Several of the design parameters discussed in the previous sections as well as the
presence of solids may affect gas-liquid oxygen transfer and that has been the object of
several studies.
Influence of the solid and liquid phases
While studying the influence of solid phase, a widespread procedure is to replace
immobilised biocatalysts (and microbial aggregates) by inert solid particles, which are
easier to handle and pose no problems concerning biomass growth and maintenance. As
regarding hydrodynamics, studies on the effect of high solids loads on gas-liquid mass
transfer are scarce, at least in situations similar to those found in flocculation bioreactors.
Verlaan and Tramper (1987) studied the gas-liquid oxygen transfer in a three-phase pilot
plant airlift bioreactor (165 L). The solid phase consisted of polystyrene or Ca-alginate
beads with a density of 1050 kg·m−^3 and diameters ranging from 2.4 mm to 2.7 mm. The
maximum solids load at which the reactor could be operated was 40% (v/v) and it was
found to provoke a decrease of kLa when compared to the value for two-phase operation,
both for polystyrene and Ca-alginate beads. A similar effect has been found in a three-
phase bubble column (Komáromy and Sisak, 1994). This decrease has been justified by a
reduction of the specific area, a, due to an increase in coalescence provoked by the
presence of the particles which cause an increase of the slurry viscosity. The viscosity
increase is apparent, however, for it is effective only at a reactor scale, as the liquid
viscosity remains the same and so the presence of solids is not expected to affect kL. Even
so, the reduction in kLa in the presence of alginate beads was slightly more significant
than for polystyrene beads and this may be related to solids wettablility, as pointed out by
the authors (being Ca-alginate classified as perfectly wetted and polystyrene as poorly
wetted). Siegel et al. (1988) also found a decrease in the overall mass transfer coefficient
with increased solids load. Nevertheless, contrary to the previous authors, they state that
the presence of non-wettable solids will have a profound (decreasing) effect on mass
transfer coefficient, while wettable solids will cause a relatively smaller decrease in mass
transfer. Correlations are presented for kLa as a function of the solids load and either the
power input per degassed reactor volume or the superficial gas velocity. The results are
classified into two groups, depending on whether the solid phase was composed of
wettable or non-wettable particles.
Similar results were found in flocculent yeast cell cultures, as it is possible to observe
a decrease of kLa values when biomass concentration increases (Sousa and Teixeira,
1996). As expected, this coincides with the behaviour observed when a non-living solid
phase was used.
Influence of design parameters
The same design parameters referred to previously will exert their influence also in the
oxygen transfer properties of a bioreactor, accordingly to Figure 13.1. The information on
this is not abundant for three-phase reactors and data are even scarcer for flocculation
bioreactors. As an example, the introduction of a draught tube in a three-phase fluidised
bed (solid phase: polyurethane foam particles with an average size of 3 mm) increased
the volumetric mass transfer rate of oxygen in the new bioreactor between one and a half
Multiphase bioreactor design 398