Less than the sum of its particles

Principles of gas-Solid Flows. First Edition
November 26, 1999

As the authors point out, gas-solid flows are endemic in natural experience and engineering practice. It is a subject that is widely taught and researched often by those we may call "chemical engineers". This book was probably written as the evolution of a course text for American undergraduate chemical engineers in the authors' institution, Ohio State University. It is presented in two parts: "Basic Relationships" and "System Characteristics". The first deals with things like particle size, collision mechanics, including momentum change and heat transfer, as well as a variety of commonly used mathematical relationships. There is some brief reference to contact mechanics and long-range interaction forces. The second part, which draws to some extent on the first, describes the characteristics and responses of particle separators, hoppers, fluidised beds and pneumatic flow systems. Particle-gas separation processes are discussed, including "wet scrubbing". The general format provides self-contained, but not heavily integrated and cross-referenced chapters. Each chapter has its own reference listing, nomenclature and "problems" (although no model answers). This compartmentalisation is further improved by sectioning and subdivision of subsidiary material within each chapter. The book is therefore rather easy to use as a reference or teaching text, providing that the content is what the teacher, student or researcher needs. For some, including the present reviewer, this may be the core problem in assessing the potential value of the book and whether or not to use or recommend it.

Let me explain first generally and then by way of examples. The book is written by chemical engineers for chemical engineers; it is actually published within the Cambridge series on chemical engineering. It seeks to do what chemical engineers do well and that is, where possible and often where impossible, to quantify through explicit equations or correlations the behaviour of complex processes in the attempt to provide accurate predictive capacity. Where this may not be viable, at least descriptions of critical transitions in system response are sought. The book is a masterful exposition of this approach and as such will certainly find a faithful following. The treatments of, for example, hopper and standpipe flow and fluidisation will contain all the numerical expressions that most undergraduates will ever need. Likewise for the chapter that deals with "basic equations". The question is whether this is what is needed by the teacher and his students, or indeed research person, and further, is it sufficient? The answers will invariably be matters of opinion. I will expand.

For at least ten years there has been a growing belief among some chemical engineers that this subject, and indeed others, could usefully profit from the introduction, both in research and teaching, of what some have chosen to call a "materials approach". What is meant can be put in a variety of ways, but in simple terms it is argued that the responses of a particulate assembly can be described at a basic level by a proper consideration of the interactions between the particles in the assembly and between the particles and their constraining walls where appropriate. The idea is, of course, not new and has an obvious precedent in the kinetic theory of gases and is now a practised methodology in, for example, solid-liquid flows. The idea is that this incorporation of firmly established interaction laws will provide a more relevant basis for process design and particle engineering. The advent of recent nano-technological methods to quantity specific interactions has been a driver here.

The book does not take this line although we do note sections, in the first part, on interparticle forces (van der Waals forces are given just over two pages) and selected contact mechanics (there are several references to Hertzian analysis but little on contact adhesion). Similarly, there is little about particle wear and disruption processes. It is also remarkable that little is recorded about capillary forces and "damp" particle systems; it is true the book is about gas-solid interactions but there is elsewhere a section on "wet scrubbing". In practice, many problems arise because of ambient humidity changes. Thus the text introduces "fundamental interaction processes" but does not practise their use. I note that there is no real reference to appropriate experimental method and in particular how system responses may be quantified by modern transducer technologies; there are many new and old methodologies available, particularly novel imaging methods. Further, there are no descriptions of numerical modelling methods and their potential applications in, say, hopper flows or indeed powder compaction; essentially these are the vehicles that may prove to be practicable, when including the interaction laws, for the prediction of the assembly behaviour.

So, if this is the approach you want, you will find this book to your taste.

Brian Briscoe is professor of interface engineering, Imperial College, University of London.

Principles of gas-Solid Flows. First Edition

Author - Liang-Shih Fan and Chao Zhu
ISBN - 0 521 58148 6
Publisher - Cambridge University Press
Price - £65.00
Pages - 557

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