An experimental study has been made of three-phase mixing and mass transfer in a small stirred vessel (T=0.204 m), which was equipped with a six-blade disc turbine. The effect of solids concentration on impeller power, gas-liquid mass transfer coefficient and gas holdup was investigated under batch-wise and continuous flow conditions, visual observations were also made of the effect that aeration had on particle suspension, vis-a-vis minimum suspension speed. Steady-state power measurements reveal that there is a complex interaction between the solid particles and gas bubbles in the immediate vicinity of the impeller. At high solids concentrations, the apparent viscosity of the three-phase suspension increases. This has a significant effect on the nature and mechanism of cavity formation occurring behind the impeller blades. In the presence of the solids, larger cavities were generally produced. Further confirmation of this is provided by unsteady-state power measurements. The size of gas cavity was evaluated from time constant values estimated from the transient power response. At very high solids concentrations (greater than 30%w/w), the cavities which were formed appear to possess characteristics which are very similar to those produced in high viscosity Newtonian and non-Newtonian systems. The gas-liquid mass transfer coefficient kLa determined using a steady-state 'gassing-out' technique. In general, both kLa and gas holdup were little affected by the presence of a small amount of solids. However, at higher solids concentrations, both of these parameters showed a rapid decrease. This was due to increased bubble coalescence and also poor dispersion of gas brought about by the increase in apparent viscosity and non-Newtonian behaviour of the suspension, With 'non-coalescing' electrolyte solution, the variation of kLa and gas holdup with solids concentration showed similar trends to those obtained for coalescing tap water. Under continuous flow operation, with solid and liquid, it was only possible to achieve iso-kinetic sampling of the vessel contents in a limited number of cases where the flow exit was located opposite the mid-plane of the impeller. For the three-phase system, iso-kinetic sampling was not achieved. Increasing slurry flow to the vessel reduced the solids holdup, but caused the impeller power consumption to increase. The solids concentration in the vessel was also higher in the three-phase case when the exit flow pipe was located above the impeller mid-plane level. Gas holdup was not significantly affected by virtue of the position of the exit flow pipe or the velocity of the exit flow.
|Date of Award||1983|