FR2957141A1 - Producing liquefied natural gas from sea, by pre-cooling natural gas by heat exchange with pre-cooling refrigerant contained in pre-cooling cycle and liquefying gas by heat exchange with liquefying refrigerant contained in liquefying cycle - Google Patents

Abstract

The process comprises partly pre-cooling natural gas by heat exchange with a pre-cooling refrigerant contained in a pre-cooling cycle (123), liquefying natural gas by heat exchange with a liquefying refrigerant contained in a liquefying cycle (100), and sub-cooling the natural gas by heat exchange with a sub-cooling refrigerant contained in a sub-cooling cycle. The liquefying refrigerant comprises nitrogen and an additional compound of which a boiling point at atmospheric pressure is -20[deg] C to -60[deg] C and a crystallization temperature is = -80[deg] C. The process comprises partly pre-cooling natural gas by heat exchange with a pre-cooling refrigerant contained in a pre-cooling cycle (123), liquefying natural gas by heat exchange with a liquefying refrigerant contained in a liquefying cycle (100), and sub-cooling the natural gas by heat exchange with a sub-cooling refrigerant contained in a sub-cooling cycle. The liquefying refrigerant comprises nitrogen and an additional compound of which a boiling point at atmospheric pressure is -20[deg] C to -60[deg] C and a crystallization temperature is = -80[deg] C. A molar proportion of nitrogen in the liquefying refrigerant is = 98.5%. The liquefying refrigerant is expanded in expanding turbines, and is partially condensed in the expanding turbines. The pre-cooling cycle, liquefying cycle and/or sub-cooling cycle comprises a compressor driven by gas turbine generating fumes. A part of heat from fume is transferred to an absorption refrigeration machine, and the gas turbine produces fume at a temperature of = 510[deg] C and is aero-derivative gas turbine. The pre-cooling refrigerant is partly cooled by the absorption refrigeration machine. The turbine gas is fed into air and the air is cooled before feeding. Independent claims are included for: (1) an installation for producing liquefied natural gas; and (2) an use of a composition comprising carbon dioxide and hydrofluorocarbon for cooling and liquefying natural gas.

Description

BACKGROUND OF THE INVENTION The present invention relates to a process for producing liquefied natural gas using a pre-cooling cycle comprising a mixture of carbon dioxide and carbon dioxide. a hydrofluorocarbon, and an installation adapted to the implementation of this process.

TECHNICAL BACKGROUND Natural gas liquefaction processes most frequently use hydrocarbons as coolants. For example, is the Air Products C3-MR cycle (APCI) based on pre-cooling by a propane cycle and liquefaction by a mixed cycle where the refrigerant is typically a mixture of methane, ethane? and propane with a few percent nitrogen. Similarly, the so-called DMR processes for "Dual Mixed Refrigerant" use two refrigeration cycles whose refrigerants are hydrocarbon mixtures.

However, these methods pose significant safety problems. In particular, among the compounds used in the above cycles, propane is among the most dangerous. Indeed, its high molar mass with respect to the air, combined with a low normal boiling temperature, generates in the event of liquid leakage clouds of high density whose dispersion in the atmosphere is difficult. The liquefaction units at sea are installed on a support, floating or not, whose dimensions are necessarily reduced compared to the space that would be available on land. This results in a containment of the facilities and a proximity of the equipment which, in the event of an incident, makes a catastrophic event plausible. The probability of occurrence of such an event and the severity of the consequences are all the greater as the quantity of hydrocarbons contained in the liquefaction units is large. R: \ 31300 \ 31360 SNP \ 31360--100305-texte_depot.doc- 8 March 2010 Since refrigeration cycles constitute a preponderant part of the hydrocarbon inventory in the processes mentioned above, there is an important interest in substituting refrigerants inert to hydrocarbon refrigerants.

Many authors have proposed alternative fluids to hydrocarbons for the liquefaction of natural gas. Nitrogen cycles based on the use of expansion turbines (usually two expansion turbines at different temperatures) are among the most frequently cited. For example, reference may be made to the following documents: Process design solutions for offshore liquefaction, M. J. Roberts, Gastech 2009; US 5,768,912 (especially Figure 4); US 6,446,465 (especially Figure 1); Designing the LNG process for the Bayu-Undan field, Offshore, September 1998; The Challenge of Floating LNG Facilities, John Sheffield, GPA Spring Meeting 2001; New FPSO design produces LNG from offshore sources, Oil and Gas Journal, August 26, 2006; The main disadvantage of the nitrogen cycle process is its high energy consumption, typically about 30% higher than that of the oil and gas journal. that of hydrocarbon cycles. In addition to the increased consumption of fuel gas, this consumption involves the implementation of a large fleet of machines whose multiplicity, potentially causing breakdowns, can affect the availability of the plant. Since nitrogen liquefaction cycles (using expansion turbines) are inherently less efficient than the so-called Pictet evaporation and condensation cycles, many authors have proposed to improve the performance of the assembly by substituting for the first stages cooling a Pictet cycle to the nitrogen cycle. Thus, Kennet et al. Proposed pre-cooling of natural gas by a Pictet cycle based on chlorodifluoromethane or R22 (Offshore liquefaction of associated gas - a suitable process for the North Sea, Offshore Technology Conference, 1981). This refrigerant is classified as Al in the ASHRAE nomenclature (that is, it is considered non-toxic and non-flammable) but it is harmful to the ozone layer and it is harmful to the ozone layer. -100305-text_depot.doc- March 8, 2010 has a significant global warming potential (GWP) (1800). As such, it is covered by the Montreal and Kyoto protocols and is now banned. Walther et al. (Liquefaction Solutions for New Offshore FPSO Developments, LNG Journal, February 2008) and A. Finn (Effective LNG Offshore Production, GPA, 2002) recommend the use of propane in pre-cooling a liquefaction cycle with nitrogen. Propane, although commonly used as a refrigerant in the petroleum industry, is highly flammable and potentially hazardous on a floating production, storage and offloading ("FPSO") system, as mentioned above. In US 5,768,912, Duber recommends upstream of the nitrogen liquefaction cycle a cooling Pictet cycle based on the use of propane, freon or ammonia. As mentioned above, propane has the disadvantage of being flammable. The family of freons, including chlorinated and fluorinated products, are considered harmful to the environment and are covered by the Montreal and Kyoto protocols. Ammonia is both flammable and toxic. Duber (WO 99/30094) and M. J. Roberts (Process design solutions for offshore liquefaction, Gastech 2009) both recommend the use of R410a (a mixture of difluoromethane and pentafluoroethane) as a pre-cooling refrigerant. This fluid is certainly non-flammable and non-toxic and has interesting thermodynamic properties, but it has a GWP of 2000 which makes it likely to ban in time. In addition, its boiling point at atmospheric pressure (NBP) of -51.6 ° C is relatively high for cryogenic applications, which limits its range of use. Fredheim et al. (WO 01/69149) provide in pre-cooling a carbon dioxide cycle which has the advantage of being non-flammable, with no effect on the ozone layer and having a low GWP. However, its low critical temperature (31 ° C) and high triple point (-56 ° C) limit its scope. There is therefore a real need to have an improved natural gas liquefaction process that does not have the above disadvantages. In particular, there is a need for a safer natural gas liquefaction process than processes using hydrocarbon cycles, and more efficient (or less energy consuming) than methods using only one cycle of nitrogen. SUMMARY OF THE INVENTION The invention relates primarily to a process for producing liquefied natural gas comprising: supplying natural gas; pre-cooling the natural gas by exchanging heat with a pre-cooling refrigerant contained in a pre-cooling cycle; the liquefaction of the pre-cooled natural gas by heat exchange with a liquefaction refrigerant contained in a liquefaction cycle; wherein the pre-cooling refrigerant comprises carbon dioxide and a hydrofluorocarbon. According to one embodiment of the above process, the hydrofluorocarbon is chosen from monofluoromethane, pentafluoroethane, 1,1,1-trifluoroethane, monofluoroethane, 1,1,1,2-tetrafluoroethane, 1,1,1,2-tetrafluoroethane, -difluoroethane and mixtures thereof, and preferably is monofluoromethane. According to one embodiment of the above method, the pre-cooling refrigerant comprises a molar content of 40 to 80% of carbon dioxide and 20 to 60% of monofluoromethane, preferably 45 to 65% of carbon dioxide. carbon and 35 to 55% monofluoromethane, and more preferably 47 to 57% carbon dioxide and 43 to 53% monofluoromethane. According to one embodiment of the above process, the liquefaction refrigerant is nitrogen or the liquefaction refrigerant is a mixture of nitrogen and an additional compound. According to an embodiment of the above process, the method comprises, after the step of liquefying the pre-cooled natural gas: subcooling the liquefied natural gas by heat exchange with a sub-cooling refrigerant contained in a subcooling cycle. According to an embodiment of the above process, the liquefying refrigerant is a mixture of nitrogen and an additional compound and the subcooling refrigerant is nitrogen.

According to one embodiment of the above process, the additional compound has a boiling point at atmospheric pressure of -85 ° C. to 0 ° C. and a crystallization temperature of less than or equal to -80 ° C., and preferably boiling temperature at atmospheric pressure from -20 ° C to -60 ° C and / or a crystallization temperature of less than or equal to -100 ° C or -120 ° C or -130 ° C or -140 ° C. According to one embodiment of the above process, the molar proportion of nitrogen in the mixture of nitrogen and additional compound is greater than or equal to 80% or greater than or equal to 85% or greater than or equal to 90% or greater or equal to 95% or greater than or equal to 97.5%; and preferably is less than or equal to 99.5% or less than or equal to 99% or less than or equal to 98.5%. According to one embodiment of the above process, the additional compound is chosen from hydrocarbons and hydrofluorocarbons, and preferably from trifluoromethane, monofluoromethane, tetrafluoroethylene, propylene, propane, monofluoroethane, perfluoropropane, isobutane, n-butane and perfluoropropene. According to an embodiment of the above method, the mixture of nitrogen and additional compound is expanded in one or more expansion turbines, and the mixture of nitrogen and additional compound is partially condensed in at least one of said turbines. relaxation. According to one embodiment of the above method, the pre-cooling cycle and / or the liquefaction cycle and / or the subcooling cycle comprise at least one compressor driven by at least one gas turbine producing fumes, at least a part of the heat of the fumes being transferred to an absorption refrigerating machine, and preferably the gas turbine producing fumes at a temperature of less than or equal to 510 ° C and / or the gas turbine is a turbine aero-derivative gas. According to an embodiment of the above method, the method comprises: a step of additional cooling of the natural gas prior to the pre-cooling of the natural gas, by means of the absorption refrigerating machine; and / or cooling the pre-cooling refrigerant, at least in part by means of the absorption refrigerating machine; and / or cooling the liquefying refrigerant, at least in part by means of the absorption refrigerating machine; and / or cooling the subcooling refrigerant, at least in part by means of the absorption refrigerating machine; and / or R: \ 31300 \ 31360 SNP \ 31360--100305-texte_depot.doc- March 8, 2010 the air supply of the gas turbine and the cooling of the air before the air supply, at least partly by means of the absorption refrigerating machine. According to one embodiment of the above process, the process is carried out at sea. The invention also relates to a liquefied natural gas production plant, comprising: a natural gas supply pipe; a pre-cooling cycle containing a pre-cooling refrigerant, said pre-cooling refrigerant comprising carbon dioxide and a hydrofluorocarbon; a pre-cooling exchanger adapted to the heat transfer between the natural gas and the pre-cooling refrigerant; a liquefaction cycle containing a liquefaction refrigerant; a liquefaction exchanger adapted to the heat transfer between the natural gas and the liquefaction refrigerant and located downstream of the pre-cooling exchanger.

According to one embodiment of the above installation, the hydrofluorocarbon is chosen from monofluoromethane, pentafluoroethane, 1,1,1-trifluoroethane, monofluoroethane, 1,1,1,2-tetrafluoroethane, , 1-difluoroethane and mixtures thereof, and preferably is monofluoromethane.

According to one embodiment of the above installation, the pre-cooling refrigerant comprises a molar content of 40 to 80% of carbon dioxide and 20 to 60% of monofluoromethane, preferably 45 to 65% of carbon dioxide and 35 to 55% monofluoromethane, and more preferably 47 to 57% carbon dioxide and 43 to 53% monofluoromethane. According to one embodiment of the above installation, the liquefaction refrigerant is nitrogen or the liquefaction refrigerant is a mixture of nitrogen and an additional compound. According to an embodiment of the above installation, the installation also comprises: a subcooling cycle containing a subcooling refrigerant; R: \ 31300 \ 31360 SNP \ 31360--100305-texte_depot.doc- March 8, 2010 a sub-cooling exchanger adapted to the heat transfer between the natural gas and the subcooling refrigerant and located downstream of the liquefaction exchanger. According to one embodiment of the above installation, the liquefaction refrigerant is a mixture of nitrogen and an additional compound and the subcooling refrigerant is nitrogen. According to one embodiment of the installation above, the additional compound has a boiling point at atmospheric pressure of -85 ° C. to 0 ° C. and a crystallization temperature of less than or equal to -80 ° C., and preferably a boiling temperature at atmospheric pressure of -20 ° C to -60 ° C and / or a crystallization temperature of less than or equal to -100 ° C or -120 ° C or -130 ° C or -140 ° C ° C. According to one embodiment of the above installation, the molar proportion of nitrogen in the mixture of nitrogen and additional compound is greater than or equal to 80% or greater than or equal to 85% or greater than or equal to 90% or greater than or equal to 95% or greater than or equal to 97.5%; and preferably is less than or equal to 99.5% or less than or equal to 99% or less than or equal to 98.5%. According to one embodiment of the installation above, the additional compound is chosen from hydrocarbons and hydrofluorocarbons, and preferably from trifluoromethane, monofluoromethane, tetrafluoroethylene, propylene, propane, monofluoroethane, perfluoropropane, isobutane, n-butane and perfluoropropene. According to one embodiment of the installation above, the liquefaction cycle comprises one or more expansion turbines, the installation being adapted to partially condense the liquefaction refrigerant in at least one of said expansion turbines. According to one embodiment of the installation above: the pre-cooling cycle and / or the liquefaction cycle and / or the subcooling cycle comprise at least one compressor; the installation comprises: compressor drive means comprising at least one gas turbine producing fumes; an absorption refrigerating machine; and means for transferring at least a portion of the heat of the flue gases from the gas turbine to the absorption refrigerating machine; R: \ 31300 \ 31360 SNP \ 31360--100305-texte_depot.doc- March 8, 2010 and preferably the gas turbine produces fumes at a temperature lower than or equal to 510 ° C and / or the gas turbine is a turbine aero-derivative gas. According to one embodiment of the installation above: the installation comprises an additional natural gas cooling device upstream of the pre-cooling exchanger, said additional cooling device of the natural gas being supplied by the refrigerating machine absorption; and / or the pre-cooling cycle comprises a cooling device for the pre-cooling refrigerant supplied by the absorption refrigerating machine; and / or the liquefaction cycle comprises a device for cooling the liquefaction refrigerant, fed by the absorption refrigerating machine; and / or the gas turbine comprises a device for cooling the air supply, supplied by the absorption refrigerating machine. According to one embodiment of the installation above, the installation is disposed at sea. The invention also relates to the use of a composition comprising carbon dioxide and a hydrofluorocarbon for cooling and liquefying the natural gas. According to one embodiment of this use, the hydrofluorocarbon is chosen from monofluoromethane, pentafluoroethane, 1,1,1-trifluoroethane, monofluoroethane, 1,1,1,2-tetrafluoroethane and 1,1-difluoroethane. and mixtures thereof, and preferably is monofluoromethane. According to one embodiment of this use, the composition comprises a molar content of 40 to 80% carbon dioxide and 20 to 60% monofluoromethane, preferably 45 to 65% carbon dioxide and 35 to 55% monofluoromethane, and more preferably 47 to 57% carbon dioxide and 43 to 53% monofluoromethane.

The present invention overcomes the disadvantages of the state of the art. In particular, it provides a safer natural gas liquefaction process than processes using hydrocarbon cycles, and is more efficient (and less consumer-friendly). of energy) that processes using only a nitrogen cycle. This is accomplished through the use of a precooling cycle comprising a mixture of carbon dioxide and at least one hydrofluorocarbon such as CH3F monofluoromethane (or R41) or the like. Indeed, the mixture of CO2 and a hydrofluorocarbon generally has a higher critical temperature than that of CO2 alone. On the other hand, this mixture can be non-flammable even if the hydrofluorocarbon is flammable, and this mixture has a GWP lower than that of the hydrofluorocarbon taken alone. According to some particular embodiments, the invention also has one or preferably more of the advantageous features listed below.

R41 has the particularity of forming a quasi-azeotrope with CO2, which makes the CO2 / R41 mixture particularly advantageous. Indeed, the slippage of this mixture (defined as the difference between the temperature of the beginning of boiling and the end of boiling point in a refrigeration cycle) is very low (less than 2 ° C). This makes the management of evaporation and condensation easier. The compositions of the liquid and vapor phases in equilibrium are very close to one another so that there is only a little enrichment in one or the other component during the evaporation: vaporization is almost constant composition which is that of the mixture. Thus, in case of leakage, the gas cloud has a similar composition to that of the initial mixture and therefore remains non-flammable, which significantly improves safety. The CO2 / R41 mixture is non-flammable and non-toxic. These benefits are paramount for use in the confined space of a floating liquefied natural gas (LNG) system. The CO2 / R41 mixture has a relatively high critical temperature (37.9 ° C for an equimolar mixture of CO2 and R41) and in particular higher than that of pure carbon dioxide; thus, the condensation of the mixture by most of the sea water is possible, if these are pumped sufficiently deeply. The boiling point (NBP) of the CO2 / R41 mixture is relatively low (-84.5 ° C to 1.013 bar for an equimolar mixture). CO2 and R41), and notably significantly lower than that of pure carbon dioxide; on the other hand the crystallization temperature of the CO2 / R41 mixture (-121 ° C for an equimolar mixture of CO2 and R41) is significantly lower than that of the triple point of carbon dioxide (-55.6 ° C). These two characteristics allow much lower pre-cooling temperatures (-80 ° C) than those obtained with carbon dioxide (-50 ° C approximately), improving the overall performance of liquefaction. From an environmental point of view, the ozone depletion potential (ODP) of the CO2 / R41 mixture is zero, and its global warming potential (GWP) is 50, which makes this mixture significantly more advantageous than Other refrigerants previously proposed, such as R410a or Freons. In addition, given the decision-making deadlines commonly associated with major liquefaction projects, their lead times, and the operating life of these plants, a period of 40 to 50 years when the decision is made to use a particular refrigerant and the end of operation of the plant that implements it. It is therefore necessary to have a fluid that is as unlikely as possible to undergo a regulatory ban related to the environment. In this respect, the ODP and GWP characteristics of the CO2 / R41 mixture are particularly advantageous. The embodiment of a three-cycle installation saves energy. The embodiment of a two-cycle installation is less complex, particularly in terms of the number of compressors. In the context of this application, the global warming potential (GWP) is defined with respect to carbon dioxide and with respect to a duration of 100 years, according to the method indicated in "The scientific assessment of ozone depletion, 2002, Report of the World Meteorological Association's Global Ozone Research and Monitoring Project. As part of this application, the ozone depletion potential (ODP) is as defined in "The Scientific Assessment R: \ 31300 \ 31360 SNP \ 31360-100305-text_depot.doc- March 8, 2010 11 of Ozone Depletion, 2002, Report of the World Meteorological Association's Global Ozone Research and Monitoring Project.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 represents a diagram of one embodiment of the installation according to the invention (two-cycle variant, with a pure nitrogen liquefaction cycle). FIG. 2 schematically represents an absorption refrigerating machine that can be used in the context of the invention. FIG. 3 represents a diagram of another embodiment of the installation according to the invention (two-cycle variant, with an enriched nitrogen liquefaction cycle). FIG. 4 represents a diagram of another embodiment of the installation according to the invention (variant with three cycles).

DESCRIPTION OF EMBODIMENTS OF THE INVENTION The invention is now described in more detail and in a nonlimiting manner in the description which follows.

General description of a two-cycle plant According to one embodiment of the invention, and with reference to FIG. 1, the natural gas is supplied by a natural gas supply pipe 101.

Upstream of this natural gas supply line 1 may be provided one or more deacidification and / or drying units for natural gas (for example molecular sieves), not shown here. The terms "upstream" and "downstream" used in this application are understood to refer to the flow direction of natural gas during the cooling and liquefying process. The natural gas enters a first heat exchanger called pre-cooling exchanger 104 which results in a temperature typically between 0 ° C and -80 ° C. A pre-cooled natural gas withdrawal line 102 brings the natural gas to a second heat exchanger, called a liquefaction exchanger 105, which it exits in a liquefied natural gas withdrawal line 103 at a temperature between -130 ° C and -160 ° C. It is then in the liquid state under a pressure typically between 35 and 70 bar. The liquefied natural gas is then expanded, eventually it undergoes denitrogenation if the gas is naturally rich in nitrogen and is sent to storage (operations not shown). The liquefaction exchanger 105 allows the transfer of heat between the natural gas and the liquefaction refrigerant. The liquefaction refrigerant may be any fluid known in the art, and especially hydrocarbon mixtures.

In the illustrated embodiment, the liquefaction refrigerant is nitrogen circulating in a liquefaction cycle 100. The operation of the liquefaction cycle 100 is based in this case on one or more expansion turbines 110, 111. The refrigerant liquefaction of the liquefaction cycle 100 is compressed in a compressor 106 and cooled to room temperature by cooling means 107. It is then further compressed in booster pumps 108, 109 coupled to the expansion turbines 110 , 111 before being cooled to room temperature by cooling means 112.

The liquefaction refrigerant is then introduced into the pre-cooling exchanger 104, cooled to the same temperature as the natural gas and recovered in a cooled liquefaction refrigerant line 113. In the illustrated embodiment, the refrigerant liquefaction is then separated into two streams in a first refrigerant pipe 114 and a second refrigerant pipe 115. The flow flowing in the first refrigerant pipe 114 is expanded in a first expansion turbine 110 (called hot turbine) to an intermediate pressure and then recovered in a first cold flow pipe 116. The flow flowing in the second refrigerant pipe 115 is introduced into the liquefaction exchanger 105 where it is cooled to a temperature typically from -70 ° C to -120 ° C then recovered in a partially cooled flow line 117. The flow flowing in the partially cooled flow line 117 is then expanded in a second expansion turbine 111 (called cold turbine) and then recovered in a second cold flow pipe 118, which opens into the liquefaction exchanger 105. The flow flowing in the first cold flow duct 116 and the flow flowing in the second flow duct R 11 will heat up in the liquefaction exchanger. 105 by condensing the natural gas by heat exchange and cooling the flow of liquefaction refrigerant from the second refrigerant fluid line 115.

They emerge respectively in a first partially heated flow line 119 and a second partially heated flow line 120, and are introduced into the pre-cooling exchanger 104 where the natural gas and the high pressure liquefaction refrigerant are pre-heated. cooled. They are then recovered respectively in a first heated flow line 121 and a second heated flow line 122 and introduced into the compressor 106 (two different stages) and the cycle starts again. The pre-cooling exchanger 104 further allows heat transfer between the natural gas and the pre-cooling refrigerant, i.e., pre-cooling the natural gas with the pre-cooling refrigerant. . It also makes it possible to ensure part of the cooling of the liquefaction refrigerant (nitrogen). The pre-cooling refrigerant circulates in the pre-cooling cycle 123, which is a Pictet cycle with evaporation and condensation.

According to the illustrated embodiment, the pre-cooling cycle 123 comprises two integrated heat exchange loops 124, 125 within the pre-cooling exchanger 104, which are at two different evaporation pressures. The pre-cooling refrigerant is compressed in a compressor 126 and condensed at a pressure below the critical pressure by cooling means 127 generally supplied by the cooling water of the plant. The liquid obtained is separated in two at a branch 129, a portion being expanded in a first expander 128 at a first pressure and then evaporated in a first exchange loop 124 integrated in the pre-cooling exchanger 104 before It is returned to the interstage of the compressor 126. The second part is expanded in a second pressure reducer 130 at a second pressure lower than the first and then evaporated in a second exchange loop 125 integrated in the pre-cooling exchanger 104. This latter vapor is returned to the suction of the compressor 126. The method according to the invention can be implemented (and the installation according to the invention can be arranged) on land or at sea, and preferably at sea, on a floating support or not. In accordance with the invention, the pre-cooling refrigerant, which is used in the pre-cooling cycle, is used in the pre-cooling cycle. a mixture comprising carbon dioxide and a hydrofluorocarbon, that is to say a compound, preferably saturated, comprising carbon, fluorine and optionally hydrogen. The number of carbon atoms of this compound is preferably from 1 to 3, and more particularly from 1 to 2. Preferably, said mixture of carbon dioxide and hydrofluorocarbon is non-flammable. The hydrofluorocarbon may especially be monofluoromethane (R41), pentafluoroethane (R125), 1,1,1-trifluoroethane (R143a), monofluoroethane (R161), 1,1,1,2-tetrafluoroethane (R134a), 1,1-difluoroethane (R152a) and mixtures thereof. The preferred hydrofluorocarbon is R41. The pre-cooling refrigerant may in particular consist of a mixture of carbon dioxide and R41, or essentially consist of a mixture of carbon dioxide and R41 (which means that it may contain, in addition to carbon dioxide and R41, one or more additives or impurities in an amount of less than or equal to 1%, preferably less than or equal to 0.5%, and more particularly less than or equal to 0.1%, in molar proportions). Advantageously, the pre-cooling refrigerant comprises, in molar proportions: from 40 to 80% of carbon dioxide and from 20 to 60% of R41, preferably from 45 to 65% of carbon dioxide and from 35 to 55% R41, and more preferably 47 to 57% carbon dioxide and 43 to 53% R41. The precooling refrigerant may in particular be the mixture known under the name ECP744, comprising 50% carbon dioxide and 50% R41, in molar proportions. The CO2 / R41 binary mixture as such has been described in the following documents: Cox, Mazur and Colbourne, Institute of Refrigeration, Proc. Inst. R. 2008-09. 1-1; GB 2438318; ù Alternative low GWP coolant blends, VDA winter 2009 MAC. This mixture has been proposed in the state of the art for automotive air conditioning. R: \ 31300 \ 31360 SNP \ 31360--100305-text_depot.doc- 8 March 2010 Variant based on the use of enriched nitrogen The description above has been made in relation to a liquefaction cycle based on the use of pure nitrogen. However, it is also possible to use enriched nitrogen as liquefaction refrigerant, that is to say a mixture comprising nitrogen and an additional compound less volatile than nitrogen, whose temperature boiling point at atmospheric pressure is between 0 ° C and -85 ° C inclusive and preferably between -20 ° C and -60 ° C inclusive, and whose crystallization temperature is less than or equal to -80 ° C, preferably less than or equal to -100 ° C or -120 ° C or -130 ° C or -140 ° C. The boiling and crystallization temperatures of the various chemical compounds contemplated in the context of the present application are known and listed, for example in the Chemical Engineer's Handbook (Perry) or in the Gas Encyclopedia (Air Liquide). In this case, advantageously, the hot expansion turbine is installed in such a way that its exhaust temperature is close to and below the condensation end temperature of the natural gas. The liquefaction refrigerant is then completely gaseous at the inlet of the hot expansion turbine but partially condensed at the exhaust. This two-phase mixture is then introduced into the liquefaction exchanger so that the vaporization of the additional compound allows the condensation of the natural gas. This embodiment makes it possible for the enthalpy curve for reheating of the mixture (enriched nitrogen) to follow as closely as possible the cooling of the natural gas, thus decreasing the temperature differences and thus allowing a decrease in the liquefaction power. . It is thus possible to substantially reduce the power required for the liquefaction of natural gas while maintaining only two expansion turbines and a single liquefaction cycle, thus a single cycle compressor. The natural gas during its cooling successively passes through a pre-cooling in the gas phase, a pseudo-condensation and a subcooling of the liquid. The pseudo-condensation usually occurs between about -50 ° C and -90 ° C. As a result, only certain compounds have a volatility appropriate to their use as additional compounds in combination with nitrogen. This volatility can be characterized by the boiling point at atmospheric pressure. The suitable temperature range is from 0 ° C to -80 ° C, preferably from -20 ° C to -60 ° C. Since the additional compound also intervenes at the coldest temperatures of the liquefying exchanger, ie from -130 ° C. to -155 ° C., this additional compound must also have a crystallization temperature lower than this operating temperature (in this case). the two-cycle embodiment). Preferably, the additional compound is a saturated or unsaturated hydrocarbon or a hydrofluorocarbon (that is to say a compound comprising carbon, fluorine and possibly hydrogen) saturated or not, optionally without a hydrogen atom, c that is, perfluorinated. It advantageously comprises from 1 to 5 carbon atoms, and more particularly from 1 to 4 carbon atoms. Table 1 below summarizes the thermodynamic properties of a number of molecules that may be used as additional compounds. Name Formula Temperature Boiling temperature (pressure atmospheric crystallization) Trifluoromethane CHF3 -81.6 ° C -155 ° C Monofluoromethane (R41) CH3F -78.4 ° C -141.8 ° C Tetrafluoroethylene C2F4 -75.6 ° C Propylene C3H8 -42 ° C -187.7 ° C Monofluoroethane (R161) C2H5F -37.1 ° C -143.2 ° C Perfluoropropane (R188) C3F8 -36.7 ° C -147.7 ° C Isobutane C4H10 -11.7 ° C -159.6 ° C N-Butane C4H10 -0.5 ° C -141.75 ° C Perfluoropropene C3F6 -29.6 ° C -156.5 ° C Table 1 List of possible additional compounds, and thermodynamic properties thereof Preferably, the molar proportion of nitrogen in the liquefaction refrigerant is greater than or equal to 80% or greater or equal to at 85% or greater than or equal to 90% or greater than or equal to 95% or

R: \ 31300 \ 31360 SNP \ 31360--100305-texte_depot.doc- 8th March 2010 greater than or equal to 97.5%; and preferably is less than or equal to 99.5% or less than or equal to 99% or less than or equal to 98.5%. The calculations show that a small amount of additional compound is sufficient to obtain the advantages offered by the invention. An additional compound content of less than or equal to 20%, or 15%, or 10% or 5% (in molar proportions) is therefore generally suitable. The embodiment in which enriched nitrogen is used in place of pure nitrogen as a liquefaction refrigerant requires some adaptations of the plant and the process. These adaptations are shown in Fig. 3. In this figure, the pre-cooling cycle is unchanged from the embodiment of Fig. 1, and is merely shown in less detail. All reference numbers that are identical to those in Figure 1 have the same meaning.

Changes to the installation and process will now be described. Firstly, the flow flowing in the first refrigerant pipe 114 is always expanded in the expansion turbine 110 (hot turbine) to an intermediate pressure, but this expansion produces a diphasic type flow, which is recovered in a first two-phase cold flow pipe 144. The flow flowing in the second refrigerant pipe 115 is introduced into the liquefaction exchanger 5 where it is cooled to a temperature typically of -70 ° C to -120 ° C and then recovered in an intermediate diphasic cold flow line 117. Instead of directly supplying the second expansion turbine 111 (cold turbine), this flow opens into a separator tank 134 in which a liquid phase and a vapor phase are separated. The liquid phase is recovered in a liquid phase pipe 135, and is expanded in a valve 136 and mixed with the flow flowing in the first diphasic cold flow pipe 144 before introduction into the liquefaction exchanger 105. The vapor phase is recovered in a vapor phase pipe 137 is introduced into the second expansion turbine 111 (cold turbine), expanded to the low pressure of the cycle and recovered in a second diphasic cold flow pipe 138 which opens into the liquefying exchanger 105 (after passing through a safety balloon 139). The flow coming from the first two-phase cold flow pipe 144 and the flow resulting from the second two-phase cold flow pipe 138 are warmed up in the second flow pipe 138. liquefaction exchanger 105 by condensing the natural gas by heat exchange and cooling the flow of liquefaction refrigerant from the second refrigerant conduit 115.

They emerge respectively in a first partially heated flow line 140 and a second partially heated flow line 141 and are introduced into the pre-cooling exchanger 104 where the natural gas and the high pressure liquefaction refrigerant are pre-cooled. . They are then introduced through a first heated flow line 142 and a second heated flow line 143 into the compressor 106 (two different stages).

Variant of a three-cycle installation It is also possible according to the invention to provide an operation based on three distinct cycles: the pre-cooling cycle; the liquefaction cycle; and a subcooling cycle, containing subcooling refrigerant.

The liquefaction refrigerant may be any fluid used in the field, including a mixture of hydrocarbons. The subcooling refrigerant may be any fluid used in the field, and especially a mixture of hydrocarbons. According to a preferred embodiment, the liquefaction refrigerant is enriched nitrogen as described above, and the subcooling refrigerant is pure or near pure nitrogen. According to this three-ring embodiment, the liquefaction cycle described above is replaced by a set of two cycles, namely the liquefaction cycle and the subcooling cycle. This embodiment is shown in FIG. 4. The liquefaction exchanger 105 of the two-cycle embodiment is here replaced by a succession of two exchangers: a liquefaction exchanger 305a and a subcooling exchanger 305b, these However, they can be integrated as two parts of the same exchanger. The cooling and the liquefaction of the natural gas are thus effected by passing it successively in a pre-cooling exchanger 304 (into which it is fed by a feed pipe of the following direction: R: \ 31300 \ 31360 SNP \ 31360 - 30305-texte_depot.doc- March 8, 2010 natural gas 301 and from which it emerges by a pre-cooled natural gas withdrawal line 302), the liquefaction exchanger 305a and the sub-cooling exchanger 305b (which it finally emerging through a liquefied natural gas withdrawal line 303).

In this three cycle embodiment, the pre-cooling cycle 351 is designed and operates in the same manner as described above. In this three cycle embodiment, the liquefaction cycle 352 operates as follows.

The liquefaction refrigerant is compressed in a compressor 327 and after a first cooling is further compressed in a blower 307 coupled to an expansion turbine 309 before being cooled to room temperature by cooling means 311.

The liquefaction refrigerant is then introduced into the pre-cooling exchanger 304, cooled to the same temperature as the natural gas and recovered in a cooled liquefaction refrigerant line 312. The fluid is then expanded in the turbine. 309 (hot turbine) to an intermediate pressure and is recovered in a diphasic flow stream 315. This fluid is then heated in the liquefaction exchanger 305a including condensing the natural gas by heat exchange. It exits in a partially heated liquefaction refrigerant line 324 and is introduced into the pre-cooling exchanger 304 where the natural gas, the high pressure liquefying refrigerant and the high pressure sub-cooling refrigerant are pre-cooled. It is then returned to the compressor 327 and the cycle begins again.

In this three cycle embodiment, the subcooling cycle 353 operates as follows. The subcooling refrigerant is compressed in a compressor 332 and after a first cooling is further compressed in a booster 308 coupled to an expansion turbine 310 before being cooled to room temperature by cooling means 350. The subcooling refrigerant is then introduced into the pre-cooling exchanger 304, cooled to the same temperature as the gas. The fluid is then cooled in the cooled subcooling refrigerant pipe 314. The fluid is then cooled in the liquefaction exchanger 305a, expanded in the expansion turbine 310 (cold turbine) to a low pressure and recovered in a reactor. This fluid is then reheated in the subcooling exchanger 305b by cooling the condensed natural gas by heat exchange, and then the liquefaction exchanger 305a in particular by condensing the natural gas by heat exchange.

It exits into a partially heated subcooling refrigerant line 323 and is introduced into the pre-cooling exchanger 304 where the natural gas, the high pressure liquefaction refrigerant and the high temperature subcooling refrigerant pressure are pre-cooled. It is then returned to the compressor 332 and the cycle starts again. In this embodiment, the additional compound present in a mixture with the nitrogen in the liquefaction cycle is subjected to lower temperatures than in the case of one or two-ring embodiments. Therefore, it is possible to use an additional compound having a higher crystallization temperature, and the choice of possible additional compounds is therefore wider. More particularly, in this embodiment, it is possible to use an additional compound having a crystallization temperature of less than or equal to -80 ° C., and preferably still less than or equal to -100 ° C. or -120 ° C.

Variant using one or more absorption and variant refrigeration machines using qaz turbines at low flue gas temperature According to a particular aspect of the invention, it is possible to use at least one absorption refrigeration machine (MFA) to make an additional cold in one or more points. MFAs are devices capable of producing a cold flow (especially cold water) from thermal energy. It is therefore possible to cool with the cold stream produced by one or more MFAs: the pre-cooling refrigerant; - the refrigerant liquefaction; - the subcooling refrigerant; R: \ 31300 \ 31360 SNP \ 31360--100305-texte_depot.doc- March 8, 2010 natural gas, for example prior to the pre-cooling stage; the air supplying a gas turbine, when such a gas turbine is present in the installation.

For example, with reference to the embodiment of FIG. 1, it is possible to optionally provide: an additional natural gas cooling device 131 upstream of the pre-cooling exchanger 104, powered by an MFA; and / or an additional cooling device of the liquefying refrigerant 132 at the outlet of the cooling means 112, fed by an MFA; and / or an additional cooling device of the precooling refrigerant 133 at the outlet of the cooling means 127, fed by an MFA. In general, the additional cooling devices 131, 132, 133 are connected to an MFA by means of a coolant (in particular water) which is heated by heat exchange with the fluid to be cooled, and which is cooled by the MFA.

An advantage of using MFAs is that the locally available cooling water may not be cold enough to allow condensation of the pre-cooling refrigerant to a temperature below the critical temperature. In this case, it is advantageous to proceed with the cooling water to a cooling of this fluid to a pressure greater than the critical pressure so as to operate according to a transcritical cycle known in the field. The fluid can then be subcooled with one or more MFAs. Transcritical cycles, however, have a lower yield than subcritical cycles in which the fluid condenses at a pressure below the critical pressure. In another embodiment of the invention and in order to restore a good efficiency of the pre-cooling cycle, the condensation of the precooling fluid is obtained at a temperature below the critical temperature by one or more MFAs. For example, if water is used as heat transfer fluid, the temperature of the coolant at the inlet of the additional cooling devices 131, 132, 133 is typically -3 ° C to 15 ° C and the temperature of the heat transfer fluid output of these devices is typically 6 ° C to 20 ° C. R: \ 31300 \ 31360 SNP \ 31360--100305-texte_depot.doc- 8th March 2010 The schematic diagram of an MFA is illustrated in figure 2. Like the mechanical refrigeration cycles, the cold is produced by low-pressure evaporation of a refrigerant in an evaporator 201 where is cooled a fluid to be cooled 202. The resulting vapor of the refrigerant is conveyed to an absorber 203 where it is dissolved in a heavy solution 204 to form a mixing solution 206 , the heat of dissolution being discharged by an auxiliary cooling system 205 (typically based on cooling water). The mixing solution 206 is pumped by a pump 207 and directed at a higher pressure to a regenerator 208. In this regenerator 208, the refrigerant is separated from the heavy solution by heating by means of a heat source 209 (generally from water vapor or hot pressurized water). The heavy solution is recycled to the absorber 203 via a recovery exchanger 210 for partially heating the mixing solution 206 before it arrives in the regenerator 208. The refrigerant in vapor form is led to the condenser 213 where it is condensed by means of an auxiliary condensation system 211. It is then returned via the valve 212 to the evaporator 201 for a new cycle There are mainly two types of MFA according to the types of binary mixtures used for their operation: the water and ammonia MFAs (the refrigerant being ammonia and the solvent water) and those water and lithium bromide (the refrigerant being water and the solvent an aqueous solution of lithium bromide). The first allows to reach temperatures of -20 to -30 ° C while the second only allows to produce chilled water at positive temperatures (> 0 ° C). Since ammonia is flammable and toxic, the water / lithium bromide binary mixture is particularly preferred for applications at sea. According to one embodiment of the invention, one or more compressors of the pre-cooling and / or liquefaction and / or subcooling cycles are driven by means of at least one gas turbine, producing fumes, and the energy supply of the MFA is provided by means of said fumes. In this context, it is particularly advantageous to use a gas turbine with a low exhaust temperature (that is to say a gas turbine producing fumes at a temperature of less than or equal to 510 ° C., preferably less than or equal to at 500 ° C, more preferably less than or equal to 490 ° C, ideally less than or equal to 480 ° C). The temperature of the fumes produced by a gas turbine is a parameter specific to each gas turbine, which can be measured according to ISO 2314. The measurement conditions are: an inlet temperature of 15 ° C, a pressure of inlet of 1013 mbar, relative air humidity at the inlet of 60%, outlet pressure of 1013 mbar. Any gas turbine used to drive a device such as a compressor offers a certain efficiency, compared to the heat produced by the combustion of the gas. Part of the thermal energy is recovered as mechanical energy by driving the turbine shaft while the balance is released to the environment as heat in the flue gas. The heat released represents a large fraction, usually more than half, of the heat of combustion and therefore offers a significant potential for recovery. According to this embodiment, the heat usually lost in the flue gases of the gas turbines is recovered by coupling with an MFA. However, the temperature required for the operation of the MFA is relatively moderate (from 100 to 200 ° C depending on the technology) while the fumes of gas turbines are available at significantly higher temperatures (from 450 to 550 ° C approximately). It is also noted that, generally, the lower the flue gas temperature, the better the efficiency of the turbine is improved. Thus, the coupling of gas turbines with high efficiency and low flue gas temperature with the MFAs makes it possible to benefit from the cold back-up produced by the MFAs since the low flue gas temperatures are well suited to the temperatures required by the MFAs. In general, gas turbines used in natural gas liquefaction plants can be of two types: industrial turbines (the most common) and aero-derived turbines. Aero-derivative turbines are distinguished mainly from industrial turbines by the origin of the gas generator, which is derived from an aircraft engine. As preferred low-temperature exhaust gas turbines in the context of the present embodiment, use is made of aero-derived turbines (or turbines derived from aeronautics, or derived from aircraft engines). Such turbines provide excellent efficiency and discharge fumes at suitable temperatures for good coupling with MFAs. R: \ 31300 \ 31360 SNP \ 31360--100305-texte_depot.doc- March 8, 2010 An industrial gas turbine conventionally comprises an air compressor, a fuel gas or liquid fuel supply, a combustion chamber and a turbine of relaxation. In operation, air is compressed in the air compressor and mixed with fuel gas or liquid fuel, which burns in the combustion chamber. The hot fumes are relaxed in the expansion turbine. The mechanical power developed by the expansion turbine is used to drive the air compressor, and the balance provides the power available for driving any device (alternator, compressor ...). An industrial turbine is always originally designed for operation in an industrial setting. An aerodynamic turbine, on the contrary, is designed from a jet engine. In a reactor, the air is compressed like an industrial turbine. It then enters the combustion chamber where it is mixed with the fuel burned there. The combustion gases are expanded in a turbine to an intermediate pressure to drive the air compressor. Hot pressurized gases are produced at the outlet of the first turbine. The assembly producing these gases (namely the air compressor, the combustion chamber and the first turbine) constitutes the gas generator. In an airplane reactor, the hot gases produced therein are expanded in a nozzle whose object is to convert the potential energy of pressure into kinetic energy. The hot gases are expelled to the rear and the aircraft, by conservation of the momentum, is propelled forward. In an air-cooled turbine, the hot gases at the output of the gas generator are expanded in a second expansion turbine with production of work that can be used for driving a compressor, an alternator or for propulsion Marine. The gases come out relaxed at atmospheric pressure. It follows that an aero-derived turbine always has two shafts, one comprising the air compressor and the first expansion turbine, the other the second expansion turbine and producing the work recovered from the turbine. This is not always the case for industrial turbines. Aero-derived gas turbines are distinguished from industrial gas turbines generally by a higher compression ratio (typically about 30: 1 vs. about 16: 1). Because of their origin, their weight is much lower than that of industrial gas turbines. As an example of industrial turbines, mention may be made of the turbines MS6001 (Frame 6), MS7001 (Frame 7) and MS9001 (Frame 9), of General R: \ 31300 \ 31360 SNP \ 31360-100305-text_depot.doc - March 8, 2010 Electric. As examples of aero-derived turbines, mention may be made of the Trent 60 and RB211 turbines of Rolls-Royce, and LM2500 and LM6000 of General Electric. Industrial gas turbines generally have average yields (typically of the order of 30 to 34% ISO) and discharge fumes at high temperature while the aero-derived turbines generally have significantly higher yields (from 38 to 42% ISO ) but lower flue gas temperatures. Table 2 below provides examples for the turbines most commonly used in the natural gas liquefaction industry. Type of Turbine Flue gas temperature (° C) MS 6001 544 MS 7001 537 MS 9001 540 LM 2500D 497 LM 6000PC 440 TRENT 60 440 RB211 6761 502 Table 2 - Characteristics of certain gas turbines

It is therefore found that the fumes of the aero-derived turbines generally have temperatures of the order of 50 ° C. to 100 ° C. lower than those of the fumes of the industrial turbines. The aero-derived turbines have temperatures that are entirely compatible with the needs of MFAs, whose driving steam temperatures are typically between 120 ° C. and 200 ° C. According to this embodiment, the drive of the compressors by means of the low exhaust temperature gas turbines can be carried out directly, by mechanical coupling, or indirectly, the low temperature exhaust gas turbines producing all or part of the electricity required for mechanically driven electric motors coupled to the compressors. Means for transferring the heat of the fumes produced are provided at the outlet of the gas turbine (s) at low exhaust temperature. These heat transfer means may comprise a heat transfer fluid circuit. According to a preferred variant, a heat recovery boiler is used. The heat of the fumes is then used to vaporize the coolant (preferably water).

EXAMPLE The following example illustrates the invention without limiting it. A process for liquefying natural gas has been modeled from the diagram of FIG. 1 (two-cycle embodiment), with the variants shown below. The gas at the liquefaction inlet is available at 70 bar and 11 ° C. Its molar composition is as follows: nitrogen: 0.04%; carbon dioxide: 50 ppm; methane: 93.28%; ethane: 6.31%; propane: 0.355%; C4 + hydrocarbons: 0.01%. The natural gas flow rate is 25364 kmol / h. Cooling water is available at 20 ° C. The pre-cooling cycle comprises a mixture of 50% CO2 and 50% R41 (ECP744). The liquefaction cycle comprises pure nitrogen. The pre-cooling cycle comprises three vaporization stages and not two as illustrated in FIG. 1. The natural gas and the nitrogen of the liquefaction cycle are cooled in the pre-cooling cycle to a temperature of -68.degree. C. The pre-cooling cycle rate is 18140 kmol / h and that of the liquefaction cycle is 120570 kmol / h. The nitrogen is expanded in the two expansion turbines 110 and 111 from the cycle HP pressure of 70 bar up to 47 and 14 bar respectively and -89 ° C and -155.4 ° C.

The natural gas, the nitrogen of the liquefaction cycle and the mixture ECP744 of the pre-cooling cycle are cooled by means of MFA up to the temperature of 12 ° C respectively at 131, 132 and 133. The high pressure of the cycle of pre-cooling is 48 bar. Its condensation takes place in exchanger 127 by the cooling water of the plant but it is sub-cooled in 133 by means of MFA. The performance of the liquefaction processes is evaluated by the specific power in kWh / t, which is the ratio of the total power consumed to the flow rate of liquefied natural gas produced. Under the conditions of the present example, the specific power is 245 kWh / t. With the same environmental conditions and a pure CO2 pre-cooling cycle, the specific power would be 285 kWh / t. Therefore, the invention allows an energy gain of about 14%. R: \ 31300 \ 31360 SNP \ 31360--100305-text_depot.doc- March 8, 2010

Claims (29)

REVENDICATIONS1. A process for producing liquefied natural gas comprising: supplying natural gas; pre-cooling the natural gas by exchanging heat with a pre-cooling refrigerant contained in a pre-cooling cycle; the liquefaction of the pre-cooled natural gas by heat exchange with a liquefaction refrigerant contained in a liquefaction cycle; wherein the pre-cooling refrigerant comprises carbon dioxide and a hydrofluorocarbon. 15 2. Process according to claim 1, in which the hydrofluorocarbon is chosen from monofluoromethane, pentafluoroethane, 1,1,1-trifluoroethane, monofluoroethane, 1,1,1,2-tetrafluoroethane, 1,1,1,2-tetrafluoroethane, 1,1,1-trifluoroethane and 1,1,1,2-tetrafluoroethane. -difluoroethane and mixtures thereof, and preferably is monofluoromethane. The method of claim 1 or 2, wherein the pre-cooling refrigerant comprises a molar content of 40 to 80% carbon dioxide and 20 to 60% monofluoromethane, preferably 45 to 65% carbon dioxide. carbon and 35 to 55% monofluoromethane, and more preferably 47 to 57% carbon dioxide and 43 to 53% monofluoromethane. 30 4. Method according to one of claims 1 to 3, wherein the liquefaction refrigerant is nitrogen or wherein the liquefaction refrigerant is a mixture of nitrogen and an additional compound. 35 5. Method according to one of claims 1 to 4, comprising, after the liquefaction step of the pre-cooled natural gas: the subcooling of the liquefied natural gas by heat exchange with a refrigerant sub-R: \ 31300 \ 31360 SNP \ 31360--100305-text_depot.doc- 8 March 2010 10 20 25cooling contained in a subcooling cycle. The method of claim 5, wherein the liquefying refrigerant is a mixture of nitrogen and an additional compound and the subcooling refrigerant is nitrogen. The process according to claim 4 or 6, wherein the additional compound has a boiling point at atmospheric pressure of -85 ° C to 0 ° C and a crystallization temperature of less than or equal to -80 ° C, and preferably a boiling point at atmospheric pressure of -20 ° C to -60 ° C and / or a crystallization temperature of less than or equal to -100 ° C or -120 ° C or -130 ° C or -140 ° C C. 8. Process according to one of claims 4, 6 or 7 wherein the molar proportion of nitrogen in the mixture of nitrogen and additional compound is greater than or equal to 80% or greater than or equal to 85% or greater or equal to at 90% or greater than or equal to 95% or greater than or equal to 97.5%; and preferably is less than or equal to 99.5% or less than or equal to 99% or less than or equal to 98.5%. 25 9. Method according to one of claims 4 or 6 to 8, wherein the additional compound is selected from hydrocarbons and hydrofluorocarbons, and preferably from trifluoromethane, monofluoromethane, tetrafluoroethylene, propylene, propane, monofluoroethane perfluoropropane, isobutane, n-butane and perfluoropropene. The process according to one of claims 4 or 6 to 9, wherein the mixture of nitrogen and additional compound is expanded in one or more expansion turbines, and wherein the mixture of nitrogen and additional compound is partially condensed in at least one of said expansion turbines. R: \ 31300 \ 31360 SNP \ 31360--100305-text_depot.doc- March 8, 2010 10 15 20 The method according to one of claims 1 to 10, wherein the pre-cooling cycle and / or the liquefaction cycle and / or the subcooling cycle comprise at least one compressor driven by at least one gas turbine. producing smoke, in which at least a portion of the heat of the smoke is transferred to an absorption refrigerating machine, and wherein, preferably, the gas turbine produces smoke at a temperature of not more than 510 ° C and / or or the gas turbine is an aerodriven gas turbine. The method of claim 11 comprising: an additional step of cooling the natural gas prior to pre-cooling the natural gas by means of the absorption refrigerating machine; and / or cooling the cooling refrigerant, at least in part by means of the absorption refrigerating machine; and / or cooling the liquefying refrigerant, at least in part by means of the absorption refrigerating machine; and / or cooling the subcooling refrigerant, at least in part by means of the absorption refrigerating machine; and / or supplying air to the gas turbine and cooling the air prior to supplying air, at least in part by means of the absorption refrigerating machine. 30 13. Method according to one of claims 1 to 12, which is implemented at sea. 14. Liquefied natural gas production plant, comprising: a natural gas supply pipe (101, 301); A pre-cooling cycle (123, 351) containing a pre-cooling refrigerant, said pre-cooling refrigerant comprising carbon dioxide and a hydrofluorocarbon; A pre-cooling exchanger (104, 304) adapted to the heat transfer between the natural gas and the refrigerant of the present invention. cooling; a liquefaction cycle (100, 352) containing a liquefaction refrigerant; a liquefaction exchanger (105, 305a) adapted to the heat transfer between the natural gas and the liquefaction refrigerant and located downstream of the pre-cooling exchanger (123, 351). Plant according to claim 14, in which the hydrofluorocarbon is chosen from monofluoromethane, pentafluoroethane, 1,1,1-trifluoroethane, monofluoroethane, 1,1,1,2-tetrafluoroethane and 1,1-difluoroethane. and mixtures thereof, and preferably is monofluoromethane. Plant according to claim 14 or 15, wherein the pre-cooling refrigerant comprises a molar content of 40 to 80% of carbon dioxide and 20 to 60% of monofluoromethane, preferably 45 to 65% of dioxide. carbon and 35 to 55% monofluoromethane, and more preferably 47 to 57% carbon dioxide and 43 to 53% monofluoromethane. Plant according to one of Claims 14 to 16, in which the liquefaction refrigerant is nitrogen or in which the liquefaction refrigerant is a mixture of nitrogen and an additional compound. An installation according to one of claims 14 to 17, further comprising: a subcooling cycle (353) containing subcooling refrigerant; a subcooling exchanger (305b) adapted for heat transfer between the natural gas and the subcooling refrigerant and located downstream of the liquefying exchanger (305a). R: \ 31300 \ 31360 SNP \ 31360--100305-text_depot.doc- March 8, 2010 10 15. 15 16. 20 25 17. 30 18. 35 The plant of claim 18, wherein the liquefying refrigerant is a mixture of nitrogen and an additional compound and the subcooling refrigerant is nitrogen. 20. Installation according to claim 17 or 19, wherein the additional compound has a boiling point at atmospheric pressure of -85 ° C to 0 ° C and a crystallization temperature of less than or equal to -80 ° C, and preferably a boiling point at atmospheric pressure of -20 ° C to -60 ° C and / or a crystallization temperature of less than or equal to -100 ° C or -120 ° C or -130 ° C or -140 ° C C. 15 21. Installation according to one of claims 17, 19 or 20, wherein the molar proportion of nitrogen in the mixture of nitrogen and additional compound is greater than or equal to 80% or greater than or equal to 85% or greater or equal to 90% or greater than or equal to 95% or greater than or equal to 97.5%; And preferably is less than or equal to 99.5% or less than or equal to 99% or less than or equal to 98.5%. 22. Installation according to one of claims 17 or 19 to 21, wherein the additional compound is selected from hydrocarbons and hydrofluorocarbons, and preferably from trifluoromethane, monofluoromethane, tetrafluoroethylene, propylene, propane, monofluoroethane perfluoropropane, isobutane, n-butane and perfluoropropene. 23. Installation according to one of claims 17 or 19 to 22, wherein the liquefaction cycle (100, 352) comprises one or more expansion turbines (110, 111, 309), the installation being adapted to partially condense the refrigerant for liquefaction in at least one of said expansion turbines (110, 111, 309). R: \ 31300 \ 31360 SNP \ 31360--100305-text_depot.doc- March 8, 2010 10 25 30 35 24. Installation according to one of claims 14 to 23, wherein: the pre-cooling cycle (123, 351) and / or the liquefaction cycle (100, 35 2) and / or the subcooling cycle. (353) comprise at least one compressor (106, 126, 327, 332); the installation comprises: compressor drive means (106, 126, 327, 332) comprising at least one gas turbine producing smoke; an absorption refrigerating machine; and means for transferring at least a portion of the heat of the flue gases from the gas turbine to the absorption refrigerating machine; And preferably wherein the gas turbine produces fumes at a temperature of 510 ° C or lower and / or the gas turbine is an aero-derived gas turbine. 25. Installation according to claim 24: comprising an additional cooling device natural gas (131) upstream of the pre-cooling exchanger (104), said additional natural gas cooling device (131) being powered by the machine absorption refrigeration; and / or wherein the pre-cooling cycle (123) comprises a pre-cooling refrigerant cooling device (133), powered by the absorption refrigerating machine; and / or wherein the liquefaction cycle (100) comprises a liquefaction refrigerant cooling device (132) fed by the absorption refrigerating machine; and / or wherein the gas turbine comprises a device for cooling the air supply, fed by the absorption refrigerating machine. 26. Installation according to one of claims 14 to 25, disposed at sea. R: \ 31300 \ 31360 SNP \ 31360--100305-text_depot.doc- March 8, 2010 20 25 30 35 27. Use of a composition comprising carbon dioxide and a hydrofluorocarbon for cooling and liquefying natural gas. 28. The use according to claim 27, wherein the hydrofluorocarbon is chosen from monofluoromethane, pentafluoroethane, 1,1,1-trifluoroethane, monofluoroethane, 1,1,1,2-tetrafluoroethane, 1,1,1-tetrafluoroethane, 1,1,1-trifluoroethane and 1,1,1,2-tetrafluoroethane. difluoroethane and their mixtures, and preferably is monofluoromethane. 29. Use according to claim 28, wherein the composition comprises a molar content of 40 to 80% of carbon dioxide and 20 to 60% of monofluoromethane, preferably 45 to 65% of carbon dioxide and 35 to 55 % monofluoromethane, and more preferably 47 to 57% carbon dioxide and 43 to 53% monofluoromethane. 20 R: \ 31300 \ 31360 SNP \ 31360--100305-text_depot.doc- March 8, 2010