Investigation of phases interactions and modification of drag models for liquid-solid fluidized bed tapered bioreactors

Fluidized bed bioreactors have been increasingly applied to wastewater treatment, since they offer advantages when compared to other reactors, such as high mass transfer and low excess of sludge per unit of reactor volume. Numerical techniques such as Computational Fluid Dynamics (CFD) can be applied to study these multiphase reactors in detail. However, in most of the available literature it is possible to find only models developed to predict gas-liquid or gas-solid interactions, even though the dynamics in anaerobic bioreactors is dominated by interactions between the liquid and solid phases. The aim of the present study is to evaluate the effect of the drag force together with the influence of the lift force and particles collisions on the prediction of bioparticles fluidization. Moreover, a modification to the drag model will be presented in order to achieve better agreement with experimental data. Simulations were performed with a two-phase, transient and turbulent approach, applied to a symmetrical three-dimensional geometry. Results show a redistribution of the bioparticles along the radial direction due to the lift force. Moreover, the use of different restitution coefficient values did not significantly affect the height of the fluidized bed. It was also observed that the usual drag models are unsuitable for predicting the height of the liquid-solid fluidized bed in the case considered, i.e. with bioparticles. A modification of the drag force with a dimensionless number provided good agreement with the experimental data under different operational conditions. (C) 2018 Elsevier B.V. All rights reserved.