RU2772630C2 - Method and system for processing gas in installation for gas storage of gas transportation tanker - Google Patents

Abstract

FIELD: gas industry.

SUBSTANCE: invention relates to a method and a system for processing gas of installation (2) for gas storage, in particular, onboard a ship. The method includes following stages. First gas (4a, 4b, 5a, 5b) is extracted in a liquid state from first tank (4) or first container (5, 500). The first gas in a liquid state is subjected to the first overcooling. The overcooled first gas in a liquid state is stored in a lower part of first tank (4) or first tank (5, 500), or the second tank or the second container to form cold reserve layer (4c, 5c, 500c) of the overcooled first gas in a liquid state at the bottom of first or second tank (4) or first or second container (5, 500).

EFFECT: creation of colling power that can be used later, wherein a cold reserve is stored at the bottom of a tank or a container for a long time.

36 cl, 6 dwg

Description

1. FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The present invention relates to a method and system for treating the gas of a gas storage facility, particularly on board a ship, such as an LPG carrier, whose plant is powered by gas derived from a cargo stored on board.

2. STATE OF THE ART

It is known in the art to transport gas in liquefied form on several types of ships to facilitate transportation 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, in fact cryogenic temperatures, so that they are in a liquid state at a pressure close to atmospheric pressure, and loaded onto specialized ships. Liquefied natural gas and liquefied petroleum gas are used as fuel for various pieces of equipment in any industry. Recently, liquefied natural gas has been used to meet the energy requirements for propulsion of ships and, in particular, ships carrying liquefied petroleum gas and liquefied natural gas, for example, to comply with new environmental regulations limiting emissions of sulfur oxides (SOx) and oxides nitrogen (NOx) in the ECA (Emission Control Area) and SECA (SOx Emission Control Area) zones.

Liquefied natural gases and liquefied petroleum gases are stored on ships in insulated tanks at very low temperatures to keep the gases in a liquid state. Tanks absorb heat inside them, which contributes to the evaporation of part of the gases in the tanks, this phenomenon is known under the abbreviation OGEI (NBOG) (Natural Boil-Off Gas) (as opposed to forced evaporation of gas or OGPI (FBOG), abbreviation to express forced boil-off gas (Forced Boil-Off Gas) Other parameters, such as the movement of gases in tanks due to sea conditions during sailing and environmental conditions, also affect the evaporation of gases. parts of containers in the headspace above the liquefied gases increase the pressure in the container.The increase in pressure can damage the containers.

The liquefied natural gas vapor is used to power the aforementioned power generation plant. In the case of natural evaporation, when the amount of gas naturally evaporated is not sufficient to meet the fuel gas demand of the plant, means such as a pump immersed in a vessel are activated to supply more fuel gas after forced evaporation. Forced evaporation is carried out, in particular, by hot water, which is heated by oil or a gas burner. During this operation, all the cold of the liquefied natural gas is lost. When the amount of vaporized gas is too large compared to the needs of the plant, the excess gas is usually flared in the gas flaring unit, resulting in loss of cargo.

In existing technology, improvements in LNG storage tanks are such that natural evaporation rates (BOR is an abbreviation for Boil-Off Rate) of liquefied gases become progressively lower. In addition, devices on board are becoming more efficient. This leads to the fact that in each of the first and second cases described above, the difference between the amount of gas generated by natural evaporation and the amount of gas required to install the vessel is very large.

With respect to liquefied hydrocarbon gases, natural evaporation of the gases is inevitable and occurs, for example, during operations of loading them into storage tanks, ship voyage or cooling of tanks as a result of heat exchange between the tanks and the external environment. Evaporation of the gases is controlled by one or more reliquefaction systems which allow the natural evaporation of the liquefied gas to be limited while maintaining it in a thermodynamic state, allowing it to be stored for a long time and at the same time controlling the pressure in the storage tank. This is because ships carrying LPG are currently unable to burn LPG vapor. Reliquefaction systems extract gas vapors from tanks, reliquefy them, and return them to a storage tank. The reliquefaction system or systems can represent a capital cost in the order of 5%-10% of the vessel's value.

The present invention offers a simple, efficient and cost-effective solution to control the natural or forced evaporation of gases in tanks or tanks, as well as the energy requirements of a storage facility, in particular on a ship, regardless of the operating conditions on the voyage, the cooling of tanks or tanks and loading liquefied gases into tanks.

3. SUMMARY OF THE INVENTION

In accordance with a first aspect, the invention provides a method for treating the gas of a gas storage plant, the plant 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 lower boiling point than the first gas, the method includes a reliquefaction step in which the vapors of the first gas flowing in the first circuit from the reservoir are reliquefied by heat exchange with the second gas in liquid state, having an inlet temperature and flowing in the second circuit, wherein the reliquefied vapors of the first gas are supplied to the reservoir, and the second gas is maintained in a liquid state at the outlet temperature after reliquefaction and returned to the container, wherein the heat exchange between the first gas and the second gas is carried out so that the outlet temperature of the reliquefied vapors of the first gas is between the first threshold value and the second threshold value.

Thus, the invention makes it possible to control the vapors of the first gas by using the cold of the second gas, which is intended to be supplied to the gas storage plant, which allows an efficient, cost-effective system while reducing NOx and SOx emissions. In particular, the reliquefaction of the vapors of the first gas by means of the second gas in the liquid state to be returned to the container allows all gas vapors generated in the storage tank of the first gas to be reliquefied at the proper temperature. The reliquefaction of the vapors of the first gas is independent of the plant's consumption. The second gas is heated after the heat exchange but remains liquid so that it can be returned to the vessel.

The method may contain one or more of the following characteristics or steps, taken alone or in combination with each other:

- the temperature difference between the temperature at the inlet of the second gas before the reliquefaction step and the temperature at the outlet of the second gas after the reliquefaction step is from 20°C to 30°C,

- the temperature at the outlet of the second gas is less than the evaporation temperature of the second gas at a pressure less than or equal to the maximum allowable storage pressure in the vessel,

the re-liquefied vapors of the first gas are introduced into the reservoir at a temperature greater than or equal to the minimum temperature that the reservoir must withstand,

- the outlet pressure of the second gas after reliquefaction of the first gas is 8 bar,

- the temperature at the outlet of the second gas is from -155°C to -105°C at a pressure of 2 to 20 bar,

- the first temperature threshold value at the exit of the first gas is essentially close to the first gas liquefaction temperature at atmospheric pressure, and the second temperature threshold value is less than the first threshold value by 10°C-40°C at atmospheric pressure,

the first threshold is in the order of -40°C and the second threshold is in the order of -50°C,

- vapors of the first gas are compressed before heat exchange,

- the second gas is extracted from the bottom of the container,

- the heat exchange in the reliquefaction step is carried out during the loading operation of the first gas or during the tank cooling operation,

- 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 facility, the system comprising:

- the reservoir in which the first gas is stored,

- a container in which the second gas is stored, the second gas having a lower boiling point than the first gas,

- the first circuit, in which flows at least part of the vapors of the first gas from the tank,

- a second circuit in which at least a part of the second gas flows in liquid state at the temperature at the inlet of the vessel, and

- a heat exchanger configured to re-liquefy at least a portion of the vapors of the first gas by heat exchange with the second gas in a liquid state, wherein the re-liquefied vapors of the first gas are supplied to the reservoir, and the second gas is maintained in a liquid state at the outlet temperature after re-liquefaction and returned into the container so that the temperature at the outlet of the vapors of the first gas is between the first threshold value and the second threshold value.

The device according to the invention may comprise one or more of the following characteristics, taken alone or in combination with each other:

- the heat exchanger is designed so that the temperature difference between the temperature at the inlet of the second gas before the reliquefaction step and the outlet temperature after the reliquefaction step is from 5°C to 55°C,

- the system contains a compressor installed before the first circuit for compressing the vapors of the first gas, which must be removed from the tank before heat exchange,

- the second circuit forms with pipes, each of which is connected to the tank and 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 vessel for the transport of liquefied gas, containing at least one system having any of the above characteristics.

According to a second aspect, the invention provides a method for treating gas from a gas storage facility, in particular on board a ship, the method comprising the steps of:

- extracting the first gas in the liquid state from the first tank or from the first tank,

- carrying out the first supercooling of the extracted first gas in the liquid state, and

storing the supercooled first gas in liquid state at the bottom of the first tank or first tank or second tank or second tank to form a cold reserve layer of the first gas in liquid state in a supercooled liquid state at the bottom of the first or second tank or first or second tank.

Thus, the subcooled first gas, which is stored at the bottom of the tank or vessel, allows a cooling capacity to be generated that can be used later, with the cold reserve stored at the bottom of the tank or vessel for a long time. The cold storage can be used, for example, to re-liquefy the vapors of the first gas in the tank and/or to reduce the pressure in the tank, if necessary. The cold reserve can also be used without the need to supply the plant or operate the heat exchangers.

The method may contain one or more of the following characteristics or steps, taken alone or in combination with each other:

- the first gas is subcooled to a temperature greater than or equal to the minimum temperature that the reservoir or container must withstand,

- a cold reserve layer is located in the first or second reservoir or the first or second reservoir under the volume of the first gas, forming the interface between two liquids,

- the supercooled first gas in a liquid state is fed into the first or second reservoir or the first or second container through a pipeline that exits at the bottom of the first or second reservoir or the first or second container,

- the first gas stored in a cold reserve layer in the first or second tank or the first or second tank is used to cool the gas in the vapor state,

- gas in vapor state is the first gas in the vapor state, located at the top of the tank or container and above the first gas in the liquid state,

- the first gas stored in the cold reserve layer is sprayed into the first or second reservoirs or the first or second containers and into the layer of the first gas in the vapor state,

- the first gas stored in the cold reserve layer is removed from the bottom of one of the tanks or containers and the first gas is re-liquefied in the vapor state by means of a heat exchanger,

- the supercooled first gas in a liquid state is stored in a cold reserve layer when the measured pressure in the tank or container is less than the first predetermined threshold pressure in the tank or container,

- the first preset threshold value is, for example, 1 to 1.05 bar absolute pressure,

- said bottom extends for less than approximately 30% of the height of the tank or tank measured from the bottom, said bottom being the lowest end of the tank or tank,

the supercooled first gas in liquid state is stored in a cold reserve bed at a temperature between the first gas liquefaction temperature minus about 5°C at atmospheric pressure and the liquefaction temperature minus about 10°C, with the first gas in liquid state remaining in the first or second reservoir, or the first or second container has a temperature higher than the liquefaction temperature of the first gas,

- the supercooled first gas in the liquid state is stored in a cold reserve layer at a temperature of from -45°C to -55°C, and the first gas in the liquid state remaining in the first or second tank or the first or second tank has a temperature greater than or equal to -42°C

- the supercooled first gas is stored in a cold reserve layer at a temperature from -160°C to -170°C, and the first gas in the liquid state remaining in the tank or vessel has a temperature greater than or equal to -160°C,

- the first supercooling of the first gas is carried out by means of a second gas, at least in a liquid state, extracted from the vessel, the second gas having a boiling point less than or equal to the boiling point of the first gas,

- the method comprises the step of vaporizing or heating the second gas, which is heated or vaporized by heat exchange during the first subcooling of the first gas for supply to the plant,

- the plant controls the flow of the second gas, which must be evaporated or heated during evaporation,

- the first supercooling of the first gas is carried out by means of the first gas extracted from the container, which is expanded and partially evaporated,

- the second gas extracted from the container is expanded and partially evaporated before heat exchange during the first subcooling,

- the second gas extracted from the container is supercooled by heat exchange with the expanded and partially vaporized second gas,

- the second subcooling of the first gas is carried out after the first subcooling,

- the second gas used for the second subcooling is removed from the bottom of the vessel or subcooled,

- the first and/or second supercooling is carried out outside the first and second tanks and/or the first and second tanks,

- heat exchange between the first gas and the second gas during the first subcooling or the second subcooling is carried out so that the subcooling temperature of the first gas is between the first threshold value and the second threshold value,

- the temperature at the outlet of the second gas after the second subcooling is from -155°C to -105°C at a pressure of 2 to 20 bar,

- the heated, vaporized or partially vaporized second gas is heated to be fed into the plant,

- the method further comprises a reliquefaction step in which the vapors of the first gas flowing in the first circuit from the reservoir are reliquefied by heat exchange with a second gas in liquid state having an inlet temperature and flowing in the second circuit, wherein the reliquefied vapors of the first gas are fed into the reservoir, and the second gas is maintained in a liquid state at the outlet temperature after reliquefaction and returned to the vessel, and the heat exchange between the first gas and the second gas is carried out so that the temperature at the outlet of the reliquefied vapors of the first gas is between the first threshold value and the second threshold value ,

the vapors of the first gas are re-liquefied when the pressure measured in the reservoir or vessel exceeds a second predetermined pressure threshold in the reservoir or vessel,

- the second threshold value is, for example, from 1 to 1.05 bar absolute pressure,

- the heated second gas is compressed for supply to the plant,

- the first gas is liquefied natural gas or liquefied petroleum gas,

- the second gas is liquefied natural gas.

The present invention also relates to a gas treatment system for a gas storage facility, in particular on board a ship, the system comprising:

- a reservoir or container in which the first gas is stored in a liquid state;

- the first heat exchanger, configured to perform the first subcooling of the first gas in the liquid state, extracted from the tank or vessel through the first pipeline, and

- a second pipeline connected to the first heat exchanger, which exits at the bottom of the tank or tank or another tank or tank for storing a supercooled first gas at the bottom of the tank or tank to form a cold reserve layer of the first gas in a liquid state.

The device according to the invention may comprise one or more of the following characteristics, taken alone or in combination with each other:

- the first gas is stored in the same reservoir or vessel from which it was extracted,

- the device contains a container in which the second gas is stored in a liquid state, and the second gas has a boiling point less than or equal to the boiling point of the first gas,

- the second gas in the liquid state flows through the second pipeline connected to the first heat exchanger to carry out the first supercooling of the first gas,

- the device contains a second heat exchanger configured to perform a second supercooling of the first gas by means of the second gas in the liquid state,

- the bottom of the tank or tank contains an outlet connected to the first end of the pipeline, and the pipeline contains a second end connected to a spray bar installed in the upper part of the tank or tank,

- a heating device through which the second gas passes, heated, vaporized or partially vaporized in the first heat exchanger,

- pressure relief means are installed in front of the first heat exchanger,

- the second heat exchanger is configured to supply the second gas at an outlet temperature of -155°C to -105°C at a pressure of 2 to 20 bar,

- the device comprises a third heat exchanger configured to re-liquefy at least a portion of the first gas vapor by heat exchange with the second gas in the liquid state, wherein the re-liquefied vapors of the first gas are fed into the tank, and the second gas is maintained in the liquid state at the outlet temperature after re-liquidation liquefaction and returned to the vessel, so that the temperature at the outlet of the vapors of the first gas is between the first threshold value and the second threshold value,

- the device comprises a fourth heat exchanger configured to partially vaporize the second gas flowing in the primary circuit and subcool the second gas flowing in the secondary circuit,

- the primary circuit is located after the pressure relief means and before the first heat exchanger (in the direction of the fluid flow in the heat exchanger),

- the secondary circuit is located in front of the second heat exchanger (in the direction of the fluid flow in the heat exchanger),

- the compressor is designed to compress the heated or vaporized second gas,

- the first gas is liquefied natural gas or liquefied petroleum gas,

- the second gas is liquefied natural gas.

The invention also relates to a vessel for the transport of liquefied gas, including at least one system having any of the above characteristics.

4. LIST OF DRAWINGS

The present invention will become better understood and other details, characteristics and advantages of the present invention will become more apparent upon reading the following description, given by way of non-limiting example and with reference to the accompanying drawings, in which:

- Figure 1 illustrates an embodiment of a gas treatment system according to the invention, which in this case is equipped with a gas storage plant, in particular on a ship,

- Figure 2 illustrates another embodiment of the gas treatment system according to the invention,

- Figure 3 illustrates another embodiment of the gas treatment system according to the invention,

- Figure 4 illustrates another embodiment of the gas treatment system according to the invention,

- Figure 5 illustrates an alternative form of the embodiment shown in Figure 4, and

- Figure 6 illustrates another embodiment of a gas treatment system according to the invention.

5. DETAILED DESCRIPTION OF THE INVENTION

Figure 1 illustrates a first embodiment of a gas treatment system 1 of a gas storage plant 2 according to the invention. The treatment system allows one or more gases to be cooled and/or the vapors of one or more gases to be reliquefied and/or one or more gases to be evaporated or heated.

In the present invention, the expression "reliquefaction" should be understood as the condensation of gas vapor, allowing it to return to a liquid state.

In the present invention, the system 1 is installed on a vessel, for example a gas transport vessel, in particular a VLGC (Very Large Gas Carrier) type vessel. Vessels of this type have a capacity of about 80,000 m ³ .

On a gas transport vessel, such as an LNG tanker, a power generation facility is provided to meet the energy requirements associated with the operation of the vessel, in particular propulsion of the vessel and/or generation of electricity for pieces of equipment on board.

The gas storage plant 2 may be a power generation plant. Such an installation mainly comprises heat engines 3, for example a ship's engine, which consumes the gas carried in the ship's tanks/tanks.

On a ship, the gas(s) is stored in a liquid state in several tanks 4 or vessels 5 at very low temperatures, in fact at cryogenic temperatures. Each reservoir 4 and container 5 may contain a gas in liquefied form or in a liquid state at a given pressure and a given temperature. One or more vessels 4 and/or vessels 5 can be connected to the plant 2 via the system 1 according to the invention. To this end, each tank and container contains a shell designed to isolate gases stored at storage temperature from the external environment.

The vessel is loaded with natural gas (NG) stored in tank 5 and hydrocarbon gases (HC) stored in one or more tanks 4. Each tank and/or tank 4, 5 may have a capacity of 1,000 to 50,000 m ³ . The number of tanks 4 and containers 5 is not limited. For example, it is from 1 to 6. In the rest of the description, the expressions "receptacle" and "reservoir" should be interpreted as "one or each container" and "one or each reservoir" respectively.

Natural gas (NG) is, for example, methane or a gas mixture containing methane. The natural gas is stored in a liquid state 5a in a container, for example, at a cryogenic temperature of the order of -160° C. and atmospheric pressure. Natural gas in liquid state or liquefied natural gas 5a is abbreviated "LNG". The container 5 also contains gas vapors 5b resulting from the evaporation of, in particular natural, LNG in the container. Evaporation or vapors 5b are abbreviated "OG" or "OGEI" in the case of natural evaporation, as opposed to "OGPI" in the case of forced evaporation. The LNG 5a is naturally stored at the bottom of the vessel 5, while the LNG exhaust gas 5b is above the N1 level of the LNG 5a in the vessel, in a space known as headspace. The LNG exhaust gas 5b in the tank is due, for example, to heat input from the environment into the tank 5 and movements of the LNG 5a in the tank 5 due to sea fluctuations.

Hydrocarbon gas (HC) contains propane, butane, propylene, ammonia, ethane, ethylene or a gas mixture containing these components. The hydrocarbon gas is stored in the liquid state 4a in the reservoir 4 at a temperature in the order of -42° C. and atmospheric pressure. Hydrocarbon gas in liquid state 4a or liquefied hydrocarbon gas is abbreviated as "LPG". The tank 4 also contains gas vapors 4b resulting from the evaporation of, in particular, the natural LPG in the tank. Similarly, the LPG 4a is naturally stored at the bottom of the tank 4, while the LPG vapor is above the N2 level of the LPG 4a in the tank in the headspace above the gas. As explained above with respect to LNG, the evaporation of LPG (OG or OGEI) in tank 4 is also due to heat from the external environment entering the tank, fluid movements during voyages (sea, LPG), during loading of LPG into tank 4 and during cooling tank to bring the temperature of the tank to the equilibrium temperature.

During the cooling in this case of the tank 4, which consists in bringing the external temperature of the tank shell to the equilibrium temperature, the liquefied gas is sprayed onto the walls of the practically empty tank. Evaporation of gas leads to the formation of cold, which is necessary for cooling the shell. During this operation, which lasts about 10 hours, there is very little LPG vapor generated due to natural evaporation (OGEI), since the tank is almost empty. On the other hand, spraying LPG onto the walls to cool them results in the formation of a large amount of LPG vapor, on the order of 10,900 kg/h. The LPG storage tank cooling operation can be used to cool LPG storage tanks.

During LPG loading, the tank contains a significant amount of EG, which is formed as a result of cooling of the tank, as well as from OGEI, formed from LPG, which is heated in the tank. Vapors formed as a result of cooling are not re-liquefied when LPG is loaded into the tank. The download operation takes approximately 18 hours. Approximately 13900 kg/h of exhaust gas is generated in the tank. The pressure in the tank during tank loading is maintained above atmospheric pressure.

In the embodiment shown in Figure 1, the system 1 shown comprises four LPG storage tanks 4 and one LNG storage tank 5. System 1 also includes a heat exchanger 6 which allows heat exchange between LNG vapor 5b, LPG vapor 4b, LPG liquid 4a and LNG liquid 5a. In the present example, the heat exchanger 6 contains several circuits or pipelines, in this case at least one first circuit 6a, one second circuit 6b, one first pipe 6c and one second pipe 6d, in which SG or CO in liquid or vapor state can flow.

The heat exchanger 6 is designed so that the first circuit 6a performs heat exchange with the second circuit 6b to maintain the LPG coming from the tank in a liquid state and at the same time re-liquefy the LPG vapors 4b coming from the tank 4. LNG at the outlet of the heat exchanger 6, in particular, the second circuit 6b is sent to tank 5, and the re-liquefied LPG vapor is sent to tank 4.

To do this, the tank 4 contains an outlet connected to the first end of the first pipeline 7, through which the LPG vapor 4b flows. The outlet of the tank 4 is located in the upper part of the tank 4, where there is a free space above the gas with vapor 4b LPG (OGEI). The first pipeline 7 is connected to the inlet of the compressor 8, which carries out the flow of LPG vapor 4b through the first pipeline 7. The latter has a second end connected to the inlet of the first circuit 6a. It is assumed that the LPG vapor is re-liquefied by heat exchange with cold from the LNG and maintains the LNG in a liquid state. The outlet of the first circuit 6a is connected to the first end of the second pipeline 9, through which the re-liquefied LPG vapor flows. The second conduit 9 has a second end which is immersed in the LPG or connected to a dip pipe 9a immersed in the tank. Alternatively, the second pipeline 9 is connected to the LPG spray boom 10. The rod 10 is located in the tank 4 in its upper part along the vertical axis in the plane shown in Figure 1, for spraying re-liquefied LPG vapor in the free space above the LPG. This causes the OGEI to recondense in the tank.

System 1 contains pumps installed in tank 5 for extracting LNG from it. In particular, the first pump 11a and the second pump 11b are immersed in the LNG and are preferably located at the bottom of the vessel 5 so that only LNG is supplied to them. The first pump 11a is connected to the first end of the third conduit 12. The first pump 11a allows the LNG to circulate in the third conduit 12. The LNG volume flow of the first pump 11a is in the order of 130 m ³ /h. The second end of the third pipeline 12 is connected to the inlet of the second circuit 6b, in which LNG 5a flows from the tank 5. The second circuit 6b contains an outlet connected to the first end of the fourth pipeline 13, through which LNG 5a also flows. The fourth conduit 13 has a second end connected to the vessel 5. The third and fourth conduits 12, 13 allow LNG to be recirculated from the vessel to the vessel through the heat exchanger 6. More precisely, the second circuit 6b and the third and fourth conduits 12, 13 form a closed circuit. LNG is removed from the tank at -160°C. The outlet temperature of the LNG and/or the outlet pressure of the LNG is controlled so that the LNG does not vaporize during heat exchange with the LPG vapor. To this end, a temperature sensor is provided, for example in the fourth pipeline 13, to monitor the temperature of the LNG returned to the vessel. Preferably, the target LNG outlet temperature is lower, for example, 5° C., than the vaporization temperature of the LNG at an acceptable storage pressure in the vessel, for example, of the order of 8 bar. The storage pressure in the LNG storage tank 5 is 2 to 20 bar. The LNG pressure at the outlet of the heat exchanger 6 must be lower than the maximum storage pressure in the tank. Thus, LNG is heated without evaporation. The temperature of the re-liquefied LPG vapor at the outlet is between the first threshold value and the second threshold value. The first LPG outlet temperature threshold is substantially close to the liquefaction temperature at atmospheric pressure, and the second temperature threshold is less than the first threshold by 10°C-40°C at atmospheric pressure. In the present example, the first threshold is -40°C, while the second threshold is in the order of -55°C. Preferably, the outlet temperature of the reliquefied gas vapor is in the order of -42°C. The heat exchange allows the LPG vapor to be reliquefied at an appropriate temperature that is not too low, in particular greater than or equal to the minimum temperature that the tank 4 must withstand. The above LPG temperatures in this example and in the following description are examples of temperatures related to propane. It should be understood that the temperatures of other LPG compounds are applicable to the invention.

The heat exchanger 6 is also designed so that the first pipe 6c performs heat exchange with the second pipe 6d for the simultaneous forced evaporation of the LNG coming from the tank and the subcooling of the LPG coming from the tank 4. In the present invention, the term "subcooling" should be understood as lowering the temperature of the liquefied gas below liquefaction temperature. The liquefied gas, for example, is subcooled to about 5° C.-20° C. below the liquefaction temperature. It should be understood that the storage of the supercooled liquefied gas in the present invention depends on the storage pressure of the liquefied gas. The vaporized LNG (OGPI) is to be fed into plant 2 and, in particular, in this case, into the ship's engine. The supercooled LPG (in liquid state) is fed into the tank 4. In particular, the first pipe 6c is configured to carry out the flow of hydrocarbon gas and, in particular, the LPG 4b in the heat exchanger 6. The first pipe 6c contains an inlet connected to one end of the fifth pipeline 14 , through which the LPG extracted from the tank flows. The other end of the fifth pipeline 14 is connected to the third pump 15, immersed in LPG. A third pump 15 is also installed at the bottom of the tank 4 to extract only the LPG and make the LPG flow through the pipeline 14. The first pipe 6c contains an outlet connected to the sixth pipeline 16, which is designed to return the supercooled LPG (in liquid state) to the tank 4. The sixth pipeline 16 can be connected to a spray boom 10, a second pipeline 9 or a dip pipe 9a to return the LPG to the tank. Preferably, the supercooled LPG is stored at the bottom of the tank 4 in a cold reserve layer 4c in the interior of the tank and at the bottom of the tank. Layer 4c can subsequently be used. Preferably, but without limitation, the second end of the conduit 9 or dip tube is located at the bottom of the tank 4 along the vertical axis in the plane shown in Figure 1 for storing supercooled LPG. Subcooling takes place outside the tank or any other tank or container. For example, subcooling does not occur in liquefied gas. 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 located under the LPG in the tank along the vertical axis relative to Figure 1, forming the interface between the two liquids. In other words, there is no baffle, auxiliary tank or compartment in the tank that separates the LPG remaining/already in the tank and the supercooled LPG stored in the reserve layer.

The second pipe 6d allows the evaporation of the LNG 5a coming from the tank 5. To this end, the second pump 11b, immersed in the LNG, is connected to the first end of the seventh pipeline 17, through which the LNG flows to the installation 2, in this case the ship's engine. The second pump 11b allows the LNG to flow in the seventh pipeline 17 with a volume flow lower than the first pump 11a. In the present example, the LNG volume flow in the seventh pipeline 17 is in the order of 4 m ³ /h. The second end of the seventh pipeline 17 is connected to the inlet of the second pipe 6d. The latter has an outlet connected to an eighth conduit 18, through which LNG vapors 5a flow, formed as a result of heat exchange with LPG, to be fed, for example, to the ship's engine. During heat exchange with evaporation and subcooling, the temperature of the LNG rises. That is, its temperature becomes higher than the liquefaction temperature at atmospheric pressure. The LNG temperature is controlled by a heating device, not shown, according to the engine specifications. The outlet pressure of the LNG, for example required for a ship's engine, is in the order of 17 bar. As for the LPG, its inlet temperature to circuit 6c is approximately 1 bar. The temperature of the supercooled LPG at the outlet is greater than or equal to the minimum temperature value that the tank or vessel must withstand. In this case, the outlet temperature is in the order of -52°C (at storage pressure in the tank).

In Figure 1, LPG vapor is removed from a tank and the re-liquefied LPG vapor is sent to another adjacent tank. Similarly, LPG taken from a tank and recooled is returned to the same tank. Of course, other configurations are possible.

In Figure 1, the heat exchanger 6 is separated from the tanks or vessel. The heat exchanger 6 is located outside the tanks and containers. The heat exchanger is not located in another tank or other container in which liquefied gas is stored.

Preferably the heat exchanger is a tube, plate or coil heat exchanger.

In the embodiment illustrated in Figure 2, system 1 comprises a plurality of heat exchangers that allow heat exchange between LNG vapor, LPG vapor, LNG and/or LPG. This system differs in particular from the first embodiment in the number of heat exchangers. In particular, in the present example, the system comprises at least two heat exchangers, hereinafter referred to as the evaporative heat exchanger 20 and the main heat exchanger 21. Figure 2 shows one tank 5 and one tank 4. Of course, the system may contain other tanks and tanks. System 1 also includes pumps 11a, 11b and 15 installed in tank 5 and tank 4. In particular, the first pump and the second pump are submerged in LNG and are preferably located at the bottom of the tank so that only LNG is supplied to them. The flow rate of the first pump is also about 130 m ³ /h and the flow rate of the second pump is about 4 m ³ /h.

The main heat exchanger 21 is configured to re-liquefy the LPG vapor 4b by exchanging heat with cold from the LNG 5a while maintaining the LNG in a liquid state. The LNG is returned to tank 5 without evaporation, and the re-liquefied LPG vapor is returned to tank 4. The main heat exchanger 21 includes a first circuit 6a and a second circuit 6b. The first circuit 6a is connected, on the one hand, to the first pipeline 7 connected to the reservoir 4, and, on the other hand, to the second pipeline 9, also connected to the reservoir 4. The first pipeline 7 is also provided with a first compressor 8 for carrying out the flow of vapors 4b LPG through the pipeline to the heat exchanger 21.

The heat exchanger 20 is configured to vaporize the LNG coming from the tank and at the same time supercool the LNG coming from the tank 4. The LNG must be subjected to forced evaporation to raise the temperature of the LNG to the temperature required, for example, for the engine of the ship fed with LNG vapor. The heat exchanger 20 includes a first pipe 6c and a 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 supplies LNG to the ship's engine. The first pipe 6c is connected, on the one hand, to the first pipeline 14 connected to the tank 4, and, on the other hand, to the sixth pipeline 16 connected to the tank 4 and, in particular, to the bottom of the tank 4.

In Figure 2, system 1 also includes a third heat exchanger, referred to as auxiliary heat exchanger 22. The latter allows for a second subcooling of the LPG via cold from the LNG and allows the LNG to be kept in a liquid state. Liquid LNG is returned to the tank, while supercooled LPG is returned to the tank.

Preferably, but not limited to, the heat exchangers 20, 21, 22 are separate from the tanks and containers.

Preferably, but not limited to, heat exchangers 20, 21, 22 are tubular, plate or coil heat 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 subcooled LPG, flows. The third circuit 6e contains an inlet connected to a ninth pipeline 23, which is connected to the tank 5. As seen in Figure 2, the ninth pipeline 23 is a bypass section of the seventh pipeline 17, which extracts LNG from the bottom of the tank 5 by means of a pump 11b. The third circuit 6e includes an outlet connected to a tenth conduit 24 which returns the LNG maintained in a liquid state to the container 5. In this exemplary embodiment, the tenth conduit 24 is connected to a section of the fourth conduit 13 returning the LNG to the container 5, for example, through a valve , for example, a three-way valve. The fourth circuit 6f contains an inlet connected to the eleventh pipeline 25, through which the LPG extracted from the bottom of the tank flows. The eleventh pipeline in this case is connected to the pipeline 16, through which the supercooled LPG flows, through the valve 29, for example, a three-way valve. The fourth circuit 6f contains an outlet connected to the twelfth pipeline 26, which is connected to the reservoir. According to this exemplary embodiment, the twelfth conduit 26 is connected to the tenth conduit section or conduit 9. The LPG supercooled by heat exchange with the LNG is sprayed into the headspace or stored at the bottom of the tank 4 in the cold reserve layer 4c. Twelfth conduit 26 may be connected to conduit 16 via valve 27. Similarly, conduit 26 may be connected to conduit 9 via valve 28. Preferably, but not limited to, valves 27, 28 are three-way valves. Pipeline 16 is connected to LPG spray boom 10 to spray LPG droplets in the headspace above the gas in tank 4 and re-condense the OGEI in tank 4. The third pump 15 is configured to flow LPG through pipelines 14, 16, 25 from the bottom of the tank to spray boom 10. With this configuration, the subcooled LPG is fed directly into the tank or boom 10 or fed into the auxiliary heat exchanger 22 for a second subcooling by LNG.

In Figure 2, the system further comprises a pipe 30 for extracting LNG vapor 5b into the tank 5 to control the pressure in the tank 5 and to supply fuel gas to the plant 2. A second compressor 31 is installed in the pipe 30 to cause the LNG vapor 5a to flow to the engine and maintain pressure in the containers. Pipe 30 is connected to a section of pipeline 18 through which heated or vaporized LNG flows to the ship's engine.

Preferably, but not limited to, a heating device 32 is located prior to the plant to adjust the temperature of the LNG to the desired temperature and ensure that all LNG is vaporized. The heating device 32 in this case is a heater.

In the third embodiment of the invention, illustrated in Figure 3, the system 1 also contains several heat exchangers. In particular, system 1 contains:

- the main heat exchanger 21, made with the possibility of re-liquefying the LPG vapor 4b by heat exchange with cold from the LNG 5a and maintaining the LNG in a liquid state,

- an evaporative heat exchanger 20 configured to vaporize the LNG coming from the tank 5 and supercool the LNG coming from the tank 4, and

- auxiliary heat exchanger 22', configured to supercool the LPG and maintain the LNG in a liquid state.

The system 1 according to this embodiment differs from the embodiment illustrated in Figure 2 in that it comprises a fourth heat exchanger 40 located before heat exchanger 20. Heat exchanger 40 is preferably, but not limited to, a vacuum evaporator (VE) for cold production. The vacuum evaporator 40 includes a primary circuit 42 which contains an inlet and an outlet. The inlet is connected to the seventh pipeline 17, through which the LNG flows from the tank. The outlet of the primary circuit 42 is connected to the first end of the conduit 44. The latter has a second end connected to the inlet of the circuit 6d of the heat exchanger 20. On the conduit 17 before the vacuum evaporator 40 means 41 pressure relief is provided. The pressure relief means 41 make it possible to obtain gas in a two-phase liquid/vapor state by lowering the pressure and temperature of the gas. The pressure relief means 41 in this case comprise an expansion valve, such as a Joule-Thomson valve. The LNG entering the pressure relief means 41 has a temperature of about -134° C. and a pressure of about 8 bar. At the outlet of the expansion valve, the LNG is cooled to a temperature of approximately -160°C at a pressure of around 1 bar. The two-phase LNG enters the vacuum evaporator 40, where heat is exchanged with the LNG removed from the vessel. In particular, the vacuum evaporator 40 includes a secondary circuit 43 which contains an inlet and an outlet. The inlet of the secondary circuit 43 is connected to a bypass pipeline 45, through which LNG flows from the tank 5. The bypass pipeline 45 comes from the seventh pipeline 17 connected to the pump 11b. Of course, conduit 45 may be connected to another pump submerged at the bottom of the vessel. The outlet of the secondary circuit is connected to a pipeline 23 returning the LNG to the bottom of the vessel 5. In this embodiment, the pipeline 23 is connected to the inlet of the circuit 6e of the heat exchanger 22'. In the vacuum vaporizer 40, the LNG flowing in the secondary circuit 43 is supercooled by recovering the latent heat of the two-phase LNG flowing in the circuit 42. The supercooled LNG (in liquid state) is fed into the vessel. The two-phase LNG flowing in the primary loop 42 is heated or vaporized and then fed to the evaporative heat exchanger 20. The temperature of the LNG at the outlet of the primary loop 42 is from -160°C to -134°C at a pressure of about 1 bar. The outlet temperature of the supercooled LNG is in the order of -160°C at a pressure of 2 to 20 bar. As the supercooled LNG flows through the heat exchanger 22', the latter is configured to keep the LNG coming from the vacuum evaporator 40 in a liquid state. This is because the LNG coming from loop 43 can exchange heat with the subcooled LPG coming from heat exchanger 20 in accordance with the mode of operation of the system described below. In this case, the LNG passing through loop 6e is heated but not vaporized.

In Figure 3, system 1 further includes a compressor 46 installed downstream of heater 32. Compressor 46 allows the vaporized LNG to be compressed to the pressure required by unit 2.

In this exemplary embodiment, subcooling takes place outside the tanks and vessel. In other words, the heat exchangers are separated from the tanks and the vessel.

In the first mode of operation (COOLING) of the gas treatment system 1 of the power generation plant 2, as illustrated in Figure 2, LNG is used to re-liquefy the LPG vapor 4b. LNG is also used to feed plant 2, in particular the ship's engine and other heat engines for power generation needs. The first mode of operation is applied during the cooling of the LPG storage tank. This is due to the fact that, as explained above, during this operation a very large amount of 4b LPG vapors (approximately 10900 kg/h) is generated. This amount of vapors 4b generated is greater than the amount of vapors 4b (OGEI) generated during the voyage of the LPG transport vessel. In the context of tank wall cooling, the fuel gas requirements of the engine are very low. The consumption of LNG vapors by plant 2 is about 500 kg/h. The system uses the main heat exchanger 21 to control the LPG vapors 4b generated during cooling. Vapors 4b of LPG are extracted from tank 4 by compressor 8, which feeds them into first pipeline 7. Vapors 4b of LPG flowing in primary circuit 6a are re-liquefied due to cold from LNG flowing in secondary circuit 6b through third pipeline 12 from the bottom of tank 5. It should be understood that the LNG at the bottom of the vessel is colder than the LNG near the N1 surface, i.e. at the interface between LNG and headspace. After re-liquefaction, the re-liquefied LPG vapors are fed into tank 4, and the LNG is maintained in a liquid state and then returned to tank 5. LPG vapors 4b enter the main heat exchanger 21 at a temperature of the order of 0°C and a pressure close to atmospheric pressure. The main heat exchanger 21 is designed so that the outlet temperature of the re-liquefied LPG vapor is between the first threshold value and the second threshold value. The first and second thresholds are considered at a pressure equal to or greater than atmospheric pressure. The temperature thresholds are greater than or equal to the minimum temperature value that the tank 4 must withstand. LPG vapor is about -50°C at a pressure equal to or greater than atmospheric pressure. Preferably, but not limited to, the outlet temperature of the reliquefied LPG vapor is -42° C. at a pressure equal to or greater than atmospheric pressure. In this way, the heat exchange is controlled so that the re-liquefied LPG vapor is not too cold.

Similarly, the heat exchange is carried out so that the temperature of the LNG outlet after reliquefaction is between the first temperature threshold and the second temperature threshold at a pressure of 6 to 20 bar. As seen in the first embodiment with reference to Figure 1, the LNG must be heated but not vaporised. The main heat exchanger 21 is configured such that the temperature difference between the LNG inlet temperature before reliquefaction and the LNG outlet temperature after reliquefaction is between 5°C and 55°C. Preferably, but not limited to, the temperature difference is 26°C. In this case, the LNG enters the main heat exchanger 21 before reliquefaction at an inlet temperature of the order of -160°C and a pressure of 2 to 20 bar. The first threshold is on the order of -155°C and the second threshold is on the order of -105°C. Preferably, but not limited to, the LNG outlet temperature is less than its vaporization temperature at a pressure less than the maximum allowable storage pressure in the vessel. The temperature is around -134°C. These values allow maximum cooling from the LNG to be transferred to the LPG vapor for reliquefaction, while preventing excessive heating of the LNG returned to the vessel and excessive cooling of the reliquefied LPG vapor. Excessively hot LNG can cause the LNG pressure in the tank to rise and exceed the allowable limits. Thus, the main heat exchanger 21 is adjusted so that the LNG and the re-liquefied LPG vapor respectively exit at the temperature required for the container or tank. During heat exchange, the LNG flow rate and the LPG vapor flow rate are respectively constant.

Since the LNG and LPG inlet and outlet temperatures are known and/or set, parameters such as LNG and LPG mass flow allow the heat exchanger 21 to be tuned for proper heat exchange.

The system can be operated such that re-liquefaction of the LPG vapor occurs when the pressure measured in the tank exceeds the set pressure in the tank.

In the first mode of operation, the system 1 also uses an evaporative heat exchanger 20 through which the LPG coming from the tank 4 and the LNG coming from the tank 5 flow to the plant 2. to be fed into unit 2. Subcooled LPG (in liquid state) is stored at the bottom of the tank, forming the next cold reserve layer 4c. This makes it possible to obtain a large available cooling capacity and, consequently, to improve the cooling efficiency of the gas in liquefied and/or gaseous form contained in the reservoir. In the present invention, the bottom of the tank 4 extends for less than about 30% of the height of the tank 4 as measured from the bottom 19. The bottom 19 is the lowest end of the tank, such as closer to the hull of a vessel when the tank is transported on an LNG tanker. In particular, the LPG drawn from the bottom of the tank by the pump passes through the heat exchanger 20 where its inlet temperature becomes approximately -42°C. The inlet temperature of the LNG withdrawn from the tank is approximately -160°C at a pressure of approximately 17 bar. After heat exchange, in which the LPG recovers the latent heat of the LNG that evaporates, the outlet temperature of the LPG is between -45°C and -55°C. The supercooled LPG is fed to the bottom of the tank, where it is stored in layer 4c at -45°C to -55°C. Preferably, the supercooled LPG is at a temperature of approximately -52° C. (at tank storage pressure). After the heat exchange, the vaporized or heated LNG has an outlet temperature of approximately 0° C. and can be further heated by the heating device 32.

Alternatively, the storage of supercooled LPG depends on the pressure in the tank. In particular, when the pressure in the tank is less than the first predetermined pressure value, for example, from 1 to 1.05 bar absolute pressure, the system controls the storage of supercooled LPG in a cold reserve layer. To this end, the pressure detection means 33 make it possible to determine the pressure inside the tank 4. The pressure detection means 33 in this case comprise a pressure sensor installed in or near the tank 4.

The LPG in the tank 4, which is above the cold storage layer 4c, for example left in the tank, has a temperature above -42°C. It is believed that the LPG storage tank contains several layers in which the LPG has different temperatures, with the coldest layers being at the bottom of the tank.

In the second mode of operation (FLIGHT) of the plant 2 gas treatment system for power generation, as illustrated in Figure 2, LNG is used to supply plant 2, for example, the ship's engine, and the LPG is subcooled to form a cold LPG reserve that will be used later for cooling LPG vapor in the tank. This mode of operation is used during the voyage of the ship, when it is necessary to handle a smaller amount of LPG vapors. This is due to the fact that the resulting vapors of LPG (OGEI) are of the order of 2700 kg/h, while the ship's engine, for example, consumes a small amount of fuel gas, of the order of 2000 kg/h. In this mode of operation, the system uses at least an evaporative heat exchanger 20, through which the LPG coming from the tank and the LNG coming from the tank flow, to effect the forced evaporation of the LNG to be supplied to the ship's engine, and an auxiliary heat exchanger 22 to form a cold reserve. . LNG is removed from the tank by means of a second pump 11b. The temperature of the LNG at the inlet to the second pipe 6d is in the order of -160°C. The LPG is drawn from the tank containing the LPG by means of a pump 15. The LPG flows through the second pipeline to the evaporative heat exchanger and enters the latter at a temperature of approximately -42°C. The LPG undergoes a first subcooling of the LPG by recovering cold from the LNG, which evaporates as a result of heat exchange in the heat exchanger 20. The heat exchange between the LPG and the LNG is carried out so that the subcooling temperature of the LPG is between the first threshold value and the second threshold value at atmospheric pressure. The evaporative heat exchanger 20 is designed to transfer the maximum amount of heat, but is limited by the temperature difference between LNG and LPG. Preferably, but not limited to, the first threshold is on the order of -40°C and the second threshold is on the order of -55°C. The supercooled LPG is stored at the bottom of the tank to form a cold LPG reserve layer or sprayed into the headspace above the gas by means of a rod 10. During the trip, the temperature of the LPG at the outlet of the heat exchanger 20 is in the order of -52°C.

Of course, as can be seen in the case of the first mode of operation, when the pressure in the tank is less than the first predetermined pressure threshold, for example, from 1 to 1.05 bar absolute pressure, the supercooled LPG is stored in a cold reserve layer.

It is considered that a cold reserve layer has already been formed, for example, during the cooling of the reservoir. The supercooled LPG is then used to cool or condense the LPG vapor in the tank. To this end, the supercooled LPG is drawn from the cold storage bed 4c and sprayed into the headspace above the gas by means of a boom 10. Alternatively, the LPG from the cold storage bed 4c is drawn from a tank outlet connected to a pipeline that is connected to a boom or heat exchanger through which the LPG vapor passes. . Thus, there is no need to run an auxiliary heat exchanger to create a cold reserve.

The LNG leaving the heat exchanger 20 is vaporized or heated by heat exchange between the LPG and the LNG. The vaporized or heated LNG is fed into the engine to power the engine. The LNG vapor that is recovered from the tank can also be fed into the engine. The vaporized or heated LNG and LNG vapor are heated so that all of the LNG is vaporized before being fed into the engine.

In the third mode of operation (LOAD) of the gas treatment system of the power generation plant, as illustrated in Figure 2, LNG is used to power the ship's engine and power generation needs, as well as to re-liquefy the LPG vapor. This mode of operation is used in particular during the loading of LPG into a tank in which a large amount of LPG vapor is generated, for example, approximately 13900 kg/h. The energy requirements of plant 2 are low, about 500 kg/h. In this mode of operation, at least two heat exchangers are used to process all the LPG vapors. Specifically, the system uses a main heat exchanger 21 to treat the LPG vapor generated during LPG loading and an evaporative heat exchanger 20 to vaporize or heat the LNG to be fed to plant 2. Thus, the heat exchangers 20, 21 operate similarly to the first mode of operation in the case of tank cooling.

In this mode of operation, the main heat exchanger 21 may not be able to control the pressure in the tank 4 due to the large amount of LPG vapor generated. Under such a scenario, when the pressure measured (by pressure detection means 33) inside the tank reaches or exceeds the second predetermined pressure threshold, the auxiliary heat exchanger 22 is activated. Thus, the task of the auxiliary heat exchanger 22 is to control the pressure inside the tank 4. LNG is drawn from the tank for heat exchange with supercooled LPG. Subcooled LPG after the first subcooling has a temperature of the order of -42°C. The temperature of -42°C is due to the fact that a small amount of LNG passes through the heat exchanger 20, in particular the second pipe 6d. This is because the engine or plant 2 determines the flow of LNG to be vaporized in the second pipe 6d. Given that plant 2 needs are low, there is very little LNG available to subcool the LPG. The plant controls the flow rate of the second gas, which must be vaporized or heated during evaporation. This implies that the amount of heat from the LNG is insufficient to significantly reduce the temperature of the LNG. Since the temperature of the LPG at the outlet of the heat exchanger 20 is not low enough, the heat exchanger 22 performs a second subcooling of the LPG. The LNG is withdrawn from the tank at a temperature of approximately -160°C and exchanges heat with the LPG, which has undergone a first subcooling in this case in the heat exchanger 20. The temperature of the subcooled LPG at the inlet is in the order of -42°C. The temperature of the LPG, supercooled for the second time, at the outlet is less than or equal to the threshold temperature value that the tank 4 must withstand. The temperature of the LPG at the outlet is about -52°C. The LPG is stored in a cold reserve bed for later use or sprayed in the headspace above the tank gas to condense or cool the LPG vapor 4b in the tank. The outlet temperature of the LNG is approximately -134°C at a pressure of about 8 bar. Thus, the LNG heats up but does not evaporate.

In the fourth mode of operation (heated LNG in the tank) of the gas treatment system 1 of the power generation plant, as illustrated in Figure 2, the system makes it possible to control the risk of heating the LNG in the tank in the event that the main heat exchanger 21 is operating (during the loading of LPG into the tank or during cooling tank). This is due to the fact that the LNG at the outlet of the main heat exchanger and/or at the outlet of the auxiliary heat exchanger is heated, i.e. has an outlet temperature of -134°C. This mode of operation uses the system shown in Figure 3, primarily in cruise mode, to cool the LNG in the tank to its cryogenic temperature. System 1 uses at least a heat exchanger 40 in which the partially vaporized LNG allows the LNG fed to the vessel to be supercooled. It is believed that the LNG stored in the tank has a temperature of approximately -134°C at a pressure of about 8 bar. LNG is removed from the tank by the second pump 11b. The LNG flows in loop 42 where it is depressurized and then partially vaporized. The temperature of the partially vaporized LNG at the inlet to the heat exchanger 40 is in the order of -160° C. at atmospheric pressure. The outlet temperature of the vaporized LNG is -134°C to -160°C at atmospheric pressure. The LNG temperature at the inlet to the heat exchanger in the second pipe 43 is in the order of -134°C and the outlet temperature is in the order of -160°C. The supercooled LNG is fed into the cold reserve layer 4c at the bottom of the vessel 5. The heat exchanger 20 supercools the LNG 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 pressure threshold, the heat exchanger 22' is activated to supercool the LPG cooled in the heat exchanger 20 for the second time. The LPG is subcooled by the LNG subcooled in the heat exchanger and passes through the heat exchanger 22'. The outlet temperature of the LNG after heat exchange in the heat exchanger 22' is in the order of -134° C. at atmospheric pressure.

The above modes of operation have been described based on Figure 2. Of course, these modes of operation can be applied to Figure 1.

Figure 4 illustrates another embodiment of a gas treatment system 1 according to the invention. The system includes LNG storage tanks, each containing LNG and LNG pairs 5b. In this case, two LNG storage tanks are shown. The pumps are also immersed in LNG in the main tank and one pump is immersed in LNG in an adjacent tank. Each pump is preferably installed at the bottom of the tank. System 1 includes a heat exchanger 50 configured to supercool LNG coming from an LNG storage tank, in this case a first tank 500A, to be stored at the bottom 190 of the same first tank 500A to form a cold reserve layer 500c at the bottom of tank 500A. Layer 500c is located in the interior of the container. The heat exchanger includes at least one first pipe 50a and one second pipe 50b. The first pipe 50a has an inlet connected to the first end of the conduit 54. The second end of the conduit 54 is connected to the first pump 51 installed at the bottom of the first vessel 500A. The conduit 54 is also connected to the spray boom 60 installed in the tank 500A through a three-way valve 67. The boom 60 is located at the top of the tank and preferably in the free space above the LNG. The first pipe 50a has an outlet connected to a conduit 56 which is connected to the bottom of the tank 500A. The conduit 56 is also connected to the spray bar 60 through a three-way valve 75a. As illustrated in Figure 4, conduit 56 exits at the bottom of the adjacent vessel, i. the second container 500B, through the three-way valve 75b, and is also connected to another rod 60 of the second container 500B through the three-way valve 75c. The second pipe 50b contains an inlet connected to the tank 500A by a conduit 57. One of the ends of the conduit 57 is connected to a second pump 52 installed at the bottom of the tank 500A. The outlet of the second pipe 50b is in this case connected to the inlet of the cylinder 70 by a pipeline 58. The outlet of the cylinder 70 at the first outlet is connected to the pipeline 56 by means of a pipe 71. The pipe 71 contains, for example, a valve 72 and a pump 73. Means 53 are installed on the pipeline 57 before the heat exchanger 50 pressure relief. This heat exchanger, as in the embodiment illustrated in Figure 3, is a vacuum evaporator. The pressure relief means 53 comprise, for example, an expansion valve (Joule-Thomson valve).

The second pipe 50b is a cold loop, and the reduced pressure LNG is to be heated as it passes through this loop to effect forced evaporation (to produce the OGTI). The first pipe 50a is a hot circuit, and the LNG coming from the vessel 500A is subject to cooling while passing through this circuit. However, the first pipe 50a does not allow the heaviest components (ethane, propane, etc.) to be evaporated. It should be understood that lowering the pressure upstream of the second pipe 50b allows the evaporation temperature to be lowered, which allows the production of HRSG by heat exchange with the LNG withdrawn from the vessel 500A and flowing in the first pipe 50a. Evaporation in order to obtain DOI requires the assistance of heat supplied from the LNG flowing in the first pipe 50a; thus, it is a source of cooling for subcooling the LNG flowing in the first pipe 50a.

The LNG from the vessel 500A is then pumped 52 to the pressure relief means 53 and then flows through the second or cold pipe 50b of the heat exchanger 50. The LNG after the pressure relief means has a temperature of -168° C. at an absolute pressure of 400 mbar. At the same time, LNG from vessel 500A is pumped by pump 51 to first or hot tube 50a of heat exchanger 50. Therefore, heat exchange between these circuits results in:

- heating the partially vaporized reduced pressure LNG to continue its vaporization, which is then fed into the cylinder 70 in this example, and

- supercooling of LNG, which is fed to the bottom of the first tank and/or the second tank for storage in it for subsequent use, or which is sprayed in the free space above the LNG by means of a rod 60.

The outlet temperature of the LNG after heat exchange in the pipe 50a is in the order of -168°C.

Storage of LNG in a cold reserve layer may depend on the pressure inside the vessel. For example, when the pressure measured (by the pressure sensor 330) in the vessel is less than the predetermined threshold pressure in the vessel, the supercooled LNG (in liquid state) is stored in the cold storage bed 500c.

Thus, cylinder 70 is designed to be fed with LNG in a two-phase liquid/vapor state from vessel 500A via heat exchanger 50. The operating pressure in cylinder 70 is less than the storage pressure of LNG in vessel 500A. The supply of LNG to the cylinder 70 may lead to additional evaporation of the LNG, which, on the one hand, leads to the formation of OHPI in the cylinder 70, as well as to supercooling of the LNG remaining in the cylinder. The cylinder allows phase separation into LNG stored at the bottom of the cylinder and LNG vapor at the top. The supercooled LNG at the outlet of the cylinder has an outlet temperature of -168°C. Cylinder 70 contains a second outlet located at its top, in which LNG vapors (OGPI) are naturally stored. The outlet of the cylinder 70 is connected to the installation 2 in this case through two compressors 61, 62.

The heat exchanger 50 also includes a third tube 50c which contains an inlet and an outlet. The inlet of the third pipe 50c is connected to the first end of the conduit 63 through which the re-liquefied LNG vapor flows. In particular, the outlet of the compressor 62 is connected to the plant 2 for supplying fuel gas thereto. A portion of the fuel gas exiting compressor 62 may be diverted and redirected to conduit 64, which may be connected to the outlet of compressor 62 via a three-way valve 65. Compressor 62 is configured to compress gas (e.g. vessel) to an operating pressure suitable for use in plant 2. Line 64 is connected to an inlet of primary circuit 66a of heat exchanger 66. the inlet of the secondary circuit 66b of the heat exchanger 66. The secondary circuit 66b contains an outlet connected to the inlet or one of the inlets of the compressor 62. The third pipe 50c contains an outlet connected to the conduit 56 by another conduit 69. An expansion valve 74 is installed on this conduit 69 to reduce the temperature of the gas by adiabatic expansion.

LNG vapor coming from vessel 500A, 500B is heated in secondary circuit 66b to be fed to plant 2 and LNG vapor at the outlet of compressor 62 is reliquefied to be fed to heat exchanger 50. In heat exchanger 50, the reliquefied gas vapor is supercooled by the cold from the LNG, flowing in pipe 50a, to be delivered to the bottom of tank(s) 500A, 500B or to spray boom 60. LNG vapor coming from tank(s) 500A, 500B can be redirected to line 64, if excess FGTL is generated, for liquefaction.

In this exemplary embodiment, subcooling takes place outside the containers. In other words, the heat exchanger 50 is separate from the tanks.

Figure 5 illustrates an alternative embodiment of the gas treatment system 1 illustrated in Figure 4. System 1 differs from the system shown in Figure 4 in that it includes a second pump 52 installed in a second vessel 500B adjacent to the first, main vessel (which shown on the right in Figure 5). The second pump 52 is located at the first end of the pipeline 80, through which flows the LNG extracted from the bottom of the second vessel 500B. The second end of the pipeline is connected to the pipeline 57, which is connected to the inlet of the second pipe 50b. In other words, the LNG is drawn from the two tanks 500A, 500B by two pumps 52. The second pump 52 allows the level of depressurization downstream of the depressurization means to be reduced by increasing the pressure and temperature. For example, in the case of two second pumps, the absolute pressure after the pressure relief means is 600 mbar and the temperature of the LNG is -164°C.

Figure 6 illustrates another embodiment of a gas treatment system in accordance with the invention. This system is similar to the embodiment illustrated in Figure 5. It is characterized in that it contains two heat exchangers 150, 150' instead of one heat exchanger 50. The first heat exchanger 150 is configured to vaporize the LNG coming from the first vessel 500A and simultaneously supercool the LNG coming from from the first tank 500A. The first heat exchanger 150 includes a first tube 150a and a second tube 150b arranged as described in the embodiment shown in Figure 4.

The second heat exchanger 150' is configured to use the subcooled LNG (in liquid state) stored in the cold reserve bed 500c, in this case coming from the first vessel 500A, to reliquefy the LNG vapor. The LNG vapors are generated as a result of natural evaporation (NGEV) of LNG not used by plant 2 for power generation, i.e. excess exhaust gas. The second heat exchanger 150' includes a third pipe 150c and a second auxiliary pipe 150b'. The third conduit 150c includes an inlet connected to conduit 163 which supplies excess LNG vapor. Specifically, the OGEI is recirculated through compressor 62 to heat exchanger 166 and through conduit 164. Third conduit 150c includes an outlet connected to conduit 169 that exits at the bottom of the vessel or each vessel 500A, 500B via a three-way valve 175b. The pipeline 169 is also connected to the spray bar 160 through a three-way valve 175a, 175c.

The second pipe 150b' contains an inlet connected to the pipe 154 through a three-way valve. The second conduit 150b' contains an outlet connected to conduit 156 via a three-way valve 180. Heat exchange takes place between the excess OHHE and the subcooled LNG coming from the vessel. The re-liquefied OGEI is fed to the bottom of the first and/or second tanks. LNG at the outlet of the second pipe 150b' is heated, but not vaporized, and returns to the bottom of the first and/or second tanks.

In this exemplary embodiment, subcooling takes place outside the containers. In other words, the heat exchangers are separated from the tanks.

Claims (42)

1. A method for treating gas from a gas storage facility (2), in particular on board a ship, comprising the steps of: - extracting the first gas (4a, 4b, 5a, 5b) in a liquid state from the first tank (4) or the first tank (5, 500), - carry out the first supercooling of the first gas in the liquid state and - store the supercooled first gas in a liquid state at the bottom of the first tank (4) or the first tank (5, 500) or the second tank or the second tank to form a cold reserve layer (4c, 5c, 500c) of the first gas in the liquid state at the bottom of the first or the second tank (4) or the first or second tank (5, 500). 2. The method according to the previous paragraph, in which the first gas is supplied to the first or second reservoir (4) or the first or second container (5, 500) through a pipeline (16, 56, 156) that exits at the bottom (19, 190) of the first or the second tank (4) or the first or second tank (5, 500). 3. The method according to any one of the preceding claims, wherein the first gas stored in the cold reserve bed (4c, 5c, 500c) of the first or second reservoir (4) or the first or second reservoir (5, 500) is used to cool the gas in vapor condition. 4. The method according to the previous paragraph, in which the gas in the vapor state is the first gas in the vapor state located in the upper part of one of the reservoirs (4) or containers (5, 500). 5. A method according to any one of the preceding claims, wherein the first gas stored in the cold reserve layer (4c, 5c, 500c) is sprayed into the first or second tanks (4) or the first or second tanks (5, 500) and into the first gas layer in the vapor state. 6. The method according to any one of paragraphs. 1-4, in which the first gas stored in the cold reserve layer (4c, 5c, 500c) is removed from the bottom of one tank from the tanks (4, 5, 500) or tanks and the first gas is re-liquefied in the vapor state by means of a heat exchanger. 7. A method according to any one of the preceding claims, wherein the subcooled first gas is stored in the cold reserve bed (4c, 5c, 500c) when the measured pressure in the reservoir or container is less than the first predetermined pressure threshold in the reservoir or container. 8. A method according to any one of the preceding claims, wherein said bottom extends for less than about 30% of the height of the tank or tank as measured from the bottom (19, 190), wherein said bottom (19, 190) is the lowest end of the tank, or containers. 9. The method according to any one of the preceding claims, wherein the supercooled first gas is stored in a cold reserve bed (4c, 5c, 500c) at a temperature between the first gas liquefaction temperature minus about 5°C at atmospheric pressure and the liquefaction temperature minus about 10°C, wherein the first gas in liquid state remaining in the reservoir or vessel has a temperature higher than the liquefaction temperature of the first gas. 10. A method according to any one of the preceding claims, wherein the supercooled first gas is stored in a cold reserve bed at a temperature of -45°C to -55°C or -160°C to -170°C, wherein the first gas is in a liquid state, remaining in one of the tanks or containers, respectively, has a temperature greater than or equal to -42°C or -160°C. 11. A method according to any one of the preceding claims, wherein the first subcooling of the first gas (4a, 4b) is carried out by means of the second gas (5a, 5b), at least in a liquid state, extracted from the container (5), the second gas having a boiling point less than or equal to the boiling point of the first gas. 12. The method according to the previous claim, comprising the step of vaporizing or heating the second gas, which is heated or vaporized by heat exchange during the first subcooling of the first gas for supply to the installation (2). 13. The method according to the previous paragraph, in which installation (2) controls the flow rate of the second gas, which is vaporized or heated during evaporation. 14. The method according to any one of paragraphs. 11-13, in which the second gas extracted from the vessel (5) is expanded and partially evaporated before heat exchange in the first supercooling. 15. The method according to any one of paragraphs. 11-14, in which the second gas extracted from the vessel is supercooled by heat exchange with the expanded and partially vaporized second gas. 16. A method according to any one of the preceding claims, comprising the step of performing a second subcooling of the first gas after the first subcooling. 17. The method according to the previous paragraph, in which the second gas used for the second subcooling is removed from the bottom of the vessel or subcooled. 18. A method according to any one of the preceding claims, wherein the first and/or second subcooling is carried out outside the first and second tanks and/or the first and second tanks. 19. A method according to any one of the preceding claims, wherein heat exchange between the first gas and the second gas at the first subcooling or the second subcooling is carried out so that the supercooling temperature at the exit of the first gas is between the first threshold value and the second threshold value. 20. The method according to any one of paragraphs. 16-19, in which the temperature at the outlet of the second gas after the second subcooling is -155°C to -105°C at a pressure of 2 to 20 bar. 21. The method according to any one of paragraphs. 11-20, in which the heated, vaporized or partially vaporized second gas is heated to be fed to the plant (2). 22. The method according to any one of paragraphs. 11-21 comprising a reliquefaction step in which vapors (4b) of the first gas flowing in the first circuit (6a) from the reservoir (4) are reliquefied by heat exchange with a second gas in liquid state having an inlet temperature and flowing in the second circuit (6b), wherein the re-liquefied vapors of the first gas are fed into the tank (4), and the second gas is maintained in a liquid state at the outlet temperature after re-liquefaction and returned to the tank (5), and the heat exchange between the first gas (4b) and the second gas (5a) is carried out so that the outlet temperature of the re-liquefied vapors (4b) of the first gas is between the first threshold value and the second threshold value. 23. The method of the preceding claim, wherein the vapors of the first gas are reliquefied when the pressure measured in the reservoir or container exceeds a second predetermined pressure threshold in the reservoir or container. 24. The method according to any one of the preceding claims, wherein the first gas is liquefied natural gas or liquefied petroleum gas. 25. The method according to any one of paragraphs. 1-24, wherein the second gas is liquefied natural gas. 26. The gas treatment system (1) of a gas storage facility, in particular on board a ship, comprising: - a reservoir or container (4, 5, 500) for storing the first gas in a liquid state; - the first heat exchanger (6, 20, 40, 50, 150) configured to perform the first subcooling of the first gas extracted from the tank or vessel (4, 5, 500) through the first pipeline (14, 54, 154), and - a second pipeline (16, 56, 156) connected to the first heat exchanger, which exits at the bottom of the tank or tank (4, 5, 500) or other tank or tank for storing the supercooled first gas at the bottom of the tank or tank or other tank, or containers for forming a cold reserve layer of the first gas in the liquid state. 27. The system according to the previous paragraph, which contains a container (5) for storing the second gas in a liquid state, and the second gas has a boiling point less than or equal to the boiling point of the first gas. 28. The system (1) according to the previous paragraph, in which the second gas in liquid state flows through the second pipeline (14) connected to the first heat exchanger (6, 20) to effect the first subcooling of the first gas. 29. System (1) according to any one of paragraphs. 26-28 which comprises a second heat exchanger (22) configured to perform a second subcooling of the first gas by means of the second gas in the liquid state. 30. System (1) according to any one of paragraphs. 26-29, in which the bottom of the tank or container contains an outlet connected to the first end of the pipeline, and the pipeline contains a second end connected to the spray bar (10, 60, 160) installed in the upper part of the tank (4) or tank (5, 500). 31. System (1) according to any one of paragraphs. 26-30, which contains a heating device (32) through which the second gas passes, heated, vaporized or partially vaporized in the first heat exchanger (20). 32. System (1) according to any one of paragraphs. 26-31, which contains pressure relief means (41, 53, 153) installed before the first heat exchanger (20, 50, 150). 33. The system according to any one of paragraphs. 26-32, in which the second heat exchanger (22) is configured to supply the second gas at an outlet temperature of -155°C to -105°C and a pressure of 2 to 20 bar. 34. System (1) according to any one of paragraphs. 26-33, in which the first gas is liquefied natural gas or liquefied petroleum gas. 35. System (1) according to any one of paragraphs. 26-34, wherein the second gas is liquefied natural gas. 36. Vessel, in particular a vessel for the transport of liquefied gas, including at least one system according to any one of paragraphs. 26-35.

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