Biomedical Engineering Reference
In-Depth Information
and minimizing the number of openings
which improves the chances for maintain-
ing sterility. All surfaces within the vessel
must be smooth and are often electropol-
ished. O-rings are used for small openings,
typically for probes used in process
control. Gaskets are used for larger open-
ings such are piping connections to the
tank and the agitator mounting flanges.
Agitators are equipped with seals to
provide integrity between the vessel and
outside environment. Double mechanical
seals, with steamquench, are often utilized
on microbial and animal cell culture appli-
cations. Vessels are designed to be free
draining and cleanable. And ultimately,
bioreactor systems must be designed to
be steamable in place. Bioreactors are
designed to be steamable from the top,
down, so condensate cannot collect and
compromise the heat kill or organisms.
Fig. 18.4
illustrates a bubble column
reactor which provides a low shear envi-
ronment for the culture. These columns
are suitable for low viscosity Newtonian
broths. The structural packing (of high
porosity) can provide high contact area,
keeping bubbles small or breaking the
bubbles. They have higher energy efficien-
cies than agitated vessels in terms of the
amount of gas transfer per unit power.
However, bubble column reactors may be
limited by foaming and bubble coalescence
and are generally less flexible than stirred-
tank bioreactors. The multistage arrange-
ment (separated by structural packing)
alleviates the bubble coalescence problem,
further increasing mass transfer efficiency.
Fig. 18.5
shows a concentric draught-
tube bioreactor with annular liquid
down flow (loop reactor or airlift reactor).
Loop reactors have intermediate charac-
teristics between bubble columns and
stirred-tank bioreactors. The fast upflow-
ing air in the center draught tube carries
Liquid Feed
Exhaust gas
Structural packing
stages
Gas Supply
Valve
Product
FIGURE 18.4
Bubble column reactor.
Liquid Feed
Exhaust gas
Gas
Supply
Valve
Product
FIGURE 18.5
Internal airlift loop reactor.
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