OA16795A - A process for liquefying natural gas with a mixture of refrigerant gas. - Google Patents

Abstract

The present invention relates to a process for the liquefaction of a natural gas comprising mainly methane, preferably at least 85% methane, the other components essentially comprising nitrogen and C-2 to C-4 alkanes. , in which said natural gas to be liquefied is liquefied by circulating said natural gas to be liquefied circulating at a pressure PO greater than or equal to atmospheric pressure (Patm.) / preferably PO being greater than atmospheric pressure, in at least 1 heat exchanger cryogenic heat (EC1, EC2, EC3) by closed-circuit countercurrent circulation in indirect contact with at least one flow of refrigerant gas remaining in the compressed gaseous state at a pressure PI entering said cryogenic exchanger at a temperature T3 'lower at T3, T3 being the liquefaction temperature of said liquefied natural gas at said pressure PO at the outlet of said cryogenic exchanger, characterized in that said refrigerant gas comprises a mixture of nitrogen and at least one other constituent selected from neon and hydrogen.

Description

Process for liquefying natural gas with a mixture of refrigerant gas

The present invention relates to a process for liquefying natural gas to produce LNG, or Liquefied Natural Gas, also called LNG. More particularly still, the present invention relates to the liquefaction of natural gas comprising predominantly methane, preferably at least 85% methane, the other main constituents being chosen from nitrogen and C-2 to C-4 alkanes. namely ethane, propane, butane.

The present invention also relates to a liquefaction installation arranged on a ship or a floating support at sea, either in the open sea, or in a protected area, such as a port, or an installation on land in the case of small or medium-sized vessels. liquefaction of natural gas.

In the case of an installation placed on a ship, the present invention relates more particularly to a gas reliquefaction process on board an LNG transport ship called a “methane tanker”, said gas to be reliquefied being the result of heating and partial evaporation. LNG contained in the tanks of said vessel, said evaporated gas, in general mainly methane, being called in English “boil off”.

Methane-based natural gas is either a by-product of oil fields, produced in small or medium quantities, usually associated with crude oil, or a major product in the case of gas fields, where it is then in combination with other gases, mainly C-2 to C-4 alkanes, CO2, nitrogen.

When natural gas is combined in small quantities with crude oil, it is usually processed and separated, then used on-site as fuel in turbines or piston engines to produce electrical energy and heat for use in processes. separation or production.

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When the quantities of natural gas are large or even considerable, it is sought to transport it so that it can be used in remote regions, generally on other continents, and to do this, the preferred method is to transport it to the cryogenic liquid state (-165 ° C) at substantially ambient atmospheric pressure. Specialized transport vessels called "LNG carriers" have very large tanks with extreme insulation so as to limit evaporation during the voyage.

The liquefaction of gas for transport is generally carried out near the production site, generally on land, and requires considerable installations to reach capacities of several million tonnes per year, the largest existing units grouping together three or four liquefaction units of 3-4 Mt per year unit capacity.

This liquefaction process requires considerable amounts of mechanical energy, the mechanical energy being generally produced on site by taking part of the gas to produce the energy required for the liquefaction process. Part of the gas is then used as fuel in gas turbines, steam turbines or reciprocating heat engines.

Multiple thermodynamic cycles have been developed in order to optimize the overall energy efficiency. There are two main types of cycles. A first type based on the compression and expansion of refrigerant fluid, with phase change, and a second type based on the compression and expansion of refrigerant gas without phase change. The term “refrigerant fluid” or “refrigerant gas” is used to refer to a gas or mixture of gases, circulating in a closed circuit and undergoing compression phases, where appropriate liquefaction, then heat exchanges with the external environment, then subsequently 30 phases of expansion, where appropriate of evaporation, and finally of heat exchanges with the natural gas to be liquefied comprising methane, which gradually cools down to reach its liquefaction temperature at atmospheric pressure, i.e. approximately -165 ° C in the case of LNG.

Said first type of cycle, with phase change, is generally used on land installations and requires a large amount of equipment and a considerable footprint. In addition, refrigerant fluids, generally in the form of mixtures, consist of butane, propane, ethane and methane, these gases being dangerous because they risk, in case of leakage, causing explosions or fires. considerable. On the other hand, despite the complexity of the equipment required, they remain the most efficient and require an energy of around 0.3 kWh per kg of LNG produced.

Many variants of this first type of process with phase change of the refrigerant fluid have been developed and each supplier of technology or equipment has its own formulation of mixtures, associated with specific equipment, both for the so-called “cascade” processes. , as for the so-called I5 “mixed cycle” processes. The complexity of the installations comes from the fact that in the phases where the refrigerant is in the liquid state, and more particularly at the level of the separators and the connecting pipes, it is necessary to install gravity collectors to bring together the liquid phase and directing it to the heart of the heat exchangers where it will then vaporize on contact with the methane to be cooled and liquefied, to obtain LNG. These devices are very bulky, but this does not pose a problem in the case of land-based installations, since it is generally simple to have a sufficient area of land to accommodate all these bulky items of equipment one beside the other. . Thus, for onshore installations, all these compression equipment, exchangers and collectors are generally installed next to each other over considerable areas of 25 to 50,000 m ² , or even more.

In US Pat. No. 6,438,994, a phase change liquefaction process is described in which the refrigerant gas only partially condenses to the liquid state because it comprises a mixture of first component selected from argon and nitrogen ( which condense towards the liquid state at the temperatures and pressures of the process) and the other component chosen from among helium and neon (which do not condense towards the liquid state).

The second type of liquefaction process, a process without phase change of the refrigerant gas, is an inverted Brayton cycle, or Claude cycle using a gas such as nitrogen. The efficiency of this second type is lower, because it generally requires an energy of the order of 0.5 kWh / kg of LNG produced, or about 20.84 kW x day / t and, on the other hand, it presents a considerable advantage in terms of terms of safety, because the refrigerant gas of the cycle, nitrogen, is inert and therefore incombustible, which is very advantageous when the installations are concentrated in a small space, for example on the deck of a floating support installed at sea open, said equipment often being installed on several levels, one above the other on a surface reduced to the strict minimum. Thus, in the event of a refrigerant gas leak, there is no danger of explosion and it is then sufficient to reinject the fraction of refrigerant gas lost into the circuit.

In addition, this method of liquefying natural gas without phase change is very advantageous in the case of floating supports, because, due to the absence of a liquid phase in the refrigerant gas, the equipment is of much simpler design.

In fact, in such installations, all of the equipment moves almost continuously to the rhythm of the movements of the floating support (roll, pitch, yaw, sheer, swing, heaving). And the management of a process with phase change involving a liquid phase of the refrigerant would be extremely delicate even for small movements of the floating support, or even almost impossible for extreme movements, whereas in fixed installations on land the problem of movements do not arise.

Despite the lower energy efficiency of the liquefaction process without phase change of the refrigerant gas, the latter remains very interesting because the equipment, mainly compressors, expansion valves, in turbines, and exchangers are much simpler than the equipment required for a liquefaction process involving phase change cycles of a refrigerant, both in terms of the technology of said equipment and of

ιί * maintenance of this equipment in a confined environment, namely a floating support anchored at sea. In addition, the operation of the installations in operation remains simpler, because this type of cycle is not very sensitive to variations in the composition of the gas to be liquefied, namely a natural gas made up of a mixture in which methane predominates. In fact, in the case of the phase change cycle of the refrigerant fluid, in order for the yields to remain optimum, the refrigerant must be adapted to the nature and composition of the gas to be liquefied and the composition of the refrigerant must be modified if necessary. over time, depending on the composition of the mixture of natural gas to be liquefied produced by the oil field.

In principle, the implementation of a cycle of the liquefaction process without phase change of the refrigerant gas such as nitrogen comprises the following 4 main elements:

- a compressor which increases the pressure of the refrigerant gas and changes it from ambient temperature at low pressure to a high temperature at high pressure,

- a heat exchanger which cools the refrigerant gas from high temperature and high pressure substantially to ambient temperature and high pressure, an expansion device, in general a decompression turbine, in which the refrigerant gas expands: its pressure drops and its temperature is very low; while, simultaneously, the mechanical energy is recovered at the level of the expansion turbine which is then generally directly reinjected at the level of the compressor which is coupled to it,

a cryogenic exchanger in which circulates on one side the refrigerant gas at cryogenic temperature, and on the other the gas to be liquefied, said refrigerant gas absorbing the calories of the gas to be liquefied, therefore heating up, while said gas to be liquefied, giving up its calories, cools down until it reaches the desired liquid state. At the end of the circulation cycle, the refrigerant gas is approximately at ambient temperature and it is then reintroduced into the compressor to perform a new closed circuit cycle.

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Throughout the cycle, the refrigerant gas remains in the gaseous state and circulates continuously as explained previously: it gradually gives up frigories, therefore gradually absorbs calories from the gas to be liquefied, namely a mixture consisting mainly of of methane and other traces of gas.

The circulation of the gas to be liquefied takes place against the current of the refrigerant gas, that is to say that said natural gas comprising methane enters substantially at ambient temperature into the exchanger at the level of the refrigerant gas outlet where the latter is then substantially at room temperature. Then, said natural gas comprising methane progresses through the exchanger towards the colder zones and transfers its calories to the refrigerant fluid: the natural gas comprising methane cools and the refrigerant gas heats up. As the natural gas methane progresses in the exchanger, its temperature drops, then at the end of its journey it liquefies and its temperature continues to drop until it reaches the temperature of T3 = -165 ° C for a gas containing 85% methane.

Throughout its journey in the heat exchanger (s), the liquefaction of natural gas takes place under PO pressure of 5 to 50 bars, generally 10 to 20 bars, in four main phases:

- phase 1: cooling of the natural gas from the ambient temperature T0 to Tl = -50 ° C approximately (this temperature depends on the composition of the natural gas),

- phase 2: liquefaction of natural gas (passage from the gaseous state to the liquid state). As natural gas is a gas mixture under a pressure PO of about a few tens of bars, this change of state ranges between Tl = -50 ° C and T2 = -120 ° C approximately,

- phase 3: once completely liquefied natural gas (LNG) is then at approximately T2 = -120 ° C, still under a pressure PO of approximately a few tens of bars. Within the exchanger (s), the LNG continues to cool to reach the temperature T3 of -165 ° C, a temperature corresponding to a liquid phase of the LNG under atmospheric pressure,

- phase 4: The liquid obtained or LNG is then depressurized to atmospheric pressure where it remains in the liquid state due to its temperature T3 less than or equal to -165 ° C, and can be transferred to a storage tank isolated, or where applicable loaded directly onto a transport vessel such as an LNG carrier.

Phase 2 consumes the most energy, because the gas must be supplied with all the energy corresponding to its latent heat of vaporization. Phase 1 consumes a little less energy, and phase 3 consumes the least energy, on the other hand it is done at the lowest temperatures, ie around -165 ° C.

The values mentioned above for Tl, T2 and T3 are suitable for a natural gas consisting of 85% methane and 15% of said other components nitrogen and C-2 to C-4 alkanes, and can vary significantly for a gas. of different composition.

In Figure 1, there is shown an installation diagram of a standard natural gas liquefaction process involving a refrigerant gas consisting of nitrogen without phase change of the refrigerant gas as described above and whose description is explained. further.

In EP 1 939 564, a process for the liquefaction of natural gas of the type without phase change is described, in which the refrigerant gas is air, that is to say a mixture of nitrogen and essentially oxygen. and argon. Oxygen and argon are gases with higher critical point temperatures than nitrogen, so this mixture of refrigerant gas (air) requires reducing the pressure of the refrigerant gas to prevent condensation to the liquid state, compared to a refrigerant gas consisting exclusively of nitrogen. As the mixture circulates at lower pressure, heat exchanges require greater flow rates and therefore pipes of larger diameters and exchangers with larger surfaces, which contributes to increasing the cost and size of the installation and reduces the overall thermodynamic efficiency of the process.

The aim of the present invention is to provide a process for the liquefaction of natural gas of the type without phase change of the refrigerant gas capable of being installed on a ship or floating support which has improved energy efficiency, namely a total energy consumed in the tank. minimum process in terms of kWh to obtain 1 tonne of LNG and / or which exhibits increased heat transfers in the exchangers and / or which makes it possible to implement a more compact and more efficient liquefaction installation.

To do this, the present invention provides a process for liquefying a natural gas comprising predominantly methane, preferably at least 85% methane, the other components comprising essentially nitrogen and C-2 to C alkanes. -4, in which said natural gas to be liquefied is liquefied by circulating said natural gas at a pressure PO greater than or equal to atmospheric pressure (Patm.), Preferably PO being greater than atmospheric pressure, in at least 1 heat exchanger cryogenic heat by circulation in a closed circuit against the current in indirect contact with at least one flow of refrigerant gas remaining in the compressed gaseous state at a pressure PI entering the said cryogenic exchanger at a temperature T3 'lower than T3, T3 being the temperature at the outlet from said cryogenic exchanger, and T3 being less than or equal to the liquefaction temperature of said liquefied natural gas at atmospheric pressure, characterized in that said refrigerated gas erant consists essentially of a mixture of nitrogen and at least one other constituent chosen from neon and hydrogen.

The term “essentially consists” here means that traces of other gases in small proportions, not generally exceeding 1%, do not disturb the process or its overall yield, and are therefore acceptable as explained below.

The inventors have in fact discovered and demonstrated that the use of a refrigerant gas comprising a mixture of nitrogen and at least one other constituent chosen from neon and hydrogen makes it possible to increase the pressure PI without causing the dew point of the mixture thus formed is reached. More particularly, at -170 ° C, the saturation pressure of a nitrogen + neon or nitrogen + hydrogen mixture is greater than that of nitrogen alone, which makes it possible to increase the pressure of the mixture without generating condensation towards the The liquid state of none of the components of the mixture, since the critical point of neon and hydrogen is lower than that of nitrogen. Thus, the refrigerant gas does not condense towards the liquid state but remains in the gaseous state although its pressure is increased very significantly. This results in an increase in the pressure P3 and an improvement in the overall thermodynamic efficiency as well as a better heat transfer within the cryogenic exchangers, which increases the energy efficiency of the process, i.e. minimizes the energy. total consumed to produce a given quantity of LNG over a given period of time, and therefore allows the implementation of more compact installations, in particular rotating machines, compressors and turbines, more compact and circulation pipes of smaller section. The refrigerant gas mixture according to the present invention is therefore particularly advantageous in the case where the process is implemented without phase change on board a floating support where space is limited.

Neon and hydrogen in fact have a liquefaction temperature lower than that of nitrogen (which is -196 ° C at Patm) so that a mixture according to the invention has at a given pressure a dew point more lower than that of nitrogen alone and therefore, at the same temperature T3 'as nitrogen alone, the pressure P1 of the mixture according to the invention is higher, resulting in a better thermodynamic efficiency and better thermodynamic efficiency. heat exchange conditions, therefore better energy efficiency of the process in terms of total energy consumed for the liquefaction of natural gas, generally expressed in kW xh / kg or in kW x day / tonne (in English kW x day / t = kW xd / t) of 30 LNG produced.

Preferably, said mixture of refrigerant gases consists of a binary mixture of nitrogen and neon. Although the nitrogen-hydrogen mixture has a better overall energy yield, neon is preferred in view of the greater risk of explosion of hydrogen and

higher pressure level P1-P2-P3 in the case of said nitrogen-hydrogen mixture, the hydrogen may also have a certain propensity to percolate through the elastomeric seals and even through the thin metal walls. Neon is therefore preferred over hydrogen because it constitutes an inert gas and presents no danger in a compact installation installed on board a floating support where space is limited.

More particularly, said mixture of refrigerant gases comprises a mixture of nitrogen and said other component (s) with a molar content of said other component (s) of not more than 50 % preferably 20 to 40%. Beyond 50%, there is a decrease in energy efficiency.

More particularly still, in the process according to the invention, said natural gas to be liquefied is liquefied by carrying out the following concomitant steps of:

(a) circulation of said natural gas to be liquefied circulating at a pressure PO greater than or equal to atmospheric pressure (Patm), preferably PO being greater than atmospheric pressure, in at least 3 cryogenic heat exchangers arranged in series including:

- a first exchanger in which said natural gas entering at a temperature T0 is cooled and exits at a temperature Tl lower than T0, then

- a second exchanger in which the natural gas is completely liquefied and leaves at a temperature T2 lower than Tl and higher than T3, and

- a third exchanger in which said liquefied natural gas is cooled from T2 to T3, and (b) circulation in a closed circuit against the current of at least a first flow of refrigerant gas in the gaseous state at a pressure PI less than P3, in indirect contact with and against the current of the natural gas flow, said first flow (SI) at a pressure PI passing through the 3 exchangers entering said third exchanger at a temperature T3 'lower than T3, then entering said second exchanger at T2 ′ less than T2, then entering said first exchanger at Tl ′ less than Tl and leaving said first exchanger at a temperature TO ′ less than or equal to TO,

- Said first flow of refrigerant gas at P1 and T3 'being obtained by expansion in at least a first regulator of at least a first part of at least one second flow of refrigerant gas compressed at P3 greater than P1 circulating at co- stream of said natural gas entering said first exchanger at TO and leaving said second exchanger substantially at T2, and

a second part of said second stream of compressed refrigerant gas P3 circulating in co-current with said natural gas entering said first exchanger at TO and leaving said first exchanger substantially at T1, is expanded in a second regulator at said pressure Pl and at a said temperature T2 ', and is recycled to join said first flow at the inlet of said second exchanger; the flow rate D2 of said second part of the second flow being greater than the flow rate DI of the first part of the second flow, and (c) said second flow compressed to P3 being obtained by compression by at least one compressor followed by at least one cooling of said first flow of recycled refrigerant gas leaving said first exchanger, preferably at least a first compressor coupled to said first expander, and (d) preferably, after step (a) the liquefied natural gas is depressurized from the pressure PO to atmospheric pressure where applicable.

The separation of the second flow S2 into two parts of different flow rates DI and D2 at the outlet of the first exchanger, preferably with

D2 greater than Dl, is advantageous because most of the energy consumed occurs in phase 2 within the second exchanger.

Thus only a minor part of the flow Dl is expanded to reach the lowest temperature of substantially -165 ° C, and passes through the third exchanger EC3 where phase 3 occurs while the flow

D = D1 + D2 of the SI circuit then passes through the cryogenic exchanger EC2 to ensure phase 2 of the liquefaction.

In one embodiment, 3 compressors are used, at least a first compressor and a second compressor arranged in parallel, comprising:

a compressor called a third compressor actuated by an engine, preferably a gas turbine, to compress from Pl to P'3 all of the first flow of refrigerant gas, P'3 being between Pl and P3, and

a first compressor coupled to the first expansion valve consisting of a turbine, to compress from P'3 to P3 at least a first part of said first flow of refrigerant gas leaving said first exchanger, and

a second compressor coupled to the second expansion valve consisting of a turbine, for compressing from P'3 to P3 at least a second part of said first flow of refrigerant gas leaving said first exchanger.

Rather than recycling after expansion a part D2 of the second flow coming from the outlet of the first exchanger, to join the first flow at the inlet of the second exchanger, in a preferred embodiment, this part D2 of the second flow is recycled to the entry of the second exchanger at an intermediate pressure P2 greater than Pl in a third independent circuit S3 and parallel to the circuit SI, that is to say in co-current of SI. Because most of the energy consumed for phase 2 of the process is transferred within said second exchanger, this makes it possible to further increase the heat transfers and the energy efficiency of the process.

More precisely, to do this, in a preferred embodiment of the process according to the invention, the said natural gas to be liquefied is liquefied by carrying out the following concomitant steps of:

(a) circulation of said natural gas to be liquefied circulating at a pressure PO greater than or equal to atmospheric pressure, preferably PO being greater than atmospheric pressure, in at least 3 cryogenic heat exchangers arranged in series, including:

- a first exchanger in which said natural gas entering at a temperature TO is cooled and exits at a temperature T1 lower than TO, then

- a second exchanger in which the natural gas is completely liquefied and leaves at a temperature T2 lower than Tl and higher than T3, and

- a third exchanger in which said liquefied natural gas is cooled from T2 to T3, and (b) closed-circuit circulation of at least two streams of refrigerant gas in the gaseous state called first and third streams respectively at different pressures PI and P2, passing through at least two said exchangers in indirect contact with and against the flow of the natural gas flow, comprising:

- a first flow of refrigerant gas at a pressure PI lower than P3 passing through the 3 exchangers entering said third exchanger at a temperature T3 'lower than T3, then entering at T2' lower than T2 in said second exchanger, then entering at Tl 'less than Tl in said first exchanger and leaving said first exchanger at a temperature TO' less than or equal to TO, said first flow of refrigerant gas at PI and T3 'being obtained by expansion in at least a first expansion valve of a first part of a second flow of refrigerant gas compressed at the pressure P3 greater than P2, said second flow circulating in indirect contact with and in co-current with said flow of natural gas, entering said first exchanger at TO and said first part the second flow leaving said second exchanger substantially at T2, and

- a third flow at a pressure P2 greater than PI and less than P3 circulating in indirect contact with and in co-current with said first flow, passing only through said second and first exchangers, entering said second exchanger at substantially a temperature T2 'and leaving said first exchanger substantially at TO ', said third flow of refrigerant gas at P2 and T2 being obtained by expansion in a second regulator of a second part of said second flow of refrigerant gas leaving said first exchanger substantially at T1, the flow D2 of said second part of the second flow being greater than the flow rate DI of the first part of the second flow, (c) said second flow of refrigerant gas compressed to pressure P3 being obtained by compression by at least two compressors and cooling of said first and third flow of refrigerant gas leaving said first exchanger at PI and respectively P2, preferably by at least two first and second compressors s arranged in series and coupled respectively to said first and second regulators consisting of gas turbines, and (d) preferably, after step (a) the liquefied natural gas leaving said third exchanger is depressurized at T3, from the pressure PO at atmospheric pressure where appropriate.

In a still preferred embodiment, means are implemented to vary said pressure P2 in a controlled manner.

More particularly, to do this, at least one compressor is coupled to a motor making it possible to vary at least the pressure P2 in a controlled manner, by supplying power in a controlled manner to said compressor, preferably at least 3%, more preferably still. from 3 to 30% of the total power supplied to all of the said compressors used. More particularly still, it is observed that when the power injected into said motor is increased, the pressure PI remains constant, the pressure P2 increases and the efficiency increases, that is to say that the energy consumption expressed in kW x day / t decreases, until it reaches a minimum, then by further increasing the power supplied by said motor, in particular beyond 30% of the total power, said energy consumption increases again.

More particularly still, it is possible to modulate and specifically control the value of the pressure P2 by mounting the two first and second compressors C1 and C2 in series and by coupling the first compressor C1, already coupled with the expansion turbine El, with a first complementary motor M1 making it possible to modulate and control the power supplied to C1 and therefore the value of the pressure P2.

This method is more particularly advantageous because it makes it possible, by modulating and specifically controlling the supply of additional power at the level of M1, to specifically control the value of the pressure P2 of said third flow, and then thus makes it possible to optimize the point of operation of the process, namely minimizing the energy consumed in particular when, as happens during operation, the composition of the natural gas to be liquefied varies.

More particularly still, it is observed that when the power injected into said first motor is increased, the pressure PI remains substantially constant, the pressure P2 increases and the efficiency increases, that is to say that the energy consumption expressed in kW x day / t decreases, until it reaches a minimum, then by further increasing the power supplied by said motor, in particular beyond 30% of the total power, said energy consumption increases again.

A conventional liquefaction unit is sized in relation to the powers of the gas turbines available, the high power turbines being commonly 25MW.

We generally seek to increase the power of the installation, and it is then possible to install two identical gas turbines in parallel to obtain double power, but we then have two lines of rotating machines, which increases the size, quantities of pipes and of course the costs.

By installing a single GT turbine of n MW and adding power less than n MW at the level of a said second motor M2, the operation of the process is identical in terms of efficiency to that using two gas turbines of n MW in parallel.

Thus, the addition of power at the level of the second motor M2, preferably by virtue of an electric motor, gives more flexibility in operation and thus allows an increase in power. On the other hand, the yield of the whole remains unchanged.

If, on the other hand, the same power is supplied to the level of the first motor M1, the overall power is always the same, but in this case the efficiency of the assembly is improved, which represents a gain in energy consumed for the same power. overall, relative to a power injection at the second engine M2.

Likewise, by installing a single n MW GT turbine and adding power to the GT turbine, less than n MW, the operation of the process is identical in terms of efficiency to that using two gas turbines of n MW in parallel.

Thus, by considering a gas turbine, the addition of power at the level of the GT turbine, gives more flexibility to the operation and thus allows an increase in power. On the other hand, the yield of the whole remains unchanged.

If, on the other hand, the same power is supplied to the level of the first motor M1, the overall power is always the same, but in this case the efficiency of the assembly is improved, which represents a gain in energy consumed for the same overall power. provided, compared to a power injection at the second engine M2.

Thus, depending on the production of natural gas, both in quantity and quality, from groundwater, a GT gas turbine, for example of 25MW, will be used advantageously, at full speed continuously, which will be completed. , or even if necessary modulate, by

- power injection at the level of the GT turbine or of the second M2 engine without changing the overall efficiency, and

- power injection at the level of the first M1 engine, which has the effect of improving the overall efficiency, until an optimum is reached, that is to say a minimum of energy consumption.

In a preferred embodiment, the two first and second compressors arranged in series are coupled respectively to said first and second expansion valves consisting of energy recovery turbines, and

- at least said first compressor is coupled to a first motor, and

- A gas turbine is coupled either to said second compressor, the latter compressing said second flow of refrigerant gas directly to P3, or to a third compressor mounted in series after the second compressor, said third compressor compressing from P'3 to P3 said second flow of refrigerant gas, said gas turbine providing the major part of the total power supplied to all of said compressors used.

In this embodiment, most of the power supplied to said compressors is injected at the level of the second and / or third compressors compressing the flow of refrigerant gas at high pressure P'3 / P3 and the recovery of energy at the level of the first and second expansion valves is reinjected at the level of the first and second compressors, compressing the refrigerant gases circulating at low and medium pressure PI and P2. In fact, the fraction of fluid passing through compressor C1 represents a small fraction of the total flow (for example 10-15%) and the energy required is of the same order of magnitude as the energy recovered by the turbine El. It is therefore interesting to couple the two. Moreover, a controlled addition of power in Cl makes it possible to improve the energy efficiency of the system by controlling PI and P2 independently of one another.

On the other hand, most of the power supplied to the compressors is injected into the compressors providing the greatest pressure (P'3, P3), which makes it possible to increase the production capacity of the process, while improving its performance. energy efficiency.

Furthermore, the implementation of said first and second compressors in series coupled to said first and second expansion valves according to the present invention also makes it possible to improve the "

h compactness of the installation which is particularly advantageous for the implementation of a process on board a floating support where space is limited.

In the present description, the term “compressor coupled to an expansion valve / turbine or engine” or “compressor actuated by an engine” (or vice versa an “expansion valve / turbine or engine coupled to the compressor”) is understood to mean the shaft of The output of the turbine or respectively of the engine drives the input shaft of the compressor, that is to say, transfers mechanical energy to the shaft of the compressor. It is therefore a matter of a mechanical coupling of the compressor to the expansion valve / turbine or respectively of the compressor to the engine.

More particularly, said motor can be either a heat engine, or preferably an electric motor, or any other installation capable of supplying mechanical energy; and the compressors are of the rotary turbine type, also called centrifugal compressor.

More preferably, in the method, two compressors connected in series are used, comprising:

(i) a first compressor coupled to said first expander, compressing from PI to P2 all of said first flow of refrigerant gas leaving said first exchanger, and (ii) a second compressor coupled to said second expander, compressing from P2 to at least P ' 3, P'3 being greater than P2 and less than or equal to P3, on the one hand the said third flow of refrigerant gas exiting at P2 from the said first exchanger, and on the other hand the said first flow of refrigerant gas compressed at P2 leaving said first compressor, to obtain said second flow of refrigerant gas at P3 and TO after cooling, and (iii) said first compressor is coupled to a first engine, and said second compressor being coupled to at least one gas turbine, said first motor allowing the pressure P2 to be varied in a controlled manner by providing k

power in a controlled manner to said first compressor, said first motor providing at least 3%, more preferably from 30% of the total power supplied to all of said compressors used, said second compressor actuated by a gas turbine providing from 97 to 70% of the total power used.

This first variant embodiment is advantageous in that it allows for the most compact installation in terms of size on board the floating support.

More particularly, three compressors are used, including two first and second compressors connected in series, comprising:

(i) a first compressor actuated by a first motor and coupled to said first expander, compressing from PI to P2 all of said first flow of refrigerant gas exiting from said first exchanger, and (ii) a second compressor actuated by a second motor and coupled said second expander, compressing from P2 to P'3, P'3 being greater than P2 and less than P3, on the one hand said third flow of refrigerant gas leaving P2 from said first exchanger, and on the other hand said first stream of refrigerant gas compressed at P2 leaving said first compressor, and (Iîi) a third compressor driven by a gas turbine to supply the major part of the energy and compress from P'3 to P3 all of the first and third streams of refrigerant gas compressed by the second compressor, to obtain said second flow of refrigerant gas at P3 and TO after cooling, and (iv) said first motor provides at least 3%, more preferably from 3 to 30% of the power total provided to all of said compressors implemented, the gas turbine coupled to said third compressor, as well as a second motor coupled to the second compressor providing together from 70% of the total power supplied to all of said compressors placed in works.

This second variant embodiment is advantageous in terms of thermodynamic efficiency and production capacity, since a maximum capacity turbine available on the market is then advantageously used as gas turbine, that is to say 25-30MW in the case of gas turbine. of turbines intended to be installed on a floating support, plus a second electric motor of 5 to 10 MW connected to the second compressor, the overall power of the second engine and third engine (gas turbine) then being 30 to 40 MW, therefore much higher to that of the largest gas turbines available on the market and intended for floating supports. Advantageously, the second engine can also be a gas turbine, preferably of identical power to the main gas turbine, which then makes it possible to achieve an overall power of 50 to 60 MW.

By varying the pressure P2, the process according to the invention makes it possible to implement a minimum total energy Ef consumed in the process of less than 21.5 kW x day / t, more particularly from 18.5 to 20.5 kW x day / t of liquefied gas product.

In general, we will operate with a gas turbine GT at full speed, which will be completed by a power supply to the first engine M1, said input being limited to less than 30% of the overall power so to optimize the output to the minimum value between 18.5 and 21.5 kW x day / t, then if necessary, the overall power will be increased by power injection at the level of the second motor M2, and concomitantly the power injected at the level will be readjusted of the first motor M1, so that said power is always substantially equal to less than 30% of the overall power so as to keep the efficiency of the installation at the optimum value of between 18.5 and 21.5 kW x day / t .

Said optimum yield of 19.75 kW x day / t for a power of the first motor M1 representing 24% of the total power is valid for a refrigerant fluid consisting of 100% nitrogen. In the case of other refrigerant gas such as neon or hydrogen or mixtures of nitrogen or nitrogen-hydrogen, the optimum efficiency as well as the power percentage vary from 18.5 to 21.5 kW x day / t depending on the refrigerant gas or mixture and the percentages of neon or hydrogen, but the advantages detailed previously remain valid and even accumulate.

According to other particular characteristics:

- the composition of the gas to be liquefied is included in the following ranges for a total of 100%:

- Methane from 80 to 100%,

- nitrogen from 0 to 20%

- ethane from 0 to 20%

- propane from 0 to 20%, and

- butane from 0 to 20%; and

- the following temperatures:

- T0 and T0 'are 10 to 35 ° C (temperature in AA), and

- T3 and T3 'are from -160 to -170 ° C (temperature in DD), and

- T2 and T2 'are from -100 to - 140 ° C (temperature in CC), and

- Tl and Tl 'are from -30 to -70 ° C (temperature in CC);

For the following pressures:

- PO is 0.5 to 5 MPa (5 to 50 bars), and

- Pl is 0.5 to 5 MPa, and

- P2 is 1 to 10 MPa (10 to lOObars), and

- P3 is 5 to 20 MPa (50 to 200 bars).

Other characteristics and advantages of the present invention will become apparent in the light of the detailed description of various embodiments which will follow, with reference to the following figures.

- Figure 1 shows the diagram of a standard double loop liquefaction process using nitrogen as refrigerant gas,

- Figure 2 shows the diagram of a liquefaction process according to the invention with a triple loop using nitrogen or a mixture comprising nitrogen as refrigerant gas, in a so-called "balanced" version,

- Figure 3 shows the diagram of a liquefaction process according to the invention with a triple loop using nitrogen or a mixture comprising nitrogen as refrigerant gas, in a so-called "compact" version,

- Figure 4 shows a diagram of cooling and liquefaction of a natural gas in the context of a liquefaction process according to the invention representing the enthalpy of natural gas and the refrigerant fluid (kJ / kg) as a function of the temperature from TO to T3,

- Figures 5 and 5A represent diagrams of the total energy consumed (Ef) in kW x day per tonne of LNG produced (kW x day / t) of a liquefaction process according to the invention using a mixture of nitrogen and neon as refrigerant gas, depending on the pressure PI and the various neon percentages of said mixture,

- Figures 5 and SB represent diagrams the total energy consumed (Ef) kW x day / t of LNG produced from a liquefaction process according to the invention using a mixture of nitrogen and hydrogen as refrigerant gas, in as a function of the pressure PI and of the various percentages of hydrogen of said mixture,

- Figure 6A shows a diagram of the total energy consumed (Ef) in kW x day / t of LNG produced from a liquefaction process according to the invention using a mixture of nitrogen and neon as refrigerant gas as a function of the pressure P2 and various neon percentages of said mixture,

FIG. 6B represents diagrams of the total energy consumed (Ef) in kW x day / t of LNG produced from a liquefaction process according to the invention using a mixture of nitrogen and hydrogen as refrigerant gas, in as a function of the pressure P2 and various percentages of hydrogen of said mixture,

- Figure 7 shows a diagram of the total energy consumed (Ef) in kW x day / t of LNG produced from LNG produced in a prior art liquefaction process (60) and a liquefaction process according to invention, using nitrogen as refrigerant gas according to the pressure level P3,

- Figure 7A shows a diagram of the total energy consumed (Ef) in kW x day / t of LNG produced from a liquefaction process according to the invention using a mixture of nitrogen and neon as refrigerant gas as a function of the pressure P3 and various neon percentages of said mixture,

FIG. 7B represents a diagram of the total energy consumed (Ef) in kW x day / t of LNG produced from a liquefaction process according to the invention using a mixture of nitrogen and hydrogen as refrigerant gas depending on pressure P3 and various percentages of hydrogen of said mixture.

Figure 1 shows the PFD (Process Flow Diagram), that is to say the flow diagram of the standard double loop process without phase change using nitrogen as refrigerant gas. The process comprises compressors C1, C2 and C3, expansion valves El and E2, intercoolers HI and H2 as well as cryogenic exchangers EC1, EC2 and EC3. The heat exchangers consist, in a known manner, of at least two fluid circuits juxtaposed but not communicating with each other at the level of said fluids, the fluids circulating in said circuits exchanging heat throughout the path within said exchanger thermal. Many types of heat exchangers have been developed for the various industries and within the context of cryogenic exchangers two types predominate in a known manner: on the one hand wound exchangers, on the other hand the so-called “brazed” aluminum plate exchangers. in English "cold box".

Exchangers of this type are known to those skilled in the art and marketed by the companies LINDE (France) or FIVE Cryogénie (France). Thus, all the circuits of a cryogenic exchanger are in thermal contact with each other to exchange calories, but the fluids which circulate therein do not mix. Each of the circuits is dimensioned to have a minimum of pressure drops at the maximum flow rate of refrigerant fluid and sufficient resistance to withstand the pressure of said refrigerant fluid existing in the loop concerned.

Conventionally, a regulator achieves a pressure drop of a fluid or a gas and is represented by a symmetrical trapezoid, the small base of which represents the inlet 10a (high pressure), and the large base represents the outlet 10b (low pressure) as illustrated in FIG. 1 with reference to the regulator E2, said regulator possibly being a simple reduction in the diameter of the pipe, or else an adjustable valve, but in the case of the liquefaction process according to the invention the regulator is generally a turbine intended to recover mechanical energy during said expansion, so that this energy is not lost.

Similarly, and in a conventional manner, a compressor increases the pressure of a gas and is represented by a symmetrical trapezoid, the large base of which represents the inlet 11a (low pressure), and the small base represents the outlet 11b ( high pressure) as illustrated in FIG. 1 with reference to compressor C2, said compressor generally being a turbine or a piston compressor, or else a scroll compressor. According to the invention, preferably (FIGS. 2 and 3) the compressors C1 and C2 are mechanically connected to a motor M1 and M2 which can be either a heat engine or an electric motor, or any other installation capable of providing mechanical energy.

The natural gas circulates in the circuit Sg and enters at AA in the first cryogenic exchanger EC1 at a temperature TO, greater than or substantially equal to the ambient temperature, and Tl = -50 ° C. approximately. In this exchanger EC1, the natural gas cools, but remains in the gas state. Then it passes to BB in the cryogenic exchanger EC2, the temperature of which is between Tl = -50 ° C approximately and T2 = -120 ° C approximately.

In this EC2 exchanger, all the natural gas liquefies to

LNG at a temperature of T2 = -120 ° C approximately, then the LNG goes to DC in the cryogenic exchanger EC3. In this EC3 exchanger, the LNG is cooled down to a temperature of T3 = -165 ° C which allows the LNG to be evacuated in the lower part in DD, then to depressurize it in EE to finally store it liquid at atmospheric pressure ambient, that is to say at an absolute pressure of approximately 1 bar (i.e. approximately O.lMPa). Throughout this natural gas journey in the Sg circuit in the various exchangers, the natural gas cools by giving up calories to the refrigerant gas, which then heats up and must permanently undergo a complete thermodynamic cycle in order to be able to extract in a continues natural gas calories entering AA.

Thus, the path of natural gas is represented on the left of the PFD, and said gas circulates from top to bottom in the circuit Sg, the temperature decreasing from top to bottom, from a substantially ambient temperature TO at the top in AA, up to a temperature T3 of about -165 ° C at the bottom in DD.

On the right side of the PFD, there is shown the thermodynamic cycle of the double-loop refrigerant gas corresponding to circuits SI and S2. For clarity of explanation, the pressure levels in the main circuits are shown in thin lines for low pressure (Pl in the SI circuit), in medium lines for intermediate pressure (P2), and in solid lines for high pressure (P3 in circuit S2).

In a conventional diagram shown in FIG. 1, phases 1, 2 and 3 are carried out by a low pressure loop P1 at very low temperature at the lower inlet of EC3.

The installation is composed of:

• an engine, generally a gas turbine GT which drives compressor C3 and provides all of the mechanical power, • 3 compressors:

- C3 which compresses the entire flow of refrigerant,

- C2 which is coupled to the turbine E2 and which compresses the portion D'2 of the total flow D, and

- Cl which is coupled to the turbine El and which compresses the complementary portion D'I of the total flow D, • of 2 turbines,

- E2 coupled directly to compressor C2, and which expands portion D2 of the total flow D, from high pressure P3 to low pressure Pl,

- El coupled directly to the compressor C1, and which expands the portion Dl of the total flow D, from high pressure P3 to low pressure Pl, • a cryogenic exchanger in three parts or 3 exchangers in series EC1, EC2 and EC3, corresponding respectively to phase 1, phase 2 and phase 3 of liquefaction, comprising three circuits, respectively SG (natural gas) and S1-S2 (refrigerant gas), • at least two coolers, HI and H2, located respectively at the outlet of the main compressor C3 (Hl) and on the high pressure loop (H2), before entering the cryogenic exchangers.

A cooler H1, H2 can consist of a water exchanger, for example a sea or river water or cold air exchanger of the fan coil or cooling tower type, such as those used in nuclear power plants.

More precisely in Figure 1, there is shown the diagram of a process and installation in which the said natural gas to be liquefied is liquefied by carrying out the following concomitant steps of:

(a) circulation of said natural gas to be liquefied circulating Sg at a pressure PO greater than or equal to atmospheric pressure (Patm), preferably PO being greater than atmospheric pressure, in 3 cryogenic heat exchangers EC1, EC2, and EC3 arranged in series including:

a first exchanger EC1 in which said natural gas entering at a temperature TO is cooled and exits BB at a temperature T1 below TO at which all the components of said natural gas are still in the gaseous state, then

- a second exchanger EC2 in which the natural gas is completely liquefied and leaves in CC at a temperature T2 lower than T1, and

- a third exchanger EC3 in which said liquefied natural gas is cooled from T2 to T3, T3 being less than T2 and T3 being less than or equal to the liquefaction temperature of said natural gas at atmospheric pressure, and (b) circulation in a closed circuit at counter-current of a first flow SI of refrigerant gas in the gaseous state compressed at a pressure Pl less than P3 in indirect contact with and against the current of the flow of natural gas Sg, said first flow SI at a pressure Pl passing through the 3 exchangers EC3, EC2, and EC1 entering in DD into said third exchanger EC3 at a temperature T3 'lower than T3 then leaving said third exchanger and entering said second exchanger EC2 in CC at a temperature T2' lower than T2, then leaving the second exchanger and entering the first exchanger EC1 at BB at a temperature Tl 'lower than Tl and exiting at AA from said first exchanger EC1 at a temperature TO' less than or equal to TO,

- Said first flow SI of refrigerant gas at Pl and T3 'being obtained by expansion in a first expansion valve El of a first part Dl of a second flow S2 of compressed refrigerant gas at P3 greater than Pl circulating in co-current of said natural gas entering at AA in said first exchanger EC1 at TO and leaving CC from said second exchanger EC2 substantially at T2, and

a second part D2 of said second stream S2 of compressed refrigerant gas P3 circulating in co-current with said natural gas entering at AA in said first exchanger EC1 at TO and exiting from said first exchanger substantially at T1 is expanded in a second regulator E2 at said pressure Pl and at a said temperature T2 ', and is recycled to join said first flow at the CC inlet of said second exchanger, and (c) said second flow S2 compressed at P3 is obtained by compression by three compressors C1, C2 , and C3 followed by at least two coolings HI and H2 of said first flow SI of recycled refrigerant gas leaving at AA from said first exchanger EC1, by a first compressor C1 coupled to said first expander El, and (d) after step ( a) the liquefied natural gas is depressurized from pressure PO to atmospheric pressure.

More precisely, in FIG. 1, 3 compressors are used, including 2 first and second compressors arranged in parallel comprising:

- a third compressor C3 actuated by an engine, preferably a gas turbine GT to compress from PI to P'3, P'3 being between PI and P3, all of said first flow of refrigerant gas coming from the outlet AA of said first EC1 exchanger, and

- a first compressor C1 coupled to the first expansion valve El consisting of a turbine, to compress from P'3 to P3, a part Dl 'of said first flow of refrigerant gas, compressed by the third compressor C3, and

- A second compressor C2 coupled to the second expansion valve E2 consisting of a turbine, for compressing from P'3 to P3 a part D2 'of said first flow of refrigerant gas compressed by the third compressor C3.

In FIG. 1, C1 and C2 are therefore arranged in parallel and operate between the medium pressure P'3 and the high pressure P3 over the entire flow coming from C3.

The refrigerant gas at the high outlet in AA of the circuit SI, at the level of the exchanger EC1 has a flow rate D: it is at the low pressure PI and at a temperature TO 'substantially lower than TO and at the ambient temperature. It is then compressed at C3 to pressure P'3 and then passes through a cooler H1. The flow rate fluid D is then separated into two parts of flow rates DI 'and D2' which respectively feed the compressors C1 (DI ') and C2 (D2') operating in parallel. The two flows at pressure P3 are then combined and then cooled substantially to ambient temperature TO while passing through the cooler H2. This overall flow D then enters the top of the cryogenic exchanger EC1 at the level of the circuit S2, then at the exit of the first level, in BB, a large part of the flow rate D2 (D2 greater than Dl) is extracted and directed. to the turbine E2 coupled to the compressor C2. The rest of the flow D1 passes through the second stage of the cryogenic exchanger EC2, then at level CC is directed to the turbine El coupled to the compressor C1.

At the outlet of the turbine El, the refrigerant gas, at a temperature T3 'lower than T3 = -165 ° C, is then directed towards the bottom of the cryogenic exchanger EC3 in the circuit SI and goes up against the current of the gas at liquefy circulating in the circuit Sg, of which it ensures the final phase 3 of the liquefaction.

The flow D2 of refrigerant gas coming from the turbine E2 is at a pressure PI and temperature T2 of about -120 ° C and is recombined within the circuit SI with the flow D1 coming from the turbine El at the level of the upper outlet of the cryogenic exchanger EC3 in CC.

The separation of the second flow S2 into two parts of different flow rates Dl and D2 at the outlet BB of the first exchanger, preferably with D2 greater than Dl, is advantageous because most of the energy consumed occurs in phase 2 within the second exchanger EC2. Thus only a minor part of the flow Dl passes through the third exchanger EC3 where phase 3 occurs, while the total flow D = D1 + D2 of the circuit SI then passes through the cryogenic exchanger EC2 to ensure phase 2 of the liquefaction (temperature from T1 = -50 ° C to T2 = -120 ° C).

The same flow D of the circuit SI finally passes through the cryogenic exchanger EC1 to ensure phase 1 of the liquefaction process (temperature from T1 = -50 ° C to TO = ambient temperature). At the upper outlet of the cryogenic exchanger EC1, the flow D of the circuit SI is at the temperature TO ′ substantially lower than the ambient temperature.

Then, the flow D is again directed to the compressor C3 to continuously perform a new cycle.

In this configuration, compressors C1 and C2 operate in parallel and must ensure the highest level of pressure in the cycle. The two compressors C1 and C2 process different refrigerant fluid flow rates, respectively DI 'and D2', and are coupled directly to the turbines El and E2 which also process different flow rates, DI and D2 respectively.

We have the relation:

DI + D2 = D = D'I + D'2, with DI different from D'I and D2 different from D'2. In practice, preferably D1 / D = 5 to 35%, preferably 10 to 25%.

Thus, in this type of installation, all of the power is injected into the system at the level of the compressor C3 (by the gas turbine GT), the power transfers at the level of the turbine compressor pairs E2-C2 and El- C1 being variable depending on the pressures in the various circuits (P1-P2-P3), the temperature levels at the inlet of the cryogenic exchangers, as well as the heat transfers within each of these said cryogenic exchangers.

Thus, such an installation has an operating point which stabilizes itself at a given level of energy consumption Ef generally expressed in kW x day / t, that is to say in kW-day per tonne of LNG produced, or again in kWh per kg of LNG produced, said operating point possibly being totally unstable. It is then very difficult to control the pressures of the high and low loops independently of each other. This may prove to be necessary in the case of variations in the composition of the natural gas to be liquefied. It is possible to modify the flows by locally constraining all or part of the Dl-D'l-D2-D'l flows, for example by creating localized pressure drops, but such arrangements lead to energy losses, therefore a drop in the overall efficiency of the liquefaction plant.

The diagram in FIG. 4 illustrates the variation in enthalpy H, expressed in kJ / kg of LNG produced, in a natural gas liquefaction process. This diagram in figure 4 is the result of a theoretical calculation relating to a natural gas comprising mainly methane (85%), the remainder (15%) consisting of nitrogen, ethane (C-2), propane (C-3) and butane (C-4).

We have represented:

- phase 1 for cooling natural gas between points AA and BB corresponding to stage EC1 of the PFD in figure 1, corresponding to temperatures between the ambient temperature TO and Tl = -50 ° C,

- phase 2 of natural gas liquefaction between points BB and CC, corresponding to stage EC2 of the PFD in figure 1, corresponding to temperatures between Tl = -50 ° C and T2 = -120 ° C,

- phase 3 for cooling the LNG between points CC and DD, corresponding to stage EC3 of the PFD in figure 1, corresponding to temperatures between T2 = -120 ° C and T3 = -165 ° C.

The curve 50 comprising triangles illustrates the variations in the enthalpy H of the fluids circulating in co-current in the circuits Sg and S2 as a function of the temperature of the gas to be liquefied comprising the methane / LNG for an ideal virtual process.

The curve 51 corresponds to the variation of the enthalpy H of the refrigerant gas circulating in the circuit SI of FIG. 1, therefore represents the energy transferred to the circuits Sg and S2 during the liquefaction process.

The surface 52 between the two curves 50 and 51 represents the overall loss of energy consumed Ef in the liquefaction process: it is therefore sought to minimize this surface so as to obtain the best efficiency. In land-based processes using refrigerant phase change processes, curve 51 is no longer rectilinear, but is much closer to theoretical curve 50, which implies less losses, therefore improved efficiency, but the The refrigerant phase change process is not suitable for liquefaction on board a floating support in a confined environment.

Figures 2 and 3 illustrate the PFD diagram of the improved process according to the invention, in which the path of the natural gas to be liquefied mainly comprising methane and traces of other gases, is identical to that of Figure 1, and s' performs in the same way within the circuit Sg, from the top (substantially ambient temperature TO) to the bottom (liquid state at T3 = -165 ° C), through three cryogenic exchangers EC1, EC2 and EC3.

In Figures 2 and 3, rather than recycling after expansion a part D2 of the second flow at the outlet of the first exchanger to join the first flow at the low inlet CC of the second exchanger as in Figure 1, this part D2 of the heat exchanger is recycled. second flow at the input CC of the second exchanger at an intermediate pressure P2 greater than PI in a third circuit S3 independent of SI, S2, SG, and parallel to SI, that is to say in co-current of SI.

Because most of the energy is consumed for phase 2 of the process within said second exchanger, this makes it possible to further increase the heat transfers and the overall energy efficiency of the process. But more importantly, it is also possible to modulate and specifically control the value of the pressure P2 by mounting the two compressors C1 and C2 in series and by coupling Cl with a motor M1 making it possible to modulate and control the additional power supplied to Cl already coupled to the turbine El, and therefore to control the value of the pressure P2 as described below.

More precisely, in FIGS. 2 and 3, there is shown the process and installation in which the said natural gas to be liquefied is liquefied by carrying out the following concomitant steps of:

(a) circulation of said natural gas to be liquefied circulating Sg at a pressure PO greater than or equal to atmospheric pressure (Patm), PO being greater than atmospheric pressure, in 3 cryogenic heat exchangers EC1, EC2, and EC3 arranged in series including :

a first exchanger EC1 in which said natural gas entering at a temperature TO is cooled and exits at BB at a temperature Tl lower than TO, temperature Tl at which all the components of the natural gas are still in the gaseous state, then

- a second exchanger EC2 in which the natural gas is completely liquefied and leaves in CC at a temperature T2 lower than T1, and

- a third exchanger EC3 in which said liquefied natural gas is cooled from T2 to T3, T3 being less than T2 and T3 being less than the liquefaction temperature of said natural gas at atmospheric pressure, and (b) closed circuit circulation of two streams SI and S3 of refrigerant gas in the gaseous state respectively called first and third flow, respectively at different pressures PI (SI) and P2 (S2), passing through two said exchangers in indirect contact with and against the current of the gas flow natural Sg, comprising:

- a first flow of refrigerant gas SI at a pressure PI less than P3 passing through the 3 exchangers EC1, EC2 and EC3 entering DD into said third exchanger EC3 at a temperature T3 'lower than T3 then leaving said third exchanger and entering the said second exchanger EC2 in CC at a temperature T2 ′ lower than T2, then leaving the second exchanger and entering the first exchanger EC1 in BB at a temperature ΤΓ less than T1 and exiting at AA from said first exchanger at a temperature TO ′ lower than TO, said first flow of refrigerant gas at PI and T3 'being obtained by expansion in a first regulator El of a part DI of a second flow S2 of refrigerant gas compressed at the pressure P3 greater than P2, said second flow S2 circulating in indirect contact with and co-current of said natural gas flow Sg entering at AA in said first exchanger EC1 substantially at TO and exiting at CC from said second exchanger EC) substantially at temperature T2, and

a third flow S3 at a pressure P2 greater than PI and less than P3 circulating in indirect contact with and in co-current with said first flow, passing only through said second and first exchangers EC2 and EC1, entering at DC into said second exchanger substantially at a temperature T2 ′ below T2 and leaving at AA from said first exchanger EC substantially at a temperature TO ′, said third flow S3 of refrigerant gas at P2 and T2 being obtained by expansion in a second expansion valve E2 of a part D2 of said second flow S2 of refrigerant gas leaving said first exchanger substantially at T1, (c) said second flow of refrigerant gas S2 compressed to pressure P3 being obtained by compressing said first and third flow of refrigerant gas leaving at AA from said first exchanger EC1 at PI and respectively P2, by two first and second compressors, respectively C1 and C2 arranged in series and coupled respectively to said first and second expansion valves El and E2 c onsisting in 15 turbines, and (d) after step (a) depressurizes the liquefied natural gas leaving at DD from said third exchanger at T3, from pressure PO to atmospheric pressure where appropriate.

More specifically, in Figure 2, we implement:

(1) three compressors C1, C2 and C3 mounted in series, comprising:

(i) a first compressor C1 coupled to said first expansion valve El, compressing from PI to P2 all of said first flow of refrigerant gas leaving at AA from said first exchanger

EC1, and (ii) a second compressor C2 coupled to said second expansion valve E2, compressing from P2 to P'3, P'3 being greater than P2 and less than or equal to P3, on the one hand said third flow S3 of refrigerant gas leaving at P2 from said first exchanger EC1, and on the other hand said first flow of refrigerant gas compressed at P2 leaving said first compressor C1, and (iii) a third compressor C3 actuated by a gas turbine GT to supply the major part energy and compress from P'3 to P3 all of the first and third streams of refrigerant gas compressed by the second compressor C2, to obtain said second stream of refrigerant gas at P3 and TO after cooling (Hl, H2), and (2) said first compressor C1 is coupled to a first motor M1, making it possible to vary the pressure P2 in a controlled manner by supplying power in a controlled manner to said first compressor C1, said first motor M1 providing at least 3%, more preferably from 3 to 30% of the potency nce provided to all of said compressors used C1, C2 and C3, the gas turbine GT coupled to said third compressor C3, as well as the second motor M2 coupled to the second compressor C2 together providing 97 to 70% of the total power supplied to all of the said compressors used C1, C2 and C3.

The installation in figure 2 is therefore composed of:

a plurality of engines, in general a gas turbine GT which drives the compressor C3 and M1-M2 engines, for example either electric or thermal, such as gas turbines, respectively connected to the compressors C1-C2,

- 3 compressors:

- C3 which compresses the entire flow of refrigerant gas D,

- C2 which is coupled to the motor M2 and to the turbine E2, and which compresses the entire flow of refrigerant gas D,

- Cl which is coupled to the motor M1 and to the turbine El, and which compresses the portion Dl of the first flow of refrigerant gas,

- 2 regulators, for example turbines,

- E2 coupled to compressor C2 and motor M2,

- El coupled to the compressor C1 and to the motor M1,

- a cryogenic exchanger in three parts or 3 exchangers in series EC1, EC2 and EC3, corresponding respectively to phases 1, 2 and 3 of liquefaction and comprising four circuits, respectively SG (natural gas) and SlS2-S3 (refrigerant gas ),

- two coolers, H1 and H2, located respectively at the outlet of the main compressor C3 (Hl) before the entry into the circuit S2 of the cryogenic exchangers, and on the high pressure loop (H2).

Compressors C1 and C2 are mounted in series.

- Cl operates between the low pressure PI and the medium pressure P2, on the portion DI of the flow of refrigerant gas coming from the turbine El circulating in the circuit SI, from the bottom to the top, through each of the three cryogenic exchangers EC3- EC2-EC1.

- C2 operates between the medium pressure P2 and the intermediate high pressure P'3 on the entire flow D, composed of the portion DI of the flow coming from the compressor C1 and the portion D2 of the flow of refrigerant gas coming from the turbine E2 circulating in circuit S3, from bottom to top, through each of the two cryogenic exchangers EC2-EC1.

The entire flow of refrigerant gas D leaving compressor C2 is cooled in a cooler H1 before returning to pressure P'3 in compressor C3, the latter being connected to an engine (GT), generally a gas turbine . Said gas turbine and the engine (M2) together provide the refrigerant gas from 70 to 97% of the overall power Q, the remainder of power being supplied to the system at the level of the engine M1, namely from 30 to 3% of the overall power Q.

At the outlet of the compressor C3, the entire flow of refrigerant gas D is at high pressure P3. The flow is then cooled in a cooler H2 before circulating in the circuit S2, from top to bottom, through each of the two cryogenic exchangers EC1-EC2.

The portion D2 of the refrigerant gas flow is taken at BB at the outlet of the cryogenic exchanger EC1 and directed towards the inlet of the turbine E2, the remainder, ie the portion DI of the refrigerant gas flow being taken. in CC at the outlet of the cryogenic exchanger EC2 and directed towards the inlet of the turbine El.

Within the compressor C3, a cooler H2 operating at pressure P'3 is installed between two compression stages, said cooler H2 treating all of the stream D.

In this method according to the invention, we have the relationships:

DI + D2 = D and preferably D1 / D2 = 1/3 to 1/20, preferably 1/4 to 1/10.

The main advantage of the device according to the invention of FIG. 2 lies in the possibility of optimizing the overall efficiency of the installations and of modifying at will the operating points of the various loops corresponding to the circuits S1-S2-S3, that is that is to say to minimize the energy consumed by increasing or decreasing the power injected into one of the compressors C1-C2-C3, or by varying the distribution of the overall power Q injected into the system. These power adjustments injected into the various compressors C1-C2-C3 have the effect of modifying the flow rates in the various loops, therefore modifying the pressures Pl, P2 & P3 as well as the mass flow rates D, DI and D2 in the various S1-S2-S3 circuits, which gives great flexibility in optimizing the operating point of the installation and therefore great ease and rapidity during process readjustments following fluctuations in the composition of the natural gas at liquefy from underground reservoirs. These variations can be significant during the life of the gas production field, which can extend over 20 to 30 years or more.

Thus, in the diagram of FIG. 4 relating to a natural gas comprising 85% methane, the remainder consisting of nitrogen, ethane (C-2), propane (C-3) and butane (C- 4), the curve 50 comprising triangles, illustrates the variations of the enthalpy H of the fluids circulating in the circuits Sg and S2 of FIG. 2 as a function of the temperature of the natural gas / LNG for an ideal virtual process.

The curve 53 corresponds to the variation of the enthalpy H of the refrigerant circulating in the circuits S1 and S3 of FIG. 2, therefore represents the energy transferred during the liquefaction process to the circuits Sg and S2 of FIG. 2.

The surface 52 between the two curves 50 and 53 represents the overall energy loss in the liquefaction process with reference to FIG. 2: it is therefore sought to minimize this surface so as to obtain the best efficiency.

During variations over time in the quality of the natural gas supplied by the gas field, and therefore in its composition, the low point 54 of the curve 50 corresponding to PO and T2 at the end of LNG liquefaction, may vary by a few%. In the conventional process of Figure 1, the corresponding point 55 of the refrigerant gas circuit remains substantially fixed, and the surface 52, therefore the efficiency of the installation can not be optimized.

On the other hand, in the device according to the invention according to FIG. 2, by playing on the distribution of the mechanical energy and in particular on the energy injected in GT, in M1 and M2, and more particularly in M1, it is possible to make advantageously vary the position of point 56, which it is thus known to move optimally in the direction of point 54, which makes it possible to reduce to a minimum the surface area of area 52 between curves 50 and 53, and thereby optimizing the performance of the liquefaction plant in real time, depending on the composition of the natural gas.

FIG. 3 represents the PFD diagram of a version of the invention exhibiting improved compactness compared to the methods and installation of FIG. 2, in which the compressor C2 is integrated on the same shaft line as the compressor C3 and is powered by the gas turbine GT representing a mechanical energy input of 85 to

95% of the total energy Q. In this configuration, the expansion turbine E2 is then connected on the one hand to the compressor C2 and on the other hand to the gas turbine GT.

In this version of FIG. 3 exhibiting greater compactness than the version described with reference to FIG. 2, however, there is less latitude to adjust the operating points of the various loops, because the power adjustments cannot then be made. than at the level of GT engines connected to C3 and M1 connected to Cl. Thus, this compact version is advantageously justified in the event of a very limited available surface, and moreover there are only two rows of rotating machine shafts and two compressors, while in the version with reference to figure 2, three lines of rotating machine shafts and three compressors must be installed, which represents a significant additional cost, but provides greater flexibility in the fine adjustment of the various loops of pressure, as well as a better final yield, therefore a better profitability of the installations in the long term, during the entire lifespan of the installations which exceeds 20 to 30 years, or even more.

In FIGS. 5 to 9 discussed below, the results of the tests in which the values of P1, P2 and P3 are varied to minimize the total energy consumed Ef in kW x day / t as a function of the variation the composition of the refrigerant gas.

In FIGS. 5-5A-5B, there is shown the diagram of the energy yield, more precisely of Ef expressed in kW x day / t, as a function of the pressure P1, and as a function of the various variants of the invention. In fact, this pressure P1 is constant for a given refrigerant gas composition, which explains why all the points of the same curve are on a line parallel to the ordinates. This pressure Pl corresponds to the lowest temperature T3 'of the device, that is to say the temperature at the low inlet of the cryogenic exchanger EC3. This pressure Pl corresponds substantially to the dew point of the refrigerant gas at a temperature T3 'substantially lower than T3 = -165 ° C, that is to say the temperature at which the LNG will remain liquid under a pressure corresponding to atmospheric pressure, i.e. substantially absolute O.lMPa, ie substantially one atmosphere.

In FIGS. 5, 5A and 5B, it is observed that by mixing nitrogen with neon or hydrogen, up to a molar proportion of 50%, the pressure PI can be increased, which is accompanied by 'a reduction in the optimum energy consumed at the stabilized operating point, and therefore a better energy efficiency of the liquefaction process.

On the other hand, on diagram 5A relating to an azoteneon mixture, the operating point in the case of the conventional process of FIG. 1 with pure nitrogen is located at 60. The curve 70 (right portion) represents the variation of the energy efficiency as a function of the power injected into the process at the level of the motor M1 with reference to FIGS. 2 and 3. The upper point W0 = 0 of the curve 70 corresponds to an unpowered motor M1, therefore providing zero power. Point W1 corresponds to a power Wl> 0 supplied by said motor M1. Likewise, the successive points of the curve correspond to increasing powers supplied to the system at the level of the motor M1, namely W4> W3> W2> Wl> W0 = 0.

Points W0 to W4 correspond to powers injected into the motor M1:

W0 = zero power,

W1 = 7% of the overall power,

W2 = 15% of the overall power,

W3 = 24% of the overall power,

W4 = 33% of the overall power.

Similarly in the diagram of FIG. 6A, the energy efficiency is shown as a function of the pressure P2, and as a function of the various variants of the invention. Curve 90 represents the process according to FIG. 2 using a refrigerant gas composed of 100% nitrogen. As in figure 5A, the upper point

W0 = 0 of curve 90 corresponds to an unpowered motor, therefore providing zero power. The point WI corresponds to a power Wl> 0 supplied by said motor M1. Likewise the following points of the curve correspond to increasing powers supplied to the system at the level of the motor M1, such as W4> W3> W2> Wl> W0 = 0: - said powers WI to W4 being identical in figures SA and 6A .

Thus, in this same FIG. 6A, it is observed that when the power W injected at the level of M1 is increased, the pressure PI remains constant, but the pressure P2 increases and the efficiency increases, that is to say that the consumption in energy expressed in kW x day / t decreases, until it reaches a minimum 90a, here substantially coincident with point W3, then said energy consumption increases again towards W4. This minimum 90a corresponds to the low point 70a of curve 70 in figure SA, for a minimum energy consumption of approximately 19.75 kW x day / t, a pressure PI of approximately 9 bars and a pressure P2 of approximately 28 bars. In comparison, the operating point W0 without energy input to the motor M1 corresponds, for a pure nitrogen process, to an energy consumption of approximately 21.25 kWxd / t, at the same pressure PI of approximately 9 bars and a pressure P2 of about 11 bars: the energy efficiency is therefore improved by 7.06%.

Similarly in the diagram of FIG. 7A, the energy efficiency is shown as a function of the pressure P3, and as a function of the various variants of the invention, in particular in the case of a neon nitrogen mixture. Points W0-W1-W2-W3-W4 correspond to the same levels of power injected into the motor M1 as previously described with reference to FIGS. 5A-6A. P3 thus represents the maximum pressure of the system at the level of circuit S3: it increases in proportion to the power injected, as well as to the percentage of neon in the refrigerant gas mixture.

Thus, an increase in the proportion of injected power W at the level of the motor M1 of figures 2-3 compared to the total injected power:

- has no influence on the PI pressure,

- increases the pressure P2,

- increases the maximum pressure P3,

- decreases the energy consumption Ef to a minimum value, for a given proportion of power W, then this energy consumption increases again beyond this said proportion.

Likewise, the use of a nitrogen-neon mixture leads to an improvement in energy performance as shown in FIGS. 5A and 6A, both in the conventional methods described with reference to FIG. 1 and in the methods described in reference to figures 2 - 3.

Thus, considering a mixture comprising 20% neon, the pressure PI is about 12.5 bars and the curve 71 of FIG. 5A represents the variations in energy consumption for the same increasing powers supplied to the system at the level of the motor M3 (W4> W3> W2> Wl> W0 = 0).

For this same neon percentage of 20%, on curve 91 of FIG. 6A, the variations in energy consumption have been represented for the same increasing powers supplied to the system at the level of the motor M1 (W4> W3> W2> Wl > W0 = 0), depending on the pressure P2. It is thus observed that when the power W injected at the level of Ml is increased, the efficiency increases, i.e. the energy consumption expressed in kWxd / t decreases, until reaching a minimum 91a, located between the points W2 and W3 of said curve 91, then said energy consumption increases again towards W4. This minimum corresponds to the low point 71a of the curve 71 of FIG. 5A, for a minimum energy consumption of approximately 19.4 kWxd / t, a pressure PI of approximately 12.5 bars and a pressure P2 of approximately 33 bars. In comparison, the operating point W0 of the same curve 91 corresponding to a 20% mixture of neon, without energy input to the motor M1 corresponds to an energy consumption of approximately 20.45 kW x day / t , at the same pressure PI of approximately 12.5 bars and a pressure P2 of approximately 17 bars, which illustrates the improvement in energy efficiency when the increase in the percentage of neon and the increase in the injected power are combined at the level of the engine Ml.

The same effects are observed for hydrogen in Figures 5B and 6B.

FIGS. 5 to 7 show performance diagrams of a conventional process and of the process according to the invention, for liquefying a natural gas consisting of 85% methane, and 15% of said other constituents.

In the diagram of FIG. 7A, the maximum pressure P3 is represented on the abscissa and the energy per unit mass of gas is on the ordinate. Energy is represented in kW x day / tonne of natural gas (1 kW x day / t = 0.024 kWh / kg). Thus, for a refrigerant gas consisting of 100% nitrogen, the operating point of the conventional process with reference to FIG. 1 is located at 60 in this FIG. 7A. On the other hand, in the process according to the invention with reference to FIGS. 2 and 3, for various nitrogen-neon mixture compositions, by injecting power at the level of the motor M1, the efficiency of the installation can be varied according to the curve 70 (20% neon) and other curves (40 50% neon). Thus, from an operating point of 45-50 bars according to the conventional method, corresponding to an energy consumption of approximately 21.3 kW x d / t, the thermodynamic efficiency can be increased by increasing the maximum pressure. Thus, as shown in this same diagram, for a refrigerant gas consisting of 100% pure nitrogen, by injecting part of the power at the level of the motor M1, and by operating at a pressure of approximately 68 bars, the consumption in energy drops to around 19.75 kWxd / t, which represents an efficiency gain of 7.28%.

In general, by operating at higher pressure, for a given mass flow rate, the volume flow rates are reduced in proportion to the increase in said pressure: - the pipes are of smaller diameter, but their mechanical resistance, therefore their thickness , their weight and cost are increased by as much: - on the other hand, the footprint is reduced accordingly, which is very interesting in the case of installations in a confined environment such as on a floating support anchored at sea, or even on an LNG carrier in the case of a boil-off reliquefaction unit. Likewise, compressors and turbines operating at higher pressure are much more compact. With regard to cryogenic exchangers, the increase in pressure also improves heat transfers, but the heat exchange surfaces are not reduced in the same proportion as in the case of pipes and compressors and turbines. On the other hand, their weight increases significantly because they must resist this increase in pressure.

Thus, overall, the method according to the invention of FIGS. 2-3 leads to installations having greater compactness and to a significant improvement in energy efficiency when the refrigerant gas is pure nitrogen, said energy efficiency being further improved. when the refrigerant gas is a mixture of nitrogen and either neon or hydrogen.

In FIG. 7A, there is shown a performance diagram of a conventional method with reference to FIG. 1, and of the method according to the invention of FIGS. 2-3 using as refrigerant gas a mixture of nitrogen and neon, in which the maximum pressure P3 is represented on the abscissa and the energy per unit mass of gas is on the ordinate. Energy is represented in KW x day per ton of natural gas (kW x d / t).

Thus, for a given composition of gas, the operating point of the conventional process with reference to FIG. 1 is located at 60 in this FIG. 7A. In the method according to the invention with reference to FIGS. 2 and 3, using a refrigerant gas composed of 100% nitrogen, by injecting power at the level of the motor M1, the efficiency of the installation can be varied according to curve 61 with an optimum operating point 62 at approximately 68 bars, corresponding to an energy consumption of approximately 19.75 kWxd / t, which represents an efficiency gain of 7.28% compared to the point

I operating 60 of the conventional method.

By using a mixture of 80% nitrogen and 20% neon as refrigerant gas, the pressure can be increased, as shown on curve 70, without the gas mixture reaching its dew point, up to an optimum value 70a of approximately 88 bars and for a minimum energy consumption of approximately 19.4 kWxd / t, which represents a gain in thermodynamic efficiency of 1.77% compared to the operating point 62 of the process according to the invention with a refrigerant gas composed of 100% nitrogen and a thermodynamic efficiency gain of 8.92% compared to the operating point 60 of the conventional process.

By using a mixture of 60% nitrogen and 40% neon as refrigerant gas, the pressure can be increased, as shown on curve 71, without the gas mixture reaching its dew point, up to an optimum value 71a of approximately 118 bars and for a minimum energy consumption of approximately 19.15 kWxd / t, which represents a gain in thermodynamic efficiency of 3.04% compared to the operating point 62 of the process according to the invention with a refrigerant gas composed of 100% nitrogen and a thermodynamic efficiency gain of 10.09% compared to the operating point 60 of the conventional process.

By using a mixture of 50% nitrogen and 50% neon as refrigerant gas, the pressure can be increased, as shown on curve 72, without the gas mixture reaching its dew point, up to an optimum value 72a of approximately 145 bars and for a minimum energy consumption of approximately 18.8 kWxd / t, which represents a gain in thermodynamic efficiency of 4.81% compared to the operating point 62 of the process according to the invention with a refrigerant gas composed of 100% nitrogen and a thermodynamic efficiency gain of 11.74% compared to the operating point 60 of the conventional process.

Likewise, as shown in the diagram of FIG. 7B, a mixture of nitrogen and hydrogen is advantageously used as refrigerant gas.

Thus, by using a mixture of 80% nitrogen and 20% hydrogen as refrigerant gas, the pressure can be increased, as shown in curve 80, without the gas mixture reaching its dew point. , up to an optimum value 80a of about 94 bars and for a minimum energy consumption of about 19.2 kWxd / t, which represents a thermodynamic efficiency gain of 2.78% compared to the operating point 62 of the process according to l The invention of Figures 2-3 with a refrigerant gas composed of 100% nitrogen, and a gain in thermodynamic efficiency of 9.86% over the operating point 60 of the conventional process of Figure 1.

By using a mixture of 60% nitrogen and 40% hydrogen as refrigerant gas, the pressure can be increased, as shown in curve 81, without the gas mixture reaching its dew point, up to 'at an optimum value 81a of approximately 140 bars and for a minimum energy consumption of approximately 18.8 kWxd / t, which represents a gain in thermodynamic efficiency of 4.81% compared to the operating point 62 of the process according to the invention Figures 2-3 with a refrigerant gas composed of 100% nitrogen and a thermodynamic efficiency gain of 11.74% over the operating point 60 of the conventional process of Figure 1.

By using a mixture of 50% nitrogen and 50% hydrogen as refrigerant gas, the pressure can be increased, as shown in curve 82, without the gas mixture reaching its dew point, up to 'at an optimum value 82a of approximately 186 bars and for a minimum energy consumption of approximately 18.7 kWxd / t, which represents a gain in thermodynamic efficiency of 5.32% compared to the operating point 62 of the process according to the invention Figures 2-3 with a refrigerant gas composed of 100% nitrogen and a thermodynamic efficiency gain of 12.21% over the operating point 60 of the conventional process of Figure 1.

Thus, an increasing percentage of complementary gas, either hydrogen or neon, added to nitrogen to constitute a refrigerant gas, radically improves the thermodynamic efficiency of the process, while allowing operation at higher pressure. , which implies more compact equipment, which is very advantageous when only very small surfaces are available, which is the case on a floating support anchored at sea, or on board an LNG carrier, in the case of reliquefaction units.

The method according to the invention uses either a mixture of nitrogen and neon, or nitrogen and hydrogen, and despite its slightly lower yield, the use of the mixture of nitrogen and neon will be preferred, because neon is an inert gas, whereas hydrogen is combustible and remains dangerous and difficult to operate, especially at high pressure in confined installations on board a floating medium. In addition, hydrogen is a gas which percolates very easily through elastomeric seals and even in certain cases through metals, especially at very high pressure, and therefore the process according to the invention based on the use of a nitrogen-hydrogen mixture does not constitute the preferred version of the invention: the preferred version of the invention remains the use as refrigerant gas of a mixture of nitrogen and neon in the devices described with reference to the various figures.

Likewise, the yield of conventional processes using nitrogen as refrigerant gas is improved, by considering a binary nitrogen-neon or nitrogen-hydrogen mixture.

Thus, as represented in the diagram of FIG. 7A, curve 75 represents the variation in the yield of a conventional process according to FIG. 1, or of its variants, as a function of the percentage of neon gas in the refrigerant gas. For a percentage of 20% neon, the operating point is at 70b, which corresponds to a maximum pressure P3 of approximately 63 bars and an energy consumption of approximately 20.45 kWxd / t, which represents a gain in efficiency thermodynamic of 3.76% compared to the operating point 60 of the same conventional process with a refrigerant gas composed of 100% nitrogen.

For a percentage of 40% neon, the operating point is at 71b, which corresponds to a maximum pressure P3 of approximately 90 bars and an energy consumption of approximately 19.70 kWxd / t, which represents a gain in efficiency thermodynamic of 7.29% compared to the operating point 60 of the same conventional process with a refrigerant gas composed of 100% nitrogen.

For a percentage of 50% neon, the operating point is at 72b, which corresponds to a maximum pressure P3 of approximately 120 bars and an energy consumption of approximately 19.35 kWxd / t, which represents a gain in efficiency thermodynamic of 8.94% over operating point 60 of the same conventional process with a refrigerant gas composed of 100% nitrogen.

Likewise with a nitrogen-hydrogen mixture comprising 20% hydrogen, as shown in FIG. 7B, the operating point is located at 80b, which corresponds to a maximum pressure P3 of approximately 68 bars and a consumption in energy of about 20.2 kWxd / t, which represents a gain in thermodynamic efficiency of 4.94% compared to the operating point 60 of the same conventional process with a refrigerant gas composed of 100% nitrogen.

For a percentage of 40% hydrogen, the operating point is at 81b, which corresponds to a maximum pressure P3 of approximately 108 bars and an energy consumption of approximately 19.8 kWxd / t, which represents a gain of thermodynamic efficiency of 6.82% compared to the operating point 60 25 of the same conventional process with a refrigerant gas composed of 100% nitrogen.

For a percentage of 50% hydrogen, the operating point is located at 82b, which corresponds to a maximum pressure P3 of approximately 150 bars and an energy consumption of approximately 19 kWxd / t, which represents a gain of thermodynamic efficiency of 10.59% compared to the operating point 60 of the same conventional process with a refrigerant gas composed of

100% nitrogen.

By way of example, a conventional liquefaction unit is dimensioned in relation to the powers of the gas turbines available, the high power turbines being commonly 25MW.

We generally seek to increase the power of the installation, and it is then possible to install two identical gas turbines (GT1 and GT2) in parallel to obtain 30MW (2xl5MW), or even 40MW (2x20MW), but we then have two rotating machine lines, which increases the overall dimensions, the quantities of pipes and of course the costs.

By installing a single 25MW GT turbine in C3 as in figure 2 and adding power to the second motor M2, for example 5MW, to obtain a total of 30MW, or 15MW to obtain a total of 40MW, the operation of the process with reference to FIG. 2 is identical in terms of efficiency to that using two gas turbines (GT1 and GT2) in parallel.

Thus, considering a GT gas turbine of 25MW, the addition of 5MW of power at the level of the motor (M2), preferably thanks to an electric motor, gives more flexibility in operation and thus allows an increase in power of 20 %. On the other hand, the efficiency of the whole remains unchanged, substantially at 21.25 kW x day / t of LNG produced as shown in the diagram of FIG. 7 at point 60.

If, on the other hand, the same power of 5MW is supplied to the level of the first motor M1, the overall power is still 30MW, but in this case the efficiency of the assembly is improved and significantly reaches the value of 19.8 kW x day / t of LNG produced, which represents a gain of 6.59% for the same overall power of 30MW, compared to a power injection of 5MW at the level of the second motor M2, as detailed previously. Said power input of 5 MW to the first motor M1 then represents 16.6% of the overall power and said efficiency (19.8 kW x day / t) corresponds substantially to point W2 of the diagram of FIG. 7.

In the same way in figure 3, by installing a single GT turbine of 25MW in C2 as in figure 3 and by adding power at the level of the GT turbine, for example 5MW to obtain a total of 30MW, or 10MW for obtain a total of 40MW, the operation of the process with reference to Fig. 2 is identical in terms of efficiency to that using two gas turbines (GT1 and GT2) in parallel.

Thus, considering a 25MW GT gas turbine, the addition of 5MW of power at the level of the GT turbine, gives more flexibility in operation and thus allows a power increase of 20%. On the other hand, the efficiency of the whole remains unchanged, substantially at 21.25 kW x day / t of LNG produced as shown in the diagram of FIG. 7 at point 60.

If, on the other hand, the same power of 5MW is supplied to the level of the first motor M1, the overall power is still 30MW, but in this case the efficiency of the assembly is improved and significantly reaches the value of 19.8 kW x day / t of LNG produced, which represents a gain of 6.59% for the same overall power of 30MW, compared to a power injection of 5MW at the level of the second motor M2, as detailed previously. Said power input of 5 MW to the first motor M1 then represents 16.6% of the overall power and said efficiency (19.8 kW x day / t) corresponds substantially to point W2 of the diagram of FIG. 7.

Thus, depending on the production of natural gas, both in quantity and quality, coming from the groundwater, a gas turbine GT, for example of 25MW, will advantageously be used at full speed continuously,

- which will be completed by power injection at the level of the GT turbine (fig. 2) or of the second motor M2 (fig. 3) without changing the overall efficiency (point WO of figure 7), and

- which will be completed, or even if necessary modulated, by power injection at the level of the first motor M1 which has the effect of improving the overall efficiency according to curve 61 of the same FIG. 7, until an optimum is reached , i.e. a minimum energy consumption of 19.75 kW x day / t corresponding substantially to point W3 of said curve 61: the energy injected into said first motor

Ml then representing in this case substantially 24% of the total energy.

In general, we will operate with a gas turbine GT at full speed, which will be completed by a power supply to the first engine M1, said input being limited to about 24% of the overall power so as to optimize the output to the minimum value of 19.75 kW x day / t, then if necessary, the overall power will be increased by power injection at the level of the second motor M2, and concomitantly the power injected at the level of the first motor M1 will be readjusted , so that said power is always substantially equal to approximately 24% of the overall power so as to keep the efficiency of the installation at the optimum value of 19.75 kW x day / t.

Said optimum yield of 19.75 kW x day / t for a power of the first motor M1 representing 24% of the total power is valid for a refrigerant fluid consisting of 100% nitrogen. In the case of nitrogen-neon or nitrogen-hydrogen mixtures, the optimum efficiency as well as the power percentage vary depending on the mixtures and the percentages of neon or hydrogen, but the advantages detailed above remain valid and even accumulate.

The invention has been described with a mixture consisting essentially of nitrogen and neon or hydrogen, helium not representing any proven interest in the process.

Indeed, traces of other gases in small proportions, not exceeding overall for example 0.5 to 1%, do not disturb the process, nor its overall yield, because they will liquefy at the low points of cold zones and will remain confined there, unless they are driven by the speed of the refrigerant gas. The only real danger would be that these liquid particles come into contact with the blades of compressors or turbines rotating at high speed, which is easy to avoid by placing the low points in privileged places and by fitting them advantageously with traps towards the top. 'atmosphere. These traps thus make it possible to quickly eliminate the unwanted residual gases and therefore to have then, after a few hours, or even a few days of operation, a truly binary mixture of refrigerant gas.

Claims (15)

1. A process for liquefying a natural gas predominantly comprising methane, preferably at least 85% methane, the other components essentially comprising nitrogen and C-2 to C-4 alkanes, in which it is liquefied said natural gas to be liquefied by circulating said natural gas at a pressure PO greater than or equal to atmospheric pressure (Patm.), preferably PO being greater than atmospheric pressure, in at least 1 cryogenic heat exchanger (EC1, EC2, EC3) by circulation in a closed circuit against the current in indirect contact with at least one flow of refrigerant gas remaining in the compressed gaseous state at a pressure PI entering the said cryogenic exchanger at a temperature T3 'lower than T3, T3 being the liquefaction temperature of said liquefied natural gas at the outlet of said cryogenic exchanger, T3 being less than or equal to the liquefaction temperature of said liquefied natural gas at atmospheric pressure, characterized in that the d it refrigerant gas consists essentially of a mixture of nitrogen and at least one other constituent chosen from neon and hydrogen. 2. Method according to claim 1, characterized in that said refrigerant gas consists of a binary mixture of nitrogen and neon. 3. Method according to claim 1 or 2, characterized in that it comprises a mixture of nitrogen and said other (s) component (s) with a molar content of said other component (s) ( s) not more than 50% preferably 20 to 40%. 4. Method according to one of claims 1 to 3, characterized in that said natural gas to be liquefied is liquefied by carrying out the following concomitant steps of: (a) circulation of said natural gas to be liquefied circulating (Sg) at a pressure PO greater than or equal to atmospheric pressure (Patm), preferably PO being greater than atmospheric pressure, in at least 3 cryogenic heat exchangers (EC1, EC2 , EC3) arranged in series including: - a first exchanger (EC1) in which said natural gas entering at a temperature TO is cooled and exits (BB) at a temperature Tl lower than TO, then - a second exchanger (EC2) in which the natural gas is completely liquefied and leaves (CC) at a temperature T2 lower than Tl and higher than T3, and - a third exchanger (EC3) in which said liquefied natural gas is cooled from T2 to T3, and (b) circulation in a closed circuit against the current of at least a first flow (SI) of refrigerant gas in the gaseous state at a pressure PI lower than P3 in indirect contact with and against the flow of the natural gas flow (Sg), said first flow (SI) at a pressure PI passing through the 3 exchangers (EC1, EC2, EC3) on entering (DD ) in said third exchanger (EC3) at a temperature T3 'lower than T3, then entering (CC) at a temperature T2' lower than T2 in said second exchanger (EC2), then entering (BB) at a temperature Tl ' less than T1 in said first exchanger (EC1) and leaving (AA) from said first exchanger (EC1) at a temperature TO 'less than or equal to TO, - Said first flow (SI) of refrigerant gas at PI and T3 'being obtained by expansion in at least a first expander (El) of at least a first part (Dl) of at least a second flow (S2) of refrigerant gas compressed at P3 greater than PI circulating in co-current with said natural gas entering (AA) in said first exchanger (EC1) at TO and exiting (CC) from said second exchanger (EC2) substantially at T2, the flow rate D2 of said second part of the second flow being greater than the flow Dl of the first part of the second flow, and - a second part (D2) of said second flow (S2) of compressed refrigerant gas P3 circulating in co-current with said natural gas entering (AA) in said first exchanger (EC1) at TO and exiting from said first exchanger substantially at T1 is expanded in a second regulator (E2) at said pressure PI and at said temperature T2 ', and is recycled to join said first flow at the inlet (CC) of said second exchanger, and (c) said second compressed flow (S2) with P3 being obtained by compression by at least one compressor (Cl, C2, C3) followed by at least 5 cooling (Hl, H2) of said first flow (SI) of recycled refrigerant gas leaving (AA) from said first exchanger (EC1), preferably at least a first compressor (C1) coupled to said first expander (El), and ( d) preferably, after step (a), the liquefied natural gas is depressurized from pressure PO to atmospheric pressure where appropriate. 5. Method according to claim 4, characterized in that three compressors are implemented, including a first compressor and a second compressor arranged in parallel, comprising: I5 - a third compressor (C3) actuated by an engine preferably a gas turbine GT to compress to P'3, P'3 being between Pl and P3, all of said first flow of refrigerant gas coming from the outlet (AA ) of said first exchanger (EC1), and - a first compressor (Cl) coupled to the first expansion valve (El) 20 consisting of a turbine, for compressing from P'3 to P3, a part (ΟΓ) of said first flow of refrigerant gas, compressed by the third compressor (C3), and - a second compressor (C2) coupled to the second expansion valve (E2) consisting of a turbine, to compress from P'3 to P3 a part 25 (D2 ') of said first flow of refrigerant gas compressed by the third compressor (C3). 6. Method according to one of claims 1 to 3, characterized in that said natural gas to be liquefied is liquefied by carrying out the following concomitant steps of: (A) circulation of said natural gas to be liquefied circulating (Sg) at a pressure PO greater than or equal to atmospheric pressure (Patm), preferably PO being greater than atmospheric pressure, in at » t at least 3 cryogenic heat exchangers (EC1, EC2, EC3) arranged in series including: - a first exchanger (EC1) in which said natural gas entering at a temperature TO is cooled and exits (BB) at a temperature Tl lower than TO, then - a second exchanger (EC2) in which the natural gas is completely liquefied and leaves (CC) at a temperature T2 lower than Tl and higher than T3, and - a third exchanger (EC3) in which said liquefied natural gas is cooled from T2 to T3, and (b) closed circuit circulation of at least two streams (SI, S3) of refrigerant gas in the gaseous state called first and third flow respectively at different pressures P1 and P2, passing through at least two said exchangers in indirect contact with and against the current of the flow of natural gas (Sg), comprising: - a first flow of refrigerant gas (SI) at a pressure Pl lower than P3 passing through the 3 exchangers (EC1, EC2, EC3) entering (DD) in said third exchanger (EC3) at a temperature T3 'lower than T3, then entering (CC) at a temperature T2 'lower than T2 in said second exchanger (EC2), then entering (BB) at a temperature Tl' lower than Tl in said first exchanger (EC1) and leaving (AA) from said first exchanger at a temperature TO 'less than or equal to TO, the said first flow of refrigerant gas at Pl and T3' being obtained by expansion in at least a first expansion valve (El) of a first part (Dl) of a second flow (S2 ) of refrigerant gas compressed at the pressure P3 greater than P2, said second flow (S2) circulating in indirect contact with and in co-current with said natural gas flow (Sg) entering (AA) in said first exchanger (EC1) at TO and said first part (Dl) of the second outgoing flow (CC) of said second exchanger (EC2) substantially at T2, and - a third flow (S3) at a pressure P2 greater than Pl and less than P3 circulating in indirect contact with and co-current of said first flow, passing only through said second and first? »Exchangers (EC2, EC1) entering (CC) in said second exchanger at a temperature T2 'lower than T2 and leaving (AA) from said first exchanger (EC1) at TO' less than or equal to TO, said third flow (S3 ) of refrigerant gas at P2 and T2 being obtained by expansion in a second expansion valve (E2) of a second part (D2) of said second flow (S2) of refrigerant gas leaving said first exchanger substantially at T1, the flow D2 of the said second part of the second flow being greater than the flow rate DI of the first part of the second flow, (c) said second flow of refrigerant gas (S2) compressed at the 10 pressure P3 being obtained by compression by at least two compressors (Cl, C2, C3) and cooling (Hl, H2) of said first and third streams (SI, S3) of refrigerant gas leaving (AA) from said first exchanger (EC1) to PI and respectively P2, preferably by at least two first and second compressors (C1, C2) arranged in series and coupled respectively to said first and second expansion valves (El, E2) consisting of turbines, and (d) preferably, after step (a), the liquefied natural gas exiting (DD) from said third exchanger at T3 is depressurized from pressure PO to atmospheric pressure, where appropriate. 20 7. Method according to claim 6, characterized in that the said pressure P2 is varied in a controlled manner, so that the energy consumed for the implementation of the method (Ef) is minimal, preferably when the composition of the natural gas to be liquefied varies. 25 8. Method according to one of claims 6 or 7, characterized in that at least one compressor (C1) is coupled to a motor (M1) making it possible to vary the pressure P2 in a controlled manner by supplying power in a controlled manner. controlled by said compressor, preferably said motor (M1) each providing at least 5%, preferably from 5 to 30%, of the total power supplied to all of said compressors used (C1, C2, C3). 9. Method according to one of claims 6 to 8, wherein we ', »· implements at least a first compressor (Cl) compressing from PI to P2 all of said first flow of refrigerant gas exiting (AA) from said first exchanger (EC1), and at least one second compressor (C2), compressing from P2 to at least P'3, P'3 being a pressure less than or equal to P3 and greater than P2, on the one hand said third flow ( S3) of refrigerant gas exiting at P2 from said first exchanger (EC1) and on the other hand said first flow of refrigerant gas compressed at P2 leaving said first compressor, to obtain said second flow of refrigerant gas at P3 and TO after cooling (Hl , H2), characterized in that: - the two first and second compressors (C1, C2) arranged in series are coupled respectively to said first and second regulators (El, E2) consistent in turbines of recovery energy, and - at less said first compressor (Cl) is coupled to a first motor (Ml), and - a gas turbine (GT) is coupled is second audit compressor, the latter compressing said second flow of refrigerant gas directly to P3, or to a third compressor (C3) mounted in series after the second compressor (C2), said third compressor compressing from P'3 to P3 said second flow of refrigerant gas, said gas turbine providing the major part of the total power supplied to all of said compressors used (C1, C2, C3). 10. The method of claim 8 or 9, characterized in that uses two compressors (C1, C2) connected in series, comprising: (i) a first compressor (Cl) coupled to said first expander (El), compressing from PI to P2 all of said first flow of refrigerant gas leaving (AA) from said first exchanger (EC1), and (ii) a second compressor ( C2) coupled to said second regulator (E2), compressing from P2 to at least P'3, P'3 being greater than P2 and less than or equal to P3, on the one hand said i third flow (53) of refrigerant gas leaving at P2 from said first exchanger (EC1) and on the other hand said first flow of compressed refrigerant gas at P2 leaving said first compressor, to obtain said second flow of refrigerant gas at P3 and TO after cooling (Hl, H2), and (iii) said first compressor (Cl) is coupled to a first engine (M1), and said second compressor (C2) being coupled to a gas turbine (GT), said first engine making it possible to vary the pressure P2 in a controlled manner by supplying power in a controlled manner to said first compressor C1, said first motor (M1) providing at least 3%, more preferably from 3 to 30% of the total power supplied to the 'set of said compressors used (C1, C2), said second compressor (C2) actuated by a gas turbine (GT) providing 97 to 70% of the total power used. 11. The method of claim 8 or 9, characterized in that three compressors (C1, C2, C3) connected in series are used, comprising: (i) a first compressor (Cl) actuated by a first motor (M1) and coupled to said first expansion valve (El), compressing from PI to P2 all of said first flow of refrigerant gas leaving (AA) from said first exchanger (EC1) , and (ii) a second compressor (C2) actuated by a second motor (M2) coupled to said second expander (E2), compressing from P2 to P'3, P'3 being greater than P2 and less than P3, by a on the one hand said third flow (53) of refrigerant gas leaving at P2 from said first exchanger (EC1), and on the other hand said first flow of compressed refrigerant gas at P2 leaving said first compressor C1, and (iii) at least one third compressor (C3) actuated by a gas turbine (GT) to supply the major part of the energy and compress from P'3 to P3 all of the first and third compressed refrigerant gas streams leaving said second compressor (C2), to obtain said second flow of refrigerant gas at P3 and TO after cooling (Hl, H2), and (iv) said motor (Ml) contributes at least 3%, more preferably from 3 to 30% of the total power supplied to all of said compressors used (Cl, C2, C3), the gas turbine (GT) coupled to said third compressor ( C3), as well as the motor (M2) coupled to the compressor (C2) together provide 97 to 70% of the total power supplied to all of the said compressors used (C1, C2, C3). 12. Method according to one of claims 1 to 11, characterized in that the composition of the gas to be liquefied is included in the following ranges for a total of 100%: - Methane from 80 to 100%, - nitrogen from 0 to 20%, - ethane from 0 to 20%, - propane from 0 to 20%, and - butane from 0 to 20%. 13. Method according to one of claims 1 to 12, characterized in that: - T0 and T0 'are 10 to 35 ° C, and - T3 and T3 'are from -160 to -170 ° C, and - T2 and T2 'are from -100 to - 140 ° C, and - Tl and Tl 'are -30 to -70 ° C. 14. Method according to one of claims 1 to 13, characterized in that: - PO is 0.5 to 5 MPa, and - Pl is 0.5 to 5 MPa, and - P2 is 1 to 10 MPa, and - P3 is 5 to 20 MPa. 15. Method according to one of claims 7 to 14, characterized in that P2 is varied until the minimum total energy Ef consumed in the process is less than 21.5 kW x day / t of liquefied gas. product, preferably 18.5 to 20.5 kW x day / t.