Modelagem e simulação numérica do escoamento trifásico de um reator UASB: influência da fração volumétrica de biogás e do tamanho das partículas de lodo

Abstract

This study aimed to describe the three-phase flow in an upflow anaerobic sludge blanket (UASB) reactor using a combined approach (numerical and experimental), considering the effect of biogas volume fraction (BVF) and sludge particle size (SP) on the hydrodynamic behavior of the system. The analysis was carried out through three dimensional numerical simulations in a computational domain representative of the reactor, using the Ansys CFX software. The modeling employed the Eulerian Eulerian multiphase approach under steady-state and turbulent flow conditions. Three scenarios were simulated with varying BVF values (0.026, 0.13, and 0.26), and based on the validated case (BVF = 0.026), the effect of varying SP diameter (from 0.5 to 5 mm) was evaluated. The experimental phase was conducted in a laboratory-scale UASB reactor, aiming to monitor its operational performance and provide input and validation data for the model. Despite the high COD/BOD ratio in the influent, the reactor exhibited satisfactory stability, although with limited efficiency in organic load removal and biogas production, due to the presence of compounds resistant to anaerobic degradation in the influent. The simulations revealed that changes in BVF did not significantly affect phase distribution or flow velocities along the reactor. However, a linear trend of increasing sludge deformation rates (SDR) was observed in the sludge zone as BVF increased. In the domain near the gas deflector, where recirculation zones were present, the highest SDR values in the system were obtained. Regarding the influence of SP size, a strong linear correlation was found between particle diameter and sedimentation velocity (r = 0.9686). SDR also increased with SP diameter, especially near the reactor walls, due to the increase in shear stress. However, in the region influenced by the gas deflector, SDR values decreased, possibly due to the reduction in the sludge-water interfacial area density and in the water velocity gradient. With respect to absolute pressure fields, no significant variations were identified due to changes in BVF or SP size. On the other hand, the apparent hydraulic head loss remained practically constant across different BVF values and was considered negligible. In contrast, the increase in SP diameter led to greater head loss, indicating higher resistance to flow. The model validation was considered adequate in predicting the amount of biogas reaching the three-phase separator, with a relative error of only 4.2% compared to the experimental data. Therefore, it is concluded that the modeling approach was suitable for representing the hydrodynamic behavior of the reactor. The results contribute to the improvement of UASB reactor design and operation, providing insights for its optimization.