Scale-up of Bioprocesses
515
a)
What diameter of the Prochem impeller will give the same unaerated power consumption as the
Rushton turbine? The values of
Np
for the Rushton turbine and the Prochem impeller are 5.2 and 1.0,
respectively.
b)
How much higher (approximately) will the A^-value be for the Prochem impeller at an aeration rate
of 0.5 wm under the conditions in problem a, (i.e. the same unaerated power consumption is
achieved)? The
k&
value for the same specific power input has been found to be identical for the two
impeller types. Assume a non-coalescing medium.
Problem 113. Design of a pilot plant bioreactor
In connection with the purchase of a new pilot plant bioreactor (41 L) to be used for penicillin fermentation, it
is desired to examine whether one of the manufacturer's standard-design bioreactors (equipped with Rushton
turbines) can be used. The dimensions of this bioreactor are specified in the table below.
Aspect ratio, d/d*
3
Tank diameter, d,
0.267 m
Stirrer diameter, 4
0.089 m
Number of impellers’
3
Maximum
stirring
speed,
N
1'max
600 rpm
The impellers are six-bladed Rushton turbines.
a.
Show that with a non-Newtonian medium, for which the rheology is described by a power law
expression (Eq. 11.21), Res is given by Eq. (1)
Re
S
P ^ d )
K k”~l
(
1
)
where
k =
10 is the constant used for calculation of the average shear rate in Eq. (11.24). Plot
Res
versus the stirring speed for a medium containing respectively 0, 20, and 40 g L'1
biomass (of the
fungus
P. chrysogenum).
The power law parameters can be taken from Fig. 11.10.
Discuss the
results.
b.
Determine the power number with
N =
600 rpm for the three cases considered in (a), using Fig.
11.10, and calculate the power input per unit volume when the bioreactor contains 25 L of medium.
Calculate the average viscosity in the bioreactor.
c.
For non-Newtonian media and the 41 L bioreactor, the following correlation was found:
ksa
0.226 -lQ-V
(
2
)
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