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Characterization of gas-liquid hydrodynamics and mass transfer in SMX static mixers

Abstract : The mixing of two or more fluids is a rather common operation in all industrial processes. The main target of the mixing is to increase the interface area between phases in order to improve mass and heat transfer and facilitate then chemical reaction. Among the multiphase mixing stands out the gas liquid dispersion. The aim of the present work is to examine a specific type of mixers, namely the static mixer. Static mixers are located into a housing or pipeline to ensure a high blending of fluids. They are usually made by a series of inserts, in turn designed by holes, helical elements and oblique blades. These elements cause local accelerations and stretching of the fluid currents to reach a high mixing efficiency. The wide applications of static mixers in numerous industrial processes require better knowledge of the hydrodynamics in these devices. The gas-liquid flow pattern through a Sulzer static mixer SMXTM mounted in a vertical cylindrical tube was investigated in this study. The main goal was to assess the performance of the Sulzer static mixer SMXTM for gas-liquid applications in industrial processes. Experimental data were collected from two main optical techniques, Backlight Shadowgraph Technique (BST) and Particle Image Velocimetry (PIV). 3D-printed static mixers were manufactured using transparent plastic in order to provide optical access. Three different liquids were used as the continuous phase, namely water, water with SDS and normal-heptane. The liquid phase was kept stagnant during the experiments. Five different lengths of mixers (with 1, 2, 5, 10 and 15 elements respectively) and several gaseous nitrogen flow rates from 1 to 10 l/h were analysed. The behaviour of the simple tube without mixing device, acting like a bubble column, was investigated as a reference, for comparison purposes with the SMXTM. Bubble diameter distributions at the inlet and outlet of the SMX mixers were evaluated. The velocity fields inside the mixers were quantified. The gas hold-up was also measured. The oxygen transfer performance in the SMX static mixer in air/water mixture was assessed by measuring the overall oxygen transferred. The mass transfer coefficient to the interfacial area kLa was determined and proved to be larger in the mixer. Volume-of-fluid numerical simulations of the mixer were performed with OpenFOAM. These 3D simulations were mainly focused on the behaviour of the SMX in an organic system at low gas flow rate (1 l/h). The numerical simulations were satisfactorily validated by experimental results. The comparison and the combination of the numerical and experimental results bring new insight into the flow pattern in a SMXTM static mixer.
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Marco Scala. Characterization of gas-liquid hydrodynamics and mass transfer in SMX static mixers. Chemical and Process Engineering. Université de Lorraine, 2019. English. ⟨NNT : 2019LORR0277⟩. ⟨tel-02558988⟩

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