## 3 Results and Discussion
### Mass Transfer Coefficient
#### 3.2.1 Influence of the Flow Rates of the Fluids on the Mass Transfer Coefficient  
The evolution of the species concentration aforementioned is characteristic of systems that work in batch mode. Nevertheless, power generation and CO\({}_{2}\) emissions are rarely interrupted, so refreshed solvents must be used to keep the concentration constant and, consequently, the same level of absorption. The way to simulate this scenario from the batch laboratory results is by calculating the mass transfer coefficients at the beginning of the experiment (time 20 min in this case) to ensure high concentration of NaOH and an almost constant concentration of CO\({}_{2}\) at the outlet gas. These results are addressed in Figures 4 and 5. Furthermore, the mass transfer coefficient for different concentrations of the NaOH and NaOH/Na\({}_{2}\)CO\({}_{3}\) mixtures obtained in the laboratory are collected in Figure 6.  
Figure 4: Mass transfer coefficient in function of the NaOH flow rate for different blue gas flow rates in the two set ups. The CO\({}_{2}\) and NaOH initial concentrations are 10% and 0.2 mol L\({}^{-1}\), respectively, for all the cases except the experimental point corresponding to 23.7 L min\({}^{-1}\) gas (membrane) and 3 L min\({}^{-1}\) of NaOH flowrate, where it was used as 15% of CO\({}_{2}\). In Figure 4, mass transfer coefficients obtained by running both a packed column and the membrane contactor are plotted as a function of the solvent flow rates for different feed gas streams. In the column, the higher the solvent flow rate and the lower the flue gas velocity, the higher the mass transfer coefficient. For the studied operation conditions, when the liquid flow rate is increased from 1 to 7 L min\({}^{-1}\) in the column, the overall mass transfer coefficient increases by 42%. When the liquid stream is increased from 1 to 3 L min\({}^{-1}\) in the contactor, the overall mass transfer coefficient enhances by 51%. The higher the liquid flow rate, the more rapidly fresh NaOH enters the column/contactor, ensuring that CO\({}_{2}\) is efficiently absorbed. Furthermore, the turbulence that is generated decreases the  
Figure 5: (**a**) Mass transfer coefficient in function of the NaOH initial concentration in the solvent for different CO\({}_{2}\) concentrations in the flue gas in the two set ups; (**b**) CO\({}_{2}\) removal efficiency. The solvent and the flue gas flow rates in the column and the membrane contactor are 5 and 33 L min\({}^{-1}\) and 1 and 23.5 L min\({}^{-1}\), respectively. Figure 6: (**a**) Mass transfer coefficient in function of the %NaOH in the liquid solution in the two set ups; (**b**) Zoom of the bottom part of Figure 6a. resistance in the liquid phase and, therefore, a better mixing of the phases within the absorber [32]. In the contactor, the membrane means an extra resistance, which is counterbalanced by reducing the liquid boundary layer by increasing the solvent flow rate. On the other hand, when the flue gas increases in the column from 33 L min\({}^{-1}\) to the double, the overall mass transfer coefficient decreases by 12%. By increasing the increment of the flue gas from 10 to 23.5 L min\({}^{-1}\) in the membrane contactor, this reduces the overall mass transfer coefficient around by 50%. While this parameter does not have a relevant effect on the column, it seems more shocking on the contactor. It is normally expected that the higher the gas flow rate, the lower the mass transfer resistance (inversely proportional to the mass transfer coefficient), as it occurs when the flue gas rises from 6.7 to 10 L min\({}^{-1}\). Nevertheless, the mass transfer coefficient is lower for a gas stream of 23.5 L min\({}^{-1}\). In this case, it was experimentally observed that bubbles appeared in the liquid phase; i.e., the other side of the membrane. This means that gas pressure overcomes the liquid one and, therefore, the gas-liquid contact was not only produced in the pores of the membrane but also in the liquid side.