Hydrodynamic Modelling of an UASB Reactor S.P. Pereira1, R.C. Leitão2, I.E. Lima Neto3, and C.A.S. Paiva1 1
Companhia de Água e Esgoto do Ceará - CAGECE, Rua Tomás Lopes, 85, Fortaleza-CE, Brasil. (E-mail:
[email protected];
[email protected]) 2 Brazilian Agricultural Research Corporation, Tropical Agroindustry National Centre. (E-mail:
[email protected]) 3 Universidade Federal do Ceará - UFC (E-mail:
[email protected]) Abstract A pilot scale UASB reactor was modelled in order to evaluate its hydrodynamic behavior. CFD simulations using COMSOL Multiphysics were carried out with a constant influent flow rate of 0.66 L/s and a hydraulic retention time of 6 h. The proposed numerical solution can represent the fluid dynamic behavior of the reactor in terms of velocity fields. In the sedimentation zone, the vertical velocity components remained approximately constant with low values (10m/h). Since this velocity is a critical design parameter, changes in reactor geometry may help to improve this distribution, which can be readily tested with the computational tool in use. Keywords CFD; hydrodynamic; simulation; UASB; wastewater.
INTRODUCTION The performance of a UASB reactor, in terms of chemical oxygen demand (COD) removal and energy yield, is usually governed by two main interrelated factors: microbiological processes and hydrodynamics (Ren et al., 2009). Sufficient mixing is important for distribution of microorganisms and nutrition, inoculation of fresh feed, homogenizing of the material and for the removal of end products of the metabolism (Thorin, 2012). Its hydrodynamics will drive the mass transfer rates, resulting in changes in biological reactions and, consequently, affecting the distribution of biomass in different reactor regions, depending on the type of flow imposed (Carvalho et al., 2008). In addition, hydraulic short-circuiting and stagnant zones may impair the efficiency of the reactors due to decreased net volume and the hydraulic retention time. This work presents the hydrodynamic behavior of an UASB, obtained by mathematical modeling. MATERIAL AND METHODS We used Computational Fluid Dynamics (CFD) tools from COMSOL Multiphysics, a commonly used software platform based on advanced numerical methods to model and simulate multiphysics problems. For this, we used the k- turbulent flow model to solve the equations of continuity, conservation of momentum and turbulence. The k- model is based on semi-empirical equations to model the turbulent kinetic energy transport (k) and its dissipation rate (ε) (Launder and Spalding, 1972). Although a multiphase model was in use, involving liquid, solid and gaseous phases simulation, in addition to the kinetics involved in anaerobic digestion, at this moment only the results regarding the liquid phase hydrodynamics are presented and discussed. The UASB geometry was specified following the Brazilian Standards recommendations (NBR 12209: 2011) and Tchobanoglous (2014), for a flowrate of 0.66 L/s (300 equivalent inhabitants) and hydraulic retention time of 6 h. This reactor is under construction and will be used for calibration and validation of this model. A three-dimensional mesh was adopted with a total of 80,347 tetrahedral elements (Figure 1).
Figure 1. UASB reactor geometry and mesh, scale in meter.
RESULTS Figure 2 shows the intensity and direction velocity. The numerical solution represents well the fluid dynamics behavior of the reactor in terms of velocity fields. In the sedimentation zone the vertical velocity components were approximately constant with low velocity gradients (10m/h) when we compare with Tchobanoglous (2014).
Figure 2. Velocity intensity (m/h) and vector direction (right).
ACKNOWLEDGEMENTS CAGECE, CNPq (process 460460/2014-5), EMBRAPA and UFC.
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