## 3 Results and Discussion
### Mass Transfer Coefficient
#### 3.2.2 Influence of the Gas and Liquid Composition on the Mass Transfer Coefficient  
Regarding the composition of the flue gas and the absorbent, Figure 5a displays the mass transfer coefficient calculated in both set ups when the NaOH initial concentration ranges from 0.2 to 1 mol L\({}^{-1}\) and 10-15% CO\({}_{2}\) in the flue gas is used. Here, the overall mass transfer coefficient for 0.5 and 1 mol L\({}^{-1}\) of NaOH is one order of magnitude higher than that obtained for 0.2 mol L\({}^{-1}\) (also plotted in Figure 4). Thus, the effect caused by the variation of the flow rate shown in the previous section is negligible compared to the important effect of the NaOH concentration. High NaOH concentration increases the overall mass transfer coefficient due to the larger availability of hydroxide groups to react with CO\({}_{2}\), which increases the absorption capacity of the solvent [33]. Regarding the flue gas composition, no effect is observed in the membrane contactor but it is noticeable in the column. In Figure 5a, both column profiles followed a parallel trend. Mass transfer coefficients are higher when the amount of CO\({}_{2}\) is lower in the flue gas. Again, more hydroxides remain in the feed tank because less reactions took place with the contaminant and, therefore, more OH\({}^{-}\) ions are able to chemically absorb with CO\({}_{2}\) while passing through the packing, ensuring a high value of the coefficient. Similar mass transfer coefficients were obtained in both configurations when the concentration of the solvent was 0.2 M. Here, it should be remembered that the mass transfer in the column involves only the resistances of the gas and liquid boundaries but, in the membrane, a third extra resistance is present: the physical barrier [34]. In this case, the third resistance of the membrane does not mean a strong disadvantage compared with the absorption column. This specific scenario shows a hypothetic competition in terms of mass transfer but the CO\({}_{2}\) removal efficiency, plotted in Figure 5b, displays better results in the case of the conventional equipment, especially when 15% of CO\({}_{2}\) is treated. On the other hand, in the experiments with 0.5 and 1 mol L\({}^{-1}\) of NaOH, the mass transfer coefficient values revealed an important difference between the packed column and the membrane contactor. It can be concluded that the most important variable affecting the mass transfer is the concentration of the solvent. Figure 5b presents the CO\({}_{2}\) removal efficiency for the same group of experiments. Upslope trends obtained agree with the mass transfer coefficient profiles. The highest efficiencies were found in the column, where 97% of the CO\({}_{2}\) was eliminated when 1 mol L\({}^{-1}\) of initial NaOH was used for capturing both 10% and 15% of the CO\({}_{2}\) in the flue gas (75% elimination with the membrane) and more than 80% removal was reached with 0.5 mol L\({}^{-1}\). The lowest removal was obtained with the column when the concentration of the solvent was 0.2 mol L\({}^{-1}\), while the removal efficiency in the membrane contactor overcame the 50% for the same case. In summary, good removal efficiencies were obtained, similar to the literature previously reported [31], which may satisfy industrial requirements. Furthermore, it can be concluded that the concentration of NaOH is the most responsible parameter in absorption. The mass transfer coefficients and the removal efficiency showed in Figures 4 and 5 must be considered conservative because they were calculated at time = 20 min, in which the initial concentration of NaOH had already decreased. Indeed, a removal efficiency of up to 97% was also obtained in the membrane contactor when the value at time 0 was used; i.e., few seconds after starting the experiment. In order to show a more realistic overview of results, according to the real sorbent concentration, Figure 6 displays the mass transfer coefficient of all the experiments made in the function of the percentage of the NaOH still present in the solution.