SYSTEM AND METHOD FOR TREATING GAS RESULTING FROM THE EVAPORATION OF A CRYOGENIC LIQUID

Abstract

The proposed system comprises a supply line for at least one engine, on which line is situated a first compression unit (3) for said gas and a bypass to a return line on which are successively situated cooling means (10) and reliquefaction means (30). The cooling means successively comprise a second compression unit (11, 12, 13) and a heat exchanger (17). Downstream of the second compression unit (11, 12, 13) a bypass to a loop (18, 20, 21) comprises first expansion means (14), the loop rejoining the return line upstream of the second compression unit (11, 12, 13) after having passed through the heat exchanger (17) in the opposite direction with respect to the gas fraction not bypassed via the loop.

Claims

1-16. (canceled)

17. A system for the supply of an engine on the basis of a gas arising from the evaporation of a cryogenic liquid and for the reliquefaction of this gas, said system comprising: a supply line for at least one engine, on which line is situated a first compression unit (3) for said gas and a bypass to a return line on which are successively situated cooling means (10) and first expansion means (30), characterized in that the cooling means successively comprise a second compression unit (11, 12, 13) and a heat exchanger (17) as well as downstream of the second compression unit (11, 12, 13) a bypass to a loop (18, 20, 21) comprising second expansion means (14), the loop rejoining the return line upstream of the second compression unit (11, 12, 13) after having passed through the heat exchanger (17) in the opposite direction with respect to the gas fraction not bypassed via the loop.

18. The supply and reliquefaction of claim 17, characterized in that it comprises a recycling line (35) making it possible to send a non-reliquefied fraction of the gas exiting the first expansion means (30) to the supply line (2) for the engine upstream of the first compression unit (3).

19. The supply and reliquefaction system of claim 18, characterized in that the recycling line (35) passes through the heat exchanger (17).

20. The supply and reliquefaction system of claim 17, characterized in that the bypass is performed within the heat exchanger (17).

21. The supply and reliquefaction system of claim 17, characterized in that the first expansion means comprise an expansion valve (30) emerging in a balloon (40) intended to separate the liquid formed and the unliquefied gas fraction.

22. The supply and reliquefaction system of claim 21, characterized in that the upper part of the balloon (40) is linked to the heat exchanger (17) in such a way that the gas originating from the balloon (40) enters the exchanger (17) on the same side as the bypass, and in that the lower part of the balloon (40) is linked to a cryogenic liquid tank (1).

23. The supply and reliquefaction system of claim 17, characterized in that the second compression unit comprises several compression stages (11, 12, 13) each with a compression wheel, in that the second expansion means comprise an expansion turbine (14), and in that each compression wheel and the expansion turbine (14) are associated with one and the same mechanical transmission (15).

24. The supply and reliquefaction system of claim 17, characterized in that it furthermore comprises means (62) for injecting gas into the bypass loop of the circuit.

25. The supply and reliquefaction system of claim 24, characterized in that the means (62) for injecting gas into the bypass loop comprise a pump (60) for cryogenic liquid, a vaporizer (63) and a control valve (64).

26. The supply and reliquefaction system of claim 17, characterized in that it furthermore comprises a collector for the recovery of the evaporated gases of a set of cryogenic liquid tanks (1), and in that the collector is linked directly, that is to say in particular with no intermediate device for heat exchange with another gas pipe, to the first compression unit (3).

27. A ship for transporting cryogenic liquid, characterized in that the ship comprises a supply and reliquefaction system of claim 17.

28. The ship of claim 27, characterized in that said ship is a methane carrier.

29. A method for managing a gas stream arising from the evaporation of a cryogenic liquid, in which: said gas stream being compressed within a first compression unit before being sent either to an engine, or to reliquefaction means, the gas fraction sent to the reliquefaction means passes through cooling means (10) and then expansion means (30) and finally through a separator (40) from which the liquid part is sent to a cryogenic liquid tank (1), characterized in that the cooling means are means of mechanical refrigeration within which: a gas stream is compressed in a second compression unit (11, 12, 13), and then cooled within a heat exchanger (17) before being expanded in such a way that a gas fraction reliquefies, after its compression, the gas stream is separated into a first gas stream part and a second gas stream part, the first part of the gas stream is cooled and then sent to the reliquefaction means so as to be at least partially liquefied, and the second part of the stream of the gas is fed into a loop (18, 20, 21) in which said second gas stream part is expanded, and then is used to cool the first part of the gas stream before rejoining the gas stream so as to be compressed again in the second compression unit (11, 12, 13).

30. The method of claim 29, characterized in that the gas arising from the evaporation is compressed without prior heat exchange with another gas pipe.

31. The method of claim 29, characterized in that the unliquefied gas exiting the first expansion means (30) is conducted by a recycling line (35) upstream of the first compression unit (3).

32. The method of claim 31, characterized in that the unliquefied gas exiting the first expansion means (30) passes through the heat exchanger (17) before being compressed again in the first compression unit (3).

Description

[0037] Details and advantages of the present invention will became more clearly apparent from the description which follows, given with reference to the appended schematic drawing in which:

[0038] FIGS. 1 to 5 are each a schematic view of a cryogenic liquid tank associated with a system for recovering the gas evaporating from said tank for on the one hand the supply of at least one engine and on the other hand the reliquefaction of the gas not consumed by said engine(s).

[0039] In each of the appended figures, a tank 1 is illustrated. Throughout the subsequent description, it will be assumed that this is a Liquefied Natural Gas (or LNG) tank from among several other similar tanks on board a seagoing ship of methane carrier type.

[0040] The numerical values in the description which follows are given by way of purely illustrative and wholly non-limiting numerical examples. They are adapted to the treatment of LNG on board a ship but can vary, in particular if the nature of the gas changes.

[0041] The tank 1 stores the LNG at a temperature of the order of 163 C. which corresponds to the customary storage temperature of LNG at a pressure close to atmospheric pressure. This temperature depends of course on the composition of the natural gas and the storage conditions. The atmosphere around the tank 1 being at a much higher temperature than that of the LNG, although the tank 1 is very well insulated thermally, heat is added to the liquid which warms up and vaporizes. The volume of the evaporating gas being much more significant than that of the corresponding liquid, the pressure in the tank 1 therefore tends to increase as time passes and as heat is added to the liquid.

[0042] To avoid reaching overly significant pressures, the gas which evaporates is withdrawn as and when from the tank 1 (and from the other tanks of the ship) and is collected from several tanks toward a main pipe 2.

[0043] In the systems illustrated in the drawing, provision is made to use the gas which has evaporated to supply at least one engine (not represented) on board the ship and to reliquefy the surplus gas. The aim here is to avoid losing the evaporated gas and therefore either to use it for the propulsion of the ship, or to recover it and return it, in the liquid phase, to the tank 1.

[0044] To be used in an engine of the ship, the gas must firstly be compressed. This compression is then carried out within a first compression unit 3 which can be, as illustrated in the drawing, multi-staged. This unit, by way of illustrative and wholly non-limiting numerical example, takes the pressure of the gas collected in the main pipe 2 from a pressure substantially equal to atmospheric pressure to a pressure of the order of 15 to 20 bar (1 bar=10.sup.5 Pa).

[0045] After this first compression step, the gas passes into an intermediate cooler 4 in which it is cooled without appreciably modifying its pressure. The gas which has been reheated during its compression is at a temperature of the order of 40 to 45 C. on exiting the intermediate cooler (these values are given purely by way of illustration).

[0046] The gas thus compressed and cooled can then be sent through an injection pipe 5 to an engine on board the ship. This may be an engine for the propulsion of the ship or for other uses (auxiliary generator, etc.). The main pipe 2 and the injection pipe 5 form a line for supplying the engine with gas evaporated from the tanks 1.

[0047] The gas requirements at the level of the engine(s) of the ship are often lower than the production of gas by evaporation in all the tanks which are on board the ship. The gas not used in the engine(s) is then sent to a reliquefaction unit comprising in particular a mechanical cooling unit 10.

[0048] The cooling unit 10 comprises at its inlet a valve 6 intended in particular to control the pressure of the gas in the injection pipe 5, and then a main circuit and a loop, both of which will be described hereinafter.

[0049] The main circuit makes it possible on the basis of the gas (which is at a pressure of the order of a few bar to about 50 bar-non-limiting values-) to obtain gas at a temperature such that it passes into the liquid phase before returning to the tank 1.

[0050] The main circuit of the cooling unit 10 comprises firstly a multi-staged compressor, here comprising 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 one and the same transmission 15 with shafts and gears. The line between the compression stages in the figures symbolizes the mechanical link between them.

[0051] After this second compression (the gas bypassing the supply line having already been compressed in the first compression unit 3), the gas passes into an intermediate cooler 16. Its pressure is then a few tens of bars, for example about 50 bar, and its temperature is again of the order of 40 to 45 C.

[0052] The gas thus compressed is then cooled within a multi-stream exchanger 17. The gas flows in this exchanger 17 in a first direction. The fluids flowing in the opposite direction (with respect to this first direction) and used to cool it will be described later.

[0053] On exiting the exchanger 17, the compressed gas cooled to a temperature of the order of 110 to 120 C. becomes liquid and is sent, still at a pressure of the order of a few tens of bars (for example about 50 bar) through an insulated pipe 22 to expansion means. In the illustrated embodiment corresponding to a preferred embodiment, an expansion valve 30 is used to further cool the reliquefied gas and to lower its pressure.

[0054] After expansion through the expansion valve 30, a methane-rich liquid and a nitrogen-rich gas (since natural gas is not composed solely of methane) are obtained at the same time. The separation of this liquid phase and of this gaseous phase is carried out within a balloon 40 in which the pressure is of the order of a few bar, for example between 3 and 5 bar.

[0055] The gas of the balloon 40 is preferably returned to the main pipe 2. In this way, it is mixed with the primary stream and will thus be partially used as fuel in the engine(s), or will pass back into the cooling unit 10. The gas originating from the balloon 40 being cold, it can be used to cool the compressed gas in the exchanger 17. Provision is therefore made to make it flow in the opposite direction in this exchanger 17 before making it return to the main pipe 2 through a linking pipe 35.

[0056] If, for diverse reasons, in particular during transient phases, the gas of the balloon 40 cannot be recycled to the main pipe 2, provision is made to send it to a flare stack or a combustion unit. A set of valves 31, 32 controls the sending of the gas from the balloon 40 to the main pipe 2 through the linking pipe 35 or to a combustion unit.

[0057] The liquid recovered at the bottom of the balloon 40 is for its part intended to return to the tank 1. Depending on the operating conditions, the liquid may be sent directly to the tank 1 (passage controlled by a valve 33), or with the aid of a pump 41 (passage controlled by a valve 34).

[0058] The return of the liquid originating from the balloon 40, directly or via the pump 41, to the tank 1 is done by way of an insulated pipe 36.

[0059] In the cooling unit 10, as mentioned above, is also situated a loop. This loop begins with a bypass pipe 18 which separates the gas stream downstream of the multi-staged compressor 11, 12, 13 into a first stream, or main stream, which corresponds to the main circuit described previously, and into a second stream, or bypass stream.

[0060] The bypass pipe 18 is preferably linked to the main circuit at the level of the exchanger 17. The gas which therefore enters the bypass pipe 18 is at high pressure (about 50 bar in the numerical example given) and at an intermediate temperature of between 40 C. and 110 C.

[0061] The gas bled off by the bypass pipe 18 is expanded within expansion means formed in the preferred embodiment retained in the drawing by an expansion turbine 14. The latter is, in the preferred embodiment illustrated in the drawing, mechanically linked to the three compression wheels corresponding to the stages 11, 12 and 13 of the multi-staged compressor of the cooling unit 10. The transmission 15 by shafts and gears links the expansion turbine 14 and the compression wheels of the multi-staged compressor. This transmission 15 is symbolized by a line linking in the figures the expansion turbine 14 to the stages 11, 12 and 13.

[0062] The gas is expanded for example to a pressure level which corresponds to its pressure level on entering the cooling unit 10, i.e. about 15 to 20 bar. Its temperature drops below 120 C. This gas stream is then sent into the exchanger 17 in the opposite direction so as to cool the gas of the main circuit, firstly the portion 19 situated downstream of the bypass pipe 18 and then the portion upstream of this bypass pipe 18. On exiting the exchanger 17, the gas regains temperatures of the order of 40 C. and can be reinjected into the main circuit of the cooling unit, upstream of the multi-staged compressor through a return pipe 21.

[0063] An open cooling loop is thus made, which uses as gas for the cooling the same gas as that which must be liquefied.

[0064] In the variant embodiment of FIG. 2, with respect to the embodiment of FIG. 1, provision is made to preserve the gas exiting from the balloon 40 into the cooling unit 10 by injecting it into the return pipe 21 through a linking pipe 35b rather than sending it to the collector 2. This embodiment is to be envisaged in particular in cases where the first compression unit 3 does not have the capacity to treat the nitrogen-rich gas originating from the balloon 40.

[0065] This variant embodiment of FIG. 2 can be combined with one or with several of the variants which will be described hereinafter with reference to FIGS. 3 to 5.

[0066] In FIG. 3, provision is made to modify the configuration of the system downstream of the expansion turbine 14 and of the exchanger 17. Instead of sending the expanded gas exiting the exchanger 17 to the inlet of the first stage 11 of the multi-staged compressor of the cooling unit 10, it is proposed here to recycle this gas stream either directly into the main pipe 2, or to make it enter, at an intermediate level, the first compression unit 3. Valves 23 and 24 make it possible to control the flowrate of gas which on exiting the exchanger 17 is sent either to the main pipe 2, or into the first compression unit 3.

[0067] By virtue of this configuration, it is possible to obtain a greater ratio of pressures at the level of the expansion turbine 14 than that of the multi-staged compressor of the cooling unit 10.

[0068] FIG. 4 illustrates the fact that the proposed system makes it possible to supply various types of engines. It is possible with the first compression unit 3 to provide various pressure levels so as to suit various types of engines. If for example the pressure in the injection pipe 5 is very high, for example greater than 250 bar, in order to supply a high-pressure gas injection engine, then it is also possible to supply the cooling unit 10 from an intermediate stage of the first compression unit 3 rather than from the injection pipe 5.

[0069] Finally, FIG. 5 illustrates means that can be implemented to facilitate the chilling of the cooling unit 10 and therefore its starting. The embodiment presented in FIG. 5 allows such starting without influencing the flowrate of gas in the injection pipe 5 supplying an engine or the like. Provision may be made for example for the valve 6 to be closed when chilling the cooling unit 10.

[0070] FIG. 5 thus makes provision to supply the loop with gas directly from the tank 1. For this purpose, a pump 60 makes it possible to bleed off some liquid from the tank 1 so as to feed it to an injection system 62 via an infeed duct 61. Within the injection system 62, a vaporizer 63 enables the liquid bled off from the tank 1 to be made to pass into the gaseous phase. A valve 64 is thereafter provided to regulate the injection of the gas obtained at the vaporizer outlet and to control the quantity of gas injected into the loop and thereby to regulate the chilling of the cooling unit 10. FIG. 5 provides for injection at the level of the return pipe 21 but another injection point could be chosen.

[0071] Provision may also be made, were it necessary, to bleed off the Liquefied Natural Gas (presence of an arrow) on the infeed pipe 61.

[0072] The system proposed here thus provides an open loop of refrigerant gas corresponding to the refrigerated gas with production of cold at two different temperatures, a temperature of about 120 C. on exiting the expansion turbine and a temperature of about 160 C. on exiting the expansion valve. The system is independent of the engines situated on board the ship which are supplied by the evaporated gas. It allows liquefaction to be carried out solely on the basis of the evaporated gas, independently of any other exterior source of cold.

[0073] In the loop, the production of cold is permanently adapted to the load at the level of the reliquefaction means and can be regulated over a wide span by acting on the second compression unit. It is thus possible to adapt the production of cold required for the reliquefaction and to carry out the energy balancing of the system.

[0074] Under steady state conditions, no gas discharge, or gas combustion, is to be envisaged.

[0075] During its starting, the chilling within the cooling loop can be managed as with a closed loop. The cooling unit does not have any influence on the first compression unit which is also used to supply the engines (or other generators). When the loop is cold, it can remain idle and be used in open loop as soon as an excess of evaporated gas has to be liquefied.

[0076] The proposed system makes it possible to limit the head losses of the gas evaporating from the tank(s). This gas is collected and sent directly to the inlet of the first compression unit. The head loss is that which is unavoidable, created by the infeed of the gas through the main pipe. It is limited and makes it possible to avoid having in all the operating conditions of the system an inlet of the first compression unit which is depressurized.

[0077] It is clear furthermore that the proposed system does not require any nitrogen treatment unit or similar. Its structure is simplified through the use of a refrigerating gas of the same nature as the gas to be refrigerated and liquefied.

[0078] Of course, the present invention is not limited to the embodiments of the systems and methods described hereinabove by way of non-limiting examples but it also relates to all the variant embodiments within the capabilities of the person skilled in the art within the framework of the claims hereinafter.