Results and Discussion

3.1 Regime identification

The objective was to find a threshold for the transient gas superficial velocity and study the effect of viscosity on this threshold. BSD characteristics and higher order statistics (skewness and kurtosis) were used to identify the operation regimes. Figure 3 shows the probability density function (PDF) of bubble size in aqueous solutions of glycerin (G1 and G3, see Table 1) at various superficial gas velocities. Figure 3 shows that as the gas superficial velocity exceeds 27.6 mm/s in both cases the PDF shape changes from a bell shape into a spike shape; bubble column literature24,40 attributed the aforementioned shift in PDF shape to regime alternation from homogenous to heterogeneous regime. In addition, Figure 3a shows that the lower gas superficial velocities (USG ≤ 27.6 mm/s) exhibit a bimodal shape in the PDF, this feature has been reported in studies of the homogenous operation regime in the bubble column literature.15,21,36,37 In homogenous bubbly flow with no bubble breakage and coalescence events, the injection condition (i.e. pore size and gas superficial velocity) determines the bubble size. The pore size distribution on the sparger is discrete and non-monodisperse, which explains the deviation of bubble size distribution from a truly Gaussian distribution. Higher order statistics (skewness and kurtosis) of the BSD were obtained for further inspection of the operation regime shift. Table 2 presents the Sauter mean diameter (d32 ), (arithmetic) mean diameter (d10 ), standard deviation (RMS), as well as higher order statistics of BSD in G1 at the gas superficial velocities tested. Skewness (S ) and kurtosis (κ ) of the bubble size distribution in Table 2 show a significant deviation from a Gaussian-distribution (S ~0, κ~3) when the gas superficial velocity exceeds USG = 27.6 mm/s.