Xiao Zhang, Chemical Engineering and Bioengineering, Washington State University, 2720 University Drive, Centre for Bioenergy and Bioproducts, Richland, WA 99354, Maobing Tu, School of Forestry and Wildlife Sciences, Auburn University, 3301 School of Forestry and Wildlife Sciences, Auburn, AL 36849, Mike Paice, Pulp and Paper Research Institute of Canada (Paprican), 570 Boul. St-Jean, Pointe Claire, QC H9R 3J9, Canada, and Zhihua Jiang, FPInnovations Paprican Division, 570 St-Jean Boul, Pointe Claire, QC H9R 3J9, Canada.
ABSTRACT
Knot rejects obtained from pulp screening after sulphite pulping are difficult to dewater, which makes landfilling expensive and burning inefficient. The rejects were found to contain up to 50% cellulose which is very susceptible to enzymatic hydrolysis to glucose. In addition, we found that hydrolysis at high substrate consistency was possible using a peg mixer. Knot rejects were hydrolyzed at 20% consistency in a laboratory peg mixer with 10 FPU/g of Novozymes Celluclast enzyme. The thick slurry was liquefied within the first hour of mixing and resulted in a glucose concentration of over 100 g/L after 24 hours of reaction. This solution was fermented by yeast to give an ethanol concentration of over 5%. The laboratory results were confirmed at pilot scale with a mortar mixer (high consistency) or stirred tank reactor (medium consistency) at the 400 L and 6000 L scales, respectively. It was found that washing the knots with ammonia resulted in increased glucose conversion. Enzyme costs could be lowered by separating the enzyme from the hydrolyzed sugars and recycling to the subsequent batch of substrate. The combination of high-consistency hydrolysis and enzyme recycling minimizes capital investment, energy requirements, and enzyme costs, which are significant factors in the overall economic viability of cellulose conversion to ethanol.
