The Dynamics of Fine Powders PDF

1 General Introduction NOTATION Ap Surface area of electrodes (m 2 ) C Capacity (F) Cp Capacity of a pair of particles (F) es Capacity of astring of particles (F) H Bed height (m) Ho Packed bed height (m) I Electrical current through powder bed (A) L Distance...

1 General Introduction NOTATION Ap Surface area of electrodes (m 2 ) C Capacity (F) Cp Capacity of a pair of particles (F) es Capacity of astring of particles (F) H Bed height (m) Ho Packed bed height (m) I Electrical current through powder bed (A) L Distance between inserted electrodes (m) N Number ofparallel strings (m- 2 ) M Time between two collisions (s) ~ Vo Potential difference over electrodes (V) ~ Vs Potential difference over string of particles (V) ap Average particle size (m) Electrical current density (A m - 2) is Electrical current during a collision (A m - 2S - 1) Distance between electrodes (m) n Number of particles in astring (-) m N umber of cOllisions per particle per second (s - 1) qn Electrical charge on a particle (C) Veo Superficial gas velocity (m s - 1) 2 THE DYNAMICS OF FINE POWDERS Vmf Superficial gas velo city at minimum fluidization (m s - 1) Vbp Superficial gas velocity at which the first bubbles appear (m S-l) (Xc Maximum tilting angle (-) e Porosity of powder bed (-) Pd Partiele density (kg m - 3) 1.1 POWDERS: WHAT ARE THEY? Powders appear to be an ill-defined group of substances. The scientific literature on powders does not provide any evidence of what is or should be covered by the term, nor can a elear-cut definition be found. In the large international dictionaries such as the Encyclopedia Britannica, the Encyclopedia Americana, 'Webster', etc., a powder is stated to be: (1) matter in a finely divided state: particulate matter; (2) apreparation in the form of fine partieles, especially for medical use; (3) any of various solid explosives (gun powder). It is interesting to note that the French Dictionnaire Encyclo- pedique Quillet gives the following definition: poudre = poussiere, petites particules de terre dessechee, qui se Zevent au moindre vent. Only the Dutch WinkZer Prins Encyclopedie mentions an upper limit of the partiele size of the individual particles, viz. 100 11m. Probably the general conception of a powder is that of a collection of small discrete solid partieles in elose contact with each other, the (empty) space between the partieles being usually filled with gas so that the bulk density of a powder is always considerably lower than the density of the individual partieles. However, this definition also covers a heap of pebbles wh ich no one would call a powder. Apparently a criterion concerning the maximum partiele size should be added. If one considers cement,  GENERAL INTRODUCTION 3 flour, potato starch, cracking catalyst, sand, and gravel, one will probably agree that the first four materials definitely are powders and the last one certainly is not. Whether one would call sand a powder probably depends on the partiele size and on personal Vlews. When the astronaut Neil Armstrong returned to the Earth from his trip on the surface of the Moon, he stated: 'The surface is fine and powdery. I can kick it up loosely with my toe. It does adhere in fine layers like powdered charcoal to the sole and inside of my boots. I only go in a small fraction of an inch, but I can see the footprints of my boots and the treads in the fine sandy partieles.' These words elearly show that the behaviour of powders de- pends on the circumstances. In wh at respects are those on the Moon different from those on the Earth? (1) The gravitational force on the surface of the Moon is only one-sixth of that on the Earth. (2) There is no gas on the Moon. The latter aspect means that any water brought there would evaporate and disappear immediately, hence powders on the Moon will always be perfect1y dry so that cohesion between the separate partieles due to liquid bridges will be zero. On the other hand the cohesion due to Van der Waals forces remains the same and this means that the ratio of the effective cohesion force to the gravitation al force (which in chapter 3 will be called the cohesion number) might not be too different from that of wet sand at the Earth. What Armstrong said about cohesion of the fine sand might point in this direction. The absence of gas on the Moon, of course, also means that friction forces only result from direct contact between par- tieles. The motion of partieles swept up in one way or another is not hindered by viscous friction forces with a surrounding me- dium. The subject of this book is confined to the mechanics and dynamics of fine powders in more or less elose packings (porosity 4 THE DYNAMICS OF FINE POWDERS < O' 7 under terrestrial t circumstances) for which the cohesion number is >0·1. In most cases this means that the average particle size is < 200 Ilm. 1.2 POWDER BEHAVIOUR Powders show a typical and often unpredictable behaviour which is quite different from the behaviour of gases, liquids and solid matter. It has even been suggested that powders constitute a fourth aggregation state. This, however, is nonsense since the thermo- dynamic properties of powders show a linear relations hip with the thermodynamic properties of the component phases: solid and gas. The behaviour of fine powders under terrestrial circumstances is mainly controlled by two mechanisms: (1) the particle-particle interaction when the partieles are in close contact and which results in friction and cohesion between the particles. (2) the solid-gas interaction which is of a two-fold nature: (a) a hydrodynamic interaction by viscous forces, and (b) a physico-chemical interaction via gas adsorption to the solids surface, which at high press ures affects the cohesion forces between the particles. The importance of solid-gas interaction was mentioned earlier by Bruff and Jenike (1967) and by McDougall (1969), in both cases in connection with solids discharge from hoppers. These two papers failed, however, to draw much notice. Generally, powder scientists did not recognize the effect which ambient gas can have on powder operations. On the other hand investigators of fluidization, in most cases, denied the role of particle-particle interaction during fluid- ization on the basis of the assumption that while fluidized the solid particles are free floating. A first paper on this subject (Rietema, tThe term 'terrestrial circumstances' implies that the powder experiences the action of some kind of conservative force (such as the gravitational force) and the presence of a surrounding viscous gas.  GENERAL INTRODUCTION 5 1967) was received with much scepticism. Up till now this has led to the remarkable situation that only incidentally has exchange of experience taken pi ace between the two groups of investigators (Rietema, 1984). One of the most remarkable properties of powders is that they can flow when a certain critical yield stress is exceeded. Many powders even flow quite easily. Before flow sets in from the packed state the powder will expand a little so that the mobility of the separate particles is increased. It is well known that a paste with the same solids concentration but in which the space between the particles is filled with a liquid, has much poorer flow properties than a powder. This is mainly due to the very low compressibility of a liquid which strongly hinders the necessary dilatation of the particle assembly before flow sets in. Another striking property of most powders is their capacity to be fluidized by agas being blown upwards through them at such a rate that they expand. After what has been said above it will be clear that in this fluidized state the higher the expansion, and hence the higher the flow rate of the fluidization gas, the better the powder flows. A third property of powders is that the particles tend to co he re in a more or less dense swarm which can only relatively slowly be diluted. This swarm coherence is partly due to the cohesion between the particles and is partly the consequence of the fact that dilution of the powder must be accompanied by inward penetra- tion of gas which is hindered by viscous forces and hence creates an outwardly directed pressure gradient. Swarm coherence is also the reason why in fluidization at excessive gas rates a major part of the gas rises rapidly as empty gas envelopes, the so-called bubbles. 1.3 POWDERS IN INDUSTRY Powders are very frequently met both in daily life and in industry. Because of their use in all domestic housekeeping (sugar, salt, 6 THE DYNAMICS OF FINE POWDERS flour, instant coffee, washing powder, etc.) everyone is familiar with powders and their great variation in behaviour. Of much greater economic significance is the production of powders in industry. The food industry pro duces many different powders, such as corn starches and potato starches, milk powder and many other spray-dried products. In the pharmaceutical industry many me- dicines are produced in tablets by compression of powders. The ceramics industry makes all kinds of products via sintering after compression in moulds. The electronics industry applies floures- cent and magnetic powders in various widely used products. Electric power plants more and more revert to the burning of powdered coal to produce the necessary thermal energy. The chemical industry, finally, is probably the largest user of powders with its widespread use of catalyst powders, while on the other hand many of the final chemical products, notably plastics and other polymers, are delivered as powders. 1.4 POWDER OPERATIONS From closer examination of the appearance and the behaviour of powders it can easily be concluded that there is no uniform behaviour of powders. This also follows from the numerous operations carried out with powders: - storage of powders in hoppers and bins - transportation of powders from the store to the process appar- atus - grinding or milling of the powder to improve its accessibility for further processing - mixing of different powders to realize a product of higher quality - compression of powders in moulds in order to obtain a pre- formed solid product - drying of powders at the end of processes in which the separate powder particles are precipitated from a wet suspension  GENERAL INTRODUCTION 7 - granulation of powders to obtain larger grains which can be more easily processed - classification of powders in fractions of different average par- ticle size or density - fluidization by blowing gas upwards through a powder bed in order to improve the contact between the powder particles and the fluidization gas, e.g. in chemical processes 1.5 ORIGIN AND PRODUCTION OF POWDERS The origin of powders is mostly of three kinds: (1) occurrence in living nature such as fine plant seeds, pollen, spores, flour, starch, etc.; (2) geological: alluvial sands and sediments, dry clays; (3) industrial production, which is the most frequent source by far. Industrial production can be further subdivided into four cat- egones: (1) the small-scale route, especially for medical purposes; (2) the chemical process route involving chemie al precipitation in the liquid phase followed by concentration and finally drying; (3) the mineral dressing route involving (sea) mining, breaking, crushing, classification or separation, sieving, etc.; (4) metallurgical processes in which powders gene rally are an intermediate product which by compression and sintering are transformed into the final product. The number of different ways of producing powders is surpris- ing. One of the oldest pieces of apparatus used in the production of powders-be it in sm all quantities only-is the well-known mortar. It was mainly used by alchemists and pharmacists. Flour is one of the oldest powders produced by grinding of seeds between millstones. The principle of grinding and milling now- adays is further worked out in a whole variety of modern mach- meso 8 THE DYNAMICS OF FINE POWDERS (a) (b) Fig. 1.1. Photographs of 4 powders: (a) spent cracking catalyst, magnifica- tion 310 x; (b) fresh cracking catalyst, magnification 310 x ;  GENERAL INTRODUCTION 9 (c) (d) Fig. 1.1-contd. (c) polypropylene, magnification 303 x; (d) potato stareh, magnification 860 x. 10 THE DYNAMICS OF FINE POWDERS Quite different products such as milk powder and cracking catalyst are generally obtained by spray-drying. Many modern polymers are produced as a suspension of small particles in water, e.g. by suspension polymerization, which after concentration, coagulation and drying yields a powder. Other products are made by precipitation from a solution and subse- quent filtration and drying. Cement owes its constitution to the burning of limestone and clay at high temperatures. A remarkable powder is potato starch. The individual grains are produced and grow by natural processes in the cells of the potato and have a perfecdy smooth surface (see Fig. 1.1). After grating the potato in special machines the starch grains are disclosed and, finally, washed out by some kind of sink-and-float process. 1.6 ABOUT THIS BOOK This book is mainly confined to the study of fine powders dur- ing their treatment in processes such as mixing, grinding, fluidiza- tion, etc. In all these processes the powder is in constant motion, generally at some degree of expansion. One can only understand the behaviour of a fine powder during treatment by considering the dynamics of the powder that should be conceived as a two-phase system of solid particles and a gas. In these dynamics the interparticle forces playamajor role. Depending on the nature of these interparticle forces (to be discussed in Chapter 4) and on the fluid dynamics of the two-phase system (to be discussed in Chapters 5 and 6), fine powders can be classified in three categories which are mainly based on the behaviour of the powder during fluidization. This classification, however, is also of relevance in other powder operations (to be discussed in Chapter 11). According to Geldart (1973) these three categories are as fol- lows: (1) Category A. Powders that at low gas velocities can be fluidized in a stable expanded state (homogeneous fluidization) but that