Fundamental Bioengineering

2Experimentally Determined Rates of Bio-Reactions

John Villadsen

Summary

Rates of bioreactions are introduced as measured terms in steady-state mass balances for a continuous stirred tank reactor (CSTR). Both mass balances and the reaction rates have the same form for enzymatically catalyzed reactions and for reactions with living cells. In cell reactions, the rate of biomass formation is included through a separate mass balance. Reactants absorbed in the liquid phase from a gas phase are treated separately, and it is shown how experimental errors can lead to errors in the calculated rates. The black-box model for a cell-reaction stoichiometry is introduced and the yield coefficients are defined. Finally, different methods of controlling the CSTR at steady state are discussed.

2.0 Introduction

The rate of an enzymatically catalyzed bioreaction, or of a reaction that involves living cells (microbial, animal, or plant cells), can be determined experimentally in a bioreactor. The bioreactors used in academic research or in an industrial R&D department to obtain reaction rates are normally glass vessels of 0.5–5 l working volume V. A typical laboratory reactor is shown in Figure 2.1. It is well stirred either by an internal mechanical stirrer (a hydrofoil or a turbine) or by a magnetic stirrer, operated from the outside of the reactor. In all cases, the mixing of liquid feed into the medium volume V is supposed to be good enough to ensure that there is no spatial variation of ...