Particle Technology and Engineering

An Engineer's Guide to Particles and Powders: Fundamentals and Computational Approaches

Jonathan P.K. Seville, Chuan-Yu Wu

• Publisher: Butterworth-Heinemann
• Year: 2016
• Print ISBN: 9780080983370
• eBook ISBN: 9780080983448
• Topic: Technik & Maschinenbau
• Subtopic: Industrielle & technische Chemie

Particle Technology and Engineering presents the basic knowledge and fundamental concepts that are needed by engineers dealing with particles and powders. The book provides a comprehensive reference and introduction to the topic, ranging from single particle characterization to bulk powder properties, from particle-particle interaction to particle-fluid interaction, from fundamental mechanics to advanced computational mechanics for particle and powder systems.The content focuses on fundamental concepts, mechanistic analysis and computational approaches. The first six chapters present basic information on properties of single particles and powder systems and their characterisation (covering the fundamental characteristics of bulk solids (powders) and building an understanding of density, surface area, porosity, and flow), as well as particle-fluid interactions, gas-solid and liquid-solid systems, with applications in fluidization and pneumatic conveying. The last four chapters have an emphasis on the mechanics of particle and powder systems, including the mechanical behaviour of powder systems during storage and flow, contact mechanics of particles, discrete element methods for modelling particle systems, and finite element methods for analysing powder systems.This thorough guide is beneficial to undergraduates in chemical and other types of engineering, to chemical and process engineers in industry, and early stage researchers. It also provides a reference to experienced researchers on mathematical and mechanistic analysis of particulate systems, and on advanced computational methods.- Provides a simple introduction to core topics in particle technology: characterisation of particles and powders: interaction between particles, gases and liquids; and some useful examples of gas-solid and liquid-solid systems- Introduces the principles and applications of two useful computational approaches: discrete element modelling and finite element modelling- Enables engineers to build their knowledge and skills and to enhance their mechanistic understanding of particulate systems

Chapter 1

Introduction

Abstract

Particle technology is the study of discrete elements—usually solid particles—the way in which they behave in isolation and the way in which they interact to produce a collective effect. The ultimate goal of particle technology is to obtain predictive relationships between individual particle properties and their collective behavior (i.e., behavior in bulk), which can then be used to design formulations, processes, and particulate products. This introductory chapter gives a philosophical overview of the development and applications of particle technology.

Keywords

Collective behavior; Discrete element; Particle technology; Particulate products; Predictive relationship

Particle technology is the study of discrete elements—usually solid particles—the way in which they behave in isolation and the way in which they interact to produce a collective effect (Seville, 2001). The ultimate goal of particle technology is to obtain predictive relationships between individual particle properties and their collective behavior (i.e., behavior in bulk), which can then be used to design formulations, processes, and particulate products. Although much progress has been made toward achieving that goal, there is much still to do!

1.1. What Are Particles?

Particles are endlessly fascinating, both to scientists and engineers and to children who build sand castles on the beach. Particles surround us—they form the foundations beneath our buildings, the soil in which we grow our food, very often that food itself. They are suspended in the air that we breathe and can cause us great damage when we do so. They are extracted from the ground to obtain metals and other mineral products. They make up many of the products we consume and the wastes derived from them and are liberated in the incineration of that waste. It is estimated that over two-thirds of all chemical products are sold as, or have passed through, a particulate form. Much energy is expended in their processing. For example, crushing and grinding of minerals is estimated to use 3–4% of the world's electricity.

The first point to make about particles is the extraordinary range of scales that they occupy (Fig. 1.1)—everything from large molecules to small bricks is generally lumped into this category. The ratio between the size of the earth and the size of a football is roughly the same as that between a football and a typical nanoparticle. As in most branches of human endeavor, it is useful to have some human length scales from which to take reference: the diameter of a human hair is of order 0.1 mm (or 100 μm), while the diameter of a human blood cell is 6–8 μm. An important physical reference from the point of view of experimental observation is the wavelength of visible light—centered at about 0.5 μm.

Figure 1.1 Particles cover a wide range of scales. (Adapted from Seville et al., 1997.)

Size has a profound effect on particle properties. Consider, for example, the family of atmospheric dispersions of water in air: rain (diameter 1 mm–1 cm); drizzle (100 μm–1 mm); and fog (1–100 μm). One obvious difference between them is the effect of gravity: the motion of raindrops is driven by gravity; they fall quickly to the ground, whereas fog droplets remain in dispersion until a rise in temperature or wind movement removes them. Another notable difference concerns the interaction with light: rain is relatively easy to see through; fog is not. Suspended particles of a size close to that of the wavelength of light are strong light-scatterers and reduce visibility. This suggests, of course, a method for measuring the concentration of particles, which is considered in Chapter 3.

Particles are not normally found singly, but in very large numbers, so that the scale of one particle is usually many orders of magnitude smaller than the scale of the container or process in which they sit. A “particulate solid” or “bulk solid” is an assembly of particles that may be surrounded by a continuous fluid phase—a gas (such as air) or a liquid. They may or may not be in contact with each other.

It would make it much easier to predict the behavior of particles if we could consider them as large molecules, for which many analytical predictive approaches have been developed, but this is bound to be an oversimplification. Particle-fluid systems present inherent experimental and theoretical difficulties (Grace, 1986) for which there are no analogies in molecular systems:

• particle shape

• particle size distribution

• surface effects, such as contamination with oxide layers

• regimes of motion, causing profound changes in drag with changes in Reynolds number¹

in addition to secondary effects such as:

• Brownian² motion

• electrostatic charge distributions

Particle systems also display a “memory” of previous processing, resulting, for example, in their sticking together (agglomeration) or breaking apart (attrition), accompanied by what may be a profound change in properties.

The divided state is a constant theme in philosophical discussion—how can we exert our individuality within the collective behavior of society? More to the point, given a large collection of individual people (or particles) how can we predict their collective behavior? The collective behavior of people is the province of economics and politics; particles are a little (but not much) easier to deal with.

Taking this theme further, we are surrounded by a different sort of particle—the information dot: the mark of the ink jet or the laser, the paint particle, the phosphor dot, the light-emitting diode, the tiny units of display which collectively register an effect, provided of course that we are far enough away to make sense of the resulting pattern. There is a long line of artists who have painted in this way, including, most famously, Georges Seurat,³ the impressionist originator of “pointillism,” who based his technique on a scientific study of color analysis and visual perception.

The scientist and natural philosopher Pierre-Simon Laplace⁴ believed that the universe is a predictable machine (like a great clock) and that if only we could calculate the behavior of all the parts, we could predict the future. Isaac Newton⁵ had made this seem possible, by showing that the motion of a set of particularly large particles obeys simple laws. At the same time, Newton knew much about matters at the scale of the wavelength of light and postulated a world of subvisible particles behaving according to analogous sets of laws. We now know, of course, that molecular systems do not behave in this way and their collective behavior can only be predicted in a statistical sense. Famously, Werner Heisenberg⁶ pointed out that at the molecular scale not only can you not predict the future, but also you cannot even know the present exactly.

What has this to do with particles, which are, for the most part, large enough that at least we know where they are, even if we must take a scanning electron microscope to look at them? The answer is that computational particle technology has now reached a critical point: it appears to be able to predict anything, if only the sponsors will buy us large enough computers. Maybe there is no further need for our approximate and semi-empirical methods. Can this be true?

The purpose of this book is to introduce the newcomer to the fundamentals of particle technology and to explain the basic principles behind the emerging computational approaches to the subject.

1.2. What Is Known?

A short answer to this question might be: “Quite a lot about isolated spheres and spheres in contact; much less about the behavior of nonspherical particles and particles in assemblies.”

The problem of the drag on an isolated sphere at low Reynolds numbers was solved exactly in the late nineteenth century by George Stokes⁷—perhaps the most impo...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Chapter 1. Introduction
• Chapter 2. Bulk Solid Characterization
• Chapter 3. Particle Characterization
• Chapter 4. Particles in Fluids
• Chapter 5. Gas–Solid Systems
• Chapter 6. Liquid–Solid Systems
• Chapter 7. Mechanics of Bulk Solids
• Chapter 8. Particle–Particle Interaction
• Chapter 9. Discrete Element Methods
• Chapter 10. Finite Element Modeling
• Index