A system and method for inter-stage cooling of BOG compressors at LNG receiving terminals

Abstract

This invention relates to a system and method for interstage cooling of a BOG compressor in an LNG receiving terminal. The system includes: a BOG compressor, a coiled-tube heat exchanger, a pressurization and vaporization system, and a control system. The BOG compressor includes an inlet buffer, a cylinder, and an exhaust buffer. The control system includes: temperature sensors, a temperature transmitter, a PLC control loop, a DCS control system, a TCV control valve, and a condition selector. The system uses BOG as a heat source and LNG as a cold source. Heat exchange occurs in the coiled-tube heat exchanger. The cooled BOG is then delivered to the next stage of the BOG compressor, while the heated LNG is delivered to the pressurization and vaporization system. A temperature sensor is installed on the outlet pipeline of the coiled-tube heat exchanger. The temperature signal is transmitted to the temperature transmitter, then through the PLC control loop to the DCS control system, and finally fed back to the TCV control valve located on the LNG pipeline. This controls the LNG flow rate into the coiled-tube heat exchanger, thus achieving cooling capacity control.

Description

Technical Field

[0001] This invention specifically relates to a system and method for interstage cooling of BOG compressors in LNG receiving terminals. Background Technology

[0002] LNG is natural gas that exists in a liquid form at cryogenic temperatures, with a storage temperature of approximately -162°C. Cryogenic liquid LNG is unusable and requires pressurization and vaporization before it can be burned and utilized. The BOG (Boiled Gas) compressor is the most critical rotating equipment in an LNG receiving terminal. It pressurizes the BOG produced at the terminal, causing the temperature of the pressurized BOG to rise. Therefore, conventional BOG compressors have air coolers or water-cooled heat exchangers after the pressurization end of each stage. The frequency of the air cooler fan is controlled by a frequency converter. If the interstage cooler is a water-cooled heat exchanger, the interstage BOG temperature is controlled by adjusting the amount of circulating water entering the heat exchanger.

[0003] LNG contains a vast amount of cold energy, and recovering this energy has considerable economic and social benefits. However, the BOG (Boiled Gas) between stages in the BOG compressor requires cooling; otherwise, the temperature of the pressurized BOG will accumulate, causing the next stage of BOG compression to malfunction. Conventional cooling methods, whether water-cooled or air-cooled, rely on electrical energy conversion to cool the BOG between stages, which is detrimental to the overall energy consumption of the receiving terminal. Neither of these two methods considers the entire receiving terminal; without considering energy conservation as a whole, the energy waste is staggering.

[0004] With the rapid development of LNG receiving terminals in China in recent years, how to rationally and economically utilize the cryogenic energy of LNG has become a focus of attention for all construction parties. Japan is one of the world's leading countries in cryogenic energy utilization. Domestically, the Putian and Rudong terminals have built cryogenic LNG air separation units, and the Zhoushan LNG receiving terminal has also built one complete set of units each for cryogenic power generation and cryogenic ice making. After years of practice and research, several technologies for LNG cryogenic energy utilization have matured, and new utilization schemes are constantly being proposed.

[0005] Besides energy conservation and emission reduction, LNG cold energy utilization technology can also drive the development of related cold chain industries, such as power generation equipment, air separation, light hydrocarbon recovery, cryogenic pulverization, seawater desalination, refrigeration, and dry ice manufacturing. Given the excellent environmental and economic benefits of cryogenic LNG, and the need for sustainable economic, social, and environmental development, developing cold energy utilization facilities at LNG receiving terminals is a requirement of the times, a necessity for development, and a green project that benefits the country and its people.

[0006] Traditional LNG receiving terminals have interstage heat exchangers between BOG compressor stages, typically using air coolers. This heat exchange method is environmentally friendly, using motors to drive fans to control the temperature of the BOG between stages. However, the scale of energy utilization in this method is limited by the size of the fans and the local ambient temperature, so it is generally not very large. Another type of BOG compressor interstage heat exchanger is a water-cooled heat exchanger, which uses a closed-loop water cooling method. This heat exchange method uses a motor to drive a circulating water pump to continuously supply cooling water to the water-cooled heat exchanger to control the temperature of the BOG between stages.

[0007] When implementing the above-mentioned cooling method for the interstage BOG of the BOG compressor in actual engineering cases, the following problems generally exist: 1. The control of the BOG compressor is considered only, without taking into account the overall energy utilization of the LNG receiving terminal; 2. The processing capacity of interstage air coolers is limited, usually requiring multiple units, which occupies a large area and involves a large investment. 3. Interstage water coolers require regular replenishment of circulating water, resulting in a complex system, high investment, and high maintenance costs; 4. Both economic and environmental benefits are affected to some extent.

[0008] Chinese patent document CN104913196A discloses a BOG (Bottle-Off Gas) processing technology for normal operation of an LNG receiving terminal. The process steps are as follows: 1) BOG compression: BOG from the LNG storage tank at a temperature of -150 to -140°C is fed into a low-pressure BOG compressor, pressurized to 0.5 to 0.8 MPaG, with an outlet temperature of 0 to 5°C, and then enters the LNG / BOG heat exchanger; 2) LNG / BOG heat exchange: LNG from the low-pressure LNG inlet pipeline of the LNG low-pressure pump is transported into the LNG / BOG heat exchanger. The BOG from the low-pressure BOG compressor outlet undergoes heat exchange, ultimately condensing into a liquid state at a temperature of -140 to -155°C; 3) BOG condensate: The condensed BOG condensate enters a BOG condensate buffer tank; simultaneously, a nitrogen pipeline is installed in the BOG condensate buffer tank to maintain a pressure of 0.6 MPaG in the buffer tank, and a regulating valve three is installed in the nitrogen pipeline to control the nitrogen replenishment; 4) Pressurization pump: The condensate from the BOG condensate buffer tank enters the pressurization pump, is pressurized to 1.0 MPaG, and then sent to an LNG tanker for loading, completing the BOG processing. When two or more low-pressure BOG compressors are installed, the two or more low-pressure BOG compressors are connected in parallel on the pipeline. This patent utilizes LNG to directly condense the gas at the BOG compressor outlet, while simultaneously using argon gas for pressure maintenance. Summary of the Invention

[0009] This invention aims to solve the aforementioned technical problems and provides a BOG cooling process system suitable for interstage BOG compressors in LNG receiving terminals. This system can control the BOG temperature between BOG compressor stages within a reasonable range, while simultaneously utilizing the heat from the interstage BOG to heat the LNG, thereby achieving effective energy cascade utilization.

[0010] This invention is achieved through the following technical solution: In a first aspect, the present invention provides a system for interstage cooling of a BOG compressor in an LNG receiving terminal. The system includes: a BOG compressor, a coiled tube heat exchanger, and a pressurization and vaporization system; wherein, there are at least two stages of inlet buffers, at least two stages of cylinders, and at least two stages of exhaust buffers. The inlet pipeline of the BOG compressor is sequentially connected to the first-stage inlet buffer, the first-stage cylinder, and the first-stage exhaust buffer. The outlet of the first-stage exhaust buffer is connected via a pipeline to the inlet of one tube side of the coiled tube heat exchanger. The outlet of the coiled tube heat exchanger is sequentially connected to the second-stage inlet buffer, the second-stage cylinder, and the second-stage exhaust buffer. The outlet inlet pipeline of the second-stage exhaust buffer is connected to the inlet of the other tube side of the coiled tube heat exchanger, until the outlet of the tube side of the coiled tube heat exchanger connected to the outlet of the last stage exhaust buffer is connected via a pipeline to the exhaust port of the BOG compressor. The low-pressure LNG inlet pipeline is connected to the shell side inlet of the coiled tube heat exchanger, and the shell side outlet of the coiled tube heat exchanger is connected via a pipeline to the inlet of the pressurization and vaporization system.

[0011] The aforementioned system also includes a control system, which comprises: a temperature sensing instrument, a temperature transmitter, a PLC control loop, a DCS control system outlet, a TCV control valve, and an operating condition selector. The temperature sensing instrument is installed on the tube-side outlet pipeline of the wound-tube heat exchanger and is connected to the temperature transmitter. The temperature transmitter is connected to the DCS control system via a PLC control loop. The TCV control valve is installed on the low-pressure LNG inlet pipeline and is connected to the DCS control system via a PLC control loop. The DCS control system is equipped with an operating condition selector.

[0012] Each stage of the wound-tube heat exchanger outlet pipeline is equipped with temperature detection instruments, temperature transmitters, and PLC control circuitry. The BOG temperature signals from each stage are transmitted to the DCS control system in the central control room, which then calculates the corresponding LNG quantity to cool the BOG at each stage, ensuring the BOG temperature reaches the set value. In the central control room, the interstage temperature signals of each BOG compressor are fed back to the DCS controller, which calculates and superimposes the required LNG for each stage to determine the LNG flow rate. Since the heated LNG requires further pressurization and vaporization, the LNG flow rate can be appropriately amplified; that is, under the condition of achieving BOG compressor interstage temperature control, the LNG flow rate entering the wound-tube heat exchanger is not considered a critical control value. The feedback LNG flow rate is sent to the TCV valve actuator to control the TCV valve opening. When the temperature of any stage of the BOG exceeds the set value, the TCV valve opening must be increased. The PLC controls the valve opening on the LNG inlet pipeline based on the temperature detected on the outlet pipeline of each stage of the wound-tube heat exchanger. Operators in the central control room can also monitor the corresponding LNG flow rate through the flow monitoring device installed on the LNG pipeline, and further adjust the required LNG flow rate to optimize the control system.

[0013] In the aforementioned system, the BOG compressor includes a first intake buffer, a second intake buffer, a third intake buffer, a first-stage cylinder, a second-stage cylinder, a third-stage cylinder, a first exhaust buffer, a second exhaust buffer, and a third exhaust buffer. The BOG compressor's intake line is sequentially connected to the first intake buffer, the first-stage cylinder, and the first exhaust buffer. The outlet of the first exhaust buffer is connected via a pipeline to the inlet of the first tube side of a coiled tube heat exchanger. The outlet of the first tube side of the coiled tube heat exchanger is sequentially connected via a pipeline to the second intake buffer, the second-stage cylinder, and the second exhaust buffer. The outlet of the second exhaust buffer is connected via a pipeline to the inlet of the second tube side of the coiled tube heat exchanger. The outlet of the second tube side of the coiled tube heat exchanger is sequentially connected via a pipeline to the third intake buffer, the third-stage cylinder, and the third exhaust buffer. The outlet of the third exhaust buffer is connected to the inlet of the third tube side of the coiled tube heat exchanger via a pipeline. The outlet of the third tube side of the coiled tube heat exchanger is connected to the exhaust port of the BOG compressor via a pipeline. The low-pressure LNG inlet pipeline is connected to the shell side inlet of the coiled tube heat exchanger. The shell side outlet of the coiled tube heat exchanger is connected to the inlet of the booster gasification system via a pipeline. A first temperature measuring instrument is installed on the outlet pipeline of the first tube side of the coiled tube heat exchanger, and the first temperature measuring instrument is connected to a first temperature transmitter. A second temperature measuring instrument is installed on the outlet pipeline of the second tube side of the coiled tube heat exchanger, and the second temperature measuring instrument is connected to a second temperature transmitter. A third temperature measuring instrument is installed on the outlet pipeline of the third tube side of the coiled tube heat exchanger, and the third temperature measuring instrument is connected to a third temperature transmitter.

[0014] According to an embodiment of the present invention, the heat exchange area of ​​the coiled tube heat exchanger can be adjusted by regulating the LNG flow rate and thus controlling the total heat exchange, thereby controlling the temperature between BOG compressor stages. Typically, cryogenic BOG compressors in LNG receiving terminals are two-stage compressors, while ambient temperature BOG compressors are three-stage compressors. This patent's embodiment is applicable to interstage cooling of BOG compressors with ≥2 stages. Taking an ambient temperature BOG compressor as an example, the pressure entering the ambient temperature BOG compressor is 0.6 MPa. After the first stage of compression, the pressure rises to 1.5 MPa, corresponding to a BOG temperature of 130°C, which can be cooled to 25°C or lower by the coiled tube heat exchanger. After the second stage of compression, the pressure rises to 4.0 MPa, corresponding to a BOG temperature of 118°C, which can be cooled to 25°C or lower by the coiled tube heat exchanger. After the third stage of compression, the pressure rises to 10 MPa, corresponding to a BOG temperature of 116°C, which can be cooled to 25°C by the coiled tube heat exchanger. The implementation method proposed in this invention can not only reasonably control the temperature of BOG to prevent it from becoming too high, thus affecting the selection of facility materials and safety, but also effectively utilize the available cold energy in the LNG receiving terminal, maximizing the saving of energy consumption during the BOG compression process.

[0015] Among them, the wound-tube heat exchanger is a multi-tube-pass single-shell wound-tube heat exchanger. The cold flow of cryogenic LNG flows through the shell side, while multiple streams of BOG flow through their respective individual tube sides. The length of the heat exchanger tubes is calculated using the following formula: L = πdnA Calculate the length of the heat exchange tube; where d n A is the inner diameter of the heat exchange tube, and A is the required total heat transfer area.

[0016] In the above system, the tubes of the wound-tube heat exchanger are made of at least one of aluminum, stainless steel, copper, and carbon steel, preferably aluminum or stainless steel, and more preferably aluminum. The heat exchange tubes of the wound-tube heat exchanger are internally corrugated and externally threaded tubes. The diameter of the heat exchange tubes in the wound-tube heat exchanger is 8~15mm, the wall thickness is 0.8~1mm, the gap between heat exchange tube layers is ≥ outer diameter + 0.8~1mm, and the gap between adjacent heat exchange tubes in the same layer is ≥ outer diameter + 3mm. A bimetallic thermometer is used for temperature detection, and an integrated temperature transmitter is used; the TCV control valve adjusts the LNG flow rate according to the BOG exhaust temperature.

[0017] In the aforementioned system, temperature sensors, transmitters, and PLC control circuits are installed on the outlet pipelines of the wound-tube heat exchangers at each stage. The BOG temperature signals between each stage are transmitted to the DCS control system in the central control room, which then calculates the corresponding LNG quantity to cool the BOG at each stage, ensuring the BOG temperature reaches the set value. In the central control room, the interstage temperature signals of the BOG compressors at each stage are fed back to the DCS controller, which calculates and superimposes the required LNG for each stage to determine the LNG flow rate. Since the heated LNG requires further pressurization and vaporization, the LNG flow rate can be appropriately amplified; that is, under the condition of achieving interstage temperature control of the BOG compressors, the LNG flow rate entering the wound-tube heat exchanger is not considered a critical control value. The feedback LNG flow rate is sent to the TCV valve actuator to control the opening of the TCV valve. When the temperature of any interstage BOG exceeds the set value, the TCV valve opening must be increased. The PLC controls the opening of the valve on the LNG inlet pipeline based on the temperature detected on the outlet pipelines of the wound-tube heat exchangers at each stage. Operators in the central control room can also monitor the corresponding LNG flow rate through the flow monitoring device installed on the LNG pipeline, and further adjust the required LNG flow rate to optimize the control system.

[0018] Since the amount of BOG generated during the unloading and loading of LNG at LNG receiving terminals is several times that of normal operations, the required LNG flow rate will also increase significantly. This invention sets the control system to an override control mode, allowing operators in the central control room to select the corresponding operating condition button based on the operating conditions of the receiving terminal. This allows for coarse control of the BOG compressor interstage temperature and fine adjustment of the LNG using the temperature detection control loop, thereby achieving precise control of the interstage BOG temperature.

[0019] Secondly, the present invention also provides a method for interstage cooling of a BOG compressor in an LNG receiving terminal using the above-mentioned system, comprising the following steps: BOG (temperature generally -10~-20℃, pressure 0.3~0.9MPag) enters the first-stage intake buffer, first-stage cylinder, and first-stage exhaust buffer sequentially through the inlet of the BOG compressor for first-stage compression. The BOG after first-stage compression (temperature 120~140℃, pressure 1.2~1.8MPag) enters the coiled-tube heat exchanger through the first-pass inlet. LNG (temperature -164~-160℃, pressure 0.16~0.52MPag) from the low-pressure LNG inlet pipeline enters the coiled-tube heat exchanger through the shell-side inlet. The BOG, as a heat source, and the LNG, as a cold source, are cooled in the coiled-tube heat exchanger. Heat exchange takes place in a tubular heat exchanger. After heat exchange, the BOG (temperature ≤28℃) leaves the coiled tube heat exchanger through the outlet of the first tube pass. According to the number of compression stages of the compressor, it enters the next stage intake buffer, the next stage cylinder, and the next stage exhaust buffer for one or more stages of compression. This results in the BOG after the last stage of compression having a temperature of 110-125℃ and a pressure of 8.0~12MPag. Then, it enters the coiled tube heat exchanger to exchange heat with LNG. After heat exchange, the BOG (temperature ≤28℃) and the BOG after the last stage of heat exchange (temperature 22~28℃) are discharged through the exhaust port of the BOG compressor. The LNG after heat exchange (temperature 150~140℃) is transported to the pressurized vaporization system (pressurized to a temperature of 0~5℃ and a pressure of 0.5~0.8MPag).

[0020] In one specific embodiment, the method of interstage cooling of the BOG compressor in an LNG receiving terminal using the above-described system includes the following steps: BOG (temperature typically -10~-20℃, pressure 0.3~0.9MPag) enters the first intake buffer, first-stage cylinder, and first exhaust buffer sequentially through the inlet of the BOG compressor for first-stage compression. The BOG after first-stage compression (temperature 125-135℃, pressure 1.2~1.8MPag) enters the coiled-tube heat exchanger through the first tube-side inlet. LNG (temperature typically -164~-160℃, pressure typically 0.16~0.52MPag) from the low-pressure LNG inlet pipeline enters the coiled-tube heat exchanger through the shell-side inlet. The BOG, acting as a heat source, and the LNG, acting as a cold source, exchange heat in the coiled-tube heat exchanger. The BOG after heat exchange (temperature ≤28℃) enters the first exhaust buffer. The BOG exiting the tube-side outlet leaves the coiled tube heat exchanger and sequentially enters the second intake buffer, the second stage cylinder, and the second exhaust buffer for secondary compression. The BOG after secondary compression (temperature 110-125℃, pressure 3.5~4.5MPag) then enters the coiled tube heat exchanger to exchange heat with LNG. The BOG after heat exchange (temperature ≤28℃) leaves the coiled tube heat exchanger and sequentially enters the third intake buffer, the third stage cylinder, and the third exhaust buffer for tertiary compression. The BOG after tertiary compression (temperature 105-122℃, pressure 8~12MPag) then enters the coiled tube heat exchanger to exchange heat with LNG. Finally, the BOG after heat exchange (temperature 22~28℃) is discharged through the exhaust port of the BOG compressor. The LNG after heat exchange (temperature 150~140℃) is transported to the pressurized vaporization system (pressurized to a temperature of 0~5℃ and a pressure of 0.5~0.8MPag).

[0021] The operating principle is as follows: BOG (Bottled Gas) from each stage of the cryogenic BOG compressor serves as the heat source, while LNG from the in-tank pump outlet serves as the cold source. Heat exchange occurs between the two in a coiled-tube heat exchanger. The cooled BOG then enters the next stage of the BOG compressor, while the heated LNG directly enters the pressurized vaporization system. Temperature sensors are installed on the BOG outlet lines of each stage of the coiled-tube heat exchanger. The detected temperature signals are transmitted to temperature transmitters, which then transmit the signals via a PLC control loop to the DCS control system in the central control room. The signals are then fed back to the TCV control valve on the low-pressure LNG inlet line, thereby controlling the LNG flow rate into the coiled-tube heat exchanger and ultimately achieving cooling capacity control. Furthermore, in the operating condition selector, operators can directly select the corresponding operating condition based on the receiving station's current operating conditions. Fine-grained control is achieved through the TCV control loop.

[0022] This invention uses a coiled tube heat exchanger as a key device for interstage cooling of the BOG compressor, which cools the interstage BOG while also providing primary energy for heating cryogenic LNG.

[0023] The control principle is as follows: BOG compressor interstage temperature control is achieved by controlling the amount of LNG entering the coiled tube heat exchanger through temperature detection instruments, temperature transmitters, a DCS control system, and a regulating valve (TCV). This controls the BOG temperature between stages. Since heating the cryogenic LNG is a primary heating process for LNG vaporization, the goal is simply to provide heat energy to the LNG as much as possible. As long as LNG doesn't vaporize, it can be safely delivered downstream. Therefore, the BOG compressor interstage temperature control is the primary control variable, while the amount of heated LNG is a secondary variable. Because the heated LNG needs further pressurization and vaporization, the LNG flow rate can be appropriately increased. That is, under the condition of achieving BOG compressor interstage temperature control, the LNG flow rate entering the coiled tube heat exchanger is not a critical control value. When any interstage BOG fails to reach the set value, the flow rate of the cold source LNG needs to be further increased. Additionally, a condition selector is installed on the central control room's DCS control system, allowing arbitrary selection of operating conditions for override control of the BOG compressor interstage temperature. The operators in the central control room will select the appropriate operating condition through the operating condition selector to control the LNG flow rate into the coiled tube heat exchanger, especially during the unloading and loading operations at the receiving station, when the BOG flow rate will be several times higher than normal. This override control mode will greatly increase the opening of the TCV valve, so that the LNG flow rate into the coiled tube heat exchanger will also be several times higher than normal, thereby keeping the BOG temperature between the BOG compressor stages within a reasonable range.

[0024] The technical solution of the present invention has the following advantages: (1) This invention is applicable to the system of interstage cooling of BOG compressor in LNG receiving station. It takes into account the energy consumption of LNG receiving station as a whole, makes full use of the thermal energy between BOG compressor stages and the cold energy of LNG, so that the energy can be utilized in a cascade manner, thus maximizing the energy saving and consumption reduction effect. (2) The present invention is applicable to the interstage cooling system of BOG compressors in LNG receiving terminals. Considering the overall facility investment of LNG receiving terminals, it simplifies the interstage cooling system of BOG compressors and reduces equipment investment as a whole. (3) This invention is applicable to the interstage cooling system of BOG compressor in LNG receiving station. It develops an operating condition control mode. Coarse operation can be completed by the operating condition control system, and fine control is completed by the temperature control loop. The two complement each other and optimize the operation control. (4) This invention is applicable to the interstage cooling system of BOG compressor in LNG receiving terminals. A DCS control system has been developed, which is suitable for frequent switching of multiple operating conditions in LNG receiving terminals. (5) This invention is applicable to the interstage cooling system of BOG compressors in LNG receiving terminals. The control system is simple, all equipment and instruments are mature products, and the implementation is highly feasible. It is applicable to all LNG receiving terminal projects. (6) The present invention is applicable to the interstage cooling system of BOG compressor in LNG receiving terminals. It is not only a new technology for the utilization of cold energy in large LNG receiving terminals, but also a new energy-saving system technology applicable to receiving terminals. Cold and heat energy are utilized to save energy and reduce emissions. (7) This invention utilizes LNG to cool the gas between stages of the BOG compressor so that the gas temperature at the outlet of each stage of the BOG compressor is not too high. By using a coiled heat exchanger to replace the original cooler between each stage of the BOG compressor, the maximum utilization of cold and heat energy can be achieved. At the same time, a working condition selector is provided in the DCS system in the central control room. The working condition selector is equipped with a control button in the DCS system. It is connected to the field actuator through the DCS system via optical cable. In specific implementation, the specific working condition can be manually selected according to the specific requirements of various operating conditions of the LNG receiving station, so as to facilitate the intelligent control of the BOG compressor. Attached Figure Description

[0025] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings, wherein: Figure 1 This is an overall structural diagram of the system for interstage cooling of the BOG compressor in LNG receiving terminals, as shown in Example 1. Figure 2 This is a structural diagram of the cooling system of a traditional BOG compressor in Comparative Example 1; Among them, 1—BOG compressor, 2-1—first intake buffer, 2-2—second intake buffer, 2-3—third intake buffer, 3-1—first stage cylinder, 3-2—second stage cylinder, 3-3—third stage cylinder, 4-1—first exhaust buffer, 4-2—second exhaust buffer, 4-3—third exhaust buffer, 5—winding tube heat exchanger, 6—pressurized vaporization system, 7-1—first temperature measuring instrument, 8-1—first temperature transmitter, 7-2—second temperature measuring instrument, 8-2—second temperature transmitter, 7-3—third temperature measuring instrument, 8-3—third temperature transmitter, 9—DCS control system, 10—TCV control valve, 11-1—first stage air cooler, 11-2—second stage air cooler, 11-3—third stage air cooler. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] In the description of the invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installed," "equipped with," "connected," etc., should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, a direct connection, or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0028] This invention is applicable to interstage cooling systems for BOG compressors in LNG receiving terminals, such as... Figure 1 As shown, the system includes: a BOG compressor 1, a coiled tube heat exchanger 5, and a pressurized vaporization system 6; wherein, the BOG compressor 1 includes at least two stages of intake buffers, at least two stages of cylinders, and at least two stages of exhaust buffers. The intake pipeline of the BOG compressor 1 is sequentially connected to the first-stage intake buffer, the first-stage cylinder, and the first-stage exhaust buffer. The outlet of the first-stage exhaust buffer is connected via a pipeline to the inlet of one tube side of the coiled tube heat exchanger 5. The outlet of the one tube side of the coiled tube heat exchanger 5 is sequentially connected to the second-stage intake buffer, the second-stage cylinder, and the second-stage exhaust buffer. The outlet inlet pipeline of the second-stage exhaust buffer is connected to the inlet of the other tube side of the coiled tube heat exchanger 5, until the outlet of the tube side of the coiled tube heat exchanger 5 connected to the outlet of the last stage exhaust buffer is connected via a pipeline to the exhaust port of the BOG compressor 1. The low-pressure LNG inlet pipeline is connected to the shell side inlet of the coiled tube heat exchanger 5, and the shell side outlet of the coiled tube heat exchanger 5 is connected via a pipeline to the inlet of the pressurized vaporization system 6.

[0029] In a preferred embodiment, the system further includes a control system comprising: a temperature sensing instrument, a temperature transmitter, a PLC control loop, a DCS control system 9, a TCV control valve 10, and a condition selector. The temperature sensing instrument is installed on the pipe outlet of the coiled tube heat exchanger 5 and is connected to the temperature transmitter. The temperature transmitter is connected to the DCS control system via a PLC control loop. The TCV control valve 10 is installed on the low-pressure LNG inlet pipe and is connected to the DCS control system 9 via a PLC control loop. The DCS control system 9 is equipped with a condition selector, and the selection options on the condition selector include unloading, loading onto trucks, loading onto ships, pressurization, and external transmission.

[0030] In another preferred embodiment, such as Figure 1 As shown, the BOG compressor 1 includes a first intake buffer 2-1, a second intake buffer 2-2, a third intake buffer 2-3, a first-stage cylinder 3-1, a second-stage cylinder 3-2, a third-stage cylinder 3-3, a first exhaust buffer 4-1, a second exhaust buffer 4-2, and a third exhaust buffer 4-3. The intake pipeline of the BOG compressor 1 is sequentially connected to the first intake buffer 2-1, the first-stage cylinder 3-1, and the first exhaust buffer 4-1. The outlet of the first exhaust buffer 4-1 is connected via a pipeline to the inlet of the first tube side of the coiled tube heat exchanger 5. The outlet of the first tube side of the coiled tube heat exchanger 5 is connected via a pipeline to the second intake buffer 2-2, the second-stage cylinder 3-3, the third exhaust buffer 4-3, and the third exhaust buffer 4-3. 2. The second exhaust buffer 4-2 is connected. The outlet of the second exhaust buffer 4-2 is connected to the inlet of the second tube side of the coiled tube heat exchanger 5 via a pipeline. The outlet of the second tube side of the coiled tube heat exchanger 5 is connected to the third intake buffer 2-3, the third-stage cylinder 3-3, and the third exhaust buffer 4-3 in sequence via a pipeline. The outlet of the third exhaust buffer 4-3 is connected to the inlet of the third tube side of the coiled tube heat exchanger 5 via a pipeline. The outlet of the third tube side of the coiled tube heat exchanger 5 is connected to the exhaust port of the BOG compressor 1 via a pipeline. The low-pressure LNG inlet pipeline is connected to the shell side inlet of the coiled tube heat exchanger 5. The shell side outlet of the coiled tube heat exchanger 5 is connected to the inlet of the booster gasification system 6 via a pipeline.

[0031] In yet another preferred embodiment, the temperature sensing instruments include a first temperature sensing instrument 7-1, a second temperature sensing instrument 7-2, and a third temperature sensing instrument 7-3; the temperature transmitters include a first temperature transmitter 8-1, a second temperature transmitter 8-2, and a third temperature transmitter 8-3, such as... Figure 1 As shown, a first temperature measuring instrument 7-1 is installed on the first tube outlet line of the wound tube heat exchanger 5, and the first temperature measuring instrument 7-1 is connected to a first temperature transmitter 8-1. A second temperature measuring instrument 7-2 is installed on the second tube outlet line of the wound tube heat exchanger 5, and the second temperature measuring instrument 7-2 is connected to a second temperature transmitter 8-2. A third temperature measuring instrument 7-3 is installed on the third tube outlet line of the wound tube heat exchanger 5, and the third temperature measuring instrument 7-3 is connected to a third temperature transmitter 8-3.

[0032] In another preferred embodiment, the tubes of the wound-tube heat exchanger 5 are made of at least one of aluminum, stainless steel, copper, and carbon steel, preferably aluminum or stainless steel, and more preferably aluminum; the heat exchange tubes of the wound-tube heat exchanger 5 are internally corrugated and externally threaded tubes; the diameter of the heat exchange tubes of the wound-tube heat exchanger 5 is 8~15mm, the wall thickness is 0.8~1mm, the gap between heat exchange tube layers is ≥ outer diameter of tube + 0.8~1mm, and the gap between adjacent heat exchange tubes in the same layer is ≥ outer diameter of tube + 3mm. A bimetallic thermometer is used as the temperature detection instrument, and an integrated temperature transmitter is used as the temperature transmitter; the TCV control valve adjusts the LNG flow rate according to the BOG exhaust temperature.

[0033] This invention employs the aforementioned system for interstage cooling of a BOG compressor in an LNG receiving terminal, comprising the following steps: BOG (temperature typically -10~-20℃, pressure 0.3~0.9MPag) enters the BOG compressor inlet via pipeline sequentially into the first intake buffer, the first-stage cylinder, and the first exhaust buffer for first-stage compression. The compressed BOG (temperature 125-135℃, pressure 1.2~1.8MPag) enters the coiled-tube heat exchanger through the first-tube inlet. LNG (temperature typically -164~-160℃, pressure typically 0.16~0.52MPag) from the low-pressure LNG inlet pipeline enters the coiled-tube heat exchanger through the shell-side inlet. The BOG, acting as a heat source, and the LNG, acting as a cold source, exchange heat in the coiled-tube heat exchanger. The BOG (temperature ≤28℃) exits through the first-tube outlet. The BOG (Boiled Gas) undergoes two-stage compression: a second intake buffer, a second-stage cylinder, and a second exhaust buffer. The compressed BOG (temperature 110-125℃, pressure 3.5-4.5 MPa) then enters the same heat exchanger to exchange heat with LNG. The compressed BOG (temperature ≤28℃) exits the heat exchanger and undergoes three-stage compression: a third intake buffer, a third-stage cylinder, and a third exhaust buffer. The compressed BOG (temperature 105-122℃, pressure 8-12 MPa) then enters the same heat exchanger to exchange heat with LNG. Finally, the compressed BOG (temperature 22-28℃) is discharged through the exhaust port of the BOG compressor. The compressed LNG (temperature 150-140℃) is then transported to a pressurized vaporization system (pressurized to 0-5℃, pressure 0.5-0.8 MPa).

[0034] According to an embodiment of the present invention, the interstage temperature of a BOG compressor can be controlled by adjusting the heat exchange area of ​​a coiled tube heat exchanger. Typically, cryogenic BOG compressors in LNG receiving terminals are two-stage compressors, while ambient temperature BOG compressors are three-stage compressors. This patent's embodiment is applicable to interstage cooling of BOG compressors with ≥2 stages. Taking an ambient temperature BOG compressor as an example, the pressure entering the ambient temperature BOG compressor is 0.6 MPa. After the first stage of compression, the pressure rises to 1.5 MPa, corresponding to a BOG temperature of 130°C, which can be cooled to 25°C or lower by the coiled tube heat exchanger. After the second stage of compression, the pressure rises to 4.0 MPa, corresponding to a BOG temperature of 118°C, which can be cooled to 25°C or lower by the coiled tube heat exchanger. After the third stage of compression, the pressure rises to 10 MPa, corresponding to a BOG temperature of 116°C, which can be cooled to 25°C by the coiled tube heat exchanger. The implementation method proposed in this invention can not only reasonably control the temperature of BOG to prevent it from becoming too high, thus affecting the selection of facility materials and safety, but also effectively utilize the available cold energy in the LNG receiving terminal, maximizing the saving of energy consumption during the BOG compression process. Example 1

[0035] like Figure 1As shown, this embodiment applies to the interstage cooling system of the BOG compressor in an LNG receiving terminal, including: BOG compressor 1, coiled tube heat exchanger 5, pressurized vaporization system 6, and control system; wherein, BOG compressor 1 includes a first inlet buffer 2-1, a second inlet buffer 2-2, a third inlet buffer 2-3, a first-stage cylinder 3-1, a second-stage cylinder 3-2, a third-stage cylinder 3-3, a first exhaust buffer 4-1, a second exhaust buffer 4-2, and a third exhaust buffer 4-3, and the inlet pipeline of BOG compressor 1 is sequentially connected to the first inlet buffer 2-1, the first-stage cylinder 3-1, the first exhaust buffer 4-3, and the third exhaust buffer 4-3. 1. Connection: The outlet of the first exhaust buffer 4-1 is connected via pipeline to the inlet of the first tube side of the wound-tube heat exchanger 5. The outlet of the first tube side of the wound-tube heat exchanger 5 is connected via pipeline to the second inlet buffer 2-2, the second stage cylinder 3-2, and the second exhaust buffer 4-2 in sequence. The outlet of the second exhaust buffer 4-2 is connected via pipeline to the inlet of the second tube side of the wound-tube heat exchanger 5. The outlet of the second tube side of the wound-tube heat exchanger 5 is connected via pipeline to the third inlet buffer 2-3, the third stage cylinder 3-3, and the third exhaust buffer 4-3 in sequence. The outlet of the third exhaust buffer 4-3 is connected via pipeline to the inlet of the third tube side of the wound-tube heat exchanger 5. The third tube outlet of the wound-tube heat exchanger 5 is connected to the exhaust port of the BOG compressor 1 via a pipeline. The low-pressure LNG inlet pipeline is connected to the shell-side inlet of the wound-tube heat exchanger 5. The shell-side outlet of the wound-tube heat exchanger 5 is connected to the inlet of the booster gasification system 6 via a pipeline. The control system includes: temperature detection instruments, temperature transmitters, PLC control loops, DCS control system 9, TCV control valve 10, and operating condition selector. A first temperature detection instrument 7-1 is installed on the first tube outlet pipeline of the wound-tube heat exchanger 5. The first temperature detection instrument 7-1 and the first temperature transmitter 8-1 are connected. A second temperature sensor 7-2 is installed on the outlet pipeline of the second tube pass. The second temperature sensor 7-2 is connected to the second temperature transmitter 8-2. A third temperature sensor 7-3 is installed on the outlet pipeline of the third tube pass of the wound-tube heat exchanger 5. The third temperature sensor 7-3 is connected to the third temperature transmitter 8-3. The first to third temperature transmitters 8-1, 8-2, and 8-3 are connected to the DCS control system 9 through a PLC control loop. The TCV control valve 10 is installed on the low-pressure LNG inlet pipeline and is connected to the DCS control system 9 through a PLC control loop. The DCS control system 9 is equipped with a condition selector.

[0036] Among them, the tube material of the wound tube heat exchanger 5 is aluminum; the heat exchange tube of the wound tube heat exchanger 5 is an internally wavy externally threaded tube; the diameter of the heat exchange tube of the wound tube heat exchanger 5 is 10mm, the wall thickness is 1mm, the gap between heat exchange tube layers is ≥ outer diameter of tube + 1mm, and the gap between adjacent heat exchange tubes in the same layer is ≥ outer diameter of tube + 3mm.

[0037] This embodiment employs the above-described system for interstage cooling of the BOG compressor at an LNG receiving station, comprising the following steps: BOG (temperature 25°C, pressure 0.6 MPa) enters sequentially through pipelines from the inlet of BOG compressor 1 into the first inlet buffer 2-1, the first stage cylinder 3-1, and the first exhaust buffer 4-1 for first-stage compression. The compressed BOG (temperature 130°C, pressure 1.5 MPa) enters the coiled-tube heat exchanger 5 through the first-pass inlet. LNG (temperature -164 to -160°C, pressure 0.16 to 0.52 MPa) from the low-pressure LNG inlet pipeline enters the coiled-tube heat exchanger 5 through the shell-side inlet. The BOG, acting as a heat source, and the LNG, acting as a cold source, exchange heat in the coiled-tube heat exchanger 5. The BOG (temperature 25°C or lower), after heat exchange, exits the coiled-tube heat exchanger 5 through the first-pass outlet. The BOG (boiler gas) sequentially enters the second intake buffer 2-2, the second stage cylinder 3-2, and the second exhaust buffer 4-2 for secondary compression. The BOG after secondary compression (temperature 118℃, pressure 4.0MPag) then enters the coiled tube heat exchanger 5 to exchange heat with LNG. The BOG after heat exchange (temperature 25℃ or lower) leaves the coiled tube heat exchanger 5 and sequentially enters the third intake buffer 2-3, the third stage cylinder 3-3, and the third exhaust buffer 4-3 for tertiary compression. The BOG after tertiary compression (temperature 116℃, pressure 10MPag) then enters the coiled tube heat exchanger 5 to exchange heat with LNG. Finally, the BOG after heat exchange (temperature 25℃) is discharged through the exhaust port of the BOG compressor 1. The LNG after heat exchange (temperature -150~-140℃) is transported to the pressurized vaporization system 6 (temperature 0~5℃, pressure 0.5~0.8MPag).

[0038] As can be seen from the above embodiments, this embodiment is applicable to the system of interstage cooling of BOG compressors in LNG receiving terminals. By setting up a coiled tube heat exchanger between the BOG compressor stages, the thermal energy between the BOG compressor stages and the cold energy of LNG are fully utilized, enabling energy cascade utilization. According to calculations, when cooling 24t / h of BOG, the average energy saving is 4155kw / h, and the energy saving ratio is close to 51%, thus maximizing the energy saving and consumption reduction effect. By setting up a working condition selector in the DCS control system, the operator can directly select the corresponding working condition according to the working condition of the receiving terminal. Fine control is achieved by the TCV control loop. The two complement each other and optimize the operation control. Comparative Example 1

[0039] like Figure 2As shown, a conventional BOG compressor cooling system includes: a BOG compressor 1, an air cooler 11, a booster vaporization system 6, and a control system; wherein, the BOG compressor 1 includes a first intake buffer 2-1, a second intake buffer 2-2, a third intake buffer 2-3, a first-stage cylinder 3-1, a second-stage cylinder 3-2, a third-stage cylinder 3-3, a first exhaust buffer 4-1, a second exhaust buffer 4-2, and a third exhaust buffer 4-3; the air cooler 11 includes a first-stage air cooler 11-1, a second-stage air cooler 11-2, and a third-stage air cooler 11-3; the control system includes: a first temperature sensor 7-1, a first temperature transmitter 8-1, a second temperature sensor 7-2, a second temperature transmitter 8-2, a third temperature sensor 7-3, and a third temperature transmitter 8-3; the intake pipeline of the BOG compressor 1 is sequentially connected to the first intake buffer 2-1, the first-stage cylinder 3-1, the second exhaust buffer 2-2, the third exhaust buffer 4-3, the first exhaust buffer 4-1, the second exhaust buffer 4-2, and the third exhaust buffer 4-3; the air cooler 11 includes a first-stage air cooler 11-1, a second-stage air cooler 11-2, and a third-stage air cooler 11-3; the control system includes: a first temperature sensor 7-1, a first temperature transmitter 8-1, a second temperature sensor 7-2, a second temperature transmitter 8-2, a third temperature sensor 7-3, and a third temperature transmitter 8-3; the intake pipeline of the BOG compressor 1 is connected to the first intake buffer 2-1, the first-stage cylinder 3-1, the second exhaust buffer 4-2, the third exhaust buffer 4-3, the third exhaust buffer 4-3, the first exhaust buffer 4-1, the second exhaust buffer 4- The first exhaust buffer 4-1 is connected. The outlet of the first exhaust buffer 4-1 is connected to the first-stage air cooler 11-1 via a pipeline. Then, it is connected in sequence to the second intake buffer 2-2, the second-stage cylinder 3-2, and the second exhaust buffer 4-2. The outlet of the second exhaust buffer 4-2 is connected to the second-stage air cooler 11-2 via a pipeline. Then, it is connected in sequence to the third intake buffer 2-3, the third-stage cylinder 3-3, and the third exhaust buffer 4-3. The outlet of the third exhaust buffer 4-3 is connected to the exhaust port of the BOG compressor 1 via a pipeline. The first temperature measuring instrument 7-1 is installed on the first exhaust buffer 4-1 and connected to the first temperature transmitter 8-1. The second temperature measuring instrument 7-2 is installed on the second exhaust buffer 4-2 and connected to the second temperature transmitter 8-2. The third temperature measuring instrument 7-3 is installed on the third exhaust buffer 4-3 and connected to the third temperature transmitter 8-3.

[0040] As can be seen from the above comparison, the traditional BOG compressor cooling system transmits temperature signals to the DCS control system in the central control room through temperature detection instruments, temperature transmitters, and PLC control loops installed in the exhaust buffers between each stage of the BOG compressor. The temperature signals are then converted into action signals for the actuators, which are sent to the air coolers between each stage. The frequency of the fan rotation is adjusted by the frequency converter to regulate the motor speed, thereby achieving the purpose of cooling the BOG. ​​This process consumes a lot of energy.

[0041] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A system for interstage cooling of BOG compressors in LNG receiving terminals, characterized in that, The system includes: a BOG compressor (1), a coiled tube heat exchanger (5), and a pressurized vaporization system (6); wherein, the BOG compressor (1) includes at least two stages of intake buffers, at least two stages of cylinders, and at least two stages of exhaust buffers, the intake pipeline of the BOG compressor (1) is sequentially connected to the first stage intake buffer, the first stage cylinder, and the first stage exhaust buffer, the outlet of the first stage exhaust buffer is connected via a pipeline to the inlet of one tube side of the coiled tube heat exchanger (5), and the outlet of the one tube side of the coiled tube heat exchanger (5) is... It is connected in sequence to the secondary intake buffer, the secondary cylinder, and the secondary exhaust buffer. The outlet inlet of the secondary exhaust buffer is connected to the other tube inlet of the coiled tube heat exchanger (5), until the tube outlet of the coiled tube heat exchanger (5) connected to the outlet of the last stage exhaust buffer is connected to the exhaust port of the BOG compressor (1) via a pipeline. The low-pressure LNG inlet pipeline is connected to the shell inlet of the coiled tube heat exchanger (5), and the shell outlet of the coiled tube heat exchanger (5) is connected to the inlet of the booster gasification system (6) via a pipeline.

2. The system according to claim 1, characterized in that, The system also includes a control system, which includes: a temperature detection instrument, a temperature transmitter, a PLC control loop, a DCS control system (9), a TCV control valve (10), and a condition selector. The temperature detection instrument is installed on the pipeline at the outlet of each tube of the coiled tube heat exchanger. The temperature detection instrument is connected to the temperature transmitter. The temperature transmitter is connected to the DCS control system through a PLC control loop. The TCV control valve is installed on the low-pressure LNG inlet pipeline and is connected to the DCS control system (9) through a PLC control loop. The DCS control system (9) is equipped with a condition selector.

3. The system according to claim 2, characterized in that, The BOG compressor (1) includes a first intake buffer (2-1), a second intake buffer (2-2), a third intake buffer (2-3), a first-stage cylinder (3-1), a second-stage cylinder (3-2), a third-stage cylinder (3-3), a first exhaust buffer (4-1), a second exhaust buffer (4-2), and a third exhaust buffer (4-3). The intake pipeline of the BOG compressor (1) is connected sequentially to the first intake buffer (2-1), the first-stage cylinder (3-1), and the first exhaust buffer (4-1). The outlet of the first exhaust buffer (4-1) is connected via a pipeline to the inlet of the first tube side of the coiled tube heat exchanger (5). The outlet of the first tube side of the coiled tube heat exchanger (5) is connected via a pipeline to the second intake buffer (2-2), the second-stage cylinder (3-3), the third-stage cylinder (3-3), the fourth-stage cylinder (3-2), the fifth-stage cylinder (3-3), the sixth-stage cylinder (3-3), the seventh-stage cylinder (3-1), the eleventh-stage cylinder (3-2), the eleventh-stage cylinder (3-3 ...2), the eleventh-stage cylinder (3-3), the eleventh-stage 3-2) The second exhaust buffer (4-2) is connected. The outlet of the second exhaust buffer (4-2) is connected to the inlet of the second tube side of the coiled tube heat exchanger (5) via a pipeline. The outlet of the second tube side of the coiled tube heat exchanger (5) is connected to the third intake buffer (2-3), the three-stage cylinder (3-3), and the third exhaust buffer (4-3) via a pipeline in sequence. The outlet of the third exhaust buffer (4-3) is connected to the inlet of the third tube side of the coiled tube heat exchanger (5) via a pipeline. The outlet of the third tube side of the coiled tube heat exchanger (5) is connected to the exhaust port of the BOG compressor (1) via a pipeline. The low-pressure LNG inlet pipeline is connected to the shell side inlet of the coiled tube heat exchanger (5). The shell side outlet of the coiled tube heat exchanger (5) is connected to the inlet of the booster gasification system (6) via a pipeline.

4. The system according to claim 3, characterized in that, A first temperature measuring instrument (7-1) is installed on the first tube outlet line of the coiled tube heat exchanger (5), and the first temperature measuring instrument (7-1) is connected to the first temperature transmitter (8-1). A second temperature measuring instrument (7-2) is installed on the second tube outlet line of the coiled tube heat exchanger (5), and the second temperature measuring instrument (7-2) is connected to the second temperature transmitter (8-2). A third temperature measuring instrument (7-3) is installed on the third tube outlet line of the coiled tube heat exchanger (5), and the third temperature measuring instrument (7-3) is connected to the third temperature transmitter (8-3).

5. The system according to any one of claims 1-4, characterized in that, The tube material of the wound tube heat exchanger (5) is at least one of aluminum, stainless steel, copper and carbon steel, and the heat exchange tube of the wound tube heat exchanger (5) is an internally wavy externally threaded tube.

6. The system according to claim 5, characterized in that, The tubes of the wound tube heat exchanger (5) are made of aluminum or stainless steel.

7. The system according to claim 5, characterized in that, The diameter of the heat exchange tubes in the coiled tube heat exchanger (5) is 8~15mm, the wall thickness is 0.8~1mm, the gap between heat exchange tube layers is ≥ outer diameter of tube + 0.8~1mm, and the gap between adjacent heat exchange tubes in the same layer is ≥ outer diameter of tube + 3mm.

8. The system according to any one of claims 1-4, characterized in that, The temperature detection instrument (7) is a bimetallic thermometer, and the temperature transmitter (8) is an integrated temperature transmitter; the TCV control valve (10) adjusts the LNG flow rate according to the BOG exhaust temperature.

9. A method for interstage cooling of a BOG compressor in an LNG receiving terminal, characterized in that, The system for interstage cooling of a BOG compressor in an LNG receiving terminal, as described in any one of claims 1-8, comprises the following steps: BOG enters the BOG compressor through the inlet via pipeline sequentially into a first-stage inlet buffer, a first-stage cylinder, and a first-stage exhaust buffer for first-stage compression. The compressed BOG (temperature, for example, 120~140℃, pressure, for example, 1.2~1.8MPag) enters a coiled-tube heat exchanger through the first-side inlet. LNG from the low-pressure LNG inlet pipeline (for example, temperature, -164 to -160℃, pressure, 0.16~0.52MPag) enters the coiled-tube heat exchanger through the shell-side inlet. The BOG, acting as a heat source, and the LNG, acting as a cold source, exchange heat in the coiled-tube heat exchanger. The compressed BOG... (For example, at a temperature ≤28℃) The BOG exits the coiled tube heat exchanger through the first tube outlet and sequentially enters the next stage intake buffer, the next stage cylinder, and the next stage exhaust buffer according to the number of compression stages of the compressor for one or more next-stage compressions, so that the temperature of the BOG after the last stage compression is 110-125℃ and the pressure is 8.0~12MPag. Then it enters the coiled tube heat exchanger to exchange heat with LNG. The BOG after heat exchange (temperature ≤28℃) and the BOG after the last stage heat exchange (for example, at a temperature of 22~28℃) are discharged through the exhaust port of the BOG compressor. The LNG after heat exchange (for example, at a temperature of 150~140℃) is transported to the pressurization and vaporization system (for example, pressurized to a temperature of 0~5℃ and a pressure of 0.5~0.8MPag).

10. The method according to claim 9, characterized in that, Includes the following steps: BOG (Boiled Gas) enters the BOG compressor inlet via pipeline, sequentially passing through the first intake buffer, first-stage cylinder, and first exhaust buffer for primary compression. The compressed BOG (temperature can be 125-135℃, pressure can be 1.2~1.8MPag) then enters the coiled-tube heat exchanger through the first tube-side inlet. LNG (temperature typically -164~-160℃, pressure typically 0.16~0.52MPag) from the low-pressure LNG inlet line enters the coiled-tube heat exchanger through the shell-side inlet. The BOG, acting as a heat source, exchanges heat with the LNG, acting as a cold source, within the coiled-tube heat exchanger. The BOG (temperature ≤28℃) then exits the coiled-tube heat exchanger through the first tube-side outlet and sequentially enters the second intake buffer, second-stage cylinder, and second exhaust buffer for secondary compression. After compression, the BOG (temperature 110-125℃, pressure 3.5~4.5MPag) enters the coiled tube heat exchanger to exchange heat with LNG. After heat exchange, the BOG (temperature ≤28℃) leaves the coiled tube heat exchanger and enters the third intake buffer, the third-stage cylinder, and the third exhaust buffer for third-stage compression. After third-stage compression, the BOG (temperature 105-122℃, pressure 8~12MPag) enters the coiled tube heat exchanger to exchange heat with LNG. Finally, the BOG (temperature 22~28℃) is discharged through the exhaust port of the BOG compressor. The LNG (temperature 150~140℃) is transported to the pressurized gasification system (pressurized to a temperature of 0~5℃ and a pressure of 0.5~0.8MPag).

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