Experimental Study On Macro-instabilities And Mass Transfer In Gas-Liquid Stirred Tanks Using Computational Fluid Dynamics

CFD simulation was carried out in this paper on the macro- and meso-characteristics of a n industrial stirred reactor with triple impellers. Polyether polyol (PPG) was chosen in this study as a research medium. multiply impellers The phenomemon was investigated which is called macro-instability(MI) in the stirred tanks, a kind of large-scale, low-frequency because of the complicated and transient flow fluid. The influences of operation conditions such as speed、the clearance of off-bottom and other factors was mainly studied on MI.1. Two types of multiple impellers (multiple impellers one which is a HEDT impeller as bottom and two WHu impellers as middle and upper;the multiple impellers two that is the HEDT as bottom and XDT impeller as middle and XYK impeller as upper) was used in stirred tank. The result showed that, CFD could capture the turbulent flow characteristics in the stirred tank, the turbulent kinetic energy and velocity about multiple impellers two is less than multiple impellers one. Multiple impellers one is more suitable for the actual production of the polyether polyols system.2. The mass transfer in a gas-liquid stirred tank reactor was investigated using multiphase computational fluid dynamics coupled with population balance method (CFD-PBM) about PPG-air. CFD model numerical considerations results were evaluated in terms of local gas holdup, bubble diameter, mass transfer coefficients. The result showed that, liquid flow velocity predictions were strongly influenced by numerical considerations, local gas holdup and bubble diameter showed two peaks distribution, The simulation and experimental results agreed well with the overall gas holdup about Grid 2. Grid 2 mesh was more suitable for polyether polyols liquid -air systems, which provided a reference for further calculations.3. The fluid flow in stirred tank is complicated and transient. The pressure signal for macro-instabilities were analyzed and the approach of power spectral density was proved to be suitable for analyzing low-frequency signal with peak value. The macro- instability(MI) phenomamon in the stirred tank was investagated with single and double impellers. Effects of the impeller installation method and rotation speed on the Macro-instabilities frequency were discussed.The results showed that, for a single and double impellers, macro-instability frequency distribution of the main frequency was ranged into 0.3-0.5Hz. With the increase of speed, the stirred tank fluid pulsation overall was enhanced, location of monitor sites were unrelated with macro-instability frequency. The results helped to understand the phenomenon of chaotic mixing fluids,to avoid a stirred tank mechanical strain.