kinetics and Reactor Design

if the catalyst is non-decaying and heat effects are negligible, the
fixed bed (integral) reactor is the best choice because of its ease of
construction and operation. The pulse reactor is most satisfactory in
systems that do not strongly adsorb or where the adsorbed species do
not relatively alter the reaction paths. In cases where the reaction
system is extremely critical, more than one reactor type is used in
determining the reaction rate law parameters.

LOOP REACTORS

A loop reactor is a continuous steel tube or pipe, which connects
the outlet of a circulation pump to its inlet. Reactants are fed into the
loop, where the reaction occurs, and product is withdrawn from the
loop. Loop reactors are used in place of batch stirred tank reactors in
a variety of applications including chlorination, ethoxylation, hydro-
genation, and polymerization. A loop reactor is typically much smaller
than a batch reactor producing the same amount of product. Mass
transfer is often the rate-limiting step in gas-liquid reactions, and a
loop reactor design increases mass transfer, while reducing reactor size
and improving process yields. An example is an organic material that
has been chlorinated in a glass-lined batch stirred tank reactor, with
chlorine fed through a dip pipe. Replacing the stirred tank reactor with
a loop reactor, with chlorine fed to the recirculating liquid stream
through an eductor, reduced reactor size, increased productivity, and
reduced chlorine usage. Figure 4-25 shows a schematic of a loop
reactor system. Table 4-5 compares the advantages of a loop reactor
to a batch stirred tank reactor.

GUIDELINES FOR SELECTING BATCH PROCESSES

Douglas [5] gives an excellent review in selecting a batch process
in favor of a continuous process. The factors that favor batch opera-
tions are summarized as follows:

• Production rates:
— Sometimes batch process, if the plants have production capacity
6
6
less than 10 × 10 lb/yr (5 × 10 kg/hr).
— Usually batch process, if the plants have production capacity
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less than 1 × 10 lb/yr (0.5 × 10 kg/hr).
— Where multiproduct plants are produced using the same pro-
cessing equipment.