Scale-up behaviour in stirred square flocculation tanks

In this study, non-intrusive measurements of local floc size distributions were performed in square tanks of different sizes (5, 7.3, 28 and 560 l). The scale-up behaviour of the floc size distribution was followed with both spatial and temporal resolution. The data was collected in situ using particle image velocimetry and image analysis. The analysis of the data was done using a connected component labelling technique. It was found that the reproducibility of the flocculation system was adequate. The results show that there are large spatial differences in the mean size and the shape of the floc size distribution within the tanks but that the differences decrease as tank size increases from 5 to 28 l. However, increasing the size further to 560 l increases the spatial variation. This indicates a change in the dynamics of the flocculation between different parts of the tank as size changes. The dynamics between two important growth mechanisms were investigated as the size of the vessels and the energy input changed. The first mechanism was orthokinetic flocculation in the impeller stream and the second mechanism was differential settling growth in the corner jet. It was found that the changes in spatial variation of the steady-state mean size and the local variation of the mean size can be explained by the difference in the relative importance between the two mechanisms as the size of the vessel and energy input change. A consequence of this is that, for a certain impeller-tank configuration and a certain energy input, there is a certain tank size that decreases the spatial variation in the system, thus, narrowing the size distribution produced in the tank.