US20110176981A1

US20110176981A1 - Absorbent solution based on a tertiary or hindered amine and on a particular activator and method for removing acid compounds from a gaseous effluent

Info

Publication number: US20110176981A1
Application number: US13/056,446
Authority: US
Inventors: Marc Jacquin; Julien GRANDJEAN; Thierry Huard
Original assignee: IFP Energies Nouvelles IFPEN
Current assignee: IFP Energies Nouvelles IFPEN
Priority date: 2008-07-28
Filing date: 2009-07-21
Publication date: 2011-07-21
Also published as: JP2011528993A; CN102105207A; CA2731061C; FR2934172A1; WO2010012884A1; WO2010012883A2; US8845787B2; EP2318120A1; AU2009275767B2; EP2310110B1; CA2731061A1; FR2934172B1; WO2010012885A1; EP2310110A2; US20110185901A1; AU2009275767A1; WO2010012883A3; EP2318119A1

Abstract

The invention relates to the removal of acid compounds from a gaseous effluent In an absorption method using an aqueous solution comprising a tertiary amine or a sterically hindered amine in admixture with a primary or secondary amine meeting the general formula as follows:

The invention is advantageously applied to the treatment of natural gas and of gas of industrial origin.

Description

FIELD OF THE INVENTION

The present invention relates to the removal of acid compounds from a gaseous effluent.
The present invention relates to the treatment of acid gases (H₂S, CO₂, COS, CS₂, mercaptans, etc.) by means of an aqueous solution of tertiary or sterically hindered amine, formulated with a primary or secondary amine meeting general formula (I).
The invention is advantageously applied to the treatment of natural gas and of gas of industrial origin.

BACKGROUND OF THE INVENTION

Treatment of Gas of Industrial Origin

The nature of the gaseous effluents that can be treated is varied, non-limitative examples thereof are syngas, combustion fumes, refinery gas, Claus tail gases, biomass fermentation gases, cement plant gases and blast furnace gases.
All these gases contain acid compounds such as carbon dioxide (CO₂), hydrogen sulfide (H₂S), carbon oxysulfide (COS), carbon disulfide (CS₂) and mercaptans (RSH), mainly methylmercaptan (CH₃SH), ethylmercaptan (CH₃CH₂SH) and propylmercaptans (CH₃CH₂CH₂SH).
For example, in the case of combustion fumes, CO₂is the acid compound to be removed. In fact, carbon dioxide is one of the greenhouse gases widely produced by human activities and it has a direct impact on atmospheric pollution. In order to reduce the amounts of carbon dioxide discharged to the atmosphere, it is possible to capture the CO₂contained in a gaseous effluent.
Treatment of Natural Gas
In the case of natural gas, three main treating operations are considered: deacidizing, dehydration and stripping. The goal of the first stage, deacidizing, is to remove acid compounds such as carbon dioxide (CO₂), as well as hydrogen sulfide (H₂S), carbon oxysulfide (COS), carbon disulfide (CS₂) and mercaptans (RSH), mainly methylmercaptan (CH₃SH), ethylmercaptan (CH₃CH₂SH) and propylmercaptans (CH₃CH₂CH₂SH). The specifications generally admitted for deacidized gas are 2% CO₂, or even 50 ppm CO₂, the natural gas being thereafter subjected to liquefaction; 4 ppm H₂S and 10 to 50 ppm volume of total sulfur. The dehydration stage then allows to control the water content of the deacidized gas in relation to the transport specifications. Finally, the natural gas stripping stage allows to guarantee the dew point of the hydrocarbons in the natural gas, here again according to transport specifications.
Deacidizing is therefore often carried out first, notably in order to remove the toxic acid gases such as H₂S in the first stage of the chain of processes and thus to avoid pollution of the various unit operations by these acid compounds, notably the dehydration section, the condensation and separation section intended for the heavier hydrocarbons.
Acid Compounds Removal by Absorption
Deacidizing gaseous effluents, such as natural gas and combustion fumes for example, as well as syngas, refinery gas, Claus tail gas, biomass fermentation gas, cement plant gas and blast furnace gas, is generally carried out by washing with an absorbent solution. The absorbent solution allows to absorb the acid compounds present in the gaseous effluent. In general terms, for the treatment of acid effluents comprising acid compounds such as, for example, H₂S, mercaptans, CO₂, COS, SO₂, CS₂, using separation agents comprising amine functions is interesting because of their performances and of their ease of use in aqueous solution.
An essential aspect of the operations for treating industrial gas or fumes by a solvent is the absorption stage. For example, in the case of CO₂capture, the absorbed CO₂reacts with the amine present in solution according to a reversible exothermic reaction known to the person skilled in the art and leading to the formation of hydrogen carbonates, carbonates and/or carbamates, allowing removal of the CO₂from the gas to be treated. Similarly, for the removal of H₂₅from the gas to be treated, the absorbed H₂S reacts with the amine present in solution according to a reversible exothermic reaction known to the person skilled in the art and leading to the formation of hydrosulfide.
Another essential aspect of the operations for treating industrial gas or fumes by a solvent is the separation agent regeneration stage. Regeneration through expansion and/or distillation and/or entrainment by a vaporized gas referred to as “stripping gas” is generally provided depending on the absorption type (physical and/or chemical).
Problems
Aqueous solutions of tertiary amines are generally preferred by the person skilled in the art for removing the acid compounds present in a gas, because they generally exhibit a high acid gas capture capacity and a high stability.
It is well known to the person skilled in the art that tertiary or sterically hindered amines have a slower CO₂and COS capture kinetics than non-hindered primary or secondary amines. In a review of 1987, Sartori et al., Sep. and Purification Methods, 16 (2), 171-200, have shown the advantages of various hindered amines, either in terms of CO₂capture capacity for moderately hindered amines, or by lowering the reactivity towards CO₂for severely hindered amines. Severely hindered amines like tertiary amines afford an advantage when the CO₂and COS concentrations are below the desired specifications because their low reactivity towards CO₂is used to achieve selective removal of H₂S. However, when the CO₂and the COS are above the desired specifications, using an aqueous solution of tertiary or hindered amine may not be satisfactory because it would require gigantic absorption column sizes to reach the specifications. In order to overcome this problem, document WO-89/11,327 aims to mix the tertiary amines with a primary amine so as to activate CO₂absorption.
This primary or secondary amine allows to dope the CO₂capture kinetics at the top of the absorption column where the CO₂and/or COS partial pressure is the lowest (references: Aroonwilas, A.; Veawab A.; Characterization and Comparison of the CO₂Absorption Performance into Single and Blended Alkanolamines in a Packed Column; Ind. Eng. Chem., Res. 43 2228-2237 & van Loo, S.; van Elk, E. P.; Versteeg, G. F.; The removal of carbon dioxide with activated solutions of methyl-diethanol-amine; Journal of Petroleum Science and Engineering 55 (2007) 135-145). Thus, adding some weight percents of activator allows to considerably reduce the size of the absorption columns while keeping the thermodynamic and physico-chemical properties of the absorbent solution of tertiary or hindered amine.
Thus, absorbent solutions made up of a tertiary amine and of some weight percents of activator are commonly used. U.S. Pat. No. 6,852,144, which describes a method of removing acid compounds from hydrocarbons, can be mentioned by way of example. The method uses a water-methyldiethanolamine or water-triethanolamine absorbent solution containing a proportion of a compound belonging to the following group: piperazine and/or methylpiperazine and/or morpholine. Patent JP-08,257,353, which describes a method of removing CO₂from fumes, can also be mentioned. The method uses a water-bis(2-dimethylaminoethyl)ether absorbent solution containing for example 2-methylaminoethanol or piperazine.
Another important aspect to be taken into account is the stability of the activators, When deacidizing the gaseous effluents, the absorbent solution, and notably the activator, is degraded either through thermal degradation or through side reaction with the acid gases to be captured, and with other compounds contained in the gaseous effluent, such as oxygen, the SOx and the NOx contained in industrial fumes for example.
These degradation reactions affect the proper functioning of the method: absorbent solution efficiency decrease, corrosion, foaming, etc. Due to these degradations, it is necessary to carry out purification of the absorbent solution by distillation and/or ion exchange and to provide make-up amine. By way of example, the make-up amine added in a post-combustion CO₂capture method using a 30 wt. % monoethanolamine absorbent solution represents 1.4 kg amine per ton of CO₂captured, which significantly increases the operating cost of a capture unit (reference: Chapel, D. G.; Mariz, C. L.; Recovery of CO₂from Flue Gases: Commercial Trends; presented at The Canadian Society of Chemical Engineers annual meeting, Oct. 4-6, 1999, Saskatoon, Saskatchewan, Canada).
It is difficult to find, for an aqueous solution of a tertiary or hindered amine, an activating compound that, added in a proportion of some weight percents, significantly increases the CO₂and COS capture kinetics, thus allowing to remove the acid compounds below specifications at lesser cost in any type of effluent. Furthermore, it is difficult to find an activating compound of high stability. The present invention provides an activator family that combines a good chemical stability and an excellent capacity of accelerating CO₂and COS absorption within a formulation containing tertiary amines and/or sterically hindered amines.

SUMMARY OF THE INVENTION

In general terms, the present invention provides an absorbent solution for absorbing the acid compounds contained in a gaseous effluent, the solution comprising:

- water,
- at least one absorbent compound selected from among tertiary amines and sterically hindered amines,
- at least one activator selected from among primary amines and secondary amines of formula:

wherein
n=1 or 2, preferably n=1
each group R1, R2, R3, R4, R5, R6, R7 and R is selected independently among one of the elements of the group made up of: a hydrogen atom, a linear or branched or cyclic alkyl group with 1 to 12 carbon atoms, an aryl group, a hydroxyalkyl group or a linear or branched or cyclic ether-oxide group with 1 to 12 carbon atoms.
According to the invention, group R can be linked by R3, R4, R5, R6 or R7 to the aromatic ring of formula (I), so as to form a cycle. For example, group R is linked by R3 or R7 to the aromatic ring of formula (I) so as to form a heterocycle with 5 to 6 atoms.
The activator can be selected from among Benzylamine, N-MethylBenzylAmine, N-EthylBenzylAmine, α-MethylBenzylAmine, α-EthylBenzylAmine, TetraHydrolsoQuinoline, Isolndoline and PhenethylAmine.
The tertiary or sterically hindered amine can be selected from among DiEthylEthanolAmine, DiMethylEthanolAmine, Diisopropanolamine, Methyl-DiEthanolAmine, TriEthanolAmine, 2-Amino-2-MethylPropan-1-ol, bis(2-dimethylamino-ethyl)ether, TetraMethyl-1,2-EthaneDiAmine, TetraMethyl-1,3-PropaneDiAmine, TetraMethyl-1,6-HexaneDiAmine and PentaMethylDiPropyleneTriAmine.
The absorbent solution according to the invention can comprise a physical solvent.
For example, the absorbent solution can comprise:

- at least 10 wt. % water,
- between 10 and 90 wt. % tertiary or sterically hindered amine,
- between 1 and 50 wt. % activator.

The present invention also describes a method for absorbing the acid compounds contained in a gaseous effluent, comprising an absorption stage wherein the gaseous effluent is contacted with an absorbent solution comprising:

wherein
n=1 or 2, preferably n=1
each group R1, R2, R3, R4, R5, R6, R7 and R is selected independently among one of the elements of the group made up of: a hydrogen atom, a linear or branched or cyclic alkyl group with 1 to 12 carbon atoms, an aryl group, a hydroxyalkyl group or a linear or branched or cyclic ether-oxide group with 1 to 12 carbon atoms.
In the method according to the invention, the absorbent solution laden with acid compounds, obtained at the end of the absorption stage, can be subjected to a regeneration operation, the regeneration stage comprising at least one of the following operations:

- expansion of the absorbent solution laden with acid compounds,
- heating of the absorbent solution laden with acid compounds.

The acid compound absorption stage can be carried out at a pressure ranging between 1 and 120 bars, and at a temperature ranging between 30° C. and 90° C.
The regeneration stage can be carried out at a pressure ranging between 1 and 10 bars, and at a temperature ranging between 100° C. and 180° C.
The gaseous effluent treated by means of the method according to the invention can comprise one of the following elements: natural gas, syngas, combustion fumes, refinery gas, Claus tail gases, biomass fermentation gases, cement plant gases, incinerator fumes.
The acid compounds can consist of at least one of the compounds: CO₂and COS.
The present invention is of interest for reducing the size of absorption columns when it is desired to remove the CO₂and/or the COS contained in a gas.
Furthermore, the primary or secondary amine compounds meeting the general formula:
exhibit increased stability in relation to known activators, thus limiting problems linked with absorbent solution degradation.

BRIEF DESCRIPTION OF THE SOLE FIGURE

Other features and advantages of the invention will be clear from reading the description hereafter, with reference to FIG. 1 given by way of example and showing a flow sheet of an acid gas effluent treating method.

DETAILED DESCRIPTION

The aqueous solutions of tertiary or sterically hindered amines activated by a primary or secondary amine meeting the general formula as follows:
are of interest in all the methods of treating acid gases (natural gas, combustion fumes, etc.).
The present invention aims to remove the acid compounds from a gaseous effluent by using aqueous solutions of tertiary amines or sterically hindered amines activated by a primary or secondary amine meeting general formula (I):
Nature of the Gaseous Effluents
The absorbent solutions according to the invention can be used to deacidize the following gaseous effluents: natural gas, syngas, combustion fumes, refinery gas, Claus tail gas, biomass fermentation gas, cement plant gas, incinerator fumes. These gaseous effluents contain one or more of the following acid compounds: CO₂, H₂S, mercaptans, COS, CS₂.
Combustion fumes are produced notably by the combustion of hydrocarbons, biogas, coal in a boiler or for a combustion gas turbine, for example in order to produce electricity. These fumes are at a temperature ranging between 20° C. and 60° C., at a pressure ranging between 1 and 5 bars, and they can comprise between 50 and 80% nitrogen, between 5 and 40% carbon dioxide, between 1 and 20% oxygen, and some impurities such as SOx and NOx if they have not been removed downstream of the deacidizing process.
Natural gas predominantly consists of gaseous hydrocarbons, but it can contain some of the following acid compounds: CO₂, H₂S, mercaptans, COS, CS₂. The proportion of these acid compounds is very variable and it can reach up to 40% for CO₂and H₂S, up to 1000 ppm for COS. The temperature of the natural gas can range between 20° C. and 100° C. The pressure of the natural gas to be treated can range between 10 and 120 bars.
Composition of the Absorbent Aqueous Solution
The absorbent solution advantageously comprises 10 to 90 wt. % tertiary or sterically hindered amine, preferably 20 to 60 wt. % and more preferably 30 to 50 wt. % tertiary or sterically hindered amine.
In the present description, what is referred to as a tertiary amine is an organic compound comprising one or more amine functions free of N—H bonds. In the present description, what is referred to as a sterically hindered amine is an organic compound comprising one or more hindered or tertiary amine functions.
The hindered amine functions can be primary or secondary.
In the case of a primary amine function, this function is considered to be hindered if the α carbon (i.e. adjacent to the nitrogen atom) is quaternary (i.e. free of CH bond). An example of a hindered primary monoamine known to the person skilled in the art is 2-amino-2-methylpropanol.
In the case of a secondary amine function, this function is considered to be hindered if the sum of the number of CH bonds for the two a carbons (i.e. adjacent to the nitrogen atom) is less than or equal to 3. An example of a hindered secondary monoamine is diisopropanolamine, which has two tertiary a carbons (i.e. having each a single CH bond), thus having a sum of the number of CH bonds for the two a carbons equal to 2.
Preferably, the tertiary or sterically hindered amine is selected from the group made up of:

- DiEthylEthanolAmine,
- DiMethylEthanolAmine,
- Diisopropanolamine,
- MethylDiEthanolAmine,
- TriEthanolAmine,
- 2-Amino-2-MethylPropan-1-ol,
- bis(2-dimethylaminoethyl)ether
- TetraMethyl-1,2-EthaneDiAmine,
- TetraMethyl-1,3-PropaneDiAmine,
- TetraMethyl-1,6-HexaneDiAmine,
- PentaMethylDiPropyleneTriAmine.

The absorbent solution comprises a non-zero proportion, below 50 wt. %, preferably below 30 wt. %, more preferably below 15 wt. %, of an activator selected from among the primary or secondary amines meeting general formula (I) as follows:
wherein
n=1 or 2, preferably n=1
each group R1, R2, R3, R4, R5, R6, R7 and R is selected independently among one of the elements of the group made up of: a hydrogen atom, a linear or branched or cyclic alkyl group with 1 to 12 carbon atoms, an aryl group, a hydroxyalkyl group or a linear or branched or cyclic ether-oxide group with 1 to 12 carbon atoms.
Preferably, R1, R2 and R can be selected independently from among the hydrogen atom and linear alkyl groups, preferably methyl or ethyl groups. In this case, R3, R4, R5, R6 and R7 are each a hydrogen atom.
According to an embodiment, group R is linked to no other element.
According to another embodiment, group R can be linked by R3, R4, R5, R6 or R7 to the aromatic ring of formula (I), so as to form a cycle. In this case, preferably, group R can be linked by R3 or R7 to the aromatic ring of formula (I) so as to form a heterocycle with 5 to 6 atoms.
For example, the activator is selected from the group made up of:
Preferably, the activator is selected from among: 1,2,3,4-TetraHydrolsoquinoline, Benzylamine, N-MethylBenzylAmine, α-MethylBenzylAmine. An excellent activator is selected from among: 1,2,3,4-TetraHydrolsoquinoline, Benzylamine.
The absorbent solution can contain at least 10 wt. % water, generally between 10 and 90 wt. % water, more preferably at least 50 wt. %, for example between 60 and 70 wt. % water.
Of course, the sum of the percentages of the various components of the absorbent solution according to the invention is 100%.
The absorbent solution according to the invention is particularly interesting in case of CO₂capture in industrial fumes or for treatment of natural gas containing CO₂above the desired specification. Indeed, for this type of applications, one wants to increase the CO₂capture kinetics in order to reduce the absorption column height.
The absorbent solution according to the invention is particularly interesting in case of CO₂capture in industrial fumes or for treatment of natural gas containing COS above the desired specification. Indeed, for this type of applications, one wants to increase the COS capture kinetics in order to reduce the absorption column height.
In an embodiment, the absorbent solution can comprise other organic compounds. Thus, the absorbent solution according to the invention can contain organic compounds non reactive towards the acid compounds (commonly referred to as physical solvents), which allow to increase the solubility of at least one or more acid compounds of the gaseous effluent. For example, the absorbent solution can comprise between 5 and 50 wt. % physical solvent such as alcohols, glycol ethers, lactames, N-alkylated pyrrolidones, N-alkylated piperidones, cyclotetramethylenesulfone, N-alkylformamides, N-alkylacetamides, ether-ketones or alkyl phosphates and derivatives thereof. By way of non-limitative example, it can be methanol, tetraethylene-glycoldimethylether, sulfolane or N-formyl morpholine.
Method of Removing the Acid Compounds from a Gaseous Effluent (FIG. 1)
The implementation of an absorbent solution for deacidizing a gaseous effluent is achieved schematically by carrying out an absorption stage, followed by a regeneration stage. The absorption stage consists in contacting the gaseous effluent containing the acid compounds to be removed with the absorbent solution in an absorption column C1. The gaseous effluent to be treated (˜1) and the absorbent solution (˜4) are fed into column C1. Upon contacting, the organic compounds provided with an amine function of the absorbent solution (˜4) react with the acid compounds contained in the effluent (˜1) so as to obtain a gaseous effluent depleted in acid compounds (˜2) that leaves the top of column C1 and an absorbent solution enriched in acid compounds (˜3) that leaves the bottom of column C1. The absorbent solution enriched in acid compounds (˜3) is sent to an exchanger E1 where it is heated by stream (˜6) coming from regeneration column C2. The absorbent solution laden with acid compounds and heated at the outlet of exchanger E1 (˜5) is fed into distillation column (or regeneration column) C2 where regeneration of the absorbent solution laden with acid compounds takes place. The regeneration stage thus notably consists in heating and possibly in expanding the absorbent solution enriched in acid compounds in order to release the acid compounds that leave the top of column C2 in gas form (˜7). The regenerated absorbent solution, i.e. depleted in acid compounds (˜6), leaves the bottom of column C2 and flows into exchanger E1 where it yields heat to stream (˜3) as described above. The regenerated and cooled absorbent solution (˜4) is then recycled to absorption column C1
The acid compound absorption stage can be carried out at a pressure ranging between 1 and 120 bars, preferably between 20 and 100 bars for treating a natural gas, preferably between 1 and 3 bars for treating industrial fumes, and at a temperature ranging between 20° C. and 100° C., preferably between 30° C. and 90° C., more preferably between 30° C. and 60° C. In fact, the method according to the invention involves an excellent acid compound absorption capacity when the temperature in absorption column C1 ranges between 30° C. and 60° C.
The regeneration stage of the method according to the invention can be carried out by thermal regeneration, optionally complemented by one or more expansion stages.
Considering the high stability of the N,N,N′,N′-tetramethylhexane-1,6-diamine used, it is possible to regenerate the absorbent solution according to the invention at high temperature in a distillation column. In general, the thermal regeneration stage is performed at a temperature ranging between 100° C. and 180° C., preferably between 130° C. and 170° C., and at a pressure ranging between 1 and 10 bars. Preferably, regeneration in the distillation column is conducted at a temperature ranging between 155° C. and 165° C., and at a pressure ranging between 6 and 8.5 bars in cases where one wants to reinject the acid gases. Regeneration in the distillation column is preferably carried out at a temperature ranging between 115° C. and 130° C. and at a pressure ranging between 1.7 and 3 bars in cases where the acid gas is sent to the atmosphere or to a downstream treating process such as a Claus process or a tail gas treating process.

EXAMPLES

Example 1

Activation for CO₂Absorption

Two series of tests for CO₂absorption by absorbent solutions according to the invention, comprising an aqueous solution of a tertiary monoamine (MethylDiEthanolAmine here) for the first series and of a tertiary diamine (TetraMethylHexaneDiAmine here) for the second series, are conducted. These tertiary amines are formulated with secondary amines meeting general formula (I) (TetraHydrolsoQuinoline and N-MethylBenzylAmine here).
The same tertiary amines can furthermore be activated with activators known to the person skilled in the art, such as MonoEthanolAmine or 2-HydroxyEthylpiperazine. The same CO₂absorption tests are thus carried out by way of comparison using absorbent solutions consisting of aqueous solutions of MethylDiEthanolAmine activated by 2-HydroxyEthylpiperazine and of TetraMethylHexaneDiAmine activated by MonoEthanolAmine.
In each test, a CO₂-containing gas is contacted with the absorbent liquid in a vertical falling film reactor provided, in the upper part thereof, with a gas outlet and a liquid inlet and, in the lower part thereof, with a gas inlet and a liquid outlet. A gas containing 10% CO₂and 90% nitrogen is injected through the gas inlet at a flow rate ranging between 10 and 40 NI/h and the absorbent liquid is fed into the liquid inlet at a flow rate of 1 l/h. A CO₂-depleted gas is discharged through the gas outlet and the CO₂-enriched liquid is discharged through the liquid outlet.
The absolute pressure and the temperature at the liquid outlet are 1 bar and 40° C. respectively.
For each test, one measures the absorbed CO₂stream between the gas inlet and outlet as a function of the incoming gas flow rate: for each gas flow rate set value: 10-15-20-25-30-35-40 NI/h, the incoming and outgoing gas is analysed by means of infrared absorption techniques in the gas phase in order to determine its CO₂content. The global transfer coefficient Kg characterizing the absorption rate of the absorbent liquid is deduced from this series of measurements by carrying out two series of increasing-decreasing flow rate tests within the flow rate range.
The operating conditions specific to each test and the results obtained are given in the table hereunder.


Composition of the aqueous absorbent solution

Tertiary amine

Activator

CO2 relative

Concentration

10⁷× KG

absorption

Nature	(wt. %)	Nature	(wt. %)	(Pa⁻¹· m⁻²· mol · s⁻¹)	rate

MDEA	40	HEP	8	3.2	1
MDEA	40	THIQ	8	5.4	1.7
TMHDA	30	MEA	5	7.1	1.0
TMHDA	30	MBA	5	7.9	1.1
TMHDA	30	THIQ	5	9.1	1.3

The above results highlight the improved CO₂absorption rate of the absorbent solutions according to the invention in relation to those of the control absorbent solutions containing the same tertiary amine and activators known to the person skilled in the art.

Example 2

Activation for COS Absorption

Two COS absorption tests using absorbent solutions according to the invention, comprising on the one hand an aqueous solution of a tertiary monoamine (MethylDiEthanolAmine here) activated by TetraHydrolsoQuinoline and, on the other hand, an aqueous solution of a tertiary diamine (TetraMethylHexaneDiAmine here) activated by TetraHydrolsoQuinoline, are conducted. These tests are compared with absorbent solutions comprising, in aqueous solution, DiEthanolAmine, MethylDiEthanolAmine and MethylDiEthanolAmine activated by piperazine, an activator known to the person skilled in the art for its COS removal performances (see document U.S. Pat. No. 6,852,144 for example).
For each test, the COS absorption by the aqueous solution is measured in a closed reactor of Lewis cell type. 200 g solution are fed into the closed reactor whose temperature is set at 40° C. Four successive carbon oxysulfide injections are carried out at a pressure between 100 and 200 mbar in the vapour phase of the reactor whose volume is 200 cm³. The gas phase and the liquid phase are stirred at 100 rpm and entirely characterized from the hydrodynamic point of view. For each injection, one measures the carbon oxysulfide absorption rate by pressure variation in the gas phase. A global transfer coefficient Kg is thus determined using a mean of the results obtained for the 4 injections.
The operating conditions specific to each test and the results obtained are given in the table hereunder.


Composition of the aqueous absorbent liquid

Amine

Activator

COS relative

Concentration

10⁸× KG

absorption

Nature	(wt. %)	Nature	(mol/kg)	(Pa⁻¹· m⁻²· mol · s⁻¹)	rate

DEA	40			5.22	1
MDEA	40			0.11	0.02
MDEA	40	PZ	0.38	9.75 +/− 0.35	1.87
MDEA	40	THIQ	0.38	9.11	1.75
TMHDA	35	THIQ	0.38	29.8	5.71

The above results highlight the COS absorption rate of the absorbent liquids according to the invention comparable to the MethylDiEthanolAmine-piperazine solution known to the person skilled in the art, which is improved in relation to those of the control absorbent solutions without activator.

Example 3

Relative Stability of the Activators

The activators of general formula (I) have the specific feature of being very resistant to the degradations that may occur in a deacidizing unit.
Aqueous amine solutions are degraded within closed reactors, heated to a temperature T, and brought under pressure with a partial pressure PP of different gases (CO₂, O₂and N₂). The liquid phase is stirred by means of a bar magnet. After 15 days, a sample of the liquid phase is taken and analysed using various techniques, notably gas chromatography. The table below gives the degradation rate TD of the absorbent solution, under various conditions, for a 15-day duration, defined by the equation below:
$TD (%) = \frac{[Amine] - [Amine] °}{[Amine] °}$
where (Amine) is the amine concentration in the degraded sample and (Amine) is the amine concentration in the non-degraded solution.
The lower the degradation rate TD, the more the amine can be considered to be stable.
The table hereafter gives the degradation rate TD of various aqueous solutions of activators according to the invention, such as TetraHydrolsoQuinoline and N-MethylBenzylamine meeting general formula (I), and of various aqueous solutions of activators known to the person skilled in the art, for a temperature of 140° C., on the one hand in the presence of CO₂and on the other hand in the presence of O₂.


		T		PP_CO2=	PP_O2=
Amine	Concentration	(° C.)		20 bar	4.2 bar

THIQ	4M	140	Degradation	5%	10%
N-MBzA	4M	140	rate	11%	9%
DEA	4M	140		93%	22%
MEA	4M	140		42%	21%

This example shows that using primary or secondary amines meeting general formula (I) as activators of a tertiary amine in an absorbent solution allows to obtain a low degradation rate in relation to the activators of the prior art (DiEthanolAmine and MonoEthanolAmine), in the presence of CO₂and in the presence of oxygen contained, for example, in combustion fumes.

Claims

1) An absorbent solution for absorbing acid compounds contained in a gaseous effluent, the solution comprising:

water,

at least one absorbent compound selected from among tertiary amines and sterically hindered amines,

at least one activator selected from among primary amines and secondary amines of formula:

wherein

n=1 or 2,

each group R1, R2, R3, R4, R5, R6, R7 and R is selected independently among one of the elements of the group made up of: a hydrogen atom, a linear or branched or cyclic alkyl group with 1 to 12 carbon atoms, an aryl group, a hydroxyalkyl group or a linear or branched or cyclic ether-oxide group with 1 to 12 carbon atoms.

2) An absorbent solution as claimed in claim 1, wherein group R is linked by R3, R4, R5, R6, R7 to the aromatic ring of formula (I) so as to form a cycle.

3) An absorbent solution as claimed in claim 2, wherein group R is linked by R3 or by R7 to the aromatic ring of formula (I) so as to form a heterocycle with 5 or 6 atoms.

4) An absorbent solution as claimed in claim 1, wherein the activator is selected from among Benzylamine, N-MethylBenzylAmine, N-EthylBenzylAmine, α-MethylBenzylAmine, α-EthylBenzylAmine, TetraHydrolsoQuinoline, Isolndoline and PhenethylAmine.

5) An absorbent solution as claimed in claim 1, wherein the absorbent compound is selected from among DiEthylEthanolAmine, DiMethylEthanolAmine, Diisopropanolamine, Methyl-DiEthanolAmine, TriEthanolAmine, 2-Amino-2-MethylPropan-1-ol, bis(2-dimethylamino-ethyl)ether, TetraMethyl-1,2-EthaneDiAmine, TetraMethyl-1,3-PropaneDiAmine, Tetra-Methyl-1,6-HexaneDiAmine and PentaMethylDiPropyleneTriAmine.

6) An absorbent solution as claimed in claim 1, comprising a physical solvent.

7) An absorbent solution as claimed in claim 1, comprising:

at least 10 wt. % water,

between 10 and 90 wt. % absorbent compound,

between 1 and 50 wt. % activator.

8) A method for absorbing the acid compounds contained in a gaseous effluent, comprising an absorption stage wherein the gaseous effluent is contacted with an absorbent solution comprising:

water,

wherein

n=1 or 2,

9) A method as claimed in claim 8, wherein the absorbent solution laden with acid compounds, obtained at the end of the absorption stage, is subjected to a regeneration operation, the regeneration stage comprising at least one of the following operations:

expansion of the absorbent solution laden with acid compounds,

heating of the absorbent solution laden with acid compounds.

10) A method as claimed in claim 8, wherein the acid compound absorption stage is carried out at a pressure ranging between 1 bar and 120 bars, and at a temperature ranging between 30° C. and 90° C.

11) A method as claimed in claim 8, wherein the regeneration stage is carried out at a pressure ranging between 1 and 10 bars, and at a temperature ranging between 100° C. and 180° C.

12) A method as claimed in claim 8, wherein the gaseous effluent comprises one of the following elements: natural gas, syngas, combustion fumes, refinery gas, Claus tail gases, biomass fermentation gases, cement plant gases, incinerator fumes.

13) A method as claimed in claim 8, wherein the acid compounds comprise at least one of the compounds: CO₂and COS.