Flowability measurement of pulverized and granulated materials using vibrating tube method

To develop a method for measuring the flowability of MOX (mixed oxide of uranium and plutonium) particles used in the simplified MOX pellet fabrication process, the flowability of model particles has been investigated by the vibrating tube method. As model particles, pulverized ZrO₂ and granulated WO₃ were used. To prepare a variety of samples, coarse particles of 106–250 μm in diameter were mixed with fine particles smaller than 45 μm in diameter in different concentrations. The prepared particles were put into the vibrating tube and the amplitude of vibration was increased at a constant rate for a period of time and then decreased. The mass of particles discharged from the tube was measured at constant time intervals. From the experimental data, the relationships between the mass flow rate and the vibration acceleration (i.e. flowability profiles) were obtained. Two factors (i.e. ‘critical vibration acceleration’ to make the particles flow and ‘characteristic mass flow rate’) were selected to analyze the flowability profiles. The hysteresis of the flowability between increasing and decreasing vibration accelerations was also evaluated.     

Measurement of flowability of lubricated powders by the vibrating tube method

Background: The evaluation of lubricity or flowability of pharmaceutical powders is important for consistent production and quality control of drug products. However, there have been only a few studies on quantitative measurements of the properties of lubricated powders.

Method: Magnesium stearate (MgSt) and sodium stearyl fumarate (SSF) were used as lubricants. Lubricated powders were prepared by adding lubricants to spray-dried lactose under different conditions. To evaluate flowability, the vibrating tube method was used. In this method, the vibration amplitude of the tube is increased at a constant rate, and the mass of the powder discharged from the tube is recorded. Flowability profiles, i.e. the relationships between the mass flow rate and vibration acceleration, were obtained experimentally. To characterize static and dynamic friction properties of powders, critical vibration acceleration required to make powder particles flow and the average mass flow rate were determined.

Results: Addition of 0.5% MgSt was sufficient for the reduction of static friction between particles. Blending time of the lubricants had little effect on the average mass flow rate of lubricated powders. On the other hand, addition of SSF resulted in an increase in static friction at the beginning of blending, and after a certain blending time, flowability improved. The combination of MgSt and SSF improved both static and dynamic friction properties irrespective of the blending time.

Conclusion: The vibrating tube method can be used to evaluate the flowability properties of lubricated powders, and the experimental results provide useful information on the production of pharmaceutical solid dosage forms.     

Effects of main particle diameter on improving particle flowability for compressed packing fraction in a smaller particle admixing system

Particle flowability can be improved by admixing particles smaller than the original particles (main particles). However, the mechanisms by which this technique improves flowability are not yet fully understood. In this study, we examined compressed packing in a particle bed, which is affected by particle flowability. To estimate the mechanism of improvement, we investigated the effects of the main particle diameter on the improvement of compressed packing fractions experimentally.The main particles were 397 and 1460 nm in diameter and the admixed particles were 8, 21, 62, and 104 nm in diameter. The main and admixed particles were mixed in various mass ratios, and the compressed packing fractions of the mixtures were measured. SEM images were used to analyze the coverage diameter and the surface coverage ratio of the admixed particles on the main particles. The main particle packing fraction was improved as the diameter ratio (=main particles/admixed particles) increased. This was explained by a linked rigid-3-bodies model with leverage. Furthermore, the actual surface coverage ratio at which the most improved packing fraction was obtained decreased with increasing main particle diameter. This was explained by the difference in the curvature of the main particle surface.     

Physical and electrochemical study of cobalt oxide nano- and microparticles

Cobalt oxide nanocrystals of size 17–21 nm were synthesized by a simple reaction between cobalt acetate (II) and dodecylamine. On the other hand, micrometric Co₃O₄ was prepared using the ceramic method. The structural examination of these materials was performed using powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM and HRTEM). XRD studies showed that the oxides were pure, well-crystallized, spinel cubic phases with a-cell parameter of 0.8049 nm and 0.8069 nm for the nano and micro-oxide, respectively. The average particle size was 19 nm (nano-oxide) and 1250 μm (micro-oxide). Morphological studies carried out by SEM and TEM analyses have shown the presence of octahedral particles in both cases. Bulk and surface properties investigated by X-ray photoelectron spectroscopy (XPS), point zero charge (pzc), FTIR and cyclic voltammetry indicated that there were no significant differences in the composition on both materials. The magnetic behavior of the samples was determined using a vibrating sample magnetometer. The compounds showed paramagnetic character and no coercivity and remanence in all cases. Galvanostatic measurements of electrodes formed with nanocrystals showed better performance than those built with micrometric particles.     

Optimization of electrolyte concentration and voltage for effective formation of Sn/SnO2 nanoparticles by electrolysis in liquid

This work investigates the optimum experimental conditions required for the synthesis of Sn nanoparticles (Sn-NPs) via surfactant-free direct-current electrolysis using KCl as the electrolyte. Metallic Sn wire was used as a cathode, which was melted by the local concentration of current upon the application of a direct-current voltage. The effect of electrolyte concentration was analyzed by varying the concentration from 0.01 to 1.0 M, under constant electric power of 40 W. Results indicated that the applied voltage required for plasma generation increased with a decrease in the electrolyte concentration and the particle size decreased at high applied voltage with low electrolyte concentration; particles with a mean diameter of 258.5 nm formed at 0.05 M. However, coarse Sn₆O₄(OH)₄ crystals were precipitated at a concentration of 0.01 M. Therefore, the optimum concentration required for the formation of smaller particles was determined to be 0.05 M. Subsequently, the effect of voltage was analyzed by varying the applied voltage from 70 to 190 V. As a result, the effective production energy of 45 W h/g was obtained at voltages ranging from 110 to 130 V.     

Erosion in the tube entrance region of an air-cooled heat exchanger

Erosion in the tube entrance region of a typical air-cooled heat exchanger is numerically predicted. The erosion rates are obtained for different flow rates and particle sizes assuming low particle concentration. The erosion prediction is based on using a mathematical model for simulating the fluid velocity field and another model for simulating the motion of solid particles. The fluid velocity model is based on the solution of the time-averaged governing equations of 3-D turbulent flow while the particle-tracking model is based on the solution of the governing equation of each particle motion taking into consideration the viscous and gravity forces as well as the effect of particle rebound behavior. The computational model was validated against available experimental data and the comparison resulted in a good agreement. The investigation covered particle sizes from 10 to 350 μm and inlet flow velocities from 0.18 to 4.5 m/s. The results show that the location and number of eroded tubes depend mainly on the particle size and flow velocity at the header inlet. The total rate of erosion was found to increase exponentially with flow velocity. At high flow velocities, the maximum total erosion rate results from large particles and the effect is reversed at low velocities. Similarly, the tube penetration rate was found to increase with the increase of flow velocity for all particle sizes. At the typical velocity of 1.1 m/s, the minimum tube lifetime was caused by the 350 μm particles and the maximum was caused by the 200 μm particles. Based on the obtained results, it is well established that erosion cannot be totally avoided so long as solid particles are present in the fluid. However, the threshold velocity below which erosion is negligible can be accurately defined if an acceptable lifetime (or penetration rate) is defined.     

Synthesis of magnetite octahedrons from iron powders through a mild hydrothermal method

Magnetite (Fe₃O₄) octahedral particles were fabricated from iron powders through a simple one-step alkali-assisted hydrothermal process. The crystallinity, morphology, and structural features of the as-prepared magnetite particles were investigated using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). The values of saturation magnetization (M_s) and coercivity (H) of the magnetite octahedrons characterized on a vibrating sample magnetometer (VSM) are 89.81 emu/g and 70.6 Oe, respectively. The concentration of NaOH and the reaction temperature played a key role in the formation of the magnetite octahedrons.     

Lattice Boltzmann simulation of flow past a non-spherical particle

Lattice Boltzmann method was used to predict the fluid-particle interaction for arbitrary shaped particles. In order to validate the reliability of the present approach, simulation of flow past a single stationary spherical, cylindrical or cubic particle is conducted in a wide range of Reynolds number (0.1 < Re_p < 3000). The results indicate that the drag coefficient is closely related to the particle shape, especially at high Reynolds numbers. The voxel resolution of spherical particle plays a key role in accurately predicting the drag coefficient at high Reynolds numbers. For non-spherical particles, the drag coefficient is more influenced by the particle morphology at moderate or high Reynolds numbers than at low ones. The inclination angle has an important impact on the pressure drag force due to the change of projected area. The simulated drag coefficient agrees well with the experimental data or empirical correlation for both spherical and non-spherical particles.     

Directed growth of nanoarchitected hybrid phosphor particles synthesized at low temperature

Sub-micronic particles are of considerable interest for a wide variety of applications, such as catalyst or optical ceramics, due to their unique properties determined by size, composition and structure. In this work, we have reported a simple, rapid, single-step aerosol processing for the continuous synthesis of nanostructured particles having homogeneous composition and narrow size distributions, good crystallinity and fluorescence response. This paper presents the synthesis, optimization and characterization of hybrid Ag@Y₂O₃:Eu (9 at.% Eu³⁺) phosphor particles by means of spray pyrolysis method from water solutions of common nitrates precursors. The effect of silver concentration on particle structure, morphology and functional properties was specially evaluated. The as-prepared samples were additionally heated from 800 to 1200 °C/12 h under constant argon flow to avoid the silver oxidation. It was evident the cubic phase with Ia-3 symmetry as the principal one in all as-prepared and thermally treated samples. For the case high silver nitrate concentrations in precursor solutions a minority crystalline phase having Fm3m symmetry was identified. The luminescence emission spectra have been taken after excitation at 235 nm wavelength. It is evident the increase in the emission caused by the presence of metallic silver nanoparticles onto Y₂O₃:Eu³⁺ particle surface. It was also determined the silver concentration influence on the fluorescence response.     

Size-controlled hydrothermal synthesis of monodispersed BaZrO3 sphere particles by seeding

Monodispersed barium zirconate (BaZrO₃) fine particles with a spherical shape have been synthesized by hydrothermal reactions using barium hydroxide and a Zr-triethanolamine (TEOA) complex. The particle mean diameter was gradually controlled in a range from 0.20 μm to 3.5 μm by change in the added amount of seed nanoparticles. The mechanistic characterization of the seed-mediated BaZrO₃ sphere synthesis revealed that the particle growth obeyed a simple seed growth mechanism. In the present system, utilization of the Zr-TEOA complex played an important rule to prevent uncontrollable nucleation to form the uniform BaZrO₃ fine particles with narrow size distributions.     

Particle size measurement of standard reference particle candidates and theoretical estimation of uncertainty region

In order to confirm reliable particle size measurement technique and to prepare standard reference particles for calibrating particle size measurement devices, experimental and theoretical studies have been conducted about particle size measurement of 0.1–1 μm silica particles. The microscopic method with sample size greater than 90,000 particles was conducted for the size measurement.Theoretical equation of uncertainty region over all particle diameter range is newly proposed and compared with computer simulation. Previous paper [H. Masuda, K. Iinoya, Theoretical study of the scatter of experimental data due to particle size distribution, J. Chem. Eng. 4(1) (1971) 60–67] reported the uncertainty region only for mass median diameter, but this paper presents the uncertainty region for all particle size range. The uncertainty region increases with the increase in particle diameter and also increases as the sample size decreases. Theoretical uncertainty region agreed with the results of computer simulation.     

Modified supercritical antisolvent method with enhanced mass transfer to fabricate drug nanoparticles

The main aim of this study was to modify the supercritical antisolvent precipitation method to enhance the mass transfer in order to prepare smaller nanoparticles of drugs. The supercritical antisolvent apparatus was customized by introducing a titanium horn in the precipitation chamber for generation of the ultrasonic field for enhanced mass transfer and the method was called supercritical antisolvent with enhanced mass transfer (SAS-EM). The effects of flow rate, ultrasonic amplitude, drug concentration and flow time on the particle size were investigated. The results showed that increasing the flow rate, incrementing the ultrasonic power up to an optimum point, decreasing the drug concentration and reducing the flow time helped to achieve smaller quercetin particles in the range of 120–450 nm. It is also shown that there is a tradeoff between the particle size and the yield; therefore the process parameters can be selected based on the particle size requirement. DSC studies suggested that the crystallinity of SAS-EM prepared quercetin nanoparticles decreased as compared to original quercetin powder. The dissolution of SAS-EM prepared nanoparticles increased significantly in comparison with the original quercetin powder. However, there was no significant difference in the dissolution of various quercetin nanoparticles samples prepared by the SAS-EM process. The best dissolution percent achieved was 75% for the smallest size sample prepared at the flow rate of 5 ml/min, power supply of 200 W, drug concentration of 10 mg/ml, and flow time of 4 min.     

Low temperature synthesis of nanocrystalline calcium aluminate compounds with surfactant-assisted precipitation method

Nanocrystalline calcium aluminates with different CaO:Al₂O₃ and surfactant/metal ion molar ratios were prepared by wet chemical synthesis method using Poly (ethylene glycol)-block-poly(propylene glycol)-block poly(ethylene glycol) (PEG–PPG–PEG, MW:5800) as surfactant. X-ray diffraction (XRD) and N₂ adsorption–desorption results showed that the increase in CaO:Al₂O₃ ratio decreased the specific surface area and increased the particle sizes of prepared samples while the surfactant/metal ion molar ratios were kept constant. These analyses also declared that for the sample with CaO:Al₂O₃ = 1:2 (CA2) addition of polymeric surfactant increased the specific surface area and decreased the crystallite size. Scanning electron microscopy (SEM) results confirmed that size of particles for CaO:Al₂O₃ = 1:6 (CA6) sample are smaller than CA2. Transmission electron microscopy (TEM) revealed no particular particle shape for the CA2 sample but it showed the high degree of crystallinity and single phase for the prepared sample at 1100 °C.     

Measurement of particle size distribution of silica nanoparticles by interactive force apparatus under an electric field

This paper describes the measurement of particle size distribution of silica nanoparticles by interactive force apparatus (IFA) under an electric field in order to suggest the application of the apparatus to the measurement of particle size distribution. The results were compared with results obtained from size measurement by dynamic light scattering. D₅₀ measured by IFA was closer to the average particle size determined by TEM (5 nm). Also, when compared the results under three different supply voltage, (1) the results at 0.01 and 0.02 V were almost identical while (2) these results were different from the one at 0.04 V. The results indicate that breakage of coagulated particles possibly occur due to electric breakdown. The distribution measured by IFA (D₅₀ = 5–7 nm) was larger than the one measured by DLS (D₅₀ = 1 nm). The electric breakdown was explained by curve fitting of three different particle size distribution functions with particle size distribution obtained from IFA measurement.     

Controllable synthesis of nickel dendritic crystals induced by magnetic field

We demonstrated in this paper a simple and easy method for the preparation of dendritic nickel crystals in an external magnetic field in boiling ethylene glycol (EG) solution. The structural features and morphology of the sample were investigated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The values of saturation magnetization (M_s) and coercivity (H_c) of the dendritic crystals characterized by using a vibrating sample magnetometer (VSM) are 170.3 emu g^?1 and 50.7 Oe, respectively. It was clear that the external magnetic field was the most important factor for controlling the morphology of the product.     

Convection induced by vibrating rod in fine-powder bed

In this study, vibration-induced convection was studied experimentally using a fine powder with a mass median particle diameter of 8 μm. A cylindrical rod arranged vertically in a powder container was vibrated horizontally with simple harmonic motion at a frequency of 300 Hz using a piezoelectric vibrator. For a vibration amplitude of 10 μm, particles around the cylindrical rod were consolidated to a certain extent due to gravity; however, for a vibration amplitude of 70 μm or more, a pair of convection rolls formed on both sides of the vibrating rod. The strength of the convection was quantified from the particle velocity distribution in the convection rolls, and the relationship between the convection strength and vibration amplitude was elucidated. In addition, the air-pressure distribution in the powder bed was measured showing that the convection strength correlates with the characteristic positive pressure, i.e., the average value of positive pressure measurements. Elliptical motion and circular motion as well as simple harmonic motion were applied to the cylindrical rod by adding two harmonic motions in directions orthogonally crossing each other with a phase difference of π/2 rad. The convection of the particles varied according to the Lissajous trajectory of the cylindrical rod. Even for simple harmonic motion, heaps of a pair of convection cells overlapped each other. In the case of elliptical motion, the overlapping portion of the heaps became larger. In the case of circular motion, the two heaps were integrated into one circular heap, and there were no effects of the circumferential angle on the particle velocity and the characteristic positive pressure.     

Induction-activated self-propagating,high-temperature synthesis of nickel aluminide

The self-propagating high-temperature synthesis (SHS) of nickel aluminide from elemental powders using an induction-activated method was investigated. The method could provide the high heating rates required for the surface layers of a compressed specimen to reach the ignition point, and develop a stable, exothermic reaction front traveling throughout the sample. The temperature history of the samples was recorded during the process to evaluate the combustion temperature; first light criterion was used to estimate the time-to-ignition. Temperatures higher than 2000 K were reached during the process, which shows that the released heat is sufficient for melting the NiAl product (T_m = 1911 K). As a result, high-density NiAl products were obtained. The effect of such parameters as sample green density, nickel particle size, and sample aspect ratio on the combustion temperature and ignition time was studied. XRD as well as SEM analyses showed that regardless of samples’ initial conditions, the main phase detected in the products was NiAl.     

Probing fluid flow using the force measurement capability of optical trapping

Interest in microfluidics is rapidly expanding and the use of microchips as miniature chemical reactors is increasingly common. Microfluidic channels are now complex and combine several functions on a single chip. Fluid flow details are important but relatively few experimental methods are available to probe the flow in confined geometry. We use optical trapping of a small dielectric particle to probe the fluid flow. A highly focused laser beam attracts particles suspended in a liquid to its focal point. A particle can be trapped and then repositioned. From the displacement of the trapped particle away from its equilibrium position one estimates the external force acting on the particle. The stiffness (spring constant) of the optical trap is low thus making it a sensitive force measuring device. Rather than using the optical trap to position and release a particle for independent velocimetry measurement, we map the fluid flow by measuring the hydrodynamic force acting on a trapped particle. The flow rate of a dilute aqueous electrolyte flowing through a plastic microchannel (W × H × L = 5 mm × 0.4 mm × 50 mm) was mapped using a small silica particle (1 μm diameter). The fluid velocity profile obtained experimentally is in very good agreement with the theoretical prediction. Our flow mapping approach is time efficient, reliable and can be used in low-opacity suspensions flowing in microchannels of various geometries.     

Eulerian–Lagrangian method for three-dimensional simulation of fluidized bed coal gasification

A comprehensive three-dimensional numerical model has been developed to simulate the coal gasification in a fluidized bed gasifier. The methodology is based on the multiphase particle-in-cell (MP-PIC) model, which uses an Eulerian method for fluid phase and a discrete particle method for particle phase. Dense particulate flow, mass and heat transfer, homogeneous and heterogeneous chemistry between phases and within the fluid mixture are considered. The dynamics of the particle phase is calculated by solving a transport equation for the particle distribution function (PDF) f. Particle collisions and chemical reactions are solved on a grid cell with particle properties mapped from discrete particles to the grid. Solid mass consumed or produced in reactions changes the size of particles. Simulations were carried out in a coal gasifier with a height of 2.0 m and a diameter of 0.22 m at atmosphere. The calculated product gas compositions compare well with the experimental data. The formation of flow patterns, profiles of particle species and gas compositions, distributions of reaction rates and consumption of carbon mass were investigated under different operating conditions.     

Analysis of triboelectric charging of particles due to aerodynamic dispersion by a pulse of pressurised air jet

Triboelectric charging of powders causes nuisance and electrostatic discharge hazards. It is highly desirable to develop a simple method for assessing the triboelectric charging tendency of powders using a very small quantity. We explore the use of aerodynamic dispersion by a pulse of pressurised air using the disperser of Morphologi G3 as a novel application. In this device particles are dispersed by injection of a pulse of pressurised air, the dispersed particles are then analysed for size and shape analysis. The high transient air velocity inside the disperser causes collisions of sample particles with the walls, resulting in dispersion, but at the same time it could cause triboelectric charging of the particles. In this study, we analyse this process by evaluating the influence of the transient turbulent pulsed-air flow on particle impact on the walls and the resulting charge transfer. Computational Fluid Dynamics is used to calculate particle trajectory and impact velocity as a function of the inlet air pressure and particle size. Particle tracking is done using the Lagrangian approach and transient conditions. The charge transfer to particles is predicted as a function of impact velocity and number of collisions based on a charge transfer model established previously for several model particle materials. Particles experience around ten collisions at different velocities as they are dispersed and thereby acquire charges, the value of which approaches the equilibrium charge level. The number of collisions is found to be rather insensitive to particle size and pressure pulse, except for fine particles, smaller than about 30 µm. As the particle size is increased, the impact velocity decreases, but the average charge transfer per particle increases, both very rapidly. Aerodynamic dispersion by a gas pressure pulse provides an easy and quick assessment of triboelectric charging tendency of powders.