## 3 Results and Discussion
### Effect of the Hydrodynamics of the Reactor Airlift on the Microalga Growth
\begin{table}
\begin{tabular}{c c c c c} \hline
**Culture** & **Biomass (g\(\cdot\)L\({}^{-1}\))** & \(\mu_{max}\) (d\({}^{-1}\)) & \(P\) (g\({}_{\text{gel}}\) L\({}^{-1}\)-d\({}^{-1}\)) & \(Fc\) (g\({}_{\text{CO2}}\) L\({}^{-1}\) h\({}^{-1}\))** \\ \hline C1 & 3.8 & 0.22 & 0.29 & 0.53 \\ C2 & 3.27 & 0.19 & 0.27 & 0.49 \\ C3 & 3.4 & 0.13 & 0.22 & 0.41 \\ C4 & 1.55 & 0.15 & 0.09 & 0.23 \\ C5 & 1.85 & 0.22 & 0.16 & 0.28 \\ C6 & 3.25 & 0.29 & 0.25 & 0.46 \\ C7 & 4.73 & 0.24 & 0.44 & 0.80 \\ C8 & 4.79 & 0.21 & 0.37 & 0.69 \\ \hline \end{tabular}
\end{table} 
Table 4: Productivity \(P\), CO\({}_{2}\) Fixation rate \(Fc\) and \(\mu_{max}\) of the cultures. Figure 4: Effect of the flow rate on the growth of \(S\). _dimorphus_ and the pH. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Culture** & **_Qg_ & _Ugr_ & _Ulr_ & _Ulld_ & _Ub_ & & & **(y)** \\
**(L-min\({}^{-1}\))** & **(m-s\({}^{-1}\))** & **(m-s\({}^{-1}\))** & **(m-s\({}^{-1}\))** & **(m-s\({}^{-1}\))** & **(m-s\({}^{-1}\))** & **(m-s\({}^{-1}\))** & **(m-s\({}^{-1}\))** & **(l/s)** \\ \hline C1 & 0.137 & 0.0018 & 0.00078 & 0.00032 & 0.367 & 0.0049 & 3121 & 902 & 8.07 \\ C2 & 0.275 & 0.0036 & 0.0015 & 0.00065 & 0.439 & 0.0082 & 6241 & 1805 & 9.42 \\ C3 & 0.412 & 0.0054 & 0.00234 & 0.00098 & 0.337 & 0.016 & 9362 & 2708 & 10.5 \\ \hline \end{tabular}
\end{table} 
Table 3: Effect of the superficial gas velocity _Ugr_ on the hydrodynamic parameters in each of the airlift bioreactors. The global shear rate profile against riser length is shown in Figure 5. It can be seen that in the position of the bubbler there are the highest shear rate (at 10 cm height of the bioreactor) and as the flow of bubbles increases, the shear rate decreases until reaching an average value at a height of 20 cm along the riser tube. As is logical, the global shear rate (\(\gamma\)) increases with the increase of the superficial gas velocity for the cultures in the following order: C3 \(>\) C2 \(>\) C1. This same behavior is observed with the volumetric mass transfer coefficients (\(KLa\)) for CO\({}_{2}\) and O\({}_{2}\) (Figure 6). This means that at higher superficial gas velocities a higher mass transfer of CO\({}_{2}\) could be expected. [PERSON] et al., 2014 [42], using an Airlift reactor with external loop, obtained values of \(KLa\) (O\({}_{2}\)) of 9 to 26 h\({}^{-1}\) at different superficial gas velocities. Also, [PERSON] et al., 2011 [43] used an external Airlift loop reactor, cultivating _Chlamydomonas reinhardtii_ obtained a \(KLa\) of 14.4 h\({}^{-1}\). Figure 5: Global shear rate in the riser tube in function of the length of the reactor for the three gas flows studied. Figure 6: Effect of the superficial gas velocity \(Ugr\) on the volumetric mass transfer coefficient of CO\({}_{2}\) (\(KLa_{\text{CO2}}\)) calculated by the balance of CO\({}_{2}\) and \(KLa\) (O\({}_{2}\)) using the dynamic method. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Culture** & _Ugr_ **(m\(\cdot\)s\({}^{-1}\))** & **[PERSON] [29]** & **[PERSON] et al. [40]** & **[PERSON] et al. [41]** & **Present Work** \\ \hline C1 & 0.0018 & 6 & 5 & 8 & 8 \\ C2 & 0.0036 & 8 & 8 & 14 & 9.4 \\ C3 & 0.0054 & 12 & 12.5 & 20.8 & 10.5 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Comparison of global shear rates \(\gamma\) in [1/s] from other authors respect the values we obtained in cultures C1, C2, and C3 as a function of _Ugr_. [PERSON] et al., 2010 [44], employing a flat panel Airlift reactor with _Spirulina_ sp., obtained a _KLa_ of 31.27 h\({}^{-1}\). [PERSON] et al., 1998 [45], using a concentric tube Airlift reactor with a culture of _Phaodactylum tricornutum_ obtained a _KLa_ of 28.8 h\({}^{-1}\) for a value of superficial gas velocity of 0.0018 m\(\cdot\)s\({}^{-1}\) and biomass of 4 g\(\cdot\)L\({}^{-1}\). These values of _KLa_ (O\({}_{2}\)) were higher than our results (1.54 to 3.71 h\({}^{-1}\)).