Figure 2. Schematic illustration of the integral heat exchange within
the catalytic reactor in which at least a portion of the thermal
combustion of the fuel takes place in the expansion zone of the
catalytic reactor to counteract the cooling effect of the expansion of
the gases.
The preferred materials are aluminum-containing steels. These steels
contain sufficient dissolved aluminum so that, when oxidized, the
aluminum forms alumina whiskers, crystals, or a layer on the steel's
surface to provide a rough and chemically reactive surface for better
adherence of the washcoat. The washcoat may be applied using an
approach, for instance, the application of gamma-alumina, zirconia,
silica, or titania materials or mixed sols of at least two oxides
containing aluminum, silicon, titanium, zirconium, and additives such as
barium, cerium, lanthanum, chromium, or a variety of other components.
For better adhesion of the washcoat, a primer layer can be applied
containing hydrous oxides such as a dilute suspension of pseudo-boehmite
alumina. The primed surface may be coated with a gamma-alumina
suspension, dried, and calcined to form a high surface area adherent
oxide layer on the metal surface. Most desirably, however is the use of
a zirconia sol or suspension as the washcoat. Other refractory oxides,
such as silica and titania, are also suitable. Most preferred for some
platinum group metals, notably palladium, is a mixed zirconia-silica sol
where the two have been mixed prior to application to the support.
Silica appears to allow the zirconia to maintain the catalyst's
stability, for instance, its activity, for a long period of time. The
washcoat may be applied in the same fashion one would apply paint to a
surface, for instance, by spraying, direct application, and dipping the
support into the washcoat material. Aluminum structures are also
suitable for use in this design and may be treated or coated in
essentially the same manner. Aluminum alloys are somewhat more ductile
and likely to deform or even to melt in the temperature operating
envelope of the process. Consequently, they are fewer desirable supports
but may be used if the temperature criteria can be met.
The catalyst structure is comprised of a series of adjacently disposed
catalyst-coated and catalyst-free channels for passage of a flowing
reaction mixture wherein the channels at least partially coated with
catalyst are in heat exchange relationship with adjacent catalyst-free
channels and wherein the catalyst-coated channels have a configuration
which forms a more tortuous flow passage for the reaction mixture than
the flow passage formed by the catalyst-free channels. For convenience
herein the terms "catalyst-coated channels" or "catalytic channels" in
the catalyst structures may refer to single channels or groupings of
adjacent channels which are all coated with catalyst on at least a
portion of their surface, in effect a larger catalytic channel
subdivided into a series of smaller channels by catalyst support walls
or pervious or impervious barriers which may or may not be coated with
catalyst. Similarly, the "catalyst-free channels" or "non-catalytic
channels" may be a single channel or grouping of adjacent channels which
are all not coated with catalyst, that is, a larger catalyst-free
channel subdivided into a series of smaller channels by catalyst support
walls or pervious or impervious barriers which are not coated with
catalyst. In this regard, increased tortuosity of the flow passages
formed by the catalyst-coated channels means that the catalyst-coated
channels are designed such that at least a portion of the reaction
mixture entering the catalyst-coated channels will undergo more changes
in direction of flow as it traverses the length of the channel than will
any similar portion of reaction mixture entering the catalyst-free
channels. In practice, the increased tortuosity of the flow passage in
the catalyst-coated channels can be accomplished by a variety of
structural modifications to the channels including periodically altering
their direction and changing their cross-sectional area along their
longitudinal axis while the catalyst-free channels remain substantially
straight and unaltered in cross-sectional area. Preferably the
tortuosity of the catalyst-coated channels is increased by varying their
cross-sectional area though repeated inward and outward bending of
channels walls along the longitudinal axis of the channels or through
the insertion of flaps, baffles or other obstructions at a plurality of
points along the longitudinal axes of the channels to partially obstruct
and divert the direction of reaction mixture flow in the channels.