(573c) Kinetic Characterization of Oligomer Formation and Degradation in Dilute Acid Pretreatment of Lignocellulosic Biomass from Forests

Introduction

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 need to
understand oligomer kinetics is further justified because oligomer sugars are
not as fermentable as monomer sugars.  The objective of this research is to
develop an experimental procedure in order to determine kinetic parameters that
can be fitted to the oligomeric intermediate model above for hardwood,
softwood, and herbaceous species (Aspen, Balsam, and Switchgrass respectively).
The kinetic parameters will be used in follow-on research to predict the
effects of dilute acid hydrolysis conditions on the dynamics of oligomer
formation and degradation and modify reactor design to achieve minimum oligomer
and maximum monomer sugar concentrations.

Methods

Previous
experiments in our lab using a transient temperature method showed significant
oligomer sugar formation early in the reaction time for xylose, glucose,
mannose, galactose, and arabinose. As time in the reaction increases, oligomer
concentration decreases as they are hydrolyzed to monomer sugars. The research
to be reported here extends this early work using isothermal small-scale
tubular reactors (3/8? OD, 0.035? wall, ~6 mL working volume).

A constant temperature bath at a temperature between 150 and 200 °C is
achieved using a suitable silicon oil, Dow Corning 550®.  Once thermal
equilibrium has been achieved, a basket with 10 stainless steel tubular
reactors  containing the dilute acid solution and the biomass (.25, 0.50, and 1%
H₂SO₄, temperature of 150°C, 160°C and 175°C, 10 % biomass
solids, biomass species aspen, balsam, switchgrass) is submerged in the bath.  This
design of experiments will cover a broad range of key reaction parameters to
characterize the kinetics of hydrolysis and identify conditions for minimum
oligomer sugar concentrations.

According to predictions made using heat transfer simulation
software, COMSOL Multiphysics? the time necessary for the contents of
the tubular reactor to reach the target temperature by conduction is on the
order of 4 minutes.  Additionally, an analytical solution to a simplified
limiting case (well mixed reactors) predicts a heat transfer period on the
order of 10 seconds.  In order to obtain composition data at isothermal
conditions this heat transfer time must be kept short by mixing.

The tubular reactors are retrieved at different times from
the silicon oil bath and immediately placed in an ice bath in order to stop the
reaction.  The first sample is taken at a pre-established heat transfer time,
and the measured compositions for that sample will be the initial conditions.
This new set up allows retrieving the remaining 9 tubular reactors with high
time resolution, which is increasingly important as the experiments are
undertaken at the higher end of the desired temperature range and the reaction
times decrease exponentially.

The hydrolyzate from each tubular reactor sample will be analyzed for
sugar concentrations via an HPLC, and oligomer concentrations are to be determined
by a secondary hydrolysis in the same fashion as in previous studies from our
lab [5].

Once the kinetic parameters for the three step kinetic model are
determined for the mentioned species, the prediction for concentration profiles
at different temperatures and acid concentrations will be compared to
experimental data and the validity of the model and its predictions will be determined.

 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.