Triboelectric charging of monodisperse particles in fluidized beds

To investigate the interplay between particle charging and hydrodynamics in fluidized beds, models for triboelectric charging and electrostatic forces were built into a computational fluid dynamics‐discrete element method model. Charge transfer was governed by the difference in effective work function between contacting materials as well as the electric field at the point of contact. Monodisperse particles were fluidized with an effective work function difference between the particles and the conducting walls. For smaller work function differences, hydrodynamics were not changed significantly as compared with an uncharged case. In these simulations, the average charge saturated at a value much lower than the value anticipated based on the work function difference, and a unimodal distribution of charges was observed. For larger work function differences, particles stuck to walls and bed height oscillations due to slugging were less pronounced. For these cases, a bimodal distribution of charges emerged due to effects from strong electric fields. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1872–1891, 2017     

Particle‐resolved direct numerical simulation of gas–solid dynamics in experimental fluidized beds

Particle‐resolved direct numerical simulations (PR‐DNS) of a simplified experimental shallow fluidized bed and a laboratory bubbling fluidized bed are performed by using immersed boundary method coupled with a soft‐sphere model. Detailed information on gas flow and individual particles’ motion are obtained and analyzed to study the gas–solid dynamics. For the shallow bed, the successful predictions of particle coherent oscillation and bed expansion and contraction indicate all scales of motion in the flow are well captured by the PD‐DNS. For the bubbling bed, the PR‐DNS predicted time averaged particle velocities show a better agreement with experimental measurements than those of the computational fluid dynamics coupled with discrete element models (CFD‐DEM), which further validates the predictive capability of the developed PR‐DNS. Analysis of the PR‐DNS drag force shows that the prevailing CFD‐DEM drag correlations underestimate the particle drag force in fluidized beds. The particle mobility effect on drag correlation needs further investigation. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1917–1932, 2016     

Investigation of electrostatic charge distribution in gas–solid fluidized beds

Over the past few decades there have been numerous attempts to measure electrostatic charges in gas–solid fluidized bed reactors; these charges have been prone to cause reactor downtime from electrostatic phenomena. In this study, a new system was developed that aimed to quantifying the electrostatic charge generation in three key areas within a gas–solid fluidized bed simultaneously: the bed particles, the particles that adhered to the column wall, and the particles that were entrained from the column. A unique online Faraday cup method was used to measure the electrostatic charge of the particles. The system was operated with dry air at two fluidizing gas velocities, one in the bubbling and the other in the slugging flow regime. An industrial polyethylene resin with a wide particle size range was utilized in all experiments. Results showed the occurrence of bi-polar charging in both flow regimes with entrained fines being mainly positively charged, whereas the bed particles and those attached to the column wall carrying a net negative charge. The charge-to-mass ratio (q/m) of the entrained fines in the bubbling regime was significantly higher than in the slugging regime. It was discovered that particles with a certain size range were predominantly adhering to the column wall with a significantly higher q/m than the other bed particles. These findings led to a proposed mechanism for the migration of particles within the fluidization column due to the effect of electrostatic charge generation.     

Experimental investigation of electrostatic effect on particle motions in gas‐solid fluidized beds

The excess accumulation of charges in the fluidized bed has a severe impact on hydrodynamics. Due to lack of effective experimental methods, electrostatic effects on hydrodynamics have mostly been studied using numerical simulation. By injecting a trace of liquid antistatic agents into a fluidized bed, charges were controlled and electrostatic influences on particle motions were investigated. The average particle–wall impact angles are acquired by developing multiscale wavelet decomposition of acoustic emission signals. The impact angles are significantly influenced by both charge levels and gas velocities. If the electric force is reduced and/or fluid drag is increased, friction dominates the particle–wall interactions. Under a larger gas velocity where fluid drag dominates, charges elimination causes no significant variation in particle impact angles, but particle velocities increase as well as at lower gas velocities. In addition, existence of electrostatic charges influences the ranges of bubble growing zone and jet impacting zone. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3628–3638, 2015     

外加电场下气固流化床的数值模拟

工业流化床中通常存在显著的静电效应,会影响流化床的流体力学特性,并常常带来一定的安全隐患,而引入外加电场可以达到"化害为利"的作用。为了获知外加电场下流化床内的流体力学特性,本文采用CFD耦合静电模型及解耦求解的方式,研究了外加电场下气固流化床内的轴向电势分布和颗粒的运动规律。研究结果表明,传统流化床的轴向电势分布呈Z形,与实验结果一致,引入直流电场后,颗粒自身产生的电场正电势区域明显增大,而负电势区域也有小幅增大。直流电场可以使得壁面处小颗粒的聚集减少,但在床中心出现明显的颗粒聚集。在交流电场下,小颗粒在壁面处的分布比正常流化床时的稀疏。通过对颗粒自身产生电场的分析,发现近分布板区和料位区的电场方向不同,近分布板区的电场指向床中心,而料位区的电场指向壁面。     

Monte Carlo modeling of fluidized bed coating and layering processes

A stochastic modeling approach based on a Monte Carlo method for fluidized bed layering and coating is presented. In this method, the process is described by droplet deposition on the particle surface, droplet drying and the formation of a solid layer due to drying. The model is able to provide information about the coating coverage (fraction of the particle surface covered with coating), the particle‐size distribution, and the layer thickness distribution of single particles. Analytical solutions for simplified test cases are used to validate the model theoretically. The simulation results are compared with experimental data on particle‐size distributions and layer thickness distributions of single particles coated in a lab‐scale fluidized bed. Good agreement between the simulation results and the measured data is observed. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2670–2680, 2016     

An adhesive CFD‐DEM model for simulating nanoparticle agglomerate fluidization

Nanoparticles are fluidized as agglomerates with hierarchical fractal structures. In this study, we model nanoparticle fluidization by assuming the simple agglomerates as the discrete element in an adhesive (Computational Fluid Dynamics—Discrete Element Modelling) CFD‐DEM model. The simple agglomerates, which are the building blocks of the larger complex agglomerates, are represented by cohesive and plastic particles. It is shown that both the particle contact model and drag force interaction in the conventional CFD‐DEM model need modification for properly simulating a fluidized bed of nanoparticle agglomerates. The model is tested for different cases, including the normal impact, angle of repose (AOR), and fluidization of nanoparticle agglomerates, represented by the particles with the equivalent material properties. It shows that increasing the particle adhesion increases the critical stick velocity, angle of repose, and leads from uniform fluidization to defluidization. The particle adhesion, bulk properties, and fluidization can be linked to each other by the current adhesive CFD‐DEM model. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2259–2270, 2016     

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．     

Towards monitoring electrostatics in gas–solid fluidized beds

The electrostatic charge distribution in a lab‐scale 2‐D fluidized bed of 900 µm glass beads was determined using arrays of induction probes, and the influence of relative humidity and superficial gas velocity was examined. The bubble presence, relative humidity, and superficial gas velocity were found to influence charge separation. Bipolar charging was observed; the net charge build‐up was found to be negligible. Moreover, the system was monitored by applying the attractor comparison method to the electrostatic charge signals from an induction probe. It was concluded that this approach can indeed be used to monitor changes in the electrostatic behaviour.     

A soft‐sphere‐imbedded pseudo‐hard‐particle model for simulation of discharge flow of brick particles

A novel hybrid approach of soft‐sphere‐imbedded pseudo‐hard‐particle model is proposed to cope with the complex collision of nonspherical particles. In this approach, the boundary of a host hard particle is covered by a series of soft‐spheres, which are allowed to oscillate about the equilibrium position according to the position, orientation, and shape configuration of the host particle. The collision processes are twofold: as a predictive process, particle‐particle interaction takes place through the collision between the distributed soft‐spheres, which causes subspheres to deviate from the equilibrium positions; as a corrective process, relaxation is superposed to allow the soft‐spheres to move back toward the equilibrium positions quickly. Consequentially, this process generates the force and torque on the host particle and determines its movement. Finally, after validation, this new model is used to explore the effects of aspect ratio and base angle on the discharge of brick particles in hoppers. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3562–3574, 2016     

CFD–DEM simulation of tube erosion in a fluidized bed

The erosion of the immersed tubes in a bubbling‐fluidized bed is studied numerically using an Eulerian–Lagrangian approach coupling with a particle‐scale erosion model. In this approach, the motion of gas and particles is simulated by the CFD–DEM method, and an erosion model SIEM (shear impact energy model) is proposed to predict the erosion of the tubes. The model is validated by the good agreement of the simulation results and previous experimental data. By analyzing the simulation results, some characteristics of the tube erosion in the fluidized bed are obtained, such as the distribution of the erosion rate around the tube, the variation of the erosion rate with the position of the tube, the effect of the friction coefficient of particles on the erosion, the relationship between the maximum and the average erosion rate, etc. The microscale behavior of particles around the tubes is also revealed and the linear relationship between the erosion and the shear impact energy is confirmed by the simulation results and experiment. The agreement between simulation and experiment proves that the microscale approach proposed in this article has high accuracy for predicting erosion of the tubes in the fluidized bed, and has potential to be applied to modeling the process in other chemical equipment facing solid particle erosion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 418–437, 2017     

Experimental and numerical study on a bubbling fluidized bed with wet particles

As liquid bridge between particles acts an important role in the particle system, it is of considerable significance to analyze the flow hydrodynamics of wet particles in fluidized beds, which will improve the reactor design and process optimization. Thus, experimental and numerical investigations on wet particles in a bubbling fluidized bed are conducted in current work. On experimental side, particle image velocimetry (PIV) technology is employed with a designed bubbling fluidized bed. The silicone oil is used in this work because it is nonvolatile and transparent. On numerical side, a modified discrete element method (DEM) numerical method is developed by compositing an additional liquid‐bridge module into the traditional soft‐sphere interaction model. Most of the physical parameters are chosen to correspond to the experimental settings. Good agreements of particle velocity are found between the DEM simulation and PIV measurement. The performance of different liquid contents and superficial gas velocities are examined. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1970–1985, 2016     

The role of conductive pathways in the conductivity and rheological behavior of poly(methyl methacrylate)–graphite composites

Up to now, research on the dynamic process of conductive network formation has tended to focus on composite particles with one‐dimensional geometry, such as carbon black and carbon nanotubes. However, studies on this subject based on fillers with two‐dimensional structure, such as graphite, are rare in the literature. In this work, the dynamic percolation and rheological properties of poly(methyl methacrylate) (PMMA)–graphite composites under an electric field were investigated. The activation energies of conductive network formation and polymer matrix mobility were calculated from the temperature dependence of the percolation time and the zero‐shear viscosity. It was found that the activation energy calculated from the zero‐shear viscosity was not influenced by the electric field in the concentration range investigated, but the electric field had an effect on the activation energy calculated from the percolation time. This finding emphasizes that the electrical and rheological properties have different physical origins. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43810.     

Combination of the direct-forcing fictitious domain method and the sharp interface method for the three-dimensional dielectrophoresis of particles

In this paper, we combine the direct-forcing fictitious domain (DF/FD) method and the sharp interface method (SI) to resolve the problem of particle dielectrophoresis in three dimensions. The flow field is solved with the DF/FD method. The electric field governed by a Laplace equation with a jump coefficient across the particle surface is solved with the sharp interface (SI) method, and the dielectrophoretic force on particles is then calculated with the Maxwell stress tensor (MST) method. The main feature of our method is that both hydrodynamic and dielectrophoretic forces on particles are calculated with the interface-resolved methods instead of the point-particle model. The accuracy of the SI/MST method for the dielectrophoretic force without the consideration of the flow is validated via two problems: the electrostatic force on the particle in a non-uniform electric field, and the electrostatic force between two particles. The capability of the proposed DF/FD-SI/MST method is demonstrated with two numerical examples: the aggregation of particles due to the conventional dielectrophoretic force, and the motion of particles due to both conventional and traveling wave dielectrophoretic forces.     

Numerical studies of the effects of fines on fluidization

Euler‐Lagrange simulations of fluidized beds of Geldart Group A particles containing different levels of fines are performed in periodic domains with various domain‐averaged solid volume fractions. Bubble‐like voids readily form when no fines are added. Introducing fines does not reduce bubble sizes if van der Waals force between particles is not accounted for. In contrast, the addition of van der Waals force produces significant changes. With no fines, bubbles are found to be suppressed at sufficiently high solid volume fractions, corresponding to the expanded bed regime for Group A particles. With the addition of fines, bubbles can be suppressed at lower solid volume fractions. With more fines added, bubbles can be suppressed at even lower solid volume fractions. When bubbles are suppressed, the system is found to be in a stable solid‐like regime. In this regime, forces on each particle are balanced, and the particle velocity fluctuations are dampened. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2271–2281, 2016     

静电引发剂对气固流化床内静电分布的影响

气固流化床内静电荷的积累会导致颗粒团聚、静电放电、结片甚至爆炸等严重后果,在工业流化床反应器内控制静电的水平具有重要的现实意义。针对LLDPE-N₂体系,对静电引发剂的加入提出了电势-电负性（δU-χ_i）假设,即金属离子的电负性χ_i很大程度上决定了金属氧化物与聚乙烯颗粒接触分离后的电性,χ_i与静电引发剂添加前后静电电势的变化呈负相关。将静电引发剂分成负静电引发剂、正静电引发剂和非活泼液体静电引发剂3类,推测了3类静电引发剂加入后对于床内静电势分布的影响,并设计了相应的在气固流化床内添加静电引发剂的实验。实验结果显示负静电剂的加入,使得整床的静电水平显著升高;正静电引发剂的少量加入,流化床内的静电水平有明显降低;非活泼液体静电引发剂的注入,流化床内的静电水平显著下降。实验结果与利用δU-χi假设预测的结果相吻合,可为生产过程中静电引发剂的添加提供指导。     

Solids volume fraction measurements on riser flow using a temporal‐histogram based DIA method

In this article we introduce a temporal histogram‐based method for digital image analysis (DIA) of pseudo‐2D fluidized bed risers. This method enables an accurate whole field measurement of the solids volume fraction in lab‐scale pseudo‐2D riser flows by successfully removing image imperfections and merely accounting for the particles’ intensity. Moreover, the new correlation between normalized intensity and solids volume fraction that is proposed in this work enables a quantitative approach for solids concentration measurements by DIA techniques. This technique can be easily adjusted with experimental settings and shows a great stability against adverse imaging conditions. The combination of this parameter‐free method with particle image velocimetry under riser flow conditions has been successfully applied, enabling the experimental acquisition of full field hydrodynamic data. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2681–2698, 2016     

A simple method to generate spontaneous chemisorption of metallic particles mediated by carboxylate groups from silylated oligomeric poly(amide‐imide)s

The present work describes oligomeric poly(amide‐imide)s (PAIs) containing several l ‐amino acidic residues and two silicon atoms in their repeat unit, whose carboxylate terminal group was chemisorbed onto metallic particles (Cu, Ag or Au) previously deposited in controlled conditions via physical vapor deposition. Thus, for each prepared polymer–metallic hybrid, the surface morphology, particle size distribution, and percentage of organic material, silicon and metal were studied using scanning electron microscopy and energy‐dispersive X‐ray spectroscopy. The results show that the hybrids are formed probably via electrostatic interaction between the carboxylate anions of the PAIs and nanoparticle cations. This bridging ligand was visualized using Raman spectroscopy and corroborated with X‐ray diffraction. Optical studies and resistivity measurements (conductivity) of each hybrid were developed using UV‐visible and the four‐point probe method, respectively. X‐ray photoelectron spectroscopy was used to study the oxidation states of the metallic particles at surface level. Thus, a simple and spontaneous protocol is proposed for the preparation of metallic particles stabilized in situ by an oligomer, a procedure that takes place from seconds to a few minutes. Finally, particle diameters were measured using atomic force microscopy in order to study possible agglomeration of the metallic particles with time. © 2017 Society of Chemical Industry     

Prediction of particle charging in a dilute pneumatic conveying system

When particles are transported in pipelines, they acquire electrostatic charges as they come into contact with the pipe wall. Charged particles can cause problems such as particle agglomeration, blockage, and explosion. Understanding the particle charge can help to prevent these issues. This study investigates a technique for predicting the particle charge in a straight pipe of any given length, as well as the pipe length at which electrostatic equilibrium occurs, through experimentation in a short 1‐m pipe section. Experimentation with five different types of particles and four pipe wall materials at longer pipe lengths were used to validate the technique. This predictive technique is applicable to a range of particle shapes and sizes under the restriction that charge transfer is due to impact charging. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2308–2316, 2013     

Electrostatics and gas phase fluidized bed polymerization reactor wall sheeting

Reduction of electrostatic charge accumulation in fluidized bed polymerization reactors can reduce the frequency of reactor wall sheeting incidents and decrease the cost of operating fluidized bed polymerization reactors. The accumulation of excess electrostatic charge in fluidized bed polymerization reactors causes the fluidized polymer particles to adhere to the reactor wall and form wall sheets. Reactor wall sheets are described in order to characterize the problem. Electrostatic forces are compared to other forces influencing fluidization, such as drag forces and van der Waals forces. Literature values of measured particle electrostatic charges are compared to the maximum theoretical values predicted by Gauss’ law applied to the particles and to the entire fluidized bed. Electrostatic charge mitigation techniques are reviewed and future research areas are suggested based on the theoretical analysis and identified knowledge gaps.