Catalytically active heating elements, their manufacture and use

Abstract

The present invention relates to a catalytically active heating element, its manufacture, and its use in the production of hydrocyanic acid (HCN). The underlying problem is to provide a thermally stable, catalytically active heating element capable of simultaneously electrically heating and chemically catalyzing the BMA process. It is particularly desirable for the heating element to be thermally and mechanically stable and maintain its catalytic activity during continuous industrial operation. The heating element according to the present invention has a layer structure (A, B, C) consisting of (A) silicon carbide (SiC), (B) aluminum nitride (AlN), and (C) platinum (Pt). The silicon carbide (SiC) functions as an electrical heating resistor. The platinum (Pt) functions as a catalyst. Between the platinum (Pt) and the silicon carbide (SiC), a protective layer of aluminum nitride (AlN) is arranged. This prevents the platinum (Pt) from alloying with the silicon carbide (SiC) during operation.

Description

[Technical Field]

[0001] The present invention relates to catalytically active heating elements, their manufacture and their use in hydrocyanic acid production.

[0002] Hydrocyanic acid (HCN), the simplest nitrile, is an important synthetic unit in organic chemistry. Traditionally, it is used in metal extraction and metal processing. On an industrial scale, hydrocyanic acid production is often carried out according to the Andrussow or BMA process.

[0003] The introduction of hydrocyanic acid production technology Gail, E., Gos, S., Kulzer, R., Loroesch, J., Rubo, A., Sauer, M., Kellens, R., Reddy, J., Steier, N. and Hasenpusch, W. (2011). Cyano Compounds, Inorganic. In Ullmann's Encyclopedia of Industrial Chemistry, (Ed.), https: / / doi.org / 10.1002 / 14356007.a08_159.pub3 can be seen in.

[0004] In the BMA process (BMA = hydrocyanic acid from methane and ammonia), hydrocyanic acid is produced from methane (CH4) and ammonia (NH3) in a strongly endothermic reaction requiring relatively high reaction temperatures of 1000-1300 °C. In contrast to the Andrussow process, the BMA process is carried out in the absence of oxygen.

[0005] The energy required for the BMA process is provided in a separate combustion chamber by the combustion of heated gases. In this case, due to the minimum temperature required for the hydrocyanic acid reaction, only a portion of the heating energy used can be utilized for the reaction itself. The need to use fossil energy carriers to provide the reaction enthalpy, combined with the low energy yield of hydrocyanic acid, results in significant CO2 emissions.

[0006] As an alternative energy source, HCN can be produced using electrical energy instead of fossil fuels. When using electricity from renewable sources, this process is potentially nearly CO2-neutral. Furthermore, electrically heated BMA processes have advantages over fossil fuel heated BMA processes, for example in terms of operational costs. Better energy efficiency can be expected by avoiding the inevitable energy losses on the fuel gas side due to the high required minimum reaction temperature, Faster start-up and shutdown cycles are achieved because there is no need to use refractory materials in the reactor lining, It is known that a more uniform temperature operation allows for higher yields, thereby reducing the specific consumption of methane and ammonia for hydrocyanic acid production; i.e., with a uniform temperature distribution, significantly higher yields can be achieved while simultaneously reducing the formation of by-products. There are further advantages.

[0007] In terms of investment costs, electrically heated BMA equipment is cheaper than thermally heated equipment. The absence of fuel and exhaust gas chambers allows for a more compact construction approach and higher space-time yield, Similarly, cost-effective modular connectivity is possible The advantages are seen.

[0008] Finally, the electrically operated BMA method is more sustainable and The resulting hydrogen-containing residual gas can, if necessary, replace natural gas as heating gas in downstream processes, thereby achieving further CO2 reductions. The hydrogen in the resulting residue gas has a significantly lower CO2 footprint than hydrogen produced from fossil hydrocarbons in a steam reformer and can be used as a feedstock for further chemical reactions, possibly after necessary purification.

[0009] For all these reasons, there is interest in developing an electrically operated BMA process capable of producing hydrocyanic acid on an industrial scale.

[0010] Various concepts are known for producing HCN in electrically heated reactors.

[0011] On the one hand, the application of electrically heated fixed-bed reactors for hydrocyanic acid production has been described, in which the catalyst bed can be heated by induction. See, for example, WO 2017186437.

[0012] Alternatively, structured catalyst bodies made of electrically conductive materials, so-called monoliths, are used, as described in DE 10317197, WO 2019228798 or WO 2021 / 063799. In the cited publications, the reactants are guided through catalyst-coated channels in an electrically heated structure.

[0013] Similarly, WO 2022017900 describes a catalytically active heating element manufactured by additive manufacturing, which is suitable for use in various endothermic reactions, including hydrocyanic acid. The heating element includes a metallic conductive core coated with a ceramic coating. A catalytically active layer is then deposited on the ceramic coating. In the context of the Andrussow process, the catalytically active layer contains Pt, Co, or SnCo. However, information regarding the composition of the ceramic layer is lacking for hydrocyanic acid production. In the context of steam reforming, ceramic layers made of Al2O3, ZrO2, MgAl2O4, or CaAl2O4 are mentioned, on which catalytically active materials made of Ni, Ru, Rh, or Ir are deposited.

[0014] A fundamental drawback of additively fabricated heating elements is the limited choice of materials for the metal core.

[0015] The use of catalytic heating rods in the production of hydrocyanic acid by the BMA reaction is described in Dutch Patent Application No. 121661 and WO 9615983: here, graphite or silicon carbide tubes are used as current-conducting elements, the inner surface of which is coated with platinum as a catalyst.

[0016] Silicon carbide tubes directly coated with platinum catalyst are not a favorable combination for the BMA reaction, as it is known that in the temperature range relevant to the BMA process, a eutectic mixture between silicon carbide and platinum is formed: LL Xu, J. Wang, HS Liu, ZP Jin: Thermodynamic assessment of the Pt-Si binary system. Calphad, Volume 32, Issue 1, 2008, Pages 101-105. https: / / doi.org / 10.1016 / j.calphad.2007.07.010.

[0017] As a result, the platinum forms an alloy with the silicon and the catalytic coating loses adhesion at the high reaction temperatures required in the BMA process. Catalysis is impaired.

[0018] US Patent Application Publication No. 20170106360 also uses catalytic heating rods, where the heating rod itself is made of a catalytic material, or the heating rod is coated with a catalyst, or a separating layer, a so-called washcoat, is first applied, and then the catalyst is applied as a further layer. In the case of a rod made of silicon carbide (SiC) onto which platinum (Pt) is directly applied as a catalyst for the BMA reaction, a eutectic mixture between SiC and Pt may also form. Therefore, when combining a platinum-containing catalyst with a heating rod made of silicon carbide for the BMA process, a separating layer is essential between the heating rod and the catalyst. US Patent Application Publication No. 20170106360 also describes such a structure with a separating layer ("washcoat") made of the material Al2O3. However, the thermal expansion coefficient of Al2O3 (approximately 800°C at 600°C) is low. * 10 -6 K -1 ) is the thermal expansion coefficient of silicon carbide (approximately 5 at 600°C). * 10 -6 K -1 ) is known to be significantly larger than the Al2O3 separation layer. It is therefore expected that the Al2O3 separation layer will peel off from the silicon carbide at high temperatures and / or temperature changes. Overall, the catalytically active heating elements known so far are not convincing.

[0019] The object of the present invention is therefore to provide a thermally stable, catalytically active heating element that can simultaneously electrically heat and chemically catalyze the BMA process. In particular, it is desirable for the heating element to be thermally and mechanically stable and maintain its catalytic activity during continuous industrial operation. Equivalent catalytic heating rods are known from US Patent Application Publication No. 2017 / 314441 and EP Patent No. 1945345.

[0020] The problem is that the heating element a) a first electrical terminal; b) a second electrical terminal; c) a solid or hollow core containing silicon carbide, the core conductively connecting the first terminal to at least the second terminal; d) a protective coating deposited on the core, the protective coating containing aluminum nitride; e) a catalytic system deposited on the protective coating, the catalytic system containing platinum This problem is solved by a heating element having the following characteristics:

[0021] Such a heating element is a first object of the present invention.

[0022] The heating element according to the present invention has a layer structure A, B, C consisting of (A) silicon carbide, (B) aluminum nitride, and (C) a platinum-containing catalyst. The silicon carbide acts as an electrical heating resistor. Between the catalyst layer and the silicon carbide, a protective layer of aluminum nitride is arranged. This prevents alloying of the platinum with the silicon carbide during operation. Aluminum nitride has a thermal expansion coefficient similar to that of silicon carbide (approximately 500°C at 600°C). * 10 -6 K -1 ), stresses caused by differential thermal expansion in the layer structure can be neglected. Aluminum nitride (AlN) behaves chemically neutral in the hydrocyanic acid reaction and therefore does not inhibit the reaction.

[0023] Preferably, the catalytic coating is deposited only on the protective coating, thereby preventing the formation of a eutectic mixture between SiC and Pt.

[0024] The protective coating and catalytic coating are ideally designed to be very thin compared to the core. Specifically, the volume v1 of the protective coating and / or the volume v2 of the catalytic coating should be smaller than the volume v0 of the core. The core requires a correspondingly larger volume v0 to be able to carry a large current despite its high specific electrical resistance.

[0025] The heating elements can be designed in various shapes, from hollow or solid material, as cylindrical tubes if desired. The tubes can be curved. The heating elements have electrical terminals and can be operated with direct current or alternating current in two or three phases.

[0026] The manufacture of the heating element according to the invention is a second subject of the present invention. a) providing a core comprising silicon carbide; b) providing a coating agent containing aluminum and nitrogen; c) providing a platinum-containing catalyst system; d) coating the core with a coating agent to obtain a protective coating adhered to the core, the coating comprising aluminum nitride; e) coating the protective coating with a catalyst system to deposit the catalyst system on the protective coating. At least includes.

[0027] In accordance with the present invention, a protective coating and then a catalytic coating are deposited on the core in that order.

[0028] According to the present invention, the protective coating contains aluminum nitride. Therefore, the coating agent must contain aluminum and nitrogen. Aluminum and nitrogen may be present in elemental or compound form, either by themselves or with each other. Preferably, the coating agent contains aluminum nitride dispersed in a dispersing medium.

[0029] The application of the protective coating then takes place purely physically in the coating process. Various methods are conceivable for coating the core: the simplest method is the dipping method, in which the core is dipped into the coating agent and then pulled out again. Similarly, spraying methods are conceivable. Printing, sputtering, roll application or brushing are further possible methods, but are only of limited suitability.

[0030] In either case, the mixture is then dried so that the dispersing medium evaporates and the aluminum nitride is deposited on the silicon carbide.

[0031] Alternatively, reactive methods can be used. For this purpose, a system containing aluminum, preferably in metallic form, as the first component is used as the coating agent. As the second component, the system contains nitrogen, preferably as a gas or nitrogen-containing gas.

[0032] For the coating, aluminum is first deposited on the core and then exposed to nitrogen. In the simplest case, this is done by exposing the aluminum-coated core to an atmosphere containing gaseous nitrogen or a nitrogen-containing gas. The nitrogen reacts with the aluminum in the presence of the core to form aluminum nitride. If necessary, the atmosphere is heated to allow the aluminum and nitrogen to react to form aluminum nitride. Thus, aluminum nitride is formed in situ directly on the silicon carbide core.

[0033] The atmosphere can be heated by supplying an electric current to the silicon carbide core. The first component can also contain a dispersion medium in which the aluminum is dispersed. The aluminum coating is correspondingly carried out by depositing the dispersion. The dispersion medium can be evaporated by drying in a nitrogen atmosphere and / or by electrically heating the core. Alternatively, metallic aluminum can be sputtered onto the core or deposited from the vapor phase.

[0034] In all coating methods, it is important that the electrical terminals are not coated, since AlN is an electrical insulator. This would mean that electrical connections would no longer be possible. The first option to avoid this is to provide the core with the first and second electrical terminals, then provide the core with a protective coating, followed by a catalytic coating. In this case, care must be taken to ensure that the electrical terminals are not coated. To do this, for example, the terminals can be masked during coating.

[0035] Alternatively, the first and second electrical terminals are applied to the core only after the core has been coated with the protective coating, in which case the core can be, for example, completely coated, and then the coating is again partially removed from the core to expose the electrical terminals.

[0036] A third subject of the invention is a heating element obtainable by the method according to the invention, which is characterized by the layer structure described and by the layer quality and layer adhesion produced by the coating method.

[0037] The heating element according to the invention can be used to heat endothermic chemical reactions that can be catalyzed by platinum. Temperatures up to about 1400°C are possible.

[0038] Preferably, the heating element is used in the production of hydrocyanic acid or other nitriles.

[0039] The use of the heating element according to the invention in the production of hydrocyanic acid is therefore likewise a subject of the present invention.

[0040] In particular, the heating element is used in the electrically heated BMA process in which hydrocyanic acid is synthesized from ammonia and methane in the absence of oxygen.

[0041] A further subject matter is therefore a method for producing hydrocyanic acid using a heating element according to the invention, said method comprising: a) providing a reactor comprising at least one heating element according to the present invention; b) supplying to the reactor a reactant gas mixture containing at least ammonia and methane, wherein the reactant gas mixture has an oxygen content of less than 2% by volume or the reactant gas mixture is oxygen-free; c) supplying an electric current to the heating element; d) withdrawing the product gas mixture containing at least hydrocyanic acid from the reactor. At least includes.

[0042] Because of the low oxygen content or the preferred absence of oxygen, the process is not an Andrussow process but an electrically heated BMA process, called the E-BMA process.

[0043] In addition to hydrocyanic acid, the product gas mixture may also contain by-products or unreacted reactants.

[0044] Preferably, the process is heated only electrically, i.e., no heat energy is provided to enable the endothermic reaction, although this does not exclude preheating the reactants by a non-electrical heat source external to the reactor.

[0045] Preferably, the reaction is catalyzed solely by the electric heating element, which means that no further catalyst is provided in the reactor apart from the catalytic system deposited on the heating element according to the invention.

[0046] A plurality of heating elements according to the present invention may be provided within the reactor.

[0047] The invention will now be explained in more detail on the basis of the drawings. [Brief explanation of the drawings]

[0048] [Figure 1] 1 is a schematic cross-sectional view of a heating element according to the present invention; [Figure 2] 1 is a schematic diagram of a method implementation according to the invention;

[0049] A heating element 10 according to the invention is shown diagrammatically in Figure 1. It comprises a core 11 made of silicon carbide (SiC), on which is deposited a protective coating 12 made primarily of aluminum nitride (AlN). Deposited on the protective coating 12 is a platinum (Pt)-containing catalyst system 13. The catalyst system 13 is separated from the core 11 by the protective layer 12.

[0050] The protective coating 12 and the catalytic system 13 completely surround the core 11 except at two locations where the heating element 10 has a first electrical terminal 14 or a second electrical terminal 15. The protective coating 12 is non-peelably adhered to the core 11, and the catalytic system 13 is non-peelably adhered to the protective coating 12.

[0051] 1, the core 11 may also be designed as a hollow tube, which is first provided on the inside with a protective coating 12 and then with a catalytic system 13 (not shown). The catalytically active coating is therefore on the inside of the tube.

[0052] The heating element 10 is contacted to a voltage source 17 (not shown in FIG. 1) by means of two terminals 14, 15. The heating element may also have a third electrical terminal (not shown) to allow three-phase operation.

[0053] FIG. 2 shows the method sequence in three steps, from top to bottom.

[0054] A reactor 16 is prepared with a heating element 10 disposed therein and filled with a reactant gas mixture (NH3 + CH4). The heating element 10 is connected to a voltage source 17 and a voltage is applied to it. Due to the ohmic resistance of the silicon carbide, the core 11 heats up and heats the reactor 16 from the inside. The platinum contained in the catalyst system 13 converts the reactant gas mixture (NH3 + CH4) into a product gas mixture (HCN + H2). The primary product gas mixture (HCN + H2) is withdrawn from the reactor 16 together with by-products and unreacted reactants.

[0055] Working Example: The present invention will now be described in more detail based on examples.

[0056] motive The goal of this experiment was to electrically heat a reactor 16 for HCN production to temperatures above 1100 °C using SiC heating elements. The heating elements 10 were located directly within the reaction gas phase. Therefore, the reaction proceeded directly on the surface of the heating elements 10, requiring a catalyst coating on this surface. At the required temperatures, alloy formation occurred between platinum, the main component of the BMA catalyst, and the element material (SiC), thereby significantly impeding the BMA reaction. To prevent this alloy formation, a protective layer was deposited on the heating elements 10, thus preventing contact between Pt and Si. AlN (aluminum nitride) was identified as a suitable barrier layer because the expansion coefficients of AlN and SiC are in a comparable range.

[0057] Experiment Description In this experiment, the SiC / AlN system was investigated in an experimental reactor. A SiC tube with dimensions of 22 mm outer diameter, 17 mm inner diameter, and 2100 mm length was coated with AlN. To do this, the AlN was introduced into a paint matrix containing binders, adhesion promoters, rheological additives, and solvents. The inside of the tube was coated using a suitable immersion method. To do this, one side of the tube was closed with a stopper, and the primer paint was poured in through the second opening. After closing the second opening, also using a stopper, the tube was rotated to completely coat the inner surface. The excess material was then poured out, and the primer was dried by passing nitrogen through it. After a drying period of 24 hours, the tube was installed in the experimental reactor, and the primer was baked in a nitrogen flow (heating rate: 100 K / h, target temperature: 1150 °C, hold time: 2 hours). After complete cooling, the inside of the tube was re-coated with primer to achieve a sufficient layer thickness, and the baking process was repeated.

[0058] Coverage amount: 28.6g Layer thickness: approximately 30 μm (calculated value).

[0059] The tubes were then coated with a platinum-containing catalyst and the synthesis performance was investigated in the experimental reactor 16. The primary goal of the experiment was to evaluate the synthesis behavior over the run time. To that end, the equipment was operated at a reactant gas load of about 60 mol / h, in ammonia excess, at a temperature of 1180° C., for a period of about 170 hours.

[0060] result The yields were greater than 80% based on ammonia and 90% based on methane over longer periods, and were therefore comparable to standard tubes made of corundum. The synthesis behavior was found to be comparable to that of standard tubes over the time period investigated.

[0061] Coating Method The selected coating method for the primer is the easiest option to coat a single pipe without much effort. Coating by spraying is also possible and has been successfully practiced. [Explanation of symbols]

[0062] 10 heating elements 11 cores 12 Protective Coating 13 Catalytic Systems 14 Electrical first terminal 15 Electrical second terminal 16 Reactor 17 Voltage Source AlN Aluminum Nitride CH4 methane CH4+NH3 reactant mixture H2 Hydrogen HCN Hydrogen cyanide HCN+H2 product mixture NH3 ammonia Pt platinum SiC Silicon carbide

Claims

1. A heating element (10), comprising: a) a first electrical terminal (14); b) a second electrical terminal (15); c) a solid or hollow core (11) containing silicon carbide, the core (11) conductively connecting the first terminal (14) to at least the second terminal (15); d) a protective coating (12) deposited on said core (11); e) a catalyst system (13) deposited on said protective coating (12), said catalyst system (13) containing platinum; A heating element (10) comprising at least A heating element (10) characterized in that the protective coating (12) contains aluminum nitride.

2. 2. The heating element (10) of claim 1, wherein the catalytic system (13) is deposited only on the protective coating (12).

3. The volume v of the protective coating (12) １ and / or the volume v of said catalyst system (13) ２ is the volume v of the core (11). ０ 3. The heating element (10) according to claim 1 or 2, wherein the heating element (10) is smaller than

4. A method for manufacturing a heating element (10), comprising the steps of: a) providing a core (11) containing silicon carbide; b) providing a coating agent containing aluminum and nitrogen; c) providing a platinum-containing catalyst system (13); d) coating the core (11) with the coating agent to obtain a protective coating (12) deposited on the core (11) containing aluminum nitride; e) coating said protective coating (12) with said catalyst system (13) to deposit said catalyst system (13) on said protective coating (12). The method has at least the following:

5. 5. The method of claim 4, wherein the coating agent is a dispersion containing a dispersing medium and aluminum nitride dispersed therein.

6. 6. A method according to claim 5, characterized in that the dispersion is sprayed onto the core (11) and then dried.

7. 6. A method according to claim 5, characterized in that the core (11) is immersed in the dispersion and then dried.

8. 5. The method of claim 4, wherein the coating agent is a system comprising two components, a first component containing aluminum and a second component containing nitrogen, the aluminum and nitrogen reacting in the presence of the core (11) to form aluminum nitride.

9. 10. Use of a heating element (10) according to any one of claims 1 to 3 or a heating element (10) produced according to any one of claims 4 to 8 in the production of nitriles, in particular hydrocyanic acid.

10. 10. Use according to claim 9, wherein the heating element (10) is used to provide thermal energy and catalyze an endothermic reaction.

11. 1. A method for producing hydrocyanic acid, comprising: a) providing a reactor (16) containing at least one heating element (10); b) supplying to said reactor (16) a reactant gas mixture containing at least ammonia and methane, said reactant gas mixture having an oxygen content of less than 2% by volume or said reactant gas mixture being oxygen-free; c) supplying an electric current to said heating element (10); d) withdrawing a product gas mixture containing at least hydrocyanic acid from said reactor (16).

1. A method comprising:

10. A method according to claim 1, wherein the heating element (10) provided is a heating element (10) according to any one of claims 1 to 3 or a heating element (10) manufactured according to any one of claims 4 to 8.

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