A system for re-liquefying LNG boil-off gas

Abstract

The present invention discloses a re-liquefaction system for LNG boil-off gas, belonging to the technical field of LNG storage and transportation. The re-liquefaction system includes a compressor unit, a first cooler, a second cooler, a compression-expansion integrated unit, a first heat exchanger and a second heat exchanger; the compressor unit is used to compress the refrigeration medium; the first cooler and the second cooler are used to cool the compressed refrigeration medium; the compression-expansion integrated unit is used to compress the refrigeration medium and expand the cooled refrigeration medium; the first heat exchanger and the second heat exchanger are used for heat exchange, and at least one high-frequency ultrasonic generator for generating vibration is arranged on the first branch, the second branch and the third branch. The re-liquefaction system for LNG boil-off gas provided by the embodiment of the present invention compensates for the mechanical loss during the compression process or the expansion process by multi-stage compensation of the cooling medium pressure, and improves the refrigeration capacity of the re-liquefaction system.

Description

Technical Field

[0001] The present invention belongs to the technical field of LNG storage and transportation, and particularly relates to a system for re-liquefying LNG boil-off gas. Background Art

[0002] Compared with the pipeline transportation of natural gas, since the maritime transportation of LNG (Liquefied Natural Gas) does not require the laying of long transportation pipelines and can flexibly transport natural gas to all parts of the world, it has the advantages of flexibility, diverse origins and destinations. During the transportation of any LNG ship, even when the heat insulation performance of the liquid cargo tank is very good, part of the LNG will inevitably evaporate into BOG (boil-off gas). The generation of BOG will cause the pressure in the liquid cargo tank to rise and damage the structure of the liquid cargo tank. If the BOG is directly discharged into the atmosphere, it will also cause direct economic losses and greenhouse hazards.

[0003] Currently, LNG is mainly re-liquefied by a re-liquefaction system that compresses and then expands and cools the refrigeration medium to liquefy the LNG boil-off gas. However, due to mechanical losses and other reasons, the output power of the compressor and expander is reduced, resulting in a decrease in the pressure of the cooling medium at the outlet of the compressor or expander, and ultimately leading to a decrease in the refrigeration capacity of the re-liquefaction system. Summary of the Invention

[0004] In view of the above defects or improvement requirements of the prior art, the present invention provides a system for re-liquefying LNG boil-off gas, aiming to compensate for mechanical losses during the compression or expansion process by multi-stage compensation of the pressure of the cooling medium, and improve the refrigeration capacity of the re-liquefaction system.

[0005] To achieve the above object, the present invention provides a system for re-liquefying LNG boil-off gas, which includes a compressor unit, a first cooler, a second cooler, a compression-expansion integrated unit, a first heat exchanger and a second heat exchanger;

[0006] The compressor unit is used to compress the refrigeration medium;

[0007] The first cooler and the second cooler are used to cool the compressed refrigeration medium;

[0008] The compression-expansion integrated unit is used to compress the refrigeration medium and expand the cooled refrigeration medium;

[0009] The first heat exchanger is used to exchange heat between the cooled refrigeration medium and the heat-exchanged refrigeration medium, and the second heat exchanger is used to exchange heat between the LNG boil-off gas and the expanded refrigeration medium;

[0010] The refrigeration medium flows through the compressor unit, the first cooler, the compression end of the compression-expansion unit, the second cooler, the first heat exchanger, the expansion end of the compression-expansion unit, the second heat exchanger and the first heat exchanger in sequence through pipelines and then returns to the inlet of the compressor unit. First branches, second branches and third branches are correspondingly arranged in parallel at the outlet of the compressor unit, the outlet of the compression end of the compression-expansion unit and the inlet of the expansion end of the compression-expansion unit, and at least one high-frequency ultrasonic generator for vibrating the refrigeration medium is arranged on the first branch, the second branch and the third branch.

[0011] Optionally, the re-liquefaction system further includes a pneumatic driver group for doing work on the expansion end of the compression-expansion unit. The outlet of the high-frequency ultrasonic generator located on the third branch, the pneumatic driver group and the inlet of the compressor unit are communicated in sequence.

[0012] Optionally, a high-frequency ultrasonic generator is arranged between the outlet of the cold end of the first heat exchanger and the inlet of the compressor unit.

[0013] Optionally, a Venturi tube is arranged between the outlet of the high-frequency ultrasonic generator and the inlet of the compressor unit, and the throat of the Venturi tube is communicated with the outlet of the pneumatic driver group.

[0014] Optionally, a flow regulating valve is arranged at the inlet of the pneumatic driver group, and a check valve and a pressure reducing valve are arranged at the outlet of the pneumatic driver group in sequence.

[0015] Optionally, the total outlet pressure of the compressor unit is 1.5 - 1.6 Mpa, the total outlet pressure of the compression end of the compression-expansion unit is 1.7 - 1.8 Mpa, and the total inlet pressure of the expansion end of the compression-expansion unit is 1.8 - 1.9 Mpa.

[0016] Optionally, a flow regulating valve and a check valve are arranged on each of the first branch, the second branch and the third branch.

[0017] Optionally, the compressor unit includes a plurality of compressors arranged in parallel with each other, and the compression-expansion unit includes a plurality of compression-expansion machines arranged in parallel with each other.

[0018] Optionally, rupture disk safety valves are communicated at the outlet of the second cooler and the outlet of the cold end of the first heat exchanger.

[0019] Optionally, one end of the third branch is communicated with the inlet of the expansion end of the compression-expansion unit, and the other end of the third branch is communicated with the inlet of the hot end of the first heat exchanger.

[0020] Optionally, the reliquefaction system further includes a gas generator for preparing a cooling medium, and an outlet of the gas generator is communicated with an inlet of the compressor unit through a flow regulating valve.

[0021] Optionally, the refrigeration medium is one or several of He, N2, H2, and Ne.

[0022] As long as the above-mentioned improved technical features do not conflict with each other, they can be combined with each other.

[0023] Generally speaking, compared with the prior art, the beneficial effects of the above technical solutions conceived by the present invention include:

[0024] For an LNG boil-off gas reliquefaction system provided by an embodiment of the present invention, a refrigeration medium at normal temperature and pressure (referring to the relative state during the cycle) is compressed by a compressor unit to become a cooling medium at high temperature and medium pressure, then cooled by a first cooler to become a cooling medium at normal temperature and medium pressure, and then compressed by a compression end of a compression-expansion integrated unit to become a refrigeration medium at high temperature and high pressure, and then cooled by a second cooler to become a refrigeration medium at normal temperature and high pressure, and then expanded by an expansion end of the compression-expansion integrated unit to become a cooling medium at low temperature and low pressure. At the same time, in a first heat exchanger, heat exchange occurs between the refrigeration medium cooled by the second cooler and the refrigeration medium heat-exchanged through a second heat exchanger; in the second heat exchanger, heat exchange occurs between the LNG boil-off gas and the expanded refrigeration medium, so as to finally recool the LNG boil-off gas through the cooling medium at low temperature and low pressure.

[0025] Furthermore, since a first branch, a second branch, and a third branch are correspondingly arranged in parallel at an outlet of the compressor unit, an outlet of the compression end of the compression-expansion integrated unit, and an inlet of the expansion end of the compression-expansion integrated unit, and at least one high-frequency ultrasonic generator for vibrating the refrigeration medium is arranged on the first branch, the second branch, and the third branch, the cooling medium compressed by the compressor unit is ultrasonically vibrated by the high-frequency ultrasonic generator on the first branch, so that the vibration of the cooling medium molecules is accelerated and the movement speed is increased, and thus the total pressure at the final outlet of the compressor unit can be increased, so as to achieve the first-stage compensation for the mechanical loss of the compressor unit.

[0026] Similarly, the high-frequency ultrasonic generator on the second branch vibrates the cooling medium compressed at the compression end of the compression-expansion unit by ultrasonic waves, causing the cooling medium molecules to vibrate faster and the movement rate to increase, resulting in an increase in the total pressure at the final outlet of the compression end of the compression-expansion unit, thereby compensating for the mechanical loss at the compression end of the second-stage compensation compression-expansion unit; the high-frequency ultrasonic generator on the third branch vibrates the cooling medium before the inlet of the expansion end of the compression-expansion unit by ultrasonic waves, causing the cooling medium molecules to vibrate faster and the movement rate to increase, resulting in an increase in the total pressure at the final outlet of the expansion end of the compression-expansion unit, thereby compensating for the mechanical loss at the expansion end of the third-stage compensation compression-expansion unit. Furthermore, by finally compensating for the mechanical loss during the multi-stage compensation compression process or expansion process, the refrigeration capacity of the reliquefaction system is improved.

[0027] That is to say, a reliquefaction system for LNG boil-off gas provided by an embodiment of the present invention compensates for the mechanical loss during the compression process or expansion process by multi-stage compensation of the cooling medium pressure, and improves the refrigeration capacity of the reliquefaction system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic structural diagram of a reliquefaction system for LNG boil-off gas provided by an embodiment of the present invention.

[0029] In all the drawings, the same reference numerals represent the same technical features, specifically:

[0030] 1. Compressor unit; 101. Compressor; 2. First cooler; 3. Second cooler; 4. Compression-expansion unit; 41. Compression-expansion machine; 411. Compression end; 412. Expansion end; 5. First heat exchanger; 6. Second heat exchanger; 7. High-frequency ultrasonic generator; 71. Venturi tube; 8. Pneumatic driver group; 801. Pneumatic driver; 9. Flow regulating valve; 10. Check valve; 11. Pressure reducing valve; 12. Bursting disc safety valve; 13. Gas generator; 14. Expansion joint; 15. Monitoring module; 1001. First branch; 1002. Second branch; 1003. Third branch. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0032] In the description of the present invention, it should be understood that the orientation or positional relationships indicated by the terms "center", "longitudinal", "transverse", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. are based on the orientation or positional relationships shown in the drawings. These are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the present invention.

[0033] In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one of such features. In the description of the present invention, the meaning of "a plurality" is at least two, such as two, three, etc., unless otherwise specifically and clearly defined.

[0034] In the present invention, unless otherwise clearly specified and defined, terms such as "installed", "connected", "joined", "fixed", etc. shall be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or integrated; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the communication inside two elements or the interaction relationship between two elements, unless otherwise clearly defined. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0035] In the present invention, unless otherwise clearly specified and defined, the first feature being "on" or "under" the second feature may be that the first and second features are in direct contact, or the first and second features are indirectly in contact through an intermediate medium. Moreover, the first feature being "above", "over" and "on top of" the second feature may be that the first feature is directly above or obliquely above the second feature, or merely indicates that the first feature has a higher horizontal height than the second feature. The first feature being "under", "beneath" and "underneath" the second feature may be that the first feature is directly below or obliquely below the second feature, or merely indicates that the first feature has a lower horizontal height than the second feature.

[0036] Embodiment 1

[0037] Figure 1 is a schematic structural diagram of a structure for an LNG boil-off gas re-liquefaction system provided by an embodiment of the present invention. As Figure 1 shown, the re-liquefaction system includes a compressor unit 1, a first cooler 2, a second cooler 3, a compression-expansion integrated unit 4, a first heat exchanger 5, and a second heat exchanger 6.

[0038] The compressor unit 1 is used to compress the refrigeration medium.

[0039] The first cooler 2 and the second cooler 3 are used to cool the compressed refrigeration medium.

[0040] The compression-expansion integrated unit 4 is used to compress the refrigeration medium and to expand the cooled refrigeration medium.

[0041] The first heat exchanger 5 is used to generate heat exchange between the cooled refrigeration medium and the heat-exchanged refrigeration medium, and the second heat exchanger 6 is used to generate heat exchange between the LNG evaporation gas and the expanded refrigeration medium.

[0042] The refrigeration medium flows through the compressor unit 1, the first cooler 2, the compression end 411 of the compression-expansion integrated unit 4, the second cooler 3, the first heat exchanger 5, the expansion end 412 of the compression-expansion integrated unit 4, the second heat exchanger 6 and the first heat exchanger 5 in sequence through the pipeline and then returns to the inlet of the compressor unit 1. The outlets of the compressor unit 1, the outlet of the compression end 411 of the compression-expansion integrated unit 4, and the inlet of the expansion end 412 of the compression-expansion integrated unit 4 are correspondingly provided with a first branch 1001, a second branch 1002 and a third branch 1003 in parallel, and at least one high-frequency ultrasonic generator 7 for vibrating the refrigeration medium is provided on the first branch 1001, the second branch 1002 and the third branch 1003.

[0043] For a LNG evaporation gas re-liquefaction system provided by an embodiment of the present invention, the refrigeration medium at normal temperature and pressure (referring to the relative state during the cycle) is compressed by the compressor unit 1 to become a high-temperature medium-pressure cooling medium, then cooled by the first cooler 2 to become a normal-temperature medium-pressure cooling medium, then compressed by the compression end 411 of the compression-expansion integrated unit 4 to become a high-temperature high-pressure refrigeration medium, then cooled by the second cooler 3 to become a normal-temperature high-pressure refrigeration medium, and then expanded by the expansion end 412 of the compression-expansion integrated unit 4 to become a low-temperature low-pressure cooling medium. At the same time, in the first heat exchanger 5, heat exchange is generated between the refrigeration medium cooled by the second cooler 3 and the refrigeration medium heat-exchanged through the second heat exchanger 6; in the second heat exchanger 6, heat exchange is generated between the LNG evaporation gas and the expanded refrigeration medium, so that the LNG evaporation gas is finally re-cooled by the low-temperature low-pressure cooling medium.

[0044] Furthermore, since the outlets of the compressor unit 1, the outlet of the compression end 411 of the compression-expansion integrated unit 4, and the inlet of the expansion end 412 of the compression-expansion integrated unit 4 are respectively provided with a first branch 1001, a second branch 1002, and a third branch 1003 in parallel, and at least one high-frequency ultrasonic generator 7 for vibrating the refrigeration medium is provided on the first branch 1001, the second branch 1002, and the third branch 1003. Thus, the high-frequency ultrasonic generator 7 on the first branch 1001 vibrates the cooling medium compressed by the compressor unit 1 ultrasonically, causing the cooling medium molecules to vibrate faster and the movement rate to increase, which can also increase the total pressure at the final outlet of the compressor unit 1, thereby realizing the first-level compensation for the mechanical loss of the compressor unit 1.

[0045] Similarly, the high-frequency ultrasonic generator 7 on the second branch 1002 vibrates the cooling medium compressed by the compression end 411 of the compression-expansion integrated unit 4 ultrasonically, causing the cooling medium molecules to vibrate faster and the movement rate to increase, increasing the total pressure at the final outlet of the compression end 411 of the compression-expansion integrated unit 4, thereby realizing the second-level compensation for the mechanical loss of the compression end 411 of the compression-expansion integrated unit 4; the high-frequency ultrasonic generator 7 on the third branch 1003 vibrates the cooling medium before the inlet of the expansion end 412 of the compression-expansion integrated unit 4 ultrasonically, causing the cooling medium molecules to vibrate faster and the movement rate to increase, increasing the total pressure at the final outlet of the expansion end 412 of the compression-expansion integrated unit 4, thereby realizing the third-level compensation for the mechanical loss of the expansion end 412 of the compression-expansion integrated unit 4. Furthermore, the mechanical loss during the multi-stage compensation compression process or expansion process is ultimately compensated, improving the refrigeration capacity of the re-liquefaction system.

[0046] That is to say, a re-liquefaction system for LNG boil-off gas provided by an embodiment of the present invention compensates for the mechanical loss during the compression process or expansion process by multi-stage compensating the pressure of the cooling medium, improving the refrigeration capacity of the re-liquefaction system.

[0047] It is easy to understand that the compression-expansion integrated unit 4 is a structure formed by installing a compressor (corresponding to the compression end 411) and an expander (corresponding to the expansion end 412) on a common rotating shaft. That is, at this time, the mechanical energy output by the expander is transmitted to the compressor through the common rotating shaft, improving the energy utilization efficiency.

[0048] It should be noted that the re-liquefaction system provided by the present invention uses multiple high-frequency ultrasonic generators 7 to gradually compensate the pressure of the cooling medium in multiple stages at different positions, rather than only setting a single high-power high-frequency ultrasonic generator 7, which avoids the problem that the temperature of the cooling medium rises sharply after the molecular vibration is too fast, and finally leads to a significant increase in its internal energy while meeting the pressure increase, resulting in energy waste. That is to say, through multi-stage gradual compensation, the temperature rise of the cooling medium can be reduced, and the energy utilization rate is improved.

[0049] In addition, the number of high-frequency ultrasonic generators 7 arranged correspondingly on each branch (the first branch 1001, the second branch 1002, and the third branch 1003) can be 1, 2, etc., and the present invention does not limit this.

[0050] Exemplarily, a flow regulating valve 9 and a check valve 10 are provided on each of the first branch 1001, the second branch 1002, and the third branch 1003. Among them, the check valve 10 plays a role in preventing the backflow of the cooling medium, while the flow regulating valve 9 plays a role in regulating the flow rate of each branch.

[0051] Exemplarily, the first cooler 2 and the second cooler 3 can be shell-and-tube coolers or plate coolers. The cooling medium exchanges heat with seawater in both the first cooler 2 and the second cooler 3 to achieve temperature reduction. In addition, a plurality of monitoring modules 15 are provided in the present re-liquefaction system. Each monitoring module 15 is arranged at the inlet or outlet of the corresponding device. The monitoring module 15 includes a pressure sensor, a pressure display, a temperature sensor, a temperature display, etc., so as to measure or display the pressure or temperature of each pipeline correspondingly, which is convenient for the monitoring and maintenance of the re-liquefaction system.

[0052] In this embodiment, the total outlet pressure of the compressor unit 1 is 1.5 - 1.6 Mpa, the total outlet pressure of the compression end 411 of the compression-expansion integrated unit 4 is 1.7 - 1.8 Mpa, and the total inlet pressure of the expansion end 412 of the compression-expansion integrated unit 4 is 1.8 - 1.9 Mpa, so as to ensure the pressure of the cooling medium after compression and expansion through the high-frequency ultrasonic generator 7.

[0053] It is easy to understand that when the output power corresponding to the compression end 411 of the compressor unit 1, the compression-expansion integrated unit 4 or the expansion end 412 of the compression-expansion integrated unit 4 is lower, the corresponding compression efficiency or expansion efficiency is lower. At this time, the corresponding mechanical compensation is greater, that is, the output power of the corresponding high-frequency ultrasonic generator 7 is greater at this time, so as to ensure the total pressure after compression or expansion. Therefore, in this embodiment, the liquefaction system further includes a plurality of control components. The control component includes a power meter, a data processor and a controller. The power meter, the data processor, the controller and the high-frequency ultrasonic generator 7 are electrically connected in sequence. The power meter is used to measure the output power of the compressor unit 1 (taking the compressor unit 1 as an example for illustration). The data processor analyzes and calculates according to the output power of the compressor unit 1 and finally outputs it to the controller. The controller then completes the control of the output power of the high-frequency ultrasonic generator 7 to ensure that the total pressure at the outlet of the compressor unit 1 reaches the required value.

[0054] It should be noted that the above-mentioned total pressure is the total pressure formed after each branch converges into the pipeline, corresponding to the pressures at positions A, B, and C respectively. Figure 1 in

[0055] Continue to refer to Figure 1 , the re-liquefaction system further includes a pneumatic driver group 8. The pneumatic driver group 8 is used to do work on the expansion end 412 of the compression-expansion integrated unit 4. The outlet of the high-frequency ultrasonic generator 7 located in the third branch 1003, the pneumatic driver group 8 and the inlet of the compressor unit 1 are connected in sequence.

[0056] In the above embodiment, the high-pressure refrigerant obtained by pressurizing through the high-frequency ultrasonic generator 7 in the third branch 1003 is used to drive the pneumatic driver group 8, so as to realize doing work on the expansion end 412 of the compression-expansion integrated unit 4, and further provide additional power compensation for the expansion end 412. It can not only compensate for the mechanical loss of the expansion end 412 of the compression-expansion integrated unit 4, but also increase the pressure output by the expansion end 412 to increase the cooling capacity.

[0057] Furthermore, a flow regulating valve 9 is provided at the inlet of the pneumatic driver group 8, and a check valve 10 and a pressure reducing valve 11 are sequentially provided at the outlet of the pneumatic driver group 8. Among them, the flow regulating valve 9 can adjust the flow rate of the refrigerant entering the pneumatic driver group 8 to avoid excessive flow rate. The check valve 10 plays a role in preventing backflow, and the pressure reducing valve 11 can reduce the pressure of the cooling medium flowing back to the inlet of the compressor unit 1 to avoid excessive pressure of the flowing-back cooling medium.

[0058] In addition, in an implementation manner of the present invention, a high-frequency ultrasonic generator 7 is provided between the outlet corresponding to the cold end of the first heat exchanger 5 and the inlet of the compressor unit 1. The high-frequency ultrasonic generator 7 can pressurize the refluxed cooling medium, increase the pressure of the cooling medium entering the compressor unit 1, thereby reducing the compression ratio of the compressor unit 1 and reducing the energy consumption of the compressor unit.

[0059] Exemplarily, a check valve and a flow regulating valve are further provided between the outlet corresponding to the cold end of the first heat exchanger 5 and the high-frequency ultrasonic generator 7.

[0060] Furthermore, a Venturi tube 71 is provided between the outlet of the high-frequency ultrasonic generator 7 and the inlet of the compressor unit 1, and the throat of the Venturi tube 71 is communicated with the outlet of the pneumatic driver group 8.

[0061] It is easy to understand that according to Bernoulli's principle, the contraction section of the Venturi tube 71 can form a negative pressure, thereby increasing the pressure difference between the inlet and the outlet of the pneumatic driver group 8. By increasing the pressure difference, the gas potential energy corresponding to the cooling medium is increased (i.e., the cooling medium is quickly sucked into the Venturi tube 71), and more potential energy is converted into the kinetic energy driven by the pneumatic driver group 8, improving the power input of the pneumatic driver group 8 and the utilization rate of the internal energy of the cooling medium.

[0062] Exemplarily, the nozzle of the high-frequency ultrasonic generator 7 is communicated with the throat of the Venturi tube 71. At this time, the nozzle of the high-frequency ultrasonic generator 7 can be regarded as the contraction section, and the outlet of the Venturi tube 71 is the diffuser section. The inner diameter of the throat is smaller than the average inner diameter of the nozzle and the average inner diameter of the diffuser section. That is to say, in this re-liquefaction system, the high-frequency ultrasonic generator 7 and the Venturi tube 71 are integrated together to form a composite structure.

[0063] In this embodiment, the compressor unit 1 includes a plurality of compressors 101 arranged in parallel, and the compression-expansion integrated unit 4 includes a plurality of compression-expansion integrated machines 41 arranged in parallel. Among them, the flow rate can be increased by the plurality of parallel compressors 101 and the plurality of parallel compression-expansion integrated machines 41, thereby increasing the cooling capacity of the system.

[0064] Exemplarily, the compressor unit 1 includes 3 compressors 101, the compression-expansion integrated unit 4 includes 3 compression-expansion integrated machines 41, the pneumatic driver group 8 is 3 pneumatic drivers 801, and the 3 pneumatic drivers 801 correspond to the 3 compression-expansion integrated machines 41 one by one.

[0065] In addition, a plurality of expansion joints 14 are sequentially provided at the inlets and outlets of the compressors 101 and at the inlets and outlets of the compression ends 411 of the compression-expansion integrated unit 4. The expansion joints 14 can ensure the normal connection between the pipelines under thermal expansion and contraction.

[0066] Exemplarily, the number of expansion joints 14 corresponding to the inlet and outlet of the compressor 101 and the inlet and outlet of the compression end 411 of the compression-expansion unit 4 is 2.

[0067] In order to ensure that the pressures at the outlet of the second cooler 3 and the outlet of the cold end corresponding to the first heat exchanger 5 are within a preset range, rupture disk safety valves 12 are connected to the outlet of the second cooler 3 and the outlet of the cold end corresponding to the first heat exchanger 5, respectively, for safety protection to prevent excessive pressure.

[0068] It is easy to understand that, as Figure 1 , the pipeline corresponding to the hot end in the first heat exchanger 5 is pipeline a, whose inlet ( Figure 1 the lower end) is connected to the outlet of the second cooler 3, and whose outlet ( Figure 1 the upper end) is connected to the inlet of the expansion end 412 of the compression-expansion unit 4. The pipeline corresponding to the cold end in the first heat exchanger 5 is pipeline b, whose inlet ( Figure 1 the upper end) is connected to the outlet of pipeline d corresponding to the cold end of the second heat exchanger 6, and whose outlet ( Figure 1 the lower end) is connected to the inlet of the compressor unit 1, so as to realize the heat exchange of the cooling medium between pipeline a and pipeline b. The pipeline corresponding to the hot end in the second heat exchanger 6 is pipeline c, whose inlet and outlet are connected to the LNG evaporation gas. The pipeline corresponding to the cold end in the second heat exchanger 6 is pipeline d, whose inlet ( Figure 1 the upper end) is connected to the outlet of the expansion end 412 of the compression-expansion unit 4, and whose outlet ( Figure 1 the lower end) is connected to the inlet of pipeline b, so as to realize the heat exchange of the cooling medium between pipeline c and pipeline d.

[0069] Exemplarily, the outlet of pipeline d in the second heat exchanger 6 is directly connected to the inlet of pipeline b in the first heat exchanger 5, so as to avoid setting pipelines between the two heat exchangers, thereby reducing costs and energy losses.

[0070] In this embodiment, one end of the third branch 1003 is connected to the inlet of the expansion end 412 of the compression-expansion unit 4, and the other end of the third branch 1003 is connected to the inlet of the hot end corresponding to the first heat exchanger 5 (i.e., the inlet of pipeline a).

[0071] In the above embodiment, by connecting the other end of the third branch 1003 to the outlet of the second cooler 3, the cooling medium heated after ultrasonic treatment by the high-frequency ultrasonic generator 7 can be heat-exchanged and cooled through the first heat exchanger 5, so that the cooling medium flowing into the inlet of the expansion end 412 of the compression-expansion unit 41 has a relatively high pressure and a relatively low temperature (i.e., ensuring the cold quantity).

[0072] Similarly, one end of the first branch 1001 is connected to the outlet of the compressor unit 1, and the other end of the first branch 1001 is connected to the inlet of the first cooler 2. One end of the second branch 1002 is connected to the outlet of the compression end 411 of the compression-expansion integrated unit 4, and the other end of the second branch 1002 is connected to the inlet of the second cooler 3. Thus, the cooling medium heated after being ultrasonically treated by the high-frequency ultrasonic generator 7 can also be cooled by the first cooler 2 or the second cooler 3, ensuring that the total pressure at the outlet of the compressor unit 1 and the total pressure at the outlet of the compression end 411 of the compression-expansion integrated unit 4 are relatively high while having a relatively low temperature (also ensuring the cooling capacity).

[0073] In this embodiment, the re-liquefaction system further includes a gas generator 13 for preparing a cooling medium. The outlet of the gas generator 13 is connected to the inlet of the compressor unit 1 through a flow regulating valve 9, so that the cooling medium prepared by the gas generator 13 is input into the system, and the flow regulating valve 9 can adjust the flow rate of the incoming cooling medium.

[0074] Exemplarily, the refrigeration medium is one or more of He, N2, H2, and Ne.

[0075] Exemplarily, the inlet pressure of the compressor 101 in the compressor unit 1 can be 0.6 - 0.7 Mpa.

[0076] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A system for re-liquefying LNG boil-off gas, characterized in that, The re-liquefaction system includes a compressor unit, a first cooler, a second cooler, a compression-expansion integrated unit, a first heat exchanger, and a second heat exchanger; The compressor unit is used to compress the refrigerant medium; The first cooler and the second cooler are used to cool the compressed refrigerant medium; The compression-expansion integrated unit is used to compress the refrigerant medium and expand the cooled refrigerant medium; The first heat exchanger is used to effect heat exchange between the cooled refrigerant medium and the heat-exchanged refrigerant medium, and the second heat exchanger is used to effect heat exchange between the LNG evaporation gas and the expanded refrigerant medium; The refrigerant medium flows through the compressor unit, the first cooler, the compression end of the compression-expansion integrated unit, the second cooler, the first heat exchanger, the expansion end of the compression-expansion integrated unit, the second heat exchanger, and the first heat exchanger in sequence through pipelines and then returns to the inlet of the compressor unit. First branches, second branches, and third branches are correspondingly arranged in parallel at the outlet of the compressor unit, the outlet of the compression end of the compression-expansion integrated unit, and the inlet of the expansion end of the compression-expansion integrated unit, and at least one high-frequency ultrasonic generator for vibrating the refrigerant medium is arranged on the first branch, the second branch, and the third branch.

2. The re-liquefaction system for LNG boil-off gas according to claim 1, wherein The re-liquefaction system further includes a pneumatic driver group, which is used to do work on the expansion end of the compression-expansion integrated unit, and the outlet of the high-frequency ultrasonic generator located on the third branch, the pneumatic driver group, and the inlet of the compressor unit are connected in sequence.

3. The one for the LNG boil-off gas re-liquefaction system according to claim 2, wherein, A high-frequency ultrasonic generator is arranged between the outlet of the first heat exchanger corresponding to the cold end and the inlet of the compressor unit.

4. A re-liquefaction system for LNG boil-off gas according to claim 3, characterized in that, A venturi tube is arranged between the outlet of the high-frequency ultrasonic generator and the inlet of the compressor unit, and the throat of the venturi tube is communicated with the outlet of the pneumatic driver group.

5. A re-liquefaction system for LNG boil-off gas according to claim 2, characterized in that A flow regulating valve is arranged at the inlet of the pneumatic driver group, and a check valve and a pressure reducing valve are arranged in sequence at the outlet of the pneumatic driver group.

6. The one kind of LNG boil-off gas re-liquefaction system according to claim 1, wherein The total outlet pressure of the compressor unit is 1.5 - 1.6 Mpa, the total outlet pressure of the compression end of the compression-expansion integrated unit is 1.7 - 1.8 Mpa, and the total inlet pressure of the expansion end of the compression-expansion integrated unit is 1.8 - 1.9 Mpa.

7. A re-liquefaction system for LNG boil-off gas according to claim 1, wherein, Flow regulating valves and check valves are arranged on the first branch, the second branch, and the third branch.

8. A re-liquefaction system for LNG boil-off gas according to claim 1, characterized in that, The compressor unit includes a plurality of compressors arranged in parallel with each other, and the compression-expansion integrated unit includes a plurality of compression-expansion integrated machines arranged in parallel with each other.

9. A re-liquefaction system for LNG boil-off gas according to claim 1, characterized in that, Burst disk safety valves are communicated at the outlet of the second cooler and the outlet of the first heat exchanger corresponding to the cold end.

10. A re-liquefaction system for LNG boil-off gas according to any one of claims 1 to 9, characterized in that One end of the third branch is communicated with the inlet of the expansion end of the compression-expansion integrated unit, and the other end of the third branch is communicated with the inlet of the first heat exchanger corresponding to the hot end.

11. A re-liquefaction system for LNG boil-off gas according to any one of claims 1 to 9, characterized in that, The re-liquefaction system further includes a gas generator, which is used to prepare a cooling medium, and the outlet of the gas generator is communicated with the inlet of the compressor unit through a flow regulating valve.

12. A re-liquefaction system for LNG boil-off gas according to any one of claims 1 to 9, characterized in that, The refrigerating medium is one or more of He, N2, H2, and Ne.

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