Laser induced plasma spectroscopy is beginning to find useful applications in the real time in situ detection of particulates and aerosols suspended in gases. Although the technique is almost 40 years old, it has become increasingly practical for this application during the last decade due to the development of more reliable lasers and optical detection systems, an improved understanding of the physical processes involved, and new data collection and analysis strategies. There is a pressing need for such detection techniques for industrial process and atmospheric monitoring. In this review, we examine literature concerned with the process of optical breakdown by focused lasers in gases, the role of particles in this process, and the characterization of particles using laser induced plasmas.
Powder electrostatics remains an integral and challenging aspect of powder processing. During powder handling operations, such as particle size reduction, mixing and powder transfer processes, particles invariably develop electrostatic charge due to particle-particle and particle-surface interactions. Triboelectrification of powders is a complex phenomenon as most powders are organic crystals and behave as insulators under ambient conditions. However, it is generally accepted that charging occurs as a result of electron transfer between materials of different electrical properties. Factors influencing charging properties include particle size and shape, nature and work function of the contacting surface and the particulate material, area and frequency of contact, surface purity, and atmospheric conditions. Consequences of charge generation upon particle dynamics and powder behavior are often unpredictable. The standard method for measurement of electrostatic charge is the Faraday pail or well, with various application-specific modifications. Electrostatics, regarded by many as a nuisance and hazard, plays an important and ever emerging role in many industrial applications, including powder coating, xerography, and pharmaceutical processing.
The exigency for an inhalation system that does not contribute to ozone-depletion, but is suitable for the delivery of both small molecules and macromolecules, has led to the booming interest in powder aerosols delivery in the last decade. Successful delivery of drugs via powder aerosol depends on both the powder formulation and aerosol inhaler. This review focuses on the emerging technologies of these two determinants in optimising powder aerosol delivery.
The unique characteristics of supercritical fluids can be utilised to overcome the drawbacks associated with the conventional techniques for the generation of pharmaceutical powders. These characteristics include high diffusivity compared with those of liquids, relatively high solvent density and solvent power intermediate between those of gases and liquids. Such characteristics make it possible to use supercritical fluids either as solvents or antisolvents. The application of supercritical fluid technology to various pharmaceutical processes such as micronisation of a wide spectrum of pharmaceutical compounds, encapsulation and polymer impregnation, coprecipitation of pharmaceutical compounds and liposome formation, has been explored. The present review critically discusses the novel supercritical fluid techniques that are currently available and the applications of such techniques to pharmaceutical powder systems.
Phase-Doppler anemometry (PDA) is a well-established optical measuring technique for the nonintrusive and simultaneous determination of size and velocity of spherical particles in various engineering processes. The design of a phase-Doppler system includes simulations based on the Mie theory, which describe the scattering of a plane wave by a spherical particle. Although the greater number of particles in technical processes or in the natural environment are non-spherical, the real shape is neglected and the particles are assumed to be perfectly spherical, with smooth surfaces, and to consist of homogeneous materials. In practice, it was found that the size-response of a phase-Doppler system deviates from the theoretical prediction when these assumptions are not fulfilled. In this paper, optical arrangements and adaptations of theories of light scattering will be discussed which utilize these deviations to provide even more information on the nature of the particles being examined.
A parameter, defined by Gabaude et al. in a previous work 1), was further investigated for its ability to evaluate flow properties of particulate materials. It is based on the volume reduction of the powder bed under low compaction pressure. Compaction data and the packing coefficient (Ct=[(H0-Hp)/H0].100) were analysed over a [0-1] MPa pressure range. Comparison of the Ct value with the flowability index (i) determined by Jenike shear cell measurements demonstrated the reliability of this packing coefficient to assess powder flow properties using small quantities of powder (<1g). This method appeared to be helpful for an adequate determination of flowability in the early stages of drug development. Finally, because the packing assessment is performed in a compression die, improvement in the prediction of the weight variation of tablets is expected, as compared with other methods of flowability assessment.
The present work reports new progress in the development of a multiple light scattering technique for investigating calcium carbonate precipitation and grinding processes. The optical analyser Turbiscan was used for the on-line analysis of light transmission and backscattering from a concentrated calcium carbonate dispersion. From a multiple scattering model based on the photon diffusion approximation, the size and mean refractive index of crystals or nuclei agglomerates are derived from measurements of the backscattered light flux. The time evolution of the particle mean diameter was determined with the on-line Turbiscan during the grinding process. In the precipitation process, the nucleation, growth and agglomeration of fine particles were investigated using this instrument. The crystal or nuclei agglomerate sizes derived from light flux measurements are compared with direct size determination by means of scanning electron microscopy. The multiple scattering technique was found to be highly sensitive to operating conditions (feed flow rate, calcium carbonate volume fraction, reactant concentration and stirring rate), and proved to be a powerful way of controlling precipitation or grinding processes and of optimising operating time.
The influence of carbon dioxide on ring and ball formation in a pilot-scale rotary kiln has been investigated. The balls were found to have a higher specific surface area and a higher content of residual carbonate compared with the rings and dusts: this was probably due to a lower heat transfer rate. The influence of carbon dioxide on sintering proved to be different for the two lime muds investigated. One of them reached a minimum in specific surface area at about 10 weight-% and the other decreased its specific surface area with increasing residual carbonate. The strength of the balls increased with increasing residual carbonate. At constant temperature and partial pressures of carbon dioxide, the strength of the balls increases with time and the specific surface area decreases. Also, as the partial pressure of carbon dioxide increases, the reduction of the specific surface area becomes slower and the increase in strength becomes more rapid.
This work establishes the link between direct measurement of powder flow in a horizontal mixer and shaft torque. Three agitator designs were used: one featured six long flat blades, one a long flat blade and the third, four series of short paddles. With the single long flat blade agitator, the amount of powder moved by the blade was related to the torque. The relationship between torque and nondimensional mean tangential velocity was demonstrated using phase diagrams. These remained unchanged with increase of agitator speed. The fluctuations of the torque with the long flat blade agitator were logically related to those for the single blade. The torque on the short-paddle agitator exhibited larger fluctuations which were strongly affected by the level of fill and the agitator speed. Powder cohesion had a significant influence on the mean and standard deviation of the instantaneous torque. Instantaneous torque measurement may be seen as a valuable method for process control.
Many studies have been made on the theoretical and practical aspects of the several adhesion forces which determine the properties of bulk solids of fine and finest particles. The aim of this work is the defined modification, especially minimisation, of the adhesion forces measured at the model adhesion system sphere-plate by the centrifuge method and AFM. The sphere diameter, surface roughness and chemical surface properties were varied in a controlled manner. In the observed diameter region (several nanometres to several tens of micrometres) and ambient conditions (25 to 80% relative humidity), the adhesion can be described as a combination of van der Waals forces and capillary forces according to well-known theories 1, 2). Because of the combination of the force spectroscopy with the imaging of surface topography in atomic force microscopy (AFM), it is possible to interpret measured forces as a function of the locally resolved curvature of the surface, which can be influenced by cleaning or etching procedures and surface modification or deposition, respectively.
A real-time cumulative mass deposition tester (MDT) was fabricated and tested to study the deposition profiles of powders in complex-shaped dies. The deposition behavior of two different powders (manganese zinc ferrite – MZF1 and microcrystalline cellulose – MCC2) was studied in an "E" shaped die. The feed shoe deposition method was used to deposit the powders in the die via two different fill directions. The MDT was used to develop real-time fill trends, deposition profiles, contour plots and databases of spatial powder mass distribution in the die. From the results, it was ascertained that both fill direction and die shape affect a powder's deposition behavior. It was observed that the average MCC powder masses deposited in both fill directions were roughly one-seventh the average MZF powder mass. Also, the average voids volume fraction of MCC was roughly double the average voids volume fraction of MZF. Due to its favorable particle characteristics (spherical-shaped particles and low cohesion), the MZF powder filled better compared to MCC. In the case of MZF, the coefficient of variation (COV) values for powder mass were mostly in the range of 5-20%, compared to 25-50% for MCC. Therefore, MCC is expected to be a more difficult powder to process due to its poor flowability and large variations in powder masses during deposition. Overall, the experimental results clearly show that the MDT is an effective and reliable tool that can be used for the real-time measurement of powder deposition profiles in a die.
The combustion process is an important powder preparation process by which several hundred compounds may be prepared. The process is, in the simplest sense, the exploitation of an exothermic and usually very rapid chemical reaction to produce a useful material. It is a versatile process to synthesize any kind of single phase, multiphase or composite on a nanophase-scale. This review presents the thermochemical concepts and aspects of the mechanism that occurs under conditions of the combustion and examples of powders synthesized by this process.
Software has been developed and extended to allow finite element (FE) modeling of ceramic powder compaction using a cap-plasticity constitutive model. The underlying, general-purpose FE software can be used to model even the most complex three-dimensional (3D) geometries envisioned. Additionally, specialized software has been developed within this framework to address a general subclass of axisymmetric compacts that are common in industry. The expertise required to build the input deck, run the FE code, and post-process the results for this subclass of compacts is embedded within the specialized software. The user simply responds to a series of prompts, evaluates the quality of the FE mesh that is generated, and analyzes the graphical results that are produced. The specialized software allows users with little or no FE expertise to benefit from the tremendous power and insight that FE analysis can bring to the design cycle. The more general underlying software provides complete flexibility to model more complicated geometries and processes of interest to ceramic component manufacturers but requires significantly more user interaction and expertise.
The maximum explosion pressure of a deflagration is often regarded as a thermodynamic quantity and it is believed to be insensitive to the flow conditions of the combustible mixture involved. While this is true for premixed gases, the present work demonstrates that the opposite is the case when powder-air mixtures are ignited to deflagration. In order to illustrate this disparate behavior, experiments were conducted with methane-air and cornstarch-air mixtures of a fixed fuel to air ratio, at varying turbulence levels. The maximum explosion pressure of cornstarch-air mixtures was observed to increase by a factor of 1.5 when the turbulence level of the dust cloud was varied. An explanation for this behavior is proposed by considering the effect of turbulence on the liberation of volatile matter from the particles.
The authors propose a new system for accurate wet-type centrifugal classification that employs an almost rigidly rotating through-flow. This flow can be produced within a rapidly rotating double-walled container, without producing any turbulent fluctuation. We have conducted both batch and continuous classification using a double-walled cylinder-sphere container in classification experiments and in theoretical and numerical flow analyses. The batch type exhibited decreasing cut size and increasing classification accuracy when the Ekman number decreased (i.e., rotation rate increased) while the Rossby number remained constant, or when the Rossby number decreased (i.e., through-flow rate decreased) and the Ekman number remained constant. With the continuous type, achieved by extracting the coarse product continuously, the cut size decreased as the extraction rate increased, but the sharpness index was almost as high as that for the batch type when the extraction rate was below the threshold value, which was reduced when the Ekman and Rossby numbers decreased.
†This report was originally printed in J. Soc. Powder Technology, Japan. 36(12), 891-896 (1999) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
The present work describes the behavior of fine particles in a laterally vibrated box under microgravity and shows the effects of vibrating conditions and particle concentration on particle motion characteristics. During the 10 seconds of microgravity provided by a drop capsule, particles 51 μm in mass median diameter rose along walls vibrating at 75 to 150 Hz and dispersed into the box's central region. Propagation of momentum from the vibrating walls to the particles was controlled by frequency and particle concentration in the dispersed phase. With a small charge, static clusters were formed due to the restricted propagation of momentum. But with a high particle concentration or at a high frequency, the enhanced propagation resulted in the formation of dynamic clusters, which were transitional assemblies of moving particles. It was found that there was a saturated concentration of dispersed particles in the fully developed vibrating state. High particle concentration caused supersaturation and the formation of new beds from dispersed particles.
†This report was originally printed in J. Soc. Powder Technology, Japan. 36(10), 742-749 (1999) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
We experimentally and numerically examine the flow characteristics of powder jet and entrained air. A phase-Doppler anemometer is used for the measurement of axial velocity profiles of particle and entrained air. It is found that the axial velocity profile of the entrained air takes a maximum at the center-line and decreases toward outer edge. A flow region of the entrained air extends transversely into a particle free region. The center-line velocity of entrained air at first increases with increasing distance from the orifice outlet and then decreases after it remains a plateau. The transverse dispersion of the entrained air is very small as compared with that of a single-phase turbulent jet. The numerical simulation is performed based on the Lagrangian modeling for particles and Eulerian modeling for air flow. We consider particle-particle collision, gravity force, drag force and transverse force due to the particle spin and to the velocity gradient of airflow, and apply a k-ε model. The present simulation qualitatively explains our measurement in terms of velocity profiles and dispersion ofparticle and air.
†This report was originally printed in J. Soc. Powder Technology, Japan. 37(3), 160-167 (2000) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
We investigated an effect of fly ash particle size on the synthesis of zeolites from coal fly ash, proposed and formulated the formation mechanism of zeolite. It is found that the treatment time for the zeolites to synthesize decreases with a decrease in feed fly ash particle size. When the feed size is small, both phillipsite and hydroxysodalite are synthesized by hydrothermal treatment with NaOH solution. Only phillipsite is synthesized when large fly ash particles are used. The zeolite from the fly ash having 2.1 μm mass median diameter indicates the maximum NH4+-adsorption capacity. The change in liquid ion concentrations calculated by the newly proposed model agrees with the experimental results. It is assumed that dissoluble silica is contained by about twice the mass of dissoluble alumina in the large fly ash particles, while in the case of small ones, they are almost equal.
†This report was originally printed in Kagaku Kogaku Ronbunshu, 25(6), 987-992 (1999) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
An optical sensor and adhesive fine powders were used to quantitatively evaluate the mixing performance of a tumbling fluidized bed granulator with an opposed pulsed jet (PJ) assembly. The effects of operating conditions were investigated in terms of the ratio of light intensity reflected from the powder mixture to that in the ultimately dispersed state, ηI. Mixing performance was then compared with that of other mixer types in terms of mixing degree ηI and processing energy consumption. The granulator's mixing performance was equal to or better than that of a high-speed stirring mixer.
†This report was originally printed in J. Soc. Powder Technology, Japan. 36(10), 756-767 (1999) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Focusing on the manufacturing process of the electrode plate of lithium ion secondary batteries, this research aims to establish an evaluation method for the dispersal of positive electrode materials and to identify the kneading and dispersing methods required to form high quality film (plate). Our research revealed that we can make quantitative evaluation of the dispersion of anode particles by measuring the film's volume resistivity rate, glossiness, film thickness and density and by organically examining these measured values. We were also able to suggest that the dispersion of the anode particle in paste form can be evaluated by measuring the particle size distribution and electric conductivity of the paste. We found that if thickener is added in stages, we can produce a plate of thin film thickness and good volume resistivity rate with high dispersion of anode particles.
†This report was originally printed in J. of Japan Soc. of Powder and Powder Metallurgy, 47(1), 97-102 (2000) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Japan Society of Powder and Powder Metallurgy.
Stress analysis on the compaction of ceramic powder was performed by the finite element method (FEM) to predict the optimum conditions. For the purpose of practical application, the powder bed was treated as an elastoplastic material, and the constitutive equation was derived from DruckerPrager's yield function expressed in terms of invariants of stress tensors and Hill's anisotropic parameters on stress in the powder bed. The powder bed has a multiform bulk density distribution, along with discontinuous deformation behavior during compression, which change the mechanical characteristics of the powder bed during compaction. It is therefore necessary to treat powder characteristics as variation associated with the progress of powder compaction. In this paper, Young's modulus and the strain-hardening rate are expressed as functions of minor principal stress and strain. These functions can be determined by a triaxial compression test. Hill's anisotropy parameters, which are induced during the compaction process, were numerically obtained by simulating the compacting behavior of particles using the particle element method (PEM). The problem of powder compaction was analyzed on the basis of the proposed constitutive model. The calculated results of the nonlinear stress-strain relation and stress distribution during powder compaction agreed well with the measured values. Results showed that the proposed procedure offers information that is useful in deciding the optimum conditions for powder compaction.
† This report was originally printed in Kagaku Kogaku Ronbunshu, 26(1), 23-30 (2000) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.