Microreaction technology industrial prospects IMRET 3 proceedings of the third international conference on microreaction technology

Preface Since the First International Conference on Microreaction Technology, held in Frankfurt I M., Germany in spring 1997, an extremely rapid development in fabrication and use of mic

Microreaction Technology: Industrial Prospects

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W Ehrfeld (Ed.)

Microreaction Technology: Industrial Prospects

Institut flir Mikrotechnik Mainz GmbH

Carl-Zeiss-StraBe 18-20

55129 Mainz

Germany

ISBN-13: 978-3-642-64104-6

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Microreaction technology: industrial prospects; proceedings ofthe Third International

Conference on Microreaction Technoloy 1 IMRET 3 W Ehrfeld (ed.) - Berlin; Heidelberg; New York; Barcelona; Hong Kong; London; Milan; Paris; Singapore; Tokyo:

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Preface

Since the First International Conference on Microreaction Technology, held in Frankfurt I M., Germany in spring 1997, an extremely rapid development in fabrication and use of microreactors for a large variety of processes took place Within only a few years time, the concepts of microreaction technology have found worldwide attention both in industry and basic R&D Various applications

in process engineering, screening purposes and basic research have been identified and investigated thoroughly In many cases, microreaction systems already have left the state of lab-scale R&D, finding their way to industrial processing and commercialization

Consequently, main focus of this years IMRET 3, again held in Frankfurt I M

on April 18 - 21, 1999, turned out to be on industrial prospects of this new nology and its neighboring fields This symposium, like its 1998 predecessor, held

tech-in New Orleans, LA, was organized by Battelle Memorial Institute, the American Institute of Chemical Engineers (AIChE), the Society for Chemical Engineering and Biotechnology (DECHEMA) and the Institute of Microtechnology Mainz (lMM) The conference, featuring 84 contributions, was a successful forum for discussion of recent developments in design and fabrication of microreactors as well as applications in chemical production, combinatorial chemistry, catalysis, biotechnology and power generation

The lectures and discussions once more showed a broad range of opinions regarding microreaction technology These ranged from general scepticism concerning applicability of microreactors to recommendation of their use as tool for process optimization and convincing statements with respect to their utilization in industry However, the first examples for successful industrial exploitation of microreaction systems seem to have a positive effect on the dissemination of microreactor concepts These pioneering activities indicate the beginning of a new era in chemistry and reaction engineering characterized by increasing flexibility in production and R&D

The book at hand comprises the full papers of the contributions to IMRET 3, giving a comprehensive overview on the present state-of-the-art in microreaction technology We thank all contributors to this conference and hope, this book will prove to be a valuable and useful guide to scientists and researchers in this exciting new field in reaction engineering, chemistry and biotechnology

September 1999

R.S Wegeng, M.K Drost, D.L Brenchley

W Ehrfeld, V Hessel, S Kiesewalter, H LOwe, Th Richter, J Schiewe

Part 2

Design and Production of Microreactor Systems

Lecture Session

Fabrication and Application of Silicon-based Microchannels 36

J.G.E Gardeniers, R W Tjerkstra, A van den Berg

Macroporous and Mesoporous Silicon: New Materials for Microfluidic and Microreaction Devices 45

V Lehmann

The Effect of Shape Variation in Microlamination on the Performance of

High-Aspect-Ratio, Metal Microchannel Arrays 53

B.K Paul, R.B Peterson, W Wattanutchariya

Fabrication of Microchannel Chemical Reactors Using a Metal Lamination Process 62

D W Matson, P.M Martin, D C Stewart A.L Y Tonkovich, M White,

J.L Zilka, G.L Roberts

Poster Session

Microreactor Technology for Biological Applications 72

G.M Greenway, T McCreedy

VIII

Laserprocessing for Manufacturing Microfluidic Devices 80

E Bremus, A Giltner, D Hellrung, H Hocker, F Legewie, R Poprawe,

M Wehner, M Wild

Functional Coatings for Microstructure Reactors and Heat Exchangers 90

M Fichtner, W Benzinger, K Haas-Santo, R Wunsch, K Schubert

Polymer Nanowell Plates with Variable Well Slope Angles 102

H Becker, T Klotzbucher

Micro Molding of Fluidic Devices for Biochemical Applications 113

M Niggemann, W Ehrfeld, L Weber

Part 3

Microreactors in Combinatorial Chemistry

Lecture Session

Combinatorial Organic Compound Libraries on Continuous Surfaces:

Towards Chemical Chips 124

L Germeroth, U Reineke, K Dietmeier, C Piossek, N Heine, D Scharn,

T Ast, M Schulz, H Matuschewski, A Kramer, J Schneider-Mergener,

A Ziogas, H Lowe, M Kupper, W Ehrfeld

High Temperature HCN Generation in an Integrated Microreaction System 151

V Hessel, W Ehrfeld, K Golbig, C Hofmann, St Jungwirth, H Lowe,

Th Richter, M Storz, A Wolf, O Worz, 1 Breysse

Micro Mixing Effects in Continuous Radical Polymerization 165

T Bayer, D Pysalt, O Wachsen

Novel Liquid Phase Microreactors for Safe Production of Hazardous

Specialty Chemicals 171

T.M Floyd, M W Losey, S.L Firebaugh, KF Jensen, M.A Schmidt

Experiences with the Use of Microreactors in Organic Synthesis 181

H Krummradt, V Koop, 1 Stoldt

A Microstructured Reactor for the Catalytic Partial Oxidation of Methane to Syngas 187

J Mayer, M Fichtner, D Wolf, K Schubert

Expansion of Microreactor Capabilities through Improved Thermal

Management and Catalyst Deposition 197

A.J Franz, S.K Ajmera, S.L Firebaugh, K.F Jensen, MA Schmidt

Synthesis of Ethylene Oxide in a Microreaction System 207

H Kestenbaum, A Lange de Oliveira, W Schmidt, F Schuth, W Ehifeld,

K Gebauer, H Lowe, Th Richter

Selective Reactions in Microchannel Reactors 213

A Kursawe, E Dietzsch, S Kah, D Honicke, M Fichtner, K Schubert,

G WieJ3meier

Periodic Operation in Microchannel Reactors 224

MA Liauw, M Baerns, R Broucek, O V Buyevskaya, 1.-M Commenge,

1.-P Corriou, L Falk, K Gebauer, H.1 Hefter, D.-V Langer, H Lowe,

M Matlosz, A Renken, A Rouge, R Schenk, N Steinfeldt, St Walter

Poster Session

Micro-Reactor Synthesis: Synthesis of Cyanobiphenyls Using a Modified

Suzuki Coupling of an Aryl Halide and Aryl Boronic Acid 235

V Skelton, G.M Greenway, SJ Haswell, P Sty ring, D.O Morgan

Optimization of Reaction-Separation Networks via Mass Integration on the /l-Scale 243

F Stepanek, M Marek

Modelling of Gas-Liquid Catalytic Reactions in Microchannels 253

P Angeli, D Gobby, A Gavriilidis

Simultaneous Screening of Catalysts in Microchannels: Methodology and Experimental Setup 260

T Zech, D Honicke, A Lohf, K Golhig, Th Richter

Palladium Membrane Microreactors 267

A.J Franz, K.F Jensen, MA Schmidt

A Micro Packed-Bed Reactor for Chemical Synthesis 277

M W Losey, M.A Schmidt, K.F Jensen

x

Parallel Synthesis and Testing of Heterogeneous Catalysts 287

U Rodemerck, P Ignaszewski, M Lucas, P Claus, M Baerns

Single Flow Electrochemical Microreactor Application to Furan

Microporous Silica Structures for the Immobilisation of Catalysts and

Enhancement of Electroosmotic Flow (EOF) in Micro-Reactors 346

N.G Wilson, T McCreedy

A New Hydraulic Stroke Amplifier for Microfluidic Components 353

N Schwesinger, S Pobering

Part 6

Microreactors for Energy Generation and Storage

Lecture Session

Microchannel Chemical Reactors for Fuel Processing Applications

II Compact Fuel Vaporization 364

AL Y Tonkovich, S.P Fitzgerald, JL Zilka, M.J LaMont, Y Wang,

D.P VanderWiel, R.S Wegeng

Microstructured Catalysts for Methanol-Steam Reforming 372

P Pfeifer, M Fichtner, K Schubert, M.A Liauw, G Emig

Fuel Cells for Low Power Applications 383

C Hebling, A Heinzel, D Golombowski, T Meyer, M Muller, M Zedda

Poster Session

Recent Developments in Microtechnology-Based Chemical Heat Pumps 394

M.K Drost, M Friedrich, C Martin, J Martin, R.J Cameron

Miniaturised Direct Methanol Fuel Cell with a Plasma Polymerised

Rapid PCR in Flow-Through Si Chip Thermocyc\ers 410

S Poser, R Ehricht, T Schulz, S Uebel, U Dillner, J M Kohler

Disposable Electrophoresis Chip for High Throughput Analysis of

Biomolecules 420

R Konrad, W Ehrfeld, H.-J Hartmann, P Jacob, M Neumann,

R Pommersheim, I Sommer, J Wolfrum

A Micromachined Analysis System for Rapid Protein Identification 430

S Ekstrom, P Onneifjord, M Bengtsson, J Nilsson, T Laurel!,

G Marko-Varga

XII

Simulation of Biochemical Reaction Kinetics in Microfluidic Systems 441

V.B Makhijani, 1 Raghavan, A Przekwas, A.1 Przekwas

Microfluidic Devices on Polymer Substrates for Bioanalytical Applications 451

1' Lin, D W Matson, D.E Kurath, 1 Wen, F Xiang, WD Bennett,

1 Lerchner, A Wolf, A Weber, R Hiittl, G Wolf, 1.M Kohler, M Zieren

Isothermal Biochemical Amplification in Miniaturized Reactors with

Integrated Microvalves 479

R Brautigam, D Steen, R Ehricht, 1.S McCaskill

Microfluidic Filtration Chip for DNA Extraction and Concentration 488

A Przekwas, D Wang, V.B Makhijani, A.1 Przekwas

A Capillary Force Filled Auto-Mixing Device 506

R U Seidel, D.1' Sim, W Menz, M Esashi

Fast Heating and Cooling for High Temperature Chemical Microreactors 514

Ch Alepee, R Maurer, L Paratte, L Vulpescu, Ph Renaud, A Renken

Gas / Liquid Microreactors for Direct Fluorination of Aromatic Compounds Using Elemental Fluorine 526

V Hessel, W Ehifeld, K Golbig, V Haverkamp, H Lowe, M Storz,

Ch Wille, A.E Guber, K liihnisch, M Baerns

Solvent Extraction and Gas Absorption Using Microchannel Contactors 541

WE TeGrotenhuis, R.J Cameron, V V Viswanathan, R.S Wegeng

Poster Session

Polymer Membranes for Product Enrichment in Microreaction Technology 550

B Schiewe, A Vuin, N Gunther, K Gebauer, Th Richter, G Wegner

Compact Heat Exchangers 556

L.A Luo, U D'Ortona, D Tondeur

Optimizing the Geometry of a Catalytic Enzyme Microreactor in Porous

Silicon 566

M Bengtsson, 1 Drott, T Laurell

High Aspect Ratio Silicon Micromachined Heat Exchanger 573

1 Bengtsson, L Wallman, T Laurel!

Comparison of Two Microvalve Designs Fabricated in Mild Steel 578

B.K Paul, T Terhaar

Integrated Microfluidics / Electrochemical Sensor System for

Field-Monitoring of Toxic Metals 588

Y Lin, D W Matson, WD Bennett, K.D Thrall, C Timchalk

Thermoelectrical Measurement System for Chemical Instrumentation 597

S Beij3ner, T Elbel, 1.M Kohler, M Zieren

Electrically Heated Microstructure Heat Exchangers and Reactors 607

1 Brandner, M Fichtner, K Schubert

Analytical Module for In-line IR Spectroscopy of Chemical Reactions in

Microchannels 617

A.E Guber, W Bacher

A Microstructure Reactor for Gas Purification 625

R Wunsch, M Fichtner, K Schubert

A Flexible Multi-Component Microreaction System for Liquid Phase

Reactions 636

Th Richter, W Ehifeld, V Hessel, H Lowe, M Storz, A Wolf

XIV

Fast Response Heating Module for Temperature Programmed GC Analysis

in Microreaction Systems 645

1 Schiewe, W Ehifeld, T Hang, H Lowe, Th Richter, X.L Yan,

A.A Kurganov, KK Unger

A Silicon-based Microreaction System for Analytical Applications 654

P Woias, K Hauser, E Yacoub-George, B Hillerich

Designing and Constructing Microplants 664

H Fink, M.J Hampe

A Modular Microreactor Design for High-Temperature Catalytic Oxidation Reactions 674

G Veser, G Friedrich, M Freygang, R Zengerle

Microstructured Reactor for Consecutive Heterogeneous / Homogeneous Gas PhaseReactions 687

Th Richter, W Ehifeld, D Erntner, K Gebauer, K Golbig, H Lowe,

A Lange de Oliveira, W Schmidt, F Schuth

Author Index

Ajmera, S.K 197 Fichtner, M 90, 187,213, Alepee, Ch 514 372,607,625

Angeli, P 253 Fiehn, H 302

Ast, T 124 Fink, H 664

Bacher, W 617 Firebaugh, S.L 171, 197 Baerns, M 224, 287, 526 Fitzgerald, S.P 364

Bayer, T 165 Floyd, T.M 171

Becker, H 102 Franz, A.I 197, 267 BeiSner, S 597 Freygang, M 674

Bengtsson, J 573 Friedrich, G 674

Bengtsson, M 430, 566 Friedrich, M , 394

Bennett, W.D 451,588 Gardeniers, J.G.E 36

Benzinger, W 90 Gavriilidis, A 253

Brandner, J 607 Gebauer, K 207,224,550,687 Brautigam, R 479 Germeroth, L 124

Bremus, E 80 GiIlner, A 80

Brenchley, D L 2 Girault, H.H 294

Breysse, J 151 Gobby, D 253

Broucek, R 224 Golbig, K 151,260,526,687 Buyevskaya, O.V 224 Golombowski, D 383

Cameron, R.I 394,541 Greenway, G.M 72, 235 Claus, P 287 Guber, AE 526,617 Commenge, J.-M 224 Gunther, N 550

Corriou, J.-P 224 Haas-Santo, K 90

D'Ortona, U 556 Hampe, M.I 664

Degerman, E 320 Hang, T 645

Dietmeier, K 124 Hartmann, H.-J 420

Dietzsch, E 213 Haswell, S.I 235

DiIlner, U 410 Hauser, K 654

Doring, M 312 Haverkamp, V 526

Drost, M.K 2, 394 Havlica, J 336

Drott, J 566 Hebling, C 383

Ehrfeld, W 14,113,136,151,207, Hefter, H.J 224

420, 526, 636, 645, 687 Heine, N 124

Ehricht, R 410,479 Heinzel, A 383

Ekstrom, S 430 Hellrung, D 80

Elbel, T 597 Hessel, V 14, 151,526,636 Emig, G 372 Hillerich, B 654

Erntner, D 687 Hocker, H 80

Esashi, M 506 Hofmann, C 151

Falk, L 224 Honicke, D 213, 260 Ferrigno, R 294 Huttl, R 469

XVI

Ignaszewski, P 287

Jacob, P 420

Hihnisch, K 526

Jensen, K.F 171, 197,267,277 Johansson, T 320

Jungwirth, St 151

Kah, S 213

Kestenbaum, H 207

Kiesewalter, S 14

Klotzbucher, T 102

Kohler, J.M 410, 469, 597 Konrad, R 420

Koop, U 181

Kosek, J 336

Kramer, A 124

Krumrnradt, H 181

Kupper, M 136

Kurath, D.E 451

Kurganov, A.A 645

Kursawe, A 213

LaMont, MJ 364

Lange de Oliveira, A 207, 687 Langer, O.-U 224

Laurell, T 320, 430, 566, 573 Legewie, F 80

Lehmann, V 45

Lerchner, J 469

Liauw, M.A 224, 372 Lin, Y 451,588 Lindner, J 336

Lohf, A 260

Losey, M.W 171,277 Lowe, H 14, 136, 151,207, 224, 526, 636, 645, 687 Lucas, M 287

Luo, L.A 556

Makhijani, V.B 441,488 Marek, M 243, 336 Marko-Varga, G 430

Martin, C 394

Martin, J 394

Martin, P.M 62, 451 Matlosz, M 224

Matson, D.W 62, 451, 588 Matuschewski, H 124

Maurer, R 514

Mayer, J 187

McCaskill, J.S 479

McCreedy, T 72,346 Menz, W 506

Mex, L 402

Meyer, T 383

Meyer, W 312

Morgan, D.O 235

Muller, J 402

Muller, M 383

Neumann, M 420

Niggemann, M 113

Nilsson, J 320, 430 Nilsson, S 320

Noack, A 500

Onnerfjord, P 430

Paces, M 336

Palmer, BJ 327

ParaUe, L 514

Paul, B.K 53, 578 Peterson, R.B 53

Petersson, M 320

Pfeifer, P 372

Piossek, C 124

Pobering, S 353

Pommersheim, R 420, 500 Poppe, R 461

Poprawe, R 80

Poser, S 410

Przekwas, A 441,488 Przekwas, A.J 441, 488 Pysall, D 165

Raghavan, J 441

Rector, D.R 327

Reid, V 294

Reineke, U 124

Renaud, Ph 514

Renken, A 224,514 Richter, Th (AMTEC) 260

Richter, Th (IMM) 14, 151,207, 550, 636, 645, 687 Roberts, G.L 62

Rodemerck, U 287

Rouge, A 224

Santesson, S 320

Scharn, D 124

Schenk, R 224 Wang, Y 364

Schiewe, B 550 Wattanutchariya, W 53

Schiewe, J 14, 645 Weber, A 469

Schmidt, M.A 171, 197, 267, 277 Weber, L 113

Schmidt, W 207,687 Wegeng, R.S 2,364,541 Schneider-Mergener, J 124 Wegner, G 550

Scholz, S 500 Wehner, M 80

Schubert, K 90, 187,213, Wen,J 451

372, 607, 625 Wenschuh, H 124

Schulz, M 124 White, M 62

Schulz, T 410 WieBmeier, G 213

Schuth, F 207,687 Wild,M 80

Schwesinger, N 353 Wille, Ch 526

Seidel, R.U 506 Wilson, N.G 346

Sevcikova, H 336 Woias, P 654

Sim, D.Y 506 Wolf, Andrej 151,636 Skelton, V 235 Wolf, Antje 469

Smith, R.D 451 Wolf, D 187

Snita, D 336 Wolf, G 469

Sommer, 1 420 Wolfrum, J 420

Steen, D 479 Worz, O 151

Steinfeldt, N 224 Wunsch, R 90, 625 Stepanek, F 243 Xiang, F 451

Stewart, D.C 62 Yacoub-George, E 654

Stoldt, J 181 Yan, X.L 645

Storz, M 151, 526, 636 Zech, T 260

Styring, P 235 Zedda, M 383

TeGrotenhuis, W.E 541 Zengerle R 674

Terhaar, T 578 Zieren, M 469, 597 Thrall, K.D 588 Zilka, J.L 62,364 Timchalk, C 588 Ziogas, A 136

Tjerkstra, R W 36

Tondeur, D 556

Tonkovich, A.L.Y 62, 364 Uebel, S 410

Unger, K.K 645

Van den Berg, A 36

VanderWiel, D.P 364

Veser, G 674

Viswanathan, V.V 541

Vuin, A 550

Vulpescu, L 514

Wachsen, O 165

Wallman, L 573

Walter, St 224

Wang, D 488

Part 1

Opening Lectures

of Chemical and Thermal Systems in the 2rt

Century

Robert S Wegeng, M Kevin Drost, David L Brenchley

Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352 USA

Abstract

The 21 st century holds great promise for development of Micro Chemical and Thermal Systems (MICRO-CATSTM) The quest for miniaturization will lead to greater process intensification Miniaturizationgreatiy reduces the resistances to heat and mass transfer When heat and mass transfer rates are increased by orders of magnitude, revolutionary changes occur in technology

Miniaturization of electronics over the past few decades has transformed the way

we live The invention of the transistor meant that small, lightweight portable radios could be carried anywhere Not long ago the idea of chips was revolutionary; today chips make hand-held computers, cell phones, and many other hand-held electrical devices possible These are examples of distributed technology made possible by miniaturization

Miniaturization of chemical and thermal systems will take us on a similar technological journey Already demands are driving the development of MICRO-CATS The quest for miniature systems is leading to man-portable cooling, automotive fuel processing, and in situ resource utilization for space exploration The 21 5t century will see great progress in the development and use of engineered nanosystems In electronics, the concept of "molecular electronics" is seen as a way

to further increase the power The same will occur in MICRO-CATS We envision using engineered nanosystems to create more powerful microscale devices with highly functional surfaces that incorporate enzymes and chemical catalysts These surfaces will make the device or system more efficient and reliable; they will be able

to repair, renew, or replace themselves; eliminate or heal themselves from surface fouling; and carry out step-by-step chemical reactions Molecular coatings will create a molecular assembly line By incorporating enzymes, reactions that are con-ventionally carried out at high temperatures and pressures can be achieved at ambient conditions Finally, in the 215t century, the missions for outer space will create opportunities for MI CRO-CA TS to be major parts of the successful exploration of space The future for MICRO-CATS is as wide open as outer space itself!

Keywords: miniaturization, microscale systems, reactors, heat pumps

W Ehrfeld (ed.), Microreaction Technology: Industrial Prospects

© Springer-Verlag Berlin Heidelberg 2000

to heat and mass transfer Imagine heat and mass transfer rates increased by factors

of 10 to 100 or more Furthermore, when many components and functions are combined and integrated into one miniature system, good things happen In fact, things happen that are almost too good to be true! That is where the age of miniaturization in MICRO-CATS is heading in the 21 sl century Process intensification opens the way for revolutionary changes Many things we can think

of today and some things we haven't yet imagined will happen in the 21s1 century For instance,

• Today, we adjust the room thermostat Tomorrow, our clothing will be manufactured with microsystems that have the capability to automatically heat and cool us

• Today, internal combustion engines move us around Tomorrow, microsystems will allow different fuels and power systems to be used in our vehicles

• Today, we launch almost everything that is needed on a space mission Tomorrow, we "will live off the land" as we explore the universe Microsystems will harvest raw materials and process them into what we need

• Today, we are able to transplant some organs Tomorrow, we will implant artificial organs composed of microsystems

As this conference on process miniaturization demonstrates, work is under way

at many research institutions and industries to fabricate microcomponents and systems that perform the same standard unit operations that are present in large chemical processing plants Although miniaturization is in its early stages, it is driven by considerable incentive and the opportunity to revolutionize heat exchangers, heat pumps, reactors, gas absorbers, solvent extractors, fuel processors, pumps, valves, compressors, and other similar components and systems In the foreseeable future, we anticipate the assembly of compact, chemical processing and energy conversion systems that range from smaller than a cubic centimeter to several cubic meters in size [1,2,3,4,5]

The quest for miniaturization is marching forward The changes that have occurred in electronics in the 20lh century enable us to envision what to expect from micro chemical and thermal systems in the 21 sl century The advances in microelectronics have transformed computers, communication systems, and appliances, to name a few The transistor is now about 50 years old This invention, along with other innovations, provided for process intensification The transistor radio changed things because it was small and light enough to carry anywhere This created possibilities we didn't have before

Miniaturization of components on computer chips has made it possible to have powerful hand-held computers, cellular telephones, global positioning systems, and many other electrical devices In fact, the telephone booth may in one more generation be found only in museums Everyone will have compact, low cost personal cell phones or something even better A few years ago the idea of putting

so many components and functions on one microchip was revolutionary Today, chips are an integral part of almost every electrical device

The advances in microelectronics spurred the development of microelectro mechanical systems (MEMS) Scientists working in this field have developed silicon actuators/levers,gears and gear reduction units, diaphragm pressure sensors, motors, transmissions, locks, mirrors, hinges, and more Some of these devices have been combined to yield intricate mechanical systems-on-a-chip Efforts are moving toward integrated microelectronic/micromechanical systems-on-a-chip; the acceleration sensor for deploying air bags is an example ofthe commercialization of micromachined systems-on-a-chip

The strategy of miniaturized distributed systems is powerful; the potential impact worldwide is enormous For example, this is probably one of the most significant strategies for the eradication of poverty Until now, the countries with the resources, large-scale technologies and infrastructure, and skilled labor enjoyed the highest standards of living But distributed processing can change that equation For instance, some third-world countries will have communications without telephone poles and underground cables Microtechnology presents new possibilities for poor countries to increase their standard of living using distributed micro chemical and thermal systems With these technologies, all countries can have higher productivity, power, and communications anywhere, without needing to build expensive infrastructures So miniaturization brings with it new hope for the 21 st century

2 Evidence for Success

The evidence for success of MICRO-CATS can be found in their demonstrated performance They have higher heat transfer rates and faster reaction rates and can

be operated with lower pressure drops than conventional systems Some of the successful results for microreactors and solvent extraction are presented below Researchers at Pacific Northwest National Laboratory (PNNL) have investigated several microchannel reactor units One, an integrated microchannel stainless steel combustor/evaporator, generates heat using the heat of reaction obtained from the gas-phase combustion of methane or hydrogen [6] The technical objective is to demonstrate that a microchannel combustor can produce at least 25 watts of thermal energy per square centimeter of heat transfer profile area and transfer that energy to

a cooling unit This heat transfer rate is approximately 20 times higher than that of

a conventional water heater The most important problem for the microcombustors

is maintaining the flame in a small volume There are also problems maintaining the desired temperature in the combustion chamber and efficiently removing the heat and the combustion products

5

The overall size of the system is 41 x 60 x 20 mm Tests were conducted on micromachined combustor channels 300 microns wide, 500 microns deep, and 35

mm long; results showed that the system achieved heat fluxes greater than 30 W/cm2

and combustion efficiencies greater than 85% (Combustion efficiency is the ratio of the heat transferred to the water relative to the heating value of the fuel.) In this system, oxygen (or air) and fuel were introduced into the combustion chamber where

an ignition wire initiated combustion Microchannels were fabricated on the under side of the burner to provide cooling during the combustion process The com-bustion products then entered a microchannel heat exchanger that enhances heat transfer between the combustion products and the combustor solid surfaces Flame quenching problems increased as the combustor size decreased, indicating that flame quenching will set a limit on the degree of miniaturization possible in this type of combustor

PNNL researchers have also investigated catalytic microchannel reactors made from high-temperature metal alloys for several applications Engineered catalysts provide large surface areas, high heat-transfer rates, and low pressure drops While slow kinetics is a current paradigm in conventional reactor systems, fast kinetics is possible in microchannel reactors While multisecond reactions may be necessary

in conventional reactors, millisecond reactions occur in microchannel reactors This improvement is attributed to the diffusion lengths, which are reduced by at least two orders of magnitude, and to heat transfer, which prevents "hot spots" and "cold spots" and enables high throughputs

A methane partial oxidation reactor was operated with fast kinetics at 900°C to produce carbon monoxide and hydrogen Methane conversion efficiencies were more than 85% with 11 millisecond residence time and 100% with 25 millisecond residence time The use of catalysts in microchannel reactors is so promising that PNNL is now developing engineered microstructures to find those best suited to microchemical reactors

At PNNL, we are investigatingmicrochannelreactors using the Sabatierreaction and the reverse water-gas shift reaction The Sabatier reaction combines carbon dioxide with hydrogen to produce methane propellant and water vapor In the reverse water-gas shift reaction, carbon dioxide combines with hydrogen to produce more fuel and water An electrolysis unit dissociates the water into oxygen and hydrogen Tests showed that the Sabatier reaction catalyst provides 85% conversion

at 250°C with a reactor residence time of 0.1 second The residence time for the reverse water gas shift catalyst was even shorter These successful results are very promising for the development of MICRO-CATS for NASA's Mars missions PNNL researchers have designed and tested microchannel devices for efficient contacting of two liquids in solvent extraction [7] Recent results are being presented

by PNNL researchers at this conference [8] Previous efforts have tested such devices as gas absorbers This solvent-extraction work is a step in the development

of compact efficient devices for chemical separations The microchannel contactor results in substantially higher throughput per total system volume compared with conventional technologies

Solvent extraction requires mtlmate contact of two immiscible liquids to facilitate mass transfer of one or more solutes from one fluid to another The architecture in this work consists of two micromachined channels separated by a contact plate The channels are separated by a micromachined contactor plate of25-micron-thick Kapton substrate that has a matrix of uniform holes 25 microns in diameter The solvent and feed streams can be operated co-currently or counter-currently This arrangement allows for intimate contact of the two immiscible fluids

as they flow through very thin channels smaller than the normal mass-transfer boundary layer

The mass transfer resistance has contributions from each flow stream, from the contactor plate, and from the interface The pressures inside the contactor must be carefully controlled to prevent breakthrough of the fluids through the holes in the contactor plate If the interface is immobilized, the two liquid films can flow in counter-current directions, allowing for a more effective separation in the contactor PNNL researchers are investigating reducing the overall size of the equipment and improve operating efficiency by reducing the thickness of the films and improving mass transfer efficiency in the contactor plate Tests were conducted using microcontactor channels 1 cm wide, 10 cm long and 200 to 500 microns high Results with cyclohexane-water feed and cyclohexane solvent indicate that the micromachined contact plates worked at least as well as commercial microporous membranes and have significant potential for improved performance Also, with channel heights of less than 300 microns, diffusion through the contactor plate is the limiting factor in mass transfer

Further evidence for success is our ability to model chemical and thermal processes at the microscale PNNL researchers have developed a lattice-Boltzmann simulation capability for modeling the behavior of microscale fluidic systems In fact, this work is being presented in a paper at this conference [9] Complex fluid dynamics problems in this size range are not now amenable to being modeled by conventional simulation methods, which do not properly account for the importance

of surface forces at fluid-gas, fluid-fluid, and fluid-solid interfaces However, our lattice-Boltzmann modeling capability overcomes these limitations This is critically important for supporting the design and testing of MICRO-CA TS

3 Progress Toward the Future

The quest for micro chemical and thermal systems is being driven by some important needs One is the need for miniature analytical systems for DNA, toxic chemicals, and pathogens such as HIV Such instruments are needed in the fields of medicine, forensics, agriculture, and environmental control Other prominent needs driving development include 1) military requirements for human-portable heating, cooling, and power generating units; 2) transportation demands for hydrogen rich streams for fuel cells; and 3) chemical processing systems capable of processing the rocks, soil, and atmosphere on planets such as Mars These are just examples While we cannot

be absolutely certain what the potential for microtechnology might be, we can

7

speculate about areas where the demand caused by technical problems or social needs will draw us forward At the present at PNNL, we are pursuing major opportunities for MICRO-CATS in buildings, carbon management, environmental restoration, industrial chemical processing, military, space exploration and transportation More details on these applications are found on our web site: http://www.pnl.gov/microcats

At PNNL, we have several systems under development but just three are described in this paper: man-portable cooling, automotive fuel processing, and in situ resource utilization for space exploration These three technologies demonstrate the diversity of potential applications

3.1 Man-Portable Cooling

PNNL researchers are working on microsystems to provide man-portable cooling The market is driving this work because there is a need to cool people who work in hazardous environments such as 1) soldiers in certain field operations, 2) emergency response workers for environmental cleanup, firefighters and other emergency workers, and 3) industrial workers in hot, unforgiving environments A cooling device is needed for use with protective clothing, and it must be compact, lightweight, and able to operate for extended periods of time We are developing absorption heat pumps using a lithium bromide and water solution as the working fluid As this technology is successfully developed, we envision spin-off applications with slightly larger versions for cooling shipping containers and air conditioning vehicles and even commercial or residential structures

In this conference, a poster session is being given by PNNL researchers describing the status of compact, heat-actuated heat pump development [10] (see Figure I) Although the unit is for energy conversion and not for chemical processing and manufacturing, it is of interest because it includes micro chemical components that perform unit operations of interest to chemical engineers It is a chemical heat pump that includes microchannel heat exchangers plus an assembly

of components making up a thermochemical compressor The components in this unit include a microchannel gas absorber and a microchannel gas desorber The high performance of this compact heat pump system is possible because of the rapid heat and mass transport available in microchannels Current estimates suggest a size reduction of a factor of 60 compared with conventional heat pumps is possible The weight of the system will be reduced by at least a factor of 2 The man-portable version of this system would operate by combustion of a liquid hydrocarbon fuel; the total system weight, including the fuel, for the man-portable system is estimated to be less than ten pounds

3.2 Automotive Fuel Processing

Automotive fuel processing is another application where the potential exists for significant market demand for microtechnology Some automotive manufacturers plan to introduce fuel-cell-powered automobiles into the market in the next few

Electronics

Figure l Mesoscopic Heat-Actuated Heat Pump years The fuel cells will generate electricity to drive the electric motors that move the vehicles These vehicles will still carry liquid hydrocarbon fuels, plus a key additional item: an onboard fuel processing plant to produce hydrogen for the fuel cells

The proton exchange membrane (PEM) fuel cell is the electricity generator for the baseline system being developed by the U.S Department of Energy (with U.S automotive manufacturers) in their Partnership for the Next Generation of Vehicles The fuel processor is a critical reactor technology for the deployment of the PEM fuel cells, which operate using hydrogen Figure 2 shows the fuel vaporizer portion ofthe automotive fuel processing system

The fuel processor produces hydrogen rich streams from gasoline or methanol fuel using a heterogeneous, catalytic microchannel chemical reactor This microreactor unit takes advantage of both the heat and mass transport effects in microchannels and is thus able to provide a compact system with high throughput

It is a multi-step process involving fuel vaporizer, primary conversion reactor to produce synthesis gas, water gas shift reactor, and CO cleanup reactor The fuel processor has an occupied volume of less than 0.3 liters to support a 50 kWe output from a fuel cell At this conference, PNNL researchers are presenting a paper that describes the development of this fuel processor (II)

9

111\11\\1\1\1\1\"1\11

1

\ I I 1 I \ I I

Figure 2 Microchannel Gasoline Vaporizer for 50-Kwe Fuel Processing System

3.3 In Situ Resource Utilization

Microtechnology has attractive applications for supporting space exploration and orbit-based research For such missions, payload size and weight is of paramount importance Compact, lightweight microsystems cost less to launch and leave room

on the spacecraft for other important equipment and supplies Before the advent of microsystems, it was not possible to carryall fuels and other chemicals from Earth and it is not cost-effective to do so now So the strategy is to "live off the land" in space exploration

NASA wants to use compact chemical processing plants in its Mars missions They are interested in reducing the costs associated with robotic and human missions

to Mars and have determined that this can be accomplished by substituting process technology for propellants and oxygen for the return voyage MICRO-CATS can produce fuel and oxygen from Martian atmospheric gases for NASA astronauts during their stay on Mars and for the return trip home

So NASA's plan is to include lightweight, chemical systems as part of its Mission to Mars program to process indigenous space materials to produce propellants, oxygen, and other chemicals The program uses a phased approach, with the following elements:

Robotic Missions

Mars ISPP Precursor (MIP) mission

Mars PUMPP

Mars Sample Return mission

Mars Sample Return mission

Produces and uses ISPP propellant and oxygen Propellants and 02 produced before crew departs Earth

Extraterrestrial chemical processing plants will need to be compact, lightweight, efficient, and able to operate reliably for prolonged periods in reduced gravity environments This presents unique challenges because conventional chemical process equipment relies heavily on gravity to operate effectively If such systems are to be efficient and reliable, a fundamental understanding of operation in non-Earth environments is required

We are working with the In Situ Resource Utilization (ISRU) program at NASA's Johnson Space Center to develop compact systems to produce fuel and oxygen from the Martian atmosphere So far, the microchannel technologies have been conceived to convert carbon dioxide to methane and water by reacting it with hydrogen using the Sabatier process and reverse water gas shift reactions These processes use carbon dioxide from the atmosphere to produce methane, water, and carbon monoxide Hydrogen must be carried from earth for use in these reactions Oxygen can be produced from the water in an electrolysis unit The carbon monoxide can be further reacted with hydrogen to produce fuel

4 MICRO-CATS in the 21 st Century

The future is here for MICRO-CATS because many opportunities are now within our reach We see three important fronts on the horizon of the 21'1 Century:

• The use of engineered nanosystems to produce "highly functional" surfaces

• The marriage of enzymes with highly functional surfaces

• Greater demands for MICRO-CATS to perform in the extreme and harsh conditions found on space missions

4.1 Engineered Nanosystems

In the 21 sl century, engineered nanosystems will be used to develop MICRO-CATS Why? Because engineered systems are often more powerful than natural ones They

is being presented in a paper at this conference [12] Looking ahead, we believe that when engineered nanosystems (e.g., molecular systems) are included with microsystems, significant additional improvements in process technology may result Highly functional surfaces result from changes in surface properties that make the device or system more efficient, more effective, or more reliable These surfaces may have the ability to repair, renew, or replace themselves They could be surfaces that control or eliminate fouling-imagine a surface that can protect and heal itself from surface fouling They could carry out a series of step-by-step chemical reactions We envision using molecular coatings to create a molecular assembly line

In the 21 sl century, the use of highly functional surfaces will become routine Custom-designed catalysts will be developed They will be designed and constructed to provide higher catalyst activity and greater specificity When engineered and fabricated at the nanoscale, microsystems are more effective This

is what we can expect in the 2151 century

4.2 Enzymes and Functional Surfaces

All of life is full of microchemical systems These have been working and developing for millions of years Things like membranes, cells, lungs, kidneys, and hearts operate because they contain viable, effective, and reliable microtechnology They involve chemical reactions, and virtually all of these chemical reactions require catalysts to arrange the reactants in energetically favorable orientations so that chemical products will be formed Man-made chemical catalysts, such as platinum compounds, only crudely arrange reactants to form products, and, consequently, our chemical plants require high temperatures, pressures, and pH extremes to complete reactions In contrast, cells require their catalysts to function under the mild conditions required for sustaining life Nearly all reactions in cells are catalyzed by enzymes Unlike chemical catalysts, enzymes are frequently catalytically perfect; i.e., once the enzyme binds its substrate, the reaction product is formed, sometimes

at thousands of reactions per second With the development of functional surfaces and the use of enzymes, many chemical processes that are not effected under high temperature and pressure conditions may be accomplished with enzymes and normal temperature and pressure conditions Furthermore, the microdevices will then provide a molecular assembly line capability such that molecular changes occur in

a sequenced step-by-step manner These reactions will be dramatically different and probably dramatically better

More than 5000 useful enzymatic reactions have been characterized to date, and many of these reactions are understood in molecular detail because the three-

dimensional structures of the enzymes and reactants have been determined DOE's genome sequencing programs and x-ray diffraction beam lines have been essential

in identifying enzymes and detennining their three-dimensional structures Recombinant DNA technology makes it possible to produce virtually any enzyme in large quantities by isolating the gene, modifying the gene to enhance enzymatic properties, inserting the gene in an organism designed to express large quantities of protein, and then purifying the resulting protein Recombinant enzymes provide attractive alternatives to constructing energy-intensive chemical plants that use crude catalysts

A long-tenn goal is to someday manufacture artificial enzymes with highly functional surfaces The structural and mechanistic insights elucidated from biological enzymes would provide the template for an advanced version of existing highly functional surfaces designed to mimic enzymes Even ifthe artificial enzyme

is orders of magnitude less efficient than the biological enzyme, the artificial enzyme might still be sufficient in most chemical applications and significantly less expensive than current crude catalysts

4.3 Future in Space

The space age arrived more than 40 years ago, but it slowed when the costs became huge to carry out increasingly complex and ambitious space missions Other priorities prevailed, and budgets for space exploration programs were not increased But now, MICRO-CATS may help provide the way to accomplish missions at lower costs In addition, and equally important is the fact that space exploration is one of the applications where microtechnology is better than conventional technology The extreme harsh environments coupled with the space mission requirements

of compactness, light weight and high reliability present a quantum level change in requirements for MICRO-CATS Can the system endure the rigors of temperature extremes? Does it operate under low-gravity or zero-gravity conditions? Can it be

on stand-by for long periods and then operate reliably when needed? The rigors of developing technology for NASA and others may also spur the development for many other applications in the future

5 Literature Cited

1 Wegeng R, C Call, and MK Drost "Chemical System Miniaturization." February 1996 PNNL SA-27317, AICHE 1996 Spring National Meeting, New Orleans Available on the world wide web: http://www.pn1.gov/microcats

2 Ehrfeld W, V Hessel, H Mobius, T Richter, V Hessel, and K Russow February

1995 "Potentials and Realizations of Microreactors." Proceedings of the DECHEMA Workshop on Microsystem Technology for Chemical and Biological Microreactors

13

3 Ehrfeld W, C Gartner, K Golbig, V Hessel, R Konrad, H Lowe, T Richter, and

C Shulz February 1997 "Fabrication of Components and Systems for

Chemical and Biological Microreactors." Microreaction Technology

-Proceedings of the First International Conference on Microreaction Technology

4 Lerou J, M Harold, J Ryley, T Ashmead, C O'Brien, M Johnson, J Perrotto,

C Blaisdell, T Rensi, and J Nyquist February 1995 "Microfabricated Minichemical Systems: Technical Feasibility." Proceedings of DECHEMA

Workshop, Microsystem Technology for Chemical and Biological Microreactors

5 Wegeng R, MK Drost, T Ameel, and R Warrington December 1995 "Energy Systems Miniaturization Technologies, Devices, and Systems." Proceedings of the International Symposium on Advanced Energy Conversion Systems and Related Technologies, RAN 95

6 Drost MK, CJ Call, JM Cuta, and RS Wegeng 1999 "Microchannel Integrated

Evaporator/ Combustor Thermal Processes." Journal of Microscale

Thermophysics Engineering (accepted for publication)

7 TeGrotenhuis WE, R Cameron, MG Butcher, PM Martin, and RS Wegeng

1998 "Micro Channel Devices for Efficient Contacting of Liquids in Solvent

Extraction." Separation Science and Technology PNNL-SA-28743 Also

available on the world wide web: http://www.pnl.gov/microcats

8 TeGrotenhuis WE, R Cameron, VV Viswanathan, and RS Wegeng April 1999

"Solvent Extraction and Gas Absorption Using Microchannel Contactors." Third International Conference on Microreaction Technology Also available on the world wide web: http://www.pnl.gov!microcats

9 Rector DR and BJ Palmer April 1999 "Simulation of Chemical Separation Process Using the Lattice-Boltzmann Method." Third International Conference

on Microreaction Technology Also available on the world wide web: http://www pnl.gov /microcats

10 Drost M, M Friedrich, C Martin, J Martin, and R Cameron April 1999

"Recent Developments in Microtechnology-Based Chemical Heat Pumps." Third International Conference on Microreaction Technology Also available

on the world wide web at: http://www.pnl.gov/microcats

11 Tonkovich A, J Zilka, M LaMont, S Fitzgerald, D Vanderwiel, Y Wang, and

RS Wegeng April 1999 "Microchannel Reactors for Automotive Fuel Processors." Third International Conference on Microreaction Technology Also available on the world wide web at: http://www.pnl.gov/microcats

12 Matson DW, PM Martin, DC Stewart, A Y Tonkovich, GL Roberts, and

M White April 1999 "Fabrication of Microchannel Chemical Reactors Using

a Metal Lamination Process." Third International Conference on Microreaction Technology Available on the world wide web at: http://www.pnl.gov/microcats

Acknowledgment

Pacific Northwest National Laboratory is operated by Battelle for the U.S Department of Energy under Contract DE-AC06-76RLO 1830

in Process Engineering

W Ehrfe1d, V Hessel, S Kiesewalter, H Lowe, Th Richter, J Schiewe

Institut flir Mikrotechnik Mainz GmbH, Carl-Zeiss-Str 18-20, D-55129 Mainz, Germany,

e-mail: wehrfeld@imm-mainz.de

Abstract

Microreaction technology offers several advantages for implementation in process engineering and biochemistry Its application fields can be divided into two major areas, namely the conversion of matter and data acquisition In the following, it will be shown that the advantages of microreaction systems, like increased yields and selectivities, process intensification as well as economical and ecological aspects, are unique properties and can be deduced to their basic characteristics In this context, short pathways and response times, large surfaces, small volumes and defined flow characteristics have to be mentioned

Today, a variety of microreactor systems as well as components already are available in srnalllot production In this paper, focussing on recent developments

in microreaction technology at IMM, the development of a variety of components for unit operations as well as their combination in integrated microreaction sys-tems for special industrial purposes will be discussed

1 Introduction

Meanwhile, it is widely accepted, that microreactors offer advantages, opening up the gate to a new chemistry including access to new process regimes as well as remarkable process improvements First prototypes of complex integrated systems currently are tested in industry, not only as tools for process development but as instruments for production, process intensification and mobile as well as station-ary energy production Moreover, microreactor components of high versatility like micromixers and heat exchangers attract special attention Some of these devices meanwhile are available in small scale production at low cost Micro-mixers for example, have been applied successfully to a growing number of pro-cesses like emulsion and foam formation, multiphase reactions and polymeriza-tion processes, just to mention a few

Furthermore, completely new aspects for chemical process engineering emerge, leading to new concepts in chemistry and reaction engineering Without any doubt, an enormous application potential for microreaction technology can be predicted This potential is known and accepted all over the world and competition concerning advantages in development and production using microreaction systems already has started The dynamic of these technological developments and changes to be expected in chemistry and process engineering can be compared to the innovative developments that took place in biotechnology

W Ehrfeld (ed.), Microreaction Technology: Industrial Prospects

© Springer-Verlag Berlin Heidelberg 2000

15

in recent years Due to economic necessities, a change in paradigms will lead to a fast dissemination of microreaction technology in the near future

Fig I: Microreactor components, fabricated by means of LlGA-technology

Potentials and advantages of microreaction technology have been discussed intensively within the past few years The unique properties of micro reactors can

be put down to their small characteristic dimensions and the ability to combine a large number of system components within small volumes These characteristics affect the chemical process most effectively, e.g by establishing strong gradients with respect to physical properties along the reaction pathway or enhanced heat management [1, 2]

Basically, microreactor applications can be divided into two distinct categories The major field regarding reaction engineering can be seen in conversion of matter within miniaturized reaction systems and their components Various processes in chemistry and pharmacy have been investigated so far with respect to production of chemicals in microreaction systems on a scale of grams, kilograms

or even tons In these fields, microreactors already are regarded as a competitive technology and open up economic alternatives to conventional process engineering

In addition, microreaction systems are applicable for data acquisition utilizing the chemical process itself as a tool for identification of new qualities and properties of matter Using small quantities of reactants, huge amounts of data can

be generated for screening purposes by parallel processing These systems already are used for many applications in pharmacy and biochemistry, e.g in high throughput screening for lead discovery, medical technology and materials sciences Consequent miniaturization increases the amount of information generated per unit volume permitting increased efficiency and fast screening efforts

2.1 Fundamental properties of microreactors

Regardless of the categories of microreactor applications as described above, microreaction systems offer a number of inherent and fundamental advantages [2]

Short pathways and response times

With respect to dimensions along the reaction path, microstructurization yields strong gradients in temperature, concentration, density of the reactants or pressure allowing fast changes in reaction conditions regarding transport of heat or matter Thus, microreactors enable new process regimes not accessible in conventional systems due to safety aspects as well as processes involving fast processing of unstable intermediates Moreover, these properties of microreaction systems enable short response times regarding the chemical process

Large surfaces and small volumes

The large surface to volume ratios of microreaction systems enable a significant enlargement in surface area available for transport of heat and matter Thus, high transfer rates and efficient processes permit rapid changes in temperature, efficient heat management or fast mixing processes At the same time, decreasing the system's volume significantly increases the information density obtainable in case

of screening applications Additionally, reduction of the amount of substances and chemicals employed in the process is advantageous regarding safety and ecologi-cal issues as well as aspects of sustainable development

Fig 2: Small series production of microreactor components and integrated systems

Defined flow characteristics

In microchannels, laminar flow conditions are preferred compared to turbulent flow conditions, facilitating theoretical description of the flow process as well as

an increase in reliability of simulations and theoretical predictions Moreover, special flow characteristics like the highly ordered "hexagon flow" of gas ! liquid dispersions only can be observed in microchannels [3] Several advantages, like

17

narrow residence time distributions and uniform mass transport kinetics are unique to microreaction systems

2.2 Applications of microreaction systems

2.2.1 Process engineering aspects

Based on the inherent potentials of microreactors regarding process engineering aspects, the following advantages and improvements in chemical and biochemical engineering are accessible using microreaction technology

Isothermal processing of exothermal reactions

The huge surface to volume ratios in micro channels have a strong impact on reaction conditions concerning heat transport phenomena For example, efficient removal of excess heat of reaction gives access to new reaction regimes unique to microreactors and enables isothermal reaction conditions even for highly exothermal reactions This was proven at BASF for a liquid multiphase process in

an integrated microreaction system developed by IMM [4) In this case, isothermal processing led to a 10 % increase in yield compared to the technical process [5]

Moreover, efficient heat removal is advantageous as well for processes employing highly reactive species, e.g the direct fluorination of aromatics using elemental fluorine This reaction, being hard to control on a larger scale for its high exothermicity and often explosive process conditions, can be performed safely and at considerable yields in microreaction systems [6)

Chemical processing within explosive regimes

Quench distances regarding explosive gas phase processes in most cases exceed the small dimensions and cross sections of microchannels Therefore, gas phase reactions within the explosion limits of the reactants can be carried out safely in microreactors This, inter alia, was proven for the ethylene oxide synthesis in a microreaction system [7] using pure ethylene / oxygen mixtures at an oxygen content up to 85 % as well as the explosive reaction of hydrogen and oxygen [8] Thus, wide ranges of parameters become accessible for optimization of process conditions

New process regimes in microreactors

Besides isothermal processing and explosive reaction regimes, other new process regimes in microreactors attract special attention regarding investigations of feasibility, specific process conditions and reaction engineering aspects as well Here, a lot of new fields to explore can be found and the capability of microreactors for development of new processes has not been exploited thoroughly yet Questions still open concern the fast processing of unstable intermediates as well as photochemical reactions or consecutive reaction sequences in microreactors, only to name a few

2.2.2 Sustainable development

Besides engineering aspects as discussed above, there is a certain impact of microreaction technology regarding sustainable development and related economical and ecological aspects as well

Waste reduction and saving of resources

Chemical processes in microreaction systems and their advantages and ments e.g on yields and selectivities lead to purer products Therefore, less by-products have to be separated and fewer resources are needed for a certain amount

improve-of product Moreover, for most screening applications, e.g in drug discovery, catalyst or material development, reduction of the amount of reactants yield in dramatic cost saving

Increased safety

Small volumes and hold-up as well as efficient heat management in microreactors significantly reduce the effort necessary concerning process safety Without any doubt, today's industrial standards regarding safety are on a very high level Nevertheless, microreactors can help to reduce costs in safety measures and allow the performance of reactions not applicable on a large scale for safety reasons

Process optimization in micro reactors

As already shown in several applications on a lab- as well as industrial scale, information on process optimization in microreactors allow predictions on optimal processing on a technical scale as well This holds for parameters like residence time, transport of heat and matter and flow conditions Often, these information can save years of development and costs regarding process engineering aspects Thus, users in microreaction technology can gain a valuable lead over competitors

on the world market

Numbering-up instead of scaling-up

Traditionally, the development of production processes goes along with a consuming scaling-up process that often takes two to five years Due to their small dimensions, microreactors are suitable for use on a lab-scale Multiplication of the number of units easily gives access to higher production capacities while the specific reaction conditions of each microreactor unit is kept unchanged Thus, years of development can be spared, saving time as well as money

time-Smaller production units and distributed production

Conventional plants require a certain size for economical production tors do not obey the rule of decreasing costs with increasing plant size In micro-reaction technology, the size of each production unit, available in large scale production at low costs, correlates to the absolute number of identical sub-units needed Therefore, microreactors show a high flexibility in output capacity and

Microreac-19

can be used for decentralized small size production as well as production on-site and on-demand [9]

Fig 3: Small series production of micromixers

2.2 3 Microreactors for data acquisition

Applications of microreactors for data acquisition in pharmaceutical, medical and materials science mainly arise from the large number of independent units to be integrated within small volumes Highly parallel screening processes in micro-systems are capable of generating enormous amounts of information at minimized input of substances, energy and time Furthermore, screening processes in micro-reaction systems are quick due to the small volumes, short distances and response times In the following, the main fields of application for microreactors in data acquisition as well as their main features are mentioned briefly

High Throughput Screening (HTS)

• Fast identification of substances with specific properties

• Fast processing of huge numbers of samples by parallel processing at a high degree of automation

• Similar processing of each sample at a high degree of repeatability

Combinatorial synthesis and materials science

• Identification of correlation e.g between chemical structure and desired properties

• Fast generation of huge numbers of samples, combined in substance libraries e.g by combinatorial synthesis

• Parallel examination of specific properties

• Fast on-site monitoring of analytical parameters possible

• Applications in pollution control, medical diagnostics, biotechnology and food stuff analysis

Since the early beginning of microreaction technology, several microreactor components have been reported on, mostly concerning mixing processes [10] and heat exchanger devices [1, 11] Still there are a lot of activities in this field, now mainly dealing with industrial applications as well as questions of high throughput and cheap mass production of microreactor components

21

High throughput micromixers

The caterpillar mixer consists of two parts having an identical microstructure as shown in figure 4 Two fluids enter the mixer and are laminated vertically and split horizontally in two partial streams that are again laminated in the vertical plane Consecutive mixing stages following this split and recombine principle double the number of fluid lamellae formed and simultaneously decrease their width by a factor of two After passage of six to eight stages a typical width of each fluid lamella of approximately 2 Ilm is reached, enabling fast and efficient mixing by diffusion The caterpillar mixer's design minimizes the pressure loss and enables volume flow rates up to 50 IIh

Another type of mixer, the so-called "star laminator" (see figure 5) comprises a stack of metal foils Here, a system of alternating lamellae are formed in a central boring by feeding both fluids radially through star-like formed structures The width of the lamellae is determined by the width of the metal foils stacked, that is

20 or 50 Ilm The star laminator was designed to enable flow rates up to 100 1Ih at

low pressure drops

Fig 5 Star laminator consisting of thin metal foils and suitable for high volume flows Material: stainless steel, process: wire-EDM

New processes in micromixers

A new approach for the generation of liquid / liquid emulsions or gas / liquid dispersions is given by the use of micromixers, characterized by a static opera-tional principle In micromixers comprising interdigital channel structures, the formation of dispersions with small droplet sizes and very narrow size distribu-tions at low operating pressures can be observed, offering significant advantages

as compared to standard dispersing techniques Moreover, the use of micro mixers enables an easy transfer of R&D results on a lab scale to the production scale Increasing the number of mixing units working in parallel ("numbering up"), easily circumvents scale-up problems

In the mixing elements used, dispersions are formed in a defined way by multiple splitting of two phases resulting in an alternating arrangement of small flow lamellae that decompose due to strong periodic velocity gradients Main parameters with influence on the mean droplet size of the dispersions formed, are the total volume throughput, the ratio of flow rates with respect to the different phases as well as the microchannel dimensions [12] Typical mean droplet diameters for oil-in-water emulsions are in the range of 4 to 50 !-1m At large differences in the flow rates of both phases, the droplet size is smaller than the microchannel width because of fragmentation due to parabolic velocity profiles and the effect of hydrodynamic focussing

In case of gas / liquid dispersions, the bubble size depends on the liquid flow rate and the ratio between the flow rates of liquid and gas flow Mean bubble sizes were found in the range of 120 to 800 !-1m for the model system argon in glycerol / water with surfactant added A continuous, well ordered flow behavior was found when using low to intermediate viscosities, low surface tensions and small liquid

to gas flow ratios [3J

Fig 6 Stainless steel heat exchanger structures, realized by a punching process

3.2 Heat exchangers

Based on previous developments concerning countercurrent flow heat exchangers [1,2J, current activities focus on mass production techniques for cost-efficient realization of these important microreactor components A five-step punching technique was employed for the production of microstructured stainless steel foils showing parallel fluid guiding channels 500 !-1m wide at a wall thickness of

300 !-1m The foils are stacked upon each other using separation foils A 90°-angle

3.3 Analytical components

Miniaturized analytical components are crucial for the widespread and efficient use of microreaction technology in order to fully exploit the advantages of microreactors for chemical processing In this context, integrated analytical components are needed for:

• Enhanced and fast process control using analytical data

• Fast and efficient determination of product distribution

• Quality control of products

Moreover, suitable miniaturized processes for product separation are of main interest These processes and components are important features with respect to increased automation and process control in microreaction systems

Fast and miniaturized GC-analysis

In order to monitor the reactant and product concentrations of a gas phase micro reactor for ethylene oxide synthesis, a miniaturized device for fast GC-analysis was developed in a joint effort with University of Mainz, Germany The introduction of short, packed capillary columns applied to the specific analytical problem of the separation of permanent gases, light hydrocarbons and their oxidation products significantly improved the separation efficiency compared to conventional, coated capillaries used in gas chromatography [13]

Additional developments dealt with the improvement of the heating and cooling process in GC-temperature programming in order to further increase analysis speed Results achieved with a 2D-test module for fast heating of short packed GC-columns clearly show the capability of this device for speeding up analysis time For example, using product mixtures of the ethylene oxide synthesis, retention time for the ethylene oxide peak was decreased to 80 seconds, a factor of four compared to conventional GC-analysis

Permeation device

A miniaturized module for high pressure product enrichment using membrane principles was developed in co-operation with the Max-Planck-Institute for Polymer Research in Mainz, Germany [14] Various micro structured supports, stacked upon each other and realized by photolithographie, electro forming and etching techniques assure a sufficient pressure stability of the system Nickel structures with a thickness of 80 11m and containing highly ordered honeycomb-like openings (60 11m in diameter) support the membrane material The module's

layers are incorporated within a pressure-proof stainless steel containment Currently, experimental characterization is under way

Fig 7 Components of a membrane module for product enrichment in microreactors

3.4 Standardized modular concepts

Standardization of microreactor components and their flexible arrangement to modular systems significantly enhances the availability of specific combinations

of unit operations, e.g for specific R&D-activities This functional variation of micro reaction systems is important for the widespread use of microreactors in R&D, education and small scale production, e.g of fine chemicals These stan-dardized components can be divided into three major groups

Basic modules

These basic units are the main part of any kind of reactor combination They represent unit operations of reaction engineering, e.g mixing, heat and mass transfer or separation processes as well as catalyst carriers and reaction units

Accessory modules

In order to guarantee flexibility in the system's layout regarding vanatlOn in process conditions and reaction sequences, a number of different components for connection and distribution of fluids are needed These assure interconnection between the system's components and to in- and outlet channels as well as fluid distribution to a certain number of microchannels and collection of individual reactant streams