(552a) Conversion of Glucose to Hydrogen Gas by Supercritical Water within a Microchannel Reactor

Glucose, with molecular formula C₆H₁₂O₆,
is obtained from the hydrolysis of cellulose or starch found in renewable carbohydrate
feedstocks such as lignocellulosic biomass or corn.  Glucose is a potential
renewable feedstock for fuel-cell hydrogen production.   The enthalpy of
reaction for the reforming of glucose to hydrogen and carbon dioxide in water
is highly endothermic (+620 kJ/mol at 25 ^oC).  Furthermore, significant
rates of hydrogen production are not realized until the reaction temperature is
at least 600 ^oC, and water is the in the supercritical state (221 bar,
374 ^oC).  A continuous-flow microchannel reactor, which uses 100
micron diameter channels to provide high rates of heat transfer that are
proportional to the inverse of the channel diameter, is ideal for driving the
this endothermic reaction and for rapidly heating the aqueous glucose feed
solution up to the reaction temperature.  Glucose was non-catalytically
gasified to a mixture of hydrogen, carbon dioxide, carbon monoxide, and methane
in supercritical water at 240 bar and temperatures of 600 ^oC or
higher within two different microchannel reactor configurations.   The first
microchannel reactor configuration was simply a 2.0 m serpentine 314 stainless
steel HPLC tube imbedded within a heating block.  Inner diameters of the tubing
ranged from 127 to 508 microns.  Aqueous glucose solution was pumped directly
into the microchannel reactor by an HPLC pump.  The reactor effluent was cooled
to 25 ^oC in a shell-tube heat exchanger and then stepped down from
240 to 1.0 bar pressure to separate out the gas and liquid products.  For
example, at 600 ^oC, 240 bar, 0.1 M glucose feed solution
concentration, and a nominal fluid fluid residence time of 28 sec within a 508
micron large-diameter tube, the gas composition was 36.1 %H₂, 50.2 %CO₂,
12.8 %CH₄, and 0.46 % CO for a hydrogen yield of 3.3 mol H₂ /
mol glucose fed.  Glucose conversion was 100% at all fluid residence times (1.4
to 28 sec), but only at residence times of 30 sec and greater was all the
carbon in glucose was converted to gaseous products.  At lower residence times,
organic acids were found in the liquid phase, suggesting that glucose was
converted to organic acid intermediates in sub-critical water which subsequently
gasified to H₂, CO₂, CO and CH₄ in concert
with the water gas-shift and methanation reactions at supercritical conditions. 
Gas phase product selectivity was a function of both fluid residence time and
the inner diameter of the tube.  This suggests that microchannel reactor
configurations for enhancing the heat transfer rate could provide process
intensification, enhance hydrogen product yield, and suppress CO formation. 
Towards this end, a second microchannel reactor configuration, which consists
of a parallel array of 100 by 200 micron channels, is presently being
fabricated in 314 stainless steel by a combination of micromachining, chemical
etching, and hotpress microlamination bonding techniques.  Spacing between
microchannels was optimized to accommodate material stresses at 600 ^oC
and 240 bar.  The final component of this study will compare hydrogen
productivity from glucose reforming in supercritical water within single
microchannel reactor versus the scaleable, parallel-array microchannel reactor.