An apparatus and method for helium extraction and light hydrocarbon recovery from dual-cycle refrigerated LNG

Abstract

This invention relates to a device and method for dual-cycle refrigeration LNG helium extraction and light hydrocarbon recovery, belonging to the field of cryogenic liquefaction separation and natural gas helium extraction technology. The device integrates a mixed refrigerant refrigeration cycle and a nitrogen refrigeration cycle, and sequentially includes a feed gas pretreatment unit, a light hydrocarbon recovery unit, a denitrification liquefaction unit, and a helium extraction unit. The mixed refrigerant cycle provides cooling capacity for feed gas cooling liquefaction, light hydrocarbon recovery, and natural gas denitrification processes, while the nitrogen cycle provides deep cooling capacity for extracting crude helium from nitrogen-rich gas. In terms of method, the purified feed gas sequentially undergoes pretreatment, light hydrocarbon recovery, denitrification, and helium extraction steps to simultaneously produce LNG, crude helium, LPG, and stabilized light hydrocarbons. This invention has advantages such as high process energy efficiency, high helium and propane recovery rates, high purity of co-produced products, and reliable operation, realizing the high-value comprehensive utilization of natural gas resources.

Description

Technical Field

[0001] This invention relates to the fields of cryogenic liquefaction separation and natural gas helium extraction technology, and particularly to an apparatus and method for helium extraction and light hydrocarbon recovery from LNG using a dual-cycle refrigeration system. Background Technology

[0002] Helium is an important rare gas that plays an indispensable role in aerospace, semiconductor, medical and military fields. Most helium is extracted from oil and natural gas, but natural gas has a low helium content. Helium can be extracted from nitrogen-rich gas obtained by removing nitrogen during the production of LNG products, which greatly improves the helium recovery rate and is more economical.

[0003] In the prior art, such as the integrated device for co-production of light hydrocarbon recovery and LNG natural gas helium extraction disclosed in Chinese Patent Publication No. CN117053497A (publication date November 14, 2023), a nitrogen expansion cycle refrigeration process is used, which has a low helium recovery rate, a low propane recovery rate in LPG products, and high power consumption, resulting in low economic benefits.

[0004] For example, the Chinese Patent Publication No. CN112179048A (publication date January 5, 2021) discloses a co-production system and method for recovering and extracting helium from light hydrocarbons in helium-poor natural gas. It uses LNG flash vapor as a refrigerant in a compression cycle refrigeration process, which results in a low helium recovery rate and low helium purity in the crude helium gas, making it unsuitable for extracting helium from helium-rich natural gas.

[0005] This invention provides an apparatus and method for dual-cycle refrigeration natural gas helium extraction and light hydrocarbon recovery. It adopts a mixed refrigerant and nitrogen refrigeration cycle system to produce LNG and co-produce crude helium, liquid nitrogen, LPG and stable light hydrocarbons. This not only reduces resource waste and environmental pollution, but also maximizes the added value of the products. Summary of the Invention

[0006] The purpose of this invention is to provide a device and method for helium extraction and light hydrocarbon recovery from dual-cycle refrigerated natural gas, which is simple in process, flexible in adjustment, reliable in operation, easy to operate, and low in energy consumption. It aims to achieve the deep and comprehensive utilization of natural gas resources, and while efficiently producing LNG, co-produce high-purity crude helium, liquid nitrogen, LPG and stable light hydrocarbons, so as to maximize the economic value of the feedstock gas.

[0007] This invention is achieved using the following technical solution: a dual-cycle refrigeration LNG helium extraction and light hydrocarbon recovery device, comprising a feed gas supply unit for providing feed gas to be processed; a light hydrocarbon separation unit connected to the feed gas supply unit for separating liquefied petroleum gas and stabilized light hydrocarbons from the feed gas; a liquefied natural gas production unit connected to the feed gas supply unit and the light hydrocarbon separation unit for separating liquefied natural gas from the feed gas; a helium extraction unit connected to the liquefied natural gas production unit for extracting crude helium from a nitrogen-rich stream; a mixed refrigerant refrigeration unit for providing cooling capacity to the feed gas supply unit, the light hydrocarbon separation unit, the liquefied natural gas production unit, and the helium extraction unit; and a nitrogen refrigeration unit for providing cooling capacity to the helium extraction unit.

[0008] Furthermore, the light hydrocarbon separation unit includes an ethane removal tower and a liquefied petroleum gas (LPG) tower connected in sequence; the LPG production unit includes a nitrogen removal tower; the helium extraction unit includes a helium extraction tower; the feed gas supply unit includes a main heat exchanger, a feed gas cryogenic separator, a DHX tower, a cryogenic liquid booster pump, and a mixed hydrocarbon booster pump; the main heat exchanger of the feed gas supply unit is provided with a feed gas channel connected to the cryogenic feed gas separator; the liquid phase outlet of the cryogenic feed gas separator is connected to the feed inlet of the ethane removal tower via the cryogenic liquid booster pump and the main heat exchanger, and its gas phase outlet is connected to the feed inlet of the DHX tower; the top gas phase outlet of the ethane removal tower... The main heat exchanger connects to the top inlet of the DHX tower, and its bottom liquid outlet connects to the inlet of the liquefied petroleum gas tower. The bottom liquid outlet of the DHX tower is connected to the inlet of the deethaner via the mixed hydrocarbon booster pump. Its top gas outlet is divided into two paths: one connects to the bottom inlet of the denitrification tower, and the other connects to the top inlet of the denitrification tower via the main heat exchanger. The top gas outlet of the denitrification tower is connected to the inlet of the helium extraction tower, and its bottom liquid outlet is connected to a liquefied natural gas output pipeline. The top gas outlet of the helium extraction tower is connected to a crude helium output pipeline, and its bottom liquid outlet is connected to a liquid nitrogen output pipeline.

[0009] Furthermore, the DHX tower has a regulating valve a on its top feed line; the deethaner tower has a regulating valve b on its bottom reboiler steam line; the deethaner tower has a regulating valve c on its bottom liquid phase outlet line; the denitrification tower has a regulating valve h on its top feed line; the denitrification tower has a regulating valve i on its bottom feed line; the deliquescent natural gas output line connected to the bottom liquid phase outlet of the denitrification tower has a regulating valve j; and the helium extraction tower has a regulating valve l on its bottom liquid phase outlet line.

[0010] Furthermore, the mixed refrigerant refrigeration unit includes a mixed refrigerant compressor system, a refrigerant separator, and multiple refrigerant channels disposed in the main heat exchanger; the outlet of the mixed refrigerant compressor system is connected to the inlet of the refrigerant channel of the main heat exchanger to provide high-pressure refrigerant; the inlet of the refrigerant separator is connected to the outlet of the high-pressure gaseous refrigerant channel of the main heat exchanger; the gaseous outlet of the refrigerant separator is connected to the reflux refrigerant channel via the high-pressure gaseous refrigerant channel II of the main heat exchanger; the liquid outlet of the refrigerant separator is connected to the refrigerant inlet of the bottom reboiler of the helium extraction tower via the high-pressure liquid refrigerant channel II of the main heat exchanger; the refrigerant outlet of the bottom reboiler of the helium extraction tower is connected to the reflux refrigerant channel; and the outlet of the reflux refrigerant channel is connected to the inlet of the mixed refrigerant compressor system.

[0011] Furthermore, the interstage liquid phase outlet of the mixed refrigerant compressor system is connected to the reflux refrigerant channel via the medium-pressure liquid phase refrigerant channel of the main heat exchanger, and a regulating valve p is provided on the connecting pipeline; the final stage liquid phase outlet of the mixed refrigerant compressor system is connected to the reflux refrigerant channel via the high-pressure liquid phase refrigerant channel I of the main heat exchanger, and a regulating valve o is provided on the connecting pipeline; a regulating valve n is provided on the outlet pipeline of the high-pressure liquid phase refrigerant channel II; a regulating valve r is provided on the pipeline connecting to the outlet of the high-pressure gas phase refrigerant channel II; and a bypass pipeline is provided on the bottom reboiler of the helium extraction tower, and a regulating valve q is provided on the bypass pipeline.

[0012] Furthermore, the nitrogen refrigeration unit includes a nitrogen compression system, a subcooler, a helium extraction tower top condenser, and nitrogen channels disposed in the main heat exchanger and the subcooler; the outlet of the nitrogen compression system is connected to the inlet of the medium-pressure liquid nitrogen channel of the subcooler via the medium-pressure nitrogen channel of the main heat exchanger; the outlet of the medium-pressure liquid nitrogen channel is divided into two paths, one path is connected to the inlet of the reflux liquid nitrogen channel of the helium extraction tower top condenser, and the other path is merged with the outlet of the reflux liquid nitrogen channel via a regulating valve S; the outlet of the reflux liquid nitrogen channel is connected to the inlet of the nitrogen compression system via the reflux liquid nitrogen channel of the subcooler and the reflux nitrogen channel of the main heat exchanger in sequence.

[0013] Furthermore, a regulating valve m is installed on the pipeline connecting to the inlet of the reflux liquid nitrogen channel.

[0014] Furthermore, the top of the liquefied petroleum gas (LPG) tower is equipped with an LPG tower top condenser and an LPG tower top reflux tank; the gas phase outlet of the LPG tower top reflux tank is connected to a fuel gas pipeline, and a regulating valve d is installed on the pipeline; the liquid phase outlet of the LPG tower top reflux tank is divided into two paths by the LPG tower liquid reflux pump, one path is connected to the top reflux port of the LPG tower, and the other path is connected to the LPG product pipeline via an LPG product cooler, and a regulating valve e is installed on the product pipeline; the bottom of the LPG tower is equipped with an LPG tower bottom reboiler, and a regulating valve f is installed on its steam pipeline; the bottom liquid phase outlet of the LPG tower is connected to the stable light hydrocarbon product pipeline via a stable light hydrocarbon product cooler, and a regulating valve g is installed on the pipeline.

[0015] Furthermore, the DHX tower, the ethane removal tower, the liquefied petroleum gas tower, the denitrification tower, and the helium extraction tower are packed towers or plate towers.

[0016] Furthermore, the refrigerant used in the mixed refrigerant refrigeration unit includes one or more of nitrogen, methane, ethane, propane, butane, and isopentane.

[0017] A method for helium extraction and light hydrocarbon recovery from LNG using a dual-cycle refrigeration system, characterized by comprising the following steps: S1. After cooling and gas-liquid separation, the liquid phase of the raw gas is pressurized and reheated before entering the de-ethanizer for distillation and separation, and the gas phase enters the DHX tower for distillation and separation. S2. The gas phase at the top of the deethanizer is cooled and returned to the top of the DHX tower. The liquid phase at the bottom of the DHX tower is pressurized and returned to the top of the deethanizer. The liquid phase at the bottom of the deethanizer is sent to a liquefied petroleum gas tower for distillation and separation to obtain liquefied petroleum gas and stable light hydrocarbons. S3. A portion of the gas phase at the top of the DHX tower enters the bottom of the denitrification tower, and the other portion, after cooling, enters the top of the denitrification tower for distillation and separation to obtain nitrogen-rich gas and liquefied natural gas. S4. The nitrogen-rich gas is cooled and then enters a helium extraction tower for distillation and separation to obtain crude helium and liquid nitrogen. S5. A mixed refrigerant refrigeration cycle provides cooling capacity for the cooling and liquefaction of the feed gas in steps S1 to S4, as well as for the distillation processes of the de-ethanizer, the denitrification, and the helium stripping tower. S6. Provide cooling for the distillation process of the helium extraction tower described in step S4 through a nitrogen refrigeration cycle.

[0018] Furthermore, in step S1, the raw gas is cooled to about -70°C through the raw gas I channel in the main heat exchanger and then enters the raw gas cryogenic separator for gas-liquid separation. The separated liquid phase is pressurized to about 1.87 MPa.G by the cryogenic liquid booster pump and then returned to the raw gas cryogenic liquid channel of the main heat exchanger to be reheated to about 15°C before entering the middle of the de-ethane tower.

[0019] Furthermore, the feed gas II channel of the top gas phase of the deethaner is cooled to about -70°C and then enters the top of the DHX tower after returning to the main heat exchanger; the liquid phase at the bottom of the DHX tower is pressurized to about 1.85 MPa.G by a mixed hydrocarbon booster pump and then returns to the top of the deethaner.

[0020] Furthermore, in step S2, the bottom temperature of the deethaner is controlled to be approximately 76°C by adjusting the steam flow rate of the reboiler at the bottom of the deethaner.

[0021] Furthermore, in step S3, the liquefied petroleum gas obtained from the top of the liquefied petroleum gas tower is cooled to 60-65°C by the condenser at the top of the liquefied petroleum gas tower and then enters the reflux tank at the top of the liquefied petroleum gas tower for gas-liquid separation. Part of the separated liquid phase is returned to the top of the liquefied petroleum gas tower, and the other part is cooled to about 40°C by the LPG product cooler and then output as a product. The separated gas phase is output as fuel gas. The temperature of the bottom of the liquefied petroleum gas tower is controlled to 148°C by controlling the amount of steam in the reboiler at the bottom of the liquefied petroleum gas tower.

[0022] Furthermore, in step S4, the nitrogen-rich gas obtained from the top of the denitrification tower is cooled to approximately -166°C in the top condenser of the denitrification tower and then enters the top reflux tank of the denitrification tower for separation. Its gas phase enters the nitrogen-rich gas I channel of the subcooler and is cooled to approximately -179°C before entering the helium extraction tower. The liquefied natural gas obtained from the bottom of the denitrification tower is subcooled to approximately -162°C through the LNG channel of the main heat exchanger before being output.

[0023] Furthermore, in step S5, the crude helium gas obtained from the top of the helium extraction tower is cooled to approximately -186.5°C in the top condenser of the helium extraction tower and then enters the top reflux tank of the helium extraction tower for separation. Its gas phase is then reheated sequentially through the crude helium gas II channel of the subcooler and the crude helium gas I channel of the main heat exchanger before being output.

[0024] The apparatus and method for helium extraction and light hydrocarbon recovery from dual-cycle refrigerated LNG as described in this invention have the following advantages: 1. Low energy consumption: The condensation and liquefaction of raw gas and nitrogen, the recovery of light hydrocarbons, the denitrification of natural gas and the liquefaction and subcooling of LNG are provided by a mixed refrigerant refrigeration cycle, which provides a cold source in the temperature range of -162~40℃. The extraction of crude helium from nitrogen-rich gas is provided by a nitrogen cycle, which provides a cold source in the temperature range of -187~-162℃. This process realizes different distribution of cold capacity in different temperature ranges, reducing the energy consumption of the entire unit.

[0025] 2. The process has a high feed gas utilization rate and can adapt to different load conditions. It can produce crude helium, liquid nitrogen, LPG and stable light hydrocarbons while producing LNG. The process has the advantages of complete component extraction and high feed gas utilization rate.

[0026] 3. High propane recovery rate in LPG products: Because the condensate from the deethaner column and the gas phase from the cryogenic separator of the feed gas come into countercurrent contact within the DHX column, undergoing simultaneous heat and mass transfer, most of the C3 components in the gas phase of the cryogenic separator are condensed. Due to the vaporization and refrigeration of the C1 and C2 light components in the condensate, the temperatures of both the gas phase at the top and the liquid phase at the bottom of the DHX column are lower than the feed temperature, fully utilizing the cooling capacity. This results in a propane recovery rate in LPG products exceeding 98%.

[0027] 4. High helium recovery rate, high helium content in crude helium product, and high purity of liquid nitrogen product: Due to the inclusion of a helium extraction tower, a top condenser of the helium extraction tower, and a bottom reboiler in the process, the cooling capacity after liquid nitrogen throttling is used as the cold source for the top condenser of the helium extraction tower, and the mixed refrigerant is used as the heat source for the bottom reboiler of the helium extraction tower, resulting in a helium recovery rate greater than 99%, a helium content greater than 76% in the crude helium product, and a nitrogen content greater than 99% in the liquid nitrogen product.

[0028] 5. High purity of LNG products: Due to the presence of a denitrification tower and a denitrification tower top condenser in the process, the cooling capacity after throttling of the mixed refrigerant is used as the cooling source for the denitrification tower top condenser. The temperature at the bottom of the denitrification tower is controlled by adjusting the opening of regulating valves h and i, ensuring that the nitrogen content in the LNG product is less than 1%.

[0029] 6. LPG products must meet the quality requirements of "Liquefied Petroleum Gas" (GB11174-2011): the (C3+C4) hydrocarbon content in the LPG product must be higher than 95%, and the C5 and C6 hydrocarbon content must be lower than 95%. + Hydrocarbon content is less than 3%.

[0030] 7. In addition, the device has the advantages of convenient maintenance, reliable operation, safety and reliability, and wide applicability. Attached Figure Description

[0031] 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. The drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0032] Figure 1 This is a schematic diagram of the system structure of the present invention; In the diagram, 1-Main heat exchanger, 2-Raw gas cryogenic separator, 3-DHX tower, 4-Cryogenic liquid booster pump, 5-Mixed hydrocarbon booster pump, 6-Ethane removal tower, 7-Ethane removal tower bottom reboiler, 8-Liquefied petroleum gas tower, 9-Liquefied petroleum gas tower top condenser, 10-Liquefied petroleum gas tower top reflux tank, 11-Liquefied petroleum gas tower liquid reflux pump, 12-LPG product cooler, 13-Liquefied petroleum gas tower bottom reboiler, 14-Stable light hydrocarbon product cooler, 15-Denitrification tower, 16-Denitrification tower top condenser, 17-Denitrification tower top reflux tank, 18-Subcooler, 19-Helium stripping tower, 20-Helium stripping tower top condenser, 21-Helium stripping tower top reflux tank, 22-Helium stripping tower bottom reboiler, 23-Nitrogen compression system, 24-Mixed refrigerant compressor system, 25-Refrigerant separator; 101-Control valve a, 102-Control valve b, 103-Control valve c, 104-Control valve d, 105-Control valve e, 106-Control valve f, 107-Control valve g, 108-Control valve h, 109-Control valve i, 110-Control valve j, 111-Control valve k, 112-Control valve l, 113-Control valve m, 114-Control valve n, 115-Control valve o, 116-Control valve p, 117-Control valve q, 118-Control valve r, 119-Control valve s; 1101-Raw Gas I Channel, 1102-Raw Gas Cryogenic Liquid Channel, 1103-Raw Gas II Channel, 1104-Raw Gas III Channel, 1105-LNG Channel, 1106-Return Nitrogen I Channel, 1107-Medium Pressure Nitrogen I Channel, 1108-Raw Helium I Channel, 1109-Return Refrigerant I Channel, 1110-High Pressure Gas Phase Refrigerant Channel I, 1111-Medium Pressure Liquid Phase Refrigerant Channel I, 1112-High Pressure Liquid Phase Refrigerant Channel I, 1113-High Pressure Gas Phase Refrigerant Channel II, 1114-High Pressure Liquid Phase Refrigerant Channel II; 1201 - Nitrogen-rich gas channel I, 1202 - Reflux liquid nitrogen channel I, 1203 - Medium-pressure liquid nitrogen channel I, 1204 - Crude helium gas channel II; 1301 - Nitrogen-rich gas II, 1302 - Refrigerant return channel III; 1401 - Crude Helium III Channel, 1402 - Reflux Liquid Nitrogen II Channel. Detailed Implementation

[0033] 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, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0034] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention. Example

[0035] like Figure 1As shown in the figure, this embodiment provides a dual-cycle refrigeration LNG helium extraction and light hydrocarbon recovery device, which mainly includes a main heat exchanger 1, a raw gas cryogenic separator 2, a DHX tower 3, a cryogenic liquid booster pump 4, a mixed hydrocarbon booster pump 5, an ethane stripper 6, a ethane stripper bottom reboiler 7, a liquefied petroleum gas (LPG) tower 8, an LPG tower top condenser 9, an LPG tower top reflux tank 10, an LPG tower liquid reflux pump 11, an LPG product cooler 12, an LPG tower bottom reboiler 13, a stabilized light hydrocarbon product cooler 14, a denitrification tower 15, a denitrification tower top condenser 16, a denitrification tower top reflux tank 17, a subcooler 18, a helium extraction tower 19, a helium extraction tower top condenser 20, a helium extraction tower top reflux tank 21, a helium extraction tower bottom reboiler 22, a nitrogen compression system 23, a mixed refrigerant compressor system 24, and a refrigerant separator 25. The main heat exchanger 1 is equipped with the following channels: raw material gas I channel 1101, raw material gas cryogenic liquid channel 1102, raw material gas II channel 1103, raw material gas III channel 1104, LNG channel 1105, reflux nitrogen I channel 1106, medium-pressure nitrogen I channel 1107, crude helium I channel 1108, reflux refrigerant I channel 1109, high-pressure gas phase refrigerant channel I1110, medium-pressure liquid phase refrigerant channel I1111, and high-pressure liquid phase refrigerant channel I1112. The supercooler 18 is equipped with a nitrogen-rich gas channel I 1201, a reflux liquid nitrogen channel I 1202, a medium-pressure liquid nitrogen channel I 1203, and a crude helium gas channel II 1204; the denitrification tower top condenser 16 is equipped with a nitrogen-rich gas channel II 1301 and a reflux liquid nitrogen channel III 1302; and the helium extraction tower top condenser 20 is equipped with a crude helium gas channel III 1401 and a reflux liquid nitrogen channel II 1402. The purified raw material gas is connected to the inlet of raw material gas channel I 1101. The outlet of raw material gas channel I 1101 is connected to the inlet of raw material gas cryogenic separator 2. The bottom liquid phase outlet of raw material gas cryogenic separator 2 is connected to the inlet of cryogenic liquid booster pump 4. The top gas phase outlet of raw material gas cryogenic separator 2 is connected to the bottom inlet of DHX tower 3. The outlet of cryogenic liquid booster pump 4 is connected to the inlet of raw material gas cryogenic liquid channel 1102. The outlet of raw material gas cryogenic liquid channel 1102 is connected to the middle inlet of deethanizer 6. The top gas phase outlet of deethanizer 6... The outlet of the feed gas channel II (1103) is connected to the inlet of the feed gas channel II (1103), and the outlet of the feed gas channel II (1103) is connected to the top feed inlet of the DHX tower 3. The bottom liquid outlet of the DHX tower 3 is connected to the inlet of the mixed hydrocarbon booster pump 5, and the outlet of the mixed hydrocarbon booster pump 5 is connected to another feed inlet in the middle of the deethaner 6. The top gas phase outlet of the DHX tower 3 is divided into two parts: one part of the gas is connected to the bottom feed inlet of the denitrification tower 15, and the other part of the gas is connected to the inlet of the feed gas channel III (1104), and the outlet of the feed gas channel III (1104) is connected to the top feed inlet of the denitrification tower 15.The reboiler outlet of the deethanizer 6 is connected to the mixed hydrocarbon inlet of the bottom reboiler 7 of the deethanizer 6; the mixed hydrocarbon outlet of the bottom reboiler 7 of the deethanizer 6 is connected to the reboiler return port of the deethanizer 6; the bottom outlet of the deethanizer 6 is connected to the middle inlet of the liquefied petroleum gas (LPG) tower 8; the bottom liquid phase outlet of the LPG tower 8 is connected to the condensate oil inlet of the stabilized light hydrocarbon product cooler 14; the condensate oil outlet of the stabilized light hydrocarbon product cooler 14 is connected to the pipeline from the stabilized light hydrocarbon product cooler 14 to the stabilized light hydrocarbon storage tank outside the boundary; the top vapor phase outlet of the LPG tower 8 is connected to the LPG inlet of the top condenser 9 of the LPG tower; and the LPG outlet of the top condenser 9 of the LPG tower is connected to the top reflux tank of the LPG tower. The feed inlet of LPG tower 8 is connected to the gas phase outlet at the top of the LPG tower top reflux tank 10, which is connected to the external fuel gas pipeline. The liquid phase outlet at the bottom of the LPG tower top reflux tank 10 is connected to the inlet of the LPG tower liquid reflux pump 11. The outlet of the LPG tower liquid reflux pump 11 is divided into two parts: one part is connected to the top reflux port of the LPG tower 8, and the other part is connected to the LPG inlet of the LPG product cooler 12. The LPG outlet of the LPG product cooler 12 is connected to the external LPG to LPG storage tank pipeline. The reboiler material outlet of the LPG tower 8 is connected to the stable light hydrocarbon inlet of the LPG tower bottom reboiler 13. The LPG tower bottom reboiler 13... The stable light hydrocarbon outlet is connected to the reboiler material return port of the liquefied petroleum gas tower 8. The top gas phase outlet of the denitrification tower 15 is connected to the inlet of nitrogen-rich gas II 1301. The outlet of nitrogen-rich gas II 1301 is connected to the feed inlet of the denitrification tower top reflux tank 17. The bottom liquid phase outlet of the denitrification tower top reflux tank 17 is connected to the top reflux port of the denitrification tower 15. The top gas phase outlet of the denitrification tower top reflux tank 17 is connected to the inlet of nitrogen-rich gas I channel 1201. The outlet of nitrogen-rich gas I channel 1201 is connected to the middle feed inlet of the helium stripping tower 19. The bottom liquid phase outlet of the denitrification tower 15 is connected to the inlet of the LNG channel 1105. The outlet of the LNG channel 1105 is connected to the LNG-to-LNG storage tank pipeline outside the boundary. The top gaseous outlet of the helium extraction tower 19 is connected to the inlet of the crude helium III channel 1401. The outlet of the crude helium III channel 1401 is connected to the inlet of the top reflux tank 21 of the helium extraction tower. The bottom liquid phase outlet of the top reflux tank 21 is connected to the top reflux port of the helium extraction tower 19. The top gaseous outlet of the top reflux tank 21 is connected to the inlet of the crude helium II channel 1204. The outlet of the crude helium II channel 1204 is connected to the inlet of the crude helium I channel 1108. The outlet of the crude helium I channel 1108 is connected to a crude helium pipeline outside the boundary. The bottom liquid phase outlet of the helium extraction tower 19 is connected to a liquid nitrogen pipeline outside the boundary. The inlet of the medium-pressure nitrogen I channel 1107 is connected to the outlet of the nitrogen compression system 23.The outlet of the medium-pressure nitrogen I channel 1107 is connected to the inlet of the medium-pressure liquid nitrogen I channel 1203. The outlet of the medium-pressure liquid nitrogen I channel 1203 is connected to the inlet of the reflux liquid nitrogen II channel 1402. The outlet of the reflux liquid nitrogen II channel 1402 is connected to the inlet of the reflux liquid nitrogen I channel 1202. The outlet of the reflux liquid nitrogen I channel 1202 is connected to the inlet of the reflux nitrogen I channel 1106. The outlet of the reflux nitrogen I channel 1106 is connected to the inlet of the nitrogen compression system 23. The inlet of the intermediate-pressure liquid refrigerant channel I1111 is connected to the interstage liquid outlet of the mixed refrigerant compressor system 24. The outlet of the intermediate-pressure liquid refrigerant channel I1111 is connected to one of the inlets of the return refrigerant channel I 1109. The inlet of the high-pressure liquid refrigerant channel I1112 is connected to the final stage liquid outlet of the mixed refrigerant compressor system 24. The outlet of the high-pressure liquid refrigerant channel I1112 is connected to one of the inlets of the return refrigerant channel I 1109. The high-pressure gaseous refrigerant channel I1... Inlet 110 is connected to the final gas phase outlet of the mixed refrigerant compressor system 24. The outlet of the high-pressure gas phase refrigerant channel I1110 is connected to the inlet of the refrigerant separator 25. The top gas phase outlet of the refrigerant separator 25 is connected to the inlet of the high-pressure gas phase refrigerant channel II1113. The outlet of the high-pressure gas phase refrigerant channel II1113 is divided into two parts: one part connects to the inlet of the return refrigerant channel III 1302, and the other part merges with the outlet of the return refrigerant channel III 1302. After combination, it is connected to one of the inlets of the reflux refrigerant I channel 1109. The bottom liquid phase outlet of the refrigerant separator 25 is connected to the inlet of the high-pressure liquid phase refrigerant channel II 1114. The outlet of the high-pressure liquid phase refrigerant channel II 1114 is connected to the refrigerant inlet of the helium stripping tower bottom reboiler 22. The refrigerant outlet of the helium stripping tower bottom reboiler 22 is connected to one of the inlets of the reflux refrigerant I channel 1109. The outlet of the reflux refrigerant I channel 1109 is connected to the inlet of the mixed refrigerant compressor system 24.

[0036] Preferably, a regulating valve a101 is installed on the feed pipeline at the top of the DHX tower 3.

[0037] Preferably, the steam pipeline of the deethaner bottom reboiler 7 is equipped with a regulating valve b102 for regulating the temperature at the bottom of the deethaner 6.

[0038] Preferably, a regulating valve C103 is installed on the liquid phase outlet pipeline at the bottom of the deethaner 6 to regulate the liquid level at the bottom of the deethaner 6.

[0039] Preferably, a regulating valve d104 is installed on the gas phase outlet pipeline at the top of the liquefied petroleum gas tower top reflux tank 10 to regulate the pressure at the top of the liquefied petroleum gas tower 8.

[0040] Preferably, the outlet pipeline of the LPG product cooler 12 is equipped with a regulating valve e105 for regulating the liquid level at the bottom of the liquefied petroleum gas tower top return tank 10.

[0041] Preferably, the steam pipeline of the reboiler 13 at the bottom of the liquefied petroleum gas tower is equipped with a regulating valve f106 for regulating the temperature at the bottom of the liquefied petroleum gas tower 8.

[0042] Preferably, a regulating valve g107 is installed on the liquid phase outlet pipeline at the bottom of the liquefied petroleum gas tower 8 to regulate the liquid level at the bottom of the ethane removal tower 6.

[0043] Preferably, a regulating valve h108 is installed on the feed pipeline at the top of the denitrification tower 15.

[0044] Preferably, a regulating valve i109 is installed on the feed pipeline at the bottom of the denitrification tower 15 to regulate the temperature at the bottom of the denitrification tower 15.

[0045] Preferably, a regulating valve j110 is installed on the outlet pipeline of the LNG channel 1105 to regulate the liquid level of the denitrification tower 15.

[0046] Preferably, a regulating valve k111 is installed on the inlet pipeline of the reflux refrigerant III channel 1302 to regulate the top temperature of the denitrification tower 15.

[0047] Preferably, a regulating valve l1112 is installed on the bottom outlet pipeline of the helium extraction tower 19 to regulate the liquid level of the helium extraction tower 19.

[0048] Preferably, a regulating valve m113 is installed on the inlet pipeline of the reflux liquid nitrogen II channel 1402 to regulate the top temperature of the helium extraction tower 19.

[0049] Preferably, a regulating valve n114 is installed on the pipeline at the outlet of the high-pressure liquid refrigerant channel II1114.

[0050] Preferably, a regulating valve o115 is installed on the pipeline at the outlet of the high-pressure liquid refrigerant channel I1112.

[0051] Preferably, a regulating valve p116 is installed on the pipeline at the outlet of the medium-pressure liquid refrigerant channel I1111.

[0052] Preferably, the bypass line of the reboiler 22 at the bottom of the helium extraction tower is equipped with a regulating valve q117 for regulating the temperature at the bottom of the helium extraction tower 19.

[0053] Preferably, a regulating valve r118 is installed on the pipeline at the outlet of the high-pressure gaseous refrigerant channel II1113.

[0054] Preferably, a regulating valve s119 is installed on the bypass line of the liquid nitrogen pipeline of the helium extraction tower top condenser 20.

[0055] Preferably, the reboiler 22 at the bottom of the helium extraction tower can be built-in or external.

[0056] Preferably, the DHX tower 3, the ethane removal tower 6, the liquefied petroleum gas tower 8, the nitrogen removal tower 15, and the helium extraction tower 19 are packed towers or plate towers.

[0057] A method for helium extraction and light hydrocarbon recovery from dual-cycle refrigerated natural gas includes the following steps: S1. Purified natural gas, after removing impurities H2O and CO2, enters feed gas I channel 1101 and is cooled to ~70℃ by the reflux cold stream. The feed gas cryogenic separator 2 performs gas-liquid separation. The liquid at the bottom is pressurized to ~1.87 MPa.G by cryogenic liquid booster pump 4 and then enters feed gas cryogenic liquid channel 1102 to be reheated to 15℃ before entering the middle of the deethaner 6 for rectification separation. The top gas phase directly enters the lower part of DHX tower 3. The liquid at the bottom of DHX tower 3 is pressurized to ~1.85 MPa.G by mixed hydrocarbon booster pump 5 and then enters the top of deethaner 6 for rectification separation. The top gas phase of DHX tower 3 is divided into two parts: one part enters the bottom of denitrification tower 15 for rectification separation after passing through regulating valve i109, and the other part enters feed gas III channel 1104, is cooled to -130℃, and then enters the top of denitrification tower 15 for rectification separation.

[0058] S2. The vapor phase from the top of the deethanizer 6 enters the feed gas II channel 1103 and is cooled to -70℃. After being depressurized by regulating valve a101, it enters the top of the DHX tower 3 for rectification and separation. The mixed hydrocarbon liquid at the bottom of the deethanizer 6 enters the middle of the liquefied petroleum gas tower 8 for rectification and separation after passing through regulating valve c103. The steam flow rate of the reboiler 7 at the bottom of the deethanizer 6 is controlled by regulating valve b102, thereby controlling the bottom temperature of the deethanizer 6 to ~76℃.

[0059] S3. The mixed hydrocarbon liquid is separated by distillation in LPG tower 8, yielding LPG at the top. This LPG is then cooled to 60-65°C in LPG tower top condenser 9 and enters LPG tower top reflux tank 10 for gas-liquid separation. The bottom liquid is pressurized by LPG tower liquid reflux pump 11 and divided into two parts: one part flows back to the top reflux port of LPG tower 8, and the other part enters LPG product cooler 12, where it is cooled to 40°C. After passing through regulating valve e105, it is sent to the LPG storage tank. The top gas phase is sent to the fuel gas pipeline network through regulating valve d104. The steam flow rate in LPG tower bottom reboiler 13 is controlled by regulating valve f106, thereby controlling the bottom temperature of LPG tower 8 to ~148°C.

[0060] S4. After distillation in denitrification tower 15, nitrogen-rich gas is obtained at the top. This nitrogen-rich gas is cooled to ~166°C in nitrogen-rich gas II 1301 and enters the top reflux tank 17 of the denitrification tower for gas-liquid separation. The low-temperature liquid at the bottom returns to the top reflux port of denitrification tower 15. The nitrogen-rich gas at the top enters nitrogen-rich gas I channel 1201 and is cooled to -179°C before being sent to helium stripping tower 19 for distillation separation. The LNG separated at the bottom of denitrification tower 15 enters LNG channel 1105, is subcooled to -162°C, and then sent to the LNG storage tank through regulating valve J110.

[0061] S5. Nitrogen-rich gas is distilled in helium stripping tower 19, yielding crude helium at the top. This crude helium is cooled to ~186.5℃ in crude helium III channel 1401 and then enters the top reflux tank 21 for gas-liquid separation. The cryogenic liquid at the bottom returns to the top reflux port of helium stripping tower 19. The crude helium at the top is continuously reheated to 36℃ through crude helium II channel 1204 and crude helium I channel 1108 before being connected to the external crude helium pipeline. Liquid nitrogen is obtained at the bottom of helium stripping tower 19, and the liquid level in tower 19 is controlled by regulating valve 1112.

[0062] S6. The condensation and liquefaction of feed gas and nitrogen, light hydrocarbon recovery, natural gas denitrification, and LNG liquefaction and subcooling are provided with cooling capacity by a mixed refrigerant refrigeration cycle. First, the medium-pressure liquid refrigerant from the mixed refrigerant compressor system 24 is subcooled to ~-60°C in the medium-pressure liquid refrigerant channel I1111 of the main heat exchanger 1. After being throttled and depressurized by regulating valve p116, it enters one of the inlets of the return refrigerant channel I109. The high-pressure liquid refrigerant from the mixed refrigerant compressor system 24 is subcooled to ~-60°C in the high-pressure liquid refrigerant channel I1112 of the main heat exchanger 1. After being throttled and depressurized by regulating valve o115, it enters the return refrigerant channel I. One of the inlets of 1109, the high-pressure gaseous refrigerant in the mixed refrigerant compressor system 24, is cooled to ~60°C in the high-pressure gaseous refrigerant channel I1110 of the main heat exchanger 1, and then enters the refrigerant separator 25 for gas-liquid separation. The liquid separated at the bottom enters the high-pressure liquid refrigerant channel II1114 and is subcooled to ~-130°C. After being throttled and depressurized by the regulating valve n114, it enters the bottom reboiler 22 of the helium stripping tower to provide a heat source for the reboiler of the helium stripping tower 19. 2. A bypass pipeline is set up, and the temperature of the bottom of the helium extraction tower 19 is controlled by regulating valve q117, thereby controlling the nitrogen content of the liquid nitrogen in the bottom of the helium extraction tower 19 to be higher than 99.89%. The liquid nitrogen in the bottom of the helium extraction tower 19 is cooled to -131℃ by the cryogenic liquid and enters one of the inlets of the reflux refrigerant channel 1109. The high-pressure gaseous refrigerant from the top of the refrigerant separator 25 enters the high-pressure gaseous refrigerant channel II 1113 and is cooled and condensed to about -163℃. It is divided into two parts. One part is throttled and depressurized by regulating valve r118 and enters one of the inlets of the reflux refrigerant channel 1109. The other part is throttled and depressurized by regulating valve k111 and enters the reflux refrigerant III channel 1302. After merging with the reflux refrigerant after regulating valve r118, the mixed refrigerant enters the reflux refrigerant I channel 1109 and absorbs heat to evaporate. After evaporating into gas and reheating to room temperature, it exits the main heat exchanger 1 and then returns to the mixed refrigerant compressor system 24 to complete the mixed refrigerant refrigeration cycle.

[0063] S7. The extraction of crude helium from nitrogen-rich gas is achieved by using nitrogen circulation to provide cooling. First, the medium-pressure nitrogen from the nitrogen compression system 23 is cooled to approximately ~162°C in the medium-pressure nitrogen I channel 1107, turning into liquid nitrogen. It then enters the medium-pressure liquid nitrogen I channel 1203 for further cooling to approximately ~186°C, dividing into two parts. One part of the liquid nitrogen is throttled and depressurized by the regulating valve m113 before entering the reflux liquid nitrogen II channel 1402, controlling the temperature of the hydrogen-rich gas entering the top reflux tank 21 of the helium extraction tower between ~-186.5°C, providing a cold source for the separation of crude helium at the top of the helium extraction tower 19. The other part of the liquid nitrogen is throttled and depressurized by the regulating valve s119, and then merges with the liquid nitrogen returning from the reflux liquid nitrogen II channel 1402 before entering the reflux liquid nitrogen I channel 1202 to be reheated to ~165°C. After being reheated to room temperature in the reflux nitrogen I channel 1106, it exits the main heat exchanger 1 and then returns to the inlet of the nitrogen compression system 23 to complete the nitrogen refrigeration cycle.

[0064] Preferably, when LPG tower 8 is under maintenance or malfunctioning, LNG production can be carried out using DHX tower 3, deethaner 6, and denitrification tower 15, and crude helium production can be carried out using helium extraction tower 19. In this case, LPG tower 8 is not operating, and the mixed hydrocarbon liquid at the bottom of deethaner 6 is directly sent to an off-site non-conforming product storage tank. The rest is carried out according to the steps in Example 1.

[0065] Preferably, the mixed refrigerant includes one or more combinations of nitrogen, methane, ethane, propane, butane, and isopentane.

[0066] Preferably, the pressure range at the outlet of the mixed refrigerant compressor unit can be ~3.2 MPa.G.

[0067] Preferably, the outlet pressure range of the nitrogen compressor can be ~1.5 MPa.G.

[0068] This embodiment can be implemented using the purified feed gas parameters shown in the table below. The parameters and composition of the resulting LNG, crude helium, liquid nitrogen, LPG, and stable light hydrocarbons are shown in Table 1 below: Table 1 Parameters and Components of Example 1

[0069] Example 2: When the LPG tower (8) is under maintenance or malfunctioning, the DHX tower (3), deethaner (6), and denitrification tower (15) can be used for LNG production, and the helium extraction tower (19) can be used for crude helium production. At this time, the LPG tower (8) is not working, and the mixed hydrocarbon liquid at the bottom of the deethaner (6) is directly sent to the off-site non-conforming product storage tank. The rest is carried out according to the steps in Example 1.

[0070] This embodiment can be implemented using the purified feed gas parameters shown in the table below. The parameters and composition of the resulting LNG, crude helium, liquid nitrogen, and mixed hydrocarbons are shown in Table 2 below. Table 2 Parameters and Components of Example 2

[0071] The above embodiments describe the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Modifications and variations made by those skilled in the art without departing from the spirit and scope of the invention should be within the protection scope of the appended claims.

Claims

1. A dual cycle refrigeration LNG helium extraction and light hydrocarbon recovery apparatus, characterized in that, It includes a feed gas supply unit for supplying feed gas to be processed; and a light hydrocarbon separation unit connected to the feed gas supply unit for separating liquefied petroleum gas and stabilized light hydrocarbons from the feed gas. A liquefied natural gas (LNG) production unit, connected to the feed gas supply unit and the light hydrocarbon separation unit, is used to separate LNG from the feed gas; a helium extraction unit, connected to the LNG production unit, is used to extract crude helium from a nitrogen-rich stream; and a mixed refrigerant refrigeration unit is used to provide cooling to the feed gas supply unit, the light hydrocarbon separation unit, the LNG production unit, and the helium extraction unit. And a nitrogen refrigeration unit, used to provide cooling for the helium extraction unit.

2. The dual cycle refrigeration LNG helium extraction and light hydrocarbon recovery apparatus of claim 1, wherein, The light hydrocarbon separation unit includes an ethane removal tower (6) and a liquefied petroleum gas tower (8) connected in sequence; the liquefied natural gas production unit includes a denitrification tower (15) and a denitrification tower top condenser (16), wherein the denitrification tower top condenser (16) is provided with a nitrogen-rich gas II (1301) and a reflux refrigerant III channel (1302); the helium extraction unit includes a helium extraction tower (19); the feed gas supply unit includes a main heat exchanger (1), a feed gas cryogenic separator (2), a DHX tower (3), a cryogenic liquid booster pump (4), and a mixed hydrocarbon booster pump (5); the feed gas supply The main heat exchanger (1) of the unit is provided with a raw gas channel connected to the raw gas cryogenic separator (2); the liquid phase outlet of the raw gas cryogenic separator (2) is connected to the inlet of the deethaner (6) via the cryogenic liquid booster pump (4) and the main heat exchanger (1), and its gas phase outlet is connected to the inlet of the DHX tower (3); the top gas phase outlet of the deethaner (6) is connected to the top inlet of the DHX tower (3) via the main heat exchanger (1), and its bottom liquid phase outlet is connected to the inlet of the liquefied petroleum gas tower (8); the DHX tower (3) The bottom liquid outlet of the tower is connected to the feed inlet of the deethaner (6) via the mixed hydrocarbon booster pump (5). Its top gas outlet is divided into two paths: one connected to the bottom feed inlet of the denitrification tower (15), and the other connected to the top feed inlet of the denitrification tower (15) via the main heat exchanger (1), where it is cooled to -130°C. The top gas outlet of the denitrification tower (15) is connected to the feed inlet of the helium extraction tower (19), and its bottom liquid outlet is connected to a liquefied natural gas output pipeline. The top gas outlet of the helium extraction tower (19) is connected to crude helium. The output pipeline has a liquid nitrogen output pipeline connected to the liquid phase outlet at the bottom of the tower; the gas phase outlet at the top of the denitrification tower (15) is connected to the inlet of nitrogen-rich gas II (1301), the outlet of nitrogen-rich gas II (1301) is connected to the feed inlet of the denitrification tower top reflux tank (17), the liquid phase outlet at the bottom of the denitrification tower top reflux tank (17) is connected to the top reflux port of the denitrification tower (15), the gas phase outlet at the top of the denitrification tower top reflux tank (17) is connected to the inlet of nitrogen-rich gas I channel (1201), and the outlet of nitrogen-rich gas I channel (1201) is connected to the feed inlet in the middle of the helium extraction tower (19).

3. The dual cycle refrigeration LNG helium extraction and light hydrocarbon recovery apparatus of claim 2, wherein, A regulating valve a (101) is provided on the top feed line of the DHX tower (3); a regulating valve b (102) is provided on the steam line of the reboiler (7) at the bottom of the deethaner (6); a regulating valve c (103) is provided on the bottom liquid phase outlet line of the deethaner (6); a regulating valve h (108) is provided on the top feed line of the denitrification tower (15); a regulating valve i (109) is provided on the bottom feed line of the denitrification tower (15); a regulating valve j (110) is provided on the liquefied natural gas output line connected to the bottom liquid phase outlet of the denitrification tower (15); and a regulating valve l (112) is provided on the bottom liquid phase outlet line of the helium extraction tower (19).

4. The apparatus for dual-cycle refrigeration LNG helium extraction and light hydrocarbon recovery according to claim 2, characterized in that, The mixed refrigerant refrigeration unit includes a mixed refrigerant compressor system (24), a refrigerant separator (25), and multiple refrigerant channels disposed in the main heat exchanger (1); the outlet of the mixed refrigerant compressor system (24) is connected to the refrigerant channel inlet of the main heat exchanger (1) to provide high-pressure refrigerant; the inlet of the refrigerant separator (25) is connected to the outlet of the high-pressure gas phase refrigerant channel (1110) of the main heat exchanger (1); the gas phase outlet of the refrigerant separator (25) is connected to the high-pressure gas phase refrigerant channel II of the main heat exchanger (1) via the high-pressure gas phase refrigerant channel II (1110) of the main heat exchanger (1). 1113) is connected to the reflux refrigerant channel (1109); the liquid phase outlet of the refrigerant separator (25) is connected to the refrigerant inlet of the bottom reboiler (22) of the helium-lifting tower (19) via the high-pressure liquid phase refrigerant channel II (1114) of the main heat exchanger (1); the refrigerant outlet of the bottom reboiler (22) of the helium-lifting tower is connected to the reflux refrigerant channel (1109); the outlet of the reflux refrigerant channel (1109) is connected to the inlet of the mixed refrigerant compressor system (24), the mixed refrigerant refrigeration unit being used to provide -162 The cooling capacity in the ~40℃ temperature range is provided by the refrigerant provided by the mixed refrigerant refrigeration unit for the reboiler (22) at the bottom of the helium extraction tower (19), and the cooling source of the condenser (16) at the top of the denitrification tower (15) is provided by the refrigerant after throttling in the mixed refrigerant refrigeration unit; the interstage liquid phase outlet of the mixed refrigerant compressor system (24) is connected to the reflux refrigerant channel (1109) via the medium-pressure liquid phase refrigerant channel (1111) of the main heat exchanger (1), and a regulating valve p (116) is provided on the connecting pipeline; the mixed refrigerant compressor system (24) is connected to the reflux refrigerant channel (1109) via the medium-pressure liquid phase refrigerant channel (1111) of the main heat exchanger (1), and a regulating valve p (116) is provided on the connecting pipeline; the mixed refrigerant compressor system (24) is connected to the reflux refrigerant channel (1109) for the reboiler (22) at the bottom of the helium extraction tower (19), and the reflux refrigerant channel ... The final liquid phase outlet of the compressor system (24) is connected to the reflux refrigerant channel (1109) via the high-pressure liquid phase refrigerant channel I (1112) of the main heat exchanger (1), and a regulating valve o (115) is provided on the connecting pipeline; a regulating valve n (114) is provided on the outlet pipeline of the high-pressure liquid phase refrigerant channel II (1114); a regulating valve r (118) is provided on the pipeline connecting to the outlet of the high-pressure gas phase refrigerant channel II (1113); a bypass pipeline is provided on the bottom reboiler (22) of the helium extraction tower, and a regulating valve q (117) is provided on the bypass pipeline.

5. The apparatus for dual-cycle refrigeration LNG helium extraction and light hydrocarbon recovery according to claim 1, characterized in that, The nitrogen refrigeration unit includes a nitrogen compression system (23), a subcooler (18), a helium extraction tower top condenser (20), and nitrogen channels disposed in the main heat exchanger (1) and the subcooler (18). The outlet of the nitrogen compression system (23) is connected to the inlet of the medium-pressure liquid nitrogen channel (1203) of the subcooler (18) via the medium-pressure nitrogen channel (1107) of the main heat exchanger (1). The outlet of the medium-pressure liquid nitrogen channel (1203) is divided into two paths: one path is connected to the inlet of the reflux liquid nitrogen II channel (1402) of the helium extraction tower top condenser (20), and the other path is connected to the reflux liquid nitrogen II channel (1402) via the regulating valve s (119). The outlets of the liquid nitrogen II channel (1402) converge; the outlet of the reflux liquid nitrogen II channel (1402) is connected to the inlet of the nitrogen compression system (23) via the reflux liquid nitrogen I channel (1202) of the subcooler (18) and the reflux nitrogen I channel (1106) of the main heat exchanger (1) in sequence. The nitrogen refrigeration unit is used to provide deep cooling capacity in the temperature range of -187~-162℃. The cold source of the helium tower top condenser (20) of the helium tower (19) comes from the liquid nitrogen provided by the nitrogen refrigeration unit; a regulating valve m (113) is provided on the pipeline connected to the inlet of the reflux liquid nitrogen II channel (1402).

6. The apparatus for dual-cycle refrigeration LNG helium extraction and light hydrocarbon recovery according to claim 2, characterized in that, The liquefied petroleum gas tower (8) is equipped with a liquefied petroleum gas tower top condenser (9) and a liquefied petroleum gas tower top reflux tank (10) at the top. The gas phase outlet of the liquefied petroleum gas tower top reflux tank (10) is connected to a fuel gas pipeline, and a regulating valve d (104) is installed on the pipeline. The liquid phase outlet of the liquefied petroleum gas tower top reflux tank (10) is divided into two paths by a liquefied petroleum gas tower liquid reflux pump (11), one of which is connected to the top reflux port of the liquefied petroleum gas tower (8). Another path is connected to the liquefied petroleum gas product pipeline via the LPG product cooler (12), and the product pipeline is equipped with a regulating valve e (105); the bottom of the liquefied petroleum gas tower (8) is equipped with a liquefied petroleum gas tower bottom reboiler (13), and its steam pipeline is equipped with a regulating valve f (106); the bottom liquid phase outlet of the liquefied petroleum gas tower (8) is connected to the stable light hydrocarbon product pipeline via a stable light hydrocarbon product cooler (14), and the pipeline is equipped with a regulating valve g (107).

7. A method for helium extraction and light hydrocarbon recovery from LNG using a dual-cycle refrigeration system according to any one of claims 1-6, characterized in that, Includes the following steps: S1. After the raw gas is cooled and separated into gas and liquid, its liquid phase is pressurized and reheated before entering the de-ethanizer (6) for distillation and separation, and its gas phase enters the DHX tower (3) for distillation and separation. S2. The top gas phase of the deethanizer (6) is cooled and returned to the top of the DHX tower (3). The bottom liquid phase of the DHX tower (3) is pressurized and returned to the top of the deethanizer (6). The bottom liquid phase of the deethanizer (6) is sent to the liquefied petroleum gas tower (8) for distillation and separation to obtain liquefied petroleum gas and stable light hydrocarbons. The bottom temperature of the deethanizer (6) is controlled to be 76°C by adjusting the steam flow rate of the reboiler (7) at the bottom of the deethanizer. S3. Part of the gas phase at the top of the DHX tower (3) enters the bottom of the denitrification tower (15), and the other part enters the top of the denitrification tower (15) after cooling for distillation separation to obtain nitrogen-rich gas and liquefied natural gas. The liquefied petroleum gas obtained at the top of the liquefied petroleum gas tower (8) is cooled to 60~65°C by the condenser (9) at the top of the liquefied petroleum gas tower and then enters the reflux tank (10) at the top of the liquefied petroleum gas tower for gas-liquid separation. Part of the separated liquid phase is returned to the top of the liquefied petroleum gas tower (8), and the other part is cooled to 40°C by the LPG product cooler (12) and then output as a product. The separated gas phase is output as fuel gas. The temperature of the bottom of the liquefied petroleum gas tower (8) is controlled to 148°C by controlling the amount of steam in the reboiler (13) at the bottom of the liquefied petroleum gas tower. S4. The nitrogen-rich gas, after cooling, enters the helium stripping tower (19) for distillation separation to obtain crude helium and liquid nitrogen. The nitrogen-rich gas obtained at the top of the denitrification tower (15) is cooled to -166°C in the denitrification tower top condenser (16) and then enters the denitrification tower top reflux tank (17) for separation. Its gas phase enters the nitrogen-rich gas I channel (1201) of the subcooler (18) and is cooled to -179°C before entering the helium stripping tower (19). The liquid nitrogen obtained at the bottom of the denitrification tower (15) Natural gas is subcooled to -162°C through the LNG channel (1105) of the main heat exchanger (1) and then output. The mixed refrigerant refrigeration cycle provides cooling capacity in the temperature range of -162°C to 40°C. The condensation cold source at the top of the helium extraction tower (19) comes from the liquid nitrogen of the nitrogen refrigeration cycle. The reboiling heat source at the bottom of the helium extraction tower (19) comes from the refrigerant of the mixed refrigerant refrigeration cycle. The condensation cold source at the top of the denitrification tower (15) comes from the refrigerant after throttling in the mixed refrigerant refrigeration cycle. S5. A mixed refrigerant refrigeration cycle provides cooling capacity for the cooling and liquefaction of the raw gas in steps S1 to S4, as well as for the distillation processes of the de-ethanizer (6), the denitrification tower (15), and the helium stripping tower (19). S6. Provide cooling for the distillation process of the helium extraction tower (19) described in step S4 by means of a nitrogen refrigeration cycle.

8. The method for helium extraction and light hydrocarbon recovery from dual-cycle refrigerated LNG according to claim 7, characterized in that, In step S1, the raw gas is cooled to -70°C through the raw gas I channel (1101) in the main heat exchanger (1) and then enters the raw gas cryogenic separator (2) for gas-liquid separation. The separated liquid phase is pressurized to 1.87 MPa.G by the cryogenic liquid booster pump (4) and then returned to the raw gas cryogenic liquid channel (1102) of the main heat exchanger (1) to be reheated to 15°C before entering the middle of the de-ethane tower (6).

9. The method for helium extraction and light hydrocarbon recovery from dual-cycle refrigerated LNG according to claim 8, characterized in that, The gas phase at the top of the deethaner (6) is cooled to -70°C in the feed gas II channel (1103) of the main heat exchanger (1) and then enters the top of the DHX tower (3); the liquid phase at the bottom of the DHX tower (3) is pressurized to 1.85 MPa.G by the mixed hydrocarbon booster pump (5) and then returns to the top of the deethaner (6).

10. The method for helium extraction and light hydrocarbon recovery from dual-cycle refrigerated LNG according to claim 7, characterized in that, In step S5, the crude helium gas obtained from the top of the helium extraction tower (19) is cooled to -186.5°C in the top condenser (20) of the helium extraction tower and then enters the top reflux tank (21) of the helium extraction tower for separation. Its gas phase is then reheated by the subcooler (18) and the main heat exchanger (1) in sequence before being output. The nitrogen refrigeration cycle provides deep cooling capacity in the temperature range of -187 to -162°C.

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