secado industrial

Foreword to the First Edition
The Handbook of Industrial Drying fills an important of drying systems, different applications of principles,
need and is of immeasurable value in the field of and different products. The book provides excellent
drying. Academics, students, and industry people— coverage of the cross-disciplinary nature of drying by
from sales to research—can learn much from the utilizing well-known authors from many countries of
combination of principles and practices used through- the world. Dr. Mujumdar and his associates have as-
out. The presentation of principles does not over- sembled an excellent up-to-date handbook.
whelm the coverage of equipment and systems. More The common thread throughout the book is the
appropriate theories will develop as a result of the movement of heat and moisture as well as the move-
description of equipment and systems. For example, a ment and handling of products. Also included are
description of dryers, particularly industrial dryers, is instrumentation, sensors, and controls that are im-
lacking in many research articles; this handbook pro- portant for quality control of products and efficiency
vides such information. of operation. The emphasis on the design of equip-
The authors have distilled much information from ment to expedite these processes in an economical
extensive literature to provide generic information as manner is appropriate and useful.
contrasted with details of a specific drying system of a The word handbook is sometimes used dispara-
particular manufacturer. The users can extrapolate gingly to describe a reference for quick answers to
the use of drying systems, by design and management, limited questions or problems. In that sense this book
to a variety of products. As a special feature, a com- is more than a handbook—the knowledge base pro-
plete listing of books written on the subject of drying vided permits the user to build different systems for
is included. products other than those covered.
The authors, a blend of students, faculty, and those
in industry, represent experience with different kinds Carl W. Hall

ß 2006 by Taylor & Francis Group, LLC.

Foreword to the Second Edition
The second edition of the Handbook of Industrial New material has been added to provide the latest
Drying continues the tradition of the editor and the information on minimizing environmental impacts,
publisher as international leaders in providing infor- increasing energy efficiency, maintaining quality con-
mation in the field of industrial drying. The authors are trol, improving safety of operation, and improving
knowledgeable of the subjects and have been chosen the control of drying systems. New sections or chap-
from among the world’s authorities in industry, aca- ters have been added to cover in detail microwave
demia, government, and consulting. Some 50 authors drying; infrared drying; impinging stream dryers;
from 15 countries have written 43 chapters plus 3 ap- use of superheated steam and osmotic dehydration;
pendices. There are 21 new chapters, plus 2 new appen- and drying of biotechnological materials, tissue and
dices. All chapters have been updated or revised. There towels, peat, coal, and fibrous materials.
is over 60% new material, making this edition practic- The information in this book can be categorized
ally a new volume. as product related, equipment related, and the rela-
The mark of an outstanding handbook is that it tionship between the two—the system of drying. For
provides current information on a subject—in this products not specifically covered, or for the design
case multidisciplinary in nature—understandable to of dryers not detailed, users can select closely related
a broad audience. A balanced approach of covering applicable information to meet many needs. The user
principles and practices provides a sound basis for the may want to pursue a subject in considerably more
presentations. Students, academics, consultants, and detail. Pertinent references, but not voluminous over-
industry people can find information to meet their whelming bibliographies, are included at the end of
needs. Researchers, designers, manufacturers, and each chapter. An appendix devoted to an annotated
sales people can benefit from the book as they con- bibliography is also included.
sider elements or components related to drying as well
as the system itself. Carl W. Hall


Foreword to the Third Edition
The Handbook of Industrial Drying, as a result of the many practical data referring to the selection of in-
great success of its first and second editions, has dustrial dryers, description of drying equipment, in-
gained high reputation among readers interested in dustrial drying technologies, recent developments in
the process of drying. In the last three decades we R&D in drying as well as future trends. Over 60% of
have observed a growing interest in the multidisciplin- the chapters are new and some 40% revised. A few
ary subject of drying which had resulted in a major chapters have been deleted from the second edition
increase of research activity, publication of several due to space limitations. New sections have been
monographs, book series, technical papers, inter- added to encompass the latest data on drying of
national journals, several drying conference series in several materials (foods, wood, herbal medicines,
almost all continents, etc. Today drying R&D con- sludge, grain, nano size products, fish and seafood,
tinues worldwide at a pace unmatched in any earlier etc.); some dryer types (rotary, indirect, drum, fluid-
period. To keep abreast with all these scattered ized, flush and pneumatic, etc.) with a strong general
sources of information in a broad area like drying is approach to energy, environmental safety, control
extremely difficult for most readers in academia and and quality aspects. So practically, this edition can
industry alike. be treated as a truly new Handbook of Industrial
So, the third edition of the Handbook, nearly a Drying based on the latest achievements in the drying
decade after the second edition, will play a very im- area.
portant role in providing comprehensive, updated Finally, having in mind the international charac-
information and a view of the current state of the ter of the authors, this Handbook gives readers a
art in industrial drying as a more cohesive whole. chance to get acquainted in considerable detail with
This third edition continues the style of the two the literature sources published not only in English
previous ones; the authors are international leaders but also in other languages. Key relevant references
and generally recognized world authorities from aca- are included at the end of each chapter.
demia, industry, and R&D laboratories from many I am confident that this third edition of the Hand-
countries. It maintains the essential interdisciplinary book will be of great help to the broad audience from
character addressing a broad academic and industrial academia and in the application, progress and future
readership. This book gives the possibility for self- trends in drying R&D on a global scale.
study and of finding a clear overview of the funda-
mentals and practical information in broad aspects Czesław Strumiłło
and problems of drying technology. It is like having Lodz Technical University
one’s own private ‘‘consultant on the desk.’’ Lodz, Poland
The topics chosen are constructed to give a quick
and clear overview of the fundamental principles and


Preface to the First Edition
Drying of solids is one of the oldest and most com- tion and selection of dryers, process calculation
mon unit operations found in diverse processes such schemes, and basic experimental techniques in drying.
as those used in the agricultural, ceramic, chemical, For detailed information on the fundamentals of dry-
food, pharmaceutical, pulp and paper, mineral, poly- ing, the reader is referred to various textbooks in this
mer, and textile industries. It is also one of the most area.
complex and least understood operations because of The volume is divided into four major parts. Part I
the difficulties and deficiencies in mathematical de- covers the basic principles, definitions, and process cal-
scriptions of the phenomena of simultaneous—and culation methods in a general but concise fashion. The
often coupled and multiphase—transport of heat, second part is devoted to a series of chapters that de-
mass, and momentum in solid media. Drying is there- scribe and discuss the more commonly used industrial
fore an amalgam of science, technology, and art (or dryers. Novel and less prevalent dryers have been ex-
know-how based on extensive experimental observa- cluded from coverage; the reader will find the necessary
tions and operating experience) and is likely to remain references in Appendix B, which lists books devoted to
so, at least for the foreseeable future. drying and related areas in English as well as other
Industrial as well as academic interest in solids languages. Part III is devoted to the discussion of cur-
drying has been on the rise for over a decade, as rent drying practices in key industrial sectors in which
evidenced by the continuing success of the Biennial drying is a significant if not necessarily dominant
Industrial Drying Symposia (IDS) series. The emer- operation. Some degree of repetition was unavoidable
gence of several book series and an international since various dryers are discussed under two possible
journal devoted exclusively to drying and related categories. Most readers will, however, find such infor-
areas also demonstrates the growing interest in this mation complementary as it is derived from different
field. The significant growth in research and develop- sources and generally presented in different contexts.
ment activity in the western world related to drying Because of the importance of gas humidity meas-
and dewatering was no doubt triggered by the energy urement techniques, which can be used to monitor
crunch of the early 1970s, which increased the cost of and control the convective drying operation, Part IV
drying several-fold within only a few years. However, includes a chapter that discusses such techniques.
it is worth noting that continued efforts in this area Energy savings in drying via the application of energy
will be driven not only by the need to conserve energy, recovery techniques, and process and design modifica-
but also by needs related to increased productivity, tions, optimization and control, and new drying tech-
better product quality, quality control, new products niques and nonconventional energy sources are also
and new processes, safer and environmentally superior covered in some depth in the final part of the book.
operation, etc. Finally, it is my pleasant duty to express my sin-
This book is intended to serve both the practicing cerest gratitude to the contributors from industry and
engineer involved in the selection or design of drying academia, from various parts of the world, for their
systems and the researcher as a reference work that continued enthusiasm and interest in completing
covers the wide field of drying principles, various this major project. The comments and criticisms re-
commonly used drying equipment, and aspects of ceived from over 25 reviewers were very valuable
drying in important industries. Since industrial dryers in improving the contents within the limitations of
can be finely categorized into over 200 variants and, space. Many dryer manufacturers assisted me and
furthermore, since they are found in practically all the contributors directly or indirectly, by providing
major industrial sectors, it is impossible within limited nonproprietary information about their equipment.
space to cover all aspects of drying and dryers. We Dr. Maurits Dekker, Chairman of the Board, Marcel
have had to make choices. In view of the availability Dekker, Inc., was instrumental in elevating the
of such publications as Advances in Drying and the level of my interest in drying so that I was able to
Proceedings of the International Drying Symposia, undertake the major task of compiling and editing a
which emphasize research and development in solids handbook in a truly multidisciplinary area whose
drying, we decided to concentrate on various practical advancement depends on closer industry–academia
aspects of commonly used industrial dryers following interaction and cooperation. My heartfelt thanks
a brief introduction to the basic principles, classifica- go to Chairman Mau for his kindness, continuous


encouragement, and contagious enthusiasm through- of numerous chapters. Without the assistance of my
out this project. coauthors, it would have been impossible to achieve
Over the past four years, many of my graduate the degree of coverage attained in this book. I wish to
students provided me with enthusiastic assistance in record my appreciation of their efforts. Indeed, this
connection with this project. In particular, I wish to book is a result of the combined and sustained efforts
thank Mainul Hasan and Victor Jariwala for their of everyone involved.
help and support. In addition, Purnima and Anita
Mujumdar kindly word-processed countless drafts Arun S. Mujumdar


Preface to the Second Edition
The second edition of the Handbook of Industrial made to each of the four parts to eliminate some of
Drying is a testimonial to the success of the first the weaknesses of the first edition. For example, an
edition published in 1987. Interest in the drying oper- extensive chapter is added in Part I on transport
ation has continued to increase on a truly global scale properties needed for dryer calculations. Chapters
over the past decade. For example, over 1500 papers on infrared drying and the novel impinging stream
have been presented at the biennial International dryers are added to Part II. Part III contains the
Drying Symposia (IDS) since its inception in 1978. largest enhancement with ten new chapters while
Drying Technology—An International Journal pub- Part IV is completely new except for the chapter on
lished some 2000 pages in seven issues in 1993 humidity measurements.
compared with just over 300, only a decade earlier. A two-volume set of this magnitude must depend
The growth in drying R&D is stimulated by the need on the direct and indirect contributions of a large
to design and operate dryers more efficiently and number of individuals and organizations. Clearly it
produce products of higher quality. is impossible to name them all. I am grateful to all the
A handbook is expected to provide the reader contributors for the valuable time and effort they
with critical information and advice on appropriate devoted to this project. The companies and publishers
use of such information compiled in a readily access- who have permitted us to reproduce some of their
ible form. It is intended to bring together widely copyrighted artwork are acknowledged for their sup-
scattered information and know-how in a coherent port. Appropriate credits are given in the text where
format. Since drying of solids is a multidisciplinary applicable. Exergex Corporation, Brossard, Quebec,
field—indeed, a discipline by itself—it is necessary to Canada provided all the secretarial and related assist-
call on the expertise of individuals from different ance over a three-year period. Without it this revision
disciplines, different industrial sectors, and several would have been nearly impossible.
countries. A quick perusal of the list of contributors Over the past two years most of my graduate stu-
will indicate a balanced blend of authorship from dents and postdoctoral fellows of McGill University
industry as well as academia. An attempt has been have provided me with very enthusiastic assistance in
made to provide the key elements of fundamentals various forms in connection with this project. In par-
along with details of industrial dryers and special ticular, I wish to express my thanks to Dr. T. Kudra for
aspects of drying in specific industries, e.g., foods, his continued help in various ways. Purnima, Anita,
pulp and paper, and pharmaceuticals. and Amit Mujumdar kindly word-processed numer-
The first edition contained 29 chapters and 2 appen- ous chapters and letters, and helped me keep track of
dixes; this one contains 43 chapters and 3 appendixes. the incredible paperwork involved. The encourage-
Aside from the addition of new chapters to cover topics ment I received from Dr. Carl W. Hall was singularly
missing from the first one, a majority of earlier chapters valuable in keeping me going on this project while
have been updated—some fully rewritten with new handling concurrently the editorial responsibilities
authorship. This edition contains over 60% new up- for Drying Technology—An International Journal and
dated material. Thus, this book will be a valuable addi- a host of other books. Finally, the staff at Marcel
tion even to the bookshelves that already hold the first Dekker, Inc., have been marvellous; I sincerely appre-
edition. ciate their patience and faith in this project.
This revised and expanded edition follows the
same general organization as the first with additions Arun S. Mujumdar


Preface to the Third Edition
From the success of the second edition of the Hand- and products in any single resource. However, I be-
book of Industrial Drying the need for an updated and lieve we have covered most of the commonly used
enhanced edition is realized at this time. Interest in drying equipment and ancillaries, as well as addressed
industrial drying operations has been growing con- industrial sectors where drying is a key operation. In
tinuously over the last three decades and still shows this edition for the first time we have covered several
no signs of abatement. This unit operation is central new topics relevant to drying, e.g., risk analysis, crys-
to almost all industrial sectors while exposure to its tallization, and frying. We have also covered new and
fundamentals and applications is minimal in most emerging drying technologies in adequate detail.
engineering and applied science curricula around the This book is organized in much the same way as
world. The escalating interest in drying is evidenced the earlier editions. The main difference is the wider
by the large number of international, regional, and coverage of topics. Once again, a deliberate attempt is
national conferences being held regularly around the made to cover most industrial sectors and make the
world, which are devoted exclusively to thermal and content useful to industry as well as academia. Stu-
nonthermal dehydration and drying. Although decep- dents and instructors in many disciplines will find the
tively simple, the processes involved are still too com- content useful for teaching, design, and research. It is
plex to be described confidently in mathematical particularly useful for researchers who wish to make
terms. This means that the design and analyses of their findings relevant to real-world needs.
industrial dryers remain a combination of science, As energy costs escalate and environmental
engineering, and art. It is necessary to have both impact becomes a serious issue in the coming decade,
know-how and know-why of the processes involved it is clear that the significance of drying for industry
to improve the design and operation of dryers. This will rise. It is hoped that industry will encourage
book represents a comprehensive compendium of col- academia to include the study of drying, both as a
lected knowledge of experts from around the world. basic and as an applied subject, as an essential part of
We are grateful to them for contributing to this effort. engineering and technical curricula. Industry–univer-
As in the earlier editions, we have a blend of sity cooperation and active collaboration is essential
academic and industry-based authors. The academics to gaining in-depth knowledge of drying and dryers.
were carefully selected to ensure they also have indus- I believe that the rising energy costs and demand for
trial background so that readers can reliably utilize enhanced product quality will drive drying R&D.
the knowledge embedded in this book. Nevertheless, Although no truly disruptive drying technology ap-
we need to include information and resources avail- pears on the horizon today, it is likely to happen
able in the public domain; despite our best intentions within the next decade. This book addresses some
and high degree of selectivity, we cannot assume re- of the new technologies that have the potential to
sponsibility for validity of all the data and informa- be disruptive.
tion given in this book. Readers must exercise due Production of a massive handbook such as this
diligence before using the data in an industrial design one is a collective effort of scores of dedicated and
or operation. enthusiastic individuals from around the globe. In-
About two thirds of this book contains new material deed, this book embodies a result of globalization.
written by new authors using recent literature. A few Aside from the authors and referees, numerous staff
topics from the second chapter are deleted. Numerous members initially at Marcel Dekker, New York, and
chapters are totally rewritten with new authorship. At then at Taylor & Francis, Philadelphia, have helped
least ten new chapters have been added to make the move this project along over a period of nearly five
coverage encyclopedic. I believe that individuals and years. Purnima Mujumdar, as usual, played a pivotal
libraries who have the second edition in their collection part in bringing this project to a successful closure.
should keep that as an independent reference. The ma- Without her enthusiastic volunteer effort it is highly
terial in it is still relevant since the shelf-life of drying unlikely this book would have seen the proverbial end
technologies is rather long—several decades! of the tunnel. A number of my postgraduate students
As some 50,000 materials are estimated to require at McGill, National University of Singapore, and
drying on varying scales, it is obvious that it is im- indeed many overseas institutions also assisted in
possible to pretend to cover all possible dryer types various ways for which I want express my gratitude.


The encouragement I received regularly from Dr. Carl NDC, IWSID, etc. I thank the authors for their
Hall was instrumental in keeping the project alive patience and effort in making this third edition a
and kicking over very long periods, especially since valuable reference work.
it competed for my leisure time used to edit Drying
Technology—An International Journal and several
other books, as well as organizational effort for Arun S. Mujumdar
many drying-related conferences such as IDS, ADC, Singapore


Editor
Arun S. Mujumdar is currently professor of mechan- journal Drying Technology—An International Journal.
ical engineering at the National University of Singa- He is also the editor of over 50 books including
pore, Singapore, and adjunct professor of chemical as the widely acclaimed Handbook of Industrial Drying
well as agricultural and biosystems engineering at (Marcel Dekker, New York) now undergoing third
McGill University, Montreal, Canada. Until 2000, he enhanced edition. His recent book, Mujumdar’s Prac-
was professor of chemical engineering at McGill. He tical Guide to Industrial Drying, has already been trans-
earned his B.Chem.Eng. with distinction from UDCT, lated into several languages including Chinese,
University of Mumbai, India, and his M.Eng. and Indonesian, French, Vietnamese, and Hungarian.
Ph.D., both in chemical engineering, from McGill. Dr. Mujumdar has lectured in 38 countries across
He has published over 300 refereed publications in 4 continents. He has also given professional develop-
heat/mass transfer and drying. He has worked on ment courses to industrial and academic audiences in
experimental and modeling projects involving almost the United States, Canada, Japan, China, and India.
all physical forms of wet products to be dried in at Details of his research activities and interests in drying
least 20 different drying configurations, many of can be found at www.geocities.com/AS_Mujumdar.
which were his original ideas that were later carried He has been instrumental in developing the
forward by others. He has supervised over 40 Ph.D. then-neglected field of drying into a major multi-
students and over 30 postdoctoral researchers at and interdisciplinary field on a truly global scale.
McGill, National University of Singapore, as well as Thanks to his missionary efforts, often carried out
in several other countries. Dr. Mujumdar has won single-handedly before the field received worldwide
numerous international awards and honors for his recognition, engineers and scientists around the
distinguished contributions to chemical engineering world have been able to pursue their interests in
in general, and to drying as well as heat and mass this exciting field, which provides a kaleidoscope
transfer in particular. Founder/program chairman of challenging research opportunities for innov-
of the International Drying Symposium (IDS) and ation. He is aptly called the Drying Guru—a label
cofounder of the sister symposia ADC, IADC, NDC he was first given during the presentation of the
series, he is a frequent keynote speaker at major esteemed Joseph Janus Medal of the Czech Acad-
international conferences and a consultant in drying emy of Sciences in Prague in 1990 to honor his
technology for numerous multinational companies. countless contributions to chemical engineering
He serves as the editor-in-chief of the premier archival and drying technologies.


Contributors
Janusz Adamiec Mainul Hasan
Faculty of Process and Environmental Engineering Department of Mining and
Lodz Technical University Metallurgical Engineering
Lodz, Poland McGill University
Montreal, Quebec, Canada
Irene Borde
Department of Mechanical Engineering Masanobu Hasatani
Ben-Gurion University of the Negev Department Mechanical Engineering
Be’er Sheva, Israel Aichi Institute of Technology
Toyota, Japan
Roberto Bruttini
Criofarma-Freeze Drying Equipment Li Xin Huang
Turin, Italy Department of Equipment Research
and Development
Research Institute of Chemical Industry
Wallace W. Carr
of Forest Products
School of Polymer, Textile, and Fiber Engineering
Nanjing, People’s Republic of China
Georgia Institute of Technology
Atlanta, Georgia
James Y. Hung
Hung International
Stefan Cenkowski Appleton, Wisconsin
Biosystems Engineering
University of Manitoba ´ ´
Laszlo Imre
Winnipeg, Manitoba, Canada Department of Energy
Budapest University of Technology
Guohua Chen Budapest, Hungary
Department of Chemical Engineering
The Hong Kong University of Science Yoshinori Itaya
and Technology Department of Chemical Engineering
Clear Water Bay, Kowloon Nagoya University
Hong Kong Nagoya, Japan

D.K. Das Gupta Masashi Iwata
Defense Food Research Lab Department of Chemistry
Mysore, India and Biochemistry
Suzuka National College
Sakamon Devahastin of Technology
Department of Food Engineering Suzuka, Japan
King Mongkut’s University of
Technology Thonburi K.S. Jayaraman
Bangkok, Thailand Defense Food Research Lab
Mysore, India
Iva Filkova´
Faculty of Mechanical Engineering Digvir S. Jayas
(retired) University of Manitoba
Czech Technical University Winnipeg, Manitoba, Canada
Prague, Czech Republic


Chua Kian Jon Andrzej Lenart
Department of Mechanical and Production Department of Food Engineering and
Engineering Process Management
National University of Singapore Faculty of Food Technology
Singapore Warsaw Agricultural University (SGGW)
Warsaw, Poland
Peter L. Jones
EA Technology Ltd. Avi Levy
Capenhurst, United Kingdom Department of Mechanical Engineering
Ben-Gurion University of the Negev
Rami Y. Jumah Be’er-Sheva, Israel
Jordan University of Science and Technology Piotr P. Lewicki
Irbid, Jordan Department of Food Engineering and
Process Management
´
Władysław Kaminski Faculty of Food Technology
Faculty of Process and Environmental Warsaw Agricultural University (SGGW)
Engineering Warsaw, Poland
Lodz Technical University
Lodz, Poland
Athanasios I. Liapis
Department of Chemical and Biological Engineering
Roger B. Keey
University of Missouri-Rolla
Department of Chemical
Rolla, Missouri
and Process Engineering
University of Canterbury
Marjatta Louhi-Kultanen
Christchurch, New Zealand
Lappeenranta University of Technology
Lappeenranta, Finland
Chou Siaw Kiang
Department of Mechanical and Production
Dimitris Marinos-Kouris
Engineering
National University of Singapore
National Technical University of Athens
Singapore
Athens, Greece
Magdalini Krokida
Adam S. Markowski
Department of Chemical Engineering Faculty of Process and Environmental Engineering
Athens, Greece
Lodz, Poland
Tadeusz Kudra
Z.B. Maroulis
CANMET Energy Technology Center
Varennes, Quebec, Canada
Athens, Greece
Chung Lim Law
School of Chemical and Environmental ´ ´
Karoly Molnar
Engineering
Department of Chemical Equipment/Agriculture
Faculty of Engineering and
Technical University of Budapest
Computer Science
Budapest, Hungary
University of Nottingham
Selangor, Malaysia
Shigekatsu Mori
H. Stephen Lee Nagoya University
Alcoa Technical Center
Nagoya, Japan
Monroeville, Pennsylvania


Arun S. Mujumdar Osman Polat
Department of Mechanical and Production Procter & Gamble International Division
Engineering Cincinnati, Ohio
National University of Singapore
Singapore Vijaya G.S. Raghavan
Department of Agricultural and Biosystems
Hyunyoung Ok Engineering
School of Polymer, Textile and Fiber Macdonald Campus of McGill University
Engineering St. Anne de Bellevue, Quebec, Canada
Georgia Institute of Technology
Atlanta, Georgia M. Shaﬁur Rahman
Department of Food Science and Nutrition
Vassiliki Oreopoulou College of Agriculture and Marine Sciences
Department of Chemical Engineering Sultan Qaboos University
National Technical University of Athens Muscat, Sultanate of Oman
Athens, Greece
Cristina Ratti
Zdzisław Pakowski Soils and Agri-Food Engineering (SGA)
Faculty of Process and Environmental Laval University
Engineering Quebec City, Quebec, Canada
Lodz, Poland Shyam S. Sablani
Department of Food Science and
Elizabeth Pallai Nutrition College of Agriculture and
Research Institute of Chemical and Process Marine Sciences
Engineering Sultan Qaboos University
Pannon University of Agricultural Sciences Muscat, Sultanate of Oman
Veszprem, Hungary
´
Virginia E. Sanchez
Seppo Palosaari Departamento de Industrias
Department of Chemical Engineering Facultad de Ciencias Exactas y Naturales
Kyoto, University Universidad de Buenos Aires
Kyoto, Japan Buenos Aires, Argentina

´
Patrick Perre G.D. Saravacos
French Institute of Forestry, Agricultural Department of Chemical Engineering
and Environmental Engineering (ENGREF) National Technical University of Athens
Nancy, France Athens, Greece

Jerzy Pikon´ Robert F. Schiffmann
Silesian Technical University R.F. Schiffmann Associates, Inc.
Gliwice, Poland New York, New York

Ana M.R. Pilosof Zuoliang Sha
Departamento de Industrias College of Marine Science and Engineering
Facultad de Ciencias Exactas y Naturales Tianjin University of Science and Technology
Universidad de Buenos Aires Tianjin, People’s Republic of China
Buenos Aires, Argentina
Mompei Shirato
Dan Poirier Department of Chemical Engineering (retired)
Aeroglide Corporation Nagoya University
Raleigh, North Carolina Nagoya, Japan


Shahab Sokhansanj Baohe Wang
Department of Chemical & Biological Engineering Dalian University of Technology
University of British Columbia Dalian, People’s Republic of China
Vancouver, British Columbia, Canada
Richard J. Wimberger
Venkatesh Sosle Spooner Industries Inc.
Department of Agricultural and Biosystems Depere, Wisconsin
Engineering
Macdonald Campus of McGill University Roland Wimmerstedt
St. Anne de Bellevue, Quebec, Canada Center for Chemistry and Chemical
Engineering
Czesław Strumiłło Lund University of Technology
Faculty of Process and Environmental Engineering Lund, Sweden
Lodz, Poland
Po Lock Yue
Tibor Szentmarjay
Hong Kong University of Science and Technology
Testing Laboratory of Environmental Protection
Clear Water Bay, Kowloon
Veszprem, Hungary
Hong Kong
Zbigniew T. Sztabert
_
Romuald Zyłła
Chemical Industry Institute (retired)
Warsaw, Poland Faculty of Process and Environmental
Engineering
Wan Ramli Wan Daud Lodz Technical University
Department of Chemical Engineering Lodz, Poland
Universiti Kebangsaan Malaysia
Sebangor, Malaysia


Table of Contents

Part I Fundamental Aspects
1 Principles, Classiﬁcation, and Selection of Dryers
Arun S. Mujumdar
2 Experimental Techniques in Drying
´ ´
Karoly Molnar
3 Basic Process Calculations and Simulations in Drying
Zdzisław Pakowski and Arun S. Mujumdar
4 Transport Properties in the Drying of Solids
Dimitris Marinos-Kouris and Z.B. Maroulis
5 Spreadsheet-Aided Dryer Design
Z.B. Maroulis, G.D. Saravacos, and Arun S. Mujumdar

Part II Description of Various Dryer Types
6 Indirect Dryers
Sakamon Devahastin and Arun S. Mujumdar
7 Rotary Drying
Magdalini Krokida, Dimitris Marinos-Kouris, and Arun S. Mujumdar
8 Fluidized Bed Dryers
Chung Lim Law and Arun S. Mujumdar
9 Drum Dryers
Wan Ramli Wan Daud
10 Industrial Spray Drying Systems
´
Iva Filkova, Li Xin Huang, and Arun S. Mujumdar
11 Freeze Drying
Athanasios I. Liapis and Roberto Bruttini
12 Microwave and Dielectric Drying
Robert F. Schiffmann
13 Solar Drying
´ ´
Laszlo Imre
14 Spouted Bed Drying
Elizabeth Pallai, Tibor Szentmarjay, and Arun S. Mujumdar
15 Impingement Drying
Arun S. Mujumdar
16 Pneumatic and Flash Drying
Irene Borde and Avi Levy
17 Conveyor Dryers
Dan Poirier


18 Infrared Drying
Cristina Ratti and Arun S. Mujumdar
19 Superheated Steam Drying
Arun S. Mujumdar
20 Special Drying Techniques and Novel Dryers
Tadeusz Kudra and Arun S. Mujumdar

Part III Drying in Various Industrial Sectors
21 Drying of Foodstuffs
Shahab Sokhansanj and Digvir S. Jayas
22 Drying of Fish and Seafood
M. Shaﬁur Rahman
23 Grain Drying
Vijaya G.S. Raghavan and Venkatesh Sosle
24 Grain Property Values and Their Measurement
Digvir S. Jayas and Stefan Cenkowski
25 Drying of Fruits and Vegetables
K.S. Jayaraman and D.K. Das Gupta
26 Drying of Herbal Medicines and Tea
Guohua Chen and Arun S. Mujumdar
27 Drying of Potato, Sweet Potato, and Other Roots
Shyam S. Sablani and Arun S. Mujumdar
28 Osmotic Dehydration of Fruits and Vegetables
Piotr P. Lewicki and Andrzej Lenart
29 Drying of Pharmaceutical Products
Zdzisław Pakowski and Arun S. Mujumdar
30 Drying of Nanosize Products
Baohe Wang, Li Xin Huang, and Arun S. Mujumdar
31 Drying of Ceramics
Yoshinori Itaya, Shigekatsu Mori, and Masanobu Hasatani
32 Drying of Peat and Biofuels
Roland Wimmerstedt
33 Drying of Fibrous Materials
Roger B. Keey
34 Drying of Textile Products
Wallace W. Carr, H. Stephen Lee, and Hyunyoung Ok
35 Drying of Pulp and Paper
Osman Polat and Arun S. Mujumdar
36 Drying of Wood: Principles and Practices
´
Patrick Perre and Roger B. Keey
37 Drying in Mineral Processing
Arun S. Mujumdar
38 Dewatering and Drying of Wastewater Treatment Sludge
Guohua Chen, Po Lock Yue, and Arun S. Mujumdar


39 Drying of Biotechnological Products
´
Janusz Adamiec, Władysław Kaminski, Adam S. Markowski, and Czesław Strumiłło
40 Drying of Coated Webs
James Y. Hung, Richard J. Wimberger, and Arun S. Mujumdar
41 Drying of Polymers
Arun S. Mujumdar and Mainul Hasan
42 Drying of Enzymes
´
Ana M.R. Pilosof and Virginia E. Sanchez
43 Drying of Coal
´
Jerzy Pikon and Arun S. Mujumdar

Part IV Miscellaneous Topics in Industrial Drying
44 Dryer Feeding Systems
Rami Y. Jumah and Arun S. Mujumdar
45 Dryer Emission Control Systems
Rami Y. Jumah and Arun S. Mujumdar
46 Energy Aspects in Drying
Czes law Strumil"" Peter L. Jones, and Romuald Zyłła
" lo,
47 Heat Pump Drying Systems
Chou Siaw Kiang and Chua Kian Jon
48 Safety Aspects of Industrial Dryers
Adam S. Markowski and Arun S. Mujumdar
49 Control of Industrial Dryers
Rami Y. Jumah, Arun S. Mujumdar, and Vijaya G.S. Raghavan
50 Solid–Liquid Separation for Pretreatment of Drying Operation
Mompei Shirato and Masashi Iwata
51 Industrial Crystallization
Seppo Palosaari, Marjatta Louhi-Kultanen, and Zuoliang Sha
52 Frying of Foods
Vassiliki Oreopoulou, Magdalini Krokida, and Dimitris Marinos-Kouris
53 Cost-Estimation Methods for Drying
Zbigniew T. Sztabert and Tadeusz Kudra


Part I
Fundamental Aspects


1 Principles, Classiﬁcation,
and Selection of Dryers
Arun S. Mujumdar

CONTENTS

1.1 Introduction ............................................................................................................................................... 4
1.2 External Conditions (Process 1)................................................................................................................. 5
1.2.1 Vapor–Liquid Equilibrium and Enthalpy for a Pure Substance Vapor–Pressure Curve................ 6
1.2.1.1 The Clausius–Clapeyron Equation................................................................................... 6
1.2.1.2 Enthalpy ........................................................................................................................... 6
1.2.1.3 Heat Capacity................................................................................................................... 7
1.2.2 Vapor–Gas Mixtures ...................................................................................................................... 8
1.2.3 Unsaturated Vapor–Gas Mixtures: Psychrometry in Relation to Drying ...................................... 9
1.2.3.1 Dry Bulb Temperature ..................................................................................................... 9
1.2.3.2 Dew Point......................................................................................................................... 9
1.2.3.3 Humid Volume ................................................................................................................. 9
1.2.3.4 Enthalpy ........................................................................................................................... 9
1.2.4 Enthalpy–Humidity Charts .......................................................................................................... 10
1.2.4.1 Adiabatic Saturation Curves .......................................................................................... 11
1.2.4.2 Wet Bulb Temperature ................................................................................................... 12
1.2.5 Types of Psychrometric Representation ....................................................................................... 13
1.3 Internal Conditions (Process 2)................................................................................................................ 13
1.3.1 Moisture Content of Solids .......................................................................................................... 14
1.3.2 Moisture Isotherms....................................................................................................................... 14
1.3.2.1 Sorption–Desorption Hysteresis..................................................................................... 15
1.3.2.2 Temperature Variations and Enthalpy of Binding ......................................................... 16
1.3.3 Determination of Sorption Isotherms........................................................................................... 16
1.4 Mechanism of Drying .............................................................................................................................. 17
1.4.1 Characteristic Drying Rate Curve ................................................................................................ 18
1.5 Classiﬁcation and Selection of Dryers ..................................................................................................... 20
1.5.1 Heating Methods .......................................................................................................................... 21
1.5.1.1 Convection ..................................................................................................................... 21
1.5.1.2 Conduction..................................................................................................................... 22
1.5.1.3 Radiation........................................................................................................................ 22
1.5.2 Temperature and Pressure of Operation....................................................................................... 22
1.5.3 Conveying of Material in Dryer ................................................................................................... 22
1.6 Effect of Energy Costs, Safety, and Environmental Factors on Dryer Selection .................................... 24
1.7 Design of Dryers ...................................................................................................................................... 26
1.8 Guidelines for Dryer Selection................................................................................................................. 26
1.9 Conclusions.............................................................................................................................................. 29
Acknowledgment.............................................................................................................................................. 30
Nomenclature ................................................................................................................................................... 31
References ........................................................................................................................................................ 31


1.1 INTRODUCTION (evaporation), mechanical dewatering operations
such as filtration, centrifugation, sedimentation, super-
Drying commonly describes the process of thermally critical extraction of water from gels to produce ex-
removing volatile substances (moisture) to yield a tremely high porosity aerogels (extraction) or so-called
solid product. Moisture held in loose chemical com- drying of liquids and gases by the use of molecular
bination, present as a liquid solution within the solid sieves (adsorption). Phase change and production of a
or even trapped in the microstructure of the solid, solid phase as end product are essential features of the
which exerts a vapor pressure less than that of pure drying process. Drying is an essential operation in the
liquid, is called bound moisture. Moisture in excess of chemical, agricultural, biotechnology, food, polymer,
bound moisture is called unbound moisture. ceramics, pharmaceutical, pulp and paper, mineral
When a wet solid is subjected to thermal drying, processing, and wood processing industries.
two processes occur simultaneously: Drying is perhaps the oldest, most common and
most diverse of chemical engineering unit operations.
1. Transfer of energy (mostly as heat) from the Over 400 types of dryers have been reported whereas
surrounding environment to evaporate the sur- over 100 distinct types are commonly available. It
face moisture competes with distillation as the most energy-intensive
2. Transfer of internal moisture to the surface of unit operation due to the high latent heat of vapor-
the solid and its subsequent evaporation due to ization and the inherent inefficiency of using hot air as
process 1 the (most common) drying medium. Several studies
report national energy consumption for industrial dry-
The rate at which drying is accomplished is gov- ing operations ranging from 10–15% for United
erned by the rate at which the two processes proceed. States, Canada, France, and U.K. to 20–25% for
Energy transfer as heat from the surrounding envir- Denmark and Germany. The latter figures have been
onment to the wet solid can occur as a result of obtained recently based on mandatory energy audit
convection, conduction, or radiation and in some data supplied by industry and hence are more reliable.
cases as a result of a combination of these effects. Energy consumption in drying ranges from a low
Industrial dryers differ in type and design, depending value of under 5% for the chemical process industries
on the principal method of heat transfer employed. In to 35% for the papermaking operations. In the United
most cases heat is transferred to the surface of the wet States, for example, capital expenditures for dryers
solid and then to the interior. However, in dielectric, are estimated to be in the order of only $800 million
radio frequency (RF), or microwave freeze drying, per annum. Thus, the major costs for dryers are in their
energy is supplied to generate heat internally within operation rather than in their initial investment costs.
the solid and flows to the exterior surfaces. Drying of various feedstocks is needed for one or
Process 1, the removal of water as vapor from the several of the following reasons: need for easy-to-
material surface, depends on the external conditions handle free-flowing solids, preservation and storage,
of temperature, air humidity and flow, area of ex- reduction in cost of transportation, achieving desired
posed surface, and pressure. quality of product, etc. In many processes, improper
Process 2, the movement of moisture internally drying may lead to irreversible damage to product
within the solid, is a function of the physical nature quality and hence a nonsalable product.
of the solid, the temperature, and its moisture con- Before proceeding to the basic principles, it is
tent. In a drying operation any one of these processes useful to note the following unique features of drying,
may be the limiting factor governing the rate of dry- which make it a fascinating and challenging area for
ing, although they both proceed simultaneously research and development (R&D):
throughout the drying cycle. In the following sections
we shall discuss the terminology and some of the basic . Product size may range from microns to tens of
concepts behind the two processes involved in drying. centimeters (in thickness or depth)
The separation operation of drying converts a . Product porosity may range from 0 to 99.9%
solid, semisolid, or liquid feedstock into a solid prod- . Drying times range from 0.25 s (drying of tissue
uct by evaporation of the liquid into a vapor phase paper) to 5 months (for certain hardwood species)
through application of heat. In the special case of . Production capacities may range from 0.10 kg/h
freeze drying, which takes place below the triple to 100 tons/h
point of the liquid that is removed, drying occurs . Product speeds range from 0 (stationary) to
by sublimation of the solid phase directly into the 2000 m/min (tissue paper)
vapor phase. This definition thus excludes conversion . Drying temperatures range from below the triple
of a liquid phase into a concentrated liquid phase point to above the critical point of the liquid


. Operating pressure may range from fraction of a before it is transported away by the carrier gas (or by
millibar to 25 atm application of vacuum for nonconvective dryers).
. Heat may be transferred continuously or inter- Transport of moisture within the solid may occur
mittently by convection, conduction, radiation, by any one or more of the following mechanisms of
or electromagnetic fields mass transfer:

Clearly, no single design procedure that can . Liquid diffusion, if the wet solid is at a tempera-
apply to all or even several of the dryer variants is ture below the boiling point of the liquid
possible. It is therefore essential to revert to the . Vapor diffusion, if the liquid vaporizes within
fundamentals of heat, mass and momentum transfer material
coupled with knowledge of the material properties . Knudsen diffusion, if drying takes place at very
(quality) when attempting design of a dryer or an- low temperatures and pressures, e.g., in freeze
alysis of an existing dryer. Mathematically speaking, drying
all processes involved, even in the simplest dryer, are . Surface diffusion (possible although not proven)
highly nonlinear and hence scale-up of dryers is gen- . Hydrostatic pressure differences, when internal
erally very difficult. Experimentation at laboratory vaporization rates exceed the rate of vapor
and pilot scales coupled with field experience and transport through the solid to the surroundings
know how for it is essential to the development of a . Combinations of the above mechanisms
new dryer application. Dryer vendors are necessarily
specialized and normally offer only a narrow range Note that since the physical structure of the dry-
of drying equipment. The buyer must therefore be ing solid is subject to change during drying, the mech-
reasonably conversant with the basic knowledge of anisms of moisture transfer may also change with
the wide assortment of dryers and be able to come up elapsed time of drying.
with an informal preliminary selection before going
to the vendors with notable exceptions. In general, 1.2 EXTERNAL CONDITIONS (PROCESS 1)
several different dryers may be able to handle a given
application. Here the essential external variables are temperature,
Drying is a complex operation involving transient humidity, rate and direction of airflow, the physical
transfer of heat and mass along with several rate form of the solid, the desirability of agitation, and the
processes, such as physical or chemical transform- method of supporting the solid during the drying
ations, which, in turn, may cause changes in product operation [1]. External drying conditions are espe-
quality as well as the mechanisms of heat and mass cially important during the initial stages of drying
transfer. Physical changes that may occur include when unbound surface moisture is removed. In cer-
shrinkage, puffing, crystallization, and glass transi- tain cases, for example, in materials like ceramics and
tions. In some cases, desirable or undesirable chem- timber in which considerable shrinkage occurs, exces-
ical or biochemical reactions may occur, leading to sive surface evaporation after the initial free moisture
changes in color, texture, odor, or other properties of has been removed sets up high moisture gradients from
the solid product. In the manufacture of catalysts, for the interior to the surface. This is liable to cause over-
example, drying conditions can yield significant dif- drying and excessive shrinkage and consequently high
ferences in the activity of the catalyst by changing the tension within the material, resulting in cracking and
internal surface area. warping. In these cases surface evaporation should be
Drying occurs by effecting vaporization of the retarded through the employment of high air relative
liquid by supplying heat to the wet feedstock. As humidities while maintaining the highest safe rate of
noted earlier, heat may be supplied by convection internal moisture movement by heat transfer.
(direct dryers), by conduction (contact or indirect Surface evaporation is controlled by the diffusion
dryers), radiation or volumetrically by placing the of vapor from the surface of the solid to the surround-
wet material in a microwave or RF electromagnetic ing atmosphere through a thin film of air in contact
field. Over 85% of industrial dryers are of the con- with the surface. Since drying involves the interphase
vective type with hot air or direct combustion gases as transfer of mass when a gas is brought in contact with
the drying medium. Over 99% of the applications a liquid in which it is essentially insoluble, it is neces-
involve removal of water. All modes except the di- sary to be familiar with the equilibrium characteristics
electric (microwave and RF) supply heat at the of the wet solid. Also, since the mass transfer is usu-
boundaries of the drying object so that the heat ally accompanied by the simultaneous transfer of
must diffuse into the solid primarily by conduction. heat, due consideration must be given to the enthalpy
The liquid must travel to the boundary of the material characteristics.


1.2.1 VAPOR–LIQUID EQUILIBRIUM AND 1.2.1.1 The Clausius–Clapeyron Equation
ENTHALPY FOR A PURE SUBSTANCE
Comprehensive tables of vapor-pressure data of com-
VAPOR–PRESSURE CURVE mon liquids, such as water, common refrigerants, and
When a liquid is exposed to a dry gas, the liquid others, may be found in Refs. [2,3]. For most liquids,
evaporates, that is, forms vapor and passes into the the vapor–pressure data are obtained at a few discrete
gaseous phase. If mW is the mass of vapor in the temperatures, and it might frequently be necessary to
gaseous phase, then this vapor exerts a pressure over interpolate between or extrapolate beyond these
the liquid, the partial pressure, which, assuming ideal measurement points. At a constant pressure, the
gas behavior for the vapor, is given by Clausius–Clapeyron equation relates the slope of the
vapor pressure–temperature curve to the latent heat
mW of vaporization through the relation
PW V ¼ RT or PW VW ¼ RT (1:1)
MW
dP0W DHW
The maximum value of PW that can be reached at any ¼ (1:2)
0
dT T(VW À VL )
temperature is the saturated vapor pressure PW. If the
vapor pressure of a substance is plotted against tem- where VW and VL are the speciﬁc molar volumes of
perature, a curve such as TC of Figure 1.1 is obtained. saturated vapor and saturated liquid, respectively,
Also plotted in the ﬁgure are the solid–liquid equilib- and DHW is the molar latent heat of vaporization.
rium curve (melting curve) and the solid–vapor (sub- Since the molar volume of the liquid is very small
limation) curve. The point T in the graph at which all compared with that of the vapor, we neglect VL and
three phases can coexist is called the triple point. For substitute for VW from Equation 1.1 to obtain
all conditions along the curve TC, liquid and vapor
may coexist, and these points correspond with the
DHW
saturated liquid and the saturated vapor state. Point d ln P0 ¼
W dT (1:3)
C is the critical point at which distinction between the RT 2
liquid and vapor phases disappears, and all properties
Since DHW could be assumed to be a constant over
of the liquid, such as density, viscosity, and refractive
short temperature ranges, Equation 1.3 can be inte-
index, are identical with those of the vapor. The
grated to
substance above the critical temperature is called a
gas, the temperature corresponding to a pressure at
each point on the curve TC is the boiling point, and DHW
ln P0 ¼ À
W þ constant (1:4)
that corresponding to a pressure of 101.3 kPa is the RT
normal boiling point.
and this equation can be used for interpolation. Al-
ternatively, reference-substance plots [6] may be con-
structed. For the reference substance,

DHR
pcrit d ln P0 ¼
R dT (1:5)
L C RT 2

Dividing Equation 1.3 by Equation 1.5 and integrat-
ing provides
Solid Liquid
MW DHW
ln P0 ¼
W ln P0 þ constant
R (1:6)
MR DHR
Pressure

The reference substance chosen is one whose vapor
pressure data are known.
Vapor
T
1.2.1.2 Enthalpy

t crit All substances have an internal energy due to the
Temperature
motion and relative position of the constituent
FIGURE 1.1 Vapor pressure of a pure liquid. atoms and molecules. Absolute values of the internal


energy, u, are unknown, but numerical values relative vapor,’’ however, cut across the constant pressure
to an arbitrarily defined baseline at a particular tem- lines and show the enthalpies for these conditions at
perature can be computed. In any steady flow system temperatures and pressures corresponding to the
there is an additional energy associated with forcing equilibrium vapor pressure relationship for the sub-
streams into a system against a pressure and in for- stance. The distance between the saturated vapor and
cing streams out of the system. This flow work per saturated liquid curves, such as the distance VÀL
unit mass is PV, where P is the pressure and V is the corresponds to the latent heat of vaporization at a
specific volume. The internal energy and the flow temperature T. Both T and VÀL are dependent on
work per unit mass have been conveniently grouped pressure, the distance VÀL decreases and becomes
together into a composite energy called the enthalpy H. zero at the critical temperature TC. Except near the
The enthalpy is defined by the expression critical temperature, the enthalpy of the liquid is al-
most independent of pressure until exceedingly high
H ¼ u þ PV (1:7) pressures are reached.

and has the units of energy per unit mass (J/kg or N 1.2.1.3 Heat Capacity
m/kg).
The heat capacity is defined as the heat required to
Absolute values of enthalpy of a substance like the
raise the temperature of a unit mass of substance by a
internal energy are not known. Relative values of
unit temperature. For a constant pressure process, the
enthalpy at other conditions may be calculated by
heat capacity CP is given by
arbitrarily setting the enthalpy to zero at a convenient
reference state. One convenient reference state for
@Q
zero enthalpy is liquid water under its own vapor CP ¼ (1:8)
pressure of 611.2 Pa at the triple-point temperature @T P
of 273.16 K (0.018C).
The isobaric variation of enthalpy with tempera- where the heat flow Q is the sum of the internal energy
ture is shown in Figure 1.2. At low pressures in the change @u and the work done against pressure P @V.
gaseous state, when the gas behavior is essentially Equation 1.8 may be expanded as follows:
ideal, the enthalpy is almost independent of the pres-
sure, so the isobars nearly superimpose on each other. @u @V @H
CP ¼ þP ¼ (1:9)
The curves marked ‘‘saturated liquid’’ and ‘‘saturated @T P @T P @T P

Lines of
constant
pressure
Low
pressure
High pressure
Vapor
Saturated vapor
Critical point
Relative enthalpy

V

Saturated liquid

L

T TC
Temperature

FIGURE 1.2 Typical enthalpy–temperature diagram for a pure substance.

ß 2006 by Taylor Francis Group, LLC.

The slope of the isobars of Figure 1.2 yields the heat mW
Y¼ (1:14)
capacities. mG
In drying calculation, it is more convenient to use
the mean values of heat capacity over a ﬁnite tem- The total mass can be written in terms of Y and mG as
perature step:
mG þ mW ¼ mG (1 þ Y ) (1:15)
ð T2
DQ 1
CP ¼ ¼ CP dT (1:10) Using the gas law for vapor and air fractions at
DT P (T2 À T1 ) T1 constant total volume V and temperature T,

Second-order polynomials in temperature have been PG V PW V
found to adequately describe the variation of CP with mG ¼ MG and mW ¼ MW (1:16)
RT RT
temperature in the temperature range 300–1500 K [4],
but for the temperature changes normally occurring Thus,
in drying the quadratic term can be neglected.
Thus if PW MW
Y¼ (1:17)
P G MG
CP ¼ a þ bT (1:11)
Using Dalton’s law of partial pressures,
then from Equation 1.10,
P ¼ PW þ PG (1:18)
1
C P ¼ a þ b(T1 þ T2 ) ¼ CP (Tav ) (1:12)
2 and
The mean heat capacity is the heat capacity evaluated PW MW
at the arithmetic mean temperature Tav. Y¼ (1:19)
P À PW MG
From Equation 1.9 and Equation 1.10, the en-
thalpy of the pure substance can be estimated from When the partial pressure of the vapor in the gas
its heat capacity by equals the vapor pressure of the liquid, an equilibrium
is reached and the gas is said to be saturated with
H ¼ CPu (1:13) vapor. The ideal saturated absolute humidity is then
where u denotes the temperature difference or excess PW MW
over the zero enthalpy reference state. Heat capacity YS ¼ (1:20)
P À P 0 MG
W
data for a large number of liquids and vapors are
found in Ref. [5].
The relative humidity c of a vapor–gas mixture is a
measure of its fractional saturation with moisture and
1.2.2 VAPOR–GAS MIXTURES is deﬁned as the ratio of the partial pressure of the
0
vapor PW to the saturated pressure PW at the same
When a gas or gaseous mixture remains in contact temperature. Thus c is given by
with a liquid surface, it will acquire vapor from the
liquid until the partial pressure of the vapor in the gas PW
mixture equals the vapor pressure of the liquid at the c¼ (1:21)
P0
W
existing temperature. In drying applications, the gas
frequently used is air and the liquid used is water. Equation 1.19 may now be written as
Although common concentration units (partial pres-
sure, mole fraction, and others) based on total quan- MW cP0 W
tity of gas and vapor are useful, for operations that Y¼ (1:22)
MG P À cP0W
involve changes in vapor content of a vapor–gas mix-
ture without changes in the amount of gas, it is more
For water vapor and air when MW ¼ 18.01 kg/kmol
convenient to use a unit based on the unchanging
and MG ¼ 28.96 kg/kmol, respectively, Equation
amount of gas.
1.22 becomes
Humid air is a mixture of water vapor and gas,
composed of a mass mW of water vapor and a mass
mG of gas (air). The moisture content or absolute cP0W
Y ¼ 0:622 (1:23)
humidity can be expressed as P À cP0W


1.2.3 UNSATURATED VAPOR–GAS MIXTURES: reduced to an infinitesimal amount below TD, the
PSYCHROMETRY IN RELATION TO DRYING vapor will condense and the process follows the sat-
uration curve.
If the partial pressure of the vapor in the vapor–gas While condensation occurs the gas always remains
mixture is for any reason less than the vapor pressure saturated. Except under specially controlled circum-
of the liquid at the same temperature, the vapor–gas stances, supersaturation will not occur and no vapor–
mixture is said to be unsaturated. As mentioned earl- gas mixture whose coordinates lie to the left of the
ier, two processes occur simultaneously during the saturation curve will result.
thermal process of drying a wet solid, namely, heat
transfer to change the temperature of the wet solid
and to evaporate its surface moisture and the mass 1.2.3.3 Humid Volume
transfer of moisture to the surface of the solid and its
subsequent evaporation from the surface to the sur- The humid volume VH of a vapor–gas mixture is the
rounding atmosphere. Frequently, the surrounding volume in cubic meters of 1 kg of dry gas and its
medium is the drying medium, usually heated air or accompanying vapor at the prevailing temperature
combustion gases. Consideration of the actual quan- and pressure. The volume of an ideal gas or vapor
tities of air required to remove the moisture liberated at 273 K and 1 atm (101.3 kPa) is 22.4 m3/kg mol. For
by evaporation is based on psychrometry and the use a mixture with an absolute humidity Y at TG (K) and
of humidity charts. The following are definitions of P (atm), the ideal gas law gives the humid volume as
expressions used in psychrometry [6].
1 Y T 1
1.2.3.1 Dry Bulb Temperature VH ¼ þ 22:4
M G MW 273:14 P

This is the temperature of a vapor–gas mixture as 1 Y T
ordinarily determined by the immersion of a therm- VH ¼ 0:082 þ
M G MW P (1:24)
ometer in the mixture.
When the mass of dry gas in the vapor–gas mixture is
1.2.3.2 Dew Point multiplied by the humid volume, the volume of the
This is the temperature at which a vapor–gas mixture vapor–gas mixture is obtained. The humid volume at
becomes saturated when cooled at a constant total saturation is computed with Y ¼ YS, and the specific
pressure out of contact with a liquid (i.e., at constant volume of the dry gas can be obtained by substi-
absolute humidity). The concept of the dew point is tuting Y ¼ 0. For partially saturated mixtures, VH
best illustrated by referring to Figure 1.3, a plot of the may be interpolated between values for 0 and 100%
absolute humidity versus temperature for a fixed pres- saturation at the same temperature and pressure.
sure and the same gas. If an unsaturated mixture
initially at point F is cooled at constant pressure out
of contact of liquid, the gas saturation increases until 1.2.3.4 Enthalpy
the point G is reached, when the gas is fully saturated.
Since the enthalpy is an extensive property, it could be
The temperature at which the gas is fully saturated
expected that the enthalpy of a humid gas is the sum
is called the dew point TD. If the temperature is
of the partial enthalpies of the constituents and a term
to take into account the heat of mixing and other
effects. The humid enthalpy IG is defined as the en-
100% thalpy of a unit mass of dry gas and its associated
Relative D 50% moisture. With this definition of enthalpy,
100% saturation
Relative curves
Absolure humidity

saturation 75% IG ¼ HGG þ YHGW þ DHGM (1:25)
curves 25%
Pressure

50%
where HGG is the enthalpy of dry gas, HGW is the
25%
G F enthalpy of moisture, and DHGM is the residual en-
thalpy of mixing and other effects. In air saturated
with water vapor, this residual enthalpy is only
TD À0.63 kJ/kg at 608C (333.14 K) [3] and is only 1% of
Temperature Temperature
HGG; thus it is customary to neglect the influences of
FIGURE 1.3 Two forms of psychrometric charts. this residual enthalpy.


It is sometimes convenient to express the enthalpy T0 and TD, C PW is the mean capacity of the moisture
in terms of specific heat. Analogous to Equation 1.13, vapor evaluated between TD and TG, and DHVD is the
we could express the enthalpy of the vapor–gas mix- latent heat of vaporization at the dew point TD. The
ture by value of DHVD can be approximately calculated from
a known latent heat value at temperature T0 by
IG ¼ C PY u þ DHV0 Y (1:26)
DHVD TD À TC 1=3
À (1:32)
C PY is called the humid heat, defined as the heat DHV0 T0 À TC
required to raise the temperature of 1 kg of gas and
its associated moisture by 1 K at constant pressure. where TC is the critical temperature. Better and more
For a mixture with absolute humidity Y, accurate methods of estimating DHVD are available in
Refs. [5,7].
C PY ¼ C PG þ C PW Y (1:27)

where C PG and C PW are the mean heat capacities of 1.2.4 ENTHALPY–HUMIDITY CHARTS
the dry gas and moisture, respectively. Using Equation 1.23, Equation 1.25, and Equation
The path followed from the liquid to the vapor 1.28, the enthalpy–humidity diagram for unsaturated
state is described as follows. The liquid is heated up to air (c 1) can be constructed using the parameters c
the dew point TD, vaporized at this temperature, and and u. In order to follow the drying process we need
superheated to the dry bulb temperature TG. Thus access to enthalpy–humidity values. There seems to be
no better, convenient, and cheaper way to store these
HGW ¼ C LW (TD À T0 ) þ DHVD data than in graphic form. The first of these enthalpy–
(1:28)
þ C PW (TG À TD ) humidity charts is attributed to Mollier. Mollier’s
original enthalpy–humidity chart was drawn with
However, since the isothermal pressure gradient (DH/ standard rectangular coordinates (Figure 1.4), but
DP)T is negligibly small, it could be assumed that the in order to extend the area over which it can be
final enthalpy is independent of the vaporization path read, art oblique-angle system of coordinates is chosen
followed. For the sake of convenience it could be for IG ¼ f(Y).
assumed that vaporization occurs at 08C (273.14 K), In the unsaturated region, it can be seem from
at which the enthalpy is zero, and then directly super- Equation 1.30 that IG varies linearly with the humid-
heated to the final temperature TG. The enthalpy of ity Y and the temperature TG. If zero temperature
the vapor can now be written as (08C) is taken as the datum for zero enthalpy, then

HGW ¼ C PW (TG À T0 ) þ DHV0 (1:29) IG ¼ C PG u þ Y (C PW u þ DHV0 ) (1:33)

where u is the temperature in degree Celsius.
and the humid enthalpy given by

IG ¼ C PG (TG À T0 )
(1:30)
þ Y (C PW (TG À T0 ) þ DHV0 ) Isotherms shown as dotted lines

0.10 Unsaturated gas
Using the definition for the humid heat capacity, 0.2
Equation 1.30 reduces to
0.5
Humid enthalpy, kJ/kg

IG ¼ C PY (TG À T0 ) þ DHV0 Y (1:31) 1.00 Saturated gas
Relative
In Equation 1.31 the humid heat is evaluated at (TG þ humidity
T0)/2 and DHV0, the latent heat of vaporization at 08C CpwqY y
(273.14 K). Despite its handiness, the use of Equation
1.31 is not recommended above a humidity of 0.05 kg/
135°
kg. For more accurate work, it is necessary to resort
Isenthalpic lines
to the use of Equation 1.28 in conjunction with Equa-
Humidity, Y
tion 1.25. In Equation 1.28 it should be noted that
C LW is the mean capacity of liquid moisture between FIGURE 1.4 An enthalpy–humidity diagram for a moist gas.


The isotherms (u ¼ constant) cut the ordinate temperature different from those at the entrance. The
(Y ¼ 0) at a value C PGu (the dry gas enthalpy). If operation is adiabatic as no heat is gained or lost by
the isenthalpic lines (IG ¼ constant) are so inclined the surroundings. Doing a mass balance on the vapor
that they fall with a slope ÀDHV0, and if only D HV0Y results in
were taken into account in the contribution of vapor
to the vapor–gas enthalpy, then the isotherms would GV ¼ GG (Yout À Yin ) (1:34)
run horizontally, but because of the contribution of
C PWu Y, they increase with Y for u 08C and de- The enthalpy balance yields
crease with Y for u 08C. Contours of relative hu-
midity c are also plotted. The region above the curve IGin þ (Yout À Yin )ILW ¼ IGout (1:35)
c ¼ 1 at which air is saturated corresponds to an
unsaturated moist gas; the region below the curve Substituting for IG from Equation 1.31, we have
corresponds to fogging conditions. At a fixed tem-
perature air cannot take up more than a certain C PYin (Tin À T0 ) þ DHV0 Yin þ (Yout À Yin )C LW (TL À T0 )
amount of vapor. Liquid droplets then precipitate
due to oversaturation, and this is called the cloud or ¼ C PYout (Tout À T0 ) þ DHV0 Yout (1:36)
fog state. Detailed enthalpy–humidity diagrams are
available elsewhere in this handbook and in Ref. [10]. Now, if a further restriction is made that the gas and
A humidity chart is not only limited to a specific the liquid phases reach equilibrium when they leave
system of gas and vapor but is also limited to a the system (i.e., the gas–vapor mixture leaving the
particular total pressure. The thermophysical proper- system is saturated with liquid), then Tout ¼ TGS,
ties of air may be generally used with reasonable IGout ¼ IGS, and Yout ¼ YGS where TGS is the adiabatic
accuracy for diatomic gases [3], so that charts devel- saturation temperature and YGS is the absolute hu-
oped for mixtures in air can be used to describe the midity saturated at TGS. Still further, if the liquid
properties of the same moisture vapor in a gas such as enters at the adiabatic saturation temperature TGS,
nitrogen. Charts other than those of moist air are that is, TL ¼ TGS, Equation 1.36 becomes
often required in the drying of fine chemicals and
pharmaceutical products. These are available in C PYln (Tln À T0 ) þ DHV0 Yln
Refs. [3,8,9].
þ (YGS À Yln )C LW (TGS À T0 )
¼ C PYGS (TGS À T0 ) þ DHV0 YGS (1:37)
1.2.4.1 Adiabatic Saturation Curves

Also plotted on the psychrometric chart are a family or substituting for C PG from Equation 1.27
of adiabatic saturation curves. The operation of adia-
batic saturation is indicated schematically in Figure C PYln (Tln À T0 ) þ Yln C PWln (Tln À T0 ) þ DHV0 Yln
1.5. The entering gas is contacted with a liquid and as
a result of mass and heat transfer between the gas and þ (YGS À Yln )C LW (TGS À T0 )
liquid the gas leaves at conditions of humidity and ¼ C PGGS (TGS À T0 ) þ C PWGS YGS (TGS À T0 )
þ DHV0 YGS (1:38)

y=I Assuming that the heat capacities are essentially con-
stant over the temperature range involved, C PGin ¼
C PGGS ¼ C PG and C PWin ¼ C PWGS ¼ C PW. Further
Y Adiabatic saturation path
subtracting Yin C PWTGS from both sides of Equation
A 1.38 and simplifying, we have

Bulk air B C PY (Tin À TGS )
humidity ¼ (YGS À Yin )
Â [(C PW (TGS À T0 ) þ DHV0 À C LW (TGS À T0 )]
TW TG
(1:39)
Wet bulb Dry bulb
Temperature T
From Figure 1.2 the quantity in square brackets is
FIGURE 1.5 A temperature–humidity diagram for moist air. equal to DHVS, and thus,


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