Fluidity and Flow Properties of Fine Powders

ABSTRACT

Fluidization and/or flow properties of many fine powders (d₅₀ < 50 μm), including pharmaceutical powders, toners, powder paints, and ceramic powders, are of critical importance. Particles in this range behave as cohesive powder because of the relatively large inter-particle forces (electrostatic, van der Waals', and liquid bridge forces), compared to the hydrodynamic force exerted on the particles by the fluid flowing around the particles. Flow additives, mechanical agitation, and other forces such as acoustic and electromagnetic, are often applied for good fluidization and uniform dilute phase flow. In this paper, we present a brief discussion and experimental data on fluidization properties, fluidity, and flow behavior of several fine powders as functions of particle size distribution, relative humidity, relative concentration of flow additives, and the frequency and amplitude of mechanical agitation. Electrostatic charging, dependent upon the chemical composition and electrical conductivity of the particles, and its influence upon the flow properties are also presented.     

Dynamic behavior of selected ceramic powders

An experimental setup has been developed on the continuous recording of the stress profiles in ceramic powders subject to shock loading with manganin gauges. A series of plate impact experiments on highly porous ceramic powders such as Al₂O₃, SiC and B₄C were conducted at the laboratory's single stage powder gun facility. The relationship between shock wave velocity and particle velocity was measured to obtain the Hugoniot data. Plate impact onto powder sample experiments were conducted at loading stresses ranging from 1.6 to 4.2 GPa. The experimental results show that the shock wave speeds in various ceramic powders vary between 1 and 2 km/s. Linear Hugoniot relations between shock velocity (D) and particle velocity (u) are observed. The loading stress–specific volume form of Hugoniot relations (P–V) was constructed using the data from quasistatic compression test results, Hopkinson bar dynamic compression test results and powder gun plate impact experiment results. The P–V diagram shows that the crush strength of ceramic powders is comparable to the loading stress level. The B₄C and SiC powders with bigger particle size more easily reach the solid state Hugoniot than the powders with smaller particle size at the same loading condition. In the case of Al₂O₃, the material shows less sensitivity to particle size difference at the same level of loading rate as compared to B₄C and SiC.     

Influence of air velocity and powder bed height on local density and float–sink of spheres in a gas–solid fluidized bed

Depending on their density, large objects will either float or sink in a gas–solid fluidized bed due to the liquid–like properties and density of the fluidized bed. The float–sink technology has been applied to dry density separations in industry. It is important for optimized industrial application to understand how the air velocity and the powder bed height affect the float–sink as the key operating factors. In this study, we investigated the float–sink of spheres of various density by varying the air velocity and the powder bed height. Also, we obtained the local fluidized bed density and the buoyancy force working on the sphere at various heights. We used the weight of a stainless-steel sphere in the fluidized bed to estimate the local fluidized bed density and the buoyancy force based on Archimedes principle. We found that the spheres float–sink behavior changes dramatically with the air velocity and the powder bed height and that the spheres float–sink behavior is correlated to ΔF = F_b – F_g, where F_b is the buoyancy force and F_g is the gravity force acting on the sphere. We also found that the fluidized bed density is not constant as a function of height when the air velocity is relatively large; the local fluidized bed density is interestingly either minimal at approximately mid-height or surprisingly, gradually increases with height within the fluidized bed at higher air velocities. The possible reasons are discussed by considering the local variation of the motion of air bubbles and the fluidized medium which affect the fluid force acting on the sphere in the fluidized bed.     

Fluidization behavior of glass beads under different vibration modules

The expansion of free bubbling gas fluidized beds has been investigated experimentally in a two-dimensional perspex-walled bed. Glass beads were fluidized with dried air at varying gas velocities, while the bed was vibrated at different frequencies, amplitudes and directions to study their effects on the fluidization quality. The experimental results showed that the particle flow pattern depends on the vibration direction, especially at superficial gas velocities less than the minimum fluidization velocity U_mf. The effect of horizontal vibration on fluidization behavior of glass beads exists at superficial gas velocities less than U_mf, while the effect of vertical vibration on fluidization behavior still exists even at higher superficial gas velocities than U_mf.     

Experimental investigation of bubble and particle motion behaviors in a gas-solid fluidized bed with side wall liquid spray

Bubble and particle motion behaviors are investigated experimentally in a gas solid fluidized bed with liquid spray on the side wall. The particles used in the experiment are classified as Geldart B particles. The results reveal that when the fluid drag force is less than the liquid bridge force between particles, liquid distribute all over the bed. Bubble size increases as the increase of inter-particle force, then decreases owing to the increase of particle weight with increasing liquid flow rate. When the fluid drag force is greater than the liquid bridge force, liquid mainly distribute in the upper part of the bed. And it is difficult for the wet particles to form agglomerates. Bubble size decreases with increasing liquid flow rate due to the increasing of minimum fluidization velocity. Besides, the acoustic emission (AE) measurements illustrate that the liquid adhesion and evaporation on particles could enhance the particles motion intensity. Consequently, the bubble and particle behaviors change due to the variation in fluidized gas velocity and liquid flow rate should be seriously considered when attempting to successfully design and operate the side wall liquid spray gas solid fluidized bed.     

Design of powder metallurgy aluminium alloys for applications at elevated temperatures Part 1 Microstructure of high pressure gas atomized powders

Abstract

A method is described for designing powder metallurgy rapidly solidified aluminium alloys using experimental and/or calculated nucleation maps which give the microstructure of gas atomised powders as a function of powder particle size and alloy composition. This method was used to predict the compositions of Al–Cr–Zr–Mn alloys for which the <45 μm sizefraction of the gas atomised powders exhibits a microstructure with or without Al₁₃Cr₂ intermetallic particles. Powders were produced by high pressure gas atomisation and were examined using analytical electron microscopy. The microstructures observed were in excellent agreement with those predicted. The powders exhibited four distinct microstructures with increasing powder particle diameter: (i) segregation free, (ii) cellular α aluminium, (iii) α aluminium plus fine spherical precipitates rich in chromium and manganese, and (iv) α aluminium plus Al₁₃Cr₂ primary intermetallic particles. The solidification of these powders is discussed in terms of solidification front velocity controlled by external heat flow and by the initial undercooling. Particles less than 10 μm in diameter undercool significantly before solidification. Segregation free microstructures occur in the fine <1 μm) particles, where the solidification front velocity exceeds the absolute stability velocity.

MST/1247a     

Effect of flow pulsation on fluidization degree of gas-solid fluidized beds by using coupled CFD-DEM

In this paper, the effect of inlet flow type on fluidization of a gas-solid fluidized bed was studied by using numerical simulations. Gas-solid fluidized beds are widely used in processes such as heating, cooling, drying, granulation, mixing, segregating and coating. To simulate the gas-particle flows, the unresolved surface CFD‐DEM was used considering Eulerian–Lagrangian approach. The fluid phase was modeled by computational fluid dynamics (CFD) while the solid phase was solved by discrete element method (DEM), and the coupling between gas and solid phases was considered to be four-way. The uniform and pulsed flows were injected through three nozzles located at the bottom of a rectangular bed. Three types of pulsed flow were considered: sinusoidal, rectangular and relocating. The fluidized bed behavior was discussed in terms of minimum fluidization velocity (MFV), pressure drop, bubble formation, bed expansion, particles velocity and, gas-solid interaction and particle contact forces. The results of different simulations indicated that the minimum fluidization velocity of the beds fluidized by pulsed flows was decreased by up to 33%. The influence of the pulsation amplitude on the minimum fluidization velocity was more significant than that of the pulsation frequency. The bed expansion and particles average velocity were increased by the pulsed flows, while the pressure drop and interaction force were decreased. As the pulsation frequency increased, the pressure drop and gas-solid interaction force increased, although size of the bubbles and bed expansion decreased. It was also observed that in large vibration frequencies, the bubbles became more regular. In the sinusoidal flow, the velocity and contact force between the particles were initially increased by frequency and in larger frequencies they were decreased.     

The influence of storage relative humidity on aerosolization of co-spray dried powders of hygroscopic kanamycin with the hydrophobic drug rifampicin

The purpose of this study was to investigate the influence of storage humidity on in vitro aerosolization and physicochemical properties of co-spray dried powders of kanamycin with rifampicin. The powders were stored for one-month in an open Petri dish at different relative humidities (RHs) (15%, 43%, and 75%) and 25?±?2?°C. The in vitro aerosolization (fine particle fraction, FPF) of the powders was determined by a next generation impactor (NGI). The moisture content, particle morphology and crystallinity of the powders were determined by Karl Fischer titration, scanning electron microscopy, and X-ray powder diffractometry, respectively. At all RH, the FPF of hydrophobic rifampicin-only powder was unaffected but the FPF of hygroscopic kanamycin-only powder significantly decreased even at 43% RH. The kanamycin-only particles fused together, crystallized and formed hard cakes at 75% RH. The aerosolization of kanamycin and rifampicin in the combination powders remained unaffected at 15% and 43% RH, but aerosolization significantly decreased at 75% RH. Enrichment of the surface of the particles with hydrophobic rifampicin did not protect the combination powders from moisture uptake but it prevented particle agglomeration up to 43% RH. At 75% RH, the moisture uptake led to agglomeration of the particles of the combination powder particles and consequently an increase in aerodynamic diameter. Further studies are required to investigate how rifampicin enrichment prevents particle agglomeration, the possible mechanisms (e.g. particle interactions due to capillary forces or electrostatic forces) for the changes in the aerosolization and changes in surface composition during storage.     

The hydrodynamics of liquid-solid and gas-liquid-solid inverse fluidized beds with bioparticles

The hydrodynamic characteristics, such as minimum fluidization velocity (U_lmf for liquid-solid (LS) system and U_g,if for gas-liquid-solid (GLS) system) and bed expansion ratio (BER), of liquid-solid and gas-liquid-solid inverse fluidized beds (LSIFB and GLSIFB) with bare particles and particles with biofilm were investigated. In the LSIFB system, U_lmf and BER of the bare particles were independent of the solids loading. For bioparticles, the increase of the biofilm thickness reduced U_lmf and increased BER, suggesting that the fluidizability increases with the presence of the biofilm. In the GLSIFB system, the initial fluidization gas velocity (U_g,if) and the complete fluidization gas velocity (U_g,cf) both increased with increasing particle diameter and decreasing particle density under fixed superficial liquid velocities. Biofilm attachment led to a decrease of both U_g,if and U_g,cf, and an increase of bed expansion, again suggesting increased fluidizability with the presence of biofilm.     

Aluminum integral foam castings with microcellular cores by nano-functionalization

The goal of the present work is the refinement of the pore morphology of aluminum integral foam castings. Integral foam molding, a modified high pressure die casting process, is used where a mixture of melt and blowing agent particles (magnesium hydride, MgH₂) is injected at high velocity into a permanent steel mold. At the mold surface, decomposition of the blowing agent and pore formation is suppressed due to the high solidification rate whereas solidification of the core is much slower allowing blowing agent decomposition, pore nucleation, and growth. Blowing agent particles not only act as gas suppliers but also represent pore nuclei. Thus, microcellular foam cores can be attained by increasing the number of MgH₂ particles. But increasing the number of powder particles by powder milling strongly decreases the flowability and strong particle agglomeration as a result of the increasing cohesive forces leads to inhomogeneous foams. Flowability of the powder can be restored by coating it with SiO₂-nano-particles resulting in a homogeneous microcellular foam morphology.     

Influence of interparticle forces on selective wet agglomeration

The influence of interparticle forces between primary particles, which include interaction force between dispersed particles in liquid calculated by DLVO theory and two kinds of forces caused by a liquid bridge of bridging liquid between particles, on selective wet agglomeration was investigated on the basis of the relationships between the results of separation efficiency obtained in author’s previous study and these interparticle forces. The first step of selective wet agglomeration is the collision between bridging liquid droplets and objective particles to be agglomerated. This collision is mainly influenced by the interparticle force calculated by DLVO theory. Incorporation of two objective particles, the second step in the agglomeration process, is influenced by liquid bridge force between objective particles. Growth to pellet-type agglomerates, the third step in the agglomeration process, is thought to be influenced mainly by aggregation force in the agglomerates by entry suction potential. The results of this study showed that selective wet agglomeration under the experimental conditions used in this study is influenced greatly by liquid bridge force and entry suction potential, which play major roles in the second step and third step, respectively, of the selective wet agglomeration process.     

Adhesion force approximation at varying consolidation stresses for fine powder under humid conditions

Interparticle adhesion forces in fine powders are greatly influenced by varying relative humidity (RH) conditions. The present study estimated the interparticle adhesion forces developed in corn starch powder under humid conditions at varying applied consolidation stresses using tensile strength determination approach. Shear test was used to determine tensile strength of powder at 1–9 kPa consolidation pressures and extrapolated values of tensile strength at zero stress were used for force estimation in non-consolidated powders. A strong dependence of interparticle adhesion force on consolidation and RH conditions was observed, mainly due to alteration in the number of adhesive contacts and contact area. The results indicated that, at low consolidation and high RH, capillary force is the prevailing force contributing to the total interparticle adhesion in contrast to higher consolidation conditions where load induced contact force plays a dominant role. Furthermore, for nonconsolidated samples, the adhesion forces registered a steep jump above 60% RH which was primarily attributed to dominance of the liquid bridge forces. Also, forces determined from tensile strength approach and those predicted theoretically, as a summation of individual forces, yielded a similar trend. Overall, a simple and effective approach for interparticle force estimation of consolidated as well as loosely packed powders under varying humidity conditions is presented here.     

Measurement methods for quantifying powder flowability and velocity in cold spray systems

Four powders with varying bulk densities, Al, Al₂O₃, Sn, and Cu, were used to determine quantifiable relationships between powder flowability, mass flow rate, and powder velocity with particle morphology and particle distribution in a cold spray system. High particle density results in good powder flowability, specifically when the powders are spherical relative to irregular morphology. Particle velocity during cold spray, measured with a double disk rotary system, increases non-linearly with an increase of inlet pressure. The increase in mass flow rate from the hopper and the resulting mass output of the cold spray system shows a consequence of good powder flowability. Conversely, a high mass flow rate decreases the particle velocity during the cold spray process, with better flowability leading to decreases on the order of 10% in particle velocity in the cold spray system. The described methods, proposed tools, and findings can be easily made with cost-effective and on-the-spot measurements.     

Prediction ability of a new minimum bubbling criterion

The recently developed minimum bubbling criterion of Brandani and Zhang [19] for a prediction of the minimum bubbling point was validated using an experimental determination of the minimum bubbling points of spherical rigid non-porous powders with various particle size distributions. These powders include a narrow size cut powder, a “natural” size distribution powder and a “bimodal” size distribution powder. The minimum bubbling points were correctly identified using the ε_d and U_d characteristic curves, obtained from a correct interpretation of 1-valve and 2-valve bed collapse curves using the bed collapse model, developed by Cherntongchai and Brandani in [21]. In order to enhance the prediction ability of the stability criterion, an appropriate drag force correlation was introduced into the criterion. Then, it was pointed out that the characteristic parameter of the criterion has a strong dependence on the voidage term as an exponential function. As a result, a simple empirical correlation is proposed. The new stability criterion was, then, tested against a detailed comparison of 700 minimum bubbling points taken from literature. The criterion can very well predict the minimum bubbling voidage for various operating conditions of rigid non-porous materials and predict fairly well the minimum bubbling velocity.     

Gas flow influences on bubbling and flow characteristics of fluidized bed with immersed tube under electrostatic effects

Industrial bubbling fluidized beds are used to fluidize particles. When particles are fluidized, electrostatic effects will cause the particles to form obvious agglomerates, thus reducing fluidization performance. For better fluidization performance, internal component immersed tubes are usually placed in fluidized bed to limit the bubble size and reduce particle agglomerates. Meanwhile, pulsed gas flow can increase particle disturbance, which is also an effective method to reduce particle agglomerates. In this paper, the CFD-DEM model under electrostatic effects is constructed to research the bubbling and flow characteristics in fluidized beds. Firstly, particle mixing qualities with and without the immersed tube are compared. Then, the effects of different superficial gas velocities are investigated with an immersed tube. Finally, different frequencies are applied to study the energy loss and flow characteristics around the immersed tube. The results show that the addition of the immersed tube can reduce bubble size to facilitate particle mixing. Due to the obstruction of the immersed tube, the bubbles are generated near the wall. As the superficial gas velocity increases, the larger bubbles are generated. Moreover, the electrostatic force applied to the particles varies periodically with the frequency of incoming pulsed gas flow, with fluctuations maximal at 2.5 Hz.     

Pressure Drop and Gas Holdup Studies in a Spout-Fluid Bed

Spout-fluid beds are used for a variety of processes involving particulate solids. They are employed where the particle agglomeration, dead zones, and sticking of particles to the vessel are the common problems in conventional spouted beds. Applications involved are granulation, coating, drying, combustion, and gasification. In this study, experimental studies have been carried out in a cylindrical Perspex column (0.094 m internal diameter and 1.217 m height) using glass beads and air. The effects of initial bed loading, spout velocity, and background (fluidization) velocity on pressure drop and gas holdup have been investigated. It is found that the minimum spout-fluidizing velocity increases with increase in initial bed loading. The pressure drop and gas holdup increase with increasing bed loading. In spout-fluid bed condition, at a constant spout velocity, as the background gas velocity increases, the gas holdup increases, and it is found to be high for smaller bed loading and is low for larger bed loading at higher velocities. The fountain height increases as spouting velocity increases and it decreases with initial bed loading. The total velocity required to fluidize the particles in spout fluidization is lower in comparison to spouted beds and fluidized beds.     

Study on the flow behavior of irregular plastic particles in a spout-fluid bed with a draft tube

This work presents an experimental investigation on the hydrodynamic performance of a draft tube spout-fluid bed with irregular particles. Nonmetal particles from waste printed circuit boards (NPCBs) were used as a spouting solid, and polypropylene (PP) particles were selected as an assistant to fluidization particles. The flow pattern, minimum spouting velocity (U_ms), and minimum spout-fluidization velocity (U_msf) were investigated under different operating conditions. The irregular cohesive particles from NPCBs showed poor flowability and channeling, which restrained stable spouting in the spout-fluid bed. The quality of fluidization and spouting improved when greater than 40?wt.% PP particles were added into the NPCB/PP mixtures. The mechanism was that the PP particles accelerated the movement of NPCB particles. Meanwhile, lower density differences between NPCB and PP particles decreased the segregation of the mixtures. The minimum spouting velocity decreased with an increase in fluidization gas velocity and the ratio of NPCB particle in the NPCB/PP mixtures. Two flow patterns, unstable spouting and unstable spouting fluidization, were observed over a large range of gas velocity. The ranges of gas velocity in these two flow patterns enlarged with the increase in mass fraction of NPCB particles within the NPCB/PP mixtures.     

Superfast densification of nanocrystalline oxide powders by spark plasma sintering

Spark plasma sintering (SPS) is a newly discovered old technique which recently has been used for superfast densification of ceramic powders. Simultaneous application of pulsed high dc current densities and load is the necessary condition for rapid and full densification of ceramic powders by SPS. Commercial nanocrystalline magnesium oxide (nc-MgO) and yttrium aluminum garnet (nc-YAG) powders were densified to optical transparency using spark plasma sintering at distinctly different homologous temperatures (0.3 T _m for nc-MgO and 0.7 T _m for nc-YAG). The observed microstructure, density and grain size evolutions versus the SPS temperature were analyzed. The enhanced densification of the nc-MgO powder at the present SPS conditions was related to plastic deformation followed by diffusion processes. Densification of nc-YAG powder was described by the formation of viscous layer at the particle surfaces and corresponding densification by grain rotation and diffusion through the liquid phase. Densification by normal grain growth takes place at higher relative densities, regardless of the material.     

Increasing the amorphous yield of {(Fe0.6Co0.4)0.75B0.2Si0.05}96Nb4 powders by hot gas atomization

The synthesis of metallic glasses requires high cooling rates leading to product size limitations of a few millimeters when using conventional casting techniques. One way to overcome these size limitations is powder metallurgy. Melt atomization and the subsequent powder processing can result in larger, amorphous components as long as no crystallization takes place during powder consolidation.An iron-based glass-forming alloy {(Fe_0.6Co_0.4)_0.75B_0.2Si_0.05}₉₆Nb₄ was formed through both ambient room and high temperature inert gas atomization at various melt flow rates (close-coupled atomization). The use of hot gas generally decreases the droplet size and hence leads to an increased cooling rate and amorphous fraction of the atomized powders.Hot gas atomization results in a lower gas consumption, a smaller gas-to-melt mass flow ratio (GMR), smaller particles and a smaller geometric standard deviation.Particles atomized in ambient temperature were fully amorphous up to a particle size fraction of 90?µm. Larger particle size fractions resulted in a higher crystalline fraction. According to the XRD and DSC analyses, hot gas atomization has only a very small influence on the cooling rate and the amorphous fraction. However, the amorphous yield is significantly increased using hot gas atomization.     

Research on the powder classification and the key parameters affecting tablet qualities for direct compaction based on powder functional properties

Direct compaction (DC) is the ideal method for tablet production. However, DC requires the highest quality powder functional properties, such as good flowability, compressibility, compactibility, and appropriate elasticity. In the present study, 24 types of natural plant product (NPP) powders, prepared using three commonly used methods, and 18 types of fillers were evaluated for their fundamental and functional properties. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) were used to classify the powders and analyze the characteristics of each category of powder, based on functional properties. Partial least squares (PLS) regression models were established to predict the tensile strength (TS), ejection force (EF), disintegration time (DT), and solid fraction (SF) based on the powder’s functional properties. The results show that: (i) Except for the direct pulverization powder, the fundamental properties among the NPP powders were similar; however, the functional properties were quite different; (ii) The powders could be classified well based on their functional parameters (w, a, k_G, y₀, k_a, k_b, b, k_FES, k_E3). The order of powders suitable for DC is Group 4 > Group 5 > Group 2 > Group 3 > Group 1; (iii) The compressibility and compactibility of the powder are beneficial to the TS and EF of the tablet. The compressibility of the powders correlated negatively with DT; nevertheless, the compactibility correlated positively with DT and SF. The true density (ρ_true), median particle size (D_0.5) of the powders correlated negatively with SF. Overall, this study systematically evaluated the properties of commonly used NPP powders and fillers and found the key properties that affect the quality of tablets.