WO2021160572A1

WO2021160572A1 - Use of ultrasound for cleaning wall-flow filter substrates

Info

Publication number: WO2021160572A1
Application number: PCT/EP2021/052998
Authority: WO
Inventors: Manuel GENSCH; Antje OLTERSDORF
Original assignee: Umicore Ag & Co. Kg
Priority date: 2020-02-10
Filing date: 2021-02-09
Publication date: 2021-08-19
Also published as: EP4103822A1; DE102020103292A1; CN115066541A

Abstract

The invention relates to the cleaning of dry deposits of coating on wall-flow filter plugs by means of ultrasound.

Description

Using ultrasound to clean wall flow filter substrates

description

The present invention is directed to ultrasonic (US) cleaning of dry powder coated wall flow filter substrates.

The exhaust gas from internal combustion engines in motor vehicles typically contains the pollutant gases carbon monoxide (CO) and hydrocarbons (HC), nitrogen oxides (NO _x ) and possibly sulfur oxides (SO _x ), as well as particles that are largely made up of solid carbon-containing particles and possibly adhering organic agglomerates . These are known as primary emissions. CO, HC and particles are products of the incomplete combustion of the fuel in the engine's combustion chamber. Nitrogen oxides are formed in the cylinder from nitrogen and oxygen in the intake air when the combustion temperatures exceed 1200 ° C. Sulfur oxides result from the combustion of organic sulfur compounds, which are always contained in small quantities in non-synthetic fuels. Compliance with the statutory exhaust emission limits for motor vehicles in Europe, China, North America and India in the future requires the extensive removal of the pollutants mentioned from the exhaust gas. To remove these emissions, which are harmful to the environment and health, from the exhaust gases of motor vehicles, a large number of catalytic exhaust gas cleaning technologies have been developed, the basic principle of which is usually based on the fact that the exhaust gas to be cleaned is via a flow-through or a wall-flow (wall). flow) honeycomb body with a catalytically active coating applied to it. The catalyst promotes the chemical reaction of various exhaust gas components with the formation of un harmful products such as carbon dioxide, water and nitrogen.

The flow or wall flow honeycomb bodies just described are also referred to as catalyst carriers, carriers or substrate monoliths, as they carry the catalytically active coating on their surface or in the walls forming this surface. The catalytically active coating is often applied to the catalyst carrier in a so-called coating process in the form of a suspension (washcoat). Many such processes have been published in the past by car exhaust catalyst manufacturers for this purpose (EP1064094B1, EP2521618B1,

W010015573A2, EP1136462B1, US6478874B1, US4609563A, WO9947260A1,

JP5378659B2, EP2415522A1, JP2014205108A2). Alternatively, a coating can also be carried out by applying dry powder. This method seems to offer advantages, in particular for the application of dry powder to the inlet side of a wall flow filter, as it allows the targeted adaptation of a desired filtration efficiency to be achieved while the exhaust gas back pressure is not increased excessively (US8388721B2; EP2727640A1; US82711580AB2; EP2502502661A1; EP2502502661A1; US9745227B2; W018115900A1).

During the coating of wall-flow filters with dry powders, deposits in the form of precipitations can form on the plugs of the inflow surface. These deposits are disadvantageous for various reasons. On the one hand, these can become detached during the further processing of the coated wall-flow filter and lead to contamination; on the other hand, this can result in disadvantageous product properties (increased back pressure, exceeding the maximum length specification, visual evaluation).

The cleaning of equipment using ultrasound is well known. The treatment usually takes place in a so-called ultrasonic bath. The use of ultrasonic waves for dry cleaning is less common. For example, US6474349B1 describes the cleaning of pipes from dirt and impurities by means of ultrasound.

The removal of dust from flat surfaces by means of ultrasound is known from US4364167. However, here a pulsating air flow is superimposed with an ultrasound and thus used for cleaning.

The object of the present invention is to remove the deposits created by the coating of the wall flow filter on the plug on the inlet side of the coating by a process step downstream of the dry powder coating of wall flow filter substrates. During this cleaning of the precipitated material, the distribution of the coated material in the wall flow filter itself should remain unaffected, so that the desired properties of the correspondingly manufactured filters are not negatively influenced.

To solve the problem, it is proposed to use ultrasound for contactless cleaning from dry precipitates on the stopper of a wall-flow filter substrate (2) coated with a dry powder. With the specialist known coating of a wall flow filter substrate with a dry powder often remains of the powder on the plug of the wall flow filter in the entrance area. They make themselves noticeable by more or less solid piles of powder on the said stopper (Fig. 4). These interfere with the subsequent process steps. Due to the pressure fluctuations caused by the ultrasonic waves, the material deposited there can be removed selectively and successfully from the plug surface or substrate surface. The pressure fluctuations ensure that the deposited material is atomized after coating on the substrate surface. The sound waves are preferably generated with a loudspeaker. The process is contactless, which avoids / reduces possible mechanical damage or other influencing of the wall flow filter. The ultrasonic apparatus (5) can be integrated very easily into a coating system for wall flow filters.

All ceramic materials customary in the prior art can be used as wall flow monoliths or wall flow filter substrates. Porous wall-flow filter substrates made of cordierite, silicon carbide or aluminum titanate are preferably used. These wall-flow filter substrates have inflow and outflow channels, the outflow-side ends of the inflow channels and the inflow-side ends of the outflow channels being closed off with gas-tight “plugs”, offset from one another. The exhaust gas to be cleaned, which flows through the filter substrate, is forced to pass through the porous wall between the inflow and outflow channels, which results in an excellent particle filter effect. The filtration properties for particles can be designed through the porosity, pore / radius distribution and thickness of the wall. The porosity of the uncoated wall flow filter is usually more than 40%, generally from 40% to 75%, especially from 50% to 70% [measured according to DIN 66133 - latest version on the filing date]. The average pore size (mean pore diameter; d50 of the Q3 distribution) of the uncoated filter is at least 7 μm, e.g. B. from 7 pm to 34 pm, preferably more than 10 pm, particularly more preferably from 10 pm to 25 pm or very preferably from 15 pm to 20 pm [measured according to DIN 66134, latest version on the filing date].

According to the invention, the dry wall-flow filter substrate is loaded with a dry powder, which is brought into the input area of the filter according to methods common to those skilled in the art (see literature above). It is given before given a dry powder-gas aerosol in the inflow channel of the dry Wall flow filter substrate spent. The use of a method and apparatus as described in DE102018111246A1 is particularly preferred.

The application of the dry powder-gas aerosol to the dry, possibly previously conventionally wet-technically, catalytically coated wall-flow filter substrate causes the powdery components, in particular high-melting metal compounds, preferably oxides, to follow the flow of the gas on the surface of the inflow channels in the Separate the filter and, if necessary, in the pores of the filter. In principle, the person skilled in the art knows how to produce an aerosol from a corresponding powder and a gas in order to then pass it through the wall-flow filter to which the powder is to be applied. To produce a corresponding wall flow filter, a dry filter on its input surface is advantageously contacted with a dry powder-gas aerosol by dispersing the powder in a gas, then passing it into a gas stream and without further supplying a gas into the inlet side or inflow channels of the filter sucks or presses. For reasons of health and safety, sucking is preferable to pressing.

As gases for the production of the aerosol and for introduction into the filter, all gases which are suitable for the present purpose to the person skilled in the art can be used. The use of air is very particularly preferred. However, other reaction gases can also be used, which can either develop an oxidizing (e.g. O2, NO2) or a reducing (e.g. H2) activity towards the powder used. The use of inert gases (e.g. N2) or noble gases (e.g. He) can also prove to be advantageous for certain powders. Mixtures of the gases listed are also conceivable.

Powders, which are preferably used in the present invention to generate the aerosol, are sufficiently known to the person skilled in the art. As a rule, these are high-melting metal compounds that are commonly used as carrier materials for catalysts in the automotive exhaust area. Corresponding metal oxide, metal sulfate, metal phosphate, metal carbonate or metal hydroxide powders are preferably used. The use of an aerosol, which is a mixture of air and a metal oxide powder, is very particularly preferred. The metals that are suitable for the metal compounds are, in particular, those selected from the group of alkali metals, alkaline earth metals or earth metals or transition metals. Those metals are preferably selected from the group calcium, magnesium, strontium, barium, Aluminum, silicon, titanium, zirconium, cerium are used. As mentioned, these metals can preferably be used as oxides. The use of cerium oxide, titanium dioxide, zirconium dioxide, silicon dioxide, aluminum oxide or mixtures or mixed oxides thereof is very particularly preferred. The term mixed oxide (solid solutions of one metal oxide in at least one other) is also understood here to mean the use of zeolites and zeotypes. In the context of the invention, zeolites and zeotypes are defined as in WO2015049110A1.

According to the invention, the powders used here can be used as such as described above. However, the use of dry metal compounds, in particular oxide powders, which have a catalytic activity with regard to exhaust gas aftertreatment, is also conceivable. As a result, the powder itself can also be catalytically active in terms of reducing the pollutants in the exhaust gas of an internal combustion engine. All activities known to the person skilled in the art can be used for this, such as TWC, DOC, SCR, LNT or soot burn-off accelerating catalysts. Generally, the powder will have the same catalytic activity as any catalytic coating of the filter. This further increases the overall catalytic activity of the filter compared to filters that are not coated with catalytically active powder. In this respect it can be the case that, for example, aluminum oxide impregnated with a noble metal is used for the production of the powder-gas aerosol. A three-way activity with a coating comprising palladium and rhodium and an oxygen storage material such as cerium-zirconium oxide in this context is preferred. It is also conceivable that catalytically active material is used for the SCR reaction. The powder can consist, for example, of zeolites or zeotypes exchanged with transition metal ions. The use of iron and / or copper-exchanged zeolites is very particularly preferred in this context. CuCHA (copper-exchanged chabazite; http://europe.iza-structure.org/IZA-SC/frame- work.php? STC = CHA) or CuAEI (http: //europe.iza-structure.org/IZA-SC/frame- work.php? STC = AEI) is used. An activity of the powder can also advantageously lie in the improved soot combustion.

The surface of the particles in the powder is preferably very high. The high-melting compound advantageously has a BET surface area of> 50 m ² / g, more preferably> 70 m ² / g and very preferably> 100 m ² / g to preferably a maximum of 1000 m ² / g. The BET surface area is determined in accordance with DIN ISO 9277: 2003- 05 (Determination of the specific surface area of solids by gas adsorption according to the BET method - latest version on the filing date). A high outer surface offers an excellent separation surface for the particles, especially soot particles in the nanometer range.

The particle diameter in the aerosol should be small. This can be expressed in that the ratio of the mean particle diameter (d50 - Q3 distribution; measured with the Tornado dry dispersion module from Beckmann in accordance with the latest ISO 13320-1 on the filing date) in the dry aerosol and the mean pore diameter of the wall flow filter (d50 - Q3 distribution; measured in accordance with DIN 66134 - latest version on the filing date) between 0.03 - 2, preferably between 0.05 - 1.43 and very particularly preferably between 0.05 - 0.63.

In order to be able to deposit the powder sufficiently well on and / or in the surface of the filter wall on the inlet side of the filter, a certain suction power or pressure power is necessary. The person skilled in the art can get an idea of the respective filter and the respective powder in orienting experiments. It has been shown that the aerosol (powder-gas mixture) preferably with a speed of 5 m / s to 60 m / s, more preferably 10 m / s to 50 m / s and very particularly preferably 15 m / s up to 40 m / s is sucked through the filter, as this corresponds to the later exhaust gas velocities. In this way, an advantageous adhesion of the applied powder is also achieved. The flow velocities mentioned were measured on the substrate surface upstream of the wall flow filter. The measuring device is in contact with the substrate surface. The speeds mentioned are measured in the middle (R = 0) of the wall flow filter. A vane anemometer is used to measure the speeds. The specialist knows how to do this.

The amount of powder in the filter depends on the type of powder and the dimensions of the filter and can be determined in preliminary tests by a person skilled in the art under the given boundary conditions (not too high exhaust gas pressure). As a rule, and preferably according to the invention, the loading of the filter with the powder is less than 50 g / l based on the filter volume. The value is preferably not more than 30 g / l, very particularly preferably not more than 20 g / l. Naturally, the desired increase in filtration efficiency forms a lower limit. Particularly preferred in this one The relationship is when the amount of powder remaining in the filter is below 10g / l.

In the context of the present invention, dry accordingly means the exclusion of the presence of a liquid, in particular water. For powder coating in particular, the preparation of a suspension of the powder in a liquid for atomization in a gas stream should be avoided. A certain amount of moisture may be tolerable for both the filter and the powder, provided that the achievement of the goal - the finely distributed deposition of the powder in or on the input surface - is not adversely affected. As a rule, the powder is free-flowing and dispersible due to the input of energy. The moisture of the powder or the filter at the time of exposure to the powder should be less than 20%, preferably less than 10% and very particularly preferably less than 5% (measured at 20 ° C and normal pressure ISO 11465 latest version on the filing date) . The bulk density of the powder is preferably 50-1000 g / L, more preferably 100-700 g / L and very preferably 150-400 g / L.

If the powder is brought into the wall-flow filter substrate in this way, there is always a deposition or precipitation of powder on the plug of the substrate surface of the wall-flow filter substrate on the inlet side. According to the invention, these deposits are removed with an ultrasound device. Ultrasound devices which are suitable for the present purpose are sufficiently known to the person skilled in the art. Basically, such devices for generating ultrasound (5), which can be used advantageously to solve the problem described here, are composed of various components that are known to the person skilled in the art (https://de.wikipedia.Org/w/ index. php? title = Sweat% C3% 9Fen&oldid=196249986). The ultrasonic signals are generated in a generator (https: //www.sonikks.de/de/funktionsweise-von-ultraschallgeneratoren). The ultrasonic signals are converted into mechanical vibrations in a converter (https://www.sonikks.de/de/ultra- schall-produkte / schwingsysteme-converter), which are preferably emitted to the environment via a sonotrode (https: // www. sonikks.de/de/ultraschall-produkte/so- notroden), translated. Depending on the required amplitude, a booster (https://de.wiki pedia.orq / w / index.php?title=Booster (welding% C3% 9Ftechnik )&oldid=188417221) can be switched between the converter and the sonotrode, which changes the amplitude of the sound wave according to the intended use. The sound waves required for cleaning are preferably generated using a sonotrode (https: //de.wi kipedia.orq / w / index.php? Title = Sonotrode & ol- did = 187253666). The sound waves are transmitted from the radiating surface of the sonotrode to the surrounding medium before they hit the precipitation. In the present case, this medium is preferably air. The sonotrode itself can be designed in such a way that an amplitude modification of the sound wave also takes place here. The sonotrode is advantageously designed so that its resonance frequency is in the frequency range given below.

Different sonotrode shapes are known from other applications such as ultrasonic welding. Sonotrodes can be designed as rectangles, full cones, hollow cones and other shapes. For the task described here, a sonotrode shape is advantageous which focuses the sound pressure on a small volume in order to generate the highest possible sound pressure densities there. Tests have shown that a full cone sonotrode can generate the highest sound pressures at a focal point in front of the sound radiation surface. Their use is therefore preferred in the present case.

For cleaning the plug surface of the wall flow filter, the sonotrode can be directed directly at the surface to be cleaned, as shown in FIG. The distance from the center point of the sonotrode emitting surface and the substrate surface (h3 in FIG. 2) in the case of direct ultrasound irradiation is between 1 mm and 100 mm, preferably between 2 mm and 60 mm and very particularly preferably between 5 mm and 30 mm. It is advantageous here if the sound hits the substrate surface at a certain angle of inclination in order to clean off the plug deposits. The angle of inclination is understood to mean the angle between the inflow surface of the substrate, the substrate surface, and the normal of the radiating surface of the sonotrode. (Fig. 2) is gebil det. A person skilled in the art can determine the optimal angle of inclination. This should preferably be between 20 ° and 80 ° and very preferably 45 ° to 80 °.

The sound pressure generated should hit the surface to be cleaned as compactly as possible. In order to give the sound a direction for this purpose, a few devices are known to those skilled in the art. In the present case, the ultrasound can advantageously be generated by means of a device for generating ultrasound (5) having a sonotrode (1), and reflectors can also be used, via which the ultrasound is reflected onto the substrate surface to be cleaned (FIGS. 1, 5 ). Regardless of the absorption and scattering of obstacles, the sound pressure is reduced by the uniform expansion in all spatial directions according to the 1 / r ² law. By using reflectors that interrupt the even propagation of sound in all directions, the drop in sound pressure on the substrate surface can be reduced. This leads to an increased sound pressure on the substrate surface and thus more efficient cleaning.

In particular, if the sonotrode cannot be aimed directly at the surface to be cleaned as shown in FIG. 2, it has proven to be particularly advantageous to radiate the sound waves in a volume limited by reflectors in order to reduce the sound pressure due to the spherical sound propagation to reduce. The portion of the sound waves that does not propagate in the direction of the wall flow filter surface is reflected back onto it (Fig. 5). This increases the range of the sound waves and increases the sound pressure density. The use of appropriately designed reflectors is therefore preferred in the present case.

The sound is reflected at the interface between air and reflector material. For this, the reflector material must be more soundproof than air. Furthermore, the material of the reflector should be made in such a way that as little sound as possible is absorbed and scattered, since otherwise it cannot be used for cleaning off powder residues. Low scattering and absorption can be achieved with smooth, hard surfaces. On the other hand, the absorption of the sound in the resonator depends on the frequency of the sound waves. The higher the frequency of the sound wave, the stronger the absorption. This results in both reduced emissions and the range into the wall flow filter is smaller. Due to the short range in the wall flow filter, the distribution of the coated material remains almost unchanged, which is to be preferred here.

To solve the problem described, two arrangements of sonotrode and reflector have proven to be particularly advantageous. FIG. 5 shows a system in which the reflector is designed as a spherical shell. The direction of emission of the sonotrode is directed towards the center of the reflector. The distance between the center of the sonotrode emitting surface and the center of the reflector (h4, Figure 5) is based on the diameter of the wall flow filter and is between 30mm and 410mm. The diameter of the spherical shell reflector must be adapted to the distance to the sonotrode. This can be done in experimental trials. The distance from the center of the The spherical shell reflector to the substrate surface is between 20mm and 200mm, preferably between 30mm and 150mm and very particularly preferably between 40mm and 120mm.

In the case of direct irradiation with ultrasound, the surface to be cleaned must be completely irradiated in the plane with the sonotrode if the entire surface is to be cleaned. However, the complexity of the overall system can be reduced using the device for plug cleaning shown in FIG. In particular, if, for example for ap parative reasons, the sonotrode cannot reach the entire substrate surface to be cleaned equally well with the ultrasound, it has proven to be particularly advantageous if the device for generating ultrasound (5), including the sonotrode (1 ), is fixed and the ultrasound is reflected by a movable reflector (6) onto the substrate surface to be cleaned. As a result, every point on the substrate surface can then be reached very easily. Alternatively, both the sonotrode (1) and the reflector (6) can be movably mounted.

An arrangement made up of a half-tube reflector (3) attached above the sonotrode (1) has also proven to be a more advantageous embodiment (FIG. 1). The half-tube reflector can also be attached to the sonotrode holder. The distance between the center axis of the sonotrode emitting surface and the half-tube reflector (h1, FIG. 1) is between 2mm and 100mm, preferably between 5mm and 80mm and most preferably between 10mm and 70mm. The diameter of the half-tube reflector is between 10 mm and 50 mm, preferably between 15 mm and 45 mm and very particularly preferably between 20 mm and 40 mm.

At the end of the half-tube reflector there is a tail tube reflector (see FIG. 1, number 4). The tailpipe reflector (4) can be attached at a variable distance. Depending on the preferred type of wave, the reflector should be attached to the end of the tube (4) at a certain distance from the sonotrode. For the formation of standing waves, the distance between the tailpipe reflector (4) must be a multiple of half the wavelength. Standing mechanical waves are characterized by fixed (stationary) minima and maxima of the amplitudes (https: //de.wi kipedia.orq / w / index.php? Title = standing wave & oldid = 179416243). In contrast, progressive sound waves, which are also called progressive waves, can also be used for the Solution of the task described can be used. These waves are characterized by non-stationary minima and maxima of the amplitudes. For the progressive waves, there is no need to maintain a special distance between the tailpipe reflector (4) and the center of the sonotrode emitting surface. The preferred distance of the tailpipe reflector is to be determined from experiments for the respective substrate dimensions. It is usually between 30mm and 410mm, which corresponds to the usual wall flow filter substrate dimensions.

In a particularly advantageous embodiment, the device for generating ultrasound (5) is designed to be movable in such a way that it - including the cavity reflector - can be guided over or around the substrate surface to be cleaned (FIG. 3). The substrate surface of the wall flow filter substrate to be cleaned has a certain extent. The substrate can be square, round or oval. In any case, it is advantageous if each point on the surface is subjected to the sound pressure from a wide variety of sides. This leads to a better cleaning of the stopper from the coating with the powder. This can advantageously be achieved in that the described device for plug cleaning (5) is guided around the substrate during the action of sound (FIG. 3). In addition to moving along the circumference of the substrate, the device shown for plug cleaning (5) is rotated around the mounting axis (see position 8 in FIG. 3) so that ultrasound can be applied to all areas of the surface to be cleaned. The travel path of the device shown for stopper cleaning (5) can be between 10% and 100% of half the circumference. The rotation (8) can take place within an angle of e.g. 50 °. For the translational movement, the holder of the So notrode (1) can be on a rail and is moved by a linear motor. The person skilled in the art can accomplish the design of this embodiment.

It is obvious that the ultrasound devices must be able to generate a corresponding sound pressure. If too little sound pressure is generated, the powder deposits cannot be removed sufficiently well. If too much sound pressure is generated, the substrate may be damaged. It has proven to be advantageous if the sound pressure of the ultrasound is 1 kPa to 100 kPa (measured at a distance from the center point of the sonotrode radiating surface between 2 mm and 10 mm). The sound pressure is more preferably between 7 kPa - 70 kPa and very particularly 10 kPa - 50 kPa. The frequency of the sound is preferably between 10 kHz and 60 kHz, preferably between 15 kHz and 55 kHz and very preferably between 20 kHz and 50 kHz. These values apply both to the direct irradiation of the ultrasound on the substrate surface and to the reflective procedure as described above.

In addition to the cleaning of the plug surfaces according to the invention, the sound pressure can lead to a dispersion of the deposits. In the present invention, the wall flow filter to be cleaned is advantageously clamped in a coating device during the ultrasonic treatment according to the invention. During the ultrasonic treatment, the material to be cleaned is advantageously sucked and / or pressed into the wall-flow filter by applying an air stream. This improves the utilization of the coating material.

The present invention is described with reference to the following figures.

1: a schematic illustration of the ultrasonic structure consisting of the sonotrode (1), half-tube reflector (3), end-tube reflector (4) and holder (5) of the ultrasonic system for cleaning the flow surface of a wall-flow filter (2); Also shown in the figure is the distance between the center point of the sonotrode radiating surface and the half-tube reflector (h1) and the distance between the center point of the sonotrode radiating surface and the substrate surface of the wall flow filter (h2); the center line of the sonotrode (dashed) does not have to be parallel to the surface of the wall flow filter but can be inclined to the surface at an angle between 0 ° and 20 °.

2 shows the ultrasound structure without reflectors, consisting of the sonotrode (1) and the holder of the ultrasound system (5) for cleaning the inflow surface of a wall flow filter (2); the angle of inclination α specifies the orientation of the radiation direction to the face of the flow surface of the wall flow filter substrate (2); The distance between the sonotrode and the substrate surface to be cleaned is also shown (h3).

Fig. 3: Schematic representation of the travel path (7) of the arrangement of sonotrode (1), half-tube reflector (3), tailpipe reflector (4) for cleaning the wall-flow filter surface (2), in addition to the travel path (7), the required rotation ( 8) the sonotrode holder (5) shown to clean all areas of the surface; the start position is shown in full black, the position of the device shown for plug cleaning is slightly grayed out.

Fig. 4: Representations of the flow surfaces of powder-coated wall-flow filters without subsequent ultrasonic cleaning of the precipitates (left) and with subsequent ultrasonic cleaning (right).

Fig. 5: Schematic representations of the arrangement consist of sonotrode (1), spherical shell reflector (6), substrate surface to be cleaned (2) and sonotrode holder (5), the direction of propagation of the sound is shown in dashed lines. Example:

In the present example, ultrasound was used to remove powder from the plugs on the upstream side of a wall-flow filter loaded with a dry powder. The cleaning effect was examined with direct and indirect ultrasonic radiation. In this context, direct ultrasound irradiation is understood to mean the arrangement as shown in FIG. The Sontrode is aimed directly at the surface to be cleaned. The surface to be cleaned is scanned with the sonotrode. The wall flow filter is clamped in a vacuum device so that the cleaned off powder is sucked into the filter. In the case of the indirect arrangement, the sonotrode is directed towards a reflector. The arrangement corresponds to that in FIG. 5. The sonotrode remained fixed and the surface was cleaned by moving the spherical shell reflector. The resonance frequency of the sonotrode was 35 kHz. Substrates with a diameter of 5.2 "and a length of 4 were used. After the ultrasonic treatment, the upstream side of the wall flow filter is powder-free (see FIG. 4). The powder distribution in the substrate was not influenced by the ultrasound. The functional properties (filtration efficiency and back pressure) remain almost unchanged compared to the reference. The d50 value of the powder used was d50 = 3.5 pm (d50 - Q3 distribution; measured with the Tornado dry dispersion module from Beckmann in accordance with the latest ISO 13320-1 on the filing date).

Table 1: Characteristics of the wall-flow filters from Example 1 determined from an engine test stand

Claims

1. Use of ultrasound for the contactless cleaning of dry precipitation on the stopper of a wall-flow filter substrate coated with a dry powder (2).

2. Use according to claim 1, characterized in that the ultrasound is generated using a sonotrode (1) which emits the sound in air before it hits the precipitates.

3. Use according to one of the preceding claims, characterized in that the distance from the center point of the sonotrode emitting surface and the substrate surface (h3) is 1mm - 100mm.

4. Use according to claim 3, characterized in that the sound hits the substrate surface at an angle of inclination (a) of 20 ° to 80 °.

5. Use according to one of the preceding claims 1 or 2, characterized in that the ultrasound is generated by means of a device for generating ultrasound (5) having a sonotrode (1) and additionally reflectors are used over which the ultrasound is transmitted the substrate surface to be cleaned is reflected.

6. Use according to claim 5, characterized in that the device for generating ultrasound (5) is stationary and the ultrasound is reflected by a movable reflector (6) onto the substrate surface to be cleaned.

7. Use according to claim 5, characterized in that a cavity reflector (3) is used and the distance from the center of the sonotrode emitting surface to the end tube reflector (4) is 30 mm - 410 mm.

8. Use according to claim 7, characterized in that the device for generating ultrasound (5) is designed in such a way that it can be guided over or around the substrate surface to be cleaned.

9. Use according to one of the preceding claims, characterized in that the sound pressure of the ultrasound is 1 kPa to 100 kPa at a distance of 2mm to 10mm from the radiating surface of the sonotrode.

10. Use according to one of the preceding claims, characterized in that the ultrasound has a frequency between 10 kHz and 60 kHz.

11. Use according to one of the preceding claims, characterized in that during the ultrasonic treatment the precipitates are sucked and / or pressed into the wall-flow filter by applying an air stream.