514
Chapter 11
This is a nice example of a reactor design based on a preliminary identification and separation of the different
subtasks to be carried out by the system (see Table 11.1).______________________________________
P R O B L E M S
Problem
11.1.
Scale-up without maintaining geometrical similarity.
In example 11.2, the scale-up of a stirred tank bioreactor from 1
L to 1
m3
was considered. In the example
it was found that the mixing time increased by a factor of 5, if the same geometry was maintained during
scale up. Suppose now instead that the diameter of the 1
m3
reactor is chosen to 0.187 m (i.e. the reactor
geometry is changed), but the requirement of a constant
P/V
(= 5.95 kW m ”3) is maintained.
a)
How much larger is the mixing time in the large-scale bioreactor now?
b)
What value of
P/V
would give the same mixing time in the large-scale reactor as the small-scale
reactor?
Problem
112,
Exchanging impellers
One drawback of the traditional Rushton impeller is that the ratio between aerated and unaerated power
consumption falls rapidly with an increased aeration rate. The hydrofoil impeller, typically has a lower power
number, but the aerated power consumption falls less rapidly with increasing aeration rate.
For stirring
speeds close to 300 rpm the ratio between aerated and unaerated power consumption is given in the figure for
a Rushton turbine and a Prochem impeller.
You are replacing a Rushton turbine in a 100
reactor (<?== i nx
d}=
1
m) by a Prochem impeller. Assume
that the stirring rate should be maintained the same.