Gas Dispersion And Mixing Characteristics Of Co-axial Mixers In Viscous Systems

Gas-liquid mixing reactors are widely used in chemical, food, cosmetics, biochemical, sewage treatment and polymer industries, because of their advantages of flexible adjustment, stable operation, high efficiency of heat and mass transfer, high levels of backmixing and efficient contacting between liquid and gas phase. It is necessary to investigate their gas dispersion and mixing characteristics, in order to understand the mechanisms and to guide industrial operation. A lot of researchers have done tremendous work to investigate the gas-liquid dispersion and mixing characteristics. But former researches are mainly focused on water/low-viscosity liquid-air systems, the mixers used are mainly single-shaft mixers, and local parameters’ researches in viscous systems are relative rare.In fact, in real industrial productions, the systems are more complicated, with high or changing viscosities, in which the newly developed co-axial mixers in recent years have been proved to be more advantageous. So it is necessary to investigate the gas dispersion and mixing characteristics of co-axial mixers in relatively high viscous systems.In this paper, innovative outer anchor impeller and gas aeration system were designed and built. During the experiment process, the comparison of the gas dispersion characteristics between co-axial mixers and single-shaft mixer was first conducted. Then gas dispersion and mixing characteristics of co-axial mixers were experimentally researched in gas-liquid mixing tank. And the effects of operating conditions (rotating speed, gas flow rate, rotating mode), mixer combinations (three combinations:outer anchor impeller+Rushton/SBT-6/PBT-6) and viscosities on them were also investigated. At the same time, CFD method was adopted, from which liquid and gas flow field, local gas holdup, bubble size and shear rate were obtained.Results showed that under the counter-rotation mode, the gas dispersion characteristics of co-axial mixers were better than that of traditional single-shaft mixer. Under the same conditions, among the three mixer combinations, when the inner impeller was Rushton, gas dispersion and mixing performance were the best. And counter-rotation mode was better than co-rotation mode. As the viscosity increased, the total gas holdup increased, while the gas dispersion in the whole tank was badly influenced resulting in lower local gas holdup near the tank wall region. The higher the rotating speed, the better the gas dispersion and mixing performance, the greater the total gas holdup and the local gas holdup in the near wall region. The effect of gas flow rate was relatively complicated. As the gas flow rate rose, gas cavities behind the blade became bigger, which was bad for the uniform gas dispersion in the whole tank. But under the experimental conditions, the rotating speed was always higher than the flooding point speed, so gas dispersion conditions were good. So, in such occasions, as the gas flow rate rose, total gas holdup and local gas holdup rose.The numerically simulated results and experimental results were compared, which showed good agreement. The simulated results are helpful for us to understand the gas dispersion and mixing mechanisms.