ARR - Annual Report
EXECUTIVE SUMMARY
The main objective of the project is to study the mixing and elutriation of fines in a bed containing mostly coarse particles, eg. 40 /,um or 70 um fines in a bed of 480 um coarse particles.
In this year’s work programme, experiments were carried out to investigate the following:
- Qualitative study of the mechanisms of fines transport in the particulate phase of a two - dimensional bed.
- Measurement of gas flow through a two - dimensional bubble.
- Measurement of the axial and radial distributions of fines in a bubbling cylindrical bed.
Photographic evidence showed that there are two mechanisms mainly responsible for the upward transport of fines (i) fines carried up within the bubble void and (ii) fines moving with the wake of the bubble. Both mechanisms contribute to the ejection of fines from the bed surface; however, depending on the ratio of the fines terminal velocity to the minimum fluidisation velocity of the coarse particles, one of the mechanisms is more dominant than the other. If the ratio is much less than unity then mechanism (i) is dominant but if the ratio is greater than unity then mechanism (ii) is dominant. The presence of fines within the bubble void can make a significant contribution to the overall gas conversion in a fluidised bed reactor. Thus the presence of fines needs to be considered when designing fluidised bed reactors.
The concentration of fines within the bubble void is a function of the velocity of flow through the bubble which is proportional to the incipient fluidising velocity of the coarse material. The gas flow through a two - dimensional bubble was measured using a Laser Doppler Velocimeter, and the throughflow velocity was found to be about the same as the minimum fluidisation velocity.
The axial and radial distributions of 70 um fines in a bed of 480 um particles were measured in a 0.28 m diameter bed in batch and continuous modes of operation. In both cases it was found that the concentration of fines is higher at the top and lower at the bottom of the bed. The existence of such a profile indicates that the upward fines transport rate is primarily induced by bubbles; the bubbles are effective in transporting fines upwards as compared with the slow downwards movement of coarse particles over the whole cross section of the bed. As would be expected, the radial fines concentration gradient is much smaller than the axial concentration gradient.
It is postulated that there are three mechanisms by which fines mix in the bed (i) upwards mainly in the wake and void of the bubble (ii) downwards in the bulk circulation (iii) downwards by eddy diffusion. These mechanisms give rise to the observed concentration profile with high fines concentration at the top of the bed which promotes elutriation of fines from the bed surface.
For the future, attention will be focused on developing theoretical work based on the above mixing mechanisms in order to predict the fines concentration profile and the fines elutriation rate.
Theory arising from the above should be helpful in interpreting the effect of temperature on elutriation, to be measured at Bradford.
Summary
The purpose of this research project is to examine the existence of the grinding limit of fineness or the equilibrium size of product powder by in-liquid grinding using media mills, and to find the factors which determine the ultimate sizes. Furthermore, the rate of fine and ultrafine grinding in liquids by the media mills was investigated mainly from the viewpoints of the mechanical grinding conditions.
It was confirmed using a planetary ball mill with very high grinding rate that the equilibrium particle size and the negative grinding phenomena do exist even in in-liquid grinding, as far as the sizes are evaluated by laser scattering- diffraction method. The equilibrium size reduced with decreasing ball size (3mm to 0.5mm) and was well correlated with the force exerting on a single ball by the maximum centrifugal acceleration in the mill pot. On the other hand, the limit size determined in terms of absorption method was found independent of the grinding conditions within most of the present experimental range.
The particle size distributions of products ground in the specific surface area by BET gas water by the media mills were well presented by the Rosin- Rammler equation. Their distribution constants n showing the sharpness of distribution were considerably higher than those usually obtained by the dry grinding with larger balls and found dependent on the ball size.
It was made clear that there is an optimum size of balls for the grinding of certain feed particles and for other mill's conditions. The in-liquid grinding with the planetary mill at higher frequency with larger balls produced products with agglomerates, the amount of which was possibly evaluated by the deviation of size distribution from Rosin-Rammler distribution.
Executive Summary
The primary objective of the Caltech program supported by IFPRI is to understand the dynamics of particles which do not coalesce immediately upon coagulation and, through that understanding, to guide the development of processes for production of particles with particular properties. Theoretical and experimental investigations of the dynamics of aggregate aerosols have been undertaken with IFPRI support. Following on our studies of the properties of agglomerate particles, we have developed models to describe the kinetics of agglomeration. The kinetics of agglomeration are determined by the mobilities of the agglomerate particles and by their collision cross sections. The lower density of agglomerates has competing effects on the coagulation kinetics as compared with dense particles of equal mass: (i) the aerodynamic drag on the particles is increased due to the larger size of the particle of the same mass; (ii) the low density tends to increase the collision cross section of the agglomerate over that of the dense sphere. We have used fractal scaling concepts to evaluate the relationship between these measures of particle size and the particle structure.
In this report, we first examine the scaling that determines the asymptotic form of the size distribution, the so-called self-presenting particle size distribution. A dynamic exponent is defined that describes the rate of growth of the mean particle size. The dynamic exponent is shown to pass through a minimum in the transition regime, behavior that has not previously been described. Essential features of this asymptotic solution are observed experimentally, although direct comparison between experiment and theory is complicated by the transition between coalescing coagulation and agglomeration that took place in our experiments at about the same particle size as the transition between the free molecular and continuum size regimes.
A more complete description of aerosol agglomeration was made in numerical solutions to the coagulation equation using a modified sectional code. Comparison of the collision frequency functions of spheres and agglomerate particles reveals that the predominant effect is the increased collision cross section of the agglomerate, leading to a dramatic increase in the collision frequency of agglomerates as compared to dense particles of the same mass. Simulations that assume that particles coalesce completely up to a limiting size and do not coalesce at all beyond that size exhibit a broadening of the size distribution at that transitional size. Experimental evidence of that broadening is provided. The comparisons at this point can be considered only provisional in as much as direct experimental validation of the collision frequency function is still lacking. This experimental evaluation is the key objective of the research under this program for the coming year.
Executive Summary
IFPRI would like to have two series of spherical particles as standard reference materials for the calibration of particle size measurement instruments, especially for light diffraction types. One of the two series are light transparent and another is light opaque (nearly black). These particle size ranges are 3 to 30 um, 10 to 100 um, and 150 to 650 um respectively. The submicron size materials may be manufactured in Europe and/or the United States of America.
The RCR committee in EC has already reference materials of irregular non-spherical shape, composed of silicate sand. Therefore, the BCR also would like to have these spherical standard powders.
This 1st year of the project has been devoted to survey many spherical materials which are suitable as the requested standard particles.
The 5 kinds of glass beads have been manufactured as transparent standard materials, of which the size ranges are 3 to 30 um, 10 to 1.00 um and 150 to 650 um. Two series of glass beads have been examined for their physical properties, especially air bubble contained percentages on number basis. As the results, the Soda-Lime-Silicate one (LU) is found to be preferable for a larger size range of 150 to 650 um, and the Barium-Titanate one (MB) is good for two smaller size ranges of 3 to 30 um and 1.0 to 100 um.
The glassy carbon beads have been manufactured as light opaque standard materials for the size range of 3 to 30 um.
Resin beads also have been produced as a comparative sample of smooth surface particles, which are half opaque and not appropriate as standard materials.
Their size distributions should be near logarithmic-normal (Gaussian) as the standard. Physical characterizes, including size distributions and shape indexes, have been measured by use of various methods in several cooperative Japanese laboratories. These measured results look like to be reasonable and allowable as standard materials.
The second year project is to manufacture large amounts (7.5 and 10 kgs) of the standard materials in the smaller size ranges (3 to 30 um and 10 to 100 um) of both light transparent and opaque ones, one by one in series. Their particle size distributions and shape indexes will be measured at various Japanese laboratories.
INTRODUCTION
A problem of widespread industrial and theoretical importance is the separation of fine solids from liquids. The suspended solids are consolidated under the influence of a body force applied to the particles, for example, gravitational force in gravity thickening or an applied pressure in a pressure filter. Such mechanical dewatering processes use much less energy than evaporative drying.
Gravitational thickening, both batch and continuous, has received quite a lot of attention in the literature. However little mathematical analysis has been done on the other common type of dewatering process, pressure filtering. This technique relies on the removal of liquid by expression, that is, compression of suspended material with drainage. A simple example is a cylinder containing the material, compressed by a piston at one face and only the liquid is allowed to pass through a porous membrane at the other face. Specifying the fluid expression rate or applied pressure are two modes of operation of such filters. Some simple linear models for constant expression rate and constant applied pressure, and a nonlinear problem for stable suspensions have been studied. Models incorporating the yielding solid rheological properties of flocculated suspensions will be discussed here. A full understanding of the dependence and sensitivities of filter presses to the various physical parameters in the process will be of value in designing more efficient presses, and ultimately to optimizing the performance of pressure filters. Further, some of the material properties of these suspensions, for example the yield stress, may be able to be determined from a simple test experiment using filter presses, as has been described for batch centrifuge settling.
Buscall and White discussed the rheological properties of concentrated suspensions and they and Howells et al applied it to settling under gravity of a flocculated suspension in a closed bottom container. Such suspensions have also been studied by us in gravity thickeners. The particles of the suspension interact directly with one another to give rise to a local particle pressure, which is the effective stress tensor. When electrolyte or polymer flocculants have been added to the suspension connected aggregate structures of many particles are produced held together by van der Waals or polymer-bridging forces. Once the average particle volume fraction is high enough that a network of connected particles is formed, the suspension takes on the properties of a solid (albeit flimsy) structure. In particular, compressive stresses on the suspension can be transmitted via the network throughout the system and the structure then possesses the ability to support itself. In a flocculated system above this volume fraction, the particle pressure should be more properly thought of as a network pressure. When such a network has formed throughout the system, we are free to increase the particle pressure by applying some sort of external compression to the network for example, push on it with a piston, or increase the gravitational forces in a centrifuge. As this process is applied, the network structure will resist further compression and the particle pressure will increase until the compressive forces become so strong that the structure will begin to deform irreversibly. The rheological property to describe this is the compressive yield stress, which is defined as the value of the network pressure at which the flocculated suspension at volume fraction will no longer resist compression elastically, and will start to yield and so irreversibly consolidate.
This compressive yield stress is an implicit function of the strength of the interparticle bridging forces and possibly the previous shear history of the system, which will determine the primary floe size and internal structure. In Buscall and White, it is shown how the equilibrium bed height measurements in a centrifuge can be used to measure the compressive yield stress. Power law curves of the type with various values of n or m have been fitted to experimental systems. Here is called the gel point, and is the value below which cannot be experimentally distinguished from zero. It may be considered the volume fraction at which all the primary floes become interconnected. This concept of a network pressure is used later in the kinetic description of consolidation.
A one-dimensional model for cylindrical filter presses will be established, using the rheological properties discussed above. The analysis can then be divided into the two distinct modes of operation - specified fluid expression rate or specified applied piston pressure. In the first mode approximate analytic results can be obtained for typical profiles. When the applied piston pressure is specified, the fluid flux and piston position must be solved for as part of the moving boundary problem. Some analytic results for small times along with a numerical method for solving the full transient solution are given. These two modes of operation show fundamental differences and these are discussed in the conclusion. Depending on the qualities of the filter cake and the speed of operation, one mode of operation may be more suitable than another. The case in which the suspension is initially fully networked, and the other case when it is initially unnetworked are both considered. The properties exhibited by the two cases are quite different and will be discussed separately.
Background I
In the annual report for 1990 we described the experimental setup developed for characterizing group A powders at room temperature; results were reported on the influence which mean particle size and addition of fines fraction had on bed expansion, Umf, Umb, deaeration rates and times, and bubble sizes. It is generally assumed that if tests are conducted at high temperature, the powder behaviour will be predicted simply by using the appropriate physical properties of the gas, adjusted for temperature, in correlations based on data gathered at room temperature with different gases. However, it is our hypotheses that interparticle forces play a major role in the behaviour of group A powders, particularly these which are at the finer end of the group, and that temperature will alter these forces. Thus, the powder behaviour change at high temperature may not be predicted by hydrodynamics effects alone.
Temperature effects on incipient fluidization velocity Umf and bed voidage at incipient fluidization point and bubble sizes have been extensively studied on group B and group D powders. In this work, temperature effects on some fluidization characteristics of group A powders are reported: these are incipient fluidization velocity mf, and the voidage of loosely settled bed cS, incipient bubbling U velocity Umb, and bed expansion.
Executive Summary
This IFPRI project investigates the formation mechanisms of uniform, submicron inorganic particles. Due to poor understanding of the physical chemistry governing particle nucleation and growth, this project has undertaken to determine if there are underlying physical or chemical themes that give rise to uniform precipitates. Previous work has focussed on spheres produced from the hydrolysis and condensation of silicon alkoxides. Here the necessity of achieving colloidal stability among the growing particles was hypothcsized as an essential step in the formation of process, Studies on the silicon alkoxide system indicated that particle growth occurred through the agglomeration of primary particles produced at a rate independent of the presence of larger particles and that unifomlity was the result of size dependent rates of aggregation.
In this annual report we review studies on a second alkoxide system. Here the formation of titanium hydrous oxide particles are discussed. The results of these studies demonstrate that a mechanism similar to that observed for silica is operable. In addition the necessity of maintaining colIoidal stability among the large particles is emphasized. This is demonstrated through experiments showing that early in precipitation reaction a constant number of colloidalIy stable particles is formed. If the particle surface potential is less than 12mV for the ionic conditions investigated, late in the reaction agglomerates are formed and become fused. If the surface potential is greater than 12mV and the reacting solution is not sheared, uniform particles are formed. However, a critical shear rate exists where for particles with surface potentials greater than 12mV, agglomerates are formed from uniform particles of a particular size. These results are interpreted in terms of particle interactions consisting of van der Waals, electrostatic and short range repulsive interactions, Agglomeration of uniform particles is hypothcsized to be the result of the combined action of the formation of a shallow minimum in pair interaction potential for particles of a partiuar size, and the continued precipitation of reactive material.
Summary
The object of this work is to extend our earlier research on the flow of gas and particles through ducts to the situation in which the flow is turbulent. Such flows are of great technical importance in pneumatic transport of particulates and in the circulation of particulate materials within chemical processes. Examples of the latter type include the riser reactors and standpipes which form components of the catalyst circulation loop in catalytic crackers, used in the refining of oil, and the long standpipes used in certain coal liquefaction plants. In all these systems the particles are found to distribute themselves in a very non-uniform way over the width of the duct. Unless the nature of this distribution can be predicted, it is not possible to calculate the pressure drop along the duct or other quantities, such as the statistical distribution of particle residence times, which are needed in the design of these systems.
The key to making useful predictions is, therefore, to understand and quantify the mechanism responsible for the distribution of particle concentration over the cross section of the duct. In earlier work, sponsored by TFPRI, we have established that collisions among the particles can endow the assembly of particles with properties analogous to those of a molecular fluid, and that this can generate a distribution of the particles which is, in many ways, realistic. However, the behavior is found to be very sensitive to mechanical properties of the particles which affect the elasticity of collisions, and this is not at all realistic behavior for most systems of technical interest.
The present work examines the effect of turbulent flow on the cross sectional distribution of the particles, and indicates that there will be sharply enhanced concentration in the vicinity of the walls of the duct, as observed in practice. In contrast to the findings of our earlier work the predicted distribution of particle concentration appears to be rather insensitive to the properties of the particles or the parameters of the turbulence model used. It must be emphasized that these conclusions are tentative, since difficulties encountered in generating numerical solutions of the governing equations have not yet been entirely overcome, but the results obtained so far are encouraging.
Abstract
New data are reported on the formation of filter cakes under pressure, where the particles are largely finer than about 10pm and can be subject to the influence of inter-particle forces. The data are analysed through two consecutive mechanisms, filtration and consolidation. Process design parameters for each mechanism are obtained from the models, which will allow calculations to be made from small scale experiments. The magnitude and dependence of the constitutive parameters on pressure, concentration of solids in the feed suspension, pH (and surface charge), particle size and shape, and the nature of the particle-particle interactions are shown through the experimental results. Some data have also been obtained for the filtration of mixtures of particles.