The ability to interrogate the dynamic internal characteristics of processing plants by using conventional instrumentation is severely limited for most practical conditions. The results of recent work, employing non-intrusive electrical sensors to obtain 2-dimensional images of component concentration profiles in cross-sections through process equipment, is making industry aware of new prospects for improving the design and control of many processes. The range and status of sensing techniques and other enabling technology for the new subject of "Process Tomography" is outlined in this paper. This is followed by a discussion of the applications and future technical challenges which must be considered if process tomographic techniques are to find widespread application, with a particular emphasis on electrically-based sensor technology.
The action of mechanical energy on solids effects changes to their structure. The resulting structural defects lead to a storage of energy in the solid, which may raise its chemical reactivity. Investigations on the mechanically induced reactivity of solids have been carried out with the specific aim of influencing reaction processes in order to obtain high yields, high reaction velocities or selective reactions. On the basis of detailed structural investigations on mechanically treated quartz and hydrargillite, the interaction of the stressed solid with the surroundings has been explored (titanite formation, zeolite formation, polymorphous transformation, gas adsorption, Na+-elimination and hydration of amorphous Al2O3). The investigations reveal that the size reduction which occurs during mechanical activation is not the decisive reason for reactivity enhancements, but that these are caused primarily by structural changes. Apart from the structural properties of solids, however, their chemical properties as well as those of the surroundings have an influence on their reactivity.
In this paper, a one-layer regular solution model for the interface crystal mother phase is treated from first principles. From this model, the concepts of thermal roughening and kinetic roughening are derived. These concepts correspond to a roughening transition under the influence of the temperature and a driving force for crystallization, respectively. The results of the simple theory are compared with recent advanced theories inspired by computer-simulation studies leading to the theory of roughening transition and kinetic roughening. It is shown how these theories can be integrated with the crystallographic morphological theory of Hartman and Perdok. Examples of mainly organic crystals are reviewed. Observed growth forms are interpreted by means of the theories discussed above.
Vertically flowing gas-solids suspensions have been investigated both theoretically and experimentally. The theoretical approach suggested the possibility of self-similar flow patterns for the particles at low solids concentration. The experiments carried out on two different installations with four different solids confirmed the existence of a so-called similar profiles regime bounded at dilute-phase flow conditions. At a constant gas velocity, a sudden transition occurs for a critical solids concentration. Beyond this limit particle concentration increases faster with increasing solids rate. These similarity properties have been eventually extended to the temperature profiles, which allowed the derivation of a general wall-to-suspension heat transfer equation containing two shape factors summarizing the flow hydrodynamics. This equation proved to be in excellent agreement with experimental results. Moreover, beyond the flow regime transition, i.e. in dense-phase flow, the same equation applies with modified shape factors.
It is well known that the characteristics of friction and wear of ceramics are extremely sensitive to the environment. It is possible to divide the mechanism of the environmental effects into three major steps: The formation of the adsorbed layers, the change of the surface properties and the formation of tribochemical reaction products. Under actual sliding conditions, the influence of tribochemical reaction plays an effective role. An attempt have been made to use this effect to activate an excellent lubrication. This work deals with the relationships between the environment and the tribological characteristics of ceramics.
Effects of various mechanical pre-treatments and grinding aids on the pore structure of cast green compacts with and without post-compression, and lightly sintered body of alpha alumina were studied. Alumina powders were mechanically activated by a vibration mill, using grinding balls made of polyamide-coated steel (PS) and sintered Si3N4 (SN). Milled products were redispersed by subsequent wet grinding with and without an additive, dextran. PS balls brought about compacts and sintered bodies denser than those treated by SN balls. Wet-grinding and redispersion by a centrifugal ball mill with dextran resulted in by far a better dispersion and hence denser compacts. Dextran lead to the smaller average pore, but broader pore size distribution. The preferential importance of state of dispersion in the green compacts over the mechanical activation was revealed in the present system.
X-ray computed tomography (CT) is an ideal technique for investigating the internal structure of multiphase materials in a noninvasive and nondestructive manner. CT technology used in conjunction with specialized algorithms and advanced computer facilities can be used to provide quantitative information in addition to being an ideal medium for scientific visualization. This paper reviews some advanced reconstruction algorithms and examines two cases in which CT has been successfully used: (1) coal washability analysis and (2) density gradient determination in an air-sparged hydrocyclone (ASH) flotation.
The paper outlines and exemplifies a multi-dimensional multi-phase Computational Fluid Dynamics (CFD) model for the various processes that occur in fluidized bed reactors. The model is based on the Eulerian description of the two phases: gas and particles. This means that separate conservation equations are set up for these phases. Calculations are shown for some examples that include flow patterns in circulating beds.
A new technique for measuring the flow of granular materials continuously is described. Slot Flow Metering utilizes the flow from vertical slots to determine flow rate. A Slot Flow Meter (SFM) consists of a hopper with one or more vertical slots in its sides supported by a means for weighing the hopper's contents, eg. a load cell. Solids flow rate can be correlated against the apparent weight of solids in the hopper. Recent experimental and theoretical work on the flow of granular materials from vertical slots is reviewed and a simple in-situ calibration technique is described. This is tested against direct calibration (bucket and stop-watch) with a pilot-scale unit located in an experimental circulating fluidized bed. The industrial prototype, installed in a Urea plant, is described.
Electrically conductive composite particles were prepared by dry coating of various types of electrically conductive fine powders on insulator resin. The course of composite formation and the state of dispersion were investigated by scanning electron microscope (referred to as SEM observation), X-ray elementary analysis and electrical property characterization. Combining SEM observation and electrical property characterization determined the course of composite formation. The state of fine particle dispersion of the surface of a single composite particle could be quantitatively evaluated by calculating the coefficient of variance for the fine particles on the basis of elemental analysis of composite particles. Experiments also revealed that composite particles with excellent electric conductivity can be obtained, provided that the state of fine particle dispersion and coating on the core particle is good.
Dense and uniform packing of ultra-fine powders, the primary particle size which ranges from 1nm to 0.1 μm, is very difficult because of strong and sparse aggregation. In this paper, the aggregating process of ultra-fine powder is examined by Brownian dynamic methods, and the effects of interaction between the primary particles and the initial concentration on aggregate shape and size is analyzed quantitatively by Fractal dimension and Weibull's distribution function. Furthermore, the packing of aggregates, which are of various shapes and are composed of different numbers of particles, is investigated by computer simulation. The effect of the geometrical characteristics and size distribution of aggregates on the porosity and distribution of micropore diameters in a packed bed is analyzed. When many aggregates consist of hundreds of particles, the porosity exceeds 94%. The porous structure is obtained by bridging between several aggregates. The wide spaces in ultra-fine powder composed by bridging are confirmed experimentally by mercury porosimetry. The effect of shape and size distribution of aggregates on packed bed structures is remarkable in the case of weakly aggregated powder, which means that the number of constituent particles is below 10.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 17(4), 837-845 (1991) 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 free settling behavior of cylindrical particles with densities ranging from 1.18 to 8.03 g/cm3 in stagnant water was observed with the aid of cameras and a stroboscope. The particles used ranged from 0.3 to 1.0 cm in diameter, and their size ratios of the diameter to the length were from 1.1 to 9.0. Particles having a size ratio over 1.2 sank with a periodic wobbling motion in the range of 700 to 15000 Reynolds number. This motion was simply expressed by a harmonic equation composed of several parameters. The parameters were expressed as functions of the size ratio, the density ratio between the particle and water and the Reynolds number. In the regular wobbling motion of the particles, the relationship between the drag coefficient and Reynolds number showed a hysteresis loop, of which the ranges of variation depended on the size of the particles, the density ratio and the Reynolds number.
†This report was originally printed in J. Soc. Powder Technology, Japan. 27(10), 673-679 (1990) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
Experimental and theoretical studies have been conducted on the measurement of the contact potential difference between a powder bed and a metal. A new model was proposed for the analysis of the experimental data. The model analysis indicated that accurate contact potential differences are obtained only when the electric charge of the powder bed is assumed to be zero. It was found that the absolute value of the contact potential difference decreased with an increase in relative humidity. The contact potential changed with the particle surface-modification by methanol vapor. However the original contact potential difference was obtained again when the methanol evaporated completely from the particle surface.
†This report was originally printed in J. Soc. Powder Technology, Japan. 28(4), 226-231 (1991) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Powder Technology, Japan.
A mathematical model based on an ideal LDV (Laser Doppler Velocimeter) signal indicates that the phase and amplitude of a light scattering particle vibrating sinusoidally within the interference fringes can be obtained from the Fourier integrals of the signal. Although actual LDV signals are distorted, it is found empirically that the use of a Hanning window is very effective for the removal of the undesirable effect of distortion. Hence the size and electrostatic charge of a particle being excited in an AC electric field can be calculated using the Fourier analysis of the LDV signal. A computer simulation shows that the phase can be determined very accurately, say, within 0.1º error if so desired. The lower limit of the amplitude measurement is ca. 0.2 μm within a few percent relative error. Therefore the present method is expected to provide a convenient device for measuring the size and the charge of particles of around 1 μm in diameter.
†This report was originally printed in FUNSAI No.35, 11-20 (1991) in Japanese, before being translated into English with the permission of Hosokawa Micron Corporation, Japan.
Effects of particle properties and number of particles on the parameters of sound from the collision between particles and a circular plate is discussed, in relation to measuring particle size and flow rate by the parameters of the impact sound. The sound pressure is proportional to the number of particles colliding with the plate, and the characteristic frequency of the sound relates to the particle diameter. Moreover, particle size distribution can be measured by the intensity of the characteristic frequency component of each particle diameter. These results show that the particle size distribution and the mass flow rate of particles can be measured instantaneously by the parameters of impact sound between particles and a circular plate.
†This report was originally printed in KAGAKU KOGAKU RONBUNSHU 16(5), 1067-1073 (1990) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of the Soc. Chemical Engineers, Japan.
A size-selective procedure has been developed in order to obtain samples of asbestos fibers of different lengths for use in biological experiments. Short chrysotile fibers less than 5 μm in length are widely suspected to be less carcinogenic than longer fibers. The bulk sample of U.I.C.C, standard asbestos (Chrysotile B) was aerosolized using a 2- component fluidized bed. It was then separated in a dry state by wire screens (200 mesh and 635 mesh) and a virtual impactor with a cut-off point of 2 μm. 0.1 g (sufficient weight for in-vitro tests) of a short-fiber fraction that passed through the wire screens and then onto the fine side of the virtual impactor, and 1.5 g of a medium fraction that passed through the 200 mesh screen but did not pass through the 635 mesh screen, were obtained from about 20 g of raw material. The length distribution of the short fiber fraction was 81% of fibers less than 5 μm long and 96 % of fibers less than 10 μm long. X-ray diffraction analysis revealed that the crystallinity of the separated chrysotile did not change markedly. The present dry and mild separation process that was developed thus successfully avoids any artificial change in the physicochemical properties of asbestos fibers.
†This report was originally printed in J. Soc. Powder Technology, Japan. 27(12), 804-810 (1990) 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 Mechanical properties and microstructure of Al2O3/ W composites have been investigated. These composites were fabricated by hot-pressing a mixture of γ- or α- phase alumina (Al2O3) and fine tungsten (W) powders. In these composites, fine W particles were located both within the Al2O3 matrix grains and at the grain boundaries. The fracture strength of Al2O3/5vol%W composites sintered at 1400ºC was 700 MPa, which is approximately 1.6 times higher than that of monolithic-Al2O3 prepared by way of the same processing method. But, the strength decreased with an increase in the sintering temperature. Although the interactions between cracks and W dispersion were partly observed, the fracture toughness was not significantly enhanced. It is believed that the presence of nanostructures do not sufficiently act on the mechanical properties of the composites because the grain boundary of the Al2O3 matrix where large W particles are located is not significantly strengthened by dispersion.
†This report was originally printed in Journal of the Japan Society of Powder and Powder Metallurgy, 38(3), 326-330 (1991) in Japanese, before being translated into English by KONA Editorial Committee with the permission of the editorial committee of Japan Society of Powder and Powder Metallurgy.