Multiphase Bioreactor Design

perforated plates 1980

which links the pile of plates, causes them to rise and fall, and thus generates the
pulsation. The efficiency of such equipment depends on the geometry of the plate, its
amplitude, the speed at which the plates moves and the flow velocity of each phase,
factors which also control the intensity of the axial mixing.
Mass transfer is improved when the agitation speed increases and, for a given
frequency, both an increase in the amplitude of the pulsation and a decrease in the
diameter of the plate holes improve extraction efficiency (Baird et al., 1989). Besides,
these systems present other advantages, such as limited energy demand, smaller risk of
preferential ways in the dispersed phase, reliability of scale-up and the possibility of
working with phases of similar density. It is also possible to achieve an effect similar to
that of the light phase by introducing an inert gas in pulsed flow (Miñana et al., 1985).
The application of pulsation in hydrometallurgy (for mineral, metallurgical and
nuclear industries), in which the speed required for extraction is high, allows an increase
in the overall mass transfer coefficients to be obtained, especially when operating at large
pulsation amplitudes (Golding and Lee, 1981). Göebel and Fortuin (1986) used a pulsed
packed-bed column to improve the contact between a dispersed solid phase, fed at the top
of the column, and a pulsed liquid phase, using a system of reciprocating pumps. Mak et
al. (1992) determined the relation between the solid phase flow and the axial dispersion
coefficient of the liquid phase. Pulsation must take place at a relatively high speed for a
good distribution of the liquid phase.
The use of pulsing perforated plates columns is also of interest in processes involving
a contact between gas and liquid phases (absorption). The overall mass transfer
coefficient increases with the intensity of the pulsation and the average velocity of the gas
due to the better contact between the phases (Yang et al., 1986). Baird and Garstang
(1972) indicated that the mass transfer rate always improves, either as a result of a
vibration or a pulsation.
In filtering processes through membranes, efficiency diminishes because of clogging,
caused by polarisation of the filtered particles on the membrane surface. Until now, the
devices commonly used to minimise this problem caused strong turbulence at the surface
of the membrane. In this context, Finnigan and Howell (1989) determined that pulsation
in an ultrafiltration system, used for the clarification of juices of high solid contents,
increases the filtration capacity.

Biochemical reactors

In aerobic transformations carried out in fixed-bed bioreactors by immobilised
microorganisms, the limited availability of dissolved oxygen in the fermenting medium is
a major obstacle. Pulsing flow helps to diminish external resistance to the oxygen
transfer. Ghommith et al. (1982) studied the production of acetic acid in a pulsing reactor
by Acetobacter acetii immobilised on a ceramic support, the pulsation being generated
through a piston pump installed at the bottom of the reactor (similar to Figure 11.1-b).
For SCP production, where the requirements of oxygen are also high, a mechanical
pulsation system (similar to the piston) joined to a column of perforated plates, has been
used successfully. This system presents an oxygen transfer effectiveness as high as air-lift

Pulsing bioreactors 331