CA1070621A

CA1070621A - Exhaust gas filter

Info

Publication number: CA1070621A
Application number: CA253,657A
Authority: CA
Inventors: Kenneth Campbell; Ting-Man Li
Original assignee: Associated Octel Co Ltd
Current assignee: Innospec Ltd
Priority date: 1975-06-04
Filing date: 1976-05-28
Publication date: 1980-01-29
Also published as: AU1446476A; IE42838L; DK239276A; BE842588A; JPS51148665A; FR2313557B1; IE42838B1; LU75065A1; DE2624686A1; FR2313557A1; GB1498130A; SE7606310L; NL7605912A; IT1061064B

Abstract

ABSTRACT OF THE DISCLOSURE

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

Description

lQ7()6Zl FIELD OF INVENTION
This invent~on relates to exhaust gas filters for internal combusion engines using leaded ruels, and to a catalyst medium for use therein.

BACKGROUND AND PRIOR ART
Increasing public concern over atmospheric pollution by exhau~t 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 a~d in-expensive filter for motor vehicle exhaust gases to reduce the lead content thereof to within environmentally acceptable lim~ts.
, .
Also oi concern are the levels of resldual com-bustlble materials, e.g. carbon monoxide, h~drogen and hydrocarbon~, contained in such exhaust gases as a result of incomplete combustlon o~ th~ fu~ . To deal with this latter problem a variety of catalytic after-burners have been pr~posed using a varlety of different oxidation catalysts. In accordance with one series of proposals, catalysts have been proposed for promoting the oxidation of combustible materials in exhau~t gases from interna.l combustion engines, which catalysts comprise a film of alumina deposited over a support of extended surface area and impregnated with an oxidation catalyst, e.g. a salt or compound, especially an oxide, .~ .
,

2.
.

~070621 of a metal from Groups I-VIII of the Per~odic Table, and especially salts or oxides of transition metals ~uch as chromium, vanadium, manganese, cobalt, iron, and many others. Such cataly~ts are disclosed in detall in U.S.
Patent Specific~tion Nos. 3,231,520, 3,240,693, 3,362,783 and 3,410,651.

Since many such oxidation catalysts are poisoned by lead there i8 yet further need of an effective filter for the remov~l of lead compounds from motor vehicle exhau~t gases prior to contact with such oxidation cataly~ts. In one method of tackling this problem, i.e.
the removal of lead ~rom motor vehicle exhaust gases, a filter has been proposed comprising a fllm of alumina deposited, as above described, over a support of extended surface area and impregnated with a phosphorus-containing compound, e.g. an alkali metal or alkaline earth metal phosphate, particular}y an acid phosphate, which react~ with the lead compounds entrained in the exhau~t gas to produce reaction products which are retained in the alumina film.

This type of exhaust gas filtration media i8 described in detail in U.S. Patent Speci~ication No. 3,227,659, and is generally used in sequence with an oxidation catalyst medium of the type de~cribed above, and as more particularly described in said U.S. Patent Speci~ication No. 3,227,659, or as described in U.S.
Patent Speci~ication No. 3,495,950 which disclose~ a ;

. - . . . .
.

10706~1 :
particular construct~on of a catalytic unlt for fltting to a vehicle exhaust æystem.

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 oP relatively high cost, which re~ults partly from the initial cost of the ph~sphorus compound used and partly from the multlstage process of manu~ac-ture which requires the alumina first to be deposited on the substrate, flrst as a hydrated form which i8 then converted to gamma or eta alumina by calcination, and thereafter to be lmpregnated wlth the phosphorus-contain-ing compound. Usually this is done using an aqueous phosphate solutlon ~ollowed by redrylng. Qulte apart from the time taken in carrying out the procee~, the necesslty for two separate drying steps contributes substantially to the energy requirements oP the process and to the increased cost oP the Pinal product. In many cases al~o it is found that two or more separate immer-sions in thephosphate solution, each followed by a separate drying step, are necessary in order to obtain a satisPactory concentration of phosphorus in the ~in~l product, and this, of course, adds to the final c03t.

In accordance with U.K. Patent Specification No. 1,271,710, an alumina-coated substrate of the type disclosed inter alia in U.S. Patent Specificatlon No. 3,227,659 and 3,231,520, but without any impregnant, 4.
.; - .
. . . .

10706Z~

is proposed as a filter medium for the treatment of exhaust gases from leaded motor fuels to remove lead there~rom prior to venting or prior to passage through catalytic converter unit containing an oxidation catalyst. Whilst the unimpregnated alumlna proposed in U.K. Patent 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 motorway driving, a substantial guantity of the lead retained in t~e filter at low engine speeds is purged from the alumina and re-emitted in the ~iltered exhaust gas stream. In othor words, the bare alumina filter is not effective to extract and retain the lead under all conditions oi! driving.

Other filter media and processes have also been proposed for the removal of lead from exhaust gases both o~ vehicular and non-vehicular origin. In accordance wlth U.K. Patent No. 1,349,887, for example, it is proposed to remove various impurities,including lea~, 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 whicn 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 ~or theAmolten salt mixture including alumina, magnesia and other : rei'ractory mater1al6, which may ba used in the form o~ a 5.

.. . ..

\ - ~;
10706Zl packed bed e.g. of saddles or Raschig rings. A
vehicle exhaust gas filter employing this principle is disclosed in U.K. 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 carb-onates, 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 there-on 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 mix-ture, thereby to melt or to keep the salt mixture . , ~ 20 : molten, before passing over the surface of the molten : mixture thereby to entrain the molten mixture into the '. demister unit. In the demister unit, alumlna 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.

According to one aspect of the present invention 3o ,' . ' .

. .

there is provided a method of preparing a filter medium for use in a filter according to claim 1, which comprises forming on a support member of ex-tended surface area a coating layer of calcined alumina, impregnating said layer with an aqueous solution of an alkali metal carbonate or borate, and drying the impregnated layer to deposit there-in said alkali metal carbonate or borate, the concentration of said alkali metal carbonate or borate in said aqueous solution being such as to deposit in said layer from 3 to 60% by weight of said alkali metal carbonate or borate, based on the weight of the calcined alumina.
In accordance with a further aspect of the present invention, therefore, there is provided an exhaust gas filter for internal combustion en-gines using leaded fuels, said filter comprising, as the filter medium, a support member of extenaed surface area and a coating layer of calcined alumina . 20 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.

' DETAILED DESCRIPTION

In the filters of this invention the amount of .

. .
: , ~ ., , '' ' :

107V6Zl carbonate or borate deposited in the alumina is not critical but is governed largely by practical cons~dera-tions; 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 effectivc surface area of the alumina, with consequent reduction of its absorption capacity. Based on these practical considerations, the amount o~ carbonate will generally be from 3-6~% by weight, based on the weight of the alumina, preferably 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 su~strate for the alum$na may be of any material, metal or non-metal, capable of providing a physical support for the alumina and which is phys~cally, , ~ chemically and dimensionally stable under conditions of ; use, which conditions generally involve elevated temperatures of 700C or more. The preferred materials for the substrate are stee~, stainless steel, nickel and titanium. The substrate may be of any 8.

:
.

-10706Zl configuration which provides an extended surface on which the alumina can be deposited. Suitable configurations - for the substrate include, sheets, tubes, meshe~, balls, plates, saddles, wires, filaments etc. The pre~erred substrate is a stainle~s 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 inventlon the substrate i8 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 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 500C 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 ha~e an advantage over the above described phosphates in that they have a greater solubility in wate~ particularly potassium carbonate, and therefore a su~ficient concen-tration of the salt can usually be deposited ~n the alu-~ mina in a single immersion. In certain cas2s, for - example, where only a relatively low carbonate concen- -- :
, 9.

, , 10706Zl tration is required, e.g. 15-20%, a significant adva~tage of potassium carbonate over potassium phosphate is that it can be simultaneously ~recipitated on the support with the A~umina trihydrate from a mixed solution of alkali metal carbonate and alkali metaI aluminate.
A suitable coprecipitation technique comprises preparing an aqueous sodium aluminate solution containing 2.0M
sodium hydroxide and 1.0M aluminium in solution.
~eating the solution to 90C and adding potassium carbon-ate to the heated solution whilst stirring until the so-lution ls 4.0M with respect to potassium c~rbonate. me 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 p~e-ipitation. After the desired coating thickness has built up, the substrate is removed, washed with water, dried, and calcined.

Suitable alkali metal carbonates and borates useful in accordance with this invention include sodium, ~20 potassium and lithium and the corresponding borates. By reason of cost and efficiency potassium carbonate is preferred.
,:
Usually, but not necessarily, the filter of this invention will be used $n tandem with a catalytic converter unit containing an oxidation catalyst, for example, of the type described in U.S. Patent Speci ica-tions me~tioned here~nbefore. ~owever, the oxidation 10.
.
:
. .

- \ ~
10706Zl catalyst as such does not ~orm any part of this ~nventio~
and need not therefore be described herein in any detail.

Tn addition to the filter medium itsel~, the present invention extends, of course, to a filter unit ~or 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 im~regnated 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 condltions, the following experiment was carrled out.

EXPERIMENT
In this experiment four identical cars were fitted with exhaust gas filters, one comprising unim-pregnated alumina, one comprising alumina impregnated with phosphate, one comprising alumina impregnated with potassium carbonate and one comprising alumina impregnated with sodium borate.
~' The filters were constructed by stripping out the sound absorbing material from inside the silencers o~

1~.

:

spaced zones with stainless steel wire wool coated with the filter medium. The volume of the front ~ilter (i.e.
nearest the engine) was 6.85 litres and the volume of the rear filter was 5.5 litres.

The ~ilters were prepared by first of all depositing a layer of alumina approxlmately 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 calcina-tion, in one case, into an aqueous solution of sodium phosphate, and in the second case into an aqueous solution of potaæsium carbonate and then drying the impregnated alumina. A third filter was prepared in similar manner but without impregnation of the alumina. The filter compos1tions on the three cars were as follows:
., .

,. . . .
.. . . .

, . . .
~,,. . : .

, . . .
:. .
~ 12.

,, .

.. . ...... .
.

~ ,j ~0706Zl Car 1 Car 2 I Car 3 Front filter _ ~_ ~olume 6.85 l.6.85 l. 6.85 l.
Wt. wire wool 685 gms.685 gms. 685 gms.
Wt. alumina 1410 gms.1295 gms. 1415 gms.
Wt. Na3P04 235 gms. ; _ _ Wt. K2C03 _ 240 gms. _ Rear filter Volume 5.5 l. 5.5 l. 5.5 l.
Wt. wire wool 550 gms. 550 gms. 550.gms.
Wt. alùmina 1068 gms. 1155 gms. 1165 gms.
Wt. Na3P04 175 gms. _ _ Wt. K2C03 _ 225 gms. _ The cars fitted with ~ilters were run on a road simulator to,give journeys of 3000 miles o~ city-urban type, stop-start driving, followed by wide open throttle acceleration ~rom idle to 70 m.p.h. and a sustained period of maximum speed to simulate motorway ~'~
driving. This seguence was repeated 5 times to give a ' 20 total o~ approximately ~5,000 miles operation. ' :
The lead emisæions from the tail pipe were monitored over the whole test period by means-of continuously recording absolute lead monitbrs attached to the tail pipe.
, . ' ' .
.
13.

~. . . .. . . . ..
- . - . . :
.
,~ ': .. . .

The results obtained were as follows:
Weight of lead emitted at purge*
(gms) Filter Medium Mileage Alumina Alumina Alumina alone ~ Na3P04 ~ K2C3

3,000 7.3 4.2 3.5 6,000 21.0 9.0 5.1 - 9,000 38.0 12.7 1~.8 12,000 67.0 22.4 14.7 15,000 49.1 22 5 29.9 ~otal lead emitted during purge 182.4 70.8 65.0 tests (gms) Total lead emitted overall test (gms) 205.9 99.2 114.0 - Total lead input to ~ilter**(gms) 1 1374 1538 1513 Total filter efficiency*** 85~o 93% 92.5~o * i.e. weight of lead emitted during the hlgh speed portion of the cycle.
** calculated from total fuel inflow, assuming total lead emiæsion ~rom engine equals total lead input to engine.
; ~
*** % of ingoing lead retained by filter.

Comparison of the above results shows that whereas alumlna 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 as a result of purging of ~ absorbed lead irom tho filt~rs. In contrast, the .' :
~ 14.

~ 07~6Zl phosphate and carbonate impregnated iilters 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 emisslon in the case of phosphate and 57~ in the case of carbonate indicating the even greater retention powers of the carbonate and borate impregnated filters than the phosphate impregnated filter.
. ~
In bench experiments using an exhaust gas filter 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 o~ 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 demonstrat-ing a similar effect to that obtained with potassium oarbonate a- the 1 ~regn~t.

,, .
1 ' .
.
, 15 .

. .
....... . . ..

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An exaust 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 according 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.

5. A method of preparing a filter medium for use in a filter according to claim 1, which comprises forming on a sup-port member of extended surface area a coating layer of calcined alumina, impregnating said layer with an aqueous solution of an alkali metal carbonate or borate, and drying the impregnated layer to deposit therein said alkali metal carbonate or borate, the concentration of said alkali metal carbonate or borate in said aqueous solution being such as to deposit in said layer from 3 to 60% by weight of said alkali metal carbonate or borate, based on the weight of the calcined alumina.