Thread guide for a knitting machine for precious metal nets

Abstract

Yarn guide (13) for a flat knitting machine for knitting precious metal wire, characterized in that the yarn guide is equipped at the wire outlet with at least one deflecting roller (14) over which the precious metal wire is deflected, so that the deflecting roller or rollers are guide rollers which have a radially circumferential groove (15) in the center of their outer running surface which prevents the wire, precious metal wire, from slipping off the roller, the groove (15) having a width of 0.2 to 2 mm and wherein the groove (15) has a round cross-section and the radius of curvature (16) of this cross-section is greater than the groove width.

Description

[0001] The invention relates to a knitting machine for knitting precious metal nets for the catalytic oxidation of ammonia, in particular for the oxidation to NO, as used for nitric acid production.

[0002] Precious metal-catalyzed gas reactions such as the oxidation of ammonia with atmospheric oxygen in nitric acid production (Ostwald process) or the conversion of ammonia with methane in the presence of oxygen to hydrogen cyanide (Andrussow process) have long been of considerable industrial importance, as they are used to produce basic chemicals for the chemical industry and fertilizer production on a large scale (Andreas Jess, Peter Wasserscheid: Chemical Technology; Wiley-VCH Verlag, Weinheim 2013, Chapter 6.4).

[0003] At the core 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. 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.

[0004] 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 advisable to arrange several precious metal meshes one behind the other and combine them into a mesh stack.

[0005] Fig. shows, using the example of catalytic ammonia oxidation (Ostwald process), a schematic representation of the reactor with the mesh stack installed therein, the function of which is described below: In the reaction zone (2) of the flow reactor (1), the mesh stack (3), consisting of several consecutive catalyst meshes (4) on the inlet side and downstream separation and getter meshes (5), is arranged in a plane perpendicular to the flow direction. This mesh stack is held in position by clamping.

[0006] The reaction gas (ammonia-air oxygen mixture with an ammonia content of 9 - 13 vol.%) (6) flows through the mesh stack (3) under atmospheric or elevated pressure, whereby the 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 (2): 4 NH3 + 5O2 (air) → 4 NO + 6 H2O

[0007] Undesirable side reactions are the oxidation of ammonia to nitrogen (N2) and nitrous oxide (N2O), whereby the former only reduces the yield of NO, but the latter is also a strong greenhouse gas: 4 NH3 + 3 O2 (air) → 2 N2 + 6 H2O 4 NH3 + 4 O2 (air) → 2 N2O + 6 H2O

[0008] The NO in the outflowing reaction gas mixture subsequently reacts with the excess atmospheric oxygen to form NO2: 2NO + O2 → 2 NO2

[0009] An undesirable side reaction is the formation of nitrous oxide: 2 NO + ½ O2 → 2 N2O

[0010] The NO2 in turn reacts with water in a downstream absorption to form nitric acid, which is used in fertilizer production, for example: 3 NO2 + H2O → 2 HNO3 + NO

[0011] Precious metal wires made of platinum, rhodium, or alloys of these metals with other precious or base metals are used to manufacture the precious metal meshes. Typical alloys are platinum-rhodium or platinum-palladium-rhodium alloys containing 88 to 97 wt.% platinum. Platinum is required to achieve the highest possible ammonia conversion; rhodium improves selectivity to NO, thereby reducing nitrous oxide emissions and increasing mechanical strength [GR Maxwell: "Synthetic Nitrogen Products - A Practical Guide to the Products and Processes," Springer Science + Business Media, Inc. 2005, page 220]. Palladium, in turn, is used to reduce precious metal costs by replacing platinum, depending on precious metal prices.

[0012] Flat knitting machines are used for knitting precious metal nets. The structure of the flat knitting machine is described in Fig. The flat knitting machine has a front (8) and a rear needle bed (9), into which the latch needles (10) are installed. The thread or precious metal wire is fed through a thread guide (13). The following description is limited to knitting precious metal wire.

[0013] If several thread guides are used, several precious metal wires can be knitted synchronously. The latch needles move through different positions depending on the machine's programming. The programming therefore determines the structure of the knitted fabric via the movement of the latch 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 one another (single-bed fabric). In addition, spacer knits (double-bed fabric) can be formed, in which the precious metal wire alternately forms stitches or tucks at the front and back (EP1358010 B2). The knitted fabric is knitted downwards between the two needle beds (11). This is done by successively knocking off the individually formed stitches via the knock-off position and knock-off bar edge (12).

[0014] Knitted precious metal nets offer several advantages over woven precious metal nets, which is why they are preferred in industrial applications today. Firstly, the knitting technique offers the possibility of high flexibility in terms of knitting patterns, the thickness of the precious metal wire used, and the resulting surface weight. Secondly, the precious metal knits can be produced more economically, as the knitting technique requires shorter setup times than the weaving technique. This, in particular, results in significantly reduced precious metal binding during production.

[0015] Precious metal mesh 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 latch needles per given width. The key terms are defined below:

[0016] Gold, silver and the platinum metals (Ru, Rh, Pd, Os, Ir, Pt) are considered precious metals.

[0017] Catalyst networks are those networks whose catalytic activity is used for the conversion of ammonia with oxygen.

[0018] Getter nets are the nets that are installed in the reaction gas flow downstream of the catalyst nets in order to capture volatile platinum oxide for recycling by alloying with the palladium of the getter nets and thus minimize platinum loss.

[0019] The term precious metal nets refers to the entirety of the catalyst and getter nets.

[0020] Separation nets are nets made of high-temperature-resistant steel, which are installed between the precious metal nets to prevent the precious metal nets from sintering together.

[0021] The mesh stack consists of the catalyst meshes on the inlet side of the mesh stack, and optionally the getter meshes on the outlet side of the mesh stack, as well as the separation meshes, which are installed between the precious metal meshes if necessary.

[0022] A flow reactor is the reactor that ensures that the reaction gas is passed over the mesh stack installed in it.

[0023] The knitting process is carried out using latch needles. One latch needle is required for each wale in a knitted fabric.

[0024] The precious metal wire is fed to the latch needles via the thread guide. Precise positioning is important so that the latch needles can grip the precious metal wire and prevent insertion errors, which could result in a knitting defect in the product.

[0025] Needle bed, single-bed fabric, double-bed fabric, pile thread: The needle bed is the element of the flat-knitting machine over which the latch needles are guided. The flat-knitting machine typically has a front and a rear needle bed, each of which can knit a single fabric (single-bed fabric). If these two fabrics are knitted together using pile threads during the knitting process, the resulting fabric is a double-bed fabric.

[0026] A loop is created by connecting two loops. A tuck is a loop that is inserted from one needle bed to the opposite needle bed in a reverse-to-reverse weave. The terms 'loop,' 'loop,' and 'reverse-to-reverse weave' are familiar to those skilled in the art.

[0027] Course and wale: The stitches formed consecutively by the same precious metal wire are collectively referred to as a course. The parallel rows of stitches are held together by the interlacing of the stitches of a wale.

[0028] During the knitting process, the precious metal wire is subjected to bending stress and friction. In particular, the thread guide (13), which is responsible for feeding the precious metal wire in the correct position to the latch needles (10), places a strain on the precious metal wire, as the precious metal wire is deflected by approximately 90° at the wire outlet via an eyelet and runs over the eyelet edge at high speed. This leads to damage to the precious metal wire surface and possibly even to the precious metal wire breaking and to wear of the thread guide eyelet. Furthermore, due to its flexural rigidity, 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.

[0029] The object of the present invention is therefore to reduce wear on the precious metal wire and the thread guide and to improve the positioning of the precious metal wire for gripping by the latch needles. Therefore, precious metal wire is always referred to as the starting material below. If necessary, steel wire or yarn can also be knitted.

[0030] The task is solved by a thread guide for a flat knitting machine for the knitting of precious metal wire, in which the thread guide is equipped with one or more deflection rollers at the wire outlet, over which the precious metal wire is deflected.

[0031] Fig. shows an example of a thread guide according to the invention. The deflection rollers (14) are clearly visible.

[0032] A pulley 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 pulley, thereby minimizing wear on the precious metal wire and thus reducing the risk of wire breakage.

[0033] The thread guide is moved back and forth above the latch needles, so that the exit direction of the precious metal wire changes by 180° at each end of the knitting bed used. The thread guide is preferably equipped with two deflection pulleys at the end, with their axes running parallel, and the precious metal wire exits the thread guide between the deflection pulleys. Thus, the precious metal wire is guided over a deflection pulley in both directions of movement without the thread guide having to be rotated.

[0034] The pulley should rotate as smoothly and quickly as possible to minimize friction between the precious metal wire and the pulley. One thread guide variant is therefore equipped with pulleys with plain bearing bushings. Teflon or bronze are suitable materials for the plain bearing bushings, as they run with low friction and ensure a long service life. Plain bearings have the advantage over roller bearings in that they are less sensitive to shocks and vibrations and are less susceptible to contamination. Furthermore, their design is very simple.

[0035] However, the pulleys are preferably equipped with roller bearings, as these are technically more complex but can further reduce the friction between the precious metal wire and the pulley. Ball bearings are particularly preferred, as they keep the bearings stable in the axial direction and can also reduce the friction generated by the axial forces acting on the pulley.

[0036] Steel is particularly suitable as a rolling bearing material because of its wear resistance. For the same reason, steel is also very suitable for the idler pulley. In this case, the idler pulley can also serve as the outer ring of the rolling bearing.

[0037] Preferably, the guide pulley or pulleys have a radial groove in the center of their outer running surface, which prevents the wire or thread from slipping off the pulley. Slipping of the wire 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.

[0038] Preferably, the groove has a round cross-section with a radius of curvature greater than the groove width. This prevents damage to the wire surface caused by the edges running on either side of the groove, thus preventing potential wire breakage.

[0039] The invention also includes a flat-bed knitting machine for knitting precious metal wire, wherein the yarn guide according to the invention is used and installed in the flat-bed knitting machine in such a way that the axes of the deflection rollers are arranged at right angles to the plane of movement of the yarn guide. This ensures that the precious metal wire always runs at right angles to the axis of the deflection roller, and this arrangement minimizes friction between the precious metal wire and the deflection roller, as well as the risk of slipping off the deflection roller. Legend: 1 flow reactor 2 Reaction zone 3 net stacks 4 catalyst networks 5 getter nets 6 Reaction gas 7 products 8 front needle bed 9 rear needle bed 10 latch needles 11 knitted product 12 teeing edge 13 thread guides 14 pulleys 15 grooves 16 Radius of curvature of the groove QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 4028916 C2

[0003] EP 0364153 B1

[0003] DE 4206199 C1

[0003] EP 1358010 B2

[0013] Cited non-patent literature

[0000] Andreas Jess, Peter Wasserscheid: Chemical Technology; Wiley-VCH Verlag, Weinheim 2013, Chapter 6.4

[0002] G. R. Maxwell: „Synthetic Nitrogen Products - A Practical Guide to the Products and Processes“, Springer Science + Business Media, Inc. 2005, Seite 220

[0011]

Claims

[1] Thread guide (13) for a flat knitting machine for the knitting of precious metal wire, characterized by that the thread guide at the wire outlet is equipped with at least one deflection roller (14) over which the precious metal wire is deflected, so that the deflection roller or the deflection rollers are guide rollers which have a radially circumferential groove (15) in the middle of their outer running surface, which prevents the wire, precious metal wire from slipping off the roller, the groove (15) has a width of 0.2 to 2 mm and wherein the groove (15) has a round cross-section and the radius of the curvature (16) of this cross-section is greater than the groove width. [2] Thread guide according to claim 1, characterized bythat the thread guide (13) is equipped at the end with two deflection rollers (14) which are arranged centrally in the same plane and whose axes run perpendicular to this plane and parallel to each other, so that the precious metal wire exits the thread guide between the deflection rollers. [3] Thread guide according to claim 1 or 2, characterized by that the pulley or pulleys are equipped with plain bearings. [4] Thread guide according to claim 3, characterized by that the pulley is equipped with Teflon or bronze bushings as plain bearings. [5] Thread guide according to claim 1 or 2, characterized by that the pulley or pulleys are equipped with rolling bearings. [6] Thread guide according to claim 5, characterized by that the rolling bearing(s) are ball bearings. [7] Thread guide according to claim 5 or 6, characterized by that the rolling bearing(s) are made of steel. [8] Thread guide according to one or more of the preceding claims, characterized by that the pulley is made of steel. [9] Thread guide according to one or more of the preceding claims, characterized by that the pulley is the outer ring for the rolling bearing. [10] Thread guide according to one of the preceding claims, characterized by that the axes of the deflection rollers are arranged at right angles to the plane of movement of the thread guide.

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