Particle Apparent Density Research Articles

Quality Evaluation of RDX Crystalline Particles by Confined Quasi‐Static Compression Method

AbstractA confined, quasi‐static uniaxial compression method, named as the compressive stiffness test (CST), is promoted to evaluate the quality of different RDX crystalline particles which are obtained from commercial and reprocessed lots. This method is based on the fact that the fragmentation or fracture behavior in the compression process could be correlated to the external/internal defects of targeted crystalline particles. The quality difference between the commonly used RDX and reprocessed RDX can be easily discriminated by compaction curves and the defined initial secant modulus can be used as a quantitative parameter to grade RDX particles. The results show that the RDX recrystallized from mixed solvents and spheroidized by treatment with solvent is of much higher quality than the commonly used RDX, and the particle size as well as external characteristics exert limited effects on the compaction curves, but the key factor is internal defects, which affect the coherence strength of RDX crystalline particles. Qualitative optical image analysis and quantitative particle apparent density measurements by liquid density gradient tubes also give consistent results with those from the CST.

Aerodynamic Characteristics of Popcorn Ash Particles

Popcorn ash particles are fragments of sintered coal fly ash masses that resemble popcorn in low apparent density. They can travel with the flow in the furnace and settle on key places such as catalyst surfaces. Computational fluid dynamics (CFD) models are often used in the design process to prevent the carryover and settling of these particles on catalysts. Particle size, density, and drag coefficient are the most important aerodynamic parameters needed in CFD modeling of particle flow. The objective of this study was to experimentally determine particle size, shape, apparent density, and drag characteristics for popcorn ash particles from a coal-fired power plant. Particle size and shape were characterized by digital photography in three orthogonal directions and by computer image analysis. Particle apparent density was determined by volume and mass measurements. Particle terminal velocities in three directions were measured in water and each particle was also weighed in air and in water. The experimental data were analyzed and models were developed for equivalent sphere and equivalent ellipsoid with apparent density and drag coefficient distributions. The method developed in this study can be used to characterize the aerodynamic properties of popcorn-like particles.

An atomic force microscopy approach for assessment of particle density applied to single spray-dried carbohydrate particles

To evaluate an atomic force microscopy (AFM) approach for effective density analysis of single spray dried carbohydrate particles in order to investigate the internal structure of the particles. In addition, the AFM method was compared to an established technique, that is gas pycnometry. Resonant frequency AFM analysis was employed for determination of the mass of individual particles of spray-dried lactose, mannitol, and a mixture of sucrose/dextran (4:1). The effective particle density was calculated using the diameter of the spherical particles obtained from light microscopy. The apparent particle density was further analyzed with gas pycnometry. It was observed by microscopy that particles appeared either "solid" or "hollow." A solid appearance applied to an effective particle density close to the true density of the material, whereas a density around 1 g/cm(3) corresponded to a hollow appearance. However, carbohydrates, which crystallized during spray drying, for example, mannitol appeared solid but the average effective particle density was 0.95 g/cm(3), indicating a continuous but porous structure. AFM measurements of effective particle density corroborate the suggestion of differences in particle structure caused by the varying propensity of carbohydrates to crystallize during spray drying, resulting in mainly either amorphous hollow or crystalline porous particles.

In situ coating—An approach for particle modification and encapsulation of proteins during spray-drying

In this paper, we present a method for in situ coating of individual protein particles in a respirable size. The aim of the coating was to influence the particle/powder properties, and to reduce or prevent surface-induced conformational changes of the protein, during spray-drying, which was the method used for simultaneously preparing and coating particles. The investigated formulations included bovine serum albumin (BSA), trehalose and either of the two non-ionic polymers, hydroxypropyl methylcellulose (HPMC) and poly(ethylene oxide)–poly(propylene oxide) triblock co-polymer (Poloxamer 188). Complete protein coating as measured by electron spectroscopy for chemical analysis (ESCA) was achieved at a polymer concentration of approximately 1% of the total solids weight, and could be predicted from the dynamic surface tension at the air/water interface, as measured by the pendant drop method. Further, particle properties such as: size, dissolution time, powder flowability, and apparent particle density, as measured by gas pycnometry, were affected by the type and concentration of the polymer. In addition, the particle surface morphology could possibly be correlated to the surface elasticity of the droplet surface during drying. Moreover, an extensive investigation (Fourier transform infrared spectroscopy, circular dichroism and size exclusion chromatography) of the structural effects of protein encapsulated in a polymeric coating suggested that in situ coating provide particulate formulations with preserved native conformation and with a high stability during rehydration.

Combining size distribution and chemical species measurements into a multivariate receptor model of PM2.5

We introduce an extended receptor model, implemented with the multilinear engine ME2, which combines simultaneous but separate filter‐based species information with size‐resolved particle volume information. Our chemical data set consisted of 24‐hour filter measurements reported by the EPA Speciation Trends Network at Beacon Hill in Seattle, Washington, from February 2000 to June 2003. We measured the particle size distribution at this site from December 2000 to April 2002 using a differential mobility particle sizer (DMPS) and an aerodynamic particle sizer (APS). The combined model extends the traditional chemical mass balance approach by including a simultaneous set of conservation equations for both particle mass and volume, linked by a unique value of apparent particle density for each source. The model distinguished three mobile source features, two consistent with previous identifications of “gasoline” and “diesel” sources, and an additional minor feature enriched in EC, Fe and Mn and ultrafine particle mass that would have been difficult to interpret in the absence of particle size information. This study has also demonstrated the feasibility of defining missing mass as an additional variable, and thereby providing additional useful model constraints and eliminating the posthoc regression step that is traditionally used to rescale the results.

Preparation and properties of soft magnetic particles based on Fe 3O 4 and hollow polystyrene microsphere composite

Magnetorheological (MR) fluid is a novel material, of which the apparent viscosity and other rheological properties of the fluids can be controlled by the intensity of the magnetic field applied on the MR fluids. However, low sedimentation stability, due to the large difference of apparent density between soft magnetic particles and carriers in MR fluids, is one of the impediments to their wider applications. A new method to prepare the lighter magnetizable composite particles, with hollow polystyrene (PS) microsphere as core and Fe 3O 4 as shell, was reported. The morphology, magnetic properties, apparent density, sedimentation stability, and magnetorheological properties of the composite particles were studied. The mean diameter of the hollow soft magnetic composite particles is about 250 nm, while that of the hollow PS microsphere is about 150 nm according to the results of transmission electron microscopy (TEM). The saturation magnetization ( B s) of the soft magnetic composite particles is 0.21 T, and the coercive force ( H c) of the soft magnetic composite particles is 8.03 kA m −1. The apparent density of the hollow magnetizable particles is around 1.5 g ml −1, which is only one-fifth or one-fourth of that of normal soft magnetic particles based on pure Fe or Fe 3O 4 powder. The MR fluids based on such kind of soft magnetic composite particles show promoted sedimentation stability and magnetorheological properties, which are attributed to the novel hollow structures.

Pluronic-g-poly(acrylic acid) copolymers as novel excipients for site specific, sustained release tablets

Potential utility of copolymers comprising Pluronic ® (PEO–PPO–PEO) surfactants covalently conjugated with poly(acrylic acid) (PAA) as excipients for sustained-release tablets was explored. Apparent particle density, particle size distribution, Carr index, thermal stability, and compression behavior of the Pluronic–PAA copolymers were characterized. Tablets prepared by direct compression of blends of Pluronic–PAA copolymers were evaluated on the basis of their thermomechanical profile, crushing strength, friability, and drug release properties. Small molecular weight drugs of aqueous solubility decreasing in the order theophylline > hydrochlorothiazide > nitrofurantoin were incorporated to the tablets. For comparison purposes, tablets were also prepared from PAA of Carbopol ® 71G (C71G), and mixtures of C71G and Pluronic ® F127, with each of the above three drugs. The Pluronic–PAA aggregates are stabilized by hydrophobic associations between poly(propylene oxide) (PPO) segments in aqueous solutions, and thus require higher ionization of the carboxylic groups to overcome the associations and swell. The swelling pattern of the Pluronic–PAA copolymers is more dramatically pH-dependent than that of Carbopol lacking any hydrophobic associations. The drug retention in and release from the Pluronic–PAA based tablets is profoundly pH-dependent and hence specific to the pH exceeding that of the p K a > 5 of these copolymers. Theophylline- and hydrochlorotiazide-containing tablets made with Pluronic–PAA copolymers showed a reduced release rate under acidic conditions compared to the neutral or alkaline conditions, while the opposite pattern was observed with the Carbopol-based tablets due to the different pH-dependent swelling behavior of the polymers. Nitrofurantoin-containing tablets showed a remarkably low drug release rate owing to the strong hydrophobic character of nitrofurantoin and of its complexes with the copolymers. Integrity of the nitrofurantoin-containing tablets was maintained during the 24 h release test. Zero-order kinetics of the cumulative release profile of all drugs under study was observed with the Pluronic–PAA as a tablet excipient. Adequate mechanical properties, the self-assembling behavior, and the pH-sensitiveness of the Pluronic–PAA copolymers make them promising excipients for tablets with preferential delivery into a neutral to alkaline pH environment.

Particle Size and Density in Spray Drying—Effects of Carbohydrate Properties

The purpose of this study was to examine some fundamental aspects of the particle formation during spray drying, related to particle size and density. Particles were prepared in a laboratory spray dryer from carbohydrates with different solubility and crystallization propensity, such as lactose, mannitol, and sucrose/dextran 4:1. The feed concentrations ranged from 1% w/w to saturated and the size of droplets and particles were measured by laser diffraction. Particles were also characterized by various microscopy techniques (i.e., scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), and light microscopy), differential scanning calorimetry (DSC), gas adsorption, and gas pycnometry. As demonstrated larger particles could be obtained by either increasing the droplet size during atomization; increasing the concentration of the feed solution; or decreasing the solubility of the solute. The apparent particle density, measured by gas pycnometry, was found negatively correlated to the feed concentration. Due to the nonlinear relationship between the feed concentration and the particle size, it was concluded that higher solids load would cause an increase in the effective particle density and that the reduction in the apparent particle density was a result of a gradually less permeable particle surface. Further, the crystallization propensity of the carbohydrate influenced the particle formation and resulted in either hollow or porous particles.

Aerosol size standards in the nanometer size range: II. Narrow size distributions of polystyrene 3–11 nm in diameter

A technique for generating charged aerosols of polystyrene (pSty) with narrow size distributions has been developed. It is based on electrospraying commercial narrow mass standards of pSty dissolved in l-methyl-2-pyrrolidone (NMP) seeded with the newly synthesized salt dimethyl ammonium formate. This salt imparts a much larger electrical conductivity than previously known NMP electrolytes, leading to higher quality sprays with greatly reduced attachment of impurities. Controlling the solute concentration enables forming polystyrene particles containing from one up to more than ten single polystyrene molecules, whereby 4 mass standards with molecular weights from 9200 up to 96,000 g/mol yield particles covering densely the diameter range from 3 to 11 nm. Combined mobility and mass measurement with a differential mobility analyzer and a mass spectrometer in tandem are carried out with a pSty sample 9200 amu in molecular weight. They fix directly the mass versus mobility relation near 9200 amu, and indirectly for the other standards and their clusters. The apparent particle density resulting from mobility versus mass data agrees with the bulk density of the polymer, indicating that the particles are dense and spherical. Although these standards have been studied only in gaseous suspension, their injection in liquids such as water where pSty is insoluble should keep them spherical.

Improving the Separation Efficiency of the Knelson Concentrator using Air Injection

ABSTRACT Recent test data have shown that the Knelson concentrator can provide a more efficient separation than froth flotation when cleaning coal in the 150 × 25 µm particle size range. To further improve the performance of this enhanced gravity separator, air bubbles were injected into the feed stream in an attempt to decrease the apparent density of the coal particles due to the formation of bubble-coal particle agglomerates. The test data showed that air injection had a significant impact on operating characteristics and separation performance. Coal recovery was increased by 10 to 20 absolute percentage points, while a 2 percentage point drop in the minimum product ash content was also achieved.

Quantitative characterization of particle internal defects in particle lots

Internal defects in particles affect the physical properties of particulate systems. The accurate and quantitative characterization of particle lot is a key point but few tools are available for particle internal defect measurements. An experimental set-up is presented to record the coupled particle apparent density distribution (PADD) and particle size distribution (PSD) of a particle lot. The data accuracy is discussed in details and is illustrated with various measurements on hexogen lots. Finally, an application to the study of hexogen crystallization is presented. It uses the plot of the volume fraction of inclusions in particles as a function of the particle sizes. These data are computed from the PADD and PSD measurements. They give information about the crystallization process.

Variability of apparent particle density of an urban aerosol.

The day to day and diurnal variation of apparent particle density was studied using highly time-resolved measurements of particle number distribution and fine-particle mass concentration. The study was conducted in Erfurt, Germany, from January 1, 1999, to November 22, 2000. A setup consisting of a differential mobility particle spectrometer and a laser aerosol spectrometer was used for particle number distribution measurements. PM2.5 particle mass was measured in parallel on an hourly basis using a tapered element oscillating microbalance (TEOM) and on daily base by using a Harvard marple impactor (HI). For the estimation of the mean apparent density of particles, number size distributions were converted into volume size distributions, assuming that the particles were spherically shaped. The volume size distributions were integrated over the range of 10 nm to 2.03 microm Stokes equivalent diameter to obtain volume concentrations. Mean apparent particle density was calculated as ratio of mass concentration and volume concentration. The mean apparent particle density, determined from HI and number size distribution on a daily basis was 1.6 +/- 0.5 g cm(-3). We found a strong day-to-day variation of apparent density ranging from 1.0 to 2.5 g cm(-3) (5th and 95th percentile). Furthermore, the apparent density showed pronounced diurnal pattern both in summer and in winter and also on weekdays and weekends. The apparent density was lowest in the morning and highest in the afternoon. The mean apparent density on an hourly basis was 1.4 +/- 0.5 and 1.5 +/- 0.5 g cm(-3) for PM2.5TEOM and corrected PM2.5TEOM using regression equation between daily mass concentration of HI and TEOM, respectively. The strong diurnal variation of apparent particle density was associated predominantly with the vertical temperature inversion and with traffic intensity. Thus, the apparent particle density depends on the physical particle properties and might be related to chemical composition of the sampled particle.

Light scattering properties of marine particles in coastal and open ocean waters asrelated to the particle mass concentration

Variations in the spectral scattering coefficient of marine particles [bp(λ)] were measured at 241 locations in oceanic (case 1) and coastal (case 2) waters around Europe. The scattering coefficient at 555 nm normalized to the dry mass of particles [bmp (555)] was, on average, 1.0 and 0.5 m2 g‐1 in case 1 and case 2 waters, respectively. Spectral variations in bp(λ) were, on average, small in all investigated waters. To understand the observed variations in particle scattering, we performed calculations based on the Mie theory with various values for the slope for the Junge‐type size distribution, refractive index, and apparent density of particles (dry weight/wet volume). The latter two were varied according to the type of particles (organic versus mineral) and, for organic ones, as a function of the water content. The higher bmp(555) values in case 1 waters are mainly due to the low apparent density, which results from the organic nature of particles and their elevated water content. In all investigated coastal regions, except for the Baltic Sea, the low bmp(555) values can be explained by the dominant presence of mineral particles, characterized by a high density that counterbalances the effect of a higher refractive index. In the Baltic Sea, bmp(555) was similar to values found in other coastal waters despite the fact that particles were dominantly organic, which may result from higher absorption relative to scattering. A smaller than expected increase of bp(λ) toward short wavelengths is attributed to significant absorption that increases toward the shorter wavelengths and reduces scattering, whether particles are living, detrital, or mineral. Our analyses suggest that the determination of bmp may be significantly sensitive to the porosity of the filter used to assess the dry mass of particles.

Influence of crystal habit on the compression and densification mechanism of ibuprofen

Ibuprofen was recrystallized from several solvents by two different methods: addition of a non-solvent to a drug solution and cooling of a drug solution. Four samples, characterized by different crystal habit, were selected: sample A, sample E and sample T, recrystallized respectively from acetone, ethanol and THF by addition of water as non-solvent and sample M recrystallized from methanol by temperature decrease. By SEM analysis, sample were characterized with the respect of their crystal habit, mean particle diameter and elongation ratio. Sample A appears stick-shaped, sample E acicular with lamellar characteristics, samples T and M polyhedral. DSC and X-ray diffraction studies permit to exclude a polymorphic modification of ibuprofen during crystallization. For all samples micromeritics properties, densification behaviour and compression ability was analysed. Sample M shows a higher densification tendency, evidenciated by its higher apparent and tapped particle density. The ability to densificate is also pointed out by D 0’ value of Heckel's plot, which indicate the rearrangement of original particles at the initial stage of compression. This fact is related to the crystal habit of sample M, which is characterized by strongly smoothed coins. The increase in powder bed porosity permits a particle–particle interaction of greater extent during the subsequent stage of compression, which allows higher tabletability and compressibility.

Plasma densification of agglomerated Cr2O3 powder for thermal spray coating

Double plasma flame treatments were carried out on spray dried Cr2O3 agglomerated powders to increase their apparent density. The powders that were subjected to the first densification treatment didn't show the entirely melted state, and were fully melted only after the second plasma treatment. Plasma densification resulted in powder size decreasing as well as apparent density of particles and also resulted in the fluidity increasing due to the powder melting and surface smoothing effects. However, some parts of the particles after the second treatment showed a hollowed structure, especially for a particle size above 30 µm. The influence of the thermal conductivity of powder and the gas pressure within aggregates exposed to the plasma flame in the particle densification process was discussed in detail. The powder density strongly influenced the structure of plasma sprayed coatings. The dense coatings with high hardness and high bond strength was achieved in the coatings produced from Cr2O3 powders after plasma densification.

Structural and Percolation Parameters of Porous and Powder Systems

Methods for calculating the structural and percolation parameters of powder systems are proposed. It is shown that estimating the structural properties of porous and powder systems should take into account the size of interparticle contacts, the threshold concentration of the solid phase (conducting component), and the apparent density of particles. Analysis is made of how the porosity affects the statistically representative thickness of a heterogeneous material (at this thickness, the heterogeneity of the material most clearly shows itself: the percolation threshold ceases to rise with increasing material thickness). The calculated results are collated with the observed data.

Surface Treatment and Facilitated Cleaning of Stainless Steel by Ozonized Air.

Surface treatment of stainless steel by ozonized air was investigated over the range of 0.01 to 0.2% (v/v) ozone (O3), using nonporous stainless steel particles. Ozonized air contained nitrogen oxides (NOx) at a NOx/O3 ratio of approximately 0.04. When stainless steel particles were treated with ozonized air, nitric acid was formed on their surfaces depending on the O3 concentration. As a result of ozonized air treatment, the adsorption affinity of water vapor for stainless steel particles decreased and the absolute values of the apparent surface charge density (σapp) of stainless steel particles decreased over the pH range of 3 to 10, resulting in the lower adsorption affinity of bovine serum albumin (BSA). The σapp curves of stainless steel particles treated with ozonized air at 0.15 to 0.2% O3 were very similar to the curves of the particles treated with 10 and 30% HNO3. These results indicated that nitric acid formed during ozonized air treatment brought about the modification of the surface charge of stainless steel particles. Ozonized air pretreatment of stainless steel particles fouled with BSA or calcium hydrogenphosphate markedly facilitated their removal during subsequent alkali cleaning or rinsing through O3 oxidation or descaling with nitric acid.

Characterizing fuels for atmospheric fluidized bed combustion

A complete methodology for characterizing coal combustion in atmospheric fluidized bed reactors is presented. The methodology comprises studies of fragmentation and particle size variations during combustion, necessary to allow an accurate determination of kinetic parameters and attrition rates. Samples of three different carbonaceous materials (a medium-ash lignite, a medium-ash anthracite and a graphite) were pyrolyzed in N 2 and partially burned in air in a bench-scale fluidized bed reactor (5 cm i.d.) at different operating conditions (0.68 mm < d 0 < 3.50 mm, 750°C < T b < 950°C and U = 0.13 m/s). The samples were retrieved from the sand-bed by means of a basket. The particle size distribution, apparent density and number of particles were evaluated by Image Analysis. Thus, the evolution of these parameters with burnoff was determined for different materials. Additionally, the sphericity factors were calculated. Combustion studies were carried out in batch experiments in the laboratory-scale, fluidized bed reactor at the same operating conditions. The reactor outlet concentrations of O 2, CO 2, and CO were monitored continuously. The results indicate that only anthracite particles experienced both primary (due to devolatilization) and secondary (during char combustion) fragmentation. Graphite particles underwent secondary fragmentation, whereas lignite particles did not significantly vary in number during combustion. Size and density variations during combustion suggest that graphite particles burn under regime II, intraparticle diffusion being the rate controlling step. On the other hand, anthracite and lignite particles developed an ash layer, which may control combustion. The attrition constants of the medium-ash materials (lignite and anthracite) were found to be very low (0.3 × 10 −7 and 0.8 × 10 −7, respectively), whereas that of graphite was much higher (17.6 × 10 −7), due mainly to peripheral percolation during combustion.

Accurate Measurement of Density of Colloidal Latex Particles by Sedimentation Field-Flow Fractionation

In this study the density of colloidal latex particles was determined to four figure accuracy by subjecting the colloid to a series of sedimentation field-flow fractionation runs in which the density of the aqueous carrier solution was systematically varied. Accuracy was gained by using high-field strengths which made possible the use of carriers with densities only 0.0005-0.05 g/cm 3 removed from the isopycnic particle density. Both positive and negative density differences were used, yielding independent measurements of particle densities that differed on average by only 0.0003 g/cm 3 . At the high accuracy level achieved, a trend was discerned in which the apparent or isopycnic density of polystyrene latex particles increases systematically with size, gaining ∼0.002 g/cm 3 over the diameter range 0.26-0.46 μm. This trend is likely attributable to interfacial effects, particularly to an interfacial volume increment generated by structural rearrangements relative to the bulk phases at the particle-aqueous interface. A comparison of the data with theory yields, after correcting for Young-Lapace particle compression, a specific interfacial volume (volume increment per unit area of interface) of ∼1.7 A and an ambient bulk polystyrene density of 1.0530 ± 0.0003 g/cm 3 . Suggestions are made for improving the work and extending it to the measurement of small second-order effects associated with adsorption, swelling, and polymer relaxation in the glassy state.

Measurement of the apparent density of porous particles by a powder characteristics tester

A new technique to determine the apparent density of porous particles has been developed. The density of coarse particles of silica gel, active carbon and a molecular sieve was determined using fine dense powders as pycnometer fluids. The apparent density of fine porous particles of adsorbents and cracking catalyst was measured by a comparative method. Both measurements require that the bulk density of particulate solids are accurately determined. Good agreement was found between the results of the new technique and those obtained by other methods. The limitations are also discussed.