Bulk coarse particle arching phenomena in a moving bed with fine particle presence

Bridging or arching of flowing solids particles is a serious hazard in the operation of moving bed systems. The mechanics of the arching has been extensively analyzed in the context of particle discharge from a hopper with conical geometry by considering the particulate layer stress distribution. However, bridging can also occur in a moving bed system with cylindrical geometry during the continuous mass flow of solids particles. Experimental work conducted in this study reveals that the appearance of solids bridging is normally accompanied by the presence of fine particles in the coarse moving particles as well as by the countercurrent interstitial gas flow. In this study, a stress analysis of the layered particles distributed in a cylindrical, vertical moving bed that flows downward opposing to upward flow of the interstitial gas is developed to quantify the bridging phenomenon. The analysis takes into account of the effects of presence of fine powder in the coarse particle flows and properties, such as particle‐size distribution, bed voidage, and interstitial gas flow rate. The experimental validation of the present stress analysis for moving bed systems with varied fine and coarse particle concentration distributions, and interstitial gas velocities is also conducted. The stress distributions of the particles under flowing and arching conditions are obtained. An arching criterion is formulated, which indicates that the critical radius of the standpipe to avoid arching phenomenon is only related to the property of the bulk solids in the present geometric configuration of the flow system. © 2014 American Institute of Chemical Engineers AIChE J, 60: 881–892, 2014     

Spontaneous inter-particle percolation: A kinematic simulation study

Spontaneous inter-particle percolation is a mechanism of particle segregation which arises when a particle falls through the voids among large particles due to gravity or other applied force, the size difference being such that this can occur without the need for applying strain. It is a phenomenon arising in some packed bed processing operations and in some dispersers for particles. Here the mechanism is modelled by computer simulation. The model uses particle sizes, a coefficient of restitution and also, to cater for less elastic materials, a coefficient of friction. This yields percolation velocities quantitatively consistent with prior laboratory studies, showing the correct dependence on coefficient of restitution and particle size. The simulation clarifies the dependence of radial dispersion on packing height. The radial distributions are well described only at higher coefficients of restitution, pointing to the need for a better means of describing collisions between percolating and packed particles.     

对焦炭塔塔鼓变形失效的机理分析

焦炭塔是延迟焦化反应的反应釜,是延迟焦化装置的重要组成部分,其长期安全的运行是炼油企业取得高效益的前提和保障.但由于工作条件的恶劣,焦炭塔普遍存在着塔体变形、裙座及塔体焊接开裂等问题,严重影响着焦炭塔的安全运行.究其原因目前主要有以下几种情况:高温蠕变的结果;低周热疲劳的结果;高温蠕变与低周热疲劳共同作用的结果冷;急热温差热应力引起的局部塑性变形.通过对高温蠕变与低周热疲劳的产生条件以及损坏特征的仔细研究,并辅助以各种试验的结果,用排除法确定了焦炭塔的腰鼓变形失效的原因为急冷、急热温差热应力导致的局部塑性变形,并且总结出了其变形失效的规律和防治的具体方法.     

Clustering in gas-fluidized riser flows of flexible fibers

Clustering of flexible fibers in riser flows is investigated using a hybrid approach of Discrete Element Method and Computational Fluid Dynamics. Unlike spherical particles, the flexible fibers possess elongated shape, undergo significant deformation, and dissipate kinetic energies through rapid fiber deformation. The present studies show that these distinct features have significant impacts on the cluster characteristics of the fibers. A larger fiber aspect ratio leads to larger number and sizes of agglomerates, while it causes a reduction in heterogeneity of solids distribution due to the more dilute clusters with reduced packing densities. As the fibers become more flexible, the heterogeneity increases, and denser clusters are obtained. More significant effects of the fiber flexibility on the clustering are observed for the fibers with larger aspect ratios. The increased energy dissipation through the rapid fiber deformation enhances the clustering by augmenting the number and size of the agglomerates.     

微液滴制备及其在静止流体中的浮升过程

采用自制微流控共轴流装置制备出粒径dp=100~600 mm且球形度良好的单一粒径微液滴,对大豆油及甲苯两种微液滴在静止流体中的浮升过程和速度进行了研究. 结果表明,生成液滴的粒径dp主要由微通道锥口直径Dt (40~450 mm)确定;既定锥口直径下,液滴粒径随连续相流量Qc增大稳定减小,而增加分散相流量只改变液滴产生频率(生成颗粒群),对粒径影响微弱,建立了dp与Qc的关联式,关联度大于0.99. 由于表面张力作用导致微液滴具有弹性球行为,微液滴的液滴阻力系数CD与雷诺数Re的关系与刚性颗粒一致;因尾流效应颗粒群中各颗粒在交替追赶中前行,导致颗粒群浮升速度高于单颗粒,颗粒群比单颗粒浮升速度高约30%.     

Molecular structure modulated properties of azobenzene-substituted polydiacetylene LB films: Chirality formation and thermal stability

Langmuir-Blodgett (LB) films of three novel azobenzene-substituted diacetylene monomers (DA1, DA2 and DA3) were fabricated and their optical and chiroptical properties were investigated in detail by ultraviolet-visible (UV-vis) spectra and circular dichroism (CD) spectra. Achiral DA1 molecules could form chiral LB films through overcrowded packing of the azobenzene moiety, while achiral DA2 and DA3 molecules not. When exposed to left-or right-handed circular polarized UV light (CPUL), striking left- or right-handed (opposite) CD signals for azobenzene chromophores and polydiacetylene chains were observed for the polymerized DA1 (PDA1) and DA2 (PDA2) LB films. However, DA3 LB films could hardly be polymerized in this case, and only striking opposite CD signals for azobenzene chromophores could be observed. It was demonstrated that the intermolecular steric hindrance and irregular arrangement of azobenzene chromophores were not favorable for the topo-polymerization and chirality formation of polydiacetylenes backbone. Further, the effects of thermal treatment on the supramolecular chirality of above three LB films were studied. Strong collective noncovalent interactions (π-π stacking) were believed to be responsible for the thermal stability of chiral supramolecular assemblies.     

Cross-stream migration of nonspherical particles in second-order fluid flows: Effect of flow profiles

We investigate the cross-stream migration of spheroidal particles in a weakly viscoelastic fluid under various unbound parabolic flows (circular and elliptic tube flow, as well as 2D slit flow). The viscoelastic fluid investigated is a second-order fluid under the co-rotational limit. Our theory finds that the different flow profiles do not significantly alter the migration speed of particles toward the flow centerline, but drastically alter their orientation dynamics. At long times, prolate particles align along the flow direction in a circular tube flow, but undergo log-rolling in a 2D slit flow. We explain the origin of these orientation dynamics, and explain why such behavior can give rise to a similar cross-stream migration velocity. We lastly examine particle motion in an unbound elliptic tube flow with unequal velocity curvature in the two lateral directions. We quantify the conditions under which the long-time particle orientation changes from log-rolling to flow-alignment.     

Computational and Experimental Studies of Flexible Fiber Flows in a Normal-Stress-Fixed Shear Cell

Flow properties of flexible fibers are poorly understood compared to those of rigid particles. In this study, a Schulze ring shear tester is used to measure the flow properties of fibers made of cut fishing wire and cut rubber cord, which have different levels of flexibility. For a comprehensive study, the discrete element method is employed to simulate flexible fiber flows. The simulations are validated by comparing with the experimental measurements. Studies show that an increase in fiber bending modulus leads to a reduction in the deformation and solid volume fraction, but it has no effect on the shear stress with the same normal stress. An increase in fiber-fiber friction coefficient, below a critical value of 0.8, can augment the angle of internal friction. The contact stiffness, contact damping coefficient, and bond damping coefficient only have limited impact on the shear flow behavior in the ranges considered. © 2018 American Institute of Chemical Engineers AIChE J, 65: 64–74, 2019     

Some Remarks on the Removal of Adhered Particles by Oscillating Air Flows

On the background of technical concepts for the removal of particles from moving textiles by linear as well as oscillating air flows, some theoretical remarks on particle adhesion forces and the hydrodynamical and acoustic forces acting on particles are given. As the main removal forces in linear air flows, drag and lift are taken into consideration and analytical expression derived from the Bernoulli equation. For a hypothetical example of particles in the size range from below 0.5 to 7 μm on a cotton fiber, hydrodynamic forces are compared to van der Waals interaction, electrostatic forces and capillary forces. In this work, a special focus is placed on the application of high-frequency oscillating air flows which results in a direct coupling of ultrasound to the particles. The theoretical calculation of resulting forces showed that the detachment threshold is determined not only by sound intensity level, but also by the frequency of the ultrasonic wave. If capillary forces act, the coupling of ultrasound alone does not suffice to detach particles within the considered size range.     

Numerical simulation of flows in beadless disperser by finite difference lattice Boltzmann method

A new beadless disperser, an ultra-thin film high-shear disperser (UFHD), was developed, and the flow in its dispersion part (a flow between rotating and stationary rings) was simulated using the finite difference lattice Boltzmann method (FDLBM). The efficiency of the FDLBM for this kind of flow field was verified. The FDLBM possesses some distinctive features such as relatively easy implementation of boundary conditions in complicated geometries, high efficiency in parallel processing, and flexible reproduction of the interface between multiple phases, that make the method very suitable for simulating complex flows in the disperser, in which the fluid consists of a mixture of solvent and particles, and the deformation, breakup and coalescence of suspended particles occur. It was shown that the performance of the disperser can easily be simulated and elucidated by the FDLBM.     

A DNS Study of Aerosol Deposition in a Turbulent Square Duct Flow

A particle-laden turbulent flow through a square duct was simulated using a direct numerical solution of the Navier-Stokes equations coupled with Langrangian particle tracking. Computations of particle transport were employed to elucidate the mechanisms by which particles with varying inertia deposit to the walls of a square duct. Gravity was neglected and a one-way coupling was assumed between the particles and the fluid. The computational results demonstrate that, although the aerosol penetration through a square duct is not significantly different than through a circular pipe, there exist differences in the transport and deposition mechanisms. Most notably, the off-axis secondary flows unique to the square duct preferentially deposit higher-inertia particles closer to the corners of the duct. By contrast, the same secondary flows act to suppress the deposition of lower-inertia particles to the duct corners by efficiently transporting them back towards the duct core before deposition can occur.     

A particle-based model for the transport of erythrocytes in capillaries

This paper presents a two-dimensional (2D) particle-based model for the red blood cell, and uses it to compute cell deformation in simple shear and pressure-driven flows. The cell membrane is replaced by a set of discrete particles connected by nonlinear springs; the spring law enforces conservation of the membrane area to a high accuracy. In addition, a linear bending elasticity is implemented using the deviation of the local curvature from the innate curvature of the biconcave shape of a resting red blood cell. The cytoplasm and the external liquid are modeled as homogeneous Newtonian fluids, and discretized by particles as in standard smoothed particle hydrodynamics (SPH) solution of the Navier-Stokes equations. Thus, the discrete particles serve not only as a numerical device for solving the partial differential equations, but also as a means for incorporating microscopic physics into the model. Numerically, the fluid flow and membrane deformation are computed, via the particle motion, by a two-step explicit scheme. The model parameters are determined from experimental measurements of cell viscosity and elastic moduli for shear, areal dilatation and bending. In a simple shear flow, the cell typically deforms to an elongated shape, with the membrane and cytoplasm undergoing tank-treading motion. In a Poiseuille flow, the cell develops the characteristic parachute shape. These are consistent with the experimental observations. Comparison with prior computations using continuum models shows quantitative agreement without any fitting parameters, which is taken to be a validation of the particle-based model and the numerical algorithm.     

A study of photooptical processes in photosensitive cholesteric azobenzene-containing polymer mixture under an action of the polarized and nonpolarized light

New liquid crystalline cholesteric mixture consisting of azobenzene-containing side-chain copolymer and the chiral low-molar-mass dopant (5%) was prepared and its properties were studied. It was shown that freshly prepared spin-coated thin films are optically isotropic. However, the films possess a noticeable circular dichroism with a maxima corresponding to the absorbance of the nonchiral azobenzene fragments. This phenomenon is an evidence for the existence of the helical supramolecular structure elements in the mixture films. An annealing of the films at temperatures above the glass transition leads to the strong increase in the circular dichroism due to formation of the perfect cholesteric phase. UV light irradiation results in the E-Z isomerization of azobenzene groups and the significant drop in circular dichroism values (almost to zero) both for the fresh and the annealed films that is associated with the transition from cholesteric to the fully isotropic state. An action of polarized UV and visible light leads to the photoorientation of azobenzene groups perpendicular to the electric vector of the polarized light. Kinetic features of the photoorientation processes in cholesteric phase in comparison with nematic systems were studied. It has been experimentally found that the helical order to some extent prevents the photoorientation of the side groups. Upon UV irradiation of the thick, planarly-oriented films of the mixture at temperatures higher than glass transition, the selective light reflection maximum is shifted to the long-wavelength spectral region. All photoinduced phenomena occurring in the mixture are thermally reversible and annealing of the films at temperatures above glass transition results in the back recovery to the cholesteric nonoriented state and to the initial helix pitch value.     

Localized particle fracture during compression of materials expected to undergo plastic deformation

When powders are compressed in a die during tablet making, various stages of compaction occur, starting with rearrangement of the powder to its closest packing, followed by eventual plastic deformation or brittle fracture. Here, scanning electron microscopy has been utilized to further characterize all stages of compression. It is shown that materials, such as sodium chloride, which undergo some eventual plastic deformation undergo some form of fracture during the earlier stages of compaction due to the creation of high localized stresses acting on individual particles.     

Modeling of Coriander Seeds Drying in an Impingement Dryer

The aim of this work is to develop a mathematical model to estimate the batch drying curve of coriander seeds in an impingement dryer and to study the axial movement of a seed in a transparent prototype impingement dryer. The apparatus is a horizontal acrylic transparent cylinder with a slight slope to induce the axial and rotational movement of particles. Gas enters tangentially downwards through a narrow slot arranged all along the dryer, flows in a counterclockwise circular motion in the chamber—in crossflow with respect to the solids—and is discharged through an upper lengthwise expansion chamber. As a result of gas drag, the particles advance in a rotational-helicoidal motion between feed and discharge.

Velocity and temperature profiles for gas in 2D turbulent flow were simulated using commercial software from Fluent Inc.^[1,^4] Maximum velocities are shown to be located close the walls; most of the gas is recirculated, and the rest is exhausted. It is assumed that particle trajectories also follow a circular motion near the walls, as observed in the transparent reproduction of the dryer operating with ambient air for small batch of solids and/or a single particle. Air velocities along this trajectory are estimated from the simulated flow field. Particle motion, heating, and drying along this path are described by unsteady momentum, heat, and mass balances when subjected to gas drag and gravity forces.

With respect to the axial trajectory of a coriander seed, for an inlet air velocity of 20 m/s at the slot the average experimental time for a complete circular cycle is 0.18 s and the simulated time is 0.21 s, whereas average experimental residence time is 1.53 s and the simulated time is 0.94 s. Differences between experimental results and simulations are due to air instability, leading to nonhomogeneous air velocity profiles along the equipment. The mathematical model is based on the assumption that air velocity profiles are homogeneous. Experimental observations indicate that the particle does not move along the equipment but sometimes moves backward (or erratically) or spins out advancing, due to an uneven air speed profile, and impacts against the wall. Finally, the drying model gives results that adjust to the batch experimental data, taking into account the deviations found with respect to the axial trajectory from a seed. This is because the model was devised exactly to predict the conduct of the system in batch operation for a particle bed, obtaining results that show the macrocospic response of the equipment (velocity and average temperature of the air). As it happens in this type of phenomenon, the drying rate in the constant period is a function of the adimensional Reynolds number.     

Synthesis of precious opal in a hydrothermal solution

The nucleation of orthosilicic acid and the growth of colloidal silica particles in hydrothermal solutions are investigated. The results obtained are compared with the data on the formation of supramolecular matrices of precious opal. Ordered supramolecular structures are experimentally grown in a model solution through the precipitation of colloidal silica particles synthesized by hydrolysis of tetraethoxysilane in an organic solvent. The type of packing of spherical particles in the supramolecular structure is studied as a function of the component concentration, particle size, and pH and temperature of the model solution. The experimental data can be used to reveal the mechanism of formation of precious opal and to design methods for preparing three-dimensional nanocomposites.     

AN EXPERIMENTAL STUDY OF PARTICLE FLOW VELOCITIES IN SOLID-LIQUID FOOD MIXTURES

The design of equipment to sterilise solid-liquid food mixtures continuously requires that the flow properties of such mixtures be understood. Little information is available on food flows, which can consist of high solids fractions of particles of a range of densities in non-Newtonian and viscous liquids. A metal detection system which can log two types of particle in the same experiment has been constructed and used in a loop flow rig to study flows of single particles and carrot-water mixtures. Results can be correlated using the particle Froude number; some preliminary analysis is presented to suggest that data can be correlated against 1/Fr_p. The effect of solids fraction on particle flow velocity has been investigated; greatest variations in the particle velocity appear for stratified flows for solids fractions between 10-20%.     

气固并流向上流动中颗粒聚集的机理

1 INTRODUCTION Cocurrent upward gas-solid flows are widely used in industrial situations such as circulating fluid- ized bed, fluid catalytic cracking and pneumatic transport. Due to its industrial relevance, many quan- titative experimental and numerical investigations have been carried out[1―4]. Investigations show that upward gas-solid flow always appears heterogenously structured and the distribution of particles is always non-uniform unless the solid phase is very dense or very dilut…