KR20230040948A - Boil-off gas re-liquefaction system, method of re-liquefaction of boil-off gas in re-liquefaction system and method of operation of boil-off gas re-liquefaction system - Google Patents

Abstract

At least one BOG compressor (12) with at least one liquefied natural gas (LNG) cargo tank (10), a boil-off gas (BOG) preheater (11), a BOG compressor cooler (13), a refrigeration cycle In the BOG reliquefaction system including a BOG recondenser (14) with a liquid phase heat for preheating BOG coming out of the LNG cargo tank (10) before the BOG is introduced into the BOG compressor (12) A separate liquid circuit in which a transfer fluid (HTF) circulates, the separate liquid circuit comprising a liquid pump (15) configured to pump the HTF, a BOG preheater (11) configured for heat exchange between the BOG and the HTF, and a BOG A boil-off gas reliquefaction system comprising a BOG recondenser (14) located downstream of a preheater (11) and a liquid trim heater (16) located downstream of the BOG recondenser (14) and configured to heat the HTF. The present invention also relates to a method of reliquefying boil-off gas (BOG) in a reliquefaction system and a method of operating a boil-off gas (BOG) reliquefaction system.

Description

Boil-off gas re-liquefaction system, method of re-liquefaction of boil-off gas in re-liquefaction system and method of operation of boil-off gas re-liquefaction system

The present invention relates to a liquefied natural gas (LNG) boil-off gas (BOG) reliquefaction system, a method of reliquefying BOG, and a method of operating the BOG reliquefaction system. More specifically, the present invention relates to a boil-off gas (BOG) reliquefaction system, reliquefaction of boil-off gas (BOG) in a reliquefaction system, as defined in the introduction to claims 1, 15 and 28. Methods and methods of operating a boil-off gas (BOG) reliquefaction system.

A common technique for transporting natural gas from a natural gas extraction site is to liquefy the natural gas at or near the site, and transport the LNG to market, often in specially designed storage tanks aboard sea vessels.

LNG is a mixture of light hydrocarbons in which methane is the main component and nitrogen is inert, and small amounts of ethane, propane, butane, and pentane may be present. Depending on its exact composition, the boiling point of LNG is about -162°C to -161°C at atmospheric pressure, and it is normally loaded, transported and unloaded in this temperature range. This requires special materials, insulation and handling equipment to handle low temperatures and evaporative vapors. Due to heat leakage, the surface of the cargo (LNG) is constantly boiling, and from the LNG vaporized natural gas, so-called boil-off gas (BOG) (mainly methane) is generated.

Equipment for the continuous reliquefaction of this BOG is well known. Installing a BOG reliquefaction system on a liquefied natural gas (LNG) carrier equipped with a dual-fuel engine provides ship operators with added flexibility to switch fuels, taking advantage of the price difference between LNG and heavy oil. State-of-the-art propulsion systems are efficient, but not all BOGs can be utilized in engines. Also, BOG is often exceeded due to slow sailing and ship maintenance operations. Instead of burning this gas in a gas combustion unit, it can be re-liquefied and returned to the cargo tank.

Re-liquefaction of BOG on LNG carriers increases cargo transport and allows owners and operators to select the optimal propulsion system and operating profile. Its advantages are, for example, a flexible fuel system, optimized operating costs and increased cargo capacity.

Typically, reliquefaction systems are used to liquefy BOG to control cargo tank pressure. Reliquefaction systems are capable of handling all BOG (100% capacity) or only excess BOG (partial liquefaction) not burned in the engine.

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

Existing BOG reliquefaction systems are based on reverse nitrogen Brayton cycle refrigeration technology. This means that there is a closed nitrogen cycle in the system for extracting heat from BOG. Nitrogen gas (N ₂ ) is used as a refrigerant to control the tank pressure by cooling and reliquefying the pressurized BOG. The re-liquefied BOG is returned to the tank.

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

WO 2011/078689 A1 discloses a gas supply system for a dual fuel or gas engine integrated with BOG reliquefaction, wherein the available cooling duty of LNG is BOG in an apparatus in which BOG or condensates of BOG and LNG exchange heat with each other. Used for cooling and condensation. The LNG is heated using available "warm" duty as cooling water from the compressor of the reliquefaction system.

BOG is preheated before entering the BOG compressor, leaving the LNG tank at a temperature generally between -140°C and -110°C. Compressor intake temperature is important when choosing a compressor. Preheating the BOG can increase the temperature at the compressor inlet. Current systems with a BOG preheater upstream of the BOG compressor have incorporated refrigerant (N ₂ ) as the preheating medium. This includes simultaneous removal of heat from nitrogen gas. N ₂ heating is not always available or sufficient, for example when reliquefaction is not carried out or fuel gas consumption is high. So, an additional heat source may be required and this is usually done using an additional BOG preheater in parallel.

Existing BOG reliquefaction systems do not provide efficient preheating of BOG, and a more efficient and simple BOG reliquefaction system is needed. More efficient and simpler preheating of BOG in a BOG reliquefaction system provides a simpler system and reduces equipment costs. For example, the more efficient preheating of BOG allows these facilities to use a less expensive type of compressor than the type commonly used for cryogenic gases. Additionally, the overall efficiency of the system will be improved over prior art solutions.

It is an object of the present disclosure to alleviate, alleviate or eliminate one or more of the drawbacks and disadvantages noted above in the prior art.

According to a first aspect, a BOG comprising at least one liquefied natural gas (LNG) cargo tank, a boil-off gas (BOG) preheater, at least one BOG compressor with a BOG compressor cooler, and a BOG recondenser with a refrigeration cycle. In the reliquefaction system, the BOG reliquefaction system includes a separate liquid circuit in which a liquid heat transfer fluid (HTF) circulates to preheat BOG from an LNG cargo tank before the BOG enters the BOG compressor, The liquid circuit of comprises a liquid pump configured to pump the HTF, a BOG preheater configured to exchange heat between the BOG and the HTF, a BOG recondenser located downstream of the BOG preheater, and a liquid trim heater located downstream of the BOG recondenser configured to heat the HTF. A boil-off gas reliquefaction system is provided.

According to some embodiments, the separate liquid circuit includes a first bypass connection from downstream of the BOG preheater to allow the HTF to bypass the BOG recondenser.

According to some embodiments, the separate liquid circuit includes a second bypass connection 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, the first portion of the HTF is passed to the BOG preheater, the second bypass connection is open to pass the second portion of the HTF through the BOG recondenser in the opposite direction from top to bottom of the BOG recondenser, and , the first bypass connection is open so that a first portion of HTF from the BOG preheater and a second portion of HTF from the BOG recondenser pass to the trim heater.

According to some embodiments, the HTF circulates through the liquid pump separate liquid circuit, through the BOG preheater, through the BOG recondenser, and through the liquid trim heater.

According to some embodiments, a portion of BOG is conducted from a BOG compressor cooler to a BOG recondenser for condensation, from which the portion of BOG is returned as liquefied gas to an LNG cargo tank.

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

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

According to some embodiments, the separate liquid circuit includes a third bypass connection upstream of the BOG pre-heater to allow a portion of the HTF to bypass the BOG pre-heater.

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

According to some embodiments, cooling medium from the BOG compressor cooler is utilized in the 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 inverse Brayton nitrogen cycle.

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

According to a second aspect, a BOG comprising at least one liquefied natural gas (LNG) cargo tank, a boil-off gas (BOG) preheater, at least one BOG compressor with a BOG compressor cooler, and a BOG recondenser with a refrigeration cycle. A method for reliquefying boil-off gas in a reliquefaction system, wherein the BOG reliquefaction method comprises a heat transfer fluid (HTF) circulating in a separate liquid circuit including a liquid pump, a BOG preheater, a BOG recondenser, and a liquid trim heater. ) into the BOG preheater by a liquid pump, preheating the BOG entering the BOG preheater from the LNG cargo tank with HTF before the BOG is transported to the BOG compressor, and cooling exiting the BOG preheater in a separate liquid circuit. A boil-off gas reliquefaction method is provided, comprising the step of transporting the HTF to a liquid trim heater.

According to some embodiments, a separate liquid circuit transports the HTF exiting the BOG pre-heater to the liquid trim heater via a first bypass connection downstream of the BOG pre-heater.

According to some embodiments, passing a first portion of the HTF through the BOG preheater and passing a second portion of the HTF through the second bypass connection through the BOG recondenser in a reverse direction from top to bottom of the BOG recondenser; Pass a first portion of HTF from the BOG preheater and a second portion of HTF from the BOG recondenser through a first bypass connection to the liquid trim heater.

According to some embodiments, a separate liquid circuit conveys the HTF exiting the BOG preheater through the BOG recondenser before entering the liquid trim heater.

According to some embodiments, a portion of the BOG is directed to a BOG recondenser for condensation, and a portion of the BOG is returned as liquefied gas from the BOG recondenser to the LNG cargo tank.

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

According to some embodiments, heat is further removed from a portion of the BOG condensed in the BOG recondenser by the HTF flowing through the BOG recondenser.

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

According to some embodiments, a portion of the HTF flowing out of the BOG preheater and upstream of the liquid pump is recycled.

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

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

According to some embodiments, the refrigeration cycle within the BOG recondenser is an inverse Brayton nitrogen cycle.

According to some embodiments, a separate liquid circuit includes an expansion tank.

According to a third aspect, a BOG material comprising at least one liquefied natural gas (LNG) cargo tank, a boil-off gas (BOG) preheater, at least one BOG compressor with a BOG compressor cooler, and a BOG recondenser with a refrigeration cycle. A method of operating a liquefaction system, the operating method comprising:

-Starting the system by pumping heat transfer fluid (HTF) circulating in a separate liquid circuit including the liquid pump, BOG preheater, BOG recondenser and liquid trim heater to the BOG preheater by the liquid pump, LNG cargo tank The BOG entering the BOG preheater from the preheater is preheated by the HTF and transported to the BOG compressor, and the cooled HTF leaving the BOG preheater is transported to the liquid trim heater in a separate liquid circuit through a first bypass connection downstream of the BOG preheater. , the system startup phase,

- pass a first part of the HTF to the BOG preheater, pass a second part of the HTF through a second bypass connection in the reverse direction through the BOG recondenser from the top to the bottom of the BOG recondenser, and pass the HTF from the BOG preheater passing a second portion of the HTF from the BOG recondenser through the first portion and the first bypass connection to the liquid trim heater to initiate a normal operating mode of the system;

circulates the HTF leaving the BOG preheater from the bottom to the top in a separate liquid circuit through the BOG recondenser before entering the liquid trim heater, and directs a portion of the BOG discharged from the BOG compressor cooler to the BOG recondenser for condensation; return said portion of BOG to the LNG cargo tank as liquefied gas in the BOG recondenser, remove heat from the portion of BOG condensed in the BOG recondenser in a refrigeration cycle connected to the BOG recondenser, and return it to HTF flowing through the BOG recondenser; A method of operating a BOG reliquefaction system comprising the step of proceeding to a normal operating mode of the system by further removing heat from a portion of the BOG condensed in a BOG recondenser by

Effects and features of the second and third aspects are similar in many respects to those described above with respect to the first aspect. Thus, the embodiments mentioned in connection with the first aspect are generally interchangeable with the second and third aspects.

The present disclosure will become apparent from the detailed description that follows. The detailed description and specific examples disclose preferred embodiments of the present disclosure by way of example only. Those skilled in the art will understand from the guidance of the detailed description that changes and modifications may be made within the scope of the present disclosure.

Accordingly, it should be understood that the disclosure disclosed herein is not limited to the specific component parts of the described devices or described method steps, as such devices and methods may vary. Also, it should be understood that terms used herein are for the purpose of describing specific embodiments and are not intended to be limiting. As used in the specification and appended claims, the articles "a", "an", "the" and "said" are intended to imply that there is more than one element unless the context clearly dictates otherwise. should be careful about Thus, for example, reference to “a unit” or “the unit” may include multiple devices and the like. Also, “comprising”, “comprising”, “including” and similar words do not exclude other elements or steps.

The term “BOG preheater” means a heat exchanger, which can be any type of heat exchanger, such as for example a shell and tube, plate/shell or plate/plate heat exchanger.

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

The term “liquid trim heater” means a heat exchanger, which can be any type of heat exchanger, such as for example a shell and tube, plate/shell or plate/plate heat exchanger.

The foregoing as well as additional objects, features and advantages of the present invention will be more fully understood by referring to the following illustrative, non-limiting detailed description of exemplary embodiments of the present invention appended hereto.
Figure 1 shows the overall process flow diagram of a BOG reliquefaction system according to the present invention with a refrigeration cycle in a BOG recondenser.
2 shows a process flow diagram of a BOG reliquefaction system in which HTF is circulated only through the BOG preheater.
Figure 3 shows a process flow diagram of a BOG reliquefaction system with an optimization function start-up mode.
Figure 4 shows a process flow diagram of a BOG reliquefaction system with an optimization function in which HTF circulates through both a BOG preheater and a BOG recondenser.
5 shows a process flow diagram of a BOG reliquefaction system in which HTF bypasses the BOG preheater.
6 shows a process flow diagram of a BOG reliquefaction system in which HTF is recycled from the BOG preheater back to the liquid pump.
In the drawings, like parts are assigned like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred exemplary embodiments of the present invention will be described with reference to the accompanying drawings shown below. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms. The disclosed embodiments are provided to fully convey the scope of the invention to those skilled in the art.

According to the present invention, the re-liquefaction capacity can be increased by reusing 'cold air' obtained by preheating BOG. The cooled HTF is used to increase the recondensation capacity of the BOG recondenser.

A first aspect of the present disclosure is shown in FIG. 1 , which includes at least one liquefied natural gas (LNG) cargo tank (10), a boil-off gas (BOG) preheater (11), and a BOG compressor cooler (13). In the BOG reliquefaction system comprising at least one BOG compressor (12) and a BOG recondenser (14) with a refrigeration cycle, the BOG reliquefaction system is an LNG cargo tank before BOG enters the BOG compressor (12) and a separate liquid circuit in which a liquid heat transfer fluid (HTF) for preheating the BOG from (10) circulates, the separate liquid circuit comprising a liquid pump (15) configured to pump the HTF, between the BOG and the HTF. A BOG preheater (11) configured for heat exchange, 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. Shows the overall layout of the BOG reliquefaction system according to the present invention showing the boil-off gas reliquefaction system characterized in that.

The HTF can bypass the BOG recondenser 14 via a first bypass connection b1 from downstream of the BOG preheater. This embodiment arises in situations where a BOG recondenser 14 is not needed to condense excess BOG back into the LNG cargo tank 10. In this case it is not necessary and not required that the HTF provide an optimization function for the BOG recondenser 14. As the BOG recondenser is bypassed, it is ensured that the BOG recondenser 14 is not supplied with excessive cool air and an imbalance occurs in the BOG recondenser 14 . If the cold HTF is not heated by the BOG recondenser 14, the liquid trim heater 16 heats the fluid, typically to about +40°C. will be. HTF always serves as a preheating medium in the BOG preheater (11).

Valves 20 and 21 are piston valves with an on/off function. Valves 18, 19, 22, 23, 24 are diaphragm valves, which can control and regulate flow through the valves as well as on/off.

2 shows a BOG reliquefaction system in which HTF is circulated only through the BOG preheater 11 without passing through the BOG recondenser 14. This mode can be used when optimization functions with HTF circulation through the BOG recondenser 14 are not required. This is the first mode used after the LNG tank 10 is filled. The HTF bypasses the BOG recondenser 14 through the first bypass connection b1. In the separate liquid circuit, valve 18 in line 103 is in the closed position, valve 19 in line 104 is in the open position, and valve 20 in line 105 is in the BOG recondenser 14 ) is in the closed position for HTF bypass.

A separate liquid circuit runs from upstream of the BOG preheater 11 through lines 108 and 105 to the BOG recondenser 14 for circulation in the reverse direction, i.e. from top to bottom through the BOG recondenser 14. ) It may include a second bypass connection (b2) leading to the upper part.

For start-up of the optimization function, the first part of the HTF is delivered to the BOG preheater (11), and the second bypass connection (b2) is passed through the BOG recondenser (14) from the top to the bottom of the BOG recondenser (14). Opened to pass the second part of the HTF in the reverse direction, the first bypass connection b1 connects 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) is open for passage.

When referring to a "first part" or a "second part" or "portion" of an HTF, one skilled in the art knows how to appropriately divide the HTF flow for different embodiments, and the exact amount/ratio of the flow depending on operating conditions. You will understand what will change. Each portion can range from 0 to 100% of the total HTF flow.

When the optimization function needs to be started, a flow of warm HTF can be pumped from the top of the BOG recondenser 14 through the BOG recondenser 14 through lines 108 and 105; , which is cooled by the refrigerant in the refrigeration circle of the BOG recondenser (14). The exit temperature at the bottom will be the same as the temperature of the cold HTF entering the BOG recondenser 14 through line 103 from the bottom of the BOG recondenser 14 during normal operation with optimization. In this way, when the optimization function is started and the HTF starts flowing from the bottom through line 103 to the BOG recondenser 14, the pipe has a temperature of about -90°C, causing the pipe to heat up the HTF and cause the HTF to become too warm. It is prevented from entering the BOG recondenser 14 in one state.

Heat is introduced into the BOG recondenser 14 by introducing warm (usually about +40° C.) HTF from the top of the BOG recondenser 14 through line 105. When the BOG recondenser 14 is not running and no warm BOG is entering the BOG recondenser 14, this additional heat can be used as a heat source to keep the BOG recondenser 14 in stable operation. do.

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

Figure 3 shows a BOG reliquefaction system with a separate liquid circuit optimization function start mode. The valve 20 is closed so that the upper right corner of the separate liquid circuit is closed. The liquid pump (15) drives the HTF through both the optimization function start connection through line (108) and the BOG preheater (11) through line (101). Valve 20 in line 105 is in the closed position and transfers a portion of the HTF flow from pump 15 to BOG recondenser 14 through line 108 and line 105 and through line 103. Valve 21 in line 108 is in the open position for delivery out of the BOG recondenser 14 through which valve 18 is in the open position and through line 104 where valve 19 is in the open position. Valve 18 is used to control the reverse flow through the BOG recondenser when starting the optimization function. Valve 21 is used to allow a portion of the flow into line 108 and the remainder of the HTF flow from pump 15 to enter BOG preheater 11 through line 101 .

4 shows that the HTF circulates through a separate liquid circuit from the liquid pump 15, through the BOG preheater 11, the BOG recondenser 14, and the liquid trim heater 16, bypass and/or reuse shows the BOG reliquefaction system in normal operating mode, not in use. In a separate liquid circuit to allow the HTF to circulate through the BOG recondenser (14) before exiting the BOG preheater (11) in line (103) and entering the liquid trim heater (16) through line (106), line (103) Valve 18 in line 104 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.

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

In normal operation, the separate liquid circuits, in order of flow in the normal direction, are: liquid pump (15), BOG preheater (11), BOG recondenser (14), liquid trimming heater (16) and optionally expansion tank (17). includes equipment

In normal operation, the liquid pump 15 pumps HTF to the BOG preheater 11, typically at +40° C., where the HTF heats the BOG. At the same time the HTF is cooled, typically to -90 °C. HTF is sent to the BOG recondenser 14 from the bottom (upstream) of the BOG recondenser 14. In the BOG recondenser 14, HTF serves as the cooling medium for the BOG and exits the BOG recondenser 14 at about +35° C. at the top (downstream).

The HTF can bypass the BOG recondenser 14 via the bypass connection b1 even in normal operating mode with optimization. Valves 18 and 19 may control the amount of HTF bypassing the BOG recondenser 14 to control the temperature exiting the top of the BOG recondenser 14, thereby BOG recondenser 14 The HTF temperature profile can be controlled so that the temperature difference between the two ends does not exceed an acceptable limit.

The liquid trim heater 16 is located on line 106 where both line 105 to the BOG recondenser 14 and line 104 bypassing the BOG recondenser are connected. A trim heater 16 is located upstream of the liquid pump 15. The liquid trim heater 16 heats the HTF sent back to the liquid pump 15 using a heating medium such as water for example. The heating medium flowing into the liquid trim heater 16 is conducted through a BOG compressor cooler 13 connected to the BOG compressor 12 for BOG cooling. There may be one or several stages of a series of BOG compressors and coolers that perform the necessary compression of BOG for the vessel's gas consumers. The BOG pressure can be eg 7 to 12 bar for gas or dual fuel engines and can be pre-compressed if the engine requires a separate compression system using high compressed gas up to 300 bar. The heating medium flow to the liquid trim heater 16 may be coupled to one or several compressor coolers depending upon reaching an appropriate heating capacity for the liquid trim heater 16 . One or more compression stages may follow connection to a reliquefaction system.

During the normal operating mode described above, a separate liquid circuit with HTF provides two different functions at once.

Preheat the BOG upstream of the BOG compressor (12) to approximately -30°C. This allows the use of non-cryogenic BOG compressors. A non-cryogenic BOG compressor means a compressor that does not have to withstand temperatures below -150°C. This allows the investment cost for the system to be reduced.

When the liquid heat transfer fluid circuit acts as a preheater, typical systems can handle BOG flows between about 1750 kg/h and up to about 5000 kg/h. The flow of the heat transfer fluid can be adjusted with the BOG flow to maintain the BOG flow temperature at the preheater outlet at about -30°C.

In existing systems using N ₂ for preheating, where N ₂ preheating is not available or sufficient, another heat source is needed, which usually runs an additional BOG preheater in parallel. The system according to the invention has only one BOG preheater, which simplifies the system and makes switching between modes much easier. The one gas/liquid and one liquid/liquid exchanger will be more compact than the at least one gas/gas and at least one gas/liquid.

An optimization function that contributes to the cooling capacity of the reliquefaction process by removing additional heat from the BOG recondenser (14), thereby increasing the efficiency of the system. Compared to existing nitrogen loop based preheat and recondensation systems for BOG, the system according to the present invention can provide a more efficient and simpler system. By transferring excess "cold air" from the BOG preheater 11 to the BOG recondenser 14 by the HTF, the required capacity from the refrigeration cycle is reduced. Therefore, optimizing reliquefaction using HTF can increase the reliquefaction capacity of the BOG recondenser 14 by about 67% compared to operation without HTF.

Once started, the optimization function is most often used while the reliquefaction system is running. And, if necessary, some HTF goes to the BOG recondenser bypass. Therefore, it is common for the mode using the optimization function to be turned on for the entire voyage of the LNG carrier. However, the optimization function may be stopped if the reliquefaction system is set on standby or stopped completely. This can happen, for example, if the fuel consumption of the main engine and other consumers is greater than the BOG produced, so the pressure in the LNG tank drops (practically all LNG vessels have engines that use boil-off gas to power the vessel). there is). The running time of the reliquefaction system may vary from vessel to vessel.

A separate liquid HTF circuit with the details described here ensures flexibility not possible with conventional preheater systems in conventional BOG reliquefaction plants.

After exiting the BOG compressor cooler (13), a portion of the BOG may be conducted through line (111) to a BOG recondenser (14) for condensation, from which a portion of the BOG is converted into liquefied gas As a result, the valve 24 is returned to the LNG cargo tank 10 through the line 112 in the open position. The BOG entering the BOG recondenser 14 may be in the range of 0 to 100% of the total amount of BOG exiting the BOG compressor cooler 13. The remainder of the BOG exiting the BOG compressor cooler 13 is not directed to the BOG recondenser for recondensation, but is directed out of the reliquefaction system to the fuel consumer.

A refrigeration cycle coupled to the BOG recondenser 14 is configured to remove heat from the portion of BOG that is condensed in the BOG recondenser 14 .

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

The separate liquid circuit may include a third bypass connection (b3) upstream of the BOG preheater (11) to allow a portion of the HTF to bypass the BOG preheater (11). The HTF may bypass the BOG preheater 11 to control the HTF temperature entering the BOG recondenser 14. 5 shows a BOG reliquefaction system in which the HTF bypasses the BOG preheater 11. The valve 22 parallel to the BOG preheater 11 in line 109 is in the open position, so the HTF bypasses the BOG preheater 11. As shown in Figure 5, at the same time the third bypass connection (b3) is open for a portion of the HTF to bypass the BOG preheater 11, the flow and/or temperature within the system components To control, the bypass connection b1 can be opened. Bypass connection b1 may be closed when bypass connection b3 is open.

The HTF is recycled downstream of the BOG preheater 11 to control the temperature of the HTF going to the BOG recondenser 14. By recycling more of the cold HTF back to the liquid pump 15, more HTF can flow through the BOG preheater 11 and the flow out of the BOG preheater 11 is maintained at the desired temperature. 6 shows a BOG reliquefaction system in which HTF is recycled. Valve 23 downstream of BOG preheater 11 in line 110 is in the open position for HTF recirculation to liquid pump 15. As shown in Figure 6, at the same time a portion of the HTF is returned to the BOG preheater 11 liquid pump 15, the bypass connection b1 will be opened to control the flow and/or temperature of system components. can Bypass connection b1 may also be closed while the HTF is being recycled.

Bypass of the BOG preheater 11 through bypass connection b3, HTF recirculation to liquid pump 15 and bypass of the BOG recondenser through bypass connection b1 can all be used simultaneously.

Valve 22 and/or valve 23 respectively control the amount of HTF that bypasses the BOG preheater 11 and the amount of HTF that is recirculated from downstream of the BOG preheater 11 back to the liquid pump 15, thereby reducing the BOG recondenser (14) can be used to control the HTF temperature.

The heat transfer fluid in the separate liquid circuit of the BOG reliquefaction system according to the present invention is a liquid heat transfer fluid suitable for heat transfer to low temperatures. HTF can be a synthetic liquid 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, for example based on hydrocarbons or silicones. An example of a suitable heat transfer fluid composition is a mixture of methyl cyclohexane and trimethyl pentane, such as the commercially available Therminol® VLT heat transfer fluid produced by Eastman. Therminol ^® VLT heat transfer fluid is a synthetic liquid fluid for use in cryogenic applications such as single fluid heating and cooling systems between -115°C and +175°C. Examples of other suitable heat transfer fluids commercially available are Caltherm UBT (silicone) for use at -100 to +260°C from Caldic, Dynalene MW (hydrocarbon for use at -112 to +163°C) from Dynalene. ) and for use at -112 to +200 ° C produced by Fragol Fragoltherm X-T9-A (silicone).

Compression of the BOG takes place in at least one BOG compressor (12) having at least one BOG compressor cooler (13) with a cooling medium. There may be multiple BOG compressors and coolers arranged in series. Preferably, the compression of BOG is performed in several stages with inter-cooling and after-cooling using a cooling medium. At least one BOG compressor 12 may be of the non-cryogenic compressor type. Examples of suitable compressors are positive displacement compressors, eg screw compressors, or dynamic compressors, eg centrifugal compressors.

Cooling medium heated from BOG compression leaving the BOG compressor cooler 13 can be reused as heating medium for the liquid trim heater 16 . Optionally, separate supplies for the BOG compressor cooler (13) and liquid trim heater (16) are also possible. The cooling medium is usually water, a water-glycol mixture or the like. The system according to the present invention, when the heat added to the HTF in the BOG recondenser (14) is not sufficient, provided that the BOG can be heated with another heat exchanger downstream of the BOG preheater (11) without the liquid trim heater (16). can be used However, a preferred solution would be a liquid trim heater 16.

separate liquid circuit It may include an expansion tank 17 connected to the HTF circuit. Expansion tank 17 can be any type of expansion tank, the purpose of expansion tank 17 is to expand the HTF.

The BOG leaving the LNG cargo tank 10 is typically at a temperature of about -140 to about -110°C as it enters the BOG preheater 11 via line 100. BOG exits the BOG preheater (11) via line (102) at a temperature of about -30° C. and then enters at least one BOG compressor (12) with at least one BOG compressor cooler (13). After compression, the BOG leaves at least one BOG compressor 12 with at least one BOG compressor cooler 13 at a temperature of about +40°C. It is then sent to the fuel consumer and/or BOG recondenser 14 via line 111 for reliquefaction. BOG is cooled and liquefied in BOG recondenser 14, entering a temperature typically of about +40°C, and at a temperature of at least about -163°C, which is a liquid without pressurization, through line 112 with valve 24 in the open position. It returns to the LNG cargo tank (10) through.

The refrigeration cycle of the BOG recondenser 14 may be a nitrogen cycle. Then the refrigerant is N ₂ do. A reverse Brayton type nitrogen cycle can be used to cool the BOG recondenser. However, other cooling cycles may be used within the scope of the present invention. The overall process flow diagram in FIG. 1 shows details of a basic inverse Brayton cycle in which the N ₂ flow is reversed in the BOG recondenser 14 . Although this cycle is described as one compressor stage with one compressor 25 and after-cooler 26, two or more compression stages may be used. In the refrigeration cycle, N ₂ flows in the cyclo line 113, passes through the compressor 25 and after-cooler 26, flows through the BOG recondenser 14, and then exits the BOG recondenser 14 to the expander ( 27) and flows in the opposite direction again through the BOG recondenser (14).

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

A second aspect of the present invention relates to at least one BOG compressor (12) having at least one liquefied natural gas (LNG) cargo tank (10), a boil-off gas (BOG) preheater (11), and a BOG compressor cooler (13). ), a method for re-liquefying boil-off gas in a BOG re-liquefaction system including a BOG re-condenser 14 having a refrigeration cycle, the BOG re-liquefaction method comprising: a liquid pump 15, a BOG preheater 11, Pumping a heat transfer fluid (HTF) circulating in a separate liquid circuit comprising a BOG recondenser (14) and a liquid trim heater (16) into a BOG preheater (11) by a liquid pump (15), BOG to BOG Preheating the BOG flowing into the BOG preheater 11 from the LNG cargo tank 10 into HTF before being transported to the compressor 12, and the cooled HTF exiting the BOG preheater 11 in a separate liquid circuit as a liquid It relates to a boil-off gas reliquefaction method comprising transporting to a trim heater (16).

2, HTF circulating in a separate liquid circuit is supplied by a liquid pump 15. Pumping to BOG preheater (11) via line (101), LNG cargo via line (100) by HTF in BOG preheater (11) before delivering BOG to BOG compressor (12) via line (102) Preheating the BOG entering the BOG preheater (11) from the tank (10), the cooled HTF leaving the BOG preheater (11) in the line (103) with the valve (18) in the closed position, and the valve (19) open direct transfer to liquid trim heater 16 via line 104 and line 106 in position.

The embodiment of the method shown in FIG. 3 passes a first portion of the HTF to the BOG preheater 11 and passes a second portion of the HTF through the second bypass connection b2 to the BOG recondenser 14 at the top. Pass the first part of HTF from the BOG preheater (11) and the second part of the HTF from the BOG recondenser (14) to the trim heater (16) through the first bypass connection (b1) pass This passes the HTF through line 108 with valve 21 in the open position and to the top of the BOG recondenser 14 with line 105 and valve 20 in the closed position, and the BOG recondenser 14 ) is returned to the liquid trim heater 16 via line 103 with valve 18 in the open position and at the bottom through line 104 with valve 19 in the open position. This is the beginning of the optimization function of the circuit.

Figure 4 shows the HTF leaving the BOG preheater 11 from the bottom (upstream) through the line in a separate liquid circuit with valve 18 in the open position and valve 19 in line 104 in the closed position. Transport to enter through BOG recondenser (14) and exit BOG recondenser (14) from the top (downstream) via line (105) with valve (20) open and trim liquid via line (106) One embodiment of the method is shown, entering the heater 16 and exiting the liquid trim heater 16 via line 107. As described for the system above, this is the normal mode of operation using optimization features.

After the BOG leaves the BOG compressor cooler 13, a portion of the BOG may flow through line 111 to the BOG recondenser 14 for condensation, from which a portion of the BOG is converted into liquefied gas. via line 112 with valve 24 in the open position. It can be returned to the LNG cargo tank (10).

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

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

5 is part of the HTF It shows an embodiment of how the third bypass connection (b3) is opened to bypass the BOG preheater (11). HTF flows through line 109 with valve 22 in the open position for bypass of BOG preheater 11. As shown in FIG. 5, at the same time as the third bypass connection (b3) is opened so that a part of the HTF bypasses the BOG preheater 11, the bypass connection (for flow and / or temperature control in the system components) b1) is can be opened Bypass connection b1 may be closed when bypass connection b3 is open.

6 shows an embodiment of a method comprising passing HTF through line 110 with valve 23 in an open position following BOG preheater 11 to recirculate HTF upstream of liquid pump 15. show As shown in FIG. 6, a portion of the HTF is recycled downstream of the BOG preheater 11 back to the liquid pump 15, while the bypass connection b1 controls the flow and/or temperature of system components. can be opened for Bypass connection b1 may also be closed during recycling of the HTF.

Bypass of the BOG preheater 11 through bypass connection b3, HTF recirculation to liquid pump 15 and bypass of the BOG recondenser through bypass connection b1 can all be used simultaneously.

Valve 22 and/or valve 23 respectively control the amount of HTF bypassing the BOG preheater 11 and the amount of HTF recirculated downstream of the BOG preheater 11 back to the liquid pump 15, thereby reducing the BOG recondenser (14) can be used to control the HTF temperature.

As described above for the reliquefaction system, in the reliquefaction method according to the present invention, the heat transfer fluid in the separate liquid circuit is a liquid heat transfer fluid suitable for heat transfer to low temperatures. The HTF may be a synthetic liquid fluid as specified 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 present invention, the compression of BOG is performed in at least one BOG compressor cooler 13 and at least one BOG compressor 12 with a cooling medium. Preferably, the compression of BOG is performed in several stages with inter-cooling and post-cooling using a cooling medium. At least one BOG compressor 12 may be of the non-cryogenic compressor type as specified 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 present invention, 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. there is. Optionally, it is also possible to feed the BOG compressor cooler 13 and the liquid trim heater 16 separately. The cooling medium is usually 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 leaving the LNG cargo tank 10 is generally between about -140 and about -110 when entering the BOG preheater 11 via line 100. is at a temperature of °C. 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) with at least one BOG compressor cooler (13). After compression, the BOG leaves at least one BOG compressor (12) with at least one BOG compressor cooler (13) at a temperature of approximately +40° C., then via line (111) for reliquefaction to the fuel consumer and/or or to the BOG recondenser (14). The BOG is cooled and liquefied in the BOG recondenser 14 and enters at a temperature of about +40°C, typically liquid at least without pressurization, at a temperature of about -163°C via line 112 with valve 24 in the open position. Back to the LNG cargo tank (10).

As described above for the reliquefaction system, the refrigeration cycle of the BOG recondenser 14 in the method according to the present invention may be a nitrogen cycle. However, other cooling cycles may be used within the scope of the present invention. An inverse Brayton nitrogen cycle may advantageously 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 is a boil-off gas (BOG) reliquefaction system comprising at least one liquefied natural gas (LNG) cargo tank (10), a BOG preheater (11), and at least one BOG compressor (12) with BOG It's about how to make it work. This method transfers heat transfer fluid (HTF) circulating in a separate liquid circuit including a liquid pump (15), a BOG preheater (11), a BOG recondenser (14) and a liquid trim heater (16) to a liquid pump (15). In this step, the system is started by pumping to the BOG preheater 11 by preheating the BOG flowing into the BOG preheater 11 from the LNG cargo tank 10 by the HTF, transporting it to the BOG compressor 12, and BOG and transporting the cooled HTF leaving the BOG preheater (11) to the liquid trim heater (16) in a separate liquid circuit via a first bypass connection (b1) downstream of the preheater (11). After the system is started, a first portion of the HTF is passed through the BOG preheater (11) and a second portion of the HTF is passed through the BOG recondenser (14) from the top to the bottom of the BOG recondenser (14) in the reverse direction to the second bi Pass through the pass connection (b2), the first part of the HTF from the BOG preheater (11) and the second part of the HTF from the BOG recondenser (14) through the first bypass connection (b1) liquid trim heater (16), the normal operating mode of the system is initiated. After the normal operating mode is started, the HTF leaving the BOG preheater (11) is circulated from bottom to top in a separate liquid circuit through the BOG recondenser (14) before entering the liquid trim heater (16) and condensed in the BOG compressor A part of the BOG discharged from the cooler 13 is led to the BOG recondenser 14, and the part of the BOG is returned to the LNG cargo tank 10 as liquefied gas in the BOG recondenser 14, and the BOG recondenser ( 14) to remove heat from a portion of the BOG condensed in the BOG recondenser 14, and from a portion of the BOG condensed in the BOG recondenser 14 by the HTF flowing through the BOG recondenser 14. By further removing the heat, the normal operating mode of the system is initiated.

One skilled in the art will recognize that the present invention is not limited to the preferred embodiments described above. Those skilled in the art will further recognize that modifications and variations are possible within the scope of the appended claims. For example, further modifications to the disclosed embodiments may be understood and effected by those skilled in the art when practicing the claimed disclosure from a study of the drawings, disclosure, and appended claims.

Claims (28)

At least one liquefied natural gas (LNG) cargo tank (10), a boil-off gas (BOG) preheater (11), at least one BOG compressor (12) with a BOG compressor cooler (13), a BOG with a refrigeration cycle. In the BOG reliquefaction system comprising a recondenser (14), the BOG reliquefaction system uses a liquid heat transfer fluid ( HTF) circulates, 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, and a BOG preheater 11 ) a boil-off gas reliquefaction system comprising a BOG recondenser (14) located downstream and a liquid trim heater (16) located downstream of the BOG recondenser (14) configured to heat the HTF. 2. Boil- according to claim 1, characterized in that the separate liquid circuit comprises a first bypass connection (b1) from downstream of the BOG preheater (11) to allow the HTF to bypass the BOG recondenser (14). Off-gas reliquefaction system. 3. The second bypass connection of claim 2, wherein the separate liquid circuit runs from upstream of the BOG preheater (11) to the top of the BOG recondenser (14) to circulate in the opposite direction through the BOG recondenser (14). A boil-off gas reliquefaction system comprising (b2). 4. The method of claim 3, wherein the first part of the HTF is delivered to the BOG preheater (11) and the second bypass connection (b2) allows the second part of the HTF to pass through the BOG recondenser (14) to the BOG recondenser (14). is opened to pass in the opposite direction from top to bottom, and the first bypass connection (b1) is such that the first part of the HTF from the BOG preheater (11) and the second part of the HTF from the BOG recondenser are trim heater (16). ) Boil-off gas reliquefaction system, characterized in that open to pass through. 2. The method of claim 1, characterized in that the HTF circulates through the liquid pump (15) separate liquid circuit, through the BOG preheater (11), through the BOG recondenser (14) and through the liquid trim heater (16). a boil-off gas reliquefaction system. 7. The process according to any one of the preceding claims, wherein a portion of the BOG is conducted from the BOG compressor cooler (13) to a BOG recondenser (14) for condensation, from which the portion of the BOG is liquefied. Boil-off gas re-liquefaction system, characterized in that the return to the LNG cargo tank (10) as the gas. 7. The boil-off gas reliquefaction system according to claim 6, characterized in that the refrigeration cycle connected to the BOG recondenser (14) is configured to remove heat from a portion of the BOG condensed in the BOG recondenser. 8. Boil-off gas reliquefaction system according to claim 6 or 7, characterized in that the HTF flowing from the bottom to the top through the BOG recondenser (14) removes heat from a portion of the BOG condensed in the BOG recondenser (14). . 2. The separate liquid circuit according to any of the preceding claims, characterized in that the separate liquid circuit comprises a third bypass connection (b3) upstream of the BOG preheater (11) to allow part of the HTF to bypass the BOG preheater (11). Boil-off gas reliquefaction system. 10. Boil-off gas reliquefaction system according to any one of the preceding claims, characterized in that part of the HTF is recirculated back from the BOG preheater (11) upstream of the liquid pump (15). 10. A boil-off gas reliquefaction system according to any one of the preceding claims, characterized in that the cooling medium from the BOG compressor cooler (13) is utilized in the liquid trim heater (16). 10. A boil-off gas reliquefaction system according to any one of the preceding claims, characterized in that at least one BOG compressor (12) is of the non-cryogenic compressor type. 7. A boil-off gas reliquefaction system according to any one of the preceding claims, characterized in that the refrigeration cycle in the BOG recondenser (14) is an inverse Brayton nitrogen cycle. 2. A boil-off gas reliquefaction system according to any one of the preceding claims, characterized in that the separate liquid circuit comprises an expansion tank (17). At least one liquefied natural gas (LNG) cargo tank (10), a boil-off gas (BOG) preheater (11), at least one BOG compressor (12) with a BOG compressor cooler (13), a BOG with a refrigeration cycle. A method for reliquefying boil-off gas in a BOG reliquefaction system comprising a recondenser (14), the BOG reliquefaction method comprising: a liquid pump (15), a BOG preheater (11), a BOG recondenser (14) and Pumping a heat transfer fluid (HTF) circulating in a separate liquid circuit comprising a liquid trim heater (16) into a BOG preheater (11) by a liquid pump (15), transporting the BOG to a BOG compressor (12) Preheating the BOG introduced from the LNG cargo tank 10 into the BOG preheater 11 with HTF, and transporting the cooled HTF exiting the BOG preheater 11 to the liquid trim heater 16 in a separate liquid circuit Boil-off gas reliquefaction method comprising the step of. 16. Boil- Off-gas reliquefaction method. 17. The method of claim 16, wherein the first part of the HTF is passed through the BOG preheater (11), and the second part of the HTF through the second bypass connection (b2) is reversed from the top to the bottom of the BOG recondenser (14) passes through the BOG recondenser 14, and the first part of HTF coming out of the BOG preheater 11 and the second part of HTF coming out of the BOG recondenser 14 through the first bypass connection b1 are liquid Boil-off gas reliquefaction method characterized in that it passes through the trim heater (16). 16. Boil-off gas ash according to claim 15, characterized in that the HTF exiting the BOG preheater (11) is transported through the BOG recondenser (14) before entering the liquid trim heater (16) in a separate liquid circuit. liquefaction method. 19. The method according to any one of claims 15 to 18, wherein a portion of the BOG is directed to a BOG recondenser (14) for condensation, and a portion of the BOG from the BOG recondenser (14) is converted to liquefied gas in an LNG cargo tank ( Boil-off gas reliquefaction method characterized by returning to 10). 20. A method according to claim 19, characterized in that heat is removed from a portion of the BOG condensed in the BOG recondenser (14) in a refrigeration cycle connected to the BOG recondenser (14). 21. Boil-off gas according to claim 19 or 20, characterized in that heat is further removed from a portion of the BOG condensed in the BOG recondenser (14) by HTF flowing through the BOG recondenser (14). reliquefaction method. 22. Boil-off according to any one of claims 15 to 21, characterized in that part of the HTF bypasses the BOG preheater (11) via a third bypass connection (b3) upstream of the BOG preheater (11). Gas reliquefaction method. 23. A method according to any one of claims 15 to 22, characterized in that part of the HTF flowing out of the BOG preheater (11) and flowing upstream of the liquid pump (15) is recycled. 24. A method according to any one of claims 15 to 23, characterized in that as heating medium in the liquid trim heater (16) a cooling medium from a BOG compressor cooler (13) is used. 25. A method according to any one of claims 15 to 24, characterized in that at least one BOG compressor (12) is of the non-cryogenic compressor type. 26. A method according to any one of claims 15 to 25, characterized in that the refrigeration cycle in the BOG recondenser (14) is a reverse Brayton nitrogen cycle. 27. Method according to any one of claims 15 to 26, characterized in that the separate liquid circuit comprises an expansion tank (17). At least one liquefied natural gas (LNG) cargo tank (10), a boil-off gas (BOG) preheater (11), at least one BOG compressor (12) with a BOG compressor cooler (13), a BOG ash with a refrigeration cycle. A method (13) of operation of a BOG reliquefaction system comprising a condenser (14), said method of operation comprising:
- heat transfer fluid (HTF) circulating in a separate liquid circuit comprising a liquid pump (15), a BOG preheater (11), a BOG recondenser (14) and a liquid trim heater (16) by a liquid pump (15) In the step of starting the system by pumping to the BOG preheater 11, the BOG flowing into the BOG preheater 11 from the LNG cargo tank 10 is preheated by HTF and then transported to the BOG compressor 12, and the BOG preheater ( 11) system start-up, transporting the cooled HTF leaving the BOG pre-heater 11 to the liquid trim heater 16 in a separate liquid circuit via the downstream first bypass connection b1;
- Pass the first part of the HTF to the BOG preheater 11, and pass the second part of the HTF from the top to the bottom of the BOG recondenser 14 through the BOG recondenser 14 in the reverse direction to the second bypass connection b2 ), the first part of the HTF from the BOG preheater (11) and the second part of the HTF from the BOG recondenser (14) through the first bypass connection (b1) to the liquid trim heater (16) pass, to start the normal operating mode of the system;
circulates the HTF leaving the BOG preheater (11) from the bottom to the top in a separate liquid circuit through the BOG recondenser (14) before entering the liquid trim heater (16) and discharged from the BOG compressor cooler (13) for condensation A portion of the BOG to be induced into the BOG recondenser 14, returning the portion of the BOG to the LNG cargo tank 10 as liquefied gas in the BOG recondenser 14, and a refrigeration cycle connected to the BOG recondenser 14 by removing heat from a portion of the BOG condensed in the BOG recondenser 14 and further removing heat from the portion of the BOG condensed in the BOG recondenser 14 by HTF flowing through the BOG recondenser 14. , A method of operating a BOG reliquefaction system comprising the steps of proceeding to a normal operating mode of the system.

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