Evaporated gas reliquefaction system, method for reliquefying evaporated gas in a reliquefaction system, and method for operating the evaporated gas reliquefaction system.

Abstract

This disclosure relates to a BOG reliquefaction system, a method for reliquefying BOG in a reliquefaction system, and a method for operating a BOG reliquefaction system. The BOG reliquefaction system includes at least one liquefied natural gas (LNG) container, a BOG preheater, at least one BOG compressor having a BOG compressor cooler, and a BOG recondenser having a refrigeration cycle. The BOG reliquefaction system includes a separate liquid circuit with circulation of a liquid phase heat transfer fluid (HTF) to preheat the BOG from the LNG container before it enters the BOG compressor. The separate liquid circuit includes: a liquid pump configured to pump the HTF; a BOG preheater configured to perform heat exchange between the BOG and the HTF; a BOG recondenser located downstream of the BOG preheater; and a liquid trimming heater located downstream of the BOG recondenser configured to heat the HTF.

Description

Technical Field

[0001] This invention relates to a liquefied natural gas (LNG) evaporated gas (BOG) reliquefaction system, a method for reliquefying BOG, and a method for operating a BOG reliquefaction system. More specifically, this disclosure relates to an evaporated gas (BOG) reliquefaction system as defined in the preamble of claims 1, 15, and 28, a method for reliquefying evaporated gas (BOG) in a reliquefaction system, and a method for operating an evaporated gas (BOG) reliquefaction system. Background Technology

[0002] Common technologies used to transport natural gas from extraction sites include liquefying natural gas at or near the site, and transporting LNG to the market in specially designed storage tanks (usually placed on ships sailing overseas).

[0003] LNG is a mixture of light hydrocarbons, primarily methane, and nitrogen as an inert component, and may contain trace amounts of ethane, propane, butane, and pentane. Depending on its precise composition, LNG has a boiling point of approximately -162°C to -161°C at atmospheric pressure, and is typically loaded, transported, and unloaded within this temperature range. This requires specialized materials, insulation, and handling equipment to cope with the low temperatures and evaporation vapors. Due to heat leakage, the surface of the cargo (LNG) continuously boils, generating evaporated natural gas from the LNG, known as boil-off gas (BOG)—primarily methane.

[0004] Facilities for the continuous reliquefaction of BOG are well-known. Installing a BOG reliquefaction system on liquefied natural gas (LNG) vehicles with dual-fuel engines allows ship operators greater flexibility in switching between fuels to take advantage of price differences between LNG and heavy fuel oil. Existing propulsion systems are inefficient, and not all BOG can be used in engines. Furthermore, slow navigation and berthing operations often result in BOG overruns. Instead of burning the gas in a gas combustion unit, the gas can be reliquefied and returned to the cargo container.

[0005] Reliquefying BOG on LNG carriers increases cargo transport capacity and allows owners and operators to choose the optimal propulsion system and operating mode. Advantages include, for example, flexible fuel systems, optimized operating costs, and increased cargo transport capacity.

[0006] Typically, reliquefaction systems are used to control cargo box pressure by liquefying BOG. ​​They are capable of handling all BOG (100% capacity) or only the excess BOG that is not burned in the engine (partial liquefaction).

[0007] The composition, flow rate, temperature, and pressure of a BOG can vary. These fluctuations require a system capable of handling the process conditions fed into the system.

[0008] Existing BOG reliquefaction systems are based on reverse nitrogen Brayton cycle recooling technology. This means that a closed nitrogen cycle exists within the system to extract heat from the BOG. ​​Nitrogen (N2) is used as the refrigerant, with the purpose of controlling the tank pressure by cooling and reliquefying the pressurized BOG. ​​The reliquefied BOG is then returned to the tank.

[0009] WO 2009 / 136793A1 discloses a gas supply system for a dual-fuel or gas-fired engine integrated with a BOG reliquefaction facility. The two systems for BOG reliquefaction and gas supply are “independent” components. The cold load is removed from the LNG via an external heat source and is not used.

[0010] WO 2011 / 078689A1 discloses a gas supply system for a dual-fuel or gas engine integrated with a BOG reliquefaction unit, wherein an available cold load in LNG is used to cool and condensate the BOG in a device in which the BOG or its condensate exchanges heat with the LNG. The LNG is heated by using an available “warm” load from a compressor in the reliquefaction system as cooling water.

[0011] BOG leaves the LNG tank at temperatures typically between -140°C and -110°C and is preheated before entering the BOG compressor. When selecting a compressor, the temperature at the compressor's suction inlet is crucial. Preheating the BOG allows for higher temperatures at the compressor inlet. Existing systems with BOG preheating upstream of the BOG compressor already incorporate refrigerant (N2) as the preheating medium. This involves simultaneously removing heat from the nitrogen. N2 heating is not always available or sufficient, for example, if reliquefaction is not operating and / or reliquefaction is under high fuel gas consumption. An additional heat source may then be required, which is typically accomplished using an additional BOG preheater in parallel.

[0012] Existing BOG reliquefaction systems do not provide effective preheating of BOG, and a more efficient and simpler BOG reliquefaction system is needed. More efficient and simpler preheating of BOG in a BOG reliquefaction system would provide a simpler system and thus reduce equipment costs. For example, more efficient preheating of BOG would make it possible to use a type of compressor in such an installation that is cheaper than those typically used for cryogenic gases. Furthermore, the overall system efficiency would be improved compared to existing technology solutions. Summary of the Invention

[0013] The purpose of this disclosure is to mitigate, reduce or eliminate one or more of the aforementioned defects and disadvantages in the prior art.

[0014] According to a first aspect, an evaporative gas (BOG) reliquefaction system is provided, the BOG reliquefaction system comprising at least one liquefied natural gas (LNG) tank, a BOG preheater, at least one BOG compressor having a BOG compressor cooler, and a BOG recondenser having a refrigeration cycle, wherein the BOG reliquefaction system includes a separate liquid circuit having circulation of a liquid phase heat transfer fluid (HTF) to preheat the BOG from the LNG tank before it enters the BOG compressor, the separate liquid circuit comprising: a liquid pump configured to pump the HTF; a BOG preheater configured to perform heat exchange between the BOG and the HTF; a BOG recondenser located downstream of the BOG preheater; and a liquid trimming heater located downstream of the BOG recondenser configured to heat the HTF.

[0015] According to some implementations, the separate liquid circuit includes a first bypass connection downstream of the BOG preheater for bypassing the HTF to the BOG recondenser.

[0016] According to some implementations, the separate liquid circuit includes a second bypass connection that leads from upstream of the BOG preheater to the top of the BOG recondenser for circulating through the BOG recondenser in the opposite direction.

[0017] According to some implementations, a first portion of the HTF is conveyed to the BOG preheater, and a second bypass connection is opened to convey a second portion of the HTF in the opposite direction from the top to the bottom of the BOG recondenser through the BOG recondenser, and a first bypass connection is opened to convey the first portion of the HTF from the BOG preheater and the second portion of the HTF from the BOG recondenser to the trimmer heater.

[0018] According to some implementations, the HTF circulates from the liquid pump through a separate liquid circuit via the BOG preheater, through the BOG recondenser, and through the liquid trimmer heater.

[0019] According to some implementations, a portion of the BOG is directed from the BOG compressor cooler to the BOG recondenser for condensation, and said portion of the BOG is returned from the BOG recondenser as liquefied gas to the liquefied natural gas (LNG) container.

[0020] According to some implementations, the refrigeration cycle connected to the BOG recondenser is configured to remove heat from that portion of the BOG condensed in the BOG recondenser.

[0021] According to some implementations, the HTF flowing from bottom to top through the BOG recondenser removes heat from that portion of the BOG condensed in the BOG recondenser.

[0022] According to some implementations, the separate liquid circuit includes a third bypass connection upstream of the BOG preheater for bypassing a portion of the HTF through the BOG preheater.

[0023] According to some implementations, a portion of the HTF is recirculated from the BOG preheater back upstream of the liquid pump.

[0024] According to some implementations, the cooling medium from the BOG compressor cooler is used in the liquid trimmer heater.

[0025] According to some implementations, at least one BOG compressor is a non-cryo compressor type.

[0026] According to some implementations, the refrigeration cycle in the BOG recondenser is a reverse Brayton nitrogen cycle.

[0027] According to some implementations, the separate liquid circuit includes an expansion tank.

[0028] According to a second aspect, a method is provided for reliquefying evaporative gas (BOG) in a reliquefaction system, the reliquefaction system including at least one liquefied natural gas (LNG) tank, a BOG preheater, at least one BOG compressor having a BOG compressor cooler, and a BOG recondenser having a refrigeration cycle, wherein: the method includes: pumping a heat transfer fluid (HTF) circulating in a separate liquid circuit including a liquid pump, the BOG preheater, the BOG recondenser, and a liquid trimming heater to the BOG preheater via the liquid pump; preheating the BOG by the HTF before transferring the BOG from the LNG tank into the BOG preheater to the BOG compressor; and transferring the cooled HTF leaving the BOG preheater in the separate liquid circuit to the liquid trimming heater.

[0029] According to some embodiments, the method includes: transferring HTF leaving the BOG preheater in a separate liquid circuit to the liquid trimming heater via a first bypass connection downstream of the BOG preheater.

[0030] According to some embodiments, the method includes: conveying a first portion of HTF to a BOG preheater, and conveying a second portion of HTF in the opposite direction from top to bottom through the BOG recondenser via a second bypass connection, and conveying the first portion of HTF from the BOG preheater and the second portion of HTF from the BOG recondenser to a liquid trimming heater via a first bypass connection.

[0031] According to some implementations, the method includes: transferring the HTF leaving the BOG preheater in a separate liquid circuit through the BOG recondenser before entering the liquid trimmer.

[0032] According to some implementations, the method includes: directing a portion of the BOG to a BOG recondenser for condensation, and returning the portion of the BOG as liquefied gas to a liquid natural gas (LNG) container from the BOG recondenser.

[0033] According to some embodiments, the method includes: removing heat from a portion of BOG condensed in the BOG recondenser in a refrigeration cycle connected to the BOG recondenser.

[0034] According to some implementations, the method includes removing additional heat from the portion of BOG condensed in the BOG recondenser by means of HTF flowing from bottom to top through the BOG recondenser.

[0035] According to some implementations, a portion of the HTF is connected to the BOG preheater via a third bypass upstream of the BOG preheater.

[0036] According to some implementations, the method includes: recirculating a portion of the HTF flowing out of the BOG preheater to upstream of the liquid pump.

[0037] According to some embodiments, the method includes using a cooling medium from a BOG compressor cooler as a heating medium in a liquid trimming heater.

[0038] According to some implementations, at least one BOG compressor is a non-cryo compressor type.

[0039] According to some implementations, the refrigeration cycle in the BOG recondenser is a reverse Brayton nitrogen cycle.

[0040] According to some implementations, the separate liquid circuit includes an expansion tank.

[0041] According to a third aspect, a method is provided for operating a boil-off gas (BOG) reliquefaction system, the BOG reliquefaction system comprising at least one liquefied natural gas (LNG) container, a BOG preheater, at least one BOG compressor having a BOG compressor cooler, and a BOG recondenser having a refrigeration cycle, wherein the method includes:

[0042] - The system is started by the following steps: a heat transfer fluid (HTF) circulating in a separate liquid loop, including a liquid pump, a BOG preheater, a BOG recondenser, and a liquid trimming heater, is pumped to the BOG preheater via the liquid pump; the BOG entering the BOG preheater from the LNG container is preheated by the HTF before being transferred to the BOG compressor; and the cooled HTF leaving the BOG preheater in the separate liquid loop is transferred to the liquid trimming heater via a first bypass connection downstream of the BOG preheater.

[0043] - To initiate the normal operating mode of the system, the following steps are performed: a first portion of the HTF is fed to the BOG preheater, and a second portion of the HTF is fed in the opposite direction from the top to the bottom of the BOG recondenser via a second bypass connection; and the first portion of the HTF from the BOG preheater and the second portion of the HTF from the BOG recondenser are fed to the liquid trimming heater via a first bypass connection.

[0044] - Operate the system in normal operating mode by the following steps: circulate the HTF (Heated Turbine Flow Unit) leaving the BOG preheater in the separate liquid circuit through the BOG recondenser from bottom to top before entering the liquid trimmer; guide a portion of the BOG leaving the BOG compressor cooler to the BOG recondenser for condensation, and return that portion of the BOG as liquefied gas to the LNG container from the BOG recondenser; remove heat from that portion of the BOG condensed in the BOG recondenser in the refrigeration cycle connected to the BOG recondenser; and remove additional heat from that portion of the BOG condensed in the BOG recondenser by the HTF flowing through the BOG recondenser.

[0045] The effects and features of the second and third aspects are largely similar to those described above in conjunction with the first aspect. The implementation methods mentioned in the first aspect are mostly compatible with the second and third aspects.

[0046] This disclosure will become apparent from the detailed description given below. The detailed description and specific examples disclose preferred embodiments of this disclosure by way of illustration only. Those skilled in the art will understand from the guidance of the detailed description that changes and modifications can be made within the scope of this disclosure.

[0047] Therefore, it should be understood that the disclosure herein is not limited to specific components of the described device or steps of the described method, as such devices and methods can be modified. It should also be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. It should be noted that, as used in the specification and appended claims, unless the context clearly indicates otherwise, the articles “a,” “an,” “the,” and “described” are intended to mean the presence of one or more elements. Thus, for example, a reference to “a unit” or “the unit” can include several devices, etc. Furthermore, the words “comprising,” “including,” “containing,” and similar terms do not exclude additional elements or steps.

[0048] The term "BOG preheater" refers to a heat exchanger, which can be any type of heat exchanger, such as a shell-and-tube heat exchanger, a plate / shell heat exchanger, or a plate / plate heat exchanger.

[0049] The term "BOG recondenser" refers to a heat exchanger, which can be any type of heat exchanger, such as a brazed plate / finned heat exchanger.

[0050] The term "liquid conditioning heater" refers to a heat exchanger, which can be any type of heat exchanger, such as a shell-and-tube heat exchanger, a plate / shell heat exchanger, or a plate / plate heat exchanger. Attached Figure Description

[0051] The above-mentioned and other objects, features and advantages of this disclosure will be more fully understood through the following illustrative and non-limiting detailed description of exemplary embodiments of this disclosure, taken in conjunction with the accompanying drawings.

[0052] Figure 1 A general process flow diagram of a BOG reliquefaction system with a refrigeration cycle in a BOG recondenser according to the present invention is shown.

[0053] Figure 2 The process flow diagram of the BOG reliquefaction system is shown, in which HTF is circulated only through the BOG preheater.

[0054] Figure 3 A flow chart of the BOG reliquefaction system with optimal start-up mode is shown.

[0055] Figure 4 The diagram shows the process flow of the BOG reliquefaction system with the optimization function enabled, where the HTF cycle passes through both the BOG preheater and the BOG recondenser.

[0056] Figure 5The process flow diagram of the BOG reliquefaction system, in which the HTF bypass BOG preheater is shown.

[0057] Figure 6 The diagram shows the process flow of the BOG reliquefaction system in which HTF is recirculated from the BOG preheater back to the liquid pump.

[0058] Similar parts in the accompanying drawings are given similar reference numerals. Detailed Implementation

[0059] This disclosure will now be described with reference to the accompanying drawings, in which preferred exemplary embodiments of the disclosure are shown. However, this disclosure may be implemented in other forms and should not be construed as limited to the embodiments disclosed herein. The disclosed embodiments are provided to fully communicate the scope of this disclosure to those skilled in the art.

[0060] Using this invention, reliquefaction capacity can be increased by reusing the "cold" obtained from preheated BOG. ​​Cooled HTF is used to increase the recondensation capacity of the BOG recondenser.

[0061] The first aspect of this disclosure is... Figure 1 As shown in the figure, Figure 1 The overall layout of a BOG reliquefaction system according to the present invention is shown, illustrating an evaporative gas (BOG) reliquefaction system comprising at least one liquefied natural gas (LNG) container 10, a BOG preheater 11, at least one BOG compressor 12 having a BOG compressor cooler 13, and a BOG recondenser 14 having a refrigeration cycle. The BOG reliquefaction system includes a separate liquid loop having circulation of a liquid phase heat transfer fluid (HTF) to preheat the BOG from the LNG container 10 before it enters the BOG compressor 12. The separate liquid loop includes: a liquid pump 15 configured to pump the HTF; a BOG preheater 11 configured for heat exchange between the BOG and the HTF; a BOG recondenser 14 located downstream of the BOG preheater 11; and a liquid trimmer 16 located downstream of the BOG recondenser 14 configured for heating the HTF.

[0062] The HTF can bypass the BOG recondenser 14 from downstream of the BOG preheater via a first bypass connection b1. This implementation occurs when it is not necessary for the BOG recondenser 14 to return excess BOG condensation to the LNG container 10. In this case, the HTF is not required / needed to provide optimization for the BOG recondenser 14. Bypassing the BOG recondenser ensures that the BOG recondenser 14 is not supplied with excess cold that would cause an imbalance in the BOG recondenser 14. When the cold HTF is not heated by the BOG recondenser 14, the liquid trimmer 16 heats the fluid to approximately +40°C. The HTF will always act as the preheating medium in the BOG preheater 11.

[0063] Valves 20 and 21 are piston valves with on / off functions. Valves 18, 19, 22, 23, and 24 are diaphragm valves, which can control and regulate the flow through the valve not only by opening / closing.

[0064] Figure 2 A BOG reliquefaction system is shown in which the HTF circulates only through the BOG preheater 11 and not through the BOG recondenser 14. This mode can be used when the optimized function of HTF circulation through the BOG recondenser 14 is not required. This is the first mode used after the LNG tank 10 is filled. The HTF bypasses the BOG recondenser 14 via a first bypass connection b1. Valve 18 in line 103 in the separate liquid loop is in the closed position, valve 19 in line 104 is in the open position, and valve 20 in line 105 is in the closed position to allow the HTF to bypass the BOG recondenser 14.

[0065] The separate liquid circuit may include a second bypass connection b2 that extends from upstream of the BOG preheater 11 via lines 108 and 105 to the top of the BOG recondenser 14 for circulation in the opposite direction (i.e., from top to bottom) through the BOG recondenser 14.

[0066] To activate the optimization function, the first portion of the HTF is conveyed to the BOG preheater 11, and the second bypass connection b2 is opened to convey the second portion of the HTF in the opposite direction from the top to the bottom of the BOG recondenser 14, and the first bypass connection b1 is opened to convey the first portion of the HTF from the BOG preheater 11 and the second portion of the HTF from the BOG recondenser to the trimmer heater 16.

[0067] When referring to the HTF as “first part”, “second part”, or “part”, those skilled in the art will understand how the HTF streams can be appropriately divided for different implementations, and that the exact number / ratio of streams will vary due to operating conditions as understood by those skilled in the art. Each part can range from 0% to 100% of the total HTF streams.

[0068] When the optimization function needs to be activated, warm HTF can be pumped from the top of the BOG recondenser 14 through lines 108 and 105, and cooled by the refrigerant in the refrigeration cycle within the BOG recondenser 14. During normal operation with the optimization function activated, the outlet temperature at the bottom will be the same as the temperature of the cold HTF entering the BOG recondenser 14 from the bottom via line 103. Thus, when the optimization function is activated, the piping will have a temperature of approximately -90°C, and the HTF will begin to flow into the BOG recondenser 14 from the bottom via line 103, thereby preventing the piping from heating the HTF and preventing overheating of the HTF as it enters the BOG recondenser 14.

[0069] Heat is drawn into the BOG recondenser 14 by introducing warm (typically around +40°C) HTF from the top of the BOG recondenser 14 via line 105. This additional heat can be used as a heat source in the absence of BOG reliquefaction and warm BOG entering the BOG recondenser 14, thereby ensuring that the BOG recondenser 14 maintains stable operation.

[0070] When the entire BOG recondenser 14 is cooled, the additional heat from the HTF introduced into the top of the BOG recondenser 14 via line 105 can also be used to reheat the entire BOG recondenser 14 after a trip.

[0071] Figure 3A BOG reliquefaction system with an optimized start-up mode featuring a separate liquid loop is shown. Valve 20 is closed to ensure the upper right corner of the loop is closed. Liquid pump 15 drives the HTF not only via line 108 through the optimized start-up connection but also via line 101 through the BOG preheater 11. Valve 20 in line 105 is in the closed position and valve 21 in line 108 is in the open position to allow a portion of the HTF flow from pump 15 via lines 108 and 105 to exit the BOG recondenser 14, and via line 103 with valve 18 in the open position, and via line 104 with valve 19 in the open position. Valve 18 is used to control a portion of the backflow through the BOG recondenser during optimized start-up. Valve 21 is used to allow a portion of the flow into line 108, while the remaining HTF flow from pump 15 via line 101 enters the BOG preheater 11.

[0072] Figure 4 A BOG reliquefaction system in normal operating mode is shown, wherein the HTF circulates through a separate liquid loop—from the liquid pump (15) through the BOG preheater (11), through the BOG recondenser (14), and through the liquid trimmer (16), without using bypass and / or recirculation. In the separate liquid loop, valve 18 in line 103 is in the open position, valve 19 in line 104 is in the closed position, and valve 20 in line 105 is in the open position, such that the HTF circulates through the BOG recondenser 14 after exiting the BOG preheater 11 in line 103 and before entering the liquid trimmer 16 via line 106.

[0073] The normal operating mode is the mode using the optimization function. The optimization function can also be called the optimizer function, optimizer / optimization mode, or simply optimizer. In the optimization function mode, the HTF enters the BOG recondenser 14 from the bottom (also called upstream) and exits from the top (also called downstream). The HTF can flow through the BOG recondenser 14 in one or more connected flow channels.

[0074] Under normal operation, the individual liquid circuit includes the following equipment in the normal flow sequence: liquid pump 15, BOG preheater 11, BOG recondenser 14, liquid trimmer 16, and optional expansion tank 17.

[0075] Under normal operation, liquid pump 15 pumps HTF, typically at +40°C, to BOG preheater 11, where the HTF heats the BOG. ​​Simultaneously, the HTF is typically cooled to -90°C. The HTF is introduced into BOG recondenser 14 from the bottom (upstream). In BOG recondenser 14, the HTF acts as a cooling medium for the BOG and exits from the top (downstream) of BOG recondenser 14 at approximately +35°C.

[0076] HTF can also bypass BOG recondenser 14 via bypass connection b1 in normal operating mode with optimized functions. Valves 18 and 19 can control the amount of HTF bypassing BOG recondenser 14 to control the temperature of the top outlet of BOG recondenser 14 and thus control the HTF temperature distribution so that the temperature difference across BOG recondenser 14 does not exceed the allowable limit.

[0077] A liquid trimming heater 16 is located in line 106, with line 105 connecting to the BOG recondenser 14 and line 104 bypassing the BOG recondenser both connected to line 106. The trimming heater 16 is located upstream of the liquid pump 15. The liquid trimming heater 16 uses a heating medium, such as water, to heat the HTF returned to the liquid pump 15. The heating medium flowing into the liquid trimming heater 16 is directed through a BOG compressor cooler 13 connected to the BOG compressor 12 to cool the BOG. ​​One or more of the following steps may be present: the BOG compressor and cooler in series perform the required BOG compression for the gas consumer on board. For gas or dual-fuel engines, the BOG pressure can be, for example, 7 bar to 12 bar, or pre-compression may be performed if the engine uses high-compression gas up to 300 bar, thus requiring a separate compression system. The heating medium flowing into the liquid trimming heater 16 may be connected to one or more compressor coolers, depending on the appropriate heating capacity to be achieved by the liquid trimming heater 16. One or more compression stages can be connected after the reliquefaction system.

[0078] During normal operation as described above, the individual liquid circuit with HTF simultaneously provides two different functions:

[0079] Preheating the BOG upstream of BOG compressor 12 to approximately -30°C enables the use of a non-cryogenic BOG compressor. A non-cryogenic BOG compressor means a compressor that does not need to withstand temperatures below -150°C. This reduces the investment cost for the system.

[0080] Using a liquid heat transfer fluid loop intended for preheating, a typical system can handle BOG flows ranging from approximately 1750 kg / h to approximately 5000 kg / h. The flow rate of the heat transfer fluid can be adjusted according to the BOG flow to maintain the temperature of the BOG flow at the preheater outlet at approximately –30°C.

[0081] In existing systems using N2 for preheating, and when N2 preheating is unavailable or insufficient, an additional heat source is required, which is typically done in parallel with an additional BOG preheater. The system according to the invention has only one BOG preheater, which simplifies the system and makes switching between modes much easier. A gas / liquid and a liquid / liquid exchanger will also be more compact compared to at least one gas / gas and at least one gas / liquid exchanger.

[0082] The optimized function, achieved through the removal of additional heat in the BOG recondenser 14, contributes to higher cooling capacity for reliquefaction, thereby improving system efficiency. Compared to conventional nitrogen-based preheating and recondensing systems for BOG, the system according to the invention provides a more efficient and simpler system. By transferring excess “cold” from the BOG preheater 11 to the BOG recondenser 14 via the HTF, the capacity required for the refrigeration cycle is reduced. Therefore, using HTF to optimize reliquefaction can increase reliquefaction capacity by up to approximately 67% compared to operation without HTF in the BOG recondenser 14.

[0083] Once started, the optimization function will be most frequently used as long as the reliquefaction system is running. And, if necessary, some HTF will be introduced into the BOG recondenser bypass. Therefore, the optimized mode is often in use throughout the LNG carrier's voyage. However, the optimization function can be stopped if the reliquefaction system is set to standby or completely shut down. This can happen, for example, in cases of pressure drop in the LNG tank, because fuel consumption by the main engine and other consumables exceeds BOG generation (in fact, all LNG carriers have engines that use evaporative gas for ship propulsion). The operating time of the reliquefaction system can vary from vessel to vessel.

[0084] The separate liquid HTF loop, with its technical details disclosed herein, ensures a flexibility that is not possible with conventional preheater systems in existing BOG reliquefaction facilities.

[0085] After leaving the BOG compressor cooler 13, a portion of the BOG can be directed via line 111 to the BOG recondenser 14 for condensation, and said portion of the BOG, as liquefied gas, returns from the BOG recondenser 14 to the LNG container 10 via line 112, in which valve 24 is in the open position. The BOG directed to the BOG recondenser 14 can range from 0% to 100% of the total BOG leaving the BOG compressor cooler 13. The remaining portion of the BOG leaving the BOG compressor cooler 13 (not directed to the BOG recondenser 14 for recondensation) is directed out of the reliquefaction system to the fuel consumer.

[0086] The refrigeration cycle connected to the BOG recondenser 14 is configured to remove heat from this portion of the BOG condensed in the BOG recondenser 14.

[0087] When the optimization function is running, the HTF flowing from bottom to top through the BOG recondenser 14 removes heat from that portion of the BOG condensed in the BOG recondenser 14.

[0088] A separate liquid circuit may include a third bypass connection b3 upstream of the BOG preheater 11 for bypassing a portion of the HTF through the BOG preheater 11. The HTF may bypass the BOG preheater 11 to control the HTF temperature entering the BOG recondenser 14. Figure 5 A BOG reliquefaction system in which the HTF bypasses the BOG preheater 11 is shown. Valve 22 in line 109, parallel to the BOG preheater 11, is in the open position to allow the HTF to bypass the BOG preheater 11. Figure 5 As shown, when the third bypass connection b3 is opened to allow a portion of the HTF to bypass the BOG preheater 11, the bypass connection b1 can be opened to control the flow and / or temperature in the system components. The bypass connection b1 can also be closed when the bypass connection b3 is open.

[0089] HTF can be recirculated downstream of BOG preheater 11 to control the temperature of the HTF in BOG recondenser 14. By recirculating a large amount of cold HTF back to liquid pump 15, a large HTF flow rate is ensured through BOG preheater 11, and the outflow from BOG preheater 11 is maintained at the desired temperature. Figure 6 A BOG reliquefaction system in which HTF is recirculated is shown. In line 110, valve 23 downstream of the BOG preheater 11 is in the open position for recirculating HTF back to the liquid pump 15. Figure 6As shown, while a portion of the HTF is recirculated from downstream of the BOG preheater 11 back to the liquid pump 15, bypass connection b1 can be opened to control the flow rate and / or temperature in the system components. During the recirculation of the HTF, bypass connection b1 can also be closed.

[0090] The bypass connection b3 bypasses the BOG preheater 11, recirculates the HTF back to the liquid pump 15, and the bypass connection b1 bypasses the BOG recondenser, all of which can be used simultaneously.

[0091] Valve 22 and / or valve 23 can be used to control the HTF temperature of BOG recondenser 14 by respectively controlling the amount of HTF bypassing BOG preheater 11 and the amount of HTF recirculated from downstream of BOG preheater 11 back to liquid pump 15.

[0092] The heat transfer fluid in the individual liquid loop of the BOG reliquefaction system according to the invention is a liquid-phase heat transfer fluid suitable for transferring heat downwards to a cryogenic temperature. The HTF can be a synthetic liquid-phase fluid. Examples of suitable heat transfer fluids are cryogenically synthesized heat transfer fluids. Examples of suitable heat transfer fluids are heat transfer fluids based on, for example, hydrocarbons or silicones. Examples of suitable heat transfer fluid compositions are mixtures of methylcyclohexane and trimethylpentane, such as commercially available ones produced by Eastman. Heat transfer fluid. Heat transfer fluids are synthetic liquid-phase fluids used in single-fluid heating and cooling systems for extremely low-temperature applications, such as between -115°C and +175°C. Other suitable commercially available heat transfer fluids include Caltherm UBT (silicone) manufactured by Caldic for use between -100°C and +260°C, Dynalene MW (hydrocarbon) manufactured by Dynalene for use between -112°C and +163°C, and Fragoltherm X-T9-A (silicone) manufactured by Fragol for use between -112°C and +200°C.

[0093] The compression of BOG is performed using a cooling medium in at least one BOG compressor 12, which has at least one BOG compressor cooler 13. Several BOG compressors and coolers may be arranged in series. Preferably, the compression of BOG is performed in multiple stages, either during or after cooling with a cooling medium. The at least one BOG compressor 12 may be a non-cryo compressor type. Examples of suitable compressors are positive displacement compressors such as screw compressors, or dynamic compressors such as centrifugal compressors.

[0094] The heated cooling medium from the BOG compressor leaving the BOG compressor cooler 13 can be reused as the heating medium for the liquid trimming heater 16. Alternatively, it can be supplied separately to both the BOG compressor cooler 13 and the liquid trimming heater 16. The cooling medium is typically water, a water-glycol mixture, or the like. When the heat added to the HTF in the BOG recondenser 14 is insufficient, the system according to the invention can be used without the liquid trimming heater 16, provided that the BOG can also be heated using another heat exchanger downstream of the BOG preheater 11. However, the preferred solution is the liquid trimming heater 16.

[0095] A separate liquid circuit may include an expansion tank 17 connected to the HTF cycle. The expansion tank 17 may be of any type, and the purpose of the expansion tank 17 is to allow the HTF to expand.

[0096] The BOG leaving LNG container 10 is typically at a temperature of about -140°C to about -110°C when it enters BOG preheater 11 via line 100. The BOG leaves BOG preheater 11 at a temperature of about -30°C via line 102 and then enters at least one BOG compressor 12 having at least one BOG compressor cooler 13. After compression, the BOG leaves at least one BOG compressor 12 having at least one BOG compressor cooler 13 at a temperature of about +40°C and is then carried via line 111 to a fuel consumer and / or BOG recondenser 14 for reliquefaction. The BOG enters at a temperature of typically around +40°C, is cooled and liquefied in BOG recondenser 14, and returns to LNG container 10 at a temperature of at least about -163°C via line 112, in which valve 24 is in the open position, to become liquid without pressurization.

[0097] The refrigeration cycle in the BOG recondenser 14 can be a nitrogen cycle. The refrigerant will then be N2. A reverse Brayton nitrogen cycle can be used to cool the BOG recondenser. However, other refrigeration cycles can also be used within the scope of this invention. Figure 1 The overall process flow diagram illustrates the details of a basic reverse Brayton cycle with opposing N2 flows in the BOG recondenser 14. The cycle is shown as having one compressor stage, including compressor 25 and subsequent cooler 26, but more than one compressor stage can be used. In the refrigeration cycle, N2 flows in the cycle through line 113, through compressor 25 and subsequent cooler 26, then through the BOG recondenser 14, and after leaving the BOG recondenser 14, again flows in the opposite direction through expander 27 before flowing through the BOG recondenser 14.

[0098] BOG reliquefaction systems can be installed on ships, offshore facilities, or onshore facilities such as LNG receiving terminals.

[0099] A second aspect of this disclosure discloses a method for reliquefying evaporative gas (BOG) in a reliquefaction system comprising at least one liquefied natural gas (LNG) tank 10, a BOG preheater 11, at least one BOG compressor 12 having a BOG compressor cooler 13, and a BOG recondenser 14 having a refrigeration cycle, wherein the method comprises: pumping a heat transfer fluid (HTF) circulating in a separate liquid circuit including a liquid pump 15, the BOG preheater 11, the BOG recondenser 14, and a liquid trimming heater 16 into the BOG preheater 11 via the liquid pump 15; preheating the BOG by the HTF before transferring the BOG from the LNG tank 10 into the BOG preheater 11 to the BOG compressor 12; and transferring the cooled HTF leaving the separate liquid circuit of the BOG preheater 11 to the liquid trimming heater 16.

[0100] Figure 2 An embodiment of the method shown includes pumping HTF circulating in a separate liquid loop to BOG preheater 11 via liquid pump 15 through line 101; preheating the BOG in BOG preheater 11 with HTF before transferring the BOG from LNG container 10 to BOG compressor 12 via line 102; and directly transferring the cooled HTF leaving BOG preheater 11 to liquid trimmer 16 via line 103 where valve 18 is in the closed position, line 104 where valve 19 is in the open position, and line 106.

[0101] Figure 3 An embodiment of the method shown includes conveying a first portion of the HTF to the BOG preheater 11, and conveying a second portion of the HTF in the opposite direction from the top to the bottom of the BOG recondenser 14 via a second bypass connection b2, and conveying the first portion of the HTF from the BOG preheater 11 and the second portion of the HTF from the BOG recondenser 14 to the trimmer heater 16 via a first bypass connection b1. The optimizer function of the individual liquid loop is activated by the following steps: conveying the HTF via line 108, in which valve 21 is in the open position, to the top of the BOG recondenser 14 via line 105, in which valve 20 is in the closed position; and returning the HTF from the bottom of the BOG recondenser 14 via line 103, in which valve 18 is in the open position, to the liquid trimmer heater 16 via line 104, in which valve 19 is in the open position.

[0102] Figure 4 An embodiment of the method for transferring HTF from the BOG preheater 11 in a separate liquid circuit through the BOG recondenser 14 is shown: entering from the bottom (upstream) via line 103 with valve 18 in the open position and valve 19 in line 104 in the closed position, and exiting from the top (downstream) of the BOG recondenser 14 via line 105, where valve 20 is in the open position, before entering the liquid trimmer 16 via line 106 and exiting the liquid trimmer 16 via line 107. As described above for the system, this is the normal operating mode with optimized features in use.

[0103] After the BOG leaves the BOG compressor cooler 13, a portion of the BOG can flow via line 111 to the BOG recondenser 14 for condensation, and from the BOG recondenser 14 via line 112 in which valve 24 is in the open position, the portion of the BOG is returned as liquefied gas to the LNG container 10.

[0104] The refrigeration cycle connected to the BOG recondenser 14 removes heat from that portion of the BOG condensed in the BOG recondenser 14.

[0105] When the optimization function is in use, the extra heat from that portion of the BOG condensed in the BOG recondenser 14 is removed by the HTF flowing from bottom to top through the BOG recondenser 14.

[0106] Figure 5 An embodiment of the method is shown in which the third bypass connection b3 is opened to allow a portion of the HTF to bypass the BOG preheater 11. The HTF flows via line 109 in which valve 22 is in the open position to bypass the BOG preheater 11. Figure 5 As shown, while the third bypass connection b3 is open to allow a portion of the HTF to bypass the BOG preheater 11, the bypass connection b1 can be opened to control the flow rate and / or temperature in the system components. The bypass connection b1 can also be closed when the bypass connection b3 is open.

[0107] Figure 6 An embodiment of the following method is illustrated, which includes: delivering HTF via line 110 after BOG preheater 11, wherein valve 23 is in the open position, to recirculate the HTF upstream of liquid pump 15. Figure 6 As shown, since a portion of the HTF is recirculated from downstream of the BOG preheater 11 back to the liquid pump 15, the bypass connection b1 can be open to control the flow rate and / or temperature in the system components. The bypass connection b1 can also be closed during the HTF recirculation.

[0108] The bypass connection b3 bypasses the BOG preheater 11, recirculates the HTF back to the liquid pump 15, and the bypass connection b1 bypasses the BOG recondenser, all of which can be used simultaneously.

[0109] Valve 22 and / or valve 23 can be used to control the HTF temperature of BOG recondenser 14 by respectively controlling the amount of HTF bypassing BOG preheater 11 and the amount of HTF recirculated from downstream of BOG preheater 11 back to liquid pump 15.

[0110] As described above for the reliquefaction system, the heat transfer fluid in the individual liquid loop of the method for reliquefaction according to the invention is a liquid-phase heat transfer fluid suitable for downward heat transfer to a low temperature. The HTF can be a synthetic liquid-phase fluid as described above for the system according to the invention.

[0111] As described above for the reliquefaction system, in an embodiment of the method according to the invention, the compression of BOG is performed in at least one BOG compressor 12 having at least one BOG compressor cooler 13 and a cooling medium. Preferably, the compression of BOG is performed in multiple stages having intermediate cooling and subsequent cooling using a cooling medium. The at least one BOG compressor 12 may be a non-cryogenic compressor type as described above for the system according to the invention.

[0112] As described above for the reliquefaction system, in embodiments of the method according to the invention, the heating / cooling medium exiting the BOG compressor cooler 13 from the BOG compression can be reused as the heating medium for the liquid trimming heater 16. Alternatively, the BOG compressor cooler 13 and the liquid trimming heater 16 can also be supplied separately. The cooling medium is typically water, a water-glycol mixture, or the like.

[0113] As described above for the reliquefaction system, in the method according to the invention, when the BOG leaving the LNG container 10 enters the BOG preheater 11 via line 100, it is typically at a temperature of about -140°C to about -110°C. The BOG leaves the BOG preheater 11 via line 102 at a temperature of about -30°C and then enters at least one BOG compressor 12 having at least one BOG compressor cooler 13. After compression, the BOG leaves the at least one BOG compressor 12 having at least one BOG compressor cooler 13 at a temperature of approximately +40°C and is then carried via line 111 to the fuel consumer and / or BOG recondenser 14 for reliquefaction. The BOG, typically entering at about +40°C, being cooled and liquefied in the BOG recondenser 14, and returning to the LNG container 10 via line 112, where valve 24 is in the open position, at a temperature of at least about -163°C, becomes liquid without pressurization.

[0114] As described above for the reliquefaction system, in the method according to the invention, the refrigeration cycle in the BOG recondenser 14 can be a nitrogen cycle. However, other refrigeration cycles can also be used within the scope of the invention. Advantageously, a reverse Brayton nitrogen cycle can be used.

[0115] As described above for the reliquefaction system, in the method according to the invention, a separate liquid circuit may include an expansion tank 17.

[0116] A third aspect of the invention relates to a method for operating a boil-off gas (BOG) reliquefaction system, the system comprising at least one liquefied natural gas (LNG) tank 10, a BOG preheater 11, at least one BOG compressor 12 having a BOG compressor cooler 13, and a BOG recondenser 14 having a refrigeration cycle. The method comprises: firstly starting the system by the following steps: pumping a heat transfer fluid (HTF) circulating in a separate liquid circuit including a liquid pump 15, the BOG preheater 11, the BOG recondenser 14, and a liquid trimming heater 16 into the BOG preheater 11 via the liquid pump 15; preheating the BOG by the HTF before transferring the BOG from the LNG tank 10 into the BOG preheater 11 to the BOG compressor 12; and transferring the cooled HTF leaving the separate liquid circuit of the BOG preheater 11 to the liquid trimming heater 16 via a first bypass connection b1 downstream of the BOG preheater 11. After the system has been started, the normal operating mode of the system is started by the following steps: the first part of the HTF is transferred to the BOG preheater 11 and the second part of the HTF is transferred in the opposite direction from the top to the bottom of the BOG recondenser 14 through the second bypass connection b2; and the first part of the HTF from the BOG preheater 11 and the second part of the HTF from the BOG recondenser 14 are transferred to the trimmer 16 through the first bypass connection b1. When the normal operating mode has been initiated, the system is operated in normal operating mode by the following steps: circulating the HTF leaving the BOG preheater 11 in the separate liquid circuit through the BOG recondenser 14 from bottom to top before entering the liquid trimmer 16; guiding a portion of the BOG leaving the BOG compressor cooler 13 to the BOG recondenser 14 for condensation, and returning said portion of the BOG as liquefied gas to the LNG container 10 from the BOG recondenser 14; removing heat from that portion of the BOG condensed in the BOG recondenser 14 in the refrigeration cycle connected to the BOG recondenser 14; and removing additional heat from that portion of the BOG condensed in the BOG recondenser 14 by the HTF flowing through the BOG recondenser 14.

[0117] Those skilled in the art will recognize that this disclosure is not limited to the preferred embodiments described above. They will also recognize that modifications and variations can be made within the scope of the appended claims. For example, additionally, by studying the drawings, this disclosure, and the appended claims, those skilled in the art can understand and implement various variations of the disclosed embodiments in practicing the claimed disclosure.

Claims

1. A BOG reliquefaction system, said BOG reliquefaction system comprising at least one liquefied natural gas (LNG) tank (10), a BOG preheater (11), at least one BOG compressor (12) having a BOG compressor cooler (13), and a BOG recondenser (14) having a refrigeration cycle, characterized in that, The BOG reliquefaction system includes a separate liquid loop with circulation of a liquid phase heat transfer fluid (HTF) to preheat the BOG from the LNG container (10) before it enters the BOG compressor (12). The separate liquid loop includes: a liquid pump (15) configured to pump the HTF; a BOG preheater (11) configured to perform heat exchange between the BOG and the HTF; a BOG recondenser (14) located downstream of the BOG preheater (11); and a liquid trimming heater (16) located downstream of the BOG recondenser (14) configured to heat the HTF, wherein a cooling medium from the BOG compressor cooler (13) is used in the liquid trimming heater (16).

2. The evaporative gas (BOG) reliquefaction system according to claim 1, characterized in that, The separate liquid circuit includes a first bypass connection (b1) downstream of the BOG preheater (11) for bypassing the HTF to the BOG recondenser (14).

3. The evaporative gas (BOG) reliquefaction system according to claim 2, characterized in that, The separate liquid circuit includes a second bypass connection (b2) that leads from upstream of the BOG preheater (11) to the top of the BOG recondenser (14) for circulating through the BOG recondenser (14) in the opposite direction.

4. The BOG reliquefaction system according to claim 3, characterized in that, The first portion of the HTF is transferred to the BOG preheater (11), and the second bypass connection (b2) is opened to transfer the second portion of the HTF from the top to the bottom of the BOG recondenser (14) in the opposite direction, and the first bypass connection (b1) is opened to transfer the first portion of the HTF from the BOG preheater (11) and the second portion of the HTF from the BOG recondenser (14) to the liquid trimming heater (16).

5. The BOG reliquefaction system according to claim 1, characterized in that, The HTF is circulated from the liquid pump (15) through the BOG preheater (11), through the BOG recondenser (14), and through the liquid trimmer (16) via the separate liquid circuit.

6. The evaporative gas (BOG) reliquefaction system according to claim 1, characterized in that, A portion of the BOG is directed from the BOG compressor cooler (13) to the BOG recondenser (14) for condensation, and the portion of the BOG is returned from the BOG recondenser (14) as liquefied gas to the LNG container (10).

7. The evaporative gas (BOG) reliquefaction system according to claim 6, characterized in that, The refrigeration cycle connected to the BOG recondenser (14) is configured to remove heat from the portion of the BOG condensed in the BOG recondenser (14).

8. The BOG reliquefaction system according to claim 6, characterized in that, The HTF flowing from bottom to top through the BOG recondenser (14) removes heat from the portion of the BOG condensed in the BOG recondenser (14).

9. The evaporative gas (BOG) reliquefaction system according to any one of claims 1 to 8, characterized in that, The separate liquid circuit includes a third bypass connection (b3) upstream of the BOG preheater (11) for bypassing a portion of the HTF through the BOG preheater (11).

10. The evaporated gas (BOG) reliquefaction system according to any one of claims 1 to 8, characterized in that, A portion of the HTF is recirculated from the BOG preheater (11) back upstream of the liquid pump (15).

11. The evaporative gas (BOG) reliquefaction system according to any one of claims 1 to 8, characterized in that, The at least one BOG compressor (12) is a non-cryo compressor type.

12. The evaporated gas (BOG) reliquefaction system according to any one of claims 1 to 8, characterized in that, The refrigeration cycle in the BOG recondenser (14) is a reverse Brayton nitrogen cycle.

13. The evaporated gas (BOG) reliquefaction system according to any one of claims 1 to 8, characterized in that, The separate liquid circuit includes an expansion tank (17).

14. A method for reliquefying evaporative gas (BOG) in a reliquefaction system, said reliquefaction system comprising at least one liquefied natural gas (LNG) container (10), a BOG preheater (11), at least one BOG compressor (12) having a BOG compressor cooler (13), and a BOG recondenser (14) having a refrigeration cycle, characterized in that, The method includes: pumping a heat transfer fluid (HTF) circulating in a separate liquid circuit comprising a liquid pump (15), the BOG preheater (11), the BOG recondenser (14), and a liquid trimming heater (16) into the BOG preheater (11) via the liquid pump (15); preheating the BOG by the HTF before transferring the BOG from the LNG container (10) into the BOG preheater (11) to the BOG compressor (12); and transferring the cooled HTF leaving the BOG preheater (11) in the separate liquid circuit to the liquid trimming heater (16), wherein a cooling medium from the BOG compressor cooler (13) is used as the heating medium in the liquid trimming heater (16).

15. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to claim 14, characterized in that, The HTF exiting the BOG preheater (11) in the separate liquid circuit is transferred to the liquid trimmer (16) via a first bypass connection (b1) downstream of the BOG preheater (11).

16. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to claim 15, characterized in that, The first portion of the HTF is transferred to the BOG preheater (11), and the second portion of the HTF is transferred in the opposite direction from the top to the bottom of the BOG recondenser (14) via the second bypass connection (b2), and the first portion of the HTF from the BOG preheater (11) and the second portion of the HTF from the BOG recondenser (14) are transferred to the liquid trimming heater (16) via the first bypass connection (b1).

17. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to claim 14, characterized in that, The HTF leaving the BOG preheater (11) in the separate liquid circuit is transferred through the BOG recondenser (14) before entering the liquid trimmer (16).

18. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to claim 14, characterized in that, A portion of the BOG is directed to the BOG recondenser (14) for condensation, and the portion of the BOG is returned as liquefied gas from the BOG recondenser (14) to the LNG container (10).

19. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to claim 18, characterized in that, In the refrigeration cycle connected to the BOG recondenser (14), heat is removed from the portion of the BOG condensed in the BOG recondenser (14).

20. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to claim 19, characterized in that, Additional heat is removed from the portion of the BOG condensed in the BOG recondenser (14) by the HTF flowing through the BOG recondenser (14).

21. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to any one of claims 14 to 20, characterized in that, A portion of the HTF bypasses the BOG preheater (11) via a third bypass connection (b3) upstream of the BOG preheater (11).

22. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to any one of claims 14 to 20, characterized in that, A portion of the HTF flowing out of the BOG preheater (11) is recirculated upstream of the liquid pump (15).

23. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to any one of claims 14 to 20, characterized in that, The at least one BOG compressor (12) is a non-cryo compressor type.

24. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to any one of claims 14 to 20, characterized in that, The refrigeration cycle in the BOG recondenser (14) is a reverse Brayton nitrogen cycle.

25. The method for reliquefying evaporated gas (BOG) in a reliquefaction system according to any one of claims 14 to 20, characterized in that, The separate liquid circuit includes an expansion tank (17).

26. A method for operating a boil-off gas (BOG) reliquefaction system, said BOG reliquefaction system comprising at least one liquefied natural gas (LNG) container (10), a BOG preheater (11), at least one BOG compressor (12) having a BOG compressor cooler (13), and a BOG recondenser (14) having a refrigeration cycle, characterized in that, The method includes: The system is started by the following steps: a heat transfer fluid (HTF) circulating in a separate liquid circuit including a liquid pump (15), the BOG preheater (11), the BOG recondenser (14), and a liquid trimmer (16) is pumped to the BOG preheater (11) via the liquid pump (15); the BOG is preheated by the HTF before being transferred from the LNG container (10) into the BOG preheater (11) to the BOG compressor (12); and the cooled HTF leaving the BOG preheater (11) in the separate liquid circuit is transferred to the liquid trimmer (16) via a first bypass connection (b1) downstream of the BOG preheater (11). The normal operating mode of the system is initiated by the following steps: a first portion of the HTF is transferred to the BOG preheater (11), and a second portion of the HTF is transferred in the opposite direction from top to bottom through the BOG recondenser (14) via a second bypass connection (b2); and the first portion of the HTF from the BOG preheater (11) and the second portion of the HTF from the BOG recondenser (14) are transferred to the liquid trimming heater (16) via the first bypass connection (b1). The system is operated in normal operating mode by the following steps: the HTF exiting the BOG preheater (11) in the separate liquid circuit is circulated from bottom to top through the BOG recondenser (14) before entering the liquid trimmer (16); a portion of the BOG exiting the BOG compressor cooler (13) is directed to the BOG recondenser (14) for condensation, and the portion of the BOG is returned as liquefied gas from the BOG recondenser (14) to the LNG container (10); heat is removed from the portion of the BOG condensed in the BOG recondenser (14) in the refrigeration cycle connected to the BOG recondenser (14); and additional heat is removed from the portion of the BOG condensed in the BOG recondenser (14) by the HTF flowing through the BOG recondenser (14).

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