These characteristics are especially useful in continuous three-phase reactors, where
solids retention is a very important issue. It was possible to retain much more biomass
during continuous fermentation with a highly flocculent strain of S. cerevisiae (NRRL
Y265) by the introduction of an enlarged top section in a concentric tube type airlift
bioreactor (Sousa et al., 1994a). The results were encouraging, reaching total biomass
concentrations of 78 kg·m−^3 (dry weight). In the absence of such an enlarged section, the
biomass would have been mostly dragged out of the bioreactor, since most of the flocs
would have not the time to settle and remain inside the vessel.
Downcomer and riser dimensionsThe downcomer to riser cross sectional area ratio (Ad/Ar) and the height of those sections
also have a very important influence in the performance of the reactor. Studies on three-
phase reactors are available and are a very useful contribution, as they provide insight
into the possible behaviour of flocculating systems, despite the fact that the solid phase in
such studies is usually not composed of cell flocs but plastic, gel or glass beads, typically
used as models. Effects on the general hydrodynamic behaviour are to be expected with
consequences on liquid circulation velocity, dispersion coefficient and gas and solid
holdup in each section of the bioreactor, which are directly influencing overall
parameters such as circulation and mixing times.
Kennard and Janekeh (1991) studied the mixing in a three-phase concentric tube airlift
bioreactor using 2.5−4 mm Perlag® solid particles. Although Ad/Ar has not been varied,
they have been able to conclude that this parameter might severely affect the operation of
an airlift reactor. In fact, the decrease of Ad/Ar ratio and the increase of reactor height
cause an increase of the liquid velocity with obvious consequences on the maximum solid
hold-up with which the reactor can deal without the occurrence of stalling.
Solid particles of diverse polymeric materials (around 2.8 mm in diameter and with
density values ranging from 1202 kg·m−^3 to 1381 kg·m−^3 ) were used to assess the
importance of the reactor size in three-phase internal and external loop airlift reactors’
behaviour (Kochbeck et al., 1992). Both the liquid velocity and the dispersion coefficient
were found to increase with reactor height, which is in agreement with other experimental
works (Verlaan and Tramper, 1987). Also, Lu et al. (1995) found an increase of the
liquid velocity with the riser height in three-phase internal loop airlift reactors with
alginate beads.
Vicente and Teixeira (1995) investigated the effects of the Ad/Ar ratio and the riser
height on mixing and circulation times and on critical (minimum) airflow rate in a three-
phase airlift with an enlarged degassing zone. Alginate beads with two different density
values (1016 kg·m−^3 and 1048 kg·m−^3 ) were used as the solid phase. An optimal
(minimum) value for the circulation time and for the critical airflow rate was obtained for
a Ad/Ar ratio of 3.8. The minimum mixing time was obtained for a riser to downcomer
height ratio of 0.80.
Other design characteristicsThe design of other parts of the bioreactor may also affect its performance. In fact, there
are studies on air sparger’s design and location (Pollard et al., 1996), on the top and
Flocculation bioreactors 393