Powder Flow

The fluidised dense-phase (FDP) conveying of powders and low-velocity slug-flow (LVSF) of granular bulk solids are the most common and popular modes of dense-phase used in industry. However, the accurate prediction of conveying performance still is not possible from first principles and relies heavily on empiricism.

The main aim of this project is to develop the necessary understanding, databases, guidelines and models for the purpose of predicting accurate optimal operating conditions for the two modes of dense-phase. However, as was mentioned in the original research grant application, it was unlikely that both the FDP and LVSF sections could be completed thoroughly in a single 3-year period (ie due to the amount of work involved). Hence, top priority was given initially to the LVSF section of the project, although some progress was made also with the FDP section of work. However, with the 3-year extension to the research grant, a substantial amount of work now can be completed in the FDP section, as well as completing particular outstanding issues in the LVSF section.

Several difficulties were encountered during the course of the first 3 years of the project (eg unexpected results and phenomena) and delayed progress in certain areas. In some cases, it was not possible to complete particular scheduled tasks (eg testing aluminium and mild steel pipes with a wide range of granular solids). In other cases, it was necessary to pursue new research issues (eg rotary valve air leakage, new pipe friction and stress transmission testers). However, in terms of achieving the main goals, there is no doubt that the project has been successful in terms of improved understanding and the development of new databases and models for the prediction of LVSF performance. For example, the new transport boundary and pressure drop models have been found quite accurate for the poly pellet type materials tested to date and also have been able to explain some of the interesting and unexpected phenomena encountered during the experimental stages of the project.

Unfortunately, due to the various problems and delays to date, as well as the new discoveries and developments, the numerous pipe wall materials and bulk solids planned originally for the LVSF section were not able not been tested, preventing further confirmation of model accuracy and validity. Additional work is planned over the next 12 months for this purpose (eg testing at least one other granular material with properties different to the poly pellets).

A significant amount of additional time will be needed for the expected relatively more complex FDP section of work. For example, only one product and a few different pipelines were able to be tested by the end of the initial 3-year period. The 3-year extension will allow this to be pursued in greater detail (eg with other powders and pipeline configurations), as well as the commencement of investigations into modelling techniques.

This Annual Report summarises the research progress and major achievements to date, as well the forward plan for the next 12 months.

The aim of this Lancaster University-Bradford University collaborative project was to understand the forces between a variety of dry materials at the single-particle level, to relate these to complementary bulk powder flow measurements, and hence to assess how far such-single particle data are able to predict flow behaviour of real value to chemical engineers. In particular, the objectives may be summarised as follows:

• At Lancaster, to investigate the forces acting between single dry particles in simple model systems, using atomic force microscope technology;
• to acquire a force-curve data bank, using mostly materials already well studied in bulk cohesion testers or of particular interest to IFPRI members;
• at Bradford and elsewhere, to standardise the bulk cohesion measurements in an annular shear cell, and to compare the results with those obtained using a variety of other testers;
• at Bradford and Lancaster, to clarify the role of particle size and morphology, relative humidity and powder-wall adhesion effects, in the bulk flow behaviour of cohesive powders.

For the single-particle work it was necessary to design and construct the required humidity control system within which the atomic force microscope could be operated. Much of the data took the form of normal force as a function of separation between the two surfaces. In addition we broke new ground in devising a reliable method of measuring lateral force (friction) at the single-particle level. To help confirm the basis of theoretical interpretation, simple model systems were studied first, followed by cohesive powders of current interest. Values of pull-off force were surprisingly similar for a range of materials, and strong humidity-dependence was the exception rather than the rule. In the case of alumina, no particle size effects were apparent, in contrast with cohesion test results. Surface treatments of glass or silica-based materials produced clear differences, but not in the case of titania. Clear increases in adhesion were seen for particle-wall contacts in alumina and limestone (in agreement with suggestions from cohesion test results), but not with most of the other materials studied.

The data obtained by means of our new single-particle friction technique gave a wealth of information on linear and non-linear load-dependence, the nature of the inter-particle contact (single- or multi-asperity), the occurrence of stick-slip behaviour, and the relevance or otherwise of adhesion in determining friction. In general, the friction technique was considerably more effective in detecting differences in the behaviour of different materials than the normal force curve technique.

The Warren Spring-Bradford Cohesion Tester – an annular shear tester - was selected to measure the bulk cohesion of the selected powders. Further study of the topic of bulk powder testers, however, revealed not only that many shear testers are available today but also that the design, measurement procedure and interpretation are topics of great discussion and controversy. For this reason, the programme of bulk powder measurement (unconfined yield strength versus major consolidation stress) were extended to form an experimental study of three different shear testers for measuring the flow properties of bulk solids. A large amount of flowability and cohesion data has enabled us to present a rigorous qualitative comparison of five different types of testing device.

We have attempted a quantitative, if somewhat over-simplified, link between the single-particle and bulk studies. This involves, in the first instance, the simulation of yield loci, making allowance for particle size effects in any approximation of average force per particle in the bulk cohesion experiments. Thus, we have attempted to predict, from single particle normal and friction forces, the results of bulk experiments by suitable scaling, and compare them with the actual bulk data. This has involved deriving a suitable model, and has enabled us to determine the limitations and advantages of the two contrasting testing techniques (atomic force microscope-based and bulk). The single-particle AFM technique is good for the rapid screening of many powders for sensitivity to humidity, and appears to be well suited to studying particle-wall friction: this is important in powder flow where the internal cohesion of the powder is high in comparison with its adhesion to walls (leading to “plug flow”). The roughness studies possible with the AFM are also highly relevant to wall friction and the internal cohesion of powders. However, with the bulk cohesion, particle size effects and consolidation effects, the AFM fails to see many phenomena of interest to chemical engineers simply because they appear to be controlled overwhelmingly by particle geometry rather than the adhesion of single contacts. The single-particle type of experiment is found to explore only the first part of the corresponding bulk experiment, near the origin of the data plots. By plotting the bulk cohesion data as average force/particle rather than as force/unit area, most of the differences between the two types of data disappear. For the finest particles (e.g. 2-4 µm limestone), the average forces per particle coincide between the two types of experiment. Fortunately, many fine powders of interest have particle sizes in the range where overlap occurs and interesting comparisons can be made. Thus, the two experimental approaches appear to be converging at small particle sizes and low consolidation loads, just where we might expect the contacts between individual particles in both studies to be single- or few-asperity contacts.

In the pull-off experiments with the AFM, a particle (or group of particles) is pulled away from another particle or group, so that the results are analogous to experiments in which the tensile strength of the powder is measured or deduced. Analysis of the results suggest that the pull-off force is a material, not a particle, property. To explain why the bulk tensile strength falls much less steeply with particle size than expected by simple scaling arguments, it is necessary to assume that the larger particles exhibit multi-asperity contacts. In general, it may be true that friction is more relevant to the particle-wall interface, and adhesion more relevant to the internal shear or cohesion of powders.

The fluidised dense-phase (FDP) conveying of powders and low-velocity slug-flow (LVSF) of granular bulk solids are the most common and popular modes of dense-phase used in industry. However, the accurate prediction of conveying performance still is not possible from first principles and relies heavily on empiricism.

The main aim of this project is to develop the necessary understanding, databases, guidelines and models for the purpose of predicting accurate optimal operating conditions for the two modes of dense-phase. However, as was mentioned in the original research grant application, it was unlikely that both the FDP and LVSF sections could be completed thoroughly in a single 3-year period (i.e. due to the amount of work involved). Hence, top priority was given initially to the LVSF section of the project, although some progress was also made with the FDP section of work. However, with the 3-year extension to the research grant a substantial amount of work can now be completed in the FDP section, as well as tying off some loose ends from the LVSF section.

Several difficulties have been encountered during the course of the project (e.g. unexpected results and phenomena) and have delayed progress in various areas. In some cases, it was not possible to complete certain scheduled tasks (e.g. testing aluminium and mild steel pipe and wide range of granular solids). In other cases, it was necessary to pursue new work (e.g. rotary valve air leakage, new pipe friction and stress transmission testers). However, in terms of achieving the main goals, there is no doubt that the project will be successful in terms of improved understanding and the development of new databases and models for the prediction of LVSF performance. Unfortunately, due to the various problems and delays to date, the full range of pipe wall materials and bulk solids will not be able to be tested – such work is necessary to confirm the accuracy and validity of the new models (e.g. majority of work to date has concentrated on poly pellets). Also, a significant amount of additional time will be needed for the relatively more complex FDP section of work (e.g. only one product and a few different pipelines were able to be tested by the end of the initial 3-year period). The 3-year extension is allowing a more concentrated effort in this area, as evidenced by the extensive testing completed over the past 12 months.

This Annual Report summarises the research progress and major achievements to date, as well the forward plan for the next 12 months.

Based on the Fokker-Planck-Equation a new model was developed to calculate the reduction of periodic concentration fluctuations entering a continuous mixer. The calculations and the experimental results show:

1. The average residence time normalized with the period length of the entering mass flow fluctuations is the main influencing parameter of the mixing quality.
2. The mixing process can be described very well with a system of Fokker-Planck-Equations.

By the development of an air-cleanable cowl for the Near Infrared (NIR) Spectrometer VECTOR 22/N it is possible to determine in-line the mixing quality at the end of the continuous mixer every 1.7 sec. The NIR Spectrometer can be used for the analysis of any powder mixture, the only requirement is that the tracer component contains C-H-, O-H- or N-H-groups which absorb the NIR radiation. Because the analysis results can only be as good as the calibration results the calibration was optimised and an apparatus for dust free, easier and faster calibration of the NIR Spectrometer was developed.

For calcium carbonate with an average diameter of 2 μm it could have been shown that the pulsation of a volumetric feeder with a single proportioning device can be reduced tremendously by the use of different dosing tube attachments. The best results were obtained with a self-developed rotating star attachment. The pulsations were reduced to a tenth of the pulsations of a standard dosing tube.

EXECUTIVE SUMMARY

This project’s objective is to bring unique experimental insight to the detailed interactions between a gas and dispersed particles. By informing recent theories for those interactions, this work will benefit a wide array of industrial processes involving gas-solid suspensions.

The research is made possible by our development of an axisymmetric Couette cell producing shearing flows of gas and agitated solids in the absence of gravitational accelerations (Fig. 1). The facility will permit gas-particle interactions to be studied over a range of conditions where the suspension is steady and fully-developed.

Unlike Earth-bound flows where the gas velocity must be set to a value large enough to defeat the weight of particles, the duration and quality of microgravity on the Space Station will permit us to achieve suspensions where the agitation of the particles and the gas flow can be controlled independently by adjusting the gas pressure gradient along the flow and the relative motion of the boundaries.

We will carry out two series of experiments in space, due to take place in 2007. In the first series, which we call “viscous dissipation experiments,” we will characterize the viscous dissipation of the energy of the particle velocity fluctuations, when there is no relative mean velocity between gas and solids. To do so, we will reduce the boundary speed in successive tests until the inertia of the solid particles becomes small enough for the particle motion to be affected by viscous forces in the gas. By evacuating the cell partially, we will also investigate the role of the molecular mean free path in dissipating the particle agitation.

In a second series of tests, which we call “viscous drag experiments,” we will impose a gas pressure gradient on the shearing cell sketched in Fig. 1. The gradient will induce a relative velocity between the two phases, while the shearing will set the solids agitation independently. These Viscous Drag Experiments will be unique in exploring a regime where particle velocity fluctuations are determined by a mechanism other than interactions with the gas. In this regime, we will measure the dependence of the drag coefficient on the solid volume fraction and agitation of the solid particles. Partially evacuation will also allow us to test the effects of particle Reynolds number on the drag coefficient.

In June 2000, this project passed the crucial “Science Concept Review,” where a panel of scientists evaluated the feasibility and importance of our investigation. This significant milestone strengthened NASA’s commitment toward our experiments.

In this final year of the IFPRI grant, we tested the prototype shear cell on the KC-135 microgravity aircraft and on the ground. On the aircraft, we demonstrated the accuracy of our capacitance probe system to record solid volume fractions; we obtained a large data base of digital images with metal and ceramic spheres that will be used to develop further the computer vision software; and we gained confidence in the ability to design the experimental system. The tests on the ground allowed us to demonstrate the measurement of the mean volume flow rate using an isokinetic section of the channel, and their data verified the accuracy of our gas-solid theory. This year, we also continued to support NASA’s development of the granular flow module that will run our experiment on the Space Station. Because the term of this project is longer than the three years of the IFPRI grant, we have not yet completed all experiments, which will await launch to the International Space Station.

However, we have already achieved the following:

• We have specified the conditions of all tests (ARR 30-06).
• We have developed theories to predict their outcome based upon best available knowledge of drag coefficients and constitutive relations (ARR 30-06 and ARR 30-07).
• We have developed computer vision software to measure the velocity of the solids (ARR 30-06); a new capacitance probe system for recording solid volume fractions (ARR 30-07); a new isokinetic technique to evaluate the mean gas flow rates (ARR 30-07); and we have proposed a tracer technique to measure the velocity of the gas (ARR 30-06).
• We have manufactured a prototype of the cell, which we tested this year on the KC-135 microgravity aircraft and in the laboratory, thus demonstrating feasibility of the experiments.

This Final Report summarizes our progress to date, with some detail on this year’s activities, and it includes all papers written on this research during the grant period in the Appendix.

SUMMARY

The fluidised dense-phase (FDP) conveying of powders and low-velocity slug-flow (LVSF) of granular bulk solids are the most common and popular modes of dense-phase used in industry. However, the accurate prediction of conveying performance still is not possible from first principles and relies heavily on empiricism.

The main aim of this project is to develop the necessary understanding, databases, guidelines and models for the purpose of predicting accurate optimal operating conditions for the two modes of dense-phase. However, as was mentioned in the original research grant application, it was unlikely that both the FDP and LVSF sections could be completed thoroughly in a single 3-year period (i.e. due to the amount of work involved). Hence, top priority was given initially to the LVSF section of the project, although some progress was also made with the FDP section of work. However, with the 3-year extension to the research grant a substantial amount of work can now be completed in the FDP section, as well as tying off some loose ends from the LVSF section.

Several difficulties have been encountered during the course of the project (e.g. unexpected results and phenomena) and have delayed progress in various areas. In some cases, it was not possible to complete certain scheduled tasks (e.g. testing aluminium and mild steel pipe and wide range of granular solids). In other cases, it was necessary to pursue new work (e.g. rotary valve air leakage, new pipe friction and stress transmission testers). However, in terms of achieving the main goals, there is no doubt that the project will be successful in terms of improved understanding and the development of new databases and models for the prediction of LVSF performance. Unfortunately, due to the various problems and delays to date, the full range of pipe wall materials and bulk solids will not be able to be tested – such work is necessary to confirm the accuracy and validity of the new models (e.g. majority of work to date has concentrated on poly pellets). Also, a significant amount of additional time will be needed for the relatively more complex FDP section of work (e.g. only one product and a few different pipelines were able to be tested by the end of the initial 3-year period). The 3-year extension is allowing a more concentrated effort in this area, as evidenced by the extensive testing completed over the past 12 months.

This Annual Report summarises the research progress and major achievements to date, as well the forward plan for the next 12 months.

Executive Summary

A review of the literature relating to flow aids, their application and their testing has been undertaken with the objective of improving both the speed and efficiency of the selection procedure.

The selection of a suitable flow aid is comparable to arranging a marriage and potentially just as difficult. Finding a legal and lasting coupling is always hard but in the case of flow aids it is traditionally complicated by being a last minute arrangement when the host powder can no longer flow on its own.

A dossier of commercially available aids and their properties has been compiled and successful instances of both historical and current liaisons delivered. Such a record is useful but cannot provide a guide to all future relationships.

A more fundamental selection procedure requires a careful knowledge of the tendency of the partners to form a stable bond together while at the same time moving freely within a powder society. The complex interplay of the forces between the flow aid and the bulk powder has been documented and needs to be understood if a correct choice of aid is to be made.

Current developments put an additional demand on the flow aid partner. When added to a multi-component mixture the aid has the potential to displace other active ingredients and damage mixture quality. Pre-mixing or sequential addition may then be necessary. 'Add-on' qualities are also in demand with the aid being asked not only to promote flow but also to add some colour, a perfume or some additional quality to the bulk mixture. Such demands add complexity to the selection procedure but give added value to the flow aids having the required flexibility of application.

The effectiveness of a flow aid addition to a process will almost certainly require practical assessment and suitable methods are reviewed as are methods of conditioning the bulk powder.

In the interests of better marriage guidance a logic structure is proposed which will sequentially pose questions about the host powder, the flow aid and the method of testing.

Executive Summary

The main aim of this project was to develop the necessary understanding, databases, guidelines and models for the purpose of predicting accurate optimal operating conditions for two modes of dense-phase conveying: low-velocity slug-flow (LVSF) of granules and fluidised dense-phase (FDP) of powders. During the first 3 years of the project, top priority was given initially to LVSF, although some progress also was made with the FDP section of work. The second 3-year period of the project allowed a substantial amount of work to be completed in the FDP section, as well as some interesting investigations emanating from the LVSF work.

Several difficulties were encountered during the course of the project (e.g. unexpected results and phenomena) and these delayed progress in various areas. In some cases, it was not possible to complete certain planned tasks (e.g. testing different pipe wall materials for the LVSF section and also a wide range of bulk solids for the LVSF and FDP sections, further testing and modelling for the FDP section). In other cases, it was necessary to pursue new research activities (e.g. rotary valve air leakage, new pipe wall friction and stress transmission testers, comparisons with other FDP models). However, in terms of achieving the overall goals, there is no doubt that the 6-year project was successful, as measured by the following achievements:

• improved understanding of both LVSF and FDP;
• development of new and comprehensive databases for both LVSF and FDP;
• development of guidelines and new fundamental models for the prediction of LVSF performance (e.g. horizontal pipeline pressure drop, unstable boundaries);
• evaluation of and development of guidelines for FDP performance (e.g. minimum transport conditions, empirical modelling of powder flow frictional properties, scaleup accuracy of new and existing models);
• as well as some new and exciting (additional) developments (e.g. new particle/bulk property testers; novel comparisons of blow tank and high-pressure rotary valve feeder performance for LVSF; new on-line instrumentation to monitor LVSF performance; evaluating the scale-up stability and accuracy of existing FDP models).

1 Introduction

The Powder Flow Working Group was charged by IFPRI to produce a review of research challenges on the flow of powders. It includes academics (Robert Behringer, Michel Louge, James McElwaine, Robert Pfeffer, Sankaran Sundaresan, and Jeorg Schwedes), industrial advisors (Karl Jacob, Thomas Halsey, James Michaels, and Paul Mort), and IFPRI officials (Nikolaas de Jaeger, Roger Place). The group held a meeting on October 21 and 22, 2004 in Newark, NJ to kick-off the activity. Triantafillos (Lakis) Mountziaris represented the NSF.

The group produced two principal deliverables. First, it wrote a series of short articles that are collected in this report. Second, for the NSF’s benefit, it outlined broad research directions that the agency should consider to advance knowledge on powder flow that is relevant to industrial problems.

This report is organized from the small scale to the flow scale. Its articles summarize a state of understanding and recommend further work. We begin with considerations of interparticle forces; we then discuss the role of compressibility and cohesion in setting bulk properties; we focus on stresses; we explore similarity and scaling; we sketch numerical techniques; and we delineate regimes of granular flows. We close by discussing chute flows and by considering effects of the interstitial fluid.

The present research is oriented towards the grand challenge to understand general powder dynamics and the ultimate goal of the work is to develop a quantitative description of active flows of fine powders. The study is centered on the “intermediate” regime of flow where both frictional and inertial effects are important and where fluctuations of strain rate and stress are significant. The main application is in the area of small-size, rough and/or cohesive powders that are industrially relevant. The novelty of the project is to study a relatively large range of materials and flow geometries to gain meaningful insight as opposed to limiting the work to a single “relevant” flow device using a “standard” powder. Extensive previous work mainly in the Physics literature on smooth glass beads and hard, metal spheres have done little to shed light on the behavior of industrially important powders that are usually non-spherical, rough, fine, cohesive and compressible.

We report on a series of materials from simple (round beds) to complex (fine, odd-shaped and slightly cohesive), used in a set of judiciously chosen equipment with interchangeable boundary conditions and measure stresses and stress fluctuations as a function of geometry and shear rate. The goal is to develop constitutive equations for powder flows and to use them in continuum-type theoretical models to predict flow patterns, velocity distributions and forces on boundaries such as stationary walls and moving pedals.

The approach follows the earlier work of Tardos, (1997), Tardos et al., (2003), and Tardos and Mort, (2005) and applies it to the geometry of the Couette device and to more complex flows such as hopper (funnel) and centripetal (“spheronizer” and high-shear mixer) flows that are relevant to storing, feeding, mixing and granulating powders.