![Need for Advanced Drying Technologies 5
to the conventional spray dryer of the same throughput.
In general, the feedstock to be dried contains both surface and internal
moisture. The rate at which the surface moisture can be removed depends
only on the external heat and mass transfer rates since the controlling resis-
tance to drying rate lies outside the material being dried. Thus, enhanc-
ing external convective heat and mass transfer rates by increasing the
gas velocity and gas temperature or reducing gas humidity will lead to
increased drying rates for a purely convective (or direct) dryer. Any action
that enhances external (gas-side) resistance will yield an increase in the
drying rate. Of course, there are exceptions, for example, intense drying
may cause case-hardening and reduce drying rates or it may cause extreme
shrinkage and cracking, which are undesirable phenomena. Thus, an
increase of free-stream turbulence, application of mechanical vibration, or
responsible for the augmentation are different (see Chapter 13).
Above a critical temperature, commonly termed the inversion tempera-
ture, the rate of evaporation of the surface moisture is higher in super-
heated steam (SHS) drying than in hot-air drying (see Chapter 7). This is
due to the superior thermal properties of SHS. At lower temperatures, the
reduced temperature difference between the drying medium and the dry-
ing surface for SHS results in a lower drying rate for the latter. In purely
convective air-drying, the surface temperature is equal to the wet bulb
temperature corresponding to the air humidity and dry bulb tempera-
ture, whereas for SHS drying, it is the saturation temperature of steam,
that is, 100°C for atmospheric pressure.
Enhancement of the falling rate period of drying, which requires faster
achieve. In general, attempts to do so result in a change in product qual-
pulsation resulting in cavitation; the successive generation of high- and
(e.g., microwave [MW] or radio-frequency radiation) can heat up volu-
metrically the polar liquid to be vaporized (e.g., water). This practically
eliminates the resistance to transfer heat into the material; the transport
of moisture out through the material is also enhanced to some extent due
to the higher mobility of moisture at higher temperatures as well as due to
internal pressure gradient toward the material surface. The same mecha-
nism is responsible for the marginally increased drying rates observed in
SHS drying.
Another possible way of intensifying the drying rate involves increas-
ing the effective interfacial areas for heat and mass transfer. For example,
CRC_73877_Ch001.indd 5
CRC_73877_Ch001.indd 5 12/29/2008 2:20:18 PM
12/29/2008 2:20:18 PM
rates, so that the dryer volume can be reduced significantly as compared
sonic fields is also known to increase the drying rates, but the mechanisms
transport of heat and moisture through the material, is more difficult to
ity. Application of an ultrasonic field can cause high-frequency pressure
low-pressure fields causes rapid vaporization and enhanced transport
of the liquid through the material. The use of an electromagnetic field
in an impinging stream configuration, the impingement zone generated
oscillation of flow yields higher drying rates. Application of ultrasonic or
© 2009 by Taylor & Francis Group, LLC](https://image.slidesharecdn.com/81813-250321112828-825fa7f0/85/Advanced-drying-technologies-2nd-Edition-Tadeusz-Kudra-22-320.jpg)
![CORA LEE.
Would you hear the story told
Of the controversy bold,
That this day I did behold,
In a court of low degree,
Where his Honor sat like fate,
To decide betwixt the state
And a wanton villain’s mate,
Named Cora Lee?
The bold chief of stars was near,
As a witness to appear.
(By his order, Cora dear
Was languishing below.)
And for counsel she had got
A descendant of old Wat—
Noted for his daring plot,
Some years ago.
It was he commenced the fuss,
“For,” said he, “by this and thus,
Here I smell an animus[1]
As strong as musk of yore;
And it’s my condensed belief,
That in language terse and brief,
I can trace it to the chief,
E’en to his door.”
Then to all it did appear
That the chief was seized with fear;
To the lawyer he drew near,
And to him muttered low:
“I could never think that ye
Would be quite so hard with me;
You had better let me be,
And travel slow.”](https://image.slidesharecdn.com/81813-250321112828-825fa7f0/85/Advanced-drying-technologies-2nd-Edition-Tadeusz-Kudra-68-320.jpg)
Advanced drying technologies 2nd Edition Tadeusz Kudra
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- 5. Second Edition Advanced Drying Technologies CRC_73877_FM.indd i CRC_73877_FM.indd i 1/8/2009 11:31:06 AM 1/8/2009 11:31:06 AM © 2009 by Taylor & Francis Group, LLC
- 6. CRC Press is an imprint of the Taylor & Francis Group, an informa business Boca Raton London New York Tadeusz Kudra Arun S. Mujumdar Second Edition Advanced Drying Technologies CRC_73877_FM.indd iii CRC_73877_FM.indd iii 1/8/2009 11:31:08 AM 1/8/2009 11:31:08 AM © 2009 by Taylor & Francis Group, LLC
- 7. CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2009 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4200-7387-4 (Hardcover) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher can- not assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copy- right.com (http://www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that pro- vides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data Kudra, Tadeusz, 1945- Advanced drying technologies / authors, Tadeusz Kudra, Arun S. Mujumdar. -- 2nd ed. p. cm. “A CRC title.” Includes bibliographical references and index. ISBN 978-1-4200-7387-4 (hardcover : alk. paper) 1. Drying. I. Mujumdar, A. S. II. Title. TP363.K755 2009 660’.28426--dc22 2008052002 Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com CRC_73877_FM.indd iv CRC_73877_FM.indd iv 1/8/2009 11:31:08 AM 1/8/2009 11:31:08 AM © 2009 by Taylor & Francis Group, LLC
- 8. v Contents Preface to the First Edition...............................................................................xi Preface to the Second Edition....................................................................... xiii Authors..............................................................................................................xv Part I General Discussion: Conventional and Novel Drying Concepts 1 1 Need for Advanced Drying Technologies............................................ 3 1.1 Why New Drying Technologies? ....................................................3 1.2 1.3 Multistage Dryers..............................................................................7 References .................................................................................................... 9 2 versus Novel Technologies .................................................................... 11 References .................................................................................................. 17 3 Innovation and Trends in Drying Technologies............................... 19 3.1 Introduction...................................................................................... 19 3.2 Innovation: Types and Common Features................................... 19 3.3 Development of Improved Drying Technologies ....................... 23 3.4 Trends in Drying Research and Development............................ 24 References .................................................................................................. 26 Part II Selected Advanced Drying Technologies 27 4 Drying on Inert Particles ....................................................................... 29 4.1 Introduction...................................................................................... 29 4.2 Drying Mechanism and Process Considerations ....................... 29 4.3 Modeling and Scale-Up .................................................................. 38 4.4 Selected Dryers with Inert Particles .............................................42 References .................................................................................................. 51 5 Impinging Stream Drying..................................................................... 55 5.1 Basic Features................................................................................... 55 CRC_73877_FM.indd v CRC_73877_FM.indd v 1/8/2009 11:31:08 AM 1/8/2009 11:31:08 AM Intensification of Drying Rates........................................................4 Classification and Selection Criteria: Conventional © 2009 by Taylor & Francis Group, LLC
- 9. vi Contents 5.2 Hydrodynamics and Heat Transfer .............................................. 62 5.2.1 Penetration Depth and Oscillation Time.........................63 5.2.2 Pressure Drop......................................................................65 5.2.3 Heat Transfer........................................................................ 66 5.3 Drying ............................................................................................... 67 5.4 Some Recent Studies on ISD .......................................................... 71 References .................................................................................................. 73 6 Drying in Pulsed Fluid Beds.................................................................77 References .................................................................................................. 87 7 Superheated Steam Drying ................................................................... 89 7.1 Introduction...................................................................................... 89 7.2 Basic Principles of Superheated Steam Drying........................... 91 7.4 Quality Considerations................................................................... 94 7.5 Superheated Steam Drying of Selected Products ....................... 97 7.5.1 Drying of Sludges................................................................ 97 7.5.2 Drying of Coal ..................................................................... 98 7.5.3 Drying of Beet Pulp .......................................................... 101 7.5.4 Drying of Lumber ............................................................. 103 7.5.5 Drying of Pulp................................................................... 106 7.5.6 Drying of Peat.................................................................... 108 7.5.7 Drying of Paper and Tissue..............................................111 7.5.8 Drying of Wood Particles and Wood Wafers................ 112 7.5.9 Low-Pressure Drying with Superheated Steam ............113 7.5.10 Miscellaneous Applications..............................................113 7.6 Utilization of Exhaust Steam ........................................................114 7.7 Closing Remarks.............................................................................117 References .................................................................................................118 8 Airless Drying........................................................................................ 123 References ................................................................................................ 127 9 Drying in Mobilized Beds................................................................... 129 References ................................................................................................ 140 10 Drying with Shock Waves................................................................... 143 Appendix: Process Calculations and Equipment Design ................. 153 References ................................................................................................ 159 11 Vacu Jet Drying System.........................................................................161 References .................................................................................................167 CRC_73877_FM.indd vi CRC_73877_FM.indd vi 1/8/2009 11:31:08 AM 1/8/2009 11:31:08 AM 7.3 Classification and Selection of Superheated Steam Dryers....... 93 © 2009 by Taylor & Francis Group, LLC
- 10. Contents vii 12 Contact-Sorption Drying ..................................................................... 169 12.1 General Characteristics ............................................................... 169 12.2 Mechanism of Contact-Sorption Drying.................................. 171 12.3 Characteristics of Sorbents/Carriers......................................... 179 12.4 Technology of Contact-Sorption Drying .................................. 184 12.4.1 Drying of Particulate Materials on Inert Sorbent .................................................................... 184 12.4.2 Drying in a Fluid Bed of Inert Sorbent........................ 185 12.4.3 Sorption Drying Using Filler Materials....................... 187 12.4.4 Contact-Sorption Freeze-Drying .................................. 194 References ................................................................................................ 197 13 Sonic Drying........................................................................................... 199 13.1 Basic Characteristics of Sound................................................... 199 13.2 Sound Generation ........................................................................ 203 13.3 Mechanism of Sonic Drying....................................................... 205 13.4 Drying Kinetics............................................................................ 210 13.5 Sound-Assisted Dryers................................................................ 215 References ................................................................................................ 221 14 Pulse Combustion Drying...................................................................225 14.1 Principle of Pulse Combustion...................................................225 14.2 Pulse Combustors: Design and Operation............................... 229 14.2.1 Basic Types of Pulse Combustors................................. 229 14.2.1.1 Quarter-Wave (Schmidt) Pulse Combustor........................................................ 229 14.2.1.2 Helmholtz Pulse Combustor ......................... 230 14.2.1.3 Rijke-Type Pulse Combustor ......................... 230 14.2.2 Valve and Valveless Pulse Combustors....................... 230 14.2.2.1 Flapper and Reed-Type Valves...................... 231 14.2.2.2 Rotary Valves ................................................... 232 14.2.2.3 Aerodynamic Valves.......................................234 14.3 Pulse Combustion in Drying ..................................................... 236 14.4 Numerical Simulation of Pulse Combustion Drying ............. 250 References ................................................................................................ 258 15 Heat Pump Drying ................................................................................ 263 15.1 Introduction.................................................................................. 263 15.2 Principle of Heat Pump Operation............................................ 263 15.3 Low-Temperature Heat Pump Drying...................................... 272 15.4 Chemical Heat Pump Drying..................................................... 278 15.5 New Developments and Trends in Heat Pump Drying......... 286 References ................................................................................................ 288 CRC_73877_FM.indd vii CRC_73877_FM.indd vii 1/8/2009 11:31:09 AM 1/8/2009 11:31:09 AM © 2009 by Taylor & Francis Group, LLC
- 11. viii Contents 16 Fry-Drying .............................................................................................. 293 References ................................................................................................ 297 Part III Selected Techniques for Drying and Dewatering 299 17 Mechanical Thermal Expression........................................................ 301 References ................................................................................................306 18 Displacement Drying............................................................................ 307 References .................................................................................................314 19 Vapor Drying.......................................................................................... 315 References ................................................................................................ 319 20 Slush Drying .......................................................................................... 321 References ................................................................................................ 325 21 Atmospheric Freeze-Drying................................................................ 327 References ................................................................................................ 335 22 Spray-Freeze-Drying............................................................................. 337 References ................................................................................................ 341 23 Refractance Window.............................................................................343 References ................................................................................................346 24 References ................................................................................................ 350 Part IV Hybrid Drying Technologies 351 25 Microwave–Convective Drying with Cogeneration ...................... 353 References ................................................................................................ 362 26 Microwave–Vacuum Drying ............................................................... 363 References ................................................................................................ 369 27 Filtermat Drying.................................................................................... 371 References ................................................................................................ 373 28 Spray-Fluid Bed–Vibrated Fluid Bed Dryer..................................... 375 Reference.................................................................................................. 377 CRC_73877_FM.indd viii CRC_73877_FM.indd viii 1/8/2009 11:31:09 AM 1/8/2009 11:31:09 AM Carver-Greenfield Process................................................................... 347 © 2009 by Taylor & Francis Group, LLC
- 12. Contents ix 29 Combined Filtration and Drying ....................................................... 379 29.1 Introduction.................................................................................. 379 29.2 Pressure Filtration and Drying .................................................. 381 29.3 Centrifuge Dryer..........................................................................384 29.4 Vapor Pressure Dewatering........................................................ 385 29.5 Steam Pressure Filtration............................................................ 390 29.6 Jet Stream Centrifugation ........................................................... 392 29.7 Microwave-Assisted Filter Drying............................................ 393 References ................................................................................................ 395 30 Radio-Frequency Drying with 50 Ω Technology............................ 397 References ................................................................................................ 405 31 Radio Frequency–Assisted Heat Pump Drying .............................. 407 References ................................................................................................ 412 32 Radio Frequency–Vacuum Drying..................................................... 413 References ................................................................................................ 417 33 Miscellaneous Hybrid Technologies................................................. 419 33.1 Combined Microwave and Superheated Steam Drying ........ 419 33.2 Combined Infrared and Convection Drying ........................... 420 33.3 Infrared Heat Pump Drying....................................................... 420 33.4 Heat Pump Superheated Steam Drying ................................... 421 33.5 Infrared–Microwave Freeze-Drying .........................................422 References ................................................................................................423 Part V Other Techniques 425 34 Special Drying Technologies.............................................................. 427 34.1 Drying by Alternating High- and Low-Pressure Operations..................................................................................... 427 34.2 Through-Air Rotary Dryer ......................................................... 429 34.3 Drying with Bed Mixing.............................................................434 34.4 Drying at Ambient Air Temperature ........................................ 438 34.5 Vacuum-Superheated Steam Drying and Granulation .......... 441 34.6 Drying in Rotating-Jet Spouted Bed..........................................445 34.7 Drying with Sol–Gel Transformations........................................ 448 34.8 Supercritical Fluid Technology to Produce Dry Particles...... 452 References ................................................................................................ 453 CRC_73877_FM.indd ix CRC_73877_FM.indd ix 1/8/2009 11:31:09 AM 1/8/2009 11:31:09 AM © 2009 by Taylor & Francis Group, LLC
- 13. xi Preface to the First Edition Drying is a ubiquitous operation found in almost all industrial sectors, ranging from agriculture to pharmaceuticals. It is arguably the oldest, most common, most diverse, and most energy-intensive unit operation— coincidentally it is also one of the least understood at the microscopic level. Drying technology is an amalgamation of transport phenomena and material science as it deals not only with the removal of a liquid to produce a solid product but also with the extent to which the dry product meets the necessary quality criteria. About two decades ago, developments in drying occurred at a remark- of progress. Spurred by the energy crisis, consumer demand for better quality, and initiation of the biennial International Drying Symposium series, advances on both the fundamental and applied fronts began by leaps and bounds. Literally, thousands of technical papers of archival interest were published and made widely available. This had a synergistic effect on promoting further advances in the truly inter- and multidisci- This book is a direct outcome of the phenomenal growth in drying lit- erature as well as in new drying hardware. It is now virtually impossible for academic and industry personnel to keep abreast of the developments and evaluate them logically. Therefore, the main objective of this book is to provide an evaluative overview of the new and emerging technol- ogies in drying, which are not readily accessible through conventional literature. We have attempted to provide a glimpse of the developments that have taken place in the past two decades and the directions toward which we see these technologies heading. We have included some well- established new technologies that are already commercialized, such as bed dryers, and laboratory curiosities such as the displacement drying of wood (displacing water with the more volatile alcohol). We hope that some of the laboratory curiosities of today will lead to truly revolutionary evaluation of current technologies will hopefully lead to new ideas. coin. It is important to know what drives innovative ideas to the market- place. Here, we have also tried to look at the process of innovation and compare the innovative technologies with the more conventional ones, noting that novelty per se is not the goal of innovation. As can be seen readily from a cursory look at this book’s Contents, we have included dryers for all types of materials—from slurries and CRC_73877_FM.indd xi CRC_73877_FM.indd xi 1/8/2009 11:31:09 AM 1/8/2009 11:31:09 AM ably slow pace. Indeed, one wondered if the field showed any visible signs plinary field of drying technology. novel drying technologies in the future; a systematic classification and the superheated steam drying of pulp in flash or pressurized fluidized Innovation and knowledge are often called the flip sides of the same © 2009 by Taylor & Francis Group, LLC
- 14. xii Preface to the First Edition suspensions to continuous sheets such as paper and textiles. We have covered from low-tech, low-value products such as waste sludge to high- tech advanced materials, biotechnology products, and ceramics. We have included production rates that range from fractions of a kilogram per hour (some pharmaceuticals) to tens of tons per hour (paper, milk, etc.). Furthermore, we have dealt with drying processes that are completed in a fraction of a second (e.g., tissue paper) to several months (certain species of wood in large-dimension pieces). Thus, the scope is broad and, as the Finally, no new technology will see the light of the day without being appropriately supported by research and development (R&D). We have therefore tried to identify loopholes in our current knowledge regarding drying and dryers, which will provide new challenges to the new genera- tion of academic and industrial researchers, eventually leading to better drying technologies. Dr. Tadeusz Kudra Dr. Arun S. Mujumdar CRC_73877_FM.indd xii CRC_73877_FM.indd xii 1/8/2009 11:31:10 AM 1/8/2009 11:31:10 AM reader will find out, the range of innovations is truly breathtaking. © 2009 by Taylor & Francis Group, LLC
- 15. xiii Preface to the Second Edition As noted in the “Preface to the First Edition” of this book, drying is a ubiquitous operation found in almost all industrial sectors ranging from agriculture to pharmaceuticals. Drying technology involves the coupling of transport phenomena and material science, as it deals not only with the removal of liquid to produce a solid product, but also with the devel- opment of necessary quality criteria in the dried product. Often, what is optimal for heat and mass-transfer rates is not appropriate for drying wet material. Selection of optimal dryers or, more appropriately, drying systems, is a complex task because of the diverse physical and chemical characteristics of both the wet material and the dry product; possibilities of heat supply by convection, conduction, radiation; and radiation includ- well as the quality, costs, energy, and environmental constraints. Several hundred types of dryers have been examined in the literature; thus, select- ing the right dryer is very challenging indeed. The rising global population, combined with their aspirations for enhanced standards of living, will continue to place very high demands on all its operations. As a particularly energy-intensive unit operation, dry- ing technologies can be expected to be re-examined in the years to come. Because most dryers still use fossil fuels, there is a serious environmen- tal impact of very large-scale drying operations. The rapidly escalating energy costs and the potential for energy shortages provide incentives for increased attention to use renewable energy sources such as solar, wind energy, and biomass for agricultural and industrial drying operations. This second edition is a result of the increased industry interest in the technical literature on drying technologies. Over the past three decades, there has been an explosive growth in the technical literature dealing with both basic and applied aspects of thermal drying. Therefore, as with form, an evaluative overview of the new and emerging technologies in drying, which is not readily accessible in the conventional literature such as handbooks or textbooks. Here, we have attempted to provide to the reader a glimpse at the key developments that have taken place in the past two decades. We have included some well-established relatively new technol- ogies that are already commercialized, such as the superheated steam tory curiosities such as the displacement drying of wood where water is displaced with the more volatile alcohol. We hope that both incremental CRC_73877_FM.indd xiii CRC_73877_FM.indd xiii 1/8/2009 11:31:10 AM 1/8/2009 11:31:10 AM ing volumetric heat generation by microwave or radio frequency fields, as energy resources. This will require industry to be more energy-efficient in the first edition, the main objective of this book is to provide, in a capsule drying of pulp in flash- or pressurized-fluidized bed dryers, and labora- © 2009 by Taylor & Francis Group, LLC
- 16. xiv Preface to the Second Edition and radical innovations in drying technologies will lead to improved drying technologies in the coming decades. whole range of wet materials ranging from liquids through semisolids to solids. Thus, the scope of coverage is broad and, as the reader will national industrial energy consumption attributed to industrial drying in developed nations, it is very important for industry and academia alike to seek better ways of thermal dehydration. As the quality of living in the populous nations such as China, India, Indonesia, and Brazil rises, the energy demand for agricultural and industrial drying will necessar- This book provides a panorama of ideas that can be applied toward this effort. We hope that this new edition of Advanced Drying Technologies will also help trigger further industry interest in supporting R&D in this important area and also in joining hands with academia to carry out cost-effective R&D in a collaborative manner. Such interaction is key to the effective and rapid transfer of technology from R&D laboratories to industrial practice. Dr. Tadeusz Kudra Dr. Arun S. Mujumdar CRC_73877_FM.indd xiv CRC_73877_FM.indd xiv 1/8/2009 11:31:10 AM 1/8/2009 11:31:10 AM As was the case with the first edition, we have covered dryers for the find out, the range of innovations is truly breathtaking. With 10–20% of ily rise. We hope that this will also lead to more efficient drying systems. © 2009 by Taylor & Francis Group, LLC
- 17. xv Authors Dr. Tadeusz Kudra is senior research scientist at CANMET Energy Technology Centre, Varennes, Quebec, Canada; formerly associate pro- fessor at Lodz Technical University (Lodz, Poland), adjunct professor at McGill University (Montreal, Canada), Laval University (Québec, Canada), and University of Ottawa (Ottawa, Canada). He is the author of more than 200 papers, 40 conference presentations, 28 book chapters, and 4 books including Drying: Principles, Applications and Design and Thermal Processing of Biomaterials. He is the holder of 12 patents and the recipient of numer- ous honors, including the prestigious International Drying Symposium (IDS) awards for Distinguished Professional Contribution to Drying Technology (IDS’96) and for Excellence in Drying: Transferring Fundamental Results into Practice (IDS’2004). He delivered more than 30 keynote and invited lectures at conferences and professional courses. He is also an honorary professor at Tianjin University of Light Industry (China) and Tambov State Technical University (Russia). Dr. Kudra is a member of several international committees, advisory panels, and boards for jour- nals and conferences in the area of drying. His research has focused on advanced drying technologies and energy aspects in drying. Dr. Kudra received his MSc (1969) and PhD (1975) from the Lodz Technical University in Poland. Dr. Arun S. Mujumdar is professor of mechanical engineering and director of the Minerals, Metals and Materials Technology Centre at the National University of Singapore (NUS). He is also an adjunct professor at McGill University, Montreal, Canada, where he was professor of chemical engineering before moving to NUS in 2000. Dr. Mujumdar is well known worldwide for his extensive and impactful contributions to drying R&D, professional service to the community through the establishment of the IDS series, and for mentoring more than 45 PhD students and 30 post- doctoral fellows. He is the editor-in-chief of Drying Technology—an inter- national journal. He has edited 50 books on drying as well as on heat and mass transfer including the Handbook of Industrial Drying (Taylor & Francis/CRC Press, Boca Raton, Florida, 2007), now in its third edition. He has authored more than 360 papers and 100 book chapters and has delivered more than 300 conference presentations and 60 keynote and plenary lectures. Being the winner of numerous international awards, Dr. Mujumdar was recently conferred with Doctor Honoris Causa by the Technical University of Lodz, Poland, and awarded a Platinum Jubilee Distinguished Visiting Professorship by the University of Mumbai CRC_73877_FM.indd xv CRC_73877_FM.indd xv 1/8/2009 11:31:10 AM 1/8/2009 11:31:10 AM © 2009 by Taylor & Francis Group, LLC
- 18. xvi Authors Institute of Chemical Technology (MUICT). His research theme has of innovation in drying technologies through mathematical modeling. Dr. Mujumdar earned his BChemEng in Chemistry with distinction from the University of Mumbai, India, and obtained his MEng and PhD degrees from McGill University, Montreal, Canada. CRC_73877_FM.indd xvi CRC_73877_FM.indd xvi 1/8/2009 11:31:10 AM 1/8/2009 11:31:10 AM focused on the development of novel dryers including intensification © 2009 by Taylor & Francis Group, LLC
- 19. Part I General Discussion: Conventional and Novel Drying Concepts This part provides a general discussion of the need for new (advanced) drying technologies, objectives of drying research and development drying technologies, as well as some thoughts on innovation and R&D will have to refer to the literature cited for details. The objectives of this part of the book are to provide a concise introduction to our philosophy and to assist in using the information provided here. CRC_73877_S001.indd 1 CRC_73877_S001.indd 1 12/16/2008 2:27:32 PM 12/16/2008 2:27:32 PM (R&D), classification and selection criteria for conventional and advanced needs. All of these topics are covered briefly; hence, the interested reader © 2009 by Taylor & Francis Group, LLC
- 20. 3 1 Need for Advanced Drying Technologies 1.1 Why New Drying Technologies? Authors of a book such as this must answer a fair question. It is true that we already have scores of conventional dryers with well-established records of performance for drying most materials. However, not all of these dry- ing technologies are necessarily optimal in terms of energy consumption, quality of dried product, safety in operation, ability to control the dryer in the event of process upsets, ability to perform optimally even with large changes in throughput, ease of control, and minimal environmen- tal impact due to emissions or combustion of fossil fuels used to provide energy for drying. Most drying technologies were developed empirically over sustained periods of time, often by small vendors of drying equip- ment with little access to research and development (R&D) resources— environmental considerations as well as quality demands were not very stringent. Indeed, many have been upgraded satisfactorily to meet the leg- islative and competitive restrictions. Perhaps most are already designed and operated at their asymptotic limit of performance. However, if for any reason we wish to exceed their current performance in a cost-effective way, we need to look for alternative technologies with a higher asymptotic by thermodynamic constraints. The majority of novel drying technologies, which evolved through a process of evolutionary incremental improvements, were built to offset some or all of the limitations faced in operating conventional dryers. The new technologies may even start at a performance level below that of a conventional dryer. From this point of view, it is not a fair comparison: novel versus conventional might be like comparing apples and oranges. We urge our readers not to be judgmental at this stage and rule on novel mance at this time, since not much effort has yet been devoted to a greater study of such technologies. Rather, they should study their potential and compare their predicted asymptotic limits of performance. Even on this scale, some of these technologies may not turn out to be commercially suc- cessful in the long run and may disappear. However, we must give new CRC_73877_Ch001.indd 3 CRC_73877_Ch001.indd 3 12/29/2008 2:20:16 PM 12/29/2008 2:20:16 PM human or financial. They were also designed at a time when energy and limit to performance, which is necessarily below the maximum defined benefits are typically also incremental rather than dramatic. Some of the dryers simply because they do not have a significantly superior perfor- © 2009 by Taylor & Francis Group, LLC
- 21. 4 Advanced Drying Technologies 30–40 years; a lifetime cost is the only way to really make a proper choice criterion in the selection of a dryer. whereas another may need one to look outside the conventional set of dryers. One must set the criteria for selection and then see which one suits them better and is more cost-effective. There is a cost associated with the the full scale. Most companies shun this risk and are prepared to pay a higher cost for a conventional technology—the premium is often consid- ered an insurance premium rather than a cost. In some cases, new drying technologies are sought simply because the current technologies have a limit in terms of the production rates possible. For example, today’s modern newsprint machine is limited by the dryer speed. One can make the wet paper sheet faster than it can be dried cost effectively on the current multicylinder dryers. For higher speeds, entirely new drying concepts are being evaluated. This is a complex and expen- sive task in view of the very high investment and operating costs required at the mill scale. In the following sections, we will review two evolutionary types of rates and multistaging of convective dryers. It is obvious that reduction of the size of the dryer will lead to a reduction in the initial capital cost. Although this should not be a deciding factor in the selection of an individual dryer, as only 10–15% of the life-cycle cost of a direct dryer is typically due to the initial capital cost of the drying system, it is still an important consideration as it can reduce the space requirement, duct sizes, size of ancillary equipment, etc., as well. One must intensify the drying rates without adversely affecting product qual- ity to make the equipment smaller. Reduction of capital and operating costs of dryers clearly depends on the feasibility to enhance drying rates within the limits of product quality requirements. Higher drying rates translate into smaller physical size of the dryer as well as the associated ancillary equipment. Generally, it is also CRC_73877_Ch001.indd 4 CRC_73877_Ch001.indd 4 12/29/2008 2:20:18 PM 12/29/2008 2:20:18 PM ideas a chance—some of them definitely will emerge as victors and those between conventional and new dryers. Novelty should not be the chief choosing them will be the beneficiaries. Note that dryers have a lifetime of A conventional dryer may be admirably suited for a specific application, risk accompanying the technology not verified at the pilot scale and even advances in drying technologies, specifically the intensification of drying 1.2 Intensification of Drying Rates intensified heat and mass transfer results in high volumetric evaporation reflected in lower running costs. An example is drying of liquid feeds in a fluidized or spouted bed of inert particles (see Chapter 4) where highly © 2009 by Taylor & Francis Group, LLC
- 22. Need for Advanced Drying Technologies 5 to the conventional spray dryer of the same throughput. In general, the feedstock to be dried contains both surface and internal moisture. The rate at which the surface moisture can be removed depends only on the external heat and mass transfer rates since the controlling resis- tance to drying rate lies outside the material being dried. Thus, enhanc- ing external convective heat and mass transfer rates by increasing the gas velocity and gas temperature or reducing gas humidity will lead to increased drying rates for a purely convective (or direct) dryer. Any action that enhances external (gas-side) resistance will yield an increase in the drying rate. Of course, there are exceptions, for example, intense drying may cause case-hardening and reduce drying rates or it may cause extreme shrinkage and cracking, which are undesirable phenomena. Thus, an increase of free-stream turbulence, application of mechanical vibration, or responsible for the augmentation are different (see Chapter 13). Above a critical temperature, commonly termed the inversion tempera- ture, the rate of evaporation of the surface moisture is higher in super- heated steam (SHS) drying than in hot-air drying (see Chapter 7). This is due to the superior thermal properties of SHS. At lower temperatures, the reduced temperature difference between the drying medium and the dry- ing surface for SHS results in a lower drying rate for the latter. In purely convective air-drying, the surface temperature is equal to the wet bulb temperature corresponding to the air humidity and dry bulb tempera- ture, whereas for SHS drying, it is the saturation temperature of steam, that is, 100°C for atmospheric pressure. Enhancement of the falling rate period of drying, which requires faster achieve. In general, attempts to do so result in a change in product qual- pulsation resulting in cavitation; the successive generation of high- and (e.g., microwave [MW] or radio-frequency radiation) can heat up volu- metrically the polar liquid to be vaporized (e.g., water). This practically eliminates the resistance to transfer heat into the material; the transport of moisture out through the material is also enhanced to some extent due to the higher mobility of moisture at higher temperatures as well as due to internal pressure gradient toward the material surface. The same mecha- nism is responsible for the marginally increased drying rates observed in SHS drying. Another possible way of intensifying the drying rate involves increas- ing the effective interfacial areas for heat and mass transfer. For example, CRC_73877_Ch001.indd 5 CRC_73877_Ch001.indd 5 12/29/2008 2:20:18 PM 12/29/2008 2:20:18 PM rates, so that the dryer volume can be reduced significantly as compared sonic fields is also known to increase the drying rates, but the mechanisms transport of heat and moisture through the material, is more difficult to ity. Application of an ultrasonic field can cause high-frequency pressure low-pressure fields causes rapid vaporization and enhanced transport of the liquid through the material. The use of an electromagnetic field in an impinging stream configuration, the impingement zone generated oscillation of flow yields higher drying rates. Application of ultrasonic or © 2009 by Taylor & Francis Group, LLC
- 23. 6 Advanced Drying Technologies by the collision of opposing gas–particle streams is one of high shear and high turbulence intensity (see Chapter 5). If a pasty or sludge-like material and increase the interfacial area of drying. The drying rate is further inten- proportional to the particle or droplet size, all other things being equal. When it is permissible, use of mechanical dispersers or mixers within the dryers results in more rapid drying. An obvious means of intensifying drying rates is to increase the convec- can yield very high drying rates. It may be possible to use adsorbent particles so that the heat-transfer medium can also effectively enhance the mass-transfer potential by lowering the gas humidity concurrently (see Chapter 12). been tested. These are discussed in some detail in this book. It should be noted that not all ideas might be applicable in a given situation as most of these also result in changes in product quality. There is an increase in the complexity of the equipment, as well. A careful technoeconomic evalua- tion is necessary before one may justify use of enhancement techniques in a given application. The application areas for some of these enhancement techniques are given in Table 1.1. TABLE 1.1 Techniques for Enhancement of Drying Rates Drying Period Constant Rate Only Both Falling Rate Period Enhance free-stream turbulence Increase interfacial area for heat and mass transfer Apply oscillation, vibration Dielectric heating Dielectric heating Two-phase (gas–particle) drying medium Superheated steam drying Electrokinetic phenomena pressure level Synergistic effects CRC_73877_Ch001.indd 6 CRC_73877_Ch001.indd 6 12/29/2008 2:20:18 PM 12/29/2008 2:20:18 PM is dispersed in it, the turbulence field tends to de-agglomerate the lumps sified by the fact that the heat and mass transfer rates are nearly inversely rate severalfold while removing surface moisture. A gas–solid suspen- drying (SD), recirculation of fines can result in better drying rates. immersion drying (e.g., mixing hot inert particles with wet particles) Finally, as particle-to-particle heat transfer is more efficient (provided sufficient contact area) than between a gas and particles, the use of Most of the drying-rate intensification concepts mentioned here have Apply ultrasonic field Acoustic field of high sound tive heat/mass transfer rate when feasible. Use of impinging flow configu- ration rather than a parallel flow configuration can increase the evaporation sion flow yields higher heat-transfer rate than a single-phase gas flow. For and geometric parameters as well as particle loading in the gas. In spray impinging gas–particle flows, the heat-transfer rate is two to three times higher than for gas flow alone; the enhancement ratio depends on the flow © 2009 by Taylor & Francis Group, LLC
- 24. Need for Advanced Drying Technologies 7 1.3 Multistage Dryers If a material has both surface and internal moistures, that is, both the so- called constant and falling rate periods exist in batch drying, it is logical to believe that for optimal drying, the drying conditions, and even the type of a dryer in some cases, should be different to remove these two distinctively different types of moisture. For cost reasons, often it is pref- erable to choose a single dryer to accomplish the entire drying by vary- ing the drying conditions spatially for continuous dryers and temporally for batch dryers, that is, the dryer type is the same. Zoning of the dryers beds, continuous vibrated beds, tunnel dryers, etc., to ensure optimal dry- ing; this is especially true for heat-sensitive materials that could be dried under intense conditions only while surface moisture is being removed. In the falling rate, the drying conditions must be made less intense to ensure that the material temperature remains below the critical tempera- ture above which the material starts to deteriorate (change its color, tex- ture, activity, solubility, etc.). However, for large production rates and for certain materials, it is cost-effective to employ two different dryer types for removal of surface and internal moistures. Removal of surface moisture is generally a more rapid process requir- ing shorter dwell time in the dryer, whereas internal moisture removal is a slower process requiring a longer dwell time and hence a larger dryer. ers, etc. For longer residence times, one could employ through circulation, reduce the overall cost of drying. Indeed, this is a well-established com- mercial process for drying coffee, detergents, skim milk, etc. SD is an expensive drying process requiring a very large spray chamber size if the entire drying is to be accomplished in the spray dryer alone. On the contrary, if all of the surface moisture is removed along with a small part of the internal moisture in the spray chamber, one can employ a small For successful multistage drying, it is important that the wet feed the drying times for the two-stage dryer concept become attractive. In some CRC_73877_Ch001.indd 7 CRC_73877_Ch001.indd 7 12/29/2008 2:20:18 PM 12/29/2008 2:20:18 PM bed (or vibrated bed) can be used to instantize (agglomerate) the fine material has both the types of moisture in significant amounts, so that cases, the first stage may be used simply to remove the surface moisture, along their length is commonly used in conveyer, continuous fluidized Dryers suited for surface moisture removal are fluid bed, flash, spray dry- fluid bed, packed bed (or tower), continuous tray dryers, etc. Relative to spray or flash dryers, which have residence times on the order of 1–45 s, fluid bed or vibrated bed dryers have much longer dwell times. Thus, a spray dryer can be followed with a fluid or vibro-fluidized bed dryer to fluid bed—even as an integral part of the conical bottom of the spray chamber—and the overall dryer becomes cost-effective. Indeed, the fluid powder produced by the spray dryer. Such hybrid dryers are presented briefly elsewhere in this book. © 2009 by Taylor & Francis Group, LLC
- 25. 8 Advanced Drying Technologies so that the product becomes nonsticky and suitable for processing in a and hot air impingement as the second stage is used to obtain softer paper although both stages have comparable drying rates and comparable dry- ing times (in fractions of a second). TABLE 1.2 Selected Examples of Two-Stage Drying Stage 1 Stage 2 Advantages Applications Spray dryer t ∼ O 10 s Fluid bed dryer t ∼ O 10 min Reduces overall size of dryer, hence better technoeconomics Spray Fluidizer (Niro) Product is granulated (instantized), if necessary Drying of slurries, for example, coffee, detergent, and milk Spray dryer t ∼ O 10 s t ∼ O 10 min Reduces overall size of dryer, hence better technoeconomics Drying of coffee, milk, etc. Product is granulated (instantized), if necessary Spray dryer t ∼ O 10 s Through circulation conveyer dryer with temperature zoning Drying at moderate conditions for heat-sensitive materials; high-sugar content sticky solids Filtermat—commercial name—can handle drying of juices, for example, orange Flash dryer t ∼ O 1–10 s Fluid bed dryer t ∼ O 10 min Surface moisture dryer; internal moisture removed in long-residence time Fluid bed dryer t ∼ O 1 min Tower/packed bed dryer t ∼ O 10 h Surface moisture residence time obtained in a tall tower Polymer suspension Through dryer t ∼ O 0.1 s Impingement dryer t ∼ O 0.1 s Through dryer helps produce a structure of tissue paper that is soft Drying of tissue paper, exceptional application for two-stage drying. The same order of residence times and drying rates in each stage Note: O, on the order of; t, dwell time in dryer. CRC_73877_Ch001.indd 8 CRC_73877_Ch001.indd 8 12/29/2008 2:20:18 PM 12/29/2008 2:20:18 PM paper drying, a two-stage process with through drying as the first stage conventional fluid bed, for example. In some special cases such as tissue Vibrofluid bed dryer removed in flash removed fast in a fluid bed fluid bed—long © 2009 by Taylor & Francis Group, LLC
- 26. Need for Advanced Drying Technologies 9 Sometimes, a long residence time is needed to accomplish some physical or chemical reactions, which are much slower than the drying kinetics, for example, crystallization of polyethylene terephthalate (PET) resin is accomplished at a tall tower, whereas the initial drying of surface moisture process. Table 1.2 lists selected commercially viable two-stage drying tech- nologies. Some of these technologies and three-stage dryers are covered elsewhere in this book. It is important to note that the multistage dry- ers represent nothing but an intelligent combination of well-established conventional technologies. However, such a combination usually offers unique advantages not possible with the component technologies sepa- rately (Table 1.3) (Mujumdar and Passos, 2000). Reference Mujumdar, A. S. and Passos, M. L. 2000. Drying: Innovative technologies and trends in research and development. In: Developments in Drying. A. S. Mujumdar and S. Suvachittanont (Eds.). Kasetsart University Press, Bangkok, Thailand, pp. 235–268. TABLE 1.3 Multistage Drying in the Dairy Industry: Combination of Conventional Technologies Dryer Energy Savings Powder Characteristics One-stage: spray dryer Reference value Nonagglomerated (∼0.2 mm) Wide size distribution Two-stage: spray dryer + ∼18% Instantized agglomerated powder Nondusting Three-stage: spray dryer + ∼30% Agglomerated and granulated Narrow size distribution Source: From Mujumdar, A. S. and Passos, M. L., Developments in Drying., Kasetsart University Press, Bangkok, Thailand, 2000, pp. 235–268. CRC_73877_Ch001.indd 9 CRC_73877_Ch001.indd 9 12/29/2008 2:20:18 PM 12/29/2008 2:20:18 PM Significant fraction of fines Small fines fraction internal fluid bed fluid bed + external fluid bed Good flowability is done in a small fluid bed dryer in a two-stage drying–crystallization © 2009 by Taylor & Francis Group, LLC
- 27. 11 2 Conventional versus Novel Technologies Mujumdar and Menon (1995) as well as Mujumdar (2000) provided criteria that are important in making an appropriate selection. It is noted that one should select a drying system—including predrying and post- as its operating conditions. More information on this subject along with detailed information on numerous conventional industrial dryers can be found in the handbook by Mujumdar (2007). Table 2.1 summarizes the key criteria often used in classifying dryers. Table 2.2 is a typical checklist for selection of industrial dryers. In addi- tion, the following information should be considered in specifying pos- sible dryer types for a given application. As a minimum, the following quantitative information is necessary to arrive at a suitable dryer: Dryer throughput; mode of feedstock production (batch/ • continuous) Physical, chemical, and biochemical properties of the wet feed • feed characteristics Upstream and downstream processing operations • Moisture content of the feed and product • Drying kinetics, sorption isotherms • Quality parameters (physical, chemical, and biochemical) • biohazards Value of the product • Need for automatic control • Toxicological properties of the product • Type and cost of fuel, cost of electricity • CRC_73877_Ch002.indd 11 CRC_73877_Ch002.indd 11 12/26/2008 2:36:37 PM 12/26/2008 2:36:37 PM Classification and Selection Criteria: detailed classification schemes for industrial dryers along with numerous A finer classification is also possible, but is not relevant here. as well as desired product specifications; expected variability in drying equipment—that can influence the choice of the dryer itself as well • Turndown ratio, flexibility in capacity requirements • Safety aspects, for example, fire and explosion hazards, © 2009 by Taylor & Francis Group, LLC
- 28. 12 Advanced Drying Technologies Environmental regulations • Space in plant • Mujumdar (2000) presents cases where the choice of dryer is also gov- erned by the quantity produced and the quality of the dried material. Table 2.3 compares possible types of conventional and new drying tech- nologies for various physical forms of wet materials. This list is not all- inclusive; it is given only for illustrative purposes. One can arguably place some of the dryers from the new to the conventional category, as their use becomes more commonplace. As expected, there is a preference by industry to use conventional dryers due to their mature status and familiarity. Dryer vendors also prefer such technologies due to the low risk factor in design and scale-up. Also, the cost of developing new technologies may discourage offering quotes involving TABLE 2.1 Criterion Types Mode of operation Batch Continuousa Heat input type Convection,a conduction, radiation, heat-transfer modes Intermittent or continuousa Adiabatic or nonadiabatic State of material in dryer Stationary Moving, agitated, or dispersed Operating pressure Vacuuma Atmospheric Drying medium (convection) Aira Superheated steam Flue gases Drying temperature Below boiling temperaturea Above boiling temperature Below freezing point Relative motion between drying medium and drying solids Concurrent Countercurrent Number of stages Singlea Multistage Residence time Short (<1 min) Medium (1–60 min) Long (>60 min) a Most common in practice. CRC_73877_Ch002.indd 12 CRC_73877_Ch002.indd 12 12/26/2008 2:36:38 PM 12/26/2008 2:36:38 PM Classification of Dryers electromagnetic fields, combination of guaranteed performance. New drying technologies must offer significant advantages over the existing ones to find industrial acceptance. Mixed flow © 2009 by Taylor & Francis Group, LLC
- 29. 13 Legislative requirements may change this picture in the future in many parts of the world. For example, the imposition of carbon tax and severe restrictions on the emission of greenhouse gases—particularly CO2—will force industry to consider superheated steam (SHS) drying where it is feasible. High fuel costs and high insurance rates reigning safe operation may also make steam drying more attractive in the future. frames include combinations of well-known conventional technologies as noted earlier. Use of heat pumps, multistage operation, better control TABLE 2.2 Typical Checklist for Selection of Industrial Dryers Physical form of feed Granular, particulate, sludge, crystalline, liquid, pasty, suspension, solution, continuous sheets, planks, odd shapes (small/large) Sticky, lumpy Average throughput Kilograms per hour (dry/wet); continuous Kilograms per batch (dry/wet) Expected variation in throughput (turndown ratio) Small High Fuel choice Oil Gas Electricity Predrying and postdrying operations (if any) Preforming, backmixing, grinding, milling, screening, standardizing For particulate feed products Mean particle size Size distribution Particle density Bulk density Rehydration properties Inlet/outlet moisture content Dry basis Wet basis Chemical/biochemical/ microbiological activity Active Inactive Heat sensitivity Melting point Glass transition temperature Sorption/desorption isotherms Shape, hysteresis Equilibrium moisture content Drying time Drying curves Effect of process variables Special requirements Material of construction Corrosion Toxicity Nonaqueous solution Flammability limits Fire hazard Color/texture/aroma requirements (if any) Foot print of drying system Space availability for dryer and ancillaries CRC_73877_Ch002.indd 13 CRC_73877_Ch002.indd 13 12/26/2008 2:36:38 PM 12/26/2008 2:36:38 PM Classification and Selection Criteria New technologies that are likely to find acceptance over shorter time at optimum conditions, model-based control, etc., will find—and indeed © 2009 by Taylor & Francis Group, LLC
- 30. 14 Advanced Drying Technologies have already found—many applications. With government incentives, use of renewable energy for drying may prove cost-effective, especially if the fossil fuel costs double in the next decade as is currently projected. Indeed, use of solar energy and wind energy to offset the need for energy from fossil fuels and electricity may be worth considering even now at several geographical locations around the world. The selection criteria for new technologies remain the same as those for conventional ones with the possible exception of risk management. With time, the risk factor will decrease and such technologies will become mainstream technologies. each type of gas–solid contactor. With the new devices, often the data available in the literature are obtained at laboratory scale—only in a few tain. One must objectively evaluate the potential offered by the new tech- TABLE 2.3 Conventional versus Innovative Drying Techniques Feed Type Dryer Type New Techniquesa Liquid suspension Drum Fluid/spouted beds of inert particles Spray Vacuum belt dryer Pulse combustion dryers Paste/sludge Spray Spouted bed of inert particles Drum Fluid bed (with solids backmixing) Paddle Superheated steam dryers Particles Rotary Flash Vibrated bed Fluidized bed (hot air or combustion gas) Ring dryer Jet-zone dryer Yamato rotary dryer Continuous sheets (coated paper, paper, textiles) Multicylinder contact dryers Combined impingement/radiation dryers Impingement (air) Combined impingement and through dryers (textiles, low-basis-weight paper) Impingement and microwave or radio frequency a New dryers do not necessarily offer better technoeconomic performance for all products. CRC_73877_Ch002.indd 14 CRC_73877_Ch002.indd 14 12/26/2008 2:36:38 PM 12/26/2008 2:36:38 PM Tables 2.4 and 2.5 compare the features of conventional and modified them, one must know and compare the specific merits and demerits of cases, it may be pilot scale. The scale-up is, therefore, difficult and uncer- nology and, if justified, carry out a systematic pilot-scale study. Often, it Spray/fluid bed combination Superheated steam fluid bed dryer Pulsated fluid bed fluidized bed and spouted bed dryers, respectively. To choose between © 2009 by Taylor & Francis Group, LLC
- 31. 15 may be possible to scale up the heat and mass transfer characteristics. experimental testing is therefore a necessity. One advantage of the so- called "Digital Big Bang" is that advanced mathematical models of vari- ous dryers can be used as a tool to foster innovation. Such models, once validated, can be used to evaluate novel designs and optimize operating parameters to reduce the cost as well as the risk involved in using novel drying technologies. Since we do not yet have a general drying theory and to precise mathematical description, this approach has a long way to go TABLE 2.4 Fluidized Bed Dryers: Conventional versus Innovative Concepts Conventional Innovative Convective heat transfer Convection + conduction (immersed heaters in bed) Constant gas temperature Variable gas temperature (vibration/agitation) Used for drying of particles Drying pastes, slurries using inert media Air/combustion gas as drying medium Air drag resisted by gravity generated by rotation) Multistage with different dryer types bed TABLE 2.5 Spouted Bed Dryers: Conventional versus Innovative Concepts Conventional Innovative Pneumatic spouting Mechanical spouting (screw, vibration) Single spout Multiple spouts spouting Constant gas temperature Variable gas temperature Drying particles Drying pastes, slurries using inert media Moving spout (rotation, oscillation) Convective drying Combined convection and conduction Axisymmetric Two-dimensional, annular, hexagonal, etc. CRC_73877_Ch002.indd 15 CRC_73877_Ch002.indd 15 12/26/2008 2:36:38 PM 12/26/2008 2:36:38 PM Classification and Selection Criteria Spatially fixed spout However, the quality of the dried product is difficult to predict: actual before industry can benefit from it. Steady gas flow Pulsed gas flow Pneumatic fluidization Mechanically assisted fluidization Superheated steam for fluidization/drying Centrifugal fluid beds (artificial gravity Single-stage/multistage fluid beds Simultaneous fluidization of entire Moving fluidization zone (pulsating fluidized bed) Constant gas flow/continuous Variable gas flow /pulsed gas flow the flow conditions in most dryers are extremely complex to be amenable © 2009 by Taylor & Francis Group, LLC
- 32. 16 Advanced Drying Technologies It is important to reemphasize that general statements regarding the superiority of otherwise different dryers, whether conventional or novel, are not possible. A dryer may be better for one product but not for another. relative cost of fossil fuel and electricity can change the cost-effectiveness of some dryers. Hence, readers should guard against copying dryers or drying systems from one geographic location to another as their cost-ef- requirements. Finally, Table 2.6 lists the attributes of the conventional conveyor (or apron) dryer and compares them with some innovative concepts. Note provide some potential advantages, but need to be tested at both laboratory and pilot scales. For a more detailed discussion, the reader is referred to Mujumdar (2000). TABLE 2.6 Conveyor (or Apron) Dryers: Conventional versus Innovative Concepts Conventional Innovative Fixed layer thickness Variable layer thickness along length (between zones) Fixed (within each zone) temperature Variable gas temperature Superheated steam as drying medium zones Fix bed—no mixing along bed depth nonhomogeneity in drying rates) Single-stage conveyor dryer moisture followed by conveyor dryer (reduce attrition, etc.) Continuous heating Tempering zone between heating zones (interrupted drying when internal heat/mass transfer resistance is high) Purely convective heating Combined convective and microwave heating to reduce drying time Atmospheric pressure Vacuum or high pressure (with steam drying) Fixed total pressure Oscillating pressure between low and atmospheric (when convective heat is supplied) CRC_73877_Ch002.indd 16 CRC_73877_Ch002.indd 16 12/26/2008 2:36:39 PM 12/26/2008 2:36:39 PM two fixed zones) Operating conditions chosen affect dryer performance significantly. The that many of the new concepts are proposed here for the first time; they do Fixed gas flow (within each zone) Variable gas flow along length Hot air or flue gases as drying medium Mix or mechanically agitate bed between fixed Unidirectional gas flow Reverse drying air flow direction between bed zones (e.g., vibrated or fluid bed between Air flow in bed thickness direction only Air flow in cross-flow direction between zones of conventional axial flow (to reduce Use of flash or fluid bed to remove surface fectiveness is typically influenced by local conditions including legislative © 2009 by Taylor & Francis Group, LLC
- 33. 17 References Practical Guide to Industrial Drying. S. Devahastin (Ed.). Exergex, Brossard, QC, pp. 23–36. Mujumdar, A. S. (Ed.). 2007. Handbook of Industrial Drying. 3rd edition. CRC Press, Boca Raton, FL. and selection of dryers. In: Handbook of Industrial Drying. A. S. Mujumdar (Ed.). Vol. 1, 2nd edition. Marcel Dekker, New York, pp. 1–39. CRC_73877_Ch002.indd 17 CRC_73877_Ch002.indd 17 12/26/2008 2:36:39 PM 12/26/2008 2:36:39 PM Classification and Selection Criteria Mujumdar, A. S. 2000. Classification and selection of dryers. In: Mujumdar’s Mujumdar, A. S. and Menon, A. S. 1995. Drying of solids: Principles, classification, © 2009 by Taylor & Francis Group, LLC
- 34. 19 3 Innovation and Trends in Drying Technologies 3.1 Introduction As an operation of prehistoric origin, drying is not normally associated with innovation. As whatever products need to be dried currently are dried using existing technologies, it is often hard to justify the need for innovation and the concomitant need for research and development (R&D) in drying and that drying is able to attract as opposed to some of the exotic bioseparation processes, which on an economic scale may be an order of magnitude less titles of which contain the words “dryer,” “drier,” or “drying”—are issued patents is being issued per year in some of the other key unit operations such as membrane separations, crystallization, adsorption, and distillation. A negative correlation appears to exist between the current level of indus- trial interest and the level of academic research activity, at least as measured by the number of publications in the archives of literature. describing types of innovation, and then identifying the need for innova- tion in drying as well as the features common to some of the novel drying technologies. At the outset, it is important to recognize that novelty per se nically superior and cost-effective compared to the current technology. In some instances, the newer technologies may offer advantages over the con- 3.2 Innovation: Types and Common Features It is interesting to begin with Webster Dictionary’s meaning of innovation, which is as follows: Innovation, n. • The introduction of something new • A new idea, method, or device CRC_73877_Ch003.indd 19 CRC_73877_Ch003.indd 19 12/29/2008 2:26:15 PM 12/29/2008 2:26:15 PM significant. It is interesting to note, however, that about 250 patents—the by the U.S. Patent Office every year. Only 10% or less of this number of U.S. It is instructive to start this discussion with a definition of innovation, is not adequate justification for embracing new technology; it must be tech- ventional ones only for specific products or specific rates of production. dewatering. This is reflected in the relatively low level of R&D resources © 2009 by Taylor & Francis Group, LLC
- 35. 20 Advanced Drying Technologies Notice that it does not use adjectives such as “better,” “superior,” “improved,” “more cost-effective,” and “higher quality” to qualify as an innovation. In our vocabulary, however, we are not interested in innova- tion for the sake of novelty or even originality of concept but for the sake of some other positive technoeconomic attributes. be more appropriate here: “A process that begins with an invention, pro- ceeds with development of the invention, and results in the introduction of the new product, process or service in the marketplace.” To make it into a free marketplace, the innovation must be cost-effective. What are the motivating factors for innovation? For drying technologies, one or more of the following attributes may call for an innovative replace- ment of existing products, operations, or processes: • New product or process not made or invented heretofore • Higher capacities than current technology permits • Better quality and quality control than currently feasible • Reduced environmental impact • Safer operation • Lower cost (overall) ucts, etc. Innovation is crucial for the survival of industries with short time scales (or life cycles) of products/processes, that is, a short half-life (less than 1 year, as in the case of most electronic and computer products). For longer half-lives (e.g., 10–20 years—typical of drying technologies), innovations come slowly and are less readily accepted. The management of innovation depends on the stage it is at. Thus, • Initially, value comes from rapid commercialization. • Later, value comes from enhancing the product, process, or service. • At maturity, value may come from discontinuing and embracing newer technology. It is important to recognize when a current technology is due for replacement. Note that management must be agreeable to discontinue a currently via- ble technology in the interest of the company’s future if the technology has reached its asymptotic limit of performance. This principle applies to all technologies. Numerous studies have appeared in the literature on the fundamental aspects of the process of innovation. One of the models of the innovation process assumes a linear progress from (a) discovery of laws of nature to CRC_73877_Ch003.indd 20 CRC_73877_Ch003.indd 20 12/29/2008 2:26:17 PM 12/29/2008 2:26:17 PM The following definition given by Howard and Guile (1992) appears to • Better efficiency (resulting in lower cost) • Better control, more flexibility, ability to handle different prod- © 2009 by Taylor & Francis Group, LLC
- 36. Innovation and Trends in Drying Technologies 21 (b) invention to (c) development of a marketable product or process, in this order. It is well known, however, that some of the truly remarkable revo- lutionary technologies evolved well before the fundamental physics or chemistry responsible for their success was worked out. True innovation is most likely to be a nonlinear—even chaotic—trial-and-error, serendipi- although one could presumably encourage creativity or try to remove blockages in the process of creativity. istics. A list of the quality parameters of innovations in general (Howard and Guile, 1992) is as follows: • Innovation establishes an entirely new product category. existence. technology. • Innovation is a modest improvement in existing product/ process. Innovations trigger technological changes, which may be revolutionary or evolutionary. From our experience, we know that the latter are more common. They are often based on adaptive designs, have shorter gesta- tion periods and shorter times for market acceptance, and are typically a result of “market-pull”—something the marketplace demands, that is, a need exists currently for the product or process. These usually result tion of the dominant design is an example). Revolutionary innovations, on the contrary, are few and far between, have longer gestation periods, may have larger market resistance, and are often a result of “technology- push,” where the development of a new technology elsewhere prompts the design of a new product or process for which market demand may have to be created. They are riskier and often require larger R&D expen- ditures as well as sustained marketing efforts. The time from concept to market can be very long for some new technologies. It is well known that copter took to the air. The idea of using superheated steam (SHS) as the drying medium was well publicized over 100 years ago, yet its real com- not fully. In fact it is not fully understood even today. A recent example of this long gestation period is the Condebelt drying process for high-ba- sis-weight (thick grades) paperboard proposed and developed by the late Dr. Jukka Lehtinen of Valmet Oy in Finland (Lehtinen, 1998). It took a full 20 years of patient, expensive, and high-quality R&D before the process CRC_73877_Ch003.indd 21 CRC_73877_Ch003.indd 21 12/29/2008 2:26:17 PM 12/29/2008 2:26:17 PM tous process. Therefore, it is difficult to teach innovation in a logical sense What may be classified as innovation can represent different character- • Innovation is the first of its type in a product category already in • Innovation represents significant improvement in existing from a linear model of the innovation process (an intelligent modifica- the concept of a helicopter appeared some 500 years before the first heli- mercial potential was first realized only about 50 years ago, and that too © 2009 by Taylor & Francis Group, LLC
- 37. 22 Advanced Drying Technologies teams of such organizations must be truly farsighted to permit successful implementation of a revolutionary process. It is natural to inquire whether it is possible to predict or even estimate the best time when the marketplace requires an innovative technology or the mature technology of the day is ripe for replacement. Foster’s well- known “S-curve”, shown in Figure 3.1 (Foster, 1986), which gives a sig- moid relationship between product or process performance indicators and resources devoted to develop the corresponding technology, is a valu- able tool for such tasks. When the technology matures (or is saturated in some sense), no amount of further infusion of R&D resources can enhance the performance level of that technology. When this happens (or even somewhat sooner), the time is right to look for alternate technologies— which should not be incremental improvements on the dominant design but truly new concepts—that, once developed to their full potential, will yield a performance level well above that of the current one. As proven by Foster with the help of real-world examples, the performance-versus- effort (resources) curve occurs in pairs when one technology is replaced by another. They represent discontinuity when one technology replaces another and industry moves from one S-curve onto another. As indicated in Figure 3.1, most well-established drying technologies are very close to their asymptotic performance level if they are well designed and operated under optimal conditions. Table 3.1 lists examples of some new drying technologies that were developed through technology-push versus market-pull. In some cases, a sharp distribution of grouping in just two types is not possible because a “market-pulled” development may require a “technology-push” to succeed. Current level (most conventional dryers) Saturation level Efforts (resources) Performance FIGURE 3.1 Foster’s S-curve. CRC_73877_Ch003.indd 22 CRC_73877_Ch003.indd 22 12/29/2008 2:26:17 PM 12/29/2008 2:26:17 PM was first deployed successfully. The vision required by the management © 2009 by Taylor & Francis Group, LLC
- 38. Innovation and Trends in Drying Technologies 23 3.3 Development of Improved Drying Technologies tionary or revolutionary process. Most developments follow the evolu- tionary path involving incremental improvements to offset one or more of the limitations of the contemporary technology. Such technologies are more readily accepted by industry since the risk associated with the adoption of such technologies is generally minimal and the cost- combinations of traditional technologies necessitated by changes in the marketplace. The following list illustrates the evolutionary developments that have be traced for most other dryer types, as well. Rotary dryer 2. Internal heaters (tubes or coils) or external heating of the shell to 3. Direct drying by air injection into the rolling bed of particles in the rotating shell through tubes connected to a central header (Yamato dryer) TABLE 3.1 Examples of New Drying Technologies Developed through Technology-Push and Market-Pull Technology-Pusha Market-Pullb Microwave/RF/induction/ultrasonic drying Superheated steam dryers—enhanced energy environmental impact, safety, etc. Pulse combustion drying—PC developed for propulsion and later for combustion applications Impulse drying/Condebelt drying of paper (also need technology-push) Vibrating bed dryers—originally developed for conveying solids economics of spray drying Impinging streams (opposing jets)— originally developed for mixing, combustion applications a Technology originally developed for other applications and applied to drying. b Developed to meet current or further market demand. CRC_73877_Ch003.indd 23 CRC_73877_Ch003.indd 23 12/29/2008 2:26:17 PM 12/29/2008 2:26:17 PM efficiency, better-quality product, reduced Intermittent drying—enhance efficiency New developments in any field may occur as a result of either an evolu- to-benefit ratio is favorable. Often, the new technologies are intelligent occurred over the past five decades in two commonly used industrial dry- improve efficiency and capacity Combined spray-fluid bed dryers—to improve ers: rotary and flash dryers. Similar evolutionary development trends can 1. Purely convective, axial gas flow © 2009 by Taylor & Francis Group, LLC
- 39. 24 Advanced Drying Technologies Flash dryer 1. Single-pass, vertical, round, insulated tube (adiabatic) 2. Single-pass, jacketed tube for increasing heat input, faster drying (nonadiabatic) 3. Flash dryer tubes of variable cross sections along its length (with delayed chambers) dryer tubes to process particles with broader size distribution and cohesive particles prone to form lumps 5. Use of SHS as carrier gas–adiabatic/nonadiabatic designs 6. Use of inert carrier particles in a pneumatic tube to dry slurries 3.4 Trends in Drying Research and Development about the direction drying technologies will take in the next several past three or four decades. As the general standard of living around the world rises along with the population of the world, it is obvious that the need for drying technologies will increase. New demands will be legislative measures, utilization of renewable energy for drying, and better-quality products at lower total costs. Currently, the major driving force for innovative drying techniques is the need to produce better- quality products at higher throughputs. If the price of fossil fuels rises rapidly and the scenarios proposed regarding the impending shortage of oil and the resulting skyrocketing price of oil, then the R&D in dry- energy-saving measures that are not cost-effective now would become very attractive if the price of oil doubles or triples in the next one or two decades. In general, drying techniques designed to enhance quality are very pharmaceuticals, nutraceuticals, and some foods) can be dried at low temperatures and under vacuum, albeit at higher costs. As noted else- where in this book, two-stage, hybrid heat pump dryers or microwave (MW)-assisted vacuum dryers can compete with freeze-drying (F-D) pro- cesses to produce a high-quality dried product at a lower cost. However, these processes are still very expensive for drying of low-value products. CRC_73877_Ch003.indd 24 CRC_73877_Ch003.indd 24 12/29/2008 2:26:17 PM 12/29/2008 2:26:17 PM 4. Multipass, automatic, aerodynamic classification in ring-shaped It is extremely difficult, if not impossible, to make definitive statements made on better energy efficiency, lower environmental impact through ing will again be driven by the need to enhance efficiency. Some of the product-specific. For example, high-valued, heat-sensitive products (e.g., Also, scale-up to very high production rates is difficult at this time. decades. Most of the developments in this field have occurred in the © 2009 by Taylor & Francis Group, LLC
- 40. Innovation and Trends in Drying Technologies 25 This book focuses on new drying technologies. Where possible, the merits and limitations of various new technologies are proposed in the literature, and novel technologies marketed by vendors around the world are evaluated as objectively as possible. For proprietary reasons, some key details could not be located in some instances. Almost with- out exception, two key pieces of information are not reported by most authors, that is, the cost-effectiveness of their proposed innovations and the objective comparison with competing current technologies. Readers will have to make such judgments carefully if they wish to use this information in practice. Many of the processes may be protected by patents, as well. The main goals of new drying technologies are to • Produce better-quality product • Operate at higher capacities, safely, and with good control • Operate at lower total cost by lowering capital as well as running costs (energy, maintenance, emissions, etc.) One or more of these objectives can be reached in several possible ways. The following is a short list: • Use of indirect heating mode, where feasible • Use of heat pumps to save energy • Use of hybrid dryers • Use of multistage dryers • Use of new gas–solids contactors • Use of SHS as a drying medium where possible • Use of enhancement techniques such as application of acoustic or • Use of better combustion techniques such as pulse combustion Note that there is a cost associated with any additional complexity in the drying process. It is imperative to make a technoeconomic evalua- tion of conventional but more complex as well as newer (advanced) dry- depend on the • Value and production rate of the product • Cost of electricity/fossil fuels (depends on time and geographic location of the plant) • Risk assessment due to uncertainties of scale-up and life-cycle cost evaluation CRC_73877_Ch003.indd 25 CRC_73877_Ch003.indd 25 12/29/2008 2:26:17 PM 12/29/2008 2:26:17 PM ultrasonic fields ing technologies before a final choice is made. The outcome will often © 2009 by Taylor & Francis Group, LLC
- 41. 26 Advanced Drying Technologies • Choice of vendors, delivery times, and performance guarantees • Expected variability in product characteristics as well as produc- are concerned References Foster, R. 1986. Innovation—The Attacker’s Advantage. Summit Books, New York, 316p. New York, 154p. Lehtinen, J. 1998. Condebelt board and paper drying. Drying Technol., 16(6): 1047–1073. CRC_73877_Ch003.indd 26 CRC_73877_Ch003.indd 26 12/29/2008 2:26:17 PM 12/29/2008 2:26:17 PM Other factors may also need to be considered for specific applications. Howard, W. G. and Guile, B. R. (Eds). 1992. Profiting from Innovation. Free Press, tion rates as some techniques are not flexible as far as capacities © 2009 by Taylor & Francis Group, LLC
- 42. Part II Selected Advanced Drying Technologies This part covers a number of relatively new but not commonly used drying techniques and technologies. Most have found commercial application for selected products in some countries. These technologies have demonstrated their potential to compete with conventional technologies and surpass them in performance in many instances, for example, superheated steam drying, drying of suspensions on inert particles, and heat pump drying. With greater awareness and industrial interest, several of these will become common technologies within the next decade or two. CRC_73877_S002.indd 27 CRC_73877_S002.indd 27 11/13/2008 2:43:47 PM 11/13/2008 2:43:47 PM © 2009 by Taylor & Francis Group, LLC
- 43. 29 4 Drying on Inert Particles 4.1 Introduction Drying of liquid materials on inert solid carriers is a relatively new commer- cial technology to produce powders from solutions, suspensions, slurries, and pastes. Although this technique was developed in the former Union of Soviet Socialist Republics (USSR) in the 1950s and used for industrial of biological origin (e.g., Kutsakova et al., 1964; Reger et al., 1967; Minchev et al., 1968; Anonymous, 1992), it was not widespread, mostly because of the language barrier. Over the past two decades, however, drying on inert particles has found a renewed interest mainly because of its ability to pro- duce powders even from the coarsely dispersed liquid feed at evaporation 1983; Adamiec et al., 2007; Kudra and Mujumdar, 2007; Reyes et al., 2008). Extensive studies, carried out in Poland, Brazil, England, New Zealand, and Australia, have resulted in several pilot units and custom-made instal- dryers with inert particles have recently been marketed by companies such as Carrier Vibrating Equipment Co., United States, and Euro-Vent, England, as well as PROKOP INOVA in the Czech Republic that offer dryers utilizing a swirling bed of inert particles (Kutsakova et al., 1990, 1994). Recently, the idea of drying of liquids sprayed on the surface of inert particles has been extended to drying of highly wet materials such as the granules of pressed yeast (70% moisture content), which were dried as a 4.2 Drying Mechanism and Process Considerations rials such as swirling stream dryers, impinging stream dryers, or pneu- CRC_73877_Ch004.indd 29 CRC_73877_Ch004.indd 29 12/26/2008 3:00:06 PM 12/26/2008 3:00:06 PM drying of pigments, fine chemicals, pharmaceuticals, and certain materials rates competitive to spray, drum, and film-rotary dryers (Strumillo et al., matic dryers (Figure 4.1). Independent of the hydrodynamic configuration, lations (e.g., Anonymous, 1986; Grbavcic et al., 1998). In addition, fluid bed fluidized mixture with inert polyethylene beads (Alsina et al., 2005). Drying on inert particles is typically performed in a variety of fluid beds (classical fluid bed, spouted bed, spout-fluid bed, jet spouted bed, vibrated fluid bed, cyclone dryer, etc.) as well as in other dryers for dispersed mate- © 2009 by Taylor & Francis Group, LLC
- 44. 30 Advanced Drying Technologies FIGURE 4.1 with inner conveyor screw, (d) spouted (jet spouted) bed, (e) vortex bed, (f) swirling streams, (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) Feed Air Air + powder Inert particles CRC_73877_Ch004.indd 30 CRC_73877_Ch004.indd 30 12/26/2008 3:00:08 PM 12/26/2008 3:00:08 PM Basic configurations of dryers with inert carriers: (a) Fluid bed, (b) spout-fluid bed, (c) fluid bed (g) vibrofluidized bed, (h) rotary dryer, (i) pneumatic dryer, and (j) impinging stream dryer. © 2009 by Taylor & Francis Group, LLC
- 45. Drying on Inert Particles 31 the principle behind this technology lies in dispersing the liquid feed the sole hydrodynamic impact of the hot air stream or by the combined impact of an air stream and a mechanical device such as a screw con- veyor, a vibrator, or lifters (Flick et al., 1990; Kudra et al., 1989; Pallai et al., 2007; Erdesz and Ormos, 1986; Kudra and Mujumdar, 1989, 2007; Pan et al., ferrite (Kovalev et al., 1989). Depending on the hydrodynamic conditions, the liquid coat on the par- ticle surface dries by convective heat transfer from hot air and by contact heat transfer due to sensible heat stored in the inert particles. When the coat is dry enough to be brittle, it cracks because of particle-to-particle and particle-to-wall collisions and peels off from the surfaces of inert particles. Because of intense attrition, a dry product is discharged from the dryer produced, especially when drying brittle materials of biological origin. Figure 4.2 presents the idealized mechanism of drying on inert particles, which boils down to the following sequence of kinetic processes: heating of inert particles, coating with dispersed liquid, drying of the coat, and cracking and peeling-off the dry product. Because of continuous supply coats at the same time not only the material-free particles but also par- ticles with a dry but not peeled-off material and particles with a partially dry layer. Thus, quasi-equilibrium is established between the individual rates of the component processes. Stable operation of the dryer requires the combined rate of drying/peeling-off to be greater than the rate of coat- ing. Otherwise, the wet coat would build up on the inert particles and the bed would eventually collapse eventually. The bed would also collapse with excessive saturation of exhaust air (Schneider and Bridgwater, 1989). Another condition for stable operation of the dryer with inert particles stems from the material properties—no elastic shell should be formed on the solid carrier at any stage of drying as impact due to particle collisions particles made of two bimetallic canopies, which change their shape when subject to temperature changes during drying, could facilitate cracking of a dry shell (Dmitriev et al., 1989). Fibrous materials (e.g., pulp and paper sludge), which could bridge solid particles and therefore immobilize the bed, are also not good candidates for drying on inert particles. The bed can also collapse when drying sticky materials such as meat-rendering sludge with excessive fat content. In such a case, the melted fat acts as a binder, CRC_73877_Ch004.indd 31 CRC_73877_Ch004.indd 31 12/26/2008 3:00:08 PM 12/26/2008 3:00:08 PM magnetic field if they are made of ferromagnetic material such as barium with the exhaust air as a fine powder of rounded particles. When chip- of the liquid feed and a definite material residence time, the liquid spray might not be sufficient to crack the shell. Here, the almond-shaped inert over the surface of an inert solid carrier. This carrier is fluidized either by 2000; Limaverde et al., 2000). Particles can also be fluidized by an external ping due to the impact of inert particles prevails attrition, small flakes are Small flakes can also be obtained when using inert particles with a corru- grooves on the particle surface (Kutsakova et al., 1985). gated surface. The size of the flakes is then proportional to the size of the © 2009 by Taylor & Francis Group, LLC
- 46. 32 Advanced Drying Technologies which immobilizes particles and traps dry meat powder inside the dryer. This problem can be solved when altering the process by contact-sorption drying (see Chapter 12). In this particular case, the meat-rendering sludge with fat content up to 30% w/w was successfully dried in the jet spouted bed dryer when using either calcium carbonate or wheat bran in the mass ratio of 4.6 and 2.5%, respectively (Amazouz and Benali, 2000). An alter- native solution to the problem of hydrodynamic stability is the use of a hydrocyclone to remove excess fat before thermal drying (Kudra, 2000). Studies on drying of a single 8 mm ceramic particle coated with a 0.6– 0.8 mm layer of the pasty pigment have shown that drying kinetics are typical for convective drying of capillary porous material (Leontieva et al., 2002). The clearly marked plateau of the material temperature cor- responds well with the wet bulb temperature of the air stream during the FIGURE 4.2 Process schematic and idealized mechanism for drying of liquids on inert particles. Outlet gas + dry powder Dry powder Inert particles Wet feed (paste, slurry) Hot gas Wet coat Inert particle Convection Conduction Drying mechanism—idealized sequence of processes Heating Coating Drying Peel-off I II III IV Heat transfer mode CRC_73877_Ch004.indd 32 CRC_73877_Ch004.indd 32 12/26/2008 3:00:08 PM 12/26/2008 3:00:08 PM © 2009 by Taylor & Francis Group, LLC
- 47. Drying on Inert Particles 33 material temperature during drying lies in-between the wet bulb tem- perature and the exhaust air temperature. This is because not all inert particles in the bed are always fully covered with the wet coat; therefore, heat conduction supplements convective drying and thus the condition for the wet bulb temperature does not hold. Also, the material tempera- ture at the moment of peel-off is higher than the wet bulb temperature because removal of a dry coat starts at a certain moisture content, which extracted literature data (Leontieva et al., 2002) to combine the curve rep- resenting the kinetics of the peel-off process with the drying and tem- perature curves. The same conclusion holds for other literature data as well. For exam- ple, Figure 4.4 presents the relationship between the equilibrium material (bed) temperature and inlet and outlet air temperatures for the drying of egg melange in a vortex bed dryer with forced pulsation of inert particles (Kutsakova and Utkin, 1989). Clearly, the material temperature is higher than the respective wet bulb temperature. At the same time, the material temperature is lower than the outlet air temperature, and this difference tends to be larger at lower inlet air temperatures. These data are consis- tent with measurements by Markowski (1992), who found the difference between the particle surface temperature and the wet bulb temperature to ture is lower by 15°C than the temperature of the outlet air. Moisture content (% w.b.) Drying curve Peel-off curve Temperature curve Time (s) Material temperature (ºC) Peeled-off mass (g) 80 60 40 20 0 0 200 400 600 20 40 60 80 0.05 0.04 0.03 0.02 0.01 FIGURE 4.3 Drying, material temperature, and peel-off curves for drying R-salt on inert particles. (Adapted from Leontieva, A. I. et al., Drying Technol., 20(4&5), 729–747, 2002.) CRC_73877_Ch004.indd 33 CRC_73877_Ch004.indd 33 12/26/2008 3:00:09 PM 12/26/2008 3:00:09 PM first drying period (Figure 4.3). An analysis of literature data on drying in is close to the final one. This can be seen in Figure 4.3, in which we have spouted and fluidized beds of inert particles indicates, however, that the be at least 20°C. Also, Grbavcic et al. (1998) found that fluid bed tempera- © 2009 by Taylor & Francis Group, LLC
- 48. 34 Advanced Drying Technologies Assuming that drying the relatively thin layer of the wet material on inert carriers is externally controlled, Kutsakova and Utkin (1987, 1989) proposed the following equation for the moisture evaporation rate: dX dt 1 X R T 273 P T ,X P i S S p g ⫽ ⫹ ⫹ ⫺ ( ) ( ) ( ) (4.1) where R = 462 J/(kg K) is the gas constant for water vapor, PS and Pg P are the water vapor pressures at the material surface and in the gas core, respec- on the surface of inert particles given by the mass of the wet material that adheres to the unit surface area of inert particles. TS represents the average gas temperature in the boundary layer at the surface of the inert particle. Integration of Equation 4.1 gives the following relationship for drying time: t R T X I T P I T P dX P T X P S i p g p g S p g X X i f ⫽ ⫽ ⫹ ⫹ ⫺ 273 1 ( ) ( ) ( ) ( ) ( ) ∫ , , , (4.2) 65 70 75 80 85 90 95 55 60 65 70 75 80 210C 220C 250C 240C 230C Exhaust gas temperature (C) Material temperature (C) FIGURE 4.4 Material temperature versus exhaust and inlet air temperatures. (Extracted from Kutsakova, V. E. and Utkin, Yu. V., Trans. VUZOV. Food Technol., 5, 92–93 (in Russian), 1989.) CRC_73877_Ch004.indd 34 CRC_73877_Ch004.indd 34 12/26/2008 3:00:09 PM 12/26/2008 3:00:09 PM tively, and φ is the parameter that quantifies the distribution of the wet coat © 2009 by Taylor Francis Group, LLC
- 49. Drying on Inert Particles 35 water vapor pressure at the material surface with the average temperature of the inert particles and the material moisture content. For drying in a vortex bed of inert particles, Kutsakova and Utkin (1987) developed the following experimental equation: P T X A BX DX T T F S p C E p r , exp ( ) ( ) ⫽ ⫺ ⫺ ⫺ ⫺ ⫹ ⫹ 5073 1 273 1 273 where the parameters in Equation 4.3 for selected protein-based materials are given in Table 4.1. The average temperature of the bed (Tp T T ) can be determined experimen- tally or can be calculated from the following relationship (Kutsakova, 2004; Kutsakova and Utkin, 1987, 1989): T T T W c a r c G X a r nh W p eq out g g p p p i p g ⫽ ⫽ ⫽ ⫹ ⫹ ⫺ ⫺ ⫺ 1 3 1 1 4 2 exp( ) ( ) ( ) c ca (4.4) where Teq T T is the equilibrium temperature of evaporation (°C), h is the gas- 2 K), G is the dryer throughput with respect to the dry product (kg/s), and n is the number of inert par- ticles in the dyer. The mean temperature of inert particles with an accuracy of 3°C can also be determined from the following empirical formula (Kutsakova, 2004): T T b p out ⫽ ⫺ (4.5) The constant b varies from 10 to 20, depending on the drying material; for protein hydrolyzate b = 15, whereas for casein b = 10. TABLE 4.1 Parameters in Equation 4.3 Material A B C D E F Tr T T (°C) Protein hydrolyzate 11.53 1.027 −0.8 4.605 −1.8 1.0 120 Skim milk 21.53 0.053 −1.0 0.313 −2.3 0.032 (112 − Tp) 100 Whey 11.53 0.00378 −1.7 6.620 −1.7 0.032 (112 − Tp) 100 Source: Extracted from Kutsakova, V. E. and Utkin, Yu. V., J. Appl. Chem. USSR, 60(5), 1077–1081, 1987. CRC_73877_Ch004.indd 35 CRC_73877_Ch004.indd 35 12/26/2008 3:00:10 PM 12/26/2008 3:00:10 PM To calculate the drying time, one should first identify the variation of the to-particle heat-transfer coefficient (W/m © 2009 by Taylor Francis Group, LLC
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- 51. propriety of firing, as I generally shoot low, and such an error in my aim could hardly have proved otherwise than disastrous. There was no use striving to make the bird loosen his hold by hooting. If there had been any virtue in that sort of demonstration the old woman would hardly have been raised above the eaves of her shanty, for she was screaming in a manner that would have made a Modoc blush. The only thing that suggested itself, and that rather hurriedly, was to get out my pencil and paper and take a sketch as she appeared passing over her cottage in the vulture’s talons. The blood, which at first forsook her cheeks through fear, was almost instantly forced back into her visage again by the pendant position of her head. She beat the empty tin pan which she still retained in her hand, but the voracious and hunger-pinched vulture had no notion of relinquishing his hold on account of noise. On the contrary, he seemed to enjoy it, and with many a sturdy twitch and flap, and many an airy wheel, he still held his way toward a rugged promontory situated at the head of the valley. Fortunately, when he was twenty feet from the ground and about eighty rods from the cottage, the calico dress and undergarments in which mainly his talons were fastened, gave out, and the liberated woman dropped on hands and knees in the muddy bed of the creek, over which the bird was passing at the time. While hovering over her, about to pounce down upon her and try the elevating business again, a sheep-herder who had seen the bird approaching the cottage, gave him a dose of buckshot, which broke one wing and left him at the mercy of his captor.
- 52. FOR BETTER OR FOR WORSE. Jonathan.—“I hain’t got no tongue for soapin’ of ye, Susan Jane. I mean business, I do. Will ye hev me?” Susan Jane.—“I don’t know much about ye, Jonathan Junkit, but I’m willin’ to risk it, anyhow. Yer’s my hand. I’m yourn.” Old Volume. This afternoon I attended a private wedding on Howard Street. I may safely term it “marriage in high life,” as the combined height of the couple was something over twelve feet. The groom was a bachelor, who for many a year had stood around the fire like the half of a tongs, very good as a poker, but not worth standing room as a picker up. He looked as though it wouldn’t require much advice to make him —even at the eleventh hour—prove recreant to his vows, and back out from under the yoke the reverend gentleman was about to place upon his neck. His companion, however, was no novice in the business in which she was engaged. She was fearlessly putting forth upon that sea on which she had twice been wrecked, but she was nothing loth to try it again. Were she only skilled in navigation as well as in embarkation, she would have been the one to send on expeditions to either the North or South Pole, as the case might be.
- 53. THE TRYING MOMENT. It was truly encouraging to the timorous and uninitiated, to see with what a broad smile she regarded her husband that was to be; and with what a readiness she responded to the momentous question propounded by the minister. And when they stood as husband and wife, her Milesian face lighted up with irrepressible joy, until it beamed like a Chinese lantern. Her emotions went far to convince me that there is in those matrimonial fields a balm for every ill; a perfect bliss worthy the seeking, even at the risk of receiving the bruised spirit, if not the bruised head.
- 54. ODE ON A BUMBLE-BEE. Oh, busy, breezy bumble-bee, A fitting theme in you I see! At once you backward turn my gaze To orchard, mead, and pasture days, To watch your movements to and fro With wondering eyes, as years ago. Come, let me set my mark on thee, As thou hast oft remembered me, When with a seeming special zeal You hastened to affix your seal. I’ve heard your gruff good-morrow ring When meeting kinsfolk on the wing; Now coming zig-zag, light and airy, Now going laden, straight and wary; Still mindful of the spider’s snare And kingbird, pirate of the air. I’ve seen you upward turn your eye, When clouds began to fleck the sky,
- 55. The winds to chafe the village pond, And thunder rumble far beyond And threaten storm, ere you could fill Your honey sack, so empty still. I’ve heard you whining forth your grief When rain commenced to pelt the leaf, And made you take the shortest road That brought you to your dark abode. I’ve marked your grumbling when you found The working bee had been around; Had left his bed and waxen door And reached the field an hour before; For still, with early bird, or bee, Or man, the maxim does agree They all must be content to find What early risers leave behind. Against the bell I’ve heard you storm, Because it kept your burly form From passing in the honeyed way, That open to the emmet lay. Thus human folk are oft denied What, in their judgment, or their pride, They should enjoy, though kept instead For meaner things that creep ahead. I know how apt you are to cling To locks of hair, to hide and sing, And keep the victim still in doubt Just where the mischief will break out; I know full well your angry tone, And how you stab to find the bone; With what a brave, heroic breast Ye strike for queen and treasure chest, Like Sparta’s sons, at duty’s call, Compelled to win, or fighting fall; Not fearing odds, nor counting twice, Ye fix your bayonet in a trice, And charge upon the nearest foe, And break the ranks where’er you go. For not the stroke of halberdier Nor thrust of Macedonian spear Can check your onset when you fly With full intent to do or die! Beneath your straight and rapid dart The foe will tumble, turn, depart, And leave you victor, to report
- 56. Your doings at the Queen Bee’s court. And proudly may you bare your brow, In presence of your sovereign bow, And tell her why you came so late, Thus panting, to the palace gate; And show your limbs of wax bereft, Your right arm crushed, and sprained the left, Your twisted horn, exhausted sting, Your wounded scalp and tattered wing, But how, in spite of every ill, You struck for independence still, Until the acre lot was free Of all that would molest the bee. ’Tis said that youngsters have a knack To take you prisoner by the back; To catch you by the wings, in haste, A piece above the belted waist, And hold you thus, to struggle there, And use your sting on empty air. But once I tried, and once I missed, For you’re a great contortionist, And somehow turn, and manage still To plant your poison where you will. Ah, they are wise, who meddling cease, And let you go your way in peace! Though many things may slip my mind Before the narrow bed I find, In fancy’s field I’d often see The busy, burly bumble-bee.
- 57. DUDLEY AND THE GREASED PIG. Boil-stricken Job had his comforters, who, despite his timely injunction, “Oh, lay your hands upon your mouths, and thereby show your wisdom,” would still drum in his ear, “Hear us, for we will speak.” Poor old Falstaff had his evil genius in Bardolph, his impecunious follower, with his “Lend me a shilling.” And I have my burdensome “Jim Dudley,” with his “Let me tell you a story.” I was kept awake last night listening to his crazy yarn about the “greased pig,” as if I cared anything about his villainous adventures. “Oh, yes, that scrape with the greased pig? I never told you about it, eh? It’s worth heerin’, for that was a tearin’ old race, and I came mi’ty nigh gettin’ shoved out of the village on account of it, too, now, I can tell ye. Down on me? Wall, I reckon you’d think so if you heered the hollerin’ that was gwine on for awhile arter that race, some cryin’ one thin’ and some another. ‘Tar and feather the cheat,’ one would holler. “‘Lynch the blamed humbug!’ another would shout. “‘Put him in a sack and h’ist him over the bridge!’ would come from another quarter. “A doctor was never so down on a patent medicine as they were on me arter that race, especially Parson Coolridge, who was one of the principal sufferers, yer see. “It was May Day amongst ’em, and the hull village seemed to be out thar enjoyin’ ’emselves. They had sack races and wheelbarrow races. That was the day blindfold Tom Moody ran the wheelbarrow through the grocer’s window, and Old Shulkin knocked him down with a ham, and a dog ran away with it. He charged Tom with the ham in the bill, along with the broken winder.
- 58. “They had a greased pole standin’ thar with a ten-dollar greenback tacked on top of it, but no person could get within ten feet of the bill. The hungry crowds were standin’ around all day gazin’ longin’ly up at the flutterin’ greenback, like dogs at a coon in a tree-top. “I didn’t try the pole, but when they brought out the greased pig—a great, slab-sided critter, jest in good condition for racin’,—I got sort o’ interested in the performance. His tail was more’n a foot long, and it was greased until it would slip through a feller’s fingers like a newly caught eel. “Several of the boys started arter him, but they’d jest make one catch, and before they were certain whether they had hold of it, they would go one way and the hog would go another. And then the crowd would holler. “I was standin’ thar a leanin’ over the fence watchin’ of ’em for some time, and I see the pig was in the habit of formin’ a sort of ring with his tail; leastwise he’d lap it over so that it e’enmost formed a knot—all it lacked was the end wanted drawin’ through. I cal’lated that a feller with pooty nimble fingers could make a tie by jest slippin’ his fingers through the ring and haulin’ the end of the tail through. That would make a plaguey good knot, and prevent his hand from slippin’ off. Arter thinkin’ over it for some time I concluded if I could git up a bet that would pay for the hardships that a feller would be likely to experience, I would try a catch anyhow. “So I ses to Jake Swasey, who stood alongside of me, ‘Jake, I believe that I kin hold that pig until he gins out.’ “‘Hold?’ he ses, surprised like and raisin’ his eyebrows just that way; ‘what’s the matter of ye? hain’t ye slept well? Ye mout as well try to hold old Nick by the tail as that big, slab-sided critter.’ “‘Wal, now, jest wait a bit,’ ses I; so I went on and told him what I cal’lated to do, and arter he looked awhile, he ses, ‘Wal, go ahead, Jim, I’ll back ye. I reckon we can git any amount of odds so long as we keep the knot bus’ness to ourselves.’ “So pullin’ off my coat I gin it to Jake to hold, and jumpin’ on the fence, I hollered, ‘I’ll bet ten to twenty that I kin freeze to the pig’s tail till he gins out!’ “Great fish-hooks! you ought to have seen ’em a-rustlin’ towards me. I couldn’t see anythin’ but hands for five minutes, as they were
- 59. holdin’ of ’em up, and signalin’, an’ a-hollerin’, ‘I’ll take that bet, Dudley, I’ll take that bet!’ I got rid of what money I had about me pooty soon, and Jake Swasey was jest a-spreadin’ out his greenbacks like a paymaster, and arter he exhausted his treasury he started arter his sister to git what money she had. I hollered to him to come back— I was fearin’ he’d tell her about the knot bus’ness; but he wasn’t no fool and knowed too well what gals are to trust her with any payin’ secret. “Old Judge Perkins was thar, jolly as a boy on the last day of school. Wal, he was holdin’ of the stakes, and his pockets were crammed chockfull of greenbacks. He was a pooty good friend of mine, and couldn’t conceive how in thunder I was a-gwine to get my money back. JUDGE PERKINS. “Beckonin’ of me one side—‘Dudley,’ ses he, kind of low that way, and confidentially like, ‘I know you’re as hard to catch as an old trout with three broken hooks in its gill; but I can’t help thinkin’ a greased pig’s tail is a mi’ty slippery foundation to build hopes on.’ “‘Never mind, Judge,’ ses I, winkin’, ‘I can see my way through.’ “‘Yes, Dudley,’ he ses, a-shakin’ of his head dubious like, ‘that’s what the fly ses when he’s a-buttin’ his head against the winder.’ “‘Wal,’ ses I, ‘without the tail pulls out, I cal’late to travel mi’ty close in the wake of that swine for the next half hour;’ and with that I moved off to where the pig was standin’ and listenin’ to all that was gwine on.
- 60. “I fooled round him a little until I got betwixt him and the crowd, and when he flopped his tail over as I was tellin’ ye, I made one desperate lunge, and made a go of it the fust time. I jest hauled the end through while he was turnin’ round, and grabbin’ hold above my hand, rolled it down into the tightest knot you ever sot eyes on. It was about two inches from the end of the tail, and he scolloped around so amazin’ lively nobody could see it. The crowd allowed I was hangin’ on the straight tail, and they didn’t know what to make of the performance anyhow. “‘Go it, piggy,’ I ses to myself, just that way, ‘I guess it’s only a question of endurance now, as the gal said when she had the flea under the hot flat-iron.’ “The gate was open, and arter a few circles around the lot, the hog p’inted for it, and away he went, pig fust and I arter. He ran helter- skelter under old Mother Sheehan, the fruit woman, jest as she was comin’ through the gateway with a big basket of apples on each arm. I did hate like snakes to hoist the old lady, bounce me if I didn’t! I would ruther have run around a mountain than do it, ’cause you see she had jest been gittin’ off a bed of sickness that came nigh shroudin’ her, and she wasn’t prepared for a panic, by any means. I did my best to swing the critter around and git him off the notion of goin’ through, but his mind was made up. Thar was plenty of room outside for him to pass along without disturbin’ the old lady, but a hog is a hog, you know—contrary the world over. Besides, he allowed he could brush me off by the operation, but I wasn’t so easily got rid of. The money was up, you see, and I had no choice but to follow where he led and stick to the rooter till he gin out. ‘Where thou goest, I will go,’ I ses to myself, rememberin’ the passage in the Scriptures, and duckin’ my head to follow him. I scrouched down as low as I could and keep on my feet; for I cal’lated, do my best, the old woman would git elevated pooty lively.
- 61. BAD FOR THE FRUIT BUSINESS. “She hollered as though a whole menagerie—elephants, kangaroos, snakes and all—had broke loose. Her sight wasn’t any too clear, and the whole proceedin’s had come upon her so sudden that she didn’t exactly know what sort of an animal was thar. She would have been satisfied it was a hog if it hadn’t taken so long to git through. I followed so close to his hams that she reckoned we both made one animal. The hog gin a snort when he started in to run the blockade, and she ses to herself, ‘Thar goes a big hog,’ but about the time she reckoned he had got out on the other side, I come a humpin’ and a boomin’ along in my shirt-sleeves, and gin her a second boost, throwin’ the old woman completely off her pins and out of her calculations at once. “She did holler good, thar’s no mistake about that. “The crowd hoorayed and applauded. The older ones of course sympathized with the poor old woman; but they could do nothin’ more, ’cause the whole catastrophe come as sudden as an earthquake and nobody seemed to be to blame. I wasn’t, and they all could see that plain enough. The young uns went for the scattered apples, but the pig and I kept right on attendin’ to business. Now and agin he’d double back towards the crowd, and they’d commence scatterin’ every which way, trampin’ on each other’s feet. Si Grope, the cashiered man-of-wars-man, stepped on Pat Cronin’s bunion, and he responded by fetchin’ the old salt a welt in the burr of the ear, and at it they went, tooth and nail, right thar. A few stopped to see fair play,
- 62. but the heft of the crowd, about three hundred, kept right on arter me and the hog. “Jake Swasey managed to git up pooty nigh to us once and hollered, ‘How are you makin’ it, Jim?’ “‘Fustrate,’ I answered; ‘I cal’late to stick to this swine through bush and bramble till I tire him out.’ “‘That’s the feelin’,’ he shouted, and with that we left him behind. The old judge was a puffin’ and a blowin’, strivin’ his best to keep up, and for some time he actewally led the crowd, but he didn’t hold out very long, but gradewelly sank to the rear. BOW-LEGGED SPINNY. “Rod Munnion, the tanner, stumbled and fell while crossin’ the street. His false teeth dropped out into the dirt, and while he was scramblin’ on all fours to git ’em ag’in, a feller named Welsh, who was clatterin’ past, slapped his foot down and bent the plate out of all shape. Munnion snatched ’em up ag’in as quick as the foot riz, and wipin’ ’em on his overalls as he ran, chucked ’em back into his mouth ag’in, all twisted as they were. They did look awful though, stickin’ straight out from his mouth, and pressin’ his lip chock up ag’inst his nose. You couldn’t understand what he was sayin’ any more than if he was Chinnook.
- 63. “Bow-legged Spinny, the cabbagin’ tailor, was thar. He met the crowd while carryin’ home Squire Lockwood’s new suit, and catchin’ the excitement of the moment, tossed the package into Slawson’s yard, and it bounded into the well quicker than ‘scat.’ He didn’t know it though, but hollered to the old woman, as he ran past the window, to look arter the package until he got back. Not seein’ any package she allowed he was crazy as a cow with her head stuck in a barrel, and flew to boltin’ of her doors pooty lively. He had been once to the Lunatic Asylum, you see, and they were still suspicious of him. “The crowd thought to head us off by takin’ down a narrow lane, and it was while they were in that, that they began to surge ahead of Judge Perkins. He was awful quick tempered, and pooty conceited, and when bow-legged Spinny was elbowin’ past him he got mad. Catching the poor stitcher by the coat tail, he hollered: ‘What! a miserable thread-needle machine claimin’ precedence?’ and with that he slung him more’n ten feet, landin’ him on his back in a nook of the fence. “That was the day they buried old Mrs. Redpath, that the doctors disagreed over. Dr. Looty had been doctorin’ her for some time for bone disease. He said her back-bone war decayin’. He didn’t make much out of it though, and they got another doctor. The new feller said he understood the case thoroughly; he ridiculed the idea of bone disease, and went to work doctorin’ for the liver complaint. He said it had stopped workin’ and he was agwine to git it started ag’in. I reckon he’d have accomplished somethin’ if she had lived long enough, but she died in the meantime. When they held a post- mortem, they found out the old woman, some time in her life, had swallered a fish-bone which never passed her stomach, and eventually it killed her. “‘Thar,’ ses Dr. Looty, ‘what did I tell ye? You’ll admit, I reckon, my diagnosis of the disease was right arter all, only I made a slight error in locatin’ the bone!’ “‘Bone be splintered!’ ses the other feller, ‘hain’t I bin workin’ nigher the ailin’ part than you?’ So they went on quackin’ thar and disagreein’ over her until old Redpath got mad and hollered, ‘You old melonheads, isn’t it enough that I’m a widderer by your fumblin’ malpractice, without havin’ ye wranglin’ over the old woman!’ So he put ’em both out, and chucked their knives and saws arter ’em.
- 64. “But as I was sayin’, that was the day of the funeral, and while it was proceedin’ from the church to the buryin’ ground with Parson Coolridge at the head, with his long white gown on, we hove in sight comin’ tearin’ down to’ards the parsonage. The minister was a feller that actewelly doted on flowers. When he wasn’t copyin’ his sermons’ he was fussin’ around among the posies. He had his gardin chock full of all kinds of plants and shrubs. Thar you could see the snapdragon from Ireland, the fu-chu from China, the snow-ball from Canada, the bachelor’s button from Californy, and every kind you could mention. “He had noticed the gardin gate was open when the funeral passed, and it worried him considerable. So when he heered the hootin’ and hollerin’, and got sight of the crowd surgin’ down the street, and see the pig and I pointin’ in the direction of the house, he couldn’t go ahead nohow. “Turnin’ around to the pall bearers who were puffing along behind him, he ses, ‘Ease your hands a minit, boys, and let the old woman rest ’till I run back and see if that Dudley is agwine to drive that hog into my gardin. Confound him!’ he contin’ed, ‘he’s wuss to have around the neighborhood than the measles.’ With that he started back on the run, his long, white gown a-flyin’ away out behind, the most comical lookin’ thing you ever see. And he could run, that Parson Coolridge, in a way that was astonishin’. I reckon he hadn’t stirred out of a walk before for thirty years, and yit he streaked it over the ground as though it was an every-day occurrence. “His j’ints cracked and snapped with the unusual motion, like an old stairs in frosty weather, but he didn’t mind that so long as he could git over the ground. He was thinkin’ of his favorite plants and the prospect of their gittin’ stirred up and transplanted in a manner he wasn’t prepared to approve. He did jerk back his elbows pooty spiteful, now I can tell you. He tried to make the gateway fust, and put in his best strides. But when he saw he couldn’t, he hollered, ‘Keep that hog out of my gardin, Dudley, or I’ll take the law of ye.’
- 65. NIP AND TUCK. “‘Don’t git wrathy, Parson Coolridge,’ I shouted. ‘I can’t prevent the pig from gwine in. I have hold of the rudder, but I’ll be boosted if I can steer the ship.’ With that, through the openin’ we went, pig fust and me arter, and the hul crowd a clatterin’ behind us. The judge was amongst ’em, but got left in the hind end of it, where the women were a-trottin’. The Parson’s flowers went down with broken necks quicker than lightnin’. It wasn’t more’n ten seconds until they were six inches under ground, for the hog kept a circlin’ around and the hoorayin’ crowd follerin’ arter, payin’ no more attention to the Parson than if he had been a young ’un a-runnin’ around. When they saw the crowd, the pall bearers and most of the people who were jest follerin’ the remains through sympathy, turned back on the run and left the mourners standin’ thar by the coffin. “Oh! it was the most excitin’ time the village ever seed. The ground was too soft in the gardin for the pig to git around well, and pooty soon he gin out. I was awful tired, too, and was hangin’ a dead weight on him for the last ten minutes. “When the boys see the knot on the tail you ought to hear ’em a- hollerin’, ‘Bets off! bets off!’ They were set on claimin’ a foul, and surrounded the old judge demandin’ thar money. “But, as the crowd was increasin’ and the Parson was e’enmost crazy, the judge told ’em to come with him to the Court-house—he wouldn’t decide nothin’ in the gardin. As the hog couldn’t walk, the judge took his tobacco knife and cut the tail off and took it along with him to introduce as proof. He decided in my favor. He said that I had
- 66. held on to the tail and touched nothin’ else, and if I managed to tie a knot while runnin’ I had performed a feat never before heard of in the country, so he paid over the money. “But Parson Coolridge was the most worked up of any of ’em. He had legal advice on the matter, but the lawyer told him to gin it up, for the judge was on my side. Besides, he shouldn’t have left the gate open, if he didn’t want the pig to go in thar. Arter a while he gin up the notion of suin’ me, but while he stopped in the village he never got over it. MORE LIGHT ON THE SUBJECT. “The boys had pictures chalked up on the fences and shop doors, so that wherever you’d look you’d see sketches of the Parson runnin’ back from the funeral, and me a holdin’ on to the pig’s tail. He paid out more’n ten dollars in small sums to one boy, hirin’ him to go round and rub out the pictures wherever he’d happen to see ’em. But every time the Parson would start out through the village, thar on some fence or door, or side of a buildin’, would be the same strikin’ picture of him, a streakin’ it to head off the hog, so he would start the rubbin’-out boy arter that one. “One evenin’ he happened to ketch that selfsame little rascal hard at work chalkin’ out the identical sketch on the cooper’s shop door, and the Parson was so bilin’ mad he chased him all over the village. The young speculator had bin carryin’ on a lively business, but arter
- 67. that discovery thar was a sudden fallin’ away in his income. I tell ye it made a plag’y stir thar for awhile, and I reckon if Judge Perkins hadn’t been on my side I’d have been obliged to git out of the place.”
- 68. CORA LEE. Would you hear the story told Of the controversy bold, That this day I did behold, In a court of low degree, Where his Honor sat like fate, To decide betwixt the state And a wanton villain’s mate, Named Cora Lee? The bold chief of stars was near, As a witness to appear. (By his order, Cora dear Was languishing below.) And for counsel she had got A descendant of old Wat— Noted for his daring plot, Some years ago. It was he commenced the fuss, “For,” said he, “by this and thus, Here I smell an animus[1] As strong as musk of yore; And it’s my condensed belief, That in language terse and brief, I can trace it to the chief, E’en to his door.” Then to all it did appear That the chief was seized with fear; To the lawyer he drew near, And to him muttered low: “I could never think that ye Would be quite so hard with me; You had better let me be, And travel slow.”
- 69. Then the lawyer quit his chair As if wasps were buzzing there, And with quite a tragic air, Addressed his Honor thus— “At your hands I claim protection. Keep your eyes in this direction, Take cognizance of his action, This animus!” Then arose the chief of stars, And his visage shone like Mars, When he recks not battle scars, But charges to the fray. And his hand began to glide To his pocket deep and wide, Where a weapon well supplied In waiting lay. THE CHIEF. “Ho!” he cried, “you shyster hound, If you go on nosing round Till an animus you’ve found, My dear sir, hearken you: I will open, by my soul! In your carcass such a hole, You will think a wagon pole Has run you through. “You would prate about the law? You would magnify a flaw?
- 70. You would touch me on the raw? So now, sir, say no more! Keep a padlock on your jaw, Not a sentence, or I’ll draw, And I’ll scatter you like straw Around the floor!” Now the Judge’s face grew red As a turkey gobbler’s head When a scarlet robe is spread On the lawn or fence. “I adjourn the court,” he cried, “’Till that animus has died, And is buried head and hide Far from hence.” Then the rush was for the door; From the corridors they pour,— Three old women were run o’er Within the justice hall; And above the tramp and patter, And the cursing and the chatter, And the awful din and clatter, Rose their squall. When the open air was gained, Then the epithets were rained, And the passer’s ear was pained With profanity flung loose, Back and forth the wordy pair, Shameless swapped opinions there; ’Till all parties got their share Of vile abuse. When the man of “briefs” would flee, Chieftain followed like a bee, Or a shark a ship at sea When hunger presses sore; ’Till, enraged, the lawyer, he Cried, “If fight you want of me, Wait with patience minutes three, Not any more; “’Till I hasten up the stair To my office, and prepare, Like yourself for rip and tear,
- 71. And piling bodies dead. Then, if you can blaze it faster, Carve designs for probe or plaster, Quicker work a soul’s disaster, Just waltz ahead.” But alas! his hasty tongue, Vulgar name or sentence flung, And the chieftain’s pride was stung Down to the marrow bone. Now upon him, head and tail, Pitched policemen, tooth and nail, Hot as bees when they assail A lazy drone. And upon the evening breeze Rose the “begorras” and the “yees” Of a dozen Mulroonees, As they roughly hale The poor lawyer through the street, Sometimes lifted from his feet, Sometimes o’er the noddle beat, Toward the jail. Now upon a truss of straw, Lies the counsellor-at-law, Wishing Satan had his paw On wily Cora Lee. For himself to grief is brought, While the animus he sought Running is, as free as thought, Or like his fee. 1. Private enmity towards the prisoner.
- 72. A BRILLIANT FORENSIC EFFORT. Having learned that a highly-educated and respectable lady of this city had instituted a suit in one of our courts for the purpose of obtaining a divorce from her husband, I stepped into the hall of justice to learn how the case progressed. The fact of a young wife demanding a separation in a country like this, which is proverbial for its separations, is nothing to be wondered at, and I was considerably surprised, on reaching the court room, to find it so full of people that I could hardly gain admittance. I was not so much astonished at the great rush, however, when informed by the bailiff that the ground on which the lady rested her case was that her husband snored. As I entered, the plaintiff’s lawyer commenced addressing the court. He entered into the case with the spirit and fire of a Clay or a Webster. After reviewing and commenting largely upon the testimony given in the case, he ended his argument in the following words:— THE ADVOCATE.
- 73. “Now, sir, whatever other people may think of this application, I take a bold stand, regardless whose corns or bunions I tread upon, so long as I put my foot down where it belongs. We have too many snorers among us. They are in our places of amusement, introducing groans and thunder where none were intended in the play. We find them in our places of worship, breaking forth in the midst of the pastor’s prayer, or while he is picturing to the congregation the wreck of ages and the crash of worlds. I maintain that this application is a righteous one; that it is a shot in the right direction, which will in all likelihood eventually bring down the game; and were I a judge invested with power to decide a peculiar case of this kind, I would show no hesitation, but grant the plaintiff her natural and very reasonable request more readily than if the grounds on which she sued for a separation were drunkenness or desertion. “The absurdity of an irascible wife seeking a divorce from a husband because he indulges too freely in the flowing bowl must be apparent to all. She rushes into the crowded court room, and, figuratively speaking, catches the astonished justice by the ear, as Joab in the extremity of his distress laid hold upon the horns of the altar, and requests him to sever the chafing bonds with his legal shears. Again: what a pitiable lack of discretion that woman exhibits who appeals to the court merely because her husband deserts her, leaving her to pursue the even tenor of her way. Why, in nine cases out of ten this is a ‘consummation devoutly to be wished;’ she is left untrammeled, and has no husband to support. “I will not allude to the many other failings which wreck the home and put out the cheerful light of many a hearthstone. “But, sir, it is with no ordinary thrill of pride that I espouse the cause of the woman who seeks a divorce from a snoring husband. I say, and I may remark that I say it boldly, that I rejoice it was reserved for me to raise my voice in her defence. I hold that a man who with malice aforethought takes from her peaceful home a tender and confiding maiden without first informing her of his trouble, commits a grave and unpardonable crime. The dogs of justice should be loosened at his heels to hound him from Puget’s Sound to Passamaquoddy Bay. He should be made to repent his villainous act. Think how the tender nerves of a sensitive creature must be shocked on being awakened by such an outburst. Picture to yourself her
- 74. husband, not breathing her name in words of love, but lying flat on his back, and snoring with the vehemence of a stranded porpoise. “Now, sir, I ask what mercy should be shown the monster who has himself shown none? He has doomed a fair representative of that sex whose presence civilizes ours, to an ever new affliction and a life of perpetual wakefulness. What course can she pursue? There are but two roads. Which shall she take? One leads to the court room and the other leads to the cemetery. She must either be freed from her husband or go down to an untimely grave, perhaps to have her place quickly filled by another unsuspecting victim. No, your Honor; this man, and I regret to say it, this husband and father, should not be permitted to destroy the peace and bright prospects of more than one female. Let it be known to the world that he has ruined the hopes of a loving wife, let it be blazoned upon the housetops and upon the fences that he snores; then let him get another mate, if he can. “The wife should not only have a divorce from the deceptive monster, but she should have the custody of the children. She deserves them by virtue of her long suffering and patience, while he who has so heartlessly deceived her cannot be competent to guide their little feet aright in the dangerous walks of life. On behalf of this sorrowing wife, all other wives, and of the wives yet to be, who are ripening into womanhood around our hearths, I cry separation! In the name of confidence betrayed, of hopes blasted, and of a life aged before its time, I repeat, separation! separation!” He sank into his seat, and despite the order of the bailiff for “silence in court,” generous applause swept throughout the room.
- 75. The judge took occasion to compliment the lawyer for his able argument, and said it was the greatest forensic effort he had listened to since he assumed the responsibilities of his office. The prayer was granted and the children awarded to the plaintiff.
- 76. VISITING A SCHOOL. Accepting an invitation extended by the principal of an uptown school, I visited that institution to-day. The masses of young humanity a person finds in these temples of instruction is something amazingly impressive. Eight or nine hundred scholars are attending the one school on which I bestowed my attentions to-day. HEAD OF HIS CLASS. FOOT OF HER CLASS.
- 77. This article must be embellished with a faithful sketch of the boy who stood at the head of his class. How he felt at that moment, I couldn’t say, never having any experience in the position myself. He looked happy and confident, however, and snapped eagerly at the words as they fell from the teacher’s lips, much as a hungry dog does at the crumbs falling from a table. But my sympathies were decidedly with the little contortionist who stood mournfully at the foot of her class. I knew how that was myself. I had been “yar,” and I regretted I wasn’t a ventriloquist, that I might from afar whisper in her ear, and assist her over some clogging syllables. If she could have gone into the yard, where I noticed a scholar of the senior class throwing herself in a delirium of joy, brought about by a skipping-rope, she would probably have acquitted herself in a creditable manner, and won the praise of all, for however inferior a person may be to another in some matters, when they can choose their game they often reverse the order, and peradventure the poor stammering scholar could have skipped the skirts off those jogging ahead of her in the common speller.
- 78. THE REJECTED SUITOR. Not often does a sadder sight Wake sympathetic strain, Than glimpse of some rejected wight Whose suit has proved in vain; Who often pinched necessities For bouquets, sweet and rare, For tickets to the carnival, The opera, or fair; A SUITOR NON- SUITED. Whose pocket oft was visited The candy box to fill; The dollar spent that should have gone To pay his laundry bill. Especially the case is sad, If he who seeks a wife Has, step by step, encroached upon The shady side of life.
- 79. The fly no darker prospect views That in the inkstand peers, Than he, whose unrequited love Must leak away in tears. At such a time how ill the smile Becomes the rival face; The “ha, ha, ha’s!” the winks and nods, Seem sadly out of place. And then comparisons are drawn At the expense, no doubt, Of him whose overflowing cup Seems full enough without. While he who moves away, alas! Of every grace so free, To criticism opens wide The door, as all may see. His mind is not reflecting now On fashions, style, or art, On proper pace, or rules of grace; But on his slighted heart. He now but sees his promised joys All foundering in his view, His castles tumbling down, that high In brighter moments grew. To know that now those ruby lips Another’s mouth will press, And now that soft and soothing hand Another’s brow caress,— Oh, dark before, and dark behind, And full of woe and pain Is life to him, whose heavy loss Makes up a rival’s gain. The gravel-walk beneath his feet Cannot too sudden ope’, To gather in the wretch, who mourns The death of every hope. The swallows, whispering in a row, Seem mocking at his tear, And in the cawing of the crow He seems to catch a sneer; The cattle grazing in the field
- 80. Awhile their lunch delay, To gaze at him, who moves along In such a listless way. Perhaps he’ll know a thousand griefs Ere death has laid him low. Perhaps, beside an open grave, He’ll shed the tear of woe; Perhaps he’ll turn him from the sods That hide a mother’s face, A father’s smile, a brother’s hand, Or sister’s buried grace; But there can hardly come a time When life will look so drear, Or can so little reason show Why he should linger here.
- 81. A NIGHT OF TERROR. I am not the oldest inhabitant, and don’t know what sort of storms they used to have here before the flood; but I’ll wager a corner lot against a plug of tobacco, that this section, for the last twenty years, has not snoozed through a rougher night than the one just past. It would have been a glorious night for a revivalist to stir up the masses. Converts would have crowded in like grists to a mill after harvest. Since the last great earthquake I have not felt so much concern about my future state as I did about twelve o’clock last night. I arose from bed, and went to rummaging books, trying to find the description of a storm that would equal ours. I found the tempest that Tam O’Shanter faced the night he discovered the witches, and the one in which King Lear was cavorting around, bare-headed, and that which made Cæsar take an account of stock and turn to interpreting dreams, and jumbled them all together; but the product was unequal to the fury that was raging without. There was no more similarity than a baby’s rattle bears to a Chinese gong. A ROUSING EVENT.
- 82. Then I fished out the storm that howled while Macbeth was murdering Duncan, and tumbled it in with the others. This addition made things about even. The “lamentations heard i’ the air” of Macbeth’s tempest were a fair precedent of the clamorous uproar from the fire bell in the City Hall tower. Only an earthquake was lacking to enable us to say, “The earth was feverous, and did shake,” or boast a night outvieing four of the roughest on record, all woven into one. It had one good effect, however—one for which poison and boot- jacks have been tried in vain: it did silence the dogs and cats. Their midnight carousals were as rare as they were in Paris just before the capitulation. Quarrelsome curs postponed the settlement of their little differences and defiant barks until such times as they would be able to discover themselves whether they barked or yawned, and cats sought other places besides a fellow’s window-sill to express opinions about each other or chant their tales of love. I know the rain is refreshing, the wind purifying, the lightning grand, and the thunder awe-inspiring; but as the poor land-lubber advised, when he was clinging to the spar of the wrecked vessel, “Praise the sea, but keep on land,” so I say to those people who want to prick up their willing ears, like a war-horse, to catch the sublime rumble of heaven’s artillery, or sit by their window and blink at the blazing sky, like a bedazzled owl at a calcium light; but I know one individual who could have got along quite as well if there had raged no war of the elements. He would have slept soundly and never mourned for what he had lost.
- 83. MY DRIVE TO THE CLIFF. I am wofully out of humor, and what is worse, out of pocket, and have just been settling a bill for repairs to a buggy which was knocked out of kilter on the Cliff House road the other day. At the present writing I feel that it will be some time before I take the chances of injuring another. The moon may fill her horn and wane again, the seals howl, and the ocean roar, but I will hardly indulge in the luxury of a drive to the beach for many a day to come. I had a couple of ladies with me. Splendid company ladies are—so long as they have unlimited confidence in your skill as a driver. But they try one’s patience after they lose faith, and want to get the lines in their own hands every time you chance to run a wheel into the ditch, or accidentally climb over a pig or calf. Those who were with me on that occasion are not particularly loud in their praise of my driving. The fact is, I didn’t acquit myself in a manner calculated to draw down encomiums in showers upon my head. I drove a span that day. They were called high-strung animals. But I don’t like high-strung horses any more. If they would only run along the track like a locomotive, I could hold the ribbons as gracefully as anybody; but I am very much opposed to all of their little by-plays. This getting scared at a floating thistle-down, or grasshopper swinging on a straw, is something I don’t approve of in a horse. There is no reason in it; no profit accrues from it. But my trotters were frightened at different objects at the same moment—one at a snail peacefully pursuing his way across the road, and the other at a butterfly winging his wabbling flight along the ditch. At once they became unmanageable, and vied with each other in extravagant antics. From the first the ladies had no very exalted opinion of my manner of handling the lines. Even before we were well under way I had the misfortune to run down a calf. Then a
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