Device, facility and method for keeping liquified gas storage part at low temperature

Abstract

To provide a device and a method for keeping a liquified gas storage part at low temperature.SOLUTION: A device 1 comprises a cryogenic refrigerator 3, sub-cool circuits 4, 5 having a suction pump 17 installed in a liquified gas storage part 2, and a heat exchanger 6 for exchanging heat between the sucked sub-cool circuit and the refrigerator 3. The sub-cool circuit has at least one injection end 7 for injecting fluid cooled in the heat exchanger into a storage part. This device further comprises a boil-off gas recovery pipe 8 connected to the storage part 2 to recover boil-off gas and having an upstream end. The recovery pipe comprises a downstream end 18 connected to a consumption part. This device comprises a bypass pipe 9 and a valve 10 enabling to transfer the boil-off gas to the sub-cool circuit from the recovery pipe. The bypass pipe has a first end connected to the recovery pipe, and a second end connected to the sub-cool circuit. The second end of the bypass pipe is connected to the sub-cool circuit at an upstream side of the heat exchanger.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to devices, facilities and methods for keeping liquefied gas stores cool.

[0002] The present invention particularly relates to facilities and methods for keeping liquefied gas reservoirs, for example for liquefied natural gas, at low temperature and / or under pressure.

[0003] The invention more particularly relates to a device for keeping a liquefied gas reservoir, for example for liquefied natural gas, at a low temperature, the device comprising a cryogenic refrigerator and a subcooling circuit comprising a set of tubes, the subcooling circuit intended to be placed at the bottom of the liquefied gas reservoir and configured to draw in liquefied gas, and a heat exchanger for exchanging heat between the drawn in subcooling circuit and the refrigerator, the subcooling circuit comprising at least one injection end configured to inject a fluid cooled in the heat exchanger into the reservoir, the device further comprising a boil-off gas recovery pipe having an upstream end intended to be connected to the top of the reservoir for recovering the boil-off gas, the recovery pipe comprising a downstream end intended to be connected to a consumer of the boil-off gas, for example a burner and / or a motor, the device comprising a bypass pipe and a set of valves configured to allow the transfer of the boil-off gas from the recovery pipe to the subcooling circuit, the bypass pipe having a first end connected to the recovery pipe and a second end connected to the subcooling circuit.

[0004] Boil-off gases from the cryogenic fluids in storage can be recovered by direct re-liquefaction. This solution involves cryogenic compression of these boil-off gases to ambient temperature and then sending them to a liquefaction system.

[0005] Another function involves re-liquefying the boil-off gas by transferring cooling power from a refrigerator to the cryogenic liquid by subcooling the cryogenic liquid and returning it to storage. [Background technology]

[0006] Document WO2019020742A1 describes a facility for treating boil-off gas or for cooling liquefied gas.

[0007] Cooling heat exchangers can become clogged by precipitation of the heaviest components (which are less soluble at lower temperatures). This is because cryogenic fluids are not necessarily pure components, but can be mixtures. This means that some components of the mixture can solidify during subcooling. These solid particles can clog the refrigerator exchangers, thereby degrading the system's performance.

[0008] There are different known solutions to reduce or limit such clogging, however they are not satisfactory for both the treatment of the boil-off gas and / or the sub-cooling of the stored liquefied gas. Summary of the Invention

[0009] One object of the present invention is to overcome all or part of the above-mentioned shortcomings of the prior art.

[0010] To this end, the device according to the invention is essentially characterised in that, in other respects in accordance with its generic definition given in the preamble above, the second end of the bypass pipe is connected to the subcooling circuit upstream of the heat exchanger.

[0011] Further, an embodiment of the present invention comprises: - the valve set comprises a flow control valve on the bypass line; - the flow control valve is a control valve configured to transfer the boil-off gas flow to the subcooling circuit in order to increase the temperature of the fluid entering the heat exchanger by a given value, for example 5°C to 25°C; - a flow control valve configured to transfer the boil-off gas stream to a subcooling circuit to maintain a temperature of a fluid entering the heat exchanger below a saturation temperature of the liquefied gas; - the flow control valve is a control valve configured to transfer the boil-off gas flow to the subcooling circuit in order to keep the temperature of the fluid leaving the heat exchanger above a given value, for example above -160°C and / or at a value equal to the temperature of the liquefied gas in the reservoir and / or at a value equal to the temperature of the liquefied gas sucked in at the suction end, - the recovery pipe includes at least one compressor, and a first end of the bypass pipe is connected to the recovery pipe downstream of the compressor; - the recovery pipe comprises a plurality of compressors in series, and the first end of the bypass pipe is connected to the recovery pipe downstream of the intermediate compressor, i.e. upstream of the last compressor in the series; - the compressor is configured to supply a gas flow to the bypass pipe at a pressure greater than the pressure of a liquid suctioned at the suction end and supplied to the subcooling circuit; - the device includes a mixing unit for mixing the boil-off gas into the subcooling circuit, the mixing unit being, for example, located at the junction between the second end of the bypass pipe and the subcooling circuit, the mixing unit comprising at least one of an indirect heat exchanger with injection, a gas injector into the liquid, a static mixer, an upstream injector, a filtration system, a condensate pot with random packing or structured packing, - the bypass pipe comprises a pre-cooling unit in heat exchange with the boil-off gas transferred to the sub-cooling circuit, the pre-cooling unit being configured, for example, to cool the transferred boil-off gas stream to an intermediate temperature between the temperature of the boil-off gas in the recovery pipe and the temperature of the liquefied gas, - the recovery pipe comprises a heat exchanger enabling heat exchange between the boil-off gas in the recovery pipe and the boil-off gas in the bypass pipe; - the bypass pipe comprises a bypass pipe and a set of valves for controlling the flow of boil-off gas in the bypass pipe that is allowed to flow through a heat exchanger on the recovery line; the suction end comprises a suction pump.

[0012] The invention also relates to a liquefied gas storage facility, e.g. for liquefied natural gas, e.g. a ship carrying liquefied gas, comprising at least one liquefied gas reservoir and a device for keeping a fluid contained in the reservoir at a low temperature, the low temperature maintenance device having any one of the characteristics described above or below.

[0013] The present invention also relates to a method for keeping a liquefied gas store at a low temperature using a device according to any one of the preceding claims, the method comprising pumping liquefied gas from a cryogenic store, cooling the pumped liquefied gas and re-injecting the cooled liquefied gas into the store, the method comprising the steps of recovering boil-off gas from the store and injecting and mixing the boil-off gas with the liquefied gas before the pumped liquefied gas is cooled.

[0014] This makes it possible to increase the temperature of the fluid at the input of the subcooling circuit by injecting boil-off gas, preventing crystallization of heavy hydrocarbons in the heat exchanger 6, since the subcooler is preferably configured so that the temperature of the fluid being cooled never falls below the initial temperature of the subcooled liquid.

[0015] According to other possible features, the injecting and mixing step is configured to increase the temperature of the pumped liquefied gas before cooling the pumped liquefied gas by a given value and / or to keep the temperature of the cooled liquefied gas below a given threshold and / or at the temperature of the stored or pumped liquefied gas.

[0016] The invention may also relate to any alternative device or method comprising any combination of the above or below features falling within the scope of the claims.

[0017] Other particular features and advantages will become apparent from the following description, which is provided with reference to the following figures.

[0018] The invention will be better understood on reading the following description, given purely by way of example with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0019] [Figure 1] 1 is a schematic diagram illustrating an example of the structure and operation of a facility according to a first embodiment of the present invention. [Diagram 2] FIG. 11 is a schematic diagram illustrating an example of the structure and operation of a facility according to a second embodiment of the present invention. [Diagram 3] FIG. 11 is a schematic diagram illustrating an example of the structure and operation of a facility according to a third embodiment of the present invention. [Figure 4] FIG. 13 is a schematic diagram illustrating an example of the structure and operation of a facility according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Like numbers refer to like elements throughout the drawings.

[0021] In this detailed description, the following embodiments are examples. Although the description refers to one or more embodiments, this does not mean that the features apply only to a single embodiment. Individual features of different embodiments can also be combined and / or substituted to provide other embodiments.

[0022] The illustrated device 1 for keeping a liquefied gas reservoir 2 cool can be used to keep, for example, a liquefied natural gas tank, for example on a ship, cool and / or under pressure.

[0023] The device 1 comprises a cryogenic refrigerator 3 and a subcooling circuit 4, 5 comprising a set of tubes.

[0024] The subcooling circuits 4, 5 are intended to be placed at the bottom of the liquefied gas reservoir 2 and comprise at least one suction end 4 configured to draw in liquefied gas. As illustrated, the suction end 4 preferably comprises a suction pump 17.

[0025] The sub-cooling circuits 4, 5 further comprise a heat exchanger 6 for exchanging heat between the suctioned sub-cooling circuits 4, 5 and the refrigerator 3. This heat exchanger 6 is preferably located outside the store 2.

[0026] The refrigerator may in particular comprise or consist of a cryogenic liquefier or refrigerator in which a cycle gas (helium, nitrogen or any other pure gas or mixture) undergoes a thermodynamic cycle (compression, cooling, expansion, heating) to generate at least one end a cooling power that can be transferred by heat exchange. For example, the refrigerator is a reverse Brayton cycle refrigerator, more specifically a turbo-Brayton refrigerator using turbomachinery (at least one compressor, one turbine on bearings and a magnetic motor combined on a single shaft). The subcooling circuit 4, 5 comprises at least one injection end 5, 7 configured to inject the fluid cooled in the heat exchanger 6 into the reservoir 2.

[0027] In the illustrated example, the subcooling circuits 4, 5 comprise two injection ends 5, 7. The first end opens into the upper part of the reservoir in the form of one or more nozzles, making it possible to inject a cooled fluid into the gas phase of the reservoir 2 in order to recondense the vapor and control the pressure within the reservoir 2.

[0028] The second end opens, for example, into the lower part of the reservoir 2 for cooling the liquid phase.

[0029] According to the invention, an ejector (injector) at the bottom of the reservoir 2 preferably returns liquefied gas and mixes it into the liquid phase. The injected liquid is not further subcooled and is at the same temperature as the liquid in the reservoir 2.

[0030] The device 1 further comprises a boil-off gas recovery pipe 8 having an upstream end connected to the top of the storage 2 and configured to recover the boil-off gas. The recovery pipe 8 preferably comprises at least one compression unit 11 (typically at least one compressor) for the recovered boil-off gas. The recovery pipe 8 comprises downstream at least one downstream end 18 intended to be connected to a consumer of the boil-off gas, for example a burner and / or a motor. This compression unit 11 makes it possible to keep a constant pressure in the storage 2 and to supply the motor with gas. The excess combustible boil-off gas can for example be conveyed to a combustion flare.

[0031] The device 1 comprises a bypass pipe 9 and a set of valves 10 configured to allow the transfer of boil-off gas from the recovery pipe 8 to the sub-cooling circuits 4, 5. The bypass pipe 9 preferably has a first end connected to the recovery pipe 8 downstream of the compression unit 11 and a second downstream end connected to the sub-cooling circuits 4, 5 upstream of the heat exchanger 6. For example, the set of valves comprises a flow control valve 10 on the bypass pipe 9.

[0032] This makes it possible to obviate clogging, for example by injecting extracted boil-off gas, for example at ambient temperature, after compression.

[0033] This allows for the injection of relatively hot gas upstream of the chiller / liquefier cooling. This gas, carried by the bypass pipe 9, can be mixed into the cryogenic liquid stream withdrawn from storage via the suction pipe 4 of the subcooling circuit.

[0034] The withdrawn gas mixed with the liquid flow is preferably at least at a pressure higher than the delivery pressure of the liquid suction pump 17 located at the bottom of the cryogenic fluid reservoir 2 .

[0035] As illustrated, the device 1 may include a mixing unit 12 for the boil-off gas in the subcooling circuits 4, 5. This mixing unit 12 is configured to partially or completely mix the gas in the liquid and is located, for example, at the junction between the downstream end of the bypass pipe 9 and the subcooling circuits 4, 5. This mixing unit 12 comprises, for example, at least one of an indirect heat exchanger with injection, a gas injector into the liquid, a static mixer, an upstream injector, a filtration system, a condensate pot with random or ordered packing, or any other suitable unit or some combination of these techniques.

[0036] The flow control valve 10 is, for example, a control valve that may be configured to transfer the boil-off gas stream to the sub-cooling circuits 4, 5 in order to increase the temperature of the fluid entering the heat exchanger 6 by a given value, for example between 5°C and 25°C.

[0037] For example, the flow control valve 10 may be configured to transfer the boil-off gas stream to the sub-cooling circuits 4, 5 to keep the temperature of the fluid entering the heat exchanger 6 below the saturation temperature of the liquefied gas. This allows the mixture to remain in the liquid phase. A temperature margin, for example 2°C below saturation, may be provided to ensure that gas bubbles do not enter the heat exchanger 6.

[0038] The gas streams mixed upstream of the heat exchanger 6 can be controlled, for example, by calculating the saturation temperature of the fluid (the bubble point temperature, which is the temperature at which the first vapor bubbles form).

[0039] This saturation temperature may be based on the typical composition of the stored fluid.

[0040] This saturation temperature can be adjusted, for example, according to a correlation with the pressure parameter, making it possible to control the injection of boil-off gas by the flow control valve 10 until the output temperature from the mixer 12 is at a temperature with a safety margin relative to the saturation temperature (the output temperature from the mixer 12 is equal to the saturation temperature of the fluid without the safety margin).

[0041] Alternatively or in combination, the flow control valve 10 can be controlled to keep the temperature of the fluid leaving the heat exchanger 6 above a given value, for example above -160°C, and / or at a value equal to the temperature of the liquefied gas in the reservoir and / or at a value equal to the temperature of the liquefied gas sucked in at the suction end 4.

[0042] For example, the temperature of the fluid may be measured at the output of pump 17 and / or in reservoir 2. The flow rate may be regulated by flow control valve 10 to ensure that the temperature of the fluid at the output of cooling heat exchanger 6 is equal to the temperature of the liquid in reservoir 2.

[0043] The increased pressure of the pumped liquid makes it possible to increase its saturation point, which allows the gas to be completely or partially dissolved in the cryogenic liquid upstream of the liquefaction device.

[0044] This dissolution of the boil-off gas into the liquid is caused by an increase in the temperature of the cryogenic liquid.

[0045] The amount of gas injected via the bypass pipe 9 makes it possible to choose to control the temperature at the input and / or output of the refrigerator (at the input and / or output of the heat exchanger 6). This control can be performed by a single flow control valve 10. The temperature of the fluid to be cooled can thus be measured at the input and / or output of the heat exchanger 6 by one or more suitable temperature sensors 19.

[0046] The refrigerator 3 reduces the temperature of the resulting liquid or gas / liquid mixture before returning it to the reservoir 2 (at the bottom of the reservoir and / or at the top via one or more booms or nozzles).

[0047] The temperature of the fluid at the output of the cooling heat exchanger 6 is therefore controlled so that no or very little solidification occurs inside the heat exchanger 6. This temperature is therefore at least equal to the initial temperature of the cryogenic liquid pumped from the reservoir 2. The amount of gas injected via the bypass pipe 9 may also make it possible to adjust the pressure drop inside the heat exchanger 6.

[0048] The device 1 can also be used in a "mixed" mode, i.e. a mode that combines, on the one hand, the recovery, compression and liquefaction of the reinjected boil-off gas and, on the other hand, the partial subcooling of the cryogenic liquid before returning it to the storage 2.

[0049] The device 1 can alternatively be operated in a subcooling mode, in which the injection of relatively hot boil-off gas through the bypass pipe 9 is stopped (valve 10 is closed), or in a liquefaction mode by injecting boil-off gas through the bypass pipe 9 (valve 10 is open).

[0050] This provides a flexible system offering two operating modes: switching from one mode to the other can be decided at will by an operator, depending for example on the risk of blockage of the heat exchanger 6, and / or automatically via detection or prediction (intelligence) by detecting a blockage (increased pressure difference at the ends of the heat exchanger 6, temperature drop, etc.).

[0051] The embodiment of Fig. 2 differs from that of Fig. 1 in that the bypass pipe 9 comprises a pre-cooling unit 13 for heat exchange with the boil-off gas transferred to the sub-cooling circuits 4, 5. This pre-cooling unit 13 may for example be configured to cool the transferred boil-off gas stream to an intermediate temperature between the temperature of the boil-off gas in the recovery pipe 8 and the temperature of the liquefied gas pumped from the storage 2.

[0052] This allows optimizing or increasing the amount of compressed gas that can be injected through the external pre-cooling unit 13 at an intermediate temperature between the temperature of the cryogenic liquid and the ambient temperature. For example, this pre-cooling unit 13 may comprise or consist of at least one of a refrigeration cycle using a refrigerant fluid, a compression system, and a valve-based, orifice-based, or turbine-based expansion system.

[0053] The embodiment of Figure 3 differs from the embodiment of Figure 1 in that the recovery pipe 8 comprises a heat exchanger 16 that allows for heat exchange between the boil-off gas in the recovery pipe 8 and the boil-off gas in the bypass pipe 9. For example, the bypass pipe 9 comprises a bypass pipe 14 and a set of valves 15 for controlling the flow of the boil-off gas in the bypass pipe 9 that is allowed to flow through the heat exchanger 16 on the recovery pipe 8.

[0054] This makes it possible to pre-cool the boil-off gas by recovering cooling power from the boil-off gas withdrawn from the storage 2 upstream of the compression.

[0055] One or more control valves may be used to selectively control the temperature of the gas at the input of the compression unit, the temperature of the fluid at the input of the chiller (entering the heat exchanger 6), or the temperature of the fluid at the output of the heat exchanger 6. This control may be used to ensure proper operation of the compression unit 11 and to make the most of the recovered cooling power while preventing the formation of solids in the heat exchanger 6 of the chiller 3. This makes it possible to optimize / increase the amount of compressed boil-off gas that can be injected via the bypass pipe 9.

[0056] The embodiment of FIG. 4 differs from that of FIG. 1 in that the recovery pipe 8 comprises a number of compressors 11 in series and a first upstream end of the bypass pipe 9 is connected to the recovery pipe 8 downstream of the intermediate compressor 11, i.e. upstream of the last compressor in the series.

[0057] This means that the boil-off gas is not recovered at the output of the last compressor, but at the output of the first or subsequent compression stage. However, the pressure of the recovered gas is preferably greater than the pressure of the liquid pumped from the storage with which it is mixed. The temperature of this boil-off gas in the intermediate compression stages is relatively lower than the temperature at the output of the last compressor. In this case, the amount of boil-off gas that can be mixed with the liquid can be higher than in the embodiment of FIG. 1. This embodiment can also replace the pre-cooling unit 13 of the embodiment of FIG. 2.

[0058] The device therefore allows optimal management of the boil-off gas and maintaining low temperatures (especially subcooling) of the liquefied gas reservoir, for example for methane, without the formation of solid deposits in the cooling heat exchanger 6. The invention makes it possible to prevent or limit degradation of the performance of the device.

[0059] Boil-off gas injected upstream of the heat exchanger 6 of the refrigerator 3 heats the liquid before it is cooled. The refrigerator 3 then brings this mixture to, for example, the temperature of the fluid in the reservoir 2.

[0060] The withdrawn and mixed boil-off gas 9 is fully or partially liquefied when it is returned to the storage 2. The lowest temperature reached by the fluid in the heat exchanger 6 does not allow it to reach the solidification condition.

[0061] The described solution allows the conversion of conventional subcooling systems by adapting them to direct reliquefaction units in the presence of heavy hydrocarbon impurities. The solution prevents the crystallization of heavy hydrocarbons during cooling.

Claims

1. A device for keeping a liquefied gas reservoir (2) at a low temperature, for example for liquefied natural gas, comprising a cryogenic refrigerator (3) and a subcooling circuit (4, 5) comprising a set of tubes, said subcooling circuit (4, 5) intended to be placed at the bottom of the liquefied gas reservoir (2) and comprising a suction end (4) adapted to suction said liquefied natural gas and a heat exchanger (6) for exchanging heat between a fluid suctioned by said subcooling circuit (4, 5) and said cryogenic refrigerator (3), said subcooling circuit (4, 5) comprising at least one injection end (5, 7) adapted to inject a fluid cooled in said heat exchanger (6) into said liquefied gas reservoir (2), said device (1) being a boil-off device. a boil-off gas recovery pipe (8) having an upstream end intended to be connected to an upper portion of the liquefied gas storage (2) for recovering boil-off gas, said boil-off gas recovery pipe (8) having a downstream end (18) intended to be connected to a consumer of boil-off gas, for example a burner and / or a motor, said device (1) comprising a bypass pipe (9) and a set of valves (10) configured to allow the transfer of boil-off gas from said boil-off gas recovery pipe (8) to said sub-cooling circuit (4, 5), said bypass pipe (9) having a first end connected to said boil-off gas recovery pipe (8) and a second end connected to said sub-cooling circuit (4, 5), A device, characterized in that the second end of the bypass pipe (9) is connected to the subcooling circuit (4, 5) upstream of the heat exchanger (6).

2. 2. The device according to claim 1, characterized in that the set of valves (10) comprises a flow control valve on the bypass pipe (9).

3. 3. The device according to claim 2, characterized in that the flow control valve (10) is a control valve configured to transfer a boil-off gas stream to the sub-cooling circuit (4, 5) in order to increase the temperature of the fluid entering the heat exchanger (6) by a given value, for example between 5°C and 25°C.

4. 4. The device according to claim 2 or 3, characterized in that the flow control valve (10) is a control valve configured to transfer a boil-off gas stream to the sub-cooling circuit (4, 5) in order to keep the temperature of the fluid entering the heat exchanger (6) below the saturation temperature of the liquefied natural gas.

5. A device according to any one of claims 2 to 4, characterized in that the flow control valve (10) is a control valve configured to transfer a boil-off gas stream to the sub-cooling circuit (4, 5) in order to keep the temperature of the fluid leaving the heat exchanger (6) above a given value, for example above -160°C and / or at a value equal to the temperature of the liquefied natural gas in the liquefied gas storage and / or at a value equal to the temperature of the liquefied natural gas sucked at the suction end (4).

6. 6. The device according to claim 1, characterized in that the boil-off gas recovery pipe (8) comprises at least one compressor (11) and the first end of the bypass pipe (9) is connected to the boil-off gas recovery pipe (8) downstream of the compressor (11).

7. 7. The device according to claim 6, characterized in that the boil-off gas recovery pipe (8) comprises a plurality of compressors (11) in series, and the first end of the bypass pipe (9) is connected to the boil-off gas recovery pipe (8) downstream of an intermediate compressor (11), i.e. upstream of the last compressor in the series.

8. 8. A device according to claim 6 or 7, characterized in that the compressor (11) is configured to supply a gas flow to the bypass pipe (9) at a pressure greater than the pressure of the liquid suctioned at the suction end (4) and delivered to the subcooling circuit (4, 5).

9. 9. The device according to any one of claims 1 to 8, characterized in that the device comprises a mixing unit (12) for mixing the boil-off gas into the sub-cooling circuit (4, 5), the mixing unit (12) being for example located at the junction between the second end of the bypass pipe (9) and the sub-cooling circuit (4, 5), the mixing unit (12) comprising at least one of the following: an indirect heat exchanger with injection, a gas injector into the liquid, a static mixer, an upstream injector, a filtration system, a condensate pot with random packing or with ordered packing.

10. 10. The device according to claim 1, characterized in that the bypass pipe (9) comprises a pre-cooling unit (13) for heat exchange with the boil-off gas transferred to the sub-cooling circuit (4, 5), the pre-cooling unit (13) being configured for example to cool the transferred boil-off gas flow to an intermediate temperature between the temperature of the boil-off gas in the boil-off gas recovery pipe (8) and the temperature of the liquefied natural gas.

11. The device according to any one of claims 1 to 10, characterized in that the boil-off gas recovery pipe (8) comprises a heat exchanger (16) enabling heat exchange between the boil-off gas in the boil-off gas recovery pipe (8) and the boil-off gas in the bypass pipe (9).

12. 12. The device according to claim 11, characterized in that the bypass pipe (9) comprises a diversion pipe (14) and a set of valves (15) for controlling the flow of boil-off gas in the bypass pipe (9) that is allowed to flow through the heat exchanger (16) on the boil-off gas recovery pipe (8).

13. A device according to any one of the preceding claims, characterized in that the suction end (4) is equipped with a suction pump (17).

14. A liquefied gas storage facility, for example for liquefied natural gas, for example a ship carrying liquefied gas, comprising at least one liquefied gas reservoir (2) and a device for keeping a fluid contained in said liquefied gas reservoir at a low temperature, said device for keeping a fluid at a low temperature (1) being a device according to any one of claims 1 to 13.

15. A method for keeping a liquefied gas store (2) at a low temperature using a device according to any one of claims 1 to 13, comprising pumping liquefied gas from a cryogenic store, cooling the pumped liquefied gas and re-injecting the cooled liquefied gas into the liquefied gas store, the method comprising the steps of recovering boil-off gas from the liquefied gas store and injecting and mixing the boil-off gas with the pumped liquefied gas before it is cooled.

16. 16. The method according to claim 15, characterized in that the step of injecting and mixing is configured to increase the temperature of the pumped liquefied gas before cooling the pumped liquefied gas by a given value and / or to keep the temperature of the cooled liquefied gas below a given threshold and / or to keep it at the temperature of the stored or pumped liquefied gas.

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