Fundamental Bioengineering

3Redox Balances and Consistency Check of Experiments

John Villadsen

Summary

A black-box stoichiometry for a single bioreaction is introduced using element balances to calculate some of the yield coefficients. Using a redox balance on the stoichiometry is shown to reduce the computational work. In a number of examples, redox balances are used to check experimentally determined yield coefficients. The concept of a theoretical maximum yield of a bioreaction is defined and used on several industrially important processes. A given experimental stoichiometry is split into several elementary stoichiometries with the purpose of finding potential process improvements. Finally, redox balancing is used to analyze typical bio-remediation processes.

3.1 Black-Box Stoichiometry Obtained in a CSTR Operated at Steady State

The stoichiometry (1) of Example 2.1 and Eq. (3.1) quantitatively describes how much biomass, ethanol, CO₂, and glycerol is produced by conversion of one C-mol glucose at a given steady-state operation condition, that is, at given values of temperature, pH, and dilution rate D = v/V.

(3.1) $$\text{C}{\text{H}}_2\text{O}+0.01658\text{N}{\text{H}}_3\to 0.1381\text{C}{\text{H}}_{1.74}{\text{O}}_{0.6}{\text{N}}_{0.12}+0.5160\text{C}{\text{H}}_3{\text{O}}_{0.5}+0.2759\text{C}{\text{O}}_2+0.070\text{C}{\text{H}}_{8/3}\text{O}+0.037{\text{H}}_2\text{O}\text{.}$$

In the particular Example 2.1 on the anaerobic cultivation of the yeast Saccharomyces cerevisiae at T = 30 °C and pH = 6.5, the stoichiometry appears to hold quite well for dilution rates between 0.05 and 0.42 h⁻¹ which is close to the washout dilution rate. In most other cases, a stoichiometry like ...