Monday, May 4, 2009
9-24
Reaction kinetics and selective product removal during high-solids enzymatic saccharification
The rate of enzymatic hydrolysis of cellulose biomass decreases as the reaction progresses, especially when high-solids substrates are used. Due to the high cost of enzyme and need to increase the rate and extent of biomass conversion, it is imperative to identify and attenuate the predominant causes of rate reduction while maximizing enzyme efficiency. Consequently, an experimental design was used to determine the relative effects of four factors: (1) product sugar inhibition, (2) reduced substrate reactivity, (3) unproductive enzyme binding, and (4) enzyme inactivation. Dilute-acid pretreated corn stover (PCS) was used as the lignocellulosic substrate. The effect of product sugar inhibition was determined by using initial rate experiments with initial glucose concentrations ranging from 0 to 50 g/L. Substrate reactivity was estimated by using pre-saccharified, washed PCS at initial cellulose conversions of up to 50%. Unproductive enzyme binding was estimated by comparing enzymatic adsorption and cellulolytic activity on the model substrate Avicel in the presence of commercially-available lignin. Lastly, the effect of enzyme inactivation was investigated by intermittently deactivating the enzyme and reloading fresh enzyme during saccharification. Preliminary results suggest that the rate reduction is predominantly due to glucose inhibition, although diminished substrate reactivity and enzyme inactivation may be significant.
Additionally, a custom ultrafiltration membrane reactor was evaluated for the processing of high-solids biomass. This configuration facilitates the selective removal of inhibitory products from cellulase enzymes and insoluble solids. Finally, the transient product concentration and total mass of eluted sugar from the reactor were modeled using kinetic data.
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