DE102008003044B4 - Emission control system for improved exhaust gas purification by convective mixing - Google Patents

Abstract

An exhaust gas purification system (100) comprising a monolithic catalyst element (103), a second catalyst element (101) and a mixer element (102), characterized in that the mixer element (102) is arranged upstream of the monolithic catalyst element (103) in the direction of flow of the exhaust gas and that the second catalyst element (101) is arranged in front of the mixer element (102), wherein the mixer element (102) is an open-pore foam body and has pores with a diameter of 100 μm to 2500 μm.

Description

The The present invention relates to an exhaust gas purification system comprising monolithic catalyst element and a mixer element comprises, wherein the mixer element in the flow direction the exhaust gas arranged in front of the monolithic catalyst element is. The invention further relates to a method for cleaning a Exhaust gas and the use of the emission control system.

Exhaust gas purification systems are currently widely used for the purification of exhaust gases from gas, diesel or gasoline engines. Constantly growing requirements in emission control are forcing manufacturers, operators and their suppliers to make permanent improvements to the emission control systems so that legally prescribed emission limits can be met. Different catalysts are available as cleaning systems on the market. Thus, in addition to SCR catalysts (selective catalytic reduction) and oxidation catalysts for stationary applications so-called diesel oxidation catalysts (DOC), SCR catalysts for automotive applications, 3-way catalysts or diesel particulate filters, which catalytically oxidize soot particles, as exhaust gas purification systems individually or in used different combinations. When contacting the exhaust gas emitted from a drive unit with the catalytically active material of the catalyst element, a reaction of pollutants in the exhaust gas takes place. These pollutants include carbon monoxide, hydrocarbons, nitrogen oxides (NO _x ) or soot particles.

modern Emission control systems often consist of the combination (series connection) of several of these catalysts to the required purification levels to achieve. In such emission control systems can often also several layers (repetitions) of the same catalyst installed be. For removal of nitrogen oxides and CO from stationary gas engine applications for example, they are common three layers of SCR catalyst followed by two layers of oxidation catalyst used.

The used catalysts distinguished in unsupported catalysts and coating catalysts become. The full catalysts consist of more than 50% of a catalytic active material, whereas coating catalysts consist of a Consist catalyst carrier body, which may consist of a metal or a ceramic, wherein the surface the catalyst carrier body with a coating is provided. In the case of coating catalysts become the catalyst carrier body with high surface area porous Coated metal oxides. This layer or the applied Suspension is called washcoat and serves as a carrier layer for the catalytically active material, which is usually a precious metal such as platinum, Palladium, rhodium, silver, gold, o. A. or metal oxides of copper, Iron, manganese, o. A. or combinations of the aforementioned precious metals or metal oxides.

The Washcoat is washed by means of a so-called washcoat suspension, d. H. a slurry of metal oxides in a fluid medium the catalyst carrier body applied. Usually will follow the applied washcoat suspension dried and calcined. The washcoat coating is then treated with the catalytically active Components impregnated and by heating activated. Alternatively you can contain the catalytically active components in the coating suspension or previously applied to the metal oxide particles of the washcoat suspension have been.

in the The prior art describes various catalyst carrier bodies. Generally, between wall-flow filters (English: Flow Through Trap, FTT), which act as depth filters, and monolithic carrier bodies. The latter are characterized by long, parallel channels, which are open at both ends. The individual channels are through thin, partly porous Walls from each other separated. In the case of the so-called structured monolithic support body, the channels Have internals or connections with each other.

monolithic Carrier body include so-called honeycomb body (English: honey comb), which are also called monoliths. The mode of action is monolithic carrier body that of a surface filter: the to be cleaned gas stream flows parallel to the channel surface and the pollutant removal from the exhaust stream is made by contact with the channel surface. In the case of catalytically coated channels, the removal is carried out by chemical reaction in the coating.

Furthermore Are there carrier bodies with foam or nonwoven structures. Depending on the size of the structures These moldings act as a surface filter (size Pores) or depth filter (small pores).

monoliths can for example, be constructed of a ceramic material, for example Cordierite. The walls have a strength from 0.2 mm to 0.3 mm. Within the channels, the flow is often laminar.

In the prior art monoliths are known in addition to ceramic supports, which are constructed of a metallic catalyst support. These metallic monoliths consist of partially structured films, with which a channel structure is produced is arranged by alternately smooth and corrugated films, so-called smooth layers or corrugated layers. Within the channels, the flow is also often laminar.

advantages monolithic catalysts compared to those whose cleaning effect As a depth filter is done, the lower pressure loss in the Flow through with the exhaust gas and a significantly reduced susceptibility to blockages by the particle load, such as soot or ash particles, in the exhaust stream. The lower pressure loss is to be regarded as an advantage, since with increasing Pressure loss a reduction of the available engine power or the Motor efficiency goes along.

One Problem is with monolithic carrier bodies the poor contact of the exhaust gas with the channel walls. A more effective contacting of the exhaust gas stream to be cleaned with the catalyst surface can be achieved by cross-mixing of the exhaust gas flow over the channel cross-section. The realization of cross mixing is technically extremely difficult. In metallic monoliths, a partial mixture is within through the channels Induces perforations of the metal foils. For ceramic carriers structuring in the form of perforations hardly possible.

at the implementation of components of a gas stream using a Catalyst plays not only the chemical reaction but also the transport speed, with which the gas molecules to be converted to the catalytically active Centers are transported, an important role. For monolithic Catalysts are transported transversely, d. H. radial, to the axial direction in the channels and second, in the pore structure of the catalysts. It puts themselves in the channel interiors as a result the forming flow profile (often laminar) a radial concentration profile, which leads to a reduction the effective transport of the gas molecules across the flow direction in the channel and thus leads to a reduction in sales.

at Emission control systems consisting of several series connected Catalyst elements remain, which form when flowing through the channels radial concentration profiles also largely exist between the elements, as a concentration compensation in the radial direction mainly by Diffusion takes place for the diffusion process but due to the short residence time between the situation is too little time.

By Magnification of the Dwell time between the layers, it is possible, a better concentration balance to achieve. The one for it required additional Space is often not to disposal, especially in automotive applications. Kick the exhaust, however not completely mixed into a monolithic catalyst element, this leads to a lower degree of purification compared to completely mixed exhaust gas. The mixing of the exhaust gas to be treated in the exhaust gas purification system represents a difficult technical problem.

task Therefore, the present invention has provided the provision of an exhaust gas purification system a catalyst element with which achieves an improved catalytic activity is, further wherein the flow resistance can be kept low in the exhaust stream.

Is solved This object is achieved by an exhaust gas purification system comprising a monolithic Catalyst element, a second catalyst element and a mixer element, wherein the mixer element in the flow direction is arranged in front of the monolithic catalyst element and wherein arranged the second catalyst element in front of the mixer element is, wherein the mixer element is an open-pored foam body and pores having a diameter of 100 microns to 2500 microns. One to clean Exhaust gas flow is first through the mixer element and then through the monolithic Catalyst element guided, being this guide the exhaust gas stream, a convective mixing of the exhaust gas before entry effected in the catalyst element. In this way it is achieved the existence Exhaust gas flow in the combined system is more mixed and the to be cleaned exhaust more effectively contacted the surface.

It is advantageous when the exhaust gas flow in the mixer element turbulent guided becomes. In a turbulent exhaust system in the mixer element is a significantly increased mixing by convective transport in the turbulent flow causes. But also with laminar guidance the flow of exhaust gas in the mixer element can be controlled by suitable flow guidance, i. H. convective, an increased mixing can be achieved. To It is necessary to control the exhaust gas flows through the geometry of a each channel of the mixer element to divide into multiple streams and changed them in the mixer element merge.

The terms turbulent and laminar flow are to be understood here as they are generally known from the teaching of fluid mechanics. It will be understood by those skilled in the art that the question of whether turbulent or laminar flow is present also depends on the flow rate and the type of fluid flowing. This means that a fluid flow in space can be performed mostly laminar as well as turbulent. If in the context of this invention of turbulent or laminar flow in the Filter elements, ie the mixer element or the catalyst element is mentioned, this always refers to the leadership of a typical exhaust gas from internal combustion engines, which is performed with - for the exhaust gas technology - typical space velocities.

The EP 0 537 968 A1 describes a catalyst system consisting of a mixer arranged between two catalyst elements, but no porous foam body. However, that is the EP 0 537 968 A1 underlying task to correspondingly mix supplied hydrogen gas in the mixer with the exhaust stream to achieve optimal reduction of NO _x . However, in the system according to the EP 0 537 968 A1 continues the problem of how an exhaust gas stream can be mixed convergent with low flow resistance.

From the US 2004/0187483 A1 Also known are devices and methods for producing H2 and CO in an oxygen-containing gas stream. Furthermore, devices and methods for removing NO _x from oxygen-containing gas streams are described, in particular oxygen-rich exhaust gas streams of a lean-burn engine such as a diesel engine. The problem of optimum mixing of the gas stream is not apparent from this document.

The WO 2006/128712 describes an exhaust gas treatment apparatus comprising a particle separator, an SCA catalyst for selective reduction of nitrogen oxides and an ammonia generator for generating ammonia as a selective reducing agent for reducing nitrogen oxides, wherein the particle separator is disposed in the main exhaust pipe, the ammonia generator in a first auxiliary pipe Connected to the main exhaust line, so that the gas flow is produced in the ammonia generator, can be passed through the SCR catalyst. However, the problem of convergent mixing with increasing catalyst capability is not addressed here either.

The DE 198 14 585 A1 describes an exhaust system for an internal combustion engine in which a plurality of bends are merged in a collector. The pipe leading from the collector serves as a feed tube to a catalyst. According to the invention, a mixer is provided in a housing of the catalyst, through which the exhaust gas stream entering the catalyst through the feed tube is geometrically and temporally homogenized. This prevents damage to catalyst elements caused by inhomogeneous exhaust gas flows. The mixer of DE 198 14 585 A1 has the task of homogenizing the flow distribution over the cross section of the exhaust pipe, so that caused by local overstress are excluded. In addition, by the mixer according to the DE 198 14 585 A1 Compressive stresses are compensated and continues to serve as a filter by knocking down unburned combustion mixture and there can burn without causing damage. The problem of the convergent mixing of an exhaust gas flow with low flow resistance is in the DE 198 14 585 A1 but not addressed.

Surprisingly was found that the inventive combination from a monolithic catalyst element and a mixer element a higher one Cumulative emission reduction shows as the sum of accumulated Pollutant reductions of the individual elements. Through this system combination will be a surprise and achieved high synergy effect.

The Invention can be Advantageously use, in the exhaust gas cleaning system according to the invention, the Amount of catalytically active components, for example precious metals, on the surfaces the catalyst elements is saved, wherein in the exhaust gas purification system according to the invention a cumulative emission reduction is achieved, that of Exhaust gas purification systems of the prior art corresponds. thanks of the surprising Synergieeffektes, by the inventive system combination of mixer element and monolithic catalyst element is achieved in a smaller amount of catalytically active species accumulated the same Pollutant reduction can be achieved.

The Invention leaves continue to take advantage of by the exhaust gas cleaning system according to the invention the size of the monolithic catalyst element is reduced, wherein in the emission control system according to the invention a cumulative te Pollutant reduction is achieved, at least that of emission control systems of the prior art. Thanks to the surprising synergy effect, due to the system combination of mixing element and monolithic Catalyst element is achieved, so can at a smaller size than at conventional systems at least the same accumulated pollutant reduction can be achieved.

According to one preferred embodiment the mixer element is an open-pored foam body. Open porous foam bodies an economically advantageous way, the exhaust stream to convectively mix in the mixer element in a small footprint, without additional to produce a high pressure loss. The open-pore foam body can a ceramic or metallic material.

In a further particularly preferred embodiment of the emission control system according to the invention the open-pore foam body has pores with a diameter of 100 microns up to 2500 μm, preferably 200 microns up to 800 μm on. The above values are the arithmetic Average pore size. Furthermore the pores have a high degree of crosslinking. With such pores On the one hand there is an intensive mixing, whereby the pores however, are not too small, so the flow resistance is at a low Level is maintained.

In a preferred embodiment The invention is the foam body a ceramic foam. ceramic foams are known in the art. They have a three-dimensional reticular Skeletal structure and contain a variety of flow channels, the from coherent Bubbles are formed.

In a further preferred embodiment, the open-pore foam body is a metal foam. Generally, metal foams are known for use as catalyst support material. For example, the DE-A-102004014076 , hereby fully incorporated by reference, a metal foam as a carrier for a washcoat coating, which in turn is impregnated with precious metals. The precious metals represent the catalytically active component for the conversion of pollutants.

Of the Term "metal foam" means a foam material of any metal or any one Alloy of metals, which may contain further impact substances, such as carbides, etc. may contain. The metal foams have a variety of pores, which are interconnected gas permeable are so that that Exhaust gas can be passed through the foam material.

The open-pored foam body of the emission control system according to the invention particularly preferably has a density of 0.1 to 0.9 g / cm ³ . This density is the bulk density of the uncoated foam, ie it is the quotient of the mass of the foam and its outer volume determined, which results from the outer dimensions of the body. This density therefore includes an average density of the foam material with the spaces between it. Foams with this density and the pore diameters mentioned above are particularly stable.

According to one preferred embodiment of In the invention, the metal foam comprises a nickel-containing alloy or an alloy of iron, chromium and aluminum. The expert are in particular two metal foams known, which are used in the field of emission control systems can. In the first case, this is the so-called Inco foam and in the other case, the so-called Pankl foam. Both alloys have proven to be particularly resistant at high temperatures. Furthermore, the production of the metal foam with the desired easy to accomplish mean pore diameters in these alloys.

Instead of the metal foam may in a further preferred embodiment the mixer element comprise a non-woven structure.

Prefers is also an exhaust gas purification system according to the invention, in which a further catalyst element is arranged in front of the mixer element. This embodiment allows a further cross-mixing of the exhaust stream in the catalyst elements. Particularly preferred is an exhaust gas purification system, wherein the second Catalyst element in the exhaust gas flow direction is arranged in front of the mixer element.

In a preferred embodiment the emission control system according to the invention the mixer element contacts the upstream catalyst element. With this feature of the invention is a higher mechanical stability of the most thinly cut Connected mixer element. Both arrangements, d. H. apart to the following and contacting the upstream catalyst element, have advantages for The invention.

Especially advantageous is the arrangement of the mixer element with a distance before the subsequently arranged catalyst element. Preferably, the distance is of the mixer element 5 to 100 mm, particularly preferably 10 to 50 mm. This creates a gap in which a convective mixture the exhaust stream takes place. This convective mixture is found a distance instead, which does not include another mixer element. This will add an extra convective mixing while saving material. The arrangement with a distance also has the advantage that the Elements are easier to install, or in the case of catalyst poisoning can be easily removed and replaced.

In a further preferred embodiment is The relationship from mean pore diameter of the mixer element to the width of the upstream catalyst element between 1 to 30 and 1 to 1, more preferably between 1 to 20 and 1 to 5.

The inner surface of the mixer element can be provided with a catalytically active coating. Primarily, however, the mixer element is intended to serve for mixing the exhaust gas flow.

The inner surface the monolith is provided with a catalytically active coating. It goes without saying that under an inner surface in the frame this invention the surface should be meant along which the exhaust stream is guided. Thus, if necessary, different reactions are catalyzed can, can the catalyst elements from each other different coatings exhibit. The catalyst elements may be coating catalysts or unsupported catalysts.

Usually coatings on catalyst support bodies by Washcoat suspensions applied. These washcoat suspensions include metal oxides selected from Alumina, silica, iron oxide, titania, ceria, zirconia and aluminosilicate. The coating suspensions may also a mixed oxide or a mixture of said metal oxides contain. The aluminosilicate is more preferably a zeolite. To coating the washcoat, drying and calcining the coating has a large surface on which the desired catalyzed Reaction takes place.

The Washcoat suspensions include in a further advantageous embodiment catalytically active components of the invention. As a particularly advantageous have platinum, palladium, silver, gold, rhodium, ruthenium, Nickel, cobalt, iron, vanadium, tungsten, manganese or copper, their Mixtures or their oxides proved. If necessary, the catalytically active metals also subsequently on the coating be applied.

task The present invention further provides a method for cleaning an exhaust gas stream with high efficiencies.

The Task is solved by a method of purifying an exhaust gas stream using an emission control system according to the invention, in which an exhaust gas flow through a mixer element and then through a downstream catalyst element is passed.

It were surprisingly Synergy effects found when an exhaust gas through a mixer element, is guided in particular by an open-cell foam body, in which an intensive mixing the exhaust gas takes place.

Further preferred embodiments the method according to the invention arise analogously from the dependent claims of the emission control system according to the invention.

In a further preferred embodiment be multiple combinations of mixer element with downstream Catalyst element arranged to build the exhaust gas purification system.

Especially The emission control system is suitable for use in diesel oxidation catalysts. Furthermore, the exhaust gas purification system according to the invention finds Use as a catalyst for the reduction of nitrogen oxides, as an oxidation catalyst for the reduction the content of CO and hydrocarbons from exhaust gases of stationary internal combustion engines.

The invention is further illustrated by the following 1 to 4 and their description, without this being to be understood as limiting. Show it

1 an inventive exhaust gas purification system,

2 to 4 : Sales of pollutants by catalysts of the invention

1 shows a preferred embodiment of an emission control system according to the invention 100 comprising three elements 101 . 102 and 103 and a catalyst housing 104 , Essential to the invention are the elements 102 and 103 , The Elements 101 and 103 represent monolithic catalysts. The element 102 is according to the invention, the mixer element through which the exhaust stream is convectively mixed. This results in a more intensive contacting of the exhaust gas flow with the walls of the catalyst element 103 instead of. An exhaust stream, represented by the dashed arrows, is introduced into the exhaust gas purification system 100 introduced, penetrates the catalyst elements 101 and 103 and the mixer element 102 and then leaves the exhaust purification system cleaned.

2 to 4 show the temperature-dependent conversion of a gas consisting of 200 ppmv propane and 800 ppmv CO in air, which is contacted at a rate of 120,000 h ^-1 with a standard catalyst (standard) or the catalysts 1 to 5 (cat 1 to 5). All catalysts include monolithic catalyst elements of coating catalysts provided with an alumina-based coating and with platinum as the catalytically active species. The gas is passed through the standard catalyst at various temperatures and the concentration of the mixture of propane and CO is determined by Ver let the catalyst measured and the conversion determined. The standard catalyst is a system composed of two monolithic catalyst elements, wherein the monolithic catalyst elements are not spaced apart from each other, ie they are in surface contact with each other.

at 2 the catalyst 1 (Cat 1) is a system consisting of two monolithic catalyst elements, which further comprises a foam plate made of silicon carbide. This foam plate has a thickness of 25 mm and has pore sizes of 35 ppi. The foam plate is arranged between the two monolithic catalyst elements and all elements are in surface contact. Catalyst 2 (Cat 2) comprises the same elements as Catalyst 1, with the foam plate again sandwiched between the monolithic catalyst elements and in face contact with the front monolithic catalyst element with the front and rear monolithic catalyst elements spaced 6 cm apart. Catalyst 3 (Cat 3) has the same structure as Catalyst 1, wherein instead of a 25 mm thick foam plate three 25 mm thick foam plate are available.

3 shows experimental results of the standard catalyst according to the 2 in comparison to a catalyst (Cat 4), which consists of two monolithic catalyst elements, between which an Incoschaumplatte is arranged. All catalyst elements are in surface contact with each other.

4 shows experimental results of the standard catalyst according to the 2 in comparison to a catalyst (Cat 5), which consists of two monolithic catalyst elements, which occupy a distance of 6 cm.

All Experiments show that both a distance of two catalyst elements as well as the presence of foam boards as mixer elements to a height Sales of harmful gas leads.

Claims (18)

Emission control system ( 100 ) comprising a monolithic catalyst element ( 103 ), a second catalyst element ( 101 ) and a mixer element ( 102 ), characterized in that the mixing element ( 102 ) in the flow direction of the exhaust gas in front of the monolithic catalyst element ( 103 ) and that the second catalyst element ( 101 ) in front of the mixer element ( 102 ), wherein the mixer element ( 102 ) is an open-pore foam body and has pores with a diameter of 100 microns to 2500 microns. Exhaust gas purification system according to claim 1, characterized that the open-pore foam body a ceramic foam or a metal foam comprises. Exhaust gas purification system according to claim 2, characterized in that that the metal foam is a nickel-containing alloy or a Comprising iron / chromium / aluminum alloy. Exhaust gas purification system according to claim 2, characterized in that the ceramic foam comprises silicon carbide. Exhaust gas purification system according to one of claims 1 to 2, characterized in that the open-pore foam body has a density of 0.1 to 0.9 g / cm ³ . Emission control system according to one of the preceding Claims, characterized in that the inner surface of the open-pored foam body with a catalytically active coating is provided. Exhaust gas purification system according to one of the preceding claims, characterized in that the mixing element ( 102 ) and the second catalyst element ( 101 ) are in contact with each other. Exhaust gas purification system according to one of the preceding claims, characterized in that the mixing element ( 102 ) and the monolithic catalyst element ( 103 ) are spaced from each other. Exhaust gas purification system according to claim 8, characterized in that the mixing element ( 102 ) and the monolithic catalyst element ( 103 ) are arranged at a distance of 5 to 100 mm, preferably 10 to 50 mm. Exhaust gas purification system according to one of the preceding claims, characterized in that the ratio of the average pore diameter of the mixer element ( 102 ) to the width of the channels of the second catalyst element ( 101 ) is between 1 to 1 and 1 to 30, preferably between 1 to 5 and 1 to 20. Exhaust gas purification system according to one of the preceding claims, characterized in that the monolithic catalyst element ( 103 ) and the second catalyst element ( 101 ) are provided with a catalytically active coating, the metal oxides selected from the group consisting of alumina, silica, iron oxide, titano xid, ceria, zirconia and an aluminosilicate, or a mixed oxide selected from the above metal oxides. Emission control system according to claim 11, characterized characterized in that the aluminosilicate is a zeolite. Exhaust gas purification system according to one of claims 11 or 12, characterized in that the monolithic catalyst element ( 103 ) and the second catalyst element ( 101 ) have mutually different coatings. An exhaust purification system according to any one of claims 10 to 13, characterized in that the coatings platinum, palladium, Rhodium, ruthenium, nickel, cobalt, silver, gold, iron, vanadium, manganese, Tungsten or copper, their mixtures or their oxides. A method for purifying an exhaust gas stream with an exhaust gas purification system according to any one of claims 1 to 14, wherein an exhaust gas flow through a mixing element ( 102 ) and then through a monolithic catalyst element ( 103 ). Method according to claim 15, characterized in that the mixing element ( 102 ) and the monolithic catalyst element ( 103 ) are spaced from each other. A method according to claim 16, characterized in that the distance between the mixer element ( 102 ) and monolithic catalyst element ( 103 ) Is 5 to 100 mm, preferably 10 to 50 mm. Use of the exhaust gas purification system according to a the claims 1 to 14 as a diesel oxidation catalyst, as a catalyst for the reduction of Nitrogen oxides or as an oxidation catalyst for the reduction of CO and hydrocarbon emissions from the exhaust gases of stationary internal combustion engines.