FR2937887A1 - PROCESS FOR THE PURIFICATION OF GASEOUS MIXTURES CONTAINING MERCAPTANS AND OTHER ACIDIC GASES - Google Patents

Abstract

The invention relates to a method for purifying a gaseous mixture containing mercaptans and other acidic gases comprising a step of contacting said gaseous mixture with an absorbing solution comprising an alkanolamine, an ethoxylated polyethyleneimine and water. The invention also relates to a composition adapted to the implementation of this method.

Description

The present invention relates to a process for the purification of gaseous mixtures, in particular of natural gas, containing mercaptans and other acid gases, as well as an absorbent solution. for carrying out said method.

TECHNICAL BACKGROUND In the context of the production of natural gas (containing mainly methane) or liquefied natural gas, it is necessary to purify said natural gas from a pool of a number of contaminants, the first of which is so-called acid gases, ie carbon dioxide (CO2), hydrogen sulphide (H2S), mercaptans (R-SH), carbonyl sulphide (COS) and carbon disulfide (CS2). Carbon dioxide and hydrogen sulphide may represent an important part of the gaseous mixture from a natural gas deposit, typically from 3 to 70% (in molar concentration). COS is present in smaller amounts, typically ranging from 1 to 50 ppm by volume. Among the contaminants to be purified are mercaptans, molecules of formula R-SH where R is an alkyl group. The total amount of mercaptans in a gas mixture from a natural gas production site may represent a few hundred ppm by volume. The two main mercaptans concerned are methylmercaptan and ethylmercaptan, but other R: `Patents 28400 2 843 L-081103-text .. depot.doc- 3 November 2008

Mercaptans (especially C3SH to C6SH molecules) may also be present, generally at lower concentrations. Many methods currently exist to deacidify and demercaptanize (simultaneously or sequentially) natural gas, using solvents likely to absorb chemically and / or physically (by dissolution) mercaptans and / or other acid gases.

Among the processes currently in effect on an industrial scale, the so-called Sulfinol process consists in eliminating the H2S, CO2, COS, CS2 gases and the mercaptans from natural gas using a solvent consisting of a mixture of sulfolane, water and an amine (such as diisopropanolamine or methyldiethanolamine). Another example is the so-called Selexol process, which uses a solvent based on polyethylene glycol dimethyl ether. Many other variants have been proposed, using alternative solvents. By way of example, mention may be made of solvents based on alkanolpyridine (US Pat. No. 4,360,363). One can also cite the document WO 2007/083012, which proposes a solvent based on an alkanolamine and a thioalkanol. DE 10 2005 043 142 discloses a deacidification process of a fluid stream comprising the use of a solution containing a polymeric amine, a monomeric amine and water, the polymeric amine being for example a polyethylene imine. However, there is still a real need to develop other solvents that can effectively absorb, preferably simultaneously, mercaptans and other acid gases present in a gas mixture. In particular, there is a need to develop solvents which make it possible to carry out deacidification and demercaptanization processes for gaseous mixtures with solvent flow rates lower than the flow rates currently used (at gas mixture flow rate R: `Patents 2840028431-- 081103-text depot_doc- November 3, 2008

equivalent), and more generally at a lower cost than the solvents currently used.

SUMMARY OF THE INVENTION The invention therefore firstly relates to a process for purifying a gaseous mixture containing mercaptans and other acidic gases comprising a step of contacting said gaseous mixture with an absorbing solution comprising an alkanolamine, a polyethylene- ethoxylated imine and water. According to one embodiment, said gaseous mixture is natural gas. According to one embodiment, the gaseous mixture comprises methyl mercaptan and / or ethyl mercaptan, and optionally one or more gases chosen from hydrogen sulfide, carbon dioxide and carbonyl sulfide. According to one embodiment, the alkanolamine is chosen from monoethanolamine, diethanolamine, triethanolamine, diisopropanolamine, methyldiethanolamine, activated methyldiethanolamine, sterically hindered amines and mixtures thereof. According to one embodiment, the average molecular weight of the ethoxylated polyethyleneimine is between 500 and 106 g / mol, preferably between 500 and 105 g / mol. According to one embodiment, the ethoxylated polyethyleneimine contains between 50 and 60% of carbon, between 9 and 11% of hydrogen, between 1 and 33% of nitrogen, and between 1 and 40 of oxygen, and preferably contains between 50 and 60% of carbon, between 9 and 11% of hydrogen, between 1 and 15% of nitrogen, and between 10 and 40% of oxygen, the quantities being expressed in mass.

According to one embodiment, the absorbent solution comprises: from 20 to 60% by mass of diethanolamine, preferably from 30 to 50, ideally about 40%; A: Patents 28400 28431--081103-Text Deposit_Doc- November 3, 2008

from 20 to 70% by weight of water, preferably from 30 to 60, preferably from 40 to 50%; and 5 to 40% by weight of ethoxylated polyethylene imine, preferably 5 to 30, preferably 10 to 20. According to one embodiment, contacting the gaseous mixture with the absorbent solution is carried out in a column of absorption at a temperature of from about 40 to about 100 ° C, preferably about 50 to about 90 ° C. According to one embodiment, the gas mixture is brought into contact with the absorbent solution at a gas mixture flow rate of between 0.23 × 106 Nm 3 / day and 56 × 106 Nm 3 / day and at a flow rate of absorbent solution of between 800 and 100,000 m3 / day. According to one embodiment, the process according to the invention further comprises a regeneration step of the absorbent solution loaded with mercaptans and other acidic gases, preferably at a regeneration pressure of between 0 and 20 bar and more particularly between 1 and 20 bar. 2 bar, and preferably at a temperature between 100 and 140 ° C. According to one embodiment, the concentration of mercaptans contained in the gaseous mixture is lowered to a value of less than about 5 ppm and / or the concentration of hydrogen sulphide contained in the gaseous mixture is lowered to a value of less than about 4 ppm and / or the concentration of carbon dioxide contained in the gaseous mixture is lowered to less than about 50 ppm and / or the concentration of carbonyl sulfide in the gaseous mixture is lowered to less than about 1 ppm. The subject of the invention is also a composition comprising an alkanolamine; some water ; and an ethoxylated polyethyleneimine. According to one embodiment, the composition comprises: R: Patents 2840028431--081103-textedepot.doc- November 3, 2008

from 20 to 60% by weight of alkanolamine, preferably from 30 to 50%, ideally around 40%; from 20 to 70% by weight of water, preferably from 30 to 60%, ideally from 40 to 50%; and 5 to 40% by weight of ethoxylated polyethylene imine, preferably 5 to 30%, most preferably 10 to 20%. According to one embodiment, the alkanolamine is chosen from monoethanolamine, diethanolamine, triethanolamine, diisopropanolamine, methyldiethanolamine, activated methyldiethanolamine, sterically hindered amines and mixtures thereof. According to one embodiment, the ethoxylated polyethyleneimine is as described above. The invention also relates to the use of ethoxylated polyethyleneimine for purifying a gaseous mixture containing the invention above, thanks to mercaptans and other acid gases. allows to fill the need expressed here-the development of a new hybrid solution constituted polyethylene-imine effectively the one of a mixture of alkanolamine, water and ethoxylée, allowing to co-absorb mercaptans and the other acid gases contained in a gaseous mixture.

This hybrid solution makes it possible to implement a process for purifying a gaseous mixture containing acid gases, by bringing the gaseous mixture into contact with the hybrid solution, and optionally regenerating the solution if it is used in a loop.

According to some particular embodiments, the invention also has the advantageous features listed below. The invention makes it possible to take advantage of the low vapor pressure of the ethoxylated polyethylene imine, which ensures a good stability of the compound in the liquid state. Compared to the document DE 10 2005 043 124, which proposes the use of polyethylene- imine for R: vBrevets`2 8400 28431-081 103-text_depot.doc- 3 November 2008

deacidify a fluid stream, it is found that the use of ethoxylated polyethyleneimine, in the context of the present invention, provides a higher efficiency, as will be apparent from the examples below. Using the ethoxylated polyethyleneimine in combination with an alkanolamine makes it possible to combine the strong absorbing capacity of the alkanolamine with regard to hydrogen sulfide and carbon dioxide in particular with the strong absorbing capacity of the ethoxylated polyethyleneimine. to mercaptans. Thus, the gas mixture can be effectively purified in a single step, or provide a better finishing treatment of the gas mixture.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 represents the absorption isotherm of methyl mercaptan at 50 ° C respectively for: a control solution obtained by mixing 40% of diethanolamine, 40% of water and 20% of polyethyleneimine (curve 1); a control solution obtained by mixing 40% of diethanolamine and 60% of water (curve 2); a solution according to the invention obtained by mixing 40% of diethanolamine, 40% of water and 20% of type A ethoxylated polyethyleneimine (curve 3); a solution according to the invention obtained by mixing 40% of diethanolamine, 40% of water and 20% of type B ethoxylated polyethyleneimine (curve 4); and a solution according to the invention obtained by mixing 40% of diethanolamine, 40% of water and 20% of type C ethoxylated polyethyleneimine (curve 5). R:, Patents2840028431--081103-test depotdoc-3November 2008

The abscissa shows the amount of methyl mercaptan in g per kg of solution loaded and, on the ordinate, the partial pressure of methyl mercaptan in bar. FIG. 2 represents the absorption isotherm of methyl mercaptan at 50 ° C. in the presence of carbon dioxide respectively for: a control solution obtained by mixing 40% of diethanolamine, 40% of water and 20% of sulfolane (curve 1 ); a control solution obtained by mixing 40% of diethanolamine and 60% of water (curve 2); a solution according to the invention obtained by mixing 40% of diethanolamine, 40% of water and 20% of type A ethoxylated polyethyleneimine (curve 3); and a solution according to the invention obtained by mixing 40% of diethanolamine, 40% of water and 20% of type C ethoxylated polyethyleneimine (curve 4).

The abscissa shows the amount of methyl mercaptan in g per kg of solution loaded and, on the ordinate, the partial pressure of methyl mercaptan in bar.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION The invention is now described in more detail and in a nonlimiting manner in the description which follows.

Gas to be treated. The invention allows the treatment of gaseous mixture, and in particular according to a preferred embodiment, natural gas. This contains mercaptans, especially methyl mercaptan and / or ethyl mercaptan, in typical volume amounts ranging from 0 to 400 ppm. The gaseous mixture also comprises other acid gases, in particular hydrogen sulphide and / or carbon dioxide and / or carbonyl sulphide, all in volume quantities typical of: less than 50% of H 2 S, less than 50% CO2 and between 0 and 100 ppm COS. R. Patents 28-100 2843 I - 081103-text _depotdoc- November 3, 2008

Although the invention is particularly useful for treating a natural gas containing mercaptans, it should be noted that the invention applies more generally to the purification of any gaseous mixture containing acid gases, with or without mercaptans. In addition to the field of natural gas treatment, the invention is also applicable, for example, to the treatment of synthesis gas, liquefied petroleum gas, Claus effluent gas, refinery gas, fumes, fuels, gaseous stream formed during the oxidation of organic substances, the gas stream that forms during composting and storage of waste containing organic substances or the gaseous stream that forms during the bacterial decomposition of organic substances.

Absorbent solution. The invention makes use of a novel composition or absorbent solution, in a conventional method of absorption / regeneration. The new solution provides chemical and physical absorption depending on the components to absorb. In general, the absorbent solution according to the invention comprises (or even consists of). about 20 to about 60% by weight of an alkanolamine; preferably about 30 to about 45%; about 20 to about 70% by weight of water; preferably about 30 to about 60% by weight of water; and about 5 to about 40% by weight of an ethoxylated polyethylene imine; advantageously about 5 to about 30%. A preferred solution comprises (or even consists of). about 40% diethanolamine, about 40 to about 50% water, and about 10 to about 20% ethoxylated polyethyleneimine. To make this absorbent solution, the alkanolamine is mixed with an ethoxylated polyethyleneimine composition and water. The composition of polyethyleneimine R` Patents 28400 28431--081103-texte_depot.doc- 3 November 2008

ethoxylated can comprise water in a typical amount of 0 to 50%, preferably 1 to 40%. This water contributes to the total amount of water in the absorbent solution. Thus, the absorbent solution according to the invention can be obtained by mixing: - about 20 to about 60% by weight of the alkanolamine; preferably about 30 to about 45%; about 20 to about 60% by weight of water; preferably about 30 to about 50% by weight of water; and about 10 to about 40% by weight of the ethoxylated polyethyleneimine composition; advantageously about 10 to about 30%.

Preferably, the absorbent solution can be obtained by mixing about 40% diethanolamine, about 40% water, and about 20% of the ethoxylated polyethyleneimine composition. Diethanolamine (DEA) is the compound of formula HN (CH2-CH2OH) 2r which is the preferred alkanolamine. In addition to DEA, other examples of alkanolamines that can be used in the process according to the invention include monoethanolamine (MEA), triethanolamine (TEA), diisopropanolamine (DIPA) and methyldiethanolamine (MDEA), or even activated methyldiethanolamine. (For example methyldiethanolamine enriched with hydroxyethylpiperazine or piperazine or primary amine or secondary amine) or sterically hindered amines. It is also possible to use mixtures of the alkanolamines indicated above. In general, the ethoxylated polyethyleneimine used in the context of the invention has the general formula of the following formula (I): ## STR2 ##. doc- November 3, 2008 (I) In this formula, Xi has the following formula (II). R3 X7 has the following formula (III): X3 represents a group R3 or a group X1R3 or a group X2R3, X1 and X2 being of the respective formulas (II) and (III) defined hereinafter -above. The index i (in the group Yi) varies from 1 to m. The groups Yi in the successive units of the group X1 may be identical or different from each other. In all cases, each Y 1 represents a group R 12 or a group X 1 or a group X 2, X 1 and X 2 being of the respective formulas (II) and (III) defined above. Indeed, the ethoxylated polyethyleneimine is generally prepared by reacting polyethyleneimine with epoxides (or alkylene oxides such as ethylene oxide in the case of ethoxylation). Not all nitrogen atoms in polyethyleneimine react with these epoxides, hence the possible variability of the Yi group. The groups R1 to R12 each independently represent a hydrogen atom or a linear or branched alkyl group comprising from 1 to 12 carbon atoms or a group X2 = I8C1O1NR101R: Patents2 8400 28431--0811 03-text_offer.doc- 3 November 2008

cycloalkyl comprising from 1 to 12 carbon atoms or an aryl group comprising from 1 to 12 carbon atoms. The groups R1 to R12 may or may not contain heteroatoms such as oxygen, sulfur, nitrogen, phosphorus, silicon or halogens to form, for example and in a non-exhaustive manner, alcohol, ether, acid and ester functional groups. , amides, amines, phosphates, sulfides, thiols, thioesters, thioacids, carbonates, carbamates, ureas, xanthates, siloxanes. m and n each independently represent an integer of between 1 and 1000. Preferably, the average molecular weight of the ethoxylated polyethyleneimine is between 500 and 106 g / mol and even more preferably between 500 and 105 g / mol . The coefficients m and n are then chosen so as to respect this constraint as regards the average molecular mass. Preferably, the groups R1 to R12 do not contain heteroatoms.

Preferably, more than 50% or more than 60% or more than 70% or more than 80% or more than 90% or more than 95% of the R 1 to R 12 groups are hydrogen atoms. More preferably, essentially all R 1 to R 12 groups represent hydrogen atoms.

Preferably, the ethoxylated polyethyleneimine used in the context of the invention has the following elemental analysis (water not included): between 50 and 60% of carbon, between 9 and 11% of hydrogen, between 1 and 33 % nitrogen, and between 1 and 40% oxygen.

Even more preferably, the ethoxylated polyethyleneimine used in the context of the invention has the following elemental analysis (water not included): between 50 and 60% of carbon, between 9 and 11% of hydrogen, between and 15% nitrogen, and between 10 and 40% oxygen.

The preferred composition of the absorbent solution according to the invention (40% of DEA, 40% of water and 20% of ethoxylated polyethyleneimine) results from a compromise: in fact the more the absorbent solution contains R: 2840028431--081103-text_dcpot.doc- November 3, 2008

ethoxylated polyethyleneimine, the greater the solubility of the mercaptans, which is favorable for the purification of the gaseous mixture; in return, the more the absorbent solution contains ethoxylated polyethylene imine, the lower the surface tension of the solution, and the higher the viscosity of the solution, which is unfavorable to the transfer of mercaptans and other acid gases into the solution . However, the adverse effect of viscosity can be counteracted by adjusting the operating temperature. According to a particular embodiment of the invention, the absorbent solution according to the invention may comprise one or more additional compounds. In this case, the absorbent solution may comprise, for example: from 20 to 60% by weight of alkanolamine as described above, preferably from 30 to 50%, preferably around 40%; from 20 to 70% by weight of water, preferably from 30 to 60, preferably from 40 to 50%; and from 5 to 40% by weight of ethoxylated polyethyleneimine as described above and additional compounds, preferably from 5 to 30, more preferably from 10 to 20 Additional compounds may for example be selected from physical or chemical solvents capable of absorbing one or more acid gases selected from mercaptans, hydrogen sulfide, carbon dioxide and carbonyl sulphide.

As a further compound, it is preferable to choose a thioalkanol having the formula RS-C2-4-OH where R is any group, for example an alkyl group or an alcohol group or a thiol group or an alkylthioalkanol group, the group containing up to at 6 carbon atoms. In particular, it is possible to choose ethylene-dithioethanol, of formula (HO-CH 2 -CH 2) -S- (CH 2 -CH 2) -S- (CH 2 -CH 2 -OH), or thiodiethylene glycol, also called thiodiglycol, of formula S (CH 2 -CH2-OH) 2 or the R °. Patents. 28400 28431u081103-text deposit.doc- 3 November 2008

méthylthioéthanol. These compounds are indeed effective solvents for mercaptans and other acid gases.

Absorption and regeneration process

The invention uses a conventional regeneration absorption method but with a new absorption solution. The absorption step may be carried out in an absorption column at a temperature of from about 40 to about 100 ° C, preferably from about 50 to about 90 ° C. The pressure in the column may be between 1 to 150 bar, preferably between 40 and 100 bar. As column, one can use all types of useful columns, and in particular a perforated tray column, a valve column, a column caps or a packed column. The absorption is carried out by contacting the gaseous mixture with the absorbing solution at a gas mixture flow rate of, for example, between 0.23 × 106 Nm 3 / day and 56 × 106 Nm 3 / day and at a solution flow rate. absorbent for example between 800 and 50000 m3 / day. As for the regeneration step of the absorbent solution, it is carried out conventionally by heating and separation of the mercaptans and other acid gases absorbed from the solution in a regeneration column. Indeed, the amine solution loaded with H2S, CO2 and RSH (so-called rich amine) from the bottom of the absorption column is sent into an intermediate pressure flash drum. The gases resulting from the expansion of the rich amine are used as fuel gas. The rich amine is then reheated and optionally partially vaporized in an amine / amine exchanger by the hot amine of the bottom of the regenerator, and then fed to the regeneration column.

At the bottom of the regeneration column, a reboiler generates steam that flows backwards in the column, causing the acidic components H2S, CO2 and RSH. A: vBrevets`28400'2 843 1--08 1 1 0 3-text_depot_doc- 3 November 2008

This desorption is favored by the low pressure and the high temperature prevailing in the regenerator. At the top of the column, the acid gases are cooled in a condenser. The condensed water is separated from the acid gas in a reflux flask and returned either to the top of the regeneration column or directly to the lean amine solution tank. The regenerated amine (also called poor amine) is then recycled to the absorption step.

It should be noted that a semi-regenerated mode of operation can also be envisaged. The process according to the invention makes it possible to achieve appreciable separation performance, and in particular to lower the concentration of mercaptans to a value of less than about 5 ppm, the concentration of hydrogen sulphide to a value of less than about 4 ppm. the carbon dioxide concentration is less than about 50 ppm and the carbonyl sulphide concentration is less than about 1 ppm.

The treated natural gas then undergoes a dehydration step and can then be available for the gas distribution network. It can also undergo cryogenic treatment to produce liquefied natural gas. Alternatively, it is possible to provide several deacidification steps: the gaseous mixture to be treated can then be brought into contact with a first absorbent solution in a first column, then with a second absorbent solution in a second column (for the finishing treatment) . In this case, the first or the second or both absorbent solutions are solutions according to the invention, as described above. Each absorbent solution then flows in a separate circuit and independently undergoes regeneration. For example, it may be appropriate to use a first absorbent solution containing a binary mixture consisting of an alkanolamine as described above and water, and a second absorbent solution consisting of a ternary mixture, according to the invention. . Indeed, the R: Patents 2840028431--081103-test deposit_doc- November 3, 2008

demercaptanization enabled by the absorbent solution according to the invention is particularly suitable as a finishing treatment of the gaseous mixture.

EXAMPLES The following examples illustrate the invention without limiting it. The absorption isotherm at 50 ° C. of methyl mercaptan was determined by three absorbent solutions according to the invention and three control solutions, either in the absence of CO2 (results shown in FIG. 1) or in the presence of CO2 at 500. mbar (results shown in Figure 2). Experimental Device: The absorbent solution is circulated through a 1.2 L jacketed reactor using a positive displacement pump. At the outlet of this pump, an exchanger is immersed in the thermostatic bath to maintain the reactor at a constant temperature, to compensate for heat losses due to the passage of fluid in the pump. The introduction of the gaseous mixture is controlled by regulating mass flowmeters, the pressure being kept constant by pressure regulation. A circulation of the gases in the reactor is ensured by taking them in the upper part and by bubbling them into the absorbing solution using a disperser placed at the bottom thereof. The entire gas flow path, including the portion leading to chromatographic sampling, is thermostatically controlled to avoid possible condensation. The sampling output is recycled to the reactor to avoid changing the system pressure. Protocol: the absorbent solution is first introduced into the reactor. Then we introduce a certain amount of gas, we wait for the stabilization of the pressure. Once equilibrium is reached, the measurements are taken. Composition of the control solutions: R: Patents 2840028431--0811 03-texte_depot.doc- November 3, 2008

First control: obtained by mixing 40% of DEA and 60% of water (curve 2 in Figure 1 and in Figure 2). Second control: obtained by mixing 40% of DEA, 20% of sulfolane and 40% of water (curve 1 in FIG. 2). Third control: obtained by mixing 40% of DEA, 20% of polyethyleneimine and 40% of water (curve 1 in FIG. 1).

Composition of the Solutions According to the Invention: Solution A: obtained by mixing 40% of DEA, 40% of water and 20% of ethoxylated polyethyleneimine of type A (curve 3 in FIG. 1 and FIG. 2) Solution B: obtained mixing 40% of DEA, 40% of water and 20% of ethoxylated type B polyethyleneimine (curve 4 in Figure 1) Solution C: obtained by mixing 40% DEA, 40% water and 20% % of type C ethoxylated polyethyleneimine (curve 5 in FIG. 1 and curve 4 in FIG. 2) In all the solutions, the diethanolamine is DEA BRENTAG SA 99.8% .The sulfolane used in the second reference solution is the SDS sulfolane of the CARLO ERBA Group, reference 932-02-48 The polyethyleneimine used in the third control solution is a non-ethoxylated Aldrich polyethyleneimine having an average molecular weight of 1200 g / mol. ethoxylated type A is an ethoxylated polyethylene-imine mass average molecular weight of 13000 g / mol, having a water content of 20% and having the following elemental analysis (water not included): carbon 53.8%, hydrogen 9.0%, nitrogen 1.8% and oxygen 35, 4%. Ethoxylated polyethyleneimine type B is marketed by Aldrich; it presents a mass RBrevets 28400 28431--081103-text deposit dot: - 3 November 2008

average molecular weight of 70000 g / mol and a water content of between 35 and 40. Ethoxylated polyethylene imine type C is an ethoxylated polyethylene imine with an average molecular weight of 60000 g / mol, which contains a water content of 1% and which has the following elemental analysis (water not included): carbon 53.5 hydrogen 9.9 nitrogen 4.1% and oxygen 32.5 It is found that the solutions according to the invention based on ethoxylated polyethyleneimine have better absorption capacity of methyl mercaptan than the control solutions, in the absence or in the presence of carbon dioxide. In particular, absorption by the solutions according to the invention is more effective than with a conventional amine solvent, a hybrid solvent containing sulfolane or a hybrid solvent containing a non-ethoxylated polyethyleneimine. R Patents 28400 28431u081103-Text Deposit.doc- 3 November 2008 1. 5 10 2. 3. 15 4. 5. 6.35

Claims (1)

REVENDICATIONS1. A method of purifying a gaseous mixture containing mercaptans and other acid gases comprising a step of contacting said gaseous mixture with an absorbent solution comprising an alkanolamine, an ethoxylated polyethyleneimine and water. The process of claim 1, wherein said gaseous mixture is natural gas. The process of claim 1 or 2 wherein the gaseous mixture comprises methyl mercaptan and / or ethyl mercaptan, and optionally one or more gases selected from hydrogen sulfide, carbon dioxide and carbonyl sulfide. Process according to one of Claims 1 to 3, in which the alkanolamine is chosen from monoethanolamine, diethanolamine, triethanolamine, diisopropanolamine, methyldiethanolamine, activated methyldiethanolamine, sterically hindered amines and mixtures thereof. Process according to one of Claims 1 to 4, in which the average molecular weight of the ethoxylated polyethyleneimine is between 500 and 1 g / mol, preferably between 500 and 105 g / mol. Process according to one of Claims 1 to 5, in which the ethoxylated polyethyleneimine contains between 50 and 60% of carbon, between 9 and 11% of hydrogen, between 1 and 33% of nitrogen, and between 1 and 40 % oxygen, and preferably RBrevets 28400 2843 1--081 I03-text_dot.doc- 3 November 2008 contains between 50 and 60% of carbon, between 9 and 11% of hydrogen, between 1 and 15% of nitrogen, and between 10 and 40% oxygen, the amounts being expressed by weight. 7. Method according to one of claims 1 to 6, wherein the absorbent solution comprises: 20 to 60% by weight of diethanolamine, preferably 30 to 50%, ideally about 40 to 20 to 70% by weight of water, preferably 30 to 60%, ideally 40 to 50%; and 5 to 40% by weight ethoxylated polyethylene imine, preferably 5 to 30%, most preferably 10 to 20%. 8. Process according to one of claims 1 to 7, in which the contacting of the gaseous mixture with the absorbing solution is carried out in an absorption column at a temperature of between approximately 40 and approximately 100 ° C. preferably about 50 and about 90 ° C. 9. Purification process according to one of claims 1 to 8, wherein the contacting of the gaseous mixture with the absorbing solution is carried out at a gaseous mixture rate of between 0.23x106 Nm3 / day and 56x106 Nm3 / day and at a flow rate of absorbent solution of between 800 and 100,000 m3 / day. 10. Method according to one of claims 1 to 9, further comprising a regeneration step of the absorbent solution loaded with mercaptans and other acid gases, preferably at a regeneration pressure between 0 and 20 bar and more particularly between 1 and 2 bar, and preferably at a temperature of from 100 to 140 ° C. 11. Process according to one of claims 1 to 10, in which the concentration of mercaptans contained in the gaseous mixture is lowered to a value of less than approximately 5 ppm and / or the concentration of hydrogen sulphide contained in the gaseous mixture is reduced to less than about 4 ppm and / or the concentration of carbon dioxide contained in the gaseous mixture is lowered to less than about 50 ppm and / or the concentration of carbonyl sulfide contained in the gaseous mixture is lowered to a value less than about 1 ppm. 12. Composition comprising: - an alkanolamine; - some water ; and an ethoxylated polyethyleneimine. 15. The composition of claim 12 comprising from 20 to 60% by weight of alkanolamine, preferably from 30 to 50, ideally about 40 °. from 20 to 70% by weight of water, preferably from 30 to 60, preferably from 40 to 50%; and from 5 to 40% by weight of ethoxylated polyethylene imine, preferably from 5 to 30, most preferably from 10 to 16. The composition of claim 12 or 13, wherein the alkanolamine is selected from monoethanolamine, diethanolamine, and the like. triethanolamine, diisopropanolamine, methyldiethanolamine, methyldiethanolamine Reactivated, sterically hindered amines and mixtures thereof. 15. Composition according to one of claims 12 to 14, wherein the ethoxylated polyethyleneimine is as described in claim 5 or 6. 16. Use of ethoxylated polyethyleneimine for purifying a gaseous mixture containing mercaptans and other gases acids. RBrevets 2840028431--081103-text_depot.doc- November 3, 2008