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- Herausgeber: Wiley-VCH
- Kategorie: Fachliteratur
- Sprache: Englisch
Faster, cheaper and environmentally friendly, these are the criteria for designing new reactions and this is the challenge faced by many chemical engineers today.
Based on courses thaught by the authors, this advanced textbook discusses opportunities for carrying out reactions on an industrial level in a technically controllable, sustainable, costeffective and safe manner.
Adopting a practical approach, it describes how miniaturized devices (mixers, reactors, heat exchangers, and separators) are used successfully for process intensification, focusing on the engineering aspects of microstrctured devices, such as their design and main chracteristics for homogeneous and multiphase reactions. It adresses the conditions under which microstructured devices are beneficial, how they should be designed, and how such devices can be integrated in an existing chemical process. Case studies show how the knowledge gained can be applied for particular processes.
The textbook is essential for master and doctoral students, as well as for professional chemists and chemical engineers working in this area.
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Ähnliche
Table of Contents
Cover
Related Titles
Title Page
Copyright
Preface
List of Symbols
Commonly Used Symbols
Common Indices
Dimensionless Numbers
Abbreviations
Chapter 1: Overview of Micro Reaction Engineering
1.1 Introduction
1.2 What are Microstructured Devices?
1.3 Advantages of Microstructured Devices
1.4 Materials and Methods for Fabrication of Microstructured Devices
1.5 Applications of Microstructured Devices
1.6 Structure of the Book
1.7 Summary
References
Chapter 2: Basis of Chemical Reactor Design and Engineering
2.1 Mass and Energy Balance
2.2 Formal Kinetics of Homogenous Reactions
2.3 Ideal Reactors and Their Design Equations
2.4 Homogenous Catalytic Reactions in Biphasic Systems
2.5 Heterogenous Catalytic Reactions
2.6 Mass and Heat Transfer Effects on Heterogenous Catalytic Reactions
2.7 Summary
2.8 List of Symbols
References
Chapter 3: Real Reactors and Residence Time Distribution (RTD)
3.1 Nonideal Flow Pattern and Definition of RTD
3.2 Experimental Determination of RTD in Flow Reactors
3.3 RTD in Ideal Homogenous Reactors
3.4 RTD in Nonideal Homogeneous Reactors
3.5 Influence of RTD on the Reactor Performance
3.6 RTD in Microchannel Reactors
3.7 List of Symbols
References
Chapter 4: Micromixing Devices
4.1 Role of Mixing for the Performance of Chemical Reactors
4.2 Flow Pattern and Mixing in Microchannel Reactors
4.3 Theory of Mixing in Microchannels with Laminar Flow
4.4 Types of Micromixers and Mixing Principles
4.5 Experimental Characterization of Mixing Efficiency
4.6 Mixer Efficiency and Energy Consumption
4.7 Summary
4.8 List of Symbols
References
Chapter 5: Heat Management by Microdevices
5.1 Introduction
5.2 Heat Transfer in Microstructured Devices
5.3 Temperature Control in Chemical Microstructured Reactors
5.4 Case Studies
5.5 Summary
5.6 List of Symbols
References
Chapter 6: Microstructured Reactors for Fluid–Solid Systems
6.1 Introduction
6.2 Microstructured Reactors for Fluid–Solid Reactions
6.3 Microstructured Reactors for Catalytic Gas-Phase Reactions
6.4 Hydrodynamics in Fluid–Solid Microstructured Reactors
6.5 Mass Transfer in Catalytic Microstructured Reactors
6.6 Case Studies
6.7 Summary
6.8 List of Symbols
References
Chapter 7: Microstructured Reactors for Fluid–Fluid Reactions
7.1 Conventional Equipment for Fluid–Fluid Systems
7.2 Microstructured Devices for Fluid–Fluid Systems
7.3 Flow Patterns in Fluid–Fluid Systems
7.4 Mass Transfer
7.5 Pressure Drop in Fluid–Fluid Microstructured Channels
7.6 Flow Separation in Liquid–Liquid Microstructured Reactors
7.7 Fluid–Fluid Reactions in Microstructured Devices
7.8 Summary
7.9 List of Symbols
References
Chapter 8: Three-Phase Systems
8.1 Introduction
8.2 Gas–Liquid–Solid Systems
8.3 Gas–Liquid–Liquid Systems
8.4 Summary
8.5 List of Symbols
References
Index
End User License Agreement
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Guide
Cover
Table of Contents
Preface
Chapter 1: Overview of Micro Reaction Engineering
List of Illustrations
Figure 1.1
Figure 1.2
Figure 1.3
Figure 1.4
Figure 1.5
Figure 2.1
Scheme 2.1
Scheme 2.2
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Figure 2.7
Figure 2.8
Figure 2.9
Figure 2.10
Figure 2.11
Figure 2.12
Figure 2.13
Figure 2.14
Figure 2.15
Figure 2.16
Figure 2.17
Figure 2.18
Figure 2.19
Figure 2.20
Figure 2.21
Figure 2.22
Figure 2.23
Figure 2.24
Figure 2.25
Figure 2.26
Figure 2.27
Figure 2.28
Figure 2.29
Figure 2.30
Figure 2.31
Figure 2.32
Figure 3.1
Figure 3.2
Figure 3.3
Figure 3.4
Figure 3.5
Figure 3.6
Figure 3.7
Figure 3.8
Figure 3.9
Figure 3.11
Figure 3.10
Figure 3.12
Figure 3.13
Figure 3.14
Figure 3.15
Figure 3.16
Figure 3.17
Figure 3.18
Figure 3.19
Figure 3.20
Figure 3.21
Figure 3.22
Figure 3.23
Figure 3.24
Figure 3.25
Figure 3.26
Figure 3.27
Figure 3.28
Figure 3.29
Figure 3.30
Figure 3.31
Figure 3.32
Figure 3.33
Figure 3.34
Figure 3.35
Figure 3.36
Figure 3.37
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
Figure 4.7
Figure 4.8
Figure 4.9
Figure 4.10
Figure 4.11
Figure 4.12
Figure 4.13
Figure 4.14
Figure 4.15
Figure 4.16
Figure 4.17
Figure 4.18
Figure 4.19
Figure 4.20
Figure 4.21
Figure 4.22
Figure 4.23
Figure 4.24
Figure 4.25
Figure 4.26
Figure 4.27
Figure 4.28
Figure 4.29
Figure 4.30
Figure 4.31
Figure 4.32
Figure 4.33
Figure 4.34
Figure 5.1
Figure 5.2
Figure 5.3
Figure 5.4
Figure 5.7
Figure 5.5
Figure 5.6
Figure 5.8
Figure 5.9
Figure 5.10
Figure 5.11
Figure 5.12
Figure 5.13
Figure 5.14
Figure 5.15
Figure 5.16
Figure 5.17
Figure 5.18
Figure 5.19
Figure 5.20
Figure 5.21
Figure 5.22
Figure 5.23
Figure 5.24
Figure 5.25
Figure 5.26
Figure 5.27
Figure 5.28
Figure 5.29
Figure 5.30
Figure 5.31
Figure 5.32
Figure 5.33
Figure 5.34
Figure 5.35
Figure 6.1
Figure 6.2
Figure 6.3
Figure 6.4
Figure 6.5
Figure 6.6
Figure 6.7
Figure 6.8
Figure 6.9
Figure 6.10
Figure 6.11
Figure 6.12
Figure 6.13
Figure 6.14
Figure 6.15
Figure 6.16
Figure 7.1
Figure 7.2
Figure 7.3
Figure 7.4
Figure 7.5
Figure 7.6
Figure 7.7
Figure 7.8
Figure 7.9
Figure 7.10
Figure 7.28
Figure 7.11
Figure 7.12
Figure 7.13
Figure 7.14
Figure 7.15
Figure 7.16
Figure 7.17
Figure 7.18
Figure 7.19
Figure 7.20
Figure 7.21
Figure 7.22
Figure 7.23
Figure 7.24
Figure 7.25
Figure 7.26
Figure 7.27
Figure 8.1
Figure 8.2
Figure 8.3
Figure 8.4
Figure 8.5
Figure 8.6
Figure 8.7
Figure 8.8
Figure 8.9
Figure 8.10
Figure 8.11
List of Tables
Table 1.1
Table 2.1
Table 2.2
Table 3.1
Table 3.2
Table 3.3
Table 3.4
Table 4.1
Table 4.2
Table 5.1
Table 5.2
Table 5.3
Table 5.4
Table 5.5
Table 5.6
Table 5.7
Table 6.1
Table 6.2
Table 6.3
Table 6.4
Table 7.1
Table 7.2
Table 7.3
Table 7.4
Table 7.5
Table 7.6
Table 7.7
Table 7.8
Table 7.9
Table 7.10
Table 7.11
Table 7.12
Table 7.13
Table 7.14
Table 8.1
Table 8.2
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Madhvanand N. Kashid, Albert Renken, and Lioubov Kiwi-Minsker
Microstructured Devices for Chemical Processing
The Authors
Dr. Madhvanand N. Kashid
Ecole Polytechnique Fédérale de Lausanne
EPFL-SB-ISIC-GGRC
1015 Lausanne
Switzerland
and
Syngenta Crop Protection Monthey SA
Route de l'Ile au Bois
1870 Monthey
Switzerland
Prof. Dr. Albert Renken
Ecole Polytechnique Fédérale de Lausanne
EPFL-SB ISIC-LGRC, Station 6
1015 Lausanne
Switzerland
Prof. Dr. Lioubov Kiwi-Minsker
Ecole Polytechnique Fédérale
EPFL-SB ISIC-LGRC, Sation 6
1015 Lausanne
Switzerland
All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany
All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law.
Print ISBN: 978-3-527-33128-4
ePDF ISBN: 978-3-527-68519-6
ePub ISBN: 978-3-527-68518-9
Mobi ISBN: 978-3-527-68523-3
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Preface
This book is written based on the potential use of microstructured devices in chemical equipment and the intensification of chemical processes. The term “microstructured devices” is coined based on their characteristic dimensions that are in the submillimeter range and on their different types such as mixers, reactors, heat exchangers, and separators. Owing to the small characteristic dimensions, diffusion times are short and the influence of transport phenomena on the rate of chemical reactions is efficiently reduced. Heat transfer is greatly enhanced compared to conventional systems, allowing a strict control of temperature and concentration gradients leading to an improved product yield and selectivity. In addition, safe reactor operation is possible under unconventional conditions such as high reaction temperatures and reactant concentrations. As a consequence, novel process windows can be opened, but not accessible with traditional systems. Therefore, microstructured devices are versatile tools for the development of sustainable chemical processes.
This book focuses on reaction engineering aspects, such as design and characterization, for homogeneous and multiphase reactions. On the basis of chemical reaction engineering fundamentals, it addresses the conditions under which these devices are beneficial, how they should be designed, and how such devices can be integrated or applied in a chemical process.
Designed as a pedagogical tool with target audience of university students and industrial professionals, it seeks to bring readers with no prior experience of these subjects to the point where they can comfortably enter into the current scientific and technical developments in the area. However, this book does not include the cross-disciplinary subjects such as fabrication techniques of these devices, integration of sensors and actuators, and their use for biological applications.
To facilitate comprehension, the topics are developed beginning with fundamentals in chemical reaction engineering with ample cross-referencing. The understanding of concepts is facilitated by clear descriptions of examples, supplied by exercises including solutions, and provided by figures and illustrations.
Finally, the authors want to highlight the complexity of microreaction engineering in particular. Therefore, this book must be viewed as a tool for stimulation of novel and meaningful solutions for the complex chemical reaction realities. It is also important to note that the growing interests and complementary developments of this subject require periodic updates.
Lausanne, Switzerland
Madhvanand Kashid,
May 2014
Albert Renken,
Lioubov Kiwi-Minsker
List of Symbols
Commonly Used Symbols
This is a list of commonly used symbols. Besides, there are some special symbols used for each chapter which are listed chapterwise.
Symbols
Significance
Unit
A
Exchange or surface area
m
2
a
Specific interfacial area or catalytic surface area per reactor volume
m
2
m
−3
A
cs
Cross-section area
m
2
Bo
Bond number
—
Bo
Bodenstein number
—
Bi
m
,
Bi
th
Biot number (mass), Biot number (thermal)
—
C
Dimensionless concentration
—
Ca
Capillary (=) or Carberry (=) number
—
c
i
Concentration of molecule
A
i
mol m
−3
c
p
Heat capacity of fluid or mixture
J kg
−1
K
−1
DaI
First Damköhler number
—
DaII
Second Damköhler number
—
DaII
mx
Second Damköhler number for mixing
—
D
ax
Axial dispersion coefficient
m
2
s
−1
De
Dean number
—
D
eff
,
D
m
Effective molecular diffusion coefficient, molecular diffusion coefficient
m
2
s
−1
d
h
Hydraulic diameter
m
d
t
Diameter of channel (or tube)
m
E
,
E
a
Intrinsic activation energy, apparent activation energy of reaction
j
J mol
−1
f
Ratio of residual concentration to initial
—
Fo
Fourier number
—
g
Gravitational acceleration
m
2
s
−1
H
Height
m
h
Heat transfer coefficient
W m
−2
K
−1
Ha
Hatta number
—
J
i
Molar flux of species i
mol m
−2
s
−1
k
,
k
r
, k
j
Reaction rate constant for homogeneous and quasi-homogenous, constant of heterogenous reaction, constant of reaction
j
variable (s
−1
(mol m
−3
)
−(
n
−1)
)
k
0
Pre-exponential or frequency factor
variable (s
−1
(mol m
−3
)
−(
n
−1)
)
K
C
Reaction equilibrium constant
variable
K
thermodynamic equilibrium constant
—
k
G
Mass transfer coefficient in gas phase
m s
−1
k
GL
Mass transfer coefficient in gas–liquid system
m s
−1
k
L
Mass transfer coefficient in liquid phase
m s
−1
Volumetric mass transfer coefficient
s
−1
k
m
Mass transfer coefficient of heterogeneous reactions
m s
−1
k
ov
Overall mass transfer coefficient
m s
−1
L
, L
c
,
L
e
,
L
t
Length, characteristic length, length of entrance zone, length of tube or channel
m
Mass flow rate
kg s
−1
Nu
Nusselt number
—
Reaction order with respect to species
A
i
—
n
Overall reaction order
—
No of moles of molecule
A
i
mol
Molar flow rate of molecule
A
i
mol s
−1
p
Pressure
Pa
P
i
Rate of production
mol s
−1
Pr
Prandtl number
—
Pe
Péclet number
—
Q
Energy
J
Rate of heat flow
W
,
,
Specific heat rate, of reaction, of heat exchange/transfer
J m
−3
s
−1
R
Ideal gas law constant
J mol
−1
K
−1
R
Radius
m
Re
Reynolds number
—
R
i
Overall reaction/transformation rate of molecule
A
i
mol m
−3
s
−1
r
j
,
r
eff
Rate of reaction/transformation of reaction
j
, effective reaction rate
mol m
−3
s
−1
r
ads
,
r
des
Rates of adsorption, of desorption
—
S
k
,
i
Selectivity of product
k
with respect to reactant
i
—
s
k
,
i
Instantaneous selectivity of product
k
with respect to reactant
i
—
Se
Semenov number
—
Sc
Schmidt number
—
Sh
Sherwood number
—
T
,
T
b
,
T
s
Temperature, bulk temperature, surface temperature
K
t, t
c
, t
D
, t
r
,
t
m
,
t
mx
,
t
ax
,
t
D,
ax
,
t
D,
rad
Time, characteristic cooling time, diffusion time, reaction time, mass transfer time, mixing time, axial dispersion time, axial molecular diffusion time, radial diffusion time
s
Mean residence time
s
U
Overall heat transfer coefficient
W m
−2
K
−1
U
i
Internal energy
J
U
v
Overall volumetric heat transfer coefficient
W m
−3
K
−1
u
,
u
b
, u
(
r
),
u
G
,
u
L
Superficial velocity, velocity of gas bubble (slug), velocity at radial position
r
, superficial flow velocity of gas phase, superficial velocity of liquid phase
m s
−1
V
,
V
R
Volume, internal (reaction) volume
m
3
Volumetric flow rate
m
3
s
−1
W
Width
m
,
,
Rate of work done, by flow, by shaft
J s
−1
X
Conversion
—
Y
k
,
i
Yield of product
k
with respect to reactant
i
—
Z
Dimensionless length
—
z
Length
m
Greek symbols
α
Thermal diffusivity
m
2
s
−1
Prater number
—
δ
(
z
)
Dirac pulse
—
Film thickness, catalytic layer or boundary layer
m
γ
Arrhenius number
—
Shear rate
s
−1
Δ
Symbol of difference
—
Δ
G
Gibbs free energy
J mol
−1
Δ
H
r
, Δ
H
a
Heat of reaction, heat of adsorption
J mol
−1
Δ
p
Pressure drop
Pa
Entropy
J mol
−1
K
−1
Adiabatic temperature rise
K
Specific power dissipation
W kg
−1
p
,
bed
Porosity of catalyst pallet, of randomly packed bed
—
η
Efficiency factor
—
θ
Dimensionless time
—
λ
,
λ
eff
,
λ
f
,
λ
wall
Thermal conductivity, effective, of fluid, of wall
W m
−1
K
−1
μ
Dynamic viscosity
Pa s
Kinematic viscosity
m
2
s
−1
Stoichiometric coefficient of species
i
in reaction
j
—
Geometric factor
—
ρ
Density
kg m
−3
σ
Interfacial tension
N m
−1
τ
,
τ
PFR
,
Residence time, of plug flow reactor, of reactor, residence time referred to reaction volume
s
Common Indices
Subscript
0
Initial value
∞
Asymptotic or infinite value
app
Apparent or observed
av
Average
Ax
Axial
b
Bulk
c
Cooling
cap
Hemispherical cap
cat
Catalyst
eff
Effective
eq
Equilibrium
ex
External
film
Wall film
gen
General
I
Phase I
II
Phase II
in
Inlet
max
Maximum
min
Minimum
out
Outlet
op
Optimum
ov
Overall
P
Pallet
s
Surface
v
Volumetric
Superscript
0
Values at standard conditions
Dimensionless Numbers
Dimensionless number
Significance
Definition
Adiabatic temperature rise
Property of reaction mixture, represent temperature rise in worst case and is independent of reactor type/reaction rate
Arrhenius number
Relative importance of activation temperature (
E
/
R
) to system bulk temperature (
T
b
)
Biot number (mass)
Relates external mass or heat transfer rates at catalyst pallet surface to diffusion or conduction inside the pallet
Biot number (thermal)
Bodenstein number
Ratio of convective transport rate to (axial) diffusion transport rate
Carberry number
It gives effective reaction rate over mass transfer rate in catalytic reactions where no internal (pellet) mass and heat transfer resistances are considered
Capillary number
Used in fluid–fluid systems. It is ratio of viscous forces to surface tension acting across an interface, that is, interfacial tension
First Damköhler number
Used to set design criteria – ratio of residence time in the reactor to the characteristic reaction time
Second Damköhler number
Used to set design criteria – ratio of reaction rate to mass transfer rate
Second mixing Damköhler number
Used to set design criteria – ratio of reaction rate to mixing rate
Dean number
Used to characterize the flow in curved channels – it is product of
Re
and square root of channel diameter to curvature radius
Efficiency (reactor) factor (fluid–fluid system)
Ratio of effective reaction rate and the maximal rate referred to the reactor volume corresponding to the maximum concentration in the reacting phase
Effectiveness factor (porous catalyst)
Ratio of effective reaction rate and the rate of reaction at bulk concentration and temperature
Effectiveness factor (mass transfer) or trade-off index
Used to access mass transfer performance with energy input
Euler number
It is ratio of pressure drop in a given reactor length to kinetic energy.
Fourier number
It is ratio of residence time to diffusion time
Hatta number
Used for fluid–fluid systems and signifies whether the reaction takes place in the bulk or near the interface (of reaction phase). It is ratio of reaction rate to interfacial mass transfer rate
Nusselt-number
Use to characterize relative importance of convective heat transfer over conductive heat transfer
Peclet number
Ratio of rate of convection to rate of diffusion/dispersion
Prandtl number
Used to characterize momentum and heat diffusion – ratio of momentum (viscous) diffusion to molecular diffusion
Prater number
Ratio of maximum temperature difference catalyst center and surface temperature to the surface temperature
Reynolds number
Most commonly used to characterize the fluid flow – gives relative importance of inertial forces over viscous forces
Reynolds number (particle)
Reynolds number (foam)
Schmidt number
Used to characterize momentum and mass diffusion – ratio of momentum (viscous) diffusion to molecular diffusion
Sherwood number (particle)
Use to characterize relative importance of convective mass transfer over diffusional mass transfer
Sherwood number
Thiele modulus
Ratio of characteristic diffusion time in the catalyst and the characteristic reaction time
first order reaction;
Weisz modulus
Used to measure influence of transport process on reaction kinetics experimentally – ratio of effective reaction rate to (effective) diffusion rate
Bond number
Relates body forces to surface tension forces
First Damköhler number (mass transfer)
Ratio of residence time in the reactor to the characteristic mass transfer time
Abbreviations
BSTR
Batchwise-operated stirred tank reactor
CSTR
Continuously-operated stirred tank reactor
CVD
Chemical vapor deposition
LIGA
Lithography, galvanization, and molding
MASI
most abundant surface intermediate
MSR
Microstructured reactors
PFR
Plug flow reactor
PRL
Power rate law
PVD
Physical vapor deposition
RTD
Residence time distribution
SMF
Sintered metal fiber
SLPC
Supported liquid phase catalyst
SCR, SAR, SHR
Serpentine channel reactor, split and recombine reactor, staggered herringbone reactor
Chapter 1Overview of Micro Reaction Engineering
This chapter is a comprehensive introduction to the field of micro reaction engineering – an increasingly relevant and rapidly expanding segment of Chemical Reaction Engineering and Process Intensification. Here emphasis is placed on the definition of the term “micro-reactor,” which is often used in various contexts to describe different equipments such as micro-mixers and micro-heat-exchangers. The more well-recognized term is microstructured devices. The advantages and limitations of these microstructured devices are compared to conventional chemical production equipments.
1.1 Introduction
Every industrial process is designed to produce a desired product in the most economical way. The large-scale production of chemicals is mostly carried out using different equipments, such as mixers, reactors, and separators with typical dimensions up to a few meters. The process classification is often referred to as “scale” and depends on the volume and quality of the product. The classifications are bulk chemicals, intermediates, and fine chemicals processes. The bulk chemicals are produced in large quantities in dedicated production units. The intermediate scale products and fine chemicals are produced in the plants mostly dominated by batch processing. Batch reactors are flexible and can be easily shared between multiple products. Therefore, they are considered to be suitable over centuries and there has been no radical change in the batch processing technology. However, in many cases conventional equipment is not sufficiently efficient. In this context, there is a need to develop chemical industries implementing sustainable technology.
There are two main approaches to reach this target: chemical and engineering. In the first one, the improvements are achieved by alternative synthesis and processing routes, for example, developing highly selective catalysts and using special reaction media – a typical chemical approach. In the second one, the mass- and heat-transport rates are improved, for example, by increasing the specific interfacial area and thus reducing the diffusion path lengths. This in turn helps to enhance the safety by virtue of the lower hold-up and superior temperature control, even for strongly exothermic reactions. In addition to this, the reactor performance is enhanced operating reactors dynamically [1, 2], and using non-conventional energy sources. This overall development is often referred to as “Process Intensification”, which can be defined in various ways depending on the application involved. However, a generic definition summarising the above discussion is given in the following [3]: “Any chemical engineering development that leads to a substantially smaller, cleaner and more energy efficient technology is Process Intensification!” Micro-technology is one of the powerful tools to attain the goals of process intensification.
1.2 What are Microstructured Devices?
The concept of process intensification using miniaturized equipments was pioneered by Professor Ramshaw and his group at Imperial Chemical Industries (ICI), UK, in the late 1970s, who considered how one might reduce equipment size by several orders of magnitude while keeping the same production rate [3]. The objectives were to reduce cost (smaller equipment, reduced piping, low energy, increased reactivity – higher yields/selectivity, reduced waste, etc.), to enhance safety (low hold-up and controlled reaction conditions), to make a compact size of the plant (much higher production capacity and/or number of products per unit of manufacturing area), and to reduce plant erection time and commissioning time (time to market). These miniaturized systems are the chemical processing systems in three-dimensional structures with internal dimension in submillimeter range. They are referred to as microstructured devices, microstructured reactors, ormicroreactors, and the research field is referred to as “microreactor” or “microreaction” technology.
The advantages and limitations of these devices come from the dimensions increasing greatly the transport processes and the high specific surface area (surface to volume ration). This is described in the following subsection.
1.3 Advantages of Microstructured Devices
1.3.1 Enhancement of Transfer Rates
Let us consider Fourier's law to describe the influence of transfer scales on heat transfer rates. For simplicity, Fourier's law for the flux in one-dimensional space can be written as
where Q is the heat energy (J), λ is thermal conductivity (W mK−1), and A is the heat transfer surface area (m2). The temperature gradient is the driving force for heat transfer. From Equation 1.1, for a given temperature difference, a decrease in the characteristic dimension results in an increase in these gradients and thus in higher heat transfer rates. The same analogy of concentration and momentum gradient could be applied to mass and momentum transfer resulting in higher mass transfer rates.
Besides the effect of decreasing linear dimensions on the corresponding gradients, the effective surface area for exchange processes has to be considered. Let us integrate Equation 1.1 for a unit volume of reactor:
where U (= ) is the overall heat transfer coefficient (W·m−2 K−1) and a is the specific surface area (surface area per unit volume, m2·m−3). For a circular tube, , where dt is the tube diameter. Thus, with decreasing characteristic dimensions, the specific surface area of the system increases leading to higher overall performances.
The surface to volume ratio for microdevices can be as high as 50 000 m2 m−3 [4]. For comparison, the specific surface area of typical laboratory and production vessels seldom exceed 100 m2 m−3. Moreover, because of the laminar flow regime within microcapillaries, the internal heat transfer coefficient is inversely proportional to the channel diameter. Therefore, overall heat transfer coefficients up to 25 000 W m−2 K−1 can be obtained, exceeding those of conventional heat exchangers by at least 1 order of magnitude [5]. Indeed, conventional heat exchangers have overall heat transfer coefficients of less than 2000 W m−2 K−1 [6]. Similar performance enhancement could be realized by the miniaturization for mass transfer leading to efficient mixing. For multiphase systems within microdevices, the interfacial surface to volume ratio between the two fluids is notably increased. Indeed, the miniaturized systems possess high interfacial area up to 30 000 m2 m−3. The traditional bubble columns do not exceed a few 100 m2 m−3 [7].
The characteristic time of chemical reactions, tr, which is defined by intrinsic reaction kinetics, can vary from hours (for slow organic or biological reactions) to milliseconds (for high temperature oxidation reactions) (Figure 1.1). When the reaction is carried out in an eventual reactor, heat and mass transfer interfere with the reaction kinetics.
Figure 1.1 Time scale of chemical and physical processes [8].
(Adapted with permission from Elsevier.)
The transfer rates presented above results in the characteristic time of physical processes (heat/mass transfer) in conventional reactors ranging from about 1 to 102 s. This means that relatively slow reactions (tr 10 s) are carried out in the kinetic regime, and the global performance of the reactor is controlled by the intrinsic reaction kinetics. The chemical reactor is designed and dimensioned to get the required product yield and conversion of the raw material. The attainable reactant conversion in the kinetic regime depends on the ratio of the residence time in the reactor to the characteristic reaction time (tr).
Depending on the kinetics and the type of the reactor, the residence time should be several times higher than the characteristic reaction time to get conversions >90% [9, 10].
For fast chemical reactions, the characteristic reaction time is in the same order of magnitude as the characteristic time for the physical processes (Figure 1.1). The performance of a conventional reactor is influenced in this case by mass and/or heat transfer. For very fast reactions, the global transformation rate may be completely controlled by transfer phenomena. As a result, the reactor performance is diminished as compared to the maximal performance attainable in the kinetic regime, and the product yield and selectivity is very often reduced.
To avoid mass and heat transfer resistances in practice, the characteristic transfer time should be roughly 1 order of magnitude smaller compared to the characteristic reaction time. As the mass and heat transfer performance in microstructured reactors (MSR) is up to 2 orders of magnitude higher compared to conventional tubular reactors, the reactor performance can be considerably increased leading to the desired intensification of the process. In addition, consecutive reactions can be efficiently suppressed because of a strict control of residence time and narrow residence time distribution (discussed in Chapter 3). Elimination of transport resistances allows the reaction to achieve its chemical potential in the optimal temperature and concentration window. Therefore, fast reactions carried out in MSR show higher product selectivity and yield.
The relative heat and mass transfer performance of microstructured reactors with respect to conventional reactors is depicted in Figure 1.2. As can be seen, both in terms of heat and mass transfer, as explained above, microstructured devices offer superior performance.
Figure 1.2 Benchmarking of microstructured reactors.
(Adapted from Ref. [11]. Copyright © 2009, John Wiley and Sons.)
A simplified algorithm for a single step homogenous reaction that could help in choosing conventional and microstructured devices based on kinetics, thermodynamics, and transport rates is presented in Figure 1.3. Here tr, ΔHr, and tmxare characteristic reaction time, heat of reaction, and characteristic mixing time, respectively. In the case of heterogenous reactions, mixing time would be replaced by characteristic mass transfer time. For a thermodynamically favored reaction, the chemical kinetics could be obtained for different operating conditions such as temperature, pressure, concentrations rendering a reaction rate equation allowing process optimization.
Figure 1.3 An algorithm showing choice of reactor based on reaction kinetics, thermodynamics, and mixing rates for a homogeneous reaction.
As described before, the limiting factor can be the intrinsic kinetics, the thermodynamics, or the heat and mass transfer of the reacting system. The characteristic reaction time of the reaction is then obtained for the operating conditions where the reaction can be operated under temperature control and the product is not decomposed. If the characteristic reaction time is less than 1 s and the heat of reaction is more than −50 kJ mol−1, the use of microstructured devices is proposed. However, even if the reaction time is high and heat of reaction is relatively low, the microstructured devices could be used to enhance the mixing leading to higher productivity.
1.3.2 Enhanced Process Safety
Process safety is an important issue for chemical industry in general and for exothermic reactions and reactions involving hazardous chemicals in particular. High hold-up of reactants in conventional batch reactors leads to very high impact in the case of accidents. A common approach to handle fast exothermic reactions is through dilution of the reactants by solvents or using semibatch mode, which is the slow addition of one of the reactants.
Microstructured devices are safer than conventional devices because of the small amount of reactants and products inside the reactor. Indeed, in case of failure, the small amount of eventual toxic chemicals released can easily be neutralized [6]. The high heat transfer performance of microdevices allows rapid heating and cooling of the reaction mixture, avoiding hot or cold spots and providing nearly isothermal conditions [5]. Under the predominant laminar regime, the volumetric heat transfer resistance at the reactor microchannel side is proportional to the square of the reactor diameter. In principle, by using the strong dependence of the heat transfer rates on the reactor diameter, any exothermic reaction can be controlled by adjusting the reactor diameter [12].
1.3.3 Novel Operating Window
In the case of slow reactions, the transformation rate is limited by intrinsic kinetics. A drastic increase of the temperature allows exponential acceleration of the reaction rate in agreement with the Arrhenius Law. Moreover, the pressure can be advantageous to accelerate reactions, to shift equilibrium, to increase gas solubility, to enhance conversion and selectivity, to avoid solvent evaporation, and to obtain single-phase processes [8, 13]. The overall transformation rate of such reactions could be significantly increased in these novel operating windows.
Using microstructured devices, these reactions could be performed in novel operating windows under more aggressive conditions than in conventional devices. The pressure can easily be increased to several hundred bars because of the small reaction volumes and low mechanical stress. The microdevices allow an easy control of process parameters such as pressure, temperature, and residence time. Thus, an unconventional operating window, that is, high temperature, pressure, and concentrations could be used within microstructured devices even in explosive and thermal runaway regimes [8].
Operating MSR under novel process windows, the key performance parameters can be increased by a few orders of magnitude. A few examples are presented here. In the case of esterification of phthalic anhydride with methanol 53-fold higher reaction rate between 1 and 110 bar for a fixed temperature of 333 K was observed [14]. A multiphase (gas/liquid) explosive reaction of oxidation of cyclohexane under pure oxygen at elevated pressure and temperature (>200 °C and 25 bar) in a transparent silicon/glass MSR increased the productivity fourfold. This reaction under conventional conditions is carried out with air [15]. Another example is for the synthesis of 3-chloro-2-hydroxypropyl pivaloate: a capillary tube of 1/8 in. operated at 533 K and 35 bar, superheated pressurized processing much above the boiling point, allowed to decrease reaction time 5760-fold as compared to standard batch operation [16]. The condensation of o-phenylenediamine with acetic acid to 2-methylbenzimidazole in an MSR is an impressive example of the reduced reaction time from 9 weeks at room temperature to 30 s at 543 K and 130 bar [17].
1.3.4 Numbering-Up Instead of Scale-Up
Microstructured devices bring in fundamental changes in the approach toward the step from laboratory to industrial scale. Conventionally, the size of the laboratory reactor or flask is upgraded to a few cubic meters to meet the target productivity through different steps including pilot scale studies. This involves cost and time expense scaling up. The numbering-up (also referred to as scale-out) concept consists of an increase in the number of parallel operating units preserving the advantages of MSR, particularly their high surface to volume ratio. This approach is simpler and faster than the conventional processes (no redesign and pilot plant experiments), thus, decreasing considerably the time between discovery and production and hence shortens the time to market. The break-even point of the cash flow curve could be reached at an earlier point of time, which renders the whole concept more appealing. Moreover, the numbering-up strategy allows to adapt the production to the market demand by increasing or decreasing the number of units as well as an earlier start of production resulting in a lower cost.
There are two ways of numbering-up of microstructured devices: internal and external (Figure 1.4). For external numbering-up, multiple identical units are operated in parallel. The advantage is that each single unit is independent of the others and performs as the developed lab-scale unit. However, as each unit will need individual equipment (such as pumps, tubing, flow meters), the costs of external numbering-up are considerable.
Figure 1.4 Schematics of Numbering-up of microstructured reactors: (a) external numbering-up, (b) internal numbering-up [18].
(Adapted with permission from Elsevier.)
When numbering-up is carried out internally, the amount of equipment is reduced and thus the cost is lower. The fluids in this case are contacted in a mixing zone and subsequently are distributed into the reaction channels, where conditions are similar to the lab-scale single channel device. The plates or chips fabricated or the standard microtubes that are used as MSR are assembled in two types of geometries: monolith geometry and multiplate geometry. In the former case the inlet stream is distributed simply between all the channels through a large distributor, while in the second case, the inlet stream is first divided into different plates/layers and then distributed into channel plates.
The main problem for internal numbering-up to overcome is the equal distribution of fluids to the multiple channels. Equal distribution is indispensable to obtain identical reaction conditions in each channel to ensure high reactor performance and safety. The eventual maldistribution in the microchannels leads to a broad residence time distribution and can even result in the clogging of some of the channels, thus affecting product yield and selectivity [18].
The flow nonuniformities generally occur because of two reasons: a poor reactor design and manufacturing tolerances. The manufacturing tolerances usually cause variations of local temperature in the range of about 5%; the former reason can cause flow ratios in different channels more than a factor of 4 [19].
A further disadvantage of internal numbering-up is the absence of reaction control in the mixing zone. As soon as both reactants are contacted, reaction is initiated and heat is generated. However, heat can be efficiently removed from the channels placed after the distribution section. Possible solutions to overcome the mentioned problems are discussed in chapter 5.
1.4 Materials and Methods for Fabrication of Microstructured Devices
The steps in the selection and use of suitable microstructured devices for a particular chemical production are depicted in Figure 1.5. Majority of steps corresponds to the procedure that is followed for conventional equipments. However, more emphasis is placed on fabrication techniques as it involves structures in micro-, nanometer scale requiring very precise fabrication techniques. In addition, they should be able to accommodate, either individually or combined, these structures and sensors that are required to control the process.
Figure 1.5 Steps in the selection and use of microstructured devices for chemical production.
Different materials such as metals, glass, polymers, and ceramics are used to fabricate the microstructured devices. Various techniques such as etching, lithography, electroplating, molding, polymer microinjection molding and embossing are applied to make the microscale channels of different cross-sectional geometries (e.g., circular, rectangular, square).
Some of the commonly used techniques to fabricate complex structures of microdevices are summarized briefly in the following.
The
LIGA
(German acronym Lithographie, Galvanoformung und Abformung of Lithography, Electroplating, and Molding) process
: The LIGA process can be expected to be superior to other methods for fabrication microstructures with high aspect ratios (ratio of height to width) and to produce microstructures of complex shapes [21]. This technique is suitable for a broad range of materials, including metals, alloys, polymers, as well as ceramics and composites to fabricate net-shaped high aspect ratio components. In this technique a layer of photo resisting resin is deposited on a base plate and exposed to high-energy X-rays to the area that is to be etched [22]. The irradiated portions of the photoresist are dissolved and eliminated with a solvent obtaining a three-dimensional structure. The metal is deposited on this structure by electroforming. This metal structure can either be used as it is or it can serve as a mold for making a large number of parts through injection molding or stamping. This technique also uses an inexpensive Ultraviolet (UV) light source, but precision is an issue in this case.
Micro
-EDM
(Electrical Discharge Machining)
: This technique uses electric discharge or sparks created between a workpiece and an electrode in dielectric fluid. When the workpiece and the electrode are separated by a small gap, called as spark gap, a pulsed discharge occurs, which removes material from the workpiece through melting and evaporation [23]. It is possible to etch as fine as 50 µm on the surface of the contacting plate using the programmable movement of the electrode [24]. A metal form obtained with another process can also be used as an electrode in order to obtain the desired surface on the substrate.
Wet etching
: This is a highly selective technique and involves chemical reactions. The metal plate to be etched is first covered with a resist using spin-coating or lamination technique. The resist is further structured via different techniques, for example, it is irradiated with UV light through a mask giving a structured resist layer partly covering the metal. Etching of the metal in the uncovered area with subsequent removal of resist gives microstructures [25].
Dry etching
: In this technique, an ion beam is directed on the surface to be etched. This process is easy to use, but it is rather expensive [26] and limited to a certain materials (e.g. silicon), which is not always suitable for a given application.
Recently a novel manufacturing technique for microstructured reactors was proposed [27]. This technique is based on a cheap and resource-efficient production of structured plates by using roll embossing. The stacked plates are joined by laser welding or vacuum brazing. The method allows manufactoring microstructured reactors in a wide range of throughput, pressure and temperature for homogeneous and multi-phase reactions.
1.5 Applications of Microstructured Devices
Microstructured devices have been successfully used for the continuous processing of fast and highly exothermic reactions [28] and chemical transformations involving toxic, sensitive, and explosive chemicals such as nitration [29], hydrogenations [30, 31], polymerization [32, 33], oxidation [34], halogenations [4], alkylation [35], tetrazole synthesis [36], and reaction of diazomethane [37]. With newly developed online monitoring techniques, MSR can be used as a powerful laboratory tool to investigate the reaction mechanism and kinetics.
1.5.1 Microstructured Reactors as Research Tool
Over the past two decades, MSR have been used in the laboratory as well as in industries. Such activities are regularly reported on various scientific meetings and conferences. One of the most important conferences, IMRET (International Conference on Microreaction Technology), was started in 1997 (Frankfurt, Germany) and is being followed up successful till date. Information on this research area is available in the form of books [7, 38–40]. In addition to the usual update through scientific journal papers, quite a few reviews are published on MSR [4, 38, 41–46]).
Microstructured devices are replacing conventional flasks or laboratory reactors as a laboratory tool. Let us take the example of an investigation of reaction mechanism and kinetics of fast and highly exothermic reactions. It is conventionally carried out using the measurements from either continuous online monitoring (e.g., stopped-flow technique [47]) or offline analysis [48]. Two difficulties arise in the former case: integration of efficient cooling to suppress temperature rise because of spectroscopic devices and separation of intermediates for identification. Therefore, only offline method combined with sample quenching (freeze/chemical) can allow catching intermediates providing information about reaction mechanism. The quenching for offline analysis is done either by adding an agent to the continuous stream [36] (referred to as quenched-flow technique) or to a container that collects the sample. MSR have been successfully used to overcome such problem. A slug-flow MSR binds both reactants in the dispersed microliter droplets, which are carried by an inert fluid within a microcapillary. Each isolated slug acts as a micro-batch reactor. Because of the small size of the reactor assembly, it can be incorporated into commercially available calorimeters with precise temperature control. This concept has been applied for the investigation of enzymatic reaction kinetics [49], bromination of styrene [50], hydrolysis of p-nitrophenyl acetate [51], high-throughput catalyst screening [52], and cyclization of Pseudoionone [53].
Besides, there are several reports on mixing and kinetics studies involving handling of hazardous reactants using both invasive and noninvasive techniques.
1.5.2 Industrial/Commercial Applications
The prominent industries that benefited from microstructured devices are pharmaceutical, specialty, and fine chemical industry. A few micro-plants have been developed and successfully tested in the laboratory as well as on commercial sites Table 1.1. The industries that are working in this area are Cellular Process Chemistry Systems GmbH, Fraunhofer ICT-IMM, Micronit Microfluidics, Ehrfeld BTS GmbH, Systanix, Inc., Styrris, to name but a few. In 2005 Fraunhofer ICT-IMM, Germany, has made successful runs of a continuous microstructured nitroglycerine plant (capacity 15 kg h−1) at the site of Xi'an Chemical Industrial Group HAC in China [54]. This plant consists of three main parts: the mixing of sulfuric acid with nitric acid (both highly concentrated fuming liquids), the reactor, and the phase separation (washing and purification devices). On demand, glycerine and the acid mixture are fed separately into an MSR where mixing occurs within milliseconds. Such micro-plant has also been developed for the polymerization of Methyl methacrylate [32] and showed significant improvement in the control of molecular weight because of superior heat transfer efficiency. The plant constructed by numbering-up eight tubes (microreactors) was continuously operated for 6 days without any problems.
Table 1.1 Examples of industries/institutes active in microprocess engineering
MSR activities
Company/institute
MSR/mixer design and fabrication, process development
Fraunhofer ICT-IMM Mainz [57, 58], Forschungszentrum Karlsruhe GmbH [59], Ehrfeld Mikrotechnik BTS [60], Microinnova Engineering GmbH [61]
MSR design and fabrication, development of laboratory systems
Mikroglas GmbH [62], Mikronit microfluidics [63], Little Things Factory [64], Syrris [65]
Engineering services of MSR
Bayer Technology Services [66], Alfa Laval [67]
Development of MSR materials
Corning [68, 69]
MSR process development and demonstration of industrial production
Merck [70], SK Chemicals [71], Ampac Fine Chemicals [72], Phoenix Chemicals [73], Clariant GmbH [74], DSM [69, 75, 76], Lonza [43, 44], Sigma-Aldrich [77]
Microreactor technology is an important part of the decentralized mobile plant concept. Such plants have already been developed for biological applications (e.g., miniature analytical thermal cycling instrument, MATCI, to amplify and detect DNA via the polymerase chain reaction in real-time [6]). The objective behind chemical mobile plants is to reduce the risk associated with transporting hazardous chemicals. Rather than transporting hazardous chemicals, a distributed production strategy may be used with economic manufacturing on consumers' site, as is currently performed for oxygen and nitrogen [7]. Recently, a chemical plant has been developed by Ehrfeld Mikrotechnik BTS GmbH, Germany, in a briefcase, which can be used for a variety of applications.
Microstructured devices have several applications in pharmaceutical and fine chemicals where production amounts are often less than a few metric tons per year [55]. The processes that rely on batch or semibatch mode could be operated continuously with multiple advantages over the batch processes. A review on the benefits of such devices for pharmaceutical industrial processes cites that 50% of the reactions in fine chemical/pharmaceutical industry could benefit from a continuous process [43]. In drug industry, there is always a strong time pressure to bring new molecules on the market to maximize the profit because of the manufacturing patent life of 20 years [56]. The process development time could be reduced significantly (Table 1.1).
1.6 Structure of the Book
The purpose of this book is to present the engineering aspects of microstructured devices. It addresses these questions: under which conditions microstructured devices are beneficial; how the devices should be designed; and, finally, in which way microstructured devices can be integrated in a chemical process. It also includes several theoretical and practical design examples on which the industry personnel have been working for several years. Some of these examples are included as exercise for the master and doctoral students in the curriculum at Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland.
This book is divided into two parts: homogenous and multiphase (heterogenous) systems. Prior to explaining homogenous systems, a chapter on the fundamentals of chemical reaction engineering is presented, which is the basis for the discussions throughout the book. Homogeneous reactions part consists of three chapters that concentrate on mixing, residence time distribution, and heat management. The chapters in the second part elaborate on fluid–solid reactions, fluid–fluid reactions, and three-phase reactions. Each chapter contains the introduction, the types of devices used in a particular application, the basic design equations, the examples depicting the design methodology for a particular application, and chemical examples.
1.7 Summary
In this chapter, the microstructured devices are introduced underlying their potential benefits for the process industries. The reduced scale facilitates the temperature control giving an opportunity to maintain the temperature within any window required. Enhanced (heat/mass) transfer rates allow control of highly exothermic and hazardous reactions. It also increases production rates and thus reduces the total processing volume. In addition, microreactors can be simply numbered up for large-scale production, avoiding the problem of scale-up of conventional reactors.
Thus, these devices are suitable when the following case-specific drawbacks are faced in the conventional processing options:
severe transport limitations (heat or mass transfer)
low yields and high wastes because of multistep reactions
safety issues for hazardous materials
poor control of reaction parameters
failure to meet market quality demand.
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