Dedicated to Professor Kurt Dialer on the occasion of his 70th birthdayCompared to chemical reactors, there are additional requirements to be considered when selecting and designing a bioreactor. The cultivated organisms and most of the desired products are to a greater or lesser extent sensitive to temperature, pH and shear stress. Furthermore, sterilization often plays an important role in maintaining a monoseptic process. Many design procedures for chemical reactors can be applied but requirements which are relevant to the biological system are sometimes decisive. Consequently, it is useful to first consider the essential features of the organisms with respect to the selection and design of bioreactors. Attention must be paid to the fermentation medium because its flow behaviour can range from slightly viscous and Newtonian to highly viscous, non-Newtonian. Many papers deal with low viscosity fermentation broths [ 1 -51. However, little is known about the design of bioreactors for highly viscous non-Newtonian liquids. Fundamentals for the design of bioreactors are presented. It has been attempted to apply theoretical equations suitable for both, low viscosity Newtonian and high viscosity nonNewtonian fermentation broths. Apart from the calculation procedure for OTR-values, the difference between global and local values within bioreactors is demonstrated by means of velocity and shear stress profiles. Special requirements for bioreactors such as prevention of excessive foaming, shear sensitivity of micro-organisms and cell-lines and effective sterilization are also discussed. Fig. 1 shows the principles of a bioreactor which can be operated batchwise or continuously. The culture medium and nutrients are fed into the reactor and the fermentation broth is aerated in order to supply the organism with oxygen. On the other hand, CO, and H,O are discharged from the reactor. During the fermentation process, cells multiply and produce the desired intra-or extracellular bioproducts. As a rule, it is necessary to maintain certain conditions such as concentrations of various components, temperature, pH, etc. for efficient Bio -(s tirred-.f ield-,solid sto te-.membrane-ireactors I I CO,.H,O.O, bioconversion. Mixing eliminates the differences between these parameters. Sufficient oxygen must be supplied by generating a large interfacial area per unit volume between the liquid and the gas phase. It is also necessary to remove heat in order to maintain a favourable temperature.
Requirements of the System
LiquidsWith respect to the selection of a bioreactor, the most important properties of a fermentation broth are its viscosity and its tendency to foam. Due to high organism concentrations and high viscosities of substrates or bioproducts, the liquids often exhibit non-Newtonian behaviour. It is common practice to describe the rheological features of such liquids by the law of Ostwald-de Waele'): shows that, with the exception of a Newtonian fluid with m = 1 and K = v, this viscosity is not constant in a bioreactor. Since there...
