OA16683A - Natural gas liquefaction process with triple closed refrigerant gas circuit. - Google Patents

Abstract

The subject of the present invention is a process for liquefying a natural gas mainly comprising methane, in which 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 in 3 exchangers heaters connected in series in which it is cooled to T3, T3 being less than or equal to the liquefaction temperature of said natural gas at atmospheric pressure, and (b) closed circuit circulation of at least: - a first flow of refrigerant gas at a pressure PI less than P3 entering the third exchanger and leaving said first exchanger, said first flow being obtained by expansion in a first regulator of a first part of a said second flow at P3 greater than P2, said second flow circulating at co-current of said natural gas stream entering said first exchanger and leaving said second exchanger, and - a third flow at a pressure P2 greater than PI and i n less than P3 circulating co-current with said first flow, entering said second exchanger and exiting from said first exchanger, obtained by expansion in a second regulator of a second part of said second flow leaving said first exchanger, (c) said second flow of refrigerant gas at pressure P3 being obtained by compression by at least two first and second compressors arranged in series and coupled to said first and second expansion valves.

Description

Natural gas liquefaction process with triple closed circuit 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 arranged on a ship, the present invention relates more particularly to a process for re-liquefying gas on board a LNG transport ship called a “methane tanker”, said gas to be re-liquefied being the result of reheating and partial evaporation of the LNG contained in the tanks of said vessel, said evaporated gas, in general predominantly methane, being called in English "boîl 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.

When the amounts of natural gas are important, even 'ί ¹ large, we try to transport your mannered was able to use in remote areas, usually on other continents and, for this, the preferred method is to transport as a cryogenic liquid (-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 facilities 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 of 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 necessary 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 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 the event of leakage, causing explosions or considerable fires. . 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. , than for the so-called “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 connection pipes, it is necessary to install gravity collectors to bring together the liquid phase and direct 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 any problems in the case of land-based installations, since it is generally easy to have a sufficient land area to house all these bulky 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.

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 energy of the order of 0.5 kWh / kg of LNG produced, i.e. about 20.84 kW x day / t and, on the other hand, it presents a considerable advantage in terms of safety, because the refrigerant gas of the cycle, nitrogen, is inert and therefore incombustible, '>

which is very interesting when the installations are concentrated in a small space, for example on the deck of a floating support installed in the open sea, said equipment often being installed on several levels, one above the other on a small surface to the bare minimum. Thus, in the event of a refrigerant gas leak, there is no danger of explosion and it is then sufficient to reinject the lost refrigerant gas fraction 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 weak movements of the floating support, or even almost impossible for extreme movements, whereas in fixed installations on land the problem of movements does 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 fluid, both in terms of technology of said equipment and maintenance of this equipment in a confined environment, namely a floating support anchored at sea. In addition, the operation of the installations operation remains simpler, because this type of cycle is insensitive to variations in the composition of the gas to be liquefied, namely a natural gas consisting 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 then 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.

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 formed mostly 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, in general 10 to 20 bars, in four main phases:

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

- phase 2: liquefaction of natural gas (transition from gaseous state 20 to liquid state). As natural gas is a gaseous mixture under a pressure PO of approximately 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 obtained liquid 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 tank of isolated storage, 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 of the process is explained later.

In US 2011/0113825 and WO 2005/071333, a natural gas liquefaction process is described in which said natural gas to be liquefied is liquefied by circulating said natural gas in 3 cryogenic heat exchangers by circulation in a closed circuit of 3 gas streams refrigerant remaining in the compressed gaseous state without phase change in which 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, in 3 cryogenic heat exchangers arranged in series, including:

- a first exchanger (101/5) in which said natural gas entering at a temperature T0 is cooled and exiting at a temperature Tl lower than T0, then

- a second exchanger (102/6) in which the natural gas is completely liquefied and leaves at a temperature T2 lower than Tl and higher than T3, T3 being lower than the liquefaction temperature of the

LNG, and

- a third exchanger (103/7) in which said liquefied natural gas is cooled from T2 to T3, and (b) closed-circuit circulation of two streams of refrigerant gas in the gaseous state called first and third streams respectively at pressures different PI and P2, passing through 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 less 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 a first expansion valve (112/9) of a first part (122 / 16B) of a second flow of refrigerant gas (22/15) compressed at a pressure P3 greater than P2, said first part of the second flow circulating in indirect contact with and co-current with said flow of natural gas , entering said first exchanger at TO and 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 expansion valve (111/8) of a second part (121/17) of said second flow of refrigerant gas (22 / 15) leaving said first exchanger substantially at T1, (c) said second flow of refrigerant gas compressed to pressure P3 being obtained by compression by three or four compressors, and cooling of said first and third flow of refrigerant gas leaving said first exchanger to PI and respectively P2.

In US 2011/0113825, two first and second compressors 113 and 114 arranged in series compress the refrigerant gas from the first 5 and third streams to P'3 and two other compressors 115a and 115b arranged in parallel compress from P'3 to P3.

In WO 2005/071333, two compressors 2 and 3 mounted in series compress said first stream 16d to P ' ₃ then a third compressor 4 mounted in series with the two first compressors 10 compresses said first and third streams to P ₃ .

In the minutes of the “24th International Conference and Exhibition for the LNG” of May 25, 2009, on behalf of Olve Skjeggedal et al. in the review GASTECH 2009, a process as described above with a triple flow of closed-circuit refrigerant gas is described, in which said first and third flows are compressed to P'3 by two compressors connected in series and two other compressors connected in series. series compress said first and third stream at P3 to give said second stream.

The process described above is advantageous over that of FIG. 1 in that, first of all, rather than recycling after expansion a part D2 of the second stream at the outlet of the first exchanger to join the first stream at the end. 'inlet of the second exchanger, this part D2 of the second flow is recycled at the inlet of the second exchanger at an intermediate pressure P2 greater than PI in a third flow 53 independent and parallel to SI, that is to say in co-current of SI. And, because most of the energy is consumed for phase 2 of the process within said second exchanger, this makes it possible to increase the heat transfers and the energy efficiency of the process.

However, in the embodiment of US 2011/0113825, all the external power supplied to said first compressor 113 and second compressor 114 mounted in series relates to the flow of refrigerant gas circulating at low and medium pressures PI and P2, the recovery of energy at the level of the turbines 111 and 112 being reinjected at the level of the two compressors 115a and 115b mounted in parallel compressing the high pressure refrigerant gas P'3 / P3, no other additional external power being supplied to the level of said compressors in parallel 115a and 115b. The two compressors in parallel 115a and 115b are supplied only respectively by the two energy recovery turbines 111 and 112.

The pressure levels PI and P2 of the gases leaving the turbines 112 and 111 are different and therefore the flow rates passing through the regulators 111 and 112 are different and in particular in practice in a ratio of 10-20% of the total flow for the flow rate of the flow from expander 112 against 80-90% for the flow rate from expander 111. As a result, compressor 115b only recovers 10-20% of the total power recovered compared to 80-90% of power recovered at the level compressor 115a. The result of this disparity in power supplied to the two compressors 115a and 115b mounted in parallel, a major difficulty in stabilizing the operation of the circuit. Indeed, the operation of two compressors in parallel can lead to pumping phenomena, that is to say that one of the compressors takes precedence over the others by disturbing their inlet and outlet pressures: there is there is then a risk of the lower capacity compressor (s) operating in "turbine mode". This mode of operation is to be avoided absolutely since all or part of the fluid then rotates in a loop between the compressors, one in compressor mode, the other or others in "turbine mode": the compression process is then radically disturbed, or even stopped and the overall efficiency of the installation then collapses.

The stabilization of the operation of the circuit can be carried out conventionally by means of control valves upstream and / or downstream of said compressors 115a and 115b mounted in parallel, and / or upstream and / or downstream of said turbines 111 and 112 to control the compressors flow rates and operation. However, these control valves generate losses in load, and therefore in energy, which greatly affects the overall efficiency sought and / or the production capacity of the installation.

In WO 2005/071333 and in the review of the GASTECH 2009 review cited above, all the compressors are mechanically coupled to the same power source, all of the power being supplied in a non-differentiated manner between the different compressors.

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, wherein 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 (EC1, EC2, EC3) by closed-circuit circulation in counter-current in indirect contact with at least one flow of refrigerant gas remaining in the gaseous state compressed at a pressure PI entering said cryogenic exchanger at a temperature T3 'lower at T3, T3 being the temperature at the outlet of said cryogenic exchanger, and T3 being less than or equal to the liquefaction temperature of said liquefied natural gas at atmospheric pressure, in which l edit natural gas to be liquefied by carrying out the following concomitant steps:

(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 below 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 less 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 to the pressure P3 greater than P2, said second flow circulating in indirect contact with and co-current with said flow of natural gas, entering said first exchanger at TO and said first part of said 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 second flow preferably being greater than the flow rate DI of the first part of 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 P2 respectively, a first compressor compressing from PI to P2 the total ity of said first flow of refrigerant gas leaving said first exchanger, and at least one second compressor, 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 of refrigerant gas leaving at P2 from said first exchanger 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, said second compressor being mounted in series with said first compressor, characterized in that:

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

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

- at least one 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 said second flow of refrigerant gas from P'3 to P3,

- Said gas turbine providing the major part of the total power supplied to all of said compressors used.

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 as the output shaft 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 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 to the refrigerant gas; and the compressors are of the rotary turbine type, also called a centrifugal compressor.

Preferably, after step (a), the liquefied natural gas leaving said third exchanger is depressurized at T3, from pressure PO to atmospheric pressure where appropriate.

The method according to the invention is advantageous over the method described in US 2011/0113825 in that all the compressors are connected in series without requiring flow control with flow control valves to stabilize the operation of the installation. In fact, in the method according to the invention, there is no separation of flows in the compression chain. It follows that the regulation of the flow rate of flow and / or energy at the level of the various compressors is obtained essentially by the regulation of the power input at the level of said first and second engines and said gas turbine. It is not essential to implement control valves at said compressors and said turbine because said first and second expansion valves are coupled to said first and second compressors mounted in series and are therefore not coupled to compressors mounted in series. parallels as in US 2011/0113825.

On the other hand, in the present invention, 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.

In addition, 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 compactness of the installation, which is particularly advantageous for the implementation of a process. aboard a floating support where space is limited.

The method according to the invention with reference to FIGS. 2 - 3 is advantageous over that of FIG. 1 in that, first of all, rather than recycling after expansion a part D2 of the second stream at the outlet of the first exchanger for join the first flow at the inlet of the second exchanger, this part D2 of the second flow at the inlet of the second exchanger is recycled at an intermediate pressure P2 greater than PI in a third independent flow S3 and parallel to SI, that is to say say to co-current of SI. And, because most of the energy is consumed for phase 2 of the process within said second exchanger, this makes it possible to increase the heat transfers and the energy efficiency of the process.

On the other hand, the method according to the invention is advantageous with respect to WO 2005/071333 and the method described in the GASTECH 2009 review cited above in that it makes it possible to vary said pressure P2 in a controlled manner. that the energy consumed for the implementation of the method (Ef) is minimal. Indeed, according to the present invention, it is possible to modulate and control specifically the value of the pressure P2 by providing a differentiated power at the level of said first compressor thanks to said first motor, making it possible to modulate and control the power supplied to the various compressors in a differentiated manner and therefore to vary the value of P2.

Thus, according to an original characteristic of the present invention, said pressure P2 is varied in a controlled manner by supplying power in a controlled manner to said first compressor with said first motor, so that the energy consumed for switching on implementation of the process (Ef) is minimal, preferably when the composition of the natural gas to be liquefied varies.

This process is more particularly advantageous because it thus makes it possible, by modulating and specifically controlling the value of the pressure P2 of said third flow, to modify and optimize the operating point of the process, namely to minimize the energy consumed and therefore to increase the efficiency in particular. when, as happens during operation, the composition of the natural gas to be liquefied varies.

More particularly, said first engine provides at least 3%, preferably from 3 to 30% of the total power supplied to all of said compressors used, said gas turbine providing from 97 to 70% of the total power brought.

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 power of the gas turbines available, high power turbines commonly being 25MW, or even 30MW when they are intended to be installed on a floating support. Fixed gas turbines installed on land can reach maximum powers of 90-100MW.

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.

Thus, depending on the production of natural gas, both in quantity and quality, from groundwater, a GT gas turbine, for example of 25MW, will advantageously be used, 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 / or

- 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 first variant embodiment of the method, two compressors connected in series are used, comprising:

(i) at least a first compressor, preferably a said 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) at least a second compressor, from preferably said second compressor coupled to said second expansion valve, 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 third flow of refrigerant gas exiting at P2 from said first exchanger , 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, and iii) said first compressor is operated by a first motor, said second compressor being actuated by at least one said gas turbine.

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

In a second variant embodiment, three compressors connected in series are used, 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 regulator, 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 at P2 from said first exchanger, and on the other hand said first flow of refrigerant gas compressed at P2 exiting from said first compressor, and (iii a third compressor operated 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 total power supplied to the in seems to be said compressors implemented, the gas turbine coupled to said third compressor, as well as said second motor coupled to the second compressor together providing 97 to 70% of the total power supplied to all of said compressors implemented.

This second variant embodiment is advantageous in terms of thermodynamic efficiency and production capacity because a maximum capacity turbine available on the market can then be used advantageously as a gas turbine, that is to say 25-30MW in the case of gas turbine. turbines intended to be installed on a floating support, plus a second electric motor for example of 5 to 10 MW connected to the second compressor, the overall power of the second motor and third motor (gas turbine) then being 30 to 40MW, therefore vastly superior 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.

The method according to the invention makes it possible, by varying the pressure P2 by supplying energy to said first compressor using said first motor, to implement a minimum total energy Ef consumed in the method of less than 21.5 kW x day / t, more particularly of

18.5 to 20.5 kW x day / t of liquefied gas produced.

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 of 18.5 to 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 of the engine will be readjusted 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 18.5 to 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 gases such as neon or hydrogen or nitrogen-neon or nitrogen-hydrogen mixtures, the optimum efficiency as well as the power percentage vary from 18.5 to 21.5 kW x day / t depending on the gas or the mixture and percentages of neon or hydrogen, but the advantages detailed previously remain valid and even accumulate.

More particularly, said refrigerant gas comprises nitrogen.

In an alternative embodiment, said refrigerant gas consists of a single gas chosen from nitrogen, hydrogen and neon.

Preferably, neon is preferred in view of the greater risk of explosion of hydrogen and the fact that hydrogen may have a certain propensity to percolate through elastomeric seals and even through thin metal walls. .

According to other particular characteristics:

- the composition of the natural 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

- PI 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).

The present invention also provides an installation on board a ship or floating support for implementing a method according to the invention, characterized in that it comprises:

- at least 3 said cryogenic heat exchangers in series comprising at least:

- A first counter-current circulation duct capable of circulating a first flow of refrigerant gas in the compressed gaseous state at PI passing through the 3 third, second and first exchangers successively,

a second co-current circulation duct capable of circulating a said second flow of refrigerant gas in the gaseous state compressed at P3 passing only successively passing through said first and second exchangers,

a third countercurrent circulation duct of said refrigerant gas suitable for circulation to circulate a said third flow of refrigerant gas in the gaseous state compressed at P2 passing only successively through said second and first exchangers in countercurrent manner,

- a fourth duct capable of circulating said natural gas to be liquefied passing successively through the first, second and third exchangers,

- a first regulator between the outlet of said second duct and the inlet of said first duct,

- a second regulator between (i) a bypass of said second duct located between said first and second exchanger and (ii) the inlet of said third duct, and

- a first compressor at the outlet of said first duct coupled to a turbine constituting said first expander,

a second compressor at the outlet of said second duct coupled to a turbine constituting said second expander, said second compressor being mounted in series with said first compressor, in particular at the outlet of said first compressor, and

a conduit for circulating all of the gas compressed at P2 by the first compressor to the second compressor thus mounted in series with said first compressor, and

- at least a first motor coupled to said first compressor, capable of providing at least 3%, preferably 3 to 30% of the total power supplied to all of said compressors used.

More particularly still, a said installation comprises + only at least two compressors mounted in series, comprising:

(i) at least one said first compressor coupled to said first expander, capable of compressing from PI to P2 all of said first flow of refrigerant gas leaving said first exchanger, and (ii) at least one second compressor coupled to said second expander, capable of in compressing from P2 to P3, on the one hand said third flow of refrigerant gas leaving at P2 from said first exchanger and, on the other hand, said first flow of compressed refrigerant gas at P2 leaving said first compressor, to obtain said second flow of gas refrigerant at P3 and TO after cooling, and (iii) a said first motor coupled to a said first compressor, and a gas turbine coupled to a said second compressor, said first motor being able to provide at least 3%, more preferably from 3 to 30% of the total power supplied to all of said compressors used, and (iv) said gas turbine coupled to said second compressor being able to supply from 97 to 70% of the total power supplied.

More particularly still, an installation according to the invention comprises:

only three compressors mounted in series comprising:

(i) a said first compressor coupled to said first expander and to a said first motor, and (ii) a said second compressor coupled to said second expander and to a said second motor, and (iii) a third compressor coupled to a gas turbine capable of supplying the major part of the energy and capable of compressing at P3 all of the first and third streams of refrigerant gas compressed by said second compressor, to obtain said third stream of refrigerant gas at P3 and TO after cooling, and (iv ) said first motor being able to provide at least 3%, more preferably from 3 to 30% of the total power supplied to all of said compressors used, the gas turbine coupled to said third compressor, as well as said second motor coupled to the second compressor being able to provide together from 97 to 70% of the total power supplied to all of said compressors used.

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 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 triple loop liquefaction process according to the invention 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 T0 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 5B show 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 of! A 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. Numerous 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 so-called “brazed” aluminum plate exchangers. called 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 Figure 1 with reference to the regulator E2, said regulator can be a simple reduction in the diameter of the pipe, or an adjustable valve, but in the case of the liquefaction process according to the invention the regulator is in general 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 even a scroll compressor. According to the invention, preferably (Figure 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 exchanger EC2, all of the natural gas liquefies into LNG at a temperature of T2 = -120 ° C approximately, then the LNG passes to CC 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 (PI 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 PI 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 total flow D, from high pressure P3 to low pressure PI,

El coupled directly to the compressor C1, and which expands the DI portion of the total flow D, from high pressure P3 to low pressure PI, • 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 specifically in Figure 1, there is shown the diagram of a process and installation in which 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 lower than TO at E 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 PI 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 PI 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 exiting from the second exchanger and entering the first exchanger EC1 at BB at a temperature ΤΓ less than T1 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 PI and T3 'being obtained by expansion in a first regulator El of a first part DI of a second flow S2 of refrigerant gas compressed at P3 greater than PI circulating in co-current of said gas natural 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 leaving said first exchanger substantially at T1 is expanded in a second regulator E2 at said pressure PI and at a said temperature T2 ′, and is recycled to join said first flow at the DC 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 expansion valve El, and (d) after step (a) the liquefied natural gas is depressurized from the 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 P2T to P'3, P'3 being between PI and P3, the whole of the first flow of refrigerant gas coming from the outlet at AA of said first exchanger EC1, and

a first compressor C1 coupled to the first expansion valve El consisting of a turbine, for compressing from P2 to P'3, 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 low pressure PI and at a temperature ΤΌ appreciably lower than TO and at 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 Dl 'and D2' which respectively feed the compressors C1 (Dl ') and C2 (D2') operating in parallel. The two streams at pressure P3 are then combined and then cooled substantially to ambient temperature TO by 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 ii 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 DI 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 DI and D2 at the outlet BB of the first exchanger, preferably with D2 greater than D1, is advantageous because most of the energy consumed occurs in phase 2 within the second exchanger EC2. Thus only a minor part of 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 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 T0 = 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 Dl 'and D2', and are coupled directly to the turbines El and E2 which also process different flow rates, respectively Dl and D2.

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 T0) 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 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 below 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 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 Tl' lower than Tl 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 expansion valve 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 to 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 con residing in turbines, and (d) after step (a) the liquefied natural gas exiting at DD from said third exchanger at T3 is depressurized, 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 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 C1, and (iii) a 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 streams of refrigerant gas compressed by the second compressor C2, to obtain said second flow 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 total power supplied to it. 'set 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 the set of 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 refrigerant D, who compresses the entire gas flow C2 who is coupled to the motor M2 and at the turbine E2, and coolant D, who compresses the entire flux gas Cl who is coupled to the motor Ml and at the

turbine El, and which compresses the portion DI 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, HI 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 supply 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 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 refrigerant gas flow portion D2 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 DC at the outlet of 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 PI, P2 & P3 as well as the mass flows 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 figure 2 as a function of * * 4

natural gas / LNG temperature 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 having an improved compactness compared to the method and installation of FIG. 2, in which the compressor C2 is integrated on the same shaft line as the compressor C3 and is actuated 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 compressor C2 and to the other hand to the GT gas turbine.

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 Figures 5 to 9 discussed below, we reproduce the results of the tests in which the values of PI, 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 PI, and as a function of the various variants of the invention. In fact, this pressure PI 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 PI corresponds to the lowest temperature T3 'of the device, ie the temperature at the low inlet of the cryogenic exchanger EC3. This pressure PI 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 FIG. 5A, the upper point W0 = 0 of curve 90 corresponds to an unpowered motor, therefore providing zero power. Point W1 corresponds to a power Wl> 0 supplied by said motor M1. Similarly, the following points on 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 W1 to W4 being identical in FIGS. 5A and 6A .

Thus, in this same FIG. 6A, it is observed that when Ton increases the power W injected at the level of M1, the pressure PI remains constant, but the pressure P2 increases and the efficiency increases, that is to say that the energy consumption expressed in kW x day / t decreases, until reaching 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 FIG. 5A, 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 given proportion of power W.

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 about 12.5 bars and a pressure P2 of about 17 bars, which illustrates the improvement in energy efficiency when the increase in the percentage of neon is combined with the increase in power injected into the engine Ml.

The same effects are observed for hydrogen in the figures

5B and 6B.

Figures 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, and 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 an anchored floating support at sea, or 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 process 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 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 tonne 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 process 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 the 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 operating point 60 of the conventional process.

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 invention with a refrigerant gas composed of 100% nitrogen and a thermodynamic efficiency gain of 8.92% over 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 on curve 80, without the gas mixture reaching its

I

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 the invention of Figures 2-3 with a refrigerant gas composed of 100% nitrogen, and a thermodynamic efficiency gain of 9.86% compared to 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 of figures 2-3 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 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 thermodynamic efficiency gain of 5.32% compared to the operating point 62 of the process according to the invention of figures 2-3 with a refrigerant gas composed of 100% nitrogen and a thermodynamic efficiency gain of 12.21% compared to 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% compared to the operating point 60 of the same conventional process with a refrigerant gas composed of 100% nitrogen.

In the same way with a nitrogen-hydrogen mixture comprising 20% hydrogen, as represented in FIG. 7B, the operating point is located at 80b, which corresponds to a maximum pressure P3 of approximately 68 bars and energy consumption. of approximately 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 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 with respect 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 5 (2x20MW), but we then two lines of rotating machines, 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 ΙΟ M2, for example 5MW, to obtain a total of 30MW, or 15MW to obtain a total of 40MW, the Operation of the method 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 15 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 assembly 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 5MW 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

4, * »power at the GT turbine, for example 5MW to obtain a total of 30MW, or 10MW to obtain a total of 40MW, the operation of the process with reference to figure 2 is identical in terms of efficiency to that using two gas turbines (GT1 and GT2) 5 in parallel.

Thus, considering a GT gas turbine of 25MW, 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 substantially 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 5MW 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, from groundwater, a gas turbine GT, for example 25MW, 25 at full speed continuously, will advantageously be used,

- 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 supplemented, 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 reaching a optimum, i.e. a minimum of

i

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 M1 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.

1/12

FIG. 1

2/12 i D, P3, To

FIG. 2

3/12

P3, To, D

FIG. 3

H (KJ / kg)

4/12

FIG. 4

Ef (KWxd / t)

21.5

20.5

19.5

18.5

60 ^ "

K ^wo t— \ H2 wM ^x

70 ^

70a

W3 ^ rW2, W4 *

x Do

-4I-X ---- x -------------------------------- x

X _____________

J

--- '' ~ U— —i—! | 50% N2CiU ^

-------- U --------- r- ¹ - ¹ ri 20 2530

I 1

100% I

---- ’i—

80% N2

II

---------------------------------- i --------------- ---------- i ------------------ Ii ___- Xi—.

1015 —r ^- in rx>

FIG. 5

Ef (KWxd / t)

21.5

20.5

19.5

18.5

60- r ^ W0 = 0 \ 70- \ \ wo = o W1- 1 71- W2.W4-1 I ΓΜ < 70a < X X " <W0 = 0 100% N2 ^ I ^ -71 to 72 A '-M ’73 · ^ ^ W1 80% N2 ^ 60% N2 ^ ^{r? ia} w? [W2, W4 73a 50% N2 P1 (bar) I 5 *] 10 I 15 20 25 30 35 40 45

6/12

FIG.5A

Ef (KWxd / t)

21.5

60% N2 ^ T i

------------- rL

---------- 1

50% N2 “V | i

30

100% N2

1 --------------- ί | 60% N2 ^

------------------ 1 i i

1 -, -------- 1

Pi (bar)

7/12

FIG. 5B

Ef (KWxdrt)

8/12

FIG.6A

Ef (KWxd / t)

FIG.6B

9/12

Ef (KWxd / t)

FIG. 7

10/12

Ef (KWxd / t)

FIG.7A

11/12

Ef (KWxd / t)

FIG.7B

12/12

Claims (17)

1. A process for liquefying a natural gas predominantly comprising methane, preferably at least 85% methane, the other components essentially comprising nitrogen and 5 C-2 to C-4 alkanes, in which one liquefies 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 gaseous state compressed at a pressure PI entering said cryogenic exchanger at a temperature T3 'lower than T3, T3 being the temperature of liquefaction 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, in which it is liquefied said natural gas to be 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), from 20 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 exiting (BB) at a 25 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 the first and third flow 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 (Sg), comprising: - a first flow of refrigerant gas (SI) at a pressure PI less 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, said first flow of refrigerant gas at PI 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 gas refrigerant compressed to 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 (AA) in said first exchanger (EC1) at TO and said first part (Dl) of the second flow (S2) leaving (CC) from said second exchanger (EC2) substantially at T2, and - a third flow (S3) at a pressure P2 greater than PI 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 regulator (E2) of a second part (D2) of said second flow (S2) of refrigerant gas leaving said first exchanger substantially at T1, the flow rate D2 of said second part of the second flow being preferably greater than the flow rate Dl of the first part of second flow (c) said second flow of refrigerant gas (S2) compressed to pressure P3 being obtained by compression by at least two compressors (Cl, C2, C3) and cooling (Hl, H2) of said first and third flows (SI, S3) outgoing refrigerant gas (AA) from said first exchanger (EC1) to PI and P2 respectively, a first compressor I 4 "compressing from PI to P2 all of said first flow of refrigerant gas leaving (AA) from said first exchanger (ECl), 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 compressed refrigerant gas at P2 exiting from said first compressor, to obtain said second flow of refrigerant gas at P3 and TO after cooling (Hl, H2), process in which: - the two first and second compressors (C1, C2) arranged in series are coupled respectively to said first and second expansion valves (El, E2) consisting of energy recovery turbines, and - at least said first compressor (C1) is coupled to a first motor (M1), said method being characterized in that: - Said first motor makes it possible to vary the power supplied to said first compressor in a differentiated manner with respect to the power supplied to the other compressors, and - a gas turbine (GT) is coupled either to said second 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 said second flow of refrigerant gas from P'3 to P3, said gas turbine providing the major part of the total power supplied to all of said compressors used (Cl, C2, C3), - said first motor (M1) providing at least 3%, preferably from 3 to 30% of the total power supplied to all of said compressors used (Cl, C2), said gas turbine (GT) providing from 97 to 70% of the total power supplied. * 2. Method according to claim 1, characterized in that one »I varies said pressure P2 in a controlled manner by supplying power in a controlled manner to said first compressor with said first motor, so that the energy consumed for the implementation of the method (Ef) is minimal. 3. Method according to claim 2, characterized in that the pressure P2 is increased by increasing the power injected to the first compressor with the first motor, the pressure PI remaining substantially constant. 4. Method according to claim 2 or 3, characterized in that said pressure P2 is varied in a controlled manner by supplying power in a controlled manner to said first compressor with said first motor when the composition of the natural gas to be liquefied varies. . 5. Method according to one of claims 1 to 4, characterized in that two compressors (C1, C2) connected in series are used, comprising: (i) a said first compressor 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 said second compressor (C2 ) coupled to said second expansion valve (E2), compressing from P2 to P3, 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 compressed refrigerant gas at P2 leaving said first compressor, to obtain said second flow of refrigerant gas (S2) at P3 and TO after cooling (H 1, H2), and (Hi) said first compressor (Cl) is operated by a first motor (Ml) , said second compressor (C2) being operated by at least one said gas turbine (GT). 6. Method according to one of claims 1 to 4, characterized in that three compressors (C1, C2, C3) connected in series, comprising: (i) a said first compressor (C1) actuated by a said first motor (M1) and 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 ( ECl), and (ii) a said second compressor (C2) actuated by a said second motor (M2) and coupled to said second expander (E2), compressing from P2 to P'3, P'3 being greater than P2 and less than 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 compressed refrigerant gas at P2 leaving said first compressor C1, and (iii) a third compressor (C3) actuated by a said gas turbine (GT) to supply the major part of the energy and to compress to P3 all of the first and third streams of refrigerant gas leaving the second compressor (C2), to obtain said third flow of refrigerant gas at P3 and TO after cooling (Hl, H2), and (iv) said first motor (Ml) contributes at least 3%, preferably 3 to 30% of the total power supplied to all of said compressors used (Cl, C2, C3), said gas turbine (GT) coupled to said third compressor ( C3), as well as said second motor (M2) coupled to the compressor (C2) together providing 97 to 70% of the total power supplied to all of said compressors used (C1, C2, C3). 7. Method according to one of claims 1 to 6, characterized in that said refrigerant gas comprises nitrogen. 8. Method according to one of claims 1 to 7, 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%, I t - nitrogen from 0 to 20%, - ethane from 0 to 20%, - propane from 0 to 20%, and - butane from 0 to 20%. 9. Method according to one of claims 1 to 8, characterized in that: - T0 and TO '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. 10. Method according to one of claims 1 to 9, characterized in what: - PO is of 0.5 at 5 MPa, and - PI is of 0.5 at 5 MPa, and - P2 is of 1 to 10 MPa, and - P3 is of 5 to 20 MPa. 11. Method according to one of claims 1 to 10, 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 produced. , preferably 18.5 to 20.5 kW x day / t. 12. Method according to claims 1 to 11, characterized in that it is implemented on board a floating support. 13. Installation on board a floating support for implementing a method according to one of claims 1 to 12, characterized in that it comprises: - at least 3 so-called cryogenic heat exchangers (EC1, EC2, EC3) in series comprising at least: "* - a first counter-current circulation duct capable of circulating a first flow (SI) of refrigerant gas in the gaseous state compressed to PI passing through the 3 third, second and first exchangers successively (EC3, EC2, EC1), - a second co-current circulation duct capable of circulating a said second flow (S2) of refrigerant gas in the gaseous state compressed at P3 passing only successively passing through said first and second exchangers (EC1, EC2) , - a third countercurrent circulation duct of said refrigerant gas suitable for circulation to circulate a said third flow (S3) of refrigerant gas in the gaseous state compressed to P2 passing through countercurrently only successively said second and first exchangers (EC2, EC1), - a fourth duct (Sg) capable of circulating said natural gas to be liquefied passing successively through the first, second and third exchangers (EC1, EC2, EC3), - a first regulator (El) between the outlet of said second duct and the inlet of said first duct, - a second regulator (E2) between (i) a bypass (BB) of said second duct located between said first and second exchanger and (ii) the inlet (CC) of said third duct, and - a first compressor (C1) at the outlet of said first duct, coupled to a turbine constituting said first expander, a second compressor at the outlet of said second duct, coupled to a turbine constituting said second expansion valve, and said second compressor being mounted in series with said first compressor, and - a conduit for circulating all of the gas compressed at P2 by the first compressor (C1) to the second compressor (C2) thus mounted in series with said first compressor, and - at least one compressor (C1) coupled to a first motor (M1) capable of providing at least 3%, preferably from 3 to 30%, of the total power supplied to all of said compressors placed in - t work (C1, C2, C3), said first motor making it possible to vary the power supplied to said first compressor in a differentiated manner with respect to the power supplied to the other compressors, and - a gas turbine (GT) coupled either to said second 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), the said third compressor compressing said second flow of refrigerant gas from P'3 to P3, said gas turbine being able to provide the major part of the total power supplied to all of said compressors used (C1, C2, C3). 14. Installation according to claim 13, characterized in that it comprises only two compressors (C1, C2) mounted in series, comprising: (i) a said first compressor (Cl) coupled to said first expander (El), able to compress from PI to P2 all of said first flow of refrigerant gas leaving (AA) from said first exchanger (EC1), and (ii) a said second compressor (C2) coupled to said second expansion valve (E2), capable of compressing from P2 to at least P'3, P'3 being a pressure greater than P2 and less than or equal to P3, on the one hand said third flow ( 53) 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), and (iiî) a said first motor (M1) coupled to a said first compressor (C1), and at least one gas turbine (GT) coupled to said second compressor (C2), said first motor being able to provide at least 3%, more preferably from 3 to 30% of the total power supplied to all of said compressors used e (Cl, C2), and (îv) said gas turbine (GT) coupled to said second compressor being able to deliver from 97 to 70% of the total power supplied. 15. Installation according to claim 14, characterized in that it comprises only three compressors (C1, C2, C3) mounted in series comprising: (i) a said first compressor (C1) coupled to said first expander (El) and to a first motor (M1), and (ii) a said second compressor (C2) coupled to said 10 second expansion valve (E2) and to a second engine (M2), and (iii) a third compressor (C3) coupled to a gas turbine (GT) capable of supplying the major part of the energy and capable of compressing at P3 all of the first and third streams of refrigerant gas compressed by the second compressor (C2), for 15 obtaining said third flow of refrigerant gas at P3 and TO after cooling (Hl, H2), and (iv) said first motor (M1) being able to provide at least 3%, more preferably from 3 to 30% of the total power supplied to all said compressors used 20 (Cl, C2, C3), and (v) the gas turbine (GT) coupled to said third compressor (C3), as well as said second motor (M2) coupled to the second compressor (C2) being able to supply together 97 to 70% of the total power supplied to all said 25 compressors used (Cl, C2, C3).