Impact of floating suspended solids on the homogenisation of the liquid phase in dual-impeller agitated vessel

The study relates to the liquid phase mixing in dual-impeller agitated contactors of internal diameter 0.32 m in presence of floating solid particles. High-density polyethylene particles and tap water were used as the solid and liquid phase in all experiments. Mechanically agitated tank was provided with two down-pumping pitched blade turbines (PTD). The mixing time has been measured by conductivity measurements using a pulse technique, whereas the minimum impeller speed for the complete drawdown of floating solid particles, NJS, was determined using Joosten visual method. The effects of a number of variables such as floating solids concentrations, the impeller diameter, the off-bottom impeller clearance as well as the spacing between impellers on the critical impeller speed, mixing time and power consumption values were studied in detail. Previous investigations on this subject have been limited to single-impeller configuration only. The dual-impeller reactor behaviour has been found to be very complex and their design must be considered with a great care. Obtained results show that the presence of floating solids in the liquid significantly affects the mixing time of the liquid phase. The analysed parameters are a strong function of the flow patterns structure created by dual-impeller configuration applied. Using experimental data dimensionless correlation is proposed to predict mixing time beyond complete suspension regime.