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Madhvanand N. Kashid, Albert Renken,
and Lioubov Kiwi-Minsker
Microstructured Devices for Chemical
Processing
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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
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V
Contents
Preface XI
List of Symbols XIII
1 Overview of Micro Reaction Engineering 1
1.1 Introduction 1
1.2 What are Microstructured Devices? 2
1.3 Advantages of Microstructured Devices 2
1.3.1 Enhancement of Transfer Rates 2
1.3.2 Enhanced Process Safety 5
1.3.3 Novel Operating Window 7
1.3.4 Numbering-Up Instead of Scale-Up 7
1.4 Materials and Methods for Fabrication of Microstructured
Devices 9
1.5 Applications of Microstructured Devices 10
1.5.1 Microstructured Reactors as Research Tool 11
1.5.2 Industrial/Commercial Applications 11
1.6 Structure of the Book 13
1.7 Summary 13
References 14
2 Basis of Chemical Reactor Design and Engineering 19
2.1 Mass and Energy Balance 19
2.2 Formal Kinetics of Homogenous Reactions 21
2.2.1 Formal Kinetics of Single Homogenous Reactions 22
2.2.2 Formal Kinetics of Multiple Homogenous Reactions 24
2.2.3 Reaction Mechanism 25
2.2.4 Homogenous Catalytic Reactions 26
2.3 Ideal Reactors andTheir Design Equations 29
2.3.1 Performance Parameters 29
2.3.2 Batch Wise-Operated Stirred Tank Reactor (BSTR) 30
2.3.3 Continuous Stirred Tank Reactor (CSTR) 35
2.3.4 Plug Flow or Ideal Tubular Reactor (PFR) 39
2.4 Homogenous Catalytic Reactions in Biphasic Systems 45
VI Contents
2.5 Heterogenous Catalytic Reactions 49
2.5.1 Rate Equations for Intrinsic Surface Reactions 50
2.5.1.1 The Langmuir Adsorption Isotherms 51
2.5.1.2 Basic Kinetic Models of Catalytic Heterogenous Reactions 53
2.5.2 Deactivation of Heterogenous Catalysts 57
2.6 Mass and Heat Transfer Effects on Heterogenous Catalytic
Reactions 59
2.6.1 External Mass and Heat Transfer 60
2.6.1.1 Isothermal Pellet 60
2.6.2 Internal Mass and Heat Transfer 69
2.6.2.1 Isothermal Pellet 69
2.6.2.2 Nonisothermal Pellet 77
2.6.2.3 Combination of External and Internal Transfer Resistances 79
2.6.2.4 Internal and External Mass Transport in Isothermal Pellets 79
2.6.2.5 The Temperature Dependence of the Effective Reaction Rate 81
2.6.2.6 External and Internal Temperature Gradient 82
2.6.3 Criteria for the Estimation of Transport Effects 83
2.7 Summary 84
2.8 List of Symbols 86
References 87
3 Real Reactors and Residence Time Distribution (RTD) 89
3.1 Nonideal Flow Pattern and Definition of RTD 89
3.2 Experimental Determination of RTD in Flow Reactors 91
3.2.1 Step Function Stimulus-Response Method 92
3.2.2 Pulse Function Stimulus-Response Method 93
3.3 RTD in Ideal Homogenous Reactors 95
3.3.1 Ideal Plug Flow Reactor 95
3.3.2 Ideal Continuously Operated Stirred Tank Reactor (CSTR) 95
3.3.3 Cascade of Ideal CSTR 96
3.4 RTD in Nonideal Homogeneous Reactors 98
3.4.1 Laminar Flow Tubular Reactors 98
3.4.2 RTDModels for Real Reactors 100
3.4.2.1 Tanks in Series Model 100
3.4.2.2 Dispersion Model 101
3.4.3 Estimation of RTD in Tubular Reactors 105
3.5 Influence of RTD on the Reactor Performance 107
3.5.1 Performance Estimation Based on Measured RTD 108
3.5.2 Performance Estimation Based on RTDModels 110
3.5.2.1 Dispersion Model 111
3.5.2.2 Tanks in Series Model 112
3.6 RTD in Microchannel Reactors 115
3.6.1 RTD of Gas Flow in Microchannels 117
3.6.2 RTD of Liquid Flow in Microchannels 118
3.6.3 RTD of Multiphase Flow in Microchannels 122
Contents VII
3.7 List of Symbols 126
References 127
4 Micromixing Devices 129
4.1 Role of Mixing for the Performance of Chemical Reactors 129
4.2 Flow Pattern and Mixing in Microchannel Reactors 136
4.3 Theory of Mixing in Microchannels with Laminar Flow 137
4.4 Types of Micromixers and Mixing Principles 143
4.4.1 Passive Micromixer 144
4.4.1.1 Single-Channel Micromixers 144
4.4.1.2 Multilamination Mixers 146
4.4.1.3 Split-and-Recombine (SAR) Flow Configurations 148
4.4.1.4 Mixers with Structured Internals 149
4.4.1.5 Chaotic Mixing 149
4.4.1.6 Colliding Jet Configurations 150
4.4.1.7 Moving Droplet Mixers 151
4.4.1.8 Miscellaneous Flow Configurations 153
4.4.2 Active Micromixers 154
4.4.2.1 Pressure Induced Disturbances 154
4.4.2.2 Elektrokinetic Instability 155
4.4.2.3 Electrowetting-Induced Droplet Shaking 156
4.4.2.4 Ultrasound/Piezoelectric Membrane Action 156
4.4.2.5 Acoustic Fluid Shaking 157
4.4.2.6 Microstirrers 157
4.4.2.7 Miscellaneous Active Micromixers 158
4.5 Experimental Characterization of Mixing Efficiency 158
4.5.1 Physical Methods 158
4.5.2 Chemical Methods 159
4.5.2.1 Competitive Chemical Reactions 159
4.6 Mixer Efficiency and Energy Consumption 171
4.7 Summary 172
4.8 List of Symbols 173
References 173
5 Heat Management by Microdevices 179
5.1 Introduction 179
5.2 Heat Transfer in Microstructured Devices 181
5.2.1 Straight Microchannels 181
5.2.2 Curved Channel Geometry 189
5.2.3 Complex Channel Geometries 191
5.2.4 Multichannel Micro Heat Exchanger 191
5.2.5 Microchannels with Two Phase Flow 193
5.3 Temperature Control in Chemical Microstructured Reactors 195
5.3.1 Axial Temperature Profiles in Microchannel Reactors 197
5.3.2 Parametric Sensitivity 201
VIII Contents
5.3.3 Multi-injection Microstructured Reactors 212
5.3.3.1 Mass and Energy Balance in Multi-injection Microstructured
Reactors 213
5.3.3.2 Reduction of Hot Spot in Multi-injection Reactors 218
5.4 Case Studies 221
5.4.1 Synthesis of 1,3-Dimethylimidazolium-Triflate 221
5.4.2 Nitration of Dialkyl-SubstitutedThioureas 222
5.4.3 Reduction of Methyl Butyrate 223
5.4.4 Reactions with Grignard Reagent in Multi-injection Reactor 224
5.5 Summary 226
5.6 List of Symbols 226
References 228
6 Microstructured Reactors for Fluid–Solid Systems 231
6.1 Introduction 231
6.2 Microstructured Reactors for Fluid–Solid Reactions 232
6.3 Microstructured Reactors for Catalytic Gas-Phase Reactions 233
6.3.1 Randomly Micro Packed Beds 233
6.3.2 Structured Catalytic Micro-Beds 235
6.3.3 Catalytic Wall Microstructured Reactors 238
6.4 Hydrodynamics in Fluid–Solid Microstructured Reactors 239
6.5 Mass Transfer in Catalytic Microstructured Reactors 243
6.5.1 Randomly Packed Bed Catalytic Microstructured Reactors 244
6.5.2 Catalytic FoamMicrostructured Reactors 245
6.5.3 Catalytic Wall Microstructured Reactors 246
6.5.4 Choice of Catalytic Microstructured Reactors 253
6.6 Case Studies 255
6.6.1 Catalytic Partial Oxidations 255
6.6.2 Selective (De)Hydrogenations 257
6.6.3 Catalytic Dehydration 259
6.6.4 Ethylene Oxide Synthesis 259
6.6.5 Steam Reforming 260
6.6.6 Fischer–Tropsch Synthesis 261
6.7 Summary 261
6.8 List of Symbols 262
References 262
7 Microstructured Reactors for Fluid–Fluid Reactions 267
7.1 Conventional Equipment for Fluid–Fluid Systems 267
7.2 Microstructured Devices for Fluid–Fluid Systems 268
7.2.1 Micromixers 269
7.2.2 Microchannels 271
7.2.2.1 Microchannels with Inlet T, Y, and Concentric Contactor 271
7.2.2.2 Microchannels with Partial Two-Fluid Contact 271
Contents IX
7.2.2.3 Microchannels with Mesh or Sieve-Like Interfacial Support
Contactors 271
7.2.2.4 Microchannels with Static Mixers 272
7.2.2.5 Parallel Microchannels with Internal Redispersion Units 272
7.2.3 Microstructured Falling Film Reactor for Gas–Liquid
Reactions 272
7.3 Flow Patterns in Fluid–Fluid Systems 273
7.3.1 Gas–Liquid Flow Patterns 273
7.3.1.1 Bubbly Flow 273
7.3.1.2 Taylor Flow 274
7.3.1.3 Slug Bubbly Flow 279
7.3.1.4 Churn Flow 279
7.3.1.5 Annular and Parallel Flow 280
7.3.2 Liquid–Liquid Flow Patterns 280
7.3.2.1 Drop Flow 281
7.3.2.2 Slug Flow 281
7.3.2.3 Slug-Drop Flow 282
7.3.2.4 Deformed Interface Flow 282
7.3.2.5 Annular and Parallel Flow 283
7.3.2.6 Slug-Dispersed Flow 283
7.3.2.7 Dispersed Flow 283
7.4 Mass Transfer 284
7.4.1 Mass Transfer Models 285
7.4.2 Characterization of Mass Transfer in Fluid–Fluid Systems 286
7.4.3 Mass Transfer in Gas–Liquid Microstructured Devices 287
7.4.3.1 Mass Transfer in Taylor Flow 287
7.4.3.2 Mass Transfer in Slug Annular and Churn Flow Regime 292
7.4.3.3 Mass Transfer in Microstructured Falling Film Reactors 293
7.4.4 Mass Transfer in Liquid–Liquid Microstructured Devices 296
7.4.4.1 Slug Flow (Taylor Flow) 296
7.4.4.2 Slug-Drop and Deformed Interface Flow 297
7.4.4.3 Annular and Parallel Flow 297
7.4.4.4 Slug-Dispersed and Dispersed Flow 298
7.4.5 Comparison with Conventional Contactors 299
7.5 Pressure Drop in Fluid–Fluid Microstructured Channels 300
7.5.1 Pressure Drop in Gas–Liquid Flow 301
7.5.2 Pressure Drop in Liquid–Liquid Flow 304
7.5.2.1 Pressure Drop – Without Film 304
7.5.2.2 Pressure Drop – With Film 305
7.5.2.3 Power Dissipation in Liquid/Liquid Reactors 307
7.6 Flow Separation in Liquid–Liquid Microstructured Reactors 307
7.6.1 Conventional Separators 308
7.6.2 Types of Microstructured Separators 308
7.6.2.1 Geometrical Modifications 309
7.6.2.2 Wettability Based Flow Splitters 310
X Contents
7.6.3 Conventional Separator Adapted for Microstructured Devices 315
7.7 Fluid–Fluid Reactions in Microstructured Devices 315
7.7.1 Examples of Gas–Liquid Reactions 317
7.7.1.1 Halogenation 317
7.7.1.2 Nitration, Oxidations, Sulfonation, and Hydrogenation 318
7.7.2 Examples of Liquid–Liquid Reactions 319
7.7.2.1 Nitration Reaction 319
7.7.2.2 Transesterification: Biodiesel Production 320
7.7.2.3 Vitamin Precursor Synthesis 320
7.7.2.4 Phase Transfer Catalysis (PTC) 321
7.7.2.5 Enzymatic Reactions 322
7.8 Summary 323
7.9 List of Symbols 324
References 325
8 Three-Phase Systems 331
8.1 Introduction 331
8.2 Gas–Liquid–Solid Systems 331
8.2.1 Conventional Gas–Liquid–Solid Reactors 331
8.2.2 Microstructured Gas–Liquid–Solid Reactors 333
8.2.2.1 Continuous Phase Microstructured Reactors 333
8.2.2.2 Dispersed Phase Microstructured Reactors 334
8.2.2.3 Mass Transfer and Chemical Reaction 336
8.2.2.4 Reaction Examples 341
8.3 Gas–Liquid–Liquid Systems 346
8.4 Summary 347
8.5 List of Symbols 347
References 348
Index 351
XI
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 tem-
perature 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 charac-
terization, 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, inte-
gration of sensors and actuators, and their use for biological applications.
To facilitate comprehension, the topics are developed beginning with fun-
damentals in chemical reaction engineering with ample cross-referencing. The
understanding of concepts is facilitated by clear descriptions of examples, sup-
plied by exercises including solutions, and provided by figures and illustrations.
XII Preface
Finally, the authors want to highlight the complexity of microreaction engineer-
ing 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 develop-
ments of this subject require periodic updates.
Lausanne, Switzerland Madhvanand Kashid,
May 2014 Albert Renken,
Lioubov Kiwi-Minsker
XIII
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 m2
a Specific interfacial area or catalytic surface
area per reactor volume
m2 m−3
Acs Cross-section area m2
Bo Bond number —
Bo Bodenstein number —
Bim, Bith Biot number (mass), Biot number (thermal) —
C Dimensionless concentration —
Ca Capillary (=) or Carberry (=) number —
ci Concentration of molecule Ai molm−3
cp Heat capacity of fluid or mixture J kg−1 K−1
DaI First Damköhler number —
DaII Second Damköhler number —
DaIImx Second Damköhler number for mixing —
Dax Axial dispersion coefficient m2 s−1
De Dean number —
Deff, Dm Effective molecular diffusion coefficient,
molecular diffusion coefficient
m2 s−1
dh Hydraulic diameter m
dt Diameter of channel (or tube) m
E, Ea Intrinsic activation energy, apparent
activation energy of reaction j
Jmol−1
f Ratio of residual concentration to initial —
Fo Fourier number —
g Gravitational acceleration m2 s−1
H Height m
(continued overleaf)
XIV List of Symbols
Symbols Significance Unit
h Heat transfer coefficient Wm−2 K−1
Ha Hatta number —
Ji Molar flux of species i molm−2 s−1
k, kr, kj Reaction rate constant for homogeneous and
quasi-homogenous, constant of
heterogenous reaction, constant of reaction j
variable (s−1
(molm−3)−(n−1))
k0 Pre-exponential or frequency factor variable (s−1
(molm−3)−(n−1))
KC Reaction equilibrium constant variable
K thermodynamic equilibrium constant —
kG Mass transfer coefficient in gas phase m s−1
kGL Mass transfer coefficient in gas–liquid
system
ms−1
kL Mass transfer coefficient in liquid phase m s−1
kLa Volumetric mass transfer coefficient s−1
km Mass transfer coefficient of heterogeneous
reactions
m s−1
kov Overall mass transfer coefficient m s−1
L, Lc, Le, Lt Length, characteristic length, length of
entrance zone, length of tube or channel
m
m Mass flow rate kg s−1
Nu Nusselt number —
ni Reaction order with respect to species Ai —
n Overall reaction order —
ni No of moles of molecule Ai mol
ni Molar flow rate of molecule Ai mol s−1
p Pressure Pa
Pi Rate of production mol s−1
Pr Prandtl number —
Pe Péclet number —
Q Energy J
Q Rate of heat flow W
q, qr , qex Specific heat rate, of reaction, of heat
exchange/transfer
Jm−3 s−1
R Ideal gas law constant Jmol−1 K−1
R Radius m
Re Reynolds number —
Ri Overall reaction/transformation rate of
molecule Ai
molm−3 s−1
rj, reff Rate of reaction/transformation of reaction j,
effective reaction rate
molm−3 s−1
rads, rdes Rates of adsorption, of desorption —
Sk, i Selectivity of product k with respect to
reactant i
—
sk, i Instantaneous selectivity of product k with
respect to reactant i
—
Se Semenov number —
List of Symbols XV
Symbols Significance Unit
Sc Schmidt number —
Sh Sherwood number —
T , Tb, Ts Temperature, bulk temperature, surface
temperature
K
t, tc, tD, tr, tm,
tmx, tax, tD, ax,
tD, 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
t Mean residence time s
U Overall heat transfer coefficient Wm−2 K−1
Ui Internal energy J
Uv Overall volumetric heat transfer coefficient Wm−3 K−1
u, ub, u(r),
uG, uL
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 , VR Volume, internal (reaction) volume m3
V Volumetric flow rate m3 s−1
W Width m
W , Wf , Ws Rate of work done, by flow, by shaft J s−1
X Conversion —
Yk, i Yield of product k with respect to reactant i —
Z Dimensionless length —
z Length m
Greek symbols
𝛼 Thermal diffusivity m2 s−1
𝛽 Prater number —
𝛿(z) Dirac pulse —
𝛿 Film thickness, catalytic layer or boundary
layer
m
𝛾 Arrhenius number —
�� Shear rate s−1
Δ Symbol of difference —
ΔG Gibbs free energy Jmol−1
ΔHr, ΔHa Heat of reaction, heat of adsorption Jmol−1
Δp Pressure drop Pa
ΔS Entropy Jmol−1 K−1
ΔTad Adiabatic temperature rise K
𝜀 Specific power dissipation Wkg−1
𝜀p, 𝜀bed Porosity of catalyst pallet, of randomly
packed bed
—
𝜂 Efficiency factor —
𝜃 Dimensionless time —
𝜆, 𝜆eff, 𝜆f,
𝜆wall
Thermal conductivity, effective, of fluid, of
wall
Wm−1 K−1
(continued overleaf)
XVI List of Symbols
Symbols Significance Unit
𝜇 Dynamic viscosity Pa s
𝜈 Kinematic viscosity m2 s−1
𝜈i,j Stoichiometric coefficient of species i in
reaction j
—
𝜁 Geometric factor —
𝜌 Density kgm−3
𝜎 Interfacial tension Nm−1
𝜏, 𝜏PFR, 𝜏R 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
List of Symbols XVII
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
ΔTad = (−ΔHr )cb𝜌cp
Arrhenius
number
Relative importance of activation
temperature (E/R) to system bulk
temperature (Tb)
𝛾 = E
RTb
Biot number
(mass)
Relates external mass or heat transfer rates at
catalyst pallet surface to diffusion or
conduction inside the pallet
Bim = tDtm
=L2cDe
kmap
Biot number
(thermal)
Bith = h⋅L𝜆e
Bodenstein
number
Ratio of convective transport rate to (axial)
diffusion transport rate
Bo = u⋅LDax
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
Ca = 𝜂exDaII
Capillary
number
Used in fluid–fluid systems. It is ratio of
viscous forces to surface tension acting across
an interface, that is, interfacial tension
Cai =ub⋅𝜇i
𝜎
First
Damköhler
number
Used to set design criteria – ratio of
residence time in the reactor to the
characteristic reaction time
DaI = 𝜏tr
Second
Damköhler
number
Used to set design criteria – ratio of reaction
rate to mass transfer rate
DaII = tmtr
Second
mixing
Damköhler
number
Used to set design criteria – ratio of reaction
rate to mixing rate
DaIImx =tmx
tr
Dean number Used to characterize the flow in curved
channels – it is product of Re and square root
of channel diameter to curvature radius
De = Re(
dhR′′
)0.5Efficiency
(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
𝜂 = reffrmax
Effectiveness
factor
(porous
catalyst)
Ratio of effective reaction rate and the rate of
reaction at bulk concentration and
temperature
𝜂p = JeffJs
=Decs∕L⋅𝜑 tanh(𝜑)
krcsL
= tanh𝜑
𝜑
(continued overleaf)
XVIII List of Symbols
Dimensionless
number
Significance Definition
Effectiveness
factor (mass
transfer) or
trade-off
index
Used to access mass transfer performance
with energy input
𝜂m = DaImEu
=kmaR⋅L
us⋅
𝜌⋅u2sΔp
Euler number It is ratio of pressure drop in a given reactor
length to kinetic energy.
Eu = Δp𝜌⋅u2
Fourier
number
It is ratio of residence time to diffusion time Fo = 𝜏tD
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
Ha =√
tmtr
=
𝛿II
√k′rDi,II
=√k′rDi,II
kL,II
Nusselt-
number
Use to characterize relative importance of
convective heat transfer over conductive heat
transfer
Nu = h⋅dh𝜆
Peclet
number
Ratio of rate of convection to rate of
diffusion/dispersion
Peax =u⋅dtDax
(tube)
Peax =u⋅dp
εbed Dax
(packed bed)Prandtl
number
Used to characterize momentum and heat
diffusion – ratio of momentum (viscous)
diffusion to molecular diffusion
𝑃𝑟 = 𝜈𝛼= 𝜈
𝜆∕(𝜌cp)
Prater
number
Ratio of maximum temperature difference
catalyst center and surface temperature to
the surface temperature
𝛽 = ΔTmax
Ts=
(−ΔHr )csTs
De
𝜆e
Reynolds
number
Most commonly used to characterize the
fluid flow – gives relative importance of
inertial forces over viscous forces
Re = 𝜌udt𝜇
Reynolds
number
(particle)
Rep = (u dp)𝜈
Reynolds
number
(foam)
Refoam = u⋅ds⋅𝜌𝜇
Schmidt
number
Used to characterize momentum and mass
diffusion – ratio of momentum (viscous)
diffusion to molecular diffusion
Sc = 𝜈Dm
Sherwood
number
(particle)
Use to characterize relative importance of
convective mass transfer over diffusional
mass transfer
Shp = dp km
Dm
Sherwood
number
Sh = km⋅dhDm
List of Symbols XIX
Dimensionless
number
Significance Definition
Thiele
modulus
Ratio of characteristic diffusion time in the
catalyst and the characteristic reaction time
𝜑2 = tDtr
= L2
Dek
𝜑 = L
√krD; first
order reaction;
𝜑gen =Vp
Ap
√krc
(n−1)s
De⋅√
n+12
Weisz
modulus
Used to measure influence of transport
process on reaction kinetics
experimentally – ratio of effective reaction
rate to (effective) diffusion rate
𝜓2s = tD
tr,eff=
R2sphere
De
csrp,eff
=
𝜂p𝜑2s
𝜓2gen = tD
tr,eff=(
Vp
Ap
)2n+12
rp,eff
De cs=
𝜂p𝜑2gen
Bond number Relates body forces to surface tension forces BO =𝜌gd2
h
𝜎First
Damköhler
number
(mass
transfer)
Ratio of residence time in the reactor to the
characteristic mass transfer time
DaIm = 𝜏Rtm
=kmaR⋅L
u
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
1
1
Overview 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 classi-
fications 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
Microstructured Devices for Chemical Processing, First Edition.Madhvanand N. Kashid, Albert Renken and Lioubov Kiwi-Minsker.© 2015 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2015 by Wiley-VCH Verlag GmbH & Co. KGaA.
2 1 Overview of Micro Reaction Engineering
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, or microreactors, 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
dQ
dt= Q = 𝜆 ⋅ A ⋅
dT
dz(1.1)
1.3 Advantages of Microstructured Devices 3
where Q is the heat energy (J), 𝜆 is thermal conductivity (WmK−1), and A is the
heat transfer surface area (m2). The temperature gradient dT∕dz 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 gradi-
ents, the effective surface area for exchange processes has to be considered. Let us
integrate Equation 1.1 for a unit volume of reactor:
Q
V= q = U ⋅
A
V⋅ ΔT = U ⋅ a ⋅ ΔT (1.2)
where U (= k∕Δz) 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, a = 4∕dt , where dt is the tube diameter. Thus, with decreasing characteris-
tic dimensions, the specific surface area of the system increases leading to higher
overall performances.
The surface to volume ratio formicrodevices can be as high as 50 000m2 m−3 [4].
For comparison, the specific surface area of typical laboratory and production
vessels seldom exceed 100m2 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 000Wm−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 2000Wm−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 interfa-
cial area up to 30 000m2 m−3. The traditional bubble columns do not exceed
a few 100m2 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 andmass transfer interfere with
the reaction kinetics.
The transfer rates presented above results in the characteristic time of phys-
ical 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).
4 1 Overview of Micro Reaction Engineering
10–3 10–2 10–1 100 101 102 103 104
Time scale (s)Mixing, heat transferconventional reactors
Characteristic intrinsic reaction time (examples)
Mixing, heat exchangeμ-structured reactors
Mass and heat transport influenced
Char. time, transport phenomena
Meta
l/halo
gen e
xchange
Grignard
keto
ne a
dditio
n
Hydro
lysis
Low
-T G
rignard
additio
n
Alk
yla
tion,
nitra
tion
SN
2 r
eactions
Many o
rganic
reactions
Conventional
org
anic
synth
esis
Figure 1.1 Time scale of chemical and physical processes [8]. (Adapted with permission
from Elsevier.)
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 com-
pletely 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.
A simplified algorithm for a single step homogenous reaction that could help in
choosing conventional and microstructured devices based on kinetics, thermo-
dynamics, and transport rates is presented in Figure 1.3. Here tr, ΔHr, and tmx
1.3 Advantages of Microstructured Devices 5
Static mixer
Static mixer/ plate exchanger
Plate heatexchanger
Rotatingpacked bed
Pulsedcolumn
Injector
Stirred tank
Heat transfer
Ma
ss t
ran
sfe
r
Loop reactor
Spinning discreactor
Micro-structuredreactorc2
s2 s1
c4
hb
c3
c1 = L
Figure 1.2 Benchmarking of microstructured reactors. (Adapted from Ref. [11]. Copyright ©
2009, John Wiley and Sons.)
are characteristic reaction time, heat of reaction, and characteristic mixing time,
respectively. In the case of heterogenous reactions, mixing timewould 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.
As described before, the limiting factor can be the intrinsic kinetics, the thermo-
dynamics, 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 reac-
tion is more than −50 kJmol−1, the use of microstructured devices is proposed.
However, even if the reaction time is high and heat of reaction is relatively low,
themicrostructured devices could be used to enhance themixing 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
6 1 Overview of Micro Reaction Engineering
A1 + A2 → A3Chemical kineticsthermodynamics
If (tr < 10s)and/or
(–ΔHr > 50 kJ mol–1)
If (tm < 1s)
No
Yes
Microstructured devices
Conventional devices
No
Yes
Figure 1.3 An algorithm showing choice of reactor based on reaction kinetics, thermody-
namics, and mixing rates for a homogeneous reaction.
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 predomi-
nant 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 Advantages of Microstructured Devices 7
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 parame-
ters 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 333K was
observed [14]. A multiphase (gas/liquid) explosive reaction of oxidation of cyclo-
hexane 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 533K and 35 bar, superheated pressurized processing
much above the boiling point, allowed to decrease reaction time 5760-fold as com-
pared to standard batch operation [16].The condensation of o-phenylenediamine
with acetic acid to 2-methylbenzimidazole in anMSR is an impressive example of
the reduced reaction time from 9weeks at room temperature to 30 s at 543K 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 labora-
tory reactor or flask is upgraded to a few cubic meters to meet the target produc-
tivity 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
8 1 Overview of Micro Reaction Engineering
is simpler and faster than the conventional processes (no redesign and pilot plant
experiments), thus, decreasing considerably the time between discovery and pro-
duction 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 oper-
ated 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.
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 distri-
bution of fluids to the multiple channels. Equal distribution is indispensable to
One oulet from each distributor goes toone of the following channel
Product collection and analysis
Microchannels
Distributors
Mixingzone
Microchannels
Product collection andanalysis
(a) (b)
Figure 1.4 Schematics of Numbering-up of microstructured reactors: (a) external
numbering-up, (b) internal numbering-up [18]. (Adapted with permission from Elsevier.)
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