FR2953146A1 - Absorbent solution, useful for absorbing acidic compounds contained in a gaseous effluent, comprises at least one amine, water, and degradation-inhibitor compound having thiourea structure to reduce the degradation of the amine - Google Patents

Abstract

Absorbent solution (A) comprises: (a) at least one amine; (b) water; and (c) at least one degradation-inhibitor compound to reduce the degradation of the amine, where the degradation-inhibitor compound has thiourea structure of formula (R2-NR1-C(=S)-NR3-R4) (I). Absorbent solution (A) comprises: (a) at least one amine; (b) water; and (c) at least one degradation-inhibitor compound to reduce the degradation of the amine, where the degradation-inhibitor compound has thiourea structure of formula (R2-NR1-C(=S)-NR3-R4) (I). R1-R4 : H, 1-8C hydrocarbon, 5-8C aryl or heterocyclic group, 1-8C hydroxyalkyl, 1-8C alkoxy-1-8C alkyl, keto-1-8C alkyl, group of formula (-C(=NH)-NH 2), group of formula (-NH-C(=S)-NH 2), group of formula (-C(=S)-NX-Y1), or group of formula (-NX-Y1); and X, Y1 : H, 1-8C hydrocarbon or 5-8C heterocyclic or aryl group.

Description

The present invention relates to the field of the deacidification of a gaseous effluent. More specifically, 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.

The 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 desulfurization, imposes specific constraints on the absorbent solution, in particular a thermal and chemical stability, especially with regard to the impurities of the effluent, namely essentially oxygen, SOx and 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, for example, of 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 being degraded 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 compared to a fresh solution of amine, - an increase in the density of the absorbing 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 degradation products 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 absorbing 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 is added in the absorbent solution, compounds whose role is to prevent or limit the degradation of 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 deacidification of natural gas, particularly oximes. US Patent 7056482 cites additives used to limit the degradation of absorbent solutions used for the capture of CO2, in particular thiosulphates and sulphites.

In general, the present invention proposes 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 aqueous solution. The present 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 for limiting the degradation of said amine, the inhibiting compound having the following general formula: R1 R3 R2 / NCNR4 IIS 25 in each of the radicals R 1, R 2, R 3 and R 4 is indifferently chosen from: • a hydrogen atom, • a hydrocarbon radical containing from 1 to 8 carbon atoms, • an aromatic or heterocyclic radical of 5 to 8 carbon atoms A hydroxyalkyl radical comprising from 1 to 8 carbon atoms, NH a radical corresponding to the general formula:

NH2 • a radical corresponding to the general formula: X

CNùY

I I S 10 • a radical corresponding to the general formula: / X \ Y

in which each of the radicals X and Y is chosen indifferently from: • a hydrogen atom, • a hydrocarbon radical containing from 1 to 8 carbon atoms, • an aromatic or heterocyclic radical containing from 5 to 8 carbon atoms,

According to the invention, at least one of the radicals R 1, R 2, R 3, R 4, X and Y may be a hydrocarbon group containing between 1 and 8 carbon atoms and may contain, in addition, at least one compound selected from a heteroatom, a halogen and a nitrile function. At least one of the radicals R1, R2, R3, R4, X and Y may be an aromatic or heterocyclic radical of 1 to 8 carbon atoms and may further comprise at least one function selected from the group: a hydroxyl function, an alkoxyalkyl radical, the alkoxy functional group comprising from 1 to 8 carbon atoms, the function of alkyls containing from 1 to 8 carbon atoms, and a ketoalkyl radical containing 1 to 8 carbon atoms. a radical corresponding to the general formula: NH 2 -N hydroxyalkyl function, an alkoxyalkyl function, a nitrile function and a halogen. At least two radicals selected from R 1, R 2, R 3 and R 4 may be hydrocarbon groups connected by a covalent bond to form a ring of 5 to 9 atoms. The two radicals X and Y may be hydrocarbon groups connected by a covalent bond to form a ring consisting of 5 to 9 atoms. The solution can 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 chosen from the group containing: thiourea, N-methylthiourea, N-ethylthiourea, N-propylthiourea, N-allylthourea, N-butylthiourea, N-phenylthiourea, N-benzylthiourea N, N'-dimethylthiourea, N, N'-diethylthiourea, N, N'-diisopropylthiourea, N-N'-dibutylthiourea, N-N'-ditertiobutylthiourea, N, N'-diallylthiourea, N, N'-dihexylthiourea, N, N'-dicyclohexylthiourea, N, N'-diheptylthiourea, N, N'-dioctylthiourea, N, N'-di-3-pyridylthiourea, N, N'diydicyanovinylthiourea , N, N'-difurfurylthiourea, 1,1'-thiocarbonyldiimidazole, morpholine-4-carbothioamide, acetylthiourea, 2-imino-4-thiobiuret, dithiobiuret, 2,5-dithiobiurea, N , N, N ', N'-tetramethylthiourea, thiocarbohydrazide, thiosemicarbazide, 4-methylthiosemicarbazide, 4-ethylthiosemicarbazide, 2-thiohydantoin, ethylenethiourea (or 2-imidazolidinethione), 1-m ethyl-2-imidazolidinethione and 1,3-dimethylimidazolidine-2-thione. The amine may be selected from the group containing: N, N, N ', N', N "-pentamethyldiethylenetriamine, piperazine, monoethanolamine, diethanolamine, methyldiethanolamine, diisopropanolamine, diglycolamine, a salt of the In the case where the amine is monoethanolamine, the degradation inhibiting compound may be chosen from N, N'-dimethylthiourea and N, N'-diethylthiourea, and the absorbing solution may contain at least one of the following: minus 39% weight of monoethanolamine.

The invention also describes a process for absorbing acidic compounds contained in a gaseous effluent, wherein the gaseous effluent is brought into contact with an absorbent solution according to the invention as described above.

In the process according to the invention, 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, tail gas from the Claus process, biomass fermentation gas, cement gas and incinerator fumes.

The gaseous effluent may comprise at least 500 ppm by volume of oxygen. Other features and advantages of the invention will be better understood and will become clear from reading the description given below, with reference to the appended figures in which FIGS. 1 and 2 represent the NH 3 content in the gas treated with a absorbent solution depending on the addition or not of additives inhibiting degradation in the absorbent solution and depending on the amine content 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 thioureas. The compounds which inhibit degradation according to the invention correspond to the following general formula: ## STR1 ## in which each of the radicals R 1, R 2, R 3 and R 4 is indifferently chosen from: • an atom of hydrogen, a saturated or unsaturated, linear, branched or cyclic hydrocarbon radical of 1 to 8 carbon atoms, which may comprise a heteroatom, a halogen, and which may be linked to a nitrile function, an aromatic or heterocyclic radical of 5 to 8 carbon atoms, which may be bonded to a hydroxyl group, a hydroxyalkyl group, and / or an alkoxyalkyl group, to a halogen, and / or to a nitrile function, a hydroxyalkyl radical comprising from 1 to 8 carbon atoms, carbon, saturated or unsaturated, linear, branched or cyclic, • an alkoxy radical (from 1 to 8 carbon atoms) -alkyl (from 1 to 8 carbon atoms) • a ceto-alkyl radical from 1 to 8 carbon atoms A radical corresponding to the general formula NH • a radical corresponding to the general formula S NH) NH 2 • a radical corresponding to the general formula IIS • a radical corresponding to the general formula In which each of the radicals X and Y is chosen indifferently from: • a hydrogen atom, A saturated or non-saturated, linear, branched or cyclic hydrocarbon radical of 1 to 8 carbon atoms, which may optionally contain heteroatoms, halogens, and which may contain hydroxyl, ketone, carboxylic or nitrile functions, an aromatic or heterocyclic radical of 5 to 8 carbon atoms, which can be bonded to a hydroxyl group, a hydroxyalkyl group, and / or an alkoxyalkyl group, a halogen, and / or a nitrile function, According to one embodiment, in the definition of the general formula of the compounds inhibiting degradation according to the invention, the radicals R1, R2, R3 and R4 are independent. However, according to another embodiment, in the case where two of R1, R2, R3 and R4 are hydrocarbon radicals, these two elements can be linked by a covalent bond to form a heterocycle of 5, 6, 7 , 8 or 9 atoms. According to one embodiment, in the definition of the general formula of the degradation inhibitor compounds according to the invention, the radicals X and Y are independent. However, according to another embodiment, the radicals X and Y are hydrocarbon radicals connected by a covalent bond to form a heterocycle of 5, 6, 7, 8 or 9 atoms.

The degradation inhibitor compounds according to the invention may also exist in their so-called tautomeric form when this is permitted, 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, the gases of fermentation of biomass, cement gas, 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 2%, 3% or 5% by volume of oxygen, up to a content which generally does not exceed 20% of oxygen volume. . The implementation of an absorbent solution for deacidifying a gaseous effluent is generally carried out by performing 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 can 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 The MEA is more particularly used for the decarbonation of combustion fumes The organic compounds may also be salts of amino acids such as the salts of glycine or taurine which are especially used for the capture of CO2 in the combustion fumes In addition, the absorbent solution according to the invention may contain compounds which absorb physically 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 inhibiting 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: increase of the capacity and the rate of absorption of the acidic compounds by the absorbing solution resulting in a reduction of the investment costs and the operating costs of the industrial unit. Indeed, as shown hereinafter in Example 3, 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 amine concentration is defined as weight percent in water prior to 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 thiourea family, the following degradation inhibiting additives are preferably used: thiourea, N-methylthiourea, N-ethylthiourea, N-propylthiourea, N-allylthiourea, butylthiourea, N-phenylthiourea, N-benzylthiourea, N, N'-dimethylthiourea, N, N'-diethylthiourea, N, N'-diisopropylthiourea, N-N'-dibutylthiourea, N-N ditertiobutylthiourea, N, N'-diallylthiourea, N, N'-dihexylthiourea, N, N'-dicyclohexylthiourea, N, N'-diheptylthiourea, N, N'-dioctylthiourea, N, N'-di- 3-pyridylthiourea, N, N'diydicyanovinylthiourea, N, N'-difurfurylthiourea, 1,1'-thiocarbonyldiimidazole, morpholine-4-carbothioamide, acetylthiourea, 2-imino-4-thiobiuret, dithiobiuret, 2,5-dithiobiurea, N, N, N ', N'-tetramethylthiourea, thiocarbohydrazide, thiosemicarbazide, 4-methylthiosemicarbazide, 4-ethylthiosemicarbazide, 2-thiohydant oine, ethylenethiourea (or 2-imidazolidinethione), 1-methyl-2-imidazolidinethione and 1,3-dimethylimidazolidine-2-thione.

The compounds of the invention listed in the preceding paragraph may also be associated with an acid or a base to form a salt where permitted by chemistry.

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

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 CO 2 of the combustion fumes, it is preferable to use one the following compounds: thiourea, N, N'-dimethylthiourea and N, N'-diethylthiourea, as well as their salts, for example the sodium, potassium or ammonium salts.

Preferably, according to the invention, N, N'-dimethylthiourea or N, N'-diethylthiourea is used to limit the degradation of an amine, in particular MEA, in aqueous solution used to deacidify a gaseous effluent. , particularly 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.

The degradation tests of an amine in aqueous solution are carried out according to the following procedure. 100 g of MEA solution (monoethanolamine) 30% by weight in deionized water are placed in a glass reactor surmounted by a condenser to prevent evaporation of the water. The reactor is heated to 80 ° C in an electric heating block. The solution is stirred at 1000 rpm by a magnetized 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 makes it possible to determine the residual concentration of MEA and therefore the rate of degradation defined by: degradation rate = / 100 [MEA] final * 100 [MEA] initial ~ Table 1 gives the degradation rates of an aqueous solution of MEA (monoethanolamine) 30% by weight, in the presence of a degradation inhibiting additive or not and subjected to a gas containing nitrogen, CO2 and containing or not oxygen: • Case n ° 1: without oxygen and without additive • Case n ° 2: in the presence of oxygen and without additive • Case n ° 3: in the presence of oxygen and in the presence of a conventional antioxidative additive, sodium sulphite (Na2SO3). Case No. 4: in the presence of oxygen and in the presence of an additive according to the invention, N, N'-dimethylthiourea. Case No. 5: in the presence of oxygen and in the presence of an additive according to the invention, the N, N'-diethylthiourea CAS content in 02 Name of the additive Degradation rate 1 0% - <3% 2 21% - 70% 3 21% sodium sulfite (Na2SO3) 71% 4 21% N, N'-dimethylthiourea <3% 21% N, N'-diethylthiourea <3% Table 1: Comparison of degradation rates of MEA 30% weight obtained in water at 80 ° C in the 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 a conventional anti-oxygen additive such as sodium sulphite, the degradation of the MEA in the presence of oxygen is not decreased 4. 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. EXAMPLE 2

In particular, the amines can be degraded by the oxygen generating 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 with regard to the protection of the environment.

FIG. 1 shows a monitoring of the concentration of ammonia in the gas leaving the reactor in cases 2, 3, 4 and 5 defined in example 1. [A] corresponds to the concentration of ammonia in ppm volume in the gas of output of the reactor, t represents the time expressed in days. Cases 2, 3, 4 and 5 are respectively represented by curve 2 in continuous line, curve 3 with squares, curve 4 with triangles and curve 5 with crosses. The ammonia concentration in the gas leaving the reactor is determined by an online Fourier Transform Infra-Red spectrometry analysis. In the case of degradation inhibitor additives according to the invention, the NH 3 content is less than 10 ppm throughout the duration of the test (see curves 4 and 5) whereas it reaches close to 2000 ppm in the case of an anti-dopant additive. conventional oxygen (see curve 3), sodium sulphite and almost 2500 ppm without inhibitor additive (see curve 2). This example therefore clearly shows that the degradation inhibitor additives according to the invention are effective in reducing the emissions of volatile degradation compounds. Therefore, in an industrial process using an absorbent solution containing degradation inhibitor additives according to the invention, the emissions of volatile compounds at the head of the absorber will be much lower than in the absence of inhibitory additives.

EXAMPLE 3 This example shows that the use of the amine degradation inhibiting additives allows the increase of the amine concentration without increasing the degradation. This example gives the results obtained with an aqueous solution of MEA at 40% by weight. The degradation tests of an amine in aqueous solution are carried out according to the following procedure. 100 g of 40 wt% MEA solution in deionized water are placed in a glass reactor surmounted by a condenser to prevent evaporation of the 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. The tests are conducted either in the absence or in the presence of a degradation inhibiting additive incorporated in the 0.25% strength aqueous amine solution. 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. The gas thus contains CO2, nitrogen and oxygen, the oxygen content in the gas being about 21%. 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 makes it possible to determine the residual concentration of MEA. An average rate of degradation over the duration of the test can therefore be calculated: [MEA] ,,, tia, e ù [MEA] final average speed of degradation = * mass of solution duration test Where De even a degradation rate can be calculated: 5 degradation rate = 1 [MEA] final J * 100 [MEA] initial Table 2 shows the mean degradation rates of the MEA obtained under the same conditions in case 6 40 wt.% MEA without additive and No. 2 MEA 30 wt.% without additive as defined in Example 1. CAS Content 02 [MEA] in% weight Average% degradation rate (g / day) 2 21% 30% 2.99 6 21% 40% 3.72 Table 2: 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 2 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. Table 3 gives the degradation rates of an aqueous solution of MEA 40% by weight, in the presence of a degradation inhibitor additive or not:

Case # 6: without additive Case # 7: in the presence of an additive according to the invention, the N, N'-diethylthiourea CAS content in 02 Name of the additive Degradation rate 6 21% - 66 % 7 21% N, N'-diethylthiourea <3% Table 3: Comparison of degradation rates of MEA 40% by weight obtained in water at 80 ° C. in the absence and in the presence of a degradation inhibitor additive according to the invention.

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%.

In conclusion, 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 4 This example shows that the use of amine degradation inhibiting additives allows the increase of the amine concentration without increasing the emissions of volatile degradation products. This example gives the results obtained with an aqueous solution of MEA at 40% by weight. FIG. 2 shows a monitoring of the ammonia concentration in the gas leaving the reactor in cases 6 and 7 defined in example 3. [A] corresponds to the concentration of ammonia in ppm volume in the reactor outlet gas. , t represents the time expressed in days. Cases 6 and 7 are respectively represented by curve 6 with rounds and curve 7 with black diamonds. The ammonia concentration in the gas leaving the reactor is determined by an online Fourier Transform Infra-Red spectrometry analysis. In the case of the degradation inhibitor additive according to the invention, the NH3 content is less than 10 ppm for the duration of the test whereas it rapidly exceeds 2000 ppm without inhibiting additive. 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. Consequently, in an industrial process using an absorbent solution containing degradation inhibitor additives according to the invention, the emissions of volatile compounds at the head of the absorber will be much less than in the absence of inhibitory additives even if the concentration of amine is increased relative to the concentration commonly used by those skilled in the art.

Claims (13)

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 inhibiting compound corresponding to the following general formula: R ~ R3 II R2 / C ~ N ~ R4 IIS in which each of the radicals R1, R2, R3 and R4 is indifferently chosen from: • a hydrogen atom, • a radical hydrocarbon containing from 1 to 8 carbon atoms, an aromatic or heterocyclic radical of 5 to 8 carbon atoms, a hydroxyalkyl radical comprising from 1 to 8 carbon atoms, an alkoxy-alkyl radical, the alkoxy functional group comprising 1 to 8 carbon atoms, the function of alkyls having from 1 to 8 carbon atoms, • a ketoalkyl radical containing 1 to 8 carbon atoms, and a radical corresponding to the general formula: NH 25 • a radical corresponding to the formula g NH 2 • a radical corresponding to the general formula: ## STR2 ## a radical corresponding to the general formula: ## STR2 ## in which each of the radicals X and Y is chosen indifferently from: a hydrogen atom A hydrocarbon radical containing from 1 to 8 carbon atoms, an aromatic or heterocyclic radical containing from 5 to 8 carbon atoms, 2) Absorbent solution according to claim 1, wherein at least one of the radicals R1, R2, R3, R4, X and Y is a hydrocarbon group containing between 1 and 8 carbon atoms and further contains at least one compound selected from a heteroatom, a halogen and a nitrile function. 3) absorbent solution according to one of claims 1 and 2, wherein at least one of the radicals R1, R2, R3, R4, X and Y is an aromatic or heterocyclic radical of 1 to 8 carbon atoms and comprises, in addition, at least one function selected from the group: a hydroxyl function, a hydroxyalkyl function, an alkoxyalkyl function, a nitrile function and a halogen. 4) Absorbent solution according to one of claims 1 to 3, wherein at least two radicals selected from R1, R2, R3 and R4 are hydrocarbon groups connected by a covalent bond to form a ring consisting of 5 to 9 atoms. 5) Absorbent solution according to one of claims 1 to 4, wherein the two radicals X and Y are hydrocarbon groups connected by a covalent bond to form a ring consisting of 5 to 9 atoms. 2130 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 degradation. 7) Absorbent solution according to one of the preceding claims, wherein the degradation inhibiting compound is selected from the group containing: thiourea, N-methylthiourea, N-ethylthiourea, N-propylthiourea, N-allylthiourea, N-butylthiourea, N-phenylthiourea, N-benzylthiourea, N, N'-dimethylthiourea, N, N'-diethylthiourea, N, N'-diisopropylthiourea, N-N'-dibutylthiourea, N- N'-Ditertiobutylthiourea, N, N'-diallylthiourea, N, N'-dihexylthiourea, N, N'-dicyclohexylthiourea, N, N'-diheptylthiourea, N, N'-dioctylthiourea, N, N ' -di-3-pyridylthiourea, N, N'diydicyanovinylthiourea, N, N'-difurfurylthiourea, 1,1'-thiocarbonyldiimidazole, morpholine-4-carbothioamide, acetylthiourea, 2-imino-4-thiobiuret, dithiobiuret, 2,5-dithiobiurea, N, N, N ', N'-tetramethylthiourea, thiocarbohydrazide, thiosemicarbazide, 4-methylthiosemicarbazide, 4-ethylthiosemicarbazide, 2-t hydantoin, ethylenethiourea (or 2-imidazolidinethione), 1-methyl-2-imidazolidinethione and 1,3-dimethylimidazolidine-2-thione. 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 N, N'-dimethylthiourea and N, N'-diéthylthiourée. 10) Absorbent solution according to one of claims 8 and 9, comprising at least 39% by weight of monoethanolamine.30 11) Process for absorbing acidic compounds contained in a gaseous effluent, wherein the gaseous effluent is brought into contact with an absorbent solution according to one of claims 1 to 10. 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.