Patent Publication Number: US-10760851-B2

Title: Simplified method for producing a methane-rich stream and a C2+ hydrocarbon-rich fraction from a feed natural-gas stream, and associated facility

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 13/879,743, filed Jun. 5, 2013, which is a 35 U.S.C. § 371 national phase conversion of PCT/FR2011/052439, filed Oct. 19, 2011, which claims priority of French Patent Application No. 10 58573, filed Oct. 20, 2010, the content of each of these applications are incorporated by reference herein. The PCT International Application was published in the French language. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method for producing a methane-rich stream and a C 2   +  hydrocarbon-rich fraction from a dehydrated feed natural-gas stream, the method being of the type comprising the following steps: 
     cooling the feed natural-gas stream advantageously at a pressure greater than 40 bars in a first heat exchanger, and introducing the cooled feed natural-gas stream into a separator flask; 
     separating the cooled natural-gas stream in the separator flask and recovering an essentially gaseous light fraction and an essentially liquid heavy fraction; 
     forming a turbine input flow from the light fraction; 
     dynamically expanding the turbine input flow in a first expansion turbine and introducing the expanded flow into an intermediate portion of a splitter column; 
     expanding the heavy fraction and introducing the heavy fraction into the splitter column, the heavy fraction recovered in the separator flask being introduced into the splitter column without passing through the first heat exchanger; 
     recovering, at the bottom of the splitter column, a bottom C 2   +  hydrocarbon-rich stream intended to form the C 2   +  hydrocarbon-rich fraction; 
     sampling at the head of the splitter column a methane-rich head stream; 
     heating up the methane-rich head stream in a second heat exchanger and in the first heat exchanger and compressing this stream in at least one first compressor coupled with the first expansion turbine and in a second compressor for forming a methane-rich stream from the compressed methane-rich head stream; 
     sampling in the methane-rich head stream a first recirculation stream; and 
     passing the first recirculation stream into the first heat exchanger and into the second heat exchanger in order to cool it down, and then introducing at least one first portion of the first cooled recirculation stream into the upper portion of the splitter column. 
     Such a method is intended to be applied for building new units for producing a methane-rich stream and a C 2   +  hydrocarbon fraction from a feed natural-gas, or for modifying existing units, notably in the case when the feed natural-gas has a high ethane, propane and butane content. 
     Such a method also applies to the case when it is difficult to apply cooling of the feed natural-gas by means of an outer cooling cycle with propane, or to the case when the installation of such a cycle would be too expensive or too dangerous, such as for example in floating plants, or in urban regions. 
     Such a method is particularly advantageous when the unit for fractionating the C 2   +  hydrocarbon cut which produces the propane intended to be used in the cooling cycles is too far away from the unit for recovering this C 2   +  hydrocarbon fraction. 
     The separation of the C 2   +  hydrocarbon fraction from a natural gas extracted from the subsoil gives the possibility of satisfying both economic imperatives and technical imperatives. 
     Indeed, the C 2   +  hydrocarbon fraction recovered from natural gas is advantageously used for producing ethane and liquids which form raw materials in petrochemistry. Further, it is possible to produce from a C 2   +  hydrocarbon cut, C 5   +  hydrocarbon cuts which are used in oil refineries. All these products may be economically valued and contribute to the profitability of the facility. 
     Technically, the requirements of natural gas marketed in a network include, in certain cases, a specification at the level of the calorific value which has to be relatively low. 
     Methods for reducing C 2   +  hydrocarbon cuts generally comprise a distillation step, after cooling the feed natural-gas in order to form a methane-rich head stream and a C 2   +  hydrocarbon-rich bottom stream. 
     In order to improve the selectivity of the method, sampling a portion of the methane-rich stream produced at the head of the column after compression and reintroducing it after cooling into the column head are known for forming a reflux of this column. Such a method is for example described in US 2008/0190136 or in U.S. Pat. No. 6,578,379. 
     Such methods give the possibility of obtaining ethane recovery of more than 95% and in the latter case, even more than 99%. 
     Such a method however does not give entire satisfaction when the feed natural-gas is very rich in heavy hydrocarbons, and notably in ethane, propane and butane, and when the inlet temperature of the feed natural-gas is relatively high. 
     In these cases, the amount of cooling to be provided is large, which requires the addition of an additional cooling cycle if maintaining good selectivity is desired. Such a cycle consumes energy. Further, in certain facilities, notably floating facilities, it is not possible to apply such cooling cycles. 
     An object of the invention is therefore to obtain a method for recovering C 2   +  hydrocarbons which is extremely efficient and highly selective, even when the content of these C 2   +  hydrocarbons in the feed natural-gas increases significantly. 
     SUMMARY OF THE INVENTION 
     For this purpose, the subject-matter of the invention is a method of the aforementioned type, comprising the following steps: 
     forming at least one second recirculation stream obtained from a methane-rich head stream downstream from the splitter column; 
     forming a dynamic expansion stream from the second recirculation stream and introducing the dynamic expansion stream into an expansion turbine for producing frigories. 
     The method according to the invention may comprise one or several of the following features, taken individually or according to all technically possible combination(s): 
     the formation of the turbine input flow includes the division of the light fraction into the turbine input flow and into a secondary flow, the method comprising the cooling of the secondary flow in the second heat exchanger and introducing the cooled secondary flow into an upper portion of the splitter column; 
     the second recirculation stream is introduced into a stream located downstream from the first heat exchanger and upstream from the first expansion turbine in order to form the dynamic expansion stream; 
     the second recirculation stream is mixed with the turbine input flow from the separator flask in order to form the dynamic expansion stream, the dynamic expansion turbine receiving the dynamic expansion stream formed by the first expansion turbine; 
     the second recirculation stream is mixed with the cooled natural-gas stream before its introduction into the separator flask, the dynamic expansion stream being formed by the turbine input flow from the separator flask; 
     the second recirculation stream is sampled in the first recirculation stream; 
     the method comprises the following steps:
         sampling a stream in the methane-rich head stream before its passing into the first compressor and into the second compressor;   compressing the sampling stream in a third compressor, and   forming the second recirculation stream from the compressed sampling stream from the third compressor, and after cooling.       

     the method comprises the passing of the sampling stream into a third heat exchanger and into a fourth heat exchanger before its introduction into the third compressor, and then the passing of the compressed sampling stream into the fourth heat exchanger, and then into the third heat exchanger in order to feed the head of the splitter column, the second recirculation stream being sampled in the cooled compressed sampling stream, between the fourth heat exchanger and the third heat exchanger; 
     the sampling stream is introduced into a fourth compressor, the method comprising the following steps:
         sampling a secondary diversion stream in the cooled compressed sampling stream from the third compressor and from the fourth compressor;   dynamically expanding the secondary diversion stream in a second expansion turbine coupled with the fourth compressor;   introducing the expanded secondary diversion stream into the sampling stream after its passing into the third compressor and into the fourth compressor;       

     the second recirculation stream is sampled in the compressed methane-rich head stream, the method comprising the following steps:
         introducing the second recirculation stream into a third heat exchanger;   separating the feed natural-gas stream into a first feed flow and into a second feed flow;   establishing a heat exchange relationship of the second feed flow with the second recirculation stream in the third heat exchanger;   mixing the second feed flow after cooling in the third heat exchanger with the first feed flow, downstream from the first exchanger and upstream from the separator flask;       

     the method comprises the following steps:
         sampling a secondary cooling stream in the compressed methane-rich head stream, downstream from the first compressor and upstream from the second compressor;   dynamically expanding the secondary cooling stream in a second expansion turbine and passing of the expanded secondary cooling stream into the third heat exchanger for establishing a heat exchange relationship thereof with the second feed flow and with the second recirculation stream;   reintroducing the expanded secondary cooling stream into the methane-rich stream before its passing into the first compressor and into the second compressor;   sampling a recompression fraction in the cooled methane-rich stream, downstream from the introduction of the expanded secondary cooling stream and upstream from the first compressor and from the second compressor;   compressing the recompression fraction in at least one compressor coupled with the second expansion turbine and reintroducing the compressed recompression fraction into the compressed methane-rich stream from the first compressor and from the second compressor;       

     the second recirculation stream is derived from the first recirculation stream in order to form the dynamic expansion stream, the dynamic expansion stream being introduced into a second expansion turbine distinct from the first expansion turbine, the dynamic expansion stream from the second expansion turbine being reintroduced into the methane-rich stream before its passing into the first heat exchanger; 
     the method comprises the following steps:
         sampling a recompression fraction in the heated-up methane-rich head stream from the first exchanger and from the second heat exchanger;   compressing the recompression fraction in a third compressor coupled with the second expansion turbine;   introducing the compressed recompression fraction into the compressed methane-rich stream from the first compressor;       

     the method comprises the diversion of a third recirculation stream advantageously at room temperature, from the at least partly compressed methane-rich stream, advantageously between two stages of the second compressor, the third recirculation stream being successively cooled in the first heat exchanger and in the second heat exchanger before being mixed with the first recirculation stream in order to be introduced into the splitter column; 
     the C 2   +  hydrocarbon-rich bottom stream is pumped and is heated up by heat exchange with a counter-current of at least one portion of the feed natural-gas stream, advantageously up to a temperature less than or equal to the temperature of the feed natural-gas stream before its passing into the first heat exchanger; 
     the pressure of the C 2   +  hydrocarbon-rich stream after pumping is selected for maintaining the C 2   +  hydrocarbon-rich stream after its heating up in the first heat exchanger, in liquid form; 
     the molar flow rate of the second recirculation stream is greater than 10% of the molar flow rate of the feed natural-gas stream; 
     the temperature of the second recirculation stream is substantially equal to the temperature of the cooled natural gas stream introduced into the separator flask; 
     the pressure of the third recirculation stream is less than the pressure of the feed natural-gas stream and is greater than the pressure of the splitter column; 
     the molar flow rate of the third recirculation stream is greater than 10% of the molar flow rate of the feed natural-gas stream; 
     the molar flow rate of the sampling stream is greater than 4%, advantageously greater than 10% of the molar flow rate of the feed natural-gas stream; 
     the temperature of the sampling stream after passing into the third heat exchanger is less than that of the cooled feed natural-gas stream feeding the separator flask; 
     the molar flow rate of the secondary diversion stream is greater than 10% of the molar flow rate of the feed natural-gas stream; 
     the molar flow rate of the secondary cooling stream is greater than 10% of the molar flow rate of the feed natural-gas stream; 
     the pressure of the expanded secondary cooling stream is greater than 15 bars; 
     the ratio between the ethane flow rate contained in the C 2   +  hydrocarbon-rich fraction and the ethane flow rate contained in the feed natural-gas is greater than 0.98; 
     the ratio between the C 3   +  hydrocarbon flow rate contained in the C 2   +  hydrocarbon-rich fraction and the C 3   +  hydrocarbon flow rate contained in the feed natural-gas stream is greater than 0.998. 
     The subject-matter of the invention is also a facility for producing a methane-rich stream and a C 2   +  hydrocarbon-rich fraction from a dehydrated feed natural-gas stream, consisting of hydrocarbons, nitrogen and CO 2 , and advantageously having a molar C 2   +  hydrocarbon content of more than 10%, the facility being of the type comprising: 
     a first heat exchanger for cooling the feed natural-gas stream advantageously circulating at a pressure of more than 40 bars, 
     a separator flask, 
     means for introducing the cooled feed natural-gas stream into the separator flask, the cooled feed natural-gas stream being separated in the separator flask in order to recover an essentially gaseous light fraction and an essentially liquid heavy fraction; 
     means for forming a turbine input flow from the light fraction; 
     a first dynamic expansion turbine for the turbine input flow; 
     a splitter column; 
     means for introducing the expanded flow into the first dynamic expansion turbine in an intermediate portion of the splitter column; 
     a second heat exchanger; 
     means for expanding and introducing the heavy fraction into the splitter, laid out so that the recovered heavy fraction in the separator flask is introduced into the splitter column without passing through the first heat exchanger; 
     means for recovering, at the bottom of the splitter column, a C 2   +  hydrocarbon-rich bottom stream intended to form the C 2   +  hydrocarbon-rich fraction; 
     means for sampling at the head of the splitter column, a methane-rich head stream; 
     means for introducing the methane-rich head stream into the second heat exchanger and into the first heat exchanger for heating it up; 
     means for compressing the methane-rich head stream comprising at least one first compressor coupled with the first turbine and a second compressor for forming the methane-rich stream from the compressed methane-rich head stream; 
     means for sampling in the methane-rich head stream a first recirculation stream; 
     means for passing the first recirculation stream into the first heat exchanger and then into the second heat exchanger in order to cool it down; 
     means for introducing at least one portion of the first cooled recirculation stream into the upper portion of the splitter column; 
     the facility comprising: 
     means for forming at least one second recirculation stream obtained from the methane-rich head stream downstream from the splitter column; 
     means for forming a dynamic expansion stream from the second recirculation stream; 
     means for introducing the dynamic expansion stream into an expansion turbine for producing frigories. 
     In an embodiment, the means for forming a dynamic expansion stream from the second recirculation stream comprise means for introducing the second recirculation stream into a stream circulating downstream from the first heat exchanger and upstream from the first expansion turbine in order to form the dynamic expansion stream. 
     In another embodiment, the means for forming the turbine input flow include means for dividing the light fraction into the turbine input flow and into a secondary flow, the facility comprising means for passing the secondary flow into the second heat exchanger for cooling it down and means for introducing the cooled secondary flow into an upper portion of the splitter column. 
     By «room temperature», is meant in the following the temperature of the gas atmosphere prevailing in the facility in which the method according to the invention is applied; This temperature is generally comprised between −40° C. and 60° C. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 block diagram of a first facility according to the invention, for applying a first method according to the invention; 
         FIG. 2  is a view similar to  FIG. 1  of an alternative of the facility of  FIG. 1 ; 
         FIG. 3  is a view similar to  FIG. 1  of a second facility according to the invention, for applying a second method according to the invention; 
         FIG. 4  is a view similar to  FIG. 1  of a third facility according to the invention, for applying a third method according to the invention; 
         FIG. 5  is a view similar to  FIG. 1  of a fourth facility according to the invention, for applying a fourth method according to the invention; 
         FIG. 6  is a view similar to  FIG. 1  of a fifth facility according to the invention, for applying a fifth method according to the invention; 
         FIG. 7  is a view similar to  FIG. 1  of a sixth facility according to the invention, for applying a sixth method according to the invention; 
         FIG. 8  is a view similar to  FIG. 1  of a seventh facility according to the invention, for applying a seventh method according to the invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a first facility  10  for producing a methane-rich stream  12  and a C 2   +  hydrocarbon-rich fraction  14  according to the invention, from a feed natural-gas  15 . This facility  10  is intended for application of a first method according to the invention. 
     The method and the facility  10  are advantageously applied in the case of the building of a new unit for recovering methane and ethane. 
     The facility  10  from upstream to downstream comprises a first heat exchanger  16 , a separator flask  18 , a first expansion turbine  22  and a second heat exchanger  24 . 
     The facility  10  further comprises a splitter column  26  and, downstream from the column  26 , a first compressor  28  coupled with the first expansion turbine  22 , a first air cooler  30 , a second compressor  32  and a second air cooler  34 . The facility  10  further comprises a column bottom pump  36 . 
     In the example illustrated in  FIG. 1 , the facility  10  further includes a second expansion turbine  132  and a third compressor  134 . 
     In all the following, a stream circulating in a conduit and the conduit which conveys it will be designated by the same references. Further, unless indicated otherwise, the mentioned percentages are molar percentages and the pressures are given in absolute bars. 
     Further, for numerical simulations, the yield of each compressor is 82% polytrophic and the yield of each turbine is 85% adiabatic. 
     A first production method according to the invention, applied in the facility  10  will now be described. 
     The field natural gas  15  is, in this example, a dehydrated and decarbonated natural gas comprising by moles, 0.3499% of nitrogen, 80.0305% of methane, 11.3333% of ethane, 3.6000% of propane, 1.6366% of i-butane, 2.0000% of n-butane, 0.2399% of i-pentane, 0.1899% of n-pentane, 0.1899% of n-hexane, 0.1000% of n-heptane, 0.0300% of n-octane and 0.3000% of carbon dioxide. 
     The feed natural gas  15  therefore more generally comprises by moles, between 10% and 25% of C 2   +  hydrocarbons to be recovered and between 74% and 89% of methane. The C 2   +  hydrocarbon content is advantageously greater than 15%. 
     By decarbonated gas, is meant a gas for which the carbon dioxide content is lowered so as to avoid crystallization of carbon dioxide, this content being generally less than 1 molar %. 
     By dehydrated gas, is meant a gas for which the water content is as low as possible and notably less than 1 ppm. 
     Further, the hydrogen sulfide content of the feed natural-gas  15  is preferentially less than 10 ppm and the content of sulfur-containing compounds of the mercaptan type is preferentially less than 30 ppm. 
     The feed natural-gas has a pressure of more than 40 bars and notably substantially equal to 62 bars. It further has a temperature close to room temperature and notably equal to 40° C. The flow rate of the feed natural-gas stream  15  in this example is 15,000 kg·mol/h. 
     The feed natural-gas stream  15  is first of all introduced into the first heat exchanger  16  where it is cooled and partly condensed at a temperature above −50° C. and notably substantially equal to −24.5° C. in order to provide a cooled feed natural-gas stream  40  which is entirely introduced into the separator flask  18 . 
     In the separator flask  18 , the cooled feed natural-gas stream  40  is separated into a gaseous light fraction  42  and a liquid heavy fraction  44 . 
     The ratio of the molar flow rate of the light fraction  42  to the molar flow rate of the heavy fraction  44  is generally comprised between 4 and 10. 
     Next, the light fraction  42  is separated into a flow  46  for feeding the first expansion turbine and into a secondary flow  48  which is successively introduced into the heat exchanger  24  and in a first static expansion valve  50  for forming a cooled and at least partly liquefied expanded secondary flow  52 . 
     The cooled expanded secondary flow  52  is introduced at an upper level N 1  of the splitter column  26  corresponding in this example to the fifth stage from the top of the splitter column column  26 . 
     The flow rate of the secondary flow  48  represents less than 40% of the flow rate of the light fraction  42 . 
     The pressure of the secondary flow  52 , after its expansion in the valve  50  is less than 20 bars and notably equal to 16 bars. This pressure substantially corresponds to the pressure of the column  26  which is more generally greater than 15 bars, advantageously comprised between 15 bars and 25 bars. 
     The cooled expanded secondary flow  52  comprises a molar ethane content of more than 5% and notably substantially equal to 9.5 molar % of ethane. 
     The heavy fraction  44  is directed towards an expansion valve  66  which opens depending on the liquid level in the separator flask  18 . 
     The totality of the heavy fraction  44  is introduced into the column  26 , without entering a heat exchange relationship with the feed gas  15 , in particular, upstream from the separator flask  18 . The heavy fraction  44  does not pass through the first heat exchanger  16 . 
     Advantageously, the heavy fraction  44  is not separated either between the flask  18  and the column  26 . 
     The foot fraction  44 , after having been expanded at the pressure of the column  26 , is then introduced to a level N 3  of the column located under the level N 1 , advantageously located at the twelfth stage of the column  26  starting from the head. 
     An upper reboiling stream  70  is sampled at a bottom level N 4  of the column  26  located under the level N 3  and corresponding to the thirteenth stage starting from the head of the column  26 . This reboiling stream is available at a temperature above −55° C., in this example −53° C., and is passed into the first heat exchanger  16  so as to be partly vaporized and to exchange heat power of about 2,710 kW with the upper streams circulating in the exchanger  16 . 
     The partly vaporized liquid reboiling stream is heated up to a temperature of more than −40° C. and notably equal to −35.1° C. and sent to the level N 5  located just below the level N 4 , and corresponding to the fourteenth stage of the column  26  from the head. 
     A second intermediate reboiling stream  72  is collected at a level N 6  located under the level N 5  and corresponding to the seventeenth stage starting from the head of the column  26 . This second reboiling stream  72  is sampled at a temperature of more than −25° C., notably at −21.4° C. in order to be sent into the first exchanger  16  and to exchange a heat power of about 1,500 kW with the other streams circulating in this exchanger  16 . 
     The partly vaporized liquid reboiling stream from the exchanger  16  is then reintroduced at a temperature of more than −20° C. and notably equal to −13.7° C. at a level N 7  located just below the level N 6  and notably at the eighteenth stage from the head of the column  26 . 
     Further, a third lower reboiling stream  74  is sampled in the vicinity of the bottom of the column  26  at a temperature of more than −10° C. and notably substantially equal to −3.3° C. at a level N 8  advantageously located at the twenty-first stage starting from the head of the column  26 . 
     The lower reboiling stream  74  is brought as far as the first heat exchanger  16  where it is heated up to a temperature of more than 0° C. and notably equal to 3.2° C. before being sent to a level N 9  corresponding to the twenty-second stage starting from the top of the column  26 . This reboiling stream exchanges heat power of about 2,840 kW with the other streams circulating in the exchanger  16 . 
     A C 2   +  hydrocarbon-rich stream  80  is sampled in the bottom of the column  26  at a temperature of more than −5° C. and notably equal to 3.2° C. This stream comprises less than 1% of methane and more than 98% of C 2   +  hydrocarbons. It contains more than 99% of C 2   +  hydrocarbons from the feed natural-gas stream  15 . 
     In the illustrated example, the stream  80  contains by moles, 0.52% of methane, 57.80% of ethane, 18.5% of propane, 8.4% of i-butane, 10.30% of n-butane, 1.23% of i-pentane, 0.98% of n-pentane, 0.98% of n-hexane, 0.51% of n-heptane, 0.15% of n-octane, 0.54% of carbon dioxide, 0% of nitrogen. 
     This liquid stream  80  is pumped into the column bottom pump  36  and is then introduced into the first heat exchanger  16  so as to be heated up therein up to a temperature of more than 25° C. while remaining liquid. It thus produces the C 2   +  hydrocarbon-rich fraction  14  at a pressure of more than 25 bars and notably equal to 31.2 bars, advantageously at 38° C. 
     A methane-rich head stream  82  is produced at the head of the column  26 . This head stream  82  comprises a molar content of more than 99.1% of methane and a molar content of less than 0.15% of ethane. It contains more than 99.8% of the methane contained in the feed natural-gas  15 . 
     The methane-rich head stream  82  is successively heated up in the second heat exchanger  24 , and then in the first heat exchanger  16  in order to provide a methane-rich head stream  84  heated up to a temperature below 40° C. and notably equal to 30.8° C. 
     In this example, a first portion of the stream  84  is compressed once in the first compressor  28  and is then cooled in the first air cooler  30 . 
     The obtained stream is then compressed a second time in the second compressor  32  and is cooled in the second air cooler  34  in order to provide a compressed methane-rich head stream  86 . 
     The temperature of the compressed stream  86  is substantially equal to 40° C. and its pressure is greater than 60 bars and is notably substantially equal to 63.1 bars. 
     The compressed stream  86  is then separated into a methane-rich stream  12  produced by the facility  10 , and into a first recirculation stream  88 . 
     The ratio of the molar flow rate of the methane-rich stream  12  to the molar flow rate of the first recirculation stream is greater than 1 and is notably comprised between 1 and 20. 
     The stream  12  includes a methane content of more than 99.0%. In this example, it consists of 99.18 molar % of methane, 0.14 molar % of ethane, 0.43 molar % of nitrogen and 0.24 molar % of carbon dioxide. This stream  12  is then sent into a gas pipeline. 
     The first methane-rich recirculation stream  88  is then directed towards the first heat exchanger  16  in order to provide the first cooled recirculation stream  90  at a temperature of less than −30° C. and notably equal to −45° C. 
     A first portion  92  of the first cooled recirculation stream  90  is then introduced into the second exchanger  24  so as to be liquefied therein before passing through the flow rate control valve  95 . The thereby obtained stream forms a first cooled and at least partly liquefied portion  94  introduced to a level N 10  of the column  26  located above the level N 1 , notably at the first stage of the column from the head. The temperature of the first cooled portion  94  is more than −120° C. and notably equal to −113.8° C. Its pressure, after passing into the valve  95  is substantially equal to the pressure of the column  26 . 
     According to the invention, a second portion  96  of the first cooled recirculation stream  90  is sampled for forming a second methane-rich recirculation stream. 
     This second portion  96  is expanded in an expansion valve  98  before being mixed with the turbine input flow  46  in order to form a flow  100  for feeding the first expansion turbine  22  intended to be dynamically expanded in this turbine  22  in order to produce frigories. 
     The feed flow  100  is expanded in the turbine  22  in order to form an expanded flow  102  which is introduced into the column  26  at a level N 11  located between the level N 1  and the level N 3 , notably at the tenth stage starting from the head of the column at a pressure substantially equal to 16 bars. 
     The dynamic expansion of the flow  100  in the turbine  22  allows 3,732 kW of energy to be recovered which for a fraction of more than 50% and notably equal to 99.5% stem from the turbine input flow  46  and for a fraction of less than 50% and notably equal to 0.5% from the second recirculation stream. 
     The flow  100  therefore forms a dynamic expansion stream which, by its expansion in the turbine  22 , produces frigories. 
     In the example illustrated in  FIG. 1 , the method further comprises the sampling of a fourth recirculation stream  136  in the first recirculation stream  88 . This fourth recirculation stream  136  is sampled in the first recirculation stream  88  downstream from the second compressor  32  and upstream from the passage of the first recirculation stream  88  in the first exchanger  16  and in the second exchanger  24 . 
     The molar flow rate of the fourth recirculation stream  136  represents less than 80% of the molar flow rate of the first recirculation stream  88  sampled at the outlet of the second compressor  32 . 
     The fourth recirculation stream  136  is then brought as far as the second dynamic expansion turbine  132  so as to be expanded to a pressure below the pressure of the splitter column  26  and notably equal to 15.4 bars and for producing frigories. The temperature of the fourth cooled recirculation stream  138  from the turbine  132  is thus less than −30° C. and notably substantially equal to −43.1° C. 
     The fourth cooled recirculation stream  138  is then reintroduced into the methane-rich head stream  82  between the outlet of the second exchanger  24  and the inlet of the first exchanger  16 . Thus, the frigories generated by the dynamic expansion in the turbine  132  are transmitted by heat exchange into the first exchanger  16  to the feed natural-gas stream  15 . This dynamic expansion allows recovery of 2,677 kW of energy. 
     Further, a recompression fraction  140  is sampled in the heated-up methane-rich head stream  84  between the outlet of the first exchanger  16  and the inlet of the first compressor  28 . This recompression fraction  140  is introduced into the first compressor  134  coupled with the second turbine  132  so as to be compressed up to a pressure of less than 30 bars and notably equal to 22.6 bars and to a temperature of about 68.2° C. 
     The compressed recompression fraction  142  is reintroduced into the cooled methane-rich stream between the outlet of the first compressor  38  and the inlet of the first air cooler  30 . 
     The molar flow rate of the recompression fraction  140  is greater than 20% of the molar flow rate of the feed gas stream  15 . 
     As compared with a facility in which the totality of the first recirculation stream  90  is reinjected into the column  26 , the method according to the invention gives the possibility of obtaining ethane recovery identical, greater than or equal to 99%, while notably reducing the power to be provided by the second compressor  32  from 19,993 kW to 18,063 kW. 
     The improvement in the yield of the facility is illustrated by Table 1 hereafter. 
                                         TABLE 1                           Flow rate of                       the stream               136 recycled       Pressure of           Ethane   to the turbine   Power of the   the column           recovery   132   compressor 32   26           % mol   kg · mol/h   kW   bars                                                            99.00   0   19993   14.20           99.00   1000   19268   14.65           99.00   2000   18697   15.00           99.00   3000   18283   15.40           99.00   4000   18063   15.90                        
Temperature, pressure and molar flow rate examples of the various streams are given in Table 2 below.
 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Temperature 
                 Pressure 
                 Flow rate 
               
               
                   
                 Stream 
                 (° C.) 
                 (bars) 
                 (kg · mol/h) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 12 
                 40.0 
                 63.1 
                 12088 
               
               
                   
                 14 
                 38.0 
                 31.2 
                 2912 
               
               
                   
                 15 
                 40.0 
                 62.0 
                 15000 
               
               
                   
                 40 
                 −24.5 
                 61.0 
                 15000 
               
               
                   
                 42 
                 −24.5 
                 61.0 
                 12597 
               
               
                   
                 44 
                 −24.5 
                 61.0 
                 2403 
               
               
                   
                 46 
                 −24.5 
                 61.0 
                 8701 
               
               
                   
                 52 
                 −110.2 
                 16.1 
                 3896 
               
               
                   
                 80 
                 3.2 
                 16.1 
                 2912 
               
               
                   
                 82 
                 −112.4 
                 15.9 
                 13278 
               
               
                   
                 84 
                 30.8 
                 14.9 
                 17278 
               
               
                   
                 86 
                 40.0 
                 63.1 
                 17278 
               
               
                   
                 88 
                 40.0 
                 63.1 
                 5190 
               
               
                   
                 90 
                 −45.0 
                 62.6 
                 1190 
               
               
                   
                 94 
                 −113.8 
                 16.1 
                 1145 
               
               
                   
                 96 
                 −45.0 
                 62.6 
                 45 
               
               
                   
                 100 
                 −24.6 
                 61.0 
                 8746 
               
               
                   
                 102 
                 −76.2 
                 16.1 
                 8746 
               
               
                   
                 138 
                 −43.1 
                 15.4 
                 4000 
               
               
                   
                 142 
                 68.2 
                 22.6 
                 7218 
               
               
                   
                   
               
            
           
         
       
     
     In an alternative  10 A of the first facility  10  illustrated in  FIG. 2 , the facility is without the second dynamic expansion turbine  132  and the third compressor  134  coupled with the second dynamic expansion turbine  132 . 
     The totality of the heated-up head stream  84  from the first heat exchanger  16  is then introduced into the first compressor  28 . Also, the totality of the first recirculation stream  88  is introduced into the first heat exchanger  16  in order to form the stream  90 . 
     The facility and the method applied in this facility  10 A are moreover similar to the first facility  10  and to the first method according to the invention. 
     A second facility  110  according to the invention is illustrated in  FIG. 3 . This second facility  110  is intended for applying a second method according to the invention. 
     Unlike the first method according to the invention and its alternative illustrated in  FIG. 2 , the second portion  96  of the first cooled recirculation stream  90  forming the second recirculation stream is reintroduced, after expansion in the control valve  98 , upstream from the column  26 , into the cooled natural gas stream  40 , between the first exchanger  16  and the separator flask  18 . 
     In this example, this second stream  96  contributes to the formation of the light fraction  42 , as well as to the formation of the flow for feeding the first expansion turbine  22 . 
     Moreover, in this example, the flow  100  is exclusively formed by the feed flow  46 . 
     This arrangement, which may be applied to the whole of the described methods gives the possibility of further slightly improving the yield of the facility. 
     A third facility  120  according to the invention is illustrated in  FIG. 4 . 
     This third facility  120  is intended for applying a third method according to the invention. 
     Unlike the first facility  10  and its alternative  10 A, the second compressor  32  of the third facility  120  comprises two compression stages  122 A,  122 B and an intermediate air coolant  124  interposed between both stages. 
     Unlike the first method according to the invention and its alternative illustrated in  FIG. 2 , the third method according to the invention comprises the sampling of a third recirculation stream  126  in the heated-up methane-rich head stream  84 . This third recirculation stream  126  is sampled between both stages  122 A,  122 B at the outlet of the intermediate coolant  124 . Thus, the stream  126  has a pressure of more than 30 bars and a temperature substantially equal to room temperature. 
     The ratio of the flow rate of the third recirculation stream to the total flow rate of the heated-up methane-rich head stream  84  from the first heat exchanger  16  is less than 0.15 and is notably comprised between 0.08 and 0.15. 
     The third recirculation stream  126  is then successively introduced into the first exchanger  16 , and then into the second exchanger  24  so as to be cooled to a temperature of more than −110.5° C. 
     This stream  128 , obtained after expansion in a control valve  129 , is then reintroduced as a mixture with the first portion  94  of the first cooled recirculation stream  90  between the control valve  95  and the column  26 . 
     A reduction in the consumed power is observed, about 3% of which is due to liquefaction at a medium pressure of the third recirculation stream  126 . 
     A fourth facility  130  according to the invention is illustrated in  FIG. 5 . This fourth facility  130  is intended for the application of a fourth method according to the invention. 
     The fourth method according to the invention differs from the alternative of the first method according to the invention in that it comprises the sampling of a third recirculation stream  126  in the heated-up methane-rich head stream  84 , like in the third method according to the invention. 
     As described earlier for the method of  FIG. 4 , the third recirculation stream  126  is then successively introduced into the first exchanger  16 , and then into the second exchanger  24  so as to be cooled to a temperature of more than −109.7° C. 
     This stream  128 , obtained after expansion in a control valve  129 , is then reintroduced as a mixture with the first portion  94  of the first cooled recirculation stream  90  between the control valve  95  and the column  26 . 
     In this alternative of the fourth method, almost the whole of the first cooled recirculation stream  90  from the first exchanger  16  is introduced into the second exchanger  24 . The flow rate of the second portion  96  of this stream illustrated in  FIG. 5  is quasi-zero. 
     In this alternative, the second recirculation stream is then formed by the fourth recirculation stream  136  which is brought as far as the dynamic expansion turbine  132  for producing frigories. 
     Further, the application of this alternative of the method according to the invention does not require provision of a conduit with which a portion of the first cooled recirculation stream  90  may be diverted towards the first turbine  22 , so that the installation  130  may be without one. 
     A fifth facility  150  according to the invention is illustrated in  FIG. 6 . This fifth facility  150  is intended for application of a fifth method according to the invention. 
     This facility  150  is intended for improving an existing production unit of the state of the art, as for example described in the American patent U.S. Pat. No. 6,578,379, by keeping constant the power consumed by the second compressor  32 , notably when the C 2   +  hydrocarbon content in the feed gas  15  substantially increases. 
     The initial feed natural-gas  15  in this example and in the following examples is a dehydrated and decarbonated natural gas mainly consisting of methane and of C 2   +  hydrocarbons, comprising by moles 0.3499% of nitrogen, 89.5642% of methane, 5.2579% of ethane, 2.3790% of propane, 0.5398% of i-butane, 0.6597% of n-butane, 0.2399% de i-pentane, 0.1899% of n-pentane, 0.1899% of n-hexane, 0.1000% of n-heptane, 0.0300% of n-octane, 0.4998% of CO 2 . 
     In the example shown, the C 2   +  hydrocarbon fraction always has the same composition which is the one indicated in table 3: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 Ethane 
                 54.8494 
                 Mol % 
               
               
                   
                 Propane 
                 24.8173 
                 Mol % 
               
               
                   
                 i-Butane 
                 5.6311 
                 Mol % 
               
               
                   
                 n-Butane 
                 6.8815 
                 Mol % 
               
               
                   
                 i-Pentane 
                 2.5026 
                 Mol % 
               
               
                   
                 n-Pentane 
                 1.9810 
                 Mol % 
               
               
                   
                 C6+ 
                 3.3371 
                 Mol % 
               
               
                   
                 Total 
                 100 
                 Mol % 
               
               
                   
                   
               
            
           
         
       
     
     The fifth facility  150  according to the invention differs from the alternative  10 A of the first facility illustrated in  FIG. 2  in that it comprises a third heat exchanger  152 , a fourth heat exchanger  154  and a third compressor  134 . 
     The facility  150  is further without any air cooler at the outlet of the first compressor  28 . The first air cooler  30  is located at the outlet of the second compressor  32 . 
     However it comprises a second air cooler  34  mounted at the outlet of the third compressor  134 . 
     The fifth method according to the invention differs from the alternative of the first method according to the invention in that a sampling stream  158  is sampled in the methane-rich head stream  82  between the outlet of the splitter column  26  and the second heat exchanger  24 . 
     The sampling stream flow rate  158  is less than 15% of the flow rate of the methane-rich head stream  82  from the column  26 . 
     The sampling stream  158  is then successively introduced into the third heat exchanger  152 , so as to be heated up to a first temperature below room temperature, and then in the fourth heat exchanger  154  so as to be heated up to substantially room temperature. 
     The first temperature is further less than the temperature of the cooled feed natural-gas stream  40  feeding the separator flask  18 . 
     The thereby cooled stream  158  is passed into the third compressor  134  and into the cooler  34 , in order to cool it down to room temperature before being introduced into the fourth heat exchanger  154  and forming a cooled compressed sampling stream  160 . 
     This cooled compressed sampling stream  160  has a pressure greater than or equal to that of the feed gas stream  15 . This pressure is less than 63 bars. The stream  160  has a temperature of less than 40° C. This temperature is substantially equal to the temperature of the cooled feed natural gas stream  40  feeding the separator flask  18 . 
     The cooled compressed sampling stream  160  is separated into a first portion  162  which is successively passed into the third heat exchanger  152  so as to be cooled therein substantially down to the first temperature, and then in a pressure control valve  164  for forming a first cooled expanded portion  166 . 
     The molar flow rate of the first portion  162  represents at least 4% of the molar flow rate of the feed natural-gas stream  15 . 
     The pressure of the first cooled expanded portion  166  is substantially equal to the pressure of the column  26 . 
     The ratio of the molar flow rate of the first portion  162  to the molar flow rate of the cooled compressed sampling stream  160  is greater than 0.25. The molar flow rate of the first portion  162  is greater than 4% of the molar flow rate of the feed natural-gas stream  15 . 
     A second portion  168  of the cooled compressed sampling stream is introduced after passing into a static expansion valve  170 , as a mixture with the flow  46  feeding the first turbine  22  in order to form the flow  100  for feeding this turbine  22 . 
     Thus, the second portion  168  forms the second recirculation stream according to the invention which is introduced into the turbine  22  in order to produce frigories therein. 
     As an alternative (not shown), the second portion  168  is introduced into the cooled feed natural gas stream  40  upstream from the separator flask  18 , as illustrated in  FIG. 3 . 
     It is thus possible to keep the second compressor  32 , without modifying its size, for a production facility receiving a richer gas in C 2   +  hydrocarbons, without degrading the recovery of ethane. 
     A sixth facility according to the invention  180  is illustrated in  FIG. 7 . This sixth facility  180  is intended for applying a sixth method according to the invention. 
     This sixth facility  180  differs from the fifth facility  150  in that it further comprises a fourth compressor  182 , a second expansion turbine  132  coupled with the fourth compressor  182 , and a third air cooler  184 . 
     Unlike the fifth method, the sampling stream  158  is introduced, after its passing into the fourth exchanger  154 , successively into the fourth compressor  182 , in the third air cooler  184  before being introduced into the third compressor  134 . 
     Further, a secondary diversion stream  186  is sampled in the first portion  162  of the cooled compressed sampling stream  160  before its passing into the third exchanger  152 . 
     The secondary diversion stream  186  is then conveyed as far as the second expansion turbine  132  so as to be expanded down to a pressure of less than 25 bars, which lowers its temperature to less than −90° C. 
     The thereby formed expanded secondary diversion stream  188  is introduced as a mixture into the sampling stream  158  before its passing into the third exchanger  152 . 
     The flow rate of the secondary diversion stream is less than 75% of the flow rate of the stream  160  taken at the outlet of the fourth exchanger  154 . 
     It is thus possible to increase the C 2   +  content in the feed stream without modifying the power consumed by the compressor  32 , or modifying the power developed by the first expansion turbine  22 , while minimizing the power consumed by the compressor  134 . 
     A seventh facility  190  according to the invention is illustrated in  FIG. 8 . This seventh facility is intended for applying a seventh method according to the invention. 
     The seventh facility  190  differs from the second facility  110  by the power of a third heat exchanger  152 , by the presence of a third compressor  134  and of a second air cooler  34 , and by the presence of a fourth compressor  182  coupled with a third air cooler  184 . Further, the fourth compressor  182  is coupled with a second expansion turbine  132 . 
     The seventh method according to the invention differs from the second method according to the invention in that the second recirculation stream is formed by a sampling fraction  192  taken in the compressed methane-rich head stream  86 , downstream from the sampling of the first recirculation stream  88 . 
     The sampling fraction  192  is then conveyed as far as the third heat exchanger  152 , after passing into a valve  194  for forming an expanded cooled sampling fraction  196 . This fraction  196  has a pressure of less than 63 bars and a temperature below 40° C. 
     The flow rate of the sampling fraction  192  is less than 1% of the flow rate of the stream  82  taken at the outlet of the column  26 . 
     The feed natural-gas stream  15  is separated into a first feed flow  191 A conveyed as far as the first heat exchanger  16  and into a second feed flow  191 B conveyed as far as the third heat exchanger  152 , by flow rate control with the valve  191 C. The feed flows  191 A,  191 B, after their cooling in the respective exchangers  16 ,  152 , are mixed together at the outlet of the respective exchangers  16  and  152  in order to form the cooled feed natural gas flow  40  before its introduction into the separator flask  18 . 
     The ratio of the flow rate of the feed flow  191 A to the flow rate of the feed flow  191 B is comprised between 0 and 0.5. 
     The sampled fraction  196  is introduced into the first feed flow  191 A at the outlet of the first exchanger  16  before its mixing with the second feed flow  191 B. 
     A secondary cooling stream  200  is sampled in the compressed methane-rich head stream  86 , downstream from the sampling of the sampling fraction  192 . 
     This secondary cooling stream  200  is transferred as far as the dynamic expansion turbine  132  so as to be expanded down to a pressure below the pressure of the column  26  and to provide frigories. The expanded secondary cooling stream  202  from the turbine  132  is then introduced, at a temperature below 40° C. into the third exchanger  152  in order to be heated up by heat exchange with the flows  191 B and  192  up to substantially room temperature. 
     Next, the heated-up secondary cooling stream  204  is reintroduced into the methane-rich head stream  84  at the outlet of the third exchanger  16 , before passing into the first compressor  28 . 
     Further, a recompression fraction  206  is sampled in the heated-up methane-rich head stream  84  downstream from the introduction of the heated-up secondary cooling stream  204 , and is then successively passed into the fourth compressor  182 , into the third air cooler  184 , into the third compressor  134 , and then into the second air cooler  34 . This fraction  208  is then reintroduced into the compressed methane-rich head stream  86  from the second compressor  32 , upstream from the sampling of the first recirculation stream  88 . 
     The compressed methane-rich stream  86  from the cooler  30  and receiving the fraction  208  is advantageously at room temperature. 
     The seventh method according to the invention gives the possibility of keeping the compressor  32  and the turbine  22  identical when the ethane content and those of C 3   +  hydrocarbons in the feed gas increase, while obtaining a recovery of ethane of more than 99%. 
     Further, the yield of this method is improved as compared with that of the sixth method according to the invention, for constant C 2   +  hydrocarbon content. This is all the more true since the C 2   +  hydrocarbon content in the feed gas is significant. 
     In an alternative (not shown), the light fraction  42  from the separator flask  18  is not divided. The totality of this fraction then forms the turbine input flow  46 , which is sent towards the first dynamic expansion turbine  22 .