the organic phase density is larger, this liquid has to be introduced at the top of the
column, and both liquids have to be separated at the bottom of the column.
Another assumption is that the three-phase fluidised-bed bioreactor is stable, which
means that the fluidised bed is in a hydrodynamic equilibrium; particles remain in the
column and the droplets do not coalesce in the fluidised bed. Experiments with different
kinds of gel beads have shown that a stable fluidised bed exists, at least, for gel beads
with a density of 1060 kg/m^3 and a diameter of 2.3 mm. Applying gel beads with a
density of 1010 kg/m^3 and a diameter of 2 mm in a three-phase fluidised bed resulted in
the wash out of the beads at any organic flow rate larger than 10−^4 m^3 /(m^2 s). It can be
concluded that a stable three-phase fluidised bed is possible for particles with a settling
velocity larger than 5.0 cm/s. Coalescence of droplets can be prevented by carefully
adjusting both flows.
AlternativesBelow two different alternatives are given for operating a three-phase liquid-liquid-solid
system, see also Figure 12.1.
Counter-current water flowA three-phase fluidised bed with both liquids flowing co-currently is not possible for
each type of particle. For particles with a terminal settling velocity larger than 5.0 cm/s a
stable bed is possible, but for particles with a velocity smaller than 2 cm/s a stable bed
could not be obtained (van Zessen, unpublished). We do not know whether a stable
fluidised bed is possible for particles with a terminal settling velocity between 2 and 5.0
cm/s.
A stable three-phase fluidised bed is possible for those particles with a settling
velocity smaller than 2 cm/s, if a different strategy is followed. In operating this strategy,
droplets rise from bottom to top and medium flows counter-currently, i.e. from top to
bottom, see Figure 12.3. A limited version of this strategy would be a case in which there
is no medium flow.
For bubble columns, it is known that gel beads can be fluidised—kept in suspension—
by the rising gas bubbles. As a liquid-liquid-solid three-phase fluidised bed is the liquid
analogue of a gas-liquid-solid three-phase fluidised bed, it should be possible to create a
droplet column with solids kept in suspension. Indeed, it has been possible to keep two
different kinds of K-carrageenan gel beads in suspension in a droplet column (van
Zessen, unpublished). These gel beads had a density of 1007.4 kg/m^3 and 1005.4 kg/m^3
and a diameter of 1.97 mm and 3.12 mm. However, to maintain the gel beads in
suspension a minimal solvent flux was required. Experimental data on hydrodynamic
parameters, such
Design of liquid-liquid-solid fluidised-bed bioreactors 357