Process and apparatus for decomposing ammonia

Abstract

The process for decomposing ammonia is improved by using heat generated in a compression unit used to compress PSA off-gas that is recycled to the PSA unit to preheat liquid ammonia prior to vaporization and decomposition. The heat is transferred using a heat transfer fluid, such as an aqueous solution containing about 50% to about 60% by weight of a glycol, e.g., ethylene glycol or propylene glycol.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Non-Provisional Application No. 17 / 990,837, filed November 21, 2022, which is incorporated herein by reference.

[0002] The present invention is in the field of ammonia decomposition to produce hydrogen, and specifically relates to a process and apparatus for the production of hydrogen gas from liquid ammonia. [Background technology]

[0003] The global interest in renewable energy, and the use of this renewable energy to generate "green" hydrogen, has fueled interest in converting "green" hydrogen to "green" ammonia, as ammonia is easier to transport over distances of hundreds or thousands of miles. In particular, while the transportation of liquid hydrogen is not currently commercially possible, the transportation of ammonia in the liquid state is currently practiced.

[0004] For use in commercial fuel cells, ammonia must be converted back to hydrogen according to the following reaction:

number

[0005] This is an endothermic process, i.e., one that requires heat, and therefore higher temperatures favor product production. The standard heat of reaction (per mole of ammonia) at 1 bar and 0°C is 45.47 kJ / mol. The endothermic nature of the process influences the need for a furnace.

[0006] This process is known as decomposition (or sometimes "dissociation") and is usually carried out over a catalyst. The gas produced (or "cracked gas") is a mixture of hydrogen (H) and nitrogen (N), but because the decomposition reaction is an equilibrium reaction, some residual ammonia is also present. The amount of ammonia in the cracked gas, commonly referred to as "ammonia slip," can be varied by changing the temperature and pressure at which the ammonia is decomposed, with higher temperatures and pressures favoring conversion, thereby reducing ammonia slip.

[0007] Currently, in most applications of crackers, the hydrogen and nitrogen mixture is utilized as is. However, because ammonia can be harmful to fuel cells, this stream can be used directly in fuel cells with suitable removal of the ammonia, such as by washing with water. However, when hydrogen is used in vehicle fueling, the nitrogen present presents a penalty to the process. Fuel to vehicle fueling systems is compressed to significant pressures of up to 900 bar. This means that nitrogen, which is merely a diluent in the process, is also compressed, requiring power, storage, increasing anode gas purge requirements, and reducing efficiency. Therefore, when hydrogen is used in vehicle fueling, it is beneficial to separate the hydrogen and nitrogen.

[0008] There are many examples of ammonia decomposition processes in the art (e.g., GB977830A, JP5330802A, CN111957270A, US2020 / 0398240A, and KR2022 / 0085469A).

[0009] Additionally, GB1142941A discloses a process for producing fuel gas interchangeable with city gas. Ammonia is decomposed to form a mixture of hydrogen and nitrogen, and the mixture is then concentrated by adding a gas with a higher heating value than the mixture, such as methane, propane, or butane, or a mixture thereof. Liquid ammonia is pumped as a cold liquid and vaporized by a closed hot water circuit. The ammonia vapor is superheated by heat exchange with flue gas from the furnace and then decomposed 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 to the catalyst-filled tubes of the furnace. The purified cracked gas is concentrated with propane and / or butane to produce a city gas product.

[0010] GB1142941A discloses that the decomposition of ammonia in a direct-fired tubular furnace in the presence of a suitable catalyst is preferred. However, this reference also discloses that other decomposition processes can be used instead. In this context, GB1142941 mentions that the process involves heating ammonia to a suitable temperature, then passing the ammonia through an unheated bed of ammonia decomposition catalyst to decompose a portion of the ammonia into hydrogen and nitrogen, and cooling the gas. Unconverted ammonia can be recovered as described above. Alternatively, the gas mixture can be reheated and passed through a second bed of catalyst to further reduce the ammonia content, and this can be repeated as many times as desired.

[0011] GB 1353751A discloses a process in which ammonia is decomposed in two stages in a heated reactor tube at pressures ranging from 20 atm to 300 atm. In the first stage, gas at a temperature ranging from 450 to 800°C is passed through a layer of nickel-, iron-, or cobalt-containing catalyst prepared by co-precipitation with aluminum oxide and magnesium oxide or magnesium-aluminum spinel supports. The first-stage catalyst can alternatively consist of either an iron-impregnated ceramic material or iron promoted with potassium oxide impregnated on a preformed support made of magnesium oxide and aluminum oxide. After the first stage, the gas then passes through a layer of doubly or triply promoted iron catalyst at a temperature ranging from 450 to 600°C, forming the second stage.

[0012] GB 1353751A specifically mentions the difficulty of nitriding the metal of the reactor tube at high temperatures and ammonia concentrations. Metal nitriding is a recognized problem in ammonia cracking units, which can lead to brittle fracture of the reactor vessel or tubes. The amount of nitriding depends on the partial pressures of ammonia, nitrogen, and hydrogen, as well as the temperature of the system. Nitridation is most difficult at higher temperatures and higher partial pressures of ammonia and nitrogen, with ammonia being a greater risk at typical ammonia decomposition temperatures (nitriding by nitrogen is unlikely until temperatures become excessively high for ammonia decomposition, e.g., above 1000°C). The exact extent to which the materials of construction form nitrides is currently unknown. However, there is data indicating that below 650°C, the degree of nitriding should be acceptable, depending on the materials of construction selected. GB 1353751A does not illustrate processes in which the reactor tube is heated to temperatures above 600°C, likely because the authors did not actually address the nitriding problem at higher temperatures.

[0013] WO 2022 / 189560A discloses an ammonia decomposition process involving a combustion reactor with tubes filled with an iron catalyst. Liquid ammonia is removed from storage, pumped, preheated, and evaporated to form ammonia gas, which is heated by heat exchange with the decomposed gas. The heated ammonia gas is further heated by heat exchange with flue gas in the convection section of the combustion reactor and then fed to two adiabatic reactors in series (with interstage heating for the flue gas) where it is partially decomposed. The partially decomposed gas is then heated by heat exchange with flue gas in the convection section before being fed to the catalyst-filled tubes of the combustion reactor to decompose the remaining ammonia.

[0014] US11287089A discloses a hydrogen fueling system in which ammonia is decomposed on-site into hydrogen and nitrogen in an ammonia cracker operating at a pressure in the range of 5 bar to 40 bar, and the hydrogen is compressed to a pressure of at least 30 MPa (300 bar) and stored ready for distribution to vehicles. When dispensed, the compressed gas from storage is cooled to a temperature in the range of -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 cooling system. If the ammonia feed is liquid, at least a portion of the duty required to vaporize the liquid ammonia is provided by the heat exchange fluid. Typically, vaporization of the liquid ammonia is performed at standard or subatmospheric pressure. US11287089A illustrates a system that produces 7.5 tons of hydrogen gas per day.

[0015] However, in general, there remains a need for improved processes for the production of hydrogen from ammonia, particularly processes that are more efficient in terms of energy consumption and / or have higher levels of hydrogen recovery and / or reduce or eliminate the need to burn fossil fuels. Summary of the Invention

[0016] According to a first aspect of the present invention, there is provided a process for decomposing ammonia, the process comprising: warming a heat transfer fluid to produce a warmed heat transfer fluid; pumping liquid ammonia to produce pumped liquid ammonia; preheating the pumped liquid ammonia to produce preheated liquid ammonia; vaporizing the preheated liquid ammonia to produce ammonia gas; heating the ammonia gas to produce heated ammonia gas; combusting a fuel with an oxidant gas in a furnace to heat catalyst-containing reactor tubes and produce flue gas; The present invention provides a process for producing ammonia gas, or a partially decomposed ammonia gas derived therefrom, by feeding the heated ammonia gas, or a partially decomposed ammonia gas, into a catalyst-containing reactor tube to decompose the ammonia and produce a decomposed gas containing hydrogen gas, nitrogen gas, and residual ammonia gas; cooling the decomposed gas to produce a cooled decomposed gas; and recovering hydrogen gas from the cooled decomposed gas in a pressure swing adsorption (PSA) unit to produce a PSA off-gas containing hydrogen gas as a product, as well as nitrogen gas, residual hydrogen gas, and residual ammonia gas. A portion of the PSA off-gas is compressed in a compression unit to produce a compressed PSA off-gas that is recycled to the PSA unit, and at least a portion of the duty required to warm the heat transfer fluid is provided by cooling the compression unit using a heat transfer fluid. In addition, a portion of the duty required to preheat the pumped liquid ammonia is provided by heat exchange with the warmed heat transfer fluid.

[0017] The inventors have determined that using heat from a PSA off-gas compression unit to provide a portion of the duty required to preheat the liquid ammonia allows for savings in cooling water and achieves a lower temperature at which the compressed PSA off-gas allows for power savings. The lower temperature may mean that water and ammonia condense from the compressed gas which can be used in the SCR.

[0018] However, the inventors recognize that if the heat exchange fluid is cooled to such a low level, there may not be enough heat exchange fluid flow to provide the required intercooling. Therefore, the optimal solution may actually be to operate at a higher temperature. Keeping the temperature above the point where water and ammonia condense in the off-gas compressor uses more power than a potential lower temperature cycle, but avoids the complications of dealing with two-phase flow.

[0019] The operating temperature of the heat exchange fluid is consistent throughout a variety of operating scenarios, including normal design and start-up nitrogen recirculation modes, minimizing control requirements.

[0020] The inventors also recognize that typically cooling duties (e.g., compressor cooling and cracked gas cooling) must be balanced with heating (e.g., ammonia heating). Thus, heat from an external source, e.g., an electric heater, will typically be required to maintain heating of the heat transfer fluid when the PSA off-gas compressor is not operating, and in some embodiments, extra cooling of the recirculated glycol may be required when the PSA off-gas compressor is operating, which may be provided by, for example, cooling water or an air cooler.

[0021] According to a second aspect of the present invention, there is provided an apparatus for decomposing ammonia, the apparatus comprising: a source of liquid ammonia; a pump having an inlet in fluid flow communication with the source of liquid ammonia and an outlet for pumped liquid ammonia; a furnace having at least one inlet for fuel and oxidant gas in fluid flow communication with at least one burner, an ammonia feed inlet in fluid flow communication with the pump, and a catalyst-filled 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 for cracked gas; a furnace having a radiant section and a convection section in fluid flow communication with the radiant section and having an outlet for flue gas; a PSA unit having an inlet in fluid flow communication with the outlet for cracked gas in the radiant section of the furnace, an outlet for hydrogen gas, and an outlet for PSA off-gas; and a compression unit having an inlet in fluid flow communication with an outlet for PSA off-gas in the PSA unit, and an outlet for compressed PSA off-gas in fluid flow communication with an inlet of the PSA unit. The apparatus further comprises at least one cooler located within the compression unit and arranged to cool the compression unit by heat exchange against a heat transfer fluid; a preheating unit located downstream of the pump and arranged to preheat the pumped liquid ammonia by heat exchange against a heat transfer fluid; and at least one heat exchanger located upstream of the ammonia feed inlet of the furnace for vaporizing the preheated liquid ammonia by heat exchange with the cracked gas to heat the ammonia gas.

[0022] The apparatus of the second aspect of the invention is particularly suitable for carrying out the process of the first aspect of the invention. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a simplified flowsheet of an ammonia decomposition process incorporating the present invention. [Figure 2] 1 is a simplified flow sheet of one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] Unless otherwise specified, amounts of ingredients given in parts per million (or ppm) are calculated by weight. Additionally, unless otherwise specified, all percentages are calculated in moles. Furthermore, unless otherwise specified, any reference to pressure is a reference to absolute pressure.

[0025] In the context of the present invention, the activity of a catalyst will be understood to refer to the rate of conversion of ammonia at a given partial pressure for a given amount of catalyst over a given period of time at a particular temperature and total pressure. The units used to define the activity of a heterogeneous catalyst are moles of ammonia converted per gram of catalyst (including substrate, if present) per second (or moles g -1 s - 1).

[0026] The phrase "in fluid flow communication" will be understood to mean that piping or other suitable conduits are used to transport a fluid from one designated location to another. During its journey between two locations, the fluid may flow through one or more other units that may be designed and / or arranged to change the physical conditions of the fluid, such as its temperature (e.g., heat exchangers) and / or pressure (e.g., compressors or pumps), or its composition, through reaction of components (e.g., catalytic reactants) within the fluid. The phrase "in direct fluid flow communication" will be understood to mean that the fluid flows directly from one location to the other, i.e., does not flow through another such unit during its journey, and therefore there is essentially no change in the composition or physical condition of the fluid.

[0027] The term "superatmospheric pressure" will be understood to mean a pressure significantly higher than atmospheric pressure, such as a pressure of at least 5 bar, for example a pressure of at least 10 bar, or a pressure of at least 20 bar or at least 30 bar. Typically, the pressure is not more than 60 bar.

[0028] The term "upstream" will be understood to mean in the opposite direction to fluid flow during normal operation, and the term "downstream" will be interpreted accordingly.

[0029] Liquid ammonia can be supplied at ambient pressure from either a pipeline or, more typically, from a refrigerated storage tank. Water is often added to the ammonia to prevent stress corrosion cracking in the storage tanks, trucks, and ships used to transport the ammonia. The presence of water in the ammonia feed transforms the feed into a multi-component stream, and evaporation of the feed stream would then require higher temperatures to achieve complete evaporation.

[0030] A typical composition of the ammonia feed is shown in Table 1. [Table 1]

[0031] Oil may be present in the ammonia due to the boil-off gas compressor used to store ammonia, either in local storage tanks, at the production site, or any other intermediate storage tanks. The presence of oil is problematic because it presents a risk of blockage and / or contamination. This can lead to poor performance of heat exchangers or reduced catalytic 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 the 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 short-chain hydrocarbons that can react with any water present to form carbon monoxide, hydrogen, and methane.

[0032] Inert gases appear to be harmless except that they may become product hydrogen. In this regard, helium may be present in ammonia derived from natural gas, but ammonia from renewable hydrogen does not contain helium.

[0033] Liquid ammonia is typically removed from storage and pumped from the storage pressure (e.g., about 1 bar) to a pressure in the range of about 5 bar to about 60 bar, for example, about 10 bar to about 50 bar, such as about 10 bar to about 30 bar or about 40 bar to about 50 bar. The temperature of the liquid ammonia is increased slightly from the storage temperature (e.g., about −34° C.) to about −32° C. When the liquid ammonia is removed from a pipeline, the temperature of the liquid ammonia is usually higher, for example, about +10° C.

[0034] The pumped (superatmospheric pressure) liquid ammonia is then ideally preheated to its boiling point by suitable heat integration within the process. Part of the preheating is achieved using a heat transfer fluid circuit in which heat from the PSA off-gas compressor, for example, heat from the intermediate and final cooling of the PSA off-gas compressor, is recovered using a heat transfer fluid such as an aqueous solution of glycol, for example, an aqueous solution containing about 50% to about 60% by weight of a glycol such as ethylene glycol or propylene glycol, optionally together with heat from the flue gas and / or cracked gas, and used to preheat the liquid ammonia. If such integration is not possible, such as when the compressor is not operating, heat from an external source such as an electric heater is typically used to preheat the ammonia.

[0035] The preheated liquid ammonia is then evaporated and the ammonia gas is further heated before being delivered to the catalyst-containing reactor tube or adiabatic reaction unit. In this regard, the ammonia gas is superheated, i.e., heated to a temperature above its boiling point, above 350°C, to ensure a useful reaction rate in the catalyst-containing reactor tube or adiabatic reaction unit.

[0036] The duty for vaporization and further heating of the preheated liquid ammonia can be provided by heat exchange with cracked gas, flue gas, or a combination of both cracked gas and flue gas. In a preferred embodiment, the cracked gas heats and vaporizes the preheated liquid ammonia by heat exchange, and the ammonia gas is then further heated by heat exchange with the flue gas.

[0037] The heated ammonia gas can be delivered directly to the catalyst-containing reactor tube, i.e., without first partially decomposing a portion of the ammonia.

[0038] The reactor tube of the furnace is filled with at least one ammonia decomposition catalyst. Many metals that catalyze the decomposition of ammonia are known in the art. These metals include transition metals from 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) can also be used.

[0039] The activity of some of these metals as catalysts for ammonia decomposition has been reported by Masel et al. (Catalyst Letters, vol. 96, Nos. 3-4, July 2004) to vary in the following order: Ru>Ni>Rh>Co>Ir>Fe>>Pt>Cr>Pd>Cu >>Te

[0040] The metals can be unsupported but are usually supported on a suitable support, typically a metal oxide support such as silica (SiO), alumina (AlO), zirconia (ZrO), or a mixed metal oxide support such as spinel (MgAlO) or perovskite (CaTiO). Alternatively, the metals can be supported on a zeolite.

[0041] As will be appreciated by those skilled in the art, the activity of a supported metal catalyst will typically depend in part on the loading of catalytically active metal on the support. In this regard, metal loading will vary according to specific requirements, but is typically within the range of about 0.1 wt. % to about 70 wt. %. For more active metals, such as ruthenium, the loading may be toward the lower end of the range, e.g., about 0.1 wt. % to about 10 wt. %, or about 0.2 wt. % to about 5 wt. %. For less active metals, such as nickel, the loading may be toward the upper end of the range, e.g., about 20 wt. % to about 65 wt. %.

[0042] Supported metal catalysts can be unpromoted, as is well known in the art, or can be promoted with at least one other metal, such as one or more of Group 1 metals, e.g., lithium (Li), sodium (Na) and potassium (K), Group 2 metals, e.g., magnesium (Mg) and calcium (Ca), or Group 13 metals, e.g., aluminum (Al), to improve activity.

[0043] Any known conventional catalyst for ammonia decomposition can be used in the present invention. Suitable catalysts are disclosed in US2015 / 0217278A, Masel et al. (supra), Lamb et al. (Int. J. Hydrogen Energy, 44 (2019) pp3726-3736), and Boisen et al. (J. Catalysis 230 (2005) pp309-312).

[0044] Bimetallic catalysts, or catalysts containing two catalytically active metals, are also suitable for use in the present invention. Examples include complex metals or metal alloys or metal nanoclusters supported on perovskites, complex oxides or nitrides, or mixed oxides or mixed nitrides disclosed in US 2021 / 0001311 A, such as CoNi-MgSrCeO4 and 1 wt. % K-CoNi-MgSrCeO4.

[0045] The catalyst of the present invention typically comprises, for example, contains, or consists of at least one metal-based catalyst. The catalytically active metal is typically selected from the transition metals of the periodic table. Suitable transition metal-based catalysts have an activity at a temperature in the range of 475°C to 600°C suitable for achieving a reaction rate at least 0.2 times, or at least 0.5 times, or at least equal to, or at least 2 times, or at least 5 times the rate calculated according to the following equation 9 proposed by Lamb et al.: r=8.73 exp[-76710 / RT].(P NH3 ) 0.28 .(P H2 ) -0.42 .(1-β 2 ) During the ceremony, "r" is the reaction rate (or "activity") of the catalyst; "RT" is the ideal gas constant "R" (8.314 J mol -1 K -1 ) multiplied by the temperature "T" in Kelvin scale, P NH3 is the partial pressure of ammonia, P H2 is the partial pressure of hydrogen, β is defined in the paper as (see Equation 5 proposed by Lamb et al.)

number

[0046] The inventors recognize that Equation 9 of Lamb et al. can be extrapolated to temperatures outside of 475°C to 600°C, for example, within the range of 450°C to 700°C.

[0047] Transition metals that may be particularly suitable for use as the key catalytically active metal of the catalyst in the catalyst bed of the reactor tube are selected from chromium, manganese, iron, cobalt, nickel, ruthenium, and copper, e.g., iron, cobalt, nickel, and ruthenium, with the inventors realizing that nickel and ruthenium are typically the most suitable metals for use in this regard.

[0048] In some embodiments, the reactor tubes of the furnace are filled with only a single ammonia decomposition catalyst, for example, a nickel-based catalyst.

[0049] In other embodiments, the reactor tube is packed with at least two different ammonia decomposition catalysts having different activities. In these embodiments, a more active catalyst, e.g., a ruthenium-based catalyst, can be located downstream of a less active catalyst, e.g., a nickel-based catalyst, to ensure that the decomposition reaction approaches equilibrium. Alternatively, the more active catalyst can be located upstream of the less active catalyst to help control the reactor tube wall temperature in the region where the ammonia temperature and partial pressure are highest, thereby controlling tube nitridation. In these alternative embodiments, a second more active catalyst, e.g., another ruthenium-based catalyst, can be located downstream of the less active catalyst to ensure that the decomposition reaction approaches equilibrium.

[0050] The activity of a more active catalyst is typically at least 50% greater than the activity of a less active catalyst. However, the difference in relative activity is usually substantially greater than 50%. In this regard, the activity of a more active catalyst is usually at least 2-fold (i.e., 50 times), or at least 3-fold, or at least 4-fold, or at least 5-fold greater than the activity of a less active catalyst. In some embodiments, the activity of a more active catalyst is at least 10-fold (i.e., an order of magnitude), or at least 15-fold greater than the activity of a less active catalyst.

[0051] In some preferred embodiments, the catalyst in the upstream layer is a ruthenium-based catalyst and the catalyst in the downstream layer is a nickel-based catalyst. If a third catalyst is present downstream of the nickel-based catalyst in these embodiments, that catalyst is preferably a ruthenium-based catalyst, but it can be a different ruthenium-based catalyst than the first catalyst, e.g., it has a different support and / or catalyst loading and / or is promoted with a different metal, if both are promoted.

[0052] Thus, the term "ruthenium-based catalyst" refers to a catalyst containing ruthenium as the only (or at least significant) catalytically active metal, i.e., the metal responsible for catalyzing the decomposition reaction. Ruthenium may be the only metal in the catalyst, or alternatively, one or more other metals may be present, for example, in a material supporting the ruthenium. The terms "nickel-based catalyst" and "iron-based catalyst" are intended to be interpreted accordingly.

[0053] Suitable ruthenium- and nickel-based catalysts may be supported, for example, on alumina (as disclosed in Lamb et al. or Masel et al.) or spinel (as disclosed in Boisen et al.), and may optionally be promoted with a Group 1 or Group 2 metal.

[0054] As mentioned above, water is typically present in ammonia as a contaminant. Water can be removed from ammonia if a catalyst that cannot tolerate water, such as an iron-based catalyst, is used in the reactor tube. However, in preferred embodiments, water is not removed to save capital and operating costs and reduce energy consumption. In these embodiments, catalysts that cannot tolerate water, such as iron-based catalysts, are typically not used. Instead, the catalyst in the reactor tube can tolerate up to 1 mol% water in the ammonia feed. Such catalysts include nickel-based and ruthenium-based catalysts.

[0055] In some preferred embodiments, a ruthenium-based catalyst is used in the first layer within each tube, where faster reaction rates allow metal temperatures to be kept within the 660°C design limit. In these embodiments, the second layer of the tube contains a lower cost, but less active, nickel-based catalyst. In this way, the catalyst is layered to use the endotherm of the reaction within the tube to keep the tube metal cool in areas where combustion is most intense on the outside of the tube. Because ruthenium is more catalytically active, it generates a stronger endotherm to cool the inner tube wall in areas of high ammonia concentration on the process side, protecting the tube from excessive nitriding caused by high ammonia concentration and high temperature.

[0056] The combustion process in the furnace is preferably at least partially internally fueled, i.e., at least a portion of the fuel is either ammonia or off-gas generated during the recovery of hydrogen from cracked gases or a mixture of the two. However, the use of hydrocarbon trim fuels increases the carbon intensity of the process, but trim fuels such as C1-C3 hydrocarbons or natural gas can be used when required. However, it is generally desirable to minimize or even eliminate the use of such trim fuels to reduce the carbon intensity of the process.

[0057] The oxidant gas is typically air, but may be oxygen-enriched gas or pure oxygen, where appropriate.

[0058] The feed to the catalyst-filled reactor tube of the furnace can be at a temperature of up to about 800°C, provided that the reactor wall material can withstand the higher temperatures. For lower temperature cycles, the feed is typically at a temperature in the range of about 400°C to about 600°C, or about 450°C to about 550°C, e.g., about 500°C. For higher temperature cycles, the feed can be at a temperature in the range of about 500°C to about 800°C, or about 600°C to about 700°C, e.g., about 650°C.

[0059] The decomposition temperature and pressure typically require that the ammonia slip in the reactor tube is 3 mol % or less, for example, from about 0.5 mol % to about 1.5 mol %.

[0060] In some embodiments, the heated ammonia gas is partially decomposed in an adiabatic reaction unit comprising at least one catalyst bed to produce partially decomposed ammonia gas for delivery to a catalyst-filled reactor tube.

[0061] The mole fraction of ammonia in the gas passing through the adiabatic reaction unit is typically reduced by at least 20%, e.g., at least 25%, or at least 30%, or even at least 35%, e.g., about 40%, and potentially up to about 50%. Stated another way, the mole fraction of ammonia can be reduced from 1 (or nearly 1) in the heated ammonia gas to an amount in the range of about 0.5 to about 0.8, 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, e.g., about 0.6, in the partially decomposed ammonia gas.

[0062] An adiabatic reaction unit may be incorporated into the process design to improve overall efficiency, specifically by using heat available in the flue gas to provide heat for the adiabatic decomposition 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, often avoiding temperatures above about 660°C due to material concerns.

[0063] The primary design parameter for an adiabatic reactor unit is the inlet temperature. Because the ammonia decomposition reaction is endothermic, a higher inlet temperature allows for greater conversion in the unit. However, higher temperatures place greater strain on the materials of construction and catalyst. The inlet temperature is typically in the range of about 350°C to about 800°C, and for lower temperature cycles, it can be in the range of about 400°C to about 600°C, or about 400°C to about 450°C. For higher temperature cycles, the inlet temperature can be in the range of about 500°C to about 700°C, or about 550°C to about 650°C.

[0064] Due to the high temperatures and ammonia concentrations, the decomposition reactor vessel, e.g., the adiabatic reactor and reactor tubes in a furnace, must typically be constructed from materials that are resistant to ammonia and / or nitriding. Suitable materials include nickel-based alloys containing at least 40% or at least 50% nickel by weight. Such alloys typically have up to 90% or up to 80% nickel by weight. The alloys typically contain one or more other metals selected from chromium, cobalt, molybdenum, and iron.

[0065] 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.

[0066] The Unified Numbering System (UNS) is an alloy designation system widely accepted in North America. Each UNS number is associated with a specific metal or alloy and defines its specific chemical composition, or in some cases, specific mechanical or physical properties.

[0067] Other suitable materials include cobalt-based alloys such as UNS R30188. In addition, high-temperature alloys with low resistance to ammonia nitriding, such as UNS N08811, or cast alloys such as HPNb, HP Micro-Alloyed, MA-1 (MetalTek International, USA), may be suitable, especially when surface modified or coated with a corrosion-resistant layer such as aluminiding, aluminiding followed by pre-oxidation, or a ceramic coating. Nitridation-resistant alloys can also be used with surface modifications or coatings for improved performance.

[0068] An adiabatic reaction unit typically comprises one or more adiabatic reactors, each comprising a catalyst bed, and the adiabatic reactor or reactors may be made from one or more of the ammonia-resistant and / or ammonia nitridation-resistant materials described above.

[0069] In a preferred embodiment, the adiabatic reaction unit comprises one or more adiabatic reactors, for example, two, three, four, five, or six reactors, with interstage heating where appropriate. The reactors may be arranged in series or parallel, or a combination of series and parallel, depending on the process requirements. However, in a preferred embodiment, the adiabatic reaction unit has two such reactors arranged in series, with interstage heating of the intermediate partially decomposed ammonia gas by heat exchange against the cracked gas and / or flue gas.

[0070] Each adiabatic reactor has a bed containing at least one catalyst suitable for decomposing ammonia. Any conventional ammonia decomposition catalyst can be used in the adiabatic reactor or in the bed of each adiabatic reactor. Suitable catalysts are discussed above in the context of the reactor tube catalysts.

[0071] In embodiments in which water is removed from ammonia prior to decomposition, a water-intolerant catalyst, such as an iron-based catalyst, may be used in the adiabatic reactor. However, in preferred embodiments, water is not removed to save capital and operating costs and reduce energy consumption. In these embodiments, the catalyst bed of the adiabatic reaction unit does not contain an iron-based catalyst; instead, a water-tolerant catalyst, i.e., a metal-based catalyst that can tolerate the presence of up to 1 mol% water, is preferred. In this regard, either a nickel-based catalyst or a ruthenium-based catalyst, or a combination of a nickel-based catalyst and a ruthenium-based catalyst, may be used in the bed of the adiabatic reactor unit.

[0072] As noted above, ruthenium-based catalysts tend to be more active than nickel-based catalysts, but are more expensive. Therefore, further optimization is possible through catalyst selection and, if more than one type of catalyst is used, through the ordering of the catalyst layers within the bed of the adiabatic reaction unit.

[0073] In some embodiments having two adiabatic reactors in series, the catalyst bed of the first reactor comprises (e.g., contains or consists of) a single layer of a first catalyst, e.g., a ruthenium-based catalyst, and the catalyst bed of the second reactor comprises (e.g., contains or consists of) an upstream layer of a second catalyst, e.g., a nickel-based catalyst, that is typically less active than the first catalyst, and a downstream layer of a third catalyst, e.g., a ruthenium-based catalyst, that is typically more active than the second catalyst.

[0074] In these embodiments, the first and third catalysts can be the same. Alternatively, the first and third catalysts can be different, for example, containing different catalytically active metals, the same catalytically active metals but on different supports, or the same catalytically active metals on the same support but with different loadings of catalytically active metals.

[0075] In other embodiments having two adiabatic reactors in series, the catalyst bed of the first reactor comprises (e.g., contains or consists of) a single layer of a first catalyst, e.g., a nickel-based catalyst, and the catalyst bed of the second reactor comprises (e.g., contains or consists of) an upstream layer of a second catalyst, e.g., a nickel-based catalyst, typically having similar activity as the first catalyst, and a downstream layer of a third catalyst, e.g., a ruthenium-based catalyst, typically having higher activity than the first and second catalysts.

[0076] In these embodiments, the first and second catalysts can be the same. Alternatively, the first and second catalysts can be different, for example, containing different catalytically active metals, the same catalytically active metals but on different supports, or the same catalytically active metals on the same support but with different loadings of catalytically active metals.

[0077] In both sets of 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%, e.g., about 50% to about 70%, or about 60%, of the total volume of the bed. In the absence of another layer of catalyst, the volume of the downstream layer of the third catalyst in the bed of the second reactor can be about 10% to about 60%, e.g., about 30% to about 50%, or about 40%, of the total volume of the bed.

[0078] However, in other embodiments, the bed of the or each adiabatic reactor contains a nickel-based catalyst which may or may not be the same as the catalyst used in the reactor tubes.

[0079] The inventors have recognized that ruthenium-based catalysts are not only sufficiently tolerant of water, but also capable of cracking hydrocarbon oils into shorter hydrocarbons, such as methane, along with carbon monoxide and hydrogen. Thus, the use of these catalysts in an adiabatic reaction unit can eliminate the need for an upstream dedicated unit for removing oil from liquid ammonia.

[0080] It is also known that at high temperatures, sintering of the catalyst reduces its activity and life. In this regard, those skilled in the art will recognize the need to balance improved conversion against higher vessel costs and shorter catalyst life.

[0081] Heat from the cracked gas and flue gas is typically used to heat the feed streams to the adiabatic reaction unit and furnace, thereby reducing the overall energy consumed by the process. In this regard, the temperature of the cracked gas will depend on the cycle being operated.

[0082] At lower temperature cycles, the temperature of the cracked gas may be up to about 700°C, e.g., typically about 550°C to about 700°C, or about 600°C to about 650°C. The temperature of the flue gas may be up to about 750°C at its highest point. However, the temperature drops due to heat leaks, and is typically about 600°C to about 700°C, at which point the heat can be efficiently utilized.

[0083] At higher temperature cycles, the temperature of the cracked gas can be up to about 750°C, e.g., typically about 650°C to about 750°C, or about 675°C to about 725°C. The temperature of the flue gas can be up to about 840°C at its highest point. However, the temperature drops due to heat leaks, and is typically about 700°C to about 800°C, at which point the heat can be efficiently utilized.

[0084] When an adiabatic reaction unit is used to partially decompose heated ammonia gas and the partially decomposed gas is heated to the feed temperature of the catalyst-filled reactor tube of the furnace, at least a portion of the duty required to heat the partially decomposed gas is provided by heat exchange with the decomposed gas. In preferred embodiments, the decomposed gas is not used to heat another process fluid prior to heating the partially decomposed gas. Some of this heating duty may be provided in another way, for example, by heat exchange with flue gas. However, all of this heating duty is preferably provided by the decomposed gas.

[0085] In a preferred embodiment, the composition of the ammonia typically remains at least substantially unchanged from the liquid ammonia in storage to the heated ammonia gas being delivered to the adiabatic reaction unit. Oil present in the liquid ammonia may be removed at some point before the partial decomposition of the ammonia, although if a ruthenium-based catalyst is used, 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.

[0086] When the partially cracked gas is heated by heat exchange with the cracked gas, the temperature of the cracked gas is reduced. The cracked gas is then further cooled, typically by providing at least a portion of the heating duty required to produce heated ammonia gas from liquid ammonia. After cooling, hydrogen can be recovered from the cracked gas as a product. Recovery is achieved in a pressure swing adsorption (PSA) process, optionally with one or more permselective membranes.

[0087] PSA off-gas, which includes nitrogen gas, residual ammonia, and residual hydrogen, is generated in the PSA process. The PSA off-gas is separated into two portions. The first portion of the PSA off-gas is compressed in a compression unit and recycled to the PSA process to improve hydrogen recovery. The second portion is typically preheated and then sent to a furnace as fuel.

[0088] However, during certain periods, such as when the off-gas compressor is offline for repair or service, or when it is determined that the plant should operate at a lower carbon intensity and therefore requires lower PSA recovery to increase the hydrogen in the PSA off-gas going to the burner to allow for a reduction in the natural gas used by the plant, the compressor may not be operational or may only operate at partial load. During these periods, the "missing" portion of the duty required to warm the heat transfer fluid is typically provided from an external source, such as an electric heater.

[0089] The compression unit is typically a multi-stage compression unit with intercooling and aftercooling using a heat transfer fluid such that the heat transfer fluid flows through the or each intercooler, which removes the heat of compression from the PSA off-gas being compressed, and through an aftercooler, which removes the heat of compression from the compressed PSA off-gas.

[0090] The compression unit also typically includes one or more sub-coolers, including a lubricant oil cooler and, in the case of a positive displacement compressor, a cooler for cylinder cooling. In such embodiments, the heat transfer fluid may also flow through the sub-cooler to provide lubricant oil cooling duty and / or cylinder cooling duty. The sub-cooling duty may be 10-30%, e.g., 20-25%, of the total cooling duty required by the compression unit.

[0091] The heat transfer fluid may also be used to help cool the cracked gas prior to hydrogen recovery. In this regard, after the cracked gas has provided its share of duty for heating and vaporizing the ammonia feed, the cracked gas is further cooled by heat exchange with a heat exchange fluid in a cracked gas heat exchanger to produce cooled cracked gas ready for hydrogen recovery in a PSA system.

[0092] The heat exchange fluid used to cool the compression unit and cracked gas is then typically cooled before being returned to the compression unit and cracked gas heat exchanger. Cooling is achieved in part by heat exchange with liquid ammonia. The remainder of the cooling duty is provided by an air cooler or by heat exchange with a cooling liquid such as water. The cooled heat exchange fluid is then typically returned to the compression unit and cracked gas heat exchanger.

[0093] Thus, the heat exchange fluid is typically circulated around a circuit, or "closed loop," which usually includes a pump to move the fluid around the circuit and may include a storage tank for the heat exchange fluid. Excess heat may be removed from the heat transfer fluid using at least one water and / or air cooler as part of the circuit.

[0094] An aspect of the present invention is #1. A process for decomposing ammonia, warming the heat transfer fluid to produce a warmed heat transfer fluid; pumping the liquid ammonia to produce pumped liquid ammonia; preheating the pumped liquid ammonia to produce preheated liquid ammonia; vaporizing the preheated liquid ammonia to produce ammonia gas; heating the ammonia gas to produce heated ammonia gas; combusting a fuel with an oxidant gas in a furnace to heat catalyst-containing reactor tubes and produce flue gas; delivering the heated ammonia gas, or partially decomposed ammonia gas derived therefrom, to a catalyst-containing reactor tube to cause decomposition of the ammonia and produce a decomposed gas comprising hydrogen gas, nitrogen gas, and residual ammonia gas; cooling the cracked gas to produce a cooled cracked gas; recovering hydrogen gas from the cooled cracked gas in a pressure swing adsorption (PSA) unit to produce a PSA off-gas comprising hydrogen gas as a product, as well as nitrogen gas, residual hydrogen gas, and residual ammonia gas; a portion of the PSA off-gas, e.g., up to about 55%, is compressed in a compression unit to produce compressed PSA off-gas that is recycled to the PSA unit; and a heat transfer fluid is used to cool the compression unit, thereby providing at least a portion of the duty required to warm the heat transfer fluid, e.g., up to about 95%; This includes processes in which a portion of the duty required to preheat the pumped liquid ammonia, for example up to about 80%, is provided by heat exchange with a warmed heat transfer fluid.

[0095] In embodiments with about 95% hydrogen recovery, the portion of the PSA off-gas compressed in the compression unit may be in the range of about 40% to about 50%, e.g., about 46%, and the portion of the duty required to warm the heat transfer fluid provided by using the heat transfer fluid to cool the compression unit may be in the range of about 80% to about 90%, e.g., about 87%. These figures are lower for embodiments where the PSA off-gas is bypassed from the compression unit and directed to the furnace burners, resulting in lower hydrogen recovery.

[0096] In embodiments where the liquid ammonia is removed from storage, the portion of the duty required to preheat the pumped liquid ammonia, provided by heat exchange with the warmed heat transfer fluid, may be in the range of about 70% to about 80%, e.g., about 75%. However, the liquid ammonia is typically stored at a temperature of −34° C. For embodiments where the liquid ammonia is removed from a pipeline, the number will be less because the temperature of the liquid ammonia in this case is generally higher, e.g., about +10° C., and therefore requires less preheating. #2. The process described in #1, where liquid ammonia is pumped from a storage vessel. #3: The process of #1 or #2, wherein a majority of the duty required to cool the compression unit, for example, up to about 85%, is the duty required to remove the heat of compression from the PSA off-gas. The remainder of the duty required to cool the compression unit is auxiliary cooling duty, for example, lubricant oil cooling duty, and, in the case of a positive displacement compressor, cylinder cooling duty. #4. A process according to any one of #1 to #3, wherein the compressed PSA off-gas is at or above its dew point temperature. #5. A process described in any of #1 to #4, wherein the duty required to warm the heat transfer fluid during periods when the compression unit is operating at partial load or not operating at all is replaced by an external source. #6. The process of #5, wherein the external source is an electric heater. #7. A process according to any of #1 to #6, wherein any remaining portion of the duty required to warm the heat transfer fluid is provided by heat exchange with cracked gas and / or flue gas. #8. A process according to any of #1-#7, wherein excess heat is removed from the heat transfer fluid using an air cooler and / or cooling water. #9. The process described in #1, where the heat transfer fluid is pumped around the circuit. #10. A process according to any one of #1 to #9, wherein the heat transfer fluid is an aqueous solution of glycol. #11. The process of #10, wherein the aqueous solution contains about 50% to about 60% by weight, for example, about 55% by weight, of glycol. #12. The process described in #10 or #11, wherein the glycol is selected from the group consisting of ethylene glycol and propylene glycol. #13. A process described in any of #1 to #12, wherein the heated heat transfer fluid is above 60°C, typically at a temperature of about 80°C or less, e.g., about 65°C. #14. A process described in any of #1 to #13, wherein a further portion of the PSA off-gas, for example the remaining portion, is preheated and fed to a furnace as fuel.

[0097] The portion of the furnace's total fuel requirement provided by the PSA off-gas varies depending on the degree to which hydrogen is recovered in the PSA unit. For example, if hydrogen recovery is about 95%, the PSA off-gas may account for only about 30% to about 40%, e.g., about 34%, of the total fuel requirement. As hydrogen recovery decreases, the PSA off-gas sent to the burners in the furnace increases in proportion as it contains more hydrogen. For example, if hydrogen recovery is about 85%, the PSA off-gas may account for about 60% to about 70%, e.g., about 66%, of the total fuel requirement. #15. The process of any of #1-#14, comprising partially decomposing heated ammonia gas in an adiabatic reaction unit comprising at least one catalyst bed to produce partially decomposed ammonia gas for delivery to a catalyst-filled reactor tube. #16. An apparatus for decomposing ammonia at superatmospheric pressure, a source of liquid ammonia; a pump having an inlet in fluid flow communication with a source of liquid ammonia and an outlet for pumped liquid ammonia; A furnace, a radiant section comprising at least one inlet for fuel and oxidant gas in fluid flow communication with the at least one burner, an ammonia feed inlet in fluid flow communication with the pump, and a catalyst filled 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 for cracked gases; a furnace comprising: a convection section in fluid flow communication with the radiant section, the convection section comprising an outlet for flue gas; a PSA unit including an inlet in fluid flow communication with an outlet for cracked gas in the radiant section of the furnace, an outlet for hydrogen gas, and an outlet for PSA off-gas; a compression unit having an inlet in fluid flow communication with an outlet for PSA off-gas in the PSA unit, and an outlet for compressed PSA off-gas in fluid flow communication with the inlet of the PSA unit; The device, one or more coolers located within the compression unit and arranged to cool the compression unit by heat exchange against a heat transfer fluid; a preheating unit located downstream of the pump and arranged to preheat the pumped liquid ammonia by heat exchange against a heat transfer fluid; and at least one heat exchanger located upstream of the ammonia feed inlet of the furnace for vaporizing preheated liquid ammonia by heat exchange with the cracked gas to heat the ammonia gas.

[0098] "Coolers" within a compression unit include intercoolers, aftercoolers, and coolers providing auxiliary cooling, such as lube oil cooling and / or cylinder cooling. #17 The apparatus of #16, wherein the source of liquid ammonia is a storage vessel. #18. An apparatus as described in #16 or #17, comprising an external heater for heating the heat transfer fluid. #19. The apparatus according to #18, wherein the external heater is an electric heater. #20. An apparatus described in any of #16 to #19, comprising at least one heat exchanger arranged to heat a heat transfer fluid by heat exchange against cracked gas and / or flue gas. #21. An apparatus described in any of #16 to #20, comprising at least one air cooler and / or at least one water cooler arranged to cool the heat transfer fluid. #22. An apparatus described in any of #16 to #21, comprising a circuit for a heat transfer fluid, the circuit comprising a pump for moving the heat transfer fluid around the circuit. #23. An apparatus described in any of #16 to #22, wherein an outlet for the PSA off-gas in the PSA unit is in fluid flow communication with at least one inlet for fuel and oxidant gas in the radiant section of the furnace, and the apparatus comprises at least one heat exchanger located upstream of the at least one inlet for the fuel and oxidant gas and arranged to preheat the PSA off-gas, for example, by heat exchange against cracked gas and / or flue gas. #24. An adiabatic reaction unit for partially decomposing heated ammonia gas, the unit comprising: an inlet for heated ammonia gas in fluid communication with a 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 partially decomposed ammonia gas; An apparatus as described in any of #16-#23, wherein the outlet for the partially decomposed ammonia gas is in fluid flow communication with the ammonia feed inlet of the furnace.

[0099] The invention will now be described, by way of example only, with reference to the following drawings:

[0100] In Figure 1, liquid ammonia stream 2 at about -32°C is taken from storage (not shown) and fed to pump P101 where it is pumped to produce pressurized liquid ammonia stream 4 at a pressure of about 46 bar and preheated in heat exchanger E271 by heat exchange with a heat transfer fluid, in this case typically a glycol solution of about 55% by weight ethylene glycol or propylene glycol in water, to produce preheated liquid ammonia stream 6 at about 55°C. An electric heater may be used to ensure that the temperature of the glycol solution being fed to heat exchanger E271 is sufficient to preheat the liquid ammonia to the required temperature.

[0101] The preheated liquid ammonia in stream 6 is further heated by heat exchange in heat exchanger E312 to produce further heated liquid ammonia stream 8. The further heated liquid ammonia in stream 8 is then vaporized by heat exchange in heat exchanger E311 to produce ammonia vapor stream 10. The ammonia vapor in stream 8 is then superheated by heat exchange in heat exchanger E310 to produce heated ammonia gas stream 12 at about 260°C.

[0102] The heated ammonia gas in stream 12 is further heated by heat exchange in heat exchanger E2102 to produce superheated ammonia gas stream 14 at about 420° C. Heat exchanger E2102 is shown as a single unit for convenience, however, in reality there may be two separate heat exchangers with a selective catalytic reactor (SCR) located in between.

[0103] Superheated ammonia gas in stream 14 is fed to first adiabatic reactor vessel C141 at about 420°C and about 43 bar and passed through a bed of ruthenium-based catalyst. A portion of the ammonia gas is decomposed over the catalyst to form intermediate gas stream 16, which contains some decomposed ammonia. The mole fraction of ammonia in the gas passing through first adiabatic reactor vessel C141 is reduced from approximately 1 to about 0.9.

[0104] The intermediate gas, at about 360°C, is then heated by heat exchange in heat exchanger E2103 to produce superheated intermediate gas stream 18, which is then fed to second adiabatic reactor vessel C142 at about 590°C and passed through beds containing an upstream layer of nickel-based catalyst and a downstream layer of ruthenium-based catalyst to produce partially decomposed ammonia gas stream 20. The mole fraction of ammonia in the gas passing through second adiabatic reactor vessel C142 decreases from about 0.9 to about 0.6.

[0105] The catalyst bed in the second adiabatic reactor vessel C142 has two layers, with a layer of nickel-based catalyst on top of a layer of ruthenium-based catalyst, to more efficiently use heat and therefore maximize ammonia conversion. Catalyst volume was also optimized, minimizing the volume of ruthenium-based catalyst by limiting the outlet temperature of the second adiabatic reactor vessel to about 390°C. The inventors found that reducing this temperature further increased the volume required for the ruthenium-based catalyst.

[0106] The ruthenium-based catalyst is the same in both the first and second adiabatic reactor vessels, however, different ruthenium-based catalysts may be used.

[0107] The partially cracked ammonia in stream 20 is heated by heat exchange in heat exchanger (or "economizer") E305 before being fed as stream 22 to catalyst-filled tubes in the radiant section F201 of the furnace (or reactor) at a pressure of about 38 bar. Heating the feed to the tubes increases the amount of cracking that can be done with heat from the burner by reducing the duty required to heat the partially cracked stream to reaction temperature. Utilizing the cracked stream from the tubes allows for efficient use of such high-temperature streams. The inlet temperature of the direct-fired tubular furnace is limited to about 500°C to limit the interior wall temperature of the cracker tubes.

[0108] Air stream 62 passes through forced draft fan K212 and is then preheated by heat exchange in heat exchanger E2141 to produce preheated air stream 64. Preheated air stream 64 is delivered to the burners (not shown) of furnace F201 in parallel with natural gas stream 70 as trim fuel. Preheating the air in this manner helps reduce fuel requirements.

[0109] The tubes in the furnace's radiant section, F201, are packed with two types of ammonia decomposition catalyst in two different layers. A ruthenium-based catalyst is used in the first layer in each tube, allowing for faster reaction rates to keep metal temperatures 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.

[0110] Cracked gas stream 24 exits the direct-fired tubular furnace radiant section F201 at approximately 640°C and is then fed to economizer E305 to provide the duty required to heat the partially cracked ammonia, thereby reducing the temperature of the cracked gas to approximately 530°C.

[0111] Economizer E305 is depicted as a shell-and-tube heat exchanger with partially cracked ammonia gas passing through the tubes and cracked gas passing through the shell side, however, this arrangement may be reversed or indeed a different style of heat exchanger may be used.

[0112] Cracked gas stream 26 is then delivered from economizer E305 to heat exchanger E310 to provide the duty required to superheat the ammonia gas, thereby further reducing the temperature of the cracked gas to approximately 389°C.

[0113] Cracked gas stream 28 is then fed from heat exchanger E310 to heat exchanger E311 to provide the duty required to vaporize additional heated liquid ammonia, thereby further reducing the temperature of the cracked gas to approximately 109°C.

[0114] Cracked gas stream 30 is then fed from heat exchanger E311 to heat exchanger E312 to provide the duty required to further heat the heated pressurized liquid ammonia, thereby further reducing the temperature of the cracked gas to about 70°C.

[0115] Each of heat exchangers E310, E311, and E312 is depicted as an individual 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 styles of heat exchangers can actually be used.

[0116] Cracked gas stream 32 from heat exchanger E312 is then further cooled by heat exchange with a heat transfer fluid in cooler E323 and then fed as stream 34 to PSA system U501 where it is separated into hydrogen gas stream 40, which is removed as product, and PSA off-gas stream 42, which contains nitrogen gas, residual hydrogen gas, and residual ammonia gas. The hydrogen gas in stream 40 may be fed to a hydrogen liquefaction unit (not shown) to produce liquid hydrogen.

[0117] All of the PSA off-gas in stream 42 can be sent directly as fuel (stream 60) for combustion in furnace F201. Alternatively, stream 42 can be split into two portions.

[0118] A first portion of the PSA off-gas in stream 44 is heated by heat exchange in heat exchanger E2112 to produce warmed PSA off-gas stream 60, which is then delivered to burners in furnace F201, along with air feed 64 and, optionally, natural gas feed 70, as needed. A minimum amount of natural gas is used as trim fuel to provide the balance of fuel required in the combustion section.

[0119] The second portion may be sent as stream 46 to a multi-stage compression unit K681 for compression. Compression unit K681 has five stages with an intercooler between each stage, with an aftercooler following the last stage. Heat is recovered from the compressed gas in the intercoolers and aftercoolers by heat exchange with a heat transfer fluid. Heat may also be recovered from the lubricating oil and, if a positive displacement compression unit is used, from the compression unit cylinder using a heat transfer fluid.

[0120] For convenience, the intercooler and aftercooler are shown as a single heat exchanger (labeled E6816A-E) that recovers heat from compressed PSA off-gas stream 48 by heat exchange with heat transfer fluid stream 52 to produce cooled compressed PSA off-gas stream 50 and warmed heat transfer fluid stream 54.

[0121] The heat transfer fluid warmed in the cooler E323 and the intercoolers and aftercoolers E6816A-E etc. is then used to provide the duty required to preheat the liquid ammonia by heat exchange in the heat exchanger E271.

[0122] The cooled, compressed PSA off-gas in stream 50 is fed to phase separator C6816, where any condensate is removed as stream 56. The compressed PSA off-gas is then recycled as stream 58 to PSA system U501 for further hydrogen recovery. In this manner, hydrogen recovery can be increased from 85% (without recycle) to 95% (with recycle).

[0123] As shown above, the process can be operated without compression unit K681, resulting in lower hydrogen recovery in PSA unit 501. Reducing the hydrogen recovery rate obviously results in less hydrogen gas product. However, reducing the hydrogen recovery rate may still be desirable because more hydrogen is in the off-gas, lowering the carbon intensity (CI) of the process, thereby reducing the need for natural gas as a trim fuel and reducing carbon dioxide emissions.

[0124] Flue gas stream 72, at about 686°C, passes from radiant section F201 through convection section 90 of furnace F201, where it first provides the duty required to heat intermediate gas from stream 16 in heat exchanger E2103, thereby reducing the temperature of the flue gas, and then uses it (as stream 74) to provide the duty required to further heat heated ammonia gas from stream 12 in heat exchanger E2102, thereby further reducing the temperature of the flue gas. Thus, the flue gas provides heating duty in a direction opposite the flow of feed gas to radiant section F201 of the direct-fired tubular furnace.

[0125] The cooled flue gas (as stream 76) is then used to provide the duty required to heat 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 duty required to heat PSA off-gas from stream 44 in heat exchanger E2112, thereby further cooling the flue gas.

[0126] The cooled flue gas exits the convection section 90 of the direct-fired tubular furnace F201 as stream 80 at approximately 121°C, i.e., above the dew point of water, passes through suction fan K211, and then exits the process as stream 82. All of the useful energy has been extracted from the flue gas at this point, which can optionally be immediately released to the atmosphere, after further processing if required, depending on its composition.

[0127] Oil can be present in liquid ammonia, either where ammonia is generated, in amounts up to about 5 ppm from a boil-off gas compressor (not shown) used in conjunction with an ammonia storage tank (not shown), or at the site where the ammonia is decomposed, or indeed anywhere it travels between the two locations. The presence of oil in ammonia can cause problems because ammonia decomposition catalysts may not tolerate oil. Therefore, it may be desirable to remove the oil before the ammonia comes into contact with the catalyst. Oil can be removed by passing the ammonia through a bed of activated carbon.

[0128] If oil is removed from the ammonia, an oil removal unit (not shown) can be located in stream 2 (i.e., in the feed line to pump P101), stream 4 (i.e., between pump P101 and glycol heater E271), stream 6 (i.e., between glycol heater E271 and heat exchanger E312), stream 8 (i.e., between heat exchanger E312 and heat exchanger E311), or stream 10 (i.e., between heat exchanger E311 and heat exchanger E310).

[0129] Figure 1 shows the process side of the flowsheet, and Figure 2 shows the glycol system involved in the process depicted in the flowsheet of Figure 1. Features common to both figures have been given the same reference numbers.

[0130] About 39.9 tons / hour of 57.5 wt% ethylene glycol in water at about 30°C is removed from buffer tank T311 as stream 110 and pumped with pumps G331A / B (two pumps in parallel) to produce pumped stream 112 at about 5 bar. Stream 112 is then distributed to a heat exchanger that uses this stream as a cooling medium.

[0131] Stream 112 may be split into two portions: streams 113 and 114. Stream 113 is discussed below. Stream 114 is itself split into three portions: streams 136, 116, and 120.

[0132] Stream 136 removes approximately 4.3 tons / hour of Glycol Solution Stream 114, passes through valve V214, and is sent as Stream 138 to heat exchanger E323, which cools cracked hydrogen-rich Stream 32 from 69°C to 50°C and raises the temperature of the glycol stream to approximately 63°C (Stream 140). The optimum temperature for Stream 140 is a compromise between temperature increase and heat exchanger size. If the glycol flow rate is reduced, the temperature of Stream 140 decreases, increasing the required temperature differential in Exchanger E323 and reducing its size, but the effect of this is to reduce the temperature of Recycled Glycol Stream 142, reducing the temperature differential in Exchanger E271 and increasing its size.

[0133] Stream 116 removes approximately 17.1 tonnes / hour of glycol solution stream 114 and uses it to cool the intercoolers and aftercoolers of compressor K681 of exchangers E6816A-E (see FIG. 1, not shown in FIG. 2) from the discharge temperature of each stage, e.g., about 120°C, to about 40°C for the intercoolers and about 50°C for the aftercoolers, temperatures determined in the examples as optimal for PSA feed. The streams are typically cooled to keep them above the dew point to prevent the formation of an ammonia-rich condensate stream. If the compressors are intercooled and aftercooled below the dew point, a phase separator then cools the NO 2 . xIt is necessary to separate the ammonia-rich condensate, which can be used in the SCR, to provide some of the ammonia used to eliminate the

[0134] Stream 120 removes approximately 18.5 tons / hour of glycol solution stream 114 and uses it for other cooling duties required by the compressor, such as cooling lubricant oil (stream 122), and auxiliary needs (stream 124), such as cylinder cooling for this example with a positive displacement compressor. The resulting warmed streams 126 and 128 / 130 can be combined to form stream 134, which is combined with stream 148 to form stream 150, which is fed to water (or air) cooler E331. Alternatively, a portion of stream 128 can be diverted by valve V212 as stream 132 and combined with stream 54 to form stream 118, which is then combined with stream 140 to form stream 142.

[0135] Heated glycol solution is used in E323, E6816A-E, and other refrigeration requirements of compressor K681 to heat the liquid ammonia feed from about −32° C. to about 53° C. To do this, the glycol solution needs to be sufficiently warm, preferably above 60° C.

[0136] In some of the coolers, the glycol solution is heated to a temperature significantly lower than that required in stream 142, and as a result, the stream shown as stream 134 is diverted to E331 rather than to ammonia heat exchanger E271. E331 is a heat exchanger that uses cooling water (or air, in the case of air-cooled exchangers) to cool the glycol solution fed to it to about 35° C. However, the majority of the recycled glycol solution (about 31.3 tons / hr) is fed through stream 142 and used to heat the ammonia feed.

[0137] 2, there is too much "hot" glycol in stream 142, so the excess is diverted through stream 144 to stream 148, which combines with stream 134 and is cooled in exchanger E331 as stream 150 to produce cooled stream 152. Stream 146 is cooled when stream 148 mixes with stream 152. Valve V202 is a bypass that can be used to control the temperature of stream 154 by controlling the flow of the bypass glycol solution so that the resulting stream 154 is at the required temperature. However, in the following example, valve V202 is closed, so streams 146 and 148 have zero flow.

[0138] Valve V202 is present in case the cooling water (or air temperature if the E331 is air cooled) drops, thereby increasing the duty of the E331 and reducing the temperature of stream 152. There is also a flow control for the flow to the compressor aftercooler, shown as valve V206, which is used to control the flow of "cold" glycol solution to maintain a gas temperature of 40°C at the discharge of the intercooler.

[0139] When compression unit K681 is operating, the temperature of stream 100 is, for example, above 60°C, sufficiently high to heat the feed ammonia stream. However, if compression unit K681 is not operating because it is out of service or turned off to reduce the hydrogen recovery of the PSA to reduce the carbon intensity of the cracking process, or because K681 is operating at a reduced speed (or part load), reducing the heat available from the compressor's aftercooler and again reducing the PSA recovery but not to the minimum recovery that would be achieved by turning off the compressor, extra heat can be provided by using electric heater E332. The ammonia could be heated directly by the electric heater. However, using a heater in the glycol system instead allows for better, smoother operation of the glycol system and allows the glycol system to operate continuously regardless of the operating state of compressor unit K631.

[0140] Feed ammonia stream 4 is heated to about 53° C. at about −31° C. by cooling glycol solution stream 104 from about 65° C. to about 28° C. The cooled stream 106 is then combined with stream 154 at 35° C. to produce stream 108 at about 30° C., which is fed to buffer tank T331.

[0141] FIG. 2 depicts several valves that demonstrate how the cycle can be controlled.

[0142] Valve V204 can be used to bypass the cooling stream to compression unit K681 and return "cold" ammonia to vapor 134 via stream 133, such as when compression unit K681 is not operating. The "cold" glycol solution travels through streams 134, 148 and 144 (countercurrent to those shown) to join stream 100 and travels via stream 102 through valve V200 to heater E332.

[0143] Valves V208, V210, V206, and V214 can be used to distribute the "cold" glycol solution to heat exchangers requiring cooling based on control of the temperature of the resulting "hot" glycol solution (increasing the flow rate to an exchanger generally reduces the "hot" glycol temperature coming out of the exchanger).

[0144] Valve V212 is used to bypass glycol solution exiting the exchanger whose temperature is too low and would adversely affect the temperature of the recycle glycol solution (i.e., reduce the glycol solution below a temperature that would allow the feed ammonia to be heated to 53°C).

[0145] Valve V200 is for controlling the division of heated glycol solution between E271 and E331, diverting glycol solution in excess of that required by E271.

[0146] Valve V202 is used to bypass E331 to allow temperature control of stream 154.

[0147] In this example, the cooling duty required for the glycol system (intercooler and aftercooler) to function plus the auxiliary duty of compressor K681 is greater than the liquid ammonia heating duty for the "hot" glycol solution, thus requiring the auxiliary cooling provided by E331. When the load on the compressor is reduced, such as when the compressor is turned down, the duty of E331 is reduced, ultimately requiring heating of the glycol solution provided by heater E332.

[0148] The invention will now be illustrated by the following non-limiting examples. [Example]

[0149] The process depicted in Figure 1 has been simulated by computer (Aspen Plus, ver. 10, Aspen Technology, Inc., Massachusetts, USA) for a plant designed to produce 30 tonnes / day of hydrogen (Stream 40).

[0150] The activity of the ruthenium- and nickel-based catalysts in the adiabatic reactor and tubes was modeled using the kinetic equation No. 9 given by Lamb et al. (Int. J. Hydrogen Energy, 44 (2019) pp3726-3736) as a basis. For the purposes of the simulation, the activity of the ruthenium-based catalyst follows the kinetic equation, while the activity of the nickel-based catalyst was assumed to be 20% of that predicted by the kinetic equation.

[0151] The results are depicted in Table 2. [Table 2-1] [Table 2-2]

[0152] The results show that with an ammonia slip (Stream 24) of 1.33 mole % from the cracker (tube furnace 201) and 95% hydrogen recovery in the PSA unit, 7626 kg / hr of ammonia (Stream 2) is required as feed, and 472.7 kg / hr of natural gas (Stream 70) is required as fuel to fire the cracker, in addition to the PSA off-gas (Stream 60).

[0153] For a given hydrogen recovery and ammonia slip, the effect of using cracked gas (instead of flue gas) to provide the duty required to heat the partially cracked gas (Stream 20) to the feed temperature of the catalyst packed tubes of the cracker (F201) is to reduce the overall carbon intensity (CI) of the process.

[0154] In an alternative process in which the glycol system is replaced by a cooling water system that achieves the same purpose, a heater, probably an electric heater, would be required to heat the ammonia, requiring an additional 0.86 MW of power and resulting in an increase in the carbon intensity of the process.

[0155] Although the invention has been described with reference to the preferred embodiments depicted in the drawings, it will be understood that various modifications can be made within the spirit and scope of the invention as defined in the following claims.

[0156] In this specification, unless expressly indicated otherwise, the word "or" is used to mean an operator that returns a true value if either or both of the specified conditions are met, as opposed to the "exclusive or" operator, which requires that only one of the conditions be met. The word "comprising" is used to mean "including" and incorporates "consisting of," but not exclusively.

[0157] All prior teachings above are incorporated herein by reference. Any acknowledgement of a prior-published document herein is not an admission or representation that the teachings were common general knowledge in Australia or elsewhere at the date thereof.

Claims

1. 1. A process for decomposing ammonia, comprising: warming the heat transfer fluid to produce a warmed heat transfer fluid; pumping the liquid ammonia to produce pumped liquid ammonia; preheating the pumped liquid ammonia to form preheated liquid ammonia; vaporizing the preheated liquid ammonia to produce ammonia gas; heating the ammonia gas to produce heated ammonia gas; combusting a fuel with an oxidant gas in a furnace to heat catalyst-containing reactor tubes and produce flue gas; delivering the heated ammonia gas, or partially decomposed ammonia gas derived therefrom, to the catalyst-containing reactor tubes to cause decomposition of the ammonia and produce a decomposed gas comprising hydrogen gas, nitrogen gas, and residual ammonia gas; cooling the cracked gas to produce a cooled cracked gas; recovering hydrogen gas from the cooled cracked gas in a pressure swing adsorption (PSA) unit to produce a PSA off-gas comprising hydrogen gas as a product, as well as nitrogen gas, residual hydrogen gas, and residual ammonia gas; a portion of the PSA off-gas is compressed in a compression unit to produce compressed PSA off-gas that is recycled to the PSA unit, and the heat transfer fluid is used to cool the compression unit, thereby providing at least a portion of the duty required to warm the heat transfer fluid; A process wherein a portion of the duty required to preheat the pumped liquid ammonia is provided by heat exchange with the warmed heat transfer fluid.

2. 10. The process of claim 1, wherein the liquid ammonia is pumped from a storage vessel.

3. 10. The process of claim 1, wherein a majority of the duty required to cool the compression unit is the duty required to remove heat of compression from the PSA off-gas.

4. 10. The process of claim 1, wherein the compressed PSA off-gas is at or above its dew point temperature.

5. 10. The process of claim 1, wherein at least a portion of the duty required to warm the heat transfer fluid during periods when the compression unit is operating at part load or not at all is provided from an external source.

6. The process of claim 5 wherein the external source is an electric heater.

7. 10. The process of claim 1, wherein any remaining portion of the duty required to warm the heat transfer fluid is provided by heat exchange with cracked gases and / or flue gases.

8. 10. The process of claim 1, wherein any excess heat is removed from the heat transfer fluid using an air cooler and / or cooling water.

9. The process of claim 1 , wherein the heat transfer fluid is pumped around a circuit.

10. 10. The process of claim 1, wherein the heat transfer fluid is an aqueous solution of glycol.

11. The process of claim 10, wherein the aqueous solution comprises from about 50% to about 60% by weight of glycol.

12. 11. The process of claim 10, wherein the glycol is selected from the group consisting of ethylene glycol and propylene glycol.

13. 10. The process of claim 1, wherein the heated heat transfer fluid is at a temperature above 60°C.

14. 10. The process of claim 1, wherein an additional portion of the PSA off-gas is preheated and delivered to the furnace as at least a portion of the fuel.

15. 10. The process of claim 1, comprising partially decomposing the heated ammonia gas in an adiabatic reaction unit comprising at least one catalyst bed to produce the partially decomposed ammonia gas for delivery to a catalyst-filled reactor tube.

16. 1. An apparatus for decomposing ammonia, comprising: a source of liquid ammonia; a pump having an inlet in fluid flow communication with the source of liquid ammonia and an outlet for pumped liquid ammonia; A furnace, a radiant section comprising at least one inlet for fuel and oxidant gas in fluid flow communication with at least one burner, an ammonia feed inlet in fluid flow communication with said pump, and a catalyst filled reactor tube having an upstream end in fluid flow communication with said ammonia feed inlet and a downstream end in fluid flow communication with an outlet for cracked gas; a convection section in fluid flow communication with the radiant section, the convection section including an outlet for flue gas; a PSA unit comprising an inlet in fluid flow communication with an outlet for the cracked gas in the radiant section of the furnace, an outlet for hydrogen gas, and an outlet for PSA off-gas; a compression unit comprising an inlet in fluid flow communication with an outlet for the PSA off-gas in the PSA unit, and an outlet for compressed PSA off-gas in fluid flow communication with the inlet of the PSA unit; The device, at least one cooler located within the compression unit and arranged to cool the compression unit by heat exchange against a heat transfer fluid; a preheating unit located downstream of the pump and arranged to preheat the pumped liquid ammonia by heat exchange against the heat transfer fluid; and at least one heat exchanger located upstream of the ammonia feed inlet of the furnace for vaporizing preheated liquid ammonia by heat exchange with the cracked gas to heat ammonia gas.

17. 17. The apparatus of claim 16, wherein the source of liquid ammonia is a storage vessel.

18. 17. The apparatus of claim 16, comprising an external heater for warming the heat transfer fluid.

19. 20. The apparatus of claim 18, wherein the external heater is an electric heater.

20. 17. Apparatus according to claim 16, comprising at least one heat exchanger arranged to warm the heat transfer fluid by heat exchange against cracked gases and / or flue gases.

21. 17. The apparatus of claim 16, comprising at least one air cooler and / or at least one water cooler arranged to cool the heat transfer fluid.

22. 17. The apparatus of claim 16, comprising a circuit for the heat transfer fluid, the circuit comprising a pump for moving the heat transfer fluid around the circuit.

23. 17. The apparatus of claim 16, wherein an outlet for PSA off-gas in the PSA unit is in fluid flow communication with the at least one inlet for fuel and oxidant gas in the radiant section of the furnace, and the apparatus comprises at least one heat exchanger located upstream of the at least one inlet for fuel and oxidant gas and arranged to preheat the PSA off-gas by heat exchange against cracked gas and / or flue gas.

24. an adiabatic reaction unit for partially decomposing heated ammonia gas, said unit comprising: an inlet for heated ammonia gas in fluid communication with said pump; and at least one catalyst bed having an upstream end in fluid communication with said inlet and a downstream end in fluid communication with an outlet for partially decomposed ammonia gas; 17. The apparatus of claim 16, wherein the outlet for partially decomposed ammonia gas is in fluid flow communication with the ammonia feed inlet of the furnace.

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