LNG (Liquefied Natural Gas) device for recycling methane through BOG (Boil Off Gas) denitrification by low-temperature membrane method and application

Abstract

The invention discloses an LNG (Liquefied Natural Gas) device for recovering methane through BOG (Boil Off Gas) denitrification by a low-temperature membrane method, which comprises a compressor, an aftercooler, a heat exchanger, a primary membrane separator and a secondary membrane separator, and the temperature of gas entering the two-stage membrane separator is-20 DEG C to-40 DEG C. According to the device, the low-temperature working condition is stably maintained by gas through cooperative pretreatment of the compressor, the aftercooler and the heat exchanger, and the separation selectivity of the membrane to methane and nitrogen is remarkably improved; by means of the two-stage membrane separation design, more than 75% of nitrogen in BOG can be efficiently removed, and the methane recovery rate of more than 85% is achieved. The device effectively reduces the nitrogen introduction amount of recovered BOG, ensures that the nitrogen content of a final LNG product is lower than 1mol%, meets the GB / T38753-2020 standard requirement, fundamentally avoids the problem of BOG emission due to nitrogen accumulation, greatly reduces methane loss, has the advantages of high separation efficiency, high resource utilization rate and high operation stability, and meets the BOG recovery treatment requirement of an LNG device.

Description

Technical Field

[0001] This invention relates to the field of BOG recycling technology, and more particularly to an LNG unit for nitrogen removal and methane recovery from BOG using a cryogenic membrane process. Background Technology

[0002] Natural gas, after being pressurized and cryogenically treated, becomes liquid LNG (Liquefied Natural Gas). When LNG is stored in tanks, it inevitably exchanges heat with the outside environment, and some of the liquid LNG evaporates, producing boil-off gas (BOG). As BOG continues to accumulate, the pressure inside the storage container gradually increases. To ensure safety, when the pressure reaches a set limit, a safety valve automatically opens, releasing excess natural gas into the atmosphere to reduce the pressure inside the tank and prevent the risk of explosion. However, this process results in a significant loss of natural gas.

[0003] The existing conventional recovery process involves heating and pressurizing BOG gas (16-20 kPa, -160 to -140°C) with an air heater, then returning it to the LNG plant inlet. There, it mixes with incoming natural gas, undergoes further pressurization and cryogenic treatment to become liquid LNG. However, BOG contains a certain amount of nitrogen, and long-term recycling can lead to nitrogen accumulation, causing the nitrogen content in the LNG to exceed the Chinese national standard GB / T38753-2020 "Liquefied Natural Gas" requirement of <1 mol%. In this situation, only a certain amount of BOG can be released to reduce the nitrogen content in the LNG, but this results in the loss of methane from the BOG. Summary of the Invention

[0004] This invention provides an LNG unit for denitrification and methane recovery using a cryogenic membrane BOG process, in order to overcome the aforementioned problems.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: A cryogenic membrane BOG denitrification and methane recovery LNG unit, comprising: A compressor used to pressurize BOG; An aftercooler used to cool compressed high-temperature gas; A heat exchanger used for exchanging heat between multiple air streams; A primary membrane separator used to separate methane and nitrogen from BOG; A secondary membrane separator for separating methane and nitrogen from the gas on the non-permeable side of the primary membrane separator; The temperature of the gas entering the primary membrane separator and the secondary membrane separator is -20℃ to -40℃.

[0006] Furthermore, the compressor's inlet end is connected to the BOG main pipeline, and the compressor's outlet end is connected to the aftercooler via the first pipeline; The outlet of the aftercooler is connected to the inlet of the first-stage membrane separator via a second pipeline. The non-permeate side of the first-stage membrane separator is connected to the inlet of the second-stage membrane separator via a mixed gas pipeline. The permeate side of the first-stage membrane separator is connected to the first methane pipeline. The non-permeate side of the secondary membrane separator is connected to the flare system via a nitrogen pipeline, and the permeate side of the secondary membrane separator is connected to the BOG main pipeline via a second methane pipeline. The BOG main pipeline is provided with BOG branch pipelines. The inlet end of the BOG branch pipeline is connected to the BOG main pipeline, and the outlet end of the BOG branch pipeline is connected to the side of the BOG main pipeline closer to the compressor. The second pipeline, the first methane pipeline, the nitrogen pipeline, and the BOG branch pipeline all exchange heat through heat exchangers.

[0007] Furthermore, the gas temperature introduced into the primary membrane separator and the secondary membrane separator is -25℃ to -35℃.

[0008] Furthermore, both the primary membrane separator and the secondary membrane separator are equipped with composite gas separation membranes; the permeability coefficients of the different gases in the composite gas separation membranes are arranged from largest to smallest as follows: C3H8>C2H6>CO2>CH4>H2>O2>He>CO>N2.

[0009] Furthermore, the specific preparation method of the composite gas separation membrane is as follows: 70 parts of vinylmethylsiloxane-octylmethylsiloxane-dimethylsiloxane copolymer were dissolved in 930 parts of isooctane, and 4 parts of polymethylhydrosiloxane and 3 parts of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane platinum complex were added. The mixture was stirred to prepare a separation layer membrane solution. The separation layer membrane solution was uniformly coated on a polyacrylonitrile substrate membrane with a pore size of 25 nm, and the membrane was dried at room temperature to obtain the composite gas separation membrane.

[0010] Furthermore, the specific operation for air drying at room temperature is as follows: air dry at room temperature for 10 minutes, and then dry at 100℃ for 20 minutes.

[0011] Furthermore, the stirring conditions were: stirring at 40°C for 60 min.

[0012] Furthermore, the heat exchanger is a plate-fin heat exchanger.

[0013] The beneficial effects of this invention are: This invention discloses a cryogenic membrane-based BOG nitrogen removal and methane recovery LNG unit. Through coordinated pretreatment by a compressor, aftercooler, and heat exchanger, the gas entering the two-stage membrane separator is stably maintained at a cryogenic temperature of -20℃ to -40℃. This temperature condition significantly improves the membrane's selectivity for separating methane and nitrogen. The two-stage separation design, combining a primary membrane separator and a secondary membrane separator, efficiently removes over 75% of nitrogen from BOG while achieving a methane recovery rate of over 85%. It also effectively reduces the amount of nitrogen carried over from the recovered BOG, ensuring that the nitrogen content in the final LNG product is below 1 mol%, meeting the requirements of GB / T38753-2020. This fundamentally avoids the problem of BOG emissions due to nitrogen accumulation, significantly reducing methane resource loss. This unit combines the advantages of high separation efficiency, high resource utilization, and strong operational stability, making it suitable for the BOG recovery and treatment needs of LNG plants. Attached Figure Description To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1 This is a schematic diagram of the structure of an LNG device for denitrification and methane recovery using a cryogenic membrane BOG process, as disclosed in an embodiment of the present invention.

[0015] In the picture: 1. Compressor; 2. Aftercooler; 3. Heat exchanger; 4. First-stage membrane separator; 5. Second-stage membrane separator; 6. BOG main pipeline; 7. First pipeline; 8. Second pipeline; 9. Mixed gas pipeline; 10. First methane pipeline; 11. Nitrogen pipeline; 12. Second methane pipeline; 13. BOG branch pipeline. Detailed Implementation

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0017] Example: like Figure 1 The image shows a cryogenic membrane BOG denitrification and methane recovery LNG unit provided in this embodiment, comprising: Compressor 1 for pressurizing BOG; Aftercooler 2 is used to cool the compressed high-temperature gas; Heat exchanger 3 is used to exchange heat between multiple air streams; 4. Primary membrane separator for separating methane and nitrogen from BOG; A secondary membrane separator 5 is used to separate methane and nitrogen from the gas on the non-permeable side of the primary membrane separator 4; The temperature of the gas entering the primary membrane separator 4 and the secondary membrane separator 5 is -20℃ to -40℃.

[0018] Heat exchanger 3 is a plate-fin heat exchanger; plate-fin heat exchangers have high heat exchange efficiency and compact structure, and are suitable for the energy recovery requirements of BOG treatment under low temperature conditions. They can quickly achieve precise temperature control of each gas stream, ensuring that the inlet gas temperature of the two-stage membrane separator is stable within the optimal range of -25℃ to -35℃; at the same time, the equipment is small in size, which can save the space occupied by the device, reduce the energy consumption of the system operation, and ensure that the membrane separation denitrification and methane recovery efficiency are stable and meet the standards.

[0019] This invention discloses a cryogenic membrane-based BOG nitrogen removal and methane recovery LNG unit. Through coordinated pretreatment by a compressor, aftercooler, and heat exchanger, the gas entering the two-stage membrane separator is stably maintained at a cryogenic temperature of -20℃ to -40℃. This temperature condition significantly improves the membrane's selectivity for separating methane and nitrogen. The two-stage separation design, combining a primary membrane separator and a secondary membrane separator, efficiently removes over 75% of nitrogen from BOG while achieving a methane recovery rate of over 85%. It also effectively reduces the amount of nitrogen carried over from the recovered BOG, ensuring that the nitrogen content in the final LNG product is below 1 mol%, meeting the requirements of GB / T38753-2020. This fundamentally avoids the problem of BOG emissions due to nitrogen accumulation, significantly reducing methane resource loss. This unit combines the advantages of high separation efficiency, high resource utilization, and strong operational stability, making it suitable for the BOG recovery and treatment needs of LNG plants.

[0020] In a specific embodiment, the inlet end of compressor 1 is connected to the BOG main pipeline 6, and the outlet end of compressor 1 is connected to the aftercooler 2 through the first pipeline 7. The outlet end of the aftercooler 2 is connected to the inlet end of the first-stage membrane separator 4 through the second pipeline 8. The non-permeable side of the first-stage membrane separator 4 is connected to the inlet end of the second-stage membrane separator 5 through the mixed gas pipeline 9. The permeable side of the first-stage membrane separator 4 is connected to the first methane pipeline 10. The non-permeate side of the secondary membrane separator 5 is connected to the flare system via a nitrogen pipeline 11, and the permeate side of the secondary membrane separator 5 is connected to the BOG main pipeline via a second methane pipeline 12, in order to reduce methane resource waste and improve methane recovery rate. A BOG branch line 13 is provided on the main BOG pipeline. The inlet end of the BOG branch line 13 is connected to the main BOG pipeline 6, and the outlet end of the BOG branch line 13 is connected to the side of the main BOG pipeline closest to the compressor. (The BOG gas at 20 kPa and a temperature of -160°C is heated by a plate-fin heat exchanger, and its temperature rises. It then mixes with the gas in the main BOG pipeline, raising the temperature of the gas in the main BOG pipeline to meet the minimum temperature requirement at the compressor inlet. This prevents damage to the compressor due to low temperature, ensures stable equipment operation, and guarantees continuous and efficient process operation.) The second pipeline 8, the first methane pipeline 10, the nitrogen pipeline 11, and the BOG branch pipeline 13 all exchange heat through the heat exchanger 3 to achieve energy recovery and reasonable distribution of each gas stream, reduce system energy consumption, and maintain the low temperature operating conditions of the inlet gas of the first-stage membrane separator and the second-stage membrane separator at -20℃ to -40℃, ensuring membrane separation selectivity and denitrification efficiency.

[0021] BOG gas first enters the BOG main pipeline 6. A stream of BOG gas at 20 kPa and -160°C is introduced into a plate-fin heat exchanger via a BOG branch pipeline for heat exchange and temperature increase. It then flows back to the BOG main pipeline 6 to mix with the gas in the main pipeline, ensuring the BOG gas temperature in the main pipeline meets the minimum inlet temperature requirement of the compressor. The mixed BOG gas then enters the compressor 1 connected to the BOG main pipeline 6 for pressurization. After pressurization, the gas is sent to the aftercooler 2 via the first pipeline 7 for cooling. The cooled gas then enters the first-stage membrane separator 4 via the second pipeline 8. After the first membrane separation, the methane-rich gas on the permeate side is discharged and recovered through the first methane pipeline 10, while the nitrogen-rich gas on the non-permeate side is sent to the second membrane separator 5 for further separation through the mixed gas pipeline 9. The methane-rich gas on the permeate side of the second membrane separator 5 is returned to the BOG main pipeline 6 for recycling through the second methane pipeline 12, while the high-nitrogen gas on the non-permeate side is sent to the flare structure for emission through the nitrogen pipeline 11. During this process, the second pipeline 8, the first methane pipeline 10, the nitrogen pipeline 11, and the BOG branch pipelines all exchange heat through the heat exchanger 3 to achieve reasonable allocation and utilization of gas energy. Through the above-mentioned pipeline connection and heat exchange design, combined with the synergistic effect of two-stage membrane separation, not only is more than 75% of nitrogen in BOG efficiently removed, but the methane recovery rate is also increased to more than 85%, ensuring that the nitrogen content in LNG products meets the requirements of GB / T 38753-2020 standard (<1mol%). This fundamentally avoids BOG emissions caused by nitrogen accumulation, significantly reduces methane loss, and reduces operating costs through energy recovery, ultimately significantly improving the yield of LNG plants. It also has the advantages of strong process continuity, low energy consumption, and high resource utilization.

[0022] In a specific embodiment, the gas temperature introduced into the primary membrane separator 4 and the secondary membrane separator 5 is -25℃ to -35℃.

[0023] This range represents the optimal low-temperature operating condition for membrane separation, ensuring the selectivity of methane and nitrogen separation and guaranteeing efficient removal of nitrogen from BOG. ​​It avoids membrane module embrittlement caused by excessively low temperatures and separation efficiency degradation caused by excessively high temperatures, ensuring stable operation of the membrane system and extending its service life. The system's heat exchange design is adapted to reduce energy consumption, ensuring synergistic effect of the two-stage membranes, stable methane recovery rate meeting standards, and nitrogen content in LNG products strictly complying with the requirements of GB / T38753-2020 standard.

[0024] In a specific embodiment, both the primary membrane separator and the secondary membrane separator are equipped with a composite gas separation membrane; the permeability coefficients of the different gases in the composite gas separation membrane are arranged from largest to smallest as follows: The permeability coefficients of the BOG (Boiler Gas) system match the core requirement for methane recovery after nitrogen removal. Methane has a significantly higher permeability coefficient than nitrogen, enabling efficient separation of methane and nitrogen. Simultaneously, the system exhibits better permeability selectivity for impurities such as C3H8, C2H6, and CO2 that may be present in the BOG than for methane, allowing for the simultaneous removal of some light hydrocarbons and acidic gas impurities, thus improving the purity of the recovered methane. This ensures the stable operation of the two-stage membrane separation system for nitrogen removal, ultimately guaranteeing that the nitrogen content of the LNG product meets standards and improving the methane recovery rate.

[0025] In a specific embodiment, the preparation method of the composite gas separation membrane is as follows: 70 parts of vinylmethylsiloxane-octylmethylsiloxane-dimethylsiloxane copolymer were dissolved in 930 parts of isooctane, and 4 parts of polymethylhydrosiloxane and 3 parts of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane platinum complex were added. The mixture was stirred at 40°C for 60 min to prepare a separation layer membrane solution. The separation layer membrane solution was uniformly coated on a polyacrylonitrile substrate membrane with a pore size of 25 nm, dried at room temperature for 10 min, and then dried at 100°C for 20 min to obtain a composite gas separation membrane.

[0026] This device is also equipped with conventional components such as pressure sensors and temperature sensors to monitor the pressure and temperature of the device in real time, so that operators can keep abreast of the system's operating conditions and accurately adjust the operating parameters of the compressor, heat exchanger and membrane separator, ensuring stable operation of the device under optimal process conditions of low temperature and high pressure. Since the pressure sensor and temperature sensor components are common components in this field, their more specific principles and logic will not be elaborated here.

[0027] The composite separation membrane preparation method used in the embodiments of this scheme (Examples 1-2) is as follows: 700 g of vinylmethylsiloxane-octylmethylsiloxane-dimethylsiloxane copolymer was dissolved in 9300 g of isooctane, and 40 g of polymethylhydrosiloxane and 30 g of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane platinum complex were added. The mixture was stirred at 40 °C for 60 min to prepare a separation layer membrane solution. The separation layer membrane solution was uniformly coated on a polyacrylonitrile substrate membrane with a pore size of 25 nm, air-dried at room temperature for 10 min, and then dried at 100 °C for 20 min to obtain a composite gas separation membrane.

[0028] Example 1 A nitrogen gas with a nitrogen content of 10 mol% and a concentration of 2000 Nm2 is introduced into this device. 3 / h of BOG gas (BOG gas is denoted as (1)), the BOG gas is heated by heat exchange in a plate heat exchanger, and then separated by a primary membrane separator to obtain 1590 m 3 / h, and containing 3mol% N2 methane gas (methane gas is denoted as (2)). This gas is returned to the LNG storage device to reduce the nitrogen content in the LNG in the storage device from 1.5%mol to 0.8%mol, which meets the nitrogen content in the Chinese national standard GB / T 38753-2020 "Liquefied Natural Gas"; the nitrogen-rich gas (nitrogen-rich gas is denoted as (3)) after separation by the secondary separation membrane is returned to the plate-fin heat exchanger for reheating to room temperature and then enters the flare system to go to the flare. The methane-rich gas after separation by the secondary separation membrane is returned to the BOG main pipeline to improve the methane recovery rate; after separation by this device, the nitrogen removal rate of BOG reaches 75.66% and the methane recovery rate reaches 85.81%, as shown in Table 1.

[0029] Table 1 Results of BOG separation using this device in Example 1

[0030] Example 2 A nitrogen gas with a nitrogen content of 3 mol% and a concentration of 2000 Nm2 is introduced into this device. 3 / h of BOG gas (BOG gas is denoted as (1)), the BOG gas is heated by heat exchange in a plate heat exchanger, and then separated by a primary membrane separator to obtain 1700 m 3 / h, and methane gas with N2 content less than 1mol% (methane gas is denoted as (2)), this gas is returned to the LNG storage device, reducing the nitrogen content in the LNG in the storage device from 1.3%mol to 0.7%mol, which meets the nitrogen content requirements of the Chinese national standard GB / T 38753-2020 "Liquefied Natural Gas"; the nitrogen-rich gas (nitrogen-rich gas is denoted as (3)) after separation by the secondary separation membrane is returned to the plate-fin heat exchanger for reheating to room temperature and then enters the flare system to go to the flare. The methane-rich gas after separation by the secondary separation membrane is then returned to the BOG main pipeline to improve the methane recovery rate; after separation by this device, the nitrogen removal rate of BOG reaches 76.09% and the methane recovery rate reaches 86.72%, as shown in Table 2.

[0031] Table 2 Results of BOG separation using this device in Example 2

[0032] In summary, this device utilizes cryogenic membrane separation technology to efficiently separate methane and nitrogen from BOG in LNG plants, significantly improving methane liquefaction rate and effectively avoiding excessive nitrogen content in LNG. In the example, the BOG nitrogen removal rate reached over 75%, and the methane recovery rate exceeded 85%. The nitrogen content in the treated LNG was reduced to 0.7%~0.8%mol, strictly complying with the requirements of GB / T38753-2020 standard. This system is highly adaptable; the membrane system can be flexibly adjusted within the range of 10~100%, capable of handling changes in BOG volume and composition caused by fluctuations in feed gas composition. Simultaneously, it fully utilizes the low temperature of the BOG itself, maintaining the membrane unit at -30℃. Through integrated heat exchange, no additional cooling is required, achieving efficient and energy-saving membrane separation operation.

[0033] Example 3 In the natural gas liquefaction process, fluctuations in the composition of the feed gas cause fluctuations in the amount of BOG (Boiled Boiled Gas). By adjusting the effective area of ​​the composite separation membrane, large variations in the BOG volume can be accommodated. For example, in this embodiment, the preparation method of the composite separation membrane in the primary and secondary membrane separators is the same as in Example 1, but the effective area is reduced to 10% compared to Example 1 (i.e., the amount of each component is reduced to 10%). A nitrogen gas with a nitrogen content of 10 mol% and a concentration of 200 Nm2 is introduced into this device. 3 / h of BOG gas (BOG gas is denoted as (1)), the BOG gas is heated by heat exchange in a plate heat exchanger, and then separated by a primary membrane separator to obtain 1590 m 3 / h, and the methane gas with N2 content of 3.14mol% (methane gas is denoted as (2)), after being separated by the secondary separation membrane, the nitrogen-rich gas (nitrogen-rich gas is denoted as (3)) is returned to the plate-fin heat exchanger for reheating to room temperature and then enters the flare system to go to the flare. The methane-rich gas after being separated by the secondary separation membrane is then returned to the BOG main pipeline to improve the methane recovery rate. After separation by this device, the nitrogen removal rate of BOG reaches 75.63% and the methane recovery rate reaches 85.81%, as shown in Table 3.

[0034] Table 3 Results of BOG separation using this device in Example 3

[0035] As can be seen from the data in Table 3, compared with Example 1, the BOG gas volume in this example is only 1 / 10 of that in Example 1, yet it can still achieve the same nitrogen removal rate and methane recovery rate. Therefore, the operational flexibility of this recovery system can reach 10~100%.

[0036] Comparative Example 1 In this comparative example, a nitrogen gas with a nitrogen content of 10 mol% and a concentration of 2000 Nm³ was introduced into the device. 3 / h BOG gas (1), the BOG gas is heated by a plate heat exchanger, and then separated by a primary membrane separator (the temperature of the primary membrane separator is 0℃, and the composite separation membrane inside is the same as in Example 1), and 1590 m 3 / h, and the N2 content of methane gas (2) is close to 4 mol%, the nitrogen-rich gas (3) after separation by the secondary separation membrane is returned to the plate-fin heat exchanger for reheating to room temperature and then enters the flare system to the flare. The methane-rich gas after separation by the secondary separation membrane is returned to the BOG main pipeline (the temperature of the secondary membrane separator is 0℃, and the composite separation membrane inside is the same as in Example 1). After separation by this device, the nitrogen removal rate of BOG is 77.81%, and the methane recovery rate is only 65.10%, as shown in Table 4.

[0037] Table 4. Results of BOG separation after being processed by this device in Comparative Example 1

[0038] As can be seen from the data in Table 4, compared with the membrane separator at low temperature (Examples 1-3), although the nitrogen removal rate of Comparative Example 1 is slightly improved, the methane recovery rate is reduced by about 20%. Therefore, this separation device is more suitable for use at low temperature, and can achieve a higher methane recovery rate, thereby improving the overall economic benefits.

[0039] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An LNG unit for nitrogen removal and methane recovery via cryogenic membrane BOG process, characterized in that, include: Compressor (1) used to pressurize BOG; An aftercooler (2) is used to cool compressed gas. Heat exchanger used to exchange heat between multiple air streams (3); (4) A primary membrane separator for separating methane and nitrogen from BOG. A secondary membrane separator (5) is used to separate methane and nitrogen from the gas on the non-permeable side of the primary membrane separator (4). The temperature of the gas entering the primary membrane separator (4) and the secondary membrane separator (5) is -20℃ to -40℃.

2. The LNG unit for methane recovery via cryogenic membrane BOG denitrification according to claim 1, characterized in that, The inlet end of the compressor (1) is connected to the BOG main pipeline (6), and the outlet end of the compressor (1) is connected to the aftercooler (2) through the first pipeline (7). The outlet end of the aftercooler (2) is connected to the inlet end of the first-stage membrane separator (4) through the second pipeline (8). The non-permeable side of the first-stage membrane separator (4) is connected to the inlet end of the second-stage membrane separator (5) through the mixed gas pipeline (9). The permeable side of the first-stage membrane separator (4) is connected to the first methane pipeline (10). The non-permeate side of the secondary membrane separator (5) is connected to the flare system via a nitrogen pipeline (11), and the permeate side of the secondary membrane separator (5) is connected to the BOG main pipeline via a second methane pipeline (12). A BOG branch pipeline (13) is provided on the BOG main pipeline (6). The inlet end of the BOG branch pipeline (13) is connected to the BOG main pipeline (6), and the outlet end of the BOG branch pipeline (13) is connected to the side of the BOG main pipeline (6) near the compressor (1). The second pipeline (8), the first methane pipeline (10), the nitrogen pipeline (11), and the BOG branch pipeline (13) all exchange heat through the heat exchanger (3).

3. The LNG unit for denitrification and methane recovery using a cryogenic membrane BOG process according to claim 1, characterized in that, The gas temperature introduced into the primary membrane separator (4) and the secondary membrane separator (5) is -25℃ to -35℃.

4. The LNG unit for denitrification and methane recovery using a cryogenic membrane BOG process according to claim 1, characterized in that, Both the primary membrane separator (4) and the secondary membrane separator (5) are equipped with composite gas separation membranes; the permeability coefficients of the different gases in the composite gas separation membranes are arranged from largest to smallest as follows: C3H8>C2H6>CO2>CH4>H2>O2>He>CO>N2.

5. The LNG unit for denitrification and methane recovery using a cryogenic membrane BOG process according to claim 4, characterized in that, The specific preparation method of the composite gas separation membrane is as follows: 70 parts of vinylmethylsiloxane-octylmethylsiloxane-dimethylsiloxane copolymer were dissolved in 930 parts of isooctane, and 4 parts of polymethylhydrosiloxane and 3 parts of 1,3-divinyl-1,1,3,3-tetramethyldisiloxane platinum complex were added. The mixture was stirred to prepare a separation layer membrane solution. The separation layer membrane solution was uniformly coated on a polyacrylonitrile substrate membrane with a pore size of 25 nm, and the membrane was dried at room temperature to obtain the composite gas separation membrane.

6. The LNG unit for denitrification and methane recovery using a cryogenic membrane BOG process according to claim 5, characterized in that, The specific steps for air drying at room temperature are as follows: air dry at room temperature for 10 minutes, and then dry at 100℃ for 20 minutes.

7. The LNG unit for denitrification and methane recovery using a cryogenic membrane BOG process according to claim 5, characterized in that, The stirring conditions were: stirring at 40℃ for 60 min.

8. The LNG unit for denitrification and methane recovery using a cryogenic membrane BOG process according to claim 1, characterized in that, The heat exchanger (3) is a plate-fin heat exchanger.

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