FIELD OF THE INVENTION  
         [0001]    The present invention relates to a process for dehydrating and fractionating a low-pressure natural gas.  
         BACKGROUND OF THE INVENTION  
         [0002]    French patent FR-B-2,605,241 describes a process for treating a natural gas or a refinery gas containing water so as to eliminate at least part of the water contained in said gas, wherein:  
           [0003]    a) the gas is contacted with a recycled liquid phase containing water and a solvent (methanol for example); the resulting aqueous liquid phase is discharged and a gas containing, in the vapour state, water and the major part of the solvent is recovered,  
           [0004]    b) said gas is cooled so as to condense a liquid phase essentially consisting of water and solvent, and  
           [0005]    c) the non-condensed gas is separated from this liquid phase, which forms the phase recycled to stage (a).  
           [0006]    French patent FR-B-2,787,870 notably describes a process for fractionating a high-pressure natural gas (pressure above about 5 MPa) wherein at least part of the gas is expanded so as to be used as a cooling agent, expansion being carried out before the fractionating operation, and for gas purification (scrubbing), this operation being carried out by means of a technique allowing simultaneously distillation and heat exchange (for example in an exchanger-dephlegmator). This prior document also describes integration of the fractionating process with a dehydration process using methanol.  
           [0007]    Now, there are cases where the natural gas, after coming out of the well or passing through various installations or equipments, is at a pressure lower than about 5 MPa To fractionate these gases (i.e. extract the condensates or NGL), a new process has been discovered which also uses a simultaneous heat exchange and distillation operation (for example in an exchanger-dephlegmator type device) which, integrated with a dehydration stage, affords the advantage of being very simple and of requiring reduced investment costs.  
         SUMMARY OF THE INVENTION  
         [0008]    The invention thus proposes a new process allowing both dehydration and fractionation (stripping) of a low-pressure wet natural gas containing notably hydrocarbon constituents referred to as &lt;&lt;heavy&gt;&gt; constituents (C 3   + ), hydrocarbon constituents referred to as &lt;&lt;light &gt;&gt; constituents (C 1  and C 2 ) and water.  
           [0009]    The process of the invention, applied to a low-pressure (P 0  below about 5 MPa) we natural gas containing constituents referred to as &lt;&lt;heavy &gt;&gt; constituents and constituents referred to as &lt;&lt;light &gt;&gt; constituents, can generally be defined by the fact that it comprises in combination at least the following stages:  
           [0010]    a) at least a fraction of the wet gas at a temperature T 0  is contacted with an aqueous liquid phase L&#39; 1  containing methanol, said gas carrying along substantially all of the methanol contained in said aqueous phase L&#39; 1 ,  
           [0011]    b) the gas from stage (a) is cooled to a temperature T 1  lower than temperature To, which produces a gas phase G 1  at equilibrium with a hydrocarbon-containing liquid phase L 1  containing C 3   +  and an aqueous liquid phase L&#39; 1  containing methanol,  
           [0012]    c) aqueous liquid phase L&#39; 1  is sent to stage (a), and  
           [0013]    d) gas phase G 1  is fractionated by distillation carried out by continuous thermal exchange with a cooling fluid, which allows to extract, on the one hand, the &lt;&lt;light)&gt;&gt; constituents in the gas form (phase G 2 ) and, on the other hand, the &lt;&lt;heavy&gt;&gt; constituents in the form of condensates (phase L 2 ).  
           [0014]    To carry out the thermal exchange in stage (d), it is possible to use an external cooling fluid such as a propane cycle. However, according to a preferred embodiment of the process of the invention, the cooling fluid consists of gas G 2  itself after expansion. In this case, the expanded gas is heated by the thermal exchange of stage (d); it can also be heated by thermal exchange with the gas entering stage (b). It is recompressed thereafter in order to be exported.  
           [0015]    More particularly, refrigeration of the gas in stage (b) can be achieved successively with cooling water or an air-cooled exchanger, then by exchange with the cooling fluid of stage (d), and finally, if necessary, by means of an external coolant such as a propane cycle.  
           [0016]    Preferred and advantageous embodiments of the process according to the invention are described hereafter.  
           [0017]    The hydrocarbon liquid phase L 2  formed in stage (d) and comprising mainly C 3   +  condensates can also contain lighter hydrocarbons (C 1  and C 2 ) and require stabilization. It can then be sent to a stabilization stage (e) in order to obtain stabilized condensates L 3  and a gas phase G 3 , to be fractionated, which is then sent back to fractionating stage (d).  
           [0018]    Small amounts of methanol which it may be desirable to recover generally remain in the C 3   +  condensates from stage (d) (phase L 2 ) or in the stabilized C 3   +  condensates from stabilization stage (e) (phase L 3 ). A condensate washing stage (f) can therefore be carried out using an aqueous phase; the aqueous phase having thus extracted the methanol can then be partly regenerated, in a regeneration stage (g), for example by stripping with the remaining fraction of the initial wet natural gas that has not been contacted with the aqueous liquid phase L&#39; 1  in stage (a). The aqueous phase thus partly regenerated in regeneration stage (g) is used again to extract the methanol from the C 3   +  condensates in washing stage (f).  
           [0019]    In the latter case, after stripping of the methanol, the natural gas fraction considered is mixed with the fraction that has been contacted with the aqueous liquid phase L&#39; 1  in stage (a).  
           [0020]    During the fractionating stage of the process according to the invention, methanol losses can occur. To compensate for these losses, extra methanol can be added, for example in the gas upstream from cooling stage (b) and/or at the start of the distillation operation of stage (d) and/or during the stage of expansion of gas G 2 .  
           [0021]    [0021]FIG. 1 appended hereafter illustrates the process according to the invention including the preferred embodiments described above. The process is described more in detail hereafter in connection with this figure.  
         DETAILED DESCRIPTION  
         [0022]    The low-pressure (below about 5 MPa) wet gas at a temperature ranging from 0° C. to 60° C., flowing in through line  1 , is partly sent through line la into the upper section T 1  of a stripping column T. At the top of the upper section, an aqueous liquid phase containing methanol delivered by pump P 1  from the bottom (&lt;&lt;boot&gt;&gt;  10 ) of a separating drum B 1 , described hereunder, is injected through a line  2 .  
           [0023]    A methanol-enriched gas is extracted at the top of column T through line  3   a  and an aqueous phase greatly depleted in methanol (generally less than 100 ppm) is extracted in the middle of the column, at the bottom of upper section T 1 , through a line  4 . The other part of the gas flowing in through line  1  is sent through line  1   b  to lower section T 2  of the column to regenerate the methanol-containing wash water coming from condensate wash column T 4  (described hereafter) and introduced at the top of lower section T 2  through a line  5  coming, through a pump P 3 , from the bottom of wash column T 4 .  
           [0024]    Methanol-enriched gas is extracted in the upper part of section T 2  of the column through a line  3   b  and methanol-depleted wash water is extracted at the bottom of column T through a line  6  in order to be sent to wash column T 4 .  
           [0025]    The methanol-enriched gases coming from lines  3   a  and  3   b  are mixed together and the resulting stream is sent, at temperature T 0  and pressure P 0 , into a heat exchanger E 1  where it is cooled by heat exchange, for example with cooling water or sea water, or air. The cooled gas sent through line  7  is then cooled in a second heat exchanger E 2 . The heat exchange occurs for example with at least part of the scrubbed gas from the fractionating and purification process, described hereafter, this gas circulating through line  14 , then line  8 . This gas is thereafter compressed in compressor C 1  to be exported through line  9 . If necessary, after cooling in exchanger E 2 , this gas can be cooled in an external-coolant exchanger E 3 , then it is sent to separating drum B 1  of a heat exchange-distillation device comprising a dephlegmator D 1  with two passes. Drum B 1  is provided with a &lt;&lt;boot &gt;&gt;  10  allowing to collect phase L&#39; 1  (water+methanol), which is sent to the top of column T by pump P 1  and through line  2 . A hydrocarbon liquid phase L 2  consisting of C 3   +  condensates and of part of the C 1  and C 2  is also separated. This liquid phase L 2  is sent by pump P 2  and through line  11  to a stabilization column T 3 . Finally, a gas phase G 1  flowing up through the reflux pass of dephlegmator D 1  is also separated. In this pass, the condensed liquid flows down into drum B 1 .  
           [0026]    Scrubbed gas G 2  flowing out at the top of dephlegmator D 1  is sent through line  12  to an expander X 1  in which it is expanded. The expanded gas is then sent through line  13  into the second pass of dephlegmator D 1  where it circulates downwards and serves as a cold source for the dephlegmator. As it flows down in the dephlegmator, the scrubbed gas warms up. It is then heated, after leaving the dephlegmator through line  14 , by heat exchange with the incoming gas in exchanger E 2 . It is then recompressed by compressor C 1  (driven by expander X 1 ) prior to being exported through line  9 .  
           [0027]    Liquid phase L 2 , mainly consisting of the condensed C 3   + , is sent through pump P 2  and line  11  to stabilization column T 3 , reboiled for example by means of an exchanger E 4 , a hot-oil exchanger for example. A the top of this column, a gas phase G 3  mainly consisting of C 1  and C 2  is discharged through a line  15  and sent back to drum B 1 .  
           [0028]    At the bottom of column T 3 , a liquid mixture L 3  essentially consisting of C 3   +  and containing only a small amount of light products (C 1  and C 2 ) is collected through line  16 . This stabilized condensate L 3  contains methanol that has to be recovered in order to reduce as much as possible the methanol losses in the process. This liquid stream is passed through a heat exchanger E 5  arranged after reboiler E 4 , where it is cooled, then it is fed through line  17  into the lower section of wash column T 4 . In this column T 4 , the C 3   +  condensates are washed by means of the methanol-depleted water introduced at the top through line  6 . The methanol-freed condensate is recovered through line  18  at the top of column T 4  and the methanol-containing wash water is recovered at the bottom of the column prior to being sent through pump P 3  and line  5  to the lower section T 2  of column T, where it is stripped by the gas fed through line  1   b  into said lower section T 2  of said column T.  
           [0029]    Extra methanol can be added, if necessary, before cooling stage (b) (dotted arrow in FIG. 1).  
           [0030]    Furthermore, gas G 2  flowing out of dephlegmator D 1  has been considerably cooled and, although it contains methanol, hydrate formation is possible. It can be useful, to prevent clogging risks, to provide an injection of methanol at the inlet of the expander and/or at the top of the distillation pass of the dephlegmator.  
           [0031]    In some cases, one is led to inject amounts of methanol that are larger than the amounts that can be discharged with the outgoing effluents of the process (i.e. the scrubbed gas and the condensates). This leads to an accumulation of methanol in liquid hydrocarbon phase L 2 , this methanol being thereafter found in the wash water recovered at the bottom of column T 4 , or an accumulation of methanol in the aqueous liquid phase at the bottom of the cold section, in practice at the bottom of separating drum B 1 . It is then advantageous to carry out an additional stage of methanol recovery.  
           [0032]    It is first possible to recover the excess methanol from the condensate wash water.  
           [0033]    In this case, the process considered is defined in the same way as the previous dehydration and fractionating process, but it furthermore comprises the following stages.  
           [0034]    After condensate washing stage (f), all or part of the water+methanol liquid phase coming from the bottom of wash column T 4  is sent to a distillation stage (h) allowing to recover, at the top, practically pure methanol and, at the bottom, a methanol-depleted aqueous liquid phase L&#39; 2  which, after cooling, is sent to regeneration stage (g) or recycled to condensate washing stage (f).  
           [0035]    Thus, as illustrated in FIG. 2, all or part of the water+methanol liquid phase coming from the bottom of wash column T 4  is sent to a distillation stage wherein practically pure methanol is recovered at the top and sent to a storage point prior to being reinjected at the various cold points of the process. A methanol-depleted water+methanol mixture is also recovered and sent to the top of part T 2  of stripping column T.  
           [0036]    Thus, as illustrated in FIG. 2, at least part of the water+methanol liquid phase coming from the bottom of wash column T 4  is sent no longer to the top of lower section T 2  of stripping column T (to be contacted with a fraction of the wet natural gas to be treated), but, through line  19 , to distillation column T 5 , reboiled for example by means of an exchanger E 6 . The top vapour of column T 5 , flowing out through line  20 , consists of methanol of high purity, for example 95-97% by mole. It is condensed in exchanger E 7  and the resulting liquid phase is collected in drum B 2 . This liquid phase is partly sent by pump P 4  through line  21  to the top of column T 5  as liquid reflux. The other part is sent through line  22  to a methanol storage point so as to be re-used later, for example reinjected at the various cold points of the process, or used in any other independent process. A liquid phase consisting of a methanol-depleted water+methanol mixture is recovered at the bottom of distillation column T 5  and either sent, after cooling in an exchanger E 8 , through line  23  and pump P&#39; 3  to the top of the upper section T 2  of stripping column T through line  5 , or sent back through line  6  to the top of wash column T 4 .  
           [0037]    It is also possible to recover the excess methanol from the aqueous liquid phase at the bottom of the cold section, in practice at the bottom of separating drum B 1 .  
           [0038]    The process considered in this embodiment of the invention is defined in the same way as the previous dehydration and fractionating process, but it furthermore comprises the following stages.  
           [0039]    Before the stage of sending (c) aqueous liquid phase L&#39; 1  to stage (a), at least part of aqueous liquid phase L&#39; 1  from stage (b) is sent to a distillation stage (h) allowing to collect, at the top, practically pure methanol and, at the bottom, a methanol-depleted aqueous liquid phase L&#39; 2  which, in stage (c), is sent to stage (a).  
           [0040]    Thus, if we refer to FIG. 3, at least part of the water+methanol liquid phase coming from the bottom  10  of separating drum B 1  and containing excess methanol is sent no longer to the top of stripping column T (to be contacted with the fraction of the wet natural gas to be treated), but through line  19  to distillation column T 5 , reboiled by means of an exchanger E 6  for example. The top vapour of column T 5 , flowing out through line  20 , consists of methanol of high purity, for example 95-97% by mole. It is condensed in exchanger E 7  and the resulting liquid phase is collected in drum B 2 . This liquid phase is partly sent by pump P 4  through line  21  to the top of column T 5 , as liquid reflux. The other part is sent through line  22  to a methanol storage point so as to be re-used later, for example by reinjection at the various cold points of the process, or in any other independent process. A liquid phase consisting of a methanol-depleted water+methanol mixture is collected at the bottom of distillation column T 5  and sent through line  23  and pump P&#39; 1  to the top of the upper section T 1  of stripping column T, as defined above, after cooling in an exchanger E 8 . Practically pure methanol is separated at the top and sent to a storage point, and a methanol-depleted water-methanol liquid mixture is collected at the bottom. This mixture is sent to stripping column T.  
           [0041]    The two particular embodiments described above can be implemented separately or coupled.  
           [0042]    The examples given hereafter illustrate the process according to the invention without limiting the scope thereof. 
       
    
    
     EXAMPLE 1  
       [0043]    The wet gas at a temperature of 50° C. and at a pressure of 2.8 MPa flows in through line  1  (see FIG. 2) at a flow rate of 8867 kmol/h Its composition, given in percent by volume, is as follows:  
                                                       Co 2     1.58%           Methane   78.94%           Ethane   9.92%           Propane   4.89%           Butanes   2.39%           C 5 -hydrocarbons   1.44%           Water   0.47%           Nitrogen   0.37%.                      
 
         [0044]    A first fraction of the gas (6207 kmol/h) is sent to upper section T 1  of stripping column T. An aqueous liquid phase (83 kmol/h) containing methanol (71% by mole) coming from &lt;&lt;boot &gt;&gt;  10  at the bottom of separating drum B 1  is injected at the top of upper section T 1 . A methanol-enriched (0.95 % by mole) gas (6249 kmol/h) is extracted at the top of section T 1  and an aqueous phase (40.8 kmol/h) containing 83 ppm by mole of methanol is extracted at the bottom of upper section T 1 .  
         [0045]    The other fraction of the gas (2660 kmol/h) is sent to lower section T 2  of stripping column T to regenerate part of the methanol-containing (21.7% by mole) wash water from condensate wash column T 4  (28.1 kmol/h), introduced at the top of lower section T 2 . The other part of the condensate wash water (23 kmol/h) is sent to column T 5 . The column comprises 15 theoretical plates. A vapour phase containing 99.9% by mole of methanol and 0.1% by mole of water is recovered at the top. This vapour is condensed and sent to the storage point (4.8 kmol/h). An aqueous phase with 99% by mole of water and 1% by mole of methanol (18 kmol/h) is recovered at the bottom of the column and sent to wash column T 4 .  
         [0046]    Methanol-enriched (0.23% by mole) gas is extracted (2665 kmol/h) in the upper part of section T 2  of column T and methanol-depleted (15 ppm by mole) wash water is extracted at the bottom of section T 2  (23 kmol/h) to be sent to wash column T 4 .  
         [0047]    The methanol-enriched gases from sections T 1  and T 2  are mixed together and the resulting stream receives extra methanol (FLOW RATE ?). This stream, at a temperature of 44° C. and a pressure of 2.7 MPa, is sent to heat exchanger El where it is cooled (with cooling water or sea water) to a temperature of 35° C. The gas is cooled further to a temperature of +1° C. in heat exchanger E 2 , by exchange with the scrubbed gas from the fractionating and purification process.  
         [0048]    After cooling in exchanger E 2 , the gas is cooled in an external-coolant exchanger E 3  to a temperature of −32° C., then it is sent to separating drum B 1  of the heat exchange-distillation device comprising a two-pass dephlegmator D 1 . The water+methanol phase L&#39; 1  (71% by mole of methanol) is collected (83 kmol/h) in &lt;&lt;boot &gt;&gt;  10  of separating drum B 1  and brought back to a pressure of 2.7 MPa by pump P 1  prior to being sent to the top of section T 1  of column T. A hydrocarbon liquid phase L 2  consisting of C 3   +  condensates and of part of the C 1  and C 2  is also separated (1485 kmol/h). This liquid phase L 2  is brought to a pressure of 3.5 MPa by pump P 2  and sent to stabilization column T 3 . Finally, a gas phase G 1  flowing up through the reflux pass of dephlegmator D 1  is separated (710 kmol/h). In this phase, the condensed liquid flows down towards drum B 1 . At the top of the reflux pass, methanol is injected at a flow rate of 5 kmol/h.  
         [0049]    The scrubbed gas flowing out of the top of dephlegmator D 1  is sent to an expander X 1  where it is expanded to a pressure of 0.1 MPa. Its temperature is −92° C. Extra methanol is injected at this point at a flow rate of 0.1 mol/h. The expanded gas is sent into the second pass of dephlegmator D 1  where it circulates downwards and serves as a cold source for the dephlegmator. As it flows down in the dephlegmator, the scrubbed gas warms up to a temperature of 4° C. It is thereafter heated to 25° C. after leaving the dephlegmator by heat exchange with the incoming gas in exchanger E 2 . The scrubbed gas is then recompressed to a pressure of 1.4 MPa by compressor C 1 , driven by expander X 1 , then to 7 MPa by means of an additional compressor, and finally exported.  
         [0050]    The scrubbed gas has the following volume composition:  
                                                       Co 2      1.73%           Methane   87.28%           Ethane   10.88%           Propane    0.11%           Methanol   16 ppm           Water    2 ppm.                      
 
         [0051]    Liquid phase L 2  is sent to stabilization column T 3 , reboiled by means of a hot-oil exchanger E 4  to a temperature of 99° C. A gas phase G 3  mainly consisting of the C 1  and C 2  is discharged at the top of this column at a temperature of −16° C. and sent back to drum B 1 .  
         [0052]    A liquid mixture mainly consisting of C 3   +  and containing only a small amount of light products (C 1  and C 2 ), in particular less than 2% by mole of C 2 , is collected at the bottom of column T 3 .  
         [0053]    The stabilized condensate collected at the bottom of column T 3  contains methanol (1.49% by mole) that has to be recovered to reduce as much as possible methanol losses in the process. This liquid stream is cooled to a temperature of 35° C. in heat exchanger E 5  arranged after reboiler E 4 , then it is introduced into the lower section of wash column T 4 , where the C 3   +  condensates are washed by means of the methanol-depleted water introduced at the top of column T 4 . The methanol-freed condensate is recovered at the top of column T 4  and the methanol-containing wash water, which is recovered at the bottom of the column, is brought by pump P&#39;to a pressure of 2.7 MPa and partly sent to lower section T 2  of column T, where it is stripped by the gas fed into said lower section T 2  of said column T (28.1 kmol/h), the other part (23 kmol/h) being sent to column T 5  described above.  
         [0054]    The condensate recovered has the following mass composition:  
                                                       Ethane     2.0%           Propane    53.5%           Butanes    27.5%           C 5   +  hydrocarbons    17.0%           Methanol    650 ppm           Water   1000 ppm.                      
 
       EXAMPLE 2  
       [0055]    The layout of Example 1 and its implementation are repeated, but the aqueous liquid phase collected in &lt;&lt;boot&gt;&gt;  10  of separating drum B 1  (71 kmol/h) with a methanol concentration of 55% by mole is sent to distillation column T 5  for recovery of the methanol (see FIG. 3). The temperature at the bottom of column T 5  is 96° C. and the temperature at the top is 81 ° C. The pressure is 0.2 MPa at the bottom and 0.18 MPa at the level of the condenser. The column comprises 15 theoretical plates.  
         [0056]    A vapour phase containing 99% by mole of methanol and 1% by mole of water is collected at the top. This vapour is condensed to a liquid, sent to a storage point. The effluent at the bottom (8.8 kmol/h) is a water-methanol mixture whose methanol concentration is 30% by mole. It is brought to a pressure of 0.28 MPa by pump P&#39; 1  and sent to the top of upper section T 1  of stripping column T.
Process for dehydrating/fractionating a low-pressure wet natural gas containing <<heavy >> constituents and <<light >> constituents, wherein: a) at least a fraction of the wet gas at temperature T 0 is contacted with an aqueous liquid phase L' 1 containing methanol, said gas carrying along substantially all of the methanol contained in said phase L' 1, b) the gas from stage (a) is cooled to a temperature T 1 lower than temperature T 0, producing a gas phase G 1 at equilibrium with a hydrocarbon-containing liquid phase L 1 containing C 3 + and an aqueous liquid phase L' 1 containing methanol, c) phase L' 1 is sent to stage (a), and d) said phase G 1 is fractionated by distillation carried out by continuous thermal exchange with a cooling fluid, so as to extract the <<light >> constituents (gas phase G 2) and the <<heavy >> constituents (condensed phase L 2).