Figure 5. Velocity contour plots of flow in 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 gas-phase reaction rate profiles along the fluid centerline of the catalytic reactor are presented in Figure 6 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 method takes advantage of a sequence of fuel combustion zones, and improves the system by conducting at least a portion of the thermal combustion in a gas expansion zone of the catalytic reactor. Part of the thermal combustion may occur in both of these types of expansion areas. As a result, a continuous reheating effect occurs within the catalytic reactor and this system provides for the highly efficient use of the fuel, greater than if no combustion occurs in the expansion zone of the catalytic reactor, without the formation of undesirable amounts of nitrogen oxides during such combustion and the development of excessively high temperatures. Part of the catalytically supported thermal combustion of the fuel-air mixture is conducted upstream of the catalytic reactor by combustion of the mixture while passing through an insufficient amount of catalyst to effect complete combustion of the fuel prior to passing through the expansion zone. The partially combusted effluent from the catalyst, with or without substantial intermediate but incomplete further combustion, is introduced into a gas expansion zone of the catalytic reactor so that the partially combusted effluent is further thermally combusted while undergoing expansion in the catalytic reactor. Catalytically-supported thermal combustion in the catalytic reactor is achieved by contacting at least a portion of the carbonaceous fuel intimately admixed with air with a solid oxidation catalyst having an operating temperature substantially above the instantaneous auto-ignition temperature of the fuel-air admixture. At least a portion of the fuel is combusted in the catalytic reactor under essentially adiabatic conditions. Combustion in the catalytic reactor is characterized by the use of a fuel-air admixture having an adiabatic flame temperature substantially above the instantaneous auto-ignition temperature of the admixture but below a temperature that would result in any substantial formation of oxides of nitrogen. The adiabatic flame temperature is determined at catalyst inlet conditions. The resulting effluent is characterized by high thermal energy useful for generating power and by low amounts of atmospheric pollutants. Where desired, combustible fuel components, for instance, un-combusted fuel or intermediate combustion products contained in the effluent from the catalytic zone, or fuel-air admixture which has not passed through a catalytic zone, may be combusted in a thermal zone following the catalytic zone. Sustained catalytically-supported, thermal combustion occurs at a substantially lower temperature than in conventional adiabatic thermal combustion and therefore it is possible to operate without formation of significant amounts of oxides of nitrogen. Combustion in the catalytic reactor is no longer limited by mass transfer as in the case of conventional catalytic combustion, and at the specified operating temperatures the reaction rate is substantially increased beyond the mass transfer limitation, for instance, at least about 8 times greater than the mass transfer limited rate. In the catalytic reactor, reaction rates of up to about 80 or more times the mass transfer limited rate may be attainable. Such high reaction rates permit high fuel space velocities which normally are not obtainable in catalytic reactions. One can employ, for instance, at least an amount of fuel equivalent in heating value to about 200 pounds of ethane per hour per cubic foot of catalyst, and this amount may be at least several times greater, for instance, an amount of fuel equivalent in heating value to at least about 800 pounds of ethane per hour per cubic foot of catalyst. There is, moreover, no necessity of maintaining fuel-to-air ratios in the flammable range, and consequently loss of combustion due to variations in the fuel-to-air ratio is not the problem it is in conventional combustors [71, 72]. When the fuel-air ratio in the cooling channels is within the limits of flammability, it is preferred that the cross-stream area for flow through the cooling channels is not increased after the cooling stream enters the conduits [73, 74]. The cross-stream area of the cooling passages may in fact be decreased to increase the cooling flow velocity for greater resistance to flashback or pre-ignition in the cooling portion of the reactor. The minimum residence time for pre-ignition to occur in the conduits is dependent upon fuel type, fuel-air ratio, temperature, and pressure, and can be measured experimentally or calculated on the basis of elementary chemical reaction rates, if known. Gas temperatures in the cooling portion of the reactor may rise to near the material limit of the catalyst and substrate, resulting in very short ignition delay times. To prevent pre-ignition in the cooling portion of the reactor, the gas residence time in the cooling portion of the reactor should be less than the ignition delay time. Increasing the cooling flow velocity within the cooling passages reduces the residence time of the cooling fluid within the reactor, and reduces the cooling fluid’s propensity for pre-ignition. Similarly, decreasing the velocity of the fluid in the exothermic catalytic reaction channel provides increased residence time for reaction in a given length reactor, while the cooling fluid’s residence time within the same length reactor remains at a smaller value, allowing a reduced propensity for pre-ignition of the cooling fluid. When applied to the catalysis of highly exothermic reactions, the catalyst structures are typically monolithic-type structures comprising a heat resistant support material composed of a plurality of common walls which form a multitude of adjacently disposed longitudinal channels for passage of a gaseous reaction mixture wherein at least a portion of the channels are coated on at least a part of their interior surface with a catalyst for the reaction mixture, catalyst-coated channels, and the remaining channels are not coated with catalyst on their interior surface, catalyst-free channels, such that the interior surface of the catalyst-coated channels are in heat exchange relationship with the interior surface of adjacent catalyst-free channels and wherein the catalyst-coated channels differ in configuration from the catalyst-free channels such that the desired reaction is promoted in the catalytic channels and suppressed in the non-catalytic channels of the catalytic reactor [75, 76]. In cases where the catalyst structure is employed in a catalytic combustion or partial combustion process, the critical difference in the design of the catalytic versus non-catalytic channels will insure more complete combustion of the fuel in the catalytic channels and minimum combustion in the non-catalytic channels over a wider range of linear velocity, inlet gas temperature and pressure [77, 78]. The highly exothermic process can be carried out in a single catalytic reaction zone employing the catalyst structure or in multiple catalytic reaction zones using catalyst structures designed specifically for each catalytic stage. In most cases the catalytic reaction zone will be followed by a homogeneous combustion zone in which the gas exiting from the earlier catalytic combustion zone is combusted under non-catalytic, non-flame conditions to afford the higher gas temperature. The homogeneous combustion zone is sized to achieve substantially complete combustion and to reduce the carbon monoxide level to the desired concentration.