INTRODUCTION
According to their operation mode, the multistage bubble column reactors sectionalised
by internal plates represent a class of reactors with dispersed gas phase, distinguished by
the energy input with gas compression and by spatially distributed energy dissipation in
the reactor (Schügerl, 1983). Although it has been commonly acknowledged that
sectionalisation of bubble column reactors can significantly improve their mass transfer
characteristics and, at the same time, substantially reduce the degree of backmixing in
contacted phases (Schügerl et al., 1977; Shah et al., 1978; Shah et al., 1982; Deckwer,
1985), the multistage bubble column reactors have been only scarcely applied to
chemical or biotechnological processes in aerated slurry systems. This type of slurry
reactor has been received little coverage even in scientific literature. Accordingly, the
multistage bubble column reactors were not even mentioned in the comprehensive
treatment of reactors for gas-liquid-solid systems published by Shah (1979), and they
were touched only marginally in the respective part of the more recent book on
heterogeneous reactions authored by Doraiswamy and Sharma (1984). The Solvay
towers, used in soda production (Shah et al., 1978), thus still represent the proverbial
exception proving the rule, regarding the industrial application of sectionalized bubble
columns for g-l-s systems. In the bioreactors area, performance of a laboratory-scale
multistage tower fermentor was studied by Prokop and co-workers (1969) and the
application prospects of staged bubble column fermentors were subsequently discussed
e.g. by Sittig and Heine (1977) and by Schügerl (1980, 1983). On the industrial scale,
various modifications of internal-loop airlift reactors with dual-flow plates in the riser
have been reportedly used for SCP production by the Japanese companies Kanefuchi and
Mitsubishi Co. (Schügerl, 1983) and, most notably, a similar construction principle has
been employed in the 1500 m^3 pressure cycle fermenter designed for the ICI PRUTEEN
plant (Westlake, 1986). These applications have not, however, found many followers and
the multistage tower fermenters were not even listed in the review paper of Mersmann
and co-workers (1990), devoted to the selection and design of aerobic bioreactors.
Apparently, it is the fear of plate holes choking and/or solid phase sedimenting on
internal plates which has been primarily responsible for the reluctant approach to the
application of staged bubble columns for systems with a suspended solid phase. While
these phenomena may, indeed, represent a definite threat, namely in rapidly sedimenting
systems, sectionalized bubble columns can be, according to our opinion, advantageously
employed for pseudohomogeneous (non-sedimenting) suspensions of fine particles and/or
in cases of small density differences between the solid and the liquid phases. Obviously,
this latter condition is generally fulfilled in biological systems. Furthermore, the danger
of plates choking can be to a large degree circumvented by the use of plates with
downcomers, i.e. with the separate passage of gas and slurry phases.
Our previous study (Vlaev and Zahradník, 1987) demonstrated superior energy
effectiveness of sectionalised bubble columns in comparison with other types of tower
reactors for aerated slurry systems, including single-stage bubble column, tower reactor
with an ejector distributor, and multistage rotating-discs reactor. In the following, the
advantageous features of multistage bubble column reactors for aerated slurry systems
will be illustrated with the results of the experimental study (Zahradník et al., 1992)
aimed at examining the effect of selected construction and operating variables (number of
Multiphase bioreactor design 2