PARTICLE FILTER WITH A PLURALITY OF COATINGS

Abstract

The invention relates to a wall-flow filter, to a method for the production and the use of the filter for reducing harmful exhaust gases of an internal combustion engine. The wall-flow filter was produced by exposing the filter at least twice successively to a powder-gas aerosol.

Claims

1. Wall-flow filter for reducing the harmful substances in the exhaust gas of an internal combustion engine, characterized in that on its inlet side, the dry filter has been exposed in a targeted manner at least twice successively to different dry powder/gas aerosols, each of which has at least one high-melting compound.

2. Wall-flow filter according to claim 1, characterized in that the filter was catalytically coated prior to being exposed to the first powder/gas aerosol.

3. Wall-flow filter according to claim 1, characterized in that during the first exposure, the powder precipitates in the pores of the filter walls and fills them at least up to the inlet surface and thereby does not form a cohesive layer on the walls of the filter over the entire length of the filter.

4. Wall-flow filter according to claim 1, characterized in that the total amount of powder remaining in the filter is below 100 g/l.

5. Wall-flow filter according to claim 1, characterized in that the final powder coating has an increasing concentration gradient over the length of the filter from the inlet side to the outlet side.

6. Wall-flow filter according to claim 5, characterized in that the concentration gradient is created such that in a region near the inlet side and in a region in the center of the filter, less than 40% of the wall surface of the inlet channel are respectively coated with powder, while in a region near the outlet side, more than 40% of the wall surface of the inlet channel are coated with powder.

7. Wall-flow filter according to claim 1, characterized in that when filter substrates with square channels are used, the powder coating in the vicinity of the corners of the channels is thicker than in the corresponding center of the inlet surface.

8. Wall-flow filter according to claim 1, characterized in that the aerosols are a mixture of air and a high-melting powder selected from the group consisting of a metal oxide, metal sulfate, metal phosphate, metal carbonate, or metal hydroxide powder, or mixtures thereof.

9. Wall-flow filter according to claim 1, characterized in that at least one of the powders is likewise catalytically active with regard to reducing the harmful substances in the exhaust gas of an internal combustion engine.

10. Wall-flow filter according to claim 1, characterized in that the first powder has a mean particle diameter (d50) of > 1/10 and <3 of the mean pore diameter (d50) of the filter.

11. Wall-flow filter according to claim 1, characterized in that the first powder has a tamped density of <200 kg/m.sup.3.

12. Wall-flow filter according to claim 1, characterized in that the second or the further powders have a mean particle diameter (d50) < 1/10 of the mean pore diameter.

13. Wall-flow filter according to claim 1, characterized in that the second or the further powders have a tamped density of between 50 kg/m.sup.3 and 1200 kg/m.sup.3.

14. Wall-flow filter according to claim 1, characterized in that the filter has a catalytically active powder zone in the last third of the filter in the inlet channel.

15. Method for producing a wall-flow filter according to claim 1 for reducing the harmful substances in the exhaust gas of an internal combustion engine, wherein a dry filter is exposed in a targeted manner on its inlet surface at least twice successively to different dry powder/gas aerosols, each having at least one high-melting compound, characterized in that the powders are dispersed one after the other in the gas, then guided into a gas stream, and sucked into the inlet side of the filter without further supply of a gas.

16. Method for producing a wall-flow filter according to claim 15, characterized in that the aerosols are sucked through the filter at a velocity of 5 m/s to 60 m/s.

17. Method according to claim 15, characterized in that the dispersion of the powders in the gas is in each case effected by at least one of the following measures: Dispersion by means of compressed air Dispersion by ultrasound Dispersion by sieving Dispersion by “in-situ milling” Dispersion by blower Dispersion by expansion Dispersion in the fluidized bed.

18. Method according to claim 15, characterized in that at least one partial gas stream is extracted downstream of the suction device and, before the powder addition, is added to the gas stream which is sucked through the filter.

19. Method according to claim 15, characterized in that a defined powder distribution over the filter cross section is adjusted by an accelerated flow upstream of the filter.

20. Method according to claim 15, characterized in that the powders are vortexed before flowing into the filter in such a way that deposits of the powders on the inlet plugs of the wall-flow filter are prevented as much as possible.

21. A method of reducing harmful exhaust gases of an internal combustion engine comprising passing the exhaust gases through the wall-flow filter according to claim 1 for reducing harmful exhaust gases of an internal combustion engine.

22. The method according to claim 21, characterized in that the filter is used in an exhaust system together with one or more catalytically active aggregates selected from the group consisting of nitrogen oxide storage catalyst, SCR catalyst, three-way catalyst, and diesel oxidation catalyst.

Description

FIGURES

[0117] FIG. 1: An image of a wall-flow filter wall dusted with a powder, wherein the powder is seated in the pores. The powder 100, in this case a non-pyrogenic powder having a d50 of 3 μm, forms the first layer and is deposited in the pores and in the pore inputs of the porous filter substrate 200. If the amount of powder is increased, an on-wall layer would additionally form.

[0118] FIG. 2: Schematic drawing of a wall-flow filter inlet channel which has been exposed twice to a powder according to the invention. In this case, the amount of powder has an increase in the flow direction. The first powder 302 is deposited in this schematic representation in the pores of the porous filter substrate 303. The volumetric flow distribution 304 in the wall flow results in increased deposition at the end of the filter, delimited by the plug 300. The second powder 301 is deposited on the powder 302. In this case, too, a gradient of the amount of powder is found. The powder 302 predominantly takes over the filtration, the powder 301 contains, for example, noble metals and forms a catalytically active layer, as is required for soot burn-off.

[0119] FIG. 3: A schematic drawing of a wall-flow filter pore, which according to the invention was exposed twice to a powder. At least the first powder is seated in the pores. The first powder 312 blocks the pore of the porous filter 303, but the powder 312 is pyrogenic and thus highly porous. It produces only a small pressure loss for the gas stream 310 but nevertheless has a good filtering effect for finer particles. The second powder 311 is clearly finer in this case and is filtered out of the gas stream 310 by the powder 312 during production. The powder 312 has the filtration task, and the powder 311 has the task, for example, of accelerating the soot burn-off.

[0120] FIG. 4: A schematic drawing of a wall-flow filter with powder penetration during coating. The catalytically active powder 333 is not sufficiently filtered out of the gas stream 330 by the filter substrate 303. The gas stream 331 contains significant amounts of the catalytically active material 333.

[0121] FIG. 5: A schematic drawing of a wall-flow filter with the structure of a barrier layer of powder which prevents the second powder from penetrating through, see FIG. 6. The coarser pyrogenic material 332 is filtered out of the gas stream 330 through the porous matrix of the filter 303. A porous filter cake is formed on the cell walls and in the pores. The exhaust air stream 331 is virtually free of particles.

[0122] FIG. 6: A schematic drawing of a wall-flow filter with the structure of a functional powder layer on a barrier layer of powder. The filter cake, formed of the porous coarser particles 332, in the pores and on the cell walls of the filter 303 now filters the finer catalytically active particles 333 from the gas stream 330. The exhaust gas stream 331 is now virtually free of particles.

[0123] FIG. 7: A schematic drawing of an advantageous device. The powder 420 or 421 is mixed with the gas under pressure 451 through the atomizer nozzle 440 in the mixing chamber with the gas stream 454 and then is sucked or pushed through the filter 430. The particles that have penetrated are filtered out in the exhaust filter 400. The blower 410 provides the necessary volumetric flow. The exhaust gas is divided into an exhaust 452 and a warm cycle gas 453. The warm cycle gas 453 is mixed with the fresh gas 450.

[0124] FIG. 8: A schematic representation of the chamber for dispersing the powders with 2 dispersing nozzles, one for each powder. The powder 1500 is conveyed via the dispersing nozzle 1410 with a gas under pressure 1400 into the chamber 2000 where it is mixed with the transport gas 1200. This mixture 1300 then flows to the filter to be coated. Subsequently, the powder 1800 in the dispersing nozzle 1910 is also dispersed with a gas under pressure 1900 and transported into the chamber 2000 where it is mixed with the transport gas 1200. This mixture 1300 then flows to the filter to be coated.

[0125] FIG. 9: A schematic representation of the chamber for dispersing the powders with 1 dispersing nozzle but for two different powders. The powder 1500 is conveyed via the dispersing nozzle 1910 with a gas under pressure 1900 into the chamber 2000 where it is mixed with the transport gas 1200. This mixture 1300 then flows to the filter to be coated. Subsequently, the powder 1800 in the dispersing nozzle 1910 is also dispersed with a gas under pressure 1900 and transported into the chamber 2000 where it is mixed with the transport gas 1200. This mixture 1300 then flows to the filter to be coated. This variant can be selected if the properties of the powders, such as the flowability, permit it.

[0126] FIG. 10: A schematic drawing of a wall-flow filter with the structure of a functional powder layer on a barrier layer of powder. The filter cake, formed of the porous coarser particles 332, in the pores and on the cell walls of the filter 303 now filters the finer catalytically active particles 334 out of the gas stream 330. The exhaust gas stream 331 is now virtually free of particles. A high gas velocity during coating and the selection of relatively coarse or heavier particles 334 forms an on-wall zone made of catalytically active material which is particularly pronounced in the last third of the filter. Since, as a rule, 50% of the volumetric flow passes through the wall, this powder is catalytically very active.

[0127] FIG. 11: A schematic drawing of a wall-flow filter according to the invention with the structure of a functional powder layer on a barrier layer of powder. The filter cake in the pores and on the cell walls of the filter 303 formed of the porous coarser particles 332 that have picked up the finer catalytically active particles 334. The particles 334 are composed, for example, of aluminum oxide with a higher coating of particles. The soot 335 in the exhaust gas stream is now deposited following the exhaust gas volumetric flow exactly where the catalytically active particles have also been deposited. The soot burn-off now starts earlier and the pressure loss and thus the gasoline consumption are lower over time.

[0128] FIG. 12: A schematic drawing shows a preferred form of an embodiment of the invention for a filter having different permeabilities. The two washcoat zones 336 and 337, which may be located on the wall, in the wall, or both in and on the wall, significantly reduce the permeability in the illustrated example.

[0129] The filtering powder layer 332 and the catalytically active powder particles 334 are then in the high permeability range. They are very effective there with regard to filtration as well as with regard to catalysis because a significant portion of the exhaust air stream runs there. The pressure loss in this design is surprisingly low.

[0130] FIG. 13: A schematic drawing shows another preferred form of an embodiment of the invention for a filter having different permeabilities. The washcoat zone 337, which may be on the wall, in the wall, or both in and on the wall, significantly reduces the permeability in the illustrated example. The filtering powder layer 332 and the catalytically active powder particles 334 are then in the high permeability range. They are very effective there with regard to filtration as well as with regard to catalysis because a significant portion of the exhaust air stream runs there. The pressure loss in this design is surprisingly low.