EP0818661B1 - Improved process and apparatus for cooling and liquefaction of natural gas - Google Patents

Abstract

The method involves compressing a coolant mixture in a pre-final stage (1A) through a series of stages of a compression unit (1). The mixture is condensed partially by cooling, and then is separated to produce a vapour fraction and a liquid fraction. The vapour fraction is cooled and partially condensed, and then passed through a final compression stage (1C) to obtain a high-pressure vapour. Part of the high-pressure vapour fraction and liquid fraction is then cooled, expanded and circulated through a first heat exchanger (5). During the vapour fraction's cooling and partial condensation stage, the vapour fraction obtained from the separation of the condensed mixture is cooled by passing it through a second heat exchanger (18) with the coolant. The condensed mixture is separated in a first separator (14). The vapour fraction from it is condensed in the second heat exchanger, and then passed through a second separator (15) to produce a vapour and a liquid fraction. The vapour fraction is sent to the final compression stage, and the liquid fraction to the first heat exchanger.

Description

The present invention relates to a method and an installation for cooling a fluid and applies in particular to the liquefaction of natural gas.

WO-A-94 24500 describes a process in which compresses in at least two stages, in an installation of the integral incorporated waterfall type, a refrigerant mixture composed of constituents of different volatilities, and, after at least each of the intermediate stages of compression (i.e. stadiums preceding the last upper story pressure) the refrigerant mixture is partially condensed, at least some of the condensed fractions as well as the high pressure gas fraction being cooled, expanded (or expanded) and linked to heat exchange with the fluid to be cooled, then compressed again, the gas from the penultimate compression stage being by elsewhere distilled in a distillation apparatus which cools the head with a liquid having a temperature below a so-called "reference" temperature, or "ambient", to form on the one hand the liquid condensate of this penultimate compression stage and, on the other hand, a vapor phase which is sent to the last stage of compression.

Preferably, this same publication provides for partially cool and condense the overhead vapor the distillation apparatus, by heat exchange (in a heat exchange unit with two plate heat exchangers arranged in series) with at least said fractions relaxed, to obtain a vapor phase and a phase liquid, and cool the camera head distillation with the liquid phase thus obtained, the phase vapor constituting said phase which is sent to the last compression stage.

Note that in the present description, as in WO-A-94 24500, the pressures in question are absolute pressures.

In addition, the refrigerant mixture which we already have spoken, should be considered to consist of a certain number of fluids including, among others, nitrogen and hydrocarbons such as methane, ethylene, ethane, propane, butane, pentane, etc ...

We will also define "ambient temperature" as the thermodynamic reference temperature corresponding to coolant temperature (water or air in particular) available on the process use site and used in the cycle, increased by the temperature difference that we fix, by construction, at the exit of the devices refrigerants of the installation (compressor, exchanger, ...). In practice, this difference will be around 1 ° C to 20 ° C, and preferably around 3 ° C to 15 ° C.

• ledit fluide (liquide) destiné au refroidissement de cette tête soit lui-même refroidi à une température inférieure à ladite température "de référence" ou "ambiante" (voire même inférieure à la température du fluide de refroidissement utilisé sur le site dans les échangeurs),
• et que la différence de température entre cette température "ambiante" et la température du fluide (liquide) destiné au refroidissement de la tête du distillateur soit comprise entre environ 20°C et 55°C, et typiquement de 30°C à 45°C.

Typiquement, la température du fluide de refroidissement disponible sur le site (air, eau de mer ou de rivière ...) sera comprise environ entre -20°C et + 45°C.It should also be noted now that if a distillation apparatus is used, it will be advantageous to cool the head with a fluid (liquid) so that:

• said fluid (liquid) intended for cooling this head is itself cooled to a temperature below said "reference" or "ambient" temperature (or even lower than the temperature of the cooling fluid used on site in the exchangers) ,
• and that the temperature difference between this "ambient" temperature and the temperature of the fluid (liquid) intended for cooling the head of the still is between approximately 20 ° C and 55 ° C, and typically from 30 ° C to 45 ° C .

Typically, the temperature of the cooling fluid available on site (air, sea or river water, etc.) will be between approximately -20 ° C and + 45 ° C.

As interesting as the process and the installation are WO-A-94 24500, it turned out, however, that one can still get a gain in overall mechanical energy used for the desired cooling and improve efficiency thermodynamics of this cooling operation, everything particularly if it is a question of liquefying natural gas, this with installation reliability and profitability potentially better.

a) on comprime le mélange frigorigène dans un avant-dernier étage (lA, 1B) parmi plusieurs étages d'une unité de compression (1, 1'),

b) on sépare le mélange comprimé pour obtenir une fraction vapeur et une fraction liquide,

c) on refroidit et on condense partiellement ladite fraction vapeur, et on refroidit ladite fraction liquide, en faisant circuler respectivement ladite fraction vapeur par des premiers passages (57, 135') et ladite fraction liquide par des seconds passages (93, 137'), dans des seconds moyens d'échange thermique (18) indépendants des premiers, en échange de chaleur avec un fluide réfrigérant circulant en circuit fermé, pour obtenir respectivement une fraction vapeur condensée et une fraction liquide refroidie, puis on refroidit et on fait circuler la fraction liquide refroidie dans au moins des premiers moyens (5) d'échange thermique,

d) on sépare la fraction vapeur condensée pour obtenir une fraction vapeur et une fraction liquide résultantes,

e) on envoie la fraction vapeur résultante vers le dernier étage de compression (1C), pour obtenir une fraction vapeur haute pression,

f) on expanse la fraction liquide refroidie ayant circulé dans les premiers moyens d'échange thermique et la fraction vapeur haute pression, avant de les faire circuler dans lesdits premiers moyens d'échange thermique en échange de chaleur avec le fluide à refroidir, puis de les recycler en tant que mélange frigorigène dans l'avant-dernier étage de compression.

The solution proposed in the invention to tend towards these objectives consists of a method having the following steps:

a) the refrigerant mixture is compressed in a penultimate stage (1A, 1B) among several stages of a compression unit (1, 1 '),

b) the compressed mixture is separated to obtain a vapor fraction and a liquid fraction,

c) said vapor fraction is partially cooled and condensed, and said liquid fraction is cooled, by circulating said vapor fraction respectively by first passages (57, 135 ') and said liquid fraction by second passages (93, 137') , in second heat exchange means (18) independent of the first, in exchange for heat with a refrigerant circulating in a closed circuit, to obtain respectively a condensed vapor fraction and a cooled liquid fraction, then the device is cooled and circulated. liquid fraction cooled in at least the first heat exchange means (5),

d) the condensed vapor fraction is separated to obtain a resulting vapor fraction and a liquid fraction,

e) the resulting vapor fraction is sent to the last compression stage (1C), to obtain a high pressure vapor fraction,

f) the cooled liquid fraction having circulated in the first heat exchange means and the high pressure steam fraction are expanded, before circulating them in said first heat exchange means in exchange for heat with the fluid to be cooled, then recycle them as a refrigerant mixture in the penultimate compression stage.

The mechanical energy necessary for the operation of second means of cooling should, according to calculations, be less than 10% of the total mechanical energy necessary for the entire installation, allowing for example to drive these second means by a motor electric from the launch engine of the turbine to gas from the refrigerant compression unit, then used as a generator.

Furthermore, with such a method applied to the natural gas liquefaction, natural gas production liquefied could be increased by more than 10% compared to the two-stage compression solution of WO-A-94 24500.

Due to the addition of the second means of cooling, compared to the solution of WO-A-94 24500, the cost of investment in equipment for a production of LNG given, will likely be increased. However, the gain in piping can be significant.

Also note that the technology of the exchanger warm of the first cooling means is also simplified. The invention makes it possible to offload partially from their thermal work, part of said first means of heat exchange, allowing optimize other elements of the cycle.

If a distillation column is used, a first optimization of the cooling of his head goes by elsewhere be possible, compared to what is expected in WO-A-94 24500.

• de séparer dans ledit appareil de distillation le mélange comprimé,
• de condenser dans lesdits seconds moyens d'échange thermique la fraction vapeur issue de cet appareil de distillation, pour obtenir une fraction vapeur condensée,
• de faire passer dans un séparateur la fraction vapeur condensée, pour obtenir ladite fraction vapeur résultante et ladite fraction liquide résultante,
• d'envoyer la fraction vapeur résultante dans le dernier étage de compression,
• et de renvoyer la fraction liquide résultante dans la tête de colonne de l'appareil de distillation, pour la refroidir.

For this, we advise, during steps b), c), d) and e) above:

• to separate the compressed mixture in said distillation apparatus,
• to condense in said second heat exchange means the vapor fraction from this distillation apparatus, to obtain a condensed vapor fraction,
• passing the condensed vapor fraction through a separator to obtain said resulting vapor fraction and said resulting liquid fraction,
• to send the resulting vapor fraction to the last compression stage,
• and returning the resulting liquid fraction to the column head of the distillation apparatus, to cool it.

Note that in place of the distillation apparatus, a another separator can be used.

• on sépare le mélange comprimé dans un premier séparateur,
• on condense la fraction vapeur issue dudit premier séparateur dans les seconds moyens d'échange thermique, pour obtenir la fraction vapeur condensée,
• on fait passer dans un second séparateur ladite fraction vapeur condensée, pour obtenir la fraction vapeur et la fraction liquide résultantes,
• on envoie la fraction vapeur résultante dans ledit dernier étage de compression,
• et on envoie la fraction liquide résultante vers lesdits premiers moyens d'échange thermique.

In that case :

• the compressed mixture is separated in a first separator,
• the vapor fraction from said first separator is condensed in the second heat exchange means, to obtain the condensed vapor fraction,
• said condensed vapor fraction is passed through a second separator to obtain the resulting vapor fraction and liquid fraction,
• the resulting vapor fraction is sent to said last compression stage,
• and the resulting liquid fraction is sent to said first heat exchange means.

• de faire circuler la fraction liquide issue de l'appareil de distillation ou du premier séparateur dans les seconds moyens d'échange thermique, sensiblement entre les extrémités chaude et froide desdits moyens d'échange thermique,
• et d'admettre la fraction liquide refroidie, en partie intermédiaire d'un premier échangeur chaud parmi deux échangeurs de chaleur disposés en série, l'un chaud, l'autre froid, appartenant auxdits premiers moyens d'échange thermique.

Preferably, in either case, we also advise:

• circulating the liquid fraction from the distillation apparatus or the first separator in the second heat exchange means, substantially between the hot and cold ends of said heat exchange means,
• and admitting the cooled liquid fraction, in the intermediate part of a first hot exchanger among two heat exchangers arranged in series, one hot, the other cold, belonging to said first heat exchange means.

• à l'extérieur des seconds moyens d'échange thermique, on fait circuler le fluide réfrigérant dans un cycle de réfrigération en circuit fermé, soit à un étage unique de compression, soit à deux étages successifs de compression, avec, en sortie du réfrigérant final (23 sur la figure 1), une condensation totale du fluide réfrigérant;
• si le fluide à refroidir est du gaz naturel, avant d'admettre ce gaz naturel dans lesdits premiers moyens d'échange thermique, on le fait circuler d'abord dans lesdits seconds moyens d'échange thermique et, avant ou après sa circulation dans ces seconds moyens, on fait passer le gaz naturel dans une unité de dessiccation ;
• entre les étapes e) et f) précitées, on refroidit la fraction vapeur haute pression après le dernier étage de compression, et on la fait circuler dans les seconds moyens d'échange thermique, pour la refroidir encore par échange de chaleur avec le fluide réfrigérant avant de l'envoyer dans les premiers moyens d'échange thermique,
• en sortie du dernier étage de compression de ladite unité de compression, on refroidit la fraction vapeur haute pression et on l'envoie dans une entrée intermédiaire d'un premier échangeur chaud, parmi deux échangeurs disposés en série, l'un chaud, l'autre froid, constituant lesdits premiers moyens d'échange thermique ;
• entre les étapes a) et b) susmentionnées, on fait circuler le mélange comprimé dans les seconds moyens d'échange thermique ;
• on fait circuler isolément un fluide caloporteur dans les seconds moyens d'échange thermique ;
• dans l'hypothèse où le gaz à refroidir est du gaz naturel,
  • avant de faire circuler ce gaz naturel dans les premiers moyens d'échange thermique, on lui fait subir une dessiccation,
  • et, après dessiccation, le gaz naturel sec passe, à l'intérieur des premiers moyens d'échange thermique, d'abord dans une première partie d'un premier échangeur chaud parmi deux échangeurs disposés en série, l'un chaud, l'autre froid, appartenant auxdits premiers moyens d'échange thermique, puis dans une partie dudit second échangeur de ces premiers moyens d'échange thermique, avant de passer dans une unité de fractionnement extérieure auxdits premiers moyens d'échange thermique.

In addition to the above, the method of the invention may also include one or more of the following characteristics:

• outside the second heat exchange means, the refrigerant is circulated in a refrigeration cycle in a closed circuit, either at a single compression stage, or at two successive compression stages, with, at the outlet of the final refrigerant (23 in FIG. 1), total condensation of the refrigerant;
• if the fluid to be cooled is natural gas, before admitting this natural gas into said first heat exchange means, it is first circulated in said second heat exchange means and, before or after its circulation in these second means, the natural gas is passed through a drying unit;
• between steps e) and f) above, the high pressure steam fraction is cooled after the last compression stage, and it is circulated in the second heat exchange means, to cool it further by heat exchange with the refrigerant before sending it to the first heat exchange means,
• at the outlet of the last compression stage of said compression unit, the high-pressure steam fraction is cooled and sent to an intermediate inlet of a first hot exchanger, from two exchangers arranged in series, one hot, the other cold, constituting said first heat exchange means;
• between steps a) and b) above, the compressed mixture is circulated in the second heat exchange means;
• a heat transfer fluid is circulated in isolation in the second heat exchange means;
• assuming that the gas to be cooled is natural gas,
  • before circulating this natural gas in the first heat exchange means, it is subjected to drying,
  • and, after drying, the dry natural gas passes, inside the first heat exchange means, first in a first part of a first hot exchanger among two exchangers arranged in series, one hot, the other cold, belonging to said first heat exchange means, then in part of said second exchanger of these first heat exchange means, before passing into a fractionation unit external to said first heat exchange means.

Note again that there may not be a device refrigerant between the compressor output of the penultimate floor and the entrance to the separation device (still in particular), and so that the mixture is not condensed compressed refrigerant before separating it in step b). So, we will then carry out the process then on the basis of art previous EP-A 117 793 with, in this case, circulation of the liquid fraction resulting from the separation of the compressed mixture in heat exchange means (marked 4A, 10, in EP-A-117 793) distinct from said "first means of exchange thermal "(marked 11, 15 in EP'793), before traveling herself.

The invention also relates to an installation for cooling according to claim 18, in particular for gas liquefaction natural, which can be used for the implementation of process presented above.

• des seconds moyens d'échange thermique comportant des premiers passages et des seconds passages indépendants des premiers pour y faire circuler respectivement ladite fraction vapeur et ladite fraction liquide en échange de chaleur avec le fluide réfrigérant, et
• un fluide réfrigérant circulant dans un circuit fermé.

It is thus intended that said means for cooling the installation of the invention comprise:

• second heat exchange means comprising first passages and second passages independent of the first for circulating therein respectively said vapor fraction and said liquid fraction in exchange for heat with the refrigerant, and
• a refrigerant circulating in a closed circuit.

This characteristic and others appear in the claims 18 to 27 below.

Les figures 1, 2, 3, 4, 5, 6 et 7 représentent autant de modes de réalisation possibles de l'installation de l'invention qu'il y a de figures.

A more detailed description of the invention will now be given, with reference to the accompanying drawings, in which:

Figures 1, 2, 3, 4, 5, 6 and 7 show as many possible embodiments of the installation of the invention as there are figures.

Natural gas liquefaction facility represented in the figures, and in particular in FIG. 1, includes in particular a cycle compression unit 1 to two compression stages 1A, 1C, each stage discharging by via a line 2A, 2C in a condenser or refrigerant, respectively 3A, 3C, cooled with water or air, the available fluid used having typically a temperature of the order of + 25 ° C to + 35 ° C; separation means identified as a whole 4, interposed between the two compression stages 1A and 1C of so as to supply the high pressure stage 1C with a steam fraction from these separation means; a first heat exchange unit 5 comprising two heat exchangers in series, namely an exchanger "hot" 6 and a "cold" exchanger 7; a separator pot intermediate 8; and a storage of liquefied natural gas (LNG) 10.

The separation means 4 can be consisting either of a distillation apparatus 12, the upper head 12a is cooled by a liquid from a separator 13 (Figures 1 to 5 and 7), or by two separator pots 14, 15, the vapor fraction of the distillation apparatus 12 or the first separator 14 circulating in the associated separator (respectively 13, 15) before being admitted to the input of the compression stage high pressure 1C.

Assuming the use of a column distillation 12, the output of the condenser 3A communicates with the lower part of tank 12b of the column distillation 12 and the bottom of the separator 13 is connected by gravity or by pump, via a siphon 16 and an adjustment valve 17, at the head 12a of the column 12.

In accordance with an important characteristic of the invention, the gas liquefaction installation natural further includes, on the different modes of realization of Figures 1 to 7, a second exchange unit thermal 18 constituting a second refrigerant group, independent of the first, 5.

• de refroidir la fraction vapeur issue des premiers moyens de séparation 12 ou 14, avant qu'elle passe dans les seconds moyens de séparation 13, 15,
• de refroidir la fraction liquide issue desdits premiers moyens de séparation 12, 14, avant de l'envoyer dans le premier 6, des deux échangeurs de la première unité d'échange thermique 5,
• d'assurer un refroidissement d'un circuit auxiliaire 19 (figures 1,2 et 4 à 7) dans lequel circule soit du pentane, soit du gaz naturel avant décarbonatation et dessiccation (c'est-à-dire relativement humide),
• ou encore, par le circuit 20 de la figure 3, de refroidir du gaz naturel déjà sec mais non encore fractionné, avant de l'envoyer dans la première unité d'échange thermique 5 pour le liquéfier, avec élimination intermédiaire d'hydrocarbures en C2+, dans l'unité de fractionnement 75.

This second refrigerant group has in particular the role, in combination or as an alternative:

• to cool the vapor fraction from the first separation means 12 or 14, before it passes into the second separation means 13, 15,
• cooling the liquid fraction from said first separation means 12, 14, before sending it into the first 6, from the two exchangers of the first heat exchange unit 5,
• to provide cooling of an auxiliary circuit 19 (FIGS. 1, 2 and 4 to 7) in which either pentane or natural gas circulates before decarbonation and drying (that is to say relatively wet),
• or, by circuit 20 of FIG. 3, to cool natural gas already dry but not yet fractionated, before sending it into the first heat exchange unit 5 to liquefy it, with intermediate elimination of hydrocarbons in C2 + , in the fractionation unit 75.

Regarding the auxiliary circuit 19, it can go to the hottest part of the exchanger 18 which is then used to cool from + 40 ° C to + 20 ° C approximately the heat transfer fluid which circulates there, this fluid (if it does not not natural gas) that can be used to refrigerate a other part of the installation, for example natural gas crude intended to be dried before processing in installation.

In the heat exchanger 18, the fluid circulating in each of the cooling circuits above is cooled by indirect heat exchange with a coolant, such as a "pure" fluid, or mixture binary or ternary, circulating in a closed circuit in the regenerating cycle 21 or 21 '.

In Figures 1, 3, 4, 5 and 7, the circuit regeneration 21 comes as a refrigeration cycle with two compression stages, including a lower stage 1D pressure (of the order of 2.5 to 3.5 bars) and a stage of 1E high pressure compression (operating at approximately 6 to 8 bars), possibly a refrigerant 22, and a condenser 23 condensing the mixture in circulation.

This mixture can in particular comprise about 60% butane and about 40% propane. A "pure" fluid can however be used, as an alternative.

The mixture leaving the high pressure stage 1E is fully condensed in condenser 23, such so it's a liquid mixture that's allowed to the hot upper end (around 40 ° C) of the exchanger 18.

Substantially half the axial length (axis 18a) of the exchanger, part of the cooled mixture until around 20 ° C is released at 25, while the remaining part continues to flow to the end lower cold side of the exchanger, to come out at 26 aux around 8 ° C and be relaxed at 27 at low pressure of the cycle before being reintroduced axially through the cold lower dome 28a of the exchanger in passages 29 where the low pressure liquid mixture is vaporized before to come out laterally at 31 substantially at mid-length axial of the exchanger and be admitted in the lower floor 1D pressure.

At the output of the 1D compression stage, the refrigerant mixture, in gaseous state, can be cooled in the refrigerant 22, before being admitted at the inlet of the high pressure stage 1E, mixed with the part of the binary mixture that we recovered in 25, relaxed to a intermediate cycle pressure at 32, reintroduced into exchanger 18 for axial circulation about half the length of the exchanger, from so as to be vaporized in the axial passages 33, the vaporized mixture emerging axially through the dome higher "hot" 28b before being mixed in 35 with the part of the mixture in the gaseous state from stage 1D.

Exchangers 6, 7 and 18 are preferably plate exchangers, these plates preferably being equipped with fins (or waves). These exchangers which are for example metallic plates and aluminum fins.

Specifically concerning the two exchangers 6, 7, they can be brazed or welded coaxially butt end, in series, for a counter-current circulation of fluids put in heat exchange relation, and can have the same length.

They also have passages between the plates necessary for the operation which will be described below.

Before that, it will be noted however that at the place end-to-end connection 40 "on domes" between the exchanger "cold" 7 and the "hot" exchanger 6, the return passages, 41 for exchanger 7 and 42 for exchanger 6 (in which the refrigerant mixture circulates against the current of traffic in the other passages of these exchangers) communicate with each other directly in the area intermediate 40, as had already been planned in WO-A-94 24500.

Note that such a direct passage at 40 between the upper dome 7a of the exchanger 7 and the lower dome 6b of exchanger 6, on at least most of the section of the two exchangers, can only be achieved by avoiding a two-phase redistribution at cutoff 40, as moreover in WO-A-94 24500.

With an installation as presented above, the refrigerant mixture consisting of hydrocarbons in C1 to C6 and nitrogen, leaves in the gaseous state from vertex 6a (so-called "hot" end) of the exchanger 6 (via the passages 42) and arrives via the recycling line 46, at the suction of the first compression stage 1A.

This gas mixture is then compressed to a first intermediate pressure Pi, typically of the order from 12 to 20 bars, then cooled to + 30 ° C to + ° 40 ° C approximately in 3A, with partial condensation, and separated in a vapor fraction and a liquid fraction in the device distillation 12.

The tank liquid in column 12 (recovered in 12b) constitutes a first suitable coolant to provide essential refrigeration of the exchanger hot 6, after cooling in the exchanger 18.

For this, this tank liquid is allowed (to around 30 ° C to 40 ° C) towards the "hot" end 28b of the exchanger 18 in which it circulates, as far as its "cold" end 28a, to come out at around 47 of 8 ° C, this cooled liquid fraction then being introduced at approximately the same temperature at the location an intermediate lateral entry 48, substantially mid-length of the hot exchanger 6, to come out of again laterally towards its "cold" end 6b, at around -20 ° C to -40 ° C, be relaxed (or undergo a expansion) at low cycle pressure (2.5 to 3.5 bar) in an expansion valve 50 and be reintroduced under two-phase form, always at the cold end 6b of the same exchanger, via the inlet side box 52 and a suitable dispensing device, to be vaporized in the low pressure passages 42 of the exchanger.

Overhead steam from the distillation column 12, recovered at the outlet of the head 12a, circulates as to it, as illustrated in Figures 1 to 5 and 7, between substantially the hot ends 28b and cold ends 28a of exchanger 18, with inlet and outlet to both ends at 53 and 55 respectively, so as to be cooled and partially condensed in passages 57 from the exchanger to an intermediate temperature lower than said "ambient" temperature, for example + 5 ° C to + 10 ° C, then introduced into the separator pot 13. In practical, the temperature reached may even (possibly) be lower than the temperature of the "coolant" available on site.

The liquid phase recovered at the base of the separator 13 returns, via the siphon 16 and of the adjustment valve 17, at the head of the column 12 for the cool, while the vapor phase of the separator is compressed at the high pressure of the cycle (of the order of 40 to 45 bar) at 1C and then reduced to + 30 ° C to + 40 ° C in the 3C refrigerant. In this case, the temperature of the head of column 12 will therefore be lower than said temperature "ambient", or even at the temperature of the "fluid cooling "available on the site, even if we would have could imagine that this temperature is higher, in particular by removing the 3A refrigerant and operating as in EP-A-117 793, that is to say with a direct passage from compression stage 1A at the entrance to the distiller 12.

This cooled high pressure steam fraction in the 3C refrigeration device substantially up to the so-called "ambient" temperature (at a fixed temperature difference in the definition on page 2), is next to again cooled from the hot end 6a to the end cold 6b (therefore around 30 ° C to -30 ° C) in the passages high pressure 59 of exchanger 6, with inlet and outlet respectively in 61 and 63, then separated into fractions liquid and vapor, in 8.

Note that the temperature and the pressure (+ 5 ° C to + 10 ° C, 12 to 20 bar) of the cooling of the head of the column 12 allows obtain a single-phase gas both at the outlet of 3C and at 40, just at the exit of the exchanger 7.

Le liquide recueilli à la base du séparateur 8 est sous-refroidi dans la partie chaude de l'échangeur 7, dans des passages 65, sorti de l'échangeur en partie intermédiaire (en 67) aux environs de -120°C, détendu à la basse pression du cycle, par exemple dans une vanne de détente 69, et réintroduit latéralement en 70, toujours en partie intermédiaire de l'échangeur, dans les passages retour basse pression 41 de celui-ci.

The refrigeration of this cold exchanger 7 is obtained by means of the high pressure fluid, as follows:

The liquid collected at the base of the separator 8 is sub-cooled in the hot part of the exchanger 7, in passages 65, taken out of the exchanger in the intermediate part (at 67) at around -120 ° C., expanded at the low pressure of the cycle, for example in an expansion valve 69, and reintroduced laterally at 70, still in the intermediate part of the exchanger, in the low pressure return passages 41 thereof.

The vapor fraction from separator 8 is, meanwhile, cooled, condensed and sub-cooled (up to around -160 ° C) from the hot end to the cold end of the exchanger 7 and the liquid thus obtained is expanded at the low pressure of the cycle in an expansion valve 71 and reintroduced into the exchanger 7, parallel to the axis 5a, at through the lower "cold" dome 7b, to be vaporized in the cold part of the low pressure passages 41, then combined with two-phase fluids (mainly liquids) relaxed admitted by intermediate entry 70, for a return to line 46.

Processed natural gas, for example arriving at a temperature of the order of 20 ° C. after drying, via a pipe 73 is, in part, admitted directly into the apparatus 75 for removing hydrocarbons in C2 + and, for its remaining part, admitted laterally in 77, substantially mid-length of exchanger 6, to be cooled to towards the cold end 6b in passages 79, before come out laterally towards this end, at 81, this portion cooled (approximately -20 ° C to -40 ° C) then being admitted in unit 75.

• les produits qui risqueraient de cristalliser lors de la liquéfaction (c'est-à-dire essentiellement les C6+),
• les produits en C2 à C5 nécessaires au maintien de la composition au gaz de cycle,
• et éventuellement les quantités de produits à extraire pour que le gaz naturel liquéfié soit conforme aux spécifications requises par les utilisateurs,
• et on produit la majeure partie du "fuel gaz" nécessaire à la production d'énergie mécanique de l'installation, directement à la pression requise.

In unit 75, natural gas admitted to it is extracted:

• the products which might crystallize during liquefaction (i.e. essentially C6 +),
• the products in C2 to C5 necessary for maintaining the cycle gas composition,
• and possibly the quantities of products to be extracted so that the liquefied natural gas meets the specifications required by users,
• and most of the "fuel gas" necessary for the production of mechanical energy from the installation is produced directly at the required pressure.

The remaining mixture leaving at 83 is then admitted in 85, near the "hot" dome 7a of the exchanger "cold" 7, to circulate near its end cold 7b, in passages 87 while being liquefied and sub-cooled to come out in 89, around -160 ° C, before being stored, in liquid form (LNG), at 10, after being relaxed.

Note that preferably the essential (approximately 90%) of the decarbonated and dry natural gas (GN) flow admitted through line 73 will circulate in passages 79, only at plus approximately 10% therefore being admitted directly into the separation installation 75.

With such a disposition and thanks in particular to the load shedding obtained from the exchanger 6 by compared to what is described in WO-A-94 24500, it is expected a gain of around 10% overall energy, as well that a discharge from the exchanger 6 of approximately half of its thermal work, 40 to 50% of natural gas can be further processed in such a defined size exchanger.

as shown in Figures 1, 2 and 4, it may be desirable to relax some of the cold liquids in liquid turbines or "expanders" 91 provided in parallel with expansion valves 69 and / or 71.

Note that in practice, we will install n heat exchangers 6 and 7, in parallel, as well as n exchangers 18 also in parallel.

Note also that the expanders provided on the circulation paths of liquids may particular be used to drive pumps (not shown), the one providing the most power being that arranged in parallel with valve 69, the valves do not preferably for fine adjustment or trigger (expansion) of the liquid in question, in the event of failure of the (turbo-) corresponding expander.

In Figure 2, the common elements with the Figure 1 have been identified in the same way (similarly for the other figures).

The main difference between Figures 1 and 2 consists of the arrangement of the closed circuit 21 'of the liquid refrigerant, circulating in the second unit heat exchange 18.

Indeed, in this figure 2, it is a cycle with a compression stage 1E 'therefore comprising a single high pressure compressor (of the order of 6.5 to 7.5 bars).

In circuit 21 ', there will preferably be a ternary mixture, for example composed of ethane, butane and propane.

At the outlet of compressor 1E ', the mixture under its vapor form is (totally) condensed in the 23 'condenser to be admitted 24' towards the end hot 28b of the exchanger 18 in which it circulates longitudinally (parallel to axis 18a) up to the cold end 28a, near which it emerges laterally in 26 'around 8 ° C to 10 ° C to be slackened by valve 27 up to around 2.5 to 3.5 bar.

The refrigerant mixture thus cooled and relaxed is then reinjected through the cold dome 28a, parallel to axis 18a, against the current of the others circulation passages, in vaporization passages 33 'to exit coaxially through the "hot" dome 28b and always be introduced in vapor form around 30 ° C to 40 ° C at the inlet of compressor 1E '.

Note that the use of a ternary mixture provides a larger temperature gradient that the binary mixture used in circuit 21 of Figures 1, 4, 5 and 7.

The 21 'circuit, which we also find in Figure 6, is simpler than circuit 21 but has an energy handicap of around 15 to 20% per compared to this circuit, i.e. approximately 1.5 to 2% over the cycle complete installation.

In Figure 3, the refrigerant mixture of installation cycle, in its liquid fraction from tank liquid from the distillation apparatus 12, after cooling substantially between the hot ends 28b and cold 28a of the exchanger 18 in the passages corresponding 93, then sub-cooling in a part cold from the "hot" heat exchanger 6 in the passages 95 of this exchanger, undergoes expansion in an expansion valve 97, before being sent to the separator 9.

The gas (via 99a) and liquid (via 99b) are then injected separately into the passages return of the cycle, spraying at low pressure.

More specifically, the vapor fraction is injected laterally at the cut 40, while that the liquid fraction is injected slightly more downstream, near the cold end 6b of the exchanger 6, via the lateral injection path 101 leading to 42.

Comparable treatment of the liquid fraction from cycle separator 8 and expanded in the valve expansion 69 after having circulated in passages 65, to be sub-cooled, is performed in the third cycle separator 103.

Thus, the gaseous and liquid from this separator are injected separately by separate injections, 105 and 107 respectively, substantially at the same intermediate level of the passages vaporization cold 41 of the exchanger 7, that is to say therefore further upstream of the return passages of the mixture refrigerant vaporized at low pressure as arrivals injection of steam and liquid fractions arriving from 99a and 99b.

Still in Figure 3, note that the gas natural (GN), after decarbonation and drying, is admitted for its main part (about 90%) in 77 ', in intermediate part of the exchanger 6, after having circulated in conduits 20 in exchange for heat in the exchanger 18 to be cooled by indirect heat exchange with the coolant circulating in the 21 "circuit that we will present below.

After having passed in the passages 79 ' up to the cold end 6b of the exchanger 6, this gas natural thus sub-cooled leaves 81 'from exchanger 6 to pass into the exchanger 7, via an injection 109, before exiting through an intermediate outlet 111, after have been sub-cooled in passages 113, to a temperature of about -40 ° C to -60 ° C, the gas thus sub-cooled passing through the separation installation 75, its fraction which leaves in 83 then being reinjected laterally at 115 in the intermediate part of the exchanger 7 to circulate in cold passages 117 to around -160 ° C and thus be liquefied, before exit at 89 ', substantially at the exit 89 of the previous figures, then go through the valve expansion 119 (which could also be an expander) and finally be stored in the storage unit 10, after relaxation.

Note that at outlet 81 ', part of the gas can be delivered to the separation unit 75, via the line 82, without passing there through the exchanger 7.

If we are interested now in the 21 "circuit of the refrigerant used in the exchanger 18, we note that in addition to circuit 21 of Figure 1 (which it takes characteristics) the 21 "circuit includes a circuit additional 121, connected in parallel, at input, between outlet 25 and the expansion valve 32 and, at the outlet, between condenser 22 (or low pressure condenser outlet 1D) and the mixing connection 35.

The circuit 121 thus connected comprises a additional exchanger 123 in which circulates between its cold end 123a and its warmer end 123b, the binary liquefied refrigerant mixture leaving 25 and relaxed in 125 in an expansion valve, before being sprayed in passages 127, between the cold ends and hot heat exchanger 123, against the flow of a relatively wet natural gas (before drying), allowed in 129 and therefore circulating in the opposite direction to the vaporized fluid in 127, inside passages 131, before being introduced into a desiccation unit (not shown), then possibly to be introduced at the entry "GN" 73 to leave either in line 20 or directly to the separation installation 75.

• du fait de la circulation de la fraction vapeur haute pression sortant de 3C, avant que cette fraction vapeur parvienne à l'entrée latérale d'injection 61 de l'échangeur 6,
• et dans la manière dont le mélange frigorigène comprimé sortant du condenseur 3A est admis dans le distillateur 12, du fait qu'un refroidissement du mélange sortant de 3A est prévu en dessous de la température "ambiante" (et même éventuellement en dessous de la température du fluide de refroidissement disponible sur le site) avant entrée dans la colonne 12, ceci par circulation dans l'échangeur 18.

The installation of FIG. 4 thus differs only from that of FIG. 1:

• due to the circulation of the high pressure steam fraction leaving 3C, before this steam fraction reaches the lateral injection inlet 61 of the exchanger 6,
• and in the manner in which the compressed refrigerant mixture leaving the condenser 3A is admitted into the distiller 12, since cooling of the mixture leaving 3A is provided below the "ambient" temperature (and even possibly below the temperature of the coolant available on the site) before entering the column 12, this by circulation in the exchanger 18.

In FIG. 4, it is thus noted that at the output of the 3C refrigerant, the high pressure steam fraction is admitted in 133 towards the "hot" end 28b of the exchanger 18 to be cooled to an intermediate zone of the axial length of the exchanger, before coming out for be admitted into exchanger 6, via the injection inlet 61.

The passages left free following those 135 reserved for said high pressure steam fraction in the exchanger 18, are used here to condense the steam fraction from head 12a of the column distillation 12 (spray passages marked 135 ') before this condensed vapor fraction is separated into 13.

A partition of the lengths of the passages has also used to cool, in the part the less cold from exchanger 18 (passages 137), the mixture compressed two-phase leaving the 3A condenser, before admit it as a low input 12c from the distillation 12 (around 10 ° C to 15 ° C below "ambient" temperature), the complementary part of passages 137 (marked 137 ') located in the more heat exchanger 18 used to cool the liquid tank recovered in 12b, before admitting it into the entry lateral injection 48 of the exchanger 6.

Note that the traffic in the passages 137 of the partially condensed two-phase mixture and tablet provides an entry temperature into the first part 12 of the separation means 4 which can therefore be different from (less than) the "temperature ambient ", or even at the temperature of the cooling available on site.

And this cooling of the tank temperature of the still 12 allows a temperature of cut (in 40) lower than in the other cases.

Also note that the circulation of high pressure steam fraction in passages 135 allows to obtain in 61 an inlet temperature of this fraction steam in exchanger 6, of the order of 25 ° C to 30 ° C that we can adapt and which can in particular be lower than the inlet temperature in 61 of the installation of FIG. 1, typically of the order of 40 ° C, that is to say close to the so-called "ambient" temperature (or the temperature of the "coolant").

Although this has not been illustrated, the intermediate cooling, in passages 137 of the partially condensed and compressed two-phase mixture, between the condenser 3A and the first unit (12 or 14) of the separation means 4, could be provided on installation with two associated separators 14, 15 of the figure 6.

But before coming back to this solution of the figure 6, note that in figure 5, the cycle gas high pressure passing through 2C and possibly partially condensed in 3C is cooled by ten degrees (i.e. typically about 40 ° C to about 30 ° C) in passages 139 of exchanger 18 located on the side of the "hot" dome 28b of it, before coming out laterally at 141, then to be injected as before at 61 in exchanger 6.

The advantage of such cooling that one can control by adapting the operation of the exchanger 18, is to reach between entrance 61 and the pipe recycling 46, a temperature difference of less than approximately 20 ° C, and therefore to obtain an exit from the cooling around 20 ° C, fairly close to the point dew of the refrigerant mixture used, this cooling only around 10 ° C in passages 139 avoiding liquefy the high pressure vapor phase before inject it in 61.

From an energy point of view, this version of figure 5 appears potentially one of the most interesting.

For the other characteristics, the installation of figure 5 corresponds to that of the figure 1 (the forecast of an expander 91 in parallel with the expansion valve 69 being optional).

In FIG. 6, the distillation column 12 has therefore been replaced by a separator 14.

The liquid fraction recovered around 8 ° C in the lower part of the second separator 15 is transmitted towards the intermediate input 48, and a priori directly, without going through the exchanger 18.

In 143, this liquid fraction from separator 15 meets the duct 145 used for the liquid fraction recovered from separator 14, after circulation substantially between the "hot" ends 28b and "cold" 28a of the exchanger 18, in the passages of indirect cooling 147.

Adjustment valves, respectively 149 and 151, allow to adapt the flow rate of liquid fractions from separators 14 and 15, respectively.

The circulation of the liquid fraction of the separator 14 in the passages 147 allows passage its temperature of around 40 ° C around 8 ° C, temperature at which the liquid fraction of the separator 15 is recovered, due to its circulation in the passages 153 of exchanger 18, substantially in the same indirect heat exchange conditions as the fraction liquid flowing through passages 147.

In view of this, and as has already been indicated, the vapor fraction having circulated in the passages 153 against the current (as for 147 in particular) passages 33 'of the cooling circuit 21', east condensed, so as to be introduced in this form in the separator 15, the vapor fraction recovered in 15a being admitted to the inlet of the high compressor pressure 1C.

In view of the above, we will have understood that the "liquid" entry of the exchanger 6, at 48, takes place around 8 ° C on the installation in Figure 6.

The installation of figure 7 differs from that of Figure 1 only (except the prediction of expander 91 in parallel with the valve trigger 69) because of the forecast not of two but of three compression stages on the compression unit of cycle 1 '.

So in this figure 7, between the input 12c of the distillation apparatus 12 and the condenser outlet 3A, a separator 155, a pump 157, a intermediate stage of compression 1B driving back into 2B in a 3B condenser whose output communicates with the input 12c of the distillation apparatus 12.

As already described in WO-A-94 24500, this intermediate compression stage and its accessories allows to separate in 155 in a vapor fraction and a liquid fraction the refrigerant mixture compressed into 1A and partially condensed into 3A, with cooling up to a temperature of + 30 ° C to + 40 ° C.

The vapor phase from separator 155 is compressed at a second intermediate pressure Pi typically of the order of 12 to 20 bar, in 1B, while the liquid fraction recovered from the same separator 155 is carried by pump 157 at the same pressure Pi and injected in line 2B (or possibly leaving the partial condenser 3B).

The mixture of the two phases in this conduct is then cooled and partially condensed into 3B, then distilled in 12.

Note that such a compression unit 1 'to three stages of compression could be used in other versions of the installation of the invention.

Moreover, and more generally, the peculiarities of such figure can be applied in the species to another, indifferently.

Regarding the use of separators 9 and 103, this could also be applied in any other case.

Similarly, the circulation of gas natural in passages 79 'then 113 can be provided on the other figures than Figure 3, insofar as the send temperature to unit 75 is different from the cut-out temperature at 40.

Claims (27)

1. A process for refrigerating a fluid to be cooled, notably for liquefying natural gas, in first heat exchanging means, by using a refrigerating mixture circulating in an endless circuit, the process comprising :

   a) compressing the refrigerating mixture in a penultimate stage (1a,1b) of a plurality of stages of a compression unit (1,1'),

   b) separating (12,14) the compressed refrigerating mixture for obtaining a vapour fraction and a liquid fraction,

   c) cooling and partially condensing the vapour fraction and cooling the liquid fraction by circulating said vapour fraction through first channels (57, 135) and said liquid fraction through second channels (93,137'), in second heat exchange means independent from first heat exchange means, said circulation through the second heat exchange means comprising a heat exchange between the vapour and liquid fractions and the refrigerating fluid which circulates in an endless circuit (21,21'), for obtaining respectively a condensed vapour fraction and a cooled liquid fraction, and then cooling the cooled liquid fraction in the first heat exchange means (5),

   d) separating (13,15) the condensed vapour fraction for obtaining a resultant vapour fraction and a resultant liquid fraction,

   e) sending the resultant vapour fraction to the final compression stage (1c), for obtaining a high pressure vapour fraction,

   f) expanding the cooled liquid fraction issued from the first heat exchange means and the high pressure vapour fraction, before circulating them through the first heat exchange means, in a heat exchange with the fluid to be cooled, and then recycling them to the penultimate stage of said compression unit, as the refrigerating mixture.
2. The process according to Claim 1, characterized in that, between steps a) and b), the compressed refrigerating mixture issued from the penultimate stage of compression is cooled by a refrigerating fluid which is present on the site.
3. The process according to Claim 1 or claim 2, characterized in that during step b) and step d):

   the compressed refrigerating mixture is separated in a first separator (14),

   the condensed vapour fraction is separated in a second separator for obtaining the resultant vapour fraction and the resultant liquid fraction.
4. The process according to Claim 3, characterized in that, before admitting the resultant liquid fraction into the first heat exchange means (5), this resultant liquid fraction is combined with the cooled liquid fraction having passed into said second heat exchange means (18).
5. The process according to Claim 1 or claim 2, characterized in that during step b), step d) and step e):

   the compressed refrigerating mixture is separated in a distillation apparatus (12),

   the condensed vapour fraction is separated in a separator (13), for obtaining said resultant vapour fraction and said resultant liquid fraction,

   and, the resultant liquid fraction is returned to the column head (12a) of the distillation apparatus, to cool it.
6. The process according to anyone of Claims 3 to 5, characterized in that:

   the liquid fraction issued from the distillation apparatus (12) or from the first separator (14) is circulated in the second heat exchange means (18), between a hot end (28b) and a cold end (28a) thereof,

   and said cooled liquid fraction is admitted in an intermediate part of a first, hot exchanger (6) of two heat exchangers arranged in series, one hot and the other cold, belonging to said first heat exchange means (5).
7. The process according to anyone of the preceding claims, characterized in that the refrigerating fluid is circulated in an endless circuit refrigeration cycle (21) comprising two successive compression stages (1D,1E) and, on emerging from the highest compression stage of the two (1E), the refrigerating fluid is totally condensed.
8. The process according to anyone of the Claims 1 to 6, characterized in that the refrigerating fluid is circulated in an endless circuit refrigeration cycle (21') which comprises a single compression stage and, on emerging from this single compression stage, the refrigerating fluid is totally condensed.
9. The process according to anyone of the preceding claims, characterized in that, between steps e) and f):

   the high pressure vapour fraction is cooled after the final compression stage (1c) of said compression unit (1,1'),

   and said condensed, high pressure vapour fraction is circulated in the second heat exchange means (18), in order to cool it further by heat exchange with the refrigerating fluid before sending it into the first heat exchange means (5).
10. The process according to anyone of Claims 5 and 9, characterized in that:

    for refrigerating one more time the cooled, high pressure vapour fraction, the latter is circulated between a hot end (28b) of the second heat exchange means (18) and an intermediate part thereof,

    and the vapour fraction issued from the distillation apparatus (12) is circulated between said intermediate part and a cold end (28a) of said second heat exchange means (18), before sending it into said separator (13).
11. The process according to anyone of Claims 5 to 9, characterized in that the vapour fraction and the liquid fraction derived from the distillation apparatus (12) are circulated between a hot end and a cold end of the second heat exchange means (18), before they are admitted respectively into said separator (13) and into said first heat exchange means (5).
12. The process according to anyone of the preceding Claims, characterized in that, between the steps a) and b), the compressed refrigerating mixture is circulated in the second heat exchange means (18).
13. The process according to anyone of the preceding Claims, characterized in that:

    the fluid to be cooled is natural gas,

    before circulating the natural gas in the first heat exchange means (5), it is subjected to drying,

    and, after drying, the dried natural gas passes, inside the first heat exchange means (5), firstly into a first part of a first, hot exchanger (6) of a first and a second exchangers arranged in series, one hot and the other cold (7), belonging to said first heat exchange means, then into a part of said second exchanger of the first heat exchange means, before passing into a fractionating unit (75) arranged outside the first heat exchange means.
14. The process according to anyone of the preceding Claims, characterized in that:

    the fluid to be cooled is natural gas,

    before admitting the natural gas into said first heat exchange unit (5), it is passed successively:

    into third heat exchange means (123) in order to cool it by heat exchange with the refrigerating fluid,

    then, into an intermediate drying unit.
15. The process according to Claim 14, characterized in that the dried natural gas derived from the intermediate drying unit is circulated in the second heat exchange means (18), before being admitted into the first heat exchange means (5).
16. The process according to anyone of Claims 1 to 14, characterized in that:

    the fluid to be cooled is natural gas, and

    before admitting the natural gas into the first heat exchange means (5), it is circulated firstly in the second heat exchange means (18) and, before or after this circulation in the second heat exchange means, the natural gas is subjected to drying.
17. The process according to anyone of the preceding Claims, characterized in that:

    the fluid to be cooled is natural gas,

    the natural gas is subjected to drying before it is admitted, in order to cool it, into a first, hot exchanger (6) of a first and a second exchangers arranged in series, one hot and the other cold (7), belonging to said first heat exchange means (5),

    at least a part of the natural gas is cooled in a first part of the second, cold exchanger (7),

    the natural gas is then passed into a fractionating unit (75) in order to obtain a fractionary resultant compound,

    and said fractionary resultant compound is circulated in a second part of the second, cold exchanger (7), in order to liquefy and under-cool it.
18. A cooling installation for refrigerating a fluid to be cooled, the installation comprising an endless circuit for a refrigerating mixture, and including:

    a compression unit (1,1') comprising a plurality of compression stages arranged in series, including a final compression stage (1c) comprising an outlet for a high pressure vapour fraction, and a penultimate compression stage (1A, 1B), for compressing at least a part of the refrigerating mixture,

    first separating means (12,14) arranged between the penultimate compression stage (1A,1B) and the final compression stage (1C), for obtaining a separation of the refrigerating mixture issued from the penultimate compression stage into a vapour fraction and a liquid fraction,

    cooling means (18), for cooling said vapour fraction and said liquid fraction issued from the first separating means, said cooling means comprising :

    second heat exchanging means (18) comprising first channels (57,135') and second channels (93,137') separate from the first channels, for circulating therein respectively the vapour fraction and the liquid fraction in a heat exchange with a refrigerating fluid and thus obtaining a condensed vapour fraction and a cooled liquid fraction,

    the refrigerating fluid which circulates in a separate endless circuit (21, 21', 21") passing through the second heat exchanging means,

    second separating means (13,15) for separating the condensed vapour fraction in a resultant liquid fraction and a resultant vapour fraction, said second separating means comprising an outlet for said resultant vapour fraction communicating with an inlet of the final compression stage,

    expanding means (50, 69, 71),

    first heat exchanging means (5) enclosing:

    a first passage (79) for circulating there through the fluid to be cooled,. the first passage having an inlet and an outlet,

    a second passage (59,65) having an outlet communicating with the expanding means and inlets for the refrigerating mixture, said inlets communicating with an outlet of the second heat exchanging means, for circulating the cooled liquid fraction, and with an outlet of the final compression stage, for circulating the high pressure vapour fraction, and,

    a third passage (41,42), for returning the refrigerating mixture to the compression unit, said third passage having an outlet communicating with an inlet of the compression unit and an inlet communicating with the expanding means.
19. The installation according to Claim 18, characterized in that the first separating means comprise a separator (14).
20. The installation according to Claim 18, characterized in that the first separating means comprise a distillation apparatus (12).
21. The installation according to anyone of Claims 18 to 20, characterized in that the second separating means comprise a separator (13,15).
22. The installation according to anyone of Claims 18 to 21, characterized in that the second separating means (13,15) comprise an outlet for a liquid fraction communicating with the cooled, liquid fraction inlet (48) of the first heat exchanging means (5).
23. The installation according to anyone of Claims 18 to 22, characterized in that:

    the first separating means (12,14) comprise an inlet communicating with an outlet of a condenser (3A),

    and this communication between the outlet of the condenser and the inlet of the first separating means (12,14) passes through the second heat exchanging means (18).
24. The installation according to anyone of Claims 18 to 23, characterized in that the refrigerating fluid circulates in a refrigerating cycle (21") including:

    the second heat exchanging means (18),

    and third heat exchange means (123) through which pass, in a heat exchange, the refrigerating fluid and the fluid to be cooled.
25. The installation according to anyone of Claims 18 to 24, characterized in that the communication between the outlet of the final compression stage (1C) and the inlet of the first exchanging means for the high pressure vapour fraction passes through the second heat exchange means (18).
26. The installation according to anyone of Claims 18 to 25, characterized in that it comprises a refrigerating fluid circuit for the refrigerating fluid which passes through the second heat exchange means (18).
27. The installation according to anyone of Claims 18 to 26, characterized in that it additionally comprises separate heat exchange means (3A, 3B) for a heat exchange with a separate cooling fluid, the separate heat exchange means being disposed between the outlet of said penultimate compression stage and the inlet of the first separating means (12,14), so as to cool the refrigerating mixture derived from the penultimate compression stage before introducing it into the first separating means.

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