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US-12616967-B2 - Precious metal mesh for catalyzing gas-phase reactions

US12616967B2US 12616967 B2US12616967 B2US 12616967B2US-12616967-B2

Abstract

The invention relates to a precious metal mesh which is knitted on a flat-bed knitting machine, having at least two float stitches per wale, thus having a significantly higher density for a given latch needle density of the flat-bed knitting machine than the precious metal mesh according to the prior art.

Inventors

  • Dirk BORN
  • Dieter Prasch
  • Artur WISER

Assignees

  • UMICORE AG & CO. KG

Dates

Publication Date
20260505
Application Date
20211116
Priority Date
20201117

Claims (17)

  1. 1 . A method for the catalytic oxidation of ammonia, comprising the following steps of providing one or more precious metal meshes, wherein at least one of the provided one or more precious metal meshes is a knitted precious metal mesh that comprises n wires per row and at least two, at most (n−1) float stitches per wale, wherein: n>3; installing a mesh stack containing said knitted precious metal mesh in a flow reactors; supplying a gas containing ammonia and oxygen at a temperature in the flow reactor of between 500° C. and 1300° C. and a pressure of between 0.1 MPa and 1.4 MPa.
  2. 2 . The method according to claim 1 , characterized in that said knitted precious metal mesh comprises n wires per row and (n−1) float stitches per wale, wherein: n≥3.
  3. 3 . The method according to claim 1 , characterized in that n≥4.
  4. 4 . The method according to claim 1 , characterized in that all n wires are of the same composition and diameter.
  5. 5 . The method according to claim 1 , characterized in that said knitted precious metal mesh comprises precious metal wires of platinum or a platinum alloy.
  6. 6 . The method according to claim 5 , characterized in that the precious metal wires consist of a platinum alloy with at least 75% platinum.
  7. 7 . The method according to claim 1 , characterized in that said knitted precious metal mesh comprises precious metal wires of palladium or a palladium alloy.
  8. 8 . The method according to claim 1 , characterized in that said knitted precious metal mesh comprises precious metal wires consist of a palladium alloy with at least 75% palladium.
  9. 9 . The method according to claim 1 , characterized in that said knitted precious metal mesh comprises at least one precious metal wire of platinum or a platinum alloy, and at least one precious metal wire of palladium or a palladium alloy.
  10. 10 . The method according to claim 1 , characterized in that one of the one or more precious metal meshes has precious metal wires that consist of platinum or a platinum alloy and is used as the outermost precious metal mesh on the gas inlet side of the mesh stack and is thus the first of the precious metal meshes through which the reaction gas flows.
  11. 11 . The method according to claim 1 , characterized in that one of the one or more precious metal meshes has at least one precious metal wire that consists of platinum or a platinum alloy, and at least one precious metal that consists of palladium or a palladium alloy and is used as the outermost precious metal mesh on the gas outlet side of the catalyst mesh stack and is thus the last of the catalyst meshes through which the reaction gas flows.
  12. 12 . The method according to claim 1 , characterized in that at least one of the one or more precious metal meshes has precious metal wires consisting of palladium or a palladium alloy and is used on the gas outlet side of the precious metal mesh stack as a getter mesh and the reaction gas thus flows through said mesh downstream of the catalyst meshes.
  13. 13 . The method according to claim 1 , wherein installing the mesh stack comprises installing, in addition to said knitted precious metal mesh, one or more catalyst meshes arranged on the gas inlet side of the mesh stack and getter meshes arranged on the gas outlet side of the mesh stack.
  14. 14 . The method according to claim 1 , wherein said knitted precious metal mesh is provided on a gas inlet side of the mesh stack as to be an outermost member of the mesh stack.
  15. 15 . The method according to claim 1 , wherein said knitted precious metal mesh comprises both one or more platinum containing wires and one or more palladium containing wires and is provided on a gas outlet side of the mesh stack as an outermost member of the mesh stack.
  16. 16 . The method according to claim 1 , wherein said providing of one or more precious metal meshes includes providing a plurality of said knitted precious metal meshes, and the method further comprising installing said knitted precious metal meshes in the mesh stack.
  17. 17 . The method according to claim 1 , wherein said knitted precious metal mesh comprises three float stitches.

Description

The invention relates to knitted precious metal meshes having a novel knitting structure for catalytic oxidation of ammonia, in particular for oxidation to NO, as used for nitric acid production. 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 give hydrocyanic acid (Andrussow process) have long been considered extremely important from an industrial perspective; after all, they provide base chemicals for the chemical industry and for fertilizer production on a large industrial scale (Andreas Jess, Peter Wasserscheid: Chemical Technology; Wiley-VCH Verlag, Weinheim 2013, Chapter 6.4.) At the center of these heterogeneously catalyzed gas reactions are precious metal catalysts in the form of gas-permeable spatial structures, on or in which the reaction takes place. Here, precious metal meshes in the form of woven fabrics (DE4028916 C2) or knitted fabrics (EP0364153 B1, DE4206199 C1) made of fine precious metal wires have been established for some time now. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically shows a reactor with mesh stack. FIG. 2 shows the structure of the flat-bed knitting machine. FIG. 3 shows a knitted fabric example. FIG. 4 shows a precious metal mesh knitting with a stitch formed with the same wire at each latch needle position. FIG. 5 shows a precious metal mesh knitting with a stitch formation of two wires (wire A and wire B) with one wire representing a float stitch. FIG. 6 shows an example of a knitted fabric with three float stitches. The catalyst grids are typically arranged in a flow reactor in a plane perpendicular to the flow direction of the gas mixture. Conical arrangements are also known. A plurality of precious metal meshes are expediently arranged one behind the other and combined to form a mesh stack. FIG. 1 schematically shows the reactor with the mesh stack installed therein, whose function is described below, in an example of catalytic ammonia oxidation (Ostwald process): In the reaction zone (2) of the flow reactor (1), the mesh stack (3), which consists of a plurality of catalyst meshes (4) arranged one after the other on the inlet side, and of downstream separating and getter meshes (5), is arranged in a plane perpendicular to the flow direction. This mesh stack is held in its position by clamping. The reaction gas (ammonia-atmospheric oxygen mixture having an ammonia content of 9-13 vol. %) (6) flows through the mesh stack (3) at atmospheric or increased pressure, wherein ignition of the gas mixture takes place in the inlet region and the combustion reaction giving nitrogen monoxide (NO) and water encompasses the entire reaction zone (2): 4NH3+5O2(air)→4NO+6H2O Undesirable side reactions are the oxidation of the ammonia to nitrogen (N2) and nitrous oxide (N2O), wherein the former only reduces the yield of NO, but the latter is also a powerful greenhouse gas: 4NH3+3O2(air)→2N2+6H2O 4NH3+4O2(air)→N2O+6H2O The NO in the outflowing reaction gas mixture subsequently reacts with the excess atmospheric oxygen to give NO2: 2NO+O2→2NO2 An undesired side reaction here is the formation of nitrous oxide: 2NO+½O2→2N2O The NO2 in turn reacts in a downstream absorption with water to give nitric acid, which is fed, for example, to fertilizer production: 3NO2+H2O→2HNO3+NO Precious metal wires made of platinum, rhodium or of alloys of said metals with other precious or non-precious metals are used for the production of the precious metal meshes. Platinum-rhodium or platinum-palladium-rhodium alloys having 88 to 97 wt % platinum are typical here. Platinum is required to achieve the highest possible ammonia conversion, and rhodium improves the selectivity to NO, thereby reducing the emission of nitrous oxide, and increases the mechanical strength [G. R. Maxwell: “Synthetic Nitrogen Products—A Practical Guide to the Products and Processes”, Springer Science+Business Media, Inc. 2005, page 220]. In turn, palladium is used, depending on precious metal prices, to reduce the precious metal costs by replacing platinum. Flat-bed knitting machines are used for the knitting of precious metal meshes. The structure of the flat-bed knitting machine is illustrated in FIG. 2. The flat-bed knitting machine has a front (8) and a rear needle bed (9) in which the latch needles (10) are installed. The latch needles pass through different positions depending on the programming of the machine. The programming thus specifies the structure of the knitted fabric. An example of a knitted fabric is shown in FIG. 3: the line in which the yarn or wire runs is referred to as a row (a); the stitches interlaced with one another using a latch needle form a wale (b). A special feature of the flat-bed knitting machine with respect to other fabric-forming machines is that knitted fabrics can be formed synchronously and independently of one another on the