Experimental Measurement And Numerical Simulation Of Hydrodynamic Characteristics In Gas-liquid And Gas-liquid-solid Stirred Tanks

Gas-liquid and gas-liquid-solid stirred tank reactors have been widely used in chemical,petrochemical,environmental,biochemical and metallurgical industries.Interactions between dispersed phases and continuous phase,even among dispersed phases,become more and more complicated along with the increased number of phases.To date,the knowledge of local fluid dynamic properties and the accurate universal mathematical models for such systems are unavailable,which makes it difficult to scale up the reactors confidently.A series of experiments on gas-liquid and gas-liquid-solid stirred reactors have been conducted based on the telecentric photographic multiphase measurement which is developed by our research group.New fluid dynamics data in gas-liquid and gas-liquid-solid stirred reactors have been obtained with this technique,and on this basis the accurate universal mathematical models on bubble breakage and coalescence will be developed.Firstly,due to the weak reflectivity of water and bubbles to incident light,the photographic measurement is encountered with a great challenge in identifying distinctly the bubbles profiles from the water background in poorly-illuminated multiphase reactors.Combined the photosensitive characteristic curves with the particle scattering principle,the bubble images that were hardly recognizable become clear and recognizable by adding light-scattering trace submicron oxide particles.Four representative oxide powders,i.e.,TiO2,Al2O3,CuO and SiO2,are chosen as scattering particles.Numerical calculations are carried out through single-particle and multi-particles models based on non-coherent scattering hypotheses to guide the selection and optimization of particle types and additive concentrations,these guidelines are validated by corresponding experiments.The results show that TiO2 particles have obvious advantages.This particle scattering approach works well without too much interference and loss of measurement accuracy.Then the intrusive photograph measurement is used to probe local hydrodynamics,bubble size distribution(BSD),and holdup distributions(BHD)in a gas-liquid stirred tank reactor(STR).This method shows more accurate for judging the flow regime in gas-liquid STRs.Under typical industrial conditions,20%-40%of bubbles are less than 0.3 mm and the Sauter bubble size is about 1.3 mm found by this method.Consequently,the obtained bubble size data are smaller than those by capillary suction probes.Besides,a large number of small bubbles are found in the turbine plane regions and the lower bulk flow regions,and the sizes of bubbles increase with the increase of the vertical ordinate.Under the operating conditions of this work,the gas holdup is low in the lower bulk flow regions.The BHD above the turbine plane is relatively uniform along the axial direction and the radial direction as well except for the holdups near the wall are slightly lower.The maximum value appears at the position near the upper vortex center with the lowest pressure.Finally,the intrusive photograph measurement has also been applied to obtain BSDs,BHDs and solid holdup distributions in gas-liquid-solid STRs.It can be found that the downward movement trend of solids is weakened due to the influence of gases where the gas holdup is high.The presence of solid increases the drag force between gas and liquid.The parameters of the bubble breakup and coalescence model are tested with the measured data,and then a population balance model(PBM)suitable for gas-liquid-solid systems is established.The new model can better predict the BSDs in such systems.It can also be found that the breaking frequency increases due to the presence of solids,indicating that the solids will aggravate the breakup of bubbles.Besides,the coalescence efficiency has an increased tendency,so the presence of solids also aggravates the coalescence of bubbles.The local hydrodynamic data based on this work can provide an important basis for validating the modeling of multiphase stirred reactors.Also,the developed model of bubble behavior provides a basis for numerical simulation design and scale-up of industrial stirred reactors.