A method for coproducing helium and ethane from low-helium natural gas

Abstract

The application discloses a method for extracting helium from low-helium-containing natural gas and co-producing ethane, relates to the technical field of helium extraction from natural gas, and specifically relates to a method for extracting helium from low-helium-containing natural gas and co-producing ethane, which comprises the following steps: raw gas decarburization: CO2 in pressurized raw gas is absorbed by activated MDEA solution, and the CO2 content is removed to below 50 ppm; raw gas dehydration and mercury removal: dehydration and mercury removal are performed by using a three-tower isobaric molecular sieve dehydration and mercury removal process, and a purified gas with a water content of less than or equal to 1 ppm and a mercury content of less than 0.01 mu g / m3 is obtained; natural gas helium extraction. The application adopts three external mixed refrigerant refrigeration cycle systems such as a mixed refrigerant precooling process without expansion refrigeration, the system has rich regulation means, can provide matched cold energy for a crude helium extraction process, is flexible in operation, and is good in adaptability to raw materials; the natural gas helium extraction is simultaneously used for co-producing ethane and other light hydrocarbon products, and the energy consumption of a demethanizer gradient multi-stream side line extraction process device is greatly reduced.

Description

Technical Field

[0001] This invention relates to the field of natural gas helium extraction technology, and more particularly to a method for helium extraction under conditions where the helium content in the raw natural gas is low. It also relates to a method for extracting ethane from natural gas, specifically a method for helium extraction and ethane production from low-helium-content natural gas, which is particularly suitable for conditions where the raw natural gas has a low helium content and low pressure energy. Background Technology

[0002] Helium extraction methods from natural gas mainly include membrane separation, pressure swing adsorption (PSA), and cryogenic separation. Membrane separation offers advantages such as low investment and low energy consumption; however, compared to cryogenic separation, it suffers from a higher helium loss rate and is currently primarily used for initial helium enrichment. PSA is mainly used in the purification of crude helium products. Cryogenic separation comprises two methods: multi-stage flash distillation and cryogenic distillation. This method is technologically mature and boasts high helium recovery rates. Multi-stage flash distillation is often coupled with natural gas liquefaction processes. While its process is simple, it results in a lower volume fraction of crude helium. Cryogenic distillation, compared to multi-stage flash distillation, has a more complex process but offers higher helium recovery rates and higher volume fractions of crude helium. Based on my country's current helium resource situation, a combined helium extraction method—initially enriching with membrane separation, further extracting crude helium with cryogenic distillation, and then purifying the crude helium with PSA—offers advantages such as high helium recovery rates, low total investment, and low energy consumption.

[0003] Currently, the main ethane recovery technologies used include liquid subcooling, gas subcooling, and partial dry gas recirculation (RSV) processes. Liquid subcooling can handle feed gases with high CO2 content without requiring dedicated CO2 removal facilities; however, the ethane recovery rate is low, approximately 80% to 86%. Gas subcooling is relatively simple, but energy consumption is high, and the ethane recovery rate is also low, only around 80%. Theoretically, partial dry gas recirculation (RSV) processes can achieve 99% ethane recovery, but this is only suitable for feed gases with high C2 content and CO2 content <1.5%. The most representative example is the Partial Dry Gas Recirculation (RSV) process from Ortloff Corporation in the United States. The principle is based on gas subcooling (GSP), where a portion of the high-pressure external dry gas is condensed after heat exchange with the overhead gas from the demethanizer. This condensed gas is then subcooled and throttled before being fed back into the top of the demethanizer to provide reflux. The refluxed external dry gas can then be used to distill the overhead gas phase, increasing the ethane recovery rate to over 95%.

[0004] In response to the low helium content of domestic raw natural gas and the gradual decrease in pressure on natural gas gathering and transmission pipelines due to declining production capacity, this project explores a comprehensive helium extraction approach for natural gas by expanding the scope of cryogenic technology. It innovatively proposes an integrated technology for cryogenic helium extraction from natural gas, along with the co-production of light hydrocarbons and LNG. Three proprietary technologies have been developed: "utilization of cold energy extracted from the side stream of the demethanizer pre-cooled MRC tower," "high-efficiency helium enrichment through two-stage MRC refrigeration," and "novel catalytic oxidation dehydrogenation." These technologies can, depending on resource market conditions, achieve maximum recovery of light hydrocarbon products such as ethane, LPG, and stabilized light hydrocarbons while simultaneously completing cryogenic helium extraction from natural gas, effectively reducing overall energy consumption and improving the overall utilization rate of natural gas resources.

[0005] In a published Chinese patent application, publication number CN115654839A, titled "An apparatus and method for cryogenic helium extraction from natural gas," although cryogenic helium extraction can be achieved through the combined setup of equipment such as a primary enrichment tower and a secondary enrichment tower, the method shown in this prior art is not suitable for helium extraction from low-helium-content natural gas, and the efficiency of helium extraction from low-helium-content natural gas is low. Summary of the Invention

[0006] (a) Technical problems to be solved

[0007] To address the shortcomings of existing technologies, this invention provides a method for helium extraction and ethane production from low-helium-content natural gas, thus solving the problems mentioned in the background section.

[0008] (II) Technical Solution

[0009] To achieve the above objectives, the present invention is implemented through the following technical solution: a method for helium extraction and ethane production from low-helium natural gas, comprising the following steps: Step 1, decarbonization of raw gas: CO2 is removed from the raw gas using an MDEA decarbonization device to obtain decarbonized raw gas.

[0010] Step 2: Raw material gas dehydration and mercury removal: The decarbonized raw material gas obtained in Step 1 is dehydrated using a three-tower isobaric molecular sieve and mercury removed using sulfur-impregnated activated carbon; during dehydration, the pressure difference between the raw material gas and the regenerated gas is controlled to be greater than or equal to 70 kPa through flow control, and the amount of regenerated gas used is about 10% of the amount of raw material gas; the purified gas is obtained after being filtered by a dust filter when exiting the tower.

[0011] Step 3, Helium Extraction from Natural Gas: The helium-containing feed gas purified in Step 2 is pre-cooled to -104°C to -108°C in a pre-cooling box and then used as a heat source for the reboiler at the bottom of the first-stage enrichment tower to recover the cooling capacity at the bottom of the tower, reducing the temperature to -106°C to -111°C. After recovering the cooling capacity, the helium-containing feed gas passes through the first-stage enrichment tower, where most of the nitrogen and helium are separated from the top of the tower. The gas phase at the top of the first-stage enrichment tower is cooled to -155°C to -160°C by a first-stage top cooler, and then separated by a first-stage top separator to obtain primary crude helium with a helium content of 20% to 40%. The liquid phase at the bottom of the first-stage enrichment tower is separated into methane and C2+ components by a demethanizer. The gas phase at the top of the demethanizer is dry gas for export, and after recovering the cooling capacity in a pre-cooling box to 15°C to 40°C, it is then exported. The obtained primary crude helium is cooled to -175°C to -182°C by the feed cooler of the secondary enrichment tower. Most of the helium is then separated from the top of the secondary enrichment tower. The gaseous phase at the top of the secondary enrichment tower is cooled to -192°C to -195°C by the secondary tower top cooler and then separated by the secondary tower top separator to obtain secondary crude helium. The secondary crude helium is then cooled to -60°C to -70°C by the secondary tower top cooler before entering the subsequent helium enrichment unit to produce high-purity helium. The liquid phase at the bottom of the secondary enrichment tower is cooled to -168°C to -178°C by the feed cooler of the secondary enrichment tower, and then cooled to 15°C to 40°C by a pre-cooling box before being vented at a high point.

[0012] Step 4, Ethane Recovery: The liquid phase component from the demethanizer in Step 3 is pressurized and then enters the deethanizer. The gas phase at the top of the deethanizer enters the main cold box and is cooled to 0°C to 3°C. After separation and pressurization to 3200 kPa, it is divided into two streams. The first stream, accounting for 48% to 52%, is cooled to -93°C to -97°C in the main cold box and then throttled to 150 kPa as ethane product. The other stream, accounting for 48% to 52%, is returned to the deethanizer as top reflux.

[0013] Step 5, condensate fractionation: The bottom liquid phase from the ethane stripper in Step 4 is used as feed to the LPG tower, where C3, C4, and C5+ components are separated. The top gas phase from the LPG tower is cooled, separated, and pressurized to 1700 kPa·a to 1900 kPa·a before being divided into two streams. The first stream, accounting for about 40%, is used as LPG product, while the other stream, accounting for about 60%, is returned to the LPG tower as reflux. The liquid phase from the LPG tower is cooled and used as a stabilized light hydrocarbon product.

[0014] Optionally, the precooling cold box, the first-stage tower top cooler, and the second-stage tower top cooler can be external mixed refrigerant refrigeration compressor units, with the precooling cold box using a precooling mixed refrigerant, the first-stage tower top cooler using a first-stage mixed refrigerant, and the second-stage tower top cooler using a second-stage mixed refrigerant.

[0015] Optionally, the precooling refrigerant mixture includes methane, isopentane, nitrogen, and ethylene; the primary refrigerant mixture includes methane, propane, isopentane, and ethylene; and the secondary refrigerant mixture includes methane and nitrogen.

[0016] Optionally, based on the temperature differences between the reabsorption section, rectification section, and stripping section of the demethanizer, different amounts of raw materials are extracted in a gradient for cold energy recovery and utilization. The demethanizer is equipped with three liquid side streams for cold energy recovery, and the side streams are returned to the demethanizer after the cold energy is recovered in the pre-cooling box.

[0017] (III) Beneficial Effects

[0018] This invention provides a method for helium extraction and ethane production from low-helium-content natural gas, which has the following beneficial effects:

[0019] 1. This invention, while completing the cryogenic extraction of helium from natural gas, is also applicable to the extraction of helium from low-helium natural gas, achieving the maximum recovery of light hydrocarbon products such as ethane, liquefied petroleum gas, and stabilized light hydrocarbons. It effectively reduces the overall energy consumption of the equipment, improves the overall utilization rate of natural gas resources, and achieves a theoretical helium recovery rate of >99%, a theoretical ethane recovery rate of >95%, and a theoretical C3+ recovery rate of >99%.

[0020] 2. This invention employs three external mixed refrigerant refrigeration cycle systems, namely a pre-cooling cold box, a first-stage tower top cooler, and a second-stage tower top cooler. The system has a variety of adjustment methods, can provide matching cooling capacity for the crude helium extraction process from natural gas, is flexible in operation, and has good adaptability to raw materials. By optimizing the mixed refrigerant formula and the refrigeration pressure of the compressor unit, the energy consumption of the unit is reduced.

[0021] 3. This invention uses a gradient cooling capacity setting in the demethanizer to extract three liquid phase side streams. The side streams are then returned to the demethanizer after the cooling capacity is recovered in a pre-cooling box. By controlling key parameters such as the number of liquid phase side streams extracted from the demethanizer, the extraction location, and the extraction rate, comprehensive utilization of cooling capacity is achieved, ensuring optimal energy efficiency and reducing the overall energy consumption of the unit. The energy consumption per unit of crude helium product is <30kW.h / Nm3.

[0022] 4. By optimizing the operating parameters such as feed temperature, operating pressure, top cooling temperature, and bottom reboiling temperature of the primary and secondary enrichment towers, the helium content of the primary crude helium tower reaches 20% to 40%, and the helium content of the secondary crude helium tower reaches 60% to 80%, with a theoretical helium yield of >95%. This invention effectively improves the helium yield while reducing the load on the subsequent crude helium refining unit.

[0023] 5. The specific implementation of the method shown in this invention can achieve a C2 yield of over 95%, and a C3 yield of... + The recovery rate reaches over 99%, the product yield is high, and the economic benefits are significant. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the process structure of a method for helium extraction and ethane production from low-helium natural gas according to the present invention;

[0025] Figure 2 for Figure 1 Enlarged structural diagram of region A in the middle;

[0026] Figure 3 for Figure 1 Enlarged structural diagram of region B in the middle;

[0027] Figure 4 for Figure 1 Enlarged structural diagram of region C in the middle;

[0028] Figure 5 for Figure 1 A magnified structural diagram of region D in the middle;

[0029] Figure 6 for Figure 1 Enlarged structural diagram of region E in the middle;

[0030] Figure 7 for Figure 2 A magnified structural diagram of region A1 in the middle. Detailed Implementation

[0031] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0032] Please see Figures 1 to 7 The present invention provides a technical solution: a method for helium extraction and ethane production from low-helium natural gas, comprising the following steps: Step 1, decarbonization of raw gas: CO2 is removed from the raw gas using an MDEA decarbonization device to reduce the CO2 content in the raw gas to below 50 ppm, thereby obtaining decarbonized raw gas.

[0033] Step Two: Raw Gas Dehydration and Mercury Removal: The decarbonized raw gas obtained in Step One is dehydrated using a three-tower isobaric molecular sieve and mercury removed using sulfur-impregnated activated carbon. During dehydration, the pressure difference between the raw gas and the regenerated gas is controlled to be greater than or equal to 70 kPa through flow control, and the amount of regenerated gas used is approximately 10% of the raw gas volume. After exiting the tower, the gas is filtered through a dust filter to obtain purified gas with a water content of less than or equal to 1 ppm and a mercury content of less than 0.01 μm / m³.

[0034] Step 3, Helium Extraction from Natural Gas: The helium-containing feed gas (with a helium content of 0.03% to 0.3% and a pressure of 1800 kPa·a to 2600 kPa·a) purified in Step 2 is pre-cooled to -104°C to -108°C in a pre-cooling box and then used as a heat source for the reboiler at the bottom of the first-stage enrichment tower to recover the cooling capacity at the bottom of the tower, reducing the temperature to -106°C to -111°C. After recovering the cooling capacity, the helium-containing feed gas passes through the first-stage enrichment tower, where most of the nitrogen and helium are separated from the top of the tower. The gas phase at the top of the first-stage enrichment tower (temperature of -130°C to -134°C and helium content of 2% to 4%) is cooled to -155°C to -160°C by the first-stage tower top cooler and then separated by the first-stage tower top separator to obtain primary crude helium (with a helium content of 20% to 40%). The bottom liquid phase of the primary enrichment column is separated into methane and C2+ components by a demethanizer. The top gas phase of the demethanizer, which is dry gas for export (temperature -107℃ to -112℃, methane content 97% to 99%, ethane content 0.1% to 2%, nitrogen content <1%, helium content <10ppm), is pre-cooled to 15℃ to 40℃ before being exported. The obtained primary crude helium is cooled to -175℃ to -182℃ by the feed cooler of the secondary enrichment column. Most of the helium is then separated from the top of the secondary enrichment column. The top gas phase of the secondary enrichment column (temperature -185℃ to -190℃, helium content 50% to 55%) is cooled to -192℃ to -195℃ by the top cooler of the secondary column and separated by the top separator of the secondary column to obtain secondary crude helium (helium content 60% to 80%). The obtained secondary crude helium... Helium is cooled to -60°C to -70°C by the secondary column top cooler before entering the subsequent helium enrichment unit to produce high-purity helium. The liquid phase at the bottom of the secondary enrichment column (temperature -178°C to -185°C, methane content 8% to 10%, nitrogen content 88% to 90%, helium content <0.5%) is cooled to -168°C to -178°C by the secondary enrichment column feed cooler, and then cooled to 15°C to 40°C by the pre-cooling box before being vented at a high point.

[0035] Step 4, Ethane Recovery: The liquid phase component from the demethanizer in Step 3 is pressurized and then enters the deethanizer. The gas phase at the top of the deethanizer (temperature 3℃ to 8℃, ethane content 96% to 98%) enters the main cold box and is cooled to 0℃ to 3℃. After separation and pressurization to 3200 kPa.a, it is divided into two streams. The first stream, accounting for 48% to 52%, is cooled to -93℃ to -97℃ in the main cold box and then throttled to 150 kPa.a as ethane product. The other stream, accounting for 48% to 52%, is returned to the deethanizer as top reflux.

[0036] Step 5, condensate fractionation: The bottom liquid phase from the ethane stripper in Step 4 (temperature 122℃ to 126℃, pressure 2600 kPa·a to 3000 kPa·a) is used as feed to the LPG tower and separated into C3, C4, and C5+ components. The top gas phase of the LPG tower is cooled, separated, and pressurized to 1700 kPa·a to 1900 kPa·a before being divided into two streams. The first stream, accounting for about 40%, is used as LPG product, and the other stream, accounting for about 60%, is returned to the LPG tower as reflux. The liquid phase from the LPG tower is cooled and used as a stabilized light hydrocarbon product.

[0037] Specifically, the precooling cold box, the first-stage tower top cooler, and the second-stage tower top cooler can use an external mixed refrigerant refrigeration compressor unit. The precooling cold box uses a precooling mixed refrigerant, the first-stage tower top cooler uses a first-stage mixed refrigerant, and the second-stage tower top cooler uses a second-stage mixed refrigerant.

[0038] More specifically, the pre-cooling mixed refrigerant includes methane, isopentane, nitrogen, and ethylene; the primary mixed refrigerant includes methane, propane, isopentane, and ethylene; and the secondary mixed refrigerant includes methane and nitrogen.

[0039] Specifically, based on the temperature differences between the reabsorption section, rectification section, and stripping section of the demethanizer, different amounts of raw materials are extracted in a gradient for cold energy recovery and utilization. The demethanizer is equipped with three liquid side streams for cold energy recovery, and the side streams are returned to the demethanizer after the cold energy is recovered in the pre-cooling box.

[0040] In use: Raw material gas decarbonization: The MDEA decarbonization device is used to remove CO2 from the raw material gas, reducing the CO2 content to below 50ppm, and obtaining decarbonized raw material gas.

[0041] Raw gas dehydration and mercury removal: The decarbonized raw gas is dehydrated using a three-tower isobaric molecular sieve and mercury removed using sulfur-impregnated activated carbon. During dehydration, the pressure difference between the raw gas and the regenerated gas is controlled by flow rate control to maintain a pressure ≥70 kPa, and the amount of regenerated gas used is approximately 10% of the raw gas volume. After exiting the tower, the gas is filtered through a dust filter to obtain purified gas with a water content ≤1 ppm and a mercury content <0.01 μm / m3.

[0042] The helium-containing feedstock natural gas 6 (flow rate 5255 kmole / h, pressure 2200 kPa.a, temperature 40℃, helium content 0.18%), after being purified by decarbonization, dehydration and mercury removal, is pre-cooled in a pre-cooling box and then used as a heat source for heat exchange in the reboiler at the bottom of the first-stage enrichment column to reduce the temperature to -109℃. After being rectified in the first-stage enrichment column (operating pressure 1800 kPa.a to 2100 kPa.a), the gas phase at the top of the column (temperature -131.5℃) is cooled to -158℃ by the first-stage column top cooler and separated by the first-stage column top separator to obtain primary crude helium (helium content 24%). The primary crude helium is cooled to -177°C by the feed cooler of the secondary enrichment column, and then further refined in the secondary enrichment column (operating pressure 400 kPa·a to 600 kPa·a). The vapor phase at the top of the column (temperature -187°C) is cooled to -193°C by the secondary column top cooler, and then separated by the secondary column top separator to obtain secondary crude helium (helium content 70.5%). The secondary crude helium is then cooled to -64°C by the secondary column top cooler before entering the subsequent helium enrichment unit to produce high-purity helium.

[0043] Please see Figure 2 and Figure 7 The bottom liquid phase of the primary concentration tower is separated by distillation in the demethanizer (operating pressure 1800 kPa·a to 2100 kPa·a). The demethanizer has three side streams: Stream 8 plate 1 (extraction rate 1600 kmole / h, temperature -108.7℃), Stream 9 plate 1 (extraction rate 2100 kmole / h, temperature -105.7℃), and Stream 24 plate 1 (extraction rate 250 kmole / h, temperature -16.9℃). After recovering the cold energy in the pre-cooling box, the streams are returned to the demethanizer, namely Stream 8 plate 3 (temperature -86℃), Stream 9 plate 1 (temperature -55℃), and Stream 24 plate 3 (temperature 5℃).

[0044] The precooling cold box, primary overhead cooler, and secondary overhead cooler shown employ external mixed refrigerant compressor units. The precooling mixed refrigerant includes methane (40% to 60%), isopentane (5% to 15%), and ethylene (15% to 30%). The primary mixed refrigerant includes methane (25% to 45%), propane (10% to 30%), isopentane (5% to 15%), nitrogen (10% to 30%), and ethylene (10% to 20%). The secondary mixed refrigerant includes methane (10% to 25%) and nitrogen (75% to 90%). The compressors are: precooling mixed refrigerant compressor (power approximately 4437 kW), primary mixed refrigerant compressor (power approximately 3454 kW), and secondary mixed refrigerant compressor (power approximately 77 kW).

[0045] The liquid phase component of the demethanizer is pressurized and then enters the deethanizer. The gas phase at the top of the tower (temperature 3℃ to 8℃, ethane content 96% to 98%) enters the main cold box and is cooled to 0℃ to 3℃. After separation and pressurization to 3200 kPa·a, it is divided into two streams. The first stream, accounting for 48% to 52%, is cooled to -93℃ to -97℃ in the main cold box and then throttled to 150 kPa·a as ethane product (yield 117.4 kmol / h, temperature approximately -93.21℃). The other stream, accounting for 48% to 52%, is returned to the deethanizer as top reflux.

[0046] The bottom liquid phase of the ethane stripper (temperature 122℃ to 126℃, pressure 2600 kPa·a to 3000 kPa·a) is used as feed to the LPG tower, where it is separated into C3, C4, and C5+ components. The top gas phase of the LPG tower is cooled, separated, and pressurized to 1700 kPa·a to 1900 kPa·a before being divided into two streams. The first stream, accounting for about 40%, is used as LPG product (yield of 33.9 kmol / h, pressure of about 1800 kPa·a), while the other stream, accounting for about 60%, is returned to the LPG tower as reflux. The liquid phase of the LPG tower is cooled and used as a stabilized light hydrocarbon product (yield of about 21.15 kmol / h, pressure of about 1500 kPa·a).

[0047] In the above embodiment, the deep-purified raw natural gas has a processing capacity of 300×10⁴ Nm³ / d, a helium content of 0.18%, a primary crude helium concentration of 24%, a secondary crude helium concentration of 70.46% (yield of 13.33 kmole / h), a theoretical helium yield of 99.18%, a total energy consumption of approximately 7968 kW, and a unit crude helium product consumption of 24.85%. The ethane content in the raw gas is 2.29%, and ethane is produced simultaneously with helium extraction (yield of 117.4 kmol / h).

[0048] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for helium extraction and ethane production from low-helium-content natural gas, characterized in that, Includes the following steps: Step 1: Raw gas decarbonization: CO2 is removed from the raw gas using an MDEA decarbonization unit to obtain decarbonized raw gas. Step 2: Raw gas dehydration and mercury removal: The decarbonized raw gas obtained in Step 1 is dehydrated using a three-tower isobaric molecular sieve and mercury removed using sulfur-impregnated activated carbon. During dehydration, the pressure difference between the raw gas and regenerated gas is controlled by flow rate control to ensure it is greater than or equal to 70 kPa. The regenerated gas volume is 10% of the raw gas volume. The purified gas exiting the tower is filtered through a dust filter. Step 3: Helium extraction from natural gas: The helium-containing raw gas purified in Step 2 is pre-cooled to -104°C to -108°C in a pre-cooling box and then used as a heat source for the reboiler at the bottom of the first-stage enrichment tower to recover the cooling energy at the bottom of the tower, reducing the temperature to -1°C. The helium-containing feed gas, after recovering its cold energy, is heated from 06℃ to -111℃. Most of the nitrogen and helium are separated from the top of the primary enrichment tower. The gas phase at the top of the primary enrichment tower is cooled to -155℃ to -160℃ by a primary tower top cooler, and then separated by a primary tower top separator to obtain primary crude helium with a helium content of 20% to 40%. The liquid phase at the bottom of the primary enrichment tower is separated into methane and C2+ components by a demethanizer. The gas phase at the top of the demethanizer is dry gas for export and is pre-cooled to 15℃ to 40℃ after recovering its cold energy in a pre-cooling box before being exported. The obtained primary crude helium is cooled to -175℃ to -182℃ by a secondary enrichment tower feed cooler, and then most of the helium is separated from the top of the secondary enrichment tower. The gas phase at the top of the column is cooled to -192°C to -195°C by a secondary column top cooler, and then separated by a secondary column top separator to obtain secondary crude helium. The obtained secondary crude helium is then cooled to -60°C to -70°C by a secondary column top cooler before entering the subsequent helium enrichment unit to produce high-purity helium. The liquid phase at the bottom of the secondary enrichment column is cooled to -168°C to -178°C by a secondary enrichment column feed cooler, and then cooled to 15°C to 40°C by a pre-cooling box before being vented at a high point. Step four, ethane recovery: The liquid phase component from the demethanizer in step three is pressurized and enters the deethanizer. The gas phase at the top of the deethanizer enters the main cold box and is cooled to 0°C to 3°C, then separated and pressurized to 3200 kPa. The product is divided into two streams. The first stream, accounting for 48% to 52%, is cooled to -93°C to -97°C in the main cold box and then throttled to 150 kPa·a as ethane product. The second stream, accounting for 48% to 52%, is returned to the de-ethanizer as top reflux. Step five, condensate fractionation: the bottom liquid phase from the de-ethanizer in step four is used as feed to the LPG tower, where C3, C4, and C5+ components are separated. The top gas phase of the LPG tower is cooled, separated, and pressurized to 1700 kPa·a to 1900 kPa·a before being divided into two streams. The first stream, accounting for 40%, is used as LPG product, and the second stream, accounting for 60%, is returned to the LPG tower as reflux. The liquid phase from the LPG tower is cooled and used as a stable light hydrocarbon product.

2. The method for helium extraction and ethane production from low-helium natural gas according to claim 1, characterized in that: The precooling cold box, the first-stage tower top cooler, and the second-stage tower top cooler all use external mixed refrigerant refrigeration compressor units. The precooling cold box uses precooling mixed refrigerant, the first-stage tower top cooler uses first-stage mixed refrigerant, and the second-stage tower top cooler uses second-stage mixed refrigerant.

3. The method for helium extraction and ethane production from low-helium natural gas according to claim 2, characterized in that: The precooling mixed refrigerant includes methane, isopentane, nitrogen, and ethylene; the primary mixed refrigerant includes methane, propane, isopentane, and ethylene; and the secondary mixed refrigerant includes methane and nitrogen.

4. The method for helium extraction and ethane production from low-helium natural gas according to claim 1, characterized in that: Based on the temperature differences in the reabsorption section, rectification section, and stripping section of the demethanizer, different amounts of raw materials are extracted in a gradient for cold energy recovery and utilization. The demethanizer is equipped with three liquid side streams for cold energy extraction. After the side streams are pre-cooled in a cold box to recover cold energy, they are returned to the demethanizer.

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