Title of Invention	A PROCESS AND PLANT FOR SEPARATING A COOLED MIXTURE UNDER PRESSURE CONTAINING METHANE, C2 AND HIGHER HYDROCARBONS
Abstract	The present invention relates to method and an installation for separating a gas mixture and gases obtained by said installations The invention concerns a method and an installation for cryogenic separation of constituents of a natural gas under pressure (14) with a first phase separator (B1) whereof the constituents of each of the phases are separated in a distillation column (C1). Part of the gas fraction (5) derived from the head of the column (C1) is recycled at the last stage thereof the method further comprises diverting (9) part of a first head fraction (3) derived from the first phase separator. Additionally, the method comprises separating a first base fraction (4) derived from the first separator, in a second separator (B2). The invention also relates to the other embodiments. (fig.1)

Title of Invention

A PROCESS AND PLANT FOR SEPARATING A COOLED MIXTURE UNDER PRESSURE CONTAINING METHANE, C2 AND HIGHER HYDROCARBONS

Abstract

The present invention relates to method and an installation for separating a gas mixture and gases obtained by said installations The invention concerns a method and an installation for cryogenic separation of constituents of a natural gas under pressure (14) with a first phase separator (B1) whereof the constituents of each of the phases are separated in a distillation column (C1). Part of the gas fraction (5) derived from the head of the column (C1) is recycled at the last stage thereof the method further comprises diverting (9) part of a first head fraction (3) derived from the first phase separator. Additionally, the method comprises separating a first base fraction (4) derived from the first separator, in a second separator (B2). The invention also relates to the other embodiments. (fig.1)

Full Text	Method and installation for separating a gas mixture containing metheme by distillation, and gases obtained by this separation The present invention relates, in general and according to a first of its aspects, to a separation process for separating the constituents of natural gas into a first gas fraction, rich in methane and essentially containing no C2 and higher hydrocarbons, and a second gas fraction, rich in C2 and higher hydrocarbons and essentially containing no methane. More specifically, the invention relates, according to its first aspect, to a process for separating a cooled mixture under pressure, containing methane and C2 and higher hydrocarbons, into a light final fraction enriched in methane and a heavy final fraction enriched in C2 and higher hydrocarbons, comprising a first step (I) in which (la) the cooled mixture under pressure is separated in a first tank into a relatively more volatile first top fraction and a relatively less volatile first bottom fraction, in which (lb) the first bottom fraction is introduced into a middle part of a distillation column, in which (Ic) the heavy final fraction enriched in C2 and higher hydrocarbons is collected as second bottom fraction in a low part of the column, in which (Id) the first top fraction, after having been expanded in a turbine, is introduced into a high part of the distillation column, in which (le) a second top fraction enriched in methane is collected in the high part of the column, in which (If), to obtain the light final fraction, the second top fraction then undergoes compression and cooling and in which (Ig) a first tap-off fraction is tapped off the light final fraction, this process comprising a second step (II) in which (Ila) the first tap-off fraction, after it has been cooled and liquefied, is introduced into the high part of the distillation column. Such a process is known from the prior art. Thus, patent US-5 881 569 discloses a process according to the preamble described above. Extraction of the ethane contained in natural gas maybe carried out using known processes, as described in the patents US-4 140 504, US-4 157 904, US-4 171 964 and US-4 278 547. Although the processes described in those documents do have a certain benefit, in practice they make it possible to obtain at best only a degree of ethane recovery of around 85%. They involve liquid/gas separators, heat exchangers, expanders (usually in the form of turbines) , compressors and distillation columns. More recently, other processes have been made public, for example by patents US-4 649 063, US-4 854 955, US-5 555 748 and US-5 568 737. Although these more recent processes can allow quite satisfactory ethane and other hydrocarbon extraction yields to be obtained, these processes require, for obtaining fractions enriched in methane or in C2 and higher hydrocarbons, relatively high energy consumption. In this context, the present invention aims to reduce the energy consumption during production of fractions enriched in methane or in C2 and higher hydrocarbons, while maintaining very high extraction yields compared to the processes of the prior art. For this purpose, the process of the invention, which moreover is in accordance with the generic definition given in the above preamble, is essentially characterized in that it furthermore includes a third step in which the first bottom fraction is subjected to a plurality of sub-steps comprising warming, passage through a second tank and a separation into a relatively more volatile third top fraction and a relatively less volatile third bottom fraction, in which the third bottom fraction is introduced into the middle part of the distillation column and in which the third top fraction, after it has been cooled and liquefied, is introduced into the high part of the distillation column. Another process, as described in patent US-5 566 554, uses two liquid-gas separators, a liquid fraction collected at the bottom of the first separator of which is heated and then introduced into a second separator, This technique makes it possible in particular to improve the extraction of the methane contained in the bottom fraction issuing from the first separator and above all to use the expansion of this bottom fraction to cool, in a heat exchanger, the stream of natural gas to be treated that enters the plant. However, this known process does not make it possible to obtain a high ethane extraction level since the amount of reflux generated by the technique is low and the ethane content of this reflux is relatively high. The present invention overcomes these problems by the use of two means. Firstly, the invention provides for a portion of the methane-rich fraction from the top of the column to be tapped off and, after being compressed and cooled, reintroduced into the last stage of the column. This makes it possible to obtain a sufficient amount of reflux of excellent quality, since the C2 content is very low, for example less than 0.1 mol%. Secondly, the invention provides for a portion of the first top fraction issuing from the first separator to be tapped off into the column before the expansion step in the turbine. This second tapped-off fraction is cooled and liquefied before being introduced into the column. Proceeding in this way makes it possible to limit the amount of recycled and liquefied gas mentioned above and reduce the associated compression costs. The invention may furthermore provide for a second tap-off fraction to be tapped off from the first top fraction and for this second tap-off fraction, after it has been cooled and liquefied, to be introduced into the high part of the distillation column. According to one possible method of implementing the invention, the second tap-off fraction is cooled and partially condensed and then separated in a third tank into a relatively more volatile fourth top fraction, which is cooled and liquefied and then introduced into the high part of the distillation column, and a relatively less volatile fourth bottom fraction, which is warmed and then separated in a fourth tank into a relatively more volatile fifth top fraction, which is cooled and liquefied and then introduced into the high part of the distillation column, and a relatively less volatile fifth bottom fraction which is warmed and then sent into the second tank. The invention may furthermore provide for the lower part of the distillation column to comprise a plurality of stages connected in pairs to a lateral reboiler or to a plurality of lateral reboilers. The invention may also provide, in order to obtain the light final fraction, for the second top fraction, after it has left the distillation column, to be successively warmed, compressed a first time in a first compressor coupled to the expansion turbine, compressed a second time in a second compressor, and cooled. The invention may also provide for the high part of the distillation column to comprise at least two successive stages, the first of which is the lowest one, and for the fifth top fraction to be introduced above the first stage. The invention may also provide for the high part of the distillation column to comprise at least three successive stages, the first of which is the lowest one, and for the fifth top fraction to be introduced above the second stage. The invention may also provide for the high part of the distillation column to comprise at least two successive stages, the first of which is the lowest one, and for the second tap-off fraction to be introduced above the first stage. The invention may also provide for the high part of the distillation column to comprise at least three stages, the first of which is the lowest one, into which column the first tap-off fraction is introduced into a low part of the last stage, and for the third top fraction to be introduced below the last stage. Finally, the invention may provide for the third top fraction to be introduced at the first stage of the high part of the distillation column. The invention may also provide for the middle part of the distillation column to comprise at least two successive stages, the first of which is the lowest one, and into which column the third bottom fraction is introduced, at least at the first stage, and for the first top fraction to be introduced above the first stage. According to a second of its aspects, the invention relates to a gas enriched in methane obtained by the present process and to a liquefied gas enriched in C2 and higher hydrocarbons obtained by the present process. According to a third of its aspects, the invention relates to a plant for separating a cooled mixture under pressure, containing methane and C2 and higher hydrocarbons, into a light final fraction enriched in methane and a heavy final fraction enriched in C2 and higher hydrocarbons, comprising means for carrying out a first step (I) in which (la) said cooled mixture under pressure is separated in a first tank into a relatively more volatile first top fraction and a relatively less volatile first bottom fraction, in which (lb) the first bottom fraction is introduced into a middle part of a distillation column, in which (Ic) the heavy final fraction enriched in C2 and higher hydrocarbons is collected as second bottom fraction in a low part of the column, in which (Id) the first top fraction, after having been expanded in a turbine, is introduced into a high part of the distillation column, in which (le) a second top fraction enriched in methane is collected in the high part of the column, in which (If), to obtain the light final fraction, the second top fraction then undergoes compression and cooling and in which (Ig) a first tap-off fraction is tapped off the light final fraction, this plant comprising means for carrying out a second step (II) in which (Ila) the first tap-off fraction, after it has been cooled and liquefied, is introduced into the high part of the distillation column, this plant comprising means for carrying out a third step (III) in which (IIIa) the first bottom fraction is subjected to a plurality of sub-steps comprising warming, passage through a second tank and separation into a relatively more volatile third top fraction and a relatively less volatile third bottom fraction, in which (IIIb) the third bottom fraction is introduced into the middle part of the distillation column and in which IIIc) the third top fraction, after it has been cooled and liquefied, is introduced into the high part of the distillation column. The invention will be more clearly understood and other obj ectives, features, details and advantages thereof will become more clearly apparent during the description that follows, with reference to the appended schematic drawings given solely by way of nonlimiting example, in which: figure 1 is a schematic diagram showing the operation of a plant according to one possible embodiment of the invention; and figure 2 is a schematic diagram showing the operation of a plant according to another preferred embodiment of the invention. The following symbols may be seen in particular in both these figures: FC, standing for flow controller; GT, standing for gas turbine; LC, standing for liquid level controller; PC, standing for pressure controller; SC, standing for speed controller; and TC, standing for temperature controller. For the sake of clarity and concision, the lines used in the plants of figures 1 and 2 will be identified by the same reference symbols as the gas fractions that flow therein. Referring to figure 1, the plant shown is intended to treat a dry natural gas in order in particular to isolate from it, on the one hand, a fraction composed mainly of methane, essentially containing no C2 and higher hydrocarbons, and, on the other hand, a fraction composed mainly of C2 and higher hydrocarbons, essentially containing no methane. Dry natural gas 14 is firstly separated into a fraction 15, which is cooled in a heat exchanger El, and a fraction 16, which is sent into a line. The flow of the fraction 16 is controlled by a control valve 17, the opening of which varies depending on the temperature of a fraction 45. On leaving the exchanger El, the fraction 15 is mixed with the fraction 16 to give a cooled fraction 18. The fraction 18 is then introduced into a liquid/gas separator tank Bl in which this fraction 18 is separated into a relatively more volatile first top fraction 3 and a relatively less volatile bottom fraction 4. The first top fraction 3 is expanded in a turbine Tl in order to deliver an expanded fraction 19 which is introduced into the middle part of a distillation column C1. Next, on the one hand, the heavy final fraction 2 enriched in C2 and higher hydrocarbons collects in a low part of the distillation column C1 as second bottom fraction 2. This heavy final fraction 2 is transported in a line having a controlled-opening valve 60, the opening of which depends on the level of liquid contained at the bottom of the column C1. On the other hand, a second top fraction 5 enriched in methane collects in a high part of the distillation column CI. This second top fraction 5 is then warmed in the exchanger El in order to deliver the warmed fraction 2 0 and then undergoes a first compression in a first compressor Kl coupled to the turbine Tl in order to deliver a compressed fraction 21. The fraction 21 then undergoes a second compression in a second compressor K2 supplied by a gas turbine, the speed of which is controlled by a speed controller slaved to a pressure controller connected to the line carrying the second top fraction 5, in order to deliver another compressed fraction 22. The latter is then cooled by air in a heat exchanger Al in order to deliver a compressed and cooled fraction 23. The fraction 23 is then divided into a first tap-off fraction 6 and a light final fraction 1 enriched in methane. The first tap-off fraction 6 is then cooled and liquefied in the heat exchanger El in order to give a cooled fraction 24 which is conveyed in a line having a control valve 25, whose opening depends on the flow rate, and is then introduced into the high part of the distillation column Cl. A second tap-off fraction 9 is tapped off the first top fraction 3 and cooled and liquefied in the heat exchanger El in order to deliver a cooled fraction 26. The latter is sent in a line having a control valve 27, with an opening that depends on the flow rate, and is then introduced into the high part of the distillation column Cl. The first bottom fraction 4 is transported in a line having a control valve 2 8, whose opening depends on the level of liquid in the bottom of the separator tank Bl. The first bottom fraction 4 is then warmed in the exchanger El in order to deliver a warmed fraction 29. The fraction 29 is then introduced into a liquid/gas separator tank B2 in order to be separated into a relatively more volatile third top fraction 7 and a relatively less volatile third bottom fraction 8. The third bottom fraction 8 is transported in a line having a control valve 30 whose opening depends on the level of liquid in the bottom of the separator tank B2. The third bottom fraction 8 is then introduced into the middle part of the distillation column C1. The third top fraction 7 is cooled and liquefied in the exchanger El in order to give a cooled fraction 31. The latter is conveyed in a line having a control valve 32, whose opening is controlled according to the pressure, and is then introduced into the distillation column CI. The distillation column C1 has, in its low part, several stages which are connected in pairs by warming circuits 33, 34, 35 which are individually connected to the heat exchanger El. Each of these warming circuits constitutes a lateral reboiler. The temperature of the fluid flowing in each of these circuits 33, 34, 35 is regulated by means of controlled-opening valves positioned on branch-off lines that do not pass through the exchanger El. The opening of these valves is controlled by temperature controllers connected to the lines. These controllers, respectively 36, 37 and 38, are positioned downstream of the zone in which the fractions are mixed after they have passed through the exchanger El and/or the branch-off lines. Referring now to figure 2, it may be seen that most of the elements contained in figure 1 are repeated in figure 2, except in particular for the addition of a circuit having two separation tanks. Thus, in the same way as in figure 1, the plant shown is intended to treat a dry natural gas, in particular to isolate from it, on the one hand, a fraction mainly composed of methane, containing essentially no C2 and higher hydrocarbons, and, on the other hand, a fraction mainly composed of C2 and higher hydrocarbons, essentially containing no methane. Dry natural gas 14 is firstly separated into a fraction 15 which is cooled in a heat exchanger El and into a fraction 16 which is sent into a line. The flow of the fraction 16 is regulated by a control valve 17 whose opening varies according to the temperature of fraction 45. On leaving the exchanger El, the fraction 15 is mixed with the fraction 16 to give a cooled fraction 18. The fraction 18 is then introduced into a liquid/gas separator tank Bl in which this fraction 18 is separated into a relatively more volatile first top fraction 3 and a relatively less volatile first bottom fraction 4. The first top fraction 3 is expanded in a turbine Tl in order to deliver an expanded fraction 19 which is introduced into the middle part of a distillation column C1. The heavy final fraction 2 enriched in C2 and higher hydrocarbons then collects, on the one hand, in a low part of the distillation coliimn C1 as second bottom fraction 2. This heavy final fraction 2 is transported in a line having a controlled-opening valve 60 whose opening depends on the level of liquid contained in the bottom of the column C1. On the other hand, a second top fraction 5 enriched in methane collects in a high part of the distillation column C1. This second top fraction 5 is then warmed in the exchanger El in order to deliver a warmed fraction 20 and then undergoes a first compression in a first compressor Kl coupled to the turbine Tl in order to deliver a compressed fraction 21. The fraction 21 then undergoes a second compression in a second compressor K2 supplied by a gas turbine, the speed of which is regulated by a speed controller slaved to a pressure controller connected to the line carrying the second top fraction 5, in order to deliver another compressed fraction 22. The latter is then cooled by air in a heat exchanger Al in order to deliver a compressed and cooled fraction 23. The fraction 23 is then divided into a first tap-off fraction 6 and a light final fraction 1 enriched in methane. The first tap-off fraction 6 is then cooled in the heat exchanger El in order to give a cooled fraction 24 which is conveyed in a line having a control valve 25, whose opening depends on the flow rate, and is then introduced into the high part of the distillation column C1. A second tap-off fraction 9 is tapped off the first top fraction 3 and cooled in the heat exchanger El in order to deliver a cooled fraction 26. The latter is conveyed in a line which, unlike in figure 1, has a control valve 39 whose opening depends on the flow rate. The cooled fraction 26 is then introduced into a liquid/gas separator tank B3 in order to be separated into a relatively more volatile fourth top fraction 10 and a relatively less volatile fourth bottom fraction 11. The collected fourth top fraction is then cooled in the exchanger El in order to give a cooled and liquefied fraction 40. The cooled and liquefied fraction 40 is then conveyed in a line having a control valve 27 whose opening depends on the flow rate and is then introduced into the high part of the distillation column C1. The fourth bottom fraction 11 is transported in a line having a control valve 41 whose opening depends on the level of liquid in the bottom of the separator tank B3. The fourth bottom fraction 11 is then warmed in the exchanger El in order to give a warmed fraction 42. This warmed fraction 42 is separated in a fourth tank B4 into a relatively more volatile fifth top fraction 12 and a relatively less volatile fifth bottom fraction 13. The fifth top fraction 12 is cooled and liquefied in the exchanger El in order to produce a cooled and liquefied fraction 43. The latter is then transported in a line having a control valve 44 whose opening depends on the pressure in the line and is then introduced into the high part of the distillation column C1. The relatively less volatile fifth bottom fraction 15 is transported in a line having a valve 62 whose opening is controlled by a controller that controls the level of liquid contained in the tank B4. The first bottom fraction 4 is transported in a line having a control valve 28 whose opening depends on the level of liquid in the bottom of the separator tank Bl, The first bottom fraction 4 and the fifth bottom fraction 13 are then joined together to give a mixed fraction 63 which is warmed in the exchanger El in order to deliver a warmed fraction 29. The fraction 29 is then introduced into a liquid/gas separator tank B2 in order to be separated into a relatively more volatile third top fraction 7 and a relatively less volatile third bottom fraction 8. The third bottom fraction 8 is transported in a line having a control valve 30 whose opening depends on the level of liquid in the bottom of the separator tank B2. The third bottom fraction 8 is then introduced into the middle part of the distillation column C1. The third top fraction 7 is cooled and liquefied in the exchanger El in order to give a cooled and liquefied fraction 31. The latter is conveyed in a line having a valve 32 whose opening is controlled according to the pressure and is then introduced into the distillation column C1. The distillation column C1 comprises, in its low part, several trays that are connected in pairs by warming circuits 33, 34, 35 which are connected individually to the heat exchanger El. Each of these warming circuits constitutes a lateral reboiler. The temperature of the fluid flowing in each of these circuits 33, 34, 35 is regulated by means of controlled-opening valves positioned on tap-off lines that do not pass through the exchanger El. The opening of these valves is controlled by temperature controllers connected to the lines. These controllers, 36, 37 and 38 respectively, are positioned downstream of the zone in which the fractions are mixed after they have passed through the exchanger El and/or the tap-off lines. The ethane extraction process using a plant according to diagram 1 allows more than 99% of the ethane contained in a natural gas to be recovered. According to a modeling of the plant of diagram 1 in operation, the charge of dry natural gas (14) at 24°C and 62 bar, the flow rate of which is 15000 kmol/h, and composed of 0.4998 mol% CO2, 0.3499 mol% N2, 89.5642 mol% methane, 5.2579 mol% ethane, 2.3790 mol% propane, 0.5398 mol% isobutane, 0.6597 mol% n-butane, 0 .2399 mol% isopentane, 0.1899 mol% n-pentane, 0 .1899 mol% n-hexane, 0.1000 mol% n-heptane and 0.0300 mol% n-octane, is cooled down to -42°C and partially condensed to 61 bar in the heat exchanger El in order to form the fraction 18. The liquid and gas phases are separated in the tank Bl. The first top fraction 3, which is a 13776 kmol/h stream, is divided into two streams: (a) the main stream 45, which has a flow rate of 11471 kmol/h, is expanded in the turbine Tl to a pressure of 23.20 bar. The dynamic expansion allows 3087 kW of energy to be recovered and allows this stream to be cooled down to a temperature of -83.41°C. This stream 19, which is partially condensed, is sent into the column Cl. The stream 19 enters this column at a stage 46 which is the tenth stage starting from the highest stage of the column Cl. Its intake pressure is 23.05 bar and its temperature is -83.57°C; (b) the 2305 kmol/h secondary stream 9, which is liquefied and cooled down to -101.40°C in the exchanger El in order to form the fraction 26. This fraction 26, which comprises 4.55 mol% ethane, is expanded to 23.20 bar at a temperature of -101.68°C and is then introduced into a stage 47 of the column C1, which is the fifth stage starting from the highest stage of the column. The first bottom fraction 4 from the tank Bl, the flow rate of which is 1224 kmol/h and which comprises 54.27 mol% methane and 13.24 mol% ethane, is expanded to a pressure of 40.0 bar and then warmed in the exchanger El from -52.98°C to -38.00°C in order to obtain the fraction 29. The latter is introduced into the separation tank B2. The top fraction 7 issuing from the tank B2, the flow rate of which is 439 kmol/h and the ethane content is 6.21 mol%, is cooled from -38.00°C to -101.40°C and liquefied in order to obtain the fraction 31. The latter is then expanded to 23.2 bar at -101.47°C and then introduced into the column C1 at a stage 48 which is the sixth stage starting from the highest stage of the column. The bottom fraction 8, the flow rate of which is 784 kmol/h and the ethane content is 17.18 mol%, is expanded to 23.2 bar and -46.46°C and then introduced into the column C1 at a stage 49 which is the twelfth stage starting from the highest stage of the column. The column C1 produces the top fraction 5 at a pressure of 23 bar and a temperature of -103.71°C with a flow rate of 15510 kmol/h. This top fraction 5 contains no more than 0.05 mol% ethane. The top fraction 5 is warmed in the exchanger El in order to deliver a fraction 2 0 at a temperature of 17.96°C and a pressure of 22.0 bar. This fraction 20 is compressed in the compressor Kl coupled to the turbine Tl. The power recovered by the turbine is used to compress the fraction 2 0 in order to give the compressed fraction 21 at a temperature of 38.80°C and a pressure of 27.67 bar. The latter fraction is then compressed in the main compressor K2 in order to give the fraction 22 at a pressure of 63.76 bar and a temperature of 118.22°C. The compressor K2 is driven by the gas turbine GT. The fraction 22 is then cooled in the air cooler Al in order to deliver the fraction 23 at a temperature of 40.00°C and a pressure of 63.06 bar. The fraction 23 is then separated, on the one hand, into the main fraction 1 with a flow rate of 13510 kmol/h, which is then sent into a gas pipe to be subsequently delivered to industrial customers, and, on the other hand, into the tap-off fraction 6 with a flow rate of 2000 kmol/h. The fraction 1 is composed of 99.3849 mol% methane and 0.0481 mol% ethane, 0.0000 mol% propane and higher alkanes, 0.1785 mol% CO2 and 0.3885 mol% N2. The tap-off fraction 6 is recycled into the heat exchanger El in order to deliver the fraction 24 cooled to -101.40°C at 62.06 bar. The fraction 24 is then expanded to 23.2 bar at -104.18°C in order then to be introduced into the column C1 at a stage 50 which is the first stage starting from the highest stage of the column. The column C1 produces, at the bottom, the second bottom fraction 2 which contains 99.18% of the ethane contained in the dry natural gas charge 14 and 100% of the other hydrocarbons initially contained in this charge 14. This fraction 2, available at 19.16'°C and 23.2 bar, contains 3.4365 mol% CO2, 0.0000 mol% N2, 0.5246 mol% methane, 52.4795 mol% ethane, 23.9426 mol% propane, 5.4324 mol% isobutane, 6.6395 mol% n-butane. 2.4144 mol% isopentane, 1.9114 mol% n-pentane, 1.9114 mol% n-hexane, 1.0060 mol% n-heptane and 0.3018 mol% -n-octane. The column C1 is provided with lateral reboilers in its low part, which is located below the stage where the fraction 8 is introduced, and comprises a plurality of stages. Thus, the liquid collected on a tray 52, available at a temperature of -52.67°C and a pressure of 23.11 bar, situated below a stage 51 which is the thirteenth stage starting from the highest stage of the column, is conducted into the lateral reboiler 33. The latter is formed by a circuit integrated into the exchanger El, the flow rate of which is 2673 kmol/h. This lateral reboiler 33 has a thermal power of 3836 kW. The liquid collected on the tray 52 is then warmed to -19.79°C and then sent back into the column C1 onto a tray 53 which corresponds to the bottom of the fourteenth stage starting from the highest stage of the column. The liquid withdrawn from the tray 52 is composed in particular of 24.42 mol% methane and 44.53 mol% ethane. Likewise, the liquid collected on a tray 55, available at a temperature of 2.84°C and a pressure of 23.17 bar, located below a stage 54 which is the nineteenth stage starting from the highest stage of the column, is conducted into the lateral reboiler 34. The latter is formed by a circuit integrated into the exchanger El the flow rate of which is 2049 kmol/h. This lateral reboiler 34 has a thermal power of 1500 kW. The liquid collected on the tray 55 is then warmed to ll.Ol°C and then sent back into the column C1 onto a tray 56 which corresponds to the bottom of the twentieth stage starting from the highest stage of the column. The liquid withdrawn from the tray 55 is composed in particular of 2.84 mol% methane and 57.29 mol% ethane. Finally, the liquid collected on a tray 58, available at a temperature of 13.32°C and a pressure of 23.20 bar, located below a stage 57 which is the twenty-second stage starting from the highest stage of the column, is conducted into the bottom reboiler of the column or the lateral reboiler 35. The latter is formed by a circuit integrated into the exchanger El, the flow rate of which is 1794 kmol/h. This lateral reboiler 35 has a thermal power of 1146 kW. The liquid collected on the tray 58, composed in particular of 0.93 mol% methane and 55.89 mol% ethane, is then warmed to 19 - 16°C and then sent back into the bottom of the column C1 in a vessel 59 which corresponds to the bottom of the twenty-third stage starting from the highest stage of the column. The liquid leaving the tray 58 has the same composition as the product at the bottom of the column 59 and as the product 2 withdrawn from the bottom of the column C1. All of the heat exchanges take place in the cryogenic exchanger El, which is preferably composed of a battery of brazed aluminum plate exchangers. The ethane extraction process using a plant according to diagram 2 allows more than 99% of the ethane contained in a natural gas to be recovered. According to a modeling of the plant of diagram 2 in operation, the dry natural gas 14, at a temperature of 24°C and a pressure of 62 bar, the flow rate of which is 15000 kmol/h, and composed of 0.4998 mol% CO2, 0.3499 mol% N2, 89.5642 mol% methane, 5.2579 mol% ethane, 2.3790 mol% propane, 0.5398 mol% isobutane, 0.6597 mol% n-butane, 0.2399 mol% isopentane, 0.1899 mol% n-pentane, 0.1899 mol% n-hexane, 0.1000 mol% n-heptane and 0.0300 mol% n-octane, is cooled down to -42°C and partially condensed to 61 bar in the heat exchanger El in order to form the fraction 18- The liquid and gas phases are separated in the tank Bl. The first top fraction 3, which is a 13776 kmol/h stream, is divided into two streams: (a) the main stream 45, with a flow rate of 11471 kmol/h, which is expanded in the turbine Tl to a pressure of 23.20 bar. The dynamic expansion allows 3087 kW of energy to be recovered and allows this stream to be cooled down to a temperature of -83.41°C. This stream 19, which is partially condensed, is sent into the column C1. It enters this column at a stage 46 which is the tenth stage starting from the highest stage of the column C1. Its intake pressure is 23.05 bar and its temperature is -83.57°C; (b) the secondary stream 9, with a flow rate of 2305 kmol/h, which is liquefied and cooled down to -62,03°C in the exchanger El in order to form the fraction 26. This fraction 26, which comprises 4.5 mol% ethane, is expanded to 46 bar at a temperature of -72.68°C and is then introduced into the third separator tank B3 where the vapor and liquid phases are separated into the fourth top fraction 10 and the fourth bottom fraction 11. The fourth top fraction 10, the flow rate of which is 1738 kmol/h, comprises 96.15 mol% methane and 2.61 mol% ethane. This fraction is then liquefied and cooled to -101.4°C in the exchanger El in order to give the fraction 40. The fraction 40 is then expanded to 23.2 bar at a temperature of -102.99°C in order to be introduced into the column C1 at a stage 47 which is the fifth stage starting from the highest stage of the column. The fourth bottom fraction 11, the flow rate of which is 567 kmol/h, comprises 82.11 mol% methane and 10.48 mol% ethane. This fraction is then warmed in the exchanger El to a temperature of -55-OO°C and a pressure of 44.50 bar in order to be introduced into the fourth separator tank B4 where the liquid and gas phases are separated into the fifth top fraction 12 and the fifth bottom fraction 13. The fifth top fraction 12, the flow rate of which is 420 kmol/h, comprises 91.96 mol% methane and 6.05 mol% ethane. This fraction is then liquefied and cooled to -101.4°C in the exchanger El in order to give the fraction 43. The fraction 43 is then expanded to 23.2 bar at a temperature of -101.57°C in order to be introduced into the column C1 at a stage 61 which is the sixth stage starting from the highest stage of the column. The fifth bottom fraction 13, the flow rate of which is 146 kmol/h, comprises 53.85 mol% methane and 23.22 mol% ethane- This fraction is then mixed with the first bottom fraction 4 to give the fraction 63. The fraction 63 is then warmed in the exchanger El from -53.70°C to -38.00°C and at a pressure of 39.5 bar in order to give the fraction 29. The first bottom fraction 4 from the tank Bl, the flow rate of which is 1224 kmol/h, and which comprises 13.24 mol% ethane, is expanded to a pressure of 40 bar before being mixed with the fraction 13. The fraction 29 is then introduced into the separation tank B2. The top fraction 7 issuing from the tank B2, the flow rate of which is 494 kmol/h and the ethane content of which is 6.72 mol%, is cooled from -38°C to -101.4°C and liquefied in order to obtain the fraction 31. This fraction is then expanded to 23.2 bar and then introduced into the column C1 at a stage 48 which is the seventh stage starting from the highest stage of the column. The bottom fraction 8, the flow rate of which is 876 kmol/h and the ethane content is 18.58 mol%, is expanded to 23.2 bar and -46.76°C and then introduced into the column C1 at a stage 49 which is the twelfth stage starting from the highest stage of the column. The column C1 produces the top fraction 5 at a pressure of 23 bar and a temperature of -103.61°C, with a flow rate of 15308 kmol/h. This top fraction 5 contains no more than 0.05 mol% ethane. The top fraction 5 is warmed in the exchanger El in order to deliver the fraction 20 at a temperature of 17.48°C and a pressure of 22 bar. This fraction 20 is compressed in the compressor Kl coupled to the turbine Tl. The power recovered by the turbine is used to compress the fraction 20 in order to give the compressed fraction 21 at a temperature of 38.61°C and a pressure of 27.76 bar. This fraction is then compressed in the main compressor K2 to give the fraction 22 at a pressure of 63.76 bar and a temperature of 117.7°C. The compressor K2 is driven by the gas turbine GT. The fraction 22 is then cooled in the air cooler Al in order to deliver the fraction 23 at a temperature of 40-00°C and a pressure of 63.06 bar. The fraction 23 is then separated, on the one hand, into the main fraction 1 with a flow rate of 13517 kmol/h, which is then sent into a gas pipe to be subsequently delivered to industrial customers, and, on the other hand, into the tap-off fraction 6 with a flow rate of 1790 kmol/h. The fraction 1 is composed of 99.3280 mol% methane and 0.0485 mol% ethane, 0.0000 mol% propane and higher alkanes, 0.23 53 mol% CO2 and 0.3882 mol% N2. The tap-off fraction 6 is recycled into the heat exchanger El in order to deliver the fraction 24 cooled to -101.4°C at a pressure of 62.06 bar. The fraction 24 is then expanded to 23.2 bar for a temperature of -104.17°C in order thereafter to be introduced into the column C1 at a stage 50 which is the first stage starting from the highest stage of the column. The column C1 produces, at the bottom, the second bottom fraction 2 which contains 99.18% of the ethane contained in the dry natural gas charge 14 and 100% of the other hydrocarbons initially contained in this charge 14. This fraction 2, available at 19.90°C and 23 .2 bar, contains 2.9129 mol% CO2, 0.0000 mol% N2, 0.5274 mol% methane, 52.7625 mol% ethane, 24.0733 mol% propane, 5.4620 mol% isobutane, 6.6758 mol% n-butane, 2.4276 mol% isopentane, 1.9218 mol% n-pentane, 1.9218 mol% n-hexane, 1.0115 mol% n-heptane and 0.3034 mol% n-octane. Column C1 is provided with lateral reboilers in its low part, which is located below the stage at which the fraction 8 is introduced, and comprises a plurality of stages. Thus, the liquid collected on a tray 52, available at a temperature of -51.37°C and a pressure of 23.11 bar, located below a stage 51 which is the thirteenth stage starting from the highest stage of the column, is conducted into the lateral reboiler 33. The latter is formed by a circuit integrated into the exchanger El, the flow rate of which is 2560 kmol/h. This lateral reboiler 33 has a thermal power of 3465 kW. The liquid collected on the tray 52 is then warmed to -19.80°C and then sent back into the column C1 onto a tray 53 which corresponds to the bottom of the fourteenth stage starting from the highest stage of the column. The liquid withdrawn from the tray 52 is composed in particular of 23.86 mol% methane and 45.10 mol% ethane. Likewise, the liquid collected on a tray 55, available at a temperature of 3.48°C and a pressure of 23.17 bar, located below a stage 54 which is the nineteenth stage starting from the highest stage of the column, is conducted into the lateral reboiler 34. The latter is formed by a circuit integrated into the exchanger El, the flow rate of which is 2 044 kmol/h. This lateral reboiler 34 has a thermal power of 1500 kW. The liquid collected on the tray 55 is then warmed to 11.71°C and then sent back into the column C1 onto a tray 56 which corresponds to the bottom of the twentieth stage starting from the highest stage of the column. The liquid present on the tray 55 is composed in particular of 2.92 mol% methane and 57.92 mol% ethane. Finally, the liquid collected on a tray 58, available at a temperature of 14.09°C and a pressure of 23.20 bar, located below a stage 57 which is the twenty-second stage starting from the highest stage of the column, is conducted into the bottom reboiler of the column or a lateral reboiler 35. The latter is formed by a circuit integrated into the exchanger El, the flow rate of which is 1788 kmol/h. This lateral reboiler 35 has a thermal power of 1147 kW. The liquid collected on the tray 58 is then warmed to 19.90*°C and then sent back into the bottom 59 of the column C1. The liquid withdrawn from the tray 58 is composed in particular of 0.94 mol% methane and 56.35 mol% ethane. In the case of the use of a plant according to the process described in diagram 2, for an ethane recovery identical to that obtained during the use of a plant according to diagram 1, a reduction in the power of the compressor K2 from 12 355 kW to 12130 kW is obtained. Likewise, a reduction in the flow rate of gas recycled into the circuit comprising the fraction 6 from 2000 kmol/h to 17 90 kmol/h makes it possible to reduce the heat exchanges when cooling the fraction 6 in order to obtain the fraction 24. A reduction in the carbon dioxide content of the C2+ cut is also obtained: according to diagram 1: 3.43 65 mol%; according to diagram 2: 2.9129 mol%, This lower CO2 content thus makes it possible to facilitate a subsequent treatment intended to remove at least some of the carbon dioxide present in the C2 cut withdrawn from the bottom of the column C1. The invention is therefore useful for limiting energy expenditure during the production of purified gases. This objective is achieved while still allowing high selectivity in separating methane and the other constituents during operation of the process. Thus, the results obtained by the invention offer major advantages, consisting of substantial simplification and savings in the construction and the technology of the equipment and of the methods employed, together with the quality of the products obtained by these methods. WE CLAIM : 1. A process for separating a cooled mixture under pressure, containing methane and C2 and higher hydrocarbons, into a light final fraction (1) enriched in methane and a heavy final fraction (2) enriched in C2 and higher hydrocarbons, comprising a first step in which said cooled mixture under pressure is separated in a first tank (Bl) into a relatively more volatile first top fraction (3) and a relatively less volatile first bottom fraction (4), the first bottom fraction (4) is introduced into a middle part of a distillation column (C1), the heavy final fraction (2) enriched in C2 and higher hydrocarbons is collected as second bottom fraction (2) in a low part of the column, the first top fraction (3), after having been expanded in a turbine (Tl), is introduced into a high part of the distillation column, a second top fraction (5) enriched in methane is collected in the high part of the column, to obtain the light final fraction (1), the second top fraction (5) then undergoes compression and cooling, and a first tap-off fraction is tapped off the light final fraction (1), this process comprising a second step in which the first tap-off fi-action (6), after it has been cooled and liquefied, is introduced into the high part of the distillation column, characterized in that it includes a third step in which the first bottom fraction (4) is subjected to a plurality of sub-steps comprising warming, passage through a second tank (B2) and a separation into a relatively more volatile third top fraction (7) and a relatively less volatile third bottom fraction (8), the third bottom fraction (8) is introduced into the middle part of the distillation column, and the third top fraction (7), after it has been cooled and liquefied, is introduced into the high part of the distillation column. 2. The process as claimed in claim 1, wherein a second tap-off fraction (9) is tapped off from the first top fraction (3) and in that this second tap-off fraction (9), after it has been cooled and liquefied, is introduced into the high part of the distillation column. 3. The process as claimed in claim 2, wherein said second tap-off fraction (9) is cooled and partially condensed and then separated in a third tank (B3) into a relatively more volatile fourth top fraction (10), which is cooled and liquefied and then introduced into the high part of the distillation column, and a relatively less volatile fourth bottom fraction (11), which is warmed and then separated in a fourth tank (B4) into a relatively more volatile fifth top fraction (12), which is cooled and then introduced into the high part of the distillation column, and a relatively less volatile fifth bottom fraction (13) which is warmed and then sent into said second tank. 4. The process as claimed in any one of the preceding claims, wherein the lower part of the distillation column comprises a plurality of stages connected in pairs to a lateral reboiler or to a plurality of lateral reboilers. 5. The process as claimed in any one of the preceding claims, wherein in order to obtain the light final fraction (1), the second top fraction (5), after it has left the distillation column, is successively warmed, compressed a first time in a first compressor (Kl) coupled to the expansion turbine (Tl), compressed a second time in a second compressor (K2) and cooled. 6. The process as claimed in claim 3, wherein the high part of the distillation column comprises at least two successive stages, the first of which is the lowest one, and the fifth top fraction (12) is introduced above he first stage. 7. The process as claimed in claim 3, wherein the high part of the distillation column comprises at least three successive stages, the first of which is the lowest one, and the fiflh top fraction (10) is introduced above the second stage. 8. The process as claimed in claim 2, wherein the high part of the distillation column comprises at least two successive stages, the first of which is the lowest one, and the second tap-off fraction (9) is introduced above the first stage. 9. The process as claimed in any one of the preceding claims, wherein the high part of the distillation column comprises at least three stages, the first of which is the lowest one, into which column the first tap-off fraction (6) is introduced into a low part of the last stage, and in that the third top fraction (7) is introduced below the last stage. 10. The process as claimed in any one of the preceding claims, wherein the third top fraction (7) is introduced at the first stage of the high part of the distillation column. 11. The process as claimed in any one of the preceding claims, wherein the middle part of the distillation column comprises at least two successive stages, the first of which is the lowest one, and into which column the third bottom fraction (8) is introduced, at least at the first stage, and in that the first top fraction (3) is introduced above the first stage. 12. The gas enriched in methane, obtained by the process as claimed in one of the preceding claims. 13. The liquefied gas enriched in C2 and higher hydrocarbons, obtained by the process as claimed in one of claims 1 to 11. 14. A plant for separating a cooled mixture under pressure, containing methane and C2 and higher hydrocarbons, into a light final fraction (1) enriched in methane and a heavy final fraction (2) enriched in C2 and higher hydrocarbons, comprising means for carrying out a first step in which said cooled mixture under pressure is separated in a first tank (Bl) into a relatively more volatile first top fraction (3) and a relatively less volatile first bottom fraction (4), the first bottom fraction (4) is introduced into a middle part of a distillation column (CI), the heavy final fraction (2) enriched in C2 and higher hydrocarbons is collected as second bottom fraction (2) in a low part of the column, the first top fraction (3), after having been expanded in a turbine (Tl), is introduced into a high part of the distillation column, a second top fraction (5) enriched in methane is collected in the high part of the column, to obtain the light final fraction (1), the second top fraction (5) then undergoes compression and cooling and a first tap-off fraction (6) is tapped off the light final fraction (1), this plant comprising means for carrying out a second step in which the first tap-off fraction (6), after it has been cooled and liquefied, is introduced into the high part of the distillation column, characterized in that it comprises means for carrying out a third step in which the first bottom fraction (4) is subjected to a plurality of sub-steps comprising warming, passage through a second tank (B2) and separation into a relatively more volatile third top fraction (7) and a relatively less volatile third bottom fraction (8), the third bottom fraction (B) is introduced into the middle part of the distillation column and the third top fraction (7), after it has been cooled and liquefied, is introduced into the high part of the distillation column.

Full Text

Method and installation for separating a gas mixture containing metheme by distillation, and gases obtained
by this separation
The present invention relates, in general and according to a first of its aspects, to a separation process for separating the constituents of natural gas into a first gas fraction, rich in methane and essentially containing no C2 and higher hydrocarbons, and a second gas fraction, rich in C2 and higher hydrocarbons and essentially containing no methane.
More specifically, the invention relates, according to its first aspect, to a process for separating a cooled mixture under pressure, containing methane and C2 and higher hydrocarbons, into a light final fraction enriched in methane and a heavy final fraction enriched in C2 and higher hydrocarbons, comprising a first step (I) in which (la) the cooled mixture under pressure is separated in a first tank into a relatively more volatile first top fraction and a relatively less volatile first bottom fraction, in which (lb) the first bottom fraction is introduced into a middle part of a distillation column, in which (Ic) the heavy final fraction enriched in C2 and higher hydrocarbons is collected as second bottom fraction in a low part of the column, in which (Id) the first top fraction, after having been expanded in a turbine, is introduced into a high part of the distillation column, in which (le) a second top fraction enriched in methane is collected in the high part of the column, in which (If), to obtain the light final fraction, the second top fraction then undergoes compression and cooling and in which (Ig) a first tap-off fraction is tapped off the light final fraction, this process comprising a second step (II) in which (Ila) the first tap-off fraction, after it has been cooled and liquefied, is introduced into the high part of the distillation column.

Such a process is known from the prior art. Thus, patent US-5 881 569 discloses a process according to the preamble described above.
Extraction of the ethane contained in natural gas maybe carried out using known processes, as described in the patents US-4 140 504, US-4 157 904, US-4 171 964 and US-4 278 547. Although the processes described in those documents do have a certain benefit, in practice they make it possible to obtain at best only a degree of ethane recovery of around 85%. They involve liquid/gas separators, heat exchangers, expanders (usually in the form of turbines) , compressors and distillation columns.
More recently, other processes have been made public, for example by patents US-4 649 063, US-4 854 955, US-5 555 748 and US-5 568 737. Although these more recent processes can allow quite satisfactory ethane and other hydrocarbon extraction yields to be obtained, these processes require, for obtaining fractions enriched in methane or in C2 and higher hydrocarbons, relatively high energy consumption.
In this context, the present invention aims to reduce the energy consumption during production of fractions enriched in methane or in C2 and higher hydrocarbons, while maintaining very high extraction yields compared to the processes of the prior art.
For this purpose, the process of the invention, which moreover is in accordance with the generic definition given in the above preamble, is essentially characterized in that it furthermore includes a third step in which the first bottom fraction is subjected to a plurality of sub-steps comprising warming, passage through a second tank and a separation into a

relatively more volatile third top fraction and a relatively less volatile third bottom fraction, in which the third bottom fraction is introduced into the middle part of the distillation column and in which the third top fraction, after it has been cooled and liquefied, is introduced into the high part of the distillation column.
Another process, as described in patent US-5 566 554, uses two liquid-gas separators, a liquid fraction collected at the bottom of the first separator of which is heated and then introduced into a second separator, This technique makes it possible in particular to improve the extraction of the methane contained in the bottom fraction issuing from the first separator and above all to use the expansion of this bottom fraction to cool, in a heat exchanger, the stream of natural gas to be treated that enters the plant.
However, this known process does not make it possible to obtain a high ethane extraction level since the amount of reflux generated by the technique is low and the ethane content of this reflux is relatively high.
The present invention overcomes these problems by the use of two means.
Firstly, the invention provides for a portion of the methane-rich fraction from the top of the column to be tapped off and, after being compressed and cooled, reintroduced into the last stage of the column. This makes it possible to obtain a sufficient amount of reflux of excellent quality, since the C2 content is very low, for example less than 0.1 mol%.
Secondly, the invention provides for a portion of the first top fraction issuing from the first separator to be tapped off into the column before the expansion step

in the turbine. This second tapped-off fraction is cooled and liquefied before being introduced into the column. Proceeding in this way makes it possible to limit the amount of recycled and liquefied gas mentioned above and reduce the associated compression costs.
The invention may furthermore provide for a second tap-off fraction to be tapped off from the first top fraction and for this second tap-off fraction, after it has been cooled and liquefied, to be introduced into the high part of the distillation column.
According to one possible method of implementing the invention, the second tap-off fraction is cooled and partially condensed and then separated in a third tank into a relatively more volatile fourth top fraction, which is cooled and liquefied and then introduced into the high part of the distillation column, and a relatively less volatile fourth bottom fraction, which is warmed and then separated in a fourth tank into a relatively more volatile fifth top fraction, which is cooled and liquefied and then introduced into the high part of the distillation column, and a relatively less volatile fifth bottom fraction which is warmed and then sent into the second tank.
The invention may furthermore provide for the lower part of the distillation column to comprise a plurality of stages connected in pairs to a lateral reboiler or to a plurality of lateral reboilers.
The invention may also provide, in order to obtain the light final fraction, for the second top fraction, after it has left the distillation column, to be successively warmed, compressed a first time in a first compressor coupled to the expansion turbine, compressed a second time in a second compressor, and cooled.

The invention may also provide for the high part of the distillation column to comprise at least two successive stages, the first of which is the lowest one, and for the fifth top fraction to be introduced above the first stage.
The invention may also provide for the high part of the distillation column to comprise at least three successive stages, the first of which is the lowest one, and for the fifth top fraction to be introduced above the second stage.
The invention may also provide for the high part of the distillation column to comprise at least two successive stages, the first of which is the lowest one, and for the second tap-off fraction to be introduced above the first stage.
The invention may also provide for the high part of the distillation column to comprise at least three stages, the first of which is the lowest one, into which column the first tap-off fraction is introduced into a low part of the last stage, and for the third top fraction to be introduced below the last stage.
Finally, the invention may provide for the third top fraction to be introduced at the first stage of the high part of the distillation column.
The invention may also provide for the middle part of the distillation column to comprise at least two successive stages, the first of which is the lowest one, and into which column the third bottom fraction is introduced, at least at the first stage, and for the first top fraction to be introduced above the first stage.

According to a second of its aspects, the invention relates to a gas enriched in methane obtained by the present process and to a liquefied gas enriched in C2 and higher hydrocarbons obtained by the present process.
According to a third of its aspects, the invention relates to a plant for separating a cooled mixture under pressure, containing methane and C2 and higher hydrocarbons, into a light final fraction enriched in methane and a heavy final fraction enriched in C2 and higher hydrocarbons, comprising means for carrying out a first step (I) in which (la) said cooled mixture under pressure is separated in a first tank into a relatively more volatile first top fraction and a relatively less volatile first bottom fraction, in which (lb) the first bottom fraction is introduced into a middle part of a distillation column, in which (Ic) the heavy final fraction enriched in C2 and higher hydrocarbons is collected as second bottom fraction in a low part of the column, in which (Id) the first top fraction, after having been expanded in a turbine, is introduced into a high part of the distillation column, in which (le) a second top fraction enriched in methane is collected in the high part of the column, in which (If), to obtain the light final fraction, the second top fraction then undergoes compression and cooling and in which (Ig) a first tap-off fraction is tapped off the light final fraction, this plant comprising means for carrying out a second step (II) in which (Ila) the first tap-off fraction, after it has been cooled and liquefied, is introduced into the high part of the distillation column, this plant comprising means for carrying out a third step (III) in which (IIIa) the first bottom fraction is subjected to a plurality of sub-steps comprising warming, passage through a second tank and separation into a relatively more volatile third top fraction and a relatively less volatile third

bottom fraction, in which (IIIb) the third bottom fraction is introduced into the middle part of the distillation column and in which IIIc) the third top fraction, after it has been cooled and liquefied, is introduced into the high part of the distillation column.
The invention will be more clearly understood and other obj ectives, features, details and advantages thereof will become more clearly apparent during the description that follows, with reference to the appended schematic drawings given solely by way of nonlimiting example, in which:
figure 1 is a schematic diagram showing the operation of a plant according to one possible embodiment of the invention; and
figure 2 is a schematic diagram showing the operation of a plant according to another preferred embodiment of the invention.
The following symbols may be seen in particular in both these figures: FC, standing for flow controller; GT, standing for gas turbine; LC, standing for liquid level controller; PC, standing for pressure controller; SC, standing for speed controller; and TC, standing for temperature controller.
For the sake of clarity and concision, the lines used in the plants of figures 1 and 2 will be identified by the same reference symbols as the gas fractions that flow therein.
Referring to figure 1, the plant shown is intended to treat a dry natural gas in order in particular to isolate from it, on the one hand, a fraction composed mainly of methane, essentially containing no C2 and higher hydrocarbons, and, on the other hand, a fraction

composed mainly of C2 and higher hydrocarbons, essentially containing no methane.
Dry natural gas 14 is firstly separated into a fraction 15, which is cooled in a heat exchanger El, and a fraction 16, which is sent into a line. The flow of the fraction 16 is controlled by a control valve 17, the opening of which varies depending on the temperature of a fraction 45. On leaving the exchanger El, the fraction 15 is mixed with the fraction 16 to give a cooled fraction 18. The fraction 18 is then introduced into a liquid/gas separator tank Bl in which this fraction 18 is separated into a relatively more volatile first top fraction 3 and a relatively less volatile bottom fraction 4.
The first top fraction 3 is expanded in a turbine Tl in order to deliver an expanded fraction 19 which is introduced into the middle part of a distillation column C1. Next, on the one hand, the heavy final fraction 2 enriched in C2 and higher hydrocarbons collects in a low part of the distillation column C1 as second bottom fraction 2. This heavy final fraction 2 is transported in a line having a controlled-opening valve 60, the opening of which depends on the level of liquid contained at the bottom of the column C1. On the other hand, a second top fraction 5 enriched in methane collects in a high part of the distillation column CI. This second top fraction 5 is then warmed in the exchanger El in order to deliver the warmed fraction 2 0 and then undergoes a first compression in a first compressor Kl coupled to the turbine Tl in order to deliver a compressed fraction 21. The fraction 21 then undergoes a second compression in a second compressor K2 supplied by a gas turbine, the speed of which is controlled by a speed controller slaved to a pressure controller connected to the line carrying the second top fraction 5, in order to deliver another compressed

fraction 22. The latter is then cooled by air in a heat exchanger Al in order to deliver a compressed and cooled fraction 23.
The fraction 23 is then divided into a first tap-off fraction 6 and a light final fraction 1 enriched in methane. The first tap-off fraction 6 is then cooled and liquefied in the heat exchanger El in order to give a cooled fraction 24 which is conveyed in a line having a control valve 25, whose opening depends on the flow rate, and is then introduced into the high part of the distillation column Cl.
A second tap-off fraction 9 is tapped off the first top fraction 3 and cooled and liquefied in the heat exchanger El in order to deliver a cooled fraction 26. The latter is sent in a line having a control valve 27, with an opening that depends on the flow rate, and is then introduced into the high part of the distillation column Cl.
The first bottom fraction 4 is transported in a line having a control valve 2 8, whose opening depends on the level of liquid in the bottom of the separator tank Bl. The first bottom fraction 4 is then warmed in the exchanger El in order to deliver a warmed fraction 29. The fraction 29 is then introduced into a liquid/gas separator tank B2 in order to be separated into a relatively more volatile third top fraction 7 and a relatively less volatile third bottom fraction 8.
The third bottom fraction 8 is transported in a line having a control valve 30 whose opening depends on the level of liquid in the bottom of the separator tank B2. The third bottom fraction 8 is then introduced into the middle part of the distillation column C1. The third top fraction 7 is cooled and liquefied in the exchanger El in order to give a cooled fraction 31. The latter is

conveyed in a line having a control valve 32, whose opening is controlled according to the pressure, and is then introduced into the distillation column CI.
The distillation column C1 has, in its low part, several stages which are connected in pairs by warming circuits 33, 34, 35 which are individually connected to the heat exchanger El. Each of these warming circuits constitutes a lateral reboiler. The temperature of the fluid flowing in each of these circuits 33, 34, 35 is regulated by means of controlled-opening valves positioned on branch-off lines that do not pass through the exchanger El. The opening of these valves is controlled by temperature controllers connected to the lines. These controllers, respectively 36, 37 and 38, are positioned downstream of the zone in which the fractions are mixed after they have passed through the exchanger El and/or the branch-off lines.
Referring now to figure 2, it may be seen that most of the elements contained in figure 1 are repeated in figure 2, except in particular for the addition of a circuit having two separation tanks.
Thus, in the same way as in figure 1, the plant shown is intended to treat a dry natural gas, in particular to isolate from it, on the one hand, a fraction mainly composed of methane, containing essentially no C2 and higher hydrocarbons, and, on the other hand, a fraction mainly composed of C2 and higher hydrocarbons, essentially containing no methane.
Dry natural gas 14 is firstly separated into a fraction 15 which is cooled in a heat exchanger El and into a fraction 16 which is sent into a line. The flow of the fraction 16 is regulated by a control valve 17 whose opening varies according to the temperature of fraction 45. On leaving the exchanger El, the fraction 15 is

mixed with the fraction 16 to give a cooled fraction 18. The fraction 18 is then introduced into a liquid/gas separator tank Bl in which this fraction 18 is separated into a relatively more volatile first top fraction 3 and a relatively less volatile first bottom fraction 4. The first top fraction 3 is expanded in a turbine Tl in order to deliver an expanded fraction 19 which is introduced into the middle part of a distillation column C1. The heavy final fraction 2 enriched in C2 and higher hydrocarbons then collects, on the one hand, in a low part of the distillation coliimn C1 as second bottom fraction 2. This heavy final fraction 2 is transported in a line having a controlled-opening valve 60 whose opening depends on the level of liquid contained in the bottom of the column C1. On the other hand, a second top fraction 5 enriched in methane collects in a high part of the distillation column C1. This second top fraction 5 is then warmed in the exchanger El in order to deliver a warmed fraction 20 and then undergoes a first compression in a first compressor Kl coupled to the turbine Tl in order to deliver a compressed fraction 21. The fraction 21 then undergoes a second compression in a second compressor K2 supplied by a gas turbine, the speed of which is regulated by a speed controller slaved to a pressure controller connected to the line carrying the second top fraction 5, in order to deliver another compressed fraction 22. The latter is then cooled by air in a heat exchanger Al in order to deliver a compressed and cooled fraction 23.
The fraction 23 is then divided into a first tap-off fraction 6 and a light final fraction 1 enriched in methane. The first tap-off fraction 6 is then cooled in the heat exchanger El in order to give a cooled fraction 24 which is conveyed in a line having a control valve 25, whose opening depends on the flow

rate, and is then introduced into the high part of the distillation column C1.
A second tap-off fraction 9 is tapped off the first top fraction 3 and cooled in the heat exchanger El in order to deliver a cooled fraction 26. The latter is conveyed in a line which, unlike in figure 1, has a control valve 39 whose opening depends on the flow rate. The cooled fraction 26 is then introduced into a liquid/gas separator tank B3 in order to be separated into a relatively more volatile fourth top fraction 10 and a relatively less volatile fourth bottom fraction 11.
The collected fourth top fraction is then cooled in the exchanger El in order to give a cooled and liquefied fraction 40.
The cooled and liquefied fraction 40 is then conveyed in a line having a control valve 27 whose opening depends on the flow rate and is then introduced into the high part of the distillation column C1.
The fourth bottom fraction 11 is transported in a line having a control valve 41 whose opening depends on the level of liquid in the bottom of the separator tank B3. The fourth bottom fraction 11 is then warmed in the exchanger El in order to give a warmed fraction 42. This warmed fraction 42 is separated in a fourth tank B4 into a relatively more volatile fifth top fraction 12 and a relatively less volatile fifth bottom fraction 13.
The fifth top fraction 12 is cooled and liquefied in the exchanger El in order to produce a cooled and liquefied fraction 43. The latter is then transported in a line having a control valve 44 whose opening depends on the pressure in the line and is then

introduced into the high part of the distillation column C1.
The relatively less volatile fifth bottom fraction 15 is transported in a line having a valve 62 whose opening is controlled by a controller that controls the level of liquid contained in the tank B4.
The first bottom fraction 4 is transported in a line having a control valve 28 whose opening depends on the level of liquid in the bottom of the separator tank Bl, The first bottom fraction 4 and the fifth bottom fraction 13 are then joined together to give a mixed fraction 63 which is warmed in the exchanger El in order to deliver a warmed fraction 29. The fraction 29 is then introduced into a liquid/gas separator tank B2 in order to be separated into a relatively more volatile third top fraction 7 and a relatively less volatile third bottom fraction 8.
The third bottom fraction 8 is transported in a line having a control valve 30 whose opening depends on the level of liquid in the bottom of the separator tank B2. The third bottom fraction 8 is then introduced into the middle part of the distillation column C1. The third top fraction 7 is cooled and liquefied in the exchanger El in order to give a cooled and liquefied fraction 31. The latter is conveyed in a line having a valve 32 whose opening is controlled according to the pressure and is then introduced into the distillation column C1.
The distillation column C1 comprises, in its low part, several trays that are connected in pairs by warming circuits 33, 34, 35 which are connected individually to the heat exchanger El. Each of these warming circuits constitutes a lateral reboiler. The temperature of the fluid flowing in each of these circuits 33, 34, 35 is regulated by means of controlled-opening valves

positioned on tap-off lines that do not pass through the exchanger El. The opening of these valves is controlled by temperature controllers connected to the lines. These controllers, 36, 37 and 38 respectively, are positioned downstream of the zone in which the fractions are mixed after they have passed through the exchanger El and/or the tap-off lines.
The ethane extraction process using a plant according to diagram 1 allows more than 99% of the ethane contained in a natural gas to be recovered.
According to a modeling of the plant of diagram 1 in operation, the charge of dry natural gas (14) at 24°C and 62 bar, the flow rate of which is 15000 kmol/h, and composed of 0.4998 mol% CO2, 0.3499 mol% N2, 89.5642 mol% methane, 5.2579 mol% ethane, 2.3790 mol% propane, 0.5398 mol% isobutane, 0.6597 mol% n-butane, 0 .2399 mol% isopentane, 0.1899 mol% n-pentane, 0 .1899 mol% n-hexane, 0.1000 mol% n-heptane and 0.0300 mol% n-octane, is cooled down to -42°C and partially condensed to 61 bar in the heat exchanger El in order to form the fraction 18. The liquid and gas phases are separated in the tank Bl. The first top fraction 3, which is a 13776 kmol/h stream, is divided into two streams:
(a) the main stream 45, which has a flow rate of 11471 kmol/h, is expanded in the turbine Tl to a pressure of 23.20 bar. The dynamic expansion allows 3087 kW of energy to be recovered and allows this stream to be cooled down to a temperature of -83.41°C. This stream 19, which is partially condensed, is sent into the column Cl. The stream 19 enters this column at a stage 46 which is the tenth stage starting from the highest stage of the column Cl. Its intake pressure is 23.05 bar and its temperature is -83.57°C;
(b) the 2305 kmol/h secondary stream 9, which is liquefied and cooled down to -101.40°C in the exchanger El in order to form the fraction 26. This fraction 26,

which comprises 4.55 mol% ethane, is expanded to 23.20 bar at a temperature of -101.68°C and is then introduced into a stage 47 of the column C1, which is the fifth stage starting from the highest stage of the column.
The first bottom fraction 4 from the tank Bl, the flow rate of which is 1224 kmol/h and which comprises 54.27 mol% methane and 13.24 mol% ethane, is expanded to a pressure of 40.0 bar and then warmed in the exchanger El from -52.98°C to -38.00°C in order to obtain the fraction 29. The latter is introduced into the separation tank B2.
The top fraction 7 issuing from the tank B2, the flow rate of which is 439 kmol/h and the ethane content is 6.21 mol%, is cooled from -38.00°C to -101.40°C and liquefied in order to obtain the fraction 31. The latter is then expanded to 23.2 bar at -101.47°C and then introduced into the column C1 at a stage 48 which is the sixth stage starting from the highest stage of the column.
The bottom fraction 8, the flow rate of which is 784 kmol/h and the ethane content is 17.18 mol%, is expanded to 23.2 bar and -46.46°C and then introduced into the column C1 at a stage 49 which is the twelfth stage starting from the highest stage of the column.
The column C1 produces the top fraction 5 at a pressure of 23 bar and a temperature of -103.71°C with a flow rate of 15510 kmol/h. This top fraction 5 contains no more than 0.05 mol% ethane.
The top fraction 5 is warmed in the exchanger El in order to deliver a fraction 2 0 at a temperature of 17.96°C and a pressure of 22.0 bar. This fraction 20 is compressed in the compressor Kl coupled to the turbine

Tl. The power recovered by the turbine is used to compress the fraction 2 0 in order to give the compressed fraction 21 at a temperature of 38.80°C and a pressure of 27.67 bar. The latter fraction is then compressed in the main compressor K2 in order to give the fraction 22 at a pressure of 63.76 bar and a temperature of 118.22°C. The compressor K2 is driven by the gas turbine GT. The fraction 22 is then cooled in the air cooler Al in order to deliver the fraction 23 at a temperature of 40.00°C and a pressure of 63.06 bar.
The fraction 23 is then separated, on the one hand, into the main fraction 1 with a flow rate of 13510 kmol/h, which is then sent into a gas pipe to be subsequently delivered to industrial customers, and, on the other hand, into the tap-off fraction 6 with a flow rate of 2000 kmol/h. The fraction 1 is composed of 99.3849 mol% methane and 0.0481 mol% ethane, 0.0000 mol% propane and higher alkanes, 0.1785 mol% CO2 and 0.3885 mol% N2.
The tap-off fraction 6 is recycled into the heat exchanger El in order to deliver the fraction 24 cooled to -101.40°C at 62.06 bar. The fraction 24 is then expanded to 23.2 bar at -104.18°C in order then to be introduced into the column C1 at a stage 50 which is the first stage starting from the highest stage of the column.
The column C1 produces, at the bottom, the second bottom fraction 2 which contains 99.18% of the ethane contained in the dry natural gas charge 14 and 100% of the other hydrocarbons initially contained in this charge 14. This fraction 2, available at 19.16'°C and 23.2 bar, contains 3.4365 mol% CO2, 0.0000 mol% N2, 0.5246 mol% methane, 52.4795 mol% ethane, 23.9426 mol% propane, 5.4324 mol% isobutane, 6.6395 mol% n-butane.

2.4144 mol% isopentane, 1.9114 mol% n-pentane, 1.9114 mol% n-hexane, 1.0060 mol% n-heptane and 0.3018 mol% -n-octane.
The column C1 is provided with lateral reboilers in its low part, which is located below the stage where the fraction 8 is introduced, and comprises a plurality of stages.
Thus, the liquid collected on a tray 52, available at a temperature of -52.67°C and a pressure of 23.11 bar, situated below a stage 51 which is the thirteenth stage starting from the highest stage of the column, is conducted into the lateral reboiler 33. The latter is formed by a circuit integrated into the exchanger El, the flow rate of which is 2673 kmol/h. This lateral reboiler 33 has a thermal power of 3836 kW. The liquid collected on the tray 52 is then warmed to -19.79°C and then sent back into the column C1 onto a tray 53 which corresponds to the bottom of the fourteenth stage starting from the highest stage of the column. The liquid withdrawn from the tray 52 is composed in particular of 24.42 mol% methane and 44.53 mol% ethane.
Likewise, the liquid collected on a tray 55, available at a temperature of 2.84°C and a pressure of 23.17 bar, located below a stage 54 which is the nineteenth stage starting from the highest stage of the column, is conducted into the lateral reboiler 34. The latter is formed by a circuit integrated into the exchanger El the flow rate of which is 2049 kmol/h. This lateral reboiler 34 has a thermal power of 1500 kW. The liquid collected on the tray 55 is then warmed to ll.Ol°C and then sent back into the column C1 onto a tray 56 which corresponds to the bottom of the twentieth stage starting from the highest stage of the column. The liquid withdrawn from the tray 55 is composed in particular of 2.84 mol% methane and 57.29 mol% ethane.

Finally, the liquid collected on a tray 58, available at a temperature of 13.32°C and a pressure of 23.20 bar, located below a stage 57 which is the twenty-second stage starting from the highest stage of the column, is conducted into the bottom reboiler of the column or the lateral reboiler 35. The latter is formed by a circuit integrated into the exchanger El, the flow rate of which is 1794 kmol/h. This lateral reboiler 35 has a thermal power of 1146 kW. The liquid collected on the tray 58, composed in particular of 0.93 mol% methane and 55.89 mol% ethane, is then warmed to 19 - 16°C and then sent back into the bottom of the column C1 in a vessel 59 which corresponds to the bottom of the twenty-third stage starting from the highest stage of the column. The liquid leaving the tray 58 has the same composition as the product at the bottom of the column 59 and as the product 2 withdrawn from the bottom of the column C1.
All of the heat exchanges take place in the cryogenic exchanger El, which is preferably composed of a battery of brazed aluminum plate exchangers.
The ethane extraction process using a plant according to diagram 2 allows more than 99% of the ethane contained in a natural gas to be recovered.
According to a modeling of the plant of diagram 2 in operation, the dry natural gas 14, at a temperature of 24°C and a pressure of 62 bar, the flow rate of which is 15000 kmol/h, and composed of 0.4998 mol% CO2, 0.3499 mol% N2, 89.5642 mol% methane, 5.2579 mol% ethane, 2.3790 mol% propane, 0.5398 mol% isobutane, 0.6597 mol% n-butane, 0.2399 mol% isopentane, 0.1899 mol% n-pentane, 0.1899 mol% n-hexane, 0.1000 mol% n-heptane and 0.0300 mol% n-octane, is cooled down to -42°C and partially condensed to 61 bar

in the heat exchanger El in order to form the fraction 18- The liquid and gas phases are separated in the tank Bl. The first top fraction 3, which is a 13776 kmol/h stream, is divided into two streams:
(a) the main stream 45, with a flow rate of 11471 kmol/h, which is expanded in the turbine Tl to a pressure of 23.20 bar. The dynamic expansion allows 3087 kW of energy to be recovered and allows this stream to be cooled down to a temperature of -83.41°C. This stream 19, which is partially condensed, is sent into the column C1. It enters this column at a stage 46 which is the tenth stage starting from the highest stage of the column C1. Its intake pressure is 23.05 bar and its temperature is -83.57°C;
(b) the secondary stream 9, with a flow rate of 2305 kmol/h, which is liquefied and cooled down to -62,03°C in the exchanger El in order to form the fraction 26. This fraction 26, which comprises 4.5 mol% ethane, is expanded to 46 bar at a temperature of -72.68°C and is then introduced into the third separator tank B3 where the vapor and liquid phases are separated into the fourth top fraction 10 and the fourth bottom fraction 11.
The fourth top fraction 10, the flow rate of which is 1738 kmol/h, comprises 96.15 mol% methane and 2.61 mol% ethane. This fraction is then liquefied and cooled to -101.4°C in the exchanger El in order to give the fraction 40. The fraction 40 is then expanded to 23.2 bar at a temperature of -102.99°C in order to be introduced into the column C1 at a stage 47 which is the fifth stage starting from the highest stage of the column.
The fourth bottom fraction 11, the flow rate of which is 567 kmol/h, comprises 82.11 mol% methane and 10.48 mol% ethane. This fraction is then warmed in the exchanger El to a temperature of -55-OO°C and a

pressure of 44.50 bar in order to be introduced into the fourth separator tank B4 where the liquid and gas phases are separated into the fifth top fraction 12 and the fifth bottom fraction 13.
The fifth top fraction 12, the flow rate of which is 420 kmol/h, comprises 91.96 mol% methane and 6.05 mol% ethane. This fraction is then liquefied and cooled to -101.4°C in the exchanger El in order to give the fraction 43. The fraction 43 is then expanded to 23.2 bar at a temperature of -101.57°C in order to be introduced into the column C1 at a stage 61 which is the sixth stage starting from the highest stage of the column.
The fifth bottom fraction 13, the flow rate of which is 146 kmol/h, comprises 53.85 mol% methane and 23.22 mol% ethane- This fraction is then mixed with the first bottom fraction 4 to give the fraction 63. The fraction 63 is then warmed in the exchanger El from -53.70°C to -38.00°C and at a pressure of 39.5 bar in order to give the fraction 29.
The first bottom fraction 4 from the tank Bl, the flow rate of which is 1224 kmol/h, and which comprises 13.24 mol% ethane, is expanded to a pressure of 40 bar before being mixed with the fraction 13.
The fraction 29 is then introduced into the separation tank B2. The top fraction 7 issuing from the tank B2, the flow rate of which is 494 kmol/h and the ethane content of which is 6.72 mol%, is cooled from -38°C to -101.4°C and liquefied in order to obtain the fraction 31. This fraction is then expanded to 23.2 bar and then introduced into the column C1 at a stage 48 which is the seventh stage starting from the highest stage of the column.

The bottom fraction 8, the flow rate of which is 876 kmol/h and the ethane content is 18.58 mol%, is expanded to 23.2 bar and -46.76°C and then introduced into the column C1 at a stage 49 which is the twelfth stage starting from the highest stage of the column.
The column C1 produces the top fraction 5 at a pressure of 23 bar and a temperature of -103.61°C, with a flow rate of 15308 kmol/h. This top fraction 5 contains no more than 0.05 mol% ethane.
The top fraction 5 is warmed in the exchanger El in order to deliver the fraction 20 at a temperature of 17.48°C and a pressure of 22 bar. This fraction 20 is compressed in the compressor Kl coupled to the turbine Tl. The power recovered by the turbine is used to compress the fraction 20 in order to give the compressed fraction 21 at a temperature of 38.61°C and a pressure of 27.76 bar. This fraction is then compressed in the main compressor K2 to give the fraction 22 at a pressure of 63.76 bar and a temperature of 117.7°C. The compressor K2 is driven by the gas turbine GT. The fraction 22 is then cooled in the air cooler Al in order to deliver the fraction 23 at a temperature of 40-00°C and a pressure of 63.06 bar.
The fraction 23 is then separated, on the one hand, into the main fraction 1 with a flow rate of 13517 kmol/h, which is then sent into a gas pipe to be subsequently delivered to industrial customers, and, on the other hand, into the tap-off fraction 6 with a flow rate of 1790 kmol/h. The fraction 1 is composed of 99.3280 mol% methane and 0.0485 mol% ethane, 0.0000 mol% propane and higher alkanes, 0.23 53 mol% CO2 and 0.3882 mol% N2.

The tap-off fraction 6 is recycled into the heat exchanger El in order to deliver the fraction 24 cooled to -101.4°C at a pressure of 62.06 bar. The fraction 24 is then expanded to 23.2 bar for a temperature of -104.17°C in order thereafter to be introduced into the column C1 at a stage 50 which is the first stage starting from the highest stage of the column.
The column C1 produces, at the bottom, the second bottom fraction 2 which contains 99.18% of the ethane contained in the dry natural gas charge 14 and 100% of the other hydrocarbons initially contained in this charge 14. This fraction 2, available at 19.90°C and 23 .2 bar, contains 2.9129 mol% CO2, 0.0000 mol% N2, 0.5274 mol% methane, 52.7625 mol% ethane, 24.0733 mol% propane, 5.4620 mol% isobutane, 6.6758 mol% n-butane, 2.4276 mol% isopentane, 1.9218 mol% n-pentane, 1.9218 mol% n-hexane, 1.0115 mol% n-heptane and 0.3034 mol% n-octane.
Column C1 is provided with lateral reboilers in its low part, which is located below the stage at which the fraction 8 is introduced, and comprises a plurality of stages.
Thus, the liquid collected on a tray 52, available at a temperature of -51.37°C and a pressure of 23.11 bar, located below a stage 51 which is the thirteenth stage starting from the highest stage of the column, is conducted into the lateral reboiler 33. The latter is formed by a circuit integrated into the exchanger El, the flow rate of which is 2560 kmol/h. This lateral reboiler 33 has a thermal power of 3465 kW. The liquid collected on the tray 52 is then warmed to -19.80°C and then sent back into the column C1 onto a tray 53 which corresponds to the bottom of the fourteenth stage starting from the highest stage of the column. The

liquid withdrawn from the tray 52 is composed in particular of 23.86 mol% methane and 45.10 mol% ethane.
Likewise, the liquid collected on a tray 55, available at a temperature of 3.48°C and a pressure of 23.17 bar, located below a stage 54 which is the nineteenth stage starting from the highest stage of the column, is conducted into the lateral reboiler 34. The latter is formed by a circuit integrated into the exchanger El, the flow rate of which is 2 044 kmol/h. This lateral reboiler 34 has a thermal power of 1500 kW. The liquid collected on the tray 55 is then warmed to 11.71°C and then sent back into the column C1 onto a tray 56 which corresponds to the bottom of the twentieth stage starting from the highest stage of the column. The liquid present on the tray 55 is composed in particular of 2.92 mol% methane and 57.92 mol% ethane.
Finally, the liquid collected on a tray 58, available at a temperature of 14.09°C and a pressure of 23.20 bar, located below a stage 57 which is the twenty-second stage starting from the highest stage of the column, is conducted into the bottom reboiler of the column or a lateral reboiler 35. The latter is formed by a circuit integrated into the exchanger El, the flow rate of which is 1788 kmol/h. This lateral reboiler 35 has a thermal power of 1147 kW. The liquid collected on the tray 58 is then warmed to 19.90*°C and then sent back into the bottom 59 of the column C1. The liquid withdrawn from the tray 58 is composed in particular of 0.94 mol% methane and 56.35 mol% ethane.
In the case of the use of a plant according to the process described in diagram 2, for an ethane recovery identical to that obtained during the use of a plant according to diagram 1, a reduction in the power of the compressor K2 from 12 355 kW to 12130 kW is obtained. Likewise, a reduction in the flow rate of gas recycled

into the circuit comprising the fraction 6 from 2000 kmol/h to 17 90 kmol/h makes it possible to reduce the heat exchanges when cooling the fraction 6 in order to obtain the fraction 24.
A reduction in the carbon dioxide content of the C2+ cut is also obtained:
according to diagram 1: 3.43 65 mol%;
according to diagram 2: 2.9129 mol%,
This lower CO2 content thus makes it possible to facilitate a subsequent treatment intended to remove at least some of the carbon dioxide present in the C2 cut withdrawn from the bottom of the column C1.
The invention is therefore useful for limiting energy expenditure during the production of purified gases. This objective is achieved while still allowing high selectivity in separating methane and the other constituents during operation of the process.
Thus, the results obtained by the invention offer major advantages, consisting of substantial simplification and savings in the construction and the technology of the equipment and of the methods employed, together with the quality of the products obtained by these methods.

WE CLAIM :
1. A process for separating a cooled mixture under pressure, containing methane and C2 and higher hydrocarbons, into a light final fraction (1) enriched in methane and a heavy final fraction (2) enriched in C2 and higher hydrocarbons, comprising a first step in which
said cooled mixture under pressure is separated in a first tank (Bl) into a relatively more volatile first top fraction (3) and a relatively less volatile first bottom fraction (4),
the first bottom fraction (4) is introduced into a middle part of a distillation column (C1),
the heavy final fraction (2) enriched in C2 and higher hydrocarbons is collected as second bottom fraction (2) in a low part of the column,
the first top fraction (3), after having been expanded in a turbine (Tl), is introduced into a high part of the distillation column,
a second top fraction (5) enriched in methane is collected in the high part of the column,
to obtain the light final fraction (1), the second top fraction (5) then undergoes compression and cooling, and
a first tap-off fraction is tapped off the light final fraction (1), this process comprising a second step in which
the first tap-off fi-action (6), after it has been cooled and liquefied, is introduced into the high part of the distillation column, characterized in that it includes a third step in which
the first bottom fraction (4) is subjected to a plurality of sub-steps comprising warming, passage through a second tank (B2) and a separation into a relatively more volatile third top fraction (7) and a relatively less volatile third bottom fraction (8),
the third bottom fraction (8) is introduced into the middle part of the distillation column, and
the third top fraction (7), after it has been cooled and liquefied, is introduced into the high part of the distillation column.

2. The process as claimed in claim 1, wherein a second tap-off fraction (9) is tapped off from the first top fraction (3) and in that this second tap-off fraction (9), after it has been cooled and liquefied, is introduced into the high part of the distillation column.
3. The process as claimed in claim 2, wherein said second tap-off fraction (9) is cooled and partially condensed and then separated in a third tank (B3) into a relatively more volatile fourth top fraction (10), which is cooled and liquefied and then introduced into the high part of the distillation column, and a relatively less volatile fourth bottom fraction (11), which is warmed and then separated in a fourth tank (B4) into a relatively more volatile fifth top fraction (12), which is cooled and then introduced into the high part of the distillation column, and a relatively less volatile fifth bottom fraction (13) which is warmed and then sent into said second tank.
4. The process as claimed in any one of the preceding claims, wherein the lower part of the distillation column comprises a plurality of stages connected in pairs to a lateral reboiler or to a plurality of lateral reboilers.
5. The process as claimed in any one of the preceding claims, wherein in order to obtain the light final fraction (1), the second top fraction (5), after it has left the distillation column, is successively warmed, compressed a first time in a first compressor (Kl) coupled to the expansion turbine (Tl), compressed a second time in a second compressor (K2) and cooled.
6. The process as claimed in claim 3, wherein the high part of the distillation column comprises at least two successive stages, the first of which is the lowest one, and the fifth top fraction (12) is introduced above he first stage.
7. The process as claimed in claim 3, wherein the high part of the distillation column comprises at least three successive stages, the first of which is the lowest one, and the fiflh top fraction (10) is introduced above the second stage.

8. The process as claimed in claim 2, wherein the high part of the distillation column comprises at least two successive stages, the first of which is the lowest one, and the second tap-off fraction (9) is introduced above the first stage.
9. The process as claimed in any one of the preceding claims, wherein the high part of the distillation column comprises at least three stages, the first of which is the lowest one, into which column the first tap-off fraction (6) is introduced into a low part of the last stage, and in that the third top fraction (7) is introduced below the last stage.
10. The process as claimed in any one of the preceding claims, wherein the third top fraction (7) is introduced at the first stage of the high part of the distillation column.
11. The process as claimed in any one of the preceding claims, wherein the middle part of the distillation column comprises at least two successive stages, the first of which is the lowest one, and into which column the third bottom fraction (8) is introduced, at least at the first stage, and in that the first top fraction (3) is introduced above the first stage.
12. The gas enriched in methane, obtained by the process as claimed in one of the preceding claims.
13. The liquefied gas enriched in C2 and higher hydrocarbons, obtained by the process as claimed in one of claims 1 to 11.
14. A plant for separating a cooled mixture under pressure, containing methane and C2 and higher hydrocarbons, into a light final fraction (1) enriched in methane and a heavy final fraction (2) enriched in C2 and higher hydrocarbons, comprising means for carrying out a first step in which said cooled mixture under pressure is separated in a first tank (Bl) into a

relatively more volatile first top fraction (3) and a relatively less volatile first bottom fraction (4), the first bottom fraction (4) is introduced into a middle part of a distillation column (CI), the heavy final fraction (2) enriched in C2 and higher hydrocarbons is collected as second bottom fraction (2) in a low part of the column, the first top fraction (3), after having been expanded in a turbine (Tl), is introduced into a high part of the distillation column, a second top fraction (5) enriched in methane is collected in the high part of the column, to obtain the light final fraction (1), the second top fraction (5) then undergoes compression and cooling and a first tap-off fraction (6) is tapped off the light final fraction (1), this plant comprising means for carrying out a second step in which the first tap-off fraction (6), after it has been cooled and liquefied, is introduced into the high part of the distillation column, characterized in that it comprises means for carrying out a third step in which the first bottom fraction (4) is subjected to a plurality of sub-steps comprising warming, passage through a second tank (B2) and separation into a relatively more volatile third top fraction (7) and a relatively less volatile third bottom fraction (8), the third bottom fraction (B) is introduced into the middle part of the distillation column and the third top fraction (7), after it has been cooled and liquefied, is introduced into the high part of the distillation column.

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Patent Number

223170

Indian Patent Application Number

913/CHENP/2003

PG Journal Number

47/2008

Publication Date

21-Nov-2008

Grant Date

05-Sep-2008

Date of Filing

10-Jun-2003

Name of Patentee

TECHNIP FRANCE

Applicant Address

170 PLACE HENRI REGNAULT, LA DEFENSE 6, 92400 COURBEVIOE,

Inventors:

#	Inventor's Name	Inventor's Address
1	PARADOWSKI HENRI	32 RUE SERPENTE, F-95000 CERGY,

PCT International Classification Number

F25J3/02

PCT International Application Number

PCT/FR01/03982

PCT International Filing date

2001-12-13

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	00/16238	2000-12-13	France