512
Chapter 11
Table 11.10. Characteristic times during a penicillin fermentation carried out in a41-L pilot plant bioreactor.
Characteristic
Growth phase (s)
Production phase (s)
time
tjn
1.5
1.5
tgm
60.3
60.3
totr
22.2
22.2
tsa
95.6
126.7
toe
11.5
12.4
tsc
189.3
102.2
was intended to keep the glucose concentration at a constant level by controlling the feed addition. Also, the
characteristic times for oxygen supply
t^,
and oxygen consumption,
t^,
are of the same order of magnitude.
This indicates that oxygen limitations may occur if the oxygen supply for some reason fails or the oxygen
requirements increase (e.g., due to an increasing feed addition of glucose). Thus the gas liquid mass transfer
seems to be the limiting regime for this process, and this conclusion would be even more pronounced in a
production-scale vessel.
It is understandable that
rules o f thumb
(i.e. practical “one-liners”) are much desired by the plant
engineer. Although all parameters cannot be maintained constant (cf. Table 11.8), there has been a
widespread belief that one could (and perhaps even should!) choose one to be maintained constant
during scale-up. Most often the specific stirrer power input, or the
k&
value, has been suggested for
this purpose. However, there are strong reasons to caution against these simple scale-up criteria.
There is very little theoretical support that a successful scale-up will result from such simplistic
procedure (Oldshue, 1997). In fact, it is more than likely that in the successfully scaled-up process
no parameter values will be exactly the same as in the small-scale or pilot-scale reactor. In fact, not
only may the parameter values change, a completely different reactor design may also be preferable
(see Example 11.6). Fig. 11.16 is offered as a final help for the difficult task of scale-up. If the
qualitative procedure of Figure 11.16 is followed
one may hope to avoid most of the pitfalls
associated with scale-up.
Identify important
physical processes
and microbial
reactions
J Need for scale-down
)
experiments?
Figure 11.16. Important principal steps during scale-up.
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