WO2017103533A1

WO2017103533A1 - Method for liquefying natural gas by means of a refrigerant mixture cycle using a refrigerant distillation column provided with a reboiler

Info

Publication number: WO2017103533A1
Application number: PCT/FR2016/053520
Authority: WO
Inventors: Laurent Benoit; Pierre-Yves DUCLOS; Hicham GUEDACHA; Michel BEN BELGACEM - STREK
Original assignee: Engie
Priority date: 2015-12-17
Filing date: 2016-12-16
Publication date: 2017-06-22
Also published as: EP3390937A1; FR3045797A1; WO2017103533A4; FR3045799A1; FR3045799B1

Abstract

The present invention relates, in general, to a method for liquefying a gas including mostly methane, in which method said gas to be liquefied circulates in a primary circuit from a source (1) to a tank (2) for liquefied gas, and in which a refrigerant mixture circulates in a closed secondary circuit, said method using, in particular, a main cryogenic exchanger, three compressors (21; 22; 23) and three aftercoolers (31; 32; 33), a distillation column (12) and a reboiler (13). The method according to the present invention is characterised in that the heat energy provided to the reboiler (13) comes directly from the overheating caused by the compression of the second compressor (22), said heat energy being carried to the reboiler (13) by means of an intermediate loop (40) connecting two heat exchangers (40a; 40b), the first heat exchanger (40a) recovering the heat energy released by the compression carried out in said second compressor (22), which heat energy is retransmitted via the intermediate loop (40) to the second heat exchanger (40b) in order to transmit same to the reboiler (13). The present invention also relates to a liquefaction facility implementing the method of the invention.

Description

PROCESS FOR THE LIQUEFACTION OF NATURAL GAS USING A REFRIGERANT MIXTURE CYCLE WITH DISTILLER COLUMN WITH REFRIGERANT WITH REBOUILLEUR Technical Field

The present invention relates to a gas liquefaction process. Even more particularly, the present invention relates to the liquefaction of gas comprising predominantly nitrogen and C 1 -C 5 hydrocarbons, namely methane, ethane, propane, butane and pentane.

The present invention also relates to a gas liquefaction plant.

The methane-based gas is either a by-product of the oil fields or a major product in the case of gas fields, where it is then in combination with other gases, mainly C 1 to C 6 hydrocarbons, CO2 and nitrogen.

When the quantities of gas are important, we try to transport it. The preferred method is to transport it as a cryogenic liquid (-160 ° C) stored at substantially atmospheric pressure.

The liquefaction of gas for transport requires considerable facilities to reach capacities of several million tons per year.

Gas liquefaction processes require considerable amounts of mechanical energy. Multiple thermodynamic cycles have been developed to optimize overall energy efficiency. There are two main types of cycles. A first type based on the compression and expansion of coolant, with phase change, and a second type based on the compression and expansion of refrigerant gas without phase change. The term "refrigerant fluid", "refrigerant gas" or "refrigerant mixture" means a gas or a mixture of gases circulating in a closed circuit and undergoing compression phases, where appropriate liquefaction, and then exchanges of heat with the external medium. then, then relaxation phases, if necessary evaporation, and finally heat exchanges with the gas to be liquefied comprising methane, which gradually cools to reach its liquefaction temperature at atmospheric pressure, this is at say about -160 ° C in the case of liquefied natural gas, or LNG.

This first type of cycle, with phase change, is generally used on installations with a large production capacity requiring a larger quantity of equipment. In addition, the cooling fluids, generally in the form of mixtures, consist of pentane, butane, propane, ethane and methane, these gases being dangerous because they risk, in case of leakage, to cause explosions or considerable fires. On the other hand, despite the complexity of the equipment required, these phase change liquefaction processes remain the most efficient.

It should be noted that certain phase change processes currently used comprise a distillation column for separating the heavy and light fractions of the refrigerant mixture at a well-defined stage of the process. It should also be noted that to optimize the use of such a distillation column, it must consume a significant amount of energy, which alters the profitability of such a liquefaction process.

Indeed, the distillation columns used today, and applied to liquefy gas, implement a reflux distillation to separate the heavy and light fractions of the refrigerant mixture. Such reflux, compared to the columns which do not use it, or in other words compared to a simple separating flask, makes it possible to better separate the light and heavy fractions from the cooling mixture, that is to say to better adapt the composition of the different fractions. Moreover, this better adaptation of the composition of the refrigerant mixture fractions makes it possible to consume less energy.

However, this consumption gain is slightly reduced by the fact that the use of a distillation column using reflux induces additional cooling in the reflux cycle of the distillation column because part of the cooled reflux required for separation tends to trap some of this cold. However, this energy consumption remaining significant (typically 2 to 5% of the overall energy consumption of the liquefaction plant), it is appropriate to implement an innovative process to further reduce this energy consumption.

In addition, other problems generally identified by the liquefaction processes currently used are related to their costs, their operating expenses (OPEX) as well as their capital investment expenditures (CAPEX). Today, there is therefore a particular need to improve such liquefaction processes to minimize these costs.

Description

The object of the present invention is therefore to provide an improved phase-change gas liquefaction process, which makes it possible in particular to solve at least the problems mentioned above, and particularly to reduce the energy impact of such a process.

To do this, the subject of the present invention is a process for liquefying a gas comprising predominantly methane, wherein said gas to be liquefied flows in a primary circuit from a source to a tank for liquefied gas, and in which a mixture refrigerant circulates in a closed secondary circuit, said method comprising the following phases:

a cooling / liquefaction phase in which said gas to be liquefied is cooled and liquefied following its circulation in a main cryogenic exchanger comprising at least three stages, arranged in cascade, in which said gas is in direct contact with a refrigerant mixture whose composition evolves according to the cooling stage:

The first cooling being carried out in a first stage between the ambient temperature and a temperature T 0 equal to or greater than -60 ° C.,

The second cooling being carried out in a second stage between T0 and a temperature T1 between -140 ° C. and -100 ° C., and

The third cooling being carried out in a third stage between the temperature T1 and a temperature T2 of between -165 ° C. and -140 ° C.,

at the end of the cooling phase, a treatment phase is performed to both partly recondense the cooling mixture and separate it into different fractions in order to adapt its composition to the cooling requirements of each stage of the main cryogenic exchanger said processing step comprising steps of:

Compressing and cooling said refrigerant mixture leaving the first stage of said main cryogenic exchanger by successive passages in at least one first compressor, then a first aftercooler which partially condenses the mixture, then in a separator tank in which the liquid fraction of the cooling mixture is separated from its vapor fraction;

• the liquid fraction of the refrigerant mixture is pumped by a pump, while the vapor fraction of the refrigerant mixture is compressed by a second compressor, then cooled in a first heat exchanger, the liquid fraction and vapor fraction flows being at the same pressure;

• at least partly separate the compressed refrigerant mixture, leaving the second compressor and the first heat exchanger, in a distillation column with a reboiler, a light fraction and a heavy fraction;

Supplying said reboiler with heat energy to generate a stream of vapor and extract at least a portion of the volatile compounds initially included in the bottom stream;

• cooling or postcooling, with the environment, said heavy fractions leaving the reboiler by routing them through a second aftercooler before introducing them into the first stage of the main cryogenic exchanger;

Injecting said light fractions coming from said distillation column into the first stage of the main cryogenic exchanger by forwarding them in advance to a third compressor and a third aftercooler.

According to the present invention, said method being characterized in that the heat energy supplied to the reboiler comes directly from the overheating caused by the compression of said refrigerant mixture in said second compressor, this heat energy being fed to the reboiler using a intermediate loop connecting two heat exchangers, the first heat exchanger recovering the heat energy released by the compression performed in said compressor, which is retransmitted by the intermediate loop to the second heat exchanger to transmit it to the reboiler. For the purposes of the present invention, the term "light fraction" means a fraction comprising vapors rich in dinitrogen (N ₂ ), methane (Ci) and ethane (C ₂ ).

By heavy fraction is meant in the sense of the present invention a fraction comprising propane (C ₃ ), butane (Q) and pentane (Cs) -rich vapors.

It should be noted that the treatment phase occurs after the cooling mixture has effectively cooled all the stages, that is to say at the outlet of said main cryogenic exchanger (on the hot temperature side or near the ambient temperature).

It should be noted that, according to the invention, the refrigerant mixture circulates through all stages of the main cryogenic exchanger co-current of the gas to be liquefied before moving against the current of the gas to be liquefied by going up all the stages previously crossed. When the flow of refrigerant mixture passes through all cooling stages co-current of the gas to be liquefied, it does not influence the temperature and the state of the latter. Indeed, the refrigerant mixture descends all the stages of the main cryogenic exchanger to cool itself and reach a temperature of the order of -160 ° C. On the other hand, when the refrigerant mixture passes through all these stages countercurrently with the gas to be liquefied along an exchange line, it cools the gas to be liquefied by exchanging directly with it heat energy to cool the gas. gas to be liquefied.

It should be noted moreover that, according to the invention, the gas to be liquefied is truly in direct contact only with a single flow of the refrigerant mixture, that which is countercurrent to the flow direction of the gas to be liquefied. The refrigerant mixture, before circulating against the current of the gas to be liquefied to cool and liquefy the latter, circulates in co-current of the gas to be liquefied.

For the purposes of the present invention, the term "exchange line" means the line according to which the gas to be liquefied and the mixture of refrigerant interchange with each other. the heat energy to cool the gas to be liquefied. This exchange line is located in the main cryogenic exchanger throughout its cooling stages.

It should also be noted that the distillation column used in the context of the invention is provided with a reboiler at the bottom of the column, so that at the outlet of the reboiler, the flow of partially vaporized heavy fractions is reinjected into the column. to distill.

In the process according to the invention, the use of a distillation column makes it possible to separate the light and heavy fractions of the cooling mixture, however, without resorting to reflux, but rather to a reboiler fed by an energy coming from a module integrated in the liquefaction process according to the invention. According to the invention, the integrated module is the second compressor. Such a power supply is intended to reduce the energy consumption of the liquefaction process insofar as the energy released by this compressor feeds the reboiler to optimize the separation of the heavy and light fractions of the refrigerant mixture.

In this way, with the aid of such an energy recycling, the impact on the energy consumption of the distillation column is reduced by 1 to 5% compared to the already existing processes and putting particularly into effect a reflux. This energy gain is related to the fact that the process according to the invention recovers energy already produced and existing to separate the fractions of the refrigerant mixture in the distillation column instead of using an energy external to the liquefaction process.

It should also be noted that the amount of recoverable heat at the second compressor, and more particularly at the second aftercooler, is approximately equal to 85 ° C. It should also be noted that the temperature necessary for the reboiler to function properly must be between 50 ° C. and 70 ° C., and preferably of the order of 65 ° C., given that the fluid which undergoes this type of heating is at a pressure typically between 13 and 18 bar. As a result, the temperature level produced by this compressor is sufficient for the reboiler to operate properly.

Moreover, there is no problem to recover the amount of heat necessary for the smooth operation of the reboiler at the second compressor. Indeed, the available heat output from compression overheating is typically equal to 5 to 7 times that required reboiler.

In a first variant embodiment, said liquid fraction of the refrigerant mixture issuing from said separator flask is reinjected directly at the outlet of said first heat exchanger.

In a second variant embodiment, said liquid fraction of the refrigerant mixture issuing from said separator flask is reinjected directly into the first stage of the main cryogenic exchanger.

This second embodiment makes it possible to improve the energy performance of the hottest section of the exchange line, at the cost however of a slight complexity. In fact, more particularly, this second embodiment consists in further fractionating the heavy section of the refrigerant mixture by recovering the liquid fraction of the refrigerant mixture which has condensed at room temperature as soon as it leaves the first aftercooler.

However, in this second variant, it is necessary to also split the first cooling, or first stage, of the main cryogenic exchanger. More precisely, in this configuration, it is necessary to add an additional stage comprised between the ambient temperature and a temperature T1 typically between 0 and 10 ° C.

By the hottest section of the exchange line is meant in the sense of the present invention the first stage of the main cryogenic exchanger. Advantageously, the gas to be liquefied contains methane in a molar proportion of at least 80%.

Advantageously, the refrigerant mixture is liquid air or a mixture comprising essentially nitrogen and alkanes or C 1 -C 5 hydrocarbons with a following typical composition (the compositions being in molar fraction):

- N ₂ : 5%,

- Ci: 30%,

- C ₂ : 40%,

- C ₃ : 5%,

- i and n C.4: 5%, and

i and n Cs: 15%.

It should be noted that these mole fractions can vary typically up to 50% depending on the composition and the pressure of the gas to be liquefied.

The present invention also relates to a gas liquefaction plant comprising a primary circuit connected to a gas source and to a tank for liquefied gas, the installation comprising:

at least one main cryogenic exchanger comprising at least three stages arranged in cascade for cooling and liquefying the gas flowing in the primary circuit,

a closed secondary circuit in which a cooling mixture circulates, said secondary circuit comprising at least:

A first compressor and a first aftercooler upstream of a separator tank,

A second compressor located between said separator tank and a distillation column,

A reboiler integrated in said distillation column, and

A third compressor and a third aftercooler at the top of said distillation column. According to the present invention, the installation further comprises an intermediate loop connecting two heat exchangers, the first heat exchanger recovering the heat energy released by the second compressor, which is retransmitted by the intermediate loop to the second heat exchanger to transmit it to the reboiler .

Brief description of the figures

Other features and advantages of the present invention will become apparent in light of the detailed description of various embodiments which follows, with reference to the following figures:

FIG. 1 represents a first embodiment of the installation according to the invention, in which the liquid fraction of the refrigerant mixture issuing from the separator tank is reinjected directly at the outlet of the second compressor; and

FIG. 2 represents a second embodiment of the installation according to the invention, in which the liquid fraction of the refrigerant mixture coming from the separator tank is reinjected into the first stage of the main cryogenic exchanger.

Preferred embodiment

The embodiments presented below are given for information only and are not limiting.

FIG. 1 describes a liquefaction plant for a gas according to the invention which makes it possible to liquefy gas comprising predominantly nitrogen and hydrocarbons such as methane, ethane, propane, butane and pentane. In the installation according to the invention, the gas is liquefied as a result of direct heat exchanges along an exchange line which crosses a main cryogenic exchanger. In this first preferred embodiment, the main cryogenic exchanger comprises three stages 61, 62, and 63 disposed in cascade.

The first stage 61 of this main cryogenic exchanger is located between the ambient temperature and a temperature T0 greater than or equal to -60 ° C. The second stage 62 is located between the temperature T0 and a temperature T1 between -140 ° C and -100 ° C. And, the third stage 63 is located between the temperature T1 and T2 between -165 ° C and -140 ° C.

After the gas to be liquefied has circulated through the first stage 61 of the exchanger, it passes through a separation system SR, typically a partial distillation column, to adjust the composition of the gas to be liquefied so that it is consistent with natural gas by taking any surplus heavy hydrocarbons (C ₂ , C ₃ , C ₄ , C 5) in order to adjust its calorific value and its Wobbe index. Thus, the gas leaving this separation system SR is a gas in accordance with the specifications of the natural gas. After passing through this separation system SR, the gas to be liquefied passes through the second stage 62 of the main cryogenic exchanger and then through the third stage 63 of the exchanger. At the outlet of the third stage 63, a liquid 2, corresponding to the liquefied gas, is recovered in a tank 2 at a temperature below -160 ° C. and at atmospheric pressure.

At the outlet of each stage of the main cryogenic exchanger, the gas to be liquefied is cooled. This cooling is achieved by heat exchange with a refrigerant mixture circulating in each stage of the main exchanger countercurrent of the gas to be liquefied. It should be noted that the composition of the refrigerant mixture evolves according to the cooling stage of the main exchanger in which it circulates. It should be noted that the concentration of volatile compounds N ₂ , Ci and C ₂ of the cooling mixture increases when it crosses, co-current of the gas to be liquefied, the different stages 61, 62 and 63 of the main cryogenic exchanger.

More specifically, at least a portion of the refrigerant mixture circulates against the current of the gas to be liquefied along an exchange line through which the heat energy of the refrigerant mixture is directly transmitted to the gas to be liquefied. Indeed, the heat energy of at least a portion of the refrigerant mixture is first transmitted to the gas to be liquefied in the third stage 63 of the main cryogenic exchanger, then in the second stage 62, before traveling through the first stage 61. After having left the third stage 63 through an orifice 61E3, the cooling mixture leaves the main cryogenic exchanger (on the hot temperature side, or close to the ambient temperature) to be treated, by a treatment phase, in a closed secondary circuit for both recondenser and separate into different fractions in order to adapt its composition to the cooling needs of each of the aforementioned stages.

The following elements refer to the treatment phase of the refrigerant mixture circulating in a closed secondary circuit.

After having left the hot temperature side of the main cryogenic exchanger via an orifice 61E3, the refrigerant mixture passes through a first compressor 21, then a first aftercooler 31. The combination of this first compressor 21 and this first pos-cooler has the effect of preparing the refrigerant mixture for its subsequent passage in a separator tank 11. In fact, this first compressor 21 has the effect of compressing the vapor contained in the refrigerant mixture, and thus increasing the pressure and the temperature of the mixture . This first aftercooler 31 has the effect of cooling the refrigerant mixture leaving the first compressor 21. Such compression followed by such cooling thus makes it possible to obtain a more liquid mixture than if it came out of the main heat exchanger (on the warm side), which facilitates subsequent separation in a separator flask 11 of the refrigerant mixture in two fractions: a liquid fraction MCO and a vapor fraction VCO.

After passing through the first compressor 21 and the first aftercooler 31, the refrigerant mixture is introduced into a separator tank 11. This separator tank 11 has the effect of separating the refrigerant mixture into the liquid fraction MCO and the vapor fraction VCO. Typically, the liquid fraction MCO is rich in heavy elements such as C ₃ , C ₄ and C 5, and the vapor fraction VCO is rich in volatile compounds such as N 2, Ci and C ₂ . This separator balloon 11 is placed between the first aftercooler 31 and a second compressor 22 to separate the two different fractions, or two phases, of the refrigerant mixture at this stage of the liquefaction device because, for reasons of optimization and feasibility, it is necessary that only the VCO vapor fraction (gas phase) is compressed in the second compressor 22.

Then, the VCO vapor fraction of the refrigerant mixture passes into the second compressor 22, which is intended to raise the pressure of the VCO vapor fraction to facilitate its condensation at later stages of the process, then in a first heat exchanger 40a which has for the purpose of both cooling the superheated VCO fraction during compression and recovering some of this heat at the reboiler 13.

It should be noted that the amount of heat energy resulting from the compression at the level of the second compressor 22 typically corresponds to five or even seven times the amount of heat necessary to produce evaporation of the flow leaving a distillation column 12 at the level of the reboiler 13 placed at the bottom of the distillation column 12. It should also be noted that this amount of heat produced by the second compressor 22 strongly depends both on the precise composition of the compressed stream and the polytropic efficiency of the compressor. This amount of generated heat typically represents about twice the energy required for compression.

The MCO liquid fraction from the separator tank 11 is in turn pumped by a pump 50 and injected at the outlet of this first heat exchanger 40a. In particular, the MCO liquid fraction is injected at the outlet of the first heat exchanger 40a because at this stage of the installation, it is only desired to compress the VCO vapor fraction because a fluid containing both a liquid phase can not be compressed in a compressor. and a gas phase. This is why the liquid fraction MCO is compressed using a pump 50.

After leaving the first heat exchanger 40a and having been connected, at the intersection A, to the flow of liquid fraction MCO, the refrigerant mixture then passes into a distillation column 12 provided with a reboiler 13 located at the bottom of Distillation column 12. The combination of this distillation column 12 and this reboiler 13 makes it possible to separate the cooling mixture into two distinct fractions: a light fraction FLE leaving the top of the distillation column 12, and a heavy fraction FLO coming out. reboiler 13.

Before passing through the reboiler 13, the flow leaving the distillation column 12 is introduced into a heat exchanger 40b to be vaporized. This heat exchanger 40b is powered by the heat energy created by the second compressor 22 and recovered by the first heat exchanger 40a. This heat energy is conveyed to the heat exchanger 40b located upstream of the reboiler 13 by means of an intermediate loop 40 which connects these two heat exchangers 40a and 40b.

This reboiler 13 makes it possible to separate the vaporized stream leaving the heat exchanger 40b at the bottom of the distillation column 12 in order to obtain, at the exit of the reboiler 13, on the one hand, a heavy fraction FLO predominantly composed of propane-rich vapors (C ₃ ), butane (Q) and pentane (C5), and on the other hand a stream of FVA vapor rich in light compounds: N _{2 /} C 1, C ₂ and C ₃ . Typically the heavy fraction FLO no longer comprises at this stage N ₂ and its molar fraction in methane (Ci) is very low (less than 2%, or even less than 1%).

The vapor flow FVA is then reinjected into the distillation column

12 to increase the separation power of heavy and light compounds of the refrigerant mixture. The flow of heavy fraction FLO passes, meanwhile, in a second aftercooler 32 before being introduced on the hot side of the first stage 61 of the main cryogenic exchanger, while the light fraction FLE of the refrigerant mixture, composed predominantly of dinitrogen (N ₂ ), methane (Ci) and ethane (C ₂ ) leaving the top of the distillation column 12 then passes into a third compressor 23 and necessarily into a third aftercooler 33 so that the fraction light FLE liquefies and so that it is at a sufficiently hot temperature when it is introduced into the exchange line, that is to say when it is introduced on the hot side of the main cryogenic exchanger via a port 61E1 . It should be noted that this third aftercooler 33 is therefore disposed upstream of the first stage 61 of the main cryogenic exchanger for reasons of energy efficiency.

The light FLE and heavy FLO fractions of the refrigerant mixture are thus reinjected into the main cryogenic exchanger, and more particularly to the hot side of the first stage 61. Each of these fractions are introduced into this first stage by taking different orifices: the light fraction FLE the refrigerant mixture is introduced through a first orifice 61E1, and the heavy fraction FLO is introduced through a second orifice 61E2.

The heavy fraction FLO passes through this first stage 61 co-current of the gas to be liquefied without exchanging heat energy with the latter and thus entering a second orifice 61E2 and then emerging through the cold side of this first stage 61 by an orifice 61S2. After having left this first stage 61, the heavy fraction FLO is expanded, in order to bring the fraction FLO to its bubble temperature, by a first expander d1 and then connected, at the intersection B, at least to a part flow of refrigerant mixture circulating countercurrent of the gas to be liquefied and leaving the second stage 62 through the orifice 62E3. At this intersection B, the entire refrigerant mixture is reconstituted. After this connection, the refrigerant mixture is introduced into the first stage 61 of the main cryogenic exchanger by circulating against the current of the gas to be liquefied by entering a port 61S3 and leaving through a third port 61E3 located on the warm side of the first stage 61 of the cryogenic exchanger. During the passage of the refrigerant mixture in this first stage against the current of the gas to be liquefied, the refrigerant mixture transmits heat energy to the gas to be liquefied. It should be noted that 40% of the overall cooling of the gas to be liquefied is carried out in the first stage 61, 50% of the overall cooling is carried out in the second stage 62 and 10% of the overall cooling is carried out in the third stage 63.

The light fraction FLE leaves from the first stage 61 through a port 61S1 through the first stage 61 co-current of the gas to be liquefied and then passes into an additional separator tank 14 to separate the flow of light fraction FLE refrigerant mixture in two distinct streams: a first lightest fraction flow FLE1 and a second largest fraction stream FLE2. This separating separator tank 14 makes it possible, on the one hand, to separate the heavier fraction FLE2 from the refrigerant mixture, the latter being intended for cooling the second stage 62, of the lightest fraction FLE1 of the cooling mixture intended for the cooling of the third stage 63. On the other hand, this separating separator balloon 14 also makes it possible to separate the flow of light fraction FLE into two streams, FLE1 and FLE2 in a simple way to have a good technical-economic compromise. The first flow of the lightest fraction FLE1 passes in the second stage 62 of the main cryogenic exchanger entering through a 62E1 orifice and out through a 62S1 orifice. And the second largest fraction flow FLE2, meanwhile, passes into the second stage 62 entering through a 62E2 orifice and out through a 62S2 orifice. These two flows FLE1 and FLE2 pass through the second stage 62 at the co-current of the gas to be liquefied. After exiting through the orifice 62S2, the heaviest fraction flow FLE2 is expanded by a second expander d2, and then connected, at the intersection C, to at least a portion of the flow of the refrigerant mixture circulating counter-flow. gas stream to be liquefied and leaving the third stage 63 through the orifice 63E2. After this connection, the resulting flow is reinjected into the second stage 62 by entering through a 62S3 orifice, and out through a 62E3 orifice while flowing in this second stage against the current of the gas to be liquefied to exchange heat energy. with the latter. This flow is then connected, at the intersection B, FLO heavy fraction flows out of the first expander dl to reconstitute the entire refrigerant mixture.

The first stream of the lightest fraction FLE1 leaving the second stage 62 through a port 62S1 then passes through a third stage 63 at the cocurrent of the gas to be liquefied by entering through an orifice 63E1, and out through a port 63S1. After having left this third stage 63, the first lightest fraction flow FLE1 is expanded in a third expander d3, then reinjected into an orifice 63S2 of this third stage 63 to circulate in this third stage against the current of the gas. to liquefy and thus transmit heat energy to the gas to be liquefied. Then, this first lightest fraction flow FLE1 exits from this third stage 63 via a port 63E2 before being connected, at the intersection C, to the heaviest fraction flow FLE2 coming out of the second expander d2. At this intersection C, the light fraction FLE of the refrigerant mixture is reconstituted. Then, the reconstituted FLE light fraction of the refrigerant mixture passes through the second stage 62 countercurrently with the gas to be liquefied by entering through a port 62S3 and exiting through a port 62E3.

Then the reconstituted FLE light fraction is connected, at the intersection B, to the heavy fraction FLO of the refrigerant mixture leaving the first expander d1. In this way, the entire refrigerant mixture is well reconstituted, as mentioned before, and passes through the first stage 61 against the current of the gas to be liquefied by entering through a port 61S3 and out through a port 61E3 before returning to the phase treatment.

FIG. 2 describes a second embodiment of a liquefaction plant of a gas according to the invention which differs from the first, in particular in that the main cryogenic exchanger comprises four stages 61 ', 62', 63 'and 64 in cascade, and in that the MCO liquid fraction from the separator tank 11 is, in turn, pumped by a pump 50 and injected into an orifice 61E3 'of the hot side of the first stage 61' of the main cryogenic exchanger .

More specifically, in this second preferred embodiment, the first stage 61 of the main cryogenic exchanger used in the first preferred mode is divided into two stages: a first stage 61 'and a second stage 62'. The first stage 61 'of the cryogenic exchanger used in this second preferred embodiment is located between ambient temperature and a temperature T1 typically between 0 and 10 ° C. And the second stage 62 'is located between a temperature Tl' and a temperature T0 between Tl 'and -60 ° C. Then, the third stage 63 ', corresponding to the second stage 62 of the main cryogenic exchanger of the first preferred embodiment, is located between the temperature T0 and a temperature T1 between-140 ° C and -100 ° C. And, the fourth stage 64 ', corresponding to the third stage 63 of the main cryogenic exchanger of the first preferred embodiment, is located between the temperature T1 and T2 between -165 ° C and -140 ° C. It should be noted that in this second embodiment, the injection of the liquid fraction MCO directly at the level of the first stage 61 'of the main cryogenic exchanger makes it possible to improve the energy performance of the hottest section of the line. exchange. In fact, more particularly, this second variant of embodiment consists in making an additional fractionation of the heavy section of the refrigerant mixture by recovering at the outlet of the first aftercooler 31, the liquid fraction MCO of the refrigerant mixture which has condensed at temperature. room.

After the liquefied gas has circulated through the first two floors

61 'and 62' of the exchanger, it passes through a separation system SR which has the same functions as that of the first embodiment.

After passing through this separation system SR, the gas to be liquefied passes through the third stage 63 'of the main cryogenic exchanger and then through the fourth stage 64' of the exchanger. At the outlet of the fourth stage 64 ', a liquid 2, corresponding to the liquefied gas, is recovered in a tank 2 at a temperature below -160 ° C. and at atmospheric pressure.

It should be noted that, apart from the introduction of the liquid fraction MCO in the first stage 61 'of the main cryogenic exchanger, the refrigerant mixture treatment phase of this second embodiment is the same as that of the first mode of cooling. production.

The disparities between these two embodiments reside in particular in the configuration of the main cryogenic exchanger. More particularly, the liquid fraction MCO is injected into the first stage 61 'of the cryogenic exchanger and crosses it at the co-current of the gas to be liquefied by entering through an orifice 61Έ3' and exiting through an orifice 61'S3 '. Then, the liquid fraction MCO is expanded by a pressure reducer of 1 before being connected, at the intersection A ', to a part of the cooling mixture. exiting through a 62Έ3 'orifice of the second stage 62' and circulating in countercurrent of the gas to be liquefied. After this connection, the entire refrigerant mixture is reconstituted.

The heavy fraction FLO of the refrigerant mixture, after passing through the second aftercooler 32, for its part, circulates in the first stage 61 'of the main cryogenic exchanger at the co-current of the gas to be liquefied by entering through an orifice 61E'2 before exiting through an orifice 61S'2. Then, this heavy fraction FLO is injected into the second stage 62 'entering through a 62Έ2' orifice. The heavy fraction FLO passes through this second stage 62 'always co-current of the gas to be liquefied and out through an orifice 62' S2 'before being relaxed by a second expander of 2. After this expansion, at an intersection B ', the heavy fraction FLO is connected to at least a part of the refrigerant mixture circulating against the current of the gas to be liquefied and exiting through a 63Έ3 orifice of the third stage 63' of the main cryogenic exchanger.

The light fraction FLE of the refrigerant mixture leaving the third aftercooler 33 is, for its part, introduced into the first stage 61 'of the main cryogenic exchanger via an orifice 61ΕΊ. Then, this light fraction FLE passes through the first stage 61 'co-current of the gas to be liquefied and out through an orifice 61S1' before being injected into the second stage 62 'through a hole 62ΈΊ. This light fraction FLE circulates in this second stage 62 ', always at the co-current of the gas to be liquefied without exchanging heat energy with the latter, and exits through an orifice 62'S'l. Then, this light fraction FLE passes into an annex separator tank 14 which separates the stream of light fraction FLE into two distinct streams: a first stream of the lightest fraction FLE1 and a second stream of the largest fraction FLE2.

The first lightest fraction flow FLE1 passes into the third stage 63 'of the main cryogenic exchanger entering through a 63'El orifice and exiting through a 63'S1 orifice. And the second most fractional flow Heavy FLE2, meanwhile, passes into the third stage 63 'entering through a 63Έ2 orifice and out through a 63'S2 orifice. These two flows FLE1 and FLE2 pass through the third stage 63 'co-current of the gas to be liquefied. After having left via the orifice 63'S2, the heavier fraction flow FLE2 is expanded by a third expander of 3 and then reinjected into the third stage 63 'entering through an orifice 63'S3 and then exiting through a 63Έ3 orifice while circulating in this third stage against the current of the gas to be liquefied to exchange heat energy with the latter. This flow is then connected, at the intersection B ', to the flows of heavy fraction FLO leaving the second expander of 2.

The first flow of the lightest fraction FLE1 leaving the third stage 63 'through an orifice 63'S1 passes through a fourth stage 64' cocurrent of the gas to be liquefied by entering through a hole 64'El, and out through a hole 64'S1. After having left this fourth stage 64 ', the first lightest fraction flow FLE1 is expanded in a fourth expander of 4, and then re-injected into an orifice 64'S2 of this fourth stage 64' to circulate in this fourth stage 64 against the current of the gas to be liquefied and thus transmit heat energy thereto. Next, this first lightest fraction flow FLE1 exits this fourth stage 64 'via a 64Έ2 orifice before being connected, at the intersection C, to the heaviest fraction flow FLE2 coming out of the third expander. 3. At this intersection C, the light fraction FLE of the refrigerant mixture is reconstituted.

Then, the reconstituted FLE light fraction of the refrigerant mixture passes through the third stage 63 'counter-current of the gas to be liquefied by entering through a 63'S3 orifice and exiting through a 63Έ3 orifice.

Then the reconstituted FLE light fraction is connected, at the intersection B ', to the heavy fraction FLO of the refrigerant mixture leaving the second expander of 2. In this way, the light FLE and heavy FLO fractions of the refrigerant mixture are connected. All light fractions FLE and heavy FLO then crosses the second stage 62 'against the current of the gas to be liquefied by entering through a 62'S3' orifice and out through a 62Έ3 'orifice before being connected, at the intersection A' to the MCO liquid fraction leaving the first I expander. In this way, as mentioned before, the entire refrigerant mixture is reconstituted.

Then, this reconstituted refrigerant mixture passes through the first stage 61 'in countercurrent of the gas to be liquefied by entering through an orifice 61'S4' and exiting through an orifice 61Έ4 'before being treated in the secondary circuit closed by the phase treatment.

Examples

The following examples illustrate the invention without, however, limiting its scope. These numerical simulations make it possible to calculate, for the method according to the invention, the following parameters:

the mechanical power used by the process, in kW;

- the specific consumption in kWh / t of LNG produced, the specific consumption in kWh / t being defined as follows:

Mechanical power used (in kW) _ ^

Mass flow rate of LNG produced (in t / h)

the energy consumption gain compared to the known method of the prior art comprising neither a reboiler nor an intermediate loop connecting two heat exchangers;

by comparison with the results that would be obtained with a known method according to the prior art having neither a reboiler nor an intermediate loop connecting two heat exchangers so as to reuse the heat energy released by a compressor of the liquefaction device . In particular, the numerical simulations were carried out with respect to the method described in the French patent FR2703762 (illustrated on the Figure 1 of FR2703762), it is a liquefaction process of natural gas type SMR (Single Mixed Refrigerant) representative of a good compromise between simplicity of design and energy consumption performance. As a pure closed cycle, it does not consume liquid nitrogen to cool the natural gas.

These numerical simulations were carried out under the following conditions:

• Entry gas to liquefy:

o Pressure: 48 bar,

o Temperature: 30 ° C

o Mass flow: 10.18 t / h

o Composition (mol%):

^■ N ₂ : 0.79%

^■ Ci: 91.21%

^■ C ₂ : 7.89%

^■ 0: 0.11%

^■ C ₄₊ : 0

• Minimum cooling temperature of the refrigerant with the surrounding environment: 30 ° C

• Polytropic compressor efficiency assumptions: 85%

• LNG subcooling temperature: -158 ° C.

To perform such numerical simulations, the simulation tool that has been used is aspen hysys V7.3, and the thermodynamic model (or equation of state) is: SRK LK-1 (soave Redlich-Kwong Lee Kesler

1) ·

The results of the simulations are presented in Table 1 below: Table 1

These results show that, compared to a process involving neither a reboiler nor an intermediate loop connecting two heat exchangers, the method according to the invention makes it possible to reduce the mechanical power required for liquefaction by more than 6% compared to standard method of the patent FR2703762, which allows on the one hand a reduction in operating costs related to energy consumption, and on the other hand a reduction in the size of the compressor and therefore their associated capital cost.

Claims

A method of liquefying a gas comprising predominantly methane, wherein said gas to be liquefied flows in a primary circuit from a source (1) to a tank (2) for liquefied gas, and wherein a refrigerant mixture circulates in a a closed secondary circuit, said method comprising the following phases:

a cooling / liquefaction phase in which said gas to be liquefied is cooled and liquefied following its circulation in a main cryogenic exchanger comprising at least three stages (61, 62, 63), arranged in cascade, in which said gas is in direct contact with a refrigerant mixture whose composition evolves according to the cooling stage:

The first cooling being carried out in a first stage (61) between the ambient temperature and a temperature T 0 equal to or greater than -60 ° C.,

The second cooling being carried out in a second stage (62) between T 0 and a temperature T1 between -140 ° C. and -100 ° C., and

The third cooling being carried out in a third stage (63) between the temperature T1 and a temperature T2 between -165 ° C. and -140 ° C.,

Compressing and cooling said cooling mixture leaving the first stage (61) of said main cryogenic exchanger by passages in at least one first compressor (21), then a first aftercooler (31) which partially condenses the mixture and then in a separator tank (11) in which the liquid fraction (MCO) of the refrigerant mixture is separated from its fraction steam (VCO);

the liquid fraction (MCO) of the refrigerant mixture is pumped by a pump (50), while the vapor fraction (VCO) of the refrigerant mixture is compressed by a second compressor (22), then cooled in a first heat exchanger (40a), the streams of liquid fraction (MCO) and vapor fraction (VCO) being at the same pressure;

at least partly separating the compressed refrigerant mixture leaving the second compressor (22) and then the first heat exchanger (40a) in a distillation column (12) provided with a reboiler (13) in a light fraction (FLE) and a heavy fraction (FLO);

supplying said reboiler (13) with heat energy to generate a vapor stream (FVA) and extract at least a portion of the volatile compounds initially included in the bottom stream (12);

cooling or post-cooling, with the environment, said heavy fractions (FLO) leaving the reboiler (13) by routing them through a second aftercooler (32) before introducing them into the first stage (61) of the main cryogenic exchanger; injecting said light fractions (FLE) from said distillation column (12) into the first stage (61) of the main cryogenic exchanger by forwarding them in a third compressor (23) and a third aftercooler (33). ; said method being characterized in that the heat energy supplied to the reboiler (13) derives directly from the overheating caused by the compression of said cooling mixture in said second compressor (22), said heat energy being supplied to the reboiler (13) at the using an intermediate loop (40) connecting two heat exchangers (40a; 40b), the first heat exchanger (40a) recovering the heat energy released by the compression performed in said compressor (22), which is retransmitted by the intermediate loop (40) to the second heat exchanger (40b) to transmit it to the reboiler (13).

2. Method according to claim 1, wherein said liquid fraction (MCO) of the refrigerant mixture from the separator tank (11) is reinjected directly to the outlet of said first heat exchanger (40a).

3. The method of claim 1, wherein said liquid fraction (MCO) of the refrigerant mixture from the separator tank (11) is reinjected directly into the first stage (61) of the main cryogenic exchanger.

4. Method according to any one of claims 1 to 3, wherein the gas to be liquefied contains methane in a molar proportion of at least 80%.

5. Process according to any one of claims 1 to 4, wherein the cooling mixture is a mixture comprising essentially nitrogen and C 1 -C 5 alkanes.

6. Liquefaction plant for a gas comprising a primary circuit connected to a source (1) of gas and a tank (2) for liquefied gas, the installation comprising:

at least one main cryogenic exchanger comprising at least three stages (61, 62, 63) arranged in cascade for cooling and liquefying the gas flowing in the primary circuit,

A first compressor (21) and a first aftercooler (31) upstream of a separator tank (11),

A second compressor (22) located between said separator tank (11) and a distillation column (12),

A reboiler (13) integrated in said distillation column (12), and

A third compressor (23) and a third aftercooler (33) at the top of said distillation column (12), characterized in that the installation further comprises an intermediate loop (40) connecting two heat exchangers ( 40a; 40b), the first heat exchanger (40a) recovering the heat energy released by the second compressor (22), which is retransmitted by the intermediate loop (40) to the second heat exchanger (40b) for transmission to the reboiler (13). ).