DE102008003044A1 - Exhaust gas cleaning system for use as e.g. diesel oxidation catalyzer in otto engine, has mixer element arranged in flow direction of exhaust gas before monolithic catalyst element and including pores with specific diameter - Google Patents

Abstract

The system (100) has a mixer element (102) e.g. open-porous foam body, arranged in flow direction of exhaust gas before a monolithic catalyst element (103). The body is made of metal foam or ceramic foam that is made of silicon carbide. The foam body provides density of 0.1 to 0.9 gram per centimeter cube. The mixer element includes pores with diameter of 200 Pico meter to 800 Pico meter. An internal surface of the foam body is provided with a catalytic active coating. An independent claim is also included for a method for cleaning an exhaust gas stream with an exhaust gas control system.

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 of the exhaust gas is arranged in front of the monolithic catalyst element. The invention further relates to a method for purifying an 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 Exhaust gas purification systems often consist of the combination (Series connection) of several of these catalysts to the required To achieve cleaning levels. In such emission control systems can often also have multiple layers (repetitions) of the same Catalyst be installed. For removal of nitrogen oxides and CO stationary gas engine applications become common, for example three layers of SCR catalyst followed by two layers of oxidation catalyst used.

The Catalysts used in solid catalysts and A distinction can be made between coating catalysts. The full catalysts consist of more than 50% of a catalytically active material, whereas coating catalysts consist of a catalyst carrier body consist of a metal or a ceramic, wherein the surface of the catalyst carrier body provided with a coating. In the case of coating catalysts be the catalyst carrier body with high surface coated porous 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 foregoing Precious metals or metal oxides.

The Washcoat layer is prepared by means of a so-called washcoat suspension, d. H. a slurry of the metal oxides in a fluid medium applied to the catalyst carrier body. Usually Subsequently, the applied Washcoat- suspension dried and calcined. The washcoat coating will after that impregnated with the catalytically active components and activated by heating. Alternatively, the catalytically active components in the coating suspension or previously applied to the metal oxide particles of the washcoat suspension have been.

in the State of the art are various catalyst carrier body described. Generally, between wall-flow filters (English: Flow Through Trap, FTT), which act as depth filters, and monolithic Carrier bodies differ. Draw the latter characterized by long, parallel channels, the 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 carrier body The ducts can be internals or connections have one another.

monolithic Carrier bodies comprise so-called honeycomb bodies (English: honey comb), which are also called monoliths. The mode of action monolithic carrier body is 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 are removed by chemical reaction in the coating.

About that In addition, there are carrier body with foam or nonwoven structures. Depending on the size of the structures, these shaped bodies act as a surface filter (large pores) or depth filter (small pores).

monoliths For example, they can be constructed from a ceramic material be, for example, cordierite. The walls have one Thickness of 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 the exhaust gas and a significantly reduced Susceptibility to blockage by the particle load, For example, 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 in available engine power or the motor efficiency.

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

at the implementation of components of a gas stream using a Catalyst plays not only the chemical reaction but also the transport speed, with the gas molecules to be reacted to the catalytic active centers are important. For monolithic catalysts transport to a transversely, d. H. radial, to the axial direction in the channels and the other in the pore structure of the catalysts. It turns into the Channel interiors as a result of the forming flow profile (often laminar) a radial concentration profile, this leads to a reduction of the effective transport of the gas molecules transverse to the flow direction in the channel and thus to a Reduction of sales leads.

at Emission control systems consisting of several series connected Catalyst elements exist that remain when flowing through also in the channels forming radial concentration profiles exist between the elements for the most part, since a concentration compensation takes place in the radial direction mainly by diffusion, for the diffusion process but due to the short residence time there is not enough time between the layers.

By Increasing the residence time between the layers it is possible, a better concentration balance to achieve. The additional required for this Space is often not available, especially in automotive applications. Kick the exhaust, however not completely mixed in a monolithic catalyst element a, this leads to a lower degree of purification in comparison to completely mixed exhaust. The mixing of the treating exhaust gas in the exhaust gas purification system makes a difficult technical problem.

task The present invention therefore was the provision of an exhaust gas purification system, where effective convective mixing becomes more effective Contacting of the exhaust gas to be cleaned with monolithic catalysts leads.

Solved This object is achieved by an exhaust gas purification system comprising monolithic catalyst element and a mixer element, wherein the mixer element in the flow direction in front of the monolithic Catalyst element is arranged. An exhaust gas stream to be cleaned is first through the mixer element and then passed through the monolithic catalyst element, wherein this guidance of the exhaust flow convective mixing causes the exhaust gas before entering the catalyst element. In this way it is achieved that an exhaust gas flow in the combined system is more mixed and that too cleaning exhaust more effectively contacted the surface.

It is advantageous when the exhaust gas flow in the mixer element turbulent to be led. In a turbulent exhaust system in the mixer element is a significantly increased mixing by causes the convective transport in the turbulent flow. But even with laminar guidance of the exhaust stream in the Mixer element can by suitable flow guidance, d. H. convective, an increased mixing can be achieved. To It is necessary to control the exhaust gas flows through the geometry of each channel of the mixer element into several partial streams divide and merge these in the mixer element.

The Terms turbulent and laminar flow are like this to understand how they are generally derived from the teaching of fluid mechanics are known. The skilled person is clear that the question of whether a turbulent or laminar flow is present, also from the flow rate and the type of flowing Fluids depends. This means that a fluid flow In the room mostly laminar as well as turbulent are led can. 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 on driving a typical exhaust gas from internal combustion engines, that with - for the exhaust technology - typical Space speeds is performed.

Surprisingly was found that the inventive Combination of a monolithic catalyst element and a Mixer element a higher cumulative emission reduction shows as the sum of the accumulated pollutant reductions the individual elements. This system combination becomes a surprising and achieved high synergy effect.

The Invention can be used advantageously by in the exhaust gas purification system according to the invention the amount of catalytically active components, for example precious metals, Saved on the surfaces of the catalyst elements is, 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 the surprising synergy effect, by the inventive System combination of mixer element and monolithic catalyst element can be achieved with a smaller amount of catalytically active Species the same cumulative emission reduction can be achieved.

The Invention can also be used advantageously by in the exhaust gas purification system according to the invention the size of the monolithic catalyst element is reduced, wherein in the inventive Emission control system achieved a cumulative pollutant reduction at least that of state-of-the-art emission control systems the technique corresponds. Thanks to the surprising synergy effect, due to the system combination of mixing element and monolithic Catalyst element is achieved, so with a smaller size at least the same as conventional systems Pollutant reduction can be achieved.

According to one preferred embodiment, the mixer element is a open-pored foam body. Open-pore foam body represent an economically advantageous option the exhaust gas flow in the mixer element convective in a small footprint to mix, without additionally a high pressure loss to create. The open-pored foam body may be a ceramic or metallic material.

In a further particularly preferred embodiment of the inventive exhaust gas cleaning system has the open-pore foam body has pores with a diameter from 100 μm to 2500 μm, preferably 200 μm up to 800 μm. The above values are around the arithmetic mean of the pore size. About that In addition, the pores have a high degree of crosslinking. In such On the one hand, pores cause intensive mixing, whereby However, the pores are not too small, so the flow resistance is kept at a low level.

In a preferred embodiment of 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 that are contiguous 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 The term "metal foam" means a foam material any metal or any alloy of Metals, optionally other impact substances, such as carbides, etc. can contain. The metal foams have one Variety of pores on each other, gas permeable are connected so that the exhaust gas through the foam material can be passed through.

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 the quotient of the mass of the foam and its outer volume, which results from the outer dimensions of the body, is determined. 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 the invention comprises the metal foam a nickel-containing alloy or an alloy of iron, Chrome and aluminum. The skilled worker is in particular two metal foams known, which are used in the field of emission control systems can. This is the case in the first case mentioned Inco foam and the other case to the so-called Pankl foam. Both alloys have been found to be extra at high temperatures proven consistently. Furthermore, the production of the metal foam with the desired mean pore diameters in these Alloys easy to accomplish.

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 further cross mixing the exhaust stream in the catalyst elements. Especially preferred is an exhaust gas purification system in which the second catalyst element arranged in the exhaust gas flow direction in front of the mixer element is.

In a preferred embodiment of the invention Emission control system contacts the mixer element upstream Catalyst element. With this feature of the invention is a higher mechanical stability of the mostly 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. Preferred is the distance of the mixer element 5 to 100 mm, particularly preferred 10 to 50 mm. This creates a gap in which a convective Mixture of the exhaust stream takes place. This convective blend takes place on a route that is not another mixer element includes. This will create an extra convective Ensures mixing while saving material. The arrangement with a distance also has the advantage that the Elements can be installed more easily, or in the case a catalyst poisoning easier to remove and replaced can be.

In a further preferred embodiment the ratio of mean pore diameter of the mixer element to the width of the upstream catalyst element between 1 at 30 and 1 to 1, more preferably between 1 in 20 and 1 in 5.

The inner surface of the mixer element can be catalytically be provided active coating. First and foremost, however the mixer element for a mixing of the exhaust gas stream serve.

The The inner surface of the monolith is catalytic provided active coating. It goes without saying that under an inner surface in the context of this invention the surface should be meant, along which the exhaust gas flow led becomes. So that optionally catalyses different reactions can be, the catalyst elements have mutually different coatings. The catalyst elements can Coating catalysts or full catalysts.

Usually become coatings on catalyst carrier bodies applied by washcoat suspensions. These washcoat suspensions include Metal oxides selected from alumina, silica, Iron oxide, titanium oxide, cerium oxide, zirconium oxide and aluminosilicate. The Coating suspensions may also be a mixed oxide or contain a mixture of said metal oxides. The aluminosilicate a zeolite is particularly preferred. After coating the washcoat suspension, the Drying and calcining the coating gives it a great deal Surface on which the desired catalyzed Reaction takes place.

The Washcoat suspensions include in a further advantageous Embodiment of the invention catalytically active components. Particularly advantageous are platinum, palladium, silver, gold, Rhodium, ruthenium, nickel, cobalt, iron, vanadium, tungsten, manganese or copper, their mixtures or their oxides proved. Possibly The catalytically active metals can also be retrofitted applied to the coating.

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

The The object is achieved by a method for cleaning a Exhaust gas stream using an inventive Exhaust gas purification system, in which an exhaust gas flow through a mixer element and subsequently through a downstream catalyst element is directed.

It Surprisingly, synergy effects were found when an exhaust gas through a mixer element, in particular through an open-pore foam body, is guided, in which an intensive mixing of the exhaust gas takes place.

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

In In another preferred embodiment, several are used Combinations of mixer element with downstream catalyst element arranged to build the emission control system.

In particular, the exhaust gas cleaner is suitable system for use in diesel oxidation catalysts. Furthermore, the exhaust gas purification system according to the invention is used as a catalyst for the reduction of nitrogen oxides, as an oxidation catalyst for reducing 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 measured after leaving the catalyst and the conversion is 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 leads to a high sales of harmful gas.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• - DE 102004014076 A [0026]

Claims (22)

Emission control system ( 100 ) comprising a monolithic catalyst element ( 103 ) 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 ) is arranged. Exhaust gas purification system according to claim 1, characterized in that the mixing element ( 102 ) is an open-pored foam body. An exhaust purification system according to claim 1 or claim 2, characterized in that the open-pore foam body a ceramic foam or a metal foam comprises. Exhaust gas purification system according to claim 3, characterized that the metal foam is a nickel-containing alloy or a Comprising iron / chromium / aluminum alloy. Exhaust gas purification system according to claim 3, characterized the ceramic foam comprises silicon carbide. An exhaust gas purification system according to any one of claims 1 to 3, characterized in that the open-pore foam body has a density of 0.1 to 0.9 g / cm ³ . Exhaust gas purification system according to one of the preceding claims, characterized in that the mixing element ( 102 ) Has pores with a diameter of 100 microns to 2500 microns, preferably 200 microns to 800 microns. Emission control system according to one of the preceding Claims, characterized in that the inner surface of the open-pore foam body with a catalytically active Coating is provided. Exhaust gas purification system according to one of the preceding claims comprising a second catalyst element ( 101 ). Exhaust gas purification system according to one of the preceding claims, characterized in that the second catalyst element ( 101 ) in front of the mixer element ( 102 ) is arranged. 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 12, 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 comprising metal oxides selected from the group consisting of alumina, silica, iron oxide, titania, ceria, zirconia and an aluminosilicate, or a mixed oxide selected from the above metal oxides. Emission control system according to claim 15, characterized characterized in that the aluminosilicate is a zeolite. Exhaust gas purification system according to one of claims 15 or 16, characterized in that the monolithic catalyst element ( 103 ) and the second catalyst element ( 101 ) have mutually different coatings. Exhaust gas purification system according to one of the claims 14 to 17, characterized in that the coatings are platinum, Palladium, rhodium, ruthenium, nickel, cobalt, silver, gold, iron, Vanadium, manganese, tungsten or copper, their mixtures or their Include oxides. A method for purifying an exhaust gas stream with an exhaust gas purification system according to one of claims 1 to 18, wherein an exhaust gas flow through a mixing element ( 102 ) and then through a monolithic catalyst element ( 103 ). Method according to claim 19, characterized in that the mixing element ( 102 ) and the monolithic catalyst element ( 103 ) are spaced from each other. A method according to claim 20, characterized ge indicates 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 of claims 1 to 18 as a diesel oxidation catalyst, as a catalyst for the reduction of nitrogen oxides or as an oxidation catalyst to reduce CO and hydrocarbon emissions from the exhaust gases of stationary combustion engines.