Fluid Dynamics Of Multi-Phase Stirred Reactors At Elevated Temperature

Multi-phase stirred reactors are widely used in many process industries, such as chemical, petrochemical, biochemical, pharmaceutical and water treatment etc. A lot of exothermic reactions were carried out in such stirred tanks, which leads to the operation temperature higher than the ambient temperature. However, little attention has been paid on the systems operated at elevated temperature. Previous works have compared the global properties of cold and hot systems and radical differences were found. Agitator relative power demand (ratio of the gassed power to ungassed power, RPD) is greater at high temperatures while retained gas fractions are substantially reduced. Unfortunately, the quantitative relations between temperature and the global properties have not reported yet. Due to lack of the studies on the meso-and micro-characteristics of the multi-phase stirred tank, it is difficult to understand the mechanism of gas dispersion and solid suspension. Therefore, experimental study and CFD simulation were carried out in this paper on the macro-and meso-characteristics of multi-phase stirred reactor at different temperatures.Systematic studies were carried out in a triple-impeller stirred tank of 0.476 m diameter with dished base. The aspect ratio of the liquid is 1.8. The impeller combination consisted of a half elliptical disk turbine (HEDT) as the bottom and two up-pumping wide-blade hydrofoils (WHU) above HEDT. The effect of temperature on the global properties, including power consumption, total gas holdup and critical impeller speed for just complete off-bottom solid suspension, was studied quantitatively. Results show that RPD increases for the higher temperature, but the effect of temperature on RPD becomes weaker when more solids added in the system. The total gas holdup decreases apparently with the increasing of temperature, which also becomes less at higher solid concentration. Empirical correlations of the effect of temperature on the global properties were also presented in this paper.Though the global characteristics are necessary for the industrial design, it is difficult to evaluate the gas dispersion in different region of the tank, not mention to the mechanism of gas dispersion without the meso-characteristics. Therefore, the meso-characteristics, including the local void fraction, bubble size and gas-liquid interfacial area in the system at different temperatures, were measured by using dual electrical conductivity probe in this work. Results show that the voidage decreases evidently with the increasing of temperature, which is in good agreement with the results for overall gas holdup. The overall Sauter mean bubble size in the hot systems is about 21% larger than that in cold conditions, which accounts for the lower gas holdup in hot system.The distribution of local void fraction and bubble size has close relationship with the flow field produced by the multi-impeller in the stirred tank. The complicated flow pattern stirred by triple-impeller configuration (HEDT+2WHU) was investigated with PIV technique. Results show that 4 loops are formed in the half vessel and the region with high turbulent kinetic energy located at the discharging flow of the bottom impeller. A pair of trailing vortices formed behind each blade of the bottom impeller moved towards the wall. Single trailing vortex formed behind each blade of the middle and top impeller moved towards the liquid surface. Energy is transported to the bulk of the tank by the movement of vortices.Gas-liquid flow in the multi-impeller stirred tank is so complicated that it takes lots of time and effort to carry out the experimental research. Therefore, the computational fluid dynamics (CFD) method was used to study the flow and the gas dispersion. The Euler-Euler two-flow model, standardκ-εturbulent model and Multiple Frames of Reference (MFR) method were used in the simulation. A Population Balance Model (PBM) model has been implemented using the commercial CFD Code. Bubble breakup and coalescence have been modeled fundamentally using isotropic turbulence theory. The local gas holdup distribution, bubble size distribution and local volumetric mass transfer coefficient predicted by CFD are in agreement with the experimental results.The results of local void fraction distribution show that the voidage in the region above the top impeller is much higher than that in other regions, which is not benefit for some industrial process uniform gas dispersion requested. In order to optimize the void fraction distribution by the design of impeller configuration, experimental and CFD methods were used to investigate the influence of top impeller diameter on power consumption, total gas holdup and local void fraction. Results show that, there is an extreme maximum voidage of about 50% just above the top impeller while the ratio of the top impeller diameter to the tank diameter, Dtop/T, is up to 0.5. The maximum voidage decreases with the decreasing of Dtop/T and almost disappears when Dtop/T is as small as 0.33. The results are of importance to the optimum of industry aerated multi-impeller stirred reactors.