Experimental and theoretical analysis of axial dispersion in the liquid phase in external-loop airlift reactors

A model has been developed for the prediction of the axial dispersion coefficient in the riser of an external-loop airlift reactor. It takes into account both radial velocity and gas hold-up profiles as a function of flow regime, but also the two-phase turbulence. It is based on the mixing length model developed by [Vial et al., 2002. Chem. Eng. Sci. 57, 4745–4762] for the flow field prediction and uses a turbulent diffusion coefficient to estimate the influence of two-phase turbulence on mixing. The relations established using experimental data from an airlift reactor have been validated experimentally using a second reactor. A comparison with theoretical dispersion coefficients deduced from CFD calculations and correlations from the literature is also provided. The results show that in all the hydrodynamic regimes, dispersion stems from bubble-induced turbulence, despite the presence of a nearly parabolic liquid velocity profile in the homogeneous regime and marked liquid hold-up profiles in churn-turbulent flow.