Abstract
The flow field in two-phase boiling and gas-liquid Stirred Tank Reactors, STR, was examined in relation to cavity formation and the associated Relative Power Demand, RPD. The cavitation process has been classified in various stages for both radial and axial impellers operating in boiling water. Experiments done in a boiling reactor of 0.45 m diameter agitated by radial and axial impellers showed that the Relative Power Demand, RPD, can be correlated well with the help of a new formulation of Cavitation number named the Cavagitation number. Differences in RPD between gas-sparged and boiling reactors detected during the experiments imply that there are differences in the mechanism of cavity formation in the two systems. These differences are explained by a consistent physical model. The model was shown to be adequate for most of the features of both boiling and sparged reactors reported in the literature. The flow field in a two-phase boiling reactor has been investigated for the first time in the open literature using Laser Doppler Velocimetry, LDV, in conditions where the RPD was as low as 0.55. Measurements gave important data of the way the flow field is changed by the presence of the second phase which forms stable cavities behind impeller blades. The results also quantified the associated flow field changes in the impeller discharge stream and in the bulk of the reactor. The two-phase flow field was quantitatively described with the help of a simple model derived on the basis of energy balances calculated around the impeller. The model estimates liquid flow rates and mean liquid velocities in the two-phase system using data of the corresponding single-phase system coupled with information about the RPD. Knowledge of the void fraction seems to improve the model predictions which were shown to be in fairly good agreement with the data of mean flow rates that exist in the literature. LDV measurements done in a boiling stirred tank of 0.45 m diameter showed that flow rates can be predicted within an accuracy of around 10% and mean liquid velocities with an accuracy better than 25%, even without taking into account void fraction. Model predictions can be used as input values for numerical codes available for the calculation of flows in two-phase stirred tank reactors. Application of the model is independent of geometry and scale.