BUBBLE GENERATION FROM IMPELLER GAS CAVITIES IN NON-NEWTONIAN LIQUIDS
Raj Parthasarathy, Vinko Momiroski, Sati N. Bhattacharya
RMIT
AUSTRALIA

Keywords: non-Newtonian, Agitation, Gas-Cavities

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The efficiency of gas-liquid stirred vessels depends to a large extent on the gas-liquid interfacial area available for the transfer processes. The creation of large interfacial area requires small average bubble size and a high gas holdup. Both of them can be achieved usually by generating small bubbles. Many researchers have recognized that the size and shape of the ventilated gas cavities formed behind the impeller blades determine the bubble size distribution. However, there is a significant lack of experimental data on bubble size distribution at various cavity regimes. The present work aims to establish a fundamental understanding on the relationship between the cavity configurations and the size distribution of bubbles produced by the impeller.

Experiments were carried out in a 0.4 m diameter baffled vessel using Carboxy Methyl Cellulose (CMC) solutions as the liquid phase. The most concentrated solution used had a zero shear viscosity of 900 mPa.s. Impellers used were Rushton (RT), Hollow Blade (HBT), and 45° Pitched Blade (PBT) turbines. Two different underwater probes containing video camera were employed to determine the bubble size distribution in the impeller zone and impeller gas cavities. It has been found that, for the same-gassed impeller power consumption, the HBT leads to smallest average bubble size as compared to other impellers over a wide range of operating conditions. Gas cavities behind the HBT impeller blades under these conditions are identified to be of vortex type. In general, operating conditions leading to the formation of vortex type cavities are found to lead to relatively smaller bubbles for all the impellers. Further data and analysis are presented in the paper.