EP3433557B1 - System for treating a gas produced by the evaporation of a cryogenic liquid and for supplying a gas engine with pressurised gas - Google Patents

Description

The present invention relates to a system and a method for treating gas resulting from the evaporation of a cryogenic liquid and for supplying pressurized gas to a gas engine.

The field of the present invention is more particularly the maritime transport of cryogenic liquids and even more particularly of Liquefied Natural Gas (LNG). However, the systems and methods which will be proposed later could also find applications in terrestrial installations.

If we consider liquefied natural gas, it has, at ambient pressure, a temperature of the order of -163 ° C (or less). During maritime transport of LNG, the latter is placed in tanks on a ship, an LNG carrier. Although these tanks are thermally insulated, thermal leaks exist and the external environment brings heat to the liquid contained in the tanks. The liquid therefore heats up and evaporates. Given the size of the tanks on an LNG carrier, depending on thermal insulation conditions and external conditions, several tonnes of gas can evaporate per hour.

It is not possible to keep evaporated gas in the vessel's tanks for safety reasons. The pressure in the tanks would increase dangerously. It is therefore necessary to allow the gas which evaporates to escape from the tanks. The regulations prohibit discharging this gas (if it is natural gas) into the atmosphere as it is. It must be burnt.

To avoid losing this gas which evaporates, it is also known, on the one hand, to use it as fuel for the engines on board the ship transporting it and, on the other hand, to reliquefy it to put it back in. the reservoirs from which it comes.

In order to reliquefy the gas which has evaporated, it is known practice to cool this gas in order to bring it back again to temperature and pressure conditions allowing it to return to the liquid phase. This cold intake is the most often carried out by heat exchange with a refrigerant circuit comprising, for example, a loop of refrigerant fluid such as nitrogen.

In addition, some LNG carriers use the natural gas they transport as fuel to ensure their propulsion. There are several types of engine that run on natural gas. The present invention relates more particularly to those which are supplied with natural gas in a high pressure gaseous phase. To then supply the propulsion engine of the LNG carrier, gas is pumped out of a liquefied natural gas tank on board the LNG carrier, then is pressurized using a pump before being vaporized in order to be able to power the motor.

The document EP-2 746 707 A1 is interested in a natural gas evaporating from liquefied natural gas storage tanks, typically arranged on board a seagoing vessel, which is compressed in a compressor with several compression stages. At least part of the compressed natural gas stream being sent to a liquefier, which typically operates according to a Brayton cycle, in order to be reliquefied. The temperature of the compressed natural gas coming from the final stage is reduced to a value below 0 ° C by passing through a heat exchanger. The first compression stage here functions as a cold compressor, and the resulting cold compressed natural gas is used in the heat exchanger so as to provide the necessary cooling of the flow from the compression stage. Downstream of its passage through the heat exchanger, the cold compressed natural gas circulates through the remaining stages of the compressor. If desired, part of the compressed natural gas can be used as fuel to supply the engines of the ocean-going vessel. In an alternative embodiment (§ [0026]), it is planned to cool the compressed gas in the gaseous state. before its liquefaction with partly compressed liquid before it is expanded for use in an engine or turbine.

The presence of a refrigerant loop with nitrogen in the Brayton cycle, or any other refrigerant gas distinct from the fluid to be refrigerated, implies providing specific equipment for the refrigerant. So, for example, when a nitrogen refrigerant circuit is provided on board a ship (or elsewhere), a nitrogen treatment (purification) unit is necessary to allow its use in the cryogenic field. It is also advisable to provide a specific tank, valves and other devices for regulating the circulation of nitrogen.

When the natural gas supplying the engines of the LNG tanker is taken directly from the tanks of the vessel, it is preferable to have a high efficiency in terms of liquefaction because the consumption of gas in the gas phase is then limited.

The object of the present invention is therefore to provide an optimized system making it possible to reliquefy gas which has evaporated and to supply a gas engine under high pressure. Preferably, the proposed system will make it possible to optimize the quantity of liquid recovered with regard to the portion of gas to be reliquefied. Advantageously, the proposed system could also be used on board a ship such as an LNG carrier. Preferably, the system will operate without using a refrigerant such as nitrogen or the like to avoid having two separate circuits with fluids of different natures. The proposed solution will also preferably not be more expensive to produce than the solutions of the prior art.

To this end, the present invention provides a system for treating a gas resulting from the evaporation of a cryogenic liquid and for supplying pressurized gas to a gas engine, said system comprising, on the one hand, from upstream to downstream, a reliquefaction unit with compression means, a first heat exchanger and expansion means, and, on the other hand, a pressurized gas supply line comprising from upstream to downstream a pump for putting liquid under pressure and high pressure vaporization means.

According to the present invention, the pressurized gas supply line has, upstream of the vaporization means, a bypass for supplying a second heat exchanger between, on the one hand, liquid under pressure from the supply line and , on the other hand, a line of the reliquefaction unit downstream of the first heat exchanger and upstream of the expansion means, characterized in that the gas resulting from the evaporation of a cryogenic liquid, then compressed and cooled is condensed at least partially within the first heat exchanger (17).

The proposed solution makes it possible to create a synergy between the reliquefaction of the gas which has evaporated and the production of gas under pressure to supply an engine, for example a MEGI engine. Indeed, on the one hand there are needs to cool the gas and on the other hand there are needs to heat the liquid before vaporizing it. The second proposed exchanger thus makes it possible both to limit the (cooling) requirements of the reliquefaction unit and the (heat) requirements of the high pressure gas supply line. In an original manner, it is proposed here to “sub-cool” the condensed gas. Indeed, after the first exchanger, the compressed gas is sufficiently cooled to condense and be found mainly in the liquid phase under pressure. This pressurized liquid must then be relaxed in order to be able to be reintroduced into the reservoirs which are substantially at atmospheric pressure (just a little above to prevent air from entering the interior). During this expansion, part of the condensed gas is vaporized. By cooling the condensed gas before expansion, therefore being in the liquid phase, this gas is subcooled and this makes it possible to limit, during expansion, the portion of condensed gas which revaporates.

To further optimize the use of the cold source coming from the flow of pressurized liquid intended to be vaporized to supply an engine, the bypass can supply a cooling system downstream of the second exchanger. It may for example be a third exchanger mounted in series with and downstream of the second exchanger and / or a heat exchanger mounted in parallel with the second exchanger.

In the system described above, provision can be made for the bypass to supply, in addition to the second exchanger, one or more exchangers for cooling gas before its reliquefaction.

A particular variant of a system as described above provides that it further comprises, downstream of the expansion means, a balloon separating the gas phase from the liquid phase in the expanded fluid; that a line conducts the gas phase to a collector to mix it with the gas resulting from the evaporation of the cryogenic liquid, and that the bypass supplies a heat exchanger to cool the gas phase before its introduction into the collector.

The system described above is particularly well suited to a reliquefaction unit which uses as coolant the same fluid as the fluid to be liquefied. In this advantageous variant, said unit thus comprises for example, downstream of its compression means, a bypass to a loop comprising second expansion means, and the loop joins the circuit upstream of the compression means after passing through the first heat exchanger. heat in the opposite direction to the fraction of gas in the circuit not diverted by the loop. In this embodiment, provision is preferably made for the compression means to include several compression stages each with a compression wheel, for the second expansion means to include an expansion turbine and for each compression wheel and the expansion turbine are associated with the same mechanical transmission. Provision may also be made for the system, with such a reliquefaction unit, to further include a third heat exchanger between pressurized liquid derived from the supply line and gas between the compression means and the second expansion means. . This third exchanger makes it possible to increase the exchanges and thus to optimize the system. As mentioned above, according to a first variant embodiment, the third heat exchanger can be mounted in parallel with the second heat exchanger and according to another alternative variant embodiment, the third heat exchanger may be mounted in series with the second heat exchanger.

The present invention also relates to a ship, in particular an LNG carrier, propelled by a gas engine, characterized in that it comprises a system for treating a gas resulting from the evaporation of a cryogenic liquid and for supplying gas. under pressure from a gas engine as described above.

Finally, the present invention proposes a method for treating a gas flow resulting from the evaporation of a cryogenic liquid and for supplying a high pressure gas to an engine, said gas flow being first of all compressed. then cooled and condensed at least partially within a first heat exchanger before being expanded, and the supply of gas under high pressure being carried out by pressurizing cryogenic liquid then by vaporizing it, and after its compression, the flow of liquid under pressure being separated into a first part of liquid flow and a second part of liquid flow, and the first part of the liquid flow being used to cool compressed and condensed gas within a second exchanger before expansion of the condensed gas, and the second part of the liquid flow receiving the first part of the liquid flow after the latter has cooled compressed gas, the whole of the liquid flow then being vaporized.

In this process it is advantageously provided that more than half, and preferably at least 90% by mass of the compressed gas is condensed before being cooled within the second exchanger.

In order to increase the efficiency of the reliquefaction, it is advantageously provided that the flow of pressurized liquid is also used to cool gas before it is condensed.

In a method as described above, it is advantageously provided that part of the compressed gas is taken from within the first exchanger to be expanded within an expansion turbine, and that the expanded gas is introduced into the first exchanger. against the current to cool the compressed gas and cause it to condense. In this way, the fluid to be reliquefied is also used as refrigerant fluid and it is then not necessary to provide a refrigerant circuit using another fluid to allow reliquefaction.

Details and advantages of the present invention will appear better from the following description, made with reference to the appended schematic drawing in which:
The figures 1 to 8 are each a schematic view, according to several variants, of a cryogenic liquid reservoir associated with a system for recovering the gas evaporating from said reservoir, with a system for treating part of the gas recovered to liquefy it and with a line high pressure gas supply to a gas engine.

In each of the appended figures, a reservoir 1 is illustrated. Throughout the remainder of the description, it will be assumed that this is a Liquefied Natural Gas (or LNG) tank among several other similar tanks on board an LNG-type ocean-going vessel.

The numerical values in the description which follows are given by way of purely illustrative numerical examples and in no way limiting. They are suitable for the treatment of LNG on board a ship but may vary, in particular if the nature of the gas changes.

The tank 1 stores the LNG at a temperature of the order of -163 ° C which corresponds to the usual storage temperature of LNG at a pressure close to atmospheric pressure. This temperature obviously depends on the composition of the natural gas and the storage conditions. The atmosphere around tank 1 being at a much higher temperature than that of LNG, although tank 1 is very well thermally insulated, calories are added to the liquid which heats up and vaporizes. The volume of gas evaporating being much greater than that of the corresponding liquid, the pressure in the reservoir 1 therefore tends to increase as time passes and as calories are added to the liquid.

To avoid reaching excessively high pressures, the gas which evaporates is gradually withdrawn from tank 1 (and from the other tanks of the ship) and is found in a manifold 2 connected to several tanks. In the remainder of the description, the gas which has evaporated is called “gas” even when it is subsequently reliquefied. It can thus be distinguished from LNG which is taken in liquid form from the tanks to supply an engine.

In the systems illustrated in the drawing, provision is made to use the gas which has evaporated as a source of energy on board the ship (for example to produce electricity) and to reliquefy the surplus gas. The aim here is to avoid losing the evaporated gas and therefore either using it on board the ship, or recovering it and returning it, in the liquid phase, to the tank 1. In addition, there is provided a dc line. 'high pressure gas supply to a gas engine of the MEGI engine type from liquid LNG taken from the ship's tanks.

To be used on board the ship, the gas evaporated from the tanks must first be compressed. This compression is then carried out within a first compression unit 3 which can be, as illustrated in the drawing, multistage. This unit, by way of illustrative and in no way limiting numerical example, takes the pressure of the gas collected in the manifold 2 from a pressure substantially equal to atmospheric pressure to a pressure of the order of 15 to 20 bar (1 bar = 10 ⁵ Pa).

After this first compression step, the gas passes through a intercooler 4 in which it is cooled without significantly modifying its pressure. The gas which has been reheated during its compression is at a temperature of the order of 40 to 45 ° C. at the outlet of the intercooler (these values are given purely by way of illustration and apply in particular for natural gas). The gas thus compressed and cooled can then be sent in the gaseous phase via a pipe 5 to a generator on board the ship.

The gas requirements at the ship's generator (s) are often less than the "production" of evaporative gas in all tanks that are on board the ship. The gas not used in the generator (s) is then sent to a reliquefaction unit 10.

The reliquefaction unit 10 comprises at its inlet a valve 6 intended in particular to control the pressure of the gas in line 5, then a main circuit and a loop which will be described below.

The main circuit allows from the gas (in the gas phase and which is at a pressure of the order of a few bars to about 50 bar - non-limiting values -) to obtain gas in the liquid phase which can return to the tank 1 .

The process for obtaining this gas in liquid phase to be returned to the reservoir is conventional. This involves compressing the gas, cooling it to condense it and then expanding it so that it regains the pressure prevailing in the reservoirs. This way of doing things is classic in the field of cryogenics.

There is thus in the main circuit first of all a multistage compressor comprising here three successive stages with the references 11, 12 and 13. Each stage is formed by a compression wheel and the three compression wheels are driven by the same one. transmission 15 with shafts and pinions. The line between the compression stages in the figures symbolizes the mechanical connection between them. In the embodiment illustrated in figure 1 , the gas arriving in the multistage compressor arrives in the second stage 12 of this compressor. Depending on the system, it may as well arrive at the first - as illustrated in the other figures of the drawing - or at the third (or more generally ^nth stage) of this compressor.

After this second compression, the gas passes into an intercooler 16. Its pressure is then a few tens of bars, for example. example about 50 bar, and its temperature is again around 40 to 45 ° C.

The gas thus compressed is then cooled and condensed within a first multi-flow exchanger 17. The gas circulates in this first exchanger 17 in a first direction. The fluids circulating in the opposite direction (with respect to this first direction) and used to cool it will be described below.

At the outlet of the first exchanger 17, the compressed gas cooled to a temperature of the order of -110 to -120 ° C. is mainly (almost entirely) in the liquid phase and is sent, always at a pressure of the order of a few tens of bars (for example around 50 bar) via an insulated pipe 22 to an expansion valve 30.

Expansion through condensed gas expansion valve 30 provides both methane-rich liquid phase gas and nitrogen-rich gas phase gas. The separation of this liquid phase and this gas phase is carried out within a balloon 40 in which the pressure is of the order of a few bars, for example between 3 and 5 bar.

The gas in the gaseous phase of the balloon 40 is preferably returned to the manifold 2. In this way, it can be used either as fuel in a generator, or to pass back into the reliquefaction unit 10. This gas being cold, it can be used either as fuel in a generator. used to cool and condense the compressed gas in the first exchanger 17. It is therefore planned to make it circulate in the opposite direction in this first exchanger 17 before returning it to the manifold 2.

If the gas in the gaseous phase of the balloon 40 for various reasons, in particular during transient phases, cannot be recycled to the collector 2, provision is made to send it to a flare or a combustion unit. A set of valves 31, 32 controls the sending of the gas in the gaseous phase from the balloon 40 respectively to the manifold 2 via a connection pipe 35 or to a combustion unit (not shown).

The gas in the liquid phase recovered at the bottom of the tank 40 is for its part intended to return to the tank 1. Depending on the operating conditions, the gas in the liquid phase can be sent directly to the tank 1 (passage controlled by a valve 33 ), or using a pump 41 (passage controlled by a valve 34).

The return of the gas in liquid phase from the balloon 40, directly or by the pump 41, to the reservoir 1 is effected by means of an insulated pipe 36 provided here with a valve 54, for example a valve. stop.

In the reliquefaction unit 10, cooling of the compressed gas in the multistage compressor (stages 11, 12 and 13) should be ensured. This cooling is usually done using a separate thermodynamic machine, operating for example according to a Brayton cycle, and using nitrogen as refrigerant. It is possible to use in the reliquefaction unit 10 such a refrigeration machine which then cools and condenses the gas within the first exchanger 17. However, it is proposed here, as mentioned above, to provide this reliquefaction unit a cooling loop using natural gas as refrigerant. This loop begins with a branch pipe 18 which separates the gas flow downstream of the multistage compressor (stages 11, 12, 13) into a first flow, or main flow, which corresponds to the main circuit described above, and into a second flow, or derivative flow.

The bypass pipe 18 is preferably connected to the main circuit at the level of the first exchanger 17. The gas in the gas phase which therefore enters the bypass pipe 18 is at "high pressure" (approximately 50 bar in the numerical example given ) and at an intermediate temperature between 40 ° C and -110 ° C.

The gas taken by the bypass pipe 18 is expanded within expansion means formed by an expansion turbine 14. This expansion turbine 14 is, in the preferred embodiment illustrated in the drawing, mechanically connected to the three compression wheels. corresponding to stages 11, 12 and 13 of the multistage compressor of the reliquefaction unit 10. The transmission 15 by shafts and pinions connects the expansion turbine 14 and the compression wheels of the multistage compressor. This transmission 15 is symbolized by a line connecting in the figures the expansion turbine 14 to the stages 11, 12 and 13.

The gas is expanded for example to a pressure level which corresponded to its pressure level when entering the reliquefaction unit 10, or about 15 to 20 bar. Its temperature drops below -120 ° C. This gas flow (gas phase) is then sent into the first exchanger 17 in the opposite direction to cool and condense the pressurized gas from the main circuit, first of all in a portion 19 located downstream of the bypass pipe 18 then in a portion of this main circuit in the first exchanger 17 upstream of this bypass pipe 18. At the outlet of the first exchanger 17, the expanded gas returns to temperatures of the order of 40 ° C and can be reinjected in the gas phase into the main circuit of the reliquefaction unit, upstream of the multistage compressor via a return line 21.

An open cooling loop is thus produced which uses as gas for cooling the same gas as that which is to be liquefied.

As indicated above, the illustrated system also has a gas supply line under (high) pressure to a gas engine, for example a MEGI type engine (not illustrated). This supply line starts from a tank 1. It is first of all supplied by an immersed pump 50 which supplies cryogenic liquid (LNG) to a pipe 51 to lead it to a high pressure pump 48. The high pressure liquid is then carried by a pipe 56 in a vaporizer 61, for example carrying out a heat exchange with water vapor, to produce vapor (natural gas in the gaseous phase) under high pressure which can then supply an engine of the MEGI type by a supply line 62.

Note in the figures the presence of a bypass 57 on the pipe 56. This bypass 57 will supply liquid under pressure, still in the liquid phase, a second exchanger 60 intended to sub-cool the condensate leaving the first exchanger 17 in the main circuit of the reliquefaction unit 10. This second exchanger 60, in the embodiment illustrated on figure 1 , is here provided to make a heat exchange between on one side the pressurized liquid of the pipe 56 supplying the MEGI motor (or other) and derived by the bypass 57 and on the other hand the condensate in the pipe isolated 22 between the first exchanger 17 and the expansion valve 30.

By way of merely illustrative and non-limiting numerical example, the liquid derived in the bypass 57 is located at approximately -150 ° C upstream of the second exchanger 60 and emerges from the latter for example at -140 ° C. (still in the liquid phase). In the insulated pipe 22, the condensed gas leaving the first exchanger 17 passes for example from -120 ° C to -135 ° C.

In the embodiment of the figure 1 , the regulation of the flow in the pipe 56 and the bypass 57 is provided by means of a valve 55 placed on the line 56 upstream of the bypass 57 and another valve 59 integrated in the bypass 57 (illustrated in downstream of the second exchanger 60, but the person skilled in the art understands that this valve 59 could equally be placed upstream of this second exchanger 60). A valve 58, manually or automatically controlled, is also provided between the two points of connection of the bypass 57 with the pipe 56.

Finally, we notice on the figure 1 (and the following ones) the presence of a junction 52 provided with a valve 53 between the insulated pipe 36 and the pipe 51. This junction 52 allows the liquid coming from the reliquefaction unit 10 to pass directly to the pipe 51 and therefore to the high pressure pump 48 without going back through a tank 1. It is thus clearly possible to limit the pressure drops and the heat losses.

The figure 2 illustrates an alternative embodiment of the system of the figure 1 with two modifications totally independent of each other. Provision is made here first of all, as already mentioned above, to inject the compressed gas in the first compression unit 3 into the first stage 11 of the multistage compressor of the reliquefaction unit. Then, provision is made for the regulation at the second heat exchanger 60 in a slightly different manner. Instead of adjusting the exchanges in the exchanger by varying the flow rates in bypass 57 ( figure 1 ), provision is made here to vary the flow rates passing through the exchanger at the level of the insulated pipe 22. It is thus provided in the embodiment of the figure 2 to pass through the second exchanger 60 between 0% and 100% of the flow (mixture between gas and liquid phase but mainly in liquid phase) circulating in the insulated pipe 22. To do this, a bypass pipe 66 short-circuits the second exchanger 60. A three-way valve 65 is provided upstream of the second exchanger 60 to regulate the flow of the insulated pipe 22 passing through the second exchanger 60 and that passing through the bypass pipe 66. Other means of regulation could be considered (such as for example at bypass 57, with a valve upstream of the bypass line and a valve in the bypass line and / or in the circuit branch containing the second exchanger). In this embodiment, provision is made to be able to isolate also the second exchanger 60 from the supply line of the MEGI engine (pipe 56). For this purpose, the embodiment of the figure 2 simply provides for providing each branch of the bypass 57, an upstream branch and a downstream branch of the second exchanger 60, with a valve 64a and 64b, respectively, manually or controlled.

In the alternative embodiment of the figure 3 , provision is made to simplify the structure of the first exchanger 17 (this simplification could also be proposed in the other variant embodiments of the invention). Here the connecting pipe 35 between the tank 40 and the collector 2 no longer passes through the first exchanger 17, the structure of which is therefore simplified. Thanks to the exchanges carried out within the second exchanger 60, it is possible to obtain good reliquefaction of the gases evaporated in the reliquefaction unit 10 with a first exchanger 17 of simpler structure and therefore of reduced cost price.

In the embodiment of this figure 3 , another regulation of the flows in the bypass 57 is proposed. In this variant, a valve 63 is arranged between the two points of connection of the bypass 57 with the pipe 56 of the engine supply line (not shown).

On the figure 4 , provision is made to pass all the evaporated gas recovered in the reservoirs 1 through the manifold 2, first of all in the first compression unit 3 then in the reliquefaction unit 10.

The figures 5 and 6 illustrate embodiments implementing a third heat exchanger 70 to cool the gas in the gaseous phase entering the refrigeration open loop of the reliquefaction unit 10. The exchange is here carried out between the liquid from line 56 and the compressed gas in the gaseous phase and already partially cooled from the branch pipe 18.

In the embodiment of the figure 5 , the third exchanger 70 is mounted in parallel with the second exchanger 60, while in the embodiment of the figure 6 , the third exchanger 70 is mounted in series with (and downstream of) the second exchanger 60.

The figure 7 proposes an embodiment in which four heat exchangers 80a-d are provided in various places of the main circuit of the reliquefaction unit 10 to cool the gas still in the gaseous phase before liquefying it. The exchanger 80a is here intended to cool the compressed gas in the first stage 11 of the multistage compressor before it enters the second stage 12 of this compressor. The exchanger 80b is disposed in a similar manner between the second stage 12 and the third stage 13. Another exchanger 80c is disposed downstream of the multistage compressor, before or after the intercooler 16 and before the first exchanger 17. Finally, it is proposed here to also have a heat exchanger 80d on the connecting pipe 35 to cool the gas returning to the manifold 2.

This embodiment is intended to be illustrative (and not limiting) of the various possibilities for positioning exchangers supplied with cryogenic liquid under high pressure. These exchangers may be four in number, or more, or much less. They are preferably mounted in parallel as illustrated, the exchangers 80n forming an exchange system mounted in series with the second exchanger 60. Other assemblies (series or parallel) can be envisaged. It is also possible to provide exchangers on the open loop cooling circuit.

Finally the figure 8 is attached to illustrate that the pressurized liquid (still liquid phase) in line 56 can also be used, in part, to cool other elements within a cooling system 90 on board the ship. The liquid used for the cooling system 90 is preferably disposed downstream of the second exchanger 60 so that the liquid from the line 56 taken from the bypass 57 primarily serves for cooling at the level of the reliquefaction unit 10. The cooling system can be for example an air conditioning unit, industrial refrigeration, ....

The variations provided in the various embodiments can be combined in various ways to achieve other embodiments according to the present invention but not illustrated.

The system proposed here achieves cooperation between a liquefaction unit and a high pressure gas supply, for example for the supply of a MEGI type motor. A synergy is created between these two sub-systems, one having cooling needs to liquefy a gas and the other requiring energy to vaporize liquid at high pressure. The system as proposed makes it possible to increase the efficiency of the reliquefaction unit, that is to say to increase the proportion of the evaporated gas which is reliquefied, to limit the cooling requirements to be supplied to carry out the reliquefaction of evaporated gas and at the same time to limit the energy needs to obtain a high pressure gas to supply an engine (MEGI engine or other system operating with gas under high pressure).

The system proposed here is particularly well suited to a reliquefaction unit having an open loop of refrigerant gas corresponding to refrigerated gas with production of cold at two different temperatures, a temperature of around -120 ° C at the outlet of the expansion turbine. and a temperature of about -160 ° C at the outlet of the expansion valve.

The system is independent of the engines on board the vessel which are powered by evaporated gas. There can be two different types of gas engines, one being supplied by the high pressure supply line and the other by the evaporated gas compressed by the first compression unit. The system also allows, from the evaporated gas, independently of any other external cold source, to achieve liquefaction.

In the bypass created on the high pressure gas supply line, the cold production can be matched to the load of the reliquefaction unit and can be regulated over a wide range.

The proposed system does not require a nitrogen treatment unit or the like. Its structure is simplified by the use of a refrigerant gas of the same nature as the gas to be refrigerated and to be liquefied and which also serves as fuel for an engine (or similar).

Of course, the present invention is not limited to the embodiments of the systems and methods described above by way of nonlimiting examples, but it also relates to all the variant embodiments within the reach of those skilled in the art in the context of of the following claims.

Claims (14)

1. System for treating a gas produced by the evaporation of a cryogenic liquid and for supplying a gas engine with pressured gas, said system comprising, on the one hand, from upstream to downstream, a reliquefaction unit (10) with compression means (11, 12, 13), a first heat exchanger (17) and expansion means (30), and, on the other hand, a pressurized gas supply line comprising from upstream to downstream a pump (48) to pressurize liquid and vaporization means (61) under high pressure, the pressurized gas supply line having, upstream of the vaporization means (61), a bypass (57) for feeding a second heat exchanger (60) between, on the one hand, pressurized fluid from the supply line (56) and, on the other hand, a line (22) of the reliquefaction unit (10) downstream of the first heat exchanger (17) and upstream of the expansion means (30),
   characterized in that the gas resulting from evaporation of a cryogenic liquid, then compressed and cooled is at least partially condensed in the first heat exchanger (17).
2. System according to claim 1, characterized in that the bypass (57) feeds downstream of the second exchanger (60) a cooling system.
3. System according to claim 2, characterized in that it comprises a third exchanger (70) mounted in series with and downstream of the second exchanger (60).
4. System according to one of claims 1 or 2, characterized in that it comprises a heat exchanger (70) mounted in parallel with the second exchanger (60).
5. System according to one of claims 1 to 4, characterized in that the bypass (56) further feeds the second exchanger (60), one or more heat exchangers for cooling the gas prior to its liquefaction.
6. System according to one of claims 1 to 5, characterized in that it comprises downstream of the expansion means (30) a balloon (40) separating the gaseous phase from the liquid phase in the expanded fluid, in that a line leads the gaseous phase to a manifold for mixing it with the gas from the evaporation of the cryogenic liquid, and in that the bypass (56) feeds a heat exchanger (80dd') for cooling the gaseous phase prior to introduction into the manifold (2).
7. System according to one of claims 1 to 6, characterized in that the reliquefaction unit comprises downstream of the compression means (11, 12, 13) a bypass to a loop having second expansion means (14), and in that the loop joins the circuit upstream of the compression means (11, 12, 13) after having passed through the first heat exchanger (17) in the opposite direction to the gas fraction of the circuit not bypassed by the loop.
8. System according to claim 7, characterized in that the compression means comprise several compression stages (11, 12, 13) each with a compressor wheel, in that the second expansion means comprise an expansion turbine (14), and in that each compressor wheel and the expansion turbine (14) are associated with a same mechanical transmission (15).
9. System according to claim 3 as well as according to one of claims 7 or 8, insofar as they depend on claim 3, characterized in that the third heat exchanger (70) exchanges heat between pressurized liquid derived from the supply line (56) and gas between the compression means (11, 12, 13) and the second expansion means (14).
10. Vessel, in particular LNG carrier, propelled by a gas engine, characterized in that it comprises a system for treating a gas produced by the evaporation of a cryogenic liquid and for supplying a gas engine with pressurized gas according to one of claims 1 to 9.
11. Method for treating a gas flow produced by the evaporation of a cryogenic liquid and for supplying an engine with gas under high pressure,
    said gas flow being first compressed then cooled in a first heat exchanger (17) before being expanded, and
    the supply in gas under high pressure being achieved by pressurizing cryogenic liquid then vaporizing it,
    characterized in that after its compression, the flow of pressurized liquid is separated into a first part of liquid flow and a second part of liquid flow,
    in that the first part of the liquid flow is used to cool compressed and condensed gas within a second exchanger (60) prior to expansion of the condensed gas, and
    in that the second part of the liquid flow receives the first part of the liquid flow after the latter has cooled compressed gas, the entire liquid flow then being vaporized, characterized in that the gas resulting from the evaporation of a cryogenic liquid and subsequently compressed and cooled is at least partially condensed within the first heat exchanger.
12. Method according to claim 11, characterized in that for more than half, and preferably at least 90% by mass of the compressed gas, is condensed before being cooled within the second exchanger (60).
13. Method according to one of claims 11 or 12, characterized in that the flow of pressurized fluid is also used to cool gas before it is condensed.
14. Method according to one of claims 11 to 13, characterized in that part of the compressed gas is taken from the first heat exchanger to be expanded within an expansion turbine (14), and in that the expanded gas is introduced into the first heat exchanger (17) counter currently to cool the compressed gas and cause its condensation.

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