4。2。 Gas hold-up

4。2。1。 Gas dispersion patterns

Gas hold-up is a direct indication of the effective gas–liquid interfa- cial area for mass transfer of the stirred vessel。 Experimentally captured gas dispersion images at the operating conditions studied here are firstly displayed below to test the superiority of DRT in dispersing the gas。 Fig。 3 indicates that the gas is in the fully dispersed regime for both impellers working under the experimental conditions。 What in common for both impellers is that, little gas is dispersed near the vessel bottom。 A large amount of bubbles are observed in the region adjacent to the impeller, whilst a small number is present in regions between the upper and lower impeller, especially when the impeller rotational speed is low。 At larger speed, the situation can be improved。 By compar- ison, although there are no obvious visual differences, a more uniform dispersion state is achieved when DRT is employed。 But at the most

extreme conditions studied (N = 700 r·min−1, Q = 0。6 m·h−1), this

advantage seems to disappear。

4。2。2。 Gas hold-up distribution

Fig。 4 shows the gas hold-up distributions obtained from the CFD simulations at t = 30 s。 Results obtained under two operating condi- tions (N = 600 r·min− 1, Q = 0。6 m3·h−1 and N = 300 r·min−1, Q = 0。4 m3·h−1) are presented。 When the impeller rotational speed is small, the gas mainly dispersed in the impeller discharge flow regions whether SRT or DRT is employed。 The situation is much better with the

increase of the impeller rotational speed。 Note that for both impellers the gas hold-up in region between the two adjacent impellers are not so high compared with other regions in the vessel。 This may be due to the fact that the impeller spacing is too large。 With this configuration, the upper and lower impellers act independently。 According to the study of Pan et al。 [34] and Khopkar and Tanguy [35], this belongs to the parallel flow pattern, which has been proved to be adverse for gas dispersion。 Studies show that there is pressure difference between each side of the impeller blade。 The disc of the Rushton type impeller has an advantage in preventing gas from passing through the lower shear region and forces the gas flow through the high shear impeller tip region [36–38]。 However, dispersion of the accumulated gas remains to be a problem。 This will be solved in our future work by retrofitting the standard  Rushton impeller。

In addition, quantitative comparisons of the time-averaged gas hold- up under different operating conditions were also performed。 Here, eight radial positions, i。e。, 20、40、60 and 80 mm from the impeller shaft in the left and right half of the vertical plane passing through the

axis were selected。 Distributions of the average gas volume fractions in the axial direction are presented in Figs。 5 and 6。 At all the radial positions, the gas hold-up is generally higher when DRT is used。 The av- erage gas volume fractions are listed in Table 2。 An average increase

(a1) N=300 r·min-1, Q=0。4 m·h -1       (b1) N=300 r·min-1,  Q=0。4 m·h-1

(a2) N=500 r·min-1,  Q=0。4 m·h -1 (b2) N=500 r·min-1,  Q=0。4 m·h-1

(a3) N=700 r·min-1,  Q=0。6 m·h-1 (b3)  N=700 r·min-1,  Q=0。6 m·h-1

Fig。 3。 Gas dispersion images of (a) SRT and (b) DRT。

N=600 r·min-1, Q=0。6 m·h-1

(a)