Void Fraction Distributions in Cold-gassed and Hot-parged Three Phase Stirred Tanks with Multi-impeller

Vertical distributions of void fraction in gas-liquid and gas-liquid-solid stirred tanks have been measured in a fully baffled dished base vessel of 0.48 m diameter, using a conductivity probe. The impeller configuration (a hollow half elliptical blade dispersing turbine below two up-pumping wide blade hydrofoils, identified as HEDT+2WHU) recommended in previous work has been used in this work. The operating temperatures were 24°C and 81°C, identified as cold and hot respectively. The effects of superficial gas velocity, agitator speed and the corresponding power input on the local void fraction in two-phase systems are investigated and discussed. Results show that the increasing of agitator speed or gas flow rate leads to an increase in local void fraction at the majority of measurement points in both cold and hot systems. However, the uniformity of gas dispersion does not always increase as the raising of agitator speed and power input. In either cold or hot sparged conditions, the two- and three-phase systems have similar vertical profiles for void fraction, with maxima in similar locations; however, the void fractions are significantly lower in hot sparging than with cold. In cold operation the presence of particles leads to a lower void fraction at most points, although the local void fractions increase a little with the addition of solid particles at high temperature, in good agreement with the global gas holdup results, and the possible reasons are discussed in this paper. This work can give a better understanding of the differences between cold-gassed and hot-sparged three phase stirred tanks.