Figure 5. Contour plots of temperature, methanol mole fraction, and
hydrogen mole fraction under nominal operating conditions for the heat
integrated reactor.
The contour plots of water mole fraction, oxygen mole fraction, and
carbon dioxide mole fraction under nominal operating conditions are
presented in Figure 6 for the heat integrated reactor. A reaction
chamber has dimensions of height, width and length. The length of the
reaction chamber is typically longer. Typically, the sides of the
reaction chamber are defined by reaction chamber walls. These walls are
preferably made of a hard material such as a ceramic, an iron-based
alloy, or high temperature nickel-based superalloys. More preferably,
the reaction chamber walls are comprised of stainless steel which is
durable and has good thermal conductivity. In addition to thermal
transfer between adjacent reaction chambers, in some cases, a reaction
chamber can be in thermal contact with a microchannel heat exchanger.
This combination of reaction chambers and heat exchangers can result in
high rates of thermal transfer. The catalyst could also be applied by
other methods such as wash coating. On metal surfaces, it is preferred
to first apply a buffer layer by chemical vapor deposition or thermal
oxidation, which improves adhesion of subsequent wash coats. Preferred
reactors and methods of conducting reactions in integrated reactors can
be characterized by their properties. The devices may be made of
materials such as plastic, metal, ceramic and composites, depending on
the desired characteristics. Walls separating the device from the
environment may be thermally insulating. However, the walls separating
adjacent exothermic and endothermic reaction chambers should be
thermally conductive.
Introduction of laterally distributed combustion fuel and air in co-flow
with endothermic reactant flow concentrates the heat transfer at the
endothermic reactor inlet, where the concentration gradient is highest,
thus obtaining superior results over systems that distribute the
combustion fuel evenly over the entire surface of the combustion
catalyst. Although the examples with distributed combustion still
exhibit excellent heat flux in comparison to conventional steam
reformers. The present design could use alternate exothermic reactions,
such as oxidation reactions, including partial oxidation reaction, to
drive an endothermic reaction. During operation, a reactant enters a
combustion or reaction chamber in a bulk flow path flowing past and in
contact with a porous material or porous catalyst. A portion of the
reactant molecularly transversely diffuses into the porous catalyst and
reacts to form a product or products, and then the products diffuse
transversely into the bulk flow path and out of the reactor. A short
heat transport distance is desired for good performance. These short
heat transport distances, combined with preferred reactor
configurations, can provide surprisingly high volumetric productivity
and low pressure drop. The reactor designs suffer from a fundamental
limitation resulting from the flow configuration in which a reacting
stream flows parallel to a heat transfer surface through which the
majority of heat is transferred perpendicular to the direction of fluid
flow. Regardless of the reaction taking place in the reaction channels,
its reaction rate will vary along the flow length of that channel due to
changes in concentration and temperature. Balancing the heat
requirements of an endothermic reaction with heat generated by an
exothermic reaction flowing parallel to and on the opposite side of a
separating plate is extraordinarily difficult since the endothermic
reaction is likely to have a very different dependence upon
concentration and temperature than the endothermic reaction. Along the
flow length of the plate that divides these reactions, the heat flux
through the plate that is perpendicular to fluid flow will vary due to
temperature and reaction rate differences along the flow length of the
plate. Since the thermally coupled reactions are so closely coupled,
neither reaction can run at a significantly different reaction rate at
any point along the channel length. Thus, each reaction will exhibit a
peak in reaction rate at nearly the same position within the reactor
with slower reaction rates before and after this peak, which leads to
the need for a long reactor channel to ensure complete conversion. A
specific example of this reaction rate problem encountered in the
parallel flow arrangement is demonstrated by attempts to drive
endothermic steam reforming with exothermic combustion in microchannel
and alternating parallel plate reactors. A convenient way to supply heat
is to couple the endothermic reaction with an exothermic combustion
reaction in the heat exchange channels. Thus, the stacked reactor
becomes an alternating series of endothermic and exothermic reactors
separated by thin heat exchange walls. Unfortunately, the combustion
reaction is difficult to control with convenient combustion catalysts
and fuels, and most of the combustion occurs near the fuel inlet. This
uneven combustion results in uneven heat transfer to the endothermic
reaction and poor overall reactor performance. The geometry allows
intimate thermal contact whilst keeping the streams from becoming mixed.
The reactor body is constructed by modification of a substantially rigid
and essentially nonporous monolith honeycomb. Prior to modification the
monolith consists of a honeycombed body having a matrix of thin walls
defining a multiplicity of discrete channels which pass through the body
of the structure from one face to the opposing face. The monolith is
modified in such a way as to produce a rigid body containing at least
two discreet process flow paths which have a number of dividing walls in
common. A channel is defined as any individual passageway through the
monolith body and a flow path is the group of channels used for a single
reaction. The inlets to the first reaction flow paths and the inlets to
the second reaction flow paths are in the inlet manifold such that the
reactants run in a co-current configuration. The reactants also leave
the reactor in the same outlet manifold via the first reaction outlets
and the second reaction outlets. The fuel processor is described using
the steam reformation of methane and oxidation of methane reactions to
illustrate the concept. The inlets and outlets may be arranged for a
countercurrent flow of the reactants.