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Oral Presentation 3-05 A Comprehensive Kinetic Model for Dilute Acid Hydrolysis of Cellulose Qian Xiang, Jun-Seok Kim, and Y. Y. Lee Department of Chemical Engineering 230 Ross Hall Auburn
University, AL 36849 Telephone: (334) 844-2019; Fax: (334) 844-2063; E-mail:
yylee@auburn.edu The known kinetic models of dilute-acid
hydrolysis of cellulose suggest that glucose yield above 70 % is
unattainable. Among the reasons cited
for the limited yields are that glucose is decomposed and that soluble
oligomers are irreversibly modified into a form that cannot be reverted to
glucose. In our recent work, however,
we have demonstrated that above 90% yield of glucose is obtained using
bench-scale bed-shrinking flow-through (BSFT) reactors. These experimental results contradict the
existing kinetic models. We have
therefore made further investigation of the fundamental aspects of the reaction
kinetics and mechanism involving cellulose hydrolysis. Paying special attention to the fact that
the reaction is heterogeneous, we find that the acid-catalyzed hydrolysis is
controlled not only by the temperature and the acid concentration but also by
the physical state of the cellulose.
Under low temperature and acid conditions the cellulose structure stays
in stable crystalline form, the prevailing reaction mode thus being endwise
hydrolysis. Under those circumstances,
glucose becomes the main sugar product.
Consequently, the conventional two sequential first-order kinetics
becomes applicable. However, when
temperature and/or acid concentration is raised to a certain level, the
cellulose structure becomes unstable most likely by breakage of hydrogen
bonding (HB), the primary force that holds the cellulose chains. Once the crystalline structure of the
cellulose is disrupted, acid molecules can then penetrate into the inner layers
of the cellulose chains. In support of
this hypothesis, we have experimentally verified that a substantial amount of
oligomers are formed as reaction intermediates under extremely low acid and
high temperature conditions. We also
find that the breakage of HB occurs within a narrow range of temperature,
showing a rather abrupt devastation of cellulose supramolecular structure. On the basis of these findings, a
comprehensive kinetic model is proposed that includes a parallel reaction
pathway (cellulose-oligomers-glucose) along with other previously known
reactions. This model is in full
compliance with our recent experimental data obtained under various reaction
conditions and reactor types.
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