Figure 6. Gas-phase reaction rate profiles along the fluid centerline of the catalytic reactor with an integral heat exchange structure in an array of longitudinally disposed, adjacent reaction passageways or channels which are either catalyst-coated or catalyst-free.
The reaction rate contour plots in the gas phase of the catalytic reactor are illustrated in Figure 7 with an integral heat exchange structure in an array of longitudinally disposed, adjacent reaction passageways or channels which are either catalyst-coated or catalyst-free. In preferred catalyst structures, the channels coated with catalyst differ from the catalyst-free channels by having an average hydraulic diameter which is lower than the average hydraulic diameter of the catalyst-free channels and by having a higher film heat transfer coefficient than the catalyst-free channels. More preferably, the catalyst-coated channels have both a lower hydraulic diameter and a higher film heat transfer coefficient than the catalyst-free channels. Thus, for the catalyst structures, the average hydraulic diameter can be determined by first finding the hydraulic diameter for all of the catalyst-coated channels in the structure by calculating the average hydraulic diameter for any given channel over its entire length and then determining the average hydraulic diameter for the catalyst-coated channels by totaling up all of the calculated hydraulic diameters for the individual channels, multiplied by a weighing factor representing the fractional open frontal area for that channel. Following the same procedure, the average hydraulic diameter for the catalyst-free channels in the structure can also be determined. The finding that the catalyst-coated channels most advantageously have a lower average hydraulic diameter than the catalyst-free channels can be explained, in part, by the fact that the catalyst-coated channels desirably have a surface to volume ratio which is higher than that of the catalyst-free channels, since hydraulic diameter bears an inverse relationship to surface to volume ratio. Further, in the catalyst structures, the difference in average hydraulic diameter of the catalyst-coated channels and catalyst-free channels gives an indication that the catalyst-free channels, on average, must be more open channeled and therefore, the gas flow through these channels is less affected by changes in the channel diameter than the catalyst-coated channels, again, in part, because of the higher surface to volume ratios in the catalyst-coated channels. Preferably, the numeric ratio of the average hydraulic diameter of the catalyst-coated channels to the average hydraulic diameter of the catalyst-free channels, that is, average hydraulic diameter of catalyst-coated channels divided by average hydraulic diameter of catalyst-free channels is between about 0.08 and about 0.88 and, most preferably, the ratio of average hydraulic diameter of catalyst-coated channels to catalyst-free channels is between about 0.2 and 0.8. The film heat transfer coefficient is a dimension-less value, which is measured experimentally by flowing gas, for instance, air or air-fuel mixtures, at a given inlet temperature through an appropriate test structure having the specified channel geometry and temperature and measuring the outlet gas temperature. Since the gas composition, flow rates, pressures and temperatures in the catalytic and non-catalytic channels of the catalyst structure are very similar, the film heat transfer coefficient provides useful means of characterizing the different flow geometries provided by the various flow channel configurations which distinguish the catalyst-coated channels from the catalyst-free channels of the catalyst structure. Since these different flow geometries, in turn, are related to the tortuosity of the flow path formed by the channels, the film heat transfer coefficient provides some measure of tortuosity as it is employed in the catalyst structures. While one could conceive of a variety of methods to measure or otherwise determine the film heat transfer coefficient in the catalyst structures, one convenient method would involve constructing an experimental test structure, for instance, a solid thick metal structure, with internal space machined to simulate the desired channel shape; and then to test it in environments where the wall temperature is essentially constant from inlet to outlet or varies from inlet to outlet and is measured at several points along the channel length in the structure. The catalyst structure is particularly useful when equipped with appropriate catalytic materials for use in a combustion or partial combustion process wherein a fuel, in gaseous or vaporous form, is typically partially combusted in the catalyst structure followed by complete homogeneous combustion downstream of the catalyst. With the catalyst structure, it is possible to obtain more complete combustion of fuel in the catalytic channels with minimum combustion in the non-catalytic channels over a wider range of linear velocities, gas inlet temperatures and pressures than has here-to-for been possible with catalyst structures of the previous designs, including those employing integral heat exchange. Accordingly, the design also encompasses an improved catalyst structure for use in the combustion or partial combustion of a combustible fuel, as well as a process for combusting a mixture of a combustible fuel and air or oxygen-containing gas, using the catalyst structure.