IE42838B1 - Exhaust gas filter - Google Patents

Abstract

1498130 Purifying exhaust gases ASSOCIATED OCTEL CO Ltd 2 June 1976 [4 June 1975] 24157/75 Heading C1A [Also in Division B1] A gas filter for reactively removing lead compounds from the exhaust of internal combustion engines using leaded fuels comprises, as filter medium, a support member of extended surface area and a coating layer of calcined alumina formed over it, the layer being impregnated with 3-60% by weight, based on weight of alumina, of an alkali metal carbonate or borate. The alumina may be impregnated with potassium carbonate and supported on stainless steel wire wool.

Description

This Invention relates to exhaust gas filters for internal combusion engines using leaded fuels, and to a catalyst medium for use therein.

Increasing public concern over atmospheric pollution by exhaust fumes from motor vehicles, especially with reference to the lead content thereof, and increasingly stringent governmental regulations on the maximum permitted lead content of motor vehicle exhaust gases have created an urgent need for an efficient and inexpensive filter for motor vehicle exhaust gases to reduce the lead content thereof to within environmentally acceptable limits.

Also of concern are the levels of residual combustible materials, e.g. carbon monoxide, hydrogen and hydrocarbons, contained in such exhaust gases as a result of incomplete combustion of the fuel. To deal with this latter problem a variety of catalytic afterburners have been proposed using a variety of different ojcidation catalysts, Xn accordance with one series of proposals, catalysts have been proposed for promoting the ossidation of combustible materials in exhaust gases from internal combustion engines, which catalysts comprise a film of alumina deposited over a support of extended surface area and impregnated with an oxidation catalyst, ©,g. a salt or compound, especially an oxide, 2« 43838 of a metal from Groups I-VIII of the Periodic Table, and especially salts or oxides of transition metals such as chromium, vanadium, manganese, cobalt, iron, and many others. Such catalysts are disclosed in detail in U.S.

Patent Specification Nos. 3,231,520, 3,240,693, 3,362,783 and 3,410,651.

Since many such oxidation catalysts are poisoned by lead there is yet further need of an effective filter for the removal of lead compounds from motor vehicle exhaust gases prior to contact with such oxidation catalysts. In one method of tackling this problem, i.e. the removal of lead from motor vehicle exhaust gases, a filter has been proposed comprising a film of alumina deposited, as above described, over a support of j5 extended surface area and impregnated with a phosphoruscontaining compound, e.g. an alkali metal or alkaline earth metal phosphate, particularly an acid phosphate, which reacts with the lead compounds entrained in the exhaust gas to produce reaction products which are retained in the alumina film.

This type of exhaust gas filtration media is described in detail in U.S. Patent Specification No. 3,227,659, and is generally used in sequence with an oxidation catalyst medium of the type described above, and as more particularly described in said U.S. Patent Specification No. 3,227,659, or as described in U.S.

Patent Specification No. 3,495,950 which discloses a 338 particular construction of a catalytic unit for fitting to a vehicle exhaust system.

Whilst the phosphorus-containing alumina described above is an effective medium for the removal of lead from exhaust gases, it does have certain disadvantages, in particular that of relatively high cost, which results partly from the initial cost of the phosphorus compound used and partly from the multistage process of manufacture which requires the alumina first to he deposited on the substrate, first as a hydrated form which is then converted to gamma or eta alumina hy calcination, and thereafter to he impregnated with the phosphorus-containing compound. Usually this is done using an aqueous phosphate solution followed by redrying. Quite apart from the time taken in carrying out the process, the necessity for two separate drying steps contributes Substantially to the energy requirements of the process and to the increased cost of the final product. In many cases also it is found that two or more separate immersions in the-phosphate solution, each followed by a separate drying step, are necessary in order to obtain a satisfactory concentration of phosphorus in the final product, and this, of course, adds to the final cost In accordance with British Patent Specification No. 1,271,710, an alumina-coated substrate of the type disclosed inter alia in U.S. Patent Specification No. 5,227,659 and 3,251,520, but without any impregnant, 4, is proposed as a filter medium for the treatment of exhaust gases from leaded motor fuels to remove lead therefrom prior to venting or prior to passage through catalytic converter unit containing an oxidation catalyst. Whilst the unimpregnated alumina proposed in British Patent Specification No. 1,271,710 is effective to remove lead from vehicle exhausts at low engine speeds, tests have shown that, at sustained high engine speeds e.g. during L() motorway driving, a substantial quantity of the lead retained in the filter at low engine speeds is purged from the alumina and re-emitted in the filtered exhaust gas stream. In other words, the hare alumina filter is not effective to extract and retain the lead under all X5 conditions of driving.

Other filter media and processes have also been proposed for the removal of lead from exhaust gases both of vehicular and non-vehicular origin. In accordance with British Patent Specification No. 1,349,887, for example, it is proposed to remove various impurities,including lead, from exhaust gases, e.g. flue gases and vehicle exhaust gases, by passing the exhaust gas through a mesh, e.g. of stainless steel wire, the surface of which is coated with a molten salt mixture, comprising a mixture of alkali metal carbonates, which acts to entrap the lead and other particulate impurities in the exhaust gas stream. Other substrates are suggested for the. molten salt mixture including alumina, magnesia and other refractory materials, which may be used in the form of a ’838 packed bed e.g. of saddles or Raschig rings. A vehicle exhaust gas filter employing this principle is disclosed in British Patent Specification No. 1,343,680. In accordance with that proposal the vehicle exhaust gases are directed onto or over a salt mixture, comprising a mixture of metal carbonates, the melting point of the mixture being such that the mixture is solid at room temperature but is molten or melts at the temperature of the exhaust gas to be filtered. Small quantities of the salt mixture are entrained in the exhaust gas stream and carried through into a demister unit comprising, for example, a stainless steel mesh, which is wetted by the molten salt mixture to form a fused coating thereon which traps the lead and other impurities. In a particular arrangement, the exhaust system is so constructed that the incoming exhaust gases pass in indirect heat exchange relationship with the salt mixture, thereby to melt or to keep the salt mixture molten, before passing over the surface of the molten mixture thereby to entrain the molten mixture into the demister unit. In the demister unit, alumina may be used in place of the wire, e.g. stainless steel, mesh. As will be apparent, an exhaust system of this type is cumbersome, very dependent upon exhaust gas velocity and temperature, and somewhat inefficient particularly until the appropriate operating temperature has been established. 4283? In accordance with the present invention we have discovered that the purging of extracted lead from an alumina filter at high temperature, e.g. during motorway driving, can be substantially eliminated by impregnating the alumina with an alkali metal carbonate or borate. Moreover, the carbonates and borates are cheaper and/or easier to incorporate into the alumina than the corresponding phosphate, are substantially independent of the temperature and velocity of the exhaust gas stream and do not I'cquirc any warm-up period. in accordance with the present invention, therefore, there is provided an exhaust gas filter for internal combustion engines using leaded fuels, said filter comprising, as the filter medium, a support member of extended surface area and a coating layer of calcined alumina formed over said surface area, said alumina layer having impregnated therein from 3-60% by weight, based on the weight of the aLumina, ol’ an alkali metal carbonate or borate.

In the filters of this invention the amount of - 7 42838 carbonate or borate deposited in the alumina is not critical but is governed largely by practical considerations; too little will result in inadequate reaction with the volatile lead compounds in the exhaust gases, resulting in lower extraction efficiencies at high speed, and at lower speeds, although there will be significant absorption of the volatile lead compounds on the alumina, the absorbed lead compounds will be revolatilised at higher speeds, when the exhaust gas temperature rises, with resultant purging of the absorbed lead compounds from the filter. Too high a concentration of carbonate will reduce the effective surface area of the alumina, with consequent reduction of its absorption capacity. Based on these practical considerations, the amount of carbonate will generally be from 3-60% by weight, based on the weight of the alumina, preferahly 15-50%.

As already indicated, the impregnated alumina will be supported on a substrate of extended surface area, as taught, for example, in U.S. Specification No.3,227,659. The substrate for the alumina may be of any material, metal or non-metal, capable of providing a physical support for the alumina and which is physically, chemically and dimensionally stable under conditions of use, which conditions generally Involve elevated temperatures of 700°C or more. The preferred materials for the substrate are steel, stainless steel, nickel and titanium. The substrate may be of any configuration which provides an extended surface on which the alumina can be deposited. Suitable configurations for the substrate include, sheets, tubes, meshes, balls, plates, saddles, wires, and filaments. The preferred substrate is a stainless steel wire wool.

The thickness of the alumina layer deposited on the substrate is not critical to this invention. Usually, however, the alumina layer will have a thickness of from 0.25 mm to 2.5 mm but thicker or thinner layers can be used.

In preparing the filter medium used in this invention the substrate is first coated with alumina, for example, in the manner taught in U.S. Patent Specification No. 3,227,659, i.e. by immersing the substrate in an jj aqueous sodium or potassium aluminate solution so as to deposit on the substrate a layer of alumina trihydrate which is subsequently converted by calcination e.g. by heating to 500°C to form a layer of gamma or eta alumina. The alumina is then impregnated with an aqueous solution of the alkali metal carbonate or borate. Here the carbonates and borates have an advantage over the above described phosphates in that they have a greater solubility in watea; particularly potassium carbonate, and therefore a sufficient concen25 tration of the salt can usually be deposited in the alumina in a single Immersion. In certain cases, for example, where only a relatively low carbonate conceng® 38 tration is required, e.g. 15-20%, a significant advantage of potassium carbonate over potassium phosphate is that it can he simultaneously precipitated on the support with the alumina trihydrate from a mixed solution of alkali metal carbonate and alkali metal aluminate.

A suitable coprecipitation technique comprises preparing an aqueous sodium aluminate solution containing 2.0M sodium hydroxide and 1.0M aluminium in solution.

Heating the solution to 90°C and adding potassium carbonate to the heated solution whilst stirring until the solution is 4.0M with respect to potassium carbonate. The substrate, e.g. stainless steel wire wool is immersed in the hot sodium aluminate/potassium carbonate solution and the solution seeded with a small amount of aluminium powder to initiate precipitation. After the desired coating thickness has built up, the substrate is removed, washed with water, dried, and oalcined.

Suitable alkali metal carbonates and borates useful in accordance with this invention include sodium, potassium and lithium and the corresponding borates. By reason of coat and efficiency potassium carbonate is preferred.

Usually, but not necessarily, the filter of this invention will be used in tandem with a catalytic converter unit containing an oxidation catalyst, for example, of the type described in U.S. Fatent Specifications mentioned hereinbefore. However, the oxidation catalyst as such does not form any part of this invention and need not therefore be described herein in any detail.

In addition to the filter medium itself, the present invention extends, of course, to a filter unit for a vehicle exhaust system, such unit comprising a housing having an inlet and an outlet for a vehicle exhaust, and located in the housing in a position exposed to the exhaust gases as they pass therethrough a filter medium as above described and containing the substrate, the alumina layer in which is impregnated the said alkali metal carbonate or borate.

In order to demonstrate the lead retention capacity of carbonate impregnated alumina, as compared with phosphate, impregnated alumina and alumina itself, under purge conditions, the following experiment was carried out.

EXPERIMENT In this experiment three identical cars were fitted with exhaust gas filters, one comprising unim20 pregnated alumina, one comprising alumina impregnated With phosphate and one comprising alumina impregnated with potassium carbonate.

The filters were each constructed by stripping out the sound absorbing material from inside a conventional silencer and packing each silencer in two longitudinally spaced zones with stainless steel wire wool coated with the filter medium. The volume of the front filter (i.e. nearest the engine) was 6.85 litres and the volume of the rear filter was 5.5 litres.

The filters were prepared by first of all depositing a layer of alumina approximately 1 mm thick on the wire wool by the technique described in U.S. Patent 3,227,659, immersing the coated wire wool, after coating and calcination, in one case, into an aqueous solution of sodium phosphate, and in the second case into an aqueous solution of potassium carbonate and then drying the impregnated alumina. A third filter was prepared in similar manner but without impregnation of the alumina. The filter compositions on the three cars were as follows: Car 1 Car 2 Car 3 Front filter Volume 6.85 1. 6.85 1. 6.85 1. Wt. wire wool 685 gms. 685 gms. 685 gms. Wt. alumina 1410 gms. 1295 gms. 1415 gms. Wt. Na3P04 235 gms. - wt. k2co3 - 240 gms. - Rear filter Volume 5.5 1. 5.5 1. 5.5 1. Wt. wire wool 550 gms. 550 gms. 550.gms. Wt. alumina 1068 gms. 1155 gms. 1165 gms. Wt. Na5P04 175 gms. - - Wt. K2CO3 — 225 gms.

The cars fitted with filters were run on a road 15 simulator to give journeys of 3000 miles of city-urban type, stop-start driving, followed by wide open throttle acceleration from idle to 70 m.p.h. and a sustained period of maximum speed to simulate motorway driving. This sequence was repeated 5 times to give a 2o total of approximately 15,000 miles operation.

The lead emissions from the tail pipe were monitored over the whole test period by means of continuously recording absolute lead monitors attached to the tail pipe. 13. 48838 The results obtained were as follows: Weight of lead emitted at purge* (gms) Filter Medium Mileage Alumina alone Alumina + KajPO^ Alumina -+ K2C03 5,000 7.3 4.2 3.5 3,000 21.0 9.0 5.1 9,000 38.0 12.7 11.8 12,000 ., 57.0 22.4 14.7 15,000 49.1 22.5 29.9 Total lead emitted during purge tests (gms) 182.4 70.8 65.0 Total lead emitted overall test (gms) 205.9 99.2 114.0 Total lead input to filter**(gms) 1374 1538 1513 Total filter efficiency*** 85% 93% 92.5% * i.e. weight of lead emitted during the high speed portion of the cycle. ** calculated from total fuel inflow, assuming total lead emission from engine equals total lead input to engine. *** % of ingoing lead retained by filter.

Comparison of the above results shows that whereas alumina itself is a reasonably effective filter for lead removal under normal, low-speed driving conditions, some-88% of the total lead-emission occurs during high speed driving , i.e. during the purge.tests, as a result .'of purging of absorbed lead from the filters. In contrast, the phosphate and carbonate impregnated filters improve not only the overall lead absorbtion but substantially reduce the amount of lead subsequently purged from the filter in periods of high speed operation, this being 71% of the total emission in the case of phosphate and 57% in the case of carbonate indicating the even greater retention powers of the carbonate impregnated filter than the phosphate impregnated filter.

In bench experiments using an exhaust gas filter 10 constructed substantially as described above but using an equivalent amount of sodium borate as an impregnant in place of the sodium phosphate (Car 1) and potassium carbonate (Car 2) and simulating a similar cycle of high and low speed driving there have been shown overall extraction and retention efficiences in excess of 90% of the lead input to the filter, as opposed to the 85% obtained with un-impregnated alumina, thereby demonstrating a similar effect to that obtained with potassium carbonate as the impregnant.

Claims (4)

1. CLAIMS!1. An exhaust gas filter for internal combustion engines using leaded fuels, said filter comprising, as the filter medium, a support member of extended surface area and a coating layer of calcined alumina formed over said surface area, said alumina layer having impregnated therein from 3-60% by weight, based on the weight of the alumina, of an alkali metal carbonate or borate.

2. A filter according to claim 1, wherein the alumina is impregnated with from 15-50% by weight of said carbonate or borate.

3. A filter acoording to claim 1, wherein the alumina is impregnated with potassium carbonate.

4. A filter according to claim 3, wherein the support member .is a stainless steel wire wool.