A compression-expansion integrated system and BOG reliquefaction method thereof

Abstract

The present invention proposes a compression-expansion integrated system and its BOG reliquefaction method, including a BOG precooler for precooling the BOG gas from the upper part of the LNG storage tank to a low temperature; a compression-expansion integrated machine, which is arranged in a low-temperature cavity and includes at least one set of compressor components and at least one set of expander components, the compressor components compress the precooled BOG gas, and the expander components expand and cool the compressed BOG gas; a multi-stage heat exchanger, one end of which is connected to the discharge port of the expander component, for heat exchange and cooling the expanded BOG; wherein the compressor component and the expander component are connected by low-temperature bearings and magnetic coupling transmission, and are arranged in the same low-temperature cavity. This application realizes efficient reliquefaction treatment of BOG gas through compression-expansion integrated design and adaptive control, and the system energy consumption is reduced by about 40% compared with the traditional process, which greatly improves the cold recovery efficiency and operation stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of liquefied natural gas storage and transportation, and particularly relates to a compression-expansion integrated system and a BOG reliquefaction method thereof. Background Art

[0002] Liquefied natural gas (LNG), a highly efficient and clean energy source, is subject to widespread volatilization during transportation, receiving, and refueling, forming what is known as BOG (Boil-Off Gas). To prevent BOG accumulation from causing overpressure in storage tanks or wasting energy, BOG recovery and treatment are typically required. Existing BOG treatment processes include the following typical methods:

[0003] 1. Normal temperature compression + throttling refrigeration (e.g., patent CN119393663A): BOG is compressed at room temperature and then cooled by throttling, allowing some of the gas to be liquefied and recovered. However, this method suffers from low cold recovery efficiency (generally less than 30%) and introduces significant heat during the compression process, resulting in poor overall energy efficiency.

[0004] 2. External nitrogen circulation expansion refrigeration (e.g., patent CN110325807A): This method uses an external nitrogen circulation system for expansion refrigeration to provide the required cooling capacity for the BOG. ​​However, this method is highly dependent on an external nitrogen source, has high system complexity, and high operating costs, making it unsuitable for miniaturized or mobile applications.

[0005] 3. Systems with separate compression and expansion equipment: In most solutions currently on the market, the compressor and expander are independent devices, arranged in normal temperature and low-temperature environments respectively. This not only leads to a long gas flow path and large cooling loss, but also requires additional heat exchangers, pipelines and control units. The overall system structure is complex, the number of failure points increases, and it occupies a large space.

[0006] Furthermore, during cryogenic equipment operation, traditional mechanical seals often fail due to the freezing of lubricating oil, limiting the system's stability and lifespan. Furthermore, the selection and preparation of cryogenic materials also directly impact the mechanical strength and airtightness of the device in environments below -160°C.

[0007] In summary, existing BOG treatment technologies generally have the following defects and shortcomings: low cold recovery efficiency and poor system energy efficiency; complex equipment structure and large space occupation, which are not conducive to integrated and modular deployment; insufficient sealing and material reliability, which affects the long-term stable operation of the system; strong dependence on external cold sources and high operating costs.

[0008] Therefore, it is urgent to propose a BOG treatment system with high structural integration, high cold recovery efficiency, stable operation and no reliance on external cold sources to meet the application requirements of LNG storage and transportation equipment in terms of efficiency, compactness and safety. Summary of the Invention

[0009] In view of the above technical problems existing in the prior art, the present invention proposes a compression-expansion integrated system and a BOG reliquefaction method thereof to solve the above technical problems.

[0010] According to a first aspect of the present invention, a compression-expansion integrated system is provided, comprising:

[0011] BOG precooler, used to precool BOG gas from the top of the LNG storage tank to a low temperature;

[0012] The compression-expansion integrated machine is arranged in a low-temperature cavity and includes at least one set of compressor components and at least one set of expander components. The compressor components compress the pre-cooled BOG gas, and the expander components expand and cool the compressed BOG gas.

[0013] A multi-stage heat exchanger, one end of which is connected to the discharge port of the expander assembly, is used to perform heat exchange cooling on the expanded BOG;

[0014] The compressor and expander components are connected via cryogenic bearings and magnetic coupling, and housed within the same cryogenic chamber. By integrating the compressor and expander within the same cryogenic chamber and utilizing magnetic coupling and cryogenic bearings for transmission, thermal bridging and cooling losses are significantly reduced, resulting in a highly efficient and compact cryogenic BOG treatment core device.

[0015] In a specific embodiment, a BOG circulation pipeline is also included to return a portion of the cooled and liquefied BOG to the LNG storage tank. The remaining portion, after heat exchange in a heat exchanger, is returned to the BOG precooler to precool new BOG entering the system. This BOG circulation pipeline enables self-circulating cooling, reducing reliance on external cooling sources while improving precooling efficiency and overall system energy efficiency.

[0016] In a specific embodiment, an adaptive control module is also included, which uses a PID algorithm to dynamically adjust the compression ratio of the compressor assembly and the speed of the expander assembly. By introducing the adaptive control module and using the PID algorithm, the system can dynamically adjust operating parameters based on operating conditions, maintaining optimal energy efficiency and enhancing intelligence and adaptability.

[0017] In a specific embodiment, the adaptive control module selects the minimum of two PID outputs and adjusts the expander speed through a ramp filter. The two minimum PID outputs, plus a predetermined deviation, serve as the maximum opening limit for the two PID controllers. The adaptive control module simultaneously adjusts the compression ratio of the compressor assembly and the speed of the expander assembly. During speed PID regulation, if the compressor ratio PID adjustment loop reaches the compression ratio set range, the speed PID will switch to the compression ratio PID adjustment loop. During compression ratio PID regulation, if the speed adjustment loop reaches the set range, the compression ratio PID adjustment will switch to the speed PID adjustment loop. The compression ratio of the compressor assembly is set between 3:1 and 8:1, and the expander speed is set between 1000 and 5000 rpm. By setting reasonable ranges for the compression ratio and expander speed, the equipment operates within a safe, economical, and efficient range, further enhancing the system's practical feasibility and stability.

[0018] In a specific embodiment, the integrated compressor / expander utilizes a three-stage compression and two-stage expansion structure. This allows for multi-stage cold recovery and compression efficiency optimization, improving the segmented energy efficiency and temperature control accuracy of the BOG treatment process.

[0019] In a specific embodiment, the compressor assembly's impeller is made of stainless steel via laser sintering, while the expander assembly's rotor is made of Inconel alloy. This configuration improves the equipment's mechanical strength, corrosion resistance, and lifespan in ultra-low temperature conditions.

[0020] In a specific embodiment, the sealing structure of the compression and expansion system is a non-contact magnetic seal. This configuration avoids the problem of traditional lubricating oil seals solidifying and failing at low temperatures, thereby improving the system's operational reliability and long-term stability.

[0021] In a specific embodiment, the multi-stage heat exchanger includes two heat exchange channels. One channel, high-pressure BOG, undergoes primary expansion before entering the first heat exchange channel for cooling and liquefaction, and the liquefied BOG is returned to the LNG storage tank. The other channel, high-pressure BOG, undergoes secondary expansion before entering the second heat exchange channel for heat exchange and temperature increase, where it is used in the BOG pre-cooling cycle. The two heat exchange channels allow for cold energy diversion and tiered utilization, enabling efficient BOG reliquefaction and pre-cooling, resulting in higher cold energy recovery efficiency and further reduced energy loss.

[0022] According to a second aspect of the present invention, a method for BOG reliquefaction using the above-mentioned integrated compression and expansion system is proposed, comprising:

[0023] S1: BOG gas is drawn from the top of the LNG storage tank and introduced into the BOG precooler for preliminary precooling, cooling the temperature to -160°C;

[0024] S2: The pre-cooled BOG enters the compression-expansion integrated unit for compression;

[0025] S3: The compressed high-pressure BOG enters the first-stage expansion module for expansion and cooling. One path of the expanded BOG enters the multi-stage heat exchanger for heat exchange, and the other path enters the second-stage expansion module for further cooling.

[0026] S4: The BOG enters the first and second heat exchange channels of the multi-stage heat exchanger for heat exchange. A portion of the BOG is liquefied and returned to the LNG storage tank, while the other portion is used to pre-cool the incoming BOG gas to form a cooling circuit.

[0027] S5: The control system adjusts the compression ratio and expander speed in real time according to the system operating status to maintain the stability of the system cooling efficiency and keep the tank pressure balanced.

[0028] In a specific embodiment, BOG is cooled to -160°C by a pre-cooling module and then enters a compression-expansion system. A compressor assembly compresses the BOG gas to 15 bar. During the expansion process, the compressed high-pressure BOG gas expands to 3 bar and its temperature drops to -180°C. The cooling energy of the low-temperature gas after expansion is recovered through a plate-fin heat exchanger.

[0029] The present invention's integrated compression and expansion system and BOG reliquefaction method achieve the following comprehensive technical effects by integrating compression and expansion in the low-temperature section and combining multi-stage cold recovery and heat exchange, adaptive control, reliable low-temperature structural materials, and sealing technology:

[0030] Significantly improve energy efficiency: The system's cooling recovery rate can reach over 85%, and power consumption is reduced by 40% compared to traditional processes;

[0031] Compact structure and simplified system: The number of equipment is reduced by about 50%, making it suitable for space-constrained scenarios such as LNG ships and fueling stations;

[0032] Self-circulating cooling capacity reduces external dependence: no external nitrogen source or cryogen is required;

[0033] Strong operational stability: the material is adapted to ultra-low temperatures, the sealing structure is reliable, and it can operate without any trouble for 10,000 hours;

[0034] Strong adaptability: PID control realizes dynamic adjustment to adapt to different gas volumes and tank status changes. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings are included to provide a further understanding of the embodiments and are incorporated into and constitute a part of this specification. The accompanying drawings illustrate the embodiments and, together with the description, serve to explain the principles of the present invention. Other embodiments and many of the expected advantages of the embodiments will be readily apparent as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale with respect to each other. Like reference numerals designate corresponding similar parts.

[0036] Figure 1 A schematic diagram of the system framework of a compression-expansion integrated system according to an embodiment of the present invention is shown;

[0037] Figure 2 A detailed diagram of the internal structure of a compression-expansion integrated machine according to a specific embodiment of the present invention is shown;

[0038] Figure 3 A flow chart of a method for BOG reliquefaction using a compression-expansion integrated system according to an embodiment of the present invention is shown.

[0039] The meaning of the numbers in the figure: 1. LNG storage tank; 2. BOG gas; 3. BOG precooler; 4. Compression expansion machine; 5. Multi-stage heat exchanger; 6. Impeller; 7. Rotor; 8. Magnetic sealing structure. DETAILED DESCRIPTION

[0040] The present invention will be further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are intended only to illustrate the invention and are not intended to limit the invention. It should also be noted that, for ease of description, only portions relevant to the invention are shown in the accompanying drawings.

[0041] It should be noted that, in the absence of conflict, the embodiments and features of the embodiments of the present invention can be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0042] While the above describes specific embodiments of the present invention, the scope of protection of the present invention is not limited thereto. Any modifications or substitutions that can be readily conceived by a person skilled in the art within the technical scope disclosed herein are intended to be encompassed within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope of protection of the claims.

[0043] Figure 1 FIG. 1 shows a schematic diagram of a system framework of a compression-expansion integrated system according to an embodiment of the present invention. Figure 1As shown, the system includes an LNG tank 1, a BOG precooler 3, a compressor / expander 4, and a multi-stage heat exchanger 5. The LNG tank 1 stores liquefied natural gas (LNG), and the boil-off gas formed in its upper chamber is the BOG gas 2. The BOG precooler 3 is located between the LNG tank 1 and the compressor / expander 4, providing initial cooling for the BOG gas 2 drawn from the LNG tank 1 to improve subsequent compression efficiency. The compressor / expander 4 is the core component of the present invention, integrating compression and expansion functions and comprising a compressor assembly and an expander assembly housed within a cryogenic chamber. The multi-stage heat exchanger 5 is connected to the compressor / expander 4, receiving the expanded low-temperature BOG gas and recovering the cold energy through a plate-fin structure for BOG reliquefaction. The multi-stage heat exchanger 5, the BOG precooler 3, and the LNG tank 1 form a closed-loop system with self-recovery of cold energy through a circulation pipeline. The optimized flow path shortens the BOG flow path and reduces cold energy loss. The compressor assembly and expander assembly are connected via cryogenic bearings and a magnetic coupling transmission system and housed within the same cryogenic chamber. By integrating the compressor and expander into the same low-temperature cavity and using magnetic coupling and low-temperature bearings for transmission, the thermal bridge effect and cooling loss can be significantly reduced, realizing a high-efficiency and compact low-temperature BOG treatment core equipment.

[0044] In a specific embodiment, an adaptive control module is also included for dynamically adjusting the compression ratio of the compressor assembly and the speed of the expander assembly using a PID algorithm. By introducing an adaptive control module and adopting a PID algorithm, the system can dynamically adjust operating parameters according to operating conditions, maintain the system operating at the optimal energy efficiency point, and improve intelligence and adaptability. In a preferred embodiment, the compression ratio of the compressor assembly is within the range of 3:1 to 8:1, and the speed of the expander is within the range of 1000-5000 rpm. By setting a reasonable range for the compression ratio and expander speed, the equipment operates within a safe, economical, and efficient range, further enhancing the practical feasibility and stability of the system.

[0045] The present invention adopts a dual PID parallel regulation method, taking the BOG gas phase pressure at the top of the LNG storage tank and the expander speed as controlled objects, respectively, to form a dual PID control loop.

[0046] The minimum of the two PID outputs is selected and then passed through a ramp filter to adjust the expander speed. Simultaneously, the minimum of the two PID outputs, plus a certain opening deviation, serves as the maximum opening limit for both PID controllers. This achieves adaptive adjustment of the two PID control processes, enhancing the smoothness and stability of dual PID control. This adaptive dual PID control algorithm simultaneously adjusts the compressor's compression ratio and the expander speed, thereby simultaneously controlling the BOG compression rate and the system's cooling capacity. When in speed PID control, if the compressor ratio PID control loop approaches the compression ratio setpoint, the speed PID will switch to the compression ratio PID control loop, preventing excessive compression ratio fluctuations. Conversely, if the speed control loop approaches the setpoint during compression ratio PID control, the compression ratio PID will switch to the speed PID control loop, preventing excessive speed increases or decreases, thereby improving system robustness.

[0047] The system sets the compression ratio adjustment range to to , and the expander speed range is 1000 to 5000 rpm. By adjusting these two key parameters in real time, the system can maintain efficient and stable operation despite varying BOG loads, tank pressure fluctuations, or external disturbances, ensuring full BOG cooling recovery and precise control of tank pressure.

[0048] In a preferred embodiment, the integrated compression and expansion machine adopts a three-stage compression and two-stage expansion structure. By setting up the three-stage compression and two-stage expansion structure, multi-stage cold recovery and compression efficiency optimization are achieved, thereby improving the segmented energy efficiency and temperature control accuracy of the BOG treatment process.

[0049] In a preferred embodiment, the multi-stage heat exchanger 5 includes two heat exchange channels. One channel of high-pressure BOG undergoes primary expansion before entering the first heat exchange channel for cooling and liquefaction, with the liquefied BOG returned to the LNG storage tank. The other channel of high-pressure BOG undergoes secondary expansion before entering the second heat exchange channel for heat exchange and temperature increase, where it is used in the BOG pre-cooling cycle. The two heat exchange channels allow for cold energy diversion and tiered utilization, enabling efficient BOG reliquefaction and pre-cooling, resulting in higher cold energy recovery efficiency and further reduced energy loss.

[0050] Figure 2 FIG. 1 shows a detailed diagram of the internal structure of a compression-expansion integrated machine according to a specific embodiment of the present invention. Figure 2As shown, the internal structure of the compression and expansion machine 4 includes a low-temperature cavity for forming a low-temperature working environment. An impeller 6, a rotor 7 and a magnetic sealing structure 8 are provided in the low-temperature cavity. The impeller 6 is the core component of the compressor assembly and is used to pressurize the BOG. ​​In a preferred embodiment, the impeller 6 is made of stainless steel and is formed by laser sintering to adapt to low-temperature and high-pressure working conditions. The rotor 7 is used to drive the expander to work. In a preferred embodiment, its material is Inconel alloy, which has good low-temperature strength and corrosion resistance. The magnetic sealing structure 8 is arranged at the end of the rotating shaft. In a preferred embodiment, a non-contact structure is adopted, which can effectively prevent the lubricating oil from solidifying at low temperatures, thereby improving the sealing reliability and system life.

[0051] Continue to refer Figure 3 , Figure 3 FIG. 1 shows a flow chart of a method for BOG reliquefaction using a compression-expansion integrated system according to an embodiment of the present invention. Figure 3 As shown, the method includes the following steps:

[0052] S1: BOG gas is drawn from the top of the LNG storage tank and introduced into the BOG precooler for preliminary precooling;

[0053] S2: The pre-cooled BOG enters the compression-expansion integrated unit for compression;

[0054] S3: The compressed high-pressure BOG enters the first-stage expansion module for expansion and cooling. One path of the expanded BOG enters the multi-stage heat exchanger for heat exchange, and the other path enters the second-stage expansion module for further cooling.

[0055] S4: The BOG enters the first and second heat exchange channels of the multi-stage heat exchanger for heat exchange. A portion of the BOG is liquefied and returned to the LNG storage tank, while the other portion is used to pre-cool the incoming BOG gas to form a cooling circuit.

[0056] S5: The control system adjusts the compression ratio and expander speed in real time according to the system operating status to maintain the stability of the system cooling efficiency and keep the tank pressure balanced.

[0057] In a specific embodiment, BOG is cooled to -160°C by a pre-cooling module and then enters a compression-expansion system. A compressor assembly compresses the BOG gas to 15 bar. During the expansion process, the compressed high-pressure BOG gas expands to 3 bar and its temperature drops to -180°C. The cooling energy of the low-temperature gas after expansion is recovered through a plate-fin heat exchanger.

[0058] The compression and expansion integrated system of the present application is used to process the BOG gas generated by liquefied natural gas (LNG) carriers during navigation. The system has a designed processing capacity of 500 kg / h, and the initial state of the BOG gas being processed is -150°C and 2 bar. The BOG gas first enters the pre-cooling module for heat exchange cooling, and the temperature is further reduced to -160°C to reduce the heat load and power consumption during the compression process. The pre-cooled gas enters the compression and expansion integrated machine with a three-stage compression structure for pressurization treatment, and the final pressure is increased to 15 bar and the temperature is approximately -165°C. Subsequently, the high-pressure BOG is diverted and sent to the two-stage expansion module for expansion and cooling treatment. The first-stage expansion reduces the pressure to approximately 3 bar, further reducing the gas temperature to -180°C. The released cold energy is recovered through a plate-fin heat exchanger and used to pre-cool the BOG gas newly entering the system, forming a cold energy self-recovery loop. The expander speed in the system is dynamically adjusted by the adaptive control module according to the LNG tank liquid level signal, so that the system can maintain a stable tank pressure when the gas volume fluctuates. Actual test results show that the system's unit processing power consumption is 0.25 kWh / kg BOG, which is approximately 40% lower than the traditional process that relies on nitrogen expansion cooling source (power consumption of approximately 0.42 kWh / kg), significantly improving energy efficiency and reducing operating costs.

[0059] In the description of the present invention, it should be understood that the terms "upper", "lower", "inside", "outside" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. The word "comprising" does not exclude the presence of elements or steps not listed in the claims. The word "one" or "an" preceding an element does not exclude the presence of a plurality of such elements. The simple fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used for improvement. Any reference signs in the claims should not be interpreted as limiting the scope.

Claims

1. A compression and expansion integrated system, characterized in that: include: BOG precooler, used to precool BOG gas from the top of the LNG storage tank to a low temperature; The compression-expansion integrated machine is arranged in a low-temperature cavity and includes at least one set of compressor components and at least one set of expander components. The compressor components compress the pre-cooled BOG gas, and the expander components expand and cool the compressed BOG gas. a multi-stage heat exchanger, one end of which is connected to the discharge port of the expander assembly, for heat exchange cooling of the expanded BOG; The compressor assembly and the expander assembly are connected via a low-temperature bearing and a magnetic coupling transmission, and are arranged in the same low-temperature cavity; The invention also includes an adaptive control module for dynamically adjusting the compression ratio of the compressor assembly and the speed of the expander assembly through a PID algorithm. The adaptive control module selects two PID output minimum values ​​and adjusts the speed of the expander assembly through a ramp filter. The two PID output minimum values ​​plus a predetermined deviation opening serve as a maximum opening limit reference for the two PID controllers. The adaptive control module adjusts the speed of the expander assembly while adjusting the compression ratio of the compressor assembly. When in speed PID adjustment, if the compression ratio PID adjustment loop reaches the compression ratio set range value, the speed PID will switch to the compression ratio PID adjustment loop. When in compression ratio PID adjustment, if the speed adjustment loop reaches the set range value, the compression ratio PID adjustment will switch to the speed PID adjustment loop. The compression ratio of the compressor assembly is within the range of 3:1 to 8:1, and the speed of the expander assembly is within the range of 1000-5000 rpm.

2. The compression and expansion integrated system according to claim 1, characterized in that: It also includes a BOG circulation pipeline for returning part of the cooled and liquefied BOG to the LNG storage tank, and the other part returns to the BOG precooler after heat exchange in the heat exchanger to precool new BOG gas entering the system.

3. The compression and expansion integrated system according to claim 1, characterized in that: The integrated compression and expansion machine adopts a three-stage compression and two-stage expansion structure.

4. The compression and expansion integrated system according to claim 1, characterized in that: The impeller of the compressor assembly is made of stainless steel and is manufactured by laser sintering; the rotor of the expander assembly is made of Inconel alloy material.

5. The compression and expansion integrated system according to claim 1, characterized in that: The sealing structure of the compression and expansion integrated system is a non-contact magnetic seal.

6. The integrated compression and expansion system according to claim 1, characterized in that: The multi-stage heat exchanger includes two heat exchange channels. One channel of high-pressure BOG undergoes primary expansion and enters the first heat exchange channel for cooling and liquefaction, and the liquefied BOG is returned to the LNG storage tank. The other channel of high-pressure BOG undergoes secondary expansion and enters the second heat exchange channel for heat exchange and temperature increase, and is then used in a BOG pre-cooling cycle.

7. A method for BOG reliquefaction using the integrated compression and expansion system according to any one of claims 1 to 6, characterized in that: include: S1: BOG gas is drawn from the upper part of the LNG storage tank and introduced into the BOG precooler for preliminary precooling to -160°C; S2: The pre-cooled BOG enters the compression-expansion integrated machine for compression; S3: The compressed high-pressure BOG enters the first-stage expansion module for expansion and cooling. One path of the expanded BOG enters the multi-stage heat exchanger for heat exchange, and the other path enters the second-stage expansion module for further cooling. S4: The BOG enters the first and second heat exchange channels of the multi-stage heat exchanger for heat exchange. A portion of the BOG is liquefied and returned to the LNG storage tank, while the other portion is used to pre-cool the incoming BOG gas to form a cooling circuit. S5: The control system adjusts the compression ratio and the speed of the expander assembly in real time according to the system operating status to maintain the stability of the system cooling efficiency and keep the tank pressure balanced.

8. The method according to claim 7, characterized in that The BOG is cooled to -160°C by the pre-cooling module and then enters the compression-expansion integrated system. The compressor assembly compresses the BOG gas to 15 bar. The compressed high-pressure BOG gas expands to 3 bar during the expansion process, and the temperature drops to -180°C. The cooling energy of the expanded low-temperature gas is recovered through a plate-fin heat exchanger.

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