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secado industrial

  1. 1. 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.
  2. 2. ß 2006 by Taylor & Francis Group, LLC.
  3. 3. 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 ß 2006 by Taylor & Francis Group, LLC.
  4. 4. ß 2006 by Taylor & Francis Group, LLC.
  5. 5. 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 ß 2006 by Taylor & Francis Group, LLC.
  6. 6. ß 2006 by Taylor & Francis Group, LLC.
  7. 7. 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 ß 2006 by Taylor & Francis Group, LLC.
  8. 8. 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 ß 2006 by Taylor & Francis Group, LLC.
  9. 9. 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 ß 2006 by Taylor & Francis Group, LLC.
  10. 10. ß 2006 by Taylor & Francis Group, LLC.
  11. 11. 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. ß 2006 by Taylor & Francis Group, LLC.
  12. 12. 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 ß 2006 by Taylor & Francis Group, LLC.
  13. 13. 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. ß 2006 by Taylor & Francis Group, LLC.
  14. 14. ß 2006 by Taylor & Francis Group, LLC.
  15. 15. 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 ß 2006 by Taylor & Francis Group, LLC.
  16. 16. 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 Department of Chemical Engineering 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 Department of Chemical 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 National Technical University of Athens Lodz Technical University Athens, Greece Lodz, Poland Tadeusz Kudra Z.B. Maroulis CANMET Energy Technology Center Department of Chemical Engineering Varennes, Quebec, Canada National Technical University of Athens 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 Department of Chemical Engineering H. Stephen Lee Nagoya University Alcoa Technical Center Nagoya, Japan Monroeville, Pennsylvania ß 2006 by Taylor & Francis Group, LLC.
  17. 17. 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 Technical University 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 ß 2006 by Taylor & Francis Group, LLC.
  18. 18. 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 Technical University Lodz, Poland Po Lock Yue Department of Chemical Engineering 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 ß 2006 by Taylor & Francis Group, LLC.
  19. 19. 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 ß 2006 by Taylor & Francis Group, LLC.
  20. 20. 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 ß 2006 by Taylor & Francis Group, LLC.
  21. 21. 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 ß 2006 by Taylor & Francis Group, LLC.
  22. 22. Part I Fundamental Aspects ß 2006 by Taylor & Francis Group, LLC.
  23. 23. ß 2006 by Taylor & Francis Group, LLC.
  24. 24. 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 ß 2006 by Taylor & Francis Group, LLC.
  25. 25. 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 ß 2006 by Taylor & Francis Group, LLC.
  26. 26. . 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. ß 2006 by Taylor & Francis Group, LLC.
  27. 27. 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 ß 2006 by Taylor & Francis Group, LLC.
  28. 28. 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.
  29. 29. 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 ß 2006 by Taylor Francis Group, LLC.
  30. 30. 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. ß 2006 by Taylor Francis Group, LLC.
  31. 31. 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. ß 2006 by Taylor Francis Group, LLC.
  32. 32. 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, ß 2006 by Taylor Francis Group, LLC.

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