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.     

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.     

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。     

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。     

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.     

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.     

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。     

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.     

Controllable preparation and formation mechanism of BSA microparticles using supercritical assisted atomization with an enhanced mixer

Supercritical fluid assisted atomization introduced by a hydrodynamic cavitation mixer (SAA-HCM) was used to prepare bovine serum albumin (BSA) microparticles. Water was used as the sole solvent. A hydrodynamic cavitation mixer was applied to improve mass transfer and achieve a continuous near-thermodynamic-equilibrium solubilization of SC-CO₂ in the liquid solution. Under the different conditions, the prepared BSA microparticles had various morphologies, such as corrugated particles, smooth hollow spherical particles and cup particles, with particle diameters ranging from 0.3 to 5 μm. The microparticle formation process was elucidated with the shell formation and central bubble mechanism. Compared to native BSA, BSA microparticles did not show significant change in primary structure, according to the results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The secondary structure of BSA was characterized by Fourier transform infrared spectroscopy (FT-IR). No new peaks were observed after SAA-HCM processing. In addition, the crystalline structure of the BSA microparticles was demonstrated to be amorphous because of the sudden supersaturation in the precipitation process. The SAA-HCM process is expected to be a promising technique for producing microparticles suitable for pulmonary delivery of therapeutic macromolecules.     

A general correlation for pressure drop in a Kenics static mixer

A new pressure drop correlation in a Kenics static mixer has been developed. Pressure drop data were generated from computational fluid dynamics (CFD) calculations, avoiding the experimental limitations in obtaining comprehensive data enough for developing a reliable pressure drop correlation. Dimensional analysis reveals that the pressure drop characteristic of the Kenics static mixer can be described by three dimensionless groups, i.e., the friction factor, Reynolds number (Re), and aspect ratio of a mixing element (AR). A systematic graphical analysis led to a single master curve governing the pressure drop behavior of the Kenics static mixer, which had never been achieved before. We derived a pressure drop correlation fitting well with the obtained master curve in a general form into which the AR effect on the pressure drop is directly incorporated. Unlike the already existing correlations available in the literature, the correlation proposed in this study can cover the whole range of Re from laminar to turbulence. The reliability of the proposed correlation was validated by the comparison with various pressure drop data reported in the literature.     

双转子连续混炼机不同转子的混炼模拟

采用三维绘图软件PRO／E对双转子连续混炼机的转子进行了结构设计,利用GAMBIT对其进行网格划分及边界条件设定,并运用有限元流体分析软件POLYFLOW对转子部分的混炼过程进行了模拟分析,比较了转子转过不同角度时沿轴向距离的剪切速率、平均速度、黏度以及压力。分析得出,螺棱交汇点位置在其长度方向1／3处的转子存在着很明显的反向压力,即可以产生明显的有利于混炼的反向流动。该研究将有助于优化转子结构,从而进一步提高混炼效果。     

SK型静态混合器湍流速度脉动特性

以SK型静态混合器为研究对象,运用激光多普勒测速仪对管内的速度脉动进行测量。结果表明:湍流时在第1个元件内的轴向速度脉动较小,进入第2个混合元件后轴向脉动均方根可达平均流速的0.5—0.7倍且基本达到稳态;每个元件的入口有1个均方根可达到0.8—1.0倍平均流速的轴向速度脉动尖峰;流体离开混合元件后,各个方向上的速度脉动都迅速衰减,经过1个混合元件长度后脉动速度均方根衰减到平均流速的0.3倍左右,流过4—5个混合元件长度后基本衰减到混合器入口处的速度脉动水平。     

Rheology and phase separation of polymer blends with weak dynamic asymmetry

Rheological methods have been frequently used to study the phase separation behavior of partially miscible polymer blends. Usually the binodal temperature can be determined from the failure of time-temperature superposition (TTS) principle in isothermal experiments, or the deviation of the storage modulus from the apparent extrapolation of modulus in miscible regime in non-isothermal experiments. However, these methods are shown in this work to be not widely applicable even in blends with weak dynamic asymmetry due to the thermo-rheological complexity. A rheological model which is an integration of the double reptation model and the self-concentration model is found to describe the linear viscoelasticity of miscible blends quite satisfactorily, from which it is possible to follow the contribution from the miscible blends even in the two phase regime. Then, the binodal temperature is readily defined as the deviation of experimental data from such model prediction for miscible blends. Such method is successfully applied in a model polymer blend (poly(methyl methacrylate)/poly(styrene-co-maleic anhydride), PMMA/SMA) with weak dynamic asymmetry.     

Chaotic mixing in a barrier‐embedded partitioned pipe mixer

Inspired by the partitioned pipe mixer (PPM), a barrier‐embedded partitioned pipe mixer (BPPM) is designed and analyzed using a numerical simulation scheme. The BPPM is a static mixer, composed of orthogonally connected rectangular plates with a pair of barriers, which divide, stretch, and fold fluid elements, leading to chaotic mixing via the baker's transformation. The aspect ratio of the plate (α) and the dimensionless height of the barrier (β) are chosen as design parameters to conduct a parameter study on the mixing performance. The flow characteristics and mixing performance are analyzed using the cross‐sectional velocity vectors, Poincaré section, interface tracking, and the intensity of segregation. The results indicate that several designs of the BPPM significantly enhance the PPM's mixing performance. The best BPPMs are identified with regard to compactness and energy consumption. © 2017 American Institute of Chemical Engineers AIChE J, 64: 717–729, 2018