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3.5 Agitated Slurry Reactors 137

For single-impeller systems, the correlation of Calderbank can be used (Bouaif i et al .,
onen 2001; Laakk et al ., 2005):

35 h 0.5
d bub 4.15 L G 0.0009
P  g  25 (3.263)
35
L  V  L  

These correlations have been derived for Ruhston turbines, but they can be applied for flat-
blade impellers as well (Laakkonen et al ., 2005). SI units should be used in these correlations.

Double-impeller combinations Bouaifi et al v wing correlations . (2001) deried the follo
for stirred gas–liquid reactors with various combinations of double impellers. The
impellers used were the lightning axial flow impeller (A-310), the four 45° pitched blade
turbine pumping down (PBTD) and the Rushton disk turbine (RDT). Furthermore, the tank
fles,
was a dish-bottom cylindrical tank equipped with four baf while the gas was intro-
duced by a ring sprager. The gas-flow rate ranged from 0.54 to 2.62 L/s, whereas the rota-
tional speed was from 1.66 to 11.67 s –1 . The gas holdup is
P  g  0.24
h G C  V  L   u sG 0.65 (3.264)

where h G is in %, P / V L in W/m 3 , u sG 0.018 m/s, and C is between 22.4 and 24.8.
g
The gas holdup was found to be lower than 10% as in the case of simple impeller conf ig-
urations (Panja and Phaneswara Rao, 1993). This correlation holds for operating condi-
tions beyond the complete dispersion point, corresponding to the situation of
homogeneous gas dispersion in which bubbles reach the bottom of the tank. It should be
noted that in the case of agitated v it is dificult to propose a general correlation to f
essels,
predict gas holdup, because it is dependent on the stirrer configuration and sparger type
(Bouaifi et al ., 2001). Furthermore, gas holdup is expected to be influenced by the solids
in three-phase systems. The Sauter mean diameter is affected by the specific power con-
sumption, but not by the gas flow rate (Bouaif i et al ., 2001). For double-impeller combi-
nations (Bouaif i et al ., 2001),

P  g  0.20
d 10.1 10 3 (3.265)
bub  V  L  

where d bub is in m. SI units are used with this correlation.

The effect of solids
In general, the addition of solids influences the b ubble diameter due to the greater num-
ber of bubble collisions resulting in increased coalescence, and thus, in an increase in b ub-
ble size (Panja and Phaneswara Rao, 1993).