Study of inclusion re-entrainment in a filter bed

Filtration is widely used in various processes such as water treatment and aluminum casting for the removal of unwanted impurities called inclusions. The deposited inclusions can re-enter the flow as a result of the unfavorable hydrodynamic conditions within the system or flow instabilities, such as flow stop/start periods during casting cycles in the aluminum industry. In this project, the re-entrainment of inclusions was studied as a function of filter-bed length, particle size, inlet inclusion concentration, and inlet velocity. A physical model using water containing PVC particles as inclusions was built. Experiments were carried out under continuous-flow as well as interrupted-flow conditions in this pilot-scale filter. It was found that the smaller bed particles and longer bed length enhance the deposition and reduce the re-entrainment of inclusions. Increasing the inlet velocity has a negative effect on the deposition and increases the re-entrainment. A one-dimensional mathematical filtration model has also been developed, and its predictions were compared with the experimental data from the pilot filter and the plant. This article presents the experimental study, its results, and the comparison between model predictions and experimental data. Applicability of the model to aluminum filtration is also illustrated.