3. Results and discussion
The temperature and oxygen mole fraction contour maps in the
micro-structured heat-exchanger reactor are illustrated in Figure 1 for
hydrogen production by steam methanol reforming. Chemical reactions that
produce heat and those that take up heat form two very important classes
of reactions. Some highly exothermic reactions, reactions with a large
but negative heat of reaction, require heat to be removed from a system
to prevent overheating. One example is the partial oxidation of ethylene
to produce ethylene oxide, an important intermediate in the production
of ethylene glycol. This reaction oxidizes ethylene over a catalyst to
produce ethylene oxide and heat. If the reaction temperature is too
high, ethylene oxide will decompose to carbon dioxide and water. In
order to reduce degradation into undesired products, the reaction
temperature must be held under control by removing heat produced by the
partial oxidation. Conversely, endothermic reactions, those with a
positive heat of reaction, do not produce heat but require heat for the
reaction to proceed. Steam reforming of hydrocarbons is an endothermic
reaction of considerable interest for hydrogen production as a fuel for
fuel cells. Steam reforming produces hydrogen and carbon monoxide when
heat is added to a catalytic reactor containing steam and hydrocarbons.
Although exothermic and endothermic reactions are easy to implement, to
do so with a compact and simple reactor design is challenging due to the
limitations of heat transfer between the reaction and the outside of the
reactor. One aspect in building compact reactors with adequate thermal
exchange requires a provision for high interfacial area between the
reaction stream and the reactor body. Microchannel technology is capable
of high heat and mass transfer coefficients between a bulk reaction
fluid and the catalytic heat exchange surface. Alternating channel
parallel plate designs can be employed in applications for thermally
coupling endothermic steam reforming with combustion in neighboring
channels. Such designs enable orders of magnitude size reduction over
conventional shell-and-tube steam reformers. Enclosed parallel flow
channels are typically formed by stacking plates separated by spacers,
and fitting the stack with appropriate headers so that alternating
channels contain the reforming reaction with exothermic combustion in
the intermediate channels. Microchannel reactors exchange heat between
chemically reacting fluid streams where flow is parallel to and on
opposite sides of a thermally conductive separating plate.