Boil off gas (BOG) reliquefaction large refrigerating system based on reverse Brayton cycle and operation method of BOG reliquefaction large refrigerating system

Abstract

The invention discloses a BOG reliquefaction large refrigerating system based on an inverse Brayton cycle and an operation method of the BOG reliquefaction large refrigerating system. The system comprises a large reverse Brayton cycle refrigeration system and a BOG reliquefaction system. The large reverse Brayton cycle refrigerating system comprises an expansion machine, 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; wherein an outlet of the liquefied natural gas storage device is sequentially connected with the mechanical pump and a second inlet of the fourth heat exchanger, a second outlet of the fourth heat exchanger is connected with an inlet of the spray tower, and an outlet of the spray tower is connected with an inlet of the liquefied natural gas storage device. Energy loss in the phase change process is avoided through the inverse Brayton cycle, and compared with a traditional BOG treatment system, the overall operation cost can be reduced.

Description

Technical Field

[0001] The present invention relates to a refrigeration system, and in particular to a large-scale BOG reliquefaction refrigeration system based on a reverse Brayton cycle and an operation method thereof. Background Art

[0002] During the storage and transportation of liquefied natural gas (LNG), a large amount of boil-off gas (BOG) is inevitably generated. Traditional BOG treatment methods often rely on direct combustion or simple compression recovery, which not only wastes energy but also has negative impacts on the environment. Existing BOG reliquefaction systems suffer from low refrigeration efficiency, high equipment energy consumption, and poor system stability, making it difficult to meet the demand for efficient BOG treatment in large-scale LNG storage and transportation scenarios. Furthermore, conventional refrigeration systems often use traditional cycle methods, such as vapor compression refrigeration cycles and absorption refrigeration cycles. When faced with large-scale refrigeration needs, they suffer from low energy utilization, bulky equipment, and high operating costs, making it impossible to achieve economical and efficient refrigeration cycles and BOG reliquefaction.

[0003] Therefore, the development of an efficient, energy-saving and stable large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle and its use method has become a key technical problem that needs to be urgently solved in the industry. Summary of the Invention

[0004] To address the aforementioned technical issues, the present invention relates to a large-scale BOG reliquefaction refrigeration system based on a reverse Brayton cycle and its operating method. These systems aim to address the low efficiency, high energy consumption, and poor stability of existing BOG treatment and refrigeration systems, thereby achieving efficient BOG reliquefaction and energy-saving and stable operation of the refrigeration system.

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

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

[0007] The large-scale reverse Brayton cycle refrigeration system includes 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 to the first inlet of the fourth heat exchanger, the first outlet of the fourth heat exchanger is connected to the inlet of the third heat exchanger, and the outlet of the third heat exchanger is connected to the inlet of the first compressor; the outlet of the first compressor is connected to the inlet of the first heat exchanger, and the outlet of the first heat exchanger is connected to the inlet of the second compressor; the outlet of the second compressor is connected to the inlet of the second heat exchanger, the outlet of the second heat exchanger is connected to the inlet of the third heat exchanger, and the outlet of the third heat exchanger is connected to the inlet of the expander;

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

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

[0010] The large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle also includes an auxiliary cooling system, which is used to cool the first heat exchanger and the second heat exchanger. The outlet of the first heat exchanger is connected to the auxiliary cooling system, and the outlet of the second heat exchanger is connected to the auxiliary cooling system.

[0011] Among them, the interfaces of the expander and the first compressor are both connected to displacement compensation devices to prevent the equipment from causing excessive vibration and structural damage to the system due to high-speed operation.

[0012] Wherein, a vacuum port and an air supply port are provided on the pipeline at the inlet end of the first compressor, and the vacuum port and the air supply port are connected to the vacuum equipment and the air supply equipment respectively.

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

[0014] Among them, the valves used on the low-temperature pipelines in the large-scale BOG reliquefaction refrigeration system are all low-temperature valves.

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

[0016] The aforementioned method for operating a large-scale BOG reliquefaction refrigeration system based on a reverse Brayton cycle specifically includes the following steps: when the large-scale reverse Brayton cycle refrigeration system is in operation, the system is first evacuated using a vacuum pump to remove non-refrigeration gas from the system; the system is then charged with refrigerant through a gas supply device; the refrigerant sequentially flows into a first compressor for compression and temperature increase, dissipates heat and cools through a first heat exchanger; then enters a second compressor for secondary compression, and continuously cools through a second heat exchanger and heat exchanger C; then enters an expander for expansion and pressure reduction and temperature reduction, with the generated cooling energy being transferred to the BOG reliquefaction system via a fourth heat exchanger; and the refrigerant, having absorbed heat and temperature through the fourth heat exchanger, returns to the first compressor, completing a refrigeration cycle.

[0017] Furthermore, during the operation 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 supercooling to form supercooled liquefied natural gas. The supercooled liquefied natural gas is sprayed into the liquefied natural gas storage device through the spray tower to reliquefy the boil-off gas in the liquefied natural gas storage device.

[0018] The supercooling degree of the supercooling treatment is ≥5℃.

[0019] The beneficial effects of the present invention are as follows:

[0020] 1. In the present invention, the expander and first compressor are assembled into an integrated structure via bearings, enabling energy recovery and conversion. During the refrigeration cycle, the energy generated by the expander's expansion work can be directly used to drive the first compressor, reducing external energy input, improving system energy efficiency, and lowering operating costs.

[0021] 2. A large reverse Brayton cycle refrigeration system works in conjunction with the BOG reliquefaction system. LNG from the LNG storage unit is pumped to the fourth heat exchanger for subcooling. The LNG is then sprayed into the LNG storage unit in a mist-like form through a spray tower, allowing the BOG to fully contact the subcooled LNG for condensation and liquefaction. This design effectively utilizes cooling capacity and improves BOG reliquefaction efficiency.

[0022] 3. A displacement compensation device is installed at the interface between the expander and the first compressor, effectively preventing excessive vibration and structural damage caused by high-speed operation, ensuring long-term stable operation of the system. Furthermore, the use of cryogenic valves on the cryogenic pipelines, along with vacuum and air supply openings at key locations, further enhance the system's stability and reliability under complex operating conditions such as low temperatures and high pressures.

[0023] 4. In large-scale reverse Brayton cycle refrigeration systems, one or more mixtures of fluids such as nitrogen, helium, neon, and argon can be used as refrigerants. Appropriate refrigerants can be selected according to different usage scenarios and needs, which expands the application range of the system and improves the versatility and adaptability of the system.

[0024] 5. The reverse Brayton cycle avoids energy loss during phase change, which can reduce overall operating costs compared to traditional BOG treatment systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the structure of a large reverse Brayton cycle refrigeration system of the present invention;

[0026] Figure 2 Schematic diagram of the structure of the BOG reliquefaction system of the present invention.

[0027] The reference numerals are as follows:

[0028] 1- expander, 2- first motor, 3- first compressor, 4- second compressor, 5- second motor, 6- auxiliary cooling system, 7- bearing, 8- liquefied natural gas storage device, 9- mechanical pump, 10- spray tower, ex1- first heat exchanger, ex2- second heat exchanger, ex3- third heat exchanger, ex4- fourth heat exchanger, s1- vacuum equipment, s2- air supply equipment, 1a- expander outlet; 1b- expander inlet; 3b- first compressor inlet; 3a- first Outlet of the 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 cryogenic valve; v302- second cryogenic valve; v303- third cryogenic valve; v304- fourth cryogenic valve; v305- fifth cryogenic valve 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-refrigerant outlet of the auxiliary cooling system for cooling the first heat exchanger; sc2-refrigerant inlet of the auxiliary cooling system for cooling the first heat exchanger; sc3-refrigerant outlet of the auxiliary cooling system for cooling the second heat exchanger, sc4-refrigerant inlet of the auxiliary cooling system for cooling the second heat exchanger. DETAILED DESCRIPTION

[0029] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0030] See also Figure 1 and Figure 2 As shown, this embodiment provides a large-scale BOG reliquefaction refrigeration system based on a reverse Brayton cycle, which is composed of a large-scale reverse Brayton cycle refrigeration system and a BOG reliquefaction system.

[0031] See also Figure 1 The large reverse Brayton cycle refrigeration system includes 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 as follows: the outlet 1a of the expander 1 is connected to the first inlet of the fourth heat exchanger ex4, the first outlet of the fourth heat exchanger ex4 is connected to the inlet of the third heat exchanger ex3, and the outlet of the third heat exchanger ex3 is connected to the inlet 3b of the first compressor 3; the outlet 3a of the first compressor 3 is connected to the inlet of the first heat exchanger ex1, and the outlet of the first heat exchanger ex1 is connected to the inlet of the second compressor 4; the outlet of the second compressor 4 is connected to the inlet of the second heat exchanger ex2, the outlet of the second heat exchanger ex2 is connected to the inlet of the third heat exchanger ex3, and the outlet of the third heat exchanger ex3 is connected to the inlet 1b of the expander 1.

[0033] See also Figure 2 The BOG reliquefaction system includes a liquefied natural gas storage device 8, a mechanical pump 9, and a spray tower 10. The connections are as follows: the outlet 8a of the liquefied natural gas storage device 8 is sequentially connected to the mechanical pump 9 and the second inlet EJ4 of the fourth heat exchanger ex4; the second outlet EC4 of the fourth heat exchanger ex4 is connected to the inlet of the spray tower 10; and the outlet of the spray tower 10 is connected to the inlet 8b of the liquefied natural gas storage device 8.

[0034] Better, see Figure 1 The expander 1 and first compressor 3 are assembled via bearings 7 to form an integrated structure, enabling energy recovery and conversion. During the refrigeration cycle, 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 lowering operating costs.

[0035] Preferably, the large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle further includes 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 is connected to the auxiliary cooling system 6, and the outlet of the second heat exchanger ex2 is connected to the auxiliary cooling system 6. The auxiliary cooling system 6 in this embodiment can adjust the refrigerant type according to the required refrigeration temperature. In this embodiment, the auxiliary cooling system 6 is a cooling water cooling device, and its refrigerant is deionized water.

[0036] See also Figure 1 In this embodiment, there are two auxiliary cooling systems 6, one for cooling the first heat exchanger ex1 and the other for cooling 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 connected to 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 connected to the refrigerant inlet sc4 of the auxiliary cooling system for cooling the second heat exchanger ex2.

[0037] The interfaces of the expander 1 and the first compressor 3 are both connected to displacement compensation devices to prevent excessive vibration and structural damage to the system caused by high-speed operation of the equipment. Figure 1 A first displacement compensating device b1 is provided near the inlet 3b of the first compressor 3, a second displacement compensating device b2 is provided near the outlet 3a of the first compressor 3, a third displacement compensating device b3 is provided near the inlet 1b of the expander 1, and a fourth displacement compensating device b4 is provided near the outlet 1a of the expander 1. The displacement compensating devices in this embodiment are metal expansion joints.

[0038] The inlet pipeline of the first compressor 3 is provided with a vacuum port and an air supply port, which are connected to vacuum equipment s1 and air supply equipment s2, respectively. A first ball valve v101 is installed on the pipeline connecting the vacuum equipment s1 and the vacuum port; a second ball valve v102 is installed on the pipeline connecting the air supply equipment s2 and the air supply port.

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

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

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

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

[0043] See also Figure 1 and Figure 2 The large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle of the present invention is further provided with a plurality of temperature sensors. Specifically, a first temperature sensor T1 is provided between the first compressor 3 and the first heat exchanger ex1, a second temperature sensor T2 is provided between the first heat exchanger ex1 and the second compressor 4, a third temperature sensor T3 is provided between the third heat exchanger ex3 and the expander 1, a fourth temperature sensor T4 is provided between the expander 1 and the fourth heat exchanger ex4, a fifth temperature sensor T5 is provided between the fourth heat exchanger ex4 and the third heat exchanger ex3, a sixth temperature sensor T6 is provided between the third heat exchanger ex3 and the first compressor 3, a seventh temperature sensor T7 is provided between the liquefied natural gas storage device 8 and the fourth heat exchanger ex4, an eighth temperature sensor T8 is provided between the fourth heat exchanger ex4 and the spray tower 10, and a ninth temperature sensor T9 is provided between the second heat exchanger ex2 and the third heat exchanger ex3.

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

[0045] According to the above connection relationship, the various components of the large reverse Brayton cycle refrigeration system and the BOG reliquefaction system are installed and connected to ensure that the connections between the components are tight and well sealed to avoid leakage.

[0046] Perform an airtight test on the system. The system pipeline is pressurized to 1.5 MPa and maintained for 24 hours. The pressure drop is required to be ≤0.1%. Use a helium mass spectrometer leak detector to detect welding points and flange connections. The leakage rate is required to be ≤1×10 -9 Pa·m 3 / s.

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

[0048] 1. Operation of large reverse Brayton cycle refrigeration system

[0049] S1: Check the vacuum and air supply openings on the inlet pipe of the first compressor 3 to ensure they are unobstructed. Connect the vacuum equipment s1 and the air supply equipment s2 and debug them to ensure they are working properly.

[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 vacuum pumping equipment S1 to vacuum the large reverse Brayton cycle refrigeration system to remove the non-refrigeration working fluid gas in the system and pump the system to 10 -3 Pa. Turn off the vacuum pump s1, start the gas supply device s2, fill the system with helium to 0.2 MPa, replace the system three times, and then fill it with a mixture of nitrogen and helium to 0.5-0.7 MPa;

[0052] S4: Start the first motor 2 and the second motor 5, driving the first compressor 3 and the second compressor 4 to begin operation. 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. Both compressors adopt an isobaric distribution design with a compression ratio of 1.2-1.6. The refrigerant first enters the first compressor 3 for compression, which increases its temperature. The compressed refrigerant flows into the first heat exchanger ex1 and is cooled by the auxiliary cooling system 6, reducing its temperature to 20-30°C. The cooled refrigerant enters the second compressor 4 for secondary compression, further increasing its pressure and temperature. The recompressed refrigerant then passes through the second heat exchanger ex2 and the third heat exchanger ex3, continuing to cool down to -145--150°C. The cooled refrigerant enters the expander 1 for expansion, reducing its pressure and temperature, generating a large amount of cooling capacity. The cooling energy generated by expander 1 is transferred to the LNG in the BOG reliquefaction system via the fourth heat exchanger ex4, supercooling the LNG. At this point, the refrigerant temperature is between -161°C and -165°C. A portion of the heated refrigerant flowing out of the fourth heat exchanger ex4 is transferred through the third heat exchanger ex3 to pre-cool the compressed refrigerant, which has been cooled by the second heat exchanger ex2. The refrigerant then returns to the first compressor 3, completing one refrigeration cycle.

[0053] 2. Operation of BOG reliquefaction system

[0054] Open the fourth and fifth cryogenic valves v304 and v305, and use mechanical pump 9 to pump LNG from liquefied natural gas storage unit 8 to the fourth heat exchanger ex4 for subcooling, producing subcooled LNG. The temperature difference between the LNG inlet and outlet is controlled between 6°C and 8°C, with a subcooling degree of ≥5°C.

[0055] Subcooled LNG passes through spray tower 10 and is sprayed into liquefied natural gas storage unit 8 using multi-stage atomizing nozzles, reliquefying the BOG within. The pressure of liquefied natural gas storage unit 8 is maintained at 0.12-0.15 MPa. When the pressure exceeds 0.15 MPa, the spray rate is automatically increased by 10-20%.

[0056] Regularly perform maintenance and upkeep on the system, such as cleaning the heat exchanger and replacing seals, to ensure long-term and stable operation of the system.

[0057] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0058] Any portions not described in detail in this specification are known in the art. The above embodiments are provided for illustrative purposes only and are not intended to limit the scope of the present invention. The scope of the present invention is defined by the appended claims. Any equivalent substitutions and modifications that do not depart from the spirit and principles of the present invention are intended to be encompassed within the scope of the present invention.

Claims

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

2. The large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle according to claim 1 is characterized in that: The expander and the first compressor are assembled through bearings to form an integrated structure.

3. The large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle according to claim 1, characterized in that: The large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle also includes an auxiliary cooling system, which is used to cool the first heat exchanger and the second heat exchanger. The outlet of the first heat exchanger is connected to the auxiliary cooling system, and the outlet of the second heat exchanger is connected to the auxiliary cooling system.

4. The large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle according to claim 1, characterized in that: The interfaces of the expander and the first compressor are both connected to displacement compensation devices to prevent excessive vibration and structural damage to the system due to high-speed operation of the equipment.

5. The large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle according to claim 1, characterized in that: A vacuum port and an air supply port are provided on the pipeline at the inlet end of the first compressor, and the vacuum port and the air supply port are connected to a vacuum device and an air supply device respectively.

6. The large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle according to claim 1, characterized in that: The refrigerant used in the large-scale reverse Brayton cycle refrigeration system is a mixture of one or more of nitrogen, helium, neon and argon.

7. The large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle according to claim 1, characterized in that: The valves used on the low-temperature pipelines in the large-scale BOG reliquefaction refrigeration system are all low-temperature valves.

8. The large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle according to claim 1, characterized in that: The surfaces of the first heat exchanger, the second heat exchanger, the third heat exchanger, the fourth heat exchanger and the low-temperature pipelines of the large-scale BOG reliquefaction refrigeration system based on the reverse Brayton cycle are all provided with a thermal insulation layer.

9. A method for operating a large-scale BOG reliquefaction refrigeration system based on a reverse Brayton cycle according to any one of claims 1 to 8, characterized in that: When a large reverse Brayton cycle refrigeration system is in operation, the system is first evacuated using a vacuum pump to remove non-refrigerating working fluid gases from the system. The system is then filled with refrigerant through a gas supply device. The refrigerant flows into the first compressor for compression and temperature increase, then dissipates heat and cools down through the first heat exchanger. The refrigerant then flows into the second compressor for secondary compression, and continues to cool down through the second heat exchanger and heat exchanger C. After that, it enters the expander for expansion, decompression and temperature reduction. The generated cooling capacity is transferred to the BOG reliquefaction system by the fourth heat exchanger. The refrigerant after absorbing heat and heating up in the fourth heat exchanger returns to the first compressor, completing a refrigeration cycle.

10. The method for operating a large-scale BOG reliquefaction refrigeration system based on a reverse Brayton cycle according to claim 9, characterized in that: During the operation of the BOG reliquefaction system, a mechanical pump pumps the liquefied natural gas in the liquefied natural gas storage device to the fourth heat exchanger for subcooling, forming subcooled liquefied natural gas. The subcooled liquefied natural gas is sprayed into the liquefied natural gas storage device through a spray tower to reliquefy the boil-off gas in the liquefied natural gas storage device. Among them, the supercooling degree of supercooling treatment is ≥5℃.

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