Methods and equipment for ammonia cracking

Abstract

This invention relates to a method and apparatus for cracking ammonia gas at extra-atmospheric pressure in a catalyst-filled reactor tube within a furnace. Each tube has an upstream layer of a first catalyst and a downstream layer of a second catalyst, the first catalyst being more active than the second catalyst. The higher activity of the upstream catalyst reduces the temperature of the outer wall of the tube in the region of the burner flame and the temperature of the inner wall of the tube in the region with the highest ammonia molar fraction. This reduces the nitriding of the tube metal in this region.

Description

Technical Field

[0001] This invention belongs to the technical field of producing hydrogen from cracked ammonia, and preferred embodiments specifically relate to a method and apparatus for producing hydrogen from liquid ammonia. Background Technology

[0002] Global interest in renewable energy and the use of it to produce “green” hydrogen has fueled interest in converting “green” hydrogen into “green” ammonia, as ammonia is easier to transport over distances of hundreds or thousands of miles. Specifically, transporting liquid hydrogen is currently commercially impossible, but transporting liquid ammonia is feasible.

[0003] For use in commercial fuel cells, ammonia must be converted back into hydrogen according to the reaction.

[0004]

[0005] This is an endothermic process, meaning it requires heat; therefore, higher temperatures will favor product formation. The standard heat of reaction (per mole of ammonia) at 1 bar and 0°C is 45.47 kJ / mol. The endothermic nature of this process dictates the requirements for the furnace.

[0006] This process is called cracking (or sometimes "dissociation") and is typically carried out over a catalyst. The resulting gas (or "cracked gas") is a mixture of hydrogen (H2) and nitrogen (N2), although some residual ammonia is present as the cracking reaction is an equilibrium reaction. The amount of ammonia in the cracked gas, often referred to as "ammonia slip," can be altered by changing the temperature at which the ammonia is heated; higher temperatures favor conversion, thus reducing ammonia slip.

[0007] Currently, in most applications of crackers, a mixture of hydrogen and nitrogen is used as is. However, since ammonia can be a poison for fuel cells, this stream can be used directly for the fuel cell, provided that ammonia is properly removed, for example, by washing with water. However, if hydrogen is used for vehicle refueling, the presence of nitrogen adversely affects the process. Fuel in vehicle refueling systems is compressed to fairly high pressures—up to 900 bar. This means that nitrogen, which is only used as a diluent in the process, is also compressed, consuming power and storage volume, and increasing anode gas purging requirements, thus reducing efficiency. Therefore, separating hydrogen and nitrogen is beneficial when hydrogen is used for vehicle refueling.

[0008] There are many examples of ammonia cracking methods in the prior art, such as GB977830A, JP5330802A, CN111957270A, US2020 / 0398240A and KR2022 / 0085469A.

[0009] Furthermore, GB1142941A discloses a method for producing fuel gas interchangeable with city gas. Ammonia is cracked to form a mixture of hydrogen and nitrogen, which is then enriched by adding a gas with a higher calorific value than the mixture, such as methane, propane, or butane, or a mixture thereof. Liquid ammonia is pumped as a cold liquid and evaporated through a closed hot water loop. After being superheated by heat exchange with flue gas from the furnace, the ammonia vapor is cracked over a suitable catalyst in the tubes of a direct-fired tubular furnace. The cracked gas is washed with water to recover residual ammonia, which is ultimately recycled to the ammonia feed in the catalyst-filled tubes of the furnace. The purified cracked gas is enriched with propane and / or butane to produce city gas products.

[0010] GB 1142941A discloses that the cracking of ammonia in a direct-fired tubular furnace in the presence of a suitable catalyst is preferred. However, the reference also discloses that other cracking methods can be used alternatively. In this regard, GB 1142941 mentions heating ammonia to a suitable temperature and then passing it through an unheated ammonia cracking catalyst bed to crack a portion of the ammonia into hydrogen and nitrogen, cooling the gases in the process. Unconverted ammonia can be recovered as described above. Alternatively, the gas mixture can be reheated and passed through a second catalyst bed to further reduce the ammonia content, and this can be repeated multiple times as needed.

[0011] GB1353751A discloses a method in which ammonia at pressures ranging from 20 atm to 300 atm is cracked in two stages within a heated reactor tube. In the first stage, gas at a temperature ranging from 450 to 800°C passes through a layer of catalyst containing nickel, iron, or cobalt, which is generated by co-precipitation with a support of alumina and magnesia or magnesium aluminate spinel. The catalyst for the first stage can alternatively consist of an iron-impregnated ceramic material or iron impregnated on a pre-formed support composed of magnesia and alumina and promoted by potassium oxide. After the first stage, gas at a temperature ranging from 450 to 600°C passes through a double- or triple-promoted iron catalyst layer to form the second stage.

[0012] WO2022 / 189560A discloses an ammonia cracking method involving a combustion reactor with tubes filled with an iron catalyst. Liquid ammonia is drawn from a storage tank, pumped, preheated, and evaporated to form ammonia gas, which is heated by heat exchange with cracked gas. The heated ammonia gas is further heated by heat exchange with flue gas in a convection section of the combustion reactor and then fed into two adiabatic reactors in series (for interstage heating with flue gas), where it is partially cracked. The partially cracked gas is then heated by heat exchange with flue gas in a convection section and then fed into the catalyst-filled tubes of the combustion reactor to crack the remaining ammonia.

[0013] US11287089A discloses a hydrogen refueling system in which ammonia is cracked in-situ into hydrogen and nitrogen in an ammonia cracker operating at pressures ranging from 5 bar to 40 bar, and in which hydrogen is compressed to a pressure of at least 30 MPa (300 bar) and stored for distribution to vehicles. When distributed, the compressed gas from the storage tank is cooled to a temperature ranging from -40°C to 5°C by heat exchange with a heat exchange fluid (such as D-limonene, FP40, or a water / glycol mixture) circulating around a closed loop. The heat exchange fluid is cooled by heat exchange with the ammonia feed to the cracker and can be further cooled in a conventional refrigeration system. In the case where the ammonia feed is liquid, at least a portion of the load required for evaporating the liquid ammonia is provided by the heat exchange fluid. The evaporation of liquid ammonia typically occurs at atmospheric pressure or below. US11287089A illustrates a system that produces 7.5 tons / day of hydrogen.

[0014] However, there is still a need for improved methods of producing hydrogen from ammonia, and particularly for methods that are more energy efficient and / or have higher levels of hydrogen recovery and / or reduce or eliminate the need to burn fossil fuels. Summary of the Invention

[0015] According to a first aspect of the invention, a method for cracking ammonia is provided, the method comprising: providing heated ammonia gas at extra-atmospheric pressure; burning fuel in a furnace with an oxidant gas to heat a catalyst-containing reactor tube and generate flue gas, each tube including an upstream layer of a first catalyst and a downstream layer of a second catalyst; feeding the heated ammonia gas or partially cracked ammonia gas derived therefrom into the catalyst-containing reactor tube to induce cracking of the ammonia and generate a cracked gas comprising hydrogen, nitrogen and residual ammonia gas, wherein the first catalyst is more active for cracking ammonia than the second catalyst.

[0016] The use of different catalyst layers with varying activities in the reactor tubes of an ammonia cracker is not novel. In known methods, a more active catalyst is typically used in a reactor tube layer downstream of a less active catalyst layer, usually in the final section of the catalyst bed at the tube outlet, to ensure near equilibrium.

[0017] However, the inventors have found that it is beneficial to use a more active catalyst in the upstream layer of the reactor tube (typically the first section of the catalyst bed at the inlet of the tube) because it reduces the nitriding of the metals in the reactor tube.

[0018] Metal nitriding is a recognized problem in ammonia crackers. The amount of nitriding depends on the partial pressures of ammonia, nitrogen, and hydrogen, as well as the process temperature. Nitriding is most severe at higher temperatures and higher partial pressures of ammonia and nitrogen, with ammonia being more dangerous at typical ammonia cracking temperatures (nitrogen-induced nitriding is unlikely before temperatures become too high for ammonia cracking, such as 1000°C or higher). The exact extent to which the structural materials form nitrides is currently unclear. However, data suggest that below 650°C, the degree of nitriding should be acceptable, depending on the materials chosen for the structure. Operating ammonia crackers at high pressures (e.g., 10 to 50 bar) increases the partial pressures of ammonia and nitrogen, and can lead to more serious failures if nitriding results in brittle fracture of the reaction vessel or tubing.

[0019] This problem is particularly important in combustion reactors, where the temperature around the reactor tubes containing the process-side catalyst is significantly higher than 650°C.

[0020] In a top-fired furnace, the burner is located at the top, and the burner flame extends downwards to a portion of the reactor tube's length. Ammonia feed is introduced to the top of the tube and flows downwards, i.e., co-current with the flue gas, reaching the bottom of the tube. Because the top of the tube is closest to the burner flame, this region of the tube has the highest heat flux. The top of the tube also has the highest partial pressure of ammonia on the process side. Therefore, this region of the tube has the highest nitriding potential and is thus of greatest interest.

[0021] Placing a more active catalyst in the first section of the catalyst bed in the reactor tube results in stronger endothermic reaction, absorbing more heat from the tube's metal and channeling it into the cracking process itself. This lowers the temperature of the inner wall of the tube and keeps it below a predetermined limit, such as 700°C (or 660°C to allow for a margin). Lowering the inner wall temperature in this way reduces nitriding of the tube's metal wall.

[0022] According to a second aspect of the invention, a furnace, typically a top-fired furnace, is provided for cracking heated ammonia under ultra-atmospheric pressure. The furnace includes a radiant section comprising at least one fuel and oxidant gas inlet for fluid flow communication with at least one burner, an ammonia feed inlet, and a catalyst-containing reactor tube having an upstream end in fluid flow communication with the ammonia feed inlet and a downstream end in fluid flow communication with an outlet of cracked gas. Each tube includes an upstream layer of a first catalyst and a downstream layer of a second catalyst. A convection section is also provided, in fluid flow communication with the radiant section and including an outlet of flue gas. The first catalyst is more active for cracked ammonia than the second catalyst.

[0023] The catalyst bed within each reactor tube typically contains two or three layers (or sections) of catalyst. Beds with more than three layers of catalyst are also suitable for specific applications.

[0024] The upstream layer is typically the first layer of the catalyst bed within each reactor tube and will include, for example, contain or consist of, the first catalyst. The length of the upstream layer may be at least 20%, at least 25%, at least 30%, or at least 35% of the total length of the catalyst bed. Typically, the length of the upstream layer does not exceed 50% of the length of the catalyst bed. Typically, the upstream layer extends beyond the tip of the burner flame.

[0025] The downstream layer is typically the second layer of the catalyst bed and will include, for example, contain or consist of a second catalyst. While an intermediate layer may exist between the upstream and downstream layers, this is not usually the case. In the case of only two layers in the catalyst bed, the length of the downstream layer will constitute the remainder of the bed. However, in some embodiments, a third catalyst layer will exist downstream of the second catalyst layer. The length of the third catalyst layer typically does not exceed 10% of the total bed length. Therefore, in the case of three layers in the catalyst bed, the downstream layer will again constitute the remainder of the bed.

[0026] Ideally, the third catalyst is more active than the second catalyst, so that, if present, the third catalyst layer can approach equilibrium.

[0027] The furnace described in the second aspect of the invention is particularly suitable for carrying out the method described in the first aspect of the invention.

[0028] According to a third aspect of the invention, an apparatus is provided for cracking heated ammonia under ultra-atmospheric pressure, the apparatus comprising a liquid ammonia source; a pump in fluid communication with the liquid ammonia source for pumping the liquid ammonia; and a furnace according to the second aspect, wherein the ammonia feed inlet of the furnace is in fluid communication with the pump, wherein the apparatus further comprises at least one heat exchanger arranged for preheating the liquid ammonia upstream of the pump; and at least one heat exchanger arranged for evaporating the pumped liquid ammonia and heating the ammonia by heat exchange with flue gas and / or cracked gas located between the pump and the ammonia feed inlet of the furnace.

[0029] The apparatus described in the third aspect of the invention is particularly suitable for carrying out the method described in the first aspect of the invention. Attached Figure Description

[0030] Figure 1 A simplified flow chart of the ammonia cracking process of the present invention can be used;

[0031] Figure 2 This is a graph showing how various temperatures and ammonia molar fraction change as a function of cracker length for a comparative example using a nickel-based catalyst as the sole catalyst; and

[0032] Figure 3 This is a graph showing how various temperatures and ammonia molar fractions vary as a function of cracker length for embodiments of the invention using an upstream layer of a ruthenium-based catalyst and a downstream layer of a nickel-based catalyst. Detailed Implementation

[0033] Unless otherwise stated, the amounts of components given in parts per million (ppm) are by weight. Furthermore, unless otherwise stated, all percentages are calculated in moles. Additionally, unless otherwise stated, any reference to pressure is a reference to absolute pressure.

[0034] In the context of this invention, catalyst activity will be understood as the conversion of ammonia at a given partial pressure for a given amount of catalyst over a given time period at a specific temperature and total pressure. The unit used to define the activity of heterogeneous catalysts is the number of moles (or mol·g) of ammonia converted per second per gram of catalyst (including the substrate, if present). -1 s -1 ).

[0035] The expression "fluid flow connectivity" will be understood to mean that a pipe or other suitable conduit will be used to transport fluid from one specified location to another. During transit between the two locations, the fluid may flow through one or more other units that may be designed and / or arranged to alter the physical state of the fluid, such as temperature (e.g., heat exchangers) and / or pressure (e.g., compressors or pumps), or to change the composition of the fluid through reactions of components within the fluid (e.g., catalytic reactors). The expression "direct fluid flow connectivity" will be understood to mean that the fluid flows directly from one location to another, i.e., without passing through another such unit during its transit, and therefore the composition or physical state of the fluid remains substantially unchanged.

[0036] The term "extra-atmospheric pressure" will be understood to mean a pressure significantly higher than atmospheric pressure, such as at least 5 bar, for example at least 10 bar, or at least 20 bar or at least 30 bar. Typically, the pressure does not exceed 60 bar.

[0037] The term "upstream" will be understood to refer to the opposite direction of fluid flow during normal operation. The term "downstream" will be interpreted accordingly.

[0038] Typical compositions of ammonia feed are shown in Table 1.

[0039] ammonia 99.5-99.8 wt% Maximum water content 0.5wt% Minimum water content 0.2wt% Oil 5ppm maximum Inert gases (e.g., Ar) <10ppm iron <1ppm

[0040] Table 1

[0041] Oil may be present in ammonia due to the evaporative gas compressor used for storing ammonia in local tanks, at the production site, or any other tanks in between. The presence of oil is a problem because it poses a risk of blockage and / or contamination. This can lead to poor heat exchanger performance or reduced catalyst activity in the reactor. Therefore, if present, the oil may need to be removed in some way. In this regard, oil can be removed by passing liquid ammonia through a bed of activated carbon. However, in a preferred embodiment, the catalyst used in the adiabatic reaction unit cracks the oil into shorter-chain hydrocarbons, which can react with any water present to form carbon monoxide, hydrogen, and methane.

[0042] Inert gases are not expected to be a problem unless they end up in the product hydrogen. In this regard, helium can be present in ammonia derived from natural gas, but ammonia derived from renewable hydrogen will not contain helium.

[0043] Liquid ammonia is typically pumped from storage pressure (e.g., about 1 bar) to pressures ranging from about 5 bar to about 60 bar, for example, from about 10 bar to about 50 bar, such as from about 10 bar to about 30 bar or from about 40 bar to about 50 bar. The temperature of liquid ammonia is slightly increased from storage temperature (e.g., about -34°C) to about -32°C.

[0044] The pumped liquid ammonia (under extra-atmospheric pressure) is then preheated to its boiling point, ideally through appropriate thermal integration during the process. Preferably, partial preheating is achieved using a heat transfer loop, wherein a heat transfer fluid such as an aqueous solution of a glycol (e.g., comprising about 50 wt% to about 60 wt% of a glycol, such as ethylene glycol or propylene glycol) is used to recover, for example, heat from the intercooling and postcooling of the PSA exhaust gas compressor, optionally along with heat from the cracked gas and / or flue gas, and is used to heat the liquid ammonia. If such integration is not possible, for example if the compressor is not operating, an electric heater may be required to preheat the ammonia.

[0045] The preheated liquid ammonia is then evaporated, and the ammonia gas is further heated before being fed into the catalyst-containing reactor tube or adiabatic reaction unit. In this respect, the ammonia gas is typically superheated, i.e., heated to temperatures above its boiling point, reaching temperatures above 350°C, to ensure an effective reaction rate in the catalyst-containing reactor tube or adiabatic reaction unit.

[0046] The heating and evaporation load of the preheated liquid ammonia can be provided by cracked gas, flue gas, or a combination of both. In a preferred embodiment, cracked gas is used to heat and evaporate the preheated liquid ammonia via heat exchange, and then the ammonia is further heated by heat exchange with the flue gas.

[0047] Heated ammonia can be fed directly into the reactor tube containing the catalyst, without first partially cracking some of the ammonia.

[0048] The reactor tubes of the furnace are filled with at least two different ammonia cracking catalysts with different activities, wherein the first (higher activity) catalyst is located upstream of the second (lower activity) catalyst. In some embodiments, the bed in each reactor tube has only two layers, an upstream layer of the first catalyst and a downstream layer of the second catalyst. However, in other embodiments, the bed may have a third catalyst layer downstream of the second catalyst layer, wherein the third catalyst is more active than the second catalyst, which allows for a shorter tube. The catalytically active metals of the first and third catalysts may be different, although in a preferred embodiment the metal is the same, such as ruthenium.

[0049] The activity of the first (or third) catalyst is typically at least 50% higher than that of the second catalyst. However, the difference in relative activity is typically much greater than 50%. In this respect, the activity of the first (or third) catalyst is typically at least two times (i.e., twice), or at least three times, or at least four times, or at least five times higher than that of the second catalyst. In some embodiments, the activity of the first (or third) catalyst is at least ten times (i.e., an order of magnitude) or at least fifteen times higher than that of the second catalyst.

[0050] Numerous metals are known in the art for catalyzing the cracking of ammonia. These metals include transition metals, such as those in Group 6 of the periodic table, such as chromium (Cr) and molybdenum (Mo); Group 8, such as iron (Fe), ruthenium (Ru), and osmium (Os); Group 9, such as cobalt (Co), rhodium (Rh), and iridium (Ir); Group 10, such as nickel (Ni), palladium (Pd), and platinum (Pt); and Group 11, such as copper (Cu), silver (Ag), and gold (Au). Metalloids such as tellurium (Te) may also be used.

[0051] Masel et al. (Catalyst Letters, Vol. 96, Nos. 3-4, July 2004) reported that the activity of some of these metals as catalysts for ammonia cracking varies in the following order:

[0052] Ru>Ni>Rh>Co>Ir>Fe>>Pt>Cr>Pd>Cu>>Te

[0053] Metals can be unsupported, but are usually supported on suitable supports, typically metal oxide supports such as silicon dioxide (SiO2), aluminum oxide (Al2O3), zirconium oxide (ZrO2), or mixed metal oxide supports such as spinel (MgAl2O4) or perovskite (CaTiO3).

[0054] As those skilled in the art will understand, the activity of a supported metal catalyst will typically depend in part on the amount of catalytically active metal loaded on the support. In this respect, the metal loading will vary depending on specific requirements, but is generally in the range of about 0.1 wt% to about 70 wt%. For more active metals (e.g., ruthenium), the loading can be closer to the lower limit of this range, for example, about 0.1 wt% to about 10 wt% or about 0.2 wt% to about 5 wt%. For less active metals (e.g., nickel), the loading can be closer to the upper limit of this range, for example, about 20 wt% to about 65 wt%.

[0055] The supported metal catalyst may be unpromoted or promoted with at least one other metal to improve its activity, such as one or more Group 1 metals, such as lithium (Li), sodium (Na) and potassium (K); Group 2 metals, such as magnesium (Mg) and calcium (Ca); or Group 13 metals, such as aluminum (Al).

[0056] Any conventional catalyst known for ammonia cracking can be used in this invention. Suitable catalysts are disclosed in US2015 / 0217278A, Masel et al. (above), Lamb et al. (International Journal of Hydrogen Energy, 44 (2019), pp. 3726-3736), and Boisen et al. (J. Catalysis, 230 (2005), pp. 309-312).

[0057] Bimetallic catalysts or catalysts containing two catalytically active metals are also suitable for this invention. Examples include composite metals or metal alloys or metal nanoclusters supported on perovskites, composite oxides or nitrides, or mixed oxides or mixed nitrides disclosed in US2021 / 0001311A, such as CoNi-MgSrCeO4 and 1 wt% K-CoNi-MgSrCeO4.

[0058] The catalysts of this invention typically comprise metal-based catalysts, such as catalysts containing a metal base or composed of a metal-based catalyst. The catalytically active metal is typically selected from transition metals of the periodic table. Suitable transition metal-based catalysts typically exhibit activity at temperatures ranging from 475°C to 600°C suitable for achieving a reaction rate at least 0.2 times, at least 0.5 times, at least equal to, at least twice, or at least five times the rate calculated according to Equation 9 proposed by Lamb et al., i.e.,

[0059] r = 8.73exp[-76710 / RT].(P) NH3 ) 0.28 .(P H2 ) -0.42 .(1-β 2 )

[0060] in:

[0061] “r” represents the reaction rate (or “activity”) of the catalyst;

[0062] "RT" is the ideal gas constant "R" (8.314 J mol). -1 K -1 Multiply by the temperature "T" in Kelvin;

[0063] P NH3 It is the partial pressure of ammonia;

[0064] P H2 It is the partial pressure of hydrogen gas;

[0065] β is defined in the paper as (see equation 5 proposed by Lamb et al.):

[0066]

[0067] P N2 It is the partial pressure of nitrogen; and

[0068] K e It is the equilibrium constant of the reaction (see equations 6 and 7 proposed by Lamb et al.).

[0069] The inventors have recognized that Equation 9 of Lamb et al. can be extrapolated to temperatures beyond 475°C to 600°C, for example, in the range of 450°C to 700°C.

[0070] The transition metals that may be particularly suitable for use as catalysts in catalyst beds in reactor tubes are selected from chromium, manganese, iron, cobalt, nickel, ruthenium and copper, such as iron, cobalt, nickel and ruthenium.

[0071] The catalytically active metal of the first (higher activity) catalyst is preferably ruthenium, and the catalytically active metal of the second (lower activity) catalyst is typically nickel.

[0072] In a preferred embodiment, the first catalyst in the upstream layer is a ruthenium-based catalyst, and the second catalyst in the downstream layer is a nickel-based catalyst. In these embodiments where a third catalyst is present, the catalyst is preferably a ruthenium-based catalyst, although this catalyst may be a different ruthenium-based catalyst from the first catalyst, for example, with a different support and / or catalyst loading, and / or, if both are promoted, with different metals.

[0073] Therefore, the term "ruthenium-based catalyst" refers to a catalyst containing ruthenium as the sole (or at least primary) catalytically active metal (i.e., the metal responsible for the catalytic cracking reaction). Ruthenium may be the only metal in the catalyst, or alternatively, one or more other metals may be present, for example, in a ruthenium-supported material. The terms "nickel-based catalyst" and "iron-based catalyst" should be interpreted accordingly.

[0074] Suitable ruthenium-based and nickel-based catalysts can be supported on, for example, alumina (as disclosed by Lamb et al. or Masel et al.) or spinel (as disclosed by Boisen et al.), optionally promoted by group 1 or group 2 metals.

[0075] As mentioned above, water is typically present as a contaminant in ammonia. Water can be removed from ammonia, in which case water-intolerant catalysts, such as iron-based catalysts, can be used in the reactor tubes. However, in preferred embodiments, water removal is not performed to save money and operating costs and reduce energy consumption. In these embodiments, water-intolerant catalysts, such as iron-based catalysts, are not used. Instead, the catalyst in the reactor tubes will be able to tolerate up to 1 mol% water in the ammonia feed. Such catalysts include nickel-based and ruthenium-based catalysts.

[0076] In a preferred embodiment, a ruthenium-based catalyst is used in the first layer within each tube, allowing for a faster reaction rate that keeps the metal temperature within the design limit of 660°C. In these embodiments, the second layer of the tube comprises a lower-cost but less active nickel-based catalyst. In a preferred embodiment, the catalysts are layered in this manner to utilize the endothermic reaction within the tube to keep the tube metal cool in the most intense combustion region on the outer side of the tube. Due to the higher catalytic activity of ruthenium, it generates stronger endothermic reaction, keeping the inner tube wall cooler in the high ammonia concentration region on the process side, which protects the tube from over-nitriding caused by high ammonia concentration and high temperature.

[0077] The combustion process in the furnace is preferably fueled at least partially internally, i.e., at least part of the fuel is ammonia or exhaust gas generated during hydrogen recovery from cracked gases, or a mixture of both. However, trimmed fuels, such as C1 to C3 hydrocarbons or natural gas, may be used as needed, although the use of hydrocarbon trimmed fuels increases the carbon intensity of the process. However, it is generally desirable to minimize or even eliminate the use of such trimmed fuels to reduce the carbon intensity of the process.

[0078] The oxidizing gas is usually air, but depending on the circumstances, it can also be oxygen-enriched gas or pure oxygen.

[0079] If the reactor wall material can withstand higher temperatures, the feed temperature to the catalyst-filled reactor tubes of the furnace can reach up to approximately 800°C. For lower temperature cycles, the feed is typically in the range of approximately 400°C to approximately 600°C or approximately 450°C to approximately 550°C, for example, at approximately 500°C. For higher temperature cycles, the feed can be in the range of approximately 500°C to approximately 800°C or approximately 600°C to approximately 700°C, for example, at approximately 650°C.

[0080] Cracking temperature and pressure typically determine that ammonia slip in the reactor tubes does not exceed 3 mol%, for example, from about 0.5 mol% to 1.5 mol%.

[0081] In some embodiments, heated ammonia is partially cracked in an adiabatic reaction unit comprising at least one catalyst bed to produce partially cracked ammonia for feeding into a reactor tube filled with catalyst.

[0082] The molar fraction of ammonia in the gas passing through the adiabatic reaction unit is typically reduced by at least 20%, for example, at least 25%, or at least 30%, or even at least 35%, for example, about 40%, and possibly as high as about 50%. In other words, the molar fraction of ammonia can be reduced from 1 (or almost 1) in heated ammonia to an amount in the range of about 0.5 to about 0.8 in partially cracked ammonia, or to an amount in the range of about 0.5 to about 0.7, or to an amount in the range of about 0.55 to about 0.65, for example, to about 0.6.

[0083] Adiabatic reaction units can be incorporated into the process design to improve overall efficiency, particularly by using the heat available in the flue gas to heat the adiabatic cracking process within the unit. In this regard, the temperature around the adiabatic reaction unit is typically optimized to maximize heat recovery from the flue gas, while avoiding temperatures above approximately 660°C due to material considerations.

[0084] The key design parameter for an adiabatic reaction unit is the inlet temperature. Because ammonia cracking is endothermic, a higher inlet temperature allows for greater conversion rates within the unit. However, higher temperatures place greater demands on structural materials and catalysts. Inlet temperatures typically range from approximately 350°C to approximately 800°C, and for lower-temperature cycles, inlet temperatures can range from approximately 400°C to approximately 600°C, or from approximately 400°C to approximately 450°C. For higher-temperature cycles, inlet temperatures can range from approximately 500°C to approximately 700°C, or from approximately 550°C to approximately 650°C.

[0085] Due to the high temperatures and ammonia concentrations, cracking reactor vessels, such as reactor tubes in insulated reactors and furnaces, must typically be constructed of materials resistant to ammonia and / or nitriding. Suitable materials include nickel-based alloys comprising at least 40 wt% or at least 50 wt% nickel. Such alloys typically have no more than 90 wt% or no more than 80 wt% nickel. The alloy will usually include one or more other metals selected from chromium, cobalt, molybdenum, and iron.

[0086] Specific examples of suitable nickel-based alloys include UNS N06600, N06625, N06601, N06617, N06025, N06230, N07214, and N08811. In some embodiments, austenitic nickel-chromium-based superalloys, such as Inconel, may be used.

[0087] The Uniform Numbering System (UNS) is a widely accepted alloy naming system in North America. Each UNS number is associated with a specific metal or alloy and defines its specific chemical composition, or in some cases, its specific mechanical or physical properties.

[0088] Other suitable materials include cobalt-based alloys, such as UNS R30188. Additionally, high-temperature alloys with lower resistance to nitriding, such as UNS N08811, or cast alloys like HPNb, HP microalloyed, and MA-1 (MetalTek International, USA), may be suitable, especially when surface-modified or coated with corrosion-resistant layers such as aluminizing; aluminizing followed by pre-oxidation; or ceramic coatings. Nitriding-resistant alloys can also be used in conjunction with surface modifications or coatings to improve performance.

[0089] An adiabatic reaction unit typically comprises one or more adiabatic reactors, each of which includes a catalyst bed. The adiabatic reactor may be made of one or more of the ammonia-resistant and / or ammonia-nitriding-resistant materials listed above.

[0090] In a preferred embodiment, the adiabatic reaction unit will include two or more adiabatic reactors, such as two, three, four, five, or six reactors, with interstage heating as appropriate. Depending on process requirements, the reactors may be arranged in series, in parallel, or a combination of series and parallel. However, in a preferred embodiment, the adiabatic reaction unit will have two such reactors arranged in series, wherein the intermediately cracked ammonia is interstage heated by heat exchange with the cracked gas and / or flue gas.

[0091] Each adiabatic reactor has a bed comprising at least one catalyst suitable for ammonia cracking. Any conventional ammonia cracking catalyst may be used in this adiabatic reactor or in the bed of each adiabatic reactor. Suitable catalysts are discussed in the context of reactor tube catalysts.

[0092] In embodiments where water is removed from ammonia prior to cracking, water-sensitive catalysts, such as iron-based catalysts, can be used in the adiabatic reactor. However, in preferred embodiments, water removal is not performed to save money and operating costs and reduce energy consumption. In these embodiments, the catalyst bed of the adiabatic reactor unit does not contain an iron-based catalyst; instead, a water-resistant catalyst is used, i.e., a metal-based catalyst resistant to the presence of up to 1 mol% water is preferred. In this regard, nickel-based catalysts or ruthenium-based catalysts, or a combination of nickel-based and ruthenium-based catalysts, can be used in the bed of the adiabatic reactor unit.

[0093] As mentioned above, ruthenium-based catalysts tend to be more active than nickel-based catalysts, but are more expensive. Therefore, further optimization is possible by selecting the catalyst, and if more than one type of catalyst is used, by arranging the catalyst layers within the bed of the adiabatic reaction unit.

[0094] In some preferred embodiments having two adiabatic reactors in series, the catalyst bed of the first reactor comprises a monolayer of a first catalyst, such as a ruthenium-based catalyst, for example, containing a monolayer of a first catalyst, such as a ruthenium-based catalyst, or consisting of a monolayer of a first catalyst, such as a ruthenium-based catalyst, and the catalyst bed of the second reactor comprises an upstream layer of a second catalyst, such as a nickel-based catalyst, and a downstream layer of a third catalyst, such as a ruthenium-based catalyst, for example, containing an upstream layer of a second catalyst, such as a nickel-based catalyst, and a downstream layer of a third catalyst, such as a ruthenium-based catalyst, or consisting of an upstream layer of a second catalyst, such as a nickel-based catalyst, and a downstream layer of a third catalyst, such as a ruthenium-based catalyst, wherein the second catalyst is typically less active than the first catalyst, and the third catalyst is typically more active than the second catalyst.

[0095] In these embodiments, the first catalyst and the third catalyst may be the same. Alternatively, the first catalyst and the third catalyst may be different, for example, containing different catalytically active metals, or containing the same catalytically active metal but on different supports, or containing the same catalytically active metal on the same support but at different loadings.

[0096] In other preferred embodiments having two adiabatic reactors in series, the catalyst bed of the first reactor comprises a monolayer of a first catalyst, such as a nickel-based catalyst, for example containing a monolayer of a first catalyst, such as a nickel-based catalyst, or composed of a monolayer of a first catalyst, such as a nickel-based catalyst, and the catalyst bed of the second reactor comprises an upstream layer of a second catalyst, such as a nickel-based catalyst, and a downstream layer of a third catalyst, such as a ruthenium-based catalyst, for example containing an upstream layer of a second catalyst, such as a nickel-based catalyst, and a downstream layer of a third catalyst, such as a ruthenium-based catalyst, or composed of an upstream layer of a second catalyst, such as a nickel-based catalyst, and a downstream layer of a third catalyst, such as a ruthenium-based catalyst, the second catalyst typically having similar activity to the first catalyst, and the third catalyst typically being more active than the first and second catalysts.

[0097] In these embodiments, the first catalyst and the second catalyst may be the same. Alternatively, the first catalyst and the second catalyst may be different, for example, containing different catalytically active metals, or containing the same catalytically active metal but on different supports, or containing the same catalytically active metal on the same support but at different loadings.

[0098] In both preferred embodiments, the volume of the upstream layer of the second catalyst in the bed of the second reactor can be about 40% to about 90% of the total bed volume, for example, about 50% to about 70% or about 60%. In the absence of another catalyst layer, the volume of the downstream layer of the third catalyst in the bed of the second reactor can be about 10% to about 60% of the total bed volume, for example, about 30% to about 50%, or about 40%.

[0099] The inventors have recognized that ruthenium-based catalysts are not only sufficiently resistant to water, but also capable of cracking hydrocarbon oils into shorter hydrocarbons, such as methane, as well as carbon monoxide and hydrogen. Therefore, using these catalysts in adiabatic reaction units eliminates the need for dedicated upstream units for removing oil from liquid ammonia.

[0100] It is also known that catalyst sintering at higher temperatures reduces catalyst activity and lifetime. In this regard, those skilled in the art will recognize the need to balance improved conversion with higher container costs and shorter catalyst lifetime.

[0101] The heat from the cracked gases and flue gas is typically used to heat the feed stream to the adiabatic reaction unit and furnace, thereby reducing the total energy consumed in the process. In this respect, the temperature of the cracked gases will depend on the operating cycle.

[0102] In lower-temperature cycles, the temperature of the cracked gas can reach as high as about 700°C, typically from about 550°C to about 700°C, or from about 600°C to about 650°C. The temperature of the flue gas can reach as high as about 750°C at its highest point. However, due to heat leakage, the temperature drops, and at the point where its heat can be effectively utilized, the temperature is typically from about 600°C to about 700°C.

[0103] In higher-temperature cycles, the temperature of the cracked gas can reach as high as approximately 750°C, for example, typically from approximately 650°C to approximately 750°C, or from approximately 675°C to approximately 725°C. The temperature of the flue gas can reach as high as approximately 840°C at its highest point. However, due to heat leakage, the temperature decreases, and at the point where its heat can be effectively utilized, the temperature is typically from approximately 700°C to approximately 800°C.

[0104] In a case where an adiabatic reaction unit is used to partially crack heated ammonia and the partially cracked gas is heated to the feed temperature of the catalyst-filled reactor tubes of the furnace, at least some of the load required to heat the partially cracked gas is provided by heat exchange with the cracked gas. In a preferred embodiment, the cracked gas is not used to heat another process fluid before heating the partially cracked gas. Some of this heating load can be provided in another manner, for example, by heat exchange with flue gas. However, all of this heating load is preferably provided by the cracked gas.

[0105] In a preferred embodiment, the composition of the ammonia typically remains at least substantially unchanged from the stored liquid ammonia to the heated ammonia gas fed into the adiabatic reaction unit. Oil present in the liquid ammonia may be removed sometime before the partial cracking of the ammonia, although ruthenium-based catalysts are used therein, the oil does not need to be removed. However, water is typically not removed, so any water present in the liquid ammonia will also be present in the heated ammonia gas.

[0106] In cases where the partially cracked gas is heated through heat exchange with other cracked gas, the temperature of the cracked gas decreases. The cooled cracked gas is then typically further cooled by providing at least a portion of the heating load required to generate heated ammonia from liquid ammonia. After cooling, hydrogen can be recovered from the cracked gas as a product. Recovery can be achieved in a pressure swing adsorption (PSA) process, or by using one or more selectively permeable membranes, or by a combination of PSA and membrane separation. In a preferred embodiment, hydrogen recovery is achieved only in the PSA process, i.e., without the use of membrane separation.

[0107] In embodiments using the PSA process, waste gas is generated comprising nitrogen, residual ammonia, and residual hydrogen. This waste gas can be preheated and used as fuel for combustion in a furnace. Alternatively, while a portion of the waste gas can be used as fuel, another portion can be compressed and returned to the PSA process to improve hydrogen recovery.

[0108] Aspects of the present invention include:

[0109] #1. A method for cracking ammonia, the method comprising:

[0110] Provides heated ammonia gas under ultra-atmospheric pressure;

[0111] In a furnace, fuel is burned with oxidant gas to heat a catalyst-containing reactor tube and produce flue gas. Each tube includes an upstream layer of a first catalyst and a downstream layer of a second catalyst.

[0112] The heated ammonia gas, or partially cracked ammonia gas derived therefrom, is fed into the catalyst-containing reactor tube to induce cracking of the ammonia and produce cracked gases including hydrogen, nitrogen, and residual ammonia.

[0113] The first catalyst is more active for cracked ammonia than the second catalyst.

[0114] #2. The method according to #1, wherein the first catalyst is a ruthenium-based catalyst.

[0115] #3. The method according to #1 or #2, wherein the second catalyst is a nickel-based catalyst.

[0116] #4. The method according to any one of #1 to #3, wherein the catalyst-containing reactor tube includes a third catalyst layer downstream of the second catalyst layer, the third catalyst being more active for cracked ammonia than the second catalyst.

[0117] #5. The method according to #4, wherein the third catalyst contains the same catalytically active metal as the first catalyst.

[0118] #6. The method according to #4 or #5, wherein the third catalyst is a ruthenium-based catalyst.

[0119] #7. The method according to any one of #1 to #6, the method comprising:

[0120] Pumping liquid ammonia containing at least 0.1 mol% water to produce pumped liquid ammonia;

[0121] The pumped liquid ammonia is preheated to produce preheated liquid ammonia;

[0122] Evaporating the preheated liquid ammonia to produce ammonia gas; and

[0123] The ammonia gas is heated to produce the heated ammonia gas under atmospheric pressure.

[0124] Water from the liquid ammonia is present in the heated ammonia gas.

[0125] #8. The method according to #7, wherein at least some, preferably most, such as more than 50% and optionally up to 80%, of the heating load required to provide the heated ammonia is provided by heat exchange with the cracked gas.

[0126] #9. The method according to #7 or #8, wherein water is present in the heated ammonia in an amount not exceeding 1 mol.%.

[0127] #10. The method according to any one of #1 to #9, wherein the catalyst-containing reactor tube does not contain an iron-based catalyst.

[0128] #11. The method according to any one of #1 to #10, the method comprising partially cracking the heated ammonia in an adiabatic reaction unit comprising at least one catalyst bed to produce partially cracked ammonia for feeding into a reactor tube filled with catalyst.

[0129] #12. The method according to #11, wherein the catalyst bed of the adiabatic reaction unit comprises at least one catalyst selected from nickel-based catalysts and ruthenium-based catalysts.

[0130] #13. The method according to #11 or #12, wherein the catalyst bed of the adiabatic reaction unit does not include an iron-based catalyst.

[0131] #14. A furnace for cracking heated ammonia gas, the furnace comprising:

[0132] A radiant section, comprising at least one fuel and oxidant gas inlet in fluid communication with at least one burner, an ammonia feed inlet, and a catalyst-containing reactor tube having an upstream end in fluid communication with the ammonia feed inlet and a downstream end in fluid communication with the outlet of cracked gas, each tube comprising an upstream layer of a first catalyst and a downstream layer of a second catalyst; and

[0133] The convection section is in fluid flow communication with the radiation section and includes the flue gas outlet;

[0134] The first catalyst is more active for cracked ammonia than the second catalyst.

[0135] #15. The furnace according to #14, wherein the first catalyst is a ruthenium-based catalyst.

[0136] #16. The furnace according to #14 or #15, wherein the second catalyst is a nickel-based catalyst.

[0137] #17. The furnace according to any one of #14 to #16, wherein each catalyst-containing reactor tube includes a third catalyst layer downstream of the second catalyst layer, the third catalyst being more active for cracked ammonia than the second catalyst.

[0138] #18. The furnace according to #17, wherein the third catalyst contains the same catalytically active metal as the first catalyst.

[0139] #19. The furnace according to #17 or #18, wherein the third catalyst is a ruthenium-based catalyst.

[0140] #20. The furnace according to any one of #14 to #19, wherein the catalyst-containing reactor tube does not contain an iron-based catalyst.

[0141] #21. An apparatus for cracking heated ammonia gas, said apparatus comprising:

[0142] Liquid ammonia source;

[0143] A pump in fluid communication with the liquid ammonia source is used to pump liquid ammonia; and

[0144] The furnace according to any one of #14 to #20, wherein the ammonia feed inlet is in fluid communication with the pump.

[0145] The device further includes:

[0146] At least one heat exchanger is arranged upstream of the pump for preheating liquid ammonia; and

[0147] At least one heat exchanger is arranged to evaporate the pumped liquid ammonia and heat the ammonia gas by heat exchange with flue gas and / or cracked gas located between the ammonia feed inlet of the pump and the furnace.

[0148] #22. The apparatus according to #21, the apparatus comprising an adiabatic reaction unit for partially cracking heated ammonia under atmospheric pressure, the unit comprising an inlet for heated ammonia under atmospheric pressure in fluid communication with the pump, at least one catalyst bed having an upstream end in fluid communication with the inlet and a downstream end in fluid communication with an outlet for the partially cracked ammonia.

[0149] The ammonia feed inlet of the furnace is in fluid communication with the outlet fluid of the partially cracked ammonia gas from the adiabatic reaction unit.

[0150] The invention will now be described by way of example only with reference to the accompanying drawings.

[0151] exist Figure 1 In this process, a liquid ammonia stream 2 at approximately -32°C is removed from a reservoir (not shown) and fed into pump P101, where it is pumped to generate a pressurized liquid ammonia stream 4 at approximately 46 bar. This pressurized liquid ammonia stream is preheated in heat exchanger E271 by exchanging heat with a heat transfer fluid (in this case, an aqueous solution of glycol, typically approximately 55 wt% ethylene glycol or propylene glycol) to produce a preheated liquid ammonia stream 6 at approximately 55°C. An electric heater can be used to ensure that the temperature of the glycol solution fed into heat exchanger E271 is sufficient to preheat the liquid ammonia to the required temperature.

[0152] The preheated liquid ammonia in stream 6 is further heated by heat exchange in heat exchanger E312 to produce a further heated liquid ammonia stream 8. The further heated liquid ammonia in stream 8 is then evaporated by heat exchange in heat exchanger E311 to produce an ammonia vapor stream 10. The ammonia vapor in stream 8 is then superheated by heat exchange in heat exchanger E310 to produce a heated ammonia stream 12 at approximately 260°C.

[0153] The heated ammonia gas in stream 12 is further heated by heat exchange in heat exchanger E2102 to produce a superheated ammonia gas stream 14 at approximately 420°C. For convenience, heat exchanger E2102 is shown as a single unit. However, in reality, there may be two separate heat exchangers with a selective catalytic reactor (SCR) located between them.

[0154] Superheated ammonia gas from stream 14 is fed into a first adiabatic reactor vessel C141 at approximately 420°C and approximately 43 bar, and passes through a ruthenium-based catalyst bed. Some of the ammonia gas is cracked over the catalyst to form an intermediate gas stream 16 containing some cracked ammonia. The mole fraction of ammonia in the gas passing through the first adiabatic reactor vessel C141 decreases from almost 1 to approximately 0.9.

[0155] Before being heated by heat exchange in heat exchanger E2103 to generate a superheated intermediate gas stream 18, the intermediate gas is at approximately 360°C and then fed into a second adiabatic reactor vessel C142 at approximately 590°C, passing through a bed comprising an upstream layer of nickel-based catalyst and a downstream layer of ruthenium-based catalyst to generate a partially cracked ammonia stream 20. The molar fraction of ammonia in the gas passing through the second adiabatic reactor vessel C142 decreases from approximately 0.9 to approximately 0.6.

[0156] The catalyst bed of the second adiabatic reactor vessel C142 has two layers: a ruthenium-based catalyst layer and a nickel-based catalyst layer, to utilize heat more efficiently and thus maximize ammonia conversion. The catalyst volume was also optimized by minimizing the volume of the ruthenium-based catalyst by limiting the outlet temperature of the second adiabatic reactor vessel to approximately 390°C. The inventors found that lowering this temperature further increased the required volume of the ruthenium-based catalyst.

[0157] The ruthenium-based catalyst is the same in both the first and second adiabatic reactor vessels. However, different ruthenium-based catalysts can be used.

[0158] The partially cracked ammonia in stream 20 is heated via heat exchange in heat exchanger (or "economizer") E305 before being fed as stream 22 at a pressure of approximately 38 bar into the catalyst-filled tubes in the radiant section F201 of the furnace (or reactor). By reducing the load required to heat the partially cracked stream to the reaction temperature, heating the feed to the tubes increases the amount of cracking that can be accomplished using heat from the burner. Utilizing the cracked stream from the tubes allows for efficient use of this high-temperature stream. The inlet temperature of the direct-fired tubular furnace is limited to approximately 500°C to limit the inner wall temperature of the cracker tubes.

[0159] Airflow 62 passes through forced draft fan K212 before being preheated by heat exchange in heat exchanger E2141 to produce preheated airflow 64. The preheated airflow 64 can be mixed with natural gas flow 70 as a fine-tuning fuel and fed into the burner (not shown) of furnace F201. Preheating the air in this way helps reduce fuel demand.

[0160] The tubes in the radiant section F201 of the furnace are filled with two different types of ammonia cracking catalysts in two distinct layers. A ruthenium-based catalyst is used in the first layer within each tube, allowing for a faster reaction rate and maintaining the metal temperature within the design limit of approximately 660°C. The second layer in the tube downstream of the first layer contains a lower-cost but less active nickel-based catalyst.

[0161] The cracked gas stream 24 exits the radiant section F201 of the direct-fired tubular furnace at approximately 640°C and is then fed into the heat saver E305 to provide the load required to heat the partially cracked ammonia, thereby reducing the temperature of the cracked gas to approximately 530°C.

[0162] The E305 heat exchanger is described as a shell-and-tube heat exchanger, in which partially cracked ammonia gas passes through the tubes and cracked gas passes through the shell side. However, this arrangement can be reversed, or in fact, different types of heat exchangers can be used.

[0163] The cracked gas stream 26 is then fed from the heat exchanger E305 to the heat exchanger E310 to provide the load required to superheat the ammonia, thereby further reducing the temperature of the cracked gas to about 389°C.

[0164] The cracked gas stream 28 is then fed from heat exchanger E310 to heat exchanger E311 to provide the load required for evaporating the liquid ammonia to be further heated, thereby further reducing the temperature of the cracked gas to about 109°C.

[0165] The cracked gas stream 30 is then fed from heat exchanger E311 to heat exchanger E312 to provide the load required for further heating of the heated pressurized liquid ammonia, thereby further reducing the temperature of the cracked gas to about 70°C.

[0166] Each of the heat exchangers E310, E311, and E312 is depicted as a separate shell-and-tube heat exchanger, with ammonia passing through the tubes and cracked gas passing through the shell side. However, this arrangement can be reversed. Alternatively, the heat exchangers can be combined into a single shell-and-tube heat exchanger, or different types of heat exchangers can actually be used.

[0167] The cracked gas stream 32 from heat exchanger E312 is then further cooled by heat exchange with a heat transfer fluid in cooler E323, and is then fed as stream 34 into PSA system U501, where it is separated into hydrogen stream 40 and PSA waste stream 42. The hydrogen stream is removed as a product, and the PSA waste stream includes nitrogen, residual hydrogen, and residual ammonia. The hydrogen in stream 40 can be fed into a hydrogen liquefaction unit (not shown) to produce liquid hydrogen.

[0168] All PSA exhaust gas in stream 42 can be directly fed as fuel (stream 60) for combustion in furnace F201. Alternatively, stream 42 can be divided into two parts.

[0169] The first portion of the PSA exhaust gas in stream 44 is heated by heat exchange in heat exchanger E2112 to produce a warm PSA exhaust gas stream 60, which is then fed into the burner in furnace F201 along with air feed 64 and, optionally, natural gas feed 70. A minimum amount of natural gas is used as a fine-tuning fuel to provide the required fuel balance in the combustion zone.

[0170] The second part can be fed as stream 46 into the multi-stage compression unit K681 for compression. Compression unit K681 has five stages, with an intercooler between each stage and an aftercooler after the last stage. Heat is recovered from the compressed gas in the intercooler and aftercooler through heat exchange with a heat transfer fluid. Heat can also be recovered from the lubricating oil, and in the case of a positive displacement compression unit, from the cylinder of the compression unit using the heat transfer fluid.

[0171] For convenience, the intercooler and aftercooler are represented by a single heat exchanger (marked as E6816A-E) that recovers heat from the compressed PSA exhaust gas stream 48 by exchanging heat with the heat transfer fluid stream 52 to produce a cooled compressed PSA exhaust gas stream 50 and a heated heat transfer fluid stream 54.

[0172] The heat transfer fluid heated in cooler E323, as well as in intercoolers and aftercoolers E6816A-E, is then used to provide the load required to preheat liquid ammonia by heat exchange in heat exchanger E271.

[0173] The cooled, compressed PSA exhaust gas in stream 50 is fed into phase separator C6816, where any condensate is removed as stream 56. The compressed PSA exhaust gas is then recycled as stream 58 back to PSA system U501 for further hydrogen recovery. In this way, the hydrogen recovery rate can be increased from 85% (without recirculation) to 95% (with recirculation).

[0174] As mentioned above, the process can operate without the compression unit K681, resulting in a lower hydrogen recovery rate in the PSA unit 501. Reduced hydrogen recovery obviously leads to less hydrogen product. However, the lower hydrogen recovery rate may still be desirable because with more hydrogen present in the exhaust gas, the process's carbon intensity (CI) decreases, thereby reducing the need for natural gas as a fine-tuning fuel and reducing CO2 emissions.

[0175] A flue gas flow 72 at approximately 686°C flows from the radiant section F201 to the convection section 90 of the furnace F201. In this convection section, it first provides the load required to heat the intermediate gas from flow 16 in heat exchanger E2103, thereby reducing the flue gas temperature. This flue gas is then used (as flow 74) to provide the load required to further heat the heated ammonia from flow 12 in heat exchanger E2102, thereby further reducing the flue gas temperature. Thus, the flue gas provides the heating load in a direction counter-current to the feed gas flow of the radiant section F201 of the direct-fired tubular furnace.

[0176] The cooled flue gas (as stream 76) is then used to provide the load required to heat the air from stream 62 in heat exchanger E2142, thereby further reducing the temperature of the flue gas. The further cooled flue gas (as stream 78) is then used to provide the load required to heat the PSA exhaust gas from stream 44 in heat exchanger E2112, thereby further cooling the flue gas.

[0177] The cooled flue gas, at a temperature of approximately 121°C (above the condensation point of water), leaves the convection section 90 of the direct-fired tubular furnace F201 as stream 80, passes through the induced draft fan K211, and then exits the process as stream 82. At this point, all the actual energy has been extracted from the flue gas, and it can now be discharged into the atmosphere, optionally after further treatment, depending on its composition, if desired.

[0178] Where ammonia is produced, or at the ammonia cracking site, or virtually anywhere transported between two sites, oil from an evaporative gas compressor (not shown) used in conjunction with an ammonia storage tank (not shown) may be present in liquid ammonia at levels as high as about 5 ppm. The presence of oil in ammonia can cause difficulties because ammonia cracking catalysts may be intolerant of oil. Therefore, it may be desirable to remove the oil before the ammonia is exposed to the catalyst. Oil can be removed by passing the ammonia through a bed of activated carbon.

[0179] If oil is to be removed from ammonia, the oil removal unit (not shown) can be located in flow 2 (i.e., in the feed line to pump P101), flow 4 (i.e., between pump P101 and ethylene glycol heater E271), flow 6 (i.e., between ethylene glycol heater E271 and heat exchanger E312), flow 8 (i.e., between heat exchangers E312 and E311), or flow 10 (i.e., between heat exchangers E311 and E310).

[0180] Example

[0181] Comparative example

[0182] Figure 1The process described has been simulated using a computer (Aspen Plus, ver. 10, Aspen Technology, Inc., Massachusetts, USA) to design a plant that produces 30 tons / day of hydrogen (flow 40).

[0183] The reactor was simulated to have 60 tubes, each with an inner diameter of 4.313 inches (0.11 m), and contained a 40-foot (12.2 m) catalyst bed containing a nickel-based catalyst as the sole catalyst.

[0184] Based on rate equation 9 given by Lamb et al. (International Hydrogen Energy, 44 (2019), pp. 3726-3736), the activities of ruthenium-based catalysts in an adiabatic reactor and nickel-based catalysts in both the adiabatic reactor and reactor tubes were modeled. For simulation purposes, it was assumed that the activity of the ruthenium-based catalyst conformed to the rate equation, but the activity of the nickel-based catalyst was 20% of the activity predicted by the rate equation.

[0185] Various temperatures and the mole fraction of ammonia as a function of cracker length are plotted on... Figure 2 The results show that the tube has a maximum outer wall temperature of approximately 750°C and a maximum inner wall temperature of approximately 724°C.

[0186] Example 1

[0187] The simulation according to the comparative example was repeated using the arrangement of the catalyst in the reactor tube according to the invention. In this respect, each tube was simulated as having an upstream layer of 15 feet (4.6 m) of ruthenium-based catalyst and a downstream layer of 25 feet (7.6 m) of the same nickel-based catalyst as in the comparative example.

[0188] All other characteristics of the simulation remained unchanged, including the arrangement of the catalyst in the bed of the adiabatic reactor and the assumed activities of the two types of catalysts.

[0189] Various temperatures and ammonia mole fractions as a function of cracker length are plotted on... Figure 3 The results showed that the tube had a maximum outer wall temperature of 668°C and a maximum inner wall temperature of 648°C, both of which were significantly lower than the equivalent values ​​of 750°C and 724°C from the comparative example, respectively.

[0190] At lower temperatures, the degree of nitriding of the metal forming the tube will decrease significantly.

[0191] The simulated heat and mass balance data for Example 1 are provided in Table 2.

[0192]

[0193]

[0194] Table 2

[0195] Data shows that, for the recovery rate of 1.33 mol% ammonia slip (stream 24) from the cracker (tube furnace 201) and 95% hydrogen from the PSA unit, in addition to the PSA exhaust gas (stream 60), 7626 kg / h of ammonia (stream 2) and 472.7 kg / h of natural gas (stream 70) are required as feed to ignite the cracker.

[0196] For a given hydrogen recovery rate and ammonia slip, using cracked gas (instead of flue gas) to provide the load required to heat the partially cracked gas (stream 20) to the feed temperature of the catalyst-filled tube of the cracker (F201) has the effect of reducing the total carbon intensity (CI) of the process.

[0197] Although the invention has been described with reference to preferred embodiments illustrated in the accompanying drawings, it should be understood that various modifications are possible within the spirit or scope of the invention as defined by the appended claims.

[0198] In this specification, unless otherwise explicitly stated, the word "or" is used to indicate an operator that returns a true value when one or both of the stated conditions are met, as opposed to the operator "XOR," which requires only one of the conditions to be met. The word "including" is used to mean "contains" and is incorporated into "consisting of," rather than simply meaning "consisting of."

[0199] All existing teachings above are incorporated herein by reference. Acknowledgment of any previously published document herein should not be construed as an acknowledgment or representation that the teachings of that document were common knowledge in Australia or elsewhere on the date of publication of that document.

Claims

1. A method for cracking ammonia, the method comprising: Provides heated ammonia gas under ultra-atmospheric pressure; In a furnace, fuel is burned with an oxidant gas to heat a catalyst-containing reactor tube and generate flue gas. Each tube includes an upstream layer of a first catalyst and a downstream layer of a second catalyst, wherein the first catalyst is a ruthenium-based catalyst and the second catalyst is a nickel-based catalyst. The heated ammonia gas, or partially cracked ammonia gas derived therefrom, is fed into the catalyst-containing reactor tube to induce cracking of the ammonia and produce cracked gases including hydrogen, nitrogen, and residual ammonia. The first catalyst is more active for cracked ammonia than the second catalyst.

2. The method of claim 1, wherein the catalyst-containing reactor tube includes a third catalyst layer downstream of the second catalyst layer, the third catalyst being more active for cracked ammonia than the second catalyst.

3. The method according to claim 2, wherein the third catalyst contains the same catalytically active metal as the first catalyst.

4. The method according to claim 2, wherein the third catalyst is a ruthenium-based catalyst.

5. The method according to claim 1, wherein the method comprises: Pumping liquid ammonia containing at least 0.1 mol% water to produce pumped liquid ammonia; The pumped liquid ammonia is preheated to produce preheated liquid ammonia; The preheated liquid ammonia is evaporated to produce ammonia gas; as well as The ammonia gas is heated to produce the heated ammonia gas under atmospheric pressure. Water from the liquid ammonia is present in the heated ammonia gas.

6. The method of claim 5, wherein at least some of the heating load required to provide the heated ammonia is provided by heat exchange with the cracked gas.

7. The method of claim 5, wherein water is present in the heated ammonia in an amount not exceeding 1 mol%.

8. The method according to claim 1, wherein the catalyst-containing reactor tube does not contain an iron-based catalyst.

9. The method of claim 1, wherein the method comprises partially cracking the heated ammonia in an adiabatic reaction unit comprising at least one catalyst bed to produce the partially cracked ammonia for feeding into a reactor tube filled with catalyst.

10. The method according to claim 9, wherein the catalyst bed of the adiabatic reaction unit comprises at least one catalyst selected from nickel-based catalysts and ruthenium-based catalysts.

11. The method according to claim 9, wherein the catalyst bed of the adiabatic reaction unit does not include an iron-based catalyst.

12. A furnace for cracking heated ammonia gas, the furnace comprising: The radiant section includes at least one fuel and oxidant gas inlet in fluid communication with at least one burner, an ammonia feed inlet, and a catalyst-containing reactor tube having an upstream end in fluid communication with the ammonia feed inlet and a downstream end in fluid communication with the outlet of cracked gas. Each tube includes an upstream layer of a first catalyst and a downstream layer of a second catalyst, wherein the first catalyst is a ruthenium-based catalyst and the second catalyst is a nickel-based catalyst. as well as The convection section is in fluid flow communication with the radiation section and includes the flue gas outlet; The first catalyst is more active for cracked ammonia than the second catalyst.

13. The furnace of claim 12, wherein each catalyst-containing reactor tube includes a third catalyst layer downstream of the second catalyst layer, the third catalyst being more active for cracked ammonia than the second catalyst.

14. The furnace according to claim 13, wherein the third catalyst has the same catalytically active metal as the first catalyst.

15. The furnace according to claim 13, wherein the third catalyst is a ruthenium-based catalyst.

16. The furnace according to claim 12, wherein the catalyst-containing reactor tube does not contain an iron-based catalyst.

17. An apparatus for cracking heated ammonia gas, said apparatus comprising: Liquid ammonia source; A pump connected in fluid flow to the liquid ammonia source is used to pump liquid ammonia; as well as According to claim 12, the furnace, wherein the ammonia feed inlet is in fluid communication with the pump. The device further includes: At least one heat exchanger is arranged upstream of the pump to preheat liquid ammonia. and At least one heat exchanger is arranged to evaporate the pumped liquid ammonia and heat the ammonia gas by heat exchange with flue gas and / or cracked gas located between the ammonia feed inlet of the pump and the furnace.

18. The apparatus of claim 17, further comprising an adiabatic reaction unit for partially cracking heated ammonia under atmospheric pressure, the unit comprising an inlet for heated ammonia under atmospheric pressure in fluid communication with the pump, and at least one catalyst bed having an upstream end in fluid communication with the inlet and a downstream end in fluid communication with an outlet for the partially cracked ammonia. The ammonia feed inlet of the furnace is in fluid communication with the outlet fluid of the partially cracked ammonia gas from the adiabatic reaction unit.

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