Multiphase Bioreactor Design

lift, fluidised or reciprocating bioreactors seemed better suited to particular applications
(Brauer, 1991; Fontana et al., 1992; Gilson and Thomas, 1993).
Several different fermentation and enzymatic processes in which pulsation may
prevent operational problems, facilitating the improvement of efficiency and control of
multiphase bioreactors, are presented. The pulsed phase was gas or liquid depending on
the particular application.

Liquid-phase Pulsing Bioreactors

Fixed and expanded bed bioreactors, using immobilised enzymes or microorganisms
have been widely applied because of their simple design and operation (Tyagi et al.,
1992). However, these systems present some drawbacks (mass transfer limitation, gas
retention, dead volumes), which have proved difficult to solve. The application of a pulse
to the bioreactor has been assayed as an useful partial solution to the above mentioned
problems.

Enzymatic hydrolysis of starch by immobilised glucoamylase on chitin

slabs

The hydrolysis of starch by immobilised glucoamylase is the first step in industrial HFCS
production. When processing high viscosity solutions in packed-bed bioreactors, the
diffusion of the substrate can represent the limiting factor of the overall reaction rate. In
the hydrolysis of starch, viscosity plays a definitive role, which in some cases led authors
to confuse this effect with an apparent inhibition by substrate (Miranda et al., 1991). In
order to overcome this difficulty, a pulsing flow was applied to the reaction bed.
Experiments were conducted in two equal glass upflow continuous packed-bed reactors
(L/D=2.3), with and without pulsation, operated at residence times (r) of 0.33–1.8 h
(Sanromán et al., 1991).
Since the reaction rate corresponds well with the Michaelis-Menten model (Miranda et
al., 1991) and that, after determining RTD curves, the flow model corresponds to an
almost ideal plug flow reactor, equation 3 describes properly the behaviour of the system.

(3)

The kinetic parameters obtained are presented in Table 11.2 and as can be observed, an
increase in the maximum hydrolysis rate (Vmax) and a slight reduction in KM are obtained
when a pulsing reactor is used, thus improving the final efficiency of the process.
Figure 11.5 shows a comparison between the substrate consumption rate obtained in a
packed bed bioreactor, pulsed and non-pulsed, both operated at a HRT of one hour for
three substrate concentration in the feeding stream. As can be seen, in all cases a higher
substrate consumption rate is obtained with the pulsed system, the effect of pulsation
being more significant at higher substrate concentration, when liquid viscosity is eight
times higher than the corresponding to 50 g L−^1.

Multiphase bioreactor design 338