WO2009071767A2 - Method for removing mercaptans by adsorption on a molecular sieve with reactive regeneration of the sieve - Google Patents

Abstract

Method for removing the mercaptans contained in a gas, in which: a) the gas is circulated through an acidic adsorbent solid SA1 in such a way as to adsorb a part of the mercaptans contained in the gas and to obtain a mercaptan-loaded acidic adsorbent solid and a mercaptan-depleted gas (4), b) a liquid (9) containing olefins is circulated through said mercaptan-loaded acidic adsorbent solid SA3 in such a way that the mercaptans react with the olefins so as to form sulfides, and then c) a flushing gas (15) is passed through the acidic adsorbent solid SA2 in such a way as to expel an effluent comprising a portion of said liquid.

Description

 PROCESS FOR REMOVING MERCAPTANS BY ADSORPTION ON A MOLECULAR SIEVE WITH REACTIVE REGENERATION OF THE SIEVE

The present invention relates to the field of mercaptan removal by solid acid adsorbent. The invention proposes to use acidic adsorbent solids for the removal of mercaptans. These acidic adsorbent solids are used in natural gas processing applications.

In the case of the treatment of natural gas, the elimination of the mercaptans consists of an adsorption step, using for example a 13X zeolite, to carry out the desulphurization, the pore sizes of these zeolites permitting adsorption of the mercaptans. The processes used are then processes of the TSA ("Thermal Swing Adsorption") type, for which the adsorption takes place at ambient or moderate temperature, typically between 20 and 60 ^° C., and the high temperature desorption, typically between 200 and 350 ⁰ C, under purge gas purge, which may be in particular a portion of the purified gas, generally between 5 and 20% of the feed gas flow, or a portion of the light fraction C1, C2 or C1 + C2 purified gas after fractionation. The pressure is either kept substantially constant throughout the cycle, or lowered during the regeneration phase so as to promote regeneration. At the end of this adsorption purification step, the gas is at the total sulfur specifications.

The disadvantage of these adsorbent solids lies in the production during the regeneration phase of a gaseous effluent rich in mercaptans, which in turn requires treatment. The regeneration gas of these adsorbent solids, containing a large quantity of mercaptans, must be treated before being recycled, for example by treatment with a basic solution (sodium hydroxide or potassium hydroxide solution), with the well-known limitations due to the low solubility of mercaptans in an aqueous solution, or by treatment with a physical solvent having a high mercaptan absorption capacity. The recycling of this gas fraction is preferably carried out upstream of the glycol dehydration unit and / or upstream of the adsorption purification units.

Other sieve regeneration techniques are based on the adsorption selectivity of different compounds. The documents FR 2 861 403 and FR 2 884 154 recommend the use of a displacement agent, for example of the hydrocarbon cut type, which may or may not be diluted in a purge gas. The documents FR 2 868 962 and FR 2 882 941 propose the use of steam, possibly diluted in a purge gas, as displacement agent. These techniques enable the thermally fragile mercaptans to be displaced under mild temperature conditions, and a desorption step of the TSA-type displacement agent, therefore at a high temperature, is then necessary to regenerate the adsorbent solid.

The present invention proposes to use an acidic adsorbent solid which can be regenerated using an olefin reacting with the mercaptans retained in said solid. The reaction products obtained can be easily removed by a hydrocarbon fluid under advantageous thermodynamic conditions.

The present invention describes a process for removing mercaptans contained in a gas, for example a natural gas, in which the following steps are carried out successively and alternately: a) the gas is circulated through an acidic adsorbent solid so as to adsorb a part of the mercaptans contained in the gas and to obtain a mercaptan-loaded acidic adsorbent solid and a gas depleted in mercaptans, b) a liquid containing olefins is passed through said mercaptan-loaded acidic adsorbent solid so that the mercaptans react with the olefins to form sulphides and then c) a purge gas is passed through the through the acidic adsorbent solid so as to discharge an effluent comprising a portion of said liquid.

According to the invention, the acidic adsorbent solid may be chosen from the family of faujasite X or Y zeolites, the zeolites comprising a Brønsted acid site concentration of between 50 and 600 μmol / g. Alternatively, the acidic adsorbent solid may be chosen from one of the following families: the family of activated aluminas, the family of amorphous silica-aluminas and the family of mesoporous silicas and in which the acidic adsorbent solid has a concentration in acidic sites of Brönsted between 10 and 60 μmol / g.

At least one of steps b) and c) may be carried out at a temperature of between -50 ^° C. and 100 ^° C.

The liquid may comprise olefins having from three to twenty carbon atoms. Preferably, the liquid may comprise olefins having more than five carbon atoms. The liquid may comprise olefins in solution in hydrocarbons having at least five carbon atoms.

In step b), it is possible to evacuate a liquid cup comprising sulphides and it is possible to regenerate said liquid cut.

The effluent obtained in step c) can be separated into a gaseous phase and a liquid phase. It is possible to regenerate said liquid phase.

Other features and advantages of the invention will be better understood and will become clear from reading the description given below with reference to FIG. 1 which schematizes the operating principle of the invention. The principle of the invention is based on the addition reaction of a mercaptan on an olefin in the presence of an acidic adsorbent solid:

This reaction is given for information only and is in no way limiting of the present invention. Indeed, this addition reaction is not substantially modified by the length of the alkyl chain of the mercaptan.

The gaseous mercaptans retained in the acidic adsorbent solid are brought into contact with a liquid containing between 0.1% and 100% by weight of olefins having between 3 and 20 carbon atoms, and preferably between 8 and 15 carbon atoms. This contact is in the presence of an acidic adsorbent solid. According to the invention, the adsorbent solid used for the capture of mercaptans provides the acidity necessary for the reaction. The gaseous mercaptans will chemically react by addition on the olefin to form sulfides. The sulphides formed solubilize preferentially in the liquid phase. The acidic adsorbent solid is then regenerated. The liquid remaining in the porosity of the acidic adsorbent solid can be removed, for example, using a flushing gas. Thus the sulphides are evacuated to a regeneration zone.

Acidic acid adsorbent is understood to mean in particular mineral solids such as zeolites, activated aluminas, silicas, and amorphous silica-aluminas, which exhibit acidity. The acidity of acidic adsorbent solids can be quantified by the Br ennsted acid site content.

The acidity of the acidic adsorbent solids can be determined by techniques known to those skilled in the art. For example, the thermal desorption, coupled with infrared spectroscopy, of basic probe molecules, such as pyridine, can be monitored by following the intensity of the absorption band at about 1545 cm ^-1. Bronsted low (desorption temperatures between 150 ° C and 250 ° C), Bronsted means sites (desorption temperatures between 250 ⁰ C and 350 ⁰ C) and strong (desorption temperatures between 350 ° C and 450 ⁰ VS).

Quantification of Bronsted acid sites, determined by thermal desorption of pyridine coupled to infrared spectroscopy, is determined by the following formula:

LS n = ε.M with: n: the concentration of acid sites per unit mass of solid I: infrared absorption peak area (band at about 1545 cm ^-1 ) S: surface of the pellet used in Infrared ε: molar extinction coefficient (ε = 1, 3 for the Brönsted acid sites)

M: Mass of the pellet

The acidity of the adsorbent solid must be sufficient to allow the mercaptan addition reaction on the double bond of the olefin under mild temperature conditions, typically below 100 ^° C.

The acidity of the adsorbent solid should not be too high so as not to lead to the coupling reaction of two molecules of olefins with each other. In addition, the acidic adsorbent solid must be capable of adsorbing the mercaptans of natural gas, which implies a sufficiently polar surface.

The concentration of Bronsted acid sites is preferably between 50 and 600 μmol / g for zeolites faujasite X or Y, and preferably between 10 and 60 μmol / g for activated aluminas, silicas and silica-aluminas.

The acidic adsorbent solid used in the present invention may be chosen from the family of zeolites of faujasite type. Zeolites must adsorb light mercaptans from natural gas. They are selected from the faujasite structure, commonly known to those skilled in the art as zeolite (or molecular sieves) X or Y.

The molar Si / Al ratio may be between 1, 2 and 50, and preferably between 2.4 and 50, and preferably between 2.4 and 25. The exchange cation of zeolites X or Y may be H +, or mixed

Na + / H +, whose H + exchange rate may be greater than 20%.

The specific surface B. AND. these zeolites is typically between 500 and 800 m ² / g.

X or Y zeolites can be shaped using a clay-type binder, the content of which is typically between 5 and 25% by weight.

X or Y zeolites may also be shaped using an alumina-type binder, the content of which is typically between 5 and 70% by weight, and preferably between 5 and 50% by weight.

The concentration of Bronsted acid sites of zeolites X or Y may be chosen between 50 and 600 μmol / g. The H ⁺ acid site concentration is directly a function of the aluminum content of the zeolite X or Y, because of the electro-neutrality of the structure.

The acidic adsorbent solid used in the present invention may be chosen from amorphous silica-aluminas.

The amorphous silica-aluminas have a specific surface area of between 100 and 600 m ² / g, and preferably between 150 and 600 m ² / g. The silica content is typically between 10% and 95% by weight, and preferably between 30 and 95% by weight or between 50% and 90% by mass. The concentration of Brόnsted acid sites of the amorphous silica-aluminas may be chosen between 10 and 60 μmol / g.

The acidic adsorbent solid used in the present invention may be chosen from the family of activated aluminas.

The activated aluminas preferably have a specific surface area of between 150 and 350 m ² / g.

The concentration of Brόnsted acid sites of the activated aluminas may be chosen between 10 and 60 μmol / g.

The acidic adsorbent solid used in the present invention may be chosen from the family of mesoporous silicas, whose specific surface area is between 150 and 500 m ² / g.

The concentration of Brόnsted acid sites of the mesoporous silicas can be chosen between 10 and 60 μmol / g.

The invention is particularly applicable in the natural gas processing chain. In the case of natural gas, there are three main operations: deacidification, dehydration and degassing. The first operation aims to eliminate acid compounds such as CO ₂ , I ¹ H ₂ S, COS and mercaptans, mainly methyl mercaptan, ethyl mercaptan and propyl mercaptan. The generally accepted specifications for the deacidified gas are 2% CO ₂ , 4 ppm H ₂ S, and 20 to 50 ppm total sulfur. The dehydration step then controls the water content of the deacidified gas against transport specifications. Finally, the degassing stage of natural gas makes it possible to guarantee the hydrocarbon dew point of the natural gas, again depending on transport specifications.

With reference to FIG. 1, natural gas arriving via line 1 is firstly deacidified in section ZA, according to the absorption techniques for example described in document FR 2 897 066: the deacidification operation for removing carbon dioxide (CO ₂ ) or hydrogen sulphide (H ₂ S) often consists in washing the gas with an absorbent solution, for example an aqueous alkanolamine solution. The basicity of the alkanolamine molecules allows a selective elimination of the most acidic impurities such as CO ₂ or I ¹ H ₂ S. The acidity of the mercaptans is on the other hand not sufficient to allow their complete elimination. The absorbent solution is regenerated in the RE1 unit, the acid gases being evacuated via the conduit 2. The deacidified gas 3 is then dehydrated in the DH unit and then sent to a unit containing an SA1 acidic adsorbent solid for the elimination of the mercaptans.

The gas 4 derived from the SA1 acidic adsorbent solid is introduced into the fractionation unit F to be fractionated and separated into different sections: C1 (line 5), C1-C2 (line 6), C3-C4 (line 7) and C5 + (leads 8).

The adsorption unit SA1 operates alternately with the adsorption units SA2 and SA3. The acidic adsorbent solid used in units SA1, SA2 and SA3 is described above. With reference to FIG. 1, the second unit containing the adsorbent solid SA3 is in the regeneration phase. A liquid hydrocarbon fraction containing between 0.1% by weight and 100% of an olefin is sent via line 9 to unit SA3 containing the mercaptan saturated acidic adsorbent solid. The olefins used comprise at least three carbon atoms. In order to limit the entrainment losses in the gaseous effluent, olefins having more than 5 carbon atoms, preferably more than 8, are preferably used. Olefins having a single double bond are preferably chosen. However, di-olefins may incidentally be used in place of olefins. Branched olefins are more reactive but present a greater risk of oligomerization. The olefin can be chosen in order to limit the risk of loss by oligomerization. The olefins can be used pure, in admixture, or diluted in a mixture of hydrocarbons having at least 5 carbon atoms. These hydrocarbons may have a carbon chain of the same length as the olefin used. The contact between the olefin and the SA3 acidic adsorbent solid can be achieved under the conditions of olefin availability. For example, during regeneration, the pressure in SA3 is between 1 and 100 bar, and a temperature between -50 ⁰ C and 100 ⁰ C, preferably between 0 ⁰ C and 60 ⁰ C. The acid adsorbent solid used for mercaptan capture brings the necessary acidity to the reaction.

Since the chemical reaction between the mercaptan and the olefin is a balanced reaction, the control of the sulphide content in the liquid cut-out in SA3 through line 10 makes it possible to control the efficiency of the conversion of mercaptans to sulphides. At the outlet of unit SA3 containing the acidic adsorbent solid, the liquid hydrocarbon fraction loaded with sulphides can be stored or regenerated in unit RE2 according to known and described techniques, in particular in documents FR 2 896 509 and FR 2 873 711 .

In FIG. 1, the liquid cut 10 is regenerated in the RE2 unit which processes all or a fraction of the liquid feed so as to regenerate the olefin and release the mercaptans via the conduit 11. The liquid containing the regenerated olefin is recycled through line 9 into SA3.

In general, an olefin booster via line 12 makes it possible to maintain the olefin concentration in the liquid hydrocarbon fraction, in order to compensate for the losses due to the process.

The third unit containing SA2 adsorbent solid works in phase purge after passing the liquid hydrocarbon cut. Scavenging gas flowing through line 15 is used to flush the liquid. A separating flask B at the outlet of the purge phase of the adsorbent solid makes it possible to separate the flushing gas 13 and the liquid phase 14 which can be recycled to the regeneration section RE2.

In the context of the treatment of a natural gas, this flushing gas may be a fraction of the gas obtained at the end of the fractionation section F. Typically, 5 to 15% of the fraction C1 discharged through the duct 5 of F are used for this purge. The gas can be recovered via line 13 at the separator tank B downstream of the acidic adsorbent solid SA2 in the purge phase. This gas 13 is saturated with heavy hydrocarbons and can then be returned to the fractionation section F, in order to recover the heavy hydrocarbons in the C5 + cut. It is possible to carry out a distillation step in zone D of the C5 + cut discharged through line 8. The distillation makes it possible to recover the heavier hydrocarbons and traces of olefins via line 15. These heavy hydrocarbons can be recycled, for example by being mixed with the stream of regenerated olefins circulating in line 9.

When the invention is applied to the removal of mercaptans in a liquefaction plant of natural gas, the flushing gas may be a portion of the gaseous fraction obtained during the final expansion of the liquefied natural gas (this gaseous fraction is commonly called "boil"). -off gas "). Boil-off gas is mainly composed of methane and nitrogen.

When the SA1 acidic adsorbent solid is saturated with mercaptans, or when the SA3 acidic adsorbent solid is freed of mercaptans, or when the SA.sub.2 acidic adsorbent solid is freed of the products of the reaction between the mercaptans and the olefins, the role of SA1, SA2 and SA3: SA1 in the adsorption phase takes the place of SA3 in the regeneration phase which takes the place of SA2 in the purge phase which takes the place of SA1. The examples described below make it possible to illustrate the operation and the embodiment of the invention.

In all the examples, the operating conditions are as follows. Adsorption of ethyl mercaptan (EtSH):

The adsorption column used has an internal diameter of 1 cm and a length of 15 cm. The corresponding volume is 11.8 cm ³ .

The acidic adsorbent solid is introduced into this column, the mass is between 6 and 10 g depending on the density. Prior to adsorption, the adsorbent solids of zeolite or silica / alumina type are activated for 16 hours at

38O ⁰ C under nitrogen sweep at 2 Nl / h, the polymeric resin is activated only

12O ⁰ C.

The adsorption of ethyl mercaptan is carried out at room temperature, the gas flow rate is set at 2 Nl / h. The gas used consists of EtSH diluted in nitrogen at a concentration of 2000 molar ppm.

At the outlet of the column, the gas is analyzed every 10 minutes, by sampling 250 μl in a sampling loop. The analysis is carried out by gas chromatography with FID detection.

The amount of ethyl mercaptan adsorbed is calculated by material balance from the piercing curve.

Extraction of EtSH:

The entire acid adsorbent solid contained in the column after adsorption is added in a mixture of 55 ml, consisting of dodecane (90% mas) and dodecene-1 (10% mas), or about 41 g. The total sulfur analysis present in the supernatant hydrocarbon solution, after extraction for 16 hours, at a temperature of between 50 and 55 ^° C., is carried out by Fluorescence X. At the same time, a chromatographic analysis with specific detection of sulfur by chemiluminescence is carried out. , so as to identify the recovered compounds. Finally an analysis Chromatographic coupling mass spectrometry allows to identify the heavy molecules formed after extraction.

EXAMPLE 1 The solid used is a polymeric resin of stryrene-divinylbenzene type functionalized with Amberlyst-15wet acid (Rohm & Haas), the specific surface area is 42.5 m ² / g.

9.5 g of resin are placed in the column. The average EtSH piercing time is 20 minutes, which corresponds to an adsorbed amount of 0.04% mas, ie 4 mg.

Due to the low adsorption capacity of Amberlyst-15wet, the adsorption of mercaptans is very low and, therefore, the extraction of adsorbed mercaptans has not been achieved. This type of solid is not interesting in the context of the invention.

Example 2

The solid used is a dealuminic acid zeolite Y of the Wessalith type

DaY (Degussa). The Si / Al ratio of this zeolite is greater than 100, and the specific surface area is 606 m ² / g. 7.7 g of solid are placed in the column. The average EtSH piercing time is 150 minutes, which corresponds to an adsorbed amount of 0.3% by weight, ie 23 mg of EtSH or 11.9 mg of S.

The adsorption capacity of the solid remains low because of the Si / Al ratio which is too high and which hinders the adsorption of the mercaptans on the solid. In addition, the low acidity of the solid does not allow the addition of mercaptans to olefins.

Due to the low adsorption capacity, the extraction of adsorbed mercaptans has not been performed. This type of solid is not interesting in the context of the invention. Example 3

The solid used is an acid silica-alumina with 30% silica. The Si / Al molar ratio of this solid is 0.38, the specific surface area is 397 m ² / g, the macroporous volume is 0.01 1 cm ³ / g. 7.4 g of solid are placed in the column. The mean EtSH piercing time is 806 minutes, which corresponds to an adsorbed quantity of 2.0% by weight, ie 150 mg of EtSH, ie 77.4 mg of S.

After extraction, the total sulfur content in the solvent is 453 ppm S mass, ie 18.6 mg of sulfur recovered. The extraction yield is 24.2%.

Chromatographic analysis coupled with mass spectrometry reveals compounds whose molar mass corresponds to an addition of EtSH on the C12 olefin.

Example 4

The solid used is a zeolite Y acid (Si / Al = 10.5) deposited on alumina (40%). The molar Si / Al ratio of the zeolite is 10.5, the specific surface area is 831 m ² / g.

7.4 g of solid are placed in the column. The mean etching time of EtSH is 2900 minutes, which corresponds to an adsorbed quantity of 7.2% by weight, ie 530 mg of EtSH, ie 273.5 mg of S.

After extraction, the total sulfur content in the solvent is 2519 ppm S, or 103.3 mg of sulfur recovered.

The extraction yield is 38.2%. Chromatographic analysis coupled with mass spectrometry reveals compounds whose molar mass corresponds to an addition of EtSH on the C12 olefin.

Example 5 The solid used is an acid Y zeolite (Si / Al = 48.8) deposited on alumina (20%). The molar Si / Al ratio of the zeolite is 48.8, the specific surface area is 858 m ² / g.

6.5 g of solid are placed in the column. The average EtSH piercing time is 120 minutes, which corresponds to an adsorbed quantity of 0.34% mas, ie 22 mg of EtSH, ie 11.35 mg of S.

After extraction, the total sulfur content in the solvent is 250 ppm S mass, 10.3 mg of sulfur recovered.

The extraction yield is 9.1%. Chromatographic analysis coupled with mass spectrometry reveals compounds whose molar mass corresponds to an addition of EtSH on the C12 olefin.

Claims

1) Process for removing mercaptans contained in a gas in which the following steps are carried out successively and alternately: a) the gas is circulated through an acidic adsorbent solid (SA1) so as to adsorb a portion of the mercaptans contained in the gas and to obtain a mercaptan-loaded acidic adsorbent solid and a mercaptan depleted gas (4); b) a liquid (9) comprising olefins is passed through said mercaptan-loaded acidic adsorbing solid (SA3); the mercaptans react with the olefins to form sulphides, and then c) a purge gas (15) is passed through the acidic adsorbent solid (SA2) so as to discharge an effluent comprising a portion of said liquid. 2) Process according to claim 1, wherein the acidic adsorbent solid is selected from the family of faujasite zeolites X or Y, the zeolites having a Br ennsted acid site concentration of between 50 and 600 μmol / g. 3) The method of claim 1, wherein the acidic adsorbent solid is selected from one of the following families: the family of activated aluminas, the family of amorphous silica-aluminas and the family of mesoporous silicas and wherein the solid adsorbent acid has a concentration in Br sitesnsted acid sites of between 10 and 60 μmol / g. 4) Method according to one of claims 1 to 3, wherein performing at least one of steps b) and c) at a temperature between -50 ⁰ C and 100 ⁰ C. 5) Method according to one of claims 1 to 4, wherein the liquid comprises olefins having from three to twenty carbon atoms. 6) Process according to claim 5, wherein the liquid comprises olefins having more than five carbon atoms. 7) Process according to claims 5 and 6, wherein the liquid comprises olefins in solution in hydrocarbons having at least five carbon atoms. 8) Method according to one of the preceding claims, wherein in step b) discharges, acid adsorbent solid, a liquid cup (10) comprising sulfides and regenerates (RE2) said liquid cut. 9) Method according to one of the preceding claims, wherein is separated (B) the effluent obtained in step c) into a gas phase (13) and a liquid phase (14). 10) Process according to claim 9, wherein said liquid phase is regenerated. 1 1) Method according to one of the preceding claims, wherein the gas is a natural gas.