Figure 4. Effect of reactor length on the methanol and hydrogen mole
fraction profiles along the length of the micro-structured
heat-exchanger reactor for hydrogen production by steam methanol
reforming.
The hydrogen and carbon dioxide mole fraction contour maps in the
micro-structured heat-exchanger reactor are illustrated in Figure 5 for
hydrogen production by steam methanol reforming. The steam reforming
catalyst is involved in the production of hydrogen, typically hydrogen
gas. Most often, the steam reforming catalyst is involved in the
production of hydrogen from methanol and water. In some instances, the
methanol steam reforming reaction can be represented by two reactions,
namely, a first cracking reaction followed by a water gas shift
reaction. When the steam reforming catalyst is coated on a monolith
substrate, a suitable amount is provided to produce hydrogen. While the
steam reforming catalyst may be in the form of a powder or pressed into
pellets, in small-scale applications involving miniature devices and
reactors with mesoscale and microscale features, including channels and
other miniature device structures, pellets are not feasible due to their
size. And both pellets and powders are not often mechanically stable in
smaller configurations. In this connection, concerns over attrition,
clogging of channels, sufficient adherence to the substrate, and
stability during vibration are raised. Thus, while a steam reforming
catalyst in the form of a powder or pellets is appropriate in larger
devices, diminished performance may result when using the steam
reforming catalyst in the form of a powder or pellets in miniature
devices and reactors. The steam reforming catalyst is formed on or in a
ceramic monolith substrate to provide for a miniaturized chemical
reactor including a porous ceramic material having a catalyst
immobilized within or upon the porous ceramic material. In miniature
devices and reactors, it may be more desirable to have an immobilized
support that retains the high porosity and surface area possible with
bulk powders. When the steam reforming catalyst is formed on or in a
ceramic monolith substrate, the immobilized catalyst is positioned in
such a way as to allow reactants to intimately contact the immobilized
catalyst, while not degrading the catalytic activity of the catalyst.
When fabricating a monolithic steam reforming catalyst system using
multilayer ceramic structures, the reactors that constitute the system
which typically include a post fire deposition of a catalyst, do not
provide for selective deposition of the catalyst material post firing.
This is because, in part, it is difficult to deposit catalyst into small
channels. In many instances, since the structure is fired prior to
introduction of the steam reforming catalyst, the steam reforming
catalyst is not able to be positioned where it is needed so as to
provide optimum temperature profiles as desired. In order to make steam
reforming catalyst system containing multilayer ceramic structures, a
green multilayer ceramic structure is coated with the steam reforming
catalyst described herein containing yttrium, palladium, a metal oxide
and cerium, and optionally zinc, and then the coated green multilayer
ceramic structure is fired to provide a steam reforming catalyst system.
This steam reforming catalyst system may have a honeycomb structure, and
may or may not contain spacers between individual reactors. The ceramic
substrate may be porous ceramic substrate.