Multiphase Bioreactor Design

Figure 12.3 Alternative process lay-

out: water flows counter-current with

respect to the droplets flow

as hold-up, were not determined; only visual observations were made on this operating
strategy. Provided the solvent flux was higher than a minimal value, this three-phase
system was stable for at least 24 hours.
As the droplet column with suspended gel beads worked well without any water flow,
we also tried to operate the set-up with water flowing counter-currently, see Figure 12.3.
The water flux puts a downward directed force on the gel beads, whereas the force put on
the gel beads by the organic solvent flux is directed upwards. Obviously, when the water
flux becomes too high, the gel beads will settle. Again, we only made visual observations
for this set-up. These observations can be summarised as follows:

− at a fixed water flux, the organic solvent flux has to have a minimal value in order to
make this system work. Increasing the organic solvent flux resulted in a larger bed
height, i.e. a lower gel-bead hold-up;
− at a fixed organic solvent flux, higher than the minimum flux required for stable
operation, the water flux can be increased to a maximum value. At higher values the
system will not work, i.e. gel beads settle, and droplets coalesce.

This operating strategy gives a couple of advantages over a co-current strategy. In
extraction processes counter-current operation is always better than co-current operation,
as there is always a higher driving force for mass transfer. In co-current operation the gel-
bead hold-up decreases with an increasing medium flux, whereas in counter-current
operation the gel-bead hold-up increases with an increasing medium flux. It is also
observed that at a higher organic solvent flux a higher water flux can be applied; this
means that the throughput of substrate can be higher in counter-current operation.

Loop reactors

Another type of bioreactor is the so-called loop reactor: these reactors consist of two
tubes connected to each other; internal and external designs exist (van ‘t Riet and
Tramper, 1991). In order to function, there has to be a difference in density of the mixture

Multiphase bioreactor design 358