RU2719258C2 - System and method of treating gas obtained during cryogenic liquid evaporation - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: engine supply system based on gas formed during cryogenic liquid evaporation and repeated liquefaction of said gas includes a supply line for at least one engine. First compressor unit (3) is directly supplied with gas formed at evaporation of cryogenic liquid. Retraction into return line, on which cooling means (10) and first expansion means (30) are serially arranged. Cooling facilities include series-arranged second compressor unit (11, 12, 13) and heat exchanger (17), and also downstream of second compressor unit (11, 12, 13), retraction into circuit (18, 20, 21), including second expansion means (14). Circuit is repeatedly connected to the return line upstream of the second compressor unit (11, 12, 13) after passing through heat exchanger (17) in the direction opposite to the gas fraction direction not discharged through the circuit.

EFFECT: technical result is increased efficiency of liquefied gas storage.

16 cl, 5 dwg

Description

The present invention relates to a system and method for processing gas obtained by evaporation of a cryogenic liquid.

More specifically, the present invention relates to the field of maritime transport of cryogenic liquids, and even more specifically, liquefied natural gas (LNG). However, the systems and methods that are proposed below may also find application in onshore installations.

At ambient temperature, liquefied natural gas has a temperature of about -163 ° C (or less). When transporting LNG by sea, it is placed in tanks on a ship. Although these tanks are thermally insulated, heat leaks occur and heat is transferred from the external environment to the liquid in the tanks. Thus, the liquid heats up and evaporates. Given the size of the tanks on the methane transport vessel, depending on the thermal insulation conditions and environmental conditions, several tons of gas per hour can evaporate.

Evaporated gas cannot be stored in the tanks of a ship for safety reasons. Tank pressure can dangerously increase. Therefore, it is necessary to ensure the release of gas that evaporates from the tank. According to the regulations, the emission of gas into the atmosphere (if it is natural gas) is prohibited. It is subject to burning.

It is also known that in order to avoid the loss of this evaporating gas, it is, on the one hand, used as fuel for engines on board the ship transporting it, and on the other hand, it is liquefied again to put it back in tanks in which he was.

The present invention relates to the supply of engines on board a ship based on vaporizing gas. It is known that when the consumption of engines significantly exceeds the “natural” evaporation of gas in tanks, part of the gas is taken to evaporate it, and then engines are supplied with it. However, more specifically, the present invention relates to the re-liquefaction of gas vaporized in tanks or tanks for cryogenic liquids and, more specifically, in tanks or tanks of a vessel for transporting methane, when the vaporization of gas exceeds the consumption of ship engines.

EP 2933183 relates to a liquefied gas processing system for a ship, which includes a liquefied natural gas storage tank and an engine that consumes liquefied natural gas contained in the storage tank as fuel. The liquefied gas processing system presented in EP 2933183 includes: a liquefied gas storage tank, an engine that consumes liquefied gas contained in the storage tank as fuel, and a fuel supply pipe that allows the liquefied gas to evaporate and supply the generated gas into the engine as fuel. The engine receives a supply of combustible gas, which is compressed to a low pressure.

In all the embodiments proposed in EP 2933183, the gas to be re-liquefied is cooled before being re-liquefied by the gas stream that exits the tanks, before it is compressed and transported to the engine (s). The heat exchanger, designated 21, is always found in drawings 1-17.

This heat exchanger 21 creates significant pressure losses in the gas stream that evaporates from the tanks. Therefore, under certain operating conditions, evaporated gas can enter the compressor at a lower pressure than atmospheric pressure. This means that there is a risk of air suction and mixing with gas.

Another disadvantage of the system presented in this prior art document is that it does not balance the production and consumption of cold. The amount of gas consumed by the engine (s) is largely independent of the amount of vaporized gas. Thus, the exchange in the heat exchanger 21 cannot be modeled depending, in particular, on the requirements for re-liquefaction in terms of cold.

To re-liquefy a gas that has evaporated, it is known to cool this gas in order to return it to the temperature and pressure conditions, which ensure its return to the liquid phase. This addition of cold is usually carried out by heat exchange with a refrigerant circuit, including, for example, a refrigerant such as nitrogen.

So, in document EP 1120615 describes a device intended for use on ships for re-compression of compressed steam. The re-compression is carried out in a closed cycle in which the working medium is compressed in at least one compressor, cooled in a first heat exchanger, expanded in a turbine and reheated in a second heat exchanger in which the compressed steam is at least partially condensed. The device includes a first subsystem including a second heat exchanger, and a second subsystem including a first heat exchanger, a compressor, and an expansion turbine. Two subsystems are located respectively on two platforms.

In document WO 2014/095877, natural gas vaporized from tanks for storing liquefied natural gas, usually located on board a marine vessel, is compressed in a compressor that has several stages including compression stages. At least one portion of the compressed natural gas stream is sent for re-liquefaction to a fluidizer, typically operating in accordance with a Brighton cycle. The temperature of the compressed natural gas from the final stage is reduced to below 0 ° C by passing through a heat exchanger. The first compression stage performs the function of a compressor operating at a low temperature, and the obtained cold compressed natural gas is used in a heat exchanger to perform the necessary cooling of the stream coming from the compression stage. Downstream, after passing through the heat exchanger, cold compressed natural gas flows through the remaining stages of the compressor. If desired, part of the compressed natural gas can serve as fuel, and it is fed into the engine of a seaworthy vessel.

The presence of a refrigeration circuit with nitrogen or any other gaseous refrigerant other than the cooled fluid causes the need for special equipment for the refrigerant. So, for example, when a nitrogen refrigerant circuit is provided on board the vessel (or elsewhere), an installation is necessary for the treatment (purification) of nitrogen to allow its use in the cryogenic sector. It is also necessary to provide a special tank, valves and other devices for regulating the flow of nitrogen.

Thus, the present invention proposes an optimized system that provides on board a vessel transporting liquefied natural gas, the possibility of supplying gas evaporating from tanks for storing the vessel, an engine based on natural gas, and re-liquefaction of gas that has evaporated and was not consumed by the engine. This system does not use any cooling liquid of a different nature than the gas used to supply the engine, and limit the pressure loss above the flow from the compressor used to supply the engine. Mostly the production of cold can be brought into line with the amount of gas to be re-liquefied.

For this purpose, the present invention provides a supply system based on the gas generated by the evaporation of the cryogenic liquid and re-liquefaction of this gas, said system including a supply line for at least one engine on which the first compressor unit for said gas is located, and an outlet to a return line on which cooling means and first expansion means are arranged in series.

In accordance with the present invention, the cooling means include a second compressor unit and a heat exchanger arranged in series, and also, downstream of the second compressor unit, a branch to the circuit including the second expansion means, the circuit being reconnected to the return line upstream of the second the compressor unit after passing through the heat exchanger in the direction opposite to the direction of the gas fraction, which is not diverted through the circuit.

Thus, a mechanical cooling circuit is proposed that avoids the use of gas that evaporates from tanks as a source of cold for cooling part of the gas before it is liquefied. Thus, the gas evaporated from the tanks can be supplied directly to the first compressor unit without loss of pressure (or with the maximum restriction of these pressure losses). Moreover, this cooling circuit works independently of other neighboring systems, and thus, it can work almost like a closed circuit of another refrigerant. Expansion tools allow you to quickly change the fluid pressure from high pressure to reduced pressure, and in each case they can include an expansion turbine, or an expansion valve, or throttle bore, or any other equivalent system.

In such a supply and re-liquefaction system, a recirculation line is preferably provided allowing the non-liquefied re-fraction of the gas leaving the first expansion means to be supplied to the supply line for the engine upstream of the first compressor unit. Preferably, the recycle line passes through a heat exchanger.

In the cooling unit, the outlet is preferably carried out in the heat exchanger so that the exhaust gas stream is already partially cooled, and then to enter the second expansion means.

In one embodiment of such a supply and re-liquefaction system, the first expansion means, for example, include an expansion valve with a cylinder outlet for separating the resulting liquid and the non-liquefied gas fraction. The cylinder provides the possibility of separation of gas and liquid and processing gas and liquid downstream in different ways. In such an embodiment, it is provided that the upper part of the cylinder is connected to the heat exchanger so that the gas leaving the cylinder enters the heat exchanger from the same side on which the outlet is located, and that the lower part of the cylinder is connected to the cryogenic liquid tank.

In a particularly advantageous embodiment of the processing system, the second compressor unit includes several compression stages, each of which comprises a compressor wheel, the second expansion means include an expansion turbine, and each compressor wheel and an expansion turbine are connected to the same mechanical transmission. This embodiment allows for a compact design. In addition, the work extracted at the level of the expansion turbine can be immediately transferred to the compressor wheels, which helps to achieve a good energy efficiency for the system.

To facilitate starting the cooling unit, this system may include additional means of introducing gas into the bypass circuit of the cooling unit. Thus, the cooling unit actually becomes autonomous and can be controlled as if it were a closed loop. Means for introducing gas into the bypass circuit include, for example, a cryogenic liquid pump, an evaporator, and a control valve.

The present invention also relates to:

- a supply and re-liquefaction system, as described above, further comprising a manifold for extracting evaporated gases from the cryogenic liquid tank set, the manifold being connected directly, in particular, in other words, without an intermediate device for heat exchange with another gas pipe, to the first compressor unit , and

- a vessel for transporting cryogenic liquid, in particular, a vessel for transporting methane equipped with such a supply and re-liquefaction system.

Finally, the invention provides a method for controlling the flow of gas generated by the evaporation of a cryogenic liquid, in which:

the specified gas stream is compressed in the first compressor unit, before it is fed either to the engine or to the means of re-liquefaction,

the gas fraction supplied to the re-liquefaction means is passed through cooling means and then expansion means, and finally through a separator, from which the liquid part is sent to the cryogenic liquid tank.

In accordance with the present invention, cooling means are mechanical cooling means in which:

the gas stream is compressed in a second compressor unit and then cooled in a heat exchanger before it is expanded so that the gas fraction is re-liquefied,

after compression, the gas stream is divided into the first part of the gas stream and the second part of the gas stream,

the first part of the gas stream is cooled and then fed to the re-liquefaction means for at least partially liquefying it, and

the second part of the gas stream is fed into a circuit in which the specified second part of the gas stream is expanded and then used to cool the first part of the gas stream, before re-combining the gas stream for re-compression in the second compressor unit.

In such a method of controlling the flow of gas generated by the evaporation of a cryogenic liquid, it is preferable to provide compression of the gas generated by the evaporation without prior heat exchange with another gas pipe. This allows you to limit the pressure loss before the gas enters the first compressor unit.

The non-liquefied gas leaving the first expansion means can be passed through a recirculation line upstream of the first compressor unit. In this case, in order to increase energy efficiency, the non-liquefied gas leaving the first expansion means preferably passes through a heat exchanger before it is again compressed in the first compression unit.

Details and advantages of the present invention will become more apparent from the description below, presented with reference to the accompanying schematic drawings, where:

in each of FIG. 1-5 is a schematic view of a cryogenic liquid tank connected to a system for extracting gas evaporating from said tank, on the one hand, for delivery to at least one engine, and on the other hand, for re-liquefying gas not consumed by said engine ( yami).

Tank 1 is shown on each accompanying drawing. In the following description, it is assumed that this is a tank for liquefied natural gas (or LNG) from a number of other similar tanks on board a marine vessel for transporting methane.

The numerical values given below in the description are presented solely as illustrative and completely non-limiting numerical examples. They correspond to the processing of LNG on board the vessel, but may change, in particular, in case of a change in the nature of the gas.

In the tank 1, LNG is stored at a temperature of the order of -163 ° C, which corresponds to the usual storage temperature of LNG at a pressure close to atmospheric. This temperature, of course, depends on the composition of natural gas and storage conditions. Since the environment around the tank 1 is at a much higher temperature than the temperature of the LNG, although the tank 1 is very thermally insulated, heat enters the liquid, which is heated and evaporates. Since the volume of the vaporized gas is much larger than the volume of the corresponding liquid, the pressure in the tank 1 increases over time and as heat enters the liquid.

To avoid excessively high pressure, the vaporizing gas is periodically removed from the tank 1 (and from other tanks of the vessel) and collected from several tanks to the main pipe 2.

In the systems shown in the drawings, the use of vaporizing gas is provided for supplying at least one engine (not shown) on board the vessel and for re-liquefying the excess gas. In this case, the goal is to avoid the loss of evaporated gas, and therefore either use it to move the vessel, or extract it and return it to the tank 1 in the liquid phase.

To use gas in the ship’s engine, the gas must first be compressed. Then this compression is carried out in the first compressor unit 3, which, as shown in the drawings, is multi-stage. In this block, only as an illustrative and completely non-limiting numerical example, the pressure of the gas collected in the main pipe 2 is adjusted 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 stage of compression, the gas is supplied to the intercooler 4, in which it is cooled without a significant change in its pressure. A gas that has been reheated during its compression is at a temperature of about 40 to 45 ° C. at the outlet of the intercooler (these values are given solely as an illustration).

Thus, the compressed and cooled gas is then supplied through an injection pipe 5 to the engine on board the vessel. This may be an engine designed to propel the vessel or for other purposes (auxiliary generator, etc.).

The main pipe 2 and the injection pipe 5 form a line for supplying the engine with gas evaporated from the tank 1.

The gas requirements for the engine (s) of the vessel are often lower than the "production" of gas by evaporation in all tanks that are on board the vessel. Gas that is not used in the engine (s) is then supplied to a re-liquefaction unit, including, in particular, a mechanical cooling unit 10.

The cooling unit 10 includes an inlet valve 6, in particular, for regulating the gas pressure in the injection pipe 5, and then the main cycle and circuit, as described below.

The main cycle makes it possible to obtain from a gas (which is under pressure from several bar to about 50 bar (5 MPa), non-limiting values) a gas of such temperature that it goes into the liquid phase before returning to tank 1.

The main cycle of the cooling unit 10 primarily includes a multi-stage compressor, in this case comprising three successive stages, designated 11, 12 and 13. Each stage consists of a compressor wheel, and three compressor wheels are driven by the same gear 15 with shafts and gears. The line between the compression steps in the drawings indicates the mechanical connection between them.

After this second compression (gas passing the supply line is already compressed in the first compressor unit 3), the gas is supplied to the intercooler 16. Its pressure is then several tens of bars, for example about 50 bar (5 MPa), and its temperature is again approximately from 40 to 45 ° C.

Then, the compressed gas is thus cooled in a multi-threaded heat exchanger 17. Gas enters the given heat exchanger 17 in a first direction. Fluids flowing in the opposite direction (relative to this first direction) and used to cool it are described below.

At the outlet of the heat exchanger 17, the compressed gas cooled to a temperature of about -110 to -120 ° C becomes liquid, and it is supplied, still at a pressure of several tens of bars (for example, approximately 50 bar (5 MPa)), through an insulated pipe 22 in expansion tools. In the embodiment according to the preferred embodiment, an expansion valve 30 is used to further cool the re-liquefied gas and reduce its pressure.

After expansion by means of expansion valve 30, methane-rich liquid and nitrogen-rich gas are simultaneously obtained (since natural gas does not consist solely of methane). The separation of this liquid phase and this gas phase is carried out inside the cylinder 40, in which the pressure is of the order of several bars, for example, from 3 to 5 bar (from 0.3 to 0.5 MPa).

The gas from the cylinder 40 is preferably returned to the main pipe 2. In this way, it is mixed with the primary stream, and therefore, it is partially used as fuel in the engine (s) or sent back to the cooling unit 10. Since the gas coming from the cylinder 40 is cold, it can be used to cool the compressed gas in the heat exchanger 17. Therefore, it is provided to pass it in the opposite direction in this heat exchanger 17 before it is returned to the main pipe 2 through the connecting pipe 35.

If, for various reasons, in particular during the transitional stages, the gas from the cylinder 40 cannot be returned to the main pipe 2, it is supplied to the flare or to the combustion unit. Using a set of valves 31, 32 control the flow of gas from the cylinder 40 into the main pipe 2 through the connecting pipe 35 or in the combustion unit.

The liquid extracted from the bottom of the cylinder 40, in turn, is intended to return to the tank 1. Depending on the operating conditions, the liquid can be sent to the tank 1 directly (the passage is regulated by valve 33), or by pump 41 (the passage is regulated by valve 34) .

The return of fluid from the cylinder 40, directly or through the pump 41 to the tank 1 is carried out through an insulated pipe 36.

In the cooling unit 10, as described above, a circuit is also located. This circuit begins with an outlet pipe 18, which separates the gas stream downstream of the multi-stage compressor 11, 12, 13 into a first stream, or a main stream, which corresponds to the main cycle described previously, and a second stream, or bypass stream.

The outlet pipe 18 is preferably connected to the main cycle at the level of the heat exchanger 17. The gas, which thus enters the outlet pipe 18, is under "high pressure" (about 5 MPa (50 bar) in the above numerical example) and at an intermediate temperature of 40 ° C to -110 ° C.

The gas passed through the exhaust pipe 18 is expanded in expansion means formed in the preferred embodiment stored in the drawing from the expansion turbine 14. The latter in the preferred embodiment illustrated in the drawing is mechanically connected to three compressor wheels corresponding to stages 11, 12 and 13 of the multi-stage compressor block 10 cooling. Transmission 15 connects the expansion turbine 14 and the compressor wheels of a multi-stage compressor via shafts and gears. This gear 15 is indicated by a line connecting the expansion turbine 14 with the steps 11, 12 and 13 in the drawings.

The gas is expanded, for example, to a pressure that corresponds to its pressure at the inlet to the cooling unit 10, i.e. from about 1.5 to 2 MPa (15 to 20 bar). Its temperature drops below -120 ° C. Then this gas stream is directed to the heat exchanger 17 in the opposite direction to cool the gas of the main cycle, first the part 19 located downstream of the outlet pipe 18, and then the part upstream of this outlet pipe 18. At the outlet of the heat exchanger 17, the gas acquires a temperature of about 40 ° C, and it can be reintroduced into the main cycle of the cooling unit, upstream of the multi-stage compressor, through the return pipe 21.

Thus, an open cooling circuit is obtained in which the same gas as the gas to be liquefied is used as gas for cooling.

In the embodiment of FIG. 2, compared with the embodiment of FIG. 1, it is envisaged that the gas exiting the cylinder 40 to the cooling unit 10 is retained by introducing it into the return pipe 21 through the connecting pipe 35b without sending it to the manifold 2. This embodiment should be provided, in particular, in cases where the first compressor unit 3 does not have the capacity to process nitrogen-rich gas coming from cylinder 40.

This embodiment of FIG. 2 can be combined with one or more of the options described later in this document with reference to FIG. 3-5.

In FIG. 3, it is planned to change the configuration of the system downstream of the expansion turbine 14 and heat exchanger 17. Instead of supplying the expanded gas leaving the heat exchanger 17 to the inlet of the first stage 11 of the multistage compressor of the cooling unit 10, in this case, recirculation of this gas stream is proposed, or directly to the main pipe 2, or, at an intermediate level, into the first compressor block 3. Valves 23 and 24 allow you to adjust the gas flow, which when leaving the heat exchanger 17 is directed either to the main pipe 2, or to rvy compressor unit 3.

Due to this configuration, it is possible to obtain a greater pressure ratio at the level of the expansion turbine 14 than this ratio of the multi-stage compressor of the cooling unit 10.

FIG. 4 illustrates the fact that the proposed system makes it possible to power various types of engines. It is possible to provide different pressure levels with the first compressor unit 3 to suit different types of engines. If, for example, the pressure in the injection pipe 5 is very high, for example, more than 25 MPa (250 bar), for delivery to the engine with direct injection of high pressure gas, it is also possible to supply the cooling unit 10 from the intermediate stage of the first compressor unit 3, rather than from the injection pipe 5.

Finally, in FIG. 5 shows means that can be implemented to facilitate cooling of the cooling unit 10 and, therefore, to start it. The embodiment of FIG. 5 allows such a start to be made without affecting the gas flow rate in the injection pipe 5 supplying the engine or the like. For example, it can be provided that, when cooling the cooling unit 10, the valve 6 is closed.

Thus, in FIG. 5, gas is supplied directly from the tank 1. For this purpose, the pump 60 allows some of the liquid to be discharged from the tank 1 so that it can be supplied to the introduction system 62 through the supply pipe 61. In the introduction system 62, the evaporator 63 allows the liquid discharged from the tank to pass 1, into the gaseous phase. Then, a valve 64 is provided for controlling the introduction of gas received in the evaporator at the outlet and controlling the amount of gas introduced into the circuit, and thereby controlling the cooling of the cooling unit 10. In FIG. 5 shows the introduction at the level of the return pipe 21, but another injection point may be selected.

Also, if necessary, can be provided for bleeding liquefied natural gas (the presence of arrows) from the supply pipe 61.

Thus, the system proposed in this invention makes it possible to provide an open cooling gas circuit corresponding to the gas to be cooled, with obtaining cold at two different temperatures, a temperature of approximately -120 ° C at the outlet of the expansion turbine and a temperature of approximately -160 ° C at the outlet of the expansion valve . The system is independent of the engines on board the ship and powered by vaporized gas. It allows liquefaction only on the basis of vaporized gas, regardless of any other external source of cold.

In the circuit, the production of cold is constantly adapted to the load at the level of the re-liquefaction means, and it can be regulated in a wide range by acting on the second compressor unit. Thus, it is possible to regulate to obtain the cold necessary for re-liquefaction and to adjust the energy balance of the system.

Under steady-state conditions, no gas discharge or gas burning is provided.

During start-up, cooling inside the cooling circuit can be arranged as in the case of a closed circuit. The cooling unit has no effect on the first compressor unit, which is also used to power engines (or other generators). When the circuit is cold, it can remain “at rest”, and it can be used in open loop mode as soon as it is necessary to liquefy the excess of evaporated gas.

The proposed system allows you to limit the pressure loss of gas evaporating from the tanks (s). This gas is collected and fed directly to the inlet of the first compressor unit. Inevitable pressure loss occurs when gas is supplied through the main pipe. It is limited, and this makes it possible to avoid that the pressure at the inlet of the first compressor unit decreases in all operating conditions of the system.

In addition, it is obvious that the proposed system does not require any installation for processing nitrogen or a similar installation. Its design is simplified by using a re-liquefied gas of the same nature as the gas that is cooled and liquefied.

Of course, the present invention is not limited to the embodiments of the systems and methods described herein by way of non-limiting examples, but it also applies to all embodiments within the scope of those skilled in the art within the scope of the appended claims.

Claims (23)

1. A system for supplying an engine based on the gas generated by the evaporation of a cryogenic liquid and re-liquefying this gas, including a supply line for at least one engine on which the first compressor unit (3) is located, which is directly supplied with gas generated by the evaporation of the cryogenic liquid, and withdrawal to a return line on which cooling means (10) and first expansion means (30) are arranged in series, characterized in that the cooling means include a second room in series a spring block (11, 12, 13) and a heat exchanger (17), as well as, downstream of the second compressor block (11, 12, 13), an outlet to the circuit (18, 20, 21), including second means (14) expansion, and the circuit is reconnected with the return line upstream of the second compressor unit (11, 12, 13) after passing through the heat exchanger (17) in the direction opposite to the direction of the gas fraction that is not discharged through the circuit. 2. The supply and re-liquefaction system according to claim 1, characterized in that it includes a recirculation line (35) providing the possibility of supplying an un-liquefied gas fraction leaving the first expansion means (30) to the upstream engine supply line (2) from the first compressor unit (3). 3. The supply and re-liquefaction system according to claim 2, characterized in that the recirculation line (35) passes through the heat exchanger (17). 4. The supply and re-liquefaction system according to one of claims 1 to 3, characterized in that the outlet to the circuit (18, 20, 21) is made within the heat exchanger (17). 5. The supply and re-liquefaction system according to one of claims 1 to 4, characterized in that the first expansion means include an expansion valve (30) that extends into the cylinder (40), designed to separate the resulting liquid and non-liquefied gas fraction. 6. Supply and re-liquefaction system according to claim 5, characterized in that the upper part of the cylinder (40) is connected to the heat exchanger (17) so that the gas leaving the cylinder (40) enters the heat exchanger (17) with the same the side on which the outlet is located, and the lower part of the cylinder (40) is connected to the cryogenic liquid tank (1). 7. The supply and re-liquefaction system according to one of claims 1 to 6, characterized in that the second compressor unit includes several compression stages (11, 12, 13), each of which contains a compressor wheel, the second expansion means include an expansion turbine (14 ), and each compressor wheel and expansion turbine (14) are connected to the same mechanical transmission (15). 8. The supply and re-liquefaction system according to one of claims 1 to 7, characterized in that it further includes means (62) for introducing gas into the loop loop. 9. The supply and re-liquefaction system according to claim 8, characterized in that the means (62) for introducing gas into the bypass circuit include a cryogenic liquid pump (60), an evaporator (63) and a control valve (64). 10. The supply and re-liquefaction system according to one of claims 1 to 9, characterized in that it further includes a collector for extracting evaporated gases from the set of tanks (1) for cryogenic liquid, and the collector is connected directly, in other words, in particular, without intermediate device for heat exchange with another gas pipe, with the first compressor unit (3). 11. A vessel for transporting cryogenic liquid, characterized in that it includes a supply system and re-liquefaction according to one of claims 1 to 10. 12. The vessel according to claim 11, characterized in that said vessel is a vessel for the transport of methane. 13. A method of controlling the flow of gas generated by the evaporation of a cryogenic liquid, in which: said gas stream is directly compressed in the first compressor unit before it is supplied either to the engine or to the means for re-liquefaction, the gas fraction supplied to the re-liquefaction means is passed through cooling means (10) and then expansion means (30) and, finally, through a separator (40) from which the liquid part is supplied to the cryogenic liquid tank (1), characterized in that the cooling means are mechanical cooling means, where the gas stream is compressed in a second compressor unit (11, 12, 13) and then cooled in a heat exchanger (17) before it is expanded so that the gas fraction is re-liquefied, after compression, the gas stream is divided into the first part of the gas stream and the second part of the gas stream, the first part of the gas stream is cooled and then fed to the re-liquefaction means for at least partially liquefying it, and the second part of the gas stream is fed to the circuit (18, 20, 21), in which the specified second part of the gas stream is expanded and then used to cool the first part of the gas stream, before re-combining the gas stream for re-compression in the second compressor block (11, 12, 13). 14. The method of controlling the flow of gas generated during the evaporation of a cryogenic liquid according to claim 13, characterized in that the gas coming from the evaporator is compressed without prior heat exchange with another gas pipe. 15. A method of controlling the flow of gas generated by the evaporation of a cryogenic liquid according to claim 13 or 14, characterized in that the un-liquefied gas coming from the first expansion means (30) is sent via an upstream recirculation line (35) from the first compressor unit (3). 16. The method of controlling the flow of gas generated during the evaporation of a cryogenic liquid according to claim 15, characterized in that the un-liquefied gas coming from the first expansion means (30) is passed through a heat exchanger (17) before it is re-compressed in the first compressor unit (3 )

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