Formation and Growth of Crystal Bridges in Bulk Solids

Solidification and caking of bulk solids often occurs during storage or while being transported to the customer. To investigate the formation and growth of solid bridges between two discrete particles, the modification of the contact region between these two particles stored in a climatic chamber is examined. The effect of load, temperature, relative humidity, and storage time on the formation of a bridge is analyzed. The objective is to describe the behavior of crystalline or salt‐like granules under real storage conditions.     

Three‐dimensional numerical simulation of upflow bubbling fluidized bed in opaque tube under high flux solar heating

Solid particles can be used as a heat transfer medium in concentrated solar power plants to operate at higher temperature and achieve higher heat conversion efficiency than using the current solar heat transfer fluids that only work below 600°C. Among various particle circulation concepts, the dense particle suspension (DPS) flow in tubes, also called upflow bubbling fluidized bed (UBFB), was studied in the frame of the CSP2 FP7 European project. The DPS capacity to extract heat from a tube absorber exposed to concentrated solar radiation was demonstrated and the first values of the tube wall‐to‐DPS heat transfer coefficient were measured. A stable outlet temperature of 750°C was reached with a metallic tube, and a particle reflux in the near tube wall region was evidenced. In this article, the UBFB behavior is studied using the multiphase flow code NEPTUNE_CFD. Hydrodynamics of SiC Geldart A‐type particles and heat transfer imposed by a thermal flux at the wall are coupled in two‐dimensional unsteady numerical simulations. The convective/diffusive heat transfer between the gas and dispersed phase, and the inter‐particle radiative transfer (Rosseland approximation) are accounted for. Simulations and experiments are compared here and the temperature influence on the DPS flow is analyzed. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3857–3867, 2018     

Growth and breakup of a wet agglomerate in a dry gas–solid fluidized bed

Using CFD‐DEM simulations, a wet agglomerate of particles was placed in a void region of a dry vigorously fluidized bed to understand how wet agglomerates grow or breakup and how liquid spreads when agglomerates interact with dry fluidized particles. In the CFD‐DEM model, cohesive and viscous forces arising from liquid bridges between particles were modeled, as well as a finite rate of liquid bridge filling. The liquid properties were varied between different simulations to vary Bond number (surface tension forces/gravitational forces) and Capillary number (viscous forces/surface tension forces) in the system. Resulting agglomerate behavior was divided into regimes of (i) the agglomerate breaking up, (ii) the agglomerate retaining its initial form, but not growing, and (iii) the agglomerate retaining its initial form and growing. Regimes were mapped based on Bo and Ca. Implications of agglomerate behavior on spreading of liquid to initially dry particles were investigated. This article identifies a new way to map agglomerate growth and breakup behavior based on Bo and Ca. In modeling both liquid forces and a finite rate of liquid transfer, it identifies the complex influence viscosity has on agglomeration by strengthening liquid bridges while slowing their formation. Viewing Ca as the ratio of bridge formation time to particle collision and separation time capture why agglomerates with high Ca struggle to grow. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2520–2527, 2017     

Modelling crystal growth between potash particles near contact points during drying processes. Part II: Analysis,results, and comparison with experimental data

Caking of bulk granular materials is a serious problem that affects many industries including the mineral processing industry. Caking occurs when bulk materials undergo wetting and drying cycles and it has been thought that it occurs due to the formation of new crystal bridges between individual particles. In the first paper in this series a mathematical model is developed for the crystal formation process that occurs at the contact point between two particles. In Part II, numerical simulations of the model are used to determine the effects of changes for several independent parameters in this model: initial moisture content; rate of evaporation of the salt solution from the particle surface; relative size of the contact region compared to the initial film thickness of salt solution; and supersaturation levels near the contact point. Non‐dimensional graphical curves of these simulations are used to compare the effects of each parameter for the deposition of salt crystals near the contact point. These results, when compared to data for cake strength in potash specimens which were obtained for various initial moisture contents, drying rate, and chemical composition of the particle surfaces, show good qualitative agreement even though cake strength and mass recrystallization near a contact point are different physical phenomena. The numerical results show that the mass of crystal deposition near the contact point will increase with increased initial moisture content and decreased evaporation rate. It is also found that variations in the degree of supersaturation near the contact point causes significant variations in the crystal mass deposition near the contact point.     

Jamming rheology of model cementitious suspensions composed of comb‐polymer stabilized magnesium oxide particles

The colloidal microstructure of concentrated suspensions containing anionic comb‐polymer‐stabilized magnesium oxide (MgO) particles in water was analyzed by shear rheometry for indications of changes in particle microstructure based on particle size and comb‐polymer usage. As the suspensions were sheared at different rates, jamming in the sheared MgO suspensions was observed as shear stress overshoots. The shear‐induced evolution of the suspension's microstructure was strongly related to the perceived interactions between neighboring MgO particles in the suspension. In the jammed state, interactions are believed to be enhanced by the formation of entanglements between opposing comb‐polymer side‐chains. Steric repulsion between side‐chains was lessened for large particles on account of their diameters, which further enabled side‐chain entanglement during close particle contact under shear. Suspensions with relatively wide particle size distributions (0.5–400 μm) were theorized to form hydrocluster aggregates, while suspensions with narrower particle size distributions (0.5–40 μm) most likely resulted in networked microstructures under the influence of the chain entanglements from the adsorbed comb‐polymer. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40429.     

高湿黏性颗粒在聚四氟乙烯微孔膜滤料表面沉积特性的数值模拟

基于聚四氟乙烯(PTFE)微孔膜滤料扫描电镜(SEM)图像,建立PTFE微孔膜滤料微观结构模型,采用计算流体力学和离散单元法(CFD?DEM)耦合的方法对黏性颗粒在微孔膜滤料表面沉积特性进行模拟,引入液桥力模型,忽略范德华力的作用,统计计算域内颗粒的受力情况,分析了不同表面能条件下3~6 ?m粒径颗粒在微孔膜滤料表面的沉积特性,将模拟结果与黏附效率的经验公式进行对比。结果表明,黏附效率与经验值、颗粒受力与液桥力模型的相对误差均在6%以内,CFD?DEM耦合计算方法可用于模拟不同环境湿度条件下的颗粒沉积;过滤风速、粒径与黏性是影响沉积形态的重要因素,提高过滤风速及增大颗粒粒径与黏性,颗粒更易在滤料表面形成稳定的树突结构,黏附效率及含尘压降增加。环境相对湿度影响两物体间液桥体积,接触力影响颗粒沉积,当增加表面能与液桥体积时,接触力及液桥力均相应增加,根据受力平衡原理,环境相对湿度对颗粒沉积影响很大。     

A model to predict liquid bridge formation between wet particles based on direct numerical simulations

We study dynamic liquid bridge formation, which is relevant for wet granular flows involving highly viscous liquids and short collisions. Specifically, the drainage process of liquid adhering to two identical, non‐porous wet particles with different initial film heights is simulated using Direct Numerical Simulations (DNS). We extract the position of the interface, and define the liquid bridge and its volume by detecting a characteristic neck position. This allows us building a dynamic model for predicting bridge volume, and the liquid remaining on the particle surface. Our model is based on two dimensionless mobility parameters, as well as a dimensionless time scale to describe the filling process. In the present work model parameters were calibrated with DNS data. We find that the proposed model structure is sufficient to collapse all our simulation data, indicating that our model is general enough to describe liquid bridge formation between equally sized particles. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1877–1897, 2016     

Sintering Experiments on Sapphire Spheres

The plastic-flow theory of sintering between particles has been suggested and verified by several investigators. Tiny spheres in contact should develop a lens or bridge between them due to capillary action. Such contact lenses have been observed by others in materials such as NaCl, Cu, and stabilized ZrO₂. They were unable to find contact lenses in pure ZrO₂ or Al₂O₃. A simple technique has been developed whereby a contact lens or bridge is obtained between 40-mil spheres of sapphire by heating them on a thin molybdenum saucer held on a molybdenum block by induction in a vacuum (or in hydrogen).     

The effect of liquid bridge model details on the dynamics of wet fluidized beds

Wet fluidized beds of particles in small periodic domains are simulated using the CFD‐DEM approach. A liquid bridge is formed upon particle‐particle collisions, which then ruptures when the particle separation exceeds a critical distance. The simulations take into account both surface tension and viscous forces due to the liquid bridge. We perform a series of simulations based on different liquid bridge formation models: (1) the static bridge model of Shi and McCarthy, (2) a simple static version of the model of Wu et al., as well as (3) the full dynamic bridge model of Wu et al. We systematically compare the differences caused by different liquid bridge formation models, as well as their sensitivity to system parameters. Finally, we provide recommendations for which systems a dynamic liquid bridge model must be used, and for which application this appears to be less important. © 2017 American Institute of Chemical Engineers AIChE J, 64: 437–456, 2018     

Predicting the performance of granulation binders through micro-mechanistic observations

The selection of an appropriate polymeric binder to be used to agglomerate drug with excipients is a critical issue for the development of high shear wet granulation processes for pharmaceutical tablet systems. The aim of the study reported here is to determine the potential for successful granulation through measurement and prediction of the interactions of the polymer solutions with individual drug particles. A novel micro-force balance (MFB) has been used to measure the forces exerted by axially strained liquid bridges of hydroxypropyl methylcellulose (HPMC) and polyvinylpyrrolidone (PVP) formed between either two paracetamol crystals or between a micro-pipette and a single paracetamol crystal. Video images were obtained simultaneously of the separation sequences for analysis of bridge geometry and contact angles. It was found that the formation of liquid bridges and their ability to bond the particles together depends on the wetting behaviour of the liquid on the particles. Binders that dewet the solid surfaces during separation are shown to produce weaker adhesion forces.     

Synthesis of hydroxypropylated debranched pea starch with high substitution degree in an ionic liquid,and its characterization and properties

Hydroxypropylation of debranched pea starch (DPS) has been carried out effectively in an ionic liquid, 1-butyl-3- methylimidazolium chloride, in order to shorten the long time required by starch being normally hydroxypropylated and improve the characteristics of pea starch (PS). As a result, hydroxypropylated debranched pea starch (HDPS) with molar substitution up to 1.34 has been obtained in homogeneous system within 3 h, which was much less than time (18 h) required by normal hydroxypropylation of starch. Based on the synthesis, HDPS was further characterized by infrared spectroscopy, X-ray diffraction, scanning electron microscopy and transflective polarizing microscope, respectively, and some of its properties were also compared with those of PS, DPS and hydroxypropylated pea starch (HPS). The experimental results indicated that the crystalline structure of PS belonged to a C-type; and one of DPS was between B-type and C-type, whereas HDPS structure was almost completely amorphous. The debranching and hydroxypropylation evidently influenced the pasting behavior and thermal properties of PS. The morphology and size of DPS and HDPS particles were remarkably different from those of PS owing to hydroxypropylation and debranching. The peak intensity of –OH groups in DPS and HDPS was evidently weakened by debranching compared with FTIR spectra of PS. The debranching resulted in the reduction in swelling power of DPS, but the hydroxypropylation led to the increase in the swelling power of DPS and HDPS.     

Evaluation of apparent viscosity of liquid-containing porous particles by the intrusion method using a conical rotor

The apparent viscosity of a wet powder consisting of porous silica particles and a viscous liquid was evaluated by means of a newly developed powder rheometer in which a rotating conical rotor with grooves on the surface intrudes semi-statically into the powder bed. Using this rheometer, the relationship between the shear torque and the depth of intrusion, i.e., the torque characteristic curve, was measured under various conditions. The shear force acting on a contact point between the particles was estimated from the torque characteristic curve. Above the critical liquid amount in which the pores of the particles were filled with the liquid, the shear force increased with an increase in the thickness of the liquid film formed on the particle surface regardless of the pore volume. From the change of shear force with the physical properties of the liquid, it was clear that the shear force is closely related to the liquid bridge force acting on the contact points between the particles. The apparent viscosity coefficient of the wet powder was determined from the shear rate dependence of the shear force. At the relatively high liquid amounts corresponding to funicular and capillary states, the apparent viscosity coefficient increased sharply with the thickness of the liquid film since the viscosity of the liquid strongly affected the shear flow of the wet powder. Subtle changes of the apparent viscosity due to the liquid amount and the physical properties of the liquid can be sensitively detected by using the rotary-intrusion method.     

Studying mechanical microcontacts of fine particles with the quartz crystal microbalance

To study micromechanical adhesion, glass particles were deposited on a quartz crystal microbalance (QCM). Beforehand, a 160 nm-thick film of polystyrene (PS) had been spin-coated on the gold surface of the QCM. Shifts in the resonance frequency were monitored versus the oscillation amplitude. The aim was to analyse how QCM experiments reflect the state of adhesion. During oscillation, the motion of the particles and the induced frequency shift of the QCM are governed by a balance between inertial and contact forces. In order to vary the relative strength of the two, the diameter of the particles was varied between 5 and 20 μm. The adherence of the particles could be increased by annealing the PS film at 150 °C. Annealing led to the formation of a PS meniscus. For a semi-quantitative interpretation we have to take into account that the particles show a distribution of coupling constants.The vibration of the QCM changes the micromechanical contact between QCM surface and particles. There is an instantaneous and a long-term effect. Instantaneously, the oscillation induces partial slip. Under an oscillating load, part of the contact ruptures, which decreases the effective stiffness of the contact. In addition, there are long-term memory effects. The vibration of the QCM can lead to a consolidation and an increased coupling. However, it can also break the contact and even lead to detachment. Particles deform the PS surface and induce damage due to inertial forces.     

Experimental study and numerical simulation on the formation of microstructure in cementitious materials at early age

The formation of microstructure in early age cement paste and concrete was examined with an ultrasonic experimental set-up. Research parameters included the influence of curing temperature (isothermal curing at 20, 30 and 40 °C), water/cement ratio (0.40, 0.45 and 0.55) and amount of aggregate. In parallel with the experiments, the cement hydration model HYMOSTRUC was utilized to simulate the formation of the microstructure. In this study, the cement paste was considered as a four-phase system consisting of water, unhydrated cement, hydration products and that part of the hydration product that causes the contact between the hydrating cement grains (so called “bridge volume”). A correlation has been found between the growth of bridge volume calculated with the model and the changes in the pulse velocity. It is believed that ultrasonic pulse velocity (UPV) measurements can represent a valuable tool to investigate the development of the microstructure at early age.     

A new computational method for studying heat transfer in fluid bed reactors

Effective thermal conductivity (ETC) is an important parameter describing the thermal behaviour of packed beds with a stagnant or dynamic fluid, and has been extensively examined in the past decades. Recently, an approach of coupled discrete particle simulation (DPS) and computational fluid dynamics (CFD) has been extended to predict the ETC, allowing the elucidation of the underlying heat transfer mechanisms at a particle scale. However, because of the sensitivity of heat transfer to particle–particle contact, a large Young's modulus and small time step have to be employed in the DPS to generate accurate results, resulting in a high computational cost. This paper proposed a method to overcome this problem. It is done by introducing a correction coefficient in the calculation of the particle–particle contact radius between colliding particles. The treatment is first implemented in our recent DPS-CFD modeling of the heat transfer in gas fluidization, and is validated by comparing the predicted ETC with literature data. The effects of model parameters, particle size, and bed average temperature on ETC are also analyzed.     

Study on the modification of epoxy resin by a phosphorus‐ and silica‐containing hybrid

A novel phosphorus‐ and silica‐containing hybrid (DPS) was synthesized by the reaction between diethyl phosphate (DEP) and polyhedral oligomeric siloxanes (POS) formed by hydrolysis condensation of 3‐glycidoxypropyltrimethoxysilane (GPTMS). The novel phosphorus‐ and silica‐containing hybrid was characterized by the flourier transform infrared spectroscope (FT‐IR), silicon nuclear magnetic resonance, and gel permeation chromatography (GPC). Then, the determination of the activation of the reaction between epoxy resin and phosphorus‐, and silica‐containing hybrids was studied by differential scanning calorimeter (DSC). In the presence of catalyst, the activation energies of the curing reaction were 63.3 and 66.7 kJ/mol calculated by Kissinger model and Ozawa model respectively. The thermal and flame retardant properties of the cured epoxy modified by DPS were determined by differential scanning calorimeter (DSC), thermal gravimetric analysis (TGA), and limited oxygen index (LOI). The results revealed that those properties were improved in comparison with unmodified epoxy resin. In addition, scanning electron microscopy (SEM) was used to investigate the morphology of the cured epoxy resin modified by DPS. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Ni‐Coated SiC Particles: Synthesis and Densification

The mechanofusion process, a dry particle coating route, has been successfully applied to coat micrometric SiC particles with submicrometric Ni filaments. In a first step, the mechanofusion parameters were optimized to form a continuous Ni coating onto SiC particles. In a second step, the Ni‐coated SiC particles were sintered by hot isostatic pressing. The temperature and pressure cycles were determined to ensure a good densification of the material. Such a densification process leads to the formation of a δ‐Ni₂Si bilayer at the SiC/Ni interface; the inner δ‐Ni₂Si layer in contact with SiC being more rich in carbon than the one in contact with the matrix. From X‐ray diffraction, wavelength‐dispersive X‐ray spectrometry and scanning electron microscopy characterizations, a mechanism is proposed to explain the microstructure of the end‐product.     

The Limits of Fine and Coarse Particle Flotation

The flotation behaviour of quartz particles was studied over the particle size range from 0.5 µm to 1000 µm and for advancing water contact angles between 0° and 83°. Flotation was performed in a column and in a Rushton turbine cell. Particle contact angle threshold values, below which the particles could not be floated, were identified for the particle size range 0.5–1000 µm, under different hydrodynamic conditions. The flotation response of the particles, either in a column or in a mechanically agitated cell with a similar bubble size, was comparable. Turbulence plays a role, as does bubble‐particle aggregate velocity and bubble size. The stability of the bubble‐particle aggregate controls the maximum floatable particle size of coarse particles. For fine particles, the flotation limit is dictated by the energy required to rupture the intervening liquid film between the particle and bubble. Flotation of very fine and large particles is facilitated with small bubbles and high contact angles. These results greatly extend our earlier observations and theoretical predictions.     

Preparation and characterization of porous titanium dioxide sulfonated polystyrene composite microspheres with amphiphilicity

Porous particles with amphiphilicity were prepared by a nonpolymeric pore‐formation process with the sulfonation of polystyrene microspheres. Nano titanium dioxide (TiO₂) particles were then grafted onto the surface via a sol–gel method to finally form the composite particles. The effects of the mass ratio of ethanol (EtOH) to water, temperature, and solubility parameter on the pore‐formation process is discussed in detail. The morphology, porous structure, and wetting properties of the particles were studied by scanning electron microscopy, transmission electron microscopy, Fourier transform infrared (FTIR) spectroscopy, and contact angle measurement. The results show that porous sulfonated polystyrene (SP) microspheres could be fabricated at 60°C with a 1 : 1 mass ratio of EtOH–water and a solubility parameter of 29.69 MPa^1/2. The TiO₂ particles were determined to be grafted onto the SP microspheres by physical‐bond interaction on the basis of FTIR analysis. The contact angles for both water (aqueous‐phase) and various organic solvent (oil‐phase) droplets with different polarities on the surface of compressed tablets of TiO₂–SP powder were all lower than 30°; this indicated excellent amphiphilicity in the composite particles. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Capillary-bridge forces between solid particles: Insights from lattice Boltzmann simulations

Liquid capillary-bridge formation between solid particles has a critical influence on the rheological properties of granular materials and, in particular, on the efficiency of fluidized bed reactors. The available analytical and semi-analytical methods have inherent limitations, and often do not cover important aspects, like the presence of non-axisymmetric bridges. Here, we conduct numerical simulations of the capillary bridge formation between equally and unequally sized solid particles using the lattice Boltzmann method, and provide an assessment of the accuracy of different families of analytical models. We find that some of the models taken into account are shown to perform better than others. However, all of them fail to predict the capillary force for contact angles larger than π/2, where a repulsive capillary force attempts to push the solid particle outward to minimize the surface energy, especially at a small separation distance. We then apply the most suitable model to study the impact of capillary interactions on particle clustering using a coupled lattice Boltzmann and Discrete Element method.