MONOLITHIC ELEMENT WITH REINFORCED CORNERS FOR THE FILTRATION OF PARTICLES

Abstract

The invention relates to a honeycomb monolithic element for the filtration of particles, characterized in that the peripheral channels forming part of the outer wall of said element are configured to help in the formation of an outer wall whose inner face is substantially flat over the whole length of the element and in that said flat wall also has an increased thickness at the corners, so that, on a cross section, the ratio R of the thickness Ec of the wall, measured along the bisecting line of the angle at a corner of the element, over the minimal thickness Emin of said wall is greater than 1.5.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 schematizes a cross section of a monolithic element according to WO 05/063462;

FIG. 2 represents schematically, in cross section, the corner portion of monolithic elements according to the invention;

FIG. 3 illustrates a cross section of a monolithic element according to a first embodiment of the invention, in which the outer walls of the element have an inner face that is substantially flat over the whole length of the element, said outer walls also having a continuous and constant increased thickness at the corners;

FIG. 4 schematizes a cross section of a monolithic element according to a second embodiment of the invention in which the outer walls of the element have an inner face that is substantially flat over the whole length of the element, said outer walls also having a continuous and decreasing increased thickness from the corners toward the center of the sides of the element;

FIG. 5 represents a cross section of a monolithic element according to a third embodiment of the invention in which a continuous and decreasing increased thickness of the corners, in a nonlinear manner, results from a curvature radius R₁ of the outer wall at the inner face that is greater than the radius R₂ of the outer face; and

FIG. 6 illustrates a cross section of a monolithic element combining the features of the aforementioned second and third embodiments of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views.

According to well known techniques, all the monolithic elements are advantageously obtained by extruding a friable paste, for example made of silicon carbide, to form a porous honeycomb structure.

Without this being considered restrictive, the extruded porous structure has the shape of monolithic blocks 1 whose external shape is that of a rectangular parallelepiped extending along a longitudinal axis between two upstream and downstream faces, as shown in FIG. 1. Its cross section is substantially square. A plurality of adjacent channels 2, 3, whose main axis is parallel with the longitudinal axis of the block, open onto the ends of the elements 1.

In a known manner, but not shown in the figures, the extruded porous structures are alternately stopped 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. Each channel 2 or 3 thus defines an internal volume delimited by side walls 4, a stopping plug (not shown in the figures) placed either on the upstream face for an outlet channel, or on the downstream face for an inlet channel and an aperture opening alternately toward the downstream face or the upstream face, so that the inlet channels 2 and outlet channels 3 are in fluid communication through the side walls 4.

In a known manner and not shown in the figures, the monolithic elements 1 are then assembled, by bonding by means of a joint cement of a ceramic nature, for example also silicon carbide-based, into a filtration structure or an assembled filter. The assembly thus constituted may then be machined to take, for example, a round or ovoid cross section, then for example covered with a coating cement to give it a seal. The result of this is an assembled filter capable of being inserted into an exhaust line, according to well known techniques. In operation, the stream F of the exhaust gases enters the filter through the inlet channels 2, then passes through the filtering side walls 4 of these channels to join the outlet channels 3.

For more details on the structure of monolithic elements and of their assembly for the formation of a filter, it is possible for example to refer to applications WO 05/063462 or else WO 05/016491.

According to the embodiment described in application WO 05/063462 and illustrated in FIG. 1, the sets of inlet channels 2 and of outlet channels 3 are nested in one another so as to form, in cross section, a checkerboard pattern in which said inlet channels alternate with said outlet channels, in the upward direction (direction y) and in the widthwise direction (direction x).

In the embodiment shown in FIG. 1 and according to the invention, the cross section of the inlet channels 2 is different from that of the outlet channels 3. Thus, the cross sections of the inlet channels 2 are larger than those of the outlet channels 3, in order to increase the overall volume of the inlet channels at the expense of that of the outlet channels. The soot storage capacity per individual element 1 is thus advantageously increased.

The inlet and outlet channels are therefore delimited by nonflat wall elements 4, concave on the side of the inlet channels and convex on the side of the outlet channels.

Preferably, the wall elements succeed one another, in cross section and along a horizontal row (along the x axis) or vertical row (along the y axis) of channels, to define a sinusoidal or wavy shape. The wall elements undulate for example substantially by a half sine wave period over the width of a channel.

The channels located on the periphery of an element 1 are called peripheral channels 8. The channels of which no portion forms part of the outer wall 7 are called inner channels 9.

The walls 4 of the peripheral channels comprise a portion 5 forming part of the outer wall 7 of the element, that is to say in contact with the outside of the element and an inner wall portion 6, shared with adjacent channels 8 or 9.

The walls of the inner channels 9 are characterized by a thickness e which is preferably substantially constant in all the volume of the element. Preferably, the inner wall portion 6 of the peripheral channels 8 also has this thickness e.

The structure according to the prior art is characterized by a globally reinforced outer wall 7 whose inner face 7′ is not flat but also has a surface of the wavy type. The outer peripheral wall 7 of the monolithic element is thus reinforced by an average and periodic increased thickness of material placed evenly on its four faces and extending over the whole length L of said element, from its upstream face to its downstream face, as shown in the cross section of FIG. 1.

FIG. 2 represents schematically, in cross section, the reinforced corner part of monolithic elements according to the invention. More precisely, FIG. 2 illustrates different profiles of the increased thicknesses E_c at the corners. According to the invention, the ratio of E_c, measured as illustrated in FIG. 2 along the bisecting line of the angle at a corner over the minimal thickness of the outer wall of the element E_min, is greater than 1.5. Such a feature advantageously leads to a better compromise between the thermomechanical strength and the total weight of the element.

As shown in FIG. 2, said increased thickness E_c in the corner may advantageously be defined:

on one hand, according to a curvature radius R₁ of the inner face of the outer wall at a corner of the element,

on the other hand, according to the shape of the outer face of the outer wall at said corner.

According to a first example illustrated by FIG. 2, the element has, in the corner, outer surfaces substantially rounded according to a curvature radius R₂, defining an increased thickness E_C1 in the corner. The curvature radius R₂ is, according to the invention, preferably less than or equal to R₁. Without departing from the context of the invention, R₂ could however also be greater than R₁.

According to a second example also illustrated by FIG. 2, the corners of the element may also have substantially flat outer surfaces, typically in the form of bevels, which define an increased thickness E_C2 in the corner.

As shown in FIG. 2, to characterize the elements according to the invention, also defined is a maximal increased thickness E′ of the walls “except corner”, that is to say independent of the reinforcement effect associated with the difference of the curvature radii in the corner.

This dimension E′ may be characterized by:

either a constant value over the whole length d₁′ of said increased thickness (E′₁ in FIG. 2),

or an increasing and regular variation in the direction of the corner over the whole length d₂′ of the increased thickness (maximal value E′₂ in FIG. 2).

According to the invention, in this latter case, the regular increase of said increased thickness is measured by an angle α lying between 0.5 and 45°, for example lying between 1 and 10°.

FIG. 3 illustrates a first embodiment of the invention. In all the figures, the elements and parts of the same type are indicated by the same reference numbers. With reference to FIG. 2, in this embodiment, the angle α is equal to 0 and the curvature radius R₁ of the inner face of the wall is equal to the curvature radius R₂ of the outer face. Without departing from the context of the invention, it is clearly evident that the rounded corner of the outer face or the inner face could be replaced by a bevel.

The monolithic element conventionally comprises a central part whose inlet channels 2 and outlet channels 3 have a cross section whose shape accords with the teaching of application WO 03/064132. According to this embodiment, the wall elements succeed one another, in cross section and following a horizontal or vertical row of channels, to define a sinusoidal or wavy shape, as already represented in FIG. 1. The wall elements undulate by a half sine wave period over the width of a channel.

Typically, the density of channels of the central and peripheral parts lies between 6 and 1800 cpsi, preferably between 90 and 400 cpsi (cells per square inch, 1 cpsi being equal to 1 cell/6.45 cm²).

According to the embodiment illustrated by FIG. 3, the outer walls 7 of the element have an inner face 7′ that is substantially flat over the whole length of the element. Such a configuration of the outer wall 7 is obtained in a known manner, without difficulty, by the use of an appropriate extrusion die. The peripheral channels 8 have, according to this embodiment, an inner wall portion 6, shared with adjacent channels 8 or 9, whose shape and thickness is identical to that of the inner channels 9 and a flat portion 5 forming part of the outer wall 7 of the element. This flat portion 5 has a minimal thickness E_min, for example measured in the middle of a side of the element, greater than the average thickness e of the inner walls by a factor at least equal to 1.2, preferably greater than 1.4.

According to this embodiment, the outer walls 7 differ not only by the flatness of their inner face 7′ but also by the presence of an increased thickness 10 present at the corners 11 of the element. According to the first embodiment of the invention illustrated by FIG. 3, this increased thickness extends over the whole length L of the element and for each side of the element over a distance d from the corners. The increased thickness according to this embodiment is characterized in that it is continuous and constant over its whole length L and its whole width d.

FIG. 4 illustrates a second embodiment of the invention identical to the preceding embodiment but for which the increased thickness 10′ present at the corners 11 is not constant over its length d but decreases in a substantially regular manner along the side of the element, from the corner 11, where the thickness is maximal, and in the direction of the middle of the side, where the thickness of the outer wall is minimal, as illustrated by the cross section of FIG. 4.

Without departing from the context of the invention, it is clearly evident that the rounded corner of the outer face or even the inner face could be replaced by a bevel.

In this second embodiment, the angle α thus formed (see FIG. 2) lies between 0.5 and 45° and the curvature radius R₁ of the inner face of the outer wall as defined with reference to FIG. 2 is equal to that of the curvature radius R₂ on the outer face.

FIG. 5 illustrates a third embodiment of the invention in which the element is essentially reinforced at its corners. The corners of the element are rounded and the curvature radius R₂ of the outer face of the outer wall is less than the curvature radius R₁ of the inner face of the outer wall.

FIG. 6 illustrates a fourth embodiment of the invention, in which the curvature radius R₂ is less than the curvature radius R₁ and in which the increased thickness 10′ extends over the sides of the element along a width d, in a decreasing and substantially regular manner from the corner and in the direction of the middle of the sides in a manner similar to the embodiment illustrated by FIG. 4.

According to the invention, the reinforcement of the corners preferably extends over the whole length L of the element, from the upstream face to the downstream face.

In the case where the corners have a rounded shape, the width d of the reinforcement is advantageously determined, according to the invention, as a function of the curvature radii R₂ and R₁. Typically, the radii R₂ and R₁ are chosen so that the width d of the reinforcement lies between 0.5 and 8 times the width of a cell, preferably between 1 and 6 times the width of a cell.

The porous material forming the individual elements is preferably made of silicon carbide recrystallized at a temperature lying between 2100 and 2400° C.

The invention and its advantages will be better understood on reading the following examples given purely as illustrations.

All the elements of the following examples have been synthesized in a conventional manner and as described in Example 1:

EXAMPLE 1

According to the Prior Art

According to the techniques of the art described, for example, in patent applications EP 816 065, EP 1 142 619, EP 1 455 923 or else WO 2004/090294, a first population of monolithic honeycomb-shaped elements made of silicon carbide have been synthesized.

To do this, the following elements are mixed in a mixer:

3000 g of a mixture of silicon carbide particles more than 98% pure and having a particle size such that 70% by weight of the particles has a diameter greater than 10 micrometers, the median diameter of this particle size fraction being less than 300 micrometers. Within the meaning of the present description, the median diameter is the diameter of the particles below which 50% by weight of the population is found.

150 g of an organic binder of the type derived from cellulose.

Water is added and mixing takes place until a smooth paste is obtained whose plasticity allows extrusion, the die being configured to obtain monolithic blocks whose channels and outer walls have a wavy structure illustrated by FIG. 1 and according to the teaching of application WO 05/064132.

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

The channels of each face of the monolith are then alternately stopped according to well known techniques, for example described in application WO 2004/065088.

The monolithic block is then baked at a temperature rise of 20° C./hour until a temperature of the order of 2200° C. is reached which is maintained for 2 hours.

The material obtained has an open porosity of 47% and an average pore distribution diameter of the order of 15 μm.

The structural features of the elements thus obtained are given in Table 1 below.

The inner face of the outer wall has a wavy surface such that the thickness of the outer wall varies between a minimum of 544 microns and a maximum of 767 microns. The average thickness of the outer wall is approximately 660 microns.

For the formation of the filter, the elements originating from one and the same mix are then assembled together by bonding by means of a cement with the following chemical composition: 72% by weight of SiC, 15% by weight of Al₂O₃, 11% by weight of SiO₂, the rest consisting of impurities, mainly Fe₂O₃ and alkaline metal oxides and alkaline earths. 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 of the order of 2.1 W/m.K at ambient temperature and its measured open porosity is approximately 38%.

The assembly is then machined, in order to produce cylinder-shaped assembled filters.

EXAMPLE 2

According to the Invention

The synthesis technique previously described is repeated identically, but the die this time is adapted so as to produce monolithic blocks characterized by a wavy disposition of the inner channels, outer walls whose inner face is substantially flat, and a constant and continuous increased thickness of the corners, according to the embodiment illustrated by FIG. 3. This example makes it possible to illustrate an embodiment in which α=0 and R₁=R₂.

The main structural features of the elements obtained according to this example are listed in Table 1.

EXAMPLE 3

According to the Invention

The synthesis technique previously described is also repeated identically, but the die this time is adapted so as to produce monolithic blocks characterized by a wavy disposition of the inner channels, outer walls whose inner face is substantially flat, and a constant and continuous increased thickness of the corners, according to the embodiment illustrated by FIG. 4. This example makes it possible to illustrate an embodiment in which α>0 and R₁=R₂. The main structural features of the elements obtained according to this example are listed in Table 1.

EXAMPLE 4

The Invention

The synthesis technique previously described is also repeated identically, but the die this time is adapted so as to produce monolithic blocks characterized by a wavy disposition of the inner channels, outer walls whose inner face is substantially flat, and an increased thickness of the corners according to the embodiment illustrated by FIG. 5. This example makes it possible to illustrate an embodiment in which α=0 and R₁>R₂.

The main structural features of the elements obtained according to this example are listed in Table 1.

EXAMPLE 5

According to the Invention

The synthesis technique previously described is also repeated identically, but the die this time is adapted so as to produce monolithic blocks characterized by a wavy disposition of the inner channels, outer walls whose inner face is substantially flat, and a continuous and decreasing increased thickness of the corners, according to the embodiment illustrated by FIG. 6 except that R₁<R₂.

The main structural features of the elements obtained according to this example are listed in Table 1 below.

	TABLE 1
	Example 1	Example 2	Example 3	Example 4	Example 5
Representative	FIG. 2	FIG. 3	FIG. 4	FIG. 5	FIG. 6 but
shape					R1 < R2
Geometry of the	wavy	Wavy	wavy	wavy	wavy
inner channels
Geometry of the	wavy	Smooth	smooth	smooth	smooth
outer walls (inner face)
Element size (mm)	37.8	37.8	37.8	37.8	37.8
Channel density (CPSI)	270	270	270	270	270
Thickness e of the	310	310	310	310	310
inner walls (μm)
Thickness Emin of	544	660	660	660	660
the outer walls (μm)
Element length (cm)	20.32	20.32	20.32	20.32	20.32
External volume of	290.3	290.3	290.3	290.3	290.3
the element (cm3)
Maximal thickness	767	847	838	660	847
E′ of the increased
thickness (μm)
outside the corner
Width d of the	0	5.2	5.2	2.0	5.2
reinforcement (mm)
Width d′ of the	0	3.4	3.4	0	3.6
reinforcement
outside the corner (mm)
R2 (mm)	1.0	1.0	1.0	1.0	1.0
R1 (mm)	NA	1.0	1.0	1.6	0.8
Angle α (degrees)	0	0	3	0	3
Thickness Ec of the	663	1198	1183	1182	1114
element in the
corner (μm)
Ec/e	2.14	3.86	3.82	3.81	3.59
Ec/Emin	1.22	1.82	1.79	1.79	1.69
E′/e	2.47	2.73	2.70	2.13	2.73
Weight (g)	189.1	191.6	190.8	190.1	190.7
Weight relative to Example 1		+1%	+1%	+1%	+1%
(NA = not applicable)

The samples of the foregoing examples were assessed by the following tests:

A—Measurement of Thermomechanical Strength:

Filters, assembled from the elements of Examples 1 to 5 according to the previously described operating mode, are mounted on an exhaust line of a 2.0 l direct injection diesel engine set to run at full power (4000 rpm) for 30 minutes and then removed and weighed in order to determine their initial weight. The filters are then reinstalled on an engine bench with a speed of 3000 rpm and a torque of 50 Nm for different periods in order to obtain a soot load of 6 g/liter (in volume of the filter).

The filters thus loaded are reinstalled onto the line to undergo a severe regeneration defined thus: after a stabilization at an engine speed of 1700 revolutions/minute at a torque of 95 Nm for 2 minutes, a post-injection is carried out with 70° of phasing for a post-injection output of 18 mm³/stroke. Once combustion of the soot is initiated, more precisely when the loss of load diminishes for at least 4 seconds, the engine speed is reduced to 1050 revolutions/minute at a torque of 40 Nm for 5 minutes in order to accelerate the combustion of the soot. The filter is then subjected to an engine speed of 4000 revolutions/minute for 30 minutes in order to eliminate the remaining soot.

The regenerated filters are inspected after being cut open to reveal any presence of cracks visible to the naked eye. The thermomechanical strength of the filter is assessed with respect to the number of cracks, a low number of cracks reflecting an acceptable thermomechanical strength for use as a particle filter.

B—Measurement of the Mechanical Strength

Breaking strength is measured at ambient temperature for each example on 30 test specimens corresponding to elements of one and the same manufacturing batch with dimensions of 20.32 cm in length and 37.8 mm in width. Mounting in 4-points flexion is carried out with a distance of 180 mm between the two bottom bearing surfaces and a distance of 90 mm between the two top punches typically according to the ASTM C1161-02.c standard. Rubber elements are placed on the punches in order to prevent initiation of compression cracks on the top face of the test specimen which might adversely affect the quality of the measurements. The speed of descent of the punch is constant, of the order of 10 mm/min. An average of the 30 measurements is considered to be representative of the mechanical strength for each example.

The main analysis and assessment data of the filters obtained according to Examples 1 to 5 are listed in Table 2 below.

	TABLE 2
	Exam-	Exam-	Exam-	Exam-	Exam-
	ple 1	ple 2	ple 3	ple 4	ple 5
Average measured	47.1	46.9	47.2	47.3	47.4
porosity (%)
Measured breaking	3900	4150	4200	4450	4500
strength (N)
Breaking	20.6	21.7	22.0	23.4	23.6
strength/weight
Calculated ratio
(n/g)
Number of cracks	5 to 10	0 to 5	0 to 4	0 to 5	0 to 5
observed in a filter
after a 6 g/l
regeneration

It can be seen in Table 2 that the monolithic elements of Examples 2 to 5 have a higher Breaking strength to Weight ratio and the filters of these examples show a better thermomechanical behavior than for Example 1.

The results imply also that it maybe possible, according to the present invention, to provide element for the formation of a filter retaining the good thermomechanical properties of the previously described filters with reinforced wall, but whose total weight is substantially reduced.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

1. A monolithic honeycomb element, comprising a plurality of adjacent ducts or channels, wherein: each of the plurality of adjacent ducts or channels is situated along an axis, the axes of the plurality of adjacent ducts or channels being substantially parallel;each of the plurality of adjacent ducts or channels is separated from adjacent ducts or channels by porous walls;each of the plurality of adjacent ducts or channels is stopped by a plug at one end;the plurality of adjacent ducts or channels comprises inlet ducts or channels being open at a gas intake face of the element, and outlet ducts or channels being open at a gas discharge face of the element, such that gas passing from the gas intake face of the element to the gas discharge face of the element passes through the porous walls;the plurality of adjacent ducts or channels comprises peripheral ducts or channels forming part of an outer wall of the element, the peripheral ducts or channels being configured to provide the element with an outer wall having an inner face that is substantially flat over an entire length of the element;the outer wall, when viewed in a cross-section substantially perpendicular to the axes of the plurality of adjacent ducts or channels, comprises at least one corner portion;the outer wall has a first cross-sectional thickness at the corner portion, the first cross-sectional thickness being measured along a line bisecting an angle between portions of the outer wall adjacent to the corner portion;the outer wall has a second cross-sectional thickness, the second cross-sectional thickness being a minimum thickness of the outer wall; anda ratio of the first cross-sectional thickness to the second cross-sectional thickness is greater than 1.5.

2. The monolithic element according to claim 1, wherein interior walls separating the plurality of adjacent ducts or channels are provided with a periodic undulation when viewed in the cross-section, such that the inlet channels have a greater cross-sectional area than the outlet channels.

3. The monolithic element according to claim 1, wherein a portion of the outer wall having the second cross-sectional thickness is located substantially centrally between two adjacent corner portions.

4. The monolithic element according claim 1, wherein: the outer wall includes an area of increased thickness including the corner portion and extending along portions of the outer wall adjacent to the corner portion; anda cross-sectional thickness of the outer wall in the area of increased thickness is continuous and constant over an entirety of the area of increased thickness.

5. The monolithic element according to claim 1, wherein: the outer wall includes an area of increased thickness including the corner portion and extending along portions of the outer wall adjacent to the corner portion; anda cross-sectional thickness of the outer wall in the area of increased thickness decreases continuously from the corner portion to locations on portions of the outer wall adjacent to the corner portion most distant from the corner portion.

6. The monolithic element according to claim 1, wherein: the outer wall includes an area of increased thickness including the corner portion and extending along portions of the outer wall adjacent to the corner portion;the inner face of the outer wall has a surface area in the area of increased thickness and a total surface area; andthe surface area in the area of increased thickness is at least one eighth of the total surface area.

7. The monolithic element according to claim 1, wherein: the outer wall includes an area of increased thickness including the corner portion and extending along portions of the outer wall adjacent to the corner portion;the inner face of the outer wall has a surface area in the area of increased thickness and a total surface area; andthe surface area in the area of increased thickness is at least one quarter of the total surface area.

8. The monolithic element according claim 1, wherein: the outer wall has an average cross-sectional thickness; anda ratio of the first cross-sectional thickness to the average cross-sectional thickness is greater than 2.7.

9. The monolithic element according claim 1, wherein: the outer wall has an average cross-sectional thickness; anda ratio of the first cross-sectional thickness to the average cross-sectional thickness is greater than 3.5.

10. The monolithic element according to claim 1, wherein the first cross-sectional thickness is between 150 and 5000 microns.

11. The monolithic element according to claim 1, wherein the first cross-sectional thickness is between 700 and 2000 microns.

12. The monolithic element according to claim 1, wherein the first cross-sectional thickness is between 1000 and 1300 microns.

13. The monolithic element according to claim 1, in which the second cross-sectional thickness is between 100 and 3500 microns.

14. The monolithic element according to claim 1, in which the second cross-sectional thickness is between 450 and 1300 microns.

15. The monolithic element according to claim 1, in which the second cross-sectional thickness is between 600 and 900 microns.

16. The monolithic element according claim 1, wherein: the outer wall has an average cross-sectional thickness; andthe average cross-sectional thickness is between 100 and 400 microns.

17. The monolithic element according claim 1, wherein: the outer wall has an average cross-sectional thickness; andthe average cross-sectional thickness is between 200 and 350 microns.

18. A filter comprising a plurality of monolithic elements according to claim 1, wherein the plurality of elements are joined by a joint cement.

19. The filter according to claim 18, wherein the plurality of elements and the joint cement comprise the same ceramic material.

20. The filter according to claim 19, wherein the ceramic material comprises silicon carbide.

21. An extrusion die for extruding a ceramic material, comprising a structure configured to form the monolithic element according to claim 1 from the ceramic material.