FILTER CANDLE HAVING A CONCENTRATION GRADIENT OF CATALYST METALS, METHOD FOR PRODUCTION THEREOF AND USE THEREOF IN AN EXHAUST GAS CLEANING METHOD

Abstract

The present invention relates to a hollow filter candle (1) which has, via impregnation of the inside with a catalyst solution, a concentration gradient of catalyst metals from the inside to the outside across the wall thickness thereof. The present invention furthermore relates to a method for producing a filter candle (1) according to the invention and the use of the filter candle (1) according to the invention for exhaust gas cleaning, in particular in waste incineration plants.

Claims

1. A filter candle for exhaust gas cleaning, the filter candle being obtainable by: a) providing a hollow filter candle body, b) impregnating an inside of the filter candle body provided in step a) with a catalyst solution comprising at least one solvent and at least one metal compound dissolved and/or dispersed in the at least one solvent to yield an impregnated filter candle body, c) drying the impregnated filter candle body obtained in step b) to yield a dried impregnated filter candle body, and d) thermally activating the dried impregnated filter candle body obtained in step c).

2. The filter candle as claimed in claim 1, wherein the filter candle body comprises or consists of inorganic fibers.

3. The filter candle as claimed in claim 2, wherein the inorganic fibers are selected from the group of ceramic fibers, glass fibers, crystalline mineral fibers, amorphous mineral fibers, mineral wood, high-temperature wool, carbon fibers, and mixtures thereof.

4. The filter candle as claimed in claim 1, wherein the filter candle has a porosity of 70 to 95%, the porosity having been ascertained through a determination of water absorption according to DIN EN 993-1:2019 DE.

5. The filter candle as claimed in claim 1, wherein the filter candle body has a collar element at one end and is closed at an opposite end.

6. The filter candle as claimed in claim 1, wherein the filter candle body is obtainable by: i) preparing an optionally aqueous suspension which comprises inorganic fibers and at least one binder system, ii) drawing the suspension prepared in step i) by suction onto a candle-shaped absorbent core for the shaping of the filter candle body, and iii) drying the filter candle body shaped in step ii).

7. The filter candle as claimed in claim 6, wherein the binder system comprises or consists of starch and at least one colloidal dispersion.

8. The filter candle as claimed in claim 1, wherein the metal of the metal compound is selected from the group comprising or consisting of platinum, palladium, ruthenium, aluminum, tungsten, titanium, vanadium, and mixtures thereof.

9. The filter candle as claimed in claim 8, wherein the metal of the metal compound is selected from the group comprising or consisting of titanium and vanadium, a mass ratio of titanium compound to vanadium compound being 100:1 to 10:1.

10. The filter candle as claimed in claim 1, wherein the catalyst solution has a total content of metal compound(s) of 1 to 10 wt %, on a total weight of the catalyst solution.

11. A filter candle for cleaning exhaust gases, comprising a hollow filter candle body and at least one catalytically active metal compound, wherein a wall of the filter candle body has a concentration gradient of the catalytically active metal compound.

12. The filter candle as claimed in claim 1, wherein the concentration of the metal compound(s) increases in a gradient from outside to inside across the wall thickness of the filter candle body.

13. The filter candle as claimed in claim 1, wherein a concentration of the catalytically active metal compound on a wall outside of the filter candle body is from 1.5 to 90% of a concentration of the catalytically active metal compound on a wall inside of the filter candle body.

14. A method for producing a filter candle as claimed in claim 1, the method comprising the following method steps in the specified order: a) providing a hollow filter candle body, b) impregnating an inside of the filter candle body provided in step a) with a catalyst solution comprising at least one solvent and at least one metal compound dissolved and/or dispersed in the at least one solvent to yield an impregnated filter candle body, c) drying the impregnated filter candle body obtained in step b) to yield a dried impregnated filter candle body, and d) thermally activating the dried impregnated filter candle body obtained in step c).

15. The method as claimed in claim 14, wherein the impregnating in step b) takes place by spraying.

16. A method of using a filter candle as claimed in claim 1 for exhaust gas cleaning.

17. An apparatus for producing a filter candle as claimed in claim 1, the apparatus comprising a reservoir vessel for the catalyst solution, said vessel being provided optionally with a stirring facility and there being a conduit which starts from the reservoir vessel and via which the catalyst solution can be conveyed by means of a conveying facility into a spraying lance which is held moveably in longitudinal direction, for the inside impregnation of the filter candle body, the apparatus further comprising a holder for the filter candle body that is equipped optionally with a drive unit for setting the filter candle body into rotation about its longitudinal axis.

18. The apparatus as claimed in claim 17, wherein in a region above the holder there is a suction withdrawal unit and/or below the holder there is a collecting vessel for excess catalyst solution.

19. The apparatus as claimed in claim 17, wherein the holder is formed by a plurality of rollers for rotatably mounting the filter candle body, at least one of the rollers being equipped with the drive unit.

20. The apparatus as claimed in claim 17, wherein the holder has a stop facility for locating the filter candle body.

Description

[0083] The present invention is elucidated below in more detail with reference to FIGS. 1 to 11 and also to the subsequent examples. In the figures,

[0084] FIG. 1 shows a longitudinal section through a filter candle 1 of the invention;

[0085] FIG. 2 shows a schematic representation of a plant for coating the filter candles with a catalyst;

[0086] FIG. 3 shows a schematic representation of a roundel having a 50 mm diameter from a filter candle of the invention with a wall thickness of 20 mm as a sample for determination of the local catalyst concentration via EDX;

[0087] FIG. 4 shows an EDX spectrum close to the inside of a roundel corresponding to FIG. 3;

[0088] FIG. 5 shows an EDX spectrum at a distance of 3 mm from the inside of the roundel from FIG. 4;

[0089] FIG. 6 shows an EDX spectrum at a distance of 8 mm from the inside of the roundel from FIG. 4;

[0090] FIG. 7 shows an EDX spectrum at a distance of 14 mm from the inside of the roundel from FIG. 4;

[0091] FIG. 8 shows a scanning electron micrograph close to the inside of the roundel corresponding to FIG. 3;

[0092] FIG. 9 shows a scanning electron micrograph at a distance of 3 mm from the inside of the roundel from FIG. 8;

[0093] FIG. 10 shows a scanning electron micrograph at a distance of 8 mm from the inside of the roundel from FIG. 8;

[0094] FIG. 11 shows a scanning electron micrograph at a distance of 14 mm from the inside of the roundel from FIG. 8.

[0095] FIG. 1 shows a longitudinal section through a schematically represented filter candle 1 of the invention. The filter candle 1 comprises a hollow filter candle body 2, which is constructed substantially of aluminum silicate fibers. The filter candle body 2 has a collar element 4 at one end thereof and is closed in a dome shape at the opposite end. The filter candle body 2 possesses a length of 4000 mm and a wall thickness of about 20 mm in the region outside the collar element 4. In the region outside the collar element 4, the inside diameter is 110 mm and the outside diameter is 150 mm. The collar element 4 finishes flush with the inside of the region outside the collar element 4 and projects beyond said collar element 4 on the outside. The collar element 4 therefore likewise possesses an inside diameter of 110 mm and a greater outside diameter of 195 mm. The collar element 4 also has a height of 30 mm and a width of 42.5 mm.

[0096] The filter candle body 2 is impregnated exclusively on the inside 3 with a catalyst solution as specified in more detail in the examples below, so producing a concentration gradient from inside to outside in the wall of the filter candle body 2.

[0097] FIG. 2 shows a schematic representation of an apparatus for impregnating a filter candle body 2 according to FIG. 1 with a catalyst. The apparatus comprises a reservoir vessel 10 for the catalyst solution, which has a stirring facility 11. Leaving from the underside of the reservoir vessel 10 is a conduit 12 through which the catalyst solution can be conveyed by means of a conveying facility, configured as pump 13, into a spraying lance 14, held moveably in longitudinal direction, for impregnating the inside of the filter candle body 2. The spraying lance 14 at its end carries a conical nozzle 14a having a nozzle diameter of 4 mm, and is coupled to a motor drive 15 with which the longitudinal-side displacement of the spraying lance 14 can be accomplished. In the conduit 12, in the portion of conduit between the reservoir vessel 10 and the pump 13, there is a three-way valve 16, which serves for flushing/cleaning.

[0098] The apparatus additionally has a holder, not presently represented, for the filter candle body 2. This holder is formed by a plurality of rotatably mounted, rubberized roller pairs, of which one roller pair is provided with a motor drive unit in order to set the roller into rotation. Speed and direction of the rotary movement are adjusted via a controller, which is not shown. A stop facility movable via a rail, and likewise not shown, ensures that the collar element 4 of the filter candle body 2 always remains at a defined point even when it is rotating due to the motor-driven rollers, independently of the length of the filter candle body 2. Above the holder for the filter candle body 2 there is a suction withdrawal unit 17 for any spray mists that emerge. Positioned below the holder is a collecting vessel 18 for collecting excess catalyst solution or for removing flushing water.

[0099] In operation of the apparatus represented in FIG. 2, a filter candle body 2 is placed onto the rubberized roller pairs of the holder, with the collar element 4 of the filter candle body 2 being located by the stop facility not presently shown. The spraying lance 14 is centered in the filter candle body 2 and advanced up to a preadjustable point in the direction of the end piece of the filter candle body 2, by means of the motor drive 15. When this point has been reached, the motor-driven roller pair is started and the filter candle body 2 is set into rotation. The pump 13 as well is engaged at this point, and the catalyst solution is consequently sprayed onto the inside 3 of the filter candle body 2. A spraying pressure of 6 bar is established here. On the backward movement of the spraying lance 14 that is initiated at the same time, out of the filter candle body 2, the spraying operation is ended before the lance tip with the conical nozzle 14a leaves the filter candle body 2. This spraying operation is carried out at least once. If required, however, this operation may be repeated twice, three, four or five times, or more often. By repeating the spraying operation it is possible to increase the concentration of the catalyst in the filter candle body, with further maintenance of the concentration gradient within the wall thickness.

[0100] The filter candle body 2 thus impregnated is then dried at 100° C. for twelve hours and subsequently subjected to a thermal activation at 500° C. for 4 h, after which the filter candle 1 is complete.

[0101] FIG. 3 shows a schematic representation of a sample from a filter candle 1 of the invention in the form of a roundel having a diameter of 50 mm and a wall thickness of 20 mm of a filter candle of the invention in longitudinal section, this roundel having been taken from the filter candle 1 using a hole saw. In accordance with the regions shown in FIG. 3, the measurements were conducted across the thickness from inside, from the center, and from outside of the 50 mm roundel. FIGS. 4 to 7 show the x-ray spectra of these samples, obtained by means of energy-dispersive x-ray spectroscopy (EDX). It is apparent that with increasing distance from the inside of the filter candle, the concentration of titanium goes down and there is therefore a concentration gradient.

[0102] This can be recognized qualitatively not least from the scanning electron micrographs in FIGS. 8 to 11. These show the images, taken by a scanning electron microscope, of the regions used in FIGS. 1 to 7. These scanning electron microscope images were taken at a resolution of 90 μm. In the samples close to the inside of the filter candle 1, the deposits of the catalytic metal compounds are clearly apparent as small crystalline structures on the relatively smooth fiber surface of the filter candle body 2, and are delimited optically by the significantly lighter color. For the samples of the outside in FIGS. 9 and 10, these structures can virtually not be seen. The concentration gradient is therefore apparent simply from SEM micrographs.

[0103] Filter Candle 1 Example

[0104] A filter candle generated in accordance with the production method described for FIG. 2 and composed of aluminum silicate wool (ASW), the impregnation having taken place with a 5 wt % catalyst suspension containing 4.84 wt % of TiO.sub.2 and 0.16 wt % of V.sub.2O.sub.5. The impregnation rate was adjusted such that 5 kg of the aforesaid catalyst suspension were applied to the inside of the hollow filter candle body with the dimensions given above for FIG. 1. The solvent used was water and the dispersant employed was polyethylene glycol. The concentration of catalyst (sum total of TiO.sub.2 and V.sub.2O.sub.5) within the wall thickness after drying was 16 wt % on the inside, 13 wt % at the center and 9 wt % on the outside, measured in each case at a third of the wall thickness.

[0105] Filter Candle 2 Example

[0106] A filter candle generated in accordance with the production method described for FIG. 2 and composed of aluminum silicate wool (ASW), the impregnation having taken place with a 7.5 wt % catalyst suspension containing 7.26 wt % of TiO.sub.2 and 0.24 wt % of V.sub.2O.sub.5. The impregnation rate was adjusted such that around 5 kg of the aforesaid catalyst suspension were applied to the inside of the hollow filter candle body with the dimensions given above for FIG. 1. The solvent used was water and the dispersants employed were polyacrylates. The concentration of catalyst (sum total of TiO.sub.2 and V.sub.2O.sub.5) within the wall thickness after drying was 19 wt % on the inside, 17 wt % at the center and 14 wt % on the outside, measured in each case at a third of the wall thickness.

[0107] Filter Candle 3 Example

[0108] A filter candle generated in accordance with the production method described for FIG. 2 and composed of aluminum silicate wool (ASW), the impregnation having taken place with a 5 wt % catalyst suspension containing 4.9 wt % of TiO.sub.2 and 0.1 wt % of V.sub.2O.sub.5. The impregnation rate was adjusted such that 6 kg of the aforesaid catalyst suspension were applied to the inside of the hollow filter candle body with the dimensions given above for FIG. 1. The solvent used was water and the dispersants employed were polyacrylates. The concentration of catalyst (sum total of TiO.sub.2 and V.sub.2O.sub.5) within the wall thickness after drying was 18 wt % on the inside, 16 wt % at the center and 12 wt % on the outside, measured in each case at a third of the wall thickness.

[0109] Filter Candle 4 Example

[0110] A filter candle generated in accordance with the production method described for FIG. 2 and composed of alkaline earth metal silicate wool (AES), the impregnation having taken place with a 5 wt % catalyst suspension containing 4.84 wt % of TiO.sub.2, 1.8 wt % of WO.sub.3 and 0.16 wt % of NH.sub.4VO.sub.3. The impregnation rate was adjusted such that around 5 kg of the aforesaid catalyst suspension were applied to the inside of the hollow filter candle body. The solvent used was water and the dispersants employed were polyacrylates. The concentration of catalyst (sum total of TiO.sub.2, WO.sub.3 and V.sub.2O.sub.5) within the wall thickness after drying was 18 wt % on the inside, 14 wt % at the center and 10 wt % on the outside, measured in each case at a third of the wall thickness.

LIST OF REFERENCE SYMBOLS

[0111] 1 Filter candle [0112] 2 Hollow filter candle body [0113] 3 Inside of filter candle body [0114] 4 Collar element [0115] 10 Reservoir vessel [0116] 11 Stirring facility [0117] 12 Conduit [0118] 13 Conveying facility/pump [0119] 14 Spraying lance [0120] 14a Conical nozzle [0121] 15 Motor drive [0122] 16 Three-way valve [0123] 17 Suction withdrawal unit [0124] 18 Collecting vessel