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Production de biohydrogène par fermentation sombre : cultures, impact des hétérogénéités spatiales et modélisation d’un bioréacteur anaérobie

Abstract : The global energy trends are currently dominated by a massive use of fossil non-renewable energy sources which are progressively depleting. In this way, the production of second-generation biohydrogen production from organic wastes by the dark fermentation process offers, therefore, an attractive solution to diversify the present energy mix. Within this framework, the aim of this work is to investigate the effect of the efficiency of the mixing process on dark fermentation. The conditions of mechanical agitation (mixer type, mixing speed) and the viscosity of the digestate (which depends on the variability of influent substrate concentration) are, indeed, among the abiotic factors that have been the most disregards up to now in this bioprocess. For example, mixing plays a key role because agitation conditions must ensure on the one hand the homogenization of the liquid phase enriched in bacteria, in organic substrate, in soluble metabolites, and in soluble biogas, and in the other hand promote liquid-to-bacteria and liquid-to-gas mass transfer. However, to reach the desired degree of mixing, two constraints must be faced: firstly, an acceptable level of mechanical stress must be maintained on the microbial consortium, and secondly, mechanical power input due to mixing must comply with the economic sustainability of the process. In this work, the combined effects of digestate viscosity and agitation conditions on the fermentative biohydrogen production in the bioreactor were studied first. Experimental results highlighted a significant effect of these factors on biohydrogen productivity which could be expressed as function of the purely hydrodynamic dimensionless Reynolds number and of the prevailing flow regime. Hydrogen production was maximized in the transition region between laminar and turbulent flow conditions. Secondly, experimental measuring methods of mixing time (conductimetric, chemical decolorization and Planar Laser Induced Fluorescence techniques) and mass transfer (dynamic deaeration/aeration) were implemented in the same conditions of viscosity and agitation conditions so as to investigate the possible limiting steps that could explain the trends observed in the mixed cultures. The results proved that mixing and liquid-gas transfer was slower than hydrogen production rate only in the laminar flow regime, while low production rate under turbulent flow conditions might stem from an interaction between turbulent eddies and bacterial aggregates. Then, the flow field in the bioreactor was simulated using a CFD (Computational Fluid Dynamics) methodology and analyzed experimentally using PIV (Particle Image Velocimetry) to determine the characteristic turbulent length scales and to compare them to the characteristic size of the bacterial aggregates. Local measurements confirmed the assumptions made from average values derived from power input data. Finally, a modified ADM1 model (Anaerobic Digestion Model N°1) was developed to simulate the biohydrogen production, accounting for lactate ions and non-ideal mixing, under batch and continuous culture conditions. Simulations fairly agree with experimental data in both modes of cultures assuming perfect mixing condition. As a conclusion, the present work as a whole confirms that digestate viscosity and mixing conditions constitute key parameters that must be considered for process optimization and for the scale-up of dark fermentation.
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Submitted on : Tuesday, December 10, 2019 - 1:44:06 PM
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2018CLFAC098_CHEZEAU.pdf
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Benoit Chezeau. Production de biohydrogène par fermentation sombre : cultures, impact des hétérogénéités spatiales et modélisation d’un bioréacteur anaérobie. Génie des procédés. Université Clermont Auvergne, 2018. Français. ⟨NNT : 2018CLFAC098⟩. ⟨tel-02402287⟩

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