IMPROVEMENTS IN CATALYTIC REACTORS

Abstract: The invention relates to improvements in catalytic reactors of a kind comprising a substrate body (1) formed by winding alternate flat and corrugated metal strips (2, 3) which form through-flow passages (4) extending axially through the substrate body (1). A mantle (6) enclosing the substrate body (1) is formed from a sheet, the thickness of which equals or is only a few times thicker than the thickness of the flat and of the corrugated metal strips (2, 3).

Description: IMPROVEMENTS IN CATALYTIC REACTORS The present invention relates to improvements in catalytic reactors of a kind comprising a metal substrate body. Generally, such a substrate body consists of alternate flat and corrugated thin metal strips (thickness = 0.05 - 0.1 mm) which 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 metal strips, said coating usually consisting of an aluminium 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, i.e. a thickness that is 10 to 30 times greater than that of the metal strips. The reason for the considerably greater thickness of the mantle is that it must be possible to fasten the mantle by welding or brazing it to on the hand the substrate body and on the other 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 thin metal strip walls 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 it is much thicker and thus has a larger mass to be heated, it will become hot and expand at a much lower rate. In consequence thereof, a considerable compression force will generate in the space between the outermost, thin layer of the substrate and the mantle, and accordingly the latter is a potential cause of deformation of the outer corrugations in the substrate with consequential destruction of the passages in that layer. When the substrate body is cooling, the opposite problem arises. The thin passage walls 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 metal strips are joined together, considerable tension will generate between the layers in the radial direction as a result of the differences in the extent of contraction between the sub- strate 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 between the outermost layers of the substrate body and the mantle the tension 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. The features characterising this mantle appear from the appended claims. The thickness of the mantle should be of the same magnitude as the thickness of the metal strip or exceed that thickness to such an insignificant degree that any compression stress that may be exerted on the metal strip corrugations or any tensile stress exerted on the brazed or welded bonds does not surpass the strength of the structure. If the thickness of the metal strips is in the range mentioned initially, i.e. 0.5 - 0.1 mm, the mantle suitably may have a thickness in the range of 0.1 - 0.5 mm. A second effect obtained by a mantle having a highly reduced thickness in comparison with prior-art mantles used within the technical field concerned is its considerably reduced mass. One consequence of the reduced size is that the mantle will be heated and cooled much quicker than do prior-art mantles . The temperature differences between the mantle and the substrate body will not be as great and therefore not either the differences in expansion and contraction between the mantle and the outermost layers of the substrate body. Furthermore, 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 appears 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 act in the same direction and increase the durability of the catalytic reactor. The thin mantle in accordance with the invention provides several other valuable advantages as will appear from the following detailed description with reference to the accompanying drawings, wherein Figs 1 and 2 are cross-sectional views of a substrate body consisting of alternate flat and corrugated flat metal strips which are wound helically upon themselves, and showing the body-enclosing mantle in open and closed positions, respectively, and Fig. 3 is a view of the catalytic reactor as seen from the side, partly surrounded by a casing, so called canning. The substrate body 1 shown in Fig. 1 in a cross- sectional view is an example of a prior-art structure wherein alternate flat and corrugated metal strips 2 and 3 are wound helically and thus form through-flow passages 4 through the substrate body 1. If these metal strips 2, 3, prior to being wound, are first oxidized followed by application thereon of a catalytic coating (wash coating) , which in itself is advantageous in order to ensure optimum reactor efficiency, the drawback arises in that once the winding of the metal strips 2, 3 is completed it becomes necessary, in order to be able to join the strips together by welding or brazing, to scrape off the coating on the outermost portion 2a of the flat metal strip 2 as well as on the neighbouring portion of the corrugated metal strip 3. In addition, also the oxidized layer must be ground away. Following application of a conventional thicker mantle about the substrate body 1 by compression moulding, the mantle is welded or brazed to its thus exposed strip portions. The disadvantages of this method are on the one hand that it is comparatively complicated and time-consuming and on the other that the coating material, which is expensive, must be removed. By the advent of the thin mantle in accordance with the teachings of the present invention the above method may be simplified. After winding of the substrate body 1, on which a coating has been applied, the metal strips 2, 3 are stapled together at the end portion 2a by means of staples, not shown, in order to prevent the metal strips 2, 3 from flexing apart. The substrate body 1 can now be removed from the machine wherein the winding operation has been performed, and a thin mantle 6 be applied about the substrate body 1 to an excellent fit, whereupon the mantle 6 is welded together along a lengthwise joint. As should be clearly apparent, this method is more convenient, quicker and consequently less expensive than conventional methods. In addition, it becomes possible to exploit other advantages offered by the thinner mantle 6, as will be described in the following. When the substrate body 1 is made from alternate flat and corrugated metal strips 2, 3 in a manner according to which tabs formed in the corrugated strips 3 are used to keep the strips together by engagement of said tabs in channels formed in the flat strips 2 by rolling, the substrate body 1 will have the appearance illustrated in Fig. 3, i.e. it exhibits peripheral impressions or grooves 7 on its circumferential surface. Because in this case the mantle 6 is thin, sharp-edged ridges 8 are easily formed therein. The ridges fit into the grooves 7 formed in the circumferential surface of the substrate body 1. The bond between the grooves and the ridges fixes the mantle 6 axially relative to the substrate body 1, as required, and this bond thus is obtained without the need for the mantle 6 to be separately fastened, such as e.g. by welding, brazing, riveting, or in any other way. As also appears from Fig. 3, the mantle 6 likewise exhibits some portions 9 which are produced by rolling and which in a manner known per se form annular protrusions 9 extending peripherally around the mantle 6. In prior-art mantles these annular protrusions have only served to form a gap or clearance 10 between the mantle 6 and an outer casing 11 (canning) in the catalytic reactor in order to reduce heat transfer to the casing 11. As a further characteristic feature of the invention, these annular protrusions 9 may be utilized in the thin mantle 6 to impart resiliency to the mantle 6 for the purpose of minimizing any axial strain that may arise between the substrate body 1 and the mantle 6 as the catalytic reactor is heated and cooled. In order to minimize any radially directed forces to which the mantle 6 may be exposed and which may be caused by the rigidity of the casing 11, the mantle 6 suitably is equipped at each end face with a collar 12 projecting away from the substrate body 1 in the prolongation of the latter. The collar 12 facilitates the operation of welding the finished catalytic reactor to the casing 11. Prior to mounting, the collar may be given a slightly conical configuration, widening in the direction away from the substrate body 1, a configuration which ensures satisfactory abutment of the collar 12 against the inner face of the casing 11 as the reactor is being fitted into the latter, a feature which further facilitates the welding operation. The attachment of the catalytic reactor to the casing 11 via a collar 12 at either side provides the additional advantage that vibrations and other movements, as also slight deformations in the casing 11, are reduced or eliminated via said flexible collars 12 and therefore will not be propagated in full to the substrate body. Consequently, this arrangement contributes to increasing the strength/durability of the reactor. As appears from the description above the thin- walled mantle 6 in accordance with the invention is instrumental in increasing the strength/durability of the catalytic reactor in several ways. A consequence of the flexible enclosure of the substrate body 1 produced by the mantle 6 is that the risk of deformation of the substrate body 1, resulting in the formation of cracks in the catalytic reactor material, is minimized and thus that the serviceable life of the catalytic reactor is increased.

Claims: CLAIMS 1. Improvements in catalytic reactors of the kind comprising a substrate body (1) of thin metal sheet formed by alternate flat and corrugated metal strips (2, 3) which are wound upon themselves so as to form through- flow passages (4) extending axially through the substrate body (1), and a mantle (6) enclosing the substrate body (1) , c h a r a c t e r i s e d in that the mantle (6) is formed from sheet metal the thickness of which equals or is only a few times greater than the thickness of the flat and of the corrugated metal strips (2, 3). 2. A catalytic reactor as claimed in claim 1, c h a r a c t e r i s e d in that at least one portion (9), produced by rolling, is formed in the mantle (6), said portion forming an annular protrusion projecting outwardly from the surface of the mantle (6) and arranged to impart resiliency to the mantle (6) for the purpose of reducing any axial forces generating between the substrate body (1) and the mantle (6) as a result of the catalytic reactor being heated and cooled. 3. A catalytic reactor as claimed in any one of the preceding claims, wherein a peripherally extending groove (7) is formed in the circumferential surface of the substrate body (1) , c h a r a c t e r i s e d in that the mantle (6) is provided with correspondingly configured ridges (8) intended to be fitted into said grooves (7) as the mantle (6) is being applied about the substrate body (1), in order to prevent axial movements of said mantle (6) relative to said substrate body (1) . 4. A catalytic reactor as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that at each one of its end faces the mantle (6) is equipped with a collar (12) projecting away from the substrate body (1) in the prolongation thereof, the mantle being arranged to be attached to a casing (11), so called canning, enclosing said catalytic reactor, by means of said collar (12) . 5. A catallytic reactor as claimed in claim 4, c h a r a c t e r i s e d in that the collar (12) widens slightly conically in a direction outwardly from the substrate body (1) .

Inventors: NILSSON SVEN MELKER, NILSSON, SVEN, MELKER
Applicants: NILSSON SVEN MELKER, NILSSON, SVEN, MELKER
Application year: 1997
Main classification: F01N3
IPC codes: B01D53, B01J19, B01J35, F01N3