Device and method for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration

Abstract

The present invention discloses a device and method for denitrifying and recovering ethane from natural gas by using a mixed refrigerant, which solves the technical problems in the prior art that when the nitrogen content in natural gas is high, the calorific value of its combustion will be affected, the energy consumption in the gathering and transportation process will be high, and the natural gas cannot be directly used as a raw material for certain chemical products and a vehicle fuel. The device comprises a main heat exchanger (1) and a low-temperature separator (2) which are connected in sequence through a cold-insulating pipeline; the top and bottom of the low-temperature separator (2) are respectively provided with a gas phase outlet and a liquid phase outlet, and the gas phase outlet and the liquid phase outlet of the low-temperature separator (2) are respectively connected to a gas phase processing line and a liquid phase processing line. The present invention provides a device and method for denitrifying and recovering ethane from natural gas by using a mixed refrigerant, which has the advantages of simple system operation, high system operation flexibility, adaptability to different load conditions, low energy consumption, reasonable investment, and can not only improve the calorific value of natural gas, but also recover liquid ethane and liquefied petroleum gas, and has great economic benefits.

Description

Technical Field

[0001] The invention relates to the technical field of natural gas processing technology, and in particular to a device and method for denitrifying and recovering ethane by using a mixed refrigerant to refrigerate natural gas. Background Art

[0002] Natural gas plays an increasingly important role in people's lives and industrial production. The efficient use of natural gas has become an important measure for many countries to adjust their energy structure. In addition to methane, natural gas also contains ethane, propane, C4 + and nitrogen and other components.

[0003] When the nitrogen content in natural gas is high, it has the following effects: ① It will affect its calorific value. The calorific value of conventional pipeline natural gas is generally 8500-9000Kcal / Nm 3 When the nitrogen content is greater than 10%, the calorific value will be lower than 7800Kcal / Nm 3 , compared with conventional pipeline transportation, the natural gas content is lower than 13%; ② It will cause high energy consumption in the pipeline gathering and transportation process. During the pipeline gathering and transportation process, the natural gas often needs to be pressurized. When the nitrogen content increases to 10%, the relative energy consumption will increase by 3-5%; ③ It cannot be directly used as a raw material for certain chemical products and vehicle fuel.

[0004] On the other hand, in long-distance natural gas pipelines, with the advancement of technology, the transmission pressure is higher; under high pressure and certain temperature conditions, liquid hydrocarbons may precipitate from the natural gas in the pipeline. Therefore, GB50251 stipulates that the gas entering the gas pipeline must be free of mechanical impurities; the general hydrocarbon dew point should be lower than the minimum transmission ambient temperature, etc. + If the hydrocarbon dew point rises by 5%, it will generally rise by 1-5°C; therefore, natural gas contains more propane and C4 + When transported in pipelines, it will affect the dew point of hydrocarbons and condense in the pipeline, affecting safe transportation.

[0005] In order to improve the quality of natural gas and meet the requirements of hydrocarbon dew point for pipeline gas, it is necessary to remove nitrogen from natural gas. + Recycle and reuse to improve the overall economic benefits of natural gas utilization. Summary of the invention

[0006] The purpose of the present invention is to provide a device and method for denitrifying and recovering ethane in natural gas using a mixed refrigerant to solve the technical problems in the prior art that when the nitrogen content in natural gas is high, it will affect its combustion calorific value, cause high energy consumption during the gathering and transportation process, and cannot be directly used as a raw material for certain chemical products and a vehicle fuel.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a mixed refrigerant refrigeration natural gas denitrification and ethane recovery device, comprising a main heat exchanger and a low-temperature separator connected in sequence through a cold-insulating pipeline; the top and bottom of the low-temperature separator are respectively provided with a gas phase outlet and a liquid phase outlet, and the gas phase outlet and the liquid phase outlet of the low-temperature separator are respectively connected to a gas phase processing circuit and a liquid phase processing circuit; wherein,

[0009] The main heat exchanger is provided with a raw gas channel I, a natural gas channel I, a raw gas channel II, a nitrogen-rich gas channel I, a natural gas channel II, a high-pressure refrigerant channel I, a high-pressure refrigerant channel II, a reflux refrigerant channel and a liquid ethane subcooling channel;

[0010] The inlet of the raw gas channel I of the main heat exchanger is connected to the inlet pipe of the raw natural gas;

[0011] The gas phase processing circuit includes a denitrification tower, a denitrification tower condenser, and a denitrification tower reflux tank which are sequentially connected by a cold-keeping pipeline along the air inlet direction; the gas phase outlet of the low-temperature separator is connected to the inlet of the raw gas channel II, and the inlet of the denitrification tower is connected to the outlet of the raw gas channel II; the gas phase outlet of the denitrification tower reflux tank is connected to the inlet of the nitrogen-rich gas channel I, and the outlet of the nitrogen-rich gas channel I is connected to an external nitrogen-rich fuel pipeline; the liquid phase outlet at the bottom of the denitrification tower is connected to the inlet of the natural gas channel II, and the outlet of the natural gas channel II is connected to an external product natural gas pipeline;

[0012] The liquid phase processing pipeline includes a demethanizer, a deethanizer, a deethanizer condenser, and a deethanizer reflux tank which are sequentially connected by a cold-insulating pipeline along the liquid inlet direction; the liquid phase outlet of the demethanizer is connected to the inlet of the middle part of the deethanizer, the gas phase outlet of the deethanizer is connected to the deethanizer condenser, the gas phase outlet of the deethanizer reflux tank is connected to the inlet of the liquid ethane supercooling channel of the main heat exchanger, and the outlet of the liquid ethane supercooling channel is connected to an external liquid ethane storage device; the gas phase outlet of the demethanizer is connected to the inlet of the natural gas channel I, and the outlet of the natural gas channel I is connected to an external product natural gas pipeline;

[0013] It also includes a mixed refrigerant refrigeration cycle loop that provides heat sources for the denitrification tower and the demethanization tower, and a propane compression condensing unit that provides a cold source for the deethanization tower;

[0014] It also includes a control device, and the main heat exchanger, low-temperature separator, denitrogenation tower, denitrogenation tower condenser, denitrogenation tower reflux tank, demethanizer, deethanizer, deethanizer condenser, and deethanizer reflux tank are electrically connected to the control device respectively.

[0015] Furthermore, the liquid phase outlet of the deethanizer is connected to a liquefied petroleum gas storage device via a cold-insulating pipeline, and a liquefied petroleum gas cooler is connected to the cold-insulating pipeline.

[0016] Furthermore, the gas phase treatment circuit also includes a denitrogenation tower reboiler for controlling the nitrogen content in the denitrogenation tower kettle, and a liquid collector 1 is built into the lower part of the denitrogenation tower, the outlet of the liquid collector 1 is connected to the inlet of the denitrogenation tower reboiler, and the outlet of the denitrogenation tower reboiler is connected to the inlet of the lower part of the denitrogenation tower; the denitrogenation tower reboiler is electrically connected to the control device.

[0017] Furthermore, the liquid phase processing circuit also includes a demethanizer side boiler for controlling the methane content in the demethanizer kettle and the liquid phase in the kettle. The middle and lower parts of the demethanizer are both equipped with liquid collectors 2, and the outlets of the two liquid collectors 2 are both connected to the inlet of the demethanizer side boiler. The outlet of the demethanizer side boiler is respectively connected to the inlet of the middle and lower parts of the demethanizer, and the demethanizer side boiler is electrically connected to the control device.

[0018] Furthermore, the liquid phase processing circuit also includes a deethanizer reboiler for controlling the ethane content in the deethanizer kettle, a liquid collector three is built in the lower part of the deethanizer, the outlet of the liquid collector three is connected to the inlet of the deethanizer reboiler, the outlet of the deethanizer reboiler is connected to the inlet of the lower part of the deethanizer, and the deethanizer reboiler is electrically connected to the control device.

[0019] Furthermore, the mixed refrigerant refrigeration cycle loop includes a mixed refrigerant compressor unit, a demethanizer side boiler, a high-pressure refrigerant channel I, a denitrification tower reboiler, a high-pressure refrigerant channel II, a denitrification tower condenser, and a reflux refrigerant channel which are sequentially connected along the refrigerant circulation direction, and the outlet of the reflux refrigerant channel is connected to the inlet of the mixed refrigerant compressor unit; the demethanizer side boiler is provided with a high-pressure refrigerant channel V, a mixed hydrocarbon channel I and a mixed hydrocarbon channel II; the denitrification tower reboiler is provided with a natural gas channel and a high-pressure refrigerant channel III; the denitrification tower condenser is provided with a nitrogen-rich gas channel II and a mixed refrigerant channel; the deethanizer condenser is provided with an ethane condensation channel and a propane evaporation channel.

[0020] Furthermore, a pressure digital controller I and a pressure regulating valve a are provided on the pipeline at the gas phase outlet at the top of the demethanizer, and the pressure digital controller I and the pressure regulating valve a are electrically connected to the control device respectively;

[0021] The low-temperature separator is provided with a liquid level digital controller II, and a liquid level regulating valve b is provided on the pipeline of the liquid phase outlet at the bottom of the low-temperature separator. The liquid level digital controller II and the liquid level regulating valve b are electrically connected to the control device respectively;

[0022] The demethanizer is provided with a liquid level digital controller III, and a liquid level regulating valve c is provided on the pipeline of the liquid phase outlet at the bottom of the demethanizer; the liquid level digital controller III and the liquid level regulating valve c are electrically connected to the control device respectively;

[0023] The denitrification tower is provided with a liquid level digital controller IV, and a liquid level regulating valve e is provided on the pipeline of the liquid phase outlet at the bottom of the denitrification tower; the digital controller IV and the liquid level regulating valve e are electrically connected to the control device respectively;

[0024] A manual regulating valve for adjusting the temperature at the top of the denitrification tower is provided on the pipeline at the outlet of the high-pressure refrigerant channel II, and the manual regulating valve is electrically connected to the control device;

[0025] A pressure digital controller V is provided on the pipeline of the gas phase outlet at the top of the denitrification tower reflux tank, and a pressure regulating valve f is provided on the pipeline of the gas phase outlet at the top of the denitrification tower reflux tank, and the pressure digital controller V is electrically connected to the pressure regulating valve f;

[0026] The deethanizer is provided with a liquid level digital controller VII, and a liquid level regulating valve h is provided on the pipeline of the liquid phase outlet of the deethanizer, and the liquid level digital controller VII is electrically connected to the liquid level regulating valve h;

[0027] A pressure digital controller VIII is arranged on the pipeline of the gas phase outlet at the top of the deethanizer reflux tank, and a pressure regulating valve j is arranged on the pipeline of the gas phase outlet at the top of the deethanizer reflux tank. The pressure digital controller VIII and the pressure regulating valve j are electrically connected.

[0028] Furthermore, the denitrification tower, demethanization tower and deethanization tower are packed towers or plate towers.

[0029] The method for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration provided by the present invention is carried out by using the aforementioned device for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration, and specifically comprises the following steps:

[0030] S1, purified gas containing methane, ethane, propane, nitrogen and a small amount of C4 + The raw natural gas enters the raw gas channel I, the purified natural gas in the raw gas channel I is cooled and partially condensed, and enters the low-temperature separator for gas-liquid separation; the gas phase separated at the top enters the raw gas channel II for cooling and condensation, and then enters the denitrification tower for denitrification; the liquid phase at the bottom of the low-temperature separator enters the demethanizer, and the liquid in the demethanizer is led to the side boiler of the demethanizer for heating to control the methane content in the demethanizer tower and the liquid phase in the bottom of the tower. The heat source of the side boiler of the demethanizer comes from the mixed refrigerant refrigeration cycle loop, and the gas phase obtained at the top of the demethanizer enters the natural gas channel I for reheating to room temperature and then is used as the product external natural gas and output through the product natural gas pipeline; the liquid phase obtained at the bottom of the demethanizer enters the middle part of the deethanizer for distillation;

[0031] S2, after being distilled through the deethanizer, an ethane-rich gas phase is obtained at its top, and after being cooled and partially condensed in the deethanizer condenser, it enters the deethanizer reflux tank for gas-liquid separation, and the gas phase is ethane with a volume percentage greater than 95%, enters the liquid ethane supercooling channel, and after being cooled, condensed and supercooled in the liquid ethane supercooling channel, it is stored in the liquid ethane storage device; the liquid phase separated by the deethanizer reflux tank is used as the reflux liquid of the deethanizer, and the cold capacity of the deethanizer tower top comes from the propane compression condensing unit; the liquid in the deethanizer bottom is drawn out to the deethanizer reboiler of the deethanizer for heating, and the ethane content of the deethanizer tower kettle is controlled. The heat source of the tower kettle is provided by the outside world, and the liquefied petroleum gas product obtained in the tower kettle is cooled to room temperature by the liquefied petroleum gas cooler and then stored in the liquefied petroleum gas storage device;

[0032] S3. The nitrogen-containing raw gas from the raw gas channel II is distilled and denitrogenated in the denitrogenation tower. The nitrogen-rich gas obtained at the top thereof, whose components are methane and nitrogen, is successively reheated through the nitrogen-rich gas channel II, the denitrogenation tower reflux tank, and the nitrogen-rich gas channel I as nitrogen-rich fuel gas, and is output through the nitrogen-rich fuel pipeline; the liquid at the bottom of the denitrogenation tower is led out to the denitrogenation tower reboiler of the denitrogenation tower for heating, and the nitrogen content of the denitrogenation tower kettle is controlled to be less than 1%. The heat source of the tower kettle is provided by the mixed refrigerant refrigeration cycle loop. The liquefied natural gas produced in the denitrogenation tower kettle, whose component is mainly methane, enters the natural gas channel II for reheating, and is used as the denitrogenated product external transmission natural gas together with the demethanization tower top gas after reheating in step S1, and is output through the product natural gas pipeline.

[0033] Furthermore, the cold energy for natural gas denitrification and methane recovery comes from the mixed refrigerant refrigeration cycle, specifically: first, the low-pressure mixed refrigerant from the reflux refrigerant channel is compressed and cooled by the mixed refrigerant compressor unit, enters the high-pressure refrigerant channel V to provide a heat source for the demethanizer, then enters the high-pressure refrigerant channel I to be partially condensed after being cooled, enters the high-pressure refrigerant channel III to provide a heat source for the denitrification tower, then enters the high-pressure refrigerant channel II to be cooled, condensed and liquefied, and after reducing pressure and throttling through the manual regulating valve d, enters the mixed refrigerant channel to provide a cold source for the top of the denitrification tower, enters the reflux refrigerant channel to provide cold energy for the positive flow channel of the main heat exchanger for rewarming, and then returns to the mixed refrigerant compressor unit to complete the closed cycle.

[0034] Based on the above technical solution, the embodiments of the present invention can at least produce the following technical effects:

[0035] The present invention provides a device and method for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration, which has the advantages of simple system operation, large system operation flexibility, adaptability to different load conditions, low energy consumption, reasonable investment, and can not only improve the calorific value of natural gas, but also recover liquid ethane and liquefied petroleum gas, and has great economic benefits. Specifically:

[0036] (1) Low energy consumption: natural gas denitrification and C2 + The refrigeration cycle of multi-component mixed refrigerant is used to provide cooling capacity. The refrigeration cycle of multi-component mixed refrigerant has the advantages of simple process, strong adaptability, easy operation and relatively low power consumption. In addition, C2 + Separation, using propane compression condensing unit to provide cooling capacity, this process achieves different temperature sections, different cooling capacity distribution, reducing the energy consumption of the whole device;

[0037] (2) High economic benefits and practical value: This process not only improves the quality of natural gas, but also can recover liquid ethane and liquefied petroleum gas, solving the problem of transporting nitrogen-rich gas;

[0038] (3) The process is simple and easy to operate. This process uses a closed multi-component mixed refrigerant refrigeration cycle and a propane compression condensing unit refrigeration cycle. The two systems are relatively independent, simple to operate and easy to start;

[0039] (4) Easy maintenance and reliable operation. This device uses fewer power equipment, which facilitates equipment maintenance and reduces equipment failures;

[0040] (5) In addition, the device has the advantages of safety, reliability and wide practicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a connection diagram of an embodiment of the present invention;

[0042] In the figure: 1. Main heat exchanger; 2. Low temperature separator; 3. Denitrification tower; 4. Denitrification tower condenser; 5. Denitrification tower reflux tank; 6. Denitrification tower reboiler; 7. Demethanizer; 8. Demethanizer side boiler; 9. Deethanizer; 10. Deethanizer condenser; 11. Deethanizer reflux tank; 12. Deethanizer reboiler; 13. LPG cooler; 14. Mixed refrigerant compressor unit; 15. Pressure digital controller I; 16. Pressure regulating valve a; 17. Liquid level digital controller II; 18. Liquid level regulating valve b; 19. Liquid level digital controller III; 20. Liquid level regulating valve c; 21. Liquid level digital controller IV; 22. Liquid level regulating valve e; 23. Manual regulating valve d; 24. Pressure digital controller V; 25. Pressure regulating valve f; 26. Liquid level digital controller VII; 27. Liquid level regulating valve h; 28. Pressure digital controller VIII; 29. ​​Pressure regulating valve j; 30. Liquid ethane storage device; 31. Propane compression condensing unit; 32. Liquefied petroleum gas storage device; 33. Nitrogen-rich fuel pipeline; 34. Product natural gas pipeline; 35. Inlet pipe; 36. Temperature digital controller VI; 37. Regulating valve g;

[0043] A1, raw gas channel I; A2, natural gas channel I; A3, raw gas channel II; A4, nitrogen-rich gas channel I; A5, natural gas channel II; A6, high-pressure refrigerant channel I; A7, high-pressure refrigerant channel II; A8, reflux refrigerant channel; A9, liquid ethane supercooling channel; B1, nitrogen-rich gas channel II; B2, mixed refrigerant channel; C1, natural gas channel; C2, high-pressure refrigerant channel III; D1, high-pressure refrigerant channel V; D2, mixed hydrocarbon channel I; D3, mixed hydrocarbon channel II; E1, ethane condensation channel; E2, propane evaporation channel. DETAILED DESCRIPTION

[0044] like Figure 1 As shown:

[0045] Example 1

[0046] A device for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration, comprising a main heat exchanger 1 and a low-temperature separator 2 connected in sequence through a cold-insulating pipeline; a gas phase outlet and a liquid phase outlet are respectively provided at the top and the bottom of the low-temperature separator 2, and a gas phase processing circuit and a liquid phase processing circuit are respectively connected to the gas phase outlet and the liquid phase outlet of the low-temperature separator 2; wherein,

[0047] The main heat exchanger 1 is provided with a raw gas channel IA1, a natural gas channel IA2, a raw gas channel IIA3, a nitrogen-rich gas channel IA4, a natural gas channel IIA5, a high-pressure refrigerant channel IA6, a high-pressure refrigerant channel IIA7, a return refrigerant channel A8 and a liquid ethane subcooling channel A9;

[0048] The inlet of the raw gas channel I of the main heat exchanger is connected to the inlet pipe of the raw natural gas;

[0049] The gas phase processing circuit includes a denitrification tower 3, a denitrification tower condenser 4, and a denitrification tower reflux tank 5 which are connected in sequence through a cold-insulating pipeline along the air inlet direction; the gas phase outlet of the low-temperature separator 2 is connected to the inlet of the raw gas channel IIA3, and the inlet of the denitrification tower 3 is connected to the outlet of the raw gas channel IIA3; the gas phase outlet of the denitrification tower reflux tank 5 is connected to the inlet of the nitrogen-rich gas channel IIA4, and the outlet of the nitrogen-rich gas channel IIA4 is connected to the external nitrogen-rich fuel pipeline 33; the liquid phase outlet at the bottom of the denitrification tower 3 is connected to the inlet of the natural gas channel IIA5, and the outlet of the natural gas channel IIA5 is connected to the external product natural gas pipeline 34;

[0050] The liquid phase processing pipeline includes a demethanizer 7, a deethanizer 9, a deethanizer condenser 10, and a deethanizer reflux tank 11 which are connected in sequence through a cold-insulating pipeline along the liquid inlet direction; the liquid phase outlet of the demethanizer 7 is connected to the inlet of the middle part of the deethanizer 9, the gas phase outlet of the deethanizer 9 is connected to the deethanizer condenser 10, the gas phase outlet of the deethanizer reflux tank 11 is connected to the inlet of the liquid ethane supercooling channel A9 of the main heat exchanger 1, and the outlet of the liquid ethane supercooling channel A9 is connected to an external liquid ethane storage device; the gas phase outlet of the demethanizer 7 is connected to the inlet of the natural gas channel IA2, and the outlet of the natural gas channel IA2 is connected to an external product natural gas pipeline 34;

[0051] It also includes a mixed refrigerant refrigeration cycle loop that provides heat sources for the denitrification tower 3 and the demethanization tower 7, and a propane compression condensing unit 31 that provides a cold source for the deethanization tower 9;

[0052] It also includes a control device, and the main heat exchanger 1, the low-temperature separator 2, the denitrogenation tower 3, the denitrogenation tower condenser 4, the denitrogenation tower reflux tank 5, the demethanizer 7, the deethanizer 9, the deethanizer condenser 10, and the deethanizer reflux tank 11 are electrically connected to the control device respectively.

[0053] The present invention provides a device and method for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration, which has the advantages of simple system operation, large system operation flexibility, adaptability to different load conditions, low energy consumption, reasonable investment, and can not only improve the calorific value of natural gas, but also recover liquid ethane and liquefied petroleum gas, with great economic benefits. Specifically: low energy consumption: denitrification of natural gas and C2 + The refrigeration cycle of multi-component mixed refrigerant is used to provide cooling capacity. The refrigeration cycle of multi-component mixed refrigerant has the advantages of simple process, strong adaptability, easy operation and relatively low power consumption. In addition, C2 + The process realizes different temperature sections and different cooling capacity distribution, which reduces the energy consumption of the whole device. It has high economic benefits and practical value: this process not only improves the quality of natural gas, but also can recover liquid ethane and liquefied petroleum gas, solving the problem of nitrogen-rich gas transmission. The process is simple and easy to operate. This process adopts a closed multi-component mixed refrigerant refrigeration cycle and a propane compression condensing unit refrigeration cycle. The two systems are relatively independent, simple to operate and easy to start. It is easy to maintain and reliable in operation. The device uses less power equipment, which is convenient for equipment maintenance and reduces equipment failures. In addition, the device also has the advantages of safety, reliability and wide practicability.

[0054] As an optional embodiment, the liquid phase outlet of the deethanizer 9 is connected to a liquefied petroleum gas storage device via a cold-keeping pipeline, and a liquefied petroleum gas cooler 13 is connected to the cold-keeping pipeline.

[0055] As an optional embodiment, the gas phase treatment circuit also includes a denitrogenation tower reboiler 6 for controlling the nitrogen content in the kettle of the denitrogenation tower 3. A liquid collector 1 is built into the lower part of the denitrogenation tower 3, and the outlet of the liquid collector 1 is connected to the inlet of the denitrogenation tower reboiler 6. The outlet of the denitrogenation tower reboiler 6 is connected to the inlet of the lower part of the denitrogenation tower 3; the denitrogenation tower reboiler 6 is electrically connected to the control device.

[0056] As an optional embodiment, the liquid phase processing circuit also includes a demethanizer side boiler 8 for controlling the methane content in the bottom of the demethanizer 7 and the liquid phase in the bottom of the tower. The middle and lower parts of the demethanizer 7 are both equipped with liquid collectors 2, and the outlets of the two liquid collectors 2 are connected to the inlet of the demethanizer side boiler 8. The outlet of the demethanizer side boiler 8 is respectively connected to the inlet of the middle and lower parts of the demethanizer 7, and the demethanizer side boiler 8 is electrically connected to the control device.

[0057] As an optional embodiment, the liquid phase processing circuit also includes a deethanizer reboiler 12 for controlling the ethane content in the bottom of the deethanizer 9, and a liquid collector three is built into the lower part of the deethanizer 9, and the outlet of the liquid collector three is connected to the inlet of the deethanizer reboiler 12, and the outlet of the deethanizer reboiler 12 is connected to the inlet of the lower part of the deethanizer 9, and the deethanizer reboiler 12 is electrically connected to the control device.

[0058] As an optional embodiment, the mixed refrigerant refrigeration cycle loop includes a mixed refrigerant compressor unit 14, a demethanizer side boiler 8, a high-pressure refrigerant channel IIA6, a denitrification tower reboiler 6, a high-pressure refrigerant channel IIA7, a denitrification tower condenser 4, and a reflux refrigerant channel A8, which are sequentially connected along the refrigerant circulation direction, and the outlet of the reflux refrigerant channel A8 is connected to the inlet of the mixed refrigerant compressor unit 14; the demethanizer side boiler 8 is provided with a high-pressure refrigerant channel VD1, a mixed hydrocarbon channel ID2 and a mixed hydrocarbon channel IID3; the denitrification tower reboiler 6 is provided with a natural gas channel C1 and a high-pressure refrigerant channel IIIC2; the denitrification tower condenser 4 is provided with a nitrogen-rich gas channel IIB1 and a mixed refrigerant channel B2; the deethanizer condenser 10 is provided with an ethane condensation channel E1 and a propane evaporation channel E2.

[0059] As an optional embodiment, a pressure digital controller Ⅰ15 and a pressure regulating valve a16 are provided on the pipeline at the gas phase outlet at the top of the demethanizer 7, and the pressure digital controller Ⅰ15 and the pressure regulating valve a16 are electrically connected to the control device respectively;

[0060] The low-temperature separator 2 is provided with a liquid level digital controller II17, and a liquid level regulating valve b18 is provided on the pipeline of the liquid phase outlet at the bottom of the low-temperature separator 2. The liquid level digital controller II17 and the liquid level regulating valve b18 are electrically connected to the control device respectively;

[0061] The demethanizer 7 is provided with a liquid level digital controller III19, and a liquid level regulating valve c20 is provided on the pipeline of the liquid phase outlet at the bottom of the demethanizer 7; the liquid level digital controller III19 and the liquid level regulating valve c20 are electrically connected to the control device respectively;

[0062] The denitrification tower 3 is provided with a liquid level digital controller IV21, and a liquid level regulating valve e22 is provided on the pipeline of the liquid phase outlet at the bottom of the denitrification tower 3; the digital controller IV21 and the liquid level regulating valve e22 are electrically connected to the control device respectively;

[0063] A manual regulating valve d for regulating the top temperature of the denitrification tower 3 is provided on the pipeline at the outlet of the high-pressure refrigerant channel ⅡA7, and the manual regulating valve d is electrically connected to the control device;

[0064] A pressure digital controller V24 is provided on the pipeline of the gas phase outlet at the top of the denitrification tower reflux tank 5, and a pressure regulating valve f25 is provided on the pipeline of the gas phase outlet at the top of the denitrification tower reflux tank 5. The pressure digital controller V24 and the pressure regulating valve f25 are electrically connected to the control device respectively;

[0065] The deethanizer 9 is provided with a liquid level digital controller VII26, and a liquid level regulating valve h27 is provided on the pipeline of the liquid phase outlet of the deethanizer 9, and the liquid level digital controller VII26 and the liquid level regulating valve h27 are electrically connected to the control device respectively;

[0066] A pressure digital controller VIII 28 is arranged on the pipeline of the gas phase outlet at the top of the deethanizer reflux tank 11, and a pressure regulating valve j29 is arranged on the pipeline of the gas phase outlet at the top of the deethanizer reflux tank 11. The pressure digital controller VIII 28 and the pressure regulating valve j29 are electrically connected to the control device respectively.

[0067] A temperature digital controller VI36 is provided on the outlet pipeline of the ethane condensation channel E1, and a regulating valve g is provided on the inlet pipeline of the propane evaporation channel E2. The temperature digital controller VI36 and the regulating valve g are electrically connected to the control device respectively; the temperature of the ethane-rich gas phase at the outlet of the ethane condensation channel E1 is measured by the temperature digital controller VI to control the opening of the regulating valve g, thereby controlling the temperature of the ethane-rich gas phase at the outlet of the ethane condensation channel E1, that is, the regulating valve g controls the amount of propane entering the propane evaporation channel E2 to control the temperature of the ethane-rich gas phase.

[0068] As an optional embodiment, the denitrification tower 3, the demethanizer 7 and the deethanizer 9 are packed towers or plate towers.

[0069] All components are connected by cold-insulated pipelines. The specific connection methods are as follows:

[0070] The inlet of the raw gas channel IA1 is connected to the inlet pipe 35 of the purified raw natural gas from the outside, the outlet of the raw gas channel IA1 is connected to the feed port 2-A of the low-temperature separator 2, and the liquid phase outlet 2-B of the low-temperature separator 2 is connected to the feed port 7-A of the demethanizer 7;

[0071] The inlet of the natural gas channel IA2 is connected to the gas phase outlet 7-B at the top of the demethanizer 7, and the outlet of the natural gas channel IA2 is connected to the external product natural gas pipeline 34;

[0072] The inlet of the raw gas channel ⅡA3 is connected to the gas phase outlet 2-C of the low-temperature separator 2, and the outlet of the raw gas channel ⅡA3 is connected to the feed port 3-A of the denitrification tower 3;

[0073] The inlet of the nitrogen-rich gas channel IA4 is connected to the gas phase outlet 5-B of the denitrification tower reflux tank 5, and the outlet of the nitrogen-rich gas channel IA4 is connected to the external nitrogen-rich fuel pipeline 33;

[0074] The inlet of the natural gas channel IIA5 is connected to the liquid phase outlet 3-B of the denitrification tower 3, and the outlet of the natural gas channel IIA5 is connected to the external product natural gas pipeline 34;

[0075] The inlet of the high-pressure refrigerant channel IA6 is connected to the outlet of the high-pressure refrigerant channel VD1, and the outlet of the high-pressure refrigerant channel IA6 is connected to the inlet of the high-pressure refrigerant channel IIIC2;

[0076] The inlet of the high-pressure refrigerant channel IIA7 is connected to the outlet of the high-pressure refrigerant channel IIIC2, and the outlet of the high-pressure refrigerant channel IIA7 is connected to the inlet of the mixed refrigerant channel B2;

[0077] The inlet of the return refrigerant channel A8 is connected to the outlet of the mixed refrigerant channel B2, and the outlet of the return refrigerant channel A8 is connected to the inlet of the mixed refrigerant compressor unit 14;

[0078] The inlet of the liquid ethane supercooling channel A9 is connected to the gas phase outlet 11-B of the deethanizer reflux tank 11, and the outlet of the liquid ethane supercooling channel A9 is connected to the external liquid ethane storage pipeline to recover the liquid ethane to the liquid ethane storage device 30 for storage;

[0079] The inlet of the nitrogen-rich gas channel ⅡB1 is connected to the gas phase outlet 3-C of the denitrification tower 3, the outlet of the nitrogen-rich gas channel ⅡB1 is connected to the feed port 5-A of the denitrification tower reflux tank 5, and the liquid phase outlet 5-C of the denitrification tower reflux tank 5 is connected to the feed port 3-D of the denitrification tower 3;

[0080] The inlet of the natural gas channel C1 is connected to the outlet 3-E of the denitrification tower 3, and the outlet of the natural gas channel C1 is connected to the inlet 3-F of the denitrification tower 3;

[0081] The inlet of the high-pressure refrigerant channel VD1 is connected to the outlet of the mixed refrigerant compressor unit 14, the inlet of the mixed hydrocarbon channel ID2 is connected to the outlet 7-F of the demethanizer 7, the outlet of the mixed hydrocarbon channel ID2 is connected to the inlet 7-G of the demethanizer 7, the inlet of the mixed hydrocarbon channel IID3 is connected to the outlet 7-D of the demethanizer 7, the outlet of the mixed hydrocarbon channel IID3 is connected to the outlet 7-E of the demethanizer 7, and the liquid phase outlet 7-C of the demethanizer 7 is connected to the feed port 9-A of the deethanizer 9;

[0082] The inlet of the ethane condensation channel E1 is connected to the gas phase outlet 9-B of the deethanizer 9, and the outlet of the ethane condensation channel E1 is connected to the feed port 11-A of the deethanizer reflux tank 11;

[0083] The inlet of the propane evaporation channel E2 is connected to the outlet of the propane compression condensing unit 31, and the outlet of the propane evaporation channel E2 is connected to the inlet of the propane compression condensing unit 31;

[0084] The liquid phase outlet 11-C of the deethanizer reflux tank 11 is connected to the inlet 9-C of the deethanizer 9, the inlet of the deethanizer reboiler 12 is connected to the outlet 9-E of the deethanizer 9, the outlet of the deethanizer reboiler 12 is connected to the inlet 9-F of the deethanizer 9, the outlet 9-D of the deethanizer 9 is connected to the inlet of the liquefied petroleum gas cooler 13, and the outlet of the liquefied petroleum gas cooler 13 is connected to the liquefied petroleum gas storage pipeline to recover the liquefied petroleum to the liquefied petroleum gas storage device 32 for storage.

[0085] Embodiment 2:

[0086] A method for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration is carried out using the device for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration described in Example 1, comprising the following steps:

[0087] S1, the purified raw natural gas containing methane, ethane, propane, nitrogen and a small amount of C4+ enters the raw gas channel IA1, in which the purified natural gas is cooled and partially condensed by the reflux mixed refrigerant, and enters the low-temperature separator 2 for gas-liquid separation; the gas phase separated at the top enters the raw gas channel IA3 for cooling and condensation, and then enters the denitrification tower 3 for denitrification; the liquid phase at the bottom of the low-temperature separator 2 enters the demethanizer 7 after passing through the liquid level regulating valve b. The middle and bottom of the demethanizer 7 are equipped with liquid collectors to collect the liquid. The liquid in the middle and bottom is led to the side boiler 8 of the demethanizer for heating to control the methane content in the demethanizer 7 and the liquid phase in the bottom of the tower. The heat source of the side boiler 8 of the demethanizer comes from the high-pressure mixed refrigerant at the outlet of the mixed refrigerant compressor unit 14. The gas phase obtained at the top of the demethanizer 7 is mainly methane and a small amount of ethane. After being reduced in pressure by the pressure regulating valve a, it enters the natural gas channel IA2 for reheating to room temperature and is used as a product for external transmission. It is output through the product natural gas pipeline 34; the main components of the liquid phase obtained at the bottom of the demethanizer 7 are ethane, propane and C4 + , after being decompressed by the liquid level regulating valve c20, it enters the middle part of the deethanizer 9 for distillation;

[0088] S2, after being distilled in the deethanizer 9, an ethane-rich gas phase is obtained at the top thereof, and after being cooled and partially condensed in the ethane condensation channel E1 of the deethanizer condenser 10, it enters the deethanizer reflux tank 11 for gas-liquid separation, and the gas phase is ethane with a volume percentage of more than 95%, and enters the liquid ethane supercooling channel A9, and after being cooled, condensed and supercooled in the liquid ethane supercooling channel A9, it is stored in the liquid ethane storage device 30; the liquid phase separated in the deethanizer reflux tank 11 is used as the reflux liquid of the deethanizer 9, and the deethanizer 9 The cooling capacity of the tower top comes from the propane compression condensing unit 31; the bottom of the deethanizer 9 is built with a liquid collector, and the liquid at the bottom of the deethanizer 9 is led to the deethanizer reboiler 12 of the deethanizer 9 for heating, and the ethane content of the deethanizer 9 is controlled to be lower than 1.5%. The heat source of the deethanizer 9 is provided by the outside, such as: 0.5Mpag saturated steam or 180℃ heat transfer oil. The liquefied petroleum gas product obtained in the tower bottom is cooled to room temperature by the liquefied petroleum gas cooler 13 and then stored in the liquefied petroleum gas storage device 32;

[0089] S3, the nitrogen-containing raw gas from the raw gas channel ⅡA3 is distilled and denitrified in the denitrification tower 3, and the nitrogen-rich gas obtained at the top thereof, which is composed of methane and nitrogen, is successively reheated through the nitrogen-rich gas channel ⅡB1, the denitrification tower reflux tank 5, and the nitrogen-rich gas channel IA4 as nitrogen-rich fuel gas, and is output through the nitrogen-rich fuel pipeline 33; the bottom of the denitrification tower 3 is built-in with a liquid collector, and the liquid at the bottom of the denitrification tower 3 is led out to the denitrification tower reboiler 6 of the denitrification tower 3 for heating, and the nitrogen content of the bottom of the denitrification tower 3 is controlled to be less than 1%. The heat source of the tower bottom is provided by the mixed refrigerant refrigeration cycle loop. The liquefied natural gas produced in the bottom of the denitrification tower 3, which is mainly composed of methane, is reduced in pressure by the liquid level regulating valve e, enters the natural gas channel ⅡA5 for reheating, and is used as the denitrified product external transmission natural gas together with the demethanization tower top gas after reheating in step S1, and is output through the product natural gas pipeline 34;

[0090] S4. The cooling capacity of natural gas denitrification and ethane recovery comes from the mixed refrigerant refrigeration cycle. First, the low-pressure mixed refrigerant from the return refrigerant channel A8, whose components are mainly methane, ethane (or ethylene), propane, butane, isopentane and nitrogen (for example, by volume, methane is 45%, ethane (or ethylene) is 20%, propane is 18%, isobutane is 2%, isopentane is 8%, and nitrogen is 7%), is compressed and cooled by the mixed refrigerant compressor unit 14, and enters the high-pressure refrigerant channel ⅤD1 provides a heat source for the demethanizer 7, then enters the high-pressure refrigerant channel IA6 to be partially cooled and condensed, then enters the high-pressure refrigerant channel ⅢC2 to provide a heat source for the denitrification tower 3, then enters the high-pressure refrigerant channel IA7 to be cooled, condensed and liquefied, and after reducing pressure and throttling through the manual regulating valve d, enters the mixed refrigerant channel B2 to provide a cold source for the top of the denitrification tower 3, enters the reflux refrigerant channel A8 to provide cold reheating for the positive flow channel of the main heat exchanger 1, and then returns to the mixed refrigerant compressor unit to complete the closed cycle.

Claims

1. A device for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration, characterized in that: The invention comprises a main heat exchanger (1) and a low-temperature separator (2) which are connected in sequence via a cold-insulating pipeline; the top and bottom of the low-temperature separator (2) are respectively provided with a gas phase outlet and a liquid phase outlet, and the gas phase outlet and the liquid phase outlet of the low-temperature separator (2) are respectively connected to a gas phase processing line and a liquid phase processing line; wherein: The main heat exchanger (1) is provided with a raw gas channel I (A1), a natural gas channel I (A2), a raw gas channel II (A3), a nitrogen-rich gas channel I (A4), a natural gas channel II (A5), a high-pressure refrigerant channel I (A6), a high-pressure refrigerant channel II (A7), a reflux refrigerant channel (A8) and a liquid ethane subcooling channel (A9); The inlet of the raw gas channel I (A1) of the main heat exchanger (1) is connected to an inlet pipe (35) of raw natural gas; The gas phase processing circuit comprises a denitrification tower (3), a denitrification tower condenser (4), and a denitrification tower reflux tank (5) which are connected in sequence along the air inlet direction through a cold-insulating pipeline; the gas phase outlet of the low-temperature separator (2) is connected to the inlet of the raw gas channel II (A3), and the inlet of the denitrification tower (3) is connected to the outlet of the raw gas channel II (A3); the gas phase outlet of the denitrification tower reflux tank (5) is connected to the inlet of the nitrogen-rich gas channel I (A4), and the outlet of the nitrogen-rich gas channel I (A4) is connected to an external nitrogen-rich fuel pipeline (33); the liquid phase outlet at the bottom of the denitrification tower (3) is connected to the inlet of the natural gas channel II (A5), and the outlet of the natural gas channel II (A5) is connected to an external product natural gas pipeline (34); The liquid phase processing pipeline comprises a demethanizer (7), a deethanizer (9), a deethanizer condenser (10), and a deethanizer reflux tank (11) which are connected in sequence along the liquid inlet direction through a cold-insulating pipeline; the liquid phase outlet of the demethanizer (7) is connected to the inlet of the middle part of the deethanizer (9), the gas phase outlet of the deethanizer (9) is connected to the deethanizer condenser (10), the gas phase outlet of the deethanizer reflux tank (11) is connected to the inlet of the liquid ethane supercooling channel (A9) of the main heat exchanger (1), and the outlet of the liquid ethane supercooling channel (A9) is connected to an external liquid ethane storage device; the gas phase outlet of the demethanizer (7) is connected to the inlet of the natural gas channel I (A2), and the outlet of the natural gas channel I (A2) is connected to an external product natural gas pipeline (34); It also includes a mixed refrigerant refrigeration cycle loop for providing a heat source for the denitrification tower (3) and the demethanization tower (7), and a propane compression condensing unit (31) for providing a cold source for the deethanization tower (9); The invention also includes a control device, wherein the main heat exchanger (1), the low-temperature separator (2), the denitrification tower (3), the denitrification tower condenser (4), the denitrification tower reflux tank (5), the demethanizer (7), the deethanizer (9), the deethanizer condenser (10), and the deethanizer reflux tank (11) are electrically connected to the control device respectively; the mixed refrigerant refrigeration cycle loop includes a mixed refrigerant compressor unit (14), a demethanizer side boiler (8), a high-pressure refrigerant channel I (A6), a denitrification tower reboiler (6), a high-pressure refrigerant channel II (A7), a denitrification tower condenser (4), a reflux tank (5), a demethanizer (7), a deethanizer (9), a deethanizer condenser (10), and a reflux tank (11) which are sequentially connected along the refrigerant circulation direction. A return refrigerant channel (A8) is provided, wherein the outlet of the return refrigerant channel (A8) is connected to the inlet of the mixed refrigerant compressor unit (14); the demethanizer side boiler (8) is provided with a high-pressure refrigerant channel V (D1), a mixed hydrocarbon channel I (D2) and a mixed hydrocarbon channel II (D3); the denitrification tower reboiler (6) is provided with a natural gas channel (C1) and a high-pressure refrigerant channel III (C2); the denitrification tower condenser (4) is provided with a nitrogen-rich gas channel II (B1) and a mixed refrigerant channel (B2); the deethanizer condenser (10) is provided with an ethane condensation channel (E1) and a propane evaporation channel (E2); A pressure digital controller I (15) and a pressure regulating valve a (16) are provided on the pipeline at the gas phase outlet at the top of the demethanizer (7), and the pressure digital controller I (15) and the pressure regulating valve a (16) are electrically connected to the control device respectively; The low-temperature separator (2) is provided with a liquid level digital controller II (17), and a liquid level regulating valve b (18) is provided on the pipeline of the liquid phase outlet at the bottom of the low-temperature separator (2), and the liquid level digital controller II (17) and the liquid level regulating valve b (18) are respectively electrically connected to the control device; The demethanizer (7) is provided with a liquid level digital controller III (19), and a liquid level regulating valve c (20) is provided on the pipeline of the liquid phase outlet at the bottom of the demethanizer (7); the liquid level digital controller III (19) and the liquid level regulating valve c (20) are respectively electrically connected to the control device; The denitrification tower (3) is provided with a liquid level digital controller IV (21), and a liquid level regulating valve e (22) is provided on the pipeline of the liquid phase outlet at the bottom of the denitrification tower (3); the digital controller IV (21) and the liquid level regulating valve e (22) are respectively electrically connected to the control device; A manual regulating valve d (23) for regulating the top temperature of the denitrification tower (3) is provided on the pipeline at the outlet of the high-pressure refrigerant channel II (A7), and the manual regulating valve d (23) is electrically connected to the control device; A pressure digital controller V (24) is provided on the pipeline of the gas phase outlet at the top of the denitrification tower reflux tank (5), and a pressure regulating valve f (25) is provided on the pipeline of the gas phase outlet at the top of the denitrification tower reflux tank (5), and the pressure digital controller V (24) and the pressure regulating valve f (25) are electrically connected; The deethanizer (9) is provided with a liquid level digital controller VII (26), and a liquid level regulating valve h (27) is provided on a pipeline at a liquid phase outlet of the deethanizer (9), and the liquid level digital controller VII (26) and the liquid level regulating valve h (27) are electrically connected; A pressure digital controller VIII (28) is provided on the pipeline of the gas phase outlet at the top of the deethanizer reflux tank (11), and a pressure regulating valve j (29) is provided on the pipeline of the gas phase outlet at the top of the deethanizer reflux tank (11), and the pressure digital controller VIII (28) and the pressure regulating valve j (29) are electrically connected.

2. The device for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration according to claim 1, characterized in that: The liquid phase outlet of the deethanizer (9) is connected to a liquefied petroleum gas storage device via a cold-keeping pipeline, and a liquefied petroleum gas cooler (13) is connected to the cold-keeping pipeline.

3. The device for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration according to claim 1, characterized in that: The gas phase treatment circuit further comprises a denitrification tower reboiler (6) for controlling the nitrogen content in the kettle of the denitrification tower (3); a liquid collector 1 is built into the lower part of the denitrification tower (3); the outlet of the liquid collector 1 is connected to the inlet of the denitrification tower reboiler (6); the outlet of the denitrification tower reboiler (6) is connected to the inlet of the lower part of the denitrification tower (3); the denitrification tower reboiler (6) is electrically connected to the control device.

4. The device for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration according to claim 1, characterized in that: The liquid phase treatment circuit further comprises a demethanizer side boiler (8) for controlling the methane content in the bottom of the demethanizer (7) and in the bottom liquid phase. The middle and lower parts of the demethanizer (7) are both provided with liquid collectors 2, the outlets of the two liquid collectors 2 are both connected to the inlet of the demethanizer side boiler (8), the outlet of the demethanizer side boiler (8) is respectively connected to the inlet of the middle and lower parts of the demethanizer (7), and the demethanizer side boiler (8) is electrically connected to the control device.

5. The device for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration according to claim 1, characterized in that: The liquid phase processing circuit further comprises a deethanizer reboiler (12) for controlling the ethane content in the kettle of the deethanizer (9); a liquid collector three is built in the lower part of the deethanizer (9); the outlet of the liquid collector three is connected to the inlet of the deethanizer reboiler (12); the outlet of the deethanizer reboiler (12) is connected to the inlet of the lower part of the deethanizer (9); and the deethanizer reboiler (12) is electrically connected to a control device.

6. The device for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration according to claim 1, characterized in that: The denitrification tower (3), the demethanization tower (7) and the deethanization tower (9) are packed towers or plate towers.

7. A method for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration, characterized in that: The device for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration according to any one of claims 1 to 6 comprises the following steps: S1. The purified raw natural gas containing methane, ethane, propane, nitrogen and a small amount of C4+ enters the raw gas channel I (A1). The purified natural gas in the raw gas channel I (A1) is cooled and partially condensed, and enters the low-temperature separator (2) for gas-liquid separation. The gas phase separated at the top enters the raw gas channel II (A3) for cooling and condensation, and then enters the denitrification tower (3) for denitrification. The liquid phase at the bottom of the low-temperature separator (2) enters the demethanizer (7). The liquid is led to the side boiler (8) of the demethanizer for heating to control the methane content in the liquid phase in the demethanizer (7) and in the bottom of the tower. The heat source of the side boiler (8) of the demethanizer comes from the mixed refrigerant refrigeration cycle. The gas phase obtained at the top of the demethanizer (7) enters the natural gas channel I (A2) for reheating to room temperature and is then output as a product natural gas through the product natural gas pipeline (34). The liquid phase obtained at the bottom of the demethanizer (7) enters the middle of the deethanizer (9) for distillation. S2, after being distilled in a deethanizer (9), an ethane-rich gas phase is obtained at the top. After being cooled and partially condensed in a deethanizer condenser (10), the gas phase enters a deethanizer reflux tank (11) for gas-liquid separation. The gas phase is ethane with a volume percentage of more than 95%, and enters a liquid ethane supercooling channel (A9). After being cooled, condensed and supercooled in the liquid ethane supercooling channel (A9), the gas phase is stored in a liquid ethane storage device (30). The liquid separated in the deethanizer reflux tank (11) is The phase is used as the reflux liquid of the deethanizer (9), and the cooling capacity of the top of the deethanizer (9) comes from the propane compression condensing unit (31); the liquid in the bottom of the deethanizer (9) is led to the deethanizer reboiler (12) of the deethanizer (9) for heating, and the ethane content in the bottom of the deethanizer (9) is controlled. The heat source of the bottom is provided by the outside. The liquefied petroleum gas product obtained in the bottom is cooled to room temperature by the liquefied petroleum gas cooler (13) and then stored in the liquefied petroleum gas storage device (32); S3, the nitrogen-containing raw gas from the raw gas channel II (A3) is distilled and denitrogenated in the denitrogenation tower (3), and the nitrogen-rich gas obtained at the top thereof, which consists of methane and nitrogen, is reheated in turn through the nitrogen-rich gas channel II (B1), the denitrogenation tower reflux tank (5), and the nitrogen-rich gas channel I (A4) as nitrogen-rich fuel gas, and is output through the nitrogen-rich fuel pipeline (33); the liquid at the bottom of the denitrogenation tower (3) is drawn into the denitrogenation tower reboiler (6) of the denitrogenation tower (3) for heating, and the nitrogen content in the bottom of the denitrogenation tower (3) is controlled to be less than 1%. The heat source of the bottom is provided by the mixed refrigerant refrigeration cycle loop. The liquefied natural gas produced in the bottom of the denitrogenation tower (3), which consists mainly of methane, enters the natural gas channel II (A5) for reheating, and is output through the product natural gas pipeline (34) as the denitrogenated product natural gas for external transmission together with the demethanization tower top gas after reheating in step S1.

8. The method for denitrification and ethane recovery of natural gas by mixed refrigerant refrigeration according to claim 7, characterized in that: The cold energy for denitrification and methane recovery of natural gas comes from the mixed refrigerant refrigeration cycle, specifically: first, the low-pressure mixed refrigerant from the reflux refrigerant channel (A8) is compressed and cooled by the mixed refrigerant compressor unit (14), enters the high-pressure refrigerant channel V (D1) to provide a heat source for the demethanizer (7), then enters the high-pressure refrigerant channel I (A6) to be partially cooled and condensed, then enters the high-pressure refrigerant channel III (C2) to provide a heat source for the denitrification tower (3), then enters the high-pressure refrigerant channel II (A7) to be cooled, condensed and liquefied, and after being reduced in pressure and throttled by the manual regulating valve d (23), enters the mixed refrigerant channel (B2) to provide a cold source for the top of the denitrification tower (3), enters the reflux refrigerant channel (A8) to provide cold energy for the positive flow channel of the main heat exchanger (1) for reheating, and then returns to the mixed refrigerant compressor unit (14) to complete the closed cycle.

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