Phase distribution phenomena in two-phase bubble column reactors

Abstract

In a recent paper, Jakobsen, Sannæs, Grevskott and Svendsen (Ind. Eng. Chem. Res. 36(10) (1997) 4052–4074) presented a review of the present status on fluid dynamic modeling of vertical bubble-driven flows. Special emphasis was placed on two-phase flows in bubble column reactors. For these multiphase reactors, the averaged Eulerian multifluid models have been found to represent a trade-off between accuracy and computational efforts for practical applications. Unfortunately, in such multifluid models constitutive relations are needed to describe the phase interaction processes. It was concluded that the general picture from the literature is that time-averaged liquid velocity fields are reasonably well predicted both with steady-state and dynamic models of this type. The prediction of phase distribution phenomena, on the other hand, is still a problem, in particular at high gas flow rates. The present paper gives an overview of the pertinent constitutive relations presented in the literature aiming at a firm mechanistic prediction of the phase distribution phenomena. This includes transversal forces, steady drag forces, surface tension effects, and hydrodynamic bubble–bubble and bubble–wall interactions. Several interaction mechanisms in the turbulence fields like the so-called turbulent mass diffusion, turbulent migration, turbulent drift velocities, anisotropic turbulent drag forces, as well as the interactions between these mechanisms, and the impacts of variations in bubble size and shape distributions are discussed. Various aspects of these relations have been questioned. It is therefore the aim of this paper to compare the capabilities of the existing Eulerian multifluid modeling concepts and parameterizations. The various approaches are evaluated using an in-house 2D Euler/Euler steady-state code. There are several reasons why we choose a steady 2D model for evaluation. First, the model has the advantage of being relatively simple, thus the computational effort required for practical applications involving chemical reactions, and interfacial heat and mass transfer is feasible. Second, the dynamic axisymmetric 2D models do not give much improvement as the flow phenomena possibly missing are believed to be 3D. That is, they do not resolve the swirling motion of bubble swarms. Third, most parameterizations presented in the literature are based on, and have so far only been applied to, 2D models. The results obtained indicate that the various phase interaction parameterizations available in the literature predict very different phase distributions in bubble columns. For operating reactors these deviations will significantly influence the predicted process performance. The results presented here thus confirm the demand for improved modeling including more accurate and stable numerical solution algorithms. Low-accuracy algorithms may totally destroy the physics reflected by the models implemented.