Diffusion in Gases and Porous Media

Abstract

Softcover reprint of the hardcover 1 st edition 1980All righ ts reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any focm or by any means, electronic, mechanical, photocopying, microftlming, recording, or otherwise, without written permission from the Publisher We wish to express our deep appreciation to Professor Mario Teruggi of the University of La Plata, Argentina, for his advice on English style and to Professor Rutherford Aris of the University of Minnesota, for his encouragement. PrefaceThe world we live in exhibits, on different scales, many phenomena related to the diffusion of gases. Among them are the movement of gases in earth strata, the aeration of soils, the drying of certain materials, some catalytic reactions, purification by adsorption, isotope separation, column chromatography, cooling of nuclear reactors, and the permeability of various packing materials.The evolution of the understanding of this subject has not always been straightforward and progressive-there has been much confusion and many doubts and misunderstandings, some of which remain to this day. The main reason for the difficulties in the development of this subject is, we now know, the lack of an understanding of the effects of walls on diffusing systems.Textbooks usually treat diffusion on two levels: at the physicochemical or molecular level, making use of the kinetic theory of gases (which while a very rigorous and well-founded theory nevertheless is valid only for systems without walls), or at the level of a transport phenomenon, a level geared toward applications. The influence of walls is usually disregarded or is treated very briefly (for example, by taking account of the Knudsen regime or by introducing a transition regime of limited validity) in a way unconnected with previous studies. As a consequence, the extensive, generalized, and well-founded knowledge of systems without walls has often been applied without sound basis to real situations, i.e., to systems with walls.Only recently has a unifying theory, the dusty gas model, been developed that correctly takes account of the influence of walls and which thus clarifies many of the problems and much of the confusion that has beset this subject. The dusty gas model shows why, for example, the fluxes of the species in a binary system are not always equal and opposite (and its predictions in this regard have been experimentally verified), it shows clearly that there is a difference between systems "without walls" and vii viii Preface systems "without wall effects," and it clarifies the nature of the total diffusive flux of a system and the nature of the coupling between the diffusive and viscous fluxes.In this book we present the subject of diffusion from the point of view of the dusty gas model, pointing out where the confusion and errors have arisen-and where they can and still unfortunately do arise if care is not taken. We will be more concerned with the foundations and preliminary questions than with the applications.In t...

Nanofluidic transport governed by the liquid/vapour interface

“…R t is dominated by collision of water molecules with pore walls in the Knudsen regime 27 , but is influenced here to some extent by collisions with air molecules as the mean free path of water molecules in air (60-100 nm) 28 is comparable to the nanopore diameter. η monotonically decreases with AR, indicating an increasing probability of a molecule starting at one end to return to the same end before reaching the other end of the pore 29,30 .…”

Coupling between geochemical reactions and multicomponent gas and solute transport in unsaturated media: A reactive transport modeling study

“…The nonequimolar flux occurs in systems with walls (soil particles) because diffusing species have different molecular weights: lighter molecules move faster than heavier molecules [Cunningham and Williams, 1980]. Although diffusive in origin, the nonequimolar flux results in a nonseparative flux in the same direction as the diffusive flux of the lightest gas species in the mixture.…”

Fluid dynamics in naturally fractured tectonic reservoirs

“…The real fractured system is heterogeneous and anisotropic, and their diffusion processes depend on fractures aperture or porous diameter (Cunningham and Williams 1980;Treybal 1980). In consequence, fast complex diffusion is reached in a nonlinear laminar flow, which may be modeled by Couette equation.…”