Chemical Engineering and Processing 50 (2011) 181–188 Contents lists available at ScienceDirect Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep Effect of some solid properties on gas–liquid mass transfer in a bubble column P. Mena a , A. Ferreira a,b , J.A. Teixeira b , F. Rocha a,∗ a LEPAE-Laboratory for Process, Environmental and Energy Engineering, Departamento de Engenharia Qumica, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal b IBB-Institute for Biotechnology and Bioengineering, Centre of Biological Engineering, Universidade do Minho, Campus de Gualtar, 4710-057 Braga, Portugal article info Article history: Received 25 June 2010 Received in revised form 18 October 2010 Accepted 30 December 2010 Available online 11 January 2011 Keywords: Bubble column Mass transfer Absorption Multiphase reactors Particles Slurries abstract The knowledge about the effects of solids on gas–liquid systems and the respective physical mech- anisms are not yet totally clarified. In this work, the effect of the solids on the mass transfer characteristics in a bubble column was studied experimentally for the systems air/water/expandable polystyrene (EPS) beads and air/water/glass beads. Volumetric liquid side mass transfer coefficient, k L a, was determined under different solid concentrations (up to 30 vol.%), superficial gas veloci- ties (up to 2.7 mm/s) and mean diameters (1100, 770 and 591 m for EPS and 9.6 m for glass beads). The presence of EPS solids affects negatively k L a being this effect more pronounced for the smaller particles. Also, a decrease in k L a occurs when the solid loading increases. Experiments done with large polystyrene particles (d p ≥ 591 m) contaminated with very fine EPS particles (d p ∼ = 0.1 m) indicate that very fine particles play an important role on gas–liquid mass transfer. Mass transfer experiments in a hollow glass spheres three-phase slurry showed a dual effect of solids loading on k L a, contrarily to what happens with the previous particles. These results can be associated with the different surface properties of the particles studied. An empirical correlation for k L a on the experimental variables was developed. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In many operations of chemical industry processes, one or more components of a gas phase are absorbed into a liquid phase. The phase contacting is often required to trigger reactions among components of the two phases. The complexity and diversity of industrial processes implied that different types of gas–liquid con- tactors were developed and constructed, such as, bubble columns, pipes/tubes, mechanical agitated tanks, packed columns, plate/tray columns, spray towers, jet (loop) reactors, tubular/ventury ejec- tors and motionless mixers [1]. Bubble columns are contactors in which a gas or a mixture of gases in the form of a dispersed phase of bubbles moves in a continuous liquid phase. In the liq- uid, a suspended or fluidized, reactive or catalytic solid can also be present. Thus, in fact, two or three-phase bubble columns exist [2]. These columns can be of different types such as sin- gle stage, multi stage, multi channel, with motionless mixers, loop reactors, jet reactors, downflow bubble columns, three-phase fluidized-bed reactors and slurry reactors. Their applications are not limited to the chemical industry; they can be found in biochem- ∗ Corresponding author. Tel.: + 351 225081678; fax: +351 225081632. E-mail address: frocha@fe.up.pt (F. Rocha). ical operations, separation of mixtures by rectification, absorption, wastewater treatment [3] and petrochemical industry. Bubble columns are also gaining increasing importance in the field of biotechnology [4]. Parameters such as phase holdup, flow regime, bubble size dis- tribution, coalescence characteristics, gas–liquid interfacial area, interfacial mass transfer coefficients, heat transfer coefficients and dispersion coefficients influence the bubble column reactors design. The mass transfer coefficient is believed to be the most important design variable, followed by the gas holdup and the axial dispersion coefficient [5]. In three phase systems, the presence of solids affects the gas–liquid mixture in different ways: bubble formation and rise [6,7], axial and radial profiles [8,9], mixing and dispersion, mass transfer [10–13], and gas holdup and flow regimes [14,15]. Despite all the research efforts, the knowledge about the effects of solids on gas–liquid systems and the respective physical mecha- nisms are not yet clarified. In the present work, volumetric mass transfer coefficients, k L a, were obtained for different gas flow rates, solids loading and size. These results were used in order to develop an empirical correlation for k L a, taking into account the different experimental variables. The influence of fine parti- cles on k L a was also analyzed, in order to clarify some unexpected results. 0255-2701/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2010.12.013