Abstract:
The invention provides a method of extracting the hydrogen sulfide contained in a gas comprising aromatic hydrocarbons, wherein the following stages are carried out: 
     a) contacting said gas with an absorbent solution so as to obtain a gas depleted in hydrogen sulfide and an absorbent solution laden with hydrogen sulfide, 
     b) heating and expanding the hydrogen sulfide-laden absorbent solution to a predetermined temperature and pressure so as to release a gaseous fraction comprising aromatic hydrocarbons and to obtain an absorbent solution depleted in aromatic hydrocarbons, said temperature and pressure being so selected that said gaseous fraction comprises at least 50% of the aromatic hydrocarbons and at most 35% hydrogen sulfide contained in said hydrogen sulfide-laden absorbent solution, 
     c) thermally regenerating the absorbent solution depleted in aromatic hydrocarbon compounds so as to release a hydrogen sulfide-rich gaseous effluent and to obtain a regenerated absorbent solution.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the sphere of deacidizing a hydrocarbon-containing gas, a natural gas for example. 
     BACKGROUND OF THE INVENTION 
     It is well known to use thermally regenerable liquid solvents for extracting the hydrogen sulfide contained in a gas, in particular in a natural gas. Examples of the most commonly used solvents are aqueous amine solutions and some physical solvents such as sulfolane, methanol, N-formyl morpholine, acetyl morpholine, propylene carbonate. Although many solvents likely to extract H 2 S also allow to extract CO 2 , some of them however show a selectivity for H 2 S over CO 2  and are therefore used when the amount of CO 2  extracted with H 2 S is to be limited. Examples of the most commonly used H 2 S selective solvents are methyldiethanolamine (MDEA), diisopropanolamine (DIPA), as well as the sterically encumbered amines and some physical solvents such as, for example, dimethyl ether polyethylene glycol or N-methyl pyrrolidone. 
     These methods generally involve a stage of extraction of the H 2 S contained in the gas to be treated by contacting this gas with the regenerated solvent in an absorber operating at the pressure of the gas to be treated, followed by a thermal regeneration stage, generally at a pressure slightly higher than the atmospheric pressure, generally between 1 and 5 bara, preferably between 1.5 and 3 bara. This thermal regeneration is generally carried out in a column equipped in the bottom with a reboiler and at the top with a condenser allowing to cool the acid compounds released by the regeneration and to recycle the condensates to the top of the regenerator as reflux. 
     When the pressure of the gas to be treated is notably higher than the atmospheric pressure, for example in the case of a natural gas that has to be treated at a pressure of the order of 70 bar, the H 2 S-rich solvent obtained at the absorber bottom can contain significant amounts of dissolved hydrocarbons. It is then common practice to carry out a stage of release of these dissolved hydrocarbons vaporized by simple expansion of the H 2 S-rich solvent. This expansion is carried out at an intermediate pressure between that of the raw gas to be treated and that of the thermal regeneration stage, typically of the order of 5 to 15 bara. A gas containing the major part of the dissolved hydrocarbons, that can be used as fuel gas, is thus separated from the H 2 S-rich solvent. This gas is sometimes washed by a stream of regenerated solvent coming from the thermal stage so as to re-absorb the acid compounds, notably the H 2 S released upon expansion. This washing of the fuel gas released by expansion is generally performed in a column placed directly on the separator drum between the gas and the expanded liquid. The solvent thus laden with H 2 S is directly mixed with the expanded solvent and sent to the thermal regeneration stage. 
     In order to reduce the heat consumptions of these methods, a stage of thermal exchange between the rich solvent after expansion and the regenerated solvent obtained hot at the bottom of the regeneration column is generally carried out. 
     Regeneration of these solvents produces a gaseous effluent rich in acid compounds, essentially containing the extracted H 2 S and CO 2 . This acid effluent is generally subjected to a treatment in order to convert the H 2 S to elementary sulfur, non-toxic and easy to transport. The most commonly used conversion method is the Claus process, notably described in documents FR-2,494,255 and FR-2,327,960, wherein the acid gas extracted undergoes partial combustion in air or oxygen-enriched air generating a stoichiometric mixture of H 2 S and CO 2  and of the elementary sulfur, recovered by condensation. This first thermal stage is generally followed by one to three catalytic conversion stages during which the H 2 S and the CO 2  react and form elementary sulfur according to the Claus reaction:
 
2 H 2 S+SO 2 ←→3/ x  S x +2 H 2 O
 
     After the catalytic stages of the Claus process, a gas still containing notable amounts of sulfur products (SO 2 , H 2 S, as well as COS, CS 2  and elementary sulfur) is obtained. In order to limit discharge of these compounds into the environment, this type of gas is generally subjected to a complementary finishing treatment. Various technologies have been proposed and used to carry out this type of finishing treatment. One of the most commonly used methods consists in converting all of the sulfur compounds of this gas to H 2 S, by reaction with reducing gases (hydrogen, CO) in the presence of a suitable catalyst. The residue gas thus obtained after this catalytic reduction stage is then washed by a solvent allowing selective extraction of the H 2 S and, after regeneration of this solvent, recycling of the H 2 S thus extracted to the thermal stage of the Claus plant. 
     It is possible to use a selective solvent, for example an aqueous MDEA solution, for washing the residue gas from the catalytic reduction stage downstream from the Claus plant. 
     When the raw gas to be treated is a natural gas containing CO 2  and notable amounts of aromatic hydrocarbons (for example some hundred ppmv), notable amounts of these compounds are found in admixture with the H 2 S in the acid gas. In fact, although the stage of expansion of the H 2 S-rich solvent obtained at the bottom of the absorber allows to release the major part of the light hydrocarbons (methane, ethane, . . . ) dissolved in the solvent at the absorber bottom, it does not allow to extract the major part of the heavier compounds, in particular the aromatic compounds whose solubility in solvents is generally much higher than that of the aliphatic hydrocarbons. An acid gas that can contain several hundred ppmv of aromatic hydrocarbons is then commonly obtained at the regenerator top. Besides, even with solvents allowing selective absorption of H 2 S over CO 2 , a certain CO 2  co-absorption is always observed. When the raw gas to be treated contains more CO 2  than H 2 S, this co-absorption can lead to an acid gas containing large or even major proportions of CO 2 . 
     The simultaneous presence of large amounts of CO 2  and of notable proportions (some hundred ppmv) of aromatic hydrocarbons in an acid gas leads to certain difficulties for conversion of the H 2 S of this gas to sulfur by means of the Claus process. In fact, dilution of H 2 S by CO 2  reduces the temperature obtained in the oven of the Claus thermal stage. This temperature reduction in turn decreases the destruction of the aromatic compounds, which are then present in notable proportions in the subsequent catalytic stages. The presence of these aromatic compounds during these catalytic stages can then cause various operating problems such as: production of coloured sulfur contaminated by carbon-containing compounds and therefore unfit for sale, clogging and activity loss of the catalysts by formation and deposition of carbon-containing compounds on the catalysts (carsuls). 
     Furthermore, when dilution of the H 2 S in the gaseous effluent produced during regeneration becomes too high, it is no longer possible to have a thermal stage in the Claus process. One may then consider treating highly diluted gases (containing only some % by volume of H 2 S) by means of direct oxidation processes wherein the acid gas and the air are directly contacted in the presence of a suitable catalyst allowing the reaction between the H 2 S and the oxygen of the air to be controlled so as to essentially produce only sulfur, but then again the presence of a large proportion of aromatic hydrocarbons makes it difficult to use these catalysts. 
     Various solutions have been proposed to overcome these drawbacks, among which:
         preheating the gases (acid gas and air) feeding the burner of the thermal stage of the Claus process. Such a preheating operation allows the temperature to be increased in the oven of the Claus plant. Depending on the CO 2  content of the gas, this solution does however not always allow to reach the temperature levels required to obtain nearly-total destruction of the aromatic hydrocarbons (of the order of 1150° C. or more), unless expensive preheating methods are implemented,   absorption of the aromatic compounds present in the acid gas on a suitable material (activated charcoal for example). This method requires an additional processing unit that may be expensive as regards investment (case of a regenerable adsorbent) or operating costs (case of non-regenerable adsorbents),   acid gas enrichment by selective re-absorption of the H 2 S it contains in a suitable solvent. This method is very efficient as regards sulfur production since it allows to obtain, on the one hand, an H 2 S-depleted gas containing most of the CO 2  and of the aromatic hydrocarbons present in the acid and, on the other hand, an H 2 S-concentrated gas depleted in aromatic hydrocarbons. It however represents a significant investment since all of the H 2 S extracted from the raw gas to be treated has to be re-absorbed.       

     The present invention provides a simple and inexpensive method that requires only a small number of additional equipments for separating the major part of the aromatic hydrocarbons co-absorbed by the solvent from the major part of the hydrogen sulfide absorbed by the solvent. 
     SUMMARY OF THE INVENTION 
     In general terms, the present invention relates to a method of extracting the hydrogen sulfide contained in a gas comprising aromatic hydrocarbons, wherein the following stages are carried out: 
     a) contacting said gas with an absorbent solution so as to obtain a gas depleted in hydrogen sulfide and an absorbent solution laden with hydrogen sulfide, 
     b) heating and expanding the hydrogen sulfide-laden absorbent solution to a predetermined temperature and pressure so as to release a gaseous fraction comprising aromatic hydrocarbons and to obtain an absorbent solution depleted in aromatic hydrocarbons, said temperature and said pressure being so selected that said gaseous fraction comprises at least 50% of the aromatic hydrocarbons contained in said hydrogen sulfide-laden absorbent solution and at most 35% hydrogen sulfide contained in said hydrogen sulfide-laden absorbent solution, (the gaseous fraction released following the expansion comprises a ratio of CO 2  content to H 2 S content greater than the ratio of CO 2  content to the H 2 S content of the absorbent solution before the expansion), 
     c) thermally regenerating the absorbent solution depleted in aromatic hydrocarbon compounds so as to release a hydrogen sulfide-rich gaseous effluent and to obtain a regenerated absorbent solution. Thermal regeneration can be achieved by distillation or steam stripping of the acid compounds. 
     According to the invention, at least part of the regenerated absorbent solution obtained in stage c) can be recycled to stage a) as absorbent solution. 
     Furthermore, at least part of the hydrogen sulfide-rich gaseous effluent obtained in stage c) can be treated by a Claus process. 
     The gaseous fraction comprising aromatic compounds obtained in stage b) can be sent to a burner of said Claus process. 
     The gaseous fraction comprising aromatic hydrocarbons obtained in stage b) as well as tail gas from said Claus process can be contacted with a second absorbent solution so as to produce a hydrogen sulfide-poor stream and a second absorbent solution enriched in hydrogen sulfide. At least part of the second hydrogen sulfide-enriched absorbent solution can be recycled to stage a) as absorbent solution. 
     Stage c) can be carried out in a regeneration column and at least part of the second hydrogen sulfide-enriched absorbent solution can be fed into said column so as to be regenerated. 
     The gaseous fraction comprising aromatic hydrocarbons obtained in stage b) can be contacted with a portion of the regenerated absorbent solution obtained in stage c) so as to obtain a hydrogen sulfide-depleted gaseous fraction and a third hydrogen sulfide-enriched absorbent solution. 
     The gaseous fraction comprising aromatic hydrocarbons obtained in stage b) can first be cooled and condensed prior to being contacted with the portion of the regenerated absorbent solution obtained in stage c). 
     The hydrogen sulfide-rich gaseous effluent obtained in stage c) can be treated by means of a hydrogen sulfide oxidation process. 
     Prior to stage c), the aromatic hydrocarbon-depleted absorbent solution can be contacted with a fraction of the hydrogen sulfide-rich gaseous effluent obtained in stage c) so that said fraction carries along part of the aromatic hydrocarbons contained in said solution. 
     Alternatively, the hydrogen sulfide-rich gaseous effluent obtained in stage c) can be partly condensed by cooling so as to produce a gas phase and a condensate and, prior to stage c), the aromatic hydrocarbon-depleted absorbent solution can be contacted with a fraction of said gas phase so that said fraction carries along part of the aromatic hydrocarbons and of the carbon dioxide contained in said solution. 
     Alternatively or as a complement to the aforementioned gaseous fraction recycles, part of the regenerated absorbent solution obtained in stage c) can be recycled hot, directly from the regenerator outlet, to stage b) as absorbent solution: the gaseous fraction released in stage b) is contacted with part of the regenerated absorbent solution obtained in stage c). The gaseous fraction released in stage b), comprising aromatic hydrocarbons and hydrogen sulfide, is thus depleted in hydrogen sulfide without any additional equipment and the absorbent solution from stage b) is accordingly enriched in hydrogen sulfide. This recycling of part of the hot regenerated absorbent solution furthermore leads to better thermal integration of the process. 
     The absorbent solution can comprise an amine in solution in water. The amine can be selected from among the group comprising methyldiethanolamine, diisopropanolamine and the sterically encumbered amines and, in stage b), said temperature can range between 80° C. and 140° C. and said pressure can range between 1.5 and 6 bara. The amine can also be selected from among the group comprising monoethanolamine and diethanolamine and, in stage b), said temperature can range between 80° C. and 140° C. and said pressure can range between 1.5 and 6 bara. 
     Alternatively, the absorbent solution can be selected from among the group made up of sulfolane, methanol, N-formyl morpholine, acetyl morpholine, propylene carbonate, dimethyl ether polyethylene glycol or N-methyl pyrrolidone. 
     According to the invention, the gas can be a natural gas. The gas can comprise at least 50 ppmv of aromatic hydrocarbons. 
     In the method according to the invention, implementation of this method is favoured by the fact that the proportion of aromatic hydrocarbons sent to the Claus process is limited. 
     When a solvent with selective H 2 S absorption over CO 2  is used, the method according to the invention also allows to obtain a substantial increase in the H 2 S concentration of the acid gas, again at a low cost, notably without having to re-absorb all or even a great fraction of the H 2 S extracted from the raw gas. This effect also contributes to improving operation of the Claus process. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Other features and advantages of the invention will be clear from reading the description hereafter, with reference to the accompanying figures wherein: 
         FIG. 1  diagrammatically shows an embodiment of the method according to the invention, 
         FIGS. 1   b ,  2 ,  2   b ,  3  and  3   b  diagrammatically show embodiment variants of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In connection with  FIGS. 1 and 1   b , the gas to be treated flows through line  1  into the bottom of an absorber  2 . The gas from which the H 2 S absorbed by the solvent injected at the absorber top through line  30  and the fractions of the co-absorbed compounds, notably the hydrocarbons and the CO 2 , are extracted is recovered at the top of absorber  2 . This absorber generally operates at temperatures close to or slightly higher than the ambient temperature, typically ranging between 20° C. and 100° C., preferably between 30° C. and 90° C., and at pressures typically ranging between 10 and 200 bar, preferably between 20 and 100 bar. 
     The solvent used in the method according to the invention is selected for its H 2 S absorption capacity. The method can notably be implemented when the solvent used is an aqueous solution of an amine or of a mixture of amines such as monoethanolamine, diethanolamine, diisopropanolamine, the sterically encumbered amines, a physical solvent such as sulfolane, methanol, N-formyl morpholine, acetyl morpholine, dimethyl ether polyethylene glycol, N-methyl pyrrolidone, propylene carbonate or a mixture of amines, of physical solvent and of water in variable proportions. The method according to the invention can be carried out with an H 2 S selective or non-selective solvent over CO 2 . 
     The gas flowing in through line  1  can be a natural gas available at a pressure ranging between 10 and 200 bar, and at a temperature ranging between 20° C. and 100° C. This gas comprises H 2 S and possibly other acid compounds such as CO 2 , COS, mercaptans. Furthermore, the natural gas comprises aromatic hydrocarbons, which are unsaturated cyclic compounds comprising an aromatic ring such as benzene, toluene or xylenes. 
     In  FIGS. 1 and 1   b , the H 2 S-rich solvent obtained at the bottom of the absorber through line  4  is expanded by an expansion means  5  and fed into a first flash drum  6 . This first expansion stage is optional for implementing the method according to the invention, but it allows to obtain, through line  7 , a gas containing the major part of the aliphatic hydrocarbons co-absorbed by the solvent. This gas is possibly washed by a fraction of the regenerated solvent and the gas thus obtained can be used as fuel gas. This washing procedure, which is optional, is not shown here. Flash drum  6  operates at a pressure that is lower than that of absorber  2  and higher than that of flash drum  12 . This pressure generally depends on the conditions of use of the fuel gas and it is typically of the order of 5 to 15 bara. This drum operates at a temperature that is substantially identical to that of the solvent obtained at the bottom of absorber  2 . 
     The H 2 S-rich solvent obtained after expansion is sent through line  8  to a preheating means.  FIGS. 1 and 1   b  show a heat exchanger  9  with the regenerated solvent obtained at the bottom of regeneration column  18 , but any other suitable preheating means can be used, provided that it allows the temperature of the H 2 S-rich solvent to be brought to the level required for partial vaporization of the compounds absorbed by the H 2 S-rich solvent. 
     The preheated H 2 S-rich solvent is fed through line  10 , after possible expansion by means of an expansion means  11 , into drum  12  where the vaporized gases and the H 2 S-rich solvent are separated. This drum  12  is operated under such temperature and pressure conditions that vaporization of a minor fraction of the H 2 S absorbed by the solvent, generally below 35%, preferably below 30%, and of a major fraction of the aromatic hydrocarbons absorbed by the solvent, above 50%, preferably above 70%, is obtained. The pressure of drum  12  is lower than that of drum  6  and higher than the atmospheric pressure, preferably ranging between 2 and 6 bara. The temperature of drum  12  ranges between that of the H 2 S-laden solvent obtained at the bottom of absorber  2  and that of the regenerated solvent obtained at the bottom of regenerator  18 . 
     It is also possible to recycle a minor fraction of the acid gas obtained at the top of reflux drum  21  or to use any other gas stream that would be available as stripping agent, injected into the bottom of drum  12 , so as to increase the proportion of aromatic hydrocarbons vaporized during this stage. Besides, alternatively or as a complement to the acid gas recycle, it is also possible to recycle a fraction of hot regenerated absorbent solution obtained at the bottom of regenerator  18  to the top of drum  12  so as to wash the vaporized gases and thus to concentrate the hydrogen sulfide in the absorbent solution sent to regeneration column  18 . 
       FIG. 1   b  completes the method described in reference to  FIG. 1  by diagrammatically showing two embodiments of the recycle of the acid gas obtained in regenerator  18  to drum  12  and an embodiment of the recycle of the hot regenerated absorbent solution obtained at the bottom of regenerator  18  to drum  12 . 
     In  FIG. 1   b , a fraction of the acid gas obtained directly at the outlet of regenerator  18  through line  42  or obtained at the top of reflux drum  21  through line  43  is injected into the bottom of drum  12 . This recycling of a fraction of the acid gas, ranging between 0 and 80% by volume of the total stream, preferably between 0 and 50% and more preferably between 10 and 35% of the total stream, allows to increase the proportion of aromatic hydrocarbons vaporized during this stage and to enrich in H 2 S said acid gas from regenerator  18 , and therefore to improve the operation of the plant intended for conversion of the H 2 S to sulfur by means of the Claus process. This H 2 S enrichment of the acid gas at the outlet of regenerator  18  is explained by the amount of CO 2  discharged through line  13  that is notably improved, which causes an increase of concentration of the residual H 2 S in the solvent obtained at the bottom of drum  12  and sent through line  15  and pump  16  to the top of regenerator  18 , and therefore an increase of concentration of the H 2 S in the acid gas released at the top of regenerator  18  to the Claus plant through lines  25  and  41 . 
     In  FIG. 1   b , a fraction of hot regenerated absorbent solution obtained at the bottom of regenerator  18  through line  27  is injected via line  44  to the top of drum  12 . In drum  12 , the hot regenerated absorbent solution is contacted with the gas released by the expansion of the acid compound-laden absorbent solution flowing in through line  11  and possibly with the stripping gases fed into  12  through line  42  and/or  43 . This recycling of a fraction of hot regenerated absorbent solution ranging between 0 and 50% by volume of the total stream, preferably between 5 and 35% by volume of the total stream, allows to wash, without any additional equipment, the gaseous fraction from drum  12  and thus to deplete this gaseous fraction in hydrogen sulfide, therefore to enrich the absorbent solution in hydrogen sulfide. The absorbent solution thus enriched in hydrogen sulfide allows to produce an acid gas that is accordingly richer in H 2 S at the outlet of regenerator  18 . 
     In  FIGS. 1 and 1   b , the solvent obtained at the bottom of drum  12  is sent through line  15  and pump  16  to the top of regenerator  18 . In this regenerator, the acid compounds absorbed by the solvent, notably H 2 S, are vaporized by stripping effect with steam generated by reboiler  26  at the regenerator bottom. These gases are collected through line  19  at the regenerator top, cooled in exchanger  20 , and the majority of the water and of the solvent contained in the regenerator top gas is condensed, separated in reflux drum  21  and recycled as reflux to the regenerator top through line  22 . The operating temperature and pressure conditions of the regenerator depend on the type of solvent used. Regenerator  18  operates at a pressure generally ranging between the atmospheric pressure and 10 bara, preferably between 1.5 and 3 bara. The temperature at the bottom of the regenerator generally ranges between 100° C. and 200° C., preferably between 110° C. and 150° C. 
     At the bottom of regenerator  18 , a stream of hot regenerated solvent is obtained through line  27  and recycled via line  28 , pump  29  and line  30  to the top of absorber  2  after heat exchange with the H 2 S-rich solvent in exchanger  9 . 
     The gases released by partial vaporization in drum  12  are fed through line  13 , possibly after condensation of the water in condenser  40  and possibly after expansion by an expansion means  14 , into a first section  32  of the thermal stage of a plant implementing the Claus process. The gases obtained after expansion in means  14  can be mixed with a fraction of the acid gas obtained at the top of reflux drum  21  through line  25 . This acid gas fraction from reflux drum  21  is possibly introduced so as to adjust the overall composition of the gas in thermal section  32 . In this section, these gases rich in aromatic hydrocarbons, possibly mixed with an acid gas fraction, are subjected to combustion in air or oxygen-enriched air supplied through line  31 . This combustion is carried out by means of suitable instruments (burner, oven, . . . ), not shown here, and the air flow rate is so determined as to obtain a temperature allowing nearly-total destruction of the aromatic hydrocarbons. This temperature must be above 1150° C., preferably above 1200° C. 
     The hot gaseous mixture thus obtained is then fed into a second combustion section  33  where it is mixed with the acid gas obtained at the top of reflux drum  21  through line  41 , with a proportion of air or of oxygen-enriched air supplied through line  34 , determined in such a way as to obtain at the outlet of the thermal stage, through line  36 , a gaseous mixture containing a stoichiometric H 2 S/SO 2  ratio (H 2 S/SO 2 =2). The air or the oxygen-enriched air and the acid gas are fed into this second combustion section through suitable means that are not shown here (burners, injection pipes, . . . ). Determination of the flow rate of air or of oxygen-enriched air injected into this second section so as to control the reaction stoichiometry is performed by the means commonly used in this type of plant, not shown here (on-line analyzer of the acid gas at the Claus plant outlet acting on a makeup air or oxygen-enriched air control valve). The reaction in the section is not disturbed by the presence of aromatic hydrocarbons because they have been separated in drum  12  and destroyed in section  32 . 
     After these combustion stages, the gas is cooled so as to produce liquid elementary sulfur through line  35  and a gas that is fed through line  36  into catalytic section  37  of the Claus plant, this section producing liquid sulfur through line  38  and a residue gas through line  39 , that can be possibly subjected to additional treatments. 
     In the second embodiment of the invention described in connection with  FIGS. 2 and 2   b , the gas to be treated is fed through line  1  into the bottom of an absorber  2 . The gas from which the H 2 S absorbed by the solvent injected at the absorber top through line  30  and the fractions of the co-absorbed compounds, notably the hydrocarbons and the CO 2 , are extracted is recovered at the top of absorber  2 . This absorber generally operates at temperatures close to or slightly higher than the ambient temperature, typically ranging between 20° C. and 100° C., preferably between 30° C. and 90° C., and at pressures typically ranging between 10 and 200 bar, preferably between 20 and 100 bar. 
     The H 2 S-rich solvent obtained at the bottom of the absorber through line  4  is expanded by an expansion means  5  and fed into a first flash drum  6 . This first expansion stage is optional for implementing the method according to the invention, but it allows to obtain, through line  7 , a gas containing the major part of the aliphatic hydrocarbons co-absorbed by the solvent. This gas is possibly washed by a fraction of the regenerated solvent and the gas thus obtained can be used as fuel gas. This washing procedure, which is optional, is not shown here. Flash drum  6  operates at a pressure that is lower than that of absorber  2  and higher than that of flash drum  12 . This pressure generally depends on the conditions of use of the fuel gas and it is typically of the order of 5 to 15 bara. This drum operates at a temperature that is substantially identical to that of the solvent obtained at the bottom of absorber  2 . 
     The H 2 S-rich solvent obtained after expansion is sent through line  8  to a preheating means.  FIGS. 2 and 2   b  show a heat exchanger  9  with the regenerated solvent obtained at the bottom of regeneration column  21 , but any other suitable preheating means can be used, provided that it allows the temperature of the H 2 S-rich solvent to be brought to the level required for partial vaporization of the compounds absorbed by the H 2 S-rich solvent. 
     The preheated H 2 S-rich solvent is fed through line  11 , after possible expansion by means of an expansion means  10 , into drum  12  where the vaporized gases and the H 2 S-rich solvent are separated. This drum  12  is operated under such temperature and pressure conditions that vaporization of a minor fraction of the H 2 S absorbed by the solvent, generally below 35%, preferably below 30%, and of a major fraction of the aromatic hydrocarbons absorbed by the solvent, above 50%, preferably above 70%, is obtained. These aromatic hydrocarbons can be benzene, toluene or xylenes. The pressure of drum  12  is lower than that of drum  6  and higher than the atmospheric pressure, preferably ranging between 2 and 6 bara. The temperature of drum  12  ranges between that of the H 2 S-laden solvent obtained at the bottom of absorber  2  and that of the regenerated solvent obtained at the bottom of regenerator  21 . 
     It is also possible to recycle a minor fraction of the acid gas obtained at the top of reflux drum  24  or to use any other gas stream that would be available as stripping agent, injected into the bottom of drum  12 , so as to increase the proportion of aromatic hydrocarbons vaporized during this stage. 
     Besides, alternatively or as a complement to the acid gas recycle, it is also possible to recycle a fraction of hot regenerated absorbent solution obtained at the bottom of regenerator  21  to the top of drum  12  so as to wash the vaporized gases and thus to concentrate the hydrogen sulfide in the absorbent solution sent to regeneration column  21 . 
       FIG. 2   b  completes the method described in reference to  FIG. 2  by diagrammatically showing two embodiments of the recycle of the acid gas obtained in regenerator  21  to drum  12  and an embodiment of the recycle of the hot regenerated absorbent solution obtained at the bottom of regenerator  21  to drum  12 . 
     In  FIG. 2   b , a fraction of the acid gas obtained directly at the outlet of regenerator  21  through line  32  or obtained at the top of reflux drum  24  through line  33  is injected into the bottom of drum  12 . This recycling of a fraction of the acid gas, ranging between 0 and 80% by volume of the total stream, preferably between 0 and 50% and more preferably between 10 and 35% of the total stream, allows to increase the proportion of aromatic hydrocarbons vaporized during this stage and to enrich in H 2 S said acid gas from regenerator  21 . This H 2 S enrichment of the acid gas at the outlet of regenerator  21  is explained by the amount of CO 2  discharged at the top of drum  12 , that is notably improved, which causes an increase of concentration of the residual H 2 S in the solvent obtained at the bottom of drum  12  and sent through line  18  and pump  19  to the top of regenerator  21 , and therefore an increase of concentration of the H 2 S in the acid gas released at the top of regenerator  21  discharged through line  25 . 
     In  FIG. 2   b , a fraction of hot regenerated absorbent solution obtained at the bottom of regenerator  21  through line  28  is injected via line  34  to the top of drum  12 . In drum  12 , the hot regenerated absorbent solution is contacted with the gas released by the expansion of the acid compound-laden absorbent solution flowing in through line  11  and possibly with the stripping gases fed into  12  through line  32  and/or  33 . This recycling of a fraction of hot regenerated absorbent solution ranging between 0 and 50% by volume of the total stream, preferably between 5 and 35% by volume of the total stream, allows to wash, without any additional equipment, the gaseous fraction from drum  12  and thus to deplete this gaseous fraction in hydrogen sulfide, therefore to enrich the absorbent solution in hydrogen sulfide. The absorbent solution thus enriched in hydrogen sulfide allows to produce an acid gas that is accordingly richer in H 2 S at the outlet of regenerator  21 . 
     In reference to  FIGS. 2 and 2   b , the solvent obtained at the bottom of drum  12  is sent through line  18  and pump  19  to the top of regenerator  21 . In this regenerator, the rest of the acid compounds absorbed by the solvent, notably H 2 S, is vaporized by stripping effect with steam generated by reboiler  27  at the regenerator bottom. These gases are collected through line  22  at the regenerator top, cooled in exchanger  23 , and the majority of the water and of the solvent contained in the regenerator top gas is condensed, separated in reflux drum  24  and recycled as reflux to the regenerator top through line  26 . The operating temperature and pressure conditions of the regenerator depend on the type of solvent used. Regenerator  21  operates at a pressure generally ranging between the atmospheric pressure and 10 bara, preferably between 1.5 and 3 bara. The temperature at the bottom of the regenerator generally ranges between 100° C. and 200° C, preferably between 110° C. and 150° C. 
     At the bottom of regenerator  21 , a stream of hot regenerated solvent is obtained through line  28  and recycled at least partly via pump  29  and line  30  to the top of absorber  2  after heat exchange with the H 2 S-rich solvent in exchanger  9 . Another part of the regenerated solvent is sent through line  31  to the top of absorber  15 . The distribution of the regenerated solvent between absorber  2  and absorber  15  can be determined by the maximum proportion of H 2 S desired in the treated gas coming from line  3 . 
     The gases released by partial vaporization in drum  12  are preferably cooled in exchanger  13  prior to being fed through line  14  into an absorber  15  to be contacted with a fraction of the regenerated solvent that is injected through line  31  at the top of absorber  15 . The liquid fraction obtained after cooling in  13  can be fed into reflux drum  24 . In absorber  15 , the most part of the H 2 S contained in the gas from drum  12  is re-absorbed by the solvent. A gas that can be incinerated is obtained on the one hand at the top of this absorber  15  through line  16  and an H 2 S-rich solvent that can be mixed with the solvent obtained at the bottom of drum  12  is obtained on the other hand at the bottom of absorber  15  through line  17 . According to the composition of this solvent, it may also be advantageous to recycle it after pumping to absorber  2 . 
     In the third embodiment of the invention described in connection with  FIGS. 3 and 3   b , the gas to be treated flows through line  1  into the bottom of an absorber  2 . The gas from which the H 2 S absorbed by the solvent injected at the absorber top through lines  29  and/or  33  and the fractions of the co-absorbed compounds, notably the hydrocarbons and the CO 2 , are extracted is recovered at the top of absorber  2 . This absorber generally operates at temperatures close to or slightly higher than the ambient temperature, typically ranging between 20° C. and 100° C., preferably between 30° C. and 90° C., and at pressures typically ranging between 10 and 200 bar, preferably between 20 and 100 bar. 
     The H 2 S-rich solvent obtained at the bottom of the absorber through line  4  is expanded by an expansion means  5  and fed into a first flash drum  6 . This first expansion stage is optional for implementing the method according to the invention, but it allows to obtain, through line  7 , a gas containing the major part of the aliphatic hydrocarbons co-absorbed by the solvent. This gas is possibly washed by a fraction of the regenerated solvent and the gas thus obtained can be used as fuel gas. This washing procedure, which is optional, is not shown here. Flash drum  6  operates at a pressure that is lower than that of absorber  2  and higher than that of flash drum  12 . This pressure generally depends on the conditions of use of the fuel gas and it is typically of the order of 5 to 15 bara. This drum operates at a temperature that is substantially identical to that of the solvent obtained at the bottom of absorber  2 . 
     The H 2 S-rich solvent obtained after expansion is sent through line  8  to a preheating means.  FIGS. 3 and 3   b  show a heat exchanger  9  with the regenerated solvent obtained at the bottom of regeneration column  15 , but any other suitable preheating means can be used, provided that it allows the temperature of the H 2 S-rich solvent to be brought to the level required for partial vaporization of the compounds absorbed by the H 2 S-rich solvent. 
     The preheated H 2 S-rich solvent is fed through line  11 , after possible expansion by means of an expansion means  10 , into drum  12  where the vaporized gases and the H 2 S-rich solvent are separated. This drum  12  is operated under such temperature and pressure conditions that vaporization of a minor fraction of the H 2 S absorbed by the solvent, generally below 35%, preferably below 30%, and of a major fraction of the aromatic hydrocarbons absorbed by the solvent, above 50%, preferably above 70%, is obtained. The pressure of drum  12  is lower than that of drum  6  and higher than that of absorber  25 , preferably ranging between 1.5 and 6 bara. The temperature of drum  12  ranges between that of the H 2 S-laden solvent obtained at the bottom of absorber  2  and that of the regenerated solvent obtained at the bottom of regenerator  15 . 
     It is possible to recycle a minor fraction of the acid gas obtained at the top of reflux drum  18  or to use any other gas stream that would be available as stripping agent, injected into the bottom of drum  12 , so as to increase the proportion of aromatic hydrocarbons vaporized during this stage. Besides, alternatively or as a complement to the acid gas recycle, it is also possible to recycle a fraction of hot regenerated absorbent solution obtained at the bottom of regenerator  15  to the top of drum  12  so as to wash the vaporized gases and thus to concentrate the hydrogen sulfide in the absorbent solution sent to regeneration column  15 . 
       FIG. 3   b  completes the method described in reference to  FIG. 3  by diagrammatically showing two embodiments of the recycle of the acid gas obtained in regenerator  15  to drum  12  and an embodiment of the recycle of the hot regenerated absorbent solution obtained at the bottom of regenerator  15  to drum  12 . 
     In  FIG. 3   b , a fraction of the acid gas obtained directly at the outlet of regenerator  15  through line  42  or obtained at the top of reflux drum  18  through line  43  is injected into the bottom of drum  12 . This recycling of a fraction of the acid gas, ranging between 0 and 80% by volume of the total stream, preferably between 0 and 50% and more preferably between 10 and 35% of the total stream, allows to increase the proportion of aromatic hydrocarbons vaporized during this stage and to enrich in H 2 S said acid gas from regenerator  15 , and therefore to improve the operation of unit  21  intended for conversion of the H 2 S to sulfur by means of the Claus process. This H 2 S enrichment of the acid gas at the outlet of regenerator  15  is explained by the amount of CO 2  discharged through line  40  that is notably improved, which causes an increase of concentration of the residual H 2 S in the solvent obtained at the bottom of drum  12  and sent through line  13  and pump  14  to the top of regenerator  15 , and therefore an increase of concentration of the H 2 S in the acid gas released at the top of regenerator  15  through line  20 . 
     In  FIG. 3   b , a fraction of hot regenerated absorbent solution obtained at the bottom of regenerator  15  through line  31  is injected via line  44  to the top of drum  12 . In drum  12 , the hot regenerated absorbent solution is contacted with the gas released by the expansion of the acid compound-laden absorbent solution flowing in through line  11  and possibly with the stripping gases fed into  12  through line  42  and/or  43 . This recycling of a fraction of hot regenerated absorbent solution ranging between 0 and 50% by volume of the total stream, preferably between 5 and 35% by volume of the total stream, allows to wash, without any additional equipment, the gaseous fraction from drum  12  and thus to deplete this gaseous fraction in hydrogen sulfide, therefore to enrich the absorbent solution in hydrogen sulfide. The absorbent solution thus enriched in hydrogen sulfide allows to produce an acid gas that is accordingly richer at the outlet of regenerator  15 . 
     In connection with  FIGS. 3 and 3   b , the solvent obtained at the bottom of drum  12  is sent through line  13  and pump  14  to the top of regenerator  15 . In this regenerator, the acid compounds absorbed by the solvent, notably H 2 S, are vaporized by stripping effect with steam generated by reboiler  32  at the regenerator bottom. These gases are collected through line  16  at the regenerator top, cooled in exchanger  17 , and the majority of the water and of the solvent contained in the regenerator top gas is condensed, separated in reflux drum  18  and recycled as reflux to the regenerator top through line  19 . The operating temperature and pressure conditions of the regenerator depend on the type of solvent used. Regenerator  15  operates at a pressure generally ranging between the atmospheric pressure and 10 bara, preferably between 1.5 and 3 bara. The temperature at the bottom of the regenerator generally ranges between 100° C. and 200° C., preferably between 110° C. and 150° C. 
     At the bottom of regenerator  15 , a stream of hot regenerated solvent is obtained through line  31  and recycled at least partly via line  33  and pump  36  to the top of absorber  2  after heat exchange with the H 2 S-rich solvent in exchanger  9 . Another part of the regenerated solvent can be sent through line  34  and pump  35  to the top of absorber  25 . The distribution of the regenerated solvent between absorber  2  and absorber  25  can be determined according to the maximum proportion of H 2 S desired in the treated gas coming from absorber  2  through line  3 . 
     The acid gas obtained at the top of reflux drum  18  through line  20  is treated in plant  21  intended for conversion of the H 2 S to sulfur by means of the Claus process. This plant produces liquid elementary sulfur through line  22 . It comprises a stage of treating the residue gases obtained at the outlet of the Claus catalytic stage by catalytic reduction to H 2 S of the sulfur compounds and cooling. The residue gas, commonly referred to as Claus tail gas, thus obtained through line  23  is sent to the bottom of absorber  25 , where the H 2 S present in this gas will be absorbed by a fraction of the cooled regenerated solvent, injected via line  34  and pump  35  at the top of absorber  25 . Operation of Claus plant  21  is improved because the proportion of aromatic compounds in the acid gas obtained after regeneration is decreased by means of the separation carried out in drum  12 . Furthermore, in cases where the expansion in drum  12  allows to vaporize a substantial amount of CO 2  contained in the solvent, the acid gas coming from the regeneration stage through line  20  is concentrated in H 2 S. This increase in the H 2 S concentration contributes to improving the operation of Claus plant  21 . 
     The gases released by partial vaporization in drum  12  are collected through line  40  and, possibly after expansion through means  41 , they are also fed to the bottom of absorber  25  where the H 2 S they contain will be re-absorbed. These gases can possibly, but not necessarily, also be cooled and condensed prior to being sent to absorber  25 . Such an option is not shown here. 
     The residue gas from sulfur recovery plant  21  possibly containing a lower proportion of H 2 S than the gas coming from drum  12  can also be optionally fed into absorber  25  at a higher level than the injection level of the gas coming from drum  12  through line  24 . 
     A solvent partly laden with H 2 S is obtained at the bottom of residue gas absorber  25 , which can be either sent back directly to regenerator  15  after heat exchange with a fraction of the regenerated solvent obtained at the bottom of regenerator  15 , or, as shown in  FIG. 3 , sent back to absorber  2  via pump  28  so as to benefit from the residual H 2 S absorption capacity of this partly laden solvent. This solvent can be sent totally or partly to the top of absorber  2 , in admixture with the regenerated solvent, via line  29 , or totally or partly to a lower level of absorber  2  via line  30 . 
     The numerical examples given hereafter illustrate the operation and the advantages of the method according to the invention. The first example is given by way of comparison and illustrates a method according to the prior art. Examples 2 and 3 illustrate the mode of operation of the method according to embodiments 3 and 1 of the invention. Examples 4 and 5 allow to compare the mode of operation of the methods described in reference to  FIGS. 3 and 3   b.    
     EXAMPLE 1 
     Comparative, According to the Prior Art 
     A natural gas whose composition is given in Table 1 is treated in an absorption column. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 3.5% 
                 mol 
               
               
                   
                 carbon dioxide 
                 4.5% 
                 mol 
               
               
                   
                 nitrogen 
                 1% 
                 mol 
               
               
                   
                 methane 
                 84.5% 
                 mol 
               
               
                   
                 ethane 
                 3% 
                 mol 
               
               
                   
                 C3 + (propane + other heavier 
                 3.35% 
                 mol 
               
               
                   
                 hydrocarbons) 
               
               
                   
                 aromatic hydrocarbons (benzene, 
                 1500 
                 ppmv 
               
               
                   
                 toluene and xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     The gas is at a temperature of 40° C. and it flows into an absorption column at a pressure of 60 bar, at a flow rate of 472 kNm 3 /h. 
     The solvent used is methyldiethanolamine diluted to 50% weight in water. The solvent flows into the absorption column at a temperature of 50° C. and at a flow rate of 600 m 3 /h. 
     The natural gas, after being treated, leaves the absorption column at a flow rate of 441 kNm 3 /h with the composition given in Table 2. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 4 
                 ppmv 
               
               
                   
                 carbon dioxide 
                 1.65% 
                 mol 
               
               
                   
                 nitrogen 
                 1.1% 
                 mol 
               
               
                   
                 methane 
                 90% 
                 mol 
               
               
                   
                 ethane 
                 3.2% 
                 mol 
               
               
                   
                 C3 + (propane + other heavier 
                 3.6% 
                 mol 
               
               
                   
                 hydrocarbons) 
               
               
                   
                 aromatic hydrocarbons (benzene, 
                 1460 
                 ppmv 
               
               
                   
                 toluene and xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     The amine solution enriched in acid compounds (hydrogen sulfide and carbon dioxide) flows from the absorption column and undergoes, in a drum, an expansion allowing to remove part of the acid compounds and of the hydrocarbons absorbed. The amine solution leaves the flash drum and is directly sent to the regeneration column. 
     The acid gas from the regeneration column has the composition given in Table 3. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 51.4% 
                 mol 
               
               
                   
                 carbon dioxide 
                 43.2% 
                 mol 
               
               
                   
                 water 
                 5% 
                 mol 
               
               
                   
                 methane 
                 0.2% 
                 mol 
               
               
                   
                 ethane + other heavier hydrocarbons 
                 0.02% 
                 mol 
               
               
                   
                 aromatic hydrocarbons (benzene, 
                 1743 
                 ppmv 
               
               
                   
                 toluene and xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     This acid gas, that constitutes the Claus inlet feed, at a flow rate of 30.4 kNm 3 /h, comprises an aromatics content of 1743 ppmv and a low H 2 S content of 51.4% mol, and, even preheated to 220° C., like the air injected, it does not allow to reach a temperature likely to reduce the aromatics content at the oven outlet to below 300 ppmv. The high aromatic hydrocarbon residual content leads to deactivation of the catalysts of the first Claus catalytic stage. 
     EXAMPLE 2 
     According to the Invention 
     Example 2 illustrates the mode of operation of the method described in reference to  FIG. 3 . 
     The same natural gas, with the same composition as in Example 1 (see Table 1), is treated in absorption column  2  under the same operating conditions as in Example 1. 
     After a first expansion in flash drum  6 , the amine solution leaves flash drum  6  through line  8 , flows through exchanger  9 , and is subjected to a second expansion in drum  12  at 2.7 bar and 104.3° C. Expansion, that allows part of the CO 2  and the major part of the aromatic hydrocarbons to be removed, releases a flash gas that leaves the drum through line  40 . This flash gas has the composition described in Table 4. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 23.1% 
                 mol 
               
               
                   
                 carbon dioxide 
                 38.2% 
                 mol 
               
               
                   
                 water 
                 38.1% 
                 mol 
               
               
                   
                 methane 
                 0.3% 
                 mol 
               
               
                   
                 ethane + other heavier hydrocarbons 
                 0.03% 
                 mol 
               
               
                   
                 aromatic hydrocarbons (benzene, 
                 2612 
                 ppmv 
               
               
                   
                 toluene and xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     This flash gas is then sent at a flow rate of 19.0 kNm 3 /h to second absorption column  25  through line  40  and it is treated with the gases coming from (Claus) section  21  according to the mode of operation described in reference to  FIG. 3 . 
     The amine solution leaves drum  12  through line  13  and it is sent to regeneration column  15 . 
     After regeneration, the amine solution leaves regeneration column  15  through line  31  prior to entering, after flowing through exchanger  9 , absorption columns  2  and  25  through lines  33  and  34 . 
     The acid gas from regeneration column  15  has the composition given in Table 5. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 62.4% 
                 mol 
               
               
                   
                 carbon dioxide 
                 32.6% 
                 mol 
               
               
                   
                 water 
                 5% 
                 mol 
               
               
                   
                 aromatic hydrocarbons (benzene, 
                 188 
                 ppmv 
               
               
                   
                 toluene and xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     This acid gas is sent through line  20  to (Claus) section  21  at a flow rate of 18.0 kNm 3 /h. This acid gas, that eventually contains only 188 ppmv aromatic hydrocarbons, versus 1743 ppmv in the acid gas of Example 1 according to the prior art, and that comprises 62.4% mol hydrogen sulfide, versus only 51.4% mol in the acid gas of Example 1 according to the prior art, is preheated to 220° C. like the air injected. The temperature reached in the oven is higher than that of Example 1 and therefore sufficient to reduce the residual aromatic hydrocarbon content at the oven outlet to less than 25 ppmv. The Claus catalyst of the first catalytic stage will therefore have a longer life and activity. 
     EXAMPLE 3 
     According to the Invention 
     Example 3 illustrates the mode of operation of the method described in reference to  FIG. 1 . 
     The same natural gas, with the same composition as in Example 1 (see Table 1), is treated in absorption column  2  according to the same operating conditions as in Example 1. 
     After a first expansion in flash drum  6 , the amine solution leaves flash drum  6  through line  8 , flows through exchanger  9 , and is subjected to a second expansion in drum  12 . Expansion, that allows part of the carbon dioxide and the major part of the aromatic hydrocarbons to be removed, releases a flash gas that leaves the drum through line  13 . This flash is carried out under the same conditions as in Example 2 and the flash gas has the same composition as in Example 2, given in Table 4. 
     The amine solution leaves drum  12  through line  15  and it enters regeneration column  18 . 
     After regeneration, the amine solution leaves regeneration column  18  through line  27  and it enters absorption column  2  through line  30  after flowing through exchanger  9 . 
     The acid gas from regeneration column  18  has the same composition as in Example 2, given in Table 5. 
     The flash gas from drum  12  is sent to a condenser  40  through line  13  in order to lower the water content. The composition of the flash gas obtained after condensation is given in Table 6. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 35.6% 
                 mol 
               
               
                   
                 carbon dioxide 
                 58.9% 
                 mol 
               
               
                   
                 water 
                 4.6% 
                 mol 
               
               
                   
                 methane 
                 0.5% 
                 mol 
               
               
                   
                 ethane + other heavier hydrocarbons 
                 0.04% 
                 mol 
               
               
                   
                 aromatics (benzene, toluene and 
                 4028 
                 ppmv 
               
               
                   
                 xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     This H 2 S-poor and aromatics-rich flash gas is sent at a flow rate of 12.3 kNm 3 /h, through line  13 , after condensation, to section  32 , i.e. to the first zone of the Claus plant oven. A fraction of the H 2 S-rich acid gas from regeneration column  18  is sent through line  25  to section  32 , i.e. the first zone of the Claus plant oven, and the major part of this acid gas is sent through line  41  to section  33 , i.e. the second zone of the Claus plant oven. 
     The flash gas mixed with a fraction of the H 2 S-rich acid gas is preheated to 220° C., as well as the air also preheated to this temperature: they are sent to the first zone of the oven through the burner, which allows to reach a temperature higher by about 250° C. in comparison to the temperature reached in Example 1 according to the prior art, thus sufficient to reduce the aromatics content at the oven outlet to less than 30 ppmv. The Claus catalyst of the first catalytic stage will therefore have a longer life and activity. 
     The fact that the acid gas obtained in Examples 2 and 3 (see Table 5) is, on the one hand, more concentrated in H 2 S and, on the other hand, poorer in CO 2  and in aromatic hydrocarbons, than the acid gas obtained in Example 1 (see Table 3) clearly shows the significance of carrying out an expansion, under the conditions according to the invention, of the solvent in drum  12  prior to regeneration. 
     EXAMPLE 4 
     According to the Invention 
     Example 4 illustrates the mode of operation of the method described in reference to  FIG. 3 . 
     The same natural gas with the same composition as in Example 1 (see Table 1) is treated in absorption column  2  under the same operating conditions as in Example 1. 
     After a first expansion in flash drum  6 , the amine solution leaves flash drum  6  through line  8 , flows through exchanger  9 , and is subjected to a second expansion in drum  12  at 2.0 bar and 100.1° C. Expansion, that allows part of the CO 2  and the major part of the aromatic hydrocarbons to be removed, releases a flash gas that leaves the drum through line  40 . This flash gas has the composition described in Table 7. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 22.2% 
                 mol 
               
               
                   
                 carbon dioxide 
                 33.0% 
                 mol 
               
               
                   
                 water 
                 44.3% 
                 mol 
               
               
                   
                 methane 
                 0.2% 
                 mol 
               
               
                   
                 ethane + other heavier hydrocarbons 
                 0.02% 
                 mol 
               
               
                   
                 aromatic hydrocarbons (benzene, 
                 2103 
                 ppmv 
               
               
                   
                 toluene and xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     This flash gas is then sent at a flow rate of 24.2 kNm 3 /h to second absorption column  25  through line  40  and it is treated with the gases coming from (Claus) section  21  according to the mode of operation described in reference to  FIG. 3 . 
     The amine solution leaves drum  12  through line  13  and it is sent to regeneration column  15 . 
     After regeneration, the amine solution leaves regeneration column  15  through line  31  prior to entering, after flowing through exchanger  9 , absorption columns  2  and  25  through lines  33  and  34 . 
     The acid gas coming from regeneration column  15  through line  20  has the composition given in Table 8. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 63.3% 
                 mol 
               
               
                   
                 carbon dioxide 
                 31.7% 
                 mol 
               
               
                   
                 water 
                 5% 
                 mol 
               
               
                   
                 aromatic hydrocarbons (benzene, 
                 132 
                 ppmv 
               
               
                   
                 toluene and xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     This acid gas is sent through line  20  to (Claus) section  21  at a flow rate of 16.2 kNm 3 /h. This acid gas, that eventually contains only 132 ppmv aromatic hydrocarbons, versus 1743 ppmv in the acid gas of Example 1 according to the prior art, and that comprises 63.3% mol hydrogen sulfide, versus only 51.4% mol in the acid gas of Example 1 according to the prior art, is preheated to 220° C. like the air injected. The temperature reached in the oven is higher than that of Example 1 and therefore sufficient to reduce the residual aromatic hydrocarbon content at the oven outlet to less than 25 ppmv. The Claus catalyst of the first catalytic stage will therefore have a longer life and activity. 
     EXAMPLE 5 
     According to the Invention 
     Example 5 illustrates the mode of operation of the method described in reference to  FIG. 3   b , with recycling of a fraction of the acid gas from the outlet of regenerator  15  through line  42  to drum  12 . 
     The same natural gas with the same composition as in Example 1 (see Table 1) is treated in absorption column  2  under the same operating conditions as in Example 1. 
     After a first expansion in flash drum  6 , the amine solution leaves flash drum  6  through line  8 , flows through exchanger  9 , and is subjected to a second expansion in drum  12  at 2.0 bar and 100.1° C. Expansion, that allows part of the CO 2  and the major part of the aromatic hydrocarbons to be removed, releases a flash gas that leaves the drum through line  40 . This flash is favoured by recycling of 20% of the acid gas stream leaving regenerator  15  through line  42  at 2.35 bar and 114.1° C. and at a flow rate of 9.9 kNm 3 /h. The gas recycled through line  42  has the composition described in Table 9. The flash gas leaving drum  12  through line  40  has the composition described in Table 10. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 9 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 22.1% 
                 mol 
               
               
                   
                 carbon dioxide 
                 8.1% 
                 mol 
               
               
                   
                 water 
                 69.8% 
                 mol 
               
               
                   
                 methane 
                 0.0% 
                 mol 
               
               
                   
                 ethane + other heavier hydrocarbons 
                 0.0% 
                 mol 
               
               
                   
                 aromatic hydrocarbons (benzene, 
                 5 
                 ppmv 
               
               
                   
                 toluene and xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 10 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 22.5% 
                 mol 
               
               
                   
                 carbon dioxide 
                 32.6% 
                 mol 
               
               
                   
                 water 
                 44.5% 
                 mol 
               
               
                   
                 methane 
                 0.2% 
                 mol 
               
               
                   
                 ethane + other heavier hydrocarbons 
                 0.02% 
                 mol 
               
               
                   
                 aromatic hydrocarbons (benzene, 
                 1738 
                 ppmv 
               
               
                   
                 toluene and xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     This flash gas is then sent at a flow rate of 30.4 kNm 3 /h to second absorption column  25  through line  40  and it is treated with the gases coming from (Claus) section  21  according to the mode of operation described in reference to  FIG. 3   b.    
     The amine solution leaves drum  12  through line  13  and it is sent to regeneration column  15 . 
     After regeneration, the amine solution leaves regeneration column  15  through line  31  prior to entering, after flowing through exchanger  9 , absorption columns  2  and  25  through lines  33  and  34 . 
     The acid gas coming from regeneration column  15  through line  20  has the composition given in Table 11. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 11 
               
               
                   
                   
               
               
                   
                 Constituents 
                 Concentration 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 hydrogen sulfide 
                 69.5% 
                 mol 
               
               
                   
                 carbon dioxide 
                 25.5% 
                 mol 
               
               
                   
                 water 
                 5% 
                 mol 
               
               
                   
                 aromatic hydrocarbons (benzene, 
                 15 
                 ppmv 
               
               
                   
                 toluene and xylenes) 
               
               
                   
                   
               
             
          
         
       
     
     This acid gas is sent through line  20  to (Claus) section  21  at a flow rate of 12.6 kNm 3 /h. This acid gas, that eventually contains only 15 ppmv aromatic hydrocarbons, versus 132 ppmv in the acid gas of Example 4 according to the invention, and that comprises 69.5% mol hydrogen sulfide, versus 63.3% mol in the acid gas of Example 4 according to the invention, is preheated to 220° C. like the air injected. As a result of the H 2 S enrichment, the temperature reached in the oven is higher than that of Example 4 and it therefore allows to improve the reduction in the residual aromatic hydrocarbon content at the oven outlet. The Claus catalyst of the first catalytic stage will therefore have a longer life and activity. Furthermore, the flow rate of the gas to be treated is decreased from 16.2 to 12.6 kNm 3 /h and therefore leads to a 22% reduction in the size of the Claus plant and in the amount of catalyst required in this plant.