A large-scale BOG reliquefaction refrigeration system based on reverse brayton cycle and operation method thereof

Abstract

The application discloses a BOG reliquefaction large refrigeration system based on reverse Brayton cycle and an operation method thereof. The system comprises a large reverse Brayton cycle refrigeration system and a BOG reliquefaction system. The large reverse Brayton cycle refrigeration system comprises an expander, a first motor, a first compressor, a bearing, a first heat exchanger, a second heat exchanger, a third heat exchanger, a fourth heat exchanger, a second compressor and a second motor. The BOG reliquefaction system comprises a liquefied natural gas storage device, a mechanical pump and a spray tower. The outlet of the liquefied natural gas storage device is connected with the mechanical pump and the second inlet of the fourth heat exchanger in sequence, the second outlet of the fourth heat exchanger is connected with the inlet of the spray tower, and the outlet of the spray tower is connected with the inlet of the liquefied natural gas storage device. The application avoids energy loss in the phase change process through the reverse Brayton cycle, and can reduce the overall operation cost compared with the traditional BOG treatment system.

Description

TECHNICAL FIELD

[0001] The present application relates to a refrigeration system, in particular to a BOG reliquefaction large-scale refrigeration system based on reverse Brayton cycle and a method for operating the same. BACKGROUND

[0002] During the storage and transportation of liquefied natural gas (LNG), a large amount of boil-off gas (BOG) will inevitably be generated. The traditional BOG treatment method mostly adopts direct combustion or simple compression recovery, which not only causes energy waste, but also has a negative impact on the environment. The existing BOG reliquefaction system has problems such as low refrigeration efficiency, large equipment energy consumption, and poor system stability, and it is difficult to meet the demand for efficient treatment of BOG in large-scale LNG storage and transportation scenarios. At the same time, conventional refrigeration systems mostly use traditional cycles such as vapor compression refrigeration cycle and absorption refrigeration cycle, which have the disadvantages of low energy utilization rate, large equipment volume, and high operating cost when facing large-scale refrigeration demand, and cannot realize economic and efficient refrigeration cycle and BOG reliquefaction treatment.

[0003] Therefore, it has become a key technical problem to be solved in the industry to develop an efficient, energy-saving, and stable BOG reliquefaction large-scale refrigeration system based on reverse Brayton cycle and a method for using the same. SUMMARY

[0004] To solve the above technical problems, the present application relates to a BOG reliquefaction large-scale refrigeration system based on reverse Brayton cycle and a method for operating the same, which aims to solve the problems of low efficiency, high energy consumption, and poor stability of existing BOG treatment and refrigeration systems, and to realize efficient reliquefaction of BOG and energy-saving and stable operation of the refrigeration system.

[0005] In order to achieve the above purpose, the present application provides the following technical solutions:

[0006] A BOG reliquefaction large-scale refrigeration system based on reverse Brayton cycle, comprising a large-scale reverse Brayton cycle refrigeration system and a BOG reliquefaction system.

[0007] The large reverse-Brayton cycle refrigeration system comprises an expander, a first motor, a first compressor, a bearing, a first heat exchanger, a second heat exchanger, a third heat exchanger, a fourth heat exchanger, a second compressor and a second motor; wherein the outlet of the expander is connected with the first inlet of the fourth heat exchanger, the first outlet of the fourth heat exchanger is connected with the inlet of the third heat exchanger, and the outlet of the third heat exchanger is connected with the inlet of the first compressor; the outlet of the first compressor is connected with the inlet of the first heat exchanger, and the outlet of the first heat exchanger is connected with the inlet of the second compressor; the outlet of the second compressor is connected with the inlet of the second heat exchanger, the outlet of the second heat exchanger is connected with the inlet of the third heat exchanger, and the outlet of the third heat exchanger is connected with the inlet of the expander.

[0008] The BOG reliquefaction system comprises a liquefied natural gas storage device, a mechanical pump and a spray tower; wherein the outlet of the liquefied natural gas storage device is connected with the mechanical pump and the second inlet of the fourth heat exchanger in sequence, the second outlet of the fourth heat exchanger is connected with the inlet of the spray tower, and the outlet of the spray tower is connected with the inlet of the liquefied natural gas storage device.

[0009] The expander and the first compressor are assembled into an integrated structure through the bearing.

[0010] The BOG reliquefaction large refrigeration system based on the reverse-Brayton cycle further comprises an auxiliary cooling system, which is used for cooling the first heat exchanger and the second heat exchanger, the outlet of the first heat exchanger is connected with the auxiliary cooling system, and the outlet of the second heat exchanger is connected with the auxiliary cooling system.

[0011] The displacement compensation devices are connected to the interfaces of the expander and the first compressor, so as to avoid excessive vibration and structural damage of the equipment caused by high-speed operation.

[0012] The vacuum extraction port and the air supplement port are connected with the vacuum extraction device and the air supplement device respectively.

[0013] The refrigeration working medium used in the large reverse-Brayton cycle refrigeration system is one or a mixture of more than one of nitrogen, helium, neon and argon.

[0014] The valves used in the low-temperature pipeline of the BOG reliquefaction large refrigeration system are low-temperature valves.

[0015] The first heat exchanger, the second heat exchanger, the third heat exchanger, the fourth heat exchanger and the low-temperature pipeline of the BOG reliquefaction large refrigeration system based on the reverse-Brayton cycle are all provided with an adiabatic layer.

[0016] The operation method of the BOG reliquefaction large refrigeration system based on the reverse Brayton cycle comprises the following steps: when the large reverse Brayton cycle refrigeration system is running, first, a vacuumizing device is used to perform a vacuumizing operation on the system to remove non-refrigerant gas in the system; then, a gas supplementing device is used to supplement refrigerant gas to the system; the refrigerant gas flows into the first compressor in sequence, is compressed and heated, and is cooled by the first heat exchanger; then, the refrigerant gas enters the second compressor for secondary compression, is continuously cooled by the second heat exchanger and the heat exchanger c; then, the refrigerant gas enters the expander for expansion, pressure reduction and temperature reduction, and the generated cold energy is transferred to the BOG reliquefaction system by the fourth heat exchanger; the refrigerant gas that is heated by the fourth heat exchanger returns to the first compressor, and a refrigeration cycle is completed.

[0017] Further, during the operation of the BOG reliquefaction system, a mechanical pump is used to pump liquefied natural gas in a liquefied natural gas storage device to the fourth heat exchanger for subcooling treatment, so as to form subcooled liquefied natural gas; the subcooled liquefied natural gas is sprayed into the liquefied natural gas storage device by a spray tower, so as to reliquefy boil-off gas in the liquefied natural gas storage device.

[0018] The subcooling degree of the subcooling treatment is greater than or equal to 5 DEG C.

[0019] The present application has the following advantages:

[0020] 1. In the present application, the expander and the first compressor are assembled by bearings to form an integrated structure, so that energy recovery and conversion can be realized. During the refrigeration cycle, the energy generated by the expansion of the expander can be directly used to drive the first compressor, so that external energy input is reduced, the energy efficiency of the system is improved, and the operation cost is reduced.

[0021] 2. The large reverse Brayton cycle refrigeration system and the BOG reliquefaction system work cooperatively, the LNG in the LNG storage device is pumped to the fourth heat exchanger for subcooling treatment, and then is sprayed into the LNG storage device in the form of mist by the spray tower, so that the BOG and the subcooled LNG are fully contacted and condensed and liquefied. This design can effectively utilize the cold energy and improve the efficiency of the BOG reliquefaction.

[0022] 3. The interface connection displacement compensation device of the expander and the first compressor effectively avoids excessive vibration and structural damage of the equipment caused by high-speed operation, and ensures long-term stable operation of the system. At the same time, low-temperature valves are used on the low-temperature pipeline, and vacuumizing and gas supplementing openings are arranged at key positions, so as to further enhance the stability and reliability of the system under complex working conditions such as low temperature and high pressure.

[0023] 4. One or more mixtures of nitrogen, helium, neon, argon and the like can be used as the refrigerant in the large reverse Brayton cycle refrigeration system, so that appropriate refrigerant can be selected according to different use scenes and requirements, the application range of the system is expanded, and the universality and adaptability of the system are improved.

[0024] 5、By avoiding the energy loss in the phase change process through the reverse Brayton cycle, the overall operating cost can be reduced compared with the traditional BOG treatment system. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 It is a schematic diagram of the structure of the large-scale reverse Brayton cycle refrigeration system of the present application;

[0026] Figure 2 It is a schematic diagram of the structure of the BOG reliquefaction system of the present application.

[0027] The reference signs are as follows:

[0028] 1-expander, 2-first motor, 3-first compressor, 4-second compressor, 5-second motor, 6-assisted cooling system, 7-bearing, 8-liquefied natural gas storage device, 9-mechanical pump, 10-spraying tower, ex1-first heat exchanger, ex2-second heat exchanger, ex3-third heat exchanger, ex4-fourth heat exchanger, s1-vacuumizing equipment, s2-gas supplementing equipment, 1a-outlet of the expander; 1b-inlet of the expander; 3b-inlet of the first compressor; 3a-outlet of the first compressor; 8a-outlet of the liquefied natural gas storage device; 8b-inlet of the liquefied natural gas storage device; EJ4-second inlet of the fourth heat exchanger; EC4-second outlet of the fourth heat exchanger; b1-first displacement compensation device; b2-second displacement compensation device; b3-third displacement compensation device; b4-fourth displacement compensation device; v301-first low-temperature valve; v302-second low-temperature valve; v303-third low-temperature valve; v304-fourth low-temperature valve; v305-fifth low-temperature valve; v101-first ball valve; v102-second ball valve; v201-first butterfly valve; v202-second butterfly valve; T1-first temperature sensor; T2-second temperature sensor; T3-third temperature sensor; T4-fourth temperature sensor; T5-fifth temperature sensor; T6-sixth temperature sensor; T7-seventh temperature sensor; T8-eighth temperature sensor; T9-ninth temperature sensor; P1-first pressure sensor; P2-second pressure sensor; P3-third pressure sensor; P4-fourth pressure sensor; P5-fifth pressure sensor; P6-sixth pressure sensor; P7-seventh pressure sensor; P8-eighth pressure sensor; sc1-outlet of the refrigeration working medium of the assisted cooling system for cooling the first heat exchanger; sc2-inlet of the refrigeration working medium of the assisted cooling system for cooling the first heat exchanger; sc3-outlet of the refrigeration working medium of the assisted cooling system for cooling the second heat exchanger, sc4-inlet of the refrigeration working medium of the assisted cooling system for cooling the second heat exchanger. DETAILED DESCRIPTION

[0029] In order to make the objects, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not used to limit the present application. In addition, the technical features involved in the various embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0030] Referring to Figure 1 and Figure 2 , the present embodiment provides a large-scale BOG reliquefaction system based on inverse Brayton cycle, which is composed of a large-scale inverse Brayton cycle refrigeration system and a BOG reliquefaction system.

[0031] Referring to Figure 1 , the large-scale inverse Brayton cycle refrigeration system comprises an expander 1, a first motor 2, a first compressor 3, a bearing 7, a first heat exchanger ex1, a second heat exchanger ex2, a third heat exchanger ex3, a fourth heat exchanger ex4, a second compressor 4 and a second motor 5.

[0032] The specific connection relationship is that the outlet 1a of the expander 1 is connected with the first inlet of the fourth heat exchanger ex4, the first outlet of the fourth heat exchanger ex4 is connected with the inlet of the third heat exchanger ex3, and the outlet of the third heat exchanger ex3 is connected with the inlet 3b of the first compressor 3; the outlet 3a of the first compressor 3 is connected with the inlet of the first heat exchanger ex1, the outlet of the first heat exchanger ex1 is connected with the inlet of the second compressor 4; the outlet of the second compressor 4 is connected with the inlet of the second heat exchanger ex2, the outlet of the second heat exchanger ex2 is connected with the inlet of the third heat exchanger ex3, and the outlet of the third heat exchanger ex3 is connected with the inlet 1b of the expander 1.

[0033] Referring to Figure 2 , the BOG reliquefaction system comprises a liquefied natural gas storage device 8, a mechanical pump 9 and a spray tower 10. The connection relationship is that the outlet 8a of the liquefied natural gas storage device 8 is connected with the mechanical pump 9 and the second inlet EJ4 of the fourth heat exchanger ex4 in sequence, the second outlet EC4 of the fourth heat exchanger ex4 is connected with the inlet of the spray tower 10, and the outlet of the spray tower 10 is connected with the inlet 8b of the liquefied natural gas storage device 8.

[0034] Preferably, referring to Figure 1 , the expander 1 and the first compressor 3 are assembled into an integrated structure through the bearing 7, which can realize energy recovery and conversion. In the refrigeration cycle process, the energy generated by the expansion work of the expander 1 can be directly used to drive the first compressor 3, reducing external energy input, improving system energy efficiency and reducing operating cost.

[0035] Preferably, the BOG reliquefaction system based on the reverse Brayton cycle further comprises an auxiliary cooling system 6 for cooling the first heat exchanger ex1 and the second heat exchanger ex2, the outlet of the first heat exchanger ex1 being connected to the auxiliary cooling system 6, and the outlet of the second heat exchanger ex2 being connected to the auxiliary cooling system 6. The auxiliary cooling system 6 in this embodiment can adjust the type of refrigerant according to the required refrigeration temperature. The auxiliary cooling system 6 in this embodiment is a cooling water cooling device, and the refrigerant is deionized water.

[0036] Referring to Figure 1 , the auxiliary cooling system 6 in this embodiment is two, which are used to cool the first heat exchanger ex1 and the second heat exchanger ex2. The refrigerant inlet of the auxiliary cooling system for cooling the first heat exchanger ex1 is sc2, and the refrigerant outlet of the auxiliary cooling system for cooling the first heat exchanger ex1 is sc1; the refrigerant inlet of the auxiliary cooling system for cooling the second heat exchanger ex2 is sc4, and the refrigerant outlet of the auxiliary cooling system for cooling the second heat exchanger ex2 is sc3. Specifically, the outlet of the first heat exchanger ex1 is in communication with the refrigerant inlet sc2 of the auxiliary cooling system for cooling the first heat exchanger ex1, and the outlet of the second heat exchanger ex2 is in communication with the refrigerant inlet sc4 of the auxiliary cooling system for cooling the second heat exchanger ex2.

[0037] The displacement compensation device is connected to the interface of the expander 1 and the first compressor 3 to avoid excessive vibration and structural damage to the system caused by high-speed operation of the equipment. Referring to Figure 1 A first displacement compensation device b1 is arranged near the inlet 3b of the first compressor 3, a second displacement compensation device b2 is arranged near the outlet 3a of the first compressor 3, a third displacement compensation device b3 is arranged near the inlet 1b of the expander 1, and a fourth displacement compensation device b4 is arranged near the outlet 1a of the expander 1. The displacement compensation device in this embodiment is a metal expansion joint.

[0038] The pipeline at the inlet end of the first compressor 3 is provided with a vacuum extraction port and a gas supplement port, and the vacuum extraction port and the gas supplement port are respectively connected to a vacuum extraction device s1 and a gas supplement device s2. A first ball valve v101 is arranged on the communication pipeline between the vacuum extraction device s1 and the vacuum extraction port; a second ball valve v102 is arranged on the communication pipeline between the gas supplement device s2 and the gas supplement port.

[0039] The refrigerant used in the large reverse Brayton cycle refrigeration system is one or a mixture of nitrogen, helium, neon, and argon, and the appropriate refrigerant can be selected according to actual needs.

[0040] The valves used in the low-temperature pipeline in the BOG reliquefaction large refrigeration system are low-temperature valves. Figure 1 and Figure 2 A first low-temperature valve v301 is arranged on the connecting pipeline between the expander 1 and the fourth heat exchanger ex4; a second low-temperature valve v302 is arranged on the connecting pipeline between the fourth heat exchanger ex4 and the third heat exchanger ex3; a third low-temperature valve v303 is arranged on the connecting pipeline between the third heat exchanger ex3 and the expander 1; a fourth low-temperature valve v304 is arranged on the connecting pipeline between the fourth heat exchanger ex4 and the liquefied natural gas storage device 8; and a fifth low-temperature valve v305 is arranged on the connecting pipeline between the fourth heat exchanger ex4 and the spray tower 10. Correspondingly, these connecting pipelines are low-temperature pipelines.

[0041] The first heat exchanger ex1, the second heat exchanger ex2, the third heat exchanger ex3, the fourth heat exchanger ex4 and the low-temperature pipeline of the BOG reliquefaction large refrigeration system based on the reverse Brayton cycle are all provided with an adiabatic layer.

[0042] Referring to Figure 1 A first butterfly valve v201 is arranged on the connecting pipeline between the outlet of the first heat exchanger ex1 and the inlet of the second compressor 4; and a second butterfly valve v202 is arranged on the connecting pipeline between the outlet of the second heat exchanger ex2 and the inlet of the third heat exchanger ex3.

[0043] Referring to Figure 1 and Figure 2 The BOG reliquefaction large refrigeration system based on the reverse Brayton cycle of the present application is further provided with a plurality of temperature sensors. Specifically, a first temperature sensor T1 is arranged between the first compressor 3 and the first heat exchanger ex1, a second temperature sensor T2 is arranged between the first heat exchanger ex1 and the second compressor 4, a third temperature sensor T3 is arranged between the third heat exchanger ex3 and the expander 1, a fourth temperature sensor T4 is arranged between the expander 1 and the fourth heat exchanger ex4, a fifth temperature sensor T5 is arranged between the fourth heat exchanger ex4 and the third heat exchanger ex3, a sixth temperature sensor T6 is arranged between the third heat exchanger ex3 and the first compressor 3, a seventh temperature sensor T7 is arranged between the liquefied natural gas storage device 8 and the fourth heat exchanger ex4, and an eighth temperature sensor T8 is arranged between the fourth heat exchanger ex4 and the spray tower 10.

[0044] Referring to Figure 1 and Figure 2The BOG reliquefaction large refrigeration system based on the reverse Brayton cycle of the application is also provided with a plurality of pressure sensors, specifically, a first pressure sensor P1 is arranged between the first compressor 3 and the first heat exchanger ex1, a second pressure sensor P2 is arranged between the first heat exchanger ex1 and the second compressor 4, a third pressure sensor P3 is arranged between the third heat exchanger ex3 and the expander 1, a fourth pressure sensor P4 is arranged between the expander 1 and the fourth heat exchanger ex4, a fifth pressure sensor P5 is arranged between the fourth heat exchanger ex4 and the third heat exchanger ex3, a sixth pressure sensor P6 is arranged between the third heat exchanger ex3 and the first compressor 3, a seventh pressure sensor P7 is arranged between the liquefied natural gas storage device 8 and the fourth heat exchanger ex4, and an eighth pressure sensor P8 is arranged between the fourth heat exchanger ex4 and the spray tower 10.

[0045] According to the above connection relationship, each component of the large reverse Brayton cycle refrigeration system and the BOG reliquefaction system is installed and connected, so that the connection between the components is tight and sealed well, and leakage is avoided.

[0046] The system is subjected to air tightness test, the pipeline of the system is pressurized to 1.5 MPa, and the pressure is maintained for 24 hours, and the pressure drop is required to be ≤0.1%. The welding points and flange connections are detected by using a helium mass spectrometer leak detector, and the leakage rate is required to be ≤1×10 -9 Pa·m 3 / s.

[0047] The operation method of the above-mentioned BOG reliquefaction large refrigeration system based on the reverse Brayton cycle is as follows:

[0048] I. Operation of the large reverse Brayton cycle refrigeration system

[0049] S1: Check the vacuumizing and air charging openings on the inlet pipeline of the first compressor 3 to ensure that they are unobstructed. Connect the vacuumizing device s1 and the air charging device s2 for debugging to ensure that they work normally.

[0050] S2: Inject an appropriate amount of cooling water into the auxiliary cooling system 6, and check whether the cooling water pipeline is normal. The initial cooling water temperature is 25℃, and the flow rate is 20 m 3 / h;

[0051] S3: Start the vacuumizing device s1 to perform vacuumizing operation on the large reverse Brayton cycle refrigeration system, and remove the non-refrigeration working gas in the system to 10 -3 Pa. Close the vacuumizing device s1, start the air charging device s2, and charge helium into the system to 0.2 MPa, replace the system for 3 times, and then charge the mixed working medium of nitrogen and helium to 0.5~0.7 MPa;

[0052] S4: Start the first motor 2 and the second motor 5 to drive the first compressor 3 and the second compressor 4 to start working. The operating frequency of the first compressor 3 is 460-490 Hz, and the operating frequency of the second compressor 4 is 480-510 Hz. The two compressors adopt equal pressure ratio distribution design, and the compression ratio is 1.2-1.6. The refrigeration working medium first enters the first compressor 3 for compression, and the temperature is increased. The compressed refrigeration working medium flows into the first heat exchanger ex1 and is cooled by the auxiliary cooling system 6 to cool the temperature of the refrigeration working medium to 20-30℃. The cooled refrigeration working medium enters the second compressor 4 for secondary compression to further increase the pressure and temperature. The again compressed refrigeration working medium successively passes through the second heat exchanger ex2 and the third heat exchanger ex3 for continuous cooling to cool the temperature of the refrigeration working medium to-145--150℃. The cooled refrigeration working medium enters the expander 1 for expansion to produce a large amount of cold energy. The cold energy produced by the expander 1 is transmitted to the LNG in the BOG reliquefaction system through the fourth heat exchanger ex4 to supercool the LNG, and at this time the temperature of the refrigeration working medium is-161--165℃. The temperature rising working medium flowing out from the fourth heat exchanger ex4 pre-cools the compressed refrigeration working medium cooled by the second heat exchanger ex2 through the third heat exchanger ex3, and then the refrigeration working medium returns to the first compressor 3 to complete a refrigeration cycle period.

[0053] II. Operation of the BOG reliquefaction system

[0054] The fourth low-temperature valve v304 and the fifth low-temperature valve v305 are opened, and the LNG in the liquefied natural gas storage device 8 is pumped to the fourth heat exchanger ex4 by the mechanical pump 9 for supercooling treatment to form supercooled LNG. The temperature difference of the LNG inlet and outlet is controlled at 6-8℃, and the supercooling degree is ≥5℃.

[0055] The supercooled LNG passes through the spray tower 10, adopts multi-stage atomizing nozzles, is sprayed into the liquefied natural gas storage device 8, and reliquefies the BOG in the liquefied natural gas storage device 8. The pressure of the liquefied natural gas storage device 8 is maintained at 0.12-0.15 MPa. When the pressure exceeds 0.15 MPa, the spraying amount is automatically increased by 10%-20%.

[0056] Periodically maintain and maintain the system, such as cleaning the heat exchanger, replacing the sealing element, etc., to ensure long-term stable operation of the system.

[0057] Those skilled in the art will readily understand that the above description is only a preferred embodiment of the present application and is not intended to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

[0058] The portions of the present application specification not described in detail are part of the general knowledge in the art, and the above examples are provided only for the purpose of describing the present application and are not intended to limit the scope of the present application. The scope of the present application is defined by the appended claims. Various equivalent substitutions and modifications made without departing from the spirit and principles of the present application should be encompassed within the scope of the present application.

Claims

1. A large scale BOG reliquefaction system based on reverse-Brayton cycle, characterized in that, The BOG reliquefaction large refrigeration system based on reverse Brayton cycle comprises a large reverse Brayton cycle refrigeration system and a BOG reliquefaction system. The large reverse Brayton cycle refrigeration system comprises an expander, a first motor, a first compressor, a bearing, a first heat exchanger, a second heat exchanger, a third heat exchanger, a fourth heat exchanger, a second compressor and a second motor; wherein the outlet of the expander is connected with the first inlet of the fourth heat exchanger, the first outlet of the fourth heat exchanger is connected with the inlet of the third heat exchanger, and the outlet of the third heat exchanger is connected with the inlet of the first compressor; the outlet of the first compressor is connected with the inlet of the first heat exchanger, and the outlet of the first heat exchanger is connected with the inlet of the second compressor; the outlet of the second compressor is connected with the inlet of the second heat exchanger, the outlet of the second heat exchanger is connected with the inlet of the third heat exchanger, and the outlet of the third heat exchanger is connected with the inlet of the expander. The BOG reliquefaction system comprises a liquefied natural gas storage device, a mechanical pump and a spray tower; wherein the outlet of the liquefied natural gas storage device is connected with the mechanical pump and the second inlet of the fourth heat exchanger in sequence, the second outlet of the fourth heat exchanger is connected with the inlet of the spray tower, and the outlet of the spray tower is connected with the inlet of the liquefied natural gas storage device.

2. The BOG reliquefaction large scale refrigeration system based on inverse Brayton cycle of claim 1, wherein, The expander and the first compressor are assembled into an integrated structure through the bearing.

3. The inverse Brayton cycle based BOG reliquefaction macro- refrigeration system of claim 1, wherein, The BOG reliquefaction large refrigeration system based on reverse Brayton cycle further comprises an auxiliary cooling system for cooling the first heat exchanger and the second heat exchanger; the outlet of the first heat exchanger is connected with the auxiliary cooling system, and the outlet of the second heat exchanger is connected with the auxiliary cooling system.

4. The inverse Brayton cycle based BOG reliquefaction macro- refrigeration system of claim 1, wherein, The displacement compensation devices are connected to the interfaces of the expander and the first compressor to avoid excessive vibration and structural damage of the equipment caused by high-speed operation.

5. The inverse Brayton cycle based BOG reliquefaction macro- refrigeration system of claim 1, wherein, A vacuum extraction port and a gas supplement port are arranged on the pipeline of the inlet end of the first compressor, and the vacuum extraction port and the gas supplement port are respectively connected with a vacuum extraction device and a gas supplement device.

6. The inverse Brayton cycle based BOG reliquefaction macro- refrigeration system of claim 1, wherein, The refrigeration working medium used in the large reverse Brayton cycle refrigeration system is one or a mixture of more than one of nitrogen, helium, neon and argon.

7. The inverse Brayton cycle based BOG reliquefaction macro- refrigeration system of claim 1, wherein, The valves used in the low-temperature pipeline of the BOG reliquefaction large refrigeration system are low-temperature valves.

8. The inverse Brayton cycle based BOG reliquefaction macro- refrigeration system of claim 1, wherein, The first heat exchanger, the second heat exchanger, the third heat exchanger, the fourth heat exchanger and the low-temperature pipeline of the BOG reliquefaction large refrigeration system based on reverse Brayton cycle are all provided with an adiabatic layer.

9. A method of operating a large scale BOG reliquefaction system based on reverse-Brayton cycle according to any of claims 1 to 8, characterized in that: During the operation of the large reverse Brayton cycle refrigeration system, the vacuum extraction device is used to perform a vacuum extraction operation on the system to remove non-refrigeration working gas in the system; then the gas supplement device is used to charge the refrigeration working medium into the system; the refrigeration working medium flows into the first compressor in sequence, is compressed and heated, and is cooled by the first heat exchanger; then the refrigeration working medium enters the second compressor for secondary compression and is continuously cooled by the second heat exchanger and the third heat exchanger; Then the refrigeration working medium enters the expander for expansion, pressure reduction and temperature reduction, and the generated cold energy is transferred to the BOG reliquefaction system by the fourth heat exchanger; the refrigeration working medium heated by the fourth heat exchanger returns to the first compressor to complete a refrigeration cycle.

10. A method of operating a large scale BOG reliquefaction system based on inverse Brayton cycle according to claim 9, characterized in that: In the working process of the BOG reliquefaction system, the mechanical pump pumps the liquefied natural gas in the liquefied natural gas storage device to the fourth heat exchanger for subcooling treatment, to form subcooled liquefied natural gas, and the subcooled liquefied natural gas is sprayed into the liquefied natural gas storage device through the spray tower, to reliquefy the evaporated gas in the liquefied natural gas storage device. The subcooling degree of the subcooling treatment is greater than or equal to 5 DEG C.

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