Experimental and theoretical analysis of the hydrodynamics in the riser of an external loop airlift reactor

A detailed description of the local hydrodynamic parameters of both liquid and gas phases is obtained in the riser of an external loop airlift reactor (EL-ALR) in order to achieve a better understanding of the local phenomena and to obtain experimental data for CFD validation. The radial and the axial evolutions of these parameters are studied. A valve placed in the downcomer is used to increase the pressure drop due to friction effects, in this section, to study the influence of downcomer and junction geometry on the local flow. A simple analytical model is proposed to estimate the radial evolution of the local shear stress in the riser. Bubble-induced turbulence is shown to prevail, which leads to a strong anisotropy of the Reynolds stress tensor. Local hydrodynamic parameters are shown to be influenced both by the overall liquid circulation, which depends essentially on reactor geometry, and by the bubble-scale phenomena which exhibit little dependence on reactor geometry. Experimental data and results from the analytical model are finally compared with simulations obtained using the CFD code FLUENT® 4.51. An acceptable agreement is achieved only in the homogeneous regime, and the inaccuracy of the simulations at high gas flow rate is shown to be due to a poor estimation of the influence of bubble-induced turbulence.