Assessment of air entrainment in stirred tanks using capacitive sensors

Abstract

Air entrainment is among the key issues in solid–liquid mixing operations. In this paper, both analytical and finite element methods are employed to study the relationship between the volume of entrained air and the changes of capacitance. With a capacitance sensor installed at the bottom of the stirred tank, an experimental platform is established, which integrates a stirring system and a capacitive detection system. The pitched-blade turbines with four blades (PBT-4) are employed as the impellers in the experimental system and made of Plexiglas. Afterwards, the experiments are conducted with an empty tank to study the influences of propellers on capacitance. The capacitance values are measured at different stirring speeds in the single-phase (deionized water) and two-phase (deionized water and glass beads) stirring systems and applied in the assessment of air entrainment. The results are as follows. In the single-phase system, there is scarcely any air entrained in stirred tanks at low stirring speeds, and air entrainment occurs after the stirring speed (N) reaches a certain value. In the two-phase system, air entrainment is not observed obviously when the solid particles are at rest, but can be found after solid particles are mixed uniformly. Nevertheless, in the transition period between the two regions above, the capacitance becomes increasingly larger along with the growth of N due to the decrease of volume fraction of solid particles in the sensing area, which hinders the assessment of air entrainment. The discussions in this paper may serve as guidelines for solid–liquid mixing processes and can be further generalized for the assessment of air entrainment in solid–liquid mixing operations under other different parameters.