Characterization of flow, mixing and particle suspension in alumina draft tube precipitators of taller aspect ratio

Alumina precipitation is a vital rate determining step in the entire Bayer process. Industry scale draft tube precipitator design (Agglomerators and Growth tanks) in Bayer alumina process has the task to uniformly suspend the alumina hydrate particles with minimum power consumption. Lane (2006) reported that during the operation, there always was a difference in particle concentrations in the overflow. In the previous work, computational fluid dynamics (CFD) modeling was carried out to investigate the reasons for the particle variations in two different aspect ratio tanks (H/T = 1.96 and 2.57) using a uniform momentum source impeller model. Also, it was concluded that more frequent upward surging of turbulent flow in the smaller aspect ratio precipitator is responsible for the higher particle concentration in the overflow. It is important to note that the overflow particle concentration will be uniform if particles are uniformly suspended in the entire precipitator volume; else gradients in particle concentration may prevail in the alumina precipitators. Brown et al. (2014) validated CFD modeling of H/T ≤ 2 aspect ratio tank using both uniform momentum source and frozen rotor impeller model. They observed differences in the draft tube region flow pattern between the uniform momentum source and frozen rotor impeller model. It is important to understand the impact of the actual draft tube flow pattern on the overall flow, mixing and particle suspension. Hence, in the present work, investigation has been made in the taller aspect ratio precipitator (H/T = 2.57) using impeller rotation model. The CFD analysis is made for both batch and continuous models whereas the previous authors carried out the analysis in batch model. The present work brings out the importance of an actual impeller rotation model against the constant mass source model using CFD analysis. Also, the work depicts the important role of draft tube slots and operation mode on changes in the annular region flow pattern and particle suspension. The batch precipitator design exhibits a non-swirl flow pattern and the same design in the continuous operation mode reveals a swirl flow pattern in the annular region of the precipitator. The present study conveys the role of impeller rotation, draft tube slots and the effect of annular region flow pattern on mixing and particle suspension.