EP1454104B1 - Method and installation for separating a gas mixture containing methane by distillation - Google Patents

Description

The present invention relates generally and according to a first aspect, a separation process for separating the constituents of natural gas in a first gas fraction, rich in methane and substantially free of hydrocarbons C ₂ and higher, and a second gas fraction, rich in C ₂ and higher hydrocarbons and substantially free of methane.

More specifically, the invention relates, in its first aspect, to a process for separating a pressure-cooled mixture containing methane and C ₂ and higher hydrocarbons into a light final fraction enriched with methane and a final enriched heavy fraction. in C ₂ and higher hydrocarbons, comprising a first step (I) in which (Ia) the pressure-cooled mixture is separated in a first flask into a first relatively more volatile head fraction and a first relatively less volatile, in which (Ib) the first bottom fraction is introduced into a middle part of a distillation column, in which (Ic) is collected, in a lower part of the column, as a second fraction of a foot, the heavy final fraction enriched in C ₂ and higher hydrocarbons, in which (Id) is introduced, after having relaxed it in a turbine, the first top fraction in a upper part of the distillation column, in which (Ie) is collected in the upper part of the column, a second methane-enriched top fraction, in which (Ie) the second head fraction is then subjected, for the obtaining the final light fraction, a compression and a cooling, and wherein (Ig) is taken from the final light fraction a first collection fraction, this method comprising a second step (II). wherein (IIa) is introduced the first withdrawal fraction, after cooling and liquefaction, in the upper part of the distillation column.

Such a method is known from the prior art. Thus, the patent US 5881569 discloses a method according to the preamble described above.

The extraction of the ethane contained in the natural gas can be carried out using known methods, as described in the patents US 4140504 , US 4157904 , US 4171964 and US 4278547 . Although the processes described in these patents have a certain interest, they do not allow to obtain, at best, in practice, a ethane recovery rate of the order of 85%. They involve liquid / gas separators, heat exchangers, regulators (usually in the form of turbines), compressors and distillation columns.

More recently, other methods have been made public, in particular by patents US 4649063 , US 4854955 , US 5555748 and US 5568737 . If these newer processes can lead to fairly satisfactory extraction yields of ethane and other hydrocarbons, these processes require, in order to obtain fractions enriched in methane or C ₂ and higher hydrocarbons, relatively high energy costs. important. US 5568737 and US 5566554 describe other methods of separation.

In this context, the present invention aims to reduce the energy consumption during the production of enriched fractions methane or hydrocarbons C ₂ and higher, while maintaining very high extraction yields compared to the processes of the prior art.

For this purpose, the subject of the invention is a method according to claim 1.

Another method, as described in the patent US 5566554 uses two liquid / gas separators of which a liquid fraction collected at the bottom of the first separator 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 resulting from the first separator and especially to use the expansion of this fraction of the foot to cool in a heat exchanger the flow of natural gas to be treated that enters in the installation.

On the other hand, this known method does not make it possible to obtain a thorough extraction of ethane because 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.

On the one hand, the invention provides for the derivation of a portion of the methane-rich column head fraction and its reintroduction to the last stage of the column after compression and cooling. This makes it possible to obtain reflux in sufficient quantity and of excellent quality, since the content of C ₂ is very low, for example less than 0.1 mol%.

On the other hand, the invention provides for the derivation to the column of a part of the first fraction of head from the first separator before the step of expansion in the turbine. This second derivative fraction is cooled and liquefied before introduction into the column. This way of proceeding makes it possible to limit the amount of recycled and liquefied gas mentioned above and to reduce the related compression costs.

The method according to the invention may comprise one or more of the features of claims 2 to 9.

The invention also relates to an installation according to claim 10.

• La figure 1 représente un schéma synoptique fonctionnel d'une installation conforme à un mode de réalisation possible de l'invention ; et
• La figure 2 représente un schéma synoptique fonctionnel d'une installation conforme à un autre mode de réalisation préféré de l'invention.

The invention will be better understood and other objects, characteristics, details and advantages thereof will appear more clearly in the following description with reference to the accompanying schematic drawings, given solely by way of non-limiting example and wherein :

• The figure 1 represents a functional block diagram of an installation according to a possible embodiment of the invention; and
• The figure 2 represents a functional block diagram of an installation according to another preferred embodiment of the invention.

In these two figures, the symbols "FC" meaning "flow controller", "GT" meaning "gas turbine", "LC" meaning "liquid level controller", "PC" meaning means "pressure controller", "SC" meaning "speed controller" and "TC" meaning "temperature controller".

For the sake of clarity and brevity, the lines used in figures 1 and 2 will be repeated by the same signs of preference as the gaseous fractions circulating there.

By referring to the figure 1 , the instillation shown is intended to treat a dry natural gas, in particular to isolate a fraction composed mainly of methane substantially free of hydrocarbons C ₂ and higher on the one hand, and a fraction composed mainly of C hydrocarbons ₂ and higher essentially free of methane, on the other hand.

Dry natural gas 14 is first separated into a fraction 15 which is cooled in a heat exchanger E1, and a fraction 16 which is sent into a heat exchanger. conduct. The circulation of the fraction 16 is regulated by a controlled valve 17 whose opening varies as a function of the temperature of a fraction 45. At the outlet of the exchanger E1, the fraction 15 is mixed with the fraction 16 to give a fraction 18 cooled. Fraction 18 is then introduced into a liquid / gas separator flask B1 in which this fraction 18 is separated into a relatively more volatile first head fraction 3 and a relatively less volatile first fraction of foot 4.

The first head fraction 3 is expanded in a turbine T1 to provide a loose fraction 19 which is introduced into the middle portion of a distillation column C1. Then, on the one hand, in a lower part of the distillation column C1, as the second bottom fraction 2, the final heavy fraction 2 enriched in C ₂ and higher hydrocarbons. This heavy final fraction 2 is transported in a pipe comprising a controlled opening valve 60 whose opening depends on the liquid level contained at the bottom of the column C1. On the other hand, a second top fraction 5 enriched in methane is collected in an upper part of the distillation column C1. This second head fraction 5 is then reheated in the heat exchanger E1 to provide the heated fraction 20, and then is subjected to a first compression in a first compressor K1 coupled to the turbine T1 to provide a compressed fraction 21. The fraction 21 is then subjected to a second compression in a second compressor K2 fed by a gas turbine whose speed is regulated by a speed controller controlled by a pressure controller connected to the pipe carrying the second head fraction 5, to provide another compressed fraction 22. The latter is then cooled by air in a heat exchanger A1 to provide a compressed and cooled fraction 23.

Fraction 23 is then divided into a first withdrawal fraction 6 and a final light fraction 1 enriched in methane. The first sample fraction 6 is then cooled and liquefied in the heat exchanger E1 to give a cooled fraction 24 which is conveyed in a pipe with a flow-controlled opening valve 25 and is introduced into the upper part of the column. C1 distillation.

A second withdrawal fraction 9 is taken from the first top fraction 3 and cooled and liquefied in the heat exchanger E1 to provide a cooled fraction 26. The latter is conveyed in a pipe comprising a controlled valve 27 with an opening dependent on the flow, and is introduced into the upper part of the distillation column C1.

The first foot fraction 4 is conveyed in a pipe which comprises a controlled valve 28, the opening of which depends on the level of liquid in the bottom of the separating balloon B1. The first bottom fraction 4 is then reheated in exchanger E1 to provide a heated fraction 29. Fraction 29 is then introduced into a liquid / gas separator flask B2 to be separated into a relatively more volatile third head fraction 7, and a relatively less volatile third foot fraction 8.

The third foot fraction 8 is conveyed in a pipe which comprises a controlled valve 30 whose opening depends on the level of liquid in the bottom of the separating drum 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 E1 to give a cooled fraction 31. The latter is conveyed in a pipe comprising a controlled valve 32 with a controlled opening as a function of the pressure, and then introduced into the distillation column C1.

The distillation column C1 has in its lower part several stages which are connected in pairs by heating circuits 33, 34, 35 which are individually connected to the heat exchanger E1. Each of these reheating circuits constitutes a side reboiler. The regulation of the fluid circulation temperature in each of these circuits 33, 34, 35 is effected by means of controlled opening valves positioned on bypass lines which do not pass through the exchanger E1. The opening of these valves is controlled by temperature controllers connected to the pipes. These controllers, respectively 36, 37, 38, are positioned downstream of the mixing zone of the fractions after their passage through the exchanger E1 and / or the bypass lines.

Now referring to the figure 2 , we observe that most of the elements contained in the figure 1 find themselves in the figure 2 , with the exception of the addition of a circuit comprising two separation balls.

So, in the same way as for the figure 1 , the plant shown is intended to treat a dry natural gas, in particular to isolate a fraction composed mainly of methane essentially free of C ₂ and higher hydrocarbons on the one hand, and a fraction composed mainly of C 2 hydrocarbons ₂ and higher essentially free of methane, on the other hand.

Dry natural gas 14 is first separated into a fraction 15 which is cooled in a heat exchanger E1, and a fraction 16 which is sent into a pipe. The circulation of the fraction 16 is regulated by a controlled valve 17 whose opening varies as a function of the temperature of a fraction 45. At the exit of the exchanger E1, fraction 15 is mixed with fraction 16 to give a cooled fraction 18. Fraction 18 is then introduced into a flask. liquid / gas separator B1 in which this fraction 18 is separated into a relatively more volatile first head fraction 3 and a relatively less volatile first foot fraction 4. The first head fraction 3 is expanded in a turbine T1 to provide a loose fraction 19 which is introduced into the middle part of a distillation column C1. Then, on the one hand, in a lower part of the distillation column C1, as the second bottom fraction 2, the final heavy fraction 2 enriched in C ₂ and higher hydrocarbons. This heavy final fraction 2 is transported in a pipe comprising a controlled opening valve 60 whose opening depends on the liquid level contained at the bottom of the column C1. On the other hand, a second top fraction 5 enriched in methane is collected in an upper part of the distillation column C1. This second head fraction 5 is then reheated in the exchanger E1 to provide a heated fraction 20, and then is subjected to a first compression in a first compressor K1 coupled to the turbine T1 to provide a compressed fraction 21. The fraction 21 is then subjected to a second compression in a second compressor K2 fed by a gas turbine whose speed is regulated by a speed controller controlled by a pressure controller connected to the pipe carrying the second head fraction 5, to provide another compressed fraction 22. The latter is then cooled by air in a heat exchanger A1 to provide a compressed and cooled fraction 23.

Fraction 23 is then divided into a first withdrawal fraction 6 and a final light fraction 1 enriched in methane. The first sample fraction 6 is then cooled in the exchanger thermal E1 to give a cooled fraction 24 which is conveyed in a pipe with a controlled valve 25 opening flow dependent, and is introduced into the upper part of the distillation column C1.

A second sampling fraction 9 is taken from the first top fraction 3 and cooled in the heat exchanger E1 to provide a cooled fraction 26. The latter is conveyed in a pipe which, unlike the figure 1 , comprises a controlled valve 39 with opening dependent on the flow. The cooled fraction 26 is then introduced into a liquid / gas separator flask B3 to be separated into a relatively more volatile fourth head fraction 10 and a relatively less volatile fourth foot fraction 11.

The fourth overhead fraction collected is then cooled in exchanger E1 to give a cooled and liquefied fraction 40.

The cooled and liquefied fraction 40 is then conveyed in a pipe comprising a controlled valve 27 with a flow-dependent opening and then introduced into the upper part of the distillation column C1.

The fourth foot fraction 11 is transported in a pipe which comprises a controlled valve 41 whose opening depends on the level of liquid in the bottom of the separator drum B3. The fourth foot fraction 11 is then reheated in the exchanger E1 to give a heated fraction 42. This heated fraction 42 is separated in a fourth balloon B4 into a relatively more volatile fifth head fraction 12 and a fifth fraction of a relatively smaller foot. volatile 13.

The fifth top fraction 12 is cooled and liquefied in the exchanger E1 to produce a cooled and liquefied fraction 43. The latter is then transported in a pipe which comprises a controlled valve 44 whose opening depends on the pressure in the pipe, then is introduced into the upper part of the distillation column C1.

The relatively less volatile fifth foot fraction 13 is conveyed in a line comprising an opening valve 62 controlled by a liquid level controller contained in the balloon B4.

The first foot fraction 4 is conveyed in a pipe which comprises a controlled valve 28, the opening of which depends on the level of liquid in the bottom of the separating balloon B1. The first bottom fraction 4 and the fifth bottom fraction 13 are then combined to give a mixed fraction 63 which is reheated in exchanger E1 to provide a heated fraction 29. Fraction 29 is then introduced into a liquid / gas separator flask B2 to be separated into a relatively more volatile third head fraction 7, and a relatively less volatile third foot fraction 8.

The third foot fraction 8 is conveyed in a pipe which comprises a controlled valve 30 whose opening depends on the level of liquid in the bottom of the separating drum 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 E1 to give a cooled and liquefied fraction 31. The latter is conveyed in a pipe having a controlled opening valve 32 as a function of the pressure, and is then introduced into the column. C1 distillation.

The distillation column C1 has in its lower part several trays which are connected in pairs by heating circuits 33, 34, 35 which are individually connected to the heat exchanger E1. Each of these reheating circuits constitutes a side reboiler. The regulation of the temperature of fluid circulation in each of these circuits 33, 34, 35 is effected by means of controlled opening valves positioned on bypass lines which do not pass through the exchanger E1. The opening of these valves is controlled by temperature controllers connected to the pipes. These controllers, respectively 36, 37, 38, are positioned downstream of the mixing zone of the fractions after their passage through the exchanger E1 and / or the bypass lines.

The ethane extraction process using an installation according to scheme 1 makes it possible to recover more than 99% of the ethane contained in a natural gas.

1. (a) le courant principal 45, qui a un débit de 11471 kmol/h, est détendu dans la turbine T1 jusqu'à une pression de 23,20 bar. La détente dynamique permet de récupérer 3087 kW d'énergie et permet de refroidir ce courant jusqu'à une température de -83,41°C. Ce courant 19, qui est partiellement condensé, est envoyé vers la colonne C1. Le courant 19 entre dans cette colonne sur un étage 46 qui est le dixième étage en partant de l'étage le plus élevé de la colonne C1. Sa pression d'entrée est de 23,05 bar et sa température est de -83,57°C.
2. (b) le courant secondaire 9 de 2305 kmol/h, qui est liquéfié et refroidi jusqu'à -101,40°C dans l'échangeur E1 pour former la fraction 26. Cette fraction 26 qui comprend 4,55 %mol d'éthane est détendue à 23,20 bar à une température de -101,68°C puis est introduite dans un étage 47 de la colonne C1 qui est le cinquième étage en partant de l'étage le plus élevé de la colonne.

According to a modeling of the operating installation of scheme 1, the load of dry natural gas (14) at 24 ° C and 62 bar, whose flow rate is 15000 kmol / h, and composed of 0.4998 mol% of CO ₂ , 0.3499 mol% of N ₂ , 89.5642 mol% of methane, 5.2579 mol% of ethane, 2.3790 mol% of propane, 0.5398 mol% of isobutane, 0.6597 mol% of n-butane, 0.2399 mol% isopentane, 0.1899 mol% n-pentane, 0.1899 mol% n-hexane, 0.1000 mol% n-heptane, 0.0300 mol% n -octane is cooled and partially condensed in the heat exchanger E1 to -42 ° C and 61 bar to form the fraction 18. The liquid and gaseous phases are separated in the flask B1. The first head fraction 3 which is a current of 13776 kmol / h, is divided into two streams:

1. (a) the main stream 45, which has a flow rate of 11471 kmol / h, is expanded in the turbine T1 to a pressure of 23.20 bar. The dynamic expansion allows to recover 3087 kW of energy and allows to cool this current up to a temperature of -83,41 ° C. This stream 19, which is partially condensed, is sent to the column C1. The stream 19 enters this column on a stage 46 which is the tenth stage starting from the highest stage of the column C1. Its inlet pressure is 23.05 bar and its temperature is -83.57 ° C.
2. (b) the secondary stream 9 of 2305 kmol / h, which is liquefied and cooled to -101.40 ° C in the exchanger E1 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 C1 column which is the fifth stage starting from the highest stage of the column.

The first bottom fraction 4 of the flask B1, whose flow rate is 1224 kmol / h and which comprises 54.27 mol% of methane and 13.24 mol% of ethane, is expanded at a pressure of 40.0 bar and is then reheated in the E1 exchanger from -52.98 ° C to -38.00 ° C to obtain the fraction 29. The latter is introduced into the separation tank B2.

The top fraction 7 from the flask B2, whose flow rate is 439 kmol / h and the ethane content is 6.21 mol%, is cooled and liquefied from -38.00 ° C. to -101.40 ° C. to obtain the fraction 31. The latter is then expanded to 23.2 bar and -101.47 ° C, and then introduced into the column C1 to a stage 48 which is the sixth floor starting from the highest stage of the column.

The bottom fraction or bottom fraction 8, whose flow rate 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 C1 column at one floor 49 which is the twelfth floor from the highest floor of the column.

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 head fraction 5 contains only 0.05 mol% of ethane.

The overhead fraction 5 is reheated in exchanger E1 to provide a fraction at a temperature of 17.96 ° C and a pressure of 22.0 bar. This fraction 20 is compressed in the compressor K1 coupled to the turbine T1. The power recovered by the turbine is used to compress the fraction to give the compressed fraction 21 at a temperature of 38.80 ° C and a pressure of 27.67 bar. This latter fraction is then compressed in the main compressor K2 to give 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. Fraction 22 is then cooled in air cooler A1 to provide 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 at 13510 kmol / h, which is then sent to a gas pipeline for delivery to industrial customers, and on the other hand to the bypass fraction 6 at the rate of 2000 kmol / h. Fraction 1 is composed of 99.3849 mol% of methane and 0.0481 mol% of ethane, 0.0000 mol% of propane and higher alkanes, 0.1785 mol% of CO ₂ and 0.3885 mol% of N ₂ .

The bypass fraction 6 is recycled to the heat exchanger E1 to provide fraction 24 cooled to -101.40 ° C at 62.06 bar. Fraction 24 is then expanded to 23.2 bar and -104.18 ° C and thereafter introduced into column C1 at a stage 50 which is the first stage starting from the highest stage of the column.

Column C1 produces in 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% of CO ₂ , 0.0000 mol% of N ₂ , 0.5246 mol% of methane, 52.4795 mol of ethane, 23.9426 % mol of propane, 5.4324 mol% of isobutane, 6.6395 mol% of n-butane, 2.4144 mol% of isopentane, 1.9114 mol% of n-pentane, 1.9114 mol% of n hexane, 1.0060 mol% n-heptane, 0.3018 mol% n-octane.

Column C1 is provided with side reboilers in its lower 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 plate 52, available at a temperature of -52.67 ° C. and a pressure of 23.11 bar, located below a stage 51 which is the thirteenth stage starting from the stage higher of the column, is conducted in the side reboiler 33. This is constituted by an integrated circuit in the exchanger E1 whose flow is 2673 kmol / h. This side reboiler 33 has a thermal power of 3836 kW. The liquid collected on the plate 52 is then warmed to -19.79 ° C. and then returned to the column C1 on a plate 53 which corresponds to the bottom of the fourteenth stage starting from the highest stage of the column. The liquid withdrawn from the plate 52 is composed in particular of 24.42 mol% of methane and 44.53 mol% of ethane.

Similarly, the liquid collected on a plate 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 floor starting from the highest stage of the column, is conducted in the side reboiler 34. This is constituted by an integrated circuit in the exchanger E1 whose flow is 2049 kmol / h. This lateral reboiler 34 has a thermal power of 1500 kW. The liquid collected on the plate 55 is then heated to 11.01 ° C. and then returned to the column C1 on a plate 56 which corresponds to the bottom of the twentieth stage starting from the highest stage of the column. The liquid withdrawn from the plate 55 is composed in particular of 2.84 mol% of methane and 57.29 mol of ethane.

Finally, the liquid collected on a plate 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 from the floor the highest of the column, is conducted in the column bottom reboiler or side reboiler 35. This is constituted by an integrated circuit in the exchanger E1 whose flow is 1794 kmol / h. This side reboiler 35 has a thermal power of 1146 kW. The liquid collected on the plate 58, composed in particular of 0.93 mol% of methane and 55.89 mol% of ethane, is then heated to 19.16 ° C. and then returned to the bottom of the column C1 in a chamber. 59 which corresponds to the bottom of the twenty-third floor starting from the highest floor of the column. The liquid leaving the plate 58 has the same composition as the bottom product of column 59 and the product 2 withdrawn at the bottom of column C1.

All heat exchange is in the cryogenic heat exchanger E1 which is preferably composed of a battery of brazed aluminum plate heat exchangers.

The ethane extraction process using an installation according to scheme 2 makes it possible to recover more than 99% of the ethane contained in a natural gas.

1. (a) le courant principal 45 d'un débit de 11471 kmol/h, qui est détendu dans la turbine T1 jusqu'à une pression de 23,20 bar. La détente dynamique permet de récupérer 3087 kW d'énergie et permet de refroidir ce courant jusqu'à une température de -83,41°C. Ce courant 19, qui est partiellement condensé, est envoyé vers la colonne C1. Il entre dans cette colonne sur un étage 46 qui est le dixième étage en partant de l'étage le plus élevé de la colonne C1. Sa pression d'entrée est de 23,05. bar et sa température est de -83,57°C.
2. (b) le courant secondaire 9, d'un débit de 2305 kmol/h, qui est liquéfié et refroidi jusqu'à -62,03°C dans l'échangeur E1 pour former la fraction 26. Cette fraction 26 qui comprend 4,5 %mol d'éthane est détendue à 46 bar à une température de -72,68°C puis est introduite dans le troisième ballon séparateur B3 où les phases vapeur et liquide sont séparées en lae quatrième fraction de tête 10 et la quatrième fraction de pied 11.

According to a modeling of the operating installation of FIG. 2, the load of dry natural gas 14, at a temperature of 24 ° C. and a pressure of 62 bar, whose flow rate is 15,000 kmol / h, and composed of 0.4998 mol% of CO ₂ , 0.3499 mol% of N ₂ , 89.5642 mol% of methane, 5.2579 mol% of ethane, 2.3790 mol% of propane, 0.5398 mol% of isobutane, 6597 mole percent n-butane, 0.2399 mole percent isopentane, 0.1899 mole percent n-pentane, 0.1899 mole percent n- hexane, 0.1000 mole percent n- heptane, 0300 mol% of n-octane is cooled and partially condensed in the heat exchanger E1 to -42 ° C and 61 bar to form the fraction 18. The liquid and gaseous phases are separated in the flask B1. The first head fraction 3 which is a current of 13776 kmol / h, is divided into two streams:

1. (a) the main stream 45 of a flow rate of 11471 kmol / h, which is expanded in the turbine T1 to a pressure of 23.20 bar. The dynamic expansion allows to recover 3087 kW of energy and allows to cool this current up to a temperature of -83,41 ° C. This stream 19, which is partially condensed, is sent to the column C1. It enters this column on a floor 46 which is the tenth floor starting from the highest floor of column C1. Its inlet pressure is 23.05. bar and its temperature is -83.57 ° C.
2. (b) the secondary stream 9, of a flow rate of 2305 kmol / h, which is liquefied and cooled to -62.03 ° C in the exchanger E1 to form the fraction 26. This fraction 26 which comprises 4, 5% mol of ethane is expanded to 46 bar at a temperature of -72.68 ° C and is then introduced into the third separator flask B3 where the vapor and liquid phases are separated into the fourth fourth head fraction and the fourth fraction of foot 11.

The fourth top fraction 10, whose flow rate is 1738 kmol / h, comprises 96.15 mol% of methane and 2.61 mol% of ethane. The latter is then liquefied and cooled to -101.4 ° C in the exchanger E1 to give the fraction 40. The fraction 40 is then expanded to 23.2 bar at a temperature of -102.99 ° C to be introduced into column C1 at a stage 47 which is the fifth stage starting from the highest stage of the column.

The fourth fraction of the foot 11, whose flow rate is 567 kmol / h, comprises 82.11 mol% of methane and 10.48 mol% of ethane. The latter is then heated in the exchanger E1 at a temperature of -55.00 ° C. and a pressure of 44.50 bar to be introduced into the fourth separator flask B4 where the liquid and gaseous phases are separated in the fifth fraction of head 12 and the fifth fraction of foot 13.

The fifth top fraction 12, whose flow rate is 420 kmol / h, comprises 91.96 mol% of methane and 6.05 mol% of ethane. The latter is then liquefied and cooled to -101.4 ° C in the exchanger E1 to give the fraction 43. The fraction 43 is then expanded to 23.2 bar at a temperature of -101.57 ° C to be introduced into column C1 at a stage 61 which is the sixth stage starting from the highest stage of the column.

The fifth foot fraction 13, whose flow rate is 146 kmol / h, comprises 53.85 mol% of methane and 23.22 mol% of ethane. The latter is then mixed with the first bottom fraction 4 to give fraction 63. The fraction 63 is then reheated in exchanger E1 from -53.70 ° C. to -38.00 ° C. and at a pressure of 39.degree. 5 bar to give fraction 29.

The first foot fraction 4 of the flask B1, whose flow rate is 1224 kmol / h and which comprises 13.24 mol% of ethane, is expanded to a pressure of 40 bar before being mixed with the fraction 13.

Fraction 29 is then introduced into separation flask B2. The top fraction 7 from the flask B2, whose flow rate is 494 kmol / h and the ethane content is 6.72 mol%, is cooled and liquefied from -38 ° C. to -101.4 ° C., to obtain fraction 31. The latter is then expanded to 23.2 bar and introduced into column C1 at a stage 48 which is the seventh stage starting from the highest stage of the column.

The bottom fraction or bottom fraction 8, whose flow rate 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 C1 column at one floor 49 which is the twelfth floor from the highest floor of the column.

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 head fraction 5 contains only 0.05 mol% of ethane.

The overhead fraction 5 is reheated in exchanger E1 to provide fraction 20 at a temperature of 17.48 ° C and a pressure of 22 bar. This fraction 20 is compressed in the compressor K1 coupled to the turbine T1. The power recovered by the turbine is used to compress the fraction to give the compressed fraction 21 at a temperature of 38.61 ° C and a pressure of 27.76 bar. This last fraction is then compressed in the main compressor K2 to give 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. Fraction 22 is then cooled in air cooler A1 to provide 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 at the rate of 13517 kmol / h, which is then sent to a gas pipeline for delivery to industrial customers, and on the other hand to the bypass fraction 6 for from 1790 kmol / h. Fraction 1 is composed of 99.3280 mol% methane and 0.0485 mol% ethane, 0.0000 mol% propane and higher alkanes, 0.2353 mol% CO ₂ and 0.3882 mol% N ₂ .

The bypass fraction 6 is recycled to the heat exchanger E1 to provide fraction 24 cooled to -101.4 ° C under a pressure of 62.06 bar. The fraction 24 is then expanded to 23.2 bar for a temperature of -104.17 ° C. and then introduced into the column C1 at a stage 50 which is the first stage starting from the highest stage of the column. .

Column C1 produces in 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% of CO ₂ , 0.0000 mol% of N ₂ , 0.5274 mol% of methane, 52.7625 mol% of ethane, 24.0733 % mol of propane, 5.4620 mol% of isobutane, 6.6758 mol% of n- butane, 2.4276 mol% of isopentane, 1.9218 mol% of n-pentane, 1.9218 mol% of n hexane, 1.0115 mol% of n-heptane, 0.3034 mol% of n-octane.

The column C1 is provided with side reboilers in its lower 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 plate 52, available at a temperature of -51.37 ° C. and a pressure of 23.11 bar, situated below a stage 51 which is the thirteenth stage starting from the stage the higher of the column, is conducted in the side reboiler 33. This is constituted by an integrated circuit in the exchanger E1 whose flow is 2560 kmol / h. This side reboiler 33 has a thermal power of 3465 kW. The liquid collected on the plate 52 is then warmed to -19.80 ° C. and then returned to the column C1 on a plate 53 which corresponds to the bottom of the fourteenth stage starting from the highest stage of the column. The liquid withdrawn from the plate 52 is composed in particular of 23.86 mol% of methane and 45.10 mol% of ethane.

Similarly, the liquid collected on a plate 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 in the side reboiler 34. This is constituted by an integrated circuit in the exchanger E1 whose flow is 2044 kmol / h. This lateral reboiler 34 has a thermal power of 1500 kW. The liquid collected on the plate 55 is then warmed to 11.71 ° C. and then returned to the column C1 on a plate 56 which corresponds to the bottom of the twentieth stage starting from the highest stage of the column. The liquid present on the plate 55 is composed in particular of 2.92 mol% of methane and 57.92 mol% of ethane.

Finally, the liquid collected on a plate 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 from the floor the highest of the column, is conducted in the column bottom reboiler or side reboiler 35. This is constituted by an integrated circuit in the exchanger E1 whose flow is 1788 kmol / h. This side reboiler 35 has a thermal power of 1147 kW. The liquid collected on the plate 58 is then heated to 19.90 ° C and returned to the bottom 59 of the column C1. The liquid withdrawn from the plate 58 is composed in particular of 0.94 mol% of methane and 56.35 mol% of ethane.

In the case of the use of an installation according to the method described in diagram 2, for a recovery of ethane identical to that obtained when using an installation according to scheme 1, a reduction in the power of the K2 compressor from 12355 kW to 12130 kW is obtained. Similarly, a reduction in the flow rate of recycled gas in the circuit comprising fraction 6 from 2000 kmol / h to 1790 kmol / h makes it possible to reduce heat exchanges during the cooling of fraction 6 to obtain fraction 24.

• Selon le schéma 1 : 3;4365 %mol
• Selon le schéma 2 : 2,9129 %mol

There is also a reduction in the carbon dioxide content of the C ₂ + cut:

• According to scheme 1: 3, 4365 mol%
• According to scheme 2: 2.9129 mol%

This lower rate of CO ₂ thus facilitates a subsequent treatment to remove at least partly the carbon dioxide present in the C ₂ cut, withdrawn at the bottom of column C1.

The invention is therefore of interest for limiting energy expenditure during the production of purified gases. This goal is achieved while allowing a high selectivity of separation of methane and other constituents during the implementation of the process.

Thus, the results obtained by the invention provide significant advantages consisting in a substantial simplification and economy in the production and the technology of the equipment and methods of their implementation as well as in the quality of the products obtained by these methods.

Claims (10)

1. Process for separation of a mixture cooled under pressure, containing methane and C2 and higher hydrocarbons, into a final methane-rich light fraction (1) and a final heavy fraction (2) rich in C2 and higher hydrocarbons, comprising a first stage (I) in which (Ia) said mixture cooled under pressure is separated, in a first flask (B1), into a relatively more volatile first top fraction (3) and a relatively less volatile first bottom fraction (4), in which (Ib) the first bottom fraction (4) is introduced into a middle part of a distillation column (C1), in which (Ic) in the lower part of the column, the final heavy fraction (2) rich in C2 and higher hydrocarbons is collected as the second bottom fraction (2), in which (Id) there is introduced, after pressure reduction in a turbine (T1) the first top fraction (3) in an upper part of the distillation column, in which (Ie), in the upper part of the column, a second methane-rich top fraction (5) is collected, in which (If), in order to obtain the final light fraction (1), the second top fraction (5) then undergoes compression and cooling, and in which (Ig) a first sample fraction (6) is taken from the final light fraction (1), this process comprising a second stage (II)in which (IIa) the first sample fraction (6) is introduced after cooling and liquefaction, into the upper part of the distillation column, the method comprising a third stage (III) in which (IIa) the first bottom fraction (4) is subjected to a number of sub-stages including heating, passage into a second flask (B2), and separation into a relatively more volatile third top fraction (7), and a relatively less volatile third bottom fraction (8), in which (IIIb) the third bottom fraction (8) is introduced into the middle part of the distillation column, and in which (IIIc) the third top fraction (7) is introduced, after cooling and liquefaction, into the upper part of the distillation column, the process comprising a step in which a second sample fraction (9) is removed from the first top fraction (3), and this second sample fraction (9) is introduced into the upper part of the distillation column after cooling and liquefaction, and a step in which said second sample fraction (9) is cooled and partly condensed, then separated into a third flask (B3) into a fourth relatively more volatile top fraction (10), which is cooled and liquefied, then introduced into the upper part of the distillation column, and a fourth relatively less volatile bottom fraction (11), which is heated, then separated in a fourth flask (B4) into a fifth relatively more volatile top fraction (12) which is cooled and then introduced into the upper part of the distillation column, and a fifth relatively less volatile bottom fraction (13) which is heated and then sent into said second flask.
2. Process according to any one of the preceding claims, characterized in that the lower part of the distillation column comprises a number of stages connected in pairs to one or more lateral reboilers.
3. Process according to any one of the preceding claims, characterized in that, in order to obtain the final light fraction (1), the second top fraction (5), after it leaves the distillation column, undergoes heating, a first compression in a first compressor (K1) coupled with a pressure reducing turbine (T1), a second compression in a second compressor (K2), and cooling.
4. Process according to claim 1, characterized in that the upper part of the distillation column comprises at least two successive stages, the first of which is the lowest, and the fifth top fraction (12) is introduced above the first stage.
5. Process according to claim 1, characterized in that the upper part of the distillation column comprises at least three successive stages, the first of which is the lowest, and the fifth top fraction (10) is introduced above the second stage.
6. Process according to claim 1, characterized in that the upper part of the distillation column comprises at least two successive stages, the first of which is the lowest, and the second sample fraction (9) is introduced above the first stage.
7. Process according to any one of the preceding claims, characterized in that the upper part of the distillation column comprises at least three stages, the first of which is the lowest, in which the first sample fraction (6) is introduced in a lower part of the last stage, and in which the third top fraction (7) is introduced below the last stage.
8. Process according to any one of the preceding claims, characterized in that the third top fraction (7) is introduced in the first stage of the upper part of the distillation column.
9. Process according to any one of the preceding claims, characterized in that the middle part of the distillation column comprises at least two successive stages, the first of which is the lowest, and in which the third bottom fraction (8) is introduced at least into the first stage, and in that the first top fraction (3) is introduced above the first stage.
10. Installation for separation of a mixture cooled under pressure containing methane and C2 and higher hydrocarbons, into a methane-rich final light fraction (1) and a final heavy fraction (2) rich in C2 and higher hydrocarbons, comprising the means to carry out a first stage (I) in which (Ia) said mixture cooled under pressure is separated in a first flask (B1) into a relatively more volatile first top fraction (3) and a relatively less volatile first bottom fraction (4), in which (Ib) the first bottom fraction (4) is introduced into the middle part of a distillation column (C1) in which (Ic), in the lower part of the column, the final heavy fraction (2) rich in C2 and higher hydrocarbons is collected as the second bottom fraction (2), in which (Id) there is introduced, after pressure reduction in a turbine (T1), the first top fraction (3) into an upper part of the distillation column, in which (Ie) a second methane-rich top fraction (5) is collected in the upper part of the column, in which (If), in order to obtain the final light fraction (1), the second top fraction (5) undergoes compression and cooling, and in which (Ig) a first sample fraction (6) is taken from the final light fraction (1), this installation comprising the means to carry out a second stage (II)in which (IIa) the first sample fraction (6) is introduced, after cooling and liquefaction, into the upper part of the distillation column, the installation comprising means to carry out a third stage (III) in which (IIIa) the first bottom fraction (4) is subjected to a number of sub-stages, including heating, passage into a second flask (B2), and separation into a relatively more volatile third top fraction (7) and a relatively less volatile third bottom fraction (8), in which (IIIb) the third bottom fraction (8) is introduced into the middle part of the distillation column, and in which (IIIc) the third top fraction (7) is introduced into the upper part of the distillation column after cooling and liquefaction, the installation comprising means for sampling a second sample fraction (9) from the first top fraction (3) and means for introducing this second sample fraction (9), after cooling and liquefaction, in an upper part of the distillation column, the installation comprising means for cooling and partly condensing the second sample fraction (9), and a third flask for separating the cooled and partly condensed second sample fraction into a fourth relatively more volatile top fraction and into a fourth relatively less volatile bottom fraction, the installation comprising means for cooling, liquefying and introducing into an upper part of the distillation column the fourth relatively more volatile top fraction, the installation comprising means for heating the fourth relatively less volatile top fraction and a fourth separating flask, for separating the fourth relatively less volatile bottom fraction into a fifth relatively more volatile top fraction and into a fifth relatively less volatile bottom fraction, the installation comprising means for cooling and introducing the fifth relatively more volatile top fraction in the upper part of the distillation column and means for heating and sending the fifth relatively less volatile bottom fraction into the second flask.

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