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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
Microchemical Engineering in Practice provides the information chemists and engineers need to evaluate the use of microreactors, covering the technical, operational, and economic considerations for various applications. It explains the systems needed to use microreactors in production and presents examples of microreactor use in different chemistries, including larger scale production processes. There are guidelines on calculating the costs and the risks of production using continuous flow microreactors. Complete with case studies, this is an essential guide for chemists and engineers interested in investigating the advantages of chemical microreactors.
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Veröffentlichungsjahr: 2011
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Contents
Cover
Half Title page
Title page
Copyright page
Preface
Chemical Engineering in A World of Severe Ecological and Economical Problems
The New Way: Sustainability
New Paradigms in Chemical Engineering
Need of Education
Aim of This Book
Contributors
Part I: Introduction
Chapter 1: Impact of Microtechnologies on Chemical Processing
1.1 Innovation: An Answer to the Challenges of Sustainable Development
1.2 Process Intensification: A New Paradigm in Chemical Engineering
1.3 Microprocessing For Process Identification
1.4 Intensified Flux of Information (R&D) vs. Intensified Flux of Material (Production)
1.5 Implementation of Microtechnologies in Chemical Processing: A Few Select Examples
1.6 Challenge of Cost Efficiency: Balance Between Capex and Opex
1.7 Perspectives
Bibliography and Other Sources
Part II: Microfluidic Methods
Chapter 2: Microreactors Constructed From Metallic Materials
2.1 Metals as Materials of Construction for Microreactors
2.2 Material Selection
2.3 Micro- and Precision Engineering Methods
2.4 Joining and Mounting Techniques
Bibliography and Other Sources
Chapter 3: Microreactors Constructed From Insulating Materials And Semiconductors
3.1 Silicon Microreactors
3.2 Glass Microreactors
Bibliography and Other Sources
Chapter 4: Micromixers
4.1 Introduction
4.2 Mixing Principles and Fluid Contacting
4.3 Technical aspects
4.4 Evaluating the Performance of A Micromixer
4.5 Multiphase Mixing
4.6 Problems and Solutions
Bibliography
Chapter 5: Microchannel Heat Exchangers And Reactors
5.1 Microchannel Heat Exchangers
5.2 Chemical Reaction and Microchannel Heat Exchangers
Bibliography
Chapter 6: Separation Units
6.1 Introduction
6.2 Membrane Separation of Gases
6.3 Absorption of Gases
6.4 Stripping of Volatile Components
6.5 Distillation of Binary Mixtures
6.6 Immiscible Phase Liquid–-Liquid Extraction
6.7 Particle Separation From Liquids
6.8 Concluding Remarks
Bibliography
Chapter 7: Calculations And Simulations
7.1 Introduction
7.2 Mechanisms and Scales
7.3 Modeling and Computation of Reacting Flows
7.4 Evaluation and Validation of Cfd Simulations
Bibliography
Part III: Peripheric Equipment
Chapter 8: Dosage Equipment
8.1 Concept and Requirements
8.2 Type of Pumps
8.3 Range of Suitability
Bibliography
Chapter 9: Micromachined Sensors for Microreactors
9.1 Introduction
9.2 Pressure Sensors
9.3 Temperature Sensors
9.4 Flow and Mass Flow Sensors
9.5 Conductometric/Amperometric Sensors
9.6 Optical Photometric and Fluorometric Sensors
9.7 Closing Remarks
Acknowledgments
Bibliography and Other Sources
Chapter 10: Automating Microprocess Systems
10.1 Automation: Why?
10.2 Sensors and Actuators for Microprocess Systems
10.3 Typical Architectures and Functionalities of Control Systems
10.4 Examples of Automated Microprocess Systems
10.5 Summary
Bibliography
Part IV: Microreaction Plants
Chapter 11: Strategies for Lab-Scale Development
11.1 Introduction
11.2 Criteria for Choosing the Correct MRT Device
11.3 Applications for MRT Plants
11.4 Further aspects of Lab-Scale Development
Acknowledgments
Bibliography
Chapter 12: Microreaction Systems for Education
12.1 Introduction
12.2 Influence of the Industrial Sector on Microreactor Education
12.3 Academic Approaches to Teaching About Microreactors
12.4 Academic Courses on Microreactors and Interdisciplinary Classes
12.5 Future Outlook
Bibliography
Chapter 13: Microreaction Systems for Large-Scale Production
13.1 Overview of Large-Scale Opportunity and Challenges
13.2 Scale-Up Considerations for Large-Scale Systems
13.3 Competing With Conventional Technology and Economy of Scale
13.4 Examples of Large-Scale Production
13.5 Future of Large-Scale Production Opportunities
Bibliography
Chapter 14: Process Intensification
14.1 Introduction
14.2 Definitions and Objectives
14.3 International State-of-the-Art
14.4 Guidelines
14.5 Case Studies
14.6 Other Intensification Principles
Bibliography
Chapter 15: Standardization in Microprocess Engineering
15.1 Introduction
15.2 The Microchemtec Standardization Initiative
15.3 Conclusion and Outlook
Bibliography
Part V: Applications
Chapter 16: Polymerization in Microfluidic Reactors
16.1 Introduction
16.2 Polymerization in A Continuous Solution Flowing Through Microchannels
16.3 Polymerization in Droplets
16.4 Conclusions
Bibliography
Chapter 17: Photoreactions
17.1 Theory
17.2 Photoreactions in Microreactors
17.3 Typical Examples of Other Research
Bibliography and Other Sources
Chapter 18: Intensification of Catalytic Process by Micro-Structured Reactors
18.1 Introduction
18.2 Characteristics of Microstructured Reactors
18.3 Microstructured Reactors for Heterogeneous Catalytic Reactions
18.4 Main Design Parameters of Catalytic Msr
18.5 Conclusions
Bibliography
Chapter 19: Microstructured Immobilized Enzyme Reactors for Biocatalysis
19.1 Introduction
19.2 Steps toward A Microstructured Immobilized Enzyme Reactor
19.3 Selected Examples From the Literature
Acknowledgments
Bibliography
Chapter 20: Multiphase Reactions
20.1 Introduction
20.2 Two-Phase Flow Regimes
20.3 Examples of Microreactors Based on Segmented Flow
20.4 Examples of Microreactors Based on Other Flow Types
20.5 Concluding Remarks
Bibliography
Index
MICROCHEMICAL ENGINEERING IN PRACTICE
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Library of Congress Cataloging-in-Publication Data:
Dietrich, Thomas R., 1963-
Microchemical engineering in practice / Thomas R. Dietrich.p. cm.Includes index.ISBN 978-0-470-23956-8 (cloth)1. Microreactors. 2. Microchemistry. I. Title.TP159.M53D54 2009660′.2832–dc222008027979
PREFACE
CHEMICAL ENGINEERING IN A WORLD OF SEVERE ECOLOGICAL AND ECONOMICAL PROBLEMS
Humanity is facing severe global problems, for example, global warming or decreasing natural resources. A typical example of this situation is the consumption of the world’s oil resources.
It took nature several million years to “produce” the oil, which we burn in a few decades. The U.S. Department of Energy reported that 93% of the U.S. energy consumption was generated from non-renewable resources (oil, coal, gas, nuclear). The other 7% were renewable energies such as wind, solar, biomass, or hydroenergy (Source: EIA, Renewable Energy Consumption and Electricity Preliminary 2007 Statistics. Table 1: US Energy Consumption by Energy Source, 2003–2007 (May 2008)). The worlds energy consumption was 2005 at 1.4 × 1014 kW/h (Source: Energy Information Administration (EIA). International Energy Annual 2005 (June–-October 2007), web site: www.eia.doe.gov/lea). It is expected to increase in 2030 to 2 × 1014 kW/h (Source: EIA, World Energy Projections Plus (2008)). It has been increasing more rapidly during the last few years due to growth in developing countries such as China and India.
In 2007 the world’s oil consumption was approximately 4 billion tons. An ESSO study estimates the world’s usable oil reserves at 181 billion tons. (ExxonMobil GmbH, Brochure “Oeldorado 2008”, http://www.esso.de/ueber_uns/info_service/pubhkationen/downloads/files/oeldorado08_de.pdf) Even without any increase in oil consumption in the future, we will run out of oil in 45 years.
To make the situation even more critical, oil is the main source for most chemical products. Pharmaceuticals, polymers, and many of our daily consumer products are made from oil. Statistics published by the German Association of the Chemical Industry (VCI: “Chemiewirtschaft in Zahlen”, 48.edition, 2006, p.98, tab.55) shows that the worldwide use of chemicals in the last 10 years doubled from 1.2 trillion in 1995 to 2.05 trillion in 2005. The chemical industry depends on oil and coal as natural resources for chemical products; therefore, it is not acceptable that most of these resources are simply “burned” to produce energy for our cars and households.
Due to these issues and to the environmental problems connected with the use of fossil resources, costs are increasing dramatically. As shown in the same statistical report (p.31, tab.12, the price of oil and other industrial resources increased from 2002 to 2005 by 120%.
THE NEW WAY: SUSTAINABILITY
The solution to these issues is described by one word: sustainability.
The report of the Brundtland Commission of the United Nations in 1987 about “Our Common Future” defined “sustainability” as a “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”. (UN Documents: “Report of the World Commission on Environment and Development: Our Common Future”. chapter I.3., paragr. 27, http://www.un-documents.net/wced-ocf.htm). Many other definitions for sustainable behavior have been published since then.
At the moment we use our natural resources, e.g. for energy production or in the chemical industry, without replacing them. Most of these resources are limited. If we work and use these resources like we do it today, they will disappear from our planet in a short period of time. We have to find processes, which do not use up resources. We have to find and work with resources, which are renewable. Only then it will be possible to protect the environment and preserve nature, so that our children and grandchildren will still have a planet where it is worth living (see also Chapter 1).
However, it is not enough only to use alternative materials and processes; it is also necessary to develop and use more efficient chemical processes to save energy and resources. In many cases conventional chemical production is not efficient enough. New technologies are needed for “Process Intensification” (see Chapter 14). One of these is microreaction technology which is described in detail in the following chapters of this book.
Microreaction technology has been studied now for more than 15 years. Starting mainly in Germany and lead by institutes, for example, the Institute of Micro-technology in Mainz and the Karlsruhe Research Center, several companies have been formed producing microfluidic modules as well as whole microreaction systems.
The main idea behind this new approach is that within microstructures all chemical and physical parameters of a reaction [e.g., mixing (see Chapter 4) and heat exchange (see Chapter 5)] can be controlled much faster and better. Due to the flexible manufacturing processes of microfluidic structures (see Chapters 2 and 3), the reactors can be designed to fit to the required chemistry–-in contrast to the conventional procedure, where the chemistry is “pressed” into a given plant. Separation procedures can be added if necessary (see Chapter 6).
It could be shown in many cases that this leads to higher selectivity and yield: the consumption of resources and solvents can be reduced; there are less byproducts and less waste; the use of energy is much more effective; and safety issues are reduced, due to the small internal volume of the microreactors.
This does not work against the interests of the chemical industry: processes with more efficiency, more yield, and more selectivity are also more cost effective. Costs for natural resources, waste deposition, and safety decrease with this technology. Moreover, a faster time-to-market gives the industry a huge additional benefit. The production can be done “on-site”, where the products are needed. Large-scale production plants and long transportation to the point-of-use can be avoided.
All this leads to much more efficient processes and makes the new chemistry “green”, helping to protect our environment. Governments all over the world have already realized the opportunities of this new technology for sustainability, safety, and environmental protection. Funding has been provided to develop the technology, especially from the German, the European and the Japanese Government. Developing countries such as China, India, or Mexico have also started developing technology.
NEW PARADIGMS IN CHEMICAL ENGINEERING
There are major differences between using a conventional reaction plant and a micro-reaction plant: using a microreactor to run a reaction under optimized conditions, the reaction parameters have to be known precisely, especially the kinetics and the thermodynamics of the reactions involved. Instead of controlling the reaction by time as in a usual batch process, they have to be controlled by the geometry of the microreactor. Therefore, one has to go through the following steps when starting a new project using microchemical engineering:
Physical and chemical data of the reactions have to be collected.Simulations can help to calculate missing parameters. Software tools are available from different suppliers (see Chapter 7).Choosing the right technology using, for example, the following criteria:Can particles be avoided or made small enough for microreactors?Is the production volume in a suitable range for microreactors?Are there issues with the yield or the selectivity in conventional plants?Is the reaction time fast enough to profit from a microfluidic system?Is a good temperature control of a microheat exchanger necessary?Is a good mixing control of a micromixer necessary?Is a good control of a dangerous reaction or dangerous components necessary?The right microfluidic modules (reactors, mixers, heat exchangers, separation modules) as well as the peripheral equipment, such as pumps (see Chapter 8) and sensors (see Chapter 9) have to be chosen. This includes the choice of material as well as the dimensions of the microfluidics.To control and optimize the reaction scheme it is necessary to have suitable control software which is available from different suppliers (see Chapter 10).After the first experiments all parameters can be corrected and a laboratory plant with all the different modules can be built and optimized (see Chapter 11).For large-scale production a scaling-up procedure and appropriate manufacturing is necessary (see Chapter 13).After the reaction has been developed, it is relatively easy to ramp-up to the production volume, either by parallelization of the microfluidic modules (“numbering-up”) or by multiplying the microstructures in a larger housing (“equaling-up”). In both cases, a production set-up can be built and installed in a much shorter time than in a conventional ramp-up procedure. This again helps saving costs and shortening the time-to-market.
NEED OF EDUCATION
Even though the advantages of this technology are well established, not many production examples are known. The main reason for this is missing know-how in the chemical industry: there are no educated staff available. Universities usually do not teach microchemical engineering and often continuous processes are not part of the curriculum.
Chemists and chemical engineers do learn the fundamentals and principles in chemistry and chemical engineering, as they did in the past. It takes a long time for new technologies to enter into standard educational programs, especially if these new ideas involve expensive equipment. Universities usually do not have enough funding for investment in new equipment. Even though they should always be at the forefront of technological advances, it is difficult to get new ideas distributed within the academic community (see Chapter 12).
Knowing this, in 2007 the German government started a process to develop lectures and practical experiments for students which should help to introduce this new approach into chemistry departments. Grants have been given for equipment to be used at universities, mainly for education, but also for internal research projects. As always, the funding was sufficient only for a small number of projects at a small number of universities. But, with their experience, which will be published and can be used by any interested person, it should be possible to influence the curricula of other universities. German equipment manufacturers profit from this program and will be able to develop and produce tools for foreign research and education institutes.
Developing countries such as India, China, or Mexico have learned from the experience of Japan and Germany and started their own programs. Mexico is going to build and renew its chemical industry, for example. It has planned to invest in the newest and most effective technology. Lead by the Technológico de Monterrey, courses on microchemical engineering will be introduced into bachelor and master studies. In parallel, cooperation with the chemical industry will bring these new ideas into practice.
It is very important to include these new technologies in the curricula of chemistry and chemical engineering schools. But it will take several years before this knowledge then reaches industry; therefore, it is necessary, to support chemists and engineers in the industry and make the new concepts available to them. This is done in different ways:
National Chemical Engineering Associations, such as DECHEMA in Germany (www.MicroChemTec.de) or AIChE in the USA (Process Development, Areas 12e) or newly founded organizations such as the MCPT in Japan (www.mcpt.jp) are organizing workshops and seminars. They are collecting information and providing help to “beginners”.Over the last 10 years the experts in this new field meet at the International Conference on Microreaction Technology (IMRET), discussing new developments and looking for partners to solve problems.There are already German, European and International Groups (ISO) working on standards in microchemical engineering. In particular, the connection of microfluidic chips from different suppliers shall be facilitated by standardized interfaces (see Chapter 15).AIM OF THIS BOOK
Much research has been done in recent years on microchemical engineering. It is not possible to summarize all the results in one book. Therefore, the aim of this book is to give an overview of the advantages and the challenges of this new technology. It also aims to provide help to new users in getting started. Information is summarized to enable the reader to decide on the right reactor material as well as for the suitable dimensions. Examples in different fields are given, for example, for polymerizations (see Chapter 16), for photoreactions (see Chapter 17), for catalytic reactions (see Chapter 18), for enzymatic reactions (see Chapter 19), or for multiphase reactions (see Chapter 20). Cited literature will be helpful to gain deeper insight into certain subjects.
It is the hope of the authors that this book will help to accelerate the introduction of this new technology into the chemistry and chemical engineering departments of universities, into research and development institutes, and into the chemical industry–-to the economical and ecological benefit of the world.
THOMAS R. DIETRICH
Mainz, March 2008
CONTRIBUTORS
JOËLLE AUBIN, Laboratoire de Génie Chimique UMR 5503 CNRS, 5, rue Paulin Talabot, BP-1301, 31106 Toulouse Cedex 1, France, E-mail: [email protected]
ALEXIS BAZZANELLA, Dechema e.V., Theodor-Heuss-Allee 25, 60486 Frankfurt, Germany, E-mail: [email protected]
DIETER BOTHE, Lehrstuhl für Mathematik CCES, Center for Computational Engineering Science, Pauwelsstr. 19 52074 Aachen, E-mail: bothe@mathcces. rwth-aachen.de
ERIC A. DAYMO, Velocys, Inc., 7950 Corporate Boulevard, Plain City, OH 43064
JAN DZIUBAN, The Wrolaw University of Technology, ul. Janiszewiskiego 11–17, 50372 Wrolaw, Poland, E-mail: [email protected]
JOHN EDWARD ANDREW SHAW, 45 Colne Avenue, West Drayton, Middlesex UB7 7AL, United Kingdom, E-mail: [email protected]
ANDREAS FREITAG (Glass), mikroglas chemtech GmbH, Galileo-Galilei-Str. 28, 55129 Mainz, Germany, E-mail: [email protected]
J. G. E. (HAN) GARDENIERS, MESA+ Institute for Nanotechnology, University of Twente, P.O.Box 217, 7500 AE Enschede, The Netherlands, E-mail: [email protected]
ASTERIOS GAVRIILIDIS, Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom, E-mail: [email protected]
MARK GEORGE KIRBY, Heatric, Division of Meggit (UK) Ltd., 46 Holton Road, Holton Heath, Poole, Dorset, BH16 6LT, United Kingdom, E-mail: [email protected]
FRANK N. HERBSTRITT, Ehrfeld Mikrotechnik BTS GmbH, Mikroforum Ring 1, D-55234 Wndelsheim, Germany, E-mail: [email protected]
TEIJIRO ICHIMURA, Department of Chemistry and Materials Science, Guraduate School of Science and Engineering, Tokyo Institute of Technology, W4-17, 2-12-1 Ohokayama, Meguro, Tokyo 152-8551, Japan, E-mail: [email protected]
JEAN-MARC COMMENGE, GPM, LSGC-ENSIC, 1 rue Grandville, BP 20451, F-54000 NANCY, France, E-mail: [email protected]
JEAN F. JENCK, ENKI Innovation, 3 chemin des Balmes, F-69110 Sainte-Foy, France, E-mail: [email protected]
ASIF KARIM, BASF AG, 67056 Ludwigshafen, Germany, E-mail: asif.karim@ basf.com
DIRK KISCHNECK, Microinnova Engineering GmbH, Reininghausstrasse 13a, 8020 Graz, E-mail: [email protected]
L. KIWI-MINSKER, Ecole Polytechnique Fédéral de Lausanne, SB-ISIC-GGRC, Station 6, CH-1015 Lausanne, Switzerland, E-mail: [email protected]
EUGENIA KUMACHEVA, Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S 3H6, Canada; Institute of Biomaterials and Biomedical Engineering, Department of Materials Science and Engineering, University of Toronto, 4 Taddle Creek Road, Toronto, Ontario, M5S 3G9, Canada; Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, Ontario M5S 3E5, Canada, E-mail: [email protected]
HAB. LAURENT FALK, GPM, LSGC-ENSIC, 1 rue Grandville, BP 20451, F-54000 NANCY, France, E-mail: [email protected]
MARCEL A. LIAUW, ITMC, RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany, E-mail: [email protected]
WOLFGANG LOTH, BASF AG, 67056 Ludwigshafen, Germany, E-mail: [email protected]
THOMAS MÜLLER-HEINZERLING, Siemens AG, Automation & Drives, Competence Center Chemical, Cement, Glass, G.-Braun-Str. 18, D-76187 Karlsruhe, Germany, E-mail: [email protected]
MICHAEL MATLOSZ, GPM, LSGC-ENSIC, 1 rue Grandville, BP 20451, F-54000 NANCY, France, E-mail: [email protected]
YOSHIHISA MATSUSHITA, Department of Chemistry and Materials Science. Guraduate School of Science and Engineering, Tokyo Institute of Technology. W4-17.2-12-1 Ohokayama, Meguro, Tokyo 152-8551, Japan
BERND NIDETZKY, Institute of Biotechnology and Biochemical Engineering, Graz University of Technology, Petersgasse 12, A-8010 Graz, Austria
BERND NIDETZKY, Institute of Biotechnology and Biochemical Engineering, Graz University of Technology, Petersgasse 12, A-8010 Graz, Austria, E-mail: bernd. [email protected]
ZHIHONG NIE, Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S 3H6, Canada
ALBERT RENKEN, Ecole Polytechnique Fédéral de Lausanne, SB-ISIC-GGRC, Station 6CH-1015 Lausanne, Switzerland, E-mail: [email protected]
SVEND RUMBOLD, Heatric, Division of Meggit (UK) Ltd., 46 Holton Road, Holton Heath, Poole, Dorset, BH16 6LT, United Kingdom, E-mail: [email protected]
KOSAKU SAKEDA, Department of Chemistry and Materials Science, Guraduate School of Science and Engineering, Tokyo Institute of Technology, W4-17, 2-12-1 Ohokayama, Meguro, Tokyo 152-8551, Japan
NORBERT SCHWESINGER (Silicium), LS TEP FG Mikrostrukturierte mechatronische SystemeTU München, Arcisstraße 21,80333 München, Germany, E-mail: [email protected]
TADASHI SUZUKI, Department of Chemistry and Materials Science, Guraduate School of Science and Engineering, Tokyo Institute of Technology, W4-17, 2-12-1 Ohokayama, Meguro, Tokyo 152-8551, Japan
MALENE S. THOMSEN, Institute of Biotechnology and Biochemical Engineering, Graz University of Technology, Petersgasse 12, A-8010 Graz, Austria
ANNA LEE Y. TONKOVICH, Velocys, Inc., 7950 Corporate Boulevard, Plain City, OH 43064, E-mail: [email protected]
DINA E. TREU, Germany, E-mail: [email protected]
CATHERINE XUEREB, Laboratoire de Génie Chimique UMR 5503 CNRS, 5, rue Paulin Talabot, BP-1301, 31106 Toulouse Cedex 1, France, E-mail: [email protected]
HONG ZHANG, Department of Chemistry, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S 3H6, Canada
PART I
INTRODUCTION
CHAPTER 1
IMPACT OF MICROTECHNOLOGIES ON CHEMICAL PROCESSING*
JEAN F. JENCK
1.1 INNOVATION: AN ANSWER TO THE CHALLENGES OF SUSTAINABLE DEVELOPMENT
Sustainability was defined in 1987 by the World Commission on Environment and Development as “a development that meets the needs of the present without compromising the ability of the future generations to meet their needs.” A classical approach is to say that development is sustainable so long as it takes care of the three P’s of people, planet, and profit.
Has much been done in this direction, having a look at the current status of the chemical industry? Published under the umbrella of the European Chemical Federation, a tutorial review “Products and Processes for a Sustainable Chemical Industry” [1] shows that industrial sustainable chemistry is not an emerging trend, but already a reality through the application of “green” chemistry and engineering expertise.
On the other side, most of the basic pieces of equipment usually operated by the chemical industry are century-old designs; for instance, stirred vessels and impellers geometry do not seem to have changed since 1554 when G. Agricola pictured them in his book Re Metallica (Fig. 1.1).
The chemical industry progressed by building large plants, due to the economy of scale: Investment costs rise less than production capacity and rough estimates generally use the following formula (“gamma rule”):
FIGURE 1.1 Process engineering in the 16th century (from Stankiewicz with permission [2]).
(1.1)
As depreciation figures tend to impact less and less standard manufacturing costs as production capacity increases, there has been a tendency to always make bigger manufacturing units.
However, “the classical world-scale plant is being phased-out,” as stated by the president of corporate engineering at BASF, and it has been suggested that chemical engineering now follow the opposite direction, as a result of these trends:
Need for a paradigm change in plant engineeringPressure of time-to-marketNeed for modular plant techniques“Microprocess engineering will have a role on plant philosophy more than on absolute size” [3]A recent statement by the board of directors at Linde was in line with these trends [4].
1.2 PROCESS INTENSIFICATION: A NEW PARADIGM IN CHEMICAL ENGINEERING
Process intensification (PI), where the motivation is “doing more with less,” is a design methodology aiming to minimize diffusion phenomena (mass and/or energy transfer). Its first goal is to build smaller, more compact, and cheaper production plants. PI started in the late 1970s when Colin Ramshaw developed the “Higee” technology at ICI. We can today bet on a future chemical plant based on modular elements to run a flexible miniplant [5–7]. The ongoing change introduced by process intensification may be depicted saying that the process, thus far, resulted from the optimization and balancing of four constraints:
Chemistry kinetics
Mass transfer
Heat transfer
Hydrodynamics
Through process intensification, transfer rates are maximized and the process is basically governed by chemical kinetics. It is no longer limited by diffusion. Fick law coefficients are maximized and global apparent kinetics closely approach intrinsic chemical kinetics [8].
Initially, the goal of the PI approach was to build “smarter” production plants relying on eco-efficient processes. Additional goals have materialized. On the one hand, running a chemical reaction in currently difficult–-if not impossible conditions–-will become possible. On the other hand, an emerging goal is to closely control the properties of products by mastering their production process. With process intensification, adapting the process to the chemical reaction becomes the leitmotiv with the following provisions:
Adapting the size of equipment to the reaction
Replacing large, expensive, inefficient equipment by smaller, more efficient, and cheaper equipment
Choosing the technology that best suits each step
Combining sometimes multiple operations in fewer pieces of equipment
To alleviate diffusion limitations, four principles of PI allow us to approach the intrinsic kinetics of phenomena:
1. Multifunctionality where unit operations are combined in a single piece of equipment
2. Alternative solvents (and even suppression of solvents) to increase the thermodynamic potential of reagents (activity and/or diffusivity)
3. Reduction of size by miniaturization, frequently using “microtechnologies” for equipment (microreactors, micromixers, microseparators), monitoring (micro-sensors), and control (microvalves)
4. Alternative energy fields: electromagnetic (microwaves, HF, photons), acoustic (ultrasounds), electric, and gravitational [9]. Beyond the process intensification strategy, some driving forces have been clearly identified:
Process safety
Continuous process replacing a batch process
On-site on-demand production
1.2.1 Process Safety
The signs given by the chemical industry to the general public are that it does not always learn from its past:
The AZF explosion that killed 32 people and injured more than 2,000 in Toulouse, France, on September 21, 2001, was a reoccurrence of the BASF accident in Oppau, Germany, exactly 80 years earlier.
In Bhopal, India, on December 3, 1984, a cloud of 41 tons of MIC rose, killing thousands. A later report showed that an inventory of 10 kg of MIC would have been sufficient to run the plant [10].
Big inventories definitely are unsafe (Fig. 1.2). Smaller inventories and in-process volumes would have significantly lowered the magnitude of these accidents. Based on this point of view, everything should be done to lower in-process amounts of material and, consequently, hazardous material inventories. In so doing, the occurrence of large-scale accidents should become less likely. The in-process volume reduction will mainly result from a philosophical change, from a batch process to continuous process.
FIGURE 1.2 Accidents in the chemical industry (from Stankiewicz with permission [11]).
1.2.2 Continuous Process Replacing a Batch Process
Reaction can be more easily controlled in extreme conditions (low temperature, high pressure, etc.) when small amounts of material have to be instantaneously handled. Therefore, rather than running the reaction in a vessel of a few cubic meters, much better process control results by running the same reaction in a continuous process where much smaller amounts of hazardous materials and energy are instantaneously involved.
So doing brings about not only safer control of the process especially when the reaction media has to be kept in cryogenic conditions, but also a more specific reaction. A well-known and documented example is the Meck KgaA vitamin H process (Fig. 1.3). Resulting from all previous comments, as well as the philosophy of miniaturization, flexibility will bring about new facility concepts.
FIGURE 1.3 Transition from batch to continuous mode.
1.2.3 On-Site On-Demand Production
Making manufacturing units smaller, we can envisage delocalized productions, close to the customer, with much lower inventories (Fig. 1.4). The “on the road” manufacturing unit is no longer a dream. Online production devices have been designed in the following cases:
FIGURE 1.4 Modular skid-mounted production unit (from Green (2005) with permission [12]).
Interox produces 1 ton per day peroxysulfuric acid (Caro acid) in a 20-cm3 tubular reactor at 1-s residence time.
Kvaerner provides modular phosgene COCl2 generators, point-of-use and skid-mounted.
Online on-demand generators have been designed for hydrogen cyanide, chlorine dioxide, ethylene oxide, etc.
Now that we have reviewed the driving forces of process intensification, one question comes to mind: What commercial venture on offer would support any process engineering intended to include process-intensified technology in the design?
1.2.4 Commercial Offer of Intensified Devices
Several types of equipment already exist, and their performance may be described in terms of their efficiency with regard to mass and heat transfers. Microreactors are special in their performances (Fig. 1.5). Intensified devices are already commercially available. A few examples appear in Figs. 1.6 through 1.9. Applications are illustrated for some of them in Fig. 1.10.
FIGURE 1.5 Transfer performances (from Fleet (2005) with permission [13]).
FIGURE 1.6 Compact heat exchanger (courtesy BHR Group).
FIGURE 1.7 Oscillating baffled reactor.
FIGURE 1.8 Spinning disk.
FIGURE 1.9 Twin shaft mixer/kneader (LIST AG).
FIGURE 1.10 Open plate reactor from Alfa-Laval for aromatic nitration.
Monolith loop reactor (MLR) technology has experienced some developments in the case of supported catalysts, enzymes, or cells. An example of the industrial development of loop reactors is that advanced by Air Product which runs hydrogenations (Figs. 1.11 and 1.12).
FIGURE 1.11 Monolith loop reactor.
FIGURE 1.12 Monolithic catalyst.
Retrofitting a stirred tank reactor to replace slurry catalysts also increases productivity by a factor of 10 to 50. “Intensive” engineering may lead to the design of many external loop reactors.
Process intensification may be considered a new approach of process engineering based on the fact that mass and heat transfers are no longer limitations and that actual kinetics are now close to intrinsic kinetics. Having stressed the fact that commercial technologies are already commercially available, we now consider the specific contribution of microprocessing to process intensification.
1.3 MICROPROCESSING FOR PROCESS IDENTIFICATION
1.3.1 Supply of Microstructured Components
Today, mierotechnologies are commercially available, thanks to world-class manufacturers and engineering firms such as IMM, Velocys, BTS Ehrfeld, Microinnova, Siemens, CPC, mikroglas, FZK, Heatric, Dai Nippon Screen, IMT, etc. This list is by no means exhaustive; a few examples are given in Fig. 1.13.
FIGURE 1.13 Microdevices.
Microchannels with characteristic dimensions between 0.1 and 1 mm enable compact operations by reducing transport distances compared to conventional technologies where tube diameters fall in the range 10 to 100 mm.
Yole Development (http://www.yole.fr) has identified 21 microtechnology suppliers worldwide. Their locations are distributed among the major economic zones according to the chart shown in Fig. 1.14.
FIGURE 1.14 Microtechnology suppliers (from Yole Development with permission [14]).
The field is constantly evolving not only in terms of materials and technologies, but also in terms of players: New companies start their activity; others stop it and reposition, raising their business targets.
1.3.2 System Integration: Selected Examples
Modularity becomes possible and opens new routes for unit engineering, as companies offer commercial systems where microelements are assembled. Here are four examples, in increasing order of integration:
Example 1: Bayer Technology Services Modules are clamped on a plate to assemble a miniplant (Fig. 1.15). With a base price of ~500 € per module, a complete miniplant costs about 50–100 k€.
FIGURE 1.15 BTS Ehrfeld microplant.
Example 2: Dainippon Screen See Fig. 1.16.
FIGURE 1.16 Dainippon microplant [15].
Example 3: Hitachi Ltd By mounting several microreactors in parallel, a pilot plant has been developed to produce up to 72 tons of chemicals per year (Fig. 1.17).
FIGURE 1.17 Hitachi microplant [16].
Example 4: Siemens Automation and Drives SiProcess is a process system for chemical syntheses, based on microprocess technology through a combination of modularity and automation (Fig. 1.18). The modules are compact and easily
FIGURE 1.18 SiProcess modules [17].
FIGURE 1.19 SiProcess flow diagram.
exchanged, and the system is designed so that end users can also insert their own components. The electronics of each module are connected to a higher-level system of automation. The system can be configured according to customer requirements using the modules shown in Fig. 1.19:
A) distributes the raw materials, as well as a solvent and a cleaning agent.
PU meters the raw materials.
RE chemical reaction (mixing and heating) occurs in a microreactor.
DL is a delay loop to complete the reaction.
PC controls pressure in the system.
SA is a sampling and quenching module.
1.4 INTENSIFIED FLUX OF INFORMATION (R&D) VS. INTENSIFIED FLUX OF MATERIAL (PRODUCTION)
The usual way to industrialize a new product or new process starts with results gathered at the laboratory scale. Critical parameters are worked out in the pilot plant. This set of information is then used to design and build the production unit. Microtechnologies are now changing the picture: The numbering-up principle is making industrialization move from the conventional scale-up to the scale-out (Fig. 1.20).
FIGURE 1.20 Scale-out (from Renken (2006) with permission [18]).
1.5 IMPLEMENTATION OF MICROTECHNOLOGIES IN CHEMICAL PROCESSING: A FEW SELECT EXAMPLES
1.5.1 Example 1
An interesting example of the pilot testing of microtechnologies has been unveiled by Degussa in association with Uhde. The objective of project Demis® was to run an epoxidation reaction to obtain propylene oxide from propylene and gaseous H2O2. See Figs. 1.21 and 1.22. Among the conclusions of this positive pilot testing, an important one was the verification of the numbering-up principle as a way to scale up the process.
FIGURE 1.21 Demis® microprocessing.
FIGURE 1.22 Demis® reactor.
1.5.2 Example 2: Fine Chemicals Plant in a “Shoe Box”
Forschungszentrum Karlsruhe (FZK) and DSM paricipated in a collaboration that led to a manufacturing “box” that is 65 cm high, weighs 290 kg, and has a 1,700 kg per hour throughput of liquid chemicals. “Micro” in its interior, as the device is made of micromixers and several ten thousands of microchannels, it can remove reaction heat of up to several 100 kW (Fig. 1.23).
FIGURE 1.23 FZK microreactor [19].
It was announced at the 2006 ACHEMA in Frankfurt, Germany, that such a device was now in permanent operation at DSM Fine Chemicals in Linz, Austria. It was validated after a 10-week demonstration showing that 300 tons of high-value product could be manufactured with a better yield than with the conventional process and improved process safety conditions.
1.5.3 Examples 3 and 4: Radical Polymerizations
Idemitsu Kosan claims that its polymerization pilot (with a size of 3.5 × 0.9 m) produces up to 10 tons per year (Fig. 1.24). It is still unclear which type of radical polymerization is handled in these microtechnologies. Figure 1.25 provides images of reaction units made up of microchannels. “Has the plant been scaled up at the industrial level?” remains an open question, but this pilot plant proves that micro-technologies can handle viscous flows, as already demonstrated previously by Siemens-Axiva in its Corapol process (Fig. 1.26).
FIGURE 1.24 Polymerization microplant in Japan [20].
FIGURE 1.25 Microchannels for polymerization.
FIGURE 1.26 Polymerization in microreactor [21].
1.5.4 Example 5: Nitroglycerin Microstructured Pilot Plant
In May 2005 Xian Chemicals started nitroglycerin (NG) production on a pilot plant level (15 kg/h NG, >100L/h) in China (Fig. 1.27). Xian Chemicals invested ~5 M € in a facility, developed by IMM in Mainz, Germany [22–23].
FIGURE 1.27 Trinitroglycerin synthesis.
The finished material is used as medicine for acute cardiac infraction. Thanks to a microengineering philosophy in implementation:
The product quality is at its highest grade.The plant operates safely and is fully automated.Environmental protection is ensured by advanced wastewater treatment and a closed cycle.The Xian nitroglycerin microplant team appears in Fig. 1.28.
FIGURE 1.28 Xian nitroglycerin microplant team [24].
1.5.5 Example 6: Pharmaceutical Chemistry
Cellular process chemistry (CPC) is the basis of the Cytos® pilot system; it includes 10 (+1 spare) microreactors for a cost of ~1.2 M€. The microreactor capacity follows the user requirements on a contract basis.
FIGURE 1.29 CPC Cytos® microplant [25].
As an example, a commercial, large-scale, multiproduct plant near Leipzig, Germany, has been running since mid-2006, producing high added-value chemicals (niche applications for pharmaceuticals) with a range from 1 to 100 kg. Synthacon has started production of a multipurpose unit with 20 tons per year capacity.
Sigma-Aldrich has installed a standard Cytos® in Buchs, Switzerland. Many of Sigma-Aldrich’s catalog products are produced under typical lab conditions in flasks up to 20 L. Out of the 2,000 compounds in this portfolio, about 800 could be produced in microreactors with little or no process modification. See Fig. 1.29.
1.6 CHALLENGE OF COST EFFICIENCY: BALANCE BETWEEN CAPEX AND OPEX
Based on the known examples of microtechnology implementations, the University of Eindhoven in the Netherlands has arrived at an interesting synthetic view of the field (Fig. 1.30). Microtechnologies have a rather high investment cost, at least as long, as there is no mass production of microdevices. How is the Capex cost offset by savings on operating costs, Opex?
1.6.1 Aniline by Hydrogenation of Nitrobenzene
Aniline is produced in a highly exothermic process. Following current practice, it is run in tubular fixed beds. This brings up several issues: poor performances, renewal
FIGURE 1.30 Process and plant engineering (courtesy Professor V. Hessel).
of catalyst that requires the operator to unload the old and reload the new, frequent catalyst regenerations. The microreactor technology (MRT) solution works:
Has an immobilized catalyst
Involves a lower hydrogen recycle rate
Avoids by-product formation because of better temperature control
Eliminates any previously necessary catalyst unloading and loading
Thus significantly lowers downtime
A cost analysis, conducted by CMD International in 2002 for a 50 kt per year unit, led to savings of about 200 kUS€ per year in favor of the MRT option. Would 5 US€ per t be a high enough reduction of the manufacturing costs to vindicate the MRT investment (and risk)?
1.6.2 Direct Hydrogen Peroxide
UOP arrived at a basic engineering quotation for a 160 kt per year plant operating with microstructures that showed operation in the explosive regime at low pressure is the least capital-intensive (Fig. 1.31). Explosive conditions are also those that guarantee significantly lower variable costs (Fig. 1.32).
FIGURE 1.31 Global cost of hydrogen peroxide plant [26].
FIGURE 1.32 UOP hydrogen peroxide (copyright UOP LLC; all rights reserved).
1.6.3 Fine Chemicals
Lonza published the results of their detailed analysis of different type of reactions (Fig: 1.33). Type A are very fast and mixing-controlled; type B are rapid and kinetically controlled; type C are slow, but with a safety or quality issue. Of 86 reaction campaigns carried out at Lonza, 50% could benefit from a continuous process.
FIGURE 1.33 Typology of reactions [27].
Concerning capital expenditures, microreactor costs are as high as or higher than traditional technology costs. However, this cost is compensated by high operating savings when the reaction is run continuously. As raw material costs contribute to 30 to 80% of the total manufacturing cost, higher product yield and quality may have a significant impact. In addition, an automated process reduces QA/QC and labor costs.
FIGURE 1.34 Operational costs.
1.6.4 Microreactor Process Cost Incentives
An assessment of microreactor process operation costs was made at the University of Iena, Germany based on the following:
Rough cost estimate grounded in a few similar estimates and a real case
Production scenarios in the field of fine chemicals, averaged to one plot
Only consideration of the reaction to the crude product with no purification [28]
The study’s conclusions are clearly depicted in the following charts: operation costs as shown in Fig. 1.34 and capital expenditures as shown in Fig. 1.35.
FIGURE 1.35 Capital expenditures.
Other available examples also stress the fact that run in inappropriate conditions, microtechnologies may be more expensive than regular technologies. However, micro-reactors open the way to such conditions that reaction selectivity may be significantly improved, making the operation cost drastically lower. Another defining example is the switch from batch to continuous operation to avoid cryogenic reaction conditions.
Great care must be taken to use the optimized performances of the microreactors in assessing their operating costs. Other than that, capital expenditures do not seem to be very different from regular process investment costs.
1.7 PERSPECTIVES
1.7.1 Opportunities for Microprocessing in Intensified Formulation
For fine chemicals and advanced materials, microprocessing implemented as part of product engineering will yield new properties because of the narrower distribution of molecular weight, particle diameter, etc. The challenge is to ensure a flow regime with as low an axial dispersion as possible. Some examples of these trends were presented at the 2006 AIChE Spring Meeting:
Organic nano particles (Paper No. 98a by Fuji)
Ultrafine powders (Paper No. 84a by EPFL and TechPowder, and Paper No. 98e by Microinnova)
Block copolymers (Paper No. 62d by the University of Strasbourg)
Complex emulsions (Paper No. 140g by Unilever)
Those in the field of emulsion and L-L dispersion are particularly concerned about microtechnologies because of their intrinsic advantages over conventional techniques:
Higher energy efficiency, with therefore milder operating conditions
Narrower droplet size distribution
Controllability of droplet size
Velocys is developing a micromembrane emulsification technology (Figs. 1.36 and 1.37). IMM is using micromixers for cream synthesis (Fig. 1.38). In solid formulation, significantly narrower distributions of particle sizes can also be obtained. The efficiency of segmented flow microreactors is illustrated here: For copper oxalate in Fig. 1.39 and for, mono-disperse silica nanoparticles in Fig. 1.40.
FIGURE 1.36 Velocys microchannel emulsification.
FIGURE 1.37 Emulsion microprocessing unit.
FIGURE 1.38 Microprocessing for cosmetics.
FIGURE 1.39 Segmented flow tubular reactor [29].
FIGURE 1.40 Microchannels for nanoparticles [30].
1.7.2 Quantitative Assessment of Eco-Efficiency
Current studies focus on the much needed life-cycle assessment and positive impacts of microtechnology operations. A reaction operated in a microreactor process and eco-efficient is shown in Fig. 1.41. The LCA clearly indicates significant ecological advantages achieved with the continuous synthesis in the microreactor (Cytos® Lab System) vs. the macro-scale discontinuous batch process (10-L double-wall reactor).
FIGURE 1.41 Synthesis of m-methoxy-benzaldehyde [31–32].
1.7.3 Market Segmentation of Microprocessing
Yole Development (http://www.yole.fr) see two different routes of development for microprocessing:
In Chemicals
New technology to develop and produce very high-quality molecules in fine chemicals
High-end differentiation of some chemical companies in a competitive environment, in which China continues its leading position in some market segments
Higher production yield and enhanced safety conditions
With MRT advancing it, the change from a batch to continuous flow process
In Pharmaceuticals (same drivers as for chemicals, but with some specificity)New reaction conditions leading to new drugs
Increase in drug development pipeline profitability: More molecules will go through toxicological testing (Phases I to IV)
To conclude, we note these trends in the perspectives for microtechnologies:
Technology validation is still to be assessed.The need for a better understanding of financial benefits continues to exist.It is clear today that microtechnology is decisive for sustainable chemistry, as new routes are reconsidered. The approach is also emerging in the pharmaceutical industry, bringing benefit to the product development process. It also turns out to be useful for on-site applications (cosmetics, drugs, and testing). Whatever development route we consider, engineering methodologies and holistic approaches will be required.
BIBLIOGRAPHY AND OTHER SOURCES
1. J. Jenck, F. Agterberg, and M. Droesher, Green Chem., 6 (2004), 544–556.
2. Image courtesy A. Stankiewicz, TU Delft, July 2003.
3. Dr. S. Deibel, CHE Manager, 2 (2006).
4. Dr. A. Belloni, Process, 13(4) (2006), 64.
5. C. Ramshaw, “Process Intensification and Green Chemistry,” Green Chem., 1 (1999), G15–G17.
6. A. I. Stankiewicz and J. A. Moulijn, “Process Intensification: Transforming Chemical Engineering,” Chem. Eng. Prog., 1 (2000), 22–34.
7. R. Jachuck, “Process Intensification for Responsive Processing,” Trans. IChemE, 80, Part A (April 2002).
8. R. Bakker, Reengineering the Chemical Processing Plant, Marcel Dekker, New York, 2003.
9. R. Jashuck and J. F. Jenck, AIChE Process Development Symposium, Palm Springs, CA, June 12, 2006.
10. D. Hendershot, CEP, 2000.
11. Image courtesy A. Stankiewicz, TU Delft, July 2003.
12. Image courtesy A. Green, BHR Group, Sept. 2005.
13. Image courtesy S. Fleet, BRITEST, March 2005.
14. Image courtesy Yole Development, Lyon, France.
15. Photos in Chemical & Engineering News, Dec. 18, 2006, p. 38.
16. 71st Annual Meeting SCE, Tokyo, Japan, March 28–29, 2006. http://www.hqrd.hitachi.co.jp/global/news_pdf_e/mer1060327nrde_microreactor.pdf.
17. http://www.siemens.com/siprocess.
18. Image courtesy A. Renken, EPF, Lausanne, Switzerland, Feb. 10, 2006.
19. http://www.fzk.de/idcplg?IdcService=FZK&node=1298&documentID_050873.
20. Proc. IMRET, 8, Atlanta, GA, April 2005.
21. T. Bayer, D. Pysall, and O. Wachsen, Proc. IMRET, 3, 2000.
22. http://www.imm-mainz.de/upload/dateien/PR%2020050405e.pdf?PHPSESSID=e8b7ef919907a581959f42cc890a8511.
23. Chemie Ingenieur Technik, 5 (May 2005), 77.
24. Chemical & Engineering News (May 2005>), cover story.
25. http://www.cpc-net.com/cytosls.shtml.
26. P. Pennemann, V. Hessel, and H. Löwe, Chem. Eng. S., 59 (2004), 4789–4794.
27. D. M. Roberge, L. Ducry, N. Breler, P. Cretton, and B. Zimmerman, Chem. Eng. Tech., 28(3) (2005), 318–323.
28. U. Krtschil, V. Hessel, D. Kralisch, G. Kreisel, M. Küpper, and R. Schenk, “Cost Analysis of a Commercial Manufacturing Process of a Fine Chemical Using MicroProcess Engineering,” CHIMIA, 60(9) (2006).
29. http://ltp.epfl.ch/page17388.html.
30. S. A. Khan, et al., Langmuir, 20 (2004), 8604–8611.
31. D. Kralisch and K. Kreisel, Chem. Ing. Tech., 77(6) (2005), 62–69.
32. D. Kralisch and G. Kreisel, AIChE Spring Meeting 2006, Paper No. 23g.
* Adapted from a lecture given by Jenck and coworkers at the Spring 2006 AIChE Meeting, Topical T1 Applications of Microreactor Engineering, Paper No. 23a, Orlando, Florida, April 24, 2006.
PART II
MICROFLUIDIC METHODS
CHAPTER 2
MICROREACTORS CONSTRUCTED FROM METALLIC MATERIALS
FRANK N. HERBSTRITT
2.1 METALS AS MATERIALS OF CONSTRUCTION FOR MICROREACTORS
Aside from frequently scientific lab applications, where glass, semiconductor materials or plastics often play a domineering role (e.g., “Lab-on-a-Chip”), metal is probably the most important and most widely used category of materials of construction for components used in the field of microprocess engineering. This is not just attributable to designers’ general familiarity with these materials. Unlike any other category of materials, metals–-of course, also including a large variety of metallic alloys–-combine a number of properties that are necessary in the construction of (micro) mechanical equipment. They are as follows:
Best All-round Workability No other category of materials can be processed in as many ways as metals: Cutting techniques, electrical discharge machining, reducing and separating laser processes as well as form etching techniques allow precise shaping with great geometric latitude over a broad range down to the micrometer level. Stamping, casting, and forming processes make it possible to economically produce even complex building elements in large numbers. With galvanic, gas-phase, or vacuum-based deposition techniques, manifold functional coatings can be produced and, in combination with lithographic structuring techniques (LIGA), highly precise microstructures may be generated. Finally, a broad array of welding, soldering, and diffusion joining techniques make it possible to create high-strength inter-metallic bonds whose thermal and chemical stability often approach that of the base material.
High Mechanical Strength In many cases, glass and ceramic materials outperform metals with regard to tensile and compression strength. However, particularly where safety-relevant applications are concerned but also in terms of general use, the yield point or breaking elongation of a material is also of crucial importance for its applicability. Metals and especially metal alloys are highly ductile and elastic. They are therefore as equally well suited for the construction of heavy-duty pressure vessels as for the construction of filigree-type microstructure elements that must on a regular basis withstand robust handling (e.g., during cleaning operations) in laboratories, pilot plants, or production units.
High Thermal Stability With regard to stability, most metallic materials relevant for process engineering applications (e.g., stainless steel, nickel-based alloys, or titanium) can without major restrictions be used under conditions exposing them to cryogenic temperatures up to about 400 to 700°C. Heat-resistant and high-temperature steels are available for applications that expose them to temperatures up to approx. 800 to 1,100°C, as far as corrosion stress is strictly limited. In those instances where more severe corrosion attack must be expected, refractory metals (particularly zirconium, tantalum, tungsten, and molybdenum) and their alloys cover a temperature range that may–-with certain reservations regarding stability–-clearly exceed 1,500°C. Unlike many brittle-rigid materials, such as most types of glass and many ceramics, nearly all metallic materials are highly resistant to temperature change. However, it must be considered, particularly when the sizing of safety-relevant design elements is concerned, that many metals are less ductile when exposed to low temperatures and have diminishing yield points as well as increasing corrosion sensitivity when exposed to high temperatures.
Good Chemical Resistance Aside from a few precious metals, whose prices alone preclude in most cases their use in the manufacture of microstructure elements, none of the metallic materials are as chemical-resistant as fluorine polymers, most types of glass, and many ceramics. However, metallic materials of construction as a whole cover virtually the complete range of relevant applications in the chemical industry. Indeed, careful material selection is particularly important in this area.
2.2 MATERIAL SELECTION
With regard to the requirements a material of construction must meet, microprocess engineering is at first glance not basically different from classic process engineering. Function-relevant criteria, such as corrosion resistance, mechanical strength, and the temperature range in which a material can be used, must be considered to the same extent as the more economically important criteria of price, availability, and workability. Depending on the intended application, additional criteria, for example, heat conductivity, electrical or magnetic properties, wetability or bio-compatibility, may need to be examined as well. No single material fulfills all these criteria to a completely ideal extent (Fig. 2.1). For example, corrosion resistance to a certain medium may only be attainable with a particularly expensive or poorly workable material or certain component geometries may only be achievable by certain manufacturing methods to which, in turn, only a few materials lend themselves. But during the course of the history of chemical process technology, a broad and versatile assortment
FIGURE 2.1 Comparative evaluation of metallic materials of construction with regard to criteria important for the construction of microreactors.
of metallic materials has evolved, with each of them covering a specific range of applications and many of them preferably used also for the construction of microprocessing elements.
Selecting one material from this assortment will therefore in most cases require a compromise–-in the instance of the construction of conventional equipment as well as for the construction of microreactors. However, individual criteria will sometimes be evaluated differently for the construction of a microreactor than, for example, the construction of an agitated tank with a volume of several cubic meters. The price and strength of a material will in most cases play a lesser role if it is to be used to build a microreactor as compared to the construction of an agitated tank, because already in principle considerably less material is needed for the microreactor and any additional material necessary to attain some increased pressure resistance will only be of minor further impact on the device’s cost. On the other hand, considerably stricter standards regarding corrosion resistance and workability of a material must in many cases be applied in the field of microprocess engineering than in the field of classical unit construction.
Although the material erosion of several 0.01 mm per year is acceptable on a conventional pressure vessel or pipeline for most applications, this rate of erosion would make many microstructure elements unusable within a few weeks or months. Moreover, in microreactor design, the use of corrosion-protective coatings, as it is practiced in classical unit construction, is normally not possible because, to assure durable protection, the thickness of these coatings often reaches the order of magnitude of the characteristic dimensions of microstructures. Even more so than in classical unit design, the corrosion resistance of a material in microprocess engineering is therefore in many cases the highest-ranking KO criterion for its use. The other criteria listed may have to be subordinated. This applies especially to the price of a material, albeit within certain limits. The extremely widely resistant precious metals, for example, gold or platinum and their alloys (which will resist considerably higher mechanical stress), are normally too expensive even for the construction of microstructure components.
As compared to these metals, which are actually only attacked by very few substances, the corrosion resistance of practically all technically and economically relevant materials for equipment in the area of process technology is based on the formation of a dense, stable corrosion layer (passive layer, generally oxidic), which protects the material under it from further chemical destruction. Therefore, corrosive attack on these materials is in most cases accompanied by damage to this passive layer, which may, for example, be caused by oxidizing or reducing effects, attack by acids or bases, or the formation of complex compounds that are soluble in the attacking medium. Metallic materials of construction that are protected by passivation therefore always have a more or less limited range of resistance and are sometimes preferably attacked by a whole category of media (pH value, oxidation potential, presence of certain ions).
Table 2.1 gives an overview of a number of metallic materials of construction that are of particular interest for microreactor design [1]. They are briefly introduced in the following paragraphs.
Austenitic stainless steels (e.g., AISI 316Ti) cover a solid basic spectrum of process engineering applications, particularly in the foodstuffs and pharmaceutical fields as well as in chemical applications with redox-neutral to slightly oxidizing and slightly acidic to basic organic and aqueous solutions. They are not resistant to many concentrated acids, reducing media, and halides, particularly chlorides, which cause increasing hole corrosion [2]. These steels are preferred for the manufacture of (especially modular) microreactor systems for laboratories and experimental plants with broader application spectra because of their low prices and–-compared to other corrosion-resistant materials–-good workability by all precision engineering and microprocess engineering methods (except LIGA) as well as by a large number of welding techniques.
In addition to nickel, Hastelloy® Alloy C-276 (registered trademark of Haynes International, Inc.) contains chrome and molybdenum and small amounts of tungsten and iron. It belongs to the broad category of nickel-based materials with higher–-in some cases, mutually complementary–-resistance to a wider spectrum of media, particularly chloride-containing and slightly reducing substances, in comparison to stainless steel. Alloy C-276 covers practically the entire resistance spectrum of A4 stainless steels and significantly expands it to include halogens (except fluorine and, conditionally, chlorine), halogenides, some mineral acids, and reducing aqueous media. Like most nickel-based materials of construction, Alloy C-276 has a strong tendency to work-harden (see hardness values in Table 2.1). Although, on the one hand, this benefits robustness, particularly of thin-walled construction components,
TABLE 2.1 Comparison of the Physical Properties of Metals of Particular Importance for Microreactor Construction
it makes processing, especially by machining, markedly more difficult. On the other hand, microstructuring by electrical discharge machining or laser cutting of this material is possible with practically equal precision as stainless steel, albeit at somewhat greater expense. Aside from the price of the material itself, the processing costs usually make these components about 1.5 to 3 times more expensive than those made of stainless steel. Microstructuring by form etching, however, is not possible, at least not by methods that are generally commercially available.
Monel® Alloy 400 (registered trademark of Special Metals Corp.) has a somewhat broader chemical resistance than pure nickel, but is not by far as chemical-resistant as Hastelloy C-276. However, compared to nickel, it is somewhat less expensive, can be used throughout a somewhat larger temperature range, and, due to work-hardening, can reach clearly higher strength values (especially 0.2% yield point; see Table 2.1). Compared to stainless steels, it is considerably more resistant to chloride-containing media and has better heat conductivity. Among other applications, this makes the material interesting for the construction of heat exchangers. Its workability is similar to that of Hastelloy C, with the exception that microstructuring by form etching is possible with Monel.
Nickel, commercially available at 99.2% purity as Alloy 200 or, with reduced carbon content, as Alloy 201, has a still narrower overall chemical resistance spectrum than stainless steel. However, unlike stainless steel, it is resistant to chlorine and hydrogen chloride (including wet hydrogen chloride; Alloy 201 is resistant to dry gases at temperatures up to 550°C), chlorides, fluorides, and etching alkalis (in concentrated solutions up to the melting point). Moreover, the heat conductivity of nickel as a pure metal is higher than that of the alloys at still relatively good strength values. The workability of nickel is similar to that of Monel. With regard to the LIGA process, nickel and some of its alloys are especially important in the field of microprocess engineering because of their processability by galvanic deposition techniques.
Titanium and its alloys have a similarly broad chemical resistance spectrum as Hastelloy C-276. However, the resistance properties of the two materials are in certain ways complementary. Titanium materials, in particular, are resistant to nitric acid and various soda solutions at practically all concentrations that would lead to corrosion attack on Hastelloy C, whereas the nickel-based alloy is the superior material when it comes to resistance to halogens. The mechanical strength of titanium materials is to a considerably higher degree dependent on the concentration of commonly present contaminants (e.g., Fe, O, N, C, or H) in the metal than that of the alloyed materials discussed here. This is the reason for differentiating among four grades even of the unalloyed material. In addition, numerous titanium alloys are technically relevant, of which especially those with an addition of a few tenths of a percent of palladium are particularly important for chemical applications because of their improved corrosion resistance. Although problematic reshaping properties as well as the elaborate and expensive welding requirements of titanium (welding is possible only in an inert atmosphere) impose narrow limits on its use in classical equipment construction, its use in microprocess engineering has so far been more limited by its relatively high price and poor availability as well as the difficult machining characteristics of the material.
2.3 MICRO- AND PRECISION ENGINEERING METHODS
