with the volume of each bubble \(V_{Bubble,\ \ i}\) and the volume of
one impeller compartment \(V_{\text{Compartment}}\) (Figure 1). To
distinguish between the ratio of the actual gas passing the impeller to
the gas circulating in the compartment and bypassing the impeller, the
gas hold-up in the volume of each impeller was calculated. Eq.2 was
applied replacing\(\ V_{\text{Compartment}}\) by\(V_{\text{Impeller}}\), the volume of the cylinder around each
impeller, with the dimensions of the impeller diameter and the impeller
blades height (Figure 1). To examine the local gas load of each
impeller, the effective aeration rate was introduced. This rate is equal
to the radial gas flow leaving the impeller region as dispersed gas. The
flow of gas dispersed by each impeller was estimated by multiplying the
velocities of the bubbles in the lateral surface of the impellers
cylinder with the void fraction of the respective cells. Thus, this key
figure reflects the axial aeration at the impeller by the gas bubbles
recirculating from top and below of the impeller, and the radial gas
dispersion process.