## 4 Results and Discussions
### Effects of Variables on the Indexes  
Figure 6 presents a plot of \(E_{F}\) versus \(\gamma\) for different mixed amines. According to the trend, \(E_{F}\) decreases with an increase in \(\gamma\), while mixed amine A5 (MEA + PZ) yielded a higher \(E_{F}\) at the same \(\gamma\), suggesting that PZ can promote mass transfer and, hence, the absorption efficiency. There are two points that affect the \(\gamma\) and hence \(E_{F}\) value; one is the resident time of CO\({}_{2}\) gas and the other is the solvent molar flow rate. A lower residence time meaning a higher gas flow rate leads to reduce \(E_{F}\), while the E\({}_{\rm F}\) can be increased when a higher molar flow rate was used. Nonetheless, the \(E_{F}\) is also affected by temperature and pH, which are not included in Figure 6a. Therefore, a regression was required to assess the influences of \(\gamma\), \(T\), and pH on the \(E_{F}\). The results are shown below:  
\[E_{F}(\%)=3.0532\times 10^{2}\exp(-\frac{4137}{R({\rm JK}^{-1}{\rm mol}^{-1} )T({\rm K})})\gamma^{-0.1801}pH^{0.09684}. \tag{33}\] \begin{table}
\begin{tabular}{c c c c c c} \hline
**Level** & **A** & **B** & **C** & **D** & **E** & **F** \\ \hline
1 & 38.1559 & 37.9573 & 37.7625 & (38.9978) & 36.5413 & 37.784 \\
2 & 37.9301 & 38.1093 & 37.4206 & 38.7922 & 37.9967 & 37.6308 \\
3 & 37.3404 & 37.6729 & 37.7078 & 38.343 & 38.262 & 37.674 \\
4 & 37.8643 & (38.2346) & 37.9977 & 37.2163 & 38.7237 & 38.2035 \\
5 & (38.7104) & 37.9347 & (38.4925) & 36.9414 & (38.7678) & (38.6857) \\ DELTA & 1.37 & 0.5617 & 1.0719 & 2.0564 & 2.2265 & 1.0549 \\ RANK & 3 & 6 & 5 & 2 & 1 & 4 \\ \hline \end{tabular}
\end{table} 
Table 7: S/N ratio analysis for E\({}_{\rm F}\) giving optimum condition and parameter significance. \begin{table}
\begin{tabular}{c c c} \hline
**Indicators** & **Optimum Condition** & **Parameter Sequence** \\ \hline \(E_{F}\) (No. 26) & A5B4C5D1E5F5 & E \(>\) D \(>\) A \(>\) F \(>\) C \(>\) B \\ \(R_{A}\) (No. 27) & A2B2C3D5E4F5 & D \(>\) C \(>\) E \(>\) F \(>\) A \(>\) B \\ \(K_{\rm C}\)\(a\) (No. 28) & A5B3C3D5E5F4 & E \(>\) A \(>\) D \(>\) F \(>\) C \(>\) B \\ \(\phi\) (No. 29) & A1B4C1D5E1F4 & E \(>\) C \(>\) D \(>\) A \(>\) B \(>\) F \\ \hline \end{tabular}
\end{table} 
Table 8: Optimum condition and parameter sequence for all indicators. \begin{table}
\begin{tabular}{c c c c c} \hline
**No. ** & \(E_{F}\) (\%) & \(R_{A}\times 10^{4}\) **(mol/s-L)** & \(K_{C}\)**\(a\) **(1/s)** & \(\phi\) **(mol-CO\({}_{2}\)/L-mol-Solvent)** \\ \hline
1–25 & 56.58–100.0 & 4.4153–18.9500 & 0.1195–0.9139 & 0.0433–0.2923 \\ E\({}_{\rm F}\)  
The R\({}^{2}\) was found to be 0.7453 (>0.5), indicating that the powder law model is suitable for \(E_{F}\) and that the results are reliable. \(E_{F}\) increased with an increase in t and pH and decreased with an increase in \(\gamma\). Figure 6b is a plot of calculated values versus measured values; almost all data were within 10% error, showing the confidence of Equation (33). A similar regression was also performed for \(R_{A}\) and \(K_{G}a\), but the R squares for both were <0.5 showing the differences in solvents. Due to this, the regressions could be carried out for different solvents and were expressed as: \(\xi=\alpha pH^{a}\gamma^{b}t^{c}\). Tables 10 and 11 show both parameters and R\({}^{2}\) for different solvents. A2 and A3 were excluded because R\({}^{2}\) < 0.5, and \(R_{A}\) decreased with an increase in \(\gamma\) for A1 and decreased with an increase in t for A5. In addition, \(R_{A}\) increased with an increase in \(pH\), \(\gamma\), and \(t\), respectively. According to Figure 7a, a plot of calculated data versus measured data showed that the error limit for most data was within 20%. A2 and A3 showed some scattering data; conversely, for A1, \(K_{G}a\) decreased with an increase in \(\gamma\) and \(K_{G}a\) and increased with an increase in pH, \(\gamma\), and t, respectively. Figure 7b shows that a plot of calculated data versus measured data showed that the errors of most data were within 20%. Some scattering data were also observed for A2 and A3.