METHOD AND SYSTEM FOR PROCESSING GAS IN A GAS STORAGE FACILITY FOR A GAS TANKER

Abstract

The invention relates to a gas treatment method and system of a gas storage facility (2), in particular on board a ship, the method comprising the following stages: an extraction of a first gas (4a, 4b, 5a, 5b,) in the liquid state from a first tank (4) or first vessel (5; 500), a first subcooling of the first gas in the liquid state, and storage of the subcooled first gas in the liquid state in the lower part of the first tank (4) or of the first vessel (5; 500) or of a second tank or of a second vessel, so as to constitute a reserve layer of cold (4c, 5c, 500c) of the subcooled first gas in the liquid state at the bottom of the first or second tank (4) or of the first or second vessel (5; 500).

Claims

1. A gas treatment method of a gas storage facility, in particular on board a ship, the method comprising the following stages: an extraction of a first gas in the liquid state from a first tank or first vessel, a first subcooling of the first gas in the liquid state, and storage of the subcooled first gas in the liquid state in the lower part of the first tank or of the first vessel or of a second tank or of a second vessel, so as to constitute a reserve layer of cold of the first gas in the liquid state at the bottom of the first or second tank or of the first or second vessel.

2. The method as claimed in claim 1, characterized in that the first gas is transferred into the first or second tank or first or second vessel via a pipeline which emerges in the bottom of the first or second tank or first or second vessel.

3. The method as claimed in claim 1, characterized in that the first gas stored in the reserve layer of cold of the first or second tank or first or second vessel is used to cool a gas in the vapor state.

4. The method as claimed claim 3, characterized in that the gas in the vapor state is the first gas in the vapor state located in the upper part of one of the tanks or vessels.

5. The method as claimed in claim 1, characterized in that the first gas stored in the reserve layer of cold is sprayed into the first or second tanks or first or second vessels and into the layer of the first gas in the vapor state.

6. The method as claimed in claim 1, characterized in that the first gas stored in the reserve layer of cold is extracted from the bottom of one tank of the tanks or vessels and reliquefies the first gas in the vapor state through a heat exchanger.

7. The method as claimed in claim 1, characterized in that the subcooled first gas is stored in the reserve layer of cold when a measured pressure in the tank or vessel is less than a first predetermined pressure threshold value of the tank or of the vessel.

8. The method as claimed in claim 1, characterized in that said lower part extends over approximately less than 30% of the height of the tank or vessel, measured from its bottom, said bottom being the lowermost end of the tank or vessel.

9. The method as claimed in claim 1, characterized in that the subcooled first gas is stored in the reserve layer of cold at a temperature between a liquefaction temperature of the first gas less approximately 5° C. at atmospheric pressure and a liquefaction temperature less approximately 10° C., the first gas in the liquid state remaining in the tank or in the vessel being at a temperature greater than the liquefaction temperature of the first gas.

10. The method as claimed in claim 1, characterized in that the subcooled first gas is stored in the reserve layer of cold at a temperature of between −45° C. and −55° C. or between −160° C. and −170° C., the first gas in the liquid state remaining in one of the tanks or vessels being respectively at a temperature of greater than or equal to −42° C. or −160° C.

11. The method as claimed in claim 1, characterized in that the first subcooling of the first gas is carried out with a second gas at least in the liquid state extracted from a vessel, the second gas having a boiling point less than or equal to that of the first gas.

12. The method as claimed in claim 11, characterized in that it comprises a vaporization or heating of the second gas which is heated or vaporized by heat exchange during the first subcooling of the first gas, so as to supply the facility.

13. The method as claimed in claim 12, characterized in that the facility controls a flow rate of the second gas which has to be vaporized or heated during the vaporization.

14. The method as claimed in claim 11, characterized in that the second gas extracted from the vessel is expanded and partially vaporized before the heat exchange during the first subcooling.

15. The method as claimed in claim 11, characterized in that the second gas extracted from the vessel is subcooled by heat exchange with the expanded and partially vaporized second gas.

16. The method as claimed in claim 1, characterized in that it comprises a second subcooling of the first gas after the first subcooling.

17. The method as claimed in claim 16, characterized in that the second gas used for the second subcooling is extracted from the bottom of the vessel, or is subcooled.

18. The method as claimed in claim 1, characterized in that the first and/or second subcooling is carried out outside the first and second tanks and/or first and second vessels.

19. The method as claimed in claim 16, characterized in that the heat exchange during the first subcooling or the second subcooling between the first gas and the second gas is carried out so that a subcooling outlet temperature of the first gas is between a first threshold value and a second threshold value.

20. The method as claimed in claim 16, characterized in that the outlet temperature of the second gas after the second subcooling is between −155° C. and −105° C. at a pressure of between 2 and 20 bars.

21. The method as claimed in claim 11, characterized in that the heated, vaporized or partially vaporized second gas is heated to supply the facility.

22. The method as claimed in claim 21, characterized in that it comprises a reliquefaction stage in which vapors of the first gas moving in a first circuit from the tank are reliquefied by heat exchange with the second gas in the liquid state having an inlet temperature and moving in a second circuit, the reliquefied vapors of the first gas being transferred into the tank and the second gas being maintained in the liquid state at an outlet temperature after the reliquefaction and taken back to the vessel, the heat exchange between the first gas and the second gas being carried out so that an outlet temperature of the reliquefied vapors of the first gas is between a first threshold value and a second threshold value.

23. The method as claimed in claim 22, characterized in that the vapors of the first gas are reliquefied when a pressure measured in the tank or vessel is greater than a second predetermined pressure threshold value of the tank or vessel.

24. The method as claimed in claim 1, characterized in that the first gas is a liquefied natural gas or a liquefied petroleum gas.

25. The method as claimed in claim 1, characterized in that the second gas is a liquefied natural gas.

26. A gas treatment system of a gas storage facility, in particular on board a ship, the system comprising: a tank or vessel in which a first gas in the liquid state is stored; a first heat exchanger configured in order to carry out a first subcooling of the first gas extracted from the tank or vessel, by a first pipeline, and a second pipeline connected to the first heat exchanger emerges in the lower part of the tank or vessel or of another tank or vessel, so as to store the subcooled first gas at the bottom of the tank or vessel or of the other tank or vessel in order to form a reserve layer of cold of the first gas in the liquid state.

27. The system as claimed in claim 26, characterized in that it comprises a vessel in which a second gas in the liquid state is stored, the second gas having a boiling point less than or equal to that of the first gas.

28. The system as claimed in claim 27, characterized in that the second gas in the liquid state moves in a second pipeline connected to the first heat exchanger so as to carry out the first subcooling of the first gas.

29. The system as claimed in claim 26, characterized in that it comprises a second heat exchanger configured in order to carry out a second subcooling of the first gas with the second gas in the liquid state.

30. The system as claimed in claim 26, characterized in that the bottom of the tank or vessel comprises an outlet connected to a first end of a conduit, the conduit comprising a second end coupled to a spray bar installed in the upper part of the tank or vessel.

31. The system as claimed in claim 26, characterized in that it comprises a heating device in which the second gas heated, vaporized or partially vaporized in the first heat exchanger moves.

32. The system as claimed in claim 26, characterized in that it comprises depressurization means mounted upstream of the first heat exchanger.

33. The system as claimed in claim 26, characterized in that the second heat exchanger is configured so as to provide the second gas at an outlet temperature of between −155° C. and −105° C. at a pressure of between 2 and 20 bars.

34. The system as claimed in claim 26, characterized in that the first gas is a liquefied natural or liquefied petroleum gas.

35. The system as claimed in claim 26, characterized in that the second gas is a liquefied natural gas.

36. A ship, in particular a liquefied gas transport ship, comprising at least one system as claimed in any claim 26.

Description

4. LIST OF THE FIGURES

[0094] A better understanding of the invention will be obtained and other details, characteristics and advantages of the present invention will become more clearly apparent on reading the description which follows, given by way of nonlimiting example and with reference to the appended drawings, in which:

[0095] FIG. 1 represents an embodiment of a gas treatment system according to the invention which in this instance equips a gas storage facility, in particular on a ship,

[0096] FIG. 2 represents another embodiment of a gas treatment system according to the invention,

[0097] FIG. 3 represents another embodiment of a gas treatment system according to the invention,

[0098] FIG. 4 illustrates another embodiment of a gas treatment system according to the invention,

[0099] FIG. 5 is an alternative form of the embodiment of FIG. 4, and

[0100] FIG. 6 illustrates another embodiment of a gas treatment system according to the invention.

5. DETAILED DESCRIPTION OF THE INVENTION

[0101] FIG. 1 shows a first embodiment of a gas treatment system 1 of a gas storage facility 2 according to the invention. This treatment system makes possible the cooling of one or more gases and/or a reliquefaction of vapors of one or more gases and/or the vaporization or heating of one or more gases.

[0102] In the present invention, the term “reliquefaction” is understood to mean the condensation of the vapors of a gas making it possible to bring it back to a liquid state.

[0103] In the present invention, the system 1 is installed on a ship, such as a gas transport ship, in particular of the VLGC (Very Large Gas Carrier) type. Ships of this type have a capacity of the order of 80 000 m.sup.3.

[0104] In a gas transport ship, for example of the LNG tanker type, an energy production facility is provided in order to supply the energy needs of the operation of the ship, in particular for the propulsion of the ship and/or the production of electricity for the items of equipment on board.

[0105] The gas storage facility 2 can be the energy production facility. Such a facility commonly includes heat engines 3, such as the engine of the ship, which consumes gas originating from the gas cargo transported in the vessels/tanks of the ship.

[0106] On this ship, the gas(es) are stored in the liquid state in several tanks 4 or vessels 5 at very low temperature, indeed even at cryogenic temperatures. The tanks 4 and the vessels 5 can each contain a gas in the liquefied form or in the liquid state at a predetermined pressure and a predetermined temperature. One or more tanks 4 and/or vessels 5 of the ship can be connected to the facility 2 by the system 1 according to the invention. Each tank and vessel for this purpose comprises a jacket intended to isolate the gases stored at their storage temperature from the external environment.

[0107] The ship is loaded with natural gas (NG) stored in a vessel 5 and petroleum gases (PG) stored in one or more tanks 4. Each tank and/or vessel 4, 5 can have a capacity of between 1000 and 50 000 m.sup.3. The number of tanks 4 and vessels 5 is not limiting. It is, for example, between 1 and 6. In the continuation of the description, the terms “the vessel” and “the tank” should be interpreted respectively as “the or each vessel” and “the or each tank”.

[0108] Natural gas (NG) is, for example, methane or a gas mixture comprising methane. Natural gas is stored in the liquid state 5a in the vessel, for example at a cryogenic temperature of the order of −160° C. at atmospheric pressure. Natural gas in the liquid state or liquefied natural gas 5a bears the abbreviation “LNG”. The vessel 5 also comprises gas vapors 5b resulting from an evaporation, in particular natural, of the LNG in the vessel. The evaporation or vapor 5b is denoted by the sign “BOG” or “NBOG” for natural evaporation, unlike “FBOG” for forced evaporation. The LNG 5a is stored, naturally, at the bottom of the vessel 5, while the LNG BOG 5b is located above the level N1 of LNG 5a in the vessel, known as gas headspace. The LNG BOG 5b in the vessel is due to the heat inputs from the external environment into the vessel 5 and to movements of the LNG 5a within the vessel 5 due to movements of the sea, for example.

[0109] Petroleum gas (PG) comprises propane, butane, propylene, ammonia, ethane, ethylene, or a gas mixture comprising these components. Petroleum gas is stored in the liquid state 4a in the tank 4 at a temperature of the order of −42° C. at atmospheric pressure. Petroleum gas in the liquid state 4a or liquefied petroleum gas bears the abbreviation “LPG”. The tank 4 also comprises gas vapors 4b which result from an evaporation, in particular natural, of the LPG in the tank. Likewise, the LPG 4a is stored, naturally, at the bottom of the tank 4, while the LPG gas vapors are located above the level N2 of the LPG 4a in the tank, in the gas headspace. As was explained above for LNG, the evaporation of LPG (BOG or N BOG) in the tank 4 is also due to the heat inputs from the external environment into the tank, to fluid movements during voyages (sea, LPG), during the loading of the LPG into the tank 4 and during the cooling of the tank in order to bring the temperature of the tank back to an equilibrium temperature.

[0110] During the cooling, in this instance of the tank 4, which consists in bringing the ambient temperature of the jacket of the tank back to an equilibrium temperature, the liquefied gas is sprayed on the walls of the virtually empty tank. The evaporation of the gas generates the cold necessary for the cooling of the jacket. During this operation, which lasts about 10 h, there are very few LPG vapors produced by natural evaporation (NBOG) since the tank is virtually empty. On the other hand, the spraying of LPG on the walls in order to cool them generates a large amount of LPG vapors, of the order of 10 900 kg/h. This operation of cooling the LPG tanks can be applied to the cooling of LNG vessels.

[0111] During the loading of the LPG, the tank comprises a significant amount of BOG which originates from the cooling of the tank and also from the NBOG generated by the LPG which heats up in the tank. The vapors due to the cooling are not reliquefied by the LPG loaded into the tank. The loading operation lasts approximately 18 h. Approximately 13 900 kg/h of BOG is generated in the tank. The pressure in the tank is maintained above atmospheric pressure during the loading of the tank.

[0112] In the embodiment represented in FIG. 1, the system 1 represented comprises four LPG tanks 4 and one LNG vessel 5. The system 1 also comprises a heat exchanger 6 which makes possible heat exchanges between the LNG vapors 5b, the LPG vapors 4b, the liquid LPG 4a and the liquid LNG 5a. In the present example, the heat exchanger 6 comprises several circuits or pipes, in this instance at least one first circuit 6a, one second circuit 6b, one first pipe 6c and one second pipe 6d, in which NG or PG move in the liquid or vapor state.

[0113] The heat exchanger 6 is configured so that the first circuit 6a exchanges heat with the second circuit 6b in order to maintain the LNG coming from the vessel in the liquid state and to reliquefy LPG vapors 4b coming from the tank 4 simultaneously. The LNG at the outlet of the heat exchanger 6, in particular of the second circuit 6b, is sent to the vessel 5 and the reliquefied LPG vapors are sent to the tank 4.

[0114] For this, the tank 4 comprises an outlet which is connected to a first end of a first pipeline 7 in which LPG vapors 4b move. The outlet of the tank 4 is located in the upper part of the tank 4 where the gas headspace with the LPG vapors 4b (NBOG) is located. The first pipeline 7 is connected to an inlet of a compressor 8 which ensures the movement of the LPG vapors 4b in the first pipeline 7. The latter comprises a second end which is connected to an inlet of the first circuit 6a. The LPG vapors are intended to be reliquefied by heat exchange with the cold of the LNG and in order to keep the LNG in the liquid state. An outlet of the first circuit 6a is connected to a first end of a second pipeline 9 in which the reliquefied LPG vapors move. The second pipeline 9 comprises a second end which is immersed in the LPG or which is connected to a dip pipe 9a immersed in the tank. Alternatively, the second pipeline 9 is connected to an LPG spray bar 10. The bar 10 is arranged in the tank 4 and in the upper part of it, along a vertical axis in the plane of FIG. 1, so as to spray the reliquefied LPG vapors in the gas headspace of the LPG. This makes it possible to force the recondensation of the NBOG in the tank.

[0115] The system 1 comprises pumps which are installed in the vessel 5 in order to extract the LNG from it. In particular, a first pump 11a and a second pump 11b are immersed in the LNG, and are preferably located at the bottom of the vessel 5 in order to ensure that they are only supplied with LNG. The first pump 11a is connected to a first end of a third pipeline 12. The first pump 11a makes it possible to force the circulation of the LNG in the third pipeline 12. The flow rate by volume of the LNG of this first pump 11a is of the order of 130 m.sup.3/h. The second end of this third pipeline 12 is connected to an inlet of the second circuit 6b in which LNG 5a coming from the vessel 5 moves. The second circuit 6b comprises an outlet connected to a first end of a fourth pipeline 13 in which LNG 5a also moves. The fourth pipeline 13 comprises a second end which is connected to the vessel 5. The third and fourth pipelines 12, 13 allow recirculation of the LNG from the vessel to the vessel through the heat exchanger 6. More precisely still, the second circuit 6b and the third and the fourth pipelines 12, 13 form a closed circuit. The LNG is extracted from the vessel at a temperature of −160° C. The outlet temperature of the LNG and/or the outlet pressure of the LNG are controlled in order for the LNG not to vaporize during the heat exchange with the LPG vapors. For this, a temperature sensor is provided, for example on the fourth pipeline 13, in order to control the temperature of the LNG returned to the vessel. Advantageously, the predetermined outlet temperature of the LNG is lower, for example by 5° C., than the evaporation temperature of the LNG at an authorized storage pressure value of the vessel, for example of the order of 8 bars. The storage pressure of the vessel 5 in order to contain the LNG is between 2 and 20 bars. The outlet pressure of the LNG from the heat exchanger 6 must be lower than the maximum storage pressure of the vessel. The LNG is thus heated without being vaporized. The outlet temperature of the reliquefied LPG vapors is between a first threshold value and a second threshold value. The first threshold value for outlet temperature of the LPG gas is substantially close to its liquefaction temperature at atmospheric pressure and the second threshold temperature is less than the first threshold value by 10° C. to 40° C. at atmospheric pressure. In the present example, the first threshold value is −40° C., whereas the second threshold value is of the order of −55° C. Advantageously, the outlet temperature of the reliquefied gas vapors is of the order of −42° C. This heat exchange allows the LPG vapors to be reliquefied at an appropriate temperature which is not too cold, in particular which is greater than or equal to a minimum temperature value which has to be withstood by the tank 4. The abovementioned temperature values for the LPG in this example and in the continuation of the description are examples of temperatures related to propane. It is understood that the temperature values of the other compounds of LPG apply to the invention.

[0116] The heat exchanger 6 is also configured so that the first pipe 6c exchanges heat with the second pipe 6d in order to carry out a forced evaporation of the LNG coming from the vessel and a subcooling of the LPG coming from the tank 4 simultaneously. In the present invention, the term subcooling is understood to mean a lowering of the temperature of the liquefied gas below its liquefaction temperature. The liquefied gas is, for example, subcooled by approximately 5° C. to 20° C. below its liquefaction temperature. It is understood that the storage of the subcooled liquefied gas, in the present invention, depends on the storage pressure of the liquefied gas. The vaporized LNG (FBOG) is intended to supply the facility 2 and in particular, in this instance, the engine of the ship. The subcooled LPG (in the liquid state) is sent to the tank 4. In particular, the first pipe 6c is configured in order to cause petroleum gas and in particular LPG 4b to move in the heat exchanger 6. The first pipe 6c comprises an inlet which is connected to one of the ends of a fifth pipeline 14 in which LPG extracted from the tank moves. The other end of the fifth pipeline 14 is connected to a third pump 15 immersed in the LPG. This third pump 15 is also installed in the bottom of the tank 4 in order to withdraw only LPG and to cause the LPG to move in this pipeline 14. The first pipe 6c comprises an outlet which is connected to a sixth pipeline 16 which is intended to return subcooled LPG (in the liquid state) to the tank 4. The sixth pipeline 16 can be connected to the spray bar 10 or to the second pipeline 9, or even to the dip pipe 9a for returning the LPG to the tank. Preferably, the subcooled LPG is stored at the bottom of the tank 4 in a reserve layer of cold 4c located in the interior space of the tank and in the lower part of the tank. This layer 4c can be used subsequently. Preferably, but nonlimitingly, the second end of the pipeline 9 or that of the dip pipe is located in the lower part of the tank 4, along a vertical axis in the plane of FIG. 1, in order to store the subcooled LPG therein. The subcooling takes place outside the tank or any other tank or vessel. The subcooling is not immersed in a liquefied gas, for example. In addition, the reserve layer of cold 4c is located in the interior space of the tank, at the bottom of the tank. The reserve layer of cold is below the LPG of the tank, along a vertical axis with respect to FIG. 1, forming a liquid-liquid interface. In other words, there is no partition, subtank or compartment in the tank which separates the LPG remaining/already in the tank and the subcooled LPG stored in this reserve layer.

[0117] The second pipe 6d makes possible vaporization of the LNG 5a coming from the vessel 5. For this, the second pump 11b, which is immersed in the LNG, is connected to a first end of a seventh pipeline 17 in which the LNG moves to the facility 2, in this instance the engine of the ship. The second pump 11b makes possible the movement of the LNG in the seventh pipeline 17 at a flow rate by volume lower than that of the first pump 11a. In the present example, the flow rate by volume of the LNG in the seventh pipeline 17 is of the order of 4 m.sup.3/h. A second end of the seventh pipeline 17 is connected to an inlet of the second pipe 6d. The latter comprises an outlet which is connected to an eighth pipeline 18 in which LNG vapors 5a formed by heat exchange with the LPG move, in order to supply, for example, the engine of the ship. During this vaporization-subcooling heat exchange, the temperature of the LNG is raised. That is to say, its temperature is above its liquefaction temperature at atmospheric pressure. The temperature of the LNG is corrected by a heating device, not represented here, according to the specifications of the engine. The outlet pressure of the LNG, for example required by the engine of the ship, is of the order of 17 bars. As regards the LPG, its inlet temperature in the circuit 6c is approximately 1 bar. The outlet temperature of the subcooled LPG is greater than or equal to a minimum temperature value which has to be withstood by the tank or vessel. In this instance, the outlet temperature is of the order of −52° C. (at storage pressure in the tank).

[0118] In FIG. 1, the LPG vapors are extracted from a tank and the reliquefied LPG vapors are sent to another adjacent tank. Likewise, the LPG extracted from a tank and subcooled is returned to the same tank. Of course, other arrangements are possible.

[0119] In FIG. 1, the heat exchanger 6 is separate from the tanks or vessel. The heat exchanger 6 is positioned outside the tanks and vessels. The heat exchanger is not located in another tank or another vessel where liquefied gas is stored.

[0120] Advantageously, the heat exchanger is a tube-type, plate-type or coil-type exchanger.

[0121] In the embodiment illustrated in FIG. 2, the system 1 comprises several heat exchangers which allow heat exchanges between the LNG vapors, the LPG vapors, the LNGs and/or the LPGs. This system differs in particular from the first embodiment by the number of heat exchangers. In particular, in the present example, the system comprises at least two heat exchangers hereinafter referred to as evaporative heat exchanger 20 and main heat exchanger 21. In FIG. 2, a single vessel 5 and a single tank 4 are represented. Of course, the system can comprise other vessels and tanks. The system 1 also comprises the pumps 11a, 11b and 15 which are installed in the vessel 5 and in the tank 4. In particular, a first pump and a second pump are immersed in the LNG, and are preferably located at the bottom of the vessel in order to ensure that they are only supplied with LNG. The flow rate of the first pump is also approximately 130 m.sup.3/h and the flow rate of the second pump is approximately 4 m.sup.3/h.

[0122] The main heat exchanger 21 is configured in order to reliquefy the LPG vapors 4b by heat exchange with the cold of the LNG 5a and in order to maintain the LNG in the liquid state simultaneously. The LNG is returned to the vessel 5 without being vaporized and the reliquefied LPG vapors are returned to the tank 4. The main heat exchanger 21 comprises the first circuit 6a and the second circuit 6b. The first circuit 6a is connected, on the one hand, to the first pipeline 7 coupled to the tank 4 and, on the other hand, to the second pipeline 9 also coupled to the tank 4. A first compressor 8 is also provided on the first pipeline 7 in order to ensure the movement of the LPG vapors 4b in the pipeline to the heat exchanger 21.

[0123] The heat exchanger 20 is configured in order to vaporize the LNG coming from the vessel and to subcool the LPG coming from the tank 4 simultaneously. The LNG must undergo a forced evaporation in order to raise the temperature of the LNG to the temperature required, for example for the engine of the ship, which has to be supplied with LNG vapors. The heat exchanger 20 comprises the first pipe 6c and the second pipe 6d. The second pipe 6d is connected, on the one hand, to the seventh pipeline 17 connected to the vessel and, on the other hand, to the eighth pipeline 18 which transfers the LNG to the engine of the ship. The first pipe 6c is connected, on the one hand, to the first pipeline 14 coupled to the tank 4 and, on the other hand, to the sixth pipeline 16 coupled to the tank 4, and in particular at the bottom of the tank 4.

[0124] In FIG. 2, the system 1 also comprises a third heat exchanger referred to as auxiliary heat exchanger 22. The latter makes possible a second subcooling of the LPG with the cold of the LNG and makes it possible to maintain the LNG in the liquid state. The LNG in the liquid state is returned to the vessel and the subcooled LPG is returned to the tank.

[0125] Advantageously, but nonlimitingly, the heat exchangers 20, 21, 22 are separate from the tanks and vessels.

[0126] Advantageously, but nonlimitingly, the heat exchangers 20, 21, 22 are tube-type, plate-type or coil-type exchangers.

[0127] The auxiliary heat exchanger 22 comprises a third circuit 6e in which LNG moves and a fourth circuit 6f in which LPG, in particular sub-cooled LPG, moves. The third circuit 6e comprises an inlet coupled to a ninth pipeline 23 which is connected to the vessel 5. As can be seen in FIG. 2, the ninth pipeline 23 is a bypass portion of the seventh pipeline 17 which extracts the LNG from the bottom of the vessel 5 by means of the pump 11b. The third circuit 6e comprises an outlet which is connected to a tenth pipeline 24 which returns the LNG maintained in the liquid state to the vessel 5. In this implementational example, the tenth pipeline 24 is coupled to a portion of the fourth pipeline 13 returning the LNG to the vessel 5, for example by a valve, such as a three-way valve. The fourth circuit 6f comprises an inlet which is coupled to an eleventh pipeline 25 in which LPG extracted from the bottom of the tank moves. The eleventh pipeline is in this instance coupled to the pipeline 16 in which subcooled LPG moves and by a valve 29, such as a three-way valve. The fourth circuit 6f comprises an outlet which is coupled to a twelfth pipeline 26 which is connected to the tank. According to this implementational example, the twelfth pipeline 26 is coupled to a portion of the tenth pipeline or to the pipeline 9. The LPG subcooled by heat exchange with the LNG is sprayed into the gas headspace or is stored at the bottom of the tank 4 in the reserve layer of cold 4c. The twelfth pipeline 26 can be connected to the pipeline 16 by a valve 27. Likewise, the pipeline 26 can be connected to the pipeline 9 by a valve 28. Preferably, but nonlimitingly, the valve(s) 27, 28 are three-way valves. The pipeline 16 is connected to an LPG spray bar 10 in order to spray LPG droplets into the gas headspace of the tank 4 and to force the recondensation of NBOG in the tank 4. The third pump 15 is configured in order to force the movement of LPG in the pipeline(s) 14, 16, 25 from the bottom of the tank as far as the spray bar 10. Due to this configuration, the subcooled LPG is transferred directly into the tank or to the bar 10 or is transferred to the auxiliary heat exchanger 22 for a second subcooling with LNG.

[0128] In FIG. 2, the system additionally comprises a pipe 30 for extracting the LNG vapors 5b in the vessel 5 so as to control the pressure of the vessel 5 and to supply the facility 2 with fuel gas. A second compressor 31 is mounted on this pipe 30 in order to ensure the movement of the LNG vapors 5a to the engine and to maintain the pressure in the vessel. This pipe 30 is connected to the portion of pipeline 18 where heated or vaporized LNG moves to the engine of the ship.

[0129] Advantageously, but nonlimitingly, a heating device 32 is positioned upstream of the facility so as to adjust the temperature of the LNG to the required temperature and to ensure that all the LNG is vaporized. The heating device 32 is in this instance a heater.

[0130] In a third embodiment of the invention illustrated in FIG. 3, the system 1 also comprises several heat exchangers. In particular, the system 1 comprises: [0131] the main heat exchanger 21, which is configured in order to reliquefy the LPG vapors 4b by heat exchange with the cold of the LNG 5a and in order to maintain the LNG in the liquid state, [0132] the evaporative heat exchanger 20, which is configured in order to vaporize the LNG coming from the vessel 5 and to subcool the LPG coming from the tank 4, and [0133] the auxiliary heat exchanger 22′, which is configured in order to subcool the LPG and to maintain the LNG in the liquid state.

[0134] The system 1 of this embodiment differs from the embodiment illustrated in FIG. 2 in that it comprises a fourth heat exchanger 40 arranged upstream of the heat exchanger 20. The heat exchanger 40 is preferably, but nonlimitingly, a vacuum evaporator (VE) intended to generate cold. The vacuum evaporator 40 comprises a primary circuit 42 which comprises an inlet and an outlet. The inlet is connected to the seventh pipeline 17 in which LNG coming from the vessel moves. The outlet of the primary circuit 42 is connected to a first end of a pipeline 44. The latter comprises a second end which is connected to the inlet of the circuit 6d of the heat exchanger 20. Depressurization means 41 are provided on the pipeline 17 and upstream of the vacuum evaporator 40. The depressurization means 41 make it possible to obtain a gas in a two-phase liquid-vapor state by lowering the pressure and the temperature of the gas. The depressurization means 41 comprise in this instance an expansion valve, such as a Joules-Thomson valve. The LNG which enters the depressurization means 41 is at a temperature of the order of −134° C. and at a pressure of the order of 8 bars. At the outlet of the expansion valve, the LNG is cooled to a temperature of approximately −160° C. at a pressure of the order of 1 bar. The two-phase LNG enters the vacuum evaporator 40 where a heat exchange is carried out with LNG extracted from the vessel. More specifically, the vacuum evaporator 40 comprises a secondary circuit 43 which comprises an inlet and an outlet. The inlet of the secondary circuit 43 is connected to a bypass pipeline 45 in which LNG coming from the vessel 5 moves. This bypass pipeline 45 comes from the seventh pipeline 17 coupled to the pump 11b. Of course, the pipeline 45 might be connected to another pump immersed at the bottom of the vessel. The outlet of the secondary circuit is connected to the pipeline 23 returning the LNG to the bottom of the vessel 5. In this embodiment, the pipeline 23 is coupled to the inlet of the circuit 6e of the heat exchanger 22′. In this vacuum evaporator 40, the LNG moving in the secondary circuit 43 is subcooled by recovering the latent heat of the two-phase LNG moving in the circuit 42. The subcooled LNG (in the liquid state) is transferred into the vessel. The two-phase LNG moving in the primary circuit 42 is heated or vaporized and then transferred to the evaporative exchanger 20. The outlet temperature of the LNG at the outlet of the primary circuit 42 is between −160° C. and −134° C. at a pressure of the order of 1 bar. The outlet temperature of the subcooled LNG is of the order of −160° C. at a pressure of between 2 and 20 bars, When the subcooled LNG moves through the heat exchanger 22′, the latter is configured in order to maintain the LNG coming from the vacuum evaporator 40 in the liquid state. This is because the LNG coming from the circuit 43 can exchange heat with subcooled LPG coming from the heat exchanger 20 according to an operating mode of the system described below. In this case, the LNG passing through the circuit 6e is heated but not vaporized.

[0135] In FIG. 3, the system 1 additionally comprises a compressor 46 which is installed downstream of the heating device 32. This compressor 46 makes it possible to compress the vaporized LNG to the pressure required by the facility 2.

[0136] In this implementational example, the subcooling is carried out outside the tanks and the vessel. In other words, the heat exchangers are separate from the tanks and the vessel.

[0137] In a first operating mode (COOLING) of the system 1 for treatment of the gases for the energy production facility 2, as illustrated in FIG. 2, LNG is used to reliquefy the LPG vapors 4b. LNG is also used to supply the facility 2, in particular the engine of the ship and the other heat engines for energy production needs. This first operating mode is operated during the cooling of the LPG tank. This is because, as was explained above, a very large amount of LPG vapor 4b is generated during this operation (approximately 10 900 kg/h). This amount of vapor 4b generated is greater than the amount of vapor 4b (NBOG) generated during the voyage of the ship in order to transport the LPG. In the context of the cooling of the walls of the tank, the energy requirements of the engine with fuel gas are very low. The consumption of the facility 2 is of the order of 500 kg/h in LNG vapor. The system uses the main heat exchanger 21 to manage the LPG vapors 4b generated during the cooling. The LPG vapors 4b are extracted from the tank 4 by the compressor 8, which moves them in the first pipeline 7. The LPG vapors 4b moving in the first circuit 6a are reliquefied by the cold of the LNG moving in the second circuit 6b via the third pipeline 12 from the bottom of the vessel 5. It is understood that the LNG which is at the bottom of the vessel is cooler than the LNG close to the surface N1, i.e. at the interface between the LNG and the gas headspace. Following the reliquefaction, the reliquefied LPG vapors are transferred into the tank 4 and the LNG is maintained in the liquid state and then taken back to the vessel 5. The LPG vapors 4b enter the main heat exchanger 21 at a temperature of the order of 0° C. and at a pressure close to atmospheric pressure. The main heat exchange 21 is carried out so that the outlet temperature of the reliquefied LPG vapors is between a first threshold value and a second threshold value. The first and the second threshold values are considered at a pressure equal to or greater than atmospheric pressure. These temperature threshold values are greater than or equal to a minimum temperature value withstood by the tank 4. Advantageously, the first threshold value for outlet temperature of the LPG vapors 4b is −40° C. at a pressure equal to or greater than atmospheric pressure and the second threshold value for outlet temperature of the reliquefied LPG vapors is of the order of −50° C. at a pressure equal to or greater than atmospheric pressure. Preferably, but nonlimitingly, the outlet temperature of reliquefied LPG vapors is −42° C. at a pressure equal to or greater than atmospheric pressure. In this way, the heat exchange is controlled in order for the reliquefied LPG vapors not to be too cold.

[0138] Likewise, the heat exchange is carried out so that the outlet temperature of the LNG after the reliquefaction is between a first temperature threshold value and a second temperature threshold value at a pressure of between 6 and 20 bars. As was seen during the first embodiment in connection with FIG. 1, the LNG must be heated but not vaporized. The main heat exchanger 21 is configured in order for the temperature difference between the inlet temperature of the LNG before the reliquefaction and the outlet temperature of the LNG after the reliquefaction to be between 5° C. and 55° C. Preferably, but nonlimitingly, this temperature difference is 26° C. In this instance, the LNG enters the main heat exchanger 21, before the reliquefaction, at an inlet temperature of the order of −160° C. and at a pressure of between 2 and 20 bars. The first threshold value is of the order of −155° C. and the second threshold value is of the order of −105° C. Preferably, but nonlimitingly, the outlet temperature of the LNG is less than its vaporization temperature and at a pressure less than a maximum authorized storage pressure of the vessel. The temperature is of the order of −134° C. Such values make it possible to transfer a maximum of LNG cold to the LPG vapors for the reliquefaction while preventing the LNG which returns to the vessel from being too hot and the reliquefied LPG vapors from being too cold. An excessively hot LNG might cause an increase in LNG pressure in the vessel and exceed the authorized limits. Thus, the main heat exchanger 21 is adjusted in order for the LNG and the reliquefied LPG vapors to respectively exit at the temperature required in the vessel or the tank. During the heat exchange, the LNG flow rate and the LPG vapor flow rate are respectively constant.

[0139] Since the inlet and outlet temperatures of the LNG and of the LPG are known and/or predetermined, parameters such as the flow by weight of the LNG and of the LPG make it possible to configure the heat exchanger 21 for the heat exchange.

[0140] The system can operate so that the reliquefaction of the LPG vapors is carried out when the pressure measured in the tank is greater than a predetermined pressure value in the tank.

[0141] In this first operating mode, the system 1 also uses the evaporative exchanger 20 in which LPG coming from the tank 4 and LNG coming from the vessel 5 move in order to supply the facility 2. The heat exchange between the LPG and the LNG allows the subcooling of the LPG and the vaporization or heating of the LNG intended to supply the facility 2. The subcooled LPG (in the liquid state) is stored in the lower part of the tank so as to constitute a subsequent reserve layer of cold 4c. This makes it possible to obtain a greater available refrigerating power and thus to improve the efficiency of the cooling of the gas, liquefied and/or in the gas form, contained in the tank. In the present invention, the lower part of the tank 4 extends over approximately less than 30% of the height of the tank 4, measured from its bottom 19. The bottom 19 is the lowermost end of the tank, for example closer to the hull of the ship when the tank is transported on the LNG tanker. In particular, the LPG extracted from the bottom of the tank by the pump passes through the heat exchanger 20, where its inlet temperature is approximately −42° C. The inlet temperature of the LNG extracted from the vessel is approximately −160° C. at a pressure of approximately 17 bars. After the heat exchange, where the LPG recovers the latent heat of the LNG which vaporizes, the outlet temperature of the LPG is between −45° C. and −55° C. The subcooled LPG is transferred to the bottom of the tank where it is thus stored in the layer 4c at a temperature of between −45° C. and −55° C. Advantageously, the subcooled LPG is at approximately −52° C. (storage pressure in the tank). After the heat exchange, the vaporized or heated LNG is at an outlet temperature of approximately 0° C., where it can further be heated by the heating device 32.

[0142] Alternatively, the storage of the subcooled LPG is a function of the pressure in the tank. In particular, when the pressure in the tank is less than a first predetermined pressure value, for example between 1 and 1.05 bar absolute, the system controls the storage of the subcooled LPG in the reserve layer of cold. For this, pressure determination means 33 make it possible to determine the pressure inside the tank 4. The pressure determination means 33 comprise in this instance a pressure sensor installed in or near the tank 4.

[0143] The LPG in the tank 4 which is above this reserve layer of cold 4c, for example remaining in the tank, is at a temperature greater than −42° C. It is considered that the LPG tank comprises several layers in which the LPG is at different temperatures, the coldest layers being at the bottom of the tank.

[0144] In a second operating mode (VOYAGE) of the system for treatment of the gases for the energy production facility 2, as illustrated in FIG. 2, the LNG is used to supply the facility 2, such as the engine of the ship, and the LPG is subcooled so as to form a reserve of cold LPG which will be used subsequently to cool the LPG vapors in the tank. This operating mode is operated during the voyage of the ship, where a lesser amount of LPG vapors has to be managed. This is because the LPG gas vapors (N BOG) generated are of the order of 2700 kg/h, whereas the engine of the ship, for example, consumes a small amount of fuel gas, of the order of 2000 kg/h. In this operating mode, the system uses at least the evaporative heat exchanger 20, in which LPG coming from the tank and LNG coming from the vessel move, in order to carry out a forced evaporation of LNG which has to supply the engine of the ship, and the auxiliary heat exchanger 22, in order to constitute the reserve of cold. The LNG is extracted from the vessel via the second pump 11b. The inlet temperature of the LNG in the second pipe 6d is of the order of −160° C. The LPG is extracted from the tank containing the LPG by means of the pump 15. The LPG moves in the second pipeline to the evaporative exchanger and enters the latter at a temperature of approximately −42° C. The LPG undergoes a first subcooling of the LPG by recovering the cold from the LNG which vaporizes by heat exchange in the exchanger 20. The heat exchange between the LPG and the LNG is carried out so that the subcooling temperature of the LPG is between a first threshold value and a second threshold value at atmospheric pressure. The evaporative exchanger 20 is configured in order to transfer a maximum amount of heat but is limited by the temperature difference between the LNG and the LPG. Advantageously, but nonlimitingly, the first threshold value is of the order of −40° C. and the second threshold value is of the order of −55° C. The subcooled LPG is stored in the lower part of the tank so as to constitute the LPG reserve layer of cold or sprayed into the gas headspace by the bar 10. On a voyage, the outlet temperature of the LPG of the heat exchanger 20 is of the order of −52° C.

[0145] Of course, as was seen for the first operating mode, when the pressure in the tank is less than the first predetermined pressure threshold value, for example between 1 and 1.05 bar absolute, the subcooled LPG is stored in the reserve layer of cold.

[0146] It is considered that a reserve layer of cold has already formed, for example, during the cooling of the tank. This subcooled LPG is then used to cool or condense the LPG vapors in the tank. For this, the subcooled LPG is extracted from the reserve layer of cold 4c and is sprayed into the gas headspace via the bar 10. Alternatively, the LPG from the reserve layer of cold 4c is extracted from an outlet of the tank which is coupled to a conduit which is connected to the bar or to a heat exchanger through which the LPG vapors pass. It is thus not necessary to start up the auxiliary heat exchanger in order to create a reserve of cold.

[0147] The LNG at the exit of the exchanger 20 is vaporized or heated by the heat exchange between the LPG and the LNG. This vaporized or heated LNG is transferred to the engine for its supply. The LNG vapors which are extracted from the vessel also make it possible to supply the engine. The vaporized or heated LNG and the LNG vapors are heated so that all the LNG is vaporized before supplying the engine.

[0148] In a third operating mode (LOADING) of the system for treatment of the gases for the energy production facility, as illustrated in FIG. 2, the LNG is used to supply the engine of the ship and for the energy production needs, as well as to reliquefy the LPG vapors. This operating mode is operated in particular during the loading of the LPG into the tank, where a large amount of LPG vapors is produced, for example approximately 13 900 kg/h. The energy needs of the facility 2 are low, approximately 500 kg/h. In this operating mode, at least two heat exchangers are appealed to in order to treat all the LPG vapors. In particular, the system uses the main heat exchanger 21 to manage the LPG vapors generated during the loading of the LPG and the evaporative heat exchanger 20 to vaporize or heat the LNG intended to supply the facility 2. The heat exchangers 20, 21 thus operate in a similar manner to the first operating mode in the case of the cooling of the tank.

[0149] In this operating mode, it may be that the main heat exchanger 21 does not make it possible to manage the pressure in the tank 4 due to the large amount of LPG vapor generated. In this scenario, when the pressure measured (by virtue of the means for determining the pressure 33) inside the tank reaches or is greater than a second predetermined threshold pressure value, the auxiliary heat exchanger 22 is activated. Thus, the purpose of the auxiliary heat exchanger 22 is to manage the pressure inside the tank 4. LNG is withdrawn from the vessel so as to exchange with the subcooled LPG. The subcooled LPG after the first subcooling is at a temperature of the order of −42° C. This temperature of −42° C. is due to the fact that a small amount of LNG moves in the heat exchanger 20, in particular in the second pipe 6d. This is because it is the engine or the facility 2 which determines the flow rate of LNG which has to be vaporized in the second pipe 6d. Given that the needs of the facility 2 are low, a very small amount of LNG is available to carry out the subcooling of the LPG. The facility controls the flow rate of the second gas which has to be vaporized or heated during the vaporization. This implies that the amount of heat from LNG is not enough to substantially reduce the temperature of the LPG. As the temperature of the LPG at the outlet of the heat exchanger 20 is not cold enough, the heat exchanger 22 carries out a second subcooling of the LPG. The LNG is extracted from the vessel, at a temperature of approximately −160° C., and exchanges heat with LPG which has been subjected to a first subcooling, in this instance in the heat exchanger 20. The inlet temperature of the subcooled LPG is of the order of −42° C. The outlet temperature of the LPG subcooled a second time is less than or equal to a threshold temperature value which has to be withstood by the tank 4. The outlet temperature of the LPG is of the order of −52° C. This LPG is stored in the reserve layer of cold for subsequent use or is sprayed into the gas headspace of the tank in order to condense or cool the LPG vapors 4b in the tank. The outlet temperature of the LNG is approximately −134° C. at a pressure of the order of 8 bars. The LNG is thus hot but not vaporized.

[0150] In a fourth operating mode (hot LNG in the vessel), the system 1 for treatment of gases for the energy production facility, as illustrated in FIG. 2, the system makes it possible to manage the risk of heating of the LNG in the vessel in the case where the main heat exchanger 21 has operated (during the loading of LPG in the tank or during the cooling of the tank). This is because the LNG at the outlet of the main exchanger and or at the outlet of the auxiliary heat exchanger is hot, i.e. at an outlet temperature of the order of −134° C. This operating mode employs the system as represented in FIG. 3 and mainly in voyage mode in order to cool the LNG in the vessel to its cryogenic temperature. The system 1 uses at least the heat exchanger 40 where the partially vaporized LNG makes it possible to subcool the LNG which is transferred to the vessel. It is then considered that the LNG stored in the vessel is at a temperature of approximately −134° C. at a pressure of the order of 8 bars. The LNG is extracted from the vessel by the second pump 11b. The LNG moves in the circuit 42 where it was depressurized and then partially vaporized. The inlet temperature of the partially vaporized LNG in the heat exchanger 40 is of the order of −160° C. at atmospheric pressure. The outlet temperature of the vaporized LNG is between −134° C. and −160° C. at atmospheric pressure. The inlet temperature of the LNG in the heat exchanger, in the second pipe 43, is of the order of −134° C. and its outlet temperature is of the order of −160° C. The subcooled LNG is transferred into a reserve layer of cold 4c in the lower part of the vessel 5. The heat exchanger 20 subcools the LPG and vaporizes the LNG at the outlet of the heat exchanger 40.

[0151] When the pressure measured in the tank 4 is greater than or equal to the threshold pressure value, the heat exchanger 22′ is activated in order to subcool a second time the LPG which was cooled in the exchanger 20. The LPG is subcooled with the LNG which was subcooled in the heat exchanger and passes through the heat exchanger 22′. The outlet temperature of the LNG after the heat exchange in the exchanger 22′ is of the order of −134° C. at atmospheric pressure.

[0152] These above operating modes have been described on the basis of FIG. 2. It is, of course, possible for FIG. 1 to apply to these operating modes.

[0153] FIG. 4 illustrates another embodiment of the gas treatment system 1 according to the invention. The system comprises LNG vessels each comprising LNG vapors 5b and LNG. In this instance, two LNG vessels are represented. Pumps are also immersed in the LNG of a main vessel and a single pump is immersed in the LNG of the adjacent vessel. Each pump is preferably installed at the bottom of the vessel. The system 1 comprises a heat exchanger 50 which is configured in order to subcool LNG coming from the LNG vessel, in this instance first tank 500A, intended to be stored at the bottom 190 of the same first vessel 500A so as to constitute a reserve layer of cold 500c at the bottom of the vessel 500A. The layer 500c is located in the interior space of the vessel. The heat exchanger comprises at least one first pipe 50a and one second pipe 50b. The first pipe 50a comprises an inlet which is coupled to the first end of a pipeline 54. The second end of the pipeline 54 is connected to a first pump 51 mounted at the bottom of the first vessel 500A. This pipeline 54 is also connected to a spray bar 60 mounted in the vessel 500A via a three-way valve 67. The bar 60 is arranged in the upper part of the vessel and preferably in the LNG gas headspace. The first pipe 50a comprises an outlet which is coupled to a pipeline 56 which is connected to the bottom of the vessel 500A. The pipeline 56 is also connected to the spray bar 60 by a three-way valve 75a. As is illustrated in FIG. 4, the pipeline 56 emerges in the bottom of the adjacent vessel, second vessel 500B, by a three-way valve 75b, as well as at another bar 60 of this second vessel 5008 by a three-way valve 75c. The second pipe 50b comprises an inlet connected to the vessel 500A by a pipeline 57. One of the ends of the pipeline 57 is connected to a second pump 52 mounted at the bottom of the vessel 500A. The outlet of the second pipe 50b is connected in this instance to an inlet of a drum 70 via a pipeline 58. The outlet of the drum 70 is connected to the pipeline 56 by a first outlet, via a pipe 71. The pipe 71 comprises, for example, a valve 72 and a pump 73. Depressurization means 53 are mounted on the pipeline 57, upstream of the heat exchanger 50. This exchanger, as in the embodiment illustrated in FIG. 3, is a vacuum evaporator. The depressurization means 53 comprise, for example, an expansion valve (Joule-Thomson valve).

[0154] The second pipe 50b is a cold circuit, the depressurized LNG being intended to be heated by movement in this circuit so as to carry out a forced evaporation (to give FBOG). The first pipe 50a is a hot circuit, the LNG coming from the vessel 500A being intended to be cooled by movement in this circuit. The first pipe 50a may not, however, make it possible to vaporize the heaviest components (ethane, propane, and the like). It is understood that the depressurization upstream of the second pipe 50b makes it possible to lower the vaporization temperature, which makes it possible to generate FBOG from a heat exchange with the LNG withdrawn from the vessel 500A and moving in the first pipe 50a. The vaporization to give FBOG requires a contribution of heat supplied by the LNG moving in the first pipe 50a; it is thus a refrigerating source for the purpose of the subcooling of the LNG moving in the first pipe 50a.

[0155] LNG originating from the vessel 500A is thus conveyed by the pump 52 as far as the depressurization means 53 and then moves in the second or cold pipe 50b of the exchanger 50. The LNG downstream of the depressurization means is at a temperature of −168° C. and at an absolute pressure of 400 mbar. In the meantime, the LNG of the vessel 500A is conveyed by the pump 51 as far as the first or hot pipe 50a of the exchanger 50. Consequently, the exchange of heat between these circuits leads to: [0156] the heating of depressurized and partially vaporized LNG, for the purpose of continuing its vaporization, which is subsequently conveyed as far as the drum 70 in the present example, and [0157] the subcooling of LNG which supplies the bottom of the first vessel and or of the second vessel, in order to be stored therein for the purpose of subsequent use, or which is sprayed into the LNG gas headspace via the bar 60.

[0158] The outlet temperature of the LNG after the heat exchange in the pipe 50a is of the order of −168° C.

[0159] The storage of LNG in the reserve layer of cold can be a function of the pressure inside the vessel. For example, when the pressure measured (with a pressure sensor 330) in the vessel is less than a predetermined pressure threshold value in the vessel, the subcooled LNG (in the liquid state) is stored in this reserve layer of cold 500c.

[0160] The drum 70 is thus intended to be supplied with LNG in a two-phase liquid-vapor state originating from the vessel 500A via the heat exchanger 50. The operating pressure inside the drum 70 is less than the storage pressure of the LNG inside the vessel 500A. Supplying the drum 70 with LNG can lead to additional vaporization of the LNG, which is reflected, on the one hand, by the generation of FBOG in the drum 70, as well as the subcooling of the LNG remaining in the drum. The drum makes it possible to separate the phases with the LNG stored in the lower part of the drum and the LNG vapors in the upper part of it. The subcooled LNG at the outlet of the drum is at an outlet temperature of the order of −168° C. The drum 70 comprises a second outlet which is arranged in the upper part of it, where the LNG gas vapors (FBOG) are naturally stored. The outlet of the drum 70 is connected to the facility 2 via, in this instance, two compressors 61, 62.

[0161] The heat exchanger 50 also comprises a third pipe 50c which comprises an inlet and an outlet. The inlet of the third pipe 50c is connected to a first end of a pipeline 63 in which reliquefied LNG gas vapors move. In particular, the outlet of the compressor 62 is connected to the facility 2 for the purpose of supplying it with fuel gas. Part of the fuel gas exiting from the compressor 62 can be withdrawn and rerouted by a pipeline 64 which can be connected to the outlet of the compressor 62 by a three-way valve 65. The compressor 62 is configured in order to compress the gas (such as NBOG originating from the first vessel and/or second vessel) to a working pressure suitable for its use in the facility 2. The pipeline 64 is connected to an inlet of a primary circuit 66a of a heat exchanger 66. The primary circuit comprises an outlet which is connected to a second end of the pipeline 63. Each vessel 500A, 500B comprises an outlet 68 for LNG vapors 5b which is connected to an inlet of a secondary circuit 66b of the heat exchanger 66. The secondary circuit 66b comprises an outlet which is connected to the inlet or to one of the inlets of the compressor 62. The third pipe 50c comprises an outlet which is connected to the pipeline 56 by another pipeline 69. An expansion valve 74 is installed on this pipeline 69 in order to reduce the temperature of the gas by adiabatic expansion.

[0162] The LNG vapors coming from a vessel 500A, 500B are heated in the secondary circuit 66b so as to supply the facility 2, and the LNG vapors at the outlet of the compressor 62 are reliquefied in order to be conveyed to the heat exchanger 50. In this heat exchanger 50, the reliquefied gas vapors are subcooled with the cold of the LNG moving in the pipe 50a in order to supply the bottom of the vessel(s) 500A, 500B or the spray bar 60. The LNG vapors coming from the vessel(s) 500A, 500B can be rerouted in the pipeline 64 if FBOG is produced in excess, so as to also be liquefied.

[0163] In this implementational example, the subcooling is carried out outside the vessels. In other words, the heat exchanger 50 is separate from the vessels.

[0164] FIG. 5 represents an alternative embodiment of the gas treatment system 1 illustrated in FIG. 4. This system 1 differs from that of FIG. 4 in that it comprises a second pump 52 installed in the second vessel 500B adjacent to the first, main, vessel (which is on the right of FIG. 5). This second pump 52 is at a first end of a pipeline 80 in which LNG extracted from the bottom of the second vessel 500B moves. The second end of the pipeline is coupled to the pipeline 57 which is connected to the inlet of the second pipe 50b. In other words, the LNG is extracted from the two vessels 500A, 500B and with two pumps 52. This second pump 52 makes it possible to reduce the level of depressurization downstream of the depressurization means by increasing the pressure and the temperature. For example, with the two second pumps, the absolute pressure downstream of the depressurization means is 600 mbar and the temperature of the LNG is −164° C.

[0165] FIG. 6 represents another embodiment of the invention of a gas treatment system according to the invention. This system is similar to the embodiment illustrated in FIG. 5. It differs therefrom in that it comprises two heat exchangers 150, 150′ instead of a single heat exchanger 50. A first exchanger 150 is configured in order to vaporize the LNG coming from the first vessel 500A and in order to subcool LNG coming from the first vessel 500A simultaneously. The first exchanger 150 comprises the first pipe 150a and the second pipe 150b arranged as was described in the embodiment of FIG. 4.

[0166] The second heat exchanger 150′ is configured in order to use the subcooled LNG (in the liquid state) stored in the reserve layer of cold 500c coming in this instance from the first vessel 500A in order to reliquefy LNG vapors. These LNG vapors come from a natural evaporation (N BOG) of the LNG not used by the energy production facility 2, that is to say excess BOG. The second heat exchanger 150′ comprises the third pipe 150c and a second auxiliary pipe 150b′. The third pipe 150c comprises an inlet which is connected to the pipeline 163 through which LNG vapors produced in excess are conveyed. In particular, the NBOG recirculates via the compressor 62 in the heat exchanger 166 and via the pipeline 164. The third pipe 150c comprises an outlet which is connected to the pipeline 169 which emerges at the bottom of the vessel or of each vessel 500A, 500B by a three-way valve 175b. The pipeline 169 is also connected to a spray bar 160 via a three-way valve 175a, 175c.

[0167] The second pipe 150b′ comprises an inlet which is connected to the pipe 154 via a three-way valve. The second pipe 150b′ comprises an outlet which joins the pipe 156 via the three-way valve 180. A heat exchange is carried out between the excess NBOG and the subcooled LNG coming from the vessel. The reliquefied NBOG is transferred to the bottom of the first and/or second vessel(s). The LNG at the outlet of the second pipe 150b′ is heated but not vaporized and is returned to the bottom of the first and/or second vessel(s).

[0168] In this implementational example, the subcooling is carried out outside the vessels. In other words, the heat exchangers are separate from the vessels.