CA1160161A

CA1160161A - Catalytic filter for purifying diesel exhaust gas

Info

Publication number: CA1160161A
Application number: CA000367210A
Authority: CA
Inventors: Alfred Bozon; Edgar Koberstein; Hans-Dieter Pletka; Herbert Volker
Original assignee: Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 1979-12-20
Filing date: 1980-12-19
Publication date: 1984-01-10
Also published as: PL133095B1; PL228658A1; ES8106588A1; BR8008148A; ATE6035T1; ES495531A0; DE3066409D1; DE2951316A1; SU1160940A3; DD155389A5; EP0035053B1; CS247056B2; JPS5697518A; EP0035053A1

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides a metallic screen cloth system, which can be used as a filter for dust aerosols and/or as a support matrix for catalysts in exhaust gas puri-fying equipment of internal combustion engines. This system comprises superposed corrugaged or concentrina layers of screen cloth which alternate with flat layers of sheet metal or screen cloth and are piled so as to form a stack of per-meable to gas or wound spirally as a coiled body permeable to gas. On its opposite front ends the parcel or the coiled body is closed by cover means such that a closed front end portion lies opposite an open front end portion so that exhaust gas entering at open front end portions is forced to pass over the entire length of the matrix through the screen cloth meshes into channels assigned to an open opposite front end portion.

Description

~6Vl~l The present invention relates to a catalytic filter for purifying diesel exhaust gas Since the worldls oil reserves are shrinking, petroleum products and thus also fuels for internal combustion engines are becoming increasingly more expensive. Therefore, it is aim to construct enyines having a fuel consumption as low as possible. The Diesel engine having an efficiency which is higher by approximately 5% than that of engines operated with Otto fuels is an alternative solution to this problem. So-called lean concepts for Otto engines constitute an exception therefrom, but constructionally they are not yet fully developed.
Without a catalyst all the exhaust gas limiting standards applicable at present must be maintained for the majority of Diesel engines. However, this does not exclude the fact that Diesel engines frequently constitute a burden on the environment and this miyht cause a real problem in case of a percentage increase in the number of vehicles on the road.
Not only do Diesel engines emit deleterious substances such as CQ, hydrocarbons and nitric oxides, but because of their mode of operation they also emit carbon black particles and mlnute condensate droplets or a conglomerate thereof ("particulates").
These particulates are very rich in condensed polynuclear hydrocarbons, some of which are known to be carcinogenic.
Aldehydes also are among the emitted substances and contribute to the odorous annoyance of the environment.
In many countries, as for example, in the United States, it is intended to introduce limiting values for deleterious substances and particulates in Diesel exhaust gases which can no longer be attained by measures concerning the engine. In fact, by recycling the exhaust gas (EGR) it is possible, for example, to reduce the emission of NOX, but this simultaneously results in an out-of-proportion increase in the amount of the particulates.

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Ulb~L

Other solutions for reducing the emission of deleterious substances from exhaust gases of Diesel engines propose the interception of carbon black and condensate particles in particu-late traps.
However, since the temperatures of Diesel exhaust gases normally are not sufficiently high to burn off carbon black, for which temperatures of at least ~50~C are required, the exhaust gas temperature for regenerating the trap must be increased in order to avoid an accumulation of carbon black and condensate and thus clogging. This can be brought about, for example, by periodically fattening the air-fuel mixture so that higher exhaust gas temperatures are obtained. However, this is associated with additional fuel consumption and would partially cancel an advantage of the Diesel engine.
Furthermore, the ignition of an accumulation of carbon black and condensate particles results in an uncontrollable and even explosive combustion and thus in the destruction of the particulate trap or of the entire exhaust system. Of course, the volatile components, carbon monoxide and the malodorous substances, 0 such as aldehydes, contained in the Diesel exhaust gas cannot P
t~, be separated by means of traps. ~e catalytic reaction is required for their removal.
It might be possible to use a catalyst on the basis of a monolithic ceramic reinforcing body in order to remove said deleterious substances (see US Patent No. 3,597,165), but even after a short operating time these kinds of systems are gradually covered with carbon black since they are relatively remote from the engine and thus rapidly lose their activity in many cases.
However, the particulates can pass unimpededly through the 3a channels of the reinforcing body without being degraded.

Other devices have, in addition to a catalyst, a filter for separating carbon-black and condensate particles. This filter ~bO~b~

is cleaned mechanically ~y shak~ng off the particles (Bxitish Patent No, 1,425,386), However, there still remains the proble~ that the parti~cles are practically not reacted on a succeeding catalyst.
The present invention provides a screen cloth system of metal which can oe used as a highly effecti~e filter and/or as a catalyst support and forces the exhaust gas to flow through the filter surfaces.
This screen cloth system is made of high-temperature resistant and corrosion resistant metal and comprises alterna-tely at least one layer of a corrugated or concentrina s~reen cloth or of sheet metal and at least one flat, closed or per-forated cover layer. These layers are piled as a stack which is ~able to gas or wound spirally so as to form a cylindrical, oval, rectangular or polyg~ bcdy which ls ~meable to gas. This screen cloth system is characterized in that the stack or the coiled ~ closed on the opposite ends such ~hat a closed e~ porti.on is opposite an open end portion.
The flat cover layer can ~e a smooth metal sheet or a metallic screen cloth.
A U-shaped edging may be slipped over one screen cloth layer, or in case of identical geometrical shape over several screen cloth layers, on the face end and secured to said layer, said edging being adapted to the shape of the layer concerned.
For fixing the screen cloth system it has been found to be favourable to spot weld or lap weld or solder the edging to the layers. A stack or coil produced from alter-nately at least one layer of a corrugated or concentrina screen cloth and at least one flat closed perforated cover 3Q layer is suitably welded or soldered on the end face for further mechanical strengt~ening.
The screen clotfi systems thus produced can be coated ~y mean~; of conYentiOnal proce~ses, ~or ex~mple, hy electroplatin~

- 3a -or impregnating with noble metal and/or base metal. However, they can also be processed into a support matrix coated with a conventional supporting material for catalysts and impregnatable with solutions of active catalyst metals. According to an embodiment of the invention the individual layers are coated on their surfaces with a catalysis-promoting supporting material, usually a metallic oxide of high surface area, for example, active AQ2O3, such that a portion of the opening cross section of the perforated layers, i.e., of the screen cloth layers, remains intact. The filtering action is then adjusted with the aid of the mesh width.
A further embodiment having a particularly good filtering action is characterized in that the screen cloth system is coated with a catalysis-promoting metallic oxide while filling the opening cross section of the perforated layers, the oxide layer being such that it is permeable to gas. In this case the filtering action is adjusted by means of the porosity of the oxide layer.
The elements cerium, zirconium, iron, nickel, rare earths or a combination thereof can also be incorporated in this metallic oxide as catalysis-promoting additives. The intermediate layer of supporting material then is provided with the actual catalyst by means of conventional methods.
The agent used for covering the front ends is a high temperature-resistant and corrosion-resistant material. The covering agent may be a ceramic material, a weld or solder surface or a fitted and/or fastened stencil, which, when required, forms a perforated mask embracing a portion of the jacket of the parcel or of the coil.
Plugs of a ceramic material consisting of talc and sodium tetrasilicate are particularly suitable as the means for covering the face end portions. With this material the individual _4_ channels on the screen cloth s~stems descxibed are so plug-ged that a closedchannel end altern~tely opposes an open channel end, ~.e., that in ~ s~iral arran~ement of the screen cloth layers a closed front end portion is adjacent to two front end portIons and vice versa.
The end portions can also be covered with a mask resting against the front ends and~or secured thereto which, when required, may be a perforated, or slotted mask embracing a portion of the ~acket of the parcel or of the coil, said mask closing the corresponding channel ends.
The mask can also be such that in the case of a spirally wound matrix made exclusively of screen cloth alter-nately covers portions and leaves them open in the width of twice the amplitude of the corrugation or fold, open and closed portions being opposite on opposite front ends. This assures that exhaust gas entering an open front end portion cannont leave on the front end opposite this portion but must pass the matrix partitions which are permeable to gas.
The spiral opening sey-ments may extend closely around the circumference of a coil or of a parcel, but the closest front end portions must always overlap correspondingly in order to prevent a by-pass effect. A segmental division of the spirally slotted surfaces by odd numbers is also possible.
In a further em~odiment of the invention the front ends can be closed by weld or solder coating in the manner described hereinbefore.
The screen clot~ used may be a web having a width of mesh of 0.25 to Q.Q25 mm, preferabl~ 0.1 to 0.05 mm and a thick-ness of wire of 0.15 to Q.Q25 mm, preferabl~ 0.1 to Q.05 mm, particularly 0.07 mm. Met~llic ri~bons having a wall thic~-ness lower than 0.15 mm are used as closed cover ~ayers.
Width and height of the 01~ ' cells encompassed by the smooth and corrugated or folded layers f~ should not be ~ than 0.5 mm.
J
The corrugated layer may vary in shape. It is favour-able when it is sine-shaped or has the shape of an involute or has a rectanulgar or quadrangular shape. Sheet metal and screen cloth consisting of an alloy of iron, chromium, aluminium, and when required cerium or yttrium, and the usual alloying constituents, and other heat-resistant and nonscaling materials are particularly suitable for the screen cloth system used.
The screen cloth systems according to the invention, i.e., in the form of stacks or coils, can be disposed in a steel jacket and rigidly connected thereto by soldering or welding in order to stabilize them mechanically. The heat insulation of the steel jacket with high-temperature insulating material may be advantageous.
When the filter catalyst must be installed at points of colder exhaust-gas temperatures for reasons of space or when the degradation of carbon black and particulates is to be commenced as early as during the cold starting period, then it ?0 may be advantageous to install, for the filter catalyst, current supply leads for the continuous or periodic galvanic heating of the layers.
The screen cloth system according to the invention can be used in exhaust-gas purifying plants, particularly for internal combustion engines, as filters for dust and/or aerosols and/or as a support matrix for catalysts. The purification of exhaust gases from Diesel engines is a preferred field of application.
For this purpose a filter catalyst is placed in the exhaust pipe directly on the engine block. The oxidizable volatile components of the exhaust gas can be degraded by providing a suitablecatalytic coating parallel thereto. However, a standard catalyst or a catalyst according to the invention can also be connected in series to an embodiment designed exclusively as a filter unit.
The present invention will be further illustrated by way of the accompanying drawings in which:-Figure la is a cylindrical coil body in plan view,a front end portion closed with a plug being opposite on open front end portion;
Figure lb is a section of the coil body of Figure l;
Figure 2a illustrates one form of a cover mask when a wound matrix, consists entirely of screen cloth excepting the outer sheath;
Figure 2b shows a partial view of a section through coiled cylinder of Figure 2a;
Figure 2c is a similar view of another form of the cover mask when wound around a matrix;
Figure 3 illustrates various possibilities of com-bining screen cloth and sheet metal; and Figure 4 is a diagrammatic representation of a screen cloth system according to the invention with current supply leads for heating the layers galvanically.
Referring to Figure 1, the filter consists of spirally wound layers of smooth screen cloth or smooth metal foil and corrugated screen cloth. The channels thus formed are closed at one front end at 1 and open at the other front end at 2. A
plug 3 of ceramic material consisting, for example, of a hardened mixture of talc and sodium tetrasilicate, serves as the seal.
The exhaust gas 4 enters through the front end at the open channels of the filter and penetrates the porous side walls '~' 5. It leaves at the other end at the open sL~c~ L__ channels.
For use in exhaust systems of motor vehicles the coiled body is fitted into a metal jacket 6 and rigidly connected thereto.
However, as shown in Figure 2 perforated or slotted masks are ~ 7 _ also suitable as cover means for the partial closing of front end portions. The Figures 2a and 2c represent alternative forms of a slotted mask. They are used to cover segments 7 and 11 of the front ends while the non-covered segments 8, 10 on the -~ - 7a -bl opposite face end are closed. Figure 2b shows a section through a cylindrical arrangement of screen cloths, wherein the flow of the exhaust gas 4 through the screen cloth layers is shown diagrammatically. The slotted mask used for covering may be f' ~ de~ a cap 9 and is rigidly connected to the coil body by welding or soldering.
As shown in Figure 3 various possibilites of combining screen cloths 12 and metal sheets 13 exist. Thus, for example, the corrugated or folded layers may consist of screen cloth and the cover layers of smooth metal foil or the corrugated or folded layers and the cover layers may consist of screen cloth.
However, it is also possible to produce the corrugated or folded layers of sheet metal and to use a corresponding screen cloth as the cover layer.
As mentioned hereinbefore, it can be advantageous to preheat the filter catalyst via an ignition lock and timing relay prior to starting the engine and thus to degrade the amounts of carbon black and particulates encountered particularly in the cold starting phase of a Diesel engine. This can also become necessary during normal operating phases, for example, when the exhaust gas temperature is too low or when the accumulation of carbon black on the filter is too intense and an increased pressure loss thus results in the exhaust gas system.
The possibility of galvanically heating a filter catalyst according to the invention with the aid of à standard automobile battery is shown diagrammatically in Figure 4. Thus, for example, during the start the electric circuit may be switched on automatically, for example, for 30 seconds.
Subsequently either a timing relay or a differential pressure switch assumes the function of controlling the flow of current.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An arrangement of temperature and corrosion resis-tant metal comprising at least one corrugated or concertina screen netting or metal sheet layer alternating with at least one planar and undeformed cover layer, at least one of said layers being of screen netting or perforated, said alternating layers being arranged to provide a body having opposed end faces and containing a plurality of ducts defined by said alternating layers whereby said body is permeable to gas, said opposite end faces being provided with covering means whereby at one end face of said body a portion of the ducts are open and the remaining ducts are closed, and at the opposite end face of said body said portion of the ducts are closed and the remaining ducts are open.

2. Arrangement as claimed in claim 1, wherein a U-shaped edging is disposed on each opposite end face of said body and is attached to said layers therein.

3. Arrangement as claimed in claim 2, wherein said cover is welded or soldered to said layers by spot or surface techniques.

4. Arrangement as claimed in claim 1, wherein the layers of said body is welded or soldered to each other at said end faces thereof.

5. Arrangement as claimed in claim 1, wherein said body is coated with a catalyst.

6. Arrangement as claimed in claim 5, wherein said catalyst is applied to a metal oxide promoting catalysis.

7. Arrangment as claimed in claim 5 or 6, wherein said body is covered with a metal oxide promoting catalysis to an extent whereby a portion of the cross-section of apertures in the screen netting or perforated layer or layers is preserved open.

8. Arrangement as claimed in claim 5 or 6, wherein said body is coated with a metal oxide promoting catalysis whereby the cross-sections of apertures in the screen netting or perforated layer or layers are filled and wherein the oxide layer is formed to be permeable to gas.

9. Arrangement as claimed in claim 1, wherein the covering means consists of temperature and corrosion resistant material.

10. Arrangement as claimed in claim 9, wherein the covering means selected from the group consisting of a ceramic mass, a coating of welding or soldering material, an adjacent mask, a fastened mask and an adjacent and fastened mask.

11. Arrangement as claimed in claim 10, wherein said mask is a perforated mask surrounding a portion of said body.

12. Arrangement as claimed in claim 1, wherein said body is disposed in a casing which is open at its end faces.

13. Arrangement as claimed in claim 13, wherein said casing carries heat insulation material rigidly connected thereto.

14. Arrangement as claimed in claim 12, wherein said casing is steel and said body is welded or soldered thereto.

15. Arrangement as claimed in claim 1, wherein said body is provided with a current supply for the galvanic heating of said layers therein.

16. An exhaust gas purification installation comprising the arrangement as claimed in claim 1.

17. The exhaust gas purification installation as claimed in claim 16 for internal combustion engines.

18. The exhaust gas purification installation as claimed in claim 16, for purification of exhaust gases from diesel motors.

19. A filter for dust, aerosols, and mixtures thereof comprising the arrangement as claimed in claim 1.

20. A support matrix for catalysts comprising the arrangements as claimed in claim 1.