Gas-Liquid Systems in Stirred Vessels

Many heterogeneous chemical reactions are carried out in stirred vessels often equipped with six- or eighteen-blade impellers. The gas can be added either at the top by insurgement of the gas in the headspace or by introduction of gas under pressure at the bottom. In the case of surface gas entrainment the minimum stirrer speed for the starting of gas uptake is (Zehner 1996) 

The maximum gas flow rate Vq which can be distributed in the entire vessel is sometimes described by the simple equation 

However, Zehner has shown that there is an additional influence of the diameter ratio d/D. The maximum gas flow rate density Vq = Vo/(D 7r/4) which can be distributed by a six- or eighteen-blade stirrer for a given stirrer speed n is given by 

If the gas flow rate exceeds the value according to this equation the gas rises in form of large bubbles close to the center of the vessel with the result of a poor gas/ liquid interfacial area. The same would happen in bubble columns when the ratio Vq/we exceeds values of approximately 0.1. 

L m Thickness, pore length, travelling distance, macroscale 

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