# Application of Nanofluids in CO\({}_{2}\) Absorption: A Review
## 4 Classification of Nanofluids Based on Base Liquid
### Antine-Based Nanofluid
This mass transfer improvement can be achieved by the addition of fine particles to the solvent, which increases the rate of gas removal in gas-liquid mass transfer processes in the absorption columns, therefore, the equipment size will be reduced. It means that in the processes where CO\({}_{2}\) diffusion is involved, the acceleration of the absorption rate is achieved by the enhancement of mass transfer. Therefore, fine particles were added to the liquid phase to increase the mass transfer; it also increases the efficiency of gas-liquid absorption columns or decreases the required size of the equipment due to the enhancement in removal rate of the mass transfer process from gas to the liquid phase. Thus, the substitution of amine solvents with amine-based nanoparticle adsorbents that are dispersed in the liquid phase can save a significant amount of energy since these absorbents do not require high amounts of energy for heating and cooling cycles to recover the liquid solvent [111]. An overview of the recent works in the CO\({}_{2}\) absorption with amine-based nanoparticles is presented in Table 2.  
[PERSON] et al., used dispersions of Al\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), and TiO\({}_{2}\) nanoparticles in MEA base fluid to be used in CO\({}_{2}\) capture processes. TiO\({}_{2}\) showed the highest improvement in CO\({}_{2}\) absorption [112]. Similar reports are available in the literature. [PERSON] et al. carried out some experiments to determine the effect of four nanoparticles of SiO\({}_{2}\), MgO, TiO\({}_{2}\), and Al\({}_{2}\)O\({}_{3}\), on the improvement of CO\({}_{2}\) capture. They found that TiO\({}_{2}\)-MDEA nanofluids have the best CO\({}_{2}\) absorption performance among the four solutions. Since TiO\({}_{2}\) has the highest CO\({}_{2}\) adsorption capacity, its solution shows the maximum absorption performance due to the higher gradient of CO\({}_{2}\) concentration that it provides in the solution. Furthermore, the CO\({}_{2}\) absorption performance of TiO\({}_{2}\)-MEA nanofluid was better than TiO\({}_{2}\)-MDEA nanofluid; the reason was the higher rate of the chemical reaction between MEA and CO\({}_{2}\)[105]. [PERSON] et al. [96] and [PERSON] et al. [97, 98, 99] studied the impact of SiO\({}_{2}\) nanoparticles addition to MEA, DEA, and DIPA aqueous solutions, on CO\({}_{2}\) absorption rate in a stirred cell. Results indicated that the enhancement of nanoparticle concentration reduces the absorption rate due to the solution's elasticity. The absorption enhancement performance of Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) nanoparticles were assessed when they were dispersed in three different base fluids of MDEA, PZ, and MEA. The coefficient of mass transfer measured at the liquid side indicated a significant enhancement in absorption kinetic of PZ absorbent after the addition of nanoparticles [113]. [PERSON] et al., enhanced the absorption rate of CO\({}_{2}\) capture by amine solutions when nanoferofluids were used as the enhancement agent. They indicated that the addition of 0.39 vol% of nanoparticles to the base fluid results in a 92.8% enhancement in the absorption capacity compared to the base fluid [99]. The utilization of a nanofluid mixture of graphene-Oxide/MDEA in the gas sweetening process was assessed by [PERSON] et al. [114]. The addition of only 0.1 wt.% graphene oxide to the solvent could promote its absorption capacity by 9.1% which is attributed to the increased mass transfer coefficient due to the hydroxyl functional groups on the graphene oxide surface. In another study, [PERSON] et al., studied the impact of colloidal nanosilica addition to 2-amino-2-methyl-1-propanol solvent on CO\({}_{2}\) absorption performance in a stirred vessel. They found that as the concentration of nanoparticles increases the absorption rate and the volumetric mass transfer coefficient in the liquid side decreases [115]. [PERSON] et al.