Liquefied natural gas (LNG) boil-off gas reliquefaction system

Abstract

The invention discloses a reliquefaction system for LNG (Liquefied Natural Gas) boil-off gas, and belongs to the technical field of LNG storage and transportation. The reliquefaction system comprises a compressor unit, a first cooler, a second cooler, a compression and expansion integrated unit, a first heat exchanger and a second heat exchanger; the compressor unit is used for compressing a refrigerating medium; the first cooler and the second cooler are used for cooling the compressed refrigerating medium; the compression and expansion integrated unit is used for compressing the refrigerating medium and expanding the cooled refrigerating medium; the first heat exchanger and the second heat exchanger are used for heat exchange, and the first branch, the second branch and the third branch are each provided with at least one high-frequency ultrasonic generator capable of vibrating. According to the reliquefaction system for the evaporated LNG provided by the embodiment of the invention, through multi-stage compensation of the pressure of the cooling medium, mechanical loss in the compression process or the expansion process is compensated, and the refrigerating capacity of the reliquefaction system is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of LNG storage and transportation, and in particular relates to a system for reliquefying LNG boil-off gas. Background Art

[0002] Compared with natural gas transported by pipeline, offshore LNG (liquefied natural gas, Liquified Natural Gas) transport does not require the laying of long pipelines and can flexibly transport natural gas to all parts of the world, so it has the advantages of flexibility and diversified origins and destinations. During the transportation of any LNG ship, even if the insulation performance of the cargo tank is very good, LNG will inevitably partially evaporate into BOG (boil-off gas). The generation of BOG will increase the pressure of the cargo tank and damage the structure of the cargo tank. If BOG is directly discharged into the atmosphere, it will also cause direct economic losses and greenhouse hazards.

[0003] At present, LNG is mainly liquefied by expanding and refrigerating the refrigerant through the reliquefaction system after compression, thereby liquefying the LNG vapor. However, due to mechanical loss 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 compressor outlet or the expander outlet, which ultimately leads to a decrease in the refrigeration capacity of the reliquefaction system. Summary of the invention

[0004] In view of the above defects or improvement needs of the prior art, the present invention provides a system for reliquefaction of LNG boil-off gas, which aims to compensate for the mechanical losses in the compression process or expansion process through multi-stage compensation of the cooling medium pressure, thereby improving the refrigeration capacity of the reliquefaction system.

[0005] To achieve the above object, the present invention provides a system for reliquefaction of LNG boil-off gas, the reliquefaction system comprising 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 refrigerant medium; The integrated compression-expansion unit is used to compress the refrigerant medium and to expand the cooled refrigerant medium; The first heat exchanger is used to generate heat exchange between the cooled refrigerant medium and the refrigerant medium after heat exchange, and the second heat exchanger is used to generate heat exchange between the LNG boil-off gas and the refrigerant medium after expansion; The refrigerant medium flows through the compressor unit, the first cooler, the compression end of the integrated compression and expansion unit, the second cooler, the first heat exchanger, the expansion end of the integrated compression and expansion unit, the second heat exchanger and the first heat exchanger in sequence through the pipeline, and then flows back to the inlet of the compressor unit. The outlet of the compressor unit, the outlet of the compression end of the integrated compression and expansion unit, and the inlet of the expansion end of the integrated compression and expansion unit are correspondingly provided with a first branch, a second branch and a third branch in parallel, and at least one high-frequency ultrasonic generator for vibrating the refrigerant medium is provided on the first branch, the second branch and the third branch.

[0006] Optionally, the reliquefaction system also includes a pneumatic driver group, which is used to perform work on the expansion end of the compression and expansion integrated unit, and the outlet of the high-frequency ultrasonic generator located in the third branch, the pneumatic driver group and the inlet of the compressor unit are connected in sequence.

[0007] Optionally, a high-frequency ultrasonic generator is provided between the outlet corresponding to the cold end of the first heat exchanger and the inlet of the compressor unit.

[0008] Optionally, a venturi tube is provided between the outlet of the high-frequency ultrasonic generator and the inlet of the compressor group, and the throat of the venturi tube is connected to the outlet of the pneumatic driver group.

[0009] Optionally, the inlet of the pneumatic driver group is provided with a flow regulating valve, and the outlet of the pneumatic driver group is provided with a one-way valve and a pressure reducing valve in sequence.

[0010] 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 integrated compression and expansion unit is 1.7-1.8 MPa, and the total inlet pressure of the expansion end of the integrated compression and expansion unit is 1.8-1.9 MPa.

[0011] Optionally, the first branch, the second branch and the third branch are each provided with a flow regulating valve and a one-way valve.

[0012] Optionally, the compressor unit includes a plurality of compressors arranged in parallel with each other, and the integrated compression-expansion machine unit includes a plurality of integrated compression-expansion machines arranged in parallel with each other.

[0013] Optionally, the outlet of the second cooler and the outlet of the corresponding cold end of the first heat exchanger are both connected to a bursting disc safety valve.

[0014] Optionally, one end of the third branch is connected to the inlet of the expansion end of the integrated compression and expansion unit, and the other end of the third branch is connected to the inlet corresponding to the hot end of the first heat exchanger.

[0015] Optionally, the reliquefaction system further comprises a gas generator, which is used to prepare a cooling medium, and an outlet of the gas generator is connected to an inlet of the compressor unit via a flow regulating valve.

[0016] Optionally, the refrigerant is one or more of He, N2, H2 and Ne.

[0017] The above-mentioned improved technical features can be combined with each other as long as they do not conflict with each other.

[0018] In general, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects: For a system for reliquefying LNG boil-off gas provided by an embodiment of the present invention, the refrigerant at normal temperature and pressure (referring to the relative state in the cycle process) is compressed by the compressor unit to become a high-temperature medium-pressure cooling medium, then cooled by the first cooler to become a normal-temperature medium-pressure cooling medium, then compressed by the compression end of the compression-expansion integrated unit to become a high-temperature high-pressure refrigerant, then cooled by the second cooler to become a normal-temperature high-pressure refrigerant, and then expanded by the expansion end of the compression-expansion integrated unit to become a low-temperature low-pressure cooling medium. At the same time, in the first heat exchanger, heat exchange occurs between the refrigerant cooled by the second cooler and the refrigerant after heat exchange through the second heat exchanger; in the second heat exchanger, heat exchange occurs between the LNG boil-off gas and the expanded refrigerant, so that the LNG boil-off gas is finally recooled by the low-temperature low-pressure cooling medium.

[0019] Furthermore, since the outlet of the compressor unit, the outlet of the compression end of the integrated compression and expansion unit, and the inlet of the expansion end of the integrated compression and expansion unit are all provided with a first branch, a second branch, and a third branch in parallel, and the first branch, the second branch, and the third branch are provided with at least one high-frequency ultrasonic generator for vibrating the refrigerant medium, the high-frequency ultrasonic generator on the first branch is used to vibrate the cooling medium compressed by the compressor unit, thereby accelerating the vibration of the cooling medium molecules and increasing the movement rate, and the total pressure at the final outlet of the compressor unit can be increased, thereby realizing the first-level compensation for the mechanical loss of the compressor unit.

[0020] Similarly, the high-frequency ultrasonic generator on the second branch is used to perform ultrasonic vibration on the cooling medium after compression at the compression end of the integrated compression-expansion unit, so that the vibration of the cooling medium molecules is accelerated and the movement rate is increased, so that the total pressure at the final outlet of the compression end of the integrated compression-expansion unit is increased, thereby realizing the second-stage compensation for the mechanical loss at the compression end of the integrated compression-expansion unit; the high-frequency ultrasonic generator on the third branch is used to perform ultrasonic vibration on the cooling medium in front of the inlet of the expansion end of the integrated compression-expansion unit, so that the vibration of the cooling medium molecules is accelerated and the movement rate is increased, so that the total pressure at the final outlet of the expansion end of the integrated compression-expansion unit is increased, thereby realizing the third-stage compensation for the mechanical loss at the expansion end of the integrated compression-expansion unit, and finally the refrigeration capacity of the reliquefaction system is improved through multi-stage compensation for the mechanical loss in the compression process or the expansion process.

[0021] That is, the embodiment of the present invention provides a system for reliquefaction of LNG boil-off gas, which compensates for the mechanical loss in the compression process or the expansion process by compensating the cooling medium pressure at multiple stages, thereby improving the refrigeration capacity of the reliquefaction system. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a structural schematic diagram of a system for reliquefying LNG boil-off gas provided in an embodiment of the present invention.

[0023] In all the drawings, the same reference numerals represent the same technical features, specifically: 1. Compressor unit; 101. Compressor; 2. First cooler; 3. Second cooler; 4. Compression-expansion integrated unit; 41. Compression-expansion integrated machine; 411. Compression end; 412. Expansion end; 5. First heat exchanger; 6. Second heat exchanger; 7. High-frequency ultrasonic generator; 71. Venturi tube; 8. Air pressure driver group; 801. Air pressure driver; 9. Flow regulating valve; 10. One-way 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

[0024] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0025] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" 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, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0026] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0027] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0028] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0029] Example 1

[0030] Figure 1 is a schematic diagram of a structure of a LNG boil-off gas reliquefaction system provided by an embodiment of the present invention, such as Figure 1 As shown, the reliquefaction 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.

[0031] The compressor unit 1 is used to compress the refrigerant medium.

[0032] The first cooler 2 and the second cooler 3 are used to cool the compressed refrigerant medium.

[0033] The integrated compression-expansion unit 4 is used to compress the refrigerant medium and to expand the cooled refrigerant medium.

[0034] The first heat exchanger 5 is used to generate heat exchange between the cooled refrigerant medium and the refrigerant medium after heat exchange, and the second heat exchanger 6 is used to generate heat exchange between the LNG boil-off gas and the refrigerant medium after expansion.

[0035] The refrigerant 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 flows back to the inlet of the compressor unit 1. The outlet 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 refrigerant medium is provided on the first branch 1001, the second branch 1002 and the third branch 1003.

[0036] For a system for reliquefying LNG boil-off gas provided in an embodiment of the present invention, the refrigerant at normal temperature and pressure (referring to the relative state in the cycle process) 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 refrigerant, then cooled by the second cooler 3 to become a normal-temperature high-pressure refrigerant, 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 occurs between the refrigerant after being cooled by the second cooler 3 and the refrigerant after heat exchange through the second heat exchanger 6; in the second heat exchanger 6, heat exchange occurs between the LNG boil-off gas and the expanded refrigerant, so that the LNG boil-off gas is finally recooled by the low-temperature low-pressure cooling medium.

[0037] Furthermore, since the outlet of the compressor unit 1, the outlet of the compression end 411 of the integrated compression and expansion unit 4, and the inlet of the expansion end 412 of the integrated compression and expansion unit 4 are all correspondingly provided with the first branch 1001, the second branch 1002 and the third branch 1003 in parallel, and the first branch 1001, the second branch 1002 and the third branch 1003 are provided with at least one high-frequency ultrasonic generator 7 for vibrating the refrigerant medium, the high-frequency ultrasonic generator 7 on the first branch 1001 is used to vibrate the cooling medium compressed by the compressor unit 1 by ultrasonic vibration, so that the vibration of the cooling medium molecules is accelerated and the movement rate is increased, and the total pressure at the final outlet of the compressor unit 1 can be increased, thereby realizing the first-stage compensation of the mechanical loss of the compressor unit 1.

[0038] Similarly, the high-frequency ultrasonic generator 7 on the second branch 1002 is used to perform ultrasonic vibration on the cooling medium after compression at the compression end 411 of the compression-expansion integrated unit 4, so that the vibration of the cooling medium molecules is accelerated and the movement rate is increased, so that the total pressure at the final outlet of the compression end 411 of the compression-expansion integrated unit 4 is increased, thereby realizing the second-stage 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 is used to perform ultrasonic vibration on the cooling medium in front of the inlet of the expansion end 412 of the compression-expansion integrated unit 4, so that the vibration of the cooling medium molecules is accelerated and the movement rate is increased, so that the total pressure at the final outlet of the expansion end 412 of the compression-expansion integrated unit 4 is increased, thereby realizing the third-stage compensation for the mechanical loss of the expansion end 412 of the compression-expansion integrated unit 4, and finally through multi-stage compensation for the mechanical loss in the compression process or the expansion process, the refrigeration capacity of the reliquefaction system is improved.

[0039] That is, the embodiment of the present invention provides a system for reliquefaction of LNG boil-off gas, which compensates for the mechanical loss in the compression process or the expansion process by compensating the cooling medium pressure at multiple stages, thereby improving the refrigeration capacity of the reliquefaction system.

[0040] It is easy to understand that the integrated compression and expansion unit 4 is a structure in which a compressor (corresponding to the compression end 411) and an expander (corresponding to the expansion end 412) are installed 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, thereby improving the energy utilization efficiency.

[0041] It should be noted that the reliquefaction system provided by the present invention uses multiple high-frequency ultrasonic generators 7 to gradually compensate the cooling medium pressure at different positions in multiple stages, rather than only setting a single ultra-high-power high-frequency ultrasonic generator 7, thereby avoiding the problem of rapid increase in the cooling medium temperature after the molecular vibration is too fast, which ultimately leads to a significant increase in its internal energy while satisfying the pressure increase and causing energy waste. In other words, the multi-stage gradual compensation can reduce the temperature rise of the cooling medium and improve the energy utilization rate.

[0042] In addition, the number of high-frequency ultrasonic generators 7 correspondingly arranged on each branch (the first branch 1001 , the second branch 1002 , and the third branch 1003 ) may be 1 or 2, etc., and the present invention does not impose any limitation on this.

[0043] Exemplarily, the first branch 1001, the second branch 1002 and the third branch 1003 are all provided with a flow regulating valve 9 and a one-way valve 10. The one-way valve 10 prevents the cooling medium from flowing back, while the flow regulating valve 9 regulates the flow of each branch.

[0044] Exemplarily, the first cooler 2 and the second cooler 3 may be shell and tube coolers or plate coolers, and the cooling medium exchanges heat through seawater in both the first cooler 2 and the second cooler 3, thereby achieving temperature reduction. In addition, a plurality of monitoring modules 15 are provided in the reliquefaction system, and each monitoring module 15 is provided at the inlet or outlet of the corresponding equipment. 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, thereby facilitating the monitoring and maintenance of the reliquefaction system.

[0045] In this embodiment, the total outlet pressure of the compressor unit 1 is 1.5-1.6Mpa, the total outlet pressure of the compression end 411 of the compression-expansion integrated unit 4 is 1.7-1.8Mpa, and the total inlet pressure of the expansion end 412 of the compression-expansion integrated unit 4 is 1.8-1.9Mpa, thereby ensuring the pressure of the cooling medium after compression and expansion through the high-frequency ultrasonic generator 7.

[0046] It is easy to understand that when the corresponding output power of the compressor unit 1, the compression end 411 of 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, and the corresponding mechanical compensation is greater at this time, that is, the corresponding output power of the high-frequency ultrasonic generator 7 is greater at this time, thereby ensuring the corresponding total pressure after compression or expansion. Therefore, in this embodiment, the liquefaction system also includes a plurality of control components, which include 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 (using the compressor unit 1 as an example), and 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 completes the control of the output power of the high-frequency ultrasonic generator 7 to ensure that the outlet total pressure of the compressor unit 1 reaches the required value.

[0047] It should be noted that the total pressure mentioned above is the total pressure formed after each branch is merged into the pipeline, corresponding to Figure 1 The pressure at points A, B, and C in the diagram.

[0048] Continue to see Figure 1 The reliquefaction system also includes a pneumatic driver group 8, which is used to perform 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.

[0049] In the above embodiment, the high-pressure refrigerant medium obtained by pressurizing the high-frequency ultrasonic generator 7 through the third branch 1003 drives the pneumatic driver group 8, thereby achieving work on the expansion end 412 of the compression and expansion integrated unit 4, and then providing additional power compensation for the expansion end 412, which can not only compensate for the mechanical loss of the expansion end 412 of the compression and expansion integrated unit 4, but also increase the pressure output by the expansion end 412 to increase the cooling capacity.

[0050] Furthermore, a flow regulating valve 9 is provided at the inlet of the air pressure driver group 8, and a one-way valve 10 and a pressure reducing valve 11 are provided at the outlet of the air pressure driver group 8 in sequence. The flow regulating valve 9 can adjust the flow of the refrigerant medium entering the air pressure driver group 8 to avoid excessive flow, the one-way valve 10 plays a role of reflux, and the pressure reducing valve 11 can reduce the pressure of the cooling medium returning to the inlet of the compressor group 1 to avoid excessive pressure of the reflux cooling medium.

[0051] In addition, in one implementation 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 pressure ratio of the compressor unit 1 and reducing the energy consumption of the compressor unit.

[0052] Exemplarily, a one-way 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 .

[0053] 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 connected to the outlet of the air pressure driver unit 8 .

[0054] 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 outlet of the air pressure driver group 8. By increasing the pressure difference, the gas potential energy corresponding to the cooling medium is increased (that is, the cooling medium is quickly sucked into the venturi tube 71), allowing more potential energy to be converted into kinetic energy driven by the air pressure driver group 8, thereby increasing the power input of the air pressure driver group 8 and the utilization rate of the internal energy of the cooling medium.

[0055] For example, the nozzle of the high-frequency ultrasonic generator 7 is connected to the throat of the venturi tube 71. At this time, the nozzle of the high-frequency ultrasonic generator 7 can be regarded as a contraction section, and the outlet of the venturi tube 71 is a diffusion section, and the inner diameter of the throat is smaller than the average inner diameter of the nozzle and the average inner diameter of the diffusion section. That is to say, in the present reliquefaction system, the high-frequency ultrasonic generator 7 and the venturi tube 71 are integrated together to form a composite structure.

[0056] 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. The flow rate can be increased by the plurality of compressors 101 connected in parallel and the plurality of compression-expansion integrated machines 41 connected in parallel, thereby increasing the cooling capacity of the system.

[0057] Exemplarily, the compressor unit 1 includes three compressors 101 , the integrated compression and expansion unit 4 includes three integrated compression and expansion machines 41 , and the pneumatic driver group 8 includes three pneumatic drivers 801 , and the three pneumatic drivers 801 correspond one-to-one to the three integrated compression and expansion machines 41 .

[0058] In addition, the inlet and outlet of the compressor 101 and the inlet and outlet of the compression end 411 of the integrated compression and expansion unit 4 are provided with a plurality of expansion joints 14 connected in sequence, and the expansion joints 14 can ensure the normal connection between the pipelines under thermal expansion and contraction.

[0059] 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 integrated compression-expansion unit 4 is two.

[0060] In order to ensure that the pressure at the outlet of the second cooler 3 and the outlet of the cold end corresponding to the first heat exchanger 5 is within a preset range, the outlet of the second cooler 3 and the outlet of the cold end corresponding to the first heat exchanger 5 are both connected to a bursting disc safety valve 12 to provide safety protection to prevent excessive pressure.

[0061] It is easy to understand that Figure 1 The pipeline corresponding to the hot end of the first heat exchanger 5 is pipeline a, and its inlet ( Figure 1 The lower end of the second cooler 3 is connected to the outlet of the second cooler 3, and its outlet ( Figure 1 The upper end of the first heat exchanger 5 is connected to the inlet of the expansion end 412 of the compression expansion unit 4. The pipeline corresponding to the cold end of the first heat exchanger 5 is the pipeline b, and its inlet ( Figure 1 The upper end of the second heat exchanger 6 is connected to the outlet of the pipe d corresponding to the cold end of the second heat exchanger 6, and its outlet ( Figure 1 The lower end of the second heat exchanger 6 is connected to the inlet of the compressor unit 1, thereby realizing the heat exchange between the cooling medium in the pipeline a and the pipeline b; the pipeline corresponding to the hot end of the second heat exchanger 6 is the pipeline c, and its inlet and outlet are connected to the LNG boil-off gas. The pipeline corresponding to the cold end of the second heat exchanger 6 is the pipeline d, and its inlet ( Figure 1 The upper end of the compression expansion unit 4 is connected to the outlet of the expansion end 412 of the compression expansion unit 4, and its outlet ( Figure 1 The lower end of the pipeline (c) is connected to the inlet of the pipeline b, thereby realizing the heat exchange of the cooling medium between the pipeline c and the pipeline d.

[0062] Exemplarily, the outlet of the pipeline d in the second heat exchanger 6 is directly connected to the inlet of the pipeline b in the first heat exchanger 5, thereby avoiding the need to provide a pipeline between the two heat exchangers, thereby reducing costs and energy losses.

[0063] In this embodiment, one end of the third branch 1003 is connected to the inlet of the expansion end 412 of the compression-expansion integrated unit 4, and the other end of the third branch 1003 is connected to the inlet of the corresponding hot end of the first heat exchanger 5 (ie, the inlet of pipeline a).

[0064] 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 that has been heated up after ultrasonic treatment by the high-frequency ultrasonic generator 7 can be cooled by heat exchange through the first heat exchanger 5, so that the cooling medium flowing into the inlet of the expansion end 412 of the compression and expansion machine 41 has a higher pressure and a lower temperature (that is, the cooling capacity is guaranteed).

[0065] Similarly, one end of the first branch 1001 is connected to the outlet of the compressor unit 1, 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, so that the cooling medium that has been heated after ultrasonic treatment by the high-frequency ultrasonic generator 7 can also be cooled through the first cooler 2 or the second cooler 3, ensuring that the total outlet pressure of the compressor unit 1 and the total outlet pressure of the compression end 411 of the compression-expansion integrated unit 4 are high while having a lower temperature (also ensuring the cooling capacity).

[0066] In this embodiment, the reliquefaction system also includes a gas generator 13, which is used to prepare 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 prepared cooling medium is input into the system through the gas generator 13, and the flow regulating valve 9 can adjust the flow rate of the incoming cooling medium.

[0067] Exemplarily, the refrigerant is one or more of He, N2, H2 and Ne.

[0068] Exemplarily, the inlet pressure of the compressor 101 in the compressor unit 1 may be 0.6-0.7 MPa.

[0069] It will be easily understood by those skilled in the art 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 substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A system for reliquefaction of LNG boil-off gas, characterized in that: The reliquefaction system comprises 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 refrigerant medium; The integrated compression-expansion unit is used to compress the refrigerant medium and to expand the cooled refrigerant medium; The first heat exchanger is used to generate heat exchange between the cooled refrigerant medium and the refrigerant medium after heat exchange, and the second heat exchanger is used to generate heat exchange between the LNG boil-off gas and the refrigerant medium after expansion; The refrigerant medium flows through the compressor unit, the first cooler, the compression end of the integrated compression and expansion unit, the second cooler, the first heat exchanger, the expansion end of the integrated compression and expansion unit, the second heat exchanger and the first heat exchanger in sequence through the pipeline, and then flows back to the inlet of the compressor unit. The outlet of the compressor unit, the outlet of the compression end of the integrated compression and expansion unit, and the inlet of the expansion end of the integrated compression and expansion unit are correspondingly provided with a first branch, a second branch and a third branch in parallel, and at least one high-frequency ultrasonic generator for vibrating the refrigerant medium is provided on the first branch, the second branch and the third branch.

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

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

4. The system for reliquefaction of 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 group, and the throat of the venturi tube is connected to the outlet of the air pressure driver group.

5. The system for reliquefaction of LNG boil-off gas according to claim 2, characterized in that: The inlet of the pneumatic drive group is provided with a flow regulating valve, and the outlet of the pneumatic drive group is provided with a one-way valve and a pressure reducing valve in sequence.

6. The system for reliquefaction of LNG boil-off gas according to claim 1, characterized in that: The total outlet pressure of the compressor unit is 1.5-1.6Mpa, the total outlet pressure of the compression end of the integrated compression and expansion unit is 1.7-1.8Mpa, and the total inlet pressure of the expansion end of the integrated compression and expansion unit is 1.8-1.9Mpa.

7. The system for reliquefaction of LNG boil-off gas according to claim 1, characterized in that: The first branch, the second branch and the third branch are all provided with a flow regulating valve and a one-way valve.

8. The system for reliquefaction of 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 integrated compression-expansion machine unit includes a plurality of integrated compression-expansion machines arranged in parallel with each other.

9. The system for reliquefaction of LNG boil-off gas according to claim 1, characterized in that: The outlet of the second cooler and the outlet of the corresponding cold end of the first heat exchanger are both connected with a bursting disc safety valve.

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

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

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

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