Design of Fermentation Processes
341
Table 9.1. Advantages and disadvantages of different modes of operation of the stirred tank reactor.
Mode of operation
Advantages______________________ Disadvantages_________________
Versatile:
Can be used for different
High labor cost:
Skilled labor is
reactions every day.
required.
Batch
Safe:
Can be properly sterilized.
Little risk of infection or strain
mutation.
Much idle time:
Sterilization, growth
of inoculum, cleaning after the
fermentation.
Complete conversion of substrate
is possible.
Safety problems:
When filling,
emptying and cleaning.
Continuous at
steady state
Fed-batch
Works all the time:
low labor cost,
good utilization of reactor.
Often efficient:
due to the
autocatalytic nature of microbial
reactions, the productivity can be
high.
When the production of the desired
compound is catabolite repressed,
continuous production or fed-batch
is obligatory
Automation may be very appealing.
Constant product quality.
Combines the advantages of batch
and continuous operation.
Excellent for control and
optimization of a given production
criterion.
Often disappointing:
Promised
continuous production for months fails
due to:
a.
Infection, e.g., a short
interruption of the
continuous feed sterilization.
b.
Spontaneous mutation of the
microorganism to a non-
producing strain.
Very inflexible:
can rarely be used for
other productions without substantial
retrofitting.
Downstream:
The downstream
process equipment must be designed for
low volumetric rate, continuous
operation-unless holding tanks are used.
Some of the disadvantages of both
batch and continuous operation-but
advantages far outweigh the
disadvantages, and fed-batch is used to
produce both biomass (baker’s yeast),
industrial enzymes and antibiotics.
Substrates from the gas phase are transferred to the liquid phase by spargers or through high
efficiency nozzles. Coalescence of gas bubbles is minimized by forcing the gas- liquid dispersion
through static mixers or by redispersion in the vicinity of the mechanical mixers as further
discussed in Chapter 10. The numerous patents describing the placement of mechanical mixers,
of air lift reactors, and reactors with forced flow through static mixers or with rapid recirculation
of liquid, injection of gas in the high- pressure recirculation loop and dispersion of the gas liquid
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