FORAMINOUS SHEETS FOR USE IN CATALYSIS

Abstract: To improve their flexibility and/or their performance, relief patterns are applied to foraminous sheets (10) for use in catalysis, such as catalyst sheets, getter sheets and support sheets. The specification discloses various relief patterns and methods for producing the relief patterns.

Description: FORAMINOUS SHEETS FOR USE IN CATALYSIS This invention relates to catalysts, to getters for catalysts, and to supports for catalysts or getters. More particularly, the invention relates to foraminous sheets or layers which primarily act as catalysts, as getters or as supports, and to reactors or catalyst packs including one or more of such sheets or layers. Foraminous sheets or layers have a long history of use in industrial catalytic processes such as the oxidation of ammonia to form nitric oxide or in the production of hydrogen cyanide. In the simplest arrangements, a foraminous catalytic sheet of, for example, a Pt-based alloy is supported in a reactor. The sheet is oriented within the reactor in a plane substantially perpendicular to the direction of flow through the reactor, and is positioned in the path of one or more reactants which, in use of the reactor, flow through the sheet. In so doing, the reactants undergo a catalytic reaction promoted by the material that makes up the sheet. Generally, a plurality of such sheets are assembled in overlying relation to form a layered pack of sheets. This arrangement improves the yield of the reaction by increasing the catalytic surface area exposed to the reactants during their passage through the reactor. Commonly, the reactor is a vessel of circular cross-section. Foraminous sheets for use in such a reactor are also circular with a diameter selected to fit closely within the cross- section of the reactor, thereby to ensure that all reactants pass through the sheet in their passage through the reactor. For this purpose,, each foraminous sheet (or a layered pack of such sheets) is supported around its circumference by being rigidly attached to the surrounding internal annular wall of the reactor . A catalytic sheet may attain incandescent temperatures under reaction conditions, for example in the catalytic oxidation of ammonia; this leads to gradual evaporation of the valuable catalytic material that makes up the sheet. The evaporated material is entrained and swept away by the flow of reactants and reaction products, and therefore may be lost. In an effort to reduce this expensive loss of catalytic material, many reactors are equipped with one or more foraminous getter or catchment sheets of, for example, a Pd-based alloy, located downstream of the catalytic sheet or sheets. Getter sheets may alternate with or otherwise be interspersed with catalytic sheets in a pack. Gettering depends upon atoms of catalytic material impinging upon the material of the getter sheet; at suitably high temperatures, these atoms diffuse into the material of the getter sheet, and can later be recovered by processing the getter sheet. Accordingly, gettering efficiency can be enhanced by increasing the likelihood of collision between atoms of catalytic material and the getter sheet. Clearly, the likelihood of collision can be increased by reducing the size of the openings in the getter sheet (usually by placing the constituent wires or other elements closer together), or by installing further getter sheets in the reactor chamber. However, such measures restrict the flow of reactants through the reactor and so increase back pressure: as a result, there must be a compromise between excessive back-pressure and excessive loss of catalytic material. For the avoidance of doubt, it should be emphasised that getter sheets are not necessarily non-catalytic; indeed, they will often have spme inherent catalytic effect. In any event, the getter sheets may be expected to acquire an increasing catalytic effect because, in use, material recovered from the catalytic layers diffuses into the getter sheet material. Exothermic reactions such as the oxidation of ammonia to form nitric oxide generally involve reaction temperatures well in excess of 250 Celsius and usually in the range 650 to 1000 Celsius. The materials that make up the catalyst and getter sheets are, in general, mechanically weak at such high temperatures. Consequently, catalyst packs commonly include one or more foraminous support means such as sheets positioned to support the catalyst or getter sheets. The support sheets are usually of non-p.g. . (p.g.m. = platinum group metal) materials such as stainless steel, nickel-chromium alloys, the aluminium-chromium-iron alloy 'Kanthal' (Registered Trade Mark), or similar. Ceramic supports are also known. Foraminous sheets for use in catalysis take many forms. For example, it has been known for many years to use woven gauzes containing wires, strips or other elongate elements of a p.g.m. alloy, either as a catalyst or, more recently, as a getter for a catalyst. Woven support gauzes are also known. Woven gauzes are expensive to produce and can deteriorate quickly under the hostile conditions that typically prevail in catalytic reactors. Accordingly, there have been several proposals for non-woven foraminous sheets for use in the field of catalysis. For example, knitted gauzes have been proposed for catalysts, getters or supports. It has also been proposed to employ an assemblage of randomly oriented fibres for getter systems. These randomly-oriented fibres are suitably bonded together, e.g. by welding or sintering, to form a self- supporting pad which may then be compressed to form a foraminous getter sheet. Whilst the various foraminous sheets described above have an undulating or uneven surface texture if viewed closely, they are, viewed as a whole, substantially planar when supported in a reactor. A serious problem with existing foraminous sheet arrangements is that they lack flexibility. In particular, existing foraminous sheets, especially woven sheets, have little flexibility in the plane of the sheet, transverse to the direction of flow of reactants through a reactor. Under the heat of a catalytic reaction, the sheet will try to expand but may be prevented from doing so because its edges are supported by the reactor structure. Consequently, the sheet may be subjected to internal stresses. If the sheet has insufficient flexibility to relieve the internal stresses, it will distort from its original planar configuration, usually buckling to form ripple-like undulations in a so-called 'tortoiseshell' pattern. These undulations are undesirable for several reasons. Firstly, they give rise to local thickening of the foraminous sheet; these thickened regions commonly experience a higher temperature than other, thinner regions, because the thicker the sheet, the longer the residence time of reactants within the sheet. So, if the thinner regions are at the correct temperature for a given reaction, the thickened regions may be too hot. One consequence of the longer residence time is that it encourages carbon formation and deposition; deposition of carbon upon the catalytic surface seriously reduces catalytic efficiency by shrouding the catalytic surface and, ultimately, can lead to embrittlement and mechanical failure of the catalyst. Further, where there are neighbouring sheets in close proximity to one another, the sheets may weld together at the points of contact. This further reduces catalytic efficiency, by reducing the exposed catalytic surface area. A further consequence of buckling is that the sheet eventually contracts and so pulls away from the supporting structure, leaving gaps aro.und the edges through which reactants may bypass the sheet. Obviously, if by-pass occurs, the yield of the reaction can be drastically reduced. Why sheets contract in use is not entirely clear but it is evident that, when buckling occurs, the consequent deformation of the sheet cannot be fully recovered even when the sheet is returned to its original temperature. Accordingly, it is believed that the material of the sheet may undergo some plastic deformation when subjected to buckling stresses. Also, it is possible that the wires that make up a sheet may weld together when the sheet is in the buckled state, thus preventing the sheet from returning to its original planar configuration. Certainly, where there are several sheets in overlying relation, it is possible that sheets buckled into contact with one another may interlock or may even weld to one another. This obstructs free relative movement between adjacent sheets, and so may hinder recovery after deformation. It is against this background that the present invention was made. In our efforts to overcome the problems of expansion and contraction, we have adopted the principle of applying a relief pattern to a foraminous sheet suitable for use in catalysis. We expected that a relief pattern would provide some slack, movement or flexibility in the general plane of the sheet, whereby the sheet could accommodate and respond to internal stresses caused by high temperatures without buckling. Further, we expected a relief pattern to stiffen and strengthen the sheet in a direction transverse to the general plane of the sheet, thereby reducing bending or sagging of the sheet. As part of our investigations, we have found some prior art documents in the field of catalysis which propose the application of a relief pattern to a foraminous sheet. For example, European Patent Application No. 0428265A to Koch and United States Patent No. 4293447 to Inaba et al (Assignee: Hitachi) each disclose catalyst arrangements employing a corrugated foraminous sheet. In these documents, the corrugated foraminous sheet is a support or containment for a catalyst (Koch) or a substrate for a catalytic coating (Inaba). In each case, the corrugations are employed to maximise the surface area of the foraminous sheet, and extend in unidirectional arrays of parallel and straight alternating ridges and troughs, like a ploughed field. Koch and Inaba do not disclose foraminous sheets oriented transversely with respect to the flow of reactants in a catalytic reactor. Rather, the plane of the sheets is generally parallel to the flow of reactants, and the sheets do not act as a barrier which the flow must cross. Thus, the reactants need not necessarily flow through the sheet but can flow past the sheet along channels defined by the corrugations. It is clear that neither Koch nor Inaba are concerned with, or address, the problems of contraction or expansion of transversely-oriented foraminous sheets. Indeed, if anything, a corrugated relief pattern stiffens the sheet in a direction parallel to the corrugations and renders it substantially inextensible in that direction. Consequently, the corrugated relief pattern disclosed by Koch and Inaba is of little or no use in a transverse orientation within a catalytic reactor and does not provide the benefits that we seek. In one embodiment, Koch discloses a foraminous sheet having a relief pattern comprising a plurality of spaced-apart concave sections disposed in a regular array or grid or intersecting rows or columns. Each of the concave sections has a truncated pyramid shape and is connected to neighbouring concave sections by further concave sections of semi- cylindrical shape. Again, however, this pattern does not promote the flexibility of the sheet and, if anything, is calculated to stiffen it. Indeed, Koch refers to the foraminous sheet as a rigid plate, the function of which is to contain a catalytic material disposed within the concave sections. From one aspect, the present invention provides a foraminous sheet for use on catalysis, which sheet has a relief pattern 5 that is adapted to allow elastic expansion and contraction of the sheet in at least two mutually transverse directions extending parallel to the general plane of the sheet. In accordance with this invention, a foraminous sheet having a relief pattern as defined above is oriented within a 0 catalytic reactor such that the general plane of the sheet is substantially perpendicular to the direction of flow of reactants through the reactor. More specifically, the invention encompasses: a foraminous sheet for use in catalysis and having a relief pattern as 5 defined above applied thereto; a method for producing the sheet by applying the relief pattern to the sheet; the use of the sheet in catalysis; and products resulting from that use. The present invention particularly contemplates a foraminous getter sheet having a relief pattern applied thereto, and a 0 foraminous sheet of inherently catalytic material having a relief pattern applied thereto. The invention also encompasses a pack of foraminous sheets, in which at least one sheet of the pack has a relief pattern as defined above applied thereto. Further, the invention 5 includes a catalytic reactor fitted with a foraminous sheet, or fitted with a pack including one or more foraminous sheets, as defined above. The invention also includes the use of such , a reactor, and products obtained by such use. As previously mentioned, known foraminous sheets have a 0 surface texture that is often far from planar and that, accordingly, defines a relief pattern of sorts. However, the present invention is not concerned with surface texture but rather with the overall shape or configuration of a sheet. Accordingly, the invention contemplates foraminous sheets having a shape or configuration that may be described as embossed, contoured, dimpled, ridged or undulating. The invention also contemplates combinations of these shapes or configurations; for example, a sheet may have a dimpled portion and a ridged portion. In general, the relief pattern includes relatively raised regions (for example hills, peaks, protrusions, protuberances, convexities or lands) and/or relatively depressed regions (for example hollows, valleys, dimples, concavities or recesses). The raised and/or depressed regions are suitably disposed in two-dimensional arrays, being either regular, irregular or random arrays. The arrays may be linear (straight or curved), annular, spiral or concentric, and may be in straight or staggered relation to one another. The present invention is applicable to all foraminous sheets suitable for use in catalysis, including catalyst sheets, getter sheets and support sheets. As will be explained, the invention has particular advantages when applied to getter sheets; consequently, getter sheets are a preferred embodiment of the invention. The invention also contemplates several methods for making a foraminous sheet with a relief pattern. The relief pattern may be created as the sheet itself is created; for example, as a result of the knitting or weaving process adopted to make the sheet. Alternatively, the relief pattern may be applied to a sheet after the sheet has been created; generally, this involves plastically deforming an otherwise flat sheet with a press, thereby to produce an embossed surface with the desired relief pattern. A press to deform the foraminous sheet may take many forms. In general, the press comprises cooperable press members between which at least a portion of the sheet is pressed, at least one of the press members having a surface shaped to impart a desired relief pattern to the sheet. It is preferred that the respective surfaces of the press members are shaped in mutually complementary fashion, with raised areas of one surface corresponding to depressed areas of the other surface. In a variant, one surface may be flexible and resilient (for example of rubberised construction) in order to conform to the shape of the other surface when the press members are brought together. In one embodiment, at least one of the press members is a roller; preferably, both press members are rollers. Relative movement between the or each roller and the sheet develops the relief pattern as the roller traverses the sheet. For optimum simplicity, the rollers are suitably cylindrical. In an alternative embodiment, the sheet is laid upon and/or supported by one press member, and the other press member is brought into contact with the first press member to shape the sheet as desired. Where the sheet is relatively small, the press members may be substantially the same size as the sheet or possibly larger, thereby shaping the entire sheet in one pressing operation. However, where the sheet is large (in some reactors a sheet can be more than three metres in diameter), the press can be made more compact by pressing different regions of the sheet in turn until the whole sheet has been shaped as desired. The sheet may even be shaped in use in a reactor, by being caused to slump onto a suitably-shaped support surface. In order that the invention may be more readily understood, embodiments and aspects thereof will now be described, by way of example only, with reference to the accompanying drawings in which: Figures 1, 2 and 3 are schematic cross-sectional views depicting portions of foraminous sheets having various relief patterns in accordance with the invention; Figures 4, 5 and 6 are schematic plan views illustrating press dies shaped to form the relief patterns of Figures 1, 2 and 3; Figure 7 is a schematic cross-sectional view depicting a shaping apparatus for shaping a foraminous sheet in accordance with the invention; Figure 8 is a schematic cross-sectional view of another shaping apparatus; Figure 9 is a schematic perspective view showing a variant of the shaping apparatus shown in Figure 8; Figure 10 is a schematic cross-sectional view of catalytic reactor containing a foraminous sheet supported by a shaped support surface; Figure 11 is a schematic cross-sectional view corresponding to Figure 10 but showing the foraminous sheet slumped to follow the shape of the shaped support surface; Figure 12(a) is a schematic cross-sectional view showing how a conventional foraminous sheet provides a gettering function; Figure 12(b) is a schematic cross-sectional view showing a detail from a foraminous sheet according to the invention, in juxtaposition with Figure 12(a) for comparison purposes; Figure 13 is a schematic cross-sectional view of a foraminous sheet having differently-shaped portions, being a further embodiment of the invention; Figure 14 is a schematic perspective view of another foraminous sheet being an embodiment of the invention; Figure 15 is a cross-sectional view taken along line A-A of Figure 14; Figure 16 is a schematic perspective view showing a variant of the embodiment shown in Figures 14 and 15; Figure 17 is a cross-sectional view taken along line B-B of Figure 16; Figure 18 is a schematic perspective view showing a further variant of the embodiment shown in Figure 14 to 17; and Figure 19 is a cross-sectional view taken along line C-C of Figure 18. Referring firstly to Figures 1, 2 and 3 of the drawings, a foraminous sheet 10 comprises raised regions in the form of rounded hills 12 and depressed regions in the form of rounded hollows 14. The hills 12 extend above the general plane of the sheet 10, and the hollows 14 extend below the general plane of the sheet 10. The hills 12 and the hollows 14 extend smoothly into one another in alternating fashion, and are disposed in regular, straight arrays. Cross-sections through the sheet are generally sinusoidal as shown. Arrays of alternating hills 12 and hollows 14 extend into the page as illustrated, and further arrays extend across the page in orthogonal relation to the first arrays. Neighbouring arrays may be in staggered relation to one another, as shown in Figure 2, or may be in non-staggered, direct or corresponding relation to one another as shown in Figure 1 and Figure 3. The hills 12 and hollows 14 are depicted in Figure 1 as being substantially hemispherical. However, a rather shallower curvature as depicted in Figures 2 and 3 requires less deformation of the foraminous sheet. It is preferred, though not essential, that the foraminous sheet has a combination of hills and hollows. However, it is possible for the sheet to have all hills or all hollows, meaning that all relief formations extend to one side of the general plane of the undeformed sheet. Embodiments having rounded hills 12 and/or hollows 14 are currently preferred, as they minimise stress concentrations to a practical minimum and are easy to form. As will be evident to those skilled in the art, the sheet 10 is flexible in the general plane of the sheet because, in cross-section, it resembles a concertina. Thus, when the sheet 10 tries to expand under heating but is constrained from so doing by the walls of a reactor chamber (not shown), the folds or ridges of the concertina relieve the stress that would otherwise result. Basically, the expansion of the sheet 10 is accommodated by an increase in the incline of the walls that define the hills 12 or hollows 14; thus, the height of the hills 12, and the depth of the hollows 14, increases while the overall diameter of the sheet 10 remains substantially unchanged. Further, because the arrays of hills 12 and hollows 14 extend in mutually transverse, e.g. orthogonal, directions parallel to the general plane of the sheet 10, the sheet 10 is equally flexible in those directions and so accommodates expansion in more than one direction. In our experimental work, we applied a relief pattern to a foraminous sheet by pressing it into contact with an undulating surface defined by arrays of ball bearings. Figures 4, 5 and 6 show various ball-bearing arrangements to produce different relief patterns, the ball bearings together forming press dies. Of course, part-spherical or other shaped elements could be used instead of ball bearings if desired. In each of Figures 4, 5 and 6, it is envisaged that two dies are used, being brought together by a press (as shown in Figure 8, for example) around a foraminous sheet to form it into a desired shape. Solid lines represent the positions of ball bearings on a first die and dashed lines represent the positions of ball bearings on a second die. The sheet depicted in Figure 1 may be formed by the ball- bearing arrangement of Figure 5. In that arrangement, each ball bearing 16 of the first die is isolated, being spaced from its neighbour by approximately the width of a ball bearing. The ball bearings 16' of the second die are similarly arranged. The respective dies are in staggered relation such that the ball bearings of the second die fit into the gaps between the ball bearings of the first die and vice versa, thereby forming the deep undulations of the Figure 1 embodiment. In the arrangement depicted in Figure 4, the ball bearings 16 are in staggered relation, such that each ball bearing 16 is in contact with six other ball bearings 16. The ball bearings 16' of the second die are staggered with respect to the ball bearings 16 of the first die, and are positioned to fit closely into the gaps between the ball bearings 16 of the first die, as if stacked on top of the first die. This arrangement produces a foraminous sheet as depicted in Figure 2. Figure 6 depicts an arrangement in which each ball bearing 16 is in contact with four other ball bearings 16. Again, the ball bearings 16' of the second die are staggered with respect to the ball bearings 16 of the first die and are positioned to fit closely into the gaps between the ball bearings 16 of the first die, as if stacked on top of the first die. This arrangement produces a foraminous sheet as depicted in Figure 3. Of course, only one die may be used, the sheet being deformed into intimate contact with that die by any suitable tools or means. It is also possible for one die to have a resilient or flexible surface such that, when shaping a sheet, the protrusions of the other die press into the resilient or flexible surface. The resilient or flexible surface may be flat. The foraminous sheet of the present invention may be made by a variety of methods, which fall into two basic categories: those that make the sheet and simultaneously apply the desired relief pattern thereto; and those that apply the desired relief pattern to a planar sheet after the sheet has been made. As an example of the former category, it is known to knit fabrics with variable thickness, which can define a relief pattern. Examples of the latter category will now be described. Figure 7 of the drawings illustrates shaping apparatus for applying a desired relief pattern to a foraminous sheet. In Figure 7, rollers 32 are geared together for contra-rotating, synchronised movement. The rollers 32 have complementary shaped formations being part-spherical protrusions 34 and part-spherical indentations 36 angularly spaced and disposed in alternating fashion about the periphery of each roller. As can be seen, the protrusions 34 fit into the indentations 36 at the point of closest proximity between the rollers 32, but do not fill the protrusions: a small clearance remains. A plain foraminous sheet 38 is fed into the apparatus from the left hand side as illustrated and passes between the rollers 32. In so doing, the sheet is deformed by the protrusions 34 and indentations 36 to form a hill 12 or a hollow 14. As the sheet 38 continues to feed through the apparatus, the result is a linear series of alternating part-spherical hills 12 and hollows 14. It will be appreciated that similar formations 34, 36 may be provided on other parts of the rollers 32, thereby to form a two-dimensional array of hills 12 and hollows 14. Of course, it is not essential that part-spherical hills and hollows are formed: a variety of shapes may be formed by the apparatus, provided that the rollers 32 are adapted accordingly. Further, it is not essential that the hills and hollows alternate. Indeed, by providing only indentations in one roller 32 and only protrusions in the other roller 32, a series of only hills or only hollows will result. In a variant of the apparatus shown in Figure 7, one of the rollers 32 has a cylindrical resilient or flexible surface (e.g. of rubber) and the other roller has a series of protrusions which, when deforming a foraminous sheet, press into the resilient surface. Figure 8 illustrates a further shaping apparatus which comprises a first die 40 attached to a fixed bed 42, and a second die 44 attached to a movable press member 46. The dies 40, 44 have complementary shaped formations (shown much exaggerated in size) comprising protrusions 48 and depressions 50. The protrusions 48 and depressions 50 intermesh as shown when the dies 40,44 are brought together around a foraminous sheet 52 and pressed together, and deform the sheet 52 to provide the desired relief pattern. Again, many variations are possible. For example, as mentioned previously, one die may have a flat resilient or flexible surface and the other die may define an array of protrusions which, when shaping a sheet, press into the resilient surface of the first die. In Figure 8, the dies 40, 44 are substantially the same size as the sheet 52 and so can shape the entire sheet 52 in one pressing operation. However, when dealing with larger foraminous sheets in excess of, say, one or two metres in diameter, a correspondingly large pair of dies becomes cumbersome and unduly expensive. So, when shaping large foraminous sheets, it is preferred to employ shaping apparatus as illustrated in Figure 9. Referring to Figure 9, the shaping apparatus therein comprises a horizontal bed 54 having a flexible or resilient upper surface 56 on which a foraminous sheet 58 is laid. A die 60 is mounted on a press member 62 and is movable with the press member both vertically, towards and away from the bed 54, and horizontally, across the bed 54. The die 60 has an array of protrusions (not shown) on its underside, so that in a pressing operation, when lowered and pressed into contact with the sheet 58 atop the bed 54, the protrusions press into the upper surface 56 of the bed 54 and deform the sheet 58 to produce the desired relief pattern. The die 60 is substantially smaller than the sheet 58 and so, on each pressing operation, can produce the desired relief pattern on only a portion of the sheet 58. However, by repeating the pressing operation and, between each operation, moving the die 60 horizontally to a new position above an un- pressed portion of the sheet 58, the whole sheet 58 can eventually be given the desired relief pattern. In Figure 9, four pressing operations have been completed, giving one- quarter of the sheet 58 a relief pattern represented by the shaded area. The die 60 has been moved horizontally across the bed 54 and is poised to press a fresh area of the sheet 58, shown in dashed lines, adjacent the previously-pressed area. It will be evident that a total of sixteen pressing operations will be needed to complete the shaping of the sheet 58. The invention also contemplates applying a relief pattern to a foraminous sheet when the sheet is in use as a catalyst or as a getter. Specifically, as shown in Figure 10, it is envisaged that a shaped support 18 may be positioned within a catalytic reactor 20 in supporting relation behind a normally planar catalyst or getter sheet 22. When the catalytic reactor is in use and the sheet 22 attains its high operating temperature, the sheet 22 slumps into close contact with the shaped surface of the support to follow the shape thereof, as shown in Figure 11. In this way, the support imparts a desired shape, or relief pattern, to the catalyst or getter sheet. The support is suitably made of a ceramic foraminous material and is preferably composed of a plurality of relatively small blocks 18' disposed side-by-side as shown to define the desired shaped area appropriate to the size of the foraminous sheet. The blocks 18' may be in the region of 20 mm to 150 mm deep. For economy of manufacture, the blocks 18' are suitably identical to one another and co-operate with one another to define a repeating pattern across the area that supports the foraminous sheet 22. The blocks 18' are suitably shaped by means of a round-nosed cutter. The blocks 18' as illustrated form the sheet 22 into a shape analogous to that shown in Figure 1. However, of course, other shapes are possible and are envisaged within the scope of the invention. It has already been mentioned that the present invention provides special benefits when applied to getters. These benefits will now be explained with reference to Figures 12(a) and 12(b) . Figure 12(a) depicts, schematically, a row of elements 24 in a conventional getter sheet 26, which sheet 26 is supported in a reactor (not shown) . Usually, the elements 24 are wires of a Pd-based alloy, although this is not essential. Gases are taken to flow through the reactor from top to bottom as illustrated. It will, therefore, be understood that the elements 24 are disposed in a plane that is substantially perpendicular to the direction of gas flow. The elements 24 are spaced apart to define gaps through which the gases flow; the gas flow is represented by the dashed arrow lines in Figure 12(a). Obviously, the smaller the gaps, the higher the resistance to gas flow (i.e. back-pressure) will be. Conversely, if the gaps are widened, the back pressure will decrease. Minimum back-pressure is an aim of the designer, but widening the gaps in pursuit of reduced back¬ pressure can only go so far without unacceptably reducing the gettering ability of the sheet 26. In use in the reactor, the getter sheet 26 is placed downstream of one or more catalytic sheets (not shown) . Consequently, the getter sheet 26 receives a flow of gases that have already passed through the catalytic sheet(s). As previously explained, this flow generally contains particles of catalytic material lost from the catalytic sheet(s); it is the primary job of the getter sheet 26 to recover as much as possible of that valuable catalytic material from the flow. Designers are ever mindful that gettering requires the particles of catalytic material entrained in the gas flow to collide with the material that makes up the getter sheet 26. The paths of two -such particles are shown in Figure 12(a) as solid arrow lines, each of which ends in collision with a getter element and, in all likelihood, diffusion into the getter element, resulting in successful catchment. Clearly, the likelihood of collision must be maximised by maximising the surface area of gettering material presented to the gas flow. So, the designer's challenge is to maximise the surface area presented by the elements 24 while still providing sufficiently large gaps between the elements 24 for gases to flow through the getter sheet 18 without experiencing excessive back pressure. Unfortunately, the getter sheet 26 of Figure 12(a) allows little latitude for design freedom: the elements 24 can only be pushed closer together or spaced further apart, depending upon the amount of back-pressure that is acceptable to the user. The result is always a compromise based upon the relationship between back pressure and surface area. Fundamental improvement depends upon modifying that relationship. Figure 12(b) shows how a relief pattern applied to a foraminous sheet allows reduced back-pressure without seriously damaging its gettering ability (assuming for present purposes that the sheet is a getter sheet) . It will be noted that a sheet having a relief pattern is essentially a series of sheet portions inclined relative to the general plane of the sheet and thus also inclined relative to the general direction of gas flow through the sheet. Figure 12(b) schematically represents one of those portions of the sheet. Figure 12(b) shows how, by inclining at least a portion of a sheet 10 relative to the general direction of gas flow through the reactor, the spacing between elements 24 can be increased without increasing the overall width of the sheet 10 or reducing the number of elements 24. The resulting enlarged gaps present less resistance to the flow of gas through the sheet 10 than the relatively small gaps of sheet 26 shown in Figure 12(a) . It may be expected that the enlarged gaps will cause a corresponding reduction in gettering ability, but that is not the case. It will be noted that the apparent, or projected, spacing between the elements 24 is the same in Figure 12(b) as it is in Figure 12(a); this is the key to gettering ability. Indeed, it will be noted from the solid arrow lines of Figure 12(b) that the particles that collided with the elements 24 of sheet 26 in Figure 12(a) would still collide with the elements 24 of sheet 10 in Figure 12(b), despite their wider actual spacing. Further, it would be wrong to assume that the increased gas flow permitted by the sheet 10 of Figure 12(b) would lead to a corresponding increase in the amount of catalytic material that passes through the sheet 10 without colliding with an element 24. This is because the particles of catalytic material have greater density and thus have greater momentum or inertia than the gas flow in which they are entrained. So, whilst the gas flow can readily swerve sideways as shown, in order better to flow through the angled gaps between elements 24 of sheet 10, any particles entrained therein are more reluctant to change direction and so will follow a straighter course, a path that is more likely to end in collision with an element 24. The mathematical relationship between the gaps A^ of Figure 12(a) and gaps A2 of Figure 12(b) can be expressed as follows. If the portion 10 of Figure 12(b) is inclined at an angle of ΘP to the general plane of the sheet 10 (that is, (90-θ)° to the general direction of gas flow through the reactor) : ±- C*OS-Θ Consequently, A2 is always greater than A-^, by an amount that increases as the angle θ increases. A foraminous sheet having a relief pattern defining inclined portions is also of benefit where the sheet is primarily catalytic, such as a platinum-based alloy of any commonly- available composition. By means of a relief pattern, the catalytic surface area and the apparent wire density of each layer can be increased, thereby increasing the catalytic efficiency of the layer without increasing the back pressure that it causes. Indeed, the number of catalytic layers could be potentially reduced, which cuts the back pressure caused by the pack as a whole. On the other hand, if a given catalytic efficiency is to be maintained, each relief-pattern layer can generate reduced back pressure in comparison with a normal catalytic layer of the same catalytic efficiency. The sheet 26 depicted in Figure 13 shows how shapes may be combined, with different areas, portions or regions of the sheet 26 having different relief patterns applied thereto. In the embodiment illustrated, an outer annular portion 28 comprising circular or spiral ridges surrounds a central portion 30 which has a non-ridged pattern, preferably a dimpled pattern as shown in any of Figures 1 to 3. Figures 14 to 16 illustrate other relief patterns that may be applied to a foraminous sheet 10. In the variant shown in Figures 14 and 15, the hills 12 and hollows 14 are frusto-pyramidal, defining flat tops 64 and flat bottoms 66. Fully pyramidal hills 12 and hollows 14 are illustrated in Figure 16 and 17. Though feasible for forming a relief pattern, the full pyramids increase stress concentration in relation to the frusto-pyramidal shapes of Figures 14 and 15, and so are currently less favoured. In the variant shown in Figures 18 and 19, the hills 12 are pyramidal but are separated by flat regions 68 which do not extend below the general plane of the sheet. Nevertheless, the flat regions 68 may be described as valleys, relative to the hills 12. This variant is easier to form than a sheet having hills 12 and hollows 14. Whilst the sheet of Figures 18 and 19 has been described as defining an array of spaced-apart hills, it will be obvious that this depends upon the orientation of the sheet with respect to an observer. The sheet can be inverted and will then define an array of spaced-apart hollows.

Claims: CLAIMS 1. A catalytic reactor containing at least one foraminous sheet supported in an orientation transverse to the direction of flow of reactants through the reactor, wherein the foraminous sheet is a getter, a catalyst or a support and has a relief pattern applied thereto. 2. A catalytic reactor according to claim 1, wherein the foraminous sheet is a getter or a catalyst and is supported by a foraminous support having a relief pattern applied to a support surface thereof, the support being positioned to apply the relief pattern to the sheet while it supports the sheet during operation of the reactor. 3. A foraminous sheet having a relief pattern applied thereto, and being suitable for use as a getter, a catalyst or a support in transverse orientation within a catalytic reactor as defined in claim 1. 4. A foraminous sheet according to claim 3, wherein the relief pattern is adapted to impart flexibility to the sheet in at least two mutually transverse directions parallel to the general plane of the sheet. 5. A foraminous sheet according to claim 3 or claim 4, wherein the relief pattern includes raised regions and/or depressed regions, viewed in relation to the general plane of the sheet. 6. A foraminous sheet according to claim 5, wherein said regions are disposed in at least one array. '7. A foraminous sheet according to claim 5 or claim 6, wherein said regions define a plurality of isolated or separate relief formations. 8. A foraminous sheet according to claim 6 or claim 7, wherein the array is linear, annular, spiral, one-dimensional or two- dimensional. 9. A foraminous sheet according to any of claims 5 to 8, wherein said regions are rounded. 10. A foraminous sheet according to claim 9, wherein said regions are part-spherical. 11. A foraminous sheet according to any of claims 5 to 10, wherein said regions merge smoothly into neighbouring ones of said regions. 12. A foraminous sheet according to any of claims 3 to 11 and being of generally sinusoidal cross-section. 13. A foraminous sheet according to any of claims 3 to 12, at least a portion of which has a dimpled shape or configuration. 14. A foraminous sheet according to any of claims 3 to 13, and including different portions having respectively different shapes or configurations. 15. A foraminous sheet according to claim 14, wherein one of said portions surrounds another of said portions. 16. A foraminous sheet according to any of claims 3 to 15, and being primarily a getter sheet or a catalyst sheet. 17. A foraminous sheet according to claim 16, and being of palladium or a palladium-based alloy, or platinum or a platinum-based alloy. 18. A pack of foraminous sheets for use in catalysis, in which at least one sheet of the pack is as defined in any of claims 3 to 17. 19. The use of a foraminous sheet as defined in any of claims 3 to 17 or a pack as defined in claim 18, in a catalytic process. 20. Chemical products resulting from use as defined in claim 14. 21. A method for producing a foraminous sheet as defined in any of claims 3 to 17, comprising applying a relief pattern to the sheet as the sheet itself is created. 22. A method according to claim 21, wherein the relief pattern 0 is created as a result of a knitting or weaving process which creates the sheet. 23. A method for producing a foraminous sheet as defined in any of claims 3 to 17, wherein the relief pattern is applied to the sheet after the sheet has been created. 24. A method according to claim 23, comprising plastically deforming an otherwise flat sheet with a press, the press comprising cooperable press members between which at least a portion of the sheet is pressed, at least one of the press members having a surface shaped to impart a desired relief 0 pattern to the sheet thereby to produce an embossed surface having the desired relief pattern. 25. A method according to claim 24, wherein the sheet is laid upon and/or supported by one press member, and the other press member is brought into contact with the first press member to 5 shape the sheet as desired. » 26. A method according to claim 24 or claim 25, comprising shaping the entire sheet in one pressing operation. r 27. A method according to claim 24 or claim 25, comprising pressing different regions of the sheet in turn until the whole sheet has been shaped as desired. 28. A method according to claim 24, wherein at least one of the press members is a roller, relative movement between the or each roller and the sheet developing the relief pattern as the roller traverses the sheet. 29. A press for use in the method defined in any of claims 24 to 28, wherein one press member has a surface that is flexible and resilient to conform to the shaped surface when the press members are brought together. 30. A foraminous support having a relief pattern applied to a supporting surface thereof and being suitable for use in a catalytic reactor as defined in claim 2. 31. A foraminous support as defined in claim 30 and comprising a plurality of elements each defining a portion of the supporting surface. 32. A foraminous support as defined in claim 31, wherein each of said plurality of elements is identical. 33. A method for applying a relief pattern to a foraminous sheet, comprising positioning the foraminous sheet over a shaped support, and heating the foraminous sheet to cause it to slump onto the shaped support. 34. A method according to claim 33, wherein heating is performed in a catalytic reactor.

Inventors: CRANSTON JOSEPH JAMES, HEYWOOD ALAN EDWARD, SCORGIE ALAN, CRANSTON, JOSEPH, JAMES, HEYWOOD, ALAN, EDWARD, SCORGIE, ALAN
Applicants: CRANSTON JOSEPH JAMES, HEYWOOD ALAN EDWARD, PGP IND INC, SCORGIE ALAN, PGP INDUSTRIES, INC.
Application year: 1993
Main classification: B01J35
IPC codes: B01D53, B01J19, B01J35, C01B21, C01C3