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Encyclopedia of Physical Science and Technology EN002G-67 May 25, 2001 20:8

Bioreactors 265

FIGURE 24 Bioreactor operational modes: (a) a series of continuous flow well-mixed bioreactors, (b) use of different
feeds at different stages in a series of continuous flow well-mixed bioreactors, (c) a series of continuous flow well-mixed
reactors with biomass recycle.
location in a plug flow bioreactor operating at steady state; density and diameter of the viable and nonviable animal
however, the composition changes with position in the di- cells, the inclined channel sedimentor (Fig. 26b) may pref-
rection of flow. A plug flow bioreactor behaves the same as erentially retain viable cells in the bioreactor while al-
a series of many continuous flow well-mixed bioreactors lowing many of the nonviable ones to wash out. Another
(Fig. 24a). In terms of kinetics, a plug flow bioreactor is device that is commonly deployed for partial retention of
equivalent to a batch fermenter having the same residence animalcellsisthe“spinfilter”showninFig.27.Aspinfilter
time (i.e., the total time spent by the broth in the biore-
actor) as in the plug flow device. A plug flow bioreactor
for biomass production must be inoculated continuously,
or a portion of the harvested biomass must be recycled
to the inlet of the bioreactor (Fig. 25b), to prevent wash
out of the culture by the sterile feed. Multipoint feeding
is sometimes used in plug flow bioreactors, as shown in
Fig. 25c.
Continuous flow operation of a bioreactor such that the
cells are recycled or retained within the reactor, is some-
times known as perfusion culture. Many different kinds of
devices are available for cell retention and recycle. These
devices may be located within or outside the bioreactor
vessel. External sedimentation tanks (Fig. 23b) are of-
ten used to recycle biomass in wastewater treatment pro-
cesses. External and internal enhanced-rate sedimentors
FIGURE 25 Bioreactor operational modes: (a) a continuous plug
are also employed in animal cell culture bioreactors, as flow bioreactor, (b) a continuous plug flow bioreactor with biomass
shown in Fig. 26a, b. Because of the small differences in recycle, (c) multipoint feeding of a plug flow bioreactor.