LNG filling station BOG recovery system and working method thereof

Abstract

The application discloses an LNG filling station BOG recovery system and a working method thereof, and belongs to the technical field of LNG filling station BOG recovery system.The LNG filling station BOG recovery system comprises a BOG liquefaction recovery line, an LNG unloading line and an LNG filling process, wherein a liquid phase port of an LNG tank car is connected with an LNG immersion liquid pump, a gas phase port of the LNG tank car is connected with an LNG booster gasifier, and the BOG recovery system is connected with an LNG storage tank through pipelines at both ends; two turbine expansion and compression integrated machines are connected in parallel in the recovery system; the LNG storage tank is connected with an LNG filling mechanism through a pump, a valve and the like to form a loop; and a plurality of adjusting valves are arranged on each process pipeline in the system and used for safety control and switching of a working line.The working method of the system is also disclosed.The LNG filling station BOG recovery system has the advantages of recovery of unstable BOG generated under various operating conditions of the LNG filling station, fast cold start in a liquefaction process, safety and reliability, long-term economy and the like.

Description

Technical Field

[0001] This invention relates to the field of flash vapor treatment technology for LNG refueling stations, and in particular to a BOG recovery system for LNG refueling stations and its operating method. Background Technology

[0002] In recent years, with economic development, the demand for natural gas has surged. Coupled with the energy-saving and emission-reduction effects and lower transportation costs of LNG new energy vehicles, more and more large, medium and small vehicles fueled by LNG have entered the market, and the number of LNG refueling stations built in various regions has also increased accordingly.

[0003] Currently, LNG refueling stations primarily rely on LNG tank trucks for gas supply. During transportation, unloading, storage, and refueling, LNG is affected by various thermal factors, causing it to absorb heat and generate boil-off gas (BOG). The key characteristic of BOG recovery at LNG refueling stations is the significant variation in BOG generation due to different operating conditions. For example, BOG is generated from daily heat leakage in LNG storage tanks, during LNG unloading, and during LNG refueling. If not properly handled, BOG can lead to safety issues such as excessively high tank pressure, environmental pollution, and the creation of hazardous spaces around the refueling station containing flammable gases. Furthermore, BOG generation during station operation can cause economic losses for the company.

[0004] Currently, most medium and small-sized LNG refueling stations on the market are not equipped with BOG liquefaction recovery lines. When the pressure of LNG storage and transportation is too high, they are generally released. Although a few patents have designed recovery systems for BOG gas generated by LNG refueling stations, such as CN201510195184.6 (BOG recovery system for LNG refueling stations) and CN201710681258.6 (BOG gas recovery system in a city gate station), there are still some shortcomings: the designed systems are mostly designed for large LNG receiving stations and other places, without clear implementation direction, and the design process rarely considers the instability characteristics of BOG generated under different operating conditions of LNG refueling stations.

[0005] A study by Qi Chunjun et al. on the generation and recovery methods of BOG (Boiled Air Gaseous) at LNG vehicle refueling stations used the actual production process and equipment usage of an LNG vehicle refueling station in Pizhou City, Jiangsu Province as the basic data collection sample. The study measured that the daily BOG evaporation was approximately 93.09 kg when there was no unloading and no refueling; under the condition of no unloading, the daily BOG evaporation at an LNG refueling station with a daily refueling volume of 1×10⁴ Nm³ was approximately 394.41 kg; and under the condition of maximum refueling volume with unloading, the daily BOG evaporation was approximately 603.72 kg. This demonstrates that the amount of BOG generated at LNG refueling stations is highly unstable under different operating conditions, and a systematic solution for BOG liquefaction and recovery is still lacking. Summary of the Invention

[0006] Purpose of the Invention: To address the aforementioned problems, the purpose of this invention is to provide a BOG recovery system for LNG refueling stations, enabling the recovery of BOG gas under different operating conditions, improving cold start speed, enhancing safety and reliability, and increasing economic efficiency. The invention also provides its operating method.

[0007] Technical solution: A BOG recovery system for LNG refueling stations, including a BOG liquefaction and recovery line, an LNG unloading line, and an LNG refueling line;

[0008] The BOG liquefaction and recovery line includes an LNG storage tank. The top vapor port A-1 of the LNG storage tank is connected to a pressure control valve. The outlet of the pressure control valve splits into two branches. One branch, L-2, connects sequentially to regulating valve one, the first heat exchanger, the second heat exchanger, the third heat exchanger, the fourth heat exchanger, pump one, and throttle valve one before returning to the lower liquid port A-2 of the LNG storage tank. The other branch, L-1, connects sequentially to shut-off valve one, the regenerator, the compressor, and the cooler. The nitrogen cylinder group is connected to a pressure reducing valve. The outlet pipeline L-4 of the pressure reducing valve merges with the outlet pipeline L-3 of the cooler and then splits into two parallel lines. Branch circuits: One branch, L-5, is equipped with shut-off valve two; another branch, L-6, is sequentially connected to shut-off valve three, compressor, cooler, and shut-off valve four. The outlets of shut-off valve four and shut-off valve two merge and are sequentially connected to the second turbine expander, the first turbine expander, and the cooler before returning to the hot-end inlet E-5 of the regenerator. The hot-end outlet E-6 of the regenerator is connected to the hot-end inlet HX1-5 of the first heat exchanger. The hot-end outlet HX1-6 of the first heat exchanger splits into two branches: one branch, L-8, is sequentially connected to regulating valve two and the hot-end inlet HX2-5 of the second heat exchanger; the other branch, L-7, is sequentially connected to… The expansion inlet of the second turbine expander is connected to regulating valve three. The expansion outlet of the second turbine expander merges into the cold end inlet HX2-3 of the second heat exchanger. The hot end outlet HX2-6 of the second heat exchanger also splits into two branches: one branch, L-10, connects to regulating valve four and then to the hot end inlet HX3-5 of the third heat exchanger; the other branch, L-9, connects to regulating valve five and then to the expansion inlet of the first turbine expander. The expansion outlet of the first turbine expander merges into the cold end inlet HX3-3 of the third heat exchanger. The hot end outlet HX3-6 of the third heat exchanger connects to the hot end inlet HX4 of the fourth heat exchanger. -5 is connected, the hot end outlet HX4-6 of the fourth heat exchanger is connected to the second throttle valve and then connected to the cold end inlet HX4-3 of the fourth heat exchanger, the cold end outlet HX4-4 of the fourth heat exchanger is connected to the cold end inlet HX3-3 of the third heat exchanger, the cold end outlet HX3-4 of the third heat exchanger is connected to the cold end inlet HX2-3 of the second heat exchanger, the cold end outlet HX2-4 of the second heat exchanger is connected to the cold end inlet HX1-3 of the first heat exchanger, the cold end outlet HX1-4 of the first heat exchanger is connected to the cold end inlet E-3 of the regenerator, and the cold end outlet E-4 of the regenerator merges into the front end of the compressor inlet;

[0009] The LNG unloading route includes an LNG tank truck. After the bottom unloading port is connected to the pressure control valve, it splits into two branches. One branch, L-11, is connected to the inlet of the LNG booster vaporizer through regulating valve six. The outlet of the LNG booster vaporizer is connected to the gas phase port at the top of the LNG tank truck through regulating valve seven. The other branch, L-12, is connected to regulating valve eight, the LNG submersible pump, and throttle valve three in sequence, and then to the liquid phase port A-3 of the LNG storage tank.

[0010] The LNG refueling line includes an LNG vehicle. The liquid phase port A-4 at the bottom of the LNG storage tank is connected to the inlet of pump body 2 through regulating valve 9. The outlet of pump body 2 is connected to the LNG refueling machine. After the LNG refueling machine and the LNG vehicle form a loop, they are connected to the gas phase port A-5 at the top of the LNG storage tank through throttle valve 4.

[0011] Furthermore, the cold components and connecting pipelines of the first, second, third, and fourth heat exchangers, the regenerator, the first turbine expander, and the second turbine expander in the BOG liquefaction recovery line are respectively treated with high vacuum insulation by double-walled vacuum insulation pipes and / or high vacuum multilayer insulation cold boxes, or with filling insulation by perlite and / or aerogel insulation felt.

[0012] Ideally, the BOG liquefaction and recovery line undergoes a pumping and control process before startup, using a combination of vacuum pumps and molecular pumps to pump the pressure to below 0.001 Pa.

[0013] Furthermore, in the BOG liquefaction and recovery line, the liquid phase port of the LNG storage tank is connected to the booster vaporizer via a pressure control valve, and the booster vaporizer is connected to the gas phase port A-6 at the top of the LNG storage tank.

[0014] Ideally, when starting the system, high-purity nitrogen at 0.1–0.15 MPa is introduced into the BOG liquefaction and recovery line by opening the pressure reducing valve at the outlet of the nitrogen cylinder group, and then the compressor is used to pre-cool the various refrigerant components of the system.

[0015] A method for operating the above-mentioned LNG refueling station BOG recovery system includes a no-operation mode, an LNG refueling mode, and an LNG unloading mode;

[0016] No-job mode includes the following steps:

[0017] Step 1: When the LNG storage tank pressure exceeds the pressure control valve set value, the BOG liquefaction and recovery line is activated;

[0018] Step 2: Once the regulating valve is opened, the BOG passes through the first heat exchanger, the second heat exchanger, the third heat exchanger, and the fourth heat exchanger in sequence. After being cooled and subcooled, it is pumped into the LNG storage tank by the first pump body.

[0019] Step 3: When shut-off valve 1 is closed, only nitrogen gas is used for refrigeration to provide cooling capacity. The pressure reducing valve is opened, shut-off valves 3 and 4 are closed, and shut-off valve 2 is opened. After the nitrogen gas in the nitrogen cylinder group is depressurized, it passes through the compression ends of the second turbine expander and the first turbine expander, and then undergoes cooling and pre-cooling before entering the first heat exchanger for heat exchange.

[0020] Step 4: Regulating valves 3, 2, and 4 are opened, and regulating valve 5 is closed. Part of the nitrogen gas at the cold end outlet HX1-6 of the first heat exchanger enters the expansion end of the first turbine expander, expands, and then flows into the cold end inlet HX2-3 of the second heat exchanger, providing cooling capacity to the second heat exchanger and the first heat exchanger in sequence.

[0021] Step 5: The remaining nitrogen gas passes through the third heat exchanger and the fourth heat exchanger in sequence, and then enters the expansion valve II for expansion. After expansion, it passes through the cold end inlet HX4-3 of the fourth heat exchanger, providing cooling capacity to the fourth heat exchanger, the third heat exchanger, the second heat exchanger, and the first heat exchanger in sequence. After being heated by the regenerator, it flows into the front end of the compressor to complete the nitrogen expansion refrigeration cycle, thus completing the liquefaction and recovery of the small amount of BOG gas generated by the daily heat leakage of the equipment.

[0022] LNG refueling includes the following steps:

[0023] Step 1: When the LNG storage tank pressure exceeds the pressure control valve set value, the BOG liquefaction and recovery line is activated;

[0024] Step 2: When the regulating valve is opened, most of the BOG is cooled and subcooled by passing through the first heat exchanger, the second heat exchanger, the third heat exchanger, and the fourth heat exchanger in sequence, and then pumped into the LNG storage tank by the pump body. At the same time, the shut-off valve is opened, and a small portion of the BOG enters the regenerator through the shut-off valve and is heated to undergo a nitrogen-methane expansion refrigeration cycle. This portion of the BOG is compressed and cooled in sequence and mixed with nitrogen from the nitrogen cylinder group.

[0025] Step 3: Stop valves three and four are closed, and stop valve two is opened. The mixed gas passes through the compression ends of the two turbine expanders, then is cooled and pre-cooled, and then enters the first heat exchanger for heat exchange.

[0026] Step 4: Regulating valves 3, 2, and 4 are opened, and regulating valve 5 is closed. Part of the mixed gas enters the expansion end of the first turbine expander, expands, and then flows into the cold end inlet HX2-3 of the second heat exchanger, providing cooling capacity to the second heat exchanger and the first heat exchanger in sequence.

[0027] Step 5: The remaining mixed gas passes through the third heat exchanger and the fourth heat exchanger in sequence, and then enters the expansion valve II for expansion. After expansion, it passes through the cold end inlet HX4-3 of the fourth heat exchanger, providing cooling capacity to the fourth heat exchanger, the third heat exchanger, the second heat exchanger, and the first heat exchanger in sequence. After being heated by the regenerator, it flows into the front end of the compressor to complete the nitrogen-methane one-stage compression and one-stage expansion refrigeration cycle, completing the liquefaction and recovery of BOG gas generated during the refueling process at the gas station.

[0028] The LNG unloading process includes the following steps:

[0029] Step 1: When the pressure in the LNG storage tank exceeds the set value of the pressure control valve, the BOG liquefaction and recovery line is activated;

[0030] Step 2: Once the regulating valve is opened, most of the BOG passes through the first, second, third, and fourth heat exchangers in sequence, where it is cooled and subcooled before being pumped into the LNG storage tank by the pump body. At the same time, the shut-off valve is opened, and a small portion of the BOG passes through the shut-off valve and enters the regenerator for heating and nitrogen-methane expansion refrigeration cycle. This portion of the BOG is then compressed and cooled in sequence, and mixed with nitrogen from the nitrogen cylinder group.

[0031] Step 3: Shut-off valve 2 is closed, and shut-off valves 3 and 4 are opened. The mixed gas passes through shut-off valve 3, compressor, cooler, and shut-off valve 4 in sequence for secondary compression and cooling. Then it passes through the second turbine expander and the compression end of the first turbine expander in sequence for cooling and pre-cooling. Finally, it enters the first heat exchanger for heat exchange. At this time, regulating valves 3, 2, 5, and 4 are opened. After the mixed gas is heated by the first heat exchanger, part of the mixed gas enters the first turbine expander through regulating valve 3 for expansion and then flows into the cold end inlet HX2-3 of the second heat exchanger, providing cooling capacity to the second heat exchanger and the first heat exchanger in sequence.

[0032] Step 4: After the remaining mixed gas is heated by the second heat exchanger, a portion of the mixed gas enters the second turbine expander through regulating valve 5 for expansion and then flows into the cold end inlet HX3-3 of the third heat exchanger, providing cooling capacity to the third heat exchanger, the second heat exchanger, and the first heat exchanger in sequence.

[0033] Step 5: The remaining mixed gas passes through the fourth heat exchanger, then enters the expansion valve and is connected to the cold end inlet HX4-3 of the fourth heat exchanger. It provides cooling capacity to the fourth, third, second and first heat exchangers in sequence. After being heated by the regenerator, it flows into the front end of the compressor to complete the nitrogen-methane two-stage compression and two-stage expansion refrigeration cycle, thus completing the liquefaction and recovery of a large amount of BOG gas generated when unloading LNG at the gas station.

[0034] Beneficial effects: Compared with the prior art, the advantages of the present invention are:

[0035] (1) Since the system adopts the nitrogen-methane expansion refrigeration method with fast cold start to liquefy BOG, it can switch the refrigeration method at any time and quickly according to the different amounts of BOG to provide the appropriate amount of cold liquefied BOG, which has the advantage of adapting to the instability of BOG production at LNG refueling stations.

[0036] (2) Since the system can directly extract a small amount of BOG to provide the raw material methane required for refrigeration when liquefying BOG, a refrigeration method with faster cold start is adopted without increasing the amount of refrigerant, and BOG can be recovered in real time.

[0037] (3) A certain amount of high-pressure nitrogen is introduced into the forward cooling process of the system. Through the nitrogen expansion refrigeration cycle under low load, it has a pre-cooling effect on the refrigerated equipment and pipelines, and improves the liquefaction efficiency and response speed of the system during operation.

[0038] (4) Since an expansion turbine is added to the system, it can output shaft power for the compression equipment to optimize the energy consumption of the whole system and has a certain economic advantage. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of the system connection structure of the present invention. Detailed Implementation

[0040] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0041] A BOG liquefaction and recovery line for an LNG refueling station, such as Figure 1 As shown, the system includes a BOG liquefaction and recovery line, an LNG unloading line, and an LNG refueling line. The liquid phase port of the LNG tanker is connected to the LNG submersible pump, and the gas phase port of the LNG tanker is connected to the LNG booster vaporizer. The beginning and end of the BOG recovery system are connected to the LNG storage tank through pipelines. Two turbine expander-compressor units are connected in parallel in the recovery system. The LNG storage tank is connected to the LNG refueling unit in a loop through pumps, valves, etc. There are multiple regulating valves on each process pipeline in the system for safety control and switching of working lines, so as to complete the liquefaction and recovery of BOG gas generated under various operating conditions of the LNG refueling station in a targeted manner.

[0042] The top gas phase port A-1 of LNG storage tank 1 is connected to pressure control valve 2. After the valve, it splits into two branches: L-2 is connected to regulating valve 3. After regulating valve 3, it is connected to the hot end inlet HX1-1 of the first heat exchanger 5. The hot end outlet HX1-2 is connected to the hot end inlet HX2-1 of the second heat exchanger 6. The hot end outlet HX2-2 is connected to the hot end inlet HX3-1 of the third heat exchanger 7. The hot end outlet HX3-2 is connected to the hot end inlet HX4-1 of the fourth heat exchanger 8. The hot end outlet HX4-2 is connected to the inlet of pump body 9. The pump outlet is connected to throttle valve 10. After the valve, it is connected to the liquid phase port A-2 at the bottom of LNG storage tank 1.

[0043] Another branch, L-1, after pressure control valve 2 is connected to shut-off valve 4. After the valve, it connects to the cold end inlet E-1 of regenerator 11. The cold end outlet E-2 connects to the inlet of compressor 12. The compressor outlet connects to cooler 13. Nitrogen cylinder group 14 connects to pressure reducing valve 15. The outlet pipe L-4 of pressure reducing valve 15 merges with the outlet pipe L-3 of cooler 13, then splits into two branches. One branch, L-5, connects to shut-off valve 20. The other branch, L-6, connects sequentially to shut-off valve 316, compressor 17, cooler 18, and shut-off valve 419. After the outlets of valves 19 and 20 merge, they are sequentially connected to the compression ends of the second turbine expander 21 and the first turbine expander 22. The compression end outlet is connected to the inlet of cooler 23, and the cooler outlet is connected to the hot end inlet E-5 of regenerator 11. The hot end outlet E-6 is connected to the hot end inlet HX1-5 of the first heat exchanger 5. The hot end outlet HX1-6 splits into two branches: one branch, L-8, is connected to regulating valve 25 and then to the hot end inlet HX2-5 of the second heat exchanger 6; the other branch, L-7, is connected to regulating valve 3 24 and then to the first turbine expander 22. The expansion end inlet of expander 22 connects to the cold end inlet HX2-3 of the second heat exchanger 6. The hot end outlet HX2-6 of the second heat exchanger 6 is also divided into two branches: one branch, L-10, connects to regulating valve 4 27 and then to the hot end inlet HX3-5 of the third heat exchanger 7; the other branch, L-9, connects to regulating valve 5 26 and then to the expansion end inlet of the second turbine expander 21. This expansion end outlet connects to the cold end inlet HX3-3 of the third heat exchanger 7. The hot end outlet HX3-6 of the third heat exchanger 7 connects to the hot end inlet HX4 of the fourth heat exchanger 8. -5 are connected. The hot end outlet HX4-6 of the fourth heat exchanger 8 is connected to the throttle valve 28 and then connected to the cold end inlet HX4-3 of the fourth heat exchanger 8. The cold end outlet HX4-4 is connected to the cold end inlet HX3-3 of the third heat exchanger 7. The cold end outlet HX3-4 is connected to the cold end inlet HX2-3 of the second heat exchanger 6. The cold end outlet HX2-4 is connected to the cold end inlet HX1-3 of the first heat exchanger 5. The cold end outlet HX1-4 is connected to the cold end inlet E-3 of the regenerator 11. The cold end outlet E-4 flows into the front end of the compressor 12 inlet.

[0044] The unloading port at the bottom of the tank truck is connected to the pressure control valve 30. After the valve, it splits into two branches. One branch, L-11, is connected to the inlet of the LNG booster vaporizer 32 through the regulating valve 6 31. The outlet of the LNG booster vaporizer 32 is connected to the gas phase port at the top of the LNG tank truck 29 through the regulating valve 7 33. The other branch, L-12, is connected to the inlet of the LNG submersible pump 35 after being connected to the regulating valve 8 34. The outlet of the submersible pump is connected to the throttle valve 3 37. After the valve, it is connected to the liquid phase port A-3 of the LNG storage tank.

[0045] The liquid phase port A-4 at the bottom of LNG storage tank 1 is connected to the regulating valve 37 and then to the inlet of pump body 38. The pump outlet is connected to LNG refueling machine 39. After the refueling machine and LNG vehicle 40 form a circuit, they are connected to the gas phase port A-5 at the top of LNG storage tank 1 through throttle valve 41.

[0046] The main cold components and connecting pipelines of the first heat exchanger 5, the second heat exchanger 6, the third heat exchanger 7, the fourth heat exchanger 8, the regenerator 11, the first turbine expander 22 and the second turbine expander 21 in the BOG liquefaction recovery line are all equipped with high vacuum heat insulation or insulation measures such as perlite, aerogel and other filler materials.

[0047] Before the system is started, the BOG liquefaction and recovery line is evacuated. The system is evacuated to below 0.001 Pa by a combination of vacuum pump and molecular pump.

[0048] To expedite the system's hot start-up time, when starting the system, the pressure reducing valve 15 at the outlet of the nitrogen cylinder group 14 can be opened first to supply 0.1-0.15MPa high-purity nitrogen to the BOG liquefaction and recovery line, and the compressor 12 can be slowly started to pre-cool the various refrigerant components of the system.

[0049] The system is equipped with pressure reducing valve 15, shut-off valve 14, shut-off valve 316, shut-off valve 419, shut-off valve 20, regulating valve 324, regulating valve 225, regulating valve 526 and regulating valve 427 to realize the switching of nitrogen expansion refrigeration cycle, nitrogen-methane single-stage compression single-stage expansion refrigeration cycle and nitrogen-methane two-stage compression two-stage expansion refrigeration cycle.

[0050] The system is equipped with a turbine expander and compressor integrated machine during the BOG liquefaction process, which can output a portion of the shaft power at the compression end during system operation, thereby optimizing the total energy consumption of the system.

[0051] The system described above is suitable for areas where there are no urban gas pipelines or CNG refueling stations around LNG refueling stations.

[0052] The operating method of the above-mentioned LNG refueling station BOG recovery system includes:

[0053] (a) When the gas station is not operating:

[0054] A small amount of BOG gas is generated from heat leakage in LNG storage tanks and pipelines. Only nitrogen expansion refrigeration is used. When the LNG tank pressure exceeds the set value of pressure control valve 2, the BOG liquefaction and recovery line is activated, and regulating valve 3 opens. BOG is cooled and subcooled sequentially through the first heat exchanger 5, the second heat exchanger 6, the third heat exchanger 7, and the fourth heat exchanger 8 before being pumped into the LNG storage tank by pump body 9. At this time, shut-off valve 4 is closed, and only nitrogen provides cooling through the refrigeration cycle. Pressure reducing valve 15 opens, shut-off valves 16 and 19 close, and shut-off valve 20 opens. Nitrogen in nitrogen cylinder group 14 is depressurized and passes through the compression ends of two turbine expanders, then undergoes cooling and pre-cooling before entering the first heat exchanger 5 for heat exchange. At this time, regulating valve 24... When regulating valves 25 and 47 are opened and regulating valve 5 is closed, part of the nitrogen gas at the cold end outlet HX1-6 of the first heat exchanger 5 enters the expansion end of the first turbine expander 22 and expands before flowing into the cold end inlet HX2-3 of the second heat exchanger 6, providing cooling capacity to the second heat exchanger 6 and the first heat exchanger 5 in sequence. The remaining nitrogen gas passes through the third heat exchanger 7 and the fourth heat exchanger 8 in sequence, and then enters the expansion valve 28 and expands before connecting to the cold end HX4-3 of the fourth heat exchanger 8, providing cooling capacity to the fourth heat exchanger 8, the third heat exchanger 7, the second heat exchanger 6 and the first heat exchanger 5 in sequence. After being heated by the regenerator 11, it flows into the front end of the compressor 12 to complete the nitrogen expansion refrigeration cycle, realizing the liquefaction and recovery of a small amount of BOG gas generated by daily heat leakage from LNG storage tanks and pipelines.

[0055] (ii) When the gas station is in the process of refueling:

[0056] Due to factors such as heat exchange between LNG and equipment and pipelines during the process, and volume replacement in the LNG vehicle's onboard gas cylinders, the amount of BOG generated is greater than in the no-operation mode. Therefore, nitrogen-methane expansion refrigeration is activated. This process begins with LNG refueling. When an LNG-powered vehicle enters the station for refueling, regulating valve 9 37 opens, and pump 2 38 pumps the liquid LNG from the LNG storage tank into the refueling machine 39, and then refuels the LNG-powered vehicle group 41. The BOG gas generated during this process returns to the gas phase port A-5 above the LNG storage tank after passing through the throttle valve 41 along the loop. When the LNG storage tank pressure exceeds the set value of pressure control valve 2, the BOG liquefaction and recovery line is activated, regulating valve 1 3 opens, and most of the BOG is cooled and subcooled sequentially through the first heat exchanger 5, the second heat exchanger 6, the third heat exchanger 7, and the fourth heat exchanger 8 before being pumped into the LNG storage tank by pump 1 9. At the same time, shut-off valve 1 4 opens, and a small portion of the BOG enters the regenerator 11 through shut-off valve 1 4 for heating and then undergoes a nitrogen-methane expansion refrigeration cycle. This portion of BOG... OG is compressed and cooled sequentially, then mixed with nitrogen from nitrogen cylinder group 14. Shut-off valves 16 and 19 are closed, while valve 20 is open. The mixture passes through the compression ends of two turbine expanders, then undergoes cooling and pre-cooling before entering the first heat exchanger 5 for heat exchange. At this time, regulating valves 24 and 25 are open, but regulating valve 26 is closed. Part of the mixture enters the expansion end of the first turbine expander 22, expands, and then flows into the cold end inlet HX2-3 of the second heat exchanger 6. The second heat exchanger 6 and the first heat exchanger 5 provide cooling capacity. The remaining mixed gas passes through the third heat exchanger 7 and the fourth heat exchanger 8 in sequence. After entering the expansion valve 28, it expands and connects to the cold end inlet HX4-3 of the fourth heat exchanger 8, providing cooling capacity to the fourth heat exchanger 8, the third heat exchanger 7, the second heat exchanger 6, and the first heat exchanger 5 in sequence. After being heated by the regenerator 11, it flows into the front end of the compressor 12 to complete the nitrogen-methane one-stage compression and one-stage expansion refrigeration cycle, realizing the liquefaction and recovery of BOG gas generated during the refueling process of the gas station.

[0057] (iii) When the gas station is in the process of unloading LNG:

[0058] Because the BOG (Boiled Air Gathering) generated during the unloading process is significantly larger than that of the LNG refueling mode, a nitrogen-methane two-stage compression and two-stage expansion refrigeration system is adopted. This process begins when an LNG tanker enters the refueling station for unloading. Initially, the LNG in the tanker passes through pressure control valve 30 and enters branch line L-11, sequentially passing through regulating valve six 31, LNG booster vaporizer 32, and regulating valve seven 33 before returning to the vapor port at the top of the LNG tanker. The pressure inside the tanker increases, and the LNG is sent to the LNG storage tank for preservation. Later in the unloading process, the LNG in the tanker is low, and the booster vaporization unloading efficiency is low. The submersible pump is then activated for unloading. The LNG passes through pressure control valve 30... Following branch line L-12, the submersible pump delivers the remaining LNG to the LNG storage tank for storage. BOG generated during this process also enters the LNG storage tank. When the LNG storage tank pressure exceeds the set value of pressure control valve 2, the BOG liquefaction and recovery line is activated, and regulating valve 3 opens. Most of the BOG is cooled and subcooled sequentially through the first heat exchanger 5, the second heat exchanger 6, the third heat exchanger 7, and the fourth heat exchanger 8 before being pumped into the LNG storage tank by pump body 9. Simultaneously, shut-off valve 4 opens, allowing a small portion of the BOG to enter the regenerator 11 for heating and undergo a nitrogen-methane expansion refrigeration cycle. This portion of BOG is subsequently compressed and cooled, then mixed with nitrogen from nitrogen cylinder group 14. Shut-off valve 2... With valve 20 closed and shut-off valves 16 and 19 open, the gas mixture passes through shut-off valve 16, compressor 17, cooler 18, and shut-off valve 19 for secondary compression and cooling. It then passes through the compression ends of two turbine expanders for further cooling and pre-cooling before entering the first heat exchanger 5. At this time, regulating valves 24, 25, 26, and 27 are open. After heat exchange in the first heat exchanger 5, a portion of the gas mixture passes through regulating valve 24 into the first turbine expander 22 for expansion and then flows into the cold end inlet HX2-3 of the second heat exchanger 6, providing cooling to both the second heat exchanger 6 and the first heat exchanger 5. The remaining gas mixture passes through the second heat exchanger... After heat exchange 6, a portion of the mixed gas enters the second turbine expander 21 through regulating valve 5 26 for expansion and then flows into the cold end inlet HX3-3 of the third heat exchanger 7, providing cooling capacity to the third heat exchanger 7, the second heat exchanger 6, and the first heat exchanger 5 in sequence. The remaining mixed gas passes through the fourth heat exchanger 8 and then enters the throttle valve 2 28 for expansion, connecting to the cold end inlet HX4-3 of the fourth heat exchanger 8, providing cooling capacity to the fourth heat exchanger 8, the third heat exchanger 7, the second heat exchanger 6, and the first heat exchanger 5 in sequence. After being heated by the regenerator 11, it flows into the front end of the compressor 12 to complete the nitrogen-methane two-stage compression and two-stage expansion refrigeration cycle, realizing the liquefaction and recovery of a large amount of BOG gas generated during LNG unloading at the gas station.

[0059] Example 1:

[0060] Since most LNG refueling stations in my country are based on the Class III LNG standard, a 60m³ LNG cryogenic storage tank in Jiangsu Province was selected. 3 An example of a Level 3 LNG standard refueling station is provided:

[0061] Before starting the BOG liquefaction and recovery line, the pipelines and equipment are evacuated. The system is evacuated to below 0.001 Pa using a combination of vacuum pump and molecular pump. When the gas station is not in operation, the BOG generated due to heat leakage from the storage tanks and pipelines is approximately 93.09 kg / d. When the LNG storage tank pressure exceeds the set value of pressure control valve 2 by approximately 0.75 MPa, the BOG liquefaction and recovery line starts, regulating valve 3 opens, and BOG at approximately -128°C and 0.75 MPa passes sequentially through the first heat exchanger 5 and the second... After being cooled and subcooled to approximately -150°C by heat exchangers 6, 7, and 8, the LNG is pumped by pump 9 into throttling valve 10, where it is throttled to approximately -160°C and sent to the LNG storage tank. At this time, shut-off valve 4 is closed, and only nitrogen provides cooling through the refrigeration cycle. Pressure reducing valve 15 is open, shut-off valves 16 and 19 are closed, and shut-off valve 20 is open. Nitrogen in nitrogen cylinder group 14 is depressurized to approximately 0.2 MPa and sent into the refrigeration process, passing through the compression ends of two turbine expanders, and then... After cooling and precooling, the nitrogen enters the first heat exchanger 5 for heat exchange until the temperature drops to the inlet temperature of the expansion end of the first turbine expander 22. At this time, regulating valves 3 (24), 2 (25), and 4 (27) are opened, but regulating valve 5 (26) is closed. Part of the nitrogen enters the expansion end of the first turbine expander 22 and expands to the inlet pressure of the compressor 12 at approximately 0.1-0.15 MPa. Then, it flows into the cold end HX2-3 of the second heat exchanger 6, providing cooling capacity to the second heat exchanger 6 and the first heat exchanger 5 in sequence. The remaining nitrogen passes through the third heat exchanger 7 and the fourth heat exchanger in sequence. After passing through heat exchanger 8, the gas expands to approximately -165°C and 0.1-0.15 MPa via throttle valve 28 and connects to the cold end HX4-3 of the fourth heat exchanger 8. This provides cooling capacity to the fourth heat exchanger 8, the third heat exchanger 7, the second heat exchanger 6, and the first heat exchanger 5 in sequence. After being heated to approximately 15°C by regenerator 11, the gas flows into the front end of compressor 12 and is compressed to a pressure of approximately 0.2 MPa by compressor 12, completing the nitrogen expansion refrigeration cycle and realizing the liquefaction and recovery of a small amount of BOG gas generated when the LNG storage tank leaks heat.

[0062] When the gas station is in LNG refueling operation, regulating valve 37 opens, and pump 38 pumps liquid LNG from the LNG storage tank into the refueling machine 39, which then refuels the LNG-powered vehicle group 41. During this process, the BOG gas generated returns to the gas phase port A-5 above the LNG storage tank via throttle valve 41. Due to heat leakage from the storage tank, volume displacement during refueling, and heat exchange with equipment and pipelines, the BOG generation increases to approximately 394.41 kg / d. When the LNG storage tank pressure exceeds the set value of pressure control valve 2 by 1 MPa, the BOG liquefaction and recovery line is activated, regulating valve 3 opens, and most of the BOG is released at -120℃ and 0.75 MPa. BOG (Boiled Gas) is cooled and subcooled to approximately -154°C by passing through heat exchangers 5, 6, 7, and 8. It is then pumped by pump 9 into throttling valve 10, where it is throttled to approximately -160°C before being sent to the LNG storage tank. Simultaneously, shut-off valve 4 opens, allowing a small portion of BOG to pass through shut-off valve 4 and enter regenerator 11 for heating to approximately 25°C before undergoing a nitrogen-methane expansion refrigeration cycle. This portion of BOG is compressed and cooled to approximately 0.5 MPa, then mixed with nitrogen from the nitrogen cylinder group. Shut-off valves 16 and 19 close, while shut-off valve 20 opens. Without secondary compression, the mixed gas passes through the compression ends of two turbine expanders, undergoes cooling and pre-cooling, and then enters heat exchanger 5 to reach the inlet temperature of the expansion end of turbine expander 22. At this point, regulating valves 24, 25, and 27 are open, but the regulating valves... When valve 5 26 is closed, part of the mixed gas enters the expansion end of the first turbine expander 22 and expands to about 0.1 MPa, which is the inlet pressure of the compressor 12. Then it flows into the cold end inlet HX2-3 of the second heat exchanger 6, providing cooling capacity to the second heat exchanger 6 and the first heat exchanger 5 in sequence. The remaining mixed gas passes through the third heat exchanger 7 and the fourth heat exchanger 8 in sequence, and then enters the throttle valve 28 and expands to about -165°C and 0.1 MPa. It then connects to the cold end inlet HX4-3 of the fourth heat exchanger 8, providing cooling capacity to the fourth heat exchanger 8, the third heat exchanger 7, the second heat exchanger 6 and the first heat exchanger 5 in sequence. After being heated to about 15°C by the regenerator 11, it flows into the front end of the compressor 12. The mixed gas is compressed to about 0.5 MPa by the first-stage compressor 12, completing the first-stage compression and first-stage expansion nitrogen-methane expansion refrigeration cycle, realizing the liquefaction and recovery of BOG gas generated during the refueling process of the gas station.

[0063] When the gas station is unloading LNG, in the initial stage of unloading, the LNG in the tanker enters branch line L-11 after passing through pressure control valve 30, and then passes through regulating valve six 31, LNG booster vaporizer 32, and regulating valve seven 33 before returning to the vapor port on top of the LNG tanker. The pressure inside the tanker increases, and the LNG is sent to the LNG storage tank for storage. In the later stage of unloading, the LNG in the tanker is less, and the booster vaporization unloading efficiency is low. The submersible pump is started for unloading. The LNG passes through pressure control valve 30 and then goes through branch line L-12. The submersible pump sends the remaining LNG to the LNG storage tank for storage. Due to heat leakage from the storage tank, volume replacement during filling and unloading, and heat exchange with equipment and pipelines, etc., The BOG (Bottle-Off Gas) volume further increases to approximately 600 kg / day. When the LNG storage tank pressure exceeds the set value of pressure control valve 2 by 1 MPa, the BOG liquefaction and recovery line is activated, and regulating valve 3 opens. Most of the BOG at -120℃ and 0.75 MPa passes through the first heat exchanger 5, the second heat exchanger 6, the third heat exchanger 7, and the fourth heat exchanger 8 in sequence, where it is cooled and subcooled to approximately -154℃. It is then pumped by pump body 9 into throttling valve 10, where it is throttled to approximately -160℃ and sent to the LNG storage tank. Simultaneously, shut-off valve 4 opens, and a small portion of the BOG passes through shut-off valve 4 into regenerator 11 for heating and undergoes a nitrogen-methane expansion refrigeration cycle. This portion of the BOG then passes through a primary compression stage. The gas is cooled to approximately 0.5 MPa and mixed with nitrogen from the nitrogen cylinder group. Secondary compression cooling is initiated, with shut-off valve 20 closed and shut-off valves 316 and 419 opened. The mixture passes sequentially through shut-off valve 316, compressor 17, cooler 18, and shut-off valve 419 for secondary compression and cooling to approximately 1.2 MPa. It then passes sequentially through the compression ends of two turbine expanders for cooling and pre-cooling before entering the first heat exchanger 5 to exchange heat to the inlet temperature of the expansion end of the first turbine expander 22. Secondary expansion cooling is then initiated, with regulating valves 324, 25, 526, and 427 all open. Part of the mixture passes through regulating valve 324... After entering the expansion turbine 22 and expanding to approximately 0.1 MPa @ -120°C, the gas flows into the cold end inlet HX2-3 of the second heat exchanger 6, providing cooling capacity to the second heat exchanger 6 and the first heat exchanger 5 in sequence. The remaining gas mixture is heated by the second heat exchanger 6 to the expansion end inlet temperature of the second turbine expander 21. Part of the gas mixture then enters the second turbine expander 21 through regulating valve 5 26 and expands to 0.1 MPa @ -158°C before flowing into the cold end inlet HX3-3 of the third heat exchanger 7, providing cooling capacity to the third heat exchanger 7, the second heat exchanger 6, and the first heat exchanger 5 in sequence. The remaining gas mixture passes through the fourth heat exchanger 8 and then enters the throttle valve 28 to expand to approximately 0.After reaching 1 MPa and -165℃, the gas is connected to the cold inlet HX4-3 of the fourth heat exchanger 8, providing cooling capacity sequentially to the fourth heat exchanger 8, the third heat exchanger 7, the second heat exchanger 6, and the first heat exchanger 5. Then, after being heated to approximately ambient temperature (25℃) by the regenerator 11, it flows into the front end of the compressor 12, completing a two-stage compression and two-stage expansion nitrogen-methane expansion refrigeration cycle. Relying on the greater cooling capacity generated by the two expansions, the large amount of BOG gas generated during LNG unloading at the gas station is liquefied and recovered.

Claims

1. A BOG recovery system for LNG refueling stations, characterized in that: This includes BOG liquefaction and recovery lines, LNG unloading lines, and LNG refueling lines; The BOG liquefaction recovery line includes an LNG storage tank (1). The top gas phase port A-1 of the LNG storage tank (1) is connected to the pressure control valve (2). The outlet of the pressure control valve (2) is divided into two branches. One branch, L-2, is connected in sequence to the regulating valve (3), the first heat exchanger (5), the second heat exchanger (6), the third heat exchanger (7), the fourth heat exchanger (8), the pump body (9), and the throttle valve (10), and then back to the liquid phase port A-2 at the bottom of the LNG storage tank (1). The other branch, L-1, is connected in sequence to the shut-off valve (4), the regenerator (11), the compressor (12), and the cooler (13). The nitrogen cylinder group (14) is connected to the pressure reducing valve (15). The outlet pipeline L-4 of the pressure reducing valve (15) is connected to the outlet pipeline L of the cooler (13). -3 After converging, it splits into two parallel branches. One branch, L-5, is equipped with shut-off valve two (20). The other branch, L-6, is connected in sequence to shut-off valve three (16), compressor two (17), cooler (18), and shut-off valve four (19). The outlets of shut-off valve four (19) and shut-off valve two (20) merge and are connected in sequence to the first turbine expander (21), the second turbine expander (22), and the cooler (23), and then back to the hot end inlet E-5 of the regenerator (11). The hot end outlet E-6 of the regenerator (11) is connected to the hot end inlet HX1-5 of the first heat exchanger (5). The hot end outlet HX1-6 of the first heat exchanger (5) splits into two branches. One branch, L-8, is connected in sequence to regulating valve two (25), the second heat exchanger (6), and the hot end outlet HX1-6 of the first heat exchanger (5). The first heat exchanger (6) has two branches: one branch, L-10, connects to the regulating valve (27) and then to the expansion inlet of the second turbine expander (22). The expansion outlet of the second turbine expander (22) flows into the cold end inlet of the second heat exchanger (6), HX2-3. The hot end outlet of the second heat exchanger (6) is also divided into two branches: one branch, L-10, connects to the regulating valve (27) and then to the hot end inlet of the third heat exchanger (7), HX3-5; the other branch, L-9, connects to the regulating valve (26) and then to the expansion inlet of the first turbine expander (21). The expansion outlet of the first turbine expander (21) flows into the cold end inlet of the third heat exchanger (7), HX3-3. The hot end outlet of the third heat exchanger (7) connects to the fourth heat exchanger. The heat exchanger (8) is connected to the hot end inlet HX4-5. The hot end outlet HX4-6 of the fourth heat exchanger (8) is connected to the second throttle valve (28) and then connected to the cold end inlet HX4-3 of the fourth heat exchanger (8). The cold end outlet HX4-4 of the fourth heat exchanger (8) is connected to the cold end inlet HX3-3 of the third heat exchanger (7). The cold end outlet HX3-4 of the third heat exchanger (7) is connected to the cold end inlet HX2-3 of the second heat exchanger (6). The cold end outlet HX2-4 of the second heat exchanger (6) is connected to the cold end inlet HX1-3 of the first heat exchanger (5). The cold end outlet HX1-4 of the first heat exchanger (5) is connected to the cold end inlet E-3 of the regenerator (11). The cold end outlet E-4 of the regenerator (11) flows into the front end of the compressor (12) inlet. The LNG unloading route includes an LNG tanker (29), whose bottom unloading port is connected to a pressure control valve (30) and then splits into two branches. One branch, L-11, is connected to the inlet of the LNG booster vaporizer (32) through regulating valve six (31), and the outlet of the LNG booster vaporizer (32) is connected to the gas phase port at the top of the LNG tanker (29) through regulating valve seven (33). The other branch, L-12, is connected to regulating valve eight (34), LNG submersible pump (35), and throttle valve three (36) in sequence and then connected to the liquid phase port A-3 of the LNG storage tank (1). The LNG refueling line includes an LNG vehicle (40), the liquid phase port A-4 at the bottom of the LNG storage tank (1) is connected to the inlet of the pump body two (38) through the regulating valve nine (37), the outlet of the pump body two (38) is connected to the LNG refueling machine (39), and the LNG refueling machine (39) and the LNG vehicle (40) form a loop and are connected to the gas phase port A-5 at the top of the LNG storage tank (1) through the throttle valve four (41).

2. The BOG recovery system for an LNG refueling station according to claim 1, characterized in that: The cold components and connecting pipelines of the first heat exchanger (5), second heat exchanger (6), third heat exchanger (7), fourth heat exchanger (8), regenerator (11), second turbine expander (22) and first turbine expander (21) in the BOG liquefaction recovery line are respectively treated with high vacuum insulation by double-walled vacuum insulation pipes and / or high vacuum multi-layer insulation cold boxes, or filled with perlite and / or aerogel insulation felt.

3. The BOG recovery system for an LNG refueling station according to claim 1, characterized in that: Before starting the BOG liquefaction and recovery line, a vacuum process is performed, using a combination of vacuum pump and molecular pump to evacuate the pressure to below 0.001 Pa.

4. The BOG recovery system for an LNG refueling station according to claim 1, characterized in that: In the BOG liquefaction and recovery line, the liquid phase port of the LNG storage tank (1) is connected to the booster vaporizer (43) through the pressure control valve (42), and the booster vaporizer (43) is connected to the gas phase port A-6 at the top of the LNG storage tank (1).

5. The BOG recovery system for an LNG refueling station according to claim 1, characterized in that: When the system is started, 0.1 to 0.15 MPa of high-purity nitrogen is introduced into the BOG liquefaction and recovery line by opening the pressure reducing valve (15) at the outlet of the nitrogen cylinder group (14), and then the compressor (12) pre-cools down each cold component of the system.

6. A method for operating an LNG refueling station BOG recovery system as described in any one of claims 1 to 5, characterized in that, This includes no-operation mode, LNG refueling mode, and LNG unloading mode; No-job mode includes the following steps: Step 1: When the pressure in the LNG storage tank exceeds the set value of the pressure control valve (2), the BOG liquefaction and recovery line is activated; Step 2: When regulating valve 1 (3) is opened, BOG is cooled and subcooled by passing through the first heat exchanger (5), the second heat exchanger (6), the third heat exchanger (7), and the fourth heat exchanger (8) in sequence, and then pumped into LNG storage tank (1) by pump body 1 (9). Step 3: When the first shut-off valve (4) is closed, only nitrogen gas is used for refrigeration to provide cooling capacity. The pressure reducing valve (15) is opened, the third shut-off valve (16) and the fourth shut-off valve (19) are closed, and the second shut-off valve (20) is opened. After the nitrogen gas in the nitrogen cylinder group (14) is depressurized, it passes through the compression end of the first turbine expander (21) and the second turbine expander (22) respectively. Then it is cooled and pre-cooled, and enters the first heat exchanger (5) for heat exchange. Step 4: Regulating valve 3 (24), regulating valve 2 (25), and regulating valve 4 (27) are opened, and regulating valve 5 (26) is closed. Part of the nitrogen gas at the hot end outlet HX1-6 of the first heat exchanger (5) enters the second turbine expander (22) and expands at the expansion end. It then flows into the cold end inlet HX2-3 of the second heat exchanger (6) to provide cooling capacity to the second heat exchanger (6) and the first heat exchanger (5) in sequence. Step 5: The remaining nitrogen gas passes through the third heat exchanger (7) and the fourth heat exchanger (8) in sequence, and then enters the second throttle valve (28) for expansion. After expansion, it passes through the cold end inlet HX4-3 of the fourth heat exchanger (8) to provide cooling capacity to the fourth heat exchanger (8), the third heat exchanger (7), the second heat exchanger (6), and the first heat exchanger (5) in sequence. After being heated by the regenerator (11), it flows into the front end of the first compressor (12) to complete the nitrogen expansion refrigeration cycle, thus completing the liquefaction and recovery of a small amount of BOG gas generated by the daily heat leakage of the equipment. LNG refueling includes the following steps: Step 1: When the pressure in the LNG storage tank exceeds the set value of the pressure control valve (2), the BOG liquefaction and recovery line is activated; Step 2: When regulating valve 1 (3) is opened, most of the BOG passes through the first heat exchanger (5), the second heat exchanger (6), the third heat exchanger (7), and the fourth heat exchanger (8) in sequence to be cooled and subcooled before being pumped into the LNG storage tank by pump body 1 (9); at the same time, shut-off valve 1 (4) is opened, and a small portion of the BOG passes through shut-off valve 1 (4) into the regenerator (11) to be heated and then undergo a nitrogen-methane expansion refrigeration cycle. This portion of the BOG is compressed and cooled in sequence and mixed with nitrogen from the nitrogen cylinder group (14); Step 3: Stop valve three (16) and stop valve four (19) are closed, and stop valve two (20) is opened. The mixed gas passes through the compression ends of the two turbine expanders respectively, and then is cooled and pre-cooled before entering the first heat exchanger (5) for heat exchange. Step 4: Regulating valve three (24), regulating valve two (25), and regulating valve four (27) are opened, and regulating valve five (26) is closed. Part of the mixed gas enters the expansion end of the second turbine expander (22) and then flows into the cold end inlet HX2-3 of the second heat exchanger (6), providing cooling capacity to the second heat exchanger (6) and the first heat exchanger (5) in sequence. Step 5: The remaining mixed gas passes through the third heat exchanger (7) and the fourth heat exchanger (8) in sequence, and then enters the second throttle valve (28) for expansion. After expansion, it passes through the cold end inlet HX4-3 of the fourth heat exchanger (8) to provide cooling capacity to the fourth heat exchanger (8), the third heat exchanger (7), the second heat exchanger (6), and the first heat exchanger (5) in sequence. After being heated by the regenerator (11), it flows into the front end of the first compressor (12) to complete the nitrogen-methane first-stage compression and first-stage expansion refrigeration cycle, thus completing the liquefaction and recovery of BOG gas generated during the filling process of the gas station. The LNG unloading process includes the following steps: Step 1: When the pressure in the LNG storage tank exceeds the set value of the pressure control valve (2), the BOG liquefaction and recovery line is activated; Step 2: When regulating valve 1 (3) is opened, most of the BOG passes through the first heat exchanger (5), the second heat exchanger (6), the third heat exchanger (7), and the fourth heat exchanger (8) in sequence to be cooled and subcooled before being pumped into the LNG storage tank by pump body 1 (9); at the same time, shut-off valve 1 (4) is opened, and a small portion of the BOG passes through shut-off valve 1 (4) into the regenerator (11) to be heated and then undergo a nitrogen-methane expansion refrigeration cycle. This portion of the BOG is compressed and cooled in sequence and mixed with nitrogen from the nitrogen cylinder group (14); Step 3: Stop valve 2 (20) is closed, and stop valve 3 (16) and stop valve 4 (19) are opened. The mixed gas passes through stop valve 3 (16), compressor 2 (17), cooler (18), and stop valve 4 (19) in sequence for secondary compression and cooling. Then it passes through the compression end of the first turbine expander (21) and the second turbine expander (22) in sequence for cooling and pre-cooling. Then it enters the first heat exchanger (5) for heat exchange. At this time, regulating valve 3 (24), regulating valve 2 (25), regulating valve 5 (26), and regulating valve 4 (27) are opened. After the mixed gas is heated by the first heat exchanger (5), part of the mixed gas enters the second turbine expander (22) through regulating valve 3 (24) for expansion and then flows into the cold end inlet HX2-3 of the second heat exchanger (6), providing cooling capacity to the second heat exchanger (6) and the first heat exchanger (5) in sequence. Step 4: After the remaining mixed gas is heated by the second heat exchanger (6), some of the mixed gas enters the first turbine expander (21) through the regulating valve five (26) for expansion and then flows into the cold end inlet HX3-3 of the third heat exchanger (7), providing cooling capacity to the third heat exchanger (7), the second heat exchanger (6), and the first heat exchanger (5) in sequence. Step 5: The remaining mixed gas passes through the fourth heat exchanger (8), then enters the second throttle valve (28) for expansion and is connected to the cold end inlet HX4-3 of the fourth heat exchanger (8), providing cooling capacity to the fourth heat exchanger (8), the third heat exchanger (7), the second heat exchanger (6), and the first heat exchanger (5) in sequence. After being heated by the regenerator (11), it flows into the front end of the first compressor (12) to complete the nitrogen-methane two-stage compression and two-stage expansion refrigeration cycle, completing the liquefaction and recovery of a large amount of BOG gas generated when unloading LNG at the gas station.

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