Role of gas phase in the deposition dynamics of fine particles in trickle-bed reactors

Experiments were conducted to study the role of gas velocity in the capture of fine particles from non-aqueous suspensions circulated in co-current down-flow trickle flow reactors. The rate of filtration and pressure drop in the trickle bed were investigated using surfactant-stabilized kaolin-containing kerosene suspensions. It was determined that the filter coefficient was sensitive to liquid holdup and specific deposit. The initial collection efficiencies were compared with predictions based on existing theories. Agreement was generally not good with the exception for the limit of low superficial gas velocity. A general correlation establishing the relationship between the filtration rate and the liquid holdup in trickle beds was proposed to reconcile the experimental data with existing filtration theories.     

Effect of particle loading on the collection performance of an electret cabin air filter for submicron aerosols

Electret filters are composed of permanently charged electret fibers and are widely used in applications requiring high collection efficiency and low-pressure drop. We tested electret filter media used in manufacturing cabin air filters by applying two different charging states to the test particles. These charging states were achieved by spray electrification through the atomization process and by bipolar ionization with an aerosol neutralizer, respectively. Polydisperse solid NaCl particles with 0.1%, and 1% solutions or liquid dicotyl sebacate (DOS) particles were generated from an atomizer, and they were loaded on the filter media. The amount of charge, the mean particle size, and the particle material significantly affected the collection performance of the electret filter media for submicron particles. The collection efficiency of the electret filter media degraded as more particles were loaded, and showed minimum efficiency at steady state. The electret filter media captured the highly charged particles more efficiently during the transient state. At steady state, the filter media loaded with smaller NaCl particles showed lower collection efficiency. The filter media loaded with liquid DOS particles showed collection efficiency much lower than those loaded with solid NaCl particles.     

Computer Simulation of Monodisperse Aerosol Collection in Fibrous Filters

A computer program has been developed to simulate the filtration process in fibrous filters collecting monodisperse aerosol particles. The model filter is represented by an array of parallel cylindrical fibers and the Kuwabara flow field is employed to determine the particles trajectories inside the filter. The simulation model is based on the Monte-Carlo (self-driven) principle, and a sequence of uniform pairs of pseudorandom numbers is generated representing the initial locations of the approaching particles. The estimation of the initial collection efficiency through the simulation model that considers the deposited particles and the presence of dendrites is in good agreement with published experimental data. The development of the quasi three-dimensional simulation model offers a detailed information about the transient progression of the deposition process. The transient behavior of the pressure drop across the collector system (the fiber and the collected particles) and the morphology of deposit are presented and the results are in good agreement with experimental data.     

Verification and validation study of some polydisperse kinetic theories

The predictions of several polydisperse kinetic theories are presented in this study for the simple shear flow of a binary mixture of powders with varying sizes in a vacuum under zero gravity. These results are compared to published numerical data obtained using discrete molecular dynamics technique. The theory showing the most accurate results is modified so that the sum of the kinetic stresses of two or more identical solid phases adds to that of a monodisperse system with the same properties. Simulation results obtained in a three-dimensional riser for the flow of air and a binary mixture of glass beads are compared with available experimental data for each solid-phase velocity, concentration, and granular temperature. The predictions of the theory compare reasonably well with experimental data when the value of the linear particle-particle restitution coefficient is lowered to take into account the extra energy dissipation due to collisions between pairs of slightly frictional particles.     

Numerical simulation of horizontal jet penetration in a three-dimensional fluidized bed

The introduction of reactant gas as a jet into a fluidized bed chemical reactor is often encountered in various industrial applications. Understanding the hydrodynamics of the gas and solid flow resulting from the gas jet can have considerable significance in improving the reactor design and process optimization. In this work, a three-dimensional numerical simulation of a single horizontal gas jet into a cylindrical gas-solid fluidized bed of laboratory scale is conducted. A scaled drag model is proposed and implemented into the simulation of a fluidized bed of FCC particles. The gas and particles flow in the fluidized bed is investigated by analyzing the transient simulation results. The jet penetration lengths of different jet velocities have been obtained and compared with published experimental data as well as with predictions of empirical correlations. The predictions by several empirical correlations are discussed. A good agreement between the numerical simulation and experimental results has been achieved.     

Experimental Study of Electrostatic Aerosol Filtration at Moderate Filter Face Velocity

Aerosol collection efficiency was studied for electrostatically charged fibrous filters (3M Filtrete?, BMF-20F). In this study, collection efficiencies at moderate filter face velocities (0.5–2.5 m/s) representative of some high volume sampling applications was characterized. Experimental data and analytical theories of filter performance are less common in this flow regime since the viscous flow field assumption may not be representative of actual flow through the filter mat. Additionally, electrostatic fiber charge density is difficult to quantify, and measurements of aerosol collection efficiency are often used to calculate this fundamental parameter. The purpose of this study was to assess the relative influence of diffusion, inertial impaction, interception, and electrostatic filtration on overall filter performance. The effects of fiber charge density were quantified by comparing efficiency data for charged and uncharged filter media, where an isopropanol bath was used to eliminate electrostatic charge. The effects of particle charge were also quantified by test aerosols brought into the equilibrium Boltzmann charge distribution, and then using an electrostatic precipitator to separate out only those test particles with a charge of zero. Electrostatically charged filter media had collection efficiencies as high as 70–85% at 30 nm. Filter performance was reduced significantly (40–50% collection efficiency) when the electrostatic filtration component was eliminated. Experiments performed with zero charged NaCl particles showed that a significant increase in filter performance is attributable to an induction effect, where electrostatic fiber charge polarizes aerosol particles without charge. As filter face velocity increased the electrostatic filtration efficiency decreased since aerosol particles had less time to drift toward electrostatically charged fibers. Finally, experimental data at 0.5 m/s were compared to theoretical predictions and good agreement was found for both electrostatic and nonelectrostatic effects.

© 2013 American Association for Aerosol Research     

Hydrodynamic behavior of a liquid-solid fluidized-bed reactor for the bioconversion of coal particles to liquid products

A predictive mathematical model based on particle convection and dispersion is presented for a liquid fluidized bed of coal particles. The numerical representation can follow transient behavior of liquid fluidized beds that contain a defined particle-size distribution. The calculations exhibited excellent agreement when compared to experimental transient pressure-drop data from a column containing particles of Illinois No. 6 bituminous coal in the size range of 20–120 μm. In addition, the model was used to simulate the effect of periodic (hourly) liquid velocity step changes on elutriation of small particles from a given particle-size distribution of 34.5–75.5 μm. For the situation tested, the calculations indicate that (a) the column does not reach a steady state between velocity changes and (b) that higher initial particle elutriation rates decay to lower values until no more particles elutriate from the column.     

Collection efficiencies and particle size distributions from sampling cyclones — comparison of recent theories with experimental data

The grade collection efficiencies and particle size distributions from Stairmand-type cyclones are compared with predictions from four theories, viz. the models of Leith and Licht (1972), Dietz (1981) Mothes and Loffler (1988) and Li and Wang (1989). It is shown that the theory of Mothes and Loffler, which recogizes different flow regions within a cyclone, coupled with a realistic estimate of the particles turbulent diffusivity, is capable of providing good estimates of both the grade-efficiency curves and particle size distributions of the cyclone catch and/or outlet dusts. The lack of adequate theories or empirical formulae to estimate the effective turbulent diffusivity under cyclone flow seems the major factor hindering the use of the Mothes and Loffler theory for predictive purposes. High loadings and agglomeration of fines, if present, are responsible for strong departures from theoretical predictions.     

Simulation of particle deposition on filter fiber in an external electric field

Particle deposition onto a filter fiber was numerically simulated when a uniform external electric field was applied. The effects of electric field strength, particle inertia, and electrical conductivity of particles on particle deposition characteristics such as particle loading patterns and collection efficiency were qualitatively investigated. As a result, the electrostatic forces between a newly introduced particle and the already captured particles on the fiber were found to have a great influence on the particle deposition patterns compared with the results where the electrostatic forces were neglected. Conductive particles and filter fibers lead to higher collection efficiency and more linear structure of particle deposits than those of dielectrics, and the particle inertia could also be more important to the collection efficiency of a fibrous filter when electric fields are present. The simulated particle deposits obtained from this work agreed well with the existing experimental results, in which the photographs of particle loaded fibers, within an external electric field, were reported.     

超临界溶液快速膨胀法制备二氯二茂钛微粒及催化乙烯聚合

为克服茂金属催化剂得到的聚合物形态难以控制、表观密度较低、易粘釜和不适于气相淤浆聚合等缺点,以超临界溶液快速膨胀过程为手段,以期制得颗粒分布均匀的细微催化剂颗粒,继而得到形态良好的聚合物.作为超临界流体技术的基础,首先测定了二氯二茂钛在超临界丙烷中的溶解度.在此基础上,用RESS方法制得了均匀的超细催化剂颗粒,且系统考察了溶液浓度、预膨胀温度、喷嘴结构和接收距离对沉析颗粒粒径的影响.最后,将RESS所制得的催化剂颗粒进行乙烯淤浆聚合,并分析聚合物形态结构.实验结果表明,在温度为383.15～408.15 K和压力为10～35 MPa范围内,溶解度随温度的增加而增加,随压力的升高而增加,说明在该操作范围内,不存在反向区.RESS操作的结果显示,二氯二茂钛颗粒粒径随溶液浓度的增大而减小,随预膨胀温度的升高而增大,而喷嘴直径的减小和喷嘴长度的增加将使得颗粒粒径增大,而收集距离的增加将使得颗粒粒径先增加后减缓,直至不再变化.通过对原始的催化剂颗粒和RESS制得的催化剂颗粒进行乙烯淤浆聚合表征发现,相比于原始催化剂,由于烯烃催化剂的复制原理,RESS制得的催化剂颗粒的聚合物具有良好的形态.     

Effect of Concentration Gradients in Barrier Separation Cells

Abstract

Concentration gradients in the gas phase above and below the barrier in separation cells were measured. The experimental data were compared with existing theories and none of the theories adequately correlated the data over the range of parameters studied. A new, geometry-dependent, theory was formulated. This theory, which allows for mixing by diffusion above and below the barrier, gave better predictions, both for product concentrations and for permeated flow rate, than any of the existing theories, and also predicts concentration profiles. Although the theoretical model was developed in a study of separation of gases, it could be applied to other systems as well, if the proper permeability and diffusion coefficients were used.     

Adhesion-induced deformations of polyurethane substrates in contact with spherical glass particles: the effect of particle size on the radius of contact

Spherical glass particles having radii between approximately 0.5 and 100 *m were deposited onto a polyurethane substrate and the radii of contact, resulting from the adhesion forces between the particles and the substrate, were determined using SEM. For particles having radii less than approximately 5 μm, it was found that the contact radius varied as the particle radius to the 0.75 power. In addition, large menisci, presumably resulting from tensile interations, were observed. For particles having radii between 5 and 60 μm, the contact radius varied as the particle raidus to the 2/3 power. Stretching of the substrate was also observed for particles having radii of approximately 100 μm. This is probably a harbinger of the impending separation of the particle from the substrate, due to gravitational forces. The thermodynamic work of adhesion was calculated from the data and the results were compared with the predictions of several theories of particle adhesion.     

Analytic Solutions to Diffusional Deposition of Polydisperse Aerosols in Fibrous Filters

Deposition of polydisperse aerosols by Brownian diffusion was studied analytically using the penetration efficiency of monodisperse aerosols combined with the correlations among the moments of lognormal distribution functions. The analytic solutions, so obtained were validated using the exact solutions, which were applied to recalculate the filtration efficiencies of the existing experimental data for various filtration conditions. It was found that the collection efficiency of a fibrous filter should be corrected with respect to the position in the filter, if the particles are polydisperse. By considering the effect of the polydispersity of particle size, the analytic solutions showed good agreement with existing experimental data. It is believed that the present work makes it possible to determine the filtration efficiency of polydisperse aerosols in fibrous filters and to estimate errors associated with the degree of polydispersity of the particles quickly and accurately for the diffusion dominant regime.     

A mobility spectrometer for measurement of initial properties of Po

Reports in the literature on the measurement of the diffusive properties of ²¹⁸Po in the so-called unattached state yield conflicting results. It is now accepted that the ²¹⁸Po ion does not remain a single atom but interacts with trace vapours in the atmosphere, forming larger radioactive particles in the 0.5–3 nm size range. The mechanisms through which these interactions occur are unexplained. Recent application of the theory of ion-induced nucleation to the ²¹⁸Po ion showed that this theory can explain the experimental results reported in the literature qualitatively. The initial phases of ion-induced nucleation involve the formation of pure water clusters around which molecules of other condensable trace vapours may nucleate. This happens within microseconds after the formation of the ion. Methods available to study this phenomenon include diffusion screens, which do not have very high resolution, and electric mobility spectrometers, which can only provide information on charged particles, as is the case for the initial stages of the ²¹⁸Po ion's life before neutralization. Ion-induced nucleation can furthermore only occur for charged particles. This paper describes the development of a mobility spectrometer with adequate resolution to study the clustering processes that may occur. The first results of measurements of pure water clustering around ²¹⁸Po ions using the spectrometer are presented. It is found that the radius of the ion increases with relative humidity. The results are compared with predictions of two representations of the clustering theory. The theory qualitatively explains the results obtained, but there are considerable differences between the experimental measurements and the theoretical predictions, as well as between the two representations of the theory.     

Experimental Study of Particle Collection by Small Cyclones

A new set of experimental data on the particle collection characteristics of small cyclones is reported. The collection efficiency for particles ranging from 2 to 10 μm in diameter was measured systematically for nine cyclones at flow rates ranging from 8.8 to 18.4 L/min. Special emphasis was given to the effects of the exit tube size and of the cyclone body size on the particle collection efficiency. The size ratio of the exit tube to the cyclone body was varied from 0.24 to 0.80. The experimental results show that the stiffness of the particle collection cutoff with size does not change noticeably with a change in the cyclone body size while operation of a cyclone at a low flow rate can cause the particle collection characteristics to become less stiff. It was also found that the exit tube diameter influences the particle collection efficiency substantially, with results showing that as the exit tube size is decreased, the collection efficiency increases. A large cyclone body size increases the efficiency. However, when the cyclone body is increased excessively, the collection efficiency appears to decrease somewhat. The experimental data were compared with existing cyclone theories and Barth's (1956) theory was found to be in good agreement. Finally, the exit tube was found to affect substantially the pressure drop of cyclones. As the exit tube size increased, the pressure drop decreased. However, when the exit tube size was further increased until it approached the body size, the pressure drop increased again.     

Microstructural stability at elevated temperatures

The factors affecting the diffusion-controlled coarsening of microstructures at elevated temperatures are reviewed in this paper. Coarsening can occur in a variety of different ways. Perhaps the most familiar of these is Ostwald ripening of a dispersed phase, e.g. precipitates in a 2-phase alloy. However, there are other related Ostwald-ripening-types of processes, including coarsening of dispersed phases in 2-dimensions (e.g. precipitates in thin films, ‘fibers’ in directionally solidified eutectics) and coarsening of particles that grow in three dimensions in a 2-dimensional diffusion field (e.g. grain-boundary precipitates, particles on a substrate). Coarsening by fault migration, which is an important issue in the microstructural stability of directionally solidified rod and lamellar eutectics, and discontinuous coarsening, which affects the stability of cellular microstructures produced by discontinuous precipitation, eutectoid decomposition and eutectic solidification, are also discussed. The principal predictions of theories of these various types of coarsening mechanisms are described, and exemplified where possible by reference to published data. Emphasis is placed on ability of current theory to explain existing data obtained from real as well as computer simulation experiments. Possible reasons for the shortcomings of theory are discussed.     

A case study of simulating submicron aerosol filtration via lightweight spun-bonded filter media

The most common method of filtration is via fibrous nonwoven media. Fibrous filters are generally characterized by their collection efficiency and pressure drop. Traditional computational studies in this area are typically based on unrealistic 2-D geometries with the fibers simply placed in a lattice (regular array) perpendicular to the flow. The traditional approaches however, do not permit studying the relation between the 3-D structure of a filter media and its performance. In this study, for the first time, a virtual 3-D web is generated based on the fiber orientation information obtained from analyzing microscopic images of lightweight spun-bonded filter media. Pressure drop and collection efficiency of our virtual filter are simulated and compared with the previous 2-D analytical and numerical models as well as experiment. Our pressure drop calculation, unlike the previous models, showed a perfect agreement with the predictions of the Davies’ empirical equation. The collection efficiencies obtained from simulating a thin spun-bonded filter media challenged with submicron particles having diameters ranging from 50 to 500 nm showed a similar trend as that of the previous 2-D models. For the solid volume fraction (SVF), filter thickness, and the fiber and particle diameters considered in this study, we found collection efficiencies higher than that of the above mentioned 2-D models with a relatively good agreement with experimental data obtained from a TSI 8130 filter tester.     

Effect of Rigid Inclusions on the Sintering of Glass Powder Compacts

The effect of rigid inclusions on the sintering of glass powder compacts has been investigated at 600°C. The densification rates show good agreement with the rule of mixtures for inclusion volume fractions of °0.1 The transient stresses generated during sintering by the presence of the inclusions were evaluated from the sintering data. Below inclusion volume fractions of °0.12, the results are in excellent agreement with Scherer's theory for viscous sintering with rigid inclusions. At higher inclusion volume fractions, interactions between the inclusion particles lead to large deviations from theoretical predictions.     

液体与多孔固体颗粒间反应的模型研究

为探讨流休一多孔固体颗粒非催化反应模型中液固反应与气固反应的差别,本文用合成的一系列具有不同孔结构参数的多孔颗粒,考察了液固界面间的浸润现象(毛细现象)作为一种质量传递过程,对液体-多孔颗粒间的反应率～时间关系的影响,根据液体反应物在反应初始时刻进入颗粒的不同方式,定义了三类浸润条件,并对其中的两类提出了质量平衡方程和解析解.     

Influence of temperature,molecular weight,and molecular weight dispersity on the surface tension of polystyrene,polypropylene, and polyethylene. II. Theoretical

Experimental data for the surface tension of polystyrenes of different molecular weights (3400–200,000) and different molecular weight dispersities (1–3) and of different polyolefins are compared with the predictions of the Patterson–Rastogi and Dee–Sauer cell theories, which infer the surface tension from pressure–volume–temperature (PVT) data. PVT data for these polymers were obtained from the literature and experimentally and are fitted to the Flory–Orwoll–Vrij equation of state. Both theories predict that the surface tension will decrease linearly with increasing temperature and increase with molecular weight, thereby corroborating the experimental data. However, both theories underestimate the entropy change in the surface formation per unit area at a constant volume for low molecular weight and polydisperse systems and underestimate the effect of molecular weight dispersity on surface tension. Both theories feature two parameters, m and b, that quantify the enthalpic and entropic contributions to surface tension. The theoretical predictions are fitted to the experimental data for monodisperse polystyrene (with a molecular weight above the molecular weight of entanglement), polypropylene, and linear low‐density polyethylene to quantify the enthalpic contribution to surface tension. b is then evaluated as a function of molecular weight and molecular weight dispersity and is found to decrease with increasing molecular weight and to increase with increasing molecular weight dispersity, showing that end‐group excess at the surface has some effect on surface tension. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2201–2212, 2002