Showing papers in "Particulate Science and Technology in 1987"

Delivery of substances into cells and tissues using a particle bombardment process

Abstract: Biologists commonly wish to introduce a wide range of substances into living cells. Such substances include biological stains, proteins (antibodies or enzymes), and genetic material (either RNA or DNA). The cell membrane and, in the case of plant ceils, the cell wall pose formidable barriers which exclude many macromolecules. The concept of particle bombardment has been put forward as a universal mechanism for transporting substances into any living cell. An acceleration device has been designed and constructed which can accelerate small tungsten particles (1 to 4 urn in diameter) to velocities of about 1,000 to 2,000 ft/sec. We have found that these particles can penetrate cell walls and membranes and enter cells in a nonlethal manner. Thousands of cells can be penetrated simultaneously, in situ, as they occur in tissues. Particle bombardment has been shown to be effective in delivering foreign substances into a variety of plant species, including onion, tobacco, corn, and rice. This new method ...

Collection mechanisms of electret filter

Abstract: Collection efficiency of a single electret fiber, which carries permanent positive and negative charges, was studied. Theoretically, it was obtained by solving the equation of particle motion taking account of the induced and Coulombic forces simultaneously. When either induced or Coulombic forces dominates particle collection, the collection efficiency was found to be proportional to 2/5 power of induced force parameter KIn, and 3/4 power of Coulombic force parameter KC, respectively. However, when both forces are effective simultaneously, the efficiency was not expressed by the simple superposition of both effects because of negative interaction between both forces. Experimentally, collection efficiency of a single electret fiber was measured by using monodisperse sodium chloride particles ranging from 0.01 to 0.4 μm in diameter for filtration velocities from 5 to 200 cm/s, under different charging state of particles, i.e., uncharged, singly or doubly charged and charged in Boltzmann equilibriu...

Suspension of solid particles in agitated vessels

Abstract: Complete suspension of fine-grained particles is achieved at mean circulation velocities of the fluid exceeding the settling velocities of the particles by orders of magnitude. It is therefore argued that boundary layer effects are significant. In this paper through an analysis of the boundary layer flow in an agitated vessel a theoretical criterion for required minimum stirrer angular velocities is derived. The evaluation of own experiments proves its significance for Archimedes numbers Ar 40). Minimum stirrer rotation speeds can be predicted by the evaluation of two diagrams f...

Bubble characteristics in three phase fluidized beds of floating bubble breaker

Abstract: The local bubble phase holdup and vertical bubble length in three phase fluidized beds and the beds of floating bubble breaker have been studied in a 15.2 cm-ID pyrex glass column The effects of liquid velocity (1–9 cm/s), gas velocity (2–12 cm/s), particle size (1–6 mm) and the volume ratio of the floating bubble breaker to solid particles (0–30 %) on bubble properties at the different bed heights have been determined The bubble phase holdup increased with gas velocity, volume ratio of floating bubble breaker to solids and particle size along the bed height but decreased with liquid velocity. The vertical bubble length increased with gas velocity along the bed height but decreased with liquid velocity, particle size and the volume ratio of floating bubble breaker to solid particles The local bubble phase holdup and the vertical bubble length have been correlated with the experimental variables as well as dimension less groups of Froude and Weber numbers.

Particles as drug delivery systems

Abstract: Advances in genetically engineered products present an entirely new spectrum of drugs to the medical community. With these exciting new drugs come complex challenges for their delivery. The compounds are often high molecular weight proteins, many with extremely short in vivo half lives. The challenge is to deliver these new drugs to the body at a controlled rate. The rate can be uniform or programmed to a specific regimen, with time frames from hours to months. An additional challenge is posed by drugs that are highly toxic. If the drug were to home in directly on the site of action, systemic damage could be minimized. Drug loaded particles, (emulsions, liposomes or microcapsules), are being developed for both controlled and targeted drug delivery.

STOKES FLOW THROUGH A SYSTEM OF PARALLEL INFINITE CYLINDERS WITH AXES ORIENTED AT AN ANGLE TO THE DIRECTION Of MEAN FLOW

Abstract: Theoretical models based on Stokes flow of air through a fibrous filter predict a significantly higher pressure drop than experimentally measured values. This discrepancy persists even when the interaction of the flow between) neighboring fibers is accounted for. Various authors have attributed this discrepancy to the inhomogeneity of the fiber distribution within the filter and to the possibility that some fibers are partially orientation in the directon of mean flow. It has been shown that fiber density inhomogeneity does indeed contribute to this discrepancy In this paper, the effect on the flow and subsequent pressure drop when the fibers are oriented at an angle to the directon of mean flow is studied. The solution of the three dimensional equation for creeping, incompressible flow in a doubly periodic, infinite lattice of infinite circular cylinders when there is a constant mean flow whose direction makes an acute angle with the axes of the cylinders is given. If the volume fraction of fibe...

Particle velocity measurements through electrostatic field fluctuations using external probes

Abstract: Two new probes for measuring particle velocity and flowrate based on the elctrostatic field nature of a pneumatic transport system are Introduced. The first probe is based on the current of charged particles inducing a current in a coil placed perpendicular to the flow. The magnitude of the current induced appears to be related to the amount of charged particles passing through the coil and the velocity of the particles. The second probe utilizes fluctuations in the electrostatic field which are picked up by two probes placed a fixed distance along the axis of the flow. The signals from these two probes are identical in waveform but of set in time; therefore, the particle velocity can be obtained by cross-correlation techniques. Both probes are non-intrusive and realtively inexpensive.

Measurement and control of electrostatic charges on pulverized coal in a pneumatic pipeline

Abstract: Pulverized coal particles flowing in a pneumatic pipeline are naturally charged to a detectable level due to collisions with pipe walls. Systematic charge measurements of Anthracite (AN) coals, Medium Volatile Bituminous (MVB) coals and Lignite A (LIGA) coals in a grounded copper pipe have been made with an upgraded charge measuring system. The net particle charges were found to be positive, although both negative and positive charges were detected in all experiments. Effects of air humidity and conveying velocity on particle charges were examined. The mean particle charge was found in the order of 10−12 Coulomb and the charge-to-mass ratio in the order of 10−5 C/kg. Charge elimination techniques by strict humidity control and by introducing neutralizing charge carriers, such as minus 1 μm activated charcoal fines, ammonia, and piezoelectric ionized gas were explored. Effective reduction of 70 % to 85 % of particle charges was achieved. A charge neutralization mechanism was proposed to interpreta...

Absorption of infrared radiation inside an explosively boiling fine coal-water particle

Abstract: A mechanism of ultrafine coal-water slurry droplet disruption by infrared radiation is studied theoretically. The absorptive properties of the micro-exploding droplet are discussed within the framework of multilayered concentric spheres model of light absorption. The computations provide an estimation of the amount of electromagnetic energy absorbed in the coal core and the three-dimensional patterns of dissipation of this energy inside the coal-water vapor-liquid water layered particles.

Acoustic flow instruments for solid/gas flows

Abstract: Two nonintrusive acoustic flow sensing techniques are reported. One technique, passive in nature, simply measures the bandpassed acoustic noise level produced by particle/particle and particle/wall collisions. The noise levels, given in true root-mean-square voltages or in autocorrelations, show a linear relationship to particle velocity but increase with solid concentration. Therefore, the passive technique requires calibration and a separate measure of solid concentration before it can be used to monitor particle velocity. The second technique is based on the active cross-correlation principle. It measures particle velocity directly by correlating flow-related signatures at two sensing stations. The velocity data obtained by this technique are compared with measurements by a radioactive-particle time-of-flight (TOF) method. A multiplier of 1.53 is required to bring the acoustic data into agreement with the radioactive TOF result. The difference may originate from the difference in flow fields w...

Hold-up volume and mean residence time measurements in the air-sparged hydrocyclone

Abstract: In order to provide more Information regarding flow characteristics in the air-sparged hydrocyclone, experiments were designed to determine the hold-up volumes of water and air inside the unit under typical operating conditions. The parameters of slurry flow, air flow, annular opening, and frother concentration were varied, and the hold-up volumes of water and air and the corresponding residence times were determined by collection of the cyclone's contents. In addition, the residence time distributions were measured using radioactive-tracer and dye-tracer techniques. The results indicate that the air-sparged hydrocyclone operates under choked conditions, at least half-filled with water. Such a situation arises when the annular opening is ten percent of the radius which corresponds to the swirl-layer thickness developed for unrestricted swirl flow in a right-vertical cylinder. The measured mean residence time of water reporting to the underflow is in the range 0.5 to 1.0 seconds.

Modeling of solid flows in a fluidized bed with secondary tangential air injection in the freeboard

Abstract: The Vortexing Fluidized-Bed Combustion (VFBC) technique was recently developed for small- and medium-scale coal-burning boiler applications. Experimental observations showed that the general solid flows in the freeboard of a vortexing fluidized bed consisted of three successive stages: (1) spirally ascending motion before colliding the freeboard wall, (2) bouncing on the wall, and (3) sliding on the wall and exiting the freeboard. This study attempts to model these three stages of solid flows. The dimensionless governing equations for particle motion in the swirling field were presented taking into account the interactions of particle inertia, centrifugal force, viscous fluid drag, and gravity. Numerical solutions of particle velocities and trajectories were pursued, and effects of particle momentum transfer number, Froude number, and particle-wall restitution coefficient were delineated. The experimental validation of solid flows in the swirling freeboard was furnished with an 18 cm ID laborator...

Determination of the fractal dimensions of coal filter cakes and their relationship to cake permeability

Abstract: The permeabilities of various fine coal filter cakes were related to cake microstructure through micrographically measured fractal dimensions. Two other porous systems, consolidated glass beads and CaCO3 crystal filter cake, were also analyzed for comparison. A variety of coal mesh sizes were examined spanning a reasonable range of permeability The fractal dimension was determined using the dilation function of the image analyzer. This fractal dimension was then related to the Kozeny parameter, K″, using experimentally measured porosity and permeability, and micrographically measured specific surface. Finally, with K″ as a unique function of fractal dimension, permeabilities were estimated using the Carman-Kozeny equation. Calculated values were in good agreement with experimentally determined values considering the simplicity and direct nature of the approach.

Experimental study of nonwoven filter performance using second order orthogonal design

Abstract: This paper presents the results and conclusions from an experimental investigation concerning nonwoven precleaner performance in non-stationary filtration Results were obtained using filters with chemically cured polyacrylonitrile fibers having diameters of 3.5-20.5 denier, packing densities ranging from 0.0188 to 0.0612, filter thicknesses between 5.5 and 22.5 mm, and aerosol velocities from 0.24 to 2.36 m/s. The experiment was planned using second order orthogonal design. Since the precleaners were developed for ventilation systems and engines of off-road vehicles, the test dust was modelled from the properties of contaminants found in road tests.

Dynamics of agglomerate size distribution in linear shear flow fields

Abstract: A dispersive mixing model was developed and analyzed for the case when separation of the fragments following rupture of agglomerates primarily determines the dynamics of the mixing process. The model incorporates the effects of van der Waals forces within agglomerates and hydrodynamic interactions between fragments as they are convected apart from each other by the applied shear field. The essential features of the model are illustrated by examining the effect of shear on the breakage of parent agglomerates and their first five generations of fragments for a particular test case. In addition, the dynamics of the agglomerate size distribution produced by linear shear flow fields in gaps of various aspect ratios was analyzed for two types of initial conditions. These results provide insight to the design of the high shear zones in actual mixing equipment.

Interaction between pressure drop of gas and flow of medium in a moving granular bed filter

Abstract: Gradually increasing pressure drop, which ultimately leads to a rapid increase, is a problem encountered in moving granular bed filters when gas velocity is elevated during dust collection from the top gas of a blast furnace. The relationship between dust collection and pressure drop was mathematically simulated, and then a half-size test model moving granular bed filter was used to examine the conditions under which the medium can flow. The increase in gas pressure drop proceeds as a circular chain of interrelated events. When gas velocity is increased and the gas pressure gradient exceeds a certain critical value, the medium flow stalls along the gas discharge side of the bed. Dust accumulates in the stalled medium, raising the pressure gradient even further. The growth in pressure drop is thus accelerated, finally resulting in a rapid and undesirable increase that prevents further operation.

Numerical modelling of multiphase particulate flow and its application to sedimentation

Abstract: A numerical model has been developed to simulate the batch settling of a suspension of a particulate size distribution initially with a uniform concentration in a host fluid. The various particle size fractions are represented as separate phases with distinct concentration and velocities. This permits the larger particles to move to the bottom of the vessel and pack. As the particles settle all but the smallest will not penetrate the lower levels of the vessel. The packing behaviour is represented in detail via the approach of Ouchiyama and Tanaka. The numerical implementation is based upon the control volume techniques elucidated by Patankar, although the algorithm contains a number of original features. The model is compared favourably with the experimental results of Holdich, although further work is required to adequately represent the settling of compressible materials.

Flow patterns through tube bundles and particulate fouling onto cylinders

Abstract: The shear-stress distributions around cylinders in tube bundles were measured by means of the limiting current method and the flow inside the bundle was also visualized. In the case of the staggered bundle, the shear-stress distributions had two peaks by the effect of the jet streams generated at the front row spacing and the adjacent cylinder. While in the bundle with in-line arrangement, the front stagnant point was shifted about 30° downward and the traverse streams among the rows were observed. This traverse streams caused asymmetry of the shear-stress distributions with respect to the main flow axis. These results were discussed in connection with the fouling experiments investigated in dusty air. Particle deposition around cylinders could be qualitatively predicted from the shear-stress distributions, namely particles tend to deposit at positions where are less shear-stress in both tube arrangements. The size of particle deposited on the wake zone was coarser than that near the stagnant poi...

The stability of latex particles in aqueous suspensions

Abstract: Monodisperse polystyrene-DVB latices of 2, 4 and 9 µm diameter as supplied by SINTEF∗ for intended use by BCR∗∗ as particle standards with respect to both particle size and particle number density have been studied by a variety of techniques, and under a variety of storage conditions. The sizing and counting techniques employed light microscopy, electron microscopy (both transmission and scanning) and Coulter counting. Polymeric stabilizer and bacteriocide additives were employed at different levels and their effects on long term (3 years) stability monitored at different particle number densities. Accelerated ageing experiments employed included elevated and reduced temperatures, light exposure, centrifugation and freeze - thaw cycles. The latices were shown to be remarkably stable in the presence of the selected stabilizer/bacteriocide combination.

Particles and photography

Abstract: The technical and commercial developments of photography are outlined. The roles of particles in the black-and-white and color photographic processes are described in terms of the basic technology of photography. The desired properties of particles in photographic systems are also described, several illustrations of photographic particles and of particles in photographic systems are presented. An example of the properties of photographic systems that depend on the particle properties is also presented.

Improvements in the classification efficiency of a hydrocyclone with an impeller installation around the vortex-finder

Abstract: Study of flow mechanisms in hydrocyclones shows that about10 to 15 percent of the particles entering the overflow through a leakage stream are not subject to the full centrifugal force Hence an attempt was made to improve the separation efficinecy through design modifications An impeller was installed through a central hole in the vortex-finder, and a series of experiments were carried out to compare the performance of an impeller-assisted hydrocyclone with that of a normal hydrocyclone It was found that, with the introduction of the impeller, the classification efficiency was improved A computer simulation of a grinding circuit using the impeller-assisted hydrocyclone showed that the capacity of the circuit can be increased by 35 percent

Stability of concentrated colloidal dispersions in apolar media

Abstract: The stability of concentrated colloidal dispersions of submicron carbon black particles in solutions of Aerosol-OT in p-xylene is discussed. Electrostatic and electrodynamic forces compete leading to coagulation of the primary particles resulting in increasing particle radius. Growth continues until electrostatic stabilisation dominates. These stable dispersions are characterised by particle size, electrophoresis and Theological measurements. The effect of ionic strength is studied and discussed in terms of fundamental electric double layer theory. This work is of significance to practical concentrated dispersions in media where the dielectric constant is very low

Manufacture of optical waveguides using aerosols: a review

Abstract: Optical waveguides are glass filaments used to transmit light for telecommunication and other uses. To achieve high performance, inexpensive optical communication systems require: glass of exceptional purity and precise control of glass refractive index. For this reason, aerosol deposition is utilized exclusively for the manufacture of optical waveguides. The applications and operation of optical waveguides are outlined in this paper. The major manufacturing processes are discussed, with emphasis on aerosol formation and deposition.

The effective dielectric constant of dust layers

Abstract: The electrical breakdown of a dust layer usually occurs at average electric fields which appear to be too small to initiate the breakdown. This suggests that the local electric field where breakdown occurs is somehow enhanced over that of the average field across the layer. AC dielectric measurements performed on the dust layer can be used to determine an effective static dielectric constant at a given temperature. The use of this dielectric constant in an expression for the local electric field, which combines the effects of dielectric polarization and space charge in the layer, results in predicted values of the local electric field which can be more than an order of magnitude larger than the applied field. In studies involving 11 different dust species, the dielectric constant was found to depend on temperature and layer thickness, as well as the concentration of alkali and alkaline earth metals in the dust.

Role of particles and dispersions in electrophotography

Abstract: The genesis of a 30 billion dollar a year electrophotographic industry can be traced to the sole effort of Chester Carlson who demonstrated in 1938 that dry images could be produced by a new process involving the attraction of charged pigmented particles to an electrostatic image formed on a photoconducting film. Carlson's invention, a particular form of electrophotography, is known as xerography. Most electrophotographic processes are based on the deposition of charged particles in either an air or liquid medium. The marking particles are usually pigments dispersed in a polymer matrix. The various electrophotographic processes are critically dependent on the electrical and mechanical properties of particles and dispersions. We examine the role of particles and dispersions in electrophotographic processes in general, and xerographic processes in particular.

Continuous cooling of wet foundry sand using a fluidized bed

Abstract: In order to establish the optimum cooling system for hot returned sand in metal casting processes, a mathematical model including simultaneous heat and mass transfer has been constructed. The present study is especially directed towards clarifying the dynamical behavior of sand cooling in a one-pass fluidized bed. A series of continuous cooling experiments has been made for various conditions. Characteristics of both transient and steady-state bed temperature are explained reasonably by the present model. Furthermore, a possible proposal has been given on the optimal operation, which raises heat exchange efficiency in fluidization cooling of hot molding sand.

Oilwell completion fluid solids interaction with an unconsolidated oil reservoir

Abstract: This work addresses the problem of modelling, detection and evaluation of the mechanical interaction between oilwell completion fluid particles and granular reservoir rocks. These interactions contribute to a reservoir productivity reduction known as the “permeability damage” phenomenon. Fundamental concepts of cake filtration and filter medium filtration were used to formulate mathematical models of particles capturing: gradual pore blocking, screening, and straining. The models provide theoretical bases for simple diagnostic plots in which the linearity of a plot becomes an indication of a prevailing mechanism of particles' capture. Laboratory tests were run by pumping a completion fluid through synthetic simulated cores of unconsolidated sandstone gas reservoir samples from the Adriatic Sea. By using diagnostic plots and grade passing efficiency curves, the effects of total solids concentration particle size on the type and efficiency of the capture mechanism were investigated. The study revea...