Structure of powder flow in a planetary mixer during wet-mass granulation

Wet massing granulation, a widely used industrial process, is difficult to monitor and control and the structure of the flow is poorly understood. Flow patterns in a planetary mixer were investigated using positron emission particle tracking. Both dry and wet powders of a model pharmaceutical formulation were studied to develop understanding of the influence of moisture content on the flow structure during granulation. The flow structure was characterised using the distributions of the velocity components in different cross-sections of the mixer. Fourier analysis showed that the dry system is essentially dissipative and disordered whereas the wet system, being more inertial, shows signs of being more ordered with a periodic recirculation within the bowl. In both systems, radial and axial displacements are strongly correlated. For the dry system, within a central radial core region, the behaviour of the particle was determined by the rapid movement of the agitator, forming a single toroidal recycling cell. The radial and axial velocities of the tracer were up to two orders of magnitude lower than the tangential component. However, in the regions close to the wall, the particle was found to exhibit small movements dictated by the planetary rotation. For wet systems these two main regions were again observed. However, velocity field and velocity distribution showed the presence of two toroidal circulation loops, one above the other. In the wall region, the small movements governed by the planetary motion were again found, but with the amplitude of the displacements reduced by an order of magnitude.     

Power consumption and flow field characteristics of a coaxial mixer with a double inner impeller

A coaxial mixer meeting the actual demand of a system with high and variable viscosity is investigated. It has an outer wal-scraping frame and a double inner impeller consisting of a four-pitched-blade turbine and Rushton turbine. The power consumption and flow field characteristics of the coaxial mixer in laminar and transitional flow are simulated numerically, and then the distribution of velocity field, shear rate and mass flow rate are analyzed. The simulation results indicate that the outer frame has little effect on the power consumption of the double inner impeller whether in laminar or transitional flow, whereas the inner combined impeller has a great effect on the power consumption of the outer frame. Compared with the single rotation mode, the power consumption of the outer frame will decrease in co-rotation mode and increase in counter-rotation mode. The velocity, shear rate and mass flow rate are relatively high near the inner impeller in all operating modes, and only under double-shaft agitation wil the mixing performance near the free surface be improved. In addition, these distributions in the co-rotation and counter-rotation modes show little difference, but the co-rotation mode is recommended for the advantage of low power consumption.     

The effect of mixer properties and fill level on granular flow in a bladed mixer

The discrete element method was used to study the effect of mixer properties and fill level on the granular flow of monodisperse, cohesionless spheres in a bladed mixer. For fill levels just covering the span of the blades, a three‐dimensional (3‐D) recirculation zone develops in front of the blades, which promotes vertical and radial mixing. Increasing fill level reduces the size of the recirculation zone, decreases bed dilation and hinders particle diffusivities. However, above a critical fill level, the behavior of the particles within the span of the blade is found to be invariant of fill level. At low‐fill levels, the pressure within the particle bed varies linearly with bed height and can be approximated by hydrostatics. At higher fill levels, a constant pressure region develops within the span of the blades due to the angled pitch of the blades. Cylinder wall friction is shown to significantly influence granular behavior in bladed mixers. At low‐wall friction, the 3‐D recirculation zone observed for high‐wall friction conditions does not develop. High‐wall friction leads to an increase in convective and diffusive particle mixing. Shear stresses are shown to be a function of wall friction. Blade position along the vertical axis is shown to influence flow patterns, granular temperature and stress. The effect of increasing the mixer diameter at a constant particle diameter was also studied. When the mixer diameter is larger than a critical size such that wall effects are minimized, the observed granular behavior follows simple scaling relations. Particle velocities and diffusivities scale linearly with mixer size and blade speed. Normal and shear stress profiles are found to scale linearly with the total weight of the particle bed. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Expression for turbulent power draw of an in-line Silverson high shear mixer

We report the validation of an expression to accurately describe the power draw of an in-line rotor–stator mixer over a range of flow rates and rotor speeds. The expression consists of a term which reflects the power required to rotate the shaft in response to the liquid resistance and a term to reflect the power convected away from the mixing chamber. A specially commissioned high speed (12,000 rpm), instrumented 150/250 MS Silverson mixer has been developed with power draw determined from both torque and calorimetric measurements. Experiments are carried out using water over a range of independently controlled flow rates and rotor speeds with losses for both techniques carefully accounted for. For the torque measurement the value of the constants for the two terms above are Po_z=0.197 and k₁=9.35, respectively. For the calorimetric technique the measured temperature rise was similar to some of the corrections and losses over a significant range of the experimental space but nevertheless with careful experimentation constants similar to those for the torque technique were obtained, Po_z=0.229 and k₁=7.46. Allowing the calibration of the temperature probes to be a fit parameter in the regression routine increased the value of k₁=8.10 but did not affect the value of Po_z. A simple graphical method is also proposed using a dimensionless form of the expression which yielded slightly higher value of Po_z but a slightly lower value of k₁. The accuracy of both measurement techniques improves with rotor speed and the differences between the constants is attributed to the better accuracy of the torque technique at higher flow rates whereas the calorimetric technique is more accurate at low flow rates where the temperature rise is larger. Several repeats of the calorimetric technique with a reduced set of experimental points show good reproducibility. Finally at low flow rates (<10% of the maximum) the power unexpectantly increases and a modification to the expression is proposed by considering the pumping efficiency.     

Q型静态混合器内液滴群分散特性

静态混合器因具有结构紧凑、强化性能优异和连续性生产等优点被广泛应用于过程工业中,但Q型静态混合器(QSM)内多相流分散混合强化机理不完善制约了其在精细化工和原料药绿色生产中的应用。采用计算流体力学(CFD)耦合群体平衡方程对QSM内相含率α≤5%时液滴分散特性进行数值模拟,分析液液界面张力、动力黏度和相含率对液滴群d32的分散行为的影响。标准旋流静态混合器内的d32数值预测结果与实验结果有良好的一致性。模拟结果表明,在z/l=11.5处不同分散相d32减小73%~96%,RL-90-QSM对不同物性介质的分散混合具有高效性和普适性;在高雷诺数和低相含率下,不同分散相流过z/l=0~2时d32破碎速率最大,在z/l=2.5处d32减小51%~90%,d32随混合时间的增加逐渐减小且在z/l=10后趋于稳定;界面张力对混合结果的影响大于动力黏度。     

拉伸流动静态混合器混合性能的实验研究

设计了1种能够增强聚合物混合效果的拉伸流动静态混合器（EFM）,以高密度聚乙烯/聚苯乙烯（PE⁃HD/PS）作为混合体系,根据共混体系扫描电子显微镜（SEM）照片及分散相的平均粒径,研究了不同盘形结构和不同盘棱间隙（δ）（0.125~2.0 mm）下EFM的混合性能。结果表明,EFM的盘形入口结构对其混合性能影响较小,混合能力随盘棱顶端圆角半径的增大而有所提升,随δ的增大出现先降低再升高又降低的变化趋势。     

Development of mixing time correlation for a double jet mixer

BACKGROUND: Jet mixing is one of the simplest methods to achieve mixing. There have been a number of experimental studies concerned with jet mixing; some of these studies report empirical correlations. The existing correlations are not useful where there are significant deviations from the idealized conditions. Most correlations reported in the literature deal with liquid flow with single or multiple jets, whereas the effect of radial angle on mixing time was not studied. This present study investigates the effect of operating parameters on experimental mixing time in a double jet mixer. Nozzle configuration for jet1 was fixed based on earlier studies (2/3rd position, nozzle angle 45° and nozzle diameter 10 mm). Mixing times were estimated for different jet2 configurations of jet angle (30°, 45° and 60°), radial angles (60°, 120°, 180°), jet diameter (5 mm and 3 mm) and located at different tank heights (2/3rd and 1/3rd from the bottom of the tank). RESULTS: A mixing time correlation was developed in terms of all the parameters using dimensional analysis. The constants and powers of the parameters involved in the correlation developed were estimated using a least square method to calculate the straight line that best fitted the mixing time data obtained during the experiments. The effects of change in angle of inclination of jet2 (θ₂), radial angle of jet2 with respect to jet1 (Φ₂) and diameter of jet2 (d₂) on mixing time were analyzed and compared with the experimental mixing time. CONCLUSION: The correlation developed based on the dimensional analysis and least square method predicts the mixing time for a double jet mixing tank. Copyright © 2009 Society of Chemical Industry     

Characterization of transverse mixing in a screw mixer by image analysis

Transverse mixing of particles in a screw mixer is investigated by a digitized image analysis method coupled with a solidification technique. The effects of screw rotation speed, filling level, and particle size on the transverse mixing index and mixing rate constant are investigated experimentally. The results show that a decrease in screw rotation speed and filling level results in an increase in the mixing rate. Faster mixing is observed with large particles, and the mixing rate constant of coarse particles is 1.5–2 times higher than that of fine particles. The particle size difference of materials puts the particles at a risk of segregation.     

SV型静态混合器湍流阻力的初步研究

为了获得流体在SV型静态混合器中湍流流动时的流动阻力规律,提出一种新的含有SV型静态混合器重要几何结构参数的流体阻力计算模型。对于不可压缩流体,将其在SV型静态混合器中的运动分解成沿管壁与轴线方向平行和沿混合元件凹槽方向的直线运动。运用流体力学理论,分别求解出流体作2种运动时所产生的湍流流体阻力的计算式,并计入相邻混合元件交接部分的局部阻力,然后进行叠加得到流体阻力理论计算式。以水为实验介质,对SV型静态混合器流体湍流阻力进行了实验测量,与理论结论进行比较分析,得出摩擦因子λ与Re-0.2呈线性关系的结论。     

双转子连续混炼机混合过程物理模型的建立

建立了双转子连续混炼混合过程的物理模型，分析了影响其混炼段融体输送过程和混合过程的主要因素，并提出：转子混炼段融体输送量与转子螺棱几何形状，物料粘度，混炼段融体压降等有关；转子对物料的混合过程与转子组合（螺棱交汇区长度）转子螺棱几何形状，转子转速等因素有关。     

SK型静态混合器流体湍流阻力的研究

为了获得流体在SK型静态混合器中湍流流动时的流动阻力规律,提出一种新的流体阻力的计算模型。在流体不可压缩的假设前提下,将流体在SK型静态混合器中的螺旋形运动分解成轴向直线运动和环向旋转运动。在流体作湍流流动时,运用流体力学理论,分别求解出流体作2种运动时所产生的流体阻力的计算式,然后进行叠加得到SK型静态混合器湍流时流体阻力理论计算式。以水为实验介质,对SK型静态混合器流体湍流阻力进行了实验测量,回归出实验公式。与理论结论进行比较分析,得出流动摩擦系数与雷诺数的负0.25次幂呈线性关系的结论。     

风粉混合器内气固两相流动的模拟及试验研究

煤粉工业锅炉系统中风粉混合器是实现煤粉与一次风快速均匀混合的关键设备,测量及计算风粉混合器内煤粉、一次风气固两相流流场,对于优化风粉混合器结构,强化风粉混合效率及提高一次风粉的均匀稳定供给具有重要意义。笔者针对竖直结构及倾斜结构的2种风粉混合器,开展了数值计算及现场工程试验研究。基于几何拓扑学知识,采用ICEM软件针对2种风粉混合器划分了合适的三维网格;多相流理论模型中,多相连续介质模型中的双流体模型各相视为相互渗透、耦合但又保持各自运动特征的连续介质,相比于单流体模型,双流体模型考虑了固相的湍流输运以及气固两相间相互滑移引起的阻力,使得计算结果更接近实际情况;冷态双流体模型基本方程由守恒方程、相间耦合方程以及封闭方程构成,其中相间耦合方程用于表征气固相动量之间的耦合;为了探究不同停留时间下风粉混合器内气固两相的流场特征,采用非稳态数值计算方法,利用Fluent软件开展数值计算。基于两相流模型及Schilller-Naumann曵力系数模型研究了不同结构下风粉混合器内煤粉浓度分布随停留时间变化特征,采用德图testo425热敏风速仪测量了不同煤粉落料量下风粉混合器内负压变化规律。结果表明,竖直结构的风粉混合器内停留时间由0.25 s增至1 s时,混合器底部颗粒沉积的现象一直存在,即存在较长时间的颗粒流动死角区域;而对于倾斜结构的风粉混合器,当停留时间大于0.3 s,混合器内颗粒浓度基本降为0,较好避免了颗粒在混合器底部的沉积,该结构对于强化混合器内风粉混合及降低供料波动具有重要意义。不同落料量下的现场工程试验结果表明,高落料量下竖直结构的风粉混合器内平均负压偏小,几乎接近正压,且存在间断正压喷粉的现象,故该风粉混合器在高落料量下负压不足,易造成供料波动较大;高落料量下倾斜风粉混合器负压平均值仍大于-1 000 Pa,且无喷粉现象。相比于竖直结构,倾斜风粉混合器具有稳定且较宽的负压变化范围,能较好地克服供料波动大的现象。     

Spray and mixing characteristics of liquid jet in a tubular gas–liquid atomization mixer

For the design and optimization of a tubular gas–liquid atomization mixer,the atomization and mixing characteristics of liquid jet breakup in the limited tube space is a key problem.In this study,the primary breakup process of liquid jet column was analyzed by high-speed camera,then the droplet size and velocity distribution of atomized droplets were measured by Phase-Doppler anemometry (PDA).The hydrodynamic characteristics of gas flow in tubular gas–liquid atomization mixer were analyzed by computational fluid dynamics (CFD) numerical simulation.The results indicate that the liquid flow rate has little effect on the atomization droplet size and atomization pressure drop,and the gas flow rate is the main influence parameter.Under all experimental gas flow conditions,the liquid jet column undergoes a primary breakup process,forming larger liquid blocks and droplets.When the gas flow rate (Q_g) is less than 127 m~3·h~(-1),the secondary breakup of large liquid blocks and droplets does not occur in venturi throat region.The Sauter mean diameter (SMD) of droplets measured at the outlet is more than 140μm,and the distribution is uneven.When Q_g127 m~3·h~(-1),the large liquid blocks and droplets have secondary breakup process at the throat region.The SMD of droplets measured at the outlet is less than 140μm,and the distribution is uniform.When 127Q_g162 m~3·h~(-1),the secondary breakup mode of droplets is bag breakup or pouch breakup.When 181Q_g216 m~3·h~(-1),the secondary breakup mode of droplets is shear breakup or catastrophic breakup.In order to ensure efficient atomization and mixing,the throat gas velocity of the tubular atomization mixer should be designed to be about 51 m·s~(-1)under the lowest operating flow rate.The pressure drop of the tubular atomization mixer increases linearly with the square of gas velocity,and the resistance coefficient is about 2.55 in single-phase flow condition and 2.73 in gas–liquid atomization condition.     

Experimental evaluation of gas mixing with a static microstructure mixer

Mixing plays an important role in chemical reaction engineering. In the last years several types of static microstructure mixers have been developed. The characterization of microstructure mixing is difficult to perform as the dimensions are too small for conventional methods. Therefore, we report a method to characterize the mixing of two gases directly by measuring the concentration of the gases at the outlet of the mixer. The experiments have been carried out up to gas flows of 5000 ml/min STP per passage. The mixing degree and mixing length were determined as well as the mixing time was calculated. These values depend on the properties of the gases and other parameters as temperature and gas velocity. Thus complete mixing is achieved after a mixing length, i.e., the distance to the microchannel outlet, of only 300-800 μm. Corresponding mixing times are just 100-600 μs. Furthermore, discontinuities in the mixing characteristic can be explained with the results obtained. Also design parameters for a further improvement of the mixer geometry individually for various applications could be set up.     

新型静态混合器湍流特性数值模拟

结合新型静态混合器的结构特点,利用CFD软件采用标准的k-ε湍流模型对新型静态混合器内的湍流状态下的三维不可压缩流场进行数值模拟。通过研究新型静态混合器脉动速度分布的对称性及其间歇性发现:新型静态混合器内3个方向速度分量的偏斜因子和平坦因子分布具有周期性;x和z2个方向的速度概率密度分布存在较小不对称性且其平坦因子数值在2.3—5.7变化,径向偏斜因子的数量级均较轴向小1个数量级。采用新的数据处理方法计算和分析得到了不同长径比下新型静态混合器湍流流动阻力统一特性曲线及其关联式。     

脉冲式静态混合器压力降分析

测定了雷诺数Ｒｅ从６８变化到１６５１时混合器压力降与雷诺数，摩擦系数与雷诺数的关系曲线和相应的曲线拟合方程。Ｒｅ＜５５０时，摩擦系数ｆ＝１３９．６６Ｒｅ－０．７４２，当Ｒｅ＞５５０时，流动基本处于湍流状态，ｆ＝５．２Ｒｅ－０．２０３。当雷诺数增大时，混合器内元件数量的改变对摩擦系数影响不大。     

Dispersion of high-viscosity liquid-liquid systems by flow through SMX static mixer elements

The pressure drop and the dispersed phase drop size distribution have been measured for flow through SMX static mixer elements, in columns of diameter 41.18 and 15.75 mm, for a continuous phase of aqueous corn syrup and a dispersed phase of silicone oil. For single-phase flow the pressure drops were consistent with known literature correlations. In the presence of the dispersed phase the pressure drops were increased about 20% above the expected single-phase values, showing more short-term fluctuations but with no significant effect of the flow fraction of the dispersed phase. Droplet size distributions were measured by the computer-aided analysis of images from a digital camera. For shorter lengths of packing the distributions showed a significant “tail” at the large-diameter end, but as the packing length was increased the tail decreased or became non-existent. The mean drop sizes have been compared with a new model based on drop formation at equivalent point sources within the packing.     

The effect of interparticle cohesion on powder mixing in a ribbon mixer

The effect of interparticle cohesion on powder mixing in a ribbon mixer was studied by means of the discrete element method. It is shown that with an increase in the cohesion, the mixing rate and uniformity of mixing deteriorate, the coordination number increases indicating the loss of the ability of particles to be engaged in free flowing motion, and a majority of particles have a stronger tangential velocity allowing bulk angular motion of particles. Conversely, with a decrease in the cohesion, more particles have larger axial velocities, which will increase convective motion in the axial direction. When the cohesion is reduced, the number of particles having large radial stresses increases, and normal stress in the axial direction remains mostly unchanged. The ribbon mixer can mix cohesive particles in a wide range of the Bond numbers without causing large stresses. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1023–1037, 2016     

Microdynamic analysis of the particle flow in a cylindrical bladed mixer

A microdynamic study of the particle flow in a vertical axis mixer with slowly rotating flat blades has been performed by means of a modified discrete element method. The conditions are comparable to recent experiments conducted using positron emission particle tracking, with a mixer being in diameter, filled by 16,000 monosized spheres of diameter, and two blades rotating at a speed of . The dependence of flow behaviour on particle-particle and particle-wall sliding and rolling frictions is quantified and the results are used to establish the spatial and statistical distributions of microdynamic variables related to flow and force structures such as velocity, porosity, coordination number, particle-particle and particle-wall interaction forces. While the geometry and operational conditions are relatively simple, the particle flow is shown to be very complicated. There is a three-dimensional zone in front of a blade where particles have a strong recirculating flow. Increasing sliding friction coefficient or decreasing rolling friction coefficient can promote the formation of this zone. The flow and force structures of particles in the mixer are not uniform, although macroscopically steady flow is reached readily. The results show that increasing the rolling friction coefficient and, in particular, the sliding friction coefficient can increase the bed porosity and decrease the mean coordination number. The recirculating flow and the mixing kinetics are promoted by increasing the sliding friction coefficient or decreasing the rolling friction coefficient. Furthermore force arching is strong in the particle bed, with large inter-particle forces concentrating near the bottom corner just in front of the blade and propagating into the bed. Increasing the sliding or rolling friction coefficient increases the potential energy of particles in the mixer, but the kinetic energy is not sensitive to these coefficients. The increased potential energy gives increased particle-particle and particle-wall interaction forces and hence an increased torque required to drive the system. The results highlight the capacity and usefulness of numerical simulation in developing an understanding of the interplay of structure, forces, velocities and mixing in granular systems.