Three phase mixing : studies of geometry, viscosity and scale.
One-, two- and three-phase mixing systems have been extensively
studied. The experiments were performed in a range of standard
(baffled) vessel geometries of diameter varying between 0.22 - 1.83 m.
Rushton disc turbines and mixed flow impellers (both pumping directions)
were fully investigated. Water and glucose solution (~ - 120 mPas) were
For single phase systems, the work has shown that the power number
of disc turbines depends on disc thickness and scale of vessel. For the
mixed flow agitators the power number is dependent upon the blade
thickness and (D/T) ratio. Correlations enabling the ungassed power
number to be calculated as a function of these parameters are given.
For gassed systems, the power drawn by each type of impeller is
explained by local impeller hyarodynamics (cavity structure) and the
bulk flow pattern. The fiooding-Ioading transition (NF) and the
complet~ dispersion condition (NeD) have also been studied. A large
mixed flow impeller (6MFU45 ; D - T/2) with a large ring sparger is the
most energy efficient at NF and NCD speeds as compared with the other
geometries studied and correlations enabling the prediction of NF and
NCD for all geometries studied are presented. Hold-up correlations are
also given for each impeller firstly as a function of specific energy
dissipation rate and superficial gas velocity and secondly as' a function
of agitator speed and volumetric gassing rate. For each impeller, each
method is equally good statistically for scale-up but the latter is more
explicit. All impellers give approximately the same hold-up under equal
specific power inputs and superficial gas velocity but there are small
but statistically significant differences. These differences are
For solid-liquid systems, correlations in the literature for the
calculation of the minimum speed to just suspend solids, NJS ' are tested
for each system geometry with glass Ballotini particles. The
correlation proposed by Chapman et al. is shown to fit the present
experimental data best. The specific power input per unit mass (ET)JS -
constant, is proposed as a scale-up criterion for solids suspension.
Large 6MFD45 (D - T/2) is the most energy efficient for suspension but
6MFU45 (D - T/2) is only slightly worse.
In the three-phase mixing systems, the 6MFD
, D - T/2, is most
energy efficient for solid suspension (ET)JSg' at low gassing rates (up
to 1 vvm) but exhibits large flow pattern and torque fluctuations.
Above _ 1 vvm, 6MFU45 (D = T/2) becomes the most energy efficient for
solid suspension. In addition the minimum impeller speed for solid
suspension NJSg for this impeller is almost independent of gassing rate
and gives very stable flow patterns and torque. output throughout the
whole gassing range. Again (eT)JSg - const is the recommended scale-up
criterion for solids suspension under gassed conditions. Large (D -
T/2) impellers are found to be more energy efficient and correlations
for predicting N 45 45 .
for 6DT. 6MFD and 6MFU are obta~ned.
Increase in liquid viscosity has a rather small effect on gas
dispersion. Up to 120 mPas: (N) Q:I (N ) and (N ) Q:I F viscous F water CD viscous
(NCD) water uO.06 On the other hand, viscosity has a significant effect
on NJS and 3 to 5 times more energy is required for solid suspension at