Fumes Treatment System

Abstract

A fumes treatment system is provided for exhaust gases of internal combustion engines, for example. The system includes a housing inside which a series of elements are provided for intercepting the flow of fumes. The elements are such that they do not completely take up the internal cross-section of the housing and are provided with or leave such discontinuities as to create a tortuous path for the fumes to be treated.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Section 371 of International Application No. PCT/IT2007/000095, filed Feb. 14, 2007, which was published in the English language on Aug. 23, 2007, under International Publication No. WO 2007/094024 A1 and the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a system for the treatment of fumes derived from combustion; in particular, the system of the invention may find application in the treatment of fumes derived from the combustion of hydrocarbons.

The fumes emitted from the combustion of hydrocarbons are among the main causes of atmospheric pollution, and the regulations on their control and suppression are becoming more and more stringent. The main sources of such a kind of emissions are boilers of heating equipment and internal combustion engines of transport means. Reference will be made in the rest of the text to application to transport means, but the invention may be also applied in the field of boilers or central heating systems as well.

The emissions from the combustion of hydrocarbons are mainly comprised of (other than aqueous vapor) CO₂, CO, NO₂, NO, sulphur compounds, and unburned hydrocarbons. In addition, gasoline engines cause emissions of benzene (employed for many years as an antiknock substance in place of tetraethyl lead), whereas diesel engines emit a solid particulate, generally indicated in the field by the definition of “soot.” Soot is made up of particles of a size between about 50 and 1000 nanometers (nm) and formed by carbon residues generated by the incomplete combustion of the fuel, on the surface of which condensed hydrocarbons together with other substances are generally adsorbed. These particles are sufficiently thin to remain suspended in air for relatively long periods of time. In the rest of the text by the term “fumes” is meant the combination of gaseous compounds, vapors and soot.

In order to suppress all these species, vehicles are provided with gas and particulate treatment systems generally comprised of a housing (normally metallic) connected to the engine at one end and to the exhaust at the other end. Inside the housing, elements are arranged that are active in the conversion of gaseous species into less harmful species, and filters for retaining soot. The arrangement of converters and filters in the housing is such that gases and particulates have to pass through these active elements, or at least lap on them.

Particulate filters are generally periodically regenerated by causing the accumulated soot to burn, in order to avoid their clogging, which would impair their proper operation. This operation is commonly carried out according to the so-called “post injection” method, wherein the exhaust cycle of the pistons is temporarily modified in order to cause “fresh” fuel to arrive in the exhaust line, which by burning downstream of the engine increases the exhaust gas temperature up to about 650° C., thus triggering the combustion of the soot accumulated on the filter. The operations of gas conversion and soot combustion are generally facilitated by the presence, on the elements forming these systems, of oxidation catalysts that have the function of reducing the combustion temperature of undesired species (or, which is the same, the function of increasing the oxidizing efficiency at a given temperature).

The treatment systems of exhaust gases presently in use are mostly based on the use as active elements of the so-called ceramic monoliths, i.e., porous ceramic bodies that may be made, for example, of silicon carbide or cordierite. Monoliths generally have a structure known in the field as “honeycomb,” formed of a series of parallel channels. In particular, filters are formed of two series of blind channels, wherein the channels of the first series are open only at the side of the inlet of the gases to be treated and those of the second series are open only toward the side of gas exhaust. The two series are alternated so that (in a cross-section of the monolith) a channel of one series has the channels of the other series as first neighbors. With this construction, the gases are obliged to pass through the walls separating the channels, exploiting the porosities of the material, in order to go from the inlet to the outlet of the treatment system. In order to improve the treatment efficiency, the surfaces of the channels, as well as those of the pores, are generally catalyzed as described above.

However, these monoliths exhibit some problems.

First of all, gases coming from an engine are forced to pass through the porosities of the material and this leads to relatively high values of pressure drops at the two ends of the monolith (the pressure drop is usually indicated in the field as “head loss”; this definition will be adopted in the rest of this specification). The situation is worse when the channels are clogged by soot before a regeneration. A large head loss across the monolith results in a reduction of the power generated by the engine, and thus in the need for higher fuel consumption in order to have the same vehicle performance. In extreme cases, when the head loss becomes too high, it may lead to a spontaneous shutoff of the engine.

A second problem of ceramic monoliths is their inherent fragility. At every turning on and off of the engine, these monoliths are subject to sudden and intense temperature excursions between ambient temperature and temperatures of about 400° C., depending on the distance between monolith and engine. During the combustion steps of the soot, even higher temperatures are achieved, which, due to the poor thermal conductivity of the ceramics, can create strong thermal gradients in the monolith structure. Moreover, during the motion of vehicles, the monoliths are subject to intense mechanical stresses. The combination of these phenomena may lead to the formation of cracks in the monolith, which are preferential paths for the gaseous flow crossing them, thus reducing the efficiency of gas conversion or soot retention.

In addition, since in these systems the fumes must pass through the ceramic walls separating the channels, in order to have acceptable head losses at the ends of the system, monoliths having high values of total surface area are manufactured, usually equal to some square meters, depending on the application. This entails various problems: first, the size of these systems may cause positioning problems in the exhaust systems of the transport means; second, the use of relatively large amounts of material may lead to expensive fumes treatment systems, particularly when using silicon carbide for the filters and noble metals, such as platinum or palladium, as a catalyst in the gas converters.

In order to overcome these problems, the use of converters or filters has been suggested, wherein porous elements made of metallic material are used, instead of ceramic elements. These may be made of preformed thin metal plates, sintered metal powders or metal fibers packed and possibly adhered one to another by means of thermal treatments.

One type of active metal elements is those made from metal fiber meshes. The preferred material for this purpose is an alloy called Fecralloy, essentially formed of iron (the main component), aluminum, chromium and yttrium, the composition and the preparation of which are disclosed in U.S. Pat. No. 3,920,583. A suitable method for producing large amounts of Fecralloy fibers is disclosed, e.g., in U.S. Pat. No. 4,930,199. This alloy is particularly suitable for this application, as, exposing it to an oxidizing atmosphere at high temperatures (around 1000° C.), a migration of aluminum to the surface occurs with formation of a layer of alumina that is compact and highly resistant to thermal shocks, mechanical stresses and chemical attacks. As a consequence, after this first exposure to oxidizing conditions, the material becomes extremely resistant to the conditions in which it will be in the systems for the treatment of exhaust gases, regardless of how aggressive they may be.

On this thermally grown layer of alumina, a second less compact oxide layer is then preferably deposited, which, in turn, may be made of alumina, a mixed oxide of aluminum and Rare Earth elements, titanium oxide, or the like. This second oxide layer may be formed in many ways, for example by dipping the mesh into a solution containing precursors of the oxides and successive thermal treatments for evaporating the solvent, transforming the precursors into oxides and consolidating the latter. Another suitable process for this purpose is disclosed in U.S. Pat. No. 6,303,538 B1. Finally, this porous oxide layer may be made functional by a catalyst, which is generally a noble metal, such as platinum, palladium, rhodium or mixtures thereof, or oxides such as vanadium pentoxide, lanthanum manganate or cerium-zirconium mixed oxides. The preparation of a Fecralloy mesh catalyzed by a cerium-zirconium mixed oxide is disclosed, e.g., in European Patent EP 0764455 B1.

These meshes may be arranged in the housing in a direction substantially parallel to the axis thereof. In this case, bellows configurations are possible, as disclosed, e.g., in European patent application publication EP 0504422 A1, or having a cylindrical symmetry, as disclosed, e.g., in U.S. Pat. No. 4,576,799 or in European patent application publication EP 0699827 A1. With these configurations two series of spaces are generated alternately open at the gas inlet and outlet portions, similarly to what is present in the ceramic monoliths. The gases are forced to pass through the meshes in order to go from one space of the first series to one of the second series, but all the spaces of these two series are essentially equivalent to each other from the point of view of the flow characteristics, and thus the meshes in these configurations are generally all equivalent. Alternatively, the meshes may be arranged transversely to the axis of the housing, so that a series of successive spaces is created along the flow direction, as disclosed, e.g., in U.S. Pat. No. 4,900,517. In this case, it may be preferable to use meshes of different characteristics to manufacture successive elements of the series, for example having a more and more decreasing porosity, in order to try to obtain a degree of gas conversion and soot retention as homogeneous as possible over all the active elements arranged in series in the housing, as described, e.g., in U.S. Pat. No. 5,968,373.

In any event, in all the constructive solutions described in the prior art, fiber meshes are connected gas-tight to the inner walls of the housing (generally by metal supporting elements), so as not to leave side passages, so that the stream of gases and particulates is forced to pass through the meshes along the path from the inlet to the outlet of the housing. This arrangement assures a good contact between the fumes and the active elements, but causes also a high head loss.

In addition, this structure has the problem that meshes can accumulate soot, resulting in the reduction of the free area for the gas passage, clogging and increase in the head loss across the ends of the system. Finally, the inventors have verified that with a treatment system based on meshes arranged along the filter axis, of the type described in European patent application publication EP 0504422 A1, in order to have a filter with gas and particulate treatment characteristics comparable to ceramic monoliths, about 0.6-0.7 m² of metal mesh are needed. Given the high cost of these metal meshes, this leads to costs of these systems that are higher than those based on ceramic monoliths.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to overcome the problems of the prior art, and particularly to provide a system for the treatment of fumes from internal combustion engines that has a reduced head loss and/or a reduced amount of material and catalyst with respect to known systems, with the same treatment efficiency.

According to the present invention, this object is achieved by a fumes treatment system comprising a housing provided with a fumes inlet and an outlet of the treated fumes, and inside the housing a series of essentially planar flow interception elements, characterized in that the elements are formed and arranged so that:

at least two successive interception elements of the series are each provided with at least one discontinuity, such that the geometrical area of the discontinuities is between 10 and 40% of the area that the element would have if it completely occupied the internal cross-section of the housing;

for any pair of successive interception elements presenting discontinuities, the condition exists that the projection of the discontinuity of the downstream element on the upstream one, the projection being made perpendicular to the upstream element, does not superimpose with the discontinuity on the upstream element; and

the minimum distance between two discontinuities on two successive elements lies along a segment forming an angle of at least 5° with respect to a straight line perpendicular to the upstream element.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

FIG. 1 is a series of perspective views of different possible shapes of interception elements for use in the systems of the invention;

FIG. 2 is a schematic representation of geometric condition b) characterizing the invention in its most general embodiment;

FIG. 3 is a schematic representation of geometric condition c) characterizing the invention in its most general embodiment;

FIG. 4 is a schematic cross-sectional view of a preferred embodiment of the system of the invention;

FIG. 5 is a perspective view of a schematic illustration of the geometric conditions characterizing the invention in a preferred embodiment;

FIGS. 6 and 7 are partially broken away views of two possible systems of the invention comprising different types of flow intercepting elements and their mutual arrangement;

FIG. 6a is an enlarged view of a detail of FIG. 6; and

FIG. 8 is a diagrammatic view of an experimental set-up for testing the particulate filtration efficiency of filtering systems.

DETAILED DESCRIPTION OF THE INVENTION

The flow interception elements used in the systems of the invention are almost always porous bodies, and thus inherently discontinuous, but in the present specification and in the claims by discontinuity is meant discontinuities of the macroscopic type, i.e., holes in the surface of the elements, or elements having such a geometrical shape that they do not completely take up the cross-section of the housing.

The inventors have found that, contrary to what would seem intuitive and to the common manufacturing principles of prior art systems, it is possible to manufacture fumes treatment systems, wherein at least some successive flow interception elements are provided with openings or anyway with portions not contacting the housing walls, provided that these are arranged so as to impose a tortuous path to the fumes, without losing treatment efficiency (gas conversion and particulate retention), and with reduced head losses with respect to similar systems in which, in contrast, all the elements completely take up the cross-section of the housing. In fact, the increase in turbulence imposed by the tortuous path enhances the contact between the fumes and the active elements, thus compensating the reduction in efficiency caused by the presence of the discontinuities.

FIG. 1 shows several possible shapes of discontinuous elements satisfying condition a) of the invention. Element 10 is formed as a circle missing a portion, element 11 has a central hole 12, while element 13 presents a series of holes 14 near its periphery. These elements are represented as ready to fit in a housing of essentially circular or elliptical cross-section, but of course many other shapes are possible for the cross-section and consequently for the overall shape of the elements.

FIG. 2 represents condition b) of the invention. The drawing shows in cross-section a housing 25 of the system of the invention, wherein two discontinuous elements are positioned. In the drawing the flow of fumes is assumed to move from right to left, so that the element at the right is the upstream one. The downstream element is shaped as element 10 of FIG. 1, while the upstream element is shaped as element 13 with discontinuities 14 of FIG. 1. The downstream element defines with the walls of housing 25 a discontinuity indicated in the drawing by numeral 26. The projection (indicated by P in the drawing) of this discontinuity onto upstream element, drawn along lines perpendicular to the latter, has no superimpositions with discontinuities 14.

Finally, FIG. 3 represents condition c) of the invention as realized in the same system of FIG. 2. Point A in FIG. 3 represents the edge of the downstream element, while point C on the upstream element is the projection of point A onto the latter element, drawn along a line perpendicular to the upstream element. Discontinuity 14 in the upper part of element 13 in the drawing is the closest one to discontinuity 26. The segment joining point B of this closest discontinuity on element 13 to point A on the downstream element is the minimum distance between two discontinuities on the two successive elements. The angle α formed by segment A-C (lying on a straight line perpendicular to the upstream element) and segment A-B (on which lies the minimum distance between discontinuities as described above) must be at least 5°.

Though the invention may be realized by disposing the interception elements in the housing in the most varied arrangements, for instance non-perpendicular to the axis of the housing and not parallel to each other, the preferred embodiments of the invention are those in which the flow interception elements are arranged essentially perpendicular to the axis of the housing and essentially parallel to each other. In this preferred configuration, a line perpendicular to the upstream element in a pair of successive interception elements with discontinuities is essentially parallel to the axis of the housing, so that:

geometrical condition b) of the invention results in the requirement that for any given pair of successive discontinuous elements, looked at along a line parallel to the axis of the housing, no optical path exists allowing a sight through the pair of elements;

while geometrical condition c) of the invention results in the requirement that the minimum distance between two discontinuities on two successive elements lies along a segment forming an angle of at least 5° with respect to a straight line parallel to the axis of the housing.

Moreover, the invention does not necessarily require that all flow interception elements in the housing of the system be provided with discontinuities. For instance, it is possible to resort to hybrid solutions, where some elements of the series are of a traditional type, i.e., full and sealed against the inner walls of the housing, in order to increase the soot retention or the efficiency of gas treatment. However, a system wherein all of the interception elements show discontinuities is preferred in view of head loss reduction.

Finally, the invention does not require that the flow interception elements be placed equidistant in the housing or have a comparable overall size of the discontinuity. For example, it is possible to have systems where the size of the discontinuities, or the distance between successive elements, decreases from the inlet to the outlet, in order to have the fumes treatment efficiency increasing in this order. This allows achievement of a more homogeneous treatment efficiency throughout the system.

Notwithstanding the above described possible freedom of construction, from a practical standpoint the systems where all flow interception elements are essentially perpendicular to the axis of the housing and essentially parallel and equidistant to each other, and each of the elements is provided with discontinuities, represent the preferred embodiments of the invention, in particular in view of ease of manufacturing. In the following part of the description, reference will be made to these preferred conditions and embodiments, unless stated to the contrary. On the other hand, the systems described in detail in the following do not necessarily comprise flow interception elements having comparable dimension of the discontinuities.

FIG. 4 shows in cross-section a possible preferred embodiment of a system of the invention for the treatment of fumes. System 40 comprises a housing 41 generally made of metal (e.g., stainless steel) comprised of a main chamber 42 tapered at the ends in correspondence to two tubular portions 43 and 43′, respectively used to connect the system to a pipe for the fumes coming from the engine and to a pipe for the exhaust of the gases to the outside. In the drawing, the fumes cross the system from the right to the left. Chamber 42, in a view along the axis of the housing, commonly has a circular or elliptical cross-section, but other forms are also possible. In chamber 42 flow interception elements 44 are present, that do not completely take up the cross-section of chamber 42. Besides, the elements 44 are arranged in housing 41 so that the discontinuities formed by two adjacent elements with the inner wall of chamber 42 are essentially symmetrical with respect to the axis (not shown) of the housing.

By this construction, a net path for the fumes is obtained (represented by the arrows in the drawing), such that the latter are not necessarily forced to pass through the material of element 44 in order to go from the inlet to the outlet of the system. On the other hand, the tortuousness of this path results in a high turbulence of the fumes, causing them to contact the surfaces of elements 44 (both the surface facing the inlet and the one facing the outlet) and thereby the catalyst materials arranged on the surfaces and in the porosities of elements 44 in case the system 40 is used as a gas converter, or causing them to contact the soot-retaining surfaces in case the system is used as a particulate filter.

The elements 44 may be held in place by fixing elements of many types: for example, metal parts may be employed, having a height equal to the desired distance between two successive elements 44, these parts being sections of a hollow cylindrical-shaped body (where the term cylinder is used in the broad meaning of a surface generated by a straight line moving parallel to itself along a closed curve) having the external surface of shape and size essentially equal to the internal surface of chamber 42. FIG. 4 shows fixing elements 45 of this type, but such elements may be made with many different shapes, which will be evident to those skilled in the art.

FIG. 5 schematizes the geometrical condition characterizing the invention in the preferred embodiment of elements parallel to each other and perpendicular to the axis of the housing. In the drawing, two successive flow interception elements are shown, arranged in a circular cross-section housing (not shown) and having a generic shape of the discontinuities. In particular, element 50 has the shape of element 10 in FIG. 1, while in element 51 the discontinuity has the shape of a circular aperture 52, which does not reach the edges of the element and thus the walls of the housing. The straight line X-X′ represents the axis of the housing. The minimum distance that may be identified between the two discontinuities in elements 50 and 51 is represented by segment D-D′, where D and D′ are two points on the edge of the discontinuities of elements 50 and 51 respectively, while the straight line Y-Y′ is parallel to X-X′ and passes through point D′. The angle α formed by segment D-D′ and straight line Y-Y′ is the angle characterizing the systems of the invention, which must be equal to at least 5°.

Obviously, the value of this angle is determined by the size of the discontinuities on adjacent elements and by the distance between these elements. Therefore, it is possible to adopt configurations with elements having discontinuities of a greater size by moving the elements closer to one another, or to have elements being farther apart having discontinuities of a smaller size, as long as the condition α≧5° is maintained. The value of this angle is preferably greater than 15°, and more preferably greater than 30°.

In contrast, the maximum value of angle α is not fixed, but it is variable mainly depending on the material forming the flow interception elements and, in particular, depending on their porosity. In the preferred embodiments (elements parallel to each other) this maximum angle must be less than 90° (an angle of 90° would mean that two successive elements are in contact with each other). Preferably, the maximum value of angle α is not greater than 85°, because for greater values the turbulence generated in the gas flow is such as to practically suppress the advantage of head loss reduction achieved through the discontinuities.

FIG. 6 and the enlarged view of a detail thereof in FIG. 6a show in a partially broken away view a possible embodiment of the system of the invention, wherein the flow interception elements are provided with discontinuities at the internal wall of the housing. The system 60 is comprised of a circular cross-section housing 61 provided with an inlet 62 for the fumes coming from the engine and of an outlet 62′ for the treated fumes. Inside the housing flow interception elements 63 are arranged, held in place by ring-shaped fixing elements 64. The position of the latter with respect to the flow interception elements is highlighted in the enlarged view of FIG. 6a, corresponding to the portion of the drawing of FIG. 6 delimited by a dashed circle. The elements 63 are oriented in such a way that the discontinuities 65 are as far from each other as possible. In practice, this is achieved by making the chords defining the segments of the discontinuities parallel to one another and alternated on successive elements with respect to the axis of the housing.

FIG. 7 shows in a partially broken away view another possible embodiment of the system of the invention. In system 70 the flow interception elements are provided with discontinuities in the form of apertures completely included within the geometrical area of the elements, so that the apertures do not reach the inner wall of the housing 71. In the drawing, for ease of illustration, fixing elements similar to elements 64 of FIG. 6 are not shown. In this case, the flow intercepting elements are of two types: the first type, corresponding to elements 72, exhibits a central circular aperture 73; the second type, corresponding to elements 74, exhibits a series of apertures at the periphery of the element (together referred to by number 75 or 75′). The apertures on the elements of the second type are preferably equidistant and circular for ease of manufacturing (apertures of type 75), but they could also have other shapes, e.g., elongated (apertures of type 75′).

The materials with which the flow interception elements can be made may be the most varied, particularly depending on the intended use of the treatment system. If the main purpose is gas conversion, the elements will generally be rather porous in order to enable gases to penetrate the porosities and come into contact with the catalyst material deposited on the surfaces of the elements, including the inner surfaces. In the case where the intended use of the system is mainly that of a soot filter, these elements are generally less porous than those of the previous case, and they could even be made of solid metal plates, as long as they are associated with layers of an open structure material, such that it can collect the soot.

In the case where the system is a gas converter (e.g., to be used as a catalytic system immediately downstream of the engine, in the configuration known as “Close Coupled Catalyst” or CCC), the interception elements are preferably made of metallic material, e.g., Fecralloy alloy or AISI 310S steel, in the form of a fiber mesh, a porous body of sintered powders or a foam. Porous bodies in the form of metallic foams, e.g., made of Fecralloy, are sold by the Porvair Advanced Materials Company of Hendersonville, N.C., USA. The fibers may be obtained by the process disclosed in U.S. Pat. No. 4,930,199. It is also possible to employ multi-layer fiber mats in which the layers have different fiber sizes and porosities, as disclosed, for example, in European patent EP 1 450 929 B1, to be preferably employed in such an orientation that the layer facing the fumes inlet has greater size of fibers and porosity. Alternatively, it is possible to employ ceramic elements, e.g., made of silicon carbide, cordierite or aluminum titanate.

In the case where the system of the invention is to be essentially employed as a soot filter (for example downstream of a catalytic converter and in a colder position), it is possible to use in the manufacture of the interception elements all the materials previously cited for the converter, preferably with a size of the porosities less than that of the materials used for manufacturing the elements of the catalytic converter. In this case, it is further possible to use non-porous materials, e.g., a solid metal plate, if the latter are associated with materials having porosities or apertures such that they can anchor the soot. For example, it is possible to employ elements formed by two or more different layers, which may also be made of different materials, wherein the layer facing the filter outlet is manufactured with a solid material or with a material having a relatively low porosity, in order to form a soot blocking layer, whereas the layer facing the filter inlet may be comprised of fine metal nets, sintered metallic powders or metallic meshes of a porosity greater than that of the soot blocking layer, e.g., with a porosity size similar to or greater than that of the elements employed for the catalytic converter.

The invention will be further illustrated by the following examples. These non-limiting examples show some embodiments intended to teach those skilled in the art how to practice the invention and to illustrate the best mode intended to carry out the invention.

EXAMPLE 1 (COMPARATIVE)

This example relates to a gas conversion system not according to the invention.

A mesh 0.7 mm thick of sintered Fecralloy fibers having a diameter of 35 micrometers (μm) is provided. The mesh has a 90% porosity, porosity meaning the ratio of vacuum volume to the geometric volume of the mesh. Three disks having a diameter of 70 mm are cut from this mesh, and treated at 950° C. in air for 27 hours, thus obtaining the formation of alumina oxide on the surface of the fibers. The disks so treated are immersed for 15 minutes into a suspension of AlOOH particles (Disperal® suspension, sold by the SASOL Company of Milan, Italy), then withdrawn from the suspension and treated at 700° C. in air for 6 hours. A coating of porous alumina is obtained, having a weight of 12% of the weight of the starting mesh (as measured by the weight difference before and after the latter treatment). The disks are then subjected to a catalyzation process, immersing them into a platinum nitrate solution at a concentration of 80 mg/l and leaving them in the solution for 6 hours. At the end of this step, the porous alumina has almost completely absorbed the platinum compound from the solution, the disks are withdrawn from the solution and are subjected to a treatment of reduction with hydrogen at 350° C. for 3 hours. The amount of metallic platinum so deposited on the surface of the samples, measured by difference through ICP analysis of the residual platinum nitrate solution, is equal to 8% of the weight of alumina.

The three disks so produced are arranged in a cylindrical housing having a circular cross-section, made of AISI 316L steel and having an inner diameter of 70 mm, the disks being spaced 4 mm apart by spacer rings of the type shown in FIG. 6 (elements 64). The thickness of the rings is 3 mm, so that the useful diameter of the disks is 64 mm. This system does not embody the invention, as each of the disks completely takes up the cross-section of the housing.

The system so formed is employed for an efficiency test of gas conversion. The experimental set-up includes the gas treatment system, a line supplying the gas, a thermocouple at the inlet of the gas treatment system for measuring its inlet temperature, a MKS Baratron 223B differential pressure meter connected to the gas line at the two ends of the system under measure, and a Uras 14 gas analyzer by ABB S.p.A. of Sesto San Giovanni (Milan, Italy) downstream of the gas treatment system, for the continuous measurement of the concentration of carbon monoxide (CO) exiting the system.

Through the system there are passed 40 Nm³/h of a gaseous mixture comprised of dehumidified air added with 420 parts per million in volume (ppmv) of CO, such a concentration being considered in the field as representative of the gaseous emissions of an internal combustion engine. The system is heated at 350° C. in 10 minutes and left at this temperature for an additional 10 minutes, while measuring throughout the test the head loss at the ends of the system and the concentration of CO at the outlet thereof. From these measurements the “light off” temperature is further obtained, i.e., the temperature at which the conversion of CO into CO₂ reaches 50%, which is a standard parameter in the evaluation of systems for the treatment of gases emitted by the engines. The percentage of CO conversion and the head loss (ΔP) in hectoPascal (hPa) at the ends of the system at the end of the test, as well as the light off temperature, are set forth in Table 1.

EXAMPLE 2

The test of Example 1 is repeated on a system of the invention. In this case, the system includes four disks of catalyzed mesh, absolutely identical as to materials and preparation to those of Example 1, with the only difference being that in this case the disks are perforated. In particular, each disk is provided with one or more holes, such that the total area of the holes on each disk is 25% of the total area of the disk, the first and the third disks (starting from the gas inlet side) exhibiting a single central hole (disks of type 72 in FIG. 7), whereas the second and the fourth disks each have sixteen circular holes along the edge area (apertures of type 75), the centers of which are equidistant and arranged on a circle 27.5 mm in radius and concentric with the disk. With this geometric arrangement, the angle α between the discontinuities in two successive disks, as previously defined, is about 62°. This system comprises four disks instead of the three disks of the system of Example 1, in order to have the same mesh surface and the same amount and mean distribution of catalyst, so that the comparison of the two conversion tests is as homogeneous as possible (considering that the four 25% perforated disks have a surface equal to three solid disks).

The gas conversion test is carried out on this system under the same conditions described in Example 1. The results are set forth in Table 1.

	TABLE 1
	Test	CO conversion (%)	ΔP (hPa)	Light off T (° C.)
	1	>95	24.6	171
	2	>95	15.8	161

EXAMPLE 3 (COMPARATIVE)

This example relates to a particulate filtration system not according to the invention.

Two disks having a 70 mm diameter are obtained from a 1.3 mm thick mesh of Bekipor® ST XL562 material (produced by the company Bekaert S.A. of Zwevegem, Belgium), made up of three layers of metallic fibers of different diameter, in particular 17 μm on one side of the mesh, 22 μm in the central layer and 35 μm on the opposite side of the mesh, with a total porosity of 85%. The two disks are placed in a housing identical to the one used in Example 1, the side of the mesh formed of the larger fibers facing the inlet of the system. The disks are spaced 4 mm apart by a ring 3 mm thick, leaving a useful mesh surface with a diameter of 64 mm.

The system is introduced into the experimental set-up schematized in FIG. 8 and comprised of an air compressor 80, connected through a gas line L₁ to the system 81 whose properties must be measured, and to an absolute filter 82 downstream of the system (Avasan filter, by Parker Hannifin Company, Corsico, Italy), the filter being able to retain particles with a diameter greater than 10 μm. At the two ends of system 81 there are connected a MKS Baratron 223B differential pressure-meter 83 with a reading scale between 0 and 100 hPa, and two absolute pressure-meters 84 with a reading scale between 0 and 3×10⁵ Pa (by the company Ashcroft GmbH of Baesweiler, Germany), for measuring head loss values greater than 100 hPa.

Along line L₁, upstream of system 81, a by-pass line L₂ is provided, connected to line L₁ through two three-way valves V₁ and V₂. On line L₂ a loading chamber 85 for the particulate is present, made of transparent plastic, provided with an airtight lock 86. The particulate used for the test is Vulcan XC 72 R by the company Cabot Italiana S.p.A. of Ravenna, Italy, with a particle size of about 10 μm, which is one of the standard synthetic particulates employed for filtration capacity tests in the field of automotive filters.

At the beginning of the test 40 Nm³/h of dry air are flowed along line L₁. At the same time 0.2 g of particulate are loaded into chamber 85. Then, by acting on valves V₁ and V₂, the flow is deviated along line L₂, thus causing the air stream to pass through chamber 85. When the total disappearance of the particulate from the chamber is observed, the flow is deviated along line L₁ again. The procedure is repeated five times, until obtaining a total loading of 1 g of particulate on filters 81 and 82. At the end of the test, the filtration efficiency of the system is measured, as a percentage of the particulate retained by system 81 (for convenience, this percentage is calculated by the weight difference of absolute filter 82 before and after the test), and the head loss at the ends of system 81. The results of the test are set forth in Table 2.

EXAMPLE 4

The test of Example 3 is repeated but using in manufacturing the system 81 four disks of Bekipor® mesh in which circular segments have been removed in order to obtain filtering elements of the type shown in FIG. 6 (elements 63), such that the area of each of these elements is equal to 65% of the area of the full disk from which they have been obtained. The four disks are placed into the housing, spaced 4 mm apart, in the arrangement shown in FIG. 6, i.e., with the straight edges of the elements parallel to each other and alternated with respect to the axis of the housing. By this geometric construction, the angle α between two successive discontinuities, as previously defined, is about 75°. In this case four disks are used, in order to have a particulate retention efficiency comparable to that of Example 3. The results of the test are set forth in Table 2.

TABLE 2
Test	Filtration efficiency (%)	ΔP (hPa)
3	37.8	155
4	44.0	68

The comparison of the CO conversion tests (Examples 1 and 2) shows that a system of the invention having the same conversion efficiency of a system comprised of full disks, has a head loss about 35% lower and a light off temperature 10° C. lower. The comparison between filtration efficiency tests (Examples 3 and 4) shows that with similar filtration efficiencies a filter of the present invention exhibits a head loss at its ends which is about 56% lower than that of a filter made of elements that completely take up the cross-section of the housing.

It was not possible to carry out a comparison test with four full disks, because a first attempt in that way showed that the filter, getting clogged by particulate, had head loss values unacceptable for the integrity of the system (approximately over 500 hPa). Since the head loss linearly increases with the number of filtering elements, and the filters of the invention have a head loss much lower than the full disk filters, they allow a remarkable improvement of the filtration efficiency, yet remaining at acceptable head loss values for the intended applications of these systems, e.g., not greater than 200-300 hPa prior to a regeneration in the case of an automotive filter.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.

Claims

1-17. (canceled)

18. A fumes treatment system comprising a housing having a fumes inlet and an outlet for treated fumes, and a series of essentially planar flow interception elements inside the housing, wherein the elements are formed and arranged such that:

a) at least two successive interception elements of the series each has at least one discontinuity, such that a geometrical area of the discontinuity comprises between 10 and 40% of an area that the element would have if it completely occupied an internal cross-section of the housing;

b) for any pair of successive interception elements, one being upstream and one being downstream of the other and each having a discontinuity, a projection (P) of the discontinuity of the downstream element on the upstream element, wherein the projection is made perpendicular to the upstream element, does not superimpose with the discontinuity on the upstream element; and

c) a minimum distance between two discontinuities on two successive elements lies along a segment forming an angle α of at least 5° with respect to a straight line perpendicular to the upstream element.

19. The system according to claim 18, wherein the angle α is greater than 15°.

20. The system according to claim 18, wherein the angle α is greater than 30°.

21. The system according to claim 18, wherein the housing has a circular or elliptical shape in a cross-section perpendicular to an axis of the housing.

22. The system according to claim 18, wherein all of the flow interception elements have discontinuities.

23. The system according to claim 18, wherein the flow interception elements are all essentially perpendicular to an axis of the housing and essentially parallel and equidistant to each other, wherein each of the elements has discontinuities, and wherein the angle α is lower than 90°.

24. The system according to claim 23, wherein the angle α is lower than 85°.

25. The system according to claim 23, wherein the flow interception elements are retained in a desired position by hollow cylindrical metal parts having a height equal to a desired distance between two successive elements.

26. The system according to claim 23, wherein the housing has a circular cross-section, the flow interception elements exhibit discontinuities at an internal wall of the housing in a form of segments missing from a circle which would completely take up a cross-section of the housing, and the elements are oriented such that chords defining the segments are all parallel to each other and alternated in successive elements with respect to the axis of the housing.

27. The system according to claim 23, wherein two types of interception elements are present in the housing, a first type having the discontinuity in a form of a central aperture and a second type wherein the discontinuity has a form of at least one aperture in a peripheral area of the element, and wherein elements of the two types are arranged in an alternating manner along the axis of the housing.

28. The system according to claim 23, wherein the angle α between two successive flow interception elements is constant along the axis of the housing.

29. The system according to claim 23, wherein the angle α between two successive flow interception elements increases along the axis of the housing going from the inlet to the outlet of the housing.

30. The system according to claim 18, wherein the flow interception elements comprise a porous metal material having a form selected from a fiber mesh, a porous body of sintered powders, a foam, and combinations of these forms.

31. The system according to claim 30, wherein the elements comprise Fecralloy alloy or AISI 310S steel.

32. The system according to claim 30, wherein the elements are mats comprising multilayers of metal fibers, wherein the layers have different porosities and are formed of fibers of different size.

33. The system according to claim 18, wherein the flow interception elements comprise a ceramic material selected among silicon carbide, cordierite and aluminum titanate.

34. The system according to claim 18, for use as a particulate filter, wherein the flow interception elements comprise two layers of different materials, one layer of solid material and one layer of a material selected from fine metal nets, sintered metal powders and metal fiber meshes.