US20150153101A1

US20150153101A1 - Method for producing a treated natural gas, a cut rich in c3+ hydrocarbons and optionally an ethane-rich stream, and associated facility

Info

Publication number: US20150153101A1
Application number: US14/412,172
Authority: US
Inventors: Vanessa GAHIER; Fabien Gaël Léo Lacroix; Vincent Patrick Mathieu
Original assignee: Technip France SAS
Current assignee: Technip Energies France SAS
Priority date: 2012-07-05
Filing date: 2013-07-05
Publication date: 2015-06-04
Also published as: EP2870226A1; CA2878125A1; EP2870226B1; FR2992972B1; FR2992972A1; AR093223A1; RU2015103754A; MX2015000147A; RU2620601C2; WO2014006178A1; AP2015008259A0; CA2878125C

Abstract

The method includes the following steps, sampling a recycling stream (152) in a head stream (131, 140, 141) stemming from a recovery column (35), establishing a heat exchange relationship of the recycling stream (152) with at least one portion of the head stream (131) stemming from the recovery column (35), reintroducing, after expansion, the cooled and expanded recycling stream into the recovery column (35). The method includes sampling in the bottom of the recovery column (35) of at least one bottom reboiling stream (165), and establishing a heat exchange relationship of the re-boiling stream with at least one portion of the initial natural gas (13) or/and with the recycling stream (152), the bottom reboiling being ensured by the calories taken from the initial natural gas stream (13) or/and from the recycling stream (152)

Description

The present invention relates to a method for simultaneously producing a treated natural gas, a cut rich in C₃ ⁺ hydrocarbons, and under at least certain conditions of production, an ethane-rich stream, from an initial natural gas stream containing methane, ethane and C₃ ⁺ hydrocarbons, the method comprising the following steps:

- cooling and partial condensation of the initial natural gas stream in at least one upstream heat exchanger in order to form a cooled initial stream;
- separating the cooled initial gas stream into a liquid flow and into a gas flow;
- expanding the liquid flow, and introducing a stream from the liquid flow into a column for recovering C₂ ⁺ hydrocarbons at a first intermediate level;
- forming a turbine feed stream from the gas flow;
- expanding the feed stream in a dynamic expansion turbine and introducing it into the recovery column at a second intermediate level;
- recovering and compressing at least one portion of the head stream of the recovery column in order to form the natural gas and recovering the foot stream of the recovery column in order to form a liquid stream rich in C₂ ⁺ hydrocarbons;
- introducing the liquid stream to a feed level of a fractionation column provided with a head condenser, the ethane-rich stream being produced, under said production conditions, from a stream stemming from the fractionation column, the fractionation column producing a foot stream intended to form at least partly the C₃ ⁺ hydrocarbon cut;
- introducing a primary reflux stream produced in the head condenser with reflux into the fractionation column;
- producing a secondary reflux stream from the head condenser and introducing the secondary reflux stream at the head of the recovery column.

Such a method is intended for treating a natural gas stream in order to extract at least the C₃ ⁺ hydrocarbons therefrom, in order to recover liquids from the natural gas and an adjustable amount of C₂hydrocarbons.
The C₂and C₃ ⁺ hydrocarbons are extracted from the initial natural gas in order to avoid condensation during the transport or/and the handling of the gas. This condensation may lead to the production of liquid plugs in the transport facilities, which is detrimental to production. Further, these hydrocarbons may be marketed with significant merchant value, which contributes to the cost effectiveness of the facilities.
Subsequently, methods have been developed for simultaneously extracting almost all the C₃ ⁺ hydrocarbons present in the initial natural gas, and a high proportion of the ethane present in the initial gas.
However, the demand for ethane on the market is highly fluctuating, while that for C₃ ⁺ hydrocarbon cuts is relatively constant and is of considerable value.
In certain cases, it is therefore necessary to reduce the production of ethane in the method, by reducing the extraction rate of this compound in the recovery column. In this case, the extraction rate of C₃ ⁺ hydrocarbons also decreases, which reduces the cost effectiveness of the facility.
In order to overcome this problem, it is known how to provide double facilities, i.e. comprising a secondary unit optimized for producing C₃ ⁺ hydrocarbons when ethane extraction is nil. Such a secondary unit is expensive to operate and to maintain.
Patent U.S. Pat. No. 7,458,232 discloses a solution to this problem, by proposing a method which guarantees optimized extraction of C₃ ⁺ hydrocarbons, generally of more than 99%, and which nevertheless attains flexible ethane recoveries comprised, for example, between 2% and 85%, depending on the composition of the load gas.
The method described in U.S. Pat. No. 7,458,232 is therefore particularly effective and remains highly flexible. However, when the ethane extraction rate increases, energy consumption resulting from the use of compressors also increases. An improvement in the yield of the facility, notably for high ethane recovery rates, is therefore always desirable.
An object of the invention is to obtain a method with which it is possible to obtain in a flexible way ethane extraction rates which may range up to 85%, while notably reducing the energy consumption of the facility.
For this purpose, the object of the invention is an insulation of the aforementioned type, characterized in that the method includes the following steps:

- sampling a recycling stream in the head stream stemming from the recovery column;
- establishing a heat exchange relationship of the recycling stream with at least one portion of the head stream stemming from the recovery column,
- reintroducing, after expansion, the cooled and expanded recycling stream into the recovery column;

the method including the sampling in the bottom of the recovery column of at least one bottom reboiling stream, and the establishment of a heat exchange relationship of the bottom reboiling stream with at least one portion of the initial natural gas or/and with the recycling stream, the bottom reboiling being ensured by the calories taken from the initial natural gas stream or/and from the recycling stream.
The method according to the invention may comprise one or more of the following features, taken individually or according to all technically possible combinations:

- at least one portion of the head stream of the recovery column and the recycling stream are placed in a heat exchange relationship with the initial natural gas stream and with the bottom reboiling stream;
- the recycling stream stemming from the first upstream heat exchanger, the secondary reflux stream stemming from the head condenser and the head stream stemming from the recovery column are put into a heat exchange relationship in a first head heat exchanger;
- at least one side reboiling stream is sampled above the bottom reboiling stream, said or each side reboiling stream being placed in a heat exchange relationship with at least one portion of the initial natural gas stream;
- the ethane-rich current is drawn off from an intermediate level of the fractionation column located above the level for feeding the column, and below the head level of the fractionation column;
- it includes the following steps:
  - separating the initial natural gas stream into a first initial stream and a second initial stream;
  - introducing the first initial stream into the first upstream heat exchanger;
  - introducing at least one portion of the second initial stream into an auxiliary dynamic expansion turbine in order to form an auxiliary reflux stream from the effluent stemming from the auxiliary turbine;
  - introducing the auxiliary reflux stream into the recovery column;
- at least one portion of the recycling stream is compressed in an auxiliary compressor coupled with the auxiliary turbine;
- at least one portion of the head stream is compressed in an auxiliary compressor coupled with the auxiliary turbine, advantageously between a first compressor coupled with the first turbine and a second compressor,
- it includes a step for compressing at least one portion of the head current in a first compressor coupled with the first turbine, and then a step for compressing the partly compressed head stream in a second compressor, the recycling stream being sampled downstream from the second compressor.
- at least one secondary recycling stream is sampled in the recycling stream, the secondary recycling stream being introduced into a secondary expansion turbine before being reintroduced into the head stream, advantageously upstream from a passage of the head stream in the first upstream heat exchanger;
- the secondary reflux stream consists of a liquid, of a gas, or of a liquid and gas mixture stemming from the head condenser of the fractionation column;
- it includes the sampling, in the recycling stream, of a bypass stream, the bypass stream being reintroduced into a stream located upstream from the first dynamic expansion turbine;
- the liquid flow stemming from the first upstream separator flask is expanded and is introduced into a second upstream separator flask in order to form a liquid fraction and a gas fraction,
- the liquid fraction being introduced after expansion at the first intermediate level of the recovery column, the gas fraction being introduced at an upper level of the recovery column, located below the intermediate level,
- the liquid flow stemming from the first upstream separator flask being advantageously placed in a heat exchange relationship with the initial natural gas stream in order to be heated up before being introduced into the second upstream separator flask;
- it includes the establishment of a heat exchange relationship of the foot stream stemming from the recovery column with the initial natural gas stream and with the bottom reboiling stream in the first upstream heat exchanger before its introduction into the fractionation column;
- the gas flow stemming from the first separator flask is separated into the feed stream and into a reflux stream, the feed stream being intended for feeding the dynamic expansion turbine, the reflux stream being introduced, after cooling, partial or total condensation and expansion in a valve, with reflux, into the recovery column;
- it includes a step for compressing the foot stream stemming from the recovery column in a pump, before its introduction into the fractionation column.
- the method includes a step for cooling the secondary reflux stream by heat exchange with at least one portion of the head stream of the recovery column.

The object of the invention is also a facility for simultaneous production of a treated natural gas, of a cut rich in C₃ ⁺ hydrocarbons, and under at least certain conditions of production, an ethane-rich stream from an initial natural gas stream containing methane, ethane, and C₃ ⁺ hydrocarbons, the facility comprising:

- an assembly for cooling and partly condensing the initial natural gas stream comprising at least one first upstream heat exchanger in order to form a cooled initial stream;
- an assembly for separating the cooled initial current into a liquid flow and a gas flow;
- a column for recovering C₂ ⁺ hydrocarbons
- an assembly for expansion of the liquid flow, and for introducing a stream stemming from the liquid flow into the recovery column at a first intermediate level;
- an assembly for forming a stream for feeding a turbine from the gas flow;
- an assembly for expansion of the feed stream, comprising a dynamic expansion turbine and an assembly for introducing the expanded feed stream into the recovery column at a second intermediate level;
- an assembly for recovering and compressing at least one portion of the head stream of the recovery column in order to form natural gas and an assembly for recovering the foot stream of the recovery column in order to form a liquid stream rich in C₂ ⁺ hydrocarbons;
- a fractionation column provided with a head condenser,
- an assembly for introducing the liquid stream at a feed level of the fractionation column, the ethane-rich stream being able to be produced, under said production conditions, from a stream stemming from the fractionation column, the fractionation column being able to produce a foot stream intended to form at least partly the C₃ ⁺ hydrocarbon cut;
- an assembly for introducing a primary reflux stream produced in the head condenser, with reflux, into the fractionation column;
- an assembly for producing a secondary reflux stream from the head condenser and an assembly for introducing the secondary reflux stream at the head of the recovery column,

characterized in that the facility includes:

- an assembly for sampling a recycling stream in the head stream of the recovery column;
- an assembly for establishing a heat exchange relationship of the recycling stream with at least one portion of the head stream stemming from the recovery column,
- an assembly for reintroducing, after expansion, the recycling stream into the recovery column, the facility further including an assembly for sampling in the bottom of the recovery column at least one bottom reboiling stream and an assembly for establishing a heat exchange relationship of the bottom reboiling stream with at least one portion of the initial natural gas or/and with the recycling stream, reboiling being able to be ensured by the calories taken in the initial natural gas stream or/and in the recycling stream.

The facility according to the invention may comprise one or more of the following features, taken individually or according to all technically possible combinations:

- it includes a first upstream heat exchanger capable of establishing a heat exchange relationship with at least one portion of the initial natural gas stream, the bottom reboiling stream, optionally side reboiling streams, at least one portion of the head stream and the recycling stream;
- it includes a first upstream heat exchanger capable of establishing a heat exchange relationship with a first portion of the initial natural gas stream, with at least one portion of the head stream, a second upstream heat exchanger, distinct from the first upstream heat exchanger, capable of establishing a heat exchange relationship of a second portion of the initial gas stream with the bottom reboiling stream stemming from the recovery column, and a third upstream heat exchanger distinct from the first upstream heat exchanger and from the second upstream heat exchanger, the third upstream heat exchanger being capable of establishing a heat exchange relationship of at least one portion of the recycling stream with at least one portion of the head stream, the facility advantageously including an auxiliary compressor capable of compressing the portion of the recycling stream intended to be introduced into the third upstream heat exchanger;
- the facility comprises a first head heat exchanger, capable of placing in a heat exchange relationship at least one portion of the head stream, optionally with the reflux stream, and the secondary reflux stream;
- the facility comprises a second head heat exchanger, distinct from the first head heat exchanger and capable of establishing a heat exchange relationship between a second portion of the head stream and the recycling stream.

The invention will be better understood upon reading the description which follows, only given as an example, and made with reference to the appended drawings, wherein:

FIG. 1 is a functional block diagram of a first facility for applying a first method according to the invention,

FIG. 2 is a diagram similar to FIG. 1 of a second facility for applying a second method according to the invention;

FIG. 3 is a diagram similar to FIG. 1 of a third facility for applying a third method according to the invention;

FIG. 4 is a diagram similar to FIG. 1 of a fourth facility for applying a fourth method according to the invention;

FIG. 5 is a diagram similar to FIG. 1 of a fifth facility for applying a fifth method according to the invention;

FIG. 6 is a diagram similar to FIG. 1 of a sixth facility, for applying a sixth method according to the invention, the sixth facility resulting from de-bottlenecking of an existing facility.

The first facility 11 according to the invention, illustrated in FIG. 1, is intended for simultaneously producing from an initial desulfurized, dry and at least partly decarbonated natural gas stream 13, a treated natural gas 15 as a main product, a cut 17 of C₃ ⁺ hydrocarbons and an ethane-rich stream 19 with adjustable flow rate.
The term of “at least partly decarbonated” means that the carbon dioxide content in the initial natural gas stream 13 is advantageously less than or equal to 50 ppm when the treated natural gas 15 has to be liquefied. This content is advantageously less than 3% when the treated natural gas 15 is directly sent to a gas distribution network.
Also, the water content is less than 1 ppm, advantageously less than 0.1 ppm.
The facility 11 comprises a unit 21 for recovering C₂ ⁺ hydrocarbons and a unit 23 for fractionation of C₂ ⁺ hydrocarbons.
In all of the following, a liquid flow and the conduit which conveys it, will be designated by a same reference, the relevant pressures are absolute pressures and the relevant percentages are molar percentages.
The unit 21 for recovering C₂ ⁺ hydrocarbons successively comprises a first upstream heat exchanger 25, a first upstream separator flask 27, a first upstream turbine 29, coupled with a first compressor 31, a first head heat exchanger 33, and a recovery column 35 provided with at least one side reboiling circuit 37, 39 and with a side reboiling circuit 41.
In this example, the column 35 is provided with two side reboiling circuits 37, 39.
The unit 21 further comprises a second compressor 43 driven by an external energy source and a first cooler 45 placed downstream from the second compressor 43. The unit 21 also comprises a column bottom pump 47.
The fractionation unit 23 comprises a fractionation column 61. The column 61 includes at its head, a head condenser 63 and at its foot, a reboiler 65.
The head condenser 63 comprises a second cooler 67 and a first downstream separator flask 69 associated with a reflux pump 71.
A first method according to the invention applied by means of the facility 11 will now be described.
An exemplary initial molar composition of the initial desulfurized, dry and at least partly decarbonated natural gas stream 13 is given in the table below.


	Molar fraction in %

	Helium	0.0713
	CO₂	0.0050
	Nitrogen	1.2022
	Methane	85.7828
	Ethane	10.3815
	Propane	2.1904
	i-butane	0.1426
	n-butane	0.1936
	i-pentane	0.0204
	n-pentane	0.0102
	Hexane	0.0000
	Total	100.0000

More generally, the molar methane fraction in the initial natural gas stream 13 is comprised between 75% and 90%, the molar fraction of C₂ ⁺ hydrocarbons is comprised between 5% and 15%, and the molar fraction of C₃ ⁺ hydrocarbons is comprised between 1% and 8%.
The load flow rate to be treated for example is of the order of 38,000 kmol/h. The initial natural gas stream 13 has a temperature close to room temperature and notably substantially equal to 20° C., and a pressure notably greater than 35 bars.
In a particular example, the natural gas stream 13 has a temperature of 20° C. and a pressure of 50 bars absolute.
In the facility illustrated in FIG. 1, the initial natural gas stream 13 is cooled and at least partly condensed in the first upstream heat exchanger 25 in order to form a cooled initial stream 113.
The cooled initial stream 113 is introduced into the first upstream separator flask 27 in which a separation is performed between a gas phase 115 and a liquid phase 117.
The liquid phase 117 forms, after passing into an expansion valve 119, an expanded mixed phase 120 which is introduced at a first intermediate level N1 of the recovery column 35, located in the upper region of the column, above the side reboiling circuits 37 and 39.
By “intermediate level” is meant a location including distillation means above and below this level.
The gas fraction 115 is separated into a feed stream 121 and a reflux stream 123.
Advantageously, the molar flow rate of the feed stream 121 is greater than the molar flow rate of the reflux stream 123.
The feed stream 121 is expanded in the turbine 29 down to a pressure close to that of the column 35 in order to obtain an expanded feed stream 125. The stream 125 is introduced into the recovery column 35 at a second intermediate level N2, located above the first intermediate level N1.
The reflux stream 123 is partly or totally condensed in the first head heat exchanger 33, and is then expanded in an expansion valve 127 in order to form an expanded reflux stream 128. This stream 128 is introduced into the recovery column 35 at a third intermediate level N3, located above the intermediate level N2.
The pressure of the recovery column 35 is for example comprised between 12 and 40 bars.
The recovery column 35 produces a head stream 131 which is heated up in the first head heat exchanger 33 by heat exchange with the reflux stream 123 in order to form a partly heated-up head stream 139.
The stream 139 is again heated up in the first upstream heat exchanger 25 by heat exchange with the initial natural gas stream 13 in order to form a heated-up head stream 140.
The heated-up head stream 140 is then compressed in the first compressor 31, and then in the second compressor 43, in order to form a compressed head stream 141. The pressure of the stream 141 is greater than 25 bars, for example equal to 50 bars. The stream 141 is then cooled in the first cooler 45 in order to form the treated natural gas 15.
According to the invention, a recycling stream 152 is sampled in the head stream stemming from the column 35. In the example illustrated in FIG. 1, the recycling stream 152 is sampled in the compressed heated-up head stream 141, after its cooling in the first cooler 45.
The ratio of the molar flow rate of the recycling stream 152, relatively to the molar flow rate of the head stream 131 stemming from the recovery column 35 is comprised between 0% and 25%.
The recycling stream 152 is then introduced into the first upstream heat exchanger 25 so as to be cooled therein by heat exchange with at least one portion of the head stream 131. In this example, the stream 152 is placed in a heat exchange relationship with the partly heated-up head stream 139 stemming from the head heat exchanger 33, in order to form a partly cooled recycling stream 154.
The stream 154 is then introduced into the head heat exchanger 33, in order to be cooled therein by heat exchange with the head stream 131, and to form after expansion in a valve 156, a cooled recycling stream 155.
The cooled recycling stream 155 is introduced into the recovery column 35 at a level N5 located above the level N3, advantageously corresponding to the first stage starting from the top of the column 35.
The treated gas 15 contains in this example 1.36% molar of nitrogen, 96.80% molar of methane and 1.76% molar of C₂hydrocarbons.
More generally, the treated gas 15 contains more than 99% molar of the methane contained in the initial natural gas stream 13 and less than 0.1% molar of the C₃ ⁺ hydrocarbons contained in the initial natural gas stream.
The treated gas 15 contains a molar proportion varying between 2% and 85% of the C₂hydrocarbons contained in the initial natural gas stream 13, this proportion being adjustable.
The gas 15 thus comprises a content of C₆ ⁺ hydrocarbons of less than 1 ppm, a water content of less than 1 ppm, advantageously less than 0.1 ppm and a carbon dioxide content of less than 50 ppm. The treated gas 15 may therefore be directly sent to a liquefaction train in order to produce liquefied natural gas. It may also be directly sent to a gas distribution network.
In the side reboiling circuits 37 and 39, side reboiling streams 161 and 163 are extracted from the column 35 and are reintroduced therein after being heated up in the first upstream heat exchanger 25, by heat exchange with at least one portion of the initial natural gas stream 13 and at least one portion of the recycling stream 152.
Thus, an upper side reboiling stream 163 is sampled at a level N6 located under level N1, for example at the eleventh stage starting from the top of the column 35, and is then brought as far as the first heat exchanger 25. The stream 163 is then heated up in the exchanger 25 and then sent back into the column 35 at a level N7 located under the level N6.
Also, a lower side reboiling stream 161 is sampled at a level N8 located under the level N7, and is then brought into the heat exchanger 25. The stream 161 is then heated up in the heat exchanger 25 and is then reintroduced at a level N9 located under the level N8, for example at the fourteenth stage starting from the top of the column 35.
In the bottom reboiling circuit 41, a liquid bottom reboiling stream 165 is extracted in the vicinity of the foot of the column 35, below the side reboiling streams 161, 163.
According to the invention, the stream 165 is brought into the first upstream heat exchanger 25 where it is heated up by heat exchange with at least one portion of the initial natural gas stream 13 and at least one portion of the recycling stream 152. The heated up and partly vaporized bottom reboiling stream is then reintroduced into the column 35.
A bottom stream 171 rich in C₂ ⁺ hydrocarbons is extracted from the foot of the recovery column 35.
The bottom stream 171 contains more than 99% molar of C₃ ⁺ hydrocarbons contained in the initial natural gas stream 13. It has a methane content comprised between 9% and 5%.
The bottom stream 171 is pumped with the tank bottom pump 47 and introduced at an intermediate level P1 of the fractionation column 61.
In the illustrated example, the fractionation column 61 operates at a pressure comprised between 20 and 42 bars. In this example, the pressure of the fractionation column 61 is greater by at least one bar than the pressure of the recovery column 35.
A foot stream 181 is extracted from the fractionation column 61 in order to form the cut 17 of C₃ ⁺ hydrocarbons.
The extraction rate of the C₃ ⁺ hydrocarbons in the method is greater than 99%. In every case, the propane extraction rate is greater than 99%.
The ethane-rich stream 19 is directly drawn off at an intermediate level P2 located in the upper region of the fractionation column 61.
In the example illustrated in the Figures, this stream comprises 1.21% of methane, 97.77% of ethane and 1.00% of propane.
More generally, the molar ethane content in the ethane-rich stream 19 is greater than 95%, notably comprised between 96% and 100%.
The number of theoretical plates between the head of the column 61 and the upper level P2 is for example comprised between 1 and 7. The level P2 is above the feed level P1.
A second head stream 183 is extracted from the head of the column 61 and is then cooled in the second cooler 67 in order to form a second cooled and at least partly condensed head stream 185. This second stream 185 is introduced into the second separator flask 69 for producing a liquid fraction 187 and a gas fraction 188.
In the example illustrated in FIG. 1, the totality of the liquid fraction 187 is pumped in the pump 71 in order to form a primary reflux stream 190 before being reintroduced with reflux into the fractionation column 61 at a head level P3 located above the level P2.
In this case, the totality of the gas fraction 188 forms, after cooling in the head heat exchanger 33 and expansion in a valve 193, a secondary reflux stream 192.
In the head exchanger 33, the gas fraction 188 is cooled by heat exchange with the head stream 131.
In an alternative illustrated in dotted lines, the liquid fraction 187 is separated into a liquid primary reflux fraction 189 and a liquid secondary fraction 191.
The secondary liquid fraction 191, when it is present, is then mixed with the gas fraction 188 in order to form after cooling and expansion, the secondary reflux stream 192.
The secondary reflux stream 192 is introduced with reflux at a head level N4 of the recovery column 35 located between the head level N5 and the intermediate level N3.
The ethane extraction rate, and subsequently the ethane flow rate produced in the facility 11, is controlled by adjusting the flow rate of the recycling stream 152, by adjusting the pressure in the recovery column 35, by means of the compressors 43 and 31 which are of the variable rate type on the one hand, and by finally adjusting the flow rate of the secondary reflux stream 192 circulating through the expansion valve 193 on the other hand.
As shown in the table below, the flow rate of the ethane-rich stream is adjustable, practically without affecting the extraction rate of C₃ ⁺ hydrocarbons.
The method according to the invention therefore gives the possibility, with simple and inexpensive means, of obtaining a variable and easily adjustable flow rate of an ethane-rich stream 19 extracted from the initial natural gas 13, by maintaining the extraction rate of propane above 99%. This result is obtained without any significant modification of the facility in which the method is applied.


	Stream 152	Ethane	Propane	Total
Pressure	flow rate	recovery	recovery	compression
C1 (bara)	(kmol/h)	(% by moles)	(% by moles)	power (kW)

29.0	0.37	0.66	99.76	16254
26.2	1900	15.00	99.48	17622
25.4	2600	29.34	99.06	19072
24.8	4410	43.42	99.87	21389
22.5	5470	58.34	100	25861
20.7	5750	68.89	100	29554
19.1	6000	77.88	100	33136
17.9	6200	84.63	100	36183

The values of the pressures, the temperatures and flow rates in the case when the ethane recovery rate is equal to 84.99% are given in the table below.


Stream	Temperature (° C.)	Pressure (bar abs)	Flow rate (kmol/h)

13	20.0	50.0	38000
15	40.0	50.0	33634
17	86.8	33.5	978
19	11.9	33.0	3389
113	−44.0	49.8	38000
115	−44.0	49.8	36412
120	−69.5	17.8	1588
125	−81.0	17.8	30858
128	−108.5	17.8	5554
131	−101.6	17.6	38134
152	40.0	50.0	4500
154	−40.0	49.8	4500
155	−111.7	17.8	4500
171	−5.3	17.8	4376
192	−3.4	33.0	10
194	−99.0	17.8	10

When the flow rate of the ethane-rich stream 19 is reduced, the total compression power is also strongly reduced.
The facility 11 according to the invention moreover does not require the imperative use of multiflow exchangers. It is thus possible to only use exchangers with tubes and a shell.
The treated natural gas 15 includes substantially nil contents of C₅ ⁺ hydrocarbons, for example less than 1 ppm. Subsequently, if the carbon dioxide content in the treated gas 15 is less than 50 ppm, this gas 15 may be liquefied without any additional treatment or fractionation.
In the first method according to the invention, the bottom reboiling stream 165 is put into a heat exchange relationship in the first heat exchanger 25 with the recycling stream 152, with at least one portion of the head stream 131, with the initial natural gas stream 13 and with the side reboiling streams 161, 163.
This particular thermal integration of the method is beneficial in terms of yield, and does not affect the recovery of ethane, when the latter is desired.
Thus, when the recycling stream 152 is placed in a heat exchange relationship with at least one portion of the head stream 131, and when the side reboiling stream 165 is placed in a heat exchange relationship with the initial natural gas stream 13, the inventors surprisingly noticed a synergistic increase in the yield of the facility 11.
Thus, as illustrated in the table below, a 16% yield gain is observed as compared with the facility according to the state of the art while preserving a recovery rate of 85%, all the other conditions being maintained. This extremely significant gain is obtained, while maintaining very high ethane recovery.


	Ethane recovery	Total
Case	(% by moles)	power (kW)	Gain (%)

State of the art	85.01	44756	—
U.S. Pat. No. 7,458,232
Facility 11	85.00	40566	9.4
without recycling
of treated gas
Facility
11	85.04	44651	0.2
without any
integrated bottom
reboiler
Facility
11	84.99	37422	16.4

Moreover, the combined presence of the recycling of a portion of the heated gas and of an integrated bottom reboiling assembly 41 integrated into the first heat exchanger 25 surprisingly generates a larger yield gain than what is observed in the presence of either one of these steps taken individually.
Thus, when the first method is applied without any treated gas recycling stream 152, the obtained gain is 9.4%, while, when the first method 11 is applied without a bottom reboiler integrated into the heat exchanger 25, the obtained gain is 0.2%. The observed gain by sharing the aforementioned features is therefore notably greater than the sum of the individual gains obtained, demonstrating an unexpected synergistic effect, which does not affect ethane recovery.
Alternatively, the treated gas stream stemming from the first compressor 31 may be brought into a compressor 43 with two equivalent power stages, with an intermediate cooler cooling the gas to the same temperature as the cooler 45.
A second facility 201 according to the invention is illustrated by FIG. 2. The facility 201 differs from the first facility 11 in that it further includes an auxiliary expansion turbine 203 and an auxiliary compressor 205 coupled with a turbine 203. In a first embodiment, the auxiliary compressor 205 is interposed between the first compressor 31 and the second compressor 43.
A second method according to the invention is applied in the second facility 201.
Unlike the first method according to the invention, the initial natural gas stream 13 is separated into a first initial stream 207 and a second initial stream 209.
The molar flow rate of the first initial stream 207 is advantageously greater than the molar flow rate of the second initial stream 209.
Next, the first initial stream 207 is introduced into the first heat exchanger 25 so as to be cooled and partly condensed therein and to form the cooled natural gas stream 113 introduced into the first separator flask 27.
The second initial stream 209 is introduced into the auxiliary expansion turbine 203, so as to be expanded therein down to a pressure close to the operating pressure of the column 35 and to form an auxiliary reflux stream 211. The auxiliary reflux stream 211 is then introduced into the first head heat exchanger 33 so as to be cooled and partly condensed therein, and then into an expansion valve 213 for forming an expanded auxiliary reflux stream 215.
The stream 215 is then introduced into the recovery column 35 at an upper level N10 located between the level N3 and the level N4.
In the example illustrated in FIG. 2, the head stream 217 stemming from the first compressor 31 is introduced, at its outflow from the first compressor 31, into the auxiliary compressor 205, so as to be compressed at an intermediate pressure, before joining up with the second compressor 43.
The values of the pressures, temperatures, and flow rates in the case when the ethane recovery rate is equal to 85.00% are given in the table below.


Stream	Temperature (° C.)	Pressure (bar abs)	Flow rate (kmol/h)

13	20.0	50.0	38000
15	40.0	50.0	33634
17	87.7	34.0	978
19	12.6	33.5	3389
113	−50.1	49.8	35074
115	−50.1	49.8	31965
120	−79.3	16.5	3109
125	−88.8	16.5	29505
128	−110.9	16.5	2460
131	−102.9	16.3	36154
152	40.0	50.0	2520
154	−50.0	49.8	2520
155	−113.5	16.5	2520
171	−8.4	16.5	4376
192	−2.0	33.5	10
194	−100.3	16.5	10
207	20.0	50.0	35074
211	−26.3	20.9	2926
215	−107.0	16.5	2926

The application of the second method according to the invention produces a result similar to that of the first method, by the synergy observed between the establishment of a heat exchange relationship of the bottom reboiling stream 165 with the initial natural gas stream 13, taken as a combination with the presence of a recycling stream 152, put into a heat exchange relationship with at least one portion of the head stream 131.
Thus, the consumption of the method for applying the facility 201 leads to a consumed power equal to 37,588 kW, i.e. a gain of 16% as compared with the facility of the state of the art.
In an alternative of FIG. 2 (visible as dotted lines), the auxiliary compressor 205 is mounted downstream from the compressor 43 in order to compress the recycling stream 152, before introducing it into the first heat exchanger 25.
The facility and the application of the method are moreover similar to the one of FIG. 2.
A third facility 221 according to the invention is illustrated by FIG. 3. Unlike the facility 11 illustrated in FIG. 1, the facility 221 includes a second upstream separator flask 223 placed downstream from the first separator flask in order to collect the liquid phase 117 stemming from the first separator flask 27.
A third method according to the invention is applied by means of the facility 221. This third method differs from the first method according to the invention, in that the liquid phase 117 is expanded in a static expansion valve 225. This expansion is carried out down to a pressure above the operating pressure of the column 35.
The liquid phase is then expanded and introduced into the upstream separator flask 223.
A liquid fraction 227 is recovered at the bottom of the flask 223 and is expanded in a valve 229 in order to form an expanded fraction 231. The expanded fraction 231 is introduced into the recovery column 35 at level N1.
A gas fraction 233 is collected at the head of the second upstream separator flask 223. This fraction 233 is sent towards the head heat exchanger 33 so as to be cooled therein before being expanded in an expansion valve 135 in order to form an expanded fraction 237.
The expanded fraction 237 is introduced into the recovery column 35 at an intermediate level N11 comprised between the level N2 and the level N3.
The values of the pressures, temperatures and flow rates in the case when the ethane recovery rate is equal to 84.99% are given in the table below:


Stream	Temperature (° C.)	Pressure (bar abs)	Flow rate (kmol/h)

13	20.0	50.0	38000
15	40.0	50.0	33658
17	86.8	33.5	978
19	13.1	33.0	3364
113	−42.7	49.8	38000
115	−42.7	49.8	36709
117	−42.7	49.8	1291
118	−62.3	23.3	1291
125	−79.4	18.0	32325
128	−108.1	18.0	4384
131	−101.4	17.8	39758
152	40.0	50.0	6100
154	−40.0	49.8	6100
155	−111.3	18.0	6100
171	−3.5	18.0	4392
188	7.2	33.0	50
192	−98.8	18.0	50
231	−67.4	18.0	910
233	−62.3	23.3	381
237	−106.2	18.0	381

The method applied by means of a third facility 221 according to the invention leads to a total power consumed by the compressors of 35,960 kW, i.e. a gain of 19.7% relatively to the method of the state of the art.
It further allows an additional gain of 3.9% as compared with the first method according to the invention.
In an alternative of the third method, the liquid phase 117 obtained at the foot of the first separator flask 27 is introduced into the first heat exchanger 25 so as to heat it up therein, before being brought into the valve 225.
The mixture is expanded in the valve 225, before being separated in the second upstream separator flask 223.
A fourth facility 241 according to the invention is illustrated by FIG. 4. Unlike the first facility 11, the stream 171 stemming from the recovery column 35 is passed into the first heat exchanger 25 so as to be heated up therein before being introduced into the fractionation column 61.
The fourth method according to the invention therefore applies heating up of this bottom stream 171, after its passing into the pump 47.
For an ethane recovery rate of 85.00%, the total consumption is then of 34,201 kW, which provides a gain of 23.6% as compared with the facility of the state of the art. The gain is moreover 8.6% relatively to the first method according to the invention.
The values of the pressures, temperatures and flow rates in the case when the ethane recovery rate is equal to 85.00% are given in the table below:


Stream	Temperature (° C.)	Pressure (bar abs)	Flow rate (kmol/h)

13	20.0	50.0	38000
15	40.0	50.0	33656
17	86.8	33.5	976
19	12.9	33.0	3368
113	−40.1	49.8	38000
115	−40.1	49.8	37218
120	−65.8	16.2	782
125	−80.1	16.2	27578
128	−110.6	16.2	9640
131	−102.9	16.0	34051
152	40.0	50.0	395
154	−40.0	49.8	395
155	−113.9	16.2	395
171	−7.7	16.2	4354
188	5.4	33.0	10
192	−100.2	16.2	10
243	12.0	33.5	4354

A fifth facility according to the invention 251 is illustrated by FIG. 5. This facility is intended to apply a fifth method according to the invention.
Unlike the first method according to the invention, a bypass stream 253 is sampled in the recycling stream 152, advantageously downstream from the first heat exchanger 25 and upstream from the second heat exchanger 33, so as to be reintroduced into the stream located downstream from the first dynamic expansion turbine 29.
The bypass stream flow rate 253 is for example equal to 47% of the total molar flow rate of the recycling stream 152 sampled in the treated stream.
The fifth method according to the invention is moreover applied similarly to the fourth method according to the invention.
In the example of FIG. 5, the bypass stream 253 is mixed with the feed stream 121 before it is introduced into the turbine 29.
In an alternative illustrated in dotted lines, the fifth facility 251 further includes a secondary dynamic expansion turbine 255 coupled with a secondary compressor 257. A secondary recycling stream 258 is then sampled in the recycling stream 152 before its introduction into the first heat exchanger 25.
The secondary recycling stream 258 is introduced into the secondary expansion turbine 255, in order to form an expanded secondary recycling stream 261, which is reintroduced into the partly heated-up head stream 139 stemming from the first head heat exchanger 33.
Moreover, a secondary head stream 263 is sampled in the heated-up head stream 140 stemming from the first heat exchanger 25 so as to be brought as far as the secondary compressor 257 and form a compressed secondary head stream 265.
This stream 265 is then reintroduced into the compressed head stream at an intermediate pressure, stemming from the first compressor 31 upstream from the second compressor 43.
The power gain obtained relatively to the method of the state of the art is then of the order of 15.4%, for a total consumed power of 37,851 kW.
The values of the pressures, temperatures and flow rates in the case when the ethane recovery rate is equal to 85.00% are given in the table below:


Stream	Temperature (° C.)	Pressure (bar abs)	Flow rate (kmol/h)

13	20.0	50.0	38000
15	40.0	50.0	33633
17	86.8	33.5	978
19	11.9	33.0	3389
113	−47.4	49.8	38000
115	−47.4	49.8	35524
120	−74.1	17.7	2477
125	−84.8	17.7	31199
128	−108.8	17.7	6463
131	−101.7	17.5	38183
152	40.0	50.0	4550
154	−40.0	49.8	4550
155	−111.8	17.7	2412
171	−5.5	17.7	4377
188	−3.4	33.0	10
192	−99.1	17.7	10
253	−40.0	49.8	2138

A sixth facility 271 according to the invention is illustrated in FIG. 6. This facility 271 is intended for de-bottlenecking a facility as illustrated in U.S. Pat. No. 7,458,232 and initially comprising a first upstream heat exchanger 25, a first separator flask 27, a recovery column 35, a first head heat exchanger 33 and a fractionation column 61 provided with a head condenser 63.
Unlike the first facility 11 according to the invention, the facility 271 further includes a second upstream heat exchanger 273 and a third upstream heat exchanger 275, intended to be placed in parallel with the first upstream heat exchanger 25.
The facility 271 further includes an auxiliary compressor 277 intended to compress the recycling stream 152 and an auxiliary cooler 279 intended to cool the compressed recycling stream.
Moreover, the sixth facility 271 includes a second head heat exchanger 281 intended to be placed in parallel with the first head heat exchanger 33, in order to place at least one portion of the head stream 131 in a heat exchange relationship with at least one portion of the recycling stream 152.
A sixth method according to the invention is applied in the sixth facility 271. In this method, the initial natural gas stream 13 is separated into a first initial stream 207 introduced into the first upstream heat exchanger 25 and into a second initial stream 209 introduced into a second upstream heat exchanger 273.
The first initial stream 207 is then cooled in the first upstream heat exchanger 25 in order to form a first cooled initial stream 281A. Also, the second initial stream 209 is cooled in the second upstream heat exchanger 273 in order to form a second cooled initial stream 283. The streams 281A and 283 are mixed so as to form the cooled stream 113 intended to be introduced into the first upstream separator flask 27.
The side reboiling streams 161, 163 are introduced into the first heat exchanger 25 in order to be heated up therein.
Unlike the first method according to the invention, the bottom reboiling stream 165 is introduced into the second upstream heat exchanger 273 so as to be heated up therein by heat exchange with the second initial stream 209.
Also, unlike the first method according to the invention, the head stream 131 stemming from the recovery column 35 is first of all separated into a first head stream fraction 285 and a second head stream fraction 287.
The first fraction 285 is introduced into the first head heat exchanger 33 so as to be heated up therein by heat exchange with the reflux stream 123 on the one hand and with the secondary reflux stream 192 on the other hand.
The second fraction 287 is introduced into the second head heat exchanger 281.
The ratio of the molar flow rate of the first fraction 285 to the second fraction 287 is for example comprised between 0 and 20.
Next, the fractions recovered at the outlet of the heat exchangers 33, 281 are mixed again before being again separated into a first portion 289 of the heated-up head stream and into a second portion 291 of the heated-up head stream.
The first portion 289 is introduced into the first upstream heat exchanger 25 so as to be heated up therein by heat exchange with the first initial stream 207, simultaneously with the side reboiling streams 161 and 163.
The second portion 291 is introduced into the third upstream heat exchanger 275 so as to be heated up therein.
The heated-up portions 289 and 291 are then joined together in order to form the heated-up head stream 140 and are then brought to the first compressor 31.
Unlike the first method according to the invention, the recycling stream 152 is sampled in the heated head stream 140 upstream from the first compressor 31.
The ratio of the molar flow rate of the recycling stream 152 to the molar flow rate of the head stream 131 stemming from the column 35 is for example comprised between 0% and 25%.
The recycling stream 152 is then compressed in the auxiliary compressor 277, up to a pressure for example greater than 50 bars, and is then cooled in the cooler 279 in order to form a cooled compressed recycling stream 293.
The stream 293 is then successively introduced into the third upstream heat exchanger 275, and then into the second head heat exchanger 281 so as to be cooled therein, before being expanded in an expansion valve 295 and to form a cooled expanded recycling stream 297.
The stream 297 is then introduced into the recovery column 35, at the same level as the secondary reflux stream 194.
Thus, in the first upstream heat exchanger 25 initially present in the facility, a portion 207 of the initial natural gas stream 13, the side reboiling streams 161, 163 and a portion 289 of the head stream are placed in a heat exchange relationship.
In the second upstream heat exchanger 273, a second portion 209 of the initial natural gas stream 13, and the bottom reboiling stream 165 are placed in a heat exchange relationship. In the third upstream heat exchanger 275, a second portion 291 of the head stream 131, and the recycling stream 152 are placed in a heat exchange relationship.
The facility 271 according to the invention moreover does not require any imperative use of multiflow exchangers. It is thus possible to only use exchangers with tubes and a shell.
Further at the head of the column 35, the reflux stream 123, a first portion of the head stream 285, and the secondary reflux stream 192 are placed in a heat exchange relationship in the first head heat exchanger 33. In the second head heat exchanger 281, a second portion 287 of the head stream 131 and the cooled compressed recycling stream 233 are placed in a heat exchange relationship.
The facility 271 as illustrated in FIG. 6 gives the possibility of accommodating increases in the feed flow rate from 0% to 15% and more preferentially of at least 10%, by limiting to a minimum the required increase in compression power.
The values of the pressures, temperatures, and flow rates in the case when the ethane recovery rate is equal to 85.00% are given in the table below:


Stream	Temperature (° C.)	Pressure (bar abs)	Flow rate (kmol/h)

13	20.0	50.0	39900
15	40.0	50.0	35336
17	90.1	33.5	956
19	13.4	33.0	3608
113	−44.3	49.8	39900
115	−44.3	49.8	38154
120	−74.8	13.5	1746
125	−89.1	13.5	29961
128	−115.6	13.5	8193
131	−106.5	13.3	36016
140	15.9	12.9	36016
141	150.2	50.2	35336
152	15.9	12.9	680
171	−15.0	13.5	4565
192	5.0	33.0	1
194	−103.7	13.5	1
207	20.0	50.0	39900
225	−118.4	13.5	680
285	−106.5	13.3	33250
287	−106.5	13.3	2766
289	−58.6	13.1	34935
291	−58.6	13.1	1080
293	40.0	50.2	680

In the example illustrated in the figures, the ethane-rich stream 19 is directly sampled in the fractionation column 61, advantageously at an upper level P2 of the column 61 defined above.
The cut of C₃ ⁺ hydrocarbons 17 is moreover directly formed by the foot stream 181 of the column 61.
In an alternative (not shown), the C₂ ⁺ hydrocarbons are extracted from the fractionation column 61 with the foot stream 181, at the same time as the C₃ ⁺ hydrocarbons. The foot stream 181 is then introduced into a downstream fractionation column.
The ethane-rich cut 19 like the cut of C₃ ⁺ hydrocarbons 17 are then produced in the downstream fractionation column.

Claims

1. A method for simultaneously producing a treated natural gas, a cut rich in C₃ ⁺ hydrocarbons and under at least certain production conditions, an ethane-rich stream, from an initial natural gas stream containing methane, ethane and C₃ ⁺ hydrocarbons, the method comprising the following steps:

cooling and partly condensing the initial natural gas stream in at least one first upstream heat exchanger in order to form a cooled initial stream;

separating the cooled initial gas stream into a liquid flow and a gas flow;

expanding the liquid flow, and introducing a stream stemming from the liquid flow into a column for recovering C₂ ⁺ hydrocarbons at a first intermediate level;

forming a stream for feeding a turbine from the gas flow;

expanding the feed stream in a dynamic expansion turbine and introducing it the expanded feed stream into the recovery column at a second intermediate level;

recovering and compressing at least one portion of the head stream of the recovery column in order to form the natural gas and recovering the foot stream of the recovery column in order to form a C₂ ⁺-hydrocarbon-rich liquid stream;

introducing the liquid stream at a feed level (P1) of a fractionation column provided with a head condenser, the ethane-rich stream being produced under said production conditions, from a stream stemming from the fractionation column, the fractionation column producing a foot stream intended to at least partly form the C₃ ⁺ hydrocarbon cut;

introducing a primary reflux stream produced in the head condenser with reflux into the fractionation column;

producing a secondary reflux stream from the head condenser and introducing the secondary reflux stream at the head of the recovery column,

sampling a recycling stream in the head stream stemming from the recovery column;

establishing a heat exchange relationship of the recycling stream with at least one portion of the head stream stemming from the recovery column,

reintroducing, after expansion, the cooled and expanded recycling stream into the recovery column,

sampling in the bottom of the recovery column of at least one bottom reboiling stream,

establishing a heat exchange relationship of the bottom reboiling stream with at least one portion of the initial natural gas or/and with the recycling stream,

ensuring the bottom reboiling by the calories taken from the initial natural gas stream or/and from the recycling stream.

2. The method according to claim 1, including placing at least one portion of the head stream of the recovery column and the recycling stream in a heat exchange relationship with the initial natural gas stream and with the bottom reboiling stream.

3. The method according to claim 1, including putting in a heat exchange relationship in a first head heat exchanger the recycling stream stemming from the first upstream heat exchanger, the secondary reflux stream stemming from the head condenser, and the head stream stemming from the recovery column.

4. The method according to claim 1, including sampling at least one side reboiling stream above the bottom reboiling stream, and placing said or each side reboiling stream in a heat exchange relationship with at least one portion of the initial natural gas stream.

5. The method according to claim 1, including drawing off the ethane-rich stream from an intermediate level of the fractionation column located above the feed level of the column, and below the head level of the fractionation column.

6. The method according to claim 1, including the following steps:

separating the initial natural gas stream into a first initial stream and into a second initial stream;

introducing the first initial stream into the first upstream heat exchanger;

introducing at least one portion of the second initial stream into an auxiliary dynamic expansion turbine in order to form an auxiliary reflux stream from the effluent stemming from the auxiliary turbine;

introducing the auxiliary reflux stream into the recovery column.

7. The method according to claim 6, including at least one portion of the recycling stream in an auxiliary compressor coupled with the auxiliary turbine.

8. The method according to claim 1, including compressing at least one portion of the head stream in an auxiliary compressor coupled with the auxiliary turbine.

9. The method according to claim 1, including sampling, in the recycling stream, bypass stream, and reintroducing the bypass stream into a stream located upstream from the first dynamic expansion turbine (29).

10. The method according to claim 1, including expanding the liquid flow stemming from the first upstream separator flask and introducing said liquid flow into a second upstream separator flask in order to form a liquid fraction and a gas fraction,

the liquid fraction being introduced after expansion at the first intermediate level of the recovery column, the gas fraction being introduced at an upper level of the recovery column, located above the intermediate level.

11. The method according to claim 1, including establishing a heat exchange relationship of the foot stream stemming from the recovery column with the initial natural gas stream and with the bottom reboiling stream in the first upstream heat exchanger before its introduction into the fractionation column.

12. The method according to claim 1, including separating the gas flow stemming from the first separator flask into the feed stream and into a reflux stream, the feed stream being intended to feed the dynamic expansion turbine, and introducing the reflux stream being introduced, after cooling, partial or total condensation, and expansion in a valve, with reflux, into the recovery column.

13. A facility for simultaneously producing a treated natural gas, a cut rich in C₃ ⁺ hydrocarbons, and under certain production conditions, an ethane-rich stream, from an initial natural gas stream containing methane, ethane and C₃ ⁺ hydrocarbons, the facility comprising:

an assembly for cooling and partly condensing the initial natural gas stream comprising at least a one first upstream heat exchanger in order to form a cooled initial stream;

an assembly for separating the cooled initial stream into a liquid flow and into a gas flow;

a column for recovering C₂ ⁺ hydrocarbons

an assembly for expanding the liquid flow, and for introducing a stream stemming from the liquid flow into the recovery column at a first intermediate level;

an assembly for forming a stream for feeding the turbine from the gas flow;

an assembly for expanding the feed stream, comprising a dynamic expansion turbine and an assembly for introducing the expanded feed stream into the recovery column at a second intermediate level;

an assembly for recovering and compressing at least one portion of the head stream of the recovery column in order to form the natural gas and an assembly for recovering the foot stream of the recovery column in order to form a liquid stream rich in C₂ ⁺ hydrocarbons;

a fractionation column provided with a head condenser,

an assembly for introducing the liquid stream at a feed level of the fractionation column, the ethane-rich stream being able to be produced under said production conditions, from a stream stemming from the fractionation column, the fractionation column being able to produce a foot stream intended to form at least partly the C₃ ⁺ hydrocarbon cut;

an assembly for introducing a primary reflux stream produced in the head condenser with reflux into the fractionation column;

an assembly for producing a secondary reflux stream from the head condenser and an assembly for introducing the secondary reflux stream at the head of the recovery column,

an assembly for sampling a recycling stream in the head stream of the recovery column;

an assembly for establishing a heat exchange relationship of the recycling stream with at least one portion of the head stream stemming from the recovery column,

an assembly for reintroducing, after expansion, the recycling stream into the recovery column,

an assembly for sampling in the bottom of the recovery column at least one bottom reboiling stream, and

an assembly for establishing a heat exchange relationship of the bottom reboiling stream with at least one portion of the initial natural gas or/and with the recycling stream, the reboiling being able to be ensured by the calories taken from the initial natural gas stream or/and from the recycling stream.

14. The facility according to claim 13, including a first upstream heat exchanger capable of establishing a heat exchange relationship with at least one portion of the initial natural gas stream, the bottom reboiling stream, at least one portion of the head stream and the recycling stream.

15. The facility according to claim 13, including a first upstream heat exchanger capable of establishing a heat exchange relationship of a first portion of the initial natural gas stream, with at least one portion of the head stream, a second upstream heat exchanger, distinct from the first upstream heat exchanger, capable of establishing a heat exchange relationship of a second portion of the initial gas stream with the bottom reboiling stream stemming from the recovery column, and a third upstream heat exchanger distinct from the first upstream heat exchanger and from the second upstream heat exchanger, the third upstream heat exchanger being capable of establishing a heat exchange relationship of at least one portion of the recycling stream with at least one portion of the head stream, including an auxiliary compressor capable of compressing the portion of the recycling stream intended to be introduced into the third upstream heat exchanger.

16. The method according to claim 8, wherein the auxiliary compressor is coupled with the auxiliary turbine between a first compressor coupled with the first turbine and a second compressor.

17. The method according to claim 10, including placing the liquid flow stemming from the first upstream separator flask in a heat exchange relationship with the initial natural gas stream so as to be heated up before being introduced into the second upstream separator flask.

18. The facility according to claim 14, wherein the first upstream heat exchanger is capable of establishing a heat exchange relationship with side reboiling streams.

19. The facility according to claim 15, including an auxiliary compressor capable of compressing the portion of the recycling stream intended to be introduced into the third upstream heat exchanger.