# Application of Nanofluids in CO\({}_{2}\) Absorption: A Review
## 4 Classification of Nanofluids Based on Base Liquid
### Antine-Based Nanofluid  
Amine absorption of CO\({}_{2}\) is classified as a chemical process in which a gas-liquid phase mass transfer occurs. Absorption and desorption columns are used for this purpose. The gas-liquid equilibrium determines the absorption performance of the selected amine [107]. Amines have fast kinetics in CO\({}_{2}\) absorption. The water solubility is increased and the vapor pressure is reduced by the hydroxyl functional group in the amine, while the alkalinity is facilitated by the amino groups in an aqueous solution [108]. Accordingly, aqueous alkalanolamine solutions are the most common solvent used in industrial gas sweetening processes in scrubbers. Generally, the number of hydrogen atoms of an ammonia molecule substituted with other functional groups is the basis of the classifications for amines into three groups that are primary amines like MEA, secondary amines including DEA, and tertiary amines like MDEA; secondary amines have fast reactions with acidic gases like CO\({}_{2}\) with a higher rate of regeneration energy consumption in comparison to the tertiary amines [109]. The most commonly used amine solvent in large plants is the primary amine of MEA, which is highly reactive and is economically feasible. Nevertheless, this solvent is corrosive and requires a high amount of energy for regeneration. Several MEA mixtures are proposed in the literature to reduce the energy consumption of this solvent; A mixture of tertiary amines with primary amines is often desirable [110]. The favorable acid gas loading of MEDA has made it a cost-efficient solvent for gas sweetening processes. This solvent has low corrosivity and needs lesser amounts of heat for regeneration. The problem is the weak selectivity of MDEA for CO\({}_{2}\) in the presence of H\({}_{2}\)S. One way of increasing CO\({}_{2}\) selectivity is to activate it before being used in CO\({}_{2}\) separation processes; this activation can be achieved through some additives like PZ or MEA [84]. PZ is another attractive candidate for CO\({}_{2}\) absorption due to a higher reactivity than MEA. But, PZ has also some drawbacks such as higher volatility compared to MEA, higher cost-intensive implementation in CO\({}_{2}\) than MEA which is still in the development phase. Lots of papers are available in the literature that consider the efficiency, advantage, and disadvantages of amine solvents. For instance, AMP (2-Amino-2-Methyl-1-Propanol) is a primary amine with steric hindrance with higher CO\({}_{2}\) absorption capacity (due to bi-carbonate formation), furthermore, it is less corrosive compared with MEA. According to the literature, CO\({}_{2}\) is soluble in aqueous solutions containing alkalanolamines. This absorption can be further improved by the addition of PZ as an activator, similarly the additives can also enhance and accelerate the absorption of CO\({}_{2}\). On the other hand, since different species exist in different amines, the chemical stability and corrosivity of the products of CO\({}_{2}\) absorption will be different from one another. Corrosion causes some problems for the process including the reduction of equipment lifetime. Besides, the strong chemical bonds of amine solvents with CO\({}_{2}\) require high amounts of energy for the desorption process. This high energy demand of the solvent recovery process and corrosion of scrubbers used for CO\({}_{2}\) capture (water-based alkanolamine solutions) led to the idea of developing solid sorbents [10]. Many research studies suggested that the addition of promoters or the dispersion of a third phase such as solid particles, can considerably increase the gas absorption performance [33]. In other words, one way of increasing the rate of CO\({}_{2}\) absorption in CO\({}_{2}\) capture processes is to increase the CO\({}_{2}\) mass transfer.