WO2006106226A1

WO2006106226A1 - Method for purifying natural gas by mercaptan adsorption

Info

Publication number: WO2006106226A1
Application number: PCT/FR2006/000744
Authority: WO
Inventors: Michel Thomas; Eszter Toth; Fabrice Lecomte; Jean-Louis Ambrosino
Original assignee: Institut Francais Du Petrole
Priority date: 2005-04-07
Filing date: 2006-03-31
Publication date: 2006-10-12
Also published as: FR2884154A1; FR2884154B1

Abstract

A raw natural gas is dioxide in a units (DA, DH). The processed gas is subsequently purified by adsorbing mercaptans in the first chamber (AI). A part of the purified gas is heated in an EI, afterwards is introduced into a second chamber A2 in such a way that a water adsorbed by an adsorption material contained therein is removed. A stream rich in heavy hydrocarbon vapours is introduced into a third chamber (A3) containing a mercaptan-charged adsorption material. In the A3, the mercaptans are desorbed and substituted by the heavy hydrocarbons.

Description

PROCESS FOR PURIFYING NATURAL GAS BY ADSORPTING MERCAPTANS

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 contains in particular water, light hydrocarbons such as methane, ethane and propane, heavy hydrocarbons, acidic compounds such as carbon dioxide (CO ₂ ) and hydrogen sulphide (H ₂ S), and sulfur derivatives such as mercaptans. This raw natural gas must be processed to meet the various specifications required, including specifications for acid gas content, total sulfur content, and water and hydrocarbon dew point.

The raw natural gas can be treated by the 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 a part of the mercaptans. At the end of this treatment, the gas is to specification as to the content of GO ₂ , typically less than 2 mol%, and H ₂ S, typically less than 4 molar ppm.

Another gas treatment solution is to perform the deacidification by a method using an amine solvent. At the end of this treatment, the gas is to the specifications as to the content of CO ₂ , typically less than 2 mol%, and to H ₂ S, typically less than 4 molar ppm, ie a total sulfur content of the order of 6 mg S / Nm ³ . Some of the Light mercaptans, including methyl mercaptan, are removed during this step. On the other hand, the heavier mercaptans, such as ethyl, propyl and butyl mercaptan, are not sufficiently acidic to react significantly with the amines and therefore remain largely in the gas. In some cases, the mercaptan content can reach 500 ppm molar.

Then, the gas is dehydrated, for example, by a process using a solvent such as glycol, for example the process described in document FR 2 740 468. Dehydration makes it possible to lower the water content of the gas to a similar value. 60 ppm molar. In addition, an adsorption method of TSA type ("Thermal Swing

Adsorption ") on molecular sieve, for example of type 3 or 4A or 13X, or on alumina or silica gel, can be used. In this case, the water content in the gas is typically less than 1 molar ppm.

The aforementioned methods make it possible to obtain a natural gas whose contents of water, acid compounds and heavy hydrocarbons comply with commercial requirements. However, the methyl and ethyl mercaptans still predominantly remain in the gas at levels up to 200 ppm molar, or more, in sulfur equivalent. For some uses these levels of mercaptans are too high.

An object of the present invention is to provide a method for purifying natural gas, so as to obtain a molar content of mercaptans of less than 20 ppm.

It is possible to use a method of adsorptive removal of mercaptans. The conventional methods of adsorption in the gas phase are the methods commonly called TSA ("Thermal Swing Adsorption" in English), in which the adsorption step takes place at room temperature or moderate temperature typically between 20 ⁰ C and 60 ⁰ C, and the high temperature desorption (or regeneration) stage typically between 200 ⁰ C and 350 ⁰ C, under purge gas sweep (generally a part purified gas containing methane and / or ethane) whose flow rate is also 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 bringing it into contact with a basic solution (sodium hydroxide or potassium hydroxide), or it can also be sent to the torch, which is 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: the need to heat at high temperature, which causes premature aging of the adsorbent material, in particular during the desorption of thermally fragile products (under the effect of heat treatments imposed regularly during the regeneration phase of the adsorbent material the mercaptans can form reactive compounds and react with the co-adsorbed hydrocarbons to finally lead to premature aging of the adsorbent material, which can impose its frequent renewal, and therefore lead to additional cost),

use of a large quantity of purge gas, generally between 5% and 20% of the gas flow rate to be treated, the need to treat the purge gas containing the desorbed products and to recycle the purge gas.

The present invention provides a method for purifying a natural gas by adsorption of mercaptans, avoiding the disadvantages of the processes of the prior art.

The object of the present invention is, therefore, to provide an adsorption process for the extensive removal of mercaptans from a natural gas. The method proposes an adsorption step on a porous solid, under pressure and at ambient or moderate temperature, a step of desorbing the mercaptans adsorbed by a displacement agent, at ambient temperature, moderate or high and under low pressure, the agent of displacement consisting entirely or partly of heavy hydrocarbons in vapor form. Optionally, the method comprises a step of desorption and / or purge of the displacement agent, at ambient temperature, moderate or high, and under purge of purge gas under low pressure.

In general, the present invention relates to a process for purifying a natural gas containing mercaptans. The process comprises the following steps: a) contacting the natural gas with an adsorbent material so as to obtain a purified gas, the mercaptans being adsorbed by the adsorbent material, then b) contacting the adsorbent material loaded with mercaptans obtained in step a) with a flow comprising at least 80% by volume of heavy hydrocarbon vapor comprising at least five carbon atoms so as to obtain a mercaptan-loaded stream and an adsorbent material saturated with heavy hydrocarbons. According to the invention, step a) can be carried out under a pressure of between 10 bar and 100 bar and at a temperature of between 0 ^° C. and 150 ° C., and step b) can be carried out under a pressure comprised between between 1 bar and 40 bar and at a temperature between 0 ⁰ C and 300 ⁰ C.

In the process according to the invention, the following step can be carried out: c) the heavy hydrocarbon-saturated adsorbent material obtained in step b) is brought into contact with a fraction of the purified gas obtained in step a) of in order to desorb the heavy hydrocarbons from the adsorbent material and to obtain a gas loaded with heavy hydrocarbons.

Step c) can be carried out under a pressure of between 1 bar and 40 bar and at a temperature of between 0 ^° C. and 300 ^° C.

In addition, the following steps can be performed: d) the natural gas is brought into contact with a second adsorbent material so as to adsorb the water contained in the natural gas, then e) the second adsorbent material is brought into contact with a fraction of the purified gas obtained in step a) to desorb the water contained in the second adsorbent material.

Step d) can be carried out under a pressure of between 10 bar and 100 bar and at a temperature of between 0 ^° C. and 100 ^° C., and step e) can be carried out under a pressure of between 10 bar and 100 ^° C. bar and at a temperature between 100 ⁰ C and 400 ⁰ C.

According to the invention, in step b) said stream may comprise at least a portion of the purified gas obtained after contacting in step c).

The heavy hydrocarbon vapor contained in the stream obtained in step b) can be condensed and the mercaptans can be separated from the hydrocarbons. condensed. The non-condensed gas may be recycled in at least one of the following ways:

The uncondensed gas is mixed with the natural gas before step a),

The uncondensed gas is mixed with heavy hydrocarbon vapor to form at least a portion of said stream.

The purified gas obtained in step c) can be cooled to condense a portion of the water and / or hydrocarbons and the cooled gas can be recycled by mixing the cooled gas with the natural gas prior to step a).

Before step a), said natural gas can be deacidified by absorption of the acidic compounds by an absorbent solution.

According to the invention, the heavy hydrocarbons may be chosen from the group of aromatic hydrocarbons comprising between 7 and 10 carbon atoms, the group being constituted by toluene, the isomers of xylenes, ethylbenzene, mesytilene and paradiethylbenzene. According to the invention, the adsorbent materials can comprise at least one of the following materials: an activated carbon, a zeolite, a mesoporous adsorbent of activated alumina type, and a mesoporous adsorbent of silica gel type. More specifically, the adsorbent materials can comprise at least one of the following materials: an active carbon having a specific surface area of between 500 and 2500 m 2 / g, a type A zeolite, a type A zeolite, a faujasite type zeolite X a faujasite Y type zeolite, a zeolite of the MFI family, an activated alumina type mesoporous adsorbent having a BET specific surface area of between 150 m 2 / g and 800 m 2 / g, and a mesoporous silica gel type adsorbent having a BET specific surface area of between 150 m2 / g and 800 m2 / g.

The present invention makes it possible to avoid or limit losses in purified natural gas. In addition, the step of displacement by heavy hydrocarbons mercaptans, makes it easy to recover the mercaptans by simple liquid / liquid separation. In addition, losses of heavy hydrocarbons are minimized.

Other features and advantages of the invention will be better understood and will become clear from reading the description given below by way of example with reference to the drawings among which:

FIG. 1 describes the process according to the invention,

FIG. 2 represents a variant of the method according to the invention.

In FIG. 1, the natural gas to be treated arrives via line 1 and contains in particular water, CO ₂ , H ₂ S and mercaptans. The gas can be a raw natural gas directly from an oil well or a gas field. The gas may be at a pressure of between 10 bar and 100 bar.

The gas flowing in the duct 1 can be introduced into the DA deacidification unit. The gas is deacidified 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 CO ₂ content and in H ₂ S. Such processes are described in particular in documents FR 2 605 241, FR 2 636 857 and FR 2 734 083. The acidic compounds H ₂ S and CO ₂ are discharged via line 2. A portion of the mercaptans, especially methylmercaptan, is partially removed from the gas during this treatment. These mercaptans are also discharged through line 2. The gas discharged via line 3 of unit DA has a content of H ₂ S, for example of the order of 4 ppm molar and a lower CO ₂ content, for example at 2 mol%. Depending on the temperature and pressure conditions, the water content of this gas is generally between 1000 ppm and 5000 ppm molar.

The deacidified gas flowing in the duct 3 can then be sent to the dehydration unit DH. The gas is treated by a process of dehydration, for example with a glycol solution. For example, the dehydration process is that described in document FR 2 740 468. The glycol used may be triethylene glycol (TEG). At the outlet of this DH unit, a dehydrated gas is obtained, whose residual water content can be of the order of 60 molar ppm. This gas still contains mercaptans and heavy hydrocarbons. The water is discharged through line 20.

The treated gas leaving the DH unit via line 4 is depleted of water and acidic compounds H ₂ S and CO ₂ , but still contains mercaptans, in a content that may be greater than 200 ppm molar equivalent sulfur. The gas flowing in the duct 3 can directly be transferred into the duct 4.

The dehydrated and deacidified gas is then sent to an adsorption adsorption purification unit, for example on molecular sieves, in order to remove the mercaptans still present in this gas. This unit comprises at least two Al and A3 enclosures, and possibly a third chamber A2, containing a suitable adsorbent material allowing in particular the adsorption of mercaptans, such as methyl-, ethyl-, propyl-mercaptan, and higher mercaptans. . The adsorbent material contained in the enclosures A1 and A3 operates successively in adsorption, displacement and optionally regeneration mode:

in the adsorption mode, the adsorbent material adsorbs the mercaptans contained in the gas, and optionally water and heavy hydrocarbons, at ambient or moderate temperature and under high pressure, and then, in displacement mode, the mercaptans are displaced; adsorbed in adsorption mode; the mercaptans are displaced by a heavy hydrocarbon vapor, that is to say desorbed and replaced by heavy hydrocarbons, the displacement being carried out by scanning a heavy hydrocarbon vapor or a gas rich in hydrocarbons; heavy hydrocarbon vapor, at moderate or high temperature and under moderate or low pressure, then in regeneration mode, after the displacement, the desorption and purge of the heavy hydrocarbons adsorbed by the adsorbent material can be carried out by scanning a dry gas at high temperature,

then the cycle is restarted in adsorption mode of the mercaptans.

In FIG. 1, the adsorbent material contained in the enclosure Al operates in adsorption mode, the material contained in the enclosure A2 operates in regeneration mode and the material contained in the enclosure A3 operates in displacement mode.

The dehydrated and deacidified gas flowing in the duct 4 is introduced into the chamber Al. The mercaptans contained in the gas are adsorbed by the adsorbent material contained in the enclosure Al. The mercaptans, traces of water and possibly heavy hydrocarbons in particular the C ₆ ⁺ , are adsorbed by the adsorbent material in the Al chamber. At the outlet of the chamber A1, a purified gas is obtained which complies with the specifications for the contents of acid gases, total sulfur and water. The purified gas is evacuated from the chamber Al via line 5. The purified gas is obtained as long as the cycle time is less than the piercing time of the mercaptans, ie the adsorption is carried out in the enclosure Al, for example, until the material is saturated with mercaptans. Afterwards, the operating mode of the material of the enclosure Al is reversed by that of another enclosure, for example by that of the enclosure A2 whose adsorbent material is regenerated, that is to say which does not contain or little mercaptans and water adsorbed.

The temperature inside the chamber Al operating in adsorption mode is generally between 0 ⁰ C and 150 ⁰ C, preferably between 10 ⁰ C and 100 ⁰ C, and preferably between 20 ⁰ C and 70 ⁰ C. The pressure in the enclosure Al may be that of natural gas produced, typically between 10 bar and 100 bar, preferably between 30 bar and 80 bar.

The superficial gas velocity in the enclosure A1 is for example between 0.5 and 30 m / min. The purified gas is, for example, sent via line 6 to a fractionation unit in order to valorize the various cuts, for example by distillation. The purified gas can be separated to obtain methane, ethane, propane, butane and a heavy hydrocarbon fraction containing more than five carbon atoms. The purified gas may also be sent via line 6 to a place of storage or use.

In the chamber A2, the adsorbent material is loaded with heavy hydrocarbons. According to the invention, it is possible to use a part of the purified gas coming from the enclosure Al as a regeneration gas to regenerate the material contained in the enclosure A2, that is to say to evacuate the heavy hydrocarbons contained by the material in the A2 enclosure. Regeneration is performed in TSA mode. This regeneration step makes it possible to purge the adsorbent material which contains heavy hydrocarbons. The regeneration step can be carried out under pressure conditions close to or equal to those used during the displacement step described below, and possibly under temperature conditions close to or equal to those used during this displacement step. . Alternatively, another gas can be used to regenerate the adsorbent A2, for example nitrogen, CO ₂ , or hydrogen.

The increase in temperature of the material in the chamber A2 can be achieved by closed loop circulation through A2 of a quantity of gas, for example from Al, or by another type of gas. This quantity of gas is heated via a heat exchanger.

Part of the purified gas from the enclosure Al is heated in the heat exchanger E1 at a temperature of between 20 ^° C. and 350 ^° C., preferably between 50 ⁰ C and 25O ⁰ C. In addition, the purified gas is expanded in the expansion element T, for example a turbine or a valve, to a pressure below 40 bar, preferably less than 20 bar and preferred way less than 10 bar. Alternatively, the expansion in T1 can be carried out before heating in El. Next, the gas is introduced through line 7 into enclosure A2. The flow rate of gas introduced into the chamber A2 can be between 1% and 50%, and preferably between 5% and 20%, of the total flow rate of the purified gas from Al. During the regeneration stage, in the enclosure A2, the pressure is generally less than 40 bar, 20 bar or preferably less than 10 bar, the temperature may be between 20 ⁰ C and 350 ⁰ C, preferably between 50 ⁰ C and 250 ⁰ C.

Once the heavy hydrocarbons are removed from the adsorbent material of the chamber A2, the temperature in the chamber A2 is gradually lowered and the pressure is gradually raised to the temperature and pressure used to carry out the adsorption in the chamber Al For example, gas is sent from the enclosure A1 into the chamber A2 via the pipe 7, this gas not being heated by the exchanger E1 or expanded by the expansion member T1. lowering the temperature and increasing the pressure, the gas obtained at the outlet of the enclosure A2 may be recycled upstream of the enclosure A1, for example by being mixed with the gas flowing in the duct 4. This operation can also be carried out under a lower pressure, for example between 1 bar and 30 bar, preferably between 1 bar and 10 bar, so as to limit the amount of purge gas used. In this case, the gas can be used as a gas oil, for example for the utilities of the process. The temperature drop of the adsorbent material in the chamber A2 can also be achieved by closed-loop circulation of a quantity of gas, for example from Al, or by another type of gas. The amount of gas is cooled in a heat exchanger. At the outlet of enclosure A2, the purified gas flowing in line 8 is loaded with heavy hydrocarbons from the desorption of these heavy hydrocarbons adsorbed in chamber A2. The gas may be cooled in the heat exchanger E2 at a temperature of between 15 ° C. and 150 ^° C., preferably between 25 ° C. and 80 ^° C. The cooling makes it possible to condense a portion of the heavy hydrocarbons contained in the gas . The cooled gas is introduced into the gas / liquid separator B in order to recover a portion of the condensed heavy hydrocarbons by cooling.

The operating conditions of the separator B are a temperature of between 0 ^° C. and 150 ^° C., preferably between 10 ^° C. and 80 ^° C., and a pressure of between 1 bar and 100 bar, preferably between 1 bar and

30 bar. The condensed hydrocarbons are discharged from B via line 9, the gas is discharged from B via line 10.

Some or all of the gas discharged from B can be recycled. For example, this gas is compressed in K2, sent via the ducts 10 and 11 to be mixed with the gas flowing in the duct 4, and is then introduced into the chamber Al.

If the separator B operates at low pressure, the gas can be used as a gas oil, for example for the utilities of the process.

The adsorbent material contained in the enclosure A3 is loaded with mercaptans. The material may also be charged, in addition to mercaptans, with water and / or heavy hydrocarbons, in particular of the C ₆ ⁺ type. This material must be at least partially freed of mercaptans in order to be re-engaged in an adsorption cycle. According to the invention, the mercaptans are displaced by heavy hydrocarbon vapor, pure or diluted, for example by a portion of the drying gas from the enclosure A2. The heavy hydrocarbon vapor arriving via line 12 is introduced via line 13 into chamber A3. The heavy hydrocarbons used to move the mercaptans are liquid hydrocarbons under normal conditions, comprising at least five carbon atoms, and preferably at least seven carbon atoms. The hydrocarbons may be paraffinic, isoparaffinic, naphthenic or aromatic. Preferably, the hydrocarbons may be aromatic compounds preferably containing from 7 to 10 carbon atoms, for example toluene, the isomers of xylenes, ethylbenzene, mesytilene, paradiethylbenzene. The hydrocarbons may in particular be chosen from the hydrocarbons constituting the aromatic C8 cut, namely in particular the isomers of xylenes and ethylbenzene.

Prior to this displacement step, the adsorbent material of the enclosure A3 may be depressurized, for example at a pressure less than or equal to half the pressure during the adsorption step, that is to say typically less than 40 bar, and preferably less than 20 bar, and preferably less than 10 bar. The gas recovered during this depressurization phase may advantageously be used to re-pressurize another adsorber after the phase of displacement of the mercaptans and / or preferentially after the desorption phase of the displacement agent. The quantity of heavy hydrocarbons required for the step of displacement of the mercaptans adsorbed on the adsorbent in the enclosure A3 depends in particular on the degree of regeneration desired, that is to say in particular the dynamic capacity and the residual capacity. after regeneration of the adsorbent vis-à-vis the mercaptans, the temperature and pressure of this step of displacement. The amount of heavy hydrocarbons required is all the lower as the temperature is high, the total pressure low, the degree of regeneration of the weak adsorbent and the residual capacity after high regeneration. The quantity of heavy hydrocarbons sent via line 13 into chamber A3 is between 1 and 50 times the mass of mercaptans. that we want to move, preferably between 1 and 20 times, preferably between 1 and 10 times.

The heavy hydrocarbon vapor circulating in the conduit 13 may be diluted by gas coming from the enclosure A2 arriving via the conduit 17. If the heavy hydrocarbons are diluted with natural gas, the amount of heavy hydrocarbons is greater than 80 % by volume of the mixture, advantageously greater than 90% and preferably greater than 98%.

The operating conditions of the enclosure A3 are a temperature of between 0 ^° C. and 300 ^° C., and preferably between 20 ^° C. and 200 ° C. The pressure in the enclosure A3 may be chosen lower than the vapor pressure of the heavy hydrocarbons at the temperature in question (that is to say at the temperature in the enclosure A3), for example between 1 and 40 bar, preferably between 1 and 20 bar and preferably between 1 and 10 bar.

If the heavy hydrocarbons are not diluted, the pressure during the displacement step is preferably chosen so that it is between 0.2 and 0.8 times the vapor pressure, or the dew point, of the hydrocarbons. heavy at the temperature of the enclosure A3, and preferably between 0.3 and 0.6 times. If the heavy hydrocarbons are used diluted in a purge gas, the total pressure can be chosen such that the ratio between the partial pressure of the heavy hydrocarbons and the dew point pressure of these heavy hydrocarbons at the temperature of the enclosure A3, is between 0.2 and 0.8 and preferably between 0.4 and 0.6. This choice is particularly important when using an adsorbent containing mesopores, which can give rise to the phenomenon of capillary condensation, such as activated carbons and activated aluminas or silica gels. If zeolites formed with a binder are used, the mesopore content is practically zero, and it is then possible to work with a high relative pressure ratio. At the end of this displacement step, the intergrain volume of the adsorbent material of the enclosure A3 and the porous volumes of the adsorbent are filled with heavy hydrocarbons. These adsorbed hydrocarbons may optionally be removed by heating the adsorbent at high temperature under gas flushing as described with reference to the enclosure A2.

The rate of passage of the hydrocarbon vapor in the enclosure A3 is between 0.5 and 10 m / min, and preferably between 0.5 and 5 m / min.

The gaseous mixture, at the outlet of the enclosure A3 essentially consisting of mercaptans mixed with the heavy hydrocarbon vapor, is cooled in the heat exchanger E3. In the exchanger E3, the gaseous mixture is cooled to a condensation temperature of the heavy hydrocarbons, for example at a temperature between -50 ^° C. and 50 ^° C., preferably between -40 ^° C. and 0 ^° C. Then, it is sent to a gas / liquid separator S, which makes it possible to recover a liquid hydrocarbon fraction, containing most of the dissolved mercaptans and possibly a gaseous phase, especially if the vapor has been diluted in a gas.

The operating conditions of the separator S are a temperature of between -50 ^° C. and 50 ^° C., and preferably between -40 ^° C. and 0 ^° C., a pressure of between 1 and 100 bar, preferably between 10 and 80 bar.

At the outlet of the separator S, a liquid hydrocarbon fraction is recovered via the conduit 15, consisting in particular of the mercaptans, the heavy hydrocarbons, and possibly light hydrocarbons of the dissolved natural gas. This liquid hydrocarbon fraction can be depressurized at a pressure close to atmospheric pressure, then the mercaptans can be removed for example by an absorption process with a solution comprising sodium hydroxide or potassium hydroxide. This liquid fraction can also be desulfurized by a sulfrex process. The purified liquid hydrocarbon fraction thus obtained can then be recovered or recycled. The heavy hydrocarbon vapor arriving via line 12 may be diluted with a fraction of natural gas, for example all or part of the gas recovered via line 17 at the outlet of chamber A2 during the regeneration phase of the adsorbent material. This natural gas is not condensed by the cooling in exchanger E3. This gas is discharged from the separator S via the conduit 18. It can advantageously be compressed by the compressor K1, and then recycled upstream of the enclosure Al through the conduits 19 and 11 so as to be mixed with the gas flowing in the conduit 4. can also be heated by the exchanger E4, then sent through the conduit 16 to be mixed with the heavy hydrocarbon vapor arriving through the conduit 12 to dilute this steam.

In order to increase the degree of condensation of the mercaptans in the separator S, and therefore to reduce the amount of mercaptans contained in the gas recirculated via the duct 18, it is possible to produce between the outlet of the enclosure A3 and the inlet of the separator S a dehydration step of the gas leaving the enclosure A3 so as to allow operation of this separator S at lower temperature without the risk of gas hydrate formation. The dehydration of this gas can be carried out in the AS unit, for example by a glycol dehydration process (for example described with reference to the DH unit), or by an adsorption method of the TSA type on zeolite 3A or 4A. according to the rules known to those skilled in the art. Thus, according to the invention, the cooling temperature in E3 and the operating temperature of the separator S can be between -50 ^° C. and 10 ^° C., preferably between -50 ^° C. and -20 ^° C. These Operating temperatures make it possible to achieve greater condensation of desorbed mercaptans from the enclosure A3 which will be discharged from the separator via the conduit 15 with the liquid hydrocarbon phase. According to the method described in connection with FIG. 1, the enclosures, Al,

A2 and A3 operate alternately in adsorption mode (adsorption of the mercaptans contained in the gas to be treated), then in displacement mode

(displacement of mercaptans adsorbed by heavy hydrocarbons), then in regeneration mode (desorption of heavy hydrocarbons).

After a determined time, the operating mode of the material of the enclosure A1 is changed by that of the enclosure A3, that of the enclosure A2 by that of the enclosure A1, and that of the enclosure A3 by that of the A2 speaker. The change can be made when the material contained in the chamber operating in regeneration mode is sufficiently regenerated to operate in adsorption mode. The change can also be made when the material contained in the chamber operating in adsorption mode is too charged to mercaptans to continue to ensure purification of the gas to be treated. Alternatively, the method described in connection with Figure 1 can operate with two amounts of adsorbent material. While a quantity of adsorbent material undergoes the adsorption step in the chamber A1, the other quantity of adsorbent material successively undergoes the stage of displacement with the vapor of heavy hydrocarbons in the enclosure A3, then the regeneration stage or drying in the enclosure A2. Then the roles of the two adsorbents are reversed.

The duration of the different steps is typically between 1 and 24 hours, preferably between 2 and 18 hours, and preferably between 4 and 12 hours.

The process described in connection with FIG. 2 constitutes an improvement of the method described with reference to FIG. 1. The references of FIG. 2 identical to those of FIG. 1 denote the same elements. The raw natural gas arriving via line 1 is optionally de-acidified in the DA unit and then optionally dehydrated in the DH unit.

The deacidified and dehydrated natural gas is introduced into a purification unit. This unit has five speakers Al, A2, A3, A4 and A5 which operate in parallel:

a first chamber operates in adsorption mode, in particular residual water present in the gas after deacidification and dehydration, the adsorption being carried out at ambient or moderate temperature; a second chamber operates in adsorption mode, in particular mercaptans and heavy hydrocarbons; present in the gas from the first chamber, the adsorption being carried out at ambient or moderate temperature,

a third enclosure operates in regeneration mode, that is to say desorption of water by sweeping with a gas at high temperature,

a fourth enclosure operates in regeneration mode, that is to say desorption of heavy hydrocarbons by scanning with a gas at high temperature,

- A fifth chamber operates in displacement mode, in particular mercaptans adsorbed by sweeping heavy hydrocarbon vapor, the gas being at room temperature bu moderate.

In FIG. 2, the material contained in the enclosure A4 operates in adsorption mode of the water, the material contained in the enclosure Al operates in adsorption mode of the mercaptans, the material contained in the enclosures A2 operates in regeneration mode (desorption heavy hydrocarbons), the material contained in the enclosure A5 operates in regeneration mode (desorption of water), and the material contained in the enclosure A3 operates in displacement mode. In the case where the adsorbent material used for the removal of mercaptans in the enclosure A1 is hydrophobic, the enclosures A1 and A4 can be inverted. The gas flowing in the duct 3 passes first through the enclosure A1 and the enclosure A4. The treatment pressure in the Al and A4 enclosures is generally between 10 and 100 bar, preferably between 30 and 80 bar, and the temperature is between room temperature and 100 ⁰ C, and preferably between 20 ⁰ C and 70 ⁰ C The sizing of Al and A4 units can be achieved by considering the total amount of mercaptans to be adsorbed in the Al enclosure. The residual water content in the gas can be adjusted so as to saturate the adsorber A4 during the same time. cycle that the adsorber Al.

An purified natural gas evacuated via the duct 6 is obtained at the outlet of the enclosure A1, to the specifications for acid gases, total sulfur, and dew point of water.

The purified gas is then optionally sent via line 6 to a fractionation unit in order to valorize the various cuts.

Part of the purified gas, generally between 1 and 50% by volume relative to the total gas, and preferably between 2 and 30% by volume, is used as a purge gas to regenerate, in TSA mode, the adsorbent materials of the enclosures A2 and A5. . This portion of gas is heated in the exchanger E1 and then sent to the enclosures A2 and A5. Before entering the enclosure A2, the heated gas is, furthermore, expanded in the turbine Tl. The pressure in the enclosure A5 is generally between 1 and 100 bar, and preferably between 10 and 80 bar, and the temperature between 100 and 400 ⁰ C, preferably between 200 and 350 ⁰ C. The pressure in the chamber A2 is generally between 1 and 20 bar, and preferably between 1 and 10 bar, and the temperature between 100 and 400 ⁰ C, preferably between 200 and 350 ⁰ C.

At the outlet of the enclosure A5, the purified gas is loaded with water coming from the desorption of the water adsorbed in this chamber. All or part of the wet gas coming from the enclosure A5 is cooled and a gas / liquid separator B1 makes it possible to recover a part of the condensed water. The operating conditions of the separator B are a temperature between 10 and 15O ⁰ C, preferably between 15 and 8O ⁰ C, a pressure between 1 and 100 bar, and preferably between 2 and 70 bar. The operating conditions will be chosen so as to avoid any formation of natural gas hydrates in the separator. At the outlet of this separator B1, this gas, still charged with residual water, can be sent upstream of the enclosures A4 and A1 through conduits 10 and 11 after a possible recompression carried out in K2. All or part of the heavy hydrocarbon-laden gas coming from the enclosure A2 can be sent through the conduit 17 to be mixed with the heavy hydrocarbon vapor circulating in the conduit 13, this mixture being introduced into the enclosure A3.

All or part of the heavy hydrocarbon-loaded gas coming from the enclosure A2 can also be sent via the conduit 21 into the exchanger E3 and then into the separator S1.

The adsorbent material used for the removal of the mercaptans contained in the enclosure A3 is saturated, at the end of the adsorption cycle, with mercaptans and heavy hydrocarbons present in the gas, and must be at least partially regenerated in order to be again engaged in an adsorption cycle. According to the invention, this regeneration is done by using as fluid in particular hydrocarbon vapor, pure or diluted, for example by all or part of the regeneration gas of the adsorbent contained in the chamber A2, at high temperature, for example typically between 0 and 300 ° C, and preferably between 50 and 200 ^° C. The pressure for this operation must be less than the vapor pressure of the heavy hydrocarbons at the temperature in question, it is typically between 1 and 40 bar, and preferably between 1 and 20 bar, and preferably between 1 and 10 bar.

At the outlet of enclosure A3, the heavy hydrocarbon vapor contains desorbed mercaptans. A gas / liquid separator S 1 makes it possible to recover a liquid hydrocarbon fraction constituted in particular by heavy hydrocarbons and mercaptans. The operating conditions of this separator Sl are a temperature of between -50 ^° C. and 50 ^° C., and preferably between -40 ^° C. and 0 ^° C., and a pressure of between 1 and 40 bar, and preferably between 1 and 20 ^° C. bar, and preferably between 1 and 10 bar. At the end of this displacement step, the intergrain volume of the adsorber, and the porous volumes of the adsorbent used for the removal of the mercaptans, are filled with heavy hydrocarbon vapor. These adsorbed hydrocarbons may optionally be removed by heating the adsorbent at high temperature under gas flushing. This is done as described previously in the step relating to the operation of the enclosure A2.

The gas / liquid separator S 1 also makes it possible to recover the condensed heavy hydrocarbons contained in the gas arriving directly from the enclosure A2 via the conduit 21.

The duration of each of these sequences is typically between 1 and 24 hours, preferably between 2 and 18 hours, and preferably between 4 and 12 hours. The sizing of the adsorbers will be carried out according to the rules known to those skilled in the art. The superficial velocity of the vapor in the adsorber is for example between 0.5 and 30 m / min.

According to the invention, the adsorbent materials used to carry out the adsorption of the mercaptans in the Al, A2 and A3 enclosures may be hydrophilic or hydrophobic nature. The adsorbent materials used to dehydrate natural gas in enclosures A4 and A5 are hydrophilic in nature.

The hydrophobic adsorbent materials are preferably chosen from active carbons, of hydrophobic nature, and in particular those whose specific surface area is typically between 500 and 2500 Xn ² Ig, as well as zeolites, or molecular sieves, in particular of type Y, of the family of faujasites, whose silicon / aluminum molar ratio is greater than 3, preferably greater than 5 and preferably greater than 10. These Y zeolites are commonly called ultrastable Y zeolites or USY. It is also possible to choose zeolites from the MFI family, and in particular ZSM-5 zeolites that are partially or totally dealuminated, such as silicalite. The compensation cation is preferably sodium. These adsorbents are known to those skilled in the art under the name NaY. The hydrophilic adsorbent materials, optionally used to remove the mercaptans from natural gas in the enclosures A4 and A5, are preferably chosen from LTA type molecular sieves, also called zeolites, or mesoporous adsorbents of activated alumina type or silica gel. The hydrophilic adsorbent materials used to remove the mercaptans from natural gas are preferably chosen from zeolites 5A and zeolites X or Y.

Among the hydrophilic zeolites, it is possible to choose among zeolites of type A (LTA family), and in particular water-selective zeolites, in particular zeolites of type 3A and 4A, whose pore diameter is less than 0.4 nm. It is also possible to use zeolites of type 5A, or of X or Y type, such as NaX or NaY zeolites (FAU family of faujasites), whose Si / Al molar ratio is preferably less than 3. The other adsorbents that can be used for this application can be selected from activated aluminas or silica gels, and preferably those having a BET specific surface area, conventionally determined by physisorption of nitrogen at 77 K, of between 150 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. In the case of multi-beds, activated aluminas or gels. of silica are used upstream of the molecular sieves.

Claims

1) Process for purifying a natural gas containing mercaptans, in which: a) the natural gas is brought into contact with an adsorbent material so as to obtain a purified gas, the mercaptans being adsorbed by the adsorbent material, then b) the adsorbent material loaded with mercaptans obtained in step a) is brought into contact with a stream comprising at least 80% by volume of heavy hydrocarbon vapor containing at least five carbon atoms so as to obtain a stream loaded with mercaptans and an adsorbent material saturated with heavy hydrocarbons.

2) Method according to claim 1, wherein:

Step a) is carried out under a pressure of between 10 bar and 100 bar and at a temperature between 0 ^° C. and 150 ^° C.,

Step b) is carried out under a pressure of between 1 bar and 40 bar and at a temperature of between 0 ^° C. and 300 ^° C.

3) Process according to one of claims 1 and 2, wherein: c) the saturated heavy hydrocarbon adsorbent material obtained in step b) is brought into contact with a fraction of the purified gas obtained in step a) of in order to desorb the heavy hydrocarbons from the adsorbent material and to obtain a gas loaded with heavy hydrocarbons.

4) Method according to claim 3, wherein: Step c) is carried out under a pressure of between 1 bar and 40 bar, and at a temperature of between 0 ^° C. and 300 ^° C.

5) Method according to one of claims 1 to 4, wherein: d) the natural gas is contacted with a second adsorbent material so as to adsorb the water contained in the natural gas, and e) is put in contact with the second adsorbent material with a fraction of the purified gas obtained in step a) to desorb the water contained in the second adsorbent material.

The process according to claim 5, wherein:

Step d) is carried out under a pressure of between 10 bar and 100 bar and at a temperature of between 0 ^° C. and 100 ^° C.,

Step e) is carried out under a pressure of between 10 bar and 100 bar and at a temperature of between 100 ^° C. and 400 ^° C.

7) Method according to one of claims 1 to 6, wherein in step b) said stream comprises at least a portion of the purified gas obtained after contacting in step c).

8) Process according to one of claims 1 to 7, wherein the heavy hydrocarbon vapor contained in the stream obtained in step b) is condensed and the mercaptans are separated from the condensed hydrocarbons.

The process of claim 8, wherein the uncondensed gas is recycled in at least one of the following ways:

The uncondensed gas is mixed with the natural gas before step a),

The uncondensed gas is mixed with heavy hydrocarbon vapor to form at least a portion of said stream. 10) Method according to one of claims 1 to 9, wherein the purified gas obtained in step c) is cooled to condense a portion of the water and / or hydrocarbons and the cooled gas is recycled by mixing the gas cooled with the natural gas before step a).

11) Method according to one of claims 1 to 10, wherein before step a), said natural gas is deacidified by absorption of acidic compounds by an absorbent solution.

12) Method according to one of claims 1 to 11, wherein the heavy hydrocarbons are selected from the group of aromatic hydrocarbons comprising between 7 and 10 carbon atoms, the group consisting of toluene, the isomers of xylenes, the ethylbenzene, mesytilene and paradiethylbenzene.

13) Method according to one of claims 1 to 12, wherein said adsorbent materials comprise at least one of the following materials: an activated carbon, a zeolite, a mesoporous adsorbent activated alumina type, and a mesoporous adsorbent silica gel type .

14) Method according to one of claims 1 to 13, wherein said adsorbent materials comprise at least one of the following materials: an activated carbon having a specific surface area of between 500 and 2500 m ² / g, a zeolite of type A, a type 5A zeolite, a faujasite type X zeolite, a Y type faujasite zeolite, a MFI family zeolite, an activated alumina type mesoporous adsorbent having a BET specific surface area of between 150 mVg and 800 mVg, and an adsorbent mesoporous silica gel type having a BET specific surface area of between 150 m ² / g and 800 mVg.