Application of a constricted mesotube reactor with oscillatory flow to acetic fermentation

Abstract

Acetic acid bacteria are strict aerobes, capable of oxidizing ethanol to acetic acid in the presence of oxygen. Considering that acetification is an exothermal process, with high need for oxygen supply, efficient mass transfer between the gaseous and aqueous phase and efficient heat transfer in the fermentation medium, pose as the main challenges when focusing process improvement. Throughout time, several setups have been tested and used for improving efficiency in acetic fermentations, ranging from open barrel to automated acetator setups with air injection. This work demonstrates the use of a constricted oscillatory mesotube reactor for acetic fermentations, as an alternative to common acetification setups. The constricted mesotube reactor combined with oscillatory flow leads to high mass and heat transfer within the fermentation chamber, due to the combination between axial mixing of the fermentation media, promoted by pistons, and the periodic constrictions, which function as baffles. Hydrodynamic properties of this type of setup have already been described, demonstrating an improved gas diffusion in liquid media. Furthermore, this type of reactor has been tested for several chemical and biological processes, and demonstrated high potential in process improvement. In this work, acetic fermentations were performed in an oscillatory constricted mesotube reactor, using an Acetobacter sp. inoculum in a synthetic medium containing acetic acid and ethanol. Acetifications were carried out at 30ºC with air injection at 0.18 vvm. Biomass growth, total acidity and ethanol-acetic acid conversion were monitored during the process, being observed that stationary phase was reached at about 70 hours, for an increase in biomass concentration of about 0.2±0.1 g L-1 (expressed in dry weight) and a final total acidity of 4.2±0.6%, with an ethanol-acetic acid conversion efficiency of 97.4±9.28% of the theoretical yield and a GK efficiency of 96.7±16.7%. These results indicate that oscillatory flow in a constricted mesotube reactor has high potential for the application to the acetification process, due to the high yields and efficiency, attaining performance values similar to the ones reported for other common acetification setups, with a smaller air supply.