Powder Flow

We have examined the effects of powder cohesion and particle size distribution on mixing and segregation processes. As agreed in our Year 1 work plan, we have completed the development of experimental and computational procedures for this study and we have conducted an extensive characterization of non-segregating systems in order to provide a baseline for segregating mixtures (to be examined in years 2 and 3 of this project). We have met all of the stipulated milestones, summarized as follows.

1. We have built a computer-controlled lab scale double-cone blender.
2. We have developed and debugged discrete element simulation code for the double-cone blender.
3. We have simulated mixing in the double-cone over the course of up to 12 tumbler revolutions, using 15,000 monodisperse and 13,000 bidisperse particle blends.
4. We have measured the mixing rate as a function of fill level and vessel speed.
5. We have evaluated effects of filling level and vessel speed on mixing and segregation, both experimentally and computationally.
6. We have examined the effects on mixing and segregation of three-way interactions between particle size, fill level and vessel speed.

At this stage of the project, we have found the following.

1. Mixing in the double-cone occurs by a combination of radial-azimuthal convection and axial diffusion. The chief bottleneck to mixing in the double-cone is diffusion acrass the symmetry plane; we have demonstrated that judicious baffle placement can significantly improve mixing.
2. As particle sizes are reduced below about 200mu, steady and regular flow in tumbling blenders gives way to intermittent and chaotic mixing. This results in a dramatic improvement in mixing rates, overwhelmingly exceeding what would be possible by traditional mixing mechanisms. Moreover, this work demonstrates that traditional analysis cannot be applied, even qualitatively, to the study of flow and mixing of fine grains.
3. Several new segregation modes have been identified. We have charted the phase space of these modes, and we have begun an analysis of a preliminary model of segregational mechanisms which seems to show promise for developing a predictive understanding of flow and transport of polydisperse granular mixtures.

The fluidised dense-phase conveying of powders and low-velocity slug-flow 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, duidelincs and models for the purpose of predicting accurate optimal operating conditions for the fwo modes of dense-phase. This Annual Progress Report summarises the project objectives, research progress and major achievements for the period September 1998 to November 1999, and includes objectives for the second year of the project.

It is desired to understand the flow with particle clusters, because particle clustering has a definite effect on transport, phenomena in risers of circulating fluidized beds (CFBs). For example it is known that particle clustering largely increases the particle slip velocity against gas, consequently, the pcarticle residence time in the riser is largely changed from that of an isolated particle. In addition, properties of gas turbulence must be significantly modified due to the clusters. Frorn the view point of application to industrial facilities, macroscopic models for predicting flows in the risers should be developed finally. On the other hand, to develop such models, it is important to study the physics or the dynamics of the clusters from both of the experimental and numerical stand points.

According to this context, Tanaka et al., (1998) have carried out the first, year project. They applied their numerical model, in which inviscid gas and a stochastic particle-particle collision model were assumed (Tanaka et al., 1995), in the three-dimensional flows corresponding to the experiments by Louge et al. (1999). They found that their numerical model is capable of predicting the fully developped cluster flows, and examined the effects of pressurized gas condition on the flow structure. Furthermore, they evaluated the quantities for characterizing the cluster structure, such as number density distributions of cluster diameter, probability density functions of solid volume fraction, etc.

In the second year project, quantitative comparison between the sirnulation and the corresponding experiment at Cornell University was intcndcd. The collaboration between Osaka and Cornell Universities began with discussions at the Brighton Annual Meeting in July 1998. There, Tanaka asd Louge proposed to compare the predictions of the numerical simulations at Osaka with solid volume fraction mcasurcments carried out at Cornell. The Cornell group obtained solid volume fraction with an optical fiber bundle in the fully-developed region of the Cornell riser. The Osaka group then carried out the numerical simulation at the conditions of the Cornell experiments. They then compared the probability density functions and power spectra of the data sets and the corresponding simulations.

In usual industrial applications, particles do not have a uniform diameter but a wide distribution. Tarmka et al. (1995) performed Lagrangian/Eulcrian numerical simulations of two-dimensional cluster flows, and found that the spatial scale of cluster structure largely depends on the particle diameter. Therefore, it is expected that the particle size distribution may affect the flows. The Osaka group has ttaken particular care to match the particle size distribution of the experimental powders.

Tanaka then visited Cornell in March 1999 to prcscnt these comparisons and to discuss details of the simulations with Profs. Louge, Jenkins, Koch and their respcctive collaborators. The status of their collaboration was reported at the Spring TC meeting in Newark, NJ, and at, the Annual Meeting in Somerset, NJ.

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, notably those of Profs. Brady and Koch, this work will benefit a wide array of industrial processes involving gas-solid suspensions.

The research is made possible by our development of a unique experiment producing shearing flows of gas and solids in the absence of gravitational accelerations. The facility will permit gas-particle interactions to be studied over a range of conditions where the suspension is steady and fully-developed. Within that range, we shall characterize the viscous dissipation of the energy of the particle fluctuations and observe the development of localized inhomogeneities that are likely to be associated with the onset of clusters.

We are developing a microgravity flow cell in which to study the interaction of a flowing gas with relatively massive particles that collide with each other and with the moving boundaries of the cell. 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 pressure gradient along the flow and the relative motion of the boundaries.

This first annual report describes the experimental apparatus, outlines a theory and computer simulations to predict the flow, and specifies microgravity requirements for its implementation in Space.

The main aim of this Lancaster University-Bradford University collaborative project is to understand the forces between a variety of dry materials at the single particle level, to relate these to the complimentary bulk powder flow measurements, and hence assess how far such single particle data are able to predict flow behaviour of real value to chemical engineers. In particular, we are interested in (a) the role of ambient conditions such as relative humidity, and properties such as particle size, roughness and surface condition, and (b) the role of both particle-particle interactions and particle-wall interactions, in flow and adhesion behaviour.

The selection of particulate material for recent study has been determined mainly by the following criteria:

1. Availability as particles of controlled shape, size, roughness and surface chemical condition, for use as model systems for adhesion and flow studies;
2. For single particle studies, the extent to which the bulk flow and compaction properties of the material had already been studied in testers at Bradford and in laboratories elsewhere;
3. Suggestions to us from other IFPRI members for materials of particular industrial relevance and interest to the powder flow community.

The details of the force-curve technology, using a commercial AFM, the details of the bulk powder flow experiments, sample preparation, humidity control, and other experimental details have been extensively described in previous IFPRI annual reviews, and will not be repeated here. To achieve the objectives stated above, our work during the first half of this third year (December 1998-May 1999) has fallen into three main areas:

1. At Lancaster, single-particle adhesion studies in well-defined model systems comprising relatively large glass spheres and flat surfaces, and related systems, using the force-distance software available with the Topometrix Explorer AFM.
2. Adhesion studies, using force-distance curves, of more cohesive and finer powders, mainly those whose bulk cohesion properties had already been extensively studied in several testers at Bradford and elsewhere, or powders of particular industrial relevance (zeolite, hydrated alumina). In particular, we hope to relate the effects of particle size, ambient conditions, and powder-wall adhesion noted in the bulk experiments to any difference we see in the single particle or small-scale adhesion experiments.
3. At Bradford and elsewhere, the continued improvement and standardisation of the bulk powder flow experiments in the Warren Spring-Bradford Cohesion Tester (WSBCT) using a small range of well-characterised powders, and in particular by comparative studies with several other testers in different laboratories.

Many details of the work in (1) - (3) above have already been described in our previous annual report (Year 2, ending 14 November 1998) or in the report for the 1999 AGM. Only the main conclusions, and some details not discussed in the Year 2 Report, will be given in this report. Ongoing experimental work on model systems, involving the adhesion between glass spheres and flat surfaces, and fitting the experimental results to theoretical models, is being pursued in collaboration with Dr J A S Cleaver and colleagues in the Department of Chemical and Process Engineering at the University of Surrey. The program of comparative studies of bulk powder flow and cohesion using various testers is now essentially complete and includes work at Albi in the laboratory of Professor J Dodds, using a ring-type shear cell developed by Schwedes. (Once certain software problems have been solved, we plan to include also data obtained at Delft data using the bi-axial cell being developed in Professor B Scarlett’s laboratory). The force curve studies at Lancaster on cohesive powders have been directed recently towards answering questions of more particular relevance to bulk cohesion testing, such as the role of particle-wall adhesion or particle size effects, but we have an ongoing interest in explaining some of the more unusual aspects of the force curves, e.g. humidity effects and long-range forces, in terms of the fundamental interactions involved.

The main innovation since the June 1999 AGM has been the introduction of single- particle frictional force measurements using the AFM to complement the normal adhesion measurements. Frictional forces are just as important to powder flow as these adhesion forces which we have studied exclusively in the first 2 years of the project, but we had not anticipated this major development to any extent in our “forward look” from previous reports. Cohesive materials studied so far include hydrated alumina and a limestone powder used as a standard material to calibrate cohesion testers. In particular, we are attempting to correlate single particle friction- load data with the corresponding shear stress-load plots from bulk cohesion testers. To this end, we have begun discussions with theoreticians at Surrey (Professor M Ghadiri, Dr S J Anthoni, Dr A Sharif) and elsewhere who have performed distinct element analysis to construct models for the statistics of transmission of normal and shear forces through a powder assembly. These models will be essential to provide the necessary links between single particle and bulk cohesion and flow. The current project has been granted a l-year extension, and it is expected that friction and modelling studies of this type will form the bulk of the last year of the project, to November 2000.

In the second year of this project, we have continued research into mixing and segregation behaviors in tumblers and shakers. Among the key results are the following.

1. We have determined that different blenders of industrial design share common dynamical behaviors, including

(a) extremely sharp transitions between segregation states, that are observed across wide length scales.

(b) a cut-off size ratio beyond which segregation mechanisms appear to shut down.

The existence of dynamical similarity across length scales (and in different geometries) opens up the possibility to produce new scaling relations for the practical scale-up of blenders of common design. The presence of a cut-off indicates new prospects for designing products to mitigate segregational tendencies.

2. We have identified previously unreported segregation phenomena due to differential particle density in shakers and have begun study of density-induced segregation in tumblers.

3. We have established that particle shape can have a significant effect on blending behavior for coarse grains. In particular,

(a) particles with different shape but nearly identical size and density segregate in tumblers and shakers. Combined shape- and density- variations produce novel and poorly understood effects;

(b) we have identified a new regime of behavior in which shape-induced segregation is reversible -- i.e. particles of different shape can be either mixed or separated at will.

4. We have developed cellular automata models that successfully predict segregation in rotating drums, rocking drums, drums tumbled end-over-end, and V-blender shells.

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 accur’ate 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 mentioned in the original research grant application, it is unlikely that both the FDP and LVSF sections can be completed thoroughly in a single 3-year period (ie due to the amount of work involved). Hence, top priority has been given initially to the LVSF section of the project, although some progress also has been made with the FDP section of work.

Several difficulties were encountered during the course of the project (eg unexpected results and phenomena) and have delayed progress in various areas. In some cases, it was not possible to complete certain scheduled tasks (eg testing aluminium and mild steel pipe and wide range of granular solids). In other cases, it was necessary to pursue new work (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 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 (eg 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 (eg only one product and a few different pipelines will be able to be tested by the end of the initial 3-year period).

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

We describe specific progress in our understanding of granular segregation in granular and powder processing equipment of industrial importance. We have found several new results of apparent practical importance for the blending of solids.

We have:

• Established that in some applications segregation patterns shut down above a specific particle size ratio. Velocimetry measurements indicate that this cut-off occurs when larger particles sink into the cascading layer, at which point they become unable to overtake their smaller neighbors.
• Determined that segregation in tumbling blenders scales according to two distinct scaling relations depending on tumbling regime.
• Confirmed that small amounts of cohesion are sufficient to significantly inhibit segregation. When cohesion is introduced by increasing the moisture content of a blend, the amount of moisture needed to effectively mitigate segregation decreases with particle size.
• Quantitatively evaluated the effects of intensifier bars on preblending of cohesive blends of common industrial powders.
• Determined that there exist a robust and reproducible set of segregation patterns that can be found across a wide range of blender sizes and geometries and across a wide scale of particle sizes.

These segregational states can actually be accentuated by traditional strategies for improving mixing by introducing cross-flows. Moreover, the dominant segregation pattern seen at high fill levels and tumbling speeds is the pattern exhibiting the most extreme segregation throughout the highest volume of the blend.

• Developed accurate models for the development of segregation patterns based on velocity histories of cascading particles that successfully predict segregation outcomes in a several different classes of 3D blenders. These results are of direct practical importance and call for validation at all feasible scales.

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 a unique 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.

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, notably those of Profs. Brady and Koch, this work will benefit a wide array of industrial processes involving gas-solid suspensions.

The research is made possible by our development of a unique experiment producing shearing flows of gas and solids in the absence of gravitational accelerations. The facility will permit gas-particle interactions to be studied over a range of conditions where the suspension is steady and fully-developed. Within that range, we shall characterize the viscous dissipation of the energy of the particle fluctuations and record the dependence of the mean drag on granular agitation.

We are developing a microgravity flow cell in which to study the interaction of a flowing gas with relatively massive particles that collide with each other and with the moving boundaries of the cell. 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 pressure gradient along the flow and the relative motion of the boundaries.

After a literature review, this second annual report describes the new axisymmetric experimental apparatus that we have designed for this project, and it outlines theories that we will employ for the interpretation of its data.