Análise de um reator fotoquímico anular usando a fluidodinâmica computacional.

Abstract

Advanced oxidation processes are a promising technology for degradation of organic compounds resistant to conventional treatments such as phenol. Computational fluid dynamics (CFD) has recently emerged as a powerful tool that allows a deeper understanding of photochemical processes in reactor engineering by solving the coupled momentum, mass and radiation balances. This work aimed to investigate the UV/H 2 O 2 process in an annular photoreactor using CFD and a more realistic kinetic model. A progressive approach was used to develop the CFD reactor model. First, the velocity fields were determined for three volumetric flow rates (30, 60 and 100 L h 1). Two lamp diameters were considered to reflect the experimental configuration of the system. Tetrahedral meshes varying form 270,000 to 1,200,000 elements were analyzed to achieve grid independence. For accounting turbulence e↵ects, four RANS models were tested: k ✏, k !, the Shear Stress Transport (SST) and the Reynolds Stress models (RSM). The velocity field was validated through comparison to RTD experimental data. Next step was introducing the mechanism of phenol degradation proposed by Edalatmanesh, Dhib and Mehrvar (2008) into the CFD model. This kinetic model is based on dynamic equations for all species. The fluence rate field was calculated by the radial model and by solving the radiation transport equation with the discrete transfer method. Simulations reproduced experimental data spanning a wide range of initial phenol concentrations, H 2 O 2 /phenol molar ratios and three values for lamp power. It was found that the velocity field depends on the volumetric flow rate: either it maintains a swirling motion through the whole reactor or might develop like a plug flow. The k ✏ model did not represent the RTD data accurately, and the velocity field therefore, since it is not appropriate for swirling flows. The other turbulence models showed good match of RTD, especially the k ! model. Simulations of phenol degradation deviated less than 8% from experimental data. It was possible verified that, due to the swirling inlet e↵ects, reactants got concentrated close to the outer wall and migrated on the lamp direction along the reactor path. High radiation intensities close to the lamp surface created a layer around it where photolysis of H 2 O 2 took place with higher rates. • OH radicals were generated in that layer and transported towards the outer wall by convection. This caused most of phenol to be consumed in the second half of the reactor and accumulation of the radical near the lamp and the reactor outlet, since the pollutant in this area was already oxidized. The discrete transfer method predicted higher incident radiation intensity than the radial model, and higher concentrations of • OH radicals as a consequence. Satisfactory results indicated that CFD was an appropriate tool for analyzing this reactive flow.