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.