Method for extracting ethane from a primary natural gas stream and corresponding plant

Abstract

The process comprises the steps of: recovering and compressing an overhead stream (98) from a separation column (34) to produce a compressed purified natural gas stream (102); liquefying the compressed purified natural gas stream (102) in a liquefaction unit (24) to produce a pressurized liquefied natural gas stream (120); flash expanding the pressurized liquefied natural gas stream (120) and recovering it in storage (66); recovering and compressing the 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 the top stage of the separation column (34).

Description

[Technical Field]

[0001] The present invention provides a method for extracting ethane from a first natural gas stream, comprising the steps of: - cooling the first natural gas stream in at least one first upstream heat exchanger to produce a cooled natural gas stream; - separating the cooled natural gas stream into a liquid stream and a gas stream; - expanding the liquid stream and introducing at least one stream from the liquid stream into a separation column at a first level for separating methane and C2+ hydrocarbons; - generating a turbine feed stream from the gas stream; - 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; - introducing the C2+ hydrocarbon-rich bottom stream recovered from the separation column into a fractionation column and recovering an ethane stream from the fractionation column; - recovering and compressing at least a portion of the overhead stream from the separation column to produce a compressed purified natural gas stream; - liquefying the compressed purified natural gas stream in a liquefaction unit to produce a pressurized liquefied natural gas stream; - flash expanding the pressurized liquefied natural gas stream and recovering the expanded liquefied natural gas in storage; - recovering at least one stream of flash gas obtained by expansion of the pressurized liquefied natural gas stream; - compressing at least one stream of flash gas. The present invention relates to a method having the following structure: [Background technology]

[0002] Such methods are particularly for extracting ethane and C3+ hydrocarbons from primary natural gas while producing pressurized treated natural gas that is later liquefied and then expanded for storage.

[0003] Ethylene, ethane, propylene, propane and heavier hydrocarbons can be extracted from gases such as natural gas, refinery gas, and synthesis gas obtained from other hydrocarbon sources such as coal, crude oil, naphtha, etc.

[0004] Natural gas is typically predominantly methane and ethane (e.g., methane and ethane constitute at least 50 mol% of natural gas). Natural gas may also contain small amounts of heavier hydrocarbons, such as propane, butane, and pentane, as well as hydrogen, nitrogen, and carbon dioxide. Summary of the Invention [Problem to be solved by the invention]

[0005] The invention described herein more specifically relates to the recovery of ethane, propane, and heavier hydrocarbons from natural gas. While the heavy hydrocarbons in natural gas, such as ethane, propane, and butane, may be highly valued by selling them separately at high purity, they may condense during transportation or (in the case of the heaviest hydrocarbons) freeze out liquefaction exchangers.

[0006] This can lead to problems such as installing liquid plugs at the transfer facility or shutting down the liquefaction plant to unblock frozen liquefaction exchangers.

[0007] U.S. Patent No. 6,578,379 describes a highly efficient process for recovering ethane and propane from a natural gas stream. Such processes are generally carried out in a highly efficient manner, particularly to obtain very high extraction rates (e.g., greater than 99 mol%) of ethane contained in the natural gas feed stream while minimizing energy consumption.

[0008] To obtain such an extraction yield, a method is known in which a stream highly depleted in ethane is used as the main reflux, i.e., the maximum reflux of the methane-ethane separation column.

[0009] For this purpose, a recycle stream is taken from the recompressed gas from the top of the separation column for methane and ethane, which is cooled countercurrently to the gas from the top of the separation column and then expanded to produce a main reflux stream which is introduced into the top of the separation column.

[0010] However, under certain operating conditions, the quality of the main reflux stream may deteriorate in terms of temperature and / or composition.

[0011] For example, if the main reflux stream is depleted of methane, the ethane separation rate in the column will decrease, further degrading the quality of the overhead stream produced at the top of the column, further exacerbating the methane depletion in the main reflux stream. A "snowball" effect will occur, resulting in a significant reduction in ethane extraction. This situation can occur especially if liquid is trapped in the upper plate of the column.

[0012] One object of the present invention is to provide a flexible and highly efficient method for extracting ethane and C3+ hydrocarbons from a primary stream of natural gas that has no or only a small effect on ethane extraction yield when variations in the overhead quality of the separation column occur. [Means for solving the problem]

[0013] For this purpose, the subject of the present invention is - separating the compressed flash gas stream into a fuel stream and a recycle stream; - cooling and at least partially expanding the recycle stream, and then introducing the cooled and expanded recycle stream into the top stage of the separation column. a method of the type described above, characterized in that it comprises:

[0014] The method according to the invention can have one or more of the following characteristics, individually or according to any technically possible combination:

[0015] - The methane concentration in the recycle stream is higher than 90 mol%, especially higher than 95 mol%.

[0016] - The recycle stream is introduced at the first stage from the top of the separation column.

[0017] The recycle stream is introduced into a first heat exchanger where it is cooled by heat exchange with the overhead stream from the separation column.

[0018] - separating the gas stream into a turbine feed stream which is introduced into the dynamic expansion turbine and a reflux stream which is cooled and statically expanded in a second upstream heat exchanger and then introduced into the separation column at a lower position than the recycle stream;

[0019] - cooling the recycle stream by passing the recycle stream through a second heat exchanger;

[0020] - When expanding the recirculation stream, the recirculation stream is passed through a static expansion valve.

[0021] At least a portion of the compressed purified natural gas is in heat exchange relationship with a stream of flash gas in a downstream heat exchanger.

[0022] - Upstream of the liquefaction unit, a recycle stream is removed from the compressed purified natural gas stream, which is cooled and expanded and introduced into a separation column.

[0023] The pressurized liquefied natural gas stream is expanded in a dynamic or static expansion element and then introduced into a flash drum for separation into an expanded liquefied natural gas stream that is introduced into storage and a flash gas stream.

[0024] - introducing the expanded liquefied natural gas into the storage section to produce at least one stream of flash gas into the storage section;

[0025] - Introduces a pressurized liquefied natural gas stream directly into storage without flowing it through a flash drum.

[0026] - Compression of the overhead stream from the separation column is performed in at least one first compressor coupled to a dynamic expansion turbine, followed by a compression system having in series a second compressor, a cooler for the gas compressed by the second compressor, and a third compressor to produce a stream of compressed purified natural gas.

[0027] - The overhead stream from the fractionation column is cooled and partially condensed and then introduced into an overhead drum, an ethane stream is recovered at the top of the overhead drum, and the bottom stream of the overhead drum is reintroduced into the fractionation column as reflux.

[0028] The entire gas stream obtained by separation of the cooled natural gas stream forms the turbine feed stream which is sent to the dynamic expansion turbine without separation.

[0029] The present invention further provides a plant for extracting ethane from a first natural gas stream, comprising: at least one first upstream heat exchanger adapted to cool the first natural gas stream to produce a cooled natural gas stream; a separator for separating the cooled natural gas stream into a liquid stream and a gas stream; - an expansion element for the liquid flow; a separation column for 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 level; a system for generating a turbine feed stream from the gas stream; - a dynamic expansion turbine suitable for expanding the turbine feed stream and a system for introducing the expanded stream from the dynamic expansion turbine into the separation column at a second level; 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; - a system for recovering and compressing at least a portion of the overhead stream from the separation column to produce a compressed purified natural gas stream; - a liquefaction unit for the compressed purified natural gas stream, the liquefied natural gas stream being suitable for producing a pressurized liquefied natural gas stream; a flash expansion system for the pressurized liquefied natural gas flow and a recovery reservoir for the expanded liquefied natural gas; - a system for recovering at least one stream of flash gas obtained by expansion of the pressurized liquefied natural gas stream; - a system for compressing at least one stream of flash gas; - a system for separating the compressed flash gas stream into a fuel stream and a recycle stream; a system for cooling and at least partially expanding the recycle stream and introducing the cooled and expanded recycle stream into the top stage of the separation column; The present invention relates to a plant equipped with

[0030] The invention will be better understood on reading the following description, given by way of example only, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0031]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

[0032] Throughout the following, the same reference numbers are used to identify liquid streams and pipes carrying the fluids, pressures referred to are absolute pressures, and percentages referred to are mole percentages.

[0033] The described method is modeled in a process simulator. The compressor is specified to have a polytropic efficiency of 82% and the turbine is specified to have an adiabatic efficiency of 86%.

[0034] A first apparatus 10 for extracting ethane according to the present invention is shown in FIG.

[0035] The first unit 10 is configured to simultaneously produce from an initial natural gas stream 12 an ethane-rich stream 14, a C3+ hydrocarbon-rich understream 16, an expanded liquefied natural gas 18, and a fuel stream 20, advantageously for recycle in the first unit 10.

[0036] Referring to FIG. 1, the first apparatus 10 includes an ethane extraction unit 22, a liquefaction unit 24, and a flash and storage unit 26 for liquefied natural gas.

[0037] The ethane extraction unit 22 includes first and second upstream heat exchangers 28, 30, a separation drum 32, and a column 34 for separating methane and C2+ hydrocarbons. Column 34 now includes a bottom reboiler 35.

[0038] The ethane extraction unit 22 further includes a dynamic expansion turbine 36 and a second compressor 40 connected to a first compressor 38, the first compressor 38 and the second compressor 40 having downstream chillers 42 and 44, respectively.

[0039] Ethane extraction unit 22 further includes a bottoms pump 46 , a fractionation column 48 having a bottoms reboiler 50 , and a reflux system 52 , which includes a condenser 54 , a reflux drum 56 , and a reflux pump 58 .

[0040] The natural gas liquefaction unit 24 is a known unit, in particular a C3MR unit or a DMR unit.

[0041] 1, the flash-storage unit 26 includes an expansion component 60, here a dynamic expansion turbine, a flash tank 62, and a pump 64 for conveying the liquefied natural gas to storage 66. In a variant, the expansion component 60 is a static expansion valve.

[0042] The reservoir 66 is, for example, an insulated storage tank.

[0043] In this example, the flash and storage unit 26 further comprises a downstream heat exchanger 68, a suction drum 70 if appropriate, and a compression device 72 comprising multiple compressors 74 mounted in series and separated by coolers 76.

[0044] A first method according to the present invention, which is carried out by the device 10, will now be described.

[0045] The initial natural gas forming stream 12 is advantageously dry, at least partially decarbonized and desulfurized natural gas.

[0046] The term "at least partially decarbonized" means that the concentration of carbon dioxide in the initial natural gas stream 12 is advantageously 50 ppmv or less.

[0047] Likewise, the concentration of water is less than 1 ppmv, advantageously less than 0.1 ppmv.

[0048] The concentration of sulfur-containing elements, including hydrogen sulfide, is less than 10 ppmv, advantageously less than or equal to 4 ppmv.

[0049] An example molar composition of the initial natural gas stream 12 is shown in the table below.

[0050] [Table 1]

[0051] More typically, the mole fraction of methane in the initial natural gas stream 12 is in the range of 75 mol% to 95 mol%, the mole fraction of C2 hydrocarbons is in the range of 3 mol% to 12 mol%, and the mole fraction of C3+ hydrocarbons is in the range of 1 mol% to 8 mol%.

[0052] The flow rate of the initial natural gas stream 12 is for example greater than 2,000 kmol / h, for example in the range of 2,000 kmol / h to 70,000 kmol / h, in particular 55,000 kmol / h.

[0053] The initial natural gas stream 12 has a temperature close to ambient, in particular in the range of 0°C to 40°C, here 21.5°C, and a pressure advantageously higher than 35 bar, in particular higher than 70 bar, in this example 81 bar.

[0054] The initial natural gas 12 is introduced into the first heat exchanger 28 and cooled within the first heat exchanger 28. The initial natural gas 12 forms a cooled natural gas stream 80. The initial natural gas 12 is now in a supercritical state and is therefore simply cooled. In an alternative, the initial natural gas is not in a supercritical state and is at least partially condensed in the first heat exchanger 28.

[0055] The initial natural gas temperature is below -20°C, particularly within the range of -25°C to -45°C, particularly -37°C.

[0056] Stream 80 is then introduced into separation drum 32 where it is separated into a liquid stream 82 that is recovered at the bottom of separation drum 32 and a gas stream 84 that is recovered at the top of separation drum 32. The flow rate of liquid stream 82 may be zero, especially if cooled natural gas stream 80 is in a supercritical state.

[0057] Liquid stream 82 flows through static expansion valve 86 to produce a mixed expanded phase 88. The mixed expanded phase 88 has a pressure of less than 50 bar, particularly less than 30 bar, for example 28.7 bar. Mixed expanded phase 88 is introduced into separation column 34 at a bottom level N1.

[0058] Gas stream 84 is separated into a main turbine feed stream 90 and a secondary reflux stream 92 .

[0059] The molar flow rate of turbine feed stream 90 is greater than the molar flow rate of secondary reflux stream 92, and specifically is in the range of 5% to 25% of the molar flow rate of secondary reflux stream 92.

[0060] The turbine feed stream 90 is introduced into the dynamic expansion turbine 36 and expanded in the dynamic expansion turbine 36 to a pressure of less than 50 bar, particularly less than 30 bar, for example 28.7 bar.

[0061] The dynamic expansion of stream 90 can recover over 10,000 kW of energy, for example 10,865 kW of energy.

[0062] The temperature of the cooled expanded stream 94 from the dynamic expansion turbine 36 is, for example, below -70°C, in particular below -80°C, for example -80.8°C.

[0063] Cooled and expanded stream 94 is then introduced into separation column 34 at level N2 above level N1.

[0064] Reflux stream 92 is introduced into static expansion valve 96 and expanded in static expansion valve 96 to a pressure of less than 50 bar, particularly less than 30 bar, particularly 28.7 bar. Reflux stream 92 is cooled in second upstream heat exchanger 30 to a temperature of less than -80°C, particularly less than -90°C, particularly -95.8°C.

[0065] The expanded and cooled reflux stream is introduced into separation column 34 at the top of separation column 34 at level N3 above level N2.

[0066] The pressure in the separation column 34 is preferentially in the range of 10 bar to 40 bar, in particular in the range of 20 bar to 40 bar, for example substantially 28.5 bar.

[0067] Separation column 34 produces an overhead stream 98. The overhead stream 98 is heated in second upstream heat exchanger 30 and then heated countercurrently to the initial natural gas 12 in first upstream heat exchanger 28 to produce a heated overhead stream 100.

[0068] The temperature of the heated overhead stream 100 is greater than 0°C, in particular greater than 15°C, for example 17.6°C.

[0069] The heated overhead stream 100 is then compressed in a compressor 38 connected to a turbine 36 and then cooled in a cooler 42 to obtain a stream at a pressure greater than 30 bar, specifically 34.6 bar.

[0070] This stream is then recompressed in compressor 40 and then cooled in chiller 44 to produce compressed purified natural gas stream 102 for liquefaction unit 24 .

[0071] The pressure of the compressed purified natural gas stream 102 is greater than 60 bar, in particular greater than 80 bar, for example 91 bar. The temperature of the compressed purified natural gas stream 102 is greater than 0°C, in particular greater than 10°C, more particularly 21.5°C.

[0072] Here, the coolers 42, 44 are supplied with a cooling stream having a temperature of less than 10° C., in particular 7° C. The cooling stream may in particular be air or water.

[0073] The compressed purified natural gas stream 102 is rich in methane. The methane concentration of the compressed purified natural gas stream 102 is greater than 99.0 mol%, particularly 99.1 mol%. The compressed purified natural gas stream 102 has a low nitrogen concentration, particularly a nitrogen concentration less than 1.0 mol%, and a low concentration of C2+ hydrocarbons, more particularly an ethane concentration less than 0.5 mol%, substantially 0.2 mol%.

[0074] Separation column 34 produces a bottoms stream 106 rich in C2+ hydrocarbons. Bottoms stream 106 may contain, for example, more than 95 mol% of the ethane present in original natural gas 12 and 100 mol% of the C3+ hydrocarbons present in this stream.

[0075] The temperature of the underflow 106 is higher than 10° C., in particular in the range of 20° C. to 30° C., for example 23.2° C. The underflow 106 contains less than 1000 ppmv of carbon dioxide, preferentially in the range of 200 ppmv to 500 ppmv of carbon dioxide, for example 313 ppmv of carbon dioxide. The methane concentration in the underflow 106 is less than 5 mol %, for example in the range of 0 mol % to 3 mol %, in particular less than 1 mol %.

[0076] The following table shows examples of underflow 106 compositions:

[0077] [Table 2]

[0078] A first lateral reboil stream 108 is extracted from separation column 34 at a level N5 below level N1, for example, 20 stages from the top of separation column 34.

[0079] The first liquid reboil stream 108 is sent to the first heat exchanger 28 where it is heated to a temperature above 0° C., particularly 8.25° C., by heat exchange with, particularly, the original natural gas 12. The reboil stream 108 is then reintroduced into the separation column 34 at a level N6 below the level N5, for example, 21 stages from the top of the separation column 34.

[0080] Similarly, a second liquid reboil stream 110 is extracted from separation column 34 below level N6, for example, at level N7, 22 stages from the top of separation column 34, and sent to bottom reboiler 35 where it is heated to a temperature above 0° C., for example, 10.7° C. Energy in excess of 1 MW, for example, 4 MW, is supplied to second liquid reboil stream 110.

[0081] The second liquid reboil stream 110 is then returned to the separation column 34 at a level N8 below level N7, which may be, for example, 23 stages from the top of the separation column.

[0082] The underflow 106 is pumped to the pump 46 and introduced into the fractionation column 48 at an intermediate level P1.

[0083] Fractionation column 48 produces an overhead stream 112 at the top of the column comprising less than 1 mol % C3+ hydrocarbons, more specifically less than 1 mol % propane.

[0084] The overhead stream 112 is partially condensed in cooler 54 and then separated in reflux drum 56 to produce an ethane-rich stream 14 at the top and a liquid reflux stream 114 at the bottom which is reintroduced into the top of fractionation column 48 after pumping by reflux pump 58.

[0085] Ethane-rich stream 14 comprises greater than 96 mol% of the ethane contained in original natural gas 12. Ethane-rich stream 14 comprises greater than 97 mol% of the ethane.

[0086] The ethane-rich stream 14 is now gaseous. In a variant (not shown), the ethane-rich stream 14 is a liquid taken from the liquid stream 114.

[0087] The C3+ hydrocarbon stream contains less than 500 ppmv of ethane, more specifically less than 100 ppmv of ethane.

[0088] The compressed purified natural gas stream 102 is passed to the liquefaction unit 24 which produces a pressurized liquefied natural gas stream 120 in a known manner.

[0089] The pressure of the compressed natural gas stream is greater than 20 bar, in particular in the range of 20 bar to 90 bar, preferably 73 bar. The temperature of the compressed natural gas stream is less than -120°C, in particular less than -130°C, preferably -136.8°C.

[0090] The compressed liquefied natural gas 120 is introduced into an expansion element 60, here a dynamic expansion turbine. The compressed liquefied natural gas 120 is expanded to a pressure of less than 5 bar, particularly less than 2 bar, for example 1.25 bar, to produce a flashed liquefied natural gas stream 122.

[0091] The flashed liquefied natural gas stream 122 is introduced into the flash drum 62 where it is separated into an expanded liquefied natural gas stream 124 and a first stream 126 of flash gas.

[0092] The expanded liquefied natural gas stream 124 is pumped by pump 64 to storage tank 66 to produce expanded liquefied natural gas 18 .

[0093] A first stream of flash gas 126 is recovered at the top of flash drum 62. The first stream of flash gas 126 is introduced into downstream heat exchanger 68 where it is heated countercurrently against a portion of the compressed purified natural gas 102 and reintroduced into flashed liquefied natural gas stream 122 upstream of flash drum 62.

[0094] After heat exchange in the downstream heat exchanger 68, the heated flash gas stream 130 thus produced has a temperature greater than -60°C, in particular substantially 5°C. The methane concentration in the heated flash gas stream 130 is very high, for example greater than 80 mol%, for example greater than 85 mol%, in particular greater than 90 mol%. Such a methane concentration is advantageously greater than 95 mol%, in particular greater than 96 mol%, for example 96.46 mol%.

[0095] The nitrogen concentration in the heated flash gas stream 130 is less than 20 mol%, for example less than 15 mol%, in particular less than 10 mol%, and is advantageously less than 5 mol%, in particular less than 4 mol%, for example substantially 3.54 mol%.

[0096] The ethane concentration in the heated flash gas stream 130 is less than 50 ppmv, particularly less than 10 ppmv, for example 5 ppmv.

[0097] After flowing through suction flask 70, heated flash gas stream 130 is compressed in compressor 72 to a pressure greater than 25 bar, particularly greater than 30 bar, for example 60 bar, to produce compressed flash gas stream 132.

[0098] The compressed flash gas stream 132 is separated into a fuel stream 20 and a recycle stream 134 .

[0099] The fuel stream 20 is sent to the fuel gas network of the device 10 and is configured to be supplied to, for example, the gas turbine of the natural gas liquefaction unit 24 or the gas turbine of the current generation unit that is configured to be supplied to, for example, the compressor 40 of the device 10 or other equipment.

[0100] The pressure of the recirculation stream 134 is higher than 30 bar, particularly higher than 50 bar, for example 58.5 bar.

[0101] The recirculation stream 134 is continuously conveyed to the first heat exchanger 28 and then to the second heat exchanger 30 and is cooled to a temperature of less than -80°C, particularly less than -90°C, for example -95.5°C.

[0102] Thereafter, the recirculation stream 134 is expanded in the static expansion valve 136 to a pressure of less than 50 bar, particularly less than 30 bar, for example 28.7 bar, and is introduced into the separation column 34 at the top level N9, for example at the first stage from the top of the separation column 34. The level N9 is above the level N3 for introducing the expanded and cooled reflux stream.

[0103] As described above, since ethane remains in the liquefied natural gas 18 or is continuously extracted in the separation column 34 and then in the fractionation column 48, the recirculation stream 134 from the flash gas stream 126 is very rich in methane.

[0104] Therefore, the composition of the reflux introduced into the top of the separation column 34 remains very rich in methane regardless of the quality variation of the top stream 98 of the separation column 34.

[0105] Due to the presence of the new reflux, operational flexibility is further provided during the implementation of the method and also during the design phase.

[0106] Therefore, it is possible to optimize the overall energy consumption of the ethane extraction unit 22 and the liquefaction unit 24 by adjusting the parameters of the two units 22, 24 in order to best select the compressors and compressor operating modes required in the ethane extraction unit 22 and the liquefaction unit 24. This leads to a significant reduction in investment costs and also in operating costs, as shown in the following example:

[0107] In a variant (not shown), the heated overhead stream 100 is compressed at the outlet of a compressor 38 connected to a turbine 36 in a compressor unit with two compression stages of the same power, the total power being equal to the power of the compressor 40. The compressor unit has an intercooler between the compression stages to cool the gas. The configuration thus obtained saves 5.8 MW of power.

[0108] A second apparatus 140 for carrying out a second method according to the invention is shown in FIG.

[0109] The second method of the present invention is similar to the first method of the present invention, except that the second method includes removing a recycle stream 142 from the compressed purified natural gas stream 102.

[0110] Advantageously, the molar flow rate of the recycle stream 142 is less than the molar flow rate of the remaining stream 102 of compressed purified natural gas after removal of the recycle stream 142 upon introduction into the liquefaction unit 24 .

[0111] The pressure of the recycle stream 142 is greater than 50 bar, in particular greater than 80 bar, for example 90 bar. The recycle stream 142 is successively introduced into the first heat exchanger 28 and then into the second heat exchanger 30 to be cooled to a temperature below -90°C, preferentially below -95°C, for example substantially -95.4°C.

[0112] The recycle stream 142 is then expanded to a pressure of less than 50 bar, particularly less than 30 bar, particularly 28.7 bar, and introduced into the separation column 34 between the recycle stream 134 and the reflux stream 92 .

[0113] A third apparatus 150 for carrying out a third method according to the present invention is shown in FIG.

[0114] The apparatus 150 differs from the first apparatus 10 in that it includes a collection system 152 for collecting and recompressing the evaporative gas produced in the reservoir 66 .

[0115] The collection system 152 includes a guard drum 154 and a compression device 156 including a plurality of compression stages 158 separated in pairs by coolers 160 .

[0116] A second stream 162 of flash gas obtained by vaporizing the liquefied natural gas in the storage section 66 is collected at the top of the storage section 66 and then introduced into the compressor 156 where it is compressed to a pressure above 25 bar, particularly in the range of 26 bar to 70 bar, for example 60 bar.

[0117] The second stream 164 of compressed flash gas thus produced is separated into a fuel stream 20 and a recycle stream 134, which is cooled in heat exchangers 28, 30 and expanded in expansion valve 136 before being reintroduced into separation column 34.

[0118] 3, the apparatus 150 advantageously does not include an expansion component 60. The compressed liquefied natural gas 120 from the liquefaction unit 24 is introduced directly into a reservoir 66 for liquefied natural gas and flashed within the reservoir 66.

[0119] A fourth apparatus 170 for carrying out a fourth method according to the present invention is shown in FIG.

[0120] The fourth apparatus 170 differs from the first apparatus 10 in that the storage unit 66 includes a system 152 for collecting evaporative gases, similar to the third apparatus 150 .

[0121] During the fourth method of the present invention, the first stream 132 of compressed flash gas and the second stream 164 of compressed flash gas are mixed, and then the mixture is separated into the fuel stream 20 and the recycle stream 134.

[0122] As before, the recycle stream 134 is passed through heat exchangers 28, 30 and then expanded in static expansion valve 136 before being reintroduced into separation column 34.

[0123] A fifth apparatus 200 for carrying out a fifth method according to the invention is shown in FIG.

[0124] The fifth method differs from the second method shown in FIG. 2 in that the entire gas stream 84 recovered from the drum 32 forms the turbine feed stream 90 that is sent to the dynamic expansion turbine 36 without any separation.

[0125] The above-described invention allows the composition of the reflux stream of separation column 34 to be maintained substantially constant, preventing the snowball effect that would occur during fluctuations in the composition of the overhead stream 98 extracted from separation column 34 in the absence of the feed of recycle stream 134.

[0126] Therefore, the method is simple and effective in maintaining a constant concentration of extracted ethane without increasing capital or operating costs.

[0127] The energy consumption of the method is detailed in the table below.

[0128] [Table 3]

[0129] As shown in the table above, the total power consumed in the presence of reflux resulting from recycle stream 134 represents a significant reduction in power consumption and specific power divided by the flow rate of liquefied natural gas produced by the system.

Claims

1. A method for extracting ethane from an initial natural gas stream, comprising: - cooling the initial natural gas stream in at least one first upstream heat exchanger to produce a cooled natural gas stream; - separating the cooled natural gas stream into a liquid stream and a gas stream; - expanding the liquid stream and introducing at least one stream from the liquid stream at a first level into a separation column for separating methane and C2+ hydrocarbons; - producing a turbine feed stream from the gas stream; - 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; - introducing a bottoms stream rich in C2+ hydrocarbons recovered from the separation column into a fractionation column and recovering a stream of ethane from the fractionation column; - recovering and compressing at least a portion of the overhead stream from the separation column to produce a compressed purified natural gas stream; - liquefying the compressed purified natural gas stream in a liquefaction unit to produce a pressurized liquefied natural gas stream; - performing a flash expansion of the pressurized liquefied natural gas stream and recovering the expanded liquefied natural gas in a storage section; - recovering at least one stream of flash gas obtained by expanding the pressurized liquefied natural gas stream; - compressing at least one stream of the flash gas; - separating the compressed flash gas stream into a fuel stream and a recycle stream; - at least partially cooling and expanding the recycle stream and then introducing the cooled and expanded recycle stream into the top stage of the separation column. A method having the above steps.

2. The method according to claim 1, wherein the methane concentration of the recycle stream is higher than 90 mol%.

3. The method according to claim 1 or 2, wherein the recycle stream is introduced at the first stage from the top of the separation column.

4. The method according to any one of claims 1 to 3, wherein the recycle stream is introduced into the first upstream heat exchanger and cooled by heat exchange with the overhead stream from the separation column.

5. The method according to any one of claims 1 to 4, wherein the gas stream is separated into a turbine feed stream introduced into the dynamic expansion turbine and a reflux stream cooled in a second upstream heat exchanger for static expansion and then introduced into the separation column at a position lower than the recycle stream.

6. The method according to claim 5, wherein when cooling the recirculation flow, the flow of the recirculation flow is passed through the second upstream heat exchanger.

7. The method according to any one of claims 1 to 6, wherein when expanding the recirculation flow, the flow of the recirculation flow is passed through a static expansion valve.

8. The method according to any one of claims 1 to 7, wherein at least a part of the compressed purified natural gas is in a heat exchange relationship with the flow of the flash gas in the downstream heat exchanger.

9. The method according to any one of claims 1 to 8, wherein upstream of the liquefaction unit, except for the recirculation flow from the flow of the compressed purified natural gas, the recirculation flow is cooled and expanded and introduced into the separation column.

10. The method according to any one of claims 1 to 9, wherein the flow of the pressurized liquefied natural gas is expanded by a dynamic or static expansion component, and then introduced into a flash drum to be separated into the expanded liquefied natural gas introduced into the storage unit and the flow of the flash gas.

11. The method according to any one of claims 1 to 10, wherein when the expanded liquefied natural gas is introduced into the storage unit, at least one flow of the flash gas is generated in the storage unit.

12. The method according to claim 11, wherein the flow of the pressurized liquefied natural gas is directly introduced into the storage unit without passing through the flash drum.

13. The compression of the overhead stream from the separation column is performed by at least one first compressor connected to the dynamic expansion turbine, and then a second compressor, a cooler for the gas compressed by the second compressor, and a third compressor are continuously used to generate the flow of the compressed purified natural gas. The method according to any one of claims 1 to 12.

14. The method according to any one of claims 1 to 13, wherein the overhead stream from the fractionation column is cooled and partially condensed, then introduced into an overhead drum, the flow of ethane is recovered at the top of the overhead drum, and the bottom stream of the overhead drum is reintroduced into the fractionation column as reflux.

15. The method according to any one of claims 1 to 14, wherein the entire gas stream obtained by separating the cooled natural gas stream forms a turbine feed stream that is sent to the dynamic expansion turbine without separation.

16. An apparatus for extracting ethane from a first natural gas stream, - At least one first upstream heat exchanger suitable for cooling an initial natural gas stream to produce a cooled natural gas stream, - A separator for separating the cooled natural gas stream into a liquid stream and a gas stream, - An expansion component for the liquid stream, - A separation column for 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 level, - A system for generating a turbine feed stream from the gas stream, - A dynamic expansion turbine suitable for expanding the turbine feed stream, and a system for introducing the expanded stream from the dynamic expansion turbine into the separation column at a second level, - A fractionation column, a system for introducing the 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, - A system for recovering and compressing at least a part of the overhead stream from the separation column to produce a compressed purified natural gas stream, - A liquefaction unit for the compressed purified natural gas stream, suitable for producing a pressurized liquefied natural gas stream, - A system for flash expansion of the pressurized liquefied natural gas stream, and a storage unit for recovering the expanded liquefied natural gas, - A system for recovering at least one stream of flash gas obtained by expansion of the pressurized liquefied natural gas stream, - A system for compressing at least one stream of the flash gas, - A system for separating the compressed flash gas stream into a fuel stream and a recycle stream, - A system for cooling and at least partially expanding the recycle stream and introducing the cooled and expanded recycle stream into the overhead stage of the separation column A device comprising.

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