FR2923838A1 - Purifying a natural gas by reducing carbon dioxide, hydrogen sulfide and water present in the gas, adsorbing light mercaptans and contacting with an adsorbent material, and mixing a gaseous effluent with a hydrocarbon displacement agent - Google Patents

Abstract

The process for the purification of natural gas by reducing carbon dioxide, hydrogen sulfide and water present in the gas, comprises adsorbing light mercaptans and contacting with an adsorbent material at a pressure of 2-10 MPa and at -40[deg] C to 100[deg] C, mixing a gaseous effluent with a hydrocarbon displacement agent constituted of a liquid phase having five carbon atoms to enrich the gaseous effluent, passing a charge of mercaptans and the obtained mixture through the adsorbent material, and carrying out a thermal desorption using a purge gas to desorb the displacement agent. The process for the purification of natural gas by reducing carbon dioxide, hydrogen sulfide and water present in the gas, comprises adsorbing light mercaptans and contacting with an adsorbent material at a pressure of 2-10 MPa and at -40[deg] C to 100[deg] C, mixing a gaseous effluent with a hydrocarbon displacement agent constituted of a liquid phase having five carbon atoms to enrich the gaseous effluent, passing a charge of mercaptans and the obtained mixture through the adsorbent material, carrying out a thermal desorption using a purge gas to desorb the displacement agent at a pressure of 0.5-10 MPa and a temperature of 50-400[deg] C, and cooling the adsorbent material at a pressure of 0.5-10 MPa using another gaseous effluent or directly using a purifier gas. The quantity of displacement agent introduced in the adsorbent material is = 50 mass% of maximum capacity for the adsorption of the displacement agent on the adsorbent material. The gaseous effluent is constituted of a purified natural gas or any other gas, and is mixed with the hydrocarbon displacement agent at a pressure of 0.5-10 MPa and a temperature of 0-150[deg] C. The mercaptan gas is washed with the adsorbent solution, and then it is recycled. The hydrocarbon is a saturated hydrocarbon, aromatic hydrocarbon, paraffin and naphthene.

Description

The present invention relates to the purification of a natural gas. More particularly, the present invention provides a method of purification by adsorption of a natural gas to decrease the mercaptan content.

A raw natural gas containing water, heavy hydrocarbons, acidic compounds such as carbon dioxide (CO2) and hydrogen sulfide (H2S), and sulfur derivatives such as mercaptans, can be processed by processes described by the documents FR 2 605 241 and FR 2 636 857. These processes use a physical solvent such as methanol to carry out the dehydration, degassing and removal of the acidic compounds and mercaptans. At the end of this treatment, the gas is to specifications as to the content of CO2, typically less than 2 mol%, and H2S, typically 4 molar ppm. Another gas treatment solution is to perform the deacidification by a method using an amine solvent. Part of the light mercaptans, especially methyl mercaptan, is removed during this step. Heavier mercaptans, such as ethyl, propyl and butyl mercaptan, are not sufficiently acidic to react significantly with amines and, therefore, remain largely in the gas. Then, the gas is dehydrated by a process using a solvent such as glycol, for example the process described in FR 2,740,468. Dehydration enables the water content of the gas to be lowered to a value close to 60 molar ppm. . The aforementioned processes make it possible to obtain a natural gas whose water content, acid compounds and heavy hydrocarbons of the treated natural gas are in accordance with commercial requirements. However, the methyl and ethyl mercaptans still predominantly remain in the gas at levels up to 200 ppm, or more, in sulfur equivalents. For some uses these levels of mercaptans are too high. To reduce these mercaptan levels, it is possible to use a method of adsorptive removal of the mercaptans. Conventional gas phase adsorption processes are those commonly known as T.S.A. ("Thermal Swing Adsorption" in English), in which the adsorption step takes place at ambient or moderate temperature, typically between 20 ° C. and 60 ° C., and the desorption (or regeneration) stage at a high temperature. temperature, typically between 200 ° C and 350 ° C, under a flushing gas purge (generally a portion of the purified gas) whose flow rate is between 5% and 20% of the feed gas flow. The desorption gas, containing a large quantity of mercaptans, must then be treated before being recycled, for example by treatment with a basic solution (sodium hydroxide or potassium hydroxide), or may also be sent to the flare, which is not neither economically nor ecologically very interesting. The pressure is either kept substantially constant throughout the cycle, or lowered during the regeneration phase so as to promote regeneration. At the outlet of this adsorption purification step, the water content of the gas is less than 1 molar ppm, and the gas is at the total sulfur specifications. However, adsorption of mercaptans by T.S.A. conventional, used in industry and in particular for purifying a natural gas, has several disadvantages.

In particular, mention may be made of: long cycle times, generally rarely less than 4 hours, more often between 8 and 12 hours, sometimes more advantageously, due to the thermal inertia of the adsorbent material, immobilization of the adsorbent material because of the long cycle times, only the material transfer zone, constituted by the adsorption fronts of the various compounds in the adsorber, is actually used, both in adsorption and regeneration, need to heat at high temperature this leads to premature aging of the adsorbent material, in particular during the desorption of thermally fragile products (under the effect of the thermal treatments regularly imposed during the regeneration phase of the adsorbent material, the mercaptans can form reactive compounds, such as, for example, olefins, and react with the co-adsorbed hydrocarbons to eventually lead to premature aging ature of the adsorbent material, which may require its frequent renewal, and therefore lead to additional cost), use of a large amount of purge gas, generally between 5% and 20% of the flow of gas to be treated, need to treat the gas purge containing the desorbed products and recycle the purge gas.

Patent FR 2 861 403 discloses an original process for regenerating an adsorbent loaded with light mercaptans by using a hydrocarbon-type displacement agent consisting of a mixture of compounds of at least five dilute carbon atoms. in a purge gas. This displacement phase can last as long as the mercaptan concentration at the outlet of the adsorber is non-zero, the duration being fixed by the desired level of regeneration. After this phase of regeneration of the adsorbent by desorption of the mercaptans by displacement, generally carried out under mild conditions, in particular at a temperature close to ambient temperature, the adsorbent material contains the displacement agent in place of the mercaptans, at a content of corresponding approximately to the saturation of the adsorbent, ie about 20% by weight. According to this document, at the end of this regeneration step, the adsorbent material may be: once again engaged in a phase of adsorption of the mercaptans of the natural gas, the latter then in turn displacing the adsorbed displacement agent either undergo a complementary thermal regeneration step to desorb the displacement agent prior to re-use in a new adsorption phase to adsorb the light mercaptans of the natural gas. This mode of desorption of the mercaptans by displacement under mild conditions is effective, but has at least two disadvantages: if, at the end of the desorption stage of the mercaptans, the displacement agent is not desorbed thermally, at least partially, the subsequent adsorption of the mercaptans is made more difficult, on the one hand because the adsorbent is saturated with the displacement agent, and on the other hand because the adsorption selectivity is misplaced in favor of the displacement agent rather than for the mercaptans if the displacement agent is desorbed, at least partially, thermally, a complementary step is introduced in the complete cycle, thereby lengthening the total duration and immobilizing an additional amount of adsorbent.

It is therefore an object of the present invention to overcome one or more of the disadvantages of the prior art by providing a process for purifying a natural gas by adsorption of the mercaptans, with an original mode of regeneration of the adsorbent material using a introduced hydrocarbon type diluted in a purge gas, in the form of "pulse", that is to say for a limited time, less than that which would be necessary to desorb all mercaptans adsorbed, associated with a subsequent thermal regeneration with a purge gas containing no displacement agent.

For this, the purification process of a natural gas comprises the following phases: a) said gas is purified by adsorption of light mercaptans by contact with an adsorbent material, b) a gaseous effluent is mixed with a hydrocarbon displacement agent, consisting of a hydrocarbon liquid phase comprising more than five carbon atoms, so as to enrich the gaseous effluent with hydrocarbons, c) the mixture obtained in step is passed through the adsorbent material, loaded with mercaptans. b), the amount of displacement agent introduced into the adsorbent material being less than or equal to 50% by mass of the maximum adsorption capacity of the displacement agent on the adsorbent material, d) thermal desorption is carried out using a purge gas to desorb adsorbent material from the hydrocarbon displacement agent introduced in step c), e) the adsorbent material is cooled with another gaseous effluent or directly using the gas to be purified. Compared to the mode described in particular in patent FR 2 861 403, this invention has the particular advantage of reducing the amount of hydrocarbon-type displacement agent necessary for the displacement of mercaptans and substantially reduces the cycle time compared to the described mode. in the above patent. The advantages of this mode of operation are therefore: to desorbed by displacement the adsorbed mercaptans at a moderate temperature, thus avoiding aging of the adsorbent material which is too fast, these compounds being thermally fragile to limit the amount of hydrocarbon agent necessary for the complete displacement of the mercaptans, because only a limited amount of this agent is necessary, less than that corresponding to the saturation of the adsorbent 10 - to reduce the cycle times, since the thermal regeneration step can begin even before the complete desorption of the mercaptans - to decrease the cycle times, since the cooling step following that of thermal regeneration begins even before the complete desorption of the displacement agent. According to one embodiment of the invention: step a) is carried out under a pressure of between 2 MPa and 10 MPa and a temperature of between -40 ° C. and 100 ° C. in step b), flow rate of said gaseous effluent constitutes between 1 and 50% of the flow rate of said natural gas -in step b), said gaseous effluent may consist of purified natural gas or any other gas; step b) is carried out under a pressure of between 0.5 and 10 MPa and at a temperature of between 0 ° C. and 150 ° C. the displacement phase of step c) is carried out under a pressure of between 0.5 and 10 MPa and at a pressure of temperature between 0 ° C and 150 ° C - the displacement phase of step c) is carried out so that the relative pressure of said displacement agent in said gaseous effluent is less than 0.95 by choosing for example a temperature 2 ° C higher than that of step b), or a pressure 2 bar lower than that of step b), or e also by performing a complementary dilution of the enriched gas in step b) with a purge gas -on carries out the thermal regeneration phase of step d) under a pressure of between 0.5 and 10 MPa and at a temperature of between 50 ° C. and 400 ° C., the cooling step of step e) is carried out at between 0.5 and 10 MPa. In the process according to the invention, the natural gas to be purified may comprise hydrocarbons comprising at least five carbon atoms and, before step a), a fraction of said natural gas may be separated, the fraction comprising hydrocarbons comprising at least five carbon atoms, and said liquid phase of step b) may comprise said fraction. In the process according to the invention, the natural gas to be purified may comprise hydrocarbons comprising at least five carbon atoms, and it is possible to separate a fraction of the purified gas obtained in step a), the fraction comprising hydrocarbons comprising at least five hydrocarbons. carbon atoms, and said liquid phase of step b) may comprise said fraction. According to one embodiment of the invention, the mercaptan-loaded gas obtained in step c) is washed with a mercaptan absorbent solution and the washed gas is then recycled. The adsorbent material may comprise at least one of the following materials: a zeolite, an activated carbon, an activated alumina type mesoporous adsorbent, and a mesoporous silica gel type adsorbent. The adsorbent material may comprise at least one of the following materials: a type A zeolite, a faujasite type X zeolite, a Y type faujasite zeolite, an activated carbon having a BET specific surface area of between 200 m 2 / g and 10 2000 m 2 / g, an activated alumina type mesoporous adsorbent having a BET specific surface area of between 100 m 2 / g and 800 m 2 / g, and a mesoporous adsorbent of silica gel type having a BET specific surface area of between 100 m 2 / g and 800 m 2 /boy Wut.

In the process according to the invention, said hydrocarbons containing more than five carbon atoms may comprise at least one of the following compounds: a saturated hydrocarbon, an aromatic hydrocarbon, a paraffin, and a naphthene. According to one embodiment of the invention before step a), the amounts of CO2, H2S and water contained in the natural gas are reduced. The characteristics and advantages of the invention will appear more clearly on reading the description given below, with reference to the appended figures in which: FIG. 1 represents a first exemplary embodiment of the method according to the invention, FIG. a variant of the process according to the invention. The process, described in connection with FIGS. 1 and 2, comprises pretreatment, optionally dehydration, adsorption, regeneration, fractionation and optionally washing (to be confirmed). By gas containing no or little mercaptans, is meant for example either a portion of the purified gas obtained in step a), or a "boil-off gas" predominantly composed of methane and nitrogen, or any other gas having the same properties. In the following, the terms "purge gas" or gaseous effluents will be used to represent indistinctly these gaseous effluents. 1) Pretreatment The natural gas containing in particular water, CO2, H2S and mercaptans, arriving via line 1, is deacidified and dehydrated in the treatment unit 30. The gas can be a raw natural gas directly from an oil well or a gas field. In unit 30, the gas is processed by methods known to those skilled in the art. For example, the gas is treated by a process using chemical and / or physical solvents, for example based on amines and / or methanol, so as to produce a natural gas with specifications for the content of CO2 and H2S. Such processes are described in particular in documents FR 2 605 241, FR 2 636 857 and FR 2 734 083. The acidic compounds H2S and CO2 are discharged via line 2. Part of the mercaptans, in particular methylmercaptan, is partially removed from the product. gas during this treatment. These mercaptans are also discharged through line 2. The content of H2S is of the order of 4 ppm molar, that of CO2 less than 2 mol%. The deacidified gas may then be treated by a glycol dehydration process, for example described in document FR 2 740 468. The glycol used may be triethylene glycol (TEG). At the outlet of this unit, a dehydrated gas is obtained, whose residual water content may be of the order of 60 molar ppm. This gas still contains mercaptans and heavy hydrocarbons. The water is discharged through line 4. This glycol dehydration step can be replaced by an adsorption dehydration step, for example on molecular sieve 4A, according to a TSA operation, known to those skilled in the art.

The treated gas leaving unit 30 via line 5 is depleted of water and acidic compounds H2S and CO2, but still contains mercaptans, in a content that may be greater than 200 ppm molar equivalent sulfur. 2) Advanced dehydration The dehydrated and deacidified gas can undergo extensive dehydration by adsorption for example on molecular sieves. Referring to Figure 2, the gas flowing in the conduit 5 is introduced into the chamber 36 comprising a dehydrating adsorbent material. The dehydrated gas is discharged through line 37. In this case, it is preferable to choose a water-specific dehydration adsorbent material, such as, for example, a type 3A and / or 4A molecular sieve. The dehydrating adsorbent material is preferably placed in a specific enclosure different from the enclosure 31 used to remove the mercaptans. The regeneration of the dehydrating adsorbent material contained in the chamber 36 is conventionally provided by T.S.A., the purge gas being for example a fraction of the purified gas. The dehydrating adsorbent material may also be placed in the same chamber as that containing the adsorbent material used to remove the mercaptans, that is to say in the chamber 31. 3) Adsorption of mercaptans The dehydrated and deacidified gas is then sent to an adsorption adsorption purification unit on adsorbent material, for example on molecular sieves, in order to remove the mercaptans still present in this gas. This unit comprises at least one chamber 31 containing a suitable adsorbent material allowing especially the adsorption of mercaptans, such as methyl-, ethyl-, propyl-mercaptan, and higher mercaptans. The dehydrated and de-acidified gas flowing in the duct 5 (with reference to FIG. 1), or possibly flowing in the duct 37 (with reference to FIG. 2), is introduced into the enclosure 31.

A first step is thus carried out during which the gas is purified by adsorption of the light mercaptans by contacting with a first quantity of adsorbent material. At the end of this stage the adsorbent material contains in particular light mercaptans of natural gas and hydrocarbon compounds with more than five carbon atoms. This adsorption step is stopped when the concentration of mercaptans in the purified gas at the adsorber outlet reaches a non-zero limit threshold. The concentration profile in the adsorber can then be represented schematically in the following manner: the zone located near the inlet of the adsorber is essentially an equilibrium zone, in which the mercaptans and the heavier hydrocarbon compounds gas, and in particular the aromatic compounds, are coadsorbed, the respective quantities being a function of the temperature and the respective partial pressures of each of the compounds in the gas to be treated, the central zones and close to the outlet of the adsorber corresponding to the material transfer zone, in which the adsorbent material is only partially used, the length of this zone depending among other parameters of the speed of the gas in the adsorber and the adsorption kinetics of the various compounds The purified gas is evacuated from the chamber 31 by the conduit 6. The temperature inside the enclosure 31 is generally between -40 ° C and 100 ° C, advantageously between 0 ° C and 70 ° C, and preferably between 20 ° C and 60 ° C. The pressure of the chamber 31 may be that of the natural gas produced, typically between 2 MPa and 10 MPa. According to the invention, the adsorbent materials contained in the chamber 31 are preferably chosen from molecular sieves, also called zeolites, activated carbons, or mesoporous adsorbents of activated alumina or silica gel type. Among the zeolites, one can choose from zeolites of type A (LTA family), type X or Y (FAU family of faujasites) or of type MFI (ZSM-5 and silicalite), whose pore size is compatible with the size of the mercaptans to adsorb.

Among the zeolites of the family A (LTA), it is possible to choose a partially exchanged calcium zeolite 4A, preferably having a Na / Ca exchange rate of between 25% and 85% molar. Among the zeolites of the X or Y type (faujasites FAU), it is possible to choose a zeolite of the 13X or NaX type, but it is also possible to use other exchange cations, alone or as a mixture, such as for example Ca, Ba, Li , Sr, Mg, Rb, Cs, Cu, Ag ... The silicon to aluminum ratio can be between 1 and infinity, by infinity is meant the dealuminated Y zeolites. Among the MFI-type zeolites, it is possible to choose the ZSM-5 whose ratio Si / Al varies from 1 to infinity (silicalite in the latter case). The other adsorbent materials which may be used may be chosen from activated carbons, and preferably those having a BET specific surface area, conventionally determined by nitrogen physisorption at 77K, of between 200 and 2000 m 2 / g, or of activated aluminas or silica gels, and preferably those having a BET specific surface area of between 100 and 800 m 2 / g. The adsorbent materials are preferably used in a fixed bed, for example in the form of a ball or an extruded material. They can be used either alone or mixed, for example in multi-bed form. 4) Displacement of Mercaptans and Regeneration * Displacement Agent 10 At the end of the first step, a gas is mixed in a subsequent step with a hydrocarbon liquid phase comprising more than five carbon atoms, called a displacement agent. in order to enrich this gas with hydrocarbon compounds, their content in the gas depending in particular on the pressure and temperature conditions in which this mixing is carried out. This mixture will be used to displace the adsorbed mercaptans. By displacement agent is meant one or more hydrocarbon compounds, heavy hydrocarbons (C5 +), paraffins, naphthenes or aromatics. These compounds can be chosen from the hydrocarbon compounds of the C5 + fraction of natural gas, which are saturated and / or aromatic. These compounds may also be benzene, toluene, isomers of xylenes, or aromatic compounds having a ring substituted with one or more methyl and / or ethyl groups. The agent may comprise one of the abovementioned compounds or a mixture of several aforementioned compounds. The displacement agent contains at least one compound that can adsorb to the adsorbent material and has an affinity close to that of the mercaptans. For example, if a zeolite 5A is used, the displacing agent may preferably contain at least one normal paraffin. If a 13X zeolite is used, a mixture of saturated and / or aromatic hydrocarbon compounds can be used. Said aromatic compounds may belong to the BTX family.

The displacement agent can be obtained during the condensation and fractionation stage of the natural gas in the pipe 11 coming from the fractionation unit 34. The displacement agent can also be introduced in part or in full by the Annex line 12.

For example, the displacement agent introduced via the conduit 13 into the contactor 33 is constituted, on the one hand, by the C5 + hydrocarbons, preferably by the C6 + cut, and advantageously by the fraction rich in BTX type aromatic compounds derived from the fractionation unit 34 and, on the other hand, by an addition of heavy hydrocarbon compounds, in particular a C5 + cut, and preferably rich in BTX type aromatic compounds introduced via line 12. In the case where the agent of displacement is chosen from the heavy hydrocarbon compounds present in the natural gas, and in particular the BTX type aromatic compounds, this regeneration route also has the advantage of using a part of the heavy hydrocarbon compounds of the natural gas, and in particular the compounds aromatic, coadsorbed with the mercaptans, as displacement agent, thus limiting the external supply of displacement agent via the purge gas. Contacting Displacement Agent-Adsorbent The mixture containing the displacement agent is brought into contact with a quantity of adsorbent material loaded with mercaptans. The amount of hydrocarbon displacement agent introduced is less than or equal to 50% by weight of the maximum adsorption capacity of the displacement agent on the adsorbent material. This amount may be between 2 and 40% by weight of this maximum capacity, and preferably between 5 and 20% by weight. The duration of contacting therefore depends on the flow rate of the mixture and the concentration of hydrocarbon compounds with more than five carbon atoms in the mixture, and allows the introduction of a desired amount of displacement agent. The displacement of the undesirable compounds adsorbed on the adsorbent, in particular mercaptans, is carried out using a gaseous effluent or a purge gas, for example a part of the purified gas obtained during the mercaptan adsorption step, or any other gas enriched with liquid hydrocarbon compounds. In theory, if the displacement step is carried out at constant displacement agent temperature and partial pressure, the amount of hydrocarbon agent to be introduced to completely displace the adsorbed mercaptans is approximately equal, by mass, to the amount of adsorbed mercaptans. on the adsorbent material, to which must be added the amount necessary to saturate the adsorbent in the material transfer zone. This additional amount of hydrocarbon displacement agent may be important if the material transfer zone in the adsorber is spread, since a portion of the adsorbent material is only partially used in said area. Under these conditions, at the end of this step, the adsorbent material contains the hydrocarbon displacement agent, and the amount adsorbed on said material corresponds to the thermodynamic equilibrium under partial pressure and temperature conditions, generally close to saturation of the material, about 20% mass. The subsequent thermal regeneration step therefore requires a significant amount of energy since the adsorbent material is substantially saturated with displacement agent. Stopping the introduction of the hydrocarbon displacement agent necessary to move the mercaptans corresponds to the moment when the mercaptan concentration at the adsorber outlet is negligible, or deemed sufficiently low to stop this step.

The originality of the process according to the invention stems from the fact that it is possible to use a quantity of hydrocarbon agent that is significantly lower than this theoretical amount, by introducing this hydrocarbon agent for a limited time shorter than the theoretical duration which would lead to to the thermodynamic equilibrium, and therefore saturation of the adsorbent, described above, and then imposing a higher temperature and a zero partial pressure displacement agent in the purge gas, even before the concentration of mercaptans in the gas output is negligible. This quantity can be estimated for example by knowing the microporous volume of the adsorbent, determined by the conventional techniques for characterizing porous solids known to those skilled in the art, such as nitrogen porosimetry at 77 K (use by example of the t-plot method, or the Dubinin method), possibly coupled to mercury porosimetry. Knowing this pore volume, the maximum adsorbable amount of displacement agent can be estimated from the density of said displacement agent. With a faujasite zeolite adsorbent solid, for example X or Y faujasite, such as NaX (13X) or NaY zeolites, this maximum capacity is of the order of 20% by weight of the adsorbent solid. Thermal desorption Thermal desorption is then performed using a purge gas to desorb the hydrocarbon displacement agent introduced at the previous contacting step. The purge gas used contains no or very few mercaptans. The displacement and regeneration phase can be divided into two sequences: During the first sequence, the hydrocarbon displacement agent is introduced into the gaseous effluent or the purge gas, at the inlet of the adsorber, and the adsorbed mercaptans. in this zone will be progressively moved from the inlet of the adsorber to a more central zone located downstream in the adsorber. The zone located at the inlet of the adsorber will therefore contain the displacement agent. During the second sequence, corresponding to a thermal regeneration step, a hot purge gas, for example a part of the purified gas or a gas containing no mercaptans and not enriched with displacement agent, is introduced as input of the adsorber. The action of this hot gas is to thermally desorb the displacement agent adsorbed in the adsorber inlet zone, and thus to regenerate the adsorbent material in said zone. The displacement agent thus desorbed migrates into the adsorber and in turn displaces the mercaptans adsorbed in the central zone of the adsorber at room or moderate temperature, the temperature of the adsorbent material in this central zone being between ambient temperature and the temperature of the hot gas. Thus, at the same time: - the thermal regeneration of the adsorbent material located from the inlet of the adsorber towards the central zone, by thermal desorption of the hydrocarbon displacement agent under the action of the hot purge gas, the material adsorbent then being at high temperature adsorbing the hydrocarbon displacement agent previously desorbed, together with the displacement of the mercaptans in the central zone of the adsorber, at a temperature between room temperature and the temperature of the hot gas inlet the migration and adsorption of mercaptans from the central zone of the adsorber to the zone situated further downstream towards the outlet of the adsorber, which initially constituted the material transfer zone. The high temperature thermal front thus progressively advances in the adsorber, preceded by a material transfer front, at a moderate temperature, corresponding to the adsorption of the displacement agent and the displacement of the mercaptans. * Cooling The adsorbent is cooled using a gas containing no mercaptans or directly using the gas to be purified. Still according to the invention, it is possible to carry out the step of cooling the adsorbent, prior to a new adsorption cycle, in order to purify the gas loaded with mercaptans, even before the concentration of displacement agent at the outlet of the adsorbent. adsorber is zero or considered negligible, using either a purge gas containing no mercaptans, or directly the gas to be purified. In this way, the adsorber inlet cold gas will heat up by exchange with the hot adsorbent solid located in the adsorber inlet, and the calories thus transported will be used to complete the thermal desorption of the displacement agent in a zone. the adsorber located near the exit. If the gas used for this step of cooling the adsorbent is the gas to be purified, the mercaptans present in this gas will then be adsorbed directly on the solid located at the inlet of the adsorber. * Operating Conditions The pressure and temperature conditions of the mixing step between the gaseous effluent or the purge gas and the liquid displacement agent will be such that the relative pressure of said displacement agent in the purge gas is less than at 0.95 under the conditions of the displacement so as to avoid any liquid condensation of said displacement agent, especially in the mesopores of the adsorbent, and preferably less than 0.80. If the displacement agent is a mixture of liquid hydrocarbon compounds with more than five carbon atoms, the pressure and temperature conditions of the contacting step will be chosen so as to be located outside the condensation zone. of liquid of the phase envelope of the mixture constituted by the purge gas enriched with displacement agent, at the temperature and at the displacement pressure, so as to avoid any phenomenon of condensation of liquid, in particular in the pores of the adsorbent.

One way to avoid any risk of condensation of liquid may consist, for example: in mixing at a temperature a few degrees lower than the displacement temperature, if the pressure of this step does not differ more than 2 bar from the contacting step, and preferably at a temperature of 5 ° C below the contacting temperature, mixing at a pressure a few bars higher than the displacement pressure, if the temperature of these steps do not differ no more than 5 ° C., and preferably at a pressure greater than 5 bar at the pressure of the step placed in contact, - to carry out a complementary dilution of the purge gas enriched with displacement agent by purge gas, in particular if the pressures and temperatures of the mixing step and the moving step are similar and do not differ more than 2 bar and 5 ° C. In this case, the dilution purge gas flow of the enriched gas leaving the contactor will be in particular between 2 and 100% of the total flow, and preferably between 5 and 50%, and preferably between 10 and 30% by volume. The operating conditions of the process according to the invention can be as follows: the adsorption step is carried out under a pressure of between 2 MPa and 10 MPa and a temperature of between -40 ° C. and 100 ° C. at the stage mixing, the flow rate of said purge gas constitutes between 1 and 50% of the flow rate of said natural gas in the mixing step, said purge gas may consist of purified natural gas or any other gas such as for example a " Nitrogen-rich gas-beverage - the mixing step is carried out under a pressure of between 0.5 and 10 MPa and at a temperature between 0 ° C. and 150 ° C. the displacement phase is carried out under a pressure between 0.5 and 10 MPa and at a temperature of between 0 ° C. and 150 ° C., the displacement phase is carried out so that the relative pressure of said displacement agent in said purge gas is less than 0.95 and preferentially less than 0.80 by choosing for example a Temperature 2 ° C higher than that of the mixing step, or a lower pressure of 2 bar than that of the mixing step, or by performing a complementary dilution of the purge gas enriched in the mixing step by a purge gas - the thermal regeneration phase of the thermal desorption step is carried out under a pressure of between 0.5 and 10 MPa and at a temperature of between 50 ° C. and 400 ° C., preferably between 100 ° C. At 300 ° C., the cooling step was carried out under a pressure of between 0.5 and 10 MPa. According to the invention, part of the methane, or of the light C1-C2 fraction, constituting the purified gas coming from the enclosure 31 is used as a purge gas. The flow rate of purge gas can be between 1% and 50% of the flow rate of the feed gas arriving via line 1, for example between 1% and 20%, advantageously between 1 and 10%, and preferably between 1% and % and 5%. For example, part of the gas directly from the enclosure 31 and / or part of the light fraction of the gas (section C1 and / or C2) coming from the fractionation unit 34 are respectively sent through the ducts 15 and 14 into the contactor 33 gas / liquid to be charged with the moving agent.

The choice of the pressure and temperature conditions in the gas / liquid contactor 33 for charging the purge gas with displacement agent are preferably chosen in such a way that the relative pressure of the displacement agent in the contactor 33 is less than 0.95, preferably less than 0.8 so as to limit the phenomenon of capillary condensation in the mesopores of the adsorbent. The pressure in the contactor 33 may be between 0.5 MPa and 10 MPa, preferably between 2 MPa and 8 MPa, preferably between 3 MPa and 7 MPa. The temperature in the contactor 33 may be between 0 ° C and 150 ° C, and preferably between 20 ° C and 100 ° C, and preferably between 30 ° C and 80 ° C. The gas from the contactor 33, charged with the heavy hydrocarbon fraction, under the pressure, temperature and flow conditions of the contactor 33, thus constitutes the purge gas or gaseous effluent, and is sent via the conduit 18 into the enclosure 31 In the case where the adsorber contains only one or more adsorbent solids used to remove the light mercaptans from the natural gas, the adsorption step will preferably be carried out at the same time as the following steps, so as to use the compounds hydrocarbons from natural gas coadsorbed with the mercaptans at the inlet of the adsorbent bed as displacement agent. In the case where the adsorber contains both an adsorbent used to dehydrate the natural gas, such as for example an activated alumina or a molecular sieve, in particular of type 3A or 4A, and an adsorbent agent used to remove the mercaptans As for example an X-type zeolite (13X) or Y, the adsorbent solid used to carry out the dehydration of the gas will be placed upstream of the adsorbent solid used to remove the mercaptans. In this case, the adsorption step will preferably be carried out counter-current of the following steps so as to avoid the desorption of water from the adsorbent solid used to dehydrate the gas, towards the adsorbent solid used to remove the downstream mercaptans. . This mode of operation makes it possible in particular to protect the solid adsorbent used to remove the mercaptans from the water which would be produced, during the thermal regeneration phase, from the adsorbent solid used to carry out the dehydration, and which could damage said adsorbent solid used to remove the mercaptans. 5) Fractionation The purified gas is then sent to the fractionation unit 34 to separate the different sections, for example by distillation. At the outlet of the unit 34, methane (line 7), ethane (line 8), propane (line 9), butane (line 10) and a heavy hydrocarbon fraction containing more of five carbon atoms (line 11). This step is optional and is only interesting in the case of a rich gas. 6) Treatment of purge gas loaded with mercaptans The gas from the enclosure 31 through line 19 is loaded with mercaptans. This gas is treated in the washing unit 35 in order to eliminate the mercaptans as much as possible, for example by washing with an alkaline solution of sodium hydroxide or potassium hydroxide. The mercaptans are evacuated via line 21. Then, the washed gas is returned via line 20 with the raw natural gas to be processed. The washed gas may not undergo the deacidification treatment and, optionally, the dehydration treatment operated in the treatment unit 30. Thus, the washed gas is mixed with either the raw gas flowing in the pipe 1, or with the gas deacidified crude obtained in unit 30, either with the deacidified and dehydrated raw gas circulating in line 5.

It should be obvious to those skilled in the art that the present invention should not be limited to the details given above and allow embodiments in many other specific forms without departing from the scope of the invention. . Therefore, the present embodiments should be considered by way of illustration, and may be modified without departing from the scope defined by the appended claims.

Claims (9)

1) Process for purifying a natural gas comprising the following steps: a) said gas is purified by adsorption of light mercaptans by contact with an adsorbent material, b) a gaseous effluent is mixed with a hydrocarbon displacement agent, constituted by a hydrocarbon liquid phase comprising more than five carbon atoms, so as to enrich the gaseous effluent with hydrocarbons, c) the mercaptan-loaded adsorbent material is passed through the mixture obtained in step b) the quantity of displacement agent introduced into the adsorbent material being less than or equal to 50% by mass of the maximum adsorption capacity of the displacement agent on the adsorbent material, d) thermal desorption is carried out using a purge gas for desorbing, the adsorbent material, the hydrocarbon displacement agent introduced in step c), e) the adsorbent material is cooled using another gaseous effluent o u directly using the gas to be purified. 2) Process according to claim 1, wherein: step a) is carried out under a pressure of between 2 MPa and 10 MPa and a temperature of between -40 ° C. and 100 ° C. in step b), the flow rate said gaseous effluent constitutes between 1 and 50% of the flow rate of said natural gas in step b), said gaseous effluent may consist of purified natural gas or any other gas; step b) is carried out under a pressure of between 0 , 5 and 10 MPa and at a temperature between 0 ° C and 150 ° C is carried out the displacement phase of step c) under a pressure between 0.5 and 10 MPa and at a temperature between 0 ° C and 150 ° C the displacement phase of step c) is carried out so that the relative pressure of said displacement agent in said gaseous effluent is less than 0.95 by choosing for example a temperature 2 ° C higher than that of step b), or a pressure 2 bars lower than that of step b), or again feeding a complementary dilution of the gaseous effluent enriched in step b) with a purge gas, the thermal regeneration phase of step d) is carried out under a pressure of between 0.5 and 10 MPa and at a temperature of between 50 ° C. and 400 ° C., the cooling step of step e) is carried out at between 0.5 and 10 MPa. 20 3) Method according to one of claims 1 to 2, wherein the natural gas comprises hydrocarbons having at least five carbon atoms and wherein before step a), a fraction of said natural gas is separated, the fraction containing hydrocarbons having at least five carbon atoms, and wherein said liquid phase of step b) comprises said fraction. 25 4) Process according to one of claims 1 to 2, wherein a natural gas comprising hydrocarbons comprising at least five carbon atoms and in which a fraction of the purified gas obtained in step a) is purified, the fraction comprising hydrocarbons having at least five carbon atoms, and wherein said liquid phase of step b) comprises said fraction. 5) Process according to one of claims 1 to 4, wherein the mercaptan-loaded gas obtained in step c) is washed with a mercaptan absorbent solution and the washed gas is then recycled. 6) Process according to one of claims 1 to 5, wherein the adsorbent material comprises at least one of the following materials: a zeolite, an activated carbon, a mesoporous adsorbent activated alumina type, and a mesoporous adsorbent silica gel type . 7) Method according to one of claims 1 to 6, wherein the adsorbent material comprises at least one of the following materials: a type A zeolite, a faujasite X type zeolite, a faujasite Y type zeolite, an activated carbon having a BET specific surface area of between 200 m 2 / g and 2000 m 2 / g, an activated alumina type mesoporous adsorbent having a BET specific surface area of between 100 m 2 / g and 800 m 2 / g, and a mesoporous adsorbent of silica gel type possessing a BET specific surface area of between 100 m 2 / g and 800 m 2 / g. 8) Process according to one of claims 1 to 7, wherein said hydrocarbons containing more than five carbon atoms comprise at least one of the following compounds: a saturated hydrocarbon, an aromatic hydrocarbon, a paraffin, and a naphthene. 9) Method according to one of claims 1 to 8, wherein before step a) reduces the amounts of CO2, H2S and water contained in the natural gas.