Local Void Fraction And Bubble Size Distribution In A Gas-Liquid Multi-impeller Stirred Tank

The gas-liquid stirred tank reactors are widely used in various processing industries. The previous studies in stirred tank with multi-impeller mainly focus on the macro scale properties, such as gassed power demand, the critical suspension speed and the overall gas holdup, etc.. In the recent years, some researchers have studied on the local void fraction distribution of HEDT+2WHU, however, the data for the bubble size distributions are still not sufficient. The research of HEDT+2WHD agitators mainly focused on the macro- characteristic.All experiments were carried out in a dished base stirred tank of 0.48 m diameter. The impeller combination consisted of a half elliptical disk turbine (HEDT) as the bottom and two up-pumping wide-blade hydrofoils (WHU) above. The bubble size distributions and their relationship with the superficial gas rate and the agitation speed in HEDT+2WHU stirred tank were carried out, it could give a better understanding on the gas-liquid dispersion in stirred tank. The results show that the spatial distribution of bubble size of gas-liquid stirred tank system is seriously uneven. Under the room and high temperature conditions, the bubble sizes increase with the increasing of the gas rate, however, the bubble sizes increase slightly while the agitation speed decreases. Temperature also has an important influence on the bubble size distribution. The overall bubble size significantly increases with the increasing of the temperature.The distribution of the local gas holdup with HEDT+2WHD impeller was also investigated in this work. The results show that the local gas holdup also significantly increases with the increasing superficial gas rate. The comparison with the local void fraction of HEDT+2WHU shows that the average gas holdups of two different operating modes are close; however, the local gas holdup distribution of HEDT+2WHD is more uniform than that of HEDT+2WHU. Flow field distribution of two different combinations impeller measured with PIV methods can be used to understand the difference of the local void fraction for both cases.The results of this paper are helpful to the optimum design and operation of gas-liquid stirred tanks.