As stated before, biomass retention in flocculation bioreactors is strongly improved
when an enlarged top section is used as it permits better gas disengagement as well as
enhanced solids settling. Using a 6 L airlift bioreactor with an enlarged top section, it was
observed that both the critical airflow rate and the mixing time increased with the solids
load, while the circulation time presented a maximum for a load of solids between 5%
and 10% (v/v) (Vicente and Teixeira, 1995). Also, small changes in solids density (Ca-
alginate beads, 1016 kg·m−^3 to 1048 kg·m−^3 ) were shown to have a significant influence
on the critical airflow rate and on the mixing time. This is meaningful since the density
range used includes that of yeast flocs (Vicente et al., 1999), making it possible to have
an idea of how a flocculation bioreactor will behave in identical situations. Nevertheless,
the relatively small scale used (6 L) might have some influence in the results, so a 60 L
concentric tube airlift bioreactor with an enlarged top section was tested instead (Freitas
and Teixeira, 1997). The effects of solids load (Ca-alginate beads, with solids fractions
varying from 5% to 30%) and density (1016 kg·m−^3 and 1038 kg·m−^3 ) on gas and solids
hold-up in the riser and the downcomer, on circulation and mixing times and on the
interstitial liquid velocity were studied. It has been concluded that an increase in solids
load and density causes an increase in riser and downcomer solids hold-up but provokes a
decrease in the riser and downcomer gas hold-up. As before, circulation and mixing times
decrease with the increase of airflow rate. It has also been confirmed that, while
circulation time is practically independent of solids load and density, mixing time is
strongly influenced by these two variables: it increases with solids load up to 20%,
decreases with higher values and always increases with solids density. Increments of the
airflow rate, solids load and solids density also cause an increase in the interstitial
velocity.
A full characterisation of solid phase distribution inside the bioreactor was also done.
The system (the same 60 L airlift bioreactor—see Figure 13.2) was divided into eight
different sections (Freitas and Teixeira, 1998a) and, by using a special sampling device
(Freitas et al., 1997), it was observed that the solids hold-up increased, in general, from
the wall to the middle of the separator (sections 1 to 3 and 4 to S) and from the top to the
bottom of the reactor (sections 3, 5, 6 and 7/8). For all tested solids loads and gas flow
rates, solids hold-up in the separator has been found to be lower than that in the riser or in
the downcomer, thus proving the efficiency of this system in what concerns solids
retention.
The effect of the presence of ethanol in system hydrodynamics is particularly relevant,
since many applications of flocculation bioreactors deal with ethanol fermentation. When
water is replaced by an ethanol solution (10 kg·m−^3 ), a reduction in the surface tension
occurs and changes significantly the response of the reactor in terms of the gas and solids
hold-up in both riser and downcomer, the circulation and mixing times and the riser and
downcomer interstitial velocity (Freitas and Teixeira, 1998b). In fact, an increase of the
riser and downcomer gas hold-up is registered, together with the consequent decrease of
solids hold-up in those sections and the resulting decrease of riser and downcomer
interstitial velocity. However, the difference between gas and solids hold-up in the riser
and in the downcomer remains practically constant when ethanol is added; the driving
force for the circulation is thus maintained and so the presence of ethanol, while
increasing mixing time, has no measurable effect on circulation time.
Flocculation bioreactors 395