WO2011138552A1 - Gas filtration structure - Google Patents

Gas filtration structure Download PDF

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Publication number
WO2011138552A1
WO2011138552A1 PCT/FR2011/051000 FR2011051000W WO2011138552A1 WO 2011138552 A1 WO2011138552 A1 WO 2011138552A1 FR 2011051000 W FR2011051000 W FR 2011051000W WO 2011138552 A1 WO2011138552 A1 WO 2011138552A1
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WIPO (PCT)
Prior art keywords
channels
wall
walls
structure according
population
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PCT/FR2011/051000
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French (fr)
Inventor
Bernard Bouteiller
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Saint-Gobain Centre De Recherches Et D'etudes Europeen
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Publication of WO2011138552A1 publication Critical patent/WO2011138552A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D46/00—Filters or filtering processes specially modified for separating dispersed particles from gases or vapours
    • B01D46/24—Particle separators, e.g. dust precipitators, using rigid hollow filter bodies
    • B01D46/2403—Particle separators, e.g. dust precipitators, using rigid hollow filter bodies characterised by the physical shape or structure of the filtering element
    • B01D46/2418—Honeycomb filters
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B28—WORKING CEMENT, CLAY, OR STONE
    • B28B—SHAPING CLAY OR OTHER CERAMIC COMPOSITIONS; SHAPING SLAG; SHAPING MIXTURES CONTAINING CEMENTITIOUS MATERIAL, e.g. PLASTER
    • B28B3/00—Producing shaped articles from the material by using presses; Presses specially adapted therefor
    • B28B3/20—Producing shaped articles from the material by using presses; Presses specially adapted therefor wherein the material is extruded
    • B28B3/26—Extrusion dies
    • B28B3/269—For multi-channeled structures, e.g. honeycomb structures

Definitions

  • the invention relates to the field of filter structures possibly comprising a catalytic component, for example used in an exhaust line of a diesel type internal combustion engine.
  • Filters for the treatment of gases and the removal of soot typically from a diesel engine are well known in the prior art.
  • These structures most often have a honeycomb structure, one of the faces of the structure allowing the admission of the exhaust gas to be treated and the other side the evacuation of the treated exhaust gas.
  • the structure comprises, between these intake and discharge faces, a set of adjacent ducts or channels, most often of square section, with axes parallel to each other separated by porous walls.
  • the ducts are closed at one or the other of their ends to delimit inlet chambers opening on the inlet face and outlet chambers opening along the discharge face.
  • the channels are alternately closed in an order such that the exhaust gases, during the crossing of the honeycomb body, are forced to pass through the sidewalls of the inlet channels to join the outlet channels. In this way, the particles or soot are deposited and accumulate on the porous walls of the filter body.
  • porous ceramic filters are used for the filtration of gases, for example cordierite, alumina, aluminum titanate, mullite, silicon nitride, a silicon / silicon carbide mixture or silicon carbide.
  • a particulate filter in the exhaust line of an engine has the effect of increasing the pressure drop, even in the absence of soot, when the filter is not yet loaded.
  • the particulate filter is loaded with soot particles, which are deposited on the porous walls. This gradual clogging of the filter during the soot retention phase also causes an increase in the pressure drop, which results in an increase in the engine consumption, or even an overpressure that may deteriorate the combustion system.
  • the regeneration step is done by raising the temperature of the exhaust gas using a post injection, which is to inject late into the engine cycle of the fuel that will burn in the exhaust line.
  • filter elements whose shape and the internal volume of the inlet and outlet channels are different.
  • the wall portions follow one another, in cross section and following a horizontal and / or vertical row of channels, to define a wavy or wavy shape.
  • This geometry by increasing the area of the input channels to the detriment of that of the output channels, makes it possible to increase the storage volume of soot and residue, and to reduce the increase in pressure drop during operation of the filtered.
  • the subject of the invention is a structure for filtering particles-loaded gases of the honeycomb type and comprising a periodic set of longitudinal adjacent channels with axes parallel to one another separated by porous filtering wall portions. said channels being alternately plugged at one or other end of the structure so as to define inlet channels and outlet channels for the gas to be filtered, and to force said gas to pass through the walls porous separating the inlet and outlet channels, said structure being such that in transverse section:
  • the porous walls have periodic corrugations of the same period as the channels so that each of the four wall portions separating an inlet channel from an adjacent outlet channel is concave with respect to the center of said inlet channel, and so that the ratio R between the area of each input channel and the area of each output channel is greater than 1,
  • a first population of wall portions each separating an inlet channel and an adjacent outlet channel, has their concavity in a first direction and a second population has their concavity in a second direction, opposite to the first direction, each alternating wall periodically a portion of walls belonging to the first population then a wall portion belonging to the second population,
  • each porous wall has a first core, which corresponds to a continuous periodic curve of the same period as that of the channels, superimposed, at each wall portion of the first population, with the median line of said portion, and a second core , which corresponds to a continuous periodic curve of the same period that of the channels, superimposed, at the level of each wall portion of the second population, with the median line of said portion,
  • middle line is meant all the points situated in the middle of the segment of the line normal to the line at this point whose ends are the edges of the walls. This median line thus shares a portion of wall in two portions of equal thickness.
  • the structure is periodic and can be conceived as the repetition in two dimensions of a square elementary mesh whose center is the center of an input channel and the vertices are the centers of the input channels the closest.
  • channel period means the length of one side of the square mesh. This period is therefore twice the distance between an input channel and an adjacent output channel. In other words, the distance between the center of an output channel and the center of an adjacent input channel is half a period. The half-period is noted “p" in the rest of this text.
  • each wall has two offset cores, each coinciding with the median lines of half of the wall portions. Since each of the four wall portions separating an input channel from an adjacent output channel is concave with respect to the center of said input channel, it follows that each channel portion is convex with respect to the center of said output channel.
  • Each input channel is thus delimited by four concave wall portions with respect to the center of said input channel, and each output channel is delimited by four convex wall portions with respect to the center of said output channel.
  • the ratio R between the area of each inlet channel and the area of each outlet channel is therefore greater than 1. The areas are determined in the cross-sectional plane.
  • the ratio R is preferably between 1.05 and 3.0.
  • the structure obtained can be described as asymmetrical, in the sense that the overall volume of the input channels is greater than the total volume of the output channels. This configuration makes it possible to increase the area available for filtration and / or catalysis, thereby reducing the pressure drop of the filters and the soot loading slope or residues, the latter corresponding to the increase in the pressure loss. depending on the soot loading or residues.
  • the ratio between the distance d and the average thickness of a wall portion is preferably between 0.01 and 0.95, especially between 0.05 and 0.8.
  • the thickness of the wall portion is measured in the line segment joining the center of the inlet channel and the center of the adjacent outlet channel separated by said wall portion.
  • Each wall portion which separates an input channel from an adjacent output channel, is delimited by a so-called internal face, in direct contact with said input channel, and by a so-called external face, in direct contact with said output channel.
  • the inner face of each wall portion, in cross-section is in the form of a circular arc having an internal radius R 1 of finite value and a center Ci.
  • the external face of each portion of wall is in the form of an arc having an external radius R e of finite value and a center C e . Even more preferably, the centers C e and Ci are merged, the values R e and Ri being different.
  • the thickness of the wall portion corresponds to the difference between the outer radius and the inner radius.
  • the undulations of the walls have the same period as that of the channels.
  • One or the other of the souls of a wall is therefore a periodic curve, whose period (2p) corresponds to the period of the channels, and having the same amplitude (h).
  • the ratio T between the amplitude (h) and the half-period (p) is less than or equal to 0.41, in particular to 0.3.
  • the ratio T is preferably less than or equal to 0.26 and / or greater than or equal to 0.1, especially 0.14 and even 0.23. Too high a ratio may limit the volume of output channels too much, which may make filtering more difficult. Too low a ratio is too close to a conventional square-channel and flat-wall structure to fully benefit from all the advantages of the invention.
  • the section of the channels in cross-section is preferably constant over the entire length of the structure. It is also preferred that the sections of all the output channels are identical, with the possible exception of the channels located at the periphery of the filtration structure or the channels of the structures located in filter periphery. The same characteristic is also preferred for the input channels.
  • the thickness of the walls is preferably between 150 and 700 microns, especially between 200 and 500 microns, or between 250 and 400 microns. In a cross section, the thickness of the walls is preferably constant.
  • the density of channels is preferably between 1 and 280 channels per cm 2 , in particular between 15 and 65 channels per cm 2 .
  • the input and / or output channels may have at least one rounded corner, for example one, two, three or four rounded corners.
  • the porosity of the material constituting the filtering walls of the filter is preferably between 30 and 70% by volume and / or the median pore diameter is preferably between 5 and 45 ⁇ m.
  • the walls are preferably based on silicon carbide, which has a very good chemical resistance and at high temperatures.
  • the walls may also be of a material chosen from cordierite, alumina, aluminum titanate, mullite, silicon nitride, sintered metals, a silicon / silicon carbide mixture, or any of their mixtures.
  • At least part or even all of the surface of the inlet channels is preferably coated with a catalyst intended to promote the elimination of pollutant gases (such as CO, HC, NO x ) and / or soot.
  • pollutant gases such as CO, HC, NO x
  • At least one active catalytic phase comprising preferably a precious metal such as Pt, Pd, Rh and optionally an oxide selected from CeC> 2, ZrC> 2, or a mixture thereof.
  • the active principle is usually deposited according to well-known heterogeneous catalysis techniques, in the porosity of a support layer in general based on oxide with a high specific surface area, for example alumina, titanium oxide, silica, ceria or zirconium oxide.
  • the invention also relates to an assembled filter comprising a plurality of filter structures as previously described, said structures being bonded together by a cement.
  • the structures may be, in cross section, square, rectangular, triangular or hexagonal.
  • Another subject of the invention is the use of a filtration structure or of an assembled filter as previously described as a depollution device on an exhaust line of a Diesel or Petrol engine, preferably Diesel.
  • the invention finally relates to an extrusion die shaped so as to form, by extrusion of a ceramic material, a filtration structure according to the invention.
  • Figures 1 and 2 are front elevational views of a portion of the gas evacuation face of a filter according to the prior art.
  • FIG. 1 shows a portion of the evacuation face of a filtration structure according to the prior art, in particular according to the application WO 2005/016491.
  • the structure is of the honeycomb type and comprises a set of adjacent channels 11 and 12, longitudinal axes parallel to each other, and separated by porous filtering walls 13.
  • the channels 11, 12 are alternately blocked by plugs (not shown) at either end of the structure so as to define inlet channels 11 and outlet channels 12 for the gas to be filtered, and to force said gas through the porous walls 13.
  • the face shown being a gas evacuation face (rear face of the filter), the plugs (not shown) close the inlet channels 11. At the opposite side opposite (face or front face gas inlet), it is the outlet channels 12 which are plugged.
  • the dashed square represents the mesh of the 2-dimensional periodic lattice.
  • the period of the structure also called period of the channels, corresponds to the length of one side of the square.
  • FIG. 1 The structure of FIG. 1 is such that, in cross-section, the porous walls 13 have periodic corrugations of period 2p and amplitude h so that said porous walls 13 are concave with respect to the center of the inlet channels. 11 and convex relative to the center of the output channels 12.
  • the ratio R is of the order of 2.4.
  • the core 14 of the walls 13 is shown in dashed lines, and takes the form of the periodic undulations of the walls 13.
  • the walls 13 therefore have only one core 14, which coincides in every respect with the median lines of each of the wall portions constituting the wall 13.
  • a core 14 extending in a direction x intersects a core 14 extending in a y direction at a single intersection point marked "i" in the figure. This point of intersection also corresponds to the point in the middle of the line segment connecting the vertices closest to two adjacent input channels and two adjacent output channels.
  • the figure also shows the amplitude h and the period (2p) of the periodic curve formed by the core 14.
  • FIG. 2 shows the same filtration structure according to the prior art as the structure of FIG. 1.
  • Each wall portion which separates an inlet channel 11 from an adjacent outlet channel 12, is delimited by a face internal 15, in direct contact with said inlet channel 11, and an outer face 16, in direct contact with the outlet channel 12.
  • the inner face 15 is in the shape of an arc.
  • the outer face 16 is in the form of a circular arc (shown in broken lines) having an external radius R e of finite value and a center C e . Centers C e and Ci are merged.
  • Figure 3 illustrates a filtration structure according to the invention.
  • the structure is of the honeycomb type and comprises a set of adjacent channels 21 and 22, longitudinal axes parallel to each other, and separated by porous filter walls 23.
  • the channels 21 and 22 are alternately blocked by plugs (not shown) at either end of the structure so as to define inlet channels 21 and outlet channels 22 for the gas to be filtered, and so as to force said gas to pass through the porous walls 23.
  • face shown being the gas evacuation face (rear face of the filter), the plugs clog the inlet channels 21.
  • the output channels 22 At the opposite side to the contrary (front face or gas inlet face), these are the output channels 22 which are plugged.
  • the porous walls 23 have periodic corrugations of period 2p and amplitude h so that said porous walls 23 are concave with respect to the center of the inlet channels 21 and convex in their middle with respect to the center output channels 22.
  • the ratio R is of the order of 2.4.
  • the structure has two types of walls 23: walls extending in a direction x, and walls extending in a direction y. Among the portions of walls extending in the y direction (but the same reasoning can be maintained with respect to the walls extending in the x direction), it is possible to distinguish a first population of wall portions 24, each separating a inlet channel 21 and an adjacent outlet channel 22, having their concavity in a first direction (here to the left of the figure) and a second population of wall portions 25 having their concavity in a second direction, opposite the first direction (here to the right of the figure). Each wall 23 periodically alternates a portion of walls 24 belonging to the first population then a portion of wall 25 belonging to the second population.
  • Each porous wall 23 has a first core 26, which corresponds to a continuous periodic curve of the same period as that of the channels 21 and 22, superimposed on the level of each wall portion 24 of the first population, with the center line of said portion 24, and a second core 27, which corresponds to a continuous periodic curve of the same period as that of the channels 21 and 22, superimposed at the each of the wall portions 25 of the second population, with the median line of said portion 25.
  • the two webs 26 and 27 of each wall are each a translation of the other by a non-zero distance d.
  • the ratio of the distance d to the thickness of the wall is about 0.3.
  • FIG. 4 represents the same structure as that of FIG. 3.
  • Diagrammatically in this figure are two circles, an inner circle 31 of radius R 1 and an outer circle 32 of radius R e , the arcs of which respectively conform to inner and outer faces 34 and 35 of a wall portion 36.
  • the centers C e and Ci of the respectively outer 32 and inner 31 circles are merged.
  • the first population of monolithic elements or monoliths in the form bee and silicon carbide.
  • the median diameter of the pore dso is the diameter of the particles, such that 50% of the total population of the grains has a size less than this diameter, respectively.
  • a porogen of the polyethylene type in a proportion equal to 5% by weight of the total weight of the SiC grains and a methylcellulose type shaping additive in a proportion equal to 10% by weight of the total weight of the Sic grains.
  • the quantity of water required is then added and kneaded to obtain a homogeneous paste whose plasticity allows extrusion through a die configured to obtain monolithic blocks of square section and whose internal channels have a cross section. illustrated schematically in FIG. 1.
  • the half-period p of the corrugations is 1.96 mm and corresponds to half the period of the filtering structure.
  • the ratio T is 0.23.
  • the green monoliths obtained are dried by microwave for a time sufficient to bring the water content not chemically bound to less than 1 ⁇ 6 by mass.
  • the channels of each face of the monolith are alternately blocked according to well-known techniques, for example described in application WO 2004/065088.
  • the monoliths are then fired in argon according to a rise in temperature of 20 ° C / hour until a maximum temperature of 2200 ° C is reached which is maintained for 6 hours.
  • the porous material obtained has an open porosity of 47% and a median pore diameter of the order of 15 microns.
  • An assembled filter is then formed from the monoliths. Sixteen elements from the same mixture were assembled together according to conventional techniques by bonding with a cement of the following chemical composition: 72% by weight of SiC, 15% by weight of Al 2 O 3 , 11% by weight of S1O 2, the remainder consisting of impurities, predominantly Fe2 ⁇ 3 and alkali metal oxides and alkaline earth metal. The average thickness of the joint between two adjacent blocks is of the order of 1.2 mm.
  • the assembly is then machined in order to form assembled filters of cylindrical shape of about 14.4 cm in diameter. A cement of the same composition as the joint cement is deposited at the periphery of the machined filter, with a thickness of 1.3 mm.
  • open front area obtained by calculating the percentage ratio of the area covered by the sum of the sections cross-sections of the inlet channels of the front face of the monolithic unitary elements (except the walls and plugs) over the total area of the corresponding cross-section of said unitary elements.
  • the amount of storage of residues is greater the higher the percentage
  • the specific filtration surface of a unit monolithic element which corresponds to the internal surface of all the walls of the filter inlet channels expressed in m 2 , based on the volume in dm 3 of monolith.
  • the storage volume of the soot is all the higher as the specific surface thus defined is large.
  • pressure loss is meant within the meaning of the present invention the differential pressure existing between the upstream and downstream of the filter.
  • the pressure drop was measured according to the techniques of the art, for a gas flow rate of 250 kg / h and a temperature of 250 ° C on the new filters (not loaded in soot).
  • the structures whose geometry has been chosen according to the invention consequently have a lower pressure drop relative to the conventional structures, with a gain that can reach values of the order of

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  • Engineering & Computer Science (AREA)
  • Chemical & Material Sciences (AREA)
  • Manufacturing & Machinery (AREA)
  • Ceramic Engineering (AREA)
  • Mechanical Engineering (AREA)
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  • Chemical Kinetics & Catalysis (AREA)
  • Filtering Of Dispersed Particles In Gases (AREA)
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Abstract

The invention relates to a honeycomb structure for the filtration of particle-laden gases, comprising a periodic set of adjacent channels (21, 22) separated by portions of porous filter walls (23), said structure being such that in cross-section each porous filter wall (23) has: a first core (26) which corresponds to a continuous periodic curve of the same period as that of the channels, being superimposed at each wall portion (24) of a first population with the midline of said portion (24); and a second core (27) which corresponds to a continuous periodic curve of the same period as that of the channels, being superimposed at each wall portion (25) of a second population with the midline of said portion (25), each of the two cores (26, 27) of each wall (23) being a translation of the other by a non-zero distance d.

Description

STRUCTURE DE FILTRAT ION DE GAZ  GAS FILTRATION STRUCTURE
L' invention se rapporte au domaine des structures filtrantes comprenant éventuellement une composante catalytique, par exemple utilisées dans une ligne d'échappement d'un moteur à combustion interne du type diesel . Les filtres permettant le traitement des gaz et l'élimination des suies typiquement issues d'un moteur diesel sont bien connus de l'art antérieur. Ces structures présentent le plus souvent une structure en nid d'abeille, une des faces de la structure permettant l'admission des gaz d'échappement à traiter et l'autre face l'évacuation des gaz d'échappement traités. La structure comporte, entre ces faces d'admission et d'évacuation, un ensemble de conduits ou canaux adjacents, le plus souvent de section carrée, d'axes parallèles entre eux séparés par des parois poreuses. Les conduits sont obturés à l'une ou l'autre de leurs extrémités pour délimiter des chambres d'entrée s ' ouvrant suivant la face d'admission et des chambres de sortie s ' ouvrant suivant la face d'évacuation. Les canaux sont alternativement obturés dans un ordre tel que les gaz d'échappement, au cours de la traversée du corps en nid d'abeille, sont contraints de traverser les parois latérales des canaux d'entrée pour rejoindre les canaux de sortie. De cette manière, les particules ou suies se déposent et s'accumulent sur les parois poreuses du corps filtrant. The invention relates to the field of filter structures possibly comprising a catalytic component, for example used in an exhaust line of a diesel type internal combustion engine. Filters for the treatment of gases and the removal of soot typically from a diesel engine are well known in the prior art. These structures most often have a honeycomb structure, one of the faces of the structure allowing the admission of the exhaust gas to be treated and the other side the evacuation of the treated exhaust gas. The structure comprises, between these intake and discharge faces, a set of adjacent ducts or channels, most often of square section, with axes parallel to each other separated by porous walls. The ducts are closed at one or the other of their ends to delimit inlet chambers opening on the inlet face and outlet chambers opening along the discharge face. The channels are alternately closed in an order such that the exhaust gases, during the crossing of the honeycomb body, are forced to pass through the sidewalls of the inlet channels to join the outlet channels. In this way, the particles or soot are deposited and accumulate on the porous walls of the filter body.
A l'heure actuelle, on utilise pour la filtration des gaz des filtres en matière céramique poreuse, par exemple en cordiérite, en alumine, en titanate d'aluminium, en mullite, en nitrure de silicium, en un mélange silicium/carbure de silicium ou en carbure de silicium. At present, porous ceramic filters are used for the filtration of gases, for example cordierite, alumina, aluminum titanate, mullite, silicon nitride, a silicon / silicon carbide mixture or silicon carbide.
L'incorporation d'un filtre à particules dans la ligne d'échappement d'un moteur a pour effet d'augmenter la perte de charge, même en l'absence de suies, lorsque le filtre n'est pas encore chargé. Au cours du fonctionnement du moteur, le filtre à particules se charge en particules de suie, lesquelles se déposent sur les parois poreuses. Ce colmatage progressif du filtre lors de la phase de rétention des suies provoque également une augmentation de la perte de charge, qui se traduit par une augmentation de la consommation du moteur, voire une surpression pouvant détériorer le système de combustion. The incorporation of a particulate filter in the exhaust line of an engine has the effect of increasing the pressure drop, even in the absence of soot, when the filter is not yet loaded. During engine operation, the particulate filter is loaded with soot particles, which are deposited on the porous walls. This gradual clogging of the filter during the soot retention phase also causes an increase in the pressure drop, which results in an increase in the engine consumption, or even an overpressure that may deteriorate the combustion system.
II est donc nécessaire de prévoir, à la suite d'un cycle de filtration, un cycle de régénération, au cours duquel les suies sont brûlées. L'étape de régénération se fait par élévation de la température des gaz d'échappement à l'aide d'une post injection, qui consiste à injecter tardivement dans le cycle moteur du carburant qui va brûler dans la ligne d'échappement.  It is therefore necessary to provide, following a filtration cycle, a regeneration cycle, during which the soot is burned. The regeneration step is done by raising the temperature of the exhaust gas using a post injection, which is to inject late into the engine cycle of the fuel that will burn in the exhaust line.
On recherche généralement, pour des régimes moteurs équivalents, le meilleur compromis possible entre les propriétés suivantes :  For equivalent engine speeds, the best compromise between the following properties is generally sought:
- une faible perte de charge occasionnée par la structure filtrante en fonctionnement, c'est-à-dire typiquement lorsque celle-ci est dans une ligne d'échappement d'un moteur à combustion interne, aussi bien lorsque ladite structure est exempte de particules de suies (perte de charge initiale) que lorsqu'elle est chargée en particules , une augmentation de la perte de charge du filtre au cours dudit fonctionnement la plus faible possible, c'est à dire un faible accroissement de la perte de charge en fonction du niveau de chargement en suies du filtre, a small loss of load caused by the filtering structure in operation, that is to say, typically when it is in an exhaust line of an internal combustion engine, both when said structure is free of particles of soot (initial loss of charge) only when it is loaded with particles, an increase in the pressure drop of the filter during said operation as low as possible, ie a small increase in the pressure drop as a function of the soot loading level of the filter,
- une surface totale de filtration élevée, a high total filtration area,
un volume de stockage de suies important, notamment à perte de charge constante, de manière à réduire la fréquence de régénération,  a large volume of soot storage, in particular at constant pressure drop, so as to reduce the frequency of regeneration,
un volume de stockage des résidus plus important, les résidus (imbrûlés) restant dans le filtre après régénération .  a larger residue storage volume, the residues (unburnt) remaining in the filter after regeneration.
L'augmentation de la perte de charge en fonction du niveau de chargement en suies du filtre est notamment directement mesurable par la pente de chargement ÀP/MSUie s , dans lequel ΔΡ représente la perte de charge et MSUie s la masse de suie accumulée dans le filtre. The increase in pressure drop as a function of filter soot loading level is in particular directly measurable by the loading slope? P / M SU i are wherein ΔΡ represents the pressure drop and M i are the SU mass soot accumulated in the filter.
Il a été proposé, dans la demande de brevet WO 05/016491, des éléments filtrants dont la forme et le volume interne des canaux d'entrée et de sortie sont différents. Dans de telles structures, les portions de paroi se succèdent, en coupe transversale et en suivant un rang horizontal et/ou vertical de canaux, pour définir une forme ondulée ou en vague (wavy en anglais) . Cette géométrie, en augmentant l'aire des canaux d'entrée au détriment de celle des canaux de sortie, permet d'augmenter le volume de stockage des suies et des résidus, et de réduire l'augmentation de perte de charge lors du fonctionnement du filtre.  It has been proposed in the patent application WO 05/016491, filter elements whose shape and the internal volume of the inlet and outlet channels are different. In such structures, the wall portions follow one another, in cross section and following a horizontal and / or vertical row of channels, to define a wavy or wavy shape. This geometry, by increasing the area of the input channels to the detriment of that of the output channels, makes it possible to increase the storage volume of soot and residue, and to reduce the increase in pressure drop during operation of the filtered.
L' invention a pour but de réduire encore la perte de charge occasionnée par les filtres de l'art antérieur. A cet effet, l'invention a pour objet une structure de filtration de gaz chargés en particules, du type en nid d'abeilles et comprenant un ensemble périodique de canaux adjacents longitudinaux d'axes parallèles entre eux séparés par des portions de parois poreuses filtrantes, lesdits canaux étant alternativement bouchés à l'une ou l'autre des extrémités de la structure de façon à définir des canaux d'entrée et des canaux de sortie pour le gaz à filtrer, et de façon à forcer ledit gaz à traverser les parois poreuses séparant les canaux d'entrée et de sortie, ladite structure étant telle qu'en coupe transversale : The object of the invention is to further reduce the pressure drop caused by the filters of the prior art. To this end, the subject of the invention is a structure for filtering particles-loaded gases of the honeycomb type and comprising a periodic set of longitudinal adjacent channels with axes parallel to one another separated by porous filtering wall portions. said channels being alternately plugged at one or other end of the structure so as to define inlet channels and outlet channels for the gas to be filtered, and to force said gas to pass through the walls porous separating the inlet and outlet channels, said structure being such that in transverse section:
les parois poreuses présentent des ondulations périodiques de même période que celle des canaux de sorte que chacune des quatre portions de paroi séparant un canal d'entrée d'un canal de sortie adjacent est concave par rapport au centre dudit canal d'entrée, et de sorte que le rapport R entre l'aire de chaque canal d'entrée et l'aire de chaque canal de sortie est supérieur à 1,  the porous walls have periodic corrugations of the same period as the channels so that each of the four wall portions separating an inlet channel from an adjacent outlet channel is concave with respect to the center of said inlet channel, and so that the ratio R between the area of each input channel and the area of each output channel is greater than 1,
une première population de portions de paroi, séparant chacune un canal d'entrée et un canal de sortie adjacent, présente leur concavité dans une première direction et une deuxième population présente leur concavité dans une deuxième direction, opposée à la première direction, chaque paroi alternant de manière périodique une portion de parois appartenant à la première population puis une portion de paroi appartenant à la deuxième population,  a first population of wall portions, each separating an inlet channel and an adjacent outlet channel, has their concavity in a first direction and a second population has their concavity in a second direction, opposite to the first direction, each alternating wall periodically a portion of walls belonging to the first population then a wall portion belonging to the second population,
chaque paroi poreuse possède une première âme, qui correspond à une courbe périodique continue de même période que celle des canaux, se superposant, au niveau de chaque portion de paroi de la première population, avec la ligne médiane de ladite portion, et une deuxième âme, qui correspond à une courbe périodique continue de même période que celle des canaux, se superposant, au niveau de chaque portion de paroi de la deuxième population, avec la ligne médiane de ladite portion, each porous wall has a first core, which corresponds to a continuous periodic curve of the same period as that of the channels, superimposed, at each wall portion of the first population, with the median line of said portion, and a second core , which corresponds to a continuous periodic curve of the same period that of the channels, superimposed, at the level of each wall portion of the second population, with the median line of said portion,
les deux âmes de chaque paroi étant chacune une translation de l'autre d'une distance d non-nulle.  the two souls of each wall being each a translation of the other by a distance d non-zero.
On entend par ligne médiane l'ensemble des points situés au milieu du segment de la droite normale à ladite ligne en ce point dont les extrémités sont les bords des parois. Cette ligne médiane partage donc une portion de paroi en deux portions d'épaisseur égale.  By middle line is meant all the points situated in the middle of the segment of the line normal to the line at this point whose ends are the edges of the walls. This median line thus shares a portion of wall in two portions of equal thickness.
Selon une coupe transversale, la structure est périodique et peut se concevoir comme la répétition dans les deux dimensions d'une maille élémentaire carrée dont le centre est le centre d'un canal d'entrée et les sommets sont les centres des canaux d'entrée les plus proches. On entend par « période des canaux » la longueur d'un côté de la maille carrée. Cette période est donc le double de la distance entre un canal d'entrée et un canal de sortie adjacent. Autrement dit, la distance entre le centre d'un canal de sortie et le centre d'un canal d'entrée adjacent correspond à une demi-période. La demi-période est notée « p » dans la suite du présent texte.  According to a cross section, the structure is periodic and can be conceived as the repetition in two dimensions of a square elementary mesh whose center is the center of an input channel and the vertices are the centers of the input channels the closest. The term "channel period" means the length of one side of the square mesh. This period is therefore twice the distance between an input channel and an adjacent output channel. In other words, the distance between the center of an output channel and the center of an adjacent input channel is half a period. The half-period is noted "p" in the rest of this text.
Les inventeurs ont pu mettre en évidence qu'en modifiant la géométrie des filtres à parois périodiques ondulées connus de la demande WO 05/016491 susmentionnée, il était possible de réduire la perte de charge. Dans les filtres connus, les parois ondulées ne possèdent qu'une seule âme, qui coïncide avec la ligne médiane de chaque portion de paroi. Les filtres selon l'invention sont au contraire tels que chaque paroi possède deux âmes décalées, chacune coïncidant avec les lignes médianes de la moitié des portions de paroi. Chacune des quatre portions de paroi séparant un canal d'entrée d'un canal de sortie adjacent étant concave par rapport au centre dudit canal d'entrée, il en résulte que chaque portion de canal est convexe par rapport au centre dudit canal de sortie. Chaque canal d'entrée est donc délimité par quatre portions de paroi concaves par rapport au centre dudit canal d'entrée, et chaque canal de sortie est délimité par quatre portions de paroi convexes par rapport au centre dudit canal de sortie. Le rapport R entre l'aire de chaque canal d'entrée et l'aire de chaque canal de sortie est donc supérieur à 1. Les aires sont déterminées dans le plan de coupe transversale. The inventors have been able to demonstrate that by modifying the geometry of the corrugated periodic wall filters known from the above-mentioned application WO 05/016491, it was possible to reduce the pressure drop. In the known filters, the corrugated walls have only one core, which coincides with the median line of each wall portion. The filters according to the invention are on the contrary such that each wall has two offset cores, each coinciding with the median lines of half of the wall portions. Since each of the four wall portions separating an input channel from an adjacent output channel is concave with respect to the center of said input channel, it follows that each channel portion is convex with respect to the center of said output channel. Each input channel is thus delimited by four concave wall portions with respect to the center of said input channel, and each output channel is delimited by four convex wall portions with respect to the center of said output channel. The ratio R between the area of each inlet channel and the area of each outlet channel is therefore greater than 1. The areas are determined in the cross-sectional plane.
Le rapport R est de préférence compris entre 1,05 et 3,0. La structure obtenue peut être qualifiée d'asymétrique, au sens où le volume global des canaux d'entrée est supérieur au volume global des canaux de sortie. Cette configuration permet d'augmenter la surface disponible pour la filtration et/ou la catalyse, diminuant de ce fait la perte de charge des filtres et la pente de chargement en suie ou résidus, cette dernière correspondant à l'accroissement de la perte de charge en fonction du chargement en suie ou résidus.  The ratio R is preferably between 1.05 and 3.0. The structure obtained can be described as asymmetrical, in the sense that the overall volume of the input channels is greater than the total volume of the output channels. This configuration makes it possible to increase the area available for filtration and / or catalysis, thereby reducing the pressure drop of the filters and the soot loading slope or residues, the latter corresponding to the increase in the pressure loss. depending on the soot loading or residues.
Le rapport entre la distance d et l'épaisseur moyenne d'une portion de paroi est de préférence compris entre 0,01 et 0,95, notamment entre 0,05 et 0,8. L'épaisseur de la portion de paroi est mesurée dans le segment de droite joignant le centre du canal d'entrée et le centre du canal de sortie adjacent séparés par ladite portion de paroi.  The ratio between the distance d and the average thickness of a wall portion is preferably between 0.01 and 0.95, especially between 0.05 and 0.8. The thickness of the wall portion is measured in the line segment joining the center of the inlet channel and the center of the adjacent outlet channel separated by said wall portion.
Chaque portion de paroi, qui sépare un canal d'entrée d'un canal de sortie adjacent, est délimitée par une face dite interne, en contact direct avec ledit canal d'entrée, et par une face dite externe, en contact direct avec ledit canal de sortie. De préférence, la face interne de chaque portion de paroi épouse, en section transversale, la forme d'un arc de cercle ayant un rayon interne R± de valeur finie et un centre Ci. De préférence également, la face externe de chaque portion de paroi épouse la forme d'un arc de cercle ayant un rayon externe Re de valeur finie et un centre Ce . De manière encore plus préférée, les centres Ce et Ci sont confondus, les valeurs Re et Ri étant différentes. Dans ce cas, l'épaisseur de la portion de paroi correspond à la différence entre le rayon externe et le rayon interne. Each wall portion, which separates an input channel from an adjacent output channel, is delimited by a so-called internal face, in direct contact with said input channel, and by a so-called external face, in direct contact with said output channel. Preferably, the inner face of each wall portion, in cross-section, is in the form of a circular arc having an internal radius R 1 of finite value and a center Ci. Preferably also, the external face of each portion of wall is in the form of an arc having an external radius R e of finite value and a center C e . Even more preferably, the centers C e and Ci are merged, the values R e and Ri being different. In this case, the thickness of the wall portion corresponds to the difference between the outer radius and the inner radius.
Les ondulations des parois présentent la même période que celle des canaux. L'une ou l'autre des âmes d'une paroi est donc une courbe périodique, dont la période (2p) correspond à la période des canaux, et possédant une même amplitude (h) .  The undulations of the walls have the same period as that of the channels. One or the other of the souls of a wall is therefore a periodic curve, whose period (2p) corresponds to the period of the channels, and having the same amplitude (h).
De préférence, le rapport T entre l'amplitude (h) et la demi-période (p) est inférieur ou égal à 0,41, notamment à 0,3. Le rapport T est de préférence inférieur ou égal à 0,26 et/ou supérieur ou égal à 0,1, notamment 0,14 et même 0,23. Un rapport trop grand risque de limiter trop fortement le volume des canaux de sortie ce qui risque de rendre plus difficile la fabrication des filtres. Un rapport trop faible se rapproche trop d'une structure conventionnelle à canaux carrés et parois planes pour pouvoir bénéficier pleinement de tous les avantages liés à 1 ' invention .  Preferably, the ratio T between the amplitude (h) and the half-period (p) is less than or equal to 0.41, in particular to 0.3. The ratio T is preferably less than or equal to 0.26 and / or greater than or equal to 0.1, especially 0.14 and even 0.23. Too high a ratio may limit the volume of output channels too much, which may make filtering more difficult. Too low a ratio is too close to a conventional square-channel and flat-wall structure to fully benefit from all the advantages of the invention.
La section des canaux en coupe transversale est de préférence constante sur toute la longueur de la structure. Il est également préféré que les sections de tous les canaux de sortie soient identiques, à l'exception éventuelle des canaux situés en périphérie de la structure de filtration ou des canaux des structures situées en périphérie du filtre. La même caractéristique est également préférée pour les canaux d'entrée. The section of the channels in cross-section is preferably constant over the entire length of the structure. It is also preferred that the sections of all the output channels are identical, with the possible exception of the channels located at the periphery of the filtration structure or the channels of the structures located in filter periphery. The same characteristic is also preferred for the input channels.
Pour assurer une bonne capacité de filtration sans trop pénaliser la perte de charge, l'épaisseur des parois est de préférence comprise entre 150 et 700 micromètres, notamment entre 200 et 500 micromètres, voire entre 250 et 400 micromètres. Selon une coupe transversale, l'épaisseur des parois est de préférence constante.  To ensure a good filtration capacity without unduly penalizing the pressure drop, the thickness of the walls is preferably between 150 and 700 microns, especially between 200 and 500 microns, or between 250 and 400 microns. In a cross section, the thickness of the walls is preferably constant.
La densité de canaux est de préférence comprise entre 1 et 280 canaux par cm2, notamment entre 15 et 65 canaux par cm2. The density of channels is preferably between 1 and 280 channels per cm 2 , in particular between 15 and 65 channels per cm 2 .
Les canaux d'entrée et/ou de sortie peuvent posséder au moins un coin arrondi, par exemple un, deux, trois ou quatre coins arrondis.  The input and / or output channels may have at least one rounded corner, for example one, two, three or four rounded corners.
La porosité du matériau constituant les parois filtrantes du filtre est de préférence comprise entre 30 et 70% en volume et/ou le diamètre médian de pores est de préférence compris entre 5 et 45ym.  The porosity of the material constituting the filtering walls of the filter is preferably between 30 and 70% by volume and / or the median pore diameter is preferably between 5 and 45 μm.
Les parois sont de préférence à base de carbure de silicium, qui présente une très bonne résistance chimique et aux températures élevées. Les parois peuvent également être en un matériau choisi parmi la cordiérite, l'alumine, le titanate d'aluminium, la mullite, le nitrure de silicium, les métaux frittés, un mélange silicium/carbure de silicium, ou l'un quelconque de leurs mélanges.  The walls are preferably based on silicon carbide, which has a very good chemical resistance and at high temperatures. The walls may also be of a material chosen from cordierite, alumina, aluminum titanate, mullite, silicon nitride, sintered metals, a silicon / silicon carbide mixture, or any of their mixtures.
Une partie au moins, voire la totalité de la surface des canaux d'entrée est de préférence revêtue d'un catalyseur destiné à favoriser l'élimination des gaz polluants (tels que CO, HC, NOx) et/ou des suies. At least part or even all of the surface of the inlet channels is preferably coated with a catalyst intended to promote the elimination of pollutant gases (such as CO, HC, NO x ) and / or soot.
Sur la structure filtrante préalablement décrite, peut ainsi être déposée, de préférence par imprégnation, au moins une phase catalytique active, comprenant de préférence un métal précieux tel que Pt, Pd, Rh et éventuellement un oxyde choisi parmi CeC>2, ZrC>2, ou l'un de leurs mélanges. Le principe actif est habituellement déposé selon des techniques bien connues en catalyse hétérogène, dans la porosité d'une couche support en général à base d'oxyde à forte surface spécifique, par exemple l'alumine, l'oxyde de titane, la silice, l'oxyde de cérium ou de zirconium. On the previously described filtering structure, it is thus possible to deposit, preferably by impregnation, at least one active catalytic phase, comprising preferably a precious metal such as Pt, Pd, Rh and optionally an oxide selected from CeC> 2, ZrC> 2, or a mixture thereof. The active principle is usually deposited according to well-known heterogeneous catalysis techniques, in the porosity of a support layer in general based on oxide with a high specific surface area, for example alumina, titanium oxide, silica, ceria or zirconium oxide.
L'invention a également pour objet un filtre assemblé comprenant une pluralité de structures filtrantes telles que précédemment décrites, lesdites structures étant liées entre elles par un ciment. Les structures peuvent être, en section transversale, de forme carrée, rectangulaire, triangulaire ou encore hexagonale.  The invention also relates to an assembled filter comprising a plurality of filter structures as previously described, said structures being bonded together by a cement. The structures may be, in cross section, square, rectangular, triangular or hexagonal.
L'invention a encore pour objet l'utilisation d'une structure de filtration ou d'un filtre assemblé tels que précédemment décrits comme dispositif de dépollution sur une ligne d'échappement d'un moteur Diesel ou Essence, de préférence Diesel.  Another subject of the invention is the use of a filtration structure or of an assembled filter as previously described as a depollution device on an exhaust line of a Diesel or Petrol engine, preferably Diesel.
L'invention a enfin pour objet une filière d'extrusion conformée de manière à former, par extrusion d'une matière céramique, une structure de filtration selon l'invention.  The invention finally relates to an extrusion die shaped so as to form, by extrusion of a ceramic material, a filtration structure according to the invention.
Les figures 1 à 4 et les exemples non limitatifs qui suivent permettent de mieux comprendre l'invention et ses avantages.  Figures 1 to 4 and the following non-limiting examples provide a better understanding of the invention and its advantages.
Les figures 1 et 2 sont des vues de face en élévation d'une portion de la face d'évacuation des gaz d'un filtre selon l'art antérieur.  Figures 1 and 2 are front elevational views of a portion of the gas evacuation face of a filter according to the prior art.
Les figures 3 et 4 sont des vues de face en élévation d'une portion de la face d'évacuation des gaz d'un filtre selon l'invention. Sur la figure 1 est représentée une portion de la face d'évacuation d'une structure de filtration selon l'art antérieur, notamment selon la demande WO 2005/016491. La structure est du type en nid d'abeilles et comprend un ensemble de canaux adjacents 11 et 12, longitudinaux, d'axes parallèles entre eux, et séparés par des parois poreuses filtrantes 13. Les canaux 11, 12 sont alternativement bouchés par des bouchons (non-représentés ) à l'une ou l'autre des extrémités de la structure de façon à définir des canaux d'entrée 11 et des canaux de sortie 12 pour le gaz à filtrer, et de façon à forcer ledit gaz à traverser les parois poreuses 13. La face représentée étant une face d'évacuation des gaz (face arrière du filtre), les bouchons (non-représentés) bouchent les canaux d'entrée 11. Au niveau de la face opposée au contraire (face avant ou face d'admission des gaz), ce sont les canaux de sortie 12 qui sont bouchés. Figures 3 and 4 are front elevational views of a portion of the gas evacuation face of a filter according to the invention. FIG. 1 shows a portion of the evacuation face of a filtration structure according to the prior art, in particular according to the application WO 2005/016491. The structure is of the honeycomb type and comprises a set of adjacent channels 11 and 12, longitudinal axes parallel to each other, and separated by porous filtering walls 13. The channels 11, 12 are alternately blocked by plugs (not shown) at either end of the structure so as to define inlet channels 11 and outlet channels 12 for the gas to be filtered, and to force said gas through the porous walls 13. The face shown being a gas evacuation face (rear face of the filter), the plugs (not shown) close the inlet channels 11. At the opposite side opposite (face or front face gas inlet), it is the outlet channels 12 which are plugged.
Le carré en traits pointillés représente la maille du réseau périodique à 2 dimensions. La période de la structure, appelée aussi période des canaux, correspond à la longueur d'un côté du carré.  The dashed square represents the mesh of the 2-dimensional periodic lattice. The period of the structure, also called period of the channels, corresponds to the length of one side of the square.
La structure de la figure 1 est telle qu'en coupe transversale, les parois poreuses 13 présentent des ondulations périodiques de période 2p et d'amplitude h de façon à ce que lesdites parois poreuses 13 soient concaves par rapport au centre des canaux d'entrée 11 et convexes par rapport au centre des canaux de sortie 12. Le rapport R est de l'ordre de 2,4.  The structure of FIG. 1 is such that, in cross-section, the porous walls 13 have periodic corrugations of period 2p and amplitude h so that said porous walls 13 are concave with respect to the center of the inlet channels. 11 and convex relative to the center of the output channels 12. The ratio R is of the order of 2.4.
L'âme 14 des parois 13 est représentée en lignes pointillées, et reprend la forme des ondulations périodiques des parois 13. Les parois 13 ne possèdent donc qu'une seule âme 14, qui coïncide en tout point avec les lignes médianes de chacune des portions de paroi constituant la paroi 13. The core 14 of the walls 13 is shown in dashed lines, and takes the form of the periodic undulations of the walls 13. The walls 13 therefore have only one core 14, which coincides in every respect with the median lines of each of the wall portions constituting the wall 13.
Une âme 14 s' étendant selon une direction x croise une âme 14 s' étendant selon une direction y en un seul point d'intersection noté « i » sur la figure. Ce point d' intersection correspond également au point situé au milieu du segment de droite reliant les sommets les plus proches de deux canaux d'entrée adjacents et de deux canaux de sortie adjacents.  A core 14 extending in a direction x intersects a core 14 extending in a y direction at a single intersection point marked "i" in the figure. This point of intersection also corresponds to the point in the middle of the line segment connecting the vertices closest to two adjacent input channels and two adjacent output channels.
Sur la figure sont également représentées l'amplitude h et la période (2p) de la courbe périodique constituée par 1 ' âme 14.  The figure also shows the amplitude h and the period (2p) of the periodic curve formed by the core 14.
En figure 2 est représentée la même structure de filtration selon l'art antérieur que la structure de la Figure 1. Chaque portion de paroi, qui sépare un canal d'entrée 11 d'un canal de sortie adjacent 12, est délimitée par une face interne 15, en contact direct avec ledit canal d'entrée 11, et par une face externe 16, en contact direct avec le canal de sortie 12.  FIG. 2 shows the same filtration structure according to the prior art as the structure of FIG. 1. Each wall portion, which separates an inlet channel 11 from an adjacent outlet channel 12, is delimited by a face internal 15, in direct contact with said inlet channel 11, and an outer face 16, in direct contact with the outlet channel 12.
La face interne 15 épouse la forme d'un arc de cercle The inner face 15 is in the shape of an arc.
(représenté en traits pointillés) ayant un rayon interne R± de valeur finie et un centre Ci. La face externe 16 épouse la forme d'un arc de cercle (représenté en traits pointillés) ayant un rayon externe Re de valeur finie et un centre Ce . Les centres Ce et Ci sont confondus. (represented in dashed lines) having an internal radius R i of finite value and a center Ci. The outer face 16 is in the form of a circular arc (shown in broken lines) having an external radius R e of finite value and a center C e . Centers C e and Ci are merged.
La figure 3 illustre une structure de filtration selon 1 ' invention .  Figure 3 illustrates a filtration structure according to the invention.
La structure est du type en nid d'abeilles et comprend un ensemble de canaux adjacents 21 et 22, longitudinaux, d'axes parallèles entre eux, et séparés par des parois poreuses filtrantes 23. Les canaux 21 et 22 sont alternativement bouchés par des bouchons (non-représentés ) à l'une ou l'autre des extrémités de la structure de façon à définir des canaux d'entrée 21 et des canaux de sortie 22 pour le gaz à filtrer, et de façon à forcer ledit gaz à traverser les parois poreuses 23. La face représentée étant la face d'évacuation des gaz (face arrière du filtre), les bouchons bouchent les canaux d'entrée 21. Au niveau de la face opposée au contraire (face avant ou face d'admission des gaz), ce sont les canaux de sortie 22 qui sont bouchés. The structure is of the honeycomb type and comprises a set of adjacent channels 21 and 22, longitudinal axes parallel to each other, and separated by porous filter walls 23. The channels 21 and 22 are alternately blocked by plugs (not shown) at either end of the structure so as to define inlet channels 21 and outlet channels 22 for the gas to be filtered, and so as to force said gas to pass through the porous walls 23. face shown being the gas evacuation face (rear face of the filter), the plugs clog the inlet channels 21. At the opposite side to the contrary (front face or gas inlet face), these are the output channels 22 which are plugged.
En coupe transversale, les parois poreuses 23 présentent des ondulations périodiques de période 2p et d'amplitude h de façon à ce que lesdites parois poreuses 23 soient concaves par rapport au centre des canaux d'entrée 21 et convexes en leur milieu par rapport au centre des canaux de sortie 22. Le rapport R est de l'ordre de 2,4.  In cross-section, the porous walls 23 have periodic corrugations of period 2p and amplitude h so that said porous walls 23 are concave with respect to the center of the inlet channels 21 and convex in their middle with respect to the center output channels 22. The ratio R is of the order of 2.4.
La structure présente deux types de parois 23 : des parois s' étendant selon une direction x, et des parois s' étendant selon une direction y. Parmi les portions de parois s' étendant selon la direction y (mais l'on peut tenir le même raisonnement eu égard aux parois s' étendant selon la direction x) , on peut distinguer une première population de portions de paroi 24, séparant chacune un canal d'entrée 21 et un canal de sortie 22 adjacent, présentant leur concavité dans une première direction (ici vers la gauche de la figure) et une deuxième population de portions de paroi 25 présentant leur concavité dans une deuxième direction, opposée à la première direction (ici vers la droite de la figure) . Chaque paroi 23 alterne de manière périodique une portion de parois 24 appartenant à la première population puis une portion de paroi 25 appartenant à la deuxième population.  The structure has two types of walls 23: walls extending in a direction x, and walls extending in a direction y. Among the portions of walls extending in the y direction (but the same reasoning can be maintained with respect to the walls extending in the x direction), it is possible to distinguish a first population of wall portions 24, each separating a inlet channel 21 and an adjacent outlet channel 22, having their concavity in a first direction (here to the left of the figure) and a second population of wall portions 25 having their concavity in a second direction, opposite the first direction (here to the right of the figure). Each wall 23 periodically alternates a portion of walls 24 belonging to the first population then a portion of wall 25 belonging to the second population.
Chaque paroi poreuse 23 possède une première âme 26, qui correspond à une courbe périodique continue de même période que celle des canaux 21 et 22, se superposant, au niveau de chaque portion de paroi 24 de la première population, avec la ligne médiane de ladite portion 24, et une deuxième âme 27, qui correspond à une courbe périodique continue de même période que celle des canaux 21 et 22, se superposant, au niveau de chaque portion de paroi 25 de la deuxième population, avec la ligne médiane de ladite portion 25. Les deux âmes 26 et 27 de chaque paroi sont chacune une translation de l'autre d'une distance d non- nulle. Le rapport entre la distance d et l'épaisseur de la paroi est d'environ 0,3. Each porous wall 23 has a first core 26, which corresponds to a continuous periodic curve of the same period as that of the channels 21 and 22, superimposed on the level of each wall portion 24 of the first population, with the center line of said portion 24, and a second core 27, which corresponds to a continuous periodic curve of the same period as that of the channels 21 and 22, superimposed at the each of the wall portions 25 of the second population, with the median line of said portion 25. The two webs 26 and 27 of each wall are each a translation of the other by a non-zero distance d. The ratio of the distance d to the thickness of the wall is about 0.3.
La figure 4 représente la même structure que celle de la figure 3. Sont schématisés sur cette figure deux cercles, un cercle interne 31 de rayon Ri et un cercle externe 32 de rayon Re, dont les arcs épousent respectivement les faces interne 34 et externe 35 d'une portion de paroi 36. Les centres Ce et Ci des cercles respectivement externe 32 et interne 31 sont confondus. FIG. 4 represents the same structure as that of FIG. 3. Diagrammatically in this figure are two circles, an inner circle 31 of radius R 1 and an outer circle 32 of radius R e , the arcs of which respectively conform to inner and outer faces 34 and 35 of a wall portion 36. The centers C e and Ci of the respectively outer 32 and inner 31 circles are merged.
L' invention et ses avantages par rapport aux structures déjà connues seront mieux compris à la lecture des exemples non limitatifs qui suivent.  The invention and its advantages over previously known structures will be better understood on reading the nonlimiting examples which follow.
Exemple 1 (comparatif) : Example 1 (comparative):
On a synthétisé selon les techniques de l'art, par exemple décrites dans les brevets EP 816065, EP 1 142 619, EP 1 455 923 ou encore WO 2004/090294, une première population d'éléments monolithiques ou monolithes en forme de nid d'abeille et en carbure de silicium.  The first population of monolithic elements or monoliths in the form bee and silicon carbide.
Pour ce faire, de manière comparable au procédé décrit dans la demande EP 1 142 619, on mélange dans un premier temps 70% poids d'une poudre de SiC dont les grains présentent un diamètre médian dso de 10 microns, avec une deuxième poudre de SiC dont les grains présentent un diamètre médian dso de 0,5 micron. Au sens de la présente description, on désigne par diamètre médian de pore dso le diamètre des particules tel que respectivement 50% de la population totale des grains présente une taille inférieure à ce diamètre. A ce mélange est ajouté un porogène du type polyéthylène dans une proportion égale à 5% poids du poids total des grains de Sic et un additif de mise en forme du type methylcellulose dans une proportion égale à 10% poids du poids total des grains de Sic. For this purpose, in a manner comparable to the process described in application EP 1 142 619, 70% by weight of a SiC powder whose grains have a median diameter dso of 10 microns with a second SiC whose grains have a median diameter dso of 0.5 micron. For the purposes of the present description, the median diameter of the pore dso is the diameter of the particles, such that 50% of the total population of the grains has a size less than this diameter, respectively. To this mixture is added a porogen of the polyethylene type in a proportion equal to 5% by weight of the total weight of the SiC grains and a methylcellulose type shaping additive in a proportion equal to 10% by weight of the total weight of the Sic grains. .
On ajoute ensuite la quantité d'eau nécessaire et on malaxe jusqu'à obtenir une pâte homogène dont la plasticité permet l'extrusion à travers une filière configurée pour l'obtention de blocs monolithes de section carrée et dont les canaux internes présentent une section transversale illustrée schématiquement en figure 1. La demi-période p des ondulations est de 1,96 mm et correspond à la moitié de la période de la structure filtrante. Le rapport T est de 0,23.  The quantity of water required is then added and kneaded to obtain a homogeneous paste whose plasticity allows extrusion through a die configured to obtain monolithic blocks of square section and whose internal channels have a cross section. illustrated schematically in FIG. 1. The half-period p of the corrugations is 1.96 mm and corresponds to half the period of the filtering structure. The ratio T is 0.23.
Les monolithes crus obtenus sont séchés par micro-onde pendant un temps suffisant pour amener la teneur en eau non liée chimiquement à moins de 1 ~6 en masse .  The green monoliths obtained are dried by microwave for a time sufficient to bring the water content not chemically bound to less than 1 ~ 6 by mass.
Les canaux de chaque face du monolithe sont alternativement bouchés selon des techniques bien connues, par exemple décrites dans la demande WO 2004/065088.  The channels of each face of the monolith are alternately blocked according to well-known techniques, for example described in application WO 2004/065088.
Les monolithes (éléments) sont ensuite cuits sous argon selon une montée en température de 20°C/heure jusqu'à atteindre une température maximale de 2200°C qui est maintenue pendant 6 heures.  The monoliths (elements) are then fired in argon according to a rise in temperature of 20 ° C / hour until a maximum temperature of 2200 ° C is reached which is maintained for 6 hours.
Le matériau poreux obtenu, présente une porosité ouverte de 47% et un diamètre médian de pores de l'ordre de 15 micromètres. Un filtre assemblé est ensuite formé à partir des monolithes. Seize éléments issus d'un même mélange ont été assemblés entre eux selon les techniques classiques par collage au moyen d'un ciment de composition chimique suivante : 72% poids de SiC, 15% poids d'Al203, 11% poids de S1O2, le reste étant constitué par des impuretés, majoritairement de Fe2Û3 et d'oxydes de métaux alcalins et alcalino-terreux . L'épaisseur moyenne du joint entre deux blocs voisins est de l'ordre de 1,2 mm. L'ensemble est ensuite usiné, afin de constituer des filtres assemblés de forme cylindrique d'environ 14,4 cm de diamètre. Un ciment de même composition que le ciment de joint est déposé à la périphérie du filtre usiné, avec une épaisseur de 1,3 mm. The porous material obtained has an open porosity of 47% and a median pore diameter of the order of 15 microns. An assembled filter is then formed from the monoliths. Sixteen elements from the same mixture were assembled together according to conventional techniques by bonding with a cement of the following chemical composition: 72% by weight of SiC, 15% by weight of Al 2 O 3 , 11% by weight of S1O 2, the remainder consisting of impurities, predominantly Fe2Û 3 and alkali metal oxides and alkaline earth metal. The average thickness of the joint between two adjacent blocks is of the order of 1.2 mm. The assembly is then machined in order to form assembled filters of cylindrical shape of about 14.4 cm in diameter. A cement of the same composition as the joint cement is deposited at the periphery of the machined filter, with a thickness of 1.3 mm.
Les caractéristiques dimensionnelles des éléments ainsi obtenus sont données dans le tableau 1 ci-après.  The dimensional characteristics of the elements thus obtained are given in Table 1 below.
Exemples 2 à 4 (selon l'invention) : Examples 2 to 4 (according to the invention):
La technique de synthèse des monolithes décrite précédemment est reprise à l'identique, mais la filière est cette fois adaptée de manière à réaliser des blocs monolithes se caractérisant par une disposition du type de celle représentée schématiquement en figure 3.  The technique for synthesizing the monoliths previously described is identical, but this time the die is adapted so as to produce monolithic blocks characterized by an arrangement of the type shown schematically in FIG.
Les caractéristiques dimensionnelles des éléments ainsi obtenus sont données dans le tableau 1 ci-après. The dimensional characteristics of the elements thus obtained are given in Table 1 below.
Tableau 1 Table 1
Figure imgf000018_0001
Les échantillons obtenus ont été évalués et caractérisés selon les modes opératoires suivants:
Figure imgf000018_0001
The samples obtained were evaluated and characterized according to the following procedures:
Caractéristiques dimensionnelles Dimensional features
Le tableau 2 ci-après indique pour chaque exemple les caractéristiques dimensionnelles suivantes :  Table 2 below indicates for each example the following dimensional characteristics:
- l'OFA (« open front area » en anglais) ou surface de front ouverte, obtenue en calculant le rapport en pourcentage de l'aire couverte par la somme des sections transversales des canaux d'entrée de la face avant des éléments monolithiques unitaires (hormis les parois et bouchons) sur l'aire totale de la section transversale correspondante desdits éléments unitaires. Le volume de stockage des résidus est d'autant plus grand que ce pourcentage sera élevé, - the open front area (OFA) or open front area, obtained by calculating the percentage ratio of the area covered by the sum of the sections cross-sections of the inlet channels of the front face of the monolithic unitary elements (except the walls and plugs) over the total area of the corresponding cross-section of said unitary elements. The amount of storage of residues is greater the higher the percentage,
- le WALL, qui correspond au rapport, selon une coupe transversale et en pourcentage, entre la surface occupée par l'ensemble des parois d'un élément monolithique unitaire (hormis les bouchons) et l'aire totale de ladite section transversale,  - the WALL, which corresponds to the ratio, in cross-section and in percentage, between the area occupied by all the walls of a monolithic unitary element (except the plugs) and the total area of said cross-section,
- la surface spécifique de filtration d'un élément monolithique unitaire, qui correspond à la surface interne de l'ensemble des parois des canaux d'entrée filtrants exprimée en m2, rapportée au volume en dm3 de monolithe. Le volume de stockage des suies est d' autant plus élevé que la surface spécifique ainsi définie est grande. - The specific filtration surface of a unit monolithic element, which corresponds to the internal surface of all the walls of the filter inlet channels expressed in m 2 , based on the volume in dm 3 of monolith. The storage volume of the soot is all the higher as the specific surface thus defined is large.
Mesure de perte de charge : Measurement of pressure loss:
Par perte de charge, on entend au sens de la présente invention la pression différentielle existant entre l'amont et l'aval du filtre. La perte de charge a été mesurée selon les techniques de l'art, pour un débit de gaz de 250 kg/h et une température de 250°C sur les filtres neufs (non- chargés en suie) .  By pressure loss is meant within the meaning of the present invention the differential pressure existing between the upstream and downstream of the filter. The pressure drop was measured according to the techniques of the art, for a gas flow rate of 250 kg / h and a temperature of 250 ° C on the new filters (not loaded in soot).
Les résultats obtenus pour les exemples 1 à 4, directement comparables, ont été reportés dans le tableau 2. Tableau 2 The results obtained for Examples 1 to 4, which are directly comparable, have been reported in Table 2. Table 2
Figure imgf000020_0001
Figure imgf000020_0001
Les structures dont la géométrie a été choisie selon l'invention présentent par conséquent une plus faible perte de charge relativement aux structures conventionnelles, avec un gain pouvant atteindre des valeurs de l'ordre de The structures whose geometry has been chosen according to the invention consequently have a lower pressure drop relative to the conventional structures, with a gain that can reach values of the order of

Claims

REVENDICATIONS
1. Structure de filtration de gaz chargés en particules, du type en nid d'abeilles et comprenant un ensemble périodique de canaux (21, 22) adjacents longitudinaux d'axes parallèles entre eux séparés par des portions de parois poreuses filtrantes (23) , lesdits canaux (21, 22) étant alternativement bouchés à l'une ou l'autre des extrémités de la structure de façon à définir des canaux d'entrée (21) et des canaux de sortie (22) pour le gaz à filtrer, et de façon à forcer ledit gaz à traverser les parois poreuses (23) séparant les canaux d'entrée (21) et de sortie (22), ladite structure étant telle qu'en coupe transversale : 1. Filtration structure of particles-loaded gases of the honeycomb type and comprising a periodic set of adjacent longitudinal channels (21, 22) of mutually parallel axes separated by portions of porous filtering walls (23), said channels (21, 22) being alternately plugged at one or other end of the structure so as to define inlet channels (21) and outlet channels (22) for the gas to be filtered, and so as to force said gas to pass through the porous walls (23) separating the inlet (21) and outlet (22) channels, said structure being such that in cross-section:
les parois poreuses (23) présentent des ondulations périodiques de même période que celle des canaux (21, 22) de sorte que chacune des quatre portions de paroi séparant un canal d'entrée (21) d'un canal de sortie (22) adjacent est concave par rapport au centre dudit canal d'entrée the porous walls (23) have periodic corrugations of the same period as the channels (21, 22) so that each of the four wall portions separating an inlet channel (21) from an adjacent outlet channel (22) is concave with respect to the center of said inlet channel
(21) , et de sorte que le rapport R entre l'aire de chaque canal d'entrée (21) et l'aire de chaque canal de sortie(21), and so that the ratio R between the area of each input channel (21) and the area of each output channel
(22) est supérieur à 1, (22) is greater than 1,
- une première population de portions de paroi (24), séparant chacune un canal d'entrée (21) et un canal de sortie adjacent (22), présente leur concavité dans une première direction et une deuxième population (25) présente leur concavité dans une deuxième direction, opposée à la première direction, chaque paroi (23) alternant de manière périodique une portion de parois (24) appartenant à la première population puis une portion de paroi (25) appartenant à la deuxième population, chaque paroi poreuse (23) possède une première âme (26), qui correspond à une courbe périodique continue de même période que celle des canaux, se superposant, au niveau de chaque portion de paroi (24) de la première population, avec la ligne médiane de ladite portion (24), et une deuxième âme (27), qui correspond à une courbe périodique continue de même période que celle des canaux, se superposant, au niveau de chaque portion de paroi (25) de la deuxième population, avec la ligne médiane de ladite portion (25) , a first population of wall portions (24), each separating an inlet channel (21) and an adjacent outlet channel (22), has their concavity in a first direction and a second population (25) has their concavity in a second direction, opposed to the first direction, each wall (23) periodically alternating a portion of walls (24) belonging to the first population then a wall portion (25) belonging to the second population, each porous wall (23) has a first core (26), which corresponds to a continuous periodic curve of the same period as that of the channels, superimposed, at each wall portion (24) of the first population, with the line median of said portion (24), and a second core (27), which corresponds to a continuous periodic curve of the same period as that of the channels, superimposed, at each wall portion (25) of the second population, with the median line of said portion (25),
les deux âmes (26, 27) de chaque paroi (23) étant chacune une translation de l'autre d'une distance d non- nulle .  the two webs (26, 27) of each wall (23) being each a translation of the other by a non-zero distance d.
2. Structure de filtration selon la revendication 1, telle que le rapport entre la distance d et l'épaisseur moyenne d'une portion de paroi (23) est compris entre 0,01 et 0,95, notamment entre 0,05 et 0,8.  2. Filtration structure according to claim 1, such that the ratio between the distance d and the average thickness of a wall portion (23) is between 0.01 and 0.95, in particular between 0.05 and 0. 8.
3. Structure de filtration selon l'une des revendications précédentes, telle que, en coupe transversale :  Filtration structure according to one of the preceding claims, such as, in cross-section:
la face interne (34) de chaque portion de paroi épouse la forme d'un arc de cercle (31) ayant un rayon interne R± de valeur finie et un centre Ci,  the inner face (34) of each wall portion is in the form of a circular arc (31) having an internal radius R ± of finite value and a center Ci,
la face externe (35) de chaque portion de paroi épouse la forme d'un arc de cercle (32) ayant un rayon externe Re de valeur finie et un centre Ce, the outer face (35) of each wall portion is in the form of an arc (32) having an external radius R e of finite value and a center C e ,
les centres Ce et Ci sont confondus. the centers C e and Ci are merged.
4. Structure de filtration selon l'une des revendications précédentes, telle que le rapport T entre l'amplitude (h) et la demi-période (p) de la première ou deuxième âme (26, 27) est inférieur ou égal à 0,41, notamment à 0,3. 4. Filtration structure according to one of the preceding claims, such that the ratio T between the amplitude (h) and the half-period (p) of the first or second core (26, 27) is less than or equal to 0 , 41, especially at 0.3.
5. Structure de filtration selon l'une des revendications précédentes, telle que le rapport R est compris entre 1,05 et 3,0. 5. Filtering structure according to one of the preceding claims, such that the ratio R is between 1.05 and 3.0.
6. Structure de filtration selon l'une des revendications précédentes, telle que la section des canaux Filtration structure according to one of the preceding claims, such as the section of the channels
(21, 22) en coupe transversale est constante sur toute la longueur de la structure. (21, 22) in cross section is constant over the entire length of the structure.
7. Structure de filtration selon l'une des revendications précédentes, dans laquelle l'épaisseur des parois (23) est comprise entre 150 et 700 micromètres, notamment entre 200 et 500 micromètres.  7. Filtration structure according to one of the preceding claims, wherein the thickness of the walls (23) is between 150 and 700 microns, especially between 200 and 500 micrometers.
8. Structure de filtration selon l'une des revendications précédentes, dans laquelle la densité de canaux est comprise entre 1 et 280 canaux par cm2, notamment entre 15 et 65 canaux par cm2. 8. Filtration structure according to one of the preceding claims, wherein the channel density is between 1 and 280 channels per cm 2 , in particular between 15 and 65 channels per cm 2 .
9. Structure de filtration selon l'une des revendications précédentes, dans laquelle les parois (23) sont à base de carbure de silicium ou d'un matériau choisi parmi la cordiérite, l'alumine, le titanate d'aluminium, la mullite, le nitrure de silicium, les métaux frittés, un mélange silicium/carbure de silicium, ou l'un quelconque de leurs mélanges.  9. Filtration structure according to one of the preceding claims, wherein the walls (23) are based on silicon carbide or a material selected from cordierite, alumina, aluminum titanate, mullite, silicon nitride, sintered metals, a silicon / silicon carbide mixture, or any of their mixtures.
10. Structure de filtration selon l'une des revendications précédentes, dans laquelle une partie au moins de la surface des canaux d'entrée (21) est revêtue d'un catalyseur destiné à favoriser l'élimination des gaz polluants et/ou des suies.  10. Filtering structure according to one of the preceding claims, wherein at least a portion of the surface of the inlet channels (21) is coated with a catalyst for promoting the elimination of gaseous pollutants and / or soot .
11. Filtre assemblé comprenant une pluralité de structures de filtration selon l'une des revendications précédentes, lesdites structures étant liées entre elles par un ciment. 11. An assembled filter comprising a plurality of filter structures according to one of the preceding claims, said structures being bonded together by a cement.
12. Utilisation d'une structure de filtration ou d'un filtre assemblé selon l'une des revendications précédentes comme dispositif de dépollution sur une ligne d'échappement d'un moteur, notamment Diesel. 12. Use of a filter structure or an assembled filter according to one of the preceding claims as a pollution control device on an exhaust line of an engine, including diesel.
13. Filière d'extrusion conformée de manière à former, par extrusion d'une matière céramique, une structure de filtration selon l'une des revendications 1 à 10.  An extrusion die shaped to form, by extrusion of a ceramic material, a filter structure according to one of claims 1 to 10.
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