Ammonia cracking for green hydrogen

Abstract

The recovery of hydrogen from the ammonia cracking process, in which the cracked gas is purified in the PSA unit, is improved by using a membrane separator on the PSA tail gas.

Description

Background Art

[0001] Global interest in renewable energy and the use of such renewable energy to produce green hydrogen has driven interest in converting green hydrogen into green ammonia, as ammonia is simpler to transport over distances of hundreds or thousands of miles. In particular, transporting liquid hydrogen is currently not commercially possible, but transporting ammonia in a liquid state is currently feasible.

[0002] To be used in commercial fuel cells, ammonia must be converted back into hydrogen, according to a reaction.

[0003]

[0004] This is an endothermic process (i.e., a process that requires heat) and is carried out over a catalyst. This process is called cracking. The gas produced (or "cracked gas") is hydrogen (H 2 ) and nitrogen (N 2 ). Because the cracking reaction is an equilibrium reaction, there is also some residual ammonia. At present, in most applications of cracking furnaces, the hydrogen + nitrogen mixture is used as it is. However, since ammonia can be a poison for fuel cells, this stream with appropriate removal of ammonia, such as by washing with water, can be used directly in fuel cells. However, if hydrogen is used for vehicle fuel, the nitrogen present will have an adverse effect on the process. The fuel of the vehicle fuel system is compressed to a considerable pressure - up to 900 bar. This means that nitrogen (which is only a diluent in this process) is also compressed, consuming power and taking up storage capacity, and increasing the anode gas purge demand, reducing efficiency. Therefore, in the case where hydrogen is to be used for vehicle fuel, it is beneficial to purify the hydrogen + nitrogen.

[0005] Small cracking reactors or "crackers" typically use a pressure swing adsorption ("PSA") unit to separate the cracked gases and recover hydrogen and produce a PSA tail gas (or waste gas). However, these crackers are typically electrically heated, and the PSA tail gas is typically vented to the atmosphere.

[0006] PSA can be used to purify nitrogen + hydrogen as is common in hydrogen production from steam methane reforming (SMR) reactors.The cracking reaction is carried out in a tube filled with catalyst, which is heated externally by a furnace (see GB1142941).

[0007] GB1142941 discloses a method for producing town gas from ammonia. Ammonia is cracked and the cracked gas is washed with water to remove residual ammonia. The purified hydrogen / nitrogen mixture is then enriched with propane and / or butane vapor to produce town gas for distribution.

[0008] US6835360A discloses an endothermic catalytic reaction device for converting hydrocarbon feedstock and methanol into useful gases such as hydrogen and carbon monoxide. The device comprises a tubular endothermic catalytic reactor combined with a radiant combustion chamber. The resulting cracked gas is directly used in a fuel cell after passing through a gas conditioning system.

[0009] GB977830A discloses a method for cracking ammonia to produce hydrogen. In this method, hydrogen is separated from nitrogen by passing the cracked gas through a bed of molecular sieves that adsorb nitrogen. The nitrogen is then discharged from the bed and can be stored in a container.

[0010] JP5330802A discloses an ammonia cracking method, wherein ammonia and an ammonia decomposition catalyst are allowed to react at 10 kg / cm 2 (or about 9.8 bar) and a temperature of 300°C to 700°C. Hydrogen is recovered from the cracked gas using a PSA unit. The reference mentions that the desorbed nitrogen can be used to facilitate upstream processes, but no details are provided.

[0011] US2007 / 178034A discloses a method in which a mixture of ammonia and hydrocarbon feedstock is passed through a combustion steam reformer at 60°C and 3.2MPa (or about 32 bar), where the mixture is converted into a synthesis gas containing about 70% by volume of hydrogen. The synthesis gas is enriched in hydrogen in a shift reaction, cooled and condensate is removed. The resulting gas is fed to a PSA system to produce a purified hydrogen product having 99% by volume or more of hydrogen. Exhaust gas from the PSA system is supplied to the combustion steam reformer as fuel.

[0012] CN111957270A discloses a process in which ammonia is cracked in a tubular reactor within a furnace. The cracked gas is separated by adsorption to produce hydrogen and a nitrogen-rich waste gas. Using a combination of cracked gas, hydrogen product gas and / or waste gas appears to meet the fuel requirements of the furnace.

[0013] There is a general need for improved processes for producing hydrogen from ammonia, particularly processes that are more efficient in terms of energy consumption and / or have higher levels of hydrogen recovery and / or reduce or eliminate the need to burn fossil fuels.

[0014] In the following discussion of embodiments of the present invention, pressures given are absolute pressures unless otherwise stated. Summary of the invention

[0015] According to a first aspect of the present invention, there is provided a method for producing hydrogen from ammonia, comprising:

[0016] Pressurizing liquid ammonia;

[0017] heating (and optionally vaporizing) the liquid ammonia by heat exchange with one or more hot fluids to produce heated ammonia;

[0018] Combusting fuel in the furnace to heat reaction tubes containing the catalyst and to form flue gases;

[0019] supplying the heated ammonia to a reaction tube containing a catalyst to crack the ammonia into a cracked gas containing hydrogen and nitrogen;

[0020] purifying the cracked gas in a PSA unit to produce a hydrogen product gas and a PSA tail gas; and

[0021] separating the PSA tail gas or a gas derived therefrom using a membrane separator to discharge a nitrogen-rich retentate gas and recycle a hydrogen-rich permeate gas for further processing in a PSA unit and / or for blending into the hydrogen product gas,

[0022] wherein the fuel comprises one or more of ammonia, the PSA tail gas, hydrogen and methane; and

[0023] The one or more thermal fluids include flue gas and / or cracked gas.

[0024] The liquid ammonia is typically pressurized to a pressure greater than 1.1 bar (e.g., at least 5 bar or at least 10 bar). In some embodiments, the liquid ammonia is pressurized to a pressure in the range of about 5 bar to about 50 bar, or in the range of about 10 bar to about 45 bar, or in the range of about 30 bar to about 40 bar.

[0025] The liquid ammonia is typically heated to produce heated ammonia at a temperature greater than about 250° C., for example, in the range of about 350° C. to about 800° C. or about 400° C. to about 600° C. At the pressure, the liquid ammonia is typically fully vaporized to form heated ammonia vapor.

[0026] The temperature is ultimately determined by the characteristics of the catalyst, the operating pressure, and the desired "glide" (i.e., the amount of ammonia that passes through the cracking reactor without being cracked). In this regard, the process is typically operated at no more than about 4% glide, which would be the amount of glide if the cracking process were operated at 5 bar and 350° C., which is close to equilibrium. At temperatures above about 700° C., some materials of construction have problems at any appreciable pressure.

[0027] The cracking reaction takes place in a catalyst-filled reaction tube heated by a furnace. However, in theory any heterogeneously catalyzed gas reactor could potentially be used for the conversion.

[0028] There are a large number of catalysts known in the art that can be used for ammonia cracking reactions, and any of these conventional catalysts can be used in the present invention.

[0029] The primary fuel for the furnace typically comprises methane. The fuel may be pure methane, but is more likely natural gas or biogas. In some embodiments, the primary fuel is natural gas or biogas supplemented with hydrogen as an auxiliary fuel, optionally in the form of ammonia cracked gas. In these embodiments, liquid ammonia may be pumped and cracked to form cracked gas that is added to the primary fuel.

[0030] The PSA unit can operate a PSA cycle or a vacuum swing adsorption (VSA) cycle. The TSA unit can be used in combination with a PSA unit, a TSA unit for removing ammonia (see US10787367) and a PSA unit for removing nitrogen and producing a hydrogen product. Suitable PSA cycles include any cycle disclosed in US9381460, US6379431 and US8778051.

[0031] Recycling the hydrogen-rich permeate gas for further processing in a PSA unit (or, simply, "PSA") significantly improves hydrogen recovery, and the recovery can approach 100%, for example, can be about 99%.

[0032] Like hydrogen, ammonia is a "fast gas" that readily permeates through membranes used for gas separations. Some membranes, such as those composed of polyamide or polysulfone polymers, are more tolerant to ammonia. However, some membranes, such as those composed of polyimide polymers, are less tolerant to ammonia. Therefore, ammonia is typically removed upstream of the membrane separator, or its concentration is at least reduced.

[0033] Removal of ammonia can be accomplished at several different locations in the process. Ammonia can be removed from the PSA tail gas prior to separation of the PSA tail gas. Alternatively, ammonia can be removed from the cracked gas prior to purification of the cracked gas. In both cases, the removed ammonia can be recovered and recycled to the ammonia supplied to the reactor tubes containing the catalyst.

[0034] Ammonia can be removed from the gas by adsorption (e.g. by TSA) or by absorption in water (e.g. by washing the gas with water in a scrubber). The resulting ammonia-depleted gas and ammonia solution are separated, so that the ammonia-depleted gas can be further processed without the ammonia causing any difficulties. Ammonia can be recovered from the ammonia solution by stripping in a distillation column system. Such a process can be applied to the cracked gas before being supplied to the PSA unit, or alternatively to the PSA tail gas before being supplied to the membrane separator.

[0035] However, in some preferred embodiments, ammonia is condensed from the PSA tail gas and the condensed ammonia is fed to the ammonia feed to the cracker. In these embodiments, the method comprises:

[0036] compressing the PSA tail gas to produce compressed PSA tail gas;

[0037] cooling the compressed PSA tail gas by heat exchange with one or more cold fluids to condense ammonia in the compressed PSA tail gas; and

[0038] separating the condensed ammonia from the compressed PSA tail gas to produce an ammonia-depleted PSA tail gas,

[0039] The ammonia-depleted PSA tail gas is supplied as feed to the membrane separator.

[0040] The one or more cold fluids may comprise an external refrigerant, although in a preferred embodiment, the one or more cold fluids comprise pressurized liquid ammonia and / or ammonia-depleted PSA tail gas for internal heat integration of the process.

[0041] The condensed ammonia may be discharged from the process, but is preferably mixed with pressurized liquid ammonia.

[0042] The membrane separator may include only a single membrane separation unit (also referred to as a membrane stage). In these embodiments, the membrane separator does not include more than one membrane separation unit. However, in preferred embodiments, the membrane separator includes a first membrane separation unit and a second membrane separation unit connected in series. In these embodiments, the method further includes:

[0043] supplying the ammonia-depleted PSA tail gas to the first membrane separation unit to produce the hydrogen-rich permeate gas and the nitrogen-rich retentate gas including residual hydrogen for further processing in the PSA; and

[0044] The nitrogen-rich retentate gas is supplied to the second membrane separation unit to discharge the nitrogen-rich retentate gas and produce additional hydrogen-rich permeate gas that is mixed with the PSA tail gas.

[0045] Using two membrane separation units in this manner helps avoid having to use a second compressor to compress the hydrogen recycled to the PSA, thus using less power overall than involving two compressors while maintaining high hydrogen recovery (about 99%).

[0046] The nitrogen-rich retentate gas may be expanded in a turbine to recover power and then discharged to the atmosphere after appropriate treatment to remove contaminants such as ammonia. In such embodiments, the nitrogen-rich retentate gas is typically heated by heat exchange with one or more hot fluids prior to expansion. Alternatively, the nitrogen-rich retentate gas may be purified to produce a nitrogen product.

[0047] According to a second aspect of the present invention, there is provided an apparatus for producing hydrogen from ammonia, comprising:

[0048] A pump for pressurizing liquid ammonia;

[0049] at least one first heat exchanger in fluid communication with the pump, the first heat exchanger for heating (and optionally vaporizing) the liquid ammonia from the pump by exchanging heat with one or more hot fluids;

[0050] a catalyst-containing reaction tube in fluid communication with the first heat exchanger, the reaction tube for cracking the heated ammonia from the first heat exchanger to produce a first cracked gas containing hydrogen and nitrogen;

[0051] a furnace in thermal communication with the catalyst-containing reaction tubes, the furnace being configured to burn a fuel to heat the catalyst-containing reaction tubes and form a flue gas;

[0052] A PSA device in fluid communication with the catalyst-containing reaction tube, the PSA device being used to purify the cracked gas to produce hydrogen product gas and PSA tail gas;

[0053] a membrane separator in fluid communication with the PSA unit, the membrane separator for separating the PSA off-gas or gases produced therefrom to discharge a nitrogen-rich retentate gas and a hydrogen-rich permeate gas;

[0054] Wherein the apparatus comprises a conduit for feeding the hydrogen-rich gas to the PSA unit for further processing and / or a conduit for mixing the hydrogen-rich gas with the first hydrogen product gas.

[0055] The furnace may be separate from the catalyst filled reaction tubes, although the furnace and the catalyst filled reaction tubes are preferably integrated in the same unit. In a preferred embodiment, a steam methane reforming (SMR) type reactor is used, wherein the furnace comprises a radiant section through which the catalyst containing reaction tubes pass.

[0056] The apparatus typically includes an ammonia removal system upstream of the membrane separator in order to protect membranes that are not resistant to ammonia and / or to prevent ammonia from passing through the membranes together with hydrogen. The ammonia removal system may be a scrubber for removing ammonia by absorption in water, or may be a TSA unit for removing ammonia by adsorption.

[0057] The ammonia removal system may be located downstream of the PSA unit, thereby removing ammonia from the PSA tail gas.Alternatively, the ammonia removal system may be located downstream of the at least one heat exchanger and upstream of the PSA unit, thereby removing ammonia from the cracked gas.

[0058] The apparatus may further comprise an ammonia recovery system for recovering ammonia from the aqueous ammonia solution produced in the scrubbing tower. The recovery system may comprise a stripping tower (or a stripping section in a distillation tower system) for recovering ammonia from the aqueous ammonia solution.

[0059] Additionally or alternatively, the apparatus may comprise a selective catalytic reduction (SCR) reactor for removing nitrogen oxides from the flue gas, into which reactor at least part of the aqueous ammonia solution is fed to provide ammonia for the SCR reaction.

[0060] The PSA tail gas compressor is usually arranged downstream of the PSA device to compress the PSA tail gas for the membrane separator. The compressor can be composed of one or more stages, and cooling will be performed between each stage and after the final stage. Water will usually condense from the compressed PSA tail gas between stages or at the aftercooler stage. The water-containing condensate is usually removed after each cooling stage of the compressor, and a small amount of ammonia will come out of the PSA tail gas together with the condensate.

[0061] Additionally or alternatively, a hydrogen rich gas compressor is typically located downstream of the membrane separator for compressing the hydrogen rich gas for use in the PSA unit and / or for mixing with the hydrogen product gas.

[0062] In some preferred embodiments, the device comprises:

[0063] a PSA tail gas compressor in fluid communication with the PSA device, the PSA tail gas compressor being used to compress the PSA tail gas;

[0064] a heat exchanger in fluid communication with the PSA tail gas compressor, the heat exchanger being used to cool the compressed PSA tail gas by exchanging heat with one or more cooling fluids;

[0065] a phase separator in fluid communication with the heat exchanger for separating condensed ammonia from the cooled compressed PSA tail gas to form an ammonia-depleted PSA tail gas; and

[0066] A conduit for supplying ammonia-depleted PSA tail gas from the phase separator to the membrane separator.

[0067] In these embodiments, the apparatus may include a conduit for supplying condensed ammonia to the pressurized liquid ammonia.

[0068] In addition, wherein the membrane separator comprises a first membrane separation unit and a second membrane separation unit connected in series, the device may further comprise:

[0069] a conduit for supplying ammonia-depleted PSA tail gas from the phase separator to the first membrane separation unit;

[0070] a conduit for supplying the hydrogen-enriched permeate gas from the first membrane separation unit to the PSA unit for further processing;

[0071] a conduit for supplying nitrogen-enriched retentate gas from the first membrane separation unit to the second membrane separation unit; and

[0072] a conduit for supplying additional hydrogen-enriched permeate gas from the second membrane unit to the PSA off-gas.

[0073] The apparatus generally comprises a conduit for supplying nitrogen-rich retentate gas from the membrane separator to a vent, optionally together with a turbine located downstream of the membrane separator for expanding the nitrogen-rich retentate gas and recovering energy. In addition, a heat exchanger may be located upstream of the turbine for heating the nitrogen-rich retentate gas by heat exchange with one or more hot fluids.

[0074] Alternatively, a nitrogen purification unit may be in fluid communication with the membrane separator for purifying the nitrogen-rich retentate gas to produce a nitrogen product. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] Figure 1 is a process flow diagram of a first reference example of an ammonia cracking process for producing hydrogen;

[0076] Figure 2 is based on Figure 1 A process flow diagram of another reference example of an ammonia cracking process in which no hydrogen product is used as fuel

[0077] Figure 3 is based on Figure 1 and 2 A process flow diagram of another reference example of an ammonia cracking process of FIG. 1 , wherein only PSA tail gas is used as fuel;

[0078] Figure 4 is a process flow chart of a first embodiment of an ammonia cracking process for producing hydrogen according to the present invention;

[0079] Figure 5 is a process flow chart of a second embodiment of an ammonia cracking process for producing hydrogen according to the present invention; and

[0080] Figure 6 FIG. 4 is a process flow chart of a third embodiment of an ammonia cracking process for producing hydrogen according to the present invention. DETAILED DESCRIPTION

[0081] This article describes a method for producing hydrogen by cracking ammonia. The method is particularly suitable for producing so-called "green" hydrogen, which is hydrogen produced using renewable energy instead of fossil fuels. In this case, ammonia is usually produced by electrolyzing water using electricity generated by renewable energy (such as wind and / or solar energy) to produce hydrogen, which is then catalytically reacted with nitrogen (Haber process) to produce ammonia that is easier to transport than hydrogen. After reaching its destination, the ammonia is then cracked to regenerate hydrogen.

[0082] In the process of the present invention, the heat required for the reaction is usually provided by the combustion of PSA tail gas (which usually contains a certain amount of residual hydrogen and ammonia) in the furnace. If the calorific value of the PSA tail gas is lower than that of the vaporized ammonia, a portion of the product hydrogen or an alternative fuel can be used as trim fuel together with the tail gas.

[0083] Indeed, natural gas could be used as a trimming fuel together with the PSA tail gas, as is done for hydrogen in SMRs. However, there is an incentive to use a "renewable fuel" because of the desire to maintain the "green" or renewable credentials of the hydrogen so produced. This could be "renewable" ammonia from the cracking, ammonia itself or another renewable energy source such as biogas, or indeed electric heating, whether or not the electricity itself comes from a renewable source, in which case it is local electricity to the cracking process, as opposed to renewable electricity used to produce the hydrogen that is already transported in the form of ammonia.

[0084] A reference example of this process is Figure 1 , the process takes liquid ammonia from a reservoir (not shown). The ammonia to be cracked (line 2) is pumped (pump P201) as a liquid to a pressure above the desired cracking pressure (see GB1142941). According to Le Chatelier's principle, the reaction pressure is a compromise between operating pressure and conversion. There is an incentive to operate the reactor (8) at higher pressures because pumping liquid ammonia requires less power and money than compressing the product hydrogen.

[0085] The pressurized liquid ammonia (line 4) is then heated, vaporized (if it is below its critical pressure) and further heated to a temperature above 250°C via a heat exchanger (E101) using the heat available in the cracked gases leaving the reaction tubes and in the flue gases from the furnace. In the figure, the heat exchanger (E101) is shown as one heat exchanger, but in reality it will be a series of heat exchangers in a network.

[0086] The initial heating and vaporization of the pressurized liquid ammonia can alternatively be carried out for an alternative heat source (such as cooling water or ambient air). Typical reaction temperatures are above 500°C (see US2601221), palladium-based systems can operate at 600°C and 10 bar, while RenCat's metal oxide-based systems operate at less than 300°C and 1 bar. (See https: / / www.ammoniaenergy.org / articles / ammonia-cracking-to-high-purity-hydrogen-for-pem-fuel-cells-in-denmark / ). The operating pressure of the cracker is generally an optimization of several factors. Ammonia cracking into hydrogen and nitrogen is favored by lower pressures, but other factors favor higher pressures, such as power consumption (which is minimized by pumping feed ammonia rather than compressing product hydrogen) and PSA size (which is smaller at higher pressures).

[0087] Hot ammonia (line 6) enters the reaction tubes of the reactor (8) at the required pressure, where additional heat is provided by the furnace (10) to crack the ammonia into nitrogen and hydrogen. The resulting mixture of residual ammonia, hydrogen and nitrogen leaves (line 12) the reaction tubes of the reactor (8) at the reaction temperature and pressure. The reaction products are cooled in a heat exchanger (E101) against the feed ammonia (from line 4), the furnace fuel (in this case pumped ammonia from line 14, pump P202 and line 16; PSA tail gas from line 18; and product hydrogen to be used as fuel in line 20) and the combustion air (from line 22, fan K201 and line 24) to reduce the temperature to as close as possible to the temperature required for the inlet of the PSA unit (26). Any residual heat in the cracked gas mixture (line 28) is removed in a water cooler (not shown) to achieve an inlet temperature to the PSA unit (26) in the range of about 20°C to about 60°C, such as about 50°C.

[0088] The PSA product (line 30) is pure hydrogen conforming to ISO Standard 14687-hydrogen fuel quality, wherein residual ammonia <0.1ppmv and nitrogen <300ppmv at approximately reaction pressure. The product hydrogen (line 30) is further compressed (not shown) for filling into pipeline trailers (not shown) for transportation, or it can be liquefied in a hydrogen liquefier (not shown) after any desired compression. The PSA tail gas (line 18) or "purge gas" from the PSA unit (26) is shown as being heated via heat exchanger E101 using cracked gas (line 12) or furnace flue gas (line 32) leaving the reaction tubes of the reactor (8) before being sent to the (line 36) furnace as a combustion fuel. However, the PSA tail gas (line 18) can be fed directly into the furnace (10) without heating.

[0089] The resulting heated ammonia fuel (line 34) and heated hydrogen (line 40) are depicted as being mixed in a mixer (42) with (optionally) heated PSA tail gas (line 36) to produce a mixed fuel that is fed (line 44) to the furnace (10) for combustion to produce flue gas (line 32, and after cooling in E101, line 48). However, it should be noted that one or more fuels can be fed directly into the furnace without premixing. Heated air (for combustion of the fuel) is fed to the furnace (10) via line 46.

[0090] One of the purposes of the preferred embodiment of the method of the present invention is to maximize the amount of hydrogen produced by cracking renewable ammonia. This means minimizing the amount of hydrogen used as fuel, or if ammonia is used directly as fuel, minimizing the amount of ammonia. Therefore, heat integration is important in order to properly use hot flue gas and cracked gas, such as air (line 24) and ammonia (line 4) preheated to the cracker, because this reduces the amount of "fuel" to be used in the burner of the furnace (10). This leads to a higher hydrogen recovery rate because less hydrogen is lost as water in the furnace flue gas (lines 32 and 48). Therefore, for example, the generation of steam should be minimized to facilitate heat integration within the process.

[0091] Figure 1 Ammonia is shown as both a fuel (lines 34 and 44) ​​and a feed (line 6), and it also shows product hydrogen (lines 40 and 44) ​​as a fuel - in practice, it is likely that only one of these streams is used as a fuel. In this regard, Figure 2 Depicted with Figure 1 A process similar to that of , in which ammonia is used as the fuel (line 34), but not the product hydrogen. Figure 2 All other features of the process described in Figure 1 The same and common features in the present invention have been given the same reference numerals.

[0092] The inventors have realised that stable combustion of ammonia is facilitated if hydrogen is also used as a fuel, especially during start-up and preheating.

[0093] Figure 3 Depicted with Figure 2 In this process, the recovery of hydrogen from the PSA (line 30) can be adjusted to provide a tail gas (line 18) which, when combusted, will provide all the heat required for the process, thus eliminating the need for trimming the fuel. Figure 3 All other features of the process described in Figure 1 The same and common features in the present invention have been given the same reference numerals.

[0094] As discussed above, if there are viable alternative sources of renewable energy for the cracking reaction, recovery of hydrogen from the PSA tail gas may be considered to increase the net hydrogen production from the process in addition to the hydrogen produced from the PSA. Such a process may use a membrane having a selective layer that is permeable to hydrogen but relatively impermeable to nitrogen to separate hydrogen from a nitrogen-rich PSA tail gas stream ( Figure 4 ).

[0095] If membranes are used as part of the separation process, ammonia may need to be removed specifically but not exclusively, as the membrane material cannot tolerate high concentrations of ammonia, and ammonia is a fast gas and will permeate with the hydrogen and so will accumulate in the process if not removed. Ammonia can be removed upstream of the membrane by, for example, water washing or other well-known techniques for ammonia removal. Ammonia can be recovered from the aqueous ammonia solution produced in the water wash using a stripping tower, and the recovered ammonia can be recycled to the feed to the cracking reactor. This could theoretically increase the hydrogen recovery from the process to 100%. Recovery of ammonia from the cracked gas simplifies the hydrogen purification step, increases the recovery of hydrogen from the ammonia if the separated ammonia is recovered as feed, and also removes ammonia from the feed to the combustor, eliminating the contribution to NO caused by burning the ammonia. X concerns arising from.

[0096] It may also be necessary to remove water from the feed ammonia to prevent damage to the ammonia cracking catalyst. Ammonia typically has a small amount of water added to the ammonia to prevent stress corrosion cracking in the container during transportation and storage. This may need to be removed. However, the removal of water can be incorporated into the stripping tower described above. The ammonia will evaporate at the required pressure, and care should be taken in the design of the evaporator to ensure that water is also brought to the stripping tower along with the ammonia from the evaporator. This ammonia, which is mainly in the gas phase, enters the midpoint of the tower, and pure ammonia leaves from the top of the tower. The tower has a partial condenser (condensing only enough liquid for reflux), and the overhead vapor contains the feed ammonia (without water) plus ammonia recovered from the cracker gas stream.

[0097] It may be more energy efficient to first feed the cracked gas into a membrane to produce a hydrogen-rich permeate stream and a nitrogen-rich retentate stream that can be discharged. The hydrogen-rich permeate can be further purified in the PSA. A second membrane can be added to the PSA tail gas stream to further increase the overall hydrogen recovery. This configuration will greatly reduce the size of the tail gas compressor.

[0098] The use of a membrane separator to increase the recovery of hydrogen allows nitrogen to be exhausted from the process without passing through the combustion section of the process. In processes where the nitrogen stream is under pressure, it would be beneficial to expand the nitrogen to atmospheric pressure before exhausting it through an expansion turbine to recover power. If the pressurized nitrogen is heated prior to expansion using heat available in the flue gas or cracked gas stream, the amount of power recovered will be increased.

[0099] Figure 4 A process involving a membrane for recovering hydrogen from a PSA tail gas while eliminating nitrogen is depicted. Figure 4 The process and Figures 1 to 3 Features common to the processes in the present invention have been given the same reference numerals. Figure 4 Discussion of new features in .

[0100] The renewable fuel source (line 50) is heated in a heat exchanger (E101) and fed (line 52) to a furnace (10) for combustion to heat the catalyst filled tubes of the cracking reactor (8).

[0101] The cooled cracked gas (line 28) is mixed with the compressed hydrogen-rich permeate gas (line 62) to form a mixed gas, which is fed (line 64) to the PSA unit (26). The mixed gas is separated to form a hydrogen product (line 30). The tail gas (line 54) from the PSA is compressed in the first compressor (K301), and the compressed gas is fed (line 56) to the membrane separation unit (M301) to produce a hydrogen-rich permeate stream (line 58) and a nitrogen-rich retentate stream (line 60). The membrane separation unit can consist of a plurality of membrane devices arranged in parallel or in series, as determined by the feed flow rate and the desired hydrogen recovery rate. The permeate stream is compressed in a second compressor (K302) to form a compressed hydrogen-rich gas (line 62) which is recycled (line 64) to the PSA (26) along with the cooled cracked gas (line 28) to increase the overall recovery of hydrogen.

[0102] Assuming the purity of the hydrogen produced by the membrane is high enough, part (line 68) of the hydrogen permeate gas can be mixed with the hydrogen product (line 30) after compression, with the remainder being recycled (line 64) to the PSA together with the cooled cracked gas (line 28). Mixing part of the hydrogen permeate gas with the hydrogen product in this way will reduce the required size of the PSA and the power of K301 and K302.

[0103] Alternatively, all of the compressed hydrogen-rich permeate gas (line 62) may be mixed with the hydrogen product (line 30), none of which is recycled to the PSA (26).

[0104] Figure 4 There is a system for removing and recovering ammonia from the cracked gas, depicted as unit 90. The system may involve a scrubber for removing ammonia from the gas with water and a stripper (or stripping section in a distillation column system) for recovering ammonia from the water, or may involve a TSA unit for removing ammonia by adsorption (see US10787367).

[0105] However, it is also possible to condense ammonia from the compressed PSA off-gas stream using cold feed ammonia, such as Figure 5 shown. Figure 5 The process and Figures 1 to 4 Features common to the processes have been given the same reference numerals. Figure 5 Discussion of new features in .

[0106] exist Figure 5 In the process, the PSA tail gas (line 54) is compressed in the compressor (K301), and most of the water present in the PSA tail gas will remain as condensate in the separator of the intercooler and / or aftercooler (not shown) of the compressor (K301). The tail gas compressed gas (line 70) is cooled by heat exchange in the heat exchanger (E102) to condense ammonia and any residual water from the compressed gas. The cooled gas (line 72) is fed to the phase separator (74) to remove the condensate. The ammonia-poor gas (line 76) is heated by heat exchange with the compressed gas in the heat exchanger (E102), and the heated gas is supplied (line 80) to the membrane separation unit (M301). The ammonia condensate is mixed (line 78) with the cold feed ammonia (line 4), and the cold feed ammonia is also heated by heat exchange with the compressed gas in the heat exchanger (E102).

[0107] Ammonia is usually passed through the membrane along with the recycled hydrogen, so a cryogenic separator is one way to remove and recycle the ammonia to the feed stream. This process also produces a purge stream (line 60) that may contain about 98% nitrogen. If this stream contains residual ammonia, then the stream needs to be treated before it can be discharged. However, the purge stream is under pressure and can be purified to produce a product nitrogen stream.

[0108] In general, Figure 5 The process depicted in generally results in very high hydrogen recoveries from ammonia (about 99%), but does involve more compression power than other possible cycles.

[0109] Figure 6 is another configuration involving a membrane. Figure 6 The process and Figures 1 to 5 Features common to the processes of the present invention have been given the same reference numerals. Figure 6 Discussion of new features in .

[0110] exist Figure 6 In the process, the heated ammonia-poor gas is supplied (line 80) to the first membrane separation unit (M301) to recover the hydrogen-rich permeate gas, which is mixed (line 82) to the feed (line 28) to the PSA device (26) without compression. The nitrogen-rich retentate gas containing residual hydrogen is fed (line 84) from the first membrane separator (M301) to the second membrane separation unit (M302) to recover the hydrogen-rich permeate gas (line 58), which is combined with the PSA tail gas (line 54) and compressed in the compressor (K301). The nitrogen-rich retentate gas (line 60) leaves the second membrane separation unit (M302) and is discharged or purified. If there are combustible components in the nitrogen-rich retentate gas, the gas can be used as a fuel or can be burned.

[0111] Figure 6 The power demand in Figure 5 , because the process only requires a PSA tail gas compressor, and Figure 5 The process in the still needs membrane compressor. However, Figure 6 The hydrogen recovery rate in the process is still very high, for example, about 99%.

[0112] The scope of the present invention is not limited by the specific aspects or embodiments disclosed in the examples, which are intended to illustrate several aspects of the present invention, and any functionally equivalent embodiments are within the scope of the present invention. Various modifications of the present invention, in addition to those shown and described herein, will become apparent to those skilled in the art and are intended to fall within the scope of the appended claims.

[0113] The invention will now be illustrated with reference to the following inventive examples and by comparison with the following reference examples. For simulation purposes, both the inventive examples and the reference examples assume that the cleavage reaction reaches equilibrium at 11 bar and 500°C.

[0114] Reference Example 1

[0115] Figure 2 The process depicted in has been simulated by a computer (Aspen Plus, Version 10, Aspen Technology, Inc.), and the results are depicted in Table 1.

[0116]

[0117]

[0118] Table 1

[0119] In this reference example, the hydrogen recovery from ammonia is 77.18%, with a PSA recovery of 83.5%. The total power of the ammonia feed pump (P201), ammonia fuel pump (P202) and blower (K201) is about 1.36 kW.

[0120] Reference Example 2

[0121] Figure 3 The process depicted in has been simulated by computer (Aspen Plus, version 10), and the results are depicted in Table 2.

[0122]

[0123] Table 2

[0124] In this reference example, the hydrogen recovery from ammonia is 77.05%, with a PSA recovery of 79.4%. The total power of the ammonia feed pump (P201) and the blower (K201) is about 1.37 kW.

[0125] Invention Example 1

[0126] Figure 5 The process depicted in has been simulated by computer (Aspen Plus, version 10), and the results are depicted in Table 3.

[0127]

[0128] Table 3

[0129] In the present example, the hydrogen recovery from ammonia is 98.95%.In addition, the total compression power requirement (PSA exhaust compressor and membrane compressor) is about 60.59 kW.

[0130] Invention Example 2

[0131] Figure 6 The process depicted in has been simulated by computer (Aspen Plus, version 10), and the results are depicted in Table 4.

[0132]

[0133] Table 4

[0134] In the present example, the hydrogen recovery from ammonia is 98.95%. In addition, the total compression power requirement (PSA tail gas compressor only) is about 56.54 kW.

Claims

1. A method for producing hydrogen from ammonia, comprising: Pressurizing liquid ammonia; heating the liquid ammonia by heat exchange with one or more hot fluids to produce heated ammonia; Combusting fuel in the furnace to heat reaction tubes containing the catalyst and to form flue gases; supplying the heated ammonia to the catalyst-containing reaction tube to crack the ammonia into cracked gas containing hydrogen and nitrogen; Purifying the cracked gas in a PSA unit to produce hydrogen product gas and PSA tail gas; Recovering ammonia from the PSA tail gas and recycling the recovered ammonia to the ammonia supplied to the catalyst-containing reaction tube; as well as separating gases derived from the PSA tail gas using a membrane separator to discharge a nitrogen-rich retentate gas and recycle a hydrogen-rich permeate gas for further processing in the PSA unit and / or for blending into the hydrogen product gas, wherein the fuel comprises one or more of ammonia, the PSA tail gas, hydrogen and methane; and The one or more thermal fluids include the flue gas and / or the cracked gas.

2. The method of claim 1, comprising compressing the PSA tail gas before supplying it as feed to the membrane separator.

3. The method of claim 1 comprising compressing the hydrogen-rich permeate gas prior to further processing in the PSA and / or blending into the hydrogen product gas.

4. The method according to claim 1, comprising: compressing the PSA tail gas to produce compressed PSA tail gas; cooling the compressed PSA tail gas by heat exchange with one or more cold fluids to condense ammonia in the compressed PSA tail gas; as well as separating the condensed ammonia from the compressed PSA tail gas to produce an ammonia-depleted PSA tail gas, The ammonia-depleted PSA tail gas is supplied as feed to the membrane separator.

5. The method according to claim 4, wherein: The one or more cold fluids include pressurized liquid ammonia and the ammonia-depleted PSA tail gas.

6. The method according to claim 4, wherein: The condensed ammonia is combined with the pressurized liquid ammonia.

7. The method according to any one of claims 4 to 6, wherein: The membrane separator comprises a first membrane separation unit and a second membrane separation unit connected in series, and the method further comprises: supplying the ammonia-depleted PSA tail gas to the first membrane separation unit to produce the hydrogen-rich permeate gas and the nitrogen-rich retentate gas including residual hydrogen for further processing in the PSA; and The nitrogen-rich retentate gas is supplied to the second membrane separation unit to discharge the nitrogen-rich retentate gas and produce a hydrogen-rich permeate gas that is combined with the PSA tail gas.

8. The method according to any one of claims 1 to 6, wherein: The membrane separator includes a single membrane separation unit.

9. The method of any one of claims 1 to 6, further comprising expanding the nitrogen-rich retentate gas in a turbine to recover power.

10. The method of claim 9, further comprising heating the nitrogen-enriched retentate gas by heat exchanging with the one or more hot fluids prior to expanding the nitrogen-enriched retentate gas.

11. The method of any one of claims 1 to 6, further comprising purifying the nitrogen-rich retentate gas to produce a nitrogen product.

12. The method of claim 1 further comprising vaporizing the liquid ammonia by heat exchange with one or more hot fluids to produce heated ammonia.

13. An apparatus for producing hydrogen from ammonia, comprising: A pump for pressurizing liquid ammonia; at least one first heat exchanger in fluid communication with the pump, the first heat exchanger for heating the liquid ammonia from the pump by exchanging heat with one or more hot fluids to produce heated ammonia; a catalyst-containing reaction tube in fluid communication with the first heat exchanger, the reaction tube being used to crack the heated ammonia from the first heat exchanger to produce a first cracked gas containing hydrogen and nitrogen; a furnace in thermal communication with the catalyst-containing reaction tubes, the furnace being configured to burn a fuel to heat the catalyst-containing reaction tubes and form a flue gas; a cracked gas conduit for feeding cracked gas from the catalyst-containing reaction tube to at least one heat exchanger; a flue gas duct for feeding flue gas from the furnace to the at least one heat exchanger; a PSA unit in fluid communication with the catalyst-containing reaction tube, the PSA unit being used to purify the cracked gas after passing through the at least one heat exchanger to produce a hydrogen product gas and a PSA tail gas; an ammonia removal system, the ammonia removal system being located downstream of the PSA unit and being used to remove ammonia from the PSA tail gas so as to recycle it to the catalyst-containing reaction tubes; as well as a membrane separator in fluid communication with the ammonia removal system, the membrane separator for separating gases derived from the PSA off-gas to discharge a nitrogen-rich retentate gas and a hydrogen-rich permeate gas; Wherein the apparatus comprises a conduit for feeding the hydrogen-enriched permeate gas to the PSA unit for further processing and / or a conduit for combining the hydrogen-enriched permeate gas with the hydrogen product gas.

14. The apparatus according to claim 13, comprising a PSA tail gas compressor downstream of the PSA unit for compressing the PSA tail gas for use in the membrane separator.

15. The apparatus of claim 13, comprising a hydrogen-rich permeate gas compressor downstream of the membrane separator for compressing the hydrogen-rich permeate gas for the PSA unit and / or for combination with the hydrogen product gas.

16. The apparatus of claim 13, comprising: a PSA tail gas compressor in fluid communication with the PSA device, the PSA tail gas compressor being used to compress the PSA tail gas; a heat exchanger in fluid communication with the PSA tail gas compressor, the heat exchanger for cooling the compressed PSA tail gas by exchanging heat with respect to one or more cooling fluids; a phase separator in fluid communication with the heat exchanger, the phase separator for separating condensed ammonia from the cooled compressed PSA tail gas to form an ammonia-depleted PSA tail gas; A conduit for supplying ammonia-depleted PSA tail gas from the phase separator to the membrane separator.

17. Apparatus according to claim 16, comprising a conduit for supplying condensed ammonia to the pressurised liquid ammonia.

18. Apparatus according to claim 16 or claim 17, wherein: The membrane separator comprises a first membrane separation unit and a second membrane separation unit connected in series, and the device further comprises: a conduit for supplying ammonia-depleted PSA tail gas from the phase separator to the first membrane separation unit; a conduit for supplying the hydrogen-enriched permeate gas from the first membrane separation unit to the PSA unit for further processing; a conduit for supplying nitrogen-rich retentate gas from the first membrane separation unit to the second membrane separation unit; and a conduit for supplying another hydrogen-rich permeate gas from the second membrane unit to the PSA off-gas.

19. The apparatus according to any one of claims 13 to 17, wherein: The membrane separator includes a single membrane separation unit.

20. Apparatus according to any one of claims 13 to 17, comprising a conduit for supplying nitrogen-rich retentate gas from the membrane separator to a vent.

21. The apparatus of claim 20, comprising a turbine located downstream of the membrane separator for expanding the nitrogen-rich permeate gas and recovering power.

22. Apparatus according to claim 21, comprising a heat exchanger upstream of the turbine for heating the nitrogen-rich permeate gas by heat exchange with one or more hot fluids.

23. The apparatus according to any one of claims 13 to 17, comprising a nitrogen purification device in fluid communication with the membrane separator, the nitrogen purification device for purifying the nitrogen-rich retentate gas to produce a nitrogen product.

24. The apparatus of claim 13, wherein: The at least one first heat exchanger is used to vaporize the liquid ammonia from the pump by exchanging heat with one or more hot fluids to produce heated ammonia.

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