Process and apparatus for decomposing ammonia

Abstract

To provide a process and an apparatus for cracking ammonia.SOLUTION: The invention concerns a process and apparatus for cracking ammonia in which heated ammonia gas at a super-atmospheric pressure is partially cracked in at least two adiabatic reactors in series with interstage heating in which a feed temperature to a first reactor is higher than a feed temperature to a further reactor so as to produce partially cracked ammonia gas, which is then fed to catalyst-containing reactor tubes in a furnace so as to produce cracked gas comprising hydrogen gas, nitrogen gas and residual ammonia gas. Use of the adiabatic reactors enables more efficient heat integration within the process, and the higher temperature in the first reactor enables use of a nickel-based catalyst in the reactor as an alternative solution to a potential problem of the presence of oil in the ammonia.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention is in the field of decomposing ammonia to produce hydrogen gas, and preferred embodiments specifically relate to a process and apparatus for producing hydrogen gas from liquid ammonia. [Background technology]

[0002] 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.

[0003] For use in commercial fuel cells, ammonia must be converted reactively to hydrogen.

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[0004] The process is known as decomposition (or sometimes "dissociation") and is often carried out over a catalyst. The gas produced (or "cracked gas") is a mixture of hydrogen (H) and nitrogen (N) gases, but because the decomposition reaction is an equilibrium reaction, some residual ammonia is also present. The amount of ammonia in the cracked gas, generally referred to as "ammonia slip," can be varied by changing the temperature and pressure; ammonia is heated at higher temperatures and pressures to promote conversion, thereby reducing ammonia slip.

[0005] In most current cracker applications, the mixture of hydrogen and nitrogen is utilized as is. However, since 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.

[0006] There are many examples of ammonia decomposition processes in the art, such as GB977830A, JP5330802A, CN111957270A, US2020 / 0398240A, and KR2022 / 0085469A.

[0007] Additionally, GB1142941A discloses a process for producing a fuel gas interchangeable with city gas. Ammonia is decomposed to form a mixture of hydrogen and nitrogen, which is then enriched 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 cryogenic liquid and vaporized by a closed hot water circuit. The vaporized ammonia gas is superheated by heat exchange with flue gas from the furnace and then decomposed on a suitable catalyst in the tubes of a direct-fired tubular furnace. The decomposed gas is scrubbed with water to recover residual ammonia and ultimately recycled from the ammonia feed to the catalyst-filled tubes of the furnace. The purified decomposed gas is enriched with propane and / or butane to produce a city gas product.

[0008] GB1142941A discloses that the decomposition of ammonia in a direct-fired tubular furnace in the presence of a suitable catalyst is preferred. However, the reference also discloses that other decomposition processes can alternatively be used instead. In this context, GB1142941 mentions heating the ammonia to a suitable temperature and then passing the ammonia through an unheated bed of an ammonia decomposition catalyst to decompose a portion of the ammonia into hydrogen and nitrogen, cooling the gas during the process. 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.

[0009] GB 1353751A discloses a process in which ammonia at a pressure between 20 and 300 atm is decomposed in two stages in a heated reactor tube. In the first stage, gas at a temperature between 450 and 800°C is passed through a layer of nickel, iron, or cobalt-containing catalyst formed by co-precipitation with aluminum oxide and magnesium oxide or magnesium-aluminum spinel supports. Alternatively, the first-stage catalyst can 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 between 450 and 600°C, forming the second stage.

[0010] WO 2022 / 189560A discloses an ammonia decomposition process involving a combustion reactor with tubes filled with an iron catalyst. Liquid ammonia is withdrawn 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.

[0011] US11287089A discloses a hydrogen fueling system in which ammonia is decomposed on-site into hydrogen and nitrogen in an ammonia cracker operating at pressures ranging from 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 the storage 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 may 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. Vaporization of the liquid ammonia is often effected at standard or subatmospheric pressure. US11287089A illustrates a system that produces 7.5 tons of hydrogen gas per day.

[0012] 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 processes that reduce or eliminate the need to burn fossil fuels. Summary of the Invention

[0013] According to a first aspect of the present invention, there is provided a process for decomposing ammonia, the process comprising: providing heated ammonia gas at superatmospheric pressure; supplying the heated ammonia gas at a first temperature (T1) to a first adiabatic reactor comprising a catalyst bed to decompose a portion of the ammonia and produce an intermediate partially decomposed ammonia gas; heating the intermediate partially decomposed ammonia gas to produce a heated intermediate partially decomposed ammonia gas; supplying the heated intermediate partially decomposed ammonia gas, or a heated intermediate partially decomposed ammonia gas derived therefrom, to a further adiabatic reactor comprising a catalyst bed at a second temperature (T2) lower than the first temperature (T1) for decomposing a further portion of the ammonia and producing partially decomposed ammonia gas; combusting fuel with an oxidant gas in a furnace to heat reactor tubes containing the catalyst and form a flue gas; and supplying the partially decomposed ammonia gas to the reactor tubes containing the catalyst to cause further decomposition of the ammonia and produce a decomposed gas comprising hydrogen gas, nitrogen gas, and residual ammonia.

[0014] Partially cracking ammonia in an adiabatic reactor containing an ammonia cracking catalyst before completing the cracking process in the catalyst-filled tubes of a combustion furnace offers the opportunity to optimize the cracking process by fully utilizing the thermal energy in the cracked gas and flue gas, resulting in a reduction in energy consumption, increased hydrogen production, and / or reduced reliance on hydrocarbon fuels such as natural gas.

[0015] In addition, the higher feed temperature to the first adiabatic reactor allows for the use of a catalyst containing a less catalytically active metal, e.g., a nickel-based catalyst, instead of a catalyst containing a more catalytically active metal, e.g., a ruthenium-based catalyst. Thus, in preferred embodiments, the catalyst bed of the first adiabatic reactor comprises, e.g., contains, or consists of, a nickel-based catalyst. Catalysts containing less catalytically active metals, e.g., nickel, tend to be less expensive than catalysts containing more catalytically active metals, e.g., ruthenium. Thus, these embodiments are likely to be significantly less expensive than embodiments in which the bed of the first adiabatic reactor contains a ruthenium-based catalyst, especially if the catalyst bed needs to be replaced periodically due to decomposition and / or contamination by oil present in the ammonia.

[0016] The catalyst beds of the second adiabatic reactor may comprise, e.g., include, or consist of, an upstream layer of a less active catalyst, e.g., a nickel-based catalyst, and a downstream layer of a more active catalyst, e.g., a ruthenium-based catalyst. Again, due to differences in the cost of catalytically active metals, these embodiments are likely to be significantly cheaper than other arrangements involving the use of more active catalyst.

[0017] In some embodiments, it may even be possible to completely eliminate the use of a highly active catalyst, such as a ruthenium-based catalyst. In these embodiments, the second adiabatic reactor bed may contain a less active catalyst, such as a nickel-based catalyst, as the only catalyst, resulting in further cost reduction.

[0018] According to a second aspect of the present invention, there is provided an apparatus for decomposing ammonia, the apparatus comprising: a first adiabatic reactor for partially decomposing heated ammonia gas at superatmospheric pressure, the reactor comprising an inlet for the heated ammonia gas and a catalyst bed having an upstream end in fluid flow communication with the inlet and a downstream end in fluid flow communication with an outlet for the intermediate partially decomposed ammonia gas; a further adiabatic reactor for decomposing the heated intermediate partially decomposed ammonia gas, or heated intermediate partially decomposed ammonia gas derived therefrom, the further adiabatic reactor comprising an inlet in fluid flow communication with an outlet of the first adiabatic reactor and a catalyst bed having an upstream end in fluid flow communication with the inlet and a downstream end in fluid flow communication with an outlet for the partially decomposed ammonia gas; a furnace comprising a radiant section comprising catalyst-containing reactor tubes having an upstream end in fluid flow communication with an outlet of the further adiabatic reactor and a downstream end in fluid flow communication with an outlet for the cracked gas, and a convection section in fluid flow communication with the radiant section having an outlet for the flue gas; a first heat exchanger for heating ammonia gas by heat exchange with the flue gas in the convection section of the furnace, the first heat exchanger having an inlet for the ammonia gas and an outlet in direct fluid flow communication with the inlet of the first adiabatic reactor; and a second heat exchanger for heating intermediate partially decomposed ammonia gas by heat exchange with the flue gas in the convection section of the furnace, the second heat exchanger having an inlet in direct fluid flow communication with the outlet of the first adiabatic reactor and an outlet in fluid flow communication with the inlet of the further adiabatic reactor, the first heat exchanger being positioned upstream of the second heat exchanger in the convection section of the furnace with respect to the flow rate of the flue gas.

[0019] 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]

[0020] [Figure 1] 1 is a simplified flowsheet of one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0021] Unless otherwise specified, amounts of ingredients given in parts per million (or ppm) are calculated by weight. Additionally, all percentages are calculated by moles unless otherwise specified. Furthermore, any references to pressure are references to absolute pressure unless otherwise specified.

[0022] In the context of the present invention, the activity of a catalyst is 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 overall pressure. The units used to define the activity of a heterogeneous catalyst are per gram (or mole g) of catalyst (including substrate, if present) per second. -1 seconds -1 ) is the moles of ammonia converted

[0023] The phrase "in fluid flow communication" will be understood to mean that piping or other suitable conduit is used to transport a fluid from one designated location to another. During its passage between two locations, the fluid may flow through one or more other units (e.g., catalytic reactors) 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 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 passage, and therefore there is essentially no change in the composition or physical condition of the fluid.

[0024] 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.

[0025] The term "upstream" will be understood to mean the direction opposite to the flow of fluid during normal operation, and the term "downstream" will be interpreted accordingly.

[0026] The heated ammonia gas supply of the present invention typically originates from liquid ammonia, which may 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 supply transforms the supply into a multi-component stream, and evaporation of the feed stream will then require higher temperatures to achieve complete evaporation.

[0027] A typical composition of the ammonia feed is shown in Table 1. Table 1 [Table 1]

[0028] Oil may be present in ammonia in local storage tanks, at the production site, or any other storage tanks in between due to the boil-off gas compressor used for ammonia storage. The presence of oil is problematic because it poses the risk of blockage and / or contamination. This can lead to poor performance of heat exchangers or reduced catalytic activity in reactors. 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, which can react with any water present to form carbon monoxide, hydrogen, and methane.

[0029] Inert gases are not expected to be a problem other than potentially becoming product hydrogen. In this regard, helium may be present in ammonia derived from natural gas, but ammonia derived from renewable hydrogen does not contain helium.

[0030] 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 liquid ammonia is removed from a pipeline, the temperature of the liquid ammonia is often higher, for example, about +10° C.

[0031] The pumped (superatmospheric) liquid ammonia is then typically preheated to its boiling point, ideally by suitable heat integration within the process. Preferably, part of the preheating is achieved using a heat transfer circuit in which heat from intercooling and post-cooling of the PSA off-gas compressor is recovered, optionally along with heat from the flue gas and / or cracked gas, using a heat transfer fluid such as an aqueous solution of glycol, e.g., an aqueous solution containing about 50% to about 60% by weight of glycol, such as ethylene glycol or propylene glycol, and used to preheat the liquid ammonia. If such integration is not possible, such as when the compressor is not running, heat from an external source, such as an electric heater, may be required to preheat the ammonia.

[0032] The preheated liquid ammonia is typically then vaporized and the resulting ammonia gas is further heated before being fed to the adiabatic reaction unit. In this regard, the ammonia gas is typically superheated, i.e., heated to a temperature above its boiling point, above 350°C, to ensure a useful reaction rate in the adiabatic reaction unit.

[0033] 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 is used to heat and vaporize the preheated liquid ammonia by heat exchange, and the ammonia gas is then further heated by heat exchange with the flue gas.

[0034] The use of an adiabatic reactor unit allows a portion of the ammonia to be decomposed before entering the catalyst-filled reactor tubes of the furnace. The mole fraction of ammonia in the gas passing through the adiabatic reactor is typically reduced by at least 20%, e.g., at least 25%, or at least 30%, or at least 35%, or at least 40%, and / or 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 in the partially decomposed ammonia gas. In some embodiments, the mole fraction of ammonia in the partially decomposed ammonia gas is in the range of about 0.58 to about 0.62, e.g., about 0.6.

[0035] Adiabatic reactors are incorporated into the process design to improve overall efficiency, specifically by using heat available in the flue gas to provide heat for the adiabatic cracking process within the reactor. In this regard, the temperature around the adiabatic reactor is typically optimized to maximize heat recovery from the flue gas while avoiding temperatures above about 660°C due to material concerns.

[0036] The primary design parameter for an adiabatic reactor is the inlet temperature. Higher inlet temperatures allow for greater conversion within the unit because the ammonia decomposition reaction is endothermic. However, higher temperatures place greater demands on the materials of construction and catalysts. Thus, the inlet temperature to each adiabatic reactor is typically in the range of about 350°C to about 800°C, with the inlet temperature to the first adiabatic reactor being in the range of about 500°C to about 700°C, or about 550°C to about 650°C. Additionally, the inlet temperature to the additional adiabatic reactors is in the range of about 400°C to about 600°C, or about 450°C to about 550°C.

[0037] The present invention requires the use of at least two adiabatic reactors, each containing a catalyst bed. One or more additional adiabatic reactors may be included, bringing the total number of adiabatic reactors up to, for example, three, four, five, or six, with interstage heating as needed. The additional 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 preferably has two such reactors arranged in series with intermediate interstage heating of the partially decomposed ammonia gas by heat exchange against the cracked gas and / or flue gas.

[0038] 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 or each adiabatic reactor bed.

[0039] Many metals are known in the art to catalyze the decomposition of ammonia. 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.

[0040] 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 The metals can be unsupported, but are often supported on a suitable support (or substrate), typically a metal 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 typically depends in part on the loading of the 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 higher end of the range, e.g., about 20 wt. % to about 65 wt. %.

[0042] Supported metal catalysts may be unpromoted or may be promoted with at least one other metal, for example, 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), to improve activity, as is known in the art.

[0043] Any conventional catalyst known 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 with the present 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 US 2021 / 0001311 A, such as CoNi-MgSrCeO4 and 1 wt. % K-CoNi-MgSrCeO4.

[0045] The catalyst of the present invention typically comprises, e.g., 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 activity at temperatures in the range of 475°C to 600°C suitable for achieving a reaction rate of at least 0.2 times, or at least 0.5 times, or at least equal to, or at least 2 times, or at least 3 times, or at least 4 times, or at least 5 times the rate calculated according to Equation 9 proposed by Lamb et al., i.e., r=8.73exp[-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 Jmol -1 K -1 ) multiplied by the temperature in Kelvin, "T", P NH3 is the partial pressure of ammonia, P H2is the partial pressure of hydrogen, β is defined in the research paper as follows (see equation 5 proposed by Lamb et al.):

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

[0047] Transition metals that may be particularly suitable for use as the primary catalytically active metal of the catalyst in the catalyst bed of an adiabatic reactor are selected from chromium, manganese, iron, cobalt, nickel, ruthenium, and copper, e.g., iron, cobalt, nickel, and ruthenium. The inventors have found that nickel and ruthenium are typically the most suitable catalytically active metals for use in the catalyst bed of an adiabatic reaction unit.

[0048] The term "ruthenium-based catalyst" refers to a catalyst that contains ruthenium as the only (or at least predominant) catalytically active metal, i.e., the metal 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 that supports ruthenium. The terms "nickel-based catalyst" and "iron-based catalyst" are intended to be interpreted accordingly.

[0049] 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.

[0050] 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 catalyst is used, through the ordering of catalyst layers within the bed of the adiabatic reaction unit. Suitable catalysts include conventional nickel- or ruthenium-based catalysts for decomposing ammonia.

[0051] In embodiments having two adiabatic reactors in series, the catalyst bed of the first reactor typically comprises, e.g., includes or consists of a single layer of a first catalyst, e.g., a nickel-based catalyst, and the catalyst bed of the second reactor typically comprises, e.g., includes or consists of an upstream layer of a second catalyst, e.g., a nickel-based catalyst, that has similar activity to the first catalyst, and a downstream layer of a third catalyst, e.g., a ruthenium-based catalyst, that is typically more active than the first and second catalysts.

[0052] In these embodiments, the first and second catalysts can be the same. Alternatively, the first and second catalysts can be different, for example, comprising different catalytically active metals, or the same catalytically active metals on different supports, or the same catalytically active metals on the same support at different loadings.

[0053] 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. If no additional layer of catalyst is present, 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.

[0054] The inventors have recognized that ruthenium-based catalysts can crack 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.

[0055] The reactor tubes of the furnace are also filled with an ammonia decomposition catalyst. Any of the conventional ammonia decomposition catalysts listed above can be used in the reactor tubes. However, for adiabatic reactors, particularly suitable catalysts for the reactor tubes include nickel-based catalysts and ruthenium-based catalysts.

[0056] In some embodiments, particularly those in which the cracking reaction is carried out at higher temperatures, the reactor tubes may be packed with a less active catalyst, such as a nickel-based catalyst, as the only catalyst in the tubes. Less active catalysts tend to be less expensive than more active catalysts, and thus this arrangement helps to reduce overall capital costs.

[0057] However, the reactor tubes may be packed with at least two different ammonia decomposition catalysts having different activities.

[0058] In these embodiments, a more active catalyst, e.g., a ruthenium-based catalyst, may be positioned in the tube downstream of a less active catalyst, e.g., a nickel-based catalyst, to ensure that the decomposition reaction approaches equilibrium.

[0059] Alternatively, a more active catalyst, e.g., a ruthenium-based catalyst, can be positioned upstream of a less active catalyst, e.g., a nickel-based catalyst, to help control the reactor tube inner wall temperature in the region of highest ammonia temperature and partial pressure, thereby controlling tube nitridation. The catalysts are layered in this manner, and in a preferred embodiment, the endotherm of the reaction inside the tube can be used to keep the tube metal cool in the region of most intense combustion outside the tube. Because ruthenium is more catalytically active than nickel, it creates a strong endotherm that cools the inner tube walls in the high ammonia concentration region on the process side, protecting the tubes from excessive nitridation caused by high ammonia concentrations and high temperatures.

[0060] In these alternative embodiments, a second, more active catalyst, for example, another ruthenium-based catalyst, may be positioned downstream of the less active catalyst to ensure that the decomposition reaction approaches equilibrium.

[0061] 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 often substantially greater than 50%. In this regard, the activity of a more active catalyst is often at least two-fold (i.e., double), or at least three-fold, or at least four-fold, or at least five-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.

[0062] In some preferred embodiments, the catalyst in the upstream layer of the tube is a ruthenium-based catalyst and the catalyst in the downstream layer is a nickel-based catalyst. If there is a third catalyst in these embodiments downstream of the nickel-based catalyst, that catalyst is preferably a ruthenium-based catalyst, but the catalyst may be a different ruthenium-based catalyst than the first catalyst, for example, the catalysts have different supports and / or catalyst loadings and / or, if both are promoted, are then promoted with different metals.

[0063] The catalyst in the reactor tubes may be the same or different from the catalyst used in the adiabatic reaction unit.

[0064] Also, at higher temperatures, catalyst sintering is known to reduce catalyst activity and life. In this regard, those skilled in the art will recognize that improved conversion must be balanced against higher vessel costs and shorter catalyst life.

[0065] As mentioned above, water is often present in ammonia as a contaminant. Water can be removed from the ammonia, in which case a catalyst that does not tolerate water, such as an iron-based catalyst, can be used in the adiabatic reaction unit and / or reactor tube. However, water is not removed in preferred embodiments to save capital and operating costs and reduce energy consumption. In these embodiments, a catalyst that cannot tolerate water, such as an iron-based catalyst, is 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.

[0066] The combustion process within 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 hydrogen recovery from the cracked gas, or a mixture of the two. However, trim fuels such as C1-C3 hydrocarbons or natural gas may be used if desired, although using hydrocarbon trim fuels increases the carbon intensity of the process and, therefore, it is generally desirable to minimize the use of such trim fuels while maintaining or even improving hydrogen recovery.

[0067] The oxidant gas is typically air, but can be an oxygen-enriched gas or pure oxygen if desired.

[0068] The partially decomposed ammonia can be fed to the catalyst-filled tubes of the furnace at a temperature of up to about 800°C, if 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.

[0069] The decomposition temperature and pressure typically dictate that the ammonia slip in the reactor tube be no more than 3 mole percent, for example, from about 0.5 mole percent to about 1.5 mole percent.

[0070] The heat from the cracked gas and flue gas is then used to heat the feed streams to the adiabatic reactor and furnace, thereby reducing the overall energy consumed by the process. In this regard, the temperature of the cracked gas depends on the cycle being operated.

[0071] 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, due to heat leaks, the temperature drops, typically to about 600°C to about 700°C, at which point the heat can be effectively utilized.

[0072] At higher temperature cycles, the temperature of the cracked gas may 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 may be up to about 840°C at its highest point. However, due to heat leaks, the temperature drops, typically to about 700°C to about 800°C, at which point the heat can be effectively utilized.

[0073] At least a portion of the duty required to heat the partially cracked gas generated in the adiabatic reaction unit to the feed temperature of the catalyst-filled reactor tubes of the furnace is typically provided by heat exchange with the cracked gas. In preferred embodiments, the cracked gas is used directly to provide this heating duty. In other words, the cracked gas is typically not used elsewhere, for example, to heat another process fluid prior to heating the partially cracked gas. Some of this heating duty may be provided otherwise, for example, by heat exchange with flue gas. However, more than half of this heating duty, i.e., 50% or more, is typically provided by the cracked gas. In preferred embodiments, at least 75%, or at least 90%, or all of this heating duty is provided by the cracked gas.

[0074] Due to the high ammonia concentration in the feed gas, decomposition reactor vessels, such as reactor tubes in adiabatic reactors and furnaces, are typically constructed from materials resistant to ammonia and / or nitridation, especially when higher decomposition temperatures are used. 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.

[0075] Specific examples of suitable nickel-based alloys include UNS N06600, N06625, N06601, N06617, N06025, N06230, N07214, N08811. In some embodiments, austenitic nickel-chromium based superalloys such as Inconel may be used. 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.

[0076] 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 the surface is modified or coated with a corrosion-resistant layer such as aluminiding, aluminiding followed by pre-oxidation, or a ceramic coating. Nitriding-resistant alloys can also be used with surface modifications or coatings for improved performance.

[0077] In preferred embodiments, the composition of the ammonia typically remains at least substantially unchanged from the liquid ammonia in storage to the heated ammonia gas being fed to the adiabatic reaction unit. Oil present in the liquid ammonia may be removed at some point prior to partial decomposition of the ammonia, although in embodiments where the ammonia feed to the adiabatic reaction unit first encounters a ruthenium-based catalyst, oil removal is not necessary. In these embodiments, water is typically not removed, so any water present in the liquid ammonia is also present in the heated ammonia gas.

[0078] The cracked gas is cooled during heat exchange with the partially cracked gas. The cooled cracked gas is then typically further cooled by providing at least a portion of the heating duty required to generate heated ammonia gas from liquid ammonia. After cooling, hydrogen may be recovered as a product from the cracked gas. Recovery may be achieved in a pressure swing adsorption (PSA) process, 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 by a PSA process alone, i.e., without the use of membrane separation.

[0079] In embodiments using a PSA process, a PSA off-gas is generated that contains nitrogen gas, residual ammonia, and residual hydrogen. The PSA off-gas is typically split into two portions. The first portion of the PSA off-gas is typically compressed in a compression unit and recycled to the PSA process to improve hydrogen recovery. The second portion is typically preheated and then fed to a furnace as fuel.

[0080] There are several factors that affect the carbon intensity of a cracking process, two of which are the amount of ammonia that is allowed to slip through the cracker and the nature of the fuel burned in the cracker, specifically whether and if so, how much hydrocarbon, such as natural gas, is used. The inventors have discovered that for a given amount of ammonia slip, natural gas burned in the cracker, the overall carbon intensity of the process can be reduced if the cracked gas (and not the flue gas) is used to provide at least a portion of the duty required to heat the partially cracked gas to the cracker feed temperature.

[0081] Aspects of the present invention include the following. #1. A process for decomposing ammonia, providing heated ammonia gas at superatmospheric pressure; supplying ammonia gas heated at a first temperature (T1) to a first adiabatic reactor comprising a catalyst bed to decompose a portion of the ammonia and produce intermediate partially decomposed ammonia gas; heating the intermediate partially decomposed ammonia gas to produce heated intermediate partially decomposed ammonia gas; feeding the heated intermediate partially decomposed ammonia gas, or a heated intermediate partially decomposed ammonia gas derived therefrom, at a second temperature (T2) lower than the first temperature (T1) to a further adiabatic reactor comprising a catalyst bed to decompose a further portion of the ammonia and produce partially decomposed ammonia gas; burning a fuel with an oxidant gas in a furnace to heat reactor tubes containing a catalyst and form a flue gas; feeding the partially decomposed ammonia gas to a reactor tube containing a catalyst to cause further decomposition of the ammonia and produce a decomposed gas comprising hydrogen gas, nitrogen gas, and residual ammonia. #2. The process of #1, wherein the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst. #3. The process according to #1 or #2, wherein the first temperature (T1) is in the range of about 550°C to about 650°C. #4. A process described in any one of #1 to #3, wherein the catalyst bed of the further adiabatic reactor comprises at least one catalyst selected from nickel-based catalysts and ruthenium-based catalysts. #5. A process according to any one of #1 to #4, wherein the second temperature (T2) is in the range of about 450°C to about 550°C. #6. A process according to any of #1 to #5, wherein at least a portion, preferably all, of the duty required to heat the intermediate partially decomposed gas is provided by heat exchange with flue gas. #7. The process described in #6, wherein part of the duty required to provide heated ammonia gas is provided by heat exchange with the flue gas upstream of heat exchange with the intermediate partially decomposed gas relative to the flue gas flow. #8. The process of any of #1-#7, wherein the partially decomposed ammonia gas is heated to produce heated partially decomposed ammonia gas before being fed to a reactor tube containing a furnace catalyst. #9. The process of #8, wherein at least a portion, preferably all, of the duty required to heat the partially decomposed ammonia gas is provided by heat exchange with the decomposed gas. #10. Pumping liquid ammonia containing at least 0.1 mole percent water 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 pressurized ammonia gas to produce heated ammonia gas at superatmospheric pressure; A process according to any one of #1 to #9, wherein water from liquid ammonia is present in the heated ammonia gas. #11. The process of #10, wherein water is present in the heated ammonia in an amount of 1 mol % or less. #12. The process of #10 or #11, wherein at least a portion, preferably more than half, i.e., greater than 50%, of the heating duty required to provide heated ammonia gas is provided by heat exchange with the decomposed gas downstream of heat exchange with the partially decomposed ammonia gas relative to the decomposed gas flow. #13. A process described in any one of #1 to #12, wherein the catalyst-filled tubes of the furnace and the catalyst bed of the adiabatic reactor do not contain an iron-based catalyst. #14. After cooling to, for example, below 60°C, for example, about 50°C, recovering hydrogen from the cracked gas in a hydrogen recovery unit to produce hydrogen gas product and an off-gas including nitrogen gas, residual hydrogen gas, and residual ammonia gas; heating at least a portion of the off-gas to produce a heated off-gas; and supplying the heated off-gas to the furnace as at least a portion of the fuel. #15. The process described in #14, in which all of the off-gas is fed to the furnace as fuel. #16. Dividing the off-gas into a first portion and a second portion, the first portion being heated and fed into a furnace; compressing a second portion of the off-gas to produce a compressed off-gas, and recycling the compressed off-gas to a hydrogen recovery unit for further hydrogen recovery. #17.An apparatus for decomposing ammonia, a first adiabatic reactor for partially decomposing heated ammonia gas at superatmospheric pressure, the reactor comprising an inlet for the heated ammonia gas and a catalyst bed having an upstream end in fluid flow communication with the inlet and a downstream end in fluid flow communication with an outlet for the intermediate partially decomposed ammonia gas; a further adiabatic reactor for decomposing the heated intermediate partially decomposed ammonia gas, or the heated intermediate partially decomposed ammonia gas derived therefrom, the further adiabatic reactor comprising an inlet in fluid flow communication with an outlet of the first adiabatic reactor, and a catalyst bed having an upstream end in fluid flow communication with the inlet and a downstream end in fluid flow communication with an outlet for the partially decomposed ammonia gas; 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 upstream end in fluid flow communication with an outlet of the further adiabatic reactor, and a catalyst-containing reactor tube having a downstream end in fluid flow communication with an outlet for the cracked gas; and a convection section in fluid flow communication with the radiant section and including an outlet for flue gas; a first heat exchanger for heating ammonia gas by heat exchange with flue gas in the convection section of the furnace, the first heat exchanger having an inlet for ammonia gas and an outlet in direct fluid flow communication with the inlet of the first adiabatic reactor; a second heat exchanger for heating the intermediate partially decomposed ammonia gas by heat exchange with flue gas in the convection section of the furnace, the second heat exchanger having an inlet in direct fluid flow communication with the outlet of the first adiabatic reactor and an outlet in fluid flow communication with the inlet of a further adiabatic reactor; The apparatus, wherein the first heat exchanger is positioned upstream of the second heat exchanger in the convection section of the furnace relative to the flow of flue gas. #18. The apparatus of #17, wherein the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst. #19. An apparatus according to #17 or #18, wherein the catalyst bed of the further adiabatic reactor comprises a catalyst selected from a nickel-based catalyst and a ruthenium-based catalyst. #20. An apparatus as described in any of #17 to #19, comprising a further heat exchanger (or "economizer") positioned to heat the partially decomposed ammonia gas by heat exchange with the decomposed gas located between the further adiabatic reactor and the radiant section of the furnace. #21. The apparatus of #20, wherein the additional heat exchanger is a shell-and-tube type heat exchanger. #22. The apparatus of #21, wherein the shell side of the heat exchanger is in direct fluid flow communication with the outlet of the radiant section of the furnace. #23. A hydrogen recovery unit, preferably a PSA unit, for recovering hydrogen gas from the cracked gas, a first inlet in fluid flow communication with the cracked gas outlet of the radiant section of the furnace; a first outlet for hydrogen gas; a second outlet for off-gas comprising nitrogen gas, residual ammonia gas, and residual hydrogen gas in fluid flow communication with at least one inlet for fuel in the radiant section of the furnace; An apparatus described in any of #17 to #22, wherein the apparatus comprises an off-gas heater arranged to heat the off-gas and positioned between the second outlet of the hydrogen recovery unit and at least one inlet for fuel in the radiant section of the furnace. #24.A compression unit for compressing off-gas, the compressor comprising: an inlet in fluid flow communication with the second outlet of the hydrogen recovery unit; an outlet in fluid flow communication with the first inlet of the hydrogen recovery unit; The apparatus of #23, wherein the apparatus includes a valve arrangement for controlling the flow of off-gas to the compression unit and the off-gas heater. #25. An apparatus described in any one of #17 to #24, wherein the catalyst-filled tubes of the furnace and the catalyst bed of the adiabatic reactor do not contain an iron-based catalyst.

[0082] The invention will now be described, by way of example only, with reference to the drawings in which:

[0083] In Figure 1, liquid ammonia stream 2 at about -32°C is removed from storage (not shown) and fed to pump P101 where it is pumped at a pressure of about 46 bar to produce pressurized liquid ammonia stream 4, which is preheated by heat exchange in heat exchanger E271 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 fed to heat exchanger E271 is sufficient to preheat the liquid ammonia to the required temperature.

[0084] 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 ammonia gas stream 12 heated at about 260°C.

[0085] The heated ammonia gas in stream 12 is further heated by heat exchange in heat exchanger E2102 to produce further superheated ammonia gas stream 13 at about 490° 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) positioned between them.

[0086] The further heated ammonia gas in stream 13 is then heated by heat exchange in heat exchanger E2104 to produce superheated ammonia gas stream 14 at about 600°C and about 45 bar. Heat exchanger E2104 is the "first heat exchanger" as defined in the claims.

[0087] The superheated ammonia gas in stream 14 is then fed to first adiabatic reactor vessel C141 and passed through a bed of nickel-based catalyst. A portion of the ammonia gas is decomposed on 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 0.75.

[0088] The intermediate gas is at about 450°C before being heated by heat exchange in heat exchanger E2103 to produce superheated intermediate gas stream 18. Heat exchanger E2103 is the "second heat exchanger" as defined in the claims.

[0089] Stream 18 was then fed to a second adiabatic reactor vessel C142 at about 500° C. and passed through beds containing an upstream layer of a nickel-based catalyst and a downstream layer of a ruthenium-based catalyst to produce Stream 20 of partially decomposed ammonia gas at about 380° C. The mole fraction of ammonia in the gas passing through second adiabatic reactor vessel C142 decreases from about 0.75 to about 0.6.

[0090] The catalyst bed in the second adiabatic reactor vessel C142 has two layers—a layer of nickel-based catalyst over a layer of ruthenium-based catalyst—to use heat more efficiently and thus maximize ammonia conversion. Catalyst volume is also optimized; that is, the volume of the ruthenium-based catalyst is minimized by limiting the outlet temperature of the second adiabatic reactor vessel. The inventors have found that reducing this temperature below about 380°C increases the volume required for the ruthenium-based catalyst without any additional benefit.

[0091] The ruthenium-based catalyst is the same in both the first and second adiabatic reactor vessels; however, different ruthenium-based catalysts may be used.

[0092] The partially cracked ammonia in stream 20 is heated by heat exchange in heat exchanger (or "economizer") E305 before being fed as stream 22 at a pressure of about 40 bar to catalyst-filled tubes in the radiant section F201 of the furnace (or reactor). Heat exchanger E305 is a "further heat exchanger" as defined in the claims.

[0093] Heating the feed to the tubes increases the amount of cracking that can be done with the heat from the burners by reducing the duty required to heat the partially cracked stream to reaction temperature. The inlet temperature of direct-fired tubular furnaces is limited to about 500°C to limit the inside wall temperature of the cracker tubes.

[0094] Air stream 62 passes through forced draft fan K212 before being preheated by heat exchange in heat exchanger E2142 to produce preheated air stream 64. The preheated air in stream 64 is supplied 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.

[0095] The tubes in the furnace's radiant section F201 are packed with two types of ammonia decomposition catalysts in two different layers. A ruthenium-based catalyst is used in the first layer in each tube, allowing for a faster reaction rate to keep the metal temperature within the design limit of about 660°C. The second layer in the tube, downstream of the first layer, contains a lower-cost, but less active, nickel-based catalyst.

[0096] Cracked gas stream 24 exits the radiant section F201 of the direct-fired tubular furnace at approximately 640°C and is then fed to economizer E305 ("further heat exchanger" as defined in the claims) to provide the necessary duty to heat the partially cracked ammonia, thereby reducing the temperature of the cracked gas to approximately 520°C.

[0097] Economizer E305 is illustrated 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 could be reversed or indeed different types of heat exchangers could be used.

[0098] Cracked gas stream 26 is then fed from economizer E305 to heat exchanger E310 to provide the necessary duty to superheat the ammonia gas, thereby further reducing the temperature of the cracked gas to approximately 380°C.

[0099] Cracked gas stream 28 is then fed from heat exchanger E310 to heat exchanger E311 to provide the necessary duty to vaporize additional heated liquid ammonia, thereby further reducing the temperature of the cracked gas to approximately 100°C.

[0100] Cracked gas stream 30 is then fed from heat exchanger E311 to heat exchanger E312 to provide the necessary duty to further heat the heated pressurized liquid ammonia, thereby further reducing the temperature of the cracked gas again to about 60°C.

[0101] Each of heat exchangers E310, E311, and E312 is illustrated as an individual shell-and-tube heat exchanger with ammonia passing through the tubes and cracked gases passing through the shell side. However, this arrangement can be reversed for at least one, e.g., all, of these heat exchangers. Alternatively, the heat exchangers can be combined into a single shell-and-tube heat exchanger, or indeed different types of heat exchangers can be used.

[0102] Cracked gas stream 32 from heat exchanger E312 is then further cooled by heat exchange with a heat exchange 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.

[0103] 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.

[0104] 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 supplied to the burners in furnace F201 along with an air supply 64 and, optionally, when needed, a natural gas supply 70. A minimal amount of natural gas is used as trim fuel to provide the balance of fuel required in the combustion section.

[0105] The second portion may be sent as stream 46 to multi-stage compression unit K681 for compression. Compression unit K681 has five stages with intercoolers 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 used in the compression unit, or, if a positive displacement compression unit is used, from the compression unit cylinder using a heat transfer fluid.

[0106] For convenience, the intercooler and aftercooler are shown by 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.

[0107] The heat transfer fluid warmed in the cooler E323 and the intercoolers and aftercoolers E6816A-E is then used to provide the duty required to preheat the liquid ammonia by heat exchange in the heat exchanger E271.

[0108] 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 additional hydrogen recovery. In this manner, hydrogen recovery can be increased from about 85 mole % (without recycle) to about 95 mole % (with recycle).

[0109] As indicated above, the process can be operated without compression unit K681, resulting in a reduction in hydrogen recovery in PSA unit 501. Reducing hydrogen recovery obviously results in a reduction in hydrogen gas product. However, reduced hydrogen recovery may still be desirable because when more hydrogen is present in the off-gas, the carbon intensity (CI) of the process is reduced, thereby reducing the need for natural gas as a trim fuel and reducing carbon dioxide emissions.

[0110] Flue gas stream 72, at about 680°C, passes from radiant section F201 through convection section 90 of furnace F201, where it first provides the duty required to further heat ammonia from stream 13 in heat exchanger E2104 (the "first heat exchanger" as defined in the claims), thereby reducing the temperature of the flue gas used (as stream 73), and provides the duty required to heat intermediate gas from stream 16 in heat exchanger E2103 (the "second heat exchanger" as defined in the claims), thereby further reducing the temperature of the flue gas. Thus, the flue gas provides heating duty in a direction cocurrent to the flow of feed gas to radiant section F201 of the direct-fired tube furnace.

[0111] The cooled flue gas is then used (as stream 74) to provide the necessary duty to further heat the ammonia gas in stream 12 in heat exchanger E2102, thereby further reducing the temperature of the flue gas.

[0112] The further cooled flue gas in stream 76, still at a temperature of about 295°C, is then used to heat air from stream 62 in heat exchanger E2142, thereby further reducing the temperature of the flue gas. The further cooled flue gas is then used (as stream 78) to provide the necessary duty to heat the PSA off-gas from stream 44 in heat exchanger E2112, thereby further cooling the flue gas.

[0113] The cooled flue gas leaves the convection section 90 of the direct-fired tubular furnace F201 as stream 80 at approximately 123°C, i.e., above the dew point of water, passes through an induced draft fan K211, and then leaves the process as stream 82. All of the useful energy has been extracted from the flue gas at this point, and it can optionally be vented to the atmosphere after further processing if required depending on its composition.

[0114] Oil may be present in liquid ammonia 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), either where the ammonia is produced, or where the ammonia is cracked, or indeed anywhere passing between the two sites. The presence of oil in ammonia is known to cause difficulties because ammonia cracking catalysts are typically considered to be oil intolerant. At least a portion of the oil may be removed by cracking in the catalyst bed of the first adiabatic reactor, particularly if a ruthenium-based catalyst is used.

[0115] It may be desirable to remove at least some of the oil before the catalyst is exposed to ammonia. In this regard, the oil may be removed by passing the ammonia through a bed of activated carbon. If the oil is removed from the ammonia upstream of the first adiabatic reactor, then an oil removal unit (not shown) may 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 exchangers E312 and E311), or stream 10 (i.e., between heat exchangers E311 and E310).

[0116] The invention will now be illustrated by the following non-limiting examples. [Example]

[0117] The process illustrated in Figure 1 has been simulated by computer (Aspen Plus, version 10, Aspen Technology, Inc., Massachusetts, USA) for a plant designed to produce 30 tonnes / day of hydrogen (Stream 40).

[0118] The activity of ruthenium-based and nickel-based catalysts in the adiabatic reactor and tubes was modeled using the rate equation No. 9 given by Lamb et al. (Int. J. Hydrogen Energy, 44 (2019) pp3726-3736) as a basis. For the purpose of the simulation, the activity of the ruthenium-based catalyst was fitted to the rate equation, while the activity of the nickel-based catalyst was assumed to be 20% of the activity predicted by the rate equation.

[0119] The results are shown graphically in Table 2. Table 2 [Table 2-1] [Table 2-2]

[0120] The results show that with a 1.33 mol % ammonia slip (Stream 24) from the cracker (tube furnace 201) and a recovery of 95 mol % hydrogen in the PSA, 7626 kg / hr of ammonia is required as feed (Stream 2) along with 27.0 kmol / hr of natural gas (Stream 70) as fuel in addition to the PSA off-gas (Stream 60) to fire the cracker.

[0121] 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.

[0122] Although the invention has been described with reference to the preferred embodiments illustrated 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.

[0123] In this specification, unless expressly indicated otherwise, the word "or" is used in the sense of an operator that returns a value of true when either or both of the stated conditions are met, as opposed to the operator "exclusive or," which requires only one of the conditions to be met. The word "comprising" is used in the sense of "including" and incorporates "consisting of," but not exclusively.

[0124] All prior teachings set forth above are incorporated herein by reference. Any acknowledgement of a prior-published document herein should not be taken as an admission or representation that the teachings were general knowledge in Australia or elsewhere at that date. The following embodiments can be given as examples of the present invention. (Appendix 1) 1. A process for decomposing ammonia, comprising: providing heated ammonia gas at superatmospheric pressure; supplying the heated ammonia gas at a first temperature (T1) to a first adiabatic reactor comprising a catalyst bed to decompose a portion of the ammonia and produce an intermediate partially decomposed ammonia gas; heating the intermediate partially decomposed ammonia gas to produce heated intermediate partially decomposed ammonia gas; feeding the heated intermediate partially decomposed ammonia gas, or a heated intermediate partially decomposed ammonia gas derived therefrom, at a second temperature (T2) lower than the first temperature (T1) to a further adiabatic reactor comprising a catalyst bed to decompose a further portion of the ammonia and produce partially decomposed ammonia gas; burning a fuel with an oxidant gas in a furnace to heat reactor tubes containing a catalyst and form a flue gas; feeding the partially decomposed ammonia gas to a reactor tube containing the catalyst to cause further decomposition of ammonia and produce a cracked gas comprising hydrogen gas, nitrogen gas, and residual ammonia. (Appendix 2) 10. The process of claim 1, wherein the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst. (Appendix 3) 2. The process of claim 1, wherein the first temperature (T1) is in the range of about 550°C to about 650°C. (Appendix 4) 10. The process of claim 1, wherein the catalyst bed of the further adiabatic reactor comprises a catalyst selected from a nickel-based catalyst and a ruthenium-based catalyst. (Appendix 5) 2. The process of claim 1, wherein the second temperature (T2) is in the range of about 450°C to about 550°C. (Appendix 6) 2. The process of claim 1, wherein at least a portion, and preferably all, of the duty required to heat the intermediate partially decomposed gas is provided by heat exchange with the flue gas. (Appendix 7) 7. The process of claim 6, wherein a portion of the duty required to provide the heated ammonia gas is provided by heat exchange with the flue gas flow upstream of the heat exchange with the intermediate partially cracked gas. (Appendix 8) 2. The process of claim 1, wherein the partially decomposed ammonia gas is heated to produce heated partially decomposed ammonia gas before being fed to the catalyst-containing reactor tubes of the furnace. (Appendix 9) 9. The process of claim 8, wherein at least a portion, preferably all, of the duty required to heat the partially decomposed ammonia gas is provided by heat exchange with the decomposed gas. (Appendix 10) pumping liquid ammonia containing at least 0.1 mole percent water 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 pressurized ammonia gas to produce the superatmospheric pressure heated ammonia gas; 10. The process of claim 1, wherein the water from the liquid ammonia is present in the heated ammonia gas. (Appendix 11) 11. The process of claim 10, wherein the water is present in the heated ammonia in an amount of 1 mol % or less. (Appendix 12) 11. The process of claim 10, wherein at least a portion, preferably more than half, of the heating duty required to provide the heated ammonia gas is provided by heat exchange with the cracked gas stream downstream of the heat exchange with the partially decomposed ammonia gas. (Appendix 13) 10. The process of claim 1, wherein the catalyst-filled tubes of the furnace and the catalyst bed of the adiabatic reactor do not contain an iron-based catalyst. (Appendix 14) recovering hydrogen from the cracked gas in a hydrogen recovery unit after cooling to produce a hydrogen gas product and an off-gas comprising nitrogen gas, residual hydrogen gas, and residual ammonia gas; heating at least a portion of the off-gas to produce a heated off-gas; 2. The process of claim 1, further comprising supplying the heated off-gas to the furnace as at least a portion of the fuel. (Appendix 15) 15. The process of claim 14, wherein all of the off-gas is supplied as fuel to the furnace. (Appendix 16) dividing the off-gas into a first portion and a second portion, the first portion being heated and fed to the furnace; compressing the second portion of the off-gas to produce a compressed off-gas and recycling the compressed off-gas to the hydrogen recovery unit for further hydrogen recovery. (Appendix 17) 1. An apparatus for decomposing ammonia, comprising: a first adiabatic reactor for partially decomposing heated ammonia gas at superatmospheric pressure, the first adiabatic reactor comprising an inlet for heated ammonia gas and a catalyst bed having an upstream end in fluid flow communication with the inlet and a downstream end in fluid flow communication with an outlet for intermediate partially decomposed ammonia gas; a further adiabatic reactor for decomposing the heated intermediate partially decomposed ammonia gas, or the heated intermediate partially decomposed ammonia gas derived therefrom, the further adiabatic reactor comprising: an inlet in fluid flow communication with the outlet of the first adiabatic reactor; and a catalyst bed having an upstream end in fluid flow communication with the inlet and a downstream end in fluid flow communication with an outlet for partially decomposed ammonia gas; A furnace, a radiant section comprising at least one inlet for fuel and oxidant gas in fluid flow communication with at least one burner, a catalyst-containing reactor tube having an upstream end in fluid flow communication with the outlet of the further adiabatic reactor and a downstream end in fluid flow communication with an outlet for cracked gas; and a convection section in fluid flow communication with the radiant section and including an outlet for flue gases; a first heat exchanger for heating ammonia gas by heat exchange with flue gas in the convection section of the furnace, the first heat exchanger having an inlet for ammonia gas and an outlet in direct fluid flow communication with the inlet of the first adiabatic reactor; a second heat exchanger for heating intermediate partially decomposed ammonia gas by heat exchange with flue gas in the convection section of the furnace, the second heat exchanger having an inlet in direct fluid flow communication with the outlet of the first adiabatic reactor and an outlet in fluid flow communication with the inlet of the further adiabatic reactor; The apparatus, wherein the first heat exchanger is positioned upstream of the second heat exchanger in the convection section of the furnace relative to the flow of flue gas. (Appendix 18) 18. The apparatus of claim 17, wherein the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst. (Appendix 19) 18. The apparatus of claim 17, wherein the catalyst bed of the further adiabatic reactor comprises a catalyst selected from a nickel-based catalyst and a ruthenium-based catalyst. (Appendix 20) 18. The apparatus of claim 17, comprising a further heat exchanger positioned to heat partially decomposed ammonia gas by heat exchange with the decomposed gas positioned between the further adiabatic reactor and the radiant section of the furnace. (Appendix 21) 21. The apparatus of claim 20, wherein the further heat exchanger is a shell-and-tube heat exchanger. (Appendix 22) 22. The apparatus of claim 21, wherein a shell side of the heat exchanger is in direct fluid flow communication with the outlet of the radiant section of the furnace. (Appendix 23) a hydrogen recovery unit, preferably a PSA unit, for recovering hydrogen gas from the cracked gas, comprising: a first inlet in fluid flow communication with the cracked gas outlet of the radiant section of the furnace; a first outlet for hydrogen gas; a second outlet for off-gas comprising nitrogen gas, residual ammonia gas, and residual hydrogen gas in fluid flow communication with the at least one inlet for fuel in the radiant section of the furnace; 18. The apparatus of claim 17, wherein the apparatus comprises an off-gas heater positioned to heat off-gas and positioned between the second outlet of the hydrogen recovery unit and the at least one inlet for fuel in the radiant section of the furnace. (Appendix 24) a compression unit for compressing the off-gas, an inlet in fluid flow communication with the second outlet of the hydrogen recovery unit; an outlet in fluid flow communication with the first inlet of the hydrogen recovery unit; 24. The apparatus of claim 23, wherein the apparatus comprises a valve arrangement for controlling flow of off-gas to the compression unit and the off-gas heater. (Appendix 25) 18. The apparatus of claim 17, wherein the catalyst-filled tubes of the furnace and the catalyst bed of the adiabatic reactor do not contain an iron-based catalyst.

Claims

1. 1. A process for decomposing ammonia, comprising: providing heated ammonia gas at superatmospheric pressure; supplying the heated ammonia gas at a first temperature (T1) to a first adiabatic reactor comprising a catalyst bed to decompose a portion of the ammonia and produce an intermediate partially decomposed ammonia gas; heating the intermediate partially decomposed ammonia gas to produce heated intermediate partially decomposed ammonia gas; feeding the heated intermediate partially decomposed ammonia gas, or a heated intermediate partially decomposed ammonia gas derived therefrom, at a second temperature (T2) lower than the first temperature (T1) to a further adiabatic reactor comprising a catalyst bed to decompose a further portion of the ammonia and produce partially decomposed ammonia gas; burning a fuel with an oxidant gas in a furnace to heat reactor tubes containing a catalyst and form a flue gas; feeding the partially decomposed ammonia gas into a reactor tube containing the catalyst to cause further decomposition of ammonia and produce a decomposed gas containing hydrogen gas, nitrogen gas, and residual ammonia; at least a portion, preferably all, of the duty required to heat the intermediate partially decomposed gas is provided by heat exchange with the flue gas; a process wherein a portion of the duty required to provide the heated ammonia gas is provided by heat exchange with the flue gas flow upstream of the heat exchange with the intermediate partially cracked gas.

2. A process for decomposing ammonia, comprising: providing heated ammonia gas at superatmospheric pressure; supplying the heated ammonia gas at a first temperature (T1) to a first adiabatic reactor comprising a catalyst bed to decompose a portion of the ammonia and produce an intermediate partially decomposed ammonia gas; heating the intermediate partially decomposed ammonia gas to produce heated intermediate partially decomposed ammonia gas; feeding the heated intermediate partially decomposed ammonia gas, or a heated intermediate partially decomposed ammonia gas derived therefrom, at a second temperature (T2) lower than the first temperature (T1) to a further adiabatic reactor comprising a catalyst bed to decompose a further portion of the ammonia and produce partially decomposed ammonia gas; burning a fuel with an oxidant gas in a furnace to heat reactor tubes containing a catalyst and form a flue gas; feeding the partially decomposed ammonia gas into a reactor tube containing the catalyst to cause further decomposition of ammonia and produce a decomposed gas containing hydrogen gas, nitrogen gas, and residual ammonia; the partially decomposed ammonia gas is heated to produce heated partially decomposed ammonia gas before being fed to the catalyst-containing reactor tubes of the furnace; A process wherein at least a portion, preferably all, of the duty required to heat said partially decomposed ammonia gas is provided by heat exchange with said decomposed gas.

3. A process for decomposing ammonia, comprising: providing heated ammonia gas at superatmospheric pressure; supplying the heated ammonia gas at a first temperature (T1) to a first adiabatic reactor comprising a catalyst bed to decompose a portion of the ammonia and produce an intermediate partially decomposed ammonia gas; heating the intermediate partially decomposed ammonia gas to produce heated intermediate partially decomposed ammonia gas; feeding the heated intermediate partially decomposed ammonia gas, or a heated intermediate partially decomposed ammonia gas derived therefrom, at a second temperature (T2) lower than the first temperature (T1) to a further adiabatic reactor comprising a catalyst bed to decompose a further portion of the ammonia and produce partially decomposed ammonia gas; burning a fuel with an oxidant gas in a furnace to heat reactor tubes containing a catalyst and form a flue gas; feeding the partially decomposed ammonia gas to a reactor tube containing the catalyst to cause further decomposition of ammonia and produce a cracked gas comprising hydrogen gas, nitrogen gas, and residual ammonia; recovering hydrogen from the cracked gas in a hydrogen recovery unit after cooling to produce a hydrogen gas product and an off-gas comprising nitrogen gas, residual hydrogen gas, and residual ammonia gas; heating at least a portion of the off-gas to produce a heated off-gas; supplying the heated off-gas to the furnace as at least a portion of the fuel; dividing the off-gas into a first portion and a second portion, the first portion being heated and fed to the furnace; compressing the second portion of the off-gas to produce a compressed off-gas, and recycling the compressed off-gas to the hydrogen recovery unit for further hydrogen recovery.

4. 4. The process of any one of claims 1 to 3, wherein the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst.

5. The process of any one of claims 1 to 3, wherein the first temperature (T1) is in the range of about 550°C to about 650°C.

6. 4. The process of any one of claims 1 to 3, wherein the catalyst bed of the further adiabatic reactor comprises a catalyst selected from nickel-based catalysts and ruthenium-based catalysts.

7. The process of any one of claims 1 to 3, wherein the second temperature (T2) is in the range of about 450°C to about 550°C.

8. pumping liquid ammonia containing at least 0.1 mole percent water 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 pressurized ammonia gas to produce the superatmospheric pressure heated ammonia gas; 4. The process of any one of claims 1 to 3, wherein the water from the liquid ammonia is present in the heated ammonia gas.

9. 9. The process of claim 8, wherein the water is present in the heated ammonia in an amount of 1 mole percent or less.

10. 3. The process of claim 2, wherein at least a portion, preferably more than half, of the heating duty required to provide the heated ammonia gas is provided by heat exchange with the decomposed gas stream downstream of the heat exchange with the partially decomposed ammonia gas.

11. 4. The process of any one of claims 1 to 3, wherein the catalyst-filled tubes of the furnace and the catalyst bed of the adiabatic reactor do not contain an iron-based catalyst.

12. 4. The process of claim 3, wherein all of the off-gas is supplied as fuel to the furnace.

13. 1. An apparatus for decomposing ammonia, comprising: a first adiabatic reactor for partially decomposing heated ammonia gas at superatmospheric pressure, the first adiabatic reactor comprising an inlet for heated ammonia gas and a catalyst bed having an upstream end in fluid flow communication with the inlet and a downstream end in fluid flow communication with an outlet for intermediate partially decomposed ammonia gas; a further adiabatic reactor for decomposing the heated intermediate partially decomposed ammonia gas, or the heated intermediate partially decomposed ammonia gas derived therefrom, the further adiabatic reactor comprising: an inlet in fluid flow communication with the outlet of the first adiabatic reactor; and a catalyst bed having an upstream end in fluid flow communication with the inlet and a downstream end in fluid flow communication with an outlet for partially decomposed ammonia gas; A furnace, a radiant section comprising at least one inlet for fuel and oxidant gas in fluid flow communication with at least one burner, a catalyst-containing reactor tube having an upstream end in fluid flow communication with the outlet of the further adiabatic reactor and a downstream end in fluid flow communication with an outlet for cracked gas; and a convection section in fluid flow communication with the radiant section and including an outlet for flue gases; a first heat exchanger for heating ammonia gas by heat exchange with flue gas in the convection section of the furnace, the first heat exchanger comprising an inlet for ammonia gas and an outlet in direct fluid flow communication with the inlet of the first adiabatic reactor; a second heat exchanger for heating intermediate partially decomposed ammonia gas by heat exchange with flue gas in the convection section of the furnace, the second heat exchanger having an inlet in direct fluid flow communication with the outlet of the first adiabatic reactor and an outlet in fluid flow communication with the inlet of the further adiabatic reactor; the first heat exchanger is positioned upstream of the second heat exchanger in the convection section of the furnace relative to the flow of flue gas.

14. 14. The apparatus of claim 13, wherein the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst.

15. 14. The apparatus of claim 13, wherein the catalyst bed of the further adiabatic reactor comprises a catalyst selected from a nickel-based catalyst and a ruthenium-based catalyst.

16. 14. The apparatus of claim 13, comprising a further heat exchanger positioned to heat partially decomposed ammonia gas by heat exchange with the decomposed gas positioned between the further adiabatic reactor and the radiant section of the furnace.

17. 17. The apparatus of claim 16, wherein the further heat exchanger is a shell-and-tube heat exchanger.

18. 18. The apparatus of claim 17, wherein the shell side of the heat exchanger is in direct fluid flow communication with the outlet of the radiant section of the furnace.

19. a hydrogen recovery unit, preferably a PSA unit, for recovering hydrogen gas from the cracked gas, comprising: a first inlet in fluid flow communication with the cracked gas outlet of the radiant section of the furnace; a first outlet for hydrogen gas; a second outlet for off-gas comprising nitrogen gas, residual ammonia gas, and residual hydrogen gas in fluid flow communication with the at least one inlet for fuel in the radiant section of the furnace; 14. The apparatus of claim 13, wherein the apparatus comprises an off-gas heater positioned to heat off-gas and positioned between the second outlet of the hydrogen recovery unit and the at least one inlet for fuel in the radiant section of the furnace.

20. a compression unit for compressing the off-gas, an inlet in fluid flow communication with the second outlet of the hydrogen recovery unit; an outlet in fluid flow communication with the first inlet of the hydrogen recovery unit; 20. The apparatus of claim 19, wherein the apparatus comprises a valve arrangement for controlling the flow of off-gas to the compression unit and the off-gas heater.

21. 14. The apparatus of claim 13, wherein the catalyst-filled tubes of the furnace and the catalyst bed of the adiabatic reactor do not contain an iron-based catalyst.

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