Available online at www.sciencedirect.com Chemical Engineering and Processing 47 (2008) 1190–1200 Effect of solid characteristics on hydrodynamic and mass transfer in a fixed bed reactor operating in co-current gas–liquid up flow J.G. Garcia Maldonado a , D. Bastoul a , S. Baig b , M. Roustan a , G. H´ ebrard a,∗ a Laboratoire d’Ing´ enierie des Proc´ ed´ es de l’Environnement (LIPE, EA 883), D´ epartement G´ enie des Proc´ ed´ es et de l’Environnement, Institut National des Sciences Appliqu´ ees, 135 Avenue de Rangueil, 31077 Toulouse Cedex 4, France b Degr´ emont, 183, Avenue du 18 Juin 1940, 92508 Rueil Malmaison, France Received 2 June 2006; received in revised form 13 November 2006; accepted 11 February 2007 Available online 1 March 2007 Abstract In order to understand better which parameters control the mass transfer efficiencies in biofilters, hydrodynamic and mass transfer parameters have been investigated for seven different packings in a three phase fixed bed reactor operating in co-current gas–liquid up flow. For this purpose, the packing characteristics (bed porosity, particle size and particle shape) and hydrodynamics parameters have been determined. Thus, the pressure drops, global gas hold-up, static gas fraction, slip velocities and bubble size have been investigated. For each type of solid, volumetric oxygen mass transfer coefficients have been measured under different operating conditions of gas (air) and liquid (tap water). The experiments were performed in a cylindrical PVC column with co-current up-flow of gas and liquid at laboratory temperature and atmospheric pressure. Air was injected at the bottom of the column using a porous disc diffuser. The superficial gas velocity ranged from 2.3E−3 m/s to 2.5E−2 m/s, the superficial liquid velocity varied from 1.2E−3 m/s to 3.3E−3 m/s. The column of 0.15 m in diameter and 4 m height was filled with packing. The following values were determined: • Pressure drop through the bed height was measured using differential manometers. • The average values of the global gas hold-up and static gas fraction were determined in the experimental set up by a new technique which is based on a monitoring of liquid flow. • The bubble sizes and slip velocities were determined by image acquisition and data treatment systems on a particular glass cell. • The volumetric oxygen mass transfer coefficient was determined by gas and liquid mass balance techniques. These different techniques applied on the fixed bed bubble column enabled for each type of solid a complete characterisation of hydrodynamic and mass transfer to be described. These experimental tools were used to compare and evaluate the influence of solid characteristics on the reactor performances and to initiate a mass transfer model. © 2007 Elsevier B.V. All rights reserved. Keywords: Gas hold-up; Oxygen mass transfer; Pressure drop; Bubble diameter; Packing effect; Fixed bed reactor; Biofilters 1. Introduction Usually called biofilters, fixed bed reactors are mainly used in the aerobic treatment of urban wastewaters for carbon, nitrogen and suspended particle elimination. When put to this use, the liq- uid is the water to be treated, the gas phase provides the oxygen required for the biological process and the solid phase corre- ∗ Corresponding author. Tel.: +33 5 61 55 97.89; fax: +33 5 61 55 97 60. E-mail address: hebrard@insa-toulouse.fr (G. H´ ebrard). sponds both to the carrier of biomass (for carbon and nitrogen elimination) and to the filter media (for suspended solid reten- tion). Generally, the liquid and the gas are injected at co-current in the lower part of the column. The superficial gas and liq- uid velocities commonly used in biofilters are relatively low (up to 10 × 10 −3 Nm/s and 5 × 10 −3 m/s, respectively). Biofilters combine compactness and high removal efficiencies. However, these efficiencies are dependent on the amount of oxygen that it is possible to supply by aeration systems to the bacteria which are attached to the suspended particles. Thus, in order to improve the design of the biofilters, it is necessary to understand the effect 0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.02.013