## 3 Results and Discussion
### Effect of the Hydrodynamics of the Reactor Airlift on the Microalga Growth  
The effect of the flow rates of 0.05, 0.1, and 0.15 vvm on algae growth, Reynolds number (Re), superficial gas and liquid velocities (_Ugr_ and _Ulr_), gas holdup in the riser (\(\varphi_{r}\)), global shear rate (\(\gamma\)) in the riser, and on the volumetric mass transfer coefficient of CO\({}_{2}\) (_KLa_) was evaluated to help to explain these cultures. In these cultures, we observed that the growth rate of biomass was proportionately higher for the culture made with lower flow rate (Figure 4), in the following order. C1 \(>\) C2 \(\approx\) C3. As shown in Table 3, as the gas flow or the superficial gas velocity in riser (_Ugr_) increases, the velocity of the liquid in the riser (_Ulr_) and the downcomer (_Ulld_) increases. In the same way, the gas holdup \(\varphi_{r}\), and the numbers of Re increased from 3121 to 9362 (Table 3), clearly in turbulent regime. It is clear that a turbulent regime prevails (Re \(>\) 2100) within the riser. Even in the downcomer, the turbulent regime is observed at the highest flow (culture C3). With these three experiments (C1 to C3), it is found that the best growth of biomass occurs in the culture C1 and demands flow patterns with numbers of Re less than 6200 and gas holdups less than 0.0082. The culture C1 showed the maximum biomass of 3.8 g\(\cdot\)L\({}^{-1}\) (Table 4) and the maximum _Fc_ (0.53 g CO\({}_{2}\) L\({}^{-1}\cdot\)d\({}^{-1}\)). In these cultures, the pH was uncontrolled, but it was observed that it was maintained in a range of 6-7 after the second day of the culture. This behavior could be explained due to the CO\({}_{2}\) equilibrium [33] and the metabolism of the microalga itself [37]. Also, it was observed that the exponential growth phase continued beyond day 12. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Chemical Element** & **In the Monolith Structure (wt.\%)** & **In the Supported Active Phase (wt.\%)** \\ \hline O\({}_{2}\) & 50.04 & 41.26 \\ Mg & 7.32 & 5.45 \\ Al & 17.73 & 18.59 \\ Si & 24.91 & 15.18 \\ Ce & 0 & 16.2 \\ Pt & 0 & 3.33 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical analysis of the catalytic converter by EDS for the monolithic structure and the active film. Figure 3: Micrograph of the monolith structure observed with magnification of 25X (**a**), 200X (**b**) and 700X (**c**). The corresponding EDS analysis of the monolith structure is shown in (**d**); the analysis on the alumina layer is in (**e**) and the active metals Ce and Pt in (**f**). This behavior was probably due to better gas exploitation inside the reactor, derived from a lower CO\({}_{2}\) stripping caused by the low fed gas flow [38]. Meanwhile, for the cultures C2 and C3 with higher flow rates of 0.1 and 0.15 vvm, the maximum biomasses were of 3.27 and 3.4 g\(\cdot\)L\({}^{-1}\), respectively (Table 4). This decrease is explained because it is known that algae are sensitive to shear rates [25]. It has been mentioned that the Kolmogoroff model of isotropic turbulence [39] indicates that a severe damage can occur at relatively large values of the eddy length scale. This model contemplates a parameter of length or size of the eddy where the energy begins to be dissipated by the viscous resistance. The increase of superficial gas and liquid velocities (_Ugr_ and _Ulr_) produces an increase in the global shear rate (\(\gamma\)) observed at the outlet of the gas disperser (Table 3). These values of (\(\gamma\)) were close to the values reported by [PERSON] [29], [PERSON] et al. [40], and [PERSON] et al. [41], especially at low superficial gas velocities of 0.0018 and 0.0036 m/s (Table 5); however, at a greater superficial gas velocity (0.0054 m/s) the difference was significant (15%).