2.3.1. Absorption

The method of absorbing CO2 in a solvent to separate it from a gas stream has been in use on an industrial scale for more than 50 years [48], but the partial pressure of the gas streams is comparatively much higher in industrial applications. This process may, in general, be classified into physical absorption and chemical absorption. A detailed classification of the absorption-based CO2 capture technique is shown in Figure 6.

**Figure 6.** Classification of absorption processes for CO2 capture [49].

If the solvent reacts with CO2 and forms chemical compounds, then the process is known as chemical absorption. CO2 is removed from the chemical compounds later. On the other hand, the solvent does not react with CO2 if it is chemically inert. It soaks the CO2 physically. This process is called physical absorption [48]. Chemical absorption of CO2 is done in two stages. At first, the treated gas is brought into contact with the solvent stream in a counter flow. In this stage, the solvent absorbs CO2 from the gas stream. This solvent is regenerated upon heating to desorb CO2 in a stripping column. Pure CO2 is collected from the top of the column [10]. It is then compressed and stored. The regenerated CO2 lean solvent is sent back to the absorber [50]. The process is shown in Figure 7.

**Figure 7.** Schematic diagram of a CO2 absorption plant [51].

The first stage of the process is optimal at high pressures and low temperatures whereas the second stage performs best at low pressures and high temperatures [10]. Chemical absorption is more favorable for capturing CO2 at relatively low pressure. This is helpful for the post combustion process when amine or carbonate solutions are used as solvents [48].

In the case of physical absorption, organic or inorganic physical solvents are used. They do not react chemically with CO2. This operation is based on Henry's law of vapor-liquid mixture equilibrium. According to this law, the amount of a gas dissolved in a unit volume of a solvent is proportional to the partial pressure of the gas in equilibrium with the solvent at any temperature [48]. Due to this pressure dependency of the physical absorption process; it shows better performance than chemical absorption at a higher partial pressure of CO2 such as in an IGCC [11]. A physical absorption process is recommended to be used in IGCC due to the higher partial pressure of CO2 in syngas which makes it more suitable for precombustion carbon capture. Physical solvents need lower energy for regeneration which is another advantage. [52].

The downside of this process is that the capacity of solvents is best at low temperatures. Therefore, the gas stream needs to be cooled before the absorption process. This causes a reduction in efficiency [52]. The processes that are being used commercially for physical absorption are known as Selexol, Rectisol, Purisol [11]. A comprehensive comparison using Aspen plus was done on these processes [49]. For capturing CO2, Selexol was found more energy efficient than other investigated solvents. Lower consumption of energy to regenerate solvent and simple process configuration was the reason for this. David et al. [53] reported that the net efficiency would be greater than in the case with the selexol process if low-temperature CCS was applied in an IGCC.

The partial pressure of CO2 in the flue gas stream is very low in the post combustion carbon capture process. For this reason, the focus of research on this process is to find a suitable solvent. A lot of research has been done on different processes and solvents to identify a cost-effective absorption method. A summary of the advantages and disadvantages of different processes is given Table 4.


**Table 4.** Advantages and disadvantages of different absorption technology [48].
