FR3063540A1 - METHOD FOR LIQUEFACTING NATURAL GAS USING A REFRIGERATION CIRCUIT COMPRISING ONLY ONE TURBINE - Google Patents

Abstract

A process for liquefying a stream of natural gas from a feed stream (1) comprising the following steps: Step a): introducing the feed stream (1) into a pre-treatment unit (2) for the implementation of an adsorption process using a regeneration current (25); Step b): introducing the pre-treated stream (3) into a main heat exchanger (4); Step e): Introduction of the gas stream (28) from step b) into the main heat exchanger (4); Step f): Introduction of the stream (25) from step e) into the pretreatment unit (2) to serve as all or part of the regeneration stream (25) implemented in step a).

Description

Holder (s): AIR LIQUIDE, ANONYMOUS COMPANY FOR THE STUDY AND EXPLOITATION OF GEORGES CLAUDE PROCESSES Société anonyme.

Extension request (s)

Agent (s): AIR LIQUIDE.

PROCESS FOR LIQUEFACTION OF NATURAL GAS USING A REFRIGERATION CIRCUIT COMPRISING ONLY ONE TURBINE.

FR 3 063 540 - A1

Method for liquefying a stream of natural gas from a feed stream (1) comprising the following steps:

Step a): introduction of the supply current (1) into a pretreatment unit (2) for the implementation of an adsorption process using a regeneration current (25);

Step b): introduction of the pretreated current (3) into a main heat exchanger (4);

Step e): Introduction of the gas flow (28) from step b) into the main heat exchanger (4);

Step f): Introduction of the current (25) from step e) into the pretreatment unit (2) in order to serve all or part of the regeneration current (25) used in step a).

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The present invention relates to a process for liquefying a stream of hydrocarbons such as natural gas, in particular in a process for producing liquefied natural gas.

It is desirable to liquefy natural gas for a number of reasons. For example, natural gas can be stored and transported over long distances more easily in the liquid state than in gaseous form, because it occupies a smaller volume for a given mass and does not need to be stored at high pressure.

Several methods of liquefying a stream of natural gas to obtain liquefied natural gas (LNG) are known.

The efficiency and complexity of liquefaction cycles vary and either method can be used depending on the capacity, flexibility and cost envisaged for the liquefaction unit.

Typically, there are two main classes of process:

The processes where the cold necessary for the liquefaction cycle is mainly generated by the expansion of the refrigerant current in one or more turbines. The gas stream (nitrogen, methane or a nitrogen / methane mixture for example) is compressed within a set of compression machines (compressor and / or turbo expander) and is introduced into a main heat exchanger. The sub-cooled current is drawn off at one or more temperature levels. It is then introduced into one or more turbines (possibly coupled to all of the compression machines) and then returned to the main exchanger to provide all or part of the cold necessary for the liquefaction of the natural gas stream. The various arrangements possible on the basis of this principle have given rise to multiple variants which are defined here under the term of "turbine cycle".

Processes where the cold necessary for the liquefaction cycle is mainly generated by isenthalpic expansion resulting in the condensation and at least partial liquefaction of a mixture of compounds (for example nitrogen, methane, ethane, ethylene, propane, butane, pentane). Typically, a stream of refrigerants is compressed using a compressor. The stream, optionally separated into a gas phase and one or more liquid phase (s), is then introduced into a main heat exchanger where it is cooled. This (s) stream (s) are then drawn off at the cold end of the exchanger (and possibly at intermediate levels), in at least partially liquid form in order to be expanded and returned to the main exchanger. The different arrangements possible on the basis of this principle have given rise to multiple variants (including one or two refrigeration cycles) which are defined here under the term of “mixed refrigerant cycle”.

Conventionally, natural gas liquefaction trains can be divided into two distinct units:

1. A pretreatment unit which removes impurities liable to freeze from natural gas (H ₂ O, CO ₂ , sulfur derivatives, etc.)

2. The liquefaction unit which cools natural gas to a cryogenic temperature (typically -160 ° C) thanks to a refrigerant cycle and possibly subtracts heavy hydrocarbons / aromatic derivatives liable to freeze.

In addition, it is not uncommon to use natural gas as a fuel to produce the energy necessary for the operation of the liquefaction train by gas turbines / engines generating electricity or directly driving the cycle engines.

In the case of small liquefiers (typically less than 250,000 tonnes of liquefied natural gas produced per year), a possible arrangement for the natural gas liquefaction train is:

1. a pretreatment provided by adsorption where the natural gas stream is purified by a regenerative adsorbent system.

2. One liquefaction cycle per turbine cycle, typically a nitrogen cycle.

The two-turbine nitrogen cycle is a technology commonly used in industry. Document US 20140245780 describes an example.

It is necessary to remove impurities upstream of the natural gas liquefier (CO ₂ , H ₂ O in particular). An alternative to the washing treatment with amines may be adsorption by pressure and / or temperature inversion.

This separation process exploits the fact that under certain pressure and temperature conditions certain constituents of the gas (CO ₂ , H ₂ O in particular) have particular affinities with respect to a solid material, the adsorbent ( molecular sieves for example).

Adsorption is a reversible process and it is possible to regenerate the adsorbent by lowering the pressure and / or raising the temperature of the adsorbent to release the constituents of the adsorbed gas.

Thus in practice a system of separation by adsorption consists of several (minimum 2 but possibilities of going up to 5) "bottles" containing one or more layers of adsorbents as well as devices dedicated to the heating / cooling of the flow of adsorption and / or regeneration.

Compared to conventional amine washing, pre-treatment has a number of advantages:

its cost, its simplicity of operation, the possibility of avoiding a certain number of utilities (topping up with amine or demineralized water).

These advantages are particularly important for small size and / or geographically isolated natural gas liquefaction units.

In a regenerative adsorption gas treatment system consisting of at least two bottles of adsorbent, the stream of natural gas enters the pretreatment unit at room temperature and a pressure between 15 and 70 bars. On contact with the adsorbent, certain impurities (typically CO ₂ , H2O, sulfur derivatives or mercury) are stopped and the dry gas can then be sent to the liquefaction unit.

Impurities therefore accumulate in the adsorption system. It is necessary to regularly regenerate the adsorbent to release the impurities that have accumulated on its surface. This regeneration phase is ensured by the passage of a dry gas (typically the stream of dry natural gas pitted at the outlet of the pretreatment unit) but typically at a pressure lower than the pressure of the adsorption stream (typically 1 to 40 Bars) and / or high temperature (> 150 ° C).

The pre-treatment process is therefore cyclical, the stream of natural gas flipping from one bottle to another. This regeneration gas is then sent to the fuel system.

The inventors of the present invention then developed a solution allowing this gas pretreatment process to be implemented by regenerative adsorption while optimizing the refrigeration cycle necessary for the liquefaction of natural gas.

The solution object of the present invention is to take advantage of the expansion of the regeneration gas to simplify the turbine cycle by using the regeneration gas of the pretreatment to bring a supply of cold to the liquefaction train of natural gas and thus simplify the cycle.

It should be noted that there is nothing to prevent the expansion of the regeneration current from being incorporated into a mixed refrigerant cycle.

The subject of the present invention is a process for liquefying a stream of hydrocarbons such as natural gas from a feed stream comprising at least the following steps:

Step a): introduction of the feed stream into a pretreatment unit in order to adsorb, by an adsorption process using a regeneration stream, certain products liable to disturb the liquefaction (for example the components included in the stream natural feed likely to freeze during liquefaction of natural gas);

Step b): introduction at a temperature T1 of the pretreated stream from step a) into a main heat exchanger, said stream cooling by circulating against the flow of a refrigerant stream mainly comprising nitrogen and / or methane and circulating in a refrigeration cycle;

Step c): extraction of the at least partially condensed current from step b) at a temperature T2 lower than T1;

Step d): expansion of the current from step c) followed by introduction, at a temperature T3 lower than T2, into a phase separator means in order to obtain a gas flow and a liquid flow;

Step e): Introduction of the gas flow from step d) into the main heat exchanger at a level such that the temperature is close to T3 up to the level of the main heat exchanger where the temperature is equal to T1;

Step f): Introduction of the current from step e) into the pretreatment unit in order to serve all or part of the regeneration current implemented in step a).

According to other embodiments, the subject of the present invention is also:

A method as defined above, characterized in that the refrigeration cycle comprises at least one compression means and a single turbine.

A method as defined above, characterized in that said at least one compression means comprises at least one compressor with at least two compression stages driven by said turbine.

A process as defined above, characterized in that T2 is less than or equal to - 140 ° C.

A process as defined above, characterized in that the liquid stream from step d) is sent to a storage means.

A process as defined above, characterized in that T2 is between - 80 ° C and - 130 ° C.

A process as defined above, characterized in that at least part of the liquid stream from step d) is introduced into the main heat exchanger at a temperature T3 below T2 and circulates until the outlet of said heat exchanger at which temperature T4 is lowest.

A process as defined above, characterized in that T4 is less than or equal to - 140 ° C.

A process as defined above, characterized in that the part of the liquid stream which has not been introduced into the heat exchanger at temperature T3, is introduced into the heat exchanger and vaporizes in the heat exchanger heat up to temperature T1.

A process as defined above, characterized in that the part of the liquid flow which has not been introduced into the heat exchanger to be sub-cooled ie is mixed with the gas flow originating from step d) before introduction into the heat exchanger at the temperature T2 or is mixed with the flow from step e) before step f).

A method as defined above, characterized in that the refrigerant current flows in the refrigeration circuit in closed cycle.

A process as defined above, characterized in that the expansion of step d) of the natural gas stream takes place at the same pressure level as the pressure of the fuel network.

A process as defined above, characterized in that the expansion of step d) of the natural gas stream takes place at the same pressure level as the storage pressure of the liquefied natural gas.

Although the process according to the present invention is applicable to various hydrocarbon feed streams, it is particularly suitable for streams of natural gas to be liquefied. Furthermore, those skilled in the art will readily understand that, during liquefaction, the liquefied natural gas can be further treated, if desired, in particular to remove certain heavy hydrocarbons and thus avoid the risk of freezing in the main exchanger. or separately recover certain hydrocarbons more than methane (mainly ethane, propane, butane).

Indeed, other intermediate treatment steps between the gas / liquid separation and the cooling can be carried out. The stream of hydrocarbons to be liquefied is generally a stream of natural gas obtained from natural gas or petroleum reservoirs. Usually, the stream of natural gas is composed essentially of methane. Preferably, the feed stream comprises at least 60 mol% of methane, preferably at least 80 mol% of methane. Depending on the source, natural gas may contain quantities of heavier hydrocarbons than methane, such as ethane, propane, butane, pentane and hexane as well as certain aromatic hydrocarbons or heavier hydrocarbons . The natural gas stream can also contain non-hydrocarbon products such as H ₂ O, N ₂ , CO ₂ , H ₂ S and other sulfur compounds, and others which would not have been removed upstream of liquefaction.

The liquefied natural gas obtained can then be depressurized by means of a Joule-Thomson valve or by means of a turbine.

The expression natural gas as used in the present application relates to any composition containing hydrocarbons including at least methane. This includes a “crude” composition (prior to any treatment such as cleaning or washing), as well as any composition that has been partially, substantially or entirely treated for the reduction and / or elimination of one or more compounds, including, but without be limited thereto, sulfur, carbon dioxide, water, and hydrocarbons having two or more carbon atoms.

The separator can be any unit, column or arrangement suitable for separating a stream into a vapor stream and a liquid stream. Such separators are known in the state of the art and are not detailed here.

The heat exchanger can be a unit or other arrangement adapted to allow the passage of a certain number of flows, and thus allow a direct or indirect heat exchange between one or more lines of refrigerant, and one or more flows feed.

The invention will be described in more detail with reference to the figures which illustrate diagrams of particular embodiments of implementation of a method according to the invention.

FIG. 1 represents a diagram illustrating a process for liquefying natural gas preceded by a pretreatment of gas by regenerative adsorption according to the state of the art for "turbined" cycle with two turbines, conventionally used in the industry.

FIG. 2 represents a diagram illustrating a first embodiment of the method which is the subject of the present invention.

FIG. 3 represents a diagram illustrating another embodiment of the method which is the subject of the present invention.

In the figure, a stream 1 of natural gas is introduced into a pretreatment unit 2 in which the stream 1 undergoes a separation of part of at least one of the following constituents: water, CO ₂ , methanol , sulfur compounds) in order to be purified.

The pretreatment unit 2 is typically a gas treatment system by regenerative adsorption here consisting of three bottles (A, B and C) of adsorbent, the stream of natural gas 1 enters the pretreatment unit 2 at room temperature and a pressure between 15 and 70 Bars. On contact with the adsorbent, certain impurities (typically CO ₂ , H ₂ O, sulfur derivatives or mercury) are stopped and the dry gas can then be sent to a liquefaction unit.

Impurities therefore accumulate in the adsorption system. It is necessary to regularly regenerate the adsorbent to release the impurities that have accumulated on its surface. This regeneration phase is ensured by the passage of a dry gas (typically the stream of natural gas 1 'dry stitched at the outlet 2' of the pretreatment unit 2) at a pressure lower than the pressure of stream 1 (typically 1 at 40 Bars) and / or a high temperature (> 150 ° C).

At the outlet of the pretreatment unit 2, a portion of the natural gas (current Γ) is therefore derived, expanded at low pressure, possibly reheated and then returned to the pretreatment unit 2 to regenerate one or more bottles of adsorbent. Depending on the number of bottles of adsorbent chosen for the adsorption unit, the regeneration gas can represent up to 40% of the natural gas flow entering the LNG train.

The pre-treatment process is therefore cyclical, the stream of natural gas flipping from one bottle to another. This regeneration gas 1 ’is then sent to the 1” fuel system. This regeneration gas can also be recompressed and returned to the distribution network.

At the outlet of the pretreatment unit 2, the stream of natural gas 3 thus purified is introduced into a heat exchanger 4 in order to be liquefied.

In the figure, the heat exchanger 4 is preferably a cryogenic plate heat exchanger. Cryogenic heat exchangers are known in the art, and can have various arrangements of their feed stream (s) and refrigerant streams. In addition, such heat exchangers can also have one or more lines to allow the passage of other flows, such as refrigerant streams for other stages of a cooling process, for example in liquefaction processes. These other lines or flows are not shown in the figure for simplicity.

The supply stream 3 enters the heat exchanger 4 via a supply inlet 5 and passes through the heat exchanger via the line 6, then is extracted from the exchanger at the outlet 7 to provide a flow at least partially liquefied hydrocarbon 8. This liquefied stream 8 is preferably fully liquefied and even sub-cooled. For the sake of simplicity, the possible extraction of heavy hydrocarbons is not shown. When the liquefied stream 8 is liquefied natural gas, the temperature can be from about -160 ° C to -140 ° C. The liquefaction of the supply stream 1 is carried out by means of a refrigerant circuit 9. In the refrigerant circuit 9 a refrigerant, preferably nitrogen, circulates.

The cold necessary for liquefaction is thus ensured by a nitrogen 9 cycle. A nitrogen flow 10 is compressed by means of compressors 11, 11 'and / or compressors (14' and 14 ”mechanically coupled to the turbines) typically at 60 Bar abs: the high pressure nitrogen 12 enters the main exchanger 4 and is expanded by at least two turbines 13 'and 13 ”at the optimum cold level.

Typically, the inlet temperature T ’of the first turbine is from -20 ° C to -5 ° C and that T” of the second is from -100 ° C to -60 ° C. These pressure and temperature levels can vary depending on the composition of the natural gas and the refrigeration cycle chosen.

Conventionally, the 13 ’and 13’ turbines are connected to the compression stages 14 ’and 14’ by a turbo compressor system.

It should be noted that it is possible to have other purification steps between the pretreatment unit 2 and the liquefaction unit 4. These steps do not directly concern the presence of the invention are not mentioned here.

In the operating arrangement of the heat exchanger 4 shown in FIG. 1, a gaseous refrigerant stream 12 is introduced into the exchanger 4 at an inlet 15, then it passes through this inlet and cools along the line 16 through the heat exchanger 4 ,. In its passage through line 16, part 12 ’of the gaseous refrigerant stream 12 is extracted to an outlet 18 at an intermediate temperature T’. The refrigerant stream 12 ′ is then expanded, for example using the turbine 13 ′, so as to provide a first stream of refrigerant at reduced pressure 19. This stream 19 is then introduced into the heat exchanger 4 by the inlet 20 in the exchange line 21 and flows to outlet 22 which is at the same temperature level as inlet 15.

In its passage through line 16, the remaining part of the 12 ”refrigerant is extracted from the exchanger 4 at an outlet 23 at a temperature T”. The refrigerant stream 12 ′ is then expanded, for example using the turbine 13 ″, so as to provide a second stream of refrigerant at reduced pressure 24. This stream 24 is then introduced into the heat exchanger 4 by the inlet 17 in the exchange line 21 and flows to outlet 22 which is at the same temperature level as inlet 15.

In FIG. 2 (the references used to describe FIG. 1 are reused when they illustrate the same elements), the regeneration gas 25 is withdrawn, not at the outlet of the pretreatment system 2 but at a temperature level judiciously chosen at during liquefaction in order to be able, by expansion, to bring a supply of cold to the system and to simplify the nitrogen cycle 9. This simplification takes the form of the elimination of at least one turbine.

In the case of natural gas containing approximately 1% CO ₂ , the natural gas stream 1 enters a pretreatment unit 2 consisting of three bottles of adsorbents (A, B and C). After possible additional treatment steps, the stream of dry natural gas 3 enters the main exchanger 4 of the liquefaction unit via the inlet 5 at a temperature T1. During liquefaction, heavy / aromatic hydrocarbons can optionally be removed from the natural gas stream (not described here). The natural gas stream is then withdrawn fully condensed 7 at a cold temperature T2 (typically between -120 ° C and -100 ° C), then expanded at low pressure (5 bar for example) 26. This stream of natural gas 26 , colder (typically between -140 ° C and -120 ° C) and two-phase, is sent to a phase separator jar 27. The vapor fraction 28 and part of the liquid fraction 29 (approximately 5% to 25% of the total liquid flow) are introduced into the main exchanger 4 by the inputs 30 and 31 at the temperature T2 (or by a common input), while the rest 32 of the liquid is introduced by the input 33 into the heat exchanger 4 at a temperature T3 lower than T2. The liquid 32 is sub-cooled 34 to exit the exchanger 4 at a temperature T4 between -160 ° C and -140 ° C.

The streams 28 and 29 recombined before or after the exchanger 4 are then sent to the pretreatment system 2 to ensure the regeneration stream 25 of at least one adsorption bottle, which will then be sent to the fuel system 35 to be burned. Optionally to ensure that the regeneration current flow is sufficient in all circumstances, it is possible to send a backup of current 3 to the regeneration flow.

Thanks to the cold brought by currents 28 and 29, cycle 9 with nitrogen is simplified. The nitrogen stream 10 is compressed into a stream 12 (at a pressure of 50 bar for example) by a nitrogen compressor (11 and 11 ') before entering the exchanger 4. The majority 12' of the flow cycle (from 75% to 95%) is expanded in a single turbine 13 at a cold temperature T '(between -70 ° C and -60 ° C) while the rest 12 ”is expanded at the cold end 17 of l exchanger 4. The pressure of stream 12 ”is adjusted to reach the temperature desired for stream 34.

The turbine 13 is conventionally connected to a compression stage of the nitrogen compressor, by a turbo / compressor system.

Thus the present invention has the following advantages;

1. Reduce the number of equipment in the liquefaction unit by eliminating at least one turbine, thereby saving the cost of the unit and reducing the flow of nitrogen circulating in the cycle, therefore the compressor capacity of cycle.

2. Increase in the efficiency of the refrigeration cycle because the current 29 by vaporizing in the exchanger 4 brings cold by "latent heat" more thermodynamically efficient than the cold brought by nitrogen ("sensible heat"). Typically this gain represents an efficiency gain of the order of 10% compared to a conventional nitrogen cycle with two turbines.

In Figure 3, The natural gas stream 1 enters a pretreatment unit 2 consisting of at least three bottles of adsorbents (A, B, C). After possible additional treatment steps, the stream of dry natural gas 3 enters the main exchanger 4 of the liquefaction unit. During liquefaction, heavy / aromatic hydrocarbons can optionally be removed from the natural gas stream (not described here). The natural gas stream is then withdrawn fully condensed 7 at a cold temperature T2 (typically -150 ° C to -110 ° C), then expanded 26 at low pressure (1 to 5 bar for example). This stream of natural gas 26 is sent to a separator pot 27. The vapor fraction 28 (approximately 20% to 40% of the total liquid flow) is introduced into the main exchanger 4 via the inlet 30 at temperature T2 while the liquid fraction 34 is sent to a storage means.

The stream 28 is sent to the pretreatment system 2 to supply the regeneration stream 25 of at least one adsorption bottle, then is sent to the fuel system 35 to be burned. Optionally to ensure that the regeneration current flow is sufficient in all circumstances, it is possible to send a backup of current 3 to the regeneration flow.

Thanks to the cold brought by current 28, the nitrogen cycle is simplified. The nitrogen 10 is compressed 12 (40 to 50 bars for example) by a nitrogen compressor (11, 11 ') before entering the exchanger 4. The cycle flow is expanded in a single turbine 13 at one temperature T 'cold (-100 ° C to -80 ° C).

The turbine 13 is conventionally connected to a compression stage of the nitrogen compressor, by a turbo / compressor system.

1. The invention can be generalized to any type of natural gas liquefaction cycle by considering that all the natural gas used as fuel for the production of energy must be expanded and can therefore bring a contribution of cold to the unit of liquefaction of natural gas.

2. In the example described above, it is possible to further improve the efficiency of the cycle by considering a nitrogen / methane mixture instead of considering a cycle with nitrogen only. It is also possible to consider that the refrigeration cycle is an open "natural gas" turbined cycle. In contrast to the closed cycle, the open cycle typically consists of expanding natural gas in a turbine and recompressing it to a pressure lower than its initial pressure before injecting it into a distribution duct at a lower pressure.

3. It is possible to optimize this system by sending two streams of natural gas taken from the exchanger 4 at different temperature levels and expanded at the same pressure into the separator pot 27.

4. It is also possible to optimize the quantity of regeneration gas required by carrying out several expansions of natural gas at different temperatures and pressure levels.

Claims (11)

1. Method for liquefying a stream of hydrocarbons such as natural gas from a feed stream (1) comprising at least the following steps: Step a): introduction of the feed stream (1) into a pretreatment unit (2) in order to adsorb, by an adsorption process using a regeneration stream (25), certain products liable to disturb the liquefaction ; Step b): introduction at a temperature T1 of the pretreated stream (3) from step a) into a main heat exchanger (4), said stream (3) cooling by circulating against the flow of a refrigerant stream ( 12) mainly comprising nitrogen and / or methane and circulating in a refrigeration cycle (9); Step c): extraction of the at least partially condensed current (7) from step b) at a temperature T2 lower than T1; Step d): expansion of the current from step c) followed by introduction (26), at a temperature T3 lower than T2, into a phase separator means (27) in order to obtain a gas flow (28) and a liquid stream (29, 32, 34); Step e): Introduction of the gas flow (28) from step d) into the main heat exchanger (4) at a level such that the temperature is close to T3 up to the level of the main heat exchanger where the temperature is equal to T1; Step f): Introduction of the current (25) from step e) into the pretreatment unit (2) in order to serve all or part of the regeneration current (25) used in step a). 2. Method according to the preceding claim, characterized in that the refrigeration cycle (9) comprises at least one compression means (11, 11 ’) and a single turbine (13). 3. Method according to the preceding claim, characterized in that said at least one compression means comprises at least one compressor with at least two compression stages driven by said turbine (13). 4. Method according to one of the preceding claims, characterized in that T2 is less than or equal to - 140 ° C. 5. Method according to the preceding claim, characterized in that the liquid stream (34) from step d) is sent to a storage means. 6. Method according to one of claims 1 to 3, characterized in that T2 is between - 80 ° C and - 130 ° C. 7. Method according to the preceding claim, characterized in that at least a portion (32) of the liquid stream from step d) is introduced (33) into the main heat exchanger (4) at a lower temperature T3 at T2 and circulates until the outlet of said heat exchanger at which the temperature T4 is the lowest. 8. Method according to the preceding claim, characterized in that T4 is less than or equal to -140 ° C. 9. Method according to claim 7, characterized in that the part (29) of the liquid stream which has not been introduced into the heat exchanger at the temperature T3 to be cooled to the temperature T4 is introduced into the heat exchanger (4) and in order to vaporize in the heat exchanger up to temperature T1. 10. Method according to the preceding claim, characterized in that the part of the liquid stream which has not been introduced into the heat exchanger at the temperature T3 to be cooled down to the temperature T4 is mixed with the gas stream (28 ) from step d) before introduction into the heat exchanger (4) at temperature T3 in order to vaporize in the heat exchanger up to temperature T1 or else is mixed with the flow from step e) before step f). 11. Method according to one of the preceding claims, characterized in that the refrigerant current flows in the refrigeration circuit in closed cycle. 1/3