The measurement of emulsion phase voidage in gas fluidized beds of fine powders

The results of two different experimental methods, X-ray absorption and bed collapse, for the determination of the emulsion-phase voidage of gas fluidized beds are compared. It is shown that good agreement exists between them over the range of gas velocities studied, but that possible differences in voidage in an axial direction are only revealed by the X-ray technique. The bed collapse method applied to a Geldart Group A powder confirms that there is little difference between the surface settling rates of a bubbling bed and of a uniformly expanded bed of the same voidage.     

Flow properties of homogeneously aerated,expanded emulsion phase of fine powders (quasi-solid emulsion phase viscosity)

The viscosity of quasi-solid emulsion phase (μ_e) of the “homogeneously aerated, expanded emulsion phase” (HAE emulsion phase) of fine powders within the gas velocity range of minimum fluidization (U_mf) and minimum bubbling (U_mb), was obtained at ambient and elevated temperatures by measuring the strain retardation time (τ) of the HAE emulsion phase in this study.The deformation coefficient (Y) was obtained by our previously developed experimental method [Powder Technol., 81 (1994) 177].We confirmed experimentally that the HAE emulsion phase behaves exactly as quasi-elastic body, by observing the emulsion phase volume expansion and contraction within the velocity range of U_mf and U_mb. It is to be noted that U_mb>U_mf. The volume element of the unit HAE emulsion phase could be expanded or contracted by the force balance between the excessive gas drag force (above the gravitational force) and the inter-particle force of the particles of the emulsion phase. This experimental evidence enabled us to decide using the visco-elastic rheological model of the Voigt–Kelvin model body (VK model body) in terms of mathematics to analyze our experimental data. The rheological parameters, i.e., the elastic deformation coefficient (Y) and quasi-solid viscosity (μ_e) of our experiments, were obtained by using the HAE emulsion phase.Using these experimental data together with the VK model body, we developed our experimental model equation including the measured strain retardation time (τ) to obtain the “quasi-solid viscosity” of the HAE emulsion phase (μ_e). The physical meaning of this viscosity of the HAE emulsion phase developed in this study is quite different from the “apparent quasi-liquid viscosity” defined in aggregative fluidized beds, under the condition of U_mf=U_mb.The interesting experimental data correlations of comprehensive rheological parameters (μ_e, Y, σ_f) of the HAE emulsion were obtained for fine powders at ambient and elevated temperatures.     

Interactions between fine combustion droplets

Interactions between fine combustion droplets were directly observed using microscopic flow visualization and high speed photography. The observations revealed “attracting-revolving-repulsing” interactions between the droplets. Force analyses showed that the traditionally considered interparticle forces, including drag, gravitation, the Coulomb force and the van der Waals force, cannot explain these interactions. However, the induced dipoles on the droplets due to the non-uniform distribution of surface charges on the fine droplets have important influence on such interactions. Therefore, the inter-dipole forces must be taken into account in the interaction force analysis as well as the Coulomb force between the net charges. The inter-dipole force includes components in the radial and azimuthal directions and is inversely proportional to r⁴. This force causes the particles to revolve and repel each other at small distance. The combined effects of the inter-dipole force and the traditionally considered forces give a complete explanation for the particle interactions.     

Simulations of vibrated fine powders

During the past few decades, several studies have been conducted to understand the behaviour of powders in vibrated beds. This paper introduces a technique of incorporating the agglomeration and deagglomeration phenomena in the simulation of vibrated fine powders. Two-dimensional direct simulations are performed using 300 spheres 2.99 mm in diameter in a trapezoidal container vibrated vertically at an amplitude of 2.5 mm and 20 Hz frequency as preliminary conditions. Under non-cohesive conditions, the results are in agreement with those found in the literature. As a preliminary effort to predict the behaviour of cohesive fine powders under vibrated conditions, agglomeration and deagglomeration processes are modelled as the formation and destruction, respectively, of interparticle bonds during particle collisions. Two parameters used to model agglomeration and deagglomeration are the ease of cohesion and cohesivity of the powder. Dependencies of these parameters on certain physical properties of cohesive powders have been suggested. Simulation results reveal two aggregate populations, one with uniform size aggregates and another population with multi-sized aggregates. The former aggregates were more prevalent in weakly cohesive powders while the latter in highly cohesive powders. Interesting macroscopic bed behaviour such as alternating cycles of agglomeration and deagglomeration were also observed. Further work is needed in which the aerodynamic forces are taken into account and cohesion mechanisms at the particle surface are modelled.     

Preparation of barium hafnate fine powders by oxalate method

Barium hafnate (BaHfO₃) precursor was prepared by mixed barium nitrate and hafnium (IV) chloride with ammonium oxalate aqueous solution. Barium hafnate powders were obtained by decomposition of the precipitate precursor at 800 °C for 2 h. As a dispersant, polyethylene glycol (PEG6000) was used in the precipitation reaction and the influence of dispersant dosage on particle size was studied. X-ray diffraction analysis (XRD) showed the powders was pure cubic BaHfO₃ without impurities. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) demonstrated that the size distribution was uniform, and the average grain size was about 50 nm.     

Two-phase theory and fine powders

The validity of the assumptions built into the two-phase theory has been re-examined for a range of fine powders. These long-standing but often criticised assumptions are that, in the dense phase of a bubbling fluidised bed, the voidage and velocity correspond to those at incipient fluidisation.Eleven different powders were used, and average particle size varied from 12 to 67 μm. Particle density ranged between 1300 and 5200 kg/m³. Dense phase velocity and voidage were measured by the bed collapse technique, and the systematic deviations from the above assumptions demonstrated.It has been shown that the minimum fluidisation point loses meaning at high fines levels; a transitional class on the Geldart [6] powder classification diagram, termed class AC, is postulated. This class is characterised by the absence of a meaningful incipient fluidisation point, and the absence of a contraction of the bed when bubbles first begin to pass through. The dense phase voidage has been found to increase more strongly than expected, and correlates strongly with fines level. Particle density has been found to have an insignificant effect on dense phase voidage in the range covered, and the gas velocity in the dense phase has also been found to increase and correlate strongly with increasing fines level. Correlations are presented for dense phase voidage and velocity.     

可再分散乳胶粉的研究概况

可再分散乳胶粉(SWF)是近年来大力推广的一种新型建筑材料添加剂。对SWF的研究状况、产品类型、基本组分、生产过程、作用机理和应用领域等作了简单介绍,重点概括了其在瓷砖胶、保温系统及自流平砂浆等领域中的应用情况及不足之处,提出了影响SWF发展所必须解决的问题。     

3D printing of fine alumina powders by binder jetting

Additive manufacturing of ceramics is still at an early-development stage; however, the huge interest in custom production of these materials has led to the development of different techniques that could provide highly performing devices. In this work, alumina (α-Al₂O₃) components were produced by binder jetting 3D printing (BJ), a powder-based technique that enables the ex-situ thermal treatment of the printed parts. The employment of fine particles has led to high green relative density values (>60 %), as predicted by Lubachevsky-Stillinger algorithm and DEM modelling. Then, extended sintering has been observed on samples treated at 1750 °C that have reached a final density of 75.4 %. Finally, the mechanical properties of the sintered material have been assessed through bending test for flexural resistance and micro-indentation for Vickers hardness evaluation.     

Permeability techniques for characterizing fine powders

The physical significance of the so-called “surface area” values deduced from permeability studies are discussed. The uses of permeability techniques in research and development of powder technology processes are outlined. The various forms of permeameter currently available are described.     

The mixing state of fine powders measured by magnetic resonance imaging

The usefulness of magnetic resonance imaging (MRI) for the spatially resolved, quantitative characterization of mixtures of fine powders in the size range of about 10 μm is investigated. The sources of errors inherent in the signal translation process between raw MRI data and local concentrations are analyzed in detail. Possibilities for their correction are pointed out in deriving a global characteristic such as the standard deviation.The analysis of the mixing state of powders by MRI and the benefits of error correction are then illustrated by simple examples covering a wide range of compositions and mixing states. These experiments were made by mixing spherical, oil-filled (hence “visible” to MRI) melamine microcapsules in the size range of about 1-20 μm with solid melamine spheres of similar size and density. The mixture volume of typically 1 cm³ was imaged with an isotropic resolution of 235 μm, providing over 60,000 contiguous data points per measurement. The benefit of error correction is shown to be significant, provided the mixtures are homogeneous to a large extent.     

可再分散乳胶粉SWF的制备与性能研究

优选一种改性单体(V),与醋酸乙烯酯(VAc)共聚制备了可再分散乳胶粉(SWF)的原乳液,再用喷雾干燥法制取了性能优异的SWF。实验结果表明,为使SWF具有较好的粒径还原性,尽量选用粒径小的防结块剂;随着VAc单体含量的增加,SWF成膜后的拉伸强度也随之增长;由于SWF的减水作用,随着聚灰比的增大,改性砂浆的弯曲强度和压缩强度呈现增大的趋势。     

Preparation of monodispersed emulsion with large droplets using microchannel emulsification

Microchannel (MC) emulsification is a novel technique for producing monodispersed emulsions with coefficients of variation of less than 5%. To produce emulsions with large droplets, we designed three MC with large dimensions. The MC structure consists of two parts: a channel and a terrace. Terrace length was defined as the length from the exit of the MC to the end of the terrace. The MC plates used in this study have deeper channels and longer terraces. The size limit of droplets prepared by MC emulsification was studied. Monodispersed emulsions with droplets as large as 100 μm were prepared using an MC with a depth of 16 μm and a terrace length of 240 μm. The average diameter (coefficient of variation) of the emulsion droplets was 98.1 μm (2.5%). Emulsions with larger-diameter droplets were prepared using an MC with a longer terrace. The effect of the applied pressure on emulsification behavior was studied and discussed from the viewpoint of the droplet formation mechanism. At low applied pressures, droplet diameters were independent of the applied pressure, and monodispersed emulsions were produced. The pressure ranges of constant droplet diameter for large-droplet emulsions were narrower than those for the 5 to 30 μm droplet size emulsions because interfacial tension is more significant on a smaller scale compared with the other forces.     

Stability of monomer emulsion droplets and implications for polymerizations therein

The Lifshitz-Slezov-Wagner (LSW) theory of Ostwald ripening [I.M. Lifshitz, V.V. Slezov, Zh Exp Teor Fiz 35 (2(8)) (1958) 479-492; C.Z. Wagner, Elektrochemistry 65 (7/8) (1961) 581-591. [1] and [2]] and Kabalnov's extension [A.S. Kabalnov, A.V. Pertzov, E.D. Shchukin, Colloid Surf 24 (1987) 19-32 [3]] is applied to theoretically analyze the stability of single and two component monomer emulsion drops (C1—monomer, C2—additional compound with low solubility in the continuous phase), respectively. The analysis is carried out for monomers frequently used in heterophase polymerizations such as methyl methacrylate (MMA), butyl acrylate (BA), styrene (STY), tert-butyl styrene (TBS), and lauryl methacrylate (LMA) and C2 with varied solubility in the continuous phase. Conclusions are drawn from the results regarding heterophase polymerizations carried out with such monomer emulsions. The calculations show that the solubility of both C1 and C2 in the continuous phase in combination with the concentration of C2 and the average emulsion drop size as well as the width of its distribution determines essentially the stability of emulsions.     

Pressures in hoppers filled with fine powders

In a previous paper [18] the authors showed that air pressures occurred between the grains of fine powders when they were loaded into hoppers and presented data where the pore air pressure was 60% of the total stress measured at the bunker wall when the hopper was first loaded. The paper included a mathematical model to describe the dissipation with time of these pore air pressures. The model was used to scale up from a bench scale deaeration test to give the rate of air pressure dissipation in a large scale hopper, which was broadly in agreement with that observed.The present paper continues the work, but the emphasis is experimental. The total stresses measured in large scale hoppers of various configuration and dimensions when first loaded with a fine powder are demonstrated to be hydrostatic in nature. The total stresses and pore air pressures near the wall during discharge of a fine powder are then presented. Different types of discharge are considered, namely mass flow, flow from eccentric outlets, and vibration-assisted flow. The total stress is observed to fluctuate with time, but these fluctuations are generally less regular and of lower magnitude than those observed in mass flow with free-flowing materials, such as sand. The total stresses on discharge by mass flow may rise above the initial hydrostatic stresses but it is shown that for the fine particulate material considered, the hydrostatic stress represents a reasonable approximation for one of the loading cases that should be considered during design.     

Preparation of spherical yttrium oxide powders using emulsion evaporation

Water-in-oil type emulsions were used to prepare yttrium oxide powders by evaporation of the yttrium-containing aqueous phase in a hot oil bath. Emulsions were characterized with respect to emulsion type, droplet size distribution and stability. Y₂O₃ powders obtained by this method consisted of small (1–3 μm) spherical granules made up of 0·1 μm crystallites. The effects on the powder of emulsifier concentration, yttrium ion concentration, evaporation temperature and atmosphere were studied.     

Dose uniformity of fine powders in ultrasonic microfeeding

Ultrasonic microfeeding can be used in solid freeforming and pharmaceutical dosing. In this work, dose uniformity was studied because it is one of the important factors in evaluation of microfeeding. The experiments were carried out in a computer-controlled microfeeding system. The powders used were H13 tool steel, tungsten carbide (WC) and glass beads. The experimental results show that the minimum dose mass for these powders in a 0.2 mm diameter nozzle is close to 0.1 mg, 14 μg and 16 μg respectively. The probability distributions of relative mean dose mass deviation can be divided into four categories: single symmetrical peak, single positive skewed peak, single negative skewed peak or double peaks. Because the signal amplitude voltage and oscillation period are the two important factors influencing dose mass, the effects of these two factors on standard deviation, relative standard deviation and probability distribution were studied and discussed.