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Bubble trapping and coalescence at the baffles in stirred tank reactors



Turbulent flow and bubble coalescence in water at the leeward side of a baffle in a stirred tank reactor have been studied with PIV and a high-speed CCD camera. A large stationary vortex was revealed at the leeward side of the baffle. At impeller speeds of 400–600 RPM, the maximum tangential velocity at the vortex was proportional to the impeller tip speed (uθ ≈ 0.27 Utip), giving a pressure difference in the vortex of as much as a few hundred pascal. The main mechanism for coalescence in this area was trapping of bubbles in the vortex, increasing the local hold-up and coalescence probability by forcing the bubbles together. The force on the bubbles resulting from the local pressure gradient makes the bubbles move fast to the centre, within 10–20 ms. The coalescence efficiency was very high and more than 85% of the coalescence occurred within 2 ms. Bouncing of bubbles was mainly seen for large bubbles moving up in the centre of the vortex. The trapping of the bubbles was modeled successfully by a drag coefficient model corrected for nonspherical bubbles and turbulent continuous phase. © 2007 American Institute of Chemical Engineers AIChE J, 2007