(196b) Kinetic Characterization of Xylose Monomer and Oligomer Concentrations During Dilute Acid Pretreatment of Lignocellulosic Biomass From Forests

Dilute acid pretreatment achieves the saccharification of the hemicellulose portion of lignocellulosic biomass in a biomass to ethanol process.  The yield of fermentable, monomer sugars during pretreatment is a crucial factor for the maximization of the overall process efficiency. 

The kinetics of xylose formation and degradation has been investigated in more detail than that of any other sugar present in hemicellulose.  This is the case given that xylose is often the main component with percentages of total hemicellulose by dry weight ranging from 27 to 91% depending on the type of feedstock [1].  The simplest kinetic mechanisms that has been proposed for this reaction is a two-step pseudo-first order irreversible reaction with Arrhenius-type kinetic constants [2]:

However, experimental observations suggest that other models that include oligomeric intermediates and parallel reactions of slow and fast reacting hemicellulose phases (biphasic) could describe the reaction more accurately [3].  There is not a consensus on whether the biphasic behavior of hemicellulose is real or simply an artifact of experimental methods.  On the other hand, oligomer concentrations have been measured [4-6] indicating that an oligomer intermediate should be included in kinetic modeling:

The kinetics of xylose monomer and oligomer sugars under 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 were fitted to a four-step kinetic model with first-order irreversible rate constants at each step.  The maximum yield of xylose monomer for aspen and balsam show limited variability with changing reaction severity with maximum levels around 85% and 67% of initial xylan respectively, while switchgrass monomer levels varied significantly with very low yields observed at 0.25% wt. acid (30% of initial xylan) and relatively high yields at 0.75% wt. acid (80% 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 as well as the initial points 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 prior literature values.  Preliminary results of the effect of aspen solids loading on reaction kinetics show that a 50% decrease in reactor solids loading (10% to 5%) results in faster rates of reaction, particularly for the degradation of xylose monomer which increases by a factor of almost 3.

 References

1.            McMillan, J.D., Process for Pretreating Lignocellulosic Biomass: A Review. NREL/TP-421-4978. 1992.

2.            Saeman, J.F., Kinetics of Wood Sacharification: hydrolysis of Cellulose and Decomposition of Sugars in Dilute Acid at High Temperature. Industrial & Engineering Chemistry Research, 1945. 37(1): p. 43-52.

3.            Jacobsen, S.E. and C.E. Wyman, Cellulose and Hemicellulose Hydrolysis Models for Application to Current and Novel Pretreatment Process. Applied Biochemistry and Biotechnology, 2000. 84-86: p. 81-96.

4.            Yat, S.C., A. Berger, and D.R. Shonnard, (2008) Kinetic Characterization for Dilute Sulfuric Acid Hydrolysis of Timber Varieties and Switchgrass. 99(9), pp 3855-3863.

5.            Jensen, J.R., Morinelly, J., Aglan, A., Mix, A., Shonnard, D.R., Kinetic Characterization of Biomass Dilute Sulfuric Acid Hydrolysis: Mixtures of Hardwoods, Softwood, and Switchgrass, AIChE Journal, online April, 2008, http://www3.interscience.wiley.com/journal/109931315/issue

6.            Chen, R., Y.Y. Lee, and R. Torget, Kinetic and Modeling Investigation on Two-Stage Reverse-flow Reactor as Applied to Dilute-Acid Pretreatment of Agricultural Residues. Applied Biochemistry and Biotechnology, 1996. 57/58: p. 133-146.