438
Chapter 10
db.mas =
2.83'10"3 m
(7)
Again we find some deviation among the various correlations. The order of magnitude is, however, the
same, and this is often sufficient for design purposes because of the large uncertainties of all the
calculations. If the initial bubble diameter is 13.8 mm as found in Eq. (5), the breakup processes result in a
rapid disintegration of the bubbles formed at the orifice, and we therefore do not observe the large bubbles
formed right at the orifice when the dispersion is agitated._________________________________________
10.1.3. Empirical Correlations for Jt/tf
A large number of different empirical correlations for the volumetric mass transfer coefficient
kfi
have been presented in the literature (for a review see e.g. Moo-Young and Blanch, 1981). Most of
these correlations can be written in the form
ktad
(10.27)
which has a great similarity to Eq. (10.26) for the specific interfacial area. The parameters tend to
depend on the considered system, i.e. the bioreactor design. Thus for different stirrers (see Chapter
11) and different tank geometry the parameter values may change significantly, and a certain set of
parameters can be safely used only when studying a system, which resembles that from which the
parameters were originally derived. Some parameter values reported in the literature for stirred
tanks are listed in Table 10.4.
Normally, the correlation in Eq. (10.27) holds independently of whether mixing is performed
mechanically in stirred tanks or pneumatically in bubble columns. Thus for the same power input
per liquid volume, the magnitude of
kp
is approximately the same in a stirred-tank reactor and in a
bubble column. It is, however, possible to obtain much higher power input in stirred-tank reactors
Table 10.4 Parameter values for the empirical correlation given by Eq. (10.271
Medium
k
a
p
Aeitator
Reference
Coalescing
0.025
0.5
Ô.4
Six-bladed Rushton turbine
Moo Young and Blanch
(1981)
0.00495
0.4
0.593
Six-bladed Rushton turbine
Linek
et al.
(1987)
0.01
0.4
0.475
Various agitators
Moo-Young and Blanch
(1981)
0.026
0.5
0.4
Not specified
van’t Riet (1979)
Non-coalescing
0.0018
0.3
0.7
Six-bladed Rushton turbine
Moo-Young and Blanch
(1981)
0.02
0.4
0.475
Various agitators
Moo-Young and Blanch
(1981)
0.002
0.2
0.7
Not specified
van't Riet 09791
Parameter values are specified in SI units, i.e., the power input is in W m 3 and the superficial gas flow rate is in m s'1.
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