Canadian company pursues hemp-based ethanol production

Abstract

This work uses Sunn hemp fibre – a new non-food lignocellulosic energy crop, containing cellulose (75.6%), hemicelluloses (10.05%) and lignin (10.32%) – for enzymatic hydrolysis in second generation bioethanol production. We quantify the effects of reactor mixing on the kinetics of cellulase-mediated depolymerization of Sunn hemp’s cellulose to glucose and other soluble oligomers. The kinetic parameters (Km, Vmax), the nature of product inhibition (non-competitive), and the glucose inhibition constant (Kx) are experimentally determined for cellulose loading of 10–50 mg/ml, at reactor mixing speeds of 0–150 rpm. These kinetic constants are fitted to algebraic expressions that quantify their exponential decrease with increasing mixing speed. After 72 h of hydrolysis performed on 10 mg/ml of cellulose at the optimum enzyme:substrate loading of 1:15, the yields of glucose and reducing sugars decrease from 59% and 65%, respectively, at 0 rpm (i.e., no mixing) to 53.4% and 58.8%, respectively, at 150 rpm. Thus, 90% of the reduction in soluble sugar yield engendered by mixing results from the disappearance of glucose monomers that non-competitively inhibit cellulase. Better macromixing facilitates convective transport of enzymes to solid celluloses while enhancing product inhibition by rapidly homogenizing the inhibitors (glucose, cellobiose) in the reactor. Though faster mixing transforms the hydrolysis system from being mass-transfer limited to being kinetic-limited, the soluble sugar yields decrease as stronger product inhibition overpowers the rate-enhancing effects of mass transfer in well-mixed reactors. We show that batch hydrolysis performed under mass-transfer limitations maximizes sugar yields by reducing product inhibition. However, mixing affects sugar yields from cellulose extracted from recalcitrant lignocelluloses less significantly than that from pure cellulose due to the former’s high crystallinity and degree of polymerization, and low porosity.