Note: Descriptions are shown in the official language in which they were submitted.
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i-~rfi -I~ Ft A ~d S ~ A~ I I i:i PJ
Apparatus for the catalytic purification of flowincL
ases in articular exhaust ases of internal combustion
engines
The invention relates to an apparatus for the
catalytic purification of flowing gases, in particular
exhaust gases of internal combustion engines, having a
housing to be arranged in a gas flow path, which housing
contains a gas-permeable support in the form of a textile
sheet material made of heat-resistant fiber material onto
which a catalytically active material is applied.
The catalytic converters conventional for the
exhaust gas purification of motor vehicle engines, in
particular internal combustion engines are composed
throughout of ceramic monoliths in the form of extruded
cellular bodies which are coated on the surfaces swept by
the exhaust gas with a catalytically active material, in
particular platinum. These mechanically sensitive mono-
liths are accommodated in metal housings in which they
must be elastically supported, which is accompanied by
certain problems. In order to obtain a sufficiently large
surface area, before the application of the catalytically
active material they must be coated with a so-called
"washcoat'°, which, however, rapidly alters at tempera-
tures above 800°C. Because of this thermal sensitivity,
these catalytic converters cannot be arranged directly on
the engine. On the other hand, the arrangemeiat at a
relatively large distance from the engine, together with'
y the considerable mass of the monoliths, is accompanied by
i
the disadvantage that heating up the monoliths to the
1 _
n
- 2 -
operating temperature after a cold start requires a
relatively long time. During this heating-up period, the
catalytic converter is thus active only to a limited
extent. Since the exhaust gases essentially flow in
laminar flow through the cellular body of the gionolith,
the conversion rate is also restricted on principle.
Austrian Patent 61 419, furthermore, discloses
using filters for the purification of exhaust gases, in
particular of internal combustion engines, which filters
have fibers made of a heat-resistant polycrystalline
material having a crystallite size between 50 and
500 angstroms. Polycrystalline aluminum oxide or zir-
conium oxide are preferred for this fibrous material.
However, in principle, metal oxides having a temperature
stability up to 900°C can be used.
In order to decrease the temperature at which
soot particles separated out from the exhaust gas stream
are burnt, the fibers can be coated with a catalytic
material, in particular silver, bismuth, lead, uranium,
cobalt, etc. To remove undesirable gaseous components,
for example carbon monoxide or hydrocarbons, from the
exhaust gases it is also known, in the case of this
filter, to coat zirconium oxide fibers with finely
divided platinum.
The fibrous material is used in the filters in
the form of loose staple fibers, paper, woven fabric,
films, cardboard or felt, filter elements also being able
to be used in which paper (or corrugated cardboard), or
yarn, or felt produced from such fibrous material is
loosely wound up to form a filter body through which gas
CA 02132634 2002-11-15
23792-118
- 3 -
flows. Filter bodies made of loosely packed fibrous
material are problematic because the non-interlocked fibers
become free with time, whereas in the case of fibers
processed to form paper, felt, cloth or like linear textile
sheet materials, either their resistance to flow is too high
or there is the risk that the filters plug relatively
rapidly. Moreover, the surface of the incorporated fibers
is exposed to a very restricted extent, the exposed fiber
surface being further increasingly reduced in operation. At
any rate, catalytically active filters of this type have not
hitherto achieved economic importance.
The object of the invention, in contrast, is to
create an apparatus for the catalytic exhaust gas
purification ("catalytic converter") which can be used in
particular in the exhaust gas purification of internal
combustion engines and which features mechanical resistance
with high activity.
The invention provides apparatus for a catalytic
purification of flowing gas, comprising a housing adapted to
be arranged in a gas flow path, which housing contains a
gas-permeable support in the form of a knitted textile
fabric folded to form a stack of contacting superimposed
sheets in substantially parallel planes and made of heat-
resistant fibrous material on which a catalytically active
material is applied, wherein the planes of the superimposed
sheets are essentially parallel to the flow of gas such that
the flow of gas is entirely parallel to the planes of the
superimposed sheets wherein at least a substantial portion
of at least one surface of each sheet contacts at least a
substantial portion of a surface of an adjacent sheet.
CA 02132634 2002-11-15
23792-118
- 3a -
The fibrous material advantageously contains
ceramic microfibers, which are taken to mean fibers having a
diameter of 3 ~,m and more. In particular polycrystalline
mullite fibers have proved to be expedient, but other heat-
s resistant fibrous materials are also suitable. Thus the
fibrous material for certain application purposes can also
contain carbon fibers or can be composed entirely of these.
The surface area of the
213~f,3!~
- 4 -
fibers of the fibrous material is expediently between 0.2
and 0.4 m2/g and above, without this range representing a
restriction of the fibers usable in principle.
A knitted fabric whose fibers form the support
for the catalytically active material offers, as has been
shown, a high mass transfer to the fiber surfaces with a
simultaneous outstanding hydraulic configuration. It has
bulk elasticity and is insensitive to vibrations and
pulsations of the gas flow. The fibers coated with the
catalytic material are dimensionally pretensioned and
firmly incorporated in the knitted fabric. Nevertheless,
their surface is substantially open; they can move with
respect to each other to a limited extent, so that
stresses are reduced and damped. The extensively exposed
fiber surface ensures a maximum catalytic activity.
The structure of the knitted fabric forms a
versatile pore system in which resistance elements are
contained, so that even in the case of laminar through-
flow, extensive wall contact and thus high mass transfer
results in all flow regions, as is essential for good
catalytic activity. Within the knitted fabric itself, the
flow path is branched several times with the result that
the individual fibers supporting the catalytically active
material are impinged by the flow over a large surface
area.
Because of its special structure, the knitted
fabric additionally ensures outstanding mixing of an
exhaust gas stream linear with respect to concentzation
or temperature. From this there results a particular
advantage in the case of multiple cylinder internal
z~ ~~s3~
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combustion engines and the so-called lambda 1 technology.
Finally, such a knitted fabric has good noise insulation
properties, it acting, moreover, as a filter for particu-
late pollutants, for example soot particles.
The good bonding of the fibers in the knitted
fabric already mentioned has the effect that fiber
fragments possibly occurring remain substantially
anchored in the knitted fabric, while, on the other hand,
progression of damage from one damaged site which has
occurred virtually does not take place. Short fiber yarns
can therefore also be used.
The knitted fabric can be made in at least a
double-threaded manner from various fibrous materials of
which the fibers of the one and/or of the other fibrous
material can support the same or different catalytically
active material. At least one metallic thread can also be
worked into the knitted fabric, which metallic thread
gives the knitted fabric, for example, an additional
dimensional stability. It is also conceivable to work
carbon fibers having an enlarged surface area (activated
carbon fibers) into the knitted fabric, in order by this
means to create the opportunity, for example during the
cold start phase, of temporarily adsorbing unburnt hydro-
carbons occurring and of desorbing again the substances
adsorbed in this way in a subsFquent period (peak
emission).
For this purpose, the knitted fabric can contain
electrically conducting fibers which are electrically
conductively connected to connection means for an
electric power source and which thus permit direct
~~3~~34
internal heating of the knitted fabric. However, this
internal heating can be used not only for the desorption
of adsorbed substances, but in particular, also, during
the cold-start phase, to bring the knitted fabric con-
taming the catalytically active material rapidly to the
operating temperature. The electrically conducting fibers
can be carbon fibers or metal fibers in the form of metal
wires etc.
The carbon fibers, on their surface, can also
bear an electrically insulating coating, for example
Si02. For this purpose, SiC, which is converted by the
action of heat into SiOz, is applied to the fibers. Such
an SiOz coating simultaneously forms a protection against
oxidation.
The fibrous material is worked into the knitted
fabric generally in the form of yarns made from staple
fiber or continuous filaments. Yarn hanks from continuous
filaments, in the knitted fabric, essentially have only
one beginning and one end and thus offer a low surface
area for degradation. Staple fibers, in contrast, with
their fiber ends projecting from the yarn hanks give a
somewhat larger surface area and improved filter action.
Which of the two types of fiber - which if appropriate
can also be used in a mixture - is preferred in the
individual case depends on the conditions of use. It is
also conceivable in principle that the knitted fabric
contains fibrous material. in the form of rovings.
In each case, fibers having a rough surface are
to be preferred to smooth fibers. By influencing the
surface topography of the fibers, the surface, as has
-~213~634
been shown, can be increased by approximately the factor
20.
The yarn material used for the knitted fabric
should be twisted as little as possible, for precisely
which reason rovings are also suitable. In practice, yarn
having up to one twist per inch of length has proved to
be advantageous. Texturized loosened yarn is frequently
to be preferred.
When the flow passes through the knitted fabric,
the flow should find as far as possible no resistance-
free routes along the "coarse pores" due to the mesh
structure. This can be avoided by the fact that the
knitted fabric is used in such a way that the gas flows
through it essentially in the plane of the knitted
fabric. Another method comprises compacting the knitted
fabric in order to close the "coarse pores". However,
this is generally connected with an increase of the
resulting flow resistance and thus of the pressure drop
occurring at the knitted fabric.
In a knitted fabric, the loop bows still remain
curved even with high local stretching, that is the
fibers, in contrast to the conditions in woven fabrics or
in a coil, are never completely extended. This has the
consequence that the pretensioning of the fibers remains
restricted, so that the fibers remain loose with respect
to each other in the yarn hank and offer large open
active surfaces. Tn order to alter this pretensioning, it
can be expedient if the knitted fabric is held under a
predetermined tension in the direction of the wales
and/or of the courses. In,the housing passing the gas
~~J~~j~~
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stream to be purified through the catalytically active
knitted fabric, the knitted fabric can be arranged, using
differing fabrication techniques, to fill the catalytic
converter volume through which flow is to pass. Thus it
is possible that the knitted fabric is, in part, rolled
up or folded; it can also, at least in part, be pleated
or doubled. It has proved to be advantageous if the
knitted fabric has been produced by circular knitting.
The knitted tube thus formed can then be rolled up in
itself or as mentioned pleated, folded or arranged in
another manner appropriate to the purpose. It is also
conceivable in principle to twist this tube together, for
example to form a "rope" and to wind it up in a spiral
shape in this form on a spindle - possibly in several
layers ..
Since, when appropriate fibrous material is used,
for example ceramic fibers which are coated with the
catalytically active material, the knitted fabric is
resistant to high temperature, the catalytic converter
produced from this knitted fabric can also be arranged
immediately downstream of the outlet valves of an inter-
nal combustion engine. The knitted fabric ensures anchor-
ing of the fibers in the loop interlocking such that it
can withstand the exhaust gas stream pulses which are
still strongly expressed in this region, at the same time
the pressure drop caused by the knitted fabric being so
small that it does not significantly impair the function
of the internal combustion engine. The knitted fabric, in
an appropriate fabrication, can if required also be
arranged directly in the exhaust manifold of the internal
~~3~63~
_ g _
combustion engine in such a way that this - or a connect-
ing part of the exhaust pipe - if necessary forms the
housing for the catalytic converter. An arrangement of
this catalytic converter upstream of an exhaust gas
turbocharger or pressure-wave charger possibly present is
conceivable.
Finally, in principle, the catalytically active
material can be applied to the knitted fabric or to the
yarn prior to knitting. In certain cases, application to
the individual filaments before their processing to form
a yarn is also conceivable.
In the drawing, exemplary embodiments of the
subject-matter of the invention are represented. In the
figures:
Fig. 1 shows an. apparatus according to the invention
in the form of a catalytic converter intended
for arrangement in the exhaust pipe of an
internal combustion engine in axial section, in
a side view and in diagrammatic representation,
Fig. 2 shows an apparatus resembling Fig. 1 in a
modified embodiment and in a corresponding
representation,
Fig. 3 shows a knitted fabric of the catalytic
converter according to Fig. 1 or 2, which
knitted fabric is coated with a catalytically
active material, in detail. in perspective
representation showing various inflow direc-
tions,
Fig. 4 shows the knitted fabric according to Fig. 1 in
plan view in detail showing various extension
z~~~634
- 10 -
states,
Fig. 5 shows a slightly twisted yarn of the knitted
fabric according to Fig. 3 in detail in
diagrammatic representation and to a different
scale,
Fig. 6 shows a highly twisted yarn in comparison
to
Fig. 5 in detail and in a corresponding
repre-
sentation,
Fig. 7 shows a diagrammatic drawing to show the
inflow
conditions of a fiber of the knitted fabric
according to Fig. 3,
Fig. 8 shows a diagrammatic representation of
the
fundamental inflow conditions in the knitted
fabric according to Fig. 3 in a sectional
draw-
ing.
Fig. 9 shows the knitted fabric according to
Fig. 3 in
rolled form in diagrammatic perspective
repre-
sentation, showing the influx conditions
of the
coil thus formed,
Fig. 10 shows the knitted fabric according to
Fig. 3 in
pleated form in diagrammatic representation,
showing the influx conditions,
Fig. 11 shows the knitted fabric according to
Fig. 3 in
folded form, showing the influx conditions,
Fig. 12 shows the knitted fabric according to
Fig. 3 in
the form of a circular-knitted tubular
fabric
which is wound up in part. to form a ring
(torus), in diagrammatic perspective repr.esen-
tation,
Fig. 13 shows an arrangement of a plurality of
adjacent
~~~~~J~
- 11 -
rings according to Fig. 12, showing the influx
conditions in a diagrammatic sectional repre-
sentation,
Fig. 14 shows the knitted fabric according to Fig. 3 in
the form of a circular-knitted and then in part
pleated tubular fabric in perspective diagram-
matic representation, in part in section,
Fig. 15 shows the pleated circular-knitted product
according to Fig. 14 in section in a side view
showing the influx conditions and in diagram-
matic representation,
Fig. 16 shows the knitted fabric according to Fig. 3 in
the form of an at least once-folded circular-
knitted tubular fabric in perspective diagram-
matic representation,
Fig. 17 shows the folded circular-knitted fabric
according to Fig. 16 wound up helically and in
axial section in a diagrammatic side view
showing the influx conditions,
Figs. 18 show an apparatus according to the invention
to 21 in the form of a catalytic converter which is
accommodated directly in the exhaust manifold
of an internal combustion engine and which is
shown in various embodiments and diagrammatic
sectional representations
Fig. 22 shows the knitted fabric according to Fig. 3 in
a modified embodiment in detail and in diagram-
matic representation,
Fig. 23 shows a thread system of the knitted fabric
according to Fig. 22 showing the electrical
~1 ~'63~
- 12 -
connection conditions,
Figs. 24 show the catalytic converter according to
and 25 Fig. 1 in two modified embodiments using the
knitted fabric according to Figs. 22/23, each
in axial section, in a side view and in
diagrammatic representation.
In Figs. 1, 2, two conventional basic shapes of
catalysts for internal combustion engines are represented
diagrammatically. They each have a sheet metal housing 1
which is preferably cylindrical or oval in cross section,
which sheet metal housing 1 is connected at both ends via
an inlet or outlet funnel 2 or 3 to an inlet-side and an
outlet-side pipe connector piece 4 and 5 respectively
which permits the housing 1 to be arranged in the exhaust
pipe of an internal combustion engine, which is not
depicted in more detail, in such a manner that in opera-
tion the exhaust gages to be purified flow evenly through
the housing interior in the axial direction indicated in
6.
In the simple embodiment according to Fig. 1, a
gas-permeable catalyst body 8 is arranged in the housing
between two perforated plates 7, which catalyst body 8
completely fills the interior of the housing 1, In the
so-called candle design according to Fig. 2, in contrast,
a plurality of axially parallel cylindrical catalyst
units 10 are accommodated in the housing 1 attached to an
appropriately designed perforated plate 9, of which cata-
lyst units each essentially comprises a candle-shaped
housing 11 which is closed on a bottom side at 1?. and is
manufactured from perforated sheet metal 13 at the
~I3~63~
- 13 -
periphery. Each of the candle-shaped housings 11 is lined
internally with a gas-permeable cylindrical catalyst body
8a through which, in operation, exhaust gases flow
essentially radially from the interior to the exterior.
The catalyst body 8 or 8a comprises a gas-
permeable support in the form of a knitted fabric which
contains catalytically active fibrous material and is
fabricated in correspondence with the shape of the
catalyst body 8, 8a.
The knitted fabric of which a detail is represen-
ted at 14 in the Figs. 3, 4 can, as shown, be a plain
jersey knitted fabric, but other single- and multi-layer
knitting stitches are also conceivable depending on the
respective application. These knitted fabrics can also be
worked with stitch patterns, e.g. with tuck loops; they
can also have different stitching in different sections.
As can be seen by comparison of the two diagrams a and b
of Fig. 4, which show the same knitted fabric 14 once
unstretched (a) and once stretched in the direction of
the wales (b), the loop bows are still curved even in the
case of high local stretching. In contrast to the
conditions in a woven fabric or a wick-like fiber coil,
the fibers are never completely stretched. As long as the
knitted fabric is not overextended - which is to be
excluded by suitable fabrication - the fibers remain
movable with respect to each other in the yarn hank, so
that their catalytically active surface remains essen-
tially completely accessible for impingement by the
exhaust gas to be purified flowing through the catalyst
body 8, .8a.
- 14 -
The knitted fabric 14 is made from a yarn 15
which, depending on its structure, can be composed of
staple fibers (Fig. 5) or of continuous filaments or of
a mixture of both fiber types. In principle, the yarn 15
can be single- or multiple-threaded, in individual cases
so-called core-spun yarns also being suitable.
Tn general, the yarn 15 is to be twisted as
little as possible in order to expose as large as pos-
sible a fiber surface area. This can be taken without
difficulty from Figs. 5, 6, of which Fig. 5 shows a
slightly twisted yarn 15 and Fig. 6 shows a highly
twisted yarn 15. The slightly twisted yarn 15 according
to Fig. 5 has a structure which permits the exhaust gas
to flow through the yarn itself; the greater the yarn is
twisted; the higher is also the resistance to throughflow
for the exhaust gas.
Yarns having approximately 0.5 twists per inch of
length or even rovings have produced good results in
practice. Texturized loosened yarn is also advantageous.
The fibrous material from which the yarn 15 is
processed must have a heat resistance which permits it to
withstand, for long operating periods, the operating
temperatures occurring.
The relevant temperature range, according to
experience, is between approximately 200° and 800°C. With
respect to the largest possible surface area, microfibers
are advantageous, which is taken to mean fibers having a
fiber diameter of about 3 ~,m and more.
The material of which the fibers are composed is
generally a ceramic material but, depending on the
2t3~63~
- 15 -
temperature range for which the catalytic converter is
intended, other organic and inorganic materials are also
useful in principle. All metal oxides are highly suit-
able. Pure carbon fibers have also already been used
which offer a great number of advantages, for example
electrical conductivity. The surface area of the fibers
can be increased by appropriate measures. Thus, e.g.
fibers having a rough surface are generally to be pre-
ferred to smooth fibers. By influencing the surface
topography, the surface area can be increased according
to experience by about the factor 20. In the case of
carbon fibers, so-called activated carbon fibers can be
processed which have a significantly increased surface
area and can thus also be simultaneously used as an
absorbent, for example during the cold-start phase.
In general it must be stated that different yarns
or threads, which can also be composed of different
materials, can also be worked into the fabric 14 so that
to them various functions [lacuna] made single-threaded
and multi-threaded and contain, for example, at least one
metallic thread which, inter alia, gives the knitted
fabric 14 and thus the catalyst body 8, 8a a certain
dimensional stability which can be of importance, depend-
w ing on the chosen fabrication of the catalyst body 8, 8a
and the operating conditions.
The construction of the catalyst body 8, 8a from
the knitted fabric 14 can take place in various ways as
will be described in detail with reference to some
examples. In principle, however, it must be established
that, the flow through the knitted fabric 14 must take
~~eJ~~e.,.~~
- 16 -
place in such a way that the gas flow does not find any
resistance-free routes along the coarse pores or "loop
caves" present in principle in a knitted fabric because
of the loop structure. In particular, when the exhaust
gas to be purified in the catalyst body 8, 8a is not
forced to flow through a plurality of knitted layers
lying one above the other and, as a result, to follow
branched flow channels, care must generally be taken that
the flow passes through the knitted fabric 14 roughly in
the plane of the knitted fabric as is indicated in Fig. 3
by the arrows 16, 17. Throughflow transversely to the
knitted fabric plane in accordance with the arrow 18 is,
as mentioned, at least in the case of single-layered
arrangement of the knitted fabric 14, less expedient as
a simple glance at Fig. 4 shows in which the "coarse
pores", situated between the individual loop sides and
loop bows, of a knitted fabric 14 (made from a smooth
thread) are shown in principle.
In the case of throughflow in the knitted fabric
plane, in principle the flow conditions represented in
Fig. 8 are produced in which it is ensured that the
exhaust gas stream 19 on its flow path repeatedly meets
flow resistances in the form of individual fibers 20
round which flow passes over the largest possible area in
the manner to be seen in Fig. 7.
Fabrication methods for the knitted fabric 14
which are suitable for producing the catalyst body 8, 8a
supplying the catalyst volume through which flow is to
pass are~shown by way of example in Figs. 9 to 17. The
exemplary embodiments shown are not exhaustive; they can
~'I3~'~;3r~
- 17 -
also be combined together or supplemented by other
fabrication methods.
In the embodiment according to Fig. 9, the
knitted fabric 14 is rolled up to form a simple coil 21
which, for example, simply forms the catalyst body 8 of
the catalytic converter according to Fig. 1. For the
reasons already explained, it is expedient to take care
in use that the flow passes through the coil 21 in the
axial direction as indicated by an arrow 22. Transverse
throughflow (perpendicular to the axis) is possible but
less advisable.
In the embodiment according to Fig. 10, the
knitted fabric 14 is pleated, i.e. folded up to form a
stack 23 of layers lying one above the other of the
knitted fabric folded in a concertina-like manner. The
best throughflow direction is again indicated by two
arrows 22; it proceeds in the knitted fabric plane of the
individual layers. A transverse flow, as given by an
arrow 24, is likewise possible in principle but less
advantageous.
Fig. 11 shows ~an exemplary embodiment in which
the knitted fabric 14 is folded. The folding here can be
single- or multi-layered; it can proceed in the direction
of the Wales or of the courses. Folds are also possible
in which at least one fold is made in the direction of
the Wales or courses and at least one subsequent fold is
performed transversely thereto. The preferred throughflow
direction in the knitted fabric plane is given by the
arrow 22 for the fold represented; the statement made for
the exemplary embodiment according to Fig. 10 applies to
,.
~.I~'~s3~
- 18 -
a throughflow in the transverse direction thereto in
correspondence with the arrow 24.
The embodiments according to Figs. 12 to 16 start
from a knitted fabric 14 which was produced in the form
of a circular-knitted tubular fabric 140. Such a tubular
fabric permits hollow shapes to be produced simply which
are expedient for many applications. Very simple condi-
tions result ii this tubular fabric - preferably multi-
layered - is pulled in the manner of a sock onto an
appropriate candle-shaped gas-permeable housing 11
according to Fig. 2 or is inserted as a hollow cylinder
into a said housing 11.
The embodiment according to Fig. 12 offers a
larger fiber surface area active for the exhaust gases
flowing through, in which embodiment the tubular fabric
140 is wound up to form a ring (torus) 25, a cylindrical
hollow body indicated at 26 in Fig. 13 then being filled
with such wound up rings or tori 25 in order to form the
coiled body 8a. The gas throughflow direction is indica-
ted by arrows 27 ; it runs transversely to the plane of
the rings or tori 25.
Figs. 14, 15 represent an embodiment in which the
tubular fabric 140 is pleated in the depicted manner at
28. A cylindrical configuration results, the walls of
which are composed of the layers lying one above the
other of the knitted fabric 14 folded in a concertina'
like manner. This hollow cylindrical configuration can in
turn be inserted into the hollow body 26 or placed on an
appropriate candle-shaped housing 11 (Fig. 2) or inserted
into a said housing~ll. An advantage of this embodiment
- 19 -
is that in the radial throughflow direction indicated by
the arrows 27, the flow passes essentially in the plane
of the knitted fabric 14 through the layers of the
pleated folds 28.
In the embodiment according to Figs. 16, 17,
finally, the tubular fabric 140 is folded once or several
times about. an axis extending in the longitudinal direc-
tion of the tube. The folded configuration 29 can then be
wound up helically to form a filter candle which can
either be accommodated in the hollow body 26 in the
manner seen in Fig. 17 or be pulled onto a candle-shaped
housing 11 according to Fig. 2 or arranged in the said
housing 11. In this case also, the flow passes through
the layers of the folded knitted fabric 14 essentially in
the plane of the knitted fabric 14, as given by the
arrows 27.
In principle, in all these fabrication methods,
catalyst bodies 8, Sa of a circular cylindrical hollow
body, noncircular cylindrical hollow body or hollow body
of any desired shape can be produced. The, in particular,
candle-shaped catalytic converter thus formed, as rep-
resented in Fig. 2, can be used either connected in
parallel, but they can also be used connected one after
the other or stacked one inside the other. Tn this manner
a great increase of the active throughflow surface area
is possible, at the same time low pressure drops being
able to be achieved.
Whereas, in the embodiments of catalytic con-
verters as are shown in Figs. 1, 2, the catalyst body 8,
8a is accommodated in its own sheet metal housing 1,~ the
~.~ ~~1~3~
- 20 -
characteristic of the knitted fabric forming this cata-
lyst body also permits such a housing to be dispensed
with for certain applications. Examples of this are shown
in Figs. 18 to 21 which show exemplary embodiments in
which the catalytically active knitted fabric is
accommodated directly in the exhaust manifold, indicated
diagrammatically at 30, of an internal combustion engine
31.
In the embodiment according to Fig. 18, the
knitted fabric 14 is rolled up to form a coil 21 in
accordance with Fig. 14, which coil sits directly in the
exhaust manifold 30 which thus forms the "housing" for
the catalytic converter.
It is also conceivable in principle to arrange
this coil 21 at another position of the exhaust pipe in
such a way that the tubular wall thereof is utilized as
catalytic converter housing. Arrows 32 indicate the gas
throughflow direction.
The embodiment according to Figs. 19, 20 is dif-
ferentiated from that according to Fig. 18 by the fact
that a gas-permeable part-housing 33, essentially rectan-
gular in cross section and extending over the length of
the exhaust manifold 30 is introduced into the exhaust
manifold 30, which part-housing contains the knitted
fabric 14 which is pleated or folded, for example, in
accordance with Figs. 10, 11, roughly in the shape of a
stack 23. The gas throughflow direction is in turn
indicated by arrows 32.
Finally, in the embodiment according to Fig. 21,
a gas-permeable part-housing 34 extending over the length
~~~~G3~
- 21 -
of the exhaust manifold 30 is inserted into the exhaust
manifold 30, which part-housing bears a cylindrical
knitted fabric casing 35 which is, if required, pleated
in accordance with Figs. 14, 15 or wound up in accordance
with Figs. 16, 17 in such a way that, overall, the
fundamental construction results in a filter candle which
lies in the exhaust manifold 30 and through the wall of
which filter candle the flow essentially passes radially,
as indicated by the arrows 32.
The catalytically active material preferably used
is platinum if the exhaust gases of internal combustion
engines are to be purified of unburnt hydrocarbons and of
carbon monoxide. For other applications in which the
conversion of other pollutants contained in the gas
stream to be purified is to the fore, or in which the
object given is to decrease the combustion temperature,
for example for soot particles, other catalytically
w active materials can be used which sit on the fiber
surface of the knitted fabric 14 and past which the gas
stream to be purified flows. Examples are Pt, Pd, V, Rh,
etc.
When platinum is used as catalytically active
material, the platinum coating of the fibers can take
place in various ways. The coating can be performed from
the gas phase (chemical vapor deposita_on = CVD). Another
process is wet impregnation. This pracess comprises the
knitted fabric 14 being impregnated with a dilute solu-
tion of a platinum salt, whereupon the impregnated
knitted fabric is dried and then the salt is thermally
decomposed to platinum.
_. ~~J~~~f~
Depending on the reaction conditions in the
thermal decomposition of the salt, three modifications of
platinum can be formed: platinum black (platinum contain-
ing physisorbed oxygen), platinum sponge (noncrystalline
high-surface area platinum without sorbed oxygen) and
platinum metal (crystallized low-surface area platinum).
Exemplary embodiment 1:
A knitted fabric 14 corresponding to Fig. 3 is
produced from a fiber yarn of low twist (0.5 twists per
inch of length) composed of a polycrystalline mullite
fiber having a surface area of 0.2 to 4 m2/g (second
value with leaching). The fiber diameter is approximately
~m and is thus outside the region hazardous to health;
however, fibers having a fiber diameter of 5 ~,m and cor-
respondingly greater surface area can also be used.
The knitted fabric 14 was then impregnated in a
dilute solution of a platinum salt (wet impregnation),
whereupon the salt was then decomposed (calcined) at
temperatures around 800°C to give platinum sponge. The
coating rate was approximately 1 percent by weight;
however, it can be decreased to approximately 1 part per
thousand by weight.
The knitted fabric 14 was then wound up to a coil
21 corresponding to Fig. 9 and flow was passed through
axially. For this purpose, the coil 21 was inserted into
a stainless steel tube 1 in accordance with Fig. 1.
Exemplary embodiment. 2:
A knitted fabric 14 was produced from carbon
microfibers, which knitted fabric has a density of
approximately 0.5 to 1.0 g/cm'. The fibers are
243634
- z3 -
monofilaments having a fiber diameter of 6.5 Vim; 3000
monofilaments are present per hank. The flow velocity
based on standard conditions was approximately 85 mm/sec.
Tests were carried out with CO-enriched air.
Even at approximately 235°C, complete conversion
of CO into C02 resulted at a space velocity of 10,000 h''
(space velocity: throughput m' of gash per m' of catalyst
volume).
Instead of a glatinum catalyst, a rhodium cata-
lyst or a Pt/Rh catalyst could also be used.
Exemplary embodiment 3
Carbon fibers, as in exemplary embodiment 2, were
first CVD-coated with SiC. A catalytic coating was then
applied to the fibers by wet impregnation with a Pt/Pd
mixture and subsequent calcination at approximately
800°C.
A coil 21 in accordance with exemplary embodiment
1 was produced from the knitted fabric, through which
coil propane was passed axially.
At a space velocity of about 10,000 h'1, the
catalytic conversion of the propane began already at a
starting temperature of approximately 210°C; at approxi-
mately 330°C, a 100 conversion of the propane was
observed.
For example, the use of carbon fibers as a
support for the catalytically active material permits
internal heating of the catalyst body 8, 8a to be
directly performed, because the carbon fibers are elec-
trically conducting. Another possibility for such an
internal heating is that 'the knitted fabric 14 is made
~13~~ ;~
- 24 -
double-threaded, in addition to the yarn 15 which is, for
example, composed of ceramic fibers and coated with
catalytically active material, an electrically conductive
yarn 36 is used for knitting which is plaited onto the
yarn 15. An example of this is represented in Fig. 22,
for which it should be noted that, as already explained
at the outset, for purposes for which an internal heating
of the knitted fabric 14 is not required, the second yarn
36 can also be composed of an electrically insulating
material which is selected so that it gives the knitted
fabric the additional properties required for the parti-
cular application.
It is also possible to incorporate inlaid threads
37 into the knitted fabric, which inlaid threads, when
internal heating is desired, are composed of an electri-
cally conductive material, but are otherwise produced
from a material which was selected with respect to
certain properties of the knitted fabric to be achieved.
Such properties are, for example, increased dimensional
stability, improved. strength or certain elongation
properties of the knitted fabric which are different in
the direction of the courses from those in the direction
of the wales.
The electrically conducting fibers can be
connected in the manner visible in Fig. 23 with a power
source not shown there in more detail. For this purpose,
the yarn elements 36 of the knitted fabric 14 which. are
composed of these fibers are soldered or welded onto two
contact strips 39, 40 to be electrically conducting,
which in the case of carbon fibers, for example, can take
25 -
place with the aid of an appropriate prior coating of the
fibers. The contact strips 39, 40 essentially extend in
the direction of the wales, i.e. transversely to the
knitting direction. In this manner, definite resistance
conditions result and a uniform current flow through the
yarn lengths forming the individual courses results.
The same arrangement in principle obviously also
results when a double-threaded knitted fabric 14 accor-
ding to Fig. 22 is used; Fig. 23 then only shows the yarn
system composed of the yarn 36.
In order to avoid short circuits between the
individual layers of the internally electrically heated
knitted fabric 14 in the case of a multi-layer catalyst
body composed of such a knitted fabric 14 containing
electrically conducting fibers, some simple measures can
\_ be taken of which two examples are illustrated in Figs.
24, 25:
In the embodiment according to Fig. 24 which
shows a catalytic converter similar to Fig. 1, the
catalyst body 8 is composed of a pleated knitted fabric
14, for example corresponding to Fig. 10 or 14. Between
the individual layers 41, which lie one on top of the
other, of the folded knitted fabric 14 are inserted
insulating intermediate layers 42 which, for example, are
composed of mica and project from bath ends into the
catalyst body 8.
In the other embodiment according to Fig. 25, the
arrangement is of a type such that, during the folding of
the knitted fabric 14, an insulating second knitted
fabric 45, for example made of ceramic fiber or glass
w ~~~zs3~
- 26 -
fiber material, was enclosed in such a way that the two
knitted fabrics 14 are folded together in the manner
shown.
The electrical internal heating of the catalyst
body 8, 8a permits the catalytic converter to be brought
rapidly to operating temperature or, as already mentioned
at the outset, permits additional desorption processes to
proceed, for which purpose, in particular, a double-
threaded knitted fabric can also be used in which a
thread system can be composed, for example, of activated
carbon fibers which are distinguished by a particularly
high surface area and thus a high adsorption capacity.
