Table Of ContentBUTTERWORTHS SERIES IN
CHEMICAL ENGINEERING
SERIES EDITOR ADVISORY EDITORS
HOWARD BRENNER ANDREAS ACRIVOS
Massachusetts Institute of Technology The City College of CUNY
JAMES E. BAILEY
California Institute of Technology
MANFRED MORARI
California Institute of Technology
E. BRUCE NAUMAN
Rensselaer Polytechnic Institute
ROBERT K. PRUD'HOMME
Princeton University
SERIES TITLES
Chemical Process Equipment: Selection and Design Stanley M. Walas
Constitutive Equations for Polymer Melts and Solutions Ronald G. Larson
Fundamental Process Control David M. Prett and Carlos E. Garcia
Gas-Liquid-Solid Fluidization Engineering Liang-Shih Fan
Gas Separation by Adsorption Processes Ralph T. Yang
Granular Filtration of Aerosols and Hydrosols Chi Tien
Heterogeneous Reactor Design Hong H. Lee
Molecular Thermodynamics of Nonideal Fluids Lloyd L. Lee
Phase Equilibria in Chemical Engineering Stanley M. Walas
Physicochemical Hydrodynamics: An Introduction Ronald F. Probstein
Transport Processes in Chemically Reacting Flow Systems Daniel E. Rosner
Viscous Flows: The Practical Use of Theory Stuart W. Churchill
Granular Filtration
of Aerosols and Hydrosols
Chi Tien
Professor of Chemical Engineering
Syracuse University
Syracuse, New York
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Copyright © 1989 by Butterworth Publishers, a division of Reed
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Library of Congress Cataloging-in-Publication Data
Tien, Chi, 1930-
Granular filtration of aerosols and hydrosols/Chi Tien.
p. cm.—(Butterworths series in chemical engineering)
Includes bibliographies and index.
ISBN 0-409-90043-5
1. Filters and filtration. 2. Aerosols. 3. Liquids.
I. Title. II. Series.
TP156.F5T54 1989
660.2'84245—dcl9 88-23727
CIP
British Library Cataloguing in Publication Data
Tien, Chi
Granular filtration of aerosols and hydrosols.
1. Aerosols & hydrosols. Filtration
I. Title
660.2'84245
ISBN 0-409-90043-5
Butterworth Publishers
80 Montvale Avenue
Stoneham, MA 02180
10 9 8 7 6 5 4 3 21
Printed in the United States of America
To Julia, Anita, and Ellen
Preface
The information explosion our society is experiencing is, almost by necessity,
accompanied by a mushrooming of publications all seeking to document, explore,
or clarify our newfound knowledge. In this environment the author of material
dealing with what appears to be a familiar subject is often challenged to justify his or
her work, to defend against the charges of simply adding another volume to the
plethora that already exist on the topic. In preparing this book, I have been spared
this requirement. A casual search of the library and of publication catalogues
reveals that no publication focusing on granular filtration is presently available. It is
interesting to note that this void stands in contrast to the fact that granular filtration
is not only an engineering practice of long standing but has also enjoyed resurgent
attention of late, evidenced by the numerous recent studies of its research and
development.
This book is concerned with the fundamental aspects of granular filtration of
both liquid and gas suspensions. In writing this book, I have attempted to present
systematically the principles underlying the various phenomena associated with
granular filtration, especially those which can be used to provide a rational basis for
predicting the dynamic behavior of granular filtration in its entirety. I have also
tried to demonstrate that by using relatively simple and familiar knowledge from the
basic engineering sciences, one can indeed examine the granular filtration process in
some detail and it is not always necessary to treat a filter as a magic black box as is
often done.
Furthermore, in the hope of gaining a wider audience, I have deliberately kept
to a minimum the background information necessary to comprehend the material
presented. In fact, the level is consistent with what is taught in an accredited B.Sc.
degree program in chemical, civil (environmental), or mechanical engineering. Thus,
I hope the book will be useful to those beginning research or development work in
granular filtration. One could also adopt the book as a text or part of the text for
graduate courses dealing with separation technology although it is not strictly
written as a textbook.
On a phenomenological level, granular filtration involves the transfer of mass
(small particles) from a mobile to a stationary phase and is, therefore, a fixed-bed
process. On a more detailed level, problems such as particle deposition or filter
clogging all arise from the flow of suspension through porous media; their analysis
requires combined knowledge in fluid mechanics, particle mechanics, solution
chemistry, and the surface sciences. From any of these perspectives, there is no
fundamental difference between aerosol and hydrosol filtration. For this reason, this
book takes a unifying approach in its treatment of the topic. Chapters 2 through 6
provide material which is equally applicable to both systems. Even in Chapter 9,
where individual case studies are presented, the methods developed are in most
situations useful to both aerosols and hydrosols.
XI
xii Preface
It is well recognized that a unified approach to the treatment of granular
filtration of aerosols and hydrosols does not necessarily conform to current practice.
Investigators of granular filtration are invariably identified as either deep-bed
people (hydrosols) or aerosol scientists (aerosols). Admittedly, the difference
between certain relevant physical properties of water and air may indeed become
significant under certain circumstances. I have recognized and acknowledged these
differences, for example, by handling separately the discussions on collection
efficiency for aerosols (Chapter 6) and hydrosols (Chapter 7). Furthermore, because
of the tradition of treating aerosols and hydrosols separately, different termi-
nologies have been developed to describe essentially the same phenomena; for
example, both the concept of filter coefficient and that of collector efficiency are used
to describe filtration rate. To avoid further confusion, however, I have adhered to
this tradition as much as possible throughout the text.
This book is really an outgrowth of the lecture notes I have assembled during
the past fifteen years of my graduate teaching at Syracuse University. Some of these
notes were also used for two special courses I taught at the University of Leeds,
England, in the fall of 1976 and at Karlsruhe University, West Germany, in the
summer of 1982. In this connection, I would like to express my gratitude to
Professor Colin McGreavy (Leeds) and Professor Heinrich Sontheimer (Karlsruhe)
for inviting me to lecture in their respective institutions.
A substantial part of the material presented here is the result of various
research investigations on granular filtration carried out at Syracuse since 1968.
Both Professors C.S. Wang and R.M. Turian collaborated with me during parts
of this period and I am indebted to them for their contributions and their
friendships. I must, of course, acknowledge my former coworkers, particularly
professors A.C. Payatakes (University of Patras), R. Rajagopalan (University of
Houston), H. Pendse (University of Maine), H. Emi (University of Kanazawa),
H. Yoshida (University of Hiroshima), K. Ushiki (Kyushu Institute of Technology),
T. Takahashi (University of Nagoya), J. Tsubaki (University of Nagoya),
S. Vigneswaran (Asian Institute of Technology), B.V. Ramarao (SUNY College
of Environmental Science and Forestry), Drs. R. Gimbel (Engler-Bunte Institute,
Karlsruhe), M. Beizaie (University of California, San Diego), F.J. Onorato
(Celanese Research Company), R.C. Tsiang (E.I. DuPont), H.W. Chiang (Atomic
Energy of Canada, Ltd.), and my present graduate students, R. Vaidyanathan,
Y. Jung, S. Yiacoumi, C. Choo, and C. Yao, who as is customary, have carried out
or are conducting the brunt of these research activities.
Finally, I would like to thank Anne Coffey Fazekas, of Word-Wrights, Inc., for
her invaluable editorial help; S. Yiacoumi and C. Yao for their proofreading; and
Kathleen J. Datthyn-Madigan, who with her fine keyboard skills and unusual
ability to decipher difficult material and nearly unintelligible handwriting, typed
and retyped the entire manuscript.
Chi Tien
Syracuse, New York
Chapter 1
Introduction
Granular filtration is a fluid-solid separation process commonly applied to remove
minute quantities of small particles from various kinds of fluids. This engineering
practice is interesting historically as well as contemporarily. Both Sanskrit medical
lore and Egyptian inscriptions give clear evidence that granular filtration was used
for water treatment (as early as 200 B.C.), as detailed in Baker's book, The Quest for
Pure Water (1949). At the same time, there is hardly a segment of the process and
chemical industries that does not use granular filtration today. The significant
number of patents granted in recent years to gas-cleaning processes based on
granular filtration attests to its enduring utility.
The versatility of granular filtration is evident from its scope of application as
well as from the manner in which it is carried out. Either liquid or gas fluid streams
can be treated. Besides water or air, systems that may be treated by granular
filtration include such diverse substances as flue gas, combustion products, molten
metal, petrochemical feedstocks, polymers, and alcoholic or nonalcoholic bev-
erages. (For convenience, whenever distinction is necessary, granular filtration
will henceforth be referred to as hydrosol filtration or aerosol filtration, depending
on whether liquid or gas suspension is involved.) Although granular filtration is
frequently carried out in the fixed-bed mode, it may also be conducted in moving-
bed or fluidized-bed mode so that the operation is continuous.
The basic principle of granular filtration remains the same regardless of the
system being treated or the manner in which filtration is conducted. The suspension
is made to pass through a medium composed of granular substances (granular
medium). As the suspension flows through the medium, some of the particles present
in the suspension, because of the various forces acting on them, move toward and
become deposited on the surface of the granules of which the medium is composed.
Although the extent of deposition throughout the medium cannot be made uni-
form, the entire medium is to be used for particle collection.
The purpose of this monograph is to present a systematic and rational
treatment of deposition and other problems arising from the flow of fluid-particle
suspensions through granular media. Whenever possible, both aerosol and hydrosol
systems are considered as a single entity. Although the problems considered in this
text by no means constitute granular filtration research in its entirety, their studies
represent an important segment of this research field. This knowledge is essential to
the modeling, design, optimization, and control of granular filtration systems.
1
2 Introduction
1.1 GRANULAR FILTRATION AS A
FLUID-PARTICLE SEPARATION TECHNOLOGY
Fluid-particle separation technology refers to a collection of processes for removing
(as contaminants or impurities), separating (one type of particle from a mixture of
particles), concentrating, and recovering (as products) particles from fluid-particle
suspensions. As a technology, its age is probably second only to that of crushing and
grinding of solids (Purchas 1971). Although the processes classified as fluid-particle
separation are too numerous to be cited individually, it is generally accepted that
fluid-particle separation encompasses cake filtration, granular and fibrous filtration,
cartridge and membrane filtration, cycloning, thickening, flocculation, dewatering
and expression, scrubbing, and electrostatic precipitation. The technology is basic to
many manufacturing industries (chemical, mineral, and food and beverages) as well
as to pollution abatement and environment control (for example, clean rooms). It is
difficult to find any important engineering enterprise in which fluid-particle
separation is not involved.
The relationship among the various fluid-particle separation processes can be
seen from the classification scheme suggested by Tiller (1974) for liquid-solid
separation. This scheme, shown in Figure 1-1, is based on Tiller's idea that solid-
liquid separation should be viewed as a system consisting of one or more stages:
(1) pretreatment, to facilitate the operation of subsequent stages; (2) solids con-
centration, to increase the solids content of suspensions; (3) solids separation, to
separate solids and the suspending liquid; and (4) posttreatment, to improve the
quality of the recovered products (either solid or liquid).
Figure 1-1 is useful for delineating the function and field of application of
granular filtration; that is, the process is used primarily for clarifying dilute
suspensions by using the granular media as collecting bodies for particles present in
the suspension. In contrast, cake filtration is used to recover solid products from
relatively concentrated slurries. The difference between cake filtration (a subject
often included in basic engineering texts) and granular filtration is the manner in
which they operate. In the former case, the medium (or the bulk of it) through which
the treated suspension flows is composed of the solids to be recovered. The re-
sistence (that is, pressure drop) to suspension flow increases with time as a direct
result of the increase in filter-cake thickness. For granular filtration, deposition
occurs throughout the entire medium, and the pressure drop increase results when
the medium is clogged.
The differences between cake and granular filtrations and the mechanisms they
use to separate solids from liquid do not imply that the two processes embody totally
separate and distinct physical phenomena. Because particles present in a slurry to be
treated by cake filtration invariably cover a wide size range, the finer particles are, to
a large extent, removed by mechanisms like those operating in granular filtration.
Similarly, even though the purpose of conducting granular filtration is to ensure
that particle collection takes place throughout the entire filter medium, the extent of
deposition within a granular filter cannot be made uniform. Excessive pressure drop
often results from the formation and presence of filter cakes near the entrance of a
granular filter. Thus, understanding the conditions leading to filter-cake formation
is important to properly designing granular filtration systems.
1.2 Granular Filtration Versus Fibrous Filtration 3
Chemical
Flocculatlon
Coagulation
PRETREATMENT -A
Ptiysical
Crystal growth
Freezing and other physical changes
Rlter aid addition
Thickening
Hydrocycloning
SOLIDS
CONCENTRATION ~1
I— Clartfication
r- Batch
PRESS, VACUUM, -\
Recovery of Solid GRAVITY FILTERS
Particles
I— Continuous
CAKE FORMATION
SOLIDS FILTERING
SEPARATION Solid bowl
L- CENTRIFUGES-
SEDIMENTING
Perforated bowl
Clartflcation
No cake formed
Deep granular beds
Cartridges
Filtrate — Polishing
Membranes
Ultrafiltration
POST-TREATMENT
»— Cake — Washing Deliquoring
Displacement Drainage
Reslurry Mechanical
Hydraulic
FIGURE 1 -1 Stages of solid-liquid separation by Tiller {1974). {Reprinted with permission.)
1.2 GRANULAR FILTRATION VERSUS
FIBROUS FILTRATION
Fibrous filtration, generally speaking, refers to the process in which the removal of
particles from gas streams is effected by passing the streams through fibrous media
of various kinds. Depending on the manner with which the filter media are
4 Introduction
constituted, particle retention takes place either at the media surface, in the form of
filter cakes, or throughout the media. In the former case, the fibers (natural or
chemical fibers, cellulose, metal or glass fibers) are pressed together in felt or spun or
woven into cloth (fabric) such that the filter media pores are relatively small (as
compared with the size of the particles to be removed). Most of the particles are
separated in the form of filter cakes at the surface of the media, which are then
removed intermittently when the pressure drop becomes excessive. This type of fi-
brous filtration, in a sense, is similar to cake filtration used to separate particles
from solid-liquid slurries. Baghouse filters used in power utilities are a typical ex-
ample of this type of operation.
If the fibrous media are formed by packing fibers loosely, such as in ventilation
and air-conditioning applications, particle retention takes place mostly within the
interior of the media. This second type of fibrous filtration is very similar to granular
filtration. Because the physical laws governing the flow through either type of media
are the same, the methodologies used to describe either type of filtration become
almost interchangeable. This strong similarity does not imply that the method-
ologies are identical. In addition to the obvious difference in the geometries of the
entities constituting the filter media (e.g., granules or spheres in granular media
versus fibers or cylinders in fibrous media), there are also significant differences in
packing densities (or porosity), collector sizes, and mechanical strength. Because
of their small collector size (i.e., fiber diameter) and high porosity, fibrous filters
enjoy the advantages of higher single- (or unit-) collector efficiency and lower
pressure drop. Granular filters, on the other hand, can be easily regenerated, in
contrast to the difficulty of removing deposited particles for individual fibers.
Furthermore, because of the relative abundance of granular substances that are
temperature and corrosion resistant, granular filtration is more suitable for treating
high-temperature and/or corrosive gaseous streams.
1.3 GRANULAR FILTRATION VERSUS
FIXED-BED ADSORPTION
By common understanding and usage, adsorption is referred to as the process in
which certain components in a fluid phase are removed by transferring these
components from the fluid to the surface of a solid adsorbent. Usually the small
granules of adsorbent are placed in a fixed bed, and fluid is passed through the bed
until the effluent concentration reaches a certain critical value (when breakthrough
occurs) or until the adsorbent granules become nearly saturated. Thus, the
operation of fixed-bed adsorption is very similar to that of granular filtration.
Furthermore, there is reason to believe that the same types of interaction forces
(such as the London-van der Waals and double-layer forces) are responsible for
both adsorption and desorption and for deposition and reentrainment rates. Thus,
many similarities exist between adsorption and granular filtration processes in
terms of equipment configuration, mode of operation, and the respective underlying
phenomena.
Because of these similarities, the words adsorption and filtration have become
interchangeable. The removal of colloidal particles from a fluid phase to a solid
phase can be described as either adsorption or filtration (Hirtzel and Rajagopalan
1985). In engineering practice, granular carbon columns used to remove soluble
