Facility and method for refrigerating hydrogen

Abstract

The invention relates to a refrigeration facility and method comprising a circuit (2) for supplying a gas flow, the supply circuit (2) having a set of one or more heat exchangers in heat exchange with the supply circuit (2), a pre-cooling refrigerator (10) comprising a cycle circuit (11) configured to subject a cycle fluid to a cycle comprising compression (15), cooling, expansion and heating, the supply circuit (2) comprising a plurality of catalysis sections (17), including at least two catalysis sections (17) in series which are respectively adjacent to two distinct heat exchangers (6, 7, 8, 9) in series of the set of heat exchangers, the cycle circuit (11) comprising at least two phase separator vessels (13, 14) downstream of a cycle fluid expansion member (12) of the cycle circuit (11), and a set of pipes connected to the vessels (13, 14) and configured to transfer cycle fluid from the vessels (13, 14) respectively to the two heat exchangers (6, 7) adjacent to the catalysis sections (17) in series in order to provide respective cooling capacities to the supply gas, the supply gas having determined temperature levels and determined ortho / para conversion levels.

Description

Hydrogen refrigeration installation and process.

[0001] The invention relates to an installation and a method for refrigerating hydrogen, for example for the liquefaction of hydrogen.

[0002] The invention relates more particularly to a hydrogen (H2) refrigeration and optional liquefaction installation comprising a supply circuit for a flow of gaseous hydrogen to be cooled, for example, for the purpose of its liquefaction, the supply circuit having an upstream end intended to be connected to a source of gaseous hydrogen and a downstream end intended to be connected to a collection device for the cooled hydrogen and, for example, at least partially liquefied, the installation comprising a set of heat exchanger(s) in thermal exchange with the supply circuit, the installation comprising a pre-cooling refrigerator in thermal exchange with at least part of the set of heat exchanger(s) and configured to pre-cool the supply circuit from a first temperature, for example, equal to the ambient temperature, to a second intermediate temperature lower than the first temperature.for example between 80 and 110K, the installation possibly including a second cooling refrigerator in heat exchange with at least part of the heat exchanger assembly(ies) and configured to cool the supply circuit from the second temperature to a third temperature lower than the second temperature, for example to a temperature between 15 and 25K, the pre-cooling refrigerator being a cycle refrigerator of a cycle fluid comprising at least one of: O2, N2, Ar, He, Ne said pre-cooling refrigerator comprising a cycle circuit configured to subject the cycle fluid to a thermodynamic cycle comprising compression in a compression unit comprising at least one compressor, cooling in at least one heat exchanger, expansion in at least one expansion unit and heating in at least one heat exchanger,the supply circuit comprising several catalytic sections in series configured to accelerate the conversion of ortho hydrogen to para hydrogen, of which at least two catalytic sections in series are adjacent respectively to two separate exchangers in series of the exchanger assembly.

[0003] To liquefy a stream of gaseous hydrogen, it is common practice to pre-cool the hydrogen (H2) to a temperature of approximately 80 K and then cool it to around 20 K. Pre-cooling typically uses a pre-cooling cycle refrigerator. This means that a cycle gas (nitrogen or another) is subjected to a thermodynamic cycle that includes an expansion to provide cooling power. During this pre-cooling, the hydrogen to be cooled is generally subjected to catalytic conversion to accelerate the conversion of ortho hydrogen to para hydrogen. This conversion is carried out, for example, in adiabatic catalytic converters and / or within the process heat exchangers. These catalytic conversion converters represent a heat load that must be dissipated by the cycle. This heat load is greater at lower temperatures.This requires a relative increase in the pre-cooling cycle fluid flow rate and its energy consumption.

[0004] One aim of the present invention is to overcome all or part of the disadvantages of the prior art noted above.

[0005] To this end, the installation according to the invention, which also conforms to the generic definition given in the preamble above, is essentially characterized in that the cycle circuit of the pre-cooling refrigerator comprises at least two phase separator pots arranged in parallel and / or in series in the cycle circuit downstream of a cycle fluid expansion element of the cycle circuit, the cycle circuit comprising a set of pipes connected to the separator pots and configured to transfer cycle fluid from the separator pots to respectively the two heat exchangers which are adjacent to the catalysis sections in series, that is to say that the two separator pots are configured to provide respective cooling powers to the feed gas of the feed circuit having determined temperature levels and determined ortho / para conversion levels.

[0006] Furthermore, embodiments of the invention may include one or more of the following features: the installation is configured to adjust the temperatures of the cycle fluid flow transferred from the separator pots to the heat exchangers adjacent to the series catalysis sections; the adjustment of the temperatures of the cycle fluid flow transferred in the heat exchangers adjacent to the series catalysis sections is achieved by controlling the pressure in the separator pots; the installation is configured to transfer saturated cycle fluid into the heat exchangers; the cycle fluid is composed of at least one of the constituents of air: Oxygen (O2), nitrogen, argon, helium, neon, and has a molar mass greater than 16 g / mol.The cycle circuit piping assembly connected to at least one of the separator pots includes a first supply pipe connecting the outlet of a cycle circuit expansion device to a first inlet of the pot, a second pipe connecting a lower outlet of the pot to a passage in at least one heat exchanger of the heat exchanger assembly(ies), and a third pipe connecting an upper outlet of the pot to a passage in at least one heat exchanger of the heat exchanger assembly(ies). A downstream end of the second pipe and / or a downstream end of the third pipe is connected to an inlet of the compression unit. The cycle circuit piping assembly connected to at least one of the separator pots includes a liquid transfer pipe connecting a lower outlet of a first separator pot to an inlet of a second separator pot and configured to transfer liquid cycle fluid from the first separator pot to the second separator pot.The compression unit comprises several compressors in series forming several compression stages. A downstream end of the second pipe and a downstream end of the third pipe are connected respectively to the inlets of different compressors, i.e., to different compression levels. The installation includes several cycle fluid expansion devices arranged in series and / or in parallel in the cycle circuit.

[0007] The invention also relates to a hydrogen liquefaction process using an installation conforming to any one of the preceding or following characteristics and comprising a step of gradual pre-cooling of a gaseous hydrogen stream from the supply circuit in the heat exchanger assembly and a step of gradual conversion of ortho hydrogen to para hydrogen in the catalysis sections, the step of gradual pre-cooling comprising staged cooling via respectively separate heat exchangers arranged in series and cooled respectively by the phase separator pots to determined temperatures.

[0008] According to other possible features: the determined temperatures of the gradual cooling correspond respectively to the temperature levels corresponding to the ortho / para ratio of the conversion carried out by the catalytic sections; the determined temperatures of the gradual cooling are located between 80 and 110K; the process includes a step of circulating, in at least a part of the heat exchangers, a flow of cycle fluid supplied respectively by the separator pots, said flows then being returned to the compression section at determined pressure levels; the separator pots are maintained at determined pressure and temperature levels respectively adapted to the pressure levels prevailing in the compression section receiving the flow of cycle fluid supplied by the separator pots, i.e. at pressure levels similar to within 5 bar,The process includes a compression step of different fractions of the cycle fluid in the cycle circuit at respective variable compression ratios that are adjusted to minimize the energy consumption, particularly electrical, of the compression step.

[0009] The invention may also relate to any alternative device or method comprising any combination of the above or below features within the scope of the claims.

[0010] Other features and advantages will become apparent upon reading the description below, which refers to the figures in which:

[0011] is a schematic and partial view illustrating an example of the structure and operation according to a first embodiment of the invention,

[0012] is a schematic and partial view illustrating an example of the structure and operation according to a second embodiment of the invention,

[0013] is a schematic and partial view illustrating an example of the structure and operation according to a third embodiment of the invention,

[0014] is a schematic and partial view illustrating an example of the structure and operation according to a fourth embodiment of the invention, Detailed description

[0015] In all the figures, the same references refer to the same elements.

[0016] In this detailed description, the following embodiments are examples. Although the description refers to one or more embodiments, this does not mean that the features apply only to a single embodiment. Simple features from different embodiments can also be combined and / or interchanged to provide other embodiments.

[0017] With reference to the, refrigeration installation 1 may be an installation intended for the liquefaction of a flow of gaseous hydrogen.

[0018] Installation 1 includes a supply circuit 2 for a flow of gaseous hydrogen to be cooled / liquefied. The supply circuit 2 has an upstream end intended to be connected to a source of gaseous hydrogen and a downstream end intended to be connected to a collection device for the cooled (and for example at least partially liquefied) hydrogen.

[0019] Installation 1 comprises a set of heat exchangers 3, 4, 5, 6, 7, 8, 9 in heat exchange with the supply circuit 2. In addition, installation 1 comprises at least one pre-cooling refrigerator 10 in heat exchange with at least part of the heat exchanger set and configured to pre-cool the supply circuit 2 from a first temperature, for example, equal to ambient temperature, to a second intermediate temperature lower than the first temperature, for example, between 80 and 110 K. For example, installation 1 comprises several heat exchangers 3, 4, 5, 6, 7, 8, 9 or heat exchange sections arranged in series on the supply circuit. Heat exchangers 3, 4, 5, 6, 7, 8, 9 are configured for example to achieve progressive cooling of supply circuit 2 from upstream to downstream (the heat exchangers are increasingly cold from upstream to downstream).

[0020] As shown schematically, installation 1 may include a second cooling refrigerator 20 in heat exchange with at least part of the heat exchanger assembly(ies) and configured to cool the supply circuit 2 from the second temperature to a third temperature lower than the second temperature, for example, to a temperature between 15 and 25 K. This cooling refrigerator 20 may include, for example, a cycle refrigerator using a cycle gas (primarily or entirely hydrogen and / or helium). This cycle gas is subjected to a thermodynamic cycle including an expansion to produce cooling power.

[0021] The pre-cooling refrigerator 10 comprises or consists of a cycle refrigerator with a cycle fluid comprising at least one of the following: O2, N2, Ar, He, Ne (preferably hydrocarbon-free). More preferably, the cycle fluid consists of nitrogen. This pre-cooling refrigerator 10 comprises a cycle circuit 11 configured to subject the cycle fluid to a thermodynamic cycle comprising compression 15 in at least one compressor of a compression unit, cooling in at least one heat exchanger (for example, from the set of heat exchangers 3, 4, 5, 6, 7, 8, 9), expansion in at least one expansion device 12 (for example, valve(s) and / or turbine(s)) and heating in at least one heat exchanger (for example, from the set of heat exchangers 3, 4, 5, 6, 7, 8, 9). Compression unit 15 includes, for example, electrically driven compressors.

[0022] The supply circuit 2 comprises several catalytic sections 17 in series configured to accelerate the conversion of ortho hydrogen to para hydrogen. These catalytic sections 17 are located in pots in this example. However, all or part of these sections could be integrated into heat exchangers of the heat exchanger assembly(ies) 3, 4, 5, 6, 7, 8, 9.

[0023] In particular, the installation 1 includes at least two series catalysis sections 17 in the supply circuit 2 which are adjacent respectively to two separate heat exchangers 6, 7 in series of the heat exchanger assembly.

[0024] According to an advantageous feature, the pre-cooling refrigerator 10 cycle circuit 11 comprises two phase separator pots 13, 14 arranged in parallel and / or in series in the cycle circuit 11 downstream of a cycle fluid expansion device 12 of the cycle circuit 11.

[0025] Furthermore, the cycle circuit 11 includes a set of pipes connected to the separator pots 13 and 14 (which are preferably adiabatic) and configured to transfer cycle fluid from the two separator pots 13 and 14 to two heat exchangers 6 and 7, respectively, which are adjacent to the series catalysis sections 17. That is, the two separator pots 13 and 14 are configured to provide respective cooling capacities to the feed gas of the supply circuit 2, which has predetermined temperature levels and ortho / para conversion levels. The pots 13 and 14 thus ensure differentiated cooling at the various catalysis sections. This optimizes catalytic conversion at each of these catalytic conversion stages.

[0026] That is to say, the two separator pots 13 14 each provide a flow of cycle fluid (preferably liquid) to a respective heat exchanger associated with a catalysis section 17 which has a temperature and pressure determined to absorb the thermal load caused by the catalysis section 17.

[0027] This multi-stage configuration with several separator pots 13, 14 (at least two) smooths the thermal load associated with the ortho / para conversion by adjusting the operating temperature supplied by the pre-cooling cycle. Supplying such a flow rate of cycle fluid through the separator pot 13, 14 (preferably liquid due to its higher specific heat capacity) allows for a relative reduction in the cycle flow rate and therefore its consumption in the cycle circuit 11. Indeed, this staged configuration reduces the amount of liquid in the coldest pot, while also reducing the required cycle fluid flow rate. This has a positive impact on the energy consumption of the pre-cooling cycle. The various pipes in the cycle circuit 11 allow only the flow rates necessary for each temperature level to be used.This therefore reduces the total flow rate at the compressor 15 cycle, in particular from low pressure to intermediate pressures (before the final pressure level).

[0028] In the illustrated example, only two separator pots 13, 14 are associated with two catalysis sections 17 and their associated heat exchangers 6, 7. Of course, more than two separator pots 13, 14 can be used to provide greater staged cooling at the respective catalysis sections (three, four, or more, for example). The separator pots 13, 14 are associated and operate at different ortho / para conversion temperature levels (adiabatic or isothermal conversion) at each stage.

[0029] This arrangement allows for better staggering of the ortho-par conversion of hydrogen, particularly in the portion of the circuit where the hydrogen is cooled in the temperature range between 80K and 110K. In this temperature range, the ortho / par equilibrium is likely to evolve rapidly.

[0030] Preferably, the cycle fluid is a refrigerant fluid composed of air constituent(s): O2 and / or N2 and / or Ar) with a relatively low specific heat capacity (less than 2000J:kg) but a relatively high molar mass (preferably greater than 16g / mol).

[0031] In the example shown, the cycle circuit 11 comprises two expansion turbines 12 arranged in parallel and supplied respectively by two separate cycles of fluid (via parallel pipes). The two cycles of fluid supplying the two parallel expansion turbines 12 are not necessarily at different pressures. A first separator pot 13 is located downstream of the first turbine 12, while the second separator pot 14 is located downstream of the second turbine 13. The first pot 13, being further upstream (with reference to the direction of flow of the fluid to be cooled in the supply circuit 2), may be at a higher temperature / pressure than the second pot 14, which is further downstream.

[0032] As illustrated, the set of lines in the cycle circuit 11 connected to at least one of the separator pots 13, 14 preferably includes a first supply line 113, 114 connecting the outlet of the expansion device 12 to a first inlet of the corresponding pot 13, 14. This first line supplies the separator pot 13, 14 with a flow of expanded cycle fluid (liquid or two-phase, for example).

[0033] The circuit 11 further includes a second conduit 213, 214 connecting a lower outlet of the pot 13, 14 to a passage in at least one heat exchanger 6, 7, 9 of the exchanger assembly. This second conduit 213, 214 thus supplies a liquid flow to the exchanger associated with the catalytic stage 17. This flow from the second conduit 213, 214 can then "return" to all or part of the heat exchangers 7, 6, 5, 4, 3 to be heated before returning to the compression unit 15.

[0034] The circuit 11 further includes a third line 313, 314 connecting an upper outlet of the separator 13, 14 to a passage in at least one heat exchanger of the exchanger assembly. This flow from the third line 313, 314 can, for example, be returned to the compressor 15 by passing through all or part of the heat exchangers to be reheated before returning (for example, without passing through the heat exchanger 7 that first receives the liquid flow from the same separator). As illustrated, this cycle fluid flow from the third line 313, 314 can be mixed with the cycle fluid flow from the second line 213, 214 which returns to the compressor (after heat exchange with the exchanger associated with the relevant catalysis section stage).

[0035] The cycle fluid flows returned to the compression of each pot return separately to the compression unit (are not mixed).

[0036] The different flows return to the compression which can be multi-stage. That is to say that the cycle fluid flows are directed to the appropriate pressure level in the compression unit 15 (for example three pressure levels: relatively low pressure, relatively medium pressure and relatively high pressure).

[0037] That is to say, for example, if the interstage compression pressure is 2 bara, it is possible to operate a separator pot at a pressure level compatible with this return to 2 bara, for example the separator pot returns a flow at a pressure between 2.2 and 2.5 bara (depending on the pressure loss in the circuit).

[0038] Several flows at distinct pressure levels can be returned to the compression point 15 (e.g., two to four pressure levels). This allows only the flow rates required for each temperature level of the heat exchanger cooling stages to be drawn.

[0039] In this example, a single expansion device (turbine 12) is shown in the conduit that supplies each separator pot 13, 14 with an expanded flow. Of course, one or more expansion devices (turbine(s), valve(s)) could be provided in the conduit that supplies each separator pot.

[0040] The example of the first separator differs from that of the second essentially in that the first separator pot 13, located relatively further upstream, is fed by a conduit 113 comprising two expansion turbines 12 in series. The second separator pot 14 is fed by a cycle fluid flow through a separate conduit in parallel within the circuit 11, downstream of another turbine 12. As before, the two separator pots 13 and 14 are fed by parallel conduits, each having one or more expansion devices. That is to say, these two separator pots 13 and 14 are arranged in parallel branches of the cycle circuit 11.

[0041] Furthermore, according to the possibility illustrated in the figure, the heat exchanger 7 which is relatively further upstream on the supply circuit 2 which receives the liquid flow from the associated separator pot 13 is not put into heat exchange with the liquid flow coming out of the other associated separator pot 14 further downstream.

[0042] In the example of the cycle circuit 11, two parallel expansion turbines 12 supply two separator pots 13 and 14, respectively. The first separator pot 13 is supplied by an expanded fluid flow from the first of the two turbines 12, while the second separator pot 14, located further downstream, receives a flow from the outlet of the second turbine 12. The fluid flows returned by the two pots are preferably not mixed and return to the cycle (for example, to compression via respective lines and preferably dedicated passages in all or part of the heat exchangers).

[0043] The example of the first differs from that of the second essentially in that the cycle circuit 11 comprises two expansion turbines 12 in series. A first upstream pot 13 is fed by a flow from the second of the two series turbines 12. All the cycle fluid flow expanded in the first turbine 12 (upstream) is then expanded in the second series turbine 12 of the cycle circuit 11. The second pot 14 further downstream (associated with a colder downstream heat exchanger 9) receives a liquid flow supplied by the first separator pot 13. That is to say, the cycle circuit 11 includes a liquid transfer line 413 connecting a lower outlet of the first separator pot 13 to an inlet of the second separator pot 14 and configured to transfer liquid cycle fluid from the first separator pot 13 to the second separator pot 14.

[0044] As illustrated, the liquid transfer line 413 connecting a lower outlet of the first separator pot 13 to an inlet of the second separator pot 14 preferably includes a valve 514, for example, a pressure-reducing valve or any other suitable flow control device. That is, liquid contained in the upstream first separator pot 13 and produced by the turbine(s) 12 at a first pressure and temperature can be reduced to a second pressure and temperature for supply to the downstream second separator pot 14. The second pressure (for example, 1.5 bar) is lower than the first pressure (for example, 3 bar). The first temperature (for example, 90 K) is higher than the second temperature (for example, 80 K).This allows the liquid cycle fluid from the first separator pot 13 to be transferred to the second separator pot 14, resulting in a lower pressure and temperature liquid in the second separator pot 14. This also allows for staggered temperatures of the cycle fluid in pots 13 and 14, thus cooling the heat exchangers at the relevant catalytic conversion stages to different temperatures.

[0045] Thus, the first separator pot 13 can be maintained at a pressure higher than the pressure of the second separator pot 14 and the transfer can be carried out by expansion, for example by opening a valve 514 located on the transfer line 413.

[0046] The invention, while being simple in structure and inexpensive, makes it possible to reduce the thermodynamic impact of the catalytic conversion of hydrogen during its cooling.

Claims

Hydrogen (H2) refrigeration and optionally liquefaction installation comprising a supply circuit (2) for a flow of gaseous hydrogen to be cooled, for example, for the purpose of its liquefaction, the supply circuit (2) having an upstream end intended to be connected to a source of gaseous hydrogen and a downstream end intended to be connected to a collection device for the cooled and, for example, at least partially liquefied hydrogen, the installation (1) comprising a set of heat exchanger(s) (3, 4, 5, 6, 7, 8, 9) in heat exchange with the supply circuit (2), the installation (1) comprising a pre-cooling refrigerator (10) in heat exchange with at least part of the set of heat exchanger(s) and configured to pre-cool the supply circuit (2) from a first temperature, for example, equal to the ambient temperature, to a second intermediate temperature lower than the first temperature,for example between 80 and 110K, the installation (1) optionally comprising a second cooling refrigerator (20) in heat exchange with at least part of the heat exchanger assembly(ies) and configured to cool the supply circuit (2) from the second temperature to a third temperature lower than the second temperature, for example to a temperature between 15 and 25K, the pre-cooling refrigerator (10) being a cycle refrigerator of a cycle fluid consisting of nitrogen (N2), said pre-cooling refrigerator (10) comprising a cycle circuit (11) configured to subject the cycle fluid to a thermodynamic cycle comprising compression (15) in a compression unit comprising at least one compressor, cooling in at least one heat exchanger, expansion in at least one expansion unit (12) and heating (3, 4, 5, 6, 7, 8, 9) in at least one heat exchanger,the supply circuit (2) comprising several series-configured catalytic sections (17) to accelerate the conversion of ortho hydrogen to para hydrogen, of which at least two series-configured catalytic sections (17) are adjacent respectively to two separate heat exchangers (6, 7, 8, 9) in series of the heat exchanger assembly, characterized in that the cycle circuit (11) of the pre-cooling refrigerator (10) comprises at least two phase-separating pots (13, 14) arranged in parallel or in series in the cycle circuit (11) downstream of a cycle fluid expansion device (12) of the cycle circuit (11), the cycle circuit (11) comprising a set of conduits connected to the separating pots (13, 14) and configured to transfer cycle fluid from the separating pots (13, 14) to the two heat exchangers (6, 7) adjacent to the catalytic sections (17) series, that is to say that the two pots (13,14) Separators are configured to provide respective cooling capacities to the feed gas of the supply circuit (2) having determined temperature levels and determined ortho / para conversion levels. Installation according to claim 1 characterized in that it is configured to adjust the cycle fluid flow temperatures transferred from the separator pots to the heat exchangers (6, 7) which are adjacent to the series catalysis sections (17) by controlling the pressure in the separator pots (13, 14). Installation according to any one of claims 1 to 2, characterized in that it is configured to transfer saturated cycle fluid into the heat exchangers (6, 7). An installation according to any one of claims 1 to 3, characterized in that the set of pipes of the cycle circuit (11) connected to at least one of the separator pots (13, 14) comprises a first supply pipe (113, 114) connecting the outlet of a pressure-reducing device (12) of the cycle circuit (11) to a first inlet of the pot (13, 14), a second pipe (213, 214) connecting a lower outlet of the pot (13, 14) to a passage in at least one heat exchanger of the set of exchangers (3, 4, 5, 6, 7, 8, 9) and a third pipe (314) connecting a upper outlet of the pot (13, 14) to a passage in at least one heat exchanger of the set of exchangers (3, 4, 5, 6, 7, 8, 9). Installation according to claim 4, characterized in that a downstream end of the second pipe and / or a downstream end of the third pipe is connected to an inlet of the compression unit (15). Installation according to any one of claims 1 to 5, characterized in that the set of lines of the cycle circuit (11) connected to at least one of the separator pots (13, 14) includes a liquid transfer line (413) connecting a lower outlet of a first separator pot (13) to an inlet of a second separator pot (14) and configured to transfer liquid cycle fluid from the first separator pot (13) to the second separator pot (14). Installation according to claim 6, characterized in that the compression unit (15) comprises several compressors in series forming several compression stages, a downstream end of the second pipe and a downstream end of the third pipe being connected respectively to the inlets of different compressors, i.e. to different compression levels. Installation according to any one of the preceding claims, characterized in that it comprises several cycle fluid expansion devices (12) arranged in series and / or parallel in the cycle circuit (11). A hydrogen liquefaction process using an installation according to any one of the preceding claims and comprising a step of gradual pre-cooling of a gaseous hydrogen stream from the supply circuit (2) in the heat exchanger assembly and a step of gradual conversion of ortho hydrogen to para hydrogen in the catalysis sections (17), the step of gradual pre-cooling comprising staged cooling via respectively separate heat exchangers arranged in series and cooled respectively by the phase separator pots (13, 14) at determined temperatures. Method according to claim 9, characterized in that the determined temperatures of the gradual cooling correspond respectively to the temperature levels corresponding to the ortho / para ratio of the conversion carried out by the catalysis sections (17). Method according to claim 9 or 10, characterized in that the determined temperatures of the gradual cooling are located between 80 and 110K. A method according to any one of claims 9 to 11, characterized in that it comprises a step of circulating, in at least a part of the heat exchangers, a cycle fluid flow supplied respectively by the separator pots (13, 14), said flows then being returned to the compression section at determined pressure levels. Method according to claim 12, characterized in that the separator pots (13, 14) are maintained at respective determined pressure and temperature levels adapted to the pressure levels prevailing in the compression section (15) receiving the cycle fluid flows supplied by the separator pots (13, 14), i.e. at pressure levels similar to within 5 bar. A method according to any one of claims 9 to 13, characterized in that it comprises a compression step of different fractions of the cycle fluid in the cycle circuit (11) at respective variable compression rates which are adjusted to minimize the energy consumption, in particular electrical, of the compression step.

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