Influence of impeller diameter on local gas dispersion properties in a sparged multi-impeller stirred tank☆

Vertical distributions of local void fraction and bubble size in alr–water dispersion system were measured with a dual conductivity probe in a fully baffled dished base stirred vessel with the diameter T of 0.48 m, holding 0.134 m3 liquid. The impel er combination with a six parabolic blade disk turbine below two down-pumping hy-drofoil propel ers, identified as PDT+2CBY, was used in this study. The effects of the impel er diameter D, rang-ing from 0.30T to 0.40T (corresponding to D/T from 0.30 to 0.40), on the local void fraction and bubble size were investigated by both experimental and CFD simulation methods. At low superficial gas velocity VS of 0.0077 m·s?1, there is no obvious difference in the local void fraction distribution for al systems with different D/T. However, at high superficial gas velocity, the system with a D/T of 0.30 leads to higher local void fraction than systems with other D/T. There is no significant variation in the axial distribution of the Sauter mean bubble size for al the systems with different D/T at the same gas superficial velocity. CFD simulation based on the two-fluid model along with the population balance model (PBM) was used to investigate the effect of the impel er diameter on the gas–liquid flows. The local void fraction predicted by the numerical simulation approach was in reasonable agreement with the experimental data.     

Experimental study on gas–liquid dispersion and mass transfer in shear-thinning system with coaxial mixer

The effects of impeller type, stirring power, gas flow rate, and liquid concentration on the gas–liquid mixing in a shear-thinning system with a coaxial mixer were investigated by experiment, and the overall gas holdup, relative power demand, and volumetric mass transfer coefficient under different conditions were compared. The results show that, the increasing stirring power or gas flow rate is beneficial in promoting the overall gas holdup and volumetric mass transfer coefficient, while the increasing system viscosity weakens the mass transfer in a shearing–thinning system. Among the three turbines, the six curved-blade disc turbine (BDT-6) exhibits the best gas pumping capacity; the six 45° pitched-blade disc turbine (PBDT-6) has the highest volumetric mass transfer coefficient at the same unit volume power.     

Numerical optimization for blades of Intermig impeller in solid–liquid stirred tank

The multiphase flow in the solid-liquid tank stirred with a new structure of Intermig impeller was analyzed by computational fluid dynamics(CFD).The Eulerian multiphase model and standard k-ε turbulence model were adopted to simulate the fluid flow,turbulent kinetic energy distribution,mixing performance and power consumption in a stirred tank.The simulation results were also verified by the water model experiments,and good agreement was achieved.The solid-liquid mixing performances of Intermig impeller with different blade structures were compared in detail.The results show that the improved Intermig impeller not only enhances the solid mixing and suspension,but also saves more than 20% power compared with the standard one.The inner blades have relatively little influence on power and the best angle of inner blades is 45°,while the outer blades affect greatly the power consumption and the optimized value is 45°.     

搅拌槽内多层组合桨对液-液分散特性的影响

以水-煤油及水-环己烷为体系,研究Rushton涡轮桨(RT),半椭圆管涡轮(HEDT)及翼形轴流式桨(CBY)的6种不同组合桨的液-液分散特性。测定了不同输入功率时分散相体积分率沿轴向及径向的分布。结果表明:当搅拌槽内液位与槽径之比达1.5,在相同输入功率时,三层桨的液液分散性能优于两层桨,功率准数较低的CBY组合桨在输入功率0.8kW/m~3时,槽内的轴向及径向分散相体积分率达到稳定的均匀分布。而功率准数较大的RT组合桨需要在输入功率1.8kW/m~3才能达到槽内分散相的均匀分布。     

CFD modeling of gas dispersion and bubble size in a double turbine stirred tank

In the present paper, gas dispersion in a double turbine baffled stirred tank is modeled using a commercial computational fluid dynamics (CFD) code FLUENT 6.1 (Fluent Inc., USA). A bubble number density equation is implemented in order to account for the combined effect of bubble break-up and coalescence in the tank. In the proposed work, the impellers are explicitly described in three dimensions using multiple reference frame model. Dispersed gas and bubbles dynamics in the turbulent water are modeled using an Eulerian-Eulerian approach with dispersed k-ε turbulent model and modified standard drag coefficient for the momentum exchange. The model predicts spatial distribution of gas holdup, average local bubble size and flow structure. The results are compared with experimental and numerical finding reported in the literature and good agreement between the present model and measurements of Alves et al. [Gas liquid mass transfer coefficient in stirred tanks interpreted through bubble contamination kinetics. Chemical Engineering Science, 2002, 57, 487-496] is achieved.     

搅拌反应器内计算流体力学模拟技术进展

综述了计算流体力学(CFD)技术应用在搅拌反应器的进展情况。重点对搅拌反应器内流动场模拟的各种处理方法,包括"黑箱"模型法、内外迭代法、多重参考系法和滑移网格法,进行了介绍与评价,指出了各种方法所具有的特点及存在的问题。阐述了搅拌反应器内CFD技术的发展方向,并就国内的研究现状进行了简单概述。     

Experimental and modelling study of gas dispersion in a double turbine stirred tank

Gas dispersion in a double turbine stirred tank is experimentally characterised by measuring local gas holdups and local bubble size distributions throughout the tank, for three liquid media: tap water, aqueous sulphate solution and aqueous sulphate solution with PEG. For all these media, bubble coalescence generally prevails over breakage. Where average bubble size decreases, this can be attributed to the difference in slip velocity between different sized bubbles. Most of the coalescence takes place in the turbine discharge stream.A compartment model that takes into account the combined effect of bubble coalescence and breakage is used to simulate gas dispersion. The model predicts spatial distribution of gas holdup and of average bubble size, with average bubble size at the turbines as an input. Reasonable agreement between experiment and simulation is achieved with optimisation of two parameters, one affecting mainly the slip velocity, the other related mainly to the bubble coalescence/breakage balance. Different sets of parameters are required for each of the three liquid systems under study, but are independent of stirring/aeration conditions. The model only fails to simulate the smaller average bubble diameters at the bottom of the tank.     

三种湍流模型下搅拌釜内气含率特性的模拟

采用计算流体力学（CFD）方法,应用Euler-Euler双流体模型,桨叶采用多重参考系法（MFR）,与考虑气泡聚并与破碎对气泡尺寸影响的群体平衡模型（PBM）相结合,比较了标准k-ε、Realizable k-ε和RNG k-ε3种湍流模型对双层涡轮搅拌釜内气-液两相液相流场、局部气含率及气泡尺寸分布的影响。结果表明：3种湍流模型预测的液相流场流型相似,总体气含率预测值相差不大,均与实验值吻合较好。对于局部气含率,标准k-ε和RNG k-ε模型在桨叶区的预测值偏大,在接近自由液面处三者预测值均偏低,Realizable k-ε模型预测结果与实验值符合最好;对于气泡尺寸,3种湍流模型预测结果均与实验值较吻合,在靠近自由液面处预测值均偏小,气泡尺寸分布与湍流长度分布相吻合。     

Hydrodynamics and volumetric gas-liquid mass transfer coefficient of a stirred vessel equipped with a gas-inducing impeller

Hydrodynamic and mass transfer characteristics of a gas-liquid stirred tank provided with a radial gas-inducing turbine were studied. The effect of the rotation speed and the liquid submergence on global hydrodynamic and mass transfer parameters such as the critical impeller speed, the induced gas flow rate, the gas holdup, the power consumption and the volumetric gas-liquid mass transfer coefficient were investigated. The experiments are mainly conducted with air-water system. In the case of critical impeller speed determination, two liquid viscosities have been used. The volumetric gas-liquid mass transfer coefficient k_La has been obtained by two different techniques. The gas holdup, the induced gas rate and the volumetric gas-liquid mass transfer coefficient are increasing functions with the rotation speed and decreasing ones with the liquid submergence. The effects of these operating parameters on the measured global parameters have been taken into account by introducing the dimensionless modified Froude number and correlations have been proposed for this type of impeller.     

CFD modeling of gas entrainment in stirred tank systems

In the present work, CFD modeling was used to study the phenomenon of gas entrainment in stirred tank systems. Two types of impellers (DT, PBTD) were simulated. VOF method was used as surface tracking technique along with LES model to study interfacial behavior at the onset of gas entrainment. Simulations were performed to study cause of entrainment and underlying interfacial mechanism at the location of entrainment. CFD simulations clearly showed differences in onset and non onset conditions in terms of the magnitudes of interfacial turbulence. As per the predictions, phenomenon of surface aeration in stirred tank systems was characterized by exchange of momentum across the interface from water side to air side. Magnitudes of instantaneous axial velocities on air side, strain rates on air side and vorticities on air side exhibited a threshold at the onset of entrainment and reduced substantially after the onset.     

Measurement of gas and liquid flows in stirred tank reactors with multiple agitators

The gas flow in a 3:1 aspect ratio vessel agitated by triple Rushton turbines has been measured by an ultrasound Doppler probe and by means of residence time studies. Strong recirculation around each impeller is found which fits in well with the compartmentalisation found in earlier liquid mixing studies. Surprisingly, when two axial A315 impellers above a Rushton turbine were used, gas recirculation around each impeller was still found. Study of the liquid phase mixing by a decolourisation technique confirmed that the gas flow essentially destroyed the strong axial liquid flow expected. Indeed, even under unaerated conditions, compartmentalisation was found between each impeller.     

Scalar mixing in a turbulent stirred tank with pitched blade turbine: Role of impeller speed perturbation

Mixing of a passive scalar inside a pitched blade turbine (PBT) impeller stirred tank (STR) is studied using large-eddy simulation (LES) coupled with the immersed boundary method (IBM) for resolving moving interfaces. Mixing time is calculated based on the 95% homogenization of the scalar over the entire tank volume. Growth rate of the unmixed tracer volume is observed in order to identify the effects of low frequency macroinstability (MI) oscillations. Mixing time is significantly reduced when the STR flow is perturbed using a step-change in the impeller speed with a specific MI frequency. The enhancement in turbulent kinetic energy and changes in mean flow field due to the perturbation is observed. The spatio-temporal behavior of the large-scale mixing structures for the fixed impeller-speed case and the perturbed case are compared. The mechanism of mixing enhancement is further explored by observing dynamic changes in the concentration distribution and the velocity field over a perturbation cycle. Penalty in power requirement due to perturbation is calculated.