In vortex chambers, the tangential injection of the gas via multiple slots in the outer cylindrical wall and the central evacuation via a chimney generates a fast rotating flow in which particles can be suspended. Due to their inertia, the particles concentrate near the outer cylindrical wall, forming a rotating fluidized bed. High-G operation allows gas-solid slip velocities exceeding the terminal velocity of the particles in the earth gravitational field while maintaining a uniform and dense particle bed [1]. Intensified gas-solid contact can be combined with intensified gas-solid separation and particle segregation. The latter is of interest for certain applications, such as fine particle coating and granulation [2] or chemical looping or oxy-fuel combustion [3], but had not been studied in detail so far [4]. With a rotating fluidized bed of monoâ?disperse particles, intense particle bed mixing in the radial and axial directions was observed [5], but a clear indication of particle segregation followed from the separation of uncoated and coated particles in fine particle coating experiments [2]. In this study MFIX-DEM simulations are used to quantify segregation with respect to particle size and density in a vortex chamber generated rotating fluidized bed. A 15 cm diameter vortex chamber with 8 x 4 mm gas inlet slots was used. Batch operation with binary particle systems is simulated for different particle size and density ratios and starting from a well-mixed particle bed. Density ratios between 1.125 and 10 and size ratios between 1.06 and 4 have been considered. The total bed height was 2 to 4 cm, depending on the operating conditions. Both the rate of segregation and the steady state quality of segregation were analyzed. Within a relatively wide size and density range, fast (0.2-3 sec depending on the conditions) and efficient segregation takes place, with an inner and slower rotating bed of the lighter particles forming within the outer and faster rotating bed of the heavier particles, each bed only few cm high only. Contamination of the outer bed with lighter particles occurs more easily than contamination of the inner bed with heavier particles and increases with decreasing size or density ratio. With a too high size or density ratio, on the other hand, the inner bed is seen to expand, indicating a decrease of the bed rotation speed, and some bubbling in the inner bed is observed in certain cases. Too much bed expansion can lead to solids losses via the chimney, only observed in extreme cases in this study. The velocity of fine particles is seen to fluctuate due to the formation of a wavy interface between the segregated layers. Porosity plots show that vortex chambers with more than 8 slots have to be used to guarantee particle bed uniformity and improve the vortex chamber performance. Finally, the flexibility with respect to the gas flow rate is discussed, a unique feature of vortex chamber generated rotating fluidized beds.
References
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[2] Eliaers P, de Broqueville A, Poortinga A, van Hengstum T, De Wilde J. High-G, low-temperature coating of cohesive particles in a vortex chamber. Powder Technol. 2014;258:242-251.
[3] Volchkov EP, Dvornikov NA, Lukashov VV, Abdrakhmanov RKh. Investigation of the flow in the vortex chamber with centrifugal fluidized bed with and without combustion. Thermophys Aeromech. 2013;20(6):663â?668.
[4] De Wilde J, Richards G, Benyahia S. Discrete Particle Simulations of Segregation in a Biâ?Disperse Rotating Fluidized Bed in a Vortex Chamber (accepted for publication in Advanced Powder Technology, in press).
[5] De Wilde J. Gas-solid fluidized beds in vortex chambers. Chem Eng Process. 2014;85:256-290.
