circulating as well as to optimise the hydrodynamic conditions inside the reactor. All the
above-mentioned parameters are, in turn, influenced by the design of one or several of the
parts that compose an airlift. The next sections are intended to give an idea of how those
parameters are affected by reactor design and hence to give clues for the design of a
flocculation bioreactor.
The effect of solid and liquid phases will be dealt with as well, in order to show their
possible influence on reactor behaviour. Solids’ characteristics such as density, size,
shape and surface properties may change considerably the performance of the bioreactor,
the same happening with the liquid phase properties.
As one of the main features of high cell density systems is the high hold-up of the
solid phase (that can go up to 50–60% v/v of the total bioreactor volume, with the
corresponding reduction in liquid-phase volume) a significant research effort has been
devoted to optimise the design of several parts of three-phase airlift reactors, namely in
those aspects related to their use as high cell density systems. As previously stated the use
of flocculent microorganisms requires a great deal of attention when designing a reactor
in order to retain flocs with the suitable characteristics for the process (shape, density and
size, mainly).
Gas-liquid separatorThe gas-liquid separator is the region at the top of an airlift reactor where riser and
downcomer are connected. In systems operating with flocculating cultures, the design of
this zone is a key factor, as its correct dimensioning is crucial in assuring biomass
retention inside the system, particularly in continuous cultures. As a matter of fact, it has
a major influence on the entire behaviour of the reactor; the gas recirculation rate is
greatly affected by the gas-liquid separator’s geometric configuration and by the liquid
level in the separator, influencing the stability of the reactor (Siegel et al., 1986). This
section can be used to change the operating characteristics of an airlift reactor, making it
possible to achieve operating patterns ranging from those of internal loop to those of
external loop airlift reactors (Siegel and Merchuk, 1991). This may be important in
giving more flexibility to this type of reactor.
Several studies have been done, characterising the effect of the design of this section
of airlift bioreactor on hydrodynamic characteristics such as liquid circulation velocity,
gas hold-up and liquid mixing, thus showing its importance. Russel et al. (1994) carried
out yeast fermentations in a concentric tube airlift reactor and measured the influence of
the top section height, which had a significant effect on liquid mixing by decreasing the
mixing time as it was increased; liquid velocity and gas hold-up, on the other hand, were
found to be independent from the top section height. Both the geometry and the design of
that section, on the contrary, were found to have effects on hydrodynamic performance
and oxygen transfer behaviour of airlifts by changing the liquid velocity, the gas hold-up
in the downcomer, the mixing time and the overall volumetric gas-liquid oxygen transfer
coefficient (Choi et al., 1995). However, data describing how this section affects biomass
retention in high cell density systems, particularly flocculation bioreactors, are scarce.
The introduction of an enlarged degassing zone in the top section (usually just an
unbaffled extension of the riser and downcomer (Siegel and Merchuk, 1991)) of an airlift
reactor not only enhances gas disengagement but also allows for a better solids settling.
Multiphase bioreactor design 392