Vapour generation from the impellers in boiling stirred tank reactors

The vapour generation in a boiling stirred tank was examined using a 0.2 m i.d. stirred tank with the 3 kW electric heater and multiple impeller systems. Dual and triple six-flat blade disk turbines, four-pitched blade downflow disk turbines and six-concave blade disk turbines were used. With an increase in superficial vapour flow rate, the power consumption decreased. Although the vapour hold-up in the boiling systems continuously increased with increased power consumption or impeller speed, they were considerably smaller than those in the cold gas-sparged systems. Vapour was generated from the heater at lower impeller speeds. With increasing impeller speeds, rather large cavitations were formed behind the impeller blades and most vapour was generated from the top impeller rather than the lower impellers and the heater. In order to examine the change in the nucleation site, the impeller speeds at which the vapour generation from the heater stops and that from the top impeller starts, NHe and NIm, were measured. Although NIm was almost independent of the vapour generation rate and impeller designs, NHe rather steeply increased with an increase in the vapour generation rate. We measured the liquid temperatures near the heater, impeller blades and free surface. They decreased with increasing impeller speed. It was found that the difference between the vapour pressure near the heater under no impeller mixing condition (pr) and that near the vapour generation site (po) based on the measured local temperature has a dependence on impeller speed N ((pr − po) ∝ N0.24).