Method for cracking ammonia

Abstract

A process for catalytic cracking of ammonia, the process comprising: supplying an ammonia feed gas to one or more heated catalyst-containing reaction vessels disposed within an ammonia cracking reactor; and cracking ammonia in the ammonia feed gas in the one or more catalyst-containing reaction vessels to produce a hydrogen-containing stream wherein the reaction vessel or each of the reaction vessels has a wall comprised of at least a first alloy and a second alloy wherein the first alloy is more resistant to nitriding than the second alloy, and the second alloy provides mechanical support for the first alloy, and wherein at least a portion of the wall adjacent the catalyst is comprised of the first alloy.

Description

TECHNICAL FIELD

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

[0002] There is renewed interest in using hydrogen as a green, carbon-free fuel in various industrial settings. Hydrogen gas can be combusted to produce heat energy or electricity. Alternatively, hydrogen can be used to produce electrochemical energy in, for example, fuel cells.

[0003] Ammonia is of interest as a possible compound to store and transport hydrogen. Liquid ammonia has a higher hydrogen density than liquid hydrogen and can be transported using existing infrastructure already used for this purpose, such as infrastructure used to transport ammonia in the agrochemical fertilizer industry.

[0004] Once liquid ammonia is transported, it can be combusted directly or converted to hydrogen by a cracking process.

[0005] Catalytic cracking of ammonia to hydrogen and nitrogen has been known for many years. The reaction can be depicted as follows:

[0006]

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

[0008] However, a heated catalyst-containing reaction tube arranged in a furnace can react with the ammonia-containing gas or cracking gas, thereby forming undesirable metal nitrides within the tube material. This undesirable material damage mechanism, known as nitriding, can lead to accelerated failure of the reaction tube, particularly at locations within the reaction tube where the potential for nitriding is highest (e.g. locations of highest partial pressure of ammonia / nitrogen, locations of highest temperature, and / or locations where the combination of ammonia / nitrogen partial pressure and temperature results in the highest potential for nitriding).

[0009] Reaction tube failure requires complete shutdown of the ammonia cracking reactor and results in significant plant downtime. Furthermore, nitriding and failure of the reaction tube present a serious safety hazard.

[0010] It is an object of the present specification to address the foregoing problem of nitriding and to provide a process for catalytic cracking of ammonia with improved operability and increased safety. SUMMARY

[0011] One option for addressing the nitriding problem in a process for catalytically cracking ammonia is to change the operating conditions of the process to reduce the potential for nitriding within the reaction tube. For example, lowering the operating temperature and / or partial pressure of ammonia within the reaction tube. However, this can result in a decrease in the efficiency of the ammonia cracking process.

[0012] Another option for addressing the nitriding problem in a process for catalytically cracking ammonia is to change the material used to form the reaction vessel / tube. That is, to construct the reaction vessel / tube from a material that is more resistant to nitriding. However, while alloys that are resistant to nitriding are known, many of these alloys are very expensive and / or do not have the thermal and / or mechanical properties required to maintain mechanical integrity when exposed to the harsh environment of the ammonia cracking process for long periods of time. For example, due to the high temperatures and pressures required for the ammonia cracking reaction, a heat resistant material is required for containment. Deformation of the reaction tube containing the catalyst can occur, which can result in eventual failure of the reaction tube. Heat resistant materials have resistance to hot gas erosion, but also have suitable mechanical properties at high temperatures under long term load. This property is referred to as creep strength. However, while alloys with high creep strength are known, many of these alloys are susceptible to nitriding and / or are too expensive.

[0013] It has been noted that the nitriding potential of a process environment is directly related to the partial pressure of nitrogen containing species in the gas and the temperature of the gas. Since ammonia cracking is performed at pressures and temperatures known to be highly nitriding, the most resistant alloys are required for containment. These alloys have high nickel / cobalt content and are expensive, and by minimizing the nitride forming alloying elements that are valuable in containment alloys as strengthening elements, the most nitriding resistant alloys are not necessarily also suitable for resisting creep deformation. To date, most ammonia cracking reactions have been performed at low pressure and moderate scale, where the high temperature mechanical properties and use economics of nitride resistant alloys have been more acceptable. Ammonia cracking at the scale required for hydrogen production in the energy sector will require high pressure, and thus higher creep strength, but will also be highly influenced by the most economic metallurgy / design.

[0014] Since it is difficult to find a single alloy that meets all the requirements (nitriding resistance, heat / mechanical resistance, cost) of ammonia cracking at the scale required for current hydrogen production, the present specification provides a method for ammonia cracking that utilizes a reaction vessel / tube composed of at least two different alloys, where one alloy is more resistant to nitriding and the other alloy provides mechanical support under high pressure and high temperature load.

[0015] In view of the above, the present specification provides a method for catalytic cracking of ammonia, the method comprising supplying an ammonia feed gas to one or more heated catalyst-containing reaction vessels disposed within an ammonia cracking reactor, and cracking ammonia in the ammonia feed gas in the one or more catalyst-containing reaction vessels to produce a hydrogen-containing stream, the or each of the reaction vessels having a wall composed of at least a first alloy and a second alloy, wherein the first alloy is more resistant to nitriding than the second alloy, and the second alloy provides mechanical support to the first alloy, and wherein at least a portion of the wall adjacent to the catalyst is composed of the first alloy.

[0016] In the foregoing method, at least a portion of the reactor vessel wall adjacent to the catalyst is formed of an alloy that is more resistant to nitriding to reduce nitriding, while the remainder of the reactor vessel wall provides mechanical support to meet the thermal and mechanical requirements for containment under conditions of prolonged exposure to high pressure and high temperature in use.

[0017] Optionally, the second alloy has a higher creep strength and / or mechanical stability than the first alloy. However, it is also envisaged that the first alloy can be selected to have a higher creep strength than the second alloy in addition to the higher resistance to nitriding. In this case, the dual alloy system can still be preferred as a first alloy having both high resistance to nitriding and high creep strength can be very expensive and it would therefore be advantageous to limit the amount of such a first alloy. The second alloy would still provide mechanical support to the first alloy, particularly if a small amount of the first alloy is used relative to the second alloy.

[0018] The or each reaction vessel can be in the form of a reaction tube in which the catalyst is disposed. In this case, the second alloy can be in the form of the tube and the first alloy can form a coating or layer on at least a portion of the inner surface of the tube. The first alloy can be disposed in the region of the reaction vessel with the highest potential for nitriding, i.e. the region where the ammonia / nitrogen partial pressure and / or temperature is highest. This can be in the region closer to the inlet of the reaction vessel than the outlet of the reaction vessel relative to the gas flow. Alternatively or in addition, at least 50%, 60%, 70%, 80%, 90% or substantially the entire surface of the reactor vessel adjacent to the catalyst can be formed of the first alloy. The or each reaction vessel can comprise a lower wt% of the first alloy compared to the second alloy.

[0019] The first and second alloys can be disposed as layers of the reactor vessel wall, with the layer of the first alloy having a thickness that is less than the layer of the second alloy. Such a configuration can reduce nitriding of the reactor vessel while limiting the amount of nitriding-resistant alloy required.

[0020] It can also be noted that additional materials / alloys can be provided to form the walls of the reaction vessel, e.g. as additional layers. For example, a further layer of material / alloy can be provided between the first alloy and the second alloy to improve adhesion between the first alloy and the second alloy and / or to match the coefficient of thermal expansion, e.g. an alloy having a coefficient of thermal expansion intermediate between the first alloy and the second alloy.

[0021] Additionally or alternatively, an intermediate layer (e.g. pure Ni or Co) can be employed which has a low solubility for nitrogen to delay the transport of nitrogen to the mechanical support layer or which has a high proportion of nitrogen forming elements to consume and tie up nitrogen, thus also delaying the transport of nitrogen to the mechanical support layer.

[0022] Furthermore, an additional layer of an oxide or oxide forming material can be provided on at least a portion of the surface adjacent to the catalyst material to further enhance the resistance to nitrogen penetration. Such an oxide layer can be an aluminium oxide layer formed from the aluminium in the first alloy. The ammonia feed gas contains oxidizing species such as oxygen and / or water to ensure oxidizing conditions in the reaction vessel and to help maintain the oxide layer. These oxidizing species can be naturally present in the ammonia feed gas or added intentionally. In the case of high pressure, high temperature nitriding in ammonia, it is preferred to maintain water in the process stream gas as it shows a huge advantage over other means of oxide formation and ensures good repair of the oxide layer if damaged throughout the operation.

[0023] Optionally, the first alloy comprises nickel and / or cobalt, e.g. in an amount greater than 11 wt%, 15 wt%, 20 wt%, 30 wt%, 50 wt% or 75 wt% of the first alloy. While in principle up to 100 wt% nickel / cobalt can be used as the first alloy, optionally the first alloy further comprises one or more of Cr, Si or Al (e.g. in addition to Ni and / or Co, a small proportion of these elements is also comprised). 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 comprises iron, nickel and / or chromium. Optionally, the second alloy comprises at least 10 wt%, 15 wt%, 20 wt% or 30 wt% Cr. For example, the second alloy can be a Ni-based alloy or a Fe-based alloy containing a proportion of Cr.

[0024] The method as described above provides a method for ammonia cracking to produce hydrogen with improved operability, reduced downtime and increased safety and also in a cost effective manner in terms of the construction material of the reaction vessel / tube. The reaction tube(s) / vessel(s) used in the method are resistant to nitrogen penetration and to creep and / or embrittlement damage at the high temperatures and high pressures of ammonia used in the catalytic cracking of ammonia and can also be more cost competitive than tubes made entirely from a nitrogen penetration resistant alloy.

[0025] The specification also provides a reaction vessel for use in the above method. The reaction vessel contains an ammonia cracking catalyst and has a wall comprised of at least a first alloy and a second alloy, where the first alloy is more resistant to nitriding than the second alloy, and the second alloy provides mechanical support to the first alloy, and where at least a portion of the wall adjacent to the catalyst is comprised of the first alloy. In other aspects, the reaction vessel is as described with respect to the method for cracking ammonia. Also provided is an ammonia cracking reactor comprising one or more catalyst-containing reaction vessels (e.g., reaction tubes) as described herein. BRIEF DESCRIPTION OF DRAWINGS

[0026] For better understanding of the present application and to show how it can be implemented, certain embodiments of the present application will now be described, purely by way of example, with reference to the attached drawings in which:

[0027] Figure 1 A schematic view of a reaction tube is shown along its axial length x and in the plane x,y - the circular cross section of the reaction tube can be seen in the y,z plane;

[0028] Figure 2 A schematic view of a cross section of a reaction tube in the plane 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; and

[0029] Figure 3 A diagram of an ammonia cracking reactor is shown, comprising catalyst-filled reaction tubes and a burner for providing thermal energy to the reaction tubes. DETAILED DESCRIPTION

[0030] Preferred and / or optional features of the present application will now be set out. Any aspect of the present application can be combined with any other aspect of the present application, unless context demands otherwise. Any preferred and / or optional feature of any aspect can be combined, either individually or in combination, with any aspect of the present application, unless context demands otherwise.

[0031] The method of the specification comprises the step of supplying an ammonia feed gas to one or more heated catalyst-containing reaction tubes disposed within an ammonia cracking reactor. The ammonia feed gas can be sourced from any source. The ammonia feed gas can be produced by catalytic combination of hydrogen and nitrogen, for example, the ammonia feed gas can be produced by a Haber-Bosch ammonia synthesis process. The ammonia feed gas can be produced in an ammonia production facility located upstream of the ammonia cracking reactor. Alternatively, the ammonia feed gas can be provided from an ammonia gas storage facility, ammonia storage unit, ammonia storage tank, or ammonia gas pipeline.

[0032] The ammonia feed gas can be preheated prior to being supplied to the one or more catalyst-containing reaction tubes. Thus, the process can include a step of preheating the ammonia feed gas. The ammonia feed gas can be preheated to a temperature greater than 350°C, greater than 400°C, greater than 450°C, greater than 500°C, or greater than 550°C. The ammonia feed gas can be preheated to a temperature less than 1000°C, less than 950°C, less than 850°C, less than 750°C, or less than 700°C. The ammonia feed gas can be preheated to a temperature from 350°C to 1000°C, from 400°C to 950°C, from 450°C to 850°C, or from 500°C to 750°C, such as from 550°C to 700°C.

[0033] Suitable ammonia cracking reactors are known and can include a fuel combustion zone having a radiant section that includes one or more burners to which one or more fuel streams and an oxygen feed gas, such as air, oxygen enriched air, or oxygen, are fed. The radiant section can include one or more catalyst-containing reaction tubes through which the ammonia feed gas passes. Combustion of the one or more fuel streams in the one or more burners of the fuel combustion zone generates heat energy (e.g., radiant heat) that is used to heat the one or more catalyst-containing reaction tubes. There can be tens or hundreds of catalyst-containing reaction tubes in the radiant section. If desired, flue gas from the combustion of the one or more fuel streams can be used downstream of the radiant section to preheat one or more feed streams in a convection section. Reactors that include a radiant section containing reaction tubes (containing catalyst) and a convection section for preheating feeds are known in steam methane reforming and can be applied to the process for ammonia cracking of the present invention.

[0034] Alternative ammonia cracking reactors can be used. For example, where combustion of the one or more fuel streams in a fuel combustion zone is separate from the reactor that includes catalyst-containing reaction tubes. Such a reactor can be a compact reformer available from Johnson Matthey Davy Technologies Limited. Other alternative forms of reactors include plate exchangers heated by molten salt, rotating heaters such as manufactured by Coolbrook, or printed circuit heat exchangers. In certain configurations, the tubes can be fired and the area outside the tubes contains ammonia. Alternatively, the tubes can be electrically heated, inductively heated, or concentrated solar heated.

[0035] The catalyst can be any ammonia cracking catalyst. For example, a nickel catalyst and / or a ruthenium catalyst can be used. The catalyst can comprise 3 to 30 wt% nickel, preferably 8 to 20 wt% nickel, on a suitable refractory support such as alumina or metal aluminate, expressed as NiO. The catalyst can be in the form of a granular unit, which can include one or more through-holes, or can be a washcoat on a structured metal or ceramic catalyst. A particularly preferred catalyst is KATALCO® 27-2, which comprises 12% nickel expressed as NiO on cylindrical shaped granules formed from a high surface area calcium aluminate support. RTM 27-2, which comprises 12% nickel expressed as NiO on cylindrical shaped granules formed from a high surface area calcium aluminate support.

[0036] The process of the application comprises the step of cracking ammonia in an ammonia feed gas in the one or more heated catalyst-containing reaction tubes to produce a hydrogen-containing stream.

[0037] In a fired reactor, the flames will typically be greater than 1000°C. These flames can impinge on the reaction tubes during operation, but typically the reaction tubes will be heated to a temperature in the range 500°C to 1000°C. Furthermore, typically one end of the reactor tube will be hotter than the other. This can be due to variations in process gas temperature (e.g. cooler inlet gas) and / or variations in heat distribution in the radiant section.

[0038] The temperature of the ammonia feed gas at the inlet to the one or more catalyst-containing reaction tubes can be in the range 350°C to 1000°C, 400°C to 950°C, 450°C to 850°C or 500°C to 750°C (such as 550°C to 700°C). The temperature of the hydrogen-containing stream exiting the one or more catalyst-containing reaction tubes will influence the equilibrium position of the cracking reaction, and can be in the range 500°C to 950°C. When a nickel catalyst is used in the one or more catalyst-containing reaction tubes, the temperature of the hydrogen-containing stream exiting the one or more catalyst-containing reaction tubes can preferably be greater than about 700°C.

[0039] The inlet pressure of the one or more catalyst-containing reaction tubes will be set by the flowsheet design, and can be: at least 1 bar, 10 bar, 30 bar, 50 bar or 100 bar; no more than 1000 bar, 500 bar, 100 bar, 75 bar or 50 bar; or in a range defined by any combination of the lower and upper values mentioned above. Exemplary ranges include 1 to 100 bar absolute, 10 to 100 bar absolute or 30 to 75 bar absolute.

[0040] The ammonia cracking reaction produces a hydrogen-containing stream. The hydrogen-containing stream contains H2. The hydrogen-containing stream also contains nitrogen, and can further contain residual ammonia (e.g. unreacted ammonia).

[0041] The hydrogen-containing stream can comprise 40 mol% or more H2, 50 mol% or more H2, or 60 mol% or more H2. The hydrogen-containing stream can comprise 75 mol% or less H2, 70 mol% or less H2, or 65 mol% or less H2. For example, the hydrogen-containing stream can comprise 40 mol% to 75 mol% H2, 50 mol% to 70 mol% H2, or 60 mol% to 65 mol% H2.

[0042] As described in the SUMMARY, the methods of the present disclosure use a catalyst- containing reaction vessel comprised of at least a first alloy and a second alloy, where the first alloy is more resistant to nitriding than the second alloy, and the second alloy provides mechanical support to the first alloy, and where at least a portion of the wall of the reaction vessel adjacent to the catalyst is comprised of the first nitriding-resistant alloy. In configurations where the reaction vessel is in the form of a catalyst-containing tube, then the first nitriding-resistant alloy is disposed on the inner surface of the tube. Alternatively, if heating is provided by a tube and the catalyst is disposed in a reactor region outside of the tube, then the first nitriding-resistant alloy is disposed on the outer surface of the tube, i.e., the side of the tube adjacent to the catalyst.

[0043] Nitriding can cause accelerated failure of the reaction tube, particularly at locations within the reaction tube where the potential for nitriding is highest or the temperature driving nitriding is highest. This can typically be at a location towards the inlet end of the reaction tube (e.g., closer to the inlet than the outlet, optionally at the inlet), where the ammonia partial pressure is highest. However, depending on the reactor design, the location with the highest potential for nitriding can be slightly down the reaction tube from the inlet, as the temperature at the inlet location can not be the highest. By providing a nitriding-resistant alloy at least at the location where the potential for nitriding is highest, this failure mechanism can be reduced, while using another alloy material to provide mechanical support.

[0044] In the following description, configurations are described in which a catalyst is provided within a reaction tube, where one or more reaction tubes have an outer region comprising a mechanical support composition and an inner region comprising a nitriding-resistant composition. In such configurations, the outer region of the reaction tube is exposed to a heat source that is used to provide thermal energy to the catalyst-containing reaction tube(s), and the heat is used to support the endothermic ammonia cracking reaction in the ammonia cracking reactor. The inner region of the reaction tube refers to the interior of the reaction tube, which contains the catalyst used to catalyze the cracking of ammonia in the ammonia feed gas.

[0045] The reaction tube has an inlet side and an outlet side that define an axial length x of the reaction tube. It will be understood that the catalyst for the ammonia cracking reaction is disposed within the interior of the reaction tube along the axial length x. It will also be understood that the ammonia feed gas passes through the ammonia cracking catalyst disposed therein from the inlet side of the reaction tube, and a hydrogen stream exits the reaction tube via the outlet side of the reaction tube. Figure 1 The axial length x of the reaction tube is shown in the x,y plane.

[0046] The reaction tube has a cross-section in a plane perpendicular to the axial length of the tube in the y,z plane. The cross-section can take any shape and is not particularly limited. For example, the cross-section can be circular, oval, quadrilateral (e.g., square or rectangular), 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.

[0047] The outer region and the inner region have a thickness measured in a plane of the cross-section of the reaction tube. Figure 2 An example of a cross-section of a reaction tube in the y,z plane is shown in FIG. 13, where the shape of the cross-section is circular, and where “b” and “a” represent the thickness of the outer region and the inner region, respectively. Preferably, the thickness of the outer region is greater than the thickness of the inner region (i.e., b > a).

[0048] The outer region (second “mechanical support” alloy) can have a thickness of at least 1 mm, 3 mm, 5 mm, 8 mm, or 10 mm; no more than 100 mm, 50 mm, 20 mm, 15 mm, or 14 mm; or within a range defined by any combination of the above lower and upper limits. An example thickness range is 8 mm to 14 mm, optionally 10 mm to 14 mm. It should be noted, however, that these ranges are for standard combustion reactor designs, and the thickness can vary for other types of reactor configurations. For example, printed circuit heat exchangers and plate exchangers can use lower thicknesses, such as 0.1 mm to 2 mm, optionally 0.25 mm to 1 mm.

[0049] The inner region can have a thickness of at least 0.05 mm, 0.1 mm, 0.5 mm, or 1 mm; no more than 10 mm, 8 mm, or 5 mm; or within a range defined by any combination of the above lower and upper limits. An example thickness range is 1 mm to 5 mm.

[0050] The outer composition is a heat-resistant alloy, which can include iron, nickel, and / or chromium. For example, the outer composition can be a heat-resistant and corrosion-resistant alloy that includes at least 10 wt%, 15 wt%, 20 wt%, or 30 wt% chromium (e.g., a nickel-based or iron-based alloy containing that proportion of chromium). For the avoidance of doubt, the amount of any component of the outer composition or the inner composition is expressed as a weight percent of that composition.

[0051] Optionally, the amount of nickel and / or cobalt in the external composition is lower 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 will depend on the specific operating conditions for the ammonia cracking process, which can vary depending on the reactor design. For example, the external composition can 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, Super304H, 314, 347, or P91.

[0052] Heat resistant materials for temperatures above 550°C tend to be based on iron-nickel-chromium alloy systems for petrochemical applications, where economic factors have a greater influence on material selection than other factors. These alloys tend to have a chromium content of 8 to 30 wt.%, or aluminum or silicon to form a protective oxide layer. Chromium can be most commonly used because it is very soluble among nickel, iron, and cobalt, and has a low tendency to form harmful intermetallic phases. A chromium content of 20 to 30 wt.% provides the best oxidation protection up to about 1000°C. Aluminum oxide is generally more protective above 1000°C, but the alloying ratio is much lower than chromium.

[0053] Suitable creep strength and metallurgical stability / predictability allow the metal alloy component to have a longer service life before becoming prone to distortion, cracking, and ultimately losing containment or replacement. The metal alloy component can lose strength or the strength can actually increase, but the material can form voids and cracks, where the material can move over time, leading to a loss of containment.

[0054] Heat resistant alloys of the iron-nickel-chromium system suitable for use at ammonia cracking temperatures include austenitic stainless steels such as those of the 300 series alloys such as but not limited to 304, 309, 310, 314, 316, 321, 330, 347, and variants thereof. Or similarly, 200 series austenitic stainless steels, or 400 series ferritic and martensitic stainless steels. At lower temperature ranges, even alloy steels such as P91 can be considered.

[0055] At higher temperatures, alloys with increasing amounts of nickel and / or cobalt can be considered, for example Alloy 800 and variants thereof, Alloy 825, 253MA, 353MA, UNS S31035, alloys of the 600 series such as 600, 601, 602CA, 625, 690, etc., and alloys for reforming and ethylene cracking applications, etc., manufactured in a centrifugal casting process.

[0056] Alloys of the nickel, chromium, iron, molybdenum system such as C-276, C22, C2000, Alloy 59, etc. can also be considered, however the economics of simple high temperature strength and oxidation resistance applications will tend to discourage the use of these more expensive alloys.

[0057] Precipitation hardening alloys such as Alloy 718 and other nickel and chromium containing alloys optionally containing significant amounts of cobalt and / or tungsten such as Alloy 188, 230, C-276, 617, etc. can also be considered.

[0058] Nitrogen and nitrogen-containing gases are widely used in the process industries, including ammonia (NH3), as a feedstock, intermediate, product, or as a fuel. In the presence of these gases, nitrogen can react with alloying elements of the process containment shell to form nitrides. Different elements have different affinities for nitrogen and thus more or less readily form nitrides than other elements. Nickel and cobalt are known to be weak nitride formers, while aluminum and titanium are strong nitride formers, with chromium being at an intermediate level. The formation of these nitrides in the alloy composition can cause serious material problems as they can cause embrittlement, but also reduce the effective wall thickness. Common nitrides in commercial heat resistant alloys at high temperatures are Fe4N / Fe2N, CrN / Cr2N, AIN, and TiN / Ti2N.

[0059] Additionally, it has been shown that nickel-based and cobalt-based alloys have significantly higher nitrogen solubility than iron-based alloys such as stainless steels. Higher nickel / cobalt content has been shown to increase the ability of the alloy to resist nitriding and associated deleterious mechanical issues. However, nickel and cobalt are significantly more expensive than iron and the cost can continue to rise as these elements are needed for the expanding battery materials market.

[0060] New, scaled ammonia cracking applications are pushing the limits of what reactor vessel alloys are currently required, particularly in relation to the problem of simultaneously requiring economically viable creep strength plus high nitriding resistance. There are some alloys that have shown high creep strength and high nitride resistance, but these alloys contain cobalt and high levels of nickel and thus can be very expensive. Increasing the iron content reduces the cost of the tube, but iron should be avoided from contact with the nitriding environment as iron is a strong nitride former. At scale, these reactor tubes will need to be produced for ammonia cracking for the energy industry, and the economics of a bi-metallic tube can be superior to simply shifting to higher levels of cobalt and nickel in a single metal alloy solution. Thus, the present solution using more than one alloy type is considered superior to simply using a more nitriding resistant alloy and not operating at high pressure / high temperature, using a more expensive alloy, or making the containment vessel thicker.

[0061] The inner composition can be an alloy comprising nickel and / or cobalt, for example, in an amount greater than 11 wt.%, 15 wt.%, 20 wt.%, 30 wt.%, 50 wt.%, or 75 wt.% (individually or in total combined amount). While up to 100 wt.% nickel / cobalt can be used in principle, optionally, the inner composition further comprises one or more of Cr, Si, or Al. Optionally, the amount of nickel and / or cobalt in the inner composition is greater than the amount of nickel and / or cobalt in the outer composition. The inner layer serves for nitriding resistance, while saving expensive Ni and Co by not requiring the entire reactor tube to be formed of expensive Ni / Co rich alloy material. The dual alloy approach for the reactor tube also allows for more optimized compositions for the separate functions of nitriding resistance for the tube interior and mechanical support / thermal stability on the tube exterior. The exact composition of nickel / cobalt of the inner composition will depend on the nitriding potential, which depends on the temperature plus partial pressure and type of nitriding species. Some examples of suitable alloys for different operating temperatures, pressures, and nitriding potentials are given in the table below.

[0062] Exterior alloy material selection

[0063]

[0064] Interior alloy material selection

[0065]

[0066] Additional components can include any welding consumables and metal powders identical or similar to those described above for the overlay / thermal spray / powder metallurgy routes, for example, low / medium / high Ni / Co with or without Al.

[0067] The inner surface of the inner alloy can include an oxide layer on the inner alloy on the surface exposed to the process fluid. It is formed at the time of manufacture from the reaction of the alloy with oxygen in the air. If oxygen is available, the oxide layer will also grow and repair in the process environment. The formation of aluminum oxide requires less partial pressure of oxygen in aluminum containing alloys. Controlling the oxidizing species content (e.g., water and / or oxygen) in the process stream gas within the reactor tube during operation can ensure the oxidizing conditions and help maintain the protective oxide layer. Alternatively, naturally occurring oxidizing species in the process stream gas can fulfill this function. That is, in the case of high pressure, high temperature nitriding in ammonia, it is preferred to maintain water in the process stream gas as it shows a huge advantage over other means of oxide formation and ensures good repair of the oxide (if damaged) throughout the operation. The oxide layer can provide a layer that protects the reactor tube from nitriding effects of the ammonia feed gas.

[0068] ​​The internal composition can include an alloy that includes Cr, Si, and / or Al to provide greater advantage in terms of oxide formation, with Al oxide being preferred. It is noted that the present description contemplates that nitrogen permeation resistant alloys can be selected to take advantage of oxidation scales for resistance to nitrogen permeation, or optimized Ni / Co content for resistance to nitrogen permeation, or both. The preferred selection will depend on nitrogen permeation potential and economics. For example, the internal composition can be an alloy selected from Alloy 600, Alloy 601, Alloy 200, Alloy 214, or MA47P.

[0069] The outer region and the inner region of the reaction tube can be bonded together by a metallic bond. The outer region and the inner region of the reaction tube can be bonded together by a metallic bond, for example. The bond can be uniform across the axial length of the reaction tube to form a single interface between the internal composition and the external composition. In other words, the inner region and the outer region can be bonded together such that the reaction tube has one inner surface and one outer surface. The metallurgical bonding process can include, for example, extrusion bonding, explosive bonding, hot isostatic pressing, and centrifugal casting.

[0070] Optionally, the reaction tube can include an additional bonding layer between the inner layer and the outer layer. This can be advantageous for various reasons, such as better metallurgical bonding between two different alloys, better match of the coefficient of thermal expansion, etc. Optionally, the intermediate layer can be an alloy that is not a very good oxide former, but has excellent nitrogen barrier properties, such as pure Ni / Co.

[0071] The reaction tube can be manufactured using methods known in the art. For example, the outer region comprising the external composition can first be formed as a base pipe using centrifugal casting or seamless pipe rolling process, also known as pilgering. For the base pipe, the inner region comprising the internal composition can be provided by a process such as cladding or thermal spraying. Alternatively, the outer region and the inner region can be simultaneously formed into the reaction tube during pilgering using a co-extrusion or co-rolling process or by hot isostatic pressing (HIP) of powders.

[0072] In addition to the ammonia cracking methods described above, the present description provides a reaction vessel, such as a reaction tube, for catalytic cracking of ammonia, the reaction tube containing an ammonia cracking catalyst. The ammonia cracking catalyst is housed inside the reaction tube. That is, the ammonia cracking catalyst is housed inside the reaction tube along the axial length of the reaction tube. The ammonia cracking catalyst can be any catalyst known to be suitable for catalyzing the cracking of ammonia into hydrogen and nitrogen. It can be preferred that the catalyst is a nickel-based catalyst, such as nickel on an inert support (e.g., alumina, silica, or another refractory oxide). For example, the catalyst can be KATALCO 27-2 RTM Alternatively, it can be preferred that the catalyst is a noble metal catalyst, such as a ruthenium-based catalyst.

[0073] Also provided is an ammonia cracking reactor comprising one or more catalyst-containing reaction vessels (e.g., reaction tubes) as described above. Figure 3 A diagram of an ammonia cracking reactor is shown, comprising a catalyst-filled reaction tube and a burner for providing thermal energy to the reaction tube.

[0074] While the application has been particularly shown and described with reference to certain examples, it will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the scope of the application as defined by the appended claims.

Claims

1. A method for catalytic cracking of ammonia, the method comprising: supplying an ammonia feed gas to one or more heated catalyst-containing reaction vessels disposed within an ammonia cracking reactor; and cracking ammonia in the ammonia feed gas in the one or more catalyst-containing reaction vessels to produce a hydrogen-containing stream, wherein the reaction vessel or each of the reaction vessels has a wall comprised of at least a first alloy and a second alloy, wherein the first alloy is more resistant to nitriding than the second alloy, and the second alloy provides mechanical support to the first alloy, and wherein at least a portion of the wall adjacent to the catalyst is comprised of the first alloy.

2. The method of claim 1, wherein the second alloy has a higher creep strength and / or mechanical stability than the first alloy.

3. The method of claim 1 or 2, wherein the reaction vessel or each reaction vessel is in the form of a reaction tube having a catalyst disposed therein.

4. The method of 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 an inner surface of the tube.

5. The method of any preceding claim, wherein the first alloy is disposed in at least one region of the reaction vessel or each reaction vessel where the potential for nitriding is highest.

6. The method of any preceding claim, wherein the first alloy is disposed in at least one region of the reaction vessel or each reaction vessel that is closer to an inlet of the reaction vessel than an outlet of the reaction vessel with respect to a gas flow.

7. The method of any preceding claim, wherein at least 50%, 60%, 70%, 80%, 90%, or all of a surface of the reaction vessel or each reaction vessel adjacent to the catalyst is formed of the first alloy.

8. The method of any preceding claim, wherein the reaction vessel or each reaction vessel comprises a lower weight % of the first alloy compared to a weight % of the second alloy.

9. The method of any preceding claim, wherein the first alloy and the second alloy are disposed as layers of the reaction vessel wall or each reaction vessel wall, wherein the first alloy layer has a thickness that is less than the second alloy layer.

10. The method of any preceding claim, wherein the first alloy has an oxide layer disposed on a surface thereof.

11. The method of any preceding claim, wherein an intermediate adhesion layer is disposed between the first alloy and the second alloy.

12. The method of any preceding claim, wherein the first alloy comprises nickel and / or cobalt in an amount greater than 11 wt%, 15 wt%, 20 wt%, 30 wt%, 50 wt%, or 75 wt% of the first alloy.

13. The method of any preceding claim, wherein the first alloy comprises a wt% amount of nickel and / or cobalt that is greater than a wt% amount of nickel and / or cobalt in the second alloy.

14. The method of any preceding claim, wherein the first alloy comprises one or more of Cr, Si, or Al.

15. The method of any preceding claim, wherein the second alloy comprises iron, nickel, and / or chromium.

16. The method of any preceding claim, wherein the second alloy comprises at least 10 wt%, 15 wt%, 20%, or 30% chromium.

17. A reaction vessel for use in the method of any preceding claim, wherein the reaction vessel comprises an ammonia cracking catalyst and has a wall comprised of at least a first alloy and a second alloy, wherein the first alloy is more resistant to nitriding than the second alloy, and the second alloy provides mechanical support to the first alloy, and wherein at least a portion of the wall adjacent to the catalyst is comprised of the first alloy.

18. An ammonia cracking reactor comprising one or more reaction vessels according to claim 17.

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