Process and apparatus for cracking ammonia

Abstract

The invention relates to a process and apparatus for cracking ammonia, wherein heated ammonia gas at superatmospheric pressure is partially cracked in at least two adiabatic reactors in series with interstage heating, wherein the feed temperature of the first reactor is higher than the feed temperature of the further reactors, to produce partially cracked ammonia gas which is then fed to a catalyst-containing reactor tube in a furnace to produce cracked gas comprising hydrogen, nitrogen and residual ammonia gas. The use of adiabatic reactors makes the heat integration in the process more efficient, and the higher temperature in the first reactor enables the use of a nickel-based catalyst in that reactor as an alternative solution to the potential problem of the presence of oil in the ammonia.

Description

Technical Field

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

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

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

[0004]

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

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

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

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

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

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

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

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

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

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

[0015] According to a first aspect of the invention, a method for cracking ammonia is provided, the method comprising: providing heated ammonia gas at ultra-atmospheric pressure; feeding the heated ammonia gas at a first temperature (T1) into a first adiabatic reactor including a catalyst bed to crack a portion of the ammonia and produce partially cracked ammonia gas; heating the partially cracked ammonia gas to produce heated partially cracked ammonia gas; feeding the heated partially cracked ammonia gas or heated partially cracked ammonia gas derived therefrom into a further adiabatic reactor including a catalyst bed at a second temperature (T2) below the first temperature (T1) to crack a further portion of the ammonia and produce partially cracked ammonia gas; burning fuel with an oxidant gas in a furnace to heat a catalyst-containing reactor tube and form flue gas; and feeding the partially cracked ammonia gas into the catalyst-containing reactor tube to induce further cracking of ammonia to produce a cracked gas comprising hydrogen, nitrogen and residual ammonia.

[0016] Before the cracking process is completed in the catalyst-filled tube of the combustion furnace, ammonia is partially cracked in an adiabatic reactor comprising at least one ammonia cracking catalyst bed. This provides an opportunity to optimize the cracking process by making full use of the thermal energy in the cracked gas and flue gas, resulting in reduced energy consumption, increased hydrogen production, and / or reduced dependence on hydrocarbon fuels such as natural gas.

[0017] Furthermore, the higher feed temperature of the first adiabatic reactor allows for the use of catalysts containing less catalytically active metals, such as nickel-based catalysts, instead of catalysts containing more catalytically active metals, such as ruthenium-based catalysts. Therefore, in a preferred embodiment, the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst, for example, containing or composed of a nickel-based catalyst. Catalysts containing less catalytically active metals, such as nickel, are often less expensive than catalysts containing more catalytically active metals, such as ruthenium. Therefore, these embodiments may be significantly less expensive than embodiments where the bed of the first adiabatic reactor comprises a ruthenium-based catalyst, especially if the catalyst bed requires periodic replacement due to degradation and / or contamination by the oil present in ammonia.

[0018] The catalyst bed of the second adiabatic reactor may include an upstream layer of a less active catalyst, such as a nickel-based catalyst, and a downstream layer of a more active catalyst, such as a ruthenium-based catalyst. For example, it may consist of an upstream layer of a less active catalyst, such as a nickel-based catalyst, and a downstream layer of a more active catalyst, such as a ruthenium-based catalyst, or it may be composed of an upstream layer of a less active catalyst, such as a nickel-based catalyst, and a downstream layer of a more active catalyst, such as a ruthenium-based catalyst. Similarly, due to the cost differences in catalytically active metals, these embodiments may be significantly cheaper than other devices involving the use of more highly active catalysts.

[0019] In some embodiments, the use of highly active catalysts, such as ruthenium-based catalysts, may even be completely eliminated. In these embodiments, the bed of the second adiabatic reactor may include a less active catalyst, such as a nickel-based catalyst, as the sole catalyst, resulting in further cost reduction.

[0020] According to a second aspect of the invention, an apparatus for cracking ammonia is provided, the apparatus comprising: a first adiabatic reactor for partially cracking heated ammonia gas under atmospheric pressure, the reactor including an inlet for the heated ammonia gas, a catalyst bed having an upstream end in fluid communication with the inlet and a downstream end in fluid communication with an outlet of partially cracked ammonia gas; a further adiabatic reactor for cracking heated partially cracked ammonia gas or heated partially cracked ammonia gas derived therefrom, the reactor including an inlet in fluid communication with the outlet of the first adiabatic reactor, a catalyst bed having an upstream end in fluid communication with the inlet and a downstream end in fluid communication with the outlet of the partially cracked ammonia gas; a furnace; a radiant section including at least one fuel and oxidant gas inlet in fluid communication with at least one burner, and a catalyst-containing reactor tube; and so on. The reactor tube has an upstream end in fluid flow communication with the outlet of the additional adiabatic reactor and a downstream end in fluid flow communication with the outlet of the cracked gas; and a convection section in fluid flow communication with the radiation section, including an outlet of flue gas; a first heat exchanger for heating ammonia by heat exchange with flue gas in the convection section of the furnace, the first heat exchanger including an inlet for ammonia and an outlet in direct fluid flow communication with the inlet of the first adiabatic reactor; and a second heat exchanger for heating intermediate cracked ammonia 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 additional adiabatic reactor, wherein the first heat exchanger is located upstream of the second heat exchanger within the convection section of the furnace relative to the flue gas flow.

[0021] The apparatus described in the second aspect of the invention is particularly suitable for carrying out the method described in the first aspect of the invention. Attached Figure Description

[0022] Figure 1 This is a simplified flowchart of an embodiment of the present invention. Detailed Implementation

[0023] Unless otherwise stated, the amounts of components given in parts per million (ppm) are by weight. Furthermore, unless otherwise stated, all percentages are calculated in moles. Additionally, unless otherwise stated, any reference to pressure is a reference to absolute pressure.

[0024] In the context of this invention, catalyst activity will be understood as the conversion of ammonia at a given partial pressure for a given amount of catalyst over a given time period at a specific temperature and total pressure. The unit used to define the activity of heterogeneous catalysts is the number of moles (or mol·g) of ammonia converted per second per gram of catalyst (including the substrate, if present). -1 s -1 ).

[0025] The expression "fluid flow connectivity" will be understood to mean that a pipe or other suitable conduit will be used to transport fluid from one specified location to another. During transit between the two locations, the fluid may flow through one or more other units that may be designed and / or arranged to alter the physical state of the fluid, such as temperature (e.g., heat exchangers) and / or pressure (e.g., compressors or pumps), or to change the composition of the fluid through reactions of components within the fluid (e.g., catalytic reactors). The expression "direct fluid flow connectivity" will be understood to mean that the fluid flows directly from one location to another, i.e., without passing through another such unit during its transit, and therefore the composition or physical state of the fluid remains substantially unchanged.

[0026] The term "extra-atmospheric pressure" will be understood to mean a pressure significantly higher than atmospheric pressure, such as at least 5 bar, for example at least 10 bar, or at least 20 bar or at least 30 bar. Typically, the pressure does not exceed 60 bar.

[0027] The term "upstream" will be understood to refer to the opposite direction of fluid flow during normal operation. The term "downstream" will be interpreted accordingly.

[0028] The heated ammonia feed of this invention is typically generated from liquid ammonia, which can be supplied at ambient pressure via pipeline or, more commonly, from refrigerated storage tanks. Water is often added to the ammonia to prevent stress corrosion cracking in the tanks, trucks, and ships used for transporting the ammonia. The presence of water in the feed ammonia transforms the feed into a multi-component stream, requiring higher temperatures for complete evaporation of the feed stream.

[0029] Typical compositions of ammonia feed are shown in Table 1.

[0030] Components composition ammonia 99.5–99.8 wt% Maximum water content 0.5wt% Minimum water content 0.2wt% Oil 5ppm maximum Inert gases (e.g., Ar) <10ppm iron <1ppm

[0031] Table 1

[0032] Oil may be present in ammonia due to the evaporative gas compressor used for storing ammonia in local tanks, at the production site, or any other tanks in between. The presence of oil is a problem because it poses a risk of blockage and / or contamination. This can lead to poor heat exchanger performance or reduced catalyst activity in the reactor. Therefore, if present, the oil may need to be removed in some way. In this regard, oil can be removed by passing liquid ammonia through a bed of activated carbon. However, in a preferred embodiment, the catalyst used in the adiabatic reaction unit cracks the oil into shorter-chain hydrocarbons, which can react with any water present to form carbon monoxide, hydrogen, and methane.

[0033] Inert gases are not expected to be a problem unless they end up in the product hydrogen. In this regard, helium can be present in ammonia derived from natural gas, but ammonia derived from renewable hydrogen will not contain helium.

[0034] Liquid ammonia is typically drawn from a storage tank and pumped from a storage pressure (e.g., about 1 bar) to a pressure ranging from about 5 bar to about 60 bar, for example, from about 10 bar to about 50 bar, such as from about 10 bar to about 30 bar, or from about 40 bar to about 50 bar. The temperature of the liquid ammonia is slightly higher than the storage temperature (e.g., about -34°C) to about -32°C. If the liquid ammonia is drawn from a pipeline, the temperature of the liquid ammonia is typically higher, for example, about +10°C.

[0035] The pumped liquid ammonia (under extra-atmospheric pressure) is then typically preheated to its boiling point, ideally through appropriate thermal integration during the process. Preferably, partial preheating is achieved using a heat transfer loop, where a heat transfer fluid such as an aqueous solution of a glycol (e.g., comprising about 50 wt% to about 60 wt% of a glycol, such as ethylene glycol or propylene glycol) is used to recover heat, for example, from the intermediate and post-cooling of the PSA exhaust gas compressor, optionally along with heat from the flue gas and / or cracked gases, and is used to heat the liquid ammonia. If such integration is not possible, for example if the compressor is not operating, heat from an external source (such as an electric heater) may be required to preheat the ammonia.

[0036] The preheated liquid ammonia is typically then evaporated, and the resulting ammonia gas is further heated before being fed into the adiabatic reaction unit. In this respect, the ammonia gas is typically superheated, i.e., heated to temperatures above its boiling point, reaching temperatures above 350°C, to ensure an efficient reaction rate in the adiabatic reaction unit.

[0037] The evaporation of preheated liquid ammonia and the load for further heating can be provided through heat exchange with cracked gas, flue gas, or a combination of both. In a preferred embodiment, cracked gas is used to heat and evaporate the preheated liquid ammonia via heat exchange, and then the ammonia is further heated through heat exchange with flue gas.

[0038] The use of an adiabatic reactor unit allows some ammonia gas to be cracked before entering the catalyst-filled reactor tubes of the furnace. The molar fraction of ammonia in the gas passing through the adiabatic reactor is typically reduced by at least 20%, for example, at least 25%, or at least 30%, or at least 35%, or at least 40%, and / or up to about 50%. In other words, the molar fraction of ammonia can be reduced from 1 (or almost 1) in heated ammonia gas to an amount ranging from about 0.5 to about 0.8 in partially cracked ammonia gas, or to an amount ranging from about 0.5 to about 0.7, or to an amount ranging from about 0.55 to about 0.65. In some embodiments, the molar fraction of ammonia in the partially cracked ammonia gas is in the range of about 0.58 to about 0.62, for example, about 0.6.

[0039] The adiabatic reactor is incorporated into the process design to improve overall efficiency, particularly by using the 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 the recovery of heat from the flue gas, while avoiding temperatures above approximately 660°C due to material considerations.

[0040] The key design parameter for adiabatic reactors is the inlet temperature. Because ammonia cracking is endothermic, higher inlet temperatures allow for greater conversion rates within the unit. However, higher temperatures place greater demands on the materials and catalysts used in the structure. The inlet temperature of each adiabatic reactor is therefore typically in the range of approximately 350°C to approximately 800°C, with the inlet temperature of the first adiabatic reactor potentially ranging from approximately 500°C to approximately 700°C, or from approximately 550°C to approximately 650°C. Furthermore, the inlet temperatures of other adiabatic reactors range from approximately 400°C to approximately 600°C or from approximately 450°C to approximately 550°C.

[0041] This invention requires the use of at least two adiabatic reactors, each including a catalyst bed. One or more additional adiabatic reactors may be included, bringing the total number of adiabatic reactors to, for example, three, four, five, or six, with interstage heating where appropriate. Depending on process requirements, the additional reactors may be arranged in series, in parallel, or a combination of series and parallel. However, in a preferred embodiment, the adiabatic reaction unit will have two such reactors arranged in series, which preferably provide interstage heating of the intermediately cracked ammonia gas through heat exchange with the cracked gas and / or flue gas.

[0042] Each adiabatic reactor has a bed comprising at least one catalyst suitable for ammonia cracking. Any conventional ammonia cracking catalyst can be used in the adiabatic reactor or in the bed of each adiabatic reactor.

[0043] Numerous metals are known in the art for catalyzing the cracking of ammonia. These metals include transition metals, such as those in Group 6 of the periodic table, such as chromium (Cr) and molybdenum (Mo); Group 8, such as iron (Fe), ruthenium (Ru), and osmium (Os); Group 9, such as cobalt (Co), rhodium (Rh), and iridium (Ir); Group 10, such as nickel (Ni), palladium (Pd), and platinum (Pt); and Group 11, such as copper (Cu), silver (Ag), and gold (Au). Metalloids such as tellurium (Te) may also be used.

[0044] Masel et al. (Catalyst Letters, Vol. 96, Nos. 3-4, July 2004) reported that the activity of some of these metals as catalysts for ammonia cracking varies in the following order:

[0045] Ru>Ni>Rh>Co>Ir>Fe>>Pt>Cr>Pd>Cu>>Te

[0046] Metals can be unsupported, but are typically supported on a suitable support (or substrate), usually a metal oxide support such as silicon dioxide (SiO2), alumina (Al2O3), zirconium oxide (ZrO2), or a mixed metal oxide support such as spinel (MgAl2O4) or perovskite (CaTiO3). Alternatively, metals can be supported on zeolites.

[0047] As those skilled in the art will understand, the activity of a supported metal catalyst will typically depend in part on the amount of catalytically active metal loaded on the support. In this respect, the metal loading will vary depending on specific requirements, but is generally in the range of about 0.1 wt% to about 70 wt%. For more active metals (e.g., ruthenium), the loading can be closer to the lower limit of this range, for example, about 0.1 wt% to about 10 wt% or about 0.2 wt% to about 5 wt%. For less active metals (e.g., nickel), the loading can be closer to the upper limit of this range, for example, about 20 wt% to about 65 wt%.

[0048] The supported metal catalyst may be unpromoted or promoted with at least one other metal to improve its activity, such as one or more Group 1 metals, such as lithium (Li), sodium (Na) and potassium (K); Group 2 metals, such as magnesium (Mg) and calcium (Ca); or Group 13 metals, such as aluminum (Al).

[0049] Any conventional catalyst known for ammonia cracking can be used in this invention. Suitable catalysts are disclosed in US2015 / 0217278A, Masel et al. (above), Lamb et al. (International Journal of Hydrogen Energy, 44 (2019), pp. 3726-3736), and Boisen et al. (J. Catalysis, 230 (2005), pp. 309-312).

[0050] Bimetallic catalysts or catalysts containing two catalytically active metals are also suitable for this invention. Examples include composite metals or metal alloys or metal nanoclusters supported on perovskites, composite oxides or nitrides, or mixed oxides or mixed nitrides disclosed in US2021 / 0001311A, such as CoNi-MgSrCeO4 and 1 wt% K-CoNi-MgSrCeO4.

[0051] The catalysts of the present invention typically comprise at least one metal-based catalyst, such as a catalyst containing at least one metal-based catalyst or composed of at least one metal-based catalyst. The catalytically active metal is typically selected from transition metals of the periodic table. Suitable transition metal-based catalysts exhibit activity at temperatures ranging from 475°C to 600°C suitable for achieving a reaction rate at least 0.2 times, at least 0.5 times, or at least equal to, at least two times, at least three times, at least four times, or at least five times the rate calculated according to Equation 9 proposed by Lamb et al., i.e.,

[0052] r = 8.73exp[-76710 / RT].(P) NH3 ) 0.28 .(P H2 ) -0.42 .(1-β 2 )

[0053] in:

[0054] “r” represents the reaction rate (or “activity”) of the catalyst;

[0055] "RT" is the ideal gas constant "R" (8.314 J mol). -1 K -1 Multiply by the temperature "T" in Kelvin;

[0056] P NH3 It is the partial pressure of ammonia;

[0057] P H2 It is the partial pressure of hydrogen gas;

[0058] β is defined in the paper as (see equation 5 proposed by Lamb et al.):

[0059]

[0060] P N2 It is the partial pressure of nitrogen; and

[0061] K e It is the equilibrium constant of the reaction (see equations 6 and 7 proposed by Lamb et al.).

[0062] The inventors have recognized that Equation 9 of Lamb et al. can be extrapolated to temperatures beyond 475°C to 600°C, for example, in the range of 450°C to 700°C.

[0063] The primary catalytically active metal of the catalyst, which may be particularly suitable for use in catalyst beds of adiabatic reactors, is selected from chromium, manganese, iron, cobalt, nickel, ruthenium, and copper, such as iron, cobalt, nickel, and ruthenium. The inventors understand that nickel and ruthenium are generally the most suitable catalytically active metals for use in catalyst beds of adiabatic reaction units.

[0064] The term "ruthenium-based catalyst" refers to a catalyst in which ruthenium is the sole (or at least the primary) catalytically active metal (i.e., the metal responsible for the catalytic cracking reaction). Ruthenium may be the only metal in the catalyst, or alternatively, one or more other metals may be present, for example, in a ruthenium-supported material. The terms "nickel-based catalyst" and "iron-based catalyst" should be interpreted accordingly.

[0065] Suitable ruthenium-based and nickel-based catalysts can be supported on, for example, alumina (as disclosed by Lamb et al. or Masel et al.) or spinel (as disclosed by Boisen et al.), optionally promoted by group 1 or group 2 metals.

[0066] Ruthenium-based catalysts are generally more active than nickel-based catalysts, but are also more expensive. Therefore, further optimization is possible through catalyst selection, and if more than one type of catalyst is used, through the sequencing of the catalyst layers within the bed of the adiabatic reaction unit. Suitable catalysts include conventional nickel-based or ruthenium-based catalysts used for ammonia cracking.

[0067] In an embodiment having two adiabatic reactors in series, the catalyst bed of the first reactor typically comprises a single layer of a first catalyst, such as a nickel-based catalyst, or consists of a single layer of a first catalyst, such as a nickel-based catalyst, and the catalyst bed of the second reactor typically comprises an upstream layer of a second catalyst, such as a nickel-based catalyst, and a downstream layer of a third catalyst, such as a ruthenium-based catalyst, or consists of an upstream layer of a second catalyst, such as a nickel-based catalyst, and a downstream layer of a third catalyst, such as a ruthenium-based catalyst, or consists of an upstream layer of a second catalyst, such as a nickel-based catalyst, and a downstream layer of a third catalyst, such as a ruthenium-based catalyst. The second catalyst typically has similar activity to the first catalyst, and the third catalyst is typically more active than the first and second catalysts.

[0068] In these embodiments, the first catalyst and the second catalyst may be the same. Alternatively, the first catalyst and the second catalyst may be different, for example, containing different catalytically active metals, or containing the same catalytically active metal but on different supports, or containing the same catalytically active metal on the same support but at different loadings.

[0069] The volume of the upstream layer of the second catalyst in the bed of the second reactor can be about 40% to about 90% of the total bed volume, for example, about 50% to about 70% or about 60%. In the absence of another catalyst layer, the volume of the downstream layer of the third catalyst in the bed of the second reactor can be about 10% to about 60% of the total bed volume, for example, about 30% to about 50% or about 40%.

[0070] The inventors have recognized that ruthenium-based catalysts can crack hydrocarbon oils into shorter hydrocarbons, such as methane, as well as carbon monoxide and hydrogen. Therefore, using these catalysts in an adiabatic reaction unit eliminates the need for a dedicated upstream unit for removing oil from liquid ammonia.

[0071] The reactor tubes of the furnace are also filled with ammonia cracking catalyst. Any of the conventional ammonia cracking catalysts mentioned above can be used in the reactor tubes. However, for adiabatic reactors, particularly suitable catalysts for the reactor tubes include nickel-based and ruthenium-based catalysts.

[0072] In some embodiments, particularly those in which the cracking reaction occurs at higher temperatures, the reactor tube may be filled with a less active catalyst, such as a nickel-based catalyst, as the sole catalyst in the tube. Less active catalysts tend to be cheaper than more active catalysts, and therefore, this arrangement helps to reduce total capital costs.

[0073] However, the reactor tube can be filled with at least two different ammonia cracking catalysts with different activities.

[0074] In these embodiments, a more active catalyst, such as a ruthenium-based catalyst, can be located in a tube downstream of a less active catalyst, such as a nickel-based catalyst, to ensure that the cracking reaction is close to equilibrium.

[0075] Alternatively, a more active catalyst, such as a ruthenium-based catalyst, can be positioned upstream of a less active catalyst, such as a nickel-based catalyst, to help control the inner wall temperature of the reactor tubes in the region of highest ammonia temperature and partial pressure, thereby controlling the nitriding of the tubes. In a preferred embodiment, the catalysts can be stratified in this manner to utilize the endothermic reaction within the tubes to keep the tube metal cool in the region of most intense combustion outside the tubes. Because ruthenium is more catalytically active than nickel, it generates stronger endothermic reaction, which keeps the inner tube wall cooler in the high ammonia concentration region on the process side, thus protecting the tubes from over-nitriding caused by high ammonia concentration and high temperature.

[0076] In these alternative embodiments, a second, more active catalyst, such as another ruthenium-based catalyst, can be located downstream of the less active catalyst to ensure that the cracking reaction is close to equilibrium.

[0077] The activity of a more active catalyst is typically at least 50% higher than that of a less active catalyst. However, the difference in relative activity is often much greater than 50%. In this respect, the activity of a more active catalyst is typically at least two times (i.e., twice), or at least three times, or at least four times, or at least five times greater than that of a less active catalyst. In some embodiments, the activity of a more active catalyst is at least ten times (i.e., an order of magnitude greater) or at least fifteen times greater than that of a less active catalyst.

[0078] 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. In these embodiments, where a third catalyst is present downstream of the nickel-based catalyst, this catalyst is preferably a ruthenium-based catalyst, although it may be a different ruthenium-based catalyst from the first catalyst, for example, with a different support and / or catalyst loading, and / or, if both are promoted, with different metals.

[0079] The catalyst in the reactor tube may be the same as or different from the catalyst used in the adiabatic reaction unit.

[0080] It is also known that catalyst sintering at higher temperatures reduces catalyst activity and lifetime. In this regard, technicians will recognize the need to balance improved conversion with higher container costs and shorter catalyst lifetime.

[0081] As mentioned above, water is typically present as a contaminant in ammonia. Water can be removed from ammonia, in which case water-intolerant catalysts, such as iron-based catalysts, can be used in adiabatic reaction units and / or reactor tubes. However, in preferred embodiments, water removal is not performed to save money and operating costs and reduce energy consumption. In these embodiments, water-intolerant catalysts, such as iron-based catalysts, are generally not used. Instead, the catalyst in the reactor tubes will be able to tolerate up to 1 mol% water in the ammonia feed. Such catalysts include nickel-based and ruthenium-based catalysts.

[0082] The combustion process in the furnace is preferably fueled at least partially internally, i.e., at least part of the fuel is ammonia or waste gas generated during hydrogen recovery from cracked gases, or a mixture of both. However, trimmed fuels, such as C1 to C3 hydrocarbons or natural gas, can be used as needed, although the use of hydrocarbon trimmed fuels increases the carbon intensity of the process. Therefore, it is generally desirable to minimize the use of such trimmed fuels while maintaining or even improving hydrogen recovery rates.

[0083] The oxidizing gas is usually air, but depending on the circumstances, it can also be oxygen-enriched gas or pure oxygen.

[0084] If the reactor wall material can withstand higher temperatures, partially cracked ammonia can be fed into the catalyst-filled tubes of the furnace at temperatures up to about 800°C. For lower temperature cycles, the feed is typically in the range of about 400°C to about 600°C or about 450°C to about 550°C, for example, at about 500°C. For higher temperature cycles, the feed can be in the range of about 500°C to about 800°C or about 600°C to about 700°C, for example, at about 650°C.

[0085] Cracking temperature and pressure typically determine that ammonia slip in the reactor tubes does not exceed 3 mol%, for example, from about 0.5 mol% to 1.5 mol%.

[0086] The heat from the cracked gas and flue gas is then used to heat the feed stream to the adiabatic reactor and furnace, thereby reducing the total energy consumed in the process. In this respect, the temperature of the cracked gas will depend on the cycle in operation.

[0087] In lower-temperature cycles, the temperature of the cracked gas can reach as high as about 700°C, typically from about 550°C to about 700°C, or from about 600°C to about 650°C. The temperature of the flue gas can reach as high as about 750°C at its highest point. However, due to heat leakage, the temperature drops, and at the point where its heat can be effectively utilized, the temperature is typically from about 600°C to about 700°C.

[0088] In higher-temperature cycles, the temperature of the cracked gas can reach as high as approximately 750°C, for example, typically from approximately 650°C to approximately 750°C, or from approximately 675°C to approximately 725°C. The temperature of the flue gas can reach as high as approximately 840°C at its highest point. However, due to heat leakage, the temperature decreases, and at the point where its heat can be effectively utilized, the temperature is typically from approximately 700°C to approximately 800°C.

[0089] At least some of the load 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 through heat exchange with the cracked gas. In a preferred embodiment, the cracked gas is used directly to provide this heating load. In other words, the cracked gas is typically not used elsewhere, such as for heating another process fluid, before being heated. Some of this heating load may be provided in another manner, such as through heat exchange with flue gas. However, the majority, or more than 50%, of this heating load is typically provided by the cracked gas. In a preferred embodiment, at least 75%, or at least 90%, or all of this heating load is provided by the cracked gas.

[0090] Due to the high ammonia concentration in the feed gas, cracking reactor vessels, such as reactor tubes in adiabatic reactors and furnaces, are typically constructed of materials resistant to ammonia and / or nitriding, especially when higher cracking temperatures are used. Suitable materials include nickel-based alloys comprising at least 40 wt% or at least 50 wt% nickel. Such alloys typically have no more than 90 wt% or 80 wt% nickel. The alloy will usually include one or more other metals selected from chromium, cobalt, molybdenum, and iron.

[0091] 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.

[0092] The Uniform Numbering System (UNS) is a widely accepted alloy naming system in North America. Each UNS number is associated with a specific metal or alloy and defines its specific chemical composition, or in some cases, its specific mechanical or physical properties.

[0093] Other suitable materials include cobalt-based alloys, such as UNS R30188. Additionally, high-temperature alloys with lower resistance to nitriding, such as UNS N08811, or cast alloys like HPNb, HP microalloyed, and MA-1 (MetalTek International, USA), may be suitable, especially when surface-modified or coated with corrosion-resistant layers such as aluminizing; aluminizing followed by pre-oxidation; or ceramic coatings. Nitriding-resistant alloys can also be used in conjunction with surface modifications or coatings to improve performance.

[0094] In preferred embodiments, the composition of ammonia generally remains at least substantially unchanged from the stored liquid ammonia to the heated ammonia gas fed into the adiabatic reactor. Oil present in the liquid ammonia may be removed sometime before the partial cracking of the ammonia, although in embodiments where the ammonia feed to the adiabatic reactor first encounters the ruthenium-based catalyst, oil removal is not required. In these embodiments, water is generally not removed, so any water present in the liquid ammonia will also be present in the heated ammonia gas.

[0095] The cracked gas is cooled during heat exchange with partially cracked gas. The cooled cracked gas is then typically further cooled by providing at least a portion of the heating load required to generate heated ammonia from liquid ammonia. After cooling, hydrogen can be recovered from the cracked gas as a product. Recovery can be achieved in a pressure swing adsorption (PSA) process, or by using one or more selective permeation membranes, or by a combination of PSA and membrane separation. In a preferred embodiment, hydrogen recovery is achieved only in the PSA process, i.e., without the use of membrane separation.

[0096] In embodiments using the PSA process, PSA exhaust gas is generated, comprising nitrogen, residual ammonia, and residual hydrogen. The PSA exhaust gas is typically divided into two parts. The first part is usually compressed in a compression unit and recycled back into the PSA process to improve hydrogen recovery. The second part is typically preheated and then fed into the furnace as fuel.

[0097] Several factors influence the carbon intensity of cracking processes, two of which are the amount of ammonia allowed to escape through the cracker and the nature of the fuel burned in the cracker, particularly whether hydrocarbons such as natural gas are used, and if so, how much. The inventors have discovered that, for a given amount of ammonia escape and natural gas burned in the cracker, the overall carbon intensity of the process can be reduced if cracked gas (rather than flue gas) is used to provide at least some of the load required to heat the partially cracked gas to the cracker feed temperature.

[0098] Aspects of the present invention include:

[0099] #1. A method for cracking ammonia, the method comprising:

[0100] Provides heated ammonia gas under ultra-atmospheric pressure;

[0101] The heated ammonia gas at a first temperature (T1) is fed into a first adiabatic reactor comprising a catalyst bed to crack a portion of the ammonia and produce intermediately cracked ammonia gas.

[0102] Heating the intermediate cracked ammonia gas to produce heated intermediate cracked ammonia gas;

[0103] At a second temperature (T2) lower than the first temperature (T1), the heated intermediate cracked ammonia or the heated intermediate cracked ammonia derived therefrom is fed into another adiabatic reactor including a catalyst bed to crack another portion of the ammonia and produce partially cracked ammonia.

[0104] In a furnace, fuel is burned using oxidant gas to heat a catalyst-containing reactor tube and generate flue gas; and

[0105] The partially cracked ammonia gas is fed into the catalyst-containing reactor tube to induce further cracking of the ammonia, producing a cracked gas comprising hydrogen, nitrogen, and residual ammonia.

[0106] #2. The method according to #1, wherein the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst.

[0107] #3. The method according to #1 or #2, wherein the first temperature (T1) is in the range of about 550°C to about 650°C.

[0108] #4. The method according to any one of #1 to #3, wherein the catalyst bed of the additional adiabatic reactor comprises at least one catalyst selected from nickel-based catalysts and ruthenium-based catalysts.

[0109] #5. The method 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.

[0110] #6. The method according to any one of #1 to #5, wherein at least some, preferably all, of the load required to heat the intermediate cracked gas is provided by heat exchange with the flue gas.

[0111] #7. The method according to #6, wherein a portion of the load required to provide the heated ammonia is provided by heat exchange with the flue gas upstream of the heat exchange with the partially cracked gas relative to the flue gas flow.

[0112] #8. The method according to any one of #1 to #7, wherein the partially cracked ammonia is heated to produce heated partially cracked ammonia before being fed into the catalyst-containing reactor tube of the furnace.

[0113] #9. The method according to #8, wherein at least some of the load required to heat the partially cracked ammonia is preferably provided entirely by heat exchange with the cracked gas.

[0114] #10. The method according to any one of #1 to #9, the method comprising:

[0115] Pumping liquid ammonia containing at least 0.1 mol% water to produce pumped liquid ammonia;

[0116] The pumped liquid ammonia is preheated to produce preheated liquid ammonia;

[0117] Evaporating the preheated liquid ammonia to produce ammonia gas; and

[0118] Heating pressurized ammonia gas to produce the heated ammonia gas at atmospheric pressure.

[0119] Water from the liquid ammonia is present in the heated ammonia gas.

[0120] #11. The method according to #10, wherein water is present in the heated ammonia in an amount not exceeding 1 mol%.

[0121] #12. The method according to #10 or #11, wherein at least some, preferably most, i.e. more than 50%, of the heating load required to provide the heated ammonia is provided by heat exchange with the cracked gas downstream of the heat exchange with the partially cracked ammonia relative to the cracked gas stream.

[0122] #13. The method according to any one of #1 to #12, wherein the catalyst-filled tube of the furnace and the catalyst bed of the adiabatic reactor do not contain iron-based catalysts.

[0123] #14. The method according to any one of #1 to #13, the method further comprising:

[0124] After cooling to, for example, below 60°C, such as to about 50°C, hydrogen is recovered from the cracked gas in the hydrogen recovery unit to produce hydrogen products and waste gas including nitrogen, residual hydrogen and residual ammonia.

[0125] Heating at least a portion of the exhaust gas to produce heated exhaust gas; and

[0126] The heated exhaust gas is fed into the furnace as at least a portion of the fuel.

[0127] #15. The method according to #14, wherein all of the exhaust gas is fed into the furnace as fuel.

[0128] #16. The method according to #14 or #15, the method comprising:

[0129] The exhaust gas is divided into a first portion and a second portion, wherein the first portion is heated and fed into the furnace; and

[0130] The second portion of the exhaust gas is compressed to produce compressed exhaust gas, and the compressed exhaust gas is recycled to the hydrogen recovery unit for further hydrogen recovery.

[0131] #17. An apparatus for cracking ammonia, the apparatus comprising:

[0132] A first adiabatic reactor is used for the partial cracking of heated ammonia under ultra-atmospheric pressure. The reactor includes an inlet for the heated ammonia and a catalyst bed having an upstream end in fluid communication with the inlet and a downstream end in fluid communication with the outlet of the partially cracked ammonia.

[0133] An additional adiabatic reactor is used for cracking heated intermediate partially cracked ammonia or heated intermediate partially cracked ammonia derived therefrom, the reactor comprising an inlet in fluid flow communication with the outlet of the first adiabatic reactor, a catalyst bed having an upstream end in fluid flow communication with the inlet and a downstream end in fluid flow communication with the outlet of the partially cracked ammonia.

[0134] Furnace, the furnace comprising:

[0135] A radiant section, the radiant section comprising at least one fuel and oxidant gas inlet in fluid communication with at least one burner, a catalyst-containing reactor tube having an upstream end in fluid communication with the outlet of the additional adiabatic reactor and a downstream end in fluid communication with the outlet of cracked gas; and

[0136] The convection section, which is in fluid flow communication with the radiation section, includes the flue gas outlet.

[0137] A first heat exchanger is used to heat ammonia gas by exchanging heat with flue gas in the convection section of the furnace. The first heat exchanger includes an inlet for ammonia gas and an outlet in direct fluid flow communication with the inlet of the first adiabatic reactor.

[0138] A second heat exchanger is used to heat the intermediate cracked ammonia gas by exchanging heat with the flue gas in the convection section of the furnace. The second heat exchanger has 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 other adiabatic reactor.

[0139] The first heat exchanger is located upstream of the second heat exchanger within the convection section of the furnace, relative to the flue gas flow.

[0140] #18. The apparatus according to #17, wherein the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst.

[0141] #19. The apparatus according to #17 or #18, wherein the catalyst bed of the additional adiabatic reactor comprises a catalyst selected from nickel-based catalysts and ruthenium-based catalysts.

[0142] #20. The apparatus according to any one of #17 to #19, wherein the apparatus includes an additional heat exchanger (or “heat saver”) arranged to heat partially cracked ammonia by exchanging heat with cracked gas located between the additional adiabatic reactor and the radiant section of the furnace.

[0143] #21. The device according to #20, wherein the additional heat exchanger is a shell-and-tube heat exchanger.

[0144] #22. The apparatus according to #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.

[0145] #23. The device according to any one of #17 to #22, the device comprising:

[0146] A hydrogen recovery unit, preferably a PSA unit, for recovering hydrogen from cracked gases, comprising:

[0147] A first inlet for fluid flow communication with the cracked gas outlet of the radiant section of the furnace;

[0148] For the first outlet of hydrogen; and

[0149] A second outlet is used for the exhaust gas, which includes nitrogen, residual ammonia, and residual hydrogen. This second outlet is in fluid communication with at least one fuel inlet of the radiant section of the furnace.

[0150] The device includes an exhaust gas heater arranged to heat exhaust gas and located between the second outlet of the hydrogen recovery unit and the at least one fuel inlet of the radiant section of the furnace.

[0151] #24. The device according to #23, the device comprising:

[0152] A compression unit for compressing exhaust gas, the compressor comprising:

[0153] An inlet in fluid flow communication with the second outlet of the hydrogen recovery unit; and

[0154] The outlet is in fluid flow communication with the first inlet of the hydrogen recovery unit.

[0155] The device includes a valve assembly to control the flow of exhaust gas to the compression unit and the exhaust gas heater.

[0156] #25. The apparatus according to any one of #17 to #24, wherein the catalyst-filled tube of the furnace and the catalyst bed of the adiabatic reactor do not contain iron-based catalysts.

[0157] The invention will now be described by way of example only with reference to the accompanying drawings.

[0158] exist Figure 1 In this process, a liquid ammonia stream 2 at approximately -32°C is removed from a reservoir (not shown) and fed into pump P101, where it is pumped to generate a pressurized liquid ammonia stream 4 at approximately 46 bar. This pressurized liquid ammonia stream is preheated in heat exchanger E271 by exchanging heat with a heat transfer fluid (in this case, an aqueous solution of glycol, typically approximately 55 wt% ethylene glycol or propylene glycol) to produce a preheated liquid ammonia stream 6 at approximately 55°C. An electric heater can be used to ensure that the temperature of the glycol solution fed into heat exchanger E271 is sufficient to preheat the liquid ammonia to the required temperature.

[0159] The preheated liquid ammonia in stream 6 is further heated by heat exchange in heat exchanger E312 to produce a further heated liquid ammonia stream 8. The further heated liquid ammonia in stream 8 is then evaporated by heat exchange in heat exchanger E311 to produce an ammonia vapor stream 10. The ammonia vapor in stream 8 is then superheated by heat exchange in heat exchanger E310 to produce a heated ammonia stream 12 at approximately 260°C.

[0160] The heated ammonia gas in stream 12 is further heated by heat exchange in heat exchanger E2102 to produce a further heated ammonia gas stream 13 at approximately 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) located between them.

[0161] The ammonia gas, further heated in stream 13, is then heated by heat exchange in heat exchanger E2104 to produce a superheated ammonia gas stream 14 at approximately 600°C and approximately 45 bar. Heat exchanger E2104 is the “first heat exchanger” as defined in the claims.

[0162] The superheated ammonia gas in stream 14 is then fed into the first adiabatic reactor vessel C141 and passes through a nickel-based catalyst bed. Some of the ammonia gas is cracked over the catalyst to form an intermediate gas stream 16 containing some cracked ammonia. The mole fraction of ammonia in the gas passing through the first adiabatic reactor vessel C141 decreases from almost 1 to about 0.75.

[0163] The intermediate gas is at approximately 450°C before being heated by heat exchange in heat exchanger E2103 to generate a superheated intermediate gas stream 18. Heat exchanger E2103 is a “second heat exchanger” as defined in the claims.

[0164] Stream 18 is then fed into a second adiabatic reactor vessel C142 at approximately 500°C and passes through a bed containing an upstream layer of nickel-based catalyst and a downstream layer of ruthenium-based catalyst to produce a partially cracked ammonia gas stream 20 at approximately 380°C. The molar fraction of ammonia in the gas passing through the second adiabatic reactor vessel C142 decreases from approximately 0.75 to approximately 0.6.

[0165] The catalyst bed of the second adiabatic reactor vessel C142 has two layers—a ruthenium-based catalyst layer on top of a nickel-based catalyst layer—to utilize heat more efficiently and thus maximize ammonia conversion. The catalyst volume was also optimized by minimizing the volume of the ruthenium-based catalyst by limiting the outlet temperature of the second adiabatic reactor vessel. The inventors found that lowering this temperature to below 380°C increased the required volume of the ruthenium-based catalyst without any additional benefit.

[0166] The ruthenium-based catalyst is the same in both the first and second adiabatic reactor vessels. However, different ruthenium-based catalysts can be used.

[0167] The partially cracked ammonia in stream 20 is heated by heat exchange in heat exchanger (or “economizer”) E305 before being fed as stream 22 into the catalyst-filled tubes in the radiant section F201 of the furnace (or reactor) at a pressure of about 40 bar. Heat exchanger E305 is an “additional heat exchanger” as defined in the claims.

[0168] By reducing the load required to heat the partially cracked stream to the reaction temperature, heating the feed to the tubes increases the amount of cracking that can be accomplished using heat from the burner. The inlet temperature of the direct-fired tubular furnace is limited to approximately 500°C to limit the inner wall temperature of the cracker tubes.

[0169] Airflow 62 passes through forced draft fan K212 before being preheated by heat exchange in heat exchanger E2142 to generate preheated airflow 64. Preheated airflow 64 is fed together with natural gas flow 70, which serves as a fine-tuning fuel, into the burner (not shown) of furnace F201. Preheating the air in this manner helps reduce fuel requirements.

[0170] The tubes in the radiant section F201 of the furnace are filled with two different types of ammonia cracking catalysts in two distinct layers. A ruthenium-based catalyst is used in the first layer within each tube, allowing for a faster reaction rate and maintaining the metal temperature within the design limit of approximately 660°C. The second layer in the tube downstream of the first layer contains a lower-cost but less active nickel-based catalyst.

[0171] The cracked gas stream 24 exits the radiant section F201 of the direct-fired tubular furnace at approximately 640°C and is then fed into a heat exchanger E305 (as defined in the claims as an “additional heat exchanger”) to provide the load required to heat the partially cracked ammonia, thereby reducing the temperature of the cracked gas to approximately 520°C.

[0172] The E305 heat exchanger is described as a shell-and-tube heat exchanger, in which partially cracked ammonia gas passes through the tubes and cracked gas passes through the shell side. However, this arrangement can be reversed, or in fact, different types of heat exchangers can be used.

[0173] The cracked gas stream 26 is then fed from the heat exchanger E305 to the heat exchanger E310 to provide the load required to superheat the ammonia, thereby further reducing the temperature of the cracked gas to about 380°C.

[0174] The cracked gas stream 28 is then fed from heat exchanger E310 to heat exchanger E311 to provide the load required for evaporating the liquid ammonia to be further heated, thereby further reducing the temperature of the cracked gas to about 100°C.

[0175] The cracked gas stream 30 is then fed from heat exchanger E311 to heat exchanger E312 to provide the load required for further heating of the heated pressurized liquid ammonia, thereby further reducing the temperature of the cracked gas to about 60°C.

[0176] Each of heat exchangers E310, E311, and E312 is depicted as a separate shell-and-tube heat exchanger, with ammonia passing through the tubes and cracked gas passing through the shell side. However, for at least one, or all, of these heat exchangers, this arrangement can be reversed. Alternatively, the heat exchangers can be combined into a single shell-and-tube heat exchanger, or different types of heat exchangers can actually be used.

[0177] The cracked gas stream 32 from heat exchanger E312 is then further cooled by heat exchange with a heat exchange fluid in cooler E323, and is then fed as stream 34 into PSA system U501, where it is separated into hydrogen stream 40 and PSA waste stream 42. The hydrogen stream is removed as a product, and the PSA waste stream includes nitrogen, residual hydrogen, and residual ammonia. The hydrogen in stream 40 can be fed into a hydrogen liquefaction unit (not shown) to produce liquid hydrogen.

[0178] All PSA exhaust gas in stream 42 can be directly fed as fuel (stream 60) for combustion in furnace F201. Alternatively, stream 42 can be divided into two parts.

[0179] The first portion of the PSA exhaust gas in stream 44 is heated by heat exchange in heat exchanger E2112 to produce a warm PSA exhaust gas stream 60, which is then fed into the burner in furnace F201 along with air feed 64 and, optionally, natural gas feed 70. A minimum amount of natural gas is used as a fine-tuning fuel to provide the required fuel balance in the combustion zone.

[0180] The second part can be fed as stream 46 into the multi-stage compression unit K681 for compression. Compression unit K681 has five stages, with an intercooler between each stage and an aftercooler after the last stage. Heat is recovered from the compressed gas in the intercooler and aftercooler through heat exchange with the heat transfer fluid. Heat can also be recovered from the lubricating oil used in the compression unit, and in the case of a positive displacement compression unit, from the cylinder of the compression unit using the heat transfer fluid.

[0181] For convenience, the intercooler and aftercooler are represented by a single heat exchanger (marked as E6816A-E) that recovers heat from the compressed PSA exhaust gas stream 48 by exchanging heat with the heat transfer fluid stream 52 to produce a cooled compressed PSA exhaust gas stream 50 and a heated heat transfer fluid stream 54.

[0182] The heat transfer fluid heated in cooler E323, as well as in intercoolers and aftercoolers E6816A-E, is then used to provide the load required to preheat liquid ammonia by heat exchange in heat exchanger E271.

[0183] The cooled, compressed PSA exhaust gas in stream 50 is fed into phase separator C6816, where any condensate is removed as stream 56. The compressed PSA exhaust gas is then recycled as stream 58 back to PSA system U501 for further hydrogen recovery. In this way, the hydrogen recovery rate can be increased from approximately 85 mol% (without recirculation) to approximately 95 mol% (with recirculation).

[0184] As mentioned above, the process can operate without the compression unit K681, resulting in a lower hydrogen recovery rate in the PSA unit 501. Reduced hydrogen recovery obviously leads to less hydrogen product. However, the lower hydrogen recovery rate may still be desirable because with more hydrogen present in the exhaust gas, the process's carbon intensity (CI) decreases, thereby reducing the need for natural gas as a fine-tuning fuel and reducing CO2 emissions.

[0185] A flue gas flow 72 at approximately 680°C flows from the radiant section F201 to the convection section 90 of the furnace F201. In the convection section, it first provides the load required for further heating of the ammonia from stream 13 in heat exchanger E2104 (as defined in the claims as a "first heat exchanger"), thereby reducing the temperature of the flue gas. This flue gas is then used (as stream 73) to provide the load required for heating the intermediate gas from stream 16 in heat exchanger E2103 (as defined in the claims as a "second heat exchanger"), thereby further reducing the temperature of the flue gas. Thus, the flue gas provides the heating load in a direction parallel to the feed gas flow of the radiant section F201 of the direct-fired tubular furnace.

[0186] The cooled flue gas (as stream 74) is then used to provide the load required for further heating of the ammonia in stream 12 in heat exchanger E2102, thereby further reducing the temperature of the flue gas.

[0187] Then, in heat exchanger E2142, the further cooled flue gas in stream 76, still at a temperature of approximately 295°C, is used to heat the air from stream 62, thereby further reducing the temperature of the flue gas. The further cooled flue gas (as stream 78) is then used to provide the load required to heat the PSA exhaust gas from stream 44 in heat exchanger E2112, thereby further cooling the flue gas.

[0188] The cooled flue gas, at a temperature of approximately 123°C (above the dew point of water), leaves the convection section 90 of the direct-fired tubular furnace F201 as stream 80, passes through the induced draft fan K211, and then exits the process as stream 82. At this point, all the actual energy has been extracted from the flue gas, and it can now be discharged into the atmosphere, optionally after further treatment, depending on its composition, if desired.

[0189] Where ammonia is produced, or at the ammonia cracking site, or virtually anywhere transported between the two sites, oil from an evaporative gas compressor (not shown) used in conjunction with an ammonia storage tank (not shown) may be present in liquid ammonia in amounts up to about 5 ppm. The presence of oil in ammonia is known to cause difficulties because ammonia cracking catalysts are generally considered intolerant of oil. At least some of the oil can be removed by cracking in the catalyst bed of a first adiabatic reactor, especially if a ruthenium-based catalyst is used.

[0190] It may be desirable to remove at least some of the oil before the catalyst is exposed to ammonia. In this regard, the oil can be removed by passing ammonia through an activated carbon bed. The oil removal unit (not shown) can then be located in flow 2 (i.e., in the feed line to pump P101), flow 4 (i.e., between pump P101 and ethylene glycol heater E271), flow 6 (i.e., between ethylene glycol heater E271 and heat exchanger E312), flow 8 (i.e., between heat exchangers E312 and E311), or flow 10 (i.e., between heat exchangers E311 and E310).

[0191] The invention will now be illustrated by the following non-limiting examples.

[0192] Example

[0193] Figure 1 The process described has been simulated using a computer (Aspen Plus, ver. 10, Aspen Technology, Inc., Massachusetts, USA) to design a plant that produces 30 tons / day of hydrogen (flow 40).

[0194] Based on rate equation 9 given by Lamb et al. (International Hydrogen Energy, 44 (2019), pp. 3726-3736), the activities of ruthenium-based and nickel-based catalysts in adiabatic reactors and tubes were modeled. For simulation purposes, it was assumed that the activity of the ruthenium-based catalyst conformed to the rate equation, but the activity of the nickel-based catalyst was 20% of the activity predicted by the rate equation.

[0195] The results are shown in Table 2.

[0196] Table 2

[0197]

[0198]

[0199] Table 2

[0200] The results show that, in order to recover 1.33 mol% of ammonia escape (stream 24) from the cracker (tube furnace 201) and 95 mol% of hydrogen from the PSA, in addition to the PSA exhaust gas (stream 60), 7626 kg / h of ammonia (stream 2) and 27.0 kmol / h of natural gas (stream 70) are required as feed to ignite the cracker.

[0201] For a given hydrogen recovery rate and ammonia slip, using cracked gas (instead of flue gas) to provide the load required to heat the partially cracked gas (stream 20) to the feed temperature of the catalyst-filled tube of the cracker (F201) has the effect of reducing the total carbon intensity (CI) of the process.

[0202] Although the invention has been described with reference to preferred embodiments illustrated in the accompanying drawings, it should be understood that various modifications are possible within the spirit or scope of the invention as defined by the appended claims.

[0203] In this specification, unless otherwise explicitly stated, the word "or" is used to indicate an operator that returns a true value when one or both of the stated conditions are met, as opposed to the operator "XOR," which requires only one of the conditions to be met. The word "including" is used to mean "contains" and is incorporated into "consisting of," rather than simply meaning "consisting of."

[0204] All existing teachings above are incorporated herein by reference. Acknowledgment of any previously published document herein should not be construed as an acknowledgment or representation that the teachings of that document were common knowledge in Australia or elsewhere on the date of publication of that document.

Claims

1. A method for cracking ammonia, the method comprising: Provides heated ammonia gas under ultra-atmospheric pressure; The heated ammonia gas at a first temperature T1 is fed into a first adiabatic reactor including a catalyst bed to crack a portion of the ammonia and produce intermediate partially cracked ammonia gas. Heating the intermediate cracked ammonia gas to produce heated intermediate cracked ammonia gas; At a second temperature T2, which is lower than the first temperature T1, the heated intermediate cracked ammonia gas or the heated intermediate cracked ammonia gas derived therefrom is fed into another adiabatic reactor including a catalyst bed to crack another portion of the ammonia and produce partially cracked ammonia gas. In the furnace, fuel is burned with oxidant gas to heat the catalyst-containing reactor tubes and form flue gas; as well as The partially cracked ammonia gas is fed into the catalyst-containing reactor tube to induce further cracking of the ammonia, producing a cracked gas comprising hydrogen, nitrogen, and residual ammonia. T1 is in the range of 500℃ to 700℃, and T2 is in the range of 400℃ to 600℃.

2. The method according to claim 1, wherein the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst.

3. The method according to claim 1, wherein the first temperature T1 is in the range of 550°C to 650°C.

4. The method of claim 1, wherein the catalyst bed of the additional adiabatic reactor comprises a catalyst selected from nickel-based catalysts and ruthenium-based catalysts.

5. The method according to claim 1, wherein the second temperature T2 is in the range of 450°C to 550°C.

6. The method of claim 1, wherein at least some of the load required to heat the intermediate cracked gas is provided by heat exchange with the flue gas.

7. The method of claim 1, wherein the load required to heat the intermediate cracked gas is entirely provided by heat exchange with the flue gas.

8. The method of claim 6 or 7, wherein a portion of the load required to provide the heated ammonia is provided by heat exchange with the flue gas upstream of the heat exchange between the flue gas stream and the gas from the intermediate cracking process.

9. The method of claim 1, wherein the partially cracked ammonia is heated to produce heated partially cracked ammonia before being fed into the catalyst-containing reactor tube of the furnace.

10. The method of claim 9, wherein at least some of the load required to heat the partially cracked ammonia is provided by heat exchange with the cracked gas.

11. The method of claim 9, wherein the load required to heat the partially cracked ammonia is entirely provided by heat exchange with the cracked gas.

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

13. The method of claim 12, wherein water is present in the heated ammonia in an amount not exceeding 1 mol%.

14. The method of claim 12, wherein at least some of the heating load required to provide the heated ammonia is provided by heat exchange with the cracked gas downstream of the heat exchange between the partially cracked ammonia and the cracked gas stream.

15. The method of claim 12, wherein a majority of the heating load required to provide the heated ammonia is provided by heat exchange with the cracked gas downstream of the heat exchange between the partially cracked ammonia and the cracked gas stream.

16. The method of claim 1, wherein the catalyst-containing reactor tube of the furnace and the catalyst bed of the adiabatic reactor do not contain iron-based catalysts.

17. The method according to claim 1, wherein the method further comprises: The cracked gas is cooled, and after cooling, hydrogen is recovered from the cracked gas in a hydrogen recovery unit to produce hydrogen products and waste gas including nitrogen, residual hydrogen and residual ammonia. Heating at least a portion of the exhaust gas to produce heated exhaust gas; as well as The heated exhaust gas is fed into the furnace as at least a portion of the fuel.

18. The method of claim 17, wherein all of the exhaust gas is fed into the furnace as fuel.

19. The method of claim 17, wherein the method comprises: The exhaust gas is divided into a first part and a second part, wherein the first part is heated and fed into the furnace; as well as The second portion of the exhaust gas is compressed to produce compressed exhaust gas, and the compressed exhaust gas is recycled to the hydrogen recovery unit for further hydrogen recovery.

20. An apparatus for cracking ammonia, said apparatus comprising: A first adiabatic reactor is used for the partial cracking of heated ammonia under ultra-atmospheric pressure. The first adiabatic reactor includes an inlet for heated ammonia and a catalyst bed having an upstream end in fluid communication with the inlet and a downstream end in fluid communication with the outlet of the partially cracked ammonia. An additional adiabatic reactor is used for cracking heated intermediate partially cracked ammonia or heated intermediate partially cracked ammonia derived therefrom, the additional adiabatic reactor including an inlet in fluid flow communication with the outlet of the first adiabatic reactor, a catalyst bed having an upstream end in fluid flow communication with the inlet and a downstream end in fluid flow communication with the outlet of the partially cracked ammonia. Furnace, the furnace comprising: The radiant section includes at least one fuel and oxidant gas inlet in fluid communication with at least one burner, and a catalyst-containing reactor tube having an upstream end in fluid communication with the outlet of the additional adiabatic reactor and a downstream end in fluid communication with the outlet of the cracked gas. and The convection section, which is in fluid flow communication with the radiation section, includes the flue gas outlet. A first heat exchanger is used to heat ammonia gas by exchanging heat with flue gas in the convection section of the furnace. The first heat exchanger includes 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 is used to heat the intermediate cracked ammonia gas by exchanging heat with the flue gas in the convection section of the furnace. The second heat exchanger has 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 other adiabatic reactor. The first heat exchanger is located upstream of the second heat exchanger within the convection section of the furnace, relative to the flue gas flow.

21. The apparatus of claim 20, wherein the catalyst bed of the first adiabatic reactor comprises a nickel-based catalyst.

22. The apparatus of claim 20, wherein the catalyst bed of the additional adiabatic reactor comprises a catalyst selected from nickel-based catalysts and ruthenium-based catalysts.

23. The apparatus of claim 20, further comprising an additional heat exchanger arranged to heat partially cracked ammonia by exchanging heat with cracked gas located between the additional adiabatic reactor and the radiant section of the furnace.

24. The apparatus of claim 23, wherein the additional heat exchanger is a shell-and-tube heat exchanger.

25. The apparatus of claim 24, wherein the shell side of the heat exchanger is in direct fluid flow communication with the outlet of the radiant section of the furnace.

26. The apparatus of claim 20, wherein the apparatus comprises: A hydrogen recovery unit for recovering hydrogen from cracked gases, the hydrogen recovery unit comprising: A first inlet for fluid flow communication with the cracked gas outlet of the radiant section of the furnace; For the first outlet of hydrogen; and A second outlet is used for the exhaust gas, which includes nitrogen, residual ammonia, and residual hydrogen. This second outlet is in fluid communication with at least one fuel inlet of the radiant section of the furnace. The device includes an exhaust gas heater arranged to heat exhaust gas and located between the second outlet of the hydrogen recovery unit and the at least one fuel inlet of the radiant section of the furnace.

27. The apparatus of claim 26, wherein the hydrogen recovery unit is a PSA unit.

28. The apparatus of claim 27, wherein the apparatus comprises: A compression unit for compressing exhaust gas, the compression unit comprising: An inlet in fluid flow communication with the second outlet of the hydrogen recovery unit; and The outlet is in fluid flow communication with the first inlet of the hydrogen recovery unit. The device includes a valve assembly to control the flow of exhaust gas to the compression unit and the exhaust gas heater.

29. The apparatus of claim 20, wherein the catalyst-filled tube of the furnace and the catalyst bed of the adiabatic reactor do not contain iron-based catalysts.

Images