Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
2000-05-22
2001-07-10
Watkins, III, William P. (Department: 1772)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S131000, C428S117000, C428S118000, C422S180000
Reexamination Certificate
active
06258436
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a catalyst body that has a number of passages through which a medium can flow in a preferred direction.
Catalysts are used, for example, to remove nitrogen oxides, hydrocarbons, carbon monoxide and/or dioxins from the flue gas of an incineration plant. Catalysts of this nature may be configured as honeycomb catalysts. In a honeycomb catalyst, it is usual to provide a catalyst body that is in honeycomb form and has a number of passages through which a liquid and/or a gas, such as for example the flue gas, can flow in a preferred direction.
A honeycomb catalyst of this nature may be provided for reducing or oxidizing pollutants. The honeycomb catalyst which is provided as a DeNO
x
catalyst, for example, uses the so-called selective catalytic reduction (SCR) process, with an addition of a reducing agent, to reduce nitrogen oxides (NO
x
) to nitrogen (N
2
) and water (H
2
O). However, the honeycomb catalyst that is configured as a dioxin catalyst also has a certain SCR activity. For example, a dioxin catalyst oxidizes polychlorinated dibenzodioxins/dibenzofurans using molecular oxygen.
To catalytically clean the flue gas from stationary or mobile incineration plants or internal combustion engines, honeycomb catalysts are used, the catalyst bodies of which have a square passage geometry. German Patent DE 28 19 378 C2 discloses a honeycomb body with cells which are guided through and are of substantially rectangular basic shape. In this case, the longitudinal sides of a first group of the passages are disposed approximately at right angles to the longitudinal sides of a second group of the passages.
The number of channels may in this case be 10/6.452 cm
2
. Published, Non-Prosecuted German Patent Application DE 37 13 209 A1 also discloses a honeycomb body with passages of approximately rectangular cross section. A ceramic structure body with a honeycomb body whose passages are approximately rectangular in cross section is also known in principle from the reference titled “Haus der Technik (House of Technology”, Prof. E. Steinmetz (Ed.), Vulkan-Verlag Essen, (1992), p. 23.
However, with catalyst bodies of this nature there is a risk of mechanical blockage of the honeycomb openings caused by flue dust. Therefore, regular, comparatively expensive cleaning of the catalyst is required in order to ensure that the catalyst operates reliably. In addition, the production of catalyst bodies of this nature involves the problem of cracking during the manufacturing process owing to the internal stress of the catalyst bodies.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a catalyst body which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which can be operated particularly reliably with only little cleaning outlay. It is also intended for cracking during the process of manufacturing the catalyst body to be rare.
With the foregoing and other objects in view there is provided, in accordance with the invention, a catalyst body configuration, containing a catalyst body having a number of passages formed therein through which a medium can flow in a preferred direction. The catalyst body has a plurality of longitudinal sides and a plurality of shorter transverse sides defining the passages. Each of the passages is disposed at right angles to the preferred direction and is approximately rectangular in cross section, as defined by the longitudinal sides and the shorter transverse sides. A number of the passages per unit surface area lie in a range from 0.5 to 2.5 cm
−2
. The longitudinal sides of a first group of the passages are disposed approximately at right angles to the longitudinal sides of a second group of the passages. The passages are combined to form subgroups and the passages of each of the subgroups, in terms of their cross sections, form an approximately square configuration. Each of the subgroups containing in each case the passages from a same group of one of the first group and the second group.
The object is achieved by the catalyst body of the type described above in which each passage, disposed at right angles to the preferred direction, is approximately rectangular in cross section, as defined by a longitudinal side and a shorter transverse side. The number of passages lying in the range from 0.5 to 2.5 cm
−2
, and the longitudinal sides of the first group of the passages is disposed approximately at right angles to the longitudinal sides of a second group of the passages. According to the invention, the passages are combined to form subgroups, the passages of each subgroup, in terms of their cross sections, form an approximately square configuration, and each subgroup contains in each case passages from the same group.
The invention is based on the recognition that a catalyst body functions particularly reliably, even with little cleaning outlay, if the risk of mechanical blockage of the passages is kept at a low level. A catalyst body that is configured with a particularly low risk of the passages becoming blocked should also have a high catalytic activity. It has been found that in a particularly suitable catalyst body the number of passages per unit surface area lies in the range from 0.5 to 2.5 cm
−2
, but preferably in the range from 0.9 to 1.6 cm
−2
, with passages which are approximately rectangular in cross section.
The so-called AP value represents a measure of the catalytic activity of the catalyst body. This value is defined as the ratio of the geometric surface area of the catalyst body to its physical volume. At an identical AP value, a catalyst body whose passages have an approximately rectangular cross section of flow has fewer passages per unit surface area and therefore fewer passage edges than a catalyst body whose passage cross sections are approximately square. The smaller the number of passage edges, the lower the tendency of the passages to become mechanically blocked, since flue dust preferentially accumulates in the area of the passage edges. To keep the problems of internal stress when producing the honeycomb catalyst body, and the associated cracking during the manufacturing process, at a particularly low level, the longitudinal sides of the first group of the passages are advantageously disposed approximately at right angles to the longitudinal sides of the second group of the passages. Furthermore, the passages are combined to form subgroups, each subgroup containing passages from the same group. In terms of their cross sections, the passages of one subgroup form an approximately square configuration. Forming subgroups in this way allows the passages to be disposed particularly simply and flexibly in the catalyst body. Depending on what is most expedient, adjacent subgroups may be assigned to the first or second group of the passages. In both cases, a particularly stable overall structure is produced. With regard to each outer side, a catalyst body of this nature is able to withstand forces that act in the longitudinal and/or transverse directions.
In an advantageous configuration of the invention, subgroups are combined to form layers. Layers in which subgroups of the first and second groups of the passages are disposed alternately have proven particularly advantageous, since each such layer inherently exhibits a particularly low tendency to crack during the production process.
In an alternative, advantageous configuration, all the passages in one layer belong to the same group of passages.
An advantageous configuration of the passages in the catalyst body of this nature is provided by disposing the layers assigned to the first group of passages and the layers assigned to the second group of passages alternately.
The catalyst body may be configured as a supported catalyst, in which a support body is coated with a catalytically active layer. Advantageously, however, the catalyst body is configured as an unsupported extrudate. In this case, the catalyst body consists exclusively
Girschik Andreas
Kleuderlein Robert
Pajonk Gunther
Witzel Frank
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
Watkins III William P.
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