FR2498471A1 - COMPOSITION AND PROCESS FOR TREATING EXHAUST GAS - Google Patents

Abstract

THE PRESENT INVENTION RELATES TO A COMPOSITION FOR CONVERTING POLLUTANTS IN AN EXHAUST GAS INTO INFFENSIVE ENTITIES AND FOR REMOVING THE PARTICLES SUSPENDED IN THE GAS. ACCORDING TO THE INVENTION, IT CONTAINS A CATALYTIC MATERIAL, EFFECTIVE FOR CONVERSION, WHICH IS DEPOSITED ON A COARSE FILTER AND ON A THIN FILTER, THESE FILTERS INCLUDING A REFRACTORY MATERIAL EFFECTIVE FOR TRAPPING PARTICLES IN THE GAS AND BEING PLACED SO THAT THE GAS FLOWS IN SUCCESSION THROUGH THE COARSE FILTER AND THE FINE FILTER. THE INVENTION APPLIES IN PARTICULAR TO THE EXHAUST SYSTEMS OF INTERNAL COMBUSTION ENGINES.

Description

The present invention relates to the treatment of exhaust gases and

  more particularly, at the time of removal, exhaust gases from internal combustion engines, particles suspended in lead and carbon, and gaseous pollutants. Filters have been used to remove suspended solids from the exhaust gases, such as

  lead and carbon particles in exhaust gases

  internal combustion engines. As the particles accumulate in the filter, the resulting restriction to the flow of the exhaust gas increases the back pressure and reduces the filtering efficiency and the engine. In order to restore normal operation, the filter must be periodically regenerated, for example, by mechanical cleaning of this filter or by heating it and

  by burning the trapped particles of carbon.

  Particulate diesel emissions are evidenced by occasional and visible evacuations of smoke occurring during acceleration or maximum power operation. Large amounts of very small and very light carbon particles in diesel exhaust have significant difficulties in achieving a high degree of particle removal and avoiding back pressure

excessive.

  According to the method of the invention, the particles of carbon and lead are removed from the exhaust gases of internal combustion engines by passing the gases

  through a coarse filter and then through a fine filter.

  The filters include refractory material effective for trapping particles. The use of a fine filter makes it possible to trap a high percentage of particles and

  significantly reduces particulate emissions. The utili-

  The use of a relatively coarse filter to remove larger particles before the gases reach the second fine filter extends the useful life of the fine filter and reduces the rate at which the counterpressure increases.

  while particles accumulate in the filters.

  The selective filtration and the particle trapping distribution according to the invention thus offer a high trapping efficiency with low backpressure augursa-tiorns and efficient filtering during further

  long periods before regeneration is required.

  The composition according to the invention for converting one or more pollutants into an exhaust gas into harmless entities and for removing suspended particles from the gas, comprises a catalytic material> effective for the conversion which is deposited on a coarse filter and on a fine filter. The filters comprise an effective refractory material for trapping particles in the gas and are placed so that the gas flows

  in succession through the coarse filter and the fine filter.

  When the catalyst material is effective for the conversion of carbon and gaseous pollutants in the exhaust gas, the exotherm of the gaseous pollutant conversion occurs in the fine filter due to the low conversion that results from the mass transfer in the coarse filter. The exothermic improves the combustion of the carbide particles that are trapped in the fine filter and the regeneration of the

filtering ability.

  The filters may include any material that is effective in trapping particles in gases, for example, as a result of inertial impact or electrostatic attraction. Filters are usually made of a porous and refractory material that is resistant to gas temperatures and catalytic conversion of pollutants. Suitable materials which have a certain affinity for the particles and to which the particles adhere, are ceramic or refractory metal materials which have sufficient thermal and mechanical stability for use in a catalytic reactor. The metal may be, for example, steel, stainless steel, aluminum, copper or nickel. The ceramic material may be an oxide of a refractory metal, such as alumina, silica, magnesia, zirconia, titania, chromium oxide, or combinations of those such as cordierite or a silicate or carbide of a refractory metal. The filters may be in the form of beads of a refractory inorganic oxide, such as ceramic spheres or cylinders. Preferably, the filters are unitary structures of relatively large size such as ceramic monoliths, metal wools or metal meshes. A cell filter structure

  have a number of voids or pores

  connected is particularly preferred. Cells

  of such a structure form convoluted

  Therefore, there is a greater probability that a particle will be trapped and will not pass through the filter. This structure has a higher particle retention capacity and higher efficiency

filter than the other filters.

  A particularly preferred filter having a

  Cellular structure and continuous is a ceramic foam.

  In addition to the higher particle retention capacities

  and the higher filtration efficiencies, ceramic foams are particularly useful in the treatment of diesel exhaust emissions because of their low pressure loss Diup, their greater self-agitation, their greater geometric surface area and their lower density than ceramic monoliths and other filters having laterally extending flow paths. Foams also have a heat-sensitive resistance to physical and chemical degradation. The ceramic foam filter preferably used in the present invention is prepared from an open cell flexible foam material having a number of interconnected voids or pores surrounded by a core of the flexible foam material, such as

  polyurethane foam or cellulosic foam.

  The foam material is impregnated with an aqueous ceramic slurry so that the slurry coats the core. The coated foam is dried and heated to burn the organic foam and agglomerate the ceramic coating. The product is a fused ceramic foam having a number of interconnected pores surrounded by a ceramic core bonded or fused or fused to the configuration of the organic foam. Adaptive cordierite foams of various cell sizes are commercially available from Bridgestone Tire Co., Ltd., Tokyo, Japan and can be prepared according to the process described in Japanese Patent Kokai 77/77 114, published June 29, 1977. D ' Other ceramic foams which are suitable for use in the present invention are described in US Pat.

Nos. 3,893,917 and 3,962,081.

  The degree to which the filter allows the passage of suspended particles in the exhaust gas, or the trap depends on the volume of the voids or pores or the porosity and the pore size of the filter. The porosity of the filter may include voids in a unitary structure or gaps between the individual components of a particulate filter medium, such as ceramic beads. The pore size and porosity of the filters used in the invention can be varied to suit the particular gases being filtered. The filters generally have a pore size of from about 2 to about 50 pores per 25 millimeters in length. In general, the filters have a porosity, that is to say a volume of voids of the order of 80 to about 95% of the total volume occupied by the filter. Porosity is obtained from measurements of filter material density and bulk density using the formula: filter weight porosity (%) = density of filter material x volume of filter x 100 The appropriate filters also typically an air permeability of the order of 400 to about

8000 x10 cm2.

  The coarse filter is placed upstream in the gas flow through the composition and the fine filter is placed downstream with respect to the coarse filter in the gas flow through the composition. The fine filter has a larger number of cells per unit length and a smaller cell size than the coarse filter. The respective dimensions of the pores as well as the permeabilities may vary depending on the particular nature of the gas treated. The dimensions of the cells of each filter are chosen to make the best possible relative degree of particle trapping in each of the filters and distribute the trapped particles between the filters so that the pressure drop is minimal while maintaining a good efficiency trapping. The upstream filter generally has a relatively coarse pore size of about 2 to about 25 millimeters of pores and an air permeability of about 2500 to about 8000 x 10-7 cm 2. The relatively thin filter downstream generally has a pore size of about 15 to about 50 pores per 25 millimeters in length and an air permeability of about 400 to about 2500 x 10-7 cm 2. Preferably, the coarse filter is in the range of about 6 to about 20 pores per 25 millimeters in length and the fine filter is in the range of about 17 to about 30 pores per 25 millimeters in length. Multiple coarse filters having the same or different dimensions of the cells can be employed in combination with multiple fine filters having the same or different cell dimensions to vary the selective filtration and balance the pressure drop and the trapping efficiency needed for a treatment appliaettion

of a particular gas.

  Catalytic material may be deposited on the filters. The catalytic material is a metal or a catalytically active metal compound, which is effective for the conversion of one or more pollutants in the exhaust gases to harmless antites. The polluting agents may be the particulate and / or gaseous pollutants present in the exhaust gases. In general, the catalyst material is an oxidation catalyst. In the treatment of the exhaust gases of an internal combustion engine, the catalyst material can be effective for combustion of the carbon particles. Catalyst materials suitable for carbon combustion contain an element of the first

  transition series, silver, hafnium and their mixtures.

  In the present application, the elements of the first transition series are vanadium, chromium, manganese, iron, cobalt, nickel, copper and zinc. The material may be present in the form of metal, metal oxide, mixed metal oxide such as copper chromite or perovskite or other catalytically active metal compounds. Copper oxide and

chromium oxide are preferred.

  In use in the treatment of the exhaust gases of an internal combustion engine, the catalyst material is preferably also effective for the conversion of hydrocarbons, carbon monoxide and / or

  nitrogen oxide as pollutants. Such materials

  Catalytic catalysts contain a noble metal, an element of the first transition series, and mixtures thereof. The noble metals are gold, silver and platinum group metals such as platinum, palladium, rhodium, ruthenium, iridium and osmium. The material may have the shape of the metal, the oxide of the metal or other compounds

catalytically active metal.

  Platinum, palladium and oxide are preferred

  chromium for their high activity of oxidation of hydro-

  carbides at relatively low temperatures. Chromium oxide is most preferred because it is also particularly effective in the combustion of carbon particles at the diesel exhaust. In a particularly preferred embodiment of the invention, the catalytic material comprises a platinum group metal such as platinum, palladium or mixtures thereof and chromium oxide. The combination of platinum group metal and chromium oxide catalyzes carbon combustion at a considerably lower temperature than

each compound alone.

  The catalyst material may be deposited on the filters in any desirable manner from aqueous or organic solutions of a metal or complex compound or slurries of the metal or metal oxide. In

  In general, the deposit of this component is carried out in

  filtering an aqueous solution of an inorganic, water-soluble and thermally decomposable, or complex, salt of the particular metal or metals and drying the impregnated filter at a temperature in the range of about 90 to about 250; For about 2 to about 20 hours. The dried filter can then be calcined at a temperature of

  about 300 to about 7001C for about 1 to 3 hours.

  The calcination may be conducted in air or other oxidizing gases or in a reducing gas such as hydrogen if the metal form of the catalyst is desired. Typical thermally decomposable water-soluble metal compounds include acetate, chloride and nitrate. Preferably, the platinum group metal component is deposited as a sulfite complex as described in U.S. Patent No. 3,850,847 to Graham et al.

  its dispersion and surface area.

  If a surface area greater than that of the filter is desired, the catalyst material can be

  supported on a refractory and porous inorganic oxide.

  These oxides have a high total pore volume and a large surface area. In general, the surface area of the refractory oxide is at least about 2 m 2 / g, preferably in the range of about 100 to about 300 m 2 / g, and the total pore volume is at least about 0.4 cc per gram, preferably in the range of 0.5 to about 2.0 cc per gram. Surface areas

  indicated throughout this description are determined

  by the BET method with nitrogen. The total pore volumes are determined by adding water to a powder sample, to the point where a

nascent humidity.

  In general, the refractory oxide is predominantly composed of oxides of one or more Group II, III and IV metals having atomic numbers not exceeding 40. Suitable refractory and porous organic oxides may be prepared by dehydration, preferably substantially completely, the hydrated form of the oxide by calcination generally at temperatures of the order of 150 to about 800 C, while

  periods of the order of half an hour to about 6 hours.

  The preferred refractory oxide is a transition alumina, such as chi, rho, kappa, gamma, delta, eta and theta, and

  especially gamma-alumina. A particular gamma-alumina

  The most preferred embodiment can be prepared by calcining an intermediate boehite-pseudoboehmite alumina prepared according to U.S. Patent No. 4,154,812 to Sanchez et al. at a temperature of about 650 C for about 1 hour. Other suitable oxides include, for example, calcined beryllium oxide, zirconia, magnesia and mixtures of metal oxides such as boron oxide-alumina, silica-alumina

and the like.

  In a most preferred embodiment of the invention, the catalyst material comprises the diesel exhaust catalytic composition of US Patent Application No. 153,552 filed May 27, 1980, by Ernest and Welsh. entitled "Composite Diesel Exhaust Catalyst". This catalyst contains a mixture of catalytically effective amounts of at least one material selected from the group consisting of a noble metal, chromium and catalytically active compounds supported on a refractory and porous inorganic oxide and at least one bulk material selected from the group consisting of an element of the first transition series, silver, hafnium and catalytically active compounds. The bulk material can be prepared by thermally decomposing a compound of the desired metal. Typically, the acetate, nitrate, carbonate, hydroxide or chloride is heated at a temperature of the order of 450 to about 800 ° C for 1 to about 5 hours. The bulk material is slurried with the supported material and is deposited on the filters. The refractory oxide may be coated on the filter and then the catalytic material be deposited thereon. Preferably, however, the catalytic material is deposited on the refractory oxide and then the supported catalyst is deposited on the filter. For example, a suitable catalyst component may be added to an aqueous slurry of the oxide and the mixture deposited on the filter by conventional methods, such as diving or spraying.

  The coated filter is then dried at a temperature of

  in the range of 90 to about 250 ° C for about 1 to about 4 hours to remove the solvent and deposit the solids in an adherent film on the filter. The dried filter can be calcined at a temperature of from about 250 to about 800 ° C for about

about 4 hours.

  The amount of catalyst material that is coated on the filter depends on economy, size limits, and design features. The catalyst material is about 50, and preferably about 2 to about 303 based on

filter weight.

  In use, the catalytic composition is typically arranged to occupy the major portion of the cross-sectional area of a housing

  having a gas inlet and a gas outlet. The composition

  It typically has the general shape of the housing and is placed in this housing with the general direction of gas flow between the inlet and the outlet. The

  Filters may adhere to each other or be spaced apart.

  By employing the composition according to the invention for the treatment of the exhaust gases of internal combustion engines, the gases are brought into contact with the composition and the particles of lead and carbon are trapped in the filters. The carbon particles are burned together with the gaseous pollutants in the exhaust gas. Accumulated deposits of particles

  carbon can be removed periodically

  the engine to reduce airflow with constant fuel flow remaining and increase the exhaust temperature. At the resulting higher exhaust temperatures, the combustion of particulate matter is obtained quite rapidly in

  presence of the catalytic filter according to the invention.

  In addition to filtering lead and / or carbon particles from the exhaust emissions of internal combustion engines, the filter according to the invention can be

  used, for example, to reduce particulate emissions

  of other mobile powertrains as well as stationary sources such as catalytic combustion gas turbine engines using fuels

  which produce particulate pollutants.

  The present invention will be illustrated by the following examples where all parts and percentages

  are by weight unless otherwise indicated.

1 1

EXAMPLE 1

  A composition according to the invention containing the "Diesel Exhaust Catalyst" of the US patent application NI 153,502 filed May 27, 1980 by Ernest and Welsh, deposited on ceramic foam monoliths marketed by Bridgestone Tire Co., Ltd., Tokyo, Japan was prepared as follows: A hydrated boehmite-pseudoboehmite intermediate alumina powder prepared according to the process of US Patent No. 4,154,812 to Sanchez et al was air calcined at 6490C for 1 hour. The resulting gamma-alumina had a total pore volume of 1.56 to 1.68 cm3 per gram and a total volatile content (weight loss after heating for 1 hour

at 9540C) from 1.8 to 3.3%.

  3.000 g of the calcined alumina powder were impregnated with 840 ml of a solution of 260.29 g of chromic acetate in 840 ml of deionized water and 5 ml of glacial acetic acid. The impregnated powder was allowed to air dry for half an hour and then dried for 16 hours at 1350C. The powder was sieved through a 20 mesh U.S. screen and calcined at 843-871 ° C for 1 hour. The calcined chromium-alumina powder had a surface area of 107 m 2 per gram and nominally contained 10% by weight of

chromium.

  150 g of freshly prepared copper oxide were separately ball milled by decomposition of cupric acetate for 3 hours at 53 ° C. in a muffle furnace and 150 g of the chromium-alumina oxide powder with

  deionized water for 16 hours. The resulting porridge

  aunts were combined at a ratio of 1 to 1 (solid basis) and homogenized. The pH of the combined slurry was adjusted to 3.5 with nitric acid. The process was repeated four times using a weight ratio

  from 1 to 1 of copper oxide and chromium oxide

  alumina but varying the solids content in the slurry by adding additional water. Each slurry was coated on a Bridgestone ceramic foam monolith of 14.37 to 14.50 cm in diameter and 7.30 to 7.67 cm in length. The excess slurry was blown from the coated monoliths and the monoliths were then dried at 135 ° C. for 16 hours and activated for 1 hour at 428 ° C. The number of cells for 25 millimeters in length of the monoliths, the solids content of the slurries and the quantities and percentages of the catalytic material coated on the monoliths are indicated in

Table I.

Table I

  Nominal number Content in Grams% coating of cells / solids collected mm%

6 27 44.4 9.1

13 23 45.2 8.1

14 35.9 6.1

14 41.1 7.7

  Three of the 13-dimensional monoliths and one of the 30-sized monoliths were joined and placed in a cylindrical vessel. Monoliths of dimension 13 were placed at the entrance structure, the first central section adjacent to the entrance structure and the second central section adjacent to the exit structure and the dimension monolith 30 was placed at the structure of the entrance structure. exit. The composition was tested in the treatment of an exhaust gas from a bench (4 cylinders) of a 5.7 liter Oldsmobile diesel engine at a gas flow rate of 153 m3 / h. The test was carried out for about 6 hours and backpressure and weight measurements

  emissions were taken approximately every hour.

  Good average trapping efficiency and a relatively small increase in pressure on the

duration of the test.

  Another configuration was prepared where the 6-dimensional monoliths were placed at the inlet section and at the first and second central sections and the dimension 20 was used at the exit section and tested by the same method. It was found that there was a much smaller increase in pressure but a much lower average efficiency of trapping than in

the first configuration.

EXAMPLE 2

  The process of Example 1 was repeated but incorporating platinum and palladium into the powder

  chromium oxide-alumina as follows.

  320 g of the chromium alumina oxide powder of Example 1, a mixed solution of platinum and palladium prepared by sparging sulfur dioxide gas at 2 mmol per minute into 250 ml of sodium hydroxide were impregnated. deionized water for 11.0 minutes and adding 4.022 ml of a solution of palladium nitrate having a titer of 129.27 g of palladium per liter of solution. To the solution was then added 112.012 g of a solution of (NH4) 6Pt (SO3) 4 having a titer of 92.85 g of platinum per kilogram of solution and 3.2 g of dibasic ammonium citrate. The total volume of the solution was increased to 434 ml by addition of deionized water. The powder was impregnated with this volume of the solution, dried in air

  for 1 hour then in the oven for 16 hours at 1350C.

  The powder was finally activated in air for 1 hour at 538WC. The powder contained nominally 3.30% platinum

  and palladium at a weight ratio of 20 to 1.

  339 g of the activated powder and 300 g of copper oxide freshly prepared as in Example 1 were ball milled separately with deionized water at a solids content of 28% for 16 hours. Both lots were combined at a ratio of 1 to 1 (solid basis) and homogenized. Other slurries were prepared using a 1: 1 ratio of powders but varying the solids content by adding additional water. The pH of the slurries was adjusted to 3.5 with nitric acid and the slurries were coated on monoliths of Bridgestone ceramic foam having various cell sizes. Coated monoliths were dried at 135 ° C. for 16 hours and then activated for 1 hour at 42 ° C. The relevant data

are shown in Table II.

  Nominal number of cells / 25 mm solids content%

Table II

  Grams collected 47.36 43.55 46.38 46.15 44.64 43.98, 62 43.09 Coverage% 8.7 7.6 8.5 8.4 7.5 7.5 8.7 7, 4 grams of platinum and palladium 0.781 0.719 0.765 0.761 0.737 0.726 0.753 0.711 nJ, P-- -.4 The monoliths may be joined to the configurations of Example 1 and used in the

  treatment of diesel exhaust.

Claims (23)

R E V E N D I C A T I 0 N S   1. Composition for converting one or more pollutants in an exhaust gas into harmless entities and removing the particles in suspension from said gas, characterized in that it contains a catalytic material effective for conversion deposited on a coarse filter and on a fine filter, said filters comprising a refractory material effective for trapping particles in the gas and being positioned so that the gas flows in succession through the coarse filter and the fine filter.   2. A composition according to claim 1, characterized in that the aforementioned filters have a pore size of about 2 to about 50 pores for 25 mm in length, they have a porosity of the order of about 95% and they have an air permeability of   the order of 400 to about 8000 x 10-7 cm2.   3. Composition according to claim 1, characterized in that the aforesaid coarse filter has a pore size of about 2 to about 20 pores for 25 mm in length and more preferably about 6 to about 25 pores for 25 mm in length and the aforesaid fine filter has a pore size of the order of 15 to about 25 pores for 25 mm and better on the order of 17 to about 30 pores for 25 mm in length, and in that said coarse filter has an air permeability of the order of 2500 to about 8000 x 10-7 cm2 and said fine filter has an air permeability of the order of 400 to about 2500 x 10 7cm2.   4. Composition according to claim 1, characterized in that the aforementioned filters comprise   a refractory or metallic ceramic material.   5. Composition according to claim 1, characterized in that the aforementioned filters comprise a ceramic monolith, a metal wool or a metallic mesh.   6. Composition according to claim 1, characterized in that the aforementioned filters have a open cell structure.   7. Composition according to claim 5, characterized in that the aforementioned filters comprise   a monolithic structure of ceramic foam.   8. Composition according to claim 1, characterized in that the catalytic material mentioned above is   effective for the combustion of carbon particles.   9. Composition according to claim 8, characterized in that the aforementioned catalytic material comprises an element of the first series of transition,   silver, hafnium, or mixtures thereof.   10. Composition according to claim 1, characterized in that the aforementioned catalytic material contains a mixture of catalytically effective amounts of at least one supported material selected from the group consisting of a noble metal, chromium and catalytically active compounds, said material being supported on a refractory and porous inorganic oxide, and at least one bulk material selected from the group consisting of an element of the first transition series, silver,   hafnium and catalytically active compounds.   11. A composition according to claim 10, characterized in that the abovementioned supported material comprises a platinum group metal, chromium oxide or mixtures thereof, the aforementioned bulk material comprises copper oxide, chromium oxide or mixtures thereof, and the aforementioned catalytic material comprises a mixture of about 40 to about 60% by weight of a supported material comprising a platinum group metal, chromium oxide or mixtures thereof supported on a transition alumina and of the order of 40 to about 60% by weight of a bulk material comprising copper oxide.  12. Composition for collecting and removing carbon particles in the exhaust gas of internal combustion engines, characterized in that it contains a catalytic carbon combustion material deposited on a coarse ceramic foam filter having a dimension pores of the order of 2 to about 20 pores per mm of length and a fine ceramic foam filter having a larger number of pores in the range of 15 to about 50 pores per 25 mm in length, said filters being placed so that the gases flow in succession   through the coarse filter and the fine filter.   13. Composition according to claim 12, characterized in that the aforementioned catalytic material comprises an element of the first series of transition,   silver, hafnium or their mixtures.   14. A composition according to claim 12, characterized in that the aforementioned catalytic material comprises a mixture of catalytically effective amounts of at least one supported material selected from the group consisting of a noble metal, chromium and catalytically active compounds, said material being supported on a refractory and porous inorganic oxide, and at least one bulk material selected from the group consisting of an element of the first transition series, silver,   hafnium and catalytically active compounds.   15. A method for removing carbon particles and lead from the exhaust gas of internal combustion engines, characterized in that it consists in passing the gases through a coarse filter and then through a fine filter, said filters including a   effective refractory material for trapping particles.   16. A method according to claim 15, characterized in that the coarse filter has a pore size of about 2 to about 20 pores for 25 mm in length and better in the range of 6 to about 20 pores for mm. of length and the fine filter has a pore size of about 15 to about 50 pores for 25 mm in length and better in the range of 17 to about 30 pores for 25 mm length.   17.- Method according to claim 15, characterized in that the aforementioned filters comprise a refractory ceramic material or metal. 18. A process according to claim 15, characterized in that the aforementioned filters comprise a ceramic monolith, a metal wool or a metallic mesh.   19. A process according to claim 15, characterized in that the filters have a structure of open cells.   20. A process according to claim 15, characterized in that the aforementioned filters comprise   a monolithic structure of ceramic foam.   21. A process according to claim 15, characterized in that the aforesaid filters further contain a catalyst material for carbon combustion. deposited on the filters.   22.- Process according to claim 21, characterized in that the catalyst material comprises a noble metal, an element of the first series of   transition, hafnium or their mixtures.   The method of claim 21, characterized in that the catalyst material comprises a mixture of catalytically effective amounts of at least one supported material selected from the group consisting of a noble metal, chromium, and catalytically active compounds, said material being supported on a refractory and porous inorganic oxide, and at least one bulk material selected from the group consisting of an element of the first transition series, silver, hafnium and catalytically active compounds.