Note: Descriptions are shown in the official language in which they were submitted.
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PLATE CATALYST
The invention relates to a plate-type catalytic
converter, which includes a plurality of plates disposed in
a holder device and coated with a catalytically active
composition.
Such plate-type catalytic converters are used
among other purposes for reducing nitrogen oxides contained
in a gas mixture. The nitrogen oxides, together with a
reducing agent, usually ammonia NH3, introduced into the gas
mixture beforehand, are converted into water and nitrogen by
contact with the catalytically active composition by the
process of selective catalytic reduction (SCR). The
catalytically active composition with which the plates are
coated on both sides usually includes titanium dioxide and
one or more of the following additives: tungsten trioxide,
molybdenum trioxide and vanadium pentoxide. Examples of
catalysts of this kind can be found for instance in German
Patent 24 58 888.
A plate-type catalytic converter usually includes
a so-called element case as a holder device for the plates
coated with the catalytically active composition. The
catalyst plates are inserted into the element case in such a
way as to be spaced apart uniformly and oriented parallel to
one another. The element case is usually in the form of a
parallelepiped, which is open on the end surfaces, which are
the leading and trailing ends for a flow medium, such as the
aforementioned gas mixture. The planes of the catalyst
plates are oriented at right angles to the planes of the end
surfaces. The main flow direction for the gas mixture is
oriented parallel to the edges of the element case that join
opposite end surfaces together.
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A plurality of these element cases equipped with
catalytically active plates axe combined in a module, and a
plurality of modules form one level of plate-type catalytic
converters. For instance, a so-called deNOX system for
reducing the nitrogen oxides in the flue gas from a
combustion system typically has from 3 to 5 levels of such
plate-type catalytic converters.
For spacing the plates apart in an element case,
the plates typically have impressed beads, which extend
parallel to the main flow direction and divide the space
between two immediately adjacent plates into a plurality of
subchambers. These subchambers experience a substantially
laminar flow of the flow medium, so that as the flow
distance through the plate-type catalytic converter
increases, an increasingly less advantageous flow profile
for the catalytic conversion, for instance of the nitrogen
oxides with ammonia, develops in the subchambers, along with
a gas distribution that is becoming less homogeneous,
because of inadequate mixing of the components of the gas
mixture.
To improve this turbulence, static mixers are
already known that are disposed upstream of a plate-type
catalytic converter in terms of the flow direction of the
gas mixture and which disadvantageously take up a not
inconsiderable length of the region of the wake of the
mixture for making the components of the flow medium
turbulent (compare German Published Patent Application
DE-OS 41 23 161). Catalysts are also known that have a
cross channel structure of the gas channels (subchambers) to
attain high turbulence. These catalysts, however, cause a
relatively major pressure drop in a line for the flow
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medium, and in the case of gas mixtures heavily laden with
dust and particles, they stop up relatively quickly.
Catalyst plates with openings at each of which
there are two tabs bent in opposite directions away from the
applicable catalyst plate are also known from German Utility
Model G 89 O1 773Ø The tabs are shaped and dimensioned
such that they act as spacers from adjacent catalyst plates.
The bending edges at which these tabs are bent away from the
applicable catalyst are oriented parallel to the main flow
direction, so as to hinder the flow as little as possible.
Especially for catalysts in motor vehicle exhaust
systems, embodiments are also known in which catalyst
layers, that are oriented parallel to the main flow
direction of the exhaust gas and which by way of example
include coated metal plates or bands that can also be rolled
up into concentric tubes or spirals, are provided for
mechanical reasons with spacer elements that lead to
unavoidable turbulence in the exhaust gas flow. However,
such turbulence fades relatively quickly and only causes a
local disturbance to the flow, which otherwise has a laminar
course.
It is therefore the object of the invention to
disclose a plate-type catalytic converter in which the
catalytically active surface is utilized especially
uniformly and intensively for catalytic conversion, for
example of nitrogen oxides contained in a flue gas. It is
desirable for the pressure drop to be as slight as possible
and for plugging of the catalyst to be avoided as much as
possible.
In accordance with this invention, there is
provided a plate-type catalytic converter, comprising: a
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holder device; plates being coated with a catalytically
active composition, being held one above the other in said
holder device and extending along a main flow direction;
said plates including first and second adjacent plates
forming at least one reaction chamber; and said first plate
having a corrugated first structure formed of wave troughs
and wave crests oriented obliquely relative to said main
flow direction for deflecting a flow medium flowing along
said first structure from said main flow direction; and said
second plate having a second structure formed of wave
troughs and wave crests oriented parallel to said main flow
direction.
Each two of these plates which are immediately
adjacent one another, together with the lateral boundary of
the holder device, thus define a reaction chamber, which on
the leading and trailing end is bounded by the holder device
that allows the passage of the flow medium.
The wave crest pointing convexly out of the
reaction chamber of one such corrugated structure
accordingly creates a widening of the reaction chamber, the
widening is oriented obliquely to the main flow direction,
and the wave trough following this crest forms a narrowing
of the reaction chamber, so that a constriction and
expansion alternate constantly along the way from the inflow
end to the outflow end. At the constrictions, the exhaust
gas flow backs up and some of it is deflected into the
crosswise expansions of the reaction chamber. As a result,
a constantly repeated disruption of the exhaust gas flowing
along the surface of the corrugated plate is attained, and
the result is that it is virtually impossible for a laminar
flow aimed directly from the inflow side to the outflow side
to develop at this surface.
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Instead, eddies develop at the constrictions and
mix the flow medium within each reaction chamber in the wake
of the constriction. If the plate parts at which the
reaction chamber is narrowed and therefore at which the flow
medium is partly deflected out of the main flow, also have
apertures that lead to the next reaction chamber in
succession, then a partial deflection of the flow medium
into adjacent reaction chambers is also attained along with
an additional turbulence in the components of the flow
medium and a mass transfer between adjacent reaction
chambers that encompasses the entire catalyst volume.
In this way, the components contained in the flow
medium, such as nitrogen oxides and ammonia, are especially
well mixed together, and especially because of their
turbulence are often carried along the surface of the
catalyst plates. Because the flow medium is always only
partially deflected from the main flow direction by these
means, the result is a relatively slight pressure drop.
Moreover, only very limited regions result in which dust
and/or particles contained in the flow medium can be
deposited. Hence the danger of stoppage of the catalyst
remains quite low.
What is attained by the structure according to the
invention is that as a result of its deflection, the flow
medium in a reaction chamber is thoroughly mixed both
locally and macroscopically over the entire distance of the
reaction chamber, and as a result, it is also attained that
the components of the flow medium are carried especially
frequently along the surface of the plates coated with the
catalytically active composition. In this way, the
likelihood of adsorption for the undesired components of the
flow medium, such as nitrogen oxides in ammonia and a flue
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gas, is especially high. Especially for the catalytic
conversion of nitrogen oxides with ammonia, the adsorption
of the nitrogen oxides and ammonia is especially
advantageous for the catalytic reaction thereof at the
active centers of the catalytic composition. In this way,
the Sherwood number which is a standard for a flow-induced
contribution to the catalytic activity of the catalyst,
remains at a virtually constant high level along the flow
path, which means that the catalytic activity of the
catalyst along the flow path is also largely uniformly and
intensively exploited. This is in sharp contrast with
previously conventional catalysts through which the flow was
practically laminar and in which the Sherwood number drops
along the flow path. This leads to high separation
efficiency while at the same time the pressure loss is
negligible.
In a structurally simple configuration of the
plate-type catalytic converter, the above-mentioned
deflection means for deflecting the flow medium may be
deflection elements which protrude out of the plane of the
plate and with which flow openings for the flow medium in
the plate are associated. For instance, openings may be
stamped into the plate. The deflection elements can also be
form bodies mounted on the plate which mix and/or deflect
the flow medium.
In a direct further development of this
embodiment, the deflection elements are bent out of the
plane of the plate at a bend, whereby the bend angle forms
an angle a with the main flow direction which is greater
than 0° and less than 180°, and which preferably lies
between 20° and 160°. The line formed by a wave trough and
which determines the direction of the deflection forms this
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angle a with the main flow direction. The flow openings are
therefore the direct result of the deflection elements which
protrude from the plane of the plates and that are oriented
relative to the main flow direction in such a way that in a
region preceding these deflections elements and in the wake
region thereof, local pressure differences in the flow
medium are produced. These pressure differences dictate
both turbulence in the flow medium in the same reaction
chamber and mixing of the flow medium with portions of the
flow medium from adjacent reaction chambers through the flow
openings.
Due to the fact that the deflection elements are
inclined about an angle R, which is preferably between 10
and 60°, they present impact faces to the flow medium that
because of their inclination cause a high degree of
turbulence and at the same time cause only a relatively
slight pressure drop.
In order to guide the flow medium particularly
well and at least partially from one reaction chamber into
adjacent reaction chambers, it is advantageous when the
deflection elements are included against the main flow
direction, whereby the flow apertures are disposed
immediately in front of the deflection elements as seen in
the flow direction of the flow medium. It is also possible
for the deflection elements, however, to be inclined in the
main flow direction, whereby the flow apertures are then
preferably disposed behind the deflection elements as seen
in the flow direction of the flow medium.
In order to take advantage of the catalytic
activity of the plate-type catalytic converter about the
entire catalytically active surface as evenly as possible
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and to adjust the volume flow exiting the converter per unit
of cross-sectional area as evenly as possible, it is
advantageous for the deflection elements of one plate to
protrude towards different sides from the plane of the
plate. This causes the flow medium to be deflected towards
the different sides of the plates in equal parts on average.
A simple design of the plate-type catalytic
converter is obtained if the plates are spaced apart from
one another by means of the deflection elements. In this
way, large catalytically active surfaces can be attained,
referred to the total volume of the plate-type catalytic
converter, by simply stacking the catalyst plates in the
holder device, preferably in an element case.
It is advantageous when the length and the width
of the deflection elements is small compared to the length
and the width of the plates. That dimensioning of the
deflection elements makes it possible to achieve good local
mixing of the flow medium following each deflection element
and to form a flow system which extends about the entire
catalyst volume.
With regard to the structure of the plates it is
advantageous when the structure is a corrugated profile of
the plates. That corrugated profile can thereby have the
shape of a sine wave, or a rectangular or saw-tooth
corrugation, or a combination of these shapes. On
principle, any structure is hereby possible which deflects
the flow medium from the main flow direction to turbulent
flow paths.
With regard to the disposition of the plates it is
advantageous when the structure is oriented at an angle
relative to the flow direction, so that this disposition of
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the structure also produces turbulent transverse flows at
these structures relative to the main flow direction.
All afore-mentioned measures, therefore,
contribute substantially to improving and to increasing the
contribution induced by the guiding of the flow in the
catalytic activity of the plate-type catalytic converter.
Other advantageous features of the invention may be learned
from the remaining dependent claims.
Exemplary embodiments of the invention will be
described in further detail in conjunction with the
drawings, as follows:
FIG. 1 is a fragmentary, diagrammatic, partially
cut-away, perspective view of two stacked catalyst plates
having a different structure;
FIG. 2 is a partially cut-away plan view of the
catalyst plates of FIG. 1;
FIG. 3 is a sectional view taken along a line
III--III of FIG. 2;
FIG. 4 is a partially cut-away plan view of a
leading end of an element case in which the catalyst plates
of FIGS. 1-3 are installed and stacked on one another;
FIG. 5 is a partially cut-away plan view of two
catalyst plates with an identical structure, in which the
structures of immediately adjacent catalyst plates are
disposed at an angle relative to one another;
FIG. 6 is a sectional view through the catalyst
plates, which is taken along a line VI--VI of FIG. 5; and
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FIG. 7 is a fragmentary, partially cut-away view
of two catalyst plates having an identical structure and a
sinusoidal profile, in which the structures are disposed at
right angles to one another, and apertures are provided on
sloping edges of the sinusoidal profile.
Figure 1, in a partially cut-away perspective
view, shows two catalyst plates 102, 104 disposed directly
one above the other. The catalyst plates 102, 104, along
with all the subsequent catalyst plates in the other drawing
figures, are coated on both sides with a catalytically
active composition, but this is not shown in further detail
for the sake of simplicity. The catalyst plates 102, 104
define a reaction chamber 106, which is formed by a
structure 108, 110 of the catalyst plates 104 and 102,
respectively. Beads 108 are impressed into the catalyst
plate 104 as a structure and extend parallel to a main flow
direction 112 between two edges of the plate 104. The
deflection of a flow medium, which flows into the reaction
chamber 106 parallel to the main flow direction 112, is
brought about in the embodiment of Fig. 1 essentially by a
cross-channel-like structure 110 of the catalyst plate 102.
This cross-channel-like structure 110 includes
depressions 114 ("wave troughs") and protrusions 116
("wave crests"), which are oriented at an angle a of
approximately 45° from the main flow direction 112. The
depressions 114 and protrusions 116 extend between two edges
of the plate 102, with multiple changes of direction of
approximately 90° each time. This so-called open cross-
channel structure is distinguished by good turbulence of the
components of the flow medium and at the same time a very
slight pressure drop and a very slight danger of plugging
from the particles and dust contained in the flow medium.
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A plate-type catalytic converter 118, as shown in
Fig. 4 in a plan view on its leading end, can be made simply
by stacking the catalyst plates 102, 104 on one another in
alternation. Because of the especially good turbulence of
the flow medium, which for example is a nitrogen-oxide-
containing flue gas of a combustion system or an
incineration system, the nitrogen oxides, along with ammonia
introduced beforehand into the flue gas, in such a plate
catalyst 118 are carried especially often along the surface
of the catalyst plates 102, 104 coated with the
catalytically active composition, and as a result the
likelihood of adsorption for the nitrogen oxides and the
ammonia at the catalyst plates 102, 104 rises considerably
compared with plates having a laminar flow. The object of
the invention is accordingly advantageously attained in this
exemplary embodiment by means of an alternating arrangement
of plates 104 having a structure 108 solely parallel to the
main flow direction 112 and plates 102 having a structure
110 that extends at the angle a from the main flow
direction 112.
Fig. 6 shows the catalyst plates 102, 104 of
Fig. 1 in a partially cut-away plan view and once again
illustrates the arrangement of the structures 108, 110
relative to one another.
The section shown in Fig. 3, taken along the line
VII-VII of Fig. 2, makes it clear that the catalyst plates
102, 104 are spaced apart in a simple and advantageous way
by means of their structures, or in other words the beads
108 and the protrusions and depressions 116 and 114,
respectively. Moreover, as has already been the case in
Fig. 1, it is clearly shown that the reaction chamber 106
between the catalyst plates 102, 104 is not subdivided into
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individual subchambers, as is usual in the prior art, but
rather is a single chamber with only pointwise interruptions
at the points of contact of the structures 108, 114.
Fig. 4 is a detail of the plan view on the leading
end of the plate-type catalytic converter 118. This plate-
type catalytic converter 118 includes the catalyst plates
102, 104 shown in Figs. 1 to 3, which are stacked in
alternation on one another and are built into an element
case 120. In this exemplary embodiment, the element case
120 comprises thin sheets of a stainless steel, and on its
long sides 122, 124 it has (non-illustrated) guide rails,
for the catalyst plates 102, 104, which as a result are
simple to slide into the element case. The deflection
elements are preferably inclined relative to the plane of
the plates by an angle of inclination ~, which is preferably
between 10° and 60°.
Fig. 9 shows the way in which the concept of the
invention can be realized with catalyst plates 126, 128 of a
plate-type catalytic converter 130 if the catalyst plates
have as their structure a corrugated profile, such as a
sawtooth or triangular profile, or a sinusoidal profile as
shown in the exemplary embodiment. The structures 132, 134
of the catalyst plates 126 and 128, respectively, extend at
an angle to the main flow direction 112, and the directions
of the structures intersect, and the structures 132, 134
extend without changing direction between two edges of the
respective catalyst plates 126, 128. Each two catalyst
plates 126, 128, which are shown uninstalled in Fig. 5 and
are shown in an installed state in an element case 120 in
Fig. 6, along with the side walls 122, 124 of the element
case 120 shown in Fig. 4, define one reaction chamber 136.
As in the previous exemplary embodiment as well, the
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reaction chamber 136 is extended over the entire space
between two catalyst plates 126, 128, and is interrupted
only pointwise at the points of contact of the catalyst
plates 126, 128 that occur because of the structures 132,
134. By means of the structures 132, 134, the catalyst
plates 126, 128 are also spaced apart from one another, as
is best seen in Fig. 6, which shows a section taken along
the line VI-VI of Fig. 5.
Because of the corrugated structures 132, 134,
which extend at an angle to the main flow direction 112, a
flow medium flowing into the reaction chamber 136 parallel
to the main flow direction 112 is deflected from the main
flow direction 112 and rendered turbulent. As a result, the
separation rates for nitrogen oxides, for example, with
ammonia are improved, on the one hand as a result of a
highly homogeneous distribution of the flow medium
components, and on the other as the result of an increased
adsorption likelihood for the nitrogen oxides contained in
the flow medium along with the ammonia, compared with plate-
type catalytic converters having catalyst plates through
which the flow is laminar. As a consequence, the catalytic
activity of the plate-type catalytic converter 130 is
increased over plate-type catalytic converters with a
laminar flow through them, since the contribution, to the
catalytic activity of the catalytic converter, which is
induced by the course of the flow, is increased.
The component of the catalytic activity that is
induced by the course of the flow can be increased still
further if, for example on the basis of a structure and
arrangement of the catalyst plates 126, 128 relative to one
another in generally flat elements, which are inclined
relative to a main plate plane (in this case, the plane
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shown in Fig. 5) and the main flow direction 112, apertures
138 are provided, as shown in perspective, partially cut
away, in Fig. 7. The apertures are embodied as holes 138 in
the catalyst plates 126, 128, each of the holes being
disposed on the sloping edges of the sinusoidal profile 132,
134. What is attained as a result is that the flow medium
is not only deflected from the main flow direction 112 and
thus mixed within a reaction chamber 136, but moreover it
can at least partially enter adjacent reaction chambers (the
reaction chambers, not shown, here disposed above the plate
128 and below the plate 126). Accordingly, the result of
this feature is that not only are local differences in
concentration from turbulence in the flow medium compensated
for, but concentration differences that extend over the
catalyst volume within an element case 120 can also be
compensated for. The holes 138 may for instance also be
embodied as tabs or lugs that are stamped out of the
catalyst plates 126, 128 and protrude into the individual
reactions chambers. The arrangement of apertures 138 may be
made in many ways. They may be disposed in both the rising
and the falling sloping edge of the structure 132, 134, and
they may also be offset from one another in the main flow
direction 112.
As a consequence, the aforementioned turbulence of
the components of the flow medium causes the individual
components of the flow medium to be moved considerably more
often along the catalytically active surfaces of the
catalyst plates 102, 104. As a result, the likelihood of a
three-way contact between the reagents, in this case the
nitrogen oxides and the ammonia, for instance, and the
active centers of the catalyst, is increased considerably as
compared with catalyst plates that are known from the prior
art and through which the flow is only laminar. The
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absorption of the nitrogen oxides and the ammonia at the
catalytic material is especially advantageous because the
nitrogen oxides together with the ammonia are converted at
the catalytically active centers of the catalytically active
layer of the catalyst plates into nitrogen and water.
Since the height of the structures is small as
compared with the dimensions of the catalyst plates 102,
104, the pressure drop that is necessarily caused by the
deflection of the flow medium from the main flow direction
112 also remains within tolerable values. The danger of
stoppage of the reaction chambers, for instance from a flue
gas that is heavily laden with particles and dust, can also
be precluded, since because of the good microscopic
turbulence of the flow medium, no dead spaces in the flow
are created in the reaction chambers.
A plate-type catalytic converter which is
constructed in the manner described herein attains
substantially higher separation efficiency, at the same
predetermined conditions, because of the flow deflection, as
compared with plate-type catalytic converters through which
the flow is virtually exclusively laminar. Conversely, at
predetermined separation efficiency levels, this also
permits the catalyst volume of a plate-type catalytic
converter according to the invention to be chosen to be
considerably smaller than that in a plate-type catalytic
converter of the prior art through which the flow is
virtually exclusively laminar.
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