UNIVERSITY “POLITEHNICA” of BUCHAREST Faculty of Applied...

MODELLING AND SIMULATION OF CONTINUOUS CATALYTIC

DISTILLATION PROCESSES

A Thesis submitted to the

University POLITEHNICA of Bucharest and

University of Barcelona

in joint collaboration

for ACADEMIC TITLE OF DOCTOR

by

Alexandra Elena BONET RUIZ

SPECIALITY: CHEMICAL ENGINEERING

under the supervision of

Professor Grigore BOZGA Professor Joan LLORENS LLACUNA

Professor José COSTA LÓPEZ

July 2012

UNIVERSITY “POLITEHNICA” of BUCHARESTFaculty of Applied Chemistry and Material Sciences

Department of Chemical Engineering

LIST OF PAPERS

PLESU A.E., BONET J., PLESU V., BOZGA G., GALAN M.I., 2008, Residue Curves Map

Analysis for tert-Amyl Metyl Ether Synthesis by Reactive Distillation in Kinetically Controlled

Conditions with Energy –Saving Evaluation, Energy, 33, 1572-1589

BONET-RUIZ, A.-E., BONET-RUIZ, J., PLEŞU, V., BOZGA, G., LLORENS LLACUNA, J.,

COSTA LOPEZ, J., 2009, New contributions to modelling and simulation of TAME synthesis by

catalytic distillation, Chemical Engineering Transactions, 18, 959-964

BONET-RUIZ A.E., BONET J., PLESU V., BOZGA G., 2010, Environmental performance

assessment for reactive distillation processes, Resources Conservation and Recycling, 54 (5),

315-325

BONET-RUIZ, J., BONET-RUIZ, A.-E., RADU, V.-C., LLACUNA, J.L., LOPEZ, J.C., 2010,

A simplified cost function for distillation systems evaluation, Chemical Engineering

Transactions, 21, 1405-1410

BONET-RUIZ, A.-E., BONET, J., BOZGA, G., LLACUNA, J.L., PLEŞU, V., 2010, Number

of transfer units information on residue curve maps, Chemical Engineering Transactions 21,

1417-1422.

CIORNEI, C.I., BUMBAC, G., PLESU, A.E., MOTELICA, A., IVANESCU, I., TOMA, A.,

2006, Catalytic Distillation Process Modelling and Simulation for the TAEE Synthesis on

Amberlyst 35, 33th International Conference of Slovak Society of Chemical Engineering, 22 – 26

May 2006, Tatranské Matliare, Slovakia, Proceedings - Slovak Society of Chemical Engineering,

ISBN 80-227-2409-2, paper no. Le-Fr-2 251.

BUMBAC, G., MOTELICA, A., PLESU, A.E., BOZGA, G., TOMA, A., 2006, Kinetic studies

on the etherification of isobutene to fuel ether ETBE, 17th International Congress of Chemical

and Process Engineering CHISA 2006 - 9th Conference PRES2006, 27 – 31 August 2006,

Prague, Czech Republic, Proceedings - Czech Society of Chemical Engineering

BUMBAC, G., CIORNEI, C.I., PLESU, A.E., SIMION, C., TOMA, A., 2007, Kinetic Studies

on the Etherification of C5-Alkenes To Fuel Ether TAME, 34th International Conference of

Slovak Society of Chemical Engineering, 21 – 25 May 2007, Tatranské Matliare, Slovakia,

Proceedings - Slovak Society of Chemical Engineering, Po-Th-5, 039p, ISBN 978-80-227-2640-

5

BUMBAC, G., PLESU, A.E., PLESU, V., 2007, Reactive Distillation Process Analysis in

Divided Wall Column, Proceedings of ESCAPE17, May 2007, Computer Aided Chemical

Engineering, vol. 24, pg. 443-448, V. Plesu and P.S. Agachi (Editors), Elsevier.

BUMBAC, G., PLESU, V., CIORNEI, C.I., PLESU, A.E., 2007, Modelling and simulation the

process in a reactive dividing wall column for gasoline additive TAME synthesis in the oil

refinery 10th Conference on Process Integration, Modelling and Optimisation for Energy Saving

and Pollution Reduction-PRES'07, vol 1, J. Klemeš Editor, 24-27 June 2007, Ischia Island Gulf

of Naples, Italy, Chem.Eng.Trans, vol 12, ISBN 88-901915-4-6, pp79-84, 2007.

PLESU, A.E., BONET, J., CIORNEI, C.I., PLESU, V., BOZGA, G., GALAN, M.I., 2008,

Energy-Saving Issues in Reactive Distillation Schemes, Energy for Sustainable Future

EMINENT 2 Workshop, 2008, University of Pannonia, Veszprem, Hungary, 5-8 May 2008,

ISBN 978-963-9696-38-9, pp257-266.

BONET, J, GALAN, M.I., COSTA, J., MEYER, X.M., MEYER, M., PLESU, A.E.,

Representation of Residue Curve Maps with Pinch Zones, 2008, 18th European Symposium on

Computer Aided Process Engineering - ESCAPE18, 1-4 June 2008, Computer-Aided Chemical

Engineering, 18, Lyon, France.

PLESU, V., BOZGA, G., PLESU, A.E., CIORNEI, C.I., BOLOGA, V., Experimental Devices

and Modelling Tools for Sustainable Processes Development, 2008, 11th Conference on Process

Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction-

PRES2008, J. Klemeš Editor, 24-28 August 2008, Prague, vol 4, ISBN 978-80-02-02051-6,

paper J.4.1, pp1124.

PLESU, V., BOZGA, G., PLESU, A.E., CIORNEI, C.I., BOLOGA, V., 2008, Modelling and

Simulation Tools for Sustainable Processing and Production, 11th Conference on Process

Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction-

PRES2008, J. Klemeš Editor, 24-28 August 2008, Prague, vol 4, ISBN 978-80-02-02051-6,

paper P.5.101, pp1459

PLESU, A.E., BONET, J., PLESU, V., BOZGA, G., GALAN, M.I., 2008, TAME Sustainable

Production – Infinite/Infinite Analysis for Synthesis by Reactive Distillation in Chemical non-

Equilibrium Conditions, 11th Conference on Process Integration, Modelling and Optimisation for

Energy Saving and Pollution Reduction- PRES2008, J. Klemeš Editor, 24-28 August 2008,

Prague, vol 4, ISBN 978-80-02-02051-6, paper P.5.102, pp1460

PLESU, V., BILDEA, C.S., IANCU, P., BUMBAC, G., DANCIU, T.D., BONET-RUIZ, J.,

BONET-RUIZ A.E., NEGHINA, I.L., 2009, A Systematic Approach To Graduation Project In

Chemical Engineering, Proceedings Romanian International Conference on Chemistry and

Chemical Engineering, RICCCE XVI, 9 – 12 September 2009, Sinaia – Romania, paper S4. K-

02.

BONET-RUIZ A.E., BOZGA G., LLORENS LLACUNA J., COSTA LOPEZ J., 2012, The

kinetics of TAME synthesis on solid acid catalysts, U.P.B. Sci. Bul., Series B - Chemistry and

Materials Science, ISSN 1454-2331, sent to journal.

i

CONTENTS

CHAPTER 1 Introduction............................................................................... ………………..1 CHAPTER 2 Survey regarding the catalytic distillation technique and synthesis methods of

ethers used as fuel additives. Catalysts, kinetics and thermodynamics…………………………...6

2.1. The principle of reactive distillation and its technical importance ..................................... 6

2.2. The importance of tertiary aliphatic ethers C5-C7. Legislation aspects and main

representatives............................................................................................................................... 11

2.2.1. Sources of tertiary olephins .............................................................................................. 17

2.2.2. Synthesis methods for C5-C7 aliphatic tertiary ethers series............................................. 18

2.2.3. Industrially applied technologies for fuel ethers synthesis and technical-economical .... 21

2.2.4. TAME ............................................................................................................................... 23

2.3. Termodynamics of liquid phase alcohol adition to C5 tertiary olefins ............................. 29

2.4. Kinetic models for TAME synthesis reactions ................................................................. 34

2.4.1. Literature survey ............................................................................................................... 34

2.4.2. Solid catalysts for synthesis of ethers by alcohol addition to tertiary olefins................... 54

2.4.2.1. Ion exchange resins....................................................................................................... 56

2.4.2.2. Inorganic solid acid catalysts ........................................................................................ 60

2.5. Industrial TAME production technologies ....................................................................... 62

2.6. Mathematical models used in the calculation of reactive distillation processes............... 65

2.6.1. Simplified (total equilibrium) models............................................................................... 66

2.6.2. Equilibrium models........................................................................................................... 67

2.6.3. Non – equilibrium models ................................................................................................ 69

Conclusions................................................................................................................................... 74

Chapter 3.The kinetics of TAME synthesis on solid acid catalysts…………………………...…76

3.1. Experimental study of isoamylenes etherification with methanol.................................... 76

3.1.1. Materials and method........................................................................................................ 76

3.1.2. Set-up description ............................................................................................................. 78

3.1.3. Sample analysis................................................................................................................. 82

3.1.4. Operating Procedure ......................................................................................................... 85

3.1.4.1. Set-up start-up............................................................................................................... 85

3.1.4.2. Experimental procedure ................................................................................................ 87

3.1.5. Experimental results.......................................................................................................... 88

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3.1.5.1. Influence of reaction temperature ................................................................................. 88

3.1.5.2. Influence of catalyst grain size ................................................................................. 89

3.1.5.3. Effect of isoamylenes concentration......................................................................... 91

3.2. Modelling reaction kinetics for TAME synthesis ............................................................. 92

3.2.1. Comparison of experimental results with values calculated based on kinetic models

published in literature ................................................................................................................... 93

3.2.2. Parameters estimation for the kinetic model proposed ................................................. 97

Conclusions................................................................................................................................. 101

CHAPTER 4 Experimental study of vapour liquid equilibrium data ......................................... 104

4.1. Experimental device............................................................................................................. 103

4.1.1. Description of the dynamic vapour liquid equilibrium (VLE) apparatus ......................... 103

4.1.2. Operation and experimental procedure............................................................................. 109

4.1.3. Considerations related to the experimental error .............................................................. 112

4.2. Materials .............................................................................................................................. 112

4.3. Gas chromatograph analysis ................................................................................................ 113

4.4. Experimental results of the binary vapour liquid equilibrium............................................. 114

4.4.1. 2-methyl-2-butene/Methanol ............................................................................................ 114

4.4.2. 2-methyl-2-butene / TAME .............................................................................................. 119

4.4.3. 2-methyl-1-butene / TAME .............................................................................................. 123

4.4.4. Isopentane / methanol ....................................................................................................... 127

4.4.5. Summary of the performances of different thermodynamic model.................................. 131

Conclusions ………………………………………………………………………………….135

CHAPTER 5 Feasibility and simulation of TAME synthesis-separation process……………...138

5.1. Market demand and basic economic feasibility................................................................... 138

5.2. Basic physical feasibility ................................................................................................ 138

5.2.1. Introduction to Residue Curve Maps: Theory and Practice........................................ 138

5.2.2. Mathematical model for the (reactive) residue curves................................................ 143

5.2.2.1. Residual Curves and Number of Transfer Units (NTU)......................................... 147

5.2.2.2. Singular Points ........................................................................................................ 153

5.2.3. Implementation of the reactive residue curve maps calculation................................. 155

5.2.3.1. Data Input................................................................................................................ 155

5.2.3.2. Exploration of the map topology ............................................................................ 156

5.2.3.3. Residue curve calculation ....................................................................................... 157

iii

5.2.3.3. Residue curve calculation ....................................................................................... 157

5.2.3.4. Number of Transfer Units calculation .................................................................... 157

5.2.3.5. Implementation of the residue curve map model to the TAME case ..................... 158

5.2.4. Reactive residue curve maps calculated for TAME ....................................................... 160

5.2.5. Number of transfer units in the residue curve maps calculated for TAME .................... 165

5.2.6. Comparison of the reactive residue curves with the reactive distillation column profile at

infinite reflux for the TAME synthesis....................................................................................... 172

5.2.7. Analysis of the system .................................................................................................... 176

5.2.7.1. Simplification of the rigorous mathematical model....................................................... 176

5.2.7.2. The infinite/infinite analysis and the reactive residue curve maps ................................ 179

5.2.7.3. Application of the infinite/infinite analysis to non-chemical equilibrium systems ....... 181

5.2.7.4. Results of the infinite/infinite analysis to kinetically controlled reactive distillation for

TAME synthesis.......................................................................................................................... 185

5.2.7.5.Results of the infinite/infinite analysis to kinetically controlled hybrid reactive distillation

for TAME synthesis.................................................................................................................... 200

5.3. Technical assessment ...................................................................................................... 210

5.3.1. Rigorous simulation of reactive distillation column alternatives TAME synthesis ....... 210

5.3.2. Rigorous simulation taking into account the presence of isopentane inert..................... 219

5.4. Economical assessment................................................................................................... 221

5.4.1. Basic economic feasibility .............................................................................................. 221

5.4.2. Economical comparison of the reactive distillation alternatives .................................... 222

5.4.3. Techno-economic model with CO2 capture feasibility................................................... 226

5.5. Environmental performance assessment......................................................................... 227

5.5.1. Environmental Performance Index (EPI) ranking .......................................................... 228

Conclusions ………………………………………………………………………………….231

CHAPTER 6. Conclusions……………………………………………………………………..233

References……………………………………………………………………………………. 236

Appendix 1……………………………………………………………………………………. 256

iv

LIST OF FIGURES

Figure 2. 1 Typical scheme for a conventional process of (a) reaction and separation and (b)

reactive distillation.......................................................................................................................... 9

Figure 2. 2 Main tertiary ethers used at industrial scale ............................................................... 13

Figure 2. 3 Generation of tertiary alcohols by hydration of olephins........................................... 20

Figure 2. 4. Amounts of product used in various fields............................................................... 26

Figure 2. 5 The amount of TAME consumed in different EU countries ...................................... 27

Figure 2. 6 Main reactions for obtaining TAME .......................................................................... 28

Figure 2. 7 Secondary reaction – dimerization of isoamylenes .................................................. 28

Figure 2. 8 Secondary reaction – etherification (dehydration) of methanol ................................. 28

Figure 2. 9 Reactions for TAME synthesis................................................................................... 33

Figure 2. 10 The reaction network in the formation and splitting of TAME................................ 35

Figure 2. 11 Chemical structure of ion exchange resins............................................................... 57

Figure 2. 12 Mechanism of alcohols addition to olefins, in alcohol excess ................................. 59

Figure 2. 13 Mechanism of alcohols addition to olefins, without alcohol excess ........................ 59

Figure 2. 14 NexTAME process ................................................................................................... 63

Figure 2. 15 Philips etherification procedure with high conversion............................................. 64

Figure 2. 16 General structure of the plant for TAME synthesis by CDEtherol technology ....... 65

Figure 2. 17 Physical model of the reactive distillation column................................................... 66

Figure 2. 18 Simplified scheme of an equilibrium tray ................................................................ 67

Figure 2. 19. Schematic representation of non-equilibrium tray .................................................. 70

Figure 2. 20 Mass transfer in homogeneous reactive distillation ................................................. 71

Figure 2. 21 Transport processes in reactive distillation in the presence of non-porous catalysts72

Figure 3. 1 Autoclave-type reactor of the etherification plant...................................................... 79

Figure 3. 2. Batch plant scheme for kinetic studies ...................................................................... 81

Figure 3. 3 Time evolution of TAME concentration at different temperatures............................ 89

Figure 3. 4. Experimental results at 80oC for various granule sizes ............................................. 90

Figure 3. 5. Experimental results at 90oC for various granule sizes ............................................. 90

Figure 3. 6. Effect of isoamylenes concentration in the C5 fraction at 80oC. .............................. 92

Figure 3. 7. Parity diagram experimental conversion – calculated conversion ............................ 97

using kinetic model proposed by Rihko et al (1995, 1997) .......................................................... 97

Figure 3. 8. Variation in time of the experimental and calculated conversions............................ 98

Figure 3. 9. Energy of reactions scheme for the TAME synthesis ............................................... 99

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Figure 3. 10. Parity diagram experimental conversion – calculated conversion ........................ 100

Figure 4.1. Dynamic VLE apparatus used in the experimental work......................................... 103

Figure 4. 2. VLE apparatus scheme............................................................................................ 104

Figure 4. 3. VLE apparatus control panel ................................................................................... 106

Figure 4.4. VLE 2-methyl-2-butene (1) / methanol (2), x-y diagram (P=1013 mbar) ............... 114

Figure 4.5. VLE 2-methyl-2-butene (1) / methanol (2), Txy diagram (P=1013 mbar) .............. 114

Figure 4.6. VLE 2-methyl-2-butene (1) / methanol (2), x-y diagram from data available in the

literature (P=1013 mbar)............................................................................................................. 115

Figure 4.7. VLE 2-methyl-2-butene (1) / methanol (2), Txy diagram from data available in the


Figure 4.8 VLE 2-methyl-2-butene (1) / methanol (2), .............................................................. 116

a) experimental activity coefficients, b) Herington consistency test (P=1013 mbar)................. 116

Figure 4.9 VLE 2-methyl-2-butene (1) / methanol (2), correlation of the experimental data to

several thermodynamic models (P=1013 mbar) ......................................................................... 117

Figure 4.10 VLE 2-methyl-2-butene (1) / methanol (2), predictions compared with the

experimental data (P=1013 mbar)............................................................................................... 118

Figure 4.11 VLE 2-methyl-2-butene (1) / methanol (2), comparison of Aspen database with the


Figure 4.12. VLE 2-methyl-2-butene (1) / TAME (2), x-y diagram (P=1013 mbar) ................. 119

Figure 4.13 VLE 2-methyl-2-butene (1) / TAME (2), Txy diagram (P=1013 mbar)................. 119

Figure 4.14. VLE 2-methyl-2-butene (1) / TAME (2), x-y diagram from data available in


Figure 4.15. VLE 2-methyl-2-butene (1) / TAME (2), Txy diagram from data available in the


Figure 4.16 VLE 2-methyl-2-butene (1) / TAME (2), correlation to a volatility coefficient for

ideal mixtures (P=1013 mbar) .................................................................................................... 122

Figure 4.17. VLE 2-methyl-2-butene (1) / TAME (2), a) experimental activity coefficients, b)

Herington consistency test (P=1013 mbar)................................................................................. 122

Figure 4.18 VLE 2-methyl-2-butene (1) / TAME (2), correlation of the experimental data to


Figure 4.19 VLE 2-methyl-2-butene (1) / TAME (2), predictions compared with the


Figure 4.20. VLE 2-methyl-1-butene (1) / TAME (2), x-y diagram (P=1013 mbar) ................. 124

Figure 4.21 VLE 2-methyl-1-butene (1) / TAME (2), Txy diagram (P=1013 mbar)................. 124

vi

Figure 4.23 VLE 2-methyl-1-butene (1) / TAME (2), Txy diagram from data available in the


Figure 4.24 VLE 2-methyl-1-butene (1) / TAME (2), correlation to a volatility coefficient for

ideal mixtures (P=1013 mbar) .................................................................................................... 126

Figure 4. 25 VLE 2-methyl-1-butene (1) / TAME (2), a) experimental activity coefficients, b)


Figure 4. 26 VLE 2-methyl-1-butene (1) / TAME (2), correlation of the experimental data to


Figure 4.27 VLE 2-methyl-1-butene (1) / TAME (2), predictions compared with the


Figure 4.28 VLE isopentane (1) / methanol (2), x-y diagram (P=1013 mbar) ........................... 127

Figure 4.29 VLE isopentane (1) / methanol (2), Txy diagram (P=1013 mbar) .......................... 128

Figure 4.30 VLE isopentane (1) / methanol (2), x-y diagram from data available in the literature

(P=1013 mbar) ............................................................................................................................ 128

Figure 4.31 VLE isopentane (1) / methanol (2), Txy diagram from data available in the literature

(P=1013 mbar) ............................................................................................................................ 129

Figure 4.32 VLE isopentane (1) / methanol (2), a) experimental activity coefficients, b)


Figure 4.33. VLE isopentane (1) / methanol (2), correlation of the experimental data to several

thermodynamic models (P=1013 mbar) ..................................................................................... 130

Figure 4.34. VLE isopentane (1) / methanol (2), predictions compared with the experimental data

(P=1013 mbar) ............................................................................................................................ 130

Figure 4.35 VLE isopentane (1) / methanol (2), comparison of Aspen database with the


Figure 4. 36 Thermodynamic model correlation for each binary system. Root mean square error

for the temperatures. ................................................................................................................... 132

Figure 4. 37 Average temperature deviation.. ………………………………………………….133

Figure 5.1 Systematic procedure of the methodology proposed and used. ................................ 137

Figure 5.2 Rayleigh distillation scheme...................................................................................... 139

Figure 5.3 Residue curve map example. (TAME synthesis at Da=1, P=1 bar; .......................... 140

from Moreira Duarte, 2006)........................................................................................................ 140

Figure 5.4 Screen snapshot of the implemented user interface for initial concentration............ 159

Figure 5. 5 Reactive residue curve map at p = 4 bar and Da equal to 10-4 ................................. 160

Figure 5.6 Reactive residue curve map at p = 4 bar and Da equal to 10-3 .................................. 161


vii

Figure 5.1 Systematic procedure of the methodology proposed and used. ................................ 137

Figure 5.2 Rayleigh distillation scheme...................................................................................... 139

Figure 5.3 Residue curve map example. (TAME synthesis at Da=1, P=1 bar; .......................... 140

from Moreira Duarte, 2006)........................................................................................................ 140

Figure 5.4 Screen snapshot of the implemented user interface for initial concentration............ 159

Figure 5. 5 Reactive residue curve map at p = 4 bar and Da equal to 10-4 ................................. 160





Figure 5.10 Reactive residue curve map at p = 6 bar and Da equal to 10-3 ................................ 163



Figure 5.13. Number of transfer units in the residue curve map at .......................................... 167

p = 4 bar and Da equal to 10-4.................................................................................................... 167

Figure 5.14 Number of transfer units in the reactive residue curve map at.............................. 167

p = 4 bar and Da equal to 10-3..................................................................................................... 167

Figure 5. 15 Number of transfer units in the reactive residue curve map at p = 4 bar and Da

equal to 10-2................................................................................................................................. 168

Figure 5.16 Number of transfer units in the reactive residue curve map at p = 4 bar and Da

equal to 10-1................................................................................................................................. 168

Figure 5.17 Number of transfer units in the reactive residue curve map at p = 6 bar and Da equal

to 10-4 .......................................................................................................................................... 169


equal to 10-3................................................................................................................................. 169

Figure 5. 19 Number of transfer units in the reactive residue curve map at p = 6 bar and Da

equal to 10-2................................................................................................................................. 170


equal to 10-1................................................................................................................................. 170


equal to 0..................................................................................................................................... 171

Figure 5.22 Number of transfer units in the reactive residue curve map at p = 6 bar and Da equal to 0 ………………………………………………………………………………….172

viii

Figure 5.23 Reactive residue curve map (discontinue line) and infinite reflux column profiles

(continuous line) for the TAME reactive distillation at 1 bar and a value of H/V of 0.0001 sec173


(continuous line) for the TAME reactive distillation at 1 bar and a value of H/V of 0.001 sec 174


(continuous line) for the TAME reactive distillation at 1 bar and a value of H/V of 0.01 sec... 174


(continuous line) for the TAME reactive distillation at 1 bar and a value of H/V of 0.1 sec..... 175


(continuous line) for the TAME reactive distillation at 1 bar and a value of H/V of 1 sec........ 175


(continuous line) for the TAME reactive distillation at 1 bar and a value of H/V of 100 sec.... 176

Figure 5.29 Three possible ∞/∞ column profiles....................................................................... 178

Figure 5.30. Possible feed locations ........................................................................................... 181

Figure 5.31 The illustration of the reactive lever rule in composition space.............................. 183

Figure 5.32 Mass balance for simple binary distillation............................................................. 185

Figure 5.33 Bifurcation diagram of distillate compositions for the TAME synthesis at Da=0, P=1

bar and real-feed composition at the corresponding chemical equilibrium at 1 bar. .................. 187

Figure 5.34 Bifurcation diagram of distillate compositions for the TAME synthesis at Da=

0.0001, P=1 bar and real-feed composition at the corresponding chemical equilibrium ........... 189

Figure 5.35 Bifurcation diagram of distillate compositions for the TAME synthesis at Da=0.001,

P=1 bar and real-feed composition at the corresponding chemical equilibrium at 1 bar. .......... 192

Figure 5.36 Bifurcation diagram of distillate compositions for the TAME synthesis at Da=1, P=1

bar and real-feed composition at the corresponding chemical equilibrium at 1 bar. .................. 193

Figure 5.37 Bifurcation diagram of distillate compositions for the TAME synthesis at Da=0,

P=10 bar and real-feed composition at the corresponding chemical equilibrium at 10 bar. ...... 194


0.0001, P=10 bar, real-feed composition at the corresponding chemical equilibrium. ............. 195


0.001,P=10 bar and real-feed composition at the corresponding chemical equilibrium ........... 196

Figure 5 40 Bifurcation diagram of distillate compositions for the TAME synthesis at Da=0, P=1

bar and real-feed composition at the corresponding chemical equilibrium at 10 bar. ............... 197

Figure 5.41 Bifurcation diagram of distillate compositions for the TAME synthesis at Da= 0.0001,P=1 bar and real-feed composition at the corresponding chemical equilibrium……. 198

ix


0.001, P=1 bar and real-feed composition at the corresponding chemical equilibrium ............. 199

Figure 5.43 Bifurcation diagram of distillate compositions for the TAME synthesis at Da=1,

P=1 bar and real-feed composition at the corresponding chemical equilibrium at 10 bar. ........ 200

Figure 5.44 Bifurcation diagram of distillate compositions for the TAME synthesis in hybrid

distillation column, at Da=0.0001, P=10 bar and real-feed composition. .................................. 202


distillation column, at Da=0.001, P=10 bar and real-feed composition ..................................... 204


distillation column, at Da=1, P=10 bar and real-feed composition ............................................ 205


distillation column, at Da=1, P=1 bar and real-feed composition .............................................. 206

Figure 5.48 Hybrid reactive distillation column overcoming chemical equilibrium................. 208

Figure 5.49 Traditional system of reactor and distillation column at 1 bar using an excess of

isoamylene on the reactor feed. .................................................................................................. 209

Figure 5 50 Traditional system of reactor and distillation column at 10 bar using an excess of

isoamylene in the reactor feed. ................................................................................................... 209

Figure 5.51 Hybrid reactive distillation column at 10 bar using an excess of isoamylene in the

column feed................................................................................................................................. 210

Figure 5.52 Influence of the reaction extent on the performance of various configurations...... 210

Figure 5.53 Traditional system of reactor and distillation column (a); entire reactive distillation

column (b); hybrid reactive distillation column (c). ................................................................... 213

Figure 5.54 Hybrid reactive distillation column compositions profile ....................................... 216

Figure 5.55 Hybrid reactive distillation column temperature profile ......................................... 216

Figure 5.56 Distribution of the reaction extent in the hybrid reactive distillation column......... 217

Figure 5.57 Internal hybrid reactive distillation column flow rates............................................ 217

Figure 5. 58 Correlation between reboiler duty and vapour flow rate........................................ 219

Figure 5.59 Influence of distillate flow rate and reflux on reboiler duty.................................... 219

Figure 5. 60. Flowsheet scheme for RDWC............................................................................... 221

Figure 5. 61. Liquid phase composition profiles in the RDWC ................................................. 222

Figure 5. 62 Operational and investment costs for ideal mixtures K=1.8 and n=0.54. .............. 226

Figure 5. 63 Operational and investment costs crossing point for ideal mixtures. ..................... 226

Figure 5. 64 Operational and investment costs crossing point for ideal mixtures...................... 227

x

LIST OF TABLES

Table 2. 1. Allowed concentration limits for reformulated gasoline compounds......................... 14

Table 2. 2 Allowed concentration limits for ethers used in gasoline............................................ 14

Table 2. 3. Worldwide tertiary ethers production in year 2000 .................................................... 15

Table 2. 4 Tertiary ethers and gasoline properties........................................................................ 16

Table 2. 5. Typical composition of C4 fraction from pyrolysis units............................................ 17

Table 2. 6 Typical composition of C4 fraction from catalytic cracking units............................... 17

Table 2. 7 Typical composition of C5 fraction from catalytic cracking units............................... 17

Table 2. 8 Main characteristics of industrial processes for the synthesis of tertiary ethers……. 21

Table 2. 9 TAME composition produced in European Union ...................................................... 23

Table 2. 10 Physicochemical properties of tert-amyl methyl ether .............................................. 24

Table 2. 11 Companies producing TAME with capacities of over 1000 t/year ........................... 25

Table 2. 12 Amounts (produced+imported), industrial branches and areas of use for TAME..... 26

Table 2. 13 The amount of TAME consumed in different EU countries in 2002 ........................ 26

Table 2. 14 Thermodynamical data for the participants in the reaction ....................................... 31

Table 2. 15 Equilibrium constants involved in the TAME’s synthesis ........................................ 34

Table 2. 16 Kinetic parameters for the reactions involved in the synthesis of TAME................. 36

Table 2 17 Kinetic constants involved in TAME’s synthesis....................................................... 40

Table 2. 18 Rate expressions for TAME synthesis process.......................................................... 43

Table 2 19 Characteristics of Amberlyst and Purolite – type catalysts ...................................... 57

Table 2. 20 Characteristic variables of the equilibrium model ..................................................... 69

Table 2. 21 Equations of the equilibrium model........................................................................... 69

Table 2. 22 Variables of the non-equilibrium model.................................................................... 73

Table 2. 23 Equations of the non-equilibrium model………………………………………. 73

Table 3. 1 Experimental plan ........................................................................................................ 77

Table 3. 2. Parameters of the model used for calculating the theoretical values .......................... 95

Table 3. 3. Estimated parameters of the kinetic model ………………………………….100

Table 4.1. Raw materials for VLE experiments ......................................................................... 112

Table 4. 2. Estimated thermodynamic models parameters. ........................................................ 133

Table 4. 3 Maximum error temperature prediction for the estimated parameters. ………….134

Table 5.1. Case studies used in the infinite-infinite analysis for traditional system reactor +

distillation column at stoichiometric feed................................................................................... 185

xi

Table 5.2 Flow rates of the selected alternatives obtained by simulation. ................................. 214

Table 5.3 Comparison of the distillation columns and reactors for the several systems. ........... 215

Table 5. 4 Overall cost comparison of the several alternatives .................................................. 227

Table 5. 5 Results for the TAME synthesis (for 2.656 TAME Mt/h)......................................... 228

Table 5. 6 Normalised impact scores for different categories of potential environmental......... 230

Table 5. 7 Individual total PEI for each TAME process alternative (PEI/kg product)*............. 231

Table 5. 8 Factors for EPI determination of TAME process ...................................................... 232

1

CHAPTER 1

Introduction

An increasing environmental concern has promoted the use of some new products in detrimental

of some others less environmental friendly. Tert amyl methyl ether (TAME) is additive for clean

gasoline. TAME is the tertiary aliphatic ether most used nowadays in Europe as fuel additive and

therefore the present work is focused on it. The goal of this work is to contribute to the

understanding of chemical synthesis process, providing new experimental data and an original

approach taking into account the technical, economical and environmental aspects in process

design.

Chapter 2 presents the state of the art and the motivation of the present work. Several alternatives

for octane improvement of gasoline are presented from an historical perspective, until the

nowadays solution: tertiary aliphatic ethers C5-C7. The reasons for which these compounds

improve the octane number and what processes take place inside the engine are described. The

choice of various compounds as octane enhancers for gasoline throughout history is motivated

by their environmental impact and the legislation response to these evidences. A review of

several alternatives available is provided and they are discussed according to the legislative

framework. Information about their use and quantities mixed in the gasoline, properties inferred

to the gasoline, production capacities, factories and biodegradability routes in the environment

are discussed.

The reaction schemes used for tertiary aliphatic ethers synthesis are presented. The raw materials

usually employed are alcohols and the olefins contained in some light fractions from the oil

processing processes (as fluid catalytic cracking –FCC- or steam cracking). Several sources of

olefins are identified and the typical composition of these fractions is quantified. The reaction

conditions suitable for these reactions to take place and several catalysts useful for this purpose

are presented. The relative reactivity among several olefins is compared. The secondary

reactions possible to be present in the system are identified and explained in detail; the

recommended reaction conditions to avoid them are stated. TAME is produced by reaction of

methanol (MeOH) with two reactive C5 olefins, contained in C5 fraction separated in different

processes: 2-methyl-1-butene (2M1B) and 2-methyl-2-butene (2M2B). The reactions at

equilibrium are modelled and the values for the equilibrium constants are provided. Some of the

available thermodynamic models for estimation of physical-chemical properties are discussed in

Chapter 2 including the parameters used in prediction models.

2

A critical comparison of the available industrial processes for TAME production shows that

reactive distillation is a process integration/intensification technique providing very good

performance. Each process alternative is described in detail. The production capacities for

several sites around the world are tabulated indicating the licensor of the process.

The thermodynamic and chemical data as well as physical properties for this particular

component are provided. The main impurities present in industrially obtained TAME are

quantified depending on the source. The emissions of TAME to the environment from several

sources are quantified and tabulated, and effects evaluated. The evolution of TAME production

processes in Europe, main companies involved, production capacity, consumption in each

European country, and main uses are reviewed.

There are several classes of catalysts for synthesis of ethers used as ecological fuel additives as

already pointed out previously. They are classified and evaluated according to their main

properties. The ion exchange resins are discussed in more detail, as they are most applied in

industry and for this reason they are used later in this work. The reaction mechanism is discussed

showing that both 2M1B and 2M2B pass by the same intermediate carbocation tert-amyl before

obtaining final product, TAME. The catalyst review is closed by a section about inorganic solid

acid catalyst with emphasis on zeolites.

As pointed out previously, reactive distillation (RD) is a promising option for TAME synthesis.

In Chapter 2, reactive distillation is reviewed and potential advantages are underlined. The

historical framework in the use of reactive distillation is presented. Structural details on the

placement of the catalyst inside the RD column are shown. Several mathematical models useful

to describe the process are detailed.

Chapter 3 is dedicated to the thermodynamic and kinetic study for TAME synthesis process

scheme. A detailed literature review of kinetic models is provided along with the main

conclusions of main studies. An experimental kinetic study is performed using as raw materials

MeOH and isoamylenes enriched C5 fraction from FCC gasoline cut, as used in industry with

Amberlyst 35 ion exchange resin catalyst. A pressurized reactor connected to a gas

chromatograph with a FID detector is used. For an optimum tracking of operating parameters,

the set-up is endowed with a computer interface. This allows the transmission of temperature and

pressure values, determined by sensors in direct contact with work environment, to a computer

on which the momentary parameters are registered and presented in table or in graph format. To

increase the accuracy of sampling and analysis, the set-up is provided with a sealed system,

which connects the sampling device to the chromatograph where analysis of the reaction mixture

is performed at pre-established intervals. The influence of several parameters on the synthesis

process is studied: reactor temperature, catalyst size (to evaluate internal diffusion limitation),

3

and isoamylenes concentration. The activity coefficients are calculated using UNIFAC

thermodynamic model and the results are correlated to thermodynamic/kinetic models of Rihko

et al (1995, 1997). The model parameters are regressed in MATLAB and a parity diagram is

used to visualize the correlation of the results, obtaining a good quality of correlation.

Chapter 4 is devoted to experimental work and modelling of thermodynamic data, related to

vapour liquid equilibrium (VLE) for binary systems less studied in literature : MeOH / 2M2B,

2M2B / TAME, 2M1B / TAME and Isopentane / MeOH. At the beginning of the chapter, the

dynamic ebulliometer LABODEST 602 is described, providing details about the specific

operation procedure used in research. Samples analysis is performed by gas chromatography

with adapted procedure. For each of the binary mixtures studied, based on experimental data

obtained, xy and Txy diagrams are built, and compared to results published in literature. In

modelling work, experimental data is used to fit own parameters for several thermodynamic

activity models (Wilson, Van Laar, NRTL, UNIQUAC), based on computer tool implemented in

ASPEN Plus software. Then activity coefficient diagram and logarithm of activity coefficients

ratio are used to evaluate thermodynamic consistency. xy and Txy diagrams are used further to

compare : own experimental results with predictions of models (UNIFAC, UNIFAC Dortmund,

and COSMO which is an ab initio method) implemented in ASPEN Plus, along with prediction

of Wilson, Van Laar, NRTL, UNIQUAC having own fitted parameters. The root mean square

error for saturation temperatures and average temperature deviation are used to compare the

correlation of different thermodynamic models. Ideal gas phase behaviour fit correlation best.

From this chapter can be concluded that the experimental data and model parameters obtained in

this work is predict VLE for studied systems in agreement with the literature. Most of the

thermodynamic models considered provide good correlations.

In Chapter 5 a methodology to evaluate the feasibility of TAME synthesis process is proposed,

starting with fast calculations and evolving towards rigorous ones, taking into account

economical and environmental aspects, from early stages of process design. The residue curve

maps are used in the early stage to check the system behaviour and feasibility of component

separation. At the beginning of the chapter, the residue curve maps are introduced, identifying

the main topological aspects. The mathematical model to calculate the residue curves with

special features is formulated and implemented in MATLAB. Then model behaviour is

discussed. The Number of Transfer Units (NTU) is used to complement the residue curves for

checking the feasibility of separation process based on distillation. The suitability of the residue

curves for systems with reaction is discussed. Using the concept of NTUs and the mass balances

for a packed reactive distillation column operating at infinite reflux flow rate, a new expression

is obtained and compared to the results of the reactive residue curves for TAME synthesis

4

system. Above implemented software application objectives are: first, exploring the map

topology; second, the computation of the desired residue curves and third, computation of NTUs.

The residue curve maps plotted for different operation pressures of industrial interest, and for

different Damkhöler (Da) numbers are shown. They are plotted again with a smaller number of

residue curves but representing the difficulty of separation in the background by the NTUs.

Another series of maps discuss the differences between the residue curves and the curves

obtained at infinite reflux flow rate.

The infinite/infinite analysis, as a tool for process early analysis, is extended in the present work

to reactive systems, taking into account the reaction kinetics and the results obtained for the

TAME synthesis system. The infinite/infinite analysis is also applied to classical process (reactor

and separation column) as well as to a hybrid reactive distillation column. The originality of the

approach is that in all above cases, reaction kinetics is taken into account.

The results of the analysis are useful for rigorous simulation. Several alternatives are compared

according to the simulation results. A simulation taking into account the presence of isopentane

inert is provided, because this is the case industrial significance (isopentane is dominant in C5

fraction).

For comparing design alternatives, a simplified cost model is proposed and the results are

compared based on a techno-economical model available in literature. The originality of the cost

function proposed is that the goal is not to obtain a cost value, but a number proportional to the

cost. The main interest is not to calculate cost, but to optimize the process parameters.

Finally, environmental performance of TAME synthesis process is evaluated, based on potential

environmental impact (PEI), emissions, resource, and energy conservation factors. PEI is

obtained using waste reduction (WAR) algorithm as evaluation tool. For industrial processes

treating the same input (reactants) streams to provide the same product, the system boundaries

become fixed and their environmental impact depends only on the process itself. All the data

required for the environmental impact evaluation are obtained previously. From this chapter can

be concluded that the residue curve maps are a very useful tool for the analysis of the system and

that these first insights can be very useful for later rigorous simulations.

One of the main points stressed in this thesis is that the environmental and economical aspects

should be taken into account, from the early stages of any study.

The vapour liquid equilibrium experimental studies presented in this work are performed at the

Department of Chemical Engineering, Faculty of Chemistry of the University of Barcelona and

the kinetic experimental studies are performed in the Department of Chemical Engineering,

5

Faculty of Applied Chemistry and Materials Science of the University POLITEHNICA of

Bucharest.

Theoretical and experimental research developed for this doctoral program were partially funded

by the research grant TD GR 18/29.05.2007 offered by The National University Research

Council of Romania, by PN II IDEI project no. 1545 MASBIOGLI funded by Romanian National

Authority of Scientific Research, and by research fellowship to University of Barcelona partially

funded by ERASMUS program. During the stage in University of Barcelona infrastructure

created by the project CTQ 2009 – 11465 SEPBIO was intensively used.

6

CHAPTER 2

Survey regarding the catalytic distillation technique and synthesis methods of ethers used as fuel additives. Catalysts, kinetics and

thermodynamics

2.1. The principle of reactive distillation and its technical importance

Chemical processes industry is permanently subjected to fast changes, which are not exclusively

related to society demands for new products of higher quality and safer which don’t affect the

environment, but also to the dynamics of business environment (Herder, 1999 a,b). The success

depends as well on the way we face the demands regarding process design and control in a short

time interval.

In chemical industry, chemical transformations and separation – products purification processes

are usually performed sequentially. Moreover, many of the technologies of commercial interest

are based on reversible reactions, subject to thermodynamic constraints, which limit the

maximum degree of transformation towards the valuable product. In this way, the untransformed

reactants must be recovered by an efficient separation system and recycled. As the dregree of

transformation is lower, the investment and operation costs due to the two additional operations

mentioned before are higher. Economic needs as well as ecological demands are major driving

forces in improving chemical processes and plants. To meet these goals, processes have to be

intensified to get products of higher quality, to increase yield by reducing or even avoiding by-

products and to minimise energy consumption. Therefore, advanced strategies emerged for

process control and real-time optimization. A special attention must be given to the synthesis of

new unit operations which integrate different functions and units in order to enhance efficiency

at process and plant level (Grossman and Westerberg, 2000; Stankiewicz and Moulijn, 2002).

These systems are conventionally named hybrid units and are characterized by reduced costs and

high process complexity. Catalytic distillation is an example of such operation.

A conventional scheme includes chemical reactor, separator and recycle streams. Chemical

reactors have different topologies (continuous stirred tank reactor (CSTR), tubular reactor, etc)

operating in continuous, discontinuous or batch continuous modes. Operating conditions are on

wide areas (catalytic, adiabatic, single/multiple phase etc). Separation systems are based on

different principles (vapour-liquid equilibria – distillation, gas-liquid equilibria –

adsorbtion/desorption, liquid – liquid equilibria – extraction, etc.). However, most widely used is

distillation. Recycle system is important for improving conversion / yield in reaction system, to

7

limit formation of by-products, to save energy by process integration as well as to improve

controllability.

Combining reaction – separation – recycle system in one apparatus are conventionally names

hybrid units. As examples are reactive distillation, reactive extraction, reactive absorption, etc.

Economic and environmental considerations have encouraged industry to focus on technologies

based on process “intensification”. Reactive distillation is an excellent example of process

intensification. It can provide an economically and environmentally attractive alternative to

conventional multiunit flowsheets in some systems (Luyben and Yu, 2008).

The first patents for this kind of processing appeared in the 1920s, but small progress in this

domain was performed until 1980 (Malone and Doherty, 2000; Agreda and Partin, 1984) when

reactive distillation attracted more attention, being considered a good alternative to replace the

traditional sequence chemical reaction – distillation. After this date, investigation in this domain

got intensified and diversified, the distillation procedures with solid catalysed chemical reactions

(catalytic distillation) having a major extent among other processes.

Integration is performed either with the purpose to improve separation through chemical

transformation (I), or to improve the performance of the latter with the help of sepation (II). A

representative example for the first case (I) is the process of CO2 capture from synthesis gas, in

ammonia technology. For the second case (II), according to LeChatelier principle, avoidance of

the thermodynamic limitations is performed by shifting the chemical equilibrium towards the

interesting products, eliminating a product from the reaction environment. Therefore, the degree

of reactants transformation into valuable products is improved and a relaxation of constraints

occurs for the system of separation and recycle. There are many representative examples for this

case, the most known being the synthesis of methyl acetate(MeAc) by Eastman Chemical Co.

procedure and the synthesis of MTBE.

The first industrial applications of reactive separation processes are registered in the 1920s

(Backhaus, 1921a,b,c), being applications of esterification with its versions transesterification

and hydrolisys. The success of catalytic distillation in the 1980 was mainly due to the success of

MTBE as gasoline additive replacing the ones based on lead (DeGarmo et al, 1992 and Schrans

et al, 1996). Together with the methyl acetate synthesis process by homogeneous reactive

distillation, catalytic distillation became a practical example of the advantages provided by

reactive separation techniques. For the MTBE case, the economy performed in the operating and

investment costs is due to a compact process schemes made of a single unit (catalytic distillation

column) which avoids the formation of three possible azeotropes. In the methyl acetate case, the

economy performed is determined by elimination of 28 equipments needed for purification and

azeotropes avoidance which are formed between reactants and between reactants and products

8

(Agreda et al, 1990). In a classical process scheme, the investment cost associated to this big

number of equipments is added to the operating cost and to the cost of additional solvents used.

Lately, there has been a growing interest in the field, by performing experimental research of

reactive distillation applications. For example, Doherty and Malone (2001) report more than 60

reactive distillation systems studied. Stankiewicz (2003) mentions the following important

industrial processes as potential candidates to adopt the reactive distillation technology: ethers

decomposition to high purity olephins, dimerization, aromatic and aliphatic hydrocarbons

alkylation, hydroisomerization, hydrolysis, ethers dehydration to alcohols, oxidative dehydration,

carbonylation. In the field of fine chemical products technologies, Omota et al (2001, 2003)

propose an innovative process of catalytic distillation for the esterification reaction of fatty acids.

The feasibility of this process is firstly suggested by the thermodynamic analysis combined with

modelling and computer simulation (Omota et al., 2003). The design methodology proposed is

successfully applied to a representative esterification reaction in kinetic regime. Liu et al (2012)

propose a new approach that substitutes the original two-step olefinic alkylation of thiophenic

sulfur (OATS) technology with a reactive distillation (RD) column was proposed to remove

sulfur compounds from fluid catalytic cracking (FCC) gasoline.

Reactive distillation represents the synergetic effect of combining the distillation process with

chemical transformation into a single multifunctional unit. The reactive zone inside the column

is identified by the presence of catalyst (liquid or solid). The feed is done usually at its

extremities, function of the compounds volatility. The products are continuosly removed at the

top or bottom part, while the reactants are maintained inside the column. In this way, the

importance of purification or recycle system is diminished or even eliminated, which provides

economy at energy and capital level and a high conversions.

The direct advantages of reactive distillation can be exemplified considering the chemical

reaction:

DCBA

with the side reactions

ECA

FC 2

The process can take place inside a conventional reactor (Figure 2.1a), the objective being the

production of C from A and B. Together with C, D is also produced, and side-reactions also take

place which lead to a reactor effluent made of a mixture of all these compounds. A and B must

be separated and recycled, C (the most volatile) must be separated and D (the heaviest), E and F

must be treated as by-products. This leads to the need of more rectification columns to be used.

Figure 2.1b illustrates the scheme of a typical reactive distillation process. The chemical

9

transformation takes places in the reactive zone, the stripping and rectifying zones being used for

product conditioning and by-products separation. Product C is rapidly removed from the reaction

volume producing in this way the shifting of chemical equilibrium towards products, increasing

the total conversion. Diminishing the concentration of product C in reaction volume determines

the reduction of side-reactions rate, so that less by-products are produced and the necessary

separations and purifications in the classical scheme can be elimated in this way.

Rectification area

Reactive area

Stripping area

Rectification area

Reactive area

Stripping area

Figure 2. 1 Typical scheme for a conventional process of (a) reaction and separation and (b) reactive distillation

The main advantages of reactive distillation are highlighted as following:

capital economy by separation system simplification or elimination

energy saving needed for fluid recycle by improving the reactants conversion up to 100%

selectivity improvement: removing one of the products from the reaction environment or

maintaining reduced concentrations of one of the reactants leads to the reduction of side-

reaction rates and to the reduction of by-products formation

reduction of catalyst quantity needed for the same conversion degree

azeotropes avoidance: when the reactor effluent is a mixture of chemical species forming

azeotropes. The process conditions permit avoiding them.

Thermal integration: the reaction heat of an exothermal chemical transformation can be

used for vapour formation and reduction of reboiler thermal duty

Avoidance of local hotspots in the catalyst layer through liquid partial vaporization.

However, reactive distillation presents also several disadvantages:

The reactants and products must have compatible volatilities in order to maintain high

reactants concentrations in liquid phase and low products concentration in the reaction

zone

10

Optimal conditions of temperature and pressure for separation must correspond to the

ones necessary for the chemical transformation

Processing big quantities of raw materials determines issues related to liquid distribution

inside reactive distillation column packings.

This last issue determined intensive research for the design of column internals. Efficient

contacting of the reactive phase with the catalyst is necessary to avoid radial gradients, local

hotspots, maximize conversion and reduce the catalyst quantity necessary for a certain

conversion value. The catalyst particles used to diminish intra-particle diffusion effects have

dimensions in the interval 1 – 3 mm and < 1 mm in liquid phase. The elimination of possible

entrainment imposes their retention inside packings of various shapes:

Porous grains impregnated with catalyst (Buchholz et al, 1995)

Different metallic shapes: spheres, tablets, a.s.o. (Smith, 1984)

Metallic structures disposed in forms of horizontal channels filled with catalyst (van

Hasselt, 1999)

Horizontally disposed tubes containing the catalyst (Buchholz et al, 1995)

Fabric packages which contain catalyst (Johnson et al, 1994) retained inside pockets

made of two fibre glass strips

Catalyst particles disposed between metallic fibre strips in sandwhich shape (Stringaro,

1995). These structures are patented by Sulzer Co. (KATAPAK-S) and by Koch-Glitsch

(KATAMAX) and consist of two rectangular strips made of metallic fibres sealed at the

edges forming pockets of 1 – 5 cm wide. More such strips are connected in order to form

a packing cube which is mounted inside reactive distillation columns.

Inside tray columns, the catalyst is placed in vertical shapes disposed along the direction of

liquid flowing. These are completely immersed in liquid to assure an optimum contacting

between the reactive phase and the solid phase. An alternative is to place the catalyst in the tray

distributor or at its exit.

Even if most of the scientific studies deal with continous reactive distillation, many practical

applications are found also in the field of discontinous processes. The industries of fine

chemicals or specialities synthesis, such as the pharmaceutical industry, use discontinous

reactive distillation to favour the product formation of an equilibrium reaction or to avoid

secondary reactions. This technology is more versatile and appropriate compared to the

continous process to produce small quantities of valuable substances, and the flexibility of

operation offers more freedom to adapt to consumers demand on a more and more unpredictable

market. The high commercial value of finite products diminish the negative impact of energy

costs, which are higher than in the case of continous processes (Furlonge et al, 1999), on

11

economical performances. In this context, discontinous reactive distillation can be a viable

alternative to classical processes of reaction – separation (Sørensen et al, 1996). Three possible

configurations of discontinous reactive distillation columns are mentioned in literature (Guo et

al, 2003):

Discontinous reactive rectifying column: the reactive mixture is fed to the bottom, where

reaction takes place, while products are collected at the top part, having low volatilities.

Discontinous reactive stripping column: the reactants with low volatilities are introduced

in the condenser.

Side-vessel column: contains a rectifying section and a stripping section, being used

when the reactants have intermediate volatilities compared to the products which are

collected both at the top an dat the bottom of the column.

In the present thesis, reactive distillation is applied to the synthesis of gasoline additives, such as

the TAME synthesis. This introductory chapter states the problem and evidences the usefulness

of reactive distillation for this purpose.

2.2. The importance of tertiary aliphatic ethers C5-C7. Legislation aspects and main representatives

The car fuels used in engines with internal combustion originate from distillate fractions

obtained at crude-oil processing and possess usually an octane number under the interval of

values required (87–93). The tetra-ethyl-lead was firstly used in United States of America, while

in Europe, it was firstly used the alcohol as a gasoline additive. The advantage of leadded

gasoline, which provides a higher caloric capacity, made it worldwide used. One of the great

advantages of tetraethyl-lead was that it can be used in very small concentrations, while other

antiknocking agents needed to be used in higher concentrations, leading many times to the

decrease of gasoline caloric power.

The alcohol used as gasoline additive was causing the retention of humidity inside the

fuel, which leads to an effect of corrosion and rusting of fuel transport lines towards the engine.

While tetraethyl-lead is very soluble in gasoline, ethanol has a low solubility which decreases as

the humidity increases. Therefore, in time, drops or water areas can be formed inside the fuel

tank and this process creates the risk of fuel freezing. The great values of fuel humidity can also

favorise issues related to biological contamination with various bacteria which grow at the

interface water-gasoline. The antibacterian properties of tetraethyl-lead prevent the formation

and growth of such bacteria and fuel degradation. Therefore, this became an important

12

compound used worldwide a long time period as additive to increase the gasoline octane number.

It has been produced commercially since 1923 until 1986, a period during which approximately

7 million tones have been manufactured. However, the octane number enhancement by using

tetra-ethyl-lead as fuel additive induces great pollution problems, the toxic effect of Pb aerosols

in exhaust gases being well known.

In the most western countries, this additive was banned at the end of 20th century due to

issues related to air and soil pollution and the lead accumulative neurotoxicity. Starting from

2007, Pb-free gasoline was available worldwide, except countries like Yemen, Afganistan and

North Coreea which still use great quantities of Pb-based gasoline. However, tetra-ethyl-lead

remained the additive to be used in jet fuels (octanic number = 100) in piston-type engines and

until recently in fuels used for sportive car races.

Many cars manufactured before banning tetra-ethyl-lead needed modifications to be

able to function with Pb-free gasoline. These modifications can be divided in 2 categories:

‐ engines needed physical compatibility with Pb-free gasoline: the exhaust system was

modified (valve systems)

‐ modifications made to compensate the relative decrease of Pb-free gasoline octanic

number: decrease of pressure inside engines by thinning of piston cylinder head or

building piston engines which can reduce the compression rate.

The replacement of tetra-ethyl-lead started to be effective along with the introduction of

environmental protection restrictions in USA by “Clean Air Act Amendments” (CAAA) in 1990.

CAAA forbids the tetra-ethyl-lead usage and establishes the limits to maximum 1% benzene and

maximum 25% aromatic hydrocarbons in gasoline with high octane number. To cover the octane

deficit appeared by tetra-ethyl-lead banning, the only solution accepted at refineries level was the

usage of tertiary aliphatic compounds C5-C7, among which the most spread is methyl-tert-butyl-

ether (MTBE). This asymmetric ether presents superior characteristics in comparison with the

alcohols. It provides a high octane number and a series of other advantages such as reduction of

carbon monoxide content in exhaust gases, improved engine start-up in cold conditions,

chemical stability for the duration of storage, reduced toxicity (Krahenbuhl and Gmehling,

1994). However, the MTBE capacity, even in small concentrations, to modify in a not desired

way the taste and smell of water lead to new politics of MTBE usage as fuel additive. The need

to find new alternatives to replace MTBE emerged at this point and other oxygenate compounds

began to be studied: ethyl-tert-butyl-ether (ETBE), tert-amyl-methyl-ether (TAME), tert-amyl-

ethyl-ether (TAEE). The aliphatic tertiary ethers with practical use contain in their molecule two

alkyl groups, one tertiary (tert-butyl or tert-amyl) and the other primary (methyl, ethyl, and rarely

isopropyl). Figure 2.2 exhibits the most representative members of this class.

13

Superior ethers containing more than six carbon atoms have been synthesised and tested

as fuel additives as well, but their application is more restricted. Among these, the most known

are: 2-methyl-2-pentyl ethyl ether (THxEE1), 2,3-dimethyl-2-butyl ethyl ether (THxEE2), 3-

methyl-3-pentyl ethyl ether (THxEE3), 3-methyl-3-hexyl ethyl ether (THpEE), 3-methyl-3-hexyl

ethyl ether (THpME) and 3-methyl-3-pentyl methyl ether (THxME). Note also that di-isopropyl

ether is not a tertiary ether but possesses additive properties close to the tertiary ones.

Due to the ecological restrictions imposed in the last years, the tertiary aliphatic ethers

from C5-C7 series and the superior ethers have gained a great importance. These compounds

contain a tertiary radical containing 4 – 5 carbons, derived from a tertiary C4 – C5 olefine.

Nowadays, they are used as octane components for ecological gasoline preparation, successfully

replacing tetra-ethyl-lead.

CH 3 CH 2 O CC H3

C H3C H 2 C H3

CH 3 O CC H3

C H3C H3 C H3 CH 2 O C

C H3

C H3C H3

C H 3 O CCH 3

CH 3C H2 CH3

m etil tert-butil eter ( M TB E ) etil tert-butil e ter ( E TBE )

tert-am il m etil e ter ( T AM E ) te rt-am il etil e ter ( T AEE )

Methyl-tert-butyl-ether (MTBE) Ethyl-tert-butyl-ether (ETBE)

tert-amyl-methyl-ether (TAME) tert-amyl-ethyl-ether (TAEE)

CH 3 CH 2 O CC H3

C H3C H 2 C H3

CH 3 O CC H3

C H3C H3 C H3 CH 2 O C

C H3

C H3C H3

C H 3 O CCH 3

CH 3C H2 CH3

m etil tert-butil eter ( M TB E ) etil tert-butil e ter ( E TBE )

tert-am il m etil e ter ( T AM E ) te rt-am il etil e ter ( T AEE )

Methyl-tert-butyl-ether (MTBE) Ethyl-tert-butyl-ether (ETBE)

tert-amyl-methyl-ether (TAME) tert-amyl-ethyl-ether (TAEE)

Figure 2. 2 Main tertiary ethers used at industrial scale

The antiknocking properties of tertiary ethers are due to their capacity to form free

radicals, which are more stable than other alkyl radicals when colliding with other species in the

system. The most reactive group is the methylene in vicinal position of oxygen atom. These

radicals possess a greater stability due to the sp3 orbital semi-occupied with electrons that can

receive electrons from the neighbouring oxygen atom, obtaining in this way the characteristic of

a completely occupied orbital.

Due to the high concentrations of ether in fuels and to the high temperatures, other free

radicals may be obtained in the interactions between ether molecules with unstable radicals.

Gasoline combustion occurs by a free radical mechanism. During the compression of

the fuel mixture in the engine, the temperature increases and favours cracking reactions of the

alkanes presents in this mixture. Consequently, primary, secondary and tertiary radicals are

formed. Due to their higher reactivity, primary and secondary radicals are decomposing fast (by

β-splitting) generating explosions before the total compression of the fuel mixture. A way to

inhibit these early unwanted explosions is the transformation of the unstable radicals resulted

14

from alkanes during the compression into stable hydrocarbons. In this way, unstable radicals R

and ether molecules are consumed resulting R-H hydrocarbons, α-ether alkyl radicals and tert-

amyl radicals, more stable than the primary alkyl ethers. The early explosion is avoided and the

fuel mixture can be compressed until the final point.

A high ethers content in gasoline leads to the increase of octane number and to the

decrease of toxic combustion products concentration (carbon monoxide, nitrogen oxides, carbon

oxides, unburnt hydrocarbons, polycyclic hydrocarbons, etc). Due to the low freezing point and

to the water solubility, the ethers decrease the freezing point of gasoline. Consequently, they can

be used at low temperature conditions. The fuels containing ethers are compatible to all types of

existing engines and feeding systems (injection, carburization) without any modification.

(Ullmann's Encyclopedia of Industrial Chemistry)

The European regulations (Council regulation 85/536/EEC from 5 September 1985) and

North-American regulations (EPA, Clear Air Act Sun Refining, 1988, 1995) imposed restrictions

both in the gasoline’s oxygen content and in aromatic hydrocarbons content. The minimum and

maximum limits for gasoline compounds, according to the above-mentioned regulations are

presented Table 2.1 (Oktar et al., 1999).

Table 2. 1. Allowed concentration limits for reformulated gasoline compounds

Compound Minimum content Maximum content

Oxygen (as ether) 2 wt % 2.7 wt %

Benzene - 1 % (volumetric)

Total aromatic

hydrocarbons - 25 % (volumetric)

Heavy metals Total lack

Table 2.2 presents the maximum and minimum quantities of ether allowed in

reformulated gasoline according to the legislation mentioned above.

Table 2. 2 Allowed concentration limits for ethers used in gasoline

Ether

Minimum content

allowed in gasoline,

[wt %]

Maximum content

allowed in gasoline,

[wt %]

MTBE 11.000 14.850

ETBE 12.750 17.213

TAME 12.750 17.213

TAEE 14.500 19.575

15

The calculations are made based on the hypothesis that the entire oxygen content of an

ecological (reformulated) gasoline is provided by the added ether. The maximum ether

concentration in gasoline is imposed to maintain within allowed limits the quantity of toxic

compounds resulted at ether combustion (formaldehyde, acetaldehyde and their corresponding

acids). The minimum quantity of ether is imposed for complete hydrocarbons combustion (the

lowest possible quantities of carbon monoxide, nitrogen oxides, carbon, unburnt hydrocarbons,

polycyclic hydrocarbons etc in exhaust gases).

The immediate effect of tetra-ethyl-lead banning as fuel additive was the increase of

ethers consumption. To satisfy this demand, high production capacities have been implemented,

the majority of plants being able to produce hundreds of thousands of tons per year. Table 2.3

presents the worldwide production capacities in year 2000 for the main ethers. More than 95% of

ethers worldwide production is used as gasoline additive (Pääkkönen, 2003).

Table 2. 3. Worldwide tertiary ethers production in year 2000 (Pääkkönen, 2003)

Ether Worldwide production capacity (m3/year)

MTBE 33.370.000

ETBE 6.500.000

TAME 3.597.000

In December 2000, in Europe, there were 37 plants for the production of tertiary ethers

(MTBE, ETBE, TAME). The production capacities of the plants were betweem 15,000 and

600,000 t/year, the total ether production in Europe being estimated to 3,991,000 t/year.

The most used ether from the beginning of the mass production until nowadays is

methyl-tert-butyl-ether, followed by ethyl-tert-butyl-ether. The major disadvantage of these

ethers is their relatively high volatility. The vapours arrived in atmosphere are degraded slowly

and they are returned by rain in soil, in ground and surface waters, where they persist due to their

low biodegradability. In USA, important quantities of methyl-tert-butyl-ether accumulated in

ground waters, creating major problems of purification, for drinking water. Ethyl-tert-butyl-ether

has also been used in Europe, creating problems of environment contamination. However, the

phenomenon has lower amplitude due to its lower volatility and due to the lower fuel

consumption than in USA. The inconveniences associated to the usage of these ethers in gasoline

raised the problem of using other ethers, with lower vapour pressure but with similar octane

properties. Table 2.4 presents the main properties of tertiary ethers and of the gasoline that

contain them (Krahenbuhl and Gmehling , 1994; Pääkkönen, 2003).

16

It can be observed that tert-amyl-ethyl-ether presents an octane number closer to the

large scale used ethers, but its vapour pressure is considerably lower and it is produced from a

renewable raw material (ethanol).

The disadvantage of these ethers is their low degradation rate, despite their fairly low

toxicity. The natural degradation can be done chemically (in the atmosphere) or biochemically

(in soil and waters). Biochemically, the ether can be degraded until aldehydes (formaldehyde,

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