FRR - Final Report
Summary:
Over the past several years, IFPRI-supported work at Duke has focused on understanding jaming and flow properties in a quasi-2D hopper by using photoelastic particles. Two final goals of the proposed work were
- to extend studies using cohesionless photoelastic particles to similar particles, but with cohesive interactions, and
- to studies of three dimensional systems. During the final year of this project, we have addressed the goal of using cohesive particles by carrying out i) studies of particles with cohesion and ii) studies of the stress response of systems of agglomerate particles, which have the property that they can break. We have
- carried out additional studies of hopper flow for cohesionless photoelastic particles, where we have used synchronized high speed video imaging for both particle tracking and photoelastic imaging. The goals of 3) are to understand the coupling between force chains (as a measure of stresses), flow velocity, and density. A key rationale of this last set of studies is to understand the relation between the various granular states that occur for hoppers: flowing and jammed, and to understand whether shear jammed states (recently discovered at Duke) are in fact the same states that occur for jamming of a hopper. We have also developed a new approach for studying fully 3D granular systems at the particle scale, which includes the measurement of inter-particle forces and particle motion. This report provides information on the last year’s work, as well as highlights of previous results.
IFPRI support has led to the Ph.D. of Junyao Tang, now employed in industry. Two additional students, Audrey Melville, and Yiqiu Zhao, have worked on this project in the past year.
Executive summary
In this work our objective has been to develop quantitative methods for the prediction of all technically important features of the flow of gas and solid particles through ducts. Flows of this sort are of great importance in pneumatic transport of particulate material and in the circulation of particulate materials within certain chemical process plants; for example, catalytic crackers used in oil refining, and circulating fluidized beds, which are important in coal processing and a number of other chemical processes. Design of these systems is made difficult by the tendency of the particles to distribute themselves over the cross section of the duct in a highly non-uniform way. Consequently it has not been possible to predict properties such as the holdup of particles, the distribution of residence times of the particles in a given section of duct, or the gas pressure drop. Even scaling these quantities between ducts of different sizes, operated under different conditions, has been uncertain, at best.
In an earlier period of IFPRI funding (see Final Report FRR 09- 10) we developed equations of motion for gas-particle mixtures and applied them to streamline flow through ducts. The results predicted a rich variety of behavior, which conformed well with observations in systems of this type, but the quantitative predictions were unrealistically sensitive to the values of, ‘certain parameters representing physical properties of the particles. We speculated that this shortcoming was a consequence of limiting our attention to steady, streamline flows since, in most situations of technical interest, the flow is markedly unsteady, like the fast, turbulent flow of a simple liquid or gas through a pipe. Accordingly, we have extended our treatment to cover this type of flow, and we now find that the unrealistic sensitivity is suppressed, while the qualitative features of interest are conserved. There is a shortage of experimental measurements against which the quantitative predictions can be tested, but comparisons with the data presently available in the open literature are encouraging.
Executive Summary
It is now well established that meso-scale structures, whose characteristic size is on the order of a few millimeters, arise in rapid gas-solid flows. These structures significantly affect the overall flow behavior and should therefore be accounted for in CFD simulations.
Unfortunately, the meso-scale structures cannot be resolved adequately in CFD simulations of risers of practically relevant dimensions. As coarse-grid simulations of such gas-solid flows do not resolve the meso-scale structures, quantities such as the effective inter-phase drag and the net rate of dissipation of pseudo-thermal energy are not properly accounted for in the computations. Therefore, the coarse-grid simulations, which have been published in the literature, should be viewed with some skepticism, as there is little basis to argue that these results are indeed true solutions of the differential equation models one is trying to solve.
In the last four years, we have made significant progress in understanding the origin of the meso-scale structures. It is established that they arise as a result of the inertial instability associated with the relative motion between the gas and particle phases and/or inelastic collisions, both of which are local events occurring on a length-scale comparable to the size of the meso-scale structures. This allowed us to assemble a tentative sub-grid model to account for the effects of the (unresolved) meso-scale structures in coarse-grid simulations. The proposed sub-grid model interrogates the stability of uniform motion on a length scale smaller than the grid size of a coarse-grid simulation and incorporates corrections to quantities such as effective drag, etc. accordingly. In that sense, it is indeed based on the differential equation model, which one is trying to solve. The sub-grid model is internally consistent, in the sense that, as the grid size goes to zero, the sub-grid corrections become smaller and smaller. Thus, the intent of the sub-grid model is not to change the original system of differential equations one is trying to solve, but to help us simulate the macro-scale structures correctly without having to resolve the meso-scale structures. Some of the elements of the sub-grid model are speculative and remain to be tested.
We then embarked on a program of research aimed at gathering statistics on fluctuations associated with the meso-scale structures in a model two-phase flow problem, so that we can verify the validity of the speculative elements of the sub-grid model. This work is in progress and some of the initial results on fluctuation statistics are described in this report. Results obtained at different levels of solids loading manifest qualitatively similar fluctuation statistics, giving us hope that a validated sub-grid model is indeed within reach.
EXECUTIVE SUMMARY
Agglomeration is an inherent problem in almost all industrially relevent powder processing techniques. Succesful dispersion of powders often requires the total elimination of agglomerates. To achieve this, it is important to understand the nature of, and to ascertain the properties of, these agglomerates. Of particular importance is the strength of the interparticle bonds, (between primary particle units forming the agglomerate), in relation to the powder processing procedure used for their generation. The unit operation which is most closely associated with the formation of hard agglomerates during powder processing is drying. However, the various mechanisms which lead to the formation of hard agglomerates during drying have not been previously systematically studied.
The effect of capillary pressure during drying on the strength of fine powder agglomerates was investigated. Silica and titania slurries were dried at different drying rates, using both spray drying and tray drying, and the strength and strength distribution of the dry agglomerates was quantitatively measured using a calibrated ultrasonic field. The slurry surface tension was varied by using aprotic and protic solvents of different surface tension and by using mixtures of water and n-propanol or n-butanol of varying composition. Particles of four different diameter (20 nm, 28 nm, 60 nm, and 500 nm) were used to vary the effective radius of curvature between the particles.
The average agglomerate strength was found to increase with increasing surface tension and/or decreasing particle radius. Alcohol washed samples had similar agglomerate strength to samples washed with aprotic solvents of similar surface tension. Based on FTIR, TGA, and 1% NMR results, it was concluded that the role of alcohol washing, which is commonly employed in fine ceramic powder preparation to produce softer agglomerates, is that of primarily surface tension reduction and not particle surface esterification as previously presumed.
The effect of particle solubility and dissolution rate on agglomerate was studied by drying silica and titania particles from aqueous slurries with pH ranging from 2 to 12. The agglomerate strength and strength distribution was measured by a calibrated ultrasonic force and the strength was found to increase with increasing solubility and dissolution rate. Two different particle size silica powders (60 nm and 500 nm) were studied and smaller sized particles were found to form stronger agglomerates. The drying rate of the powders was varied by using both spray drying and tray drying and it was shown that slower drying leads to higher agglomerate strength.
The agglomerate strength of titania powder (insoluble in water) was found to be independent of the pH while the agglomerate strength of silica was found to depend on the PH. It was concluded that the solubility and dissolution rate are important parameters governing the strength of agglomerates.
EXECUTIVE SUMMARY
Vertical risers constitute an important class of reactors for contacting solid particles and gases in the chemical engineering, petroleum refining and power generation industries. In an effort to inform recent models of their fluid dynamics, we employed a unique facility that recycles fluidization gases of adjustable properties.
In that facility, we investigated the effects of gas density, scale and operating conditions while achieving hydrodynamic similarity with generic high-temperature risers operating at pressures of 1 and 8 atm. We interpreted our results in the upper riser using steady, fully-developed momentum balances for the gas and solid phases. The analysis showed that the “atmospheric” and “pressurized” experiments conform to distinct viscous and inertial regimes. It also provided quantitative predictions for the suspension density in the upper riser.
By recording radial profiles of volume fraction and axial gradients of gas pressure, we inferred the shear stress at the wall and found conditions where the solid recirculation produces shear stresses directed along the flow, rather than against Tt.
From a study of solid clusters, we produced a robust correlation for their descending velocity at the wall and concluded that their dynamics in that region is mainly governed by particle interactions.
We also compared our measurements of pressure losses and efficiency of a cyclone operating at high pressure and solid loading with available models. Finally, we developed new process instrumentation to record local values of the solid volume fraction in high-temperature industrial vessels and we produced an exhaustive state-of-the-art review of experimental techniques for dense gas-solid flows.
Summary
In the particle technology, it is fundamentally important to know the interaction and adhesive forces between particles and to find the correlation of those forces with the microscopic characteristics of particle surface, because these forces are the origin of many phenomena which particles exhibit in industrial processes. The aim of this project is to clarify in-situ on the molecular level the microstructures of surfaces in various solutions of industrial importance and their correlation with interaction and adhesive forces between surfaces, using not only an atomic force microscope (AFM) but also computer simulations.
The solutions employed here were classified into to two kinds of large categories: aqueous solutions and non-aqueous solutions. The investigations were conducted step by step, picking up the subjects given in Fig.1, successively. The abstracts for each subject obtained in this study are listed below as well as the reports of this year. We consider that the major important mechanisms for the correlations between microscopic characteristics of particle surface and their interaction and adhesive forces in solutions are clarified at least qualitatively. When the details of the contents are needed, they are given in the previous reports and in the papers listed at the abstracts.
EXECUTIVE SUMMARY
With the conclusion of this project, my students and I wish to thank the IFPRI member companies for their generous support and invaluable discussions during the past several years. The work summarized below, triggered by the invitation by IFPRI to submit a proposal for work in the nanorheology area, has been perhaps the most productive period during my research career.
Goals of this project
The objective of this project was systematic understanding of particle-particle nanorheology based on the single particle-particle contact of two atomically-smooth solid surfaces in molecularly-thin proximity. The main relevance was to understand the origins of suspension rheology, especially the origins of rheological anomalies that arise when interfacial films between two solid bodies are so thin that the intuition of what to expect based on bulk rheology no longer applies. Based on this understanding, we sought to develop new methods to control and manipulate the properties of their interfacial films. The premise for this work was the conviction that progress in understanding fine powder applications is impeded by difficulties in separating the overall rheology of a macroscopic-sized sample into various mechanistic subprocesses. Much is known about interparticle forces (van der Waals, electrostatic, hydrogen bonding, capillary, steric, etc.) and the information obtained in rheology experiments has often been interpreted in these terms. This project took the different approach of seeking to understand rate-dependent influences of nanorheological response. We were concerned with the rate-dependence of nanorheological responses not just in shear but also in adhesive mode.
EXECUTIVE SUMMARY
An experimental and theoretical study of the agglomeration phenomenon which causes destabilization of certain low and high temperature fluidized beds was performed. A theoretical model was proposed to determine the conditions under which defluidization occurs in fluidized beds in which cohesion forces between granules arise due to the presence of sticky fluids and/or high temperatures. Bonding mechanisms between particles such as solid-liquid bridges, viscoelastic flattening and high temperature sintering were all considered. The model, which predicts breakup of aggregates by bubble motion, was compared to limiting fluidization-defluidization (quenching) experiments performed by the authors and others. An experimental method to measure surface softening of small particles heated to high temperatures was developed by using a dilatometer to measure the surface viscosity of the particles from rate of deformation data. Experimental methods to determine the minimum sintering temperatures of a variety of granules were also presented. Lastly, experiments were performed to study the dynamic strength of a liquid bridge between two spheres coated with a liquid and moving away from one another, It was shown that the strength of the dynamic bridge was at least one order of magnitude larger than the corresponding strength of the static bridge between the two spheres. This result accounts for the relatively high gas velocities necessary to keep a bed of sticky particles in continuous fluidization.
EXECUTIVE SUMMARY
Removal of fines and ultrafines from sludges and such colloidal suspensions is often not easily achieved without flocculation using polymers. Even though the effectiveness of the polymers have been speculated in the past to depend on their conformation / orientation on the particles, there existed no information on the optimum conformation for flocculation nor reliable in-situ technique to directly monitor it. In this work, with the aim to improve quality of fines removal from liquids, conformation of polymers was investigated along with sedimentation and clarification. In addition, with a view to identify optimum flocc structures for solid-liquid separation and the mechanisms of flocc formation, structural characterization was done along with theoretical modelling of aggregation.
In order to identify methods to manipulate the adsorbed flocculent species for good flocculation, it was necessary first to identify their conformational behavior. For this purpose, we used fluorescence techniques and polymers with luminescent labels and compared their conformational behavior on the particles in suspension with its flocculation/sedimentation response. This approach led successfully to the conclusion that polymer conformation can be altered drastically by shifting the pH of the system or by adding other polymers or inorganics that can complex with it.
Characterization of floccs is a major hurdle in flocculation studies and in this regard we have adapted CAT SCAN techniques for characterizing sedimentation processes and flocc structure. Also, a Monte Carlo model was developed and applied successfully to simulate sedimentation of fine particles by considering sedimentation as a result of competition between gravitation and Brownian motion. In computer simulation of aggregation, the diffusion-limited aggregation model has been modified to generate, for the first time, realistic sparse as well as dense floccs by varying the step size of random walks.
A high resolution electrostatic probe has been designed and constructed. This laboratory system is capable of detecting charge on individual particles I it has been demonstrated to work and is able to detect bipolar charging. In the cohesion and adhesion of powders electrostatic forces play a significant role. This system therefore can be used to predict such properties of powders.
The probe has been used to measure charge on PVC particles of diameter down to 50um. The powder was processed by several different methods and the charge generated on the particles measured. Bothmonppolar and bipolar charging was observed.
Further developments on the system are necessary before it can be used in industrial systems to evaluate powder flow properties. The following work will enable a comprehensive instrument to be constructed:
- Improvements on the existing system such as increase in spatial resolution, incorporation of a micro-processor to analyse data and motorise movements of an array of probes.
- Application of probe in a laboratory powder flow system.
- Tests on a full scale pneumatic conveying system.
The final instrument will utilise electrostatic charging phenomena that is inevitably present in almost all powder handling processes to measure such flow parameters as velocity, cohesion, particle size distribution etc.