**2. Chemical absorption solvents**

### **2.1. Conventional amine-based solvents**

The amine-based chemical absorption process has been used for CO2 and H2 S removal—acid gas removal—from gas-treating plants since 1950s [4] and are considered to be by far the most developed CO2 capture process. CO2 is absorbed typically using amines to form a soluble carbonate salt. The absorber operates below 60°C and ambient pressure (step 1 in **Figure 1**) [4].

and secondary amines are the non-fully substituted alkanolamines and they have hydrogen atoms at the non-substituted valent sites on the central nitrogen, whereas the tertiary amines are fully substituted on the central nitrogen. This structural characteristic plays an important role in the acid gas removal capabilities of the various treating solvents [7]. **Table 1** shows the most

**Figure 1.** Simplified diagram of a conventional acid gas removal process using chemical absorption adapted from Ref. [5].

HO

gas and the absorption temperature [4]. The cyclic capacity, defined as the difference of the

Primary alkanolamines such as monoethanolamine (MEA) and diglycolamine (DGA), provide high chemical reactivity, favored kinetics, medium-to-low absorption capacity and acceptable stability. Monoethanolamine (MEA), the first-generation and the most well-known amine-

properties can reduce the absorber height and ensure a feasible operation. Although MEAbased scrubbing technology is suitable for acid gas removal and, in particular, post-combustion capture from fossil-fired plants flue gas, it suffers from several issues during operation, including high energy requirements for stripping: high enthalpy of reaction, low absorption capacity, oxidative and thermal degradation and piping corrosion [8]. Hence, efforts have

loading between the rich solvent and the lean solvent, is also used for easily comparison.

chemical reactivity, the absorption kinetics is expressed as the rate of the CO2

and SO2

, NOx

absorption can be evaluated from several specific proper-

STRIPPER COLUMN

loading, is defined as the amount

partial pressure in the bulk

per second. Finally, the

and low cost. These

and also under elevated tem-

/mole solvent). The maximum CO2

solubility represented by vapour-liquid

Condensador

Cooling water

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

> Flash Separate

steam

CO2

143

widely used amines for the treatment of acid gases, which will be described in detail.

absorption capacity, namely CO2

that can be absorbed per mole of solvent (mole CO2

Storage Tank

1 2

Cooling water

PURIFIED GAS

equilibrium (VLE) curves of each solvent, depending on the CO2

absorbed from the bulk gas to the bulk liquid that means mole CO2

based absorbent, is highlighted by its high chemical reactivity with CO2

perature is compared by the solvent losses under specific operating conditions.

absorption capacity can be deducted from the CO2

solvent resistance to be degrade in presence of O2

The performance of solvents for CO2

ties. In particular, the CO2

ABSORBER COLUMN

of CO2

SOUR INLET GAS

CO2

In terms of CO2

This reaction is reversible and the CO2 can be released by heating the solution with the carbonate salt in a separate stripping column. The CO2 stripping occurs at 120°C and pressures ranging between 1.8 and 3 bar [6], as illustrated in the step 2 in **Figure 1**.

Nowadays, amine-based chemical absorption came up as a potential technology that can be applied to reduce carbon dioxide emissions in industrial processes such us fossil fuels power plants, cement production and iron and steel manufacturing. Post-combustion is the nearest close-to-market and industrially developed carbon capture and storage (CCS) technology.

Specifically, the alkanolamines are volatile, cheap and safe to handle compounds and are commonly classified by the degree of substitution on the central nitrogen; a single substitution denoting a primary amine; a double substitution, a secondary amine; and a triple substitution, a tertiary amine. Each of the above-mentioned alkanolamines has at least one hydroxyl group and one amino group. In general, the hydroxyl group serves to reduce vapor pressure and increases the solubility in water, while the amine group provides the necessary alkalinity in aqueous solutions to promote the reaction with acid gases. Therefore, the molecular structures of primary

based on Henry's law and hence high CO2

142 Carbon Dioxide Chemistry, Capture and Oil Recovery

of CO2

vent. Chemical absorption occurs indeed at low CO2

and newly developed solvents mainly focused on CO2

and novel biphasic configurations and their use as CO<sup>2</sup>

capture process. CO2

The amine-based chemical absorption process has been used for CO2

between 1.8 and 3 bar [6], as illustrated in the step 2 in **Figure 1**.

**2. Chemical absorption solvents**

**2.1. Conventional amine-based solvents**

This reaction is reversible and the CO2

salt in a separate stripping column. The CO2

developed CO2

partial pressures and low-temperatures are highly

partial pressure. Low-temperature is also

capture processes based on physical

and H2

S removal—acid

capture solvents.

is absorbed typically using amines to form a soluble car-

can be released by heating the solution with the carbonate

stripping occurs at 120°C and pressures ranging

and the sol-

and par-

, SO2

recommended for its application. Chemical absorption is based on the reaction between CO2

recommended for this mechanism. Numerous solvents have been developed since the first chemical absorption process was patented in the early 1930s. However, the implementation

ticulate matter which degrade the solvents [2, 3]. This chapter summarizes both conventional

and chemical absorption. A deep revision of the solvents reported from the literature was made including primary, secondary, tertiary amines and non-amine-based solvents. Novel solvents such as sterically hindered amines and blends were discussed further. In respect to physical solvents, authors report the conventional solvents used by licensed processes such us Rectisol™, Selexol™, Sulfinol™ and Purisol™. Special attention will be paid in ionic liquids

gas removal—from gas-treating plants since 1950s [4] and are considered to be by far the most

bonate salt. The absorber operates below 60°C and ambient pressure (step 1 in **Figure 1**) [4].

Nowadays, amine-based chemical absorption came up as a potential technology that can be applied to reduce carbon dioxide emissions in industrial processes such us fossil fuels power plants, cement production and iron and steel manufacturing. Post-combustion is the nearest close-to-market and industrially developed carbon capture and storage (CCS) technology.

Specifically, the alkanolamines are volatile, cheap and safe to handle compounds and are commonly classified by the degree of substitution on the central nitrogen; a single substitution denoting a primary amine; a double substitution, a secondary amine; and a triple substitution, a tertiary amine. Each of the above-mentioned alkanolamines has at least one hydroxyl group and one amino group. In general, the hydroxyl group serves to reduce vapor pressure and increases the solubility in water, while the amine group provides the necessary alkalinity in aqueous solutions to promote the reaction with acid gases. Therefore, the molecular structures of primary

 absorption at industrial processes such as cement production, iron and steel manufacturing and fossil-fuel power plants requires novel solvent formulations that can address the main constrains limiting its deployment: the huge volume of treated gas, the low CO<sup>2</sup>

and specific compounds—solvents—that lead to form a weak bond between CO<sup>2</sup>

concentration in the flue gas and the presence of trace components such as NO<sup>x</sup>

**Figure 1.** Simplified diagram of a conventional acid gas removal process using chemical absorption adapted from Ref. [5].

and secondary amines are the non-fully substituted alkanolamines and they have hydrogen atoms at the non-substituted valent sites on the central nitrogen, whereas the tertiary amines are fully substituted on the central nitrogen. This structural characteristic plays an important role in the acid gas removal capabilities of the various treating solvents [7]. **Table 1** shows the most widely used amines for the treatment of acid gases, which will be described in detail.

The performance of solvents for CO2 absorption can be evaluated from several specific properties. In particular, the CO2 absorption capacity, namely CO2 loading, is defined as the amount of CO2 that can be absorbed per mole of solvent (mole CO2 /mole solvent). The maximum CO2 absorption capacity can be deducted from the CO2 solubility represented by vapour-liquid equilibrium (VLE) curves of each solvent, depending on the CO2 partial pressure in the bulk gas and the absorption temperature [4]. The cyclic capacity, defined as the difference of the CO2 loading between the rich solvent and the lean solvent, is also used for easily comparison. In terms of CO2 chemical reactivity, the absorption kinetics is expressed as the rate of the CO2 absorbed from the bulk gas to the bulk liquid that means mole CO2 per second. Finally, the solvent resistance to be degrade in presence of O2 , NOx and SO2 and also under elevated temperature is compared by the solvent losses under specific operating conditions.

Primary alkanolamines such as monoethanolamine (MEA) and diglycolamine (DGA), provide high chemical reactivity, favored kinetics, medium-to-low absorption capacity and acceptable stability. Monoethanolamine (MEA), the first-generation and the most well-known aminebased absorbent, is highlighted by its high chemical reactivity with CO2 and low cost. These properties can reduce the absorber height and ensure a feasible operation. Although MEAbased scrubbing technology is suitable for acid gas removal and, in particular, post-combustion capture from fossil-fired plants flue gas, it suffers from several issues during operation, including high energy requirements for stripping: high enthalpy of reaction, low absorption capacity, oxidative and thermal degradation and piping corrosion [8]. Hence, efforts have


In general, the main issues that amine-based chemical absorption has to address to be applied

• Corrosion requires the use of both inhibitors and resistant materials in their application.

Sterically hindered amines are considered a type of amines which can improve CO2

tion 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

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

According to Nicole Hüser et al. [11], a decrease up to 15% can be achieved using hindered amines.

ary 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

**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/

molecule, as it is extracted from the overall reaction.

. Based on this assumption, the maximum CO2

amines is higher than for unhindered, primary or secondary amines.



capacity.

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absorp-

145


and other impurities such as particles, HCl, HF and Hg.

is lower that the common primary and second amines.


loading using sterically hindered

) is shown in **Figure 3**. Regarding the primary or second-

as industrial scale for carbon dioxide emission mitigation are listed below:

, SOx

• High energy consumption during the solvent regeneration.

• Scale up from actual (800 t/day) to required (8000 t/day) CO2

• Degradation in the presence of O2

**2.2. Sterically hindered amine solvents**

carbon atom in the second (**Figure 2**).

the energy consumption to release CO2

The general reaction scheme of the CO2

sterically hindered amine(AmCOO<sup>−</sup>

by CO2

absorb one CO2

The system CO2

molecule of CO2

black balls represent N atoms [11].

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

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 energy requirement for solvent regeneration than MEA.

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

There are three main differences in the performance of primary and secondary amines as they are compared to tertiary amines for the CO2 separation process. Primary and secondary amines are very reactive; they form carbamate by direct reaction with CO<sup>2</sup> by Zwitterion mechanism. Therefore, these amines showed limited thermodynamic capacity to absorb CO2 due to the 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 by carbamate formation and, hence, tertiary amines show low CO2 absorption rates [9].

In general, the main issues that amine-based chemical absorption has to address to be applied as industrial scale for carbon dioxide emission mitigation are listed below:

