AU2013200759B2

AU2013200759B2 - CO2 absorption from gas mixtures using an aqueous solution of 4-amino- 2,2,6,6-tetramethylpiperidine

Info

Publication number: AU2013200759B2
Application number: AU2013200759A
Authority: AU
Inventors: Wolfgang Benesch; Helmut Bruggemann; Bernd Glockler; Axel Kobus; Jens Reich; Thomas Riethmann; Jorn Rolker; Rolf Schneider; Matthias Seiler; Hermann Winkler
Original assignee: Evonik Degussa GmbH
Current assignee: Evonik Operations GmbH
Priority date: 2009-02-02
Filing date: 2013-02-13
Publication date: 2013-07-18
Anticipated expiration: 2030-01-29
Also published as: AU2013200759A1

Abstract

CO 2 ABSORPTION FROM GAS MIXTURES USING AN AQUEOUS SOLUTION OF 4-AMINO-2,2,6,6-TETRAMETHYLPIPERIDINE CO 2 is absorbed from a gas mixture by bringing the gas mixture into contact with an absorbent 5 that comprises water and at least one amine of the formula (I), wherein R1 and R2, independently of each other, are hydrogen or an alkyl group. According to the invention, absorption media comprise sulfolane or an ionic liquid in addition to water and an amine of the formula (I). A device according to the invention for removing CO 2 from a gas mixture comprises an absorption unit, a desorption unit, and an absorption medium according to the invention that is conducted in [0 the circuit. (I)

Description

1 C0 2 absorption from gas mixtures using an aqueous solution of 4-amino-2, 2,6, 6-tetramethyl piperidine Technical field The invention relates to a method for the absorption of C02 5 -from a gas mixture and also to absorption media and a device for carrying out the method. The absorption of CO 2 from a gas mixture is of particular interest for the removal of carbon dioxide from flue gases, especially for reduction of the emission of carbon dioxide, 10 which is considered to be the main cause of what is known as the greenhouse effect, from power plant processes. Furthermore, carbon dioxide is required .for some processes and using the method according to the invention carbon dioxide can be provided as feedstock for these processes. 15 Prior art On the industrial scale, aqueous solutions of alkanolamines are typically used as absorption medium for the absorption of CO 2 from a gas mixture. The loaded absorption medium is 20 regenerated by heating, expansion to a lower pressure or stripping, whereupon the carbon dioxide is desorbed. After the regeneration process the absorption medium can be reused. These methods are described, for example, in Rolker J.; Arlt, W.I ' Abtrennung von Kohlendioxid aus 25 Rauchgasen mittels Absorption" -[Elimination of carbon dioxide from flue gases by means of absorption] in Chemie Ingenieur Technik 2006, 78, pages 416 to 424. These methods have the disadvantage that for the elimination of CO 2 by absorption and subsequent desorption 30 a relatively large amount of energy is required and that during desorption only some of the absorbed CO 2 is desorbed again, so in one cycle of absorption and desorption the 2 fraction of the alkanolamine used for the absorption of CO 2 is low. In addition, the absorption media used are highly corrosive and are subject to an interfering oxidative degradation in the absorption of CO 2 from oxygen-containing gas mixtures. US 7,419,646 describes a method for the deacidification of exhaust gases which uses an 5 absorption medium which, during the absorption of the acidic gas, forms two phases which can be separated from one another. As a reactive compound for the absorption of an acidic gas, in column 6, inter alia, mention is made of 4-amino-2,2,6,6,-tetramethylpiperidine. For the absorption of C0 2 , US 7,419,646 discloses, in column 3, lines 22 to 32, the use of N,N,N',N',N'"-pentamethyldiethylenetriamine, N,N,N',N',N' 10 pentamethyldipropylenetriamine, N,N-bis(2,2-diethoxyethyl)methylamine and N,N dimethyldipropylenetriamine as reactive compounds for absorption. The method of US 7,419,646 has the disadvantage that additional apparatuses are required for separating the two phases which result from the absorption. DD 266 799 describes a method for the purification of 4-amino-2,2,6,6-tetramethylpiperidine in is which CO 2 is introduced into a solution of 4-amino-2,2,6,6-tetramethylpiperidine in water and acetone and the salt which is precipitated is split again to CO 2 and 4-amino-2,2,6,6 tetramethylpiperidine by heating it to 90 to 200 0 C. The use of ionic liquids for the absorption of CO 2 is described in X. Zhang et al., "Screening of ionic Liquids to Capture CO 2 by COSMO-RS and Experiments", AIChE Journal, Vol. 54, pages 20 2171 to 2728. Description of the Invention According to a first aspect of the present invention, there is provided a method for the absorption of CO 2 from a gas mixture by contacting the gas mixture with an absorption medium, wherein the absorption medium comprises water and at least one amine of formula (I) 2a

NR'R

2 N H (1) where R1 and R 2 independently of one another are hydrogen or an alkyl moiety with the proviso that R and R2 are not both hydrogen. According to a second aspect of the present invention, there is provided an absorption medium 5 for the absorption of CO 2 from a gas mixture, comprising water, at least one ionic liquid and at least one amine of formula (I)

NR'R

2 N H (I) where R 1 and R 2 independently of one another are hydrogen or an alkyl moiety with the proviso 10 that R' and R 2 are not both hydrogen. According to a third aspect of the present invention, there is provided a device for separating

CO

2 from a gas mixture, said device comprising an absorption unit, a desorption unit and a 2b circulating absorption medium, wherein said absorption medium comprises water and at least one amine of formula (I) NR' R 2 N H (I) s where R1 and R 2 independently of one another are hydrogen or an alkyl moiety with the proviso that R' and R2 are not both hydrogen. It has now surprisingly been found that the disadvantages 3 of the known methods may be avoided by the use of 4-amino 2,2,6,6-tetramethylpiperidines for the absorption of CO 2 , wherein the 4-amino-2, 2,6, 6-tetramethylpiperidine can be substituted by one or two alkyl moieties on the 4-amino 5 group. The invention therefore relates to a method for the absorption of CO 2 from a gas mixture by contacting the gas mixture with an absorption medium which comprises water and at least .one amine of formula (I) .10

NR

1

R

2 N where R' and R 2 independently of one another are hydrogen or an alkyl moiety. 15 The invention also relates to an absorption medium which comprises an amine of formula (I), water and sulpholane, and also an absorption medium which comprises an amine of formula (I), water and an ionic -liquid.

The invention further relates to a device for separating 20 CO 2 from a gas mixture, said device comprising an absorption unit, a desorption unit and a circulating absorption medium which comprises water and an amine of formula (I). In the method according to the invention, the absorption of 25 CO 2 is performed by contacting a gas mixture with an absorption medium which .comprises water and at least one 4 amine of formula (I)

NRR

2 N (I) H, 5 where R' and R 2 independently of one another are hydrogen or an alkyl moiety. The -method according to the invention can in principle be carried out using any gas mixture which contains C02, in particular with combustion exhaust gases; exhaust gases 10 from biological processes such as composting processes, fermentations or sewage treatment plants; exhaust gases from calcination processes such as lime burning and cement production; residual gases from blast furnace processes of iron production; and also residual gases from chemical 15. processes, such as exhaust gases of carbon black production or hydrogen production by steam reforming. Preferably, the gas mixture is. a combustion exhaust gas, particularly preferably a combustion exhaust gas -which contains from 1 to 60% by volume of C02, in particular from 2 to 20% by 20 volume of C02. In a particularly preferred embodiment, the gas mixture is a combustion exhaust gas from a power plant process, in particular a desulphurized combustion exhaust gas from a power plant process. In the particularly preferred embodiment using a desul-phurized combustion 25 exhaust gas from a power plant process, use can be made of all desulphurization methods known. for power plant processes, preferably gas scrubbing with milk of lime or according to the Wellmann-Lord method.

5 In the method according to the invention, the absorption medium comprises at least one amine of formula (I), where

R

1 and R 2 independently of one another are hydrogen or an alkyl moiety. Preferred alkyl moieties are alkyl moieties 5 having one to .three carbon atoms, in particular methyl moieties. In a particularly preferred embodiment, R 1 and R 2 are hydrogen. Amines of formula (I) can be produced from commercially available triacetonamine-by reductive amination, i.e. by 10 reacting triacetonamine with an amine of the formula R 1

R

2 NH and hydrogen in the presence of -a- hydrogenation catalyst. Methods for the reductive amination of triacetonamine are known to those skilled in the art from the prior art, for example from EP 0 033 529 for the production of the amine 15 of formula (I) where R 1 , R 2 = H. In the method according to the invention, the absorption medium also comprises water-in addition to at least one amine of formula (I). Preferably, the weight ratio of water to amines of formula (I) in the absorption medium is in the 20 range from 10:1 to 1:10, particularly preferably in the range from 5:1 to 1:1, and in particular in the range from 4:1 to 2:1. The absorption medium preferably comprises at least 5% by weight of amines of formula (I), particularly preferably at least 10% by weight, and in particular at 25 least 25% by weight. For the method according to the invention, all apparatuses suitable for contacting a gas phase with a liquid phase can be used to contact the gas mixture with the absorption medium. Preferably, gas scrubbers or absorption columns 30 known from the prior art are used, for example membrane contactors, radial-flow scrubbers, jet scrubbers, Venturi scrubbers, rotary spray scrubbers, packed-bed columns, packing columns and tray columns. Particularly preferably, use is made of absorption columns in the countercurrent 35 flow mode of operation.

6 In the method according to the invention, the absorption of C02 is preferably carried out at a temperature of the absorption medium in the range from 0 to 70 0 C, particularly preferably 20 to 50OC. When an absorption column is used in 5 the countercurrent flow mode of operation, the temperature *of the absorption medium is particularly preferably 30 to 60 0 C at the entry into the column and 35 to 70 0 C at the exit from the column. The absorption of CO 2 is preferably carried out at a 10 pressure of the gas mixture in the range from 0.8 to 50 bar, particularly preferably 0.9 to 30. bar. In a particularly,preferred embodiment, the absorption is carried out at a total pressure of the gas mixture in the range from 0.8 to 1.5 bar, in particular 0.9 to 1.1 bar. 15 This particularly preferred embodiment enables the absorption of C02 from the combustion exhaust gas of a power plant without compression of the combustion exhaust gas. In the method according tothe invention, the absorption 20 medium can further contain one or more solvents in addition to water and at least one amine of formula (I). Preferably, an absorption medium according to the invention, further comprises sulpholane, CAS No. 126-33-0, in addition to water and at least one amine of formula (I), 25 preferably with a fraction of sulpholane of at least 5%.by weight, particularly preferably at least 10% by weight, and in particular at least 25% by weight. The weight ratio of water to sulpholane in such an absorption medium is preferably in the range from 10:1 to 1:1, particularly 30 preferably in the range from 5:1 to 2:1. The weight ratio of sulpholane to amines of formula (I) is -preferably in the range from 3:1 to 1:3, particularly preferably 2:1 to 1:2. In a further embodiment, an absorption medium according. to the invention further comprises at least one ionic liquid 7 in addition to water and at least one amine of formula (I), preferably with a fraction of ionic liquid of at least 5% by weight, particularly preferably at least 10% by weight, and in particular at least 25% by weight. The weight ratio 5 of water to ionic liquid in such an absorption medium is preferably in the range from 10:1 to 1:1, particularly preferably in the range from 5:1 to 2:1. The weight ratio of ionic liquid to amines of formula (I) is preferably in the range from 3:1 to 1:10, particularly preferably 2:1 to 10 1:5. An ionic liquid in the meaning of the invention is a salt of anions and cations or a mixture of such salts, wherein the salt or the mixture of salts has a melting point of below 100 0 C. Preferably, the ionic liquid consists of one 15 or more salts of organic cations with organic or inorganic anions. Mixtures of a plurality of salts having different organic cations-and identical anions are particularly preferred. Suitable organic cations are, in particular, cations of the 20 general formulae .(II) to (VI):

R

1

R

2

R

3

R

4 N* (II)

R

1

R

2

R

3

R

4 P+ (III)

R

1

R

2

R

3 S+ IV) RR 2N+=C (NR 3

R

4 ) (NRsR 6 ) (V) 25 RIR 2

N*=C(NR

3

R

4 ) (XR 3 ) (VI) where 1 2 3 4 5 6 R R R R Rs, R are identical or different and are hydrogen, a linear or branched aliphatic or olefinic hydrocarbon.moiety having 1 to 30 carbon atoms, a cyclo 30 aliphatic or cycloolefinic hydrocarbon moiety having 5 to 40 carbon atoms, an aromatic hydrocarbon moiety having 6 to 8 40 carbon atoms, an alkylaryl moiety having 7 to 40 carbon atoms, a linear or branched aliphatic or olefinic hydrocarbon moiety, which is interrupted by one or more groups -0-, -NH-, -NR'-, -0-C(O)-, -(0)C-0-, .-NH-C(O)-, 5 -(0)C-NH-, -(CH 3 )N-C(O)-, -(O)C-N(CH 3 )-, -S(0 2 )-O-, -O-S (0 2 )-, -S(0 2 )-NH-, -NH-S(0 2 )-, -S(0 2

)-N(CH

3 )- or

-N(CH

3 )-S(0 2 )-, and having 2 to 30 carbon atoms, a linear or .branched aliphatic or olefinic hydrocarbon moiety which is functionalized in the terminal position by OH, OR', NH 2 , 10 N(H)R' or N(R') 2 and having 1 to 30 carbon atoms, or a polyether moiety of the formula -(R -0),-R 8 which is made up blockwise or randomly, wherein R 5 is not hydrogen for cations of the formula (VI), R' is an aliphatic or olefinic hydrocarbon moiety having 1 15 to 30 carbon atoms,

R

7 is a linear or branched alkylene moiety containing 2 to 4 carbon atoms, n is from 1 to 200, preferably 2 to 60,

R

8 is hydrogen, a linear or branched aliphatic or olefinic 20 hydrocarbon moiety having 1 to 30 carbon atoms, a cyclo aliphatic or cycloolefinic hydrocarbon moiety having 5 to 40 carbon atoms, an aromatic hydrocarbon moiety having 6 to 40 carbon atoms, an alkylaryl moiety having 7 to 40 carbon atoms or a moiety -C(0)-R', 25 R 9 is a linear.or branched. aliphatic or olefinic hydrocarbon moiety having 1 to 30 carbon atoms, a cycloaliphatic or cycloolefinic hydrocarbon moiety having 5 to 40 carbon atoms, an aromatic hydrocarbon moiety having 6 to 40 carbon atoms or an alkylaryl moiety having 7 to 40 30 carbon atoms, X is an oxygen atom or a sulphur atom, and wherein at. least one, and preferably each, of the moieties 9 1 2 3 4 56 R, R, R, R, R, and R 6 is different from hydrogen. Likewise suitable are cations of the formulae (II) to (VI) in which the moieties R' and R 3 together form a 4- to 10-membered ring, preferably 5- to 6-membered ring. 5 In the cations of the formula (V) , the moieties R' to R 5 are preferably methyl groups and the moiety R 6 is preferably an ethyl group or an n-propyl group. In the cations of the formula (VI), the moieties R 1 to R 4 are preferably methyl groups. 10 Likewise suitable are heteroaromatic cations having at least one quaternary nitrogen atom in the ring which bears a moiety R' as defined above, preferably derivatives of pyrrole, pyrazole, imidazole, oxazole, isoxazole, thiazole, isothiazole, pyridine, pyrimidine, pyrazine, indole, 15 quinoline, isoquinoline, cinnoline, quinoxaline or phthalazine which are substituted on the nitrogen atom. Suitable inorganic anions are, in particular, tetrafluoro borate, hexafluorophosphate, nitrate, sulphate, hydrogen sulphate, phosphate, hydrogenphosphate, dihydrogen 20 phosphate, hydroxide, carbonate, hydrogencarbonate and the halides, preferably chloride. Suitable organic anions are, in particular, RaOSO 3 -, RaSO 3 , RaOPO 3 2 -, (RaO) 2

PO

2 -, RaPO 3 2 -, RaCOO~, RaO~, (RaCO) 2 N-, (RaSO 2

)

2 N-, NCN , Rb 3

PF

3 - and. RbBF 3 -, wherein Ra is a linear 25 or .branched aliphatic. hydrocarbon moiety having 1 to 30 carbon atoms, a cycloaliphatic hydrocarbon moiety -having 5 to 40 carbon atoms, an aromatic hydrocarbon moiety having 6 to 40 carbon atoms, an alkylaryl moiety having 7 to 40 carbon atoms or a linear or branched perfluoroalkyl moiety 30 having 1 to 30 carbon atoms and R is a perfluoroalkyl moiety having 1 to 30 carbon atoms, preferably 1 to 3 carbon atoms.

10 In a preferred embodiment, the ionic liquid comprises one or more 1,3-dialkylimidazolium salts, wherein the alkyl groups particularly preferably independently of one another are selected from methyl, ethyl, n-propyl, n-butyl and 5 n-hexyl. In a further preferred embodiment, the ionic liquid comprises one or more quaternary ammonium salts with a monovalent anion and cations of the general formula (II) where 10 R' is an alkyl moiety having 1 to 20 carbon atoms, R2 is an alkyl moiety having 1 to 4 carbon atoms, R3 is a moiety (CH 2 CHRO),-H where n is from 1 to 200 and R = H or CH 3 and R4 is an alkyl moiety having 1 to 4 carbon atoms or a 15 moiety (CH 2 CHRO)n-H where n is from 1 to 200 and R = H or is CH 3 . Methods for the production of the ionic liquids are known to those skilled in the art from the prior art. In addition to water and at least one amine of formula (I), 20 the absorption medium according to the invention can comprise both an ionic liquid and sulpholane. In the method according to the invention, preferably the above-described absorption media according to the invention are used. 25 In the method according to the invention the absorption medium can contain, in addition to the abovementioned components, also additives,- preferably corrosion inhibitors and/or additives promoting wetting. Corrosion inhibitors which can be used in the method 30 according to the invention are all substances which are known to those skilled in the art as suitable corrosion inhibitors for methods for the absorption of CO 2 using alkanolamines, in particular the corrosion inhibitors 11 described in US 4,714,597. Additives promoting wetting which are used are preferably one or more surfactants from.the group of the nonionic surfactants, zwitterionic surfactants and cationic 5 surfactants. Suitable nonionic surfactants are alkylamine alkoxylates, amidoamines, alkanolamides, alkylphosphine oxides, alkyl N-glucamides, alkyl glucosides, bile acids, alkyl alkoxylates, sorbitan esters, sorbitan ester ethoxylates, 10 fatty alcohols, ethoxylated fatty acids, ester ethoxylates and polyether siloxanes. Suitable zwitterionic surfactants are betaines, alkyl glycines, sultaines, amphopropionates, amphoacetates, tertiary amine oxides and silicobetaines. 15 Suitable cationic surfactants are quaternary ammonium salts having one or two substituents having 8 to 20 carbon atoms, in particular corresponding tetraalkylammonium salts, alkylpyridinium salts, ester quats, diamidoamine.quats, imidazolinium quats, alkoxyalkyl quats, benzyl quats and 20 silicone quats. In a preferred embodiment, the additive promoting wetting comprises one or more nonionic surfactants of the general formula R(OCH 2 CHR')mOH with m from 4 to 40, where R is an -alkyl moiety having 8 to 20 carbon atoms, an alkylaryl 25. moiety having 8 to 20 carbon atoms or a polypropylene oxide moiety having 3 to.40 propylene oxide units and R' is methyl, or preferably hydrogen. In a further preferred embodiment, the additive promoting wetting comprises a polyether-polysiloxane copolymer which 30 contains more than 10% by weight of [Si(CH 3

)

2 0] units and more than 10% by weight of [CH 2 CHR-O] units in which R is hydrogen or methyl. Particular preference is given to polyether-polysiloxane copolymers of the general formulae 12 (VII) to (IX):

(CH

3 ) 3 Si-0- [SiR- (CH 3 ) -O]n-Si (CH 3 ) 3 (VII) R 2O-Ap- [B-A]m-Aq-R 2(VIII) R 20-[A-Zp-[B-Si(CH 3 )2-Z-0-A-Z].-B-Si(CH3) 2 [Z-0-A qOI-qR2 (IX) 5 where A is a divalent moiety of the formula -[CH 2

CHR

3 -O]r-, B is a divalent moiety of the formula -[Si(CH 3

)

2 -O]s-, Z is a divalent linear or branched alkylene moiety having 2 to 20 carbon atoms, and is preferably -(CH 2

)

3 -, 10 n = 1 to 30, m = 2 to 100, p, q = 0 or 1, r = 2 to 100, s = 2 to 100, 15 from I to 5 of the moieties R3 are moieties of the general formula -Z-0-A-R 2 and the remaining moieties R 1 are methyl, R2 is hydrogen or an aliphatic or olefinic alkyl moiety or acyl moiety having 1 to 20 carbon atoms and

R

3 is hydrogen or methyl. 20 The additives promoting wetting are already known to those skilled in the art from the prior art as additives for . aqueous solutions and can be produced by methods known from the prior art. In the method according to the invention, temperature and 25 pressure in the absorption and also the composition of the 13 absorption medium are preferably chosen in such a manner that the absorption medium is present as one phase after the absorption of .C0 2 , i.e. the absorption of C02 in the absorption medium does not lead to precipitation of a solid 5 or separation of a second liquid phase. This preferred embodiment of the method according to the invention does not require any additional apparatuses for phase separation and can be carried out in the devices known from the prior art for the absorption of C02 using alkanolamines. 10 In a preferred embodiment of the method according to the invention, C02 absorbed in the -absorption medium is desorbed again by increasing the temperature and/or decreasing the pressure and the absorption medium after this desorption of C02 is reused for the absorption- of C02. 15 By means of such a Cyclic process of absorption and desorption, C02 can be separated off in whole or in part from the gas mixture and obtained separately from other components of the gas mixture. Alternatively to increasing the temperature or decreasing 20 the pressure, or in addition to a temperature increase and/or pressure reduction, desorption can also be carried out by stripping the C0 2 -loaded absorption medium with a gas. If, in the desorption of C02, water is additionally removed 25 from the absorption medium, water can also be optionally. added to the absorption medium before reuse for absorption. All apparatuses known from the prior art for desorption of a gas from a liquid can be used for the desorption. Preferably, desorption- is carried out in a desorption 30 column. Alternatively, desorption'of C02 can also be carried out in one or more flash evaporation stages. In the case of desorption by raising the temperature, the desorption of C02 is preferably carried out at a 14 temperature of the absorption medium in the range from 50 to 2000C, particularly preferably 80 to 150 0 C. The temperature in the desorption in this case is preferably at least 200C, particularly preferably at least 500C, above 5 the temperature in the absorption. In the case of desorption by reducing the pressure, the desorption of C02 is preferably carried out at a total pressure in the gas phase in the range from 0.01 to 10 bar, in particular 0.1 to 5 bar. The pressure in the desorption 10 in this case is preferably at least 1.5 bar, particularly preferably at least 4 bar, below the temperature in the absorption. In the case of desorption by raising the temperature, the pressure in the desorption of C02 can also be higher than 15 in the absorption of C02. In this embodiment the pressure in the desorption of C02 is preferably up to 5 bar, particularly preferably up to 3 bar, above the pressure in the absorption of C02. Using this embodiment, the C02 separated from the gas mixture can be compressed to a 20 higher pressure than that'of the gas mixture without using mechanical energy. A device according to the invention for separating C02 from a gas mixture comprises an absorption unit, a desorption unit and a circulating absorption medium according to the 25 invention. The apparatuses described above for the absorption in a method according to the invention are suitable as absorption unit of' the device according to the invention. The apparatuses described above for the desorption in a method according to the invention are 30 suitable as desorption unit of the device according to the invention. Preferably, the device according to the invention comprises an absorption unit and a desorption unit as are known to those skilled in the art of devices for separating C02 from a gas mixture with the use of an 35 alkanolamine.

15 The method according to the invention and the absorption media according to the invention enable, by the use of at least one .amine of formula (I) in the absorption medium, a higher degree of loading of the absorption medium with C02 5 in the absorption, wherein the degree of loading in the meaning of the invention denotes the molar ratio of C02 to amine in the absorption medium, compared with the known methods and absorption media, in particular compared with the industrially most used alkanolamines. Furthermore, the 10 absorption medium of the method according to the invention is less corrosive, less toxic and shows a higher chemisorption rate for C02 and a lower oxidative breakdown with oxygen than absorption media which contain alkanolamines. Furthermore, in the embodiment of a cyclic 15 process of absorption and desorption, compared with the known methods and absorption media, in particular compared with alkanolamines, an improved carbon dioxide differential is achieved, wherein the carbon dioxide differential in the meaning of the invention is the difference between the 20 degree of loading of the absorption medium with C02 after the absorption of C02 and the degree of loading of the absorption medium with C02 after the desorption of C02 These advantages enable a. more effective absorption of' CO2 from gas-mixtures having a low C02 partial pressure, and 25 also a decrease in size of the apparatuses and a lowering of the energy consumption compared with the methods known from the prior art. Furthermore, because of the lower corrosivity a lower amount of:corrosion inhibitors is required in the method according to the invention than in 30 the known methods. Absorption media according to the invention which, in addition to water and an amine of formula (I), also. contain sulpholane exhibit a further improved carbon dioxide differential, in particular in a method having desorption 35 of the C02 by temperature increase.

16 Using absorption media according to the invention which, in addition to water and an amine of formula (I), also contain at least one ionic liquid, the desorption of CO 2 may be carried out at higher temperatures and/or lower pressures 5 without a loss of solvent or, due to the evaporation of water, a precipitation of solid or a phase separation of the absorption medium occurring in the desorption. The examples hereinafter clarify the invention, but without limiting the subject matter of the invention. 10 17 Examples Example 1 In an apparatus for measuring gas-liquid equilibria which is thermostated and provided- with a pressure controller,--a 5 mixture of 30% by weight of 4-amino 2,2,6,6-tetramethylpiperidine and 70% by weight of water was -charged at constant temperature and brought into contact with gaseous carbon dioxide at a constant pressure, wherein pressure and temperature were varied. After 10 reaching the equilibrium state, in each case the content of absorbed C02 in the loaded absorption medium was determined and the degree of loading was calculated therefrom as molar ratio of C02 to amine in the loaded absorption medium. The studied temperatures and pressures and the degrees of 15 loading determined therefor are summarized in Table 1. Example 2 Example 1 was repeated using a mixture of 30% by weight of 4-amino-2, 2,6, 6-tetramethylpiperidine, 35% by weight of 20 sulpholane and 35% by weight of water. Example 3 (comparative example) Example 1 was repeated using a mixture of 30% by weight of monoethanolamine (MEA) and 70% by weight of water. 25 From the degrees of loading result the carbon dioxide differentials listed in Table 2 for absorption and desorption at 1.5 bar and a desorption by temperature rise from 40 to 120 0 C and the carbon dioxide differentials listed in Table 3 for absorption and desorption at 120 0 C 30 and a desorption by pressure reduction from 1.5 to 0.8 bar.

18 Table 1 Example Pressure Temperature Degree of loading [bar] [0C] [mole of C0 2 /mole of amine] 1.5 40 1.20 1 3.0 40 1.29 1 0.8 120 0.35 1 1.5 120 0.64 *2 1.5 40 1.21 2 3.0 40 1.29 2 0.8 120 0.35 2 1.5 120 0.49 3* 1.5 40 0.63 3* 3.0 40 0.70 3* 0.8 120 0.39 3* 1.5 120 0.41 *not according to the invention 19 Table 2 Example Carbon dioxide differential [mole of C0 2 /mole of amine] 1 0.56 2 0.72 3* 0.22 *not according to the invention Table 3 Example Carbon dioxide differential [mole of C0 2 /mole of amine] 1 0.29 2 0.14 3* 0.02 5 *not according to the invention

Claims

2. The method of claim 1, wherein the gas mixture is a combustion exhaust gas.
3. The method of claim 1 or 2, wherein the absorption medium is present as one phase after the absorption of CO 2 .
4. The method of any one of claims 1 to 3, wherein the absorption medium comprises water and amines of formula (I) in a weight ratio of from 10:1 to 1:10.
5. The method of any one of claims I to 4, wherein the absorption medium comprises at least 5% by weight of amines of formula (I).
6. The method of any one of claims 1 to 5, wherein CO 2 absorbed in the absorption medium is desorbed by increasing the temperature and/or decreasing the pressure and the absorption medium after this desorption of CO 2 is reused for the absorption of CO 2 . 21
7. The method of claim 6, wherein the absorption is carried out at a temperature in the range from 0 to 70*C and the desorption at a higher temperature in the range from 50 to 200*C.
8. The method of claim 6 or 7, wherein the absorption is carried out at a pressure in the range from 0.8 to 50 bar and the desorption at a lower pressure in the range from 0.01 to 10 bar.
9. An absorption medium for the absorption of CO 2 from a gas mixture, comprising water, at least one ionic liquid and at least one amine of formula (I) NR'R 2 N H (I) where R' and R2 independently of one another are hydrogen or an alkyl moiety with the proviso that R' and R 2 are not both hydrogen.
10. The absorption medium of claim 9, comprising water and ionic liquid in a weight ratio of from 10:1 to 1:1.
11. The absorption medium of claim 9 or 10, comprising ionic liquid and amines of formula (I) in a weight ratio of from 3:1 to 1:10.
12. The absorption medium of any one of claims 9 to 11, comprising water and amines of formula (I) in a weight ratio of from 10:1 to 1:10.
13. The absorption medium of any one of claims 9 to 12, comprising at least 5% by weight of amines of formula (I). 22
14. The method of any one of claims 1 to 8, wherein the absorption medium is the absorption medium of claim 9.
15. The method of any one of claims 1 to 8, wherein the absorption medium is the absorption medium of claim 10.
16. A device for separating CO 2 from a gas mixture, said device comprising an absorption unit, a desorption unit and a circulating absorption medium, wherein said absorption medium comprises water and at least one amine of formula (I) NR' R 2 N H (I) where R' and R 2 independently of one another are hydrogen or an alkyl moiety with the proviso that R and R2 are not both hydrogen.
17. The device of claim 16, wherein the absorption medium comprises water and amines of formula (I) in a weight ratio of from 10:1 to 1:10.
18. The device of claim 16 or 17, wherein the absorption medium comprises at least 5% by weight of amines of formula (I).
19. The device of any one of claims 16 to 18, wherein the absorption medium comprises at least one ionic liquid. 23
20. The device of claim 19, wherein the absorption medium comprises water and ionic liquid in a weight ratio of from 10:1 to 1:1.
21. The device of claim 19 or 20, wherein the absorption medium comprises ionic liquid and amines of formula (I) in a weight ratio of from 3:1 to 1:10.
22. A method for the absorption of CO 2 from a gas mixture by contacting the gas mixture with an absorption medium, said method as claimed in claim 1 and substantially as hereinbefore described with reference to any one of the Examples but excluding any comparative Examples.
23. An absorption medium for the absorption of CO 2 from a gas mixture, said medium as claimed in claim 9 and substantially as hereinbefore described with reference to any one of the Examples but excluding any comparative Examples.
24. A device for separating CO 2 from a gas mixture, said device as claimed in claim 16 and substantially as hereinbefore described with reference to any one of the Examples but excluding any comparative Examples. Evonik Degussa GmbH Patent Attorneys for the Applicant/Nominated Person SPRUSON & FERGUSON