**2.4. Solvent blends**

**Abbreviation AMP MDA PE**

)(CH3 )(NH2

conventional amine-based solvents is the sodium carbonate (Na2

carbonate slurry is used to provide a basic environment in which CO2

ate followed by sodium bicarbonate formation [13]. The NaHCO3

Sodium carbonate has shown a high performance in CO2

son with the MEA benchmark. It produces a high CO2

ˉ) and a reboiler duty of 3.2 MJ/kg CO2

in presence of pollutants such as SO2


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

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

Structural formulae HO-CH2

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

**Figure 3.** General reaction scheme of the CO2

146 Carbon Dioxide Chemistry, Capture and Oil Recovery

bicarbonate formation and, hence, the CO2

Chemical structure

CO3 2

CO2

CO2

4.2 MJ/kg CO2

solvents for CO2

Name 2-amino-2-methyl-1-propanol 1,8-p-menthanediamine 2-piperidineethanol

) (CH3

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

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

formation weak bonds [11]. Several sterically hindered amines are shown in **Table 2**.


NH2 -C6 H9 -C3 H6 NH2

at low absorption rates, which lead to higher absorption column height. It assumes that

) C2 H4

which can enable the cyclic capacity of amine-based

CO3

capture capacity of the solvent is improved.

rather than 0.5 mole CO<sup>2</sup>

in case MEA is used as a solvent. Furthermore, this chemical solvent can absorb

absorption [13, 14]. Despite those advantages, sodium carbonate can absorb

C2 H4


). About 30% p/p sodium

is absorbed as bicarbon-

/mole MEA and 3.5–

/ mole

precipitation enhances the

separation from flue gas in compari-

loading capacity (0.73 mole CO2

H4 OH)- 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 operating conditions and the solvent used to absorb CO2 consume a high amount of energy. In this sense, amine blends could offer potential improvements in CO<sup>2</sup> chemical absorption to reduce the regular reboiler duty and the common solvent circulation rates [14, 19].

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 (BZA) and MDEA [22, 23].

A huge number of solvent have been proposed for CO2 chemical absorption applied to carbon 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 solvent but it is almost 50 times slower than PZ. The CO<sup>2</sup> absorption rate of MEA can be significantly improved by adding small amounts of PZ as a promoter [24]. This blend improved the individual CO2 absorption rate. Potassium carbonate promoted with PZ is also considered a promising solvent [25], along with the PZ and 2-amino-2-methyl-1-propanol (AMP) blends [26]. A summary of the most promising amines blends are given below (**Table 4**).


**Table 3.** Advantages and disadvantages of CCS based on chemical absorption using Na2 CO3 [14].


allows them to offer unique molecular structures specifically designed for each application, in

their low vapor pressure must be also taking into account due to its impact on the environment. ILs are non-volatile compounds and therefore the presence of ILs in the cleaned exhaust gas

erty is also related to low energy requirements during the solvent regeneration. Several studies reported up to 15% of reduction of the specific energy consumption of MEA functionalized ILs

Despite their potential for carbon capture, there are some disadvantages that constrain their competitiveness of ILs compared to conventional solvents. ILs show relatively high viscosity, limit-

producing solvent pumping issues as well as mass transfer and operational difficulties during

between the compounds formed by the Zwitterion and dication mechanisms. Authors proposed to promote ILs by the use of non-amine functionalized ILS to avoid the formation of hydrogen bonds [35, 36]. Other proposals such as introducing ether oxygen atoms into the ILs structure and adding particular molecular solvents to provide IL-based solvents were found in the literature. For instance, the use of organic amines instead of water decrease also the viscosity without

relatively expensive in comparison with common amine-based solvents [37]. Other aspect that should be further investigated in order to address the main knowledge gaps are indicated below: • Determination of transport properties, physical properties, absorption kinetics of the best

• Development of simulation tools that can predict the chemistry of the interactions in multi-

• The evaluation of the toxicity and environmental impact derived from the use of IL and its

As it was mentioned previously, numerous ILs formulae have been developed for last 20 years. Pure ILs configurations, typically referred as room-temperature ionic liquid (RTILs), retain CO<sup>2</sup>

with conventional amine-based solvents. The next generation of ILs was defined from the combination of conventional ILs with a functionalized amine group, preferable. Based on this configu-

ILs. The development of this type of ILs enhanced the performance of IL in both biogas/natural gas treatment and CCS. **Table 5** summarizes the best performance IL reported from the literature.

capture process. According to Luo and Wang [32], the increase of the viscosity

absorption capacity [30]. Finally, it is necessary to point out that they are also

absorption is due to the formation of strong and dense hydrogen-bonded networks

ing their mass transfer capacities. Indeed, they become excessively viscous once CO2

• Efforts on developing new combinations of IL using membranes.

in order to improve the feasibility of its use as a potential CO2

by physisorption mechanism showing an unfeasible CO2

showing further improvements on the CO2

• Investigations on the task-specific IL and the mechanisms involved in CO<sup>2</sup>

ration, amine-functionalized IL, also called task-specific IL, reacts with CO<sup>2</sup>

separation step is negligible. In this sense, solvent losses associated to ILs are assumed

concentrated flue gas treatment [30, 32, 33]. Other properties such as

capture process based on these emerging solvents. This prop-

is absorbed,

Solvents for Carbon Dioxide Capture http://dx.doi.org/10.5772/intechopen.71443 149


by chemisorption

separation approach.

absorption performance in comparison

capture process derived from physisorption-based

particular for low CO2

to be completely avoided in a CO2

compared to conventional MEA-based scrubbing [33, 34].

after CO2

the overall CO2

reducing the CO2

performance ILs.

components systems.

derivatives.

after the CO2

**Table 4.** A summary of most relevant solvent blends reported from the literature [14].

Some advantages of blending these amines are listed below [29]:

