Improved ammonia cracking process and apparatus for hydrogen recovery
The use of single or two-stage PSA units with partial tail gas recirculation optimizes hydrogen separation in ammonia cracking, addressing inefficiencies and reducing compression forces, thereby enhancing hydrogen recovery and lowering operational costs.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- UOP LLC
- Filing Date
- 2022-12-01
- Publication Date
- 2026-06-01
AI Technical Summary
Existing ammonia cracking processes face inefficiencies in hydrogen separation and require excessive compression forces due to the use of single PSA units or two-stage designs with membrane separators, leading to high operational costs and energy consumption.
Implementing a single PSA unit with partial tail gas recirculation or a two-stage PSA configuration to minimize specific compressibility forces by optimizing the separation process, utilizing conventional PSA units with adsorbents like molecular sieve zeolites and water washing to enhance hydrogen recovery.
The proposed method reduces the specific compressibility force and enhances hydrogen recovery, especially at lower fuel gas loads, while maintaining high hydrogen purity and reducing overall operational costs.
Smart Images
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Abstract
Description
Technical Field
[0005]
[0001] (Priority Claim) This application claims the priority of U.S. Patent Application No. 63 / 264,974, filed on December 6, 2021, the entire content of which is incorporated herein by reference.
Background Art
[0002] Ammonia can be used as a liquid hydrogen carrier in existing transportation infrastructure. When carbon dioxide by-products from fossil fuel-based ammonia production processes are recovered and sequestered (blue ammonia), the resulting hydrogen has a low carbon footprint. During use, blue ammonia decomposes into H2 and N2 at high temperature (e.g., 850 °C) according to the following exothermic equilibrium reaction. 2NH3 = N2 + 3H2
[0003] The resulting product mixture (75 mol% H2 and 25 mol% N2) is then separated in a pressure swing adsorption (PSA) unit to recover high purity hydrogen.
[0004] Some prior art methods involve the use of a single PSA unit. Some current small-scale ammonia cracking furnaces use electric heaters, and the PSA tail gas provides additional heat to drive the process.
[0005] Other prior art methods use a two-stage configuration where a membrane separator follows the tail gas stream from the PSA unit after the PSA unit. One drawback of the two-stage design using a membrane is that additional compression is required for the membrane permeate to return the permeate to the PSA feed for recirculation. As a result, overall high specific compression force requirements occur, and an additional compressor for the permeate is required.
[0006] Therefore, there is a need for a process that improves the separation efficiency at the point of use and reduces the net cost of hydrogen recovery.
Brief Description of the Drawings
[0007] [Figure 1] This shows one embodiment of the process according to the present invention having a single PSA unit. [Figure 2] A second embodiment of the process according to the present invention, having two PSA units, is shown. [Figure 3] Figures 1 and 2 are graphs comparing the compression force versus % heat load due to hydrogen combustion for the processes shown. [Figure 4] Figures 1 and 2 are graphs comparing hydrogen production by hydrogen combustion versus % heat load for the processes shown. [Modes for carrying out the invention]
[0008] The disclosure of the present invention addresses this need by using a single PSA unit or a two-stage PSA configuration with partial tail gas recirculation to minimize the specific compressibility force (kW hr / MT H2) in the separation section of the ammonia cracking process.
[0009] This process involves the use of a single PSA unit with partial tail gas recirculation or two hydrogen PSA units in series. In a single PSA unit with partial tail gas recirculation, the decomposition effluent from the ammonia cracking reaction zone is sent to the hydrogen PSA unit either directly or after water washing in a water washing vessel. The temperature of the effluent entering the hydrogen PSA unit is 30-50°C. The decomposition effluent is separated into a high-purity, high-pressure hydrogen stream and a low-pressure tail gas stream. The high-pressure hydrogen stream can be recovered. The low-pressure tail gas stream is compressed and sent to the hydrogen PSA unit either directly or first in a water washing vessel (if present).
[0010] High-pressure hydrogen streams typically contain more than 99.0 mol% hydrogen, or more than 99.9 mol% hydrogen, or more than 99.97 mol% hydrogen. Low-pressure tail gas streams typically contain 50 mol% to 70 mol% nitrogen on a dry basis, with the remainder being hydrogen (i.e., 30 mol% to 50 mol% hydrogen).
[0011] The pressure of a high-pressure hydrogen stream is typically in the range of 2000 kPa to 6000 kPa, or 2000 kPa to 5000 kPa, or 2000 kPa to 4000 kPa, or 2000 kPa to 3000 kPa.
[0012] The pressure of a low-pressure tail gas flow is typically in the range of 100 kPa to 300 kPa, or 100 kPa to 200 kPa.
[0013] In a two-stage PSA configuration, the decomposition effluent from the ammonia cracking reaction zone is sent to the first hydrogen PSA unit either directly or after water washing in a water washing vessel. The decomposition effluent is separated into a high-purity, high-pressure hydrogen stream and a low-pressure tail gas stream. The high-pressure hydrogen stream may be recovered. The low-pressure tail gas stream is compressed and sent to the second hydrogen PSA unit, where it is separated into a second high-pressure hydrogen stream and a second low-pressure tail gas stream. The second low-pressure tail gas stream may be recovered for use as fuel gas for the ammonia cracking reactor or elsewhere in the plant. The second high-pressure hydrogen stream may be sent back to the first hydrogen PSA unit either directly or first in a water washing vessel for further separation. Alternatively, it may be recovered and optionally further processed to purify the stream as needed. The first high-pressure hydrogen stream and the first low-pressure tail gas stream are as described above for a single PSA unit. The second high-pressure hydrogen stream typically contains 60 mol% to 90 mol% hydrogen and 10 mol% to 40 mol% nitrogen. The second low-pressure tail gas stream typically contains 70 mol% to 90 mol% nitrogen on a dry basis, with the remainder being hydrogen (i.e., 10 mol% to 30 mol% hydrogen).
[0014] A two-stage hydrogen PSA design provides a lower specific compressibility compared to a single hydrogen PSA unit. For the same hydrogen recovery, the compressibility is lower with the two-stage design. Conversely, for the same compressibility, hydrogen recovery is greater with the two-stage design. This is especially true at lower fuel gas loads (higher electric heating loads) with higher H2 product recovery.
[0015] The high-pressure hydrogen stream from the second hydrogen PSA unit is recovered and optionally further processed to purify the stream as needed. Alternatively, it can be recycled back to the first hydrogen PSA unit.
[0016] The fuel bleed flow from the first low-pressure tail gas flow can be adjusted to meet the desired heating load in the ammonia cracking reaction zone. In addition, hydrogen recovery from the second hydrogen PSA unit can be adjusted to provide the desired amount of fuel gas for the ammonia cracking reaction zone.
[0017] A single-stage PSA configuration is preferred when it is desirable to provide the majority of the heating load from hydrogen combustion and minimize the heating load from electricity or auxiliary fuels. In this case, a single-stage PSA configuration offers lower capital costs and approximately the same operating costs. A two-stage PSA configuration is preferred when the goal is to maximize hydrogen recovery and provide more heating load from electricity or auxiliary fuels.
[0018] Hydrogen PSA units are conventional PSA units. For example, they may be 6-bed PSA units with three pressure equalization steps (6-1-3 cycles). The minimum number of beds is 4, and the maximum number can be 10 or more.
[0019] The hydrogen PSA unit includes an adsorbent layer for removing water, ammonia, and nitrogen. Water is present if the process includes a water washing container. Any suitable adsorbent may be used. Suitable adsorbents for nitrogen adsorption may be molecular sieve zeolites containing, but not limited to, CaA, NaX, CaX, or LiX. Suitable adsorbents for water and ammonia include, but are not limited to, silica gel, activated alumina, or activated carbon.
[0020] The hydrogen recovery rate from ammonia cracking spills is typically in the range of 80% to 98%.
[0021] The low-pressure tail gas from the 1st and 2nd hydrogen PSA typically supplies 10% - 90% of the heat load for the ammonia cracking process, and the remainder is supplied by electric heating or auxiliary fuel.
[0022] FIG. 1 shows an ammonia cracking process 100 having a single hydrogen PSA unit with partial tail gas recirculation. An ammonia feed stream 105 containing ammonia is sent to an ammonia cracking reaction zone 110. The ammonia cracking reaction zone 110 can be any suitable ammonia cracking zone. It can include an ammonia cracking reactor and related equipment such as a furnace equipped with one or more burners, electric heaters, heat exchangers, etc. Suitable ammonia cracking zones are well-known to those skilled in the art.
[0023] Heat is input into the ammonia cracking reaction zone 110 using a heat source 115. The effluent stream 120 from the ammonia cracking reaction zone 110 contains a mixture of H2 and N2 and typically has a temperature in the range of 700 - 1000 °C.
[0024] The effluent stream 120 from the ammonia cracking reaction zone 110 is heat-exchanged with the ammonia feed stream 105 in a heat exchanger 125. It can be further cooled in an optional second cooler 130.
[0025] The cooled effluent stream 135 can be introduced into an optional water washing vessel 140. A clean water stream 145 is introduced into the water washing vessel 140 to remove residual unreacted ammonia, and the used water stream 150 containing ammonia is removed from the water washing vessel 140.
[0026] The washed effluent stream 155 is sent to a hydrogen PSA unit 160 where it is separated into a high purity high pressure hydrogen stream 165 and a low pressure tail gas stream 170. The low pressure tail gas stream 170 is sent to a compressor 175. The pressure of the high pressure hydrogen stream 165 is in the range of 2000 - 6000 kPa, while the pressure of the low pressure tail gas stream is 100 - 300 kPa. The temperature of the washed effluent stream 155 entering the first hydrogen PSA unit is 30 - 50 °C.
[0027] A slip stream 180 from the low pressure tail gas stream 170 can be removed to prevent nitrogen accumulation.
[0028] The compressed tail gas stream 185 can be recycled to the water wash vessel 140.
[0029] However, this scheme results in a higher specific compression force compared to the two - stage process 200 shown in Figure 2. The ammonia feed stream 205 is sent to an ammonia cracking reaction zone 210. The ammonia cracking reaction zone 210 can be any suitable ammonia cracking zone as described above.
[0030] Heat is input to the ammonia cracking reaction zone 210 using a heat source 215. The effluent stream 220 from the ammonia cracking reaction zone 210 is heat - exchanged with the ammonia feed stream 205 in a heat exchanger 225. It can be further cooled in an optional second cooler 230.
[0031] The cooled effluent stream 235 can be introduced into an optional water wash vessel 240. A clean water stream 245 is introduced into the water wash vessel 240 and the used water stream 250 is removed from the water wash vessel 240.
[0032] The washed effluent stream 255 is sent to a first hydrogen PSA unit 260 where it is separated into a high purity high pressure hydrogen stream 265 and a low pressure tail gas stream 270. The temperature and pressure of the first hydrogen PSA unit 260 are the same as those described above.
[0033] The low-pressure tail gas flow 270 is sent to the compressor 275. The slip flow 280 from the low-pressure tail gas flow 270 can be removed.
[0034] The compressed tail gas flow 285 is sent to a second hydrogen PSA unit 290, where it is separated into a second high-pressure hydrogen flow 295 and a tail gas flow 300. The tail gas flow 300 can be used, for example, as fuel. It can provide at least a portion of the fuel for the heat source 215.
[0035] The second high-pressure hydrogen stream 295 can be returned to the first hydrogen PSA unit 260.
[0036] Examples A comparison was made between a single PSA design with partial tail gas recirculation according to the present invention, as shown in Figure 1, and one embodiment of a two-stage PSA configuration according to the present invention, as shown in Figure 2. Computer simulations of the ammonia cracking process were constructed using these two separation schemes for a fixed ammonia supply rate of 185 MT / day. In the single PSA design, the fuel gas is extracted as a slip flow upstream of the tail gas compressor.
[0037] This fuel gas provides part of the total heat load for the cracking process, with the remainder coming from electric heaters inside the cracking furnace.
[0038] The two-stage scheme involves adding a second PSA unit to the compressed tail gas and recirculating the top gas back to the first PSA feed. Fuel gas is taken out as a slip flow from the first PSA tail gas flow and mixed with the tail gas from the second PSA unit.
[0039] The simulation results are shown in Figures 3-4. For any given fuel gas thermal load, the two-stage design provides a lower specific compressive force compared to a single PSA. This is especially true at lower fuel gas loads (higher electric heating loads) with higher H2 product recovery.
[0040] Specific Embodiments The following description will be given in conjunction with specific embodiments, but it should be understood that this description is intended to illustrate the scope of the above description and the attached claims, and is not intended to limit them.
[0041] A first embodiment of the present invention is a method for producing hydrogen from ammonia, comprising: decomposing an ammonia-containing ammonia feedstream in an ammonia cracking reaction zone to produce a decomposition effluent containing hydrogen and nitrogen; separating the decomposition effluent in a first hydrogen pressure swing adsorption (PSA) unit into a first high-pressure hydrogen stream and a first hydrogen-reduced tail gas stream containing some hydrogen and nitrogen; compressing the first hydrogen-reduced low-pressure tail gas stream with a compressor to form a compressed tail gas stream; recirculating at least a portion of the compressed tail gas stream to a first hydrogen PSA unit; and recovering the first high-pressure hydrogen stream. Embodiments of the present invention are any or all of the preceding embodiments to the first embodiment of this section, further comprising separating the compressed tail gas stream in a second hydrogen PSA unit into a second high-pressure hydrogen stream and a second hydrogen-reduced tail gas stream containing some hydrogen and nitrogen, and then recirculating at least a portion of the compressed tail gas stream to the first hydrogen PSA unit. Embodiments of the present invention further include dividing a first hydrogen reduction tail gas flow into a first portion and a second portion, removing the first portion of the first hydrogen reduction low-pressure tail gas flow to remove nitrogen, and compressing the first hydrogen reduction low-pressure tail gas flow with a compressor, which includes compressing the second portion of the first hydrogen reduction low-pressure tail gas flow, and are any or all of the embodiments from the preceding embodiments of this section to the first embodiment of this section. Embodiments of the present invention further include recirculating at least a portion of the compressed tail gas flow to a first hydrogen PSA unit, which includes recirculating a second high-pressure hydrogen flow to a first hydrogen PSA unit, and are any or all of the embodiments from the preceding embodiments of this section to the first embodiment of this section. Embodiments of the present invention further include recovering a second high-pressure hydrogen flow, which are any or all of the embodiments from the preceding embodiments of this section to the first embodiment of this section. Embodiments of the present invention are any or all of the embodiments from the preceding embodiments of this section to the first embodiment of this section, further comprising washing the decomposition effluent in a water washing container to remove ammonia before separating the decomposition effluent.Embodiments of the present invention are any or all of the preceding embodiments to the first embodiment of this section, further comprising recirculating at least a portion of the compressed tail gas flow to a water washing container. Embodiments of the present invention are any or all of the preceding embodiments to the first embodiment of this section, further comprising burning at least a portion of the first hydrogen reduction tail gas flow as a heat source for the ammonia cracking reaction zone. Embodiments of the present invention are any or all of the preceding embodiments to the first embodiment of this section, further comprising burning at least a portion of the second hydrogen reduction tail gas flow as a heat source for the ammonia cracking reaction zone. Embodiments of the present invention are any or all of the preceding embodiments to the first embodiment of this section, further comprising the first hydrogen reduction tail gas flow further containing ammonia. Embodiments of the present invention are any or all of the preceding embodiments to the first embodiment of this section, further containing ammonia in the second hydrogen reduction low-pressure tail gas flow. Embodiments of the present invention are any or all of the preceding embodiments to the first embodiment of this section, further comprising preheating the ammonia supply stream together with the decomposition effluent stream. Embodiments of the present invention are any or all of the preceding embodiments to the first embodiment of this section, wherein the first high-pressure hydrogen stream has a pressure in the range of 2000 kPa to 6000 kPa and the first hydrogen reduction tail gas stream has a pressure in the range of 100 kPa to 300 kPa. Embodiments of the present invention are any or all of the preceding embodiments to the first embodiment of this section, wherein the second high-pressure hydrogen stream has a pressure in the range of 2000 kPa to 6000 kPa and the second hydrogen reduction low-pressure tail gas stream has a pressure in the range of 100 kPa to 300 kPa. Embodiments of the present invention are any or all of the preceding embodiments to the first embodiment of this section, wherein the temperature of the decomposition outflow entering the first PSA unit is in the range of 30°C to 50°C.Embodiments of the present invention include one or all of the preceding embodiments to the first embodiment of this section, wherein the combustion of a first hydrogen reduction tail gas flow and a second hydrogen reduction tail gas flow as a heat source for the ammonia cracking reaction zone provides 10% to 90% of the heat load for the ammonia cracking reaction zone. Embodiments of the present invention include one or all of the preceding embodiments to the first embodiment of this section, wherein the hydrogen recovery rate from the decomposition effluent is in the range of 80% to 98%.
[0042] A second embodiment of the present invention is an apparatus for producing hydrogen from ammonia, comprising: an ammonia cracking reaction zone having an inlet and an outlet; a first PSA unit having an inlet, a high-pressure hydrogen outlet, and a low-pressure tail gas outlet; and a compressor having an inlet and an outlet, wherein the outlet of the ammonia cracking reactor is in fluid communication with the inlet of the first PSA unit, the compressor inlet is in fluid communication with the low-pressure tail gas outlet of the first PSA unit, and the compressor outlet is in fluid communication with the inlet of the first PSA unit. Embodiments of the present invention further comprise a second PSA unit having an inlet, a high-pressure hydrogen outlet, and a low-pressure tail gas outlet, wherein the inlet of the second PSA unit is in fluid communication with the compressor outlet, and are any or all of the embodiments from the preceding embodiments of this section to the second embodiment of this section. Embodiments of the present invention are any or all of the embodiments described in the preceding section to the second embodiment described in this section, wherein the high-pressure hydrogen outlet of the second PSA unit is in fluid communication with the inlet of the first PSA unit.
[0043] Without further detail, it is expected that those skilled in the art will be able to utilize the invention to the fullest extent without departing from the spirit and scope of the invention, readily identify the essential characteristics of the invention, and make various changes and modifications to the invention to suit various uses and conditions. Accordingly, the prior preferred specific embodiments should be construed as merely illustrative and not to limit the remainder of this disclosure in any way, but are intended to cover various modifications and equivalent configurations that fall within the scope of the appended claims.
[0044] In the above, all temperatures are given in degrees Celsius, and all parts and percentages are based on weight unless otherwise specified.
Claims
1. A method for producing hydrogen from ammonia, The ammonia supply stream (105) containing ammonia is decomposed in the ammonia cracking reaction zone (110) to generate a decomposition effluent (120) containing hydrogen and nitrogen, In the first hydrogen pressure swing adsorption (PSA) unit (160), the decomposition outflow (120) is separated into a first high-pressure hydrogen flow (165) and a first hydrogen reduction tail gas flow (170) containing some of the hydrogen and nitrogen. The first hydrogen-reducing low-pressure tail gas flow (170) is compressed by a compressor (175) to form a compressed tail gas flow (185), Recirculating at least a portion of the compressed tail gas flow (185) to the first hydrogen PSA unit (165), To recover the first high-pressure hydrogen stream (165), A method comprising removing a portion (180) of the first hydrogen-reducing low-pressure tail gas flow (170) to remove nitrogen, and then compressing the first hydrogen-reducing low-pressure tail gas flow (170).
2. The method according to claim 1, further comprising separating the compressed tail gas flow (285) in a second hydrogen PSA unit (290) into a second high-pressure hydrogen flow (295) and a second hydrogen-reduced tail gas flow (300) containing some hydrogen and nitrogen, and then recirculating at least a portion of the compressed tail gas flow (285) to the first hydrogen PSA unit (260).
3. A device for producing hydrogen from ammonia, an ammonia cracking reaction zone (210) having an inlet and an outlet, A first PSA unit (260) having an inlet, a high-pressure hydrogen outlet, and a low-pressure tail gas outlet, wherein the outlet of the ammonia cracking reactor (210) is in fluid communication with the inlet of the first PSA unit (260), A compressor (275) having an inlet and an outlet, wherein the compressor inlet is in fluid communication with the low-pressure tail gas outlet of the first PSA unit (260), and the compressor outlet is in fluid communication with the inlet of the first PSA unit (260), A second PSA unit (290) having an inlet, a high-pressure hydrogen outlet, and a low-pressure tail gas outlet, wherein the inlet of the second PSA unit (290) is in fluid communication with the compressor outlet, The apparatus wherein the high-pressure hydrogen outlet of the second PSA unit (290) is in fluid communication with the inlet of the first PSA unit (260).