Synthesis ammonia device purge gas dehydrogenation helium extraction system and method thereof

Abstract

The present application relates to the technical field of vent gas dehydrogenation helium extraction, and discloses a synthetic ammonia device vent gas dehydrogenation helium extraction system, which comprises a pretreatment unit, a first heating unit, a first dehydrogenation unit, a cooling unit, a membrane filtration unit, a second heating unit, a second dehydrogenation unit and a drying unit; the pretreatment unit is communicated with the first heating unit; the first heating unit is communicated with the first dehydrogenation unit; the first dehydrogenation unit is communicated with the shell side of the second heating unit; the shell side of the second heating unit is communicated with the cooling unit; the cooling unit is communicated with the membrane filtration unit; the membrane filtration unit is communicated with the tube side of the second heating unit; the tube side of the second heating unit is communicated with the second dehydrogenation unit; and the second dehydrogenation unit is communicated with the drying unit. A method for extraction is also disclosed. The present application can make full use of the system waste heat, improve the helium extraction efficiency and reduce the energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of purge gas dehydrogenation and helium extraction technology, specifically, it relates to a purge gas dehydrogenation and helium extraction system and method for ammonia synthesis plant. Background Technology

[0002] The ammonia synthesis off-gas mainly consists of ammonia, nitrogen, hydrogen, and a small amount of methane, with ammonia being the primary component, typically accounting for over 70%, followed by hydrogen and nitrogen. To effectively utilize helium resources, the ammonia synthesis off-gas needs to be purified. Hydrogen is the most difficult component to separate because its molecules are very similar to those of helium, making it difficult to obtain high-purity helium through physical methods such as membranes and adsorbents.

[0003] In the existing technology, hydrogen is mainly processed by catalytic oxidation, or, as in the patent with publication number CN114538394A, hydrogen and helium are separated by cryogenic hydrogen dehydrogenation and cryogenic adsorption technology to extract high-purity helium.

[0004] For example, patent CN211612197U discloses a device for separating and recovering hydrogen and helium in the off-gas of ammonia synthesis. Specifically, it uses a methanation catalyst filled in a primary dehydrogenation tank to react most of the hydrogen with carbon monoxide or carbon dioxide or a mixture of the two to produce methane. The methane and helium are then initially separated by a membrane separator. Furthermore, the residual hydrogen in the helium is completely separated by reacting oxygen with hydrogen to produce water under the action of the dehydrogenation catalyst.

[0005] However, the catalytic oxidation method requires a high concentration of oxygen, which poses a safety hazard. The cryogenic hydrogen desorption and cryogenic adsorption technologies require the temperature of the hydrogen-containing crude helium gas to be reduced to between 14K and 20K, which consumes a great deal of energy for refrigeration. For the synthetic ammonia purge gas, which has a low hydrogen concentration, the cryogenic hydrogen desorption method is counterproductive.

[0006] In light of the above situation, how to efficiently, safely, and easily remove hydrogen from the ammonia synthesis purge gas to improve the purity of the collected helium is an urgent problem to be solved. Summary of the Invention

[0007] This application aims to address the technical problems of high energy consumption and difficulty in implementing hydrogen removal and helium purification in existing technologies.

[0008] Therefore, the first objective of this invention is to provide a system for dehydrogenating and extracting helium from purge gas in an ammonia synthesis unit, comprising:

[0009] The system comprises a pretreatment unit, a first heating unit, a primary dehydrogenation unit, a cooling unit, a membrane filtration unit, a second heating unit, a secondary dehydrogenation unit, and a drying unit. The pretreatment unit is connected to the first heating unit. The first heating unit is connected to the primary dehydrogenation unit. The primary dehydrogenation unit is connected to the shell side of the second heating unit. The shell side of the second heating unit is connected to the cooling unit. The cooling unit is connected to the membrane filtration unit. The membrane filtration unit is connected to the tube side of the second heating unit. The tube side of the second heating unit is connected to the secondary dehydrogenation unit. The secondary dehydrogenation unit is connected to the drying unit.

[0010] Technical principle:

[0011] The gas discharged from the pretreatment unit is heated to the primary dehydrogenation reaction temperature by the first heating unit, and then most of the hydrogen is converted by the primary dehydrogenation unit. Subsequently, the gas after primary dehydrogenation is passed into the shell side of the second heating unit, where it exchanges heat with the tube side of the second heating unit and is initially cooled. The gas after heat exchange is then passed into a cooling unit for further cooling, and then separated by a membrane filtration unit. The separated gas is heated by the second heating unit after heat exchange, and then dehydrogenated and dried by the secondary dehydrogenation unit to obtain helium.

[0012] The second objective of this invention is to provide an extraction method for the helium extraction system of the above-mentioned ammonia synthesis unit's off-gas dehydrogenation process, comprising the following steps:

[0013] S1 purge gas is compressed, deoxygenated, and dried by the pretreatment unit and then passed into the first heating unit for heating to obtain mixed gas I;

[0014] S2. Mixed gas I is introduced into the first-stage dehydrogenation unit for methanation and dehydrogenation to obtain mixed gas II;

[0015] S3. Mixed gas II is introduced into the cooling unit, and after cooling, it is introduced into the membrane filtration unit to obtain mixed gas III;

[0016] S4. Mixed gas III is introduced into the second heating unit, and after being heated, it is introduced into the secondary dehydrogenation unit for catalytic oxidation dehydrogenation. After catalytic oxidation dehydrogenation, the gas is introduced into the drying unit and dried to obtain helium.

[0017] Technical Mechanism:

[0018] (1) First, most of the hydrogen in the purge gas is converted into methane by a methanation catalyst, and then the methane is removed by cooling and membrane filtration. Further, the mixed gas III after the methane has been removed is heated and the residual hydrogen is completely oxidized by catalytic oxidation. Finally, high-purity helium is obtained after drying.

[0019] (2) Cooling is required from the methanation stage to the membrane filtration stage, and then heating is required from the membrane filtration stage to the catalytic oxidation stage. This, in conjunction with the helium extraction system mentioned above, effectively utilizes the waste heat, reduces energy consumption, and reduces the amount of expensive and efficient catalyst used in the catalytic oxidation stage.

[0020] (3) During the pretreatment of purge gas, the compressed purge gas is deoxygenated and dried to ensure that there is no risk of combustion or explosion after the hydrogen is converted into methane.

[0021] The beneficial effects of this application are as follows:

[0022] This application heats the gas discharged from the pretreatment unit to the primary dehydrogenation reaction temperature through a first heating unit, and then converts most of the hydrogen through the primary dehydrogenation unit. Subsequently, the gas after primary dehydrogenation is passed into the shell side of the second heating unit, where it exchanges heat with the tube side of the second heating unit and is initially cooled. The gas after heat exchange is then passed into a cooling unit for further cooling, and then separated by a membrane filtration unit. The separated gas is heated by the second heating unit after heat exchange, and then dehydrogenated and dried by a secondary dehydrogenation unit to obtain helium. This achieves full utilization of the system's waste heat, which improves the purity of helium and reduces energy consumption. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the dehydrogenation and helium extraction system for the ammonia synthesis unit provided by the present invention.

[0025] Icons: 1-Pretreatment unit, 11-First hydrogen concentration detector, 12-Gas compressor, 13-Deoxygenation device, 2-First heating unit, 21-Preheating device, 22-Heating device, 3-First-stage dehydrogenation unit, 31-Making gas pipe, 32-Second flow meter, 4-Cooling unit, 5-Membrane filtration unit, 6-Second heating unit, 7-Second-stage dehydrogenation unit, 8-Drying unit, 9-First flow meter, 10-Temperature detector. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0027] First, the present invention provides a system for dehydrogenation and helium extraction from purge gas in an ammonia synthesis unit, comprising a pretreatment unit 1, a first heating unit 2, a primary dehydrogenation unit 3, a cooling unit, a membrane filtration unit 5, a second heating unit 6, a secondary dehydrogenation unit 7, and a drying unit 8; the pretreatment unit 1 is connected to the first heating unit 2; the first heating unit 2 is connected to the primary dehydrogenation unit 3; the primary dehydrogenation unit 3 is connected to the shell side of the second heating unit 6; the shell side of the second heating unit 6 is connected to the cooling unit; the cooling unit 4 is connected to the membrane filtration unit 5; the membrane filtration unit 5 is connected to the tube side of the second heating unit 6; the tube side of the second heating unit 6 is connected to the secondary dehydrogenation unit 7; and the secondary dehydrogenation unit 7 is connected to the drying unit 8.

[0028] In this invention, the pretreatment unit 1 includes a first hydrogen concentration detector 11 and a gas compressor 12; the first hydrogen concentration detector 11 is located at the inlet end of the gas compressor 12; the gas compressor 12 is connected to the first heating unit 2.

[0029] In this invention, a first flow meter 32 is provided between the gas compressor 12 and the first heating unit 2; the first flow meter 32 is electrically connected to the gas compressor 12; and the gas compressor 12 is electrically connected to the first hydrogen concentration detector 11.

[0030] In this invention, a temperature detector 10 is provided between the primary dehydrogenation unit 3 and the second heating unit 6.

[0031] The hydrogen concentration is detected in advance by the first hydrogen concentration detector 11, which is electrically connected to the gas compressor 12. The gas is then compressed to the required concentration by pre-calculation. The flow rate of the compressed gas is controlled by the first flow meter 9 to avoid excessive methane concentration in the later stage, which may lead to leakage, flammability, explosion and other risks.

[0032] In this invention, the first heating unit 2 includes a preheating device 21 and a heating device 22; the pretreatment unit 1 is connected to the tube side of the preheating device 21; the preheating device 21 is connected to the heating device 22; the heating device 22 is connected to the tube side of the first-stage dehydrogenation unit 3; the tube side of the first-stage dehydrogenation unit 3 is connected to the shell side of the preheating device 21; and the shell side of the preheating device 21 is connected to the cooling unit 4.

[0033] The high-temperature mixed gas after dehydrogenation by the first-stage dehydrogenation unit 3 is introduced into the shell side of the preheating device 21 through the preheating device 21. This can cool the mixed gas, reduce the energy consumption of the cooling unit, and preheat the subsequent compressed purge gas.

[0034] In this invention, the preheating device 21, the heating device 22, and the second heating unit 6 are all tube heat exchangers.

[0035] In this invention, the primary dehydrogenation unit 3 includes a reaction tower, the gas inlet of which is connected to the gas outlet of the heating device 22, and a flow meter is provided between the reaction tower and the heating device 22; the reaction tower is also connected to carbon monoxide and carbon dioxide inlet pipes, and a gas flow meter is also provided on the inlet pipes, thereby controlling the reaction in the reaction tower, thereby controlling the concentration of methane in the reaction tower, and further ensuring the safety during the conversion.

[0036] Second, the present invention provides an extraction method for the helium extraction system of the above-mentioned ammonia synthesis unit's off-gas dehydrogenation, comprising the following steps:

[0037] S1 purge gas is compressed, deoxygenated, and dried by the pretreatment unit 1 and then passed into the first heating unit 2 for heating to obtain mixed gas I;

[0038] S2 introduces mixed gas I into the first-stage dehydrogenation unit 3 for methanation and dehydrogenation to obtain mixed gas II;

[0039] S3 introduces mixed gas II into the cooling unit, and after cooling, it is introduced into the membrane filtration unit 5 to obtain mixed gas III;

[0040] S4. Mixed gas III is introduced into the second heating unit 6. After being heated, it is introduced into the secondary dehydrogenation unit 7 for catalytic oxidation dehydrogenation. After catalytic oxidation dehydrogenation, the gas is introduced into the drying unit 8 and dried to obtain helium.

[0041] In this invention, iron powder and silica gel desiccant are used as fillers for deoxidation and drying in S1.

[0042] In this invention, the temperature of the mixed gas I in S1 is 300~400℃.

[0043] In this invention, the methanation catalyst in S2 includes at least one of Ni / Al2O3 and Ni / SiO2.

[0044] In this invention, the mixed gas II in S3 is cooled to -5~-1℃ before being introduced into the membrane filtration unit.

[0045] In this invention, the membrane filtration unit 5 in S3 adopts a multi-stage membrane filter, and the membrane used includes at least one of polycarbonate hollow fiber membrane, silicone rubber and polysulfone composite hollow fiber membrane.

[0046] By employing a multi-stage membrane filter, methane, nitrogen, and other gases in the purge gas can be separated.

[0047] In this invention, the catalytic oxidation dehydrogenation temperature in S4 is 30~60℃; the catalyst contains at least one of Pd / Al2O3 and Pd / SiO2.

[0048] <Example>

[0049] Example 1

[0050] The hydrogen concentration of the purge gas is pre-detected by a first hydrogen concentration detector 11, which is electrically connected to a gas compressor 12. The purge gas is then compressed to the required concentration based on pre-calculation. The flow rate of the compressed purge gas is controlled by a first flow meter 9. The compressed purge gas is further preheated by a preheating device 21, and then heated by a heating device 22. A temperature monitoring device is installed between the preheating device 21 (tube heat exchanger) and the heating device 22 (tube heat exchanger). The heating device 22 is equipped with a temperature controller, which is electrically connected to the temperature monitoring device. This controller effectively regulates the temperature of the heating device 22 during production, ensuring that the temperature of the mixed gas I discharged is 300℃. The mixed gas I is further passed into the first-stage dehydrogenation unit 3, where Ni / Al2O3 is used as the methanation catalyst to perform first-stage dehydrogenation at 300°C to obtain mixed gas II. It then passes through the cooling unit 4, first through a tube-and-shell heat exchanger, and then through a refrigerator to cool mixed gas II to -5°C. It is then passed through a polycarbonate hollow fiber membrane filter for multi-stage filtration of methane to obtain mixed gas III. Mixed gas III is then passed into the second heating unit 6 (tube-and-shell heat exchanger) for reheating, raising its temperature to 30°C. Using Pd / Al2O3 as the catalyst, the remaining hydrogen is fully catalytically oxidized, and after drying in a drying tower, high-purity helium is obtained.

[0051] Example 2

[0052] The difference between this embodiment and Example 1 is that Ni / SiO2 is used as the methanation catalyst, and primary dehydrogenation is carried out at 350°C.

[0053] Example 3

[0054] The difference between this embodiment and Embodiment 2 is that, and the difference between this embodiment and Embodiment 1 is that, the first-stage dehydrogenation is carried out at 400°C.

[0055] Example 4

[0056] The difference between this embodiment and Embodiment 1 is that Pd / SiO2 is used as a catalyst, and secondary dehydrogenation is carried out at 40°C.

[0057] Example 5

[0058] The difference between this embodiment and Embodiment 4 is that secondary dehydrogenation is performed at 60°C.

[0059] Example 6

[0060] The difference between this embodiment and Embodiment 1 is that a silicone rubber and polysulfone composite hollow fiber membrane filter is used to perform multi-stage filtration of methane.

[0061] <Comparative Example>

[0062] Comparative Example 1

[0063] The difference between this comparative example and Example 1 is that a silica membrane filter is used to perform multi-stage filtration of methane.

[0064] Comparative Example 2

[0065] The difference between this comparative example and Example 1 is that a molecular sieve membrane filter is used to perform multi-stage filtration of methane.

[0066] Comparative Example 3

[0067] The difference between this comparative example and Example 1 is that no secondary dehydrogenation treatment was performed.

[0068] <Experimental Example>

[0069] Helium obtained from the purification of Examples 1-6 and Comparative Examples 1-3 was used as samples for helium purity testing. The experimental results are shown in Table 1.

[0070] Table 1

[0071]

[0072] As shown in Table 1, Examples 1-6 and Comparative Examples 1-3, the purified helium has a high purity. Among them, the polycarbonate hollow fiber membrane has the best methane separation effect, followed by the silicone rubber and polysulfone composite hollow fiber membrane. The methanation catalyst can remove most of the hydrogen, but there is still some residue. The purity of helium can be further improved by secondary dehydrogenation.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A system for dehydrogenating and extracting helium from purge gas in an ammonia synthesis unit, characterized in that, The system includes a pretreatment unit (1), a first heating unit (2), a primary dehydrogenation unit (3), a cooling unit (4), a membrane filtration unit (5), a second heating unit (6), a secondary dehydrogenation unit (7), and a drying unit (8). The pretreatment unit (1) is connected to the first heating unit (2). The first heating unit (2) is connected to the primary dehydrogenation unit (3). The primary dehydrogenation unit (3) is connected to the shell side of the second heating unit (6). The shell side of the second heating unit (6) is connected to the cooling unit (4). The cooling unit (4) is connected to the membrane filtration unit (5). The membrane filtration unit (5) is connected to the tube side of the second heating unit (6). The tube side of the second heating unit (6) is connected to the secondary dehydrogenation unit (7). The secondary dehydrogenation unit (7) is connected to the drying unit (8). The first heating unit (2) includes a preheating device (21) and a heating device (22); the pretreatment unit (1) is connected to the tube side of the preheating device (21); the preheating device (21) is connected to the heating device (22); the heating device (22) is connected to the tube side of the first-stage dehydrogenation unit (3); the tube side of the first-stage dehydrogenation unit (3) is connected to the shell side of the preheating device (21); the shell side of the preheating device (21) is connected to the cooling unit (4). The extraction method used in the ammonia synthesis unit's purge gas dehydrogenation and helium extraction system includes the following steps: S1 purge gas is compressed, deoxygenated, and dried by the pretreatment unit (1) and then passed into the first heating unit (2) for heating to obtain mixed gas I; S2. Mixed gas I is introduced into the first-stage dehydrogenation unit (3) for methanation and dehydrogenation to obtain mixed gas II; S3 The mixed gas II is introduced into the cooling unit (4), and after cooling, it is introduced into the membrane filtration unit (5) to obtain mixed gas III; S4. Mixed gas III is introduced into the second heating unit (6), and after being heated, it is introduced into the secondary dehydrogenation unit (7) for catalytic oxidation dehydrogenation. After catalytic oxidation dehydrogenation, the gas is introduced into the drying unit (8) and dried to obtain helium.

2. The ammonia synthesis unit purge gas dehydrogenation and helium extraction system according to claim 1, characterized in that, The pretreatment unit (1) includes a first hydrogen concentration detector (11) and a gas compressor (12); the first hydrogen concentration detector (11) is located at the inlet end of the gas compressor (12); the gas compressor (12) is connected to the first heating unit (2).

3. The ammonia synthesis unit purge gas dehydrogenation and helium extraction system according to claim 2, characterized in that, A first flow meter (9) is provided between the gas compressor (12) and the first heating unit (2); the first flow meter (9) is electrically connected to the gas compressor (12); the gas compressor (12) is electrically connected to the first hydrogen concentration detector (11).

4. The ammonia synthesis unit purge gas dehydrogenation and helium extraction system according to claim 1, characterized in that, A temperature detector (10) is provided between the primary dehydrogenation unit (3) and the second heating unit (6).

5. The ammonia synthesis unit purge gas dehydrogenation and helium extraction system according to claim 1, characterized in that, The temperature of mixed gas I in S1 is 300~400℃.

6. The ammonia synthesis unit purge gas dehydrogenation and helium extraction system according to claim 1, characterized in that, In S2, the methanation catalyst contains at least one of Ni / Al2O3 and Ni / SiO2.

7. The ammonia synthesis unit purge gas dehydrogenation and helium extraction system according to claim 1, characterized in that, Mixed gas II in S3 is cooled to -5~-1℃ before being introduced into the membrane filtration unit.

8. The ammonia synthesis unit purge gas dehydrogenation and helium extraction system according to claim 1, characterized in that, The catalytic oxidation dehydrogenation temperature in S4 is 30~60℃; the catalyst contains at least one of Pd / Al2O3 and Pd / SiO2.

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