# Application of Nanofluids in CO\({}_{2}\) Absorption: A Review
## 4 Classification of Nanofluids Based on Base Liquid
### Methanol-Based Nanofluid  
The methanol-based nanofluids are another kind of absorbent that improves CO\({}_{2}\) absorption in synthetic natural gas (SNG) systems. The high selectivity and low cost of this absorbent and the possibility of being used for high-pressure natural gas streams have made it a good candidate for these processes. Another advantage of this absorbent is the requirement of low temperatures for the regeneration process compared to the aqueous solutions since it has smaller latent heat and a lower boiling point. Therefore, the non-aqueous alkanolamines are good candidates to enhance the CO\({}_{2}\) absorption performance. But, according to [PERSON]'s law of solubility, it is required to keep the temperature of the absorbent at about \(-20\)\({}^{\circ}\)C to be able to increase the rate of absorption [104; 104]. Therefore, it needs a lot of energy to keep methanol at such a low temperature. The addition of nanoparticles to the base fluids is a perfect way of CO\({}_{2}\) absorption enhancement. The present research efforts are summarized in Table 4.  
Accordingly, extensive investigations were done by [PERSON] et al., with nanofluids containing Al\({}_{2}\)O\({}_{3}\) dispersed nanoparticles with 0.005-0.1 vol% concentrations. They achieved the highest CO\({}_{2}\) elimination of 8.3% at nanoparticle a concentration of 0.01 vol% compared to pure methanol as the absorbent. The suggested that the reason for this enhancement is \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Researchers** & **Base Fluid** & **Nanoparticle** & **Particle Size (nm)** & **Contactor Type** & **Enhancement \%** & **Absorbent Loading** \\ \hline [PERSON] et al. [87] & Methanol & Al\({}_{2}\)O\({}_{3}\) & 40–50 & AC, T-CA & 1.2, 10 & 0.05 vol\% \\  & & TiO\({}_{2}\) & \textless{}25 & & 4.6, 5 & 0.05 vol\% \\  & & SiO\({}_{2}\) & 10–20 & & 1.1, 6 & 0.05 vol\% \\ [PERSON] et al. [105] & Methanol & Al\({}_{2}\)O\({}_{3}\) & 40–50 & Bubble column & 8.3 & 0.01 vol\% \\ [PERSON] et al. [141] & Methanol & Al\({}_{2}\)O\({}_{3}\) & 40–50 & Tray column & 9.4 & 0.05 vol\% \\ [PERSON] et al. [104] & Methanol & SiO\({}_{2}\) & 10–20 & & 9.7 & 0.05 vol\% \\ [PERSON] et al. [104] & Methanol & Al\({}_{2}\)O\({}_{3}\) & 40–50 & Bubble column & 4.5 & 0.01 vol\% \\ [PERSON] et al. [78] & Methanol & Al\({}_{2}\)O\({}_{3}\) & 40–50 & Bubble column & 26 & 0.01 vol\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Common use of methanol-based nanofluids in CO\({}_{2}\) absorption. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Researchers** & **Base Fluid** & **Nanoparticle** & **Size of Nanoparticle (nm)** & **Contactor Type** & **Enhancement \%** & **Absorbent Loading** \\ \hline [PERSON] et al. [120] & DI Water & Fe\({}_{3}\)O\({}_{4}\)-proline & 10–16 & Stirred cell reactor & 25.07 & 0.02 wt.\% \\  & Fe\({}_{3}\)O\({}_{4}\)-lysine & 13–18 & & 31.04 & 0.1 wt.\% \\  & Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)-NH\({}_{2}\) & 31–39 & & 34.23 & 0.1 wt.\% \\ [PERSON] et al. [95] & Water & Al\({}_{2}\)O\({}_{3}\) & - & WWC & 19 & 0.6 vol\% \\ [PERSON] et al. [139] & Water & TiO\({}_{2}\) & - & Packed column & 65 & 1.0 vol\% \\  & MWCNT & 10 & Bubble Column & 36 & 40 Mg/L \\ \hline \hline \end{tabular}  
* \({}^{1}\): CO\({}_{2}\) removal% *: Outside diameter = 8 nm, Inside diameter = 2.5 nm and Length= 10 μm. \({}^{2}\): Hollow fiber membrane contacts.  
\end{table} 
Table 3: _Cont._the Brownian motion that induces particle-laden flows and consequently creates the mixing effects by Al\({}_{2}\)O\({}_{3}\) nanoparticles [105]. [PERSON] et al., used a try column to study the impact of Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) on CO\({}_{2}\) absorption rate when they are dispersed in a methanol-based solution. Results of their experiments showed a 9.4% and 9.7% enhancement, respectively, in the absorption capacity of nanofluids containing Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\), when the optimum concentration of 0.05% of the nanoparticles was used [141].