**2.2. Sterically hindered amine solvents**

focused on the development of attractive solvents to achieve high absorption/desorption capacities, energy-efficient performance and oxidative and thermal stability. Furthermore, DGA presents similar properties to MEA in many aspects, except that its low vapor pressure allows its use at higher concentrations, typically between 40 and 60%wt. in aqueous solution. Secondary alkanolamines such as diethanolamine (DEA) and diisopropanolamine (DIPA), which have a hydrogen atom directly bonded to the nitrogen, shows intermediate properties compared to primary amines and they are considered as an alternative to MEA. DEA is more resistant to degrade and shows lower corrosion strength than MEA, whereas DIPA has lower

**Abbr. Name Industrial process Structural formulae Chemical structure CO2**

H4 )-O-

CHOHCH2

)2

OH 0.5

NH 0.7–1

OH)2 0.1–0.3

NH 0.43–0.22

NH2 -CH2 -CH2

(C2 H4 -NH2 )

(CH2 CH2 OH)2

(CH3

CH3 N(C2 H4

Finally, tertiary amines such as triethanolamine (TEA) or methyldiethanolamine (MDEA), that are characterized by having a high equivalent weight, which causes a low absorption

There are three main differences in the performance of primary and secondary amines as they

stable carbamates formation along the absorption process. On the other hand, tertiary amines can only form a bicarbonate ion and protonated amine by the base-catalyzed hydration of CO2 due to their lack of the necessary N─H bond [9, 10]. Hydration is slower than the direct reaction

Therefore, these amines showed limited thermodynamic capacity to absorb CO2

separation process. Primary and secondary amines

by Zwitterion mechanism.

absorption rates [9].

due to the

 **loading**

0.25–0.35

energy requirement for solvent regeneration than MEA.

are very reactive; they form carbamate by direct reaction with CO<sup>2</sup>

by carbamate formation and, hence, tertiary amines show low CO2

capacity, low reactivity and high stability.

MEA Mono

DGA Diglycol amine

DEA Diethanol amine

DIPA Diiso

MDEA Methyl

ethanol amine

144 Carbon Dioxide Chemistry, Capture and Oil Recovery

propanol amine

diethanol amine

Natural and syngas purification

Natural gas containing high concentrations of COS and CS2

ADIP, Sulfinol: refinery gas treatment

Solvents URCASOL, gas washing in Clauss

plants

Gray, C atom; white, H atom; red, O atom; dark blue, N atom.

**Table 1.** Most commonly amines used in acid gas treatment [4].

Syngas treatment (HO-C2

are compared to tertiary amines for the CO2

Sterically hindered amines are considered a type of amines which can improve CO2 absorption rates in comparison with the common primary and second amines, usually amino alcohols. A sterically hindered amine is formed by a primary or secondary amine in which the amino group is attached to a tertiary carbon atom in the first case or a secondary or tertiary carbon atom in the second (**Figure 2**).

These amines are characterized by forming carbamates of intermediate-to-low stability, introducing a bulky substituent adjacent to the amino group to lower the stability of the carbamate formed by CO2 -amine reaction. This weaker bond leads to high free-amine concentration in solution, so the energy consumption to release CO2 is lower that the common primary and second amines. According to Nicole Hüser et al. [11], a decrease up to 15% can be achieved using hindered amines.

The general reaction scheme of the CO2 -primary or secondary amine (AmH) and the CO2 sterically hindered amine(AmCOO<sup>−</sup> ) is shown in **Figure 3**. Regarding the primary or secondary reaction scheme, the symbol B represents a base that should be another amine molecule that requires to form the carbamate anion. In this case, two amine molecules are needed to absorb one CO2 molecule, as it is extracted from the overall reaction.

The system CO2 -sterically hindered amine requires only one amine molecule to capture one molecule of CO2 . Based on this assumption, the maximum CO2 loading using sterically hindered amines is higher than for unhindered, primary or secondary amines.

**Figure 2.** Molecular structure of primary amines on the left (MEA) and a sterically hindered amine on the right (AMP). 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 [11].


sodium carbonate requires the use of promotors such as primary amines to enhance its CO2 absorption rates [15–18]. The advantages and disadvantages to use sodium carbonate as an

separation process are shown in **Table 3**.

As it was indicated in previous section, the high energy penalty related to amines regeneration and solvent degradation are the most significant issues hindering a large deployment of this technology. Solvent regeneration is a high-intensive energy process. Moreover, the stripper

However, in view of taking advantage these main benefits, except its low reactivity, the addition of a small amount tertiary amines (MDEA, TEA) in primary or secondary amines aqueous solutions (MEA, DEA) to form a solvent blend enhances the overall behavior of the solvent in terms of lower energy requirements for solvent regeneration and higher resistance to solvent degradation [20, 21]. For this reason, different researchers are studying novel solvent formulations and blends, involving fast kinetic solvents such as MEA with other slow kinetic solvents like TEA, 2-amino-2-methyl-1-propanol (AMP), benzylamine

capture. The first amine was combined with faster kinetic amines was N-methyldiethanolamine (MDEA). Amines such a methanolamine (MEA), diethanolamine (DEA) and piperazine (PZ) have used as promoters for MDEA blends. It is also possible to increase the reaction rate of fast solvents by combining them with an even faster solvent. For example, MEA is a fast

nificantly improved by adding small amounts of PZ as a promoter [24]. This blend improved

a promising solvent [25], along with the PZ and 2-amino-2-methyl-1-propanol (AMP) blends

• Multi-pollutant capture system • Slow absorption rate. The solvent should be promoted

[26]. A summary of the most promising amines blends are given below (**Table 4**).

• Use of a non-hazardous and non-volatile solvent • Solid and slurry management

**Table 3.** Advantages and disadvantages of CCS based on chemical absorption using Na2

absorption rate. Potassium carbonate promoted with PZ is also considered

with increasing rate additiveti

• High pollutant removal

capture with amine-based solvent or other class of amine, like a sterically hindered

) is other non-amine-based solvents that can be used as a pro-

consume a high amount of energy.

Solvents for Carbon Dioxide Capture http://dx.doi.org/10.5772/intechopen.71443 147

chemical absorption applied to carbon

absorption rate of MEA can be sig-

CO3 [14].

chemical absorption to

absorption solvent in a CO2

The potassium carbonate (K2

(BZA) and MDEA [22, 23].

the individual CO2

compounds

moter the CO2

**2.4. Solvent blends**

amine [12].

CO3

operating conditions and the solvent used to absorb CO2

A huge number of solvent have been proposed for CO2

solvent but it is almost 50 times slower than PZ. The CO<sup>2</sup>

**Advantages Disadvantage**

• Lower fouling and corrosion issues than amine

In this sense, amine blends could offer potential improvements in CO<sup>2</sup>

reduce the regular reboiler duty and the common solvent circulation rates [14, 19].

**Figure 3.** General reaction scheme of the CO2 -amines system [11].

The use of this type of amines leads to reduce the energy requirement for the amine-based solvent regeneration up to 20% compared to conventional MEA-based scrubbing, due to the formation weak bonds [11]. Several sterically hindered amines are shown in **Table 2**.

### **2.3. Non-amine-based solvents**

Non-amine-based solvents are called to those chemical solvents which do not integrate an amine group in their structure molecular. The most relevant solvent proposed as an alternative to the conventional amine-based solvents is the sodium carbonate (Na2 CO3 ). About 30% p/p sodium carbonate slurry is used to provide a basic environment in which CO2 is absorbed as bicarbonate followed by sodium bicarbonate formation [13]. The NaHCO3 precipitation enhances the bicarbonate formation and, hence, the CO2 capture capacity of the solvent is improved.

Sodium carbonate has shown a high performance in CO2 separation from flue gas in comparison with the MEA benchmark. It produces a high CO2 loading capacity (0.73 mole CO2 / mole CO3 2 ˉ) and a reboiler duty of 3.2 MJ/kg CO2 rather than 0.5 mole CO<sup>2</sup> /mole MEA and 3.5– 4.2 MJ/kg CO2 in case MEA is used as a solvent. Furthermore, this chemical solvent can absorb CO2 in presence of pollutants such as SO2 which can enable the cyclic capacity of amine-based solvents for CO2 absorption [13, 14]. Despite those advantages, sodium carbonate can absorb CO2 at low absorption rates, which lead to higher absorption column height. It assumes that


Gray, C atom; white, H atom; red, O atom; dark blue, N atom.

**Table 2.** Examples of sterically hindered amines [12].

sodium carbonate requires the use of promotors such as primary amines to enhance its CO2 absorption rates [15–18]. The advantages and disadvantages to use sodium carbonate as an absorption solvent in a CO2 separation process are shown in **Table 3**.

The potassium carbonate (K2 CO3 ) is other non-amine-based solvents that can be used as a promoter the CO2 capture with amine-based solvent or other class of amine, like a sterically hindered amine [12].
