Process and apparatus for cleaning a gas flow

This application is a division of application Ser. No. 08/806,511, filed Feb. 24, 1997 now U.S. Pat. No. 5,965,098.

This application claims priority of German patent application 196 06 657 filed Feb. 23, 1996, the entire contents of which are expressly incorporated herein by reference.

1. Field of the Invention

The invention relates to a method and devices for scrubbing gases.

2. Description of Related Art and Summary of the Invention

For reducing nitrogen oxides from gaseous samples, catalytically active spinels and/or spinel-type compositions have been used with the general chemical formulae:

metal (I) metal (II) Al 2 O 4

and/or

metal (I) oxide.metal (II) oxide.Al 2 O 3 .

However, calculations of the catalytic effect of these compositions assumes they will be used above approximately 150° C. If these compositions are used to scrub exhaust, especially from internal combustion engines, only a slight reduction in nitrogen oxides levels is detected in the starting phase, or immediately after the engine is started, during the period when the temperature of the above-mentioned compositions is below approximately 150° C. Consequently, there is increased emission of nitrogen oxides during the starting phase if these compositions are used in vehicle exhaust systems. In addition, these compositions exhibit reduced effectiveness in removing nitrogen oxides when water and/or steam are present.

In one supported catalyst previously described, an aluminum oxide catalyst containing silver is used to decompose nitrous oxide. The method for its manufacture is discussed in PCT application WO 94/02244. The application of that catalyst to the removal of nitrogen oxides, especially at temperatures below 200° C., is not discussed in WO 94/02244.

A goal of the present invention is to develop methods, devices, and materials for use in reducing the total content of gases, especially nitrogen oxides, optionally in the presence of water and/or steam, from certain emissions, preferably at temperatures below 200° C. This particular goal is achieved through the methods and devices of this invention by employing an intermediate storage medium with gas flow channels in its interior. The intermediate storage medium is composed of a porous storage material for storing nitrogen oxide at a particular temperature range and a supporting material. The intermediate storage medium may be a component of a gas scrubbing device, such as part of a motor vehicle exhaust system.

Advantageously, a supporting material is chosen that simultaneously exhibits a storing effect for hydrocarbons in the nitrogen oxide storing temperature range of the storage material and a catalytically-active effect for nitrogen oxides above a material-specific response temperature. In choosing a supporting material and preparing the compounds and devices of this invention, copending U.S. applications Ser. Nos. 08/764,465, 08/764,464, 08/764,461, and 08/763,535, each filed Dec. 12, 1996, can be referred to and are specifically incorporated herein by reference.

Choosing a supporting material exhibiting a hydrocarbon storing effect is especially advantageous when the method is used or this intermediate storage medium is used with internal combustion engines operated on hydrocarbons, preferably motors or gasoline or diesel engines. Generally, the stored hydrocarbons are released from the supporting material at temperatures above the response temperature at which catalytic reduction of the nitrogen oxides takes place with the aid of hydrocarbons. Preferably, in this regard, the response temperature of the supporting material lies in the vicinity of or below the upper limit of the nitrogen oxide storing temperature range of the storage material, since reduction of nitrogen oxides increases when the storage effect of the storage material decreases.

The supporting material selected for use in an intermediate storage medium may comprise spinels and/or spinel-type compositions according to the formal chemical formula Cu x Me y Al z O 4 , since compositions of this type exhibit a significant catalytic effect with respect to nitrogen oxides at higher temperatures (above approximately 150° C). In this case, not only the nitrogen oxides but also the hydrocarbons are extracted from the gas at lower temperatures, so that the total emission output is reduced.

In an exhaust line of an internal combustion engine, the intermediate storage medium is logically used as a filter body that simultaneously causes the storage of NOx and optionally hydrocarbons at temperatures below the response temperature of the supporting material and causes the oxidation of hydrocarbons and the reduction of nitrogen oxides above the response temperature, with the release of the stored hydrocarbons and nitrogen oxides to the gas advantageously taking place in this temperature range. Oxidation and reduction as well as storage of the hydrocarbons are performed by the supporting material that may have additional materials and/or elements besides those mentioned, while the intermediate storage of NOx is performed by the storage material mixed with the support material.

Further logical embodiments can be derived from the descriptions below. In addition, the invention is explained in greater detail with reference to the embodiments shown in the drawings.

Other objects, advantages and novel features of the invention will be apparent to one skilled in the art from the following detailed description of the invention and in conjunction with the accompanying drawings.

In FIGS. 1, 2 , and 3 :

1 is the direction of flow through a device; 2 is an intermediate storage medium; 3 is a feed channel; 4 are through flow channels; 5 is an atomizer jet; 6 is a ring jet; 7 7 ′ and 7 ″ are partial bodies of the intermediate storage medium; 8 and 8 ′ are intermediate chambers; and 9 represents an exterior connection of the device to the exhaust line of an internal combustion engine.

FIG. 1 shows a cross section of a device for scrubbing the exhaust of an internal combustion engine, with one intermediate storage medium arrangement.

FIG. 2 is the section shown in FIG. 1 with a different intermediate storage medium arrangement.

FIG. 3 is the section shown in FIG. 1 with the intermediate storage medium divided into three parts.

FIG. 4 is a graph showing the NO concentration versus temperature.

In FIG. 1, a scrubber is shown as a device for processing gases, with intermediate storage medium ( 2 ) designed as a filter body being incorporated into an exhaust line 9 of a gasoline-powered, four-cycle engine, especially an automobile engine. Porous intermediate storage medium ( 2 ) is made of a storage material and a supporting material. The storage material may be one or more metals of Group I and/or Group VIII of the periodic chart and/or one or more of their metal compounds, especially their oxides, and/or one or more of their metal alloys. Thus, one or more of each of the elements Cu, Ag, Au, Fe, Co, Ni, Ru, Rh, Pd, Re, Os, Ir, Pt, oxides thereof and alloys thereof, can be mixed with the support material in producing an intermediate storage medium.

In one embodiment, the intermediate storage medium is a composition that contains silver, with the general chemical formula Ag.CuAl 2 O 4 that is made from the carrier material according to a method that is known from WO 94/02244. In this case silver is the storage material, while the CuAl 2 O 4 is the support material. The intermediate storage medium has the property that nitrogen oxide is bonded or stored by adsorption at temperatures below about 145° C, and released again above about 145° C. The properties of the composition are particularly interesting in that this storage of nitrogen oxide also takes place when detectible amounts of water are present in the gas being scrubbed. This surprising effect can be seen in FIG. 4 .

To perform the measurements to produce the graph shown in FIG. 4, a porous composition, extruded in pellets and composed of storage material and supporting material, is exposed to a stream of gas in a heatable reactor, the flow rate of said gas being approximately 30,000 liters per hour. The composition of the gas is as follows: Ar+800 ppm NO+800 ppm C 3 H 6 +10% O 2 +8% H 2 O. From the graph, which also shows he behavior of other spinel compositions in the presence of wager for comparison, the storage of No below 145° C. is clearly visible. In addition, the rise in NO concentration beyond she introduced amount of 800 ppm NO above 145° C. shows that the NO that was stored earlier is being released from the storage material. Since water is created during combustion, the storage properties in the presence of water is of great significance.

In the Figures, representing devices incorporating the use of methods of the invention and the compositions and devices of this invention, intermediate storage medium ( 2 ) may have as through-flow channels ( 4 ) through which the exhaust to be scrubbed flows and which are shown here as straight channels for improved clarity.

The exhaust, flowing through a through-flow channel ( 4 ), contains hydrocarbons, which, when above a certain response temperature of the catalytically effective supporting material, serve as reducing agents for the nitrogen oxides that are also present in the exhaust, in this case NO and NO 2 in particular. After a certain processing distance, however, the hydrocarbons are used up so that intermediate storage medium ( 2 ) is saturated with emissions as far as its catalytic effect is concerned.

In order for further reduction of emissions to be possible within intermediate storage medium 2 in the scrubbing system, the intermediate storage medium 2 composed of a supporting material that acts as a catalyst and composed of a storage material, has an additional channel 3 between two through-flow channels 4 and serving as a feed device for the reducing agent. Feed channel 3 is connected with a storage medium (not shown) of the feed device. Advantageously, feed channel 3 is operably connected, with suitable safety measures being taken, with the tank vent of a motor vehicle so that the gas space in the tank forms the storage medium.

Feed channel 3 in the form of a blind hole, looking in flow direction 1 of the exhaust, is closed on the gas inflow side and connected on the gas outflow side with the storage medium. Ammonia and hydrocarbons, the later for use particularly in motor vehicles, have proven to be satisfactory reducing agents.

When added, the reducing agent flows in on the gas outflow side and diffuses through the intermediate storage medium 2 by capillary-action and flows into through-flow channels 4 where it evaporates, so that the concentration of reducing agent is increased in the already partially scrubbed exhaust.

The increase in the concentration of hydrocarbons that takes place within intermediate storage medium 2 means that the emissions that remain in the pre-scrubbed exhaust are further reduced during the additional passage of the exhaust through intermediate storage medium 2 , with the exhaust being cleaned better in the entire scrubbing system. Emissions can also be reduced advantageously with regard to the concentration of carbon monoxide and/or hydrocarbons in the exhaust.

In order for the amount of reducing agent added not to be too large, the feed device optionally has a metering device (not shown) or a volumetric pump so that the amount of reducing agent added can be controlled.

Feed channel 3 can also be provided, starting at the outer circumference of intermediate storage tedium 2 and running crosswise to flow direction 1 of the exhaust up to the middle of intermediate storage medium 2 . This embodiment, not shown, has the advantage that a plurality of through-flow channels 4 with a single feed channel 3 can be simultaneously charged with reducing agent. It is also favorable in this regard for the feed channel or channels 3 to be supplied in simple fashion with reducing agent from the outer circumference of intermediate storage medium 2 .

In the case of intermediate storage medium 2 , which is designed to be porous, the feed device can preferably have jets located circumferentially on intermediate storage medium 2 , especially a ring jet 6 through which the reducing agent can escape transversely to flow direction 1 of the exhaust through intermediate storage medium 2 . The transport of the reducing agent, preferably a gas, in the direction of the exhaust flow likewise takes place through the intermediate storage medium 2 .

The reducing agent is advantageously added downstream from a processing section that corresponds to a saturation length of intermediate storage medium 2 , with a saturation length corresponding to the distance traversed by the gas after which emissions reduction, especially reduction of nitrogen oxides, becomes saturated, looking in flow direction 1 .

FIG. 2 shows another scrubbing system whose intermediate storage medium 2 likewise is located in exhaust line 9 of an internal combustion engine, especially a gasoline-powered four-cycle engine or a four-cycle engine that runs lean. To avoid superfluous repetition, in the following description of embodiments of the invention, represented by FIG. 2, only the differences between the embodiments represented by FIG. 1 will be discussed.

In the embodiments according to FIG. 2, the feed device has an atomizer jet 5 located on the gas inflow side upstream from intermediate storage medium 2 that can be traversed be the exhaust, said jet being provided to atomize the reducing agent. Atomizer jet 5 is dimensioned such that the reducing agent is atomized to form drops of a size such that a drop is completely evaporated in intermediate storage medium 2 only after it has penetrated to a given depth into intermediate storage medium 2 , said depth preferably corresponding to at least the saturation length. A drop size of this nature can be determined by one of ordinary skill in the art by empirical methods, for example. Feed of reducing agent with intermediate storage medium 2 takes place by evaporating the drops within intermediate storage medium 2 .

FIG. 3 shows other embodiments of a scrubbing system, and in this case also, to avoid superfluous repetition, only the differences between these embodiments and the ones of FIGS. 1 and 2 will be discussed.

In the embodiments according to FIG. 3, intermediate storage medium 2 is divided into three parts like a filter cascade. The partial bodies 7 , 7 ′, 7 ″ of intermediate storage medium 2 through which the exhaust can flow and which are made porous are spaced apart from one another and arranged sequentially in flow direction 1 of the exhaust. The length of a partial body 7 , 7 ′, 7 ″ measured in flow direction 1 logically corresponds to the saturation length of the supporting material of intermediate storage medium 2 .

Jets 5 and 6 that are associated with the feed device are located in intermediate chambers 8 , 8 ′ located between partial bodies 7 , 7 ′, 7 ″ of intermediate storage medium 2 . From the jets, namely an annular jet 6 that is located edgewise and flows outward radially inward into the first intermediate chamber, and atomizer jet 5 centrally located in second intermediate chamber 8 ′, 8 , the reducing agent flows into the respective intermediate chambers 8 , 8 ′ and mixes with the partially-scrubbed exhaust already there. An intermediate chamber 8 , 8 ′ can therefore be thought of as a sort of mixing chamber. The exhaust, enriched with reducing agent in intermediate chambers 8 . 8 ′, is then fed to the next partial body 7 ′ or 7 ″ for further reduction of emissions.

In the compositions of the invention and the compositions used in the methods and devices of the invention, the supporting material is a spinel. As used in the invention, a spinel is a material with the general chemical formula A a B b O 4 which has, at least microscopically, a crystallographic or crystal-like cubic lattice structure with oxygen ions that are face-centered, and tetrahedral and octahedral vacancies, in which tetrahedral vacancies the A particles and up to 50% of the B particles are located and in which octahedral vacancies the remaining B particles are located. Here, an A or B particle merely refers to the crystallographic arrangement.

In accordance with the invention, sub-stoichiometric compounds and/or compositions can also be considered spinels in which the B b O 3 functions as a matrix and which have the characteristic spinel lines in the x-ray spectrum, with the spinel of the formal composition A a B b O 4 being in a B b O 3 matrix, so that a stoichiometry of A a (1−x)B b O 4 formally results. In a material regard, the A as well as the B particles can be different from one another.

In the spinel that is used as the supporting material, the A particle is of one or more of the group A elements Mg, Ca, Mn, Fe, Ni, Co, Cu, Zn, Sn, and Ti and the B particle is of one or more group B elements Al, Ga, In, Co, Fe, Cr, Mn, Cu, Zn, Sn, Ti, and Ni. However it should be noted that none of the elements in the exclusion group Mn, Fe, and Co can be an A and a B particle simultaneously.

The following spinel-type compositions have proven to be especially advantageous: (MgCu)Al 2 O 4 ; (CuCu)Al 2 O 4 ; (CuZn)Al 2 O 4 ; (CoZn)CuAl 2 O 4 ; mixtures of (ZnCu)Al 2 O 4 with WO 3 and/or V 2 O 5 and/or TiO 2 ; and Mg 0.5 Cu 0.5 Al 2 O 4,x , Cu 0.5 Cu 0.5 Al 2 O 4 , Cu 0.5 Zn 0.5 Al 2 O 4 , CO 0.25 Zn 0.25 CuO 0.5 Al 2 O 4 , or their mixtures with 10% WO 3 and 6% V 2 O 5 and/or 84% TiO 2 and/or Al 2 O 3x . One or more spinels can be used in combination in selecting the composition of a supporting material of the invention.

It has also proven to be advantageous to react the spinels used as supporting materials with catalytically active elements, namely palladium, platinum, rhodium, ruthenium, osmium, iridium, and rhenium and/or rare earths such as lanthanum, and with cerium, vanadium, titanium, niobium, molybdenum, tungsten and/or their salts and/or their oxides. Several of these materials just mentioned or their combinations will be discussed in greater detail as examples.