Experiment on solid-liquid suspension in unbaffled stirred tank

Solid-liquid suspension performance in a high solid concentration was investigated. An unbaffled, flat bottom, cylindrical glass vessel of diameter d=0.34 m equaling to the liquid height was used, which was stirred with two different axial flow impellers, a ZHX propeller and a 45° pitched blade turbine(PBT) impeller. The spherical glass particles of 150 μm were chosen as dispersed phase. The particle volume fraction was 20%. The states of suspension were observed and recorded using a digital camera. The just-suspended speed was determined by making visual observations through the base of the vessel illuminated by a laser light source. The effects of impeller off-bottom clearance, type of axial impeller and eccentricity of the impeller in eccentric agitation operation on the solid suspension states, the just-suspended speed and power consumption were examined. The experimental results show that, either co-axial agitation or eccentric agitation, solid particles at the corners of the tank base can not suspend simultaneosly along circular direction. The suspension of solid particles in the stirred tank can be improved by decreasing impeller off-bottom clearance height. Solid particles reach the better suspension when using PBT than ZHX. The just-suspended speed and power consumption increase with increasing eccentricity in eccentric agitation operation. The eccentric agitation has no effective action to improve the suspension performance in a high solid concentration stirred tank.