Experimental Investigation And Numerical Simulation Of Local Gas Dispersion In A Stirred Tank With A Multiple Impeller

Gas-liquid stirred tank reactors are widely applied in many process industries, such as biochemical, sewage treatment, petrochemical, pharmaceutical etc. So far, the study concentrates upon experimental research for the global properties and numerical simulations of single or dual-impeller. Very few experimental and numerical simulation researches on the gas-liquid mixing in stirred tanks with multiple impellers were published.The local gas holdup and bubble size distribution were measured with double-tip conductivity probes in a multiple impeller(HEDT+2WHD) stirred tank. The effects of stirring speed and gas inlet rate on gas dispersion performance were investigated. It was found that local gas holdup showed a trend of bimodal distribution in the axial direction. The distribution of bubble size was non-uniform, the smallest bubbles were founded in the impeller discharge, bubble diameter increased in the region away from the impellers with a maximum value near the top impeller. The local gas holdup and bubble size increased with the increasing of gas rate, but the bubble size decreased while the agitator speed increase.The bubble size and local gas holdup in a stirred tank with dual turbine impellers had been simulated using computational fluid dynamics(CFD).The Euler-Euler multiphase flow model, multiple frames of reference(MFR) method were used in the simulations. A population balance model (PBM) model was implemented in order to account for the combined effect of bubble break-up and coalescence in the tank. The simulation results agree well with the experimental values which come from reference. Also the effects of different turbulent models had been studied, the standard k-ε and RNG k-ε models over predicted the local gas holdup values at the impeller regions, the realizable k-ε model gave the best result compared with the experimental values.Finally, the hydrodynamics and mass transfer in a gas-liquid triple-impeller stirred tank reactor were investigated using multiphase computational fluid dynamics coupled with population balance method. The Euler-Euler formulation of the realizable k-ε turbulence model with method of MFR was used in the simulation. The CFD model predictions of local hydrodynamic parameters were in reasonable agreement with the experimental data.