Measurements of impeller speed, power consumption and gas holdup
were combined with visual observations of the extent of gas dispersion
and particle suspension in three phase systems for a wide size range of
fully baffled, agitated vessels (tank diameters from 0.29 to 1.83 m).
Several common impeller geometries were examined and a minimum
mixing condition with regard to gas dispersion was specified for each
type. Where the impeller had previously been characterized (e. g. the
six-bladed disc turbine) in gas-liquid systems, this minimum condition
coincided with those generally accepted in the literature.
The minimum mixing condition with regard to particle suspension was
taken to be when no particle remained at rest on the base for more than
one to two seconds. A system was defined as efficiently mixed when
both gas dispersion and particle suspension criteria were. simultaneously
satisfied for the minimum power input.
The effect of well-suspended particles on the gas-liquid hydrodynamics
in the vessel was negligible, but if large quantities of
particles were settled out on the vessel base, gas dispersion was
affected. On the other hand, aeration had an adverse influence on
particle suspension, and increased impeller speeds and powers were
necessary to maintain the particles in the just-suspended state if the
system was sparged with gas. Consequently the result of aerating a
system which was operating at the ungassed just-suspended condition was
sedimentation of the solids. The severity of this sedimentation was
dependent on impeller geometry, and at one extreme could result in
complete sedimentation of all the solids for a very small change in
operating conditions. A qualitative method is presented which gives
an indication of the likely extent of sedimentation.
Of the impellers investigated, the disc turbine and mixed flow
impeller pumping upwards are shown to provide the most stable and
efficient operation at high gas rates. A tentative procedure for
designing a disc turbine agitated system is proposed and is supported
by data collected over the whole vessel size range. A novel theoretical
approach to estimating gas-liquid mass transfer coefficients is
utilised to produce data which support the use of disc turbines and
mixed flow impellers pumping upwards as the most efficient impellers
in a three phase system