Numerical analysis of liquid-solid flow in tapered and cylindrical fluidized beds for wastewater treatment and biogas production

Highlights: • Biofilm thickness is affected by the hydrodynamics of bioreactors. • Results indicate that the tapered fluidized beds are better than conventional ones. • Higher taper angles resulted in lower energy dissipation rates. • Biofilm thickness must be smaller in the lower part of the bioreactor. • Reactor project and operation can be optimized by knowing details of the flow. -- Abstract: One of the most promising technologies employed for biological wastewater treatment and biogas production in recent years is the fluidized bed bioreactor, in which microorganisms immobilized on an inert particle surface convert organic effluents to biogases like methane and hydrogen at high rates. In order to ensure an appropriate biofilm thickness, it is important to know the hydrodynamics of these bioreactors. This study is focused on the investigation of the hydrodynamic characteristics of cylindrical and tapered fluidized bed reactors and evaluation of the influence of hydrodynamic variables on biofilm formation using Computational Fluid Dynamics tools. The Eulerian-Eulerian two-phase, transient, turbulent flow model was simulated in a cylindrical fluidized bed and also in 2.5° and 5° tapered fluidized beds, under conditions found in anaerobic fluidized bed bioreactors. Simulations were performed for a wide range of liquid superficial velocities using two-dimensional axisymmetric meshes. Results indicated that the tapered fluidized beds are superior to conventional cylindrical fluidized beds since a more uniform axial distribution was obtained, thereby providing a more stable operation. Better recirculation rates and a more uniform distribution of energy dissipation rate were observed for the 2.5° tapered fluidized bed operating under 7.83 cm/s. These characteristics induce the formation of particles with uniform biofilms. The tapered fluidized bed with a 5° taper angle operating under low superficial velocities provided lower energy dissipation rates, which can result in a lower erosive effect acting on the biofilm, generating thicker biofilms. Results presented in this paper emphasize the importance of using more sophisticated models, which can predict the details of the flow. These models enabled the identification of the geometric and operational conditions that can improve the operation of fluidized bed bioreactors.