EP2755741A1

EP2755741A1 - Honeycomb element with reinforced corners

Info

Publication number: EP2755741A1
Application number: EP12773022.4A
Authority: EP
Inventors: Bernard Bouteiller
Original assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Current assignee: Saint Gobain Centre de Recherche et dEtudes Europeen SAS
Priority date: 2011-09-14
Filing date: 2012-09-12
Publication date: 2014-07-23
Also published as: WO2013038103A1; US20140356570A1; FR2979837A1; FR2979837B1; US9546115B2

Abstract

Monolithic honeycomb element (1) comprising a plurality of adjacent channels (2,3) with axes parallel to each other, separated by walls (4) made of a porous material, said element (1) having, in cross section, a polygonal, particularly square, cross section, delimited by outer wall elements (6-9), said element being characterised in that at least one corner (11) of said polygon, and preferably all the corners of the polygon, have an extra thickness e_c according to the bisector (13) of the angle at said corner (11), such that the total thickness E_c of the outer wall, also measured according to said bisector (13) of the angle at said corner (11), is greater than average thickness E of said outer walls by a factor of at least 1.43, said extra thickness e_c being obtained at least in partlby an addition (10) of porous material to the outer face of said corner.

Description

BEAN NEST ELEMENT WITH REINFORCED CORNERS

The invention relates to the field of honeycomb structures, more particularly for thermal applications, in particular heat exchangers or particulate filters used in an exhaust line of an engine for the removal of soot produced by the combustion of a diesel fuel or gasoline in an internal combustion engine. Filtration structures for soot contained in the exhaust gas of an internal combustion engine are well known in the prior art. These structures most often have a honeycomb structure, one of the faces of the structure for the admission of the exhaust gases to be filtered and the other side the exhaust of the filtered exhaust gases. The structure comprises, between the intake and discharge faces, a set of adjacent ducts or channels of axes parallel to each other separated by porous filtration walls, which ducts are closed to one or the other of their ends for delimiting input chambers s' opening according to the inlet face and outlet chambers opening along the discharge face. For a good seal, the peripheral part of the structure is most often surrounded by a coating cement. The channels or conduits are alternately closed in an order such that the exhaust gases, during the crossing of the honeycomb body, are forced to pass through the side walls 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. Most often, Filter bodies used in automotive exhaust lines are porous ceramic material, for example cordierite, aluminum titanate or silicon carbide or silicon nitride.

In known manner, during its implementation, the particulate filter is subjected to a succession of filtration phases (accumulation of soot) and regeneration (removal of soot). During the filtration phases, the soot particles emitted by the engine are retained and are deposited inside the filter. During the regeneration phases, the soot particles are burned inside the filter, in order to restore its filtration properties. The porous structure is then subjected to intense thermomechanical stresses, which can cause microcracks likely over time to cause a severe loss of filtration capacity of the unit, or even its complete deactivation. This phenomenon is particularly observed on monolithic filters of large diameter. It has indeed been observed, operating in an exhaust line, that the thermal gradient between the center and the periphery of such structures is even higher than the dimensions of the monolith are important.

To solve these problems and increase the life of the filters, it has been proposed more recently filtration structures associating several blocks or monolithic elements in honeycomb. The elements are most often assembled together by gluing by means of a glue or cement of a ceramic nature, hereinafter called seal cement. Examples of such filter structures are for example described in patent applications EP 816,065, EP 1 142 619, EP 1 455 923, WO 2004/090294 or WO 2005/063462. In order to ensure a better relaxation of the stresses in an assembled structure, it is known that the thermal expansion coefficients of different parts of the structure (filter elements, coating cement, joint cement) must be substantially of the same order. As a result, said parts are advantageously synthesized on the basis of the same material, most often silicon carbide (SiC), cordierite or aluminum titanate. This choice also makes it possible to homogenize the distribution of heat during the regeneration of the filter.

During the manufacturing process of such filtering structures, unitary elongate elements are firstly extruded from a loose slip of particles of the preceding material, most often including porogenic organic materials, and then fired in such a way as to obtain elements in particular. honeycomb, suitable for the filtration of particulate-laden gases by the porous walls that constitute them. These unitary ceramic elements or monoliths have a section in a polygonal radial section, usually of quadrangular type, in particular square.

According to other possible modes, the sections may be triangular or even more rarely hexagonal or a mixture of these different forms (triangular, quadrangular, hexagonal).

Conventionally in the process of obtaining a large honeycomb structure (whether the walls are closed off or not by plugs at one or the other end), unitary elements of square section (which are taken for reference in the following description without however being limited) are assembled by means of a seal cement first affixed to the outer walls of the unitary elements, then baked at a generally temperature typically less than 1000 ° C, so as to ensure the assembly a sufficient cohesion, without stiffening it in proportions that tend to to render its mechanical properties comparable to that of a monolithic structure of the same size.

The production of the assembled structures is therefore necessarily the result of a compromise between the desired adhesion between the unit elements and a limited value of the overall rigidity of the assembled structure finally obtained.

Such a problem is all the more crucial as the assembled structure must most often be readjusted to its external dimensions in order to adapt it to its housing in the exhaust line. Most often, the large structure must for example be in the form of an elongated cylinder, of more or less elliptical section according to the specifications imposed by each car manufacturer (here is meant by car manufacturer, the manufacturer of private vehicles but also the one that makes commercial vehicles, including trucks). To obtain such adapted shapes, it is necessary to eliminate the peripheral portion of the structures assembled from the square section elements by abrasion and finally cover the filter cut to the right dimensions by an external coating cement to smooth the outer surface according to the principles described, for example, in applications EP 1142619 A1, EP 1516659 A1 or US 2006/289501 A1.

However, during the implementation of the various solutions described in these prior publications, the applicant company has found that the assembly of monolithic ceramic honeycomb elements poses reliability problems of the process for obtaining the assembled filter, especially when one or more peripheral elements in the assembly are subjected to a transverse stress, that is to say in the plane perpendicular to the direction formed by the channels of the Honeycomb. Such a solicitation results in particular from the forces applied during the abrasion and the elimination of the peripheral portions of the assembled filter, with a view to its resizing. If the cement does not allow sufficient grip, it is common that some of the unitary elements, including the most peripheral in the assembly, then detach from the entire structure.

The surface of the ceramic honeycomb unit elements is also very smooth and the attachment of the cement nevertheless seems to require cements having a very strong adhesion. As previously described, this strong adhesion can quickly be problematic because it helps to stiffen the whole structure. In such a case, when the final assembled filter is subjected to high temperatures related to the burning of soot (typically more than 800 ° C), the expansion phenomena can then create internal mechanical stresses likely to damage and dissociate the together, as explained previously.

Solutions have been proposed to reduce the rigidity of the cement by using compositions leading to a joint material having a lower modulus of elasticity as in EP1249262. Sets with joints based on ceramic fibers are thus unveiled by EP0816065, allowing better grip.

Other solutions exist, which consist in making nonadhering zones to relax the stresses or reduce the rigidity of the assembly (FR2833857A1 or FR2853256A1). These solutions, which provide a significant improvement with regard to the previously observed tendency to separate the unit elements, remain however perfectible, in particular as regards the resistance to radial mechanical stresses exerted on the structure during the step of resizing the assembled structure.

For the purposes of the present invention and as indicated above, the term "radial mechanical stresses" means stresses contributing to dissociating the unit blocks of the assembly with respect to the main central axis of the assembly parallel to the channels of the honeycombs. The assembly can also be weakened during handling operations when the cement is not yet completely set and when the hardening is not completed. The most peripheral elements, in particular those whose essential of the initial material must be eliminated during the machining, are obviously those which have the greatest probability of detaching.

Solutions exist to eliminate the machining operation such as for example EP1977808A1 by assembling peripheral unit blocks of final shape. The problem of manipulation persists. Furthermore this solution is industrially inflexible because it assumes the management of at least three families of different unit blocks by filter form and an adaptation of the dies for each of the elements. In addition, the dies must be changed as soon as the final shape of the filter changes. Thus, on the basis of an appropriate and specific die, this solution makes it possible to achieve only honeycomb structures assembled to a fixed outer diameter and single, without possible adaptation.

The present invention therefore essentially aims to solve the previously discussed manufacturing problems and in particular to provide elements for the formation of a structure formed by the assembly of honeycombs maintaining its integrity even under strong radial mechanical stresses , of the type exercised during of its machining in order to obtain the final external dimensions of the structure.

More specifically, the present invention relates in a first aspect to a monolithic honeycomb element comprising a set of adjacent conduits parallel axes between them separated by walls made of a porous material. The element has, in a cross section, a polygonal section, in particular quadratic, delimited by external wall elements of average thickness E. It is characterized in that at least one corner of said polygon, preferably all corners of the polygon, present, according to the bisector of the angle at said corner, an excess thickness e _c , so that the total thickness E _c of the outer wall, also measured along the bisector of the angle at said corner, is greater than the average thickness E (expressed in the same unit) of said outer walls by a factor of at least 1.43. According to another essential characteristic of the present invention, said excess thickness e _c is obtained at least partly by an additional material disposed on the outer face of said corner.

According to advantageous embodiments of the present invention, which can of course possibly be combined with one another:

- The monolithic element has a section, in a cross section, of substantially quadratic or triangular shape and the corners have aperture angles at the vertex a between 60 and 120 °.

- The monolithic element has a section, in a cross section, of substantially square shape.

Said overthickness has a substantially rounded outer edge s' registering in a radius of curvature R between 0.3 and 3 mm.

Said excess thickness, in cross-section, extends over a length li and has a maximum value e ₁ according to the first wall element constituting the corner and over a length 1 ₂ and said extra thickness has a maximum value e ₂ according to the second element wall forming the corner, so that:

The lengths li and 1 ₂ described above are substantially equal.

- The oversize ei and e ₂ described above are substantially equal.

The ratio of the total thickness E _c of the external wall, also measured along the bisector of the angle at the said corner (11), to the average thickness E of the said walls is greater than or equal to 1.45; preferably greater than 1.5 and most preferably greater than 1.6.

The ratio of the total thickness E _c of the external wall, also measured along the bisector of the angle at the said corner, to the average thickness E is less than or equal to 2.8, preferably less than or equal to 2.5 and even more preferably less than 2.

- The average thickness E of the outer walls of the channels is between 100 and 1000 microns, preferably between 300 and 800 microns.

- The thickness of the inner walls of the channels is between 100 and 1000 microns, preferably between 200 and 600 microns.

- said ducts are closed by plugs to one or the other of their ends to define inlet ducts opening along a gas intake face and outlet ducts opening along a gas evacuation face, so that the gas passes through the walls porous.

- Said overthickness extends over the entire length of

1 element.

- The porous material is silicon carbide (SiC), silicon nitride or aluminum titanate, in particular silicon carbide. The porous material constituting the unitary element is, for example, recrystallized silicon carbide at a temperature of between 2100 and 2400 ° C.

According to another aspect, the present invention also relates to a structure, in particular a particle filter, obtained by assembling a plurality of monolithic elements as previously described, said elements being bonded by a joint cement.

Preferably, in such a structure, the ratio of the excess thickness ei on the average thickness of the joint cement between two constituent elements (measured according to the same unit, of course), according to said transverse plane, is less than or equal to 0, 4 and / or the ratio of the excess thickness e ₂ on the average thickness of joint cement, always according to said transverse plane, is less than or equal to 0.4. Preferably these two ratios are less than or equal to 0.4.

Preferably, the joint cement comprises grains and / or a matrix of a ceramic nature.

Advantageously, within the filter, said elements and the joint cement essentially comprise the same ceramic material, and preferably are based on silicon carbide (SiC). The invention will be better understood on reading the description of various embodiments of the invention which follow, respectively illustrated by FIGS. 1 and 2.

FIG. 1 schematizes a cross-section and in perspective of a monolithic element according to the invention.

FIG. 2 diagrammatically and in more detail voluntarily exaggerates, according to this same cross-section, the corner portion of the monolithic element according to the invention.

According to well-known techniques, all the monolithic elements are advantageously obtained by extrusion of a loose paste, for example silicon carbide, to form after baking a porous honeycomb structure. The shape of the extruder head is configured according to conventional methods, for example as described in US Pat. No. 5,761,787, for obtaining and forming honeycomb elements having the wedge thicknesses according to the invention. , as schematized in Figures 1 and 2 which follow.

Without this being considered restrictive, the extruded structure is presented according to FIG. 1 in the form of a block or monolithic unitary element 1 whose external shape is that of a rectangular parallelepiped extending along a longitudinal axis between its faces. upstream and downstream. Its cross section is substantially square. On the ends of the elements 1 open a plurality of adjacent channels 2, 3, whose main axis is parallel to the longitudinal axis L of the block.

In known manner but not shown in the figures, the extruded porous structures may be alternately plugged on their upstream face or on their downstream face by upstream and downstream plugs, respectively, to form outlet channels 3 and inlet channels 2, respectively, for the formation of filtering structures. Each channel 2 or 3 then defines an interior volume delimited by internal walls 4, a closure plug (not shown in the figures) disposed either on the upstream face for an outlet channel, or on the downstream face for a control channel. an inlet and an opening opening alternately towards the downstream face or the upstream face, such that the inlet and outlet channels 3 and 3 are in fluid communication by the internal walls 4.

In a known manner and not shown in the figures, several monolithic unit elements 1 are assembled together by bonding by means of a cement cement of a ceramic nature, for example also based on silicon carbide, into a filtration structure or filter assembled. The assembly thus formed must then be machined to take, for example, a round or ovoid section, then for example to be covered with a coating cement to seal and have a smooth outer surface.

When the monolithic elements are plugged as described above, this results in an assembled filter capable of being inserted into an exhaust line, according to well-known techniques. In operation, the flow of the exhaust gases enters the filter through the inlet channels 2, then passes through the filtering internal walls 4 of these channels to join the outlet channels 3. For more precision with regard to the structure monolithic elements and their assembly for the formation of a filter, one can for example refer to the applications EP 1142619, WO 05/063462 or WO 05/016491.

In the mode shown in Figure 1, the section transversal input channels 2 is different from that of the output channels 3. Thus, the cross sections of the input channels 2 are greater than those of the output channels 3, to increase the overall volume of the input channels at the expense of that of the exit channels. In the mode illustrated by the figures, the walls 4 follow one another in cross-section and in a horizontal (along the x-axis) or vertical (along the y-axis) row of channels to define a sinusoidal or wave shape. (wavy in English). For example, the wall elements wave substantially a half-period of sinusoid over the width of a channel.

In an application of the structure as a filter, the storage capacity of the particles per unit element 1 is thus advantageously increased. However, it would not come out of the invention if the cross sections of the input and output channels were identical and the walls 4 planar.

External wall elements 6, 7, 8, 9 of thickness E complement and surround the inner walls 4. These wall elements meet and intersect two by two along a bisector, at the corners having opening angles. at the top a = 90 ° for the square section structure shown in FIG.

FIG. 2 diagrammatically and in greater detail shows, in the same cross section, the corner portion of the monolithic element described in FIG. 1. More precisely, FIG. 2 illustrates in greater detail (and in an exaggerated manner to facilitate comprehension and reading), the profile of the wedges 11 having extra thicknesses 10. According to the invention, the filtering elements are therefore characterized, at the wedge 11, by the presence of an excess thickness 10, in the form a supplementary material disposed at the outer portion 12 of the corner 11. This extra thickness is characterized by an additional layer of material relative to the conventional configuration described in the documents of the art, illustrated for example in the application EP0816065.

More precisely, according to the conventional configuration and as represented by the dashed lines in FIG. 2, the two wall elements 6 and 7 meet to form the outer corners of the unitary element, according to straight edges having an angle of 90 °, to form outer edges along the entire length of the element.

According to the invention and as shown in FIG. 1, an additional quantity of material, of a thickness e _c measured along the bisector 13 of the angle formed by the wall elements 6 and 7, is disposed at said corner 11. According to the invention, this additional quantity of material is present on the outer side (the edge) 12 of the corner of the element such that, along said bisector 13, the value e _c contributes to the total thickness E _c of the outer wall, and in such a way that said total thickness E _c of the wall, again according to this same bisector, is greater than the average thickness E of said walls 6 and 7 by a factor of at least 1, 43, preferably at least 1, 45 or even at least 1.5, or very preferably at least 1.6.

As represented in FIG. 2, said extra thickness preferably has a rounded outer edge, in particular inscribed in a radius of curvature R between 0.3 and 3 mm, the center of the circle inscribed along the rounded outer edge being placed on said bisector 13.

According to the invention, said excess thickness extends over a length li on the first wall element 6 constituting the wedge 11 (that is to say in the X direction) and over a length 1 ₂ on the second wall element 7 constituting the corner 11 (that is to say in the Y direction). Preferably, the ratio of the lengths li and 1 ₂ is between 0.5 and 2 and very preferably is close to 1 or equal to 1. According to the invention, said excess thickness 10 has a maximum value ei vis-vis the first wall element 7 constituting the corner and a maximum value Θ2 vis-a-vis the second wall element 6 constituting the corner. Preferably, the ratio of the lengths e 1 and e 2 is between 0.5 and 2 and very preferably is close to 1 or equal to 1.

Typically, the element channel density is from 1 to about 280 c / cm ² , preferably from about 14 to about 62 c / cm ² . According to the invention, the extra thickness at the corners preferably extends over the entire length L of the element, from the upstream face to the downstream face.

The invention and its advantages will be better understood on reading the example which follows, given purely by way of illustration.

Example:

The elements according to the invention have been synthesized in a conventional manner:

More specifically, a population of monolithic elements in the form of a honeycomb and, for example, those described in patents EP 816065, EP 1142619, EP 1455 923 or WO 2004/090294, has been synthesized according to the techniques of the art, for example silicon carbide.

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 d ₅ o of 10 microns are mixed initially with a second one. SiC powder whose grains have a median diameter of ₅ o 0.5 micron. For the purposes of the present description, the median pore diameter d ₅ o is the diameter of the particles such as respectively 50% of the total population of grains has a size smaller than this diameter. 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. .

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 Figure 1. The half-period p ripples is 1.83mm.

The green monoliths obtained are dried by microwave for a time sufficient to bring the water content not chemically bound to less than 1% by weight.

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 (elements) are debonded then baked under argon according to a rise in temperature of 20 ° C / hour until reaching a maximum temperature of 2200 ° C 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.

The structural characteristics of the elements thus obtained are given in Table 1 below, in connection with the data already described in the previous description of FIGS. 1 and 2. For the formation of the filter, 16 elements (4x4) are then assembled together 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 SiO ₂ the remainder being impurities, usually Fe ₂ O _3, and alkali and alkaline-earth metal oxides. The average thickness of the joint between two adjacent blocks is of the order of 2 mm. The thermal conductivity of the joint cement after heat treatment is about 2.1 W / mK at room temperature and its measured open porosity is about 38%.

The assembly is then machined by abrasion, the most peripheral parts being eliminated in order to constitute assembled filters of cylindrical shape. A cement of the same composition as the joint cement is deposited at the periphery of the machined filter at an average thickness of 1 mm in order to smooth the outer surface of the cylindrical filters.

A plurality of assembled filters have thus been made from the unit elements.

Section Size Length Geometry Density of Elements Elements Elements Channel Channels

(section) (cm) internal (c / cm ² )

(mm x mm)

square 35.8 35.8 25.4 "Wavy" About 30

Number Thickness of Thickness Thickness of Thickness of elements final joint of the average wall E of assemblies (mm) external filter internal walls (mm) (μπι) external

(Μπι)

16 1.2 144 370 650

Thickness Thickness max Thickness max li e _c according to the total E _c ei e ₂ (mm) bisector according to (μπι) (μπι)

(μπι) bisector

(Μπι)

420 1136 300 300 10

1 ₂ Ray of

(mm) curvature R

(Mm)

10 2

Table 1 In the step of resizing the filters, unlike the filters known in the art, there was no evidence of tearing of the peripheral unit elements of the pre-assembled structures, under the forces allowing the abrasion of the peripheral parts of the filtered. Such a result clearly indicates that the polygonal filter elements according to the present invention, having a supplement of porous material on the outer faces of the corners, make it possible to significantly improve the cohesion of the filtering elements within the structure, especially when it must be subjected to significant radial mechanical stresses.

In the foregoing description, the advantages of the present invention have been mainly exposed in relation to the honeycomb structures used as particulate filters in an exhaust line of an internal combustion engine, allowing the elimination of soot produced by the combustion of a diesel fuel or gasoline. The invention is obviously not limited to such an application and also finds application in all areas where the previously discussed problems arise, particularly in the field of heat exchangers.

Claims

A monolithic honeycomb element (1) comprising a set of adjacent conduits (2, 3) with mutually parallel axes separated by internal walls (4) made of a porous material, said element (1) having, in a cross section, a polygonal section, in particular quadratic, delimited by outer wall elements (6-9), said element being characterized in that at least one corner (11) of said polygon, preferably all corners of the polygon , according to the bisector (13) of the angle at said wedge (11), has an excess thickness e _c , so that the total thickness E _c of the external wall, also measured along said bisector (13) of the angle at said corner (11) is greater than the average thickness E of said outer walls by a factor of at least 1.43, said excess thickness e _c being obtained at least partly by a supplement (10) of material on the outer face of said corner.

2. A monolithic element according to claim 1 having a section, in a cross section, of substantially quadratic or triangular shape and whose corners have aperture angles at the apex of between 60 and 120 °.

3. monolithic element according to one of claims 1 or 2, having a section in a cross section, substantially square shape.

4. monolithic element according to one of claims 1 to 3 wherein said extra thickness has a substantially rounded outer edge s' enrolling in a radius of curvature R between 0.3 and 3 mm.

5. Monolithic element according to one of the preceding claims, wherein in a cross section, said extra thickness (10) extends over a length li and has a maximum value ei according to the first wall element (7) constituting the corner and over a length 1 ₂ and has a maximum value e ₂ according to the second wall element (6) constituting the corner, and wherein:

6. monolithic element according to the preceding claim, wherein the lengths li and 1 ₂ are substantially equal and / or wherein the thicknesses ei and e ₂ are substantially equal.

7. Monolithic element according to one of the preceding claims wherein the ratio of the total thickness E _c of the outer wall, measured along the bisector of the angle at said corner (11), on the average thickness E of said walls is greater than or equal to 1.45, preferably greater than 1.5, and very preferably greater than 1.6.

8. Monolithic element according to one of the preceding claims, wherein the average thickness E of the outer walls of the channels is between 100 and 1000 microns.

9. monolithic element according to one of the preceding claims, wherein said ducts are closed by plugs at one or the other of their ends to define inlet ducts (2) s 'opening on a side d' intake of gases and outlet pipes (3) opening on a gas evacuation face, in such a way that the gas passes through the porous walls (4).

10. Monolithic element according to one of the preceding claims, wherein said thickness (10) is present over the entire length of the element.

Monolithic element according to one of the preceding claims, wherein the porous material is silicon carbide (SiC) or silicon nitride.

12. Assembled structure, in particular particulate filter, obtained by one assembly of a plurality of monolithic elements according to one of the preceding claims, said elements being bonded by a joint cement.

13. Assembled structure according to the preceding claim, wherein the ratio of the excess thickness ei on the average thickness of the joint cement between two constituent elements, according to said transverse plane, is less than or equal to 0.4 and / or in which the ratio of the excess thickness e ₂ on the average thickness of cement joint, according to said transverse plane, is less than or equal to 0.4.