Structure for electrically heatable catalytic core

Abstract: There is provided an improved electrically heatable core (10) for an electrically heatable catalytic converter (50). The core is characterized by a plurality of corrugated thin metal strips (12) secured to an electrically conductive central core (14) or tube. Alternating with the corrugated thin metal strips (12) are narrow strips (16) of brazing metal in an axially staggered pattern. These strips are spirally wound and fused to braze them together in a unified monolith. A retainer shell (18) is supplied to hold the monolith together. The core is placed in a housing (52) fitted with insulated means (26) for conducting electric current to the monolith to effect heating. The housing is then inserted in an exhaust line where it is effective to control start-up pollution, and where the monolith is constrained against telescoping.

Description: This invention relates to an improved structure for an electrically heatable catalytic core and, more particularly, to an electrically heatable catalytic core which resists telescoping in the course of a severe testing regime as described below. BACKGROUND OF THE INVENTION AND PRIOR ART The purpose of a catalytic converter is to convert pollutant materials in engine or turbine exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides etc., to carbon dioxide, nitrogen and water. Conventional catalytic converters utilize a ceramic honeycomb monolith having square or circular straight-through openings or cells with catalyst deposited in the cells, catalyst coated refractory metal oxide beads, e.g., alumina beads, or a corrugated thin metal foil monolith, e.g., ferritic stainless steel foil, having catalyst carried on or supported by the surface. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such metals. The catalyst catalyzes a chemical reaction, mainly oxidation, whereby the pollutant is converted to a harmless by-product which then passes through the exhaust system to the atmosphere. However, conversion is not efficient initially when the exhaust gases are relatively cold. To be effective at a high conversion rate, the catalyst and the surface of the converter with which the exhaust gases come in contact must be at a minimum temperature, e.g., 199°C (390 F) for carbon monoxide, 299°C (570 F) for volatile organic compounds (VOC) and 538°C (1000 F) for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. Once the exhaust system has came to its operating temperature, the catalytic converter is optimally effective. Hence, it is necessary to contact relatively cold exhaust gases with hot catalyst to effect satisfactory conversion at engine start-up. Both compression ignited (diesel) and spark ignited engines have this need. To achieve initial heating of the catalyst prior to engine start-up, there is provided an electrically heatable catalytic converter formed of a corrugated thin metal foil monolith which is connected to a voltage source, e.g., a 12 volt or 24 volt automotive battery, and power supplied, preferably before and during start-up, to elevate and maintain the temperature of the catalyst to at least about 343°C (650 F). US-A-4 928 485 discloses an electrially heatable core for an electrically heatable catalytic converter, comprising a central electrically conductive core. A monolith wound about the central core includes a plurality of thin metal strips of substantially equal length which are secured to the central electrically conductive core. The thin metal strips are corrugated and non-nesting. A steel retaining shell surrounds the spirally wound monolith. One terminal is connected to the steel retaining shell, the other to the central electrically conductive core. The disadavantage of this known device is that it is subject to telescoping. DE-U-88 12 924 describes a catalytic converter core in which a corrugated band and two adjacent smooth bands are spirally wound in order to form the structure. It is proposed to provide some fixing areas between adjacent faces of the smooth bands, which otherwise are not connected to each other to allow for thermal expansion. One possibility for achieving the fixation is to provide a brazing foil at certain distances during the coiling procedure and heat the assembly afterwards in order to fuse the brazing foils. This reference, however, does not give further details on the application of the brazing foil. EP-A-0 456 919 discloses one form of electrically heatable catalytic converter normally subject to telescoping of the core, and provides one means for offsetting the tendency to telescoping of the core in operation and ultimate destruction thereof. The present invention provides a different means for offsetting the tendency to telescoping. Because much of the disclosure of EP-A-0 456 919 is relevant to the present application, the disclosure of EP-A-0 456 919 is incorporated herein by reference thereto. Instead of utilizing a ceramic core in juxtaposition with the electrically heatable catalytic core to inhibit telescoping, the present invention utilizes inter-leaf brazing whereby the leaves are held against telescoping or axial displacement. The electrically heatable catalytic cores are normally spirally wound, or S-wound, corrugated thin metal foil as described in the aforesaid EP-A-0 456 919 with the exception that the corugated thin metal foil strips are not initially washcoated with a refractory metal oxide coating, e.g., an alumina coating, and a catalyst. This treatment comes later according to the present invention. The corrugated thin metal cores are subjected to a severe test which they must pass in order to be acceptable. This test involves oscillating (100-200 Hertz and 28 to 60 G) the device in a vertical attitude at high temperature (between 700 and 950 C; 1292 F and 1742 F, respectively) with exhaust gas from a running internal combustion engine being passed through the device. If the wound core device telescopes in the direction of gas flow or breaks up after a predetermined time, e.g., 5 to 200 hours, the device is said to fail the test. Usually, the test device will fail in 5 hours if it is going to fail. Accordingly, it is a principal object of the present invention to provide a device which will pass the foregoing test and thereby prove reliable in extreme field service. Reference may be had to U.S. Patent 3,768,982 to Kitzner dated 30 October 1973. In this patent, heat from a centrally located electric heater is transferred by conduction through a monolithic catalyst support to heat the catalyst to optimum operating temperature. Reference may also be had to U.S. Patent 3,770,389 to Kitzner dated 30 October 1990 which disclose a central electrically heated core within a ceramic monolith, heat being transmitted by conduction to the catalyst contained in the openings of the ceramic monolith. The heating core is formed of metal sheets, one corrugated, the other flat, coated with alumina and also bearing a catalyst. The metallic core is heated electrically by virtue of its own electrical resistance. Heating by conduction takes too long to solve the problem of atmospheric pollution at start-up. Moreover, the thin metal cores of the present invention do not require a flat thin metal sheet in alternating relation with a corrugated thin metal sheet. Reference may also be had to U.S. Patent 4,711,009 to Cornelison et al dated 8 December 1987 for details of a process for corrugating thin metal foil strips. The coating of the surface with a refractory metal oxide is not done in the present case or the application of the catalyst as described in that patent. These steps cannot be performed in the present case as the resulting surfaces cannot be brazed. However, the corrugating of a thin metal foil strip with a herringbone, or chevron pattern as taught therein is applicable to the present application and to that extent, the disclosure of the aforesaid patent 4,711,009 is incorporated herein by reference. The refractory metal oxide coating and the catalyst are applied in the present process by dipping after formation of the core. The composition of the washcoat and the catalyst treating solutions as taught in patent 4,711,009 are pertinent to the present process. Reference may also be had to International PCT publication numbers WO 89/10471 and WO 10470 each filed 2 November 1989. S-wound cores are disclosed in these publications. However, there is no teaching of brazing between the corrugated thin metal foil layers. In the following description, reference will be made to "ferritic" stainless steel. A suitable formulation for this alloy is described in U.S. Patent 4.414,023 dated 8 November 1983 to Aggens et al. A specific ferritic stainless steel useful herein contained 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium and praseodymium, or a mixture of two or more thereof, balance iron and steel making impurities. In the following description, reference will also be made to fibrous ceramic mat or insulation. Reference may be had to U.S. Patent 3,795,524 dated 5 March 1974 to Bowman for formulations and manufacture of ceramic fibers and mats useful herein. One such material is currently commercially available from 3-M under the registered trademark "INTERAM." BRIEF STATEMENT OF THE INVENTION The object of the invention, i.e. to provide a device which will pass the aforementioned telescoping test, is achieved by an electrically heatable core for an electrically heatable catalytic converter having the features of Claim 1 and by an electrically heatable catalytic converter having the features of Claim 11. Briefly stated, the present invention is an improved structure for an electrically heatable catalytic converter. The structure is characterized by corrugated thin metal leaves or strips extending from an electrically conducting central core in alternating relation with thin narrow brazing metal strips. The thin narrow brazing metal strips are staggered from side to side of the corrugated thin metal strips or leaves, lying adjacent first one marginal edge of a corrugated thin metal strip, and then adjacent the opposite marginal edge of the next succeeding corrugated thin metal strip. The assembly of corrugated thin metal strips and the thin narrow brazing strips is then spirally wound, or S-wound, into a tight core, and a retaining shell placed around the tightly wound core with an intervening wrapping of brazing metal and the whole unit heated to a temperature sufficient to fuse the brazing metal. Simultaneously, the interleafed brazing strips are fused, and the assembly becomes a rigid unit. The the entire unit is dipped into a washcoat of a refractory metal oxide dispersed in water, and the coating baked to tightly adhere the metal oxide, usually gamma alumina, with or without a small amount of ceria, to the metal surface. Then a noble metal catalyst is deposited on the metal oxide surface from an aqueous solution of the noble metal. The temperature is then raised to decompose the noble metal containing compound and deposit the catalyst on the surface. Alternatively, the catalyst may be dissolved in the aqueous medium of the washcoat before dipping. In use, a voltage source, usually a 12 or 24 volt automotive battery, is connected across the central conductive bar and the outer retaining shell to provide power to heat the monolith to a minimum of about 343°C (650 F) in about 10 to about 30 seconds. Power in the range of 1500 to 12000 watts may be supplied through a MOSFET power supply system such as described in EP-A-0 456 919. The resulting catalytically active electrically heatable unit now will resist telescoping during the severe test outlined above. Moreover, the current, instead of flowing through the brazing locations from one leaf to the next, will flow mainly along the length of the corrugated thin metal strip as it provides the shortest path from negative to positive. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood by having reference to the annexed drawings showing a preferred embodiment of the invention and wherein: Fig. 1 is a cross-sectional view of an electrically heatable catalytic core in accordance herewith showing the inter-leafed brazing strips in alternating relation with corrugated thin metal foil strips, and showing the general connections for supplying power to the core. Fig. 1a is a small segment of Fig. 1 showing the brazing foil layer between the spirally wound core and the binding shell Fig. 2 is a cross-sectional view showing the inter-leafing of brazing strips and corrugated thin metal foil strips, secured to a central post, prior to winding in a tight spiral form as shown in Fig. 1. Fig. 3 shows the pattern of alternation between the corrugated thin metal strips and the brazing strips, and the staggered relation of the brazing strips with respect to a transverse center plane lying perpendicular to the central post in Figs. 1 and 2. Fig. 4 is a side elevation of the electrically heatable core of Fig. 1 Fig. 5 is a cross-sectional view of a completed. electrically heatable catalytic converter in accordance with this invention. DETAILED DESCRIPTION OF THE DRAWINGS As indicated above, the present invention is in a structure for an electrically heatable catalytic core and a converter containing the same. This core is able to withstand the severe test described above without telescoping. Referring more particularly to Figs. 1 and 2, there is here shown in cross-section an electrically heatable catalytic core, generally indicated at 10. The core 10 is composed of a series of corrugated thin metal strips 12 secured to a central post 14 by any suitable means. The preferred means is spot welding. Alternatively, and as shown in the aforesaid EP-A-0 456 919 the central post 14 may be made up of a group of individual wires or rods with a thin herringbone corrugated metal strip overfolded over each wire. The individual wires with strips welded thereto are gathered together in a bundle and welded at the ends to form the central post 14. In the present and preferred case, the strips are reverse folded in the middle thereof to form two portions of equal length, and tack welded along the fold line to a stainless steel or nickel center post 14. Folding the herringbone or chevron pattern corrugated thin metal strips prevents the corrugations of one portion from nesting within the corrugations of the adjacent portion. The corrugated thin metal strips are from about 38.1 mm (1.5") to about 88.9 mm (3.5") wide and corrugated by passing between rolls having the corrugation pattern formed therein. Reference may be had to U.S. Patent 4,711,009, supra, for details of the mode of forming corrugated thin metal strips useful herein. The corrugated thin metal strips are desirably stainless steel, and preferably a ferritic stainless steel about 0.0381 to 0.0635 mm (0.0015 to 0.0025 inch) thick, e.g., 0.0406 mm (0.0016") thick. A particularly useful ferritic stainless steel contains 20% chromium, 5% aluminum, .002% to .05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium and praseodymium, or a mixture thereof, and the balance iron plus steel making impurities. Reference may be had to U.S. Patent 4,414,023 to Aggens dated 8 November 1983 for suitable ferritic stainless steel alloys useful herein. The thin stainless steel strip is corrugated continuously, preferably in a herringbone pattern, or a chevron pattern, and later cut to the desired length. The corrugations have an amplitude of from about 0.508 mm (0.02") to about 6.35 mm (0.25") and a pitch or wavelength of from about 0.762 mm (0.03") to about 6.35 mm (0.25"). The chevron or herringbone corrugations desirably have a slope of from 7 degrees to 10 degrees to a line perpendicular to the marginal edges of the thin metal strip, and a pitch of from about 19.05 mm (0.75") to about 38.1 mm (1.5") from apex to apex. Prior to coating the corrugated thin metal strips with a refractory metal oxide and catalyst, the strips are, as indicated, secured to a central post 14. Brazing strips 16 which are 9.525 mm (0.375") to 25.4 mm (1") wide, are alternately (Figs. 2 and 3) and positioned in staggered relation (Fig. 3). Thereafter, the central post 14 is twisted and the corrugated foil and brazing foil strips spirally wound into a tight cylinder as shown in Fig. 1. In order to provide better electrical contact between the binding tube 18 and the spirally wound core 20, a layer of brazing foil 22 is desirably interposed between the core 20 and the binding tube 18. This brazing foil layer has an axial length equal to the axial length of the corrugated thin metal strips 12. The tube 18 may conveniently be formed of two half shells to be welded together on assembly. The brazing foil is rolled to about 0.0254 mm (0.001") to about 0.0762 mm (0.003") thick. It is desirably a nickel/chromium/boron/silicon brazing alloy analyzing 75% to 83% nickel with a liquidus temperature of 1149 to 1260°C (2100 to 2300 F). Where platinum is to be used as the catalyst metal, phosphorus is avoided in the alloy. Other nickel-containing brazing alloys containing from 7% to 14% chromium, 3% to 4.5% iron, 3.5% to 4.5% silicon, 2% to 3% boron, balance nickel and having liquidus temperatures of above about 1149°C (2100 F) may also be used. After the core of Fig. 1 and 1a is fully assembled, it is heated to a temperature sufficient to fuse the brazing strips 16 and 22 to the contiguous members. This has the effect of preventing telescoping of the core in the direction of gas flow during a severe high frequency and high temperature test. To coat the surface of the corrugated thin metal strips, the entire unit is dipped into a slurry of a refractory metal oxide, e.g., an alumina slurry. The unit (Fig. 4) after wiping off the outer shell is then baked at a temperature sufficient to remove the liquid base (water, principally) from the coating and set to coating on the surface, e.g. about 538°C (1000 F). The unit (Fig. 4) is then dipped into an aqueous solution of a noble metal catalyst material. The porous nature of the gamma alumina coating traps and absorbs the aqueous catalyst-containing solution. Heating of the catalyst treated core to the decomposition temperature of the catalyst-containing compound yields the catalyst metal on the substrate. The noble metals useful as catalysts are palladium, platinum, rhodium, ruthenium and mixtures of two or more of such metals. Reference may be had to U.S. Patent 4,711,009 for details of catalytic solutions useful in applying the catalyst to the refractory metal oxide washcoated surface. At this point, a terminal post 26 is secured to the outside of the outer shell 18 as by welding. One of the poles of a voltage source, e.g., the positive pole of an automotive vehicle battery, is later secured to the terminal post 26 by a suitable cable. The opposite pole of the voltage source, e.g., the negative pole of an automotive vehicle battery is later attached to the central post 14 by a suitable cable. To effect rapid heating of the electrically heatable catalytic core, considerable power must be drawn from the voltage source. For details of a suitable power supply system, reference may be had to EP-A-0 456 919 supra. When the assembly as shown in Fig. 4 is completed, it is placed in a suitable housing such as shown in Fig.5. Fig. 5 show a completed electrically heatable catalytic converter, generally indicated at 50. There is provided a steel, preferably stainless steel, housing 52 having adapter end caps 54 and 56 having nipples 58 and 60 which reduce the diameter to the diameter of the exhaust pipe, not shown. The unit 10 (See Figs. 1-4) is disposed within the housing 52 which is conveniently formed of half shells welded together at the seam, and the end caps 54 and 56 later welded on. Because the outer steel binder shell 18 will become positively charged, it must be insulated from the housing 52, and to this end a flexible ceramic insulation mat 62 is interposed between the housing 52 and the steel binder shell 18. An aperture 64 is provided in the mat 62 to accommodate the positive terminal post 26 and suitable insulation means are provided for traversing the housing 52. There is provided an aperture 66 through the housing 52 into which is threadedly engaged a bushing 76 which holds an insulating sleeve 74 which in turn surrounds the post 26. A metal cap 78 provides a seat for a positive cable clamp, not shown. Curent flows to the central post 14 through a transverse bar 80 welded as at 82 to the central post 14 at its midpoint and to the end cap 54 at its extremities. Alternatively, one end of the transverse bar 80 may be extended through end cap 54 and threaded to accept a cable connector from the battery. The flow of power to the electrically heatable unit 10 is effected by suitable switching means 44. Reference may be had to EP-A-0 456 919 for the details of a suitable switching means. The temperature of the core 10 is conveniently sensed by a thermocouple 55 extending through the housing 52 with a metal bushing 59. A suitable insulating sleeve 53 and an inner tube 57 wherein the conducting leads from the junction 61 are carried spaced from each other as is known, extends into the interior of the housing 52. The leads terminate in a junction 61 which is embedded in the core 10. There have thus been provided means for securing spirally wound or S-wound corrugated thin metal foil strips against telescoping under severe testing conditions. The flow of current through the foil strip lengths is preserved because only a tortuous path is provided for current to otherwise follow. Current will prefer to flow from one end of the corrugated thin metal strip at the central post to the other at the inner periphery of the housing. The appropriate resistance in ohms is thus provided. The refractory metal oxide coating on the surface of the corrugated foil has a high dielectric strength and for all practical purposes prevents shorting between contiguous layers of foil except in the areas where the brazing metal is located. Hence, the electrical integrity and performance of the device as disclosed in the aforesaid EP-A-0 456 919 is preserved.

Claims: An electrically heatable core (10) for an electrically heatable catalytic converter (50) including (a) a central electrically conductive core (14), (b) a monolith (20) spirally wound about said central core (14) and having a plurality of corrugated thin metal strips (12) of substantially equal length secured to said central electrically conductive core (14), and (c) a steel retaining shell (18) surrounding said spirally wound monolith (20), said monolith (20) being characterized by a plurality of fused narrow brazing strips (16) having a width of from about 1/4 to about 1/10 of the width of said corrugated thin metal strips (12), and a length substantially equal to the length of said corrugated thin metal strips (12), said brazing strips (16) being in alternating relation with said corrugated thin metal strips (12) and in staggered relation with respect to a centrally located plane normal to the longitudinal axis of said central core (14), said brazing strips (16) being fused to the adjacent corrugated thin metal strips (12). An electrically heatable core as defined in Claim 1 having a current conducting binding post (26) radially extending from said steel retaining shell (18). An electrically heatable core as defined in Claim 1 having a current conducting binding post (80) attached to said electrically conducting core (14). An electrically heatable core as defined in Claim 1 wherein the corrugated thin metal strips (12) are secured to the central core (14) at their midpoints. An electrically heatable core as defined in Claim 4 wherein the corrugated thin metal strips (12) are spot welded to the central core (14). An electrically heatable core as defined in Claim 1 wherein the corrugated thin metal strips (12) are stainless steel. An electrically heatable core as defined in Claim 6 wherein the stainless steel is a ferritic stainless steel. An electrically heatable core as defined in Claim 1 wherein the corrugated thin metal strips (12) have a coating of a refractory metal oxide on at least one surface thereof. An electrically heatable core as defined in Claim 1 wherein a fused layer of thin brazing metal is disposed between the spirally wound monolith (20) and the retaining shell (18). An electrically heatable core as defined in Claim 8 wherein a noble metal catalyst is supported on the refractory metal oxide surface of the corrugated thin metal strips (12). An electrically heatable catalytic converter comprising (a) a housing (52), (b) an end cap (54, 56) at each end of said housing (52) including a nipple (58, 60) adapted to accommodate an exhaust pipe, (c) an electrically heatable core (10) as defined in Claim 1 comprising (I) a central electrically conductive core, (II) a monolith (20) spirally wound about said central core (14) including (1) a plurality of corrugated thin metal strips (12) of substantially equal length secured to said central electrically conductive core (14), and (2) a plurality of fused thin narrow brazing strips (16) having a width of from about 1/4 to about 1/10th of the width of said corrugated thin metal strips (12) and a length substantially equal to the length of said corrugated thin metal strips (12), said brazing strips (16) being in alternating relation with said corrugated thin metal strips (12) and in staggered relation with respect to a centrally located plane normal to the longitudinal axis of said central core (14), said brazing strips (16) being fused to the adjacent corrugated thin metal strips (12), (3) a refractory metal oxide coating on at least one surface of said corrugated thin metal strips (12), (4) a noble metal catalyst supported on said refractory metal oxide coating, and (III) a steel retaining shell (18) surrounding said spirally wound monolith (20); (d) means for electrically insulating said electrically heatable core (10) from said housing (52), and (e) means for supplying electrical power from a voltage source to said electrically heatable core (10).

Inventors: WHITTENBERGER WILLIAM, WHITTENBERGER, WILLIAM
Applicants: GRACE W R & CO, W.R. GRACE & CO.-CONN.
Application year: 1991
Main classification: F01N3
IPC codes: B01J35, F01N3