Suspension of buoyant particles in a three phase stirred tank

Despite many applications in the process industries, the suspension of floating particles in aerated stirred tanks has received little attention. The critical just drawdown agitator speed for different concentrations of floating particles, gas holdup and shaft power have been measured in a vessel of 0.476 m diameter with four baffles and dished base. The aspect ratio of the liquid in the tank is 1.8. Five agitator configurations, each with three impellers on a single shaft, were used in the experiments. Two impeller designs were used, a deep hollow blade (semi-ellipse) disk turbine (HEDT) and a 4-wide-blade hydrofoil impeller, the latter in either upward-pumping (WHU)(WHU) or downward-pumping (WHD)(WHD) mode. Polypropylene (PP) particles, 3 or 4 mm in size and density 900kg/m3, were used in the suspension experiments. Results show that these solids have little effect on the relative power demand (RPD, ratio of gassed power demand to that when ungassed, i.e., RPD=PG/PURPD=PG/PU). Agitators using the radial dispersing HEDT as the bottom impeller have a flatter RPD, than those when an up or down pumping hydrofoil is at the bottom. However, gas holdup decreases with increasing solid concentration. Among the five different impeller combinations, a downward pumping top impeller has a significantly higher just drawdown agitation speed and power consumption, so the WHDWHD mode is not as suitable for the drawdown of floating particles. For any given upper impeller, the just drawdown agitation speeds for different combinations are almost same, though in multi-impeller stirred tanks the gas dispersion is controlled mainly by the bottom impeller. Of these impeller combinations, those using a HEDT as the bottom impeller have the flattest RPD and the highest gas holdup at a given gas flow rate and power input. The optimal impeller combination for to achieve both the drawdown of floating particles and gas dispersion is HEDT+WHD+WHUHEDT+WHD+WHU.