MX2008006904A - Structure for the filtration of a gas based on silicium carbide with a controlled wall surface porosity - Google Patents

Abstract

The invention relates to a honeycomb-type structure for the filtration of gas loaded with particles, characterised in that the material based on silicium carbide constituting the walls of the structure has an open porosity of between 30 and 53%, a median pore diameter of between 9 and 20 ?m, an average number of open pores on the surface of the walls, having an equivalent opening surface of between 20 and 310 ?m2, which is higher than 300/mm2of wall, and a ratio between the total opening surface of said open pores and the surface of the walls which is between 15 and 30%. The invention also relates to a method for obtaining said structure.

Description

STRUCTURE FOR THE FILTRATION OF A GAS, BASED ON SILICON CARBIDE, WITH CONTROLLED POROSITY OF THE SURFACE OF THE WALL The invention relates to the field of filter structures, which may comprise a catalytic component, used for example in a gas exhaust line of a diesel-type internal combustion engine.

Filters for the treatment of gases and for removing soot particles that typically come from a diesel machine are well known in the prior art. Usually all these structures have a honeycomb structure, one of the faces of the structure allows the entrance of the exhaust gases that are going to be treated, and the other side is for the exit of the treated exhaust gases. The structure comprises, between the entrance and exit faces, an assembly of adjacent ducts or channels, whose axes are parallel to each other, separated by porous walls. The ducts are sealed at one or the other of their ends to form entrance chambers that open on the entrance face and exit chambers that open on the exit face. The channels are alternately closed in an order such that, in the course of their passage through the body of the honeycomb, the exhaust gases are forced to pass to through the side walls of the input channels to gather the output channels. In this way, particulates or soot particles are deposited and accumulated in the porous walls of the filter body.

Normally, for filtration of gas, filters made of a porous ceramic material are used, for example, made of cordierite, alumina, mullite, silicon nitride, a mixture of silicon / silicon carbide or silicon carbide. silicon.

In a known manner, during use, the particulate filter is subjected to a succession of filtration (hollin accumulation) and regeneration (elimination of hollin) phases. During the filtration phases, the hollin particles emitted by the machine are retained and deposited inside the filter. During the regeneration phases, the hollin particles burn inside the filter, to restore its filtering properties. An important criterion involved in the implementation of a filter, for example in the exhaust line of a machine, is therefore its thermomechanical resistance.

It is also known that the introduction of a filter of particles such as that previously described in the exhaust line of a machine, leads to a pressure drop that seems to deteriorate the performance parameters of the latter. Consequently, the filter must be configured in such a way that deterioration is avoided.

Another substantial criterion for the selection of the optional catalytic filter structures previously described is their soot deposition time. This time corresponds to the period of time required for the filter to reach its maximum level of filtering efficiency, when it is implemented for the first time or after a regeneration phase. It is assumed that this time depends, in particular, on the deposition of a sufficient amount of soot within the porosity of the filter, to prevent the direct passage of fine particles of soot through the walls of the filter. One of the direct consequences of a badly adapted soot deposition time is the appearance of persistent and noxious black fumes, together with the presence of soot residues at the exit of the exhaust line, in a new filter or after a phase of regeneration. Needless to say, for reasons of environmental impact, image and comfort of use, automakers would want to eliminate the occurrence of these phenomena, or at least minimize them in the vehicles equipped with said filters.

Soot deposition is a phenomenon that is little known, no doubt due to the fact that the deposited mass of soot is not measurable in real time on a filter during its use. In fact, only the deposition time, indirectly measured, is accessible based on the analysis of the concentration of particles present in the exhaust gases at the outlet of the filter.

The present invention relates to the field of silicon carbide filters, preferably obtained by sintering / recrystallization (R-SiC). Examples of catalytic filters according to the invention are described, for example, in Patent Applications EP 816 065, EP 1 142 619, EP 1 455 923 or in WO 2004/065088, to which reference will be made for a more detailed description of its structure or its mode of synthesis. The structures according to the invention may be simple monolithic structures or, preferably, more complex assembled structures, usually obtained by the association of several monolithic elements, joined by a cement known as sealant cement.

Therefore, the objective of this invention is to provide a new honeycomb structure, which possibly comprises a catalytic component, which allows solving all the aforementioned problems.

Therefore, the invention relates to a filtering structure comprising, for maximum filtration efficiency and long-term use, the following properties: a minimum pressure drop during operation, typically in an exhaust line of a machine of internal combustion; - a sufficient thermomechanical resistance to withstand the restrictions of filter operation; and an optimized filtration efficiency as soon as the filter is implemented or after a regeneration phase, resulting in a minimized time of deposition of the hollin.

This structure is especially applicable as a particulate filter in an exhaust line of a diesel or gasoline engine.

In its most general form, the present invention relates to a structure for filtering gases charged with particles, of the honeycomb type, and comprising an assembly of adjacent ducts or channels, whose axes are parallel to each other, separated by porous walls, these ducts being sealed by plugs at one or the other of their ends, so as to form entrance chambers that open on a face of gas entry and exit chambers that open on a gas outlet face, so that the gas to be filtered passes through the porous walls, characterizing the structure in which the material based on silicon carbide constituting the walls, has: - an open porosity of between 30 and 53%, preferably between 40 and 50%, and most preferably between 43 and 49%; - an average pore diameter between 9 and 20 μm, preferably between 12 and 18 μm; the average number of open pores in the surface of the walls, whose opening area is between 20 and 310 μm2, is greater than 300 per mm 2 of wall, preferably greater than 350 per mm 2 of wall; Y - the ratio of the total opening area of the open pores to the area of the walls is between 0.15 and 0.30, preferably between 0.20 and 0.27.

The term "SiC-based material" is understood, within the context of the present description, to mean that said material comprises at least 30% of SiC by weight, preferably at least 70% SiC by weight and most preferably at least 98% SiC by weight.

The term "average pore diameter" is understood, within the context of the present invention, to mean the pore diameter for which 50% by volume of the pores is equal to or less than this pore size.

The open pores in the surface of the walls, whose opening area is between 20 and 310 μm2, are, in the context of the present description, the pores for which the opening area in the channels corresponds approximately to the area of a perfect disc, whose diameter is between approximately 5 μm and approximately 20 μm.

The structure according to the invention can also include a catalytic coating for treating the polluting gases of the CO or HC type, the coating being present for example on the surface and in the porosity of the walls.

The thickness of the walls of the structures according to the invention is typically between 200 and 500 μm.

In general, the pore size distribution is of a unimodal type.

According to a preferred embodiment of the invention, the present filter structure comprises a plurality of honeycomb filtering elements joined together by a bonding cement, the number of channels typically being 7.75 to 62 per cm2, the channels having one cross section in the dimensions from 0.5 to 9 mm.

The invention also relates to a process for manufacturing the above-described SiC-based filter structures, comprising: a mixing step of the initial mixture with at least one pore-forming agent, preferably chosen from the group consisting of polyethylene, polystyrene, starch and graphite, for example as described in patent applications JP 08-281036 or EP 1 541 538. The mixing results in a homogeneous product in the form of a bonded paste. The process also includes an extrusion step of the product through a suitable die, in order to form honeycomb monoliths; a drying step of the obtained monoliths; and, optionally, an assembly step and a baking step, the method being characterized in that at least one of the parameters within the group consists of the size of the particles of the initial mixture, the nature and quantity of the forming agent (s) of pore, and the baking temperature, is controlled to obtain the structure.

According to a possible implementation method, the silicon carbide is introduced in powder form, this powder having at least two types of particle size, for example in the form of a first population of particles, whose average diameter is between 10 and 10. and 100 μm, preferably between 10 and 50 μm, and a second population of particles, whose average diameter is between 0.1 and 10 μm, preferably between 0.1 and 5 μm.

Advantageously, the baking temperature is adjusted, for the requirements of the present invention, to between 2100 and 2400 ° C, and preferably at between 2150 and 2300 ° C.

The procedure may also include, but not necessarily, a deposition step, preferably by impregnation, of a catalytic coating comprising an active catalytic phase, typically consisting of at least one precious metal such as Pt and / or Rh and / or Pd, and optionally an oxide such as Ce02, Zr02, Ce02-Zr02.

Finally, the present invention relates to the use of the structure described above as a particulate filter in an exhaust line of a diesel or gasoline engine.

The invention and its advantages will be better understood by reading the following non-limiting examples; In the following examples, the filters were synthesized by starting from an initial mixture of the following four constituents: constituent A: a first powder composed of SiC particles whose average diameter varies between 5 and 50 μm, with at least 10% by weight of the particles having a diameter greater than 5 μm, - constituent B: a second powder composed of SiC particles of average diameter d50 on the scale of between 0.1 and 10 μm, - constituent C: a pore-forming agent of the polyethylene type, and - constituent D: an organic binder of the methyl cellulose type.

EXAMPLE 1 A first particle filter was synthesized and tested. Firstly, 50 parts by weight of constituent A, composed of a powder of SiC particles with a mean diameter d50 of about 30 μm and 50 parts by weight of constituent B with an average diameter of SiC particles of about 2.5 μm, they were mixed in a mixer.

Second, 5% by weight of constituent C with respect to the total mass of constituents A and B, and 5% by weight of constituent D with respect to the total mass of constituents A and B were added to this first mixture. .

Water was added and mixing continued until obtain a uniform paste whose plasticity allowed it to be extruded through an extrusion die as honeycomb monolithic structures, whose dimensional characteristics are given in Table 1: TABLE 1 Subsequently, the obtained green monoliths were dried by microwaves for a sufficient time to bring the proportion of non-chemically bound water to less than 1% by weight. The channels were alternately closed on each face of the monolith according to well-known techniques, for example, those described in Patent Application No. WO 2004/065088. Then the monolith was baked with a temperature increase of 20 ° C / h until that a temperature of approximately 2200 ° C was reached, which was maintained for 2 hours.

Finally, a series of silicon carbide monoliths whose microstructural characteristics depended on the composition of the initial mixture and the synthesis conditions were obtained.

The elements from one and the same mixture were then joined to each other by joining them with a cement of the ceramic type and then machined to form filters of 14.4 cm in diameter according to the teachings of Patent Application EP 816 065. Filters obtained according to this example correspond to specimen 1 of Table 2.

EXAMPLES 2 TO 12 In these Examples, the filter synthesis protocol described in Example 1 was reproduced in the same way.

The differences introduced to modify the microstructural properties of the obtained monoliths were as follows: various powders whose mean particle diameters varied between 5 and 50 μm were used as constituent A, at least 10% by weight of the particles making up these powders had a diameter greater than 5 μm, - several powders with average particle size varying between 0.1 and 10 μm were used as constituent B, and - the proportions of constituents A and B varied within the following limits: constituent A: from 20 to 80%, constituent B, from 80 to 20%, to obtain a first mixture comprising exclusively (100%) constituents A and B .

Second, constituents C and D were added to each mixture A and B, in proportions within the amounts of 3 to 12% and 1 to 20% by weight, respectively, with respect to the total mass of constituents A and B.

The dimensional characteristics of the monoliths and of the filters obtained after baking and of the filters obtained after assembly, were identical to those given in Example 1.

The specimens obtained were evaluated according to three different tests: A - Hollin deposition time measurement The hollin deposition time is the time required for the deposit of a sufficient amount of hollin, in a new filter or after a regeneration, so that it reaches its maximum level of filtration efficiency .

For the measurement, the filter that was to be tested was installed in an exhaust line of a machine in a test bench. The machine used was a diesel engine with a capacity of 2.0 liters. The filter was progressively charged with soot by operating the machine at a speed of 3000 rpm at 50 Nm.

The bank was equipped at the exit with an ELPI system (Low Pressure Electric Impactor, acronym in English), known per se, which allowed to measure in real time the concentration of particles in the gas, starting from the moment it was charged the filter. It got like this a filtration efficiency curve as a function of time, being characterized by a quasi-plateau after a given test time. The plateau corresponds to a filtration efficiency greater than or equal to 99%. The period of time between the start of loading of the filter and the time from which an efficiency equal to at least 99% was obtained corresponds, according to the present invention, to the soot deposition time.

B - Measurement of the pressure drop The pressure drop, within the meaning of the present invention, is understood as the pressure differential that exists between the upstream side and the downstream side of the filter. The pressure drop was measured according to the prior art techniques for an air flow of 300 m3 / h in an ambient air stream.

C - Measurement of the thermomechanical resistance The filters were mounted on an exhaust line of a 2.0 L diesel engine running at full power (4000 rpm) for 30 minutes, then dismantled and weighed to determine its initial mass. Then the filters were reinstalled in the test bench of the machine with a speed of 3000 rpm and a torque of 50 Nm for different periods of time, to obtain soot loads between 1 g / liter and 10 g / 1 in the filter.

The filters thus loaded were again mounted on the line, to make them pass through an intense regeneration that is defined as follows: after stabilization at a machine speed of 1700 rpm at a torque of 95 Nm for 2 minutes, a post injection with 70 ° of phase adjustment for a post injection flow ratio of 18 mmVarrera. Once the combustion of the soot deposits started, more precisely when the pressure loss decreased during a period of at least 4 seconds, the speed of the machine was reduced to 1050 rpm at a torque of 40 Nm for 5 minutes to accelerate the combustion of soot deposits. Then it was subjected to the filter at a machine speed of 4000 rpm for 30 minutes, to remove the remaining hollin.

The regenerated filters were inspected after cutting to reveal the possible presence of cracks visible to the naked eye. The soot limiting mass thus measured, defined as the mass of soot for which the first cracks appear after severe regeneration, measures the thermomechanical resistance of the filters.

The microstructural characteristics of the samples were subsequently measured by various techniques well known in the art: D - Porosimetry of the material that forms the walls The open porosity of the silicon carbide that forms the walls was determined according to the conventional techniques of high pressure mercury porosimetry, with a porosimeter of the Micromeritics 9500 type. The analysis shows, for all samples tested, a unimodal distribution of pore sizes. The average pore diameter was determined using the cumulative pore volume distribution as a function of the pore size, obtained by measuring the porosimetry using the mercury porosimeter.

E - Analysis by electron scanning microscopy (SEM) The number, nature and size of the pores on the surface of the walls was determined by an automatic image processing technique on each specimen, based on photographs of a surface area of 1 mm2 of wall taken by a microscope. crawling on BSE mode (electron backscatter).

The structural data and the results of the various tests obtained in the representative specimens of all the results obtained, are given in Table 2.

In Table 2, it can be seen that specimens 1 to 5, which meet the microstructural criteria according to the invention, have satisfactory results in the various evaluation tests, suitable for being used as a particle filter in a line of exhaust from a diesel machine, that is, under the conditions of measurement, a deposition time of 10 minutes or less, combined with a pressure drop below 20 pascals and a limiting mass of hollin equal to or greater than 4 g / liter. The measurements of specimens 6 to 9, which are given as a comparison, show that the values of porosity and average pore diameter that do not agree with those described above, mean that these structures can not be used as a filter of particles.

In addition, measurements on specimens 10 to 12, which are also given as a comparison, show that the surfaces of the walls, which are not in accordance with the present invention, do not allow them to be used as a particle filter.

Claims (10)

1. Structure for filtering gases loaded with particles, of the honeycomb type and comprising an assembly of ducts or adjacent channels, whose axes are parallel to each other, separated by porous walls, the ducts being sealed with plugs at one or the other of their ends in a manner that entrance chambers are formed that open on the gas entrance face and exit chambers that open on the gas exit face, so that the gas to be filtered passes through the porous walls, distinguishing this structure in which the material based on silicon carbide constituting the walls has: an open porosity of between 30 and 53%, preferably between 40 and 50%, and most preferably between 43 and 49%; an average pore diameter between 9 and 20 μm, preferably between 12 and 18 μm; and in that: - the average number of open pores in the surface of the walls, whose opening area is between 20 and 310 μm2, is greater than 300 per mm2 of wall, preferably greater than 350 per mm2 of wall; and - the ratio of the total opening area of the open pores, to the area of the walls, is between 0.15 and

0. 30, preferably between 0.20 and 0.27.

2. Structure according to claim 1, characterized in that it also includes a coating catalytic to treat polluting gases of the Co or HC type.

Structure according to one of the preceding claims, characterized in that the thicknesses of the walls are between 200 and 500 μm.

Structure according to one of the preceding claims, characterized in that the pore size distribution is of the unimodal type.

Filtering structure according to one of the preceding claims, characterized in that it comprises a plurality of filter elements in a honeycomb joined together by means of a sealing cement.

6. Process for manufacturing a filter structure based on SiC, comprising: a mixing step of the initial mixture with at least one pore-forming agent, preferably chosen from the group consisting of polyethylene, polystyrene, starch and graphite, resulting in a homogeneous product in the form of a bonded paste; a step of extruding the product through a suitable die, in order to form honeycomb monoliths; a drying step of the obtained monoliths; and, optionally, an assembly step and a baking step, characterizing the procedure in which at least one of the parameters within the group consists of the size of the particles of the initial mixture, the nature and amount of the pore-forming agent (s), and the bake temperature, is controlled for a structure according to one of claims 1 to 5.

7. Process according to claim 6, characterized in that the silicon carbide is introduced in the form of a powder in the mixture, this powder having at least two types of particle size, for example in the form of a first population of particles, whose average diameter is between 10 and 100 μm, preferably between 10 and 50 μm and a second population of particles, whose average diameter is between 0.1 and 10 μm, preferably between 0.1 and 5 μm.

Method according to any of claims 6 and 7, characterized in that the baking temperature is between 2100 and 2400 ° C, and preferably between 2150 and 2300 ° C.

Method according to one of claims 6 to 8, characterized in that it also includes a deposition step, preferably by impregnation, of a catalytic coating comprising an active catalytic phase, typically consisting of at least one precious metal such as Pt. and / or Rh and / or Pd, and optionally an oxide such as Ce02, Zr02, Ce02-Zr02.

10. Use of a structure according to one of the claims 1 to 5, as a particulate filter in an exhaust line of a diesel or gasoline engine, and preferably a diesel engine. RESU IN The invention relates to a honeycomb structure for the filtration of gas loaded with particles, characterized in that the material based on silicon carbide constituting the walls of the structure has an open porosity of between 30 and 53%, average pore diameter of between 9 and 20 μm, an average number of open pores in the surface of the walls, which has an opening equivalent surface of between 20 and 310 μm2, which is greater than 300 / mm2 of wall, and a ratio between the total opening surface of the open pores, and the surface of the walls, which is between 15 and 30%. The invention also relates to a method for obtaining that structure.