(490ab) Mass Transfer Limitations of Enzymatic Hydrolysis of Cellulose at High Insoluble Solids Concentrations

The use of ethanol as an alternative liquid fuel has received much attention lately. Currently, ethanol is produced from crops such as corn, grain, sugar cane and sugar beets [1]. However, there is a desire to expand the current feedstocks to include lignocellulosic biomass from agricultural waste and energy designated crops such as switch grass. The fermentable sugars contained in these crops are released and converted to ethanol through a biochemical process that consists of four main steps: biomass pretreatment, enzymatic hydrolysis, microbial fermentation, and ethanol separation. The primary contributors to process costs are the pretreatment and hydrolysis steps. This is due to the recalcitrance of biomass to biodegradation. In order to improve the reduced hydrolysis rates that occur at higher conversions, an understanding of the mechanisms responsible for this reduction is needed before developing strategies for improvement. Known contributors to reduced hydrolysis kinetics include: product inhibition; unproductive binding to cellulose substrate; hemicellulose and lignin association with cellulose that blocks enzymes from cellulose substrate; irreversible enzyme adsorption to lignin; and loss of enzyme activity due to denaturation, mechanical shear, or low thermal stability [2]. In addition, fractal kinetics and enzyme jamming have been used to model hydrolysis rate profiles with good results[3]. Further work is needed to determine mass transfer limitations at high insoluble solids concentrations. Hydrolysis reactions that can operate at high solids concentrations will have a faster specific rate and will require smaller reactors to achieve the same level of production. This will allow for lower capital costs towards reactors and enzymes. This work seeks to develop a model that will allow for the quantification of the difference between predicted vs. measured hydrolysis rates in order to quantify the mass transfer limit at high solids loading.

References

1. Cantarella, M., et al., Effect of inhibitors released during steam-explosion treatment of poplar wood on subsequent enzymatic hydrolysis and SSF. Biotechnology Progress, 2004. 20(1): p. 200-206.

2. Jorgensen, H., J.B. Kristensen, and C. Felby, Enzymatic conversion of lignocellulose into fermentable sugars: challenges and opportunities. Biofuels Bioproducts & Biorefining-Biofpr, 2007. 1(2): p. 119-134.

3. Xu, F. and H.S. Ding, A new kinetic model for heterogeneous (or spatially confined) enzymatic catalysis: Contributions from the fractal and jamming (overcrowding) effects. Applied Catalysis a-General, 2007. 317(1): p. 70-81.