ABSTRACT
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 m3 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.
