Mass Transfer
435
the same concentration (Keitel and Gnken,
1982). Surface active compounds also strongly
influence the coalescence. Some (e.g., foam-stabilizing compounds like proteins) reduce the
coalescence, whereas others (e.g., antifoam agents like fatty acids) increase the coalescence.
Normally, surface-active materials reduce the mass transfer coefficient for the liquid film (i.e.,
k,)
and the overall effect of surface-active materials on the volumetric mass transfer coefficient
kp
is
therefore quite complex.
However, for foam-stabilizing compounds the increase in the specific interfacial area
a
(due to a
smaller average bubble diameter) is normally larger than the decrease in
kh
In most fermentation
media, the tendency for coalescence is smaller than for pure water. Water is therefore often called a
coalescing medium,
whereas many fermentation broths are
non-coalescing.
It is quite clear that the mechanism of coalescence is not yet fully understood (Craig
et ai,
1993).
Whereas the presence of certain ions in an aqueous medium reduces the coalescence, other ions
seem to have no influence at all.
The combined effect of bubble breakup and coalescence on the average bubble diameter is
determined by the relative rate of the two processes. If coalescence is very slow compared to bubble
breakup, the average bubble diameter is determined by the breakup process, i.e. by Eq. (10.23).
However, if the bubbles formed at the orifice are smaller than the maximum stable bubble diameter
(i.e.,
da < dbmax)
the average bubble diameter is determined by the bubble formation process, i.e.,
Eq. (10.20). On the other hand, if coalescence occurs rapidly, bubbles formed at the orifices
coalesce and grow larger until they exceed the maximum stable bubble size, after which bubble
breakup occurs. Since bubble breakup depends on the local velocities of the eddies, there are local
coalescence-breakup equilibria, resulting in a variation of bubble size throughout the bioreactor.
E
E,
k .
d>
«
E
n
'■£
*
* o
a"
A
* o
oA
«1
0,01
0,1
Ionic strength (moles L )
Figure 10.5 Mean Sauter bubble diameter as a function of ionic strength for aqueous solutions of the salts
AI2(S04)3 (■), Na2S04 (A), NaCl (o) and NaOH (* ). The data are from an air-water system in a bubble
column (Keitel and Onken (1982)).
previous page 457 Bioreaction Engineering Principles, Second Edition  read online next page 459 Bioreaction Engineering Principles, Second Edition  read online Home Toggle text on/off