Method for producing noble metal meshes for ammonia oxidation

Abstract

The invention relates to a method for producing noble metal meshes for the catalytic oxidation of ammonia. By omitting selected needles from the knitting process, the meshes can be produced in a way that more carefully preserves the material.

Description

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

[0002] Description

[0003] The present invention relates to a process for producing precious metal meshes for the catalytic oxidation of ammonia. The meshes can be produced in a more material-friendly manner by selectively omitting needles for the knitting process. Precious 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) 2N2O + 6H2O

[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 on the topic of precious metal meshes for ammonia oxidation have already been released. US5266293A, for example, describes a knitted precious metal mesh 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 fabrics 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] Similarly, metal wires place a strain on the needle hooks, which develop indentations due to friction with the wire. These indentations can later cause wires to snag and break off. This leads to defects in the net, production downtime, or even the need to produce a new net. Due to wear and tear on the needle hooks, they must be replaced after a certain production run. Besides the cost of purchasing new needles, machine downtime must also be accepted during needle replacement. These and other problems arising from the known prior art for those skilled in the art are solved by a method that is the subject of claim 1. Claims 2-7 are preferred embodiments of the method according to the invention.

[0020] By programming a flat knitting machine to selectively skip needles within a row of precious metals, and thus achieving a beneficial and surprising solution to the problem, the present invention positively influences the throughput and the quality of the nets and production during the knitting of precious metals. This is achieved in a process for manufacturing precious metal nets for ammonia oxidation, where at least one net is knitted on a flat knitting machine comprising two needle beds with precious metal-containing wires. The present invention also improves the throughput and net quality during production. The manufacturing process becomes more robust and can be carried out with fewer interruptions, which helps to reduce production costs. The replacement of worn needles, and therefore the maintenance costs of the flat knitting machine, are reduced.

[0021] 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 yarn guide or wire guide (13). If several yarn guides are used, several precious metal wires can be knitted synchronously. Depending on the machine's programming, the tongue needles pass through different positions. The programming thus determines the structure of the knitted fabric via the movement of the tongue needles.

[0022] 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-spaced fabrics (double-bed fabric) can be produced, in which the precious metal wire alternately forms stitches or loops at the front and rear (EP1358010B2). The knitted fabric 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 this could cause it to 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 / kettbereitung / gatter / fadenspanner-waechter / ; https: / / www.supertek.de / produkte-und-services / wickeltechnik / zugkraftregler).

[0027] Another advantageous component is the wire guide or yarn 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 yarn guide is mentioned, for example, in [reference to be added].

[0028] As already mentioned, the precious metal wire is subjected to bending stress and friction during the knitting process. In particular, the yarn guide (13), which guides the precious metal wire to the needles (10) in the correct position, and the wire tensioner, which must maintain tension on the wire, place 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 yarn 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, the at least one wire guide and / or the wire tensioner is therefore equipped with a deflection roller for the wire. A deflection roller preferably consists of a wheel that is mounted on an axle with minimal friction, and over which a precious metal wire is guided. This minimizes the friction between the precious metal wire and the deflection roller, thereby minimizing wear on the precious metal wire and thus reducing the risk of wire breakage. The thread guide is advantageously 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 used. Preferably, the thread guide is equipped with two deflection rollers at its end, the axes of which run parallel, with the precious metal wire exiting the thread guide between the deflection rollers.Thus, the precious metal wire is preferably guided over a deflection pulley in both directions of movement without the need to rotate the thread guide.

[0029] The guide pulley should rotate as easily and quickly as possible to minimize friction between the precious metal wire and the pulley. Therefore, an advantageous version of the thread guide is equipped with guide pulleys featuring plain bearing bushings. Teflon or bronze are suitable materials for the plain bearing bushings because they run with low friction and ensure 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.

[0030] 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 particularly suitable as a rolling bearing material because it is resistant to wear. For the same reason, steel is also very suitable for the guide roller. In this case, the guide roller can simultaneously serve as the outer ring of the rolling bearing.

[0031] 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.

[0032] 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 a person 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.

[0033] In a highly preferred embodiment, the oil is an oil-water emulsion. The oil types 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 (e.g., triethanolamine salts of fatty acids).

[0034] 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.

[0035] In this process, needles within a row being knitted are specifically excluded from the knitting of the precious metal mesh. Which needles are omitted can be programmed into the flat knitting machine. All modern flat knitting machines are equipped with a corresponding programming unit (Lit... https: / / www.stoll.com / en / products-and-solutions / software / ). The programming can preferably be done in such a way that specific needles are omitted. However, care should be taken to ensure that the needles to be omitted are not all located next to each other, as this could result in excessively large stitches in the mesh. Furthermore, the programming can be configured so that needles omitted in the previous row are not omitted again in the row being knitted. This allows for a controlled, but even, reduction of needle wear.Alternatively or cumulatively, it may also be preferable to specify in the programming how often needles should be skipped. It can therefore be advantageous to program the flat knitting machine so that, for example, only every second or every third needle is used. This can be designed, for instance, by shifting the knitting program by one needle at a time. That is, considering the width of a flat knitting machine, the second needle is not used in the first mesh. In the subsequent mesh, this same needle becomes the first needle. It may also be preferable to skip every third or fourth, or even more preferably, every third needle. That is, in the pattern sequence, needles 1 and 2 knit, but not 3; 4 and 5 knit, but not 6; and so on.

[0036] If individual needles are skipped during the knitting of a row, this reduces the stitch density. A lower stitch density implies a lower basis weight in the finished net. Therefore, the basis weight of the nets can be advantageously controlled by skipping needles using the method according to the invention. The basis weight of the precious metal nets can influence the oxidation process. A preferred embodiment of the present invention is one in which the basis weights of the nets are between 250 and 1850 g / cm². 2 , preferably 350 - 1650 g / cm² 2 and especially preferred between 400-1450 g / cm² 2 The amount of precious metal wire available for oxidation can be determined via the basis weight, and flow-related special features can be established in the network.

[0037] Alternatively or cumulatively, it is also preferred to control the basis weight of the nets by knitting with different wire thicknesses. Multi-strand knitting is particularly advantageous in this regard. This means that several wires or threads are involved in the knitting process. Wires with different precious metal compositions and / or different thicknesses can then be used simultaneously. Alternatively, several identical wires can be used at the same time. According to the invention, wires with a diameter between 50 and 125 pm, more preferably 55 and 105 pm, and most preferably 60 and 92 pm, are generally used. As already mentioned, the amount of precious metal wire available for oxidation can be determined via the basis weight, and furthermore, flow characteristics can be established within the net.

[0038] In a particularly preferred embodiment, it has proven advantageous to knit in such a way that one needle catches two wires. It has been found that catching two wires results in less wear on the needles. This is probably due to the fact that with two wires, the wires do not always rub against the same point in the needle hook, but rather at random points. Over the course of the knitting, this leads to less damage to the needle hook.

[0039] The method presented here enables the simple yet effective production of precious metal-based catalyst meshes. According to the invention, the meshes are produced with minimal wear on flat knitting machines. This results in fewer production failures and improved mesh quality. Consequently, the product quality in the subsequent ammonia production process is better. This was not anticipated given the existing state of the art.

[0040] Figures:

[0041] Fig. 1: Structure of a flow reactor for the oxidation of ammonia.

[0042] Fig. 2: Side view of a flat knitting machine for producing nets including yarn guides.

[0043] Numbering:

[0044] 1. Flow reactor

[0045] 2. Reaction zone

[0046] 3. Net stack

[0047] 4. Catalyst network stack

[0048] 5. Getter net stack

[0049] 6. Reaction gas

[0050] 7. Products

[0051] 8. front needle bed

[0052] 9. posterior needle bed

[0053] 10. Tongue needles

[0054] 11. knitted product

[0055] 12. Teeing-off edge

[0056] 13. Thread guide

[0057] 14. Flat knitting machine

Claims

Patent claims 1. Method for producing precious metal meshes for ammonia oxidation, wherein at least one mesh is knitted on a flat knitting machine having two needle beds with precious metal-containing wires and wherein the flat knitting machine is programmed such that needles within a row to be knitted are selectively omitted.

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, characterized in that only every second or every third needle is used.

4. Method according to one of the preceding claims, characterized in that the basis weight of the nets is controlled by omitting needles.

5. Method according to one of the preceding claims, characterized in that the basis weight of the nets is controlled by knitting with different wire thicknesses.

6. Method according to one of the preceding claims, characterized in that the knitting is done with multiple strands.

7. Method according to claim 6, characterized in that a needle catches two wires.

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