Multiphase Bioreactor Design

Figure 11.7 Performance of the pulsed

and non-pulsed packed bed bioreactor

for the production of ethanol by

immobilised S. cerevisiae, at a HRT of

3 h by using 100 g L−l of glucose as

substrate. Symbols:  Non pulsed and

Pulsed with a frequency of 0.008

s

− 1

.

When the residence time was high enough for pulsation to exert limited mechanical
effects, the ethanol concentration in the bioreactor reached its maximum value at a bed
height of 0.15 m, where the substrate was completely consumed. Pulsation enables
hydrodynamic behaviour to follow that of plug flow model quite closely, and partially
impedes the backmixing observed in non-pulsed operation. As a complete substrate
conversion is achieved in both cases, the difference in the results is explained not by
possible differences in external mass transfer and internal effective diffusion but by
differences in free yeast growth caused by the wash out obtained through the application
of the pulses.
Operation in a pulsed mode increases productivity significantly in the upper part of the
fermentor, when appropiate pulsations are applied. The same behaviour was observed
when working with smaller bioreactors (Sanromán et al., 1994a,b).

Gas-phase Pulsing Bioreactors

The physiological and morphological growth characteristics of many filamentous fungi
require the introduction of large amounts of gas to maintain the levels of dissolved
oxygen desirable for metabolite production. Besides, after a very short period of time, the
excessive growth of fungi gives rise to technical difficulties such as fouling of the
fermenter probes, bed spouting and bioreactor clogging due to the interconnection of
conglomerates of pellets or immobilised bioparticles (Presser and Tough, 1991). An
example of this is shown in Figure 11.8, which corresponds to the formation of
conglomerates of Phanerochaete chrysosporium pellets after a 3-day operational period,
in an expanded-bed bioreactor. As a consequence, a low productivity of metabolites and a
short bioreactor performance in stable conditions are common.

Multiphase bioreactor design 342