3. Results and discussion
The ethane mole fraction profiles along the fluid centerline of the catalytic reactor are presented in Figure 3 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. Regardless of the specific catalytic reactor configuration, the reactor should be capable of providing good contact between the oxidation catalyst and the fuel-oxidant mixture in the exothermic catalytic reaction channel. For catalytic combustion applications, it is preferred that the reactor be sized such that the reaction of the fuel-oxidant mixture in the exothermic catalytic reaction channel should proceed more than 50 percent of the way to completion before exiting. For fuel-lean mixtures, this means that more than 50 percent of the fuel entering the channel should be consumed. Most preferably, more than 80 percent of the fuel entering the exothermic catalytic reaction channel should burn before exiting. The percentage of reaction completed in the exothermic catalytic reaction channel depends both upon the flow rate of the fuel-oxidant mixture through the exothermic catalytic reaction channel and upon the physical characteristics of the catalytic reactor. The chemical composition of the fuel-oxidant mixture may also affect the percentage of reaction completed, particularly if the rate of chemical reaction is significantly limiting when compared to the rate of mass transfer to the catalyst surface. With regard to percentage of reaction completed, important physical characteristics within the exothermic catalytic reaction channel include the rate of mass transfer to the oxidation catalyst surface, the ratio of oxidation catalyst surface area to reaction channel volume, and the activity of the oxidation catalyst. The catalyst-coated substrate may be fabricated from any of various high temperature materials. High temperature metal alloys are preferred, particularly alloys composed of iron, nickel, and cobalt, in combination with aluminum, chromium, and other alloying materials. High temperature nickel alloys are especially preferred. Other materials which may be used include ceramics, metal oxides, intermetallic materials, carbides, and nitrides. Metallic substrates are most preferred due to their excellent thermal conductivity, allowing effective backside cooling of the catalyst. The support material, preferably metallic or intermetallic, may be fabricated using conventional techniques to form a honeycomb structure, spiral rolls or stacked patterns of corrugated sheet, sometimes inter-layered with sheets which may be flat or of other configuration, or columnar or other configuration which allow for the presence of adjacent longitudinal channels which are designed to present flow channels in accordance with the design criteria set forth above. If intermetallic or metallic foil or corrugated sheet is employed, the catalyst will be applied to only one side of the sheet or foil or in some cases the foil or sheet will remain uncoated depending on the catalyst structure design chosen. Applying the catalyst to only one side of the foil or sheet, which is then fabricated into the catalyst structure, takes advantage of the integral heat exchange concept, allowing heat produced on the catalyst to flow through the structure wall into contact with the flowing gas at the opposite non-catalytic wall thereby facilitating heat removal from the catalyst and maintaining the catalyst temperature below the temperature for complete adiabatic reaction. In this regard, the adiabatic combustion temperature is the temperature of the gas mixture if the reaction mixture reacts completely and no heat is lost from the gas mixture. The critical difference in the design of the catalytic versus non-catalytic channels for the catalytic structure, in its most basic terms, is that the catalytic channels are designed so that the reaction mixture flow passages defined by the catalytic channels possess a higher or increased tortuosity over the corresponding flow passages formed by the non-catalytic channels. The concept of tortuosity, as used herein, is defined as the difference between the length of the path which a given portion of reaction mixture will travel through the passage formed by the channel as a result of changes in direction of the channel and changes in channel cross-sectional area versus the length of the path traveled by a similar portion of the reaction mixture in a channel of the same overall length without changes in direction or cross-sectional area, in other words, a straight channel of unaltered cross-sectional area. The deviations from a straight or linear path, of course, result in a longer or more tortuous path and the greater the deviations from a linear path the longer the traveled path will be. When applied to the catalyst structures, differences in tortuosity between catalytic and non-catalytic channels is determined by comparing the average tortuosity of all of the catalytic channels in the structure to the average tortuosity of all of the non-catalytic channels in the structures.