Tuesday, May 3, 2011
Pulping liquors from paper production represent an underutilized resource for the production of biofuels and other chemicals. In this study, sugars (primarily xylose) in the sulfite liquor from a hardwood pulping operation are converted to ethanol in a Simultaneous Isomerization and Fermentation (SIF) system. The SIF system consists of a fermenter with an attached recirculation loop which includes yeast separation and xylose isomerization modules. Yeast separation is accomplished continuously with a vibrationally enhanced microfiltration system. Concentrated yeast slurry is returned to the fermenter. Permeate from the microfiltration membrane is continuously circulated through a reactor containing immobilized xylose isomerase. The enzyme converts a portion of the xylose to xylulose and the mixture is returned to the fermenter where the xylulose is fermented to ethanol by Schizosaccharomyces pombe (ATCC 2476). Temperature and pH are adjusted within the recirculation loop to allow both the fermentation and isomerization steps to operate at near-optimal conditions. Results from a ten liter system indicate near theoretical conversion of xylose to ethanol until accumulation of ammonium ion, used for pH correction, caused yeast stress and a reduction in yield to approximately 70% of theoretical. Using a mix of potassium hydroxide and ammonium hydroxide for pH correction reduced yeast stress. Preliminary results from a 200 liter system are also presented.