Process for breaking down ammonia

Abstract

A process for catalytic cracking of ammonia, comprising supplying an ammonia feed gas to one or more catalyst-containing reaction vessels arranged in an ammonia cracking reactor, and cracking ammonia in the ammonia feed gas in one or more catalyst-containing reaction vessels to produce a hydrogen-containing stream, wherein each of the reaction vessels has walls made of at least a first alloy and a second alloy, the first alloy being more resistant to nitriding than the second alloy, the second alloy providing mechanical support to the first alloy, and at least a portion of the wall adjacent to the catalyst being made of the first alloy.

Description

Technical Field

[0001] The present invention relates to a process for catalytic decomposition of ammonia. The present invention further relates to a reaction vessel for use in catalytic decomposition of ammonia and optionally a tubular reaction vessel.

Background Art

[0002] In various industrial environments, there is a new interest in using hydrogen as an environmentally friendly carbon-free fuel. Hydrogen can be burned to generate thermal energy or electricity. Alternatively, hydrogen can be used, for example, to generate electrochemical energy in a fuel cell.

[0003] Ammonia has attracted interest as a compound that may enable the storage and transportation of hydrogen. Liquid ammonia has a higher hydrogen density than liquid hydrogen and can be transported using existing infrastructure that is already used for the transportation of ammonia in the agrochemical fertilizer industry.

[0004] Liquid ammonia can be burned directly after being transported or can be converted to hydrogen by a decomposition process.

[0005] The catalytic decomposition of ammonia into hydrogen and nitrogen has been known for many years. The reaction can be shown as follows.

[0006]

Chemical Formula

[0007] The ammonia decomposition reaction is endothermic and can be usefully achieved by passing ammonia over a suitable catalyst in a heated catalyst-containing reaction vessel such as a catalyst-containing reaction tube heated externally and placed in a furnace. For example, such furnaces for steam reforming of natural gas or naphtha feedstocks are known.

[0008] However, reaction tubes containing heating catalysts placed in a furnace may react with ammonia-containing gases or decomposed gases to form undesirable metal nitrides within the tube material. This undesirable material damage mechanism, known as nitriding, can accelerate the failure of the reaction tube, particularly at locations within the reaction tube where the nitriding potential is highest (e.g., locations where the partial pressure of ammonia / nitrogen is highest, where the temperature is highest, and / or where the combination of ammonia / nitrogen partial pressure and temperature produces the highest nitriding potential).

[0009] A malfunction in the reaction tube necessitates a complete shutdown of the ammonia decomposition reactor, resulting in significant plant downtime. Furthermore, nitriding and malfunction of the reaction tube pose a serious safety hazard.

[0010] The objective of this specification is to provide a process for catalytic cracking of ammonia that addresses the aforementioned nitriding problems and has improved operability and increased safety. [Overview of the Initiative]

[0011] One option to address the nitriding problem in the catalytic cracking of ammonia is to modify the process operating conditions to reduce the nitriding potential in the reaction tube. For example, this could involve reducing the operating temperature and / or partial pressure of the ammonia in the reaction tube. However, this may result in a decrease in the efficiency of the ammonia cracking process.

[0012] Another option to address the nitriding problem in the process of catalytically cracking ammonia is to change the material used to form the reaction vessel / tube; that is, construct the reaction vessel / tube from a material that is more resistant to nitriding. Nitriding-resistant alloys are known, however, many of these are either very expensive and / or lack the thermal and / or mechanical properties required to maintain mechanical integrity when exposed for long periods to the harsh environment of the ammonia cracking process. For example, the containment, due to the high temperature and pressure required for the ammonia cracking reaction to occur, requires heat-resistant materials. Deformation of the catalyst-containing reaction tube can occur, which can ultimately lead to the failure of the reaction tube. Heat-resistant materials are resistant to attack by high-temperature gases but also possess favorable mechanical properties at high temperatures under long-term loading. This property is known as creep strength. Alloys with high creep strength are known, however, many of these are susceptible to nitriding and / or prohibitively expensive.

[0013] It has been noted that the nitriding potential of a process environment is directly related to the partial pressure of nitrogen-supported species in the gas and the gas temperature. Since ammonia decomposition, known to be highly nitridable, is carried out at high pressure and temperature, the most resistant alloys are required for storage. These alloys have high nickel / cobalt content and are expensive, but the alloys most resistant to nitriding, while having the highest nickel / cobalt content and minimizing the nitride-forming alloying elements in the storage alloy (which are valuable as strengthening elements), are not necessarily suitable for withstanding creep deformation. To date, most ammonia decomposition reactions have been carried out at low pressure and medium scale, where the high-temperature mechanical properties and the economics of using nitride-resistant alloys are more acceptable. Ammonia decomposition at the scale required for hydrogen production in the energy industry requires high pressure and therefore higher required creep strength, but is also heavily influenced by the most economical metallurgy / design.

[0014] Since it is difficult to find a single alloy that satisfies all the requirements for ammonia decomposition at the scale currently needed for hydrogen production (nitriding resistance, thermal / mechanical resistance, cost), this specification provides a process for ammonia decomposition utilizing reaction vessels / tubes constructed of at least two different alloys, one of which is more resistant to nitriding and the other which provides mechanical support under high pressure and high temperature loads.

[0015] In light of the foregoing, this specification provides a process for catalytic cracking of ammonia, the process comprising supplying an ammonia feed gas to one or more catalyst-containing reaction vessels arranged in an ammonia cracking reactor, and cracking ammonia in the ammonia feed gas in one or more catalyst-containing reaction vessels to produce a hydrogen-containing stream, wherein each of the reaction vessels or reaction vessels has walls composed of at least a first alloy and a second alloy, the first alloy being more resistant to nitriding than the second alloy, the second alloy providing mechanical support to the first alloy, and at least a portion of the wall adjacent to the catalyst being composed of the first alloy.

[0016] In the aforementioned process, at least a portion of the reactor vessel wall adjacent to the catalyst is formed of a more nitridation-resistant alloy to reduce nitriding, while the remaining portion of the reactor vessel wall provides mechanical support to meet the thermal and mechanical requirements for long-term storage exposed to high pressure and high temperature during use.

[0017] Optionally, the second alloy may have higher creep strength and / or mechanical stability than the first alloy. However, it is also conceivable that the first alloy may be selected to have higher creep strength than the second alloy, in addition to higher nitriding resistance. In that case, the first alloy having both high nitriding resistance and high creep strength is likely to be very expensive, and therefore it is advantageous to limit the amount of such a first alloy, so the two-alloy system may still be preferred. In particular, when a small amount of the first alloy is used compared to the second alloy, the second alloy still provides mechanical support to the first alloy.

[0018] The reaction vessel, or each reaction vessel, may be in the form of a reaction tube in which the catalyst is placed. In this case, the second alloy may be in the form of a tube, and the first alloy may form a coating or layer on at least a portion of the inner surface of the tube. The first alloy may be located in the region of the reaction vessel where the nitriding potential is highest, i.e., where the partial pressure of ammonia / nitrogen and / or temperature are highest. This may be in the region closer to the inlet of the reaction vessel than to the outlet of the reaction vessel with respect to the gas flow. Alternatively or additionally, at least 50%, 60%, 70%, 80%, 90%, or substantially all of the surface of the reactor vessel adjacent to the catalyst may be formed of the first alloy. The reaction vessel, or each reaction vessel, may contain a lower weight percent of the first alloy compared to the second alloy.

[0019] The first and second alloys may be provided as layers on the walls of the reactor vessel, such that the first alloy layer is thinner than the second alloy layer. Such a configuration can reduce nitriding of the reactor vessel while limiting the amount of nitridation-resistant alloy required.

[0020] It should also be noted that additional material / alloy may be provided, for example, as an additional layer, to form the walls of the reaction vessel. For example, to improve the adhesion and / or matching of the thermal expansion coefficients between the first alloy and the second alloy, a layer of another material / alloy, such as an alloy having an intermediate thermal expansion coefficient between that of the first alloy and that of the second alloy, may be provided between the first and second alloys.

[0021] Additionally or alternatively, an intermediate layer (e.g., pure Ni or Co) may be used, having low nitrogen solubility to delay nitrogen transport through the mechanical alloy, or having a high proportion of nitride-forming elements to consume and constrain nitrogen, thereby also delaying nitrogen transport to the mechanical support layer.

[0022] Furthermore, an additional layer of oxide or oxide-forming material may be provided on at least a portion of the surface adjacent to the catalyst material to further improve nitriding resistance. Such an oxide layer may be an aluminum oxide layer formed from aluminum in the first alloy. The ammonia feed gas contains oxidizing species such as oxygen and / or water to ensure oxidation conditions in the reaction vessel and to help maintain the oxide layer. These oxidizing species may be naturally present in the ammonia feed gas or may be intentionally added. Maintaining water in the process flow gas is preferred in the background of high-pressure, high-temperature nitriding in ammonia because it offers significant advantages over other means of oxide formation and ensures good recovery of the oxide throughout the operating period if damaged.

[0023] Optionally, the first alloy contains nickel and / or cobalt in an amount greater than, for example, 11% by weight, 15% by weight, 20% by weight, 30% by weight, 50% by weight, or 75% by weight of the first alloy. In principle, up to 100% by weight of nickel / cobalt can be used as the first alloy, but optionally, the first alloy also contains one or more of Cr, Si, or Al (for example, Ni and / or Co in small amounts). Optionally, the amount of nickel and / or cobalt in the first alloy is greater than the amount of nickel and / or cobalt in the second alloy. Optionally, the second alloy contains iron, nickel, and / or chromium. Optionally, the second alloy contains at least 10% by weight, 15% by weight, 20% by weight, or 30% by weight of Cr. For example, the second alloy may be a Ni or Fe-based alloy containing a certain percentage of Cr.

[0024] The above process provides a process for decomposing ammonia to produce hydrogen, which has improved operability, reduced downtime, and increased safety, and also decomposes ammonia in a cost-effective manner in terms of the material of the structure for the reaction vessel / tube. One or more reaction tubes / containers used in the process are resistant to nitridation, creep damage, and / or embrittlement damage under the high temperature and high pressure ammonia used in the catalytic decomposition of ammonia, and can be made to be more cost-competitive than tubes made entirely of alloys resistant to nitridation.

[0025] This specification also provides a reaction vessel for use in the above process. The reaction vessel houses an ammonia decomposition catalyst and has a wall composed of at least a first alloy and a second alloy, where the first alloy is more resistant to nitridation than the second alloy, the second alloy provides mechanical support to the first alloy, and at least a portion of the wall adjacent to the catalyst is composed of the first alloy. In other aspects, the reaction vessel is as described in relation to the process for decomposing ammonia. Also provided is an ammonia decomposition reactor comprising one or more catalyst-containing reaction vessels (e.g., reaction tubes) described herein.

Brief Description of the Drawings

[0026] For a better understanding of the present invention and to show how the present invention may be implemented, certain embodiments of the present invention are now described by way of example only with reference to the accompanying drawings. [Figure 1] A schematic view of a reaction tube in the planes x, y along the axial length x is shown, and the circular cross-section of the reaction tube can be seen in the y, z plane. [Figure 2] A schematic view of a cross-section of the reaction tube in the planes y, z is shown, where "a" represents the thickness of the inner region of the tube and "b" represents the thickness of the outer region. [Figure 3] A diagram of an ammonia decomposition reactor comprising a catalyst-filled reaction tube and a burner for providing thermal energy to the reaction tube is shown. [Modes for carrying out the invention]

[0027] The following describes preferred and / or optional features of the present invention. Any aspect of the present invention may be combined with any other aspect of the present invention unless otherwise required by context. Any preferred and / or optional feature of any aspect may be combined with any aspect of the present invention, either individually or in combination, unless otherwise required by context.

[0028] The process described herein includes supplying an ammonia feed gas to one or more reaction tubes containing a heated catalyst, which are located within an ammonia decomposition reactor. The ammonia feed gas can be derived from any source. The ammonia feed gas can be produced by a catalytic combination of hydrogen and nitrogen, for example, from the Haber-Bosch ammonia synthesis process. The ammonia feed gas can be produced in an ammonia production facility located upstream of the ammonia decomposition reactor. Alternatively, the ammonia feed gas can be supplied from an ammonia gas storage facility, an ammonia storage unit, an ammonia storage tank, or an ammonia gas pipeline.

[0029] The ammonia feed gas may be preheated before being supplied to one or more catalyst-containing reaction tubes. Therefore, the process may include a step of preheating the ammonia feed gas. The ammonia feed gas may be preheated to temperatures above 350°C, above 400°C, above 450°C, above 500°C, or above 550°C. The ammonia feed gas may be preheated to temperatures below 1000°C, below 950°C, below 850°C, below 750°C, or below 700°C. The ammonia feed gas may be preheated to temperatures between 500°C and 750°C, such as 350°C to 1000°C, 400°C to 950°C, 450°C to 850°C, or 550°C to 700°C.

[0030] A suitable ammonia decomposition reactor is known and may include a fuel combustion zone having a radiating section containing one or more burners supplied with one or more fuel streams and an oxygen supply gas, e.g., air, oxygen-enriched air, or oxygen. The radiating section may comprise one or more catalyst-containing reaction tubes through which the ammonia supply gas passes. Combustion of one or more fuel streams in one or more burners in the fuel combustion zone produces thermal energy (e.g., radiant heat) to heat one or more catalyst-containing reaction tubes. Dozens or hundreds of catalyst-containing reaction tubes may be present in the radiating section. If necessary, downstream of the radiating section, the fuel gas from the combustion of one or more fuel streams may be used to preheat one or more supply streams in a convection section. A reactor comprising a radiating section containing catalyst-containing reaction tubes and a convection section for preheating the feed is known in steam methane reforming and can be applied to this process for ammonia decomposition.

[0031] Alternative ammonia decomposition reactors may be used, for example, when the combustion of one or more fuel streams in the fuel combustion zone is divided into reactors equipped with catalyst-containing reaction tubes. Such reactors may be miniature reformers available from Johnson Matthey Davy Technologies Limited. Other alternative forms of reactors include plate changers heated by molten salt, rotary heaters such as Coolbrooks, or printed circuit heat exchangers. In certain configurations, the tubes may be combusted, and the outer regions of the tubes may contain ammonia. Alternatively, the tubes may be heated electrically, inductively, or by concentrated solar heating.

[0032] The catalyst may be any ammonia decomposition catalyst. For example, a nickel catalyst and / or a ruthenium catalyst may be used. The catalyst may contain 3 to 30% by weight of nickel, preferably 8 to 20% by weight of nickel, expressed as NiO, on a suitable refractory carrier, such as alumina or a metallic aluminate. The catalyst may be in the form of pelletized units, which may contain one or more through-holes, or it may be provided as a washcoat on a structured metal or ceramic catalyst. A particularly preferred catalyst is KATALCO, available from Johnson Matthey PLC. RTM This is 27-2, which contains 12% nickel, represented as NiO, on cylindrical pellets formed from a high-surface-area calcium aluminate support.

[0033] The process of the present invention includes the step of decomposing ammonia in an ammonia supply gas in one or more heated catalyst-containing reaction tubes to generate a hydrogen-containing stream.

[0034] In combustion reactors, flames are typically above 1000°C. These can strike the reaction tubes during operation, but typically the reaction tubes are heated to temperatures in the range of 500–1000°C. Furthermore, typically one end of the reactor tube is hotter than the other. This can be due to fluctuations in the process gas temperature (e.g., a cooler inlet gas) and / or variations in the heat distribution at the radiating end.

[0035] The temperature of the ammonia supply gas at the inlet to one or more catalyst-containing reaction tubes may be in the range of 500°C to 750°C, such as 350°C to 1000°C, 400°C to 950°C, 450°C to 850°C, or 550°C to 700°C. The temperature of the hydrogen-containing stream exiting one or more catalyst-containing reaction tubes may be in the range of 500°C to 950°C, influencing the equilibrium position of the decomposition reaction. When nickel catalysts are used in one or more catalyst-containing reaction tubes, the temperature of the hydrogen-containing stream exiting one or more catalyst-containing reaction tubes may preferably be higher than about 700°C.

[0036] The pressure inlets to one or more catalyst-containing reaction tubes are set by the flow sheet design and may be at least 1, 10, 30, 50, or 100 bar, 1000, 500, 100, 75, or 50 bar or less; or within the range defined by any combination of the aforementioned lower and upper limits. Examples of ranges include 1 to 100 bar (absolute pressure), 10 to 100 bar (absolute pressure), or 30 to 75 bar (absolute pressure).

[0037] The ammonia decomposition reaction produces a hydrogen-containing stream. The hydrogen-containing stream contains H2. The hydrogen-containing stream also contains nitrogen and may further contain residual ammonia (e.g., unreacted ammonia).

[0038] Hydrogen-containing streams may contain 40 mol% or more of H2, 50 mol% or more of H2, or 60 mol% or more of H2. Hydrogen-containing streams may contain 75 mol% or less of H2, 70 mol% or less of H2, or 65 mol% or less of H2. For example, hydrogen-containing streams may contain 40 mol% to 75 mol% of H2, 50 mol% to 70 mol% of H2, or 60 mol% to 65 mol% of H2.

[0039] As described in the abstract section, the processes of this specification use a catalyst-containing reaction vessel composed of at least a first alloy and a second alloy, wherein the first alloy is more resistant to nitriding than the second alloy, the second alloy provides mechanical support to the first alloy, and at least a portion of the wall of the reaction vessel adjacent to the catalyst is composed of the first nitride-resistant alloy. In a configuration in which the reaction vessel is in the form of a catalyst-containing tube, the first nitride-resistant alloy is provided on the inner surface of the tube. Alternatively, if heating is provided by the tube and the catalyst is located in the reactor region outside the tube, the first nitride-resistant alloy is provided on the outer surface of the tube, i.e., the side of the tube adjacent to the catalyst.

[0040] In particular, nitriding can cause accelerated failure of the reaction tube at locations within the reaction tube where the nitriding potential is highest or where the temperature driving nitriding is highest. This is typically the location towards the inlet end of the reaction tube where the ammonia partial pressure is highest (e.g., closer to the inlet than the outlet, optionally the inlet). However, since the temperature may not be highest at the inlet location depending on the reactor design, the location with the highest nitriding potential may be some distance below the inlet in the reaction tube. This damage mechanism can be mitigated by providing a nitriding-resistant alloy at least at the location with the highest nitriding potential, while simultaneously using another alloy material to provide mechanical support.

[0041] The following description illustrates a configuration in which a catalyst is provided within a reaction tube, wherein one or more reaction tubes have an external region containing a mechanical support composition and an internal region containing a nitridation-resistant composition. In this configuration, the external region of the reaction tube is exposed to a heat source used to supply thermal energy to one or more catalyst-containing reaction tubes, and this heat is used to support the endothermic ammonia decomposition reaction in the ammonia decomposition reactor. The internal region of the reaction tube refers to the inside of the reaction tube containing the catalyst used to catalytically decompose ammonia in the ammonia feed gas.

[0042] The reaction tube has an inlet and an outlet that define the axial length x of the reaction tube. It will be understood that the catalyst for the ammonia decomposition reaction is located inside the reaction tube along the axial length x. It will be further understood that the ammonia feed gas passes from the inlet side of the reaction tube over the ammonia decomposition catalyst located inside it, and the hydrogen stream exits the reaction tube through the outlet side. Figure 1 shows the axial length x of the reaction tube in the x,y plane.

[0043] The reaction tube has a cross-section in a plane perpendicular to its axial length in the y,z plane. The cross-section can take any shape and is not particularly limited. For example, the cross-section may be circular, elliptical, quadrilateral (e.g., square or rectangle), or triangular. Typically, the cross-section is circular. When the cross-section is circular, the reaction tube will be understood to be a cylindrical tube.

[0044] The outer and inner regions have thicknesses measured in the plane of the reaction tube's cross-section. An example of a cross-section of the reaction tube in the y and z planes is shown in Figure 2, where the cross-section is circular in shape, and "b" and "a" represent the thicknesses of the outer and inner regions, respectively. Preferably, the outer region has a greater thickness than the inner region (i.e., b > a).

[0045] The outer region (second “mechanically supported” alloy) may have a thickness of at least 1 mm, 3 mm, 5 mm, 8 mm, or 10 mm; 100 mm, 50 mm, 20 mm, 15 mm, or 14 mm or less; or within a range defined by any combination of the lower and upper limits. An exemplary thickness range is 8–14 mm, optionally 10–14 mm. However, it should be noted that these ranges are for standard combustion reactor designs, and the thickness may vary in other types of reactor configurations. For example, printed circuit heat exchangers and plate exchangers may use thinner thicknesses, e.g., 0.1–2 mm, optionally 0.25–1 mm.

[0046] The internal region may have a thickness of at least 0.05 mm, 0.1 mm, 0.5 mm, or 1 mm; 10 mm, 8 mm, or 5 mm or less; or within a range defined by any combination of the lower and upper limits. An example thickness range is 1 to 5 mm.

[0047] The external composition is a heat-resistant alloy that may contain iron, nickel, and / or chromium. For example, the external composition may be a heat-resistant and corrosion-resistant alloy containing at least 10% by weight, 15% by weight, 20% by weight, or 30% by weight of chromium (e.g., a nickel or iron-based alloy containing this proportion of chromium). To avoid doubt, the amount of any component of the external or internal composition is expressed as a weight percentage of the composition.

[0048] Optionally, the amount of nickel and / or cobalt in the external composition is less than the amount of nickel and / or cobalt in the internal composition. Suitable alloys for the external composition are commercially available, and the most preferred alloy may vary depending on the specific operating conditions of the ammonia decomposition process and according to the reactor design. For example, the external composition may be an alloy selected from alloy 625, alloy 800 / H / HT, alloy 20 / 32, alloy 20 / 32Nb / CT15C, HK 40, HP Nb, HP microalloy, microalloy, 25 / 35Nb, Sanicro 25, Super 304H, 314, 347, or P91.

[0049] Heat-resistant materials for use above 550°C tend to be based on iron-nickel-chromium alloy systems for petrochemical applications, with economic considerations outweighing other factors strongly influencing material selection. These alloys tend to have an 8-30 wt% chromium, or aluminum, or silicon content to form a protective oxide layer. Chromium is the most universally usable because it dissolves very readily in nickel, iron, and cobalt, which tend to form a low-grade intermetallic phase. A chromium content of 20-30 wt% provides the best protection from oxidation up to almost 1000°C. Aluminum oxide is generally more protective above 1000°C but is alloyed at much lower proportions than chromium.

[0050] Suitable creep strength and metallurgical stability / predictability allow for a longer service life of metal alloy components before they become prone to strain, cracking, and eventual loss of storage or replacement. While metal alloy components may lose strength or even increase it, the material can form voids and cracks, and the material can migrate over time, leading to loss of storage.

[0051] Suitable iron-nickel-chromium heat-resistant alloys for use at ammonia decomposition temperatures include, but are not limited to, austenitic stainless steels such as the 300 series alloys, including 304, 309, 310, 314, 316, 321, 330, 347, and their variations. Alternatively, austenitic stainless steels of the 200 series, or ferritic and martensitic stainless steels of the 400 series, may be considered. For lower temperature ranges, even alloy steels such as P91 can be considered.

[0052] At higher temperatures, alloys with increased nickel and / or cobalt content may be considered, such as alloy 800 and its variations, alloys in the 600 series including alloys 825, 253MA, 353MA, UNS S31035, 600, 601, 602CA, 625, 690, and alloys produced by centrifugal casting processes for applications such as modification and ethylene decomposition.

[0053] Nickel, chromium, iron, and molybdenum alloys such as C-276, C22, C2000, and alloy 59 may also be considered; however, due to economic considerations for simple high-temperature strength and oxidation resistance applications, the use of these more expensive alloys tends to be avoided.

[0054] In addition to alloy 718, other nickel and chromium-containing alloys containing a considerable amount of cobalt and / or tungsten may also be considered, such as precipitation-hardening alloys like alloys 188, 230, C-276, and 617.

[0055] Nitrogen and nitrogen-containing gases, including ammonia (NH3), are widely used in process industries as raw materials, intermediates, products, or fuels. In the presence of these gases, nitrogen may react with alloying elements in process containment vessels to form nitrides. Different elements have different affinities to nitrogen and therefore form nitrides more readily or less readily than other elements. Nickel and cobalt are known to be weak nitride-forming factors, while aluminum and titanium are strong nitride-forming factors, and chromium is intermediate. The formation of these nitrides in alloy components can cause embrittlement and thus lead to significant material problems, but it can also reduce the effective wall thickness. Common nitrides in commercially available heat-resistant alloys at high temperatures are Fe4N / Fe2N, CrN / Cr2N, AlN, and TiN / Ti2N.

[0056] In addition, nickel and cobalt alloys have been shown to have significantly higher nitrogen solubility than iron-based alloys such as stainless steel. Higher nickel / cobalt content has been shown to increase the alloy's ability to withstand nitriding and associated harmful mechanical problems. However, nickel and cobalt are significantly more expensive than iron, and as these elements are required to expand the battery materials market, the cost ratio is likely to continue to rise.

[0057] The new scale of ammonia decomposition applications is pushing the boundaries of what is currently needed in terms of reactor vessel alloys, particularly in relation to the parallel issues of the need for economically viable creep strength and high nitridation resistance. While several alloys have been shown to be both high creep strength and high nitridation resistance, these contain cobalt and high levels of nickel and can therefore be prohibitively expensive. Increasing the iron content reduces the cost of the tubing, but since iron is a strong nitride-forming factor, contact with the nitriding environment should be avoided. At the scale at which these reactor tubing needs to be manufactured for ammonia decomposition in the energy industry, the economics of bimetallic tubing may be preferable to simply increasing the levels of cobalt and nickel in a single metal alloy solution. Thus, this solution of using two or more alloy types is considered advantageous over simply using a more nitridation-resistant alloy and not operating at high pressure / high temperature, or using a more expensive alloy, or using a thicker containment vessel.

[0058] The internal composition may be an alloy containing nickel and / or cobalt in amounts exceeding, for example, 11% by weight, 15% by weight, 20% by weight, 30% by weight, 50% by weight, or 75% by weight (either individually or in a combined total). In principle, up to 100% by weight of nickel / cobalt can be used, but optionally, the internal composition may also contain one or more of Cr, Si, or Al. Optionally, the amount of nickel and / or cobalt in the internal composition is greater than the amount of nickel and / or cobalt in the external composition. The internal layer serves for nitridation resistance while simultaneously saving expensive Ni and Co by not requiring the entire reaction tube to be formed from expensive Ni / Co-rich alloy material. The two-alloy approach for reaction tubes also allows for the use of more optimal compositions for separate functions: nitridation resistance inside the tube and mechanical support / thermal stability outside the tube. The precise nickel / cobalt composition for the internal composition depends on the nitridation potential, which depends on temperature, plus partial pressure, and the type of nitrided species. The following table provides several examples of alloys suitable for different operating temperatures, pressures, and nitriding potentials.

[0059] [Table 1]

[0060] [Table 2]

[0061] Additional components may include overlay / thermal spray / powder metallurgy pathways, and any welding consumables and metal powders of the same or similar composition as above, for example, low / medium / high Ni / Co, and with or without Al.

[0062] The inner surface of the internal alloy may contain an oxide layer, which is located on the surface of the internal alloy exposed to the process fluid. This is formed from the reaction of the alloy with oxygen in the air during fabrication. Furthermore, the oxide layer grows and recovers in the process environment, provided oxygen is available. A lower oxygen partial pressure is required for alumina formation in aluminum-containing alloys. Controlling the oxide species content (e.g., water and / or oxygen) in the process flow gas within the reaction tube during operation can ensure oxidation conditions and help maintain the protective oxide layer. Alternatively, naturally occurring oxide species in the process flow gas may perform this function. However, maintaining water in the process flow gas is preferred in the background of high-pressure, high-temperature nitriding in ammonia, as it offers significant advantages over other means of oxide formation and ensures good recovery of the oxide throughout the operating period if damaged. The oxide layer can provide a layer that protects the reaction tube from the nitriding effect of the ammonia supply gas.

[0063] The internal composition may include an alloy containing Cr, Si, and / or Al to provide greater advantages in terms of oxide formation, with Al oxide being preferred. It should be noted that this specification anticipates that nitridation-resistant alloys may be selected to utilize oxide scale for nitridation resistance, or an optimized Ni / Co content for nitridation resistance, or both. The preferred choice depends on the nitridation potential and economics. For example, the internal composition may be an alloy selected from alloy 600, alloy 601, alloy 200, alloy 214, or MA47P.

[0064] The external and internal regions of the reaction tube can be joined together by metallic bonding. The bonding can be homogeneous throughout the entire axial length of the reaction tube, forming a single interface between the internal and external compositions. In other words, the internal and external regions can be joined together such that the reaction tube has one inner surface and one outer surface. Examples of metallurgical bonding processes include extrusion bonding, explosive bonding, hot isostatic pressing, and centrifugal casting.

[0065] Optionally, the reaction tube may include an additional bonding layer between the inner and outer layers. This may be advantageous for various reasons, such as better metallurgical bonding between two different alloys or a better match in thermal expansion coefficients. Optionally, the intermediate layer may be an alloy that is not very good as an oxide-forming factor but is an excellent nitrogen barrier, such as pure Ni / Co.

[0066] The reaction tube can be manufactured using processes known in the art. For example, the external region containing the external composition may first be formed on the base tube using a seamless tube rolling process also known as centrifugal casting or pilgering. The internal region containing the internal composition may then be positioned on the base tube by processes such as welding overlay or thermal spraying. Alternatively, the external and internal regions may be formed simultaneously on the reaction tube using a co-extrusion or co-rolling process during pilgering, or through hot isostatic pressing (HIP) of powder.

[0067] In addition to the ammonia decomposition process described above, this specification also provides a reaction vessel, such as a reaction tube, for use in the catalytic cracking of ammonia, wherein the reaction tube contains an ammonia decomposition catalyst. The ammonia decomposition catalyst is contained inside the reaction tube; that is, the ammonia decomposition catalyst is contained inside the reaction tube along its axial length. The ammonia decomposition catalyst can be any catalyst known to be suitable for catalyzing the decomposition of ammonia to hydrogen and nitrogen. The catalyst may preferably be a nickel-based catalyst, such as nickel on an inert support (e.g., alumina, silica, or another refractory oxide). For example, the catalyst is KATALCO 27-2, available from Johnson Matthey PLC. RTM This is possible. Alternatively, the catalyst may preferably be a noble metal catalyst, such as a ruthenium-based catalyst.

[0068] Also provided is an ammonia decomposition reactor comprising one or more catalyst-containing reaction vessels (e.g., reaction tubes) as described above. Figure 3 shows a diagram of an ammonia decomposition reactor comprising a catalyst-filled reaction tube and a burner for supplying thermal energy to the reaction tube.

[0069] Although the present invention has been specifically illustrated and described with reference to certain examples, it will be understood by those skilled in the art that various modifications of form and detail can be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A process for catalytic cracking of ammonia, wherein the process is Supplying ammonia supply gas to one or more reaction vessels containing a heating catalyst located within the ammonia decomposition reactor, This includes decomposing ammonia in the ammonia supply gas in one or more catalyst-containing reaction vessels to generate a hydrogen-containing stream, A process wherein the reaction vessel or each of the reaction vessels has walls composed of at least a first alloy and a second alloy, the first alloy being more resistant to nitriding than the second alloy, the second alloy providing mechanical support to the first alloy, and at least a portion of the wall adjacent to the catalyst being composed of the first alloy.

2. The process according to claim 1, wherein the second alloy has higher creep strength and / or mechanical stability than the first alloy.

3. The process according to claim 1 or 2, wherein the reaction vessel or each reaction vessel is in the form of a reaction tube in which a catalyst is arranged.

4. The process according to claim 3, wherein the second alloy is in the form of a tube, and the first alloy forms a coating or layer on at least a portion of the inner surface of the tube.

5. The process according to any one of claims 1 to 4, wherein the first alloy is provided in at least one region of the reaction vessel or each reaction vessel where the nitriding potential is highest.

6. The process according to any one of claims 1 to 5, wherein the first alloy is provided in the reaction vessel or at least one region of each reaction vessel, closer to the inlet of the reaction vessel than to the outlet of the reaction vessel with respect to the gas flow.

7. The process according to any one of claims 1 to 6, wherein at least 50%, 60%, 70%, 80%, 90%, or all of the reactor vessel adjacent to the catalyst or the surface of each reactor vessel is formed of the first alloy.

8. The process according to any one of claims 1 to 7, wherein the reaction vessel or each reaction vessel contains the first alloy in a lower weight percentage compared to the weight percentage of the second alloy.

9. The process according to any one of claims 1 to 8, wherein the first alloy and the second alloy are provided as layers of the reactor vessel or the walls of each reactor vessel, such that the first alloy layer has a thinner thickness than the second alloy layer.

10. The process according to any one of claims 1 to 9, wherein the first alloy has an oxide layer disposed on its surface.

11. The process according to any one of claims 1 to 10, wherein an intermediate adhesive layer is provided between the first alloy and the second alloy.

12. The process according to any one of claims 1 to 11, wherein the first alloy contains nickel and / or cobalt in an amount greater than 11% by weight, 15% by weight, 20% by weight, 30% by weight, 50% by weight, or 75% by weight of the first alloy.

13. The process according to any one of claims 1 to 12, wherein the first alloy contains nickel and / or cobalt in a greater weight percentage than the second alloy.

14. The process according to any one of claims 1 to 13, wherein the first alloy comprises one or more of Cr, Si, or Al.

15. The process according to any one of claims 1 to 14, wherein the second alloy comprises iron, nickel, and / or chromium.

16. The process according to any one of claims 1 to 15, wherein the second alloy comprises at least 10% by weight, 15% by weight, 20%, or 30% chromium.

17. A reaction vessel for use in a process according to any one of claims 1 to 16, wherein the reaction vessel comprises an ammonia decomposition catalyst and has walls composed of at least a first alloy and a second alloy, the first alloy being more resistant to nitriding than the second alloy, the second alloy providing mechanical support to the first alloy, and at least a portion of the wall adjacent to the catalyst being composed of the first alloy.

18. An ammonia decomposition reactor comprising one or more reaction vessels as described in claim 17.

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