FR2959944A1 - ABSORBENT SOLUTION CONTAINING PYRIDINE-DERIVED DEGRADATION INHIBITOR AND METHOD FOR ABSORPTION OF ACIDIC COMPOUNDS CONTAINED IN GASEOUS EFFLUENT - Google Patents

Abstract

The degradation of an absorbent solution comprising organic compounds provided with an amine function in aqueous solution is substantially reduced in the presence of a small amount of degradation inhibiting additives belonging to the family of pyridine derivatives including at least one substituent contains a sulfur atom. The absorbent solution is used to deacidify a gaseous effluent.

Description

The present invention relates to the field of the deacidification of a gaseous effluent. More precisely, the present invention provides compounds for reducing the degradation of an absorbent solution used to absorb the acidic compounds contained in a gaseous effluent, the absorbent solution comprising amines in aqueous solution. In particular, the invention relates to compounds used to reduce the degradation of the amines used for the deacidification of oxygen-containing gases, such as for example combustion fumes.

Deacidification of gaseous effluents, such as, for example, natural gas and combustion fumes, is generally carried out by washing with an absorbent solution. The absorbent solution makes it possible to absorb the acid compounds present in the gaseous effluent (H2S, mercaptans, CO2, COS, SO2, CS2). The deacidification of these effluents, in particular decarbonation and desulphurization, imposes specific stresses on the absorbent solution, in particular a thermal and chemical stability, particularly with regard to the impurities of the effluent, namely essentially oxygen, SOx and sulfur. NOx. The oxygen may also come into contact with the absorbent solution without necessarily being present in the gaseous effluent to be treated, as in the case of, for example, an accidental entry of air into the absorbent solution storage tanks. Absorbent solutions most used today are aqueous solutions of alkanolamines. We can cite the document FR 2 820 430 which proposes processes for deacidification of gaseous effluents. However, it is well known to those skilled in the art that these amines have the disadvantage of degrading under the conditions of implementation. In particular, the amines can be degraded by the oxygen generating amine consumption and the formation of degradation products which accumulate in the unit or, for the more volatile, which are entrained in the gaseous effluents of the process . Thus, particularly in the case of post-combustion flue gas treatment in a process using an aqueous monoethanolamine (MEA) solution, large amounts of ammonia are formed. The ammonia thus formed is entrained in the atmosphere with the treated fumes, which poses problems with regard to the protection of the environment.

In the case of CO2 capture in fumes from industrial units or from electricity or energy production in general, the degradation phenomena of the absorbent solution with amines are increased by the presence of a massive amount of oxygen in the load to be treated up to 5% by volume in general. In the case of fumes from the combined cycle with natural gas, the volume content of oxygen in the fumes can reach 15%. The degraded solution is characterized by: a decrease in the absorption of the acidic compounds of the filler relative to a fresh solution of amine, an increase in the density of the absorbent solution, as well as its viscosity, which can lead to a loss of performance, the formation of more volatile amines polluting the treated gas and the acid gas from the regeneration step: ammonia, methylamine, dimethylamine and trimethylamine, for example according to the nature of the amine used, an accumulation of products of degradation in the absorbent solution which may cause the need for treatment of the degraded solution, possible foaming problems due to degradation products. The degradation of the absorbent solution therefore penalizes the performance and the proper functioning of the gas deacidification units.

To overcome the problem of degradation, failing to limit or eliminate the presence of oxygen in the absorbent solution, compounds are added to the absorbent solution whose role is to prevent or limit the degradation phenomena of the amine compounds. , in particular the degradation caused by the oxidation phenomena. These compounds are commonly referred to as degradation inhibiting additives. The main known modes of action of the degradation-inhibiting additives are, depending on their nature, a reduction-type reaction and / or a capture, trapping and / or stabilization of the radicals formed in the absorbent solution in order to limit or prevent or interrupting reactions, including chain reactions, degradation.

US Patent 5686016 cites additives used to limit the degradation of absorbent solutions used for the deacidification of natural gas, especially oximes. US Patent 7056482 lists additives used to limit the degradation of absorbent solutions used for CO2 capture, particularly thiosulfates and sulfites.

The present invention provides a family of degradation inhibiting additives which makes it possible in particular to reduce the degradation of an absorbent solution used for the absorption of acidic compounds contained in a gaseous effluent, the absorbent solution comprising amine compounds in solution. aqueous.

In general, the invention describes an absorbent solution for absorbing the acidic compounds of a gaseous effluent, said solution comprising: a) at least one amine, b) water, c) at least one degradation inhibiting compound to limit the degradation of said amine, the degradation inhibiting compound being a derivative of pyridine, at least one carbon atom of the pyridine ring is bonded to a sulfur atom.

The degradation inhibiting compound can be in one of the following general formulas: R1 (Formula III) (Formula IV) (Formula V) R1 R1 R1 (Formula VI) (Formula VII) (Formula VIII) O SIX wherein X is selected from among a hydrogen atom, an alkaline or alkaline earth element, a monovalent or multivalent metal, an ammonium NH 4 + cation or resulting from the protonation of an amine function, a phosphonium cation, a hydrocarbon radical, comprising 1 to 12 carbon atoms, o one of the following units A, B, C, D, E or F: R5 R5 (Reason A) S (Reason B) R7 (Reason C) R R8 (Reason D) / S ( Reason E) R7 (Reason F) in which each of the radicals R1, R2, R3, R4, R5, R6, R7 and R8 is selected from: o a hydrogen atom, o a hydrocarbon radical comprising 1 to 12 carbon atoms carbon, 5 o an acid or sulphonic ester function, o an amine function W o a radical Y in which W is chosen from a sulfur atom and an oxygen atom and Y is chosen from: o a hydrogen, a radical comprising 1 to 12 carbon atoms, an alkaline or alkaline earth element, a monovalent or multivalent metal, an ammonium NH 4 + cation or resulting from the protonation of an amine function, a phosphonium cation.

According to the invention, at least one of the radicals R1, R2, R3, R4, R5, R6, R7, R8, X and Y may be a hydrocarbon group containing between 1 and 12 carbon atoms and may also comprise at least one member selected from a heteroatom and a halogen. At least one of the radicals R1, R2, R3, R4, R5, R6, R7, R8, X and Y may be a hydrocarbon group containing between 1 and 12 carbon atoms and may also comprise at least one chosen function. among the group: a hydroxyl function, a ketone function, a carboxyl function, an amine function and a nitrile function. At least two radicals selected from R1, R2, R3 and R4, or from R5, R6, R7 and R8 may be hydrocarbon radicals connected by a covalent bond to form a ring consisting of 5, 6, 7 or 8 atoms. The absorbent solution may comprise between 10% and 99% by weight of amine, between 1% and 90% by weight of water and between 5 ppm and 5% by weight of degradation inhibiting compound. The degradation inhibiting compound may be selected from the group containing 2-mercaptopyridine, 2-mercapto-6-methylpyridine, 2-mercaptopyridine N-oxide, 2-mercaptonicotinic acid, 6-mercaptonicotinic acid, 2-quinolinethiol, 5- (trifluoromethyl) -2-mercaptopyridine, 4-mercaptopyridine, 7- (trifluoromethyl) quinoline-4-thiol, 2- (2-pyridinyldisulfanyl) pyridine, 4- (4-pyridinyldisulfanyl) pyridine, a salt of 2-mercaptopyridine, a salt of 2-mercapto-6-methylpyridine, a salt of 2-mercaptopyridine N-oxide, a salt of 2-mercaptonicotinic acid, a salt of 6-mercaptonicotinic acid, a salt of 2-quinolinethiol, a salt of 5- (trifluoromethyl) -2-mercaptopyridine, a salt of 4-mercaptopyridine, a salt of 7- (trifluoromethyl) quinoline-4-thiol, a salt 2- (2-pyridinyldisulfanyl) pyridine and a salt of 4- (4-pyridinyldisulfanyl) pyridine. The amine may be selected from the group containing: N, N, N ', N', N "-pentamethyldiethylenetriamine, piperazine, monoethanolamine, diethanolamine, methyldiethanolamine, diisopropanolamine, diglycolamine, a glycine salt and a salt of taurine.

Preferably, the amine is monoethanolamine and the degradation inhibiting compound is chosen from 2-mercaptopyridine, 2-mercaptonicotinic acid, the sodium salt of 2-mercaptopyridine N-oxide, and 2-mercapto-6. methylpyridine, 5- (trifluoromethyl) -2-mercaptopyridine and 4-mercaptopyridine.

The absorbent solution may comprise at least 39% by weight of monoethanolamine.

The present invention also describes a process for absorbing acidic compounds contained in a gaseous effluent, in which the gaseous effluent is brought into contact with an aqueous solution containing at least one amine, and in which the degradation of said amine is controlled by introducing in said aqueous solution at least one pyridine-derived degradation inhibiting compound, of which at least one carbon atom of the pyridine ring is bonded to a sulfur atom.

The aqueous solution can be used to absorb acidic compounds contained in one of the effluents of the group containing the natural gas, the combustion fumes, the synthesis gases, the refinery gases, the gases obtained at the bottom of the Claus process. , biomass fermentation gases, cement gases and incinerator fumes.

The gaseous effluent may comprise at least 500 ppm by volume of oxygen. At least one degradation inhibiting compound selected from the group containing: 2-mercaptopyridine, 2-mercapto-6-methylpyridine, 2-mercaptopyridine N-oxide, 2-mercaptonicotinic acid can be introduced into the aqueous solution , 6-mercaptonicotinic acid, 2-quinolinethiol, 5- (trifluoromethyl) -2-mercaptopyridine, 4-mercaptopyridine, 7- (trifluoromethyl) quinoline-4-thiol, 2- (2-pyridinyldisulfanyl) pyridine , 4- (4-pyridinyldisulfanyl) pyridine, a salt of 2-mercaptopyridine, a salt of 2-mercapto-6-methylpyridine, a salt of N-oxide of 2-mercaptopyridine, a salt of acid 2 -Mercaptonicotinic acid, a salt of 6-mercaptonicotinic acid, a salt of 2-quinolinethiol, a salt of 5- (trifluoromethyl) -2-mercaptopyridine, a salt of 4-mercaptopyridine, a salt of 7- (trifluoromethyl) quinoline-4-thiol, a salt of 2- (2-pyridinyldisulfanyl) pyridine and a salt of 4- (4-pyridinyldisulfanyl) pyridine. To limit the degradation of the monoethanolamine in aqueous solution implemented to capture the CO2 contained in combustion fumes, is preferably added in the aqueous solution at least one degradation inhibiting compound selected from the group containing: 2-mercaptopyridine, 2-mercaptonicotinic acid, sodium salt of 2-mercaptopyridine N-oxide, 2-mercapto-6-methylpyridine, 5- (trifluoromethyl) -2-mercaptopyridine and 4-mercaptopyridine.

Other features and advantages of the invention will be better understood and will be clear from reading the description given below, with reference to FIG. 1 representing the content of NH 3 in the gas treated with an absorbent solution depending on the addition or not of additives inhibiting degradation in the absorbent solution.

In order to reduce the degradation of an absorbent solution, the inventors have shown that the degradation of an absorbent solution comprising organic compounds provided with an amine function in aqueous solution is substantially reduced in the presence of a small amount of inhibiting additives. degradation described below. The degradation inhibiting additives according to the invention are compounds belonging to the family of pyridine derivatives of which at least one substituent contains a sulfur atom. In the present description, pyridine is understood to mean cyclic compounds with 6 atoms whose ring contains 5 carbon atoms and 1 nitrogen atom, and 3 unsaturations. In the present description, the term "pyridine derivatives of which at least one substituent contains a sulfur atom" means that at least one of the carbon atoms of the pyridine ring is bonded to a sulfur atom. The degradation inhibiting compounds according to the invention may, for example, correspond to the following general formula (I): According to the position of the substituents of the ring, the general formula (I) is divided into three formulas (III) (IV) and (V) SX R4 (Formula I) (Formula III) The degradation inhibiting compounds according to the invention can also exist in their pyridine oxide form, also known as pyridine N-oxide, and in accordance with rules of organic chemistry. In this case, the degradation inhibitor compounds according to the invention may, for example, satisfy the following general formula (II): R 2 According to the position of the substituents of the ring, the general formula (II) is divided into three formulas (VI ) (VII) and (VIII): R4 (Formula II) (Formula VI) SIX (Formula VII) (Formula VIII) In formulas I to VIII, the radical X is chosen from: o a hydrogen atom, o an alkaline element, preferably sodium or potassium, an alkaline earth element, a monovalent or multivalent metal, an ammonium cation NH4 + or resulting from the protonation of an amine function, a phosphonium cation, a radical comprising 1 to 12 carbon atoms, saturated or unsaturated, linear, branched or cyclic, heterocyclic or aromatic, which may optionally contain heteroatoms and / or halogens, and which may contain hydroxyl, ketone, carboxylic, amine or nitrile functions, o a unit R '-' according to the following three units A, B and C according to the position of the substituents: R8 (Reason A) / S (Reason B) R7 (Reason C) R8 o a R unit which can be declined according to the three D, E units and F 5 according to the position of the substituents: / S (Reason E) R7 (Reason F) (Reason D) In the formulas I to VIII, as well as in the patterns A, B, C, D, E and F each of the The radicals R1, R2, R3, R4, R5, R6, R7 and R8 are chosen independently and indifferently from: a hydrogen atom, a radical containing 1 to 12 carbon atoms, saturated or unsaturated, linear, branched or cyclic, heterocyclic or aromatic optionally containing heteroatoms and / or halogens, and possibly containing hydroxyl, ketone, carboxylic, amine or nitrile functions, o an acid function or a sulphonic ester function, o an amine function, W o a radical Y with W being a sulfur atom or an oxygen atom and Y being a hydrogen atom ene, o a radical comprising 1 to 12 carbon atoms, saturated or unsaturated, linear, branched or cyclic, heterocyclic or aromatic, which may optionally contain heteroatoms and / or halogens, hydroxyls, ketones, or protonation of a o

According to a first embodiment of the invention, in the definition of the formulas I to VIII, as well as in the units A, B, C, D, E and F, degradation inhibitors according to the invention, each of the radicals R 1 R2, R3, R4, R5, R6, R7 and R8 are independent, i.e. they are not linked together. However, according to a second embodiment, in the case where two of R1, R2, R3 and R4, or of R5, R6, R7 and R8 are hydrocarbon radicals, these two radicals can be linked to form a ring. consisting of 5, 6, 7 or 8 atoms. The cycle consists of carbon atoms and possibly oxygen or nitrogen atoms. Said cycle may comprise unsaturations. The cycle respects the rules of organic chemistry. According to the invention, in the case where X is a multivalent element, it is understood that in order to respect the neutrality of the molecule, the degradation inhibiting agent according to the invention can take one of the following forms: carboxylic functions, amines or nitriles, an alkali or alkaline-earth element, a monovalent or multivalent metal, o an ammonium NH4 + cation or resulting amine function, a phosphonium cation. Xb a Xb -a with a and b being integers allowing the respect of the neutrality or the valences in the respect of the rules of chemistry The molecules of the invention can also exist in their so-called tautomeric form when this is allowed, and this, in compliance with the rules of organic chemistry.

The absorbent solutions according to the invention can be used to deacidify the following gaseous effluents: natural gas, synthesis gases, combustion fumes, refinery gases, gases obtained at the bottom of the Claus process, gases biomass fermentation, cement gases, incinerator fumes. These gaseous effluents contain one or more of the following acidic compounds: CO2, H2S, mercaptans, COS, SO2, NO2, CS2. In particular, the process according to the invention can be implemented to absorb acidic compounds contained in an oxygen-containing off-gas, such as, for example, combustion fumes. The oxygen content in the gaseous effluent may be greater than 500 ppm by volume, preferably greater than 0.5%, or even at least 1%, 3% or 5% by volume. In general, the oxygen content in the gaseous effluent remains below 20% by volume. 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. These fumes can comprise between 50% and 90% of nitrogen, between 5% and 20% of carbon dioxide. The fumes generally comprise at least 500 ppm by volume, preferably at least 1% by volume, or even at least 2%, 3% or 5% by volume of oxygen, up to a content which generally does not exceed 20% by volume. 'oxygen. The use of an absorbent solution for deacidifying a gaseous effluent is generally carried out by carrying out an absorption step followed by a regeneration step. The absorption step consists of contacting the gaseous effluent with the absorbing solution. Upon contact, the organic compounds provided with an amine function of the absorbent solution react with the acidic compounds contained in the effluent so as to obtain a gaseous effluent depleted of acidic compounds and an acid-enriched absorbent solution. The regeneration step includes heating and, optionally, relaxing, at least a portion of the absorbent solution enriched in acidic compounds to release the acidic compounds in gaseous form. The regenerated absorbent solution, that is to say depleted in acidic compounds is recycled to the absorption step. The absorbent solution according to the invention comprises organic compounds in aqueous solution. In general, the organic compounds are amines, that is to say that they contain at least one amine function. The organic compounds may be in variable concentration, for example between 10% and 99% by weight, preferably between 20% and 75% by weight, and even between 20% and 50% by weight, in the aqueous solution. The absorbent solution may contain between 1% and 90% by weight of water, preferably between 25% and 80% by weight, and even between 50% and 70% by weight of water. For example, the organic compounds are amines such as N, N, N ', N', N "-pentamethyldiethylenetriamine or piperazine, for example piperazine is used for the treatment of natural gas and for the decarbonation of combustion fumes.

The organic compounds may also be alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), diisopropanolamine (DIPA) or diglycolamine. Preferably, MDEA and DEA are commonly used for deacidification of natural gas. MEA is more particularly used for the decarbonation of combustion fumes. The organic compounds may also be amino acid salts such as glycine or taurine salts which are used in particular for the capture of CO2 in the combustion fumes.

In addition, the absorbent solution according to the invention may contain compounds which physically absorb at least partially one or more acidic compounds of the gaseous effluent. For example, the absorbent solution may comprise between 5% and 50% by weight of absorbent compounds of a physical nature such as, for example, methanol, sulfolane or N-formyl morpholine.

Another advantage of the invention lies in the fact that the use of degradation inhibitor additives according to the invention makes it possible to increase the concentration of amines commonly used by those skilled in the art and thus to increase the performance of the process : Increasing the capacity and speed of absorption of the acidic compounds by the absorbing solution resulting in a reduction of investment costs and operating costs of the industrial unit. Indeed, as shown below in Example 1, in the absence of degradation inhibiting additives, the degradation rate of the amines increases with the increase in the concentration of amines. Thus in the case for example of the use of an aqueous solution of MEA (monoethanolamine) for the capture of CO2 in the combustion fumes, the concentration of MEA is commonly limited to 30% by weight to limit the degradation of this amine. It is understood here that the concentration of amine is defined as weight percentage in water before absorption of CO2. For example, an absorbent solution used for the capture of CO2 in a combustion smoke and containing a degradation inhibitor additive according to the invention may contain more than 30% by weight and preferably more than 35% by weight of MEA, a good value. the concentration of MEA being at least equal to 39% by weight.

Among all the molecules belonging to the family of pyridine derivatives in which at least one substituent contains a sulfur atom, the following degradation inhibiting compounds are preferably used: 2-mercaptopyridine, 2-mercapto-6-methylpyridine , 2-mercapton-donin N-oxide, 2-mercaptonicotinic acid, 6-mercaptonicotinic acid, 2-quinolinethiol, 5- (trifluoromethyl) -2-mercaptopyridine, 4-mercaptopyridine, 7- ( trifluoromethyl) quinoline-4-thiol, 2- (2-pyridinyldisulfanyl) pyridine and 4- (4-pyridinyldisulfanyl) pyridine, as well as the salts of the above mentioned elements.

The salts of the degradation-inhibiting compounds according to the invention can be obtained for example by their neutralization with the aid of an alkaline or metal hydroxide or an alkali metal carbonate (preferably a sodium or potassium hydroxide or carbonate), or ammonium or with an amine present in the absorbent solution. In this case, the functions having an acidic character, such as for example the thiol functions and for example the possible carboxylic acid functions, may be partially or completely neutralized. The salts of the degradation inhibitor compounds according to the invention can also be obtained for example by their neutralization using an organic or inorganic acid. In this case, it is at least one nitrogen atom present on the molecule that is protonated.

The degradation inhibitor compounds listed in the preceding paragraph are particularly well suited to the prevention of the degradation of amines in aqueous solution implemented in a process for capturing CO2 contained in combustion fumes.

In order to limit the degradation of an absorbent solution composed of amines, in particular alkanolamines, for example monoethanolamine (MEA), in aqueous solution, in particular for capturing the CO2 from the combustion fumes, it is preferable to use one of the following compounds: 2-mercaptopyridine, 2-mercapto-6-methylpyridine, 2-mercaptopyridine N-oxide, 2-mercaptonicotinic acid, 6-mercaptonicotinic acid, 5- (trifluoromethyl) -2- mercaptopyridine, 4-mercaptopyridine, as well as their salts, for example the sodium, potassium or ammonium salts, for example the sodium salt of 2-mercaptopyridine, the potassium salt of 2-mercaptopyridine, the sodium salt of 2-mercaptopyridine N-oxide, sodium salt of 2-mercaptonicotinic acid and potassium salt of 2-mercaptonicotinic acid.

Preferably, according to the invention, 2-mercaptopyridine, 2-mercaptonicotinic acid, sodium salt of 2-mercaptopyridine N-oxide, 2-mercapto-6-methylpyridine, 5- (trifluoromethyl) are used. 2-mercaptopyridine, 4-mercaptopyridine for limiting the degradation of an amine, in particular MEA, in aqueous solution used to deacidify a gaseous effluent, especially in the context of the capture of CO2 contained in combustion fumes .

The absorbent solution according to the invention comprises an amount of degradation inhibiting additives defined by the general formula described above. The absorbent solution may comprise one or more different degradation inhibiting additives corresponding to said general formula. In addition, in the absorbent solution, the degradation inhibitor additives according to the invention may be combined with other degradation inhibitor compounds of different chemical families. According to the invention, the absorbent solution comprises between 5 ppm and 5% by weight of degradation inhibiting additives according to the invention, preferably from 50 ppm to 2% by weight, and an excellent content of degradation inhibiting additives in the solution being between 100 ppm and 1% by weight.

The examples presented below make it possible to compare and illustrate the performance of the degradation inhibitor additives according to the invention, in terms of reducing the degradation of amines in aqueous solution, reducing the emissions of volatile degradation compounds and the possibility of to increase the concentration of amines without increasing their degradation.

EXAMPLE 1 The amines of the absorbent solution can be degraded in a use according to the invention generating a consumption of the amine. This example shows that the use of amine degradation inhibiting additives can greatly limit the degradation of said amines. This example further shows that in the case of MEA (monoethanolamine) the use of degradation inhibitor additives according to the invention allows the increase in amine concentration of 30% by weight, a concentration commonly used by those skilled in the art. at 40% weight without increasing degradation.

The degradation tests of an amine in aqueous solution are carried out according to the following procedure: 100 g of solution of MEA (monoethanolamine) 30% or 40% by weight in deionized water are placed in a glass reactor surmounted by a condenser to prevent evaporation of water. The reactor is heated to 80 ° C in an electric heating block. The solution is stirred at 1000 rpm by a magnetic bar. The presence of counter blades prevents the formation of a vortex. A gas is brought into contact with the solution by means of a dip tube for 7 days at atmospheric pressure. According to the tests, the nature of the gas brought into contact with the solution is varied. Similarly, the tests are carried out either in the absence or in the presence of various additives degradation inhibitors incorporated in the aqueous solution of amine at 0.25% by weight.

When the test is conducted only in the presence of CO2 and in the absence of oxygen, the gas brought into contact with the solution is a mixture of 7NI / h of nitrogen and 0.033 Nl / h of CO2 produced in a chamber mixture. In this case, the gas comprises only CO2 and nitrogen.

When the test is conducted in the presence of CO2 and oxygen, the gas brought into contact with the solution is a mixture of 7NI / h of atmospheric air, that is to say unpurified ambient air, and 0.033 Nl / h of CO2 produced in a mixing chamber. In this case, the gas contains CO2, nitrogen and oxygen, the oxygen content in the gas being about 21%.

A gas chromatographic analysis of the solution thus degraded is carried out at the end of the test. The chromatographic method uses a polar column, a carrier gas, helium, an internal standard, triethylene glycol and FID (Flame Induced Detection) detection. This analysis

allows to determine the residual concentration of MEA. An average degradation rate over the duration of the test can therefore be calculated: [MEA], n; aare - [MEA1 final mean degradation rate = * mass of solution duration test 18 Similarly a degradation rate can be calculated: degradation rate = 1 - [MEA] final * 100 [MEA] initial Table 1 shows the mean degradation rates of MEA in the following cases:

Case No. 1: MEA 30% by weight in the presence of oxygen and without additive according to the invention

Case No. 2: MEA 40% by weight in the presence of oxygen and without additive according to the invention CAS Content of 02 [MEA] in% by weight Average rate of degradation (g / day) 1 21% 30% 2.99 2 21% 40% 3.72 Table 1: comparison of average degradation rates of MEA 30% and 40% by weight in the absence of a degradation inhibitor additive according to the invention.

Table 1 confirms that an aqueous solution of 40 wt% MEA degrades faster than a 30 wt% MEA solution. Thus, for the same duration, the mass of degraded MEA is greater in the case of an aqueous solution of MEA 40% by weight. 20

Table 2 gives the degradation rates of an aqueous solution of MEA (monoethanolamine) 40% by weight, in the presence of a degradation inhibitor additive or not and subjected to a gas containing nitrogen, CO2 and containing or no oxygen:

Case No. 2: in the Presence of Oxygen and Without Additive According to the Invention 2959944, 19-Case No. 3: Without Oxygen and Without Additive According to the Invention-Case No. 4: in the Presence of Oxygen and in Case of Oxygen presence of an additive according to the invention, 2-mercaptopyridine - Case No. 5: in the presence of oxygen and in the presence of an additive according to the invention, 2-mercaptonicotinic acid. Case No. 6: in the presence of oxygen and in the presence of an additive according to the invention, the sodium salt of the N-oxide of 2-mercaptopyridine. Case No. 7: in the presence of oxygen and in the presence of an additive according to the invention, 2-mercapto-6-methylpyridine. Case No. 8: in the presence of oxygen and in the presence of an additive according to the invention, 5- (trifluoromethyl) -2-mercaptopyridine - Case No. 9: in the presence of oxygen and in the presence of an additive according to the invention, 4-mercaptopyridine CAS content in 02 Name of the additive Degradation rate 2 21% - 66% 30% - <3% 4 21% 2-mercaptopyridine <3% 21% acid 2 -Mercaptonicotinic <3% 6 21% sodium salt of <3% 2-mercaptopyridine N-oxide. 7 21% 2-mercapto-6-methylpyridine <3% 8 21% 5- (trifluoromethyl) -2- <3% mercaptopyridine 9 21% 4-mercaptopyridine <3% Table 2: comparison of degradation rates of MEA 40% weight obtained in water at 80 ° C in different cases.

It is clear that: 1. the MEA solution is not degraded in the presence of the only CO2 in the absence of oxygen 2. the degradation of the MEA is attributable to the presence of oxygen 3 in the presence of additives according to the invention, the degradation of the MEA is brought to the same level as that observed in the absence of oxygen, that is to say considered as zero because less than the uncertainty of the measurement which is 3 %.

In conclusion, the additives according to the invention effectively combat the effect of oxygen on the degradation of MEA. It clearly appears that in the presence of an additive according to the invention, the degradation of the MEA at 40% by weight in water can be considered as zero because it is less than the uncertainty of the measurement which is 3%.

Thus, in the case of MEA, the additives according to the invention make it possible to increase the concentration of amine commonly used by those skilled in the art without increasing the degradation of the amine. EXAMPLE 2

In particular, the amines can be degraded by oxygen causing the formation of volatile products, which are entrained in the gaseous effluents of the process. Thus, for example in the case of post-combustion flue gas treatment in a process using aqueous MEA solution, large amounts of ammonia are formed. The ammonia thus formed is entrained in the atmosphere with the treated fumes, which poses problems for the protection of the environment.

This example shows that the use of amine degradation inhibiting additives greatly limits the formation of volatile products. This example further shows that in the case of MEA (monoethanolamine) the use of the degradation inhibiting additives according to the invention makes it possible to increase the amine concentration by 30% by weight, a concentration commonly used by the human being. 40% by weight without increasing the formation of volatile products. This example gives the results obtained with an aqueous solution of MEA at 40% by weight.

FIG. 1 shows a monitoring of the concentration of ammonia in the gas leaving the reactor in the cases 2, 4, 5, 6, 7, 8 and 9 defined in example 1. [A] corresponds to the concentration of ammonia in ppm volume in the reactor outlet gas, t represents the time expressed in days. Cases 2, 4, 5, 6, 7, 8 and 9 are respectively represented by curve 2 with black circles, curve 4 with white diamonds, curve 5 with white triangles, curve 6 with crosses, curve 7 with white squares, curve 8 with black triangles and curve 9 with white circles. The ammonia concentration in the gas exiting the reactor is determined by online analysis by Fourier Transform Infrared spectrometry.

In the case of degradation inhibitor additives according to the invention, the NH3 content is less than 17 ppm throughout the test, whereas it rapidly exceeds 2000 ppm without additive degradation inhibitor. It is clear that in the presence of an additive according to the invention, the ammonia emissions related to the degradation of MEA to 40% by weight in water are considerably reduced.

This example thus clearly shows, in the case of MEA, that the additives according to the invention make it possible to increase the concentration of amine commonly used by those skilled in the art without increasing the ammonia emissions. Therefore, in an industrial process using an absorbent solution containing degradation inhibiting additives according to the invention, the emissions of volatile compounds at the absorber head will be much lower than in the absence of degradation inhibitor additives even if the concentration in amine is increased relative to the concentration commonly used by those skilled in the art.

Claims (15)

CLAIMS1) Absorbent solution for absorbing the acidic compounds of a gaseous effluent, said solution comprising: a) at least one amine, b) water, c) at least one degradation inhibiting compound for limiting the degradation of said amine, the degradation inhibitor compound being a derivative of pyridine, at least one carbon atom of the pyridine ring is bonded to a sulfur atom. 2) An absorbent solution according to claim 1, wherein said degradation inhibiting compound has one of the following general formulas: ## STR2 ## wherein R 1 R 4 R 4 R 4 R 3 S, X R 4 R 3 (Formula VI) (Formula VII) Wherein X is selected from hydrogen, an alkaline or alkaline earth element, a monovalent or multivalent metal, an NH 4 + ammonium cation or resulting from the protonation of a functional group. amine, o a phosphonium cation, o a hydrocarbon radical comprising 1 to 12 carbon atoms, o one of the following units A, B, C, D, E and F: R8 (Reason A) / S (Reason B) R7 ( Reason C) R / S (Reason) R7 (Reason F) (Reason D) in which each of the radicals R1, R2, R3, R4, R5, R6, R7 and R8 is selected from: o a hydrogen atom a hydrocarbon radical comprising 1 to 12 carbon atoms, an acid or sulphonic ester function, an amine function W or a radical Y in which W is chosen from a sulfur atom and an atom. e is selected from: o a hydrogen atom, o a radical containing 1 to 12 carbon atoms, o an alkaline or alkaline earth element, o a monovalent or multivalent metal, o an ammonium cation NH4 + or resulting from the protonation of an amine function, a phosphonium cation. 3) Absorbent solution according to claim 2, wherein at least one of the radicals R1, R2, R3, R4, R5, R6, R7, R8, X and Y is a hydrocarbon group containing between 1 and 12 carbon atoms and further comprises at least one member selected from heteroatom and halogen. 4) Absorbent solution according to one of claims 2 and 3, wherein at least one of the radicals R1, R2, R3, R4, R5, R6, R7, R8, X and Y is a hydrocarbon group containing between 1 and 12 carbon atoms and further comprises at least one function selected from the group: a hydroxyl function, a ketone function, a carboxylic function, an amine function and a nitrile function. 5) Absorbent solution according to one of the preceding claims, wherein at least two radicals selected from R1, R2, R3 and R4, or from R5, R6, R7 and R8 are hydrocarbon radicals connected by a covalent bond to form a cycle consisting of 5, 6, 7 or 8 atoms. 6. Absorbent solution according to one of the preceding claims, wherein the solution comprises between 10% and 99% by weight of amine, between 1% and 90% by weight of water and between 5 ppm and 5% by weight of inhibiting compound. degradation. 7. Absorbent solution according to one of the preceding claims, wherein the degradation inhibiting compound is selected from the group containing 2-mercaptopyridine, 2-mercapto-6-methylpyridine, 2-mercaptopyridine N-oxide, 2-mercaptonicotinic acid, 6-mercaptonicotinic acid, 2-quinolinethiol, 5- (trifluoromethyl) -2-mercaptopyridine, 4-mercaptopyridine, 7- (trifluoromethyl) quinoline-4-thiol, 2- (2-pyridinyldisulfanyl) pyridine, 4- (4-pyridinyldisulfanyl) pyridine, a salt of 2-mercaptopyridine, a salt of 2-mercapto-6-methylpyridine, a salt of N-oxide of 2-mercaptopyridine a salt of 2-mercaptonicotinic acid, a salt of 6-mercaptonicotinic acid, a salt of 2-quinolinethiol, a salt of 5- (trifluoromethyl) -2-mercaptopyridine, a salt of 4-mercaptopyridine, a salt of 7- (trifluoromethyl) quinoline-4-thiol, a salt of 2- (2-pyridinyldisulfanyl) pyridine and a salt of 4- (4-pyridinyldisulfanyl) pyridine. 8) Absorbent solution according to one of the preceding claims, wherein the amine is selected from the group containing: N, N, N ', N', N "-pentamethyldiethylenetriamine, piperazine, monoethanolamine, diethanolamine, methyldiethanolamine, diisopropanolamine, diglycolamine, a salt of glycine and a salt of taurine. 9) Absorbent solution according to one of the preceding claims, wherein the amine is monoethanolamine and wherein the degradation inhibiting compound is selected from 2-mercaptopyridine, 2-mercaptonicotinic acid, the sodium salt of N- 2-mercaptopyridine oxide, 2-mercapto-6-methylpyridine, 5- (trifluoromethyl) -2-mercaptopyridine and 4-mercaptopyridine. 10) Absorbent solution according to one of claims 8 and 9, comprising at least 39% by weight of monoethanolamine. 11) Process for absorbing acidic compounds contained in a gaseous effluent, in which the gaseous effluent is brought into contact with an aqueous solution containing at least one amine, and wherein the degradation of said amine is controlled by introducing into said aqueous solution at least one degradation inhibitor compound derived from pyridine, at least one carbon atom of the pyridine ring is bonded to a sulfur atom. 12) Process according to claim 11, wherein the aqueous solution is used to absorb acidic compounds contained in one of the effluents of the group containing natural gas, combustion fumes, synthesis gas, refinery gas , tail gas from the Claus process, biomass fermentation gas, cement gas and incinerator fumes. 13) The method of claim 12, wherein the gaseous effluent comprises at least 500ppm oxygen volume. 14) Method according to one of claims 11 to 13, wherein is introduced into the aqueous solution at least one degradation inhibiting compound selected from the group containing: 2-mercaptopyridine, 2-mercapto-6-methylpyridine, N 2-mercaptopyridine oxide, 2-mercaptonicotinic acid, 6-mercaptonicotinic acid, 2-quinolinethiol, 5- (trifluoromethyl) -2-mercaptopyridine, 4-mercaptopyridine, 7- (trifluoromethyl) quinoline 4-thiol, 2- (2-pyridinyldisulfanyl) pyridine, 4- (4-pyridinyldisulfanyl) pyridine, a salt of 2-mercaptopyridine, a salt of 2-mercapto-6-methylpyridine, a salt of N 2-mercaptopyridine oxide, a salt of 2-mercaptonicotinic acid, a salt of 6-mercaptonicotinic acid, a salt of 2-quinolinethiol, a salt of 5- (trifluoromethyl) -2-mercaptopyridine, a salt of 4-mercaptopyridine salt, a salt of 7- (trifluoromethyl) quinoline-4-thiol, a salt of 2- (2-pyridinyl) diisopropyl) pyridine and a salt of 4- (4-pyridinyldi) sulfanyl) pyridine. 15) The method of claim 11, wherein to limit the degradation of monoethanolamine in aqueous solution used to capture the CO2 contained in combustion fumes is added in the aqueous solution at least one degradation inhibitor compound selected from the containing: 2-mercaptopyridine, 2-mercaptonicotinic acid, sodium salt of 2-mercaptopyridine N-oxide, 2-mercapto-6-methylpyridine, 5- (trifluoromethyl) -2-mercaptopyridine, and 4-mercaptopyridine.25