Method for extracting high-purity helium from BOG

Abstract

The invention belongs to the technical field of helium extraction, and particularly relates to a method for extracting high-purity helium from BOG (Boil Off Gas), which comprises the following steps: separating methane and nitrogen in BOG gas through a low-temperature crude extraction process to obtain crude helium; oxygen is added into the crude helium, then heating is performed, hydrogen and a small amount of methane in the crude helium react with the oxygen respectively under the action of a catalyst, the reacted gas is dried and adsorbed, and dehydrogenated crude helium is obtained; carrying out a low-temperature condensation process on the dehydrogenated crude helium to liquefy and remove nitrogen and a small amount of oxygen in the dehydrogenated crude helium to obtain refined helium; nitrogen and oxygen are removed from the refined helium through a low-temperature adsorption process to obtain high-purity helium, liquid nitrogen is adopted as a cold source in the low-temperature adsorption process, and when the liquid nitrogen is vaporized, the vaporized nitrogen is discharged through vacuumizing treatment. The pressure in the liquid nitrogen storage tank is smaller than 1 KPa, the vaporization rate of the liquid nitrogen is higher, more heat is absorbed, the adsorption efficiency of the second adsorber is improved, and consumption of the liquid nitrogen is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of helium extraction, and in particular relates to a method for extracting high-purity helium from BOG. Background Art

[0002] Helium is a rare gas that is abundant in the universe but rare on Earth. It mainly exists in the atmosphere, rocks and natural gas. Helium is usually a colorless and odorless gas. It is the only substance that cannot solidify under standard atmospheric pressure. Its special physical properties determine the special uses of helium. As a scarce strategic resource for the development of national defense and high-tech industries, helium is widely used in military industry, scientific research, petrochemicals, refrigeration, medical treatment, semiconductors, pipeline leak detection, superconducting experiments, metal manufacturing, deep-sea diving, high-precision welding, optoelectronic product production and other fields, which is of great significance to scientific research.

[0003] There are many feasible solutions for extracting helium from BOG at home and abroad, some of which have been industrially implemented, such as: cryogenic method, liquefaction method, pressure swing adsorption method, membrane separation, etc.

[0004] For example, a BOG helium extraction system and helium extraction method with publication number CN114264116A includes a cryogenic distillation unit and a palladium membrane separation and dehydrogenation unit, wherein the cryogenic distillation unit is used to separate a first-stage crude helium gas including a small amount of nitrogen, hydrogen and a small amount of oxygen from BOG by a cryogenic method, and the palladium membrane separation and dehydrogenation unit is used to remove hydrogen from the first-stage crude helium gas by a palladium membrane separation method to obtain a second-stage crude helium gas. Although the membrane separation method has great advantages in its modularity and energy saving, the membrane separation can only obtain helium with a concentration of 99%. Summary of the invention

[0005] The purpose of the present invention is to provide a method for extracting high-purity helium from BOG, so as to solve the technical problems existing in the existing methods for extracting high-purity helium.

[0006] In order to achieve the above technical purpose, the technical solution of the present invention is: A method for extracting high-purity helium from BOG comprises the following steps: S1. Separate the methane and nitrogen from the BOG gas through a cryogenic crude extraction process and a cryogenic distillation process to obtain crude helium; S2, adding oxygen to the crude helium gas, and then heating it, so that the hydrogen and a small amount of methane in the crude helium gas react with the oxygen respectively under the action of a catalyst, and the gas after the reaction is dried and adsorbed to remove water and carbon dioxide, thereby obtaining dehydrogenated crude helium gas; S3, the dehydrogenated crude helium gas is subjected to a low-temperature condensation process to liquefy and remove nitrogen and a small amount of oxygen therein to obtain refined helium gas; S4. Refined helium is subjected to a low-temperature adsorption process to remove nitrogen, oxygen and other impurities to obtain high-purity helium. Liquid nitrogen is used as a cold source during the low-temperature adsorption process. When the liquid nitrogen vaporizes, the vaporized nitrogen is discharged through vacuum treatment.

[0007] As a further improvement, in step S1, the specific process of the low-temperature crude extraction process is: deep cooling the BOG gas for heat exchange to liquefy the methane and nitrogen in the BOG gas, and then separating the liquid methane and nitrogen from the gaseous crude helium by a gas-liquid separation method.

[0008] As a further improvement, the crude helium after the low-temperature crude extraction process enters the buffer tank, and the crude helium in the buffer tank is mixed with oxygen to form a mixed gas. The mixed gas enters the reactor, which includes a hydrogen reactor and a methane reactor. In the hydrogen reactor, the oxygen in the mixed gas reacts with the hydrogen to generate water, and then enters the methane reactor. After being heated to 330~440℃ in the methane reactor, the methane and oxygen in the mixed gas react, and the reacted gas is dried and adsorbed to obtain dehydrogenated crude helium.

[0009] As a further improvement, in step S3, the specific process of the low-temperature condensation process is: deep cooling the dehydrogenated crude helium gas for heat exchange to liquefy the nitrogen and oxygen in the dehydrogenated crude helium gas, and then separating the liquid nitrogen and oxygen from the gaseous refined helium gas by a gas-liquid separation method.

[0010] As a further improvement, in step S4, the specific process of the low-temperature adsorption process is as follows: the refined helium gas is liquefied by deep cold heat exchange and then enters the low-temperature adsorber, the low-temperature adsorber is filled with molecular sieves and activated carbon, a liquid nitrogen storage tank is arranged outside the low-temperature adsorber, the liquid nitrogen as a cold source is located in the liquid nitrogen storage tank, the liquid nitrogen storage tank is connected to a vacuum system, the vacuum system includes a heat exchanger and a vacuum pump, the liquid nitrogen in the liquid nitrogen storage tank is vaporized by heat exchange and then continues to heat exchange and heat up through the heat exchanger, and then is discharged through the vacuum pump.

[0011] As a further improvement, the mixed gas first enters a preheater to be preheated to 60° C., and then enters a reactor for reaction.

[0012] As a further improvement, after the dehydrogenated crude helium is dried and adsorbed, part of the dehydrogenated crude helium is pressurized and returned to the buffer tank, and circulated for oxygenation, dehydrogenation and methane removal, and part of the dehydrogenated crude helium is pressurized and enters the low-temperature condensation process.

[0013] As a further improvement, the liquid methane and nitrogen separated by the cryogenic crude extraction system are separated into methane and nitrogen through a cryogenic distillation process.

[0014] Due to the adoption of the above technical solution, the beneficial effects of the present invention are: The method for extracting high-purity helium from BOG provided by the present invention removes a large amount of methane and nitrogen through a low-temperature crude extraction process and a low-temperature distillation process, then removes hydrogen and methane through cyclic oxygenation to remove hydrogen and methane, hydrogen and methane react with oxygen respectively to generate water and carbon dioxide, and dehydrogenated crude helium is obtained through dry adsorption, and then nitrogen and a small amount of oxygen are removed through a low-temperature condensation process, and then impurities in the refined helium are removed through a low-temperature adsorption process to obtain high-purity helium, and the purity of the high-purity helium reaches 99.999%, which is higher.

[0015] In the process of cyclic oxygenation, dehydrogenation and methane removal, part of the dehydrogenated crude helium gas after drying and adsorption is returned to the buffer tank, and the oxygenation, dehydrogenation and methane removal process is cyclically carried out. The dehydrogenated crude helium gas returned to the buffer tank is mixed with the crude helium gas in the buffer tank, which can dilute the hydrogen content in the crude helium gas in the buffer tank and remove the reaction heat in time.

[0016] The nitrogen vaporized by heat exchange in the liquid nitrogen storage tank is discharged through a vacuum pump, and the pressure in the liquid nitrogen storage tank is made less than 1KPa. The vaporization rate of liquid nitrogen is higher and it absorbs more heat, which improves the adsorption efficiency of the second adsorber and reduces the consumption of liquid nitrogen. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a process flow chart of extracting high-purity helium from BOG of the present invention; Figure 2 This is a flow chart of the cyclic oxygen addition and hydrogen removal process of the present invention; Figure 3 This is a flow chart of the equipment of the BOG extraction high-purity helium system of the present invention; Among them, 1-first heat exchanger, 2-first separation tank, 3-distillation tower, 4-buffer tank, 5-mixer, 6-preheater, 7-reactor, 8-cooler, 9-first adsorber, 10-vacuum pump, 11-circulating air compressor, 12-compressor, 13-second heat exchanger, 14-second separation tank, 15-second adsorber, 16-third heat exchanger, 17-external electric heating furnace, 18-product tank. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0019] like Figure 1-3 As shown, a method for extracting high-purity helium from BOG comprises the following steps: S1. First, methane and nitrogen in BOG gas are separated by cryogenic crude extraction process and cryogenic distillation process to obtain crude helium. Specifically, BOG gas is cryogenically heat exchanged with liquid nitrogen to liquefy methane and nitrogen therein, and then liquid methane and nitrogen are separated from gaseous crude helium by gas-liquid separation method. At this time, the crude helium contains helium, a small amount of nitrogen and hydrogen. The separated mixed gas of methane and nitrogen is separated into methane and nitrogen in cryogenic distillation process; S2, removing hydrogen and methane from the crude helium through a cyclic oxygenation and dehydrogenation and demethane process, adding oxygen to the crude helium, and then heating it, so that the hydrogen and methane in the crude helium react with the oxygen respectively under the action of a catalyst, and drying and adsorbing the gas after the reaction to remove water and carbon dioxide, thereby obtaining dehydrogenated crude helium; S3, the dehydrogenated crude helium gas is subjected to a low-temperature condensation process to liquefy and remove nitrogen and a small amount of oxygen therein to obtain refined helium gas. Specifically, the dehydrogenated crude helium gas is subjected to heat exchange with liquid nitrogen to liquefy the nitrogen and oxygen therein, and then the liquid nitrogen and oxygen are separated from the gaseous refined helium gas by a gas-liquid separation method to obtain refined helium gas. At this time, the refined helium gas contains helium, a small amount of nitrogen and a small amount of oxygen; S4. Refined helium is subjected to a low-temperature adsorption process to remove extremely small amounts of impurities such as nitrogen and oxygen to obtain high-purity helium. The purity of high-purity helium reaches 99.999%. Liquid nitrogen is used as a cold source during the low-temperature adsorption process. When the liquid nitrogen vaporizes, the vaporized nitrogen is discharged through vacuum treatment.

[0020] In this embodiment, in step S2, the crude helium after the low-temperature crude extraction process enters the buffer tank 4, and the crude helium in the buffer tank 4 is mixed with oxygen to form a mixed gas. The mixed gas enters the preheater 6 and is preheated to 60°C, and then enters the reactor 7. The reactor includes a hydrogen reactor and a methane reactor. In the hydrogen reactor, the hydrogen and oxygen in the mixed gas react to generate water, and the reaction is accompanied by heat release, so that the gas in the hydrogen reactor is heated to 72~132°C, and then enters the methane reactor. In the methane reactor, the mixed gas is heated to 330~440°C. After reaching the methane reaction temperature, the oxygen in the mixed gas reacts with methane to generate water and carbon dioxide. After the reaction is completed, the reacted gas enters the cooler 8, and after cooling in the cooler 8, enters the dry adsorption, and removes water and carbon dioxide to obtain dehydrogenated crude helium.

[0021] After the dehydrogenated crude helium gas is dried and adsorbed, part of the dehydrogenated crude helium gas is pressurized and returned to the buffer tank 4, and the oxygenation, dehydrogenation and methane removal process is carried out again. Part of the dehydrogenated crude helium gas is pressurized and transported to the low-temperature condensation process. In the circulating oxygenation, dehydrogenation and methane removal process, the dehydrogenated crude helium gas returned to the buffer tank 4 is mixed with the crude helium gas in the buffer tank 4, which can dilute the hydrogen content in the crude helium gas in the buffer tank 4 and remove the reaction heat in time. Under normal circumstances, the methane content in the dehydrogenated crude helium gas meets the process requirements. If it is detected that the methane content in the dehydrogenated crude helium gas does not meet the process standards, the circulating oxygenation, dehydrogenation and methane removal process needs to be adjusted. This is because methane is easy to solidify when the dehydrogenated crude helium gas enters the low-temperature condensation process. Therefore, in the circulating oxygenation, dehydrogenation and methane removal process, it is necessary to detect the methane content in the dehydrogenated crude helium gas to prevent methane from solidifying during the subsequent deep cold heat exchange process.

[0022] In this embodiment, the specific process of low-temperature adsorption is as follows: refined helium is liquefied by deep cold heat exchange and then enters the low-temperature adsorber, the low-temperature adsorber is filled with molecular sieves and activated carbon, a liquid nitrogen storage tank is arranged outside the low-temperature adsorber, liquid nitrogen is arranged in the liquid nitrogen storage tank as a cold source for the low-temperature adsorber, the liquid nitrogen storage tank is connected to a vacuum pumping system, the vacuum pumping system includes a third heat exchanger and a vacuum pump, during the low-temperature adsorption process, refined helium enters the low-temperature adsorber, a small amount of nitrogen, oxygen and other impurities in the refined helium are adsorbed by the molecular sieve and activated carbon in the low-temperature adsorber, at the same time, the liquid nitrogen in the liquid nitrogen storage tank exchanges heat with nitrogen and oxygen, so that the nitrogen and oxygen are always in a liquid state, the liquid nitrogen in the liquid nitrogen storage tank is vaporized by heat exchange, and then enters the third heat exchanger 16 for further heat exchange, and then is discharged through the vacuum pump, during actual use, the vacuum pump continues to work, through the vacuum pumping effect of the vacuum pump, the pressure in the liquid nitrogen storage tank is less than 1KPa, the vaporization rate of the liquid nitrogen is higher, and more heat is absorbed, thereby improving the adsorption efficiency of the low-temperature adsorber and reducing the consumption of liquid nitrogen.

[0023] The present embodiment also provides a system for extracting high-purity helium from BOG, including a cryogenic crude extraction system, a cryogenic distillation system, a circulating oxygenation, dehydrogenation and demethane removal system, a cryogenic condensation system, a cryogenic adsorption system and a vacuum system. The BOG gas first enters the cryogenic crude extraction system to remove methane and nitrogen to obtain crude helium. The crude helium enters the circulating oxygenation, dehydrogenation and demethane removal system to react hydrogen and methane in the crude helium with oxygen respectively, and then removes water and carbon dioxide by dry adsorption to obtain dehydrogenated crude helium. The dehydrogenated crude helium enters the cryogenic condensation system to liquefy and separate nitrogen and oxygen in the dehydrogenated crude helium to obtain refined helium. The refined helium enters the cryogenic adsorption system to adsorb a small amount of nitrogen, oxygen and other impurities in the refined helium at low temperature to obtain high-purity helium.

[0024] In this embodiment, the cryogenic crude extraction system includes a first heat exchanger 1 and a first separation tank 2. A BOG gas pipeline and a liquid nitrogen pipeline are arranged in the first heat exchanger 1. The BOG gas enters the BOG gas pipeline and exchanges heat with the liquid nitrogen in the liquid nitrogen pipeline, so that the methane and nitrogen in the BOG gas are liquefied, and then enters the first separation tank 2. The crude helium enters the circulating oxygenation, hydrogenation and methane removal system from the top of the first separation tank 2, and the methane and nitrogen enter the cryogenic distillation system from the bottom of the first separation tank 2.

[0025] The cryogenic distillation system comprises a distillation tower 3, in which the methane and nitrogen at the bottom of the first separation tank 2 are cryogenically distilled, and nitrogen is obtained at the top of the distillation tower, and methane is obtained at the bottom of the distillation tower.

[0026] A methane pipeline and a nitrogen pipeline are also provided in the first heat exchanger 1. In the distillation tower 3, according to the different boiling points of methane and nitrogen, methane enters the methane pipeline of the first heat exchanger 1 from the bottom of the distillation tower 3, and nitrogen enters the nitrogen pipeline of the first heat exchanger 1 from the top of the distillation tower 3. After heat exchange in the first heat exchanger 1, methane enters the subsequent recycling procedure, and nitrogen is discharged after heat exchange in the first heat exchanger 1. The energy in methane and nitrogen is recovered through the first heat exchanger 1 to improve the utilization rate of energy.

[0027] In this embodiment, the circulating oxygen addition dehydrogenation and demethane removal system includes a buffer tank 4, a mixer 5, a preheater 6, a reactor 7, a cooler 8 and a first adsorber 9. The crude helium enters the buffer tank 4 from the first separation tank 2, and then enters the mixer 5. After being mixed with oxygen in the mixer 5, a mixed gas is formed. The mixed gas enters the preheater 6, is preheated to 60° C. in the preheater 6, and then enters the reactor 7. The reactor includes a hydrogen reactor and a methane reactor. In the hydrogen reactor, the oxygen and hydrogen in the mixed gas react. At the same time, the mixed gas is heated to 72~132° C. in the hydrogen reactor, and then the mixed gas enters the methane reactor. The mixed gas is heated to 330~440° C., and the oxygen and methane in the mixed gas react. The reacted gas enters the cooler 8, and enters the first adsorber 9 after being cooled in the cooler 8. The first adsorber 9 is filled with molecular sieves, alumina and activated carbon. In the first adsorber 9, water and carbon dioxide are adsorbed to obtain dehydrogenated crude helium, and the dehydrogenated crude helium enters the low-temperature condensation system.

[0028] The first adsorber 9 includes two parallel dry adsorbers. When one dry adsorber is adsorbing, the other dry adsorber is regenerating. Automatic switching valves are arranged at the inlets of the two dry adsorbers. In actual use, the system for extracting high-purity helium from BOG also includes a carbon dioxide online analyzer. The carbon dioxide online analyzer can be arranged between the dry adsorber and the low-temperature condensation system. The carbon dioxide content in the refined helium gas is detected by the carbon dioxide online analyzer. When the carbon dioxide content in the refined helium gas is detected to be greater than 0.8ppm, the automatic switching valve automatically switches, so that the dry adsorber originally in the adsorption program is subjected to the regeneration program, and the dry adsorber that has completed the regeneration enters the adsorption program. In this way, the two dry adsorbers are automatically switched and used, and the adsorption efficiency is higher.

[0029] The outlet of the cooler 8 is connected to the bottom inlet of the dry adsorber. A regeneration inlet is also provided at the bottom of the dry adsorber. The regeneration inlet is connected to the outlet of the external electric heating furnace 17. Nitrogen is introduced into the inlet of the external electric heating furnace 17. The nitrogen is heated by the external electric heating furnace 17 and then enters the dry adsorber to regenerate the dry adsorber. A regeneration gas outlet is provided at the top of the dry adsorber, and the regeneration gas after the regeneration of the dry adsorber is discharged from the regeneration gas outlet.

[0030] In this embodiment, the circulating oxygenation, dehydrogenation and demethane removal system further includes a circulating air compressor 11 and a compressor 12 connected to the outlet of the first adsorber 9. The inlets of the circulating air compressor 11 and the compressor 12 are both provided with automatic valves. The outlet of the circulating air compressor 11 is connected to the inlet of the buffer tank 4. The outlet of the compressor 12 is connected to the low-temperature condensation system. A portion of the dehydrogenated crude helium gas at the outlet of the first adsorber 9 is transported to the buffer tank 4 through the circulating air compressor 11 to continue oxygenation and dehydrogenation, and promotes the gas to circulate in the circulating oxygenation, dehydrogenation and demethane removal system to dilute the crude helium gas in the buffer tank 4. The hydrogen content in the gas is determined by the compressor, and the other part is pressurized by the compressor and then enters the low-temperature condensation system. A methane content detection device is set at the outlet of the circulating gas compressor. The working effect of the circulating oxygen-added dehydrogenation and methane removal system is judged by detecting the methane content in the dehydrogenated crude helium gas going to the buffer tank. Under normal circumstances, the methane content in the dehydrogenated crude helium gas returned to the buffer tank meets the process standards. However, if the methane content in the dehydrogenated crude helium gas returned to the buffer tank does not meet the process standards, it means that there is a problem with the circulating oxygen-added dehydrogenation and methane removal system, which needs to be adjusted by the staff.

[0031] In this embodiment, the low-temperature condensation system includes a second heat exchanger 13 and a second separation tank 14. The second heat exchanger 13 includes a dehydrogenated crude helium gas pipeline and a liquid nitrogen pipeline. The dehydrogenated crude helium gas from the compressor 12 enters the dehydrogenated crude helium gas pipeline, exchanges heat with the liquid nitrogen in the liquid nitrogen pipeline for cooling, and the nitrogen and oxygen in the dehydrogenated crude helium gas are liquefied, and then the dehydrogenated crude helium gas enters the second separation tank 14.

[0032] A nitrogen and oxygen pipeline is also provided in the second heat exchanger 13. The liquefied nitrogen and oxygen enter the nitrogen and oxygen pipeline of the second heat exchanger 13 from the bottom of the second separation tank 14. The liquefied nitrogen and oxygen exchange heat with the dehydrogenated crude helium in the dehydrogenated crude helium pipeline, and the cold energy of the liquefied nitrogen and oxygen is recovered and then discharged from the outlet of the liquid nitrogen pipeline.

[0033] A refined helium pipeline is also provided in the second heat exchanger 13. The refined helium gas at the top outlet of the second separation tank enters the refined helium pipeline of the second heat exchanger 13, and after deep cold heat exchange with the liquid nitrogen in the liquid nitrogen pipeline, enters the cryogenic adsorption system.

[0034] In this embodiment, the low-temperature adsorption system includes a second adsorber 15, which includes two low-temperature adsorbers. The two low-temperature adsorbers are arranged in parallel. The two low-temperature adsorbers are filled with molecular sieves and activated carbon to adsorb impurities such as residual oxygen, nitrogen and neon in the refined helium gas to obtain high-purity helium. The outlet of the low-temperature adsorber is connected to the product tank 18, and the high-purity helium is pressurized at the outlet of the low-temperature adsorber and transported to the product tank 18.

[0035] In this embodiment, a liquid nitrogen storage tank is provided outside the two cryogenic adsorbers. The liquid nitrogen storage tank is filled with liquid nitrogen. The liquid nitrogen serves as a cold source to provide cold energy to the cryogenic adsorber for cryogenic adsorption. The liquid nitrogen storage tank is connected to a vacuum pumping system. The vacuum pumping system includes a third heat exchanger 16 and a vacuum pump 10. During the cryogenic adsorption process, refined helium enters the interior of the cryogenic adsorber, and a small amount of nitrogen and oxygen and other impurities in the refined helium are adsorbed by the molecular sieve and activated carbon in the cryogenic adsorber. At the same time, the liquid nitrogen in the liquid nitrogen storage tank exchanges heat with the adsorbed nitrogen and oxygen in the cryogenic adsorber, so that the nitrogen and oxygen are always in a liquid state. After the liquid nitrogen in the liquid nitrogen storage tank is vaporized by heat exchange, it enters the third heat exchanger 16 for further heat exchange, and then is discharged through the vacuum pump 10. Through the vacuum pumping effect of the vacuum pump 10, the pressure in the liquid nitrogen storage tank is less than 1KPa, the vaporization rate of the liquid nitrogen is higher, and more heat is absorbed, thereby improving the adsorption efficiency and reducing the consumption of liquid nitrogen.

[0036] The present invention uses BOG gas as a raw material, and extracts helium from the BOG gas through low-temperature rough extraction, cyclic oxygenation, hydrogenation and methane removal, low-temperature condensation, and low-temperature adsorption processes. The purity of the obtained high-purity helium reaches 99.999%. Moreover, through the effect of the vacuum pumping system, the vaporization rate of the liquid nitrogen in the liquid nitrogen storage tank is higher, and more heat is absorbed, thereby improving the adsorption efficiency of the second adsorber and reducing the consumption of liquid nitrogen.

[0037] The specific implementation of the present invention described above does not constitute a limitation on the protection scope of the present invention. Any other corresponding changes and modifications made based on the technical concept of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for extracting high-purity helium from BOG, characterized in that: The following steps are involved: S1. Separate the methane and nitrogen from the BOG gas through a cryogenic crude extraction process and a cryogenic distillation process to obtain crude helium; S2, adding oxygen to the crude helium gas, and then heating it, so that the hydrogen and a small amount of methane in the crude helium gas react with the oxygen respectively under the action of a catalyst, and the gas after the reaction is dried and adsorbed to remove water and carbon dioxide, thereby obtaining dehydrogenated crude helium gas; S3, the dehydrogenated crude helium gas is subjected to a low-temperature condensation process to liquefy and remove nitrogen and a small amount of oxygen therein to obtain refined helium gas; S4. Refined helium is subjected to a low-temperature adsorption process to remove nitrogen, oxygen and other impurities to obtain high-purity helium. Liquid nitrogen is used as a cold source during the low-temperature adsorption process. When the liquid nitrogen vaporizes, the vaporized nitrogen is discharged through vacuum treatment.

2. The method for extracting high-purity helium from BOG according to claim 1, characterized in that: In step S1, the specific process of the low-temperature crude extraction process is: deep cooling the BOG gas for heat exchange to liquefy the methane and nitrogen in the BOG gas, and then separating the liquid methane and nitrogen from the gaseous crude helium by a gas-liquid separation method.

3. The method for extracting high-purity helium from BOG according to claim 1, characterized in that: The crude helium after the low-temperature crude extraction process enters the buffer tank. The crude helium in the buffer tank is mixed with oxygen to form a mixed gas. The mixed gas enters the reactor. The reactor includes a hydrogen reactor and a methane reactor. In the hydrogen reactor, the oxygen in the mixed gas reacts with the hydrogen to generate water, and then enters the methane reactor. After being heated to 330~440℃ in the methane reactor, the methane in the mixed gas reacts with oxygen. The gas after the reaction is dried and adsorbed to obtain dehydrogenated crude helium.

4. The method for extracting high-purity helium from BOG according to claim 1, characterized in that: In step S3, the specific process of the low-temperature condensation process is: deep cooling the dehydrogenated crude helium gas for heat exchange to liquefy the nitrogen and oxygen in the dehydrogenated crude helium gas, and then separating the liquid nitrogen and oxygen from the gaseous refined helium gas by a gas-liquid separation method.

5. The method for extracting high-purity helium from BOG according to claim 1, characterized in that: In step S4, the specific process of the low-temperature adsorption process is as follows: the refined helium gas is liquefied through deep cold heat exchange and then enters the low-temperature adsorber, the low-temperature adsorber is filled with molecular sieves and activated carbon, a liquid nitrogen storage tank is arranged outside the low-temperature adsorber, the liquid nitrogen as a cold source is located in the liquid nitrogen storage tank, the liquid nitrogen storage tank is connected to a vacuum system, the vacuum system includes a heat exchanger and a vacuum pump, the liquid nitrogen in the liquid nitrogen storage tank is vaporized through heat exchange and then continues to heat and heat up through the heat exchanger, and then is discharged through the vacuum pump.

6. The method for extracting high-purity helium from BOG according to claim 3, characterized in that: The mixed gas first enters the preheater to be preheated to 60° C., and then enters the reactor for reaction.

7. The method for extracting high-purity helium from BOG according to claim 3, characterized in that: After drying and adsorption, part of the dehydrogenated crude helium gas is pressurized and returned to the buffer tank for circulation to add oxygen, remove hydrogen and remove methane, and part of the dehydrogenated crude helium gas is pressurized and enters the low-temperature condensation process.

8. The method for extracting high-purity helium from BOG according to claim 2, characterized in that: The liquid methane and nitrogen separated by the cryogenic crude extraction system are separated by cryogenic distillation process.

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