Catalytically active heating element, production and use thereof

Abstract

The invention relates to catalytically active heating elements, to the production thereof and to the use thereof in the production of hydrocyanic acid (HCN). The problem addressed by the invention is that of providing a thermally stable and catalytically active heating element with which it is possible to simultaneously electrically heat and chemically catalytically BMA processes. In particular, the heating element should be thermally and mechanically stable and maintain its catalytic activity in continuous industrial operations. The heating element according to the present invention has a layered structure (A, B, C) formed of (A) silicon carbide (SiC), (B) aluminum nitride (AlN), and (C) platinum (Pt). Silicon carbide (SiC) acts as an electrical heating resistor. Platinum (Pt) acts as a catalyst. Aluminum nitride (AlN) is arranged as a protective layer between platinum (Pt) and silicon carbide (SiC). Platinum (Pt) and silicon carbide (SiC) are prevented from alloying during the proceeding operation.

Description

Technical Field

[0001] The invention relates to catalytically active heating elements, their production and their use in the production of hydrocyanic acid. Background Art

[0002] Hydrocyanic acid (HCN), the simplest nitrile, is an important synthetic unit in organic chemistry. It is traditionally used in metal extraction and processing. The production of hydrocyanic acid is usually carried out on an industrial scale by the Andrussow process or the BMA process.

[0003] An introduction to the technology of hydrocyanic acid production can be found in: Gail, E., Gos, S., Kulzer, R., Lorösch, 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, (eds.), https: / / doi.org / 10.1002 / 14356007.a08_159.pub3

[0004] In the BMA process (BMA: = Blausäure aus Methan und Ammoniak [Synthesis of hydrocyanic acid from methane and ammonia]), hydrocyanic acid is produced from methane (CH4) and ammonia (NH3) in a strongly endothermic reaction which requires relatively high reaction temperatures of 1000° C. to 1300° C. Unlike the Andrussow process, the BMA process is carried out in the absence of oxygen.

[0005] The energy required in the BMA process is provided by the combustion of a combustible gas in a separate combustion chamber. Due to the required minimum temperature for the hydrocyanic acid reaction, only a portion of the heating energy used here can be used for the reaction itself. The combination of the need to use fossil energy carriers to provide the reaction enthalpy and the low energy yield of hydrocyanic acid leads to a significant formation of CO2.

[0006] As an alternative energy source, HCN can be produced using electricity instead of fossil fuels. When using electricity from renewable sources, the process can be largely CO2-neutral. The electrically heated BMA process has further advantages over the fossil fuel heated BMA process, for example in terms of operating costs:

[0007] • Better energy efficiency is expected by avoiding energy losses in the fuel gas due to high required minimum reaction temperature.

[0008] Faster startup and shutdown cycles as there is no need to line the reactor with refractory materials.

[0009] • A more uniform temperature operating pattern makes it possible to achieve higher yields and thus a reduction in the specific amount of methane and ammonia used for the production of hydrocyanic acid. This is because it is known that a uniform temperature distribution makes it possible to achieve significantly higher yields and at the same time lower by-product formation.

[0010] In terms of investment costs, electrically heated BMA units also have advantages over thermally heated units:

[0011] • The absence of fuel gas and flue gas spaces allows for a more compact construction and a higher space-time yield,

[0012] • Cost-effective modular interconnection is also possible.

[0013] Finally, the BMA method for electricity operation is more sustainable:

[0014] •The generated residual gas containing hydrogen may replace natural gas as a combustible gas in downstream processes, thereby achieving additional CO2 reduction.

[0015] The hydrogen in the generated residual gas has a much lower CO2 footprint than hydrogen produced from fossil hydrocarbons in a steam reformer and can be used as a raw material for further chemical reactions after possible required purification.

[0016] For all these reasons, there is interest in developing an electrically powered BMA process capable of producing hydrocyanic acid on an industrial scale.

[0017] For the production of HCN in electrically heated reactors, various concepts are known:

[0018] On the one hand, the production of hydrocyanic acid by using an electrically heated fixed-bed reactor is described, wherein the heating of the catalyst random bed can be achieved by induction; see WO 2017186437 A1.

[0019] On the other hand, structured shaped catalyst bodies, so-called monoliths, made of electrically conductive materials, as described in DE 10317197 A1, WO 2019228798 A1 or WO 2021 / 063799 A1 are also used. In the cited publications, the reactants are guided through channels of an electrical heating structure coated with a catalyst.

[0020] Similarly, WO 2022017900 A1 describes a catalytically active heating element produced by additive manufacturing, which is intended for various endothermic reactions, including in the production of hydrocyanic acid. The heating element comprises a metal conductive core with a ceramic coating. A catalytically active layer is applied to the ceramic coating. In the Andrussow process, the catalytically active layer contains Pt, Co or SnCo. However, details of the composition of the ceramic layer relevant to the production of hydrocyanic acid are lacking. In the case of steam reforming, a ceramic layer consisting of Al2O3, ZrO2, MgAl2O4, CaAl2O4 is mentioned, on which a catalytically active material consisting of Ni, Ru, Rh, Ir is applied.

[0021] A fundamental disadvantage of additively manufactured heating elements is that the choice of materials for the metal core is limited.

[0022] NL 121661 and WO 9615983 A1 describe the use of catalytic heating rods for producing hydrocyanic acid by BMA reaction: graphite or silicon carbide tubes are used as conductive elements, on the inner surface of which platinum is applied as a catalyst.

[0023] Silicon carbide tubes with directly applied platinum catalyst are not a favorable combination for the BMA reaction. This is because it is known that at the relevant temperature range for the BMA process, a eutectic forms between silicon carbide and platinum:

[0024] LL Xu, J. Wang, HS Liu, ZP Jin: Thermodynamic assessment of thePt–Si binary system, Calphad, Vol. 32, No. 1, 2008, pp. 101-105. https: / / doi.org / 10.1016 / j.calphad.2007.07.010

[0025] As a result, platinum forms an alloy with silicon and the catalytic coating loses adhesion at the high reaction temperatures required in the BMA process. The catalytic effect deteriorates.

[0026] Catalytic heating rods are also used in US 20170106360 A1, wherein the heating rod itself here consists of a catalytic material, or the heating rod is coated with a catalyst, or an isolation layer, the so-called "washcoat", is first applied and then the catalyst is applied as a further layer. In the case of directly applied platinum (Pt) silicon carbide (SiC) rods as catalyst for the BMA reaction, it is likewise possible for a eutectic between SiC and Pt to form. Therefore, in the case of a combination of a platinum-containing catalyst and a heating rod consisting of silicon carbide for the BMA process, an isolation layer between the heating rod and the catalyst is mandatorily necessary. US 20170106360 A1 also describes such a construction with an isolation layer ("washcoat") consisting of the material Al2O3. However, it is known that the thermal expansion coefficient of Al2O3 (~8*10 at 600°C) is 200°C. -6 K -1 ) is significantly higher than silicon carbide (~5*10 at 600°C) -6 K -1 ). It is therefore expected that the barrier layer comprising Al2O3 will peel off from the silicon carbide at elevated temperatures and / or temperature changes.

[0027] The catalytically active heating elements known to date are generally unconvincing. Summary of the invention

[0028] It is therefore an object of the present invention to provide a thermally stable and catalytically active heating element with which simultaneous electrical heating and chemical catalytic BMA processes are possible. In particular, the heating element should be thermally and mechanically stable and retain its catalytic activity in continuous industrial operation. Comparable catalytic heating rods are known from US 2017 / 314441 A1 and EP 1 945 345 B1.

[0029] The object is achieved by a heating element having the following characteristics:

[0030] a) a first electrical connector;

[0031] b) a second electrical connector;

[0032] c) a solid or hollow core comprising silicon carbide, wherein the core conductively connects the first contact to at least the second contact;

[0033] d) a protective coating applied to the core and comprising aluminum nitride;

[0034] e) a catalyst system applied to the protective coating, wherein the catalyst system comprises platinum.

[0035] The present invention first provides such a heating element.

[0036] The heating element according to the 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. The aluminum nitride is arranged as a protective layer between the catalyst layer and the silicon carbide. It prevents platinum and silicon carbide from forming an alloy during the operation. Since aluminum nitride has a similar thermal expansion coefficient to silicon carbide (~5 * 10 at 600 ° C), the aluminum nitride is preferably used as a protective layer. -6 K -1 ), the stresses in this layer structure caused by the different thermal expansion can be neglected. Aluminum nitride (AlN) exhibits chemically neutral behavior in the hydrocyanic acid reaction and therefore does not impair the reaction.

[0037] Preferably, the catalyst coating is applied only onto the protective coating. This prevents the formation of eutectics between SiC and Pt.

[0038] The protective coating and the catalyst coating are preferably made very thin compared to the core. Specifically, the volume v1 of the protective coating and / or the volume v2 of the catalyst coating should be smaller than the volume v0 of the core. The core requires a correspondingly larger volume v0 so that it can still conduct large currents despite its high specific resistance.

[0039] The heating element can be hollow or made of solid material and can be implemented in different shapes, possibly in the form of a cylindrical tube. The tube can be curved. The heating element has electrical connections and can be operated with two-phase or three-phase direct current and alternating current.

[0040] In a second aspect, the present invention provides the production of a heating element according to the present invention. The production comprises at least the following steps:

[0041] a) providing a core comprising silicon carbide;

[0042] b) providing a coating composition comprising aluminum and nitrogen;

[0043] c) providing a catalyst system containing platinum;

[0044] d) coating the core with the coating composition to obtain a protective coating comprising aluminum nitride attached to the core;

[0045] e) coating the protective coating with the catalyst system such that the catalyst system is attached to the protective coating.

[0046] According to the invention, a protective coating and then a catalyst coating are applied successively to the core.

[0047] According to the invention, the protective coating contains aluminum nitride. The coating composition must therefore contain aluminum and nitrogen. Aluminum and nitrogen may be present in elemental form or in the form of compounds (including compounds with themselves or with each other). The coating composition preferably contains aluminum nitride dispersed in a dispersion medium.

[0048] The protective coating is then applied by purely physical means in the coating process. Various methods are conceivable for coating the core. The simplest method is the dipping method. The core is dipped into the coating composition and removed from it again. Spraying methods are also conceivable. Printing, sputtering, roller coating or brushing may be further methods, but are only suitable in limited cases.

[0049] In all cases, drying is then carried out so that the dispersion medium evaporates and the aluminum nitride adheres to the silicon carbide.

[0050] Alternatively, a reaction method can also be used. For this purpose, the coating composition used is a system which comprises aluminum, preferably in metallic form, as a first component. As a second component, the system comprises nitrogen, preferably as a gas or as a nitrogen-containing gas.

[0051] For coating, aluminum is first applied to the core and then exposed to nitrogen. This is achieved in the simplest case by exposing the aluminum-coated core to an atmosphere containing gaseous nitrogen or a nitrogen-containing gas. In the presence of the core, the nitrogen reacts with the aluminum to provide aluminum nitride. If necessary, the atmosphere is heated to allow the aluminum and nitrogen to react to provide aluminum nitride. The aluminum nitride is then formed in situ directly on the core composed of silicon carbide.

[0052] The heating of the atmosphere can be carried out 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 coating of the aluminum is accordingly carried out by applying a dispersion. The dispersion medium can be dried with a nitrogen atmosphere and / or evaporated by electrical heating of the core. Alternatively, the metallic aluminum can be sputtered onto the core or deposited from the vapor phase.

[0053] In all coating methods it is important that the electrical connections are not coated, since AlN is an electrical insulator. This would make electrical connection no longer possible. A first possible option to prevent this is to provide the core with first and second electrical connections, then provide it with a protective coating and subsequently with a catalyst coating. It must be ensured that the electrical connections are not coated. For this purpose, the connections can be masked, for example, during the coating process.

[0054] Perhaps, only after the core has been coated with the protective coating, the core is provided with the first and second electrical connections. In this case, the core can for example be fully coated, and then the coating is partially removed from the core again to expose the electrical connections.

[0055] In a third aspect, the invention provides a heating element obtainable by the method according to the invention, which is characterized by the layer construction and the layer quality and layer adhesion produced by the coating method.

[0056] The heating element according to the invention can be used to heat endothermic chemical reactions which can be catalyzed by platinum. Temperatures of up to about 1400° C. are possible.

[0057] The heating element is preferably used in the production of hydrocyanic acid or other nitriles.

[0058] The present invention therefore likewise provides for the use of a heating element according to the invention in the production of hydrocyanic acid.

[0059] The heating element is used in particular for the electrically heated BMA process, in which hydrocyanic acid is synthesized from ammonia and methane in the absence of oxygen.

[0060] The present invention therefore further provides a method for producing hydrocyanic acid using the heating element according to the present invention. This method comprises at least the following steps:

[0061] a) providing a reactor comprising at least one heating element according to the invention;

[0062] b) supplying a reactant gas mixture containing at least ammonia and methane to the reactor, wherein the reactant gas mixture has an oxygen content of less than 2% by volume or wherein the reactant gas mixture contains no oxygen;

[0063] c) supplying electric current to the heating element;

[0064] d) removing a product gas mixture comprising at least hydrocyanic acid from the reactor.

[0065] Due to the low oxygen content or preferably the absence of oxygen, the process is not an Andrussow process but an electrically heated BMA process, referred to as an E-BMA process.

[0066] Besides hydrocyanic acid, the product gas mixture may also contain by-products or unconverted reactants.

[0067] Preferably, the process is entirely electrically heated, ie it does not provide thermal energy for carrying out the endothermic reaction. This does not exclude preheating the reactants with a non-electrical heat source external to the reactor.

[0068] Preferably, the reaction is completely catalyzed by the electrical heating element. This means that apart from the catalyst system applied to the heating element according to the invention, no further catalyst is provided in the reactor.

[0069] It is also possible to provide two or more heating elements according to the invention in a reactor. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] The present invention will now be explained in detail with reference to the accompanying drawings. In the accompanying drawings:

[0071] Figure 1 Showing: a schematic cross-sectional view of a heating element of the present invention;

[0072] Figure 2 Shown: A schematic diagram of the implementation of the method of the present invention.

[0073] The heating element 10 of the present invention is shown in Figure 1 It comprises a core 11 consisting of silicon carbide (SiC). A protective coating 12 consisting mainly of aluminum nitride (AlN) has been applied thereto. A catalyst system 13 containing platinum (Pt) has been applied on the protective coating 12. The catalyst system 13 is separated from the core 11 by the protective layer 12.

[0074] The protective coating 12 and the catalyst system 13 completely surround the core 11 except for two locations where the heating element 10 comprises a first electrical connection 14 or a second electrical connection 15. The protective coating 12 is inseparably attached to the core 11 and the catalyst system 13 is inseparably attached to the protective coating 12.

[0075] As Figure 1 As an alternative to the embodiment shown in , the core 11 can also be in the form of a hollow tube, on the inside of which the protective coating 12 is first provided and subsequently the catalyst system 13 (not shown). The catalytically active coating is correspondingly inside the tube.

[0076] The two connections 14, 15 are used to connect the heating element 10 to a voltage source 17 (at Figure 1 The heating element may also have a third electrical connection (not shown) to enable three-phase operation.

[0077] Figure 2 The method flow is schematically shown in three steps from top to bottom:

[0078] A reactor 16 is provided in which a heating element 10 is arranged and filled with a reactant gas mixture (NH3+CH4). The heating element 10 is connected to a voltage source 17 and supplied with voltage. Due to the ohmic resistance of silicon carbide, the core 11 heats up and heats the reactor 16 from the inside. The reactant gas mixture (NH3+CH4) is converted into a product gas mixture (HCN+H2) using the platinum present in the catalyst system 13. The primary product gas mixture (HCN+H2) is removed from the reactor 16 together with by-products and unconverted reactants. DETAILED DESCRIPTION

[0079] Example:

[0080] The present invention will now be explained in detail with reference to Examples.

[0081] Purpose

[0082] The aim of the experiment was to electrically heat a reactor 16 for the production of HCN to temperatures above 1100° C. using SiC heating elements, wherein the heating element 10 is arranged directly in the reaction gas phase. Since the reaction therefore takes place directly on the surface of the heating element 10, said surface must be coated with a catalyst. At the required temperature, alloy formation occurs between platinum, the main component of the BMA catalyst, and the material of the element (SiC), thereby significantly interfering with the BMA reaction. In order to avoid the formation of this alloy, a protective layer is applied to the heating element 10, thereby avoiding contact between Pt and Si. AlN (aluminum nitride) was identified as a suitable barrier layer, since the expansion coefficients between AlN and SiC are in a similar range.

[0083] Experimental Description

[0084] In this experiment, the SiC / AlN system was investigated in an experimental reactor. SiC tubes with the dimensions ØA = 22 mm, ØI = 17 mm, L = 2100 mm were coated with AlN. For this purpose, the AlN was introduced into a lacquer matrix containing binders, adhesion promoters, rheological additives and solvents. The coating of the inside of the tube was carried out in an adapted dipping method. This involves sealing one end of the tube with a stopper and filling the primer via the second opening. After the second opening has also been sealed with a stopper, the inner surface is completely coated by rotating the tube. The excess material is subsequently poured off and the primer is dried by flowing nitrogen through the tube. After a drying time of 24 hours, the tube is installed in the experimental reactor and the primer is baked in a nitrogen stream (heating rate: 100 K / h, target temperature 1150°C, holding time 2 hours). After complete cooling, in order to achieve a sufficient layer thickness, the inner surface of the tube is coated with primer again and the baking process is repeated.

[0085] Application amount: 28.6 g

[0086] Layer thickness: approx. 30 µm (calculated).

[0087] The tubes were then coated with a platinum-containing catalyst and the synthesis performance was investigated in an experimental reactor 16. The main purpose of the experiment was to evaluate the synthesis behavior over the operating duration. To this end, the apparatus was operated at a reactant gas loading of approximately 60 mol / h in excess ammonia at a temperature of 1180° C. for a period of approximately 170 h.

[0088] result

[0089] Over relatively long periods of time, the yields were greater than 80% based on ammonia and greater than 90% based on methane, thus being comparable to the levels of standard tubes made of corundum. The synthetic behavior over the period studied was comparable to that of standard tubes.

[0090] Coating method

[0091] The selected primer coating method is the simplest option for coating individual tubes at low cost and low complexity. Coating by spraying is likewise possible and has been successfully practiced.

[0092] Reference numerals

[0093] 10 Heating elements

[0094] 11 cores

[0095] 12 Protective coating

[0096] 13 Catalyst system

[0097] 14 First electrical connector

[0098] 15 Second electrical connector

[0099] 16 Reactors

[0100] 17 Voltage Source

[0101] AlN Aluminum Nitride

[0102] CH4 Methane

[0103] CH4+NH3 reactant mixture

[0104] H2 Hydrogen

[0105] HCN Hydrocyanic acid

[0106] HCN+H2 product mixture

[0107] NH3 Ammonia

[0108] Pt Platinum

[0109] SiC Silicon Carbide

Claims

1. A heating element (10), comprising at least: a) a first electrical connector (14); b) a second electrical connector (15); c) a solid or hollow core (11) comprising silicon carbide, wherein the core (11) electrically conductively connects the first connection (14) to at least the second connection (15); d) a protective coating (12) applied to the core (11); e) a catalyst system (13) applied to the protective coating (12), wherein the catalyst system (13) contains platinum, Features The protective coating (12) contains aluminum nitride.

2. The heating element (10) according to claim 1, characterized in that The catalyst system (13) is applied only to the protective coating (12).

3. The heating element (10) according to claim 1 or 2, wherein the volume v1 of the protective coating (12) and / or the volume v2 of the catalyst system (13) is smaller than the volume v0 of the core (11).

4. A method for producing a heating element (10), comprising at least the following steps: a) providing a core (11) comprising silicon carbide; b) providing a coating composition comprising aluminum and nitrogen; c) providing a catalyst system (13) containing platinum; d) coating the core (11) with the coating composition to obtain a protective coating (12) containing aluminum nitride attached to the core (11); e) coating the protective coating (12) with the catalyst system (13) such that the catalyst system (13) is attached to the protective coating (12).

5. The method according to claim 4, characterized in that The coating composition is a dispersion containing a dispersion medium and aluminum nitride dispersed therein.

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

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

8. The method according to claim 4, characterized in that The coating composition is a system comprising two components, namely a first component containing aluminum and a second component containing nitrogen, and the aluminum and nitrogen react in the presence of the core (11) to provide aluminum nitride.

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

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

11. A method for producing hydrocyanic acid, comprising the following steps: a) providing a reactor (16) comprising at least one heating element (10); b) supplying a reactant gas mixture containing at least ammonia and methane to the reactor (16), wherein the reactant gas mixture has an oxygen content of less than 2% by volume or wherein the reactant gas mixture contains no oxygen; c) supplying electric current to the heating element (10); d) removing a product gas mixture containing at least hydrocyanic acid from the reactor (16), Features The heating element (10) provided is a heating element (10) according to one of claims 1 to 3 or a heating element (10) produced according to one of claims 4 to 8.

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