The rate of oxygen transfer from the gas phase to the liquid phase is a critical process parameter for biocatalytic oxidations due to the poor water solubility of molecular oxygen and the low oxygen affinity of many of the relevant enzymes, such as oxidases. In gas–liquid systems, mechanical mixing can be used to increase the interfacial area available for mass transfer and thereby increase the volumetric mass transfer coefficient (kLa). As such, the operation of these reactions in a continuous stirred tank reactor (CSTR) may allow for better performance in a readily scalable way. Even so, achieving a high substrate conversion in a single reactor would require operation at high pressure, to improve the solubility of oxygen, as well as a high enzyme concentration. An alternative and more cost-effective means of improving the substrate conversion might be to operate a series of multiple CSTRs. As such, the oxidation of glucose to gluconic acid by glucose oxidase, coupled with catalase, was modeled in a series of identical well-mixed reactors. It was found that achieving full conversion would require an impractical number of reactors at atmospheric pressure. However, the overall conversion of the reaction could be doubled by simply using two CSTRs in series. Subsequently, experiments were carried out to validate this, and the results showed that the overall conversion was in fact tripled. This likely resulted from a higher kLa in the second reactor, which was potentially caused by the change in the media composition from the first reactor.
- Dissolved oxygen