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.

Kinetic Modelling and Experimental Studies for the Effects of Fe2+ Ions on Xylan Hydrolysis with Dilute-Acid Pretreatment and Subsequent Enzymatic Hydrolysis

“…The model proposed by Morinelly et al also found the same over-prediction of xylooligomer yields, and so the authors suggested quasiequilibrium between low-molecular-weight oligomers and xylose [32]. The reversible kinetic model was successfully applied earlier to the dilute acid hydrolysis of cellulose [37].…”

“…The differential equation system was solved symbolically as reported by Morinelly et al [32]. The equations were fitted to the corresponding data simultaneously using a quasi-Newtonian method in Excel solver.…”

“…The kinetic constants should obey the Arrhenius equation, which is a function of temperature (T, absolute temperature) and acid concentration, as shown in Equation (7) [32]. To simplify the model, the acid concentration was held constant (0.5 wt %) in the current study.…”

“…The simplest model is a linear two-step consecutive homogeneous pseudo-first-order irreversible reaction (Equation (1)) derived from the study of acid hydrolysis of cellulose [31]. This model was then modified to include oligomer intermediates and the main xylose degradation product, furfural, whose occurrence was confirmed by experimental observations (Equation (2)) [32]. Furthermore, this model was adjusted to a biphasic model of the first step, which assumed the hydrolysis of two fractions of xylan at different reaction rates [33].…”

Hot Water Pretreatment of Boreal Aspen Woodchips in a Pilot Scale Digester

“…Nabarlatz et al proposed a modified pseudo-first-order model that xylan is composed of xylose, arabinose, and acetic acid [20,21]. Other models tend to ignore the possible biphasic nature of xylan and propose more general linear sequence first-order kinetic model [22,23]. Mittal et al also investigated the mass-transfer effects on the kinetic model to predict the concentration of products [24].…”