Kinetic Modeling and Trickle-Bed CFD Studies in the Catalytic Wet Oxidation of Vanillic Acid Rodrigo J. G. Lopes, Adria ´ n M. T. Silva, and Rosa M. Quinta-Ferreira* Department of Chemical Engineering, UniVersity of Coimbra, Rua Sı ´lVio Lima, Polo II-Pinhal de Marrocos, 3030-790 Coimbra, Portugal Aiming to examine the catalytic wet air oxidation process in three-phase reactors, we investigated a slurry reactor for kinetic studies and a pilot-unit trickle-bed reactor, considering reaction aspects as well as the transport mechanisms involved in the treatment of an aqueous vanillic acid solution, which is a compound typically found in olive mill wastewater. Kinetic studies were performed to bring up the lumped kinetic parameters in terms of total organic carbon (TOC) over a Mn-Ce-O laboratory-made catalyst. A hydrodynamic model for the prediction of pressure drop and liquid holdup for a trickle-bed reactor has been developed by means of computational fluid dynamics (CFD) according to data taken from the open literature. First, single- phase flow pressure drop was studied in a region of flow rates that is of particular interest to trickle-bed reactors (10 < Re G < 400), and it was demonstrated that the Eulerian model is able to predict reasonably the pressure drop of single-phase flow over spherical particles when the Ergun equation adjusts pressure drop measurements within 10% on average. The two-phase flow operating regime is then investigated, and the CFD Eulerian model predicts very well liquid holdup in the range of gas flows studied (G ) 0.10-0.70 kg/(m 2 s)). Finally, CFD runs were performed in unsteady state for the catalytic wet air oxidation of the aqueous vanillic acid solution. TOC profiles indicated that complete reduction of organic matter was achieved at space times up to 1.5 h. Moreover, CFD demonstrated the considerable effect of temperature, whereas the air partial pressure only has a minor influence. 1. Introduction Nowadays, the increasing environmental concerns and restric- tive legislation strongly require the development of suitable technologies for treatment of polluted wastewaters. Particularly, during the production of olive oil, significant amounts of liquid effluents are generated from the water and olive juice added to the process, containing unrecoverable oil. High volumes of olive oil mill wastewater (OOMW) are produced each year (10-30 × 10 6 m 3 /year), with chemical oxygen demand (COD) charges between 45 000 and 170 000 mg/L being the large charge of phenolic compounds contained in these wastewaters and one of the major obstacles to achieve the detoxification. 1 Technologies for OOMW treatment have been reviewed elsewhere, 2,3 including several physicochemical, biological, and combined processes resulting in considerable organic load and toxicity abatement. Biological processes, aerobic and anaerobic, including anaerobic codigestion with other effluents and com- posting, are predominant in the treatment of OOMW, and it is ideally suited for low pollutant concentrations, with maximum concentrations up to 50 ppm. 2-4 Biodegradation and incineration are two typical examples of reactive destruction technologies. However, both these methodologies have their limitations and are not always the optimal solution. Incineration is ideal for highly concentrated liquid streams, typically around 350 000 ppm or higher COD content when the energy requirements become self-sustaining. 3 Neither process is optimal when the toxic organic compounds concentration falls between the extremes. Alternative destruction technologies have also been studied in the field, including noncatalytic and catalytic wet air oxidation (CWAO). The catalyst allows milder operating conditions than WAO while yielding similar if not superior kinetic performance. CWAO research has been conducted on a variety of organic compounds using numerous catalysts with different results, and it is known to have a great potential in advanced wastewater treatment facilities. 2-5 Trickle-bed reactors (TBRs) are widely used for heteroge- neous catalyzed reactions between gas and liquid reactants, such as hydrogenation, oxidation or partial oxidation, and detoxifi- cation of liquid effluents; such systems are frequently used in petrochemical and chemical processes, wastewater treatment, and biochemical and electrochemical processing. 6 In these reactors, gas and liquid phases flow cocurrently downward through a fixed bed of catalyst particles. Generally, three-phase fixed-bed reactors can operate in hydrodynamically different regimes whose boundaries depend on gas and liquid superficial velocities, catalyst bed, and fluid properties. TBRs are charac- terized by high catalyst loading per unit volume of the liquid, and the low-energy dissipation rate makes them preferable to slurry reactors because of the higher calorific capacity. Under continuous gas flow and low liquid flow rate conditions, the trickle-flow regime exists in which the liquid phase moves in drops or in small rivulets through the catalyst bed. With increasing gas and liquid loading through the bed, so-called spray, pulsing, and foaming hydrodynamic regimes are encountered. 6-8 Trickle- and pulse-flow regimes are of particular industrial interest because of the higher catalyst surface utiliza- tion. The following studies in the paper are focused on the trickling regime under unsteady-state operation conditions in order to investigate the start-up behavior of the TBR unit and to evaluate its influence in hydrodynamic parameters. Heterogeneous catalysts used in TBR reveal important advantages for the wet oxidation technology, since in homo- geneous operation, a posterior metal recovery step is needed. Several important aspects with respect to the influence of external and intraparticle mass transfer limitations and partial wetting of catalyst particles have been studied in the literature, * Corresponding author. Phone: +351-239798723. Fax: +351- 239798703. E-mail: rosaqf@eq.uc.pt; rodrigo@eq.uc.pt. 8380 Ind. Eng. Chem. Res. 2007, 46, 8380-8387 10.1021/ie070009a CCC: $37.00 © 2007 American Chemical Society Published on Web 07/21/2007