DEVICE, PLANT AND METHOD FOR KEEPING A LIQUID GAS STORAGE TANK COLD

Description

[0001] The invention relates to a device, an installation and a method for keeping a liquefied gas storage facility cold.

[0002] The invention relates in particular to a device and a method for keeping cold and / or maintaining pressure in a storage of liquefied gas, for example liquefied natural gas.

[0003] The invention relates more particularly to a device for keeping a liquefied gas storage, for example liquefied natural gas, cold, comprising a cryogenic refrigerator, a subcooling circuit comprising a set of pipe(s), the subcooling circuit comprising a suction end intended to be housed in a lower portion of a liquefied gas storage configured to draw in liquefied gas, a heat exchanger ensuring heat exchange between the suction subcooling circuit and the refrigerator, the subcooling circuit comprising at least one injection end configured to inject the cooled fluid into the storage in the heat exchanger, the device further comprising a vaporization gas recovery line having an upstream end intended to be connected to an upper portion of the storage to recover vaporization gas,the recovery line comprising a downstream end intended to be connected to a consumer of the vaporized gas, for example a burner and / or an engine, the device comprising a bypass line and a set of valve(s) configured to allow the transfer of vaporized gas from the recovery line to the subcooling circuit, the bypass line having a first end connected to the recovery line and a second end connected to the subcooling circuit.

[0004] The vaporization gases of a cryogenic fluid in storage can be recovered by direct reliquefaction. The solution involves cryogenically compressing these vapors, bringing them to ambient temperature, and then sending them to a liquefier.

[0005] Another feature is the reliquefaction of evaporation gases achieved by transferring cold power from a refrigerator to the cryogenic liquid by subcooling it, which is then returned to storage.

[0006] The document WO2019020742 A1 describes an installation providing the treatment of vaporization gas or the cooling of liquefied gas.

[0007] The cooling exchanger can become fouled due to the precipitation of heavier components (the more the cooling process, the lower the solubility). Indeed, the cryogenic fluid is not necessarily a pure compound and may be in the form of a mixture. Thus, some components of the mixture can solidify during subcooling. These solid particles can foul the refrigerator's exchanger and thereby degrade the system's performance.

[0008] Various solutions are known to reduce or limit this fouling. However, these solutions are not satisfactory for both treating vaporization gases and / or subcooling a stored liquefied gas.

[0009] One aim of the present invention is to overcome all or part of the disadvantages of the prior art noted above.

[0010] To this end, the device according to the invention, which also conforms to the generic definition given in the preamble above, is essentially characterized in that the second end of the bypass pipe is connected to the subcooling circuit upstream of the heat exchanger.

[0011] Furthermore, embodiments of the invention may include one or more of the following characteristics: The valve assembly includes a flow control valve on the bypass line. The flow control valve is a pilot-operated valve configured to transfer a flow of vaporized gas to the subcooling circuit to increase the temperature of the fluid entering the heat exchanger by a predetermined value, for example, between 5 and 25°C. The flow control valve is a pilot-operated valve configured to transfer a flow of vaporized gas to the subcooling circuit to maintain the temperature of the fluid entering the heat exchanger below the saturation temperature of the liquefied gas. The flow control valve is a pilot-operated valve configured to transfer a flow of vaporized gas to the subcooling circuit to maintain the temperature of the fluid exiting the heat exchanger above a predetermined value.for example above -160°C and / or at a value equal to the temperature of the liquefied gas in the storage and / or at a value equal to the temperature of the liquefied gas drawn into the suction end, the recovery line comprising at least one compressor, the first end of the bypass line is connected to the recovery line downstream of the compressor, the recovery line comprises several compressors in series, the first end of the bypass line is connected to the recovery line downstream of an intermediate compressor, i.e. upstream of the last compressor in series, the compressor is configured to supply a gas flow to the bypass line having a pressure greater than the pressure of the liquid drawn into the suction end and supplied to the subcooling circuit, the device includes a mixing element for the vaporization gas in the subcooling circuit,the mixing unit being located for example at the junction between the second end of the bypass line and the subcooling circuit, the mixing unit comprising at least one of the following: an indirect heat exchanger with injection, a gas injector in liquid, a static mixer, an upstream injector, a filtration system, a condenser pot with bulk or structured packing, the bypass line comprising a precooling unit in heat exchange with the vaporization gas transferred to the subcooling circuit, the precooling unit being configured for example to cool the vaporization gas flow transferred to a temperature intermediate between the vaporization gas temperature of the recovery line and the liquefied gas temperature,The recovery line includes a heat exchanger enabling heat exchange between the vaporization gas from the recovery line and the vaporization gas from the bypass line; the bypass line includes a diverter and a set of valve(s) enabling control of the flow of vaporization gas from the bypass line admitted to circulate in the heat exchanger of the recovery line; the suction end includes a suction pump.

[0012] The invention also relates to a liquefied gas storage installation, for example liquefied natural gas, for example ship, for transporting liquefied gas, comprising at least a liquefied gas storage and a device for keeping the fluid contained in the storage cold, the keeping cold device conforming to any one of the preceding or below characteristics.

[0013] The invention also relates to a method for keeping a liquefied gas storage cold using a device according to any one of the preceding claims comprising pumping liquefied gas into a cryogenic storage, cooling the pumped liquefied gas and reinjecting the cooled liquefied gas into the storage, the method comprising recovering vaporization gas from the storage and a step of injecting and mixing vaporization gas into the pumped liquefied gas before its cooling.

[0014] This allows the temperature of the fluid at the inlet of the subcooling circuit to be increased by the injection of vaporization gas in order to prevent the crystallization of heavy hydrocarbons in the heat exchanger 6 since the subcooler (refrigerator) is preferably configured to never lower the temperature of the fluid to be cooled below the initial temperature of the subcooled liquid.

[0015] Depending on other possible features, the injection and mixing stage is configured to raise the temperature of the pumped liquefied gas before cooling by a determined value and / or to maintain the temperature of the cooled liquefied gas below a determined threshold and / or at the level of the temperature of the liquefied gas in storage or pumped.

[0016] The invention may also relate to any alternative device or method comprising any combination of the above or below features within the scope of the claims. Other features and advantages will become apparent from the following description, given with reference to the figures in which: Brief description of the figures

[0017] The invention will be better understood upon reading the following description, given solely by way of example and made with reference to the accompanying drawings in which: [ Fig. 1] is a schematic view illustrating an example of the structure and operation of an installation according to a first embodiment of the invention, [ Fig. 2 ] is a schematic view illustrating an example of the structure and operation of an installation according to a second embodiment of the invention, [ Fig. 3 ] is a schematic view illustrating an example of the structure and operation of an installation according to a third embodiment of the invention, [ Fig. 4 ] is a schematic view illustrating an example of the structure and operation of an installation according to a fourth embodiment of the invention. Detailed description

[0018] In all the figures, the same references refer to the same elements.

[0019] In this detailed description, the following embodiments are examples. Although the description refers to one or more embodiments, this does not mean that the features apply only to a single embodiment. Simple features from different embodiments can also be combined and / or interchanged to provide other embodiments.

[0020] The device 1 for maintaining the cold of a storage 2 of liquefied gas illustrated can ensure the maintenance of cold and / or the maintenance of pressure for example of a liquefied natural gas tank, for example on a ship.

[0021] This device 1 includes a cryogenic refrigerator 3 and a subcooling circuit 4, 5 comprising a set of pipe(s).

[0022] The subcooling circuit 4, 5 includes at least one suction end 4 located in a lower portion of a liquefied gas storage tank 2 and configured to draw in liquefied gas. As illustrated, the suction end 4 preferably includes a suction pump 17.

[0023] The subcooling circuit 4, 5 further includes a heat exchanger 6 ensuring heat exchange between the suction subcooling circuit 4, 5 and the refrigerator 3. Preferably, this heat exchanger 6 is located outside the storage 2.

[0024] The refrigerator may, in particular, comprise or consist of a cryogenic refrigerator or liquefier in which a cycle gas (helium, nitrogen, or another pure gas or mixture) undergoes a thermodynamic cycle (compression, cooling, expansion, heating) producing a cooling capacity at at least one end that can be transferred by heat exchange. For example, the refrigerator is of the reverse Brayton cycle type, and more specifically of the Turbo Brayton type, employing turbomachinery (at least one compressor and one turbine mounted on bearings and a magnetic motor coupled on the same shaft). The subcooling circuit 4, 5 includes at least one injection end 5, 7 configured to inject the cooled fluid into the heat exchanger 6 in the storage 2.

[0025] In the illustrated example, the subcooling circuit 4, 5 includes two injection ends 5, 7. The first end opens into the upper part of the storage tank in the form of nozzle(s) to allow the cooled fluid to be injected into the gaseous phase of storage tank 2 in order to recondense vapors and / or control the pressure in storage tank 2.

[0026] A second end opens, for example, into the lower part of storage 2 to cool the liquid phase.

[0027] According to the invention, the ejector at the bottom of storage tank 2 (injector) preferably returns the liquefied gas and the mixture to the liquid phase. The injected liquid is no longer subcooled but at the same temperature as the liquid in storage tank 2.

[0028] Device 1 further includes a vaporization gas recovery line 8 having an upstream end connected to an upper portion of the storage tank 2 configured to recover vaporization gas. The recovery line 8 preferably includes at least one vaporization gas compression device 11 (typically at least one compressor). The recovery line 8 includes downstream at least one downstream end 18 intended to be connected to a vaporization gas consumer, for example, a burner and / or an engine. This compression device 11 maintains a constant pressure in the storage tank 2 and supplies gas to engines; excess combustible vaporization gas can be directed to a combustion torch, for example.

[0029] Device 1 includes a bypass line 9 and a set of valve(s) 10 configured to allow the transfer of vaporized gas from the recovery line 8 to the subcooling circuit 4, 5. This bypass line 9 has a first end connected to the recovery line 8, preferably downstream of the compression unit 11, and a second downstream end connected to the subcooling circuit 4, 5, upstream of the heat exchanger 6. For example, the set of valve(s) includes a flow control valve 10 on the bypass line 9.

[0030] This helps to prevent fouling phenomena by injection of vaporization gas taken, for example, from the outlet of the compression, for example at ambient temperature.

[0031] This allows for the injection of relatively warmer gas upstream of the cooling by the refrigerator / liquefier. This gas, supplied via bypass line 9, can be mixed with a stream of cryogenic liquid drawn from the storage via suction line 4 of the subcooling circuit.

[0032] The gas taken and mixed with the liquid stream is preferably at a pressure at least greater than the discharge pressure of the liquid suction pump 17 located at the bottom of the cryogenic fluid storage 2.

[0033] As illustrated, the device 1 may include a mixing element 12 for the vaporization gas in the subcooling circuit 4, 5. This mixing element 12, configured to achieve partial or complete mixing of the gas in the liquid, is located, for example, at the junction between the downstream end of the bypass line 9 and the subcooling circuit 4, 5. This mixing element 12 includes, for example, at least one of the following: an indirect heat exchanger with injection, a gas-in-liquid injector, a static mixer, an upstream injector, a filtration system, a condenser pot with bulk or structured packing, or any other suitable element or combination of several of these technologies.

[0034] The flow control valve 10 is, for example, a pilot-operated valve which can be configured to transfer a flow of vaporized gas to the subcooling circuit 4, 5 to increase the temperature of the fluid entering the heat exchanger 6 by a determined value, for example between 5 and 25°C.

[0035] For example, the flow control valve 10 can be configured to transfer a flow of vaporized gas to the subcooling circuit 4, 5 to maintain the temperature of the fluid entering the heat exchanger 6 below the saturation temperature of the liquefied gas. This allows the mixture to remain in the liquid phase. A temperature margin, for example 2°C below saturation, can be provided to ensure that no gas bubbles enter the heat exchanger 6.

[0036] The regulation of the mixed gas flow upstream of the heat exchanger 6 can be achieved for example by calculating a saturation temperature of the fluid (the bubble point temperature being the temperature at which the first vapor bubble is created).

[0037] This saturation temperature can be based on a typical composition of the stored fluid.

[0038] This saturation temperature can optionally be adjusted according to a correlation via a pressure parameter, for example. It is thus possible to control the injection of vaporization gas by the flow control valve 10 until the outlet temperature of mixer 10 is obtained with a safety margin relative to the saturation temperature (the outlet temperature of mixer 12 being equal to the saturation temperature of the fluid minus the safety margin).

[0039] Alternatively or in combination, the flow control valve 10 can be piloted to maintain the temperature of the fluid exiting the heat exchanger 6 above a predetermined value, for example above -160°C and / or at a value equal to the temperature of the liquefied gas in the storage and / or at a value equal to the temperature of the liquefied gas drawn into the suction end 4.

[0040] For example, it is possible to measure the temperature of the fluid at the outlet of pump 17 and / or in storage 2. The flow rate can be adjusted by the flow control valve 10 so that the temperature of the fluid at the outlet of the cooling exchanger 6 is equal to the temperature of the liquid in storage 2.

[0041] Increasing the pressure of the pumped liquid raises its saturation point. This results in the gas becoming fully or partially soluble in the cryogenic liquid upstream of the liquefier.

[0042] This dissolution of the vaporization gas in the liquid is achieved by raising the temperature of the cryogenic liquid.

[0043] The quantity of gas injected via the bypass line 9 can therefore be used to set a temperature at the refrigerator's inlet and / or outlet (at the inlet and / or outlet of the heat exchanger 6). This control can be achieved using a simple flow control valve 10. The temperature of the fluid to be cooled can then be measured at the inlet and / or outlet of the heat exchanger 6 by one or more suitable temperature sensors 19.

[0044] Refrigerator 3 reduces the temperature of the liquid or gas / liquid mixture obtained before returning it to storage 2 (at the bottom of storage and / or at the top via one or more ramps or nozzle(s)).

[0045] The temperature of the fluid exiting the cooling heat exchanger 6 is thus controlled to minimize or eliminate solidification within the heat exchanger. This temperature is therefore at least equal to the initial temperature of the cryogenic liquid pumped into storage tank 2. The amount of gas injected via the bypass line 9 can also be used to regulate the pressure drop within the heat exchanger 6.

[0046] Device 1 can also be used in "mixed" mode, i.e. by combining on the one hand, a liquefaction of the recovered vaporization gas which is compressed and reinjected and, on the other hand, a partial subcooling of the cryogenic liquid before returning to storage 2.

[0047] Device 1 can alternatively operate in subcooling mode in which the injection of relatively hotter vaporization gas is stopped via bypass line 9 (valve 10 closed) or in liquefaction mode by injection of vaporization gas via bypass line 9 (valve 10 open).

[0048] This allows for a flexible system offering both operating modes. The switch from one mode to the other can be decided, for example, arbitrarily by an operator according to the risk of blockage of the heat exchanger 6 and / or automatically via a detection or prediction (intelligence) system that detects the occurrence of a blockage (increase in pressure difference across the heat exchanger 6, drop in temperatures, etc.).

[0049] The method of implementation of the [ Fig. 2 ] differs from that of the [ Fig. 1in that the bypass line 9 includes a pre-cooling element 13 in heat exchange with the vaporization gas transferred to the sub-cooling circuit 4, 5. This pre-cooling element 13 can be configured, for example, to cool the vaporization gas flow to an intermediate temperature between the vaporization gas temperature in the recovery line 8 and the temperature of the liquefied gas pumped into storage 2.

[0050] This allows for the optimization or increase of the quantity of compressed gas that can be injected via the external pre-cooling 13, which has a temperature intermediate between the temperature of the cryogenic liquid and the ambient temperature. For example, this pre-cooling component 13 may include or consist of at least one of the following: a refrigeration cycle using a refrigerant, a compression system, and an expansion system of the valve, orifice, or turbine type.

[0051] The method of implementation of the [ Fig. 3 ] differs from that of the [ Fig. 1in that the recovery line 8 includes a heat exchanger 16 enabling heat exchange between the vaporization gas of the recovery line 8 and the vaporization gas of the bypass line 9. For example, the bypass line 9 includes a diverter line 14 and a set of valve(s) 15 for controlling the flow of vaporization gas from the bypass line 9 admitted to circulate in the heat exchanger 16 of the recovery line 8. This allows for pre-cooling of the vaporization gas by recovering cooling power from the vaporization gas taken from storage 2, upstream of the compression. One or more control valves can be used to control, as appropriate, the temperature of the gas at the inlet of the compression unit, the temperature of the fluid at the inlet of the refrigerator (at the inlet of the heat exchanger 6), or the temperature of the fluid at the outlet of the heat exchanger 6.This control can be carried out to maximize the recovered cooling power while ensuring proper operation of the compression 11 and preventing the formation of solids in the heat exchanger 6 of the refrigerator 3. This allows the amount of compressed vaporized gas injectable via the bypass line 9 to be optimized / increased.

[0052] The method of implementation of the [ Fig. 4 ] differs from that of the [ Fig. 1 ] in that the recovery line 8 comprises several compressors 11 in series and the first upstream end of the bypass line 9 is connected to the recovery line 8 downstream of an intermediate compressor 11, that is to say upstream of the last compressor in series.

[0053] That is to say, the vaporized gas is recovered at the outlet of the first or a subsequent compression stage rather than at the outlet of the last compressor. The pressure of the recovered gas is, however, preferably higher than the pressure of the liquid pumped into the storage tank with which it is mixed. The temperature of this vaporized gas at an intermediate compression stage is relatively lower than at the outlet of the last compressor. The quantity of vaporized gas that can be mixed with the liquid can thus be greater than in the embodiment of the [ Fig. 1 ]. This embodiment can also replace the pre-cooling component 13 of the embodiment of the [ Fig. 2 ].

[0054] The device thus enables optimal management of vaporization and cooling gases (including subcooling) in a liquefied gas storage system, for example, for methane, without generating a solid deposit in the cooling heat exchanger 6. The invention prevents or limits any degradation in the device's performance.

[0055] The vaporization gas injected upstream of the heat exchanger 6 of refrigerator 3 heats the liquid before it is cooled. Refrigerator 3 then returns this mixture, for example, to the temperature of the fluid in storage 2.

[0056] The vaporization gas taken from 9 and mixed is totally or partially liquefied on its return to storage 2. The coldest temperature reached by the fluid in the heat exchanger 6 does not allow the conditions for solidification to be reached.

[0057] The described solution allows for the conversion, through adaptation, of a conventional subcooling system into a direct reliquefaction system in the presence of pollutants such as heavy hydrocarbons. The solution prevents the crystallization of heavy hydrocarbons during cooling.

Claims

1. A device for keeping a storage (2) of liquefied gas cold, for example liquefied natural gas, comprising a cryogenic refrigerator (3), a sub-cooling circuit (4, 5) comprising a set of pipe(s), the sub-cooling circuit (4, 5) comprising a suction end (4) intended to be housed in a lower portion of a storage (2) of liquefied gas and configured to suck liquefied gas, a heat exchanger (6) ensuring thermal exchange between the fluid sucked by the sub-cooling circuit (4, 5) and the refrigerator (3), the sub-cooling circuit (4, 5) comprising at least one injection end (5, 7) configured to inject into the storage (2) the fluid cooled in the heat exchanger (6), the device (1) further comprising a recovery pipe (8) for vaporization gases having an upstream end intended to be connected to an upper portion of the storage (2) to recover vaporization gas, the recovery pipe (8) comprising a downstream end (18) intended to be connected to a consumer of the vaporization gas, for example a burner and / or an engine, the device (1) comprising a bypass pipe (9) and a set of valve(s) (10) configured to allow the transfer of vaporization gas from the recovery pipe (8) to the sub-cooling circuit (4, 5), the bypass pipe (9) having a first end connected to the recovery pipe (8) and a second end connected to the sub-cooling circuit (4, 5), characterized in that the second end of the bypass pipe (9) is connected to the sub-cooling circuit (4, 5) upstream of the heat exchanger (6).

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

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

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

5. The device according to any one of claims 2 to 4, characterized in that the flow control valve (10) is a piloted valve configured to transfer a flow of vaporization gas to the sub-cooling circuit (4, 5) to maintain the temperature of the fluid exiting the heat exchanger (6) above a determined value, for example above -160°C and / or at a value equal to the temperature of the liquefied gas in the storage and / or at a value equal to the temperature of the liquefied gas sucked at the suction end (4).

6. The device according to any one of claims 1 to 5, characterized in that the recovery pipe (8) comprises at least one compressor (11) and in that the first end of the bypass pipe (9) is connected to the recovery pipe (8) downstream of the compressor (11).

7. The device according to claim 6, characterized in that the recovery pipe (8) comprises several compressors (11) in series and in that the first end of the bypass pipe (9) is connected to the recovery pipe (8) downstream of an intermediate compressor (11), that is to say upstream of the last compressor in series.

8. The device according to claim 6 or 7, characterized in that the compressor (11) is configured to supply a gas flow to the bypass pipe (9) having a pressure higher than the pressure of the liquid sucked at the suction end (4) and delivered to the sub-cooling circuit (4, 5).

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

10. The device according to any one of claims 1 to 9, characterized in that the bypass pipe (9) comprises a pre-cooling member (13) in thermal exchange with the vaporization gas transferred to the sub-cooling circuit (4, 5), the pre-cooling member (13) being configured for example to cool the flow of the transferred vaporization gas to an intermediate temperature between the temperature of the vaporization gas of the recovery pipe (8) and the temperature of the liquefied gas.

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

12. The device according to claim 11, characterized in that the bypass pipe (9) comprises a bypass line (14) and a set of valve(s) (15) for controlling the flow of vaporization gas from the bypass pipe (9) admitted to circulate in the heat exchanger (16) of the recovery pipe (8).

13. The device according to any one of claims 1 to 12, characterized in that the suction end (4) comprises a suction pump (17).

14. An installation for storing liquefied gas, for example liquefied natural gas, for example a boat, for transporting liquefied gas, comprising at least one storage (2) for liquefied gas and a device for keeping the fluid contained in the storage cold, the device (1) for keeping cold being in accordance with any one of the preceding claims.

15. A method for keeping a storage (2) of liquefied gas cold using a device according to any one of claims 1 to 13, comprising pumping liquefied gas in a cryogenic storage, cooling the pumped liquefied gas and reinjecting the cooled liquefied gas into the storage, the method comprising recovering vaporization gas from the storage and a step of injecting and mixing vaporization gas into the pumped liquefied gas before its cooling.

16. The method according to claim 15, characterized in that the injection and mixing step is configured to raise the temperature of the pumped liquefied gas before cooling by a determined value and / or to maintain the temperature of the cooled liquefied gas below a determined threshold and / or at the level of the temperature of the liquefied gas in the storage or pumped.