3. Results and discussion
The contour plots of temperature, methanol mole fraction, and hydrogen
mole fraction under nominal operating conditions are presented in Figure
5 for the heat integrated reactor. 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 applied to 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. In this design, enclosed
channels are formed by stacking plates separated by spacers, and the
stack is fitted with appropriate headers so that alternating channels
contain the reaction fluid with heat exchange fluid in the intermediate
channels. The reaction channels can be filled with catalyst, and the
heat exchange channels can have a structured packing to increase the
heat exchange area. Another approach to increasing the surface area for
reaction on each side of the separating plate is to add fins or other
surface features. Indeed, this approach is adopted in plate-type reactor
designs. Although somewhat successful, the design still adds complexity
and the alternating coupled reaction chambers continue to restrict the
overall size of each chamber. All of these examples share the same
general flow geometry where thermal energy transfers between chemically
reacting fluid streams that flow parallel to and on opposite sides of a
separating plate.