*2.6.1.1. Aminosilicones*

The basic of the minimum energy requirement for CO2

**Table 5.** Summary of the best performance IL applied as solvents for CO2

1-butylpyridinium tetrafluoroborate [Bpy] [BF4

150 Carbon Dioxide Chemistry, Capture and Oil Recovery

(Trifluoromethyl sulfonyl)imide-based IL [Tf2

Tryhexyl(tetradecyl)-phosphonium

1-butyl-3-methyl-imidazolium hexafluorophosphate

bis(trifluoromethylsulfonyl)imide

Viscosity measurements below 300 K.

imidazole

Allyl-pyridinium

\*\*Pressure over 1 MPa. \*\*\*Ambient pressure.

\*

for CO2

structure of IL has on the CO2

cosity during the CO2

ing IL and its use as a CO2

make CO2

free volume available, the higher CO2

separation performance in terms of CO2

interaction between both species. In general, some studies indicated that CO2

**IL Abbreviation Field Ref. μ\***

[bmim][PF6

[Apy] [Tf2

1-butyl-3-methyl-imidazolium acetate [bmim][Ac] Biogas/natural gas

the length chain, the presence of species such as F− increase the CO<sup>2</sup>

mixed matrix membranes have shown a high potential as a CO2

**2.6. New generation solvents for carbon dioxide capture**

Most recent applications of IL involve the use of membranes for CO2

strongly depend on van der Waals forces in case small and symmetric molecular structures are provided, whereas electrostatic interactions dominates as large and asymmetric molecular structures are used. Besides the acid-base interaction also plays a key role as a mechanism


N] Biogas/natural gas upgrading

upgrading

solubility of the IL [30].

on this field demonstrate the combination of IL with membrane significantly reduces the vis-

membranes (SILM), the poly(ionic liquid)-ionic liquid composite membranes, the combination of facilitated transport membranes with IL and the incorporation of task-specific IL into

on the literature, two main mechanisms are identified for IL-based membranes, namely solution-diffusion and facilitated transport mechanism [30]. The new pathway discovered regard-

New generation solvents proposed are focused on energetic consumption reduction in order to

scale. It is well-known that most of the energy consumption takes place in the regeneration step

separation approach requires further investigation.

chemical absorption a cost-competitive technology to be deployment at CCS industrial

solubility. The amount of free space provided by means, that is,

separation.

permeability and selectivity [30]. The supported IL

absorption process and also ensures further improvements of the gas

release from ILs consist of the weak


 **(cP) CO2**

[39] — 0.66–0.84

[39] — —

] Post-combustion [34] 150 <0.05\*\*\*

] Post-combustion [38] — 0.75

N] Pre-combustion [40] 17.7–28 —

[P66614] Post-combustion [35] 223–1077 0.3–0.91

**\*\* load.**

capacity of IL. The higher

separation approach. Based

separation. Research

Aminosilicones are one of the most relevant solvents currently under investigation. Besides the absence of water in its formulation, the hybrid nature of this type of solvents (physisorbing and chemisorbing) provides a potential improvement in CO2 capture due to the possibilities that its chemical nature offers.

Perry et al. developed GAP-0 and GAP-1 aminosilicones formulated as a CO2 -philic siloxane backbone and a CO2 reactive amino group (**Figure 4**) [42, 43]. The absorption capacity of these compounds is higher than the theoretical of the selected amino group due to the physisorption phenomenon that occurs in this type of blends. However, the possibility of solid formation and the increase of viscosity during the absorption process make necessary to use cosolvents in order to avoid the above-mentioned issues.

### *2.6.1.2. Non-aqueous organic blends*

This type of solvents has been studied by some research groups including, for example, Kim et al. In this work, sterically hindered amines 2-[(1,1-dimethylethyl)amino]ethanol (TBAE) and 1-[(1,1-dimethylethyl)amino]-2-propanol (TBAP) were tested using organic compounds as solvents such as methanol and ethylene glycol [44–47]. The efficiency of this type of solvents is also revealed by Mani et al. In this work, AMP mixed with different alkanolamines (DEA, MDEA, MMEA and DIPA) and using organic solvents were analyzed [48, 49]. The tests concluded that, among other considerations, the absorption efficiency at equilibrium ranged 73–96% (**Table 6**).

**Figure 4.** GAP-0 (on the left) and GAP-1 aminosilicones (on the right). Note that gray balls represent C atoms; white balls represent H atoms; red/dark gray balls represent O atoms; dark blue/black balls represent N atoms; black small balls represent Si atoms.


**Table 6.** CO2 loading capacity at 20°C and absorption efficiency by different amines at increasing desorption temperatures. The overall amine concentration is 2.0 mol dm−3.

#### *2.6.1.3. Amines with superbase promoters*

Amines with superbase promoters might allow an increase in the CO2 capture efficiency. This type of solvents combines a primary amine and a strong non-nucleophilic base which enhances the proton transfer from the primary amino group, facilitating the carbamate formation (**Figure 5**). CO2 capture efficiency and the kinetic behavior of a primary amine using a superbase promoter could be increased over 30%. In addition, several solvents are able to work even at high temperatures (over 50°C), which make them useful in high temperature process. Nevertheless, these blends present similar issues than aminosilicones. The possibility of solid formation and the increase of viscosity during the absorption process make necessary to use organic cosolvents such as dimethylsulfoxide (DMSO), especially with nucleophilic polyamines [50, 51].

This argument is based on the optimization of the solvent volume treated in regeneration step,

**Type Absorbent Absorption Desorption Stripping Temp. (°C)**

Primary solvent DMCA 72 101 71 88 40–70

Activator A1 130 124 86 101 40–80

Blend 3:1 DMCA + DPA 89 90 80 86 40–75

**Rich loading (g/L)**

MDEA 30 59 30 43 40–70

DPA 127 88 65 78 40–80 MEA (30 wt%) 127 122 28 47 40–80

DMCA + A1 94 117 105 112 40–75 MDEA + MEA 47 62 26 48 40–75

**Cyclic loading I (g/L)**

**Cyclic loading II (g/L)**

Solvents for Carbon Dioxide Capture http://dx.doi.org/10.5772/intechopen.71443 153

**Absorption rate (g/(Lh))**

In recent years, polyamine compounds and blends have been studied in order to improve the

be recycled back to the absorption process without energy consumption. The precipitate formed

new solvent represents an alternative to the usual polyamine-water solvents although the high

Recent studies showed that some types of blended amines have the property of forming two

provides the possibility of perform selective regeneration process, being only the rich amine treated inside the regeneration reboiler. 3-(methylamino)propylamine (MAPA) and 2-(diethylamino)ethanol (DEEA) blend was studied by Bruder and Svendsen showing a promising

vapor pressure of ethanol must be considered in order minimize evaporation losses.

 absorption capacity in CCS technologies. A higher amount of amine functional groups, using water as a dissolvent, provides the polyamines higher absorption rates, but in spite of that fact, regeneration penalties and solvent circulation costs due to the high viscosity of this kind of compounds made unfeasible its application in pilot plants. Triethylenetetramine (TETA) using ethanol as dissolvent was tested by Zheng et al. [53]. In this work, solid generation occurs after

reaction with TETA. Solid phase generated and separated, containing a total of 81.8% of

absorption can be regenerated heating to 90°C and returning to liquid phase TETA. This

, allows a lower cost regeneration process due to the fact that liquid phase can

capture process compared with habitual solvents currently used for this

in capture process. This capacity of the solvent,


**Table 7.** Main properties of selected amines in 3 M aqueous solutions. Adapted from ref. [52].

stripping only the CO2

CO2

the CO2

after CO2

the captured CO2

behavior in the CO2

*2.6.2.2. Phase change amine blends*

different liquid phases after reaction with CO<sup>2</sup>

proposal as, for example, 30 wt.% MEA [54].

*2.6.2.1. TETA/ethanol blends*

#### *2.6.2. Biphasic solvents*

In the last decades, it has been assumed that biphasic mixes generation during CO2 aminebased capture processes becomes an operation issue in terms of liquid circulation and homogeneity of the solvents, especially in the regeneration step. However, recent studies support the new idea that a decrease in the energy requirements using biphasic solvents would be possible.

**Figure 5.** Reaction of CO2 -primary amines in the presence of a strong non-nucleophilic base. Note that gray balls represent C atoms; black big balls represent dimethyl groups; white balls represent H atoms; red/dark gray balls represent O atoms; dark blue/black balls represent N atoms.


**Table 7.** Main properties of selected amines in 3 M aqueous solutions. Adapted from ref. [52].

This argument is based on the optimization of the solvent volume treated in regeneration step, stripping only the CO2 -rich phase [52].

### *2.6.2.1. TETA/ethanol blends*

**Entry Amine Solvent Amine conc.** 

152 Carbon Dioxide Chemistry, Capture and Oil Recovery

Adapted from Ref. [49].

mation (**Figure 5**). CO2

polyamines [50, 51].

*2.6.2. Biphasic solvents*

**Figure 5.** Reaction of CO2

represent O atoms; dark blue/black balls represent N atoms.

The overall amine concentration is 2.0 mol dm−3.

*2.6.1.3. Amines with superbase promoters*

**Table 6.** CO2

**(wt%)**

 AMP/DEA DEGMME 18.3 31.7 73.1 91.6 AMP/MDEA EG/methanol 20.7 28.7 — 93.5 AMP/MMEA EG/methanol 16.9 43.4 76.7 95.9 AMP/MMEA EG/ethanol 16.8 40.7 — 92.6 AMP/DIPA EG/ethanol 22.6 27.3 — 93.1

Amines with superbase promoters might allow an increase in the CO2

**Loading capacity** 

**Average absorpt. efficiency and** 

capture efficiency.

amine-

**desorpt. temp (°C) 65 80**

**(wt%)**

loading capacity at 20°C and absorption efficiency by different amines at increasing desorption temperatures.

capture efficiency and the kinetic behavior of a primary amine using


This type of solvents combines a primary amine and a strong non-nucleophilic base which enhances the proton transfer from the primary amino group, facilitating the carbamate for-

a superbase promoter could be increased over 30%. In addition, several solvents are able to work even at high temperatures (over 50°C), which make them useful in high temperature process. Nevertheless, these blends present similar issues than aminosilicones. The possibility of solid formation and the increase of viscosity during the absorption process make necessary to use organic cosolvents such as dimethylsulfoxide (DMSO), especially with nucleophilic

In the last decades, it has been assumed that biphasic mixes generation during CO2

based capture processes becomes an operation issue in terms of liquid circulation and homogeneity of the solvents, especially in the regeneration step. However, recent studies support the new idea that a decrease in the energy requirements using biphasic solvents would be possible.

represent C atoms; black big balls represent dimethyl groups; white balls represent H atoms; red/dark gray balls

In recent years, polyamine compounds and blends have been studied in order to improve the CO2 absorption capacity in CCS technologies. A higher amount of amine functional groups, using water as a dissolvent, provides the polyamines higher absorption rates, but in spite of that fact, regeneration penalties and solvent circulation costs due to the high viscosity of this kind of compounds made unfeasible its application in pilot plants. Triethylenetetramine (TETA) using ethanol as dissolvent was tested by Zheng et al. [53]. In this work, solid generation occurs after the CO2 reaction with TETA. Solid phase generated and separated, containing a total of 81.8% of the captured CO2 , allows a lower cost regeneration process due to the fact that liquid phase can be recycled back to the absorption process without energy consumption. The precipitate formed after CO2 absorption can be regenerated heating to 90°C and returning to liquid phase TETA. This new solvent represents an alternative to the usual polyamine-water solvents although the high vapor pressure of ethanol must be considered in order minimize evaporation losses.

#### *2.6.2.2. Phase change amine blends*

Recent studies showed that some types of blended amines have the property of forming two different liquid phases after reaction with CO<sup>2</sup> in capture process. This capacity of the solvent, provides the possibility of perform selective regeneration process, being only the rich amine treated inside the regeneration reboiler. 3-(methylamino)propylamine (MAPA) and 2-(diethylamino)ethanol (DEEA) blend was studied by Bruder and Svendsen showing a promising behavior in the CO2 capture process compared with habitual solvents currently used for this proposal as, for example, 30 wt.% MEA [54].
