Figure 3. Effect of catalyst layer thickness on the enthalpy of reaction
in the oxidation and reforming processes of the thermally coupled
reactor for conducting simultaneous endothermic and exothermic
reactions.
The effect of catalyst layer thickness on the methanol conversion and
hydrogen yield in the oxidation and reforming processes is illustrated
in Figure 4 in the thermally coupled reactor for conducting simultaneous
endothermic and exothermic reactions. In addition to the reaction
channels, additional features such as microchannel or non-microchannel
heat exchangers may be present. Microchannel heat exchangers are
preferred. Adjacent heat transfer microchannels enable temperature in
the reaction channel to be controlled precisely to promote steam
reforming and minimize unselective reactions in the gas phase. The
thickness of a wall between adjacent process channels and heat exchange
channels is preferably 0.80 mm or less. Each of the process or heat
exchange channels may be further subdivided with parallel subchannels.
The heat exchange fluids can be gases or liquids and may include steam,
liquid metals, or any other known heat exchange fluids. Especially
preferred heat exchangers include combustors in which a fuel is oxidized
to produce heat for the steam reforming reaction. The incorporation of a
simultaneous exothermic reaction to provide an improved heat source can
provide a typical heat flux of roughly an order of magnitude above the
convective cooling heat flux. The flow of hot fluid through a heat
exchanger may be cross flow, counter-flow or co-flow. For all of the
above conditions, the approach to equilibrium conversion is the ratio of
measured hydrocarbon conversion to equilibrium hydrocarbon conversion.
The equilibrium composition or moles hydrocarbon out at equilibrium is
based upon the measured average pressure of the inlet and outlet of the
reactor zone and the inlet molar composition. The equilibrium
distribution or composition for a given temperature, pressure, and inlet
mole fraction distribution can be calculated using Gibbs free energies
with programs. The catalyst requires catalytically active surface sites
that reduce the kinetic barrier to the steam reforming reaction. The
catalyst comprises one or more of the following catalytically active
materials: ruthenium, rhodium, iridium, nickel, palladium, platinum, and
carbide of group VIB. Rhodium is particularly preferred. The
catalytically active materials are typically quite expensive. Therefore,
it is desirable to minimize the amount used to accomplish the desired
performance. The catalyst also contains an alumina support for the
catalytically active materials. An alumina support contains aluminum
atoms bonded to oxygen atoms, and additional elements can be present.
Preferably, the alumina support comprises stabilizing element or
elements that improve the stability of the catalyst in hydrothermal
conditions. Stabilizing elements typically are large, highly charged
cations. Preferably, the catalytically active materials are present in
the form of small particles on the surface of the stabilized alumina
support. The catalytically active layer is preferably disposed on a
porous substrate. Preferably, the catalyst contains an alumina layer
disposed on a thermally conductive surface. The surface could be, for
example, a porous substrate or reaction chamber walls.