in both tubes. A density difference results in a pressure difference and water will flow
between both tubes: the direction of this water flow goes from a high to a low density.
This principle has been applied in air-lift reactors (Chisti, 1989). The liquid analogue,
droplets instead of gas bubbles, the so-called liquid-impelled loop reactor, has been
described by Van Sonsbeek (1992a). This type of reactor with particles, κ-carrageenan
gel beads, has been used by Mateus et al. (1996) and Vermuë et al. (1995) for
biotransformations.
Figure 12.4 shows four possible reactor configurations for thé application of a three-
phase system in a loop reactor. First, it is assumed that the particles remain in one
column, and are not circulating. Particles circulating between both tubes are discussed
below. In parts A and B of this figure, a bed of biocatalytic particles and a spray
extraction column are separated. In part C and D, conversion and extraction are fully
integrated.
In part A droplets of an organic solvent rise from the bottom to the top of the column
as their density is lower than the water density. Consequently, pressure is lower in the
spray
Figure 12.4 Loop reactors (see text for
explanation)
column, and water flows from top to bottom in the column with particles. If the density of
the particles is higher than the water density, a packed bed will be formed. If the density
of the particles is lower than the water density, and the water flux is larger than the rise
velocity of the swarm of particles, a fluidised bed will be formed. This so-called inverse
fluidisation has been studied by Fan et al. (1982).
In part B, droplets of an organic solvent settle from the top to the column, as the
density is higher than the medium density. In this case, water flows from bottom to top in
Design of liquid-liquid-solid fluidised-bed bioreactors 359