ARR - Annual Report

A piece of equipment has been devised in which the gap promoting attrition can be varied on a controlled manner with a view to carrying out studies on the relation between attrition in process equipment and in the annular attrition cell.

A modified attrition cell was fitted with fins to simulate flow of particles through a gap and preliminary trials at constant pressure showed that a small gap clearance gave more attrition than a larger one. Constant volume testing yielded a finer product. ProvIsionally, the particle breakage mechanism is unaltered in this type of equipment.

With the cell in its normal format using extruded particles the Gwyn kinetic formulation has been found to be true whatever attrition production size, testing stress and initial particle shape is selected. A shift in breakage mechanism from bodily fracture to surface abrasion has been found at low stresses. In general, mechanisms in operation are flexical bending (for long particles), fracture across a principal dimension, chipping (which is particularly noticeable for angular particles), and surface abrasion.

This report covers the contract year 1985/86 and describes the work performed to investigate the flow of fine materials from conical hoppers. Six materials - Kale seed and five different grades of sand ranging in size from 2,28mm to 150pm - are used in the experiments, Mass flow rates and interstitial pressure profiles adjacent to the orifice are measured simultaneously. Results have been obtained for four orifice diameters on a cone of half angle 9.5’ and for six orifice diameters on a cone of half angle 15”.

Previous work in the field has been extensively reviewed. Experiments on the kale seed and the coarsest sand (d = 2.28mm) provide further evidence for the applicability of the Beverloo (196l)correlation in describing the flow of coarse granular materials. For conical hoppers, the form of the correction factor proposed by Hose and Tanaka has been verified but the value of the exponent is still in doubt. They proposed a value of -0.35 which compares with the value of n m -0.2 indicated by the results obtained in this work.

Experiments on the different size ranges indicate that for particles in the range 50 to 600pm the measured flow rates are less than those predicted by the Beverloo correlation. It is shown that this retarded flow is caused by self-generated interstitial pressure gradients which arise as the material dilates on approaching the orifice. The Crewdson equation (1977), originally developed for air-augmented flows is found to be adequate in describing these retarded flows. Below 50um cohesive arching becomes an important factor and it may even prevent flow.

Crewdson et al. investigated a theory of stress induced dilation but their efforts proved inconclusive because of the difficulty in obtaining an accurate voidage-stress relationship at the low stress levels prevailing near the orifice. This line of thought is revived as the commercially available consolidometer can be used to obtain a voidage- stress relationship for stresses less than 10kN/ms. However, experimentally determined voidage changes exceed those predicted from the theory by a factor of almost 100. This suggests that it is necessary to look elsewhere for a suitable mechanism to explain the cause of the dilation.

Possible avenues of further work, both experimental and theoretical, are presented. An important element in the future is to be the direct determination of the voidage profile, which will be attempted using the r-ray tomography method developed by Seville et al. (1986) at the University of Surrey.

It is hoped to be able to develop a correlation for the flow of fine powders in the near term using the fractional retardation W/Ws. Meanwhile in the absence of any suitable alternatives, Carleton’s (1972) correlation can be used to predict the flow of fine powders from orifices greater than 20mm. The results presented in this work suggest that an overprediction in the range 20% to 50% can be expected.

Work performed on agglomeration of particle systems in fluidized beds at both low temperatures (granulation) and high temperatures (sintaring) is described in Part I and Part II of this report, respectively.

Part I

Part I includes a detailed review of the available literature on low temperature granulation as well as a description of original work performed on the behaviour of liquid bridges between two relatively moving particles. Experimental results are given for bridge strength measurements using two small spheres in a vibrational motion. These results are then compared to a modified form of a theoretical model developed from the well-known lubrication approximation. From both, the reviewed literature as well as from the experimental and theoretical work performed on moving liquid bridges, it is clearly concluded that the fluid (binder) viscosity and its rate of change with time are two of the most important binder characteristics which ultimately determine agglomerate growth in a granulator. It is also concluded that the instrument at CCNY used to measure bridge strength as a function of particle velocity (frequency, amplitude) and binder viscosity may be an efficient tool to characterize industrial binders. Based on these findings, future work is proposed to actually correlate bridge strength measurements with agglomerate growth rates in an experimental granulator.

Part II

Part II of the report includes the description of a large number of dilatometer and defluidization experiments performed at high temperatures with a great variety of amorphous and crystalline materials. Sometimes these experiments were complemented by differential scanning calorimeter (DSC) experiments to determine characteristic temperatures of recrystallization and/or phase change. The defluidization experiments were performed on the newly constructed fluidized bed agglomerator capable of operating at temperatures up to about 1150°C. Among the materials characterized during these experiments were different polymers and glass powders, sodium chloride, sodium bromide, sodium citrate and ferrous chloride crystals and a large number of more complex materials such as a titanium dioxide ore, FCC catalyst and fly ash samples. It was clearly established that the minimum sintering temperature (and other phase transition temperatures) can be determined using dilatometry. It was also shown that fluid bed defluidization (high temperature agglomeration) always occurs at temperatures somewhat higher than the minimum sintering measured in the dilatometer. Furthermore, the behaviour of the powder during high temperature fluidization can be reliably determined from the dilatometer experiments. Future work on this project will include upgrading both the dilatometer and the fluid bed agglomerator to withstand temperatures as high as 1500°C. The new dilatometer will also enable tests to be performed under a controlled atmosphere thereby allowing study of agglomeration due to chemical reactions.

The rheology of suspensions of submicron particles depends on the particle size, shape, and concentration and the nature and magnitude of the interparticle potentials. Indeed the rheological response differs qualitatively between rigid rods and hard spheres, between submicron and macroscopic particles, between dilute and concentrated suspensions, and between stable and flocculated colloidal suspensions. Hence a useful first step in understanding the subject is to classify systems according to the size and shape of the particles and the dominant interaction potential. Our work focusses on submicron, i.e. colloidal, particles which are spherical and strives to assess the effects of concentration and interparticle potential.

For colloidal systems the rheology is closely linked to the stability or phase behavior, which also reflects the nature and magnitude of the interparticle potentials. Dramatic rheological behavior is generally related to thermodynamic non-idealities, e.g. the solid-like rest state and plastic flow associated with flocculation due to van der Waals forces or phase separation induced by dissolved polymer, the elasticity of colloidal crystals generated by long range electrostatic repulsions, and the shear thinning and shear thickening phenomena associated with disorder-order transitions in stable dispersions near close packing. Thus an initial classification of colloidal systems might take the form.

Synopsis

Flocculation and sedimentation of fine particles are investigated here with emphasis on the role of conformation of adsorbed polymers on flocculation, theoretical modelling of aggregation and sedimentation processes, and structural characterization of sedimentation and floccs.

Polymer adsorption and, more importantly, the configuration of adsorbed polymer is considered critical in flocculation/dispersion processes. We have developed a fluorescence spectroscopic technique to investigate the conformational aspects of polymer adsorption. We have also successfully applied this technique to monitor coiled/stretched transformation of adsorbed polymers and to correlate the conformational information with the flocculation performance in the same system. Characterization of floccs is a major hurdle in flocculation studies and in this regard we have initiated the characterization of sedimentation processes and flocc structure by a CAT scan.

A Monte Carlo model has been developed and applied successfully to simulate sedimentation of fine particles by considering sedimentation as the result of competition between gravitation and the Brownian motion. In computer simulation of aggregation, the diffusion-limited aggregation model has been modified to generate sparse floccs as well as dense floccs by varying the step size of random walks.

This report covers the second year of the IFPRI Suspension Rheology Project 1985-88 at the K.U.Leuven. This project deals with the prediction, and manipulation of the rheological properties and the flow-induced structure in colloidal suspensions. It consists of two sub-projects. The first deals with colloidally stable systems, the second with weakly flocculated materials.

For the stable suspensions, the effect of particle size and that of stabilizer layer deformability is under investigation. Rheological measurements have been performed on suspensions, containing sterically stabilized PMMA particles of narrow size distribution and different particle sizes. This provides reference values for the projected measurements on systems with particle size distributions. In addition the data could be used to analyse the effect of stabilizer layer deformability on the rheology. It is concluded that this effect can be represented quite accurately by modifying two factors: the volume fraction at maximum packing and the factor expressing the shear stress dependence. The latter changes with concentration but it is shown that the shape of this curve hardly depends on softness. Quantitative results for the two parameters are given. Scaling relations are still under consideration.

Yielding was investigated, using silica suspensions. Their floe structure changes reversibly with shear. Yield stresses were measured with three different techniques. It is shown that, for some samples, identical results are obtained with the different techniques. However, in other cases deviations occur because of shear history effects during the measurement. Rational prediction of this phenomenon is not possible yet. For the silica under investigation yield stress and storage modulus change with concentration according to a power law. The data agree qualitatively with recent theoretical analyses.

The attrition and breakage of particles has been examined in a cone cell which permits the clearance between a rotating blade and a wall to be varied between experiments. Breakage is considerable if the clearance between the blade and the wall is just under one particle diameter or about 1% - 2 particle diameters. This effect has been found for a number of absolute initial particle sizes and for two materials of entirely different structure namely a polycrystalline urea and catalyst beads. Breakage is not detectable if the clearance is less than 0.25 particle diameters or greater than 2.5 particle diameters. A pronounced minimum attrition rate is found at about 1.25 particle diameters. An explanation of such effects is offered in terms of the particle packing. The findings are of considerable significance for understanding how attrition occurs in processing equipment.

Studies on well characterised alumina extrudates have been conducted in the annular shear cell used in previous work for IFPRI. The cell has the advantage that the stress, strain and strain rate applied to deforming bodies of particulate solids are known. The extrudates have been made in the different strengths and have known and reproducible structures. A range of normal stresses varying by over three orders of magnitude could be imposed on the deforming bed of particles.

The less strong extrudate showed a progressive decrease in fine attrition product as the normal stress decreased which indicated a shift in dominant mode of breakage from fragmentation to abrasion, but each mechanism remained at every stress. Both photographic evidence and particle size distributions confirmed this behaviour. For the harder particles the photographic evidence was consistent with this behaviour but the particle size distributions were erratic; it is thought that there is some limitation to the use of the cell but this remains to be proved.

It was also found that re-use of broken material in the cell may affect behaviour and further is necessary to establish whether the unification of breakage data may be achieved for the full stress range as it was previously over a more restricted range. The annular shear cell is of most practical use for systems with high ambient stresses.

Work performed on agglomeration of particle systems in fluidized beds at both low temperatures (granulation) and high temperatures (sintering) is The research described in Part I and Part II of this report,. respectively. reported on was performed during the period December 1986 - November 1987. carried out from December 1987 to September 1988 is also included. A special section in Part I (7.2) contains Future work for both projects to be three years program starting in September 1988 for the low temperature granulation project only.

In Part I of this report, containing the description of theoretical and experimental work on granulation in fluidized beds, it is demonstrated that the viscosity of the binder (in addition to other properties such as surface tension, wetting, etc.) is a very important characteristic which in final analysis determines the morphology and strength of the formed granules. After a short review of theoretical and experimental procedures-relating to liquid bridge strength given in section 3, the newly developed "dynamic bridge apparatus" is described (see section 4) and its capabilities are shown. The effects of the Capillary number, viscosity, bridge volume, etc. on the' strength of an axially strained pendular bridge and comparison of theoretical and measured values are given. It was demonstrated that under conditions of low Reynolds and Capillary numbers, large bridge volumes and favorable wetting of the solid surface the theoretical and experimental data are in good agreement. It was also clearly shown (see section 5) that binders which show a high rate of strengthening with time as the solution becomes more concentrated, i.e., the viscosity increases, yield agglomerated granules as a final product from granulation while binders for which the strengthening rate is moderate or low, yield layered granules but no agregates. This result was also predicted from a simple experiment using the dynamic bridge apparatus. Additional experiments with different binders exhibiting a wide range of properties in both the dynamic bridge apparatus and in the fluidized bed granulator are given in the Appendix (see section 10).

In Part II of the report which contains work on agglomeration due to high temperature sintering, it is shown that there is a strong correlation between elongation-contraction behavior of a powder sample in the proposed future work for an additional dilatometer and the agglomeration of the same powder when fluidized. The minimum sintering temperature of m&y different materials as determined in the dilatometer was shown to correspond to the temperature at which the material will defluidize. It was also found however that, although the correlation mentioned above holds for'incipient sintering, there seems to be no direct relation between rates -of deformation in the dilatometer at temperatures beyond sintering and actual sintering rates in the fluid bed. It is therefore often necessary that both dilatometer and fluidized bed tests be performed on the same powder before a final conclusion can be drawn. A detailed description of both experimental methods mentioned above are given in sections 2 and 3 respectively, while results and discussion of different test materials are given in section 4.. This section also contains a case study of a powder undergoing a chemical reaction which induces agglomeration. It was found that if the product of the chemical reaction causes agglomeration the controlling factor in the process is the conversion rate; this type of agglomeration behavior can not be detected in the dilatometer.

Section 5 in Part II contains a critical review of existing models of agglomeration; this section also includes some directions for the development of a more realistic theoretical model. The need for some basic knowledge of the magnitude of the break-up forces in a fluidized bed is also discussed. Finally, the Appendix to Part II contains a copy of a paper on high temperature sintering presented at the annual AICHE meeting in November of 1987.

This report consists of an account of work done under IFPRI contract in Cambridge University during the year to December 1987. The principal achievements during that time are:

1. Installing pressure transducers on the flow rig and developing the associated software so as to be able to measure the very low pressure differences occurring.
2. The performance of a series of experiments on a hopper of half angle 15 degrees; the results confirming the observations tentatively reported in the last IFPRI report.
3. The development of a possible theory to explain the observation.

Also included is the programme of work proposed for the rest of the contract.

This report covers research under the auspices cf IFPRI during the period January - December 1987 following the appointment of Hr. M.C. Turner to a Research Assistantship.

The foremost objective of the research is to elucidate the fundamental description and mechanisms of granular flow using both computer simulation and experimental methods. We aim to properly understand and formally describe the diversity of flax behaviour commonly observed in processes such as fluidisation of povters and flow from hoppers.

A generalised three-dimensional computer simulations program has Seen developed, ab initio, specifically designed to integrate the equations-of-motion of colloidal-like interacting spheres under gravitational flow on an inclined plane. The technique uses the general methodology of previous studies of dense suspensions under shear flow but incorporates numerous features essential to granular flow such as boundary friction and an interparticulate coefficier: of restitution to achieve steady-state conditions.

A novel feature of the simulations is the use of "gravitational units" whereby the gravitational constant g sets tie time and energy scales. With this approach we expect to gain s overview of the different regimes of behaviour (e.g. Geldart-types X, B, C, D, etc.) found for different powders on real laboratory of engineering time scales. The basic initial computer program is now 13 'production' and preliminary results are reported.

Laboratory experiments are being carried out alongside the simulations to investigate the flow behaviour of well-characterised (spherical, monodisperse) powders under gravitational: steady-shear flow. An experimental rig comprising a rotating bed, with the provision for a fluidisation flow field, has been constructed E d is operational. Preliminary experiments are reported, for larger particles (> 5~) and no gas field, to resemble the initial simulation conditions as closely as possible.

Both the simulations and the experimental studies are being extended to investigate the effects of a cohesive inter particle potential, which becomes important for fine powder when it is large compared to gmo, the gravitational energy, and t'L2 hydrodynamic flow field where aeration plays an important role.