Continuous Very High Gravity Ethanol Fermentation And Bioreaction Engineering Strategies

A bioreactor system composed of a stirred tank bioreactor and three tubular bioreactors in series was established based on ethanol fermentation kinetics and bioreaction engineering theories. Continuous ethanol fermentation using a general yeast strain and very high gravity media was carried out. The average levels of 24.5g/L for residual glucose and 15.4% (v/v) for ethanol were achieved when the dilution rate of the bioreactor system was designated to be 0.012h^-1. Sustainable oscillations of residual sugar, ethanol and biomass concentrations were observed. Ethanol, residual glucose and biomass concentrations were validated to be homogeneous along the axial directions of these tubular bioreactors because of backmixing resulted from lower dilution rate, aeration and CO₂ produced. However, the differences of biomass concentrations in the broths overflowed from and inside the tubular bioreactors were detected. A model, CSTR with recycling stream, was applied to predict the biomass concentrations, which was validated by experimental data. Five different dilution rates were designed in order to investigate the impact of the dilution rates on the oscillations. The Hopf Bifurcation theory was used to analyze and predict the occurring of these oscillations and the dilution rates that incited these oscillations. Theoretical analysis revealed that oscillations could occur at designated dilution rates, which was validated by experimental data. The benefits of oscillations for the ethanol fermentation system were also evaluated by comparing the fermentation results with oscillations and without oscillations. It was believed that oscillations could improve the fermentation ability of yeast so that they could tolerance the stress of high ethanol concentration, and higher ethanol concentrations in the final broth, e.g., over 15%(v/v), could be achieved during continuous fermentations.