ARR - Annual Report

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.

Summary

Results from theory and experiment in the literature for the viscosity of dispersions of monodisperse hard spheres are contrasted to illustrate the effects of particle microstructure. Hard spheres comprise a simple ideal limit, with no inter-particle forces other than infinite repulsion at contact, and are achieved experimentally by either minimizing van der Waals attractions or negating them with short range repulsions. “Real” systems, with either additional longer range repulsions or significant short range attraction, will exhibit higher viscosities (but for different reasons); so the results discussed here generally represent limiting cases. A fundamental connection also exists between composites of hard particles in an incompressible, elastic continuous phase and dispersions of spheres with a corresponding microstructure. The analogy between Hookean elasticity and Stokes flow means that the static shear modulus of the former, normalized by the modulus of the continuous phase, equals the high frequency limiting relative viscosity of the latter.

For hard sphere dispersions, the balance between viscous forces and Brownian motion, as gauged by the Peclet number Pe, determines the microstructure and, hence, the viscosity. This results in a progression with isotropic equilibrium at Pe = 0, a small perturbation oriented with the principle direction of strain for Pe ccl, two dimensional anisotropy for Pe >>l, and a return to isotropy, albeit hydrodynamically dominated, at Pe = M. The corresponding viscosities vary as Pe = 0 2 Pe >> 1 I Pe << 1 I Pe = 00~ The high frequency limiting viscosity/static shear modulus depends on the degree of mechanical coupling between particles and, hence, increases in the order of face-centered cubic < body-centered cubic < random < simple cubic.

Our theoretical effort to develop means for predicting these and other aspects of the rheology of concentrated dispersions continues to progress. The tests of thermodynamic closures for handling three-body couplings through the pair potential have produced the first such predictions of viscosities exceeding that of the solvent by a couple of orders of magnitude, These pertain to both hard sphere and Yukawa inter-particle potentials and will soon be tested against Brownian dynamics simulations, The analogous closure for many-body hydrodynamics still eludes our grasp but the quest continues.

Executive Summary

Prediction, Scale Up Test Methods in Solid/Liquid Separation

Investigators: Professor L.R. White, Dr. K.A. Landman, Advanced Mineral Products Centre, Department of Mathematics, University of Melbourne, PARKVILLE VIC 3052

Overall Project Aims

• (a) To refine a rheological model which would enable accurate prediction and scale up of solid-liquid separation in flocculated suspensions.
• (b) To devise laboratory test procedures which would determine the parameters of that rheological model for any given solid/liquid system.
• (c) To model with this theoretical description, existing separation methods in commercial use with a view to increasing efficiency and economy.

Progress to Date STAGE ONE AND STAGE TWO

• (a) The basic features of the rheological model have been developed viz. the role of compressional yield stress Py(o) and the hindered settling factor r(o) in thickening processes. At this stage, shear stress has not been included in the fundamental model. Initial work on computer simulated separation has commenced.
• (b) A laboratory procedure for the measurement of P,(o) has been developed and applied in practical systems e.g. red mud.
• (c) Modelling of conventional separation processes using the fundamental model have been completed for several systems,
• (i) transient batch settling59
• (ii) steady state gravity thickener11
• (iii) batch pressure filtration, under both moderate and high pressures14
• (d) Development of a steady state laboratory test method for the direct measurement of the hindered settling factor r(o) e.g. red mud. A student with Professor D.V. Boger locally has commenced setting up a pressure cell to undertake the required measurements.
• (e) Commenced discussions on analysing Professor R.J. Wakeman’s filtration experimental results in terms of our theoretical models.
• (f) Collaboration with Exeter on a new experimental program to measure combined compression and shear effects (a filtration cell with a rotating face).

OBJECTIVES STAGE THREE

• (a) Detailed comparison of the experimental data obtained by Professor D.V. Boger locally and by Professor R.J. Wakeman at Exeter with our theoretical models.
• (b) Modification of the fundamental rheological model to include shear stress effects.
• (c) More sophisticated two dimensional (i.e. non-plug flow) modelling of gravity thickeness with the effect of shear yield stress included.
• (d) Development of computer simulation models of separation processes.

Executive Summary

This report summarizes research supported by I.F.P.R.I. at Stanford University on the use of high speed optical polarimetry measurements to characterize the dynamics and structure of fine particle suspensions. The work is divided along two lines:

1. Characterization of single particle properties.

   The optical experiments employed in the laboratory of the principal investigator provide sensitive measurements of anisotropy. In dilute suspensions, this would refer to orientation of nonspherical particles by external fields (flow, electric, or magnetic). These measurements can be used to determine a number of physical characteristics of colloidal particles, such as size and shape distributions, and dipole moments. This report summarizes a new technique whereby the first direct measurements of dipole moments for colloidal particles can be obtained. The method combines optical measurements of the average orientation angle of particles as a function of time following the simultaneous inception of a shearing flow and an electric field. In the limit of very small times following the application of flow, the convection diffusion equation governing the particle dynamics can be solved exactly and the experiment yields a determination of the induced electric dipole moment of the particles in suspension. Results are reported here on measurements made on iron oxide particles suspended in a glycerin/water mixture. It is demonstrated that this method yields measurements of the anisotropy in the particle polarizability without resorting to approximate models of particle orientation and in a manner that is insensitive to the presence of brownian motion.

2. Field induced structure in dense suspensions.

   The application of external fields onto dense suspensions of colloidal spheres can induce structural anisotropy. Optical experiments in the laboratory of the principal investigator have been used to measure anisotropy in the particle pair distribution function as a result of applying either electric or flow fields. Two problems have been investigated over the past two years: (1) electric field induced anisotropy and chain formation, and (2) the connection between structural anisotropy and shear thickening behavior in dense suspensions. The findings of this research have recently been published in two papers and preprints of the published work are included in an appendix to the annual report.

Executive Summary

Bubble Control with electric fields in fluidized beds-Part I

New results are presented for both ac and dc electric field bubble control in gas fluidized beds tested to 125 “C. Modeling includes, (a) electrostatic forces at a bubble interface for dc fields, and (b) an ac theory for inter-particle capacitive forces. Correlations for bed expansion are presented that includes the electric field strength and superficial velocity based on an extension of two-phase fluidization theory. The experimental results reflect studies that were initiated last summer with the new Trek 10 kV high voltage power supply purchased with IFPRI funds. This supply has permitted us to undertake the study of charge relaxation, which is known to influence inter-particle forces, and also to study the effect of field strength and frequency on bubble control and bed expansion to greater extent than was possible before.

The experimental variables studied to date include bed temperature, electric field strength, and field frequency. Bubble control experiments were carried out on two sizes of glass microspheres, 44-77 urn and 77-144 pm., both glasses being Class A powders using Geldart’s quality of fluidization criteria. Significant effect was observed for bubble control with simultaneous bed expansion to 15% when optimized at a frequency of 3 Hz (compared to l-2% expansion for a field-free bed). An asymptotic drop-off in bed height with increasing frequency and improved bubble control with increasing temperature can be explained by ac modeling of inter-particle forces. The reason for the temperature dependence is not yet fully understood.

DC modeling for the mrtvimum electrostatic forces at the bubble interface follow an assumed spherical bubble shape, chosen to be consistent with the Davidson model used for the fluid dynamics. It is postulated that electrostatic forces must be of the same order of magnitude as particle-fluid forces for effective bubble control. Our dc model is successful in predicting the correct magnitude of electric field strength (E) required for the onset of bubble control showing that E - O(kV/cm) as observed experimentally. However, the model is deficient in that it does not show a dependence of bubble control on superficial velocity as observed experimentally with ac fields and to a lesser extend with dc fields.

Modeling of ac fields shows that inter-particle forces depend in addition (to dc force effects) on the frequency of the field, particle contact resistance, particle surface resistivity and the capacitance of adjacent particles. The ac model needs to be perfected to include the important effect of superficial velocity based on experimental evidence.

Future modeling will include non-spherical bubble shapes and the coupling between ac and dc particle forces with bed expansion (superficial velocity effect). Our numerical program K-Fix is still being developed by Forhad Hossain and is considered to be a long-range part of our modeling program for the electrostatics and hydrodynamics in fluidized beds. Future experiments will incorporate materials of industrially importance at elevated temperature using a quartz bed. These studies will also include particle diameter and relative humidity as variables.

Elutriation control with electric fields-Part II

Fines concentration reductions of up to 96% have been measured in the freeboard of a gas tluidized bed with ac and dc fields. Experimental studies were first reported at Harrogate for dc fields. An optical method employing a helium-neon laser was developed to measure real time particle concentration in the freeboard. A theory was then developed to interpret this data in terms of the elutriation constants (ki).

In the experiments, sand fines, 3.67 to 32 pm, were tested. with ac and dc fields in bulk sand having an average diameter of 300 pm. Elutriation constants were of the order of magnitude of ki = 0.02 set-l and varied with dc field strength depending on the temperature of the bed, whereas the elutriation constants remained relatively independent of tcmperaturc up to 500 “C. Additional data are needed to obtain correlations and to expand this research to include industrial materials of different kinds and shapes.

Modeling of electrostatic forces in the bed is needed to better understand the experimental results. Two approaches for modeling particle-surface forces arc suggested, (1) equilibrium and (2) stability analysis. A closed cell model will dctcrmine the macroscopic field in which fine particles are acted upon by electrostatic forces and surface currents.

The experimental aspect of our research, Parts I and II, is being carried out by J. S. Wang, a Ph.D. student in mechanical engineering

Electrostatic powder separation-Part Ill-a

This study concluded our first year IFPRI study of triboelectric charging in a circulating tluidized bed. It demonstrated that separation of the constituents in a powder mixture, in our case coal pyrite from the parent coal, is possible using selective triboelectric charging in a circulating lluidized bed. Analysis of data showed that the single most important parameter is the difference of charge received by individual constituents. The voltage difference in the precipitator and the superficial velocity were found to be of secondary importance.

Corona discharge-Part 111-b

Corona discharge will often accompany the application of high voltages, high temperatures, and surfaces exhibiting high curvature (e.g. fine particles). It is likely that corona breakdown takes place between particles in an electrofluidized bed under conditions of sufficiently high fields. In the presence of particles, the so called corona wind will literally blow particles toward walls.

Our first study in corona discharge was to investigate modeling of the corona wind and to measure its effect as drag on a glass plate in a low Reynolds number wind tunnel, Rex < 3.6x10^3 (x = length of glass plate ). Our model was successful in predicting the expected trend in drag phenomena over a flat plate using the Karman-Pohlhausen method. However, theoretical predictions of drag were two orders of magnitude too large compared to experiments with ac corona. The reason for the large discrepancy is thought to be that the model is essentially dc whereas the experiment was for an ac corona discharge, Also the effective ionic mobilities usad in the predictions were thought to be too large for the actual conditions.

There are no immediate plans to extend this work. However, corona discharge phenomena will likely need to be included or at least understood in our future modeling of high temperature fluidized beds as well as in the freeboard region if high voltage fields or highly charged particles present.

Executive Summary

This is the first annual report of the IFPRI Suspension Rheology project 1991-1994 at the Katholieke Universiteit Leuven (Belgium). Whereas the previous projects here dealt with the flow properties of stable colloidal dispersions, the present concentrates on flocculated systems. In particular it will be attempted to generate some basic information on how to manipulate the rheology of “reversibly flocculated systems”. The latter term refers here to colloidal dispersions in which the floes can be broken down reversibly by flow. Such systems occur frequently in materials processing operations.

During this first year an existing device for probing floe structure during flow with dielectric spectroscopy has been further developed. It is now being computerized, also the measuring range has been extended to make the technique applicable to a wider range of materials. Meanwhile some exploratory dielectric work has been started on a particular dispersion, consisting of carbon black in mineral oil, to investigate the potential of the technique for less structurized systems than those used before. Time and frequency domain measurements have been combined to cover a larger part of the dielectric spectrum. In this manner a low frequency resonance between floe rotation and dielectric polarization has been identified. In addition a power law region could be detected at high frequencies. The effects of both shear rate and temperature have been studied. A general theory to calculate structural characteristics from the dielectric measurements is still lacking. A preliminary discussion of possible structural interpretations is given.

It is concluded that dielectric spectroscopy on flowing dispersions can be a quite sensitive technique to probe the changing floe structure during flow. The potential use of the technique developed here has been demonstrated. Possible experimental and data handling procedures are suggested and have been applied on a given dispersion. Awaiting a suitable model for the dielectric behaviour, the information is mainly qualitative but even that is extremely helpful while studying the rheological consequences of the variable structure.

Executive Summary

The second year project of the spherical reference materials is to manufacture 3 kinds of the transparent or opaque particles, of which the size ranges are 3 to 30um or 10 to lOOum, and also to produce preliminary samples of both glass and carbon beads within a 1 to 10um size range.

1. Transparent barium titanate glass beads, MBP 3-30', which have a 3 to 30um size range, have been produced and sent to the AEA Technology in England for the certification in February 1992. The total amount is lOkgs, instead of the contract quantity 7.5kgs.

2. Opaque glassy carbon beads, GCP3-30, which have a 3 to 30um size range, have also been produced and sent to the same AEA Technology for the certification at the same time. The total amount is also lOkgs, instead of the contract quantity 7.5kgs.

3. Transparent barium titanate glass beads, MBP 10-100, which have a 10 to lOO#m size range, have been manufactured and sent to the same AEA Technology in April 1992. The total amount is 20kgs, instead of the contract quantity lOkgs, through the courtesy of Union Company, Japan, because BCR(Bureau of Community Reference) of EC commission requested as much as possible.

4. A preliminary study of both transparent barium titanate glass beads and opaque glassy carbon beads, of which the size range is 1 to l0um, have been successfully carried out.

The physical characteristics of the above three product materials are measured in several Japanese company laboratories, and given in this report.

The above materials have been sent in one or two packages by air mail, and not split into small sample packs. Segregation in particle size could happen during transportation and also process handling. Therefore complete mixing is necessary before sample splitting, and the total quantity of a test sample also should be completely mixed before size measurement, because the measuring mass is very small. The third and forth year projects will be to manufacture following 5 kinds of spherical materials.

1. GCRlO-100: Opaque glassy carbon beads of a 10 to 100um size range. The quantity is more than 10kgs.

2. MBPl-10: Transparent glass beads of a 1 to lOum size range. The quantity is more than 5kgs.

3. GCPl-10: Opaque glassy carbon beads of a 1 to 10um size range. The quantity is more than 5kgs.

4. LBP150-650: Transparent glass beads of a 150 to 650bm size range. The quantity is more than 20kgs.

5. GCP150-650: Opaque glassy carbon beads of a 150 to 65OJm size range. The quantity is more than 20kgs.

Note+ The last symbol P in both MBP and GCP beads means the final product, which are sent to AEA in England.