Large-scale, high-solids enzymatic hydrolysis of steam-exploded poplar

Enzymatic hydrolysis at high solids loadings is key to scale-up of lignocellulosic biochemical conversion processes, because of potentially higher sugar and ethanol titers and lower hydraulic loads. However, high solids loadings can pose rheological challenges, reduce mass and heat transfer efficiency, and increase the concentration of enzyme inhibitors in the system, resulting in low conversion of glucan and xylan into fermentable sugars. In this study, ten batch enzymatic hydrolyses were conducted in a 200-liter reactor, while monitoring sugar and inhibitor profiles. The effects of enzyme cocktail, biomass loading, pre-treatment severity, and hydrolysis temperature were assessed using techno-economic indicators to evaluate the efficacy of the enzymatic hydrolysis. For similar experimental conditions, different enzyme cocktails produced distinct hydrolysis outcomes allowing cocktail optimization. In spite of a rapid initial reaction rate, fermentable sugars concentrations reached a plateau after about 48 h, indicating severe inhibition. Increased biomass loadings did not proportionally increase sugar production. Both observations indicated the presence of severe inhibition, likely endogenous. Pre-treatment at a lower severity (200 degrees C for 8 min) led to the most efficient hydrolysis, while higher severities destroyed hemicellulose and led to lower overall sugar production. Lower saccharification temperatures (30-32 degrees C) caused a 20% decrease in sugar conversion when compared to 50 degrees C operation. Strategies to mitigate inhibition will be required if high-solids enzyme hydrolysis is to be successfully scaled up to commercially relevant levels. (C) 2011 Society of Chemical Industry and John Wiley & Sons, Ltd