13-41: Continuous fermentation and microfiltration cell recycle applied to a solids-free, very high gravity lignocellulose hydrolysate

In the fermentation of lignocellulose hydrolysates to ethanol, process economics are limited by low ethanol yields and production rates, and by growth inhibition from aldehydes and organic acids.  Very high gravity fermentations offer the benefit of increasing the ethanol titer; process simulations demonstrate that a 60% reduction in distillation energy could be realized by increasing the initial glucose concentration from 40 to 100 g/L.  A parallel strategy for improving process efficiency is continuous fermentation with cell recycle.  Microfiltration cell recycle, in which cells exiting the fermentor are concentrated and returned, would allow for higher volumetric productivities by increasing the steady-state cell concentration. Previous studies have shown that high S. cerevisiae densities can improve the ethanol production rate by as much as 50 times that of a low cell density culture.  High cell densities can also improve inhibitor tolerance, especially in the case of metabolized inhibitors such as furfural.

The goal of this study was to investigate the fermentation of a very high gravity, solids-free lignocellulose hydrolysate via continuous fermentation with cell recycle.  A chemostat culture of S. cerevisiae D₅A was coupled with a cross-flow microfiltration device and fed a defined medium that mimics a concentrated lignocellulose hydrolysate.  Our kinetic model was derived from batch fermentation data, and indicates that a 110% increase in volumetric productivity can be obtained by operating at a bleed ratio of 0.2.  Discrepancies between this model and our experimental chemostat runs will be discussed, along with the effects of dilution rate and cell concentration factor.