GB2188559A

GB2188559A - Exhaust gas reactor

Info

Publication number: GB2188559A
Application number: GB08705753A
Authority: GB
Inventors: Dipl-Ing Helmut Schweers; Dipl-Ing Gunther Forster; Dipl-Betriebswirt Frank Drache
Original assignee: DRACHE KERAMIKFILTER
Current assignee: DRACHE KERAMIKFILTER
Priority date: 1986-03-14
Filing date: 1987-03-11
Publication date: 1987-10-07
Also published as: GB8705753D0; DE3608635A1

Abstract

An exhaust gas reactor comprises at least first and second open-pore ceramic foam plates which are arranged in succession in the direction of flow through the reactor. The plates of ceramic foam have a catalytically active surface layer thereon. Filter plates of uncoated ceramic foam may additionally be provided upstream and downstream of the first-mentioned plates. <IMAGE>

Description

SPECIFICATION Exhaust gas reactor and process for the production thereof The invention relates to an exhaust gas reactor, for example for use with an internal combustion engine, and a processforthe production of such a reactor.

The term exhaust gas is used in this specification in a broad sense and may therefore embrace exhaust gas from an internal combustion engine as well as exhaustorwastegasfrom other sources, such as a paint spraying booth.

One form of an exhaust gas reactor comprises a refractory carrier material in the form of a monolithic block of open pore ceramic foam, which has thereon a catalytically active surface layer.

Porous ceramic foam is produced by an open-pore plastics foam being filled with a slurry of ceramic material, and then vaporising the plastics foam at elevated temperature. The porous ceramic foam produced in that way is also of an open-pore nature and is then provided with a catalytic layer which is matched to the respective chemical process involved. Catalysts produced in that way from porousceramicfoam are subjected to further processing to produce monolithic blocks of appropriate size, and used in thatform.

According to the present invention, there is provided an exhaust gas reactor including a refractory carrier means of open-pore ceramic foam comprising at least two plates which are disposed in succession in the direction of gas flow th rough the reactor, and a catalytically active surface layer on the carrier means.

Tests have been carried out which showthatan exhaust gas reactor which is constructed from a plurality of successively disposed plates is substantially more efficient in operation than an exhaust gas reactor consisting of a monolithic block of appropriate size. Furthermore, an exhaust gas reactor comprising a plurality of plates is substantially easier to manufacture and it is also easier to provide it with a catalytically active surface layer, than a large monolithic block. It is possible to arrive at a made-to-measure construction, depending on the particular composition and temperature of the waste or exhaust gases which are to be subjected to the post-combustion procedure, by virtue ofthe plates being of suitable composition, configuration and dimensions.In addition,the reactor can be rapidly adapted to the respective operating conditions involved, particularly in the event of changes therein, by virtue ofthe insertion or replacement of plates. All in all, because of the large internal surface area oftheceramicfoam,the exhaustgasreactorcan be small in volume, in relation to the respective gas through-put thereof. A compact exhaust gas reactor of that nature is particularly important when, in connection with subsequentfittingto motor vehicles, a conventional silencer box or muffler is to be replaced by an exhaust gas reactorwhich must take account of the predetermined dimensions and conditions of installation of the respective silencer box or muffler.

Besides the very large internal surface area of a ceramic foam, relative to its external volume, a critical consideration in regard to the level of efficiency of an exhaust gas reactor in accordance with the present invention is also the high level of reaction probability which is provided by the micro-turbulent post-flow of the individual rods or fibres ofthe ceramic foam.

In accordance with a further feature of the invention, the porosity of the plates is between 10 and 60 ppi (pores per inch), and preferably between 10 and 30 ppi. The reference to pores per inch (ppi) denotes the number of pores per inch as a unit of length.

Afurther embodiment of a reactor according to the invention advantageously provides that plates of a differentconfigurationornature may be used. The different configuration or nature may bethatthe porosity of the plate#s is different, for example a plate with coarse pores is followed by one or more plates with finer pores. However the different configuration or nature may additionally or alternatively involve different catalytically active surface layers.

Furthermore and possibly in addition thereto the plates may be of different shapes and sizes in order for example to produce a given flow characteristic in aerodynamic respects.

Afurtherfeature of the invention may provide that one or more uncoated plates is or are arranged upstream and/or between and/or downstream of catalytically coated plates. In that connection the uncoated plates act in particular as a filter means. In that way, undesirable particles in the gas flowing into the reactor are filtered out of the gas flow prior to passing into the part of the reactor in which reaction occurs, and reaction products are filtered out ofthe gas flow if further uncoated plates are arranged within the coated plates ofthe reactor or downstream thereof. The pore size of the uncoated filter plates isto be matched to the size of the particles which are to be filtered out of the gas flow.If an uncoated plate with very fine pores is disposed at the downstream end of the reactor assembly, it is possible to ensure that any particles of the ceramic carrier material which come loose do not pass into the atmosphere, togetherwith constituents of the washcoat base layer and the catalyst material.

In one embodimentofa reactor, the individual plates thereof may bear against each other in a sandwich-likeconstruction.An advantageous feature ofthe invention provides however that the plates are arranged at a spacing from each other, in which respect the spacings between the plates may be the same or may vary. The spacings between the plates prevent the gases from forming preferential through4low paths, which would have the result that parts of the plates would nothavegasflowing therethrough or would have less gas flowing therethrough, so that the level of efficiency would drop, in relation to unit of surface area.If the spacing between plates is correctly chosen, turbulent effects occurbetweenthe plates, which ensurethatthegas flow is balanced and equalised out. The spacings between the plates are also to be selected having regard to the flow speeds through the reactor, the dimensions of the plates and the properties of the respective gases involved. In that connection however the spacings should be kept as small as possible, having regard to the reactor being of a compact design.

If, in accordance with a further advantageous feature of the invention, the plates are individually interchangeable, then a reactor of that nature is simpler and less expensive from the point of view of maintenance than a reactor which uses a monolithic block. In that case individual plates can be replaced for repair or exchanged for other plates, if the operating conditions alter. To remove the dust which has been produced, the above-mentioned uncoated filter plates may be exchanged or only cleaned after they have been removed.

Aparticularlyadvantageousdevelopmentwhich is also an aspectofindependentsignificanceprovides that the catalyst material used comprises at least one non-noble metal oxide.

The reaction temperature of such oxides is higher than that of noble metals which are conventionally used, for example platinum, and is between about 450 and 750 C. As a result, the level of effectiveness scarcely decreases, even under high loading and at high temperatures, so that such a reactor is particularly suitable notjustfor internal combustion engines of motor vehicles but also for stationary engines. The non-noble metal oxides are also resistant to lead poisoning phenomena and to other noxious substances such as chlorine, fluorine and sulphur, as occur in the uncleaned gases of stationary engines.

In a preferred feature of the invention, the catalytically active surface layer of a non-noble metal oxide may be arranged on what is known as a washcoat base layer of aluminium oxide in its oy-modification form. Such a washcoat base layer is known and providesfora more rapid and better reaction. In order to improve long-term stability, additives may be added to the base layer, for example compounds of elements from the rare earth group.

In orderto provide a reducing reaction, a further embodiment ofthe invention provides for using a catalyst material comprising at least one oxide ofthe following metals: iron (Fe2O3 or Fe3O4), cobalt (CoO or Cho203 or Cho304), nickel (NiO or Ni2O3),titanium (TiO2),vanadium (V2O5), chromium (Cr203), 'molybdenum (MoO3), tungsten (W03), and manganese (MnO or Mn203 or Mn304), or at least a mixture of at leasttwothereof.

To provide an oxidising reaction, the catalyst material of the reactor may comprise at least one oxide of at least one ofthefollowing metals: copper (CuO), chromium (Cr203), molybdenum (MoO3), tungsten (WO3), manganese (MnO or Mn203 or Mn3O4),vanadium (V205), iron (Fe2O3) or at least one mixture of at leasttwothereof.

In a further advantageous embodiment ofthe invention, the reactor may be of a two-stage configuration, with a reducing reaction taking place in thefirst stage and an oxrdising reaction taking place in the second stage. For example, in thefirst stage of an exhaust gas reactorfor internal combustion engines, nitrogen oxides (NO,) are reduced to harmless nitrogen at the catalyst material by means of the carbon monoxide (CO) in the exhaust gas. Then, the second stage involves oxidation of the excess carbon monoxide to form carbon dioxide, with oxygen possibly also being added, in the form of air. In other areas of use of such a reactor, it is also possible to provideforthe oxidation of other constituents, in particular organic components, forexample hydrocarbons.Such constituents occur inter alia in the waste or exhaust gases from paint spraying boothst The two stages of the reactor each have at least one plate and may be arranged in separate housings or in a common housing. At least one uncoated filter plate is desirably arranged upstream of the first stage, so such a filter can also retain soot particles which can then undergo post-combustion if the initial filter reaches a temperature above 600 C. The second stage may be followed by an uncoated fine-pore filter plate, constituting a veryfinefilterwhich retains in particular pieces of ceramic carrier material which have come away.

In another aspect, the present invention also provides a processforthe production of an exhaust gas reactor, wherein a non-noble metal oxide is added to the slurry which, as referred to in the opening part of this specification, is used to produce the ceramicfoamfrom open-pore plastics foam.

After calcination of the foam which is coated with the ceramic material, the non-noble metal oxide is then a constituent of the ceramic material and is thus also present at the surface thereof. Such a mixing effect may be used in particular when the oxide is comparatively inexpensive.

An alternative option in regard to the process for producing the exhaust gas reactor according to the invention provides that the foam which has been coated with ceramic material and pre-dried but not yet subjected to the calcination operation may be coated, preferably by a dipping operation, with a particular suspension or slurry which contains the non-noble metal oxide and optionallyfurther constituents which promote adhesion of the oxide as well as the catalytic fu nction. Besides for example titanium dioxide, the suspension or slurry may additionally contain the material for the base layer (aluminium oxide).

Anotherform of procedure for applying the catalyst material in the reactor according to the invention provides that the foam which has been coated with ceramic material and possibly the base layer and which may or may not have been subjected to the calcination operation is impregnated with a solution of a metal salt of the non-noble metal to be used, and then the metal is converted into the metal oxide bya heattreatmentsuch as calcination, and fixed on the ceramic foam.

The reactor according to the invention is inexpensive to manufacture, enjoys a wide range of adaptability and has a high level of efficiency, even at high temperatures, while being easy and simple to maintain and being capable of adapting to altering operating conditions.

Embodiments of a reactor according to the invention and the processforthe production thereof will now be described byway of example with reference to the accompanying drawing in which: Figure 7 is a diagrammatic view in longitudinal section of a first embodiment of the reactor according to the invention, and Figure2 is a basic view in longitudinal section of a two-stage reactor as a second embodiment of the invention.

Referring to the diagrammaticview in Figure 1,an exhaust orwaste gas reactor according to the invention comprises three plates 1 of open-pore ceramic foam with a catalytically active surface layer thereon, which are arranged in succession in the direction in which the gas flows through the reactor, as indicated by the arrows 10 and 11.

Disposed upstream of the first reactor plate 1 is a further plate 2 of open-pore ceramic foam, which is not coated with catalyst material. The plate 2 serves as a filter plate for particles contained in the flow of exhaust gas. Furthermore, arranged downstream of the last reactor plate 1 is a very fine filter plate 3 of open-pore ceramic foam without a catalyst coating thereon. The plate 3 has very fine pores and filters out of the gas flow particles of the ceramic base material which have come loose. The spacings between the individual plates 1,2 and 3 provide for a turbulent effect therein. Those spacings are selected to be non-uniform in such a way that the reaction is as complete as possible, the flow resistance is at a low level and the dimensions of the arrangement are not excessive.

Figure 2 is a diagrammatic view of a two-stage reactor. The two stages are arranged in separate casings or chambers 4 and 5which are in communication by way of a pipe 6. The waste or exhaust gas to be treated flows into the casing 4 in the direction indicated by the arrow 10. Arranged first in the casing 4 is an uncoated filter plate 2 which is followed at a spacing by two spaced-apart reactor plates 1 which are coated with catalytically active material. In the plates 1, the nitrogen oxides (NOx) in the gas react with the carbon monoxide (CO) which is also present therein, with the nitrogen oxides being reduced to non-toxic nitrogen. In the pipe 6, air is then added to the exhaust gas by means of a nozzle 7 from a pipe 8, for example by means of a pump inthe form of a blower.If it is assumed that the denoxed gas in the pipe 6 still has a component of about 1% CO, then about 0.5% 2 from the air must be added for complete combustion. With an oxygen component for about 20%, that means the addition of about 2.5% air. As operation is usually conducted at higher than stoichiometry, about double the amount is added in a practical situation, that is to say about 5% air.

In the casing 5, the gas again first flows through a filter 2 and then two reactor plates 1, atthe surface of which the carbon monoxide undergoes combustion to form carbon dioxide. Avery fine filter plate 2 at the outlet of the casing 5 eliminates reaction particles and fine dusts.

One embodiment of the reactor according to the invention used chromium trioxide as the metal oxide. The ceramic foam carrier material was impregnated with ammonium dichromate solution (between 3 and 25%) and then the substance was thermally decomposed to form chromium trioxide (Cr203), on the carrier material. The ammonium dichromate may also be reduced with a reducing agent such as for example ethanol or ascorbic acid, to form trivalent chromium trioxide. Fixing on the carrier material is then effected by drying and calcining. The thermal expenditure involved in such a procedure is slight.

Various forms of process may be employed for producing a reactor as described above; for example, when using a slurry of ceramic material to fill an open-pore plastics foam which is then vaporised at elevated temperature to produce a porous ceramic foam, the catalyst material such as the non-noble metal oxide referred to above may be added to the slurry used to fill the plastics foam which is then removed in the manner stated.

Alternatively, the foam may be coated with ceramic material and then impregnated with a solution of a salt of a non-noble metal,the salt then being converted into the metal oxide and fixed on the ceramic foam.

it will be appreciated that the above-described embodiments of the reactor and process according to the invention have been setforth solely byway of example thereof and that various modifications and alterations may be made therein without thereby departing from the scope of the invention as defined by the appended claims.

Claims

1. An exhaust gas reactor including a refractory carrier means of open-pore ceramicfoam, comprising at least two plates which are disposed in succession in the direction ofgasflowthrough the reactor, and a catalytically active surface layer on the carrier means.

2. A reactor according to claim 1 wherein the porosity of the plates is between 10 and 60 ppi.

3. A reactor according to claim 2 wherein the porosity of the plates is between 10 and 30 ppi.

4. A reactor according to claim 1 and comprising plates of a different configuration or nature.

5. A reactor according to claim 4wherein the porosity ofthe plates is of different sizes.

6. A reactor according to claim 4 or claim 5 wherein the plates have different catalytically active surface layers.

7. A reactor according to any one of claims 4, 5 and 6 wh erei n the plates are of different shapes.

8. A reactor according to any one ofthe preceding claims and comprising one or more uncoated plates disposed upstream and/or between and/or downstream of the catalytically coated plates.

9. A reactor according to one of the preceding claims wherein the plates are arranged at a spacing from each other.

10. A reactor according to claim 9 wherein the spacing between the plates varies.

11. A reactor according to one of the preceding claims wherein said plates are individually interchangeable.

12. A reactor according to one of the preceding claims wherein the cataylst material used includes at least one non-noble metal oxide.

13. A reactor according to one of the preceding claims wherein said catalytically active surface layer is arranged on a base layer (washcoat) of aluminium oxide (Al203) in the y-modification form.

14. A reactor according to claim 12 or 13 wherein, fora reducing reaction, the catalyst material comprises at least one oxide of at least one ofthe following metals: iron, cobalt, nickel, titanium, vanadium, chromium, molybdenum, tungsten, and manganese or a mixture of at least two thereof.

15. A reactor according to claim 12 or claim 13 wherein, for an oxidising reaction, the catalyst material comprises at least one oxide of at least one ofthefollowing metals; copper, chromium, molybdenum,tungsten, manganese, vanadium, iron ora mixture of at least two thereof.

16. A reactor according to one ofthe preceding claims which comprises at leastfirstand second stages, of such a nature that in operation a reducing reaction takes place in the first stage and an oxidising reaction takes plate in the second stage.

17. A reactor according to claim 16 wherein said first and second stages each comprise at least one plate and they are arranged in a common housing.

18. A reactor according to claim 16 or claim 17 and further including an uncoated filter plate arranged upstream of the first stage in the direction offlowtherethrough.

19. A reactor according to one of claims 16 to 18 and further comprising an uncoatedfine-porefilter plate arranged downstream of the second stage in the direction offlowtherethrough.

20. A reactor according to one of claims 16 to 19 wherein air is injected into the flow of gas through the reactor, between said first and second stages.

21. An exhaust gas reactor substantiallyas hereinbefore described with reference to Figure 1 of the accompanying drawings.

22. An exhaustgasreactorsubstantiallyas hereinbefore described with reference to Figure 2 of the accompanying drawings.

23. A process for the production of a reactor according to any one of claims 12 to 20 comprising introducing a slurry of ceramic material into an open-pore plastics foam and then removing the plastics foam, wherein said non-noble metal oxide is added to the slurry used to produce the ceramic foam from the open-pore plasticsfoam.

24. A process for producing a waste gas reactor according to any one of claims 12 to 22 comprising introducing a slurry of ceramic material into an open-pore plastics foam and then removing the plasticsfoam,wherein said non-noble metal oxide is added to a suspension, and thefoam which is coated with ceramic material and which has been pre-dried is coated with the suspension, preferably by a dipping process, and then it is subjected to calcination.

25. A process according to claim 24for producing a reactor according to claim 13 or any claim appendantthereto wherein the material (Al203) ofthe base layer is additionally mixed with the suspension.

26. Aprocessforthe production of a reactor according to any one of claims 12 to 22 comprising introducing a slurry of ceramic material into an open-pore plasticsfoam and then removing the plastics foam, wherein the foam which is coated with ceramic material and optionally a base layer and which is in a calcined or uncalcined condition is impregnated with a solution of a metal salt of a said non-noble metal and the metal salt is then converted into the metal oxide by chemical reaction and/or calcination, and fixed on the ceramic foam.

27. A process for the production of a reactor, according to any one of claims 23 to 26 and substantially as hereinbefore described.

28. An exhaust gas reactor substantially as herein described with reference to the accompanying drawings.