Method for the production of noble metal nets for ammonia oxidation

Abstract

The invention relates to a method for producing noble metal nets (11) for ammonia oxidation. The method according to the invention allows for the robust and simple production of distortion-free nets (11) using a particular winding process.

Description

[0001] Process for the production of precious metal meshes for ammonia oxidation Description

[0002] The present invention describes a method for producing precious metal meshes for ammonia oxidation. The method according to the invention allows for the robust and simple production of distortion-free meshes through a special winding process.

[0003] Noble metal-catalyzed gas reactions, such as the oxidation of ammonia with atmospheric oxygen in nitric acid production (Ostwald process) or the reaction of ammonia with methane in the presence of oxygen to form hydrogen cyanide (Andrussow process), have long been of considerable industrial importance. These processes provide basic chemicals for the chemical industry and for fertilizer production on an industrial scale (Andreas Jess, Peter Wasserscheid: Chemical Technology; Wiley-VCH Verlag, Weinheim 2013, Chapter 6.4).

[0004] At the heart of these heterogeneously catalyzed gas reactions are precious metal catalysts in the form of gas-permeable three-dimensional structures on or within which the reaction takes place. For some time now, nets in the form of woven (DE4028916C2), knitted (DE4300791A1; EP606535A1) or crocheted (EP364153B1, DE4206199C1) structures made of fine precious metal wires have become established.

[0005] The catalyst meshes are typically arranged in a flow reactor in a plane perpendicular to the flow direction of the gas mixture. Conical arrangements are also known. It is advantageous to arrange several meshes one behind the other and combine them into a mesh stack.

[0006] The reaction gas or fresh gas (ammonia-air oxygen mixture with an ammonia content of 9 - 13 vol%) flows through the mesh stack under atmospheric or elevated pressure, whereby ignition of the gas mixture takes place in the inlet area and the combustion reaction to nitrogen monoxide (NO) and water covers the entire reaction zone:

[0007] 4 NH3 + 5 O2 (air) 4 NO + 6 H2O

[0008] Undesirable side reactions include the oxidation of ammonia to nitrogen and nitrous oxide (N2O), the former merely reducing the yield of NO, while the latter is also a potent greenhouse gas: 4 NH3 + 3 O2(air) 2 N2 + 6 H2O

[0009] 4 NH3 + 4 O2 (air) 2 N2O + 6 H2O

[0010] The NO in the outflowing reaction gas mixture subsequently reacts with the excess atmospheric oxygen to form NO2:

[0011] An undesirable side effect here is also the formation of nitrous oxide:

[0012] 2 NO + 1 2O2 -> 2 N2O

[0013] The NO2 then reacts with water in a subsequent absorption process to form nitric acid, which is used, for example, in fertilizer production:

[0014] For the production of precious metal meshes, precious metal wires made of platinum, rhodium, or alloys of these metals with other precious or base metals are used. Platinum-rhodium or platinum-palladium-rhodium alloys with 88 to 98 wt.% platinum are typical. Platinum is required to achieve the highest possible ammonia conversion, while rhodium improves the selectivity for NO, thereby reducing nitrous oxide emissions and increasing the mechanical strength of the meshes (GR Maxwell: "Synthetic Nitrogen Products - A Practical Guide to the Products and Processes", Springer Science + Business Media, Inc. 2005, p. 220). Palladium, in turn, is used to reduce precious metal loss by forming more stable alloys and, depending on current precious metal prices, to lower precious metal costs by replacing less platinum.

[0015] Several patent publications have already been released on the topic of precious metal meshes for ammonia oxidation. For example, US5266293A describes a knitted precious metal fabric in which the precious metal is selected from platinum group metals, gold and silver, and alloys thereof. EP544710A1 describes a knitting process for producing precious metal meshes, which offers several advantages over weaving.

[0016] The devices used for knitting precious metal mesh are known in principle from the field of textile knitting processes and can be adapted to the conditions of wire knitting (US5188813; W09202301A; US5931023). Flat knitting machines are preferably used for knitting precious metal mesh (DE4206199C1). The flat knitting machine preferably has a front and a rear needle bed in which the tongue needles, which carry out the knitting process, are installed. The tongue needles pass through different positions depending on the machine's programming. The programming thus determines the structure of the knitted fabric. A special feature of the flat knitting machine compared to other fabric-forming machines is that knitted fabrics can be formed synchronously and independently on both the front and rear needle beds (single-bed fabric).The two knitted pieces can be joined together on one side, on both sides and / or in the middle through the knitting process (double-bed fabric).

[0017] The aforementioned DE4206199C1 relates to a process for producing gas-permeable nets made of precious metals for the catalytic oxidation of ammonia. Precious metal nets obtained by knitting wires are used. The wire diameters range from 50 to 120 pm. They can preferably be produced on flat knitting machines, with a needle pitch (distance between the needles on the flat knitting machine) between 3.63 mm and 1.81 mm and a stitch length between 2 and 6 mm. Multi-layered nets in net stacks can also be advantageously used for the oxidation reaction under consideration (EP680787A1).

[0018] Metal wires do not possess the same elasticity as other textile threads, such as polyester. This leads to mechanical stress on the wire during the knitting process, for example, of precious metal mesh on flat knitting machines. This stress is caused by jerky movements of the carriage and during the stitch formation process (pulling the needle on the wire loop). Such stress can lead to wire breakage and thus knitting defects. Therefore, the wire should be guided above the needle bed with as constant a tension as possible during knitting.

[0019] It is equally important that the precious metal mesh to be produced has a homogeneous stitch pattern, is knitted evenly, and has no bunching or distortion of the stitch rows. During the knitting process, the material sags beneath the flat knitting machine, and its own weight creates varying tensile forces across its width, which can lead to inhomogeneities that may be detrimental to its later use in the reactor. Material accumulation and bunching can also occur at the edges of the knitted fabric. In the worst case, this leads to defects in the knitting because the excess material prevents the loops from being released from the needle hook.

[0020] These and other problems arising from the known prior art for a person skilled in the art are solved by a method that is the subject of claim 1. Claims 2 to 6 are preferred embodiments of the method according to the invention.

[0021] A method for producing precious metal meshes for ammonia oxidation is proposed, wherein at least one mesh is knitted with precious metal-containing wires on flat knitting machines having two needle beds. The flat knitting machine includes at least one winding roller located below the needle beds, which winds up the precious metal mesh(es) after the knitting process. The winding speed of the winding roller(s) is controlled by the knitting speed or the knitting process itself. The knitting speed or knitting process is determined by at least one light barrier that detects the passage of the wire guide and advances the winding roller radially by 0.2–2.0 mm for each passage of the wire guide. This method advantageously and surprisingly solves the problem at hand. The present invention improves the quality and product properties of the precious metal meshes.Likewise, the manufacturing process can be more robust, meaning less wear and tear on needles and machine components due to errors and material build-up, and can be carried out without interruptions, which helps to reduce production costs. Further handling of the precious metal meshes in production is also easier, as the rolls are, for example, easier to stack and more resistant to kinking.

[0022] Flat knitting machines, or flatbed knitting machines, are already known to those skilled in the art for knitting precious metal wire (DE4206199C1). The construction of the flat knitting machine is illustrated in Fig. 2. The flat knitting machine (14) has a front (8) and a rear needle bed (9) in which the tongue needles (10) are installed. The yarn or precious metal wire is fed by a wire guide (13). If several wire guides are used, several precious metal wires can be knitted synchronously. The tongue needles pass through different positions depending on the machine's programming. The programming thus determines the structure of the knitted fabric via the movement of the tongue needles. A special feature of the flat knitting machine compared to other fabric-forming machines is that knitted fabrics can be formed synchronously on both the front and rear needle beds independently of each other (single-bed fabric).Furthermore, spaced knitting (double-bed knitting) can be created, in which the precious metal wire alternately forms stitches or loops at the front and back (EP1358010B2). The knitting is worked downwards between the two needle beds (11). This is done by successively knocking off the individual formed stitches over the knock-off position and knock-off edge (12).

[0023] Knitted precious metal meshes offer several advantages over woven precious metal meshes, which is why they are now preferred in industrial applications. Firstly, knitting technology offers a high degree of flexibility with regard to knitting patterns, the thickness of precious metal wire used, and the resulting basis weight. Secondly, knitted precious metal meshes can be produced more economically, as knitting requires shorter setup times than weaving. This results in a significantly reduced amount of precious metal used in production. Precious metal meshes of any length can be produced on flat knitting machines. However, the minimum mesh size, i.e., the density of the knitted fabric, is limited by the maximum number of needles per given width.

[0024] The precious metal meshes are stacked in the so-called mesh stack, which is placed in the reactor (Fig. 1). The term "precious metal meshes" refers to the entirety of the catalyst and getter meshes. "Separation meshes" are meshes made of high-temperature-resistant steel, which are installed between the precious metal meshes to prevent them from sintering together. The mesh stack consists of the precious metal-containing catalyst meshes on the inlet side of the mesh stack, and optionally the getter meshes on the outlet side, as well as the separation meshes, which may be installed between the precious metal meshes. In this context, "precious metals" refers to gold, silver, and the platinum group metals (Ru, Rh, Pd, Os, Ir, Pt). Preferably, the wires consist of platinum-rhodium alloys with 3 to 12 wt.% rhodium or wires of platinum-palladium-rhodium alloys with 3 to 38 wt.% palladium and rhodium.

[0025] A flat knitting machine typically has several units. At least one unit is responsible for supplying the wire yarn or yarns at the required speed for the knitting process. Those skilled in the art know which devices are advantageously available for this purpose. For example, tangential unwinders or brush unwinding systems for the so-called overhead unwinding of the wire from the respective spool can preferably be used (e.g., https: / / www.mobac.de / wickelmaschinen / flyer-und-drahtablaeufe; https: / / wiretec.ch / ; https: / / www.winding-technology.com / ; or Flexible Automation Solutions for the Production of Winding Products, author: Andreas Dobroschke, Fertigungstechnik Erlangen - ISBN 978-3-87525-317-7).

[0026] A second unit is preferably included in the device to maintain sufficient, but not excessive, tension on the wire. These wire tensioners are familiar to those skilled in the art from textile knitting. Wire tensioning devices are also known in the winding technology of metal wires. The tension on the wire can preferably be applied by tension or compression. More preferably, tensile tension is applied. The tension of the wire or wires is adjusted so that the wire does not break under tensile stress and is gently fed into the machine with a constant tensile force. This reduces the use of guide rollers and the resulting friction. However, the tension must be sufficiently high so that the wire is not too loose and does not sag, as it could otherwise become entangled in the needle bed. The same applies to the use of a pressure regulator.The tension is therefore preferably applied mechanically by pulling or pushing on a wire tensioner arm that encompasses the wire. Those skilled in the art know how this can be done (https: / / www.karlmayer.com / de / produkte / kettvorbereitung / gatter / fadenspanner-waechter / ; https: / / www.supertek.de / produkte-und-services / wickeltechnik / zugkraftregler).

[0027] Another advantageous component is the wire guide. This serves to feed the precious metal wire to the needles. Precise positioning is crucial so that the needles can grip the precious metal wire and no insertion error occurs, which would result in a knitting defect in the product. Such devices are well known to those skilled in the field of knitting machines (see, for example, the links above). An advantageous wire guide is mentioned, for example, in German standard 051705.

[0028] As already mentioned, the precious metal wire is subjected to bending stress and friction during the knitting process. In particular, the wire guide (13), which guides the precious metal wire in the correct position to the needles (10), and the wire tensioner, which must maintain tension on the wire, exert stress on the precious metal wire. This is because the precious metal wire is deflected by approximately 90° at the wire exit point via an eyelet and runs over the eyelet edge at high speed. This can damage the surface of the precious metal wire and possibly even lead to wire breakage. In any case, it contributes to wear on the wire guide eyelet. Furthermore, due to its bending stiffness, precious metal wire is more difficult to hold in position than a textile thread, which can result in insertion errors during the stitch formation process of knitting.Advantageously, at least one wire guide and / or wire tensioner is therefore equipped with a deflection roller for the wire.

[0029] A guide roller preferably consists of a wheel mounted on an axle with minimal friction, over which a precious metal wire is guided. This minimizes friction between the precious metal wire and the guide roller, thereby minimizing wear on the wire and reducing the risk of wire breakage. Advantageously, the wire guide is moved back and forth above the tongue needles, so that the exit direction of the precious metal wire changes by 180° at each end of the knitting bed. Preferably, the wire guide is equipped with two guide rollers at its end, the axes of which run parallel, with the precious metal wire exiting the guide between the guide rollers. Thus, the precious metal wire is preferably guided over a guide roller in both directions of movement without the need to rotate the wire guide.

[0030] The guide pulley should rotate as easily and quickly as possible to minimize friction between the precious metal wire and the pulley. Therefore, a particularly advantageous wire guide is equipped with guide pulleys featuring plain bearing bushings. Teflon or bronze are suitable materials for the plain bearing bushings because they offer low friction and a long service life. Plain bearings have the advantage over roller bearings of being less sensitive to shocks and vibrations, and less susceptible to contamination. Furthermore, their design is very simple.

[0031] However, the guide rollers are preferably equipped with rolling bearings, as these, although technically more complex, can further reduce the friction between the precious metal wire and the guide roller. Ball bearings are particularly preferred because they provide axial stability and can also reduce the friction generated by the axial forces acting on the guide roller. Steel is a particularly suitable material for rolling bearings because of its wear resistance. For the same reason, steel is also very suitable for the guide roller itself. In this case, the guide roller can simultaneously serve as the outer ring of the rolling bearing.

[0032] Preferably, the deflection roller(s) are guide rollers that have a radially circumferential groove in the center of their outer running surface, preventing the wire or thread from slipping off the roller. Wire slippage necessitates an interruption of the knitting process and must be avoided at all costs. This groove preferably has a width of 0.2 to 2 mm. Preferably, the groove has a circular cross-section with a radius of curvature greater than the groove width. This prevents damage to the wire surface from the edges running on both sides of the groove, thus preventing potential wire breakage.

[0033] Advantageously, after leaving the wire feeding unit and preferably after leaving the wire tensioning unit and before coming into contact with the wire guiding unit, the wire or wires are fed to a further unit of the device for the application of a lubricant. This further unit is positioned so that the at least one wire can come into contact with the lubricant. The unit discussed here can be designed according to the specifications of those skilled in the art. If in doubt, it is a container with the lubricant, e.g., a tray containing, for example, oil, through which the at least one wire is guided. By contacting the wire with a lubricant, the friction and thus the wear of the components of the device according to the invention and of the wire itself are minimized, which helps to minimize wire breakage and abrasion of the further components of the device according to the invention.

[0034] In a highly preferred embodiment, the oil is an oil-water emulsion. The types of oil mentioned above can be used. The emulsion should preferably contain 50-95 wt.%, preferably 70-90 wt.% water. Advantageously, further additives, such as emulsifiers, can be present in the emulsion. These are known to those skilled in the art (e.g., https: / / kluthe.com / magazine / emulsifiers-for-production-of-cooling-lubricants / ). Others include, for example, petroleum sulfonates, alkali soaps, and amine soaps (for example, triethanolamine salts of fatty acids).

[0035] The precious metal wire is preferably guided from the wire feeder, via a wire tensioner, to the wire guide before being picked up by a tongue needle of the flat knitting machine. The wire enters the wire tensioner at a specific speed. In a preferred embodiment, the device used here is designed such that, during operation, the entry speed of the at least one wire into the wire tensioner is between 0.05 and 2 m / s, in particular between 0.20 and 1 m / s, preferably between 0.25 and 0.75 m / s. These speeds ensure that the wire is supplied quickly enough for an efficient knitting process without the risk of breakage.

[0036] The knitting speed is correlated with the wire feed speed. The knitting speed is between 0.1 and 1 m / s, preferably 0.3 to 0.5 m / s. This reflects the movement speed of the carriage(s). The carriage is the component on the flat knitting machine that carries the wire guides across the needle bed(s).

[0037] One knitting cycle means the completion of one row of stitches per stroke, i.e., one pass of the wire guides from left to right and back again over the needle beds (front and back). Preferably, 1 to 6 rows of stitches are knitted per wire guide cycle, more preferably 1 to 4, and most preferably 1 to 3 rows. The winding roller speed is adjusted according to the number of rows of stitches knitted per wire guide cycle.

[0038] The knitting speed, or rather the progress of the knitting process, is relevant because, according to the invention, it can control the winding speed of the winding roller (15). In a preferred embodiment of the present invention, the knitting speed is therefore preferably determined, and the winding speed of the winding roller is controlled based on this. Those skilled in the art know how to proceed. The passage of the wire guide (knitting process), or equivalently, the passage of the carriage, is determined by means of one or more light barriers that detect the passage of the wire guide / carriage. Whenever the wire guide or carriage moves once, for example, from left to right (and back again) across the double needle bed and through the light barrier(s), a signal is triggered. This signal then triggers the rotation of the winding roller.In one embodiment of the invention, the speed of the winding rollers per passage of the wire guide / carriage through the light barrier(s) is a radial speed of 0.2–2.0 mm, more preferably 0.25–1.5 mm, and most preferably 0.3–1.0 mm. When winding the mesh, care must be taken to ensure that the radius of the winding increases. The mm measurement therefore refers to the radial distance over which the precious metal mesh is wound.

[0039] All of this can, of course, also be done using a computer. The computer can be programmed to optimally adjust the winding speed of the yarn spool to the knitting speed or process, and the stitch size to the type of yarn used. This ensures consistent stitch sizes and prevents distortions in the stitches that would compromise the quality of the expensive product.

[0040] Once the net is fully wound, it is removed from the machine and preferably unwound onto a table for further processing (oil removal and cutting to size). Alternatively, the winding roll(s) can also be stacked very efficiently. Furthermore, the inventive method preferably uses a winding roll that can be radially controlled in both directions to allow the knitted fabric to be unwound or discharged. This makes it easy to prepare the knitted net for further processing.

[0041] It should be noted that, according to the invention, two winding rollers can also preferably be positioned below the flat knitting machine. On the double-bed flat knitting machines, two nets can be knitted in parallel. Thus, a first net can be wound onto one winding roller and a second net onto a second winding roller, according to the invention. The above descriptions for one winding roller therefore also apply, mutatis mutandis, to a method with preferably two winding rollers below the flat knitting machine.

[0042] In one embodiment of the prior art, the winding of a knitted textile net is controlled by a tensile force (DE3707141A1). As our own tests show, such an embodiment appears to be less suitable for wire nets.

[0043] In contrast, the method presented here allows for the simple yet effective production of precious metal-based catalyst meshes. According to the invention, after knitting, the meshes are immediately wound onto at least one winding reel positioned below the needle beds of the flat knitting machine. This provides them with significantly better protection against distortion, compression, or kinking. Without such a device, the knitted fabric and its stitches become distorted, resulting in an uneven knit and an inhomogeneous basis weight due to distorted stitches. This leads to improved product quality in the subsequent ammonia production process. Furthermore, such a reel—similar to carpets—is easier to transport and handle. This was not foreseeable with the prior art.

[0044] Figures

[0045] Fig. 1: Structure of a flow reactor for the oxidation of ammonia.

[0046] Fig. 2: Side view of a flat knitting machine for producing nets including wire guides.

[0047] Fig. 3: Flat knitting machine with 2 needle beds and winding roller with yarn guide and light barrier

[0048] Fig. 4: Knitting process with two opposing needle beds

[0049] Fig. 5: Net according to the present invention with homogeneous entanglement

[0050] Fig. 6: State-of-the-art mesh with relatively inhomogeneous knitting

[0051] Numbering:

[0052] 1. Flow reactor

[0053] 2. Reaction zone

[0054] 3. Net stack

[0055] 4. Catalyst network stack

[0056] 5. Getter net stack

[0057] 6. Reaction gas

[0058] 7. Products

[0059] 8. front needle bed

[0060] 9. posterior needle bed

[0061] 10. Tongue needles

[0062] 11. knitted product

[0063] 12. Teeing-off edge

[0064] 13. Wireman

[0065] 14. Flat knitting machine

[0066] 15. Winding roller

[0067] 16. Meshes of precious metal wire

[0068] 17. Sled with 2 thread guides

[0069] 18. Light barrier example:

[0070] A wire mesh was knitted on a flat knitting machine with two needle beds (Fig. 2). The winding roller mounted under the needle beds wound the wire mesh in such a way that, when the carriage with the wire guide passed through a light barrier, the winding roller was advanced a certain radial distance (see claim 1) (Fig. 3). Fig. 5 shows the quality of such a mesh.

[0071] In contrast, a wire mesh was knitted on the same knitting machine. Here, the mesh was moved downwards by a tensile force. This simulates a tension-controlled winding process according to the prior art (e.g., DE7422930U). Figure 6 illustrates the inferior quality of this mesh. This knitting process also resulted in more frequent breakages.

Claims

Patent claims 1. A method for producing precious metal meshes for ammonia oxidation, wherein at least one mesh is knitted on flat knitting machines having two needle beds with precious metal-containing wires, and wherein the flat knitting machine has at least one winding roller which is mounted below the needle beds and which winds up the precious metal mesh(es) after the knitting process, the winding speed of the winding roller(s) being controlled by the knitting speed or by the knitting process, characterized in that the knitting speed or knitting process is determined by at least one light barrier which detects the passage of the wire guide and moves the winding roller radially by 0.2 - 2.0 mm for each passage of the wire guide through the light barrier.

2. Method according to claim 1, characterized in that the wires consist of platinum-rhodium alloys with 3 to 12 wt.% rhodium or wires of platinum-palladium-rhodium alloys with 3 to 38 wt.% palladium and rhodium.

3. Method according to one of the preceding claims 1 or 2, characterized in that the winding roller can be controlled radially in both directions in order to unwind or throw off the knitted fabric.

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