used for fluidising the biocatalytic particles. An immiscible solvent is pumped through a
sparger, and the droplets formed rise through the fluidised bed. If coalescence of the
droplets is easily achieved, which might be a prerequisite for good operation, both liquids
are separated at the top of the column and sent back to their respective storage vessels. If
separation between both liquids is not achieved by a simple one-stage gravity settler, the
two-phase mixture can be separated outside the column.
Figure 12.2 Process lay-out for three-
phase fluidised-bed bioreactor
If the biocatalyst requires oxygen, than aeration of one or both storage vessels will be an
adequate method. Temperature control of the bioreactor can be done by keeping the
temperature of both storage vessels constant. pH is controlled by keeping the pH of the
substrate storage vessel constant.
Operating the bioreactor in this manner is a batch operation; accumulation of the
product in the organic phase and depletion of the substrate will occur. A continuous
operation with respect to the substrate phase can be achieved by adding a high substrate
concentration to the storage vessel, keeping the substrate concentration constant. To
prevent overflow of the storage vessel part of the circulating medium flow has to be
purged, see also Figure 12.2.
Operating the bioreactor with a continuous supply of substrate will still result in the
accumulation of the product in the organic phase. Operating the organic phase
continuously is easily done by sending the total out-flowing organic phase to the recovery
section, and pumping purified organic phase into the column. This strategy will be
disadvantageous when the product concentration in the organic phase is low; a large
volume of organic solvent has to be clarified. A better strategy would be to allow
accumulation of the product to a certain concentration, below the equilibrium
concentration, and to send part of the out-flowing organic phase to the recovery section,
see Figure 12.2.
In the above discussion of a plant lay-out and a process operation for a three-phase
fluidised-bed bioreactor, it is assumed that both liquid phases flow upward co-currently.
This is true when the organic phase has a smaller density than the medium phase. When
Multiphase bioreactor design 356