 METHOD AND SYSTEM FOR TREATING GAS FROM A GAS STORAGE FACILITY FOR A GAS TRANSPORT SHIP
专利摘要:
The invention relates to a method and a system for gas treatment of a gas storage installation (2), the installation (2) comprising a tank (4) in which a first gas (4a, 4b) is stored and a reservoir (5) in which a second gas (5a, 5b) is stored, the second gas (5a, 5b) having a lower boiling temperature than the first gas, the process comprising a reliquefaction step in which vapors (4b) of the first gas flowing in a first circuit (6a) from the tank (4) are reliqued by heat exchange with the second liquid-state gas having an inlet temperature and flowing in a second circuit (6b) the vapors of the first reliqued gas being transferred into the tank (4) and the second gas being maintained in the liquid state at an outlet temperature after the reliquefaction and returned to the tank (5), the heat exchange between the first gas (4b) and second gas (5a) being realized so that an exit temperature of the vapors (4b) of the first reliqued gas is between a first threshold value and a second threshold value. 公开号:FR3077867A1 申请号:FR1851135 申请日:2018-02-09 公开日:2019-08-16 发明作者:Pavel Borisevich；Bernard Aoun 申请人:Gaztransport et Technigaz SARL； IPC主号:
专利说明:
METHOD AND SYSTEM FOR GAS TREATMENT OF A GAS STORAGE FACILITY FOR A GAS TRANSPORT VESSEL 1. TECHNICAL AREA The invention relates to a method and a system for treating gas from a gas storage installation, in particular on board a ship, such as a liquefied gas transport ship, the installation of which operates on gas from the cargo. stored on the ship. 2. STATE OF THE ART It is known to transport on a ship several types of gas in liquefied form in order to facilitate their transport over long distances. Examples of liquefied gas are liquefied natural gas (LNG) or liquefied petroleum gas (LPG). The gases are cooled to very low temperatures, even to cryogenic temperatures so that they are liquid at a pressure close to atmospheric pressure and load them on specialized vessels. Liquefied natural gas and liquefied petroleum gas are used as fuel for various equipment in all types of industry. Recently, liquefied natural gas has been used for the energy needs of ship operations, and in particular those transporting liquefied petroleum gases and liquefied natural gas so as to meet new environmental regulations restricting sulfur oxide (SOx) emissions. ) and nitrogen oxide (NOx) in the zones known as ECA (for Emission Control Area in English) and SECA (for SOx Emission Control Area in English) for example. These liquefied natural gases and liquefied petroleum gases are stored in thermally insulated tanks at very low temperatures on ships to maintain the gases in a liquid state. The tanks absorb heat inside them, which contributes to an evaporation of part of the gases in the tanks which is known by the acronym NBOG in English for Natural Boil-Off Gas (as opposed to forced evaporation of gas or FBOG, acronym for Forced Boil-Off Gas). Other parameters such as the movement of gases inside the tanks due to the sea state during navigation and the ambient conditions also influence the evaporation of gases. These gas vapors, which are stored in the upper part of the tanks in a gaseous sky above the liquefied gases, increase the pressure in the tank. This increase in pressure can cause the tanks to rupture. Vapors from liquefied natural gas are used to power the above-mentioned energy production facility. In the case of natural evaporation where the quantity of naturally evaporated gas is insufficient for the demand for combustible gas in the installation, means such as a pump immersed in the tank are actuated to supply more combustible gas after evaporation forced. Forced evaporation is carried out in particular from hot water which is heated by oil or a gas burner. All the cold of the liquefied natural gas is lost during this operation. When the amount of gas evaporated is too large in relation to the demand for the installation, the excess gas is generally burned in a gas combustion unit, which represents a loss of the cargo. In the current technique, the development of liquefied natural gas tanks are such that the natural evaporation rates (BOR - acronym for the Boil-Off Rate) of liquefied gases are lower and lower. Also, the machines of a ship are more and more efficient. This has the consequence, in each of the first and second cases mentioned above, that the difference is very large between the quantity of gas naturally produced by evaporation and that required by the installation of a ship. With regard to liquefied petroleum gases, natural evaporation of the gases is inevitable and takes place, for example, during loading operations in their storage tanks, during the voyage of the ship or during cooling of the tanks following heat exchanges between the tanks. and the external environment. The evaporation of the gases is managed by one or more reliquefaction system (s) making it possible to limit the natural evaporation of the liquefied gas while keeping it in a thermodynamic state allowing its storage in a sustainable manner and by controlling the pressure in the reservoir of storage. Indeed, ships carrying liquefied petroleum gas are not able today to burn the vapors of liquefied petroleum gas. Reliquefaction systems extract gas vapors from the tanks, reliquefy them, and return them to the storage tank. This or these reliquefaction systems can represent an investment in the order of 5 to 10% of the price of the ship. The present invention proposes to provide a simple, efficient and economical solution making it possible to manage the natural or forced evaporations of gas in tanks or vats and the energy needs of a storage installation in particular on a ship whatever the conditions of travel operation, cooling tanks or vats and loading liquefied gases into the tanks. 3. STATEMENT OF THE INVENTION According to a first aspect, the invention provides a method of treating gas from a gas storage installation, the installation comprising a tank in which a first gas is stored and a tank in which a second gas is stored, the second gas having a boiling temperature lower than that of the first gas, the method comprising a reliquefaction step in which vapors of the first gas flowing in a first circuit from the tank are reliquified by heat exchange with the second gas in the state liquid having an inlet temperature and flowing in a second circuit, the vapors of the first liquefied gas being transferred into the tank and the second gas being maintained in the liquid state at an outlet temperature after the liquefaction and returned to the tank, the heat exchange between the first gas and the second gas being carried out so that a temperature of s nettle of the vapors of the first reliquefied gas is between a first threshold value and a second threshold value. Thus, the invention makes it possible to manage the vapors of the first gas by using the cold of the second gas which is intended to supply the gas storage installation, which makes it possible to have an efficient, economical system while reducing the NOx emissions. and SOx). In particular, reliquefying the vapors of the first gas with the second gas in the liquid state intended to return to the tank makes it possible to reliquify all the gas vapors generated in the tank of the first gas and at the right temperature. The reliquefaction of the first gas vapors is independent of the consumption of the installation. The second gas is heated following this heat exchange but is kept liquid so that it can be returned to the tank. The process may include one or more of the following characteristics or steps, taken in isolation from one another or in combination with one another: - the temperature difference between the inlet temperature of the second gas before the liquefaction step and the outlet temperature of the second gas after the liquefaction step is between 20 ° C and 30 ° C, the outlet temperature of the second gas is lower than the vaporization temperature of the second gas at a pressure less than or equal to a maximum authorized pressure value for storage of the tank, - the vapors of the first liquefied gas are transferred into the tank at a temperature greater than or equal to a minimum temperature value to be supported by the tank, - the outlet pressure of the second gas after the reliquefaction of the first gas is 8 bars, the outlet temperature of the second gas is between -155 ° C and 105 ° C at a pressure between 2 and 20 bars, the first threshold value of the outlet temperature of the first gas is substantially close to the liquefaction temperature of the first gas at atmospheric pressure and the second threshold temperature is lower than the first threshold value of 10 ° C to 40 ° C at atmospheric pressure, - the first threshold value is around -40 ° C and the second threshold value is around -50 ° C, - the vapors of the first gas are compressed before the heat exchange, - the second gas is extracted from the bottom of the tank, - the heat exchange during the reliquefaction stage is carried out during a loading operation of the first gas or during a cooling operation of the tank, - the first gas is a liquefied petroleum gas, - the second gas is liquefied natural gas. The invention also relates to a gas treatment system of a gas storage installation, the system comprising: - a tank in which a first gas is stored, - a tank in which a second gas is stored, the second gas having a boiling temperature lower than that of the first gas, a first circuit in which at least part of the vapors of the first gas circulates from the tank, a second circuit in which at least part of the second gas in the liquid state circulates at an inlet temperature from the tank, and a heat exchanger configured to reliquefy at least part of the vapors of the first gas by heat exchange with the second gas in the liquid state, the vapors of the first liquefied gas being transferred to the tank and the second gas being kept at liquid state at an outlet temperature after the reliquefaction and returned to the tank, and so that an outlet temperature of the vapors of the first gas is between a first threshold value and a second threshold value. The device according to the invention may include one or more of the following characteristics, taken in isolation from one another or in combination with one another: - the heat exchanger is configured so that the temperature difference between the inlet temperature of the second gas before the reliquefaction step and the outlet temperature after the reliquefaction step is between 5 ° C and 55 ° C, the system comprises a compressor installed upstream of the first circuit so as to compress the vapor of the first gas to be extracted from the tank before the heat exchange, the second circuit forms with conduits each connected to the tank and to the second circuit a closed circuit, - the first gas is a liquefied petroleum gas, - the second gas is liquefied natural gas. The invention also relates to a liquefied gas transport vessel, comprising at least one system having any of the above characteristics. According to a second aspect, the invention proposes a method for treating gas from a gas storage installation, in particular on board a ship, the method comprising the following steps: - an extraction of a first gas in the liquid state from a first tank or from a first tank, - a first sub-cooling of the first gas in the extracted liquid state, and a storage of the first gas in the sub-cooled liquid state in the lower part of the first tank or of the first tank or of a second tank or of a second tank so as to constitute a cold reserve layer of the first gas in the liquid state at the bottom of the first or second tank or the first or second tank. Thus, the first sub-cooled gas which is stored at the bottom of the tank or the tank makes it possible to create a refrigerating power which can be used later, the cold reserve being stored at the bottom of the tank or the tank in a durable manner. This cold reserve can be used for example to reliquefy vapors of the first gas in the tank and / or reduce the pressure in the tank and as soon as necessary. This cold storage can also be used without the need to power the installation or operate heat exchangers. The process may include one or more of the following characteristics or steps, taken in isolation from one another or in combination with one another: - the first gas is sub-cooled to a temperature greater than or equal to a minimum temperature value to be supported by the tank or the tank, - the cold reserve layer is located in the first or second tank or first or second tank below an amount of the first gas by forming a liquid-liquid interface, the first gas is transferred into the first or second tank or first or second tank via a pipe which opens into the bottom of the first or second tank or first or second tank, - the first gas stored in the cold reserve layer of the first or second tank or first or second tank is used to cool a gas in the vapor state, - the gas in the vapor state is the first gas in the vapor state located in the upper part of the tank or tank and the first gas in the liquid state, - the first gas stored in the cold reserve layer is sprayed into the first or second tank or first or second tank and into the layer of the first gas in the vapor state, - the first gas stored in the cold reserve layer is extracted from the bottom of one of the tanks or reservoir and re-liquefies the first gas in the vapor state through a heat exchanger, - the first sub-cooled gas is stored in the cold reserve layer when a pressure measured in the tank or tank is less than a first predetermined pressure threshold value of the tank or tank, the first predetermined threshold value is for example between 1 and 1.05 bar absolute, - said lower part extends over approximately less than 30% of the height of the tank or tank measured from its bottom, said bottom being the most lower end of the tank or tank, - the first sub-cooled gas is stored in the cold reserve layer at a temperature between a liquefaction temperature of the first gas minus about 5 ° C at atmospheric pressure and a liquefaction temperature minus about 10 ° C, the first gas in the liquid state remaining in the first or second tank or first or second tank being at a temperature higher than the liquefaction temperature of the first gas, - The first sub-cooled gas is stored in the cold reserve layer at a temperature between -45 ° and -55 ° C, the first gas in the liquid state remaining in the first or second tank or first or second tank being at a temperature greater than or equal to -42 ° C, - the first sub-cooled gas is stored in the cold reserve layer at a temperature of -160 ° and -170 ° C, the first gas in the liquid state remaining in the tank or tank being at a temperature greater than or equal at -160 ° C, - the first sub-cooling of the first gas is carried out with a second gas at least in the liquid state extracted from a reservoir, the second gas having a boiling temperature lower than or equal to that of the first gas, - the process includes 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 installation, - the installation controls a flow rate of the second gas to be vaporized or heated during vaporization, the first sub-cooling of the first gas is carried out with the first gas extracted from the tank which is expanded and partially vaporized, - the second gas extracted from the tank is expanded and partially vaporized before the heat exchange during the first subcooling, the second gas extracted from the tank is sub-cooled by heat exchange with the second expanded and partially vaporized gas, a second sub-cooling of the first gas is carried out after the first sub-cooling, - the second gas used for the second sub-cooling is extracted from the bottom of the tank, or is sub-cooled, - the first and / or second sub-cooling is carried out outside the first and second tanks and / or first and second tanks, the heat exchange during the first sub-cooling or the second sub-cooling between the first gas and the second gas is produced so that a sub-cooling temperature of the first gas is between a first threshold value and a second threshold value, the outlet temperature of the second gas after the second subcooling is between -155 ° and -105 ° C at a pressure between 2 and 20 bars, - the second heated, vaporized or partially vaporized gas is reheated to supply the installation, the method further comprises a reliquefaction step in which vapors of the first gas flowing in a first circuit from the tank are reliquified by heat exchange with the second gas in the liquid state having an inlet temperature and flowing in a second circuit, the vapors of the first reliquefied gas being transferred into the tank and the second gas being maintained in the liquid state at an outlet temperature after the reliquefaction and returned to the tank, the heat exchange between the first gas and the second gas being produced so that an outlet temperature of the vapors of the first reliquefied gas is between a first threshold value and a second threshold value, - the vapors of the first gas are reliquified when a pressure measured in the tank or tank is greater than a second predetermined pressure threshold value of the tank or tank, the second threshold value is for example between 1 and 1.05 bar absolute, - the second heated gas is compressed so as to supply the installation, - the first gas is a liquefied natural gas or a liquefied petroleum gas, - the second gas is liquefied natural gas, The present invention also relates to a gas treatment system of a gas storage installation, in particular on board a ship, the system comprising: - a tank or tank in which is stored a first gas in the liquid state; a first heat exchanger configured to produce a first subcooling of the first gas extracted from the tank through a first pipe, and - a second pipe connected to the first heat exchanger opens out at the bottom of the tank or tank or of another tank or tank so as to store the first sub-cooled gas at the bottom of the tank or tank to form a reserve layer of cold of the first gas in the liquid state. The device according to the invention may include one or more of the following characteristics, taken in isolation from one another or in combination with one another: - the first gas is stored in the same tank or the same tank from which it is extracted, - the device comprises a tank in which a second gas in the liquid state is stored, the second gas having a boiling temperature lower than or equal to that of the first gas, - the second gas in the liquid state circulates in a second pipe connected with the first heat exchanger so as to produce the first sub-cooling of the first gas, - the device comprises a second heat exchanger configured to perform a second sub-cooling of the first gas with the second gas in the liquid state, - the bottom of the tank or tank comprises an outlet connected to a first end of a pipe, the pipe comprising a second end coupled to a sprayer boom installed in the upper part of the tank or tank, a heating device in which the second heated, vaporized or partially vaporized gas circulates in the first heat exchanger, depressurization means are mounted upstream of the first heat exchanger, the second heat exchanger is configured so as to supply the second gas at an outlet temperature between -155 ° and -105 ° C at a pressure between 2 and 20 bars, - The device comprises a third heat exchanger configured to reliquefy at least part of the vapors of the first gas by heat exchange with the second gas in the liquid state, the vapors of the first reliquefied gas being transferred into the tank and the second gas being maintained in the liquid state at an outlet temperature after the reliquefaction and returned to the tank, and so that an outlet temperature of the vapors of the first gas is between a first threshold value and a second threshold value, the device comprises a fourth heat exchanger configured to partially vaporize the second gas circulating in a primary circuit and to sub-cool the second gas circulating in circulating in a secondary circuit, - the primary circuit is arranged downstream of the depressurization means and upstream of the first heat exchanger (depending on the direction of circulation of the fluid in the heat exchanger), - the secondary circuit is arranged upstream of the second heat exchanger (according to the direction of circulation of the fluid in the heat exchanger), - a compressor is intended to compress the second heated or vaporized gas, - the first gas is a liquefied natural gas or a liquefied petroleum gas, - the second gas is liquefied natural gas, The invention also relates to a liquefied gas transport vessel, comprising at least one system having any of the above characteristics. 4. LIST OF FIGURES The invention will be better understood and other details, characteristics and advantages of the present invention will appear more clearly on reading the description which follows, given by way of nonlimiting example and with reference to the appended drawings, in which: FIG. 1 represents an embodiment of a gas treatment system according to the invention, which here equips a gas storage installation, in particular on a ship, FIG. 2 represents another embodiment of a gas treatment system according to the invention, FIG. 3 represents another embodiment of a gas treatment system according to the invention, FIG. 4 illustrates another embodiment of a gas treatment system according to the invention, FIG. 5 is a variant of the embodiment of FIG. 4, and FIG. 6 illustrates another embodiment of a gas treatment system according to the invention. 5. DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows a first embodiment of a gas treatment system 1 of a gas storage installation 2 according to the invention. This treatment system allows the cooling of one or more gases and / or a reliquefaction of vapors from one or more gases and / or the vaporization or heating of one or more gases. In the present invention, we mean by the term "reliquefaction" the condensation of the vapors of a gas making it possible to bring it back to a liquid state. In the present invention, the system 1 is installed on a ship, such as a gas transport ship, in particular of the VLGC type (for the English expression Very Large Gas Carrier). Ships of this type have a capacity of around 80,000 m3. In a gas transport ship, for example of the LNG tanker type, an energy production installation is provided 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 equipment. of edge. The gas storage installation 2 can be the energy production installation. Such an installation commonly includes thermal machines 3, such as the ship's engine consuming gas from the gas cargo transported in the ship's tank or tanks. On this ship, the gas or gases are stored in the liquid state in several tanks 4 or tanks 5 at very low temperature, or even at cryogenic temperatures. The tanks 4 and the tanks 5 can each contain a gas in liquefied form or in the liquid state at a predetermined pressure and temperature. One or more tanks 4 and / or tanks 5 of the ship can be connected to installation 2 by the system 1 according to the invention. Each tank and tank for this purpose comprise an envelope intended to isolate the gases stored at their storage temperature from the external environment. The ship is loaded with natural gas (GN) stored in a tank 5 and petroleum gases (GP) stored in one or more tanks 4. Each tank and / or tank 4, 5 can have a capacity of between 1,000 to 50,000 m3. The number of tanks 4 and tanks 5 is not limiting. It is for example between 1 and 6. In the following description, the terms "the tank >> and" the tank >> should be interpreted respectively as "the or each tank >> and" the or each tank ". Natural gas (GN) is for example methane or a mixture of gases comprising methane. Natural gas is stored in the liquid state 5a in the tank, for example at a cryogenic temperature of the order of -160 ° C at atmospheric pressure. Natural gas in liquid state or liquefied natural gas 5a is marked "LNG". The tank 5 also includes gas vapors 5b resulting from evaporation, in particular natural, of the LNG in the tank. Evaporation or vapor 5b is designated by the sign "BOG" or "NBOG" for natural evaporation in contrast to "FBOG" for forced evaporation. LNG 5a is stored, of course, at the bottom of tank 5 while LNG BOG 5b are located above the level N1 of LNG 5a in the tank, called gaseous sky. The LNG BOG 5b in the tank is due to heat entering the outside environment inside the tank 5 and to movements of the LNG 5a within the tank 5 due to movements of the sea for example. Petroleum gas (GP) includes propane, butane, propylene, ammonia, ethane, propylene, ethylene, or a mixture of gases comprising these compositions. 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 liquid state 4a or liquefied petroleum gas is marked "LPG". The tank 4 also includes gas vapors 4b which results from evaporation, in particular natural, of the LPG in the tank. Similarly, 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 gaseous sky. As explained previously for LNG, the evaporation of LPG (BOG or NBOG) in the tank 4 is also due to the heat inputs from the external environment inside the tank, to the movements of fluid during journeys (sea, LPG), when loading the LPG in the tank 4 and when cooling the tank to bring the tank temperature back to an equilibrium temperature. During cooling, here of the tank 4 which consists in bringing the ambient temperature of the shell of the tank to an equilibrium temperature, the liquefied gas is sprayed on the walls of the almost empty tank. The evaporation of the gas generates the cold necessary to cool the envelope. During this operation, which takes around 10 hours, there are very few LPG vapors produced by natural evaporation (NBOG) since the tank is almost empty. On the other hand, spraying LPG on the walls for cooling generates a large amount of LPG vapors in the order of 10900 kg / h. This operation of cooling the LPG tanks can be applied to cooling the LNG tank. When loading the LPG, the tank includes a significant amount of BOG which comes 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 cooling are not reliquefied by the LPG loaded in the tank. The loading operation takes around 18h. About 13,900 kg / h of BOG is generated in the tank. The pressure in the tank is maintained above atmospheric pressure when loading the tank. In the embodiment shown in FIG. 1, the system 1 shown comprises four tanks 4 of LPG and a tank 5 of LNG. The system 1 also includes a heat exchanger 6 which allows heat exchanges between the vapors 5b of LNG, the vapors 4b of LPG, the liquid LPG 4a and the liquid LNG 5a. In the present example, the heat exchanger 6 comprises several circuits or conduits, here at least a first circuit 6a, a second circuit 6b, a first conduit 6c, and a second conduit 6d, in which GN or GP circulate. liquid or vapor state. The heat exchanger 6 is configured so that the first circuit 6a exchanges heat with the second circuit 6c to keep the LNG coming from the tank in the liquid state and to re-liquefy 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 tank 5 and the liquefied LPG vapors are sent to the tank 4. For this, the tank 4 comprises an outlet which is connected to a first end of a first pipe 7 in which vapor 4a of LPG circulates. The outlet of the tank 4 is located in the upper part of the tank 4 where there is the gaseous sky with the vapors 4b (NBOG) of LPG. The first pipe 7 is connected to an inlet of a compressor 8 which circulates the LPG vapors 4b in the first pipe 7. The latter includes a second end which is connected to an inlet of the first circuit 6a. The LPG vapors are intended to be liquefied by heat exchange with the cold of the LNG and to keep the LNG in the liquid state. An outlet of the first circuit 6a is connected to a first end of a second pipe 9 in which the reliquefied LPG vapors circulate. The second pipe 9 comprises a second end which is immersed in the LPG or which is connected to a plunger 9a immersed in the tank. Alternatively, the second pipe 9 is connected to an LPG spraying boom 10. The ramp 10 is arranged in the tank 4 and in the upper part thereof, along a vertical axis in the plane of FIG. 1, so as to spray the vapors of LPG reliquefied in the gaseous sky of LPG. This allows to force the recondensation of the NBOG in the tank. The system 1 comprises pumps which are installed in the tank 5 to extract the LNG therefrom. 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 tank 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 pipe 12. The first pump 11a makes it possible to force the circulation of LNG in the third pipe 12. The volume flow of LNG from this first pump 11a is of the order of 130 m3 / h. The second end of this third pipe 12 is connected to an inlet of the second circuit 6b in which LNG 5a from the tank circulates. The second circuit 6b comprises an outlet connected to a first end of a fourth pipe 13 in which also circulates LNG 5a. The fourth pipe 13 comprises a second end which is connected to the tank 5. The third and the fourth pipes 12, 13 allow recirculation of the LNG from the tank to the tank through the heat exchanger 6. More precisely still, the second circuit 6b, the third and fourth pipes 12, 13 form a closed circuit. The LNG is extracted from the tank at a temperature of -160 ° C. The LNG outlet temperature and / or LNG outlet pressure are controlled so that the LNG does not vaporize during heat exchange with the LPG vapors. For this, a temperature sensor is provided, for example on the fourth pipe 13, to control the temperature of the LNG returned to the tank. Advantageously, the predetermined outlet temperature of the LNG is lower, for example by 5 ° C., of the evaporation temperature of the LNG than a value of authorized storage pressure of the tank, for example of the order of 8 bars. The storage pressure of the tank 5 to contain the LNG is between 2 and 20 bars. The LNG outlet pressure of the heat exchanger 6 must be lower than the maximum storage pressure of the tank. 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 the outlet temperature of the LPG gas is substantially close to its liquefaction temperature, at atmospheric pressure and, the second threshold temperature is lower than the first threshold value from 10 ° C to 40 ° C at atmospheric pressure. In the present example, the first threshold value is -40 ° while 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 adequate temperature which is not too cold, in particular which is greater than or equal to a minimum temperature value to be supported by the tank 4. The above-mentioned temperature values for the LPG in this example and in the following 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. The heat exchanger 6 is also configured so that the first conduit 6c exchanges heat with the second conduit 6d to effect forced evaporation of the LNG from the tank and sub-cooling of the LPG from the tank 4 simultaneously. In the present invention, we mean by the term subcooling a lowering of the temperature of the liquefied gas below its liquefaction temperature. The liquefied gas is for example sub-cooled from about 5 ° C to 20 ° C below its liquefaction temperature. It is understood that the storage of the sub-cooled liquefied gas, in the present invention, depends on the storage pressure of the liquefied gas. The vaporized LNG (FBOG) is intended to supply installation 2, and in particular here the ship's engine. The sub-cooled LPG is sent to the tank 4. In particular, the first conduit 6c is configured to circulate petroleum gas, and in particular LPG 4a in the heat exchanger 6. The first conduit 6c comprises an inlet which is connected to one end of a fifth pipe 14 in which circulates LPG extracted from the tank. The other end of the fifth pipe 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 so as to take only LPG and circulate the LPG in this pipe 14. The first conduit 6c comprises an outlet which is connected to a sixth pipe 16 which is intended to return LPG sub-cooled in the tank 4. The sixth pipe 16 can be connected to the spraying boom 10 or to the second pipe 9, or even to the plunger 9a to return the LPG to the tank. Preferably, the sub-cooled LPG is stored at the bottom of the tank 4 in a cold reserve layer 4c located in the interior of the tank and in the lower part of the tank. This layer 4c can be used later. Preferably, but not limited to, the second end of the pipe 9 or that of the plunger is located in the lower part of the tank 4, along a vertical axis in the plane of Figure 1 to store the LPG sub-cooled. The sub-cooling takes place outside the tank or any other tank or tank. The sub-cooling is not immersed in a liquefied gas for example. In addition, the cold reserve layer 4c is located in the interior of the tank, at the bottom of the tank. The cold reserve layer 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, under-tank or compartment in the tank which separates the LPG remaining / already in the tank and the sub-cooled LPG stored in this reserve layer. The second conduit 6d allows vaporization of the LNG 5a from the tank 5. For this, the second pump 11b, which is immersed in the LNG, is connected to a first end of a seventh line 17 in which the LNG circulates, l installation 2, here the ship's engine. The second pump 11b allows the circulation of the LNG in the seventh pipe 17 at a volume flow rate lower than that of the first pump 11a. In the present example, the volume flow rate of the LNG in the seventh line 17 is of the order of 4 m3 / h. A second end of the seventh pipe 17 is connected to an inlet of the second conduit 6d. The latter comprises an outlet which is connected to an eighth pipe 18 in which circulate LNG vapors 5a formed by heat exchange with the LPG to supply, for example the ship's engine. During this vaporization-sub-cooling heat exchange, the LNG temperature is raised. That is, its temperature is above its liquefaction temperature at atmospheric pressure. The LNG temperature is corrected by a heating device, not shown here, according to the engine specifications. The LNG outlet pressure, for example required by the ship's engine, is around 17 bars. Regarding LPG, its entry temperature in circuit 6c is around 1 bar. The outlet temperature of the sub-cooled LPG is greater than or equal to a minimum temperature value to be supported by the tank or tank. Here, the outlet temperature is around -52 ° C (at storage pressure in the tank). In Figure 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 sub-cooled is returned to the same tank. Of course, other arrangements are possible. In Figure 1, the heat exchanger 6 is separated from the tanks or tank. The heat exchanger 6 is arranged outside the tanks and the tanks. The heat exchanger is not located in another tank or another tank where liquefied gas is stored. Advantageously, the heat exchanger is a tube, plate or coil exchanger. 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 LNG and / or the LPG. 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, main heat exchanger 21. In Figure 2, a single tank 5 and a single tank 4 are shown. Of course, the system can include other tanks and vats. The system 1 also includes the pumps 11a, 11b and 15 which are installed in the tank 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 tank to ensure that they are only supplied with LNG. The flow rate of the first pump is also approximately 130 m3 / h and the flow rate of the second pump is approximately 4 m3 / h. The main heat exchanger 21 is configured to reliquefy the LPG vapors 4b by heat exchange with the cold of the LNG 5a and to keep the LNG in the liquid state simultaneously. The LNG is returned to the tank 5 without being vaporized and the liquefied 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 pipe 7 coupled to the tank 4, and on the other hand to the second pipe 9 also coupled to the tank 4. A first compressor 8 is also provided on the first pipe 7 to ensure the circulation of the LPG vapors 4b therein towards the heat exchanger 21. The heat exchanger 20 is configured to vaporize the LNG from the tank and to sub-cool the LPG from the tank 4 simultaneously. The LNG must undergo forced evaporation to raise the temperature of the LNG to the required temperature, for example for the ship's engine to be supplied with LNG vapors. The heat exchanger 20 comprises the first conduit 6c and the second conduit 6d. The second conduit 6d is connected on the one hand to the seventh line 17 connected to the tank and on the other hand to the eighth line 18 which transfers the LNG to the ship's engine. The first conduit 6c is connected, on the one hand to the first pipe 14 coupled to the tank 4, and on the other hand to the sixth pipe 16 coupled to the tank 4, and in particular to the bottom of the tank 4. In FIG. 2, the system 1 also includes a third heat exchanger called the auxiliary heat exchanger 22. The latter allows a second sub-cooling of the LPG with the cold of the LNG and to keep the LNG in the liquid state. The liquid LNG is returned to the tank and the sub-cooled LPG is returned to the tank. Advantageously, but not limited to, the heat exchangers 20, 21, 22 are separated from the tanks and reservoirs. Advantageously, but not limited to, the heat exchangers 20, 21, 22 are tube, plate or coil exchangers. The auxiliary heat exchanger 22 comprises a third circuit 6e in which LNG flows and a fourth circuit 6f in which LPG, in particular sub-cooled, circulates. The third circuit 6e comprises an input coupled to a ninth line 23 which is connected to the tank 5. As we can see in Figure 2, the ninth line 23 is a bypass portion of the seventh line 17 which extracts the LNG from the bottom of the tank 5 by means of the pump 11b. The third circuit 6e includes an output which is connected to a tenth line 24 which returns the LNG maintained in the liquid state to the tank 5. In this embodiment, the tenth line 24 is coupled to a portion of the fourth line 13 returning LNG to tank 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 pipe 25 in which circulates LPG extracted from the bottom of the tank. The eleventh pipe is here coupled to the pipe 16 in which circulates LPG sub-cooled and by a valve 29, such as a three-way valve. The fourth circuit 6f includes an output which is coupled to a twelfth line 26 which is connected to the tank. According to this exemplary embodiment, the twelfth line 26 is coupled to a portion of the tenth line or to the line 9. The LPG sub-cooled by heat exchange with the LNG is sprayed in the gaseous air or is stored at the bottom of the tank 4 in the cold reserve layer 4c. The ninth line 23 can be connected to the line 16 by a valve 27. Likewise, the line 23 can be connected to the line 9 by a valve 28. Preferably, but not limited to, the valve or valves 27, 28 are three-way valves. Line 16 is connected to an LPG spraying boom 10 to spray droplets of LPG into the gaseous sky of tank 4 and force the recondensation of NBOG in tank 4. The third pump 15 is configured to force the circulation of LPG in the pipe or pipes 14, 16, 25 from the bottom of the tank to the spray boom 10. Due to this configuration, the sub-cooled LPG is transferred directly to the tank or to the ramp 10 or is transferred to the auxiliary heat exchanger 22 for a second sub-cooling with LNG. In FIG. 2, the system further comprises a pipe 30 for extracting the vapors 5b of LNG in the tank 5 so as to control the pressure of the tank 5 and to supply the installation 2 with combustible gas. A second compressor 31 is mounted on this pipe 30 to ensure the circulation of the LNG vapors 5a towards the engine and maintain the pressure in the tank. This pipe 30 is connected to the pipe portion 18 where heated or vaporized LNG flows to the ship's engine. Advantageously, but not limited to, a heating device 32 is arranged upstream of the installation so as to adjust the temperature of the LNG to the required temperature and to ensure that all of the LNG is vaporized. The heating device 32 is here a heater. In a third embodiment of the invention illustrated in FIG. 3, the system 1 also includes several heat exchangers. In particular, the system 1 comprises: the main heat exchanger 21 which is configured to reliquefy the LPG vapors 4b by heat exchange with the cold of the LNG 5a and to maintain the LNG in the liquid state, the evaporation heat exchanger which is configured to vaporize the LNG from the tank 5 and to sub-cool the LPG from the tank 4, and the auxiliary heat exchanger 22 'which is configured to sub-cool the LPG and keep the LNG in the liquid state. 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 not limited to, a vacuum evaporator (ESV) intended to generate cold. The vacuum evaporator 40 includes a primary circuit 42 which includes an inlet and an outlet. The inlet is connected to the seventh line 17 in which LNG from the tank circulates. The outlet of the primary circuit 42 is connected to a first end of a pipe 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 pipe 17 and upstream of the vacuum evaporator 40. The depressurization means 41 make it possible to obtain a gas in a liquid-vapor two-phase state by lowering the pressure and the temperature of the gas. The depressurization means 41 here comprise 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 around 160 ° C and to a pressure of the order of 1 bar. The two-phase LNG enters the vacuum evaporator 40 where heat is exchanged with LNG extracted from the tank. More specifically, the vacuum evaporator 40 includes a secondary circuit 43 which includes an inlet and an outlet. The input of the secondary circuit 43 is connected to a bypass pipe 45 in which LNG flows from the tank 5. This bypass pipe 45 comes from the seventh pipe 17 coupled to the pump 11b. Of course, the pipe 45 could be connected to another submerged pump at the bottom of the tank. The output of the secondary circuit is connected to the line 23 returning the LNG to the bottom of the tank 5. In this embodiment, the line 23 is coupled to the input of the circuit 6e of the heat exchanger 22. In this evaporator under vacuum 40, the LNG circulating in the secondary circuit 43 is sub-cooled by recovering the latent heat of the two-phase LNG and circulating in the circuit 42. The sub-cooled LNG is transferred to the tank. The two-phase LNG circulating in the primary circuit 42 is heated or vaporized then transferred to the evaporation exchanger 20. The LNG outlet temperature at the outlet of the primary circuit 42 is between -160 and -134 ° C and a pressure of around 1 bar. The outlet temperature of the sub-cooled LNG is around -160 ° C at a pressure between 2 and 20 bars. When the sub-cooled LNG circulates through the heat exchanger 22, it is configured to maintain the LNG from the vacuum evaporator 40 in the liquid state. Indeed, the LNG from circuit 43 can exchange heat with sub-cooled LPG from 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. In FIG. 3, the system 1 also 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 installation 2. In this exemplary embodiment, the sub-cooling is carried out outside the tanks and the tank. In other words, the heat exchangers are separated from the tanks and the tank. In a first operating mode (COOLING) of the gas treatment system 1 for the installation 2 of energy production as illustrated in FIG. 2, LNG is used to reliquefy the vapors 4b of LPG. LNG is also used to supply installation 2, in particular the ship's engine and other thermal machines for energy production needs. This first operating mode is operated during the cooling of the LPG tank. In fact, as we explained previously, a very large amount of LPG vapor 4b is generated during this operation (around 10,900 kg / h). This quantity of vapor 4b generated is greater than the quantity of vapor 4b (NBOG) generated during the voyage of the ship to transport the LPG. As part of the cooling of the walls of the tank, the energy requirements of the engine with combustible gas are very low. The consumption of installation 2 is around 500 kg / h in LNG vapor. The system uses the main heat exchanger 21 to manage the LPG vapors 4b generated during cooling. The LPG vapors 4b are extracted from the tank 4 by the compressor 8 which circulates them in the first pipe 7. The LPG vapors 4b circulating in the first circuit 6a are reliquified by the cold of the LNG circulating in the second circuit 6b via the third line 12 from the bottom of the tank 5. It is understood that the LNG which is at the bottom of the tank is cooler than the LNG near the surface N1, ie at the interface between the LNG and the gaseous sky. Following the reliquefaction, the reliquefied LPG vapors are transferred to the tank 4 and the LNG is maintained in the liquid state then returned to the tank 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 second threshold values are considered to be equal to or greater than atmospheric pressure. These temperature threshold values are greater than or equal to a minimum temperature value supported by the tank 4. Advantageously, the first threshold temperature for the outlet temperature of the LPG vapors 4b is -40 ° C. at a pressure equal to or greater at atmospheric pressure and the second threshold value for the outlet temperature of the liquefied LPG vapors is of the order of -50 ° C. at a pressure equal to or greater than atmospheric pressure. Preferably, but not limited to, 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 so that the reliquefied LPG vapors are not too cold. Similarly, the heat exchange is carried out so that the outlet temperature of the LNG after the reliquefaction is between a first threshold value and a second temperature threshold value at a pressure between 6 and 20 bars. As we saw in the first embodiment in relation to Figure 1, the LNG must be heated but not vaporized. The main heat exchanger 21 is configured so that the temperature difference between the inlet temperature of the LNG before the reliquefaction and the outlet temperature of the LNG after the reliquefaction is between 5 ° C and 55 ° C. Preferably, but not limited to, this temperature difference is 26 ° C. Here, 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 between 2 and 20 bars. The first threshold value is around -155 ° C and the second threshold value is around -105 ° C. Preferably, but not limited to, the LNG outlet temperature is lower than its vaporization temperature and at a pressure below a maximum authorized storage tank pressure. The temperature is around -134 ° C. Such values make it possible to transfer a maximum of LNG cold to the LPG vapors for reliquefaction while preventing the LNG returning to the tank from being too hot and the reliquefied LPG vapors from being too cold. LNG that is too hot could cause an increase in LNG pressure in the tank and exceed the authorized limits. Thus, the main heat exchanger 21 is adjusted so that the LNG and the liquefied LPG vapors exit respectively at the required temperature in the tank or the tank. During heat exchange, the LNG flow rate and the LPG vapor flow rate are constant, respectively. Since the inlet and outlet temperatures of LNG and LPG are known and / or predetermined, parameters such as the mass flow of LNG and LPG make it possible to configure the heat exchanger 21 for the heat exchange. 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. In this first operating mode, the system 1 also uses the evaporation exchanger 20 in which LPG from the tank 4 and LNG from the tank 5 circulates to supply the installation 2. The heat exchange between the LPG and LNG allows the sub-cooling of the LPG and the vaporization or heating of the LNG intended to supply the installation 2. The sub-cooled LPG is stored in the lower part of the tank so as to constitute a cold reserve layer 4c higher. This makes it possible to obtain a greater available refrigerating power and therefore to improve the efficiency of the cooling of the gas, liquefied and / or in the form of gas, 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 lowest end of the tank, for example closer to the ship's hull when the tank is transported on the LNG carrier. In particular, the LPG extracted from the bottom of the tank by the pump goes through the heat exchanger 20 where its inlet temperature is around -42 ° C. The inlet temperature of the LNG extracted from the tank is approximately 160 ° C at a pressure of approximately 17 bars. After the heat exchange where the LPG collects the latent heat of the vaporizing LNG, the outlet temperature of the LPG is between -45 ° C and -55 ° C. The sub-cooled LPG is transferred to the bottom of the tank or it is therefore stored in layer 4c at a temperature between -45 ° and -55 ° C. Advantageously, the sub-cooled LPG is approximately 52 ° C. (storage pressure in the tank). After the heat exchange, the vaporized or heated LNG is at an outlet temperature of about 0 ° C where it can still be heated by the heater 32. Alternatively, the storage of the sub-cooled 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 sub-cooled LPG in the cold reserve layer. For this, pressure determination means 33 make it possible to determine the pressure inside the tank 4. The pressure determination means 33 here comprise a pressure sensor installed in or near the tank 4. The LPG in the tank 4 which is above this cold reserve layer 4c, for example remaining in the tank, is at a temperature above 42 ° C. We consider that the LPG tank has several layers in which the LPG is at different temperatures, the coldest layers being at the bottom of the tank. In a second operating mode (TRAVEL) of the gas treatment system for installation 2 of energy production, as illustrated in FIG. 2, LNG is used to supply installation 2 such as the engine of the vessel and the LPG is sub-cooled to form a reserve of cold LPG which will be used later to cool the LPG vapors in the tank. This operating mode is operated during the voyage of the vessel where a lesser quantity of LPG vapors must be managed. Indeed, the LPG gas vapors (NBOG) generated are of the order of 2700 kg / h while the ship's engine for example consumes a small amount of combustible gas, of the order of 2000 kg / h. In this operating mode, the system uses at least the evaporation heat exchanger 20 in which circulates LPG from the tank and LNG from the tank to carry out forced evaporation of LNG to supply the ship's engine, and the auxiliary heat exchanger 22 to constitute the cold reserve. The LNG is extracted from the tank via the second pump 11b. The inlet temperature of the LNG in the second conduit 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 latter circulates in the second pipe to the evaporative exchanger and enters it at a temperature of around -42 ° C. The LPG undergoes a first sub-cooling of the LPG by recovering the cold of 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 sub-cooling temperature of LPG is between a first threshold value and a second threshold value at atmospheric pressure. The evaporative exchanger 20 is configured to transfer a maximum amount of heat but is limited by the temperature difference between the LNG and the LPG. Advantageously, but not limited to, the first threshold value is of the order of -40 ° C. and the second threshold value is of the order of -55 ° C. The sub-cooled LPG is stored in the lower part of the tank so as to constitute the cold reserve layer of LPG or sprayed into the gaseous sky by the ramp 10. When traveling, the temperature at which the LPG leaves the heat exchanger 20 is around -52 ° C. Of course, as we have seen for the first operating mode, when the pressure in the tank is lower than the first predetermined pressure threshold value, for example between 1 and 1.05 bar absolute, the sub-cooled LPG is stored in the layer cold reserve. We consider that a cold reserve layer has already formed, for example, during the cooling of the tank. Then this sub-cooled LPG is used to cool or condense the LPG vapors in the tank. For this, the sub-cooled LPG is extracted from the cold reserve layer 4c and is sprayed into the gaseous air via the boom 10. Alternatively, the LPG from the cold reserve layer 4c is extracted from an outlet. of the tank which is coupled to a pipe which is connected to the ramp or to a heat exchanger through which the LPG vapors pass. It is therefore not necessary to start the auxiliary heat exchanger to create a cold reserve. The LNG leaving 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 that are extracted from the tank also fuel the engine. The vaporized or heated LNG and the LNG vapors are heated so that all of the LNG is vaporized before fueling the engine. In a third operating mode (LOADING) of the gas treatment system for the energy production installation, as illustrated in FIG. 2, LNG is used to power the ship's engine and for production needs of energy, as well as to reliquefy the LPG vapors. This operating mode is operated in particular when loading the LPG into the tank where a large quantity of LPG vapors is produced, for example around 13900 kg / h. The energy requirements of installation 2 are low, around 500kg / h. In this operating mode at least two heat exchangers are used to treat all the LPG vapors. In particular, the system uses the main heat exchanger 21 to manage the LPG vapors generated during loading of the LPG and the evaporative heat exchanger 20 to vaporize or heat the LNG intended to supply the installation 2. The heat exchangers 20, 21 therefore operate in a similar manner to the first mode of operation in the case of cooling of the tank. In this operating mode, the main heat exchanger 21 may not be able to manage the pressure in the tank 4 due to the large amount of LPG vapor generated. In this case, when the pressure measured (thanks to 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 taken from the tank so as to exchange with the sub-cooled LPG. The LPG sub-cooled after the first sub-cooling 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 circulates in the heat exchanger 20, in particular in the second duct 6d. Indeed, it is the engine or the installation 2 which determines the rate of LNG to be vaporized in the second conduit 6d. Since the needs of installation 2 are low, a very small quantity of LNG is available to sub-cool the LPG. The installation controls the flow rate of the second gas to be vaporized or heated during vaporization. This implies that the amount of heat from LNG is not enough to significantly 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 performs a second sub-cooling of the LPG. The LNG is extracted from the tank, at a temperature of approximately -160 ° C., and exchange of heat with LPG having undergone a first sub-cooling, here in the heat exchanger 20. The inlet temperature of the LPG sub-cooled is around -42 ° C. The outlet temperature of the LPG sub-cooled a second time is less than or equal to a threshold temperature value to be supported by the tank 4. The outlet temperature of the LPG is of the order of -52 ° C. This LPG is stored in the cold reserve layer for later use or is sprayed in the gas overhead of the tank to condense or cool the vapors 4b of LPG in the tank. The LNG outlet temperature is around -134 ° C at a pressure of around 8 bars. LNG is therefore hot but not vaporized. In a fourth operating mode (hot LNG in the tank), the gas treatment system 1 for the energy production installation, as illustrated in FIG. 2, the system makes it possible to manage the risk of heating the LNG in the tank in the case where the main heat exchanger 21 has operated (when loading LPG in the tank or when the tank is cooling). Indeed, the LNG at the outlet of the main exchanger and or at the outlet of the auxiliary heat exchanger is hot, ie at an outlet temperature of around -134 ° C. This operating mode employs the system as shown in FIG. 3 and mainly in travel mode to cool the LNG in the tank to its cryogenic temperature. The system 1 uses at least the heat exchanger 40 where the partially vaporized LNG allows sub-cooling of the LNG which is transferred to the tank. We then consider that the LNG stored in the tank is at a temperature of around -134 ° C at a pressure of the order of 8 bars. The LNG is extracted from the tank by the second pump 11b. LNG circulates in circuit 42 where it has been depressurized and then partially vaporized. The inlet temperature of the partially vaporized LNG into the heat exchanger 40 is around -160 ° C at atmospheric pressure. The outlet temperature of the vaporized LNG is between -134 and -160 ° C at atmospheric pressure. The inlet temperature of the LNG into the heat exchanger, in the second duct 43, is of the order of -134 ° C and its outlet temperature is of the order of 160 ° C. The sub-cooled LNG is transferred to a cold reserve layer 4c in the lower part of the tank 5. The heat exchanger 20 sub-cools the LPG and vaporizes the LNG at the outlet of the heat exchanger 40. When the pressure measured in the tank 4 is greater than or equal to the threshold pressure value, the heat exchanger 22 'is activated to sub-cool a second time the LPG which has been cooled in the exchanger 20. The LPG is sub-cooled with LNG which has been sub-cooled in the heat exchanger and passes through the heat exchanger 22 '. The LNG outlet temperature after the heat exchange in the exchanger 22 ’is of the order of -134 ° C and at atmospheric pressure. These above operating modes have been described on the basis of FIG. 2. It is of course possible that FIG. 1 applies to these operating modes. FIG. 4 illustrates another embodiment of the gas treatment system 1 according to the invention. The system includes LNG tanks each comprising LNG vapors 5b and LNG. Here, two LNG tanks are represented. Pumps are also submerged in LNG from a main tank and a single pump is submerged in LNG from the adjacent tank. Each pump is preferably installed at the bottom of the tank. The system 1 comprises a heat exchanger 50 which is configured to sub-cool LNG from the LNG tank, here first tank 500A, intended to be stored at the bottom 190 of the same first tank 500A so as to constitute a reserve layer of cold 500c at the bottom of the tank 500A. Layer 500c is located in the interior of the tank. The heat exchanger comprises at least a first duct 50a and a second duct 50b. The first conduit 50a includes an inlet which is coupled to the first end of a pipe 54. The second end of the pipe 54 is connected to a first pump 51 mounted at the bottom of the first tank 500A. This pipe 54 is also connected to a spray boom 60 mounted in the tank 500A via a three-way valve 67. The boom 60 is arranged in the upper part of the tank and preferably in the LNG gas overhead. The first conduit 50a includes an outlet which is coupled to a pipe 56 which is connected to the bottom of the tank 500A. The pipe 56 is also connected to the sprayer boom 60 by a three-way valve 75a. As illustrated in FIG. 4, the pipe 56 opens into the bottom of the adjacent tank, second tank 500B by a three-way valve channel 75b as well as to another ramp 60 of this second tank 500B by a three-way valve 75c. The second conduit 50b comprises an inlet connected to the tank 500A by a pipe 57. One of the ends of the pipe 57 is connected to a second pump 52 mounted at the bottom of the tank 500A. The outlet of the second conduit 50b is connected here to an inlet of a balloon 70 via a pipe 58. The outlet of the balloon 70 is connected to the pipe 56 by a first outlet to a pipe 71. The pipe 71 comprises for example a valve 72 and a pump 73. Depressurization means 53 are mounted on the pipe 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 (JouleThomson valve). The second conduit 50b is a cold circuit, the depressurized LNG being intended to be heated by circulation in this circuit so as to achieve forced evaporation (in FBOG). The first conduit 50a is a hot circuit, the LNG coming from the tank 500A being intended to be cooled by circulation in this circuit. The first conduit 50a may not allow however to vaporize the heaviest components (ethane, propane, etc.). We understand that the depressurization upstream of the second conduit 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 tank 500A and circulating in the first conduit 50a. The vaporization in FBOG requires a supply of heat supplied by the LNG circulating in the first conduit 50a, it is therefore a cooling source for the sub-cooling of the LNG circulating in the first conduit 50a. LNG from the tank 500A is thus conveyed by the pump 52 to the depressurization means 53 and then circulates in the second duct 50b or cold of the exchanger 50. The LNG downstream of the depressurization means is at a temperature of -168 ° and at an absolute pressure of 400 mbar. In the meantime, LNG from tank 500A is conveyed by pump 51 to the first conduit 50a or heat exchanger 50. Consequently, the heat exchange between these circuits results in: the heating of depressurized and partially vaporized LNG, in order to continue its vaporization, which is then conveyed to the tank 70 in the present example, and the sub-cooling of LNG which supplies the bottom of the first tank and or of the second tank for be stored there for later use or sprayed in the LNG gas overhead via the boom 50. The LNG outlet temperature after the heat exchange in duct 50a is around -168 ° C. The storage of LNG in the cold reserve layer can be a function of the pressure inside the tank. For example, when the pressure measured (with a pressure sensor 330) in the tank is less than a predetermined pressure threshold value in the tank, the sub-cooled LNG is stored in this cold reserve layer 500c. The tank 70 is thus intended to be supplied with LNG in a two-phase liquid-vapor state coming from the tank 500A via the heat exchanger 50. The operating pressure inside the tank 70 is lower than the storage pressure of the LNG inside the 500A tank. The supply of balloon 70 with LNG can lead to additional vaporization of LNG, resulting firstly in the generation of FBOG in balloon 70, as well as the sub-cooling of the LNG remaining in the balloon. The tank makes it possible to separate the phases with the LNG stored in the lower part of the tank and the LNG vapors in the upper part of it. The sub-cooled LNG leaving the tank is at an outlet temperature of around -168 ° C. The balloon 70 comprises a second outlet which is arranged in the upper part thereof where the gas vapors (FBOG) of LNG are naturally stored. The outlet of the balloon 70 is connected to the installation 2 here by two compressors 61,62. The heat exchanger 50 also includes a third conduit 50c which includes an inlet and an outlet. The inlet of the third conduit 50c is connected to a first end of a pipe 63 in which vapor of reliquefied LNG gas circulates. In particular, the output of the compressor 62 is connected to the installation 2 for its supply of combustible gas. Part of the combustible gas leaving the compressor 62 can be withdrawn and re-routed through a pipe 64 which can be connected to the outlet of the compressor 62 by a three-way valve 65. The compressor 62 is configured to compress the gas (such as NBOG coming from of the first tank and / or second tank) at an operating pressure suitable for its use in the installation 2. The pipe 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 pipe 63. Each tank 500A, 500B comprises an outlet 68 of LNG vapors 5a which is connected to an inlet of a secondary circuit 66b of the heat exchanger 66. The circuit secondary 66b comprises an outlet which is connected to the inlet or to one of the inlets of the compressor 62. The third conduit 50c comprises an outlet which is connected to the pipe 56 by another pipe 69. An expansion valve 74 is installed on this pipe 56 to reduce the temperature of the gas by adiabatic expansion. The LNG vapors from a tank 500A, 500B are heated in the secondary circuit 66b so as to supply the installation 2, and the LNG vapors at the outlet of the compressor 62 are reliquified to be conveyed to the heat exchanger 50 In this heat exchanger 50, the reliquefied gas vapors are sub-cooled with the cold of the LNG circulating in the duct 50a to supply the bottom of the tank or tanks 500A, 500B or the spraying boom 60. The LNG vapors from the tank (s) 500A, 500B can be redirected into line 64 if FBOG is produced in excess so as to be also reliquefied In this exemplary embodiment, the sub-cooling is carried out outside the tanks. In other words, the heat exchanger 50 is separated from the tanks. FIG. 5 represents an alternative embodiment of the gas treatment system 1 illustrated in FIG. 5. This system 1 differs from that of FIG. 4 in that it comprises a second pump 52 installed in the second tank 500B adjacent to the first tank, main (which is on the right of figure 5). This second pump 52 is at a first end of a pipe 80 in which LNG extracted from the bottom of the second tank 500B circulates. The second end of the pipeline is coupled to the pipeline 57 which is connected to the inlet of the second conduit 50b. In other words, the LNG is extracted from the two tanks 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. 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 Figure 5. It differs in that it comprises two heat exchangers 150, 150 'instead of a single heat exchanger 50. A first exchanger 150 is configured to vaporize LNG from the first 500A tank and to sub-cool LNG from the first 500A tank simultaneously. The first exchanger 150 comprises the first conduit 150a and the second conduit 150b arranged as described in the embodiment of FIG. 4. The second heat exchanger 150 ’is configured to use the sub-cooled LNG stored in the cold reserve layer 500c here from the first tank 500A to reliquefy LNG vapors. These LNG vapors come from natural evaporation (NBOG) of LNG not used by the energy production facility 2, that is to say excess BOG. The second heat exchanger 150 ’comprises the third duct 150c and a second auxiliary duct 150b’. The third conduit 150c includes an inlet which is connected to the pipe 163 through which is conveyed LNG vapors produced in excess. In particular, the NBOG recirculates via the compressor 62 in the heat exchanger 166 and via the line 164. The third conduit 150c includes an outlet which is connected to the line 169 which opens at the bottom of the tank or each tank 500A, 500B by a three-way valve 175b. Line 169 is also connected to a spray boom 160 via a three-way valve 175a, 175c. The second conduit 150b ’includes an inlet which is connected to the conduit 154 via a three-way valve. The second line 150b ’includes an outlet which joins line 156 via the 3-way valve 180. A heat exchange is carried out between the excess NBOG and the sub-cooled LNG coming from the tank. The reliquified NBOG is transferred to the bottom of the first and / or second tank (s). The LNG at the outlet of the second conduit 150b ’is heated but not vaporized and is returned to the bottom of the first and / or second tank (s). In this exemplary embodiment, the sub-cooling is carried out outside the tanks. In other words, the heat exchangers are separated from the tanks.
权利要求:
Claims (17) [1" id="c-fr-0001] 1. A method of treating gas from a gas storage installation (2), the installation (2) comprising a tank (4) in which a first gas (4a, 4b) is stored and a tank (5) in which a second gas (5a, 5b) is stored, the second gas (5a, 5b) having a boiling temperature lower than that of the first gas, the method comprising a reliquefaction step in which vapors (4b) of the first gas flowing in a first circuit (6a) from the tank (4) are reliquified by heat exchange with the second gas in the liquid state having an inlet temperature and flowing in a second circuit (6b), the vapors of the first gas reliquefied being transferred into the tank (4) and the second gas being maintained in the liquid state at an outlet temperature after the reliquefaction and returned to the tank (5), the heat exchange between the first gas (4b) and the second gas (5a) being produced from my negatively that an outlet temperature of the vapors (4b) of the first reliquefied gas is between a first threshold value and a second threshold value. [2" id="c-fr-0002] 2. Method according to the preceding claim, characterized in that the temperature difference between the inlet temperature of the second gas (5a) before the reliquefaction step and the outlet temperature of the second gas (5a) after the step of reliquefaction is between 5 ° C and 55 ° C. [3" id="c-fr-0003] 3. Method according to any one of the preceding claims, characterized in that the outlet temperature of the second gas (5a) is lower than the vaporization temperature of the second gas at a pressure less than or equal to a maximum authorized pressure value of tank storage (5). [4" id="c-fr-0004] 4. Method according to any one of the preceding claims, characterized in that the vapors of the first reliquefied gas are transferred into the tank (4) at a temperature greater than or equal to a minimum temperature value to be supported by the tank (4 ). [5" id="c-fr-0005] 5. Method according to any one of the preceding claims, characterized in that the outlet temperature of the second gas (5a) is between -155 ° C and -105 ° C at a pressure between 2 and 20 bars. [6" id="c-fr-0006] 6. Method according to any one of the preceding claims, characterized in that the first threshold value corresponds to the liquefaction temperature of the first gas at atmospheric pressure and the second threshold value is 10 to 40 ° C lower than the first value threshold at atmospheric pressure .. [7" id="c-fr-0007] 7. Method according to any one of the preceding claims, characterized in that the vapors (4b) of the first gas are compressed before the heat exchange. [8" id="c-fr-0008] 8. Method according to any one of the preceding claims, characterized in that the heat exchange during the reliquefaction step is carried out during a loading operation of the first gas or during a cooling operation of the tank. [9" id="c-fr-0009] 9. Method according to any one of the preceding claims, characterized in that the first gas is a liquefied petroleum gas. [10" id="c-fr-0010] 10. Method according to any one of the preceding claims, characterized in that the second gas is a liquefied natural gas. [11" id="c-fr-0011] 11. System (1) for gas treatment of a gas storage installation (2), the system comprising: - a tank (4) in which is stored a first gas (4a, 4b), - a tank (5) in which is stored a second gas (5a, 5b), the second gas having a boiling temperature lower than that of the first gas, - a first circuit (6a) in which at least part of the vapors of the first gas flows from the tank (4), a second circuit (6b) in which at least part of the second gas in the liquid state circulates at an inlet temperature from the tank (5), and - a heat exchanger (6, 21) configured to reliquefy at least part of the vapors of the first gas by heat exchange with the second gas in the liquid state, the vapors of the first reliquefied gas being transferred into the tank (4) and the second gas being maintained in the liquid state at an outlet temperature after the reliquefaction and returned to the tank (5), and so that an outlet temperature of the vapors of the first gas is between a first threshold value and a second threshold value. [12" id="c-fr-0012] 12. System (1) according to the preceding claim, characterized in that the heat exchanger is configured so that the temperature difference between the inlet temperature of the second gas before the reliquefaction step and the outlet temperature after l '' reliquefaction stage, between 5 ° C and 55 ° C. [13" id="c-fr-0013] 13. System (1) according to any one of claims 11 to 12, characterized in that it comprises a compressor (8) installed upstream of the first circuit (6a) so as to compress the vapor of the first gas to be extracted from the tank (4) before the heat exchange. [14" id="c-fr-0014] 14. System (1) according to any one of claims 11 to 13, characterized in that the second circuit forms with conduits each connected to the tank and to the second circuit a closed circuit. [15" id="c-fr-0015] 15. System (1) according to any one of claims 11 to 14, characterized in that the first gas is a liquefied petroleum gas. [16" id="c-fr-0016] 16. System (1) according to any one of claims 11 to 15, characterized in that the second gas is a liquefied natural gas. [17" id="c-fr-0017] 17. Ship, in particular for transporting liquefied gas, comprising at least one system (1) according to any one of claims 11 to 16.
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同族专利:
公开号 | 公开日 FR3077867B1|2020-01-31|
引用文献:
公开号 | 申请日 | 公开日 | 申请人 | 专利标题 JPH07119897A|1993-10-25|1995-05-12|Tokyo Gas Co Ltd|Method and device for suppressing bog generated in lpg storage tank| EP2716542A2|2011-05-31|2014-04-09|Daewoo Shipbuilding & Marine Engineering Co., Ltd.|Cold heat recovery apparatus using an lng fuel, and liquefied gas carrier including same|WO2021032925A1|2019-08-19|2021-02-25|Gaztransport Et Technigaz|System for treating gas contained within a tank for storing and/or transporting gas in the liquid state and the gaseous state, the system being fitted on a ship| FR3103227A1|2019-11-20|2021-05-21|Gaztransport Et Technigaz|Gas supply system for at least one gas consuming device fitted to a ship| WO2021123685A1|2019-12-20|2021-06-24|Gaztransport Et Technigaz|Method for estimating and adjusting an energy balance of a gas in liquid form contained in a tank|
法律状态:
2019-02-28| PLFP| Fee payment|Year of fee payment: 2 | 2019-08-16| PLSC| Publication of the preliminary search report|Effective date: 20190816 | 2020-02-28| PLFP| Fee payment|Year of fee payment: 3 | 2021-03-05| PLFP| Fee payment|Year of fee payment: 4 | 2021-11-29| PLFP| Fee payment|Year of fee payment: 5 |
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申请号 | 申请日 | 专利标题 FR1851135|2018-02-09| FR1851135A|FR3077867B1|2018-02-09|2018-02-09|METHOD AND SYSTEM FOR GAS TREATMENT OF A GAS STORAGE FACILITY FOR A GAS TRANSPORT VESSEL|FR1851135A| FR3077867B1|2018-02-09|2018-02-09|METHOD AND SYSTEM FOR GAS TREATMENT OF A GAS STORAGE FACILITY FOR A GAS TRANSPORT VESSEL| 相关专利
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