Design of a vacuum flashing system for high-solids fermentation of corn

Efficiency gains in current grain ethanol processes are limited by limitations of the yeast biocatalyst. Yeast stress, including glucose concentrations (15% w/v) produced during liquefaction and saccharification and subsequent high ethanol concentrations (18% v/v) produced during fermentation, restrict slurry solids to 32% w/w for grain ethanol processes. A system was constructed to circumvent this solids limitation by combining two technologies: (1) granular starch hydrolyzing enzyme (GSHE), which can liquefy starch simultaneously with fermentation, and (2) reduced vapor pressure to remove ethanol from high-solids fermentations. GSHE eliminates the need for a separate liquefaction step because it gradually digests raw starch to glucose, which results in lower initial glucose concentrations (5% w/w). Vacuum was applied to remove ethanol as concentrations increased to near inhibition levels. An in situ ethanol removal system was constructed to conduct fermentation at 40% solid content. The vacuum flashing process successfully removed ethanol from the fermentation broth, thereby maintaining ethanol concentration in the broth below 10% to 12% v/v, while ethanol concentration in the control experiment without vacuum stripping was above 18% v/v. The final residual glucose concentrations in the fermentation broth for vacuum and non-vacuum treatments were 1.5% and 0.1%, respectively, indicating a more complete fermentation with vacuum flash. However, ethanol yields for vacuum and non-vacuum treatments were similar, 0.288 ±0.013 L kg-1 and 0.285 ±0.013 L kg-1, respectively. Neither removal of CO2 nor repressurization with unfiltered air affected final ethanol yields.