Abstract: Solid-liquid suspension characteristics in a flat bottom, cylindrical glass vessel of diameter D = 300 mm equal to the liquid height, stirred with a ZHX propeller are investigated. The particle volume fraction is from 1% to 40%. The effects of particle volume fraction, impeller speed, solid physical properties and impeller off-bottom clearance on the suspension characteristics are analyzed in detail by visual observations using a solid-state laser and a digital camera. The experimental results show that the suspension height is dependent on particle size, solids concentration and impeller off-bottom clearance. The settled particle-bed height at the tank walls is dependent on impeller off-bottom clearance. There are two zones at the tank base where the difficulty of suspension occurs, one just below the impeller and the other at the junction of the tank base and wall. The impeller off-bottom clearance and solid concentration have influence on the position where the final solid particles to be suspended come from. The suspension height increases with the decreasing of impeller off-bottom clearance, but power consumption increases as the impeller off-bottom clearance is reduced. The impeller speed when the settled particle-bed height becomes just invisible, or suspension height reaches 85% of the liquid height can be regarded as the complete off-bottom critical suspension speed in stirred tanks for solid-liquid suspension.

Key words: Critical suspended speed, Solid-liquid suspension, Stirred tank, Suspension height