madhvanandn.kashid,albertrenken, · 2014. 8. 29. · theauthors dr. madhvanand n. kashid...

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


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


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


References 228

6 Microstructured Reactors for Fluid–Solid Systems 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


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


7.3.2.6 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


7.4.4.4 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


References 325

8 Three-Phase Systems 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


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


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


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


XVI List of Symbols


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


XVIII List of Symbols

Dimensionless

number


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


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)


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).


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


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


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].


1.3.3

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


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.)

madhvanandn.kashid,albertrenken, · 2014. 8. 29. · theauthors dr. madhvanand n. kashid...

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