the column with particles. This set-up will work if the particle density is higher than the
water density. A fluidised bed will be observed, if the water flow is higher than the
minimum fluidization velocity.
Part C of Figure 12.4 shows a three-phase mixture in one column and medium in the
other column of the loop reactor. The density of the organic solvent is less than the
medium density and solvent droplets will rise. A three-phase fluidised bed will only
exists when the density of this mixture is less than the density of the medium in the other
column. In that case water will flow from bottom to top in the three-phase column. For
example, in our laboratory experiments have been done on the hold-ups in a three-phase
system, i.e. water, dodecane and gel beads (density equals 1060 kg/m^3 ). At a water flux
of 1.8×10^2 m/s and a dodecane flux of 0.91×10^2 m/s, a gel bead hold-up of 0.20 and a
dodecane hold-up of 0.08 was measured. This gives a mixture density of 992 kg/m^3. The
density difference between both columns is in this case large enough to obtain this water
flux of 1.8×10^2 m/s (Van Sonsbeek et al., 1990).
A different situation exists when the density in the three-phase system is larger than
the density of the medium in the other column, due to a higher solids hold-up or a lower
organic solvent hold-up. In this case water will flow from top to bottom. Provided the
density of the solids is close to water, this configuration can also work (see the previous
section “Counter-current water flow”).
The column with only medium can be aerated. However, aeration results in a decrease
of the mixture density in this column, and the circulation velocity of the medium between
both columns is changed. In order for a three-phase fluidised bed to exist, the aeration
must be carried out carefully. Aeration results in an air hold-up, and hence a smaller
mixture density in the aerated column. Consequently, the density difference between both
columns becomes less, and the circulation velocity decreases. At this smaller medium
velocity a three-phase fluidised bed must still exist.
Part D of Figure 12.4 is the same as part C, but in this case the density of the organic
solvent is larger than the density of water, hence droplets settle. Water will always flow
in the direction indicated in this figure; from top to bottom in the three-phase column.
Only if the biocatalytic particles have a density smaller than water, i.e. they rise in the
column, will this configuration work.
So far it is assumed that the particles remain in a fluidised state. However, the water
circulation velocity can be so high that it exceeds the terminal settling velocity of a single
particle. In that case particles flow together with the water, and are present in both
columns. This principle is widely used in air-lift bioreactors (Heijnen et al., 1997).
PERFORMANCE OF A CONVENTIONAL BIOREACTOR AND A
THREE-PHASE FLUIDISED-BED BIOREACTOR
In the preceding section possible reactor configurations have been discussed. Whether
such a three-phase reactor will work better than a conventional reactor is the topic of this
section. We decided to compare a two-phase fluidised-bed bioreactor, the conventional
reactor, with a three-phase liquid-liquid-solid fluidised-bed bioreactor. Comparing two
different reactors, one should consider a fair comparison. This comparison can be based
on cost, but in that case one should consider total cost, i.e. cost of a total production plant.
Multiphase bioreactor design 360