Gas liquid mixing in tall vessels fitted with multiple impellers

The dispersion of air into water was studied in a tall agitated vessel (ungassed liquid depth = 2.4 x vessel diameter) fitted with four different triple impeller combinations:

1. Three identical Rushton turbines
2. One Rushton turbine + 2 pitched blade turbines (both pumping down)
3. One Rushton turbine + 2 pitched blade turbines (both pumping up)
4. One Rushton turbine + 2 LE20 impellers (both pumping down) Two vessel scales, of identical geometry, were studied:
5. 0.61 m diameter, 0.420 m3 liquid volume
6. 0.95 m diameter, 1.570 m3 liquid volume. For a range of impeller speeds and gas flow rates, measurements were made of the power draw for each impeller, gas holdup, liquid mixing rates and mass transfer rates. The impeller combinations were compared on the basis of these parameters. The performance of the individual impellers in each combination were also compared with single impellers mounted in standard geometry vessels. ; In terms of power draw and gas holdup many of the trends observed in standard geometry vessels can be directly applied to tall vessel, multiple impeller geometries. A method of estimating the gassed power draw of multiple Rushton turbines is presented. All impeller combinations studied, produced compartmentalised flow patterns. Aeration was found to have a significant effect on liquid mixing rates, and the magnitude of this effect was a function of the impeller combination used. Liquid mixing with multiple impellers was found to be very complex and geometry specific. Simple staged models, used previously in the literature, were found not to apply to the impeller combinations tested in this thesis. An economic comparison is presented, of the options available for improving mass transfer rates in an existing reactor. For the range of conditions considered, increasing the back pressure in the vessel was generally found to be a cheaper option than increasing the power input from the impellers, increasing the gas flow rate, or enriching the inlet gas.