Ammonia decomposition for green hydrogen

Abstract

The reduction in the water content of ammonia used in the ammonia decomposition process allows for the use of water-insoluble decomposition catalysts. The water removal process can also be used to recover and recirculate ammonia from the decomposition gas.

Description

Background Technology

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

[0002] For use in commercial fuel cells, ammonia must be converted back into hydrogen through a reaction.

[0003]

[0004] This is an endothermic process, meaning it requires heat, and is carried out via a catalyst. This process is known as cracking. The generated gas (or "cracking gas") is a combination of hydrogen (H2) and nitrogen (N2). Since the cracking reaction is an equilibrium reaction, some residual ammonia is also present. In most current cracker applications, the hydrogen + nitrogen mixture is utilized as is. However, since ammonia can be harmful to fuel cells, this stream, with the ammonia properly removed—for example by washing with water—can be used directly in the fuel cell. However, if hydrogen is used for vehicle fueling, the presence of nitrogen presents a disadvantage to the process. Fuel into the vehicle fueling system is compressed to a significant pressure—up to 900 bar. This means that the nitrogen, which is the diluent in the process, is also compressed, taking up power and storage volume, and increasing the anode gas purge requirements, thereby reducing efficiency. Therefore, it is advantageous for hydrogen to be used for vehicle fueling so that the hydrogen + nitrogen can be purified.

[0005] Small-scale cracking reactors, or "crackers," typically use pressure swing adsorption ("PSA") to separate cracking gases, recover hydrogen, and generate PSA tail gas (or off-gas). However, these crackers are generally electrically heated, and the PSA tail gas is typically released into the atmosphere.

[0006] As is common in hydrogen production from steam methane reforming (SMR) reactors, PSA can be used to purify nitrogen + hydrogen. The cracking reaction is carried out in packed tubes using a catalyst that is heated externally by a furnace (see GB1142941).

[0007] GB1142941 initiates a process for producing city gas from ammonia. Ammonia is decomposed, and the decomposition gas is washed with water to remove residual ammonia. The residual ammonia is recovered using a distillation column and recirculated to the decomposition process. To produce city gas for distribution, the purified hydrogen / nitrogen mixture is then enriched into propane and / or butane vapor.

[0008] US6835360A discloses an endothermic catalytic reactor for converting hydrocarbon feedstocks and methanol into useful gases, such as hydrogen and carbon monoxide. The apparatus comprises a tubular endothermic catalytic reactor combined with a radiant combustion chamber. The resulting decomposition gases pass through a gas control system and are used directly in a fuel cell.

[0009] GB977830A initiates a process for decomposing ammonia to produce hydrogen. In this process, hydrogen is separated from nitrogen by passing the decomposition gas through a bed of molecular sieves that absorb nitrogen. The nitrogen can then leave the bed and be stored in a holder.

[0010] JP5330802A discloses an ammonia decomposition process in which ammonia is brought into contact with an ammonia decomposition catalyst at a pressure of 10 kg / cm² (or about 9.8 bar) and a temperature of 300 to 700°C. Hydrogen is recovered from the decomposition gas using a PSA device. The reference mentions that desorbed nitrogen may be used to extend the upstream process, but no details are provided.

[0011] US2007 / 178034A initiates a process in which a mixture of ammonia and hydrocarbon feedstocks passes through a combustion steam reformer at 600°C and 3.2 MPa (or about 32 bar), where it is converted into synthesis gas containing about 70 vol.% hydrogen. The synthesis gas is enriched with hydrogen in a shift reaction, cooled, and condensate is removed. The resulting gas is fed into a PSA system to produce a purified hydrogen product containing 99 vol.% hydrogen or more. Off-gas from the PSA system is fed into the combustion steam reformer as fuel.

[0012] CN111957270A initiates a process in which ammonia is decomposed in a tubular reactor within a furnace. The decomposition gas is separated by adsorption to produce hydrogen gas and nitrogen-rich off-gas. The fuel demand of the furnace appears to be satisfied using a combination of decomposition gas, hydrogen product gas and / or off-gas.

[0013] US2020 / 123006 discloses a process for decomposing ammonia using heat generated by the non-catalytic partial oxidation of ammonia with gaseous oxygen. Residual ammonia is separated from the process gas and recycled for use in the oxidation process.

[0014] In general, there is a demand for improved processes for the production of hydrogen from ammonia, and specifically for 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.

[0015] In the following discussion of embodiments of the present invention, given pressures are absolute pressures unless otherwise stated.

[0016] According to a first aspect of the present invention, a method for generating hydrogen from ammonia is provided, said method comprising:

[0017] Pressurizing a liquid ammonia feed containing water as a trace impurity to produce a pressurized liquid ammonia feed;

[0018] Combusting primary fuel in a furnace to heat catalyst-containing reactor tubes and to form flue gas;

[0019] Supplying heated ammonia to catalyst-containing reactor tubes to cause the decomposition of ammonia into hydrogen gas, nitrogen gas, and decomposition gas containing residual ammonia;

[0020] Cooling the decomposition gas by heat exchange to generate cooled decomposition gas;

[0021] Scrubbing ammonia from cooled decomposition gas using water in a scrubbing column to produce ammonia-depleted decomposition gas and a water-soluble ammonia solution;

[0022] Stripping ammonia from an aqueous ammonia solution in a distillation column system to produce water-depleted ammonia feed vapor and an aqueous ammonia-depleted base liquid;

[0023] Heating water-depleted ammonia feed vapor by heat exchange with one or more high-temperature fluids to generate heated ammonia;

[0024] The method includes purifying ammonia-depletion decomposition gas in a first PSA device to produce a first hydrogen product gas and a first PSA tail gas, and

[0025] The pressurized liquid ammonia feed is characterized in that it is heated and vaporized by heat exchange with one or more high-temperature fluids to produce ammonia feed vapor that is fed into a distillation column system to remove water from the ammonia feed vapor, and

[0026] One or more high-temperature fluids include decomposition gas and / or flue gas.

[0027] The liquid ammonia feed 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 to about 45 bar, or in the range of about 30 bar to about 40 bar.

[0028] Typically, liquid ammonia feeds contain a small amount of water added to prevent stress corrosion cracking in containers during transport and storage. Liquid ammonia feeds contain water as a trace impurity in amounts ranging from, for example, about 0.1 wt.% to about 0.5 wt.%, typically about 0.2 wt.%. Water must be removed from the feed ammonia to prevent damage to the ammonia decomposition catalyst. Some catalysts, for example, iron-based catalysts, are water-insoluble and may be less compatible with water-containing feeds.

[0029] The liquid ammonia feed is heated and vaporized to generate ammonia feed vapor. The temperature of the ammonia feed vapor supplied to the distillation column system is typically in the range of about 25°C to about 90°C, ensuring that water is also transported to the distillation system along with the vaporized ammonia. The actual temperature is determined by the boiling point of ammonia at the pumped pressure. The vaporized ammonia entering the column may contain a small (e.g., 1 to 2%) water-rich liquid phase to be transported to the distillation system. Alternatively, the water-containing ammonia stream may be vaporized to leave a water-rich liquid stream that can be fed separately to the distillation system.

[0030] Water is removed from the ammonia feed vapor using a distillation column system. Since the ammonia feed has already vaporized, it can be introduced near the top of the distillation column system. The resulting water-depleted ammonia feed vapor typically contains less than 5 ppm of water, preferably less than 2 ppm, and more preferably less than 1 ppm.

[0031] The water-depleted ammonia feed vapor is typically heated to a temperature higher 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, to produce ammonia.

[0032] The temperature is ultimately determined by the identity of the catalyst, the operating pressure, and the desired "slip," that is, the amount of ammonia that passes through the decomposition reactor without decomposing. In this regard, the process typically operates with a slip of only about 4% if the decomposition process is operated at 5 bar and 350°C, which is close to equilibrium. At temperatures exceeding about 700°C, problems with some constituent materials may occur due to any significant pressure.

[0033] The decomposition reaction takes place in catalyst-filled reactor tubes heated by a furnace. However, theoretically, any heterogeneous catalyst gas reactor can potentially be used for conversion.

[0034] There are numerous catalysts known in the art to be useful for ammonia decomposition reactions, and any of these conventional catalysts may be used in the present invention. Iron-based catalysts are commonly used in the Haber-Bosch process for the production of ammonia; therefore, since both processes are equilibrium-limiting reactions, it is expected that such iron-based catalysts can be utilized for the ammonia decomposition process. However, it is well known in ammonia production processes that catalysts are poisoned by ppm levels of water and oxygen present in the feed. Therefore, if iron-based catalysts are used, it is expected that water in the feed ammonia needs to be removed.

[0035] The primary fuel for the furnace typically includes hydrogen, ammonia, cracking gas and / or PSA tail gas, but the primary fuel preferably includes methane. The fuel may be pure methane, but more likely natural gas or biogas.

[0036] A PSA device can operate a PSA cycle or a vacuum swing adsorption (VSA) cycle. Suitable PSA cycles include any of the cycles disclosed in US9381460, US6379431 and US8778051, which are incorporated herein by reference.

[0037] In a conventional PSA system with a single PSA device, hydrogen recovery is typically in the range of about 75% to 85%. However, there are two options to increase the recovery. First, the PSA tail gas can be recirculated to the first PSA. In these embodiments, the first PSA tail gas can be compressed, and the compressed PSA tail gas is recirculated to the first PSA device. Recirculating in this manner can achieve a total hydrogen recovery of about 94% to about 96%.

[0038] Alternatively, two PSA devices may be used in series, and the first PSA tail gas is further processed in the second PSA device. In these embodiments, the process comprises compressing the first PSA tail gas to produce compressed PSA tail gas; and purifying the compressed PSA tail gas in the second PSA device to produce the second PSA tail gas and the second hydrogen product gas. In this way, further processing can achieve a total hydrogen recovery of about 95% to about 97%. For example, if the first PSA device achieves 83% recovery and the second PSA achieves 80% recovery, the total recovery is 96.6%.

[0039] In these embodiments, the second hydrogen gas may be combined with the first hydrogen product gas to form a combined hydrogen product gas.

[0040] Similar to the first PSA device, the second PSA device may operate a PSA cycle or a vacuum swing adsorption (VSA) cycle. Suitable PSA cycles include any of the cycles disclosed in US9381460, US6379431, and US8778051.

[0041] PSA tail gas from the first PSA device or the second PSA device may be supplied as a secondary fuel for combustion in the furnace. Preferably, the PSA tail gas is heated by heat exchange with one or more high-temperature fluids and / or optionally mixed with a primary fuel before being supplied to the furnace.

[0042] The PSA tail gas, or gas derived therefrom, can be separated using a membrane separator to release a nitrogen-rich residue gas for further processing in the PSA device and / or mixing with the hydrogen product gas, and to recirculate the hydrogen-rich permeate gas.

[0043] Like hydrogen, ammonia is a "fast gas" that easily permeates across membranes used for gas separation. Some membranes, such as those composed of polyamide or polysulfone polymers, are more tolerant of ammonia. However, some membranes, such as those composed of polyimide polymers, are less tolerant of ammonia. Therefore, ammonia is typically removed, or its concentration is at least reduced upstream of the membrane separator.

[0044] Cooled decomposition gas is typically generated by heat exchange with pressurized liquid ammonia feed and water-depleted ammonia feed vapor, and optionally with a primary fuel source, air supply to a furnace, and / or PSA tail gas.

[0045] Ammonia is removed from the cooled decomposition gas by absorption in water, for example, by washing the gas with water in a scrubbing column. The resulting ammonia-depleted gas and aqueous ammonia solution are separated so that the ammonia-depleted gas can be further processed without ammonia causing any difficulties. Ammonia is recovered from the aqueous ammonia solution by stripping it in a distillation column system. Preferably, the aqueous ammonia solution is pumped from the scrubbing column to the distillation column system. This process can also be applied to the PSA tail gas before it is fed to a membrane separator.

[0046] Recovering ammonia from decomposition gas not only simplifies hydrogen purification steps but also increases hydrogen recovery from ammonia, as the recovered ammonia is recycled back into the ammonia feed. This removes ammonia from the feed to the burners, thereby reducing concerns regarding the generation of nitrogen oxides (NOx) caused by the combustion of ammonia.

[0047] A distillation column system typically generates overhead vapor containing ammonia and a water-soluble base liquid. A portion of the base liquid from the distillation column may be removed. Additionally or alternatively, all or part of the base liquid from the distillation column may be reboiled. In this regard, the base liquid in or from the distillation column system may be reboiled by heat exchange with one or more high-temperature fluids.

[0048] In other embodiments, the base liquid in or from the distillation column system is reboiled using an electric-driven heater. The heater may be at least partially powered by electricity generated from at least one renewable source, such as solar, wind, or tidal energy, as this reduces the carbon intensity of the process.

[0049] A water-soluble base liquid from a distillation column system can be supplied to a scrubbing column. Before entering the scrubbing column, the water-soluble base liquid is cooled by heat exchange, preferably with a coolant, preferably a water-soluble ammonia solution supplied from the scrubbing column to the distillation system.

[0050] In some preferred embodiments, overhead vapor in or from the distillation column system is partially condensed by heat exchange with the coolant to produce a condensation stream and water-depleted ammonia feed vapor. The condensation stream is then used as reflux for the distillation column system.

[0051] According to a second aspect of the present invention, an apparatus for generating hydrogen from ammonia is provided, and said apparatus:

[0052] A pump for pressurizing a liquid ammonia feed containing water as a trace impurity to produce a pressurized liquid ammonia feed;

[0053] At least one first heat exchanger fluidly connected to a pump to vaporize pressurized liquid ammonia by heat exchange with one or more high-temperature fluids to generate ammonia feed vapor;

[0054] Catalyst-containing reactor tubes fluidly connected to the first heat exchanger(s) to decompose heated ammonia from the first heat exchanger(s) to produce decomposition gas containing hydrogen gas, nitrogen gas, and residual ammonia;

[0055] A furnace that heats the catalyst-containing reactor tubes and heat-communicates with the catalyst-containing reactor tubes for the combustion of fuel to form flue gas;

[0056] A flue gas conduit for supplying flue gas from a furnace to at least one first heat exchanger(s);

[0057] A decomposition gas conduit for supplying decomposition gas to at least one first heat exchanger(s);

[0058] A scrubbing column fluidly connected to at least one first heat exchanger(s) to scrub ammonia from a decomposition gas cooled after passing through the first heat exchanger(s) using water to generate an ammonia-depleted decomposition gas and a water-soluble ammonia solution;

[0059] A distillation column system fluidly connected to a scrubbing column to strip ammonia from a water-soluble ammonia solution to produce water-depleted ammonia feed vapor and water-soluble ammonia-depleted base liquid;

[0060] An ammonia feed vapor conduit for supplying ammonia feed vapor from the first heat exchanger(s) to the distillation column system;

[0061] A water-depleted ammonia feed vapor conduit for supplying a water-depleted ammonia feed to a first heat exchanger(s) in a distillation column system for additional heating by heat exchange with one or more of the high-temperature fluids to produce heated ammonia;

[0062] A PSA device fluidly connected to a scrubbing column to purify ammonia-depletion decomposition gas to produce hydrogen product gas and a first PSA tail gas; and

[0063] It includes a hydrogen product gas conduit for removing hydrogen product gas from a PSA device.

[0064] The furnace may be separate from the catalyst-filled reactor tubes, but the furnace and the catalyst-filled reactor tubes are preferably integrated within the same unit. In preferred embodiments, a steam methane reforming (SMR) type reactor is used, wherein the furnace includes a radiation section passing through the catalyst-containing reactor tubes.

[0065] A compressor may be provided below the first PSA device to compress the first PSA tail gas to produce compressed PSA tail gas. The compressor may consist of one or more stages, and cooling will occur between each stage and after the final stage. Water will typically condense from the compressed PSA tail gas between stages or at the final cooler stage. The aqueous condensate is typically removed after each cooling stage of the compressor, and a small amount of ammonia will exit the first PSA tail gas along with this condensate.

[0066] In some preferred embodiments, the device is:

[0067] A compressor fluidly connected to a first PSA device to compress a first PSA tail gas to generate a compressed PSA tail gas; and

[0068] It includes a recirculation conduit for recirculating compressed PSA tail gas to a first PSA device.

[0069] In some alternative preferred embodiments, the device is:

[0070] A compressor fluidly connected to a first PSA device to compress a first PSA tail gas to generate a compressed PSA tail gas;

[0071] A second PSA device fluidly connected to a compressor to purify the compressed PSA tail gas to generate the second PSA tail gas and the second hydrogen gas;

[0072] A second hydrogen gas conduit for removing the second hydrogen gas from the second PSA device; and

[0073] It includes a second PSA tail gas conduit for removing the second PSA tail gas from the second PSA device.

[0074] In these embodiments, the first and second hydrogen gas conduits are combined to form a hydrogen product gas conduit.

[0075] In some preferred embodiments, a portion or all of the first PSA tail gas and / or the second PSA tail gas is recirculated and may be used as a second fuel for combustion in the furnace. In these embodiments, the device includes a conduit for supplying the first PSA tail gas and / or the second PSA tail gas to the furnace. Preferably, the conduit is fluidly connected to a first heat exchanger(s) for heating by heat exchange with one or more high-temperature fluids before combustion in the furnace.

[0076] The present invention will now be described merely by example with reference to the following drawings. Brief explanation of the drawing

[0077] FIG. 1 is a process flow diagram of a first reference example of an ammonia decomposition process for generating hydrogen; FIG. 2 is a process flow diagram of another reference example based on the ammonia decomposition process of FIG. 1, wherein no hydrogen product is used as fuel; FIG. 3 is a process flow diagram of an additional reference example based on the ammonia decomposition process of FIG. 1 and 2, wherein only PSA tail gas is used as fuel; FIG. 4 is a process flow diagram of an ammonia decomposition process including the recovery of residual ammonia from decomposition gas; and FIG. 5 is a process flow diagram of an example of an ammonia decomposition process for generating hydrogen, wherein water is removed from the feed ammonia according to the present invention. Specific details for implementing the invention

[0078] A process for generating hydrogen by decomposing ammonia is described herein. The process has a specific application in generating so-called "green" hydrogen, which is hydrogen produced using renewable energy instead of fossil fuels. In this case, ammonia is typically produced by electrolyzing water using electricity generated from 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 more easily transported than hydrogen. After reaching its destination, the ammonia is then decomposed to regenerate hydrogen.

[0079] In this invention process, the heat required for the reaction is typically provided by the combustion of PSA tail gas (usually containing some residual hydrogen and ammonia) in a furnace. If the PSA tail gas has an insufficient calorific value compared to the vaporized ammonia, a portion of the product hydrogen, or another fuel, may also be used.

[0080] In practice, natural gas can be used as fuel, along with PSA tail gas, as is done in SMRs for hydrogen. However, there are incentives for using "renewable fuels" out of a desire to maintain the "green" or renewable credentials of the hydrogen thus produced. This can be decomposed "renewable" ammonia, ammonia itself, another renewable energy source such as biogas, or electric heating—whether localized to the decomposition process or in contrast to the renewable electricity used to generate hydrogen transported in the form of ammonia, where the electricity is actually from a renewable source.

[0081] A reference example of the process is shown in FIG. 1. The process takes liquid ammonia from storage (not shown). The ammonia to be decomposed (Line 2) is pumped as a liquid at a pressure higher than the desired decomposition pressure (pump P201) (see GB1142941). The reaction pressure is a trade-off between the operating pressure and the conversion pressure according to Le Chatelier's principle. There is an incentive to operate the reactor (8) at a higher pressure because the pumped liquid ammonia requires less power and capital than compressing the product hydrogen.

[0082] Pressurized liquid ammonia (line 4) is then heated and vaporized (if it is below critical pressure) and further heated through a heat exchanger (E101) to a temperature higher than 250°C using heat available from the decomposition gas and flue gas from the furnace leaving the reaction tubes. In this drawing, the heat exchanger (E101) is shown as a single heat exchanger, but in reality, it would be a series of heat exchangers in a network.

[0083] The initial heating and vaporization of pressurized liquid ammonia can occur against alternative heat sources, such as cooling water or ambient air. Typical reaction temperatures are higher than 500°C (see US2601221), and palladium-based systems can operate at 600°C and 10 bar, whereas RenCat's metal oxide-based systems operate at 300°C and less than 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 typically the optimization of several factors. The decomposition of ammonia into hydrogen and nitrogen prefers lower pressure, but other factors, such as power consumption (minimized by pumping feed ammonia rather than compressing product hydrogen) and PSA size (smaller at higher pressures), prefer higher pressure.

[0084] High-temperature ammonia (line 6) enters the reaction tubes of the reactor (8) at a desired pressure, where additional heat is provided by the furnace (10) to decompose the ammonia into nitrogen and hydrogen. The resulting mixture of residual ammonia, hydrogen and nitrogen exits the reaction tubes of the reactor (8) at the reaction temperature and pressure (line 12). The reaction products are cooled in a heat exchanger (E101) against a combination of feed ammonia (from line 4), furnace fuel (in this case line (14), pumped ammonia from pump (P202) and line 16; PSA tail gas from line 18; and product hydrogen to be used as fuel from line 20) and combustion air (from line 22, fan (K201) and line 24) to reduce the temperature as close as possible to what is required for the inlet of the PSA device (26). Any residual heat from the decomposed gas mixture (line 28) is removed in a water cooler (not shown) to achieve an inlet temperature to the PSA device (26) in the range of about 20°C to about 60°C, e.g. 50°C.

[0085] The PSA product (line 30) is pure hydrogen that complies with ISO standard 14687 - hydrogen fuel quality - residual ammonia < 0.1 ppmv and nitrogen < 300 ppmv at approximately the reaction pressure. The product hydrogen (line 30) may be further compressed (not shown) to fill tube trailers (not shown) for transport, or it may be liquefied in a hydrogen liquefier after any required compression. The PSA tail gas (line 18) or "purge gas" from the PSA device (26) is shown to be heated through a heat exchanger (E101) using the decomposition gas (line 12) or furnace flue gas (line 32) leaving the reaction tubes of the reactor (8) before being sent to the furnace as combustion fuel (at line 36). However, the PSA tail gas (line 18) may be supplied directly to the furnace (10) without heating.

[0086] The resulting heated ammonia fuel (line 34) and heated hydrogen (line 40) are described as being combined with heated PSA tail gas (line 36) in a mixer (42) (optionally) to produce a combined fuel (line 44) that is supplied 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 of the fuels may be supplied directly to the furnace without prior mixing. Heated air (for combustion of the fuel) is supplied to the furnace (10) in line 46.

[0087] One of the objectives of the preferred embodiments of the process is to maximize the amount of hydrogen produced by decomposing renewable ammonia. This means minimizing the amount of hydrogen used as fuel, or the amount of ammonia used directly as fuel. Therefore, thermal integration is important for preheating air (line 24) and ammonia (line 4) into the decomposer, for example, to properly utilize the high-temperature flue gas and decomposition gas, as this reduces the amount of "fuel" used in the burners of the furnace (10). This leads to higher hydrogen recovery as less hydrogen is lost as water in the furnace flue gas (lines 32 and 48). Therefore, steam generation must be minimized, for example, in support of process-in-process thermal integration.

[0088] Figure 1 illustrates ammonia provided as fuel (lines 34 and 44) ​​and feed (line 6), and it also illustrates product hydrogen as fuel (lines 40 and 44)—in reality, only one of these streams is likely to be used as fuel. In this regard, Figure 2 depicts a process similar to that of Figure 1, where ammonia is used as fuel (line 34) rather than product hydrogen. All other features of the process depicted in Figure 2 are identical to those in Figure 1, and common features are given the same reference numbers.

[0089] The inventors know that stable combustion of ammonia is also possible if hydrogen is used as fuel, especially during starting and warm-up.

[0090] FIG. 3 depicts a process similar to that described in FIG. 2. In this process, the recovery of hydrogen (line 30) from the PSA can be adjusted to provide tail gas (line 18) that provides all the heat required by the process when burned, thus eliminating the need for trim fuel. All other features of the process described in FIG. 3 are the same as those in FIG. 1, and common features are given the same reference numbers.

[0091] As discussed above, if there is a viable alternative source of renewable energy for the decomposition reactions, one might consider recovering hydrogen from the PSA tail gas to increase net hydrogen production from the process in addition to the hydrogen generated from the PSA. This process may utilize membranes in series or parallel to separate hydrogen from the nitrogen-rich PSA tail gas stream.

[0092] The decomposition gas (line 12) is removed and contains a residual amount of ammonia that can be recirculated to the decomposition process. This offers two advantages: first, it simplifies the adsorption process, and second, it allows for the recovery of un-decomposed ammonia back into the process by stripping ammonia from the water in the distillation system. Ammonia needs to be removed if membranes are used as part of a separation process, as membrane materials may not tolerate ammonia, and since ammonia is a fast-moving gas and permeable to hydrogen, it will accumulate in the process if not removed. NH3 can be removed upstream of the membrane, for example, by water washing or other well-known techniques for ammonia removal. The ammonia recovered in the ammonia removal step can be returned as a feed to the decomposition process using a distillation system to recover ammonia from the water used to absorb ammonia from the decomposition gas. This can theoretically increase hydrogen recovery from the process by up to 100%. Recovering NH3 from decomposition gas simplifies hydrogen purification steps and can increase the recovery of hydrogen from ammonia if separated ammonia is recovered as a feed, and can also remove ammonia from the feed to burners, thereby at least reducing and possibly eliminating concerns regarding the generation of NOx caused by burning NH3.

[0093] FIG. 4 depicts a process involving conventional means for recovering residual ammonia from decomposition gas and recirculating the recovered ammonia into catalyst-containing reactor tubes for decomposition. Features of the process in FIG. 4 that are common to the processes of FIG. 1 through 3 are given the same reference numbers. The following is a discussion of the new features in FIG. 4.

[0094] Fuel (line 50) is heated in a heat exchanger (E101). The resulting heated fuel (line 52) is combined with heated PSA tail gas (line 36) in a mixer (42) to produce a combined fuel (line 44) that is supplied to the furnace (10) for combustion to produce flue gas (line 32 and, after cooling in E101, line 48). However, one or more of the fluids may be supplied directly to the furnace without prior mixing. Heated air is supplied to the furnace (10) in line 46.

[0095] The decomposition gas is cooled in a heat exchanger (E101) with a combination of feed ammonia (from line 4), furnace fuel (in this case from line 50; PSA tail gas from line 18), and combustion air (from line 22, fan (K201), and line 24) to reduce the temperature as close as possible to that required for the inlet of the scrubbing column (60). The inlet temperature to the scrubbing column (60) is preferably in the range of about 5°C to about 30°C, e.g., about 10°C.

[0096] The cooled decomposition gas (line 28) is fed to a scrubbing column (60) where water (line 61) is used to recover residual ammonia from the decomposition gas to produce an ammonia-depleted decomposition gas and a water-soluble ammonia solution. The water-soluble ammonia solution is transferred to a distillation column system (72) (line 70) using a pump (P301). In some embodiments, it may not be necessary to use a pump for this purpose.

[0097] The distillation column system (72) removes water from the water-soluble ammonia solution to produce water-depleted ammonia feed vapor and water-soluble ammonia-depleted base liquid. The water-depleted ammonia feed vapor is recirculated through line 76 to the catalyst-containing reactor tubes for decomposition. The ammonia-containing overhead vapor is partially condensed by a condenser (82), and the two-phase fluid is phase-separated in a separator (80) to produce the condensed liquid (line 78) and water-depleted ammonia feed vapor (line 76), which are fed to the column system (72) as reflux.

[0098] The water-soluble base liquid from the column system (72) may be removed (line 86) or reboiled. In this regard, the water-soluble base liquid (line 88) may be heated and partially vaporized in a heat exchanger (E101) before being fed to the separator (92) via line 90. Alternatively, or otherwise, the water-soluble base liquid (line 88) may be heated and partially vaporized using an electric-driven heater (not shown). The separator (92) separates the liquid and vapor portions of the reboiled water-soluble base liquid. The vapor portion is fed back to the distillation column system (72) via line 94 to provide steam for the column system. The liquid portion of the reboiled liquid, a water-rich liquid containing the majority of completely stripped ammonia, is fed to the scrubbing column (60) via line 96. Alternatively, the heated and partially vaporized water-soluble base liquid (line 90) can be fed directly back into a distillation system (72) (not shown).

[0099] The water-rich liquid (line 96) derived from the reboiled water-soluble base liquid is cooled before entering the scrubbing column (60). Preferably, cooling is achieved by heat exchange with the water-soluble ammonia solution (lines 106 and 108) in the heat exchanger (98). The cooled water-rich liquid (line 100) is further cooled by heat exchange with the coolant in the heat exchanger (102) before being supplied to the scrubbing column (60).

[0100] The ammonia depletion decomposition gas is supplied to the first PSA device (26) (line 62). The decomposition gas is separated to form the first hydrogen product gas (line 30) and the tail gas (line 54). The tail gas can be supplied back to the furnace, optionally through a heat exchanger (E101) and a mixer (42) (not shown) (line 54).

[0101] As illustrated in FIGS. 4 and 5, as an alternative, the tail gas (line 54) from the first PSA device (26) is compressed in a compressor (K301) to produce compressed PSA tail gas (line 56) which is supplied to the second PSA device (64) to produce the second hydrogen product gas (line 66) and the second PSA tail gas (line 18). The second hydrogen product gas (line 66) is combined with the hydrogen product gas (line 30) from the first PSA device (26) to produce the combined hydrogen product gas (line 68). The second PSA tail gas (line 18) is heated by heat exchange with the decomposition gas (line 12) and flue gas (line 32) in a heat exchanger (E101) and then supplied to the furnace (10) as fuel (line 36 and line 44).

[0102] Optionally, a portion of the first PSA tail gas (from line 54) may be combined with the second PSA tail gas (line 18) to produce a combined PSA tail gas that can then be used as fuel in the furnace (10).

[0103] The second PSA tail gas, or the combined PSA tail gas, may optionally be supplied directly to the furnace (10) without heating and / or mixing with the primary fuel (lines 50 and 52) (not shown).

[0104] In another embodiment having only a single PSA device (26), the compressed PSA tail gas (line 56) can be recirculated back to the first PSA device (26) for purification with cooled ammonia-depleted decomposition gas (62) (not shown).

[0105] In additional embodiments, a first portion of the first PSA tail gas (line 54) is compressed in a compressor (K301) to produce compressed PSA tail gas (line 56) and may be further processed as described above. However, a second portion of the first PSA tail gas (not shown) may be fed back into the furnace through a flow control valve (not shown). The ratio of the PSA tail gas used as fuel to the primary fuel can be controlled, thereby controlling the carbon intensity of the process.

[0106] Water will need to be removed from the feed ammonia to prevent damage to the ammonia decomposition catalyst. Some catalysts, for example, iron-based catalysts, are known to be water-insoluble and incompatible with water-containing feedstocks. Unfortunately, ammonia contains a small amount of water added to prevent stress corrosion cracking in containers during transport and storage. Once this water is removed, any suitable decomposition catalyst, including water-insoluble catalysts, can be used.

[0107] Water removal can be integrated into a distillation column system as shown in FIG. 5. The process features in FIG. 5 that are common to the processes in FIG. 1 through 4 are given the same reference numbers. The following is a discussion of additional features in FIG. 5.

[0108] In FIG. 5, the pressurized liquid ammonia feed (line 4) is heated in a heat exchanger (E101) and at least partially vaporized. The ammonia is evaporated at the required pressure to ensure that water is also transported to the distillation system with the vaporized ammonia.

[0109] The heated and vaporized ammonia feed (line 110) is usually fed to the distillation system (72) at an intermediate position of the column. The overhead vapor generated in the column system is substantially water-free (e.g., contains about 1 ppm of water) and contains ammonia from the feed along with ammonia recovered from the decomposition gas.

[0110] The recovered ammonia (line 76) is returned to a heat exchanger (E101) to be further heated before being supplied to the reaction tubes of the reactor (8) (line 6).

[0111] The present invention will now be illustrated by reference to the following examples of invention and by comparison with the following reference examples.

[0112] See Example 1

[0113] The process depicted in Fig. 2 was simulated by a computer (Aspen Technology, Inc., Aspen Plus, ver. 10), and the results are described in Table 1. This example assumes equilibrium for the decomposition reaction at 11 bar and 500°C.

[0114]

[0115] Table 1

[0116] In this reference example, hydrogen recovery from ammonia is 77.18% with a PSA recovery of 83.5%. The total power of the ammonia supply pump (P201), ammonia fuel pump (P202), and air fan (K201) is approximately 1.36 kW. Additionally, the percentage of ammonia in the PSA off-gas is 3.6492%.

[0117] See Example 2

[0118] The process depicted in Fig. 3 was simulated by a computer (Aspen Plus, ver. 10), and the results are shown in Table 2. This example assumes equilibrium for the decomposition reaction at 11 bar and 500°C.

[0119]

[0120] Table 2

[0121] In this reference example, hydrogen recovery from ammonia is 77.05% with a PSA recovery of 79.4%. The total power of the ammonia supply pump (P201) and the air fan (K201) is approximately 1.37 kW. Additionally, the percentage of ammonia in the PSA off-gas is 3.3810%.

[0122] See Example 3

[0123] The process depicted in Fig. 4 was simulated by a computer (Aspen Plus, ver. 10), and the results are shown in Table 3. For the purposes of the simulations, equilibrium for the decomposition reaction was assumed at 11 bar and 500°C.

[0124]

[0125]

[0126] In this example of the invention, the recovery of hydrogen from ammonia is 96.47%. Additionally, the percentage of ammonia in the PSA off-gas from the second PSA is 0.083%. It can be seen that the crude hydrogen has a reduced ammonia content from 1.398% to 0.024%. Not all ammonia is recovered from the decomposition gas. Since more ammonia recovery requires a higher reboiler duty in the stripping column and there is a limit to the heat available for such duty within the process without affecting the efficiency of the process, optimization can be performed.

[0127] See Example 4

[0128] The process depicted in Fig. 4 was simulated by a computer (Aspen Plus, ver. 10), and the results are shown in Table 4. For the purposes of the simulations, equilibrium for the decomposition reaction was assumed at 11 bar and 500°C.

[0129]

[0130]

[0131] In this example of the invention, the recovery of hydrogen from ammonia is 96.45%. In addition, it can be seen that the crude hydrogen has a decrease in ammonia content from 2.614% to 0.029%.

[0132] Invention Example 1

[0133] The process depicted in Fig. 5 was simulated by a computer (Aspen Plus, ver. 10), and the results are shown in Table 5. For the purposes of the simulations, equilibrium for the decomposition reaction was assumed at 11 bar and 500°C.

[0134]

[0135]

[0136] In this example of the invention, the recovery of hydrogen from ammonia is 96.41%. In addition, it can be seen that the crude hydrogen has a reduced ammonia content from 1.401% to 0.087%.

[0137] Invention Example 2

[0138] The process depicted in Fig. 5 was simulated by a computer (Aspen Plus, ver. 10), and the results are shown in Table 6. For the purposes of the simulations, equilibrium for the decomposition reaction was assumed at 21 bar and 500°C.

[0139]

[0140]

[0141] In this example of the invention, the recovery of hydrogen from ammonia is 96.41%. In addition, it can be seen that the crude hydrogen has a reduced ammonia content from 2.619% to 0.053%.

[0142] The scope of the present invention is not limited by specific aspects or embodiments disclosed in examples where functionally identical aspects and any embodiments of the present invention are intended to be within the scope of the present invention. Various modifications of the present invention other than those shown and described herein will be apparent to those skilled in the art and are intended to be within the scope of the appended claims.

Claims

Claim 1 A method for generating hydrogen from ammonia, comprising the steps of: pressurizing a liquid ammonia feed containing water as a trace impurity to produce a pressurized liquid ammonia feed; heating catalyst-containing reactor tubes and burning a primary fuel in a furnace to form flue gas; supplying the heated ammonia to the catalyst-containing reactor tubes to cause the decomposition of the ammonia into a decomposition gas containing hydrogen gas, nitrogen gas, and residual ammonia; cooling the decomposition gas by heat exchange to produce a cooled decomposition gas; scrubbing the ammonia from the cooled decomposition gas using water in a scrubbing column to produce an ammonia-depleted decomposition gas and a water-soluble ammonia solution; stripping the ammonia from the water-soluble ammonia solution in a distillation column system to produce a water-depleted ammonia feed vapor and a water-soluble ammonia-depleted base liquid; and heat exchange with one or more high-temperature fluids to produce the heated ammonia. A method comprising the steps of: heating the water-depleted ammonia feed vapor by; and purifying the ammonia-depleted decomposition gas in a first PSA device to produce a hydrogen product gas and a first PSA tail gas, wherein the pressurized liquid ammonia feed is characterized in that it is heated and vaporized by heat exchange with one or more high-temperature fluids to produce an ammonia feed vapor supplied to a distillation column system to remove water from the ammonia feed vapor, and the one or more high-temperature fluids include the decomposition gas and / or the flue gas. Claim 2 A method according to claim 1, comprising the step of replenishing the base liquid in or from the distillation column system by heat exchange with one or more high-temperature fluids. Claim 3 A method according to claim 1 or 2, comprising the step of reboiling the base liquid in or from the distillation column system using an electric drive heater. Claim 4 In paragraph 3, the method wherein the heater is at least partially powered by electricity generated from at least one renewable source. Claim 5 A method according to claim 1 or 2, comprising the step of partially condensing overhead vapor in or from the distillation column system by heat exchange to produce a condensed liquid and the water-depleted ammonia feed vapor, wherein the condensed liquid is recirculated to the distillation column system as reflux. Claim 6 A method according to claim 1 or 2, comprising the step of supplying the aqueous base liquid from the distillation column system to the scrubbing column. Claim 7 In claim 6, the water-soluble ammonia solution supplied to the distillation column system is heated by heat exchange with the water-soluble base liquid supplied to the scrubbing column. Claim 8 In claim 6, the water-soluble base liquid is further cooled by heat exchange before being supplied to the scrubbing column. Claim 9 A method according to claim 1 or 2, comprising the step of pumping the aqueous ammonia solution from the scrubbing column into the distillation column system. Claim 10 A method according to claim 1 or 2, comprising the step of compressing the first PSA tail gas to generate the first compressed PSA tail gas; and the step of recirculating the compressed first PSA tail gas to the first PSA device to further recover hydrogen gas. Claim 11 A method according to claim 1 or 2, comprising the step of compressing the first PSA tail gas to produce a compressed first PSA tail gas; and the step of purifying the compressed first PSA tail gas in a second PSA device to produce an additional hydrogen product gas and a second PSA tail gas. Claim 12 A method according to claim 11, comprising the step of burning the second PSA tail gas as a secondary fuel in the furnace. Claim 13 An apparatus for generating hydrogen from ammonia, comprising: a pump for pressurizing a liquid ammonia feed containing water as a trace impurity to produce a pressurized liquid ammonia feed; at least one first heat exchanger fluidly connected to the pump to vaporize the pressurized liquid ammonia by heat exchange with one or more high-temperature fluids to produce ammonia feed vapor; catalyst-containing reactor tubes fluidly connected to the first heat exchanger(s) to decompose the heated ammonia from the first heat exchanger(s) to produce a decomposition gas containing hydrogen gas, nitrogen gas, and residual ammonia; a furnace heat-connected to the catalyst-containing reactor tubes for combustion of fuel to heat the catalyst-containing reactor tubes and form flue gas; a flue gas conduit for supplying flue gas from the furnace to the at least one first heat exchanger(s); a decomposition gas conduit for supplying decomposition gas to the at least one first heat exchanger(s); and an ammonia-depleted decomposition gas A scrubbing column fluidly connected to at least one first heat exchanger(s) for scrubbing ammonia from a cooled decomposition gas to produce a water-soluble ammonia solution; a distillation column system fluidly connected to the scrubbing column for stripping ammonia from the water-soluble ammonia solution to produce a water-depleted ammonia feed vapor and a water-soluble ammonia-depleted base liquid; an ammonia feed vapor conduit for supplying ammonia feed vapor from the first heat exchanger(s) to the distillation column system; a water-depleted ammonia feed vapor conduit for supplying a water-depleted ammonia feed from the distillation column system to the first heat exchanger(s) for additional heating by heat exchange with one or more of the high-temperature fluids to produce the heated ammonia; and a first PSA device fluidly connected to the scrubbing column for purifying the ammonia-depleted decomposition gas to produce a hydrogen product gas and a first PSA tail gas.A device comprising a hydrogen product gas conduit for removing hydrogen product gas from the first PSA device. Claim 14 In paragraph 13, the distillation column system comprises a base reboiler thermally integrated with the first heat exchanger(s). Claim 15 In paragraph 13, the distillation column system is a device comprising an electric-driven reboiler. Claim 16 An apparatus according to any one of claims 13 to 15, wherein the distillation column system comprises an overhead condenser for partially condensing overhead vapor by heat exchange with a coolant. Claim 17 In paragraph 16, the apparatus comprises a reflux conduit for supplying the condensed liquid as reflux from the overhead condenser to the distillation column system. Claim 18 An apparatus comprising, in any one of claims 13 to 15, a water-soluble base liquid conduit for supplying a water-soluble base liquid from the distillation column system to the scrubbing column. Claim 19 In claim 18, the apparatus comprises a second heat exchanger for heating a water-soluble ammonia solution from the scrubbing column by heat exchange with a water-soluble base liquid from the distillation column system. Claim 20 In paragraph 18, the apparatus comprises a third heat exchanger for cooling a water-soluble base liquid from the distillation column system by heat exchange with a coolant. Claim 21 An apparatus according to any one of claims 13 to 15, comprising a pump for pumping a water-soluble ammonia solution from the scrubbing column into the distillation column system. Claim 22 An apparatus according to any one of claims 13 to 15, comprising: a compressor for compressing a first PSA tail gas to generate a compressed first PSA tail gas; and a recirculation conduit for recirculating the compressed first PSA tail gas to the first PSA device. Claim 23 An apparatus comprising, in any one of claims 13 to 15, a compressor for compressing a first PSA tail gas to produce a compressed first PSA tail gas; and a second PSA device fluidly communicating with the compressor to purify the compressed first PSA tail gas to produce an additional hydrogen product gas and a second PSA tail gas. Claim 24 An apparatus comprising, in any one of claims 13 to 15, a conduit for supplying a second PSA tail gas to the first heat exchanger(s) for heating by heat exchange with the one or more high-temperature fluids prior to combustion in the furnace.

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