Process and corresponding apparatus for extracting ethane from an initial natural gas stream

Abstract

A method and corresponding apparatus for extracting ethane from an initial natural gas stream. The method includes the steps of recovering and compressing an overhead stream (98) from a separation column (34) to form a compressed purified natural gas stream (102); liquefying the compressed purified natural gas stream (102) in a liquefaction unit (24) to form a pressurized liquefied natural gas stream (120); flash expanding the pressurized liquefied natural gas stream (120) and recovering in a storage vessel (66); recovering and compressing an expanded flash gas stream (126); separating the compressed flash gas stream (132) into a fuel stream (20) and a recycle stream (134); cooling and expanding the recycle stream (134) and then introducing the cooled and expanded recycle stream into an overhead stage of the separation column (34).

Description

TECHNICAL FIELD

[0001] The present invention relates to a process for extracting ethane from an initial natural gas stream, the process comprising the steps of:

[0002] cooling the initial natural gas stream in at least one first upstream heat exchanger to form a cooled natural gas stream;

[0003] separating the cooled natural gas stream into a liquid stream and a gas stream;

[0004] expanding the liquid stream and introducing at least one stream from the liquid stream into a separation column for separating methane and C2+ hydrocarbons at a first level;

[0005] forming a turbine feed stream from the gas stream;

[0006] expanding the turbine feed stream in a dynamic expansion turbine and introducing the expanded stream from the dynamic expansion turbine into the separation column at a second level;

[0007] introducing a C2+ hydrocarbon rich bottom stream recovered from the separation column into a fractionation column and recovering an ethane stream from the fractionation column;

[0008] recovering and compressing at least a portion of an overhead stream from the separation column to form a compressed purified natural gas stream;

[0009] liquefying the compressed purified natural gas stream in a liquefaction unit to form a pressurized liquefied natural gas stream;

[0010] flash expanding the pressurized liquefied natural gas stream and recovering an expanded liquefied natural gas in a reservoir;

[0011] recovering at least one flash gas stream from the expansion of the pressurized liquefied natural gas stream;

[0012] compressing the at least one flash gas stream. BACKGROUND

[0013] This process is particularly useful for extracting ethane and C3+ hydrocarbons from an initial natural gas while producing a pressurized treated natural gas which is then liquefied before being expanded for storage.

[0014] Ethylene, ethane, propylene, propane and heavier hydrocarbons can be extracted from gases such as natural gas, refinery gases and synthetic gases obtained from other hydrocarbon sources such as coal, crude oil, naphtha.

[0015] Natural gas typically contains a majority of methane and ethane (e.g., methane and ethane make up at least 50 mol% of the gas). Natural gas can also contain a more negligible amount of heavier hydrocarbons such as propane, butane, pentane and also hydrogen, nitrogen and carbon dioxide.

[0016] 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 in natural gas, such as ethane, propane and butane, can be sold separately at a high purity and thus at a high value, these hydrocarbons can condense during transport or freeze in the liquefaction exchanger (for the heaviest hydrocarbons).

[0017] The above can lead to accidents, such as the appearance of a liquid plug in the transport facilities or the shutdown of the liquefaction plant to unblock the frozen exchanger.

[0018] Document US 6 578 379 describes a very effective process for recovering ethane and propane from a natural gas stream. This process generally operates in a very effective manner, in particular in order to obtain a very high extraction yield of the ethane contained in the natural gas feed (for example greater than 99 mol%), while minimizing the energy consumption.

[0019] To obtain such extraction yields, it is known to use an ethane very lean stream as the main reflux, i.e. the highest reflux for the methane and ethane separation column.

[0020] To this end, a recycle stream is extracted from the re-compressed gas coming from the top of the methane and ethane separation column. Said recycle stream is cooled in countercurrent with the gas coming from the top of the column and then expanded to form the main reflux introduced at the top of the column.

[0021] However, under certain operating conditions, the quality of the main reflux can deteriorate in terms of temperature and / or composition.

[0022] For example, if the main reflux becomes lean in methane, the ethane separation yield in the column decreases, the quality of the overhead stream produced at the top of the column deteriorates further, thus exacerbating the methane lean in the main reflux. A "snowball" effect then occurs, resulting in a substantial decrease in the ethane extraction yield. This can occur in particular if liquid is entrapped on the upper trays of the column. SUMMARY

[0023] An object of the invention is to provide a flexible and very effective process for extracting ethane and C3+ hydrocarbons from an initial natural gas stream, in which the ethane extraction yield is not affected at all or only slightly affected when the separation column overhead stream quality fluctuates.

[0024] To this end, the subject of the invention is a process of the above type, characterized in that it comprises the following steps:

[0025] - separation of the compressed flash gas stream into a fuel stream and a recycle stream;

[0026] - cooling and at least partial expansion of said recycle stream, then introduction of the cooled and expanded recycle stream into the top stage of the separation column.

[0027] The method according to the application can comprise one or more of the following features taken separately or according to any technically possible combination:

[0028] - the methane concentration of the recycle stream is greater than 90 mol%, in particular greater than 95 mol%;

[0029] - the introduction of the recycle stream is carried out at the first stage starting from the top of the separation column;

[0030] - the recycle stream is introduced into the first upstream heat exchanger and is cooled in the first upstream heat exchanger by heat exchange with the overhead stream from the separation column;

[0031] - the method comprises separating the gas stream into a turbine feed stream introduced into a dynamic expansion turbine, and a reflux, the reflux being cooled in a second upstream heat exchanger and being introduced into the separation column at a lower level than the recycle stream after static expansion;

[0032] - the cooling of the recycle stream comprises flowing the recycle stream through a second heat exchanger;

[0033] - the expansion of the recycle stream comprises flowing the recycle stream through a static expansion valve;

[0034] - at least a portion of the compressed purified natural gas of the overhead is arranged to exchange heat with the flash gas stream in a downstream heat exchanger;

[0035] - the method comprises extracting a re-circulation stream from the compressed purified natural gas stream upstream of the liquefaction unit, the re-circulation stream being cooled, expanded and introduced into the separation column;

[0036] - the pressurized liquefied natural gas stream is expanded in a dynamic or static expansion component, then introduced into a flash tank to be separated into a flash gas stream and an expanded liquefied natural gas introduced into a reservoir;

[0037] - at least one flash gas stream is formed in the reservoir when the expanded liquefied natural gas is introduced into the reservoir;

[0038] - the pressurized liquefied natural gas stream is introduced directly into the reservoir without flowing through the flash tank;

[0039] - the compression of the overhead stream from the separation column is carried out in at least one first compressor associated with the dynamic expansion turbine, then in a compression machine comprising in sequence a second compressor, a cooler for the gas compressed in the second compressor and a third compressor;

[0040] - the overhead stream from the fractionation column is cooled and partially condensed, then introduced into an overhead tank, an ethane stream being recovered at the top of the overhead tank, the bottom stream of the overhead tank being reintroduced into the fractionation column as a reflux;

[0041] - all of the gaseous stream separated from the cooled natural gas stream is formed without separation into a turbine feed stream sent to a dynamic expansion turbine.

[0042] The application also relates to a device for extracting ethane from an initial natural gas stream, comprising:

[0043] - at least one first upstream heat exchanger adapted to cool the initial natural gas stream to form a cooled natural gas stream;

[0044] - a separator for separating the cooled natural gas stream into a liquid stream and a gaseous stream;

[0045] - means for expanding the liquid stream;

[0046] - a separation column for separating methane and C2+ hydrocarbons and a system for introducing at least one stream from the expanded liquid stream into the separation column at a first height;

[0047] - a system for forming a turbine feed stream from the gaseous stream;

[0048] - a dynamic expansion turbine adapted to expand said turbine feed stream and a system for introducing an expanded stream from the dynamic expansion turbine into the separation column at a second height,

[0049] - a fractionation column, a system for introducing a C2+ hydrocarbon-rich bottom stream from the separation column into the fractionation column, and a system for recovering an ethane stream from the fractionation column;

[0050] - a system for recovering and compressing at least a portion of an overhead stream from the separation column to form a compressed purified natural gas stream;

[0051] - a liquefaction unit for said compressed purified natural gas stream, adapted to form a pressurized liquefied natural gas stream;

[0052] - a system for flash-expanding the pressurized liquefied natural gas stream and a reservoir for recovering the expanded liquefied natural gas;

[0053] - a system for recovering at least one flash gas stream from the expansion of said pressurized liquefied natural gas stream;

[0054] - a system for compressing said at least one flash gas stream,

[0055] characterized in that:

[0056] - a system for separating the compressed flash gas stream into a fuel stream and a recycle stream;

[0057] - a system for cooling and at least partially expanding said recycle stream and introducing the cooled and expanded recycle stream into a top stage of the separation column. BRIEF DESCRIPTION OF DRAWINGS

[0058] The application will be better understood by reading the following description, given only as an example and with reference to the appended drawings, in which:

[0059] Figure 1 is a flow diagram showing a first apparatus for implementing a first method for extracting ethane according to the application;

[0060] Figure 2 is a flow diagram of a second apparatus for implementing a second method for extracting ethane according to the application, similar to the schematic shown in Figure 1

[0061] Figure 3 is a flow diagram of a third apparatus for implementing a third method for extracting ethane according to the application, similar to the schematic shown in Figure 1

[0062] Figure 4 is a flow diagram of a fourth apparatus for implementing a fourth extraction method according to the application, similar to the schematic shown in Figure 1

[0063] Figure 5 is a flow diagram of a fifth apparatus for implementing a fifth extraction method according to the application, similar to the schematic shown in Figure 2 DETAILED DESCRIPTION

[0064] Throughout the following, the same reference numbers will be used to designate the pipes of the streams and of the carrier fluid, the pressures considered are absolute pressures, and the percentages considered are molar percentages.

[0065] The described methods were modelled on a process simulator. A polytropic efficiency of 82% was defined for the compressor, and an adiabatic efficiency of 86% for the turbine.

[0066] Figure 1 A first apparatus 10 for extracting ethane according to the application is shown in

[0067] The apparatus 10 is intended to produce simultaneously, from an initial natural gas stream 12, an ethane stream 14 enriched in ethane, a bottom stream 16 enriched in C3+ hydrocarbons, an expanded liquefied natural gas 18, and a fuel stream 20, which can advantageously be reused in the apparatus 10.

[0068] With reference to Figure 1 , the apparatus 10 comprises an extraction unit 22 for extracting ethane, a liquefaction unit 24, and a flashing and storage unit 26 for the liquefied natural gas.

[0069] The extraction unit 22 comprises a first upstream heat exchanger 28 and a second upstream heat exchanger 30, a separation tank 32 and a separation column 34 for separating the methane and the C2+ hydrocarbons. Here, the separation column 34 is provided with a bottom reboiler 35.​​​​

[0070] The extraction unit 22 further comprises a dynamic expansion turbine 36 associated with a first compressor 38 and a second compressor 40, each of the first and second compressors 38, 40 being downstream of a cooler 42, 44.

[0071] The extraction unit 22 further comprises a bottom pump 46, a fractionation column 48 provided with a bottom reboiler 50 and a reflux system 52 comprising a cooler 54, a reflux tank 56 and a reflux pump 58.

[0072] The liquefaction unit 24 of the liquefied natural gas is a known unit, in particular a C3MR or DMR unit.

[0073] In the example shown, the flash and storage unit 26 comprises an expansion device 60 (here a dynamic expansion turbine), a flash tank 62 and a pump 64 for delivering the liquefied natural gas to a storage 66. In a variant, the expansion device 60 is a static expansion valve. Figure 1 The storage 66 is for example an adiabatic storage tank.

[0074] The storage 66 is for example an adiabatic storage tank.

[0075] In the example, the flash and storage unit 26 further comprises a downstream heat exchanger 68, if appropriate a suction tank 70 and a compression device 72 comprising a plurality of compressors 74 mounted in series, separated by coolers 76.

[0076] A first method according to the application will now be described, which is implemented in the installation 10.

[0077] The initial natural gas forming the initial natural gas stream 12 is advantageously dry, at least partially decarbonated, desulphurised natural gas.

[0078] The term "at least partially decarbonated" means that the concentration of carbon dioxide in the initial natural gas stream 13 is advantageously less than or equal to 50 ppmv.

[0079] Similarly, the concentration of water is less than 1 ppmv, advantageously less than 0.1 ppmv.

[0080] The concentration of sulphur elements, including hydrogen sulphide, is less than 10 ppmv, advantageously less than or equal to 4 ppmv.

[0081] Examples of the molar composition of the initial natural gas stream 12 are given in the following table.

[0082] [Table 1]

[0083] Mole fraction (%) Nitrogen 0.19 Methane 90.62 Ethane 6.56 Propane 2.05 Isobutane 0.25 n-Butane 0.23 C5+ 0.09

[0084] More generally, the molar fraction of methane in the initial natural gas stream 12 is between 75 and 95 mol%, the molar fraction of C2hydrocarbons is between 3 and 12 mol%, the molar fraction of C3+ hydrocarbons is between 1 and 8 mol%.

[0085] The flow rate of the initial natural gas stream 12 is for example greater than 2 000 kmol / h, for example between 2 000 kmol / h and 70 000 kmol / h, in particular equal to 55 000 kmol / h.

[0086] The temperature of the initial natural gas stream 12 is close to ambient temperature, in particular between 0 and 40°C, here equal to 21.5°C, the pressure of the initial natural gas stream is advantageously greater than 35 bars, in particular greater than 70 bars, in the present case equal to 81 bars.

[0087] The initial natural gas is introduced into the first upstream heat exchanger 28 to be cooled therein. It forms a cooled natural gas stream 80. The initial natural gas is supercritical here, so it is simply cooled. In a variant, the initial natural gas is not supercritical, it is at least partially condensed in the first upstream heat exchanger 28.

[0088] The temperature of the cooled natural gas stream is lower than -20°C, in particular between -25 and -45°C, in particular equal to -37°C.

[0089] The cooled natural gas stream 80 is then introduced into the separation tank 32 to be separated therein into a liquid stream 82 and a gaseous stream 84, the liquid stream 82 being recovered at the bottom of the separation tank 32, the gaseous stream 84 being recovered at the top of the separation tank 32. The flow rate of the liquid stream 82 can be zero, in particular when the cooled natural gas stream 80 is supercritical.

[0090] The liquid stream 82 flows through a static expansion valve 86 to form a mixed expansion phase 88. The pressure of the mixed expansion phase 88 is less than 50 bars, in particular less than 30 bars, for example equal to 28.7 bars. The mixed expansion phase 88 is introduced at a first height N1 of the lower part of the separation column 34.

[0091] The gaseous stream 84 is divided into a main turbine feed stream 90 and a secondary reflux stream 92.

[0092] The molar flow rate of the turbine feed stream 90 is greater than the molar flow rate of the reflux stream 92, in particular between 5 and 25% of the molar flow rate of the reflux stream 92.

[0093] The turbine feed stream 90 is introduced into the dynamic expansion turbine 36 to be expanded therein to a pressure less than 50 bars, in particular less than 30 bars, for example equal to 28.7 bars.

[0094] The dynamic expansion of the turbine feed stream 90 allows recovering more than 10 000 kW of energy, for example 10 865 kW of energy.

[0095] The temperature of the cooled expanded stream 94 coming from the dynamic expansion turbine 36 is for example lower than -70°C, in particular lower than -80°C, for example equal to -80.8°C.

[0096] The cooled expanded stream 94 is then introduced into the separation column 34 at a second height N2 located above the first height N1.

[0097] The reflux 92 is introduced into the static expansion valve 96 so as to be expanded therein to a pressure lower than 50 bars, in particular lower than 30 bars, in particular equal to 28.7 bars. The reflux is cooled in the second upstream heat exchanger 30 to a temperature lower than -80°C, in particular lower than -90°C, in particular equal to -95.8°C.

[0098] The expanded and cooled reflux is introduced into the separation column 34 at a height N3 located above the second height N2 at the top of the separation column 34.

[0099] 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.

[0100] The separation column 34 produces an overhead stream 98. The overhead stream 98 is heated in the second upstream heat exchanger 30 and then in the first upstream heat exchanger 28 in countercurrent with the initial natural gas to form a heated overhead stream 100.

[0101] The temperature of the heated overhead stream 100 is greater than 0°C, in particular greater than 15°C, for example equal to 17.6°C.

[0102] The heated overhead stream 100 is then compressed in the first compressor 38 connected to the dynamic expansion turbine 36 and then cooled in the cooler 42 to obtain a stream having a pressure greater than 30 bars, in particular equal to 34.6 bars.

[0103] It is then recompressed in the second compressor 40 and then cooled in the cooler 44 to produce a compressed purified natural gas stream 102 for the liquefaction unit 24.

[0104] The pressure of the compressed purified natural gas stream 102 is greater than 60 bars, in particular greater than 80 bars, for example equal to 91 bars. Its temperature is greater than 0°C, in particular greater than 10°C, more particularly equal to 21.5°C.

[0105] The coolers 42, 44 are here supplied with a cooling stream having a temperature lower than 10°C, in particular equal to 7°C. This cooling stream can in particular be air or water.

[0106] The compressed purified natural gas stream 102 is rich in methane. It has a methane concentration greater than 99.0 mol%, in particular equal to 99.1 mol%. It has a low nitrogen concentration, in particular less than 1.0 mol%, and a low C2+ hydrocarbon concentration, in particular an ethane concentration less than 0.5 mol%, approximately equal to 0.2 mol% of ethane.

[0107] The separation column 34 produces at the bottom a bottoms stream 106 rich in C2+ hydrocarbons. This bottoms stream 106 contains, for example, more than 95 mol% of the ethane contained in the initial natural gas, as well as 100 mol% of the C3+ hydrocarbons contained in this stream.

[0108] The temperature of the bottoms stream 106 is greater than 10°C, in particular between 20°C and 30°C, for example equal to 23.2°C. This bottoms stream contains less than 1000 ppmv of carbon dioxide, advantageously between 200 ppmv and 500 ppmv of carbon dioxide, for example 313 ppmv of carbon dioxide. The methane concentration of this bottoms stream is less than 5 mol%, for example between 0 mol% and 3 mol%, in particular less than 1 mol%.

[0109] The following table illustrates an example of the composition of the bottoms stream 106.

[0110] [Table 2]

[0111] Mole fraction (%) Nitrogen 0.0 Methane 0.21 Ethane 70.5 Propane 22.8 Isobutane 2.8 n-Butane 2.6 C5+ 0.97

[0112] A first lateral reboil stream 108 is extracted from the separation column 34 at a height N5 lower than the first height N1, for example at the 20th level from the top of the separation column 34.

[0113] The first lateral reboil stream 108 is brought to the first upstream heat exchanger 28 to be heated in the first upstream heat exchanger 28 by heat exchange, in particular with the initial natural gas, to a temperature greater than 0°C, in particular equal to 8.25°C. The first lateral reboil stream 108 is then reintroduced into the separation column 34 at a height N6 lower than the height N5, for example at the 21st level from the top of the column 34.

[0114] Similarly, a second reboil stream 110 is extracted from the separation column 34 at a height N7 lower than the height N6, for example at the 22nd level from the top of the separation column 34, to be brought to the bottom reboiler 35 so as to be heated therein to a temperature greater than 0°C, for example equal to 10.7°C. Greater than 1 MW (Megawatt), for example equal to 4 MW, of energy is provided to the second reboil stream 110.

[0115] The second reboil stream 110 is then returned to the separation column 34 at a height N8 lower than the height N7. For example, the height N8 is at the 23rd level from the top.

[0116] The underflow 106 is pumped to the pump 46 to be introduced at an intermediate height PI of the fractionation column 48.

[0117] The fractionation column 48 produces at the top a top stream 112 containing less than 1 mol% of C3+ hydrocarbons, in particular less than 1 mol% of propane.

[0118] The top stream 112 is partially condensed in the cooler 54 and then separated in the reflux drum 56 so as to form at the top an ethane-rich ethane stream 14 and at the bottom a liquid reflux 114 which is reintroduced at the top of the fractionation column 48 after being pumped by the reflux pump 58.

[0119] The ethane-rich ethane stream 14 contains more than 96 mol% of the ethane contained in the initial natural gas. It contains more than 97 mol% of ethane.

[0120] The ethane-rich ethane stream 14 is gaseous here. In a variant (not shown), the ethane-rich ethane stream 14 is liquid taken from the liquid reflux 114.

[0121] The C3+ hydrocarbon stream contains less than 500 ppmv of ethane, in particular less than 100 ppmv of ethane.

[0122] The compressed purified natural gas stream 102 is brought into the liquefaction unit 24 which produces a pressurized liquefied natural gas stream 120 in a known manner.

[0123] 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 lower than -120°C, in particular lower than -130°C, advantageously equal to -136.8°C.

[0124] The pressurized liquefied natural gas stream 120 is introduced into the expansion means 60, here into a dynamic expansion turbine. The pressurized liquefied natural gas stream is expanded to a pressure less than 5 bars, in particular less than 2 bars, for example equal to 1.25 bars, to form a flash liquefied natural gas stream 122.

[0125] The flash liquefied natural gas stream 122 is introduced into the flash tank 62 to be separated therein into an expanded liquefied natural gas stream 124 and a first flash gas stream 126.

[0126] The expanded liquefied natural gas stream 124 is pumped by means of the pump 64 into a storage tank to form the expanded liquefied natural gas 18.

[0127] The first flash gas stream 126 is recovered at the top of the flash tank 62. It is introduced into the downstream heat exchanger 68 so as to be heated there in countercurrent with a portion of the compressed purified natural gas stream 102 which is reintroduced into the flash liquefied natural gas stream 122 upstream of the flash tank 62.

[0128] After the heat exchange in the downstream heat exchanger 68, the heated flash gas stream 130 thus formed has a temperature higher than -60°C, in particular equal to approximately 5°C. It has a very high methane concentration, for example greater than 80 mol%, for example greater than 85 mol%, in particular greater than 90 mol%. This concentration is advantageously greater than 95 mol% of methane, in particular greater than 96 mol% of methane, for example equal to 96.46 mol% of methane.

[0129] It has a nitrogen concentration less than 20 mol%, for example less than 15 mol%, in particular less than 10 mol%. The said concentration is advantageously less than 5 mol%, in particular less than 4 mol%, for example equal to approximately 3.54 mol% of nitrogen.

[0130] The heated flash gas stream 130 has an ethane concentration less than 50 ppmv, in particular less than 10 ppmv, for example equal to 5 ppmv.

[0131] After flowing through the suction tank 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, for example equal to 60 bars, so as to produce a compressed flash gas stream 132.

[0132] The compressed flash gas stream 132 is separated into a fuel stream 20 and a recirculation stream 134.

[0133] The fuel stream 20 is intended to be sent into the gas network of the installation 10, so as to supply a gas turbine of the liquefaction unit 24 of natural gas for example or to supply a gas turbine of a power generation unit, for example for supplying the second compressor 40 or other equipment of the installation 10.

[0134] The recirculation stream 134 has a pressure greater than 30 bars, in particular greater than 50 bars, for example equal to 58.5 bars.

[0135] The recirculation stream is successively conveyed to the first upstream heat exchanger 28 and then into the second heat exchanger 30, so as to be cooled therein to a temperature lower than -80°C, in particular lower than -90°C, for example equal to -95.5°C.

[0136] The recirculation stream 134 is then expanded in a static expansion valve 136 to a pressure lower than 50 bars, in particular lower than 30 bars, for example equal to 28.7 bars, so as to be introduced into the separation column 34 at a top level N9 of the separation column 34, for example at the first level from the top of the separation column 34. The level N9 is located above the level N3 for the introduction of the expanded cooling reflux.

[0137] As mentioned above, the recirculation stream 134 from the first flash gas stream 126 is extremely rich in methane, since the ethane is retained in the expanded liquefied natural gas 18, or is successively extracted in the separation column 34 and then in the fractionation column 48.

[0138] Thus, whatever the fluctuations in the quality of the top stream 98 of the separation column 34, the composition of the reflux introduced at the top of the separation column 34 remains extremely rich in methane.

[0139] The presence of the new reflux provides operating flexibility not only during implementation of the method, but also during the design phase.

[0140] Thus, it is possible to optimize the energy consumption between the extraction unit 22 of ethane and the liquefaction unit 24 as a whole, to adjust the parameters of the two units, the extraction unit 22 and the liquefaction unit 24, in order to choose the compressors and their drive modes as required in the two units, the extraction unit 22 and the liquefaction unit 24, as well as possible. Thus, the investment costs are greatly reduced, and the operating costs are also greatly reduced, as will be seen in the example described below.

[0141] In a variant (not shown), the heated top stream 100 is compressed in a compression machine comprising two compression levels of the same power, the total power being equal to the power of the second compressor 40, at the outlet of the first compressor 38 associated with the dynamic expansion turbine 36. This compression machine comprises an intercooler which cools the gas between the compression levels. The arrangement thus obtained makes it possible to save 5.8 MW (megawatts) of power.

[0142] Figure 2 A second device 140 for implementing the second method according to the application is shown in Figure 2.

[0143] The second method according to the application is similar to the first method according to the application. It differs from the first method according to the application in that it comprises a step of extracting a recirculation stream 142 from the compressed purified natural gas stream 102.

[0144] The molar flow rate of the recirculation stream 142 is advantageously less than the molar flow rate of the remaining compressed purified natural gas stream 102 which, after extraction of the recirculation stream 142, will be introduced into the liquefaction unit 24.

[0145] The recirculation flow 142 has a pressure greater than 50 bars, in particular greater than 80 bars, for example equal to 90 bars. This recirculation flow is successively introduced into the first upstream heat exchanger 28 and then into the second upstream heat exchanger 30, so as to be cooled therein to a temperature lower than -90°C, preferably lower than -95°C, for example approximately equal to -95.4°C.

[0146] The recirculation flow 142 is then expanded to a pressure lower than 50 bars, in particular lower than 30 bars, in particular equal to 28.7 bars, and is introduced between the recirculation flow 134 and the backflow 92 into the separation column 34.

[0147] Figure 3 A third device 150 for implementing a third method according to the application is shown in Figure 3.

[0148] The device 150 differs from the first device 10 in that the device 150 comprises a system 152 for collecting and recompressing the boil-off gas formed in the reservoir 66.

[0149] The collection system 152 comprises a containment tank 154 and a compression device 156 comprising a plurality of compression stages 158 spaced two by two by a cooler 160.

[0150] A second flash gas flow 162 resulting from the evaporation of the liquefied natural gas in the reservoir 66 is collected at the top of the reservoir 66 and is then introduced into the compression device 156, so as to be compressed therein to a pressure greater than 25 bars, in particular between 26 bars and 70 bars, for example equal to 60 bars.

[0151] The second compressed flash gas flow 164 thus produced is separated into the fuel flow 20 and the recirculation flow 134, which, after cooling in the first upstream heat exchanger 28, the second upstream heat exchanger 30 and expansion in the static expansion valve 136, is reintroduced into the separation column 34.

[0152] Advantageously, in the example shown, the device 150 does not comprise the expansion member 60. The compressed liquefied natural gas coming from the liquefaction unit 24 is introduced directly into the reservoir 66 for the liquefied natural gas and is flashed in the reservoir 66. Figure 3 In the example shown, the device 150 does not comprise the expansion member 60. The compressed liquefied natural gas coming from the liquefaction unit 24 is introduced directly into the reservoir 66 for the liquefied natural gas and is flashed in the reservoir 66.

[0153] Figure 4 A fourth device 170 for implementing a fourth method according to the application is shown in Figure 4.

[0154] The fourth device 170 differs from the first device 10 in that the reservoir 66 is equipped, as in the third device 150, with a system 152 for collecting the boil-off gas.

[0155] In the course of implementing the fourth method according to the application, the first compressed flash gas stream and the second compressed flash gas stream 164 are mixed, then the mixture is separated into a fuel stream 20 and a recycle stream 134.

[0156] As before, the recycle stream 134 is reintroduced into the separation column 34 after flowing through the first upstream heat exchanger 28, the second upstream heat exchanger 30, then being expanded in the static expansion valve 136.

[0157] Figure 5 Fig. 5 shows a fifth apparatus 200 for implementing the fifth method according to the application.

[0158] The fifth method differs from the second method shown in that the entire gas stream 84 recovered from the tank 32 is not separated but forms the turbine feed stream 90 sent to the dynamic expansion turbine 36. Figure 2

[0159] With the application just described, it is possible to keep the composition of the reflux of the separation column 34 substantially constant, thus preventing the snowball effect that would occur if the composition of the overhead stream 98 extracted from the separation column 34 fluctuated in the absence of the supply of the recycle stream 134.

[0160] The method thus allows a constant concentration of extracted ethane to be maintained simply and effectively, without increasing the investment costs or operating costs.

[0161] The energy consumption of the method is detailed in the table below.

[0162] [Table 3]

[0163]

[0164] As indicated in the table above, the total power consumed in the presence of the reflux generated from the recycle stream 134 represents a significant reduction in the power consumed and the specified power divided by the flow of liquefied natural gas produced by the apparatus.​

Claims

1. A method for extracting ethane from an initial natural gas stream (12), comprising the following steps: - The initial natural gas stream (12) is cooled in at least one first upstream heat exchanger (28) to form a cooled natural gas stream (80); - Separate the cooling natural gas stream (80) into a liquid stream (82) and a gas stream (84); - Expand the liquid flow (82) and introduce at least one stream from the liquid flow (82) at a first height (N1) into a separation tower (34) for separating methane and C2+ hydrocarbons; -The turbine feed flow (90) is formed by the airflow (84); - The turbine feed stream (90) is expanded in the dynamic expansion turbine (36), and the expanded stream (94) from the dynamic expansion turbine (36) is introduced into the separation tower (34) at the second height (N2); - The underflow rich in C2+ hydrocarbons recovered from the separation tower (34) is introduced into the fractionation tower (48), and the ethane stream (14) is recovered from the fractionation tower (48); -Recover and compress at least a portion of the top stream (98) from the separation tower (34) to form a compressed purified natural gas stream (102); - The compressed purified natural gas stream (102) is liquefied in the liquefaction unit (24) to form a pressurized liquefied natural gas stream (120); - The pressurized liquefied natural gas stream (120) is flash-expanded and the expanded liquefied natural gas (18) is recovered in a storage tank (66); -Recover at least one flash stream from the expansion of the pressurized liquefied natural gas stream (120); - Compress the at least one flash vapor stream. Its characteristics include the following steps: - Separate the compressed flash stream into a fuel stream (20) and a recirculation stream (134); - The recirculated flow (134) is at least partially cooled and expanded, and then the cooled and expanded recirculated flow is introduced into the top stage of the separation tower (34).

2. The method according to claim 1, wherein, The methane concentration in the recirculating stream (134) is greater than 90 mol%.

3. The method according to claim 1 or 2, wherein, The introduction of the recirculation flow (134) is carried out in the first stage starting from the top of the separation tower (34).

4. The method according to claim 1 or 2, wherein, The recirculated flow (134) is introduced into the first upstream heat exchanger (28) and cooled in the first upstream heat exchanger by exchanging heat with the top flow (98) from the separation tower (34).

5. The method according to claim 1 or 2, wherein, The method includes separating the airflow (84) into a turbine feed stream (90) introduced into a dynamic expansion turbine (36) and a recirculation stream (92), which is cooled in a second upstream heat exchanger (30) and introduced into a separation tower (34) at a lower level than the recirculation stream (134) after static expansion.

6. The method according to claim 5, wherein, Cooling of the recirculation flow (134) involves passing the recirculation flow (134) through a second upstream heat exchanger (30).

7. The method according to claim 1 or 2, wherein, The expansion of the recirculation flow (134) involves causing the recirculation flow (134) to flow through the static expansion valve (136).

8. The method according to claim 1 or 2, wherein, At least a portion of the top compressed purified natural gas stream (102) is arranged to exchange heat with the flash stream in the downstream heat exchanger (68).

9. The method according to claim 1 or 2, wherein, The method includes extracting a recirculation stream (142) from a compressed purified natural gas stream (102) upstream of the liquefaction unit (24), the recirculation stream (142) being cooled, expanded and introduced into a separation tower (34).

10. The method according to claim 1 or 2, wherein, The pressurized liquefied natural gas stream (120) expands in a dynamic or static expansion unit (60) and is then introduced into a flash tank (62) to be separated into a flash stream and expanded liquefied natural gas (124) introduced into a storage tank (66).

11. The method according to claim 1 or 2, wherein, When expanded liquefied natural gas is introduced into the storage tank (66), at least one flash vapor stream is formed in the storage tank (66).

12. The method according to claim 11, wherein, The pressurized liquefied natural gas stream (120) is introduced directly into the storage tank (66) without passing through the flash tank (62).

13. The method according to claim 1 or 2, wherein, The compression of the top stream (98) from the separation tower (34) is carried out in at least one first compressor (38) connected to a dynamic expansion turbine (36), and then in a compression mechanism to form a compressed purified natural gas stream, the compression mechanism comprising a second compressor, a cooler for the gas compressed in the second compressor, and a third compressor.

14. The method according to claim 1 or 2, wherein, The top stream from the fractionation column (48) is cooled and partially condensed, and then introduced into the top tank. The ethane stream (14) is recovered at the top of the top tank, and the bottom stream from the top tank is reintroduced into the fractionation column (48) as reflux.

15. The method according to claim 1 or 2, wherein, All the airflow (84) from the cooling natural gas flow (80) is not separated and forms the turbine feed flow (90) sent to the dynamic expansion turbine (36).

16. An apparatus for extracting ethane from an initial natural gas stream (12), comprising: - At least one first upstream heat exchanger (28) is adapted to cool the initial 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); - Components of the expanding fluid flow (82); - A separation tower (34) for separating methane and C2+ hydrocarbons and a system for introducing at least one stream from an expanding liquid stream (82) into the separation tower (34) at a first height (N1); - A system in which a turbine feed flow (90) is formed by airflow (84); - A dynamic expansion turbine (36) suitable for expanding the turbine feed flow (90), and a system for introducing the expanded flow (94) from the dynamic expansion turbine (36) into the separation tower (34) at a second height (N2). - A fractionation column (48), a system for introducing a C2+ hydrocarbon-rich underflow (106) from a separation column (34) into the fractionation column (48), and a system for recovering an ethane stream (14) from the fractionation column (48); - A system that recovers and compresses at least a portion of the top stream (98) from the separation tower (34) to form a compressed purified natural gas stream (102); -The liquefaction unit for the compressed purified natural gas stream (102) is adapted to form a pressurized liquefied natural gas stream (120); - A system for flash expansion of pressurized liquefied natural gas stream (120) and a storage unit (66) for recovering expanded liquefied natural gas (18); - A system for recovering at least one flash stream from the expansion of the pressurized liquefied natural gas stream (120); -The system that compresses at least one flash vapor stream. Its features are: - A system that separates the compressed flash stream into a fuel stream (20) and a recirculation stream (134); - Cool and at least partially expand the recirculation flow (134) and introduce the cooled and expanded recirculation flow into the top stage of the separation tower (34).

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