Note: Descriptions are shown in the official language in which they were submitted.
1~76~Z2
BIFUNCTIONAL FILTER FOR THE TREATMENT OF EX~AUST GASES
BACKGROIJND OF T~E INVENTION
The present invention relates to a
bifunctional ilter for the treatment of exhaust gases.
5 More especially the present invention relates to a
bifunctional filter for the purification of exhaust
gases by ~iltration of the solid particles and cataly-
tic treatment of the gaseous contaminants. The present
invention also provides an improved process for the
10 treatment of exhaust gases. In another aspect, the
present invention discloses a use of the bifunctional
~; filter in the treatment of exhaust gases from diesel
engines.
According to the present invention, particle
~; 15 filters are provided to reduce particle emissions in
combustion processes. The filters function to filter
the solid particles, consisting essentially of soot,
from the flow of exhaust gas. Simultaneously, a
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catalytic after-treament of gaseous contaminants, such
as for example, hydrocarbons, carbon monoxide, sulfur
compounds and nitrogen oxides, may be effected by means
of a catalyst applied to the filter.
S Thus, uncoated or catalytically-active
filters may be used in the flow of exhaust gases of
combustion processes, in particular in the flow of
exhaust gases from diesel engines, to reduce the
emission of particles and contaminants.
The ceramic filter systems without catalysts
used heretofore have the disadvantage that they are
limited to particle emission only. In the case of a
catalytic coating of the filters, initially sulfate
particles are additionally formed on the catalyst by
the reaction of the sulfur oxides contained in the
exhaust gas. These are precipitated onto the filter,
together with the solid particles which consist essen-
tially of soot. With the increasing precipitation of
solid particles, pressure and temperature in the filter
increase, until the ignition temperature of the soot
particles is reached and the filter is regenerated by
burning. It is not possible, however, to obtain
complete regeneration since the codeposited sulfate
particles cannot be removed by burning. Depending on
Z5 the sulfur content of the exhaust gases, a more or less
rapid clogging of the filter pores by the sulfate
particles takes place, until the filter becomes
useless.
An arrangement is known from European Patent
P~Jb/~c~2 f /CJ~ ,rO .
Application ~c. 0,020,766 whereby the exhaust gas of
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diesel engines is treated by filtration and a
subsequent catalytic stage. For this purpose, a
sponge-like catalyst in a helical form is placed into a
hollow cylinder comprised of a metal filter fabric.
S ~owever, such an arrangement has other aggravating
disadvantages. For example, to resenerate the filter,
which in this embodiment is no longer self-
-regenerating, the difference in pressure between the
inlet and outlet side of the filter must be constantly
monitored in order to determine the point in time when
regeneration is necessary, and in order to effect this
regeneration of the filter, hot air must be passed onto
the filter by means of a supplemental burner.
SUMMARY OF T~E INVENTION
It is therefore an object of the present
invention to eliminate the disadvantages of the state
of the art and, in particular, to provide a bifunc-
tional ceramic filter, wherein clogging by sulfate
particles may be avoided and regeneration occurs
automatically.
Another object of the present invention is to
provide an improved filter for removing both solid
particles and contaminants.
Yet another object of the present invention
is the provision of an improved bifunctional filter
which finds particular application in diesel engines.
In accomplishing the foregoing objects of the
present invention, there has been provided in
accordance with one aspect of the present invention a
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bifunctional filter for the treatment of an exhaust gas
comprising a plurality of inlet chambers wherein the
exhaust gas is introduced, a plurality of outlet
chambers from which the exhaust gas is released, and a
wall separating each of the inlet and outlet chambers
wherein the wall comprises a ceramic material permeable
to gas but impermeable to particles and wherein 'he
wall further comprises a coating of a catalytically-
active substance on the side of the wall adjacent to
each of the outlet chambers.
In a preferred embodiment of the invention,
each of the inlet chambers is connected to at least one
of the outlet chambers to form a system of unilaterally
closed pipes, and more preferably, these inlet and
outlet chambers are arranged adjacently, most
preferably parallel, to each other and are connected
by the wall comprising the gas-permeable material.
In a further preferred embodiment, the filter
further comprises a gas-impermeable closure means
attached both to the end of each of the outlet chambers
initially contacted by the exhaust gas ~upstream end)
and to the opposite end ~downstream end) of each of the
inlet chambers.
In accordance with another aspect of the
present invention, there is provided a process for
treating an exhaust gas comprising the steps of
providing an exhaust gas to an inlet chamber, passing
the exhaust gas from the inlet chamber through a wall
comprising a gas-permeable ceramic material to remove
solid particles therefrom, thus forming an essentially
particle-free exhaust gas, passing the essentially
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particle-free exhaust gas through a coating on the wall
comprising a catalytically active substance to remove
gas contaminants from the exhaust gas, and removing the
exhaust gas by means of an outlet chamber.
Finally, in accordance with yet another
aspect of the present invention, there is provided a
bifunctional filter for treating exhaust gas for use in
diesel engines.
An advantageous result of the above-discussed
aspects of the present invention is the provision for
high exhaust gas flows when the inlet and outlet
chambers are adjacent to each other.
Further objects, features and advantages of
the present invention will become apparent from the
detailed description of preferred embodiments which
follows, when considered in light of the attached
figureq of drawing.
BRIEF DESC~IPTION OF T~E DRAWINGS
In the drawings:
Figure 1 is an end view of the inlet side of
the filter according to the invention:
Figure 2 is a schematic longitudinal
sectional view through the filter according to the
invention: and
Figure 3 is an expanded partial view of the
portion "A~ of the longitudinal sectional view of
Figure 2.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention provides for the
purification of exhaust gases from a combustion
process by an initial filtration of solid particles and
a subsequent catalytic treatment of gas contaminants.
As seen in Figure 1 or 2 of the drawings, the
filter comprises a plurality of pipes 1 essentially
parallel to each other having an essentially square
cross-section. The partitions 2 between the pipes 1
consist, except for the outer peripheries, of casing
surfaces common to adjacent pipes and are made of a
ceramic material permeable to gas but not permeable to
particulate material contained in such exhaust gases.
Bodies of this general type are known in the area of
catalysis, particularly under the designation
monolithic catalyst bodies.
Each pipe 1 is closed at one end by a gas-
impermeable ceramic stopper 3, wherein, in the case of
the filter with square tubular cross-sections, adjacent
pipes are closed at their opposing ends. The end view
of the inlet (and/or outlet) side of the filter then
shows the checker-board pattern illustrated in Figure
1.
Instead of bodies having square cross-
sections, tubes with polygonal, e.g., hexagonal orround cross-sections are also within the scope of the
present invention. These adjacent tubular layers are
closed at their opposing ends in a manner similar to
the square cross-sections discussed above.
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117682Z
The sealing of the adjacent pipes 1 at their
opposing ends creates the inlet chambers 4 and the out-
let chambers 5, which communicate with each other through
the casing surfaces 2 in a manner permeable to gas but im-
permeable to particles. As indicated in Figure 3 the en-
larged section "III" of Figure 2, a catalytically active
layer 6 is applied to the casing surface of the tubes 1
facing the outlet chambers 5- The catalytic treatment may
be effected by means of all known catalysts applicable to
exhaust gas treatments- According to the invention, this
treatment takes place after the filtration stage- T~e
catalyst is applied as a coating to the side facing the
outlet chamber of the gas permeable partition. The con-
cept of an "application" of catalytic to the carrier is
intended to include all conventional technical processes,
such as for example, coating with the catalyst or with
catalyqt containing masses, impregnation of the surface
layers of the partition, etc.
The exhaust gas follows the path indicated by
the arrows in Figure 1 when flowing through the filter
and is freed initially of particles at the partition 2
and is treated in the catalytically active layer 6 after
flowing through the partitions.
Example. Bifunctional filter
A porous filter of the cellular ceramic honey-
comb type is used as a substrate for a bifunctional fil-
ter. Such porous filters are commercially available*) and
described in detail in SAE Technical Paper Series 810114,
1981 (Editor: Society of Automotive Engineers, Inc.).
Of these filters the type EX-47 is best suited.
Such a filter is dipped into an aqueous solution of plati-
niumtetraminehydroxide, the concentration of which had
*) manufacturer:Corning Glass Works
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been adjusted in such a way, that the finished bi-
functional filter contained 0,075 % by weight of
platinium. Prior to dipping the side of the substrate
which is to become the inlet side of the bifunctional
filter was sealed airtight by means of a cuff surround-
ing said inlet side. Then the substrate is dipped into
said solution- After dipping, the cuff is removed and
the filter is shaken to remove excess solution. The
bifunctional filter is dried at 120C and tempered at
10 800C.
Other dipping solutions with which good results
have been attained are disclosed in DE-PS 23 04 831.
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