GAS FILTER

Abstract: A vehicle exhaust filter comprises a cylindrical housing (4, 5, 6) with inlet and outlet spigots (7 and 8) enabling it to be included in a vehicle's exhaust duct. Partition walls (13, 14, 15, 16) within the housing support four porous ceramic cylindrical filter elements, namely two upstream elements (9, 10) and two downstream elements (11, 12). The partition walls are ported in a manner such that gas flowing from inlet spigot (7) to outlet spigot (8) must pass through the cylinder wall of one or other of the upstream elements (9, 10) and, thereafter through the cylinder wall of one or other of the downstream elements (11, 12). The upstream elements hold a quantity of adsorbed potassium aluminium sulphate to convert lead residues in the exhaust into lead sulphate and the downstream elements hold a quantity of adsorbed trisodium orthophosphate or other hygroscopic salt to maintain their surfaces moist and tacky.

Description: GAS FILTER TECHNICAL FIELD The present invention, in its broadest aspects, relates to filters for the separation of entrained particulate matter from a stream of gas loaded with such matter. It was devised primarily for the filtration of waste gases, such as the exhaust emissions of internal combustion engines, for the purpose of preventing the discharge of atmospheric pollutants therewith, and is described primarily with reference to that application hereinafter. However it will be appreciated that it is applicable to the filtration of gas streams in general. BACKGROUND ART It is well known that the exhaust emissions of internal combustion engines, for example, petrol or diesel fuelled vehicle engines, frequently include soot, smoke or other particles. It is also known to provide filters in the exhaust ducts of such engines to trap those particles so as to reduce atmospheric pollution. Typical prior proposals are described in the specifications belonging to the following patents or patent applications, UK 808806 (Saunders), US 3556734 (Peterson), US 3757489 (Evanshen), US 1716479 (Bilsky), and PCT/HU91/00005 (Kapas et al.). Consideration of those specifications shows that it is known to use a body of a fibrous or porous medium as the filter element of an exhaust filter, to load such filter elements with oil to enhance their tackiness, and to impregnate the element with chemical reactants intended to neutralise specific components of the exhaust emissions. DISCLOSURE OF INVENTION It has been found that a gas filter element comprising a body of a fibrous or porous filter medium is much more effective at trapping entrained particles in a gas stream when it is newly wet with oil or other tackifying liquid. However in a relatively short time the trapped particles coat the surface of the medium and shield the liquid from contact with further particles so that the efficiency of the filtration rapidly falls off. It has been proposed to provide a reservoir of liquid within exhaust gas filters and means to continuously wash the filter element with the liquid, but this calls for a large and expensive filter and renders the task of removing and replacing the filter element, when eventually it becomes fully loaded or choked, more messy and time consuming than is the case with a dry filter. Therefore an object of the present invention is to provide a gas filter comprising a filter element that presents a wet and tacky surface to the gas stream to be filtered, but which filter does not require a reservoir of liquid for the maintenance of that surface for the effective working life of the filter element. The invention consists in a gas filter comprising a housing, having an inlet port and an outlet port, and at least one filter element, comprising a body of filter medium, within said housing, wherein at least a part of any gas stream entering said housing through said inlet port and leaving said housing through said outlet port is caused to flow through said medium, and wherein a quantity of a hygroscopic salt is adsorbed to the surface of said medium. In preferred embodiments of the invention that are suitable for use as engine exhaust gas filters, the housing is adapted to be included as part of an exhaust duct of an internal combustion engine, the filter medium is heat resistant, and the hygroscopic salt is a stable salt, that is to say, as the term "stable" is used herein, the salt in question does not react chemically with the exhaust emissions under the conditions existing in an operating internal combustion engine's exhaust duct. The preferred hygroscopic salt is trisodium phosphate (Na 3 P0 4 ). Lead oxide (PbO) is a particularly noxious atmospheric pollutant released in substantial amounts by engines operating on so called leaded fuel, that is to say fuel containing lead compounds introduced to enhance the octane rating of the fuel. Thus, in preferred embodiments of the invention the filter medium of said at least one filter element, or more preferably that of a further upstream filter element, holds a second adsorbate which is able to react with the lead oxide and convert it into lead sulphate particles that are readily trapped by said at least one filter element. That second adsorbate may be a duplex sulphate containing one and three valent metallic ions. The preferred sulphate is potassium aluminium sulphate or kalinite (KAI(S0 4 ) 2 ). The invention also extends to exhaust gas filters including only a duplex sulphate adsorbate, in that the mere conversion of the lead oxide in the exhaust emissions to lead sulphate is advantageous, because the substantially insoluble nature of the latter greatly decreases its toxicity. BRIEF DESCRIPTION OF THE DRAWINGS By way of example, an embodiment of the above described invention is described in more detail hereinafter with reference to the accompanying drawings. Figure 1 is a longitudinal sectional view of an exhaust gas filter according to the invention. Figure 2 is a cross-sectional view taken on line 2-2 of figure 1 , drawn to a larger scale. Figure 3 is a cross-sectional view taken on line 3-3 of figure 1 , drawn to a larger scale. BEST MODE OF CARRYING OUT THE INVENTION The illustrated exhaust filter comprises a sheet metal cylindrical housing of circular cross-section, comprising a curved wall 4 and two end walls 5 and 6 respectively. The end wall 5 is pierced by an inlet port surrounded by an inlet spigot 7, and the end wall 6 is pierced by an outlet port surrounded by an outlet spigot 8. The spigots 7 and 8 enable the filter to be installed as part of a motor vehicle's exhaust duct by means of conventional pipe clamps. In other embodiments the housing may be differently proportioned or shaped to suit the overall design of the exhaust system of a vehicle for which it is intended. Frequently this will require it to have an oval cross-section. Four, self-supporting, tubular filter elements 9, 10, 11 and 12 respectively, are fixedly positioned within the housing by partition walls 13, 14, 15 and 16 respectively. Those partition walls are circular metal plates that may be spot welded directly to the wall 4 or to anchorage lugs thereon, or otherwise conventionally secured in place. The filter elements 9, 10, 11 and 12 may be adhered to the respective partition walls. Alternatively they may be sleeved over and/or within short tubular projections or circular flanges projecting from the respective partition walls. Each of the filter elements 9, 10, 11 and 12 comprises a thick- walled cylinder of a porous ceramic filter medium, such that gas may flow through the medium but smoke sized particles may not. The partition walls 13 and 15 are imperforate except for a ring of holes 17 near their peripheries, whereas the walls 14 and 16 are imperforate except for two relatively larger holes 18 respectively in register with the bores of the filter elements contacting these partition walls. Thus, exhaust gas entering the housing through the inlet spigot 7 is caused to flow into the space between end wall 5 and partition wall 13, then through the holes 17 in partition wall 13 into the space between that wall and partition wall 14 surrounding the filter elements 9 and 10, then through the filter medium constituting the walls of those elements into their bores, then through the holes 18 in partition wall 14 into the space between that wall and partition wall 15. The gas then must flow via a similar path through the partition walls 15 and 16 and filter elements 11 and 12 to and through the outlet spigot 18, all as indicated by the arrows in figure 1. Therefore all of the gas must pass through the filter medium forming one or other of the upstream filter elements 9 and 10 and then through the filter medium forming one or other of the downstream filter elements 1 1 and 12 in its passage through the filter as a whole. The filter medium of the upstream elements 9 and 10 is coated with an adsorbed layer of potassium aluminium sulphate (KAI(S0 4 ) 2 ). This reacts with the lead in the exhaust gas (appearing as PbO) and converts it into insoluble lead sulphate. The filter medium of the downstream filter elements 11 and 12 is coated with an adsorbed layer of trisodium orthophosphate (Na 3 P0 4 ). That phosphate is hygroscopic and soluble in water. Thus it absorbs water from the products of combustion and this has been found to form a tacky layer covering all of the surface of the porous ceramic filter medium, including the surfaces of the cells within it. This layer is very effective in trapping particulate wastes such as lead sulphate particles, and soot or smoke particles. Furthermore, it has been discovered that the phosphate continuously migrates to the surface of the trapped matter, and being stable, is therefore able to continue functioning for the effective life of the filter medium, that is until the medium becomes clogged with trapped matter. The illustrated filter is suitable for use with an engine of large capacity. In other embodiments of the invention, more appropriate for smaller engines, the two parallel filter elements of each stage may be replaced by a single similar element. Likewise for still larger engines the two elements of each stage may be replaced by three or more elements in parallel. For very small engines a single stage, single element filter may be provided. In this instance the element may be impregnated with a trimetallic orthophosphate alone (especially for lead free fuel), or both adsorbates may be present in the one filter element. The illustrated filter may be termed a full flow filter, in that all of the exhaust emissions must flow through the filter medium before it leaves the filter. However the invention is also applicable to so called by-pass filters in which, under normal operating conditions, a major part of the flow is directed through the medium, but a more restricted path (referred to as a minor path herein) is available that by-passes the filter elements to enable the engine to continue functioning if the filter medium becomes clogged and is not replaced. The above mentioned prior art specifications disclose filters of the by-pass type to which the invention may be applied. In the present instance such a minor by-pass path could be provided, for example, by providing relatively small openings in the partition walls 14 and 16 at or near their centre points. The illustrated embodiment utilises a ceramic "sponge" for the adsorbent filter medium. A preferred way of producing such a medium is as follows. A body substantially conforming to the size and shape of the required filter element of a fairly rigid plastics foam of open porosity is moulded in conventional manner, or cut from a larger work piece of commercially available stock material. It is, of course, essential that a foam be selected in which the cells are interconnected so that gases or other fluids may pass through it. The plastics foam body is then dipped into a solution of aluminium silicate, preferably of a concentration such that it has the consistency of a thick batter. After draining the body is allowed to dry and then is fired at a temperature within the range of from 300°C to 500°C to burn out the plastics foam, leaving a green ceramic sponge. This is then cured at a temperature of from 1000°C to 1200°C to produce a hard ceramic body. A ceramic sponge body produced in this way is freely porous to gas and has a very high surface area to volume ratio, enabling it to accept and retain a considerable quantity of adsorbate. The cured ceramic sponge body may then be dipped into a, preferably boiling, solution of KAI(S0 4 ) 2 and/or Na 3 P0 4 as the case may be. Once the solution has dried the filter element may be heated to about 250°C to cause the salt or salts to bond firmly to the ceramic for transport and storage until installed in an engine exhaust filter. It should be mentioned that while such a ceramic sponge is preferred it is not essential to the invention and satisfactory exhaust gas filters according to the invention may be produced using other conventional porous or fibrous bases for the adsorbent filter medium. For example, zeolite, diatomite, attapulgite, vermiculite and like fibrous minerals are satisfactory, as are compacted metallic wools or other known heat resistant filter media able to retain fine particles. In the event that the filter is not called upon to filter hot or corrosive gas streams, for example in the case of an engine air intake filter, the adsorbent filter medium may be water resistant felt or paper. The illustrated embodiment is a one piece filter that has to be replaced in its entirety when clogged, whereas in other embodiments, an end wall of the filter housing may be removed, or it may be otherwise openable, to allow removable filter elements to be replaced. Such filter elements may be bare or in removable cartridges, depending on the overall design of the filter. Finally it is mentioned that exhaust gas filters according to the invention may include further filter elements of materials adapted to remove specific components of the exhaust emissions not retained or modified by the filter elements holding adsorbates according to the invention. For example, activated charcoal filter elements may be provided for the retention of carbon monoxide gas.

Claims: CLAIMS: 1. A gas filter comprising a housing, having an inlet port and an outlet port, and at least one filter element, comprising a body of filter medium, within said housing, wherein at least a part of any gas stream entering said housing through said inlet port and leaving said housing through said outlet port is caused to flow through said medium, and wherein a quantity of a hygroscopic salt is adsorbed to the surface of said medium. 2. An exhaust gas filter according to claim 1 wherein said housing is adapted to be included as part of an exhaust duct of an internal combustion engine, said filter medium is heat resistant, and the hygroscopic salt is one that does not react chemically with internal combustion engine exhaust emissions under the conditions existing in an operating internal combustion engine's exhaust duct. 3. A filter according to claim 1 wherein said hygroscopic salt is a trimetallic orthophosphate. 4. A filter according to claim 3 wherein said trimetallic orthophosphate is trisodium orthophosphate. 5. A filter according to any one of the preceding claims wherein a quantity of a duplex sulphate containing one and three valent metallic ions is also adsorbed to the surface of said medium. 6. A filter according to claim 5 wherein said duplex sulphate is potassium aluminium sulphate. 7. An exhaust filter according to claim 1 comprising at least one further filter element comprising a body of heat resistant filter medium with a quantity of a duplex sulphate containing one and three valent metallic ions adsorbed to its surface, and wherein said exhaust emissions are directed to said at least one further filter element before reaching said at least one filter element. 8. An exhaust filter according to claim 7 wherein said duplex sulphate is potassium aluminium sulphate. 9. A filter according to claim 1 wherein a minor flow path by- passes the filter element or elements as the case may be. 10. A filter according to claim 1 claims wherein said filter medium is a porous ceramic material. 11. A filter according to claim , 10 when said filter medium is made by a method comprising the steps of forming a body substantially conforming to the size and shape of the required filter element from a plastics foam of open porosity and interconnected cells, dipping the plastics foam body into a solution of aluminium silicate, allowing the dipped plastics foam body to drain and dry, firing the dry body at a temperature of from 300°C to 500°C to burn out the plastics material and leave a green ceramic sponge, and then curing the green ceramic sponge at a temperature of from 1000°C to 1200°C to produce a hard, porous ceramic body. 12. A filter according to claim 11 when the ceramic body is loaded with adsorbate by a method comprising the steps of dipping the ceramic body into a solution of the adsorbate, allowing the dipped body to drain and dry, then heating it to cause the adsorbate to bond firmly to the ceramic. 13. A gas filter comprising a housing, having an inlet port and an outlet port, and at least one filter element, comprising a body of filter medium, within said housing, wherein at least a part of any gas stream entering said housing through said inlet port and leaving said housing through said outlet port is caused to flow through said medium, and wherein a quantity of a duplex sulphate containing one and three valent metallic ions is adsorbed to the surface of said medium. 14. A filter according to claim 13 wherein said duplex sulphate is potassium aluminium sulphate.

Inventors: KAPAS SANDOR, KAPAS, SANDOR
Applicants: AIR SAFE EXHAUST SYST, AIR SAFE EXHAUST SYSTEMS PTY. LTD.
Application year: 1993
Main classification: B01D53
IPC codes: B01D46, B01D49, B01D53, F01N3, F01N7