CN115885145A

CN115885145A - Boil-off gas reliquefaction system, method for reliquefying boil-off gas in reliquefaction system, and method for operating boil-off gas reliquefaction system

Info

Publication number: CN115885145A
Application number: CN202080102751.8A
Authority: CN
Inventors: 谢蒂尔·莱努姆; 约恩·伊费森; 英格博格·达尔
Original assignee: Wartsila Oil and Gas Systems AS
Current assignee: Wartsila Oil and Gas Systems AS
Priority date: 2020-05-14
Filing date: 2020-05-14
Publication date: 2023-03-31
Also published as: EP4150273A1; WO2021230751A1; KR20230040948A

Abstract

The present disclosure relates to a boil-off gas (BOG) reliquefaction system comprising at least one Liquefied Natural Gas (LNG) cargo tank (10), a BOG preheater (11), at least one BOG compressor (12) with a BOG compressor cooler (13), a BOG recondenser (14) with a refrigeration cycle, wherein the BOG reliquefaction system comprises a separate liquid circuit with circulation of a liquid phase Heat Transfer Fluid (HTF) for preheating BOG from the LNG cargo tank (10) before entering the BOG compressor (12), the separate liquid circuit comprising: a liquid pump (15) configured to pump HTF; a BOG pre-heater (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 trim heater (16) located downstream of the BOG recondenser (14) configured to heat the HTF. The present disclosure also relates to methods for reliquefying boil-off gas (BOG) in a reliquefaction system and methods for operating a boil-off gas (BOG) reliquefaction system.

Description

Boil-off gas reliquefaction system, method for reliquefying boil-off gas in reliquefaction system, and method for operating boil-off gas reliquefaction system

Technical Field

The present invention relates to a Liquefied Natural Gas (LNG) boil-off gas (BOG) reliquefaction system, a method for reliquefying BOG, and a method for operating a BOG reliquefaction system. More specifically, the present disclosure relates to a boil-off gas (BOG) reliquefaction system, a method for reliquefying boil-off gas (BOG) in a reliquefaction system and a method for operating a boil-off gas (BOG) reliquefaction system as defined in the preambles of claim 1, claim 15 and claim 28.

Background

A common technique for transporting natural gas from a production site for the natural gas is to liquefy the natural gas at or near the site and transport the LNG to market in specially designed storage tanks (typically placed on overseas marine vessels).

LNG is a mixture of light hydrocarbons with methane as the major component and nitrogen as an inert component, and small amounts of ethane, propane, butane, and pentane may be present. Depending on the exact composition of the LNG, the boiling point of LNG is about-162 ℃ to-161 ℃ at atmospheric pressure, and loading, transporting, and unloading are typically performed at this temperature range. This requires special materials, insulation and handling equipment to handle the low temperatures and the boil-off vapors. Due to heat leakage, the surface of the cargo (LNG) is constantly boiling, producing vaporized natural gas, the so-called Boil Off Gas (BOG), from the LNG, mainly methane.

Facilities for continuously reliquefying the BOG are well known. The installation of a BOG reliquefaction system on a Liquefied Natural Gas (LNG) vehicle with a dual fuel engine enables the ship operator more flexibility to switch between fuels to take advantage of the price difference between LNG and heavy fuel oil. The prior art propulsion systems are efficient and not all BOG can be used in the engine. In addition, slow voyage and vessel berthing operations often result in an excessive amount of BOG. Instead of burning this gas in a gas combustion unit, the gas may be re-liquefied and returned to the cargo tank.

Reliquefying BOG on board LNG carriers causes increased cargo delivery and allows owners and operators to select the best propulsion system and operating mode. For example, advantages are flexible fuel systems, optimized operating costs and increased cargo capacity for transport.

Typically, reliquefaction systems are used to control the cargo box pressure by liquefying BOG. They have the ability to handle all of the BOG (100% capacity) or just excess BOG that is not combusted in the engine (partial liquefaction).

The BOG may vary in composition, flow, temperature, and pressure. These fluctuations require a system that can handle the process conditions fed to the system.

The existing BOG reliquefaction system is based on the counter-nitrogen Brayton cycle re-refrigeration technology. This means that there is a closed nitrogen cycle in the system for extracting heat from the BOG. Nitrogen (N) ₂ ) Used as a refrigerant for the purpose of controlling tank pressure by cooling and reliquefying the pressurized BOG. The reliquefied BOG is then returned to the tank.

WO 2009/136793A1 discloses a gas supply system for a dual fuel or gas engine integrated with a BOG reliquefaction plant. The two systems for BOG reliquefaction and gas supply are "stand-alone" components. The cold load is removed from the LNG by an external heat source and is not used.

WO 2011/078689A1 discloses a gas supply system for a dual fuel or gas engine integrated with a BOG reliquefaction device, wherein the available cooling load in LNG is used to cool and condense BOG in a plant in which BOG or condensate of BOG and LNG are heat exchanged with each other. LNG is heated by using the available "warm" load from the compressor in the reliquefaction system as cooling water.

The BOG leaves the LNG tank at a temperature typically between-140 ℃ and-110 ℃ and is preheated before entering the BOG compressor. When it comes to selecting a compressor, the temperature at the suction of the compressor is very important. Higher temperatures at the compressor inlet can be achieved by preheating the BOGAnd (4) degree. Existing systems with BOG preheating upstream of the BOG compressor have incorporated refrigerant (N) as the preheating medium ₂ ). This involves the simultaneous removal of heat from the nitrogen. N is a radical of hydrogen ₂ Heating is not always available or sufficient, for example, if the reliquefaction is not running and/or the reliquefaction is at high fuel gas consumption. Additional heat sources may then be required, and this is typically done with additional BOG preheaters in parallel.

Existing BOG reliquefaction systems do not provide for efficient preheating of BOG and a more efficient and simple BOG reliquefaction system is needed. More efficient and simple preheating of BOG in a BOG reliquefaction system would provide a simpler system and thus lower equipment costs. For example, a more efficient preheating of the BOG will make it possible to use a compressor type in such installations that is less expensive than the compressor type typically used for cryogenic gases. Furthermore, the overall efficiency of the system will be improved compared to prior art solutions.

Disclosure of Invention

It is an object of the present disclosure to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages.

According to a first aspect, there is provided a boil-off gas (BOG) reliquefaction system comprising at least one Liquefied Natural Gas (LNG) cargo tank, a BOG preheater, at least one BOG compressor with a BOG compressor cooler, a BOG recondenser with a refrigeration cycle, wherein the BOG reliquefaction system comprises a separate liquid circuit with circulation of a liquid phase Heat Transfer Fluid (HTF) for preheating BOG from the LNG cargo tank before entering the BOG compressor, the separate liquid circuit comprising: a liquid pump configured to pump HTF; a BOG pre-heater configured for heat exchange between the BOG and the HTF; a BOG recondenser downstream of the BOG preheater; and a liquid trim heater downstream of the BOG recondenser configured to heat the HTF.

According to some embodiments, the separate liquid circuit comprises: a first bypass connection downstream from the BOG preheater for bypassing the HTF to the BOG recondenser.

According to some embodiments, the separate liquid circuit comprises: a second bypass connection leading from upstream of the BOG preheater to the top of the BOG recondenser for circulation in the opposite direction through the BOG recondenser.

According to some embodiments, a first portion of the HTF is passed to the BOG preheater and a second bypass connection is opened to pass a second portion of the HTF through the BOG recondenser in an opposite direction from top to bottom of the BOG recondenser, and a first bypass connection is opened to pass the first portion of the HTF from the BOG preheater and the second portion of the HTF from the BOG recondenser to the trim heater.

According to some embodiments, HTF is circulated from the liquid pump through the BOG preheater, through the BOG recondenser, and through the liquid trim heater through a separate liquid loop.

According to some embodiments, a portion of the BOG is directed from the BOG compressor cooler to a BOG recondenser for condensation, and the portion of the BOG is returned from the BOG recondenser as a liquefied gas to a Liquefied Natural Gas (LNG) cargo box.

According to some embodiments, the refrigeration cycle connected to the BOG recondenser is configured to remove heat from the portion of the BOG condensed in the BOG recondenser.

According to some embodiments, the HTF flowing through the BOG recondenser from bottom to top removes heat from the portion of the BOG condensed in the BOG recondenser.

According to some embodiments, the separate liquid circuit comprises: a third bypass connection upstream of the BOG preheater for bypassing a portion of the HTF around the BOG preheater.

According to some embodiments, a portion of the HTF is recycled from the BOG preheater back to upstream of the liquid pump.

According to some embodiments, the cooling medium from the BOG compressor cooler is used in a liquid trim heater.

According to some embodiments, at least one BOG compressor is a non-cryogenic compressor type.

According to some embodiments, the refrigeration cycle in the BOG recondenser is an inverted brayton nitrogen cycle.

According to some embodiments, the separate liquid circuit comprises an expansion tank.

According to a second aspect, there is provided a method for reliquefying boil-off gas (BOG) in a reliquefaction system comprising at least one Liquefied Natural Gas (LNG) tank, a BOG preheater, at least one BOG compressor having a BOG compressor cooler, a BOG recondenser having a refrigeration cycle, wherein: the method comprises the following steps: pumping a Heat Transfer Fluid (HTF) circulating in a separate liquid loop comprising a liquid pump, a BOG preheater, a BOG recondenser, and a liquid trim heater into the BOG preheater by the liquid pump; preheating the BOG entering a BOG preheater from an LNG cargo tank by HTF before passing the BOG to a BOG compressor; and passing the cooled HTF exiting the BOG preheater in the separate liquid loop to the liquid trim heater.

According to some embodiments, the method comprises: HTF leaving the BOG preheater in the separate liquid loop is transferred to the liquid trim heater via a first bypass connection downstream of the BOG preheater.

According to some embodiments, the method comprises: a first portion of the HTF is passed to the BOG preheater and a second portion of the HTF is passed through the BOG recondenser in an opposite direction from top to bottom of the BOG recondenser through 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 passed to the liquid trim heater through the first bypass connection.

According to some embodiments, the method comprises: HTF leaving the BOG preheater in a separate liquid loop is passed through a BOG recondenser before entering the liquid trim heater.

According to some embodiments, the method comprises: directing a portion of the BOG to a BOG recondenser for condensation, and returning the portion of the BOG from the BOG recondenser as a liquefied gas to a Liquid Natural Gas (LNG) cargo box.

According to some embodiments, the method comprises: in the refrigeration cycle connected to the BOG recondenser, heat is removed from the portion of the BOG condensed in the BOG recondenser.

According to some embodiments, the method comprises: additional heat is removed from the portion of the BOG condensed in the BOG recondenser by the HTF flowing through the BOG recondenser from bottom to top.

According to some embodiments, a portion of the HTF bypasses the BOG preheater via a third bypass connection upstream of the BOG preheater.

According to some embodiments, the method comprises: a portion of the HTF effluent from the BOG preheater is recycled upstream of the liquid pump.

According to some embodiments, the method comprises: the cooling medium from the BOG compressor cooler is used as the heating medium in the liquid trim heater.

According to a third aspect, there is provided a method for operating a boil-off gas (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, a BOG recondenser having a refrigeration cycle, wherein the method comprises:

-starting up the system by: pumping a Heat Transfer Fluid (HTF) circulating in a separate liquid loop comprising a liquid pump, a BOG preheater, a BOG recondenser, and a liquid trim heater into the BOG preheater by the liquid pump; preheating the BOG entering a BOG preheater from an LNG cargo tank by HTF before passing the BOG to a BOG compressor; and passing the cooled HTF exiting the BOG preheater in the separate liquid loop to the liquid trim heater via a first bypass connection downstream of the BOG preheater,

-initiating a normal operation mode of the system by: passing a first portion of the HTF to the BOG preheater and a second portion of the HTF in an opposite direction from the top to the bottom of the BOG recondenser through a second bypass connection; and passing a first portion of the HTF from the BOG preheater and a second portion of the HTF from the BOG recondenser to the liquid trim heater through a first bypass connection,

-running the system in a normal operation mode by: circulating HTF exiting a BOG pre-heater in a separate liquid loop through a BOG re-condenser from bottom to top prior to entering a liquid trim heater; directing a portion of the BOG exiting the BOG compressor cooler to a BOG recondenser for condensation and returning the portion of the BOG from the BOG recondenser to the LNG cargo box as a liquefied gas; removing heat from the portion of the BOG condensed in the BOG recondenser in a refrigeration cycle connected to the BOG recondenser; and removing additional heat from the portion of the BOG condensed in the BOG recondenser by the HTF flowing through the BOG recondenser.

The effects and features of the second and third aspects are largely analogous to those described above in connection with the first aspect. The embodiments mentioned in relation to the first aspect are mostly compatible with the second and third aspects.

The present disclosure will become apparent from the detailed description given below. The detailed description and specific examples disclose preferred embodiments of the disclosure by way of illustration only. Those skilled in the art, guided by the detailed description, will appreciate that changes and modifications may be made within the scope of the disclosure.

Accordingly, it should be understood that the disclosure disclosed herein is not limited to the particular component parts of the devices described or steps of the methods described, as such devices and methods may vary. It is also to 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 the appended claims, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements, unless the context clearly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising," "including," "containing," and similar words do not exclude additional elements or steps.

The term "BOG preheater" means a heat exchanger, which may 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.

The term "BOG recondenser" means a heat exchanger, which may be any type of heat exchanger, such as a brazed plate/fin heat exchanger.

The term "liquid trim heater" means a heat exchanger, which may 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.

Drawings

The above objects, as well as other objects, features and advantages of the present disclosure will be more fully understood by reference to the following illustrative and non-limiting detailed description of exemplary embodiments of the present disclosure when considered in conjunction with the accompanying drawings.

FIG. 1 shows an overall process flow diagram of a BOG reliquefaction system having a refrigeration cycle in a BOG recondenser according to the present invention.

FIG. 2 shows a process flow diagram for a BOG reliquefaction system in which HTF is circulated only through the BOG pre-heater.

FIG. 3 shows a process flow diagram for a BOG reliquefaction system with a best-effort start-up mode.

FIG. 4 shows a process flow diagram for a BOG reliquefaction system in which the optimization function is on, with HTFs circulating through both the BOG pre-heater and the BOG post-condenser.

FIG. 5 shows a process flow diagram for a BOG reliquefaction system in which HTFs bypass the BOG preheater.

FIG. 6 shows a process flow diagram for a BOG reliquefaction system in which HTF is recycled from the BOG preheater back to the liquid pump.

Like parts in the drawings are given like reference numerals.

Detailed Description

The present disclosure will now be described with reference to the accompanying drawings, in which preferred example embodiments of the disclosure are shown. However, the present disclosure may be embodied in other forms and should not be construed as limited to the embodiments set forth herein. The disclosed embodiments are provided to fully convey the scope of the disclosure to the skilled person.

With the present invention, the reliquefaction capacity can be increased by reusing the "cold" obtained from preheating the BOG. The cooled HTF is used to increase the recondensing capacity of the BOG recondenser.

A first aspect of the disclosure is illustrated in fig. 1, which fig. 1 shows the overall layout of a BOG reliquefaction system according to the present invention, showing a boil-off gas (BOG) reliquefaction system comprising at least one Liquefied Natural Gas (LNG) cargo tank 10, a BOG preheater 11, at least one BOG compressor 12 with a BOG compressor cooler 13, a BOG recondenser 14 with a refrigeration cycle, wherein the BOG reliquefaction system comprises a separate liquid circuit with circulation of a liquid phase Heat Transfer Fluid (HTF) for preheating BOG before it enters the BOG compressor 12 from the LNG cargo tank 10, the separate liquid circuit comprising: a liquid pump 15 configured to pump HTF; a BOG preheater 11 configured for heat exchange between BOG and HTF; a BOG recondenser 14 located downstream of the BOG preheater 11; and a liquid trim (trim) heater 16 downstream of the BOG recondenser 14 configured to heat the HTF.

The HTF can bypass the BOG recondenser 14 downstream from the BOG preheater via a first bypass connection b 1. This embodiment occurs without the need for the BOG recondenser 14 to condense excess BOG back to the LNG cargo box 10. In this case, the HTF is not required/required to provide the optimization function for the BOG recondenser 14. By-passing the BOG recondenser, it is ensured that the BOG recondenser 14 is not supplied with excessive cooling which causes unbalance in the BOG recondenser 14. The liquid trim heater 16 heats the fluid to typically about +40 ℃ when the cold HTF is not heated by the BOG recondenser 14. The HTF will always act as a preheating medium in the BOG preheater 11.

The

valves

20 and 21 are piston valves having an opening/closing function. The

valves

18, 19, 22, 23 and 24 are diaphragm valves, which make it possible to control and regulate the flow through the valves not only by opening/closing.

Fig. 2 shows a BOG reliquefaction system in which HTF is circulated only through the BOG preheater 11 and not through the BOG recondenser 14. This mode may be used when it is not necessary to have an optimization function of the HTF cycle through the BOG recondenser 14. 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 b 1. The valve 18 in line 103 in the separate liquid loop is in a closed position, the valve 19 in line 104 is in an open position, and the valve 20 in line 105 is in a closed position for HTF to bypass the BOG recondenser 14.

The separate liquid loop may comprise a second bypass connection b2 leading from upstream of the BOG preheater 11 via line 108 and line 105 to the top of the BOG recondenser 14 for circulation through the BOG recondenser 14 in the opposite direction, i.e. from top to bottom.

To start the optimization function, a first part of the HTF is passed to the BOG preheater 11, and the second bypass connection b2 is opened to pass a second part of the HTF in the opposite direction from the top to the bottom of the BOG recondenser 14 through the BOG recondenser 14, and the first bypass connection b1 is opened for passing the first part of the HTF from the BOG preheater 11 and the second part of the HTF from the BOG recondenser to the trim heater 16.

When referring to a "first portion" or "second portion" or "one portion" of the HTF, one skilled in the art will understand how to properly divide the HTF flow for different implementations, and the exact number/ratio of flows will vary due to the operating conditions as understood by the skilled artisan. Each portion may be in the range of 0% to 100% of the total HTF flow.

When it is desired to initiate the optimization function, the warm HTF stream may be pumped from the top of the BOG recondenser 14 through the BOG recondenser 14 via line 108 and line 105, cooled by the refrigerant in the refrigeration cycle in the BOG recondenser 14. During normal operation with optimized function, 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 of the BOG recondenser 14 via line 103. Thus, at the start of the optimization function, the tubes will have a temperature of about-90 ℃ and the HTF will begin to flow from the bottom via line 103 into the BOG recondenser 14, preventing the tubes from heating the HTF and overheating as it enters the BOG recondenser 14.

Heat is introduced to the BOG recondenser 14 by introducing warm (typically about +40 ℃) HTF from the top of the BOG recondenser 14 via line 105. In the case where the reliquefaction of the BOG is not performed and the warmed BOG does not enter the BOG recondenser 14, the additional heat may be used as a heat source, thereby ensuring that the BOG recondenser 14 maintains stable operation.

When the entire BOG recondenser 14 is cooled, additional heat from the HTF introduced in the top of the BOG recondenser 14 via line 105 can also be used to heat the entire BOG recondenser 14 after tripping.

FIG. 3 shows a BOG reliquefaction system with optimized function start-up mode of a single liquid loop. Valve 20 closes to ensure that the upper right hand corner of the circuit is closed. Liquid pump 15 not only drives HTF through the optimized function start-up connection via line 108 but also through BOG preheater 11 via line 101. The valve 20 in line 105 is in the closed position and the valve 21 in line 108 is in the open position for passing a portion of the flow of HTF from the pump 15 to the BOG recondenser 14 via line 108 and line 105, and exits the BOG recondenser 14 via line 103 with the valve 18 in the open position, and exits the BOG recondenser 14 via line 104 with the valve 19 in the open position. Valve 18 is used to control a portion of the reflux through the BOG recondenser in the initiation of the optimization function. Valve 21 is used to allow a portion of the stream to enter line 108 while the remaining HTF stream from pump 15 enters BOG preheater 11 via line 101.

Fig. 4 shows the BOG reliquefaction system in normal operation mode, where HTF is circulated through a separate liquid loop from the liquid pump (15), through the BOG preheater (11), through the BOG recondenser (14) and through the liquid trim heater (16), and bypass and/or recirculation is not used. The valve 18 in line 103 in the separate liquid loop is in an open position, the valve 19 in line 104 is in a closed position and the valve 20 in line 105 is in an 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 trim heater 16 via line 106.

The normal operation mode is a mode in which the optimization function is used. The optimization function may also be referred to as an optimizer function or optimizer/optimization mode or simply as an optimizer. In the optimized function mode, the HTF enters the BOG recondenser 14 from the bottom (also referred to as upstream) of the BOG recondenser 14 and exits from the top (also referred to as downstream) of the BOG recondenser 14. The HTF may flow through the BOG recondenser 14 in one or several connected flow channels.

Under normal operation, the individual liquid circuit comprises the following equipment in the order of flow in the normal direction: a liquid pump 15, a BOG preheater 11, a BOG recondenser 14, a liquid trim heater 16, and an optional expansion tank 17.

Under normal operation, the liquid pump 15 pumps HTF, typically at +40 ℃, to the BOG preheater 11 where it heats the BOG. At the same time, the HTF is typically cooled to-90 ℃. The HTF is fed into the BOG recondenser 14 from the bottom (upstream) of the BOG recondenser 14. In the BOG recondenser 14, the HTF acts as a cooling medium for the BOG, and exits the BOG recondenser 14 from the top (downstream) at about +35 ℃.

The HTF can also bypass the BOG recondenser 14 via the bypass connection b1 in the normal operating mode with optimized functionality. The valves 18 and 19 may control the amount of HTF bypassing the BOG recondenser 14 to control the temperature of the top outlet of the BOG recondenser 14 and thereby the HTF temperature profile so that the temperature difference across the BOG recondenser 14 does not exceed the allowable limit.

The liquid trim heater 16 is in line 106 and both the line 105 connecting to the BOG recondenser 14 and the line 104 bypassing the BOG recondenser are connected to line 106. Trim heater 16 is located upstream of liquid pump 15. The liquid trim heater 16 uses a heating medium, such as water, to heat the HTF that is sent back into the liquid pump 15. The heating medium flowing into the liquid trim heater 16 is directed through a BOG compressor cooler 13 connected to the BOG compressor 12 to cool the BOG. There may be one or several of the following steps: the BOG compressors and coolers in series perform the required BOG compression for the gas consumers onboard the ship. For gas or dual fuel engines, the BOG pressure may be, for example, 7 to 12 bar, or pre-compression may be performed in case the engine uses high compressed gas up to 300 bar, so a separate compression system would be required. The heating medium flowing into the liquid trim heater 16 may be connected to one or several compressor coolers, depending on the appropriate heating capacity of the liquid trim heater 16 being achieved. One or more compression stages may follow the connection of the reliquefaction system.

During the normal operating mode as described above, the separate liquid circuit with HTF provides two different functions simultaneously:

the BOG upstream of the BOG compressor 12 is preheated to about-30 ℃ to enable the use of non-cryogenic (non-cryogenic) BOG compressors. By non-cryogenic BOG compressor is meant a compressor that does not have to withstand temperatures below-150 ℃. This reduces the investment cost for the system.

Typical systems can handle BOG streams between about 1750kg/h up to about 5000kg/h using a liquid heat transfer fluid loop for pre-heater purposes. The flow rate of the heat transfer fluid may be adjusted according to the BOG flow to maintain the temperature of the BOG flow at the outlet of the preheater at about-30 ℃.

In the use of N ₂ In existing systems where preheating is performed, and when N ₂ When preheating is not available or sufficient, another source of heat is required, which is usually done in parallel with an additional BOG preheater. The system according to the invention has only one BOG pre-heater, which simplifies the system and makes the transition between modes easier. Gas/liquid and liquid/liquid exchanger andat least one gas/gas will also be more compact than at least one gas/liquid.

The optimization function by additional heat rejection in the BOG recondenser 14 contributes to a higher cooling capacity for the reliquefaction process, thereby increasing the efficiency of the system. The system according to the invention can provide a more efficient and simple system compared to conventional nitrogen loop based preheating and recondensing systems for BOG. By transferring excess "cold" from the BOG preheater 11 to the BOG recondenser 14 through the HTF, the capacity required for the refrigeration cycle is reduced. Thus, optimizing reliquefaction using HTF may increase reliquefaction capacity by as much as about 67% compared to operation without HTF in the BOG recondenser 14.

Once started, the optimization function will be most common as long as the reliquefaction system is running. And will accompany some HTF into the BOG recondenser bypass if necessary. Thus, the mode with optimized function in use is often in an open state during the entire journey of the LNG carrier vessel. However, in the case where the reliquefaction system is set in a standby state or stopped completely, the optimization function may be stopped. This may occur, for example, in the event of a pressure drop in the LNG tank, because the fuel consumption by the main engines and other consumers is greater than BOG generation (virtually all LNG ships have engines that use boil-off gas for the power of the ship). The operating time of the reliquefaction system may vary from vessel to vessel.

The separate liquid HTF loop as disclosed herein with technical details of the separate liquid HTF loop ensures flexibility not possible with conventional preheater systems in existing BOG reliquefaction facilities.

After exiting the BOG compressor cooler 13, a portion of the BOG may be directed to the BOG recondenser 14 via line 111 for condensation and returned as liquefied gas from the BOG recondenser 14 to the LNG cargo box 10 via line 112 with valve 24 in the open position. The BOG directed to the BOG recondenser 14 may be in the range of 0% to 100% of the total BOG leaving the BOG compressor cooler 13. The remainder of the BOG leaving the BOG compressor cooler 13 (which is not directed to the BOG recondenser 14 for recondensation) is directed out of the reliquefaction system to the fuel consumer.

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.

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

The separate liquid loop may comprise a third bypass connection b3 upstream of the BOG preheater 11 for bypassing a part of the HTF around the BOG preheater 11. The HTF may bypass the BOG preheater 11 to control the HTF temperature entering the BOG recondenser 14. Fig. 5 shows a BOG reliquefaction system in which HTF bypasses the BOG preheater 11. Valve 22 in line 109, parallel to BOG preheater 11, is in an open position for HTF bypassing BOG preheater 11. As shown in fig. 5, the bypass connection b1 may be opened for controlling the flow and/or temperature in the system components when the third bypass connection b3 is opened such that a part of the HTF bypasses the BOG preheater 11. When the bypass connection b3 is open, the bypass connection b1 can also be closed.

The HTF may be recycled from downstream of the BOG preheater 11 to control the temperature of the HTF to the BOG recondenser 14. By recycling a large amount of cold HTF back to the liquid pump 15, thereby ensuring that a large HTF flow passes through the BOG preheater 11, the effluent stream of the BOG preheater 11 is maintained at the desired temperature. Fig. 6 shows a BOG reliquefaction system in which HTF is recycled. Valve 23 in line 110 downstream of BOG preheater 11 is in an open position for recycling HTF back to liquid pump 15. As shown in fig. 6, the bypass connection b1 may be opened to control the flow and/or temperature in the system components while 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 also be closed during said recirculation of HTF.

Bypassing the BOG preheater 11 via bypass connection b3, recycling the HTF back to the liquid pump 15 and bypassing the BOG recondenser via bypass connection b1 may also all be used simultaneously.

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

The heat transfer fluid in the separate liquid loop in the BOG reliquefaction system according to the present invention is a liquid phase heat transfer fluid adapted to transfer heat down to a low temperature. The HTF may be a synthetic liquid phase fluid. An example of a suitable heat transfer fluid is a low temperature synthetic heat transfer fluid. Examples of suitable heat transfer fluids are heat transfer fluids based on, for example, hydrocarbons or silicones. An example of a suitable composition of heat transfer fluid is a mixture of methylcyclohexane and trimethylpentane, such as that commercially available from Eastman

A heat transfer fluid. />

The heat transfer fluid is a synthetic liquid phase fluid for use in extremely low temperature applications such as single fluid heating and cooling systems between-115 ℃ and +175 ℃. Examples of other suitable commercially available heat transfer fluids are Caltherm UBT (silicone) manufactured by Caldic for use between-100 ℃ and +260 ℃, dynalene MW (hydrocarbons) manufactured by Dynalene for use between-112 ℃ and +163 ℃, and Fragiltherm X-T9-A (silicone) manufactured by Fragol for use between-112 ℃ and +200 ℃.

The compression of the BOG is accomplished with a cooling medium in at least one BOG compressor 12 having at least one BOG compressor cooler 13. There may be several BOG compressors and coolers arranged in series. Preferably, the compression of the BOG is done in or after a plurality of stages in which cooling is performed using a cooling medium. At least one BOG compressor 12 may be a non-cryogenic compressor type. Examples of suitable compressors are positive displacement compressors, such as screw compressors, or dynamic compressors, such as centrifugal compressors.

The heated cooling medium from BOG compression leaving the BOG compressor cooler 13 can be reused as heating medium for the liquid trim heater 16. Alternatively, it may be supplied separately to the BOG compressor cooler 13 and the liquid trim 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 trim heater 16, in case another heat exchanger downstream of the BOG preheater 11 can also be used to heat the BOG. However, the preferred solution is to liquid trim the heater 16.

The separate liquid circuit may comprise an expansion tank 17 connected to the HTF circulation. The expansion vessel 17 may be any type of expansion vessel and the purpose of the expansion vessel 17 is to allow the HTF to expand.

The temperature of the BOG exiting the LNG cargo box 10 when entering the BOG preheater 11 via line 100 is typically about-140 ℃ to about-110 ℃. BOG exits BOG preheater 11 via line 102 at a temperature of about-30 ℃ 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 with the at least one BOG compressor cooler 13 at a temperature of about +40 ℃ and is then taken via line 111 to a fuel consumer and/or a BOG recondenser 14 for re-liquefaction. BOG enters at a temperature typically around +40 ℃, is cooled and liquefied in the BOG recondenser 14, and is returned to the LNG cargo box 10 at a temperature of at least about-163 ℃ via line 112 with valve 24 in the open position to be liquid without pressurization.

The refrigeration cycle in the BOG recondenser 14 may be a nitrogen cycle. The refrigerant will then be N ₂ . An inverted Brayton (Brayton) nitrogen cycle may be used to cool the BOG recondenser. However, other refrigeration cycles may be used within the scope of the present invention. The overall process flow diagram in FIG. 1 shows the reversed N in the BOG recondenser 14 ₂ Details of the basic inverted brayton cycle of the flow. The cycle is shown with one compressor stage comprising a compressor 25 followed by a cooler 26, but more than one compressor may be usedAnd (4) stages. In the refrigerating cycle, N ₂ Flows in the cycle in line 113 through the compressor 25 and then the cooler 26, then through the BOG recondenser 14, and after leaving the BOG recondenser 14, again flows through the expander 27 in the opposite direction before flowing through the BOG recondenser 14.

The BOG reliquefaction system may be installed for a ship, an offshore facility, or an onshore facility such as an LNG receiving station.

A second aspect of the present disclosure shows a method for reliquefying boil-off gas (BOG) in a reliquefaction system comprising at least one Liquefied Natural Gas (LNG) cargo tank 10, a BOG preheater 11, at least one BOG compressor 12 having a BOG compressor cooler 13, a BOG recondenser 14 having a refrigeration cycle, wherein the method comprises: the Heat Transfer Fluid (HTF) circulating in the separate liquid loop comprising the liquid pump 15, the BOG preheater 11, the BOG recondenser 14 and the liquid trim heater 16 is pumped into the BOG preheater 11 by the liquid pump 15, the BOG entering the BOG preheater 11 from the LNG cargo box 10 is preheated by the HTF before being passed to the BOG compressor 12, and the cooled HTF leaving the BOG preheater 11 in the separate liquid loop is passed to the liquid trim heater 16.

The embodiment of the method shown in fig. 2 comprises pumping HTF circulating in a separate liquid loop by a liquid pump 15 via line 101 into the BOG preheater 11, preheating BOG entering the BOG preheater 11 from the LNG cargo tank 10 via line 100 in the BOG preheater 11 by HTF in the BOG preheater 11 before passing it to the BOG compressor 12 via line 102, passing cooled HTF leaving the BOG preheater 11 directly to the liquid trim heater 16 via line 103 with valve 18 in the closed position, line 104 with valve 19 in the open position and via line 106.

The embodiment of the method shown in fig. 3 comprises passing a first portion of the HTF to the BOG preheater 11 and passing a second portion of the HTF in the opposite direction from the top to the bottom of the BOG recondenser 14 through the BOG recondenser 14 via a second bypass connection b2, and passing 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 trim heater 16 via a first bypass connection b 1. Initiating an optimizer function of the individual liquid circuit by: HTF via line 108 with valve 21 in the open position is passed to the top of the BOG recondenser 14 via line 105 with valve 20 in the closed position, and HTF leaving the BOG recondenser 14 from the bottom via line 103 with valve 18 in the open position is returned to the liquid trim heater 16 via line 104 with valve 19 in the open position.

Fig. 4 shows an embodiment of the method of conveying HTF leaving the BOG preheater 11 in a separate liquid loop through the BOG recondenser 14: enters 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 exits the BOG recondenser 14 from the top (downstream) via line 105 with valve 20 in the open position before entering the liquid trim heater 16 via line 106 and exiting the liquid trim heater 16 via line 107. As described above for the system, this is the normal mode of operation with the optimization function in use.

After the BOG exits the BOG compressor cooler 13, a portion of the BOG may flow to the BOG recondenser 14 via line 111 for condensation and be returned from the BOG recondenser 14 to the LNG cargo box 10 as liquefied gas via line 112 with valve 24 in the open position.

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

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

Fig. 5 shows an embodiment of the method in which the third bypass connection b3 is opened to bypass part of the HTF by-pass the BOG preheater 11. HTF flows via line 109 with valve 22 in the open position for bypassing the BOG preheater 11. As shown in fig. 5, the bypass connection b1 may be opened to control the flow and/or temperature in the system components while the third bypass connection b3 is opened to allow a portion of the HTF to bypass the BOG preheater 11. When the bypass connection b3 is open, the bypass connection b1 can also be closed.

Fig. 6 shows an embodiment of a method comprising: HTF is delivered after the BOG preheater 11 via line 110 with valve 23 in the open position to recycle the HTF upstream of the liquid pump 15. As shown in fig. 6, at the same time, the bypass connection b1 may be open to control the flow and/or temperature in the system components, since part of the HTF is recirculated back to the liquid pump 15 from downstream of the BOG preheater 11. The bypass connection b1 can also be closed during said recirculation of HTF.

As described above for the reliquefaction system, the heat transfer fluid in the separate liquid loop in the method for reliquefaction according to the present invention is a liquid phase heat transfer fluid suitable for heat transfer down to a low temperature. The HTF may be a synthetic liquid phase fluid as described above for the system according to the invention.

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

As described above for the reliquefaction system, in an embodiment of the method according to the invention, the heated cooling medium exiting the BOG compressor cooler 13 from the BOG compression can be reused as heating medium for the liquid trim heater 16. Alternatively, the BOG compressor cooler 13 and the liquid trim heater 16 may be supplied separately. The cooling medium is typically water, a water-glycol mixture or the like.

As described above for the reliquefaction system, in the method according to the present invention, the BOG exiting the LNG cargo box 10 is typically at a temperature of about-140 ℃ to about-110 ℃ when entering the BOG preheater 11 via line 100. BOG exits BOG preheater 11 via line 102 at a temperature of about-30 ℃ 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 with the at least one BOG compressor cooler 13 at a temperature of approximately +40 ℃ and is then taken via line 111 to a fuel consumer and/or a BOG recondenser 14 for re-liquefaction. BOG enters at a temperature typically about +40 ℃, is cooled and liquefied in the BOG recondenser 14, and is returned to the LNG cargo box 10 at a temperature of at least about-163 ℃ via line 112 with valve 24 in the open position to become liquid without pressurization.

As described above for the reliquefaction system, in the method according to the present invention, the refrigeration cycle in the BOG recondenser 14 may be a nitrogen cycle. However, other refrigeration cycles may be used within the scope of the present invention. Advantageously, a reverse brayton nitrogen cycle may be used.

As described above for the reliquefaction system, in the method according to the invention the separate liquid circuit may comprise an expansion tank 17.

A third aspect of the invention relates to a method for operating a boil-off gas (BOG) reliquefaction system comprising at least one Liquefied Natural Gas (LNG) cargo tank 10, a BOG preheater 11, at least one BOG compressor 12 having a BOG compressor cooler 13, a BOG recondenser 14 having a refrigeration cycle. The method comprises the following steps: the system is first started by: the Heat Transfer Fluid (HTF) circulating in a separate liquid loop comprising the liquid pump 15, the BOG preheater 11, the BOG recondenser 14 and the liquid trim heater 16 is pumped into the BOG preheater 11 by the liquid pump 15; the BOG entering the BOG preheater 11 from the LNG cargo box 10 is preheated by HTF before passing to the BOG compressor 12; and the cooled HTF leaving the BOG preheater 11 in the separate liquid loop is passed to the liquid trim heater 16 via a first bypass connection b1 downstream of the BOG preheater 11. After the system has been started, a normal operation mode of the system is started by: passing a first portion of the HTF to the BOG preheater 11 and a second portion of the HTF in the opposite direction from the top to the bottom of the BOG recondenser 14 through a second bypass connection b 2; and a first part of the HTF from the BOG preheater 11 and a second part of the HTF from the BOG recondenser 14 are passed to the trim heater 16 through a first bypass connection b 1. When the normal operation mode has been initiated, operating the system in the normal operation mode by: the HTF leaving the BOG preheater 11 in a separate liquid loop is circulated from bottom to top through the BOG recondenser 14 before entering the liquid trim heater 16; directing a portion of the BOG exiting the BOG compressor cooler 13 to the BOG recondenser 14 for condensation and returning the portion of the BOG from the BOG recondenser 14 to the LNG cargo box 10 as a liquefied gas; removing heat from the portion of the BOG condensed in the BOG recondenser 14 in a refrigeration cycle connected to the BOG recondenser 14; and removing additional heat from the portion of the BOG condensed in the BOG recondenser 14 by the HTF flowing through the BOG recondenser 14.

Those skilled in the art realize that this disclosure is not limited to the preferred embodiments described above. Those skilled in the art will also recognize that modifications and variations can be made within the scope of the appended claims. For example, various modifications to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed disclosure, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. Boil-off gas (BOG) reliquefaction system comprising at least one Liquefied Natural Gas (LNG) cargo box (10), a BOG preheater (11), at least one BOG compressor (12) with a BOG compressor cooler (13), a BOG recondenser (14) with a refrigeration cycle, characterized in that the BOG reliquefaction system comprises a separate liquid circuit with circulation of a liquid phase Heat Transfer Fluid (HTF) for preheating BOG from the LNG cargo box (10) before entering the BOG compressor (12), the separate liquid circuit comprising: a liquid pump (15) configured to pump the HTF; the BOG pre-heater (11) configured for heat exchange between the BOG and the HTF; said BOG recondenser (14) located downstream of said BOG preheater (11); and a liquid trim heater (16) located downstream of the BOG recondenser (14) configured to heat the HTF.

2. The boil-off gas (BOG) reliquefaction system of claim 1, wherein the separate liquid loop comprises: a first bypass connection (b 1) downstream from the BOG preheater (11) for bypassing the HTF around the BOG recondenser (14).

3. The boil-off gas (BOG) reliquefaction system according to claim 2, wherein the single liquid loop comprises: a second bypass connection (b 2) leading 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. Boil-off gas (BOG) reliquefaction system according to claim 3, characterized in that a first part of the HTF is routed to the BOG preheater (11) and the second bypass connection (b 2) is opened to route a second part of the HTF in the opposite direction through the BOG recondenser (14) from the top to the bottom of the BOG recondenser (14) and the first bypass connection (b 1) is opened to route the first part of the HTF from the BOG preheater (11) and the second part of the HTF from the BOG recondenser (14) to the liquid trim heater (16).

5. The boil-off gas (BOG) reliquefaction system according to claim 1, wherein the HTF is circulated from the liquid pump (15) through the BOG preheater (11), through the BOG recondenser (14), and through the liquid trim heater (16) through the separate liquid loop.

6. Boil-off gas (BOG) reliquefaction system according to any one of the preceding claims, characterized in that a portion of the BOG is led from the BOG compressor cooler (13) to the BOG recondenser (14) to be condensed, and the portion of the BOG is returned from the BOG recondenser (14) as liquefied gas to the LNG cargo box (10).

7. The boil-off gas (BOG) reliquefaction system according to claim 6, wherein 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 boil-off gas (BOG) reliquefaction system according to any one of claims 6 to 7, wherein the HTF flowing through the BOG recondenser (14) from bottom to top removes heat from the portion of the BOG condensed in the BOG recondenser (14).

9. Boil-off gas (BOG) reliquefaction system according to any one of the previous claims, wherein the separate liquid circuit comprises: a third bypass connection (b 3) upstream of the BOG preheater (11) for bypassing a portion of the HTF around the BOG preheater (11).

10. Boil-off gas (BOG) reliquefaction system according to any one of the preceding claims, characterized in that a part of the HTF is recycled from the BOG preheater (11) back upstream of the liquid pump (15).

11. Boil-off gas (BOG) reliquefaction system according to any of the previous claims, characterized in that cooling medium from the BOG compressor cooler (13) is used in the liquid trim heater (16).

12. Boil-off gas (BOG) reliquefaction system according to any of the previous claims, characterized in that the at least one BOG compressor (12) is of a non-cryogenic compressor type.

13. Boil-off gas (BOG) reliquefaction system according to any one of the preceding claims, characterized in that the refrigeration cycle in the BOG recondenser (14) is an inverted brayton nitrogen cycle.

14. Boil-off gas (BOG) reliquefaction system according to any one of the previous claims, wherein the separate liquid circuit comprises an expansion tank (17).

15. A method for reliquefying boil-off gas (BOG) in a reliquefaction system comprising at least one Liquefied Natural Gas (LNG) cargo tank (10), a BOG preheater (11), at least one BOG compressor (12) with a BOG compressor cooler (13), a BOG recondenser (14) with a refrigeration cycle, characterized in that the method comprises: pumping a Heat Transfer Fluid (HTF) circulating in separate liquid loops including a liquid pump (15), the BOG preheater (11), the BOG recondenser (14), and a liquid trim heater (16) into the BOG preheater (11) through the liquid pump (15); preheating BOG entering the BOG preheater (11) from the LNG cargo box (10) by the HTF before passing the BOG to the BOG compressor (12); and passing the cooled HTF leaving the BOG preheater (11) in the separate liquid loop to the liquid trim heater (16).

16. Method for reliquefying boil-off gas (BOG) according to claim 15, characterized in that the HTF leaving the BOG preheater (11) in the separate liquid circuit is passed to the liquid trim heater (16) via a first bypass connection (b 1) downstream of the BOG preheater (11).

17. The method for reliquefying boil-off gas (BOG) according to claim 16, characterized in that a first part of the HTF is conveyed to the BOG preheater (11) and a second part of the HTF is conveyed in the opposite direction from the top to the bottom of the BOG recondenser (14) through a second bypass connection (b 2) through the BOG recondenser (14), and that the first part of the HTF from the BOG preheater (11) and the second part of the HTF from the BOG recondenser (14) are conveyed through the first bypass connection (b 1) to the liquid trim heater (16).

18. The method for reliquefying boil-off gas (BOG) according to claim 15, wherein the HTF leaving the BOG preheater (11) in the separate liquid loop is conveyed through the BOG recondenser (14) before entering the liquid trim heater (16).

19. The method for reliquefying boil-off gas (BOG) according to any one of claims 15 to 18, wherein a portion of the BOG is directed to the BOG recondenser (14) to be condensed, and the portion of the BOG is returned from the BOG recondenser (14) to the LNG cargo box (10) as liquefied gas.

20. The method for reliquefying boil-off gas (BOG) according to claim 19, wherein 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).

21. The method for reliquefying boil-off gas (BOG) according to any one of claims 19 to 20, wherein 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).

22. Method for reliquefying boil-off gas (BOG) according to any one of claims 15 to 21, wherein a portion of the HTF bypasses the BOG preheater (11) via a third bypass connection (b 3) upstream of the BOG preheater (11).

23. The method for reliquefying boil-off gas (BOG) according to any one of claims 15 to 22, characterized in that a portion of the HTF flowing out of the BOG preheater (11) is recycled upstream of the liquid pump (15).

24. The method for reliquefying boil-off gas (BOG) according to any one of claims 15 to 23, wherein a cooling medium from the BOG compressor cooler (13) is used as the heating medium in the liquid trim heater (16).

25. The method for reliquefying boil-off gas (BOG) according to any one of claims 15 to 24, wherein the at least one BOG compressor (12) is of the non-cryogenic compressor type.

26. The method for reliquefying boil-off gas (BOG) according to any one of claims 15 to 25, wherein the refrigeration cycle in the BOG recondenser (14) is an inverted brayton nitrogen cycle.

27. Method for reliquefying boil-off gas (BOG) according to any one of claims 15 to 26, wherein the separate liquid circuit comprises an expansion tank (17).

28. A method for operating a boil-off gas (BOG) reliquefaction system comprising at least one Liquefied Natural Gas (LNG) cargo tank (10), a BOG pre-heater (11), at least one BOG compressor (12) having a BOG compressor cooler (13), a BOG recondenser (14) having a refrigeration cycle, the method comprising:

-starting the system by: pumping a Heat Transfer Fluid (HTF) circulating in separate liquid loops including a liquid pump (15), the BOG preheater (11), the BOG recondenser (14), and a liquid trim heater (16) into the BOG preheater (11) through the liquid pump (15); preheating BOG entering the BOG preheater (11) from the LNG cargo box (10) by the HTF before passing the BOG to the BOG compressor (12); and passing the cooled HTF leaving the BOG preheater (11) in the separate liquid loop to the liquid trim heater (16) via a first bypass connection (b 1) downstream of the BOG preheater (11),

-initiating a normal operation mode of the system by: passing a first portion of the HTF to the BOG preheater (11) and a second portion of the HTF in the opposite direction from the top to the bottom of the BOG recondenser (14) through a second bypass connection (b 2); and passing a first portion of the HTF from the BOG preheater (11) and a second portion of the HTF from the BOG recondenser (14) to the liquid trim heater (16) through the first bypass connection (b 1),

-running the system in a normal operation mode by: circulating the HTF exiting the BOG preheater (11) in the separate liquid loop from bottom to top through the BOG recondenser (14) prior to entering the liquid trim heater (16); directing a portion of the BOG exiting the BOG compressor cooler (13) to the BOG recondenser (14) for condensation and returning the portion of the BOG from the BOG recondenser (14) to the LNG cargo box (10) as liquefied gas; removing heat from the 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 the portion of the BOG condensed in the BOG recondenser (14) by the HTF flowing through the BOG recondenser (14).