Cryogenic reliquefaction system and method of operation thereof

Abstract

The present disclosure relates to a cryogenic reliquefaction system and a method of operating the same. The cryogenic reliquefaction system comprises a liquefied natural gas (LNG) inlet mother pipe (20), an LNG outlet mother pipe (30), a to-be-cooled LNG branch, a first heat exchanger (40a), a second heat exchanger (40b) and a cooled LNG branch. The first heat exchanger (40a) is configured to conduct heat exchange on LNG from the first LNG inlet branch (22a) and the cooling medium expanded by the first expansion machine (50a) so as to conduct deep cooling on the LNG flowing in the first LNG inlet branch (22a). And the second heat exchanger (40b) is configured to perform heat exchange on the LNG from the second LNG inlet branch (22b) and the cooling medium expanded by the second expansion machine (50b) so as to perform deep cooling on the LNG flowing in the second LNG inlet branch (22b).

Description

Technical Field

[0001] The embodiments of this disclosure generally relate to the storage and transportation of liquefied natural gas (LNG), and particularly to cryogenic reliquefaction systems and their operating methods. Background Technology

[0002] Due to the characteristics of liquefied natural gas (LNG), even with excellent insulation in the storage tanks, LNG inevitably generates blow-off gas (BOG) during storage and transportation. For environmental protection and energy efficiency reasons, this BOG needs to be reliquefied or the LNG undergoes deep cooling to suppress it.

[0003] Various cryogenic systems have been proposed for cryogenic LNG. These systems primarily employ turbine-type Brayton coolers. For example, Chinese patent application publication CN114364930A discloses a cryogenic system with a heat exchanger cleaning function to improve LNG flow efficiency. Another example is Chinese patent application publication CN115711360, which discloses a cryogenic system with improved cooling capacity for the power unit of the expander or compressor.

[0004] However, traditional cryogenic systems are not satisfactory in terms of fault response and energy efficiency. Summary of the Invention

[0005] According to a first aspect of this disclosure, a cryogenic reliquefaction system is provided. The cryogenic reliquefaction system includes: an LNG inlet header adapted to be connected to an LNG outlet of an LNG storage tank to receive LNG to be cooled from the LNG storage tank; an LNG outlet header adapted to be connected to an LNG inlet of the LNG storage tank to transport cooled LNG back to the LNG storage tank; a branch line for the LNG to be cooled, including at least a first LNG inlet branch and a second LNG inlet branch branching from the LNG inlet header; and a first heat exchanger configured to exchange heat between LNG from the first LNG inlet branch and a cooling medium expanded by a first expander, to cryogenically cool the LNG from the first LNG inlet branch. The system includes: a second heat exchanger configured to exchange heat between LNG from the second LNG inlet branch and a cooling medium expanded by the second expander, thereby cryogenically cooling the LNG flowing in the second LNG inlet branch; and a cooled LNG branch, comprising at least a first LNG outlet branch and a second LNG outlet branch connected to the LNG outlet header, the first LNG outlet branch being connected to the first heat exchanger and configured to receive cryogenically cooled LNG from the first heat exchanger, and the second LNG outlet branch being connected to the second heat exchanger and configured to receive cryogenically cooled LNG from the second heat exchanger.

[0006] The cryogenic reliquefaction system disclosed herein has fault tolerance capabilities, ensuring continuous operation even if a component within a single refrigeration system fails. Furthermore, the cryogenic reliquefaction system disclosed herein has wide load tolerance capabilities; by adjusting the workload of each module, the system can operate efficiently across a wide load range.

[0007] In some embodiments, the system further includes: at least one first compressor connected in series with and located upstream of the first expander, wherein a cooling medium compressed by the at least one first compressor is delivered to the first expander and expanded at the first expander before being delivered to the first heat exchanger, the cooling medium circulating in a closed loop within a first cryogenic module including the first expander and the at least one first compressor; and at least one second compressor connected in series with and located upstream of the second expander, wherein a cooling medium compressed by the at least one second compressor is delivered to the second expander and expanded at the second expander before being delivered to the second heat exchanger, the cooling medium circulating in a closed loop within a second cryogenic module including the second expander and the at least one second compressor.

[0008] In some embodiments, the first heat exchanger and the first cryogenic module are implemented as a first skid, and the second heat exchanger and the second cryogenic module are implemented as a second skid.

[0009] In some embodiments, the system further includes: a first flow regulating valve disposed on the first LNG inlet branch; a second flow regulating valve disposed on the second LNG inlet branch; and a control system communicatively connected to the first and second flow regulating valves and configured to adjust the opening of the first and second flow regulating valves based on LNG parameters of the LNG storage tank, thereby regulating the amount of LNG to be cooled allocated to the first heat exchanger and the amount of LNG to be cooled allocated to the second heat exchanger. Thus, the automated operation of the cryogenic reliquefaction system can be achieved through the control system.

[0010] In some embodiments, the control system is further configured to: determine the total cooling capacity for the LNG to be cooled based on the LNG parameters; obtain a first rated cooling capacity of the first cryogenic module and a second rated cooling capacity of the second cryogenic module; and control the operating modes of the first cryogenic module and the second cryogenic module based on the total cooling capacity, the first rated cooling capacity, and the second rated cooling capacity. Thus, multiple different operating modes can be set to cope with malfunctions and / or different operating conditions of the cryogenic reliquefaction system.

[0011] In some embodiments, the control system is further configured to adjust the amount of LNG to be cooled allocated to the first heat exchanger and the amount of LNG to be cooled allocated to the second heat exchanger based on the performance curves of the at least one first compressor of the first cryogenic module and / or the performance curves of the at least one second compressor of the second cryogenic module. This allows the compressors of the cryogenic module to operate in their high-efficiency range, improving the energy efficiency ratio and saving costs.

[0012] In some embodiments, the control system is further configured to: based on the determined total cooling amount for the LNG to be cooled; determine whether to connect the first LNG inlet branch or the second LNG inlet branch to the LNG inlet header; and in response to determining that the first LNG inlet branch or the second LNG inlet branch is not connected to the LNG inlet header, stop the operation of the cryogenic module corresponding to the unconnected LNG inlet branch; and / or in response to determining that the first LNG inlet branch or the second LNG inlet branch is connected to the LNG inlet header, start the operation of the cryogenic module corresponding to the connected LNG inlet branch.

[0013] In some embodiments, the control system is further configured to: receive a fault signal from the first cryogenic module and / or the second cryogenic module; based on the fault signal, stop the operation of the cryogenic module corresponding to the fault signal; and start at least one third cryogenic module and / or increase the flow rate of LNG to be cooled allocated to the cryogenic modules in use.

[0014] In some embodiments, the system further includes: at least one third heat exchanger configured to be connected to a third LNG inlet branch branching from the LNG inlet header and configured to exchange heat between LNG from the third LNG inlet branch and a cooling medium expanded by a third expander to cryogenically cool the LNG in the third LNG inlet branch; wherein the third cryogenic module includes: at least one third compressor connected in series with the third expander and located upstream of the third expander, wherein the cooling medium compressed by the at least one third compressor is delivered to the third expander and expanded at the third expander before being delivered to the third heat exchanger, and the cooling medium circulates in a closed loop within the third cryogenic module including the third expander and the at least one third compressor.

[0015] In some embodiments, the system further includes: an LNG header bypass disposed between the LNG inlet header and the LNG outlet header; and / or two redundant branches disposed on the LNG inlet header, wherein each branch includes a shut-off valve, a filter and a regulating valve disposed in sequence; and / or shut-off valves disposed in the first LNG outlet branch and the second LNG outlet branch respectively.

[0016] According to a second aspect of this disclosure, a method for operating a cryogenic reliquefaction system is provided. The method includes: acquiring LNG parameters of an LNG storage tank; and adjusting the opening of a first flow control valve and a second flow control valve based on the LNG parameters of the LNG storage tank to adjust the amount of LNG to be cooled allocated to a first heat exchanger and the amount of LNG to be cooled allocated to a second heat exchanger; wherein the first heat exchanger is configured to exchange heat between LNG from a first LNG inlet branch and a cooling medium expanded by a first expander to cryogenically cool the LNG from the first LNG inlet branch; the second heat exchanger is configured to exchange heat between LNG from a second LNG inlet branch and a cooling medium expanded by a second expander to cryogenically cool the LNG from the second LNG inlet branch; the first flow control valve is disposed on the first LNG inlet branch, and the second flow control valve is disposed on the second LNG inlet branch.

[0017] In some embodiments, the method further includes: determining the total cooling amount for the LNG to be cooled; determining, based on the total cooling amount of the LNG to be cooled, whether to connect the first LNG inlet branch or the second LNG inlet branch to the LNG inlet main pipe; and, in response to determining that the first LNG inlet branch or the second LNG inlet branch is not connected to the LNG inlet main pipe, stopping the operation of the cryogenic module corresponding to the unconnected LNG inlet branch; and / or, in response to determining that the first LNG inlet branch or the second LNG inlet branch is connected to the LNG inlet main pipe, starting the operation of the cryogenic module corresponding to the connected LNG inlet branch.

[0018] In some embodiments, the method further includes: receiving a fault signal from a cryogenic module associated with the first expander and / or the second expander; based on the fault signal, stopping the operation of the cryogenic module corresponding to the fault signal; and activating at least one standby third cryogenic module and / or increasing the flow rate of LNG to be cooled allocated to the cryogenic module in use. Attached Figure Description

[0019] The above and other objects, features, and advantages of embodiments of the present disclosure will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings. Several embodiments of the present disclosure are illustrated in the drawings by way of example and not limitation.

[0020] Figure 1 A schematic diagram of a cryogenic reliquefaction system according to a first embodiment of the present disclosure is shown.

[0021] Figure 2 A schematic diagram of a cryogenic reliquefaction system according to a second embodiment of the present disclosure is shown.

[0022] Figure 3 A general schematic diagram of a cryogenic reliquefaction system according to a third embodiment of the present disclosure is shown; and

[0023] Figure 4 A flowchart illustrating a method for operating a cryogenic reliquefaction system according to an embodiment of the present disclosure is shown.

[0024] Throughout the accompanying drawings, identical or similar parts are indicated by the same and similar reference numerals. Detailed Implementation

[0025] Preferred embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While preferred embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art.

[0026] The term "comprising" and its variations as used herein signify an open-ended inclusion, i.e., "including but not limited to". Unless otherwise stated, the term "or" means "and / or". The term "based on" means "at least partially based on". The terms "one example embodiment" and "one embodiment" mean "at least one example embodiment". The term "another embodiment" means "at least one additional embodiment". Terms such as "upper", "lower", "front", and "rear", indicating placement or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings and are used only for the purpose of describing the principles of this disclosure, and are not intended to indicate or imply that the elements referred to must have a specific orientation, be constructed or operated in a specific orientation, and therefore should not be construed as limiting this disclosure.

[0027] During LNG transportation, heat inevitably seeps into the system, and some of the liquefied cargo evaporates. Taking marine transportation as an example, for marine equipment, factors such as available space on the ship may make process compactness, operability, and safety more important design indicators. Cryogenic reliquefaction systems based on turbo-Broughton refrigeration technology have advantages in reliability, space occupation, installation and commissioning, and maintenance costs. However, traditional cryogenic reliquefaction systems are mostly single-station, large-capacity designs. If a component within a single station fails, the system cannot operate normally; in addition, single-station designs are unsatisfactory in terms of load regulation capabilities. According to this disclosure, a cryogenic reliquefaction system and a method for operating a cryogenic reliquefaction system are provided, which can solve one or more of the above-mentioned technical problems.

[0028] Figure 1 A general schematic diagram of a cryogenic reliquefaction system 100 according to a first embodiment of the present disclosure is shown. Figure 1As shown, the cryogenic reliquefaction system 100 includes: an LNG inlet header 20 connected to the LNG outlet 12 of the LNG storage tank 10 to receive LNG to be cooled from the LNG storage tank 10; and an LNG outlet header 30 connected to the LNG inlet 14 of the LNG storage tank 10 to transport cooled LNG back to the LNG storage tank 10. In some embodiments, such as Figure 1 As shown, a first valve 75a can be installed on the LNG inlet header 20 near the LNG storage tank 10 to control the flow of LNG on the LNG inlet header 20. Similarly, a second valve 75b can be installed on the LNG outlet header 30 near the LNG storage tank 10 to control the flow of LNG on the LNG inlet header 20.

[0029] The cryogenic reliquefaction system 100 also includes a first LNG inlet branch 22a and a second LNG inlet branch 22b branching from the LNG inlet header 20. LNG from the LNG inlet header 20 can be transported to heat exchangers 40a, 40b for cooling via the first LNG inlet branch 22a and the second LNG inlet branch 22b, which serve as LNG branches to be cooled. In the illustrated embodiment, there are LNG branches to be cooled and corresponding two heat exchangers 40a, 40b. It should be understood that more LNG branches to be cooled and corresponding heat exchangers can be provided. The first heat exchanger 40a is configured to exchange heat between the LNG from the first LNG inlet branch 22a and the cooling medium expanded by the first expander 50a. This cryogenically cools the LNG flowing within the first LNG inlet branch 22a. The second heat exchanger 40b is configured to exchange heat between the LNG from the second LNG inlet branch 22b and the cooling medium after expansion by the second expander 50b, so as to deeply cool the LNG flowing in the second LNG inlet branch 22b.

[0030] The cryogenic reliquefaction system 100 also includes a first LNG outlet branch 32a and a second LNG outlet branch 32b that converge to the LNG outlet header 30. The first LNG outlet branch 32a is connected to a first heat exchanger 40a and configured to receive cryogenically cooled LNG that has been cryogenically cooled by the first heat exchanger 40a. The second LNG outlet branch 32b is connected to a second heat exchanger 40b and configured to receive cryogenically cooled LNG that has been cryogenically cooled by the second heat exchanger 40b. It should be understood that the illustrated embodiment is merely exemplary, and the number of LNG outlet branches can be configured according to the number of LNG branches to be cooled and the corresponding number of heat exchangers. The cooled LNG flowing out of each heat exchanger 40a, 40b can converge to the LNG outlet header 30 via the first LNG outlet branch 32a and the second LNG outlet branch 32b, which serve as cooled LNG branches, and then return to the LNG storage tank 10 via the LNG outlet header 30.

[0031] According to this disclosure, the LNG to be cooled can be distributed to multiple LNG cryogenic heat exchangers 40a and 40b through the manifold system of the provided cryogenic reliquefaction system 100. In this case, even if a component in a single refrigeration system within the system fails, the entire system will not be paralyzed. Compared to existing single-station technologies, if a critical component within the system fails, the entire system will stop operating. In such cases, the only way to avoid safety hazards during transportation is to burn BOG, which would result in significant economic losses and potential safety risks. On the other hand, considering that BOG production fluctuates drastically with environmental and operational conditions... When the BOG generated is small, the LNG to be cooled can be distributed to one of multiple heat exchangers 40a and 40b, and the heat exchanger can be operated in its high-efficiency zone (e.g., 60%–100% of rated load, 70%–100% of rated load, 75%–100% of rated load, or even 80%–100% of rated load). This avoids the problem of a single large-capacity system being forced to operate in a low-load zone significantly below its rated operating conditions, which would lead to a significant decrease in the system's energy efficiency ratio and high operating costs. As an example, when the BOG generated is too large, exceeding the rated operating conditions of one of the multiple heat exchangers 40a and 40b, multiple heat exchangers 40a and 40b can be started up, ensuring that the started heat exchanger operates in its high-efficiency zone. Therefore, the system's energy efficiency is significantly improved, and efficient operation is maintained over a wide load range.

[0032] Cryogenic modules used to supply refrigerant to heat exchangers can include a variety of implementations. In some embodiments, such as... Figure 1As shown, at least one first compressor 64a is connected in series with and located upstream of the first expander 50a. Cooling medium compressed by the at least one first compressor 64a is conveyed to the first expander 50a and, after expansion at the first expander 50a, is conveyed to the first heat exchanger 40a. Cooling medium flowing out of the first heat exchanger 40a then returns to the compressor for further compression. Thus, the cooling medium circulates in a closed loop within the cryogenic module comprising the first expander 50a and at least one first compressor 64a. In some embodiments, the circulating cooling medium loop may be helium, nitrogen, hydrogen, or another pure gas, or a mixture thereof.

[0033] In the illustrated embodiment, the cryogenic module includes two-stage compression, with a first-stage compressor 62a used for primary compression. In the illustrated embodiment, the first-stage compressor 62a is coaxial with the first expander 50a. It should be understood that the first-stage compressor 62a can be independently configured and not coaxial with the first expander 50a. The primary-compressed cooling medium undergoes secondary compression at the first compressor 64a, and the secondary-compressed cooling medium is then delivered to the first expander 50a to produce a subcooled cooling medium. It should be understood that the illustrated embodiment is merely exemplary; in other embodiments, primary or multiple-stage compression may be used. Each compressor also includes its own power system cooling device (not shown). In some embodiments, cooling devices may be provided between the compressors and between the compressors and the expanders. The number and arrangement of the expanders and compressors are not limited to the illustrated embodiment.

[0034] Cryogenic modules used in different heat exchangers can employ a similar arrangement. For example... Figure 1 As shown, at least one second compressor 64b is connected in series with and located upstream of the second expander 50b. Cooling medium compressed by the at least one second compressor 64b is conveyed to the second expander 50b and, after expansion at the second expander 50b, is conveyed to the second heat exchanger 40b. The cryogenic module for the heat exchanger 40b also employs two-stage compression; a second first-stage compressor 62b is used for first-stage compression, and its detailed description is omitted. The cooling medium circulates in a closed loop within the second cryogenic module, thereby deeply cooling the LNG conveyed to the heat exchanger 40b. Figure 1 The cryogenic module shown for supplying refrigerant to heat exchanger 40b employs a similar arrangement to the cryogenic module for supplying refrigerant to heat exchanger 40a. It should be understood that this is merely exemplary. Cryogenic modules for different heat exchangers may employ different arrangements and may provide the same or different cooling capacities.

[0035] In some embodiments, the first heat exchanger 40a and the first cryogenic module are implemented as a first skid 65a, and the second heat exchanger 40b and the second cryogenic module are implemented as a second skid 65b. This facilitates the installation and / or maintenance of each skid. The modular design also facilitates capacity expansion and fault maintenance.

[0036] In some embodiments, a first flow regulating valve 71a may be provided on the first LNG inlet branch 22a; and a second flow regulating valve 71b may be provided on the second LNG inlet branch 22b. The amount of LNG to be cooled distributed to each heat exchanger 40a, 40b can be controlled by adjusting the first flow regulating valve 71a and the second flow regulating valve 71b. In some embodiments, the first LNG outlet branch 32a and the second LNG outlet branch 32b may be respectively provided with a first shut-off valve 72a and a second shut-off valve 72b. In some cases, such as in the event of a fault, the first shut-off valve 72a and the second shut-off valve 72b can be conveniently closed to isolate each cryogenic module.

[0037] In some embodiments, the amount of LNG to be cooled allocated to each heat exchanger 40a, 40b can be automatically controlled by the control system 70. The control system 70 may include a processor with computing capabilities, which automatically controls the operation of the cryogenic reliquefaction system 100 by running a computer program on the processor. The control system 70 may be communicatively connected to the first flow control valve 71a and the second flow control valve 71b and configured to adjust the opening of the first flow control valve 71a and the second flow control valve 71b based on the LNG parameters of the LNG storage tank 10, thereby regulating the amount of LNG to be cooled allocated to the first heat exchanger 40a and the second heat exchanger 40b. In some embodiments, the LNG storage tank 10 may be equipped with sensors such as temperature sensors and pressure sensors to obtain the temperature and pressure parameters of the LNG within the LNG storage tank 10. These parameters provide a basis for the operation of the control system 70.

[0038] In some embodiments, the control system 70 is further configured to: determine the total cooling capacity for the LNG to be cooled based on LNG parameters; obtain a first rated cooling capacity for the first cryogenic module and a second rated cooling capacity for the second cryogenic module; and control the operating modes of the first and second cryogenic modules based on the total cooling capacity, the first rated cooling capacity, and the second rated cooling capacity. Thus, the control system 70 can allocate the workload of each cryogenic module according to its own cooling capacity and the total cooling capacity to be cooled, ensuring that each cryogenic module operates in its most efficient range (e.g., above 70% of its rated load).

[0039] In some embodiments, the control system 70 is further configured to adjust the amount of LNG to be cooled allocated to the first heat exchanger 40a and the amount of LNG to be cooled allocated to the second heat exchanger 40b based on the performance curves of at least one first compressor 64a of the first cryogenic module and / or the performance curves of at least one second compressor 64b of the second cryogenic module. Thus, the cryogenic reliquefaction system 100 can achieve more energy-efficient allocation for each cryogenic module.

[0040] In some embodiments, the control system 70 can provide a wide load range adjustment capability. Considering the potential for drastic changes in pressure and temperature within the LNG storage tank 10, the control system 70 can configure the number of cooling systems to be connected to the cryogenic process as needed, and selectively disable or enable the cooling systems. In some embodiments, the control system 70 is further configured to: determine, based on the determined total cooling capacity for the LNG to be cooled; whether to connect the first LNG inlet branch 22a or the second LNG inlet branch 22b to the LNG inlet header 20; and, in response to determining that the first LNG inlet branch 22a or the second LNG inlet branch 22b is not connected to the LNG inlet header 20, stop the operation of the cryogenic module corresponding to the unconnected LNG inlet branch; and / or, in response to determining that the first LNG inlet branch 22a or the second LNG inlet branch 22b is connected to the LNG inlet header 20, start the operation of the cryogenic module corresponding to the connected LNG inlet branch. Thus, a wide load capacity can be dynamically adapted as needed.

[0041] In some embodiments, the cryogenic reliquefaction system 100 can provide fault response capabilities. In some embodiments, the control system 70 is further configured to: receive fault signals from a first cryogenic module and / or a second cryogenic module; based on the fault signal, stop the operation of the cryogenic module corresponding to the fault signal; and activate a standby cryogenic module and / or increase the flow rate of LNG to be cooled allocated to the cryogenic module in use. The standby cryogenic module may be an unused cryogenic module or an additional cryogenic module. Thus, even if a component within the cryogenic reliquefaction system 100 fails, it can be isolated from the system, for example, by closing its inlet and outlet valves. Subsequently, the fault condition can be addressed by activating the standby system or by increasing the load on the flow control valves of the remaining modules, thus avoiding equipment downtime.

[0042] Figure 2 A general schematic diagram of a cryogenic reliquefaction system 200 according to a second embodiment of the present disclosure is shown. Figure 2 The illustrated embodiments and Figure 1The illustrated embodiments are similar, except that the cryogenic reliquefaction system 200 may also include additional functional modules. To avoid repetition, descriptions of identical parts are omitted, and the differences are emphasized. In some embodiments, such as Figure 2 As shown, an LNG header bypass 25 can also be provided between the LNG inlet header 20 and the LNG outlet header 30. A valve 75c can be installed on the LNG header bypass 25. By providing the LNG header bypass 25 and the valve 75c, pre-cooling can be provided to the manifold. In some embodiments, such as... Figure 2 As shown, two redundant branches can also be installed on the LNG inlet header 20. Each redundant branch includes a valve 76 (e.g., a shut-off valve), a filter 77, and a regulating valve 78 arranged in sequence. With this arrangement, even if the filter 77 on one branch fails, the branch containing the failed filter 77 can be shut down, while the branch with the unfailed filter can be activated, ensuring that the system is always in normal operation without the need for downtime maintenance.

[0043] Figure 3 A general schematic diagram of a cryogenic reliquefaction system 300 according to a third embodiment of the present disclosure is shown. Figure 3 The illustrated embodiments and Figure 1 The embodiments shown are similar, except that the cryogenic reliquefaction system 300 may also include additional manifold branches, additional heat exchangers, and additional cryogenic modules.

[0044] like Figure 3 As shown, the cryogenic reliquefaction system 300 may further include a third LNG inlet branch 22c branching from the LNG inlet header 20 to receive LNG to be cooled. The third LNG inlet branch 22c is connected to a third heat exchanger 40c. The LNG to be cooled undergoes heat exchange at the third heat exchanger 40c with a cooling medium expanded by the third expander 50c, and is then cryogenically cooled before being returned to the LNG outlet header 30 via the third LNG outlet branch 32c. Figure 3 As shown, the third cryogenic module includes at least one third compressor 64c connected in series with and located upstream of the third expander 50c. Cooling medium compressed by the at least one third compressor 64c is conveyed to the third expander 50c and, after expansion at the third expander 50c, is conveyed to the third heat exchanger 40c. The cooling medium circulates in a closed loop within the third cryogenic module, which includes the third expander 50c and the at least one third compressor 64c. Figure 3 In the illustrated embodiment, the compressor employs a two-stage compression process, with a third-stage compressor 62c used for primary compression. In some embodiments, a third flow regulating valve 71c may be installed on the third LNG inlet branch 22c. In some embodiments, a third shut-off valve 72c may also be installed on the third LNG outlet branch 32c.

[0045] It should be understood that, Figure 3 The embodiments shown are merely exemplary. A cryogenic reliquefaction system may include two, three, or more heat exchangers and cryogenic modules that work in conjunction with the heat exchangers. The respective heat exchangers and cryogenic modules of the cryogenic reliquefaction system according to this disclosure can be implemented as respective skid-mounted 65a, 65b, 65c.

[0046] According to this disclosure, by providing multiple heat exchangers and corresponding cryogenic modules, fault redundancy can be achieved between them. If any module fails, it can be quickly isolated, while the remaining modules can continue to operate, ensuring system continuity. This disclosure also ensures efficient operation across a wide load range. After determining the number of operating modules, by flexibly combining the number of start-stop modules and finely adjusting the load of each module, all operating compressors can always operate in their high-efficiency range, thereby achieving optimal overall system efficiency across the entire load variation range. The cryogenic reliquefaction system of this disclosure offers sufficient flexibility and scalability for operating condition adjustment, facilitating capacity expansion and maintenance.

[0047] Figure 4 A flowchart illustrating a method for operating a cryogenic reliquefaction system 400 according to an embodiment of the present disclosure is shown. At block 402, LNG parameters of the LNG storage tank 10 are acquired. At block 404, the opening degrees of a first flow control valve 71a and a second flow control valve 71b are adjusted based on the LNG parameters of the LNG storage tank 10 to regulate the amount of LNG to be cooled allocated to the first heat exchanger 40a and the amount of LNG to be cooled allocated to the second heat exchanger 40b. The first heat exchanger 40a is configured to exchange heat between LNG from the first LNG inlet branch 22a and the cooling medium expanded by the first expander 50a, so as to deeply cool the LNG from the first LNG inlet branch 22a; the second heat exchanger 40b is configured to exchange heat between LNG from the second LNG inlet branch 22b and the cooling medium expanded by the second expander 50b, so as to deeply cool the LNG from the second LNG inlet branch 22b; the first flow regulating valve 71a is installed on the first LNG inlet branch 22a, and the second flow regulating valve 71b is installed on the second LNG inlet branch 22b.

[0048] In some embodiments, the method may further include: determining the total cooling amount for the LNG to be cooled; determining, based on the total cooling amount of the LNG to be cooled, whether to connect the first LNG inlet branch 22a or the second LNG inlet branch 22b to the LNG inlet header 20; and, in response to determining that the first LNG inlet branch 22a or the second LNG inlet branch 22b is not connected to the LNG inlet header 20, stopping the operation of the cryogenic module corresponding to the unconnected LNG inlet branch; and / or, in response to determining that the first LNG inlet branch 22a or the second LNG inlet branch 22b is connected to the LNG inlet header 20, starting the operation of the cryogenic module corresponding to the connected LNG inlet branch.

[0049] In some embodiments, the method may further include: receiving a fault signal from a cryogenic module associated with the first expander 50a and / or the second expander 50b; based on the fault signal, stopping the operation of the cryogenic module corresponding to the fault signal; and starting at least one third cryogenic module and / or increasing the flow rate of LNG to be cooled allocated to the cryogenic module in use.

[0050] Although the operations are described in a specific order, this should be understood as requiring that such operations be performed in the specific order shown or in sequential order, or requiring that all illustrated operations be performed to achieve the desired result. In certain environments, multitasking and parallel processing may be advantageous. Similarly, while several specific implementation details are included in the above discussion, these should not be construed as limiting the scope of this disclosure. Certain features described in the context of individual embodiments may also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation may also be implemented individually or in any suitable sub-combination in multiple implementations.

[0051] Although the subject matter has been described using language specific to structural features and / or methodological logic, it should be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or actions described above. Rather, the specific features and actions described above are merely illustrative examples of implementing the claims.

[0052] The various embodiments of this disclosure have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A cryogenic reliquefaction system, comprising: a liquefied natural gas (LNG) import header (20) adapted to be connected to an LNG outlet of an LNG storage tank (10) to receive LNG to be cooled from the LNG storage tank (10); an LNG export header (30) adapted to be connected to an LNG inlet of the LNG storage tank (10) to deliver cooled LNG back to the LNG storage tank (10); a to-be-cooled LNG branch comprising at least a first LNG import leg (22a) and a second LNG import leg (22b) branched from the LNG import header (20); a first heat exchanger (40a) configured to exchange heat between LNG from the first LNG import leg (22a) and a cooling medium expanded by a first expander (50a) to cryogenically cool the LNG flowing in the first LNG import leg (22a); a second heat exchanger (40b) configured to exchange heat between LNG from the second LNG import leg (22b) and a cooling medium expanded by a second expander (50b) to cryogenically cool the LNG flowing in the second LNG import leg (22b); and a cooled LNG branch comprising at least a first LNG export leg (32a) and a second LNG export leg (32b) merged into the LNG export header (30), the first LNG export leg (32a) connected to the first heat exchanger (40a) and configured to receive cryogenically cooled LNG by the first heat exchanger (40a), the second LNG export leg (32b) connected to the second heat exchanger (40b) and configured to receive cryogenically cooled LNG by the second heat exchanger (40b).

2. The system of claim 1, further comprising: at least one first compressor (64a) connected in series with the first expander (50a) and upstream of the first expander (50a), the cooling medium compressed by the at least one first compressor (64a) being delivered to the first expander (50a) and, after being expanded at the first expander (50a), to the first heat exchanger (40a), the cooling medium being closed-loop circulated in a first cryogenic module comprising the first expander (50a) and the at least one first compressor (64a); and at least one second compressor (64b) connected in series with the second expander (50b) and upstream of the second expander (50b), the cooling medium compressed by the at least one second compressor (64b) being delivered to the second expander (50b) and, after being expanded at the second expander (50b), to the second heat exchanger (40b), the cooling medium being closed-loop circulated in a second cryogenic module comprising the second expander (50b) and the at least one second compressor (64b). ​ ​ 3. The system of claim 2, wherein the first heat exchanger (40a) and the first cryogenic module are implemented as a first skid, and the second heat exchanger (40b) and the second cryogenic module are implemented as a second skid.

4. The system of claim 2, further comprising: a first flow regulating valve (71a) disposed on the first LNG inlet branch (22a); a second flow regulating valve (71b) disposed on the second LNG inlet branch (22b); and a control system (70) communicatively connected with the first flow regulating valve (71a) and the second flow regulating valve (71a) and configured to adjust the opening of the first flow regulating valve (71a) and the second flow regulating valve (71b) based on LNG parameters of the LNG storage tank (10) to adjust the amount of LNG to be cooled that is distributed to the first heat exchanger (40a) and the amount of LNG to be cooled that is distributed to the second heat exchanger (40b).

5. The system of claim 4, wherein the control system (70) is further configured to: determine a total cooling amount for LNG to be cooled based on the LNG parameters; obtain a first rated cooling amount of the first cryogenic module and a second rated cooling amount of the second cryogenic module; and control the operating mode of the first cryogenic module and the second cryogenic module based on the total cooling amount, the first rated cooling amount, and the second rated cooling amount.

6. The system of claim 5, wherein the control system (70) is further configured to adjust the amount of LNG to be cooled that is distributed to the first heat exchanger (40a) and the amount of LNG to be cooled that is distributed to the second heat exchanger (40b) according to a performance curve of the at least one first compressor (64a) of the first cryogenic module and / or a performance curve of the at least one second compressor (64b) of the second cryogenic module.

7. The system of claim 5, wherein the control system (70) is further configured to: based on the determined total cooling amount for LNG to be cooled; determine whether to connect the first LNG inlet branch (22a) or the second LNG inlet branch (22b) to the LNG inlet header (20); and in response to determining that the first LNG inlet branch (22a) or the second LNG inlet branch (22b) is not connected to the LNG inlet header (20), stop operation of the cryogenic module corresponding to the unconnected LNG inlet branch; and / or in response to determining that the first LNG inlet branch (22a) or the second LNG inlet branch (22b) is connected to the LNG inlet header (20), start operation of the cryogenic module corresponding to the connected LNG inlet branch.

8. The system of any one of claims 4-7, wherein the control system (70) is further configured to: receive a fault signal from the first cryogenic module and / or the second cryogenic module; ​ based on the fault signal, stopping operation of a cryogenic module corresponding to the fault signal; and starting a backup cryogenic module and / or increasing a flow of LNG to be cooled to an in-use cryogenic module.

9. The system according to any one of claims 1-7, further comprising: at least one third heat exchanger (40c) configured to be connected to a third LNG inlet branch branched from the LNG inlet main pipe (20) and configured to exchange heat between LNG from the third LNG inlet branch and cooling medium expanded by a third expander (50c) to cryogenically cool the LNG of the third LNG inlet branch; wherein the third cryogenic module comprises at least one third compressor (64c) connected in series with the third expander (50c) and located upstream of the third expander (50c), cooling medium compressed by the at least one third compressor (64c) is delivered to the third expander (50c) and delivered to the third heat exchanger (40c) after being expanded at the third expander (50c), and the cooling medium is closedly circulated in the third cryogenic module comprising the third expander (50c) and the at least one third compressor (64c).

10. The system according to any one of claims 1-7, further comprising: an LNG main pipe bypass (25) provided between the LNG inlet main pipe (20) and the LNG outlet main pipe (30); and / or two redundant branches provided on the LNG inlet main pipe (20), wherein each branch comprises a shut-off valve, a filter and a regulating valve provided in sequence; and / or shut-off valves respectively provided on the first LNG outlet branch (32a) and the second LNG outlet branch (32b).

11. A method for operating a cryogenic reliquefaction system, comprising: obtaining LNG parameters of an LNG storage tank (10); and adjusting opening degrees of a first flow regulating valve (71a) and a second flow regulating valve (71b) based on the LNG parameters of the LNG storage tank (10) to adjust an amount of LNG to be cooled that is distributed to a first heat exchanger (40a) and an amount of LNG to be cooled that is distributed to a second heat exchanger (40b); wherein the first heat exchanger (40a) is configured to exchange heat between LNG from a first LNG inlet branch (22a) and cooling medium expanded by a first expander (50a) to cryogenically cool the LNG of the first LNG inlet branch (22a); the second heat exchanger (40b) is configured to exchange heat between LNG from a second LNG inlet branch (22b) and cooling medium expanded by a second expander (50b) to cryogenically cool the LNG of the second LNG inlet branch (22b); the first flow regulating valve (71a) is provided on the first LNG inlet branch (22a), and the second flow regulating valve (71b) is provided on the second LNG inlet branch (22b).

12. The method according to claim 11, further comprising: determining a total cooling amount for the LNG to be cooled; determining whether the first LNG inlet branch (22a) or the second LNG inlet branch (22b) is to be connected to the LNG inlet header (20) based on the total cooling amount for the LNG to be cooled; and stopping operation of the cryogenic module corresponding to the LNG inlet branch not connected to the LNG inlet header (20) in response to determining that the first LNG inlet branch (22a) or the second LNG inlet branch (22b) is not connected to the LNG inlet header (20); and / or starting operation of the cryogenic module corresponding to the LNG inlet branch connected to the LNG inlet header (20) in response to determining that the first LNG inlet branch (22a) or the second LNG inlet branch (22b) is connected to the LNG inlet header (20).

13. The method of claim 11, further comprising: receiving a fault signal from a cryogenic module in which the first expander (50a) and / or the second expander (50b) is located; stopping operation of the cryogenic module corresponding to the fault signal based on the fault signal; and starting at least one third cryogenic module and / or increasing the flow of LNG to be cooled allocated to the cryogenic module in use.

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