Multiphase Bioreactor Design

Plug flow may be achieved using packed-bed systems, in which the yeast is
immobilised in or on a particular support. However, packed-bed reactors have a number
of drawbacks: a) Mass transfer limitations, especially in the end zone, when the
concentration of substrate is the lowest; b) Inter and intra-particular accumulation in the
lower zone of the gas produced (CO2), which occurs when the bed is not sufficiently
porous; c) The formation of preferential paths which reduce the effective working
volume; and d) The compacting of the bed due to the partial disintegration of the
bioparticles (because of shear stress or the intraparticular accumulation of gas) (Núñez
and Lema, 1987).
In order to overcome these problems, the application of a pulsation in the liquid
entering the bioreactor has been proposed (Sanromán et al., 1994a,b). The EMP system
was selected as it offers operational advantages, the most important being the
maintenance of the plug flow hydrodynamics (Roca et al., 1994).

i) Effects of pulsation on bioreactor performance

The effects of pulsation on the bioreactor (V=2.1 L; H/D=2.7) performance were
analysed at three hydraulic residence times of 3, 1.4 and 0.7 h, pulsation frequencies
between 0.008 and 0.024 s−^1 , glucose concentrations of 100 and 200 g L−^1 , their
efficiency always being compared with the results obtained during the operation without
pulsation (Roca et al., 1996a).
More stable operation was obtained for longer periods when a pulsation was applied to
the feed stream, which prevented bubble gas occlusions, bed fragmentation and bed slug
formation, problems that frequently arise when operating without pulsation. At higher
glucose concentrations, higher amounts of gas are produced, and therefore pulsing at a
higher frequency facilitates more efficient gas removal. Thus, the selection of an
appropriate frequency to synchronise the pulses with gas production could help to
improve the performance of the bioreactor (Figure 11.6). The explanation of this effect is
complex as pulsation controls two different factors; mass transfer resistance and
degasification of the bed. The mechanical effect, which improves mass transfer, increases
with the amplitude of the pulse for a particular flow rate. The effect of pulsation on
degasification is more complex because it also depends on the gas production rate during
the fermentation process.
The selection of the optimum amplitude and frequency of pulsation is quite difficult
because there are many contradictory effects (Roca et al., 1996b). Higher substrate
concentrations imply the evacuation of greater volumes of gas and therefore the optimum
pulsation frequency should be higher. On the other hand, operation at higher hydraulic
residence times should imply a decrease in the optimum pulsation frequency, in order to
keep the mechanical effect of pulsation. To determine the best pulsing conditions, a
control-supervision system based on the knowledge of the process, including an optimum
search algorithm for the determination of the best frequency for each particular operation
(residence time and substrate concentration), was implemented.

ii) Efficiency

Multiphase bioreactor design 340