On mixing and aeration of Rushton turbine stirred tanks

The present study investigates the influence of the fluid properties on the mixing and aeration process of a Rushton turbine stirred tanks. Once the Rushton turbine agitated reactor is designed and developed following common standards, the effects of the viscosity, density, and surface tension on the bubbling process are evaluated. The size of single gas bubbles issued from a submerged nozzle is estimated at constant gas flow rate varying the orifice diameter, gas phase injected, and the liquid medium. Four orifice diameters are used: 0.6 mm, 1.0 mm, 2.0 mm, and 5.0 mm in diameter. As liquid medium, glycerine aqueous solutions at three different glycerine volume concentrations (20%, 40% and 60%), salt aqueous solution (300 g/L of salt) and surfactant solutions using Tween 20 at 0.01 mM and 0.1 mM concentrations, are employed for the experiments. Instead, air and argon are used to analyse the influence of the gas properties on the bubble size. Subsequently, the evaluations of the bubble frequency, the time rising, gas hold up and power consumption are performed considering all the investigated solutions. The mentioned experiments are carried out at three different impeller speed: 0, 185 and 315 RPM. It has been found that the fluid properties have an important role on the bubble size which largely influence the gas frequency, the time rising, and the gas hold up. Finally, the objective is to study qualitatively the shape of the gaseous cavities in relation with the agitator speed and the gas phase flow rate in order to understand in which flow regime the different gas cavities are present. The experiments are carried out using water as liquid medium and air for the gas phase. Two different cases are investigated. In the first case, the impeller speed was varied from 150 RPM to 600 RPM, maintaining constant the gas flow rate at 0.5 L/min. In the second case, the impeller speed has been kept constant at 500RPM and the gas flow rate varied from 0.5 L/min to 1.5 L/min.