Impinging Streams Fundamentals- Properties- Applications
Impinging Streams Fundamentals- Properties- Applications
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Impinging Streams Fundamentals- Properties- Applications
Yuan Wu College of Chemical Engineering and Pharmacy Wuhan hlstitute of Technology Wuhan 430073 PR China
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Elsevier B.V. Radarweg 29, PO Box 21 l, 1000 AE Amsterdam, The Netherlands Chemical Industry Press No. 13, Qingnianhu South Street, Dongcheng District, Beijing 10001 l, P.R. China First edition in English 2007 Copyright © 2007 Elsevier B.V and Chemical Industry Press This edition is jointly published by Elsevier and Chemical Industry Press, P.R. China No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher Permissions may be sought directly from Elsevier' s Science & Technology Rights Department in Oxford, UK: phone (+44) (0) 1865 843830; fax (+44) (0) 1865 853333; email:
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ISBN-13:978-0-444-53037-0 ISBN-10:0-444-53037-1 Impinging Streams: Fundamentals, Properties, Applications By Yuan Wu
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Foreword Impinging Streams (IS) is a novel technical method. As a scientific concept, it was first presented by Elperin in 1961, while its earliest emergence can be traced back to the development and application of the Koppers-Totzek gasifier in 1953. The original idea of IS is to send two solid-gas streams to impinge against each other at high velocity with the aim of enhancing transfer between phases. It is interesting to note that the core of the Koppers-Totzek gasifier first applied industrially effectively enhances transfer between phases. Without doubt, IS is very effective for this purpose. The results of a large number of investigations have shown that transfer coefficients between gas and solid in IS can be ten times, or even several tens of times, greater than those in traditional equipment. Because of the universality of transfer phenomena, IS has received widespread attention and has been the subject of extensive investigations. In the past 40 years and more, investigations into IS have been in three stages. The first, from 1961 to the early 1970s. was a newly established stage and the work was concentrated mainly in the former Soviet Union. Naturally, the target systems were mainly those with gas as the continuous phase, because the concept of IS was originally aimed at transfer enhancement in such systems, while the dispersed phase in IS was gradually extended to include liquid. In the second stage, from 1974 to the mid-1990s, the core of the research moved to Israel where work was mainly carried out by A. Tamir and his group. Most (about 80%) of the target systems were still those with gas as the continuous phase. Although several systems with liquid as the continuous phase, such as dissolution of salts, emulsion, extraction, etc'., were also studied, the ideas were simply analogous and the goal was still transfer enhancement. The depth and scope of investigations into the liquid-continuous phase were never comparable with those of the gas-continuous phase. The last ten years and more may be considered as the third stage. Researchers from more than twenty countries, including China, the United States, Canada, Germany, etc., have been engaged in investigations into IS, the number of investigations now overtaking those carried out in Russia and Israel. On the other hand, the emphasis at this stage changed to investigating IS with liquid as the continuous phase. As a technical method, IS can never be a universal tool. The method of gascontinuous impinging streams (GIS) is indeed very efficient in enhancing transfer. However, it has the fatal disadvantage of very short residence times (about 1 s only) in the active region and has much higher requirements for flow configuration arrangement than traditional devices. Some very fast processes, such as burning of sprayed liquid
fuel or powdery coal etc., can be carried out in GIS with greatly increased efficiency. In practice, however, many processes cannot finish instantaneously but have to last for a very long time. On the other hand, any arrangement of a multistage IS must lead to an excessively complicated system and these disadvantages, therefore, limit the application of pure IS to a considerable extent. For a long time, these problems were not fully addressed and this is the main reason for the slow progress made in the application of IS, although there might also be other reasons. One of the essential conditions for carrying out impinging streams is that there must be, at least, one continuous phase, which can be either a gas or a liquid. If a liquid is chosen, the dispersed phase should either be solid or another liquid, soluble or not. Otherwise, the employment of IS has limited use. Because of the difference in aggregation statuses, the physical properties of liquid and gas are quite different from each other. Normally, the density of liquid is of the same order as solid; it is greater than that of gas by 103 times, while liquid viscosity is about 102 times that of gas. As a result, the factors effectively enhancing transfer in GIS, e.g., very large relative velocity between phases, penetration of particles to and fro between the opposing streams, etc., no longer exist or become very weak in LIS, so that the transfer coefficient in LIS becomes essentially no different from that created by traditional methods. In fact, there already existed experimental data, including those by Tamir, showing such a situation. Unfortunately, LIS was still considered to be superior to traditional methods even in enhancing transfer, and thus few investigations were made into the property and performance differences between LIS and GIS before the 1990s. On the other hand, the high density of liquid brings new features to LIS. The discovery in the 1990s that LIS promotes micromixing very efficiently is the most important advance in the field of IS. It was also found recently that very strong pressure fluctuation exists in LIS. Such phenomena must relate to the fact that the impingement of two opposing streams of high density against each other leads to their much stronger interaction. Certainly, enhanced micromixing and pressure fluctuation are of significance for processes occurring at the molecular scale, especially those involving chemical reaction(s). The above discovery therefore enables the fields of IS application to be greatly expanded. In fact, the results of a number of investigations have shown the perfect performance and great application potential of LIS in the preparation of ultrafine products by reaction-precipitation, etc. Two books devoted to IS have previously been published. The first is "Transport Processes in Opposing Jets" by I. T. Elperin (Nauka I Tekhnica, Minsk, 1972; in Russian). It summarized the investigations in the field before 1970 and its depth and scope are consistent with the achievements made in the newly established stage of IS investigation. The second book is "Impinging-Stream Reactors: Fundamentals and Applications" by A. Tamir (Elsevier, 1994), which was translated into Chinese for my Chinese colleagues (The Chemical Industry Press of China, Beijing, 1996). It is a systematic summary of the works in the second stage of IS investigation. Based on the understanding that "almost any process in chemical engineering can be carried out by applying impinging streams", Tamir's investigations extended over almost all chemical
vi
unit operations. The topics discussed in the contents are numerous and the data are full and accurate. Without exaggeration, the book was almost an inspiration for me, even though it could not cover the achievements made in the last ten years, both in understanding and technologies, e.g., the perfect and valuable properties of LIS, etc. My investigation into IS began in 1992 and also started with GIS. Afterwards, the emphasis was diverted to LIS because of the intrinsic disadvantages of GIS and the perfect nature of LIS; consequently the latter will be one of the focal points of the discussions in this book, distinguishing it from the two books mentioned above. Science and technology continually grow and progress: and our understanding is constantly improved. This book cannot and should not be the last one related to IS. It is both believed and expected that, when the next book emerges, the industrial application of IS will have greatly evolved and become universal. This book was originally written in Chinese, and its publication was supported by the National Natural Science Foundation of China. This English edition is not simply a translation: some corrections, revisions and supplementary information have been made in order to improve its contents. I am very pleased that the publication of this English edition has enabled the book to be available to more of my colleagues, especially those from English-speaking countries. Yuan Wu Wuhan, China
January 31, 2006
vii
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Acknowledgments My investigations on Impinging Streams (IS) were twice supported by the National Natural Science Foundation of China (No29276260, 20176043); and also by the Key Laboratory of Open Investigation on Multiphase Reactions, Institute of Process Engineering, Academy of China; the Natural Science Foundation of Zhejiang Province, China: the Key Laboratory on Polymerization Reaction Engineering, Zhejiang University; and the Education Department of Hubei Province e t c . Without this support the author would not have been able to carry out the investigations involved in many projects and thus could not have written this book. I would like to express my sincere appreciation to all these supporters. During my investigation on IS. a number of my colleagues collaborated with me, undertaking sub-projects or specific research jobs to effectively push the investigations forward. They include Associate Professors Yuxin Zhou, Gao'an Wu, Jianmin Xu, Deshu Li, Chuanping Bao, Jun Yuan. Anqig Shu, Senior Engineers Xiaoping He and Lecturer Tielin Wang from Wuhan Institute of Technology; Associate Professors A'san Yang, Qin Sun, Rong Cheng, Yungen Chen and Huayan Liu from Zhejiang University of Technology; Senior Engineer Jingnian Xu from Beijing University of Chemical Technology, My PhD students for Huaiyu Sun, Qin Li, Jianwei Zhang; ME students Kai Huang, Yu Chen, Yang Xiao. Guochao Li, Zhen Chen and Fang Li, took various phases of the IS investigation as topics for their theses, making great efforts and substantially promoting the fundamental studies and technical developments. Many undergraduate students joined the investigations on IS and obtained a lot of useful experimental data. In addition, during many discussions with me, Dr. Xiaoxi Wu offered a number of helpful comments and suggestions All of the persons mentioned above have contributed significantly, and I would like express my thanks to them. I was also deeply moved that Academician Professor Yong Jin has warmly supported the publication of the Chinese edition of this book. Finally, I must mention my wife, Senior Engineer Yuqiong Huang. In order to support my work, she essentially forsook her own research and took over all the household jobs as an understanding wife and loving mother so that I could concentrate on my investigations. Her indirect contribution to this book has been invaluable. Once again, I would like to sincerely thank all those mentioned above. Yuan Wu
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Contents Foreword
........................................................................................................................
Acknowledgments
..........................................................................................................
v
ix
I n t r o d u c t i o n ....................................................................................................................
1 2 3 4 5
Part I
E n h a n c e m e n t of transfer between phases and origin of i m p i n g i n g streams ........................................................................................................ 1 Basic principles of impinging streams ........................................................ 4 Experimental evidence for enhancing transfer ........................................... 6 Other performances of impinging streams .................................................. 7 Extension of impinging stream technology ................................................ 9 5.1 Extension in flow configuration .................................................... 9 5.2 Extension in phase ...................................................................... I I Gas-Continuous
Flow of Continuous
1.1 1.2
1.3 1.4
Impinging
S t r e a m s ............................................................
P h a s e ..................................................................................
Flow characteristics .................................................................................. Velocity field in laminar impinging streams ............................................ 1.2.1 General equations ........................................................................ 1.2.2 Planar two-dimensional impinging streams ................................ 1.2.3 A x i a l - s y m m e t r i c impinging streams ........................................... 1.2.4 General three-dimensional impinging streams ............................ 1.2.5 Viscous impinging streams ......................................................... Experimental results for the flow field in impinging streams ................... Turbulent impinging streams ....................................................................
P a r t i c l e B e h a v i o r .................................................................................................
2.1
2.2 2.3
i
17 19
19 25 25 26 28 30 31 32 36 41
M o t i o n of a single particle in co-axial horizontal i m p i n g i n g streams ...... 41 2. I. 1 Qualitative description ................................................................ 41 2.1.2 Basic relationship for the particle motion ................................... 2.1.3 Solutions of the motion equation for various stages ................... 2.1.4 Residence time of the particle in the i m p i n g e m e n t zone ............. E x p e r i m e n t a l results on the behavior of a single particle in co-axial horizontal t w o - i m p i n g i n g streams ............................................................
43 45 51
Behavior of a single particle in co-axial vertical impinging streams ........ 2.3. l Description of motion p h e n o m e n a .............................................. 2.3.2 Motion equation and its solution ................................................. 2.3.3 Terminal velocity ........................................................................
56 56 57 58
xi
52
2.4
Behavior of particle crowds in impinging streams ................................... 2.4.1 Distribution of particle concentration in impinging streams ....... 2.4.2 Influence of particle concentration in feed streams ..................... 2.4.3 The influence of collision between particles ...............................
59 60 63 65
Residence Time of Particles and its Distribution ............................................. 67 3.1
3.2
3.3 3.4
3.5
Theoretical consideration ......................................................................... 3.1.1 Impinging stream device ............................................................. 3.1.2 Constituents of R T D of particles in the ISC ................................ 3.1.3 M o d e l for the overall residence time distribution ....................... M e t h o d for experimental m e a s u r e m e n t of particles' residence time distribution ................................................................................................ 3.2.1 Input signal ...................................................................................... 3.2.2 Data interpretation ........................................................................... Relationships for fitting data .................................................................... Major experimental results for R T D of particles ...................................... 3.4.1 M e a s u r e m e n t of tracer concentration .......................................... 3.4.2 C o m p a r i s o n b e t w e e n the results measured and simulated .......... 3.4.3 M e a n residence times of particles ............................................... Remarks ....................................................................................................
67 68 69 75 77 77 81 84 86 86 87 88 89
Hydraulic Resistance of Impinging Stream Devices ........................................ 91 4.1
4.2
4.3
4.4
Theoretical consideration ......................................................................... 92 4.1.1 F l o w through the accelerating tubes ............................................ 92 4.1.2 I m p i n g e m e n t b e t w e e n opposing streams .................................... 94 4.1.3 Resistance due to the structure of the IS device .......................... 95 4.1.4 Overall resistance of the IS contactor ......................................... 96 Experimental equipment and procedure ................................................... 96 4.2.1 Experimental equipment ............................................................. 96 4.2.2 Experimental procedure .............................................................. 97 M a j o r results from the experimental study ............................................... 98 4.3.1 Basic characteristics of pressure drop distribution ...................... 98 4.3.2 Resistance of accelerating tubes to pure air flow ........................ 99 4.3.3 Pressure drop due to acceleration and collisions of particles .... 100 4.3.4 Resistance due to structure of the device .................................. 102 4.3.5 M o d e l for the overall pressure drop .......................................... 103 Evaluation of p o w e r consumption and discussions related to application .............................................................................................. 105
Influence of Impinging Streams on Dispersity of Liquids ............................. 107 5.1 Statement of the p r o b l e m ........................................................................ 107 5.2 Experimental equipment and procedure ................................................. 109 5.2.1 Impinging stream device ........................................................... 109 5.2.2 M e t h o d for m e a s u r e m e n t of droplet size distribution ............... 110 5.2.3 A r r a n g e m e n t of sampling .......................................................... 110 5.3 M a j o r results of the investigation ........................................................... 111 xii
5.4
5.3.1 Size distribution of droplets ...................................................... 111 5.3.2 M e a n diameter of droplets ........................................................ 115 C o n c l u d i n g remarks ................................................................................ 117
Impinging Stream Drying ................................................................................ 119 6.1 Introduction ............................................................................................ 119 6.2
6.3
6.4
Earlier research and d e v e l o p m e n t ........................................................... 6.2.1 I m p i n g i n g stream spray drying ................................................. 6.2.2 Impinging stream drying of granular materials ......................... 6.2.3 Impinging stream drying combinations ..................................... Circulative impinging stream drying ...................................................... 6.3.1 Basic ideas for e q u i p m e n t design .............................................. 6.3.2 Structure and working principles of the dryer ........................... 6.3.3 Experimental model e q u i p m e n t scheme and procedure ............ 6.3.4 Major results of the model experiments .................................... 6.3.5 Influences of structural and operating parameters .................... Concluding remarks ................................................................................
121 121 123 128 134 134 135 137 139 142 151
Impinging Stream Absorption ......................................................................... 153 7.1 7.2
7.3 7.4
7.5
7.6
Adaptability of impinging streams for g a s - l i q u i d reaction systems ....... Earlier investigations .............................................................................. 7.2.1 M o d e l s tk)r absorption e n h a n c e m e n t ......................................... 7.2.2 Absorption equipments ............................................................. 7.2.3 Major results of the investigations ............................................ W e t desulfurization of flue gas (I) General considerations .................... W e t desulfurization of flue gas (II) Investigations in Israel ................... 7.4.1 Experimental equipment and procedure .................................... 7.4.2 Major results ............................................................................. W e t desulfurization of flue gas (III) Investigations in China ................. 7.5.1 Experimental e q u i p m e n t ........................................................... 7.5.2 Experimental scheme and procedure ......................................... 7.5.3 Data interpretation ..................................................................... 7.5.4 Results and discussion .............................................................. 7.5.5 Conclusions ............................................................................... Design of a device for large gas flow rates .............................................
153 155 155 156 160 162 164 164 166 169 169 172 174 176 186 186
Impinging Streams Combustion and Grinding .............................................. 191 8.1
M o d e l s for particles and droplets c o m b u s t i o n ........................................ 191
8.2 8.3
8.1.1 E v a p o r a t i o n - b u r n i n g equations tot a single droplet ................. 8.1.2 Burning equations for a single particle ..................................... Intensification of c o m b u s t i o n processes due to impinging streams ........ Impinging stream combustors .................................................................
8.4
8.3.1 Furnaces for gas and liquid fuels .............................................. 198 8.3.2 K o p p e r s - T o t z e k gasifier for p o w d e r y coals ............................. 199 Impinging stream grinding ..................................................................... 201 xiii
191 194 196 198
Part II Liquid-Continuous Impinging Streams .................................................... 205 Differences Between Properties of Continuous Phases and Classification of Impinging Streams ....................................................................................... 207 9.1 9.2
9.3
10
207 208 208 208 211
Micromixing In Liquid-Continuous Impinging Streams ............................... 213 10.1 10.2
10.3
10.4
10.5 10.6
11
Progress of investigation on liquid-continuous impinging streams ........ Differences between properties of continuous phases and their influences on the performance of impinging streams ............................. 9.2.1 Differences between properties of liquid and gas ..................... 9.2.2 Influences of property differences on the performance of impinging streams ..................................................................... Supplementary classification of impinging streams ...............................
Macromixing and micromixing .............................................................. Methods for investigation of mixing problems ....................................... 10.2.1 Macromixing ............................................................................. 10.2.2 Micromixing ............................................................................. Flow and macromixing in SCISR ........................................................... 10.3.1 Design ideas and basic structure of SCISR ............................... 10.3.2 Macromixing time ..................................................................... 10.3.3 Flow configuration and residence time distribution .................. Micromixing in SCISR ........................................................................... 10.4.1 Experimental equipment and procedure .................................... 10.4.2 Governing variable and its experimental measurement ............ 10.4.3 Experimental procedure ............................................................ 10.4.4 Major results for micromixing .................................................. 10.4.5 Comparison between micromixing performances of SCISR and STR .................................................................................... 10.4.6 Comparison between measured and theoretically predicted results for micromixing time ..................................................... 10.4.7 Relationship between macro- and micro-mixing ...................... Micromixing in impinging stream reactor without circulation ............... Comparison between the investigations on micromixing in LIS as concluding remarks ................................................................................
213 214 214 215 216 216 218 219 222 222 224 226 226 229 230 232 233 235
Pressure Fluctuation in the Submerged Circulative Impinging Stream Reactor .................................................................................................. 237 11.1
11.2
11.3
Investigation method of pressure fluctuation .......................................... 11.1.1 Meaning of pressure fluctuation ................................................ 11.1.2 Investigation method of pressure fluctuation ............................ Experimental equipment and procedure ................................................. 11.2.1 Experimental equipment ........................................................... 11.2.2 Measurement and control of the impinging velocity ................. 11.2.3 Arrangement of measuring points and sampling frequency ..... 11.2.4 Pre-treatment of the experimental data ..................................... Experimental results and discussion .......................................................
xiv
237 237 238 240 240 241 241 242 242
11.4 12
12.3
12.4
Qualitative analysis lbr the influences of pressure fluctuation and m i c r o m i x i n g ..................................................................................... 12.2 Crystal-growth kinetics of di-sodium phosphate ....................... 12.2.1 Basic principles ......................................................................... 12.2.2 E x p e r i m e n t a l investigation ........................................................ Kinetics of ethyl acetate saponification .................................................. 12.3.1 C h e m i c a l reaction and experimental m e t h o d ............................ 12.3.2 Major results ............................................................................. C o n c l u d i n g remarks ................................................................................
253 254 254 257 265 265 265 266
Preparation of Ultrafine Powders by Reaction-Precipitation in Impinging Streams I: "Ultrafine" White Carbon Black ............................... 269 13.1 13.2 13.3
13.4
13.5
14
242 245 246 247 249 250
Influence of Liquid-Continuous Impinging Streams on Process Kinetics .............................................................................................................. 253 12.1
13
11.3.1 Intensive fluctuation region ....................................................... 11.3.2 V o l u m e t r i c distribution of fluctuation intensity ........................ l 1.3.3 Definition of the i m p i n g e m e n t zone .......................................... 11.3.4 Influence of the impinging velocity on fluctuation intensity .... 11.3.5 Power spectrum analysis for pressure fluctuation ..................... Conclusions and discussion ....................................................................
Adaptability of liquid-continuous i m p i n g i n g streams for preparation of ultrafine powders ................................................................................ Properties of white carbon black and chemical reactions in its preparation by precipitation processes .................................................... Experimental e q u i p m e n t and procedure ................................................. 13.3.1 Experimental e q u i p m e n t ........................................................... 13.2.2 Experimental procedure ............................................................ Results and discussions .......................................................................... 13.4.1 Semi-batch operation ................................................................ 13.4.2 Continuous operation of the S C I S R .......................................... 13.4.3 C o m p a r a t i v e e x p e r i m e n t s in semi-batch operation ................... 13.4.4 Study of the final treatment of the reaction product .................. Conclusions ............................................................................................
270 271 273 273 274 275 275 278 279 280 281
Preparation of Ultrafine Powders by Reaction-Precipitation in Impinging Streams II: Nano Copper and its Surface Improvement ............ 283 14. I 14.2 14.3
14.4
Introduction ............................................................................................ Properties and main uses of nano copper ................................................ Principles and experimental m e t h o d ....................................................... 14.3.1 Chemical reactions in preparation of nano copper by reduction-precipitation ..............................................................
283 284 286
14.3.2 Results 14.4.1 14.4.2
287 288 288 290
Experimental e q u i p m e n t and procedure .................................... and discussions on the preparation of nano copper p o w d e r ....... Major results obtained in the first stage .................................... Results on the influences of various factors .............................. XV
286
14.4.3 Preparation experiments under optimal conditions ................... 296 Surface i m p r o v e m e n t of nano copper: preparation of Cu-Ag double metal powder .......................................................................................... 297 Conclusions ............................................................................................ 299
14.5 14.6
15
Preparation of Ultrafine Powders by Reaction-Precipitation in Impinging Streams III: Nano Titania ................................................................................ 301 15.1 15.2 15.3
Properties of nano titania and chemical reactions in its preparation ....... Experimental equipment and procedure ................................................. Results and discussions .......................................................................... 15.3.1 M a j o r results obtained in the first stage .................................... 15.3.2 Experiments and major results in the second stage ................... 15.3.3 Experiments of mass preparation and the results ...................... 15.3.4 Experiments of neutralization with aqua a m m o n i a ................... 15.3.5 Experiments for final optimization of conditions and the results .................................................................................. 15.3.6 Comparative experiments between SCISR and STR and the results .................................................................................. Conclusions ............................................................................................
15.4
16
313 313 314
Preparation of Ultrafine Powders by Reaction-Precipitation in Impinging Streams IV" Nano Hydroxyapatite ............................................... 317 16.1 Introduction ............................................................................................ 317 16.2
Experimental equipment and procedure ................................................. 16.2.1 E q u i p m e n t ................................................................................. 16.2.2 Procedure of the experimental operation .................................. Results and discussions .......................................................................... 16.3.1 Influences of some factors ........................................................ 16.3.2 Optimal conditions for synthesis of nano H A P ......................... 16.3.3 Characterization of nano H A P product ..................................... Concluding remarks ................................................................................
16.3
16.4 17
301 303 304 304 306 309 312
318 318 319 320 320 324 324 326
Research and Development of Liquid-Continuous Impinging Stream Devices and Application Forecasting .............................................................. 329 17.1 17.2 17.3
The vertical circulative impinging stream reactor .................................. 329 Impinging stream crystallizer ................................................................. 333 Prospects for the application of liquid-continuous impinging streams... 337
Postscript
....................................................................................................................
339
References
...................................................................................................................
341
..............................................................................................................
355
Index ...............................................................................................................
361
Nomenclature Subject
xvi
INTRODUCTION
1 ENHANCEMENT OF TRANSFER BETWEEN PHASES AND ORIGIN OF IMPINGING STREAMS Heat and mass transfer, especially mass transfer, in multiphase systems are problems commonly encountered in processing units in the chemical, petrochemical, and many other process industries. Because transfer rates significantly affect the efficiencies and technical-economic indexes of the processes, the enhancement of transfer has been a continuing topic of interest in chemical engineering since the late 1930s. A vast number of theoretical and experimental investigations have been carried out in the search for new methods of enhancing transfer between phases. According to the theory of transfer rates, the amount of heat or mass transferred per unit time can be expressed by Amount transferred per unit time =
driving force xinterface area specific resistance
(1)
Therefore one, or a combination, of the following measures can be used to increase the amount to be transferred per unit time: (1) Enhancing driving force; (2) Increasing interface area; and (3) Reducing specific resistance. All three measures are, of course, effective in principle. However, their potential to enhance transfer and the degree of difficulty in carrying them out are quite different in practice. The driving forces of heat and mass transfer are temperature and concentration gradients, respectively. To a considerable extent, they are limited by the characteristics of the specific processes involved, such as stocks, heat sources, and equipment materials, etc. In most cases only a limited increasing magnitude is permitted. Relatively, increasing the interface area, i.e., enhancing the dispersal of a liquid or a solid, is a measure that can be employed widely, and, in fact, has been applied successfully in a number processes, such as spray drying and cooling etc. However, it is also limited to an extent. For example, spray drying can only be applied in the production of powdery products, and excessive dispersion may give rise to difficulties in powder collection etc: while spray cooling is only applicable to the cases where moisture increase is permitted. On the other hand, in common equipment systems, the maximum relative velocity between phases is mostly just equal to the terminal velocity, u~, which
2
IMPINGING STREAMS
decreases sharply as the particle/droplet size reduces. This may partially offset the effect of the increase in interface area for enhancing overall transfer rate. In comparison, the reduction of specific resistance is an effective way of enhancing transfer between phases and has great potential. It is generally considered that there exist three resistances in series in transfer processes of gas-solid, gas-liquid, liquid-liquid, and liquid-solid systems: gas or liquid side resistance, the so-called external resistance, interface resistance, and internal resistance of particle/droplet. The interface resistance possibly results from the accumulation of impurities on the interface. Reduction of any one of these three types of resistance can enhance transfer processes. The overall specific resistance for heat or mass transfer is the reciprocal of the heat or mass transfer coefficient, U or K, where U and K are the common parameters characterizing heat and mass transfer rates, respectively, defined as U=~
Q
AAT
K -
nA
(2) (3)
AAC A
They represent heat and mass fluxes with unit gradients of temperature and concentration, respectively, and so can be considered as specific heat and mass transfer rates. In the last few decades, the results of a large number of investigations into drying, absorption, cooling, combustion etc, have shown that, apart from the natures of the systems involved, including dispersion degree, the major factor influencing heat and mass transfer coefficients is the relative velocity between phases, Ur. An increase in the relative velocity results in enhanced turbulence and reduced thickness of the boundary layer, and also favors surface renewing of the liquid side. As a result, the transfer resistance of the gas or liquid side is reduced. Synthesizing existing experimental results of various unit operations, it can be concluded that transfer coefficients are exponential functions of the relative velocity: t
U -///r " and K - ///r"
Depending on the substance systems involved, the types of equipment and the operating conditions, the exponent, n" or n, varies approximately in the range from 1/3 to 4/5. For example, for particles of spherical form, Ranz-Marshall [1] obtained the following relationship for prediction of the film heat transfer coefficient: Sh = 2 + 0 . 6 R e l / 2 S c 1/3
(4)
For bubbles or suspended particles smaller than 2.5 mm in size, the Levich empirical equation below was recommended by Calderbank and Moo-Yong [2] for prediction of the heat transfer coefficient:
INTRODUCTION
3
Sh_l.()l(dpurl I/3 IDaB )
(5)
All the results mentioned above lead to a simple and clear conclusion: increasing the relative velocity between phases is one of the most effective approaches to enhance transfer processes. In traditional processing devices, increase in relative velocity is limited by various factors. For example, in column equipment the operating velocity must be smaller than that of liquid-flooding; the limitation of relative velocity in common gas-solid or liquid-solid suspensions is the terminal velocity, etc. It seems that other approaches must be found in order to raise the relative velocity between phases to higher levels. The efforts to search for approaches to raising relative velocity between phases has led to the development and/or application of impinging stream contactors, and also some other devices. The original conception of impinging streams (IS) is to bring two solid-in-gas suspension streams to flow in opposite directions at a considerably high velocity and impinge against each other, yielding extremely high relative velocity at the instant of impingement, and thus greatly enhance transfer between phases. As a scientific concept, IS was first proposed by Elperin [31 in 1961; while its application can be traced back to the development and application of the Koppers-Totzek gasifier in the early 1950s [4], although the term "'impinging streams" was not used at that time. In the period from the 1960s to the early 1970s, a large number of theoretical and experimental investigations on impinging streams were carried out, mainly by Elperin and his group. On the death of Elperin, the research core was moved to Israel. Tamir [5] carried on for over 20 years from 1974 until the 1990s, and his researches extended over almost all the unit operations in chemical engineering. All the results of investigations involving transfer processes show that impinging streams can increase transfer coefficients by large amplitudes. For instance, the heat transfer coefficient obtained by Elperin [6] from the experiments of wet particles drying is as high as 5800 W.m--~.K- I while that calculated with the assumption of relative velocity being of the order of fluidizing velocity is only of the value of 470 W.m-2-K-~ Another kind of device that efficiently enhances transfer processes in gas (vapor)liquid systems is the rotating packed bed (RPB), also called "HIGEE", presented in the 1960s [7, 8]. The basic idea tbr RPB design is that extremely high relative velocity can be employed with the action of centrifugal force produced by rotating the packed bed at high speed to enhance strongly the transfer between phases. In comparison, in traditional column equipments, such as packed tower and sieve plate column etc', the permitted operating relative velocities are bounded to low levels due to the limitation of liquid-flooding. There is yet another method which also enhances transfer very efficiently, in which a stream is induced to impact a fixed wall surface, i.e., the impinging jet (IJ). Obviously, the flow configuration and the action of stream impingement of IJ are totally different from the impinging streams, although it uses also the term "impinging"
4
IMPINGING STREAMS
[9]. The impinging jet has important applications in rapid heating and cooling, drying of coating layer, reaction, and surface cleaning e t c , and investigations in that field are also very active; but it is beyond the scope of the present book. All the researches, developments, and applications of IS, RPB, and IJ show the extreme importance of increasing relative velocity for enhancing transfer between phases.
2
BASIC
PRINCIPLES
OF
IMPINGING
STREAMS
As mentioned above, the original concept of impinging streams presented by Elperin [3] is to bring two equal solid-in-gas suspension streams formed after fully accelerating solid particles by gas to flow in opposite directions at a considerably high velocity and impinge against each other at the middle point between the two accelerating tubes, as shown in Fig. 1. The gas velocity at the outlet of the accelerating tubes can be as high as 20 m.s -~ or even higher, and the particles can theoretically be accelerated to a velocity near that of gas. The impingement between the two-phase streams causes an impingement zone of high turbulence with the highest concentration of particles [5], which provides excellent conditions for heat and mass transfer. In the case where the difference in densities of the two phases, e . g . , in a solid-in-gas suspension, particles would penetrate from one stream into the opposing one, and, just at the instant of penetration into the opposite stream, the relative velocity between particles and gas flow achieves a maximum value. After that, particles are decelerated due to the friction force of the opposing gas flow until particles achieve zero velocity. Thereafter, particles are accelerated by that gas flow in the opposite direction towards the impinging plane, and then penetrate into the stream that the particles originally existed in. After several repetitions of penetration to and fro between the opposing streams, particles gradually lose their axial velocity due to dynamic energy consumption, and are finally carried by the radial gas flow to leave the impingement zone.
ug
.'i'i, t OD +~ ~
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•
•
•
•
•
•
"el
Air+Particles
eio
~
•.-
,/o!o"
• • •• •o •
.~ ~
Accelerating tube
"p
"
ug ~
',
..
//~f..-'...'/-_..'...'_..'..-'//...'...'..-'..-'..-;.;-'/...'...'..-n
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~..........+.._..._........._..._............. _...............
•
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-
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. . . . . . .
Alr+varncJes
~.,.,......,...,...., . . . . ....,.,.. ....... ._+.,..,
/
",°~o , ~ Accelerating tube ',o.o, ".[., Impinging plane Impingement zone
Ur- Up--(-- Ug)= Up+ Ug
Figure 1 Basic configuration and principles of impinging streams.
INTRODUCTI()N
5
The phenomena of penetration to and fro between the opposing streams can occur even in a homogeneous gas system. Bley et al [10] observed experimentally that, when a gaseous He stream impinges against another co-axial stream of a mixture of He and SF(, SF(, molecules penetrate deeply into the pure He stream. In the impinging streams of solid-in-liquid suspensions, penetration phenomena may occur theoretically. However, because of low operating impinging velocity and large friction resistance of the opposed stream, perceiving the penetration of particles is difficult. Elperin and Tamir considered that, in impinging streams with gas as the continuous phase, transfer between phases is enhanced by the factors below: (1) Relative velocity between particles and the opposite gas flow is greatly increased. The relative velocity round the impinging plane, u,., may be calculated roughly by u, - .p - i - u ~ ) - up + u,~
(6)
where the velocity of gas, u~, can be considered approximately as constant, while that of particles, u~,, varies from time to time during penetration. At the exact instant the particles penetrate the opposite stream, the relative velocity achieves a maximum value, and, in the idealized case of particles being accelerated up to a value equal to the velocity of gas flow, then the maximum relative velocity can be twice that of the gas velocity (see Fig. 1). At all other instants during penetration, the relative velocity between particles and the opposite gas flow are larger than the gas velocity. In impinging stream devices for gas-solid systems, the operating velocity of gas flow inside the accelerating tubes is usually higher than l0 m.s -~ or, sometimes, even above 20 m-s -~. Obviously, the relative velocities in the traditional column devices can never be comparable with that in gas-solid impinging streams. (2) The penetration of particles to and fro between the opposing streams lengthens their residence time in the region active for transfer, i.e. the impingement zone, so that, to an extent, the conditions for enhancing transfer can continue for longer. Elperin [6] observed 5 to 8 times of particle penetration to and fro between the opposing streams in his experiments. For instant processes, such as combustion of powdery coal or fine droplets of oil, such an amplitude of residence time increase is of very great significance. The resulting global behavior of the residence time increase of particles is that the concentration of particles (or droplets) within the impingement zone is much higher than in any other regions. This implies that the impingement zone has much a larger interface area per unit volume for heat and mass transfer. (3) In the impinging streams of gas-liquid systems, high relative velocity between phases and collision between droplets favor surface renewing of droplets, resulting in reduced liquid film resistance and thus increased overall mass transfer coefficient.
6
IMPINGING STREAMS
(4) Impingement between the flows of continuous phase in the opposing streams, plus the oscillation movement of particle penetration, leads to strong mixing in the impingement zone, resulting in homogenization of temperature and composition. In some cases, this favors an increase in the average driving forces of heat and mass transfer and thus promotes the transfer processes. The problems related to mixing will be discussed further in later chapters of this book. It can be seen that impingement between the streams shown in Fig. 1 is of a "soft" nature. As stated above, its flow configuration and impingement action are totally different from the impinging jet impacting on a fixed wall surface, which is a "rigid" impingement.
3 EXPERIMENTAL EVIDENCE FOR ENHANCING TRANSFER As mentioned before, in his early investigation on drying of wet particles, Elperin [6] obtained powerful evidence that impinging streams enhance heat transfer. He determined the heat transfer coefficient is as high as 5800 W.m-2-K-~, while, in comparison, that calculated by the general empirical relationship, assuming the relative velocity to be of the order of the fluidizing velocity, is only about 470 W.m-2-K-~. In addition, the relationships of heat transfer coefficient and pressure drop versus relative velocity obtained by interpretation of experimental data are h c~ u 19 and Ap ~ u 2, respectively; while, by other technical methods, usually h c~ u°~3. This suggests that the employment of impinging streams will yield much higher efficiency. Tamir [5] tested the effectiveness of impinging streams in enhancing heat transfer by introducing a partition between the two opposing streams, which separates the impinging stream dryer into two non-interacting components. The results showed that the partition causes a significant reduction in heat transfer coefficient, h. The values for h in the case without partition are larger than those with partition by 1 to 2 times where the other conditions are the same. In their investigation on circulative impinging stream drying of PVC, Huang et al. [11] measured experimentally the value for the specific volumetric evaporation coefficient to be 16x10 -4 kg-s-~-m-3.K-~, which is about 10 times that in spray dryers, and a conclusion is also obtained similar to that by Elperin according to the heat transfer coefficient predicted from the evaporation coefficient just described above. On the enhancement of mass transfer, Tamir [5] studied the absorption of acetone into water with a similar method, i.e. using a partition. The results they obtained were: under suitable operating conditions and with appropriate structural parameters, the runs without partition yield absorption rates higher than those with partition by over 4 times. The experiments on combustion of powdery coal carried out by Ziv et al. [12] are another instance that shows impinging streams enhancing transfer. They measured the temperature profiles along the direction of flame length in the two cases with and without partition, and the results showed obviously higher temperature profiles in the case without partition than with partition, suggesting that impinging streams enhancing heat and mass transfer leads to stronger combustion.
INTRODUCTION
7
It should be noted that, in all the comparative experiments made by Tamir and his group, the cases without impingement are imitated by using a partition between the two opposing streams. In those cases each stream impinges on one side of the partition, which actually plays the role of a fixed wall surface. Thus, each piece of experimental equipment used with partition is equivalent to two impinging jets. As mentioned in Section 1, impinging jets also enhance transfer between phases very efficiently. Therefore the results of their comparative experiments could not reflect fully the influence of impinging streams on heat and mass transfer. This may account for the fact that the degree of enhancing heat transfer by impinging streams Tamir obtained in the study on drying (1 to 2 times) is much lower than those by Elperin (more than 10 times). There is still much more experimental evidence for impinging streams enhancing transfer. All the evidence, both that mentioned above and that not mentioned, supports the following conclusion: impinging streams are very efficient in enhancing transfer between phases, especially those controlled by diffusion through gas-film. Because transfer phenomena are widely encountered in various processing industries, the method of impinging streams undoubtedly has great potential application.
4 OTHER PERFORMANCES OF IMPINGING STREAMS Impinging streams were first suggested for enhancing transfer between gas and solid phases; however, the results of a large number of investigations have shown that, as well as this effect, the method of impinging streams has many other functions valuable for application; of course, at the same time, it also has some disadvantages, unbeneficial to application [9]. For the processes occurring in liquid phase or multiphase systems with a liquid as the continuous phase, the mixing status has significant effects on the efficiencies. The results of investigations since the 1990s showed that impinging streams have excellent performance for mixing. The most remarkable is that, because of the special flow configuration of the two opposing streams impinging against each other, impinging streams promote micromixing very efficiently [13]. In addition, the results of investigations by the author of the present book show that in liquid-continuous impinging streams there exists a pressure fluctuation of multi frequency in the range of sub-sonic waves, and the maximum amplitude can be as large as over 1 kPa. The details will be discussed in Chapter 11 of this book. Most likely, such pressure fluctuation is one of major reasons for impinging streams promoting micromixing efficiently. Yet, the pressure fluctuation favors kinetic processes, and this has also been proved by experiments. The phenomena of impinging streams promoting micromixing and the existence of the pressure fluctuation in liquid-continuous impinging streams had not been fully considered, and had not even been discovered in the investigations on impinging streams before the early 1990s. Consequently, the application potential of impinging streams for the processes of reactions and precipitation etc. received little attention, although the stagnation jet mixer developed by Brauer [14] was mentioned in Ref. [5]. In practice, since many processes are carried out at the molecular scale, the
IMPINGING STREAMS features of impinging streams promoting micromixing and the existence of pressure fluctuation are of very great value for application. In a number of researches and developments carried out in recent years [15-19], various impinging stream reactors of different structures were used for liquid reactions, reaction-precipitation or reaction crystallization etc. of various systems, and have performed well. Among the applications mentioned, the preparation of nano or sub-micrometer materials by impinging stream reaction-precipitation is an area of great potential. The milling effect resulting from strong collisions between particles in gas-solid impinging streams is another important feature of value for application. The most outstanding advantage of this technology of milling is that no milling material is needed, so that the substance being milled can be protected effectively from pollution. In addition, because milling proceeds in gas flows at high velocity, the phenomenon of overheating is also avoided so that the technology is applicable especially to substances of thermal sensitivity. Impinging stream milling technology was applied industrially as early as the 1970s. A typical example is the Trost Jet Mill [20]. The results of a large number of investigations and applications have shown that "ultrafine" products of the order of sub-micrometer can be produced by such a technology. As stated by the author of this book in the section "Translation Illustration" of the Chinese translation of Ref. [5] (The Chemical Industry Press of China, Beijing, 1996), as a technical method impinging streams cannot be a universal tool. It also has some disadvantages which limit its application. The most obvious problem is the very short residence time of the material in the active region of the impinging stream device. As will be discussed later, in gas-solid impinging streams the average residence time of solid particles is only about 1 s. On the other hand, the flow configuration of an impinging stream device is relatively more complex so that it becomes difficult to arrange a multistage system, such as in column devices. Tamir proposed several structures of multistage impinging stream contactors (refer to Fig. 3.2 in Ref. [5]). However, from the point of view of industrial application, they are obviously impractical. Most processes of industrial interest, such as drying materials containing porous moisture and/or combined water etc., need considerably longer time, even if they are carried out under conditions of significantly enhanced heat and mass transfer. Very short residence time, plus the difficulty of arranging multistage systems, significantly limits the fields in which impinging streams alone can be applied. For the processes restricted by equilibrium, single stage impinging streams can enhance heat and mass transfer to yield higher rates although it is difficult to ensure that the requested processing degree can be achieved. For example, the final absorption fraction or reaction conversion etc. may not achieve the level expected. Nevertheless, impinging streams, as a novel technical method, has a number of superior properties, among which the features of gas-continuous impinging streams enhancing transfer between phases and liquid-continuous impinging streams promoting micromixing have considerable value for application, and have found, and are finding, more and more applications.
INTRODUCTION
5 EXTENSION OF IMPINGING STREAM TECHNOLOGY Figure 1 represents the basic principles of gas-solid impinging streams, and also its essential structure as originally designed. On the basis of the essential structure, various devices can be constructed by extending the idea of impinging streams. Two extension schemes of IS have been proposed' extension of the flow configuration and extension of the phase conditions of the substance systems involved, as described below.
5.1 Extension in flow configuration Starting with the elements necessarily included, the concept of impinging streams can be extended to include various flow configurations. Talnir et al. [5] investigated a number of impinging stream contactors with different flow configurations" and the structures of some of the contactors they studied are shown in Fig. 2. A P
A
P
A------~
A(W) A
A A
P
p+ (P+W) A(W)
l (a)
(b)
A A
(c)
A ~
A A
A
------~ o (d)
P
(c)
Q,~L )
o t~---
(f)
Figure 2 Impinging stream contactors of various configurations [5]. (a) Coaxial-horizontal two impinging streams" (b) Horizontal three impinging streams (c) Coaxial-vertical two impinging streams; (d) Curvilinear two impinging streams; (e) Curvilinear four impinging streams; (f) Four impinging streams. A--air; P--particles: W--water.
10
IMPINGING STREAMS
In addition to those shown in Fig. 2, there are many other different structures. Different impinging stream devices may have different flow configurations, although all of them contain the same essential elements: (1) the streams flow in opposite directions and impinge against each other, and (2) each stream contains at least one continuous phase. Impinging stream equipment contains two types of part: (1) Accelerating tubes, which are also the conduits for feeding fluid of continuous phase; and (2) Equipment body with separate outlet ports for continuous and dispersed phases, respectively. Referring to Tamir's work, the following classification according to various features may be applicable for various impinging stream devices with different flow configurations:
Flow of the continuous phase: Parallel: the streamlines are parallel to the axis of flow, e.g. (a), (b) and (c) in Fig. 2. Rotational: the streamlines are helicoids with respect to the axis of flow, e.g. (d), (e) and (f) in Fig. 2.
Flow of the streams inside the device: Coaxial countercurrent: two streams enter the device in opposite directions along the same axis, and flow as free jets before impingement, e.g. (a) in Fig. 2. Eccentric countercurrent: as above but different flows are not on the same axis, e.g. (b) in Fig. 2. Co-plane-rotational: two streams enter the device tangentially and counter currently with the central lines in the same plane before impingement, and then flow on the wall of the device, with streamlines of a half circle form, e.g. (d) and (f) in Fig. 2. Non-co-plane-rotational: two streams enter the device tangentially and counter currently with the central lines in different planes before impingement, and then flow on the wall of the device, with streamlines of several half circle forms, e.g. (e) in Fig. 2.
Operation modes: Continuous two-side feeding: both phases flow at steady state, and particles are injected into both streams symmetrically; all devices shown in Fig. 2. Continuous one-side feeding: both phases flow at steady state, while particles are injected only into one stream. Semi-batch: only the continuous phase flows at steady state, while particles are circulated inside the device. In addition, according to the feature and number of impingement planes, devices can also be classified as stationary, moving, and multi impingement zone, etc. Readers may refer to Ref. [5].
INTRODUCTION
II
Any modification in flow configuration is aimed at: (1) Producing some advantages in operation or performance; and (2) Making the device more suitable for some specific systems. Very often, the latter is needed in practice, and usually it can be achieved by certain modification; while the former is somewhat complex. It can be seen from the various modifications proposed that, relatively, the common advantage obtained is that the total residence time of particles is lengthened in various degrees. However, essentially, the residence time in the active region could not be lengthened. On the other hand, the total residence time of particles is lengthened in some schemes although a price must be paid: (1) The structure of the equipment must be complicated; (2) The effects of impinging streams described in Section 2 must be weakened; and (3) In the cases of gas-solid impinging streams the resistance of the system must be increased remarkably. So, not every flow configuration in Fig. 2 is of practical significance. More ideal modification schemes with more advantages and fewer disadvantages may possibly be constructed in the future with further investigations into impinging streams. Among various flow configuration schemes, co-axial two impinging streams is the most essential and simplest; while its effects of enhancing transfer between phases and mixing are most significant. On the other hand, this scheme is the key for understanding principles and application of impinging streams. Therefore the discussions in the present book will take this scheme as the major topic.
5.2 Extension in phase conditions Obviously, one of the necessary conditions to carry out impinging streams is that both the opposed streams in impingement must have, at least, one continuous phase. In the impinging streams shown in Fig. 1 the continuous phase is a gas; although a liquid can of course also be the continuous phase. If a liquid is used, the dispersed phase should be a solid or another unmixable liquid. Otherwise, the employment of impinging streams would have less sense. The properties of a liquid are quite different from those of gas. These essential differences must result in different performances of impinging streams with gas and liquid as the continuous phase, respectively. The following facts are clear: (1) Liquid (L) is normally greater in density than gas (G) by three orders of magnitude. (2) L is larger in viscosity than G by two orders, and (3) G has a considerably larger molecular free path , while the molecules of stationary L can only vibrate and/or rotate with extremely small displacement round their balanced positions. Because of these significant differences, the behavioral features of the impinging streams with a liquid as the continuous phase are quite different from those with a gas as the continuous phase. As an example, consider here the case where the dispersed phase is a solid. When a liquid is taken as the continuous phase, the relative velocity cannot be large because the densities of solid and liquid have the same order of magnitude and the friction force between phases is very large. Furthermore, the phenomena of particles penetration to and fro between the opposing streams become non obvious and fine particles tend to follow streamlines. As the result, the enhancement of heat and mass transfer become
12
IMPINGING STREAMS
very weak, as has been proved by experimental data. This aspect will be discussed further later. On the other hand, since liquid is at condensed status and has large density, the interaction between two opposing liquid streams in continuous phase will be much stronger than gas streams, although its operating velocity is usually smaller than the latter. The strong micromixing and pressure fluctuation in impinging streams with liquid as the continuous phase mentioned above would be related closely to such strong interaction between the two opposing liquid streams impinging against each other; these features have great value for application. It can be considered that the extension of continuous phase in impinging streams from only gas to include liquid is progress of major significance which brings great application potential to impinging streams. However, this has unfortunately been ignored for a long time. Since there are significant differences of properties and performances between gasand liquid-continuous impinging streams, the two kinds of impinging streams will be discussed separately in this book. In addition, the method of impinging streams can also be used for systems of single phase, such as gas-gas and liquid-liquid impinging streams etc. In fact, single phase impinging streams have great value for practical application in mixing, gas combustion, etc.
6 APPLICATION STATUS OF IMPINGING STREAMS AND LOOKING AHEAD As mentioned above, the Koppers-Totzek gasifier of powdery coal [4, 5], the Stagnation jet mixer [14] and the Trost jet mill [20] are practical examples of the successful application of impinging streams. Apart from these, very few industrial applications of impinging streams had been by the end of the last century, [9, 21 ]. The following may account for the fact that the application of impinging streams has progressed so slowly:
(1)
Incorrect selection of application objectives decentralized time and efforts of investigations. Guided by the understanding "almost any process in chemical engineering can be carried out" [5], investigations extended over almost all the unit operations in chemical engineering, even including those controlled by internal diffusion so that, essentially, impinging streams cannot play any role, such as calcination of phosphate rock etc. That is, researchers did not focus on the cases where impinging streams were likely to be applied successfully. (2) The engineering problems that are normally encountered in practical application did not receive enough attention, and thus appropriate and feasible solutions were not found. As a result, few complete set technologies have been provided for industry.
INTRODUCTION
13
(3) Investigation on impinging streams with a liquid as the continuous phase started very early although, the perfect features and the application potential of liquidcontinuous impinging streams had been ignored for so long that both quantity and depth of investigations and development in this area were insufficient.
In fact, as described in the last section, the method of impinging streams has outstanding advantages and, simultaneously, intrinsic disadvantages. It can never be expected to become a universal tool. The proper selection of application objectives based on an understanding of the properties of impinging streams, the improvement of its advantages while avoiding the disadvantages, and focusing on solving related engineering problems may be the most important things to push n impinging streams towards industrial application. Fortunately, since the 1990s, technologies employing impinging streams have received increasing attention, and investigations into them have been growing faster than before. It is reasonable to believe, therefore, that more and more technologies applying impinging streams will emerge in various processing industries in the near future. The development of applied technologies of impinging streams has tended to increase significantly in the last 10 years. The following areas may be the most promising for impinging streams application to achieve success: (1)
Preparation of ultra fine powders by reaction-precipitation in impinging streams One of the most important conditions for preparation of ultrafine particles by reaction-precipitation is to create a very high and uniform supersaturation environment for precipitation. The tact that liquid-continuous impinging streams promote micromixing effectively favors such conditions, and so has received much attention in the last ten years and more. Instant reaction- precipitation processes can be carried out in an impinging stream reactor alone. Particularly noteworthy is that such types of reactor can be used for the production of nano materials. Mahajan et al. [22] and Liu et al. [16] studied the rapid precipitation of a number of medicines in two impinging stream reactors to prepare ultrafine products and obtained satisfactory results. By reaction- precipitation in a submerged circulative impinging stream reactor (SCISR), the author of this book obtained a Titania product average-sized 5.68 nm and copper powder sized 5.1 nm, both with very narrow size distribution. The details will be discussed in the relevant chapters of Part II. Essentially, few engineering problems involved in such technologies remain to be solved for successful application.
(2) Impinging stream ~'ombustion The strong micromixing in single phase impinging streams for gas fuel and highly enhanced heat and mass transfer for sprayed liquid fuel or fine powdery coal favor their combustions considerably. The Koppers-Totzek gasifier for powdery coal mentioned above is a typical example of employing impinging streams, and has been proved to be successful. The scheme of multi-frame inclined impingement has been used in some novel cooking stoves. Recent research and developments in the area of combustion have focused on improving the structures of burning
14
IMPINGING STREAMS chambers and the arrangement of burners in order to increase combustion efficiency further.
(3~ Impinging stream drying Drying of solid normal or fine particles is a type of typical process involving parallel heat and mass transfer, and thus is an area where the application of impinging streams could be most promising. In fact, since the 1970s a large number of investigations on this topic have been carried out, and many technologies and related devices have been proposed, as will be described in Chapter 6. However, no essential progress in industrial application has been seen [21] for over four decades. The main reason for this lies in the fact that some of the engineering problems involved had not been solved in the related developments. Most of the particular materials contain both free moisture and combined or in-pore water. The former can be removed instantly under enhanced transfer conditions, provided the particles are not too large; while the latter needs a considerably longer time to be removed because porous diffusion, especially diffusion of the liquor water, is involved. Since it has the intrinsic disadvantage of a very short residence time in the active region, impinging streams alone cannot accomplish the tasks of removing both free moisture and combined or in-pore water; while the design of a multistage impinging stream device would greatly complicate the system and increase its hydraulic resistance. Some researchers have used impinging streams for drying grains. In this case, in addition to the problems above, the energy consumed in accelerating grains would be very large. The author of this book recently developed a circulative impinging stream dryer [11] which, on one hand, utilizes the feature of impinging streams enhancing transfer and, on the other, can provide arbitrary residence time for the material being dried as needed by the arrangement of circulation. It is expected to be applied industrially in the near future. (4) Impinging stream milling As already stated, the most outstanding advantages of impinging stream milling are that it is without milling material and that the milling is carried out in gas streams at high velocity so that the substance being milled can be protected effectively from pollution and overheating. The Trost jet mill was applied successfully as early as the 1970s [20]. The development of applied technologies and devices of impinging stream milling have increased tendency in recent years [23, 24]. This is also an area of application that is currently well to the fore.
(5~ Impinging stream absorption Absorption is typical process involving transfer between phases and so is another area where impinging streams may be applied successfully. However, many systems to be processed by absorption are subject to equilibrium limitations, while to arrange a multistage countercurrent system employing impinging streams, such as in a column device, is very difficult. For such systems the impinging streams method is not a good option. On the other hand, for chemical absorption systems involving fast or instant irreversible reaction(s) in liquid phase, most possibly, an
INTRODUCTION
15
impinging stream device can be successfully applied. A notable objective of application is desulfurization of flue gas from coal burning. As is well known, this is a major problem involving the protection of the human environment. The absorption of sulfur dioxide with Ca(OH)e-water suspension involves irreversible fast reactions in liquid and is a typical case of gas-film diffusion control. Berman et al [25] studied such a process in an impinging stream absorber with three coaxial cylinders. Their results are certainly positive from the point of view of SO~_removal efficiency, but from the standpoint of engineering practice, both the equipment and the system scheme are considerably complex, giving rise to difficulties tk)r industrial application. Further research and development are still needed. Recently, the author of this book developed a novel impinging stream absorption device system with a simple structure and scheme, which has been used for wet desulfurization of flue gas, yielding good results. The details will be discussed in Chapter 7. In addition to the above, it is predicted that impinging streams can also be used for some other processes, such as solvent extraction, emulsification etc., to yield good performances. In the last two decades the application potential of impinging streams has been receiving more and more attention from scientists and engineers and the subsequent research and development has increased significantly, encompassing more and more countries and regions. It can be expected, therefore, that more and more applied technologies of impinging streams will continue to emerge in various processing industries in the near future.
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PART I GAS-CONTINUOUS IMPINGING STREAMS The idea of impinging streams (IS) was originally presented for enhancing transfer processes in gas-solid systems. In the 30 years from 1961 when the concept of IS was presented by Elperin to the mid-1990s, investigations on IS were mainly concentrated on systems with a gas as the continuous phase, while, to an extent, the dispersed phase was extended to include liquid. For processes in multiphase systems, whether the dispersed phase is solid or liquid, the common characteristics of gas-continuous impinging streams (GIS) are: low viscosity of the continuous phase, large density difference between the continuous and dispersed phases, and high operating impinging velocity. These features result in the following phenomena in GIS: strong turbulence in the impingement zone, very large relative velocity between phases, and penetration of particles or droplets in the dispersed phase to and fro between the opposing streams. The latter two can be considered as the special phenomena of GIS. Without question, GIS is one the most effective methods for enhancing transfer between phases to date. Part I of this book focuses on problems relating to gas-continuous impinging streams, including basic regulations, properties, and some of its applications. It is clear that the flow of continuous phase plays a very important role in impinging streams. Part I will start with single-phase impinging streams, because, to a great extent, the flow phenomena in such impinging streams can reflect the flow of continuous phase in multiphase impinging streams. Considering the similarities in movement of liquid and gas, the discussion in this Part will also involve single phase IS of liquid.
17
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-1FLOW OF CONTINUOUS PHASE
As mentioned betbre, the concept of impinging streams (IS) was originally suggested for enhancing heat and mass transfer between a solid and a gas, and the development of the application of impinging streams were long focused mostly on multiphase systems. However, the problems involved in multiphase impinging streams are considerably complex so that it is necessary to study the behaviors of the individual phases separately. In gas-continuous impinging streams, the concentration of dispersed phase, particles or droplets, is usually very low. Particles or droplets have no significant influence on the flow of the continuous phase except that their acceleration consumes part of the dynamic energy of the flow. For investigating the basic regularities of the flow, the behavior of gas-continuous impinging streams may be considered as a simple summation of that of single phase impinging streams plus the movement of dispersed phase without significant deviation. The flow of the continuous phase is the most essential phenomenon, and is the origin of various features of impinging streams. In addition, the method of impinging streams of single phase has a number of practical uses in gas-gas or liquid-liquid mixing, gas jet burning, etc'. Therefore investigation on the flow of continuous phase is of practical significance. The discussions in this chapter relate to single phase impinging streams, the behavior of which may reflect part of those of multiphase impinging streams. Taking into account the similarities of liquid with gas, the discussions will also involve impinging streams of liquid alone: while the differences between gas- and liquidcontinuous impinging streams will be described in detail in the chapters of Part II.
1.1 FLOW CHARACTERISTICS The impingement of two single-phase flows against each other is a very complex and interesting phenomenon and it can be considered that investigations on this phenomenon are not sufficiently up to date. Powell [26] presented a Mirror Image model in 1960, in which impingement between two opposing jets at a distanced of L is considered as equivalent to the impingement of one jet on a flat plane at a distanced of L/2. In other words, all the states in the impingement zone are symmetrical with respect to the impingement plane. Nosseir et al. [27] considered that the concept of mirror
19
20
IMPINGING STREAMS
image may be correct if the jets in impingement are laminar flows; but for turbulent flows, that concept would be questionable. The results of their study on the flow field in the impingement zone showed that pressure fluctuation occurs on the impingement plane, and then fluctuation is enhanced in a feedback mechanism. This pressure fluctuation is very significant and should never be neglected. The results obtained recently by the author of this book show that the pressure fluctuation in liquidcontinuous impinging streams has fundamental influence on micromixing in the impingement zone, and thus on process kinetics. The details will be discussed in Chapter 11. The major disadvantage of the mirror image model is that the interaction between the two opposing streams in impingement was not considered. In fact, even in laminar impinging streams, such an interaction cannot be negligible. Denshchikov et al. [28, 29] studied experimentally the interaction between the opposing flows. Two slit nozzles are mounted opposite each other in a water filled tank sized lx lx0.23 m. The nozzles are in the form of a plane box. The dimensions of the outlets of the nozzles ensure that the jets from them are in two-dimensional flows. Inkstained water streams are ejected from the two nozzles and impinge against each other at the center of the tank. The experimental observations are: the flow direction of a single jet is always stable; when another jet is introduced to impinge against it, both the jets begin to deflect in opposite directions with considerably large amplitude (up to 0.05-0.2 m), and then the directions of deflection periodically vary, as shown in Fig. 1.1, where T is the period of oscillation. Apart from planar deflection, there is also a twisting of the ends of the deflected jets in a vertical direction.
t--O
I=T/4
Figure 1.1 Image of impingement between two-dimensional jets of ink-stained water [29].
FLOW OF CONTINI !OUS PHASF
2i
When the jet on the right-hand side is at the top and that on the left-hand side is at the bottom, the twisting is clockwise; while for opposite positions of the jets, it is anticlockwise. The deflection and twisting of the jets result in the formation of vortices on both sides of the outflow plane, the intensities of which depend on the parameters of the jets in impingement. The researchers considered that the collision between the two jets is associated with retardatioE~ of the liquid, resulting in an increase in pressure in the impingement zone. The experimental measurements were correlated in terms of the period T of the auto oscillation as a flmction of the operation parameters. For two equal jets ejected from the nozzles at the same velocity, the relationship they obtained is
.
T =0.34Re-~(~] ~)45 .
.
.
(1.1)
where --/2
T ,~ = - - - " V
/2 0 d
Re - - -
V
L is the distance between the nozzles, u0 is the velocity of the jet just leaving the nozzles, i . e . , impinging velocity, 6 is the transverse dimension of the nozzles, and v is dynamic viscosity of the fluid. It is clear that such significant deflection as that shown in Fig. 1.1 must be related to the properties of the liquid employed. In comparison with liquid, both the density and viscosity of a gas are much smaller, so that such a strong deflection could not be observed with gas jets. However. in principle, it is possible that such deflection phenomena could occur in gaseous single-phase impinging streams, but the degree of deflection may differ greatly from that of liquor ones. Oren e t a l . [30] also observed deflection of the flows resulting from the interaction between two opposed jets in an investigation on the impingement of cylinder-formed liquid jets with small diameters (3-8 ram) submerged in liquid. The investigation was carried out in an electro-chemical reactor of 0.07 m in diameter and 0.012 m high, and the inlets of the two jets and the two outlets for flows from the reactor are uniformly distributed along the circle and meet at right angles (see Fig. 4.2 in Ref. [5]). According to the structure of the device, most possibly, the mentioned deflection is related to the arrangement of inlets and outlets of the reactor, i . e . , the symmetry of impinging streams is disturbed by the flows toward the outlets. A notable factor is that in the experiments made by both Denshchikov e t a l . [28, 29] and Oren e t a l . [30] the cross-section areas of the jets are considerably smaller. This may be an important reason for the deflection of the jets. Since 1995, the author of the present book has organized a number of investigations, theoretical and experimental, on the properties and application of the submerged circulative impinging stream reactor (SCISR) [9, 13, 15-18, 31]. The flow configuration inside the reactor is two impinging horizontal streams, as shown in Fig.
22
IMPINGING STREAMS
1.2. The propellers on the two sides transport liquid to pass through drawing tubes 60 mm in diameter and impinge against each other at the center. The rotary speed of the propellers ranges from 400 to 1300 rpm, and, correspondingly, the velocity of liquid flow at the outlet of the drawing tube, i.e., the impinging velocity, ranges from 0.18 to 0.6 m.s -~. In experiments with water as the process material, the following global flow phenomena are observed: (1) A strong surge occurs in the impingement zone at high impinging velocity, with the appearance of the flow being significantly different from that outside this zone. (2) The two streams in impingement do not reflect, provided the drawing tubes are mounted co-axially. (3) At very high rotary speed of the propellers, horizontal oscillation movement of the impingement plane along the axis of flow is observed, the maximum displacement of which can be as large as 30-40 mm. The horizontal oscillation obviously results from the pressure fluctuation round the impingement plane. (4) In the range of normal rotary speed of the propellers the horizontal position of the impingement plane is essentially stable, but its micro oscillation can still be observed visually.
Drawing tube
Impingement zone Propeller
Figure 1.2 A simplified view of the flow configuration in the submerged circulative impinging stream reactor. On gaseous impinging streams, Becker et al. [32] made a comparative study of two cases" two opposed free jets impinging against each other and a single jet impinging on a wall plane vertical to the jet axis. The researchers found that in the laminar region the impinging streams and the impinging jet have the same configuration, except for the friction of the wall surface, and the former is equivalent to the combination of the latter with its mirror image on the wall. However, in the turbulent region, a boundary layer forms on the surface of the fixed wall impinged by the jet, which stabilizes the flow and eliminates oscillation on the plane, while in the case of two opposing jets impinging against each other the interaction between the two streams is significant. In addition, Becker et al. mapped the concentration field in impinging streams by marking them with oil condensation smoke from a generator. The results they obtained are" the concentration decreases along the axis towards the impingement plane, and also decreases towards the radial direction. These tendencies are reasonable, because the two jets mix strongly with each other on the impingement plane and mixing is most intensive at the center.
FLOW OF CONTINUOUS PHASE
23
Popiel and Trass [33] studied the flows in impingement of two opposing jets against each other under conditions of low turbulence by the smoke-wire visualization technique. They provided an excellent photograph of the flow behavior of free impinging round jets issuing into ambient air at rest through bell-shaped convergent nozzles, as shown in Fig. 1.3.
Figure 1.3 Streamlines in circle impinging streams [33]. Possibly because the mixing problem involved was not fully considered, the interaction between the two opposing streams in impingement has not received enough attention in earlier investigations. A typical example is the mirror image model proposed by Powell [26], which takes the impingement between two opposing streams as two streams, independent of each other, impinging the same rigid plane from opposite directions. In the investigation carried out by Becket [32] the mixing between the two streams was considered although the target is gaseous impinging streams, for which the mixing problem is very simple and not as important as for liquid systems, because gases have a considerably larger molecular free path. As mentioned in the Introduction, the impingement between two opposing streams is quite different from a single jet impinging on a rigid wall surface. In the impingement of two opposing streams against each other, the interface (the impingement plane) is "soft". On the other hand, the continuous phases of impinging streams of gas and liquid have common properties: they have no fixed forms and certain distances exist between molecules and/or fluid elements. Consequentially, the relative movement between molecules and/or fluid elements can occur with certain magnitude. Of course, there would be a considerable difference between the permitted relative displacements in gas and liquid. In gas-solid impinging streams, solid particles can take the oscillation movement of penetration to and fro between the opposing streams; and even in the single phase impinging streams of gaseous He-He+SiF4, the molecules SiF4 have been found to penetrate into the opposing stream deeply. The penetration phenomenon may also occur theoretically in liquor impinging streams. However, since the densities of both the two streams are large, with very small difference between them, and the friction Ibrce is also very large, the depth of penetration into the opposed stream, if it happened, would be extremely small, so that it is very difficult to observe.
24
IMPINGING STREAMS
In addition, due to the increase in pressure caused by the strong momentum transfer between the streams on the impingement plane, the fluid elements in opposing streams may collide, shear and press each other, resulting in deformation of fluid elements, and finally the elements may break up to reducing segregation scales. This action may be very important and effective for promoting mixing, especially micromixing. Figure 1.4 shows a simplified model describing Element A deforming and breaking-up into Elements A1 and A2 by pressing and shearing after passing through the impingement plane, where the lengths of the arrows represent qualitatively the velocities of the corresponding elements. Since the interaction between elements always occurs mutually, every element could be subject to deformation by shearing and pressing, and/or to breaking up. It is difficult to obtain direct experimental evidence for the model shown in Fig. 1.4, but the model would be reasonable according to the general properties of liquid and the analysis for momentum transfer involved. On the other hand, in general theory it is considered that mixing is caused by flow, turbulence and eddy diffusion etc., yielding reduced segregation scale in the fluid. When the segregation scale is reduced down to the Kolmogoroff micro scale 2, molecular diffusion takes over to achieve a complete homogeneity, i.e. the ideal micromixing status. According to such a theory, the results obtained by the author of this book on the very strong micromixing in the submerged circulative impinging stream reactor (SCISR) [ 13] may be taken as indirect evidence for the model shown in Fig. 1.4. I
|
Impingement plane t = to
Impingement plane t = tl
Impingement plane t=
t2
Figure 1.4 Model for deformation and break-up of fluid element due to pressing and shearing round the impingement plane. Summarizing the results and analysis described above, the following can be generally concluded for the flow characteristics of co-axial single phase impinging streams, at least for those with larger diameter jets: (1) Impingement does not change the axial symmetry of the flows, i.e., each of the streams does not deflect, and the related parameters, such as streamlines, density etc., are kept in axially symmetry; (2) Pressure fluctuation occurs round the impingement plane; and (3) Due to strong momentum transfer, significant interaction between the two streams occurs round the impingement plane, and, in the case of liquid impinging streams, the interactions of collision, sheafing, and pressing etc. between the streams may finally result in reduction of the segregation scale. The characteristics described in Items (2) and (3) above are important effects in micromixing and will be discussed in detail in Part II.
25
FLOW OF CO\r~ftNUOUS PHAS!~
1.2 VELOCITY FIELD IN LAMINAR IMPINGING STREAMS Powell's mirror image model has the major disadvantage that the interaction between the opposing streams in impingement was not taken into account, although it is helpful for understanding some of the global parameters and their profiles in impinging streams, such as velocity field etc; while the model itself is relatively simple.
1.2.1 General equations Based on general principles of fluid dynamics and Powell's mirror image model, Tamir [5] analyzed the velocity field in laminar impinging streams. The case considered was two gaseous coaxial impinging streams, and the distance between the outlets of the two tubes was represented by L. In establishing the model, the following assumptions were made: (1) The fluid has zero viscosity and is incompressible, i.e., ¢tg = 0 and p~ = constant; (2) The flows in impinging streams are at a steady state; (3) The flows are without rotation; and (4) The influence of gravity is negligible. Obviously, the assumption of incompressible fluid greatly simplifies the problem; since pressure drop due to impingement between the opposing streams is usually not large, this assumption would result in no significant error. With the above assumption, the equations for fluid motion can be obtained as [34]
pV ] - u 2
-~ - p [ u x ( v x u ) - - v / ;
(1.2)
The equation for flow without rotation is
Vxu =0
(1.3)
V.u-O
(1.4)
and the continuity equation is
where - v/,
- -vP
+ pg
With the assumptions of fl~ = O, p~ = constant and neglecting the effect of gravity, the motion equation (1.2) is simplified to
OVl-u -~ - - V P /,
(1.5)
26
IMPINGING STREAMS
1.2.2 Planar two-dimensional impinging streams For planar two-dimensional impinging streams, the velocity vector, u, can be represented in the well-known way, as u - iu x
+
jUy
and the equations above can be specified to be as follows.
For motion:
~)u×
~)u~
=
OP
¢)Uy buy ~)p x-"~-X" k - Uay y ~ = - - ay ~
(1.6)
For irrotational flow..
~U x
~Uy
by
Ox
=0
(1.7)
For continuity: Ou
Ou " + Y -0 0x Oy
(1 8)
Using Eqs. (1.6), (1.7) and (1.8), ux, Uy and P can be determined as a function of the coordinates x and y; while it would be more convenient to represent them in terms of the stream function N(x, y) and the velocity potential O(x, y), which are defined by the relationships below.
u
=-~,
~)Y
a0
u -=-~, ~)Y
u
Y - - t - ~OX
Uy -
a0 ~)x
(1.9)
(1 10)
Substituting Eqs. (1.9) and (1.10) into Eqs. (1.7), (1.8) and (1.6) and their integral forms yields
FLOW OF CONTINUOUS PHASE
27
v-~-o
(1.11)
V2¢~-0
(1.12)
Oy
3x O(x, y)
0(v2¢) a(v2~u)
(1.13)
Oy 2
~)x2 and, from Eq. (1.5), we have 2
2
0.5 p(u ~ + u y ) + P - const.
(1.14)
The equations above can be represented in terms of the velocity potential to yield ~p- 0.5M ( - x 2 + y2)
(1.15)
where M is a constant determined from a known velocity at some distance from the origin. From the definitions for the velocity components, Eqs. (1.9) and (1.10), using Eq. (1.13) and considering that d ~r = u~clx- u:dy, one can obtain the expressions for the velocity components below u ~ -
-Mx,
u y -
My
(1.16)
and dv uy . . . . . .
y
dx
X
I t ,,
( ~ - Const..)
(1.17)
The pressure distribution in the flow field is determined by Eqs. (1.14) and (1.16) to be P - P(t - 0 . S p M 2 (x 2 + y2 )
(1.18)
The streamlines in two-dimensional impinging streams are shown in Fig. 1.5, the slope of which demonstrates the direction of flow and is equal to the local velocity vector meeting the relationship b e l o w
28
IMPINGING STREAMS 2 )0.5 _ I1--( bl2x WHy
M (x 2 + y2 )0.5
This relationship indicates that the velocity profile is flat for large x or y; while when x and y are of the same order of magnitude, the velocity is minimal at the centerline and increases with x or y increasing. This is indicated in Fig. 1.5. For the case of neglecting viscosity, these results are reasonable, because no shearing force is imposed on the jets. Integrating Eqs. (1.18) and (1.9) leads to the streamlines equation: xy - gz / M = const.
(1.19)
/ bly--my
xy= gzA~=const.
uy,
N~~
ux=Mx ..........................................
iiiiiiiiiiii!iii t-
x
P u=M(x:+y2)°5
I
Impingement plane Figure 1.5 Streamlines of two dimensional impinging streams.
1.2.3 Axial-symmetric impinging streams For axial-symmetric impinging streams, it is more convenient to use the cylindrical coordinates. The equations above then become the following:
FLOW OF CONTINUOUS PHASE
29
For motion:
I 0(~,E 2~u) + ~2~3E~-/~ r
3(r,:)
,
_
0
(1.20)
r 2 3:,
For irrotational flow with respect to 0 axes: Ou,. , 3z
c)u~') _ 0 Or )0
(1.21)
and tbr continuity: 1 3(ru,. ) + ~ U z _ 0 r Or 3z
(1.22)
where
E-p'-
32~ 3 Or-
"
32g 1 ON + ~ 9 r Or 3z"
(1.23)
3p' u,. - + ~ 3~
(1.24)
The stream function is defined by 3gt Or '
u, =
A specific solution of the equations above is given in terms of the velocity potential 0as 0 - 0.5M (-22. 2 + r 2 )
(1.25)
where M has the same meaning as that in Eq. (1.15); and 0 meets the definition and the restrictive conditions below
II r
u~
=
a~
1 a W"
Or
r 3:
ao ~z
u0 =0
-Mr
13~u
r ar
- -2Mz
(1.26)
IMPINGING STREAMS
30
1.2.4 General three-dimensional impinging streams For general three-dimensional impinging streams it is difficult to define a stream function satisfying the continuity equation. However, if the velocity vector is defined by u = -V¢
(1.27)
then it automatically satisfies irrotational flow defined by Eq. (1.3); and the continuity and momentum equations are given by Eqs. (1.12) and (1.5), respectively. A specific solution for the velocity field satisfying the modified equations above is given by the velocity potential function below:
¢-
(1.28)
0 . 5 M ( - 2 x 2 + z 2 q- r 2 )
and the differentiation of Eq. (1.28) yields the following velocity components: 30
li x
()X
a¢ UY=Oy
-2Mx
(1.29)
= My
3 0 = Mz
Uz = ~ Z
Using the expressions for the velocity components above and Eq. (1.5), one can obtain the expression for the pressure distribution as P = P o - 0 . 5 p M 2 ( 4x2 + y2 + z2)
(1.30)
and that for streamlines as U x "bly "U z -- d x "
dy'dz
- -2x"
y"
z
(1.31)
The results of integrating Eq. (2.30) are y - C l z,
where CI and Cz are integral constants.
xy 2
-- C 2 ,
xz 2 -
C2
(1.32)
FLOW OF CONTINUOUS PHASE
31
1.2.5 Viscous impinging streams In all the above derivations in this section, the influence of viscosity is neglected so that analytical solutions for velocity and pressure profiles can be obtained. When the viscosity of fluid is taken into account, it is difficult to obtain any analytical solution. Kuts and Dolgushev [35] solved numerically the flow field in the impingement of two axial round jets of a viscous impressible liquid ejected at the same velocity from conduits with the same diameter and located very close to each other. The mathematical formulation incorporated the complete Navier-Stokes equations transformed into stream and velocity functions in cylindrical coordinates r and z, with the assumption that the velocity profiles at the entrance and the exit of the conduit were parabolic. The continuity equation is given by Eq. (1.22); and the equations for motion in dimensionless form are:
,~)u~ , .~)u~. . . ~) P u,. Or" +u~ ~)z' ~)z.'
+ 1 1 ~) (r, +~)-u~• R-e-eL-r-7~-Tr'k, Or J ~)z'2
(1.33) , au,___ u,
Or" +
, au,__~= u~
,
a
1
- -'r--;
1 a ( r u,.
a 2u~' + az '2
+--~
where the dimensionless variables are defined as
u,
,
u,. u~
, uz
uz u~
r
,
r R
7
,
z --, R
Re
Ru o v
R is the radius of the conduits; u~ the velocity at the outlet of the conduit, i . e . , the impinging velocity. The velocity components in terms of stream function were given by Eq. (1.26); while the conditions of irrotational flow were determined by Eq. (1.21). The streamlines calculated by Kuts and Dolgushev [35] for some of the operating conditions are given in Fig. 1.6(a) and (b) as functions of dimensionless axial and radial coordinates, where the measure for ~- is from the impingement plane and that for r from the flow axis. A comparison between these results and the data to be introduced later shows that the flow field configuration theoretically calculated above fits the experimental data qualitatively well. The two streamlines figures indicate that the flow in impinging streams has a layered nature and the presence of vortices was not observed. As the Reynolds number, R e , increases, the streamlines are slightly deformed in the direction along the impingement plane. However, this slight difference and the absence of vortices, despite a significant difference in R e between the figures, indicate that the assumption of ignoring the influence of viscosity made before is reasonable indeed; this assumption greatly simplifies the calculation.
32
IMPINGING STREAMS 1.0
.e=,O 'J A
~
/
/
0.5
0.5
0.005 (a)
I
I
0.5 z/R
I
(b)
0.5 z/R
(d)
0.5 r/R
0.04 0.03
-..< (c)
0.5 JR
• 0.02 0.01 0
Figure 1.6 Streamlines (a, b) and pressure profiles in axial and radial directions (c, d).
Figures 1.6(c) and (d) show the pressure profile in the impinging streams. The profile is characterized by the considerable pressure gradients in the direction of the chamber axis, as shown in Fig. 1.6(c), and also in the perpendicular plane, as shown in Fig. 1.6(d). Obviously, in the region a little distance away from the impingement plane, the pressure profile is independent of the impinging distance. A decrease in the impinging distance leads to a more rapid increase in pressure only in the impingement zone. All the above analyses for several specific cases are based on the mirror image model. As mentioned earlier, the major disadvantage of the model is that the interaction between opposing streams in impingement, including momentum transfer and the consequent pressure increase and fluctuation, was not taken into account. In addition, the flows were assumed to be irrotational in the establishment of the models. Naturally, the streamlines calculated do not show vortices existing. Although these results cannot completely reflect the flow characteristics of impinging streams, the information on velocity and pressure profiles that the researchers provided is helpful for understanding the properties and some regularity of impinging streams.
1.3 EXPERIMENTAL RESULTS FOR THE FLOW FIELD IN IMPINGING STREAMS Elperin et al. [3, 36] investigated experimentally the hydrodynamics of co-axial gassolid suspension impinging streams. The dimensions of the device they used are"
FLOW OF CONTINUOUS PHASE
33
diameter of the gas conduit do = 0.05 m; distance between the outlets of the conduits, L, variable in the range of (0.5-8)d0. The air velocity in the conduit was measured with a Pitot tube connected to a micromanometer; while the values and the direction of the velocity and the static pressure in the impingement zone were measured by a threechannel probe attached to a traverse gear. Figure 1.7 demonstrates the distributions of axial and radial velocities, along with the isobars (dotted lines) for the case of L/d = 8. This figure can be related to Fig. 1.3 demonstrating streamlines in impinging streams for reading and analysis. It can be seen that the gas stream leaving the conduit behaves like a free jet flowing into infinite space with a characteristic velocity profile. As it approaches the impingement plane (x/do = 0), the axial velocity profile is deformed and a defined extreme point is observed along the x-axis. This behavior is the result of the hydrodynamic interaction between the opposing streams and the consequent appearance of the radial component of velocity. When the impinging distance becomes even smaller (L/d < 3), the axial velocity profile deforms immediately once the gas flow is ejected from the conduit. v/R
~
I V
~
I
~'"
-3
-2
"
",, ~ - -
-1
0
\...
1
....
2
/
3
Figure 1.7 Velocity and pressure profiles in impinging streams for L/d=8. It can be seen from the isobars (dashed lines) that the highest static pressure appears near the impingement plane. The isobars have complex appearances and assume an ellipsoidal form in the region far from the x-axis. These experimental findings and the symmetry of the flow pattern in impinging streams with respect to the impingement plane are suitable for applying Eq. (1.28) for the pressure distribution in a non-viscous impinging jet far from the x-axis, where the constant pressure surfaces are ellipsoids with the main axis ratio of 0.5:1:1. The maximum pressure is observed at the point x = y=z=O.
34
IMPINGING STREAMS
G)
I
2
~D
¢~
I
I
¢,g3
10
y
c.
P/(0.5pu 0 )
©
0
I
G~
o
-1.0
Ux/U° I
~
-4d
I -2d
I
I 0
I
I 2d
I 4d
Figure 1.8 Variations of dimensionless velocity and pressure along the axis (y/d = 0). Figure 1.8 shows the variation of the dimensionless axial velocity on the flow axis ((y/R - 0) along the radial direction. This variation can be approximately represented by the hyperbolic tangent function below: bl x
e x/d ~
e - x/d
x/d
-x/d
= u o
e
( 1 . 3 4)
+e
The absolute values of the dimensionless velocity vary between 1 and 0. The minus sign in the figure indicates that the velocities are in opposite directions. Figure 1.8 also shows the variation of pressure along the radial direction. The velocity profiles in the x direction shown in this figure are different from those based on the theoretical model in Fig. 1.5. This is because the experimental profiles in the jet are affected by the drag forces of the stagnant atmosphere. The variation of the maximum pressure versus the dimensionless impinging distance L/do at a constant impinging velocity, u0, is shown in Fig. 1.9. It can be seen that, in the range of L/do - 3-8, the maximum pressure at the center of the impingement plane increases moderately as the impinging distance decreases; while, as the impinging distance decreases further, the pressure increases sharply. This is a problem connected with the design of the impinging stream device. When the impinging stream technique is applied to processes such as combustion etc., it is important to determine the characteristics of the variation of the maximal radial velocity. Figure 1.10 gives the experimental results related to this topic. In coaxial two impinging streams, after impingement the two streams are mixed with each other and then turned to be an axial flow. At the point of an axial coordinate y, the axial velocity is affected by two factors: (1) After the flow direction turning, the fluid originally in the region of r < y must flow outward through the point of axial coordinate
FLOW OF CONTINUOUS PHASE
35
v. Therefore the amount of fluid flowing outwards passing through Point y must be increased as v increases. This is a positive factor for increasing radial velocity. (2) The passage area for the radial flow increases as y increases, yielding a negative influence on the radial velocity. As a result of the combined effect of the two factors in contradiction, there must be a m a x i m u m value on the curve describing the relationship of the radial velocity versus the dimensionless radial distance y / R ; the curves in Fig. 1.10 clearly demonstrate such a situation. In the range of L / d - 3-8, the following relationship gives a good approximation to the experimental data:
(1.35) u(}
R
I
I
I
I
I
I
I
I
4 tt~
2
I
i
()
i
I
2
i
4
I
I
i
6
8
L/d
F i g u r e 1.9 Maximum static pressure vs impinging distance at the center of impingement plane.
2.0 S/d
0.5 1.0 I I 3.0 (5 4.0 ...,. 6.0 • 9.0
•
1.6
x
1.2 0.8 0.4 I
0.0 0
1
2
I
3
4
5
6
v/R
F i g u r e 1.10 Variation of maximum radial velocity with radial distance.
36
IMPINGING STREAMS
where R is the radius of the gas conduit. According to Eq. (1.35) the following inference of practical sense can be withdrawn: When y/R - 6, ur/uo = 0.0355, the radial velocity becomes negligible in comparison with the velocity of the gas flow inside the conduit. Therefore the relationship below is suggested as a criterion for the decision of the diameters of gaseous impinging stream device in design:
Did > 6
(1.36)
1.4 TURBULENT IMPINGING STREAMS The theoretical method describing turbulent impinging streams was presented first by Champion and Libby [37]; although it may not be the best, there is nothing better to date. Champion and Libby analyzed both the planar two-dimensional impinging streams and the impingement of two co-axial-cylindrical jets in which the flow is axis symmetrical. Actually, the results they obtained are applicable for both the two cases, provided the two-dimensional coordinates in planar impinging streams are replaced by the cylindrical coordinates. The jets are assumed to be ejected at an initial velocity of u0 along the x-axis, and then expanded rapidly towards the y-direction. The corresponding velocity components are ux and Uy. It is also assumed that the distance between the outlets of the conduits, i.e. the impinging distance, is very small and is equivalent to the diameter of the conduit, d0. The turbulent kinetic energy of each jet is k0, and the mean viscous dissipation is e0. It is then possible to define the following two dimensionless parameters. The first one is the integral scale of turbulence, 10, which is the measure of the turbulent degree and is defined by comparing it to half the separation distance of the jets. This ratio is
1o L/2
k~5/eo L/2
The second parameter is the measure of the turbulence intensity and is defined as
ko/u g . It was found in laboratory experiments that turbulence intensities resulting from a grid or a baffle are such that k o/U2o is of the order of 0.01 and 10/(L~) is of the order of 0.1. The fact that the two parameters are very small forms the basis of an asymptotic analysis of the model. As the quantity k o/U2o approaches zero, the ratio (L / 2) / u 0
flow time
10 / k0°5
turbulence time
is of the order of 1, i.e., it is independent of the limit process. With the two parameters defined above and from a comparison of their orders, it can be considered that the
FLOW OF CONTINUOUS PHASF
37
flows associated with closely spaced jets consist of two regions: one is the outer region between the exit and the neighborhood of the plane containing the stagnation line or point, called the stagnation plane; and the other is a thin layer centered about that plane, commonly called the impingement zone, in which adjustments of various quantities take place on each side. The so-called stagnation plane is the plane containing stagnation points and/or stagnation lines. The two-dimensional flow equations employed by Champion and Libby [37] are just the well known Reynolds stress equations [38], t
...... +_8(u'~u[) Z ~k 8~ 6= ,. , Ok ~.r
+ .8(u'~u,) . . . Ov
. .1 .OP. p Oi
(i
x,y)
(1.37)
where pu[u"k are the Reynolds stresses. The analysis involves solving the partial differential equations that are treated by asymptotic methods to solve for the intensities of the radial and axial shear stresses and for the viscose dissipation. Of principal interest are the dimensionless axial and radial intensities, respectively, defined as G~(()-,,
"~-/k ~,
Gy(()-,~
"2/k o
(1.38)
where g"= x/d is the dimensionless axial distance, and ( = 0 corresponds to the impingement plane. The initial values at ~'= 1, i.e., the exit planes, are determined from experimental data. Kostiuk [39] obtained data suitable for comparison with the theoretical analysis relating to opposing circular jets. The experiments took air as the working fluid. The two impinging jets with an exit diameter of 0.035 m were spaced 0.07 m apart, and the mean velocity at the exit plane of each jet was 9 m.s -~. The turbulence is generated by perforated plates located 0.02 m upstream of the exit planes. The perforated plates have various geometries, but all with a blockage ratio of 50%. A comparison of the dimensionless mean velocity between the results experimentally measured and calculated is shown in Fig. l . l l ; while that of the dimensionless intensity in Fig. 1.12. Note that the calculation based on the theoretical model requires a value for an adjustable parameter. From the figures, it seems that the agreement between the calculated values and the experimentally measured data is excellent. In addition, Fig. 1.12 provides the information of significance below: the intensity of the axial component exceeds that of the radial component. Kostiuk et al. [40] measured experimentally the flow field of the vertical co-axial turbulent impinging streams with a two-component Laser Doppler velocity meter. The opposing gas streams were ejected from two burner nozzles, which were designed to produce a uniform axial velocity profile at their exits. The turbulence in the flow was generated by a perforated plate located at the end of the contraction section in each nozzle. The air velocity at the exit of the nozzle was varied from 4.1 to 11.4 m.s-~; and
38
IMPINGING STREAMS
the distance b e t w e e n the nozzles f r o m 0.02 to 0.103 m. The f o l l o w i n g m a j o r results were obtained:
1.0
0.8 0
E
~) :,/~,-~
0.6
u0, m's -~ d, mm
c'~ ot'~~ a ~'-~ a
V !> []
0.4
u~, the particle will be accelerated by the gas flow to the terminal velocity and will then move towards Point 2 at that velocity. Therefore, the terminal velocity is necessary for the determination of the operational range of the gas flow velocity for a specific vertical gas-solid impinging stream system. As an example, let us now examine the specific case of particle motion at 25°C and atmosphere pressure. The related physical properties are: dp= 0.001 m, pp = 1000 kg.m -3, p ~ - 1.145 kg-m -3, and ¢t~- 1.798 Pa-s. The calculated values for the terminal velocity and the operational condition ranges are given in Table 2.2. An important conclusion that can be drawn from the data listed in the fourth column of Table 2.2 is that the Stokes regime cannot exist in co-axial horizontal impinging streams; while other regimes are applicable in this kind of impinging stream. Table 2.2 Operational range for vertical impinging streams Regime
u~, m/s
Conditions for u,.
Special conditions for u,
Stokes
30.3
u~ tim always holds.
RESIDENCE TIME OF PARTICLES AND ITS DISTRIBUTION
77
The following may be concluded theoretically from the above analysis: (1) The flow of the particles inside the impinging stream contactor has the characteristics of perfect mixing--plug flow in series; (2) Back-mixing in the impingement zone, the major active region for heat and mass transfer, is critical. This may be favorable for some processes; while it is harmful for others since back-mixing usually results in a decreased mean driving force for the processes involved; (3) The existing results of theoretical analysis and some experiments indicate that both the mean residence times of the particles in the major and the secondary active regions, the impingement zone and the space inside the accelerating tube, are very short, totally about 1 s only. This is a great limiting factor for practical application of impinging streams. The primary conclusions above are helpful for understanding possible performances of impinging stream devices.
3.2 METHOD FOR EXPERIMENTAL MEASUREMENT OF PARTICLES' RESIDENCE TIME DISTRIBUTION Theoretical analysis gives some useful information on the characteristics of residence time and its distribution of particles although experimental evidence is always important. It is well known that the measurement of residence time distribution usually employs the dynamic method [54], the so-called input-response technique. However, for measuring RTD of solid particles the input signal is a difficult and troublesome problem. The author of the present book employs an arbitrary known function as the input signal so that this problem is solved. This procedure is also applicable, in principle, to the measurements of RTD of solid materials in other devices.
3.2.1 Input signal In the measurement of residence time distribution, the most convenient and so most widely employed input signals are the impulse and the step change. For a fluid, as continuous phase, whether it is a liquid or a gas, both the signals are simple and convenient to carry out; while for solid particles, as dispersed phase, it is very difficult to input any one of the two signals usually used, and the so-called frequency response [55] is even more difficult. One may consider the facts below: could any conveyer for solid particles or powders be used to provide a step change of tracer to the following equipment under conditions of ensuring stable flow rate? Inputting tracer particles in pulse form is theoretically possible, but the amount of particles to be inputted is a problem. If the amount of tracer particles inputted is as large as is needed, the width of the pulse would be considerably increased, yielding unacceptable experimental errors;
78
IMPINGING STREAMS
while if the amount is small, the randomness of movement of the particles would conceal the regularity of the residence time distribution. Luzzatto et al. [56] studied experimentally the residence time distribution of particles in a co-axial gas-solid two impinging stream reactor with a special ejector to input radioactive-tagged particles to the reactor, and interrelated the data they obtained by Markov chains. In order to keep the flow conditions stable and, at the same time, to ensure that the input signal was close to an impulse, the number of the tagged particles they inputted to the system for each run was only about 200 particles; while, in comparison, the number of process particles in the equipment is almost a million. Since the movement of particles is highly random, one experimental run cannot yield useful data from so small a number of tracer particles, so that the researcher had to repeat the experiment many times under the same conditions to obtain a set of statistically averaged results. The experimental procedure is considerably long-winded, and the interpretation of data with Markov chains is also significantly troublesome. Obviously, if no special restriction is exerted on the input signal, the experimental measurement of residence time distribution of solid particles would become simpler and much more convenient. From observations it is found that, inside the glide tube of a hopper for solid particles feeding, the movement of particles behaves very like a plug flow, as shown in Fig. 3.5. This implies that the hopper can provide the following device with a step change with good approximation. On the other hand, it is also found by experiments that a screw feeder with good shifting performance cannot only control very stable feed flow rate but also give a very stable response to its input of a step change with good reproducibility of data. The observations described above indicate that, with good control, the concentration of the tracer particles in the out stream of the screw feeder can be determined to be a known function of time, and, furthermore, it is feasible to use such a known function as the input signal to the impinging stream equipment to be tested. In this way the experimental procedure can be greatly simplified. Of course, this scheme calls for corresponding mathematical relationship(s) for data interpretation.
Figure 3.5 Plug-flow movement of particles within feeding tube.
RESIDENCE TIME OF PARTICLES AND ITS DISTRIBUTION
79
Tracer
P r o c e s s particles
Hopper
b-----q
To a c c e l e r a t i n g tube
Figure 3.6 Feeder to IS equipment. The screw feeder used in the experiments is shown in Fig. 3.6. It is driven by a motor with very stable rotary speed and good speed-shifting performance. Two sets of screw feeders are used in the experimental system, which are mounted oppositely and symmetrically. The outlet of the glide tube of each hopper is connected directly to one particle feeding tube of the impinging stream contactor, i.e., the accelerating tube. In operation, two air streams flowing in the two accelerating tubes accelerate the particles fed by the two screw feeders, respectively, and then the particles-in-air suspension streams are ejected from the accelerating tubes towards the impingement zone. Initially the two feeders are ted with only the process particles B. Under the condition of stable flow, Feeder 1 is inputted with a step change of the tracer particles A; while Feeder 2 is continuously ted with process particles B at a stable flow rate. A certain time later, the particles of Tracer A emerge at the outlet of Feeder 1. The instant the tracer A emerges at the outlet of Feeder 1 is taken as the initial time for the residence time distribution measurement. After that, the concentration of Tracer A in the out stream of Feeder 1 varies with time , and this function of time, ~ ll(t) "-~Ai
'
is the response of Feeder 1 to its
input signal, the step change; it is also the input signal to the following equipment to be tested for RTD, which can be determined to become a known function of time. In order to protect the flow stability from turbulence caused by the input signal, the properties of the tracer used should be as close as possible to those of the process particles. In the investigation carried out by the author of this book the process particles are yellow millets; while purplish-red rape seeds are used as the tracer, the properties of which are very similar those of the millets. The properties of the process and the tracer particles are listed in Table 3.1. The concentration of the tracer is represented in terms of mass fraction, and is measured by manually separating the tracer from the process particles according to the difference in color and weighing the amount of tracer. This is laborious and time-consuming work, but it can yield reliable data. Using the procedure described above, input a step change of the tracer to the hopper and then measure the response of the screw feeder to the step change as a known function of time, which is used as the input signal to the following impinging stream device for the measurement of RTD.
80
IMPINGING STREAMS
Table 3.1 Properties of the materials used for RTD measurement Property Material Pp, kg'm -3
Pb, kg'm -3
dp X 103, m
Millets (B)
1101
661.5
1.6872
Rape seeds (A)
1172
696.5
1.6304
It is clear that the response of the screw feeder to the step change of the tracer depends on the rotary speed of the screw. For convenience, the responses at various rotary speeds are calibrated prior to the experimental measurements as the curves of the tracer concentration versus time, which represent the responses as known functions of time. For each rotary speed, the calibration was carried out two or three times, and the data were averaged. A set of typical data calibrated is shown in Fig. 3.7. It demonstrates that there is good regularity and reproducibility of the data, suggesting the calibrated curves can be used as the input signals to the impinging stream device for RTD measurement to yield sufficiently accurate results.
1.0 O
•E
~0.8
o
~0.6
•-~
~0.4
~D
A
o0.2 O t
0.0 -20
0
20
40
60 I,S
80
100
I
120
t
140
Figure 3.7 Typical response of the screw feeder to a step-change of the tracer concentration (rap = 1.914x10 -4 kg.m-3).
R E S I D E N C E T I M E OF P A R T I C L E S A N D ITS D I S T R I B U T I O N
81
3.2.2 Data interpretation The relationship for the interpretation of data measured for RTD can be easily derived with Laplace transformation [54]. However, the equations so obtained are not convenient for time-dispersed measurement because they still need Laplace transformation to deal with the data. In the following, another relationship will be derived.
re(l)[ a,
.I
Air flow
IS Device
"~1
•[" I
Air flow
mao~mBo(m~o) Out particles Figure 3.8 Principle scheme for measurement of particles RTD. Consider the equipment system schemed in Fig. 3.8, where the subscript B denotes the process particles and A the tracer. Corresponding to the impinging stream equipment with feeding on both sides, the system has two feeds, which are denoted by the superscripts (1) and (2), respectively; while it has only one out stream of particles. For convenience of operation, the tracer A is inputted into one feeding stream,
i.e.,
rt 2)
Ai -- 0. The system is assumed to be operated at a steady state, and only the particles B were fed before, mA~ = 0 for t < 0. From the instant of to = 0, the tracer particles A are fed into the equipment at an amount comparable with the particles B, and then both the particles A and B are fed into the device continuously and simultaneously. In order to keep the momentums of the two streams in balance, the operation of the screw feeders ensures that the total amounts of particles of the two feeding streams are always kept equal,
i.e.,
i.e.,
m~i'
+
m(" Bi
-
m Bi '2)
= mBi,
rn~i'
-- mAi
(3.29)
At the instant t, the residence times of all the particles of the tracer A must be < t; while the particles of B consist of two groups: in the first group all the particles are fed at instant to - 0 or later and so also have residence times < t; while the particles in the second group were fed into the device before to = 0 and so have residence times > t. If the particles in the first group are denoted by the superscript "*", then, from the definition of the residence time distribution function, F, we have [
:f:
Io (mA,, + mBo )dt
f ( t ) - I~(mAo + mBo)dt
IomAodt + Io mBodt f[) mto dt
(3.30)
82
IMPINGING STREAMS
On the other hand, one of the essential conditions for correct measurement of RTD is that the tracer particles have the properties, including RTD characteristics, very close to those of the process particles. This implies that, in the time domain of t _> 0, the residence time distribution functions of particles A and B in the same device should be approximately equal to each other, i.e. F A (t) - F B(t),
t>0
(3.31)
It should be noted that Eq. (3.31) is an approximate relationship. This is because both the individual flow rates of the particles A and B vary with time, although the total flow rate, i.e. the flow rate of A plus that of B, is stable, while the variation of particle flow rate may affect RTD. However, since the variations of the flow rates of particles A and B are not so large, the possible deviation of Function F caused by this factor may be neglected, just like the generalized (dimensionless) residence time distribution function can be extended for application in a certain range [57]. According to the definition of the F-function, the residence time distribution functions of A and B for the case under consideration can be expressed by the corresponding ratios of the amounts of particles coming out from the device to those inputted in the time interval from 0 to t, i.e. ~OmAodt F A (t) - - ~ ~ - lOmAidt (3.32) [OmBodt FB(t ) = . IomBidt Using the relationship represented by Eq. (3.31), Eq. (3.32) yields mA°d--------~t I; = m~3°d-------~t I° J'; mAidt
(3.33)
I;mBi dt
and, from the well-known mathematical theorem of proportion by addition, Eq. (3.33) becomes J'omAodt + ItomBodt
j'omAodt =~ ~omAidt + ]'; mBidt j'; mAidt t
(3.34)
The mass balance at steady state gives t dt + ]'omBidt i0mAodt + J'omBodt- J'0mtodt =!0mAi
(3.35)
83
RESIDENCE TIME OF PARTICLES AND ITS DISTRIBUTION
Substituting Eq. (3.35) into Eq. (3.34) and combining the resulting expression with Eq. (3.30) leads to
F(t)- I/~mA°d~t
(3.36)
I;mAi dt
On the other hand, the condition of the total material stably flowing suggests the following relationship holds: into - mAo + rnB{, -2[m~i ) + m~Bii)] - mti- const.
(3.37)
Dividing both the numerator and the denominator in Eq. (3.36) by Eq. (3.37) leads to
[[t (mAo /mto )dt F(t) = " fo (rn {l ' /mti )
(3.38)
while the following relationships are easy to obtain:
mA° = CAo into
and
m~i) mti
(m~
(')+ (2) ) + mBi mBi
1
Therefore Eq. (3.38) becomes
F(t) =
2IoCAo(t)dt
Ill r~til "~Ai (t)dt
(3.39)
or, in the case of time-scattered measurement, ZCAo(ti)Ati F(t) = 2 i Z Cdi ) (ti)All i
(3.40)
In the derivation above, the input signal CAi(t) is an arbitrary function of time, without any restraint condition, but, of course, it should be known; CAo is the response of the impinging stream device to CA~(t) that can be measured by sampling at the outlet of the device.
84
IMPINGING STREAMS
3.3 RELATIONSHIPS FOR FITTING DATA Equation (3.39) or (3.40) can be used simply for the determination of the overall residence time distribution, and further the mean residence time, of the particles in the whole equipment according to the data measured for the concentrations of the tracer in the streams inputted to and coming out from the device. However, as can be seen from the analysis in Section 3.1, the impinging stream equipment includes several flow regions of quite different natures so that the global residence time distribution in the whole device is not helpful for understanding the processing performance of an IS device. A combination of the theoretical model with the relationships for data interpretation makes it possible to obtain some important parameters, such as the mean residence time in the impingement zone etc., by fitting experimental data. For convenience, define the integrals below:
t -(1) I i (t) - ~oC'Ai dt (3.41) I o (t) - Io CAodt Their differentiations are d I i ( t ) _ (-,(1)
dt
"Ai (t) (3.42)
t d I o (t) _ CAo (t) L dt On the other hand, using the expressions defined above, Eq. (3.39) can be rewritten as F(t) -
21° (t) ii(t )
(3.43)
For easier simulation, the equations above are reformed further. Differentiating Eq. (3.43) and rearranging the expression leads to dF(t) = E ( t ) = 2 .-,(1)( t ) i ° (t)] d-----~ 12 (t) [cA° (t)Ii (t)--CAi
(3.44)
Thus, we have the set of differential equations consisting of Eqs. (3.41) and (3.44), including the integral terms, of which the initial conditions are I i (0)
=
0,
I o (0) = 0,
F(0) = 0
(3.45)
RESIDENCE TIME OF PARTICLEq AND ITS DISTRIBUTION
85
This set of equations can be used to determine the residence time distribution function of the particles in the equipment, F(t), according to the data measured for
C~ Ai (t)
and CAo (t). However, for simulating calculation, it is more convenient to use the differential equation related to CA 80%) to the streams, whether pure air or particles-in-air suspensions, occurs in the accelerating tubes, i.e.,-Apa c >_ 0.Sx(--ApT); (2) The presence of particles in the suspension streams does not exhibit any substantial influence on both the pressure drops resulting from i m p i n g e m e n t and caused by structural factors of the IS contactor,-Ap~m and--Apds; while (3) The presence of particles results in a significant increase in the pressure drop through the accelerating tubes.
Table 4.1 Data measured for pressure drop distribution (u0= 14.22 m.s -~, S/dac=4.0) Millets
mp/ma
--APT
Rape seeds
--( Apim+ Apd0
--Apac
--APT
--( Apim+ Apd0
-APac
Pa
Pa
%
Pa
%
Pa
Pa
%
Pa
%
0.0
87.36
15.65
17.9
71.71
82.1
83.38
12.32
14.8
71.06
85.2
0.556
1 3 3 . 6 4 13.69
10.2
119.95
89.8
142.70 15.06
10.5
127.64
89.5
0.588
1 4 0 . 8 8 16.82
11.9
124.06
88.1
144.33 15.58
10.8
128.75
89.2
0.625
1 3 9 . 9 6 15.45
ll.0
124.51
89.0
144.00 14.60
10.1
129.40
89.9
0.667
1 4 0 . 4 8 14.34
10.2
126.14
89.8
147.01 14.80
10.1
132.21
89.9
0.714
1 4 4 . 0 7 14.34
10.0
129.73
90.0
150.53 15.06
10.0
135.47
90.0
0.769
1 4 9 . 2 9 14.67
9.8
134.62
90.2
153.66 14.67
9.5
138.99
90.5
0.833
1 5 0 . 7 3 13.76
9.1
136.97
90.9
163.36 16.75
10.2
146.61
89.8
0.909
1 5 2 . 2 2 13.69
9.0
138.53
91.0
167.34 15.12
9.0
152.22
91.0
1.0
162.32
12.3
142.31
87.7
20.01
HYDRAULIC RESISTANCE OF IMPINGING STREAM DEVICES
99
A very important observation obtained from the characteristics described in Items (1) and (3) above is that the major part of the power needed for operating the impinging stream contactor is being consumed in the acceleration of the particles. It can also be seen that an experimental impinging stream contactor is desirable that has no complicated structural factor yielding additional pressure loss so that it enables one to understand fully the pressure drops caused by the acceleration of particles and the impingement between the opposing streams, which are the essential elements in operating solid-gas impinging stream devices.
4.3.2 Resistance of accelerating tubes to pure air flow The experimental results on the influences of the impinging velocity, u0, and the dimensionless impinging distance, S/d,~, on the pressure drop due to pure airflow passing through the accelerating tube are shown in Fig. 4.2. It can be seen that for the dimensionless impinging distance S/dd~, > 4 -Ap,~.~ is kept essentially constant, and S/d~c exhibits a very small effect; while in the range of S/d,,,c < 4.0, -Ap~c,~ increases slightly as S/d,,,c decreases. In addition, the influence of S/d.~ on-Ap~m exhibits the same tendency, as can be seen in Table 4.2. The latter results are consistent with those obtained by Elperin [3] for the axial profile of static pressure in an impinging stream contactor, which demonstrate that the pressure is essentially constant in the range of S/d.~c > 4 but obviously increases once S/ct.,c < 4. It is also found in the experiments that, when S/d,~, is decreased to a level below 4.0, the pressure fluctuation at the outlet of the accelerating tube is strengthened and the impingement zone becomes unstable. Therefore it can be considered that S/d,~ - 4 should be the operational lower limit for gas-solid impinging streams" and thus the pressure drop due to pure airflow passing through the accelerating tube,-At),~,, can be considered to be independent of the impinging distance in the range of practical interest, i.e., S > 4.0. 120 1 O0
~"
80
.~
6o
I
"¢' 0
X~x
X
X-X--
m~,m_,_..mm--l--m__
40 20 I
2.0
I
3.0
I
I
4.0
I
I
5.0
I
I
6.0
I
I
7.0
I
I
I
8.0
S/d,,~ Figure 4.2 Influences of air velocity and impinging distance on -Ap ...... u0, s.m-l" m9.48 xl 1.06; o12.64; /k 14.22" @15.80; D17.30.
100
IMPINGING STREAMS
On the other hand, Fig. 4.2 illustrates that the velocity of the gas flow in the accelerating tube, u0, has an important influence on-Apse,,; and such influence can also be predicted by Eq. (4.2). 0.05 0.04 ,-~= 0.03 0.02 0.01
1.2
1.5
1.8
2.1
2.4
Re ~x10- 4
Figure 4.3 Friction coefficient of airflow through the accelerating tube. [] from Ref. [20]
A measured.
Using Eq. (4.2), the values for the friction coefficient ~ are obtained by regression of the experimental data, as shown in Fig. 4.3; and, for comparison, those obtained from Ref. [20] are also given in the same figure. As can be seen, with respect to the influence of the Reynolds number on the friction coefficient, both kinds of data exhibit the same tendency; while the values obtained from Ref. [20] are higher systematically than those measured by about 50%. The reason for this is still unclear. By a linearized regression of the experimental data, we come to .n
0 000118
2 a - 0.0213 l•e a"
4.20)
The power of 0.000118 in Eq. (4.20) implies that the effect of Rea on ~ is very small, so that ,;Lacan be considered to be independent of Re~ in the range of practical interest, and a constant of 0.0214 is taken for 2a. Since the pressure drop due to pure airflow passing through the accelerating tube occupies only a very small fraction of the total across the contactor; while the values for 2, obtained from the curves of 2~ versus Rea given in Ref. [20] can also be used directly for calculation without significant error.
4.3.3 Pressure drop due to acceleration and collisions of particles The pressure drop due to acceleration and collisions of the particles on the walls and between them,-Apac,p, can be calculated with Eq. (4.12) according to the data
HYDRAULIC RESISTANCE OF IMr'[ !'q(;ING STREAM DEVICES
101
measured for the combined pressure drop through the accelerating tube, -Apse. It is found from the measured results that both the velocity of particles at the outlet of the accelerating tubes, Up0, and the particles to gas mass flow rate ratio, mp/m~, have essential influences on this pressure drop. The measured data are partly shown in Fig. 4.4.
120 Particles m p/m ~ Millets 0.588 2 Millet,, 0.769 3 Rape seeds 0.588 4 Rape seeds 0.769 !
100
80 & 60 40 20 0
3.0
I
I
I
I
3.5
4.{)
4.5
5.0 /,,to, m - S
I
I
I
5.5
6.0
6.5
7.0
-I
Figure 4.4 Influences of out-velocity of particles and mass flow rate ratio on Apse,p (S/d=4.0).
10
U
I
,
0.5
i
,
i
,
0.6
i
I
I
I
I
0.7
I
I
I
I
I
I
I
0.8
i
,
i
0.9
,
i
,
i
i
1.0
i
,
i
,
1.1
m p/m a
Figure 4.5 Part of the data measured for the local resistance coefficient ~,p (Upo=3.51-6.42 re.s-l); - - averaged over 490 sets of data: + m O r a p e seeds, S/d=4.0; ~[]/~x.omillets, S/d=5.0.
102
IMPINGING STREAMS
The local resistance coefficient (ac,p is calculated for a total of 490 sets of measured data, and the results are given in Fig. 4.5. It is clear from Fig. 4.5 that the data for (ac,p are considerably concentrated: over 85% of the values are in the range 4.3 to 6.2, and the average value is 5.34. This fact indicates that (ac,p can be considered to remain essentially constant and that, consequentially, Eq. (4.11) describes well the pressure drop behavior due to the acceleration and collisions of the particles. Therefore the combined consideration of the two kinds of collisions, i.e., the collisions of particles on the wall and between particles, is reasonable and feasible. It should be noted that, as described by Eqs. (4.5) and (4.6), for a given impinging velocity u0, the velocity of particles at the outlet of the accelerating tube depends on the length of the tube Lac, the particles to gas mass flow rate ratio mp/ma, and the mean diameter of particles dp. Therefore Eq. (4.11) actually involves all the factors affecting-Apac,p. This may account for the phenomenon that no difference between the values for the local resistance coefficient (~c,p measured for millets and rapeseeds, respectively, has been observed. Considering such general applicability, the equation below is recommended for the prediction of the pressure drop due to the movement of particles, including acceleration and collisions:
-- APac, p -
,mp, 2 2.67Pa I,~)UpO
(4.21)
ma
4.3.4 Resistance due to structure of the device As described in Section 4.2, the sub-pressure drop measured between Points B and C or between Points B' and C, -ApBc or -ApB,c, is the sum of the pressure drop across the impingement zone,-Apim, and the structural resistance-Apds, but not any individual one of them. Such a measuring arrangement is not only because the pressure fluctuation frequently occurs round the impingement zone, making accurate measurement difficult, but also because both sub-pressure drops are too small (< 10 Pa) to be measured separately. To obtain the values for these sub-pressure drops, Eq. (4.17) can be specified for the impinging stream contactor studied here as -Apds - 2 (
0.02116 4 0.05
)PaU2=O.O642Pa u2
(4.22)
Thus, for each experimental run, the pressure drop across the impingement zone, -APim, can be obtained from the average value of measured-APBc and-Ap~,c and the value of-Apds calculated with Eq. (4.22) as its indirectly measured value:
--~Pim
1
- -~ [(--@BC) + (-APB'c)I - (-APds)
(4.23)
HYDRAULIC RESISTANCE OF IMPINGING STREAM DEVICES
103
Table 4.2
Typical data measured indirectly for-Api mand (ira
U0, m-s
S/d,a~.= 6.0
-1
S/d~c = 6.7
-Api .... Pa
(,m
-Apim, Pa
~m
9.48
4.90
0.091
4.30
0.079
11.06
6.84
0.093
6.81
0.092
12.64
7.47
0.081
7.48
0.081
14.22
10.76
0.088
11.09
0.091
15.80
13.38
0.089
12.85
0.085
17.36
15.95
0.088
15.12
0.083
By way of illustration, two sets of indirectly measured data for the pressure drop across the impingement zone are listed in Table 4.2. As can be seen from the table, the mass flow rate ratio mp/ma has no effect on -Apim, and the equivalent local resistance coefficient (,m calculated with Eq. (4.13) is essentially kept constant. This implies that the pressure drop across the impingement zone is independent of the presence of particles. The value for (,m averaged over a total of 490 sets of data is equal to 0.096. So, the pressure drop across the impingement zone can be calculated with the relationship below: 9
- Ap~m - 0.048p~u~
(4.24)
4.3.5 Model for the overall pressure drop Combining the results of the theoretical analysis and the experimental investigation above, the overall pressure drop across the whole impinging stream contactor can be represented by rap. 2 2 1 2 La~ p.~uo ~-2.67p,,, ~--)Up0 + 0.048pau 0 +-~ (dsPaU0
__ A p T _ ~,~
" d~,~.
2
Substituting the known factors into the expression above and rearranging the result in the following empirical equation"
-
ApT
-
(0.048 +
~.~ Lac 2 .mp. 2 " + 0 . 5 ( d s )Pab/0 -Jr-2.67p,, ¢--)UpO 2 d~,c ma
(4.25)
104
IMPINGING STREAMS
Equation (4.25) is the overall pressure drop model for the impinging stream contactor. In this equation, the first term on the right-hand side reflects the effect of the gas flow; while the second term reflects that of the particles. It is applicable for calculating the hydraulic resistance of common gas-solid horizontal two-impinging stream equipment with a dimensionless impinging distance of S/d~c > 4. For a given length of the accelerating tube, Lac, density and mean diameter of particles, pp and dp, the out-velocity of particles Up0 in this equation is related to the impinging velocity u0, and can be calculated by integrating Eq. (4.6). In the model equation above, in addition to the dimensions of the device and the properties of the solid particles and the gas involved, only two variable parameters are included: the friction factor 2~ and the structural resistance coefficient ~s. 2~ depends mainly on the roughness of the inside wall of the accelerating tube, and its value can be obtained from common handbooks of chemical engineering. For the zinc-plated tubes used in the experimental study carried out by Wu and Wu [66], the value of 0.0214 can be taken for ~. While the structural resistance coefficient ~s is related closely to the structure of the device to be considered, and its calculation must be aimed at the specific equipment; but all the methods for calculation involved are general. Therefore the empirical model, Eq. (4.25), is universally applicable. A comparison between the data measured for the overall pressure drop across the impinging stream contactor and the corresponding values calculated with Eq. (4.25) is illustrated in Fig. 4.6. Good agreement between the results measured and calculated can be observed clearly, suggesting that the total pressure drop model established is reasonable and feasible for application. In addition, it has the advantages of universal applicability and convenience in calculation. 250
~, g,
200
# I
150
0
1 O0
;>
50 I
0
I
I
I
I
50
I
I
I
I
I
100
i
i
i
i
i
i
i
150
i
i
I
200
I
I
i
i
250
Calculated-ApT , Pa
Figure 4.6 Comparison between measured data and results calculated with Eq. (4.25) for the overall pressure drop cross the IS contactor, u0, m.s-1" n9.48 @ 11.06 e 12.64 A 14.22 x15.60 + 17.36.
HYDRAULIC RESISTANCE OF IMPINGING STREAM DEVICES
105
It is clear from Eq. (4.25) that the velocity of the airflow in the accelerating tube, i.e., the impinging velocity u0, has a fundamental influence on the hydraulic resistance of the impinging stream equipment: the overall pressure drop, -APT, increases in the form of an exponential function as u0 increases, resulting in rapidly increased power consumption. On the other hand, at lower impinging velocity the impingement between the opposing streams could not be efficient so that it is difficult to achieve the goal of enhancing transfer between phases. Therefore the appropriate selection of the value for the impinging velocity is very important in the design and operation of an impinging stream device.
4.4 EVALUATION OF POWER CONSUMPTION AND DISCUSSIONS RELATED TO APPLICATION It has been mentioned before that power consumption in the operation of impinging stream equipment is a problem of great concern, because IS involves gas flow at high velocity. The first conclusion from the investigation on the hydraulic resistance of the impinging stream contactor shown in Fig. 4.1 is that the resistance of the gascontinuous impinging stream device is not large, and, consequently, the power consumption is acceptable, provided the structure of the device is reasonably designed and the material impinging streams being applied for is properly selected, not too heavy and not too large. It is of interest to compare an impinging stream contactor with another type of processing device, although such a comparison may not be easy because of the lack of operation data. With regard to conveying of solid particles by pneumatics, the device most similar to impinging stream equipment is the pneumatic flash dryer, also simply called a flash dryer. Table 4.3 gives a relative comparison between the two kinds of devices, with particles of dp= 0.001 m and pp = 1500 kg.m -3 as the superficial material being processed. As the basis, all the parameters related to the impinging stream contactor are set to be unity. It can be seen in the table that, in common cases, impinging the stream contactor exhibits a much higher efficiency than the flash dryer, occupying less space and have lower power consumption. Generally it can be considered that power consumption should not be a problem in the application of gas-continuous impinging stream devices. On the resistance constitution of the equipment system, the major conclusions that can be drawn from the investigation are: (1) Where millets or rapeseeds are the material to be processed, the power for the operation of the impinging stream contactor is mainly (>80%) consumed in the acceleration of particles; (2) The pressure loss due to the impingement between the opposing streams is independent of the presence of solid particles.
106
IMPINGING STREAMS Table 4.3
A relative comparison between flash dryer and IS contactor dealing with the same amount of solid material Type of equipment
Impinging stream contactor
Flash dryer
1
0.3-0.5
1 (-*2u0)
0.1-0.2 (=ut P2/1:'1 , u~ can be calculated by Eq. (5.8); and (2) If PffP1 < P 2 / P l , ua should be equal to the local sonic velocity, Uc, which is written as -
(5.~o)
/ ooO gcRr :/M
where MA is molecular mass of the gas, air in this investigation. A quasi-linearized regression is made for the experimental data with Eq. (5.6), which yields K - 3200,
a - 0.09,
b - -0.32
(5.11)
with a complex correlation coefficient of r = 0.7275, which is much greater than the acceptable minimum value, 0.418, for the confidence degree of 1%. A comparison between the measured data and the results calculated by Eq. (5.6), with the values obtained by the regression for the parameters involved, is illustrated in Fig. 5.4. The standard deviation of the calculation is SD=21.06 gm. If the intrinsic difficulties in the measurement of the spray droplet sizes mentioned above are taken into account, then the results shown in Fig. 5.4 indicate that Eq. (5.6) is acceptable for fitting experimental data. The following can be seen from the regressive equation and the results shown in Fig. 5.4: 190 170 150 130
[]
[]
~D
110 ~9
I
90
A
70 50
50
70
90
110
130
150
170
190
Measured D32, m Figure 5.4 Comparison between Sauter mean diameters measured and calculated by Eq. (5.6). A A before impingement; om after impingement; oA water-air system; • A water-CO2 system.
INFLUENCE OF IMPINGING STREAMS ON DISPERSITY OF LIQUIDS
117
(1) The impinging distance, S, is not involved in the regressive equation, Eq. (5.6). Actually, in the interpretation of data, various parameters containing S, such as S itself, do~S, and (l+d,,/S) etc, were tested by introducing them into possible equations. However, no or very weak influences of them on the Sauter mean diameter of droplets, Ds:, were found in every case. Therefore S is cancelled from the final regressive equation. As mentioned above, parameter S affects the intensity of the impingement between the opposing streams to an extent, and S = oo implies no impingement. So, the fact that S does not appear in the regressive equation suggests that the impingement of the suspension streams does not affect the Sauter mean diameter of spray droplets. (2) The negative exponent on the Reynolds number o f - 0 . 3 2 indicates that Reo has a medium effect on the mean diameter. This result is about in accordance with those obtained by Gretzinger and Marshall [80] in their investigation on the external mixing pneumatic spray nozzles. (3) It is observed in the investigation that the liquid to gas mass flow rate ratio, mL/m.,, has little influence on the mean diameter of droplets, as indicated by the exponent of 0.09 on the ratio. In comparison with the results obtained by other researchers [80, 81], the influence exhibited in this investigation is much smaller. The structure of the nozzles of the type Caldyn CSL2 used in the present study being quite different from those used by the mentioned researchers may be the major reason for the difference described above. It should be noted that, generally, the properties of liquid should affect the mean diameter of spray droplets to some extent, both before and after the impingement. In the investigation on the dispersity of liquid in impinging streams described here, however, only water was tested as a process liquid; while other liquids were not. This remains to be studied further.
5.4 CONCLUDING REMARKS Gas-continuous impinging streams have a number of important applications for gasliquid systems, such as combustion of liquid fuel, absorption, etc. All these applications involve transfer processes between phases. A liquid is very close to a solid in density, and thus, as a dispersed phase, would exhibit a number of behaviors in impinging streams similar to those of a solid, such as in the residence time distribution and in the hydraulic resistance, etc. However, because of the difference in their assembly conditions, liquid and solid, as the dispersed phases, have different effects on the performances of gas-continuous impinging streams. From the aspect of macro behaviors, the most important difference is that coalescence and/or breaking up (reatomization) would occur with liquid droplets in impinging streams, but not with solid particles. As a result, the total interface area and the transfer coefficients may change, yielding effects on the transfer processes between phases. Therefore the influence of impinging streams on the dispersity of liquid is a problem that needs to be considered.
118
IMPINGING STREAMS
The author of this book investigated experimentally the influence of the impingement between two opposing liquid droplets-in-gas suspension streams on the dispersity of the liquid in an open device of horizontal two-impinging streams, with the internal mixing nozzles of Type Caldyn CSL2 as the atomizers and water-air and water-CO2 systems as the targets, and with the slide-sampling, micro-photographing and imaging analysis procedure The following results of interest were obtained: (1) The impingement between the two opposing suspension streams makes the sizes of the spray droplets uniform to an extent, yielding narrower size distribution. More intensive impingement favors the uniformization of droplet sizes more effectively. (2) Essentially, the impingement between the two opposing droplets-in-gas suspension streams does not change the mean diameter of the droplets. (3) The Reynolds number of gas flow, Re~, exhibits a medium influence on the Sauter mean diameter of the droplets, both before and after the impingement; while the liquid to gas mass flow rate ratio, mL/ma, affects the same amount very weakly. (4) The Sauter mean diameters of the spray droplets, D32, both before and after the impingement can be correlated and predicted with Eq. (5.6), which gives reasonable and acceptable fitting of the experimental data.
-6IMPINGING STREAM DRYING
6.1 INTRODUCTION In the previous chapters the essential principles and characteristics of gas-continuous impinging streams (GIS) were discussed; while this and subsequent chapters in Part I will focus on the research and development of applied technologies. The contents have been chosen to be as valuable as possible for practical application, while successful or unsuccessful experiences included can be used for reference. As mentioned, like any other technical method, the method of impinging streams (IS) cannot be a universal tool. Oil one hand, IS has the outstanding advantage of significantly enhancing heat and mass transfer between phases; while on the other, it also has its intrinsic faults. From the discussions in the previous chapters, the essential characteristics of gas-continuous impinging streams can be summarized briefly as follows:
(1) The special flow configuration is suitable for processing two-phase or multiphase systems with a gas as the continuous phase;
(2) Its effect on enhancing heat and mass transfer between phases is very significant; (3) There exists strong mixing in the major active region for transfer, i.e., the impingement zone;
(4) The residence time of materials in either the continuous or dispersed phase in the active region is very short, about 1 s only; and (5) In comparison with other existing technologies mainly used for transfer processes, such as plate column etc'., the flow configuration in the devices of impinging streams is much more complex. This increases the difficulty of putting impinging streams into practice, especially in arranging countercurrent multistage systems. Items (1) and (2) above are the most obvious advantages of gas-continuous impinging streams. Since transfer between phases is a problem often encountered in multiphase systems, these advantages provide impinging streams with a wide sphere of application in the chemical, petrochemical, and other processing industries. The characteristic described in item (3) may appear as an advantage on certain occasions, while strong mixing implies serious back-mixing, leading to lower efficiency in many processes.
119
120
IMPINGING STREAMS
The very short residence time of the materials being processed in the major active region is a serious disadvantage or fault of impinging stream technology. In various processing industries, some processes can be carried out within a short time; and, under conditions of strongly enhanced heat and mass transfer, the time interval needed may be shortened further and thus the processes can be carried out with impinging stream technology, yielding significant benefits in reducing the volume of the equipment and reducing power consumption. However, many other processes need considerably longer time, e.g., several tens of minutes, even if under conditions of significantly enhanced transfer, in order to achieve the required processing degree, e.g., certain reaction conversion, absorption or moisture removal efficiency etc., to obtain the specified technological-economic indexes. It is evident that any effective development of impinging streams application must be based on an understanding of its properties. The industrial application of impinging streams can only be developed further when researchers have an in-depth knowledge of the characteristics of IS, including its advantages and limitations, and, on this basis, can bring its superiorities into play and avoid its disadvantages and faults. On the other hand, the industrial application of a new technological method must face many practical engineering problems, in addition to those of the method itself, which must be solved appropriately with the ideas of system engineering. Otherwise, successful application is difficult, even if the technology is very nice. Drying is a typical process involving parallel heat and mass transfer, and is one of the most suitable areas for the application of gas-continuous impinging streams. This has long been one of the hot spots of investigation and so many studies on impinging stream drying have been carried out since the early 1970s [21]. It is true that drying is the unit operation process most involved in the research and development of impinging streams. Many industries involve drying and so many materials, either final or intermediate products, need to be dried. Since the various materials have quite different properties, the impinging stream dryers that have been reported are also of many different types. Although, great efforts have been made in the past over 30 years and more, unfortunately no impinging stream dryer has yet been applied industrially. The lack of in-depth understanding of the properties of impinging streams and the number of unsolved engineering problems encountered during development may account for this slow progress in the industrial application of impinging stream drying. Assimilating positive and negative the experiences obtained in the past, the author of this book has developed the "Circulative Impinging Stream Dryer", an IS device suitable for powdery and/or granular materials [11, 82]. A test with quasi-industrial equipment on a scale of 1000 tones per year has exhibited good performance, and practical application may be expected in the near future. In this chapter, the research and development that has been carried out and the proposed impinging stream drying technologies and devices will be introduced first. By summarizing and analyzing those works, and following the train of thought described above, one may gain some useful understanding. Later, the circulative impinging
IMPINGING STREAM DRYING
121
stream dryer will be discussed in detail, including the basic idea for the dryer design, its working principles, and the major experimental results, etc.
6.2 EARLIER RESEARCH AND DEVELOPMENT Since the 1970s, researchers from different countries have proposed a number of impinging stream drying technologies for drying various materials and this will be discussed briefly below. It should be noted that there are two other kinds of drying technologies incorporating the word "impingement", i.e., "Impingement drying" and "Jet impingement drying". In the tormer, the material to be dried, a solution or suspension, is coated, by ejection and impacting action, on a certain surface vertical to the jet axis and rotating at high speed, where the drying is carried out [83]. In the latter, the jet of drying medium, hot air or super heated steam, impacts continuously the surface of a thin sheet material and dries it. This method is applicable for the drying of paper, tissue, textiles, and films, etc. [62, 84]. For porous materials, jet impingement drying can combine with through-air drying [85], i.e. a part or the whole of the impinged gas flows through the material sheet to increase the drying rate. It may be more suitable to call these two types of drying "impacting drying" and "gas-jet impacting drying", respectively. Their common feature is that the stream impacts a solid surface, and neither involves impingement between opposing streams. As stated in the Section 6.1, they do not belong to the category being discussed in this book.
6.2.1 Impinging stream spray drying Leiner et al. [86] and Elperin et al. [87] proposed an impinging stream spray drying system for aluminum sulfate, as shown in Fig. 6.1. The spray drying is actually carried out in two primary drying chambers with perforated walls placed opposite each other. The hot airflows pass through the perforated walls at high velocity to enter the primary drying chambers in order to avoid the material caking on the walls. The hot airflows contact the spray droplets and partially dry them, and then carry and accelerate the particles flowing through the conduits to enter the secondary drying chamber where the impingement between the opposing hot airflows occurs and drying is finally carried out. It is reported that, since the process is controlled by external diffusion, the action of impinging streams in enhancing transfer is very efficient. The influences of the atomizing pressure, air flow velocity and the initial concentration of aluminum solution on the moisture content of the product were studied. Comparative experiments were also made with and without impingement and the results indicate that impingement can reduce the final moisture content from 18% in the case without impingement to a level below 12%. However, in this comparison the factor below was not considered: if one did not want to employ impinging streams, the structure of the equipment shown in Fig. 6.1 would obviously be unreasonable. In tact, the addition of a secondary drying chamber increases the residence time of the material to a large magnitude. This may be the major reason tor the increase in the degree of drying. No report on the industrial
122
IMPINGING STREAMS
application of the technology and equipment described above has yet been found. The application may be hindered mainly by the considerable complexity of the system. In addition, although the measure of the hot air passing through the perforated walls was employed, possibly the problem of material caking on the walls could not be completely avoided.
t Exhaust Hot air
Primarydrying 2 chamber I /
~
- ' - ........
o,u;i
•"" " '
Hot air
. :i !!....,
!!:::..]-
econ /I
. . . . . . . . . . . . .
V"!':-: 1.75, Ev somewhat increases as ~: increases is still difficult to explain precisely. The most likely reason is that part of hot air II by-passes through the annular chamber at large lower spacing, resulting in the circulative flow rate of particles decreasing inversely. From the data shown in Fig. 6.18, the dimensionless lower spacing of ~:~ 1.5 is recommended as optimal.
IMPINGING STREAM DRYING
145
16.00 ~g.s -7
-1
14.00
~.
E -7 TO
:~'
~at)
12.00
% X
>
10.00
8.00 0.00
.
.
.
.
1.25
, 1.50
1.75
2.00
2.25
~=h/d Figure 6.18 Influence of the lower distance, u0, m's-~- O 25.47" • 24.05; D 22.64; • 21.22.
6.3.5.4 Influence of impinging velocity The experimental results on the influence of the operating velocity of gas flows in the accelerating tube, i.e. the impinging velocity u0, on the volumetric evaporation intensity at various feeding rates of the material are shown in Fig. 6.19. All the experimental curves exhibit linear relationship. It may be considered that any impinging velocity in the whole range of 20-25 m.s -~ tested is feasible for operation. Of course, larger impinging velocity implies increased power consumption. The decision on the operating impinging velocity for a practical IS dryer depends on the balance between the enhancement of transfer processes and the power consumption. The influence of the impinging velocity on the hydraulic resistance of an IS device has been described in detail in Chapter 4.
6.3.5.5 Influence of the feed rate of material The results on the influence of the feed rate of wet PVC on the volumetric evaporation intensity are shown in Fig. 6.20. In the range of the feed rate, rap, tested the volumetric evaporation intensity E, increases linearly as mp increases. This is because of the increase in the surface area of wet particles as the feed rate increases. This does not, of course, imply that the feed rate can be increased infinitely. Increase in the feed rate directly suggests increased capacity of the dryer, while an impinging stream dryer has a finite capacity. At certain flow rate and temperature of hot air, there is an increase in the degrees of both the drop in temperature and the rise in humidity of the drying medium, resulting in decreased driving forces for heat and mass transfer so that the drying rate decreases and the moisture content of the product increases.
146
I M P I N G I N G STREAMS
20 S=85 mm
7
h=30 mm
16 E -7r.g3
6
A
(',4
12
×
I
18
i
I
20
i
I
22 u0, m-s
24
26
-1
F i g u r e 6.19 Influence of impinging velocity on the volumetric evaporation intensity, mp, kg.s-l: C) 0.00414; • 0.004395;/~ 0.00370; • 0.00346; A 0.00315.
9.5 S=85 mm, 9.0 -
7
h=35 mm
-
©
(3
8.5 -
8.0 ×
7.5
7.0 0.0034
I
I 0.0036
I
I 0.0038
I
I 0.0040
i 0.0042
mr,, kg/s F i g u r e 6.20 Influence of feeding rate. u0, m's-~: © 25.47; • 24.05" A 22.64; A 21.22.
IMPINGING STREAM DRYING
147
0.32
0.30
w
7
0.28
E
r-i
3.0 0.26 -
0.24 0.0028
O/
Uo= 22.64 m's-1
I
0.0032
i
I
0.0036
I
I
0.004
I
0.0044
mp, kg.s -j Figure 6.21 Influence of feed rate on moisture content of product. A set of data on the variation of moisture content of the product with the feed rate under typical operating conditions is given in Fig. 6.21. In critical cases the moisture content may be above the value specified; and in more serious cases the material in the annular chamber may completely loss its ability to flow, the system is blocked out, and finally the operation is destroyed.
6.3.5.6 Study of the arrangement of the product discharge position In the equipment shown in Fig. 6.14 two possible options for the product discharge were considered. It is reasonable that researchers would want to employ the scheme of overflow through the upper outlet. If such a scheme was feasible, the control of the material level in the annular chamber, which has an important effect on the stable operation of the device, would become very easy and convenient. The premise for putting such a scheme into practice is that the particles in the impinged suspension can be classified by gravity upon their moisture contents. That is, only the particles with moisture content lower than that specified can be carried by the radial gas flow to fly and to drop down into the discharging area outside the annular chamber; while those with higher moisture contents would drop down into the annular chamber to circulate again. The basis for this idea is that water in a porous particle does not change the diameter of the particle, so that the particles with lower moisture content have smaller density, undergo less influence of gravity, and thus can fly for a longer path.
148
IMPINGING STREAMS
Unfortunately, the experiments yield negative results: with the upper discharging port, the out-particles are obviously finer in size, the out rate is unstable, and the moisture content of the out material is higher and higher as the process proceeds. Even without analysis, it can be judged that the moisture content of the product does not meet that required. The following theoretical analysis is carried out to check the negative results above. The following assumptions are made: (1) The particles fly radially by the drag force of the gas flow and, at the same time, drop down by gravity; (2) Since the suspension is thin dilution, the interparticle action can be negligible and so the motion equations for a single particle are used; and (3) The influence of buoyancy is neglected. According to Elperin et al. [6], after impingement between the opposing streams, the radial velocity of the gas flow decays following the relationship below:
Uar = 1.5
Ua
exp - 1.52
(6.4)
With Eq. (6.4), the radial distance of the position where the maximum radial velocity of gas flow appears, rma~, can be calculated to be 1.974 times the diameter of the accelerating tube, Rac. Assume the particles are carried out of the impingement zone at r = 1.974Rac with an initial radial velocity of zero. From the force balance, the movement equations of sphere particles after leaving the impingement zone can be obtained as Horizontal direction:
d/Apr = --O.5CDrPaAp I.pr -/AarI(.pr -/gar)
mp dt
(6.5)
Vertical direction:
dupz dt
[ -mp -° oz a plUpI l.pz
(6.6)
The initial conditions of Eqs. (6.5) and (6.6) are T = 0: z = 0, b/pz- 0; ~ 1.974R, /'/pr-" 0
(6.7)
where the radial and axial drag coefficients are calculated by 24 CD~ = ~ ,
Repr
CD~ = ~
24
Repz
(6.8)
The mass of a particle is related to its diameter dp and moisture content x: Ic dp3pp ( l + x ) mp - -~
(6.9)
IMPINGING STREAM DRYING
149
where pp is the density of a dry particle. The relationship between the particle velocity and the distance the particle travels is well known as dr
it~, . . . .
dr
dz
,
Up~ = - - -
(6.10)
dt
Thus, the trajectories of particles with various diameters and moisture contents, after leaving the impingement zone, can be determined by solving simultaneously Eqs. (6.5). (6.6) and (6.10) with the initial conditions, Eq. (6.7). Because of gravity, the particles drop down as they fly outwards with gas flow so that they move along parabolas. It is clear that this flight cannot continue infinitely. Once a particle drops down to the surface of the material bed, its flight must stop. In other words, the vertical distance between the impingement plane and the surface of the material bed is the limitation of the particle's flight; and the difference between the radial distances traveled by various particles at the end of their flight is the possible maximum separating distance. The results calculated for the flight trajectories of particles with various diameters and moisture contents are shown in Fig. 6.22. The figure indicates that the radial gas flow exhibits certain classification effect for particles with various diameters. However, the moisture content of the particle has almost no effect on the flying distance. These theoretical results illustrate that the arrangement of the upper overflow discharging port is totally unfeasible. 0.()0
-5 0.04
6 7
0.08
dpx 104, m -
0.12 _
0.16
--
1
3.0
2 3 4 5
2.7 2.4 2.1 1.8
6
1.8
7 8
1.8 1.8
1
0.20 0.00
().()4
X~
0 0 0 0.06 0.04 0.02 0 J
] 0.08
I
[ 0. i 2
112
314 0.16
I 0.20
F, ITI
Figure 6.22 Result calculated for the flying trajectories of particles of various diameters and moisture contents.
150
IMPINGING STREAMS
6.3.6 A brief introduction to the quasi industrial test The results of the model experimental investigation indicate that the essential structure of the circulative impinging stream dryer designed by the author of this book is feasible for porous powdery and/or granular materials such as PVC produced by the suspension process. Its performance has achieved the expected goal: The feature of impinging streams enhancing transfer is fully utilized; while the residence time can be arbitrarily set so that the water, both free and in pores or bounded, can be removed in one device simultaneously. It can be expected that the development of its industrial application would yield obvious benefits in simplifying the system scheme and energy saving etc. In comparison with some advanced dryers, such as the whirlwind dryer, it has the additional significant advantage that the major part of the product is discharged from the bottom of the dryer, yielding a greatly reduced load for the dust collection system. On the basis of the investigation above, a quasi industrial test was carried out, as described briefly below. The equipment and system for the test were originally designed for PVC from the suspension process. Because of a requirement from the factory, the target material was then changed, and the new one was the highly chlorinated PV. The wet feed was the crystalline of highly chlorinated PV from the centrifuge containing large amounts of crystalline water and its moisture content was as high as 54 to 60% in wet basis. The other physical properties of the target material were: The mean diameter of particles dr= 350 ~tm, the density of particles pv= 920 kg.m -3, and the bulk density of dry product pb = 464 kg.m -3. The main parameters for the design were as follows: • Capacity of dry product: 0.139 kg.h -1 (1000 t-y-1) -1 • Velocity of gas flow in the accelerating tube: u0 = 25 m.s • Length of the accelerating tube: Lac - 0.6 m • Diameter of the accelerating tube: do= 0.15 m • Height of the annular chamber (cylinder): H = 0.7 m • Diameter of the annular chamber: D = 0.75 m • Impinging distance: S ~ 0.45 m (adjustable in a certain range) • Lower spacing: h ~ 0.225 m (adjustable in a certain range) • Temperature of hot air: Tg0 = 130-140°C The equipment system scheme is essentially the same as that shown in Fig. 6.14; but with two differences: (1) The orifice plates are used for metering airflow rates; and (2) Since the equipment is much larger than that used in the model investigation and therefore the feeding rate is much larger, the screw feeder for wet material feeding in the quasi industrial test is not as complex as that shown in Fig. 6.14 in structure, but is a common one. The primary operations of the equipment system yield positive results: (1) With the highly chlorinated PV crystalline from the centrifuge containing about 60% water as the feeding wet material, the final moisture content of the product from continuous
IMPINGING STREAM DRYING
151
operation of the system can achieve the specified index, i.e. no larger than 0.6%. (2) Although the capacity of the equipment cannot achieve 1000 tonne per year, as designed for PVC, for the change in the target material, its ability in terms of water evaporation exceeds the designed index by 20 to 30%. Development is still continuing; and it can be expected to be put into production in the near future.
6.4 CONCLUDING REMARKS Gas-continuous impinging streams (GIS) is a very effective technical method for enhancing heat and mass transfer between phases. Drying, being a typical process of parallel heat and mass transfer, is one of the areas where GIS can be expected to be applied successfully. Much research and development has taken place on the application of GIS in drying and many IS dryers with various structures and working principles have been proposed. Unfortunately, no successful application of GIS for drying in industry has yet been reported. Although many factors may account for this slow progress, the two most important problems that need attention are: (1) There may not be a thorough enough understanding of the properties of impinging streams, its advantages and disadvantages, resulting in improper selection of target materials and, consequently, giving unexpected results; and (2) Some engineering problems related directly to the application have not been properly solved so that no complete set of technologies can be provided to industries, delaying industrial application of GIS in the field of drying. Without any doubt, the application of GIS in the drying field is of great potential and it is reasonable to expect that drying technologies employing GIS will appear in various industries in the coming years.
This Page Intentionally Left Blank
-7IMPINGING STREAM ABSORPTION
7.1 ADAPTABILITY OF IMPINGING STREAMS FOR GAS-LIQUID REACTION SYSTEMS lkbsorption is another important unit operation process involved in the chemical, petrochemical, and a number of other processing industries. Since the discovery of the major effect of chemical reaction in promoting absorption, especially after the establishment of the systematical analysis method by Denckwerts [70], chemical absorption has been more widely applied industrially. It is clear that whenever an absorption treatment is needed in the processing industries, people always prefer where possible to employ chemical absorption. Of course, in the application of impinging streams in the area of absorption, what we are most concerned with is its application in chemical absorption too. This is the focal point of the discussion in this chapter. Chemical absorption is a kind of gas-liquid reaction and must involve transfer between phases, and thus is one of the important areas where, it is hoped, impinging streams can be applied successfully On the other hand, as mentioned before, the method of gas-continuous impinging streams has the outstanding advantage of significantly enhancing transfer between phases, while, at the same time it has the intrinsic disadvantages of very short residence time in the active region and relatively complex flow configuration, so that it cannot be applied for every gas-liquid reaction or chemical absorption system. Any suitable technology and/or equipment for a chemical absorption system is closely related to the nature of the reaction(s) involved in liquid [57]. To achieve success, the appropriate selection of the target systems of IS application must be made according to such nature, combined with the properties of impinging streams. According to Denckwerts [70]. the nature of a gas-liquid reaction system can be characterized by the parameter M, which is defined as: m =
Possible maximum reaction rate in liquid film Possible maximum rate of mass transfer through liquid film
(7.1)
Parameter M ha~ different definitions for different types of reactions, and various definitions can be found in ReE [57] or other textbooks and monographs on chemical ~eaction engineering.
154
IMPINGING STREAMS Table 7.1
Features of reactions in liquid phase at various values for M Range of M value
Feature of reaction
81 vs 6L *
Reaction region
~/M >>3
instant
61 ~ 0
at the interface
~/m > 3
fast
81 < ~
inside liquid film
x/M -- 1
middle
81 -- ~
up to liquid film
x/M ~
over liquid film
x/M ~
whole liquid phase
* ~--thickness of the reaction layer; ~--thickness of liquid film The characteristics of liquid reaction with various values for M are listed in Table 7.1. It can be seen that, in the last two cases, i.e. ~ 3, the reaction(s) in liquid proceed fast, the global processes are controlled by diffusion, and thus the measure of enhancing transfer will play a positive key role. Although the parameter M does not involve diffusion through the gas film directly, it has important referential value for the selection of the target system for impinging streams application in the area of absorption, because the diffusion resistance of a gas film has an order of magnitude comparative to that of liquid a film in most systems of practical interest. It can be concluded from the simple analysis above that impinging streams can only be used for gas-liquid reaction or chemical absorption systems involving fast reaction(s) in liquid for success. Concerning the flow configurations, it is clear that impinging streams with gas as the continuous phase is most suitable for chemical absorption, while, with liquid as the continuous phase, impinging streams cannot generally give a perfect performance. In an absorption process with gas-continuous impinging streams, the liquid is usually atomized into fine droplets. For the absorptions involving fast reaction(s) in liquid, the atomization of liquid provides a large interface area for transfer between gas and liquid. It can be expected that with the effect of impinging streams enhancing transfer, chemical absorption processes can be greatly intensified. On the other hand, the flow configuration of impinging streams is relatively complicated so that it is difficult and usually unfeasible to arrange a countercurrent multistage system, such as in a column device. In addition, there is strong mixing in the active region of an impinging stream device. Both factors are unfavorable for absorption systems involving reversible reaction(s) in liquid, even if they are fast ones.
IMPINGING STREAM ABSORPTION
155
These systems are subject to the limitations of equilibrium, and thus it is difficult to achieve the higher absorption efficiency or conversion required in a single stage impinging stream device with strong mixing. From the discussions above, the following general principle for selection of target systems for IS application can be concluded: the gas-continuous impinging streams method is especially applicable to gas-liquid reaction or chemical absorption systems involving fast-irreversible reaction(s) in liquid.
7.2 EARLIER INVESTIGATIONS Investigations on impinging stream absorption began in the mid-1980s and were mainly concentrated in Israel. Up to the 1990s the work carried out was essentially on the fundamentals and focused mainly on analyzing and verifying the enhancement of transfer by impinging streams and searching for related experimental evidence. Few researchers gave detailed consideration to the feasibility or even the possibility of impinging streams application for the target systems so that, essentially, the results obtained cannot be taken as a basis for further development. However, those works provided some referential values for later investigations.
7.2.1 Models for absorption enhancement In impinging streams with gas as the continuous phase, there exist all the factors enhancing transfer mentioned in the previous discussions related to gas-solid systems. In addition to these, Tamir [5] considered that the following factors may further enhance transfer in gas-liquid impinging streams:
(1) Because of the collisions between droplets and the shearing effect of the gas flow carrying the droplets, the original droplets may be re-atomized, yielding an increased interface area. (2) The collision, deformation of droplets, effects of shearing force between droplets and gas flow, and surface tension are factors that cause circulation of liquid at the surface of and inside droplets, favoring surface renewing and, consequently, promoting transfer between phases. However, as described in Chapter 5, because the aggregation status of liquid is different from that of solid, both re-atomization and coalescence are possible in gasliquid impinging streams with liquid as the dispersed phase. This introduces some complicated uncertainty factors. According to the results obtained, it is uncertain whether droplet re-atomization increases the interface area for transfer, because coalescence of fine droplets decreasing the interface area counteracts or even exceeds the positive effect of re-atomization. More possibly, a negative effect may be obtained, i.e., the interface area may be reduced to a certain degree.
156
IMPINGING STREAMS
Tamir [5] analyzed the effects of impinging streams enhancing physical and chemical absorption processes. To describe the enhancement of absorption, the following two enhancements were defined to account for the two factors: oscillation movement and re-atomization-coalescence of droplets, respectively E1=
Mass absorbed in the presence of oscillations Mass absorbed in the absence of oscillations
(7.2)
and E2 -
Mass absorbed in the presence of re - atomization or coalescence Mass absorbed in the absence of re- atomization or coalescence
(7.3)
Based mainly on the analytical results for single particle motion in impinging streams, Tamir derived a number of expressions for the two parameters for various flow regimes in the two cases with and without chemical reaction, in which the parameters such as the droplet size, the motion times of a particle in the accelerating and decelerating stages, particle to gas velocity ratio at the outlet of the accelerating tube, etc. were involved (see Eqs. 11.2 to 11.25 in Ref. [5]). Unfortunately, those models may contain a number of defects. Firstly, the influence of the relative velocity between phases on transfer coefficients has not been considered, while such an influence is just the most intrinsic reason for impinging streams enhancing transfer processes. Secondly, the assumption on the re-atomization and coalescence of droplets is short of both theoretical accordance and experimental evidence. These, plus the randomness of both dispersity and motion of droplets, make the models generally less general meaningful and difficult to apply. The following fact might be of interest: Tamir carried out a number of experimental studies in order to verify the effect of impinging streams enhancing absorption processes [106-108], while the results were essentially independent of the models mentioned above.
7.2.2 Absorption equipments Generally, absorption equipment with impinging streams includes two essential elements" the atomizer and the absorption chamber.
7.2.2.1 Atomizers For absorption processes carried out in impinging streams with gas as the continuous phase, atomization of the liquid is an essential operation. In earlier investigations on impinging stream absorption, all the liquids were atomized by pneumatic nozzles. The nozzles used were mainly of two types, the first being shown in Fig. 7.1a. With this atomizer, the gas contacts and mixes with the liquid outside the nozzle and is generally called the "external mixing nozzle" in industry, although Tamir and co-workers called it the "no-mixing nozzle". The second one is the internal mixing nozzle of Caldun CSL2 type, i.e. the so-called Critical nozzle [59-00, 109], which has a special structure,
IMPINGING STREAM ABSORPTION
157
as shown in Fig. 7.lb. This special structure ensures that the gas contacts and mixes fully with the liquid inside the nozzle, while yielding larger resistance as well. From the point of view of transfer, the mixing chamber inside the nozzle is also a considerably active region, in which the fresh gas contacts the fresh liquid, both with large driving forces for transfer, so that a significant part of the absorption must be carried out in it. Unfortunately, to date no data on the states of gas and liquid at the outlet of the nozzle have been measured so that the contribution of the mixing chamber to absorption cannot be determined. Gas
.... Liquid[ ,'-~.... ,%%%
f i
(a)
External mixing type
Liquid
(b) Internal mixing type Figure 7.1 Pneumatic atomizers. In general, a pneumatic nozzle can produce sprays of fine droplets to provide a large interface area for heat and mass transfer; but the power consumption for atomization is very high. In some cases, e.g., when it is used in technical equipment for environmental protection to remove harmful gases, its high power consumption may become a significant economic problem.
7.2.2.2 Structure of absorption equipment In earlier investigations aimed mainly at verifying the enhancement effects of impinging streams, the common horizontal two impinging streams was mostly employed, although some other flow configurations were also sometimes used.
158
IMPINGING STREAMS
~ Liquid
~ Liquid
~ Gas
~ Gas
(b) With a partition
(a) Normal
• Liquid
Liquid
~ Gas
Cas
i'-
....
r
....
I
'
"
I
(c) With concentric nozzles
(d) With eccentric nozzles
Figure 7.2 Two-impinging stream absorber for demonstrating enhancement. The structures of the experimental equipment used in the investigations by Tamir et al. are shown in Fig. 7.2. Figure 7.2(a) shows the common co-axial two impinging
stream absorber, in which the major part of the gas does not pass through the nozzles but enters the absorption chamber concurrently with the sprayed liquid. The absorber shown in Fig. 7.2(b) has a partition at the middle to separate the two opposing streams and the rest is completely the same as shown in Fig. 7.2(a). The researchers aimed to verify the enhancing effects of impinging streams for absorption by comparative experiments carried out in the absorbers shown in Figs. 7.2(a) and 7.2(b). The absorber shown in Fig. 7.2(c) also has the flow configuration of co-axial two impinging streams, but all the feeding gas is used as the atomization medium; it is called "concentric nozzles". In that shown in Fig. 7.2(d) all the feeding gas is also used as the atomization medium, but the two nozzles are placed eccentrically and so the device is called "eccentric nozzles". Tamir et al. [109] also studied an impinging stream absorber operated in bubbling mode, as shown in Fig. 7.3. The absorber takes liquid as the continuous phase while gas is dispersed in liquid, so it actually belongs to liquid-continuous impinging streams (LIS). The experimental results obtained showed that this flow configuration exhibits a higher absorption rate than that shown in Fig. 7.2(a). Combining them with the results
IMPINGING STREAM ABSORPTION
159
from an investigation on micromixing carried out by the author of this book [110], it might be considered that the major reason for increasing absorption rate in the absorber shown in Fig. 7.3 is that strong micromixing in the continuous liquid phase yields a decreased resistance of liquid side. Obviously, this mechanism is different from that of gas-continuous impinging streams enhancing transfer. This flow configuration is of interest for the gas-liquid reaction processes for which the bubble-bed reactor is suitable, but the other aspects of the reactor have not yet been evaluated. Bubble /
G/2
G/2
n
W/2
,-
8888-/00888 o O8o8oO O8o
_
o o° o
W/2
Figure 7.3 Bubbling impinging stream absorber.
T
G
L #-
1 4
L
Figure 7.4 Impinging stream loop reactor. Probably inspired by the fact that the Jet Loop Reactor has successfully been applied in industry, Gaddis and Vogelpohl [111] proposed an impinging stream loop reactor, as shown in Fig. 7.4. It seems that their main purpose is to lengthen residence times in the reaction vessel. The principles of the reactor's operation are somewhat similar to those of the Air Left Reactor (ALR). The only difference lies in the fact that
160
IMPINGING STREAMS
it also employs a flow configuration of impinging streams. During operation, two gas flows are fed into the nozzles on two sides through separate tubes, and then mix with the liquid flowing through the nozzles at their exits. By strong shearing force the gas is dispersed into liquid to form two gas-liquid two-phase streams of lower density, and the latter flow upwards at considerably high velocity, sucking the liquid or gas-liquid mixture from the main tube of the reactor to cause internal circulation inside the reactor. Considering the flow and mixing status, it can be seen that the liquid is not a unique continuous phase in the mixture before impingement, while the gas may also be such a phase or, at least, not a fine-dispersed phase. This is the specialty of this reactor. The difficulty in operation of the reactor is that the impinging velocity cannot achieve higher levels and so the effect of impinging streams enhancing transfer is limited. Using a cycling pump may be a possible way of solving this problem and, in fact, the researchers have done this, but it led to an increased consumption of energy and a more complicated system. In addition, Ponikarov et al. [112] studied more special impinging stream absorption equipment. It employs the flow configuration of rotating impinging streams and the impingement occurs in a collision chamber of half-circle form. This equipment appears to be of less practical interest and so will not be discussed further here.
7.2.3 Major results of the investigations The following systems were studied by the researchers mentioned above: absorption of CO2, acetone and ammonia into water, and absorption of CO2 into NaOH solution. According to the nature of the reactions in the liquid phase involved, single stage impinging streams is only applicable for the absorption of CO2 into NaOH solution; while from the point of view of economics, using NaOH to absorb CO2 is generally unfeasible. The investigations on impinging stream absorption of the systems above therefore have little practical interest. In fact, earlier investigations on this topic focused mainly on verification of impinging streams enhancing absorption and obtaining fundamental data, while little attention was paid to feasibility of application. The following are the major results obtained.
7.2.3.1 Experimental evidence for IS enhancing absorption The results of the comparative experiments on the absorption of C O 2 and acetone [107, 108] into water are: the absorption rate in the absorber without partition shown in Fig. 7.2(a) is higher than that in the absorber with a partition shown in Fig. 7.2(b), indicating the enhancing effect of impinging streams. However, as mentioned in the Introduction, the basic standard they used for comparison is unreasonable. In the absorber with a medium partition, the flow configuration becomes two jets impinging fixed wall surfaces separately, and the latter enhance transfer significantly, too. In other words, the intrinsic enhancing effect of IS should be stronger than that reported. For example, the impinging jet on a fixed wall may enhance the absorption rate by x times
IMPINGING STREAM ABSORPTION
161
than normal, and IS enhance by y times than the impinging jet on a fixed wall; so the total number of the times enhanced intrinsically by IS than the normal should be xxy.
7.2.3.1 Mass transfer coefficient Obviously, mass transfer coefficient is a topic of general interest. Tamir, Herskowits et al. [59, 106, 107, 109] studied experimentally the absorption of CO2 and acetone into water in a two impinging stream absorber operated in various modes with various atomizers. The data they measured for the volumetric mass transfer coefficient are listed in Table 7.2, which are representative among earlier investigations. As mentioned, from the point of view of practical application, impinging streams is not suitable for the systems given in Table 7.2. On the other hand, the absorption processes for which impinging streams is applicable normally involve fast reaction(s) in liquid and thus are controlled by gas-film diffusion. Therefore the most important should be the gas-film mass transfer coefficient, kG, which is absent in the table.
Table 7.2
Volumetric mass transfer coefficient in two impinging stream absorber
Nozzle type
Operation mode
Volumetric mass transfer coefficient, s-~
Ref.
CO2--H20
External mixing
Spray
0.041 < kLa 4; molar ratio Ca/S = 1.0 for pseudo flue gas without CO2; the nozzles were mounted at the outlets of the gas conduits; (3) The gas-film mass transfer coefficient, kG, was determined based on the Sauter mean diameter of spray droplets. The results show essentially no influence of initial concentration of SO2 on kc, suggesting that the process is controlled by diffusion through gas film and that the method proposed for the determination of k~ is feasible; (4) The data on the relationship between impinging velocity and gas-film mass transfer coefficient were fitted by k~ - 2 . 9 × 1 0 - 4 u ; 75821 , with the standard deviation SD = 2.45×10 -4 m-s -1, implying u0 is a strong effecting variable, and thus a very important operation variable; (5) With the impinging velocity u0 ranging from 5.53 to 16.62 m-s -1, the measured volumetric mass transfer coefficient k~a is in the range 0.577 to 1.037 s-~ and k~ from 0.00641 to 0.0416 m.s -~, showing clearly the effect of gas-continuous impinging streams enhancing mass transfer; (6) The impinging stream gas-liquid reactor has low hydraulic resistance. In the range of operation conditions tested, the pressure drop across the reactor, Ap, is round 400 Pa only.
7.6 DESIGN OF A DEVICE FOR LARGE GAS FLOW RATES The results described in Section 7.5 illustrate that the application of gas-continuous impinging stream gas-liquid reactor for the wet desulfurization of flue gas performs to good effect. Gas-liquid reaction is a large category of important reactions involved in many processing industries, among which many systems involve fast or instantaneous
IMPINGING STREAM ABSORPTION
187
reaction(s) in liquid phase. One can say almost with certainty that gas-continuous impinging streams (GIS) will find more and more important applications in the field of absorption. The major advantages of GIS over other methods for the wet desulfurization of flue gas are its high sulfur-removal efficiency, very large volumetric mass transfer coefficient which necessitates only a small device, and relatively small resistance to the streams. Therefore the application of GIS for wet FGD can be expected to yield great economic and social benefits. However, for such a purpose the related engineering problems need to be solved. From the point of view of practical application, the major nature of the wet desulfurization of flue gas lies in the fact that the flue gas to be treated has extremely large flow rates and, consequently, the amount of the absorbent to be atomized is also very large. For coal burning power plants, e.g., around 4000 m3.h-~ of flue gas per MW is generated, and a coal-burning power station with a capacity of 400 MW will exhaust over 1.6 Mm3.h -~ Correspondingly, the amount of atomized absorbent suspension needed by a wet FGD system for such a station would be around 1.6 km~.h-j. Although the GIS gas-liquid reactor shown in Fig. 7.9 performed well in the test on a small pilot plant scale, its structure is not suitable for treating extremely large amounts of gas, like practical flue gas. An extremely large-volume reactor would be needed and the amount of absorbent to be atomized would make both design and manufacture of the equipment extremely difficult, if not totally impossible. The Combined Multifunctional Impinging Stream Gas-Liquid Reactor was designed [130] to make this technology suitable for processing huge amounts of gas. The reactor's structure is shown in Fig. 7.21 with a vertical view in Fig. 7.22. It employs multiple groups of flow configurations of horizontal-coaxial four impinging streams. The reactor consists of two major parts: the tower body (1) and several groups of impinging stream components I, II, etc, mounted inside the tower body (1) at various altitudes. The tower body (1) is a vertical cylinder. Near the top of the cylinder a mesh can be installed as a foam remover 2. The cylinder has a top cover (in pan or conical shape) connected to the gas exhaust port (3), and a liquid discharge port (4) near its bottom. All groups of impinging stream components, I, II, etc. are identical in size and working principles (Fig. 7.21 shows only Groups I and II as examples). For each group there are four gas conduits (5). At the outlet of each gas conduit, one nozzle (6), or a set of nozzles is installed for atomizing liquid, either pure or containing solid particles. Liquid or solid-in-liquid suspension is supplied to nozzle (6) through the high pressure liquor feed pipeline (7). Above the four conduits, a droplets-removal damper (8), in pan or conical form, is placed. The damper (8), tower body (1), and either the bottom of tower (1) or the damper (8) below the four conduits in the upper group, form a subchamber for absorption. For each group, the four gas conduits are divided into two subgroups, with two conduits in each sub-group. The two conduits in each sub-group are placed coaxially, with the exits of the conduits facing each other. The axes of the two sub-groups of the conduits are perpendicular to each other.
188
IMPINGING STREAMS
2
1 i
II !~i/ , i,,
7 ~id
feed ~
Gas~//d, 6J
/~
J
Impingement zone
Liquidfeed
:i'/ ......... oii '
~
d
.L ~ ~
7
d°ut ~ 6 ~
4
Figure 7.21 Combined multifunctional impinging stream gas-liquid reactor. 1-tower; 2-screen foam-remover; 3-gas outlet tube; 4-liquid outlet tube; 5-gas conduit; 6-eddy pressure nozzle; 7liquid feeding tube; 8-damper.
IGF
1 3
GF Figure 7.22 Vertical view of the combined multifunctionai impinging stream gas-liquid reactor. 1-tower; 3-gas outlet tube; 5-gas conduit, 7-liquid feeding tube.
IMPINGING STREAM ABSORPTION
189
The distances from the outlets of the conduits to the center where the two axes meet are equal (See Fig. 7.22), and the distance between the exits of the two conduits in each sub-group is the "impinging distance". Depending on the requirement of the amount of liquid or suspension to be used, one nozzle or a set of nozzles can be installed inside each conduit with the exit(s) towards the same direction as the outlet of the conduit, i.e., towards the center of the absorption chamber. All groups of the impinging stream component I, II, etc. are operated concurrently. For Nozzle (3), a pressure atomization nozzle, also called the centrifugal pressure nozzle, can be employed. It is desirable to use the eddy pressure nozzle (Chinese patent, ZL00230305.1). The latter has a higher flow-rotating efficiency, and thus requires less energy to atomize the liquid or solid-in-liquid suspension. The purpose of using the screen foam-remover is to separate the gas flowing upwards from the foam or fine droplets carried by the gas flow. Such a remover has a high efficiency of foam-removal and gives small hydraulic resistance. However, if solid particles are present in the gas or liquid, or there is solid product from the chemical reaction, the solid particles may clog the mesh of the foam remover (2), resulting in blockage of the gas flow channels and increased hydraulic resistance, and also making it difficult to clean up. In this case, the foam remover screen can be replaced by an internal wet cyclone, as shown in Fig. 7.23. The wet cyclone can also achieve very high separation efficiency (>99%), but its hydraulic resistance is larger than that of the foam remover screen by about 600-800 Pa. If the requirement for dust removal is not high, it is recommended that neither foam remover screen nor the internal cyclone be used. It can be seen that in the combined multifunctional impinging stream gas-liquid reactor shown in Figs. 7.21 and 7.23, the working principles and action in enhancing transfer between phases for each sub chamber of absorption are the same as those of the reactor shown in Fig. 7.10. The first difference between the two reactors is that a pair of impinging streams is added in the direction perpendicular to the flow axis of the original pair of impinging streams. As a result, the utilization factor of the space inside the sub chamber is increased. The major active region in impinging stream equipment (the impingement zone) is only a thin layer with a diameter 8-10 times that of the gas conduit and a thickness of about a quarter to a half of the impinging distance, so when only one pair of impinging streams is used the utilization factor of the space is very low. The addition of another pair of impinging streams approximately doubles the utilization factor of the space and doubles the gas flow rate that can be processed in the sub chamber for absorption. Another feature of the combined multifunctional impinging stream gas-liquid reactor is the employment of multiple groups of impinging stream components placed one above the other, with each group including four streams. This arrangement further increases several-told the gas flow rate that can be processed in the reactor, while all the groups are operated individually without disturbing each other. The arrangement slightly increases the hydraulic resistance of the system. Compared with the existing equipment for wet FGD, the resistance of the reactor shown in Fig. 7.21 or 7.23 is still much smaller.
190
IMPINGING STREAMS GO
J
/3
9
1
::-~'-:: ,,i,\;~:: II
.:::t::..
liiiiiiiii ',:[
'.L'
I
Figure 7.23 Combined multifunctional impinging stream gas-liquid reactor with the screen foam remover (2) in Fig. 7.21 replaced by the internal cyclone (9) in this figure. According to the data for volumetric mass transfer coefficient measured in the device on a small pilot plant scale, for a certain load of flue gas to be processed, the required total volume of the reactor under consideration would be very small, only about 1/3 that of existing wet FGD equipment. In addition, the arrangement of the internal wet cyclone shown in Fig. 7.23 enables the reactor to have simultaneously high ash-removal efficiency. The reactor is especially suitable for the wet desulfurization of flue gas with hydrated lime or dilute ammonia solution as the absorbent. The design of the large-scale reactor suitable for a power station has now been accomplished and is expected to be applied industrially in the very near future.
-8IMPINGING STREAMS COMBUSTION AND GRINDING
The combustion of powdery coals and sprayed liquid fuels and the grinding and milling of solid materials are fields in which gas-continuous impinging streams have been successfully applied for many years, and so are of industrial significance. Although the author of this book has not carried out any experimental work in these two areas, because of their importance, this chapter has been included to keep the integrality of the book as one specially discussing impinging streams. Consequently, the chapter mainly introduces the reader to some representative results obtained by other researchers although, from time to time, the author's own opinion is included.
8.1 MODELS FOR PARTICLES AND DROPLETS COMBUSTION In principle, gas-continuous impinging streams (GIS) can be applied for the combustion of gases, powdery solids and sprayed liquids. Since gas-combustion is relatively simple and the process is essentially independent of the major feature of GIS, i.e., that it significantly enhances heat and mass transfer between phases, the discussions in this chapter will focus on the combustion of the latter two kinds of fuels.
8.1.1 Evaporation-burning equations for a single droplet Chemically, combustion is a violent oxidation reaction, normally occurring at high temperature. According to the Arrhenius relationship, high temperature defines the kinetics feature of extremely fast reaction(s) and, consequently, any transfer between phases must be the governing factor, so that impinging streams should be applicable. In order to increase transfer rates, a liquid fuel must first be atomized into fine droplets to create a large interface area, no matter what kind of burner is being employed. The behavior of a single droplet during burning is the foundation for understanding and analyzing the process. Barnard et al. [136] proposed that, if a liquid droplet exceeds some critical size but less than about one millimeter in diameter, the combustion takes the form of a spherical diffusion flame round the droplet and the burning rate is determined by the vaporization from the surface of the droplet. The fact
191
192
IMPINGING STREAMS
that burning is closely related to vaporization is the feature of the combustion of sprayed liquid fuel which distinguishes it from that of powdery solid fuel. Let us consider the symmetrical burning of a spherical droplet with the radius rp in surroundings without convection. Assume that there is an infinitely thin flame zone from the surface of the droplet to the radial distance rf~ [ 137], which is much larger than the radius of the droplet, rp. The heat released from the burning is conducted back to the surface to evaporate liquid fuel for combustion. Because the reaction is extremely fast, there exists no oxidant in the range of rp< r < rn; while no fuel vapor is available at r > rn. At a quasi steady state the mass flux through the spherical surface with the radius r (>rp), Mfv, can be obtained with Fick' s law as
Mfv
_
-pgDfv
dmfv
dr,
(8.~)
+ m f v M
where mfv is the mass fraction of the fuel vapor at radius r. In surroundings without convection the flux of inert gas Mg = 0, and so the total flux M = Mfv + Mg = Mrv. Thus, at the external surface of the droplet, Eq. (8.1) becomes [ dmfv dr ]s +mfv,sMfv,s
Mfv's =-pgDfv
(8.2)
From the continuity equation at the quasi steady state, we have
(8.3)
Mfv,s 4~p2 - Mfv 4err 2
Substituting Eqs. (8.1) and (8.2) into Eq. (8.3) and integrating the resulting equation from rp to rn gives the flux of fuel-burning, Mfv,~, as Pg Dfv M fv,s= ~ l n [ 1 rp
+
mfv,s - mfv 1 - mfv,s
]
(8.4)
On the other hand, the diameter of the droplet reduces continuously as the vaporization-burning proceeds. The variation of the droplet radius with time can be determined from the mass balance round the droplet itself: d 4 3 4~p2. dt (-3 n'rp PL) = M fv,s
(8.5)
Simplification of the equation results in drp
Mfv,s m
dt
PL
(8.6)
IMPINGING STREAMS COMBUSTION AND GRINDING
193
Substituting Eq. (8.4) into Eq. (8.6) and integrating the resulting equation from t=0 to t leads to r~,9
__
2 rpo - 2t P~DJv l n [ l PL
mt\1,s
mfv
]
(8.7)
1 - mt.v,,~
Let the final radius of the droplet be r p - 0, the time for complete burning of the droplet, tb, can be obtained to be 7)
r~)PL
tb =
(8.8)
2p,gDfv ln[1 - mtv'~ - mtv ] 1- mr-v,s To eliminate the variable mfv, one can use the relationship of mass balance around the neighborhood of the burning zone: (8.9)
Mmtv 110 - - M oomox oo
where O represents the mass of oxidant needed for burning unit mass of fuel, and the subscript oo denotes the states in the bulk gas outside the burning zone. Utilizing Eq. (8.9), Eq. (8.4) becomes pg Dfv M t~,~= ~ l n ( 1 5'
+
mfv,s - mfv//O 1 - mt~'.s
)
(8.10)
For the calculation of tb the data for the mass fraction of fuel vapor at the surface of the droplet, mr,., is needed. From the heat balance one can obtain ~g Mt.v,~=~ln(l+ Cpgrpo
Cpg(T~ - L ) + A H c mo× ~ / O - O s / M fv,s
)
(8.11)
where Q~ is the heat flux through the gas surrounding the droplet to evaporate liquid fuel of the mass M~-v~.If without the heating process of the droplet before burning, i.e., the droplet enters the system just at the boiling point of the fuel, then the amount of heat Q~ meets -Q~/Mt~,~AH v
(8.12)
The amounts of T~ m,,x.~ in Eq. (8.11) are normally known; while the results from analysis indicate that the temperature at the surface of the droplet is always just under
194
IMPINGING STREAMS
the boiling point of the fuel, TBp. Taking the place of Ts with TBp, then the flux of combustion can be calculated with Eq. (8.11). Using the heat balance, the expression for the time needed for complete combustion of the droplet under consideration is rewritten as
rp2oPL
tb =
(~
2 28 ln[1 + Cpg
C0g
(8.13)
- TBp~ + a / - / c / O
]
zM-/v
It is clear that intensifying combustion is just to shorten the time needed for complete combustion tb. It can be seen from Eq. (8.13) that tb is positively proportional to the square of the radius of the droplet, rp. Therefore the most effective measure for the intensification of combustion is full atomization of the liquid fuel to increase its dispersity, i.e. reducing the size of the droplet. For heavier oils fine atomization is of more importance because of their large densities PL, as can be predicted by Eq. (8.13). In practice, the combustion of atomized liquid fuel is a very complex process. The model introduced briefly above cannot be considered as completely consummated. In the derivation of Eq. (8.13) several assumptions were made, which might be unreasonable. For example, pressure is assumed to have no effect; while actually this parameter does have an effect. As the pressure approaches the critical value its influence becomes very significant. At the extremes, i.e. when the critical pressure is arrived at, the vaporization heat of the fuel becomes zero. For combustion at high pressure, Spalding [137] proposed a theoretical method for the prediction of the burning rate; and later Rosner [138] and Dominicis [139] improved the theory. In addition, You [ 140] summarized the models on the influences of the boundaries on the two sides of the interface and the internal circulation on the evaporation rate proposed by Prakash et al. [141 ], which indicates that the internal circulation promotes heating of the droplet and increases the evaporating rate during heating.
8.1.2 Burning equations for a single particle The combustion of a fine particle of solid fuel has many identical features to that of a liquid droplet; but with important differences. One of the differences is that, in addition to no evaporation existing, the combustion of particles of solid fuel is frequently controlled by chemical kinetics, i.e., diffusion-governing is not the only possibility. Let us consider the burning of an ideal spherical particle in static gas. The oxidant diffuses to the surface of the particle to react with the carbon: C + 02---~C02, while the latter diffuses out from the surface of the particle. The combustion heat is transferred to the surrounding gas partially by convection and partially by radiation. The following assumptions were made in the modeling: (1) The process is at a pseudo steady state. (2) The temperature the highest at the surface, and continuously drops down outwards from the surface of the particle; and the concentration of oxidant is highest in the bulk
IMPINGING STREAMS COMBUSTION AND GRINDING
195
gas and continuously drops down towards the particle surface, while the concentration of the product of burning exhibits the opposite profile. (3) All the transport properties of gas are uniform. The diffusion equation for the oxidant round the surface of the particle is written as
M°x" - -P~D°~
dm°x )
dr
+ M~m°x
(8.14)
s
Similarly, since M~.~= 0, there would be M s - Mo,'~
/
2
Figure 12.3 Scheme of experimental system with impinging stream crystallizer. 1-cooling tank; 2-ISC; 3-thermometer; 4-thermostat.
The temperature of the solution inside the ISC is measured with an accurate thermometer with a scale of 0.1°C so that the accuracy can be _+0.05 °C. The solution out of the ISC is sampled at the outlet of the overflow port and the sample is analyzed chemically for the concentration of Na2HPO4. The average of the concentrations of Na2HPO4 in the inlet and the outlet solutions is taken as the mean concentration of the solution inside the ISC; and its corresponding super- saturation is then determined as the difference between the concentration and the solubility at the operating temperature, ACre, while the latter is obtained from Curve 1 in Fig. 12.2. Since the amount of the solute consumed in the growth of the crystals is only about 1% of the total amount in the solution passing through the ISC, implying a very small variation of the concentration, an arithmetic mean value of the concentrations in the inlet and the outlet solutions is accurate enough for calculation. The experiments are repeated 2-3 times for each set of conditions, and the values measured for the overall crystal-growth rate coefficient are averaged. The time interval for the growth in each run ranges from 1200 to 2400 s. To ensure that the supersaturation is stable, both the amount of the crystal seeds added and the flow rate of the solution passing through the ISC are very small in comparison with the content in the ISC and, consequently, the overflow rate is also very small, so that no crystals being carried by the overflow solution are observed, i.e., no loss of the crystals needs to be considered and the measurement accuracy for the amount of the crystals after growth can be ensured to meet the requirement.
INFLUENCE OF LIQUID-CONTINUOUS IS ON PROCESS KINETICS
261
The structure of a experimental fluidized bed crystallizer (FBC) is shown in Fig. 12.4, where the crystallizer is actually a universal equipment for the measurement of crystal-growth rate. The solution enters the FBC at its bottom, and leaves the FBC by overflow. All the other parts of the experimental system are the same as shown in Fig. 12.3, and so are not shown in Fig. 12.4. The operation procedure for the FBC is the same as for the ISC. For convenience of comparison, the corresponding conditions, temperature and concentration of the solution, operated in the ISC and the FBC are rigorously controlled to be the same, with the deviation of the operating temperature no greater than 0.05 °C.
Overflow
Na2HPO4 solution
Recycling water
Figure 12.4 Fluidized-bed crystallizer (FBC). The values measured in the ISC for the overall crystal-growth rate coefficient of Na2HPO4 are listed in Table 12.2. As can be seen, the reproducibility of the data is in a reasonable range and that of most data is very good. A similar table summarizing the experimental data measured in the FBC, with the same mean diameters of crystal seeds, dp~, and under the same operating temperatures, T, was also obtained; but it is not given here. A comparison between the overall crystal-growth rate coefficients obtained from different crystallizers is of interes!, The data measured in the ISC and FBC are listed in Table 12.3. It is obvious that the values measured in the impinging stream crystallizer for the overall crystal-growth rate coefficient, K~s, are greater systematically than those measured in the fluidized bed crystallizer, KFB, by about 15 to 20%. Nothing can account for such differences, except that the strong micromixing and the pressure fluctuation occurring in liquid-continuous impinging streams promote process kinetics. The impinging stream crystallizer used in the experiments is of a horizontal structure
262
IMPINGING STREAMS
and so its operating impinging velocity, u0, is limited, because higher u0 will lead to liquid splashing and thus unstable operation. Therefore it is not sure w h e t h e r the crystal-growth rate can be increased further by increasing the impinging velocity. Table 12.2
Overall crystal-growth rate coefficient of Na2HPO4 in SCISR
doox 104
mox 10 3
m
kg
T, °C
ACnl,kg.m -3
tf, s
kg
Measured
Average
2.51
0.7802 0.7803
32.7
3.98 4.67
1800 2100
1.4041 1.6382
6.79 6.44
6.61
2.51
0.8010 0.8008
35.9
4.98 5.39
2400 2400
2.4892 2.7594
8.65 8.88
8.77
2.51
0.5715 0.4509
38.6
6.21 7.87
1800 1800
2.1931 2.3024
11.39 11.46
11.43
3.48
0.5610 0.5614
33.9
7.58 13.4
1800 1800
1.3243 2.1316
7.58 7.25
7.42
3.84
0.7401 0.7403
36.5
7.37 9.24
1800 1800
2.1415 2.1972
9.99 8.20
9.10
3.84
0.8121 0.8118
38.7
7.73 8.39
1800 1800
3.5173 3.6787
14.13 13.53
13.83
5.22
0.5428 0.7217
34.2
9.56 10.25
1800 1800
1.2459 1.7409
8.68 8.65
8.67
Conditions
0.6924 5.22
1800
1.6895
13.28
8.12 11.36 13.04 9.31 11.18 8.37
1800 2400 1800 1800 2400 2400
2.1329 3.6147 4.2943 3.9875 5.3435 1.0877
14.63 12.60 16.29 18.98 16.86 8.54
37.0
7.21 5.76 6.64 8.49
2400 2400 1800 2100
1.1422 0.9868 1.3417 1.9875
10.74 13.47 16.12 17.49
38.8
7.92 8.52
1800 1500
2.2521 2.1972
20.49 22.24
8.74
2100
1.7322
13.78
7.59 5.92 7.12 9.45
1800 2100 1800 1800
1.2355 1.3687 1.9611 2.0349
10.29 13.90 17.88 16.16
7.67
1800
1.5851
25.61
8.29 9.78
1500 1200
1.7799 1.4219
27.16 23.80
37.2
33.6
7.36
0.5425 0.5419 0.6222 0.6227
7.36
0.7507 0.7500
7.36
39.6
0.7602 8.95
8.95
0.7611 0.7624 0.9229 0.8410
33.4
36.7
0.5301 8.95
0.6209 0.5810
Klsx 10 6, m.s -j
6.78
0.6918 0.6931 0.8001 0.8415 0.8409 0.5434
5.22
mfx 10 3
39.0
13.50
17.38
10.92
16.81 21.37 12.66
17.02 25.52
INFLUENCE OF LIQUID-CONTINUOUS IS ON PROCESS KINETICS
263
T a b l e 12.3
Comparison between overall crystal-growth rate coefficients measured in the ISC and the FBC, respectively
dpoX 104, m
T, °C
3.48
5.22
7.36
8.95
K[s/KvB
Kls
KI:B
6.61
5.16
1.281
35.9
8.77
7.54
1 163
38.6
11.43
9.53
1 199
33.9
7.42
6.30
1 178
32.7 2.51
Averaged Kx I 0 f', m.s -]
36.5
9.10
7.86
1 158
38.7
13.83
11.58
1 194
34.2
8.67
7.30
1 188
37.2
13.50
10.50
1.286
39.6
17.38
14.70
1.182
33.6
10.92
8.25
1.169
37.0
16.81
14.35
1.171
38.8
21.37
19.15
1.116
33.4
12.66
9.69
1.307
36.7
17.02
14.54
1.|71
39.0
25.52
22.23
1.148
It is also interesting to examine the influence of temperature on the crystal-growth rate. For this purpose the generalized Arrhenius relationship below is used: K - K ° e x p ( - E / RT) The mean observed active energies obtained by regression of the experimental data for various crystal seeds with different mean diameters and in different crystallizers are listed in Table 12.4, where, similarly, the subscripts IS and FB denote the parameters in the impinging stream crystallizer and the fluidized bed crystallizer, respectively. T a b l e 12.4
Observed active energies for Na2tlPO4 crystal-growth in the ISC and the FBC, respectively, obtained by fitting data dptl× 104, m
Eis, kJ'mol -]
EFB, kJ'mol -j
2.51
73.87
81.38
3.48
106.02
103.21
5.22
103.31
104.27
7.36
103.44
128.86
8.95
104.01
101.06
264
IMPINGING STREAMS
The following can be observed from the table:
(1) In the small range of the crystal sizes, the observed active energy for NaePO4 crystal-growth is related to the crystal size. As can be seen the values for the active energy with the initial mean size of crystals of 0.251 mm in both the ISC and the FBC are obviously smaller, while with larger crystals the active energy remains essentially constant, about 104 kJ-mol -j, independent of the crystal size. Wu [180] observed similar phenomena in an investigation on the crystal-growth rate of Na3PO4. As has been mentioned, crystal growth involves two steps: the diffusion of solute molecules and the crystallizing reaction at the surface. So the overall process may be affected by both diffusion and reaction kinetics. The data listed in Table 12.4 indicate that with crystals initially sized 0.251 mm the influence of the diffusion is more significant, while with those initially sized 0.348 mm or larger the process is mainly affected by reaction kinetics. Since the difference between the densities of the liquid and the crystals is relatively small and the viscosity of the mother liquor is very large, the fine crystals tend to follow the streamlines, yielding small relative velocity between the crystals and the mother liquor and thus reduced liquid-film transfer coefficient. Globally, this shows an increased influence of the diffusion of solute molecules through the liquid-film with smaller sized crystals. Therefore the phenomena described above are reasonable. (2) Except for a few questionable data, the values for the observed active energy measured in the two crystallizers of different types, E~s and EFB, show little difference and can be considered to be more or less identical. On the other hand, the values measured in the impinging stream crystallizer for the overall crystalgrowth rate coefficient,/(is, are obviously and systematically larger than those in the fluidized bed crystallizer, KFB. Therefore it can be affirmed without the need for further analysis that, with the observed frequency factors, there must 0
0
be K~s > K w . From the point of view of the molecular collision theory, this suggests that more effective collisions occur in the ISC. Therefore the results given in Table 12.4 support the inference made in Section 12.1, i.e., the strong pressure fluctuation in liquid-continuous impinging streams causes a considerable part of the molecules that obtain more vibration energy converted from the flow dynamic energy to achieve higher level. In addition, the good micromixing in the LIS creates favorable conditions for the increase in collision probability. Thus, the two factors result in a larger observed frequency factor for the crystallization 0 system, K~s. The most important result obtained from the above investigation is the experimental evidence that the crystal-growth rate in the impinging stream crystallizer is higher than that in the fluidized bed crystallizer. More generally, the results certify that the crystalgrowth rate not only depends on the nature of the substance system involved and the operating conditions but is also related to the flow configuration in the device used, and thus indicates a new possible way for the enhancement of crystallization.
INFLUENCE OF LIQUID-CONTINUOUS IS ON PROCESS KINETICS
265
12.3 KINETICS OF ETHYL ACETATE SAPONIFICATION 12.3.1 Chemical reaction and experimental method For further verification that the LIS promotes process kinetics, Wu e t al. [181] studied comparatively the kinetics of ethyl acetate saponification in the submerged circulative impinging stream reactor (SCISR) and a stirred tank reactor (STR), respectively. The chemical reaction is represented by CH3COOC2Hs(A) + NaOH(B)
-
CH3OONa(R) + C2HsOH(S)
(12.10)
Chemically, the system has a very stable kinetic nature and is without by-reaction and so the kinetics data are easy to obtain. Therefore it is very suitable for the goal of the present investigation. The experiments are carried out in the SCISR with an effective volume of 3.6xl 0 -3 m 3, the structure of which is the same as that shown in Fig. 10.2, and a traditional stirred tank reactor (STR) of 0.6x10 -~ In 3 in the effective volume with dampers" both the reactors are operated in batch mode and the temperature of the reaction mixture is rigorously controlled with an accuracy of +0.05°C during the reaction in the same way as for the investigation on the crystal-growth kinetics described in the previous section. The impinging velocity in the SCISR operation is controlled at about 0.26 m-s -j, corresponding to the propeller rotary speed of 900-1000 rpm, while the STR is operated under the condition of fully agitating, which is determined by visual observation, and the rotary speed of the paddle is about 1200 rpm. As the reaction proceeds, the NaOH in the reaction mixture is consumed gradually so that the electroconductivity of the mixture drops continuously. Because the electro-conductivity of the mixture is a monodrome/'unction of the concentration of NaOH, CB, the variation of CB during the reaction can be determined continuously by measuring the electroconductivity of the mixture with an online probe, while that of the ethyl acetate is easily determined as a function of time by a molecular mass balance. The data are correlated with the well known kinetics equation below rj\ - k C A C B
(12.1 1)
12.3.2 Major results The results of correlating the experimental data measured in the range of 25-45 °C are given in Table 12.5, where the subscripts IS and ST denote the parameters determined in the SCISR and STR, respectively. The results reflected by the data in Table 12.5 are similar to those on the crystalgrowth kinetics of Na2HPO4, i.e., the values for the reaction rate constant measured in the SCISR, k~s, are systematically higher than those measured in the STR, ksT, by about 20%.
266
IMPINGING STREAMS Table 12.5
Comparative data for the rate constants in the reaction kinetics equation (12.11)
T,°C
Rate constant, m3.kmol-J.s-J
Rate constant ratio
kis
ksT
kls / ksv
25.0
0.175
0.137
1.28
35.0
0.239
0.196
1.22
45.0
0.758
0.651
1.16
The interpretation the experimental data obtained in the SCISR and STR, respectively, with the Arrhenius relationship yield the measured values for the active energies in the two reactors being E~s = 57.5 kJ-mol -~ and Esv = 60.1 kJ-mol -~. The difference between the two values is small, and is actually within the scope of experimental error so that they can be considered to be identical. These results lead to the conclusion similar to that obtained in the investigation on the crystallization kinetics of Na2HPO4, i.e., with the values for the frequency factor of the reaction represented by Eq. (12-10) measured in the two reactors, there must be k0,~s> k0,sv.
12.4 CONCLUDING REMARKS Comparative investigations were made for the crystal-growth kinetics of Na2HPO4 in the impinging stream crystallizer (ISC) and the fluidized bed crystallizer (FBC) and the kinetics of ethyl acetate saponification in the submerged circulative impinging stream reactor (SCISR) and the stirred tank reactor (STR) and the following was concluded: (1) In the ranges of the operating conditions tested, the overall crystal- growth rate coefficient of Na2HPO4 measured in ISC, K~s, is higher systematically than that measured in FBC, KVB, by 15 to 20%, while the reaction rate constant of ethyl acetate saponification measured in the SCISR, k~s, is larger systematically than that measured in the STR, ksf, by about 20%. (2) The results of correlating the data on the overall crystal-growth rate coefficient of NazHPO4 with the generalized Arrhenius relationship indicate that the values for the observed active energy obtained in both the ISC and the FBC are essentially identical. With the crystal seeds sized 0.348 mm or larger, the observed active energy is determined to be about 104 kJ.mol -~, while with the smallest crystal seeds tested, average-size 0.251 mm, the observed active energy is obviously smaller than with the larger crystal seeds. (3) The regression of the data on the kinetics of ethyl acetate saponification in the range of reaction temperature from 25 to 45 °C yields the active energy in the
INFLUENCE OF LIQUID-CONTINUOUS IS ON PROCESS KINETICS
267
SCISR E~s = 57.5 kJ-mol -~ and that in the STR EST = 60.1 kJ-mol -~. The difference between the two values is small and is within the scope of experimental error so they can be considered as identical. (4) The results of the two kinetics investigations support the theoretical inference that the efficient micromixing and the considerably strong pressure fluctuation in liquid-continuous impinging streams promote process kinetics. Obviously, the results described above are of significance. They have a certain theoretical accordance and indicate reasonable directions for the further development of LIS application. As described above, the major reasons of LIS promoting process kinetics may be that the effective micromixing increases the collision probability between molecules and that the considerably strong pressure fluctuation changes the energy distribution over the molecules, causing the part of the molecules that obtains more energy to achieve the higher level required for inducing reaction. However, the theoretical analysis above is only an inference and, limited by the current technological conditions, no direct experimental evidence can as yet be provided. Therefore, it seems that the results reported in this chapter also raise a number of new problems that require further investigation, two of which are:
(1) Influences of micromixing and pressure .fluctuation: What are the mechanisms of their effects? How can direct experimental evidence be obtained for these effects? How can these influences be quantitatively described? How can their individual contributions to the global influence be determined? (2) Real kinetics dam: To date, almost all the kinetics data on reaction systems in liquid phase or multiphase with liquid as the continuous phase have been measured in traditional stirred tank reactors. From the results reported in this chapter, it is likely that significant deviations exist in the existing kinetics data. On the other hand, the LIS device cannot yet be considered as absolutely ideal for kinetics investigation, not least because its micromixing time, tM, is not zero. What then is the ideal equipment and conditions for obtaining real kinetics data? All these topics, valuable both academically and from an application standpoint, remain to be further investigated in depth.
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-13PREPARATION OF ULTRAFINE POWDERS BY REACTION-PRECIPITATION IN IMPINGING STREAMS I: "ULTRAFINE" WHITE CARBON BLACK
Because of the change in degree of dispersal, ultrafine particles have a number of excellent characteristics that are quite different from those of corresponding solids in normal conditions" hence they have several valuable application features and so have received much attention. Consequently, the preparation technologies for ultrafine powders have become one of the hot spots of investigation. In many countries nanotechnology is becoming one of the key technologies for the 21 st century. Scientifically, the term "ultrafine powder" or "ultrafine particles" is used to describe solid products in which the particle sizes are no greater than 100 rim. The "ultrafine" white carbon black to be discussed in this chapter is the product of particles of a smaller size than those in common products, i.e., "ultrafine" here is not a scientific but a commercial term. The ultrafine powders in the scientific sense, e.g., nano copper, nano TiO2 and nano hydroxyapatite, and related topics will be discussed in later chapters. Nevertheless, the principles involved in the preparation of "ultrafine" white carbon black by impinging stream reaction-precipitation are very similar to those involved in the preparation of the nano powders mentioned above. Therefore this topic is discussed here under the overall title "Preparation of ultrafine powders". Because of their important application value, much research and development on the preparation technologies of ultrafine powders has been carried out in the last twenty years and more, and hundreds of preparation methods have been proposed. Since they are not the major topic of this book, neither a description of the classification of the methods nor an introduction to the details of the various methods will be covered here. On the other hand, reaction-precipitation methods generally have a number of advantages such as lower cost, moderate operating conditions, lower equipment requirements, convenience of operation, and normally yield good-performing products etc." thus they occupy an important position among the various methods. The liquid-continuous impinging stream (LIS) device has the features of efficient micromixing and strong pressure fluctuation and, being the major equipment in the preparation of ultrafine powders by reaction-precipitation, it has exhibited excellent performances in a number of application cases. However, of the various preparation
269
270
IMPINGING STREAMS
methods, the LIS device is only applicable for the reaction-precipitation process and not for others. It can be considered therefore that the LIS device has no superiority over other preparation technologies.
13.1 ADAPTABILITY OF LIQUID-CONTINUOUS IMPINGING STREAMS FOR PREPARATION OF ULTRAFINE POWDERS Essentially, the key process in the preparation of ultrafine powders by reactionprecipitation is crystallization from a solution. As mentioned in the previous chapter, crystallization from a solution includes two steps: nucleation and crystal-growth. Both can occur only in a supersaturated solution and spontaneous nucleation can occur only when the concentration of the solute in the solution is over the super solubility of the substance involved. The rate equation for nucleation derived from the principles of thermodynamics is represented by [182] I -16~rcr3M2 1 Np - Kp exp 3R3T3p2(lnS) 2
(13.1)
where K r, is the nucleation rate constant; S, is the relative supersaturation, S = C/Cs, and Cs is the solubility of the solute. For practical application, Eq. (13.1) is not convenient; and the empirical equation below is usually employed: Np = KpAC t'
(13.2)
where AC is the supersaturation, AC = C - Cs. It is obvious that both nucleation and crystal growth consume the solute in the solution, resulting in a decrease in supersaturation. Therefore there is competition between nucleation and crystal growth for the solute in the solution. From Eq. (13.2), the first premise for preparation of ultrafine powders by precipitation is that the crystallization must be carried out at very high supersaturation in order that nucleation in quantity can occur instantaneously to yield a large amount of crystal nucleus, so that the nucleus cannot grow significantly for lack of the solute and thus remains fine. In the preparation of ultrafine powders by reaction-precipitation, supersaturation is produced by the certain chemical reaction(s). This implies that, chemically, the necessary conditions for the preparation of ultrafine particles by reaction-precipitation are: (1) The reaction producing the solid product must be very fast so that the solute can be produced in quantity instantaneously to achieve the very high supersaturation expected; and (2) The solid product to be prepared in the form of ultrafine powder should generally have a very low solubility. The solution of the substance with larger solubility usually has a wide metastable region so that it is not easy to achieve very high supersaturation rapidly by chemical reaction. Secondly, the solid precipitating out from a solution, i.e., crystallization, must be carried out in an environment of high and uniform supersaturation otherwise the newly
PREPARATION OF "ULTRAFINE" WHITE CARBON BLACK
271
formed nucleus may grow quickly at low supersaturation leading to a product with a wide size distribution, even if the mean supersaturation is very high, because the existence of a local concentration difference is unavoidable under not completely uniform conditions. In the case of product having a wide size distribution, the mean size of the particles in the product is usually larger, because the large particles have a very large "weight" in the weighted mean size. Summarizing the discussions above, the necessary conditions for the preparation of ultrafine powders by reaction-precipitation may be concluded to be the following: (1) Very fast reaction that can yield instantaneously a quantity of the substance to be precipitated; (2) Very low solubility of the substance to be prepared in the form of ultrafine particles: (3) Extremely high supersaturation; and (4) Very uniform supersaturation. As has been seen before, liquid-continuous impinging streams (LIS) have the excellent features of efficient micromixing and very strong pressure fluctuation. These characteristics promote the reaction(s) in liquid phase in the preparation of ultrafine powders to proceed more rapidly to yield instantaneously high supersaturation on one hand; and the strong micromixing can ensure very uniform supersaturation in the solution, on the other. Therefore it can be expected that a finer product with a narrower size distribution can be prepared with an LIS device as the reaction-precipitation equipment. The results of several investigations have proved that the LIS device is one of most advantageous technical equipment for preparation of ultrafine products by reaction-precipitation.
13.2 PROPERTIES OF WHITE CARBON BLACK AND CHEMICAL REACTIONS IN ITS PREPARATION BY PRECIPITATION PROCESSES "White carbon black" is the global title for synthetic hydrated silica and silicate, scientifically termed "hydrated silicon dioxide" (SiO2.nH20), and is an important additive of rubber etc'. [1831]. Because of its white color and properties comparable with carbon black, it is termed "White Carbon Black", for which the Chinese quality standard is given in Table 13.1. Both its application value and the commercial price of white carbon black are closely related to its particle size [184], although the size index had not been included in the standard. For example, the products sized about 30 to 50 lam can be used only in production of general rubbers etc: while the "ultrafine" products sized less than 10 lam can be used as a reinforcing agent in high-grade rubber products such as meridian tyres, insulation and piezoelectric rubbers etc, because of their large specific surface area, strong linking ability, good dispersity, and good optic and mechanical properties; they are also widely used in the production of accurate foundry, high grade fillings, coatings, precise ceramics, compounded materials, and light-guide fibres etc. Accordingly, the market price tot "'ultrafine" white carbon black can be over ten times higher than the common ones.
272
IMPINGING STREAMS Table 13.1
Chinese National Standard of White Carbon Black (GB 10517-89) Item Content of SiO2, % Color Residual on mesh of 45 ~tm, %
Index > 90 Prior or equal to the standard sample < 0.5
Heating loss, %
4.0-8.0
Calcinations loss, % pH
< 7.0 5.0-8.0
Total Cu, mg.kg-j
< 30
Total Fe, mg.kg-~
< 1000
Total Mn, mg.kg-~
< 50
DBP adsorption, cm~. g-~
2.00-3.50
In the profession, people are used to using size to describe the dimension of the assembled particles, but not the primary particles. In the discussions in this chapter, this convention will be followed. There are many industrial methods for the production of white carbon black [184], e.g., the gaseous phase processes, the carbonization method, etc., among which the liquid phase processes, especially the precipitation methods, are most widely employed. The liquid phase processes cannot yield so fine a product with such high activity as those produced by gaseous phase processes; however their products can meet many requirements with very low costs and therefore have received wide attention. Among the various processes producing white carbon black of various grades, the precipitation methods occupy the dominant position. However, the common precipitation process is unable to produce a product with very fine particles, e.g., no greater than 10 ~tm. Because of the attraction of the high application value and high commercial price, a number of investigations and developments had been made in modifying the precipitation process in order to produce ultrafine white carbon black, giving rise to several processes, such as the collosol process, the gelation process, and the twice collosol process etc. The Degussa co. in Germany and the Silica co. in Japan have successfully produced products with assembled particles no greater than 10 ~tm by the process of twice collosol. However, these improved processes usually have the disadvantages and/or difficulties of complicated system schemes, harsh requirements of operation and control, and higher cost etc. It is of interest therefore to develop a simpler process with greater operating flexibility. Chen[ 16] studied experimentally the preparation of "ultrafine" white carbon black by the common (one-step) precipitation process with the submerged circulative impinging stream reactor (SCISR) developed by Wu [15] as the reaction-precipitation equipment and obtained satisfactory results.
PREPARATION OF "ULTRAFINE" WHITE CARBON BLACK
273
In the common precipitation process, the double decomposition reaction between an inorganic acid and the sodium silicate is employed to yield hydrated silica: Na20.mSiO2 + 2H + + aq = 2Na + + mSiO2.nH20$ + aq
(13.1)
As in the preparation of most ultrafine powders, an ageing stage of a certain timeinterval under the condition of continuous stirring is needed for the reacted mixture for deactivation of the surface of fine particles in order to get a stable product consisting of assembled particles. It has been mentioned before that, in addition to the effective micromixing and the strong pressure fluctuation, the SCISR has the special flow configuration of perfect mixing-plug flow in series. With the SCISR as the reactionprecipitation equipment, nucleation in quantity occurs essentially in the impingement zone, while, after nucleation in quantity, the deactivation of the surface of the newly formed fine particles may occur to an extent during the solution of very low supersaturation after nucleation, carrying the solid particles flowing through the regions essentially without mixing, the annular chamber between the drawing tube and the reactor wall and inside the drawing tubes restraining coalescence of the particles. On the other hand, the time that the suspension flows through the regions without mixing is much longer than that through the impingement zone, and thus the ageing can be carried out partially during the flow so that the time for ageing after reaction may be greatly reduced. A large amount of water is usually contained in the dictyo-structure of the hydrated silica precipitated, including both free and combined moisture. The precipitate is separated out by filtering and washed to remove impurities, mainly Na and acid radical ions. The cake is made into slurry again by stirring and the latter is then spray-dried to yield the powdery product of white carbon black. The inorganic acid used for the preparation of white carbon black can be any of the hydrochloric, sulfuric or nitric acids; that mostly used is sulfuric acid and it is also used in the present investigation as the reagent or the precipitant.
13.3 EXPERIMENTAL EQUIPMENT AND PROCEDURE 13.3.1 Experimental equipment In the investigation on preparation of "ultrafine" white carbon black the submerged circulative impinging stream reactor (SCISR) is used as the reaction- precipitation equipment, the structure of which is the same as that shown in Fig. 10.2; it also has the same effective volume of 3.6×10 -3 m 3, but the top cover is not used for convenience because the process is carried out at room temperature and under atmospheric pressure. For further understanding the performance of the SCISR by comparison, the preparation experiments are also carried out simultaneously in a stirred tank reactor (STR) with an effective volume of 0.6×10 -3 m 3, the structure of which is indicated in Fig. 13.1. In order to mimic industrial conditions, the STR is equipped with three dampers distributed uniformly along the circle; the stirrer is a flat paddle.
274
IMPINGING STREAMS
per
Figure 13.1 Stirred tank reactor for comparative experiments.
13.2.2 Experimental procedure The testing materials used in the investigation are commercial sodium silicate with the modulus of 3.3 and sulfuric acid. The following substances are studied: (1) experiments for optimizing the operating conditions for the SCISR in semi-batch operation mode; (2) experiments in the SCISR operated continuously under the optimized conditions; (3) comparative experiments for the SCISR and the STR; and (4) a study of the final treatment of the precipitate from the continuous operations of the SCISR. In semi-batch operation, the SCISR is first filled with a solution of sodium silicate with certain concentration, and then a sulfuric acid solution of a given concentration is dripped at a certain rate into the reactor to react with the sodium silicate at a controlled temperature. The reaction continues for a certain interval of time after the dripping has finished. Stirring is then stopped for ageing of the precipitate for a term, and then the precipitate is sampled and the sample is measured with a laser particle-measuring instrument of FAM type to obtain the sizes and size distribution of the particles in the wet product. The experiments of continuous operation are carried out under the optimal conditions determined by the semi-batch operation experiments. Na2SiO3 and H2SO4 solutions of given concentrations are fed into the reactor at the inlets of the drawing tubes on the two sides of the SCISR and react with each other, while the reacted mixture over the settled level overflows from the reactor through the overflow port on the upper side wall. When the operation achieves a steady state, the overflow slurry is collected in a container for ageing, and then the precipitate is separated from the liquid, sampled and measured for the size distribution. Since the investigation is aimed at the preparation of ultrafine product, the measuring terms are limited to the size and size distribution of particles in the reacted precipitate in most of the experimental runs.
PREPARATION OF "ULTRAFINE" WHITE CARBON BLACK
275
The comparative experiments are carried out only in semi-batch operation mode; the experimental conditions and the operation procedure are identical for both the SCISR and the STR, and the specific effective power inputs for the two reactors are controlled rigorously to be identical. The experimental run in the study on the final treatment of the precipitate is carried out in the SCISR operated continuously under the optimized conditions for about 48 h to accumulate a large enough amount of product for spray drying. The whole of the reacted precipitate separated out is fully mixed and is then used for the spray drying experiment.
13.4 RESULTS AND DISCUSSIONS 13.4.1 Semi-batch operation The major goal of the semi-batch experiments with the SCISR as the reactionprecipitation equipment is to determine the optimal conditions. An experimental technique of normal design is employed and a total of eight influencing factors are examined. The normal design conditions are listed in Table 13.2 where the range of ageing time is determined primarily by searching experiments. The number of temperature levels tested is only two, because complete gelation happens in the reaction mixture so that the operation is destroyed even at 60°C, and so any test at higher temperature becomes meaningless. Table 13.2
Normal design for the experimental conditions Level
Operation variable* A
B
C
D**
E
F
G
H
1
38
600
8
1
30
300
0
1800
2
60
900
9
2
60
3
--
1200
10
3
120
600
1
3600
900
2
7200
* Operating variables: A--reaction temperature, °C; B--rotary speed of propellers, rpm; C--concentration of H2SO4, kmol'm-~; D--feeding position; E--time of feeding, s; F--reacting time after feeding; G .... amount of dispersion agent, g; H--time for maturation, s. ** Feeding position: 1--inlet of drawing tube, 2--outlet of drawing tube, 3 --center of the reactor. The results of the normal-designed experiments are listed in Table 13.3, where I, II, and III denote the summations of the Sauter mean diameters, d32, at Levels 1, 2, and 3, respectively; and R is the extreme difference at a certain level. From Table 13.3 it follows that the order of influence of various factors on the mean size of the particles in the precipitate is D>C>B>F>H>G>E>A. The influences of the latter three factors, G, E, and A, can be considered as very weak, while the most serious factors leading to gelation of the reaction mixture are, in order, A and G, suggesting the reaction temperature cannot be too high and the amount of dispersion agent cannot be too large.
276
IMPINGING STREAMS
Table 13.3 Results of the normal designed experiments Variable
A
B
C
D
Run NO
E
F
G
H
Level
Results Status
1
1
1
1
3
2
2
1
2
1
1
1
3
1
3
1
2
4
1
1
2
5
1
2
6
1
7
2
d32,gm
1
2
thicken
1.455
1
2
1
normal
1.168
3
3
3
3
thicken
1.481
2
1
2
3
1
normal
1.471
2
3
3
1
1
3
normal
1.452
3
2
1
2
3
2
2
normal
1.337
1
1
3
1
3
1
3
2
thicken
1.360
8
1
2
3
2
2
3
1
1
normal
1.297
9
1
3
3
3
1
2
2
3
normal
1.667
10
2
1
1
1
1
3
1
3
gelation
1.248
11
2
2
1
2
3
2
2
2
gelation
1.360
12
2
3
1
3
2
1
3
1
gelation
1.399
13
2
1
2
3
3
3
2
1
gelation
1.463
14
2
2
2
1
2
2
3
3
gelation
1.451
15
2
3
2
2
1
1
1
2
gelation
1.430
16
2
1
3
2
2
1
2
3
gelation
1.541
17
2
2
3
3
1
3
3
2
gelation
1.420
18
2
3
3
1
3
2
1
1
gelation
1.471
I
12.688
8.538
8.111
8.035
8.404
8.35
8.353
8.269
II
12.783
8.148
8.604
8.58
8.48
8.875
8.536
8.362
HI
--
8.785
8.756
8.856
8.587
8.246
8.582
8.84
R
0.095
0.637
0.645
0.821
0.183
0.629
0.229
0.571
I + II + HI =25.471
PREPARATION OF "ULTRAFINE" WHITE CARBON BLACK
277
Taking into account the fact that the average size of the product should be as small as possible and that the operation must be stable, the optimal operating conditions determined are A~, B2, C~, D~, E~, F3, G~, and H~, i.e., the optimized conditions are: reaction temperature of 38 °C, rotary speed of the propellers 900 rpm, concentration of -3 sulfuric acid 8 kmol.m -, position tbr feeding at the inlet of the drawing tube, feeding time 30 s, reaction time after feeding 900 s, amount of the dispersion agent zero, and ageing time after reaction 1800 s. Under these conditions the SCISR operated in semibatch mode can produce a product of white carbon black with particles sizes from 0.5 to 2.0 gm, the average size ranging from 1.1 to 1.6 gm. The results relating to the influence of the feeding position indicate that the best position is at the inlet of the drawing tube. According to the principles of impinging streams, the essential condition t0r the enhancement of transfer and/or mixing is the impingement between the opposing streams at a certain impinging velocity. The material, a solution or suspension, fed either at the outlet of the drawing tube or at the center of the reactor cannot be accelerated effectively, so that it cannot mix well with other stream(s), the poorest position for material feeding leading to the poorest mixing status is at the center of the reactor. Poor mixing must result in a slow reaction and thus is unable to create a high and uniform supersaturation for precipitation. So, the results described above indicate that the only option for the feeding position is at the inlet of the drawing tube(s). The results relating to the influence of Factor C, i.e., the concentration of sulfuric acid, show that the particle size of the product tends to increase with the H2SO4 concentration increasing. The increase in H2SO4 concentration favors increasing the supersaturation caused by reaction and thus promotes nucleation, on one hand, while the change in H2SO4 concentration also affects the electrical field environment around the newly formed particles, favoring coalescence of particles, on the other. It seems that, for achieving high supersaturation, the lowest concentration of H2SO4 originally selected is high enough and any further increase would be harmful. The influence of Factor B, the rotary speed of the propellers N, actually reflects the effect of the impinging velocity, u0. For the same SCISR u0 is a monodrome function of N. It was somewhat unfortunate that during this investigation the measurement of u0 at various N could not then be carried out and so the rotary speed was used as the influencing factor; now, for the approximate dependence of u0 on N one may refer to Fig. 11.2 in Chapter 11. The results on the mean diameter of the particles versus N exhibit a turning influence, which is similar to that obtained by Chen et al. [165]. In principle, from the flow configuration in the SCISR, the increase in N enhances micromixing in the impingement zone and thus should favor nucleation in quantity, while on the contrary, at too high rotary speed the mean size of the particles increases, as indicated by the data in Table 13.3. The following three facts may account for the phenomena' (1) Too strong micromixing may lead to excessive nucleation, leading to an enhanced coalescence tendency (2) Higher rotary speed increases the collision probability between the fine particles newly formed also leading to an enhanced coalescence tendency; and (3) As the rotary speed increases, the flow rate transported by the propellers increases, suggesting that the amount (volume) taking part in the
278
IMPINGING STREAMS
reaction increases, leading to, under the same other conditions, decreased supersaturation in the reaction region. These three items imply that there exist opposite influences on the mean diameter of the particles when increasing the rotary speed of the propellers, resulting in the overall turning influence. Unfortunately, with existing technical tools it is difficult to determine exactly the reasons for the phenomena described above and further investigations are necessary. The purpose of setting further reaction time after the dripping of H2804 is finished is to exhaust the reactants and to precipitate the hydrated silica fully. During the reaction, the following three actions may occur simultaneously: continuous nucleation, coalescence of particles and deactivation of the particle surface. From the experimental data it follows that Factor F, the reaction time, also exhibits a turning influence on the mean size of the particles and a maximum value appears at a time of about 600 s. It is possible that, at that time, infirm coalescence of the particles occurred and some of the coalescent particles can be broken into individual particles under the conditions of stirring-mixing. Therefore, in order to produce a finer product, it is necessary to arrange a period of time for further reaction after the dripping of H2SO4 is finished under continuous stirring conditions. The ageing time, Factor H, exhibits a monotonous increasing influence on the mean size of the particles, suggesting that the particles of hydrated silica have a significant tendency to coalesce under stationary conditions. Therefore, the ageing time before further processing of the precipitate should not be too long. Both the Factors E, the dripping rate of H2SO4, and G, the amount of dispersion agent, exhibit monotonously increasing influences on the mean diameter of particles; but both their effects are insignificant. Globally, the extreme differences, R in Table 13.3, for various factors are not large. This is because the result of a number of investigations on this topic have been referred to so that all the conditions selected for the present study are essentially in the operational ranges.
13.4.2 Continuous operation of the SCISR To determine the optimal conditions, experiments of continuous operation are carried out at a reaction temperature of 25°C, while solutions of Na2SiO3 and H2804 are fed at the inlets on the drawing tubes on the two sides, respectively; the mean residence time of the reaction mixture in the SCISR is 900 s. At a steady state of operation, the overflow suspension is collected for ageing, and then the precipitate is separated from the liquor, the size distribution of particles in the precipitate are sampled and measured. In order to examine the possibility of increasing the concentration of the slurry for a larger capacity reactor, experiments with various concentrations of Na2SiO3 solutions are carried out; all the other conditions are the same as the optimized ones determined in the last section. The results are illustrated in Table 13.4. The measurement under the condition of the Na2SiO3 concentration being 0.84 kmol.m -3 is repeated many times, while the variation of the mean diameter of particles in the precipitate is small and is
PREPARATION OF "ULTRAFINE" WHITE CARBON BLACK
279
within the error range for the measuring instrument, suggesting the operation is very stable. The data listed in the table are the averages. From the data listed in Table 13.4 it follows that an increase in the concentration of the reaction slurry is possible, but the magnitude of the permitted increase is not large, otherwise gelation would occur, resulting in operation break-up. The reason is clear: the increase in the concentration leads to increased viscosity of the slurry, negatively affecting mixing. The most significant conclusion obtained by these experiments is that the SCISR can be operated continuously in the preparation of ultrafine white carbon black by the common (one-step) precipitation process, and the average size of the particles in the product is essentially the same as those obtained by semi-batch operation. To date, all the commercial reactors for preparation of white carbon black by precipitation processes are STRs operated in semi-batch mode. Globally the flows in these devices have a perfect-mixing feature. It is likely that the special flow configuration of the perfect mixing-plug flow in series in the SCISR is the major reason that it can be operated continuously. It is clear that continuous operation is normally superior to batch or semi-batch, especially for production on a large scale. Table 13.4
Results for different concentrations of Na2SiO3 solution Concentration of Si02, kmol.m-3
Particle size, gm d32
d,.lo
dvso
dvgo
dvmin
d .....
0.66
1.245
1.118
1.268
1.373
0.667
1.556
0.80
1.331
1.004
1.431
1.793
0.222
2.556
0.84
1.381
1.106
1.459
1.571
0.370
2.334
0.90
Gelation
13.4.3 Comparative experiments in semi-batch operation Comparative experiments are carried out between the SCISR and the STR for further verifying the good performance of the SCISR. Both the reactors are operated in semibatch mode and under the same optimized conditions as before. The structure and dimensions of the experimental reactors have been described in Section 13.2. The size distributions of the particles in the precipitates from the two reactors are illustrated in Fig. 13.2. Obviously, the product from the SCISR is finer with a narrower size distribution, i.e., more uniform in size. It should be noted that the effective volume of the experimental SCISR is six times that of the STR, suggesting the scales favor the
280
IMPINGING STREAMS
STR. Thus, these results indicate an obvious difference between the performances of the two kinds of reactor. 40
30 0 0
~
20
r~
10
_1_Jl._1
1.0
i ~
2.0
....
3.0
Particle size, ~tm Figure 13.2 Comparison between particle size distributions of white carbon black prepared in SCISR and STR, respectively. A--0.6xl0 -3 m 3STR .--3.6x10 -3 m 3 SCISR.
13.4.4 Study of the final treatment of the reaction product As mentioned above, the major goal of the present investigation is to produce white carbon black product as fine as possible, reaction-precipitation being the key operation for its production. In order to focus attention on the major problems, all the measurements of size and size distributions made above are with the reacted wet precipitates. To examine the size stability of the product during the final treatment, experiments on spray drying of the reacted wet precipitate are carried out. The reaction-precipitation takes place continuously under the optimal conditions determined in Section 13.3.1; the washed cake separated from the liquor and the washing water is made into slurry again by stirring and is then spray-dried in a tower of 500 mm in diameter with hot airflow to yield the dry product. Since there is a great difference between the capacities of the reactor and the spray dryer, the reactionprecipitation must be operated for a long time, over 48 hours, until the amount of wet precipitate collected is large enough for the spray dryer operation for, at least, 2 hours; and then the dryer can be operated. Therefore the mean time for maturation of the precipitate is very long, over 24 h and the longest can be 48 h. All the dried products collected at the bottom of the dryer, from the cyclone and the bag fitter, respectively, are put together and mixed fully, and then sampled and measured for size and size distribution. Data for the characteristic parameters of sizes are listed in Table 13.5.
PREPARATION OF "ULTRAFINE" WHITE CARBON BI,ACK
1281
From the data listed in Table 13.5 it can be seen that the Sauter mean diameter of the dried product, d32, is larger than that of the wet precipitate obtained under the same reaction conditions by about 10%, or by 0.15 gm. An obvious fact is that no matter whether at the bottom of the dryer or in the cyclone or in the bag filter, the recovery of the finer particles must be lower than that of the larger particles. These differences between the recoveries of particles of different sizes must lead to an increased mean diameter of the product. If this fact is taken into account, the sizes of the particles can be considered to be stable enough during the final treatment of the precipitate, without coalescence of particles occurring. Table 13.5 Characteristic sizes of the spray-dried product from SCISR operated continuously Characteristic size lam
d-,_~
d, 1!~
d, 50
dvg0
dvmin
d,m~,x
1.491
i.294
1.530
1.702
0.556
2.0
13.5 CONCLUSIONS Utilizing its features of efficient micromixing and very strong pressure fluctuation and the special flow configuration ot perfect mixing-plug flow in series, the submerged circulative impinging stream reactor (SCISR) is used for the preparation of "ultrafine'" white carbon black by the common (one-step) precipitation process; comparative experiments are also made between the SCISR and the traditional stirred tank reactor (STR). The following main results are obtained: ~,1) In the SCISR of 3.6x10 ~ rn ~ m effective volume, the common (one-step) precipitation process operated Jn semi-batch yields ~ultrafine" white carbon black consisting of particles sized () 5 to 2.0 gm, the average sizes ranging from 1.1 to 1~6 btm. (2) The main factors affecting the dzc and ;i~:.-, distribution of the particles in the product of white carbon black are detcrmined experimentally, for which the optimal conditions are: the reaction temperature is the common (room) temperature; the concentration of H-,SO4 solution 8 kmol.m-3; the feeding position at the inlet of the drawing tube(s): the reaction time after feeding of all the reactants 900 s; and the ageing time 1800 s. (3) The SCISR can also be operated continuously, and the product so prepared ha,, essentially the same size and size distribution a~ that obtained by operation in semi-batch mode under the s~me condition~. (4) The results of the comparative exp~:rimer.t~ operated in semi-batch m~de indicate that the product prepared with the SCISR is finer and with a narrower distribution than that from the STR of 0.,q×10 --~ m 3 in effective volume, suggesting that the performance of the SCISR i,: ,,uperior to t>,at or" the STR.
282
IMPINGING STREAMS
(5) The results of the spray drying experiment of the wet precipitate show that the particle sizes of particles in the product produced in the present investigation are stable, and no coalescence of particles during the final treatment of the reacted wet-precipitate is observed.
-14PREPARATION OF ULTRAFINE POWDERS BY REACTION-PRECIPITATION IN IMPINGING STREAMS I1: NANO COPPER AND ITS SURFACE IMPROVEMENT
14.1 INTRODUCTION As is well known, the nanometer, nm, is a measurement of materials and represents a length of 10-9 m, which is equal to the scale of about 10 atoms. The term "nano material" indicates those materials consisting of particles sized less than 100 nm in every dimension, i.e., solid materials consisting of ultrafine particles sized from 1 to 100 nm. Since the dimensions of the composition phase or the crystalline particles are near molecular size, nano materials have a number of excellent characteristics which normal materials do not possess and so can be widely used in the fields of electricity, magnetism, optics, superconductors, intelligent materials, hydrogen-storage, biomedicine, nano-medicine, functional eyes, functional ceramics, functional fibers etc., and have very high application value. As mentioned in the previous chapter, many countries, including the industrially most developed ones, have adopted nanotechnology as one of the key technologies for development in the 21st century. It is no exaggeration to say that developments in nanotechnology will yield significant and farreaching influences on science and technology, economics, military affairs, and daily life etc. in the coming few decades. Put simply, nanotechnology includes the aspects of preparation, property characterization, surface improvement, and application of nano materials. It involves many disciplines and its progress needs the cooperation of scientists and engineers from various disciplines. Obviously, among the aspects mentioned above, preparation of nano materials is the basis. If there was nano material preparation any other work related to nano materials would be meaningless, like a tree without roots. Because of their important application values, ultrafine powders have been the subject of a number of investigations and developments in the last two or three decades, and many kinds of preparation methods have been proposed. By examining the status of the research and development and the various methods proposed, it is not 283
284
IMPINGING STREAMS
difficult to see that chemical engineering, as a traditional and old discipline, has played and will continue to play a very important role in research, development and application of the technologies for the preparation of ultrafine powders. With the discovery that the features of efficient micromixing and strong pressure fluctuation existing in the submerged circulative impinging stream reactor (SCISR) favor the preparation of ultrafine particles by reaction- precipitation, in addition to that of the "ultrafine" white carbon black, the preparation technologies of several nano particles were investigated experimentally with the SCISR as the reaction-precipitation equipment, and all the investigations yielded satisfactory results. In all the processes the method of reaction-precipitation were employed because only in such processes can the SCISR exhibit its superior performance. This chapter introduces investigations into the preparation of nano copper and its surface improvement together with the major results; the preparation of the other two nano materials, Titania and hydroxyapatite, will be discussed in later chapters.
14.2 PROPERTIES AND MAIN USES OF NANO COPPER Nano copper powder has the size- effects, the quantum-tunnel effect, the surface-effect and the volumetric effect similar to other nano metal powders and thus exhibits many special properties quite different from those of normally assembled copper [185-187]. Common metallic copper is purple in color, and the melting point is 1084°C, boiling point 2582°C, and density 8920 kg.m -3. While the melting point of the nano copper with an average size of 40 nm drops to 750°C and that with an average size of 20 nm drops even more sharply to the level of 39°C, the specific surface area of nano copper increases rapidly and, consequently, the surface energy increases sharply as the particle size reduces. For example, when the average size reduces from 100 nm to 10 and 1 nm, the specific surface area increases rapidly from 6.6 mZ.g-~ to 66 and 660 mZ.g-~, respectively; the surface energy increases from 590 J-mo1-1 to 5900 and 59000 J.mol -~, respectively; and, correspondingly, its reactive and catalytic activities are greatly increased. In addition, the specific heat capacity of nano copper is twice as large as common copper and the self-diffusivity of nano copper crystalline is 1016 to 1019 times that of common copper crystalline, e.g. the diffusivity of the nano copper sized 8 nm is of the value of 2.6x10 -2° m2.s-~. Nano crystalline copper has a greater strength than that of the common copper, and exhibits plastic ductility. Its coefficient of elongation is over 5100% and the phenomenon of hardening will not appear during handling. Its hardness and yield-strength are higher than those of common copper by 50 and 12 times, respectively. Nano metallic copper tends strongly to electric neutrality and exhibits almost non electric conduction. Its electrical resistivity increases as the particle size reduces, while the thermo-coefficient of electrical resistivity decreases, and can even be a negative value, as the particle size reduces. Because of the extremely high activity of its surface, nano copper very easily adsorbs oxygen in the surrounding air and, simultaneously, it is oxidized. Also, nano copper has a very great ability for light-absorption, while its ability for light-reflex is very weak, normally lower than 1%. Nano copper powder has important applications in the following fields:
PREPARATION OF NANO COPPER AND ITS SURFACE IMPROVEMENT
285
(1) Catalysts: Nano copper can be used as a catalyst for deep splitting of long-chain hydrocarbons in the petrochemical industry. For the hydration of acrylonitrile nano copper exhibits very high catalytic activity and selectivity [188]. It is sldo highly active in both the reactions of ethyne polymerization and the catalytic oxidation of CO. The results of the investigation on the catalytic activity of nano copper particles in the polymerization of ethyne carried out by Wang et al. [189] showed that the size of the nano copper particles has an important influence on the catalytic activity: the smaller the particle size, the higher is the yield of the product. The results of the comparative experiments to assess the catalytic activities of nano Cr, Mn, Ni, Fe and y-A1203 catalysts [190] showed that for a conversion of 100%, the reaction temperature required by nano copper is the lowest, suggesting it has the highest catalytic activity. (2) Electro-conductive rubber material: Because of its great strength and much lower price than such expensive metals as silver and palladium, nano copper or coppersilver double metal powder can be used in the electronics industry to take the place of those expensive metals for the preparation of electro-conductive rubbers [191], electro-conductive slurry, and electrode materials etc.; in addition, the copper-silver double metal powder has the characteristic of antibiosis. For such use nano copper or copper-silver powder should be needle-like crystalline; the nano copper powders of sphere-like crystalline has very low electro-conductivity. (3) Additives in greases of high grade: This is one of the fields where nano copper is most successfully applied. Copper is a soft metal. The addition of oil-soluble nano copper in grease can increase the wear-resistance performance by a wide margin and can form highly efficient arc-eliminating electro-conductive grease. Also, the addition of nano copper of sphere form gives the shaft-bearing system a selfrenovating function. In an experimental study Dou et al. [192] considered that the fine particles of metallic copper exert a bearing effect, polishing effect, metallurgical effect and strengthening effect on the interface. The fine copper particles can penetrate through the interface and thus improve the surface. Under the action of mechanical motion forces, the fine particles are pressed and inlayed on the friction surface in the form of atoms to form complex metallic structures, resulting in a greatly reduced friction drag force. Xia et al. [ 193] proposed that nano copper powder takes part in lubrication in the forms of pads and balls. The polishing scratch increases very little as the loading increases, the surface is kept smooth, and the concaves on the slipping surface are filled and leveled up, yielding improved lubricating performance; the addition of 5% triethanolamine yields a composite effect [194] that greatly increases the wear and tear of the lubricating grease. Xu et al. [195] formulated several lubricants with nano copper particles of various sizes in the range 4 to 50 nm, which were prepared by gasstream grinding in fluidized bed, and tested their performances. The experimental results showed that with the nano copper particles of 4 to 15 nm the increase in lubricating efficiency was very significant. Later, Xu et al. used the nano copper powder sized about 10 nm prepared by reduction-precipitation in the experiments carried out by Chen [196] to take the place of that prepared by gas stream grinding, and the formulated lubricant exhibits very good performances: all the indexes achieve or even exceed those of all the existing lubricants.
286
IMPINGING STREAMS
14.3 PRINCIPLES AND EXPERIMENTAL METHOD 14.3.1 Chemical reactions in preparation of nano copper by reductionprecipitation Various methods for the preparation of nano copper have been proposed. Because of its feasibilities in both technology and economics and to take advantage of the strong points of liquid-continuous impinging streams, in the investigation carried out by Chen [196] the reduction-precipitation process in liquid phase was employed, i.e., a certain reducing agent is added to a solution to react with the soluble copper salt in the liquid phase, and the copper particles formed as the product of the reaction precipitate out. Several reducing agents have been used for this purpose, such as ascorbic acid, formaldehyde, hydrazine hydrate, sodium hypophosphite, potassium borohydride, sodium hyposulfate etc; while in the preparation of various nano metal powders potassium borohydride exhibited perfect performances, although its price is somewhat higher. In the investigation by Chen, a CuCI2 solution was used as the copper source and potassium borohydride, KBH4, as the reducing agent. The major chemical reaction occurring in the liquid phase can be represented by 4CUC12 -+-KBH4 + 8 K O H = 4Cu $ + 8 K C I + K B O 2 + 6 H 2 0
(14.1)
Because of the multi-valence nature of Element Cu, the simple substance copper formed may react with Cu 2+ in the solution to form the monovalence copper ion Cu +, and the latter may also react with C1- to produce precipitate: Cu @- C u 2+ -+2Cu +
(14.2)
Cu + + C1----, CuC1 $
(14.3)
2CuC1 + H20--~ 2HC1 + Cu20 ,L
(14.4)
Therefore, to increase the yield of copper, the formation of the Cu ÷ ion in liquid phase must be restrained. For this purpose a certain amount of aqua ammonia is added, as the coordinative agent, to form the coordinate complex ion Cu(NH3)42+ for restraining the reaction producing Cu +, Eq. (14.2), and also for dissolving the precipitates of CuC1 and Cu20 to keep Cu 2+ from precipitation in the form of hydroxide yielding a product with low purity under the condition of basicity. It is known by an analysis of the equilibrium constants that the concentration of Cu 2+ ion in the solution with aqua ammonia added is very low and is independent of CuCI2, but is relative to the concentration of the dissociated ammonia. On the other hand, under the condition of KBH4 excess the reaction is carried out very completely; while the existence of the coordinative agent ammonia decreases the reducibility of KBH4. Therefore both pH and the concentration of KBH4 must be enhanced.
PREPARATION OF NANO COPPER AND ITS SURFACE IMPROVEMENT
287
14.3.2 Experimental equipment and procedure The experimental equipment system for the preparation of nano copper powder by reduction-precipitation is shown in Fig. 14.1, where the submerged circulative impinging stream reactor (SCISR) has the same structure as was used in the investigations described in the previous chapters in Part II of this book, with the same effective volume of 3.6×10 -3 m~; it is also operated without the top cover; but is made of titanium for anti-corrosion of C1-~. The formulated KBH4 solution is first fed into the SCISR; then the driving motors on the propellers are turned to push the liquid so that it circulates inside the reactor and impinges against the opposite stream, while the rotary speed of the propellers is manipulated so as to achieve the required impinging velocity, u0. Water at a given temperature passes through the heat-exchanging jacket outside the reaction vessel to keep the temperature of the process material inside the reactor constant during the reaction. When both the flow and the temperature in the reactor achieve a steady state, the liquor reactants are fed into the reactor in the modes selected, which will be described later. The reactions take place for a certain interval of time after feeding the reagents, then the reaction mixture is discharged from the reactor and separated with a high-speed centrifuge; the cake is washed with water to remove the residual Cu 2+ (check with potassium ferrohexacyanide K2[Fe(CN)6] for without Cu2+), washed with acetone, and then dried at 30 °C under vacuum to yield the final product of nano copper. The make-up of the reactant solutions is an important link in the preparation of nano copper by reduction-precipitation affecting the efficiency of the process. The procedures for the two solutions are as follows: .....
t
t
Figure 14.1 Scheme of the experimental system. 1, 2-solution tank; 3-propeller; 4-drawing tube; 5-discharging port; 6-jacket for heat exchange.
288
IMPINGING STREAMS
(1) KBH4 solution: 2000 mL potassium borohydride solution is prepared for each run, its concentration being determined according to the concentration of CuCl2 to be used in the run; a little strong aqua ammonia and an appropriate amount of KOH solution with the concentration of 4 mol.L -~ are added to adjust the pH, and 10 to 40 mL of the dispersing agent is also added to the solution. (2) CuC12solution: 1500 mL of copper chloride solution is prepared for each run. The solution is made-up of solid CuClz.2H20, strong aqua ammonia, and the surfactant, 2% PVP solution. The amount of CuClz.2H20 to be added is determined from the required concentration; the amount of aqua ammonia is the stoichiometric amount for producing Cu(NH3)42+ ions; the amount of 4 mol.L -~ KOH solution > 75 mL; and the amount of the dispersing agent, 2% P V P , is in the range of 10 to 40 mL. The experiments are carried out in two stages. The major goal of the work in the first stage is to make nano copper powder with a relatively uniform size distribution; the work also aims to determine the optimal or, at least, most feasible mole ratio of CuCl2 to KBH4. In the second stage the influences of various operating parameters on the mean size and the appearance of the produced nano copper aree investigated to yield the optimal conditions. The effecting factors examined are the concentration of copper salt, the amount of PVP, the pH of the reaction mixture, the reaction temperature, and the impinging velocity (satisfied by controlling the rotary speed of the propellers). Since the global goal is to prepare finer nano copper with a narrower size distribution, the mean size of the product is always taken as one of the major criteria for analysis and comparison of the results.
14.4 RESULTS AND DISCUSSIONS ON THE PREPARATION OF NANO COPPER POWDER 14.4.1 Major results obtained in the first stage As mentioned above the major goal of this stage is to produce a qualified product. Three sets of experiments are carried out with various CuC12 to KBH4 mole ratio, CuClz:KBH4 = 1:1, 1:2 and 1:3, respectively; while the other conditions are fixed at the concentration of CuCI2 C cuc12 = 0.1 kmol.m -3, the reaction temperature T = 20 °C, the rotary speed of the propellers N = 1000 rpm, the pH of the reaction mixture pH 14, the reaction time tr = 10 rain, and the reactants are fed quickly. During each run the resulting phenomena are observed carefully; the sizes and the appearance of the product are characterized with the transmission electron mirror microscope (TEM). The major results obtained are: (1) At the CuCl2 to KBH4 mole ratio of 1:1, the product has a larger size and clumps together significantly, and a modicum of long fiber-like product is formed. The major observations are: in the earlier stage of the reaction brown particles are precipitated, then the precipitate forms gradually wadding;
PREPARATION OF NANO COPPER AND ITS SURFACE IMPROVEMENT
289
the existence of Cu ~+ is detected out. (2) With the mole ratio of 1:2, the product is an average size of 16 to 20 nm, slightly clumped together; the particles are approximately of sphere form without fiber- or needle-like appearance. The observations are: during the reaction the solution is colored black, no significant precipitation within one hour; no Cu 2+ in the precipitate is detected. (3) With the mole ratio of 1:3, the product gathers seriously; there is a needle-like appearance on the surface of the clumped particles and the sizes of the spherical particles are larger. The observations are: the reaction solution is colored brown; no Cu 2+ ion is detected in it; the precipitate appears up to two days later and oxide of a red-brown color forms on the surface of the precipitate. Most of the TEM photos taken at this stage are not given here due to restrictions of space, but that of the product obtained at CuC12:KBH4 = 1:2 is shown in Fig. 14.2. It can be seen from this figure that the particles in the product are sized approximately from 16 to 20 rim. The following results obtained in the first stage of investigation are of significance:
(1) P h e n o m e n o n observations: If the reaction solution or the precipitate e.xhibits a non-black color, there must be un-reacted Cu 2+ or the product of the re-reaction of Cu, Cu +, and thus the purity of the product must not be high; normally, the sizes and appearance of the product would not be ideal, i.e., the particles are largersized, non-sphere crystalline, and significantly clumped together. If the precipitate appears too early, the sizes of the particles in the product are usually large. These observations are helpful in preliminary judgments of whether the reaction conditions employed are good or not. (2) Optimal or feasible mole ratio" According to the mean sizes and the appearances of the products obtained it is considered that, among the three mole ratios tested, CuC12:KBH4- 1"2 is the best. The feasibility of this mole ratio has been verified by the results of repeated experiments.
50 nm
t................................ !
Figure 14.2 TEM photo of the product obtained under the condition of CuCI::KBH4 = 1:2.
290
IMPINGING STREAMS
It should be noted that, from the reaction represented by Eq. (14-1), the stoichiometric ratio of CuClz:KBH4 should be 4:1. That is, with any one among the three mole ratios KBH4 is greatly excessive. This is for the complete reaction of Cu 2+ to increase the yield of the simple substance copper. From the experimental results it follows that with the mole ratio of 1:1, i.e. the excess of KBH4 is smaller, the reaction of Cu 2+ is incomplete; while for the mole ratio 1:3, i.e. the excess of KBH4 is very large, Cu 2+ can be reacted completely, although the side-reaction leads to the formation of copper oxide and, consequently, a decreased yield of simple substance copper. So, the mole ratio of CuC12:KBH4 is an important operating condition that needs to be optimized.
14.4.2 Results on the influences of various factors The influences of various factors, rather than the mole ratio of CuC12: KBH4, are studied in the second stage of the investigation and the experimental conditions are listed in Table 14.1, where the symbols in the second column for the feeding modes denote the following: I - CuC12 solution is added rapidly to the KBH4 solution; IICuCI2 solution is added slowly to the KBH4 solution; III- KBH4 is added slowly to the CuCI2 solution; and I V - I n making up the solutions, the CuCI2 solution is without the addition of KOH, but 160 mL of 4 mol.L -~ KOH is added to the KBH4 solution, while for the reaction KBH4 solution is added slowly to the CuCl2 solution. Among the influencing variables listed in Table 14.1, the rotary speed of the propellers, N, actually reflects the influence of the impinging velocity, u0; while for convenience of operation N is taken as the operation variable. For a given SCISR u0 is a monodrome function of N, and for the reactor used in the present investigation the curve shown in Fig. 10.8 in Chapter 10 is essentially applicable for the relationship between u0 and N. The experimental results obtained in this stage are described below.
Table 14.1
Experimental conditions for study on the influences of various effects Influencing variable Level
Feeding mode*
Concentration of
CuC12, mol-L-1
Temperature °C
Rotary speed rpm
Amount of PVP, mL
pH
I
o. 1
10
600
0
11
II
0.2
20
800
20
12
III
0.3
30
1000
40
13
IV
0.4
40
1200
60
14
PREPARATION OF NANO COPPER AND ITS SURFACE IMPROVEMENT
291
14.4.2.1 Feeding mode The other operating conditions for investigation the influence of the feeding mode are: CcL,cl2- 0.1 kmol.m -3, T = 20°C, N = 1000 rpm, pH - 14, and CuC12 to KBH4 mole ratio 1:2. The major results are as follows" In Mode I (feeding time 1 rain), the results are similar to those obtained in the repeated verifying experiments mentioned in the previous section. The TEM photo of the product shows particle-clumping and the results of X-ray diffraction (XRD) analysis indicate that the product consists mainly of copper powder with a little Cu20. The X-ray spectrum of the product obtained in this feeding mode is shown in Fig. 14.3, which indicates the major part being Cu, with little Cu20 and no CuO. In Mode II (feeding time 10 min), the TEM photo of the product shows clumped particles with a little needle-like crystalline, and the data of XRD indicates that the major product is Cu20, with a little Cu only and no CuO. In Mode III (feeding time 8 rain), the TEM photo of the product shows the loose needle-like crystalline, with a modicum of clumped particles. In Mode VI, the TEM photo of the product shows particles clumping with a modicum of needle-like crystalline cluster sized about 20 nm. After analysis and comparison, feeding mode ! is considered to be the best. On the other hand, from the point of view of rapid reaction yielding very high supersaturation, Mode ! should be the best. Therefore feeding mode I is confirmed. It should be noted that, in Mode III the product essentially of needle-like crystalline can be produced under well controlled conditions, as illustrated in Fig. 14.4. In some cases, e.g. for the production of electro-conductive rubber etc, this kind of product may be required. Therefore the feeding mode III is also of practical significance.
i ~
ii ~i!~i~
i~ ~
i ~~
i
i ii~;~!
~ii'~!;
:~!i~
~iii~
Figure 14.3 X-ray spectrum of the product obtained in Feeding mode I.
~i;iiiii!¸
292
IMPINGING STREAMS
~e......
!:.~
i
Figure 14.4 Nano copper of needle-like form obtained in the feeding mode III.
14.4.2.2 Influence of CuCI2 concentration The other operating conditions for investigating the influence of CuCI2 concentration are: the reaction temperature T = 24°C, the feeding mode I, the rotary speed of the propellers N = 1000 rpm, pH = 14, CuC12 to KBH4 mole ratio 1:2, and the reaction time after feeding tr = 10 min. The major results are:
(1) Observations: At the concentration of
C C u C 1 2 -- 0.1 kmol.m -3 the reaction solution is colored brown, while for other concentrations the color of the solution is brown at the beginning and then turns to black. The higher the CuCI2 concentration, the quicker the color changes, suggesting qualitatively that, from the point of view of increasing the yield of the simple substance copper, the condition of Ccoc~2 = 0.1 kmol.m -3 would not be desirable. (2) Appearance of product from TEM: With Ccucl2- 0.1 kmol.m -3, the product is loosely clumped particles and needle-like appearance; at Ccuc~2 = 0.2 the product consists essentially of sphere particles without needle-like appearance; at Ccuc~2 = 0.3 the appearance of the product is essentially the same as for Ccuc~2 = 0.2; while for Ccuc~2 = 0.4 the product is loose particles and needle-like cluster of long length. (3) Mean sizes: The variation of the mean size with CuC12 concentration exhibits a minimum culminating point, as indicated in Fig. 14.5.
From both the mean sizes and the appearance characteristics it seems that at lower concentrations of CuCI2 solution, normally spherical particles of uniform sizes are produced; while for Ccuc~2 - 0.4 kmol.m -3 needle-like crystalline clusters possibly up to several micrometers long are formed, which consists of the growing particles in the
PREPARATION OF NANO COPPER AND ITS SURFACE IMPROVEMENT
293
system [197]. For crystallization of copper, the thermo coefficient of the supersaturation, d(AC)/dT, approaches zero and the metastable region must be extremely narrow. In the ranges of the operating conditions tested, it is very easy for the reaction solution to be in the unstable region, leading to nucleation in quantity. On the other hand, the surfaces of the nano copper particles contain unsaturated valence bounds and thus have remainder force field. At higher concentrations, interparticle adsorption would occur, resulting in assembly of particles, i.e., the condensation at particle level. These opposite effects may account for the variation in the mean size of the products shown in Fig. 14.5. According to the experimental results, the concentration of Cc,,c~2 = 0.2 kmol.m -3 is the optimal or is, at least, feasible.
/
13 //
/
E ._~ 12 ~D
//
/
/
/
/
/
/
/
100 nm
yes
[206]
N2H4.H20
50-- 500 nm
--
> 300 nm
yes
[200]
_>500 nm
- 38.84
> 500nm
yes
[199]
STR
~ 45 ~tm
- 15.22
- 45 ~tm
yes
[207]
SCISR
-- 10 nm
- 30
5-30 nm
yes
[ 196]
Ascorbic acid
KBH4
14.6 CONCLUSIONS T h e p r e p a r a t i o n of nano c o p p e r p o w d e r by reduction-precipitation in the s u b m e r g e d circulative i m p i n g i n g stream reactor ( S C I S R ) with KBH4 as the r e d u c i n g agent, strong aqua a m m o n i a as the c o m p l e x a n t , and P V P as the dispersing agent, and the surface
300
IMPINGING STREAMS
improvement of the nano copper powder so prepared for anti-oxidation were investigated experimentally. The following can be concluded: (1) The optimal conditions determined for the preparation of nano copper by reduction-precipitation are: the concentration of CuCI2 solution Ccuc~2 = 0.2 kmol-m-3; the rotary speed of the propellers N ~ 800 rpm; the reaction temperature T = 20°C; the pH of the reaction solution pH = 14; the amount of 2% surfactant PVP addition 40 mL; the mole ratio CuClz:KBH4 = 1:2; and the feeding mode I, i.e., CuCI2 solution is added quickly to KBH4. (2) The repeated experimental operation under the optimal conditions described in Item (1) can yield a stable nano copper product average-sized 5 to 10 nm with a narrow size distribution and a high content of the simple substance copper. (3) The pH of the reaction solution has an important influence on the properties and appearance of the nano copper product; the escape of ammonia in the form of bubbles during and after the reaction favors the dispersity of the particles; the rotary speed of the propellers has no significant influence on the particle size in the product in the range of speed tested. (4) With the nano copper produced in the SCISR the Cu-Ag double metal powder is prepared by the replacement reaction with AgNO3 as the reactant. The TEMdetermined results show that the product of Cu-Ag double metal powder prepared consists of particles sized 5 to 30 nm and their clumps; after storage for one month at room temperature no change in the color of the Cu-Ag double metal powder has been found, suggesting the product has anti-oxidation nature at room temperature. (5) In comparison with the nano copper powders prepared with various processes and/or devices, the products prepared in the present investigation are finer with a narrower size distribution, showing that the SCISR with the features of efficient micromixing and very strong pressure fluctuation and the special flow configuration of perfect mixing flow-plug flow in series is very suitable for the preparation of ultrafine powders by reaction-precipitation and, in fact, exhibits good performance in the present investigation.
-15PREPARATION OF ULTRAFINE POWDERS BY REACTION-PRECIPITATION IN IMPINGING STREAMS II1: NANO TITANIA
15.1 PROPERTIES OF NANO TITANIA AND CHEMICAL REACTIONS IN ITS PREPARATION Titanium dioxide (Titania) is a white powder which is inert, insoluble in water, organic and weak inorganic acids, while being slightly soluble in alkali and soluble in saturated potassium acid carbonate. It can be completely dissolved in strong sulfuric acid and hydrofluoric acid after boiling for a long time. It is thermo-stable and melts gradually at temperatures over 1800°C [208]. Titania has two main stable crystalline forms: titanic and anatase. Titanic is the most stable one, and anatase converts into titanic form at high temperatures over 915°C. From the thermodynamic data, the formation heat of anatase is 8-12 kJ-mol -~, and so it should be more stable than titanic [209]. On the other hand, the TiO2 of anatase has lower hardness, and is more suitable for use in cosmetics. Nano Titania is one of the earliest nano materials to be applied commercially. It has a number of superior properties, such as super strong scattering and anti-ultraviolet capabilities, special electromagnetism and catalysis characteristics, especially the photocatalysis ability decomposing microbes, and also extremely high surface activity [210]" and can be used as active ingredients in high-grade coatings, anti-ultraviolet cosmetics, hygiene ceramics, self-cleaning glasses, composite polymer materials, photoelectric cells, electronic ceramics, semiconductors, catalysts, etc. The application of nano Titania is still in the stage of initial development and a number of possible applications of great potential have not yet been put into practice, e.g., its use as a catalysis-active component, etc. As for other inorganic nano materials, a number of preparation methods have been proposed for nano Titania. Because of its economic advantages and its simplicity, Li [211] employed the process of TIC14 hydrolyzation-precipitation in impinging streams. In practice, the chemical process whereby TIC14 hydrolyzation yields TiO2 precipitate is very complex [212]. Since it is not the major topic of this book, the chemical reaction mechanism will not be discussed in detail here. Put simply, under
301
302
IMPINGING STREAMS
controlled conditions TIC14 is converted by hydrolyzation-ionization-hydrolyzation into TiO2 precipitate [213], as follows.
Hydrolyzation of TIC14: TIC14 + H20 ~ TiOH 3÷+ H + + 4C1-
(15.1)
Ionization of the middle product: TiOH3+___~ TiO 2++ H +
(15.2)
Hydrolyzation of the secondary ion: TiO 2++ H20 ---* TiO2 + 2H +
(15.3)
where Reaction (15.1) is a rapid reaction. When a higher concentration of TIC14 solution is added, the hydrogen ions from Reaction (15.1) may restrain the desired reactions, Reactions (15.2) and (15.3), to yield a limpid solution containing TiOH 3+. With the addition of an ammonium sulfate solution containing hydrochloric acid, at higher concentration the sulfate ion can combine with the TiO 2+ ion to produce TiOSO4 precipitate, promoting Reaction (15.2). At higher temperature, the solubility of TiOSO4 increases, and so the titanium is mainly in the form of TiO 2+' while when the temperature reaches 95°C, the precipitation, Reaction (15.2), occurs. It is clear that all three reactions, Reactions (15.1) to (15.3), produce hydrogen ions. To avoid pH variation too quickly resulting in a non uniform composition of the precipitate, it is necessary to add a certain amount of aqua ammonia to form a buffer solution so that the rate of titanium oxide formation is kept suitable for the precipitation of TiO2. In addition, to set a period of time for keeping warm after the reaction operation promotes the formation and growth of the TiO2 nucleus. Actually, during the reactions, the meta-titanic acid HzTiO3 is precipitated first, which adsorbs considerably large amounts of water and soluble impurities such as ammonium sulfate, ammonium chloride etc, and also contains some insoluble impurity particles such as oxides of iron, strontium, and calcium etc. Therefore, it is essential to fully wash the precipitate to obtain a product of high purity. The insoluble and part of the soluble impurities can be removed by multiple washings; ammonium chloride and ammonium sulfate are decomposed and released as the precipitate is later heated to 300°C and 700°C, respectively, during the calcination. The meta-titanic acid obtained directly by hydrolyzation is amorphous, with extremely unstable surface properties. For convenience of storage and application, it is usually calcined to convert it into anatase or titanic powder. During calcination, the primary particles are formed gradually as dewatering. The growth of TiO2 particles during calcination are usually in the two schemes: isothermal growth and agglomerative growth, the latter yielding larger particles outside the scope of nano materials. For the preparation of nano titanium oxide, the fully washed precipitate is usually calcined at a constant temperature, and the temperature of calcination must be controlled to avoid agglomeration, or, at least, to reduce agglomeration as much as possible. The calcination temperature for anatase is round 600°C, while for titanic crystalline is
PREPARATION OF NANO TITANIA
303
800°C. During calcination the particles grow by themselves, but the growth rate is smoothed after achieving certain sizes; and the crystalline form may also be partially changed. For example, some of the titanic crystalline may also appear in the product calcined at 600 °C for a long time.
15.2 EXPERIMENTAL EQUIPMENT AND PROCEDURE The experimental equipment system for the preparation of nano titanium oxide by TIC14 hydrolyzation-precipitation is the same as that shown in Fig. 14.2 in Chapter 14, and the submerged circulative impinging stream reactor (SCISR) is also made of titanium material for anti corrosion of C12 and CI-, with an effective volume of 3.6x 10 -~ m ~. As with the preparation of nano copper powder, the make-up of the solutions of the reactants TIC14 and (NH4)2SO4 is an important link in the preparation of nano titanium oxide. Naturally, TC14 is very easily hydrolyzed and so cannot be fed directly into the reactor. Otherwise, TIC14 would be quickly hydrolyzed and produce a large amount of precipitate and no nano product would be obtained. To solve this problem, TIC14 is first dissolved in an aqua solution of hydrochloric acid to obtain the aqua TiC14hydrochloric acid solution in order to keep the concentration of Ti 4+ high enough at the beginning of reaction. The make-up of the solution is carried out in a flask at low temperature. The flask is put into an ice-water bath, and is filled with a certain amount of deionized water and strong hydrochloric acid, and then, under stirring, a certain amount of TIC14 is slowly added from a funnel into the flask to yield a transparent TiC14-hydrochloric acid solution slight yellow in color. Another process solution is that of ammonium sulfate. The dissolution of (NH4)2SO4 is endothermal, and its solubility and dissolving rate are greatly affected by temperature, so that the use of hot water is needed. First put a certain amount of (NH4)2SO4, and then hot water into an ordinary beaker, quickly agitate the mixture to speed up dissolution of (NH4)2SO4, and a limpid, transparent solution is obtained. The experimental procedure is as the following: Put a certain amount of deionized water into the SCISR; turn on the propellers to push towards the water circulation and to make the opposing streams impinge against each other inside the SCISR; adjust the rotary speed of the propellers to control the impinging velocity; let the cold water passing through the heat exchanging jacket of the SCISR cool the contents in the reactor to keep the temperature inside the reactor constant at 2 to 3°(7. When both the flow status and the temperature in the reactor are stable, slowly add the titanium chloride solution into the reactor and then drip the aqua ammonium sulfate solution into the reactor. During mixing between the reactants the temperature is controlled to be higher than 15 °C" the initial concentration calculated after mixing is 1.0 to 1.2x 10-~ mol.m -~" while the mole ratios Tia+/H+- 15 and Ti4+/SO42- - 2/1. When full mixing is achieved, let the heating medium pass through the jacket of the SCISR to enhance the temperature inside the reactor to a certain level and keep it for a certain time for the
304
IMPINGING STREAMS
hydrolyzation-precipitation reaction; add strong aqua ammonia to adjust the pH to the given value. Then cool the reacted suspension to room temperature by cold water circulating through the jacket; leave for 12 h for ageing; separate the precipitate from the liquor by filter; wash the cake with deionized water to remove C1- ions (check with 0.1M AgNO3 solution), wash with alcohol another three times and then dry the washed precipitate at room temperature to yield the reaction product. Finally, calcine the dried reaction product at the given temperature for 3 h to get the nano Titania product. In the experimental investigation the influences of the following factors were examined: (1) (2) (3) (4) (5) (6)
The The The The The The
concentration of TIC14 solution and pH of the reaction mixture; impinging velocity (by adjusting the rotary speed of the propellers); reaction or mixing time and temperature; feeding time; feeding position; and keeping warm (ageing) time after reaction.
The experiments for the preparation of nano TiO2 were carried out in three stages as described below. In the first stage the major factors affecting the size of product were examined to determine the primary optimal operation conditions. In the second stage, on the basis of the determination of the primary optimal conditions, the influences of certain factors on both the particle size and the yield of titanium were examined for further optimization of the conditions. For arrangement of the experiments carried out in both the first and the second stages the normal design technique was employed. In the third stage the experiments were carried out for mass-preparation of nano Titania with commercial TIC14 as the raw material, instead of chemical reagents, and under the conditions optimized in the last stage. Also, some supplemental studies were made at this stage, including examination of the influence of the neutralization time (rate) with ammonia on particle size; while part of the experiments organized with the uniform design technique were for further optimizing the conditions and for examining the influence of calcination temperature on the particle size. Finally, the preparation experiments were also carried out with the traditional stirred tank reactor (STR) as the hydrolyzation- precipitation equipment for comparison.
15.3 RESULTS AND DISCUSSIONS 15.3.1 Major results obtained in the first stage The first stage of the investigation is aimed at the preparation of the product of smallest-sized particles, and the normal design technique is employed for arranging the experiments. The normal-designed operation conditions are listed in Table 15.1.
PREP/XRATION OF NANO TITANIA
305 Table 15.1
Normal-designed experimental conditions in the first stage Effecting factor*
Level
A
B
C**
D
1 1.0 600 I 60 2 1.1 900 II 120 3 1.2 1200 III 180 * A--concentration of TIC14, kmol.m -3" B--rotary speed of propellers, rpm; C--feeding point; D--feeding time, s ** I--inlet of drawing tube; II--outlet of drawing tube; III--center of reactor
Table 15.2
Results of normal-designed experiments in the first stage A
B
C
D
1
2
3
4
1
1
1
1
1
15.67
2
1
2
2
2
16.09
3
1
3
3
3
18.02
4
2
1
2
3
17.28
5
2
2
3
1
17.78
6
2
3
1
2
15.44
7
3
1
3
2
13.70
8
3
2
1
3
10.16
9
3
3
2
1
12.32
Ki
49.78
46.65
41.27
45.77
K2
50.5
44.03
45.69
45.23
K3
36.18
45.78
49.50
45.46
Ki
16.59
15.55
13.76
15.26
K~
16.83
14.68
15.23
15.08
K~
12.06
15.26
16.50
15.15
R
4.77
0.87
2.74
0.18
Run No
_
Average size, nm
T=KI+K2+ K3 =136.46
T/9= 15.16
306
IMPINGING STREAMS
The experiments are carried out following the procedure described in the previous section and the washed and dried reaction products are calcined at 600 °C for 3 h. The results are listed in Table 15.2, where the data for the average sizes listed in the sixth column are calculated from the X-ray spectrum; the X-ray spectrum shows that the products are of the crystalline form of anatase. The following can be seen from the data listed in Table 15.2: (1) the influencing significances of various factors tested on the average size of product are in the order of A > C > B > D; (2) the set of operation conditions yielding the smallest average size is A3B2C1D3, i.e., the initial concentration of TIC14 is 1.2 kmol.m -3, the rotary speed of the propellers N = 900 rpm, the feeding position at the inlet of the drawing tube, and the feeding time 180 s. The product prepared under these conditions is sized 10.16 nm. For more detailed results one may refer to Ref. [211 ]. The fact that the initial concentration of TIC14 most significantly affects the average size is easily understood: the higher concentration enhances the reaction rate to produce higher supersaturation resulting in nucleation in large amounts to yield a finer-sized product. However, the initial concentration of 1.2 kmol.m -3 cannot be confirmed to be optimal as it is the limit of the range tested, i.e., the highest concentration. The rotary speed of the propellers exhibits a turning influence on the average size of the product; this is similar to the results obtained by Chen et al. [165] in their investigation on an analogous problem in a stirred tank reactor and also similar to the results on the preparation of nano copper described in the previous chapter. It results from the mutual effect between macro- and micro-mixing, as mentioned before. The experimental results on the influence of feeding position are consistent with those obtained in the investigation on the preparation of "ultrafine" white carbon black, i.e., the inlet of the drawing tube is the best position and this can be considered as a general regularity for the SCISR being used for the preparation of ultrafine particles by reaction-precipitation and is consistent with the initial design idea. The fresh feed passes first through the drawing tube where essentially no mixing and, consequently, no reaction take place so that it keeps its initial composition of high reactant concentration. Such a feed stream enters the impingement zone and suddenly undergoes strong micromixing and thus a very rapid reaction, favoring the creation of a high and uniform supersaturation, thus promoting nucleation in huge quantity. The feeding time has no significant influence on the main size of the product. The major reaction producing meta-titanic acid precipitate occurs in the heated reaction mixture while the TIC14 solution is fed completely before heating of the mixture. These facts may account for the phenomena described above.
15.3.2 Experiments and major results in the second stage On the basis of primary determination of the optimal conditions made in the first stage, the experiments are carried out for further optimization of the operation conditions in the second stage of the investigation, and also the influences of the reaction
PREPARATION OF NANO TITANIA
307
temperature, the reaction (with keeping warm) time and the pH of the reaction mixture on both the average size and the yield of titanium are examined at this stage. The yield of titanium is calculated as the ratio of the amount of titanium in the product to that in the fed solution. The other fixed conditions are: the rotary speed of the propellers N 900 rpm, the initial concentration of TIC14 1.2 kmol.m -3. The normal design technique is still employed, and the experimental conditions so designed are listed in Table 15.3. T a b l e 15.3
Normal designed experimental conditions in the second stage Effecting factor* Level A
B
C
1
75
1.0
5.0
2
85
1.5
6.0
3
95
2.0
7.0
* A--temperature, °C; B - reaction (heating) time, s C--pH of the reaction mixture The procedure for the treatment of reaction product is the same as for the first stage. The results obtained are given in Table 15.4 (over page). The data listed in Table 15.4 illustrate the following: (1) the influencing significances of various factors tested on the average size of product are in the order of C > B > A; (2) the set of operation conditions yielding the smallest average size is A~B~C~, i.e., the reaction temperature 75°C, the reaction (keeping warm) time 1 h, and the pH of the reaction mixture 5.0. The operation under these conditions yields a product consisting of particles average-sized 7.33 nm, and the yield of titanium is as high as 98.3%. Obviously, both results are very good. According to the general principles of dissolution-crystallization, the temperature (Factor A) has contradictory influences on the mean size of the product. At higher temperature, the reaction goes taster to produce larger amounts of the substance to be precipitated, favoring enhanced supersaturation, on one hand; while, at the same time, the solubility of the substance under consideration may increase, yielding a negative influence on the supersaturation, on the other. The overall tendency of the mean size variation depends on the balance between the two contradictory factors. From the experimental results, the reaction at 75°C produces the product with the smallest-sized particles and also with a higher yield of Ti. However, because 75°C is the lowest of the temperatures tested, it cannot be verified whether an even lower temperature would be better and so further study may be needed. According to the results obtained in similar investigations with other reactors, the operation temperature determined for the hydrolyzation-precipitation was mostly higher than 75°C. Therefore this temperature can be considered to be, at least, reasonable and feasible.
308
IMPINGING STREAMS Table 15.4
Results of normal-designed experiments in the second stage A
B
C
1
2
3
1
1
1
2
1
3
Run No
Yield of Ti, %
Average size, nm
1
98.3
7.33
2
2
90.1
13.94
1
3
3
75.4
8.5
4
2
1
2
87.0
11.33
5
2
2
3
89.4
11.05
6
2
3
1
93.6
14.85
7
3
1
3
88.2
15.22
8
3
2
1
92.6
12.1
9
3
3
2
92.9
10.67
Size nm
Yield %
K1
9.92
11.56
11.43
K2
12.41
12.36
11.98
12.66
11.34
11.59
Rs
2.74
1.02
0.55
Kl
87.93
91.17
94.93
K2
90.01
90.70
90.00
K3
91.23
87.30
84.30
3.30
3.87
10.63
Rv
Tsize =
104.9
Tsize/9 = 11.7
Tyield -807.5 Tyield/9 = 89.72
The reaction (keeping warm) time (Factor B) exhibits little influence on the mean size of the product, but significantly affects the yield of titanium. However the direction of the influence is somewhat unexpected: the longer the reaction time, the lower is the yield of Ti. This indicates that re-dissolution of the precipitate obviously occurred, but, as yet, the phenomenon is difficult to explain exactly or reasonably and further investigation is needed. After all, the reaction mixture is very complex. The pH of the reaction mixture (Factor C) has the most significant effect on the yield of titanium, while exhibiting abnormal behavior: pH the yield of Ti decreases as pH increasing. Wu et al. [214] also observed similar phenomena in their investigation.
PREPARATION OF NANO TITANIA
309
The most possible reason is that the molecules of ammonia have contradictory influences on the nucleation. Wu et al. considered that, to an extent, the increase in the concentration of ammonia can speed up the peptizing; however, at excessively high concentration the molecules of ammonia may combine with the Ti 4+ ions to form a complex, causing H2TiO3 to be hydrolyzed resulting in a low yield of Ti. Also, pH has a certain influence on the mean size of the product: the higher the pH value, the greater is the mean size of the product. This is not difficult to understand: if the ammonia leads to hydrolyzation of H2TiO3, the finer particles must be hydrolyzed first, leaving the larger ones. Summarizing the results obtained in the first and second stages, the optimal operation conditions can be preliminarily determined as follows: the initial concentration of TIC14 1.2 kmol.m -~, the feeding position at the inlet of the drawing tube, the reaction temperature after feeding 75°C, the pH of the reaction mixture pH = 5, the rotary speed of the propellers N = 900 rpm, the reaction (keeping warm) time 1 h.
15.3.3 Experiments of mass preparation and the results In order to examine the operational conditions determined and to make the results more feasible for industrial application, and also to provide sample product for further application testing, experiments for mass-preparation are carried out under the optimal conditions determined above, following the procedure described in Section 15.2 and with commercial TIC14 as the raw material, the composition of which is indicated in Table 15.5. Table 15.5
Composition of the commercial TIC14 used
Component Content
TIC14 %
Fe ppm
Sr ppm
Ca ppm
Na ppm
K ppm
Mg ppm
96-99.9
6.76
6.61
4.73
