WO2016193257A1 - Method for eliminating acid compounds from a gaseous effluent with an absorbent solution made from aminoethers such as bis-(3-dimethylaminopropoxy)-1,2-ethane - Google Patents

Abstract

Method for eliminating the acid compounds contained in a gaseous effluent, comprising bringing into contact, in the absorption column C1, an effluent gas (1) with an absorbent solution (4) comprising water and at least one nitrogen compound of the following general formula: (I), in which x is equal to 0 or 1, and in which R is chosen from a hydrogen atom or a radical -(CH₂)₃-N-(CH₃)₂ when x is equal to 1, said absorbent solution not comprising Ν,Ν,Ν',Ν'-tetramethyl-l,6-hexanediamine. The nitrogen compound is, for example, bis-(3-dimethylaminopropoxy) -1,2-ethane having the following formula: (II)

Description

 PROCESS FOR REMOVING ACIDIC COMPOUNDS FROM A GASEOUS EFFLUENT WITH AN ABSORBENT SOLUTION BASED ON AMINOETHERS SUCH AS BIS- (3-DIMETHYLAMINOPROPOXY) -1,2-ETHANE

Field of the invention

The present invention relates to the field of deacidification processes of a gaseous effluent. The invention is advantageously applicable to the treatment of industrial gas and natural gas.

General Context Gas deacidification processes using aqueous solutions of amines are commonly used to remove acidic compounds present in a gas, in particular carbon dioxide (C0 ₂ ), hydrogen sulphide (H ₂ S), carbon oxysulfide (COS), carbon disulfide (CS ₂ ), sulfur dioxide (SO ₂ ) and mercaptans (RSH) such as methyl mercaptan (CH ₃ SH), ethyl mercaptan (CH ₃ CH ₂₎ SH) and propyl mercaptan (CH ₃ CH ₂ CH ₂ SH). The gas is deacidified by contact with the absorbent solution, and the absorbent solution is thermally regenerated.

These acid gas deacidification processes are also commonly referred to as "solvent washing" with a so-called "chemical" solvent, as opposed to the so-called "physical" solvent use for absorption which is not based on carrying out reactions. chemical. A chemical solvent is an aqueous solution comprising a reagent that selectively reacts with the acidic compounds (H ₂ S, CO ₂ , COS, CS ₂ , etc.) present in the treated gas to form salts, without reacting with the other compounds non-acidic gas. The treated gas after contact with the solvent is then depleted of acidic compounds, which are selectively transferred as salts into the solvent. The chemical reactions are reversible, which allows the solvent loaded with acid compounds to be subsequently deacidified, for example under the action of heat, to release on the one hand the acidic compounds in the form of gases, which can then be stored , transformed or used for various applications, and on the other hand regenerate the solvent which returns to its initial state and can be reused for a new reaction phase with the acid gas to be treated. The reaction phase of the solvent with the acid gas is commonly called the phase absorption, and that where the solvent is deacidified is called the regeneration phase of the solvent.

In general, the performance of the separation of the acidic compounds from the gas, in this context, depends mainly on the nature of the reversible reaction chosen. Conventional acid gas deacidification processes are generally so-called "amine" processes, ie they rely on the reactions of the acidic compounds with amines in solution. These reactions are part of the general framework of acid-base reactions. H ₂ S, CO ₂ or COS are, for example, acidic compounds, especially in the presence of water, whereas amines are basic compounds. The mechanisms of the reactions and the nature of the salts obtained generally depend on the structure of the amines used.

For example, US 6,852,144 discloses a method for removing acidic compounds from hydrocarbons using a water-N-methyldiethanolamine or water-triethanolamine absorbent solution containing a high proportion of a compound belonging to the following group: piperazine and / or methylpiperazine and / or morpholine.

The performance of acid gas deacidification processes by amine washing is directly dependent on the nature of the amine or amines present in the solvent. These amines can be primary, secondary or tertiary. They may have one or more equivalent or different amino functions per molecule. In order to improve the performance of the deacidification processes, amines which are always more efficient are continuously being sought.

A limitation of the absorbent solutions commonly used in deacidification applications is an insufficient selectivity of absorption of H ₂ S with respect to C0 ₂ . In fact, in certain cases of deacidification of natural gas, selective removal of H ₂ S is sought with a minimum of C0 ₂ absorption. This constraint is particularly important for gases to be treated already containing a CO ₂ content less than or equal to the desired specification. A maximum absorption capacity of H ₂ S is then sought with a maximum selectivity of absorption of H ₂ S with respect to CO ₂ . This selectivity makes it possible to maximize the quantity of gas treated and to recover an acid gas at the regenerator outlet having the highest concentration possible in H ₂ S, which limits the size of the units of the sulfur chain downstream of the treatment and guarantees a better operation. In some cases, an H ₂ S enrichment unit is needed to concentrate the acid gas in H ₂ S. In this case, the most selective amine is also sought. Tertiary amines, such as N-methyldiethanolamine, or hindered secondary amines with slow reaction kinetics with C0 ₂ are commonly used, but have limited selectivities at high H ₂ S loading rates.

It is well known to those skilled in the art that tertiary amines or secondary amines with a severe steric hindrance have a kinetics of uptake of C0 ₂ slower than primary or secondary amines uncrowded. In contrast, tertiary or secondary amines with severe steric hindrance have instantaneous H ₂ S capture kinetics, which allows for selective removal of H ₂ S based on distinct kinetic performance.

Various documents propose the use of congested tertiary or secondary amines, in particular tertiary or congested secondary diamines, in solution for deacidifying acid gases.

Among the applications of tertiary or secondary amines with a severe steric hindrance, US Pat. No. 4,405,582 describes a process for the selective absorption of sulfurized gases by an absorbent containing a diaminoether of which at least one amine function is tertiary and whose other amine function is tertiary or secondary having a severe steric hindrance, the nitrogen atom being in the latter case linked either to at least one tertiary carbon or to two secondary carbon atoms. The two amino functions and the main chain carbons may be substituted by alkyl or hydroxyalkyl radicals.

US Patent 4,405,583 also discloses a method for the selective removal of H ₂ S in gases containing H ₂ S and CO ₂ by an absorbent containing a diaminoether whose two secondary amine functions have a severe steric hindrance defined like before. The substituents of the amino functions and the carbons of the main chain may be substituted by alkyl and hydroxyalkyl radicals.

The French patent application filed by the applicant under the number 13/61 .853 describes a deacidification process using a solution of a tertiary diamine, N, N, N ', N'-tetramethyl-1, 6- hexanediamine (TMHDA), combined with a diaminoether such as bis- (3-dimethylaminopropoxy) -1,2-ethane.

Another limitation of the absorbent solutions commonly used in total deacidification applications is the kinetics of C0 ₂ or COS pickup that are too slow. In the case where the desired specifications on the C0 ₂ or in COS are very advanced, one seeks a kinetics of reaction as fast as possible so as to reduce the height of the absorption column. This pressure equipment represents a significant part of the investment costs of the process.

Whether maximum C0 ₂ and maximum COD uptake kinetics are sought in a total deacidification application, or a minimum CO ₂ capture kinetics in a selective application, it is always desirable to use an absorbent solution having the largest cyclic capacity. possible. This cyclic capacity, denoted Δα corresponds to the load ratio difference (a denotes the number of moles of acid compounds absorbed _gas aci _d n e per kilogram of absorbent solution) between the absorbent solution drawn off at the bottom of the absorption column and the absorbent solution feeding said column. Indeed, the more the absorbent solution has a high cyclic capacity, the more the absorbent solution flow rate that must be used to deacidify the gas to be treated is limited. In gas treatment processes, the reduction of the absorbent solution flow rate also has a strong impact on the reduction of investments, especially in the dimensioning of the absorption column. Another essential aspect of industrial gas treatment or flue gas treatment operations is the regeneration of the separating agent. Depending on the type of absorption (physical and / or chemical), regeneration by expansion, and / or distillation and / or entrainment by a vaporized gas called "stripping gas" is generally envisaged. The energy consumption necessary for the regeneration of the solvent can be very important, which is particularly true in the case where the partial pressure of acid gases is low, and represent a considerable operating cost for the CO ₂ capture process.

It is well known to those skilled in the art that the energy required for regeneration by distillation of an amine solution can be broken down into three different positions: the energy required to heat the absorbent solution between the head and the bottom of the regenerator, the energy necessary to lower the partial pressure of acid gas in the regenerator by vaporization of a stripping gas, and finally the energy required to break the bond between amine and C0 ₂ .

These first two positions are proportional to the flow rates of absorbent solution that must be circulated in the unit to achieve a given specification. To reduce the energy consumption associated with the regeneration of the solvent, it is therefore preferable once again to maximize the cyclic capacity of the solvent. Indeed, the more the absorbent solution has a high cyclic capacity, the more the absorbent solution flow rate that must be used to deacidify the gas to be treated is limited.

There is thus a need, in the field of gas deacidification, to provide compounds which are good candidates for the removal of acidic compounds from a gaseous effluent, including, but not limited to, selective removal of gaseous effluents. H ₂ S with respect to C0 ₂ , and which make it possible to operate at lower operating costs (including regeneration energy) and investments (including the cost of the absorption column).

Description of the invention

The inventors have demonstrated that tertiary or secondary multiamines having a severe steric hindrance are not equivalent in terms of performance for their use in absorbent solution formulations for the treatment of acid gases in an industrial process.

The subject of the present invention is the use, in the field of gas deacidification, of an aqueous absorbent solution comprising a nitrogen compound of general formula (I) below:

ladite solution absorbante ne comportant pas de N,N,N',N'-tétraméthyl-1 ,6-hexanediamine (TMHDA).

said absorbent solution not containing N, N, N ', N'-tetramethyl-1,6-hexanediamine (TMHDA).

The general formula (I) is detailed below. In particular, the subject of the present invention is the use, in the field of deacidification of gas, an aqueous absorbent solution comprising bis- (3-dimethylaminopropoxy) -1,2-ethane of formula (II) below, corresponding to the general formula (I), said absorbent solution not containing any N, N, N ', N'-tetramethyl-1,6-hexanediamine (TMHDA).

The inventors have demonstrated that the use of the compounds corresponding to this general formula (I), to which bis (3-dimethylaminopropoxy) -1,2-ethane in particular responds, in aqueous solution, said solution not containing of TMHDA, makes it possible to obtain good performances in terms of cyclic capacity for acid gas absorption and absorption selectivity with respect to H ₂ S, in particular an absorption selectivity vis-à-vis more important H ₂ S than reference amines such as N-methyldiethanolamine (MDEA) for equivalent or greater acidic acid absorption capacity.

Summary of the invention

In general, the invention relates to a process for removing acidic compounds contained in a gaseous effluent, in which an absorption step of the acidic compounds is carried out by bringing the effluent into contact with an absorbent solution comprising water and at least one nitrogen compound of the following general formula (I):

dans laquelle x est égal à 0 ou 1 , et dans laquelle R est un atome d'hydrogène ou un radical -(CH₂)3-N-(CH₃)2 quand x est égal à 1 ,

wherein x is 0 or 1, and wherein R is hydrogen or - (CH ₂ ) 3 -N- (CH ₃ ) 2 when x is 1,

 said absorbent solution not comprising N, N, N ', N'-tetramethyl-1,6-hexanediamine.

Preferably, said nitrogenous compound is bis- (3-dimethylaminopropoxy) -1,2-ethane.

According to the invention, the absorbent solution may comprise between 5% and 95% by weight of said nitrogenous compound, preferably between 10% and 90% by weight of said nitrogenous compound, and between 5% and 95% by weight of water, and preferably between 10% and 90% by weight of water.

The absorbent solution may further comprise between 5% and 95% by weight of at least one additional amine, said additional amine being either a tertiary amine or a secondary amine comprising two secondary carbons alpha to the nitrogen atom or minus a tertiary carbon alpha to the nitrogen atom.

Said additional amine may be a tertiary amine selected from the group consisting of:

 N-methyldiethanolamine;

 triethanolamine;

 diethylmonoethanolamine;

 dimethylmonoethanolamine; and

 ethyldiethanolamine.

The absorbent solution may further comprise a non-zero and less than 30% by weight amount of at least one additional amine being a primary amine or a secondary amine.

Said additional primary or secondary amine may be chosen from the group consisting of:

 monoethanolamine;

 diethanolamine;

 n-butylethanolamine;

 aminoethylethanolamine;

 diglycolamine;

 piperazine;

 1-methylpiperazine;

 2-methylpiperazine;

 - homopiperazine;

N- (2-hydroxyethyl) piperazine; N- (2-aminoethyl) piperazine;

 morpholine;

 3- (methylamino) propylamine;

 1,6-hexanediamine;

 N, N-dimethyl-1,6-hexanediamine;

 N, N'-dimethyl-1,6-hexanediamine

 N-methyl-1,6-hexanediamine; and

 N, N ', N'-trimethyl-1,6-hexanediamine.

The absorbent solution may further comprise at least one physical solvent selected from the group consisting of methanol, ethanol, 2-ethoxyethanol, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, pentaethylene glycol dimethyl ether, hexaethylene glycol dimethyl ether, heptaethylene glycol dimethyl ether, and octaethylene glycol dimethyl ether. diethylene glycol butoxyacetate, glycerol triacetate, sulfolane, N-methylpyrrolidone, N-methylmorpholin-3-one, Ν, Ν-dimethylformamide, N-formylmorpholine, N, N-dimethylimidazolidine -2-one, N-methylimidazole, ethylene glycol, diethylene glycol, triethylene glycol, thiodiglycol, propylene carbonate, tributyl phosphate.

Preferably, the absorption step of the acidic compounds is carried out at a pressure between 1 bar and 200 bar, and at a temperature between 20 ° C and 100 ° C. Preferably, an absorbent solution loaded with acid compounds is obtained after the absorption step, and at least one regeneration step of the absorbent solution loaded with acidic compounds is carried out at a pressure of between 1 bar and 10 bar and at a pressure of temperature between 100 ° C and 180 ° C.

The gaseous effluent may be chosen from natural gas, synthesis gases, combustion fumes, refinery gases, acid gases from an amine unit, gases from a tail reduction unit of Claus process, biomass fermentation gases, cement gases, incinerator fumes.

The process according to the invention can be implemented to selectively remove H ₂ S with respect to C0 ₂ from a gaseous effluent comprising H ₂ S and CO ₂ . Advantageously, the process according to the invention is implemented for the selective removal of H ₂ S with respect to C0 ₂ contained in natural gas.

Preferably, the H ₂ S is eliminated selectively with respect to CO ₂ in a ratio S of formula (a) below, said ratio being greater than 1.1. ^ 1 H ₂ S

In formula (a): r | _H2 s is the elimination efficiency of H ₂ S, said elimination efficiency of H ₂ S η _Η2δ

 ^ Gaztracted ^ ^ Gaztraitê,,

s expressing according to the following formula (a1): η _Ηι8 = 1 - (a1)

^ ^ ^ Gazbmt Gazb t 2 η _∞ is the efficiency of removal of C0 _2, said removal efficiency of C0 _2, η ₀ ο2 speaking by the formula (a2): _η ∞2 = (a2)

In the formulas (a1) and (a2), F is the molar flow rate of the gaseous effluent, raw or treated; and y is the molar fraction of H ₂ S or CO ₂ acid gas.

Preferably, the ratio S is greater than 2, and is preferably greater than 2.5.

Other objects and advantages of the invention will appear on reading the following description of examples of particular embodiments of the invention, given by way of non-limiting examples, the description being made with reference to the appended FIG. below.

Brief description of the figure

FIG. 1 represents a schematic diagram of the implementation of an acid gas treatment process. Detailed description of the invention

The present invention proposes to eliminate the acidic compounds of a gaseous effluent by implementing in aqueous solution whose composition is detailed below. Composition of the absorbent solution

The absorbing solution used for the removal of the acidic compounds contained in a gaseous effluent comprises:

 - some water;

at least one nitrogen compound corresponding to the following general formula (I):

dans laquelle x est égal à 0 ou 1 , et dans laquelle R est un atome d'hydrogène ou un radical -(CH₂)3-N-(CH₃)2 quand x est égal à 1 .

wherein x is 0 or 1, and wherein R is hydrogen or - (CH ₂ ) 3 -N- (CH ₃ ) 2 when x is 1.

According to the invention, the absorbent solution does not comprise N, N, N ', N'-tetramethyl-1,6-hexanediamine (TMHDA), of the following formula:

Preferably, the nitrogen compound of general formula (I) is bis (3-dimethylaminopropoxy) -1,2-ethane corresponding to the following formula (II):

(II) La formule (II) correspond au cas de la formule générale (I) où x est égal à 0.

(II) The formula (II) corresponds to the case of the general formula (I) where x is equal to 0.

The nitrogen compound of general formula (I) may be in variable concentration in the absorbent solution, for example between 5% and 95% by weight, preferably between 10% and 90% by weight, more preferably between 20% and 60% by weight. and most preferably between 25% and 50% by weight, inclusive. The absorbent solution may contain between 5% and 95% by weight of water, preferably between 10% and 90% by weight of water, more preferably between 40% and 80% by weight of water, and very preferably 50% by weight. 75% water, limits included.

The sum of the mass fractions expressed in% by weight of the various compounds of the absorbent solution is equal to 100% by weight of the absorbent solution.

According to one embodiment, the absorbent solution may also contain at least one additional amine which is a tertiary amine such as N-methyldiethanolamine, triethanolamine, diethylmonoethanolamine, dimethylmonoethanolamine, or ethyldiethanolamine, or which is a secondary amine having a severe steric hindrance, this congestion being defined either by the presence of two secondary carbons alpha to the nitrogen atom, or by at least one tertiary carbon alpha to the nitrogen atom. By said additional amine is meant any compound having at least one tertiary amine or secondary amine function severely congested. Said additional amine is not N, N, N ', N'-tetramethyl-1,6-hexanediamine. The concentration of said tertiary or secondary secondary amine severely congested in the absorbent solution may be between 5% and 95% by weight, preferably between 5% and 50% by weight, very preferably between 5% and 30% by weight.

According to one embodiment, the amines according to the general formula (I) can be formulated with one or more compounds containing at least one primary or secondary amine function. For example, the absorbent solution comprises up to a concentration of 30% by weight, preferably less than 15% by weight, preferably less than 10% by weight of said compound containing at least one primary or secondary amine function. Preferably, the absorbent solution comprises at least 0.5% by weight of said compound containing at least one primary or secondary amine function. Said compound makes it possible to accelerate the absorption kinetics of C0 ₂ and, in certain cases, of the COS contained in the gas to be treated.

A non-exhaustive list of compounds containing at least one primary or secondary amine function that may be included in the formulation is given below:

 monoethanolamine;

 diethanolamine;

 n-butylethanolamine;

aminoethylethanolamine; diglycolamine;

 piperazine;

 1-methylpiperazine;

 2-methylpiperazine;

 - homopiperazine;

 N- (2-hydroxyethyl) piperazine;

 N- (2-aminoethyl) piperazine;

 morpholine;

 3- (methylamino) propylamine;

 1,6-hexanediamine and all its variously N-alkylated derivatives, for example N, N'-dimethyl-1,6-hexanediamine, N, N'-dimethyl-1,6-hexanediamine, N-methyl 1,6-hexanediamine or N, N ', N'-trimethyl-1,6-hexanediamine. It is noted that the TMHDA does not fit this definition because it is not a compound having a primary or secondary amine function. According to the invention, the absorbent solution comprising a nitrogen compound of general formula (I) may comprise a mixture of additional amines as defined above.

According to one embodiment, the absorbent solution may contain organic compounds that are non-reactive with respect to acidic compounds (commonly called "physical solvents"), which make it possible to increase the solubility of at least one or more acidic compounds of the gaseous effluent. For example, the absorbent solution may comprise between 5% and 50% by weight of physical solvent such as alcohols, ethers, ether alcohols, glycol ethers and polyethylene glycol ethers, glycol thioethers, glycol esters and alkoxy esters, and the like. polyethylene glycol, glycerol esters, lactones, lactams, N-alkylated pyrrolidones, morpholine derivatives, morpholin-3-one, imidazoles and imidazolidinones, N-alkylated piperidones, cyclotetramethylenesulfones, N-alkylated alkylformamides, N-alkylacetamides, ether ketones of alkyl carbonates or alkyl phosphates and their derivatives. By way of example and without limitation, it may be methanol, ethanol, 2-ethoxyethanol, triethyleneglycoldimethylether, tetraethyleneglycoldimethylether, pentaethyleneglycoldimethylether, hexaethyleneglycoldimethylether, heptaethyleneglycoldimethylether, octaethyleneglycoldimethylether, diethylene glycol butoxyacetate, glycerol triacetate, sulfolane, N-methylpyrrolidone, N-methylmorpholin-3-one, N, N-dimethylformamide, N-formyl-morpholine, N, N-dimethyl-imidazolidin-2-one, N-methylimidazole, ethylene glycol, diethylene glycol, triethylene glycol, thiodiglycol, propylene carbonate, tributyl phosphate.

Synthesis of Nitrogen Compounds of General Formula (I)

Synthesis of bis- (3-dimethylaminopropoxy) -1,2-ethane

Bis (3-dimethylaminopropoxy) -1,2-ethane can be prepared according to various synthetic routes known to those skilled in the art.

The bis- (3-dimethylaminopropoxy) -1,2-ethane can be prepared according to the following synthetic route, cited by way of non-limiting example.

Bis (3-dimethylaminopropoxy) 1,2-ethane can be prepared in three stages, the first of which consists in the reaction of one mole of ethylene glycol with two moles of acrylonitrile, leading to 1,2-bis (2-cyanoethoxy) )ethane.

The nitrile functions of 1, 2-bis (2-cyanoethoxy) ethane are then reduced in a second step to primary amine functions by the action of hydrogen, in the presence of a suitable catalyst and sometimes ammonia to lead to bis ( 3-aminopropoxy) 1,2-ethane. The synthesis of bis (3-aminopropoxy) 1,2-ethane according to this sequence of two steps is described, for example, in US 4,313,004, US 5,075,507 and US 5,081,305.

In a third step, the two primary amine functions of bis (3-aminopropoxy) 1,2-ethane are methylated, for example by reductive amination generally using formaldehyde, in the presence of hydrogen and a suitable catalyst to lead to bis (3-dimethylaminopropoxy) 1,2-ethane.

Synthesis of the compound of general formula (I) with x = 1 and R = H

The compound of general formula (I) with x = 1 and R = H may be prepared according to different synthesis routes known to those skilled in the art.

The compound of general formula (I) with x = 1 and R = H may be prepared according to following synthesis, cited by way of non-limiting example.

The compound of general formula (I) with x = 1 and R = H may be prepared in three stages, the first of which consists in the reaction of one mole of glycerol with two moles of acrylonitrile, resulting in 3- [3- ( 2-cyanoethoxy) -2-hydroxypropoxy] propanenitrile. The nitrile functions of 3- [3- (2-cyanoethoxy) -2-hydroxypropoxy] propanenitrile are then reduced in a second step to primary amine functions by the action of hydrogen, in the presence of a suitable catalyst and sometimes of ammonia. to yield 1,3-bis (3-aminopropoxy) propan-2-ol.

In a third step, the two primary amine functions of 1,3-bis (3-aminopropoxy) propan-2-ol are methylated, for example by reductive amination, generally using formaldehyde, in the presence of hydrogen and a catalyst. suitable for yielding 2,14-dimethyl-6,10-dioxa-2,14-diazapentadecan-8-ol.

Synthesis of the compound of general formula (I) with x = 1 and R = - (CH ₂ ) ₂ -NMe

The compound of general formula (I) with x = 1 and R = - (CH ₂ ) 3 -NMe ₂ may be prepared according to different synthesis routes known to those skilled in the art.

The compound of general formula (I) with x = 1 and R = - (CH ₂ ) 3 -NMe ₂ may be prepared according to the following synthetic route, cited by way of non-limiting example.

The compound of general formula (I) with x = 1 and R = - (CH ₂ ) ₃ -NMe ₂ may be prepared in three stages, the first of which consists in the reaction of one mole of glycerol with three moles of acrylonitrile, resulting in 3 - {[1,3-bis (2-cyanoethoxy) propan-2-yl] oxy} propanenitrile.

The nitrile functions of 3 - {[1,3-bis (2-cyanoethoxy) propan-2-yl] oxy} propanenitrile are then reduced in a second step to primary amino functions by the action of hydrogen, in the presence of a suitable catalyst and sometimes ammonia to yield 3 - {[1,3-bis (3-aminopropoxy) propan-2-yl] oxy} propan-1-amine. In a third step, the three primary amine functions of 3 - {[1,3-bis (3-aminopropoxy) propan-2-yl] oxy} propan-1-amine are methylated, for example by reductive amination generally by means of formaldehyde, in the presence of hydrogen and a suitable catalyst to give 8- [3- (dimethylamino) propoxy] -2,14-dimethyl-6,10-dioxa-2,14-diazapentadecane. Nature of gaseous effluents

According to the invention, the absorbent solutions can be used to deacidify the following gaseous effluents: natural gas, synthesis gases, combustion fumes, refinery gases, acid gases from an amine unit, gases from a reduction unit at the bottom of the Claus process, biomass fermentation gases, cement gases, incinerator fumes. These gaseous effluents contain one or more of the following acidic compounds: C0 ₂ , H ₂ S, mercaptans (for example methylmercaptan (CH ₃ SH), ethylmercaptan (CH ₃ CH ₂ Si-I), propylmercaptan (CH ₃ CH ₂ CH ₂ SH)), COS, CS ₂ , S0 ₂ . The combustion fumes are produced in particular by the combustion of hydrocarbons, biogas, coal in a boiler or for a combustion gas turbine, for example for the purpose of producing electricity. By way of illustration, a deacidification process according to the invention can be used to absorb at least 70%, preferably at least 80% or even at least 90% of the CO ₂ contained in the combustion fumes. These fumes generally have a temperature of between 20 ° C. and 60 ° C., a pressure of between 1 and 5 bar and may comprise between 50% and 80% of nitrogen, between 5% and 40% of carbon dioxide, between % and 20% oxygen, and some impurities such as SOx and NOx, if they were not removed upstream of the deacidification process. In particular, the deacidification process according to the invention is particularly well adapted to absorb the CO ₂ contained in combustion fumes comprising a low partial pressure of CO ₂ , for example a CO ₂ partial pressure of less than 200 mbar.

The deacidification process according to the invention can be implemented to deacidify a synthesis gas. The synthesis gas contains carbon monoxide CO, dihydrogen H ₂ (generally in a ratio H ₂ / CO equal to 2), water vapor (generally at saturation at the temperature at which washing is carried out) and carbon dioxide C0 ₂ (of the order of ten percent). The pressure is generally between 20 and 30 bar, but can reach up to 70 bar. It may also contain sulfur (H ₂ S, COS, etc.), nitrogen (NH ₃ , HCN) and halogenated impurities.

The deacidification process according to the invention can be implemented to deacidify a natural gas. The natural gas consists mainly of gaseous hydrocarbons, but can contain several of the following acidic compounds: C0 ₂ , H ₂ S, mercaptans, COS, CS ₂ . The content of these acidic compounds is very variable and can be up to 70% by volume for C0 ₂ and up to 40% by volume for H ₂ S. The temperature of the natural gas can be between 20 ° C and 100 ° C. The pressure of the natural gas to be treated may be between 10 and 200 bar. The invention can be implemented to achieve specifications generally imposed on the deacidified gas, which are less than 2% of C0 ₂ , or even less than 50 ppm of C0 ₂ to then achieve liquefaction of natural gas and less than 4 ppm H ₂ S, and less than 50 ppm, or even less than 10 ppm, total sulfur volume.

Process for removing acidic compounds in a gaseous effluent

The use of an aqueous solution comprising at least one nitrogen compound of general formula (I), such as bis- (3-dimethylaminopropoxy) -1,2-ethane, and having no TMHDA, for deacidifying an effluent gaseous is performed schematically by performing an absorption step followed by a regeneration step, for example as shown in Figure 1.

With reference to FIG. 1, the deacidification plant of a gaseous effluent according to the invention comprises an absorption column C1 provided with gas-liquid contacting means, for example a loose packing, a structured packing or plates. The gaseous effluent to be treated is conveyed via a pipe 1 opening at the bottom of column C1. A pipe 4 allows the introduction of the absorbent solution at the top of the column C1. A pipe 2 allows the evacuation of the treated gas (deacidified), and a pipe 3 conveys the absorbent solution enriched in acidic compounds following absorption to a regeneration column C2. This regeneration column C2 is equipped with internal contacting between gas and liquid, for example trays, loose or structured packings. The bottom of the column C2 is equipped with a reboiler R1 which provides the heat necessary for the regeneration by vaporizing a fraction of the absorbing solution. The solution enriched in acidic compounds is introduced at the top of the regeneration column C2 via a line 5. A line 7 makes it possible to evacuate at the top of the column C2 the gas enriched in acid compounds released during the regeneration, and a line 6 arranged at the bottom of the column C2 makes it possible to send the regenerated absorbent solution to the absorption column C1. A heat exchanger E1 makes it possible to recover the heat of the regenerated absorbent solution issuing from column C2 in order to heat the absorbent solution enriched in acidic compounds leaving the absorption column C1. The absorption step consists of contacting the gaseous effluent arriving via line 1 with the absorbent solution arriving via line 4. During contact, the amine functions of the molecules of the absorbing solution react with the acidic compounds contained in the effluent so as to obtain a gaseous effluent depleted of acidic compounds which is discharged through line 2 at the top of column C1 and an acid-enriched absorbent solution discharged through line 3 at the bottom of column C1 to be regenerated.

The absorption step of the acidic compounds can be carried out at a pressure in the column C1 of between 1 bar and 200 bar, preferably between 20 bar and 100 bar for the treatment of a natural gas, preferably between 1 bar and 3 bar for the treatment of industrial fumes, and at a temperature in the column C1 of between 20 ° C and 100 ° C, preferably between 30 ° C and 90 ° C, or between 30 and 60 ° C.

The regeneration step consists in particular in heating and, optionally, in expanding, the absorbent solution enriched in acidic compounds in order to release the acidic compounds in gaseous form. The absorbent solution enriched in acidic compounds leaving the column C1 is introduced into the heat exchanger E1, where it is heated by the flow flowing in the pipe 6 from the regeneration column C2. The heated solution at the outlet of E1 is introduced into the regeneration column C2 via line 5.

In the regeneration column C2, under the effect of contacting the absorbent solution arriving via line 5 with the vapor produced by the reboiler, the acid compounds are released in gaseous form and discharged at the top of column C2 by the 7. The absorbent solution regenerated, that is to say depleted in acidic compounds, is discharged through line 6 and is cooled in E1, then recycled to the absorption column C1 via line 4.

The regeneration step may be carried out by thermal regeneration, optionally supplemented by one or more expansion steps. For example, the absorbent solution enriched in acidic compounds discharged through the pipe 3 can be sent to a first expansion tank (not shown) before it passes through the heat exchanger E1. In the case of a natural gas, the expansion makes it possible to obtain a gas evacuated at the top of the flask containing most of the aliphatic hydrocarbons co-absorbed by the absorbing solution. This gas may optionally be washed with a fraction of the regenerated absorbent solution and the gas thus obtained may be used as a fuel gas. The expansion flask preferably operates at a pressure lower than that of the column absorption C1 and greater than that of the regeneration column C2. This pressure is generally set by the conditions of use of the fuel gas, and is typically of the order of 5 to 15 bar. The expansion flask operates at a temperature substantially identical to that of the absorbent solution obtained at the bottom of the absorption column C1. The regeneration can be carried out at a pressure in the column C2 of between 1 bar and 5 bar, or even up to 10 bar and at a temperature in the column C2 of between 100 ° C. and 180 ° C., preferably between 10 ° C. and 10 ° C. ° C and 170 ° C, more preferably between 120 ° C and 140 ° C. Preferably, the regeneration temperature in column C2 is between 155 ° C. and 180 ° C. in the case where it is desired to reinject the acid gases. Preferably, the regeneration temperature in column C2 is between 115 ° C. and 130 ° C. in the cases where the acid gas is sent to the atmosphere or in a downstream treatment process, such as a Claus process or a tail gas treatment process.

The process according to the invention advantageously allows a selective removal of the H ₂ S vis-à-vis the CO ₂ in the gas to be treated.

Preferably, the process is implemented for the selective removal of H ₂ S with respect to CO ₂ of natural gas comprising H ₂ S and CO ₂ .

The selectivity of absorption of H ₂ S with respect to CO ₂ can be expressed by the ratio S of the elimination efficiencies of the two gases:

et

Dans ces expressions, Fdésigne le débit molaire de l'effluent gazeux, brut ou traité; y désigne la fraction molaire en gaz acide, H₂S ou C0₂. These elimination efficiencies are expressed respectively by

and

In these expressions, F denotes the molar flow rate of the gaseous effluent, raw or treated; y denotes the molar fraction of acid gas, H ₂ S or CO ₂ .

According to the invention, the elimination selectivity of H ₂ S with respect to C0 ₂ is greater than 1, 1, preferably greater than 2, and even more preferably greater than 2.5. Examples

The examples below illustrate, in a nonlimiting manner, the synthesis of a nitrogen compound of general formula (I), bis (3-dimethylaminopropoxy) -1,2-ethane, as well as some of the performances of this compound when it is used in aqueous solution, without TMHDA, to remove acid compounds, such as CO ₂ or H ₂ S, contained in a gaseous effluent by contacting the gaseous effluent with the solution.

Example 1 Synthesis of bis (3-dimethylaminopropoxy) -1,2-ethane

At the laboratory scale, the synthesis of bis (3-dimethylaminopropoxy) -1,2-ethane is carried out as follows.

90.0 g (0.51 l mol) of 1,2-bis (3-aminopropoxy) ethane and 0.5 g of a platinum on carbon catalyst are introduced into an autoclave reactor. The reactor is then purged by the introduction of hydrogen up to 5 bar followed by degassing under reduced pressure. The operation is repeated 3 times. 250 g of an aqueous solution of formaldehyde at 37% by weight are then introduced slowly. The reactor is then stirred for 5 hours at 120 ° C. under a pressure of 20 bar of hydrogen. After returning to ambient temperature, the catalyst is filtered to isolate the liquid phase. The previous operation is reproduced and then the two liquid phases are joined and distilled. 137.3 g is recovered of a product whose NMR spectrum ¹³ C below detailed is consistent with bis- (3-dimethylaminopropoxy) -1, 2-ethane.

44.7 ppm: (CH ₃ ) ₂ N-CH ₂ -CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -CH ₂ -N (CH ₃ ) ₂ 55.8 ppm: ( CH ₃ ) ₂ N-CH ₂ -CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -CH ₂ -N (CH ₃ ) ₂ 27.2 ppm: (CH ₃ ) ₂ N -CH ₂ -CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -CH ₂ -N (CH ₃ ) ₂

68.7 ppm: (CH ₃ ) ₂ N-CH ₂ -CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -CH ₂ -N (CH ₃ ) ₂

69.4ppm: (CH ₃ ) ₂ N-CH ₂ -CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -O-CH ₂ -CH ₂ -CH ₂ -N (CH ₃ ) ₂ Example 2 Absorption Capacity of the HpS of an Amine Formulation for an Acid Gas Treatment Process

The performance of the absorption capacity of H ₂ S at 40 ° C. of an aqueous solution of bis (3-dimethylaminopropoxy) -1,2-ethane according to the invention, containing 30% by weight of bis ( 3-dimethylaminopropoxy) -1,2-ethane, and not containing TMHDA, is compared with that of an aqueous solution of MDEA containing 50% by weight of MDEA, which is a reference absorbent solution for selective removal in treatment gas.

An absorption test at 40 ° C is carried out on aqueous amine solutions in a thermostatic equilibrium cell. This test consists of injecting into the equilibrium cell, previously filled with degassed aqueous amine solution, a known quantity of acid gas, of H ₂ S in this example, and then waiting for the establishment of the state of equilibrium. 'balanced. The quantities of acid gas absorbed in the aqueous amine solution are then deducted from the temperature and pressure measurements by means of material and volume balances. The solubilities are conventionally represented in the form of H ₂ S partial pressures (in bar) as a function of the feed rate in H ₂ S (in mol of H ₂ S / kg of absorbing solution and in mol of H ₂ S / mol of amine).

In the case of selective deacidification in natural gas treatment, the partial H ₂ S pressures encountered in the acid gases are typically between 0.05 and 0.15 bar, at a temperature of 40 ° C. By way of example, in this industrial range, Table 1 below compares the H ₂ S loading rates obtained at 40 ° C. for various H ₂ S partial pressures between the MDEA absorbent solution at 50.degree. % wt. and the absorbent solution of bis- (3-dimethylaminopropoxy) -1,2-ethane at 30% wt.

Table 1 At 40 ° C., whatever the H ₂ S partial pressure, the absorption capacity of the aqueous solution of bis (3-dimethylaminopropoxy) -1,2-ethane according to the invention is greater than that of the solution. of MDEA. Indeed, at a partial pressure of 0.05 bar, the loading rate in H ₂ S is 1.25 mol / kg in the absorbent solution of bis (3-dimethylaminopropoxy) -1,2-ethane and 0 , 64 mol / kg in the reference MDEA absorbent solution. At a H ₂ S partial pressure of 0.10 bar, the difference between the H ₂ S loading rates of the two absorbent solutions is 0.82 mol / kg with an absorption capacity for the absorbent solution. of bis (3-dimethylaminopropoxy) -1,2-ethane increased by 93% relative to the reference MDEA absorbent solution. At a H ₂ S partial pressure of 0.15 bar, the difference in H ₂ S loading ratio between the two absorbent solutions is still 77% in favor of the absorbing solution of bis (3-dimethylaminopropoxy) - 1, 2-ethane. It can therefore be seen that the aqueous solution of bis (3-dimethylaminopropoxy) -1,2-ethane at 30% by weight has a capacity for absorbing H ₂ S that is higher than the aqueous solution of 50% reference MDEA. weight of MDEA at 40 ° C, in the range of partial pressures in H ₂ S between 0.05 and 0.15 bar corresponding to a range of partial pressure representative of the usual industrial conditions.

Example 3: Calculation of a plate absorber. Search for a maximum selectivity of absorption of HpS with respect to CO?

The absorption step of the process according to the invention is carried out in order to treat a natural gas whose pressure at the inlet of the absorber is 71.9 bar and the temperature of 31.2 ° C. The molar composition of the natural gas at the inlet of the absorber is as follows: 85 mol% of methane, 4.9 mol% of ethane, 1, 41% of propane, 0.26% of isobutane, 0, 59% n-butane, 0.15% isopentane, 0.30% n-pentane and 0.14% n-hexane. The gas also contains 0.09% water, 2.53% nitrogen and 2.13% C0 ₂ and 2.49% H ₂ S. The specifications for the treated gas are 2 ppmv for H ₂ S and 2 mol% for C0 ₂ . Selectivity for eliminating H ₂ S from maximum C0 ₂ is therefore required.

The raw gas, at a flow rate of 19 927 kmol / h, is contacted in counter-current with an aqueous solution of regenerated amine, characterized by charge rates of 7.10 ^-4 mol of H ₂ S per mole of amine and 3.10 ^"3 mole C0 ₂ per mole of amine, in an absorber equipped with 4-pass flap trays, the spacing between trays being 60 cm. The temperature of the regenerated amine solution at the top of the absorber is 44.6 ° C. The absorber is modeled by n real trays taking into account rigorously the geometry of the trays, on each of which the acid gas flows are calculated in using the double film approach. An iterative calculation makes it possible to resolve the material and thermal balances plateau by plateau and to calculate, according to the number of plateaux n, the acid gas concentration and the temperature profiles in the absorber.

The absorber is sized by the number of trays necessary to achieve the required specification of 2ppmv H ₂ S in the treated gas. For each calculation, this number of plates n is obtained as well as the corresponding CO ₂ concentration in the treated gas.

It is recalled that the absorption selectivity of H ₂ S with respect to C0 ₂ is defined by the ratio S of the elimination efficiencies of the two gases:

J (a)

et co ₂ These elimination efficiencies are expressed respectively by

and

_Λ F Treated gas x J v G ^c a ° z ² treated _/ _- _{* \}

77co ₂ = 1 ^ - (a2)

 ^ Gazbmt ^ ^ Gazbmt

In these expressions, F denotes the molar flow rate of acid gas, crude or treated; denotes the molar fraction of acid gas, H ₂ S or CO ₂ .

Table 2 below compares the results obtained by calculation for an absorption step using an aqueous solution of MDEA containing 47% by weight of MDEA according to the reference method to those obtained by using an aqueous solution of bisphenol. (3-dimethylaminopropoxy) -1,2-ethane at 50% by weight, and not containing TMHDA, according to the method of the invention. bis- (3- MDEA

 dimethylaminopropoxy) -

Formulation

 1, 2-ethane at 50% wt. 47% weight

 according to the invention (reference)

Processed gas

Y∞ ₂ [% molsec] 1, 38 1, 26

YH ₂ S [ppm molsec] 2 2

% removal H ₂ S 100.0% 100.0%

% elimination C0 ₂ 37% 43%

Selectivity H ₂ S / C0 ₂ 2.67 2.33

Solvent flow

 300,350

[Sm ³ / h]

Diameter [m] 3.41 3.51

Number of

 12 16

 uplands

Table 2

This example illustrates the gain in selectivity of 15% obtained with the process according to the invention using a formulation of 1 bis- (3-dimethylaminopropoxy) -1,2-ethane at 50% by weight without TMHDA, compared to the process of reference. The implementation of the process according to the invention described here also makes it possible to reduce by 15% the flow rate of solvent used. This results in a decrease in the diameter of the column, from 3.51 to 3.41 m.

The number of trays needed to achieve a 2ppm H ₂ S specification in the treated gas is also reduced from 16 to 12 trays. Example 4: Calculation of a plate absorber. Search for maximum absorption capacity in acid gas (H? S and CO?)

The absorption step of the process according to the invention is carried out to treat a natural gas whose pressure at the inlet of the absorber is 66.8 bar and the temperature of 43.5 ° C. The molar composition of the natural gas at the inlet of the absorber is as follows: 84.62 mol% of methane, 2.44 mol% of ethane, 1.37% of propane, 0.31% of isobutane, 0.67% n-butane, 0.25% isopentane, 0.28% n-pentane and 0.37% n-hexane. The gas also contains 0.1% nitrogen and 5.51% C0 ₂ and 4.05% H ₂ S. The specifications for the treated gas are 2 ppmv for H ₂ S and 2 mol%. for C0 ₂ . In this case it is sought to minimize the flow of absorbent solution to implement the solvent for a total elimination H ₂ S and a removal efficiency of CO ₂ fixed at 66%.

The raw gas at a flow rate of 15025 kmol / h is contacted countercurrently with an aqueous solution of regenerated amine, characterized by charge rates of 7.10 ^-4 mol H ₂ S per mole of amine and 3.10 ^"3 moles of C0 ₂ per mole of amine, in an absorber equipped with 4-pass flap trays, the spacing between trays being 60cm. The temperature of the regenerated amine solution at the top of the absorber is 57.7 ° C. The absorber is modeled as in the previous example.

The absorber is sized by the number of trays necessary to achieve the required specification of 2% CO ₂ in the treated gas. This number of plates n is obtained for each calculation, as is the residual H ₂ S content, which is less than 2 ppm.

Table 3 below compares the results obtained by calculation for an absorption step using an aqueous solution of MDEA containing 46% by weight of MDEA according to the reference method to those obtained by using an aqueous solution of bisphenol. (2-dimethylaminopropoxy) -1,2-ethane at 50% by weight, and not containing TMHDA, according to the method of the invention. bis- (3- MDEA

 dimethylaminopropoxy) -

Formulation

 1, 2-ethane at 50% weight 46% weight

 according to the invention (reference)

Processed gas

Y∞ ₂ [% molsec] 2.0 2.0

YH ₂ S [ppm molsec] 0.2 1, 3

Solvent flow

 500 600

[Sm ³ / h]

Diameter [m] (4

 3.81 3.90

 assists)

Nb. Trays 24 28

Table 3

The implementation of the process according to the invention described here makes it possible to reduce by 17% the flow rate of absorbing solution used. This results in a decrease in the diameter of the column, from 3.90 to 3.81 m. The number of trays needed to achieve a specification of 2% C0 ₂ in the treated gas is also reduced from 28 to 24 trays.

Claims

1. A method of removing acidic compounds contained in a gaseous effluent, wherein a step of absorbing acidic compounds is carried out by contacting the effluent with an absorbent solution comprising: - water; at least one nitrogen compound of general formula (I) below:

wherein x is 0 or 1, and wherein R is a hydrogen atom or - (CH ₂ ) 3 -N- (CH ₃ ) 2 radical when x is 1, said absorbent solution not comprising N, N, N ', N'-tetramethyl-1,6-hexanediamine. The process according to claim 1, wherein said nitrogenous compound is bis (3-dimethylaminopropoxy) -1, 2-ethane of formula (II) below:

3. Method according to one of the preceding claims, wherein the absorbent solution comprises between 5% and 95% by weight of said nitrogenous compound, preferably between 10% and 90% by weight of said nitrogenous compound, and between 5% and 95% by weight. water, and preferably between 10% and 90% by weight of water. 4. Method according to one of the preceding claims, wherein the absorbent solution further comprises between 5% and 95% by weight of at least one additional amine, said additional amine being either a tertiary amine or a secondary amine comprising two carbons secondary in alpha of the nitrogen atom or at least one tertiary carbon in alpha of the nitrogen atom. The method of claim 4, wherein said additional amine is a tertiary amine selected from the group consisting of:  N-methyldiethanolamine;  triethanolamine;  diethylmonoethanolamine;  dimethylmonoethanolamine; and  ethyldiethanolamine. Method according to one of the preceding claims, wherein the absorbent solution further comprises a non-zero amount and less than 30% by weight of at least one additional amine being a primary amine or a secondary amine. The method of claim 6, wherein said additional primary or secondary amine is selected from the group consisting of:  monoethanolamine;  diethanolamine;  n-butylethanolamine;  aminoethylethanolamine;  diglycolamine;  piperazine;  1-methylpiperazine;  2-methylpiperazine;  - homopiperazine;  N- (2-hydroxyethyl) piperazine;  N- (2-aminoethyl) piperazine;  morpholine;  3- (methylamino) propylamine;  1,6-hexanediamine; N, N-dimethyl-1,6-hexanediamine; N, N'-dimethyl-1,6-hexanediamine  N-methyl-1,6-hexanediamine; and  N, N ', N'-trimethyl-1,6-hexanediamine. Method according to one of the preceding claims, wherein the absorbent solution further comprises at least one physical solvent selected from the group consisting of methanol, ethanol, 2-ethoxyethanol, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, pentaethylene glycol dimethyl ether, hexaethyleneglycoldimethylether, heptaethyleneglycoldimethylether, octaethyleneglycoldimethylether, diethyleneglycol butoxyacetate, glycerol triacetate, sulfolane, N-methylpyrrolidone, N-methylmorpholin-3-one, N, N-dimethylformamide, N-formyl- morpholine, N, N-dimethylimidazolidin-2-one, N-methylimidazole, ethyleneglycol, diethyleneglycol, triethyleneglycol, thiodiglycol, propylene carbonate, tributylphosphate. Process according to one of the preceding claims, in which the step of absorption of the acidic compounds is carried out at a pressure of between 1 bar and 200 bar, and at a temperature of between 20 ° C and 100 ° C. Process according to one of the preceding claims, in which an absorbent solution loaded with acidic compounds is obtained after the absorption step, and at least one regeneration step of said absorbent solution loaded with acidic compounds is carried out at a pressure of between 1 bar and 10 bar and at a temperature between 100 ° C and 180 ° C. Process according to one of the preceding claims, in which the gaseous effluent is chosen from natural gas, synthesis gases, combustion fumes, refinery gases, acid gases derived from an amine unit, gases from a reduction unit at the bottom of the Claus process, biomass fermentation gases, cement gases, incinerator fumes. 12. Method according to one of the preceding claims, implemented for the selective removal of H2S with respect to CO2 from a gaseous effluent comprising H2S and CO2, preferably natural gas. 13. The method of claim 12, implemented for the selective removal of H2S relative to CO 2 contained in natural gas. 14. Process according to any one of claims 12 and 13, in which the H 2 S is eliminated selectively with respect to CO2 in a ratio S of formula (a) below: S = (a) co ₂ r | _H 2s being the removal efficiency of H ₂ O, said H ₂ S removal efficiency F Gas treated x Jv Ga ² z treated r | _H 2s s expressing according to the following formula (a1): η _Ηι8 = 1 - (a1) ^ Gazbrut ^ 3 ^ Gazbri η ₀ ο2 being CO ₂ removal efficiency, said CO ₂ removal efficiency, η∞ ₂ F _Gastmity xy _G ^c ° expressing according to the following formula (a2): η _∞2 = 1  ^ Gazbmt ^ ^ Gazb brriut Celebrating the molar flow rate of gaseous effluent, raw or treated; y being the molar fraction of acid gas H ₂ S or C0 ₂ ,  said ratio S being greater than 1.1. 15. The method of claim 14, wherein the ratio S is greater than 2, and preferably greater than 2.5.