Catalytic reactors

Details Catalytic reactors Catalytic reactors

1999-09-21

2001-07-31

Langel, Wayne (Department: 1754)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Waste gas purifier

C422S198000, C422S211000, C502S439000

Reexamination Certificate

active

06267932

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to improvements in catalytic reactors of a kind comprising a metal substrate body. Generally, such a substrate body consists of flat thin metal (foil) strips alternating with corrugated thin metal (foil) strips. The thickness of the flat and thin foil strips is typically in the range of 0.05 to 0.1 mm. The strips are wound upon themselves about an axis so as to form passages extending axially through the substrate body for through-flow and catalytic purification of exhaust gases.
To achieve the desired catalytic purification, a coating (so called washcoat) is applied on the foil strips, said coating usually consisting of an aluminum oxide and noble metals (such as e.g. rhodium, platinum, palladium).
The thus-wound substrate body is provided with an enclosing metal mantle. According to prior-art technology, this mantle has a sheet thickness of between 1 and 1.5 mm. The reason for the relatively large thickness of the mantle is to make it possible to fasten the mantle, by welding or brazing, to the substrate body and to a casing (canning) surrounding the mantle.
One problem encountered in a thus-structured catalytic reactor is its deficient strength. When hot gases (up to 1000° C.) flow through the passages, the foil strips are rapidly heated, and as a result the substrate body expands axially and radially. The surrounding mantle, on the other hand, is not directly exposed to the gas flow. Since in addition thereto the mantle is much thicker and thus has a larger mass to be heated than the foil strips, it will expand at a much lower rate. In consequence thereof, a considerable compression force will be generated in the space between the mantle and the outermost, thin layer of the substrate body. That is a potential cause of deformation of the outer corrugations in the substrate body with consequential destruction of the passages in that layer.
When the substrate body is cooling, the opposite problem arises. The foil strips are cooled at a much higher rate than is the considerably thicker mantle, the latter, as already mentioned, having no direct contact with the gas flow. Consequently, the substrate body will contract much quicker than the mantle. If, in this situation, the individual foil strips are joined together, considerable tension will be generated between the layers in the radial direction as a result of the differences in the extent of contraction between the substrate body and the mantle.
In substrate bodies wherein the coating is the only bonding agent, the strength of the bond will be exceeded and cracks and gaps form, usually in a couple of layers closest to the mantle. In substrate bodies having a diameter size of about 100 mm, a gap of 1 mm may form.
Considerable tension is generated in substrate bodies of the kind wherein the layers are joined together by brazing. Particularly the tension between the outermost layers of the substrate body and the mantle will be of such a magnitude that the brazed bonds run the risk of disrupting. The strength of the catalytic reactor is seriously affected by these problems.
The difference in expansion between the mantle and the substrate body is the principal reason for the generation of compression or tensile stress. The object of the invention is to provide a catalytic reactor having a mantle which is more adaptable to the motions of the substrate body and thus is able to prevent the substrate body from being exposed to harmful mechanical compression or tensile stress.
SUMMARY OF THE INVENTION
The thickness of the mantle is made much smaller than in the prior art, so that any compression stress that may be exerted on the foil strip corrugations or any tensile stress exerted on the brazed or welded bonds does not surpass the strength of the structure. The mantle suitably has a thickness not greater than 0.8 mm, more preferably not greater than 0.5 mm.
A second effect obtained by a mantle with highly reduced thickness in comparison with thicker prior art mantles in catalytic reactors of this type is a considerably reduced mass of the mantle. One consequence of the reduced mass is that the mantle will be heated and cooled much quicker than prior art mantles. Also, the temperature differences between the mantle and the substrate body will not be as great, and therefore the differences in rates of expansion and contraction between the mantle and the outermost layers of the substrate body will not be as great. This will lead to less compression and tensile stress. The reduced difference in contraction between the outermost layers of the substrate body and the mantle owing to the quicker cooling of the latter reduces or eliminates cracking and reduces the formation of gaps between these parts. The result is a catalytic reactor having considerably improved strength/durability.
As is evident from the above, the thin mantle produces two effects. The difference in expansion and contraction is reduced, and mantle-induced compression is lessened owing to the improved ductility. Both effects cooperate to increase the durability of the catalytic reactor.


REFERENCES:
patent: 5079210 (1992-01-01), Kaji et al.
patent: 5116581 (1992-05-01), Cyron et al.
patent: 5190732 (1993-03-01), Maus et al.
patent: 5486338 (1996-01-01), Ota et al.
patent: 5494881 (1996-02-01), Machida et al.
patent: 5665669 (1997-09-01), Yamanaka et al.
patent: 8266905 (1996-10-01), None
patent: 96/34188 (1996-04-01), None
International Search Report for PCT/SE97/00480.

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