Continuous ethanol fermentation with tangential flow filtration cell recycle applied to a concentrated, solids-free softwood hydrolysate

Continuous stirred-tank fermentation with cell recycle (CSTF-CR) offers several advantages to second generation ethanol production.  In addition to improving volumetric productivity and reducing process downtime, high cell densities quickly convert metabolizable inhibitors such as furfural and HMF into less toxic intermediates, significantly reducing the steady-state concentrations of these inhibitors.  This is especially important in the fermentation of high gravity hydrolysates in which the final ethanol concentration is high and inhibitors are concentrated with the sugars.  In the present work, batch fermentations and design of experiments methodology were used to develop a kinetic model that accounts for the inhibitory effects of acetic acid, furfural, HMF and ethanol on S. cerevisiae D₅A.  The kinetic model was then used to predict CSTF-CR performance, and high cell retention (>90%) runs were used to validate the model for feeds with and without inhibitors.  Early CSTF-CR runs compared well to the model but filter fouling limited feed rates, leading to low productivities and cell morphology changes. In later runs with an improved filtration unit, ethanol productivities of 30 g/l/h for an uninhibited feed and 5-10 g/l/h for a highly inhibited feed were obtained with cell densities of 90 and 75 gDW/L, respectively. For previous batch and continuous fermentations, changes in cell morphology and reduction in cell yield were observed at high cell densities, and literature has indicated reduced cell viability can occur.  Longer runs (> 120 hours) at high cell density (≥ 100 gDW/l) were used to quantify these effects and target optimum CSTF-CR operating conditions.