Biomedical Engineering Reference
In-Depth Information
u large ¼ u small D i small
D i large ¼ 400O48:42595 rpm
¼ 8:26 rpm
(E18-1.6)
(c) For constant Reynolds number.
i
i
ðuD
Þ small ¼ðuD
Þ large
(E18-1.7)
! 2
D i small
D i large
¼ 40048:42595 2 rpm
u large ¼ u small
¼ 0:17 rpm
(E18-1.8)
Scale-up on the basis of constant P 0 /V would be the most likely choice unless the culture
was unusually shear sensitive like mammalian cells, in which case constant impeller tip
speed may be used. Scale-up based on constant Reynolds number would usually not
be used.
18.5. SCALE-DOWN
Scale-down models provide a small-scale experimental system that duplicates exactly the
same heterogeneity in environment that exists at the larger scale. At the smaller scale, many
parameters can be tested more quickly and less expensively than at the production scale.
Scale-downmodels are also used to demonstrate proof of process during technology transfers.
In many cases, scale-up will require using existing production facilities, so it is important to
replicate the production facilities at a smaller scale. They can be used to evaluate proposed
process changes for an existing operating process and can be used to estimate the system's
response (growth rate, product formation, formation of contaminating by-products) to
changes in medium composition (new supplier of raw materials), introduction of modified
production strains, use of different inoculum preparation, new antifoam agents, and for
testing for O 2 and CO 2 tolerances.
1 8.6. BIOINSTRUMENTATION AND CONTROL S
The maintenance of optimal conditions for product formation in the complex operating
environment of a bioreactor requires the control and measurement of at least a few param-
eters. Almost all fermentors have pH, temperature, and DO control. In addition, fermentor
pressure control is also common. A balance of functional benefit of control versus a signif-
icant increase in the probability of contamination must be achieved. Instrumentation must
be sterilizable, preferably with steam. They must be able to withstand high temperatures
(121 C) in the presence of high humidity (100%). Chemical sterilization may be used to
allow for temperature-sensitive devices but is less desirable. Instrumentation must be
compatible with the designed limitation of sterility. In addition to the instrumentation
identified above, the following parameters are also monitored and controlled for the
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