Scale-up of Bioprocesses
487
by external pumps in e.g. airlift feimentors. The power input for stirring is a major cost for
aerobic bioprocesses, and it depends on several factors. Most important are the stirrer rate,
N,
the
impeller diameter,
ds,
the density of the fluid,
pi,
and the viscosity of the fluid,
rj
. By dimensional
analysis (see e.g. Geankoplis, 1993), it is possible to combine these factors into dimensionless
numbers, and the power consumption for stirring is normally expressed as a function of the
dimensionless
power number, Np
(sometimes also called the Newton number) defined as
The power number can normally be expressed as a function of the Reynolds number,
Res,
defined by Eq. (11.
6
).
The variation of
Np
with
Res
is shown schematically in Fig. 11.6. The flow in a stirred tank
reactor is fully turbulent if
Res
defined in Eq. (11.6) is larger than 104. This is true in most cases
( 11.6)
10 0
turbulent
region
1
J— -----i--------- ---------- ----------
1
--------- -
1
10
1
02
1
03
1
04
1
05
Reynolds number, Re,
Fig 11.6. Graph showing schematically the variation of the power number,
Np,
with Reynolds number,
Re„
for a bioreactor equipped with one six-bladed Rushton turbine impeller and baffles
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