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Transcript of Alexandra Elena Plesu (Bonet Ruiz) Phd Thesis
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MODELLING AND SIMULATION OF CONTINUOUS CATALYTIC
DISTILLATION PROCESSES
A Thesis submitted to theUniversity POLITEHNICA of Bucharest
andUniversity of Barcelona
in joint collaboration
for ACADEMIC TITLE OF DOCTOR
by
Alexandra Elena BONET RUIZ
SPECIALITY: CHEMICAL ENGINEERING
under the supervision of
Professor Grigore BOZGAProfessor Joan LLORENS LLACUNA
Professor Jos COSTA LPEZ
July 2012
UNIVERSITY POLITEHNICA of BUCHAREST
Faculty of Applied Chemistry and Material Sciences
Department of Chemical Engineering
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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., PLEU, 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., PLEU, 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, 33thInternational Conference of Slovak Society of Chemical Engineering, 22 26
May 2006, Tatransk Matliare, Slovakia, Proceedings - Slovak Society of Chemical Engineering,
ISBN 80-227-2409-2,paper no. Le-Fr-2251.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, Tatransk Matliare, Slovakia,
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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
refinery10thConference on Process Integration, Modelling and Optimisation for Energy Saving
and Pollution Reduction-PRES'07, vol 1, J. Kleme 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, 18thEuropean Symposium on
Computer Aided Process Engineering - ESCAPE18, 1-4 June 2008, Computer-Aided ChemicalEngineering, 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, 11thConference on Process
Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction-
PRES2008, J. Kleme 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. Kleme 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,11thConference on Process Integration, Modelling and Optimisation for
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Energy Saving and Pollution Reduction- PRES2008, J. Kleme 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.
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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-C7aliphatic 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 C5tertiary 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...763.1. 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............................................................................. 1094.1.3. 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
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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 forTAME 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 CO2capture 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
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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
literature (P=1013 mbar)............................................................................................................. 116
Figure 4.8 VLE 2-methyl-2-butene (1) / methanol (2), .............................................................. 116
a) experimental activity coefficients, b) Herington consistency test (P=1013 mbar)................. 116Figure 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
experimental data (P=1013 mbar)............................................................................................... 118
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
literature (P=1013 mbar)............................................................................................................. 120
Figure 4.15. VLE 2-methyl-2-butene (1) / TAME (2), Txy diagram from data available in the
literature (P=1013 mbar)............................................................................................................. 120
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
several thermodynamic models (P=1013 mbar) ......................................................................... 122
Figure 4.19 VLE 2-methyl-2-butene (1) / TAME (2), predictions compared with the
experimental data (P=1013 mbar)............................................................................................... 123
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
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Figure 4.23 VLE 2-methyl-1-butene (1) / TAME (2), Txy diagram from data available in the
literature (P=1013 mbar)............................................................................................................. 125
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)
Herington consistency test (P=1013 mbar)................................................................................. 126
Figure 4. 26 VLE 2-methyl-1-butene (1) / TAME (2), correlation of the experimental data to
several thermodynamic models (P=1013 mbar) ......................................................................... 126
Figure 4.27 VLE 2-methyl-1-butene (1) / TAME (2), predictions compared with the
experimental data (P=1013 mbar)............................................................................................... 127
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).......................... 128Figure 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)
Herington consistency test (P=1013 mbar)................................................................................. 130
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
experimental data (P=1013 mbar)............................................................................................... 131
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
Figure 5.7 Reactive residue curve map at p = 4 bar and Da equal to 10-2
.................................. 161
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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
Figure 5.7 Reactive residue curve map at p = 4 bar and Da equal to 10-2
.................................. 161
Figure 5.8 Reactive residue curve map at p = 4 bar and Da equal to 10-1
.................................. 162
Figure 5.9 Reactive residue curve map at p = 6 bar and Da equal to 10-4
.................................. 163
Figure 5.10 Reactive residue curve map at p = 6 bar and Da equal to 10-3
................................ 163
Figure 5.11 Reactive residue curve map at p = 6 bar and Da equal to 10
-2
................................ 164Figure 5.12 Reactive residue curve map at p = 6 bar and Da equal to 10
-1................................ 164
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
Figure 5.18 Number of transfer units in the reactive residue curve map at p = 6 bar and Da
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
Figure 5.20 Number of transfer units in the reactive residue curve map at p = 6 bar and Da
equal to 10-1
................................................................................................................................. 170
Figure 5.21 Number of transfer units in the reactive residue curve map at p = 4 bar and Da
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
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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
Figure 5.24 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 0.001 sec 174
Figure 5.25 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 0.01 sec... 174
Figure 5.26 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 0.1 sec..... 175
Figure 5.27 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 1 sec ........ 175
Figure 5.28 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 100 sec.... 176Figure 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
Figure 5.38 Bifurcation diagram of distillate compositions for the TAME synthesis at Da=
0.0001, P=10 bar, real-feed composition at the corresponding chemical equilibrium. ............. 195
Figure 5.39 Bifurcation diagram of distillate compositions for the TAME synthesis at Da=
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
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Figure 5.42 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 ............. 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
Figure 5.45 Bifurcation diagram of distillate compositions for the TAME synthesis in hybrid
distillation column, at Da=0.001, P=10 bar and real-feed composition ..................................... 204
Figure 5.46 Bifurcation diagram of distillate compositions for the TAME synthesis in hybrid
distillation column, at Da=1, P=10 bar and real-feed composition ............................................ 205
Figure 5.47 Bifurcation diagram of distillate compositions for the TAME synthesis in hybrid
distillation column, at Da=1, P=1 bar and real-feed composition .............................................. 206Figure 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
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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 C4fraction from catalytic cracking units............................... 17
Table 2. 7 Typical composition of C5fraction 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 TAMEs synthesis ........................................ 34
Table 2. 16 Kinetic parameters for the reactions involved in the synthesis of TAME................. 36
Table 2 17 Kinetic constants involved in TAMEs synthesis....................................................... 40Table 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
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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
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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 alternativesfor 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 C5olefins, 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.
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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 mainproperties. 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),
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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 thermodynamicactivity 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 initiomethod) 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
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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 Damkhler (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 C 5
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,
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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 dont 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 shorttime 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 operationsmentioned 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
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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 technologiesbased 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 domaingot 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 CO2capture 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
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(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 importantindustrial 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 issuccessfully 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
FC2
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
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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 necessaryseparations 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
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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. Efficientcontacting 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
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economical performances. In this context, discontinous reactive distillation can be a viable
alternative to classical processes of reaction separation (Srensen 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, wherereaction 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 (8793). 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 formationand growth of such bacteria and fuel degradation. Therefore, this became an important
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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 20thcentury 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 beable 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 2.2exhibits the most representative members of this class.
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from alkanes during the compression into stable hydrocarbons. In this way, unstable radicals R
and ether molecules are consumed resulting R-H hydrocarbons, -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 thedecrease 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) andNorth-American regulations (EPA, Clear Air Act Sun Refining, 1988, 1995) imposed restrictions
both in the gasolines 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
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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 correspondingacids). 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(Pkknen, 2003).
Table 2. 3. Worldwide tertiary ethers production in year 2000 (Pkknen, 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; Pkknen, 2003).
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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,
acetaldehyde) and tertiary alcohols (tert-butylic, tert-amylic) by using a certain type of bacterial
cultures such as those obtained from the soils polluted with gasoline (Becker, 2004), Kharoune et
al., 2001). In the atmosphere, their degradation proceeds until formic acid and tertiary alcohols
(Kasprzyk-Hordern et al., 2004). In both cases, the first step is the oxidation of methylene group
neighbour to the etheric oxygen atom, which possesses a greater reactivity than the rest of the
aliphatic groups in the molecule. The resulted alcoxy radical has two splitting possibilities: inthe first, (a) a hydrogen atom is eliminated, and in the second, (b) a methyl group is eliminated.
The reaction proceeds mainly following the (b) alternative, fact proven in lab using oxidising
agents stronger than air (e.g. NO2). The main product in this case is the tert-amyl formiate
(Becker, 2004).
Table 2. 4 Tertiary ethers and gasoline properties
Compound
PropertyCH3OH C2H5OH MTBE ETBE TAME TAEE
Pure component vapour
pressure at 37.8C [psi]- 2.3 7.8 4 2.5 1.2
Vapour pressure at 25C of
gasoline with octane content*
[psi]
59.46 18.0 8.0 4 2 2
Octane number**of gasoline
with octane content*120 115 110 112 105 109
Boiling point [K] 338 351 328 345 358 379
Oxygen content (wt %) 50 34.7 18.2 15.7 15.7 13.8
Water solubility at 25C
[mg/L]
Totally
miscible
Totally
miscible48000 26000 20000 4000
Relative atmospheric photo-
degradation***1 3.4 2.6 8.1 7.9 8.9
* gasoline with massic oxygen content of 2.7% or higher; 1 psi = 6.89510 3Pa**octane number as average of research octane number (RON) and motor octane number (MON). Research octane number
represents the fuel octane number measured in experimental engine under low severity engine operation. Motor octane number represents the fuel
octane number measured in experimental engine with more severe operation that might be incurred at high speed or high load.***refers to the ether reaction with oxygen in position.
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2.2.1.Sources of tertiary olephins
Fuel ethers are obtained by the addition of inferior alcohols to tertiary olephins C4 C6, among
which only the tertiary ones are enough reactive for alcohol addition.
The olephins currently used in the industrial production of tertiary ethers are components of
fractions emerging from hydrocarbons cracking units. The isobutene is derived mainly from the
C4fraction of thermal cracking units (pyrolysis) and from the C4fractions of catalytic cracking
units respectively. The reactive isomers of C5and C6olefines, mainly 2-methyl-1-butene and 2-
methyl-2-butene, appear in the catalytic cracking fractions, respectively in the C5 fraction of
pyrolysis units. The typical compositions of these fractions are presented in Tables 2.5 2.7.
Table 2. 5. Typical composition of C4fraction from pyrolysis units
Compound Concentration ( wt %)
i- butane 2 5n-butane 2 5isobutene 18 321-butene 14 222-butene 5 15
butadiene 35 50
Table 2. 6 Typical composition of C4fraction from catalytic cracking units
Compound Concentration ( wt %)
i- butane 35 45
n-butane 7 12
isobutene 10 20
1-butene 9 12
2-butene 2030butadiene 0.5 1.5
Table 2. 7 Typical composition of C5fraction from catalytic cracking units
Compound Concentration ( wt %)
C4 2.51
3-methyl-1-butene 1.68
Isopentane 44.74
1-Pentene 3.56
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2-methyl-1-butene 7.02
n-pentane 3.81
Isoprene 0.21
Trans-2-pentene 6.87
Cis-2-pentene 3.662-methyl-2-butene+ trans-piperilene 8.68+0.12
2,2-dimethyl-butan + cis-piperilene 0.13+0.04
Cyclopentene 1.04
( 4-methyl + 3-methyl ) 1-pentene 0.38
Cyclopentane 0.34
2,3-dimethyl-butane + MTBE 1.38
2,3-dimethyl-1-butene 0.142-methyl-pentane 4.36
3-methyl-pentane 2.19
2-methyl-1-pentene 0.51+0.22
n-hexane 0.46
2-methyl-2-pentene 0.57
3-methyl-trans-2-pentene 0.26
3-methyl-cis-2-pentene 0.37olephins C6unidentified 1.76
Methyl-cyclopentane 1.31
Benzene 0.51
Methyl-cyclopentene 0.35
(isoheptanes+ heptenes ) 0.82
TOTAL 100.00
2.2.2.Synthesis methods for C5-C7aliphatic tertiary ethers series
The synthesis of fuel ethers is usually perfomed under pressure in liquid phase on a
strongly acidic macroporous ion exchange resin as catalyst. The conventional ion exchange
resins are sulfonated copolymers of divinylbenzene (DVB) with styrene. A tertiary ether is made
by the reaction between an alcohol and an alkene with a double bond involving at least a tertiary
carbon atom:
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may participate independently to side reactions favoured by water presence, high reaction
temperatures, important alcohol excess or isoamylene excess. The main secondary reactions are:
Generation of tertiary alcohols by hydration of olephins:
CH3 CH2 C
CH3
CH3OH
CH2 C
CH3
CH2CH3
CCH3
CH3 CH CH3 OH2
OH2 CH3 CH2 C
CH3
CH3
OH
alcool tert-amilic (T
2-metil-1-butena (2M1B)
2-metil-2-butena (2M2B)
+
+
alcool tert-amilic (TA2-methyl-1-butene (2M1B)
2-methyl-2-butene (2M2B) Tert-amylic alcohol (TAA)
Tert-amylic alcohol (TAA)
CH3 CH2 C
CH3
CH3OH
CH2 C
CH3
CH2CH3
CCH3
CH3 CH CH3 OH2
OH2 CH3 CH2 C
CH3
CH3
OH
alcool tert-amilic (T
2-metil-1-butena (2M1B)
2-metil-2-butena (2M2B)
+
+
alcool tert-amilic (TA2-methyl-1-butene (2M1B)
2-methyl-2-butene (2M2B) Tert-amyl alcohol (TAA)
Tert-amyl alcohol (TAA)
Figure 2. 3 Generation of tertiary alcohols by hydration of olephins
Formation of symmetrical ethersdue to intermolecular water elimination between two
molecules of alcohol:
C2H
5OH C
2H
5OH C2H5 O C2H5 OH2+ +
Olephins dimerizationmay be practically avoided by using a high excess of alcohol and
maintaining a strict control of reaction temperature.
C5H10 C5H10 C10H20+ Several position isomers of C10H20olefin are obtained, such as 3,3,5-trimethyl heptene
and 2,3,4,4-tetramethyl hexene are obtained. Their formation implies the addition of the tert-
amyl carbocation to the two isoamylenes, followed by the elimination of a proton. These
dimmers are formed in very small quantities (the order of tens of ppm) in conditions of industrial
reactions (temperatures between 60 80 oC, pressures of 7 8 atm). Polymerization reactions of
isoamylenes are possible if high temperatures are used. The formed polymers can block the
active centres of the catalyst, deactivating it(Boonthumtirawuti et al., 1993).Olephins isomerisation: more important in the range of C5 and C6 olephins. For
instance, in the case of isoamylenes reactive isomers, the chemical equilibrium in the
isomerisation reaction is displaced towards the formation of 2-methyl-2-butene, which is more
thermodynamically stable due to the greater level of double bond substitution. The increased
stabi