Gas-liquid mass transfer around Taylor bubbles moving in a meandering millimetric square channel was locally visualized and characterized in the present study. For that, the colorimetric technique proposed by Dietrich et al. (2013) was implemented. With this technique, the evolution of equivalent oxygen concentration fields in the liquid slugs passing through one and several bends was firstly described. In particular, it was observed how the flow structure (recirculation zones) inside the liquid slugs were twisted and split by the periodic bends (centrifugal effect), until reaching, after several bends, a uniform O2 concentration inside the liquid slugs. The influence of the “turning point”, joining two “straight” sections of meandering channel was also highlighted: a slowing down of the gas-liquid mass transfer was clearly shown. Volumetric mass transfer coefficients were determined at last by fitting the experimental axial profiles of averaged oxygen concentrations in the liquid slugs (before the turning point) with the ones predicted by a classical plug-flow model.
Abstract: The present work provides an experimental study of the effect of the glycerol concentration on removal performances in a Submerged Aerobic Fixed-Film (SAF) Reactor performing in batch mode. This study consisted in adding various initial substrate concentrations of synthetic solutions (glycerol) in order to assess their effects on biodegradation kinetics and then on bacterial cells adaptation. For this purpose, three series of tests were made. The first two series were carried out with initial Chemical Oxygen Demand concentrations respectively of 330 mgO2/L (SL) and 1120 mgO2/L (SH). The third series (SS) was carried out with gradually increasing concentrations, in which initial COD values were 480, 590 and 760 mgO2/L. It has been found that the adaptation time decreased when the initial concentration increased. But, when synthetic solutions were renewed for a new adaptation, better performances were observed. Increasing initial COD solutions (SS test) showed excellent performances; in fact, it seems combine both, the advantage of low substrate concentration (better adaptability, interesting time for adaptation – case of SL test) and of a high concentrated synthetic solutions (better treatment performance and high biodegradation kinetics constant – case of SH test).
The aim of this work is to study continuous counter-current absorption of Volatile Organic Compounds (VOCs) by an oil–water emulsion. This process enables the treatment of hydrophilic and hydrophobic VOC within a gaseous effluent emitted by chemical or food processing industries. Toluene was chosen as the pollutant in this work because of its hydrophobicity and its widespread use in chemical industries. As organic solvents for VOC absorption, vegetable oils were proposed for the treatment process to reduce the impact on the environment. The absorbing oil was selected for its good absorption capacities, its chemical and thermal stability and its low cost. To test their impact on the operational efficiency of the absorption process, numerous parameters were varied, such as liquid and gas flow rates, temperature and nozzle type. Thermal regeneration (120 °C) of the oil was proposed and tested on the experimental device. No impact on efficiency was noted, even after several regenerations. Finally a model was determined to predict the effects of operational conditions on the absorption efficiency of an emulsion.
Visualization of mass transfer is a powerful tool to improve understanding of local phenomenon. The use of an oxygen-sensitive dye (colorimetric technique) (Dietrich et al., Chem Eng Sci. 2013; 100:172–182) has showed its relevancy for locally visualizing and characterizing gas–liquid mass transfer at different scales (Kherbeche et al., Chem Eng Sci. 2013; 100: 515–528; Yang et al., Chem Eng Sci. 2016; 143:364–368). At present, the occurrence of a possible enhancement of the gas–liquid mass transfer by this reaction has not been yet demonstrated. This article aims at filling this gap by evaluating the Hatta number Ha and the enhancement factor E associated with the oxygen colorimetric reaction when implementing in milli/micro channels. For that, as data on the kinetic of the colorimetric reaction are seldom in the literature, the reaction characteristic time was first estimated by carrying out experiments in a microchannel equipped with a micromixer. The diffusion coefficients of dihydroresorufin and O2 were then determined by implementing two original optical methods in a specific coflow microchannel device, coupled with theoretical modelling. The knowledge of these parameters enabled at last to demonstrate that no enhancement of the gas–liquid mass transfer by this colorimetric reaction existed. Complementary information about the reliability of the colorimetric technique to characterize the gas–liquid mass transfer in milli/micro systems was also given.
by B. Lekhlif, G. Hébrard, N. Dietrich, A. Kherbeche, J. Echaabi
Afr. J. Environ. Sci. Technol.
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By : N. DIETRICH, J. FRANCOIS, M. JIMENEZ, A. COCKX, P. GUIRAUD, G. HEBRARD
Abstract: A fast method is proposed for determining the oxygen gas-liquid diffusion coefficient from measurements of the fluorescence quenching behind a bubble. The approach consists of capturing pictures of concentration field at micro-scale in the laminar bubble wake. The Gaussian concentration profiles measured in the wake are demonstrated to be systematically equivalent to an instantaneous plane diffusion case. It permits to evaluate accurately the gas-liquid diffusivity in a very short time around one second.
Corresponding Author: Nicolas Dietrich
E-mail: Nicolas.Dietrich@insa-toulouse.fr Phone number : 335-61-55-97-81 Fax : 335-61-55-97-60
By Mélanie Jimenez, Nicolas Dietrich, John R. Grace, Gilles Hébrard
Powerful techniques, based on the Planar Laser Induced Fluorescence (PLIF) technique, are deployed to locally visualize and quantify the impact of surfactants in wastewaters on hydrodynamics and oxygen mass transfer. Bubble diameter, aspect ratio, rise velocity, contamination angle, as well as flux, flux density, liquid side mass transfer and diffusion coefficients of transferred oxygen are determined based on these techniques applied in the wake of rising bubbles of diameter 1 mm and through planar gas/liquid interfaces. The initial experiments were performed in demineralized water containing small amounts of surfactant. Different concentrations of surfactant were added to finally reach the Critical Micelle Concentration (CMC). Bubbles have classically been found to be more spherical with a reduced rise velocity in the presence of surfactants up to the CMC. Above the CMC, these hydrodynamic characteristics were found to be almost constant, although the oxygen mass transfer decreased due to the presence of surfactants. Experimental results were markedly lower than predicted by the well-known Frössling equation with rigid surfaces. This is believed to be caused by a barrier of surfactants hindering the oxygen mass transfer at the interface. Similar hindrance of oxygen mass transfer applies to waters from sewage plants (filtered raw water and treated water), making accurate design of aeration tanks difficult.
Visualization of mass transfer around a bubble at formation in a flow-focusing microreactor of 500µm square channel.
By Lixia Yang, Nicolas Dietrich, Karine Loubière, Christophe Gourdon & Gilles Hébrard
To be submitted (2014)