Improved power and mass transfer correlations for design and scale-up of multi-impeller gas-liquid contactors

The impeller power and volumetric mass transfer coefficient were measured in a pilot-plant single-, double- and triple-impeller vessels of inner diameter 0.6 m. The experimental conditions corresponded with those used earlier in geometrically similar laboratory scale vessel of inner diameter 0.29 m Fujasová et al., 2007, Chem. Eng. Sci. 62, 1650-1669]. The same impeller types and their combinations were used as well as the experimental techniques and forms of the data treatment/correlations, which distinguish bottom and upper section behaviour. Concretely, 23 combinations of the following impeller types were used: Rushton turbine (RT), six-pitched-blade impeller pumping upwards (PBU) and downwards (PBD), Lightnin A315 (LTN) impeller, and Techmix 335 pumping upwards (TXU) and downwards (TXD). Distilled water, representing a low-viscosity coalescent batch, was used as the liquid phase.It was found that the correlations established on the basis of the laboratory scale data might be used to describe the transport characteristics in the pilot-plant vessel. The more precise correlations, based on the data from both the laboratory and the pilot-plant scale vessels have also been established. The specific powers dissipated by impellers under gassed conditions (P_g) were within the interval from 10 to 8500 W m⁻³ in the experiments. General correlations of the relative power down under aeration (P_g/P₀) are presented separately for the bottom and upper sections of the vessel. k_La were measured by dynamic pressure method in the individual vessel sections simultaneously. Their values moved within the interval from 0.002 to 0.21 s⁻¹. The best fit provided correlating the single- and the multi-impeller (double and triple) vessels data separately. Correlation of the k_La data measured in the middle height of the triple-impeller vessel, the method often used in literature, is also included.Of the triple-impeller configurations, 3RT gave the best mass transfer performance. The configurations utilizing the same impeller type have shown that the radial flow impellers provide higher (20 up to 50%) mass transfer coefficients than the axial flow impellers. The combined configurations (i.e., those with an RT impeller in the bottom section) do not achieve the mass transfer performance of 3RT. The k_La values produced by RT+2PBD and RT+2PBU were only 15-20% lower than those achieved using 3RT at the same power input. The 3LTN and RT+2LTN configurations provided the poorest mass transfer coefficients at the same power input, both being up to 40% lower than those of 3RT.