Process for reducing diesel enigne emissions

Abstract

The present invention discloses a system and a process for removing diesel engine emissions. The system comprises two honeycomb-type oxidation catalysts and a catalyzed flow-through filter configured such that the filter sandwiched between the honeycombs. The system is effective for the continuous removal of carbon monoxide and hydrocarbon particulate matter, and has high efficiency and low pressure drop.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is related to U.S. Provisional Application Ser. No. 60/495,419 filed on Aug. 15, 2003 and incorporated herein in its entirety by reference.

BACKGROUND

[0002] The present invention discloses a system and a process for removing diesel engine emissions. The system comprises two honeycomb-type oxidation catalysts and a catalyzed flow-through filter. The system is configured with the filter sandwiched between the honeycombs. The system is effective for the continuous removal of carbon monoxide and hydrocarbon particulate matter, and has high efficiency and low pressure drop. The process for removing the diesel engine emissions involves passing the emissions through the system.

[0003] Internal combustion engines function by burning fuels (hydrocarbons) at high temperatures. In theory, the products of the combustion process are CO.sub.2 and water. But, it is not uncommon that the combustion process is incomplete resulting in the formation of undesirable byproducts are formed such as carbon monoxide, hydrocarbons and soot. Other reactions occurring in internal combustion engines include the oxidation of nitrogen molecules to produce nitrogen oxides and the oxidation of sulfur to form SO.sub.2 and small percentage of SO.sub.3. Further, when the temperature decreases, the SO.sub.3 can react with H.sub.2O to form sulfuric acid. Other inorganic materials are formed as ash. The products of these reactions result in undesirable gaseous, liquid and solid emissions from internal combustion engine: gaseous emissions--carbon monoxide, hydrocarbons, nitrogen oxides, sulfur dioxide; liquid phase emissions--unburned fuel, lubricants, sulfuric acid; and, solid phase emissions--carbon (soot). The combination of liquid phase hydrocarbons, solid phase soot and sulfuric acid results in the formation of small size droplets often called total particulate matter. These emissions create pollution and are potential health risks.

[0004] Efforts have been made to develop exhaust gas cleaning catalysts for a number of years. Flow-through honeycomb oxidation catalysts have been proposed for the removal of gas phase carbon monoxide, light hydrocarbons and particulate matter. These catalysts commonly include a precious metal, such as platinum, palladium, rhodium or a combination thereof, washcoated onto an alumina, titania, zirconia, silica, zeolite or combination support. However, these honeycomb catalysts have not been especially effective for the oxidation of the soluble organic fraction (SOF) of the total particulate matter.

[0005] Because soot has a graphitic structure and normally a relatively large particle size, the soot cannot penetrate into the honeycomb catalysts pore system, but rather passes straight through unaffected. To remove the soot, and for high efficiency particulate removal in general, filtration methods must be used.

[0006] The purpose of a filter is to remove particulate matter while allowing exhaust to freely pass through the filter. However, as particulate collects on the filter, the flow of the exhaust is impeded, resulting in an increased back pressure with the filter, which results in reduced engine efficiency. When the efficiency is below acceptable performance standards, the filter must either be replaced or regenerated by burning off the particulate matter. However, burning off the particulate requires temperatures in excess of 600.degree. C., that is energy intensive and that can lead to uncontrolled light-off of the soot and temperature overshoots that can damage the filter medium. Thus, an efficient, low cost, essentially continuous regeneration of the diesel particulate filter at a lower temperature would be an improvement to the prior art.

SUMMARY OF THE INVENTION

[0007] The present invention discloses an exhaust cleaning system and a process for removing diesel engine emissions. The system comprises first and second honeycomb-type oxidation catalysts and a catalyzed flow-through filter. The honeycomb-type oxidation catalysts comprise a high surface area support impregnated with one or more precious metals. The filter is a non-wall flow type particulate filter washcoated with either a precious metal catalyst or a base metal catalyst or a combination thereof. The system is configured with the filter sandwiched between the honeycombs. The system is effective for the continuous removal of carbon monoxide and hydrocarbon particulate matter, and has high efficiency and low pressure drop.

[0008] The process for removing the diesel engine emissions involves passing the emissions through the system thereby allowing the carbon monoxide to react on the honeycomb catalyst and allowing the particulate matter to react on the filter. In an exemplary embodiment, a honeycomb--filter--honeycomb "sandwich" is mounted in a catalyst housing, the housing is mounted between an engine cylinder exhaust manifold and exhaust pipe open to the atmosphere, and diesel exhaust is passed through the housing from the exhaust manifold exiting to the exhaust pipe.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0009] The present invention discloses an exhaust cleaning system and a process for removing diesel engine emissions. Diesel exhaust is cleaned by passing through a multiple phase catalyst and filter, reducing the amount of noxious emissions normally associated with diesel engines.

[0010] The exhaust cleaning system comprises first and second honeycomb-type oxidation catalysts and a catalyzed flow-through filter. The first and second catalysts comprise a honeycomb substrate washcoated with a support material and treated with either a precious metal catalyst or a base metal catalyst or a combination thereof. Preferably, the metals are selected for their efficacy in oxidizing carbon monoxide, emission hydrocarbons, and/or total particulate matter.

[0011] The honeycomb substrate can be either ceramic or metallic, as is known in the art. Honeycomb supports are commonly used in applications for treatment of exhaust because the honeycombs create low back pressure to the exhaust flow. In the present application, suitable honeycomb substrates include those made of ceramic, cordierite, mullite, silicon carbide, alumina, titania, zirconia, silica, alumina-silica, alumina-zirconia, stainless steel, Fe--Cr--Al alloy and the combinations and other materials typically used for diesel oxidation catalysts. The honeycomb substrate is washcoated with a high surface area support, such as, without limitation, alumina, silica, titania, zirconia, alumina-silica, zeolites, and combinations thereof.

[0012] The metal catalyst can then be deposited on the washcoated honeycomb substrate. The metal catalyst can be any metal known in the art to facilitate total particulate matter oxidation, such as, without limitation an alkaline metal, an alkaline earth metal, vanadium, a precious metal, platinum, palladium, rhodium and combinations thereof. The metal can be impregnated on the support or deposited in any other manner known in the art, including depositing the metal in the form of a salt solution, incipient wetness impregnation or dipping.

[0013] For the purposes of this development, when used in the exhaust cleaning system the first and second honeycomb-type oxidation catalysts may have essentially the same compositions or may vary such that the honeycomb catalysts have different compositions, that is, each honeycomb catalyst has its own support and its own metal combination. Further, the honeycomb substrates may be the same for the first and second honeycomb catalysts or may differ as appropriate for the application and exhaust conditions. The terms "first" and "second" as applied to the honeycomb catalysts merely are for purposes of distinguishing where the catalyst is positioned relative to the filter, and is not intended to imply that the catalyst is limited to a single honeycomb catalyst. If appropriate for the application, two or more honeycombs, each having its own support and metal combination, may be combined to form the "first" and/or "second" honeycomb catalysts.

[0014] The catalyzed flow-through filter is used to enhance the oxidation of soot by lowering the soot ignition temperature and by accelerating the reaction rate. Foams, wire-mesh, and other non-wall-flow type particulate filters generate sufficient particulate matter trapping efficiency for the diesel engine applications and the filtration efficiency can be designed to operate in a range of from about 20% to about 80% soot removal. In addition, foam and wire-mesh type filters work under a principle of depth filtration, whereby the physical contact of particulate matter with the catalyst is significantly improved as compared to that of a straight channel honeycomb, and because these filters have wide pore structures, ash passes through the filter without being accumulated, thereby minimizing the pressure drop, maintaining engine efficiency and reducing the catalyst deactivation rate. As a result, less frequent cleanup is needed and the durability of the emission control system is improved. Some exemplary filter materials include, but are not limited to, ceramic, alumina, tiatania, zirconia, cordierite, mullite, silicon carbide, stainless steel, iron chromium alloy, and combinations thereof.

[0015] To reduce the temperature needed for regeneration of the filter, a catalyst is added to the filter. In the present development, the catalyst is added via conventional means, such as washcoating the filter is with high surface area support such as alumina, titania, zirconia, zeolite, and then depositing catalytic materials. Catalytic materials that are active for soot oxidation are preferred, such as, without limitation, precious metals, base metals, vanadium-comprising complexes, alkaline metals, alkaline earth metals, alkaline vanadates, MoO.sub.3--V.sub.2O.sub.5, platinum, palladium, rhodium and combinations thereof.

[0016] The exhaust cleaning system of the present development comprises at least a first and a second honeycomb oxidation catalyst and at least one non-wall flow type particulate filter. The exhaust cleaning system is mounted between an engine cylinder exhaust manifold and an exhaust pipe open to the atmosphere. In practice, the diesel exhaust is passed through the exhaust manifold, and then into the system where the exhaust first contacts the first honeycomb catalyst and then contacts the filter and then contacts the second honeycomb catalyst, and then the exhaust exits through the exhaust pipe. In an exemplary embodiment, the exhaust cleaning components are combined in a catalyst housing such that the first honeycomb catalyst is positioned at the exhaust manifold end of the housing, the second honeycomb catalyst is positioned at the exhaust pipe end of the housing, and the filter is positioned between the first and second honeycomb catalysts. In an alternative embodiment, each component has its own housing unit and the housing containing the first honeycomb catalyst is connected via piping to the exhaust manifold, the housing containing the second honeycomb catalyst is connected via piping to the exhaust pipe, and the housing containing the filter is positioned between the first and second honeycomb catalysts so as to receive exhaust from said first catalyst and send exhaust to said second catalyst. Other variations combining one honeycomb catalyst with the filter within a single housing are also expected to function as intended, provided the filter is between the honeycombs with respect to the order in which the exhaust meets each component of the system.

[0017] The exhaust cleaning system of the present development is effective for the control of emissions of any type of diesel engines generating either "wet particulates" or "dry particulates". Both soluble organic fraction and soot of total particulate matter are removed using the combination of the multiple honeycomb oxidation catalysts and the catalyzed filter. The system is particularly effective for the continuous reduction of carbon monoxide and hydrocarbon emissions and the continuous removal of particulate matter with minimum back pressure penalty on the source engine, and for diesel engines operating at high temperatures, or operating with lower quality fuels that contain high level of impurities. Because the system operates essentially continuously, there is little or no need for frequent filter clean-up and maintenance.

Claims

What is claimed is:

1. An exhaust cleaning system for treating diesel exhaust, said system comprising; (a) at least a first and a second catalyst, each comprising a honeycomb substrate washcoated with a support material and treated with a metal known in the art to facilitate total particulate matter oxidation; and (b) a catalyzed flow-through filter; wherein said exhaust cleaning system is mounted between an engine cylinder exhaust manifold and an exhaust pipe, and said diesel exhaust can pass through the exhaust manifold, and then into said cleaning system where the exhaust first contacts said first honeycomb catalyst, and then contacts said filter, and then contacts said second honeycomb catalyst, and then exits said cleaning system and enters said exhaust pipe.

2. The exhaust cleaning system of claim 1 wherein said honeycomb substrate is ceramic or metallic.

3. The exhaust cleaning system of claim 1 wherein said honeycomb substrate is selected from the group consisting of ceramic, cordierite, mullite, silicon carbide, alumina, titania, zirconia, silica, alumina-silica, alumina-zirconia, stainless steel, Fe--Cr--Al alloy, other materials typically used for diesel oxidation catalysts, and combinations thereof.

4. The exhaust cleaning system of claim 1 wherein said honeycomb substrate is washcoated with a high surface area support.

5. The exhaust cleaning system of claim 4 wherein said high surface area support is selected from the group consisting of alumina, silica, titania, zirconia, alumina-silica, zeolites, and combinations thereof.

6. The exhaust cleaning system of claim 1 wherein said metal catalyst is selected from the group consisting of an alkaline metal, an alkaline earth metal, vanadium, a precious metal, platinum, palladium, rhodium and combinations thereof.

7. The exhaust cleaning system of claim 1 wherein said first and said second honeycomb-type oxidation catalysts have essentially the same compositions.

8. The exhaust cleaning system of claim 1 wherein said first and said second honeycomb-type oxidation catalysts have different compositions.

9. The exhaust cleaning system of claim 1 wherein said filter is a particulate filter with a foam, wire-mesh, or other non-wall-flow type structure.

10. The exhaust cleaning system of claim 1 wherein said filter is made of a material selected from the group consisting of ceramic, alumina, tiatania, zirconia, cordierite, mullite, silicon carbide, stainless steel, iron chromium alloy, and combinations thereof.

11. The exhaust cleaning system of claim 1 wherein said filter comprises a high surface area support and a catalyst.

12. The filter of claim 11 wherein said high surface area support is selected from the group consisting of alumina, titania, zirconia, zeolite, and combinations thereof.

13. The filter of claim 11 wherein said catalyst is selected from the group consisting of precious metals, base metals, vanadium-comprising complexes, alkaline metals, alkaline earth metals, alkaline vanadates, MoO.sub.3--V.sub.2O.sub.5, platinum, palladium, rhodium and combinations thereof.

14. The exhaust cleaning system of claim 1 further comprising a catalyst housing having an exhaust manifold end and an exhaust pipe end, said honeycomb catalysts and said filters being contained within said housing such that said first honeycomb catalyst is positioned near said exhaust manifold end, and said second honeycomb catalyst is positioned near said exhaust pipe end, and said filter is positioned between the first and second honeycomb catalysts.

15. The exhaust cleaning system of claim 1 wherein said first honeycomb catalyst, said filter, and said second honeycomb catalyst are each contained within a housing and are arranged such that the housing containing said first honeycomb catalyst is connected via piping to an exhaust manifold, and the housing containing said second honeycomb catalyst is connected via piping to an exhaust pipe, and the housing containing said filter is positioned between the first and second honeycomb catalysts so as to receive exhaust from said first catalyst and send exhaust to said second catalyst.

16. An exhaust cleaning system for treating diesel exhaust, said system comprising; (a) at least a first and a second catalyst, each comprising a honeycomb substrate washcoated with a support material and treated with a metal known in the art to facilitate total particulate matter oxidation; and (b) a catalyzed flow-through filter; wherein said exhaust cleaning system is mounted between an engine cylinder exhaust manifold and an exhaust pipe, and said diesel exhaust can pass through the exhaust manifold, and then into said cleaning system where the exhaust first contacts said first honeycomb catalyst, and then contacts said filter, and then contacts said second honeycomb catalyst, and then exits said cleaning system and enters said exhaust pipe.

17. The exhaust cleaning system of claim 16 wherein said honeycomb substrate is selected from the group consisting of ceramic, cordierite, mullite, silicon carbide, alumina, titania, zirconia, silica, alumina-silica, alumina-zirconia, stainless steel, Fe--Cr--Al alloy, other materials typically used for diesel oxidation catalysts, and combinations thereof; and wherein said honeycomb substrate is washcoated with a high surface area support selected from the group consisting of alumina, silica, titania, zirconia, alumina-silica, zeolites, and combinations thereof; and wherein said metal catalyst is selected from the group consisting of an alkaline metal, an alkaline earth metal, vanadium, a precious metal, platinum, palladium, rhodium and combinations thereof.

18. The exhaust cleaning system of claim 16 wherein said filter is a particulate filter with a foam, wire-mesh, or other non-wall-flow type structure made of a material selected from the group consisting of ceramic, alumina, tiatania, zirconia, cordierite, mullite, silicon carbide, stainless steel, iron chromium alloy, and combinations thereof; said filter further comprising a high surface area support selected from the group consisting of alumina, titania, zirconia, zeolite, and combinations thereof, and a catalyst selected from the group consisting of precious metals, base metals, vanadium-comprising complexes, alkaline metals, alkaline earth metals, alkaline vanadates, MoO.sub.3--V.sub.2O.sub.5, platinum, palladium, rhodium and combinations thereof.

19. A process of treating diesel exhaust comprising passing said exhaust through an exhaust manifold, and then into an exhaust cleaning system where said exhaust first contacts a first honeycomb catalyst and then contacts a filter and then contacts a second honeycomb catalyst, and then said exhaust exits through an exhaust pipe.

20. The process of claim 19 wherein said first honeycomb catalyst comprises a honeycomb substrate washcoated with a support material and treated with a metal known in the art to facilitate total particulate matter oxidation, and said filter comprises a particulate filter having a high surface area support and a catalyst, and said second honeycomb catalyst comprises a honeycomb substrate washcoated with a support material and treated with a metal known in the art to facilitate total particulate matter oxidation.