Multiphase Bioreactor Design

bottom clearances (i.e. the distance between the top of the draught tube, and the liquid
surface and between the bottom of the reactor and the bottom of the draught tube,
respectively) (Merchuk, et al., 1994; Pollard et al., 1996) as well as on the introduction of
movable parts (Pollard et al., 1997; Sisak et al., 1990) or static mixers. In order to
adequately design flocculation bioreactors, these studies must be extended to high solids
loads.
In terms of flocculation bioreactor performance, the inclusion of static mixers has a
particular importance. Experiments made with a 60 L external loop airlift reactor in the
presence and in the absence of static mixers (Rüffer et al., 1995) showed that these
mixers are responsible for a reduction of the velocities of the liquid and gas phases,
increasing therefore the circulation time; there is also an increase of the radial and axial
liquid dispersion for lower aeration rates but not for higher aeration rates. As expected,
static mixers also increase the shear stresses inside the bioreactor and those increased
stresses may be used to decrease particle size if the particles are sufficiently fragile.
Knowing that, static mixers were introduced in the riser of a three-phase airlift bioreactor,
in order to diminish the floc size during fermentation with a highly flocculent strain of S.
cerevisiae (Vicente et al., 1999), and the average floc diameter decreased from 3 mm to 1
mm. As discussed later, this had favourable consequences in terms of a decrease in mass
transfer limitations and an increase in overall system productivity.

Influence of the solid and liquid phases

In order to increase biomass performance using yeast cell flocs, it is of crucial importance
to characterise the properties of the solid phase (particularly solid phase hold-up) and the
way it affects the hydrodynamics of flocculation bioreactors. However, no real data are
available for flocculation bioreactors and so far, even when systems with similar
properties have been used as models, only a few of them deal with solids loads as high as
those found in flocculation bioreactors. Studies where gel beads (with a density similar to
that of the flocs) are used as solid phase are particularly interesting, since the beads are
easy to prepare and to handle and are, therefore, a very convenient model system.
The addition of solids may promote either bubble coalescence, bubble break up, or
both, this effect being dependent on the concentration and size of the solid particles; this
has consequences on the gas-liquid mass transfer efficiency (Smith and Skidmore, 1990).
Also gas hold-up is affected: a reduction is observed when solids are added at low gas
flow rate, but no effect is reported if the gas flow rate is high (Siegel et al., 1988). In
terms of superficial liquid velocity, the effect of solids can be negligible whereas their
effect on circulation time may depend on the liquid phase; circulation time increases with
solids for distilled water but decreases e.g. for carboxymethyl cellulose (CMC) solutions.
Liquid velocity and gas hold-up decrease were related to an increase in the particle
diameter or increasing solids load (Lu et al., 1995) in an internal loop airlift (containing
Ca-alginate beads of 1 mm to 3.6 mm in diameter) and Ganzeveld et al. (1995) using split
cylinder airlift bioreactors in order to study the effect of solids load (solid phase: cell
microcarriers of 0 kg·m−^3 to 30 kg·m−^3 , with a particle size of 150 μm to 300 μm in
diameter and density of 1030 kg·m−^3 to 1050 kg·m−^3 ) showed that, in the range of the
solids load studied, increasing solids load provoked a decrease in liquid circulation
velocity and an increase in mixing time.

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