## 2 Experimental
### Experimental Set up and Operation  
The CO\({}_{2}\) capture by NaOH solution was experimentally studied by means of a membrane contactor and an absorption column connected in parallel so that both set ups can be used independently in the same installation (Figure 1). The absorption column was a packed column UOP7-MKII Gas Absorption Column, Armfield Ltd. (Ringwood, UK). The membrane contactor was the 2.5 \(\times\) 8 Extra-Flow Module\({}^{\mathrm{TM}}\)Liqui-Cel\({}^{\mathrm{\circledr}}\), Membrane GmbH (Wuppertal, Germany). Characteristics of both devices are collected in Table 1.  
In both systems a flue gas (mixture air-CO\({}_{2}\)) was contacted with an absorbent (NaOH) in counter-current mode. The composition of the gas and liquid phases were measured at the inlet and the outlet of the absorbers: CO\({}_{2}\) concentration in the gas was directly measured by sensors and plotted on the console screen. Physical and chemical absorptions took place and, therefore, new components appeared (carbonates and bicarbonates). The concentration of the hydroxides, carbonates and bicarbonates was measured at the inlet of the absorbers; i.e., whose values correspond to the composition of the feed tank--and at the outlet; i.e., after the absorption takes places. The inlet \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Packed Column**} & \multicolumn{2}{c}{**Hollow Fiber Membrane Contactor**} \\ \hline Parameters & Data from manufacturer & Parameters & Data from manufacturer \\ \hline Walls material & Acrylic & Membrane/Potting & \\  & & Material & Polypropylene/Polyethylene \\ \hline Column diameter (m) & 0.8 & Fiber i.d/o.d (\(\upmu\)m) & 240/300 \\ \hline Column packing & Raschig rings & Wall thickness (\(\upmu\)m) & 40 \\ \hline Raschig rings & \(10\times 10\) & Effective pore size (\(\upmu\)m) & 0.04 \\ dimensions (mm \(\times\) mm) & & & \\ \hline Raschig rings material & Glass & Porosity (\%) & 40 \\ \hline Column length (m) & 1.4 & Effective fiber length (m) & 0.16 \\ \hline Raschig rings specific & 440 & Effective membrane & 1.4 \\ area (m\({}^{2}\)/m\({}^{3}\)) & & surface area (m\({}^{2}\)) & 1.4 \\ \hline Packing volume (L) & 7 & Number of fibers & 10,200 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the packed column and the hollow fiber contactor. and outlet samples were taken at the same time every 20 min. The ion concentrations in the aqueous solution were indirectly calculated by titrations of the liquid samples (see Section 2.3). The gas coming out from the column/contactor; i.e., air and CO\({}_{2}\) not absorbed--is released to the atmosphere while the liquid phase works in a closed loop, returning to the sump tank during the 80 min of each experiment. In the case of the column, the absorbent was pumped to the top of the packed column (filled with glass Raschig rings, with an interfacial area of 3.1 m\({}^{2}\)) and fell through it by gravity. At the bottom, the gas stream is introduced. The CO\({}_{2}\) from the gas diffuses to the liquid phase and is chemically absorbed by NaOH in the liquid film surrounding the packing material. The column has an interfacial area and a volume of 3.1 m\({}^{2}\) and 0.7 m\({}^{3}\), respectively. Regarding the membrane contactor, the fluid-fluid contact was produced in the pores of the more than ten thousand hollow fibers (the effective interfacial area is 1.4 m\({}^{2}\)). Gas circulated inside the fibers (lumen side) and the liquid run around the fibers (shell side). The module (including the housing) occupies 0.13 m\({}^{3}\). The pressure drop caused by the liquid phase was evaluated when varying the liquid and gas flow rates in the column from 1 to 7 L min\({}^{-1}\) and from 40 to 180 L min\({}^{-1}\), respectively. Liquid and gas flow rates ranged in the membrane contactor from 1 to 3 L min\({}^{-1}\) and from 10 to 24 L min\({}^{-1}\), respectively. The values of the pressure drop in both systems are addressed in the Appendix A as supporting information.