The angle of repose of bulk corn stover particles

Lignocellulosic biomass feedstock such as corn stover, the residues left on the field after corn grain harvest, has been studied as one of the renewable feedstocks to be used for fuel ethanol conversion in the near future. The primary objective of this work was to determine the angle of repose (AoR) of bulk corn stover particles prepared to four particle sizes (chopped and particles screened through 6.4, 3.2 and 1.6 mm) at two moisture contents (dry, < 10% and wet, > 20%). The results show that particles size and feedstock moisture content were important variables that affected the angle of repose for all three angle of repose methods (piling AoR loose-base, piling AoR fixed-base and sliding AoR) investigated. In general, increasing moisture content and particle size increased the piling AoR (loose-base), piling AoR (fixed-base) and sliding angle of repose. Characterization of the flow behavior of bulk corn stover particles using the piling AoR (loose-base) and comparison with three granular bulk solids of biological origin (corn, soybean and distillers dried grains with solubles, DDGS) indicated that dry and wet stover particles of the particle size ranges tested in this study have a poor flow behavior.     

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.     

A 3D semi-digital model for the placing of granular materials

A 3D semi-digital model has been developed in order to simulate the gravitational placing of granular mixtures composed of spherical particles of different sizes allowing the optimization of mixtures proportions. The aim is to control concrete placing in formworks particularly in the presence of reinforcement bars. In this case, the representation of a large number of particles is necessary and segregation or wall effects have to be taken into account. Moreover, an easy representation of complex geometries and obstacles of arbitrary shapes should be possible. In order to respect these conditions, the used 3D semi-digital model is based on the digitization of a calculation volume and on the representation of spherical particles with real numbers. The model takes into account only steric repulsion between particles and simulates their collisions during placing by random displacements. The simplicity of the model allows the simulation of up to 2 million spherical particles without spending too much computational time. The model can therefore be used for the simulation of granular materials having a spread granulometry-like concrete.The packing density of binary mixtures of spherical particles with different size ratios and varying proportions of small and big particles is first studied. The results are compared successfully with theoretical packing density of binary mixtures without interactions. It is therefore demonstrated that the semi-digital model can be used for the simulation of binary mixtures having low size ratios (0.1), which corresponds approximately to the size ratio of fine and coarse aggregates in concrete.Secondly, digitized cylindrical obstacles are added in the systems with the aim to represent reinforcement bars in a formwork. The influence of small and big particles proportioning on the local packing density and the homogeneity of the mixture is studied. The results show that in the presence of obstacles, the optimum placing is obtained when the proportion of small particles in the mixture is higher than that without obstacles. This is due principally to the looser packing of big particles around obstacles.     

Fluid Dynamics of a Sand‐Biomass Mixture in a Spouted‐Bed Reactor for Fast Pyrolysis

The spouted‐bed reactor represents an interesting alternative to pyrolysis as compared with conventional fluid beds due to its better performance in handling coarse and irregular materials, requiring lower fluidizing flow rates and providing intense thermal contact. The fluid dynamics of a mixture of sand and sugarcane bagasse in a spouted bed was investigated. Since this process involves a mixture of solids of different sizes, shapes, and densities, particle segregation was also analyzed. The results provided significant insights about the segregation phenomenon which may cause severe operating problems during pyrolysis. Various mixture compositions were identified in which the particles exhibited good circulation in the bed.     

立磨选粉机分级环间距对分级性能的影响

分级环间距大小是影响选粉机分级性能指标的重要因素之一。通过构建不同分级环间距的立磨选粉机模型,采用Fluent软件对SMG5500型立磨选粉机不同分级环间距下的流场特性进行研究,对比分析间距大小对速度场、压力场和分级效率的影响规律,得出最优的分级环间距,并对整机进行实验验证。数值模拟结果表明:当分级环间距过小时,大部分大于80 μm的粗颗粒都能进入转笼,使产品极易出现跑粗现象;当分级环间距过大时,大部分小于80 μm的颗粒不能进入转笼,这使得选粉机的循环负荷加大,选粉效率受到了极大的限制;当分级环间距为110 mm左右时,SMG5500型立磨选粉机的分级性能最优。     

超细颗粒聚团流化的临界流化速度

在内径50 mm的流化床实验台上,测量SiO₂、Al₂O₃和TiO₂ 3种超细颗粒原生粒径从30 nm增加到5 μm的临界流化速度（U_mf）,并以Geldart A类颗粒（粒径45 μm）为参照。结果表明：3种超细颗粒的U_mf随粒径的变化规律一致,随原生粒径从30 nm增加到5 μm,U_mf逐渐增大;当颗粒粒径增加到45 μm,U_mf大幅度减小,其与原生粒径为30和200 nm时接近。对于不同材料,U_mf由大至小的顺序依次为TiO₂、Al₂O₃、SiO₂。粉体安息角测量表明：对于同种材料颗粒,原生粒径对超细颗粒的U_mf和安息角的影响规律一致,即5 μm超细颗粒的安息角最大。聚团尺寸模型计算表明：稳定流化时,聚团尺寸随原生粒径的变化趋势以及随不同材料的变化趋势均与U_mf的变化趋势一致。研究结果为超细颗粒流化临界速度预测研究奠定了基础。     

Investigation of flow properties of metal powders from narrow particle size distribution to polydisperse mixtures through an improved Hall-flowmeter

Initially, we critically examined the results provided by the improved Hall-flowmeter, based on bulk volume flow rate, with narrow particle size distribution metal powders (iron, aluminium and copper) of different sizes (fine to coarse) and shapes (nearly spherical to non-spherical). Binary and ternary mixtures of various combinations of fine (< 100 μm) and coarse (> 100 μm) metal powders at different size ratios and weight fractions were allowed to discharge from a mass flow hopper. The results show that the mass flow rate of polydisperse mixtures of metal powders is affected by four factors: the size ratio, the volume fraction of the smallest sieved fraction, the initial mass flow rate and the shape of metal powders.     

Cohesive effects in powder mixing in a tumbling blender

Rotary drums are used as mixers, dryers, kilns and granulators. In all of these systems, powder cohesion deeply affects mixing and segregation, and it is critical in process scale up. In this paper, we focus on the effect of cohesion in mixing and size segregation of binary mixtures of uniform and non-uniform sizes in a partially filled rotating drum. The cohesive force between particle is simulated using a square-well potential and the numerical model is used to characterize flow and mixing properties. The model is validated by comparison to experimental images. Results show a time-dependent spatial distribution of cohesive powder that depends on the magnitude of cohesion and friction. In uniform binary systems, as cohesion increases, the rate of mixing first increases and then decreases, however for the case of non-uniform binary systems, we observe different mixing patterns depending on the relative magnitude of forces acting between particles of same/different sizes. Unlike free flowing material, for cohesive mixtures, a higher rotation speed is found to enhance mixing performance.     

A perspective on vibration-induced size segregation of granular materials

Segregation of particulate mixtures is a problem of great consequence in industries involved with the handling and processing of granular materials in which homogeneity is generally required. While there are several factors that may be responsible for segregation in bulk solids, it is well accepted that nonuniformity in particle size is a fundamental contributor. When the granular material is exposed to vibrations, the question of whether or not convection is an essential ingredient for size segregation is addressed by distinguishing between the situation where vibrations are not sufficiently energetic to promote a mean flow of the bulk solid, and those cases where a convective flow does occur. Based on experimental and simulation results in the literature, as well as dynamical systems analysis of a recent model of a binary granular mixture, it is proposed that “void-filling” beneath large particles is a universal mechanism promoting segregation, while convection essentially provides a means of mixing enhancement.     

Segregation and mixing effects in the riser of a circulating fluidized bed

Segregation and mixing effects of binary mixtures of particles having difference in sizes and densities were studied in 0.1016 m-diameter riser of a circulating fluidized bed at gas velocities between 2.01 and 4.681 m/s and solids circulation rate between 12.5 and 50 kg/m² s. Two groups of bed materials (three quartz sand-spent fcc catalyst mixtures with different initial mass % of sand and two coal-iron mixtures, one with almost same sizes but with different densities and the other having both different sizes and densities) were used. Using local axial mass % of heavier/coarser particles and their mean sizes the extent of segregation was evaluated. The influence of operating conditions like superficial gas velocity and solids circulation rate on segregation was examined and found that with their increase segregation effects generally tend to decrease and a uniform mixture conforming to initial composition of the mixture results. Using the data available in the literature and those of the present authors an empirical correlation to obtain the extent of segregation in CFBs has been proposed.     

Comparison of different flowability tests for powders for inhalation

A series of placebo powders for inhalation was characterized regarding bulk density and powder flowability using different techniques. The powders were of the ordered mixture type and were prepared by mixing a pharmaceutical carrier grade of lactose with different fractions of intermediate sized and fine (i.e., micronized) lactose. A modified Hausner Ratio was obtained by measurement of the poured and the compressed bulk densities. Other tests investigated were the angle of repose, the avalanching behaviour using the AeroFlow, and the yield strength using the Uniaxial tester. Furthermore, the relation between ordered mixture composition and flowability was examined.Of the methods investigated, the modified Hausner Ratio discriminates well between the investigated powders and seems to have the widest measuring range. It was also found that the poured and compressed bulk densities provide information about the packing of the particles in the powders. A good correlation was obtained between the modified Hausner Ratio and the angle of repose. The AeroFlow was suitable for powders with a low percentage of fine particles, but could not discriminate between the more cohesive powders. The Uniaxial tester, on the other hand, seems to be better suited for more cohesive powders.Regarding the powder composition, addition of micronized particles has a strong influence on the flowability of ordered mixtures, while intermediate sized particles have little impact on the powder flow.     

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.     

Particle segregation in a spouted bed of binary mixtures of particles

Minimum spouting velocity and segregation behaviour of binary mixtures of particles differing in size have been studied. The experiments were carried out in a bed of 20 cm diameter at superficial gas velocities up to 1.3 U_ms by use of silica sand of four different particle sizes from 0.655 to 2.23 mm. An empirical equation was proposed for the minimum spouting velocity of binary mixtures. The effects of the particle size difference and the superficial gas velocity on segregation were investigated. Results showed that considerable radial segregation as well as axial segregation occurred even for high gas velocity under the condition of large particle size difference.     

Determining the Strength of Coarse‐Grained AlON and Spinel

The strength of two coarse‐grained (grain size > 200 μm) cubic ceramics, a magnesium aluminate spinel (MgAl₂O₄) and an AlON , along with a fine‐grained (1.5 μm) MgAl₂O₄, was determined by conducting a series of four‐point and equibiaxial flexure tests on specimens of different sizes. Weibull strength size scaling revealed a linear relationship on a log–log plot between average flexure strength and effective specimen area for the fine‐grained spinel, but a nonlinear relationship for both coarse‐grained materials. Initial fractography showed that each material had a single flaw population limiting the strength over the entire specimen size range, which does not account for the nonlinear size scaling relationship in the two coarse‐grained materials. However, further fractography revealed that in both materials there was an initial flaw and a critical flaw. The former appears to be machining/polishing damage that started the fracture process while the latter was a cleaved grain in AlON or a cracked grain boundary in the HP/HIP spinel that lead to fracture of the specimen. The difference between the initial and critical flaw size coupled with a detailed analysis of the strength as a function of test specimen thickness accounted for the nonlinear strength size scaling relationship. As a result, strength values obtained using thin test specimens can lead to an erroneous strength prediction for large components made of these ceramics. The implication of these findings is that strength tests must be conducted using appropriately thick specimens to obtain a representative strength value. If appropriately thick specimens cannot be tested, then fractography must be conducted to determine the flaw size. If the flaw size is sufficiently large, compared with the specimen thickness, then the strength must be adjusted according to a stress field correction factor to obtain a more accurate strength value.     

Segregation in beds of large particles at high velocities

In some industrial fluidized bed processes, notably coal combustion, the bed contains a very wide size range (50 – 5000 μm) of equal density particles. In others the particles change their density as the reaction proceeds, giving a bed of particles having similar sizes but densities varying by a factor of up to 2.

Experiments have been done in a bed 0.29 m diam. at velocities up to 5 m/s, using coarse particles up to 6 mm. They show that segregation by density difference can be reduced to negligible proportions by using high velocities, but that segregation by size appears to be an intrinsic feature of coarse particle systems at all velocities when a very wide size range is present. A tentative form of equation is proposed for segregation by size; the equations of Rowe are useful predictors for segregation by density difference.     

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).     

Effect of liquid addition on the bulk and flow properties of fine and coarse glass beads

The effect of water on the packing and flow properties of fine and coarse particles was experimentally investigated. Four different particle sizes of glass beads, from 5 to 275 μm, were studied with increasing water weight‐percentages. Using a FT4 Powder Rheometer, changes in bulk properties were collected as a function of water content and particle size. The results show that water content plays a significant role on the packing and flow of the particles. Small amounts of water created porous aggregates due to liquid bridging. Greater amounts of water resulted in the filling of the void‐spaces. This was indicated by an increase in basic flow energy, density, and pressure drop, with a decrease in porosity. A greater understanding of bulk properties of wetted material is useful to develop standard systems that can be used to examine the behavior of more complex situations, and implement changes to improve materials handling and processing. © 2015 American Institute of Chemical Engineers AIChE J, 62: 648–658, 2016     

复合固体推进剂中固体填料的安息角测试与仿真

复合固体推进剂含有固体颗粒较多,离散单元法是一种适合固体推进剂生产过程数值仿真的有效方法,颗粒物料的接触参数是保证离散单元法仿真精度的关键。本文以复合固体推进剂的主要组分铝粉和高氯酸铵固体颗粒为研究对象,通过实验测试获得了相关物料的安息角,利用专业离散元软件EDEM仿真模拟了安息角测试实验过程,建立了物料安息角与接触参数之间的联系。研究表明,滚动摩擦系数和滑动摩擦系数越大,安息角越大,物料流动性越差。对比仿真与实验结果,通过逆向反推法确定了物料的滑动摩擦系数和滚动摩擦系数两个关键接触参数。铝粉与高氯酸铵1∶2混合颗粒的滑动摩擦系数为0.2,滚动摩擦系数为0.05。为固体推进剂加工生产过程离散元数值仿真提供了关键基础数据。     

Influence of the Metal Particle Size on the Ignition of Energetic Materials

A theoretical study of multi‐particle ignition uses a hot spot model which calculates the temperature evolution of individual hot spots in an energetic material. It indicated that ultra‐fine hot particles would be very effective in igniting energetic materials if impinging and penetrating the solid propellant. Igniting mixtures were prepared containing ultra‐fine Ti particles which react fastest compared with coarse particle mixtures, standard B/KNO₃ and black powder. The ultra‐fine particles, however, are obviously oxidized or gasified too fast as to reach the energetic material to be initiated and longer ignition delays are found mainly compared with coarse particle mixtures. An optimized mixture of coarse and ultra‐fine particles would give an improvement of ignition delay times.