Mechanisms of solids drawdown in stirred tanks

Agitated tanks are used in several industrial processes to achieve complete drawdown of floating solids in liquids. The design requirements for this process are not completely defined, and are currently limited to heuristics regarding the use of a surface vortex and the effect of wettability on the difficulty of mixing, along with several initial studies in the literature. In this study, the effect of the type of impeller, particle size and shape, solids concentration, impeller submergence, and baffle configuration on the minimum drawdown speed (N_jd) are investigated. It was found that the formation of a large surface vortex acts to hold particles close to the surface. Suppression of the surface vortex is recommended. In baffled tanks where the formation of a large surface vortex is suppressed, the intensity of turbulence and mean circulation velocity of the liquid are responsible for solids drawdown and distribution in the tank. The submergence of the impeller relative to the liquid surface and the pumping mode of the pitched blade turbine (PBT) were found to be the controlling parameters. CFD simulations were carried out to obtain a better understanding and interpretation of the flow patterns and drawdown mechanisms for the different baffle configurations.     

Effect of baffles on fluid flow field in stirred tank with floating particles by using PIV

PIV technique was applied to elucidate the effect of baffles at different shaft positions and different impeller off‐bottom clearances on the flow field in a stirred tank with floating particles. The investigation was carried out in a cylindrical tank with a flat base, and five different baffle configurations: standard baffles, narrow baffles with a width of 15 mm, narrow baffles with a width of 10 mm, down triangular baffles and up triangular baffles. The measurements show that down triangular baffles offers several advantages over standard baffles at C = T/3: high axial and radial velocities, relatively Low critical agitation speed and power consumption for just drawdown of 1.0 vol.% floating particles. While at C = T/2 this superiority disappears and the fluid flow field is similar to that for standard baffles. The other baffles are similar in performance except for a small difference in the critical agitation speed. An off‐centred shaft helps reduce the critical just drawdown speed and the corresponding power consumption for baffle configurations considered. With down triangular baffles, the critical power consumption to draw down the floating particles for the most eccentric shaft is about 42% of that for a centred shaft. © 2012 Canadian Society for Chemical Engineering     

Numerical simulation of turbulent solid–liquid two-phase flow and orientation of slender particles in a stirred tank

In this paper, turbulent solid–liquid two-phase flow involving slender particles in a tank stirred by standard Rushton turbines is simulated with two-fluid model using the improved inner–outer iterative method. Standard k–ε model is used to deal with turbulent flow. By comparison with the case of equivalent spherical particles, it is found that the flow field of slender particles is similar to that of spherical particles. The evolution of particle orientation as it follows the liquid flow in a stirred tank is modeled directly from the rigid slender rods revolution equation. Experiments about solid–liquid two-phase flow are also performed in a baffled tank using DPIV (digital particle image velocimetry). All simulation results are compared with experiments. The comparison between simulation and experiments confirms that the results are reliable. The good agreements between simulation and experiments verify the reliability of the methods employed in this paper. The influences of impeller speed on flow field and orientations are also investigated.     

Effects of geometrical and physical factors on light particles dispersion by agitation characteristic curve

In this study, the effects of geometrical and physical factors on light particles dispersion in stirred tank were investigated by agitation characteristic curve. The experiments and CFD simulations with discrete phase model (DPM) and volume of fluid model (VOF) were conducted in this paper. Five factors, which include four geometrical factors (submergence, impeller-to-tank ratio, number of impeller blades and baffling mode) and a physical factor (liquid viscosity) were considered. For each factor, the power consumption curve and agitation characteristic curve were drawn to compare the power consumption and mixing results in the stirred tank. Characteristics of the agitation characteristic curves were compared with the previous published literatures and theories. It is found that the agitation characteristic curves reflect the tendency of power consumption and particles distribution well in stirred tank. The good agreement indicates the applicability of the agitation characteristic curves for the study of light particles distribution in stirred tank.     

Rührtechnologische Forschung zum Suspendieren und Dispergieren

This article deals with the suspension in a stirred tank and the behavior of sinking and floating particles. It is shown that a single-baffle configuration and an up-pumping axial impeller are optimal devices for floating particles. The experimental results for the analysis of particle strain and breakage in turbulent dispersions demonstrate that at the same specific power input, the stirrer with the lowest tip-speed creates the largest droplets (equal to lowest particle strain).     

Complete suspension of particles in mechanically agitated vessels

A new model is proposed to explain the mechanism of complete suspension of solid particles in cylindrical flat-bottomed stirred tanks. According to this model the suspension of the particles at rest on certain zones of the tank bottom is mainly due to turbulent eddies of a scale of the order of the particles size. The development of this hypothesis has led, with some other assumptions, to an expression which has been experimentally verified and compared with analogous expressions available in literature.     

Numerical simulation of fluid dynamics in the stirred tank by the SSG Reynolds Stress Model

The Speziale, Sarkar and Gatski Reynolds Stress Model (SSG RSM) is utilized to simulate the fluid dynamics in a full baffled stirred tank with a Rushton turbine impeller. Four levels of grid resolutions are chosen to determine an optimised number of grids for further simulations. CFD model data in terms of the flow field, trailing vortex, and the power number are compared with published experimental results. The comparison shows that the global fluid dynamics throughout the stirred tank and the local characteristics of trailing vortices near the blade tips can be captured by the SSG RSM. The predicted mean velocity components in axial, radial and tangential direction are also in good agreement with experiment data. The power number predicted is quite close to the designed value, which demonstrates that this model can accurately calculate the power number in the stirred tank. Therefore, the simulation by using a combination of SSG RSM and MRF impeller rotational model can accurately model turbulent fluid flow in the stirred tank, and it offers an alternative method for design and optimisation of stirred tanks.     

疏水缔合聚丙烯酰胺溶解槽内流固混合特性的数值模拟

利用FLUENT软件对在聚酯生产中应用的双层搅拌桨搅拌槽内疏水缔合聚丙烯酰胺AP-P4溶解过程的流场进行数值模拟分析,采用标准k-ε模型和多重参考系法(MRF)。分析了AP-P4溶解过程中刚加入聚合物颗粒时在搅拌槽内的混合情况。得到了搅拌槽内流场状况和固体颗粒的体积分数分布。并对流场的分布规律、固体颗粒体积分数分布特点加以分析,由模拟结果计算出搅拌轴的功率,为搅拌槽的设计和实际应用提供有益的结论。     

用改进的内外迭代法数值模拟Rushton涡轮搅拌槽流场

在有挡板的搅拌槽中，受搅拌桨驱动的液体在挡板的作用下会产生复杂的三维湍流流动.利用“快照”法思路和改进的内外迭代法及k–e湍流模型对Rushton涡轮有挡板的搅拌槽进行了整体数值模拟. 同文献中的实验数据进行了比较，模拟值同实验值基本吻合. 改进后的内外迭代法不依赖经验公式和实验数据，有一定的通用性.     

桨式搅拌器功率准数三种取得方法的对比

在有挡板条件下,对常用的桨式搅拌器(单层二叶平桨、二叶斜桨、四叶斜桨及双层四叶斜桨),桨槽径比为0.5—0.6,进行搅拌功率曲线的测绘。利用经验公式对功率准数进行了计算,通过关联值与实验值的对比发现,Nagata关联式在层流状态时关联值与实验值相差较小,在湍流时二者相差较大,而Kamei和Hiraoka关联式则在过渡流和湍流区与实验值比较吻合,在层流区的偏差比较大。利用计算流体力学模拟了搅拌器各种状态的功率准数值,模拟值与实验值对比发现,模拟值在不同的雷诺数时都与实验值吻合较好。     

Large Eddy Simulations of Mixing Time in a Stirred Tank

Large eddy simulations （LES） of mixing process in a stirred tank of 0.476m diameter with a 3-narrow blade hydrofoil CBY impeller were reported. The turbulent flow field and mixing time were calculated using LES with Smagorinsky-Lilly subgrid scale model. The impeller rotation was modeled using the sliding mesh technique. Better agreement of power demand and mixing time was obtained between the experimental and the LES prediction than that by the traditional Reynolds-averaged Navier-Stokes （RANS） approach. The curve of tracer response predicted by LES was in good agreement with the experimental. The results show that LES is a reliable tool to investigate the unsteady and quasi-periodic behavior of the turbulent flow in stirred tanks.     

Numerical Investigation of the Effect of Impeller Design Parameters on the Performance of a Multiphase Baffle‐Stirred Reactor

The turbulent gas‐liquid flow field in an industrial 100‐m³ stirred tank was calculated by using computational fluid dynamics based on the finite‐volume method. Turbulent effects were modeled with the shear stress transport model, and gas‐liquid bubbly flow was modeled with the Eulerian‐Eulerian approach using the Grace correlation for the drag force interphase momentum transfer. The relative motion between the rotating impeller and the stationary baffled tank was considered by using a multiple frames of reference algorithm. The effects of Rushton and pitched‐blade impeller design parameters such as blade geometry, location, and pumping direction on the mixing performance were investigated. It was found that a combination of Rushton turbines with up‐pumping pitched‐blade turbines provides the best mixing performance in terms of gas holdup and interfacial area density. The approach outlined in this work is useful for performance optimization of biotechnology reactors, as typically found in fermentation processes.     

Turbulent blend times with off-centre v-mounted agitators

Experimental measurements in a flat-bottom tank with narrow-blade hydrofoil and pitched-blade impellers are used to develop guidelines for off-centre, or eccentric, placement of vertical agitators in unbaffled tanks. The guidelines are based on providing a turbulent blend time that is no more than 20% longer than that of the same impeller operating at the same rotational speed on the centreline of a baffled tank. In addition to investigating the effect of impeller type, impeller diameter and off-bottom clearance are also considered. The results support the commonly noted rule of thumb that as off-centre distance is increased, performance in an unbaffled vessel approaches that in a baffled tank. A notable exception to this axiom occurs when a large impeller is located close to the tank base (specifically, D/T = 0.40, C/T = 0.10, and O/T = 0.25). In this case, a stable impeller tip vortex forms with both impeller types, with slow exchange of material between the vortex and bulk liquid leading to long blend times. Besides blend time decreasing with increasing off-centre distance, the uncertainty or run-to-run variation in blend time also decreases dramatically. In most cases, the pitched-blade turbine requires a smaller off-centre distance than the hydrofoil impeller to approximate the blending performance provided during baffled operation.     

轴流桨搅拌槽三维流场数值模拟

利用k -ε湍流模型预测了搅拌槽在不同操作条件下宏观速度场 ,模型成功预测了搅拌槽内速度分布 ,计算结果与实验结果吻合较好 .模型预测结果表明 ,搅拌槽内宏观流动场受搅拌桨槽径比影响较大 .对单层搅拌桨 -槽体系 ,挡板前后宏观流动场差别很大 ,在挡板以前区域 ,轴向流动较强 ,在整个r -z断面上形成一个整体循环 ;而在挡板后面区域 ,流体在桨叶安装位置高度附近转向轴心流动 ,槽体上半部区域形成二次循环区域 ,且二次循环区域内流体以向下流动为主 .     

Simulation of laminar and turbulent impeller stirred tanks using immersed boundary method and large eddy simulation technique in multi-block curvilinear geometries

Impeller stirred tanks are commonly used in the chemical processing industries (CPI) for a variety of mixing and blending technologies. Such processes require accurate modeling of the turbulent flow in the tank over a range of operating conditions (e.g. impeller speed), and in addition, require a computationally efficient solution strategy that can represent moving rigid geometric parts (impellers) in the tank. In the present study, a methodology is proposed that combines the advantages of the immersed boundary method (IBM) to represent moving rigid geometries with the efficiency of multi-block structured curvilinear meshes (to minimize wasted grid points) for the representation of overall complex domains. The IBM implementation on a multi-block curvilinear mesh is advocated for the simulations of impeller stirred tank reactors (STR) and has distinct advantages over other competing methods. In the present work, the curvilinear-IBM methodology is further combined with the curvilinear coordinate implementation of large eddy simulation (LES) technique to address the issue of modeling unsteady turbulent flows in the STR. To verify the implementation of IBM in a multi-block curvilinear geometry, a laminar STR with a stack of four pitched blade impellers on a single shaft is simulated and compared against experimental data. Verification of the combined IBM-LES implementation strategy in curvilinear coordinates is done through comparisons with the measurements of turbulent flow in a baffled STR with a single pitched blade impeller. For both laminar and turbulent STR, the predictions are in very good agreement with measurements. It is suggested here that this methodology can be reliably used as a predictive tool for the flow fields in STRs with complex geometries.     

Laser Doppler Measurements of Turbulent Parameters in Different Multiple‐Propeller Systems

In this study, laser Doppler anemometry (LDA) measurements in the r‐z plane are obtained in two stirred tanks equipped with two downward pumping propellers in the turbulent and the transient flow domains. A new approach of the hydrodynamics generated by this system consists of determining the one‐dimensional energy spectrum and the autocorrelation coefficient function at different locations in each vessel. In the turbulent flow, the integral scales, the Taylor microscales, and the Kolmogorov microscale have been determined. The dissipation rate is obtained with two methods: from the semi‐empirical expression ϵ = A k^3/2/D and from the frequency spectrum. Results show the influence of the rheological properties on the flow patterns, the influence of the blade passage frequency on the frequency spectrum, and the apparition of a low frequency characterizing the production of macro‐instabilities. Comparison of the turbulent characteristics with the values reported in the literature shows that our combination of propellers produces larger eddies than a Rushton turbine.     

通气条件下桨型对自浮颗粒悬浮的影响

考察了五十二种搅拌桨的组合对搅拌釜内自浮颗粒的气液固三相体系混合问题的功耗、气含率和釜底部的颗粒含量的影响。研究表明 ,对自浮颗粒三相体系的搅拌混合应采用多层桨 ,且上层最好用上推式桨 ;实验发现 ,当高径比为 1 .6时 ,三层桨的混合参数优于两层桨的 ;给出了优异的搅拌桨型。     

搅拌槽中液-液-固三相传质的实验研究

选择欧洲化学工程师协会(EFCE)推荐的典型液-液萃取体系正丁醇-丁二酸-水,加入不同粒径的玻璃珠构成液-液-固三相传质,以去离子水为连续相,正丁醇为分散相,溶质丁二酸从分散相向连续相传质. 利用电导率法测定液-液相传质系数,并考察了搅拌转速、固体质量百分含量、不同桨型(标准Rushton桨、上推式和下推式45°六折叶涡轮桨)、桨中心平面距槽底距离以及固体颗粒粒径对相间传质的影响. 结果表明,在高转速时,惰性固体粒子的存在强化液-液体系的传质. 随着惰性固体含量增大,液-液-固三相传质有一极大值. 粒径大于100 μm 的固体粒子对液-液体系传质系数影响很小. 三种桨中Rushton桨的对流传质效果最好.     

Computational fluid dynamic modelling of flow patterns in an oscillatory baffled column

Numerical simulations to date within the context of oscillatory flow in a baffled column have been limited to flows in a symmetrical regime, i.e. eddies are generated symmetrical to the central line of the column where the oscillatory Reynolds numbers are below 400. In this paper, 3-D computational fluid dynamic (CFD) simulation of flow patterns of oscillatory flow in a baffled column has, for the first time, been carried out and the results extended to all regimes of oscillatory Reynolds numbers covering from symmetric to asymmetric flows. The flow patterns simulated have also been validated by both direct flow visualisation and by digital particle image velocimetry measurements. The success of such CFD simulations opens doors for many potential studies, from optimisation of geometry for plug flow to suspension of particles, and from droplet breakage and coalescence to mass/heat transfer of particles.