WO1995008702A1

WO1995008702A1 - Combined hydrocarbon trap and electrically heatable converter

Info

Publication number: WO1995008702A1
Application number: PCT/US1994/009624
Authority: WO
Inventors: William A. Whittenberger
Original assignee: W.R. Grace & Co.-Conn.
Priority date: 1993-09-24
Filing date: 1994-08-24
Publication date: 1995-03-30

Abstract

There is provided an improved fluid treating system, apparatus therefor, and a method of treating a fluid, all of which are characterized by a combined electrically heatable catalytic converter (10) and a hydrocarbon trap (12) in the fluid conduit (16). The system is especially useful in treating exhaust gas from an internal combustion engine (18).

Description

COMBINED HYDROCARBON TRAP AND ELECTRICALLY HEATABLE CONVERTER

This invention relates to an exhaust gas treating system useful in treating exhaust gas containing hydrocarbon contaminants, and especially useful in treating exhaust gas from internal combustion engines which contain not only unburned hydrocarbons, but also other contaminants, e.g., carbon monoxide, ozone, etc.

In these systems, the carrier gas is not recirculated as in a closed system. Thus, ambient air (or the "carrier gas") is taken into the system from an unconfined source, e.g., the atmosphere, and used as a diluent and reactant, as in an internal combustion engine, becomes laden with pollutants, and as "exhaust" gas, passes through the system wherein the pollutants are at least substantially removed, and the pollutant-depleted gas returned to the unconfined source or atmosphere. A different volume of carrier gas is continuously supplied to the system, is diluted and at least partially reacted, is then treated, and finally is released to the unconfined source.

BACKGROUND OF THE INVENTION AND PRIOR ART Both hydrocarbon adsorbers or traps (TRAP) and electrically heatable converters (EHC) are known to the automobile industry as possible means to reduce cold start emissions of unburned hydrocarbons from vehicles. Both technologies work during the first minute after cold start, and before the main catalyst of the vehicle is hot enough to be maximally effective.

TRAPS work by adsorbing hydrocarbons, e.g., benzene, methane, cyclohexene, pentane, etc., generally a C2 to CIO hydrocarbon, at a very low temperature, and desorbing them at higher temperatures, usually on a monolith coated with a hydrophobic zeolite material, silica gel, or an activated carbon material. High silica alumina zeolites are most commonly used. Appropriate zeolite structures include X, Y, Z S -5, beta and ordenite. TRAPS are usually mounted upstream of a conventional non- electrically heated catalyst, e.g., a ceramic core catalytic converter. When designed properly, the temperature at which the hydrocarbon is desorbed is above the hydrocarbon light-off temperature of the catalytic converter so that the desorbed hydrocarbons can be oxidized under the influence of the catalyst before they escape to the atmosphere. The usual oxidation catalyst is a noble metal, e.g., platinum, palladium, rhodium, ruthenium, or a mixture of two or more of such noble metals, for example, a palladium/rhodium catalyst.

It is difficult to achieve a desorption temperature with TRAP materials that is above the light-off temperature of most hydrocarbon oxidation catalysts. That temperature is about 900°F for some hydrocarbons. Other volatile organic compounds "light-off" at about 590°F. Carbon monoxide "lights-off" at about 390°F. Also, TRAP materials are not tolerant of high temperatures occasionally encountered in exhaust streams. Depending on their application, considerable volumes of adsorbent can be required to trap hydrocarbon arising from cold start and before conventional converter light- off. TRAPS are attractive, however, because of their passive nature. EHC's work by electrically heating from a battery, for example, a metal monolith supporting a catalyst, e.g., a noble metal catalyst such as mentioned above. The heating process usually takes less than 30 seconds, e.g., 10 seconds, to reach a temperature of about 650°F. Oxidation of unburned hydrocarbons begins after the warm up period. Heat generated from the oxidation reaction is usually passed on to other downstream catalytic devices in the exhaust line so that they, in turn, heat up to hydrocarbon "light-off" temperature and begin to oxidize hydrocarbons efficiently.

The primary problem with EHC's is that they use too much energy from the power source. Attempts to down-size the heated element to reduce energy consumption result in a catalytic surface area too small to be effective, and monoliths too small to be structurally sound. However, EHC's work well if enough energy is available.

The present- invention combines EHC and adsorber/TRAP technology in various ways to achieve low cold-start hydrocarbon emissions while avoiding the pitfalls of each technology.

In the following description, reference will be made to "ferritic" stainless steel. A suitable ferritic stainless steel for use, particularly in the internal combustion engine exhaust applications hereof, is described in U.S. Patent 4,414,023 dated 9 November 1983 to Aggen. A specific ferritic stainless steel alloy useful herein contains 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 of such rare earth metals, balance iron and trace steel making impurities. A ferritic stainless steel is commercially available from Allegheny Ludlum Steel Co. under the trademark "Alfa IV."

Another metal alloy especially useful herein is identified as Haynes 214 alloy which is commercially available. This alloy and other nickeliferous alloys are described in U.S. Patent 4,671,931 dated 9 June 1987 to Herchenroeder et al. These alloys are characterized by high resistance to oxidation. A specific example contains 75% nickel, 16% chromium, 4.5% aluminum, 3% iron, optionally trace amounts of one or more rare earth metals except yttrium, 0.05% carbon and steel making impurities. Haynes 230 alloy, also useful herein, has a composition containing 22% chromium, 14% tungsten, 2% molybdenum, 0.10% carbon, and a trace amount of lanthanum, balance nickel. The ferritic stainless steels and the Haynes 214 and 230 alloys are examples of high temperature resistive, oxidation resistant (or corrosion resistant) metal alloys that are suitable for use in making thin metal strips for use in the converter and TRAP bodies hereof, and particularly for making heater strips for the EHC portions and "light-off" portions hereof. Suitable metal alloys must be able to withstand "high" temperatures of from 900°C to 1200°C (1652°F to 2012°F) over prolonged periods.

Other high temperature resistive, oxidation resistant metals are known and may be used herein. For most applications, particularly automotive applications, these alloys are used as "thin" metal strips, that is having a thickness of from about 0.001" to about 0.005", and preferably from about 0.0015" to about 0.003". Other metals may be used as "thin" metal strips for lower temperature (under 600°F) applications, such as aluminum alloy 3003.

The "thin" metal strips, corrugated or flat, are cleaned or etched and usually and desirably precoated with at least one wash coating of a refractory metal oxide to a loading of at least about 5 mg per square inch up to 30 mg per square inch, or if electrified, with a coating of "dielectric" material as described below. This coating and the catalyst may be applied during the processing step such as disclosed in U.S. Patent 4,711,009 dated 8 December 1987 to Cornelison et al, prior to assembly into a finished converter. Alternatively, the finished thin metal monolith cores may be post-coated after fabrication into a multicellular thin metal monolith converter body by, for example, a dipping and calcining process. This latter process has not been found to be as effective or economical as precoating. Further, the refractory metal oxide coating or "dielectric" may be applied by a precoating process as mentioned above, and the catalyst post-applied by a dipping and calcining process. These coating processes are known in the> art. For most purposes, the noble metal catalyst requires a support base of refractory metal oxide strongly adherent to the surface of the thin metal strips or foil strips. The catalyst may be applied simultaneously with the washcoat of refractory metal oxide.

By the term "exhaust gas" as used herein, is meant a gas generated by the application of energy to a given system, e.g., an exothermic chemical reaction, especially oxidation and partial oxidation, such as occurs in an internal combustion engine or a gas turbine. Spark ignited or compression ignited engines, including automotive and locomotive engines, on-the-road and off- the-road engines, stationary engines, power plants, boat and ship engines, lawn mowers, motor cycles, etc. generated the kind of pollutant-containing exhaust gas the devices of this invention are best suited to treat effectively.

BRIEF STATEMENT OF THE INVENTION Briefly stated, the present invention is an exhaust gas treating system which comprises an exhaust gas conduit, such as a conventional internal combustion engine exhaust pipe, and a hydrocarbon trap in said conduit, and an electrically heatable catalyst supporting multicellular metal monolith converter in said conduit. The hydrocarbon trap is preferably one that adsorbs and desorbs a hydrocarbon containing from 1 to 10 carbon atoms in the molecule. Desirably, the exhaust gas conduit also includes one or more conventional catalytic converters which may be of the ceramic core_.type, or the thin metal monolith type made up of layers of thin metal strips, e.g., alternating corrugated and flat thin metal strips wound about a central rigid member which is either separate from or. integral with the thin metal strips. There may also be present a "light-off" converter adjacent to the EHC, or integral therewith as disclosed in commonly owned application Serial No. 13,516 filed 3 February 1993 by the applicant herein, to which reference may be had.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by having reference to the annexed drawings showing preferred embodiments of the invention and wherein:

Fig. 1 is a schematic illustration of a combined EHC and hydrocarbon TRAP in conjunction with a main or conventional catalytic converter in series in an exhaust gas conduit.

Fig. 2 is a schematic illustration of another series arrangement of the TRAP and EHC with the main catalytic converter which is designed to protect the TRAP from undue heating in the exhaust line.

Fig. 3 is a schematic illustration of still another series arrangement of the TRAP and EHC with both a small catalytic converter and a main catalytic converter in the exhaust gas conduit.

Fig. 4 shows a series of layers of thin metal foil strips, corrugated on flat, in which some of the corrugated thin metal strips have a "dielectric" coating, a refractory metal oxide coating thereover, and catalyst coating, and are usually electrically heatable; and some have a "dielectric" coating along with a zeolite coating to provide an integral TRAP/EHC combination for use in a single converter body. Strips are marked A and B. Strips A or strips B, or both, can be electrically heated, together or separately. Also, the construction may be corrugated on corrugated, with, for example, a chevron or herringbone pattern to render them nonnesting. Fig. 5 shows a structure for a combined flat and corrugated thin metal strip assembly utilizing a plurality of flat thin metal strips and a single corrugated thin metal strip.

Fig. 6 shows a variation of a catalyzed cross flow heat exchanger plus a TRAP having an EHC upstream of the heat exchanger.

Fig. 7 shows in perspective view, an integrated EHC/TRAP device in accordance herewith.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, the present invention combines a hydrocarbon TRAP and an EHC in such a way as to achieve the advantages of each while minimizing the disadvantages. The EHC normally has the difficulty of requiring too much power to achieve rapid heating, or too much energy to heat a sizable monolith. In the present structures, the power and energy requirements can be minimized. The TRAP normally has the difficulty of the desorbing temperature being below the "light-off" temperature of the oxidizing catalytic converter. It also tends to be bulky and decreases in activity when exposed to high temperatures. The EHC and the TRAP in the present structures coact so that the power requirements can be reduced and the desorption temperature becomes much less critical, while reducing trap volume and thermally protecting the TRAP.

Fig. 1 shows an exhaust gas treating system including an EHC generally indicated at 10,_.and a TRAP, generally indicated at 12. There is also present in the system a conventional catalytic converter generally indicated at 14. These elements are located in an exhaust gas pipe 16 leading from a conventional internal combustion engine 18. The EHC may be a combined EHC and "light-off" converter structure assembled as one body such as that described and claimed in copending commonly owned application Serial No. 13,516, supra. Alternatively, an EHC structure such as that shown in commonly owned copending application Serial No. 880,082 filed 4 May 1992 filed by Sheller may be used in combination with the TRAP.

The EHC 10 reaches "light-off" temperature at approximately the same time that the TRAP begins desorbing. The EHC is able to heat slowly (up to 30 seconds or more) to reach "light-off" temperature and can, therefore, use relatively low power, e.g., less than 500 watts, because the TRAP 12 is adsorbing hydrocarbons, a principal pollutant in the period immediately after engine start-up. The lower power requirement allows simpler switching equipment, e.g., relays. The TRAP 12 is small because it needs to adsorb for only up to 30 seconds. Also, its desorb temperature can be lower because the "light-off" temperature of the catalyst is controlled electrically instead of by exhaust temperature. In the embodiment of Fig. 1, the TRAP/EHC combination (10-12) can be located some distance from the engine 18 to protect the TRAP materials from high exhaust temperature. Fig. 2 shows a series arrangement similar to that shown in Fig. 1. However, in Fig. 2 the TRAP 12 is additionally protected from thermal damage by the main catalytic converter 14. In this case, the EHC will require more power to reach "light-off" temperature than in the embodiment shown in Fig. 1 because of the cooler location in the exhaust line 16.

Fig. 3 is similar to Fig. 1, except that a small catalytic converter 20, not electrically heatable, is inserted in the gas conduit 16 upstream of the TRAP 12. The small catalytic converter 20 offers some thermal protection as in Fig. 2, without a severe power penalty.

In each of the above cases, the combined EHC/TRAP system solves the following problems of the individual technologies: (1) EHC 10 power levels are minimized resulting in a simplified electrical system.

(2) The desorb temperature of the TRAP 12 is no longer critical.

(3) The volume of the TRAP 12 is relatively small due to the shorter time window required with an EHC alone.

(4) It becomes possible to thermally protect the TRAP 12 by selecting relatively cool locations in the exhaust gas treatment system. Also, in each of the above systems for treating exhaust gas, the TRAPS 12 may be electrically heatable multicellular monoliths formed of thin metal strips, or electrically heatable extruded multicellular ceramic monoliths. This variation has the additional advantage that the exact instant of hydrocarbon desorption in the TRAP is controllable by the application of electrical power.

Fig. 4 shows a stack or bundle generally indicated at 30 of thin metal foil strips 32 and 34. The thin metal strips 32 are corrugated, and the thin metal strips 34 are flat and are in alternating relation with the corrugated strips 32 in the stack 30. Some of the thin metal strips 32 or 34 indicated at A are "dielectric" and refractory metal oxide and noble metal catalyst coated, and others indicated at B are coated with "dielectric" plus adsorber material. Corrugations are straight- through, that i&, they run in a straight line from marginal edge to marginal edge of the thin metal strips, preferably perpendicularly. The corrugated and flat strips 32 and 34, respectively, may be prepared by utilizing the corrugation and coating steps of the process described in U.S. Patent 4,711,009 dated 8 December 1987. In the example shown in Fig. 4, the strips 32 and 34 are coated in, for example, the aforesaid process, with a "dielectric" coating (gamma- alumina) then with a refractory metal oxide, e.g. , gamma- alumina, gamma-alumina/ceria, titania, titania/alumina, titania/ceria, zirconia, vanadia, as a wash coat and calcined. A suitable process is described and claimed in commonly owned U.S. Patent Application Serial No. 07/955,460 filed 2 October 1992 by Cornelison et al to which reference may be had. The resulting coating is strongly adherent to the alloy metal surface and provides an excellent substrate for a noble metal catalyst, such as those mentioned above. The thin metal strips 32 or 34, for example, may be coated on one side with the "dielectric" plus the refractory metal oxide/catalyst, and on the other with a "dielectric" plus a zeolite, e.g., a silica/alumina zeolite, which zeolite has a desorption temperature in excess of 600°F. Alternatively, adsorber material may be mixed directly with the catalyst composition, and coated on both sides over the "dielectric" coating. The invention also contemplates coating the upstream portion of the strips, e.g., the strips 32 to 34 with "dielectric" plus adsorber on both sides, and the downstream portion with "dielectric" plus refractory metal oxide/catalyst on both sides. Each of these latter variations has virtually the same effect as the basic Fig. 4 construction.

The provision of a "dielectric" coating has been referred to above. A typical procedure for applying a "dielectric" coating involves first coating the thin metal strip with one coat of "Catapal", which is a gamma- alumina, at a loading of 3 mg to 12 mg per square inch, followed by drying and then calcining at 1750°F up to 2000°F for anywhere from 10 seconds to 3 hours to form an insulative or tightly bonded coating. The higher the initial calcining temperature and the longer the initial calcining period, the better the insulating properties.

Strips A may be electrically heated, or strips B may be electrically heated, or both, or only certain of strips A, or certain of strips B may be electrically heated as the requirements of the system demand. Thus, when some of the thin metal strips 32 and 34 are coated with a hydrocarbon adsorbent on at least one side, and some of the thin metal strips 32 and 34 are electrically heated, there is then obtained an integrated EHC/TRAP device for enclosing in a single housing. Such an integrated device is shown in Fig. 4 wherein the flat thin metal strips 34 in alternating juxtaposed relation with the corrugated thin metal strips 32 are electrically heatable from the battery indicated at 33 when switch 35 is closed thereby connecting cables 37 and 39 to the flat thin metal strips 34. In certain embodiments of the invention, the flat thin metal strips 34 may be omitted, and the monolith constructed entirely of corrugated thin metal strips arranged so that contiguous strips 32 do not nest.

A group of the layered thin sheet metal strips including corrugated and flat strips 32 to 34, is crushed in the middle, then S-wound with a suitable.mandrel into a monolith, and encased in a suitable retainer such as shown in Fig. 7. Reference may be had to commonly owned U.S. Patent Application Serial No. 08/84426 filed 29 June 1993 by David Thomas Sheller which is incorporated herein by reference for a discussion of the layering, crushing in the central portion, S-winding and encasing in a suitable retainer shell.

Fig. 5 shows an assembly generally indicated at 38 comprising a corrugated thin metal strip 40 having overlayed three narrowed flat thin metal strips 42, 44 and 46. Several of these assemblies may be stacked one upon the other, crushed in the middle, grasped by a mandrel, and wound into a monolith in the manner such as described in the aforesaid commonly owned copending application Serial No. 08/84426 filed 29 June 1993 by Sheller, supra, to which reference may be had. Various combinations of the strips 42, 44, and 46 may be heated independently or together. By selective application of the appropriate coatings to the thin metal strips, single unit systems are created to meet demand. Thus, a wide range of systems and system performance are available through application of different coatings and electrical heatability in selected cases.

Fig. 6 shows a variation of a catalyzed heat exchanger 54 and TRAP combination reported in the literature, but modified according to this invention by the addition of an EHC 52 upstream of the heat exchanger 54. This modification permits a smaller TRAP 56, as the catalyst in the EHC lights off more quickly. Moreover, the desorb temperature of the TRAP 56 is less critical.

Fig. 7 shows an integrated EHC/TRAP device generally indicated at 70, in a single housing 72. As shown, the stack of thin sheet metal layers 32 and 34 has been crushed at 74 and S-wound. The S-wound stack is desirably wrapped in a layer of brazing foil, placed in the housing 72, and heated, as by induction heating, to a temperature sufficient to fuse the brazing foil and secure the free ends of the foil strips to the inside of the housing 72. Selected sheet metal layers, e.g., the flat strip layers 32, are attached to a battery power source, not shown, for effecting electrical heating. The remaining corrugated thin metal strips 34 are provided with a coating of a hydrocarbon adsorber, e.g., a silica/alumina zeolite. Thus, while the EHC portion is heating up to "light-off" temperature, the TRAP portion is adsorbing hydrocarbon from the gas stream, and then desorbing the hydrocarbon at a higher temperature.

Instead of trapping hydrocarbon in a unit such as described above, the same result may be achieved in the same way by coating the inside of the exhaust pipe 16 in Figs. 1-3 with hydrocarbon adsorbent material.

There has thus been provided an exhaust gas treating system which combines an EHC and a hydrocarbon TRAP in a way that utilizes the best features of each technology and minimizes the adverse effects thereof.

Claims

WHAT IS CLAIMED IS;

1. An exhaust gas treating system comprising (a) a gas conduit, (b) a hydrocarbon trap in said conduit, and (c) an electrically heatable catalyst supporting multicellular metal monolith converter in said conduit.

2. An exhaust gas treating system for a pollutant containing gas comprising (a) an exhaust gas conduit having an inlet and outlet, (b) a hydrocarbon trap for adsorbing and desorbing hydrocarbon located in said conduit between said inlet and said outlet, and (c) an electrically heatable oxidation catalyst supporting multicellular metal monolith converter in said conduit located between said inlet and said outlet for oxidizing gaseous hydrocarbon in said exhaust gas.

3. An exhaust gas treating system for a pollutant- containing exhaust gas comprising (a) an exhaust gas conduit having an inlet and an outlet, (b) a hydrocarbon trap for adsorbing and desorbing hydrocarbon located in said conduit between said inlet and said outlet, (c) an electrically heatable oxidation catalyst supporting multicellular metal monolith converter in said conduit located between said inlet and said outlet for raising the temperature of said exhaust gas to a temperature sufficient to oxidize pollutant material contained in said exhaust gas, and (d) a conventional catalytic converter in said conduit located between said inlet and said outlet for catalyzing the oxidation of pollutant material in said exhaust gas.

4. An exhaust gas treating system as defined in Claim 1 wherein the hydrocarbon trap includes a zeolite.

5. An exhaust gas treating system as defined in Claim 4 wherein the zeolite is a silica alumina zeolite.

6. An exhaust gas treating system as defined in Claim 1 wherein the hydrocarbon trap is a multicellular metal monolith in which the cells include a coating of a zeolite.

7. An exhaust gas treating system as.defined in Claim 6 wherein the multicellular metal monolith includes layers of corrugated thin metal.

8. An exhaust gas treating system as defined in Claim 7 wherein at least some of the layers of corrugated thin metal are coated on at least one side of a hydrocarbon adsorbing/desorbing layer of a zeolite.

9. An exhaust gas treating system as defined in Claim 6 wherein the zeolite has a temperature at which hydrocarbon is desorbed in excess of 600°F.

10. An exhaust gas treating system as defined in Claim 1 wherein the hydrocarbon trap is located upstream of the electrically heatable converter.

11. An exhaust gas treating system as defined in Claim 3 wherein the conventional catalytic converter is located downstream of the electrically heatable converter.

12. An exhaust gas treating system as defined in Claim 3 wherein the hydrocarbon trap is located upstream of the electrically heatable converter, and the conventional converter is located downstream of the electrically heatable converter.

13. An exhaust gas treating system as defined in Claim 3 wherein the conventional converter is located upstream of the hydrocarbon trap and the electrically heatable converter.

14. An exhaust gas treating system as.defined in Claim 3 wherein the elements (b) , (c) and (d) are located in the conduit in the order named.

15. An exhaust gas treating system as defined in Claim 3 which further includes an additional conventional catalytic converter upstream of the hydrocarbon trap.

16. An exhaust gas treating system comprising (a) a gas conduit and (b) a combined hydrocarbon trap and electrically heatable catalyst supporting multicellular metal monolith converter in said conduit, and (c) a housing for said trap and said electrically heatable converter.

17. An exhaust gas treating system as defined in Claim 16 wherein the combined hydrocarbon trap and electrically heatable converter is formed of a plurality of layers of thin metal sheets, at least some of which have a coating of a hydrocarbon adsorbing/desorbing zeolite on at least one side thereof.

18. An exhaust gas treating system as defined in Claim 17 wherein at least some of the plurality of thin metal sheets are corrugated, and the remainder are flat and disposed in contiguous alternating relation with the corrugated thin metal sheets.

19. An exhaust gas treating system as defined in Claim 17 wherein all of the thin metal sheets are corrugated in a nonnesting manner.

20. A method for treating an exhaust gas comprising the steps of passing an exhaust gas stream containing hydrocarbon through a hydrocarbon trap, adsorbing hydrocarbon with a hydrocarbon adsorbing agent from said exhaust gas stream and then desorbing said hydrocarbon from said adsorbing agent at a higher temperature, then passing said exhaust gas stream through an electrically heatable catalytic converter to raise the temperature of the exhaust gas stream to at least the hydrocarbon light- off temperature to oxidize said hydrocarbon to carbon dioxide and water, and then discharging said hydrocarbon depleted exhaust gas to the atmosphere.

21. A method as defined in Claim 20 wherein the hydrocarbon adsorbing agent is a zeolite.

22. A method as defined in Claim 21 wherein the zeolite is a silica alumina zeolite.

23. A method as defined in Claim 21 wherein the adsorbing agent is activated carbon.

24. A method as defined in Claim 21 wherein the adsorbing agent is silica gel.