An apparatus for producing liquefied natural gas by utilizing the pressure energy and cold energy of natural gas

Abstract

The present invention discloses a device for producing liquefied natural gas by utilizing the pressure energy and cold energy of natural gas, which includes an emergency cut-off valve, a natural gas filter A, a natural gas expander, a first-stage oil separator A, a second-stage oil separator A, an oil-oil heat exchanger, an oil coarse filter A, an oil pump A, an oil fine filter A, a filter B, a mixed refrigerant compressor, a first-stage oil separator B, a second-stage oil separator B, a third-stage oil separator, an activated carbon filter, an oil coarse filter B, an oil pump B, an oil fine filter B, a gas-gas heat exchanger, a natural gas pretreatment unit, a cold box, an air vaporizer A, an air vaporizer B, a BOG compressor, a first-stage oil and gas separator C, a second-stage oil and gas separator C, an oil fine filter C, an oil pump C, and an oil coarse filter element C. The advantages of the present invention compared with the prior art are as follows: it solves the problem that the pressure energy and cold energy after pressure regulation in the existing natural gas pressure regulating station cannot be utilized, and at the same time, a large amount of natural gas needs to be wasted for combustion heat compensation.

Description

Technical Field

[0001] The present invention relates to the technical field of pressure energy generation and cold energy utilization, and specifically refers to a device for producing liquefied natural gas by using the pressure energy and cold energy of natural gas. Background Art

[0002] During the pipeline transportation of natural gas in the national natural gas pipeline network, a relatively high pressure is always maintained, generally about 10 MPa. The natural gas from the national natural gas pipeline network is transported to major urban pipeline networks and industrial users. It mainly realizes pressure reduction through the pressure regulating stations in major urban pipeline networks, generally reducing from 10 MPa to about 4.0 MPa. After the pressure reduction, the natural gas is further regulated to 0.4 MPa through the urban gate stations of major urban pipeline networks and then transported to end-users. During the pressure reduction process in the natural gas pressure regulating station, a huge amount of pressure energy and cold energy can be utilized. At the same time, the temperature of the pipeline natural gas after pressure reduction is lower than -20°C, and heat must be added to the natural gas to ensure that the temperature of the natural gas after regulation through the urban gate station is higher than 5°C. The current pressure regulating station reduces the pressure of natural gas through pressure regulating valves. At the same time, it heats the natural gas in the pipeline by burning natural gas, wasting a large amount of pressure energy, cold energy and heat energy. If a method can be used to recover and utilize this pressure energy and cold energy, it will have very great social and economic benefits.

[0003] Due to the high working pressure of natural gas, strict requirements for sealing, prohibiting leakage, and high requirements for the strength of the machine body, it is difficult to find a suitable power machine for energy recovery in the existing technology. The screw expander is a power machine in which the male and female rotors of the screw are driven to rotate by high-pressure gas. Its core components are a pair of meshing male and female rotors. Its special internal structure determines that the screw expander is suitable for the recovery and utilization of the pressure energy of natural gas. It is a high-tech that is widely used specifically for recovering the pressure energy of natural gas.

[0004] After using the screw expander to recover the pressure energy, the temperature of the natural gas will also drop below -20°C. The expanded low-temperature natural gas has a huge amount of cold energy. How to utilize this cold energy and at the same time raise the temperature of the expanded natural gas above 5°C is another major difficulty in the recovery and utilization of the pressure energy and cold energy of natural gas. Summary of the Invention

[0005] The purpose of the present invention is to overcome the above technical defects, and provide a device for producing liquefied natural gas by using the pressure energy and cold energy of natural gas, and solve the problems existing in the prior art that the pressure energy and cold energy after pressure regulation of the natural gas pressure regulating station cannot be utilized, and at the same time a large amount of natural gas needs to be wasted for combustion heat addition.

[0006] To solve the above technical problems, the technical solution provided by the present invention is: a device for producing liquefied natural gas by utilizing natural gas pressure energy and cold energy, including a natural gas filter A, the natural gas filter A is connected to a natural gas expander through a first pipeline, the natural gas filter A is connected to a natural gas pretreatment unit through a second pipeline, an emergency cut-off valve is provided on the upstream pipeline of the natural gas filter A, a quick-opening valve A, a regulating valve A and a stop valve A are successively provided on the first pipeline, a quick-opening valve B, a regulating valve B and a stop valve B are successively provided on the second pipeline, the natural gas expander is connected to a first-stage oil separator A, the first-stage oil separator A is connected to an oil-oil heat exchanger through a third pipeline, the third pipeline passes through the oil-oil heat exchanger and is connected to the natural gas expander, an oil coarse filter A, an oil pump A and an oil fine filter A are successively provided on the third pipeline, the natural gas expander is connected to a mixed refrigerant compressor, the mixed refrigerant compressor is connected to a first-stage oil separator B, the first-stage oil separator B is connected to the oil-oil heat exchanger through a fourth pipeline, the fourth pipeline passes through the oil-oil heat exchanger and is connected to the mixed refrigerant compressor, a branch pipeline and an oil fine filter B are successively provided on the fourth pipeline, an oil coarse filter B and an oil pump B are successively provided on the branch pipeline, a filter B is provided upstream of the mixed refrigerant compressor, the natural gas pretreatment unit is connected to a cold box, the cold box is respectively connected to an air vaporizer A and an LNG loading arm, the air vaporizer A is connected to a BOG compressor through a pipeline, the LNG loading arm is respectively connected to an LNG tank truck and an air vaporizer B, the air vaporizer B is connected to the pipeline between the air vaporizer A and the BOG compressor, the BOG compressor is connected to a first-stage oil-gas separator C, the first-stage oil-gas separator C is connected to the BOG compressor through a fifth pipeline, an oil fine filter C, an oil pump C and an oil coarse filter element C are successively provided on the fifth pipeline, the first-stage oil-gas separator C is connected to a second-stage oil-gas separator C and a regulating valve D and a stop valve D are provided on the connecting pipeline, the first-stage oil-gas separator C is connected to the oil-oil heat exchanger, the first-stage oil separator A is connected to a gas-gas heat exchanger through a sixth pipeline, a second-stage oil separator A, a regulating valve C and a stop valve C are successively provided on the sixth pipeline, the sixth pipeline is connected to the pipeline of the second-stage oil-gas separator C, the first-stage oil separator B is connected to the gas-gas heat exchanger through a seventh pipeline, a second-stage oil separator B, a third-stage oil separator and an activated carbon filter are successively provided on the seventh pipeline.

[0007] Further, the regulating valve A is a pneumatic regulating valve or an electric regulating valve, which controls the rotation speeds of the natural gas expander and the mixed refrigerant compressor.

[0008] Further, the natural gas expander is of a single-machine single-stage screw expander structure, a single-machine double-stage screw expander structure or a centrifugal expander structure.

[0009] Further, the oil-oil heat exchanger is of a plate heat exchanger or a shell-and-tube heat exchanger structure.

[0010] Further, the mixed refrigerant compressor is of a single-stage single-screw compressor structure, a single-stage double-screw compressor structure or a centrifugal compressor structure.

[0011] Further, the gas-gas heat exchanger is of a plate heat exchanger or a shell-and-tube heat exchanger structure.

[0012] Further, after the natural gas expander expands and does work on the high-pressure natural gas from the upstream natural gas pipeline network, the subcooled natural gas and the oil-gas mixture of the lubricating oil after pressure reduction enter the first-stage oil separator A and the second-stage oil separator A for two-stage precision oil-gas separation. The oil content rate of the separated natural gas is less than 0.1 mg / m3. The separated subcooled lubricating oil enters the oil-oil heat exchanger for heat exchange and heat regeneration. The separated subcooled natural gas enters the gas-gas heat exchanger for heat exchange and heat regeneration. The natural gas after pressure reduction and heat regeneration directly enters the downstream natural gas pipeline network. After the mixed refrigerant compressor compresses and consumes work on the mixed refrigerant from the system, the superheated mixed refrigerant after pressurization and the oil-gas mixture of the lubricating oil enter the first-stage oil separator B, the second-stage oil separator B, the third-stage oil separator and the activated carbon filter for four-stage precision oil-gas separation. The oil content rate of the separated mixed refrigerant is less than 0.01 mg / m3. The separated superheated lubricating oil enters the oil-oil heat exchanger for heat exchange and cooling. The separated superheated mixed refrigerant enters the gas-gas heat exchanger for heat exchange and cooling. The pressurized and cooled mixed refrigerant enters the cold box to exchange heat with the natural gas from the upstream natural gas pipeline network.

[0013] Further, the cold box is of a plate-fin heat exchanger or a spiral-wound heat exchanger structure.

[0014] Further, the high-pressure natural gas from the upstream natural gas pipeline network enters the cold box after removing substances such as solid particles, carbon dioxide, hydrogen sulfide, water, and light oil in the natural gas through the natural gas pretreatment unit, and exchanges heat with the mixed refrigerant after heat exchange and cooling from the gas-gas heat exchanger to be cooled down to below -160°C. The purified gaseous natural gas CNG becomes liquefied natural gas LNG.

[0015] Further, the BOG compressor is of a screw compressor or a piston compressor structure.

[0016] The advantages of the present invention compared with the prior art are as follows: The present invention has the advantages of high pressure energy recovery efficiency, high cold energy utilization efficiency, high overall utilization efficiency, good economic benefits, etc. It can be widely applied to the pressure energy and cold energy recovery and utilization in the pressure regulation systems of urban gate stations of natural gas at all levels, and has very great social and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic structural diagram of an apparatus for producing liquefied natural gas by utilizing the pressure energy and cold energy of natural gas according to the present invention.

[0018] As shown in the figure: 1. Emergency cut-off valve; 2. Natural gas filter A; 3. Quick-opening valve A; 4. Regulating valve A; 5. Stop valve A; 6. Natural gas expander; 7. Primary oil separator A; 8. Secondary oil separator A; 9. Oil-oil heat exchanger; 10. Coarse oil filter A; 11. Oil pump A; 12. Fine oil filter A; 13. Filter B; 14. Mixed refrigerant compressor; 15. Primary oil separator B; 16. Secondary oil separator B; 17. Tertiary oil separator; 18. Activated carbon filter; 19. Coarse oil filter B; 20. Oil pump B; 21. Fine oil filter B; 22. Gas-gas heat exchanger; 23. Regulating valve C; 24. Stop valve C; 25. Quick-opening valve B; 26. Regulating valve B; 27. Stop valve B; 28. Natural gas pretreatment unit; 29. Cold box; 30. Air vaporizer A; 31. LNG loading arm; 32. LNG tanker; 33. Air vaporizer B; 34. BOG compressor; 35. Primary oil and gas separator C; 36. Secondary oil separator C; 37. Fine oil filter C; 38. Oil pump C; 39. Coarse oil filter element C; 40. Regulating valve D; 41. Stop valve D. Detailed implementation mode

[0019] The present invention will be further described in detail below with reference to the accompanying drawings.

[0020] A device for producing liquefied natural gas by utilizing the pressure energy and cold energy of natural gas, comprising a natural gas filter A2. The natural gas filter A2 is connected to a natural gas expander 6 through a first pipeline, and the natural gas filter A2 is connected to a natural gas pretreatment unit 28 through a second pipeline. An emergency cut-off valve 1 is provided on the upstream pipeline of the natural gas filter A2. A quick-switch valve A3, a regulating valve A4, and a stop valve A5 are successively arranged on the first pipeline. A quick-switch valve B25, a regulating valve B26, and a stop valve B27 are successively arranged on the second pipeline. The natural gas expander 6 is connected to a first-stage oil separator A7. The first-stage oil separator A7 is connected to an oil-oil heat exchanger 9 through a third pipeline. The third pipeline passes through the oil-oil heat exchanger 9 and is connected to the natural gas expander 6. An oil coarse filter A10, an oil pump A11, and an oil fine filter A12 are successively arranged on the third pipeline. The natural gas expander 6 is connected to a mixed refrigerant compressor 14. The mixed refrigerant compressor 14 is connected to a first-stage oil separator B15. The first-stage oil separator B15 is connected to the oil-oil heat exchanger 9 through a fourth pipeline. The fourth pipeline passes through the oil-oil heat exchanger 9 and is connected to the mixed refrigerant compressor 14. A branch pipeline and an oil fine filter B21 are successively arranged on the fourth pipeline. An oil coarse filter B19 and an oil pump B20 are successively arranged on the branch pipeline. A filter B13 is provided upstream of the mixed refrigerant compressor 14. The natural gas pretreatment unit 28 is connected to a cold box 29. The cold box 29 is respectively connected to an air vaporizer A30 and an LNG loading arm 31. The air vaporizer A30 is connected to a BOG compressor 34 through a pipeline. The LNG loading arm 31 is respectively connected to an LNG tanker 32 and an air vaporizer B33. The air vaporizer B33 is connected to the pipeline between the air vaporizer A30 and the BOG compressor 34. The BOG compressor 34 is connected to a first-stage oil and gas separator C35. The first-stage oil and gas separator C35 is connected to the BOG compressor 34 through a fifth pipeline. An oil fine filter C37, an oil pump C38, and an oil coarse filter element C39 are successively arranged on the fifth pipeline. The first-stage oil and gas separator C35 is connected to a second-stage oil and gas separator C36, and a regulating valve D40 and a stop valve D41 are provided on the connecting pipeline. The first-stage oil and gas separator C35 is connected to the oil-oil heat exchanger 9. The first-stage oil separator A7 is connected to a gas-gas heat exchanger 22 through a sixth pipeline. A second-stage oil separator A8, a regulating valve C23, and a stop valve C24 are successively arranged on the sixth pipeline. The sixth pipeline is connected to the pipeline of the second-stage oil and gas separator C36. The first-stage oil separator B15 is connected to the gas-gas heat exchanger 22 through a seventh pipeline. A second-stage oil separator B16, a third-stage oil separator 17, and an activated carbon filter 18 are successively arranged on the seventh pipeline.

[0021] The regulating valve A4 is a pneumatic regulating valve or an electric regulating valve, which controls the rotational speeds of the natural gas expander 6 and the mixed refrigerant compressor 14.

[0022] The described natural gas expander 6 has a structure of single-unit single-stage screw expander, single-unit double-stage screw expander or centrifugal expander.

[0023] The described oil-oil heat exchanger 9 has a structure of plate heat exchanger or shell-and-tube heat exchanger.

[0024] The described mixed refrigerant compressor 14 has a structure of single-unit single-stage screw compressor, single-unit double-stage screw compressor or centrifugal compressor.

[0025] The described gas-gas heat exchanger 22 has a structure of plate heat exchanger or shell-and-tube heat exchanger.

[0026] After the described natural gas expander 6 expands and does work on the high-pressure natural gas from the upstream natural gas pipeline network, the subcooled natural gas after pressure reduction and the oil-gas mixture of lubricating oil enter the first-stage oil separator A7 and the second-stage oil separator A8 for two-stage precise oil-gas separation. The oil content of the separated natural gas is less than 0.1 mg / m3. The separated subcooled lubricating oil enters the oil-oil heat exchanger 9 for heat exchange and heat regeneration. The separated subcooled natural gas enters the gas-gas heat exchanger 22 for heat exchange and heat regeneration. The natural gas after pressure reduction and heat regeneration directly enters the downstream natural gas pipeline network. After the described mixed refrigerant compressor 14 compresses and consumes work on the mixed refrigerant from the system, the superheated mixed refrigerant after pressurization and the oil-gas mixture of lubricating oil enter the first-stage oil separator B15, the second-stage oil separator B16, the third-stage oil separator 17 and the activated carbon filter 18 for four-stage precise oil-gas separation. The oil content of the separated mixed refrigerant is less than 0.01 mg / m3. The separated superheated lubricating oil enters the oil-oil heat exchanger 9 for heat exchange and cooling. The separated superheated mixed refrigerant enters the gas-gas heat exchanger 22 for heat exchange and cooling. The pressurized and cooled mixed refrigerant enters the cold box 29 to exchange heat with the natural gas from the upstream natural gas pipeline network.

[0027] The described cold box 29 has a structure of plate-fin heat exchanger or spiral-wound heat exchanger.

[0028] The high-pressure natural gas from the upstream natural gas pipeline network enters the cold box 29 after removing substances such as solid particles, carbon dioxide, hydrogen sulfide, water, and light oil in the natural gas through the natural gas pretreatment unit 28, and exchanges heat and cools down with the mixed refrigerant that has been heat exchange and cooled by the gas-gas heat exchanger 22 to below -160°C. The purified gaseous natural gas CNG becomes liquefied natural gas LNG.

[0029] The described BOG compressor 34 has a structure of screw compressor or piston compressor.

[0030] In the specific implementation of the present invention, high-pressure natural gas from the upstream natural gas pipeline network enters the natural gas filter 2 after passing through the emergency cut-off valve 1, and precisely filters the solid particles and moisture in the natural gas. The filtered high-pressure natural gas enters the natural gas expander 6 through the pipeline successively passing through the quick-opening valve A3, the regulating valve A4, and the globe valve A5. The structure of the quick-opening valve A3 is pneumatic or electric, used to quickly open or close the natural gas channel, playing a role in protecting the system; the structure of the regulating valve A4 is pneumatic or electric, and the rotational speed of the natural gas expander 6 is adjusted by regulating the amount of natural gas entering the natural gas expander 6, thereby adjusting the output power of the natural gas expander 6. The subcooled natural gas after pressure reduction and the oil-gas mixture of lubricating oil enter the primary oil separator A7 and the secondary oil separator A8 for secondary precision oil-gas separation. The oil content of the separated natural gas is less than 0.1 mg / m3. The separated subcooled lubricating oil enters the oil-oil heat exchanger 9 for heat exchange and heat regeneration. The lubricating oil after heat regeneration enters the oil rough filter A10 for rough filtering. The lubricating oil after rough filtering enters the oil pump A11 for pressurization. The pressurized lubricating oil enters the oil fine filter A12 for precision filtering. The precisely filtered lubricating oil enters the natural gas expander 6 for lubrication, cooling, and sealing;

[0031] The mixed refrigerant from the system is roughly filtered through the filter B13 to intercept the solid particles in the mixed refrigerant. The roughly filtered mixed refrigerant enters the mixed refrigerant compressor 14. After the mixed refrigerant compressor 14 compresses and consumes work on the mixed refrigerant from the system, the superheated mixed refrigerant after pressurization and the oil-gas mixture of lubricating oil enter the primary oil separator B15, the secondary oil separator B16, the tertiary oil separator 17, and the activated carbon filter 18 for four-stage precision oil-gas separation. The oil content of the separated mixed refrigerant is less than 0.01 mg / m3. The separated superheated lubricating oil enters the oil-oil heat exchanger 9 for heat exchange and cooling. The lubricating oil after cooling enters the oil rough filter A19 for rough filtering. The lubricating oil after rough filtering enters the oil pump A20 for pressurization. The pressurized lubricating oil enters the oil fine filter A21 for precision filtering. The precisely filtered lubricating oil enters the mixed refrigerant compressor 14 for lubrication, cooling, and sealing.

[0032] The separated subcooled natural gas enters the gas-gas heat exchanger 22 for heat exchange and heat regeneration. The natural gas after pressure reduction and heat regeneration enters the downstream natural gas pipeline network successively through the regulating valve C23 and the globe valve C24; the separated superheated mixed refrigerant enters the gas-gas heat exchanger 22 for heat exchange and cooling. The mixed refrigerant after pressurization and cooling enters the natural gas liquefaction system to exchange heat with the natural gas from the upstream natural gas pipeline network.

[0033] High-pressure natural gas from the upstream natural gas pipeline network enters the natural gas filter 2 after passing through the emergency cut-off valve 1, and precisely filters the solid particles and moisture in the natural gas. The filtered high-pressure natural gas enters the natural gas pretreatment unit 28 through the pipeline successively passing through the quick-opening valve B25, regulating valve B26, and stop valve B27. The structure of the quick-opening valve B25 is pneumatic or electric, and is used to quickly open or close the natural gas channel, playing a role in protecting the system; the structure of the regulating valve B26 is pneumatic or electric, and is used to regulate the natural gas entering the cold box 29. After the natural gas pretreatment unit 28 removes substances such as solid particles, carbon dioxide, hydrogen sulfide, water, and light oil in the natural gas, it enters the cold box 29, exchanges heat with the mixed refrigerant that has been heat-exchanged and cooled by the gas-gas heat exchanger 22, and cools down to below -160°C. The purified gaseous natural gas CNG becomes liquefied natural gas LNG.

[0034] A small part of the low-temperature natural gas that absorbs heat and vaporizes in the cold box 29 enters the air vaporizer A30 for heat recovery. The liquefied natural gas LNG in the cold box 20 enters the LNG tanker 32 through the LNG loading arm 31 for storage and transportation. A small part of the low-temperature natural gas that absorbs heat and vaporizes will be generated during the transportation and storage of the LNG tanker 32 by the LNG loading arm 31, and the low-temperature natural gas enters the air vaporizer B33 for heat recovery. The gaseous low-pressure natural gas recovered by the air vaporizer A30 and the air vaporizer B33 is pressurized by the BOG compressor 34, and the pressurized natural gas enters the primary oil-gas separator C35 and the secondary oil separator C36 for secondary precision oil-gas separation; the separated superheated lubricating oil enters the oil-oil heat exchanger 9 for heat exchange and cooling, the cooled lubricating oil enters the coarse oil filter A37 for coarse filtration, the coarsely filtered lubricating oil enters the oil pump A38 for pressurization, the pressurized lubricating oil enters the fine oil filter A39 for precision filtration, and the precisely filtered lubricating oil enters the mixed refrigerant compressor 14 for lubrication, cooling, and sealing. The oil content of the separated natural gas is less than 0.1 mg / m3, and it enters the downstream natural gas pipeline network successively through the regulating valve D40 and the stop valve D41.

[0035] The above describes the present invention and its embodiments. Such a description is not restrictive, and what is shown in the drawings is only one of the embodiments of the present invention, and the actual structure is not limited thereto. Generally speaking, if those of ordinary skill in the art are inspired by it and, without departing from the purpose of the present invention, design similar structural forms and embodiments to this technical solution without creative efforts, they should all fall within the protection scope of the present invention.

Claims

1. An apparatus for producing liquefied natural gas by utilizing the pressure energy and cold energy of natural gas, characterized in that: It includes a natural gas filter A (2), and the natural gas filter A (2) is connected to a natural gas expander (6) through a first pipeline. The natural gas filter A (2) is connected to a natural gas pretreatment unit (28) through a second pipeline. An emergency cut-off valve (1) is provided on the upstream pipeline of the natural gas filter A (2). A quick-opening valve A (3), a regulating valve A (4) and a stop valve A (5) are successively arranged on the first pipeline. A quick-opening valve B (25), a regulating valve B (26) and a stop valve B (27) are successively arranged on the second pipeline. The natural gas expander (6) is connected to a first-stage oil separator A (7). The first-stage oil separator A (7) is connected to an oil-oil heat exchanger (9) through a third pipeline. The third pipeline is connected to the natural gas expander (6) after passing through the oil-oil heat exchanger (9). An oil coarse filter A (10), an oil pump A (11) and an oil fine filter A (12) are successively arranged on the third pipeline. The natural gas expander (6) is connected to a mixed refrigerant compressor (14). The mixed refrigerant compressor (14) is connected to a first-stage oil separator B (15). The first-stage oil separator B (15) is connected to the oil-oil heat exchanger (9) through a fourth pipeline. The fourth pipeline is connected to the mixed refrigerant compressor (14) after passing through the oil-oil heat exchanger (9). A branch pipeline and an oil fine filter B (21) are successively arranged on the fourth pipeline. An oil coarse filter B (19) and an oil pump B (20) are successively arranged on the branch pipeline. A filter B (13) is provided upstream of the mixed refrigerant compressor (14). The natural gas pretreatment unit (2) is connected to a cold box (29). The cold box (29) is respectively connected to an air vaporizer A (30) and an LNG loading arm (31). The air vaporizer A (30) is connected to a BOG compressor (34) through a pipeline. The LNG loading arm (31) is respectively connected to an LNG tank truck (32) and an air vaporizer B (33). The air vaporizer B (33) is connected to the pipeline between the air vaporizer A (30) and the BOG compressor (34). The BOG compressor (34) is connected to a first-stage oil-gas separator C (35). The first-stage oil-gas separator C (35) is connected to the BOG compressor (34) through a fifth pipeline. An oil fine filter C (37), an oil pump C (38) and an oil coarse filter element C (39) are successively arranged on the fifth pipeline. The first-stage oil-gas separator C (35) is connected to a second-stage oil-gas separator C (36), and a regulating valve D (40) and a stop valve D (41) are provided on the connecting pipeline. The first-stage oil-gas separator C (35) is connected to the oil-oil heat exchanger (9). The first-stage oil separator A (7) is connected to a gas-gas heat exchanger (22) through a sixth pipeline. A second-stage oil separator A (8), a regulating valve C (23) and a stop valve C (24) are successively arranged on the sixth pipeline. The sixth pipeline is connected to the pipeline of the second-stage oil-gas separator C (36).The described primary oil separator B (15) is connected to the gas-gas heat exchanger (22) through the seventh pipeline. A secondary oil separator B (16), a tertiary oil separator (17), and an activated carbon filter (18) are successively arranged on the seventh pipeline. The regulating valve A (4) is a pneumatic regulating valve or an electric regulating valve, which controls the rotational speeds of the natural gas expander (6) and the mixed refrigerant compressor (14). The natural gas expander (6) has a structure of single-stage single-screw expander, single-stage double-screw expander, or centrifugal expander structure., 2. The device for producing liquefied natural gas by using natural gas pressure energy and cold energy according to claim 1, wherein: The described oil-oil heat exchanger (9) has a plate heat exchanger or a shell-and-tube heat exchanger structure.

3. The device for producing liquefied natural gas by utilizing natural gas pressure energy and cold energy according to claim 1, wherein: The described mixed refrigerant compressor (14) has a single-stage single-screw compressor structure, a single-stage double-screw compressor structure, or a centrifugal compressor structure.

4. The device for producing liquefied natural gas by utilizing the pressure energy and cold energy of natural gas according to claim 1, wherein: The described gas-gas heat exchanger (22) has a plate heat exchanger or a shell-and-tube heat exchanger structure.

5. A device for producing liquefied natural gas by utilizing natural gas pressure energy and cold energy according to claim 1, characterized in that: After the natural gas expander (6) expands and does work on the high-pressure natural gas from the upstream natural gas pipeline network, the subcooled natural gas after pressure reduction and the oil-gas mixture of lubricating oil enter the primary oil separator A (7) and the secondary oil separator A (8) for two-stage precise oil-gas separation. The oil content of the separated natural gas is less than 0.1 mg / m3. The separated subcooled lubricating oil enters the oil-oil heat exchanger (9) for heat exchange and heat recovery. The separated subcooled natural gas enters the gas-gas heat exchanger (22) for heat exchange and heat recovery. The natural gas after pressure reduction and heat recovery directly enters the downstream natural gas pipeline network. After the described mixed refrigerant compressor (14) compresses and consumes work on the mixed refrigerant from the system, the superheated mixed refrigerant after pressurization and the oil-gas mixture of lubricating oil enter the primary oil separator B (15), the secondary oil separator B (16), the tertiary oil separator (17), and the activated carbon filter (18) for four-stage precise oil-gas separation. The oil content of the separated mixed refrigerant is less than 0.01 mg / m3. The separated superheated lubricating oil enters the oil-oil heat exchanger (9) for heat exchange and cooling. The separated superheated mixed refrigerant enters the gas-gas heat exchanger (22) for heat exchange and cooling. The mixed refrigerant after pressurization and cooling enters the cold box (29) to exchange heat with the natural gas from the upstream natural gas pipeline network.

6. The device for producing liquefied natural gas by using natural gas pressure energy and cold energy according to claim 1, characterized in that: The described cold box (29) has a plate-fin or spiral-wound heat exchanger structure.

7. A device for producing liquefied natural gas by utilizing the pressure energy and cold energy of natural gas according to claim 1, characterized in that: The high-pressure natural gas from the upstream natural gas pipeline network enters the cold box (29) after removing solid particles, carbon dioxide, hydrogen sulfide, water, and light oil through the natural gas pretreatment unit (28), and exchanges heat with the mixed refrigerant that has been heat exchange and cooled by the gas-gas heat exchanger (22) to be cooled down to below -160°C. The purified gaseous natural gas (CNG) becomes liquefied natural gas (LNG).

8. A device for producing liquefied natural gas by utilizing natural gas pressure energy and cold energy according to claim 1, characterized in that: The described BOG compressor (34) has a screw compressor or a piston compressor structure.

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