A three-cycle mixed refrigerant natural gas liquefaction combined denitrification method

Abstract

The application discloses a kind of three-cycle mixed refrigerant natural gas liquefaction combined denitrification methods, it is related to natural gas liquefaction technical field, and natural gas is cooled after being pre-cooling heat exchanger and first liquefaction heat exchanger and is separated into low temperature separator for heavy hydrocarbon separation;Step two, based on the heavy hydrocarbon separation obtained in step one liquid phase heavy hydrocarbon is transported to heavy hydrocarbon separation device, and gaseous heavy hydrocarbon is liquefied to-100 DEG C extraction and enters denitrification tower after second liquefaction heat exchanger;Step three, nitrogen gas escaping from the top of denitrification tower is vented at high point after recovery cold quantity, and from the bottom of tower flows into supercooling heat exchanger supercooling, then enters the storage system of LNG after J-T valve throttling pressure reduction.The application is suitable for 200-400 million tons / year nitrogen-containing natural gas liquefaction method, with the advantages of wide adaptability, low energy consumption, high localization rate of key equipment, high efficiency of heat exchange equipment, high nitrogen removal depth.

Description

Technical Field

[0001] This invention relates to the field of natural gas liquefaction technology, and more specifically to a method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant. Background Technology

[0002] As an important and effective source of natural gas, LNG is clean and environmentally friendly, leading to rapid industry development and surging demand. However, due to resource and technological bottlenecks in large-scale domestic LNG production, overseas LNG resource supply has become a consensus in my country's LNG industry.

[0003] One way to classify liquefaction processes is by the number of cycles in the process, which can be divided into single-cycle processes, double-cycle processes, and triple-cycle processes. The more refrigeration cycle loops in the process, the greater the single-line LNG production capacity of that process.

[0004] Chinese invention patent application number CN115808052A discloses an ultra-large-scale nitrogen-containing natural gas liquefaction device and its process, including a precooling heat exchanger. The precooling heat exchanger is connected to a cryogenic separator through a pipeline. The gas phase outlet of the cryogenic separator is connected to a liquefaction heat exchanger through a pipeline. The liquefaction heat exchanger is connected to a denitrification tower, a cold heat exchanger, and a throttling valve through a pipeline. The precooling heat exchanger is connected to a precooling circulation system. The precooling unit in the precooling circulation system is pressurized to 2.5MPa in three stages and cooled to 20-40℃ before being diverted to the precooling heat exchanger and the three-stage throttling valve. The precooled mixed refrigerant entering the three-stage throttling valve is throttled and depressurized before entering the precooling separator and being separated into gaseous precooling refrigerant and liquid precooling refrigerant. Although the natural gas liquefaction unit and its process have advantages such as simplicity, small number of equipment, wide adaptability, low energy consumption, and suitability for ultra-large-scale nitrogen-containing natural gas liquefaction, the natural gas liquefaction unit and its process can only process natural gas of 1-2 million tons per year, which not only requires high investment but also incurs high maintenance costs.

[0005] Chinese invention patent application number CN110186251A discloses a three-cycle natural gas liquefaction plant and method suitable for ultra-large scale. Although it adopts a three-cycle liquefaction process and is suitable for the construction needs of ultra-large scale liquefaction plants with a single line capacity of 6-8 million tons / year, the gas turbine and wound tube heat exchanger used in the three-cycle natural gas liquefaction plant and method all need to be imported, resulting in low domestic production rate, high investment and high maintenance costs. In addition, the pre-cooling section uses a single pure propane medium, which has poor adaptability to changes in operating conditions or ambient temperature. The temperature remains unchanged during the condensation process, resulting in low heat exchanger efficiency and high energy consumption.

[0006] Therefore, it is necessary to propose a three-cycle mixed refrigerant natural gas liquefaction and denitrification method to solve the above problems. Summary of the Invention

[0007] The purpose of this invention is to solve the problems of high energy consumption, high investment, high maintenance costs, and low domestic production rate of existing natural gas liquefaction plants and processes.

[0008] To achieve the above objectives, the present invention specifically adopts the following technical solution:

[0009] A three-cycle mixed refrigerant natural gas liquefaction combined denitrification method includes the following steps:

[0010] Step 1: After the natural gas is cooled by the precooling heat exchanger and the first liquefaction heat exchanger, it enters the cryogenic separator for heavy hydrocarbon separation.

[0011] Step 2: The liquid heavy hydrocarbons obtained after the heavy hydrocarbon separation in Step 1 are transported to the heavy hydrocarbon separation unit, and the gaseous heavy hydrocarbons are liquefied to -100℃ through the second liquefaction heat exchanger and then extracted into the denitrification tower.

[0012] Step 3: The nitrogen gas escaping from the top of the denitrification tower is vented at a high point after recovering the cold energy, flows out from the bottom of the tower and enters the subcooling heat exchanger for subcooling, and then enters the LNG storage system after being throttled and depressurized by the JT valve.

[0013] Furthermore, in step one, a precooling mixed refrigerant flows through the precooling heat exchanger;

[0014] The precooling mixed refrigerant is made from a mixture of 88 mol% propane and 12 mol% isobutane;

[0015] The pre-cooled mixed refrigerant is pressurized in four stages by the pre-cooling unit to obtain a high-pressure refrigerant.

[0016] The high-pressure refrigerant enters the first-stage balance tank after being throttled by the first-stage throttling valve. The separated gas phase returns to the fourth-stage pressurization inlet of the precooling unit, and the liquid high-pressure refrigerant is divided into two streams: one stream flows into the precooling heat exchanger to provide cooling capacity and then returns to the fourth-stage pressurization inlet of the precooling unit; the other stream enters the second-stage balance tank after being throttled by the second-stage throttling valve to obtain the medium-pressure refrigerant.

[0017] The gas phase of the medium-pressure refrigerant, after being separated by the secondary balance tank, returns to the tertiary pressurization inlet of the precooling unit. The liquid phase of the medium-pressure refrigerant is divided into two streams: one stream flows into the precooling heat exchanger to provide cooling capacity and then returns to the tertiary pressurization inlet of the precooling unit; the other stream is throttled by the tertiary throttling valve and enters the tertiary balance tank to obtain low-pressure refrigerant.

[0018] The low-pressure refrigerant, after being separated by the three-stage balance tank, returns to the secondary pressurization inlet of the precooling unit in the gas phase. The liquid low-pressure refrigerant is divided into two streams: one stream flows into the precooling heat exchanger to provide cooling capacity and then returns to the secondary pressurization inlet of the precooling unit; the other stream is throttled by the four-stage throttling valve and enters the four-stage balance tank to obtain low-pressure refrigerant.

[0019] The low-pressure refrigerant, after being separated by the four-stage balance tank, returns to the first-stage pressurization inlet of the precooling unit in its gas phase. The liquid low-pressure refrigerant flows into the precooling heat exchanger to provide cooling capacity and then returns to the first-stage pressurization inlet of the precooling unit, thus completing the circulation of the precooled mixed refrigerant in the precooling unit.

[0020] Furthermore, in step two, the top of the denitrification tower is equipped with a condensing coil for condensing the top gas, and the bottom of the tower is equipped with a heating coil for heating natural gas and evaporating nitrogen.

[0021] Furthermore, liquefied mixed refrigerant flows through the precooling heat exchanger, the first liquefaction heat exchanger in step one, and the second liquefaction heat exchanger in step two.

[0022] The liquefied mixed refrigerant is made from a mixture of 76 mol% ethane and 24 mol% methane;

[0023] The liquefied mixed refrigerant is compressed in two stages by the liquefaction unit and then cooled by an air cooler to obtain high-pressure liquefied refrigerant;

[0024] The high-pressure liquefied refrigerant enters the precooling heat exchanger and is then divided into two streams: one stream flows into the first liquefied heat exchanger for precooling, and after being throttled by the first throttling valve, it returns to the first liquefied heat exchanger, reheats, and enters the secondary inlet of the liquefied unit; the other stream flows into the liquefied heat exchanger for precooling, and then flows into the second liquefied heat exchanger in step two for cooling, and after being throttled by the second throttling valve, it returns to the second liquefied heat exchanger to provide cooling capacity, reheats, and enters the primary inlet of the liquefied unit, completing the pressurization and refrigeration cycle of the entire liquefied unit.

[0025] Furthermore, a subcooled mixed refrigerant also circulates in the precooling heat exchanger, the first liquefaction heat exchanger, the second liquefaction heat exchanger, and the subcooling heat exchanger in steps one to three.

[0026] The supercooled mixed refrigerant is made by mixing 15 mol% ethane, 63 mol% methane and 22 mol% nitrogen.

[0027] The subcooled mixed refrigerant is compressed under low and high pressure by the refrigeration unit and then cooled by an air cooler to obtain a liquid-phase high-pressure subcooled refrigerant.

[0028] The liquid-phase high-pressure subcooled refrigerant is cooled by the precooling heat exchanger, the first liquefaction heat exchanger, the second liquefaction heat exchanger, and the subcooling heat exchanger. Then, it is throttled by the third throttling valve and divided into two streams: one stream returns to the subcooling heat exchanger to provide cooling capacity, and after reheating, it returns to the first-stage inlet of the subcooling unit; the other stream enters the top of the denitrification tower to provide a cooling source for the condensation of the condensing coils at the top of the denitrification tower, and after reheating, it returns to the first-stage inlet of the subcooling unit, completing the pressurization and refrigeration cycle of the entire subcooling unit.

[0029] Furthermore, in step one, the pressure of the natural gas is 4.85 MPa and the temperature is 40°C, the cooling temperature of the precooling heat exchanger is 10°C, and the cooling temperature of the first liquefaction heat exchanger is -60°C.

[0030] In step three, the subcooling temperature of the subcooling heat exchanger is -162°C, and the LNG is a product with atmospheric pressure and a temperature of -161.8°C.

[0031] Furthermore, the condensing temperature of the condensing coil is -155.2℃, and the heating temperature of the heating coil is -116℃.

[0032] Furthermore, the pre-cooled mixed refrigerant is pressurized to 1.29 MPa by the pre-cooling unit, cooled to 20-40°C by the air cooler, and then throttled to 0.9 MPa and 26°C by the first-stage throttling valve before entering the first-stage balance tank.

[0033] The liquid phase of the high-pressure refrigerant after separation in the primary balance tank is distributed in a 3:97 ratio. The 97% high-pressure refrigerant is throttled to 0.5MPa and 5℃ by the secondary throttling valve before entering the secondary balance tank.

[0034] The liquid phase of the medium-pressure refrigerant after separation in the secondary balance tank is distributed in a ratio of 14:86. The medium-pressure refrigerant with a ratio of 86 is throttled to 0.3MPa and temperature to -11℃ by the tertiary throttling valve before entering the tertiary balance tank.

[0035] The low-pressure refrigerant, after being separated by the three-stage balance tank, flows in two streams at a ratio of 17:83. The medium-pressure refrigerant, accounting for 83%, is throttled to 0.105 MPa and at a temperature of -38.25°C by the four-stage throttling valve before entering the four-stage balance tank.

[0036] Furthermore, the liquefied mixed refrigerant is pressurized to 4.23 MPa by a liquefaction unit and cooled to 20-40°C by an air cooler;

[0037] The high-pressure liquefied refrigerant enters the precooling heat exchanger and is precooled to -35°C. It then flows in two streams at a ratio of 37.5:62.5. The high-pressure liquefied refrigerant with a ratio of 37.5 flows to the first liquefaction heat exchanger and is precooled to -60°C. The first throttle valve throttles the flow to 1.48MPa and the temperature to -70.6°C, and the refrigerant returns to the liquefaction heat exchanger. It is then reheated to -7°C and enters the secondary inlet of the subcooling unit.

[0038] The high-pressure liquefied refrigerant, with a ratio of 62.5%, flows to the first liquefied heat exchanger for pre-cooling to -60°C. It then flows into the second liquefied heat exchanger for further cooling to -100°C. After passing through the second throttle valve, it is throttled to 1.48MPa and temperature to -109.5°C before returning to the second liquefied heat exchanger. It is then reheated to -66°C and enters the secondary inlet of the subcooling unit.

[0039] Furthermore, the low pressure is 0.31 MPa and the high pressure is 1.43 MPa;

[0040] The subcooled refrigerant is compressed to 4.23 MPa after passing through low and high pressures, and then cooled to 20-40°C using an air cooler.

[0041] The cooling temperatures of the precooling heat exchanger, the first liquefaction heat exchanger, the second liquefaction heat exchanger, and the subcooling heat exchanger are -35℃, -60℃, -100℃, and -160℃, respectively.

[0042] The third throttling valve has a throttling pressure of 0.45 MPa and a temperature of -165°C. The liquid phase high-pressure subcooled refrigerant in the heat exchanger is then divided into two streams at a ratio of 75:25 and flowed to the top of the heat exchanger and the denitrification tower, respectively. The liquid phase high-pressure subcooled refrigerant in the heat exchanger is reheated to -107°C and returned to the first-stage inlet of the subcooling unit. The liquid phase high-pressure subcooled refrigerant in the condenser coil at the top of the tower is reheated to -141°C and returned to the first-stage inlet of the subcooling unit.

[0043] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0044] 1. This invention is applicable to the liquefaction method of nitrogen-containing natural gas with a capacity of 2 million to 4 million tons per year. It has the advantages of wide adaptability, low energy consumption, high localization rate of key equipment, high efficiency of heat exchange equipment, and high nitrogen removal depth.

[0045] 2. This invention can improve the operational flexibility and adaptability of the equipment and reduce energy consumption by optimizing the liquefaction process; on the other hand, it can improve the localization rate of key equipment and save on engineering investment and operation and maintenance costs by optimizing key equipment parameters.

[0046] 3. The present invention uses three types of mixed refrigerants: pre-cooled mixed refrigerant, liquefied mixed refrigerant, and subcooled mixed refrigerant, which can adapt to changes in operating conditions or ambient temperature.

[0047] 4. The present invention adopts a four-stage high and low pressure cascade pressurization technology in the precooling section and a two-stage high and low pressure cascade pressurization technology in the liquefaction section, which can effectively save energy consumption.

[0048] 5. In this invention, when the nitrogen content in natural gas is higher than 1%, nitrogen in the raw gas can be flexibly and effectively separated during the liquefaction process; when the nitrogen content is lower than 1%, it can be bypassed without denitrification, making the device highly adaptable and the process highly flexible. Attached Figure Description

[0049] Figure 1 This is a schematic diagram of the overall process of the present invention;

[0050] Reference numerals: 1. Natural gas; 2. Precooling heat exchanger; 3. First liquefaction heat exchanger; 4. Cryogenic separator; 5. Second liquefaction heat exchanger; 6. Denitrification tower; 7. Natural gas after denitrification; 8. Subcooling heat exchanger; 9. JT valve; 10. LNG; 11. Precooling unit; 12. Liquefaction unit; 13. Subcooling unit; 14. Heavy hydrocarbons; 15. Nitrogen; 16. First throttle valve; 17. Second throttle valve; 18. Liquefied mixed refrigerant; 19. Subcooled mixed refrigerant; 20. Third throttle valve; 21. Precooled mixed refrigerant; 22. First-stage throttle valve; 23. First-stage balance tank; 24. Second-stage throttle valve; 25. Second-stage balance tank; 26. Third-stage throttle valve; 27. Third-stage balance tank; 28. Fourth-stage throttle valve; 29. ​​Fourth-stage balance tank. Detailed Implementation

[0051] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0052] Please see Figure 1 A method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant system includes the following steps:

[0053] Step 1: Natural gas 1 is cooled by precooling heat exchanger 2 and first liquefaction heat exchanger 3 and then enters low temperature separator 4 for heavy hydrocarbon separation.

[0054] Step 2: Based on the liquid phase heavy hydrocarbon 14 obtained after heavy hydrocarbon separation in Step 1, it is transported to the heavy hydrocarbon separation device. The gas phase heavy hydrocarbon 14 is liquefied to -100℃ through the second liquefaction heat exchanger 5 and then extracted into the denitrification tower 6.

[0055] Step 3: The nitrogen gas 15 escaping from the top of the denitrification tower 6 is vented at a high point after recovering the cold energy, flows out from the bottom of the tower and enters the subcooling heat exchanger 8 for subcooling, and then enters the LNG10 storage system after being throttled and depressurized by the JT valve 9.

[0056] Specifically, in step one, a precooling mixed refrigerant 21 flows through the precooling heat exchanger 2;

[0057] The precooling refrigerant mixture is made from a mixture of 88 mol% propane and 12 mol% isobutane;

[0058] The pre-cooled mixed refrigerant 21 is pressurized to 1.29MPa by the pre-cooling unit 11 in four stages and cooled to 20-40℃ by the air cooler to obtain high-pressure refrigerant. The four-stage pressurization is divided into low-low pressure section, low pressure section, medium pressure section and high pressure section, corresponding to the fourth, third, second and first stages and the pressurization inlet.

[0059] The high-pressure refrigerant is throttled to 0.9 MPa and 26°C by the first-stage throttling valve 22 and then enters the first-stage balance tank 23. The separated gas phase returns to the fourth-stage pressurization inlet of the precooling unit 11. The liquid high-pressure refrigerant is divided into two streams at a ratio of 3:97 (mol%): one stream, accounting for 3 mol%, flows into the precooling heat exchanger 2 to provide cooling capacity and then returns to the fourth-stage pressurization inlet of the precooling unit 11; the other stream is throttled to 0.5 MPa and 5°C by the second-stage throttling valve 24 and then enters the second-stage balance tank 25 to obtain the medium-pressure refrigerant.

[0060] After being separated in the secondary balance tank 25, the gas phase of the medium-pressure refrigerant returns to the tertiary pressurization inlet of the precooling unit 11. The liquid phase of the medium-pressure refrigerant is divided into two streams at a ratio of 14:86 (mol%): one stream of medium-pressure refrigerant flows into the precooling heat exchanger 2 to provide cooling capacity and then returns to the tertiary pressurization inlet of the precooling unit 11; the other stream of medium-pressure refrigerant is throttled to 0.3MPa and temperature to -11℃ by the tertiary throttling valve 26 and enters the tertiary balance tank 27 to obtain low-pressure refrigerant.

[0061] After the low-pressure refrigerant is separated by the three-stage balance tank 27, the gas phase returns to the secondary pressurization inlet of the precooling unit 11. The liquid low-pressure refrigerant is divided into two streams at a ratio of 17:83 (mol%): one stream of liquid low-pressure refrigerant with a proportion of 17 mol% flows into the precooling heat exchanger 2 to provide cooling capacity and then returns to the secondary pressurization inlet of the precooling unit 11; the other stream of liquid low-pressure refrigerant with a proportion of 83 mol% is throttled to 0.105 MPa and temperature to -38.25℃ by the four-stage throttling valve 28 and enters the four-stage balance tank 29 to obtain low-pressure refrigerant.

[0062] After being separated by the four-stage balance tank 29, the low-pressure refrigerant returns to the first-stage pressurization inlet of the precooling unit 11 in its gas phase. The liquid low-pressure refrigerant flows into the precooling heat exchanger 2 to provide cooling capacity and then returns to the first-stage pressurization inlet of the precooling unit 11, thus completing the circulation of the precooling mixed refrigerant 21 in the precooling unit 11.

[0063] Specifically, in step two, the top of the denitrification tower 6 is equipped with a condensing coil for condensing the top gas, and the condensing temperature of the condensing coil is -155.2℃. The bottom of the tower is equipped with a heating coil for heating natural gas 1 and evaporating nitrogen, and the heating temperature of the heating coil is -116℃.

[0064] Specifically, liquefied mixed refrigerant 18 flows through the precooling heat exchanger 2, the first liquefaction heat exchanger 3, and the second liquefaction heat exchanger 5 in step one.

[0065] The liquefied mixed refrigerant 18 is made from a mixture of 76 mol% ethane and 24 mol% methane;

[0066] The liquefied mixed refrigerant 18 is pressurized to 4.23 MPa after being compressed in two stages by the liquefaction unit 12, and then cooled to 20-40℃ by an air cooler to obtain high-pressure liquefied refrigerant;

[0067] After the high-pressure liquefied refrigerant is pre-cooled to -35°C in the pre-cooling heat exchanger 2, it is divided into two streams in a ratio of 37.5:62.5 mol%. One stream of 37.5 mol% high-pressure liquefied refrigerant flows into the first liquefaction heat exchanger 3 and is pre-cooled to -60°C. After pre-cooling, it is throttled to 1.48 MPa and -70.6°C by the first throttling valve 16 and then returns to the liquefaction heat exchanger 3. After being reheated to -7°C, it enters the secondary inlet of the liquid chiller unit 13. The other stream of 62.5 mol% flows into the liquefaction heat exchanger 3 and is pre-cooled to -60°C. Then it flows into the second liquefaction heat exchanger 5 in step two and is cooled to -100°C. After being cooled, it is throttled to 1.48 MPa and -109.5°C by the second throttling valve 17 and then returns to the second liquefaction heat exchanger 5 to provide cooling capacity. After being reheated to -66°C, it enters the primary inlet of the liquefaction unit 13, completing the entire pressurization and refrigeration cycle of the liquefaction unit 12.

[0068] Specifically, in steps one to three, the precooling heat exchanger 2, the first liquefaction heat exchanger 3, the second liquefaction heat exchanger 5, and the subcooling heat exchanger 8 also have a subcooled mixed refrigerant 19 flowing through them.

[0069] The supercooled mixed refrigerant 19 is made by mixing 15 mol% ethane, 63 mol% methane and 22 mol% nitrogen.

[0070] The subcooled mixed refrigerant 19 is compressed to 4.23 MPa after being compressed by the refrigeration unit 13 at a low pressure of 0.31 MPa and a high pressure of 1.43 MPa. It is then cooled to 20-40°C by an air cooler to obtain a liquid-phase high-pressure subcooled refrigerant.

[0071] After being cooled by the precooling heat exchanger 2 (-35℃), the first liquefaction heat exchanger 3 (-60℃), the second liquefaction heat exchanger 5 (-100℃), and the subcooling heat exchanger 8 (-160℃), the liquid-phase high-pressure subcooled refrigerant is throttled by the third throttling valve 20 at a pressure of 0.45 MPa and a temperature of -165℃. It is then divided into two streams at a ratio of 75:25 mol%. One stream of liquid-phase high-pressure subcooled refrigerant (75 mol%) returns to the subcooling heat exchanger 8 to provide cooling capacity. After being reheated to -107℃, it returns to the primary inlet of the subcooling unit 13. The other stream of liquid-phase high-pressure subcooled refrigerant (25 mol%) enters the top of the denitrification tower 6 to provide a cooling source for the condensation of the condensing coils at the top of the denitrification tower 6. After being reheated to -141℃, it returns to the primary inlet of the subcooling unit 13, completing the entire pressurization and refrigeration cycle of the subcooling unit 13.

[0072] Before the equipment is started, the precooling mixed refrigerant 21, liquefied mixed refrigerant 18 and subcooled mixed refrigerant 19 are respectively transported to the precooling unit 11, liquefied unit 12 and subcooling unit 13 via pipeline. As the entire unit is started, the precooling mixed refrigerant 21, liquefied mixed refrigerant 18 and subcooled mixed refrigerant 19 continue to circulate in the above-mentioned circulation mode to cool the natural gas 1.

[0073] In summary, this invention provides a method for liquefying nitrogen-containing natural gas with a capacity of 2 million to 4 million tons per year, which has the advantages of wide adaptability, low energy consumption, high localization rate of key equipment, high efficiency of heat exchange equipment, and high nitrogen removal depth.

[0074] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. The scope of patent protection of the present invention shall be determined by the claims. Similarly, any equivalent structural changes made based on the description and drawings of the present invention shall also be included within the scope of protection of the present invention.

Claims

1. A method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant system, characterized in that, Includes the following steps: Step 1: After the natural gas is cooled by the precooling heat exchanger and the first liquefaction heat exchanger, it enters the cryogenic separator for heavy hydrocarbon separation. Step 2: The liquid heavy hydrocarbons obtained after the heavy hydrocarbon separation in Step 1 are transported to the heavy hydrocarbon separation unit, and the gaseous heavy hydrocarbons are liquefied to -100℃ through the second liquefaction heat exchanger and then extracted into the denitrification tower. Step 3: The nitrogen gas escaping from the top of the denitrification tower is vented at a high point after recovering the cold energy, flows out from the bottom of the tower and enters the subcooling heat exchanger for subcooling, and then enters the LNG storage system after being throttled and depressurized by the JT valve. A liquefied mixed refrigerant flows through the precooling heat exchanger, the first liquefaction heat exchanger, and the second liquefaction heat exchanger in step one. The liquefied mixed refrigerant is made from a mixture of 76 mol% ethane and 24 mol% methane; The liquefied mixed refrigerant is compressed in two stages by the liquefaction unit and then cooled by an air cooler to obtain high-pressure liquefied refrigerant; The high-pressure liquefied refrigerant enters the precooling heat exchanger and is then divided into two streams: one stream flows into the first liquefied heat exchanger for precooling, and after being throttled by the first throttling valve, it returns to the first liquefied heat exchanger, reheats, and enters the secondary inlet of the liquefied unit; the other stream flows into the liquefied heat exchanger for precooling, and then flows into the second liquefied heat exchanger in step two for cooling, and after being throttled by the second throttling valve, it returns to the second liquefied heat exchanger to provide cooling capacity, reheats, and enters the primary inlet of the liquefied unit, completing the pressurization and refrigeration cycle of the entire liquefied unit.

2. The method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant according to claim 1, characterized in that: In step one, a precooling mixed refrigerant flows through the precooling heat exchanger. The precooling mixed refrigerant is made from a mixture of 88 mol% propane and 12 mol% isobutane; The pre-cooled mixed refrigerant is pressurized in four stages by the pre-cooling unit to obtain a high-pressure refrigerant. The high-pressure refrigerant enters the first-stage balance tank after being throttled by the first-stage throttling valve. The separated gas phase returns to the fourth-stage pressurization inlet of the precooling unit, and the liquid high-pressure refrigerant is divided into two streams: one stream flows into the precooling heat exchanger to provide cooling capacity and then returns to the fourth-stage pressurization inlet of the precooling unit; the other stream enters the second-stage balance tank after being throttled by the second-stage throttling valve to obtain the medium-pressure refrigerant. The gas phase of the medium-pressure refrigerant, after being separated by the secondary balance tank, returns to the tertiary pressurization inlet of the precooling unit. The liquid phase of the medium-pressure refrigerant is divided into two streams: one stream flows into the precooling heat exchanger to provide cooling capacity and then returns to the tertiary pressurization inlet of the precooling unit; the other stream is throttled by the tertiary throttling valve and enters the tertiary balance tank to obtain low-pressure refrigerant. The low-pressure refrigerant, after being separated by the three-stage balance tank, returns to the secondary pressurization inlet of the precooling unit in the gas phase. The liquid low-pressure refrigerant is divided into two streams: one stream flows into the precooling heat exchanger to provide cooling capacity and then returns to the secondary pressurization inlet of the precooling unit; the other stream is throttled by the four-stage throttling valve and enters the four-stage balance tank to obtain low-pressure refrigerant. The low-pressure refrigerant, after being separated by the four-stage balance tank, returns to the first-stage pressurization inlet of the precooling unit in its gas phase. The liquid low-pressure refrigerant flows into the precooling heat exchanger to provide cooling capacity and then returns to the first-stage pressurization inlet of the precooling unit, thus completing the circulation of the precooled mixed refrigerant in the precooling unit.

3. The method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant according to claim 1, characterized in that: In step two, the denitrification tower is equipped with a condenser coil at the top for condensing the top gas, and a heating coil at the bottom for heating natural gas and evaporating nitrogen.

4. The method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant according to claim 1, characterized in that: Subcooled mixed refrigerant also circulates in the precooling heat exchanger, the first liquefaction heat exchanger, the second liquefaction heat exchanger, and the subcooling heat exchanger in steps one to three. The supercooled mixed refrigerant is made by mixing 15 mol% ethane, 63 mol% methane and 22 mol% nitrogen. The subcooled mixed refrigerant is compressed under low and high pressure by the refrigeration unit and then cooled by an air cooler to obtain a liquid-phase high-pressure subcooled refrigerant. The liquid-phase high-pressure subcooled refrigerant is cooled by the precooling heat exchanger, the first liquefaction heat exchanger, the second liquefaction heat exchanger, and the subcooling heat exchanger. Then, it is throttled by the third throttling valve and divided into two streams: one stream returns to the subcooling heat exchanger to provide cooling capacity, and after reheating, it returns to the first-stage inlet of the subcooling unit; the other stream enters the top of the denitrification tower to provide a cooling source for the condensation of the condensing coils at the top of the denitrification tower, and after reheating, it returns to the first-stage inlet of the subcooling unit, completing the pressurization and refrigeration cycle of the entire subcooling unit.

5. The method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant according to claim 1, characterized in that: In step one, the pressure of the natural gas is 4.85 MPa and the temperature is 40°C. The cooling temperature of the precooling heat exchanger is 10°C, and the cooling temperature of the first liquefaction heat exchanger is -60°C. In step three, the subcooling temperature of the subcooling heat exchanger is -162°C, and the LNG is a product with atmospheric pressure and a temperature of -161.8°C.

6. The method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant according to claim 3, characterized in that: The condensing temperature of the condensing coil is -155.2℃, and the heating temperature of the heating coil is -116℃.

7. The method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant according to claim 2, characterized in that: The precooled mixed refrigerant is pressurized to 1.29 MPa by the precooling unit, cooled to 20-40°C by the air cooler, and then throttled to 0.9 MPa and 26°C by the first-stage throttling valve before entering the first-stage balance tank. The liquid phase of the high-pressure refrigerant after separation in the primary balance tank is distributed in a 3:97 ratio. The 97% high-pressure refrigerant is throttled to 0.5MPa and 5℃ by the secondary throttling valve before entering the secondary balance tank. The liquid phase of the medium-pressure refrigerant after separation in the secondary balance tank is distributed in a ratio of 14:

86. The medium-pressure refrigerant with a ratio of 86 is throttled to 0.3MPa and temperature to -11℃ by the tertiary throttling valve before entering the tertiary balance tank. The low-pressure refrigerant, after being separated by the three-stage balance tank, flows in two streams at a ratio of 17:

83. The medium-pressure refrigerant, accounting for 83%, is throttled to 0.105 MPa and at a temperature of -38.25°C by the four-stage throttling valve before entering the four-stage balance tank.

8. The method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant according to claim 1, characterized in that: The liquefied mixed refrigerant is pressurized to 4.23 MPa by the liquefaction unit and cooled to 20-40°C by the air cooler; The high-pressure liquefied refrigerant enters the precooling heat exchanger and is precooled to -35°C. It then flows in two streams at a ratio of 37.5:62.

5. The high-pressure liquefied refrigerant with a ratio of 37.5 flows to the first liquefaction heat exchanger and is precooled to -60°C. The first throttle valve throttles the flow to 1.48MPa and the temperature to -70.6°C, and the refrigerant returns to the liquefaction heat exchanger. It is then reheated to -7°C and enters the secondary inlet of the subcooling unit. The high-pressure liquefied refrigerant, with a ratio of 62.5%, flows to the first liquefaction heat exchanger for pre-cooling to -60°C. It then flows into the second liquefaction heat exchanger for further cooling to -100°C. After passing through the second throttle valve, it is throttled to 1.48MPa and temperature to -109.5°C before returning to the second liquefaction heat exchanger. It is then reheated to -66°C and enters the secondary inlet of the subcooling unit.

9. The method for combined denitrification of natural gas liquefaction using a three-cycle mixed refrigerant according to claim 4, characterized in that: The low pressure is 0.31 MPa and the high pressure is 1.43 MPa; The subcooled refrigerant is compressed to 4.23 MPa after passing through low and high pressures, and then cooled to 20-40°C using an air cooler. The cooling temperatures of the precooling heat exchanger, the first liquefaction heat exchanger, the second liquefaction heat exchanger, and the subcooling heat exchanger are -35℃, -60℃, -100℃, and -160℃, respectively. The third throttling valve has a throttling pressure of 0.45 MPa and a temperature of -165°C. The liquid phase high-pressure subcooled refrigerant in the heat exchanger is then divided into two streams at a ratio of 75:25 and flowed to the top of the cold heat exchanger and the denitrification tower, respectively. The liquid phase high-pressure subcooled refrigerant in the condenser coil at the top of the tower is reheated to -107°C and returned to the first-stage inlet of the subcooler unit.

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