Ideal Bioreactors

Abstract

In previous chapters we consider kinetics and mass transfer. In this and the following chapter the vessel in which the bioreaction takes place is the topic. Some issues concerning the actual vessels used in industry, equipped with elaborate sparger systems, patented agitator constructions, and baffles designed to fit a specific agitator performance, are briefly touched upon in Chapter 9, whereas the idealized concept of a reactor is the topic of this chapter. If we claim that a tank reactor is ideal, there is no shunt of substrate from inlet to outlet, no dead zones or clumps of undissolved solid substrate floating around. A drop of substrate is instantaneously distributed throughout the entire reactor volume, and the sparger provides an intimately mixed gas-liquid medium with no air bubbles sliding up along the reactor wall. Some laboratory reactors approach the ideal. Mixing time is on the order of 1–2 s, and the gas-liquid mass transfer rate is very high [see, e.g., Sonnleitner and Fiechter (1988)]. These reactors may be abundantly equipped with on-line measuring and control systems, and one is able to follow the effect of steep transients imposed on the microbial environment [see, e.g., Nielsen (1992)]. These units, the true bioreactors, are used for scientific investigations, to learn more about the cell metabolism, and to study the cell as the ultimate biochemical reactor. Other experiments are carried out—often without involvement of an actual fermentation—in the equipment that is going to be used for industrial production. Here the interaction between mechanical devices such as agitators, draught tubes, static mixers with or without corrugated surfaces, and a fluid of given properties can be studied. The outcome is a series of time constants for mixing, for circulation, for gas-to-liquid transport and the like. As discussed in Chapter 9, it may be hoped that both the chemical interactions between the cell and its microenvironment and the physical interactions between the cell or the cell culture and the macroenvironment will eventually be clarified in enough detail to allow a new bioprocess to be designed with only minimal scale-up problems.