EP0644996B1 - Gas cooling process and plant, especially for natural gas liquefaction - Google Patents

Abstract

In this process, which incorporates an integral cascade, the coolant mixture issuing from the penultimate stage (1B) of the compressor cycle (1) is delivered to a distillation apparatus (5) the head vapor of which is cooled (in 24) to a temperature significantly lower than the ambient temperature, then separated into two phases (in 6C); the vapor stage is supplied to the last stage (1C) of the compressor, and the liquid phase constitutes a coolant fluid for the hot part (8) of the heat exchange line (7).

Description

The present invention relates to a cooling process a fluid, especially for liquefying gas natural, as well as an integral waterfall type fluid cooling installation, where a mixture is compressed in at least two stages refrigerant composed of volatility constituents different and after at least each of the stages compression intermediates, we condense partially mixing at "room temperature", at least some of the condensed fractions as well as the high pressure gas fraction being cooled, relaxed, heat exchange relationships with the fluid to cool, then compressed again.

In accordance with the preambles of claims 1 and 9 respectively, such a method and such an installation are known from EP-A-0 117 793.

The pressures discussed below are absolute pressures.

It has long been proposed to liquefy natural gas using a cycle called "built-in cascade" refrigerator using a mixture of fluids.

The refrigerant mixture consists of a number of fluids including, among others, nitrogen, hydrocarbons like methane, ethylene, ethane, propane, butane, pentane, etc.

The mixture is compressed, liquefied and then sub-cooled to the high pressure of the cycle which is generally between 20 and 50 bars. This liquefaction can be carried out in one or more steps with separation of the condensed liquid at each step.

The liquid (s) obtained are, after their sub-cooling, relaxed at low pressure cycle, generally between 1.5 and 6 bars, and vaporized against the current of the natural gas to be liquefied and cycle gas to be cooled.

After reheating in the vicinity of the room temperature, the refrigerant mixture is new tablet until high cycle pressure.

To make operation possible, it is necessary to have a fluid capable of condense at room temperature at high pressure of the cycle. This poses a particular difficulty, from the fact that the mixture and the pressures are generally optimized for the cold part of the liquefaction facility and are unsuitable for a refrigeration also effective in the part hot, i.e. between the temperature ambient (generally around + 30 to + 40 ° C in natural gas producing regions) and a intermediate temperature of the order of - 20 to -40 ° C.

Many existing installations make so call, for the hot part, to a cycle of separate, propane or propaneethane mixture refrigeration. We thus obtain an energy expenditure relatively low specific, but at the cost of significant increase in the complexity and cost of the installation.

He is already known in EP-A-0 117 793 a cooling process using the characteristics set out in the preamble of claim 1.

But in this document, when a distillation is planned to distill the gas from of the penultimate compression stage (Figure 3), a "refrigerant" device followed by a separator provide cooling with partial return at the top column of the fluid which leaves there at the top, the rest of this fluid partially condensed and separated (i.e. the vapor fraction) being sent to the last compression stage.

However, with such an arrangement, the head of the distillation apparatus cannot be sufficiently cooled.

And it is considered that this does not constitute not an optimization of the operating cycle.

Under these conditions, the invention aims to to obtain both a specific energy of the process and a relatively small investment, in best conditions.

To do this, the invention relates to a method according to claim 1.

For the sake of clarity, we will define the "room temperature" stated at the beginning of the text as the room temperature thermodynamic reference corresponding to the coolant temperature (water in particular) available on the site and used in the cycle, increased by the difference in temperature fixed, by construction, at the outlet of the devices machine refrigerants (compressors, exchangers ...). In practice, this difference is around 3 to 10 ° C, and preferably around 5 to 8 ° C.

It will also be noted, as of now, that the device head cooling temperature distillation (corresponding substantially to the temperature of the "liquid" acting for this purpose) will between about 0 and 20 ° C, and generally between 5 and 15 ° C, for an "ambient temperature" (or inlet line temperature) around 15 to 45 ° C, and generally between 30 and 40 ° C.

• on refroidit et on condense partiellement la vapeur de tête de l'appareil de distillation par échange de chaleur avec au moins lesdites fractions détenduesque l'on fait circuler dans une ligne d'échange thermique (comprenant une partie "chaude" et une partie "froide", et on refroidit la tête de l'appareil de distillation avec la phase liquide ainsi obtenue ;
• on refroidit et on condense partiellement au voisinage de la température ambiante le gaz issu du dernier stade de compression, on détend la phase liquide obtenue, et on refroidit la tête de l'appareil de distillation au moyen de cette phase liquide détendue ;
• on opère une déphlegmation du gaz issu du dernier stade de compression pendant son refroidissement ;
• on effectue un échange de chaleur indirect entre le liquide résultant du refroidissement du gaz issu du dernier stade de compression et la vapeur de tête de l'appareil de distillation avant d'envoyer cette vapeur au dernier étage de compression et de détendre ledit liquide ;
• on pompe une partie au moins du condensat du premier stade de compression jusqu'à la pression de sortie du deuxième stade de compression, et on le mélange au gaz issu de ce deuxième stade de compression ;
• lorsque le procédé est destiné à la liquéfaction de gaz naturel contenant de l'azote, on sous-refroidit le gaz naturel liquéfié résultant de la réfrigération puis désazoté, par échange de chaleur avec du gaz naturel liquéfié non désazoté détendu ;
• lorsque le procédé est destiné à la liquéfaction de gaz naturel contenant de l'azote, on effectue une désazotation primaire du gaz naturel sous sa pression de traitement dans une colonne auxiliaire, on détend à une pression intermédiaire une partie du gaz naturel liquéfié ayant subi cette désazotation primaire, on vaporise le liquide ainsi détendu en refroidissant la tête de la colonne auxiliaire, ce qui produit un gaz combustible sous la pression intermédiaire, on envoie ce gaz combustible à une turbine à gaz d'entraínement du compresseur, et on traite le reste du gaz naturel liquéfié ayant subi la désazotation primaire ainsi que la vapeur de tête de la colonne auxiliaire dans une colonne de désazotation finale sous basse pression produisant en cuve le gaz naturel liquéfié désazoté destiné à être stocké.

The method can moreover comprise one or more of the following embodiments:

• the overhead vapor of the distillation apparatus is cooled and partially condensed by heat exchange with at least the said expanded fractions which are circulated in a heat exchange line (comprising a "hot" part and a "cold" part ", and the head of the distillation apparatus is cooled with the liquid phase thus obtained;
• the gas from the last compression stage is partially cooled and partially condensed near ambient temperature, the liquid phase obtained is expanded, and the head of the distillation apparatus is cooled by means of this expanded liquid phase;
• a gas dephlegmation from the last stage of compression is operated during its cooling;
• an indirect heat exchange is carried out between the liquid resulting from the cooling of the gas originating from the last stage of compression and the overhead vapor of the distillation apparatus before sending this vapor to the last compression stage and expanding said liquid;
• pumping at least part of the condensate from the first compression stage to the outlet pressure of the second compression stage, and mixing it with the gas from this second compression stage;
• when the process is intended for the liquefaction of natural gas containing nitrogen, the liquefied natural gas resulting from refrigeration and then denitrogenated is sub-cooled, by heat exchange with expanded non-denitrogenated liquefied natural gas;
• when the process is intended for the liquefaction of natural gas containing nitrogen, a primary denitrogenation of the natural gas is carried out under its treatment pressure in an auxiliary column, part of the liquefied natural gas having undergone this is expanded to an intermediate pressure primary denitrogenization, the liquid thus expanded is vaporized by cooling the head of the auxiliary column, which produces a combustible gas under intermediate pressure, this combustible gas is sent to a gas turbine driving the compressor, and the rest is treated liquefied natural gas having undergone the primary denitrogenation as well as the overhead vapor of the auxiliary column in a final denitrogenation column under low pressure producing in tank the denitrogenated liquefied natural gas intended to be stored.

The subject of the invention is also a installation for cooling a fluid, in particular natural gas liquefaction, intended for the work of such a process.

This installation includes the features of claim 9.

Thanks to the features of the invention, we will be able to optimize the exchange cycle and thermal and mechanical yields. Also note when leaving the high pressure stage, the fluid is then gaseous and does not condense, even if a "refrigerant" is set up.

In a particular embodiment, the heat exchange line will consist of two plate heat exchangers in series, connected to each other by end domes, end to end.

• la Figure 1 représente schématiquement une installation de liquéfaction de gaz naturel conforme à l'invention ;
• la Figure 2 représente schématiquement un autre mode de réalisation de l'installation suivant l'invention ;
• la Figure 3 représente plus en détail un élément de l'installation de la Figure 2 ;
• la Figure 4 représente schématiquement une partie d'une variante de l'installation de la Figure 1 ;
• la Figure 5 représente schématiquement une variante de la partie froide de l'installation de la Figure 1 ou de la Figure 2 ; et
• la Figure 6 est une vue partielle schématique d'une autre variante d'installation.

Examples of implementation of the invention will now be described in relation to the accompanying drawings, in which:

• Figure 1 schematically shows a natural gas liquefaction installation according to the invention;
• Figure 2 schematically shows another embodiment of the installation according to the invention;
• Figure 3 shows in more detail an element of the installation of Figure 2;
• Figure 4 schematically shows part of a variant of the installation of Figure 1;
• Figure 5 schematically shows a variant of the cold part of the installation of Figure 1 or Figure 2; and
• Figure 6 is a partial schematic view of another alternative installation.

The gas liquefaction facility natural shown in Figure 1 includes essentially: a single cycle compressor 1 to three stages 1A, 1B and 1C, each stage driving back, via a respective pipe 2A, 2B and 2C, in a respective refrigerant 3A, 3B and 3C water-cooled from sea, this water typically having a temperature of on the order of + 25 to + 35 ° C; a pump 4; a column of distillation 5 having some theoretical plateaus; of separator pots 6B, 6C, the top of which communicates respectively with the suction of stages 1B and 1C; a heat exchange line 7 comprising two heat exchangers in series, namely a "hot" heat exchanger 8 and a "cold" exchanger 9; a separator pot intermediate 10; an auxiliary liquid circuit 11 cooling ; an auxiliary heat exchanger 12; a denitrogenation column 13; and a storage of liquefied natural gas (LNG) 14.

The outlet of the 3A refrigerant leads into the separator 6, the bottom of which is connected to the suction of pump 4, while the latter flows back into the driving 2B. 3B refrigerant outlet communicates with the tank in column 5, and the bottom of the separator 6C is connected by gravity, via a siphon 15 and a valve 16, at the head of column 5.

Exchangers 8, 9 are exchangers parallelepipedic with aluminum plates possibly brazed, with counter-current circulation of the set fluids in heat exchange relationship, and have the same length. They each include the passages necessary to ensure the functioning which will be described below.

The refrigerant mixture, consisting of C1 to C5 hydrocarbons and nitrogen, comes out of the top (hot end) of the exchanger 8 in the gaseous state and via a line 17 to the suction of the first compressor stage 1A.

It is thus compressed to a first intermediate pressure P1, typically of the order of 8 at 12 bar, then cooled to + 30 to + 40 ° C in 3A and separated into two phases in the pot 6B. The sentence vapor is compressed at a second pressure intermediate P2, typically of the order of 14 to 20 bars, in lB, while the liquid phase is supplied by pump 4 at the same pressure P2 and injected into the driving 2B. The mixture of the two phases is cooled and partially condensed in 3B, then distilled in 5.

The tank liquid in column 5 constitutes a first coolant, suitable for provide essential refrigeration of the exchanger hot 8. For this, this liquid is introduced laterally, via an input box 18, in the part upper part of this exchanger, sub-cooled in passages 19 to the cold end of the exchanger, towards - 20 to - 40 ° C, taken out laterally via a box of output 20, relaxed at low cycle pressure, which is typically of the order of 2.5 to 3.5 bars, in a expansion valve 21, and reintroduced in the form two-phase at the cold end of the same exchanger via a side box 22 and a distribution device suitable for spraying in low passages pressure 23 of the exchanger.

The head vapor of column 5 is cooled and partially condensed in passages 24 of exchanger 8 up to a temperature intermediate significantly below temperature ambient, for example up to + 5 to + 10 ° C, then introduced into pot 6C. The liquid phase returns in reflux by gravity, via the siphon 15 and the valve 16, in head of column 5, while the vapor phase is compressed at high cycle pressure, typically from around 40 bars, in lC, then is reduced to + 30 to + 40 ° C in 3C. This vapor phase is then cooled from the hot end to the cold end of the exchanger 8 in high pressure passages 25, and separated into two phases in 10.

To complete the refrigeration of the exchanger 8, it is possible, as shown in line interrupted, subcool to a temperature intermediate part of the liquid collected in 6B, then remove it laterally from the exchanger, relax it at low pressure in an expansion valve 26, and the reintroduce laterally into the exchanger for the spray in the middle of the passages low pressure 23.

The refrigeration of exchanger 9 is obtained by means of the high pressure fluid, the next way.

The liquid collected in 10 is sub-cooled in the hot part of the exchanger 9, in passages 27, then exited the exchanger, relaxed at the low pressure in an expansion valve 28, reintroduced into the exchanger and vaporized in the part hot low pressure passages 29 thereof. The vapor phase from separator 10 is cooled, condensed and sub-cooled from the hot end to the cold end of the exchanger 9, and the liquid thus obtained is relaxed at low pressure in an expansion valve 30, and reintroduced at the cold end of the exchanger for be sprayed in the cold part of the lower passages pressure 29 then joined to the expanded fluid at 28.

Processed natural gas arriving at + 20 ° C, after drying, via line 31, is laterally introduced into the exchanger 8 and cooled to the cold end of it in passages 32.

At this temperature, natural gas is sent to a hydrocarbon removal device 33 in C2 to C5, and the remaining mixture, consisting mainly methane and nitrogen, with a small amount of ethane and propane, is divided in half streams: a first stream, cooled, liquefied and sub-cooled from the hot end to the cold end of the auxiliary exchanger 12 then expanded to 1.2 bar in an expansion valve 34, and a second current, cooled, liquefied and sub-cooled from hot end to cold end of the exchanger 9 in passages 35, sub-cooled again around 8-10 ° C in a coil 36 forming column reboiler 13, and expanded to around 1.2 bar in an expansion valve 37. The two relaxed currents are united then introduced at reflux at the top of column 13, which thus ensures the denitrogenation of natural gas. The liquid of the bottom of this column constitutes nitrogenous LNG produced by the facility and sent to storage 14, while the overhead steam is reheated to - 20 to - 40 ° C from the cold end to the hot end of the exchanger 12 and is sent via a line 38 to the "fuel gas" network to be burned or used in a gas turbine of the installation used to drive the compressor 1.

It should be noted that a cut additional on natural gas can be performed in the exchanger 9 at a temperature allowing recover additional quantities of hydrocarbons in C2 and C3 in the apparatus 33.

As shown, taking into account the very high flows generally implemented in such an installation, it may be desirable to relax some of the cold liquids in liquid turbines or "expanders" 39 to produce cold as well as part of the electric current necessary. Plus, the hottest part of exchanger 8 can be used to cool from + 40 at around + 20 ° C a suitable liquid, especially pentane, put into circulation in passages 40 of the exchanger by a pump 41 and serving to refrigerate another part of the installation, for example gas natural raw intended to be dried before treatment in the liquefaction plant. This circulation of liquid constitutes the refrigerant circuit 11 cited above.

The arrangement described above allows the times to accelerate the condensation of the mixture from the second compression stage 1B, thanks to injection liquid in line 2B by means of pump 4, to simplify the exchanger 8 if all of the liquid from pot 6B is pumped, and to obtain a high mixture pressure sufficiently freed from heavy products, more precisely, in the example considered, of the almost all of the C5 hydrocarbons and majority of C4 hydrocarbons, to be completely vaporized at the hot end of passages 29 of the exchanger cold 9. This has the important advantage that these passages can lead to an upper dome 42 of the exchanger 9 communicating directly with a dome lower 43 of exchanger 8, without any two-phase redistribution is necessary for cutting between the two exchangers. We can then simplify again the installation by butt welding the two exchangers 8 and 9.

We can also notice that the suction of compressor stage 1C to a relatively cold temperature is favorable to performance of it.

The cutoff at around - 20 to - 40 ° C between the two exchangers also correspond to heat exchange surfaces of the same order above and below this cut, so that we can use two exchangers 8 and 9 of maximum length under thermal performance conditions optimal, and a single separator pot 10, at the cutoff above, for the high pressure fluid.

We understand that the control of the temperature and pressure (+ 5 to + 10 ° C, 14 to 20 bars) coolant from the head of the column 5 provides a single-phase gas at the times at the outlet of the 3C refrigerant and at the outlet (at 42) of the cold exchanger 9 (- 20 ° C to - 40 ° C, 2.5 to 3.5 bars).

It should be noted that in practice, we climb n exchangers 8 in parallel, and n exchangers 9 in parallel.

The installation shown in Figure 2 does not differs from that in Figure 1 only by adding, between compression stages 1B and 1C, from another stage of lD intermediate compression, as well as the cooling of the reflux liquid from column 5.

Thus, the outlet of the 3B refrigerant opens in a 6D separator pot, the vapor phase of which supplies the 1D stage. The repression of it is cooled by a 3D refrigerant then introduced to the base from column 5. The liquid in the 6D pot constitutes a additional coolant, sub-cooled in additional passages 45 provided in the hot part exchanger 8, out of it, relaxed at low pressure in an expansion valve 46 and reintroduced in the exchanger to be vaporized in the part low pressure passages 23.

In addition, the overhead vapor from the column 5 is sent directly to the suction of the last stage of compression 1C, and the high fluid pressure is sent to the base of a dephlegmator 47 cooled by seawater runoff around tubes vertical 48. The majority of heavy products are collected at the base of the dephlegmator, relaxed in an expansion valve 49 and introduced at reflux at the head from column 5, and the top vapor from the dephlegmator as above, forms the high refrigerant pressure, which is cooled to the cold end of exchanger 8 then, after phase separation at 10, to the cold end of the exchanger 9.

Figure 3 shows a mode of realization of a heat exchanger which can be used as an intermediate refrigerant 3B. This exchanger comprises a calender 50 in which a number of vertical tubes 51 open to their two ends extend between an upper shelf 52 and a lower plate 53. Between these plates, and at the outside of the tubes, are mounted a number horizontal baffles 54. Cooling water arrives via a lower pipe 55 on the tray 52, flows upward through tubes 51 and is discharged through an upper pipe 56. The two-phase mixture carried by line 2B penetrates laterally in the grille under the plate 52 and descends along the baffles, then exits by the pipe outlet 57 of the exchanger, located a little above the tray 53.

Such an arrangement makes it possible homogenize the two-phase mixture during its cooling, and get to a high degree the advantage of accelerating condensation in the second stage of compressor 1 provided by the loop with pump 4.

Figure 4 shows another variant arrangement of the distillation column 5. In this variant, the column head vapor is heated a few degrees Celsius in an exchanger auxiliary heat 58, then sent to suction of the last compression stage 1C. The high fluid pressure, after cooling and condensing partial in 3C around + 30 to + 40 ° C, is separated in two phases in a separator pot 59. The vapor from this pot constitutes the high pressure refrigerant, while the liquid phase, after sub-cooling a few degrees Celsius in the exchanger 58, is expanded in an expansion valve 49 as in Figure 2 and then introduced under reflux at the top of column 5.

We understand that this variant can apply to an installation either three or four stages of compression. In addition, sub-cooling 58 is optional.

Whatever the embodiment considered, the denitrogenation column 13 must operate at 1.15 bar or 1.2 bar, and therefore the nitrogen-free LNG leaving the tank of this column should be relaxed to atmospheric pressure at the entrance to storage 14, which produces gas from flash. This gas, as well as the gas resulting from the inputs of heat in storage 14, must therefore be taken up and compressed by an auxiliary compressor to be distributed to the "fuel gas" network. Figure 5 shows a arrangement which eliminates this compressor auxiliary, in the event that LNG leaving exchanger 9 contains a few% of nitrogen.

For this, the LNG leaving the exchanger 9 is sub-cooled in the coil 36 of the column 13 and again sub-cooled in a heat exchanger auxiliary heat 60. The liquid is then expanded around 1.2 bar in the expansion valve 37 and the turbine 39, then divided into two streams: a stream which is vaporized in a heat exchanger 60 then introduced to a intermediate level in column 13, and a current which is sent in reflux at the head of the latter.

The tank liquid from column 13, which is LNG without nitrogen, is then, for each storage, divided into two streams, one of which is sub-cooled in the exchanger 60 while the other passes through a bypass 61 to adjust the degree of subcooling overall, the circulation of the liquid being provided by a pump 62.

In this way, it is sub-cooled liquid about 2 ° C which is sent to 14 storages, which removes virtually any flash at the entry of these storages and any evaporation due to heat inputs over time. As we understand is the difference in LNG compositions before and after denitrogenation which makes it possible to obtain such a subcooling in exchanger 60.

Likewise, the overhead vapor of column 5 is generally rich enough in methane to be recovered as "fuel gas", in the sense indicated upper. It is therefore necessary to provide another auxiliary compressor for this purpose. If more cycle compressor 1 is driven by a turbine gas, it is necessary to supply it with gas fuel under a pressure of the order of 20 to 25 bars, which leads to installing a compressor high power auxiliary. The layout of the Figure 6 shows how we can remove the need of such an auxiliary compressor.

In this Figure 6, we use a column additional 63 primary denitrogenation under pressure natural gas, fitted with an overhead condenser 64.

The portion of natural gas from the device 33 which is treated in the exchanger 12 is cooled only to an intermediate temperature T1, then is introduced into the tank of column 63, via a pipe 65 while the rest of this natural gas is only cooled in the exchanger 9 to a intermediate temperature T2 lower than T1 then introduced at an intermediate level of the same column, via a pipe 66.

The cooling of the condenser 64 is assured by expanding part of the liquid around 25 bars of the column tank in an expansion valve 67. The gas resulting from this vaporization has the same composition as the column bottom liquid, that is to say has a low nitrogen content, and therefore constitutes a combustible gas at 25 bars directly usable, via a pipe 68, in the gas turbine 69.

The rest of the column tank liquid 63 is, after sub-cooling partly in the cold part of the exchanger 9 and in the coil 36 of column 13, and partly in the cold part exchanger 12, expanded at 37, respectively at 70, and introduced to an intermediate level of the column 13. The overhead vapor of column 63, containing 30 to 35% nitrogen, is cooled and condensed in the part exchanger 9 cold, sub-cooled in that of the exchanger 12, and, after expansion in a valve trigger 71, introduced in reflux at the top of the column 13.

The nitrogen enrichment of the washing of column 13 thus obtained has the consequence that the nitrogen vapor from this column is sufficient low in methane, for example contains 10 to 15% of methane, to be vented through the pipeline 38 after heating in 12.

In total, we thus obtain two gases waste, one of which is rich in methane and under 25 bars, and powers the gas turbine, and one of which, under low pressure, is poor in methane and is not recovered.

As shown in Figure 6, a fraction of the natural gas to be treated conveyed by the line 31 can be cooled in the hot part of the exchanger 12 before being sent to the device 33.

Claims (19)

1. A method for cooling a fluid, especially for the liquefaction of natural gas, of the type having an integral incorporated cascade, in which:

   a) a refrigerant mixture consisting of constituents of different volatilities is compressed in at least two stages (1A, 1B; 1A, 1B, 1D),

   b) after at least each of the intermediate compression stages (1A, 1B; 1A, 1B, 1D), the mixture is partially condensed by means of a liquid coolant available at the site, especially water, some at least of the condensed fractions and also the high-pressure gaseous fraction being cooled (at 19 or 25), pressure-relieved (at 21 and 26 or at 21 and 46), brought into a heat exchange relationship with the fluid to be cooled (at 23 or 32), then compressed again,

   c) and the mixture derived from the penultimate compression stage (1B; 1D) is distilled in a distilling apparatus (5), the top of which is cooled with a liquid, to form firstly the condensate of this penultimate stage, and secondly a vapour phase which is sent to the last compression stage (1C) where it is compressed, before being used as a high-pressure gaseous fraction,

   characterised in that:

   at the time of step b) and at the penultimate compression stage, a cooling of the said mixture is created, before supplying the distilling apparatus (5), and

   at the time of step c), the top of the distilling apparatus (5) is cooled with the said liquid by introducing this liquid, at the top of this apparatus, at a temperature lower than the temperature of the liquid coolant.
2. A method according to Claim 1,
   characterised in that the vapour coming out of the top of the distilling apparatus (5) is cooled and partially condensed by heat exchange (at 24) with at least the said pressure-relieved fractions which are made to circulate in a heat exchange line (8), to obtain a vapour phase and a liquid phase, and the top of the distilling apparatus (5) is cooled with the liquid phase thus obtained (at 6C), the vapour phase forming the said vapour phase which is sent to the last compression stage.
3. A method according to Claim 1,
   characterised in that the gas derived from the last compression stage (1C) is cooled and partially condensed, to the proximity of the temperature of the liquid coolant (at 47, Figure 2; at 3C, Figure 4), the pressure of the liquid phase obtained is relieved (at 49), and the top of the distilling apparatus (5) is cooled with the thus pressure-relieved liquid phase.
4. A method according to Claim 3,
   characterised in that a defuseling of the gas derived from the last compression stage (1C) is operated during its cooling.
5. A method according to any one of Claims 3 to 4,
   characterised in that an indirect heat exchange is performed (at 58) between the liquid resulting from the cooling of the gas derived from the last compression stage (1C) and the vapour coming from the top of the distilling apparatus (5) before sending this vapour to the last compression stage (1C) and pressure-relieving the said liquid (at 49).
6. A method according to any one of Claims 1 to 5,
   characterised in that a part at least of the condensate of the first compression stage (1A) is pumped (at 4) to the output pressure of the second compression stage (1B), and it is mixed (at 2B) with the gas derived from this second compression stage.
7. A method according to any one of Claims 1 to 6, for the liquefaction of natural gas containing nitrogen,
   characterised in that the liquefied natural gas resulting from the refrigeration (at 7, 8) is supercooled (at 60) then denitrified (at 13), by heat exchange with the non-denitrified liquefied natural gas which is pressure-relieved (at 37).
8. A method according to any one of Claims 1 to 7, for the liquefaction of natural gas containing nitrogen,
   characterised in that a primary denitrification of the natural gas is performed (at 63) under its treatment pressure in an auxiliary column (63), a part of the liquefied natural gas is relieved to an intermediate pressure (at 67) after having undergone this primary denitrification, the thus relieved liquid is vaporised by cooling the top (64) of the auxiliary column, which produces a gas combustible under intermediate pressure, this combustible gas is sent to a gas turbine (70) for driving the compressor (1), and the remainder of the liquefied natural gas which has undergone primary denitrification as well as the top vapour of the auxiliary column (63) is treated in a final denitrification column (13) at low pressure producing in the vat the denitrified liquefied natural gas intended to be stored (at 14).
9. A fluid cooling installation, especially for the liquefaction of natural gas, comprising:

   a refrigerating circuit having an integral incorporated cascade in which a refrigerant mixture circulates and which comprises a compressor (1) having at least two stages (1A to 1C) of which at least the intermediate stage(s)(1A, 1B; 1A, 1B, 1D) is(are) provided (each) with a cooling apparatus (3A, 3B; 3A, 3B, 3D) cooled by a liquid coolant available on site, especially water, to partially condense the mixture,

   a distilling apparatus (5) supplied by the penultimate stage (1B; 1D) of the compressor and the top of which is connected to the suction of the last stage (1C) of the compressor,

   means (24, 6C: 47, 48, 49; 58, 59, 3C) for cooling the top of the distilling apparatus (5) by means of a liquid,

   and a heat exchange line (7, 8),

   characterised in that to cool the said liquid intended for the cooling of the top of the distilling apparatus (5) to a temperature lower than the temperature of the liquid coolant:

   the cooling apparatus (3B, 3D) with which the penultimate stage (1B) of the compressor is provided is situated between this penultimate stage of the compressor (1B) and the distilling apparatus (5), and

   the said cooling means for the top of the distilling apparatus (5) comprise:

   a cooling device (24, 6C; 47, 48, 49; 58, 59, 3C) capable of cooling the said liquid intended for the cooling of the top of the distilling apparatus (5) to a temperature lower than the said temperature of the liquid coolant, and

   means (15) for introducing the said cooled liquid at the top of the said apparatus.
10. An installation according to Claim 9,
    characterised in that the cooling device (24, 6C; 47, 48, 49; 58, 59, 3C) comprises condensation means (24; 47, 48; 3C) capable of cooling, to the said temperature lower than that liquid coolant the vapour phase produced in the distilling apparatus and coming out of its top, to form the liquid intended for the cooling of the top of this distilling apparatus.
11. An installation according to any one of Claims 9 and 10,
    characterised in that the cooling device comprises heat exchange passages (24) passing through the hot portion (8) of the heat exchange line (7), and a separating pot (6C), the bottom of which is connected to the top of the distilling apparatus (5) and the top to the suction of the last compression stage (1C).
12. An installation according to any one of Claims 9 to 10,
    characterised in that the said cooling device (47, 49) comprises means (3C: 47) for cooling, to the proximity of the temperature of the liquid coolant, the gas derived from the last stage (1C) of the compressor (1), and a pressure reducing valve (49) for the liquid derived from these cooling means, the output of this valve being connected to the top of the distilling apparatus (5).
13. An installation according to Claim 12,
    characterised in that the said cooling means (47) of the said gas comprise a Dephlegmator.
14. An installation according to any one of Claims 12 to 13,
    characterised in that an auxiliary heat exchanger (58) is provided to bring the liquid derived from the cooling means (47) of the said gas and the vapour coming out of the top of the distilling apparatus (5) into an indirect heat exchange relationship.
15. An installation according to any one of Claims 9 to 14,
    characterised in that a separating pot (6B) is interposed between the cooling apparatus (3A) of the first stage (1A) of the compressor (1) and the second stage (1B) of this compressor,

    and in that a pump (4) is provided, the suction of which is connected to the bottom of this separating pot (6B) and the delivery of which is connected to the delivery of the second stage of the compressor.
16. An installation according to any one of Claims 9 to 15, for the liquefaction of natural gas containing nitrogen,
    characterised in that it comprises a denitrification column (13) and a supercooling exchanger (60) suitable for supercooling the denitrified liquefied natural gas derived from the vat of this column by heat exchange with the non-denitrified natural gas which is pressure relieved (at 37).
17. An installation according to any one of Claims 9 to 15, for the liquefaction of natural gas containing nitrogen,
    characterised in that it comprises a denitrification column (63) supplied by natural gas under its treatment pressure and comprising a top condenser (64) supplied by the vat liquid of this column relieved (at 67) to an intermediate pressure, a gas turbine (69) supplied by the gas resulting from the vaporisation of this vat liquid, and a low-pressure final denitrification column (13) producing at its bottom the denitrified liquefied natural gas intended to be stored (at 14).
18. An installation according to any one of Claims 9 to 17,
    characterised in that the heat exchange line (7) is formed of two plate heat exchangers (8, 9) in series, connected end to end to one another by end domes (42, 43).
19. An installation according to any one of Claims 9 to 17,
    characterised in that the heat exchange line (7) comprises two plate heat exchangers (8, 9) in series, which are butt- welded.

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