EA046450B1 - METHOD FOR REMOVING ACIDIC GASES FROM A FLUID FLOW - Google Patents

Description

The present invention relates to the use of an absorbent, as well as a method for removing acid gases from a fluid stream. In some embodiments, the present invention relates to the selective removal of hydrogen sulfide from a fluid stream containing carbon dioxide and hydrogen sulfide.

Removing acid gases, such as CO2, H2S, SO2, CS2, HCN, COS or mercaptans, from fluid streams such as natural gas, refinery gas or synthesis gas is beneficial for various reasons. Sulfur compounds contained in natural gas can form corrosive acids, particularly when reacting with water, which is often entrained in natural gas. Therefore, in order to transport natural gas in a pipeline or for further processing in a natural gas liquefaction plant (LNG = liquefied natural gas), certain limits on the amount of sulfur impurities must be observed. In addition, various sulfur compounds are toxic and have an unpleasant odor even at low concentrations.

It is necessary to remove carbon dioxide from natural gas, since high CO2 concentrations reduce the calorific value of the gas. Moreover, CO2 combined with moisture can cause corrosion in pipes and valves.

Known methods for removing acid gases include scrubbing operations using aqueous absorbent solutions of inorganic or organic bases. When acid gases dissolve in an absorbent, ions are formed from the bases. Regeneration of the absorbent can be carried out by decompression to low pressure and/or by desorption, which involves the reverse reaction of ionic species, the release and/or desorption of acid gases using an inert fluid, for example, using steam. After the regeneration process, the absorbent can be reused.

The process in which almost complete removal of CO2 and H2S occurs is called complete absorption. While CO2 removal may be necessary to avoid corrosion and provide the gas calorific value required by the consumer, in some cases it is necessary or desirable to treat acid gas mixtures containing _CO2 and _H2S in order to selectively remove _H2 S from the mixture with minimal CO ₂ removal. For example, due to the technical characteristics of natural gas pipelines, there are more stringent limits on H ₂ S levels than on CO ₂ levels because H ₂ S is more toxic and corrosive than CO ₂ . In accordance with these characteristics, for public natural gas pipelines, the H2S content should generally not exceed 4 vol. ppm, while the CO2 content requirement is less stringent - 2 vol.%. Often, selective removal of H2S is desired so that the H2S content of the feed stream entering a sulfur recovery unit, such as a downstream Claus unit, can be higher.

Highly hindered secondary amines such as 2-(2-tert-butylaminoethoxy)ethanol (TBAEE) and tertiary amines such as methyldiethanolamine (MDEA) exhibit greater kinetic selectivity for H2S over CO2. Therefore, such amines are suitable for the selective removal of H ₂ S relative to CO ₂ from gas mixtures containing CO ₂ and H ₂ S, and, as a rule, they are used in the form of aqueous mixtures. These amines do not react directly with _CO2 ; the reaction of _CO2 with the amine and water to produce bicarbonate ion occurs slowly. Reaction kinetics allow H ₂ S to react more quickly directly with the amine groups of the sorbent to form a hydrosulfide ion in aqueous solution.

The use of hydroxyl-substituted amines (alkanolamines), such as those mentioned above, is well known since the presence of hydroxyl groups improves the solubility of the absorbent and acid gas reaction products in commonly used aqueous solvent systems, while also improving solvent circulation through a conventional absorption column setup /regeneration column by suppressing phase separation. The presence of hydroxyl groups can also reduce the volatility of the amine and therefore reduce losses of the amine during operation.

However, such benefits may in some cases come with certain challenges. While alkanolamines are effective in removing acid gases at higher pressures, it is expected that the selectivity for H ₂ S removal will be markedly reduced due to direct physical adsorption of CO ₂ into the liquid solvent and due to reaction with the hydroxyl groups of the amine compound. Although CO ₂ preferentially reacts with amine nitrogen, at higher pressures it reacts with oxygen, and at higher pressures the bicarbonate/hemicarbonate/carbonate reaction products that are formed as a result of the reaction at the hydroxyl center are stabilized, with increasing pressure there is a progressive decrease in selectivity for H2S.

Also, while the presence of hydroxyl groups has a positive effect on the solubility of amines in water, hydroxyl groups impart surfactant properties to the absorbent/acid gas reaction products, which may result in foaming problems during gas treatment plant operation.

- 1 046450

Another known problem with the use of aqueous amine mixtures in the absorption processing of gas mixtures is that phase separation can occur at temperatures in the regeneration temperature range for aqueous amine mixtures, which is typically in the range of 50 -170°C.

US 4,487,967, US 4,665,195 and US 4,894,178 relate to a process for the preparation of hindered amino ether alcohols or diaminopolyalkylene ethers in the presence of a hydrogenation catalyst.

US 2015/0027055 describes a method for selectively removing H ₂ S from a gas mixture containing CO ₂ using an absorbent containing hindered alkanolamines containing an ether end group. It has been found that the formation of an ether end group on alkanolamines and the exclusion of water allows for higher selectivity for H2S.

US 2010/0037775 describes an acid gas absorbent containing an alkylaminoalkyloxy monoalkyl ether (alcohol) and a method for selectively removing H2S from gas mixtures containing H2S and CO2 using an absorbent solution containing said monoalkyl ether.

WO 2013/181245 describes an absorbent composition that can be used for the selective removal of H ₂ S, wherein said absorbent composition includes an aqueous amine mixture, which is the product of the amination reaction of tert-butylamine and a mixture of polyethylene glycol, and an organic co-solvent selected from sulfones derived from sulfone and sulfoxides, and a strong acid to suppress phase separation.

WO 2014/001664 discloses absorbent solutions prepared from tertiary diamines belonging to the family of hindered aminoethyl morpholines. These compounds contain only tertiary amino groups, each of which contains a basic nitrogen atom.

US 2013/011314 describes compounds containing one or more diamines, the two amine functions of which are linked to each other by rings, and the amine function of which at the a-position is always tertiary and the amine function at the ω-position is always primary or secondary, and the use of such compounds for the selective removal of H ₂ S from gas containing H ₂ S and CO ₂ . The examples of compounds described contain secondary and tertiary amino groups, both of which contain a basic nitrogen atom.

WO 2017/186466 discloses a method for removing acid gases from a hindered morpholine-based amine fluid stream.

WO 2018/146233 describes a method for removing acid gases from a fluid stream obtained from the reaction of glycide alcohol derivatives with hindered amines such as tert-butylamine.

WO 2019/043099 relates to absorbent solutions obtained by reacting tert-butylamine with hydroxyethylpyrrolidone and structurally related compounds, as well as their use in gas purification.

US 2017/0320008 discloses a method for selectively removing H2S from a gas mixture simultaneously containing _H2S and _CO2 by contacting the mixture with an absorbent comprising an amine, water and at least one C|-C4 thioalcohol.

The method under consideration has high selectivity with respect to the removal of H2S, and also provides improved characteristics for the removal of other sulfur components, in particular mercaptans.

It is an object of the present invention to provide additional methods suitable for removing acid gases from fluid streams. The methods should be applicable for use in general absorption, where significant amounts of CO ₂ and H2S are removed, as well as for selective removal of hydrogen sulfide from fluid streams. The absorbents used in this method must be characterized by high cyclic ability and low volatility. Another object of the present invention was to provide a gas purification process using solvents with improved thermal stability that can be used at higher temperatures over a longer period. Another objective of the present invention was to provide a gas purification method with high selectivity for removing H ₂ S from gas mixtures containing both H ₂ S and CO ₂ , also providing the removal of other sulfur components that may additionally be included in gas mixtures containing both H 2 S and CO 2 H2S and CO2. In particular, the method of the present invention should also allow for the removal of mercaptans as part of such a selective gas purification process.

The object of the invention is achieved by a method for removing acid gases from a fluid stream, wherein the fluid stream is contacted with an absorbent to produce a fluid stream and a saturated absorbent, wherein the absorbent contains at least one diluent and a compound of general formula (I)

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wherein R1 represents C ₁ -C3 alkyl; R2 represents C ₁ -C3 alkyl; R3 is selected from hydrogen and ^-C^alkyl; R4 is selected from hydrogen and ^-C^alkyl, and n is an integer in the range of 1-4.

The compounds of general formula (I) are based on thiodiglycol and its derivatives and include thioether functionality. Compared to prior art gas purification solvents that include oxy-ester functionality, it has surprisingly been found that gas purification process using solvents containing compounds of formula (I) exhibits higher thermal stability while maintaining favorable absorption properties.

Absorbent.

The method according to the invention is carried out in the presence of an absorbent.

The absorbent contains a compound of formula (I) and at least one diluent.

Compound of formula (I).

The absorbent contains a compound of formula (I).

In formula (I),

R1 is ^-C^alkyl, preferably methyl, ethyl, propyl and isopropyl and most preferably methyl; R2 represents ^-C^alkyl; preferably methyl, ethyl, propyl and isopropyl and most preferably methyl;

R3 is selected from hydrogen and N-C^alkyl, preferably methyl, ethyl, propyl and isopropyl, and most preferably methyl;

R4 is selected from hydrogen and ^-C^alkyl, preferably methyl, ethyl, propyl and isopropyl, and most preferably methyl; and means an integer in the range 1-4, preferably 1-2, and most preferably 1.

In preferred embodiments, R1 and R2 are methyl, and R3 is hydrogen; or R1, R2 and R3 are methyl; or R1 and R2 are methyl, and R3 is ethyl. In a particularly preferred embodiment, R1, R2 and R3 are methyl.

In a particularly preferred embodiment, the compound of general formula (I) is selected from 2-[2-(tert-butylamino)ethylsulfanyl]ethanol; or N[2-(2-methoxyethylsulfanyl)ethyl]-2-methylpropan-2-amine; or N[2-(2-ethoxyethylsulfanyl)ethyl]-2-methyl-propan-2-amine; or 2-[2(isopropylamino)ethylsulfanyl]ethanol; or N[2-(2-methoxyethylsulfanyl)ethyl]propan-2-amine; or N[2-(2-ethoxyethylsulfanyl)ethyl]propan-2-amine.

In a most preferred embodiment, the compound of general formula (I) is 2-[2-(tert-butylamino)ethylsulfanyl]ethanol.

The absorbent contains preferably 10-70% by weight, more preferably 15-65% by weight and most preferably 20-60% by weight of the compound of general formula (I), based on the total weight of the absorbent.

Synthesis of compounds of formula (I).

The compounds of formula (I) are commercially available or can be prepared in a variety of ways.

In a particularly preferred embodiment, the compound of formula (I) is prepared by converting an amine of formula (II)

in which R1 represents ^-C^alkyl; R2 represents ^-C^alkyl; R3 is selected from hydrogen and ^-C^alkyl;

alcohol of formula (III)

in which R4 is selected from hydrogen and ^-C^alkyl; and n is an integer in the range 1-4; in the liquid phase and in the presence of a catalyst.

Preferably, the amine of formula (II) is tert-butylamine or isopropylamine. Most preferably, the amine of formula (II) is tert-butylamine.

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The alcohol of formula (III) is preferably 2-(2-hydroxyethylsulfanyl)ethanol (thiodiglycol), or 2-(2-methoxyethylsulfanyl)ethanol, or 2-(2-ethoxyethylsulfanyl)ethanol.

In the most preferred embodiment, the amine of formula (II) is tert-butylamine and the alcohol of formula (III) is thiodiglycol.

In yet another embodiment, n is 3 or 4 and R4 is methyl.

Preferably, the molar ratio of the amine of formula (II) to the alcohol of formula (III) is in the range of 0.8:1 to 1.2:1, more preferably 0.9:1 to 1.1:1, and most preferably 1:1.

Preferably, the reaction is carried out in the presence of a hydrogenation/dehydrogenation catalyst.

Theoretically, catalysts can contain nickel, cobalt, iron, copper, chromium, manganese, molybdenum, tungsten and/or other metals of the 8th and/or 9th and/or 10th and/or 11th group of the table Mendeleev.

More preference is given to the use of catalysts that contain at least one metal selected from the group consisting of Cu, Co, Ni, Pd, Pt, Ru, Rh, Ag, Au, Re and Ir.

More preference is given to the use of catalysts that contain at least one metal selected from the group consisting of Cu, Co, Ni, Pd, Pt and Ru.

The above catalysts can routinely be mixed with promoters, for example chromium, iron, cobalt, manganese, molybdenum, titanium, tin, alkali metals, alkaline earth metals and/or phosphorus.

The catalyst may be a supported catalyst or an unsupported catalyst.

Suitable support materials are carbon compounds such as graphite, carbon black and/or activated carbon, alumina (gamma, delta, theta, alpha, kappa, chi or mixtures thereof), silica, zirconia, zeolites, aluminosilicates or their mixtures.

In yet another particularly preferred embodiment of the present invention, catalysts of the Raney catalyst type are used.

As Raney type catalysts, Raney cobalt catalysts, Raney nickel catalysts and/or Raney copper catalysts are preferably used. Raney cobalt catalysts are most preferred.

In yet another preferred embodiment of the invention, the catalysts are prepared by reducing a catalyst precursor in which the above metals are present in the form of oxygen-containing compounds, such as their oxides, carbonates or hydrogen carbonates.

Catalyst precursors can be prepared by known methods, for example by precipitation, deposition or impregnation.

According to one particularly preferred embodiment, a supported hydrogenation/dehydrogenation catalyst containing copper, nickel and cobalt is used, wherein the catalytically active catalyst material, before its reduction with hydrogen, contains oxygen compounds of aluminum, copper, nickel and cobalt, and in the range of 0.2- 5.0 wt.% tin oxygen compounds, which are calculated on the basis of SnO. In a particularly preferred embodiment, a catalyst corresponding to the catalysts according to WO 2011/067199 is used.

In a preferred embodiment, the reaction is carried out at a temperature of 150-260°C. In a particularly preferred embodiment, the reaction is carried out at a temperature of 170-240°C. In the most preferred embodiment, the reaction is carried out at a temperature of 180-220°C.

The reaction can be carried out at pressures of 5-300 bar. In a particularly preferred embodiment, the reaction is carried out at a pressure of 50-200 bar (abs). In a particularly preferred embodiment, the reaction is carried out at a pressure of 60 - 130 bar (abs).

The conversion of the amine of formula (II) and the alcohol of formula (III) is preferably carried out in the liquid phase. Within the meaning of the present invention, the transformation is carried out in the liquid phase if either the amine of formula (II), or the alcohol of formula (III), or the solvent is in the liquid phase under the reaction conditions.

The transformation is preferably carried out in the presence of hydrogen. During the reaction, hydrogen is not consumed, but has a beneficial effect on maintaining the activity of the catalyst. The partial pressure of hydrogen is preferably in the range of 2.5-200 bar, more preferably in the range of 5-150 bar, even more preferably in the range of 10-100 bar and most preferably in the range of 20-50 bar.

The transformation can be carried out in the presence of a solvent. The solvent used may be any solvent that is inert under the reaction conditions and has sufficient solubility for the reactants and reaction products. Suitable solvents do not include functional groups that can react with the amine of formula (II) under the conditions of the amination reaction, for example hydroxyl groups. Preferably one or more solvents are water, ethers, preferably methyl tert-butyl ether, ethyl tert-butyl ether, dioxane, tetrahydrofuran (THF), proglyme, diglyme, polyglymes and in general oligo- and polypropylene diesters

- 4 046450 sideds, as well as oligo- and polyethylene oxides or mixed oligo- or polyalkylene oxides.

Suitable solvents also include appropriate mixtures of the solvents listed above.

In particular, preferred solvents are glymes, THF and water.

The amount of solvent present in the reaction mixture is generally in the range of 1-95% by weight, preferably 2.5-70%, more preferably 5-40%, based on the total weight of the reaction mixture, wherein the total weight of the reaction mixture consists of the sum masses of all components added to the transformation of the amine of formula (II), i.e. an amine of formula (II) and an alcohol of formula (III) and solvents.

The reaction can be carried out using mechanically stirred reactors, fixed bed tubular reactors and multi-tubular reactors. The reaction can be carried out in batch, semi-continuous and continuous modes, with or without recirculation of the crude reaction mixture. In a particularly preferred embodiment, the reaction is carried out continuously in a fixed bed tubular reactor.

The catalyst loading may vary in the range of 0.01-2 kg/(L-h), preferably in the range of 0.1-1.0 kg/(L-h), and in a particularly preferred embodiment in the range of 0.2-0 .8 kg/(L-h) ether of formula (II).

The reaction product contains an unreacted amine of formula (II), an alcohol of formula (III) and a compound of formula (III).

The reaction product is preferably purified by performing one or more distillation steps.

On a laboratory scale, compounds of formula (I) can also be prepared by reacting compounds of formula (IV)

(IV), wherein R4 is selected from hydrogen and ^-C^alkyl; and n is an integer in the range of 1-4, and 2-chloro-N-tert-butylethylamine hydrochloride in the presence of sodium ethanolate.

In standard laboratory synthesis, the compound of formula (IV) is dissolved in a 10 wt.% solution of sodium methoxide in ethanol. Typically, 2-chloro-N-tert-butylethylamine is added in the form of a 5-10 wt.% solution in ethanol. Mixing is usually carried out in such a way that the temperature of the resulting mixture is maintained in the range of 35-40°C. To complete the reaction, the resulting reaction mixture is usually stirred at 75°C for 90 minutes for another 6-12 hours at room temperature.

Preferably, the reaction is carried out under inert conditions, such as a high nitrogen atmosphere, and using dried solvents. The resulting suspension is usually filtered through a laboratory filter, and the filtrate is evaporated in a rotary evaporator to remove ethanol and obtain the desired products.

Diluent.

The compound of general formula (I) is diluted with a diluent, preferably an inexpensive diluent. The diluent may only have physical absorption capacity for carbon dioxide and other gas components such as H ₂ S. However, preferably the diluent reacts with acid-base chemicals that are present within the process. In particular, an aqueous diluent can be used as the diluent. Due to steric hindrance, compounds of general formula (I) do not have sufficiently nucleophilic amine sites for direct nucleophilic action on the CO ₂ molecule. Thus, oxygen in water acts as a nucleophile, forming a Brønsted acid, H ₂ CO ₃ , which is neutralized by a compound of general formula (I), acting as a Brønsted base, to form ammonium bicarbonate.

In the most preferred embodiment, the diluent is water.

Activator.

In a preferred embodiment, the absorbent contains at least one activator selected from an unhindered primary amine and/or an unhindered secondary amine. Sterically unhindered primary amine should be understood as compounds having primary amine groups to which only a primary or secondary carbon atom is bonded. Sterically unhindered secondary amine should be understood as compounds having secondary amine groups to which only primary carbon atoms are bonded. Unhindered primary amines or unhindered secondary amines act as strong CO ₂ absorption activators. Accordingly, the presence of an activator may be desirable in applications aimed at non-selective removal of acid gases, or applications in which CO ₂ removal is particularly important.

The activator preferably does not contain acidic groups, such as, in particular, phosphonic acid, sulfonic acid and/or carboxylic acid groups.

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The activator is, for example, selected from:

alkanolamines such as monoethanolamine (MEA), diethanolamine (DEA), ethylaminoethanol, 1-amino-2-methylpropan-2-ol, 2-amino-1-butanol, 2-(2-aminoethoxy)ethanol and 2-(2-aminoethoxy) ethanamine, polyamines such as hexamethylenediamine, 1,4-diaminobutane, 1,3-diaminopropane, 3(methylamino)propylamine (MAPA), N(2-hydroxyethyl)ethylenediamine, 3-(dimethylamino)propylamine (DMAPA), 3- (diethylamino)-propylamine, HH'-bis(2-hydroxyethyl)ethylenediamine. 5-, 6- or 7-membered saturated heterocycles having at least one NH group on the ring, which may contain one or two additional heteroatoms selected from nitrogen or oxygen on the ring, such as piperazine, 2-methylpiperazine , N-methylpiperazine, N-ethylpiperazine, N(2-hydroxyethyl)piperazine, N(2-aminoethyl)piperazine, homopiperazine, piperidine and morpholine.

Particular preference is given to 5-, 6- or 7-membered saturated heterocycles having at least one NH group on the ring, which may contain one or two additional heteroatoms selected from nitrogen or oxygen on the ring. Particular preference is given to piperazine.

In this preferred embodiment, where the absorbent contains an activator, the absorbent contains preferably 10-70 wt%, more preferably 15-65 wt% and most preferably 20-60 wt% activator.

Absence of sterically unhindered amines.

In another preferred embodiment, the absorbent does not contain any unhindered primary amine or unhindered secondary amine. Since unhindered primary amines or unhindered secondary amines act as strong activators of CO ₂ absorption, their presence in the absorbent can result in loss of H2S selectivity of the absorbent. Accordingly, in applications where high H2S selectivity is required, an absorbent substantially free of such compounds is preferred.

Additional hindered amines.

In one embodiment, the absorbent contains a tertiary amine or a highly hindered primary amine and/or a highly hindered secondary amine other than the compounds of general formula (I). Strong steric hindrance should be understood as a tertiary carbon atom immediately adjacent to a primary or secondary nitrogen atom. In this embodiment, the absorbent contains a tertiary amine or a highly hindered amine other than the compounds of general formula (I), typically in an amount of 5-50 wt.%, preferably 10-40 wt.% and more preferably 20-40 wt. .%, based on the total mass of the absorbent.

1. Tertiary alkanolamines such as bis(2-hydroxyethyl)methylamine (methyldiethanolamine, MDEA), tris(2-hydroxyethyl)amine (triethanolamine, TEA), tributanolamine, 2-diethylaminoethanol (diethylethanolamine, DEEA), 2-dimethylaminoethanol (dimethylethanolamine, DMEA) , 3-dimethylamino-1-propanol (N,Ndimethylpropanolamine), 3-diethylamino-1-propanol, 2-diisopropylaminoethanol (DIEA), N-bis(2hydroxypropyl)methylamine (methyldiisopropanolamine, MDIPA).

2. Tertiary amino esters such as 3-methoxypropyldimethylamine;

3. Tertiary polyamines, for example, bis-tertiary diamines, such as N,N,N',N'tetramethylethylenediamine, ^Ndiethyl-N, N'-dimethylethylenediamine, ^^^,N-tetraethylethylenediamine, ^^^,^- tetramethyl-1,3-propanediamine (TMPDA),

N^^,N-tetraethyl-1,3-propanediamine (TEPDA), N^^,^-tetramethyl-1,6-hexanediamine, N,Ndimethyl-H',N-diethylethylenediamine (DMDEEDA),

1-dimethylamino-2-dimethylaminoethoxyethane (bis[2-(dimethylamino)ethyl]ether), 1,4diazabicyclo[2.2.2]octane (TEDA), tetramethyl-1,6-hexanediamine;

and mixtures thereof.

Tertiary alkanolamines, i.e. amines having at least one hydroxyalkyl group attached to a nitrogen atom are generally preferred. Particular preference is given to methyldiethanolamine (MDEA).

Suitable highly hindered amines (i.e., amines having a tertiary carbon atom immediately adjacent to a primary or secondary nitrogen atom) other than the compounds of general formula (I) particularly include:

1) highly sterically hindered secondary alkanolamines, such as

2-(2-tert-butylaminoethoxy)ethanol (TBAEE), 2-(2-tert-butylamino)-propoxyethanol, 2-(2-tert-butylaminoethoxy)ethanol, 2-(2-(1-methyl-1-ethylpropylamino)ethoxy )ethanol, 2-(tert-butylamino)ethanol, 2-tert-butylamino-1-propanol, 3-tert-butylamino-1-propanol, 3-tert-butylamino-1-butanol, and 3-aza2,2-dimethylhexane -1,6-diol;

2) highly sterically hindered primary alkanolamines, such as

2-amino-2-methylpropanol (2-AMP); 2-amino-2-ethylpropanol; and 2-amino-2-propylpropanol;

3) highly sterically hindered amino esters, such as 1,2-bis(tert-butylaminoethoxy)ethane, bis(tert-butylaminoethyl)ether; and mixtures thereof.

Highly sterically hindered secondary alkanolamines are generally preferred

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Particular preference is given to 2-(2-tert-butylaminoethoxy)ethanol and 2-N-methylamino-2methylpropan-1-ol.

Acids.

In another preferred embodiment, the absorbent is an aqueous absorbent (meaning that the diluent contains water) and the absorbent further contains an acid.

The acid promotes the regeneration of the absorbent to low loading values and increases the efficiency of the method.

A proton equilibrium is formed between the acid and the compound of general formula (I). The position of the equilibrium is temperature dependent, and at higher temperatures the equilibrium shifts towards the free oxonium ion and/or amine salt with a lower enthalpy of protonation. At relatively low temperatures, which dominate the absorption stage, a higher pH promotes the absorption of acid gases, while at relatively high temperatures, which dominate the desorption stage, a lower pH supports the release of absorbed acid gases.

The acid dissociation constant pK _a is preferably less than 6, in particular less than 5, when determined at a temperature of 25° C. and atmospheric pressure. If acids have more than one dissociation step and, accordingly, more than one pK _a value, this requirement is met if one of the pK _a values is within the specified range. The acid is suitably selected from protic acids (Brønsted acids).

The acid is preferably added in such an amount that the pH of the aqueous solution, measured at 120° C., is from 7.9 to less than 9.5, preferably from 8.0 to less than 8.8, more preferably from 8.0 to less than 8 .5, most preferably from 8.0 to less than 8.2.

The amount of acid, in one embodiment, is 0.1-5.0 wt%, preferably 0.2-15 4.5 wt%, more preferably 0.5-4.0 wt%, and most preferably 1. 0-2.5 wt.%, based on the total mass of the absorbent.

The acid is selected from organic and inorganic acids. Suitable organic acids include, for example, phosphonic acids, sulfonic acids, carboxylic acids and amino acids. In a particular embodiment of the invention, the acid is a polybasic acid.

Suitable acids are, for example:

mineral acids such as hydrochloric acid, sulfuric acid, amidosulfuric acid, phosphoric acid, partial esters of phosphoric acid, for example mono- and dialkyl phosphates and mono- and diaryl phosphates, such as tridecyl phosphate, dibutyl phosphate, diphenyl phosphate and bis(2-ethylhexyl) phosphate; boric acid;

carboxylic acids, for example, saturated aliphatic monocarboxylic acids, such as formic acid, acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, isovaleric acid, pivalic acid, caproic acid, n-heptanoic acid, caprylic acid, 2-ethylhexanoic acid acid, pelargonic acid, caproic acid, neodecanoic acid, undecanoic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, margaric acid, stearic acid, isostearic acid, arachidic acid, behenic acid; saturated aliphatic polycarboxylic acids such as oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, decanedicarboxylic acid; cycloaliphatic mono- and polycarboxylic acids such as cyclohexanecarboxylic acid, hexahydrophthalic acid, tetrahydrophthalic acid, resin acids, naphthenic acids; aliphatic hydroxycarboxylic acids such as glycolic acid, lactic acid, mandelic acid, hydroxybutyric acid, tartaric acid, malic acid, citric acid; halogenated aliphatic carboxylic acids such as trichloroacetic acid or 2-chloropropionic acid; aromatic mono- and polycarboxylic acids such as benzoic acid, salicylic acid, gallic acid, positionally isomeric toluic acids, methoxybenzoic acids, chlorobenzoic acids, nitrobenzoic acids, phthalic acid, terephthalic acid, isophthalic acid; technical mixtures of carboxylic acids, for example, versatic acids;

sulfonic acids such as methylsulfonic acid, butylsulfonic acid, 3hydroxypropylsulfonic acid, sulfacetic acid, benzenesulfonic acid, p-xylenesulfonic acid, p-xylenesulfonic acid, 4-dodecylbenzenesulfonic acid, 1-naphthalene sulfonic acid, dinonylnaphthalene sulfonic acid and dinonylnaphthalene sulfonic acid, trifluoromethyl- or nonafluoro-p-butylsulfonic acid acid, camphorsulfonic acid, 2-(4(2-hydroxyethyl)-1-piperazine ethanesulfonic acid (HEPES);

organic phosphonic acids, for example phosphonic acids of formula (IV)

R4-PO3H (IV), in which R4 is Ci-Ci-18-alkyl, optionally substituted by up to four

- 7 046450 substituents independently selected from carboxyl, carboxamido, hydroxyl and amino.

These include alkylphosphonic acids such as methylphosphonic acid, propylphosphonic acid, 2-methylpropylphosphonic acid, tert-butylphosphonic acid, n-butylphosphonic acid, 2,3-dimethylbutylphosphonic acid, octylphosphonic acid;

hydroxyalkylphosphonic acids such as hydroxymethylphosphonic acid, 1-hydroxyethylphosphonic acid, 2-hydroxyethylphosphonic acid; arylphosphonic acids such as phenylphosphonic acid, tolylphosphonic acid, xylylphosphonic acid, aminoalkylphosphonic acids such as aminomethylphosphonic acid, 1-aminoethylphosphonic acid, 1-dimethylaminoethylphosphonic acid, 2-aminoethylphosphonic acid, 2-(Nmethylamino)ethylphosphonic acid, 3-aminopropylphosphonic acid new acid, 2- amino-propylphosphonic acid, 1-aminopropylphosphonic acid, 1-aminopropyl-2-chloropropylphosphonic acid, 2-aminobutylphosphonic acid, 3-aminobutylphosphonic acid, 1-aminobutylphosphonic acid, 4-aminobutylphosphonic acid, 2-aminopentylphosphonic acid, 5-aminopentylphosphonic acid, 2 -aminohexylphosphonic acid, 5-aminohexylphosphonic acid, 2-aminooctylphosphonic acid, 1-aminooctylphosphonic acid, 1-aminobutylphosphonic acid; amidoalkylphosphonic acids such as 3-hydroxymethylamino-3-oxopropylphosphonic acid; and phosphonocarboxylic acids such as 2-hydroxyphosphonoacetic acid and 2-phosphonobutane-1,2,4-tricarboxylic acid;

phosphonic acids of formula (V)

RO ₃ N ₂

R5--Q

PO ₃ H ₂ (V) in which Z is C ₂ -C ₅ -alkylene, cycloalkanediyl, phenylene, or C ₂ -C ₅ -alkylene interrupted by cycloalkanediyl or phenylene, Y is CH ₂ PO ₃ H ₂ , and m is 0-4, such as ethylenediaminetetra(methylenephosphonic acid), diethylenetriaminepenta(methylenephosphonic acid) and bis(hexamethylene)triaminepenta(methylenephosphonic acid);

phosphonic acids of formula (VII)

R7-NY2 (VII) in which R7 is C ₁ -C ₅ -αalkyl, C ₂ -C ₅ -hydroxyalkyl or R8, and R8 is CH2PO3H2, such as nitrilotris(methylenephosphonic acid) and 2-hydroxyethyliminobis-(methylenephosphonic acid);

aminocarboxylic acids having tertiary amino groups or amino groups having at least one secondary or tertiary carbon atom immediately adjacent to the amino group, such as α-amino acids having tertiary amino groups or amino groups having at least one secondary or tertiary atom carbon immediately adjacent to the amino group, such as \,\-dimethylglycine (dimethylaminoacetic acid), N,N-diethylglycine, alanine (2-aminopropionic acid), N-methylalanine (2-(methylamino)propionic acid), N-dimethylalanine, N- ethylalanine, 2-methylalanine (2-aminoisobutyric acid), leucine (2-amino-4-methylpentan-1-oic acid), N-methylleucine, N,N-dimethylleucine, isoleucine (1-amino-2-methylpentanoic acid), Nmethylisoleucine, N,N-dimethylisoleucine, valine (2-aminoisovaleric acid), a-methylvaline (2-amino-2-methylisovaleric acid), N-methylvaline (2-methylaminoisovaleric acid), N,Ndimethylvaline, proline (pyrrllidine-2-carboxylic acid ), N-methylproline, N-methylserine, N,Ndimethylserine, 2-(methylamino)isobutyric acid, piperidine-2-carboxylic acid, Nmethylpiperidine-2-carboxylic acid, β-amino acids having tertiary amino groups or amino groups having at least at least one secondary or tertiary carbon atom immediately adjacent to the amino group, such as 3-dimethylaminopropionic acid, N-methyliminodipropionic acid, N-methylpiperidine-3carboxylic acid, γ-amino acids having tertiary amino groups or amino groups having at least one secondary or a tertiary carbon atom immediately adjacent to the amino group, such as 4-dimethylaminobutyric acid.

or aminocarboxylic acids having tertiary amino groups or amino groups having at least one secondary or tertiary carbon atom immediately adjacent to the amino group, such as Nmethylpiperidine-4-carboxylic acid.

Among inorganic acids, preference is given to phosphoric acid and sulfuric acid, especially sulfuric acid.

- 8 046450

Among carboxylic acids, preference is given to formic acid, acetic acid, benzoic acid, succinic acid and adipic acid.

Among sulfonic acids, preference is given to methanesulfonic acid, p-toluenesulfonic acid and 2-(4-(2-hydroxyethyl)-1-piperazinyl)-ethanesulfonic acid (HEPES).

Among phosphonic acids, preference is given to 2-hydroxyphosphonoacetic acid, 2phosphonobutane-1,2,4-tricarboxylic acid, 1-hydroxyethane-1,1-diphosphonic acid, ethylenediaminetetra(methylenephosphonic acid), diethylenetriaminepenta(methylenephosphonic acid), bis(hexamethylene)triaminepenta (methylenephosphonic acid) (HDTMP) and nitrilotris(methylenephosphonic acid), among which 1-hydroxyethane-1,1-diphosphonic acid is particularly preferred.

Among aminocarboxylic acids having tertiary amino groups or amino groups having at least one secondary or tertiary carbon atom immediately adjacent to the amino group, preference is given to N.N-dimethylglycine and N-methylalanine.

More preferably, the acid is an inorganic acid. Non-aqueous organic solvent.

In one embodiment, the absorbent diluent contains at least one non-aqueous organic solvent. In particular cases, in addition to the non-aqueous organic solvent, the diluent contains only a small amount of water or practically no water. It may be desirable to limit the maximum water content of the absorbent to, for example, 20% by weight, alternatively to 10% by weight, preferably to 5% by weight or 2% by weight.

The non-aqueous organic solvent is preferably selected from the following compounds:

C4-C10 alcohols such as n-butanol, n-pentanol and n-hexanol;

ketones such as cyclohexanone;

esters such as ethyl acetate and butyl acetate;

lactones such as γ-butyrolactone, δ-valerolactone and ε-caprolactone;

amides such as tertiary carboxamides, for example N,N-dimethylformamide or Nformylmorpholine and N-acetylmorpholine;

lactams such as γ-butyrolactam, 5-valerolactam and ε-caprolactam and N-methyl-2-pyrrolidone (NMP);

sulfones such as sulfolane;

sulfoxides such as dimethyl sulfoxide (DMSO);

glycols such as ethylene glycol (EG) and propylene glycol;

polyalkylene glycols such as diethylene glycol (DEG) and triethylene glycol (TEG);

di- or mono(C1- _C4 -alkyl ether) glycols such as ethylene glycol dimethyl ether;

di- or mono(C1-C ₄ -alkyl ether)polyalkylene glycols such as diethylene glycol dimethyl ether, dipropylene glycol monomethyl ether and triethylene glycol dimethyl ether;

cycloureas such as N-dimethylimidazolin-2-one and dimethylpropylene urea (DMPU);

thioalkanols such as ethylene dithioethanol, thiodiethylene glycol (thiodiglycol, TDG) and methylthioethanol;

and mixtures thereof.

More preferably, the non-aqueous solvent is selected from sulfones, glycols and polyalkylene glycols. Most preferably, the non-aqueous solvent is selected from sulfones. A preferred non-aqueous solvent is sulfolane.

Other additives.

The absorbent may also contain additives such as corrosion inhibitors, enzymes, antifoaming agents, etc. In general, the amount of such additives is in the range of approximately 0.005-3% based on the total weight of the absorbent.

Areas of application of the absorbent.

The present invention also relates to the use of the absorbent described herein for removing acid gases from a fluid stream.

In one embodiment, the present invention also relates to the use of an absorbent described herein for non-selectively removing acid gases from a fluid stream. In this case, it is preferred that the absorbent contains at least one activator selected from an unhindered primary amine and/or an unhindered secondary amine as described above.

In yet another embodiment, the present invention relates to the use of an absorbent as described herein to selectively remove hydrogen sulfide from a fluid stream containing carbon dioxide and hydrogen sulfide. In this case, it is preferable that the absorbent contains any sterically unhindered primary amine or sterically unhindered secondary amine.

In one embodiment, the present method is a non-selective removal method

- 9 046450 removal of acid gases from a fluid stream. In this case, it is preferred that the absorbent contains at least one activator selected from an unhindered primary amine and/or an unhindered secondary amine as described above.

In yet another embodiment of the invention, the present method is a method for selectively removing hydrogen sulfide from a fluid stream containing carbon dioxide and hydrogen sulfide. In this case, it is preferable that the absorbent contains any sterically unhindered primary amine or sterically unhindered secondary amine.

In the context of the present invention, the term selectively for hydrogen sulfide should be understood as the value of the following coefficient:

[ ^mol ( ^H 2 ^S ) ^/mol ( ^CO 2)] _{liquid phase} ^{/ [m0l(H} 2 ^S)/m0l(CO 2 ^)] gaseous phase, where [μολβ(Η ₂ 8)/μολβ(ΟΟ ₂ )]κ^ _{μ phase} means the molar ratio of H2S/CO2 in the liquid phase that is in contact with the gas phase; And

[μολ^Η^/μολ^^^Ν^μ _phase means the molar ratio of H2S/CO2 in the gas phase.

In a standard gas scrubbing process, the liquid phase is the loaded absorbent at the bottom of the absorption unit and the gas phase is the fluid stream being treated.

A process is said to be selective for H2S when the value of the above factor is greater than 1. If the present process is a process for selectively removing hydrogen sulfide from a fluid stream containing carbon dioxide and hydrogen sulfide, the selectivity for hydrogen sulfide is preferably at least 1.1 , even more preferably at least 2 and most preferably at least 4.

The absorbent according to the invention can be used to treat all types of fluids. Fluids are primarily gases such as natural gas, synthesis gas, coke oven gas, cracking gas, coal gasification gas, recycle gas, landfill gas and combustion gases, and secondly liquids that are practically immiscible with the absorbent , such as LPG (liquefied petroleum gas) or NGL (wide fraction of light hydrocarbons). The method of the invention is particularly useful for treating hydrocarbon fluid streams. The hydrocarbons present are, for example, aliphatic hydrocarbons such as C ₁ -C ₄ hydrocarbons such as methane, unsaturated hydrocarbons such as ethylene or propylene, or aromatic hydrocarbons such as benzene, toluene or xylene.

The absorbent according to the invention can be used to remove acid gases, for example, CO2, H2S, SO3, _So2 , CS2, HCN, COS and mercaptans.

Other acid gases such as COS and mercaptans may also be present in the fluid stream.

The absorbent can be used to selectively remove hydrogen sulfide from a fluid stream containing carbon dioxide and hydrogen sulfide, the process providing high selectivity for H2S removal at low solvent circulation rates. The absorbent can be used in the tail gas treatment unit of a sulfur plant, in acid gas upgrading processes to improve the quality of lean off-gas from treatment units to produce a high-quality feed stream for a Claus plant, or in the treatment of associated gases and refinery gases.

In the method of the invention, the fluid stream is contacted with the absorbent during the absorption step in the absorption device, resulting in at least partial removal of carbon dioxide and hydrogen sulfide. The result is a fluid stream free of CO2 and H2S and an absorbent loaded with CO2 and H2S.

The absorber used is a scrubbing device used in conventional gas scrubbing processes. Suitable scrubbing devices are, for example, irregular packings, structured packing and tray columns, membrane contact filters, radial scrubbers, nozzle scrubbers, tube scrubbers and dynamic wet scrubbers, preferably structured packing and tray columns, more preferably tray and tray columns. irregular nozzles. The fluid stream is preferably treated with an absorbent in a countercurrent column. The fluid is typically fed to the bottom of the column and the absorbent to the top. Tray columns contain sieve trays, bubble-cup trays, or valve trays through which the liquid flows. Columns with irregular packings can be filled with bodies of various shapes. When the surface increases due to bodies of various shapes, usually approximately 25-80 mm in size, heat and mass transfer improves. Well-known examples are the Raschig ring (hollow cylinder), Pahl ring, Hiflow ring, Intalox saddle and other similar bodies. Irregular packings can be placed in the column in an ordered manner or in a random manner (as a layer). Possible materials include glass, ceramic materials, metal and plastic. Structured nozzles are a further development of ordered, irregular nozzles. They have an ordered structure. As a result, the pressure drop in the gas flow can be reduced. There are times

- 10 046450 personal designs of structured nozzles, for example braided nozzles or metal sheet nozzles. Materials such as metal, plastic, glass and ceramics can be used.

The temperature of the absorbent during the absorption stage is typically approximately 30-100°C, and when using a column - for example, 30-70°C at the top of the column and 50-100°C at the bottom of the column.

The method of the present invention may include one or more, in particular two successive absorption steps. Absorption can be carried out in several successive stages, in which case the raw gas containing acid gas components is in contact with the absorbent sub-stream at each stage. The absorbent with which the raw gas comes into contact may already partially contain acid gases; this means that such absorbent may have been previously recycled from a downstream absorption step to the first absorption step, or may be a partially regenerated absorbent. The characteristics of two-stage absorption are described in publications EP 0 159 495, EP 0 190 434, EP 0 359 991 and WO 00100271.

One skilled in the art is able to achieve high levels of hydrogen sulfide removal with a certain selectivity by varying the conditions during the absorption stage, in particular, for example, conditions such as the ratio of absorbent flow to fluid flow, the height of the absorber column, the type of internal elements of the absorber that provide contact with the absorbent, such as irregular packings, trays or structured packings, and/or residual load of regenerated absorbent. Since CO ₂ absorption occurs more slowly than H2S absorption, a longer treatment duration will absorb more CO ₂ compared to a shorter treatment duration. On the other hand, with longer treatment times, the selectivity of H2S decreases. Therefore, if a taller column is used, the absorption will be less selective. Plates or structured packings with relatively high liquid accumulation also entail a decrease in absorption selectivity. The thermal energy supplied to the regeneration stage can be used to adjust the residual load of the regenerated absorbent. With a smaller residual load of regenerated absorbent, improved absorption is ensured.

Preferably, the method includes an absorbent regeneration step, during which the absorbent containing large amounts of CO ₂ and H2S is regenerated. The regeneration step removes CO ₂ and H ₂ S and, if necessary, other acid gas components from the absorbent containing large quantities of CO ₂ and H2S, obtaining a regenerated absorbent. The regenerated absorbent is then preferably recycled to the absorption step. In general, the regeneration step includes at least one of the following: heating, decompression and desorption with an inert liquid.

The regeneration step preferably involves heating the absorbent containing a large amount of acid gas components, for example, using a boiler, a free circulation evaporator, a forced circulation evaporator or a forced circulation flash evaporator. Desorption of absorbed acid gases is carried out using steam obtained by heating the solution. An inert fluid such as nitrogen can also be used instead of steam. The absolute pressure in the desorption apparatus is generally 0.1-3.5 bar, preferably 1.0-2.5 bar. Typically, the temperature is in the range of 50-170°C, preferably 80°C-130°C, and here it is obvious that the temperature depends on the pressure. In some cases, an additional step of regenerating the regenerated absorption solvent bleed stream is necessary. When _SOx , _NOx , and CO are present in the fluid stream, thermally stable salts such as sulfates, nitrates, and formates are formed. An additional distillation step at higher temperatures may be used to reduce the concentration of these undesirable components, or heat-stable salts may be removed through an ion exchange process.

Alternatively or in addition, the regeneration step may include decompression. This concerns at least one decompression step of the absorbent containing large quantities of acid gases, when its pressure, which is high during the absorption step, is reduced to a lower pressure. Decompression can be carried out, for example, using a throttle valve and/or a decompression turbine. The regeneration process with a decompression step is described, for example, in publications US 4,537,753 and US 4,553,984.

The acid gas components can be removed during the regeneration step, for example, using a decompression column, such as a flash tank mounted vertically or horizontally, or a counterflow column with internals.

Likewise, the recovery column may be a random packed, structured packed, or tray-packed column. At the bottom of the regeneration column there is a heater, for example a forced circulation evaporator with a circulation pump. At the top of the regeneration column there is an outlet for removing released acid gases. The captured vapors of the absorption medium are condensed in a condenser and recycled into the column.

It is possible to connect several decompression columns in series, in which regeneration

- 11 046450 is carried out at different pressures. For example, in the pre-decompression column, regeneration can be carried out at high pressure, typically at a pressure of approximately 1.5 bar above the partial pressure of the acid gas components of the gas in the absorption stage, and in the main decompression column at low pressure, for example at 1- 2 bar abs. Regeneration with two or more stages of decompression is described in US 4,537,753, US 4,553,984, EP O 159 495, EP O 202 600, EP O 190 434 and EP O 121 109.

The process of the present invention, which uses compounds of formula (I), demonstrates high selectivity in the purification of gas streams containing both H ₂ S and CO ₂ . The process of the present invention further provides a high degree of removal of mercaptans and other sulfur compounds that may be present in such gas streams. The compounds of formula (I) exhibit higher thermal stability at higher temperatures and more complete regeneration of absorbent solutions, achieving lower load factors.

The present invention is illustrated in more detail by the following examples.

Example 1. Preparation of 2-[2-(tert-butylamino)ethylsulfanyl]ethanol.

1.7 g of sodium methylate was dissolved in 15 ml of dry ethanol. Mercaptoethanol was added to the solution while stirring. After completion of the mixing process, the mixture was stirred for another 15 minutes. A solution of 2.5 g of 2-chloro-N-tert-butylethylamine hydrochloride dissolved in 50 ml of dry ethanol was added dropwise to maintain the temperature in the range of 35-40°C. After completing the mixing process, the resulting suspension was heated to 75°C and then stirred for another 90 minutes. After stirring overnight at room temperature, the suspension was filtered and the filtrate was evaporated at 90 °C and 60 mbar on a rotary evaporator.

2.5 g of 2-[2-(tert-butylamino)ethylsulfanyl]ethanol were obtained. The calculated product yield was 97%. The structure of the compound was confirmed by proton magnetic resonance.

Example 2 Comparison of the properties of 2-[2-(tert-butylamino)ethylsulfanyl]ethanol (TBAESE) and 2-[2(tert-butylamino)ethoxy]ethanol (TBAEE).

A. Heat resistance.

The thermal stability of TBAESE was compared with TBAEE and MDEA with and without acid gas loading.

The cylinder (10 ml) was initially loaded with the appropriate solution (8 ml) and the cylinder was closed. The cylinder was heated to 150°C for 125 hours. In experiments carried out with acid gas loading, the loading of solutions with acid gas was 20 nm ³ /CO2 solution and 20 nm ³ /H ₂ S solution. The level of amine decomposition was calculated from the amine concentration measured by gas chromatography before and after the experiment. The results are presented in the table.

Water solution Decomposition coefficient No acid gas loading With acid gas loading 40 wt.% MDEA + 60 wt.% H ₂ O* 0.98 0.89 30 wt.% TBAEE + 70 wt.% H ₂ O* 0.99 0.92 20 wt.% TBAESE + 75 wt.% H ₂ O 0.99 0.96

* comparative example.

It is obvious that TBAESE has higher thermal stability compared to MDEA and TBAEE in aqueous solutions in the presence of acid gas loading.

b. Loading and regeneration of acid gas.

The pKa values for TBAESE and TBAEE were measured in the temperature range from 20 to 120°C. The results are presented in table. 1. For this, an aqueous solution of amine at a concentration of 0.01 mol/l was mineralized by 50% with a solution of HCl 0.005 mol/l. Thus, the measured pH value of a 50% neutralized amine solution is equal to the pKa value of the amine.

The measurement was performed in a glass vessel under nitrogen pressure to avoid loss of water and solvent.

As can be seen, the pK _a values for TBAESE and TBAEE are comparable within the measurement range and are significantly higher than the pK _{a value of} MDEA. Based on these measurements, it can be concluded that the acid gas loading and regeneration of TBAESE is comparable to TBAEE.

Thus, although TBAESE and TBAEE have similar absorption properties, TBAESE exhibits slightly higher thermal stability. This allows TBAESE to be operated at slightly higher regeneration temperatures, resulting in more complete regeneration of the absorbent.

TBAESE also combines the advantages of hindered amines, such as high selectivity for H ₂ S and thioalcohols, and high removal of other sulfur compounds that may be present in the feed gas, particularly mercaptans, in a single molecule.

Claims (11)

1. A method of removing acid gases from a fluid stream, wherein the fluid stream is contacted with an absorbent to produce a treated fluid stream and a saturated absorbent, wherein the absorbent contains at least one diluent and a compound of general formula (I) R1 R3 and R1 represents ^-C^alkyl; R2 represents ^-C^alkyl; R3 is selected from hydrogen and ^-C^alkyl; R4 is selected from hydrogen and ^-C^alkyl, and n is an integer in the range of 1-4. 2. The method according to claim 1, characterized in that each of R1, R2 and R3 represents Q-alkyl. 3. Method according to claims 1, 2, characterized in that the compound of general formula (I) is 2-[2(tert-butylamino)ethylsulfanyl]ethanol. 4. Method according to any one of paragraphs. 1-3, characterized in that the diluent contains water. 5. The method according to claim 4, characterized in that the absorbent additionally contains an acid. 6. Method according to any one of claims 1-3, 5, characterized in that the diluent contains a non-aqueous organic solvent. 7. The method according to claim 6, characterized in that the organic solvent is selected from C ₄ -C ₁₀ alcohols, ketones, esters, lactones, amides, lactams, sulfones, sulfoxides, glycols, polyalkylene glycols, di- or mono^-^- alkyl ether) of glycols, di- or mono(C1-C ₄ -alkyl ether) of polyalkylene glycols, cycloureas, thioalkanols and mixtures thereof. 8. Method according to any one of claims 1 to 7, characterized in that the absorbent contains at least one activator selected from a sterically unhindered primary amine and/or a sterically unhindered secondary amine. 9. Method according to claim 8, characterized in that the activator is piperazine. 10. A method according to any one of claims 1 to 7 for selectively removing hydrogen sulfide from a fluid stream containing carbon dioxide and hydrogen sulfide. 11. A method according to any one of the preceding claims, characterized in that the loaded absorbent is regenerated by at least one of the following actions: heating, decompression and desorption using an inert liquid.