PARTICULATE FILTERS

Abstract

Methods for treating a filter (2) for filtering particulate matter from exhaust gas are disclosed in which a dry powder (4) is sprayed towards an inlet face of the filter (2) entrained in a primary flow of gas to pass through the inlet face to contact a porous structure of the filter.

The back pressure of the filter (2) is monitored during spraying of the dry powder (4) and the spraying of the dry powder (4) is stopped when a back pressure of the filter (2) reaches a required value. The required value may equal an absolute back pressure, or a pre-determined target back pressure for the filter minus an offset pressure, or an estimated back pressure of the filter.

Claims

1. A method for treating a filter for filtering particulate matter from exhaust gas, the method comprising the steps of: a) containing a dry powder in a reservoir; b) locating a filter in a filter holder, the filter comprising a porous substrate having an inlet face and an outlet face, the inlet face and the outlet face being separated by a porous structure; c) establishing a primary gas flow through the porous structure of the filter by applying a pressure reduction to the outlet face of the filter; d) transferring the dry powder from the reservoir to a spray device located upstream of the inlet face of the filter; e) spraying the dry powder, using the spray device, towards the inlet face of the filter such that the dry powder is entrained in the primary gas flow and passes through the inlet face of the filter to contact the porous structure; and f) monitoring a back pressure of the filter during at least step e) and stopping the spraying of the dry powder towards the inlet face of the filter when:
p.sub.BP≥p.sub.target−p.sub.offset where, p.sub.BP is the back pressure of the filter; p.sub.target is a pre-determined target back pressure for the filter; and p.sub.offset is a pre-chosen offset pressure.

2. The method of claim 1, wherein, in step f), the spraying of the dry powder towards the inlet face of the filter is stopped when p.sub.BP≥p.sub.target−p.sub.offset is true for a minimum time period, t.sub.min, wherein t.sub.min≥0.5 s.

3. The method of claim 1, wherein p.sub.offset is 2-5 mbar.

4. The method of claim 1, wherein, in step f), the back pressure of the filter is monitored continuously during at least step e) by measuring the back pressure at a sample rate of ≥1 samples.Math.s.sup.−1.

5. The method of claim 1, further comprising in step c), monitoring a back pressure of the filter and passing the primary gas flow through the porous structure of the filter until the back pressure of the filter is stable before starting step d).

6. The method of claim 5, wherein the pre-determined target back pressure for the filter, p.sub.target, is relative to the stable back pressure monitored in step c).

7. The method of claim 1, further comprising the step: g) monitoring the back pressure of the filter and maintaining the primary gas flow through the porous structure of the filter after the spraying of the dry powder has been stopped until the back pressure of the filter is stable.

8. The method of claim 5, wherein the back pressure of the filter is considered stable when: i) the primary gas flow rate is within 0.5% of a predetermined flow rate; ii) the first derivative of the primary gas flow rate is ≤±0.15 m.sup.3hr.sup.−1.Math.s.sup.−1; and iii) the first derivative of the back pressure of the filter is ≤±1.5 mbar.Math.s.sup.−1.

9. The method of claim 1, wherein the back pressure, p.sub.BP, is an absolute back pressure measured relative to atmospheric pressure.

10. The method of claim 1, wherein the method uses a coating apparatus for carrying out at least steps a) to f), wherein the pre-determined target back pressure, p.sub.target, for the filter is calculated by: choosing a desired back pressure for the filter as would be measured by a testing apparatus different to the coating apparatus; establishing a calibration map for converting back pressures measured on the testing apparatus to back pressures measured on the coating apparatus; and converting the desired back pressure using the calibration map to establish the pre-determined target back pressure, p.sub.target.

11. A method for treating a filter for filtering particulate matter from exhaust gas, the method comprising the steps of: a) containing a dry powder in a reservoir; b) locating a filter in a filter holder, the filter comprising a porous substrate having an inlet face and an outlet face, the inlet face and the outlet face being separated by a porous structure; c) establishing a primary gas flow through the porous structure of the filter by applying a pressure reduction to the outlet face of the filter while monitoring a back pressure of the filter, and passing the primary gas flow through the porous structure of the filter until the back pressure of the filter is stable; d) subsequently to establishing the stable back pressure of the filter, transferring the dry powder from the reservoir to a spray device located upstream of the inlet face of the filter; e) spraying the dry powder, using the spray device, towards the inlet face of the filter such that the dry powder is entrained in the primary gas flow and passes through the inlet face of the filter to contact the porous structure while monitoring the back pressure of the filter; f) stopping the spraying of the dry powder towards the inlet face of the filter; and g) continuing to monitor the back pressure of the filter while maintaining the primary gas flow through the porous structure of the filter after the spraying of the dry powder has been stopped until the back pressure of the filter is stable.

12-16. (canceled)

17. A method for treating a filter for filtering particulate matter from exhaust gas, the method comprising the steps of: a) containing a dry powder in a reservoir; b) locating a filter in a filter holder, the filter comprising a porous substrate having an inlet face and an outlet face, the inlet face and the outlet face being separated by a porous structure; c) establishing a primary gas flow through the porous structure of the filter by applying a pressure reduction to the outlet face of the filter; d) transferring the dry powder from the reservoir to a spray device located upstream of the inlet face of the filter; e) spraying the dry powder, using the spray device, towards the inlet face of the filter such that the dry powder is entrained in the primary gas flow and passes through the inlet face of the filter to contact the porous structure; and f) monitoring a back pressure of the filter during at least step e) and stopping the spraying of the dry powder towards the inlet face of the filter when:
p.sub.est≥p.sub.target where, p.sub.target is a pre-determined target back pressure for the filter; and p.sub.est is an estimated final back pressure of the filter; wherein p.sub.est is calculated by extrapolation of measured back pressure data for the filter that is obtained during step f).

18-20. (canceled)

21. The method of claim 1, further comprising the step of calcining the filter after loading with the dry powder.

22. The method of claim 1, further comprising before step b) coating the filter with a catalyst washcoat.

23. The method of claim 1, comprising providing a maximum loading of the filter of 10 to 40 g/l.

24. The method of claim 1, wherein the dry powder has a tapped density of 1 to 3 g/cm.sup.3.

25. The method of claim 1, wherein the dry powder has a d50 (by volume) less than 10 microns.

26. The method of claim 1, wherein in step d) a secondary gas flow, separate to the primary gas flow, is used to transfer the dry powder from the reservoir to the spray device.

27-28. (canceled)

29. A filter obtainable by the method of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0127] Aspects and embodiments of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0128] FIG. 1 is a schematic diagram of an apparatus for treating a filter for filtering particulate matter from exhaust gas according to the present disclosure;

[0129] FIG. 2 is a flow diagram illustrating a method for manufacturing a filter according to the present disclosure incorporating a method for treating a filter using the apparatus of FIG. 1;

[0130] FIG. 3 is a flow diagram illustrating a method for treating a filter for filtering particulate matter from exhaust gas using the apparatus of FIG. 1;

[0131] FIG. 4 is a graph showing back pressure against time for two filters treated with different mass loadings of dry powder;

[0132] FIGS. 5 and 6 are graphs showing back pressure against time for two filters subject to two different regimes for spraying the dry powder;

[0133] FIG. 7 shows a calibration map comparing cold flow back pressure (CFBP) on a coating apparatus and a testing apparatus;

[0134] FIGS. 8 and 9 show further calibration maps comparing CFBP on a coating apparatus and a testing apparatus;

[0135] FIG. 10 shows a graph showing back pressure against time for eights filters;

[0136] FIG. 11 shows a graph showing back pressure against time for two of the eight filters of FIG. 10; and

[0137] FIG. 12 is a graph showing back pressure for one of the eight filters of FIG. 10 subjected to two different regimes for spraying the dry powder.

DETAILED DESCRIPTION

[0138] The skilled reader will recognise that one or more features of one aspect or embodiment of the present disclosure may be combined with one or more features of any other aspect or embodiment of the present disclosure unless the immediate context teaches otherwise.

[0139] An example of an apparatus for performing the methods of the present disclosure will now be described with reference to FIG. 1 which shows a schematic diagram of an apparatus 1 for treating a filter 2 for filtering particulate matter from exhaust gas. The filter 2 is of a type that comprises a porous substrate having an inlet face and an outlet face, the inlet face and the outlet face being separated by a porous structure.

[0140] The apparatus 1 comprises a reservoir 3 for containing a dry powder 4. A filter holder 5 is provided for holding the filter 2. A vacuum generator 6 is provided for establishing in use a primary gas flow through the porous structure of the filter 2 by applying a pressure reduction to the outlet face of the filter 2. A transport device 8 is provided for transporting the dry powder 4 from the reservoir 3 to a spray device 7. The spray device 7 is provided for receiving the dry powder 4 from the transport device 8 and spraying the dry powder 4 towards the inlet face of the filter 2. A controller 9 is provided which is configured to control operation of the apparatus 1.

[0141] The reservoir 3 may receive dry powder 4 from a dry powder inlet 11. The dry powder inlet 11 may be an output of an upstream bulk supply of the dry powder. For example, the dry powder inlet 11 may be a conduit connected upstream to a further reservoir of the dry powder 4. The dry powder inlet 11 may represent a manual, semi-automatic or automatic re-filling of the reservoir 3 through a lid or opening of the reservoir 3.

[0142] The reservoir 3 may comprise one or more hoppers. The reservoir 3 may comprise one hopper. In the illustrated example of FIG. 1, the reservoir 3 comprises a first hopper 12 and a second hopper 13. The second hopper 13 may be downstream of the first hopper 12 to receive dry powder 4 output from the first hopper 12. The one or more hoppers may be provided in separate housings. Alternatively, the one or more hoppers may be provided in a single housing. The one or more hoppers may comprise one or more chambers of a single container.

[0143] The reservoir 3 may comprise a dosing device 15. The dosing device 15 may dose the dry powder 4 by one or more of by weight, by volume, by particle number, by time. The dosing device 15 may be located at or near an outlet of the reservoir 3. The dosing device 15 may be located at or near an outlet of the one or more hoppers of the reservoir 3. The dosing device may be located at or near the outlet of the first hopper 12.

[0144] The dosing device 15 may be gravimetrically fed with the dry powder 4 from the reservoir 3.

[0145] The dosing device 15 may be a loss in weight feeder. The dosing device 15 may be a volumetric feeder comprising an auger or screwthread arrangement. Non-limiting examples of suitable dosing devices include the Coperion® K-iron Type K2-ML-T35 Gravimetric twin screw feeder available from Coperion GmbH, Stuttgart, Germany, the All-Fill® Series 51 Micro-Fill and the All-Fill® Series 10 gravimetric or volumetric auger fillers available from All-Fill International Ltd, Sandy, UK.

[0146] The transport device 8 transports the dry powder 4 from the reservoir 3 to the spray device 7. The transport device 8 may gravimetrically or volumetrically feed the dry powder 4 at least part way towards the spray device 7.

[0147] The transport device 8 may comprise one or more components. The transport device 8 may comprise one or more conduits, for example, passages, pipes, hoses, etc.

[0148] Where the reservoir 3 comprises more than one hopper the transport device 8 may transport the dry powder 4 between the hoppers. The transport device 8 may gravimetrically or volumetrically feed the dry powder 4 between the hoppers. The transport device 8 may comprise a first conduit 14 extending between the first hopper 12 and the second hopper 13. The first conduit 14 may extend from a first housing to a second housing. Alternatively, the first conduit 14 may extend from a first chamber to a second chamber of a single container. The dry powder 4 may be gravimetrically fed along the first conduit 14.

[0149] The transport device 8 may comprise a second conduit 16 extending from the second hopper 13 to the spray device 7.

[0150] The spray device 7 is provided for receiving the dry powder 4 from the transport device 8 and spraying the dry powder 4 towards the inlet face of the filter 2. The spray device 7 may comprise a secondary gas flow generator for generating a secondary gas flow that may be used to spray the dry powder 4 towards the inlet face of the filter 2.

[0151] The spray device 7 may further comprise one or more outlets for discharging the dry powder 4 towards the inlet face of the filter 2. The one or more outlets of the spray device may comprise an aperture size of 0.5 to 10 mm. The aperture may be circular, part-circular or slot-shaped. The one or more outlets may be one or more fixed outlets. Alternatively, the one or more outlets may be one or more mobile outlets, for example one or more oscillating outlets.

[0152] The one or more outlets may be provided in one or more nozzles. Each of the one or more nozzles may comprise one or more spray outlets. In the illustrated example of FIG. 1 a single nozzle 25 is provided which comprises a plurality of spray outlets.

[0153] The secondary gas flow generator may comprise a compressed gas generator. In the illustrated example of FIG. 1 the secondary gas flow generator comprises a compressed air generator which may comprise a compressor 22. The compressor 22 may receive air from an air inlet 21 and supply compressed air to the one or more outlets of the spray device 7 via a feed line 23. A return line 24 may be provided. Valves and controls necessary for operation may be provided as will be known to the skilled person.

[0154] An interconnection between the transport device 8 and the spray device 7 may be provided at which interconnection the dry powder 4 is transferred into the spray device 7 from the transport device 8. The interconnection may be provided at or near the one or more outlets of the spray device 7. In one example, the interconnection may be provided in the nozzle 25. Alternatively, the interconnection may be provided at or near the reservoir 3, for example at or near the second hopper 13 of the reservoir 3. In one example, the interconnection is a fluid connection between the feed line 23 and the second conduit 16. For example, the secondary gas flow of the spray device 7 may be fluidly connected with the second conduit 16 at or near an outlet of the second hopper 13 to fluidize the dry powder 4 to assist transport of the dry powder 4 along at least a portion of the second conduit 16. In another example, the dry powder 4 may be fed by gravity along the second conduit 16. The flow of the dry powder 4 down the second conduit 16 may be assisted by a suction force that draws the dry powder 4 towards an outlet of the second conduit 16. For example the spray nozzle 25 may produce a suction force to assist drawing of the dry powder 4 along the second conduit 16 and through the spray nozzle 25. For example the spray nozzle 25 may utilise the secondary gas flow from the feed line 23 to produce a suction force in a powder flow conduit of the spray nozzle 25 by means of a venturi device or similar apparatus.

[0155] In one example the spray device 7 comprises a compressed air gun. A non-limiting example of a suitable compressed air gun is the STAR Professional gravity feed spray gun 1.4 mm, part no. STA2591100C.

[0156] The filter holder 5 may function to maintain the filter 2 in a stationary position during treatment. The filter holder 5 may grip an upper and/or a lower end of the filter 2. The filter holder 5 may comprise an inflatable upper seal bladder 31 (also called an upper inflatable collar) and/or an inflatable lower seal bladder 30 (also called a lower inflatable collar) that support respective upper and lower ends of the filter 2. The inflatable upper seal bladder 31 and the inflatable lower seal bladder 30 may contact and/or engage with an exterior surface of the filter 2. Each may form a liquid or air-tight seal around the filter 2. The inflatable upper seal bladder 31 and the inflatable lower seal bladder 30 may be supported by one or more housings (e.g. supported by an internal wall of the one or more housings).

[0157] The apparatus 1 may be configured such that the filter 2 is located in the filter holder 5 in a vertical orientation with the inlet face of the filter uppermost. At least a portion of the spray device 7 may be located vertically above the inlet face. A spray direction of the spray device 7 may be co-axial with a longitudinal axis of the filter 2. The spray direction and the longitudinal axis of the filter 2 may be coincident.

[0158] The apparatus 1 may further comprise a flow conduit 10 located between the spray device 7 and the inlet face of the filter 2. The flow conduit 10 may function to constrain and channel the primary gas flow towards the inlet face of the filter 2. The flow conduit 10 may function to align the primary gas flow such that a flow direction of the primary gas flow when it contacts the inlet face of the filter 2 is normal to the inlet face.

[0159] The flow conduit 10 may be empty so as to provide an unimpeded flow path between the spray device 7 and the inlet face of the filter 2. Alternatively, the flow conduit 10 may comprise a flow conditioner interposed between the spray device 7 and the inlet face of the filter 2, the flow conditioner acting to promote dispersion of the dry powder 4. For example, the flow conditioner may comprise one or more of a static mixer, a mesh, a sieve, a baffle, and an orificed plate.

[0160] The flow conduit 10 may comprise a tube. The flow conduit 10 may comprise a cross-sectional shape that matches the cross-sectional shape of the inlet face of the filter 2. The flow conduit 10 may comprise a size that matches the size of the inlet face of the filter 2.

[0161] The spray device 7 may extend into the flow conduit 10. The one or more outlets of the spray device 7 may be located within the flow conduit 10. For example, the nozzle 25 may be located within an upper region of the flow conduit 10. The nozzle 25 may be located coincident with a longitudinal axis of the filter 2.

[0162] The inlet face of the filter 2 may be located greater than 10 cm, optionally greater than 20 cm from the spray device, for example from the nozzle 25 of the spray device 7. Particular benefit may be found when the inlet face of the filter 2 is located greater than 75 cm, optionally greater than 100 cm from the nozzle outlet of the spray nozzle 25. Additionally, or alternatively the spray device, for example from the nozzle 25 of the spray device 7, may be located at a distance from the inlet face of the filter 2 that is up to 4 times a diameter of the inlet face of the filter 2.

[0163] The vacuum generator 6 is provided for establishing in use the primary gas flow through the porous structure of the filter 2 by applying a pressure reduction to the outlet face of the filter 2. The vacuum generator 6 may comprise a vacuum cone 40 that may define a funnel that engages the outlet face of the filter 2. The inflatable lower seal bladder 30 may form a seal between the outlet face of the filter 2 and the vacuum cone 40. The vacuum generator 6 may comprise a vacuum pump 42 connected to the flow cone by a conduit 43. The vacuum pump 42 may be controlled to control the volumetric flow rate of the primary gas flow.

[0164] The vacuum generator 6 may be provided with a volumetric flow rate sensor. The volumetric flow rate sensor may be an orifice plate 44 in combination with pressure sensors 45 located along the conduit 43. The vacuum generator 6 may comprise a bypass conduit 46 extending to an intake 47.

[0165] The apparatus 1 may further comprises a pressure sensor 41 for monitoring a back pressure of the filter 2. A single pressure sensor 41 may be used. The single pressure sensor 41 may be located in the vacuum generator 6, preferably in a filter holder or other housing, for example the vacuum cone 40, of the vacuum generator.

[0166] The controller 9 controls operation of at least the vacuum generator 6 and the spray device 7. In FIG. 1 operative connections between the controller 9 and a remainder of the apparatus 1 are omitted for clarity. However, the skilled person would be aware that necessary connections of any suitable means may be provided. Such connections may be wired or wireless.

[0167] The controller 9 may be configured to control the transfer of the dry powder 4 from the reservoir 3 to the spray device 7 by the transport device 8 independently of controlling the primary gas flow produced by the vacuum generator 6. For example, the controller 9 may control operation of the dosing device 15.

[0168] The controller 9 may be configured to control spraying of the dry powder 4 towards the inlet face of the filter 2 independently of controlling the primary gas flow. Use of the term ‘independently’ in the present specification refers to the ability of the controller 9 to control each of the variables of the spraying of the dry powder 4 and the primary gas flow individually and irrespective of the status of the other variable. For example, the controller 9 may establish the primary gas flow without simultaneously spraying the dry powder 4. For example, the controller 9 may increase or decrease the rate of spraying of the dry powder 4 without altering the volumetric flow rate of the primary gas flow. For example, the controller 9 may increase or decrease the volumetric flow rate of the primary gas flow without altering the rate of spraying of the dry powder 4. For example, the controller 9 may control operation of the spray device 7 independently of controlling operation of the vacuum pump 42.

[0169] The controller 9 may be configured to operate the vacuum generator 6 to establish the primary gas flow before the dry powder 4 is transferred to the spray device 7 and sprayed towards the inlet face of the filter 2.

[0170] The controller 9 may be configured to control the secondary gas flow generator, for example the compressor 22, independently of the vacuum generator 6. The controller 9 may be configured to operate the vacuum generator 6 to maintain the primary gas flow as a continuous gas flow through the porous structure and to operate the secondary gas flow generator, for example the compressor 22, for only a portion of a period of the primary gas flow.

[0171] The controller 9 may be configured to control the vacuum generator 6 to control a level of the pressure reduction applied to the outlet face of the filter 2 independently of controlling the transport device 8 and/or the spray device 7 to control a speed or mass rate of the dry powder 4 sprayed towards the inlet face of the filter 2.

[0172] The controller 9 may be configured to stop the spraying of the dry powder 4 towards the inlet face of the filter 2 when a required value of the back pressure of the filter 2 is reached, for example as detected by the pressure sensor 41.

[0173] The apparatus 1 may be used to treat a filter with a dry powder 4 comprising or consisting of one or more refractory powders, optionally comprising one or more fumed refractory powders, and or one or more aerogels. Additionally or alternatively, the apparatus 1 may be used to treat a filter with a dry powder 4 comprising or consisting of a metal compound for forming by thermal decomposition a metal oxide. In examples the metal compound may comprise or consist of a metal hydroxide, a metal phosphate, a metal carbonate, a metal sulphate, a metal perchlorate, a metal iodide, a metal oxalate, a metal acetate, a metal chlorate or a mixture thereof.

[0174] An example of a method of treating a filter in accordance with the present disclosure will now be described with reference to FIG. 2 which shows a flow diagram illustrating a method for manufacturing a filter 2 incorporating use of the apparatus 1. By way of example only the method will be described with reference to a filter 2 provided with a catalytic coating.

[0175] In step S21 a catalytic slurry is prepared by methods as known in the art.

[0176] In step S22 a washcoat is prepared from the catalytic slurry by methods as known in the art. The washcoat may be, for example, a hydrocarbon trap, a three-way catalyst (TWC), a NOx absorber, an oxidation catalyst, a selective catalytic reduction (SCR) catalyst, a lean NOx catalyst and combinations of any two or more thereof.

[0177] In step S23 the washcoat is dosed and applied to a bare filter 2 by methods as known in the art. For example the washcoat may be applied to a first face of the filter 2 (e.g. an upper face) and an opposite, second face (e.g. a lower face) of the filter 2 may be subjected to at least a partial vacuum to achieve movement of the washcoat through the porous structure of the filter 2. The filter 2 may be coated in a single dose wherein washcoat may be applied to the filter 2 in a single step with the filter 2 remaining in a single orientation. Alternatively, the filter 2 may be coated in two doses. For example, in a first dose the filter 2 may be in a first orientation with a first face uppermost and a second face lowermost. A coating may be applied to the first face and coats a portion of the length of the filter 2. The filter 2 may then be inverted so that the second face is uppermost. A coating may then be applied to the second face in order to coat the portion of the filter 2 that was uncoated by the first dose. Beneficially, a two-dose process may allow different coatings to be applied to each end of the filter 2.

[0178] In step S24 the filter 2 may be dried.

[0179] In step S25 the filter 2 may be calcined by methods as known in the art.

[0180] In optional step S26 the back pressure of the filter 2 before treatment may be measured.

[0181] In optional step S27 the filter 2 may be placed in stock to await treatment. Thereafter, in step S28 the filter 2 may be removed from stock and passed for treatment. Alternatively, the filter 2 may be treated immediately, i.e. by proceeding directly to step S29.

[0182] In step S29 the filter 2 is treated according to the present disclosure as will be described in further detail below with reference to FIG. 3.

[0183] In step S30, after treatment, the filter 2 may be calcined.

[0184] Calcining the filter may be carried out at a temperature selected to produce thermal decomposition of the dry powder 4.

[0185] The calcination temperature may be selected to be at least 150° C., optionally at least 250° C., optionally at least 500° C.

[0186] In some embodiments it is preferred that the calcination temperature is no greater than 550° C. However, in other embodiments, the calcination temperature may be selected to be greater than 550° C. The calcination temperature may be selected to be up to 900° C., optionally up to 1150° C.

[0187] In one example the calcination temperature may be selected to be between 300° C. and 500° C. In another example the calcination temperature may be selected to be about 520° C. In another example the calcination temperature may be selected to be about 580° C. In another example the calcination temperature may be selected to be about 900° C.

[0188] The calcination may be carried out for a period of 30 to 90 minutes, optionally 30 to 60 minutes. In one example the period is about 35 minutes. In another example the period is about 60 minutes. Within the calcination, the dwell time of 1 to 15 minutes, preferably 5 to 10 minutes

[0189] In optional step S31 the back pressure of the filter 2 after treatment may be measured.

[0190] In step S32 the finished filter 2 may be readied for delivery to a customer.

[0191] FIG. 3 shows a flow diagram illustrating the treatment of step S29 of FIG. 2.

[0192] In step S29-1 the filter may be loaded into the filter holder 5. The filter 2 may be held in a stationary position during treatment. The filter 2 may be gripped by the filter holder 5 at an upper and/or a lower end of the filter 2. The inflatable upper seal bladder 31 and the inflatable lower seal bladder 30 may be inflated to contact and/or engage with the exterior surface of the filter 2. The filter 2 may be held in a vertical orientation with the inlet face of the filter uppermost. Operation of the filter holder 5, for example inflation of the inflatable upper seal bladder 31 and the inflatable lower seal bladder 30 may be controlled by the controller 9.

[0193] In step S29-2 the vacuum generator 6 may activated by the controller 9 to establish the primary gas flow through the filter 2. Preferably, the primary gas flow is established before the dry powder 4 is transferred to the spray device 7 and sprayed towards the inlet face of the filter 2. A level of the pressure reduction generated by the vacuum generator 6 may be controlled by the controller 9 independently of a speed or mass rate of the transfer of the dry powder 4 from the reservoir 3 to the spray device 7. The primary gas flow may have a volumetric flow rate of 10 m.sup.3/hr to 5,000 m.sup.3/hr, preferably 400 m.sup.3/hr to 2,000 m.sup.3/hr, preferably 600 m.sup.3/hr to 1000 m.sup.3/hr.

[0194] In step S29-3 the back pressure of the filter 2 may be measured while the primary gas flow is established but before the secondary gas flow is established. The back pressure may be measured in this step until the back pressure has stabilised.

[0195] The back pressure of the filter 2 may be considered stable when the first derivative of the primary gas flow rate is ≤±X m.sup.3hr.sup.−1.Math.s.sup.−1, wherein X=0 to 0.30, optionally X=0.10 to 0.20, X=0.15. Additionally or alternatively, the back pressure of the filter 2 may be considered stable when the first derivative of the back pressure of the filter is ≤±Y mbar.Math.s.sup.−1, wherein Y=0.5 to 3.0, optionally Y=1.0 to 2.0, optionally Y=1.5. Additionally or alternatively, the back pressure of the filter 2 may be considered stable when the primary gas flow rate is within Z % of a predetermined flow rate, wherein Z=1.5, optionally Z=1.0, optionally Z=0.5.

[0196] The back pressure may be measured by use of the pressure sensor 41. The back-pressure measurement in step S29-3 may be in addition to, or in place of the back-pressure measurement of step S26. Alternatively, the back-pressure measurement of step S26 may be used in place of the back-pressure measurement of step S29-3. The back-pressure measurement of step S26 and/or the back-pressure measurement of step S29-3 may be used by the controller 9 as a measure of a first back pressure of the filter 2 prior to treatment.

[0197] In step S29-4 the dry powder 4 is sprayed at the inlet face of the filter 2 by the spray device 7. During spraying of the dry powder 4 the dry powder 4 may be supplied to the spray device 7 by the transport device 8.

[0198] The spraying of the dry powder 4 towards the inlet face of the filter 2 is preferably controllable by the controller 9 independently of establishing and controlling the primary gas flow.

[0199] During step S29-4 the secondary gas flow, for example supplied by the compressor 22, which is separate to the primary gas flow, may be used to transfer the dry powder 4 from the reservoir 3 to the spray device 7. Preferably the secondary gas flow is controllable by the controller 9 independently of the primary gas flow. For example, the controller 9 may control operation of the compressor 22 and/or the valves and/or the nozzle 25 of the spray device 7 independently of controlling operation of the vacuum pump 42. The dry powder 4 may be sprayed towards the inlet face of the filter 2 by use of the secondary gas flow. The secondary gas flow may comprise a flow of compressed gas, preferably air.

[0200] During step S29-4 the primary gas flow is preferably maintained as a continuous flow. During step S29-4 the secondary gas flow may be applied as a single burst or a plurality of intermittent bursts.

[0201] In step S29-5 the back pressure of the filter 2 is monitored while the dry powder 4 is sprayed. The back pressure may be monitored by use of the pressure sensor 41. The controller 9 may be configured to stop the spraying of the dry powder 4 towards the inlet face of the filter 2 when the back pressure of the filter 2 reaches a required value. If the required value for the back pressure has not yet been reached, then the controller 9 be configured to return to step S29-4 and continue spraying of the dry powder 4. This feedback may be continuous and need not involve any pause in the spraying of the dry powder 4, i.e. the controller 9 may continuously monitor the back pressure of the filter 2 as spraying of the dry powder 4 is on-going. The back pressure of the filter 2 may be monitored continuously during at least step S29-5. The back pressure may be continuously monitored by measuring the back pressure at a sample rate of ≥1 samples.Math.s.sup.−1, optionally ≥5 samples.Math.s.sup.−1, optionally ≥10 samples.Math.s.sup.−1. The pressure sensor 41 may incorporate a degree of damping to reduce or eliminate the effects of transient readings that may be noise-related.

[0202] In step S29-6 the spraying of the dry powder 4 is stopped. For example, this may be achieved by the controller 9 stopping transfer of the dry powder by the transport device 8 and/or by stopping the secondary gas flow of the spray device 7. In step S29-6 the primary gas flow may be maintained through the porous structure of the filter 2 for a time period after the stopping of the spraying of the dry powder 4. The controller 9 may be configured to operate the vacuum generator 6 for a time period after the stopping of the spraying of the dry powder 4. For example, the primary gas flow may be maintained until the back pressure measured in this step has stabilised.

[0203] As above, the back pressure of the filter 2 may be considered stable when the first derivative of the primary gas flow rate is ≤±X m.sup.3hr.sup.−1.Math.s.sup.−1, wherein X=0 to 0.30, optionally X=0.10 to 0.20, X=0.15. Additionally or alternatively, the back pressure of the filter 2 may be considered stable when the first derivative of the back pressure of the filter is ≤±Y mbar.Math.s.sup.−1, wherein Y=0.5 to 3.0, optionally Y=1.0 to 2.0, optionally Y=1.5. Additionally or alternatively, the back pressure of the filter 2 may be considered stable when the primary gas flow rate is within Z % of a predetermined flow rate, wherein Z=1.5, optionally Z=1.0, optionally Z=0.5.

[0204] Optionally, in step S29-6 the quantity of dry powder 4 delivered towards the inlet face of the filter 2 may be measured. The controller 9 be configured to determine the quantity of the dry powder 4 delivered from signal outputs from the dosing device 15, for example from an output from the loss in weight feeder.

[0205] The method may be configured to deliver a maximum loading of the filter of 10 to 40 g/l, optionally 15 to 30 g/l, optionally about 20 g/l of the dry powder 4; or to deliver a maximum loading of the filter of <10 g/l, optionally <5 g/l, optionally <2 g/l of the dry powder 4.

[0206] In step S29-7 the primary gas flow through the filter 2 is stopped. This may be achieved by the controller 9 stopping the vacuum generator 6, i.e. stopping the vacuum pump 42.

[0207] Alternatively, this may be achieved by the controller operating valves of the vacuum generator 6 to divert the suction through the bypass conduit 46 to draw air in through intake 47. This may avoid the need to stop the vacuum pump 42 between treatments of successive filters 2 which may lead to a faster cycle time.

[0208] In step S29-8 the filter 2 is unloaded from the filter holder 5 by, for example, deflating the inflatable upper seal bladder 31 and the inflatable lower seal bladder 30. The filter 2 may then be removed and passed to step S30 as described above.

[0209] Returning to Step 29-5, the event triggering the stopping of the spraying of the dry powder 4 will be discussed in more detail.

[0210] When treating filters 2, the present applicant has found that the final back pressure of the filter cannot be controlled simply by controlling the mass loading of dry powder 4 applied to the filter 2. For example, FIG. 4 illustrates treatment of two filters, A and B, that have similar initial back pressures prior to coating (36 and 38 mbar). After treatment with the dry powder 4, both filters have similar back pressures of 64 and 67 mbar. However, the mass loading of the filters was very different. Filter A was treated with 2.8 g of dry powder that was sprayed over a period of 7 s. Filter B was treated with 8 g of dry powder that was sprayed over a period of 20 s. The difference in the required mass loading of dry powder to achieve a similar increase in back pressure is believe to be due to differences in the substrate properties of the filter including, but not limited to pore size and location of any applied washcoat.

[0211] Consequently, it can be beneficial to monitor the back pressure of the filter 2 during spraying of the dry powder 4 in step S29-5 and, as noted above, for the controller 9 to stop the spraying of the dry powder 4 towards the inlet face of the filter 2 when the back pressure of the filter 2 reaches a required value.

[0212] In some examples, the required value of the back pressure of the filter 2 when the spraying of the dry powder 4 is stopped may be a predetermined target back pressure, p.sub.target, of the filter 2. The pre-determined back pressure, p.sub.target, may be the desired final back pressure of the filter 2 as measured by the apparatus 1, i.e. the coating apparatus. The pre-determined back pressure, p.sub.target, may be an absolute back pressure. The absolute back pressure may be, for example, between 20-180 mbar at a flowrate of 600 m.sup.3/hr.

[0213] The present applicant has discovered that if the required value is chosen as the pre-determined back pressure, p.sub.target, then the final back pressure of the filter 2 may overshoot the target. For example, as illustrated in the example of FIG. 5 a filter was treated with a target for the pre-determined back pressure, p.sub.target, of 63 mbar. The dry powder spraying commenced at ˜17 seconds when the back pressure of the filter had stabilised and was stopped at ˜29 seconds when the back pressure of the filter, p.sub.BP, reached the target of 63 mbar. However, the back pressure of the filter continued to increase, i.e. overshot, and eventually stabilised at around 70 mbar.

[0214] Consequently, the present applicant has discovered that an improved method of treatment may be achieved by, in Step S29-5, monitoring the back pressure of the filter 2 and stopping the spraying of the dry powder 4 towards the inlet face of the filter 2 when:

p.sub.BP≥p.sub.target−p.sub.offset [0215] where, [0216] p.sub.BP is the back pressure of the filter 2; [0217] p.sub.target is a pre-determined target back pressure for the filter 2; and [0218] p.sub.offset is a pre-chosen offset pressure.

[0219] For example, FIG. 6 shows a modified treatment process where, as above, the target for the pre-determined back pressure, p.sub.target, was 63 mbar. An offset pressure, p.sub.offset, of 3 mbar was selected. The dry powder spraying commenced at ˜14 seconds when the back pressure of the filter had stabilised and was stopped at ˜37 seconds when the back pressure of the filter, pep, reached the 60 mbar, i.e. 63 mbar-3 mbar. The back pressure of the filter continued to increase after the cessation of spraying of the dry powder and eventually stabilised at around 63 mbar, i.e. at the desired final back pressure.

[0220] The pre-determined target back pressure for the filter, p.sub.target, may be an absolute back pressure of the filter 2. For example, the pre-determined target back pressure for the filter 2, p.sub.target, may be a target back pressure of 20-180 mbar. Alternatively, the pre-determined target back pressure for the filter 2, p.sub.target, may be a relative back pressure of the filter 2. For example, the pre-determined target back pressure for the filter 2, p.sub.target, may be a back pressure that is relative to an initial back pressure of the filter 2 before spraying of the dry powder 4. For example, the pre-determined target back pressure for the filter 2, p.sub.target, may be 105% to 200%, optionally 125% to 150% of the initial back pressure of the filter 2.

[0221] The spraying of the dry powder 4 towards the inlet face of the filter 2 may be stopped when p.sub.BP≥p.sub.target−p.sub.offset is true for a minimum time period, t.sub.min. The minimum time period may be t.sub.min≥0.1 s, optionally ≥0.5 s, optionally ≥1.0 s.

[0222] The p.sub.offset may be, for example 1-10 mbar, optionally 2-5 mbar, optionally 3-5 mbar. For example, such a p.sub.offset may be used in cases where the p.sub.target is 20-180 mbar.

[0223] The pre-determined target back pressure, p.sub.target, for the filter may be calculated by: [0224] choosing a desired back pressure for the filter as would be measured by a testing apparatus different to the coating apparatus 1; [0225] establishing a calibration map for converting back pressures measured on the testing apparatus to back pressures measured on the coating apparatus 1; and [0226] converting the desired back pressure using the calibration map to establish the pre-determined target back pressure, p.sub.target.

[0227] For example, FIG. 7 shows a first example of a calibration map for converting between back pressures on the coating apparatus 1 (shown on the x-axis and titled ‘Inline CFBP) and back pressures on a testing apparatus, in this case a Superflow Flow Bench 1050 available from Superflow of Sussex, Wis., USA. Back pressures on both the coating apparatus 1 and the testing apparatus are Cold Flow Back Pressures (CFBP) measured in mbar at a flowrate of 600 m.sup.3/hr. As can be seen a linear relationship may be established in this case between the back pressures on both apparatus when measured at the same flowrates. In this example, for the range of CFBP of interest, the present application has found that a calibration map in the form of a linear or quadratic fit may accurately allow conversions of CFBP on the coating apparatus 1 and the testing apparatus. In the example of FIG. 7 the flow rates at which the CFBP was measured were both 600 M.sup.3/hr on the coating apparatus 1 and the testing apparatus.

[0228] Furthermore, the present applicant has found that a single linear or quadratic fit may accurately allow conversions of CFBP on the coating apparatus 1 and the testing apparatus irrespective of the chosen flow rate or the diameter, length or substrate characteristic of the filter 2, as long as the same chosen flow rate is used on the coating apparatus 1 and the testing apparatus. For example, FIG. 8 illustrates that a calibration map in the form of a single linear or quadratic fit accurately allows for conversion of CFBP for two different filters—labelled Part 1 and Part 2 in FIG. 8. Not only that, but the same linear or quadratic fit applies when both Parts 1 and 2 are tested at a flow rate of 600 M.sup.3/hr and when both are tested at 800 M.sup.3/hr.

[0229] It may be desired to convert between CFBP on the coating apparatus 1 and the testing apparatus where different flow rates are used on each apparatus. The present applicant has discovered that this is possible by using a calibration map that comprises a set of linear or quadratic fit lines. For example, FIG. 9 illustrates a calibration map with two fit lines. The left-most fit line represents conversion of CFBP for two different filters (Part 1 and Part 2) where the flow rate on the coating apparatus 1 and the testing apparatus were both 600 M.sup.3/hr. The right-most fit line represents conversion of CFBP for the same Part 1 and Part 2 where the flow rate on the coating apparatus 1 was 800 M.sup.3/hr and the flow rate on the testing apparatus was 600 M.sup.3/hr. It will be understood that the calibration map may comprise any required number of fit lines for any required combination of coating and testing CFBP.

[0230] Use of an offset pressure, p.sub.offset, as described above may produce improved and acceptable results for the final back pressures of the filters 2. However, the present applicant has discovered that further improvements may be made. For example, the present applicant has discovered that characteristics of the filters 2 may affect the loading of the dry powder and in particular that the rate of loading of the dry powder 4 with respect to time may be affected by characteristics including the pore size of the filter 2, porosity, wall thickness and/or cell density of the filter 2. Consequently, using a fixed offset pressure, p.sub.offset, as described above may still lead to some variation in the final back pressures obtained. This is illustrated in FIG. 10 which shows the powder loading phase for 8 filters 2 that have differing pore sizes. As can be seen, the gradient of the back pressure-time response curves differ between filters. Thus, use of a fixed offset pressure would not obtain a uniform final back pressure for all 8 filters 2. For the 8 example filters of FIG. 10, the final obtained back pressures had a mean back pressure of 64.5 mbar (compared to a target of 63 mbar) and a standard deviation of 1.2 mbar.

[0231] For example, FIG. 11 illustrates this using, by way of example, filter numbers 4 and 6 from FIG. 10. In both cases the spraying of the dry powder 4 is stopped when the filter reaches 60 mbar (a target back pressure of 63 mbar minus an offset pressure of 3 mbar). Filter number 6 has a relatively shallow gradient of its response curve (i.e. it requires a relatively large amount of powder for each mbar increase in back pressure) and the final obtained back pressure is closely aligned to the target at 63.1 mbar. However, Filter number 4 has a steeper gradient of its response curve (i.e. it requires a relatively small amount of powder for each mbar increase in back pressure) and the final obtained back pressure overshoots the target and reaches 66.6 mbar.

[0232] Consequently, the present applicant has discovered that an improved method of treatment may be achieved by, in Step S29-5, monitoring a back pressure of the filter 2 during spraying of the dry powder 4 and stopping the spraying of the dry powder towards the inlet face of the filter 2 when:

p.sub.est≥p.sub.target [0233] where, [0234] p.sub.target is a pre-determined target back pressure for the filter; and [0235] p.sub.est is an estimated final back pressure of the filter; [0236] wherein p.sub.est is calculated by extrapolation of measured back pressure data for the filter that is obtained during spraying of the dry powder 4.

[0237] The estimated final back pressure of the filter, p.sub.est, may be calculated by extrapolating the measured back pressure data forwards in time for T.sub.s seconds, where T.sub.s is a settling time.

[0238] The method may be performed on the coating apparatus 1 and the settling time, T.sub.s, may be a coating apparatus-dependent variable and may be preferably independent of the filter 2. The parameter of the setting time, T.sub.s, may be chosen to characterise the performance of the coating apparatus 1. The setting time, T.sub.s, may be calculated theoretically for a coating apparatus 1. Alternatively, the setting time, T.sub.s, may be determined experimentally for a coating apparatus 1 by testing sample filters. For example experiments may be carried out iteratively to obtain a good estimate of the settling time.

[0239] The estimated final back pressure of the filter 2, p.sub.est, may be calculated by curve-fitting the measured back pressure data and extrapolating the fitted curve forwards in time. For example, during initial loading of the filter 2 with the dry powder 4, back pressure measurements may be taken, preferably at a high sampling rate. The processor 9 (or other computational resource) may then calculate a fit line that matches the recorded back pressure data and then use the fit line to extrapolate forwards in time. In some examples the fit line may be a quadratic fit line that fits all or a major proportion of the recorded back pressure measurements. In other examples the fit line may be a linear fit line that matches a current or recent portion of the recorded back pressure measurements.

[0240] FIG. 12 illustrates the application of the method to the filter number 4 referred to in FIG. 11. The settling time, T.sub.s, was calculated from examination of the results of the 8 filters in FIG. 10 as 3.5 seconds for the coating apparatus 1 used.

[0241] In FIG. 12, 4.1 refers to the filter number 4 as treated using a fixed offset pressure as described above with reference to FIG. 11. As noted above, the powder spraying was stopped when the filter reached a back pressure of 60 mbar (a target back pressure of 63 mbar minus an offset pressure of 3 mbar) and the final obtained back pressure overshoots the target and reaches 66.6 mbar.

[0242] The results marked 4.2 show the effect of using the improved method of extrapolating the measured back pressure data forwards in time for T.sub.s seconds, i.e. 3.5 seconds in this example. As can be seen the final obtained back pressure was 62.8 mbar, very close to the target of 63 mbar. For the 8 example filters of FIG. 10, the improved method can obtain final back pressures with a mean back pressure of 63.7 mbar (compared to a target of 63 mbar) and a standard deviation of 0.85 mbar, significantly improved compared to the results of the method using a fixed offset pressure.

[0243] According to the present disclosure a treated filter may be provided having one or more advantages compared to prior art filters. Preferably, but not exclusively, the treated filter may be treated according to the method of the present disclosure and or treated using the apparatus according to the present disclosure.

[0244] Further aspects and embodiments of the present disclosure are set out in the following clauses:

[0245] Clause A1. A method for treating a filter for filtering particulate matter from exhaust gas, the method comprising the steps of: [0246] a) containing a dry powder in a reservoir; [0247] b) locating a filter in a filter holder, the filter comprising a porous substrate having an inlet face and an outlet face, the inlet face and the outlet face being separated by a porous structure; [0248] c) establishing a primary gas flow through the porous structure of the filter by applying a pressure reduction to the outlet face of the filter; [0249] d) transferring the dry powder from the reservoir to a spray device located upstream of the inlet face of the filter; [0250] e) spraying the dry powder, using the spray device, towards the inlet face of the filter such that the dry powder is entrained in the primary gas flow and passes through the inlet face of the filter to contact the porous structure; and [0251] f) monitoring a back pressure of the filter during at least step e) and stopping the spraying of the dry powder towards the inlet face of the filter when:

p.sub.BP≥p.sub.target−p.sub.offset [0252] where, [0253] p.sub.BP is the back pressure of the filter; [0254] p.sub.target is a pre-determined target back pressure for the filter; and [0255] p.sub.offset is a pre-chosen offset pressure.

[0256] Clause A2. The method of clause A1, wherein, in step f), the spraying of the dry powder towards the inlet face of the filter is stopped when p.sub.BP≥p.sub.target−p.sub.offset is true for a minimum time period, t.sub.min, wherein t.sub.min≥0.1 s, optionally ≥0.5 s, optionally ≥1.0 s.

[0257] Clause A3. The method of clause A1 or clause A2, where p.sub.offset is 1-10 mbar, optionally 2-5 mbar, optionally 3-5 mbar.

[0258] Clause A4. The method of any preceding clause, wherein, in step f), the back pressure of the filter is monitored continuously during at least step e); and optionally wherein the back pressure is continuously monitored by measuring the back pressure at a sample rate of ≥1 samples.Math.s.sup.−1, optionally ≥5 samples.Math.s.sup.−1, optionally ≥10 samples.Math.s.sup.−1.

[0259] Clause A5. The method of any preceding clause, further comprising in step c), monitoring a back pressure of the filter and passing the primary gas flow through the porous structure of the filter until the back pressure of the filter is stable before starting step d).

[0260] Clause A6. The method of clause A5, wherein the pre-determined target back pressure for the filter, p.sub.target, is relative to the stable back pressure monitored in step c).

[0261] Clause A7. The method of any preceding clause, further comprising the step: [0262] g) monitoring the back pressure of the filter and maintaining the primary gas flow through the porous structure of the filter after the spraying of the dry powder has been stopped until the back pressure of the filter is stable.

[0263] Clause A8. The method of any one of clauses A5 to A7, wherein the back pressure of the filter is considered stable when: [0264] i) the primary gas flow rate is within 0.5% of a predetermined flow rate; [0265] ii) the first derivative of the primary gas flow rate is ≤±0.15 m.sup.3hr.sup.−1.Math.s.sup.−1; and [0266] iii) the first derivative of the back pressure of the filter is ≤±1.5 mbar.Math.s.sup.−1.

[0267] Clause A9. The method of any preceding clause, wherein the back pressure, p.sub.BP, is an absolute back pressure measured relative to atmospheric pressure.

[0268] Clause A10. The method of any preceding clause, wherein the method uses a coating apparatus for carrying out at least steps a) to f), wherein the pre-determined target back pressure, p.sub.target, for the filter is calculated by: [0269] choosing a desired back pressure for the filter as would be measured by a testing apparatus different to the coating apparatus; [0270] establishing a calibration map for converting back pressures measured on the testing apparatus to back pressures measured on the coating apparatus; and [0271] converting the desired back pressure using the calibration map to establish the pre-determined target back pressure, p.sub.target.

[0272] Clause B1. A method for treating a filter for filtering particulate matter from exhaust gas, the method comprising the steps of: [0273] a) containing a dry powder in a reservoir; [0274] b) locating a filter in a filter holder, the filter comprising a porous substrate having an inlet face and an outlet face, the inlet face and the outlet face being separated by a porous structure; [0275] c) establishing a primary gas flow through the porous structure of the filter by applying a pressure reduction to the outlet face of the filter while monitoring a back pressure of the filter, and passing the primary gas flow through the porous structure of the filter until the back pressure of the filter is stable; [0276] d) subsequently to establishing the stable back pressure of the filter, transferring the dry powder from the reservoir to a spray device located upstream of the inlet face of the filter; [0277] e) spraying the dry powder, using the spray device, towards the inlet face of the filter such that the dry powder is entrained in the primary gas flow and passes through the inlet face of the filter to contact the porous structure while monitoring the back pressure of the filter; [0278] f) stopping the spraying of the dry powder towards the inlet face of the filter; and [0279] g) continuing to monitor the back pressure of the filter while maintaining the primary gas flow through the porous structure of the filter after the spraying of the dry powder has been stopped until the back pressure of the filter is stable.

[0280] Clause B2. The method of clause B1, wherein the back pressure of the filter is considered stable when the first derivative of the primary gas flow rate is ≤±X m.sup.3hr.sup.−1.Math.s.sup.−1, wherein X=0 to 0.30, optionally X=0.10 to 0.20, X=0.15.

[0281] Clause B3. The method of clause B1 or clause B2, wherein the back pressure of the filter is considered stable when the first derivative of the back pressure of the filter is ≤±Y mbar.Math.s.sup.−1, wherein Y=0.5 to 3.0, optionally Y=1.0 to 2.0, optionally Y=1.5.

[0282] Clause B4. The method of any one of clauses B1 to B3, wherein the back pressure of the filter is considered stable when the primary gas flow rate is within Z % of a predetermined flow rate, wherein Z=1.5, optionally Z=1.0, optionally Z=0.5.

[0283] Clause B5. The method of any one of clauses B1 to B4, wherein the back pressure is an absolute back pressure measured relative to atmospheric pressure.

[0284] Clause C1. The method of any one of clauses A1 to A10 or B1 to B5, wherein the back pressure of the filter is measured using a pressure sensor; optionally a single pressure sensor that is located in a filter holder or other housing fluidly connected to the outlet face of the filter.

[0285] Clause D1. A method for treating a filter for filtering particulate matter from exhaust gas, the method comprising the steps of: [0286] a) containing a dry powder in a reservoir; [0287] b) locating a filter in a filter holder, the filter comprising a porous substrate having an inlet face and an outlet face, the inlet face and the outlet face being separated by a porous structure; [0288] c) establishing a primary gas flow through the porous structure of the filter by applying a pressure reduction to the outlet face of the filter; [0289] d) transferring the dry powder from the reservoir to a spray device located upstream of the inlet face of the filter; [0290] e) spraying the dry powder, using the spray device, towards the inlet face of the filter such that the dry powder is entrained in the primary gas flow and passes through the inlet face of the filter to contact the porous structure; and [0291] f) monitoring a back pressure of the filter during at least step e) and stopping the spraying of the dry powder towards the inlet face of the filter when:

p.sub.est≥p.sub.target [0292] where, [0293] p.sub.target is a pre-determined target back pressure for the filter; and [0294] p.sub.est is an estimated final back pressure of the filter; [0295] wherein p.sub.est is calculated by extrapolation of measured back pressure data for the filter that is obtained during step f).

[0296] Clause D2. The method of clause D1, wherein p.sub.est is calculated by extrapolating the measured back pressure data forwards in time for T.sub.s seconds, where T.sub.s is a settling time.

[0297] Clause D3. The method of clause D2, wherein the method is performed on a coating apparatus and the settling time, T.sub.s, is a coating apparatus-dependent variable and is preferably independent of the filter.

[0298] Clause D4. The method of any one of clauses D1 to D3, wherein p.sub.est is calculated by curve-fitting the measured back pressure data and extrapolating the fitted curve forwards in time.

[0299] Clause E1. The method of any one of clauses A1 to A10 or clauses B1 to B5 or clauses C1 to C2 or clauses D1 to D4, wherein the dry powder comprises or consists of: [0300] a) a metal compound for forming by thermal decomposition a metal oxide; [0301] b) a metal oxide; or [0302] c) an aerogel.

[0303] Clause E2. The method of clause E1, wherein the metal compound comprises or consists of a metal hydroxide, a metal phosphate, a metal carbonate, a metal sulphate, a metal perchlorate, a metal iodide, a metal oxalate, a metal acetate, a metal chlorate or a mixture thereof.

[0304] Clause E3. The method of clause E1 or E2, wherein the metal of the metal compound comprises or consists of one or more of magnesium, calcium, strontium, barium, aluminium, zirconium, manganese, lithium, iron, cobalt, nickel, copper, or gallium.

[0305] Clause E4. The method of any one of clauses E1 to E3, wherein the metal oxide of option c) comprises one or more fumed metal oxides or fumed mixed oxides, for example fumed alumina, fumed silica, or fumed titania.

[0306] Clause E5. The method of any one of clauses E1 to E4, wherein the aerogel comprises one or more of silica aerogel, alumina aerogel, carbon aerogel, titania aerogel, zirconia aerogel, ceria aerogel, a metal oxide aerogel and mixed oxide aerogels.

[0307] Clause E6. The method of any one of clauses E1 to E5, wherein the dry powder has a tapped density of 1 to 3 g/cm.sup.3, optionally 1.5 to 2.5 g/cm.sup.3, optionally about 2 g/cm.sup.3; or the dry powder has a tapped density of less than 0.10 g/cm.sup.3, optionally less than 0.08 g/cm.sup.3, optionally less than 0.07 g/cm.sup.3, optionally less than 0.06 g/cm.sup.3, optionally less than 0.05 g/cm.sup.3.

[0308] Clause E7. The method of any one of clauses E1 to E6, wherein the dry powder has a d50 (by volume) less than 10 microns, optionally less than 5 microns, optionally about 2 microns.

[0309] Clause E8. The method of any one of clauses E1 to E7, comprising providing a maximum loading of the filter of 10 to 40 g/l, optionally 15 to 30 g/l, optionally about 20 g/l of the dry powder; or providing a maximum loading of the filter of <10 g/l of the dry powder, optionally <5 g/l of the dry powder, optionally <2 g/l of the dry powder.

[0310] Clause E9. The method of any one of clauses E1 to E8, further comprising before step b) coating the filter with a washcoat, preferably a catalyst washcoat.

[0311] Clause F1. The method of any preceding clause, further comprising the step of calcining the filter after loading with the dry powder.

[0312] Clause F2. The method of clause F1, wherein the calcining is at a temperature of at least 150° C., optionally at least 250° C., optionally at least 500° C.

[0313] Clause F3. The method of clause F1 or clause F2, wherein the calcining is at a temperature no greater than 550° C.; alternatively at a temperature greater than 550° C.; optionally at a temperature up to 900° C.; optionally at a temperature up to 1150° C.

[0314] Clause F4. The method of any one of clauses F1 to F3, wherein the calcining is carried out for a period of 30 to 90 minutes, optionally 30 to 60 minutes and including a dwell time of 1 to 15 minutes, preferably 5 to 10 minutes.

[0315] Clause G1. The method of any preceding clause, wherein in step d) a secondary gas flow, separate to the primary gas flow, is used to transfer the dry powder from the reservoir to the spray device; and optionally the secondary gas flow is controllable independently of the primary gas flow.

[0316] Clause G1. The method of clause G1, wherein the secondary gas flow comprises a flow of compressed gas, preferably air.

[0317] Clause G2. The method of clause G1 or clause G2, wherein the secondary gas flow is applied as a single burst or a plurality of intermittent bursts.

[0318] Clause G3. The method of any preceding clause, comprising using a vacuum generator to establish the primary gas flow through the porous structure of the filter.

[0319] Clause G4. The method of clause G3, wherein a level of the pressure reduction generated by the vacuum generator is controllable independently of a speed or mass rate of the transfer of the dry powder from the reservoir to the spray device.

[0320] Clause G5. The method of any preceding clause, wherein the primary gas flow has a volumetric flow rate of 10 m.sup.3/hr to 5,000 m.sup.3/hr, preferably 400 m.sup.3/hr to 2,000 m.sup.3/hr, preferably 600 m.sup.3/hr to 1000 m.sup.3/hr.

[0321] Clause G6. The method of clause G5, further comprising using a pressure sensor, preferably a single pressure sensor, to monitor the back pressure.

[0322] Clause G7. The method of clause G6, wherein the pressure sensor, preferably the single pressure sensor, is located in a filter holder or other housing fluidly connected to the outlet face of the filter.

[0323] Clause G8. The method of clause G6 or clause G7, wherein the same pressure sensor, preferably the same single pressure sensor, is used to monitor the back pressure of the filter during at least steps c) and f).

[0324] Clause G9. The method of any preceding clause, wherein in step e) the dry powder is sprayed from one or more outlets of the spray device.

[0325] Clause G10. The method of clause G9, wherein the one or more outlets of the spray device comprise an aperture size of 1 to 10 mm, optionally 0.5 to 5.0 mm, optionally 1.0 to 2.5 mm, optionally 1.0 to 2.0 mm.

[0326] Clause G11. The method of clause G9 or clause G10, wherein the dry powder is sprayed from one or more fixed outlets of the spray device.

[0327] Clause G12. The method of clause G9 or clause G10, wherein the dry powder is sprayed from one or more mobile outlets of the spray device, preferably from one or more oscillating outlets.

[0328] Clause G13. The method of any preceding clause, further comprising in step e) channelling the dry powder from the spray device to the inlet face of the filter within a flow conduit.

[0329] Clause G14. The method of clause G13, wherein the flow conduit provides an unimpeded flow path between the spray device and the inlet face of the filter.

[0330] Clause G15. The method of clause G13, wherein the flow conduit comprises a flow conditioner interposed between the spray device and the inlet face of the filter, the flow conditioner acting to promote dispersion of the dry powder within the gas flow.

[0331] Clause G16. The method of clause G15, wherein the flow conditioner comprises one or more of a static mixer, a mesh, a sieve, a baffle, and an orificed plate.

[0332] Clause G17. The method of any preceding clause, wherein the inlet face of the filter is located from 10 to 80 cm, preferably 15 to 20 cm from the spray device, and or the spray device is located at a distance from the inlet face of the filter that is up to 4 times a diameter of the inlet face of the filter.

[0333] Clause G18. The method of any preceding clause, further comprising in step d) dosing the dry powder from the reservoir.

[0334] Clause G19. The method of clause G18, wherein the dosing comprises dosing by one or more of by weight, by volume, by particle number, by time.

[0335] Clause G20. The method of clause G18 or clause G19, comprising gravimetrically feeding a dosing device with the dry powder.

[0336] Clause G21. The method of any one of clauses G18 to G20, wherein the dosing uses a loss in weight feeder.

[0337] Clause G22. The method of any preceding clause, wherein in step a) the dry powder is contained in one or more hoppers.

[0338] Clause G23. The method of any preceding clause, wherein in step b) the filter is located in the holder in a vertical orientation with the inlet face uppermost.

[0339] Clause G24. The method of clause G23, wherein in step d) the spray device is located vertically above the inlet face; and preferably a spray direction of the spray device is co-axial with a longitudinal axis of the filter; and preferably the spray direction and the longitudinal axis are coincident.

[0340] Clause G25. The method of any preceding clause, wherein the porous substrate is a wall-flow filter.

[0341] Clause H1. A filter obtainable by the method of any preceding clause.

[0342] Clause H2. The filter of clause H1 which is one or more of a catalysed soot filter (CSF), a selective catalytic reduction filter (SCRF), a lean NOx trap filter (LNTF), and a gasoline particulate filter (GPF).