Electrically directly heated catalysts for endothermic ammonia cracking

Abstract

Tube bundle reactor (1) for endothermic NH3 cracking, characterized in that catalytically active internals (3) for carrying out reactions are provided in the tubes (2).

Description

[0001] Ammonia (NH3) is well-known as a raw material for fertilizers, pharmaceuticals, plastics, and many other chemicals. Currently, it is gaining increasing importance as a storage medium for hydrogen because handling the four-atom molecule, which contains one nitrogen atom (N2) and three hydrogen atoms (H2), is considerably easier in liquid form than handling liquefied hydrogen.

[0002] In CHEManager 8 / 23 (Chemistry & Life Sciences), ammonia is identified as the key to hydrogen handling. This is plausible because, while the energy density of liquid hydrogen is a respectable 1.4 kWh / L, this requires a pressure of approximately 700 bar. In contrast, the volumetric energy density of liquid ammonia is 3.2 kWh / L, requiring a pressure of only 8 bar.

[0003] NH3 production still follows the Haber-Bosch process, which is over 100 years old and involves the catalytic reaction of H2 with N2 from the air. Provided the H2 used is produced renewably, NH3 can be considered a climate-neutral energy carrier. This means that wherever renewable energy is readily available and inexpensive, NH3 can be produced from green H2 and transported over long distances, usually by sea, to major global industrial centers. There, NH3 can be delivered to large terminals, stored, or transported to local consumers. When needed, H2 can then be produced again through a process called NH3 cracking.

[0004] The splitting of NH3 to H2 and N2 is an endothermic, one-step reaction: 2NH3→N2+3H2 ΔH°=46 kJ / mol

[0005] For decades, endothermic reactions have been carried out in tube bundle reactors using ceramic catalysts. This concept is well-known, relatively simple, and can be scaled relatively easily by changing the number of tubes. However, a disadvantage of almost all fixed-bed configurations is thermal management, as a very large number of small tubes are required to achieve a reasonably uniform temperature profile within the bed and thus avoid potentially harmful hotspots and unwanted reactions. Heat is transferred from the externally heated tube wall to the interior by conduction. The temperature difference between the outer tube wall and the tube center can be up to 150 K lower radially.

[0006] This heat input can be achieved through natural gas combustion or by burning a portion of the NH3 produced. Hydrogen produced through splitting is generated. A disadvantage is that in both cases NOx is produced, and in the case of natural gas combustion, CO2 is also additionally produced.

[0007] In endothermic processes, the required heat can be transferred to the tubes of a tube bundle reactor, for example, by a very high number of ceiling burners (up to 3000). The heat from the combustion flame is then transferred by radiation to the tube wall and subsequently by conduction to the ceramic catalyst bed and the fluid flowing through it.

[0008] Although endothermic processes are state of the art, they require a very high level of peripheral effort for fuel storage, heat supply and, last but not least, for safety-relevant monitoring in the event of failure of these components.

[0009] The performance of endothermic tube bundle reactors depends heavily on the heat generation and distribution within the reactor, but, more importantly, also on the heat conduction within the ceramic catalyst supports used so far.

[0010] Due to heat transport mechanisms resulting from the insufficient thermal conductivity of the ceramic catalysts and the simultaneously occurring endothermic reactions, a different axial and simultaneously an uneven radial temperature profile develops within the reaction tubes.

[0011] Due to the significantly different coefficients of thermal expansion between the metallic tubes (high coefficient of expansion) and the ceramic catalyst bed (low coefficient of expansion), catalyst fractures and the generation of small particles repeatedly occur when such reactors are heated to the operating point and then cooled down, either intentionally or due to a malfunction. This results in pressure losses far outside the operating range. Furthermore, the necessity of selecting very small tube diameters due to the low thermal conductivity of the catalyst bed imposes additional restrictions on the size of the ceramic catalyst particles and leads to further problems related to pressure loss in these tubes. The smaller the tube diameter, the smaller the catalyst dimensions must be. This also significantly increases the risk of creeping currents along the tube wall.In addition, the varying temperatures within the reactor tubes cause further complications, such as undesirable side reactions, which are addressed or minimized through various countermeasures, such as regeneration. To regenerate the catalyst bed, the burner output must be reduced to very low levels at regular intervals, and the furnace chamber temperature can drop so low that the burners, when needed, no longer ignite spontaneously but rely on external ignition sources. For safety reasons, this necessitates that each of the hundreds of burners be equipped with flame monitoring, including appropriate instrumentation and maintenance. The issue of fouling of the sight glasses is also a key focus of plant operation.

[0012] In Sebastian Thor Wismann's (2019) article, "Electrically heated steam methane reforming," published by the Technical University of Denmark, it is explained that uniform heating of tube bundle reactors is a very complex process. The combustion temperature must be several hundred degrees higher than the target reaction temperature, and the required power output must be approximately 150 kW / m². 2 This results in less than 2% of the total volume of the burner-fired, refractory-lined reactor being available as the total volume for the catalyst. Furthermore, it is stated that conventional burner-fired tube bundle reactors require approximately one week to reach a constant operating state due to the stringent requirements regarding the heating gradients of the refractory lining.

[0013] The object of the invention is therefore to provide the heat requirement of endothermic tube bundle reactors via conductively directly or indirectly heated catalysts, without having to use hundreds, in some cases thousands of failure-prone and maintenance-intensive individual burners.

[0014] This problem is solved by filling the tubes of the tube bundle reactors with uniform metallic, oxide, or non-oxide ceramic materials in terms of shape and structure, which are electrically conductive and thus heatable to target temperatures by Joule heating, or by uniformly heating the catalyst bed with its support over a large area using resistance wire heaters embedded within the bed. These materials can also serve as catalyst supports or be directly catalytically active. The positive effect of this measure on the overall system can be significantly intensified by an additional catalytic internal coating of the individual reaction tubes.

[0015] The use of metallic, oxide, or non-oxide ceramic electrically heated catalysts ensures uniform heat transfer both axially and radially within the tubes. This allows for the use of significantly larger diameter tubes and reduced heights, resulting in very compact tube bundle reactors. A further advantage is that the elimination of numerous burners, which generate a large amount of radiant heat, allows for a considerably smaller refractory structure in these tube bundle reactors. Specifically, it becomes possible to use significantly larger quantities of simple insulating mats instead of the complex lightweight refractory bricks required for the reactor construction.

[0016] Furthermore, it is known that, for example, metallic catalyst supports have a low heat capacity and high thermal shock resistance. This, combined with the high thermal conductivity of these candidates, allows for faster and more uniform heating of the reaction apparatus to its operating temperature than with previously known catalyst supports. A person skilled in the art recognizes that faster heating of the system to its operating temperature can significantly increase capacity and thus reduce operating costs. Moreover, this type of heating within the catalyst bed, directly or indirectly at the catalyst, ensures a very uniform temperature distribution both radially and along the longitudinal axis of the tube.Furthermore, the heat transfer surface area of ​​such heating is much higher than when heating the outer surface of the pipe using a burner, so that the temperature gradient required for heat transport is smaller, thus protecting the catalyst and saving considerable energy.

[0017] Alternatively, it is of course possible to continue using the familiar ceramic catalyst bed and avoid the aforementioned disadvantages of prior art burner heating by applying heat to the ceramic catalyst bed in situ via electrical resistance wire heating or induction. This allows heat transfer between the heating wire and the catalyst bed to occur primarily through conduction. As already mentioned, the main advantage of this electrical heating is the achievement of uniform temperature profiles in both axial and radial directions.

[0018] Overall, the invention presented here can largely avoid the known susceptibility to malfunctions of previously used endothermic catalytic tube bundle reactors and significantly increase the efficiency.

[0019] Furthermore, the specific energy consumption of the catalysts is significantly lower with electrical internal heating than with natural gas heating using ceiling burners, because the considerable heat losses from exhaust gases are eliminated. This is also due to the fact that these systems can be built much more compactly. The use of electricity from renewable energy sources is particularly advantageous, especially during periods of lower demand from other consumers. Therefore, they could also be considered for decentralized applications. Electrically heated catalysts or catalyst supports are electrically conductive materials. They can consist of one or more components, each of which, due to its relatively high specific electrical resistance, fulfills the necessary property of converting electrical energy into heat energy.One of the main requirements, besides high-temperature and oxidation resistance, is that the specific resistance remains as independent as possible from the operating temperature. Alloying can enhance the positive properties of individual catalysts or catalyst supports and allow for differentiated adaptation to their use in tube bundle reactors.

[0020] Suitable electrically conductive material components (catalyst supports) from the metal category include austenitic CrFeNi alloys and ferritic CrFeAl alloys. Active catalytic substances include elements such as Ru, Ir, Rh, Ni, Co, Os, Pt, Fe, Mo, Pd, and Ag. In addition to optimal catalytic activity and sufficient selectivity, cost-effectiveness also plays an important role in the selection of the catalyst.

[0021] Advantageous directly heated catalyst geometries are uniform structures such as turbulators, cylindrical brush-like shapes, structured tubular metal wire devices.

[0022] Structured metal wire devices made of the aforementioned alloys are considered particularly suitable as heated catalyst supports. To achieve high catalytic efficiency, the wires can be formed into a variety of geometries with virtually any desired diameter, length, void ratio, and specific surface area. The catalyst supports can be arranged horizontally, vertically, or at specific angles within the reactor tubes. To achieve very long lengths, these tubular metal wire devices can be connected end-to-end.

[0023] To prevent edge penetration, the catalyst support can be flexibly shaped with a larger outer diameter than the inner diameter of the tube. This creates a compression fit during installation, slightly compressing the wires of the device at the edge and locally increasing the pressure drop, thus preventing leakage. An additional advantage of this arrangement is the self-locking of the larger diameter within the tube bundle reactor, eliminating the need for a separate retaining device for the catalyst bed.

[0024] Another advantage of this device according to the invention is that, due to the significantly smaller pressure loss compared to the ceramic catalyst carriers otherwise used, the Opex content can be kept low; in addition, it is possible to produce significantly higher specific surface areas with small overall dimensions of this equipment, so that comparatively very compact tube bundle reactors can be manufactured (Capex).

[0025] In electrically heated tube bundle reactors, heat is supplied precisely where it is needed. The inherently large surface area for mass transfer also ensures an energy-efficient and gentle energy supply using, wherever possible, renewable electricity.

[0026] Overall, this innovative heating approach enables the operation of ultra-compact tube bundle reactors without extended reactor tubes and external burners.

[0027] Further advantages, details and features of the invention will become apparent from the following description and from the drawings, which show: Fig. 1 a simplified sectional drawing through a tube bundle reactor according to the state of the art with ceramic catalysts inside the reaction tubes and ceiling burners (only 4 shown), Fig. 2 a simplified sectional drawing through a tube bundle reactor according to the invention with electrically directly heated catalysts inside the reaction tubes, Fig. 3 a simplified sketch of the structured electrically heated catalyst support based on metal wire according to the invention,

[0028] The in Fig. One tube bundle reactor, generally referred to as 1, consists of a multitude of tubes (2). Within these tubes is a ceramic catalytic bed (3). The NH3 to be treated enters (4), and after the reaction, a mixture of NH3, N2, and H2 flows out again in (5). The heat required for the endothermic reaction is generated by combustion of the combustion medium (6) within the ceiling burners (7). The exhaust gases flow out in (8). The subsequent steps are state of the art (heat exchanger, DeNOx, PSA, etc.) and are not explicitly described in more detail.

[0029] In Fig. Figure 2, also shown as Figure 1, is a tube bundle reactor constructed from a multitude of tubes (2). Highly thermally conductive, electrically directly heated catalysts (3a) are positioned within these tubes. The fluid to be treated enters (4) and exits (5) after the reaction.

[0030] In Fig.Figure 3 (3a) illustrates the electrically heated catalytic metal wire packing according to the invention. The NH3 is introduced into (4) and, after the reaction, a mixture of NH3, N2, and H2 flows out again into (5). The heat required for the endothermic reaction is provided by electrical resistance heating.

[0031] Of course, the examples described can be modified and supplemented in many ways without departing from the fundamental concept of the invention. The invention also relates to the method for optimizing catalytic reactions by using electrically heated catalytic devices. A person skilled in the art would naturally use this conductive electrical catalyst heating for other endothermic reactions such as steam reforming, propane dehydrogenation, methane pyrolysis, and so on. However, the heating can also be carried out inductively using the same electrically conductive catalysts. List of reference symbols 1 Tube bundle reactor 2 pipes 3 Catalytic packing 3a Electrically heated catalytic bed 4. NH3 entry into fission 5. Release of NH3, N2, H2 6 Combustion medium 7 burners 8 Exhaust gas QUOTES INCLUDED IN THE DESCRIPTION

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

[0000] Sebastian Thor (2019), “Electrically heated steam methane reforming”, Technical University of Denmark

[0012]

Claims

[1] Tube bundle reactor (1) for endothermic NH3 splitting, characterized by that catalytically active internals (3) are provided in the tubes (2) for carrying out reactions. [2] Tube bundle reactor (1) according to claim 1, characterized by that the catalytically active components (3a) are electrically conductive and provide the heat required for the reaction directly via conductive heating. [3] Tube bundle reactor (1) according to one of the preceding claims, characterized by that the catalytically active components (3a) are electrically conductive and provide the heat required for the reaction directly via inductive heating. [4] Tube bundle reactor (1) according to one of the preceding claims, characterized by that the energy supply for the direct heating of the catalytically active components is also provided by renewable energy sources. [5] Tube bundle reactor (1) according to one of the preceding claims, characterized bythat the energy supply for the direct heating of the catalytically active components is also provided by renewable energy sources and, if necessary, is supplied by electricity storage systems.

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