Nitrogen and helium capturing system for liquefied natural gas boil-off gas

Abstract

The invention relates to a liquefied natural gas boil-off gas nitrogen stripping and helium capturing system which can improve the helium recovery efficiency and comprises a recovery tower, a rectifying tower, a crude helium tower and a nitrogen circulating cooling device. A gas inlet of the recovery tower is communicated with the storage tank, a first tower top reflux cooler is arranged in the recovery tower, a gas phase outlet of the recovery tower is connected with a gas phase inlet of the rectifying tower, and a liquid phase outlet at the tower bottom of the recovery tower is communicated with the storage tank; a second tower top reflux cooler is arranged in the rectifying tower, a reboiler is arranged at the bottom, and a gas phase outlet of the rectifying tower is communicated with a gas phase inlet of the crude helium tower; a third tower top reflux cooler is arranged in the crude helium tower, a liquid phase outlet of the crude helium tower is communicated with an inlet of the third tower top reflux cooler, and liquid nitrogen flowing out of the crude helium tower can flow to the third tower top reflux cooler to serve as a cold source; and the nitrogen circulating cooling device is used for providing liquid nitrogen as a refrigerant for the first tower top reflux cooler and the second tower top reflux cooler and recovering nitrogen generated by vaporization of the liquid nitrogen, and providing nitrogen for the reboiler and recovering nitrogen.

Description

Technical Field

[0001] The present application relates to the technical field of boil-off gas recovery from liquefied natural gas storage tanks, and in particular to a liquefied natural gas boil-off gas nitrogen stripping and helium capture system. Background Art

[0002] Liquefied natural gas (LNG) ships are generally stored at atmospheric pressure and low temperature in atmospheric pressure tanks with cold preservation measures. Atmospheric pressure tanks are inevitably affected by atmospheric pressure fluctuations, external atmospheric temperature, low-speed operation or normal operation of the cryogenic submersible pump in the tank, etc., which cause the internal pressure of the tank to increase. The fundamental reason for the pressure increase is the evaporation gas generated inside the tank. When the tank pressure increases, this part of BOG (Boiled off Gas) evaporation gas will be discharged to maintain the normal operating pressure of the tank.

[0003] In actual operation, this part of BOG gas usually has the following characteristics: 1. The flow rate is considerable, accounting for about 8%~16% of the raw gas flow rate (normally 8%, the maximum 16% takes into account the generation and normal evaporation during the LNG unloading process). 2. BOG (Boiled off Gas) contains methane, nitrogen, and helium. 3. The components are lighter than pure methane and have a high nitrogen content. The temperature of BOG after liquefaction will be lower, possibly ranging from -165℃~-175℃, which is lower than the design temperature of the storage tank -165℃. 4. The helium content is high. The dew point temperature of methane at normal pressure is -163℃, the dew point temperature of nitrogen at normal pressure is -196℃, and helium is a high value-added rare gas with a dew point temperature of -253℃ at normal pressure. The nitrogen content in BOG (Boiled off Gas) is about 3v%~16v%, and the helium content is about 0.2v%~24v%. It is very important to extract as much helium as possible from BOG (Boiled off Gas).

[0004] The treatment of this part of BOG gas is usually carried out in the following ways:

[0005] 1) Use BOG compressor to compress, pressurize and heat the gas before sending it into the gas pipeline network as city gas or factory gas. Because BOG (Boiled off Gas) contains only nitrogen and methane, it is a clean fuel. The process is simple and the equipment is small, but the calorific value of high-nitrogen BOG (Boiled off Gas) is reduced. Moreover, a large amount of cold energy carried by BOG is consumed in the compression process of the compressor and is not utilized, resulting in waste. Moreover, it has limitations for areas where long-distance pipeline transportation is not convenient.

[0006] 2) Recondense BOG. ​​After the BOG is drawn out, it is pressurized by the BOG (Boiled off Gas) compressor. The 0.7MPag LNG cryogenic liquid from the storage tank and the 0.72MPag BOG gas from the BOG compressor are in reverse direct contact in the condenser for mass transfer and heat transfer. The BOG is condensed into LNG in the BOG condenser and then pressurized for external transmission. After the high-nitrogen BOG (Boiled off Gas) is reliquefied, the nitrogen component in the system continues to accumulate and exists in the LNG (Liquefied Natural Gas). More BOG (Boiled off Gas) will be formed in the tank truck during the unloading process. If this part of BOG returns to the BOG main pipe, it will form an unhealthy cycle and may increase the risk of overpressure in the LNG (Liquefied Natural Gas) storage tank.

[0007] 3) BOG pressurization-reliquefaction. After compression by BOG (Boiled off Gas) high and low pressure compressors, the return cold box uses a mixed refrigerant for liquefaction. In the cold box, the mixed refrigerant and BOG normal temperature gas are heat exchanged, which is an efficient recovery method for low nitrogen BOG (Boiled off Gas). After the high nitrogen BOG (Boiled off Gas) is reliquefied, the nitrogen component in the system continues to accumulate and exists in LNG (Liquefied Natural Gas). More BOG (Boiled off Gas) will be formed in the tank truck during the unloading process. If this part of BOG returns to the BOG main pipe, it will form an unhealthy cycle and may increase the risk of overpressure in the LNG (Liquefied Natural Gas) storage tank. In addition, the normal operation of the BOG compressor has the problem that the cold start before driving takes a long time, which is not convenient for rapid full load operation, causing pressure fluctuations in the BOG (Boiled off Gas) pipeline system, and even overpressure venting to the torch, resulting in environmental protection issues such as waste of resources and unnecessary carbon emissions.

[0008] 4) BOG is compressed and then distilled to extract nitrogen and two-stage flash distillation to extract helium. ) BOG is compressed and then distilled to extract nitrogen and two-stage flash distillation. All BOG (Boiled off Gas) evaporated from the liquefied natural gas atmospheric pressure storage tank is pressurized by the compressor and then cooled to a certain temperature by the refrigerant before entering the distillation tower for component separation. The top of the distillation tower is nitrogen and helium, among which the nitrogen component concentration is relatively high and the helium concentration is relatively low. The bottom of the distillation tower is liquefied natural gas LNG (Liquefied Natural Gas). The nitrogen and helium at the top of the tower are cooled to a certain temperature by the refrigerant to form gas-liquid two-phases. After a flash separation in the separator, the top of the separator is crude helium and the bottom is liquid nitrogen. This method consumes a lot of energy for BOG compressor compression, and the investment is relatively high. The nitrogen concentration in the obtained crude helium is relatively high, which affects the consumption and investment of the next step of the refined helium process, and the helium recovery rate is relatively low. Summary of the invention

[0009] Based on this, it is necessary to provide a liquefied natural gas boil-off nitrogen stripping and helium capture system for the high-nitrogen BOG gas generated in the liquefied natural gas storage tank, which has the advantages of high recovery rate and no circulation accumulation.

[0010] A liquefied natural gas boil-off nitrogen stripping and helium capture system, comprising: a recovery tower, a distillation tower, a crude helium tower, and a nitrogen circulation cooling device; the air inlet of the recovery tower is connected to a storage tank through a first pipe, a first top reflux cooler is provided in the recovery tower, the gas phase outlet at the top of the recovery tower is connected to the gas phase inlet of the distillation tower through a second pipe, and the liquid phase outlet at the bottom of the recovery tower is connected to the storage tank through a third pipe;

[0011] The distillation tower is provided with a second top reflux cooler, and a reboiler is provided at the bottom, and the gas phase outlet at the top of the distillation tower is connected to the gas phase inlet of the crude helium tower;

[0012] A third tower top reflux cooler is provided in the crude helium tower, and a liquid phase outlet of the crude helium tower is connected to an inlet of the third tower top reflux cooler through a pipeline, and liquid nitrogen flowing out of the crude helium tower can flow to the third tower top reflux cooler as a cold source;

[0013] The nitrogen circulation cooling device is used to provide liquid nitrogen as a refrigerant to the first tower top reflux cooler and the second tower top reflux cooler and recover nitrogen generated by vaporizing the liquid nitrogen, and to provide nitrogen to the reboiler and recover the nitrogen.

[0014] In some embodiments, the nitrogen cycle cooling device comprises a multi-stream heat exchanger;

[0015] The multi-stream heat exchanger is connected to the inlet of the first tower top reflux cooler through a first nitrogen pipeline, and liquid nitrogen from the multi-stream heat exchanger can enter the first tower top reflux cooler from the first nitrogen pipeline. The multi-stream heat exchanger is connected to the outlet of the first tower top reflux cooler through a second nitrogen pipeline, and nitrogen from the first tower top reflux cooler can enter the multi-stream heat exchanger from the second nitrogen pipeline;

[0016] The multi-stream heat exchanger is connected to the inlet of the second tower top reflux cooler through an eighteenth nitrogen pipeline, and the liquid nitrogen from the multi-stream heat exchanger can enter the second tower top reflux cooler from the eighteenth nitrogen pipeline. The multi-stream heat exchanger is connected to the outlet of the second tower top reflux cooler through a nineteenth nitrogen pipeline, and the nitrogen from the second tower top reflux cooler can enter the multi-stream heat exchanger from the nineteenth nitrogen pipeline;

[0017] The multi-stream heat exchanger is connected to the inlet of the reboiler through the twentieth nitrogen pipeline;

[0018] The nitrogen from the first tower top reflux cooler and the nitrogen from the second tower top reflux cooler enter the multi-stream heat exchanger for reheating and then enter the second nitrogen pipeline.

[0019] In some embodiments, the liquid phase outlet at the bottom of the distillation tower is connected to the multi-stream heat exchanger via a fourth pipe, and the liquefied natural gas from the distillation tower enters the storage tank after being supercooled by the multi-stream heat exchanger.

[0020] In some embodiments, the nitrogen cycle cooling device further comprises: a first compressor, a nitrogen expander and a gas-liquid separator;

[0021] The outlet of the first compressor is connected to the multi-stream heat exchanger through a sixth nitrogen pipeline, and the first compressor is used to introduce low-pressure nitrogen at normal temperature and pressurize the low-pressure nitrogen at normal temperature, and output high-pressure nitrogen at normal temperature;

[0022] The high-pressure nitrogen in the multi-stream heat exchanger has a first flow path and a second flow path. The multi-stream heat exchanger cools the normal-temperature high-pressure nitrogen from the sixth nitrogen pipeline and obtains low-temperature high-pressure nitrogen in two paths, one of which flows into the first flow path and the other flows into the second flow path.

[0023] The outlet of the first flow path is connected to the inlet of the nitrogen expander through the twelfth nitrogen pipeline, and the outlet of the nitrogen expander is connected to the gas-liquid separator through the fifteenth nitrogen pipeline; the nitrogen expander is used to reduce the temperature and pressure of the low-temperature and high-pressure nitrogen from the twelfth nitrogen pipeline and output it to the fifteenth nitrogen pipeline;

[0024] The multi-stream heat exchanger is capable of cooling and liquefying the low-temperature and high-pressure nitrogen in the second flow path, so that liquid nitrogen can flow out of the outlet of the second flow path, and the outlet of the second flow path is connected to the gas-liquid separator through the seventh nitrogen pipeline and the eighth nitrogen pipeline in sequence;

[0025] The top gas outlet of the gas-liquid separator is connected to the multi-stream heat exchanger through a ninth nitrogen pipeline, and the bottom liquid outlet of the gas-liquid separator is connected to the first nitrogen pipeline and the eighteenth nitrogen pipeline through a seventeenth nitrogen pipeline.

[0026] In some embodiments, the nitrogen circulation cooling device further comprises a second compressor, the multi-stream heat exchanger is connected to the inlet of the second compressor through a third nitrogen pipeline, and the outlet of the second compressor is connected to the inlet of a fifth nitrogen pipeline through a fourth nitrogen pipeline; the second compressor is used to pressurize the normal temperature and low pressure nitrogen from the third nitrogen pipeline and output it to the fourth nitrogen pipeline;

[0027] The multi-stream heat exchanger is connected to the outlet of the reboiler through the twenty-first nitrogen pipeline. The nitrogen from the reboiler can enter the multi-stream heat exchanger through the twenty-first nitrogen pipeline and enter the third nitrogen pipeline after being reheated in the multi-stream heat exchanger.

[0028] In some embodiments, the multi-stream heat exchanger is connected to the tenth nitrogen pipeline, and is used to output normal temperature and low pressure nitrogen to the tenth nitrogen pipeline;

[0029] The tenth nitrogen pipeline is connected to the inlet of the first compressor via the fifth nitrogen pipeline; the first compressor is used to pressurize the normal temperature and low pressure nitrogen from the fifth nitrogen pipeline to form normal temperature and high pressure nitrogen, and output it to the multi-stream heat exchanger via the sixth nitrogen pipeline.

[0030] In some embodiments, the liquefied natural gas boil-off nitrogen stripping and helium capture system further comprises an eleventh nitrogen pipeline and a thirteenth nitrogen pipeline, the eleventh nitrogen pipeline being connected to the inlet of the thirteenth nitrogen pipeline, and the outlet of the thirteenth nitrogen pipeline being connected to the fifth nitrogen pipeline, for introducing room temperature and low pressure nitrogen from the boundary zone into the fifth nitrogen pipeline.

[0031] In some embodiments, the liquid phase outlet of the crude helium tower is connected to a fifth pipe, and the fifth pipe is connected to the inlet of the third tower top reflux cooler through a twenty-third nitrogen pipeline.

[0032] In some embodiments, the fifth tube is also connected to the multi-stream heat exchanger through a twenty-fourth nitrogen pipeline, and liquid nitrogen from the crude helium tower can enter the multi-stream heat exchanger from the twenty-fourth nitrogen pipeline to provide cooling for the normal temperature and high-pressure nitrogen from the sixth nitrogen pipeline.

[0033] In some embodiments, a third compressor is also included, the inlet of the third compressor is connected to the second pipe, the outlet of the third compressor is connected to the gas phase inlet of the distillation tower through a sixth pipe, and the third compressor is used to pressurize the gas phase material flowing out of the recovery tower.

[0034] In the above-mentioned liquefied natural gas boil-off nitrogen stripping and helium capture system, the high-nitrogen BOG gas produced in the storage tank can be cooled in the first tower top reflux cooler, so that the main components in the BOG gas are liquefied into LNG liquid and refluxed to the storage tank, thus avoiding the accumulation of nitrogen components in the LNG liquid, thereby reducing the risk of overpressure in the storage tank and improving the safety of liquefied natural gas carriers. The remaining nitrogen-containing tail gas in the BOG gas enters the distillation tower and the crude helium tower successively, and the obtained crude helium has a high helium concentration and a high helium recovery rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a schematic diagram of the composition of the liquefied natural gas boil-off nitrogen stripping and helium capture system of the present application.

[0036] Description of reference numerals:

[0037] Recovery tower C01; distillation tower C02; crude helium tower C03;

[0038] The first tower top reflux cooler E01; the second tower top reflux cooler E03; the reboiler E04; the third tower top reflux cooler E05;

[0039] The first tube S01; the second tube S02; the third tube S03; the fourth tube S04; the fifth tube S05; the sixth tube S06; the seventh tube S06; the eighth tube S08; the ninth tube S09;

[0040] Throttle valve JT01; first regulating valve FV01; second regulating valve LV02; third regulating valve LV03; first pressure regulating valve FV01; second pressure regulating valve FV02; ​​third pressure regulating valve FV02; ​​first tail gas pressure valve PV01; second tail gas pressure valve PV02; third tail gas pressure valve PV03;

[0041] Nitrogen circulation cooling device 1; multi-stream heat exchanger E02; first compressor K01; second compressor K02; gas-liquid separator V01; nitrogen expander Ep01;

[0042] a first nitrogen pipeline N01; a second nitrogen pipeline N02; a third nitrogen pipeline N03; a fourth nitrogen pipeline N04; a fifth nitrogen pipeline N05; a sixth nitrogen pipeline N06; a seventh nitrogen pipeline N07; an eighth nitrogen pipeline N08; a ninth nitrogen pipeline N09; a tenth nitrogen pipeline N10; an eleventh nitrogen pipeline N11; a twelfth nitrogen pipeline N12; a bypass pipeline N13; a fourteenth nitrogen pipeline N14; a fifteenth nitrogen pipeline N15; a sixteenth nitrogen pipeline N16; a seventeenth nitrogen pipeline N17; an eighteenth nitrogen pipeline N18; a nineteenth nitrogen pipeline N19; a twentieth nitrogen pipeline N20; a twenty-first nitrogen pipeline N21; a twenty-second nitrogen pipeline N22; a twenty-third nitrogen pipeline N23; a twenty-fourth nitrogen pipeline N24; and a twenty-fifth nitrogen pipeline N25. DETAILED DESCRIPTION

[0043] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present application, so the present application is not limited by the specific embodiments disclosed below.

[0044] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0045] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of this application, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0046] The "low temperature" and "high temperature" used in the embodiments of the present application are relative concepts, which indicate that the temperature of high-temperature gas is higher than that of low-temperature gas, and "normal temperature" is the temperature between low temperature and high temperature; similarly, "low pressure" and "high pressure" are relative concepts, and the pressure of high-pressure gas is higher than that of low-pressure gas. Therefore, although the "low temperature", "high temperature", "normal temperature", "low pressure" and "high pressure" in the embodiments of the present application are not limited to specific values, they are clear protection scopes when used to express relative concepts.

[0047] Please refer to Figure 1 An embodiment of the present application provides a liquefied natural gas boil-off nitrogen stripping and helium capture system, which can be used for the recovery and treatment of high-nitrogen BOG gas from liquefied natural gas shipboard storage tanks, including: a recovery tower C01, a distillation tower C02, a crude helium tower C03, and a nitrogen circulation cooling device 1.

[0048] The air inlet of the recovery tower C01 is connected to the storage tank through the first pipe S01. The first top reflux cooler E01 is provided in the recovery tower C01. The gas phase outlet at the top of the recovery tower C01 is connected to the gas phase inlet of the distillation tower (C02) through the second pipe S02. The liquid phase outlet at the bottom of the recovery tower C01 is connected to the storage tank through the third pipe S03.

[0049] A second top reflux cooler E03 is provided in the distillation tower C02, and a reboiler E04 is provided at the bottom. The gas phase outlet at the top of the distillation tower C02 is connected to the gas phase inlet of the crude helium tower C03.

[0050] A third tower top reflux cooler E05 is provided in the crude helium tower C03. The liquid phase outlet of the crude helium tower C03 is connected to the inlet of the third tower top reflux cooler E05 through a pipeline. Liquid nitrogen flowing out of the crude helium tower (C03) can flow to the third tower top reflux cooler E05 as a cold source. The gas at the top of the crude helium tower C03 can flow to the downstream refining unit for refining.

[0051] The nitrogen circulation cooling device 1 is used to provide liquid nitrogen as a refrigerant to the first tower top reflux cooler E01 and the second tower top reflux cooler E03 and recover the nitrogen generated by the vaporization of liquid nitrogen, and to provide nitrogen to the reboiler E04 and recover nitrogen. Thus, a closed cycle of the refrigerant is realized without the need to set up an additional low-temperature liquid nitrogen storage tank. It should be noted that the liquid nitrogen and nitrogen in the nitrogen circulation cooling device do not contact the BOG gas in the recovery tower C01, and only heat exchange is performed.

[0052] In the present application, BOG gas from the storage tank can enter the recovery tower C01 from the first pipe S01. The BOG gas flows upward in the recovery tower C01 and contacts the low-temperature liquid coming down from the first tower top reflux cooler E01 in reverse. Mass and heat transfer are carried out in the packed tower by relying on the temperature gradient and component difference. After the heat and mass are exchanged, the LNG liquid coming out of the bottom of the tower returns to the storage tank through the third pipe S03. The remaining nitrogen-containing tail gas in the BOG gas enters the distillation tower C02 through the second pipe S02 to further separate the LNG liquid.

[0053] In the above-mentioned liquefied natural gas boil-off nitrogen stripping and helium capture system, the high-nitrogen BOG gas produced in the storage tank can be cooled in the first tower top reflux cooler E01, so that the main components in the BOG gas are liquefied into LNG liquid and refluxed to the storage tank. In this way, the accumulation of nitrogen components in the LNG liquid is avoided, thereby reducing the risk of overpressure in the storage tank and improving the safety of the liquefied natural gas carrier.

[0054] In addition, the remaining nitrogen-containing tail gas in the BOG gas enters the distillation tower C02 and the crude helium tower C03 successively. The obtained crude helium has a high helium concentration and a high helium recovery rate.

[0055] In addition, in the nitrogen circulation cooling device 1, nitrogen is the only component of the refrigerant, and there is no need to configure a mixed refrigerant, multiple refrigerant storage tanks and a matching dehydration system. The refrigeration system is a closed cycle, and there is no need to configure a low-temperature liquid nitrogen storage tank.

[0056] Please refer to Figure 1 The nitrogen circulation cooling device 1 includes a multi-stream heat exchanger E02. The multi-stream heat exchanger E02 is connected to the inlet of the first tower top reflux cooler E01 through the first nitrogen pipeline N01, and the liquid nitrogen from the multi-stream heat exchanger E02 can enter the first tower top reflux cooler E01 from the first nitrogen pipeline N01. The multi-stream heat exchanger E02 is connected to the outlet of the first tower top reflux cooler E01 through the second nitrogen pipeline N02, and the nitrogen from the first tower top reflux cooler E01 can enter the multi-stream heat exchanger E02 from the second nitrogen pipeline N02.

[0057] The multi-stream heat exchanger E02 is connected to the inlet of the second tower top reflux cooler E03 through the eighteenth nitrogen pipeline N18, and the liquid nitrogen from the multi-stream heat exchanger E02 can enter the second tower top reflux cooler E03 from the eighteenth nitrogen pipeline N18. The multi-stream heat exchanger E02 is connected to the outlet of the second tower top reflux cooler E03 through the nineteenth nitrogen pipeline N19, and the nitrogen from the second tower top reflux cooler E03 can enter the multi-stream heat exchanger E02 from the nineteenth nitrogen pipeline N19.

[0058] The multi-stream heat exchanger E02 is connected to the inlet of the reboiler E04 through the twentieth nitrogen pipeline N20. The nitrogen from the first tower top reflux cooler E01 and the nitrogen from the second tower top reflux cooler E03 enter the multi-stream heat exchanger E02 for reheating and then enter the twentieth nitrogen pipeline N20.

[0059] The liquid nitrogen from the multi-stream heat exchanger E02 provides refrigerant for the first tower top reflux cooler E01 and the second tower top reflux cooler E03. The first tower top reflux cooler E01 and the second tower top reflux cooler E03 occupy nitrogen channels, and the liquid nitrogen circulates in the nitrogen channels to transfer the cold to the corresponding gas phase substances and then converts into nitrogen and flows back to the multi-stream heat exchanger E02. The multi-stream heat exchanger E02 reheats the returned nitrogen and then flows to the reboiler E04 as the heat source of the reboiler E04, so that the nitrogen is recycled.

[0060] Please refer to Figure 1 In one embodiment, the liquid phase outlet at the bottom of the distillation tower C02 is connected to the multi-stream heat exchanger E02 through a fourth pipe S04, and the liquefied natural gas from the distillation tower C02 enters the storage tank after being supercooled by the multi-stream heat exchanger E02, thereby reducing flash evaporation losses.

[0061] Please refer to Figure 1 In one embodiment, the nitrogen circulation cooling device further includes: a first compressor K01, a nitrogen expander Ep01 and a gas-liquid separator V01.

[0062] The multi-stream heat exchanger E02 can receive multiple fluids at the same time to achieve heat exchange between multiple fluids. The flow path inside the multi-stream heat exchanger E02 can be designed as needed. (The principle of the multi-stream heat exchanger itself is an existing technology and will not be repeated here.)

[0063] The outlet of the first compressor K01 is connected to the multi-stream heat exchanger E02 through the sixth nitrogen pipeline N06. The first compressor K01 is used to introduce low-pressure nitrogen at room temperature and pressurize the low-pressure nitrogen at room temperature to obtain high-pressure nitrogen at room temperature. The high-pressure nitrogen at room temperature enters the multi-stream heat exchanger E02 through the sixth nitrogen pipeline N06.

[0064] The high-pressure nitrogen in the multi-stream heat exchanger E02 has a first flow path and a second flow path. The multi-stream heat exchanger E02 cools the normal-temperature high-pressure nitrogen from the sixth nitrogen pipeline N06 to obtain low-temperature high-pressure nitrogen. The low-temperature high-pressure nitrogen is divided into two paths, one part flows into the first flow path, and the other part flows into the second flow path.

[0065] The outlet of the first flow path is connected to the inlet of the nitrogen expander Ep01 through the twelfth nitrogen pipeline N12, and the outlet of the nitrogen expander Ep01 is connected to the gas-liquid separator V01 through the fifteenth nitrogen pipeline N15. After the low-temperature and high-pressure nitrogen enters the twelfth nitrogen pipeline N12 from the outlet of the first flow path, the nitrogen expander Ep01 cools down and reduces the pressure of the low-temperature and high-pressure nitrogen from the twelfth nitrogen pipeline N12, and outputs it to the fifteenth nitrogen pipeline N15, and then enters the gas-liquid separator V01 through the fifteenth nitrogen pipeline N15. The nitrogen after cooling and reducing the pressure by the nitrogen expander Ep01 can be used as a cold source for the nitrogen circulation cooling device.

[0066] The outlet of the second flow path is connected to the gas-liquid separator V01 through the seventh nitrogen pipeline N07 and the eighth nitrogen pipeline N08 in turn. When the low-temperature and high-pressure nitrogen flows in the second flow path, the multi-stream heat exchanger E02 can cool and liquefy the low-temperature and high-pressure nitrogen in the second flow path, so that liquid nitrogen can flow out of the outlet of the second flow path. The liquid nitrogen flowing out of the outlet of the second flow path enters the gas-liquid separator V01 through the seventh nitrogen pipeline N07 and the eighth nitrogen pipeline N08 in turn.

[0067] The top gas outlet of the gas-liquid separator V01 is connected to the multi-stream heat exchanger E02 through the ninth nitrogen pipeline N09, and the bottom liquid outlet of the gas-liquid separator V01 is connected to the first nitrogen pipeline N01 and the eighteenth nitrogen pipeline N18 through the seventeenth nitrogen pipeline N17, and then connected to the inlet of the first tower top reflux cooler E01 through the first nitrogen pipeline N01, and connected to the second tower top reflux cooler E02 through the eighteenth nitrogen pipeline N18.

[0068] After gas-liquid separation in the gas-liquid separator V01, the nitrogen cooled and pressurized by the nitrogen expander Ep01 can enter the multi-stream heat exchanger E02 through the ninth nitrogen pipeline N09 to provide cooling capacity; while liquid nitrogen flows from the bottom liquid outlet of the gas-liquid separator V01 through the first nitrogen pipeline N01 and the eighteenth nitrogen pipeline N18 into the first tower top reflux cooler E01 and the second tower top reflux cooler E02 respectively.

[0069] It is worth noting that when the nitrogen after being cooled and reduced in pressure by the nitrogen expander Ep01 enters the gas-liquid separator V01, it can meet the liquid nitrogen flowing out from the outlet of the second flow path in the gas-liquid separator V01, thereby further cooling the liquid nitrogen, so that the liquid nitrogen has a sufficiently low temperature when entering the first tower top reflux cooler E01 and the second tower top reflux cooler E02.

[0070] In the present application, the liquid nitrogen flowing out of the second flow path is used as a cold source, and the pressure range is 0.2MPag~0.4MPag. No low-temperature liquid nitrogen pump is required to provide pressure. The liquid nitrogen comes from the refrigeration cycle. The first compressor K01 provides sufficient pressure, and the nitrogen expander Ep01 provides the cold capacity to convert nitrogen into liquid nitrogen.

[0071] In addition, as nitrogen is the only component of the refrigerant, there is no need to configure a mixed refrigerant, multiple refrigerant storage tanks and a matching dehydration system. The refrigeration system is a closed cycle, and it is preferred not to configure a low-temperature liquid nitrogen storage tank.

[0072] The outlet fluid of the first compressor K01 is at room temperature, so it is considered to be equipped with a corresponding aftercooler and cooled by circulating water. The high-pressure end of the first compressor K01 is considered to be coaxially connected with the nitrogen expander Ep01, and the work output by the nitrogen expander Ep01 is used to drive the first compressor K01.

[0073] Please refer to Figure 1 In one embodiment, the nitrogen circulation cooling device includes a second compressor K02, a multi-stream heat exchanger E02 is connected to the inlet of the second compressor K02 through the third nitrogen pipeline N03, and the second compressor K02 is connected to the inlet of the fifth nitrogen pipeline N05 through the fourth nitrogen pipeline N04. The second compressor K02 is used to pressurize the low-pressure nitrogen at room temperature from the third nitrogen pipeline N03 and output it to the fourth nitrogen pipeline N04. The multi-stream heat exchanger E02 is connected to the outlet of the reboiler E04 through the twenty-first nitrogen pipeline N21, and the nitrogen from the reboiler E04 can enter the multi-stream heat exchanger E02 through the twenty-first nitrogen pipeline N21. The nitrogen from the reboiler E04 enters the multi-stream heat exchanger E02 through the twenty-first nitrogen pipeline N21, and then enters the third nitrogen pipeline (N03) after reheating; usually reheating to -110°C.

[0074] The second compressor K02 is used to pressurize the room temperature and low pressure nitrogen from the third nitrogen pipeline N03 and output it to the fourth nitrogen pipeline N04, and then enter the first compressor K01 from the fourth nitrogen pipeline N04 through the fifth nitrogen pipeline N05 for pressurization. Therefore, this part of the room temperature and low pressure nitrogen is pressurized and enters the multi-stream heat exchanger E02, which is beneficial to reduce the cooling amount required to convert this part of the room temperature and low pressure nitrogen into liquid nitrogen.

[0075] Please refer to Figure 1 In one embodiment, the multi-stream heat exchanger E02 is connected to the tenth nitrogen pipeline N10, and is used to output low-pressure nitrogen at room temperature to the tenth nitrogen pipeline N10.

[0076] The multi-stream heat exchanger E02 uses the cooling capacity provided by the nitrogen after cooling and pressure reduction from the nitrogen expander Ep01 to cool the normal temperature and high pressure nitrogen from the sixth nitrogen pipeline N06 to form liquid nitrogen. After releasing the cooling capacity, the nitrogen will heat up, so that the heated normal temperature and low pressure nitrogen can be output to the tenth nitrogen pipeline N10.

[0077] The tenth nitrogen pipeline N10 is connected to the inlet of the first compressor K01 via the fifth nitrogen pipeline N05, so that the normal temperature low pressure nitrogen from the tenth nitrogen pipeline N10 can be pressurized by the first compressor K01 to form normal temperature high pressure nitrogen, enter the sixth nitrogen pipeline N06, and then enter the multi-stream heat exchanger E02. In this way, the normal temperature low pressure nitrogen output from the multi-stream heat exchanger E02 can be returned to the multi-stream heat exchanger E02 for recycling.

[0078] The multi-stream heat exchanger E02 uses the cooling capacity provided by the nitrogen after the temperature and pressure reduction from the nitrogen expander Ep01 to cool the normal temperature and high pressure nitrogen from the sixth nitrogen pipeline N06 to form liquid nitrogen. After releasing the cooling capacity, the nitrogen will heat up. Some of the heated normal temperature and low pressure nitrogen can be output to the tenth nitrogen pipeline N10; and some of the normal temperature and low pressure nitrogen can be output to the third nitrogen pipeline N03.

[0079] Please refer to Figure 1 In one embodiment, the liquefied natural gas boil-off nitrogen stripping and helium capture system further includes an eleventh nitrogen pipeline N11 and a thirteenth nitrogen pipeline N13. The eleventh nitrogen pipeline N11 is connected to the inlet of the thirteenth nitrogen pipeline N13, and the outlet of the thirteenth nitrogen pipeline (N13) is connected to the fifth nitrogen pipeline (N05), which is used to introduce low-pressure nitrogen at room temperature from the boundary area into the fifth nitrogen pipeline N05. In this way, the nitrogen loss of the system can be supplemented to the nitrogen circulation cooling device through the eleventh nitrogen pipeline N11. Among them, the pressure of the nitrogen pipeline entering the eleventh nitrogen pipeline N11 from the boundary area is about 0.3~0.5MPag, and the temperature is normal temperature.

[0080] Please refer to Figure 1 In one embodiment, the liquid phase outlet of the crude helium tower C03 is connected to a fifth pipe S05, which is connected to the inlet of the third tower top reflux cooler E05 through the sixteenth nitrogen pipeline N16. The fifth pipe S05 is also connected to the multi-stream heat exchanger E02 through the twenty-fourth nitrogen pipeline N24, and the liquid nitrogen from the crude helium tower C03 can enter the multi-stream heat exchanger E02 from the twenty-fourth nitrogen pipeline N24 to provide cold for the normal temperature and high pressure nitrogen from the sixth nitrogen pipeline N06. Therefore, part of the liquid nitrogen at the bottom of the crude helium tower C03 provides cold for the third tower top reflux cooler E05, and the other part provides cold for the multi-stream heat exchanger E02.

[0081] The outlet of the third tower top reflux cooler E05 is connected to the multi-stream heat exchanger E02 through the twenty-second nitrogen pipeline N22. The nitrogen gas after liquid nitrogen vaporization from the third tower top reflux cooler E05 enters the multi-stream heat exchanger E02 for reheating, and then enters the third nitrogen pipeline N03 for recycling.

[0082] The liquid nitrogen from the crude helium column C03 can be discharged through the twenty-fifth nitrogen pipeline N25 after being vaporized in the multi-stream heat exchanger E02.

[0083] Please refer to Figure 1 In one embodiment, a third compressor K03 is further included. The inlet of the third compressor K03 is connected to the second pipe S02, and the outlet of the third compressor K03 is connected to the gas phase inlet of the distillation tower C02 through the sixth pipe S06. The third compressor K03 is used to pressurize the gas phase material flowing out of the recovery tower (C01).

[0084] Please refer to Figure 1 In one embodiment, the third pipe S03 is provided with a first regulating valve FV01 to regulate the pressure of LNG when it enters the storage tank. The fourth pipe S04 is provided with a second regulating valve LV02 to regulate the bottom liquid level of the second distillation tower (C02). The fourth pipe S05 is provided with a third regulating valve LV03 to regulate the bottom liquid level of the third crude argon tower (C03).

[0085] The seventh nitrogen pipeline N07 is provided with a throttle valve JT01, through which the liquid nitrogen can be depressurized and then output to the gas-liquid separator V01 through the eighth nitrogen pipeline N08.

[0086] The first nitrogen pipeline N01 is provided with a first pressure regulating valve FV01, which is used to adjust the flow rate of liquid nitrogen when entering the first tower top reflux cooler E01. The twenty-third nitrogen pipeline N23 is provided with a second pressure regulating valve FV02, which is used to adjust the flow rate of nitrogen entering the third tower top reflux cooler E05. The eighteenth nitrogen pipeline N18 is provided with a third pressure regulating valve FV03, which is used to adjust the flow rate of liquid nitrogen entering the second tower top reflux cooler E03. The twelfth nitrogen pipeline N12 is provided with a first tail gas pressure valve PV01, which is used to adjust the pressure of nitrogen entering the expander (Ep01). The second pipe S02 is provided with a second tail gas pressure valve PV02, which is used to adjust the pressure of nitrogen-containing tail gas entering the third compressor (K03). The sixth pipe S06 is provided with a third tail gas pressure valve PV03, which is used to stabilize the pressure of the crude helium output.

[0087] In this application, unless otherwise specified, the percentage of gas refers to the volume fraction.

[0088] In this application, the high nitrogen content BOG gas from the storage tank refers to BOG with a nitrogen content of 3% to 16%, a helium content of 0.2% to 24%, a temperature of -150°C to -155°C, and a pressure of 110KPa to 120KPa. The methane content of the liquid after the BOG is recovered and liquefied is about 99.3%, and the recovery rate is as high as 99.9%.

[0089] When BOG enters the recovery tower C01, it is at a slightly positive pressure, with a pressure range of 110~120KPa. The pressure drop through the recovery system is less than 5KPa. The gas pressure out of the top of the recovery tower C01 ranges from 105~115KPa, and the pressure is maintained at a slightly positive pressure. It is preferred that no BOG compressor is installed at the inlet of the recovery tower C01.

[0090] After BOG is recovered and liquefied, the LNG concentration is controlled at 99.3V%, and the temperature is controlled at -162℃~-163℃, which is higher than the design temperature of -165℃, and can be directly returned to the storage tank.

[0091] The gas at the top of the recovery tower C01 is pressurized by the third compressor K03; the pressure range after pressurization is 0.6~0.8MPa, and this part of the compressed gas is about 25% of the total BOG gas volume.

[0092] The gas pipeline S02 coming out of the first tower top reflux cooler E01 is compressed to 0.7MPa by the third compressor K03, and the temperature is about -75℃.

[0093] The N2 concentration of the gas discharged from the top of the distillation tower C02 is about 98%, and the rest is helium. The temperature is about -173°C and enters the downstream crude helium tower C03 through the seventh pipe S07.

[0094] In this application, by setting up a distillation tower C02 and a crude helium tower C03, the helium concentration in the crude helium is high, the recovery rate reaches 98~99.5%, and the helium concentration range is 78~82%.

[0095] In one embodiment,

[0096] The BOG evaporated from the top of the storage tank has a pressure of 115KPa and enters the first pipeline S01 with a pressure of 113KPa and a temperature of -153℃. The components are methane CH4: 85.8%, N2: 14.0%, He: 0.2%. It enters the bottom of the recovery tower C01. The gas flows upward and contacts with the low-temperature liquid coming down from the first tower top reflux cooler E01 in reverse. Mass and heat transfer are carried out in the packed tower by relying on the temperature gradient, and heat and mass are exchanged.

[0097] The third pipe S03 coming out of the bottom of the recovery tower C01 contains 99.3V% methane with a temperature of -163.2°C. This part of methane accounts for about 75% of the total BOG and is returned to the storage tank as liquefied natural gas LNG.

[0098] The gas coming out of the first tower top reflux cooler E01 is about 25% of the total BOG amount, with a nitrogen content of 55V%, a He content of 1V%, and the rest being methane. The temperature is about -172°C. After being compressed to 0.7MPa by the third compressor K03, the temperature is -75°C and enters the distillation tower C02.

[0099] The gas discharged from the top of the distillation tower C02 does not contain methane, and the nitrogen content is 98V%, the He content is 2V%, and the temperature is about -174.5℃. The liquid discharged from the bottom of the distillation tower C02 is methane, and the temperature is about -133℃. It is supercooled to about -160℃ through the multi-stream heat exchanger E02 and returned to the storage tank.

[0100] The tail gas from the top of the distillation tower C02 enters the crude helium tower C03. The temperature of the gas from the top of the crude helium tower C03 is about -194℃, and the nitrogen content is 18~20%. The helium content is 80~82V%, and the liquid from the bottom of the crude helium tower C03 is high-purity liquid nitrogen with a temperature of about -175℃.

[0101] In this application, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0102] In the present application, unless otherwise clearly specified and limited, a first feature being “above” or “below” a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being “above”, “above”, and “above” a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being “below”, “below”, and “below” a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0103] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it may be directly on the other element or there may be a central element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be a central element at the same time. The terms "vertical", "horizontal", "upper", "lower", "left", "right" and similar expressions used herein are for illustrative purposes only and are not intended to be the only implementation method.

[0104] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0105] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent application shall be subject to the attached claims.

Claims

1. A liquefied natural gas boil-off nitrogen stripping and helium capture system, characterized in that: include: Recovery tower (C01), distillation tower (C02), crude helium tower (C03), nitrogen circulation cooling device (1); The air inlet of the recovery tower (C01) is connected to the storage tank through a first pipe (S01), a first top reflux cooler (E01) is provided in the recovery tower (C01), the gas phase outlet at the top of the recovery tower (C01) is connected to the gas phase inlet of the distillation tower (C02) through a second pipe (S02), and the liquid phase outlet at the bottom of the recovery tower (C01) is connected to the storage tank through a third pipe (S03); The distillation tower (C02) is provided with a second top reflux cooler (E03), and a reboiler (E04) is provided at the bottom, and the gas phase outlet at the top of the distillation tower (C02) is connected to the gas phase inlet of the crude helium tower (C03); A third tower top reflux cooler (E05) is provided in the crude helium tower (C03), a liquid phase outlet of the crude helium tower (C03) is connected to an inlet of the third tower top reflux cooler (E05) through a pipeline, and liquid nitrogen flowing out of the crude helium tower (C03) can flow to the third tower top reflux cooler (E05) as a cold source; The nitrogen circulation cooling device (1) is used to provide liquid nitrogen as a refrigerant to the first tower top reflux cooler (E01) and the second tower top reflux cooler (E03) and to recover nitrogen generated by vaporization of the liquid nitrogen, and to provide nitrogen to the reboiler (E04) and to recover the nitrogen.

2. The liquefied natural gas boil-off nitrogen and helium capture system according to claim 1, characterized in that: The nitrogen circulation cooling device (1) comprises a multi-stream heat exchanger (E02); The multi-stream heat exchanger (E02) is connected to the inlet of the first tower top reflux cooler (E01) through a first nitrogen pipeline (N01), and liquid nitrogen from the multi-stream heat exchanger (E02) can enter the first tower top reflux cooler (E01) from the first nitrogen pipeline (N01); the multi-stream heat exchanger (E02) is connected to the outlet of the first tower top reflux cooler (E01) through a second nitrogen pipeline (N02), and nitrogen from the first tower top reflux cooler (E01) can enter the multi-stream heat exchanger (E02) from the second nitrogen pipeline (N02); The multi-stream heat exchanger (E02) is connected to the inlet of the second tower top reflux cooler (E03) through an eighteenth nitrogen pipeline (N18), and the liquid nitrogen from the multi-stream heat exchanger (E02) can enter the second tower top reflux cooler (E03) from the eighteenth nitrogen pipeline (N18); the multi-stream heat exchanger (E02) is connected to the outlet of the second tower top reflux cooler (E03) through a nineteenth nitrogen pipeline (N19), and the nitrogen from the second tower top reflux cooler (E03) can enter the multi-stream heat exchanger (E02) from the nineteenth nitrogen pipeline (N19); The multi-stream heat exchanger (E02) is connected to the inlet of the reboiler (E04) through the twentieth nitrogen pipeline (N20); The nitrogen from the first tower top reflux cooler (E01) and the nitrogen from the second tower top reflux cooler (E03) enter the multi-stream heat exchanger (E02) for reheating and then enter the second nitrogen pipeline (N20).

3. The liquefied natural gas boil-off nitrogen and helium capture system according to claim 2, characterized in that: The liquid phase outlet at the bottom of the distillation tower (C02) is connected to the multi-stream heat exchanger (E02) through a fourth pipe (S04), and the liquefied natural gas from the distillation tower (C02) enters the storage tank after being supercooled by the multi-stream heat exchanger (E02).

4. The liquefied natural gas boil-off nitrogen and helium capture system according to claim 2, characterized in that: The nitrogen circulation cooling device further comprises: a first compressor (K01), a nitrogen expander (Ep01) and a gas-liquid separator (V01); The outlet of the first compressor (K01) is connected to the multi-stream heat exchanger (E02) through a sixth nitrogen pipeline (N06), and the first compressor (K01) is used to introduce low-pressure nitrogen at room temperature and pressurize the low-pressure nitrogen at room temperature, and output high-pressure nitrogen at room temperature; The high-pressure nitrogen in the multi-stream heat exchanger (E02) has a first flow path and a second flow path, and the multi-stream heat exchanger (E02) cools the normal-temperature high-pressure nitrogen from the sixth nitrogen pipeline (N06) to obtain low-temperature high-pressure nitrogen in two paths, one part of which flows into the first flow path, and the other part of which flows into the second flow path; The outlet of the first flow path is connected to the inlet of the nitrogen expander (Ep01) through the twelfth nitrogen pipeline (N12), and the outlet of the nitrogen expander (Ep01) is connected to the gas-liquid separator (V01) through the fifteenth nitrogen pipeline (N15); the nitrogen expander (Ep01) is used to reduce the temperature and pressure of the low-temperature and high-pressure nitrogen from the twelfth nitrogen pipeline (N12) and output it to the fifteenth nitrogen pipeline (N15); The multi-stream heat exchanger (E02) is capable of cooling and liquefying the low-temperature and high-pressure nitrogen in the second flow path so that liquid nitrogen can flow out of the outlet of the second flow path, and the outlet of the second flow path is connected to the gas-liquid separator (V01) through the seventh nitrogen pipeline (N07) and the eighth nitrogen pipeline (N08) in sequence; The top gas outlet of the gas-liquid separator (V01) is connected to the multi-stream heat exchanger (E02) through a ninth nitrogen pipeline (N09), and the bottom liquid outlet of the gas-liquid separator (V01) is connected to the first nitrogen pipeline (N01) and the eighteenth nitrogen pipeline (N18) through a seventeenth nitrogen pipeline (N17).

5. The liquefied natural gas boil-off nitrogen and helium capture system according to claim 4, characterized in that: The nitrogen circulation cooling device further comprises a second compressor (K02); the multi-stream heat exchanger (E02) is connected to the inlet of the second compressor (K02) via a third nitrogen pipeline (N03); the outlet of the second compressor (K02) is connected to the inlet of a fifth nitrogen pipeline (N05) via a fourth nitrogen pipeline (N04); the second compressor (K02) is used to pressurize the normal temperature and low pressure nitrogen from the third nitrogen pipeline (N03) and output it to the fourth nitrogen pipeline (N04); The multi-stream heat exchanger (E02) is connected to the outlet of the reboiler (E04) through the twenty-first nitrogen pipeline (N21), and the nitrogen from the reboiler (E04) can enter the multi-stream heat exchanger (E02) through the twenty-first nitrogen pipeline (N21), and enter the third nitrogen pipeline (N03) after being reheated in the multi-stream heat exchanger (E02).

6. The liquefied natural gas boil-off stripping nitrogen and helium capture system according to claim 3, characterized in that: The multi-stream heat exchanger (E02) is connected to the tenth nitrogen pipeline (N10) and is used to output low-pressure nitrogen at normal temperature to the tenth nitrogen pipeline (N10); The tenth nitrogen pipeline (N10) is connected to the inlet of the first compressor (K01) via the fifth nitrogen pipeline (N05); the first compressor (K01) is used to pressurize the room temperature low pressure nitrogen from the fifth nitrogen pipeline (N05) to form room temperature high pressure nitrogen, and output it to the multi-stream heat exchanger (E02) via the sixth nitrogen pipeline (N06).

7. The liquefied natural gas boil-off stripping nitrogen and helium capture system according to claim 6, characterized in that: The liquefied natural gas boil-off nitrogen stripping and helium capture system further comprises an eleventh nitrogen pipeline (N11) and a thirteenth nitrogen pipeline (N13), wherein the eleventh nitrogen pipeline (N11) is connected to the inlet of the thirteenth nitrogen pipeline (N13), and the outlet of the thirteenth nitrogen pipeline (N13) is connected to the fifth nitrogen pipeline (N05), so as to introduce normal temperature and low pressure nitrogen from the boundary zone into the fifth nitrogen pipeline (N05).

8. The liquefied natural gas boil-off nitrogen and helium capture system according to claim 4, characterized in that: The liquid phase outlet of the crude helium tower (C03) is connected to a fifth pipe (S05), and the fifth pipe (S05) is connected to the inlet of the third tower top reflux cooler (E05) through a twenty-third nitrogen pipeline (N23).

9. The liquefied natural gas boil-off stripping nitrogen and helium capture system according to claim 8, characterized in that: The fifth pipe (S05) is also connected to the multi-stream heat exchanger (E02) through the twenty-fourth nitrogen pipeline (N24), and the liquid nitrogen from the crude helium tower (C03) can enter the multi-stream heat exchanger (E02) from the twenty-fourth nitrogen pipeline (N24) to provide cooling capacity for the normal temperature and high pressure nitrogen from the sixth nitrogen pipeline (N06).

10. The liquefied natural gas boil-off stripping nitrogen and helium capture system according to claim 1, characterized in that: It also includes a third compressor (K03), the inlet of the third compressor (K03) is connected to the second pipe (S02), the outlet of the third compressor (K03) is connected to the gas phase inlet of the distillation tower (C02) through a sixth pipe (S06), and the third compressor (K03) is used to pressurize the gas phase material flowing out of the recovery tower (C01).

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