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Étude du transfert de matière gaz-liquide en milieu complexe : application au traitement des eaux résiduaires urbaines

Abstract : The performance of tanks in sewage treatment plants is highly dependent on the oxygen transfer from gas to liquid phase, a vital process for the bacteria responsible for biodegradation. A precise prediction of the mass transfer in these tanks is thus necessary but very difficult, mainly because of the complexity of the liquid phase: It presents a complex rheology and surfactants composition, which drastically decrease the mass transfer coefficient. The aim of this PhD is to better understand the mechanisms responsible for the mass transfer decrease in these situations. For that purpose, the technique of Planar Laser Induced by Fluorescence with Inhibition (PLIF-I) has been used to simplify the system toward the measurement of mass transfer from a single bubble rising in a quiescent liquid. Thanks to this technique, we get access to information about (i) hydrodynamic parameters of bubbles and (ii) mass transfer. For the purpose of running an experiment in a system close to that of a real wastewater treatment plant, measurements of oxygen transfer are done in viscous media and in presence of surfactants. The presence of surfactants has been identified as the parameter that complicates the prediction of the mass transfer coefficient. This fact indicates that mechanisms of mass transfer in presence of surfactants are not yet fully understood. In order to identify the mechanisms, two strategies have been applied. First, for a better understanding of the impact of surfactants at the interface and in its surroundings, a literature review on optical techniques allowing visualization of surfactants at interfaces and in liquid phases has been done. Based on this review, a fluorescent surfactant has been synthetized and visualized in a channel. A reflection phenomenon on the interface has been observed which makes measurements impossible in this zone, but a diffusion coefficient of the surfactant in the bulk has been determined. The second approach dealt with the molecular structure of surfactants. Ten surfactants have been selected to cover a wide variety of properties such as charged surfactants: cationic and anionic, surfactants whose hydrophobic tails have different lengths and nonionic surfactants whose hydrophilic heads have different lengths. Thus, we linked (i) surfactant length, (ii) interface contamination determined by surface tension measurement and adsorption isotherms and (iii) mass transfer coefficient by PLIF-I experiment. Results showed that interface contamination and mass transfer coefficient are closely linked. The hydrophobic chain length has been identified as the parameter impacting the interface contamination and the mass transfer coefficient at constant bulk concentration. On the contrary, the hydrophilic chain length of nonionic surfactants did not impact the mass transfer coefficient, at constant bulk concentration. This study presents measurements ran with close to 300 single bubbles with different surfactants at different concentrations. This large panel allows us to compare experimental results with the Frössling correlation, which predicts mass transfer coefficients from fully contaminated bubbles with hydrodynamic parameters of bubbles. At high concentrations, it seems that this correlation finds a limitation leading to an overestimation of the experimental mass transfer coefficient. These results show that the hydrodynamics of a bubble alone cannot explain the decrease of the mass transfer coefficient of highly contaminated bubbles. Some ideas are proposed to build a model that takes into account more parameters like coverage ratio, adsorption constant and concentration, which is an interesting perspective of this work.
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Submitted on : Monday, February 28, 2022 - 6:12:00 PM
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  • HAL Id : tel-03591765, version 1

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Gaëlle Lebrun. Étude du transfert de matière gaz-liquide en milieu complexe : application au traitement des eaux résiduaires urbaines. Génie des procédés. INSA de Toulouse, 2021. Français. ⟨NNT : 2021ISAT0015⟩. ⟨tel-03591765⟩

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