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

Gregory L. Rorrer, Oregon State University, Department of Chemical Engineering, Corvallis, OR 97331, Aaron Goodwin, Chemical Engineering, Oregon State University, Corvallis, OR 97331, and Brian Paul, Department of Industrial & Manufacturing Engineering, Oregon State University, Corvallis, OR 97331.

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.