Multiphase Bioreactor Design

Figure 5.8 Design chart showing the

factors which determine the feasible

operating window for a two-liquid

phase biocatalytic process. Adapted

from Woodley and Lilly (1994).

(e.g. limitations on the power input may mean that adequate mass transfer is difficult to
achieve), thus reducing the size of the operating window as the process scale is increased.
Hence it is possible to make laboratory scale measurements of mass transfer and reaction
kinetics and use them to draw conclusions about larger scale operation. The rationale
behind this argument is that the design method presented here is based on fundamental
mass transfer-reaction concepts, which are therefore independent of scale. An added
complication is that measurements made over a flat liquid-liquid interface (laboratory
reactor) may not accurately simulate and be applicable to those made over a dispersed
liquid-liquid interface (likely industrial reactor). However, preliminary results have
indicated that this is not a problem (Woodley et al., 1991b).

Safety Considerations

In aerobic bioconversions, where volatile organic solvents are used, the potential to form
an explosive atmosphere at large scales of operation raises serious safety issues. Clearly
reactors for such processes must be housed in special facilities designed to comply with
both explosion-proof and biological containment regulations. Schmid and coworkers
(1998a) have identified several strategies to deal with explosion issues. First, high
pressure reactors can be built capable of containing the pressure generated by an air-
solvent vapour explosion. Secondly, the generation of an explosive atmosphere can be
prevented by a range of operating strategies. These include the dissolution of volatile
solvents in inert carrier solvents, reduction of the oxygen concentration in the inlet gas

Multiphase bioreactor design 138