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Communication Dans Un Congrès Année : 2010

Bubbly flow through fixed beds contactors. Experiments and modelling in the dense regime

Résumé

Multiphase reactors operated in fixed bed configuration are widely used in petrochemical industry but their hydrodynamic is still not well understood. In a previous paper (Bordas et al, 2006), a new one-dimensional model able to predict the pressure drop and the mean void fraction for bubbly flows in packed beds was proposed. The equations required closure laws accounting for the liquid-solid and the gas-liquid interactions. Compared to previous models, these laws had been completely revisited, in order to better account for the flow dynamics at the pore scale. In particular, it had been demonstrated that (i) in dilute conditions, the bubble size remains of the order of the pore (ii) and that the mean bubble dynamics is somewhat similar to that of a slug, with a relative velocity at meso-scale linearly increasing with the liquid superficial velocity. Besides, that relative velocity monotonically increases with the gas flow rate ratio, a behaviour that is tentatively attributed to the formation of preferential paths for the gas phase. (iii) Based on the motion of a bubble train in capillary tubes, the two-phase flow pressure drop fls scaled by its single-phase flow counterpart fls1φ at the same superficial liquid velocity is predicted to linearly increase with the void fraction, with a prefactor λ evolving with the Capillary number: Ψ=fls/fls1φ=1+λ(Ca)α. Considering the capillary pressure contribution to the pressure drop, one expects λ∼Ca-1/3 [Bretherton 1961]. These proposals were only partly validated and required further confirmation. Thus, new experiments have been performed at LEGI and at IFP in order to cover an enlarged range of flow conditions while gathering all the necessary information to test the model. Attention has been paid to the measuring techniques, both to understand the exact meaning of the measured quantities and to control the uncertainty. So far, the results confirm the postulated dynamic. The model presented assumes that all the bubbles contributing to the void fraction are mobile while in some circumstances, bubbles can be trapped within the bed. Fixed bubbles can also contribute to the pressure drop, and the closure proposed does not take such contributions into account, This question was addressed by way of a thorough data analysis. Data gathered on the void fraction and pressure loss shows that the proposed closures laws for the gas dynamic and two-phase flow pressure drop are still relevant for moderate pressure conditions (up to 10 bars) and gas volumetric ratio β ranging from 20% to 70%. In the range of operating conditions considered (capillary number, gas ratio β), bubbles-bubbles interactions probably limit such blockage. Those interactions would explain also why the size distribution remains controlled by the pore characteristic dimension, through a process of continuous coalescence/breakup. Using these closures, the model sensitivity was studied and its ability to predict the void fraction and pressure loss addressed.
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hal-03004616 , version 1 (13-11-2020)

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  • HAL Id : hal-03004616 , version 1

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Philippe Séchet, Alain Cartellier, Marie Laure Bordas, Christophe Boyer. Bubbly flow through fixed beds contactors. Experiments and modelling in the dense regime. 7th International Conference on Multiphase Flow (ICMF2010), May 2010, Tampa, United States. ⟨hal-03004616⟩

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