WO2011138555A1

WO2011138555A1 - Gas filtration structure

Info

Publication number: WO2011138555A1
Application number: PCT/FR2011/051003
Authority: WO
Inventors: Bernard Bouteiller
Original assignee: Saint-Gobain Centre De Recherches Et D'etudes Europeen
Priority date: 2010-05-04
Filing date: 2011-05-03
Publication date: 2011-11-10
Also published as: FR2959674A1

Abstract

The invention relates to a honeycomb structure for the filtration of particle-laden gases, comprising a periodic set of adjacent channels (21, 22), said structure being such that in cross-section each inner face (24) of an inlet channel (21) matches the shape of an inner circular arc having a radius R_i with a finite value and a centre C_i, each outer face (25) of the inlet channel (21) matches the shape of an outer circular arc having a radius R_e and a centre C_e, and, for each wall (23), the centre C_i of the inner circle is located at a non-zero distance (d) from the centre (C_e) of the outer circle.

Description

GAS FILTRATION STRUCTURE

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.

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.

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.

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.

For equivalent engine speeds, the best compromise between the following properties is generally sought:

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,

a high total filtration area,

a large volume of soot storage, in particular at constant pressure drop, so as to reduce the frequency of regeneration,

a larger residue storage volume, the residues, unburned, remaining in the filter after regeneration.

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.

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.

The object of the invention is to further reduce the pressure drop caused by the filters of the prior art. For this purpose, the subject of the invention is a structure for filtering particles-loaded gases of the honeycomb type and comprising a periodic assembly of longitudinal adjacent channels of axes parallel to each other separated by porous filtering walls, channels being alternately plugged at one or the other end of the structure so as to define inlet channels and outlet channels for the gas to be filtered, and so as to force said gas to pass through the porous walls separating the input and output channels. Said structure is such that in cross-section:

the ratio R between the area of each input channel and the area of each output channel is greater than 1,

each input channel is delimited by four walls each shared with an outlet channel, each wall having an inner face, the side of the inlet channel and concave with respect to the center of said inlet channel, and an outer face of the side the output channel,

each inner face is in the form of an internal arc having a radius Ri having a finite value and a center Ci,

each outer face is in the form of an outer circular arc having a radius R _e and a center C _e ,

for each wall, the center Ci of the inner circle is located at a non-zero distance d from the center C _e of the outer circle.

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. "Period of structure" means the length of a 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 the text.

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 their pressure drop. In the known filters, represented in FIG. 1, the centers of the outer (C _e ) and inner (Ci) circles coincide. The distance d between these centers is therefore zero. According to the invention, however, the distance d is non-zero.

Generally, all the walls of the filtering structure will have the same internal radius R 1, the same external radius R _e , and the same distance d.

Always according to a cross section, the structure according to the invention preferably has one or more of the following geometrical characteristics, according to all the possible combinations:

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 available surface area for filtration and / or catalysis, thereby reducing the pressure drop of the filters and the soot loading slope.

the radii R ± and R _e may be equal or different. They will be mostly different, R _e being greater than Ri or less than Ri, the ratio (R _e - e) / Ri is preferably different from 1, e being the thickness of the wall, measured on the axis connecting the center of an inlet channel to the center of an adjacent outlet channel . According to a first preferred mode, this ratio is greater than 1 and less than or equal to 80. High ratios correspond to output channels approaching a square shape. According to a second preferred embodiment, this ratio is less than 1 and greater than or equal to 0.1. Weaker ratios result in a filtration structure having wall portions between the thin outlet channels, which can lead to a reduction in the thermomechanical strength of the filter. Even more preferably, the ratio (R _e - e) / Ri is between 1.1 and 40, in particular between 1.3 and 30 or between 0.3 and 0.95,

the ratio between the radius of curvature R _e or Ri and the half-period (p) of the structure preferably varies from 0.2 to 40, in particular from 0.3 to 30,

preferably, the ratio between the distance d and the half-period (p) of the structure is at most 80, in particular at most 60, or even at most 40. This ratio is preferably at least 0.1 in particular 0.2 and even 0.5,

the centers C _e and Ci can be located on the same side of the wall. In this case, the outer faces of the walls are convex relative to the center of the outlet channels,

alternatively, the centers C _e and Ci can be located on either side of the wall. In this case, the outer faces of the walls are concave with respect to the center of the outlet channels.

The ratio (R _e - e) / Ri and the relative position of the centers C _e and Ci (on the same side or on both sides of the wall) are chosen according to the frontal surface open (OFA), the filtration surface and the wall surface, these different factors themselves being chosen according to the configuration of the pollution control system, in order to minimize the pressure loss of the filter.

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 filter structure or the channels of the structures located at the periphery of the filter. The same characteristic is also preferred for the input channels.

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. Similarly, the density of channels is preferably between 1 and 280 channels per cm ² , especially between 15 and 65 channels per cm ² .

The input and / or output channels may have at least one rounded corner, such as a corner, or two, three or even 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 μm.

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.

On the filtering structure previously described, it may thus be deposited, preferably by impregnation, at least one active catalytic phase, preferably comprising a precious metal such as Pt, Pd, Rh and optionally an oxide selected from Ce0 ₂ , Zr0 ₂ , or one of their mixtures. 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 to 4 and the following non-limiting examples provide a better understanding of the invention and its advantages. Figure 1 is a front elevational view of a portion of the gas evacuation face of a filter according to the prior art.

Figures 2 to 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.

The dashed square represents the mesh of the 2-dimensional periodic lattice. The period of the structure, also called period of the channels, and denoted "2p", corresponds to the length of one side of the square.

The structure of FIG. 1 is such that, in transverse section, the porous walls 13 have periodic corrugations so that said porous walls 13 have an inner face concave with respect to the center of the inlet channels 11 and an outer face convex relative to the center of the output channels 12. The ratio R is of the order of 2.4.

Each input channel 11 is delimited by four walls 13 each shared with an outlet channel 12, each wall 13 having an internal face 14, on the input channel 11 side and concave with respect to the center of said inlet channel 11 , and an outer face 15 of the outlet channel 12 side. Each inner face 14 is in the form of an internal circular arc, represented in dashed lines, having a radius Ri and a center Ci. Each external face 15 conforms to the shape an outer circular arc, represented in dashed lines, having a radius R _e and a center C _e . The centers Ci of the inner circle and C _e of the outer circle are merged, so that the distance d is zero.

The thickness e of the walls 13 corresponds here to the difference between the external radius R _e and the internal radius R 1, so that the ratio (R _e -e) / Ri is equal to 1.

FIG. 2 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. The 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 face of the gas inlet ), it is the outlet channels 22 which are plugged. In cross section, the porous walls 23 have periodic corrugations so that the porous walls 23 are concave with respect to the center of the inlet channels 21 and convex relative to the center of the outlet channels 22. The ratio R is the order of 2.4.

Each input channel 21 is delimited by four walls 23 each shared with an outlet channel 22, each wall 23 having an internal face 24, on the input channel 21 side and concave with respect to the center of said inlet channel 21 , and an outer face 25 on the outlet channel 22 side. Each inner face 24 is in the form of an internal circular arc, shown in dotted lines, having a radius Ri and a center Ci. Each outer face 25 conforms to the shape an outer circular arc, represented in dashed lines, having a radius R _e and a center C _e . The centers Ci of the inner circle and C _e of the outer circle are distinct, so that the distance d is non-zero.

The thickness e of the walls 23 is indicated in FIG. 2. The ratio (R _e - e) / Ri is about 1.9. The ratio R _e / p is about 2.3 while the Ri / p ratio is about 1.1. The d / p ratio is about 1.0.

Figure 3 shows another structure according to the invention. The input channels 21 are unchanged from those of Figure 2. In contrast, the output channels 22 have a different shape, closer to the square shape. The outer face 25 is in the form of an arc whose radius R _e is larger, so that its center C _e is located outside the figure. The ratio (R _e - e) / Ri is of the order of 22. The ratios R _e / p, Ri / p and d / p are respectively 25, 1,1 and 23.

FIG. 4 illustrates another embodiment of the invention, in which the centers C _e and Ci are located on both sides of the wall 23. In this case, the outer faces 25 of the outlet walls are concave with respect to the center of the outlet channels 22. The ratio (R _e - e) / Ri is of the order 0.9. The ratios R _e / p, Ri / p and d / p are respectively 1.2, 1.3 and 2.7.

The invention and its advantages over previously known structures will be better understood on reading the nonlimiting examples which follow.

Example 1 (comparative):

The first population of monolithic elements or monoliths in the form bee and 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 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. .

The quantity of water required is then added and kneaded to obtain a homogeneous paste whose plasticity allows the extrusion through a die configured to obtaining monolithic blocks of square section and whose internal channels have a cross section schematically illustrated in Figure 1. The half-period p ripples is 1.48 mm.

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 (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.

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 being impurities, mainly 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 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.

The dimensional characteristics of the elements thus obtained are given in Table 1 below. Examples 2 to 5 (according to the invention):

The technique for synthesizing the monoliths previously described is identical, but the die is this time adapted to produce monolithic blocks characterized by geometries of the type shown diagrammatically in FIGS. 2 to 4.

The dimensional characteristics of the elements thus obtained are given in Table 1 below.

Table 1

The samples obtained were evaluated and characterized according to the following procedures:

Dimensional features

Table 2 below indicates for each example the following dimensional characteristics:

- the open front area (OFA) or open front area, obtained by calculating the percentage ratio of the area covered by the sum of the cross sections of the input channels of the front face of the monolithic unitary elements (Except the walls and plugs) on 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 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,

the specific filtration surface of the unitary monolithic element, which corresponds to the internal surface of all the walls of the filter inlet channels expressed in m ² , relative to 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.

Measurement of pressure loss:

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 450 kg / h and a temperature of 600 ° C on the new (non-sooty) filters corresponding to the maximum power conditions of a turbo type 2L engine.

The results obtained for Examples 1 to 4, which are directly comparable, have been reported in Table 2.

Table 2

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

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 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 forcing said gas through the porous walls (23) separating the inlet (21) and outlet (22) channels, said structure being such that in cross-section:

the ratio R between the area of each inlet channel (21) and the area of each outlet channel (22) is greater than 1,

each inlet channel (21) is delimited by four walls (23) each shared with an outlet channel (22), each wall (23) having an internal face (24), on the inlet channel side (21); ) and concave with respect to the center of said inlet channel (21), and an outer face (25) on the outlet channel (22) side,

each inner face (24) is in the form of an internal circular arc having a radius Ri having a finite value and a center Ci,

each outer face (25) is in the form of an outer circular arc having a radius R _e and a center C _e , for each wall (23), the center Ci of the inner circle is located at a non-zero distance d from the center C _e of the outer circle.

2. Filtration structure according to claim 1, such that the ratio (R _e - e) / Ri is different from 1, e being the thickness of the wall (23), measured on the axis connecting the center of a input channel (21) in the center of an adjacent output channel (22).

3. Filtration structure according to the preceding claim, such that the ratio (R _e - e) / Ri is greater than 1 and less than or equal to 80.

4. Filtration structure according to claim 2, such that the ratio (R _e - e) / Ri is less than 1 and greater than or equal to 0.1.

5. Filtration structure according to one of the preceding claims, such that the ratio between the radius of curvature R _e or Ri and the half-period of the structure varies from 0.2 to 40.

6. Filtration structure according to one of the preceding claims, such that the ratio between the distance d and the half-period of the structure is at most 80.

7. Filtering structure according to one of the preceding claims, such that the ratio R is between 1.05 and 3.0.

8. Filtering structure according to one of the preceding claims, such that centers C _e and Ci are located on the same side of the wall (23).

9. Filtration structure according to one of claims 1 to 7, such that centers C _e and Ci are located on either side of the wall (23).

10. Filtering 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, and wherein the channel density is between 1 and 280 channels per cm ² , especially between 15 and 65 channels per cm ² .

11. 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.

12. 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 .

13. An assembled filter comprising a plurality of filter structures according to one of the preceding claims, said structures being bonded together by a cement.

14. 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.

Extrusion die shaped to form, by extrusion of a ceramic material, a filter structure according to one of claims 1 to 12.