Scale-up of Bioprocesses
A eration num ber,
Fig. 11.7. The ratio
shown (qualitatively) as a function of the aeration number for a six-bladed
Rushton turbine. The Froude number is given in Eq. (11.28). (Adapted from Ekato, Handbook of mixing
The cavities will grow with increasing
until eventually the impeller becomes fully immersed
in gas, a condition called flooding. The power number
gradually falls with increasing
manner shown by Fig. 11.7. Typically, a fluid foil impeller will have a significantly smaller drop
in the ratio
with increasing aeration rate than a Rushton turbine (see Problem 10.2).
An approximate empirical correlation describing the relation between aerated,
power consumption is (Michel and Miller, 1962):
where the value of
depends on the impeller type.
Power input in bubble columns
The power input by the aeration is normally small in stirred tank reactors. In a bubble column, on
the other hand, all mixing energy is supplied by the aeration. The compression power input for
aeration in a bubble column operating at moderate superficial velocities can be expressed as
(Chisti and Moo-Young, 2001)
Pcomp = gPl“sV
where g is the gravitational acceleration (9.82 m s'2).