WO2012034921A1

WO2012034921A1 - A process for the separation and capture of co2 from gas mixtures using amines solutions in anhydrous alcohols

Info

Publication number: WO2012034921A1
Application number: PCT/EP2011/065531
Authority: WO
Inventors: Maurizio Peruzzini; Fabrizio Mani
Original assignee: Consiglio Nazionale Delle Ricerche
Priority date: 2010-09-13
Filing date: 2011-09-08
Publication date: 2012-03-22
Also published as: ITFI20100190A1; IT1401668B1

Abstract

Process for the CO₂ capture from gas mixtures and for the CO₂ removal from gaseous wastes of industrial processes or combustion gases, which is carried out by bringing into contact the gas mixtures with an absorbent solution of amines in anhydrous alcohols; this process comprises the CO₂ absorption (1) at room temperature and atmospheric pressure and the CO₂ absorption (5) and amine regeneration at temperatures lower than the boiling temperature of the solution and at atmospheric pressure.

Description

TITLE

A PROCESS FOR THE SEPARATION AND CAPTURE OF C0₂ FROM GAS

MIXTURES USING AMINES SOLUTIONS IN ANHYDROUS ALCOHOLS

DESCRIPTION

Field of the invention

The present invention relates to a process for the separation and capture of carbon dioxide (C0₂) from gaseous mixtures containing several different gases. These gaseous mixtures may be produced by a number of industrial processes, such as, but not exclusively, hydrogen, steel and cement production, combustion of fossil fuels in power plants, as well as plants for waste-to-energy conversion and biomasses combustion. The process according to the invention is particularly suitable for the separation of C0₂ from CH₄ in the production of the so called "natural gas", and for the separation of C0₂ from H₂ in "steam reforming" and "coal gasification" processes, in which H₂ is obtained from methane, carbon or biomass treatment. I n all these processes, C0₂ is an inevitable waste product that must be separated from H₂ before its possible utilization.

State of the art

The costs of the processes used nowadays for capturing C0₂ are still so high that these processes are widely used only in the production of H₂ and of natural gas, but not in the removal of C0₂ from combustion gasses in electric plants using fossil fuels. It has been estimated that from 20 to 30% of a coal plant's energy production should be required to sustain an efficient capture and sequestration of the C0₂ produced. Furthermore, the capture and sequestration of C0₂ involves high costs also from an environmental viewpoint because of the C0₂ emission for sustaining the capture process and the production of the absorbent reagents used for capturing the C0₂ itself. Furthermore, as detailed below, the degradation of the absorbents along the continuous absorption-desorption cycles, significantly contributes to increase the overall operation and maintenance costs of the system.

The net balance C0₂₍ca_Ptured) - C0_2(emitted) of the entire process cannot be in any way higher than 70% (C0₂ emitted represents the overall amount of C0₂ released in the atmosphere to produce all energy -electrical, mechanical and thermal- necessary to support the entire process, from the manufacture of the reagents to the C0₂ disposal). In other words, for one metric ton of C0₂ captured, 0.3 metric tons of extra C0₂ are emitted (A. Hachiya and S. Frimpong, Environmental Issues and Management of Waste, in Energy and Mineral Production; K.R. Singhal and A.K. Mehrotra Eds, 2000, p. 275; G. Gottlicher, VGB Power Tec, 2003, 5, 96; M. Aresta, A. Dibenedetto, Catalysis Today, 2004, 98, 455; D. Aaron, C. Tsouris, Separ. Sci. & Tech., 2005, 40, 321 ; A.B. Rao, E.S. Rubin, Ind. Eng. Chem. Res., 2006, 43, 2421 ; M.S. Jassim, G.T. Rochelle, Ind. Eng. Chem. Res. 2006, 45, 2465).

Efficient processes for C0₂ capture are those where absorbents are aqueous solutions of inorganic (alkaline carbonates, ammonia) as well as organic (amines) bases. These processes invariably comprise two distinct steps. In the first step, the gaseous mixture containing C0₂ (for example, the gas mixtures from the fossil fuel post-combustion contain, besides water, 4-15% of C0₂ as well as N₂, 0₂, S0₂ and, at a much lesser extent, nitrogen oxides and CO) is chemically absorbed by an aqueous solution of either inorganic or organic base and is stored in solution as carbonate, bicarbonate or amine carbamate. In the second step of the process, the base is thermally regenerated by heating and relatively pure C0₂ is released. At the end of the process pure C0₂ may therefore be used as raw material or properly disposed in geological cavities, oceans or elsewhere.

The traditional methods for C0₂ chemical capture use as basic absorbents aqueous sol utions of ali phatic am ines , in particular primary and secondary alkanolamines due to their high boiling temperatures and great solubility in water. The main reactions between the amine (hereafter indicated as AMH) in aqueous solutions and gaseous C0₂ are

AMH + C0₂(gas) + H₂0 → AMH₂ ⁺ + HC0₃ ^" [1]

AMH + HCO₃ ^" → AMH₂ ⁺ + C0₃ ^2~ [2]

AMH + HCO₃ ^" → AMC0₂ + H₂0 [3]

Reaction [3] does not occur with tertiary amines as well as with sterically hindered primary and secondary amines. Reaction [1] is right hand shifted and is the prevailing one, whereas the reactions [2] and [3] contribute to a lesser extent to the C0₂ capture, and the lower is the AMH/C0₂ molar ratio the less they contribute. Moreover, the stability of the amine carbamate AMC0₂ in aqueous solution also determines the relative contribution of reaction [3] to the C0₂ capture.

The most investigated amines in laboratory scale experiments and, in few instances, employed at the industrial scale, are 2-aminoethanol (monoethanolamine, MEA) and its /V-methyl derivatives 2-(methylamino)ethanol (/V-methylethanolamine, MMEA) and 2-(dimethylamino)ethanol (Λ/,Λ/'-dimethylaminoethanol, DMMEA); 2,2'- iminodiethanol (diethanolamine, DEA); A/-methyl-2,2'-iminodiethanol (N- methyldiethanolamine, MDEA); 3,3'-imino-di-2-methylpropanol (diisopropanolamine, DIPA); 2-(2-aminoethoxy)-ethanol (diethyleneglycolamine, DGA); 2-amino-1-butanol (2-aminobutanol, 2AMB); 2-amino-2-methyl-1-propanol (aminoisobutyl alcohol, AMP); 2-amino-2-ethyl-1 ,3-prop a n d i o l ( a m i n o p r o p a n d i o l , A M P D ) ; a n d 2- (isopropylamino)ethanol (N-isopropylethanolamine, IPMEA) (G. Sartori, D.W. Savage, Ind. Eng. Chem. Fundam., 1983, 22, 239; P.M. Blauwhoff, et al., Chem. Eng. Sci., 1984, 39, 207; G. Astarita, et al., Gas Treating with Chemical Solvents, John Wiley & Sons, New York, 1984; D.A. Glascock, et al., Chem. Eng. Sci., 1991 , 46, 2829; O. Erga, et al., Energy Convers. Mgmt., 1995, 36, 387; C. Mathonat, et al., Ind. Eng. Chem. Res., 1998, 37, 4136; M.K. Abu-Arabi, et al., J. Chem. Eng. Data, 2001 , 46, 1 125; J-Y. Park, et al., Fluid Phase Equii, 2002, 202, 359; D. Bonenfant, et al., Ind. Eng. Chem. Res., 2003, 42, 3179; M.S. Jassin, G. T. Rochelle, Ind. Eng. Chem. Res., 2006, 45, 2465; F. Barzagli, et al., Energy Environ. Sci., 2009, 2, 322).

With the aim of improving the efficiency of C0₂ capture, aqueous solutions of blended amines have been investigated, such as AMP-MDEA, AM P-MMEA, AMP- DEA, MEA-MMEA (B.P. Mandal, et al., Chem. Eng. Sci., 2003, 58, 4137; B.P. Mandal, S.S. Bandyopadhyay, Chem. Eng. Sci., 2005, 60, 6438; D. Bonenfant, et al., Ind. Eng. Chem. Res., 2005, 44, 3720; R. Idem, et al., Ind. Eng. Chem. Res., 2006, 45, 2414; W.-J. Choi, et al., Green Chem. 2007, 9, 594; A. Aroonwilas, A. Veawab, Energy Procedia, 2009, 1, 4315; F. Barzagli, et al., Energy Environ. Sci., 2010, 2, 772). Some amine mixtures show an absorption efficiency greater than that of the single amines under the same experimental conditions. The experiments of C0₂ absorption are carried out at temperatures ranging from 20 to 40°C and pressure values ranging between 1 and 7 bar with 20-40 wt % of amines in the presence of variable amounts of reaction promoters, antifoaming and anticorrosion agents and additives for reducing amine degradation. In any case the amines progressively degrade thus loosing their absorption ability and fresh amines must be added. Moreover, the amines are not completely inert but, especially at high temperature, they may damage the reactors' steel, so that corrosion inhibitors must be added to the absorbent solution too.

In order to regenerate the free amines for their reuse, the C0₂-loaded absorbent must be heated at relatively high temperatures (1 10-140°C and pressures up to 4 bar) to force reaction [1] to go on in the opposite sense

AMH₂ ⁺ + HC0₃ ^" → AMH + C0₂(gas) + H₂0 [4]

Alternatively, reaction [4] can be carried out at lower temperatures (<100°C) and at reduced pressure (< 1 bar).

A number of patented processes are directed to remove acid gases (H₂S, HCN, S0₂, besides C0₂) by means of aqueous solutions of amines. In most processes, the employed amines are the same, namely, MEA, MM EA, DEA, M DEA, AM P, DI PA, DGA and their blends. The main differences amongst all of these processes deal with the use of different reaction activators or corrosion inhibitors, and the different experimental conditions used (temperature, pressure, amine concentration comprised between 15 and 70% by weight). Relevant effects may also be ascribed to the technical devices used to increase the gas-to-liquid exchange. A few patents exist based on specific amines displaying peculiar features ("Flexsorb PS", Exxon, USA; "KS-1", "KS-2", "KS-3", Mitsubishi Heavy Industry e Kansai Electric Power Co., Japan). Patented processes based on aqueous amines are: MEA, "Econamine FG", Fluor Eng. & Construct. Inc. (USA); "Sulfiban", Black, Sivallis & Bryson, Inc. (USA); The Dow Chemical Co. (USA), "Amine Guard-ST", Union Carbide (USA). DEA: "SN PA- DEA", Societe Nationale Petroles D'Aquitaine (France); "Amine Guard", Union Carbide (USA). MDEA: "Ucarsol HS-1 01 ", U nion Carbide (U SA) ; "aM D EA", BASF AG (Germany); "ADIP-MDEA", Shell (The Netherland); "Sulfinol-M", Shell (The Netherland); ELF Aquitaine (France); The Dow Chemical Co. (USA). DIPA: "ADIP- DIPA", "Sulfinol-D", Shell (The Netherland); DGA: "Diglycolammine", Huntsman Corporation (USA). "Alkazid", BASF AG (Germany) and "Catacab", G.F. Versteed et al., (The Netherland) deal with aqueous solutions of alkaline salts of aminoacids.

"AMISOL" (Lurgi Kohle GmbH, Germany) for the C0₂ capture combines the physical absorption of methanol with the chemical absorption of amines by using water-methanol solutions of MEA and DEA at 5-40°C with pressure greater than 10 bar. In the thermal desorption step, the overhead vapours of methanol and amines must be condensed by washing them with water or ethylene glycol before being recovered by fractionated distillation. However, the very high costs involved make this process unsuitable for C0₂ capture from gaseous mixtures coming from combustion processes or from industrial processes, and in general for the so-called "CCS" (Carbon dioxide Capture and Storage) technology.

Further processes for the C0₂ capture are based on the physical absorption of acidic gases under high pressure conditions in organic solvents, and on the desorption at low pressure. According to these processes the acidic gas is dissolved in the organic solvent without any chemical reaction between the gas and the solvent, which contrasts with the aqueous amine solutions technology. "Selexol Solvent Process" (Allied Chemical Co., USA) "Rectisol" and "Purisol" (Lurgi Kohle GmbH, Germany) "Fluor Solvent" (Fluor Eng. & Construct. I nc. USA) are based on the physical absorption of C0₂ at high pressure and its stripping at reduced pressure. Suitable solvents are, for example, chilled methanol, propylene carbonate, 1-methyl-2- pyrrolidone.

Recently, specific organic liquids have been designed for C0₂ capture (C0₂BOLs, 0₂ binding organic liquids; RTILs, room temperature ionic liquids; R. Hart et al., Tetrahedron, 2010, 1082; D.J. Heldebrant et al., Energy Procedia, 2008, 1 187; C. L. Lotta et al., Ind. Eng. Chem. Res., 2008, 47, 539; J. E. Bara et al., Ind. Eng. Chem. Res., 2008, 47, 8496; J. H. Davis, Jr. et al., J. Am. Chem. Soc, 2002, 124, 927). These organic liquids would meet some advantages over the conventional aqueous amines, namely to be high-boiling and thermally stable and to have lower heat capacity than water. However, their production cost is much higher than that of the amines and, even more important, the high viscosity of the C0₂-loaded liquid or the formation in it of solid products represent a disadvantage and prevent their industrial application. Furthermore, most of these liquids are not compatible with water and their regeneration requires heating to 80-100°C at reduced pressure.

The stripping step of the process, namely amine regeneration and pure C0₂ release, requires relatively high temperatures (typically, 1 10-140°C and pressure higher than 1 bar) due to the endothermic nature of reaction [4] so that a huge amount of energy is required to heat the aqueous solution due to both the high heat capacity (4.18 kJ kg ¹ °C ¹) and evaporation enthalpy of water (2.44 kJ g ¹).

Further energy costs are required to operate at pressure greater than the atmospheric one.

Besides this, additional costs come from the need to add fresh amine replacing the decomposed one, as well as from the addition of several "activators". It is estimated that 70% of the overall costs of the cyclic process is due to the amine regeneration step.

Failing to achieve a significant regeneration energy cut in the removal of C0₂, any CCS technology would remain highly questionable for large scale applications.

Summary of the invention

It is an aim of the present invention to reduce as much as possible the energy costs of the CCS technology, in particular the energy requirement of the stripping step, by taking advantage from a "new chemistry" for the C0₂ capture that requires less energy compared to the conventional use of amines in aqueous solutions, by operating at a lower regeneration temperature and atmospheric pressure while still maintaining a high absorption efficiency (u p to 96%) compared to the known processes.

This aim has been achieved by changing the "chemistry" of the C0₂ capture and amine regeneration with respect to that occurring when aqueous amines are used, and, in particular, by excluding water in the liquid absorbent of C0₂, which avoids the C0₂ capture in solution as the thermally stable bicarbonate, (HC0₃ ^", see reaction [1] above).

U nder anhydrous reaction conditions, carbon dioxide reacts with alcohol solutions of 2-amino-2-methyl-1-propanol (herein after referred to as AMP), which acts as a base, to give the corresponding alkyl carbonate of the alcohol (see reaction [5]; where ROH stands for an aliphatic alcohol and AMH for AMP)

AMH + C0₂ + ROH → ROCO2^" + AMH₂ ⁺ [5]

Additionally, C0₂ could react with either primary or secondary amines forming the carbamate derivative (see reaction [6])

2AMH + C0₂ → AMH₂ ⁺ + AMC0₂ ^" [6]

Reaction [5] is responsible for most of the captured C0₂ as the sterically hindered AM P does not form a stable carbamate. However, the unstable AM P carbamate is formed as an intermediate along the molecular process leading to the alkyl carbonate (reaction [7])

AMC0₂ + ROH → ROC0₂ + AMH [7]

Reaction [7] is peculiar of AMP in the absence of water, and does not occur with stable carbamates such as those of M EA, MM EA, DEA, etc. Moreover, both AMP carbamate and alkyl carbonate are unstable species than cannot be obtained in the presence of water.

The thermal decomposition of alkyl carbonates and amine regeneration (inverse of reaction [5]) occurs at lower temperatures than those of bicarbonate (inverse of reaction [1] and of stable carbamates (inverse of reaction [6]) and therefore requires less energy. Also the decomposition of unstable AMP carbamate requires less energy than the decomposition of bicarbonate. Amines such as M EA or DEA, which form stable carbamate, even in aqueous solution, are not useful as their carbamate decompose at high temperatures. Additionally, the reaction [5] increases the loading capacity of the amine as compared to reaction [6].

By replacing the aqueous solutions of the known processes with anhydrous alcohols, the energy requirements are appreciably reduced not only for the lower decomposition-regeneration temperatures of the carbon contai ni ng species i n solution, but mainly for the lower heat capacity of the alcohols employed (2.5-2.7 kJ kg^"1) with respect to water, for the lower temperature gap between absorption and desorption steps (30-60°C) with respect to aqueous solutions (80-100°C) and for the reduced solvent evaporation due to the high boiling temperature of the absorbent compared to the desorption temperatures. Moreover, also the thermal decomposition of the amine and the corrosion of the reactor is drastically reduced because of the lower temperatures employed and the absence of water.

All of the above said objectives have been pursued in the process of C0₂ capture from industrial gas mixtures according to the present invention whose essential characteristics are:

(a) in the absorption step, a gas stream containing C0₂ (in particular between 5 and 12% v/v in air) is brought into contact at 1 bar and at temperatures between 20 and 40°C with an alcoholic solution of either a single amine or a blend of two amines (in the range 1 : 1 - 1 :3 on molar scale). Under these conditions a solution of the corresponding alkyl carbonate and, to a lesser extent, of the carbamate of the amine together with the protonated amine, is produced;

(b) in the step of desorption of C0₂ and regeneration of the amine, the solution obtained in the absorption step (a) is heated at a temperature between 50 and 80°C, and preferably between 65 and 80°C, at atmospheric pressure;

(c) both absorption and desorption steps occur at the same time in two distinct reactors set at 20-40°C (the absorber) and 50-80°C, preferably 65-80°C (the desorber), respectively, connected to each other in a closed cycle, wherein the liquid is continuously circulating. A cross heat exchanger cools the hot regenerated amine solution exiting from the desorber before being recycled to the absorber and, at the same time, preheats the carbonated amine solution exiting from the absorber at 20- 40°C, before being transferred to the desorber;

(d) if the C0₂ absorption forms a heterogeneous slurry, instead of the continuous process described in (c), the absorption and the desorption steps can be carried out in two separate runs using either two different reactors or a single reactor acting as the absorber at 20-40°C in the first step and, once the C0₂ capture is completed, as the desorber at temperatures comprised between 50 and 80°C, preferably 65-80°C. In particular, either individual amines selected from between 2- amino-2-methyl-1-propanol (AMP) and 2-(2-aminoethoxy)ethanol (DGA), or mixtures of AMP with a different amine have been employed, that are, but not limited to, AMP and, respectively, 2,2'-iminodiethanol (DEA), A/-methyl-2,2'-iminodiethanol (MDEA), 3,3'-imino-di-2-methylpropanol (DIPA) and A/-methyl-2-aminoethanol (MMEA), preferably mixtures of AMP with another of the above said amines in molar ratios second amine:AMP comprised between 1 : 1 and 1 :3. The solvents employed are either simple alcohols or polyhydroxylated aliphatic alcohols. Typical examples are 1- propanol, 2-propanol, 1-butanol, 2-butanol, mono methyl, mono ethyl and mono butyl ethers of ethylene glycol, mono methyl and mono ethyl ethers of diethylene glycol, mixtures of ethylene glycol, 1 ,2-propandiol, diethylene glycol and diethyl ether of diethylene glycol with, respectively, methanol, ethanol, and 1-propanol.

The reaction of carbon dioxide with these alcohol solutions of both the individual amines and the mixtures AMP:second amine having molar ratio between 1 : 1 and 3: 1 , the overall amine concentration being comprised between 1 .5 and 4.0 mol dm^"3, and preferably being 2.0 mol dm^"3, is fast and efficient at room temperature and pressure without adding any activator or reaction catalyst. Moreover, the amine regeneration and C0₂ desorption processes occur at relatively low temperatures (50-80 °C, preferably 65-80 °C) as C0₂ is stored in solution as alkyl carbonate and unstable AMP carbamate. Additional benefits from using anhydrous organic solvents instead of water solutions, come from their heat capacity (in the range 2.5-2.7 kJ kg^"1), much lower than that of water, and from the low evaporation enthalpy of the solvents due by both the lower desorption temperature and the higher boiling temperature of the organic solvents. All of the aforementioned features should reduce the energy requirements of the desorption step. The design of the absorber, the greater reaction rates of [5] — [7] and the efficient amine regeneration, result in high C0₂ loading capacity and absorption efficiency (up to 96%).

The main novel features of the process according to the present invention are:

1) the use of the hi ndered amine AM P i n conjunction with alcohols (mixtures of ethylene glycol with either methanol or ethanol, preferably) and i n anhydrous conditions for the C0₂ chemical capture; in these conditions C0₂ is stored in solution as alkyl carbonate and unstable amine carbamate instead of stable bicarbonate;

2) the amine regeneration and C0₂ desorpti on occu r at lower temperatures than the bicarbonate decomposition in aqueous solution, due to the lower thermal stability of the C0₂ containing species in alcohol solution; 3) both absorption and desorption processes occur at atmospheric pressure;

4) less energy is required by the regeneration-desorption process due to the less temperature difference between the absorber (20-40 °C) and the desorber (50-80 °C, preferably 65-80 °C), to the less heat capacity of the alcohols with respect to water and to the low evaporation enthalpy of the solvent which could be greatly reduced by the lower desorption temperature and by the higher boiling temperature of the organic solvents;

5) the thermal decomposition of the amines is reduced owing to the lower desorption temperatures;

6) high C0₂ absorption efficiency (up to 96%) using amine solutions with a concentration of 2.0 mol dm^"3 (corresponding to 16.8-22.6% wt);

7) high loading capacity up to 45 wt% with respect to the amine;

8) the process does not require any reaction activator because the C0₂ capture is fast at room temperature and pressure; moreover, corrosion inhibitors are not necessary, or their use can be greatly reduced , due the mild experimental conditions used.

The advantages of the process according to the present invention result in a significant energy cut of the entire process of C0₂ capture with respect to the conventional processes based on the use of the same amine in aqueous solution or in water-organic solvent solutions.

On the other hand, the increased capital cost due to the use of alcohols could be compensated, at least in part, by the lesser amount of amines employed (about 50% with respect to the aqueous solutions) that maintain unaltered their reaction capacity for a longer time. Moreover, the alcohols employed are inexpensive, thermally stable and are entirely recycled.

The C0₂ capture by AMP in 1-propanol, 2-propanol, 1-butanol, 2-butanol, mono methyl ether of diethylene glycol (DEGMME) and mono ethyl ether of ethylene glycol (EGMEE), form a solid mixture containing the carbamate, [(AMPH)(AMPC0₂)], and the carbonate, [(AMPH)₂(C0₃)], of the protonated amine that are less soluble of the alkyl carbonate, [(AMPH) (RC0₃)] (R stands for the aliphatic residue of the alcohol). The formation of a precipitate allows to further decrease the desorption temperature to 50- 70°C, that is at lower temperature than it occurs when the precipitate is absent.

Brief description of the drawings

Further characteristics and advantages of the C0₂ separation and capture process from gaseous mixtures or from industrial exhausted gasses according to the present invention, will be clear from the following description of some embodiments thereof provided by way of non-limiting examples with reference to the attached drawings wherein:

Figure 1 is a schematic illustration of the reactor for C0₂ absorption process (absorber);

Figure 2 shows a flowchart of the absorption/desorption/regeneration continuous cycle used in experimental tests of the present process.

Detailed description of the invention

According to the process of the present invention, the reaction of C0₂ capture takes place into the reactor 1 - schematically reproduced in Figure 1 - charged with the absorbent solution. A sintered glass diffuser 2 is placed at the bottom of the reactor 1 and three polyethylene disks 3 are placed at regular intervals and fully immersed i n the l iquid . I n Figure 1 the liquid level is indicated with L. In the experimental model three polyethylene disks have been used, so that the gas mixture, while flowing through the diffuser 2, is split into micro-bubbles in such a way that the liquid-gas contact surface is greatly enhanced. Moreover, the three disks 3 spaced within the solution increase the liquid turbulence therefore providing the reacting liquid-gas mixture with a sufficient residence time. The gas way out is placed at the top of the reactor. In such a device the C0₂ capture is very fast -even at room temperature and atmospheric pressure- without the need of any catalyst or reaction activators.

The absorbent solution contained into the absorber is an amine such as, but not limited to, AMP (2-amino-2-methyl-1-propanol) and DGA [2-(2-aminoethoxy)ethanol], or a mixture of two different amines such as, but not limited to, AMP mixed with DEA (2,2'-iminodiethanol), MDEA (A/-methyl-2,2'-iminodiethanol) , DI PA (3,3'-imino-di-2- methylpropanol), or MMEA (A/-methyl-2-aminoethanol), dissolved into either a single alcohol solution or a mixture of two different alcohols. I n the present absorbent solution the overall amine concentration is comprised between 1.5 and 4.0 mol dm^"3.

When an AM P/second amine mixture is used, the AMP/second amine molar ratios are comprised between 1 : 1 and 3: 1 . Referring to the alcohol mixtures, the volume ratio of the two alcohols is comprised between 1 : 1 and 1 :2. During the absorption process the absorbent temperature is kept between 20°C and 40°C, preferably at 20°C.

At room temperature and about 1 bar pressure, the absorber is fed with the gas mixture (C0₂ content in the gas flow may vary in the range 5 -12%) through the porous diffuser. In order to determine the absorption efficiency, the outlet C0₂ concentration, after the absorption step, is analyzed by a gas chromatograph 4.

The carbonated amine solution exiting from the absorber (or the heterogeneous slurry, if the C0₂ absorption forms solid compounds) is continuously transferred to the desorbing reactor 5 (Figure 2) by means of the pump 7 and connecting tubes. The thermal decomposition of the products derived from the reaction of C0₂ with the absorbent, takes place in the desorber unit 5, thus regenerating the ammine and producing pure C0₂. The desorber is heated up at temperatures between 50 and 80°C, preferably between 65 and 80°C.

A thermocouple monitors the solution temperature during the desorption run. In order to sustain the decomposition kinetic, the solution is maintained under stirring. A condenser 6 cooled by water at room temperature is placed on the top of the desorber for the condensation of the overhead vapor that is refluxed to the desorber.

The pure C0₂ produced by the desorption-regeneration process can be sequestered in standard ways or directly used. The regenerated solution produced in the desorber unit 5 is continuously transferred into the absorber unit 1 by means of the pump 8 and connecting tubes. A cross heat exchanger 9 is placed between the two reactors. The heat exchanger preheats the solution exiting from the absorber before being transferred to the desorber. At the same time, it cools down the solution exiting from the desorber before being recycled to the absorber.

Ultimately, the absorption and desorption processes take place continuously, at the same time, in a closed loop. When the C0₂ absorption produces a solid substance too, an alternative batch process can be adopted consisting in a complete absorption run and a complete desorption run occurring in the two distinct reactors 1 and 5 or, alternatively, in the same reactor 1 , which works in the first stage as an absorption reactor (at 20°C) and in the second stage, after the C0₂ absorption is completed, as a desorption reactor (at 50-70°C).

The present absorbent solution is preferably either an alcohol solution of single AMP (2-amino-2-methyl-1-propanol) or mixtures of AMP with a second amine selected from DEA (2,2'-iminodiethanol), MDEA (N-methyl-2,2'-iminodiethanol), DI PA (3,3'- imino-di-2-methylprpopanol), and MMEA (N-methyl-2-aminoethanol). A further individual amine solution of preferred use according to the invention is an alcohol solution of DGA [2-(2-aminoethoxy)ethanol].

Preferred alcohols according to the invention are selected from the group consisting of 1-propanol, 2-propanol, 1-butanol, 2-butanol, mono methyl, mono ethyl and mono butyl ethers of ethylene glycol, mono methyl and mono ethyl ethers of diethylene glycol, mixtures of ethylene glycol and 1 ,2-propandiol, of either diethylene glycol, or diethyl ether of diethylene glycol, respectively, with methanol, ethanol and 1 - propanol.

For the implementation of the model for C0₂ capture here described, the adopted experimental conditions are set to get the most efficient results when the gas mixtures contains 12% C0₂ by volume in air (gas mixture flux 12-16 dm³/h at room conditions). For each test the starting conditions are: the absorber 1 contains 0,300 dm³ of the 2.0 M individual amine solution, or of the solution of a mixture of two amines. The absorber temperature is kept constant at 20°C (by means of a thermostated bath not illustrated in the Figure 1). Pores' size of the diffuser 2 is in the range16-40 μηι. During the entire process, a thermocouple (not illustrated in Figure 1) monitors the absorbent temperature. The inlet and outlet C0₂ percentages in the flue gas m ixtu re are measured every 5 m i nutes usi ng a properly calibrated gas chromatograph (indicated as 4 in Figure 2).

With reference to Figure 2, at the beginning of each test the desorber unit 5 contains 0.300 dm³ of the 2.0 M of the partially carbonated amine-alcohol solution (C0₂ absorbed in correspondence of 50% of its maximum solubility). The reactor is kept at a constant temperature using a paraffin oil bath (not illustrated in Figure 2). For each amine experiment, the desorption runs have been performed at temperatures of 65, 70, 75, and 80°C.

A thermocouple monitors the solution temperature during the desorption process. In order to sustain the desorption kinetic, the desorber is equipped with a magnetic stirrer. A condenser cooled by water at room temperature 6 is placed on the top of the desorber 5 in order to reflux the overhead vapors to the stripper. Actually, the amount of the vapors leaving the solution is low, as a consequence of the desorption temperature which is much lower than the solution boiling point.

The liquids, the carbonated and the regenerated ones, are continuously circulating in the closed loop, by means of the pumps 7 and 8. Altogether, the fluxes are comprised between 0.500 and 0.700 dm³/h. The maximum absorption efficiency - calculated for 16 dm³ h^"1 flux with 12% C0₂ mixture - is obtained in correspondence of a constant liquid flux of 0.600 dm³ h^"1.

A cross heat exchanger 9, placed between the two reactors, preheats carbonated liquid moving from the absorber 1 (at 20°C) to the desorber 5, and at the same time, cools the regenerated liquid moving from the desorber 5 (at 65-80°C) to the absorber 1 . Each experiment - performed at a constant desorption temperature - lasts between 4 and 6 hours and it is stopped when the absorption efficiency value remains constant; it means that the amount of C0₂ absorbed is equal to the amount of C0₂ desorbed, that means that the amount of absorbent solution used in the absorption step is equal to the amount of absorbent solution regenerated in the desorption step.

In the Table 1 below the following representative results are reported: the C0₂ absorption efficiency (percentage ratio of absorbed C0₂ compared to the C0₂ flowing in the absorbent) calculated for the different absorbent solutions and for each different desorption temperature; and the maximum absorption capacity of the amine alcohol solutions (the ratio between absorbed C0₂ and amount of amine, calculated as both weight % and in molar scale or "loading"). The starting concentration of the single amine used, or of the mixture of amines, is 2.0 M, and the weight percentage of the amine solutions is between 16.8 and 22.6.

Table 1

3 Molar ratio between absorbed C0₂ and starting amine; absorption carried out with pure C0₂.

b Percentage ratio between the masses of absorbed C0₂ and starting amine.

c Percentage ratio between absorbed and flowed C0₂; absorption carried out with 12% (v:v) C0₂ in air

d glycol/propanol = 1/2 (v/v)

⁶ ethylene glycol monomethyl ether

f a solid phase formed during absorption

⁹ 1 ,2-propandiol

h AMP/DEA = 2/1 on molar basis

1 ethylene glycol monoethyl ether

The C0₂ absorption efficiency (average value) of the solutions of the single amine, and of their mixtures, increases as a consequence of the increased desorption temperature, and, at a given temperature, it decreases as the solvent viscosity increases. For this reason, it is preferable not to use pure ethylene glycol, but to use it diluted with methyl, ethyl or n-propyl alcohol to provide a lower viscosity of the solution. Methanol based solutions are the most efficient, for a given amine and at a given regeneration temperature.

The C0₂ absorption process of AM P dissolved in 1-propanol, 2-propanol, 1- butanol, 2-butanol, mono methyl, and mono ethyl ether of ethylene glycol and mono methyl, mono ethyl and diethyl ether of diethylene glycol, produces solid mixtures containing carbamate [(AMPH₂)(AMPC0₂)] and carbonate [(AMPH₂)₂(C0₃)] of the protonated AMP. When using these mixtures, in alternative to the continuous process already described for the solutions (Table 1 , AMP in DEGMME), a batch process was used that comprises the absorption step and the successive desorption step in the reactors 1 and, respectively, 5, or in the same reactor 1 : in this last example the reactor 1 is used for the absorption step at 20°C, and subsequently for the desorption step at the desired temperature. The formation of heterogeneous slurries enhances the desorbing process that can be carried out at 50-70°C.

The desorption efficiency (percentage ratio between desorbed and absorbed C0₂) which is equal to the amine regeneration efficiency, was measured in function of desorption temperatures. The desorption process starts at 50°C and its rate increases as a function of the temperature: in most cases a desorption-regeneration efficiency greater than 90% was obtained at temperatures below than 70°C.

Some representative results are reported in the following Table 2.

Table 2

³ Molar ratio between absorbed C0₂ and starting amine; absorption of pure C0₂ ^b Percent ratio between the masses of absorbed C0₂ and starting amine

c Percent ratio between desorbed and absorbed C0₂

d ethylene glycol monoethyl ether

6 diethylene glycol monomethyl ether

The following examples illustrate some different embodiments of the process according to the present invention.

EXAMPLE 1 A solution of AMP 2.0 mol dm^"3 (18.5% by weight) is prepared by dissolving 107 g of AMP in a 1 : 1 mixture (volume by volume) of ethylene glycol and ethanol to the overall volume of 0.600 dm³. The absorber (Figure 1) is charged with 0.300 dm³ of the so obtained solution, and the temperature of the device is kept constant to 20°C by means of a thermostatic bath.

The remaining part of the solution, 0.300 dm³, is pre-carbonated with pure C0₂ up to 50% of its maximum solubility and afterwards it is transferred into the desorber unit. Said reactor, equi pped with a mag netic sti rrer and with a water-cooled condenser, is placed into a thermostatic oil-bath. The temperature in the desorber unit is brought to 65°C. The mixture of C0₂ and air (12% of C0₂ in volume) is continuously fed into the absorber through a diffuser at the bottom of the absorbent solution. This absorber setup allows to reduce the size of the C0₂ bubbles therefore maximizing the exchange surface between the two reacting phases. The mixture C0₂/air has an average flow of 16 dm³ h^"1 , approximately. Two peristaltic pumps allow the liquid mixtures to circulate continuously in a closed loop between the absorber and the desorber units through a cross heat exchanger.

In such an apparatus, the absorbed liquid, enriched in C0₂, is moved from the absorber to the desorber and, at the same time, the regenerated liquid is moved from the desorber to the absorber through the heat exchanger. The flow of these two liquids is kept constant at the value of 0.600 dm³ h^"1. The process was stopped when the efficiency of C0₂ absorption remains constant and the equilibrium is reached. The process is repeated with desorption temperature gradually brought to 70, 75 and 80°C. The maximum loading capacity and the absorption efficiency at the above said four values of temperature of desorption-regeneration runs, are shown in the Table 1 above reported.

The ¹³C-NMR analysis of the reaction mixtures allows to verify that most of C0₂ is stored in solution as the mono carbonate derivative of ethylene glycol (HOCH₂CH₂OC0₂ ^") and ethyl carbonate (CH₃CH₂OC0₂ ^").

EXAMPLE 2

A solution of AMP 2.0 mol dm^"3 (19.2% by weight) is prepared by dissolving 107 g of AMP in a 1 :2 mixture (v/v) of ethylene glycol and 1-propanol to the overall volume of 0.600 dm³. The absorber unit (Figure 1) is charged with part of said solution (0.300 dm³) and the temperature of the device is kept constant to 20°C by means of a thermostatic bath.

The remaining part of the solution, 0.300 dm³, is pre-carbonated with pure C0₂ up to 50% of its maximum solubility and afterwards it is transferred into the desorber unit. The absorption-desorption process is carried out as described above in Example 1 . The ¹³C-NMR analysis of reaction mixtures shows that most of C0₂ is stored in solution as mono carbonate derivative of ethylene glycol (HOCH₂CH₂OC0₂ ^").

EXAMPLE 3

A solution of AMP 2,0 mol dm^"3 (17.4% by weight) is prepared by dissolving 107 g of AMP in the monomethyl ether of diethylene glycol to the overall volume of 0.600 dm³. The absorber unit (Figure 1) is charged with part of said solution (0.300 dm³) and the temperature of the device is kept constant to 20°C by means of a thermostatic bath.

The remaining part of the solution, 0.300 dm³, is pre-carbonated with pure C0₂ up to 50% of its maximum solubility and afterwards it is transferred into the desorber unit. The absorption-desorption process is carried out as described above in Example 1 but using three different temperature values: 65 , 70 a n d 75°C. During the absorption step, a precipitated is formed, that contains a mixture of carbamate [(AMPH₂ ⁺)(AMPC0₂ ^")] and carbonate [(AMPH₂ ⁺)₂(C0₃ ^2")] of protonated AMP that are decomposed in the desorption step. The results are reported in Table 1.

EXAMPLE 4

A solution of AMP and DEA in the 1 : 1 molar ratio and overall amine concentration 2.0 mol dm^"3 (19.3% by weight) is prepared by dissolving 53.5 g of AMP and 63.1 g of DEA in a 1 : 1 (v/v) mixture of ethylene glycol and methanol to the overall volume of 0.600 dm³. The absorber unit (Figure 1) is charged with part of said solution (0.300 dm³) and the temperature of the device is kept constant to 20°C by means of a thermostatic bath.

The remaining part of the solution, 0.300 dm³, is pre-carbonated with pure C0₂ up to 50% of its maximum solubility and afterwards it is transferred into the desorber unit. The absorption-desorption process is carried out as described above in Example 1 at three different temperature values: 65, 70 and 75°C. The results are reported in Table 1. The ¹³C-NMR analysis of the reaction mixtures shows that C0₂ is stored in solution as DEA carbamate (DEAC0₂ ^"), mono carbonate of ethylene glycol (HOCH2CH2OCO2^"), and methyl carbonate (CH₃OC0₂ ^").

EXAMPLE 5

A solution of AMP and MDEA in 1 : 1 molar ratio and overall amine concentration 2,0 mol dm^"3 (20.7% by weight) is prepared by dissolving 53.5 g of AMP and 71.5 g of MDEA in a 1 : 1 mixture (v/v) of ethylene glycol and methanol to the overall volume of

0.600 dm³. The absorber unit (Figure 1 ) is charged with part of said solution (0.300 dm³) and the temperature of the device is kept constant to 20° C by means of a thermostatic bath.

The remaining part of the solution, 0.300 dm³, is pre-carbonated with pure C0₂ up to 50% of its maximum solubility and afterwards it is transferred into the desorber unit. The absorption-desorption process is carried out as described in Example 1 at three different temperature values: 70, 75 and 80°C. The results are shown in Table

1. The ¹³C-NMR analysis of the reaction mixtures shows that the most of C0₂ is stored in solution as mono carbonate of ethylene glycol (HOCH₂CH₂OCO₂ ^") and methyl carbonate (CH₃OC0₂ ^").

EXAMPLE 6

A solution of AMP and MMEA in 1 : 1 molar ratio and overall amine concentration

2,0 mol dm^"3 (16.9 % by weight) is prepared by dissolving 53.5 g of AMP and 45.1 g of MMEA in a 1 : 1 (v/v) mixture of ethylene glycol and methanol to the overall volume of

0.600 dm³. The absorber unit (Figure 1 ) is charged with part of said solution (0.300 dm³) and the temperature of the device is kept constant to 20° C by means of a thermostatic bath. The remaining part of the solution, 0.300 dm³, is pre-carbonated with pure C0₂ up to 50% of its maximum solubility, then transferred into the desorber unit. The absorption-desorption process is carried out as described above in Example

1. The results are reported in Table 1. The ¹³C-NMR analysis of the reaction mixtures allows to verify that the most of C0₂ is stored in solution as carbamate of MM EA (MMEACO2^").

EXAMPLE 7 A solution of DGA 2,0 mol dm^"3 (22.0 % by weight) is prepared by dissolving 126 g of DGA in the monoethyl ether of ethylene glycol to the overall volume of 0.600 dm³. The absorber unit (Figure 1) is charged with part of said solution (0.300 dm³) and the temperature of the device is kept constant to 20°C by means of a thermostatic bath. The remaining part of the solution, 0.300dm³, is pre-carbonated with pure C0₂ up to 50% of its maximum solubility and afterwards it is transferred into the desorber unit. The absorption-desorption process is carried out as described above in Example 1. The results are reported in Table 1 . The ¹³C-NMR analysis of the reaction mixtures allows to verify that most of C0₂ is stored in solution as carbamate of DGA (DGAC0₂ ^" ).

EXAMPLE 8

A solution of AMP 2.0 mol dm^"3 is prepared by dissolving 53.3 g of AMP in 1- propanol to the overall volume of 0.300 dm³. The so obtained solution is transferred into the absorber unit where the temperature is kept constant to 20°C and the absorption is carried out with pure C0₂ to its maximum solubility. A precipitate is obtained, that contains a mixture of carbamate [(AMPH₂ ⁺)(AMPOC0₂ ^")] and carbonate [(AMPH₂ ⁺)₂(C0₃ ^2")] of protonated amine. The reactors containing the heterogeneous mixture are placed into a heating bath and the temperature is gradually increased from 50 to 65°C. The reactor is connected to a gastight apparatus for the measure of the gas volume equipped with a pressure equalizing system at room temperature. The maximum loading capacity, measured on four absorption-desorption cycles (average loading), was 0.72 mol of C0₂ for 1 mol of the amine, corresponding to 35.6% by weight. The C0₂ desorption efficiency and the regeneration efficiency of the amine was 94%.

Claims

1. A process for the separation of C0₂ from gas mixtures and for the C0₂ removal from gaseous wastes of industrial processes or combustion gases, said process comprising a C0₂ absorption step, wherein a C0₂-containing gaseous stream is brought in contact with an absorbent solution of at least an amine, with a prevailing formation of a monoalkyi carbonate and, in a substantially lower amount, of the amine carbamate, and a C0₂ desorption step, wherein said carbonate and/or carbamate is thermally decomposed at temperatures lower than the boiling temperature of said absorbent solution, to yield pure C0₂ and the regenerated amine, said absorbent solution being a solution of at least an amine in an anhydrous alcohol or in a mixture of anhydrous alcohols.

2. The process according to claim 1 , wherein said absorbent solution is a solution of 2-amino-2-methyl-1-propanol (AMP), alone or in mixture with a second amine selected from the group consisting of 3,3'-imino-di-2-methylpropanol (DIPA), 2,2'- iminodiethanol (DEA), A/-methyl-2,2'-iminodiethanol (MDEA), and /V-methyl-2- aminoethanol (MMEA).

3. The process according to claim 2, wherein said solution of AMP in mixture with a second amine has a molar ratio AM P:second amine comprised between 3: 1 and 1 : 1.

4. The process according to claim 3, wherein said molar ratio is 1 : 1.

5. The process according to claim 1 , wherein said absorbent solution is a solution of 2(2-aminoethoxy)-ethanol (DGA).

6. The process according to claim 1 , wherein said absorbent solution has an overall amine concentration comprised between 1.5 and 4.0 mol dm^"3, preferably of 2.0 mol dm^"3.

7. The process according to claim 1 , wherein said alcohol is selected from the group consisting of 1-propanol, 2-propanol, 1-butanol, 2-butanol, mono methyl, mono ethyl and mono butyl ethers of ethylene glycol, mono methyl and mono ethyl ethers of diethylene glycol, and mixtures of ethylene glycol, 1 ,2-propandiol, diethylene glycol and diethyl ether of diethylene glycol with, respectively, methanol, ethanol, and 1- propanol.

8. The process according to claim 2, wherein said absorbent solution is a solution of AM P in 1-propanol, 2-propanol, 1-butanol, 2-butanol, mono methyl ether of diethylene glycol, and mono ethyl ether of ethylene glycol.

9. The process according to claim 5, wherein said absorbent solution is a solution of DGA in the mono ethyl ether of ethylene glycol.

10. The process according to claim 1 , wherein said C0₂ absorption step is carried out at temperature in the range 20-40°C and at atmospheric pressure.

1 1. The process according to claim 1 , wherein said C0₂ absorption step is carried out at temperature of 20°C and at atmospheric pressure.

12. The process according to claim 1 , wherein said C0₂ desorption step is carried out at temperature comprised between 50 and 80°C and at atmospheric pressure.

13. The process according to claim 1 , wherein said C0₂ containing gaseous stream is brought in contact with said absorbent solution in the shape of micro- bubbles and it is forced to flow through said solution along a winding path.

14. The process according to claim 1 , wherein the C0₂ concentration in the gaseous stream fed to said absorption step in comprised between 5 and 12% by volume.

15. The process according to any one of the previous claims, wherein the absorption and the desorption steps are carried out at the same time in two separate reactors kept at the relevant operating temperatures, following a continuous process occurring in a closed loop.

16. The process according to clai m 8, wherein the absorption step and the desorption step are carried out separately and one after the other, either in two distinct reactors or in the same reactors in two successive steps, following a batch process.