EP4244557A1

EP4244557A1 - Method for extracting ethane from an initial natural gas stream and corresponding plant

Info

Publication number: EP4244557A1
Application number: EP21810002.2A
Authority: EP
Inventors: Sylvain Vovard; Benoit LAFLOTTE
Original assignee: Technip Energies France SAS
Current assignee: Technip Energies France SAS
Priority date: 2020-11-10
Filing date: 2021-11-09
Publication date: 2023-09-20
Also published as: KR20230107567A; CN116783438A; FR3116109A1; JP2023548920A; FR3116109B1; WO2022101211A1; AU2021379956A1; US20230408191A1

Abstract

This method comprises the following steps: - recovering and compressing a top stream (98) from a separating column (34) in order to form a compressed purified natural gas stream (102); - liquefying the compressed purified natural gas stream (102) in a liquefaction unit (24) in order to form a stream (120) of pressurized liquefied natural gas; - flash expansion of the pressurized liquefied natural gas stream (120) and recovery in a storage tank (66); - recovery and compression of a flash gas stream (126) resulting from the expansion; - separating the compressed flash gas stream (132) into a fuel stream (20) and into a recycle stream (134); - cooling and expansion of the recycle stream (134), then introducing the cooled and expanded recycle stream at a top stage of the separating column (34).

Description

TITLE: Process for extracting ethane from a starting natural gas stream and corresponding installation

The present invention relates to a process for extracting ethane from a starting natural gas stream, comprising the following steps:

- Cooling of the starting natural gas stream in at least one first upstream heat exchanger, to form a cooled natural gas stream;

- Separation of the cooled natural gas stream into a liquid stream and a gas stream;

- expansion of the liquid stream and introduction of at least one stream from the liquid stream into a column for separating methane and C2+ hydrocarbons, at a first level;

- formation of a turbine feed stream from the gas stream;

- expansion of the turbine feed stream in a dynamic expansion turbine and introduction of the expanded stream from the dynamic expansion turbine into the separation column at a second level,

- introduction of a bottoms stream rich in C2+ hydrocarbons recovered from the separation column into a fractionation column, and recovery, from the fractionation column, of an ethane stream;

- recovery and compression of at least part of a top stream from the separation column to form a stream of compressed purified natural gas;

- Liquefaction of the compressed purified natural gas stream in a liquefaction unit to form a pressurized liquefied natural gas stream;

- flash expansion of the pressurized liquefied natural gas stream and recovery of expanded liquefied natural gas in storage;

- recovery of at least one flow of flash gas from the expansion of the pressurized liquefied natural gas stream;

- compression of the or each flow of flash gas.

Such a process is intended in particular to extract ethane and C3+ hydrocarbons from an initial natural gas, while producing a pressure-treated natural gas, which is then liquefied before being expanded with a view to its storage.

Ethylene, ethane, propylene, propane and heavier hydrocarbons can be extracted from gases such as natural gas, refinery gas and synthetic gases obtained from other hydrocarbon sources such as coal, crude oil, naphtha. Natural gas generally contains a majority of methane and ethane (for example, methane and ethane make up at least 50 molar percent of the gas). Natural gas may also contain more negligible quantities of heavier hydrocarbons such as propane, butanes, pentanes and also hydrogen, nitrogen and carbon dioxide.

The invention described here relates more particularly to the recovery of ethane, propane and heavier hydrocarbons from natural gas. In addition to the fact that the heavy hydrocarbons present in natural gas, such as ethane, propane and butane can be highly recovered by marketing them separately with a high purity, they risk condensing during transport or freezing in liquefaction exchangers (for the heaviest of them).

This can cause incidents, such as the arrival of liquid plugs in transport facilities or shutdowns of the liquefaction plant to unclog frozen exchangers.

US 6,578,379 describes a very efficient process for recovering ethane and propane from a natural gas stream. Such a process generally works very efficiently, in particular to obtain a very thorough extraction (for example greater than 99% molar) of the ethane contained in the natural gas feedstock, while minimizing energy consumption.

To obtain such extraction yields, it is known to use in main reflux, namely for the highest reflux of the column for separating methane and ethane, a stream very depleted in ethane.

For this purpose, a recirculation stream is taken from the recompressed gas from the head of the methane and ethane separation column. The recirculation stream is cooled counter-current to the gas coming from the column head, then is expanded to form the main reflux introduced at the column head.

However, under certain operating conditions, the quality of the main reflux is liable to deteriorate in temperature and/or in composition.

For example, if the main reflux becomes depleted in methane, the rate of ethane separation in the column decreases, and the quality of the overhead stream produced at the top of the column deteriorates further, aggravating the methane depletion of the main reflux. A “snowball” effect occurs, causing a significant decrease in the rate of ethane extraction. This may be the case in particular in the event of liquid entrainment on the upper plates of the column.

An object of the invention is to have a flexible and very efficient process for extracting ethane and C3+ hydrocarbons from a starting natural gas stream, wherein the rate of ethane extraction is not or little affected when fluctuations in the quality of the separation column head occur.

To this end, the subject of the invention is a method of the aforementioned type, characterized by the following steps:

- Separation of the compressed flash gas stream into a fuel stream and a recycling stream;

- Cooling and at least partial expansion of the recycle stream, then introduction of the cooled and expanded recycle stream to a top stage of the separation column.

The process according to the invention may comprise one or more of the following characteristics, taken separately or in any technically possible combination:

- the methane content of the recycling stream is greater than 90% molar, in particular greater than 95% molar;

- the recycling stream is introduced at a first stage starting from the top of the separation column;

- the recycle stream is introduced and cooled in the first heat exchanger by heat exchange with the overhead stream from the separation column;

- it comprises the separation of the gas stream into the turbine feed stream, introduced into the dynamic expansion turbine, and into a reflux stream, introduced into the separation column, lower than the recycling stream, after cooling in a second upstream heat exchanger and static expansion of the reflux stream;

- The cooling of the recycle stream comprises the passage of the recycle stream in the second heat exchanger;

- the expansion of the recycle stream comprises passing the recycle stream through a static expansion valve;

- at least a portion of the compressed purified overhead natural gas is placed in heat exchange relationship with the flow of flash gas in a downstream heat exchanger;

- it includes the withdrawal from the stream of compressed purified natural gas, upstream of the liquefaction unit, of a recirculation stream, the recirculation stream being cooled, expanded, and introduced into the separation column;

- the pressurized liquefied natural gas stream is expanded in a dynamic or static expansion device, then is introduced into a flash tank, to be separated into the expanded liquefied natural gas introduced into the storage, and into a flow of flash;

- at least one flash gas flow is formed in the storage, during the introduction of the expanded liquefied natural gas into the storage; - the pressurized liquefied natural gas stream is introduced directly into the storage, without passing through a flash tank;

- the compression of the overhead stream from the separation column takes place in at least a first compressor coupled to the dynamic expansion turbine then in a compression machine successively comprising a second compressor, a cooler for the gas compressed in the second compressor , and a third compressor, for forming the stream of compressed purified natural gas;

- an overhead stream from the fractionation column is cooled and partially condensed, then is introduced into an overhead flask, the ethane stream being recovered at the top of the overhead flask, the foot of the overhead flask being reintroduced under reflux in the fractionating column;

- the entire gas stream resulting from the separation of the cooled natural gas stream forms the turbine feed stream which is sent to the dynamic expansion turbine without separation.

The invention also relates to an installation for extracting ethane from a starting natural gas stream, comprising:

- At least one first upstream heat exchanger capable of cooling the starting natural gas stream, to form a cooled natural gas stream;

- a separator for separating the cooled natural gas stream into a liquid stream and a gaseous stream;

- a liquid flow expansion device;

- a column for separating methane and C2+ hydrocarbons and an assembly for introducing at least one stream from the expanded liquid stream into the separation column, at a first level;

- an assembly for forming a turbine feed stream from the gas stream;

- a dynamic expansion turbine capable of expanding the turbine feed stream and an assembly for introducing the expanded stream from the dynamic expansion turbine into the separation column at a second level;

- a fractionation column, an assembly for introducing a bottoms stream rich in C2+ hydrocarbons from the separation column into the fractionation column, and an assembly for recovering, from the fractionation column, a stream of ethane;

- an assembly for recovering and compressing at least part of a top stream from the separation column to form a stream of compressed purified natural gas; - A unit for liquefying the stream of compressed purified natural gas suitable for forming a stream of liquefied natural gas under pressure;

- a set of flash expansion of the stream of pressurized liquefied natural gas and a recovery storage of expanded liquefied natural gas;

- an assembly for recovering at least one flow of flash gas from the expansion of the pressurized liquefied natural gas stream;

- a set for compressing the or each flow of flash gas, characterized by:

- an assembly for separating the flow of compressed flash gas into a fuel stream and into a recycle stream;

- an assembly for cooling and at least partial expansion of the recycling stream, and for introducing the cooled and expanded recycling stream to a top stage of the separation column.

The invention will be better understood on reading the following description, given solely by way of example, and made with reference to the appended drawings, under which:

[Fig 1] Figure 1 is a block diagram representing a first installation for implementing a first ethane extraction process according to the invention;

[Fig 2] Figure 2 is a diagram similar to that of Figure 1, of a second installation for the implementation of a second ethane extraction process according to the invention;

[Fig 3] Figure 3 is a diagram similar to that of Figure 1, of a third installation for the implementation of a third ethane extraction process according to the invention;

[Fig 4] Figure 4 is a diagram similar to that of Figure 1, of a fourth installation for the implementation of a fourth extraction method according to the invention;

[Fig 5] Figure 5 is a diagram similar to that of Figure 2, of a fifth installation for the implementation of a fifth extraction process according to the invention.

In what follows, the same reference will designate a flow of liquid and the pipe which conveys it, the pressures considered are absolute pressures, and the percentages considered are molar percentages.

The processes described were modeled on a process simulator. Efficiencies of 82% polytropic for the compressors and 86% adiabatic for the turbines have been defined.

A first ethane extraction installation 10 according to the invention is shown in FIG. This installation 10 is intended for the simultaneous production, from a starting natural gas stream 12, of an ethane-rich stream 14, of a foot stream 16 rich in C3+ hydrocarbons, of a natural gas expanded liquefied 18, and a fuel stream 20, advantageously intended to be reused in the installation 10.

Referring to Figure 1, the installation 10 comprises an ethane extraction unit 22, a liquefaction unit 24, and a flash and liquefied natural gas storage unit 26.

The extraction unit 22 comprises first and second upstream heat exchangers 28, 30, a separator drum 32, and a column 34 for separating methane and C2+ hydrocarbons. Column 34 is here fitted with a bottom reboiler 35.

Unit 22 further comprises a dynamic expansion turbine 36 coupled to a first compressor 38, a second compressor 40, each compressor 38, 40 being provided downstream with a cooler 42, 44.

Unit 22 further comprises a bottoms pump 46, a fractionation column 48, equipped with a bottoms reboiler 50 and a reflux system 52, the reflux system 52 comprising a cooler 54, a reflux drum 56, and a reflux pump 58.

The natural gas liquefaction unit 24 is a known unit, in particular of the C3MR or DMR type.

In the example of Figure 1, the flash and storage unit 26 comprises an expansion device 60, here a dynamic expansion turbine, a flash balloon 62, and a pump 64 for conveying liquefied natural gas to a storage 66. Alternatively, the expansion device 60 is a static expansion valve.

Storage 66 is for example a thermally insulated storage tank.

The flash and storage unit 26 further comprises in this example a downstream heat exchanger 68, possibly a suction drum 70, and a compression device 72 comprising a plurality of compressors 74 mounted in series, separated from each other by coolers 76.

A first method according to the invention, implemented in the installation 10, will now be described.

The starting natural gas forming stream 12 is advantageously a desulfurized, dry and at least partially decarbonated natural gas.

The term "at least partially decarbonated" means that the carbon dioxide content in the starting natural gas stream 13 is advantageously less than or equal to 50 ppmv.

Likewise, the water content is less than 1 ppmv, advantageously less than 0.1 ppmv. The content of sulfur elements, including hydrogen sulphide, is less than 10 ppmv and advantageously less than or equal to 4 ppmv.

An example of the molar composition of the starting natural gas stream 12 is given in the table below.

[Table 1]

More generally, the molar fraction of methane in the starting natural gas stream 12 is between 75% molar and 95% molar, the molar fraction of C2 hydrocarbons is between 3% molar and 12% molar, and the molar fraction in C3+ hydrocarbons is between 1% molar and 8% molar.

The flow rate of the starting natural gas stream 12 is for example greater than 2000 kmol/h and is for example between 2000 kmol/h and 70000 kmol/h, in particular equal to 55,000 kmol/h.

The starting natural gas stream 12 has a temperature close to ambient temperature, in particular between 0° and 40° C., here equal to 21.5° C. and a pressure advantageously greater than 35 bars, in particular greater than 70 bars, in this example equal to 81 bars.

The starting natural gas 12 is introduced into the first heat exchanger 28 to be cooled there. It forms a stream 80 of cooled natural gas. The starting natural gas 12 is here supercritical, it is therefore simply cooled. In a variant, it is not supercritical and it is at least partially condensed in the first heat exchanger 28.

It has a temperature below -20°C, and in particular between -25°C and -45°C, in particular equal to -37°C.

Stream 80 is then introduced into separator drum 32, there to be separated into a liquid stream 82, recovered at the foot of separator drum 32, and a gas stream 84 recovered at the top of separator drum 32. The flow rate of liquid stream 82 can be zero, especially when the cooled natural gas stream 80 is supercritical. The liquid stream 82 passes through a static expansion valve 86, to form an expanded mixed phase 88. The pressure of the expanded mixed phase 88 is less than 50 bars, in particular less than 30 bars, and is for example equal to 28.7 bars. The expanded mixed phase 88 is introduced at a bottom level N1 of the separation column 34.

The gas stream 84 is split into a main turbine feed stream 90 and a secondary reflux stream 92.

The molar flow rate of the turbine feed stream 90 is greater than the molar flow rate of the reflux stream 92, and in particular between 5% and 25% of the molar flow rate of the reflux stream 92.

The turbine feed stream 90 is introduced into the dynamic expansion turbine 36 to be expanded there at a pressure of less than 50 bars, in particular less than 30 bars, for example equal to 28.7 bars.

The dynamic expansion of current 90 can recover more than 10,000 kW of energy, for example 10,865 kW of energy.

The temperature of the cooled and expanded stream 94 coming from the dynamic expansion turbine 36 is for example less than -70°C, in particular less than -80°C, for example equal to -80.8°C.

The cooled and expanded stream 94 is then introduced into the separation column 34 at a level N2 located above the level N1.

The reflux stream 92 is introduced into a static expansion valve 96 to be expanded there at a pressure of less than 50 bars, in particular less than 30 bars, in particular equal to 28.7 bars. It is cooled in the second upstream heat exchanger 30 to a temperature below -80°C, in particular below -90°C, in particular equal to -95.8°C.

The expanded and cooled reflux stream is introduced into the separation column 34 at a level N3 located above the level N2 at the top of the column 34.

The pressure of the separation column 34 is preferably between 10 bars and 40 bars, in particular between 20 bars and 40 bars, for example substantially equal to 28.5 bars.

The separation column 34 produces an overhead stream 98. The overhead stream 98 is reheated in the second upstream heat exchanger 30, then in the first upstream heat exchanger 28 in countercurrent with the starting natural gas warmed head 100.

The temperature of the reheated overhead stream 100 is greater than 0°C, in particular greater than 15°C, and is for example equal to 17.6°C. The heated overhead stream 100 is then compressed in the compressor 38 coupled to the turbine 36, then is cooled in the cooler 42, to obtain a stream at a pressure greater than 30 bars, in particular equal to 34.6 bars.

It is then recompressed in compressor 40 and then cooled in cooler 44 to produce a stream of compressed purified natural gas 102 for liquefaction unit 24.

The stream of compressed purified natural gas 102 has a pressure greater than 60 bars, in particular greater than 80 bars, for example equal to 91 bars. It has a temperature greater than 0°C, in particular greater than 10°C, in particular equal to 21.5°C.

The coolers 42, 44 are here fed by a cooling flow with a temperature of less than 10°C, in particular equal to 7°C. This cooling flow may in particular be air or water.

The compressed purified natural gas stream 102 is rich in methane. It comprises a methane content greater than 99.0% molar, in particular equal to 99.1% molar. It has a low nitrogen content, in particular less than 1.0 mol%, and a low content of C2+ hydrocarbons, in particular a content of less than 0.5 mol% of ethane, substantially equal to 0.2 mol% of ethane. .

The separation column 34 produces at the bottom a bottom stream 106 rich in C2+ hydrocarbons. This stream 106 contains for example more than 95 molar % of the ethane contained in the starting natural gas 10, and 100 molar % of the C3+ hydrocarbons contained in this stream.

Bottom stream 106 has a temperature above 10°C, in particular between 20°C and 30°C, for example equal to 23.2°C. It contains less than 1000 ppmv of carbon dioxide, preferably between 200 ppmv and 500 ppmv of carbon dioxide, for example 313 ppmv of carbon dioxide. It has a methane content of less than 5% molar, for example between 0% molar and 3% molar, in particular less than 1% molar.

The table below illustrates an example of the composition of the background current 106. [Table 2]

A first lateral reboiling stream 108 is extracted from the separation column 34, at a level N5 lower than the level N1, for example located at the ^20th floor from the top of the separation column 34.

The first reboiling liquid stream 108 is brought to the first heat exchanger 28, to be heated there in this exchanger 28 by heat exchange in particular with the starting natural gas 12, up to a temperature greater than 0° C., in particular equal to 8 .25°C. The reboiling stream 108 is then reintroduced into the separation column 34 at a level N6 located below the level N5, for example at the ^21st stage starting from the top of the column 34.

Similarly, a second reboiling liquid stream 110 is extracted from the separation column 34 at a level N7 lower than the level N6, for example from the ^22nd stage starting from the top of the separation column 34, to be brought to the bottom reboiler 35 in order to be heated there to a temperature above 0°C, for example equal to 10.7°C. An energy greater than 1 MW, for example equal to 4 MW, is supplied to the second reboiling liquid stream 110.

The second reboiling liquid stream 110 is then returned to the separation column 34 at a level N8 located below the level N7. This level N8 is for example located on the ^23rd floor from the top.

Underflow 106 is pumped into pump 46 to be introduced at an intermediate level P1 of fractionating column 48.

The fractionating column 48 produces at the top an overhead stream 112 containing less than 1% molar of C3+ hydrocarbons, in particular less than 1% molar of propane.

The overhead stream 112 is partially condensed in the cooler 54, then is separated in the reflux drum 56 to form the ethane-rich stream 14 at the top, and at the bottom, a liquid reflux stream 114 reintroduced at the top of the fractionation column 48, after pumping by the reflux pump 58. The ethane-rich stream 14 contains more than 96 molar % of the ethane contained in the starting natural gas 12. It contains more than 97 molar % of ethane.

The ethane-rich stream 14 is gaseous here. Alternatively (not shown), the ethane-rich stream 14 is a liquid taken from the liquid stream 114.

The C3+ hydrocarbon stream contains less than 500 ppmv of ethane, in particular less than 100 ppmv of ethane.

The stream of compressed purified natural gas 102 is fed into the liquefaction unit 24 which produces, in a known manner, a stream of pressurized liquefied natural gas 120.

The pressurized natural gas stream has a pressure greater than 20 bars, in particular between 20 bars and 90 bars, advantageously equal to 73 bars. It has a temperature below -120°C, in particular below -130°C, and advantageously equal to -136.8°C.

The compressed liquefied natural gas 120 is introduced into the expansion device 60, here in a dynamic expansion turbine. It is expanded to a pressure of less than 5 bars, in particular less than 2 bars, for example equal to 1.25 bars to form a stream of flashed liquefied natural gas 122.

The flashed liquefied natural gas stream 122 is introduced into a flash drum 62 to be separated therein into an expanded liquefied natural gas stream 124 and a first flash gas stream 126.

The expanded liquefied natural gas stream 124 is pumped into the storage 66 using the pump 64 to form the expanded liquefied natural gas 18.

The first flow of flash gas 126 is recovered at the head of the flash tank 62. It is introduced into the downstream heat exchanger 68 to be heated there against the current of a part of the compressed purified natural gas 102, which is reintroduced in the stream of flashed liquefied natural gas 122, upstream of the flash tank 62.

After heat exchange in the downstream heat exchanger 68, the heated flash gas flow 130 thus formed has a temperature greater than -60°C, and in particular substantially equal to 5°C. It has a very high methane content, for example greater than 80% molar, for example greater than 85% molar, in particular greater than 90% molar. This content is advantageously greater than 95% molar methane, in particular greater than 96% molar methane, for example equal to 96.46% molar methane.

It has a nitrogen content of less than 20% molar, for example less than 15% molar, in particular less than 10% molar. This content is advantageously less than 5% molar, in particular less than 4% molar, for example substantially equal to 3.54% molar of nitrogen.

The heated flash gas flow 130 has an ethane content of less than 50 ppmv, in particular less than 10 ppmv, for example equal to 5 ppmv.

After possibly passing through a suction balloon 70, the heated flash gas stream 130 is compressed in the compression device 72 to a pressure greater than 25 bars, in particular greater than 30 bars, and for example equal to 60 bars to produce a flow of compressed flash gas 132.

Compressed flash gas stream 132 is separated into fuel stream 20 and recycle stream 134.

The fuel stream 20 is intended to be sent to the fuel gas network of the installation 10 to supply, for example, gas turbines of the natural gas liquefaction unit 24 or those of an electric current generation unit intended for example to supply the compressor 40 or other equipment of the installation 10.

The recycling stream 134 has a pressure greater than 30 bars, in particular greater than 50 bars, for example equal to 58.5 bars.

It is conveyed successively in the first heat exchanger 28, then in the second heat exchanger 30 to be cooled there to a temperature below - 80° C., in particular below -90° C., for example equal to -95.5° C. .

This recycling stream 134 is then expanded in a static expansion valve 136 to a pressure of less than 50 bars, in particular less than 30 bars, for example equal to 28.7 bars, to be introduced into the separation column 34 at a top level N9 of column 34, for example on the first floor starting from the top of column 34. Level N9 is located above level N3 for the introduction of the expanded and cooled reflux stream.

As indicated above, the recycle stream 134 from the flash gas stream 126 is very rich in methane, since the ethane remains in the liquefied natural gas 18, or is successively extracted in the separation column 34, then in the fractionation column 48.

Thus, the composition of the reflux introduced at the top of the separation column 34 remains very rich in methane, regardless of the fluctuations in quality of the overhead stream 98 of the separation column 34.

The presence of this new reflux also provides operational flexibility during the implementation of the process, but also in the design phase. It is thus possible to globally optimize the energy consumption between the ethane extraction unit 22 and the liquefaction unit 24 to adjust the parameters of the two units 22, 24, in order to best select the compressors and their mode. drive required in the two units 22, 24. This significantly reduces the investment costs, and also the operating costs as will be seen in the example described below.

Alternatively (not shown), the heated overhead stream 100 is compressed at the output of the compressor 38 coupled to the turbine 36 in a compression machine comprising two compression stages of the same power, the total power being equal to that of the compressor 40. The compression machine has an intercooler that cools the gas between the compression stages. The resulting arrangement provides a power saving of 5.8 MW.

A second installation 140 intended for the implementation of a second method according to the invention is shown in Figure 2.

The second method according to the invention is analogous to the first method according to the invention. It differs from the first method according to the invention in that it comprises a step of taking off, from the stream of compressed purified natural gas 102, a recirculation stream 142.

The molar flow rate of the recirculation stream 142 is advantageously lower than the molar flow rate of the residual compressed purified natural gas stream 102, after withdrawal of the recirculation stream 142, upon its introduction into the liquefaction unit 22.

The recirculation stream 142 has a pressure greater than 50 bars, in particular greater than 80 bars, for example equal to 90 bars. It is introduced successively into the first heat exchanger 28, then into the second heat exchanger 30 to be cooled there to a temperature below -90° C., preferably below -95° C. and for example substantially equal to -95.4 °C.

Then, the recirculation stream 142 is expanded to a pressure of less than 50 bars, in particular less than 30 bars, in particular equal to 28.7 bars and is introduced into the separation column 34 between the recycling stream 134 and the reflux stream 92.

A third installation 150 intended for the implementation of a third method according to the invention is shown in Figure 3.

The installation 150 differs from the first installation 10 in that it comprises a system 152 for collecting and recompressing the evaporated gases formed in the storage 66. The collection system 152 comprises a protective balloon 154, and a compression device 156 comprising a plurality of compression stages 158, spaced two by two by a cooler 160.

A second flow of flash gas 162 resulting from the evaporation of the liquefied natural gas in the storage 66 is collected at the head of the storage 66, then is introduced into the compression apparatus 156 to be compressed there at a pressure greater than 25 bars , in particular between 26 bars and 70 bars, for example equal to 60 bars.

The second flow of compressed flash gas 164 thus produced is separated into the fuel stream 20 and into the recycle stream 134, which is reintroduced into the separation column 34, after cooling in the heat exchangers 28, 30 and expansion in the expansion valve 136.

Advantageously, in the example shown in Figure 3, the installation 150 has no expansion device 60. The compressed liquefied natural gas 120 from the liquefaction unit 24 is directly introduced into the storage 66 of liquefied natural gas and is flashed in storage 66.

A fourth installation 170 intended for the implementation of a fourth method according to the invention is shown in Figure 4.

The fourth installation 170 differs from the first installation 10 in that the storage 66 are equipped, like the third installation 150, with a system 152 for collecting evaporated gases.

When implementing the fourth method according to the invention, the first stream of compressed flash gas 132 and the second stream of compressed flash gas 164 are mixed, before the mixture is separated into the fuel stream 20, and the recycle stream 134.

As before, the recycle stream 134 is reintroduced into the separation column 34 after passing through the heat exchangers 28, 30, then expansion in the static expansion valve 136.

A fifth installation 200 for the implementation of a fifth method according to the invention is illustrated in Figure 5.

The fifth process differs from the second process shown in Figure 2 in that all of the gas stream 84 recovered from drum 32 forms turbine feed stream 90 sent to dynamic expander 36, without separation.

Thanks to the invention which has just been described, it is possible to maintain a substantially constant composition of the reflux of the separation column 34, which avoids the snowball effect which occurs during fluctuations in the composition of the current of head 98 extracted from separation column 34, in the absence of supply from recycle stream 134.

This process is therefore both simple and effective in maintaining a constant ethane extraction content, without increasing investment costs or operating costs. The energy consumption of the process is detailed in the following table.

[Table 3]

As shown in this table, the total power consumed in the presence of reflux generated from the recycle stream 134 significantly decreases the power. consumed and the specified power reduced to the start of liquefied natural gas produced by the installation.

Claims

1 . Process for extracting ethane from a starting natural gas stream (12), comprising the following steps:

- cooling of the starting natural gas stream (12) in at least one first heat exchanger (28) upstream, to form a cooled natural gas stream (80);

- separation of the cooled natural gas stream (80) into a liquid stream (82) and a gaseous stream (84);

- Expansion of the liquid stream (82) and introduction of at least one stream from the liquid stream (82) into a column (34) for separating methane and C2+ hydrocarbons, at a first level (N1);

- formation of a turbine feed stream (90) from the gas stream (84);

- expansion of the turbine feed stream (90) in a dynamic expansion turbine (36) and introduction of the expanded stream (94) from the dynamic expansion turbine (36) in the separation column (34) to a second level (N2),

- introduction of a bottoms stream rich in C2+ hydrocarbons recovered from the separation column (34) into a fractionation column (48), and recovery, from the fractionation column (48), of a stream of ethane (14);

- recovery and compression of at least a portion of an overhead stream (98) from the separation column (34) to form a stream of compressed purified natural gas (102);

- liquefying the compressed purified natural gas stream (102) in a liquefaction unit (24) to form a pressurized liquefied natural gas stream (120);

- Flash expansion of the pressurized liquefied natural gas stream (120) and recovery of expanded liquefied natural gas (18) in a storage (66);

- recovery of at least one flash gas stream (126; 162) from the expansion of the pressurized liquefied natural gas stream (120);

- compression of the or each flow of flash gas (126; 162) characterized by the following steps:

- separation of the compressed flash gas stream (132; 164) into a fuel stream (20) and a recycle stream (134);

- cooling and at least partial expansion of the recycle stream (134), then introduction of the cooled and expanded recycle stream to a top stage of the separation column (34).

2. Method according to claim 1, in which the methane content of the recycle stream (134) is greater than 90% molar, in particular greater than 95% molar.

3. Method according to any one of claims 1 or 2, in which the introduction of the recycle stream (134) takes place at a first stage starting from the top of the separation column (34).

4. Method according to any one of the preceding claims, in which the recycle stream (134) is introduced and cooled in the first heat exchanger (28) by heat exchange with the overhead stream (98) from the separation column (34 ).

5. A method according to any preceding claim, comprising separating the gas stream (84) into the turbine feed stream (90), introduced into the dynamic expansion turbine (36), and a reflux stream (92 ), introduced into the separation column (34), lower than the recycle stream (134), after cooling in a second upstream heat exchanger (30) and static expansion of the reflux stream (92).

6. The method of claim 5, wherein cooling the recycle stream (134) includes passing the recycle stream (134) through the second heat exchanger (30).

7. A method according to any preceding claim, wherein expanding the recycle stream (134) includes passing the recycle stream (134) through a static expansion valve (136).

8. A method according to any preceding claim, wherein at least a portion of the compressed purified overhead natural gas (102) is placed in heat exchange relationship with the flash gas stream (126) in a downstream heat exchanger ( 68).

9. Method according to any one of the preceding claims, comprising the withdrawal from the stream of compressed purified natural gas (102), upstream of the liquefaction unit (24), of a recirculation stream (142), the stream of recirculation (142) being cooled, expanded, and introduced into the separation column (34).

10. Method according to any one of the preceding claims, in which the stream of pressurized liquefied natural gas (120) is expanded in a dynamic or static expansion device (60), then is introduced into a flash balloon (62), to being separated into the expanded liquefied natural gas (124) introduced into the storage (66), and into a flow of flash gas (126). 19

11. Method according to any one of the preceding claims, in which at least one flash gas flow (162) is formed in the storage (66), during the introduction of the expanded liquefied natural gas into the storage (66).

12. The method of claim 1 1, wherein the pressurized liquefied natural gas stream (120) is introduced directly into the storage (66), without passing through a flash tank (62).

13. Process according to any one of the preceding claims, in which the compression of the overhead stream (98) coming from the separation column (34) takes place in at least a first compressor (38) coupled to the dynamic expansion turbine ( 36) then in a compression machine successively comprising a second compressor, a cooler for the gas compressed in the second compressor, and a third compressor, to form the stream of compressed purified natural gas.

14. Process according to any one of the preceding claims, in which an overhead stream (112) from the fractionation column (48) is cooled and partially condensed, then is introduced into an overhead drum (56), the stream of ethane (14) being recovered at the top of the top flask (56), the bottom of the top flask (56) being reintroduced under reflux into the fractionation column (48).

15. Process according to any one of the preceding claims, in which the entire gas stream (84) resulting from the separation of the cooled natural gas stream (80) forms the turbine feed stream (90) which is sent to the dynamic expansion turbine (36) without separation.

16. Plant for extracting ethane from a starting natural gas stream (12), comprising:

- at least one first heat exchanger (28) upstream capable of cooling the starting natural gas stream (12), to form a cooled natural gas stream (80);

- a separator for separating the cooled natural gas stream (80) into a liquid stream (82) and a gas stream (84);

- a liquid flow expansion device (82);

- a column (34) for separating methane and C2+ hydrocarbons and an assembly for introducing at least one stream from the liquid stream (82) expanded in the separation column (34), at a first level (N1 );

- an assembly for forming a turbine feed stream (90) from the gas stream (84);

- a dynamic expansion turbine (36) suitable for expanding the turbine feed stream (90) and an assembly for introducing the expanded stream (94) from the 20 dynamic expansion turbine (36) in the separation column (34) at a second level (N2),

- a fractionation column (48), an assembly for introducing a bottoms stream (106) rich in C2+ hydrocarbons from the separation column (34) into the fractionation column (48), and an assembly of recovering, from the fractionation column (48), an ethane stream (14);

- an assembly for recovering and compressing at least part of a top stream (98) from the separation column (34) to form a stream of compressed purified natural gas (102);

- a unit for liquefying the stream of compressed purified natural gas (102) capable of forming a stream (120) of pressurized liquefied natural gas;

- a set of flash expansion of the pressurized liquefied natural gas stream (120) and a storage (66) for recovering expanded liquefied natural gas (18);

- an assembly for recovering at least one flash gas stream (126; 162) from the expansion of the pressurized liquefied natural gas stream (120);

- an assembly for compressing the or each flow of flash gas (126; 162) characterized by:

- an assembly for separating the flow of compressed flash gas (132; 164) into a fuel stream (20) and into a recycle stream (134);

- an assembly for cooling and at least partial expansion of the recycling stream (134), and for introducing the cooled and expanded recycling stream to a top stage of the separation column (34).