WO2017103536A1

WO2017103536A1 - Hybrid method for liquefying a fuel gas and facility for implementing same

Info

Publication number: WO2017103536A1
Application number: PCT/FR2016/053524
Authority: WO
Inventors: Laurent Benoit; Denis FAURE-BRAC; Anna TORRES-MANSILLA; Emeline DROUET
Original assignee: Engie
Priority date: 2015-12-17
Filing date: 2016-12-16
Publication date: 2017-06-22
Also published as: FR3045795B1; FR3045795A1; FR3045798A1

Abstract

The present invention relates to a method for liquefying a methane-based fuel gas, which comprises a pre-cooling and liquefaction phase, in which fuel gas is cooled and liquefied between ambient temperature T₀ and a temperature T₂ at least as low as the bubble-point temperature of the gas by exchange of heat with a main flow of refrigerant mixture circulating countercurrent-wise with respect to the gas in a first heat exchanger, and a sub-cooling phase in which the liquefied gas is sub-cooled from the temperature T₂ down to a sub-cooling temperature T₃, by exchange of heat with a flow of initially liquid gaseous nitrogen circulating in a third open circuit, countercurrent-wise with respect to the flow of fuel gas. The present invention also relates to a liquefaction facility for implementing the method according to the invention.

Description

HYBRID PROCESS FOR THE LIQUEFACTION OF A COMBUSTIBLE GAS AND INSTALLATION FOR ITS IMPLEMENTATION

The present invention relates generally to a method and a facility for liquefying a fuel gas with a high methane content.

The global problem that the present invention seeks to solve is to liquefy gas with a high methane content (at least 80 mol%), typically natural gas from the gas transmission or distribution network, biomethane or gas evaporation. liquefied natural gas (usually referred to as LNG).

For this, it seeks to reduce the costs of implementation of the liquefaction process of the fuel gas (for example by reducing the number of equipment involved in the process and their size), while maintaining moderate operational costs (in particular in terms of consumables).

To solve such a problem, it is known to those skilled in the art different solutions, which can be grouped into three categories:

1. closed-cycle processes with phase change of the refrigerant, the latter being a pure substance or a refrigerant mixture in order to improve the efficiency. They are mainly used for large-scale LNG production (so-called "baseload"), ie several tens to several hundred tonnes / hour (t / h) of LNG produced, in a fixed station. these include processes following industrial processes: TEALARC process from Technip and Air Liquide, APCI processes including C3-MR and APX, Conoco Phillips optimized waterfall process, MFC process. Statoil-Linde, Linde's processes, Black and Veatch's PRICO process, and the LNG limited OSMR process. However, this type of process is now also proposed on a smaller scale (that is to say processes producing only a few tens or even a few tons / hour (t / h) of LNG produced. the following industrial processes: the BOC process of the Linde industrial group) for biogas liquefaction, the Kryopak SCMR and PCMR processes, the GTI MRC process, the Waertsila NewMR process, the SGTS process for biogas liquefaction, the process Erie for the liquefaction of biogas, and finally the MiniLNG process of SINTEF for the liquefaction of the evaporation of LNG;

Brayton closed-cycle or expansion processes, ie without phase change but with expansion of the refrigerant to create the cold. Because of their technical simplicity, these methods are used:

• for small LNG production. This is particularly the case for the Air Liquide process for the liquefaction of biogas, the Kryopak EXP process and the Cryostar process proposed both for biogas liquefaction and LNG evaporation;

• either for technically difficult situations where simplicity and robustness outweigh the energy performance ("offshore" production in particular, or for reliquefaction of LNG evaporation). This is particularly the case for the Mustang process, the APCI APX process, the SAIPEM multi-step relaxation process and the Gasconsult ZR-LNG process;

3. open cycle processes, in which the cold is not created but provided by an external medium, typically liquid nitrogen, and where the liquefaction of natural gas results from a simple (direct) heat exchange with the cold medium that vaporizes. This type of process is generally used in the laboratory or for very punctual applications requiring very little performance and a lot of simplicity. This is notably the case for the Chart process using liquid nitrogen as a refrigerant, currently used in South-East Asia (Indonesia in particular), and for the Hamworthy-Wärtsilà direct process, which is currently being implemented in the region. Skolvik small liquefaction plant in Finland (built in 2010), producing 55 tonnes per day of LNG.

However, these devices known from the prior art have many disadvantages. Thus, the processes closed cycle with phase change of the refrigerant have the following disadvantages:

• costs of developing or supplying non-consumable parts (costs usually referred to as CAPEX) that are high because of the large number of equipment and the complexity that such processes require,

• The same is true for operating costs (generally referred to as the OPEX acronym),

• the complexity of such processes and the risks involved (the refrigerant is generally flammable),

The size and large size of the equipment for implementing such processes, which makes them difficult to compact: indeed, in addition to the fluid to be cooled (typically natural gas), the process must incorporate a large amount of fluids cooling in intermediate cycles to obtain the desired cooling (in the end, the total mass flow rate of the refrigerant mixtures used being approximately 8 times that of the fluid to be cooled), and

• CO ₂ emissions related to gas consumption when it is used to supplement the energy of the process.

On the other hand, the complete closed cycle or Brayton closed-loop processes also have disadvantages, in part identical with those mentioned above for the phase change processes:

• high costs of development or provision of non-consumable parts (CAPEX costs) and high operating costs (OPEX costs),

• low energy performance, in other words significant energy consumption,

The size and large size of the equipment for the implementation of such processes, which makes them difficult to compact: here also, in addition to the fluid to be cooled (typically natural gas), the process must incorporate a large amount of fluids cooling in intermediate cycles to achieve the desired cooling. Given the absence of evaporation, the amount of refrigerant is even greater than in the previous case (relating to complete closed cycle processes with phase change of the refrigerant), the mass flow rate of refrigerant is this time equivalent to several dozens of times the mass flow of fluid to cool.

Finally, the open-cycle processes have disadvantages mainly related to their hardiness, and the cost of supplying the refrigerant (which is consumed because of the open cycle).

The object of the present invention is therefore to overcome all or part of the disadvantages of the prior art, by setting up a hybrid process in that it is partially closed and in which:

• on the one hand, we realize, using a closed cycle with cooling mixture, cooling the inlet gas over a temperature range from room temperature to a temperature at least as low as the bubble temperature of the gas, which is about -130 ° C depending on the specific pressure configurations and of precise composition of the gas at the liquefaction inlet,

On the other hand, the final subcooling of the inlet gas (once liquefied) is carried out, using an open cycle with liquid nitrogen, over a temperature range from a temperature of less as low as the bubble temperature of up to a subcooling temperature of about -155 ° C depending on the specific pressure and precise composition of the liquefied gas inlet and the LNG storage pressure .

More particularly, the subject of the present invention is therefore a process for liquefying a fuel gas comprising predominantly methane, in which the fuel gas circulates in a primary circuit open from a source of combustible gas to a tank for liquefied gas, while a refrigerant mixture circulates in a closed secondary circuit, the process comprising the following phases:

a pre-cooling and liquefaction phase, during which the fuel gas of initial flow rate m is cooled and liquefied between the ambient temperature To and a temperature 2 at least as low as the bubble temperature of the gas, this phase being carried out by heat exchange with a main stream of refrigerant mixture circulating countercurrently to the fuel gas stream in at least one first heat exchanger; and

a subcooling phase during which the liquefied fuel gas is subcooled from the temperature T ₂ to a subcooling temperature T ₃ , this subcooling stage being carried out in a heat exchanger from under -cooling;

said method being characterized in that the subcooling phase is carried out by heat exchange with a flow of initially completely liquid nitrogen gas from the liquid nitrogen reservoir and flowing in a third open circuit against the flow current combustible gas.

For the purposes of the present invention, the term "heat exchanger" means an exchanger physically separated from the main heat exchange line, or a subset or part of a single heat exchange zone incorporating all of the heat exchange line. heat exchange line of the considered phase.

For the purpose of the present invention, the term "heat exchange line" means the succession of fluids exchanging heat with each other in the phase under consideration.

For example, it may be considered that the first heat exchanger is part of a first heat exchange zone where the cooling phase takes place.

For the purposes of the present invention, nitrogen is understood to mean a fluid comprising at least 97 mol% of nitrogen. Advantageously, it will be possible to distribute the combustible gas at the outlet of the gas source in two sub-flows of respective flow rates mj and m ₂ = mm ₁ . Then we can inject:

• on the one hand the mi flow fuel gas sub-flow in the first heat exchanger, for liquefying and cooling it by heat exchange with the main flow of refrigerant mixture circulating countercurrent flow of fuel gas; and

On the other hand, the sub-flow of flow-rate fuel gas in an auxiliary heat exchanger, for liquefying it and cooling it by heat exchange with the stream of completely vaporized nitrogen leaving the sub-cooling heat exchanger.

Preferably, the flow rate / ¾ of the fuel gas sub-flow injected into the first heat exchanger may represent at least 80% and preferably of the order of 95% of the initial flow rate m of combustible gas at the outlet of the source of fuel. gas.

Advantageously, the phase pre ^¬ cooling and liquefaction may comprise the following additional steps:

the pre-cooling of the combustible gas from the ambient temperature To to a temperature equal to the dew point temperature of the combustible gas (with a variation margin of plus or minus 10 ° C. around this dew point temperature depending on the composition of the fuel gas), this pre-cooling phase being carried out in the first heat exchanger by heat exchange with the main stream of refrigerant mixture circulating countercurrent to the fuel gas stream;

the liquefaction of the fuel gas from the temperature ΤΊ to the temperature T ₂ , this liquefaction stage being carried out by heat exchange with a fraction of the refrigerant mixture flowing countercurrently with the flow of fuel gas, in a liquefaction heat exchanger arranged in cascade between the first heat exchanger and the subcooling heat exchanger.

According to an advantageous embodiment, the method according to the invention further comprises the steps

following:

- is compressed in a first compressor, the main refrigerant stream mixture at the outlet of the first heat exchanger, and cooled in a first post ^¬ cooling device; then

separating the main stream of refrigerant mixture compressed and cooled into light and heavy fractions in a phase separator; then

the heavy fraction originating from the first phase separator is injected into the first heat exchanger, circulating it countercurrently with the main refrigerant stream, then joining it to the main flow of the refrigerant mixture at the outlet of the first exchanger; thermal;

while the light fraction is injected successively into the first heat exchanger and the liquefaction heat exchanger, by circulating it countercurrently with the refrigerant flow in the first and second heat exchangers, with an intermediate stage at the outlet of the exchanger where the light fraction is joined to the heavy fraction to form the main stream of refrigerant mixture.

In this advantageous embodiment, an additional step is preferably carried out in which, at the outlet of the first aftercooling device, the main stream of refrigerant mixture is separated into a second phase separator into vapor and liquid fractions so that to be able to increase in pressure each fraction, with the adapted equipment.

In this advantageous embodiment, the method according to the invention may preferably comprise the following additional steps:

the initially vapor fraction is compressed and cooled to ambient temperature (and may then contain a fraction of liquid),

said compressed and cooled fraction is combined with the purely liquid fraction which has been compressed at the same pressure as the initially vapor fraction;

the assembly produces the refrigerant mixture which is two-phase and at high pressure;

the refrigerant mixture is then separated in the separator flask into a pure vapor fraction and a purely liquid heavy fraction. Advantageously, the gas to be liquefied may advantageously contain methane in a molar proportion of at least 80%. In this case, for such a gas to be liquefied, the temperature 2 will be between -140 ° C and -110 ° C, and the temperature 3 of subcooling liquefied fuel gas will be between -140 ° C and -170 ° C.

It should be noted that the aspects related to pretreatment of the gas to be liquefied are not taken into account in the process according to the invention. Standard gas cleaning solutions can therefore also be considered upstream of the pre-cooling and liquefaction phase, such as amine scrubbing, membrane scrubbing or the use of sieve-based systems. molecular.

The method according to the invention thus makes it possible to significantly lighten the pre-cooling and liquefaction phase (taking place in a closed cycle with a refrigerant mixture), since compared to a standard closed cycle, it is no longer a matter of cooling here. directly a gas up to its subcooling temperature (ie about -155 ° C) but only up to a temperature T2 at least as low as the gas bubble temperature (about-130 ° C). This reduces by about 40% its mechanical energy consumption and the size of the compression line (and ambient cooling). The process according to the invention, compared to a process using only one liquid nitrogen cycle, consumes about 5 times less liquid nitrogen (since the cooling power provided by it is mainly used to the only subcooling).

Furthermore, there is an increased flexibility of operability of the process of the invention, resulting in a gain of ΘΠνΐΓΟΠ 5 "6 of flexibility of production of liquefied natural gas (LNG) using the cold leftover from the flow of liquefied natural gas (LNG). vaporized nitrogen at the outlet of cooling to liquefy a portion of the input gas (in this case about 5%). In addition, the operational flexibility is increased compared to other processes because the use of liquid nitrogen can help to mitigate the fluctuations in performance of the closed secondary cycle.

Finally, the simplicity of the process and therefore its use are increased because of the reduction in the number of complex equipment (typically compressors and "associated aftercoolers") compared to a standard closed cycle. In addition, the process according to the invention is less sensitive to environmental conditions.

Another subject of the present invention is a liquefaction plant for a gas for implementing the method according to the invention, said installation comprising a primary circuit connected to a source of combustible gas and to a tank for liquefied gas, and a closed secondary circuit in which a cooling mixture circulates, the installation further comprising at least a first heat exchanger and at least one subcooling heat exchanger arranged in cascade for cooling and liquefying the gas flowing in the primary circuit, the first exchanger thermal being traversed by the primary and secondary circuits arranged so that the fuel gas and the refrigerant mixture still circulate against the current,

the installation being characterized in that it further comprises an open tertiary circuit in which circulates an initially completely liquid nitrogen stream from a liquid nitrogen tank, the second heat exchanger being traversed by the circuit tertiary so that the flow of nitrogen and the fuel gas circulate in countercurrent.

Advantageously, the installation according to the invention may furthermore comprise an auxiliary heat exchanger, into which a fraction η¾ of the combustible gas is injected, to liquefy it and cool it by heat exchange with the stream of completely vaporized nitrogen leaving the the subcooling heat exchanger.

The installation according to the invention has the advantage of being more compact than closed-cycle installations known from the prior art by reducing the closed secondary circuit.

Furthermore, it is not necessary to completely stop the installation according to the invention when performing the revision of the compressors. In fact, thanks to the liquid nitrogen and by temporarily degrading the main exchange line (that is to say by temporarily injecting more liquid nitrogen into the main cryogenic exchanger to partially compensate the absence closed cycle refrigerant), we can still produce, with the method according to the invention, a significant amount of LNG (about 50% of the nominal capacity). Finally, the liquid nitrogen stock required for subcooling can be used for inerting / safety applications in emergency situations, resulting in increased process safety.

Other advantages and features of the present invention will result from the description which follows, given by way of nonlimiting example and with reference to the appended figures: FIG. 1 represents a general block diagram of a preferred embodiment of the installation according to the invention, on which the different phases of the method of the invention have been indicated;

FIG. 2 represents the same general block diagram as that shown in FIG. 1, in which the different flows of refrigerant mixture in the closed secondary circuit have been indicated;

FIG. 3 represents a detailed view of the installation of FIG. 2, showing in particular the different flows of refrigerant mixture in the first and second heat exchangers;

FIG. 4 represents a general block diagram of an installation according to the prior art comprising an open cycle with liquid nitrogen.

The identical elements represented in the figures

1 to 3 are identified by identical reference numerals.

FIG. 4 is a device according to the prior art for implementing a method for liquefying a fuel gas known from the prior art operating with an open cycle with liquid nitrogen. This method serves as a point of comparison for the numerical simulations presented hereinafter in the examples.

FIG. 1 represents a general block diagram of a preferred embodiment of the installation according to the invention, on which the different phases of the method of the invention according to the invention have been indicated. level of the heat exchangers where they are carried out in the installation according to the invention. In particular, FIG. 1 shows that the process according to the invention consists in liquefying a fuel gas mainly comprising methane, by circulating it in a primary circuit I open from a fuel gas source G-1 to a tank for liquefied gas G -3, while the refrigerant mixture circulates in a closed secondary circuit II, and the flow of nitrogen gas initially completely liquid from the liquid nitrogen tank. In particular, the flow of nitrogen flows, in a third open circuit, against the flow of fuel gas

FIG. 1 shows that the process according to the invention takes place in the installation according to the invention in three successive phases, as follows:

a pre-cooling phase 1000 of the combustible gas from ambient temperature ΪΌ up to a temperature ΪΊ equal to the dew point temperature of the fuel gas;

a liquefaction phase 2000 of the fuel gas from the temperature ΤΊ to the temperature T2 at least as low as the bubble temperature of the gas; and

a sub-cooling phase 3000 during which the liquefied fuel gas is sub-cooled from the temperature T2 to a subcooling temperature T3.

FIG. 1, together with FIGS. 2 and 3, show in particular that:

the pre-cooling phase 1000 is carried out, in the first heat exchanger 100, by heat exchange with the main flow MR-1 of a refrigerant mixture circulating countercurrently to the flow of combustible gas; and

the subcooling phase 3000 is carried out in a subcooling heat exchanger 300, by heat exchange with a flow of nitrogen gas initially completely liquid from the liquid nitrogen reservoir N1 and which after having been pumped (state N-2 refrigerant flow) flows in an open tertiary circuit III, countercurrent to the flow of fuel gas;

while the intermediate liquefaction phase 2000 is carried out by heat exchange with a fraction of the refrigerant mixture MR-V-4 circulating countercurrently with the flow of fuel gas, in a liquefaction heat exchanger 200 arranged in cascade between the first exchanger 100 and the subcooling heat exchanger 300.

In the preferred embodiment of the installation according to the invention, it is advantageously, prior to the phase pre ^¬ 1000 cooling of the fuel gas evenly the fuel gas according to the invention in the following manner, as illustrated in Figures 1 and 3:

at the outlet of the gas source, the fuel gas G of flow m is distributed in two sub-flows GG1 and GP1 of respective flow rates / ¾ and m ₂ = mm ₁ ; then

injecting the fuel gas sub-flow G-G-1 flow η¾ in the first heat exchanger

100, to liquefy and cool it by heat exchange with the main flow MR-1 of a refrigerant mixture circulating countercurrently to the flow of combustible gas; and

the n-flow G-P-1 gas sub-flow is injected into an auxiliary heat exchanger 400, for liquefying and cooling it by heat exchange with the stream of completely vaporized nitrogen leaving the sub-cooling heat exchanger 300.

Preferably, the flow rate / ¾ of the fuel gas sub-flow injected into the first heat exchanger 100 represents at least 80%, and preferably of the order of 95%, of the initial flow rate m of fuel gas at the outlet of the source. of gas Gl.

Moreover, FIG. 2 more particularly illustrates the circulation of the refrigerant mixture in the closed secondary circuit II, which takes place as follows:

after passing through the first heat exchanger 100, the main flow MR-1 becomes the main flow MR-2;

- that is then compressed once out of the first heat exchanger 100 in a first compressor 12;

- It then becomes the main flow of refrigerant mixture MR-3, which is

- cooled in a first post ^¬ cooling device 13 where it becomes the MR-4 streams; then

at the outlet of the first post-cooling device 13, the MR-4 refrigerant main stream is separated into a phase separator 14 into vapor fractions MR-5-a and liquid MR-5-b, in order to pressurize each MR-5-a, MR-5b fraction. with the appropriate equipment: a pump for the liquid fraction and another compressor stage for the steam fraction;

the initially steam fraction MR-5-a is compressed to become the MR-6-a fraction, then it is cooled to ambient temperature to become the MR-7-a fraction, which may contain a liquid fraction;

the pure liquid fraction MR-5-b is compressed at the same pressure as MR-5-a to become the purely liquid fraction MR-6-b;

the compressed and cooled fraction MR-7-a is then combined with the purely liquid fraction MR-6-b;

the combination of fractions MR-7-a and MR-6-b produces the MR-8 refrigerant mixture which is two-phase and at high pressure; and

the refrigerant mixture MR-8 is then separated in a phase separator flask 17 into a pure steam fraction MR-V-1 and a purely liquid heavy fraction MR-L-1.

Finally, FIG. 3 more particularly illustrates the circulation of the refrigerant mixture in the closed secondary circuit II, at the level of the heat exchangers 100 and 200, after obtaining the purely vapor fractions MR-V-1 and a purely liquid heavy fraction MR-L- 1:

the liquid fraction MR-L-1 is injected into the exchanger 100, at the outlet of which 100 it becomes the MRL-2 fraction;

the pure steam fraction MR-V-1 is injected into the exchanger 100; at the exit of the exchanger 100, the pure vapor fraction MR-V-1 becomes the vapor fraction MR-V-2, which is injected into the exchanger 200 by circulating it countercurrently with the refrigerant flow MR-V-4

at the outlet of the exchanger 200, the light fraction MR-V-5 is combined with the heavy fraction MR-L-2 to form the main flow of the refrigerant mixture MR-1.

The purpose of closed secondary cycle II is to cool-liquefy most of the input gas (about 95% of the mass flow) from room temperature to about -130 ° C, while tertiary cycle III opens up to The main purpose of liquid nitrogen is to sub-cool the entire input gas stream, which at this stage is already liquid, that is to say at a temperature between -130 ° C and -155 ° C . At the subcooling outlet, the vaporized nitrogen (and therefore at a temperature close to -130 ° C.) can still be used to cool / liquefy a fraction of the inlet gas. Since the stream of nitrogen once vaporized has already yielded most of its cooling capacity, this flow of cooled / liquefied inlet gas is low and of the order of 5% (ie the complement of the flow treated by the cycle closed) .11 It should be noted that Secondary II has no great specificities and many possibilities may be appropriate. In the present case, a compromise is sought between simplicity and efficiency, hence the following characteristics of the secondary cycle retained:

• Although two compressors appear on the diagram, the cycle is designed so that these two stages of compressors can be contained in a single crankcase in order to reduce the cost CAPEX of the solution. In fact, as the closed cycle only cools the gas up to about - 130 ° C, an overall compression ratio of up to 8 for the refrigerant is possible and therefore is compatible with a single compressor casing; the separation of the heavy and light fractions of refrigerant can be carried out in the simplest way, that is to say by flask and at ambient temperature. But more complex approaches (use of a distillation column) are also possible.

EXAMPLES

The following examples illustrate the invention without, however, limiting its scope. These are numerical simulations carried out using the aspen hysys V7.3 simulation tool based on the SRK LK -1 thermodynamic model (Soave Redlich-Kwong Lee Kesler 1). These simulations made it possible to calculate the following parameters:

• the mechanical power used by the process in kW;

• the specific consumption in kWh / t of LNG produced (ratio of mechanical power and mass flow of LNG produced);

• the mass ratio of liquid nitrogen used compared to LNG produces liquid nitrogen;

The gain in energy consumption compared with the known method of the prior art with an open cycle with liquid nitrogen; The gain in energy consumption compared to a method known from the prior art with a closed cycle of the SMR type (as described in FR 2703762 or FR 2751059),

compared to the results that would be obtained with:

on the one hand, a known method according to the prior art comprising an open cycle with liquid nitrogen as illustrated in FIG. 3, and on the other hand, a known method according to the prior art as described in FIGS. French patents FR 2703762 or FR 2751059 are closed-cycle natural gas liquefaction processes of the SMR type ("Single Mixed Refrigerant") with fractionation of the gas in a distillation column. As a pure closed cycle, it does not consume liquid nitrogen to cool the natural gas.

These numerical simulations were carried out under the following conditions:

• Entry gas (combustible gas to be cooled):

o Pressure: 48 bar

o Temperature: 30 ° C

o Mass flow: 10.18 t / h

o Composition (mol%):

N ₂ 0.79%

C, 91, 21 ^;

C ₂ 7.89%

C ₃ 0.11%

C ₄ 0 • Minimum cooling temperature of the refrigerant with the surrounding environment: 30 ° C

• Polytropic compressor efficiency assumptions: 85%

· LNG subcooling temperature: -158 ° C.

The results of the simulations are presented in Table 1 below:

Table 1

These results show that the process according to the invention, as a semi-open or semi-closed cycle hybrid process, allows:

compared to a simple open-cycle liquid nitrogen process, to reduce liquid nitrogen consumption by more than 80%, which is the main source of operational costs for an open-cycle process,

compared to the known method of the prior art with a SMR type closed cycle (as described in FR 2703762 or FR 2751059), to reduce its energy consumption by 33% and thus to reduce the costs associated with the consumption of energy. energy but also to suppress a compression stage in the process, resulting in a significant reduction in process costs.

Claims

A method of liquefying a fuel gas comprising predominantly methane, wherein said fuel gas flows in a primary circuit (I) open from a fuel gas source (G1) to a liquefied gas tank (G-3), while a refrigerant mixture circulates in a closed secondary circuit (II), said method comprising the following phases:

a pre-cooling (1000) and liquefaction (2000) phase, during which the initial flow fuel gas m is cooled and liquefied between the ambient temperature ΪΌ and a temperature T2 at least as low as the bubble temperature of the gas, this phase (1000, 2000) being carried out by heat exchange with refrigerant mixture (MR-1, MRV-4) circulating countercurrently with the fuel gas stream in at least one first heat exchanger (100, 200); and

a sub-cooling phase (3000) during which the liquefied fuel gas is sub-cooled from the temperature T2 to a subcooling temperature T3, this subcooling phase being carried out in a heat exchanger of undercooling (300);

said method being characterized in that the subcooling phase (3000) is carried out by heat exchange with a flow of initially liquid nitrogen gas from the liquid nitrogen reservoir (Nl) and circulating in a tertiary circuit (III) open against the flow of fuel gas.

2. Method according to claim 1, in which: the fuel gas (G1) of flow rate m at the outlet of the gas source is distributed in two substreams (GG1) and (GP1) of respective flow rates / ¾ and m ₂ = mm ₁ ; then

injecting the fuel gas sub-flow (GG-1) with a flow rate η¾ into said first heat exchanger (100), for liquefying and cooling it by heat exchange with the main flow (MR-1) of the circulating refrigerant mixture -current flow of combustible gas; and the flow sub-stream GP1 is injected into an auxiliary heat exchanger (400) in order to liquefy it and to cool it by heat exchange with the stream of completely vaporized nitrogen leaving the sub-cooling heat exchanger. (300).

3. Method according to claim 2, wherein the flow / ¾ of the fuel gas sub-flow injected into the first heat exchanger (100) represents at least 80% and preferably of the order of 95% of the initial flow rate m of combustible gas at the outlet of the gas source (G1).

The method of any one of claims 1 to 3, wherein the pre-cooling (1000) and liquefying (2000) phase comprises the following steps:

- the pre-cooling (1000) of the fuel gas from ambient temperature ΪΌ to a temperature Ti equal to the dew point temperature of the fuel gas, this pre-cooling phase (1000) being carried out, in the first heat exchanger (100), by heat exchange with the main flow (MR-1) of circulating refrigerant mixture against -current flow of combustible gas;

the liquefaction (2000) of the fuel gas from the temperature ΤΊ to the temperature T ₂ , this liquefaction phase being carried out by heat exchange with a fraction of the refrigerant mixture (MR-V-4) flowing against the flow of the gas stream fuel in a liquefaction heat exchanger (200) cascaded between the first heat exchanger (100) and the subcooling heat exchanger (300).

The method according to any one of claims 4, wherein:

- is compressed in a first compressor (12), the main flow (MR-2) of the refrigerant mixture at the outlet of the first heat exchanger (100), and cooled in a first post ^¬ cooling device (13); then

the main stream of cooled and cooled refrigerant mixture (MR-8) is separated into a light fraction (MR-8)

V-1) and heavy (MR-L-1) in a phase separator (17); then

the heavy fraction (MR-L-1) originating from said first phase separator (17) is injected into the first heat exchanger (100) by circulating it countercurrently with the main refrigerant stream (MR-1), and then bringing it together (state MR-L- 2) to said main flow (MR-1) of refrigerant mixture, at the outlet of said first heat exchanger (100);

while the light fraction is successively injected into the heat exchangers (100) and then (200) respectively in forms MR-V-1 and MR-V-2, by circulating them in counterflow of the refrigerant flow (MR-1) for the exchanger (100) and (MR - V - 4) for the exchanger (200), with an intermediate stage at the outlet of the exchanger (200) where the light fraction (MR - V - 5) is joined to the heavy fraction (MR-L-2) to form the main flow of refrigerant mixture (MR-1).

The method of claim 5, wherein:

- the output of the first post ^¬ cooling device (13) is separated in a second phase separator (14), the main flow (MR-4) mixed refrigerant vapor fractions (MR-5-a) and liquid (MR-5-b) to pressure each fraction (MR-6-a, MR-6-b).

The method of claim 6 wherein:

The initially vapor fraction (MR-5-a) is compressed and becomes the fraction (MR-6-a), which is cooled to ambient temperature (MR-7-a),

said compressed and cooled fraction (MR-7-a) is combined with the purely liquid fraction (MR-6-b) which corresponds to the fraction (MR-5-b) after having been compressed at the same pressure as (MR -7-a);

the assembly produces the refrigerant mixture (MR-8) which is two-phase and at high pressure; said refrigerant mixture (MR-8) is then separated in the separator flask (17) in a purely vapor light fraction (MR-V-1) and a purely liquid heavy fraction (MR-L-1).

8. Process according to any one of claims 1 to 7, wherein the gas to be liquefied contains methane in a molar proportion of at least 80%.

9. The method of claim 8, wherein the temperature T2 is between -140 ° C and -110 ° C, and the subcooling temperature 3 of the liquefied fuel gas is between -140 ° C and -170 ° C .

10. Installation for liquefying a gas for carrying out the process as defined in any one of claims 1 to 9, said installation comprising a primary circuit (I) connected to a source (G-1) of gas fuel and a tank for liquefied gas (G-3), and a closed secondary circuit (II) in which circulates a refrigerant mixture, the installation further comprising at least a first heat exchanger (100, 200) and at least one subcooling heat exchanger (200) arranged in cascade for cooling and liquefying the gas flowing in the primary circuit (I), said first heat exchanger (100, 200) being traversed by the primary (I) and secondary (II) circuits; arranged in such a way that the fuel gas and the cooling mixture circulate therein counter-current,

said plant being characterized in that it further comprises an open tertiary circuit (III) in which circulates an initially completely liquid nitrogen stream from a liquid nitrogen tank (Nl), said subcooling heat exchanger (300) being traversed by said tertiary circuit (III) so that the flow of nitrogen and the fuel gas circulate therein countercurrently.

11. Installation according to claim 10, further comprising a third auxiliary heat exchanger (400) in which a n¾ fraction of the combustible gas is injected, for liquefying and cooling it by heat exchange with the stream of completely vaporized nitrogen coming out of the fuel. subcooling heat exchanger (300).