Liquid phase dispersion in bubble columns operating in the churn-turbulent flow regime

The objective of this paper is to develop a reliable, yet-simple, correlation for the liquid phase axial dispersion coefficient D-ax,D-L in bubble column reactors. In order to develop this correlation we carried out measurements of the centre-line liquid velocity V-L(0) and D-ax.L in bubble columns of diameter D-T=0.174, 0.38 and 0.63 m with the air-water system. The measurements were performed in the churn-turbulent regime of operation with superficial gas velocities U in the range 0.05-0.35 m/s. From the several literature correlations tested, the one proposed by Riquarts [Chem. Ing. Techn. 53, 1981, 60-61] can be recommended for estimation of V-L(0) for air-water systems. The Riquarts correlation however anticipates the dependence of V-L(0) on the kinematic viscosity of the liquid phase. Measurements of V-L(0) were also carried out in the 0.38 m diameter column with Tellus oil as the liquid phase, with a kinematic viscosity 87 times that of water. Comparison of V-L(0) measurements with water and Tellus oil shows a negligible influence of liquid viscosity, leading us to conclude that the best estimation procedure for V-L(0) is to use the Riquarts correlation with properties of the water as the liquid phase. The above conclusions are also supported by Eulerian simulations of the bubble column hydrodynamics. Our measured data on the D-ax.L and V-L(0) were combined to develop a simple correlation: D-ax.L=0.31 V-L(0) D-T; this correlation also provides a good description of experimental data from the literature. (C) 2000 Elsevier Science S.A.