Predicting flow and residence time in alumina digestion vessels

Understanding the flow and residence time behaviour in reaction vessels is of crucial importance to improve product yield or the extraction of mineral species from ores. Digestion vessels, used in the Bayer process, are an example of the latter. Transient Computational Fluid Dynamics (CFD) models were used to investigate the impacts of a variety of turbulence models, along with the effects of mesh refinement on the performance of two different vessel geometries. The main focus of the research work was the efficacy of a variety of turbulence models: two-equation (k-ε, SST), Reynolds Stress (SSG RSM) and Scale Adaptive Simulation (SAS-SST). The predicted velocity profiles are compared with high quality Laser Doppler Velocimetry (LDV) data. Residence times are calculated via a convected scalar (tracer) and Lagrangian tracking of particles. The calculated residence times are compared with salt tracer estimates from experiments. Generally, there was good agreement between experimental data and results obtained from tracing a passive scalar or tracking neutrally-buoyant particles, with the three results being closer when the SAS-SST model was used compared with the k-ε model. In the case of the strongly swirling flow caused by the tangential entry, the tracer was in closer agreement with the experimental data but the Lagrangian tracking results improved considerably with the use of the SAS-SST model. The importance of small-scale turbulence structures in determining the flow profile and residence times of the vessels is highlighted in this study.