## 4 Results and Discussion  
Figure 4a shows the predicted liquid velocity profile in the \(y=0\) m plane at a rotational speed of 500 RPM and an inlet velocity of 0.1 m/s. The loop circulation is generated in the region below the bottom impeller, which is consistent with previous observations [17]. Figure 4b shows the contour of the gas hold-up distribution. Bubbles mostly accumulate near the shaft and the impellers. Near the impellers, the bubbles move slightly to the radial direction as the impeller rotates. Although a number of bubbles are observed just above the sparger owing to the gas injection, there are only a few bubbles far away from the sparger in the region below the bottom impeller. Figure 4c shows the distribution of the dissolved carbon monoxide concentration. Although the gas hold-up is extremely low in the bottom region near the tank wall, a high concentration of dissolved carbon monoxide is observed in the entire region below the bottom impeller. The volume averaged gas hold-up and dissolved carbon monoxide concentration in each region (i.e., above the upper impeller, between two impellers and below the bottom impeller) are shown in Figure 4d,e. The gas hold-up in the region below the bottom impeller is approximately 20% lower than that above the upper impeller, while the volume-averaged dissolved carbon monoxide concentration in the region below the bottom impeller is approximately 70% higher than that above the upper impeller. Figure 3a shows the axial distribution of the turbulent dissipation rate at \(x=0.03\) m with a rotational speed of 500 RPM and an inlet velocity of 0.1 m/s. Note that \(x\) is the radial distance away from a center point of the stirred tank. A high turbulent dissipation rate is observed near the impellers. The turbulent dissipation rate at the tip of the impellers is about 10 times higher than those at other regions. Figure 3b shows the axial distribution of the gas hold-up which was high around impellers as the bubbles were dispersed from the shaft, where bubbles were continuously supplied from inlet sparger, by the impellers. Figure 3c shows the axial distribution of the volumetric mass transfer coefficient that was calculated with the turbulent dissipation rate and the gas hold-up. Figure 3d shows the axial distribution of the dissolved carbon monoxide concentration at \(x=0.03\) m. Although the volumetric mass transfer coefficient is extremely low, except near the impellers, dissolved carbon monoxide still accumulates in this region. As the density of dissolved carbon monoxide is comparable with that of the liquid, the dissolved gas near the impellers was transported by the convective flow generated in a radial direction at the tip of the impellers and downward near the tank wall. The simulations were also conducted with various rotational speeds of the impellers. As shown in Figure 4a,b, with a rotational speed of 300 RPM and an inlet velocity of 0.1 m/s, both a high volumetric mass transfer coefficient and a high concentration of dissolved carbon monoxide are observed near the shaft and the impellers, similar to the case with a rotational speed of 500 RPM. In Figure 4b, the dissolved carbon monoxide concentration in the region below the bottom impeller is about two times higher than that in the region above the upper impeller. Figure 4c shows the axial distribution and volumetric mass transfer coefficient profiles with a rotation speed of 900 RPM and an inlet velocity of 0.1 m/s. Unlike in the case with a rotational speed of 300 RPM, the volumetric mass transfer coefficient is high in the region above the upper impeller due to the wide dispersion of bubbles in the radial direction by convective flow. From Figure 4d, it can also be seen that the dissolved carbon monoxide concentration in the region above the upper impeller is higher than that in the region below the bottom of the impeller because the high mass transfer rate is affected by the high turbulent dissipation rate and high gas hold-up in the region above the upper impeller.