Note: Descriptions are shown in the official language in which they were submitted.
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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.
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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,
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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
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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,
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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
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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
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: 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
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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.
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DETAILED DESCRIPTION
In the filters of this invention the amount of
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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%.
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As already indicated, the impregnated alumina will
¦ be supported on a substrate of extended surface area,
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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,
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~ 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
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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- -
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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.
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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
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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.
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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.
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The filters were constructed by stripping out the
sound absorbing material from inside the silencers o~
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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:
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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. '
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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.
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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.
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*** % of ingoing lead retained by filter.
Comparison of the above results shows that
whereas alumlna itself is a reasonably effective filter
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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
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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.
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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.
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