ENVIRONMENTAL CONTROL SYSTEM INCORPORATING A DUAL BED REACTOR

Abstract: An environment control system (10, Fig. 2) for use in a transportation machine. The system is effective for receiving and conditioning a heated airstream (16, Fig. 2) containing organic compounds and ozone, prior to delivering the airstream to a habitable space within the machine. The system includes a dual bed reactor having an upstream portion (46, Fig. 2) which is effective for converting organic compounds within the airstream into carbon dioxide and water, and a downstream portion (48, Fig. 2) which is effective for decomposing ozone within the airstream. Both the upstream and downstream portions of the core structure include at least one fin assembly (114, Fig. 8). Each portion may include a plurality of the fin assemblies, configured as annular rings and generally concentrically disposed relative to one another. Alternatively, each portion of the reactor may comprise a single fin assembly which is wrapped upon itself in a spiral configuration. Radially adjacent ones of the fin assemblies of each portion, or radially adjacent spirals of the individual fin assembly of each portion, are brazed to one another so as to prevent nesting between radially adjacent ones of the fin assemblies or spirals. Each fin assembly of the upstream portion is anodized, with a catalyst which is effective for converting organic compounds into carbon dioxide and water being disposed on and within the resulting anodized surface layer. Each fin assembly of the downstream portion may be similarly anodized, with application of a catalyst effective for decomposing ozone, or alternatively, each fin assembly of the downstream portion may be constructed from a catalytically-active metal alloy.

Description: ENVIRONMENTAL CONTROL SYSTEM INCORPORATING A DUAL BED REACTOR CROSS-REFERENCES The subject application is a Continuation-Iπ-Part of each of the following co- pending and commonly assigned U.S. Patent Applications: Serial No. 08/271,922 filed on July 7, 1994 entitled "Catalytic Converter With Metal Core Structure Having An Integral Catalyst"; Serial No. 08/494,656 filed on June 26, 1995 entitled "Catalytic Converter With Metal Core Structure Having An Integral Silver Catalyst"; Serial No. 08/645,015 filed on May 9, 1996 entitled "Catalytic Reactor For The Decomposition Of Ozone". BACKGROUND OF THE INVENTION 1.0 Field of the Invention The present invention relates generally to environmental control systems, and more particularly, to an environmental control system incorporating a dual bed reactor which is effective for converting organic pollutants within an airstream to carbon dioxide and water and is also effective for decomposing ozone within the airstream. The environmental control system of the present invention has particular application for use in conditioning the air supplied to a passenger cabin of an aircraft. However, the environmental control system of the present invention may also be used in conjunction with other transportation machines such as military tanks. 2.0 Related Art Aircraft environmental control systems supply pressurized and conditioned air to the aircraft cabin. The temperature, pressure and relative humidity must be controlled to provide for the comfort of the flight crew and passengers within the aircraft. Modern commercial jet aircraft are t pically designed for fuel-efficient operation at relatively high altitudes of 25,000 feet or more above sea level where the ozone content of the air is relatively high. Accordingly, during operation at high altitudes the air supplied to the aircraft cabin from the environmental control system may contain ozone at a level of 1 - 3 ppmv. The presence of ozone in the air within the aircraft cabin can cause lung and eye irritation, headaches, fatigue and/or breathing discomfort. As a result, present Federal Aviation Agency (FAA) regulations limit ozone concentrations on commercial flights to 0. lppmv during a three hour time period and 0.25 ppmv maximum at any time. The use of catalytic converters to reduce or eliminate undesirable ozone in the air supplied to aircraft cabins, in situations where relatively high ozone levels are expected, is known in the art. An example of a commonly known type of catalytic convener is illustrated in U.S. Patent 4,405,507 which discloses a ceramic (cordierite) core structure support structure having a high surface area washcoat applied to the core structure support, with the washcoat being used to carry the catalyst (in this case a platinum group metal and a non-precious Group VIII metal oxide or aluminate). While catalytic converters, or reactors, of this type have found widespread use, they are subject to the following limitations. In the first instance, the washcoat is subject to attrition when exposed to continued vibration and thermal cycling, such as that which may be experienced when a converter is used in an aircraft application. Secondly, the washcoat and the catalyst carried by the washcoat may be washed off of the ceramic core structure support structure during routine maintenance cleaning. Additionally, catalytic reactors of this type typically include a plurality of spaced flow channels which are formed in the support structure along an axially extending axis, and through which the ozone-containing air flows under laminar conditions. Catalytic reactors of this type are typically mass-transfer limited, i.e. the efficiency of the reactor may be limited by the ability of the ozone molecule to diffuse to the surface of the catalyst. This requires laminar-flow reactors to be much larger, and consequently heavier, than those which may employ turbulent flow of the ozone-containing air through the reactor. It is also known in the art to utilize catalytic converters for the decomposition of ozone in conjunction with aircraft environmental control systems as shown in U.S. Patent 4,665,973 to Limberg, et al. issued May 19, 1987 and assigned to the assignee of the present invention. Limberg, et al. discloses a "washcoat" type coating 58 which is applied to the plate elements 54 of the hot pass side of a primary heat exchanger 18 in an environmental control system 10 of the type utilized in aircraft, tanks, or other vehicles. Although the environmental control system of Limberg, et al. provides for ozone decomposition while minimizing weight and space penalties associated with the corresponding environmental control system, the catalytic coating 58 is of the type described previously and is therefore subject to attrition when exposed to continued vibration and thermal cycling, such as that which may be experienced in an aircraft environment. Aircraft bleed air supply streams, used for passenger cabin make-up air, may also contain numerous organic contaminants such as engine oil vapors, engine exhaust gas, jet fuels, deicing fluids and cleaning solvents. These contaminants, if not destroyed, create unwanted odors and passenger discomfort (and may in some cases cause illness) in the aircraft passenger cabin. Additionally, oil vapors can foul, or poison a catalyst used to decompose ozone. U.S. Patent 4,755, 360 to Dickey, et al. discloses a known system for removing oil contamination from bleed air flowing from a port of an aircraft gas turbine engine to a duct leading to the cabin ventilation system of the aircraft. The system disclosed in Dickey, et al. includes a catalytic converter matrix element 16 mounted within a pipe 15. Element 16 is preferrably made by rolling together a first, smooth sheet 21 and a second, corrugated sheet 22 so as to define a parallel array of a large number of slender tubules 23. The included example in Dickey, et al. indicates that the flow through the individual tubules is laminar. Accordingly, the device disclosed in Dickey, et al. is subject to the disadvantages of laminar-flow catalytic reactors as discussed previously. Additionally, the use of the smooth sheet 21 in the roiled up configuration results in a plurality of radially spaced spirals of this sheet, which reduces the space available for the tubules (which have interior surfaces coated with a catalytically active coating 24 of finely-divided particles of metal such as platinum) of the corrugated sheet 22, thereby further reducing the efficiency of the catalytic converter matrix element 16. The foregoing illustrates limitations known to exist in present catalytic converters, or reactors of the type which may be used in an environmental control systems for conditioning air supplied to a habitable space in a transportation machine such as the passenger cabin in aircraft. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. SUMMARY OF THE INVENTION In one aspect of the present invention, this is accomplished by providing an environmental control system for use in a transportation machine, with the system being effective for receiving and conditioning a heated airstream containing organic pollutants and ozone, prior to delivering the airstream to a habitable space within the machine. According to a preferred embodiment, the environmental control system comprises a dual bed reactor which includes a metallic housing having an inlet portion effective for receiving the heated airstream, an outlet portion effective for discharging the airstream from the reactor, and a generally cylindrical portion disposed axially between and connected to the inlet and outlet portions. The reactor further comprises a thermally compliant core structure disposed within the generally cylindrical portion of the housing. The thermally compliant core structure includes an upstream portion which includes at least one catalyst which is effective for converting at least a portion of the organic pollutants within the heated airstream into carbon dioxide and water as the airstream flows through the upstream portion of the core structure. The thermally compliant core structure further includes a downstream portion which is effective for decomposing at least a portion of the ozone present within the airstream as the airstream flows through the downstream portion. In other embodiments, both the upstream and downstream portions of the thermally compliant core structure are disposed within and brazed to the generally cylindrical portion of the housing of the dual bed reactor, with each portion of the core structure including at least one fin assembly. Both the upstream and downstream portions may comprise a plurality of fin assemblies, each configured as an annular ring, with the fin assemblies being generally concentrically disposed relative to one another about a centerline axis of the reactor. Alternatively, either one, or both of the upstream and downstream portions may comprise a single fin assembly wrapped upon itself in a spiral configuration. Each fin assembly of the upstream portion includes a metallic support and an anodized surface layer integral with the metallic support. At least one catalyst, comprising at least one metal selected from the group consisting of Group VIII noble metals is disposed on and within the anodized surface layer of each fin assembly. Similarly, each fin assembly of the downstream portion of the core structure may include a metallic support and an anodized surface layer integral with the metallic support, with at least one catalyst, comprising one or more Group VHI noble metals disposed on and within the anodized surface layer. In this embodiment, the fin assemblies of the downstream portion may further include a base metal which is selected from a group consisting of Group VIII metals, Group IIIA metals, and Group VIIA metals. Palladium may comprise the Group VITI noble metal and nickel may comprise the base metal of the downstream portion. Alternatively, silver may be disposed on and within the anodized surface layer of each of the fin assemblies of the downstream portion of the core structure. The anodized surface layers of the upstream poπion and the downstream portion (of the associated embodiments) may have a thickness of at least 10 microns. The surface area of the anodized surface layer may range from about 30 m 2 /g to about 150 m 2 /g. As a further alternative, each of the fin assemblies of the downstream portion of the core structure may be constructed from a catalytically-active metal comprising a silver-containing metal alloy. The catalytically-active metal may have a composition including silver and copper and more specifically, may have a composition comprising, on a weight basis, about 55% silver, about 39% copper, about 5% zinc, and about 1% nickel. In other embodiments, the environmental control system may include a first heat exchanger having an inlet and an outlet for the flow of the airstream therethrough, with the first heat exchanger being adapted to receive a flow of cooling fluid in heat exchange, non-mixing relationship with the airstream so as to cool the airstream. The inlet of the first heat exchanger is in fluid communication with the outlet portion of the housing of the dual bed reactor. The system may further include a compressor having an inlet in fluid communication with the outlet of the first heat exchanger and an outlet for discharging the airstream therefrom, with the compressor being effective for compressing the airstream. The system may further include a second heat exchanger having an inlet and an outlet for the flow of the airstream therethrough, with the inlet of the second heat exchanger being in fluid communication with the outlet of the compressor and with the second heat exchanger being adapted to receive a flow of cooling fluid in heat exchange, non-mixing relationship with the airstream so as to provide cooling to the airstream. The system may further include a turbine rotatably coupled to the compressor, with the turbine having an inlet in fluid communication with the outlet of the second heat exchanger and an outlet which is effective for discharging the airstream therefrom. The turbine is effective for expanding and further cooling the airstream. The system may further include a third heat exchanger disposed upstream of the dual bed reactor, with the third heat exchanger having an inlet for receiving the heated airstream and an outlet in fluid communication with the inlet portion of the housing of the dual bed reactor. The third heat exchanger is adapted to receive a flow of cooling fluid in heat exchange, non-mixing relationship with the airstream so as to cool the airstream. The system may further include a means for mixing the cooled airstream discharging from the turbine with a relatively warmer airstream so as to produced a mixed airstream having a temperature which is higher than a temperature of the cooled airstream, prior to supplying the mixed airstream to the habitable space of the machine. According to a second aspect of the present invention, an alternative to prior environmental control systems is accomplished by providing a method for conditioning a heated airstream which contains organic compounds and ozone, prior to delivering the airstream to a habitable space within a transportation machine. According to a preferred embodiment, the method comprises the steps of: supplying the heated airstream to a the dual bed reactor having a housing and a thermally compliant core structure disposed within the housing, the core structure having an upstream portion and a downstream portion; converting at least a portion of the organic compounds within the airstream to carbon dioxide and water as the airstream flows through the upstream portion of the core structure; and decomposing at least a portion of the ozone within the airstream as the airstream flows through the downstream portion of the core structure. According to other embodiments, the method may further comprise the steps of: constructing the upstream portion of the dual bed reactor with at least one fin assembly; and constructing the downstream portion of the dual bed reactor with at least one fin assembly. The method may further comprise the steps of anodizing a metallic support of each of the fin assemblies of the upstream portion of the core structure so as to form an anodized surface layer integral with the metallic support and disposing a first catalyst on and within the anodized surface layer of the each of the fin assemblies of the upstream portion of the core structure, with the first catalyst being effective for converting at least a portion of the organic compounds into carbon dioxide and water. The method may further include the steps of anodizing a metallic support of each of the fin assemblies of the downstream portion of the core structure so as to form an anodized surface layer integral with the metallic support and disposing a second catalyst on and within the anodized surface layer of each of the fin assemblies of the downstream portion of the core structure, with the second catalyst being effective for decomposing at least a portion of the ozone. Alternative, the method may comprise the step of manufacturing each of the fin assemblies of the downstream portion of the core structure from a catalytically-active metal alloy. According to other embodiments, either one or both of the upstream and downstream portions of the core structure may comprise a plurality of fin assemblies, or alternatively, either the upstream portion or the downstream portion, or both, may comprise a single fin assembly. In the embodiments where the upstream portion comprises a plurality of fin assemblies, the method may further comprise the steps of configuring each of the fin assemblies as an annular ring; disposing the fin assemblies of the upstream portion in generally concentric relationship with one another within the generally cylindrical portion of the housing; inserting braze foil between radially adjacent ones of the fin assemblies of the upstream portion; and brazing the fin assemblies of the upstream portion to one another so as to prevent nesting between radially adjacent pairs of the fin assemblies of the upstream portion. In the embodiments where the downstream portion of the core structure comprises a plurality of fin assemblies, the method may further comprise similar steps as previously discussed with respect to the plurality of fin assemblies of the upstream portion. In the embodiments where the upstream portion of the core structure comprises a single fin assembly, the method may further comprise the steps of wrapping the single fin assembly upon itself in a spiral configuration having a plurality of spirals; inserting braze foil between radially adjacent ones of the spirals; and brazing the spirals of the single fin assembly to one another so as to prevent nesting between radially adjacent pairs of the spirals. The method may further comprise similar steps associated with the downstream portion of the core structure when the downstream portion also comprises a single fin assembly. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other aspects of the present invention will become more apparent from the subsequent detailed description of the invention when considered in conjunction with the accompanying drawings, wherein: Fig. 1 is a schematic view illustrating the environmental control system of the present invention; Fig. 2 is a longitudinal view illustrating the dual bed reactor included in the environmental control system of the present invention; Fig. 3 is a fragmentary isometric view illustrating a fin assembly, which is incoφorated in the dual bed reactor of the present invention; Fig. 4 is an enlarged fragmentary view illustrating a portion of the fin assembly shown in Fig. 3; Fig. 5 is a view taken along 5-5 in Fig. 3; Fig. 6 is a fragmentary isometric view illustrating a fin assembly according to an alternative preferred embodiment, which may be incorporated in the dual bed reactor of the present invention; Fig. 7 is a cross-sectional view taken along line 7-7 in Fig. 2; Fig. 8 is a cross-section view, similar to Fig. 7, illustrating the dual bed reactor according to an alternative, preferred embodiment of the present invention; Fig. 9 is an enlarged view of a portion of Fig. 7 illustrating the brazing of radially adjacent fin assemblies of one embodiment of the dual bed reactor of the present invention; Fig. 10 is a series of photo micro-graphs illustrating aluminum surfaces: unanodized (Fig 10a); anodized at 16°-20°C and calcined at 440°C (Fig. 10b); anodized at 23°-25°C and calcined at 440°C (Fig. 10c); anodized at 30°-37°C and calcined at 440°C (Fig. lOd); Fig. 11 is a series of photo micro-graphs illustrating aluminum surfaces anodized at 30°-37°C and calcined at the following temperatures: 150°C (Fig. I la); 400°C (Fig. 1 lb); and 540°C (Fig. 1 lc); Fig. 12 is an ozone destruction curve for a catalytically-active metal alloy used to construct fin assemblies according to one embodiment of the present invention and illustrates the effects of thermally activating the alloy; Fig. 1 is a series of graphs illustrating the effect of temperature on the surface composition of the catalytically-active metal alloy of the present invention; Fig. 14 is a photo micro-graph of the surface of the catalytically-active metal alloy of the present invention in an as-received condition; Fig. 15 is a photo micro-graph of the surface of the catalytically-active metal alloy of the present invention after thermal activation; Fig. 16 is an ozone destruction graph illustrating the ability of the catalytically- active metal alloy of the present invention to recover after temporary exposure to a sulfur dioxide-contaminated, ozone-containing feed air; Fig. 17 is a series of graphs illustrating the ability of the hydrocarbon destruction catalyst of the present invention to destroy or convert n-decane at various temperatures; Fig. 18 is a series of graphs illustrating the ability of the hydrocarbon destruction catalyst of the present invention to destroy n-decane in a feed air contaminated with sulfur dioxide; Fig. 19 is a series of graphs illustrating the ability of the hydrocarbon destruction catalyst of the present invention to destroy n-decane in a feed air contaminated with phosphorus; Fig. 20 is a pair of graphs illustrating the predicted performance of the upstream (hydrocarbon destruction) and downstream (ozone decomposition) portions of the dual bed reactor of the present invention. DETAILED DESCRIPTION Referring now to the drawings, Fig. 1 is a schematic view illustrating an environmental control system 10 according to a preferred embodiment of the present invention. In the illustrative embodiment shown in Fig. 1, system 10 is used in an aircraft application. However, it should be understood that system 10 may also be used in conjunction with other transportation machines such as military tanks. The aircraft (not shown) may include a plurality of engines, one of which is indicated schematically at 12. The engine 12 receives ambient air 14 and processes the air 14 in a conventional manner for the purpose of producing propulsive power for propelling the aircraft. The environmental control system 10 receives a heated airstream, indicated by flow arrows 16 in Fig. 2, via conduit 18. The airstream 16 may be contaminated by a variety of pollutants which include ozone (due to high altitude operation of the aircraft), and numerous organic contaminants or pollutants such as: engine oil vapors, engine exhaust gas, unburned jet fuels, deicing fluids such as ethylene glycol, aircraft hydraulic fluids, and cleaning solvents. Some of the organic pollutants such as engine exhaust gas and unburned jet fuels may be experienced during ground operation of engine 12 due to the presence and operation of surrounding aircraft. In the illustrative embodiment, the airstream 16 is supplied from a compressor stage, indicated at 20, of engine 12. Alternatively, the airstream 16 may be provided from other sources such as an auxiliary power unit or a ram air compressor. In typical aircraft applications, the temperature of the heated airstream 16 may range from about 149°C to about 216°C. However, in certain aircraft applications, the temperature of the heated, ozone-containing airstream 16 may be much cooler and lower temperature catalytic activity may be required to decompose the ozone present in airstream 16, which is achievable with the apparatus and method of the present invention as subsequently discussed in greater detail. The heated airstream 16 is supplied via conduit 18 to a first inlet 22 of a heat exchanger 24 of system 10. Heat exchanger 24 may comprise a pre-cooler mounted to engine 12 and may be optionally omitted from system 10. Heat exchanger 24 includes a first outlet 26 effective for discharging the airstream 16 from heat exchanger 24. Heat exchanger 24 is also adapted to receive a flow of a cooling fluid, indicated by flow arrows 28, in heat exchange, non-mixing relationship with the airstream 16 so as to cool the airstream 16. The cooling fluid 28 may comprise ambient air, or alternatively air supplied from engine 12 at a lower temperature than airstream 16, such as fan discharge air or air from a lower compressor stage of engine 12. The cooling fluid 28 is supplied to a second inlet 30 of heat exchanger 24 and discharges from heat exchanger 24 via a second outlet 31. The first outlet 26 of heat exchanger 24 is in fluid communication dual bed reactor 32 which is effective for converting at least a portion of the organic contaminants contained within airstream 16 to carbon dioxide and water and is further effective for decomposing at least a portion of the ozone present in airstream 16. The first outlet 26 of heat exchanger 24 communicates with reactor 32 via a conduit 34. The reactor 32 includes a housing 36 having an inlet portion 38, an outlet portion 40, and a generally cylindrical portion 42 disposed axially between and connected to the inlet portion 38 and the outlet portion 40. Reactor 32 further includes a core structure 44 which is disposed within and connected to the generally cylindrical portion 42 of housing 36. The core structure 44 may be directly attached, by brazing, to portion 42 of housing 36 or alternatively may be indirectly attached, with the use of an intermediate ring (not shown) as subsequently discussed in greater detail. The airstream 16 is supplied to the inlet portion 38 of housing 36 via conduit 34 and then flows through the core structure 44. The core structure 44 includes an upstream portion 46 which is effective for converting at least a portion of the organic contaminants within airstream 16 to harmless or innocuous molecules such as carbon dioxide and water, and a downstream portion 48 which is effective for decomposing at least a portion of the ozone present within airstream 16. Additional features of the dual bed reactor 32 are subsequently discussed in greater detail and the specific construction of reactor 32 comprises central features of the present invention. After the levels of organic pollutants and ozone within airstream 16 have been reduced to acceptable levels within the dual bed reactor 32, the airstream 16 discharges from reactor 32 through the outlet portion 40 of housing 36 which is in fluid communication with a first inlet 50 of a heat exchanger 52 via a conduit 54. Heat exchanger 52 also includes a first outlet 56, effective for discharging airstream 16 therefrom. Heat exchanger 52 is also adapted to receive a flow of cooling fluid, indicated by flow arrows 58, in heat exchange, non-mixing relationship with the airstream 16 so as to cool (or further cool, if heat exchanger 24 is employed) the airstream 16. The cooling fluid 58 may be supplied from the various sources discussed previously in conjunction with cooling fluid 28. It is further noted that the cooling fluids 28 and 58 may be supplied from a common source or from different sources. The cooling fluid 58 is supplied to a second inlet 60 of heat exchanger 52 and discharges from heat exchanger 52 via a second outlet 62. System 10 further includes a rotatable compressor 64 having an inlet 66 which is in fluid communication with the first outlet 56 of heat exchanger 52, via a conduit 68. Compressor 64 is rotatably coupled to a turbine 70 via a shaft 72. Compressor 64 is effective for compressing the airstream 16, which discharges compressor 64 via an outlet 74. The airstream 16 is then supplied to a first inlet 76 of a heat exchanger 78, via a conduit 80. Heat exchanger 78 further includes a first outlet 82, effective for discharging airstream 16 from heat exchanger 78. Heat exchanger 78 is also adapted to receive a flow of cooling fluid, indicated by flow arrows 84, in heat exchange, non- mixing relationship with the airstream 16 so as to provide cooling to the airstream 16. Accordingly, heat exchanger 78 is effective for off-setting at least a portion of the temperature rise caused by the compression of the airstream 16 -within compressor 64. The cooling fluid 84 is supplied to a second inlet 86 of heat exchanger 78 and discharges from heat exchanger 78 through a second outlet 88. Cooling fluid 84 may be provided from the sources discussed previously in conjunction with cooling fluids 28 and 58, and may be provided from either common or different sources as those used to supply cooling fluids 28 and 58. After discharging from outlet 82 of heat exchanger 78, airstream 16 is supplied to an inlet 90 of turbine 70 via a conduit 92. Turbine 70 is effective for expanding the airstream 16 and accordingly, for further cooling the airstream 16. Airstream 16 provides the motive force for rotating turbine 70, which in turn drives compressor 64 via shaft 72. The airstream 16 which has been cooled and has an ozone and hydrocarbon content within acceptable levels, discharges from turbine 70 through an outlet 94 and is supplied to a conditioner sub-assembly 96, which functions in a subsequently described manner, via a conduit 98. The airstream 16 is then supplied from the conditioner sub-assembly 96 to a habitable space 97, comprising a passenger compartment of the aircraft in the illustrative embodiment shown in Fig. 1, via a conduit 99. During certain operating conditions, the temperature of the airstream 16 may be below ambient temperature as airstream 16 discharges from the outlet 94 of turbine 70 such that any water vapor present in airstream 16 may be condensed into liquid water. To prevent ice formation, the environmental control system 10 includes a means for mixing the cooled airstream 16 discharging from turbine 70 with a relatively warmer airstream prior to supplying the resultant mixed airstream to the habitable space 96. The means for mixing the cooled airstream 16 with a relatively warmer airstream includes a control valve 100 and conduits 102 and 104. A portion of the airstream 16 discharging from the dual bed reactor 32 is routed through conduits 54 and 102 to an inlet of control valve 100, which establishes the desired flow rate of this airstream, and is then supplied to conduit 98 via conduit 104 with this heated airstream mixing with the cooled airstream 16 so as to produce a mixed airstream having a temperature which is greater than the temperature of the cooled airstream 16 as it discharges from the outlet 94 of turbine 70. The mixed airstream is then supplied to the conditioner sub-assembly 96 of the aircraft. Accordingly, by setting control valve 100 to a predetermined position, the temperature of the mixed airstream supplied to the conditioner sub-assembly 96 may be such that ice formation is prevented and an acceptable temperature level of the airstream is provided. The conditioner sub-assembly 96 comprises a condenser (not shown), an air/water separator (not shown) and a re-heater (not shown). The conditioner sub- assembly 96 is effective for separating any water contained within the mixed airstream prior to delivering the mixed airstream to the habitable space 97 and is further effective for increasing the temperature of the mixed airstream to an acceptable level prior to delivering the mixed airstream to the habitable space 97 of the aircraft. The specific construction of the dual bed reactor 32 of the present invention is illustrated in Figs. 2-9. The inlet portion 38 of housing 36 includes a flanged, upstream end 106 which is attached to a flanged downstream end of conduit 34 via a pair of clamps (not shown). Similarly, the outlet portion 40 of housing 36 includes a flanged, downstream end 110 which is attached to a flanged upstream end of conduit 54 via a pair of clamps (not shown). Alternatively, inlet portion 38 and outlet portion 40 of housing 36 may be attached to conduits 34 and 54, respectively, by other conventional means such as mating pairs of bolted flanges. In a preferred embodiment, inlet portion 38, cylindrical portion 42 and outlet portion 40 of housing 36 comprise a unitary construction. Additionally, housing 36 is preferrably made of stainless steel. Alternatively, housing 36 may be made of other metals such as aluminum, aluminum alloys, titanium and titanium alloys. In certain embodiments of the present invention, when the material of construction of core structure 44 is compatible with the material of construction of housing 36, with respect to brazing, the core structure 44 is directly attached, by brazing, to housing 36 as subsequently discussed in greater detail. In other embodiments which utilize a material of construction for housing 36 which does not permit brazing of the core structure 44 to housing 36, reactor 32 includes a first intermediate ring (not shown) made of a braze-compatible material which is brazed to the upstream portion 46 of the core structure 44, and a second intermediate ring (not shown) made of a braze-compatible material which is brazed to the downstream portion 48 of the core structure 44. In these embodiments, either one or both of the intermediate rings may be directly attached to housing 73 by conventional means such as rivets, or alternatively, either one or both of the intermediate rings may be attached to housing 73 with a resilient mount structure (not shown) which accommodates differential thermal growth between housing 73 and the corresponding one of the intermediate rings. The resilient mount structures may include one or more springs and may be made of an elastomeric material. Inlet portion 38 may comprise configurations other than that shown in Fig. 2, within the scope of the present invention. For instance, inlet portion 38 may comprise a diffuser which increases in diameter from a location just aft of the flanged upstream end 106 to a downstream end which is connected to the cylindrical portion 42 of housing 36. With this configuration, inlet portion 38 may further facilitate static pressure recovery so as to limit the pressure drop through system 10 so as to avoid excessive use of power by engine 12 to provide the required airflow to the habitable space 96. The upstream portion 46 of the core structure 44 includes at least one fin assembly 114. Fig. 3 is a fragmentary isometric view illustrating a portion of a single fin assembly 114, which is further illustrated in Figs. 4 and 5. Each of the fin assemblies 114 includes a metallic support 116, preferably made of aluminum or an aluminum alloy, and further includes an anodized surface layer 118 which is integral with the metallic support 116. As best seen in Fig. 4, surface layer 118 is integral with each side and edge of the metallic support 116 and therefore includes a plurality of orientations as shown in Fig. 3. As subsequently discussed in greater detail, a catalyst, which is effective for converting organic contaminants within the airstream 16 to carbon dioxide and water or is effective for decomposing ozone within the airstream 16, is disposed on and within the anodized surface layer (or layers) 118 of fin assembly 114. The downstream portion 48 of the core structure 44 includes either at least one of the fin assemblies 114 or, alternatively, at least one fin assembly 120, with a portion of a single fin assembly 120 being illustrated in Fig. 5. Fin assembly 120 is identical to fin assembly 114 except that fin assembly is constructed, or fabricated from a catalytically-active metal alloy which is effective for decomposing ozone in the airstream 16 and fin assembly 120 does not include the anodized surface layer 118 used in fin assembly 114 to support catalytically-active metals. The specific construction of fin assemblies 114 and 120 comprise central features of the present invention. According to one preferred embodiment, both the upstream portion 46 and the downstream portion 48 of the core structure 44 include a plurality of the fin assemblies 114, with each of the fin assemblies 114 being configured as an annular ring as shown in Fig. 7. In this embodiment, the fin assemblies 1 14 of the upstream portion 46 of core structure 44 are generally concentrically disposed relative to one another about an axially extending centerline axis 122 (shown in Fig. 2) of the dual bed reactor 32 and the fin assemblies 114 of the downstream portion 48 of core structure 44 are generally concentrically disposed relative to one another about axis 122. Due to the physical limitations of configuring the fin assemblies 1 14 as annular rings, the center of the space defined by the housing 36 of reactor 32 may be occupied by a small diameter support tube 124 having a finned strip 126. Alternatively, the support tube 124 may be capped at both ends. In other preferred embodiments, either the upstream portion 46 or the downstream portion 48 of the core structure 44 may alternatively comprise a single fin assembly 114 which is wrapped upon itself in a spiral, or generally helical configuration as shown in Fig. 8. In further alternative, preferred embodiments, the downstream portion 48 of the core structure 44 may comprise a plurality of the fin assemblies 120, with each of the fin assemblies 120 being configured as an annular ring and with the fin assemblies 120 generally concentrically disposed relative to one another about axis 122 in the same manner as that illustrated in Fig. 7 with respect to fin assemblies 114. In yet another alternative, preferred embodiment, the downstream portion 48 of the core structure 44 includes a single fin assembly 120 which is wrapped upon itself in a spiral, or generally helical configuration as shown in Fig. 8 with respect to fin assembly 1 14. It is noted that the views of the fin assembly 114 shown in Figs. 3-5, and the view of fin assembly 120 shown in Fig. 6 are prior to the fin assemblies 114 and 120 being configured as shown in either Figs. 7 or 8. Each of the fin assemblies 114 includes an inlet end 128 which is effective for receiving the flow of airstream 16 and an outlet end 130 effective for discharging the airstream 16 therefrom. Accordingly, the airstream 16 discharges from the upstream portion 46 of the core structure 44 through the outlet end 130 of each of the included fin assemblies 114. Similarly, each of the fin assemblies 120 includes an upstream end 132 and a downstream end 134. In the embodiments of the present invention where the downstream portion 48 of core structure 44 includes one or more fin assemblies 120, the upstream end 132 of each fin assembly 120 is effective for receiving the airstream 16 discharging from the upstream portion 46 of core structure 44, and the outlet end 134 of each of the included fin assemblies 120 is effective for discharging the airstream from the downstream portion 48 of core structure 44. Each of the fin assemblies 114 further includes a plurality of fins 136 which are arranged in an axial succession of rows 138 of the fins 136 which are arranged between the inlet end 128 and the outlet end 130 of the corresponding fin assembly 114. Similarly, each of the fin assemblies 120 further includes a plurality of fins 140 which are arranged in an axial succession of rows 142 of the fins 140 between the inlet end 132 and the outlet end 134 of the corresponding fin assembly 120. Further details of the construction of fin assemblies 114, as well as fin assemblies 120, may be appreciated with reference to Figs. 5 and 9. The following discussion in conjunction with Figs. 5 and 9 is directed to each of the fin assemblies 1 14 but is equally applicable to the corresponding elements of each of the fin assemblies 120, which differ from fin assemblies 114 only with respect to the omission of the anodized surface layer 118 of fin assembly 114 and the use of a catalytically-active metal alloy to construct fin assemblies 120 in place of the aluminum or aluminum alloy support 1 16 of fin assembly 1 14. Each of the fin assemblies 114 is made by shaping a substantially flat, relatively thin sheet of the metallic support 116 by conventional means such as stamping so as to form the plurality of the fins 136. The number of the rows 138 of fins 136 may vary with the particular application of the dual bed reactor 32. Three of the rows 138 of fins 136 are illustrated in Fig. 3 and designated as 138 A, 138B, and 138C. Rows 138A and 138B are further illustrated in the elevation view shown in Fig. 5. Each of the rows 138 includes a plurality of the fins 136 and defines a plurality of axially extending flow channels 144. Similarly, each of the rows 142 of fin assemblies 120 includes a plurality of the fins 140 and defines a plurality of axially extending flow channels 145. Each of the fins 136 includes a leading edge 146. The fins 1 6 and the flow channels 144 of each of the rows 138 are off-set by a distance x (Fig. 5) relative to the fins 136 and the flow channels 144, respectively, of axially adjacent ones of the rows 138. For instance, the fins 136 of row 138B are off-set with respect to the fins 136 of rows 138 A and 138C. The off-set x corresponds to a circumferential off-set after the fin assemblies 114, or the single fin assembly 114, have been configured as shown in Figs. 7 and 8, respectively. Consequently, each fin assembly 114 is configured so as to define a plurality of tortuous flowpaths through the axially extending flow channels 144, defined by each of the rows 138 of fins 136, for the flow of the airstream 16 between the inlet end 128 and the outlet end 130 of the corresponding one of the fin assemblies 114. This may be illustrated with reference to Fig. 3, although it should be understood that in actual use the airstream 16 does not flow through the fin assemblies 114 until they have been configured either as shown in Fig. 7 or in Fig. 8. The airstream 16 initially flows through the flow channels 144 of row 138 A, depicted in Fig. 3. After discharging from the flow channels 144 of row 138 A a portion of the airstream 16 impacts the leading edges, indicated at 146B, of the fins 136 of row 138B, prior to flowing through the flow channels 144 of row 138B. This process is repeated as the airstream 16 encounters each subsequent row 138 of the fins 136. For instance, after discharging from the flow channels 144 of row 138B, a portion of the airstream impacts the leading edges 146C of the fins 136 of row 138C. The impact of the airstream 16 with the leading edges 146 of the rows 1 8 of fins 136, adds turbulence to the airstream 16. Consequently, the use of the off-set rows 138 of fins 136 within fin assembly 114 results in a substantially turbulent flow of the airstream 16 between the inlet end 128 and the outlet end 130 of each fin assembly 114. Since airstream 16 is substantially turbulent a relatively high mass transfer is achieved between the airstream 16 and the subsequently described catalyst which is disposed on and within the anodized surface layer 118 of fin assembly 114 (or between the airstream 16 and the catalytically-active metal alloy of fin assembly 120). Accordingly, the turbulence of airstream 16 facilitates the conversion of organic contaminants which may be present within airstream 16 to carbon dioxide and water and also facilitates the decomposition of ozone present within airstream 16. As shown in Fig. 5, each of the fins 136 has a generally rectangular cross-section in a preferred embodiment, as seen in an axial view, and includes a height H, an axial depth D, a thickness t, a pair of flats F, and an axial depth. Each fin 136 approximates a full sine- wave shape as illustrated in brackets in Fig. 5. The off-set x, as well as the overall size of the fins 136 may be varied to achieve the desired flow characteristics through each fin assembly 114 for a particular application. Each fin row 138 has a fin density which may also be varied with application. According to the illustrative embodiment shown in Fig. 3, each fin row 138 has a fin density of about 28 fins/in. as measured in a lateral direction prior to configuring the fin assembly 114 as an annular ring as shown in Fig. 7, or wrapping the fin assembly 114 upon itself in a spiral configuration as shown in Fig. 8. However, the fin density of each of the fin rows 138 may vary with application. Similarly, each of the rows 142 of each fin assembly 120 has a fin density which may vary with application and is about 28 fins/in. in the illustrative embodiment shown in Fig. 6. The fin density of each of the annular fin assemblies 114 shown in Fig. 7, or of the single fin assembly 114 shown in Fig. 8, varies across the radial extent of the fins 136 due to the accordion effect caused by the particular configuration of the fin assembly 114, as subsequently discussed in further detail. The use of a plurality of fin assemblies 114, as shown in Fig. 7, or of a single spiral-wrapped fin assembly 114 as shown in Fig. 8, provides relatively light weight and compact upstream and downstream portions 46 and 48, respectively, of the core structure 44 which is effective for converting organic contaminants within airstream 16 to harmless carbon dioxide and water within the upstream portion 46 of core structure 44, and for reducing the ozone content within airstream 16 by decomposing ozone within the downstream portion 48 of the core structure 44 to acceptable levels so that the airstream 16 may be subsequently supplied to the habitable space 96. Although each fin 136 preferably has a generally rectangular shape, the fins 136 may alternatively have other geometric shapes such as a generally triangular cross-section, or a generally trapezoidal cross-section. In the embodiment shown in Fig. 7, each fin assembly 114 is configured as an annular ring by conventional means such as rolling or forming, with the fins 136 of opposite ends of each fin assembly 114 being hooked together so as to maintain the desired configuration as an annular ring. The radially outermost one of the fin assemblies 114 is brazed to the generally cylindrical portion 42 of housing 36 by inserting a thin braze foil (not shown) between the radially outer flat F of each of the fins 136 of the radially outermost fin assembly 114 and portion 42 of housing 36, and then heating the structure to the required braze temperature. In this embodiment, each of the fin assemblies 114 is brazed to radially adjacent ones of the fin assemblies 114. This brazing is illustrated in Fig. 9, which is an enlarged view of a portion of Fig. 7 and illustrates a pair of radially adjacent fin assemblies, denoted as 114A and 114B in Fig. 9. A thin foil of braze alloy, indicated at 148, is placed between the radially outer fin assembly 114 A and the radially inner fin assembly 114B. As shown in Figs. 7 and 9, the individual fins 136 of radially adjacent ones of the fin assemblies may randomly align, partially align or be misaligned. In this embodiment, each braze foil 148 is configured as a continuous concentric ring prior to brazing. Placement of one of the braze foils 148 between each radially adjacent pair of fin assemblies 114 causes the aligned ones of the fins 136 of the adjacent pair of fin assemblies 114 to be brazed to one another, after exposure to the required brazing temperature. Application of the brazing temperature causes the braze foils 148 to melt and capillary into the areas where the flats F of the aligned and radially adjacent ones of the fins 136 contact one another. More specifically, the lower flats F of the radially outer fin assembly 114A are brazed to aligned ones of the upper flats F of the radially inner fin assembly 114B. Additionally, one of the braze foils 148 may be placed between the radially outermost one of the fin assemblies 114 and the housing 36, for those embodiments where the materials of construction of housing 36 and fin assemblies 114 permit brazing to one another. This brazing of radially adjacent ones of the fin assemblies 114 prevents nesting between the radially outer one and the radially inner one of each radially adjacent pair of fin assemblies 114 so as to maintain structural integrity of the core structure 44 In a similar manner, the radially adjacent spirals of the individual fin assembly 114 shown in Fig. 8, are brazed to one another. In this embodiment, the braze foil 148 (not shown) comprises a long, continuous ribbon which is wound up at the same time that the individual fin assembly 114 is configured as a spiral. The braze foil 148 is given one additional wrap to cover the radially outermost one of the spirals of the fin assembly 114 which permits the braze alloy to attach fin assembly 1 4 directly to housing 36 for those embodiments where the material of construction of housing 36 is braze-compatible with the material of construction of fin assembly 114, or alternatively to an intermediate ring (not shown) which is subsequently attached to housing 36 by conventional means such as rivets. The method of brazing radially adjacent ones of the fin assemblies 114, shown in Fig. 7, to one another or of brazing the radially adjacent spirals of the single fin assembly 114 shown in Fig. 8 to one another, may be referred to as statistical brazing. Statistical methods predict the number of brazed fins 136 and the distribution of braze joints within the core structure 44. This information may then be used in determining the structural characteristics of the core structure 44 for predicting the service life of core structure 44. The foregoing discussion regarding the statistical brazing method of the present invention is also applicable to each of the fin assemblies 120. The quality and distribution of the braze contacts within core structure 44 is enhanced by the mismatch in circumferential fin densities which occurs at the interface between each radially adjacent pair of fin assemblies 114 and at the interface between each radially adjacent pair of fin assemblies 120. This feature of the fin assemblies 114 and 120 of the present invention is illustrated in the following discussion regarding fin assemblies 114 with reference to Figs. 7-9. In the embodiment shown in Fig. 7, the lateral width of each fin assembly 114 as measured in a direction substantially transverse to the direction of flow of airstream 16 through flow channels 144, prior to configuration as an annular ring, is sized based on an average radius of the resultant annular ring. Due to the accordion effect, or "fanning out" of the individual fins 136 of a fin assembly 114, the circumferential fin density as measured at the radially outermost portion of each fin assembly (corresponding to the radially outer flats F) is less than the circumferential fin density as measured at the radially innermost portion of the fin assembly (corresponding to the radially inner ones of the flats F). Additionally, the fin assemblies 114 are sized such that the circumferential fin density of the radially inner one of a pair of radially adjacent fin assemblies 114 is always less than the circumferential fin density of the radially outer one of the pair of fin assemblies 1 14 at the interface locations. For instance, the circumferential fin density of fin assembly 114B, as measured at location 150 of each of the fins 136 is less than the circumferential fin density of fin assembly 114A as measured at location 152 of each of the fins 136 (with locations 150 and 152 being shown in Fig. 9), further reducing the likelihood that radially adjacent fin assemblies 114 will nest with one another, thereby further enhancing the structural integrity of the core structure 44. In conventional fin structures, such as those used in heat transfer devices, splitter plates are typically inserted between the adjacent ones of fin assemblies within a stack to prevent the individual fins of the adjacent fin assemblies from nesting within one another. The resultant matrix in these conventional structures becomes very rigid and is incapable of being thermally compliant to the surrounding environment when in service. In contrast, the core structure 44 of the present invention is thermally compliant which results from the ability of radially adjacent ones of the fin assemblies 114 shown in Fig. 7, or radially adjacent ones of the spirals of the individual fin assembly 114 shown in Fig. 8, to stretch or shrink, i.e. to accordion, when the core structure 44 is required to change diameter due to its service environment. This flexibility also permits direct attachment of the core structure 44, by brazing the radially outermost one of the fin assemblies 114 shown in Fig. 7 or the radially outermost spiral of the individual fin assembly 114 shown in Fig. 8 to the housing 36 as discussed previously, without the use of an intervening thermally compliant support structure notwithstanding the incompatible coefficients of thermal expansion of the stainless steel housing 36 and that of the aluminum or aluminum alloy used to construct fin assemblies 114 or the subsequently discussed catalytically-active metal alloy used to construct the fin assemblies 120 of the present invention. Another disadvantage of the use of splitter plates in conventional fin assembly stacks is that the use of such plates detracts from the overall performance of the device by reducing the average turbulation "J" factor of the device. Also, the use of splitter plates reduces the space available for highly turbulent fin assemblies. The additional space available and the higher average turbulation combine to provide a distinct performance advantage to a non-splitter-plate device, such as the core structure 44 of the present invention. Anodization of the Downstream Portion 48 of the Core Structure 44 Anodizing is an electrolytic oxidation process which has been used to provide a surface coating on aluminum for protection or decoration of the aluminum or to create a porous layer which can be used as a catalyst support. The process generally involves establishing an electrolytic cell with the aluminum structure as the anode. Passing an electric current through the aluminum oxidizes the surface to an adherent aluminum oxide. Specific conditions found to be important for preparing an aluminum oxide catalyst support by anodizing will be found in the Examples 1-5. However, more generally the process involves immersing a section of the aluminum substrate in an acidic electrolyte, preferably sulfuric acid, but which could be other acids used in anodizing such as oxalic acid, phosphoric acid and the like. The acid concentration will be selected to provide the desired oxide thickness in an acceptable time. For the preferred sulfuric acid the concentration may be about 5 to 20 wt.%, preferably 9 to 15 wt.%. The aluminum substrate will be the anode, while the cathode may be various metals or carbon. The anode and cathode are connected to a source of direct current having voltage available up to about 15 volts, generally 8 to 15 volts. The voltage is varied to provide a constant anodizing current, typically about 9 amp/ft 2 (96.88 amp/m 2 ), selected to obtain the desired thickness. The process is exothermic and during the time required to produce the desired surface layer, say about 30 to 60 minutes, the temperature will rise from the initial temperature unless heat is removed. It has been found that the temperature of the anodizing bath should be maintained relatively constant and above ambient, preferably above about 30° to 37°C, particularly about 32°C, since lower temperatures produce alumina surface films which are less satisfactory as catalyst supports, as will be shown in Example 4. The temperature of anodization has been found to be an important factor in determining the formation of a suitable surface for catalytic applications. This is illustrated in Figure 10 where the surface is shown to be remarkably changed when the anodization temperature is increased. The aluminum was anodized, washed with water and then calcined at 440°C. When anodized at 16°-23°C (Fig. 10b), the surface appears relatively smooth and not much different from the appearance of unanodized aluminum (10a). These low temperatures are typical of those used to form protective surface layers and also in prior art catalytic applications where a very fine pore structure was produced, which is difficult to see at the 3000X magnification shown. Raising the anodization temperature to 23°-25°C (Figure 10c) produces a dramatic change in the surface appearance. A further increase to 30°-37°C produces even more change (lOd). It is this surface which has been found to provide superior catalysts as will be shown below. The surface layer should be at least 2 : m thick, preferably at least 10 : m, and have a surface area of about 30 to 150 m 2 /g in order to provide sufficient surface for subsequent application of the catalytic metals. However, the thickness will depend on the particular application and the thickness of the aluminum (or alloy) walls. Following anodizing, the aluminum substrate is washed with water to remove residual acid and then dried before the next step, i.e. the deposition of catalytic metals. It is preferred that after drying the substrate is calcined in air to temperatures above 150°C, preferably 440° to 540°C. A higher surface area is produced which is more accessible to the catalytic metals and to the reacting gases. The effect of calcination temperature is illustrated in Figure 11, where aluminum surfaces anodized at the preferred temperature range of 30-37°C are shown after calcination at various temperatures. Figure 1 la show the aluminum surface after calcining at 150°C. Figure 1 lb shows the surface after calcining at 400°C. Figure l ie shown the surface after calcining at 540°C. These may be compared with Figure lOd in which the surface was calcined at 440°C. It is evident that calcining at higher temperatures causes a significant change in the surface structure. Experience has shown that this change in structure is associated with improved catalytic performance, as is shown below. It is feasible, although not preferred, to omit the calcination step since similar temperatures are used after impregnation to decompose the compounds of the catalytic metals, as is illustrated in Example 5 below. Catalyst Application: Downstream Portion 48 of Core Structure 44 The methods used for applying the catalytic metals are familiar to those skilled in the catalyst art. In a preferred method, they may be applied as solutions of soluble compounds of the metals, either by dipping the anodized substrate into the solution or by otherwise applying an amount sufficient to deposit the desired metal loading. Following the impregnation step, the support is calcined at about 350° to 550°C for a period of 1 to 6 hours to convert the metal compounds to the metals or their oxides. Useful catalytic metals for ozone decomposition include various noble metals from Group VHI, particularly platinum and palladium, supplemented by base metals from Group VIII, preferably nickel, cobalt, and iron, or from Groups IIIB and V1TB preferably cerium and manganese. For oxidation of pollutants, the noble metals may be supplemented by base metals from Groups IB, IIB, IIIA, IV A, VA, VIA, VTIA, and VIII. Various soluble noble metal compounds familiar to those skilled in the art may be used such as halogen compounds and nitrogen compounds (e.g., palladium nitrate, ammonium chloroplatinate). For some applications chloride-free compounds provide superior results and compounds such as platinum sulfite acid (hydroxy disulfite platinum II acid) are preferred. For an ozone converter palladium and nickel are preferably used in combination. For catalytic oxidation of pollutants, platinum is generally preferred, optionally in combination with base metals such as Ni, Co, Fe, Cu, Zn, Cr, W, Mn, and the rare earths. The amount of noble metals may vary up to about 150g/ft 3 (5.3g/L) of the core structure, preferably about 25 to 75 g ft 3 (0.88 to 2.65 g/L). The base metals will often be applied in larger amounts up to about 500 g/ft 3 of the core structure (17.6g L), preferably about 200 to 400 g/ft3 (7.06 to 14.1 g/L). Example 1 Anodization: Downstream Poπion 48 of Core Structure 44 A sample section of a fin assembly having 104 : m thick aluminum suppoπ or substrate as described above and having overall dimensions measuring 1.61 in. x 1.00 in. x 0.25 in. (40.9 mm x 2.54 mm x 6.35 mm) and having a surface area of 24 in 2 (0.02 m 2 ) was immersed in a 9 wt.% H 2 SO 4 solution. A cathode having a surface area of 95 in 2 (0.06 m 2 ) made of aluminum foil was also immersed in the H 2 SO solution. The substrate and the cathode were connected to a variable source of D.C. voltage (Heath Schlumberger) and a voltage of 15 volts was applied to begin the anodization of the substrate. The process was continued for about 1 hour during which time the voltage was gradually lowered to 8 volts to maintain a current density of about 9 amps/ft 2 (96.9 amps/m 2 ). Over the 1 hour period the temperature of the electrolyte rose from 30°C to 40°C due to the exothermic reaction. Following the anodization step the anodized substrate was washed in water, dried at 150°C for 15 minutes and measured. The weight loss was found to be 13.9 wt.%. The depth of the anodized surface layer was 10 : m. The sample was then calcined at 538°C for 1 hour. After cooling the sample was dipped in deionized water, removed and the excess water blown off, and weighed. It was found that the water uptake was 7.9 wt%. Example 2 Metals Application The anodized and calcined sample of Example 1 was impregnated with a solution of Pd and Ni compounds. 30.37 g of a 10.03 wt% Pd (NO 3 ) 2 solution was added to 89.21 g of Ni(NO 3 ) 2 *6H 2 0 and then diluted to 100 mL with deionized water. 5 wt.% sugar was added to complete the solution preparation. Then the calcined substrate was dipped into the solution and agitated for 30 seconds, after which the excess solution was blown off and the sample calcined at 538°C for 1 hr. The impregnation was repeated twice. It was found that the impregnated sample had picked up 9.4% in weight. The amount of the metals deposited was found to be 418 g/ft 3 Ni (14.7 g/L) and 78 g/ft 3 Pd (2.75 g/L). Example 3 A micro reactor was assembled to test a shon section of the offset-fin core structure prepared as in Examples 1 and 2 at one million space velocity (lxlO 6 hr " '). A section of a core structure 7 fins wide and 2 sets of 7 fins deep (see Figure 3) measuring 0.5 in. x 0.25 in. x 0.2 in. (12.7 mm x 6.35 mm x 5.1 mm) was mounted in the reactor and air containing 2.4 ppm by volume of ozone was passed over the catalyst at lxlO 6 GHSV (@ STP). The ozone conversion was measured by a PCI ozone monitor (Model LC) before and after the reactor. The results are given in Table 1. Table 1 Temp. "C Ozone Conv. % 35 48 50 51 60 52 90 55 100 57 120 58 130 59 150 60 160 61 170 62 190 63 200 64 It can been seen that the conversion of ozone is not very sensitive to temperature and it is believed to be controlled by mass transfer of the ozone to the catalytic surface. Calculations indicate that if the catalyst contained 33 rows of fins rather than 2 rows, the conversion of ozone would be greater than 93%. Testing for 72 hours indicated that the catalyst performance was retained with only about 2% reduction from the fresh conversion. Example 4 (Comparative) A sample of the aluminum finned substrate was anodized as described in Example 1 except that the initial temperature of the anodization bath was maintained at a constant 2°C instead of beginning at 30°C and rising to 40°C. After anodization, the sample was washed in water and then calcined at 440°C for one hour. The sample was then impregnated with a solution of Pd and Ni compounds as described in Example 2. The amount of metals deposited was 25 g/ft 3 Pd(0.88 g/L) and 100 g/ft 3 Ni (3.53 g/L). The completed catalyst was tested following the procedure described in Example 3 to determine its ability to conveπ ozone at temperatures of 65°C, 121°C, and 185°C. A second sample of the aluminum finned substrate was anodized as in Example 1 except that the temperature of the anodization bath was maintained at a constant 20°C instead of 30°C. The sample was then finished as described above except that the calcination was at 440°C rather than 538°C. The amount of metals deposited was 25 g/ft 3 Pd (0.88 g/L) and 100 g/ft 3 Ni (3.53 g/L). The second catalyst was then tested as described in Example 3 for ozone conversion. In Table 2 the results of the tests described above for the two samples are repoπed so that the effect of the anodization bath temperature can be seen. Table 2 Inlet Temperature, Theoretical Actual Conversion. % °C Conversion, % at Anodization Temperature 32'C 20*C 65 57 54% 41% 121 60 57% 54% 185 65 65% 65% The column labeled "Theoretical Conversion" provides a reference value for comparison with the actual conversion as measured experimentally. The calculation assumes that the catalyst is able to conveπ any ozone which reaches it, that is, the chemical reaction is not limiting. The calculation then is based on the reaction rate which should be observed if mass transfer of the reactants and products to and from the catalyst is limiting (L. Hegedus Pet. Div. ACS Preprints, August 1973, p. 487- 502). This method is for straight channel core structures and has been modified by us to account for the offset-fin design. If the actual conversion measured is the same as that which the mass transfer limited calculation predicts, then it follows that mass transfer is limiting in fact and that the catalyst activity is not. Conversely, if the conversion is lower than the mass transfer limited conversion, then the catalyst activity is limiting. From a review of the data in Table 2 it can be concluded that the catalyst prepared with an aluminum surface anodized at 32°C has an activity which is essentially mass transfer limited at the three temperatures measured. However, the catalyst prepared using aluminum which was anodized at 20°C appears to be much less active since the conversion at 65°C is much lower than would be predicted for a mass transfer limited catalyst. As the temperature is increased, the catalyst activity appears to increase so that at 185°C mass transfer rather than catalyst activity is limiting. The physical structure of the anodized surface is very different as has been shown in Figures 10 and 11. It is believed that the relatively rough and highly porous surface produced by the higher temperature anodization provides a higher surface area for deposition of the catalytic metals and a more active and durable catalyst results. This inference is supported by the results of the experiments reported above. As demonstrated in Table 2 the fresh catalyst activity is affected by the condition of the alumina on the surface. We expect that the durability of the catalyst also would be affected. The catalyst prepared on a surface anodized at 20°C would be expected to have a significantly shorter life than that prepared on a surface anodized at 32°C. The expected longer lifetimes of the catalyst prepared on surfaces anodized at 32 β C are believed to be due to the high surface area of the corresponding support which, after catalyzation, results in a significantly increased number of catalytically- active sites as compared to catalysts prepared on a low-temperature anodized support. Example 5 Two samples of the aluminum finned substrate were prepared as described in Example 2 except that one (5 A) was not calcined before impregnation of the metal compounds which should produce a surface similar to that in Figure 1 la, while the second (5B) was calcined at 440°C for one hour, which produces the rough and porous surface as in Figure lOd. Both samples were heated at 440°C for 1 hour after impregnation to decompose the metal compounds and leave catalytically active metals. After testing as in Example 4 at a temperature of 185°C, it was found that both samples maintained nearly their fresh activity after 120 hours exposure to 2 ppm ozone in air, both 5 A and 5B having declined about 2-3% from the initial conversion of about 65%. It was concluded that the heating used to decompose the metal compounds was sufficient to produce a mass transfer limited catalyst and that pre-calcination before impregnation, having no detectable effect on the catalyst performance, at least in this short exposure, could be omitted. However, precalcination is preferred to assure development of the desired surface condition before the impregnation step. Anodization of the Downstream Portion 48 of Core Structure 44 Specific conditions found to be important for preparing an aluminum oxide catalyst support by anodizing, for the embodiments corresponding to the use of silver as a catalyst which is disposed on and within the anodized surface layer of each of the fin assemblies of the core structure, are set forth in Examples 6-9. However, more generally the process involves immersing a section of the aluminum substrate in an acidic electrolyte, preferably oxalic acid to avoid leaving potential catalyst poisons such as sulfiir and phosphorous on the surface. The acid concentration will be selected to provide the desired oxide thickness in an acceptable time. For the prefened oxalic acid the concentration may be about 5 to 25 wt.%, preferably 10 to 20 wt.%. The aluminum substrate will be the anode, while the cathode may be various metals or carbon. The anode and cathode are connected to a source of direct current having voltage available up to about 30 volts, generally 14 to 18 volts. The voltage is varied to provide a constant anodizing current, typically about 9.4 amp/ft 2 (101 amp/m 2 ), selected to obtain the desired thickness. The process is exothermic and during the time required to produce the desired surface layer, say about 30 to 60 minutes, the temperature will rise from the initial temperature unless heat is removed. It has been found that the temperature of the anodizing bath should be maintained relatively constant and above ambient, preferably above about 30° to 50°C, particularly about 40°C, since lower temperatures produce alumina surface films which are less satisfactory as catalyst supports. The surface layer should be at least 5 : m thick, preferably at least 10 : m, and have a surface area of about 30 to 150 m 2 /g in order to provide sufficient surface for subsequent application of the catalytic metals. However, the thickness will depend on the particular application and the thickness of the aluminum (or alloy) walls. Following anodizing, the aluminum substrate is washed with water to remove residual acid and then dried before the next step, i.e. the deposition of catalytic metals. It is preferred that after drying the substrate is calcined in air to temperatures above 400°C, preferably 427° to 482C 0 . A higher surface area is produced which is more accessible to the catalytic metals and to the reacting gases. It is feasible, although not prefened, to omit the calcination step since similar temperatures are used after impregnation to decompose the compounds of the catalytic metals. Catalyst Application: Downstream Portion 48 of the Core Structure 44 The methods used for applying the catalytic metals are familiar to those skilled in the catalyst art. In a preferred method, they may be applied as solutions of soluble compounds of the metals, either by dipping the anodized substrate into the solution or by otherwise applying an amount sufficient to deposit the desired metal loading. Following the impregnation step, the support is calcined at about 315° to 343°C for a period of 0.5 to 1.5 hours to convert the metal compounds to the metals or their oxides. The catalytic metal found to be particularly useful for ozone decomposition at lower temperatures of about -4° to 121 β C is silver. As will be seen below, silver is more resistant to poisoning at lower temperatures than the noble metals or base metals of the prior art. The amount of silver may vary up to about 500 g/ft 3 (17.7 g/L) of the core structure, preferably about 250 to 350 g/ft 3 (8.83 to 12.36 g/L). The optional palladium and/or nickel metals may be applied in amounts up to about 50 g/ft 3 of the core structure (1.77 g/L). Example 6 Anodization A sample section of 104 : thick aluminum finned substrate as described above measuring 1.61 in. x 1.00 in. x 0.25 in. (40.9 mm x 2.54 mm x 6.35 mm) having a surface area of 24 in 2 (0.02 m 2 ) was immersed in a 15 wt.% oxalic acid solution made by dissolving 99.14 g of oxalic acid dihydrate in 374.989 g of deionized water at 36°C. A cathode having a surface area of 144 in 2 (0.094 m 2 ) made of aluminum foil was also immersed in the oxalic solution. The substrate and the cathode were connected to a variable source of D.C. voltage (Heath Schlumberger) and a voltage of 15-17 volts was applied to begin the anodization of the substrate. The process was continued for about 1 hour. The cunent was maintained at 1.5 amps and the temperature maintained at 40-42°C. Following the anodization step the anodized substrate was washed in deionized water, dried at 120°C for 10-30 minutes and weighed. The sample was then calcined at 448°C for 1 hour. The weight loss was found to be 11 wt.%. The depth of the anodized surface layer was 10 : m. E^arnple 7 Metals Application The anodized and calcined sample of Example 6 was impregnated with a solution of AgNO 3 . 31.47 g of AgNO3 was diluted to 100 mL with deionized water. Then the calcined substrate was dipped into the solution and agitated for 30 seconds, after which the excess solution was removed by evaporation in a stream of air and the sample was weighed. The sample was covered and then calcined in an oven at 625°C for 1 hr. The impregnation was repeated twice. The sample was found to have 300 g/ft 3 Ag(10.6 g/L). For comparison to the silver catalyst of the invention three samples were prepared by similar methods to provide (a) 350 g/ft 3 (12.4 g/L) of 14.3 wt% Pd/85.7 wt.% Ni; (b) 186 g/ft 3 (6.58 g/L) of 53.8 wt.% Cu/46.2 wt.% Mn; and (c) 350 g/ft 3 (12.4 g/L) of 14.3 wt.% Ag/185.7 wt.% Mn. Example 8 A micro reactor was assembled to test a short section of each of the offset-fin core structures prepared as in Examples 6 and 7. A section of a core structure 7 fins wide and 2 sets of 7 fins deep (see Figure 1) measuring 0.5 in. x 0.25 in. x 0.2 in. (12.7 mm x 6.35 mm x 5.1 mm) was mounted in the reactor and air containing 2.5 ppm by volume of ozone was passed over the catalyst at 0.25 x 10 6 , 0.5 x 10 6 , and 1 x 10 6 , GHSV @ STP and at three temperatures, 66°C, 121°C, 185°C. The ozone conversion was measured by a PCI ozone monitor (Model LC) before and after the reactor. The results are given in Table 3. Table 3 - inlet = 185 β C Gas Hourly Calculated Space Mass Measured Conversion Velocity Transfer (STP) Limited Conversion (PdNi) (CuMn) (AgMn) (Ag) 250,000 95% 91% 92% 91% 93% 500,000 78% 81% 82% 82% 84% 1,000.000 62% 63% 64% 66% 65% τ iBlβt = i2rc Gas Hourly Calculated Space Mass Measured Conversion Velocity Transfer (STP) Limited Conversion (PdNi) (CuMn) (AgMn) (Ag) 250,000 90% 88% 88% 88% 89% 500.000 76% 78% 77% 78% 79% 1,000,000 60% 59% 60% 61% 60% ■ inlet = 66°C Gas Hourly Calculated Space Mass Measured Conversion Velocity Transfer (STP) Limited Conversion (PdNi) (CuMn) (AgMn) (Ag) 250,000 88% 84% 84% 83% 86% 500,000 73% 73% 73% 74% 74% 1,000,000 57% 55% 57% 56% 56% The column labeled "Theoretical Conversion" provides a reference value for comparison with the actual conversion as measured experimentally, as discussed previously in conjunction with Example 4. It can be seen that the initial conversion of all samples was substantially the same and that each was mass transfer limited. After 100 hours, the silver catalyst showed superior performance at low temperatures, as seen in Table 4 below. Table 4 Example 9 The silver catalyst of the invention was compared with a palladium-nickel catalyst, which had been shown previously to destroy ozone successfully at temperatures of about 185°C typical of some aircraft applications. The silver catalysts were made according to the procedures of Examples 6 and 7 while the palladium- nickel catalysts were made by a similar procedure except that the acid used for anodization was sulfuric rather than oxalic acid. Tests of the performance of the catalysts were carried out as in Example 8, except that small amounts of SO 2 or triethyl phosphate were added to the ozone- containing air for 5 hours to determine their effect on the catalyst, after which the ability of the catalysts to recover their activity was determined. The results are shown in Tables 5-7 Table 5 Ozone Destruction Performance ca. 2.5 ppmv Ozone in Air/ca. 1 ppmv SO) 1,000,000 GHSV (STP) PdNi-type HMT Catalyst Calculated Mass "Poisoned" "Recovered" nlet Transfer Limited Initial Conversion Conversion Conversion Conversion (5 hrs. SO 2 ) (15 hrs. Clean Feed) 66°C 57% 55% 5% 0% 121°C 60% 59% 8% 29% 185°C 62% 63% 29% 45% Table 6 Ozone Destruction Performance ca. 2.5 ppmv Ozone in Air/ca. 1 ppmv SO-. 1,000,000 GHSV (STP) Ag-type HMT Catalyst Calculated Mass "Poisoned" "Recovered" T inlet Transfer Limited Initial Conversion Conversion Conversion Conversion (5 hrs. SO 2 ) (15 hrs. Clean Feed) 66°C 57% 53% 1% 40% 121°C 60% 62% 2% 60% 185°C 62% 65% 2% 23% Table 7 Ozone Destruction Performance ca.2.5 ppmv Ozone in Air/ca. 1 ppmv TEP 1,000,000 GHSV (STP) Ag-type H IT Catalyst , Calculated Mass "Poisoned" "Recovered" Tu-Jet Transfer Limited Initial Conversion Conversion Conversion Conversion (5 hrs. SO2) (15 hrs. Clean Feed) 66°C 57% 54% 1 1% 40% 121°C 60% 62% 15% 51% 185°C 62% 64% 41% 39% It can be seen that the Pd-Ni catalyst lost activity when SO 2 was added and that although some performance was recovered at 185°C, the catalyst would not be useful at lower temperatures where recovery was poorer. The silver catalyst of the invention lost almost all of its activity when poisoned with SO 2 but recovered when SO 2 was removed. The results suggest that the silver catalyst would perform best at lower temperatures since it recovered less of its activity at the highest temperature tested. The results when TEP was added are similar to those found when SO 2 was added (see Table 7) and the same conclusions can be drawn. The silver catalyst is preferred for relatively low temperature applications. However, the palladium-nickel catalyst is prefened for high temperature uses. Manufacture of Fin Assemblies 120 of the Downstream Portion 48 of Core Structure 44 The catalytically-active metal alloy used to manufacture the fin assemblies 120 comprises a silver-containing metal alloy and, in a prefened embodiment, has a composition including silver and copper as principle constituents. An example of an alloy which the inventors have determined to be acceptable for use in the present invention has a composition comprising, on a weight basis, about 55% silver, about 39% copper, about 5% zinc and about 1% nickel. This composition conesponds to the following atomic ratios: Ag:Cu:Zn:Ni:30:36:4.5: 1. As subsequently discussed in greater detail, the inventors have identified silver as the active catalyst and accordingly, the catalytically-active metal alloy of the present invention must comprise a silver-containing metal alloy. However, it is considered to be within the scope of the present invention to utilize metals other than copper as a principle constituent in combination with silver. During initial testing of a portion of one of the fin assemblies 120, which had been constructed using the catalytically-active metal alloy having the aforementioned atomic ratios of 30:36:4.5: 1, the inventors observed that the silver-containing metal alloy was not catalytically active at low temperatures, as shown in the "ramp up" portion of Fig. 12. However, upon further testing, the inventors determined that the silver-containing metal alloy became catalytically-active for ozone decomposition after being thermally activated, as evidenced in the "ramp down" portion of Fig. 12. It is noted that the test results shown in Fig. 12 corresponded to an ozone concentration of 2.36 ppmv and a flow rate of 1,000,000 GHSV (STP). As shown in Fig. 12, after the silver-containing metal alloy has been thermally activated, the alloy is extremely active for ozone decomposition, providing mass-transfer-limited performance at temperatures as ϊow as Ϊ00°C. Accordingly, for low temperature applications, i.e. temperatures in which the ozone-containing airstream 16 is less than about 149°C the silver-containing metal alloy must be activated by calcining, or heating the alloy in air to a temperature ranging from about 149°C to about 216°C for a period of time ranging from about 30 minutes to about 60 minutes. For higher temperature applications, i.e. those in which the ozone-containing airstream 16 is at least 149°C the thermal activation step may be omitted. In an effort to gain an increased understanding of this phenomena, a detailed thermodynamic analysis of the surface composition of the fin assemblies 120 as a function of temperature was performed. Again, the composition of the silver- containing metal alloy conesponded to atomic ratios of: 30:36:4.5: 1. The results of the thermodynamic analysis, illustrated graphically in Fig. 13, show that the surface composition of the silver-containing metal alloy (which is exposed to air) varies with temperature. At ambient temperature, the equilibrium surface composition is a mixture of CuO, Ag 2 O, ZnO and NiO. As shown in Fig. 13, this composition changes with temperature. As the temperature increases, Ag 2 O decomposes, forming Ag° and O 2 . There is a strong conelation between the activation temperature required to obtain high ozone destruction activity and the predicted change in equilibrium surface composition. The results shown in Fig. 13 confirm that Ag° metal is the species which is catalytically-active for ozone decomposition. These results were further verified in the subsequently discussed Examples 10 and 1 land the associated examination of an as-received silver-containing metal alloy surface and an activated silver-containing metal alloy surface which was operated for greater than 300 hours in ozone air feeds at a variety of temperatures. Examples 10 and 11 are provided to demonstrate some of the benefits which may be achieved by manufacturing the fin assemblies 120 from the silver-containing metal alloy of the present invention. Example 10 A laboratory-scale reactor was assembled to include a section of a fin assembly having 2 rows of fins, with each row including 8 fins. (Refer to Fig. 6). Measured fin dimensions were approximately as follows: fin height was .181 in. (4.60 mm); fin thickness was 0.0036 in. (.091 mm); and fin axial depth was .177 in. (4.50 mm). The lateral fin density in each row was 16 fins/in. The fin assembly was constructed from a silver-containing metal alloy having a composition comprising about 55% Ag, about 39% Cu, about 5% Zn and about 1% Ni. The section of the fin assembly was mounted in the laboratory-scale reactor and air containing 2.3 ppm by volume ozone was flowed through the fin assembly, so as to contact multiple surfaces of the silver- containing metal alloy, at 1 x 10 6 GHSV at STP and at the following five temperatures: 122°F, 212°F, 302°F, 392°F and 482°F (50°C, 100°C, Ϊ50°C, 200°C, and 250 β C). The ozone conversion was measured, after 60 hours of operation using a PCI Ozone Monitor (Model LC) before and after the laboratory reactor. The results are presented in Table 8. Additionally, SEM-EDX analysis was performed on the surface of an "as-received" sample of the silver-containing metal alloy, as well as the surface of an aged sample. The low magnification (500x) SEM images of the "as received" and aged, or activated silver-containing fin surfaces are shown in Figs. 14 and 15, respectively. TABLE 8 Ozone Destruction Performance 1,000,000 GHSV (STP); 2.3 ppmv Ozone Inlet Predicted Mass Initial Conversion After Temperature Transfer -Limited Conversion 60 hours Conversion Operation (Hegedus Model) 250°C 46% 44% 44% 200°C 44% 40% 39% 150°C 42% 42% 45% 100°C 41% 40% 40% 50°C 38% 27% 17% It should be noted that the mass transfer-limited calculations shown in Table 8 may have an error as great as +/- 3% conversion in the range shown. The column labeled "Predicted Mass Transfer Limited Conversion" shown in Table 1 provides a reference value for comparison with the actual conversion as measured experimentally, as discussed previously in conjunction with Example 4. It may be seen from the results shown in Table 8 that mass transfer was limiting except at the temperature of 50°C. The "as-received" surface of the silver-containing metal alloy had a "white- copper" sheen. As shown in Fig. 14, the "as-received" metal alloy surface was relatively smooth in appearance with sub-micron size surface striations (possibly due to milling of the alloy during manufacture). In contrast, the activated silver-containing metal alloy surface had a dull gray appearance which was observed immediately after thermal activation. As shown in Fig. 15, the surface of the activated silver-containing metal alloy is roughened with particles which are about 1-10 microns in diameter. Further analysis of both the "as-received" and activated fin samples using EDX confirmed that the "particles" observed on the surface of the activated silver- containing metal alloy contained high concentrations of silver. In contrast, copper and oxygen were found in extremely low concentrations. This result supported the thermodynamic analysis discussed previously in conjunction with Fig. 13, verifying that silver metal is the active site for ozone decomposition. Example 11 A laboratory reactor was assembled to a test section of a fin assembly, having the same number of rows and fins, and made from the same alloy, as that described in Example 10. Durability testing was conducted to determine the ability of the silver- containing metal alloy to recover from a temporary poisoning with SO 2 , as follows. Initially, a "clean" feed air containing 2.3 ppm by volume of ozone was flowed through the plate-fin element at 1 x 10 6 GHSV at STP and at the following temperatures: 100°C, 150°C, 200°C and 250°C. After 20 hours of testing with the "clean" feed, 1 ppm by volume SO 2 was added to the ozone-containing air for a period of 5 hours, after which the aforementioned "clean" feed was used for an additional 75 hours, at each temperature. The ozone conversion was measured after 1 hour, 5 hours, 20 hours, and 60 hours of exposure to the ozone-containing air. The results of the test are shown in Table 9 and Fig. 16. SEM-EDX analysis of a poisoned alloy was conducted for comparison. Our spectroscopic analysis was in agreement with the hypothesis that the catalytically active sites were poisoned with sulfur. SEM-EDX of a poisoned catalyst showed that the sulfur was associated with regions which were high in silver content; in contrast, areas which were high in copper were relatively free of sulfur. TABLE 9 Ozone Destruction Performance 1,000,000 GHSV (STP); 2.3 ppmv Ozone Durability in l pmv SO 2 Inlet Predicted Initial After ! lhr 5hrs 20hrs 60hrs Temp Mass Transfer- Conversion 5 hours Limited ("dean" SO, Conversion feed) (Hegedus Model) 250°C 46% 49% 2% ! 29% 40% 44% 44% 200°C 44% 39% 0% | 26% 37% 39% 40% 150°C 42% 39% 1% I 32% 39% 39% 100°C 41% 43% 0% ! 36% 39% 39% 40% It is well documented that sulfur can act as a catalyst poison, attenuating performance by either "masking" sites or by converting the active material into an inactive compound. Using the present silver-containing metal alloy, SO 2 can either adsorb on the surface of the Ag particles ("masking" the sites) or the Ag and SO 2 can react in this oxidizing environment to form catalytically inactive Ag 2 SO 4 , which is extremely stable when formed. As illustrated in Table 9 and Fig. 16 (which conespond to the 200°C test, a dramatic attenuation in performance was observed as soon as the SO 2 was introduced. However, as shown in both Table 9 and Fig. 16, a rapid recovery to mass-transfer-limited performance was observed. This behavior was observed for each temperature range. Catalyst poisoning by surface adsoφtion of SO2 can be reversible depending on the strength of the catalyst-adsorbate bond. The rapid recovery in catalyst performance after SO 2 removal, shown in Table 9 and Fig. 16 demonstrates that this poisoning was reversible over the temperature range tested. Accordingly, the temporary performance attenuation observed was due to "masking" of the catalyst sites by SO 2 . These results demonstrate that the silver-containing metal alloy is not irreversibly poisoned by sulfur contaminates in ozone-containing air feeds. Evaluation of Hydrocarbon Destruction Catalysts for Upstream Portion 46 of Core Structure 44 Example 12 A series of fin assemblies were prepared by anodizing an aluminum alloy (6951) substrate (of each fin assembly) using a 15 wt% oxalic or sulfuric acid electrolyte following the procedure described in Examples 6-9. A catalyst having a loading of 50 g Pt/ft 3 (1.77 g/L) was tested for the destruction of a variety of alkanes which may be present in the trace lube oil and fuel pollutants including n-octane (CgHu), n-nonane (C9H20) and n-decane (C10H22). The catalyst was tested using different feed streams which contained 1.4% H 2 O and 100 ppmv hydrocarbon in air at 250,000 GHSV (STP). The catalyst performance (as measured in percent hydrocarbon conversion) was monitored at various operating temperatures. The data is presented in Table 10 and shows that the catalytic destruction efficiency increased as the size of the hydrocarbon increased. For example, at 204°C, the 50 g Pt/ft 3 (1.77 g/L) catalyst destroyed about 60% of the n-decane, but only about 28% of the n-heptane was converted at this temperature. The results also show that at 204°C, the measured conversions of n-nonane and n-decane were near the calculated mass transfer-limited conversions. The measured conversions at 204°C for n-heptane and n-octane were much less (i.e., more difficult to catalytically destroy) than those for the heavier compounds. Calculations and the combined data indicate that the fin assembly using the 50 g Pt/ft 3 (1.77 g/L) catalyst should provide mass-transfer limited destruction of trace lube oil or fuel pollutants in bleed air feed streams at operating temperatures as low as 204°C. Note that the microreactor tests were performed using catalyst samples applied to a fin assembly having only two offset rows of fins (0.1 inch/offset). Increasing the number of offsets in the catalyst bed will dramatically improve the mass transfer- limited performance of these catalysts. For example, a full scale bleed air purification reactor may contain a catalyst applied to a fin assembly having a significantly larger number of offset fin rows. As an illustration, such a reactor may have about 64 offset rows of fins (at .1 inch/offset which provides a 6.4 inch deep reactor). This reactor would provide 99% mass transfer-limited conversion efficiency of n-decane at 204°C and 250,000 GHSV (STP). Table 10 Bleed Air Purification Catalyst Performance Percent Hydrocarbon Conversion at Various Operating Temperatures 250,000 GHSV (STP), 100 ppmv Hydrocarbon, 1.4% HjO Operating Temperature Simulant Compound 152 β C 204°C 260°C n-Heptane 7% 28% 57% n-Octane 12% 46% 66% n-Nonane 23% 57% 68% n-Decane 37% 60% 68% Example 13 Example 12 demonstrated that the Pt-only catalyst is effective for the destruction of organic pollutants such as n-decane when applied to the fin assembly of the present invention. In this Example a series of fin assemblies were prepared following the procedure described in Example 12 to assess the percent destruction of n-decane at various temperatures for: a variety of Pt-only loadings; Pt-based alloys using another catalytically active metal in addition to Pt; and catalyticaily active metals other than Pt. The fin assemblies were catalyzed with a variety of formulations and tested for n-decane destruction. This compound can be considered an effective model compound used to simulate bleed air contaminants such as aircraft jet fuel or lube oil; i.e., n-decane is more difficult to destroy (catalytically oxidize) than the larger alkanes found in jet fuels and lube oils. The experimental catalysts were prepared by impregnating the fin assemblies with a dilute metal solution and calcining the impregnated samples at 441°C. The subsequent microreactor testing was performed using a synthetic bleed air gas stream containing approximately 100 ppmv of n-decane, 1.4% H 2 O and air passed over the catalysts at a space velocity of 250,000 hr *1 (STP). The catalyst performance (n-decane conversion efficiency) was measured at a variety of operating temperatures. The data is summarized in Table 11 and shows that the hydrocarbon destruction performance of catalysts containing metals such as Mn, Co Ni, Cu, Pd, and Ag are less active than the prefened Pt-only formulation. The Mn- only, Ag-only or Pd-only catalyst formulations showed n-decane conversions at 260 β C between 10% and 40%. Catalysts made with only Co, Ni, or Cu showed less than 10% conversion n-decane at 500°F. Adding Pt to catalysts containing Mn, Ag, or Pd improved their activity, but they were still not as active as the Pt-only catalyst formulations. The Pt-only catalysts which contained 50 g Pt/ft 3 destroyed about 60% of the n-decane in the feed gas at 204°C. Catalysts made with a nominal Pt loading of 100g/ft 3 (3.53 g L)destroyed about 66% of the n-decane at 204°C (which is near the mass transfer-limited conversion (about 67%) for catalysts containing two offset fin segments). A catalyst made with a nominal loading of 200 g/ft 3 (7.06 g/L) was found to be less effective than the catalyst made with a 100g/ft 3 (3.53 g/L) Pt loading for the destruction of n-decane. Example 13 demonstrated that the Pt-only formulations are preferred for the destruction of organic pollutants such as n-decane. Example 13 further demonstrated, for the fin assembly of the present invention (which provides high mass transfer), that the 100 g Pt ft 3 (3.53 g/L) is the most preferred. Table 11 Percent Destruction of n-Decane at Various Temperatures; Catalysts Tested at 250,000 GHSV (STP) With Two Offset Rows of Fins n-Decane Destruction, % Catalyst Description 177-C 204 β C 260 β C 160 g/ft 3 Mn < 2 <2 11.4 (5.65 g/L) 160 g/ft 3 Co < 2 <2 3.8 (5.65 g/L) 160 g/ft 3 Ni < 2 <2 3.0 (5.65 g/L) 160 g/ft 3 Cu < 2 <2 3.5 (5.65 g/L) 300 g/ft 3 Ag < 2 2.0 38.0 (10.6 g/L) 50 g/ft 3 Pd < 2 3.6 30.3 (1.77 g/L) 80 g/ft 3 Mn + 50 g/ft 3 Pt 10.1 27.8 49.3 (2.82 g/L Mn + 1.77 g/L Pt) 80 g/ft 3 Ag + 50 g/ft 3 Pt < 2 13.0 51.2 (2.82 g/L Ag + 1.77 g/L Pt) 80 g/ft 3 Pd + 50 g/ft 3 Pt 9.1 37.5 68.9 (2.82 g/L Pd + 1.77 g/L Pt) 50 g/ft 3 Pt 37.1 60.3 68.1 (1.77 g/L) 100 g ft 3 Pt 52.5 56.8 70.1 (3.53 g/L) 200 g/ft 3 Pt 50.3 54.3 56.1 (7.06 g/L) Example 14 A sample of the aluminum finned substrate was anodized as described in Example 1 to provide an anodized surface layer of about 10 : m. The anodized sample was immersed in an aqueous solution of platinum sulfite acid to provide a platinum loading after calcination at 538°C for about 1 hour of 50 g/ft 3 Pt(1.76 g/L). The resulting catalyst was tested for the oxidation of 300 ppm by volume of heptane in air at a space velocity of 1,000,000 hr '1 as described in Example 3. Heptane oxidation began at about 160°C and rose to about 12% at 200°C and to about 28% at 250°C. As indicated in Example 3 higher conversions would be obtained with lower space velocity or a greater number of fin rows. Example 15 Another sample of aluminum finned substrate prepared as in Example 4 was tested for the oxidation of carbon monoxide in air. The concentration of CO was 900 ppm by volume and the space velocity again was 1 ,000,000 hr. *1 . Oxidation began at about 125°C and rose to about 15% at 175°C, 36% at 200°C and 39% at 250°C. Example 16 Further testing focused on the evaluation of the 100 g Pt/ft 3 (3.53 g/L) bleed air purification catalyst for hydrocarbon destruction in dry and humidified feeds, with or without the presence of low concentrations of ozone. Fin assemblies were anodized as described previously in Example 12. It is important to verify catalyst performance in dry ozone-containing feeds since the air supplied to the reactor during high altitude operation of an aircraft which may utilize the reactor is very dry and contains low ppmv levels of ozone. The 100 g Pt ft 3 (3.53 g/L) bleed air purification catalyst was tested for hydrocarbon destruction efficiency at 250,000 GHSV (STP), in air feeds with and without humidification (1.4% H 2 O) and with and without ozone (2 ppmv). The test conditions and hydrocarbon conversion results are summarized in Tables 12-14. These results show that the catalyst provides improved performance in dry feeds compared to the standard 50% relative humidity tests and that about 2 ppmv ozone has little effect on catalyst performance. The results in Tables 13 and 14 also show that this catalyst is effective for the destruction of partially oxidized fuel and lube oil compounds such as valeraldehyde and 2-pentanone (possible bleed air contaminants). Table 12 Bleed Air Purification Catalyst Performance Percent Hydrocarbon Conversion at Various Operating Temperatures 250,000 GHSV(STP), 100 ppmv n-Decane (2 Offset Deep Catalyst Bed) Operating Temperature Feed Stream 177°C 204°C 260°C Dry Air 62% 66% 68% Air + 1.5 vol % H 2 O 48% 64% 70% Dry Air + 0 ppmv Ozone 61% 66% 68% Dry Air + 2.5 ppmv Ozone 37% 66% 72% Table 13 Bleed Air Purification Catalyst Performance Percent Hydrocarbon Conversion at Various Operating Temperatures 250,000 GHSV (STP) 20 ppmv Valeraldehyde (4 Offset Deep Catalyst Bed) Operating Temperatu ire Feed Stream 177 β C 204 β C 260 β C Dry Air 61% 75% 84% Air + 1.5 voi % H 2 O 56% 73% 84% Dry Air + 2 ppmv Ozone 65% 78% 85% Air + 1.5 vol% H 2 0 56% 74% 84% and 2ppmv ozone Table 14 Bleed Air Purification Catalyst Performance Percent Hydrocarbon Conversion at Various Operating Temperatures 250,000 GHSV (STP) 30 ppmv 2-Pentanone (4 Offset Deep Catalyst Bed) Operating Temperature Feed Stream 177°C 204°C 260°C Dry Air 30% 50% 70% Air + 1.5 vol % H 2 O 22% 44% 67% Dry Air + 2 ppmv Ozone 30% 51% 69% Air + 1.5 vol% H 2 0 and 25% 46% 67% 2 ppmv Ozone Example 17 Laboratory microreactor durability tests were performed on the 100 g Pt ft 3 (3.53 g/L) bleed air purification catalyst. Fin assemblies were anodized as described previously in Example 12. The 100 g Pt/ft 3 (3.53 g/L) catalyst was tested for performance durability in "clean" and "dirty" (poison-containing) synthetic bleed air feeds. The base synthetic bleed air feed contained 50 ppmv n-decane and 2.5 ppmv ozone (no humidifi cation). A sample of the 100 g Pt ft 3 (3.53 g/L) bleed air purification catalyst was tested for performance durability in the "clean" (no poison added) feed stream. The test consisted of operating the catalyst at high temperature 241°C at 398,000 GHSV (STP) for ca. 350 hours. The decane conversion vs. operating temperature performance of the catalyst was monitored at various times throughout the durability test. The data, summarized in Figure 17, show that the 100 g Pt/ft 3 (3.53 g/L) catalyst does not deactivate or suffer a loss in performance during the 350 hours of continuous operation in poison-free bleed air feeds. The durability testing was extended to evaluation of catalyst performance in "dirty" feeds. These feeds were synthetic bleed air feed streams which were polluted with n-decane/ozone and which also contained trace levels of sulfiir or phosphorus- containing compounds which are commonly known to deactivate/attenuate air purification catalyst performance. The results of the durability testing with sulfur-containing compounds added to the feed stream are presented in Figure 18. The results show that n-decane destruαion activities were not effected by the presence of 10 ppmv SO 2 when the operating temperature of the catalytic reactor was 241°C or higher. At lower operating temperatures (e.g. 171°C) a temporary loss of catalytic activity was noted when SO 2 was added to the feed stream; however, the deactivated catalyst fully recovered activity when the SO 2 was removed from the feed. It is important to note that the expected levels of SO 2 in actual service conditions are estimated to be orders of magnitude lower than those used during our accelerated laboratory studies. The results of the performance durability testing with phosphorus-containing compounds added to the feed streams are presented in Figure 19. The results show that n-decane destruction activity at 24TC dropped from about 63% to about 57% after 25 hours operation in the phosphorus-containing feed stream (0.1 ppmv triethyiphosphate (TEP) 50 ppmv decane, 2.5 ppmv ozone, no humidification). It is important that the poisoned catalyst begins to recover activity immediately after the TEP is removed from the feed stream. This performance recovery is notable as phosphorus poisoning of standard air purification catalysts is typically considered ineversible and require "washing" or other ex-situ means to achieve performance recovery. As in the sulfur poisoning test, this test represents a severe catalyst poisoning. In actual service, the level of phosphorus compounds in the bleed air is expected to be 20 or more times lower than the levels used in our accelerated tests. Using a computer model to estimate the mass transfer-limited n-decane conversion, the predicted performance of a full-scale reactor was determined by the inventors and is presented graphically in Figure 20. The dual bed reactor 32 of the present invention provides a cost efficient, relatively maintenance free, reactor for the conversion of organic pollutants into harmless carbon dioxide and water and for the decomposition of ozone. The use of at least one fin assembly, comprising a plurality of off-set rows (138 or 142) of fins (136 or 140), in both the upstream 46 and downstream 48 portions of the thermally compliant core structure 44 of the dual bed reactor 32, in conjunction with the statistical brazing method of the present invention, provides relatively high mass transfer between the associated airstream 16 and the catalyst which are present in both the upstream 46 and downstream 48 portions of the core structure 44 Additionally, the statistical brazing methods of the present invention enhance the mass-transfer of the dual bed reactor 32 of the present invention due to the avoidance of the use of splitter plates or tube plates which also minimizes the weight of the dual bed reactor 32 of the present invention which is particularly important in aircraft applications. Furthermore, the statistical brazing methods of the present invention allows the core structure 44 of the dual bed reactor 32 to expand and contract with changes in environment temperature without the use of an intervening thermally compliant structure. In selected embodiments, the core structure 44 may be brazed directly to the housing 36 of the dual bed reactor 32 which further simplifies, and cost-reduces, the dual bed reactor 32.

Claims: We Cl aim : 1. An environmental control system for use in a transportation machine, said system being effective for receiving and conditioning a heated airstream containing organic pollutants and ozone prior to delivering the airstream to a habitable space within the machine containing organic pollutants and ozone, said system comprising: a dual bed reactor; wherein said dual bed reactor comprises a metallic housing having an inlet portion effective for receiving the heated airstream, an outlet portion effective for discharging the airstream from the reactor, and a generally cylindrical portion disposed axially between and connected to said inlet and outlet portions; said dual bed reactor further comprises a thermally compliant core structure disposed within and connected to said generally cylindrical portion of said housing; said thermally compliant core structure includes an upstream portion which includes at least one catalyst which is effective for converting at least a portion of the organic pollutants present within the heated airstream into carbon dioxide and water as the airstream flows through said upstream portion; said thermally compliant core structure further includes a downstream portion which is effective for decomposing at least a portion of the ozone present within the airstream as the airstream flows through said downstream portion. 2. The environmental control system as recited in claim 1, wherein said upstream portion and said downstream portion of said thermally compliant core structure are disposed within and brazed to said generally cylindrical portion of said housing of said dual bed reactor. 3. The environmental control system as recited in claim 1, wherein: said upstream portion of said thermally compliant core structure of said dual bed reactor comprises at least one fin assembly, said at least one fin assembly of said upstream portion including a metallic support and an anodized surface layer integral with said metallic support; said at least one catalyst of said upstream portion is disposed on and within said anodized surface layer and comprises at least one metal selected from the group consisting of Group VIII metals; said downstream portion of said core structure comprises at least one fin assembly 4. The environmental control system as recited in claim 3, wherein said at least one fin assembly of said downstream portion includes a metallic support and an anodized surface layer integral with said metallic support, said downstream portion further includes at least one catalyst disposed on and within said anodized surface layer. 5. The environmental control system as recited in claim 4, wherein said at least one catalyst of said downstream ponton comprises one or more Group VIII noble metals disposed on and within said anodized surface layer 6. The environmental control system as recited in claim 5, wherein said downstream portion further includes at least one base metal disposed on and within said anodized surface layer, said base metal being selected from a group consisting Group VIII metals, Group IIIA metals, and Group VIIA metals. 7. The environmental control system as recited in claim 4, wherein. said at least one catalyst of said downstream portion comprises silver which is disposed on and within said anodized surface layer 8. The environmental control system as recited in claim 3, wherein said downstream portion of said thermally compliant core structure is constructed from a catalytically-active metal alloy which is effective for decomposing ozone present within the heated airstream 9 The environmental control system as recited in claim 3, wherein said at least one catalyst of said upstream portion comprises platinum 10. The environmental control system as recited in claim 9, wherein said metallic support of said upstream portion is constructed from a metal selected from the group consisting of aluminum and aluminum alloys, said upstream portion has a platinum loading ranging from about 88 g/L to about 7 1 g/L of said metallic support for each said fin assembly

Inventors: HOMEYER STEPHEN T, LEHANE COLLEEN A, LESTER GEORGE R, MIKOLAJCZYK SARA J, POLLITT STAN C, SNYDER JAMES R, TOM ROBERT, WALENGA JOEL T, WESTELAKEN WILLIAM, HOMEYER, STEPHEN, T., LEHANE, COLLEEN, A., LESTER, GEORGE, R., MIKOLAJCZYK, SARA, J., POLLITT, STAN, C., SNYDER, JAMES, R., TOM, ROBERT, WALENGA, JOEL, T., WESTELAKEN, WILLIAM
Applicants: ALLIED SIGNAL INC, ALLIEDSIGNAL INC.
Application year: 1997
Main classification: B01D53
IPC codes: B01D53, B01J15, B01J35