FR3040773A1 - SYSTEM AND METHOD FOR THE TREATMENT OF GAS FROM THE EVAPORATION OF A CRYOGENIC LIQUID - Google Patents

Abstract

The system for treating a gas resulting from the evaporation of a proposed cryogenic liquid comprises: a circuit on which are successively compression means (11, 12, 13) of said gas, a heat exchanger (17) and first expansion means (30), and downstream of the compression means (11, 12, 13) a branch towards a loop comprising second expansion means (14), the loop joining the circuit upstream of the means of compression (11, 12, 13) after having passed through the heat exchanger (17) against the non-loop circuit gas fraction.

Description

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

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 processes that will be proposed later could also find applications in terrestrial installations.

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

It is not possible to keep the evaporated gas in the tanks of the ship for safety reasons. The pressure in the tanks would increase dangerously. So let the evaporating gas escape from the tanks. The regulations prohibit the discharge of this gas (if it is natural gas) into the atmosphere as it is. It must be burned.

To avoid losing this evaporating gas, it is also known, on the one hand, to use it as a fuel for the engines on board the ship carrying it and, on the other hand, to reliquefy it to put it back in. the reservoirs from which it comes. The object of the present invention relates more particularly to the reliquefaction of gas which has evaporated in cryogenic tanks or tanks of liquid, and more particularly in tanks or tanks of LNG carriers.

To reliquefier the gas that has evaporated, it is known to cool this gas to bring it back into conditions of temperature and pressure allowing it to return to the liquid phase. This supply of cold is most often achieved by heat exchange with a refrigerant circuit comprising for example a refrigerant fluid loop such as nitrogen.

Thus, EP 1 120 615 discloses an apparatus for use on ships for recompressing pressurized steam. The recompression is performed in a closed cycle in which a working fluid is compressed in at least one compressor, is cooled in a first heat exchanger, is expanded in a turbine and is heated in a second heat exchanger, in which the compressed steam is at least partially condensed. The apparatus comprises a first subassembly comprising the second heat exchanger and a second subassembly including the first heat exchanger, the compressor and the expansion turbine. The two subsets are placed on two platforms respectively.

In WO 2014/095877, natural gas evaporating from liquefied natural gas storage tanks, typically located aboard an ocean going vessel, is compressed in a multi-stage compressor comprising compression stages . At least a portion of the stream of compressed natural gas is sent to a liquefier, typically operating in a Brayton cycle, to be reliqued. The temperature of the compressed natural gas from the final stage is reduced to less than 0 ° C by passing through a heat exchanger. The first compression stage operates as a low temperature compressor, and the resulting cold compressed natural gas is employed in the heat exchanger to effect the necessary cooling of the stream from the compression stage. Downstream of its passage through the heat exchanger, the cold compressed natural gas flows through the remaining stages of the compressor. If desired, a portion of the compressed natural gas can be used as fuel and feed the engines of the ocean-going vessel.

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

The object of the present invention is therefore to provide a simplified system for reliquishing gas which has evaporated. Advantageously, the proposed system can also be used on board a ship such as a LNG carrier. For this purpose, the present invention proposes a system for treating a gas resulting from the evaporation of a cryogenic liquid, said system comprising a circuit on which are successively compression means of said gas, a heat exchanger and first means of relaxation.

According to the present invention, the circuit comprises, downstream of the compression means, a branch towards a loop comprising second expansion means, and the loop joins the circuit upstream of the compression means after passing through the heat exchanger in the opposite direction to to the fraction of gas of the circuit not derived by the loop.

Thus, it is proposed to provide an open loop in which evaporated gas can circulate and which serves as a cooling loop. The operation of this cooling loop is independent of other surrounding systems and can thus function almost as a closed loop of another refrigerant. The expansion means make it possible to rapidly pass the fluid from a high pressure to a lower pressure and it may be each time an expansion turbine, or an expansion valve, or an orifice or any other equivalent system.

In such a treatment system, the bypass is preferably carried out within the heat exchanger such that the derived gas flow is already partially cooled to enter the second expansion means thereafter.

In one embodiment of such a treatment system, the expansion means comprise for example an expansion valve opening into a balloon intended to receive, on the one hand, the liquid formed by a gas expansion in the expansion valve and, on the other hand, a non-liquefied gas fraction. The balloon makes it possible to separate the gas and the liquid and makes it possible to treat the gas and the liquid differently downstream. In such an embodiment, it is proposed that the upper part of the balloon is connected to the heat exchanger so that the gas coming from the balloon enters the exchanger on the same side as the bypass, and that the lower part of the balloon is connected to a cryogenic liquid tank.

A particularly advantageous embodiment of the processing system provides that the compression means comprise several compression stages each with a compression wheel, that the second expansion means comprise an expansion turbine, and that each compression wheel and the turbine of compression are associated with the same mechanical transmission. This embodiment makes it possible to have a compact structure. In addition, the work recovered at the level of the expansion turbine can immediately be transmitted to the compression wheels thus promoting the achievement of energy efficiency for the system.

To facilitate the start of the treatment system described above, this system may further comprise means for injecting gas into the loop derived from the circuit. In this way, the cooling loop becomes truly autonomous and can be regulated as if it were a closed loop. The means for injecting gas into the loop derived from the circuit comprise, for example, a pump for cryogenic liquid, a vaporizer and a control valve.

The present invention also relates to: a system for recovering evaporated gases from a set of cryogenic liquid tanks, comprising an evaporated gas collector, a first compression unit and, at the outlet of the first compression unit, a on the other hand, a pipe conveying the compressed gas to an engine for supplying fuel and, on the other hand, a treatment system as described above, and a ship, in particular a tanker, equipped with such a fueling system. recovery of evaporated gases.

Finally, the invention proposes a method for treating a flow of gas resulting from the evaporation of a cryogenic liquid, said gas flow being first compressed and then cooled in a heat exchanger before to be expanded so that a fraction of gas is relubricated, characterized in that after its compression, the gas flow is separated into a first gas flow portion and a second gas flow portion, in that the first portion of the gas stream is cooled and then expanded to be at least partially liquefied, and in that the second portion of the gas flow is fed into a loop in which said second portion of gas flow is expanded, and is then used to cool the first part of the gas flow before joining the gas flow to be compressed again.

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

Figures 1 to 6 are each a schematic view of a cryogenic liquid tank associated with a gas recovery system evaporating from said tank and a treatment system of a portion of the recovered gas to liquefy it.

In each of the accompanying figures, a reservoir 1 is illustrated. Throughout the remainder of the description, it will be assumed that it is a tank of liquefied natural gas (or LNG) among several other similar tanks aboard an ocean-going vessel of the LNG type.

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

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

To avoid reaching too great pressures, the evaporating gas is removed as the tank 1 (and the other tanks of the ship) and is found in a manifold 2 connected to several tanks.

It is intended in the systems illustrated in the drawing to use the gas that has evaporated to supply at least one engine on board the ship and to reliquefy the surplus gas. The aim here is to avoid losing the evaporated gas and thus either to use it for the propulsion of the ship, or to recover it and send it back, in the liquid phase, into the tank 1.

To be used in a ship engine, the gas must be compressed first. This compression is then performed within a first compression unit 3 which can be, as illustrated in the drawing, multi-staged. This unit, as an illustrative numerical example and in no way limiting, carries the pressure of the gas collected in the collector 2 by a pressure substantially equal to the atmospheric pressure at a pressure of the order of 15 to 20 bar (1 bar = 105 Pa).

After this first compression step, the gas passes into an intercooler 4 in which it is cooled without substantially modifying its pressure. The gas which has been heated 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 for illustrative purposes only).

The gas thus compressed and cooled can then be sent via line 5 to an engine on board the ship. It can be a motor for the propulsion of the ship or for other uses (auxiliary generator, ...).

The gas requirements at the engine (s) of the ship are often lower than the "production" of evaporative gas in all the tanks on board the ship. The unused gas in the engine (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 (which is at a pressure of the order of a few bar to about 50 bar -non-limiting values-) to obtain liquid that can return to the tank 1.

The process for obtaining this liquid to be returned to the reservoir is conventional. It involves compressing the gas, cooling it and then relaxing it. This way of doing things is classic in the field of cryogenics.

Thus, in the main circuit, there is first of all a multi-stage compressor here comprising three successive stages with 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 to trees and gears. The line between the compression stages in the figures symbolizes the mechanical connection between them.

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

The gas thus compressed is then cooled in a multiflux exchanger 17. The gas circulates in this exchanger 17 in a first direction. Fluids circulating in the opposite direction (with respect to this first direction) and used to cool it will be described later.

At the outlet of the exchanger 17, the compressed gas cooled to a temperature of the order of -110 to -120 ° C is sent, always at a pressure of the order of a few tens of bar (for example about 50 bar) by an insulated pipe 22 to an expansion valve 30.

The expansion through the gas expansion valve 30 provides both a methane-rich liquid and a nitrogen-rich gas. 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.

The gas from the balloon 40 is preferably returned to the manifold 2. In this way, it can be used either as a fuel in an engine, or back in the reliquefaction unit 10. This gas being cold, it can be used to cool the compressed gas in the exchanger 17. It is therefore planned to make it circulate in the opposite direction in this exchanger 17 before returning it to the collector 2.

If the gas of the balloon 40 for various reasons, especially during transient phases, can not be recycled to the collector 2, it is intended to send it to a flare or a combustion unit. A set of valves 31, 32 controls the delivery of gas from the balloon 40 to the manifold 2 via a connecting line 35 or to a combustion unit.

The liquid recovered at the bottom of the flask 40 is intended to return to the tank 1. Depending on the operating conditions, the liquid can be sent directly into the tank 1 (passage controlled by a valve 33), either at the using a pump 41 (passage controlled by a valve 34).

The return of the liquid from the balloon 40, directly or through the pump 41, to the tank 1 is via an insulated pipe 36.

In the reliquefaction unit 10, as mentioned above, there is also a loop. This loop begins with a branch line 18 which separates the gas flow downstream of the multi-stage compressor 11, 12, 13 into a first flow, or main flow, which corresponds to the main circuit described above, and into a second flow, or derived flow.

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

The gas taken by the bypass line 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 the stages 11, 12 and 13 of the multi-stage compressor of the reliquefaction unit 10. The transmission 15 by shafts and gears connects the expansion turbine 14 and the compression wheels of the multi-stage compressor. This transmission 15 is symbolized by a line connecting in the figures the expansion turbine 14 to stages 11, 12 and 13.

The gas is expanded, for example, to a pressure level which corresponds to its pressure level by entering the reliquefaction unit 10, ie approximately 15 to 20 bar. Its temperature drops below -120 ° C.

This flow of gas is then sent into the exchanger 17 in the opposite direction to cool the gas of the main circuit, firstly the portion 19 located downstream of the branch line 18 and then the upstream portion of this branch line 18. At the outlet of exchanger 17, the gas regains temperatures of the order of 40 ° C. and can be reinjected into the main circuit of the reliquefaction unit, upstream of the multi-stage compressor via a return line. .

This produces an open cooling loop which uses as gas for cooling the same gas that must be liquefied.

FIG. 2 illustrates an alternative embodiment of the system of FIG. 1. There is provided here a recovery system 50 intended to recover from the cold of the gas collected in the collector 2 to participate in the cooling in the exchanger 17. A control valve 51 the gas passage of the manifold 2 in the exchanger 17 and another valve 52 is provided to allow the gas manifold 2 can be sent directly to the first compression unit 3 via a portion 2a of collector without passing through the exchanger 17. It is possible thanks to the recovery system 50 to limit the power of the reliquefaction unit 10.

In the embodiment of Figure 3, with respect to the embodiment of Figure 1, it is expected to keep the gas exiting the balloon 40 in the reliquefaction unit 10 by injecting into the return line 21 by a connecting pipe 35b rather than sending it to the manifold 2. This embodiment is to be considered in particular in cases where the first compression unit 3 does not have the capacity to treat the nitrogen-rich gas from of the ball 40.

It is of course possible to combine the recovery system 50 with the direct return of the gas from the flask into the reliquefaction unit 10 (combination of the variants of FIGS. 2 and 3). Likewise, these variant embodiments of FIGS. 2 and 3 may each be combined with the variants which will be described below with reference to FIGS. 4 to 6.

In FIG. 4, it is planned to modify the configuration of the system downstream of the expansion turbine 14 and exchanger 17. Instead of sending the expanded gas at the outlet of the exchanger to the inlet of the first stage 11 of the multistage compressor of the reliquefaction unit 10, it is proposed here to recycle this flow of gas either directly into the manifold 2, or to enter it at an intermediate level in the first compression unit 3. Valves 23 and 24 make it possible to control the flow of gas which, at the outlet of the exchanger 17, is sent either to the collector 2 or to the first compression unit 3.

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

Figure 5 illustrates the fact that the proposed system allows to feed different types of engines. It is possible with the first compression unit 3 to provide different pressure levels to suit different types of engines. If for example the pressure in the pipe 5 is very high, for example greater than 250 bar, to feed a high-pressure gas injection engine, then it is also possible to feed the reliquefaction unit 10 not from the conduit 5 but from an intermediate stage of the first compression unit 3.

Finally, FIG. 6 illustrates means that can be implemented to facilitate the cooling of the reliquefaction unit 10 and thus its startup. The embodiment shown in FIG. 6 allows such a start without influencing the flow of gas in the pipe 5 supplying an engine or the like. For example, it can be provided that during the cold setting of the reliquefaction unit 10, the valve 6 is closed.

FIG. 6 thus provides for supplying the gas loop directly from the tank 1. For this purpose, a pump 60 makes it possible to take liquid from the tank 1 to bring it to an injection system 62 via an air duct. In the injection system 62, a vaporizer 63 makes it possible to pass the liquid taken from the tank 1 in the gas phase. A valve 64 is then provided to regulate the injection of the gas obtained at the outlet of the vaporizer and to control the quantity of gas injected into the loop and in this way to regulate the cold setting of the reliquefaction unit 10. FIG. 6 provides an injection at the return line 21 but another injection point could be chosen.

It may also be provided, if necessary, to take Liquefied Natural Gas (presence of an arrow) on the supply line 61.

The system proposed here thus provides an open loop of refrigerant gas corresponding to the refrigerated gas with a production of cold at two different temperatures, a temperature of about -120 ° C. at the outlet of the expansion turbine and a temperature of about -160 ° C. ° C at the outlet of the expansion valve. The system is independent of the engines on board the ship that are powered by the evaporated gas. Only from the evaporated gas, it allows, independently of any other source of external cold, to achieve liquefaction.

In the loop, the cold production is permanently adapted to the charge of the reliquefaction unit and can be regulated over a wide range.

In steady state, no gas discharge, or gas combustion, is expected.

When it is started, the cooling in the cooling loop in the reliquefaction unit can be managed as with a closed loop. The reliquefaction unit has no influence on the compression unit which is also used to power the engines (or other generators). When the loop is cold, it can remain in "standby" and be used in open loop as soon as an excess of evaporated gas has to be liquefied.

It is clear that 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 liquefied.

Of course, the present invention is not limited to the embodiments of the systems and methods described above by way of non-limiting examples but it also relates to all the variants within the scope of the skilled person within the scope of the present invention. of the claims below.

Claims (10)

1. A system for treating a gas resulting from the evaporation of a cryogenic liquid, said system comprising a circuit on which are successively compression means (11, 12, 13) of said gas, a heat exchanger (17 ) and first expansion means (30), characterized in that the circuit comprises, downstream of the compression means (11, 12, 13), a branch towards a loop comprising 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 heat exchanger (17) against the non-loop circuit gas fraction. 2. Treatment system according to claim 1, characterized in that the bypass is carried out within the heat exchanger (17). 3. Treatment system according to one of claims 1 or 2, characterized in that the first expansion means comprise an expansion valve (30) opening into a balloon (40) for receiving, on the one hand, the liquid formed by a gas expansion in the expansion valve (30) and, on the other hand, a non-liquefied gas fraction. 4. Treatment system according to claim 3, characterized in that the upper part of the balloon (40) is connected to the heat exchanger (17) so that the gas coming from the balloon (40) enters the chamber. exchanger (17) on the same side as the bypass, and in that the lower part of the balloon (40) is connected to a reservoir (1) of cryogenic liquid. 5. Treatment system according to one of claims 1 to 4, characterized in that the compression means comprise a plurality of 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 the same mechanical transmission (15). 6. Treatment system according to one of claims 1 to 5, characterized in that it further comprises means (62) for injecting gas into the loop derived from the circuit. 7. Processing system according to claim 6, characterized in that the means (62) for injecting gas into the loop derived from the circuit comprise a pump (60) for cryogenic liquid, a vaporizer (63) and a valve (64). ) control. 8. System for recovering evaporated gases from a set of tanks (1) of cryogenic liquid, comprising an evaporated gas collector (2), a first compression unit (3) and, at the outlet of the first compression unit, on the one hand, a pipe (5) bringing the compressed gas to an engine for supplying fuel and, on the other hand, a treatment system (10) according to one of claims 1 to 7. 9. Ship, in particular LNG carrier, characterized in that it comprises a system for recovering evaporated gases according to claim 8. 10. A method of treating a flow of gas resulting from the evaporation of a cryogenic liquid, said gas flow being first compressed and then cooled in a heat exchanger (17) before being relaxed such that a gas fraction is relubricated, characterized in that after its compression, the gas flow is separated into a first gas flow portion and a second gas flow portion, in that the first portion the gas stream is cooled and then expanded to be at least partially liquefied, and in that the second portion of the gas stream is fed into a loop in which said second gas flow portion is expanded, and is then used to cool the first part of the gas flow before joining the gas stream to be compressed again.