Kinetic Characterization of Xylose Monomer and Oligomer Concentrations during Dilute Acid Pretreatment of Lignocellulosic Biomass from Forests and Switchgrass

Abstract

The kinetics of dilute acid hydrolysis for aspen, balsam, and switchgrass were investigated at various temperatures, acid concentrations, and reaction times using small-scale isothermal tubular reactors. The experimental data was fitted to a four-step kinetic model with first-order irreversible rate constants at each step. The maximum yields of xylose monomer for aspen and balsam show limited variability with changing reaction severity with maximum levels of approximately 85% and 67% of initial xylan, respectively. Switchgrass xylose monomer yields varied significantly with very low yields observed at 0.25 wt % acid (32% of initial xylan) and relatively high yields at 0.75 wt % acid (81% of initial xylan). The measured furfural levels at the point where xylose monomer is maximized, relative to initial xylan, range from 1% to 6% for aspen and balsam, and 3% to 15% for switchgrass. Excellent agreement, both quantitatively and qualitatively, is shown between the experimental monomer data and its model fit. Although the maximum xylose oligomer concentrations and the initial concentrations during oligomer formation are accurately predicted, the model with irreversible kinetic constants does not describe the oligomer data accurately at long times. The model predicts furfural levels comparable to those measured; however, the model overpredicts early data points and underpredicts late data points. Arrhenius kinetic parameters were derived for all three species at each step of the proposed model and these parameters were of the same order as literature values. Preliminary results in regards to the effect of aspen solids loading on reaction kinetics show that a 50% decrease in solids loading (10% to 5%) results in faster rates of reaction. This is particularly apparent for the degradation of xylose monomer which increases by a factor of almost 3. Several recommendations to improve the modeling of dilute acid hydrolysis reactions are discussed.