Theoretical and experimental testing of a scaling rule for air current segregation of alumina powder in cylindrical silos

Air current segregation (ACS) is one segregation phenomenon that has been identified to contribute significantly to the separation of fines (particles < 42 µm) from coarse material during the filling of industrial silos. This paper describes investigations of ACS for alumina powder based on experiments conducted in an industrial silo, in the laboratory and by computation, using the commercial computational fluid dynamics code Fluent.For the industrial silo, the extent of ACS has been measured using the accumulation of fine material at the wall as an indicator. Based on these results, modifications to the feeding system were undertaken which showed that ACS is promoted if the material is fed in a dilute form. Experiments in the laboratory confirmed this effect visually. In order to be able to compare numerically the extend of ACS, a segregation index has been developed. It was found that a dilute particle jet leads to more ACS than dense particle jet.The effects of solids feeding rate and air extraction rate on ACS have been investigated in the laboratory silo and the results clearly show that low solids feeding rates promote ACS. It was further found that an increase in the air extraction rate has a mild effect in suppressing ACS. These effects were confirmed by the Fluent simulations, which showed an unexpectedly good agreement with the experiments.     

Percolation segregation of multi-size and multi-component particulate materials

Percolation segregation of fines in multi-size and multi-component mixtures was quantified and compared with continuous mixtures. The quantification and comparison of percolated fines in binary, ternary, and quaternary vs. continuous mixtures was done at two strain rates of 0.25 and 0.5 Hz for both urea (spherical-shaped) and potash (angular-shaped). For this study, three coarse and three fines sizes for potash and three coarse and two fines sizes for urea were used for preparation of multisize and multi-component mixtures. Binary mixture samples were prepared from three mean coarse sizes with their corresponding three and two fines sizes for potash and urea, respectively. Ternary mixture samples were prepared from two coarse sizes at a time from the three available coarse size particles with their corresponding fines one each from three and two fines sizes available for potash and urea, respectively. Quaternary mixture samples were prepared from three coarse sizes with their corresponding two fine sizes (1850 μm, and 1550 μm) for potash and urea, respectively. The percent segregated fines mass and the normalized segregation rate (NSR) of fines decreased with the increase in number of coarse size components for a given fines size (from binary to quaternary), where NSR is defined as the amount of fines percolated from initial fines present in the binary mixture based on total time of operation kg/kg-h. The NSR decreased with the increase in order of mixtures, i.e., binary > ternary > quaternary at two strain rates of 0.25 Hz and 0.5 Hz (p < 0.05). Based on results, it was found that the segregation in continuous mixtures can be predicted by studying the multi-size mixtures.     

Experimental and DEM study of segregation of ternary size particles in a blast furnace top bunker model

Size segregation of pellets in the top bunker (hopper) of a blast furnace is an important factor affecting the radial distribution of the charged burden and indirectly also the distribution of gas in the shaft and cohesive zone. This paper studies size segregation of ternary size pellets during the discharging process of a hopper model through experiments and simulations. The simulations, which are based on the discrete element method (DEM), are first validated using four experimental cases applying different bunker filling methods. The effects of various variables, such as fine mass fraction, particle friction coefficients, diameter ratio of fine to coarse and filling method (random, layered or industrial filling), as well as the interaction with wall (static and rolling friction) on the segregation are investigated. The results show that even though many factors affect the extent of segregation during the discharging process, the most important factors are the filling method, diameter ratio of fine to coarse, wall-particle static and rolling friction, interparticle rolling friction as well as mass fraction of fine particles. Reducing wall-particle rolling or static friction or the fraction of fine particles decreased the extent of size segregation.     

The effects of air and particle density difference on segregation of powder mixtures during die filling

Segregation of mono-disperse binary mixtures with different particle densities during die filling in the presence of air was numerically analysed using a coupled discrete element method (DEM) and computational fluid dynamics (CFD) approach. Die filling with powders of different particle density ratios (i.e. the ratio of the heavy particles to the light particles) at various shoe speeds was simulated, in order to explore the effects of air and particle density difference on segregation. For die filling from a stationary shoe, the air can induce significant segregation by hindering the deposition of light particles (i.e., air-sensitive particles). As the particle density ratio increases, the light particles are deposited into the die at even lower speeds compared with the heavy ones due to the effect of air drag, resulting in an increase in the degree of segregation. For die filling with a moving shoe, segregation occurs due to different post-collisional velocities resulting from different particle inertia; and the degree of segregation increases as the particle density ratio increases due to the increasing difference in particle inertia. It is found that, as the shoe velocity increases, the powder flow pattern changes from nose flow dominated to bulk flow dominated and the degree of segregation generally decreases. The effect of air is limited for nose flow dominated die filling because the air can easily evacuate through the gap between the die walls and flowing powder stream. When bulk flow dominates in die filling, the air can be entrapped in the die, which has a significant impact on the powder flow and segregation behaviours. Finally, the effect of interparticle friction on segregation was investigated.     

Improving the Removal Efficiency of Cyclones by Recycle Stream

An innovative way to increase the overall efficiency of a cyclone is presented. By recycling a fraction of the output stream to the inlet duct, the efficiency of the cyclone can be improved. Experiments were conducted using bench scale (0.1 m diameter) and pilot scale (0.456 m diameter) cyclones. The overall efficiency was evaluated for both cyclones at different ranges of recycle ratios. In the presence of the recycle stream, the efficiency increases in a monotonic manner. It may approach 100 % with the use of very high recycle ratios, provided that there is agglomeration of the fine particles available in the feed stream and/or adhesion of fines to the coarse particles. From the experimental results, it is observed that the agglomeration of fine particles is the most important factor for improving the cyclone efficiency by use of a recycle stream. Moreover, the effect of the recycle stream persists, even if the inlet velocity surpasses the saltation velocity of the cyclone.     

Granular segregation in discharging cylindrical hoppers: A discrete element and experimental study

The segregation of granular materials due to differences in particle properties occurs during a variety of handling and transport processes, such as flow from a hopper. In the present work, the discrete element method (DEM) is used to investigate segregation of granular materials during discharge from a hopper. The effects of various particle properties and hopper geometries on the segregation of a spherical, bidisperse granular material during hopper discharge are studied. Particle contacts are modeled using a soft-particle model consisting of a hysteretic spring system and sliding friction. The effects of the particle diameter ratio, density ratio, fines mass fraction, hopper wall angle, hopper cross-sectional shape, and the initial fill conditions are investigated.These computational results are compared to those from a small experimental system with the same hopper dimensions and particle properties. The use of this small-scale system permits a novel, one-to-one comparison with the DEM model predictions for the purpose of model validation. The experiments utilize bidisperse glass spheres in a small, Plexiglas cylindrical hopper that is used in the ASTM International standard test for sifting segregation. Particles are discharged from either a ‘mass-flow’ or ‘funnel-flow’ hopper design and collected transiently in equal volumes until the hopper is empty. Analysis of the weight fractions of fine and coarse particles is conducted by sieving. A comparison of the computational and experimental results provides an indication of the model's success at predicting segregation during hopper discharge and the applicability of the DEM model to other granular flow systems.     

Percolation segregation of binary mixtures under periodic movement

Three strain rates of 1.0, 0.5, and 0.25 Hz were selected for studying percolation segregation in binary mixtures of urea (spherical) and potash (angular). Mixed binary samples prepared from three mean coarse sizes with their corresponding three and two fines sizes for potash and urea, respectively. Herein, three coarse mean sizes 3675, 3075, and 2580 µm and three mean fine sizes 2180, 1850, and 1550 µm were selected for tests. Percolation segregation in mixed binary sample was quantified using the primary segregation shear cell (PSSC-II). Based on experimental results, the segregated fines mass, normalized segregation rate (NSR) and segregation rate of fines for binary mixtures increased with increasing strain rate from 0.25 Hz to 1.0 Hz. The NSR decreased with decreasing strain rate from 1.0 Hz > 0.5 Hz > 0.25 Hz for size ratios 1.7, 2.0, and 2.4 (p < 0.05). At these three strain rates, for size ratio 2.0, the NSR of coarse size 3675 µm with fines size 1850 µm was smaller than the NSR of coarse size 3075 µm with fine size 1550 µm in the binary mixtures (p < 0.05). At three strain rates of 1.0, 0.5, and 0.25 Hz, the NSR for potash was higher (53%, 56%, and 46%) than the NSR for urea for the same size ratio (p < 0.05).     

进料位置与风速对旋风分级器颗粒分级效果的影响

根据旋风分级器内气流速度分布特点进行了进料区域划分,运用非稳态离散相模型和分级实验对比了3个代表性进料位置对颗粒运动轨迹及分级精度的影响,分析了1 μm和10 μm颗粒在不同区域内的受力情况。结果表明,边壁区域进料造成粗组分中细粉夹带现象严重,分级精度差;中部进料区域内流场强度大,粗颗粒受离心力强,细颗粒受轴向气流曳力大,有利于减少颗粒在分级区的停留时间,实现粗、细颗粒的快速分级,对改善分级精度有利;中心位置进料延长了粗颗粒的分级运动路程,增加了粗组分跑损的概率,模拟计算15 μm的粗颗粒进入细组分的质量分数达到11.7%。经实验验证,入口气速在10～22 m·s^-1,中部区域进料时分级后粗、细组分粒度分布曲线重合区面积最小,分级粒径比率值平均提高了25.3%,研究结果为离心气流分级设备的进料位置设计提供了一定的指导。     

Quantitative measurement of particle segregation mechanisms

Pharmaceutical, food, chemical, mining, and energy industries routinely handle and process materials with different particle sizes. Often the different size materials represent different chemical components. Pharmaceutical mixtures are typically mixtures of fine, active, with relatively coarse, incipient ingredients. Food industry mixtures combine particles with very different sizes and shapes. There is a great tendency for these dissimilar materials to separate during processing and handling. The cause of this separation depends on the size and shape of the particles in the mixture. If the fines are less than one-third the size of coarse particles and free flowing, they may percolate through the coarse matrix of particles resulting in sifting segregation. If the particle shapes are different, then the internal friction angles of individual components may be different, resulting in angle of repose segregation. Preventing segregation of a given material mixture depends on the cause of the segregation. If angle of repose segregation is the predominate mechanism, then segregation mitigation can be accomplished by limiting pile formation. However, if sifting segregation is the predominate mechanism, then limiting pile formation may not limit the segregation if the process equipment is subject to significant inter-particle motion or shear. This paper presents a method of measuring the magnitude of sifting segregation occurring in bulk material. It also includes data that relates bulk material strength to separation tendency. This relationship shows an inverse functionality between the yield strength of the material and the bulk segregation. Finally, the work presented in this paper compares segregation due to sifting and repose angle mechanisms to provide a means of differentiating between these mechanisms.     

Flow Channel Boundaries in Silos

The critical slip planes at the silo filling state are compared with the flow channel boundary during silo discharge for semi‐mass flows. The static critical slip planes are determined by using the dynamic programming method based on the stress field of granular solids stored in silos at the filling state. The flow channel boundary is estimated through the finite element analysis of the silo discharge. The results indicate that the critical slip line lies above the flow channel boundary. This characteristic can be attributed to the changeover of major principal stress directions of granular solids from the silo filling to the silo discharge. The analysis demonstrates that the silo wall friction tends to lift up the critical slip plane and flow boundary. A simple correlation is developed between the positions of critical slip planes and flow boundaries and is experimentally verified.     

Bench-scale tests to determine mechanisms of charge generation due to particle-particle and particle-wall contact in binary systems of fine and coarse particles

Bench-scale methods were utilized to determine changes in electrostatic charges and their mechanisms for various coarse and fine particles as they came into contact with each other and/or their containing vessel walls. Techniques included shaking tests and particle-copper plate contacting experiments. Electrostatic behaviour of coarse particles (glass beads and polyethylene) and fine particles (Larostat 519, glass beads and silver-coated glass beads) were investigated. Shaking tests resulted in charge separation in which the fine particles acquired significant positive charges, opposite to those carried by the large particles. In copper-plate contacting tests, charge transfer occurred between the fines and the copper plate with fines carrying away almost all of the initial charges on the plate followed by further charge separation. Charge separation was found to be the dominant charging mechanism between the coarse particle and copper plate, with the particles becoming negatively charged.     

Austragorgane und Austraghilfen

Feeders and Discharge Aids. Feeders and discharge aids have an influence on the flow behaviour of bulk solids in silos. Together with the geometric form of the silo, they are responsible for the operating conditions of a silo. Therefore, feeders and discharge aids have to be chosen and installed under consideration of the flow properties of the bulk solid to be stored. The necessary information is obtained by using the silo design method due to Jenike. This method yields the geometrical data of the hopper (wall steepness, outlet diameter) necessary for a troublefree operation (no doming and ratholing; mass flow; no segregation; uniform flow). Furthermore, the design of feeders and discharge aids plays an important role in determining their function. For example, stagnant zones will build up and cause flow problems on protruding edges and on walls which are not steep enough. For the correct function of the hopper/feeder combination it is important to make sure that the feeder fully activates the hopper outlet and stagnant zones cannot build up. This can be ensured by an appropriate feeder design (increasing capacity in direction of conveying). The driving power of a feeder has to be calculated also on the basis of the flow properties of the bulk solid to be stored.     

细颗粒对气固流化床中静电行为的影响

聚合物颗粒粒径和残余催化剂量（又称灰分）不同,会影响颗粒的功函数、接触面积和发生接触时的电荷转移数目等,故粒径和灰分含量不同的同种聚合物颗粒在相互接触时对静电的贡献并不相同,尤其是灰分含量较高的细颗粒对静电的影响至关重要．通过在f150mm的流化床冷模装置中,对聚乙烯颗粒－氮气体系进行流态化实验,分别测量了向含较大聚乙烯颗粒的流化床中添加不同粒径的同种聚乙烯细颗粒后的静电压,发现床内电压与所加细颗粒粒径、细颗粒重量分率及细颗粒中灰分含量密切相关,根据上述现象进而提出了包含颗粒粒径作用项和灰分含量作用项的细颗粒作用因子．在实验条件范围内,作用因子小于1.0的三种细颗粒的加入对床内静电压产生影响甚微．而平均粒径最小同时灰分含量最高的细颗粒加入后对床内静电压影响明显,当其作用因子小于1时,同样不会引起床内静电压的太大改变;且当作用因子小于0.5时,静电压还略有降低;而作用因子一旦大于1,床内静电压显著升高;当作用因子继续升高,静电压又有所下降,但同时由于细颗粒的含量增加,粘壁现象变得非常严重．     

Flow pattern measurement in a full scale silo containing iron ore

The flow pattern in a silo is important because it affects both the recovery of solids and the pressures on the silo wall during discharge. Wherever mass flow is not achieved, the boundary of the flow channel has significant implications for both the functional and structural design of the silo. Many techniques have been used for the study of flow patterns in model silos, but most cannot be used at full scale, and very few quality measurements at full scale have ever been made. This paper outlines a full scale experimental study in which the patterns of solids flow and the flow channel boundaries are reliably quantified.The full scale silo was specially designed, constructed and instrumented to exhibit funnel flow and to make observations of the solids flow pattern and the silo wall pressures. It had three outlets: one concentric, one fully eccentric and one in between. Three materials were used: iron ore pellets, slag fines and crushed basalt. This paper describes experiments involving iron ore pellets. The silo was seeded with radio frequency tags whose residence times were measured by detecting them on exit during discharge. The residence time data were studied to deduce the discharge flow pattern. This paper presents the results of three different flow pattern interpretation techniques: the best of them (mass-time relationships) is shown to give a very clear identification of the solids flow pattern and the flow channel boundary.     

进气方式和气速对卧轮式分级机性能的影响

传统卧轮式分级机内流场分布比较混乱，分级精度普遍不高。本文基于对分级机内气流运动规律的分析，将传统切向进风方式调整为径向进风，分别设计了百叶窗型和多孔型风筛，试验对比了传统切流风筛和上述两种径向进风方式对分级效果的影响。结果表明，径向进风方式对分级流场形成和细颗粒淘洗更有利，不仅可以提高分级机的粗粉产率，同时还可降低粗组分中的细粉夹带量，提高粗、细颗粒分离的程度，改善颗粒分级效果；百叶窗型风筛分级机的分级效果最优，气流经百叶窗风筛可对粗组分进行多层、充分扬析，减少细颗粒误入粗产品的概率，牛顿分级效率较传统切流风筛分级机平均提高约6%；此外，入口气速也对分级精度有较大的影响，但对分级粒径的影响不明显，存在临界入口气速使得综合分级效果最好。为提升涡轮分级机的颗粒分级性能提供了新思路。     

Prediction of the porosity of multi-component mixtures of cohesive and non-cohesive particles

This paper presents an experimental and theoretical study of the packing of mixtures of cohesive (fine) and non-cohesive (coarse) particles. The experimental results, produced by means of a standard funnel packing method with glass beads as experimental materials, are first used to depict the similarity between the packings of fine and coarse particles. On this basis, the so-called linear packing model is extended to estimate the porosity of mixtures of fine and coarse particles with a wide size range. Its interaction functions and equivalent packing size are determined empirically. The applicability of the resultant model is demonstrated by the good agreement between the predicted and measured results for typical packing systems, including particles with the Gaudin–Schuhmann distribution or with a mixture size distribution. Finally, the packing of particles with a lognormal distribution, involving both cohesive and non-cohesive particles, is investigated in detail. This example also demonstrates how the proposed model can be used to solve a packing problem.     

Silo Discharge: Measurement and Simulation of Dynamic Behavior in Bulk Solids

This paper deals with the experimental investigation and the numerical simulation of silo discharge processes, including dynamic interactions between the silo filling and the elastic silo walls. The discharge process is described by a system of nonlinear differential equations. Via the Finite Element Method (FEM) based on an EULERian reference frame, the deformation rate, the velocity field, the porosity, and the stress distribution can be calculated without the need of re‐meshing the FE‐grid. To compare simulation results with measured data, the numerical simulation examples are chosen to be similar to an experimental test‐silo of the Institute of Mechanical Process Engineering at the Technical University of Braunschweig. Optical measurement techniques are applied to investigate the flow profile, and load cells on the silo walls register the stress evolution, e.g., a stress peak (switch) moving from the outlet to the transition from hopper to shaft.     

大直径浅圆仓缩尺模型试验研究

大直径浅圆仓在生产过程中,由于仓底的偏心卸料使筒仓仓壁产生不均受力,会给筒仓结构的使用带来安全隐患。通过对浅圆仓缩尺模型试验,模拟筒仓生产过程的卸料状况,并对筒仓仓壁及仓底廊道顶部进行压力测试,将测试结果与理论计算进行对比,从而得出偏心卸料与静载状态的超压系数,为大直径浅圆仓设计提供依据。     

The simulation and experimental study of granular materials discharged from a silo with the placement of inserts

The simulation method of DEM and experimental method are used to investigate the flow pattern of the filling and discharging process for two-dimensional plane silos. Two kinds of inserts (conical insert and BINSERT®) are used in the silo to change the flow fields of the silo. The placement of inserts improves the flow behaviors of funnel flow type to mass flow type during discharging. The wall normal stresses are influenced by the change of the flow type. The effects of using differently shaped inserts on the flow pattern and wall stress are analyzed in this study. The controlling parameters include the silo half-angle, the orifice width, the shape of the insert and the properties of the granular materials. The simulation and experimental results are in good agreement.     

Pressure measurements in steel silos with eccentric hoppers

It is known that concentric filling and discharge are the most recommended solutions in silo design. However, different reasons such as costs, space problems and the client's specific requirements have a significant influence on the final design. Therefore, engineers sometimes need to make use of eccentric filling and discharge solutions. These designs are usually related to phenomena such as unfavorable switches in the flow regime, associated with extreme unsymmetrical pressure distributions around the silo wall, oscillations of the load, etc., which may cause failures or instabilities of silo structures, involving then high costs for the industry and, in the worst cases, accidents where human life may be endangered. Understanding about these phenomena associated with hopper eccentricities is still needed.This research work deals with pressure measurements on three steel silos with different hopper eccentricities. The results not only confirm, at a qualitative level, phenomena that have also been described in previous research, but also provide new knowledge concerning non-symmetric loads in wheat and maize silos with conical hoppers of different eccentricities. Non-symmetric components of the pressures and shifts in flow regimes have been studied and discussed in relation to load assessment for the design of silos.