A large variety of industrial processes involving catalytic reactions between liquid and gaseous components arise in wastewater treatment plants. The management of toxic and hazardous wastewater streams using long-term sustainable environmental-friendly technologies remains frequently as the common alternative for the efficient treatment of contaminated industrial and domestic water effluents. Aqueous wastes having an organic pollutant load in the range of few hundred to few thousand ppms are too dilute to incinerate but yet too toxic and concentrated for a biological treatment. Recently, these multiphase systems are linked with the application of catalytic wet air oxidation (CWAO) technology so that three-phase reactors are required for the continuous wastewater treatment operating in trickle flow regime at trickle-bed reactors (TBR), in which gas and liquid flow downward through packed beds to undergo chemical reactions. During this process, reactor scale maldistribution and incomplete external wetting of particles can occur in some extent. In large industrial units, considerable radial temperature gradients can exist when the reaction heat release and maldistribution of the liquid–gas mixture are significant. In order to prevent large temperature differences and to facilitate the temperature control, Computational Fluid Dynamics (CFD) is one of the promising and emergent methodologies to encompass an upgrade of multiphase fluid modeling to allow process engineers to predict and manipulate the desired fluid dynamics in wastewater process equipment.

In this work, a Volume of Fluid (VOF) multidimensional model for the TBR unit is presented to probe the interaction of transport phenomena and chemical reaction. The intrinsic multiphase nature poses a great problem to chemical reactor engineering and design challenges as the overall outcome of such processes depends on the inter-phase and intra-particle heat and mass transport, chemical kinetics, hydrodynamics and thermodynamics. Despite numerous laboratory studies already published in open literature, the industrial application of CWO can be accelerated with the aid of CFD that is to be intended to evaluate multiphase reacting flows.

The detailed information of momentum and mass transfer phenomena is investigated in a fixed bed reactor in which the liquid–gas flow through a catalytic bed comprised of monosized, spherical, solid particles arranged in a cylindrical container of a pilot TBR unit (50_mm internal diameter by 1.0 m length). The VOF method was used to compute velocity field as well as liquid volume fraction distributions. The multiphase flow is assumed to be vertical downward and incompressible, with the mathematical description for the flow of a viscous fluid through a three dimensional catalytic bed based on the Navier–Stokes equations for momentum and mass conservation. The current VOF model was undertaken to simulate the wetting phenomena in trickle-bed reactors providing a better understanding of its liquid distribution. The hydrodynamic validation is accomplished in terms of pressure drop and liquid holdup experimental data taken from the open literature and afterwards computational predictions for the wetting efficiency will be investigated. These computational results allow us to obtain a better understanding of the fundamental physics governing the efficiency of multiphase reactors for advanced wastewater treatment facilities and the CWAO technology deployment and scale-up in commercial-scale TBRs.