SAR - Review

Introduction

Upon IFPRI's request, this report contains a summary of the experimental and theoretical investigations on fracture and comminution at the Institute fiir Mechanische Verfahrenstechnik und Mechanik der Universitdt Karlsruhe.

In 1957, when Bans Rumpf started to build up the institute, comminution was a well established domain in engineering. Rumpf then had the idea to combine experience in engineering with the scientific methods of physics. This was the basis for a long period of successful research, lasting longer than Rumpf's life, which ended in 1976. In spite of comminution being only one of several quite different research activities in the institute, from 1957 to 1989 totaling 222 papers had been published on fracture and comminution. Most of the papers, however, are written in German and therefore widely unknown in Anglo-American countries.

Some of Rumpfs numerous PhD students were physicists. The first of them, Klaus Schbnert, initially assisted to realize Rumpfs ideas of a comminution science. Soon he became head of the fracture and comminution group which he directed for more than two decades. Under his influence most of the research in this field has been carried out at Karlsruhe. Since. 1981 Schijnert directs the Institute of Mineral Processing of the University of Clausthal.

To write this report, Klaus Schonert would have been the most appropriate person. Without his ideas and critic discussions, many of the highly complicated experiments in fragmentation and single particle comminution would not have been performed and many of the theoretical and experimental results would not have been found. It is difficult to distinguish today, which of some new ideas in fracture and comminution research were Rumpfs or Schdnerts ideas or the ideas of PhD students. Probably such a differentiation is unimportant. The period of successful research was not only the result of new ideas but even more a consequence of stimulation and leadership. Both, Rumpf and Schdnert highly motivated their co-workers.

Klaus Schlinert was not available, to write this report. The publications are available, however, and since I am knowing nearly all authors personally - some still from my time as undergraduate student - I have taken the risk to follow IFPRI's invitation to review three decades of comminution research in Karlsruhe.

This report is not a treatise on comminution. It will not describe and discuss the details of the various research projects. The objective of this review is the transfer of information about the activities of the comminution group at Karisruhe and their experimental and theoretical results. Detailed information can be found in the individual 222 papers.

In addition to the review, IFPRI wanted answers to some questions as well as recommendations for future research in comminution. These items are appended in this report.

Scope of the Report

This report covers comminution by impact as obtained if solid particles moving freely in a static or moving gas, generally air, impinge or collide with one another or with grinding elements in an impact grinding machine.

H. Rumpf /l.l/ distinguished between four types of stress:

Therefore only the second type of stress (II) describes the type of impact grinding covered in this report. With this type of stress the energy available for comminution is defined by the kinetic energy of the particle immediately before impact, which includes its mass and the relative velocity between the particle and a second body, usually taken to be of infinite mass. The final result, for example the surface area produced during comminution, is independent of the fact, whether the relative movement is mainly obtained by a high speed second body or grinding element and a slowly moving particle, or a particle impinging at high speed on the surface of a stationary solid object. Depending on the angle of approach between the two bodies and the relative movement of their centres of gravity, impact and friction dependent attrition occur simultaneously. The velocities of impact applied vary between ten and several hundred meters per second.

The word "impact" is used in the Anglo-Saxon literature also for a special kind of the first type of stress (I). If stress is applied at a comparatively low rate between the surface of two rigid bodies one defines this type of stress as: low compression /1.2, 1.3/. If however, the grinding mechanism of stamp or ball mills had to be evaluated, for example with single particle crushing tests, different so-called "impact-tests" were used, which consisted of:

• drop weight methods, and
• pendulum impact devices.

A considerable number of different test machines have been developed and an enormous amount of data published, most of which seem to have escaped present knowledge. In order to distinguish these "impact-tests" from tests performed under type of stress II, they have often been called "double impact-tests", because two solid surfaces contact the particle to be comminuted during stress loading. The main difference to impact grinding (type of stress II) has therefore to be attributed to the following facts:

1. comminution takes place with the stress being applied between two solid surfaces, and
2. the energy available for comminution depends on the mass of the drop or pendulum weights and their height of fall.

Results obtained under these circumstances are not relevant in impact grinding as described in this report. Information on drop weight and pendulum comminution tests for a variety of materials may be obtained from references /1.2 - 1.16/.

The present report covers therefore those impact grinding processes where particles moving freely in a static or moving gas, impinge or collide with one another or with so-called grinding elements or targets ‘of' much higher mass.

The report deals in chapter 2 with basic theoretical considerations, for example the exchange of energy during impact, the frequency of occurrence of direct central impacts, the maximum stress applied during impact etc.

Chapter 3 covers the test machines and results obtained in single particle impact.

Chapter 4 concentrates on the design and the performance of different impact grinding machines, for example: high speed pin and hammer mills, and fluid energy mills.

Summary

An analysis of the stress distribution near the corners of particles loaded at that corner is presented and this is seen to be a basis for the rational treatment of attrition. Incorporation of a fracture criterion enables the effect of corner angle on loads to cause local chipping to be seen. Attrition is taken to be the result of such chipping. Fracture through the bulk of the material is thought to be unlikely and that the fracture reported by Vervoorn does not always conform to the definition presented here. Rather it is seen as a larger scale but still local effect. An explanation for some of Vervoorn’s results is offered but further work needs to be done to extend the explanations.

The significance of plastic deformation is briefly considered and the origin and importance of residual stresses is discussed.

In order to study the mechanisms of fines ejection at the bed surface and the mixing of fines, two sets of experiments were carried out.

Cracking catalyst, mean size 58 um, was continuously injected into a bed containing sand (450 um). The axial and radial fines concentration in the bed and the elutriation rate were measured. If diffusional mixing process is operative, then for continuous injection of fines to the bottom of the bed, the fines concentration would be higher at the bottom and lower at the top. However, experiment showed an inverse concentration gradient of fines. An expression is developed to predict this profile by assuming that the fines migrate upwards in the bubble wake and downwards by eddy diffusion. The radial concentration profile is zero. Size analysis of the fines in the bed indicated the existence of fines stratification according to size.

Single bubble injection experiments were carried out by injecting a single bubble into a bed fluidised at Umfc. The quantity of fines ejected at the bed surface per bubble was determined. The experimental results are compared with the predictions from the bubble wake and the bubble roof ejection mechanisms. Another fines ejection or migration mechanism is suggested - the ejection of fines dispersed &thin the bubble void. This mechanism has not been previously suggested. There is reasonably good agreement between theory and experimental results.

The purpose of this state-of-the-art review is to survey the literature on inter-particle forces in dry powder systems - in particular their origin and their known influence on powder processing operations - and current research that indicates to what extent these forces can be measured directly.

Chapter headings include:

• Introduction and background
• Types of force acting between solid surfaces
• Contact mechanics (deformation associated with adhesive forces and with friction)
• Surface-electrical parameters and forces
• Recommendations for future lans (the potential most fruitful research objectives)

Results of current research lead to a number of relevant conclusions:

1. (i) while macroscopic concepts (plasticity, viscoelastic deformation, etc.) appear in general to hold at the level of a single interaction between two particles, it will become clear that there is an urgent need for more direct force measurements to be made with individual particles, where the variable effects of surface condition and topography are especially hard to predict.
2. (ii) Using proximal-probe technology, it is possible to derive useful quantitative information that relates to contact area and the nano-mechanical and adhesive properties of model particles (atomic force microscope tips) or actual particles.
3. (iii) Relatively little detail is known about changes to the van der Waals interaction that are induced by the presence of an adsorbed layer of permittivity different to that of the material of the underlying particle.
4. (iv) Examples of processes in which inter-particle forces are strongly influenced by surface-electrical properties include electrostatic precipitation, powder coating, fibre filtration, xero raphy, particle collection, and separation by means of triboelectrification. Within the general area of contact electrification, there remain many other complications, unsolved problems, and recent intruiging findings. For instance, van der Waals forces (in experiments with model particles of high curvature) are masked by a longer-range attraction. A “patch charge” model gives the most romising ex lanation and the best fit to the data, but analysis of patch charge effects arising from inhomogeneities in work function is in its infancy. An explosion of activity in proximal probe work on surface-electrical properties of surfaces in general at the nanometre level is taking place. So far, there have been few attempts to extend these techniques to work on actual particles. This now calls out to be done.

In recommending possible forward plans, a final chapter identifies two broad fundamental research objectives as being potentially the most fruitful, and involving measurements at the level of an individual particle-particle contact, but geared to the requirements of powder technology. The two topics recommended are:

• (i) study of energy-dissipative contact processes (as measured, for example, by force curve hysteresis) that underlie dynamic and frictional recesses
• (ii) a proper characterisation of surface-electrical roperties of particles and the factors that influence them. For example, what are the individual sites on the surface of a particle of photoreceptor material, that allow it to retain positive or negative charge? How may polymer powders be designed specifically for electrostatic deposition, with a view to improving the proportion of sufficently charged particles in a particular process? There is always hysteresis in the loading/unloading cycle of an individual particle-particle contact - what is the role of this hysteresis in determining the dynamic and frictional behaviour of particles?

These are just three of the many situations in which selective application of the “individual particle” experimental methods would help to answer specific questions of importance in particulate technology.

Executive Summary

• The object of this paper is to review studies of gas-solid fluidization at elevated temperatures and pressures and to draw conclusions from them that enable reliable extrapolations to be made from one set of operating conditions to another.
• Following a brief introduction the survey begins with the low velocity end of operations in the region between minimum fluidization velocity and minimum bubbling velocity and shows how correlations established at ambient temperature and pressure for these two quantities may be used to calculate their values at superambient conditions. The application of purely hydrodynamic fluid-bed stability criteria to account for the transition from the non-bubbling to the bubbling state is described and compared with the expected effect of interparticle forces on this transition.
• The effects of temperature and pressure on the dynamics of gas bubbles in powders of Groups A, B and D are considered next and areas of uncertainty in current theories of bubble motion are highlighted.
• Correlations for jet penetration are then discussed and recommendations made as to the most reliable of these.
• Circulating fluidized beds (CFB’s) operated at high velocity are then considered and it is shown that many of the observed effects in these systems at superambient conditions can be accounted for in terms of changes in the value of the terminal fall velocity, ut, of the bed particles. The effects of changes in ut on entrainment, elutriation and choking are also considered.
• The effect of increased pressure in enhancing bed-to-surface heat transfer coefficients in beds of Group A powders is shown to be due to the suppression of bubbling while in beds of Group B materials the enhancement is through an increase in the gas convective component of the transfer coefficient. The small amount of work carried out on heat transfer in CFB combustors is reviewed.
• Pressure effects on the combustion of char in bubbling beds are considered in terms of an established two-phase theory model and it is concluded that the increased rate of solids bum-out at high pressures is due to an increase in the value of the local Sherwood number thereby increasing the rate of mass transfer of oxygen to the surface of the burning particle.
• The important question of sintering leading to defluidization at elevated temperatures is then examined and attention drawn to the current lack of broadly based mechanical models to account for and predict the phenomenon.
• The state of the art in the area of scaling relationships is reviewed and it is shown that while the scaling laws for bubbling beds are by now reasonably well established the same is not so for CFB’s indicating a major area for further work. Finally conclusions are drawn and suggestions made for three projects worthy of funding support.

Particle Technology, Inc.(PTI) conducted a project for the International Fine Particle Research Institute(IFPRI) to produce 3 Kg. of spherical transparent powder in the 0.3 to 3 pm size range. Several clear glassy compositions were tried and a Barium Altino Boro Silicate(BABS) composition was chosen because it produced fairly spherical particles, apparently transparent under optical microscope. Three Kg. of this material were produced and size distribution tested at the DuPont Engineering Test Center using a Leeds And Northrup Microtrac instrument. The powder appeared to be in or near the target size range and easily dispersed in water reproducibly as indicated by repeated measurements over several hours duration.

A sample was sent to the AEA Winfrith laboratory and field emission micrographs revealed a gritty composition rather than a homogeneous one, and the particle size distribution as measured by a Malvern instrument was much coarser, however, the dispersion technique used at AEA may be improved to get a better dispersion as indicated by comparison with data from DuPont. The gritty texture continues to be a problem with the BABS powder.

Continued work at PTI indicates that higher processing temperatures and a different starting precursor composition can overcome this problem of the grittines. Several tens of grams of the new powder were prepared and a sample will be sent to AEA for evaluation after completion of our own evaluation.. PTI plans independent scale up of the process and if successful shall make a 3 Kg. sample for inclusion in the Standards Program.

EXECUTIVE SUMMARY

OBJECIVES . . . The objectives of this paper are to give an overview of chaotic time series analysis and to survey studies of application of chaos theory in gas-solids fluidization in order to identify the potentials of chaos theory in improving fluidized bed design and operation.

OVERVIEW . . .

The review begins with a brief introduction to chaos theory and gas-solids fluidization and follows with a discussion of the gamut of practical methods for the analysis of nonlinear, chaotic dynamical systems. It is described that chaos theory is a relatively new branch of science that is being developed as the nonlinear counterpart of classical linear signal processing.

Practical aspects of chaotic time series analysis are then discussed and recommendations are made as to the requirements for the measurement system, the optimum data-acquisition settings and standardized methods of data analysis.

Chaos studies in the field of bubbling and slugging fluidization were initiated about five years ago. These studies are considered next and it is shown that dynamical invariants such as correlation dimension and Kolmogorov entropy are able to quantitatively characterize the fluidized bed’s dynamics. Other analysis techniques such as principal component analysis and chaotic trajectory decomposition are then described and shown to be able to uniquely characterize different modes of dynamical bed behavior. It is found that these studies are merely prelimiiry in the sense that only specific examples are shown while so far only one study gives a more detailed impression about the dependency of chaotic invariants on varying fluidization conditions.

It is shown further that the dynamics of bubbling and slugging beds are spatially extended in the sense that the number of degrees of freedom (dimension) and the level of predictability (entropy) vary with the location in the bed. Novel methods are discussed that quantify this spatio-temporal behavior of fluidized beds in terms of the degree of dynamical coupling between different locations in the bed and of the level of predictability per unit of length.

Since two years also chaos studies in circulating fluidized beds (CFBs) operated at high gas velocity have been carried out and these have shown that Kolmogorov entropy can be used, for example, to quantify the turbulence level of flowing gas-solids suspensions as a function of fluidizing conditions in CFBs. These chaos studies of CFBs have been performed at a wide range of riser diameter and fluidizing conditions.

Two simple one-dimensional, ‘learning’ fluidized bed models that exhibit chaotic behavior are then examined. The first model describes single bubbles and bubble-to-bubble interactions and the second model is based on single particles and particle-to-particle interactions. The latter model is compared with experimental data about the dependence of entropy and dimension on gas velocity and is very well able to qualitatively reproduce the chaotic dependencies that were observed in the measurements.

OUTLOOK AND CONCLUSIONS ...

An outlook is presented on two practical applications of chaos theory in the areas of design and operation of gas-solids fluidized beds.

First, it is proposed to include the chaoticness of fluidized beds in reactor scale-up. Two different routes are considered: the first route, proposes to include chaotic similarity as an additional requirement in dimensionless scaling laws for fluidized beds, and the second route is based on establishing (empirical) scaling correlations that relate the chaoticness of the fluidized bed to bed size, fluidizing conditions and particle properties.

Secondly, it is suggested that the concepts of chaos control that have recently been developed in the literature may also be applied to interactively control the dynamics of fluidization in order to, for example, enhance conversion and selectivity by influencing gas flow and bubble size distributions.

Conclusions are drawn that focus on general observations and suggestions for further work.

SUGGESTED PROJECTS..

Finally outlines are presented for four projects that may be considered for funding by IFPRI. In these outlines it is proposed to support chaos research in the following areas:

• (A) dimensionless scaling of bubbling fluidized beds using the concept of chaotic similarity;
• (B) scaling of bubbling fluidized beds based on empirical correlations that relate chaotic dynamics to bed size and operating conditions;
• (C) chaos analysis of flow regimes in high velocity, circulating fluidized bed;
• (D) development of a chaos control technique for bubbling fluidized beds.

This review provides an overview of the tomographic sensing methods that can be utilized to study particulate behaviour and their particular attributes (speed, sensitivity, robustness etc.). The process benefits arising from such techniques are now becoming recognized -

• Detailed spatial and temporal information of components within process equipment to provide for modelling and model verification purposes (e.g. solid concentration profiles in hydrocyclones, chaos analysis of fluidized beds). Improved process knowledge provides more reliable models and hence greater confidence in scaling of process plant with consequent time and cost savings.
• Characterization of flow and porosity in structure materials (e.g. magnetic resonance imaging of porous solids). In-situ three-dimensional analysis is not available using other instrumental method.
• On-line monitoring of structured materials in a pipe (e.g. microelectrical imaging of emulsions and concentrated dispersions, moisture detection in powder flow). Rapid feedback of information provides new opportunities for control and enhanced efficiency, reduced wastage.

The review provides literature citations and sources of information which potential users may consult. Particular emphasis is placed upon electrical methods, due to their inherent suitability for widespread on-line use. Future possible measurement capabilities (component specificity, mass flow measurement etc.) of the family of electrical methods are discussed and likely dates when the technologies might be sufficiently mature for process use are provided.