Pyrolysis of Cyclic Compounds: a Combined Kinetic Modeling and Experimental Study
Sander D'hondt
Supervisor: Prof. dr. ir. Kevin Van Geem Counsellor: Florence Vermeire
Master's dissertation submitted in order to obtain the academic degree of Master of Science in Chemical Engineering
Department Of Materials, Textiles And Chemical Engineering Chair: Prof. dr. Paul Kiekens Faculty of Engineering and Architecture Academic year 2016-2017
The only time success comes before work is in the dictionary.
~ Harvey Specter
Acknowledgements
On the very outset of this master thesis, I would like to extent my sincerest appreciation and gratitude towards all the people who stood by my side during the past year. Without their active guidance, help, cooperation and encouragement, this work would not be what it is now.
First of all, I want to thank my promotor, prof. dr. ir. Kevin Van Geem, and prof. dr. ir. Guy Marin for giving me the chance to work on this very interesting and challenging subject, and for all the fascinating classes we could attend over the past years.
I am also very grateful to my counsellor, ir. Florence Vermeire, without whom I never could have brought this thesis to a successful end. Thank you for all your help with the simulations, all the feedback on the poster, presentations and writings, and for learning me to have lots of patience with Genesys.
A lot of gratitude goes to my office buddies, Victor and Cato, for all the laughs we had, all the misery we shared and all the Word conundrums we solved as a united front. I can of course also not forget our part-time office guests, Gio and Jeffrey, who made the late evenings that much more fun. Special thanks to the ladies of the ‘party cube’, Julie, Blerta and Annelies, for all the coffee, 10 ‘o clock, lunch, 4 ‘o clock, dinner and late night breaks! These made me forget the long hours in the last few weeks.
I am of course also extremely grateful to my awesome MaChT board. I have enjoyed every activity, every meeting, every discussion, every laugh, and I hope you guys did as well! I think we achieved a lot of great things this year and can proudly look back; I know I will!
Last but not least, I want to thank my parents and sister for all the support they gave me during all my years at university, and for always being there.
Sander D’hondt
June 2nd 2017
FACULTY OF ENGINEERING AND ARCHITECTURE
Laboratory for Chemical Technology Director: Prof. Dr. Ir. Guy B. Marin
Laboratory for Chemical Technology
Declaration concerning the accessibility of the master thesis
Undersigned,
Sander D’hondt
Graduated from Ghent University, academic year 2016-2017 and is author of the master thesis with title:
Pyrolysis of Cyclic Compounds: a Combined Kinetic Modeling and Experimental Study
The author(s) gives (give) permission to make this master dissertation available for consultation and to copy parts of this master dissertation for personal use. In the case of any other use, the copyright terms have to be respected, in particular with regard to the obligation to state expressly the source when quoting results from this master dissertation.
June 2nd 2017
Sander D’hondt
Laboratory for Chemical Technology • Technologiepark 914, B-9052 Gent • www.lct.ugent.be Secretariat : T +32 (0)9 33 11 756 • F +32 (0)9 33 11 759 • [email protected]
Pyrolysis of Cyclic Compounds: a Combined Kinetic Modeling and Experimental Study
Sander D’hondt
Master's dissertation submitted in order to obtain the academic degree of Master of Science in Chemical Engineering
Academic year 2016-2017
Promotor: Prof. dr. ir. Kevin M. Van Geem Counsellor: ir. Florence Vermeire
GHENT UNIVERSITY Faculty of Engineering and Architecture
Department Of Materials, Textiles And Chemical Engineering Chairman: Prof. dr. Paul Kiekens
Abstract
The thermal decomposition of mono- and polycyclic species is studied in this work by the automatic generation of a kinetic model with Genesys and validation based on experimental literature data. The chosen reference compounds are cyclohexane, methyl cyclohexane and decalin to represent the monocyclic, substituted monocyclic and polycyclic alkanes respectively. Ab initio rate coefficients for the initial decomposition reactions of methyl cyclohexane are taken from literature while the majority of the other reactions are based on rate rules. Reactions in the decalin pyrolysis mechanism are referred to similar reactions from methyl cyclohexane. The model simulated mole fractions for all compounds agree well with the experimental mole fractions for most product species, including aromatics. Reaction path analyses have been performed to get a better insight in the decomposition chemistry. Large differences exist between these components with respect to the initial decomposition pathways and formation of aromatics.
Keywords: Pyrolysis, cyclohexane, methyl cyclohexane, decalin, automatic kinetic model generation, rate of production analysis. Pyrolysis of Cyclic Compounds: a Combined Kinetic Modeling and Experimental Study
Sander D’hondt
Promotor: prof. dr. ir. K. M. Van Geem Counselor: ir. Florence Vermeire
Abstract: The thermal decomposition of mono- and polycyclic constructed manually. These reactions include ring-opening species is studied in this work by the automatic generation of a isomerizations and hydrogen abstractions, followed by C-C β- kinetic model with Genesys and validation based on experimental scissions that open the ring. Rate coefficients for these literature data. The chosen reference compounds are methyl reactions are taken from ab initio calculations on the CBS-QB3 cyclohexane and decalin to represent substituted cyclohexanes level of theory performed by Wang et al.2 The ring-opening and polycyclic species respectively. The model simulated mole isomerization reactions were computed by Zhang et al.3 with fractions for both compounds agree well with the experimental mole fractions for most product species, including aromatics. high-level quantum chemical calculations and RRKM master Reaction path analyses have been performed to get a better insight equation simulations. Next, the generation of a pyrolysis in the decomposition chemistry. Large differences exist between mechanism for the resulting C7 products is done with the use these two components with respect to the initial decomposition of Genesys. The reaction families included in Genesys are (i) pathways and formation of aromatics. intra- and intermolecular hydrogen abstraction reactions (ii) β-
Keywords: Pyrolysis, cyclohexane, methyl cyclohexane, decalin, scissions and the reverse intra- and intermolecular addition automatic kinetic model generation, rate of production analysis. reactions (iii) bond scissions and the reverse recombination reactions (iv) Diels-Alder cyclization. Rate coefficients for the I. INTRODUCTION majority of the reactions have been calculated by the group 4,5 At present, polyethylene based plastics are used in additive framework developed by Sabbe et al. Rate rules used for intramolecular hydrogen abstraction and addition reactions packaging, construction, transportation and much more. Not 6 7 only the numerous applications and advantages compared to are taken from Van de Vijver et al. and Wang et al. respectively. AramcoMech2.08 is used as a base mechanism alternatives like glass, but also the world’s rising prosperity, 9 spur the demand for plastics and other chemicals. Moreover, and the model of Sharma et al. is used for pathways towards with government policies and incentives promoting the use of aromatic species. non-fossil bases resources, the need for alternatives arises. The decalin kinetic model is fully constructed with Genesys, One alternative can be found in the use of cellulose, present using the same reaction families, extended with ring-opening in lignocellulosic biomass. Through catalytic hydro- isomerizations, and the same rate rules as in the mechanism for deoxygenation (HDO) cellulose is converted into a bio-oil methyl cyclohexane. Arrhenius parameters for the ring-opening containing straight-chain alkanes (mainly n-hexane). The so- isomerization and for initial hydrogen abstraction reactions produced light naphtha fraction is an ideal green feedstock for from decalin are based on similarities with methyl cyclohexane existing processes like steam cracking, but contains higher pyrolysis reactions. Due to the high complexity and the high amounts of oxygenates and cyclic components (cyclohexane, number of carbon atoms, the size of the automatic generated cyclopentane) than similar, fossil-derived feedstocks. To date, network increases fast. For this reason, severe constraints are these cyclic compounds, and in particular polycyclic and used in the generation of the decalin model and stereochemistry substituted monocyclic alkanes, have been disregarded to a is not considered. The final kinetic models for the pyrolysis of large extent for kinetic studies compared to paraffinic species. methyl cyclohexane an decalin contain 4,113 species and 8,333 In this study, the automatic kinetic model generation tool reactions and 3,370 species and 19,493 reactions respectively. Genesys is used to construct pyrolysis mechanisms for decalin and methyl cyclohexane, as respective reference components III. METHYL CYCLOHEXANE: RESULTS AND DISCUSSION for the polycyclic and substituted monocyclic alkanes. A. Literature reported experimental data Validation of these mechanisms is done with literature reported experimental data. The developed kinetic model for methyl cyclohexane pyrolysis is validated against literature-reported experimental data obtained by Wang et al.2 in a plug flow reactor at 1.0 bar II. KINETIC MODEL DEVELOPMENT and a temperature range between 900 K – 1250 K. The total Kinetic models for methyl cyclohexane and decalin are inlet flow, composed of argon and 2 mole % methyl constructed automatically with Genesys, which makes use of cyclohexane, was kept constant at 6.82 x 10-4 mole/s and fed to user-defined databases for the determination of a plug flow reactor with 7.0 mm inner diameter and a length of thermodynamics and kinetics of all compounds and reactions 22.7 mm. The performance of the kinetic model is compared to in the mechanism. For more information on the use of Genesys, that of models developed by Orme et al.10 and Wang et al.2 the reader is referred to specialized literature.1 The model for methyl cyclohexane pyrolysis has been built B. Model performance in a hierarchical way. First, a sub mechanism for the initial The experimental and simulated mole fractions of methyl reactions of methyl cyclohexane towards C7 species is cyclohexane as a function of temperature and of major product species as a function of conversion are compared in Figure 1. sequences consist out of consecutive C-C β-scissions or The simulated methyl cyclohexane mole fraction in the LCT intramolecular H-shift to yield more stable radicals. Ethylene is model is in better agreement with the experimental values than mainly produced after direct C-C β-scission reactions. Re- the two other models. Although ethylene and 1,3-butadiene are evaluation of the competition between β-scissions and H-shifts over and under predicted respectively, the mole fractions of the may improve the ethylene mole fraction predictions. Also a rate other main product species, including aromatic products, are in of production analysis on the major aromatic species, benzene, good agreement with the experimental data. The main aromatic is done with CHEMKIN PRO. Only a very small amount of the species is benzene, and not toluene as might be expected form methyl cyclohexyl radicals undergo successive H-abstraction the structure of methyl cyclohexane. and C-H β–scission resulting in the formation of benzene and
2.5 3.0 toluene. The main aromatic pathways include Diels-Alder Methyl cyclohexane C2H4 2.0 cyclization, propargyl recombination and H-assisted fulvene 2.0 isomerization, similar to cyclohexane pyrolysis. 1.5
1.0 IV. DECALIN: RESULTS AND DISCUSSION 1.0 Yield [mole %] 0.5 Yield [mole %] A. Literature reported experimental data 0.0 0.0 11 900 1000 1100 1200 1300 0.0 0.2 0.4 0.6 0.8 1.0 Zeng et al. have studied the pyrolysis of decalin in a plug Te mpe r at ur e [K] Conversion [-] 0.4 0.3 flow reactor at constant pressure (1 bar) in the temperature C3H6 Benzene range 920 K – 1230 K. The conversion of decalin in this 0.3 temperature interval is not complete and reaches a maximum 0.2 value of 72%. During these experiments, the gas mixture of 0.2 argon and decalin (1 mole%) with a total flow rate of 4.84 x 10- 0.1 Yield [mole %] 0.1 Yield [mole %] 4 mole/s was fed into plug flow reactor with 7.0 mm inner diameter and a length of 22.7 mm. In this part, the performance 0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 of the developed model for decalin pyrolysis is ascertained and 11 Conversion [-] Conversion [-] compared to that of a recently developed model by Zeng et al. Figure 1. Experimental (symbols) and simulated (lines) mole fraction of methyl cyclohexane as function of temperature and of ethylene, propylene B. Model performance and benzene as function of conversion. LCT (—), Wang et al.2 (····), 10 -4 Figure 3 Orme et al. (− − −). P=1.0 bar, nmethyl cyclohexane = 6.82 x 10 mole/s. The shows the experimental (symbols) and simulated error bars represent the confidence intervals as determined by Wang et al.2 (solid lines) mole fraction profile of decalin as a function of the reactor temperature and of the main product species as a C. Rate of production analysis function of conversion. The LCT model overestimates the onset As shown in Figure 2, three major consumption pathways exist temperature of decalin consumption by about 50 K. Also, after for methyl cyclohexane at these conditions: C-C scission, H- this point, the slope in the LCT-simulated mole fraction profile abstraction and ring-opening isomerization. The percentages is steeper than that of the experimental one, meaning that represent the fraction of methyl cyclohexane that is consumed decalin is consumed too fast in the LCT model. Although the by the respective paths, both near the entrance and center of the model developed by Zeng et al.11 yields a better prediction of reactor at 1050K. the onset temperature, it is also not able to accurately capture
23 1050 K, 20 cm the slope of the experimental data. 22 21 1050 K, 5 cm The most abundant species, hydrogen and ethylene, are 20
19 5 predicted accordingly, but propylene and 1,3-butadiene are 18 4 17 considerably underestimated. The mole fractions of the main 3 16 15 2 aromatic species, benzene, toluene and styrene, are simulated 14 1 13 adequately. 12 0 10 20 30 40 11 Rate of consumption [%] 1.2 1.5 10 C2H4 9 Decalin
8 1.0
7
6 0.8 1.0
5
4 0.6
3 2 0.4 0.5 Yield [mole %] 1 Yield [mole %] 0.2 0 20 40 60 80 100 Rate of consumption [%] 0.0 0.0 900 1000 1100 1200 1300 0.0 0.2 0.4 0.6 0.8 1.0 Figure 2. Rate of production analysis of the initial decomposition pathways Te mpe r at ur e [K] Conversion [-] of methyl cyclohexane at different temperatures and positions in the 0.3 0.4 reactor. Simulations are done with PFR in CHEMKIN PRO. 1050 K, 5 cm C3H6 Benzene (full black); 1050 K, 20 cm (dotted). P=1.0 bar. The percentage shows the 0.3 0.2 fraction of cyclohexane consumed by the corresponding reactions. 0.2
There is a clear preference for H-abstraction reactions at 0.1 Yield [mole %] longer distances in the reactor. This is to be expected because Yield [mole %] 0.1 of the increased radical concentration further along the axial 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 reactor coordinate. The main abstracting radicals are hydrogen Conversion [-] Conversion [-] atoms and methyl radicals. Note that the importance of the ring- Figure 3. Experimental (symbols) and simulated (lines) mole fraction of opening isomerization is very much reduced compared to decalin as function of temperature and of ethylene, propylene and benzene 11 as function of conversion. LCT (—), Zeng et al. (− − −). Simulations are cyclohexane pyrolysis. -6 A reaction path analysis of the methyl cyclohexyl radicals done with PFR in CHEMKIN PRO. P=1.0 bar, ndecalin=4.84 x 10 mole/s. The error bars represent 95% confidence intervals. towards smaller olefinic species reveals that the main reaction C. Rate of production analysis cyclohexane and methyl cyclohexane pyrolysis. Toluene and It is expected that dissimilar reactions govern the styrene as well are primarily formed through direct reactions consumption of decalin at different conditions. Therefore, a from the decalyl radicals and to a lesser extent by addition of rate of production analysis of the consumption of decalin at methyl respectively vinyl radicals to benzene. 1076 K, both near the entrance and in the center of the reactor, is executed. V. CONCLUSION The result of the ROP is presented in Figure 4 and shows that Microkinetic models for the pyrolysis of methyl cyclohexane decalin is consumed through two major channels: ring-opening and decalin have been developed automatically with the use of isomerization and H-abstraction by radical attack to yield three Genesys. Ab initio rate coefficients for the initial different decalyl radicals. The main abstracting species are decomposition reactions of methyl cyclohexane are taken from hydrogen atoms and methyl radicals. literature while the majority of the other reactions are based on rate rules. Reactions in the decalin pyrolysis mechanism are 14 1076 K, 20 cm referred to similar reactions from methyl cyclohexane. 13 1076 K, 5 cm Simulations with the kinetic model are validated against 12
5 11 experimental data reported in literature. Overall, a satisfactory
4 10 performance is observed for the two molecules. Differences in 3 9 reaction pathways between methyl cyclohexane and decalin 2 8 1 exist in both the initial decomposition pathways as well as in 7 0 10 20 30 40 the formation of aromatics. Contrary to the importance of the 6 Rate of consumption [%] 5 ring-opening isomerization that initiates the pyrolysis of
4 cyclohexane at the entrance of the reactor, methyl cyclohexane 3 pyrolysis is mainly initiated by the C-C bond scission of the 2 methyl group, while during decalin pyrolysis the ring-opening 1 isomerization reactions have an increased importance in the 0 20 40 60 80 100 Rate of consumption [%] middle of the reactor. During the pyrolysis of both compounds, Figure 4. Rate of production analysis of the initial decomposition pathways considerable amounts of aromatics are formed. In decalin of decalin at different temperatures and positions in the reactor. pyrolysis, these stem directly from the decalyl radicals, while Simulations are done with PFR in CHEMKIN PRO. 1076 K, 5 cm (full black); 1076 K, 20 cm (dotted). P=1.0 bar. The percentages show the in methyl cyclohexane pyrolysis, they are dominantly formed fraction of decalin consumed by the corresponding reactions. through reactions of smaller molecules.
Compared to methyl cyclohexane, the heavily reduced REFERENCES importance of hydrogen abstractions by methyl radicals during 1. Vandewiele N., et al., Genesys: Kinetic model construction using decalin pyrolysis stands out. This highlights the importance of chemo-informatics. Chemical engineering journal. 2012. the C-C scission during methyl cyclohexane pyrolysis, which 2. Wang Z., et al. Experimental and kinetic modeling study on acts as an additional methyl radical source. Note the still methylcyclohexane pyrolysis. Combustion and Flame. 2014. significant importance of the ring-opening isomerizations in the 3. Zhang F., et al., Kinetics of Decomposition and Isomerization of Methylcyclohexane: Starting Point for Studying Monoalkylated center of the reactor. This is opposite to the observation during Cyclohexanes Combustion. Energy & Fuels. 2013. cyclohexane and methyl cyclohexane pyrolysis. 4. Sabbe M., et al., Modeling the influence of resonance stabilization on Figure 5 displays a reaction path analysis of the consumption the kinetics of hydrogen abstractions. Physical Chemistry Chemical pathways of one of the three decalyl radicals. It is evident that Physics. 2010. 5. Sabbe M., et al., Carbon-Centered Radical Addition and β-Scission ethylene is produced through multiple pathways each Reactions: Modeling of Activation Energies and Pre-exponential consisting of a sequence of C-C β-scissions. Note the direct Factors. ChemPhysChem. 2008. path, i.e. without the involvement of any small species, towards 6. Vijver R., Automatic Ab Initio Calculations for Kinetic Model benzene. Generation of Gas-Phase Processes. 2017. 7. Wang K., et al., Reactivity–structure-based rate estimation rules for alkyl radical H atom shift and alkenyl radical cycloaddition reactions. The Journal of Physical Chemistry. 2015. 8. Metcalfe W., et al., A Hierarchical and Comparative Kinetic Modeling Study of C1−C2 Hydrocarbon and Oxygenated Fuels. International Journal of Chemical Kinetics. 2013. 9. Sharma S., et al., Modeling of 1, 3-hexadiene, 2, 4-hexadiene and 1, 4- hexadiene-doped methane flames: Flame modeling, benzene and styrene formation. Combustion and Flame. 2010. 10. Orme JP., et al., Experimental and modeling study of methyl cyclohexane pyrolysis and oxidation. J. Phys. Chem. A. Jan 2006. Figure 5 Rate of production analysis of the consumption pathways of 11. Zeng M., et al., Experimental and kinetic modeling investigation on decalin pyrolysis at low to atmospheric pressures. Combustion and one of the decalyl radicals. Simulations are done with the PFR code in Flame. 2016. CHEMKIN-PRO. T=1180 K, P=1.0 bar, center of the reactor. The percentages reflect the molar flux of the corresponding pathway divided by the total molar flux of decalin consumption.
A rate of production analysis indicates that almost 83% of the total benzene is formed by H2-elimination from 1,3- cyclohexadiene, which is a species formed almost exclusively through subsequent C-C β-scissions of the three decalyl radicals. Reactions starting from smaller species contribute to a smaller degree contrary to what was observed during i
Table of Contents
LIST OF SYMBOLS ...... V
LIST OF SPECIES ...... IX
CHAPTER 1. MEETING GROWING DEMAND ...... 1
1.1 INTRODUCTION ...... 1
1.2 FULFILLING FUTURE SUPPLY ...... 4
1.3 A GREENER ROUTE ...... 5 1.3.1 Biomass feedstock ...... 5 1.3.2 Lignocellulose conversion routes ...... 6 1.3.3 Lignin-first biorefinery ...... 7
1.4 SCOPE ...... 10
1.5 REFERENCES ...... 12
CHAPTER 2. KINETIC MODEL GENERATION AND EXPERIMENTAL SET-UP ...... 17
2.1 INTRODUCTION ...... 17
2.2 GENESYS: A TOOL FOR AUTOMATIC KINETIC MODEL GENERATION ...... 19 2.2.1 Reaction families ...... 19 2.2.2 Network generation ...... 23 2.2.3 Assignment of kinetic and thermodynamic parameters ...... 23
2.3 CHEMKIN ...... 26
2.4 EXPERIMENTAL SET-UP ...... 27 2.4.1 Feed section ...... 27 2.4.2 Reactor section ...... 28 2.4.3 Analysis section ...... 28
2.5 LITERATURE REPORTED REACTOR CONFIGURATIONS ...... 29 ii
2.6 REFERENCES ...... 30
CHAPTER 3. CYCLOHEXANE AS REFERENCE COMPONENT FOR NAPHTHENES ...... 33
3.1 INTRODUCTION ...... 33
3.2 LITERATURE SURVEY ON THE PYROLYSIS OF CYCLOHEXANE ...... 35 3.2.1 Literature reported experimental and kinetic modeling studies ...... 35 3.2.2 Decomposition reactions of cyclohexane ...... 37
3.3 KINETIC MODELING OF CYCLOHEXANE PYROLYSIS ...... 42 3.3.1 Experimental method ...... 42 3.3.2 Kinetic model development ...... 43 3.3.3 LCT experiments: results and discussion ...... 43 3.3.4 Wang experiments: results and discussion ...... 50
3.4 LITERATURE SURVEY ON SUBSTITUTED CYCLOALKANES ...... 57 3.4.1 Methyl cyclohexane ...... 57 3.4.2 Ethyl cyclohexane ...... 62 3.4.3 Dimethyl cyclohexane ...... 66
3.5 KINETIC MODELING OF METHYL CYCLOHEXANE PYROLYSIS ...... 69 3.5.1 Experimental method ...... 69 3.5.2 Kinetic model development ...... 70 3.5.3 Wang experiments: results and discussion ...... 72
3.6 CONCLUSION ...... 82
3.7 REFERENCES ...... 84
CHAPTER 4. DECALIN AS REFERENCE COMPONENT FOR POLYCYCLIC ALKANES ...... 93
4.1 INTRODUCTION ...... 93
4.2 LITERATURE SURVEY ON DECALIN PYROLYSIS ...... 95 4.2.1 Reported kinetic modeling and experimental studies ...... 95 4.2.2 Decomposition reactions of decalin ...... 96
4.3 KINETIC MODELING OF DECALIN PYROLYSIS ...... 99 4.3.1 Experimental method ...... 99 4.3.2 Kinetic model development ...... 99 4.3.3 Results and discussion ...... 103
4.4 CONCLUSION ...... 114
4.5 REFERENCES ...... 116 iii
CHAPTER 5. CONCLUSION AND FUTURE WORK ...... 119
5.1 CONCLUSIONS ...... 119
5.2 FUTURE WORK ...... 122
APPENDIX A. EXPERIMENTAL CYCLOHEXANE PYROLYSIS DATA ...... 125
APPENDIX B. USE OF RATE RULES FOR LINEAR SPECIES IN THE DECALIN MECHANISM . 131
B.1 GAV-BASED RATE COEFFICIENTS IN THE DECALIN MECHANISM ...... 131
B.2 REFERENCES ...... 133
iv
v
List of Symbols
Roman symbols
A Pre-exponential factor s"#or cm&mol"#s"#
) Single-event pre-exponential factor s"#or cm&mol"#s"#
"# Ea Activation energy J mol
E Activation energy in modified Arrhenius notation J mol"#
*+ Number of single events -
P Pressure Bar
T Temperature K
q Molecular partition function -
R Universal gas constant 8.314 J mol"#,"#
"# -./0123 Ring-strain energy J mol
Greek symbols
4 5 Eckhart tunneling coefficient -
Φ Equivalence ratio -
Δ8‡ Gibbs free energy of activation J mol"#
Δ:‡ Enthalpy of activation J mol"#
vi
Acronyms
GDP Gross Domestic Product
OECD Organization for Economic Co-operation and Development
BTU British Thermal Units
HDO Hydrodeoxygenation
ARBOREF Aromatic Bio-Refinery
GC Gas chromatograph
BSSC Bench Scale Steam Cracker
LCT Laboratory for Chemical Technology
CSTR Continuously stirred tank reactor
InChI IUPAC International Chemical Identifier
IUPAC International Union of Pure and Applied Chemistry
SMILES Simplified molecular-input line-entry system
GAV Group additive value
LOA Light oxygenates analyzer
ROA Refinery gas analyzer
GCxGC Two-dimensional gas chromatograph
FID Flame ionization detector
TOF-MS Time of flight-mass spectrometer
CIP Coil inlet pressure
COP Coil outlet pressure
PAH Polyaromatic hydrocarbons
MAH Monoaromatic hydrocarbons
TCD Thermal conductivity detector
BR Batch reactor
PFR Plug flow reactor vii
RCM Rapid cooling machine
ST Shock tube
JSR Jet stirred reactor
CSM Colorado School of Mines
POLIMI Politecnico di Milano
RRKM Rice–Ramsperger–Kassel–Marcus theory
ROP Rate of production
VUV Vacuum ultraviolet
SVUV-PIMS Synchrotron vacuum ultraviolet photoionization mass spectrometry
viii
ix
List of Species
Sub mechanism for aromatics in cyclohexane pyrolysis model
Formula Nomenclature Structure Formula Nomenclature Structure
CH& CH3 C=H= 1,3-CPD-R
CH> CH4 C=H? 1,3-CPD
C@H@ C2H2 C?H= C6H5
C@H> C2H4 C?H? FULVENE
C@H= C2H5 C?H? BENZENE
C&H& C3H3 CAHA C7H7
C&H> C3H4 CAHB TOLUENE
C&H= C3H5 CBHB STYRENE
C3H6 C&H? C H C4H5-1 > = C H C4H5-2 > =
x
Initial unimolecular and bimolecular reactions of methyl cyclohexane
Formula Nomenclature Structure Formula Nomenclature Structure
CAH#> MCH CAH#& MCHR4
CAH#& MCHR1 CAH#& CYCHEXCH2
CAH#& MCHR2 C?H## CYC6H11
CAH#& MCHR3 C?H#C CYC6H10
CAH#> 2-CH3-C6H11 CAH#& 2-CH3-C6H10
CAH#> 3-CH3-C6H11 CAH#& 3-CH3-C6H10
CAH#> 4-CH3-C6H11 CAH#& 4-CH3-C6H10
CAH#> 5-CH3-C6H11 CAH#& 4-CH3-C6H10
C H 2-C7H13-7 A #&
CAH#> C7H14-1
CAH#> C7H14-2
C H 1-C7H13-7 A #&
C H 1-C7H13-6 A #&
C H 2-C7H13-7 A #&
xi
xii
1. Chapter 1. Meeting Growing Demand Chapter 1 Meeting Growing Demand
1.1 Introduction
In the year 1933, two chemists working for the Imperial Chemical Industries (I.C.I.) were looking for new reactions under extreme pressure. Fifty different reactions were tried, all without the success they had hoped for. However, to R. Gibson and E. Fawcett’s surprise, one of the failed reactions between ethylene and benzaldehyde had produced a white, waxy substance.1 Both men probably could not have imagined the ways in which this waxy substance, nowadays known as polyethylene, would be used today and how much it would change the world.
At present, polyethylene based plastics are used in packaging, building and construction, transportation, medical supplies and much more. The applications are so numerous and plastics are often far more advantageous than alternatives like glass, such that the production of chemicals is the fastest-growing source of industrial energy usage. The demand for plastics and other chemical products remains strong. Also, more than half the energy that goes into the chemical sector is used not as fuel, but as feedstock, and thus is not impacted by the gains in efficiency that 2 Chapter 1. Meeting Growing Demand are curbing demand elsewhere. Given the scale of the current global chemical industry, it is not surprising that the industrial sector accounts for the largest direct use of energy and it’s predicted rise in the years to come.2 The fundamental forces that will continue to shape the world’s energy demand include population growth and economic expansion. By 2040, it is expected that the world’s population will have reached 9 billion and the GDP (Gross Domestic Product) will have more than doubled (Figure 1-1a).2
(a) (b) Figure 1-1 (a) Expected trend in global population and (b) global energy demand in British Thermal Units per region.2
Due to this growth, the global energy demand is predicted to rise by 25 percent from today to 2040.3 This is however not a global trend and does not hold true for each region or country separately, as can be seen in Figure 1-1b. In 2015, developed economies, i.e. members of the Organization for Economic Co-operation and Development (OECD), consumed about 40 percent of the world’s energy, despite having less than 20 percent of its population. The demand in these countries is expected to fall by 5 percent by 2040. More than half of the projected increase will occur among the nations of non-OECD Asia, which include China and India, and about 30 percent will be attributed to Key Growth countries.2 This is a group of countries whose rising population and living standards will drive strong increase in energy demand.
As mentioned before, the large increase in energy demand is also partly explained by a rising GDP. The GDP per capita is an indication of people’s income and which living standards this income can support. By 2040, the per capita income in China and India is expected to be more than three times today’s level; whilst that in Key Growth countries will almost double (Figure 1-1a).2 The Brookings Institution estimates that the number of people in the middle class will reach nearly 5 billion in 2030, up from 2 billion in 2014.4 Crossing the threshold of the middle class means people are Chapter 1. Meeting Growing Demand 3 earning enough to have spending power beyond daily necessities. They can pay for better education, health care, cars and larger houses. A consequence of all these improvements in life style is of course an increased energy use.2,5
The number of cars on the world’s roads is estimated to rise with about 80 percent by 2040 and with it of course, fuel consumption.2,3 Growth in transport demand is however only expected from outside OECD, because of the availability of advanced technologies for fuel efficiency within the OECD, as can be seen in Figure 1-2a.
(a) (b)
Figure 1-2 (a) Transportation demand by region in million barrels per oil equivalent; (b) Electricity generation per fuel type by region.2
The need for electricity is rising in all parts of the world, but - like fuel - mainly in the non-OECD countries due to their growing number of people in the middle class. Electricity must be generated through some other energy source, which makes it a secondary form of energy.6 Figure 1-2b shows that several fuels can be used for generation of electricity: coal, natural gas, nuclear, hydro, wind and solar.2 Due to the availability per region, large variations exist. The United States get about 33 percent of its electricity from coal and 33 percent form natural gas7, whilst China produces about 70 percent8 of its total electricity out of coal. Coal is however expected to make way for natural gas and other fuel types in the near future as is evident from Figure 1-2b.2
4 Chapter 1. Meeting Growing Demand
1.2 Fulfilling future supply
Thanks to advances in energy technology, the world’s energy choices have never been as plentiful or diverse as today. As shown on Figure 1-3a, it is clear that oil will remain the world’s top energy source in the foreseeable future, as it is needed for transportation and production of chemicals,
2 but that natural gas will overtake coal, driven by the need for cleaner fuels.
(a) (b)
Figure 1-3 (a) Global fuel consumption by resource in British Thermal Units. (b) Global demand for chemicals per region in British Thermal Units.2
As nuclear plants provide electricity in a reliable way, without emitting any CO2, and despite accidents in the recent past, many nations will seek to expand their nuclear capacity in the coming decades (Figure 1-3a). It is expected that the demand for nuclear energy will more than double by 2040.2,3,9 Among major oil products, diesel and jet fuel are predicted to grow rapidly whilst consumption of gasoline remains stable. Natural gas is also expected to emerge as a transportation fuel2.
Besides the need for fuel, rising prosperity also spurs the demand for plastics and other chemicals. While this growth is mainly driven by developing economies, supply growth is mostly coming from regions with advantaged feedstocks, like natural gas in North America.10 Figure 1-3b shows the expected growth in demand for chemicals in the coming decades. It is clear that alternatives for petroleum as a feedstock for the production of chemicals will grow in importance in the years to come.
Modern renewables like solar, wind and hydropower for energy generation and biomass for both energy generation and chemicals production will hence know a steep growth. With government policies and incentives promoting the use of non-fossil energy sources, renewable feedstocks will be one of the world’s fastest growing sources of energy at a rate of 2.6 percent per year.3 Chapter 1. Meeting Growing Demand 5
1.3 A greener route
1.3.1 Biomass feedstock Biomass refers to all organic material that originates from plants and can be divided into three categories: (i) lignocellulosic biomass derived from plant and wood residues, energy crops, etc. (ii) starch- and sugar-based biomass derived from corn, grain, etc. (iii) triglyceride-based biomass derived from animal fats and vegetable oils.11 Recent research focuses on the production of olefins from biomass for the use in both fuels and chemicals production.
The use of lignocellulosic biomass as a renewable feedstock for fuels and chemicals has known a tremendous increase in interest in the past few years.12-14 Its exploitation requires less land, is less energy intensive and doesn’t interfere with food demand; hence it is termed a second generation biomass. Moreover, the high oxygen-to-carbon ratio of cellulosic biomass creates ample opportunities for the production of chemicals and polymer building blocks.15,16 Lignocellulosic biomass is mainly composed out of three bio-polymers: cellulose, hemicellulose and lignin. Its typical structure is represented schematically in Figure 1-4.
Lignocellulose: 2G biomass
Hemicellulose Lignin
Cellulose
Figure 1-4 The main constituting bio-polymers in lignocellulosic biomass: hemicellulose, cellulose and lignin.
Cellulose accounts on average for some 40 wt% of the lignocellulose and is the most abundant organic polysaccharide on earth.13 Being a homopolymer of glucose, cellulose possesses a crystalline structure, which resists thermal decomposition better than hemicellulose. The thermal degradation occurs at 240-350 °C, resulting mainly in anhydrocellulose and levoglucosan.13 The main user of cellulose, at this moment, is the paper and pulp industry, but a lot of research is being devoted to its conversion into biofuels. In contrast to cellulose, hemicellulose (25 wt%) is random and amorphous with little strength. The hemicellulose differs from cellulose as it is constructed from five different pentose and glucose type units, xylose being the predominant sugar unit. Lignin has a highly irregular and branched polyphenolic structure, consisting of a tridimensional 6 Chapter 1. Meeting Growing Demand polymer of propyl-phenols that is imbedded in and bound to the hemicellulose. In this way, it provides rigidity to the structure.17,18
1.3.2 Lignocellulose conversion routes Currently, a first generation of fuels and chemicals is being produced from sugars, starches and vegetable oils (Figure 1-5). Sugar and starch are extracted from crops through hydrolysis and are subsequently fermented into bio-ethanol.19,20 Vegetable oils are triglycerides extracted from plants and are a good source for liquid transportation fuels due to their high energy density. In literature, the main focus of research is aimed at ester-based biodiesel, which is obtained by transesterification of vegetable oils/fats with methanol. A different processing route to convert vegetable oils into a high quality diesel fuel or diesel blend stock that is fully compatible with petroleum-derived diesel fuel is hence desired.
Catalytic hydrodeoxygenation (HDO) allows conversion of triglyceride-based biomass into hydrocarbon liquids by selectively removing oxygen in the form of CO2 and water. In this process, hydrogenation of unsaturated bonds is followed by fragmentation of the triglycerides into mono- and di-glycerides and free fatty acids. In a next step, these components are converted into linear n-alkanes by decarbonylation, decarboxylation and hydrodeoxygenation.21-25 The Neste NExBTL26,27 and Syntroleum Bio-Synfining28 processes are examples that apply this kind of technology. A complete overview of biomass conversion routes is presented in Figure 1-5.
Sugar and starch Lignocellulosic Triglyceride based based biomass biomass biomass
Pretreatment Fast pyrolysis
Lignin Bio-oil Extraction Cellulose
Hydrolysis Gasification
C5-C6 sugars Syngas Triglycerides & Fatty acids
Methanol synthesis FT synthesis HDO
Fermentation Methanol Paraffinic hydrocarbons
Ethanol MTO MTG Hydrocracking FCC
Dehydration Naphtha
Streamcracking
Olefins
Figure 1-5 Overview of biomass conversion pathways.
Chapter 1. Meeting Growing Demand 7
Besides the two previous routes, various conversion pathways based on cheaper and more abundant lignocellulosic feedstock are being developed. The structure of lignocellulose is totally different from that of present fuels and chemicals. It needs to be depolymerized and deoxygenated to be suitable for these applications. Deoxygenation is particularly important in the case of fuels as the presence of oxygen lowers the heat content of the molecules and usually gives them high polarity, which hinders blending with present fuels.17 For the application in the production of high-value specialty chemicals, often less deoxygenation is needed because the presence of oxygen provides valuable properties to the product. In case of steam cracking, even the smallest amounts of oxygen can already have a negative effect on for example selectivity.29
A variety of processes can be applied to convert lignocellulosic biomass into valuable fuels and chemicals. Thermochemical pathways are employed in the gasification and pyrolysis of biomass, whilst biochemical routes are exploited in hydrolyzing it to liberate the lignin and depolymerize the polysaccharides into sugars.11,30
Pyrolysis is a long standing technology, which allows to form char and oil/gas, depending on the temperature and reaction time. These can vary from 300 °C and hours, to 500 °C and minutes or seconds, on to >700 °C and fractions of a second. Typically, very high heating rates are employed, under an inert atmosphere. The so-produced pyrolysis oil is a very complex and multiphase mixture of low and high molecular weight components, including water, organic oxygenates and polymeric carbohydrate and lignin fragments.31 Pyrolysis oil is therefore unstable, highly acidic, partly water-soluble and has a low energy content (~ 17GJ/t). Hence, further upgrading, i.e. by hydro- or catalytic cracking, is required before it can be used as e.g. a transportation fuel.
Gasification uses very high temperatures (>1000 °C) to convert the biomass into a valuable product, called syngas. At these temperatures, extensive C-C bond breaking occurs, which yields a mixed gas of CO and H2, together with some components, like CO2 and CH4, that need to be removed. The resulting syngas can subsequently be converted to methanol or polymerized into a mixture of liquid hydrocarbons via Fischer-Tropsch synthesis.32
1.3.3 Lignin-first biorefinery While a lot of research has been devoted to the conversion of cellulose and hemicellulose into fuels and chemicals, technologies for lignin valorization are far less developed. The primary reason for this, is the structural complexity and heterogeneity of this aromatic biopolymer. Both in the paper industry and bioethanol production, the lignin fraction is usually employed as a low grade energy source.18,33 8 Chapter 1. Meeting Growing Demand
Publications and patents concerning lignin valorization mostly address the production of biofuels and oxygen-free hydrocarbons.34-36 While these pathways offer attractive opportunities, they receive a lot of competition from the petrochemical industry, which disposes of vast oil fields and cheap shale gas. Furthermore, the scale of biomass processes is not, and will never be, competitive with the megaton production capacity of conventional refineries and steam crackers. Hence, a better option is to focus on the production of a handful of highly valuable chemicals, instead of a broad range of products that can be produced in a much cheaper way in petrochemical plants.
It is an interesting topic for discussion whether forthcoming bio-refineries should merely focus on the strong defunctionalization of the highly functionalized bio-based macromolecules or on the milder and more selective conversion of nature’s resources. Value creation through exploitation of the original chemical structure, and hence preservation of a high atom efficiency, will be in competition with the economic viability of such a process.
Recently, a new concept for an aromatic bio-refinery (ARBOREF) has been proposed by a team from KULeuven, that applies a targeted approach to extract aromatics from renewable resources like wood and grasses.37-39 This process includes catalytic hydrogenolysis in the liquid phase, starting from raw unfractionated lignocellulose. The thermal and solvolytic disassembly of lignin (delignification) is here immediately followed by the reductive stabilization of lignin’s most reactive intermediates, such as olefins and carbonyls, into soluble and stable low-molecular weight phenolic compounds. A process overview is given in Figure 1-6.
Reductive fractionation HDO Bio-oil Carbohydrate Pulp Ru/C 3h Steam Cracking 220 C 50 bar Olefins Lignin Oil Ru/C methanol This work 3h 100 bar - 250 C Extraction w. hexane
Phenolics
Figure 1-6 Schematic overview of the aromatic bio-refinery process.
As can be seen from the flow scheme in Figure 1-6, the bio-refinery separates the lignocellulosic biomass into a lignin fraction and a (hemi)cellulose fraction. Due to its high natural abundance and uniform chemical structure with repeating C6 sugar units, cellulose should be the ideal precursor for selectively making C6 alkanes. This can be achieved through a catalytic Chapter 1. Meeting Growing Demand 9
hydrodeoxygenation (HDO) in a biphasic reactor at elevated temperature and under H2 pressure.40 Under these conditions, cellulose is converted into a HDO bio-oil containing straight- chain alkanes (mainly n-hexane). The so-produced light naphtha fraction is an ideal green feedstock for existing processes like steam cracking, but contains higher amounts of oxygenates and cyclic components (cyclohexane, cyclopentane, methyl cyclopentane) than a similar fossil- derived feedstock. This is evidenced by the GC-analysis of a cellulose based naphtha fraction presented in Figure 1-7. In total, the HDO bio-oil contains 87.46 wt. % paraffins, 9.79 wt. % napthenes and 2.24 wt. % oxygenates.
Figure 1-7 Compositional GC-analysis of HDO bio-oil.
10 Chapter 1. Meeting Growing Demand
1.4 Scope
In the past decades, intensive research has been devoted to the pyrolysis and combustion of normal and branched alkanes, like n-heptane41 and iso-octane42,43, as they are the major components in fuels and steam cracker feedstock. Cyclic alkanes on the other hand have received few attention in comparison; and hence their behavior during steam cracking is less defined and precise. Understanding the chemistry of these components during steam cracking is however essential in the search for possible optimizations. Among the class of the cyclic alkanes, two different categories can be distinguished: monocyclic and polycyclic alkanes. In this work, cyclohexane and methyl cyclohexane are considered as model components for the former. Due to its high symmetry, the decomposition chemistry of cyclohexane during pyrolysis should be relatively straightforward and is hence an ideal starting point. Methyl cyclohexane allows to ascertain the effect of substituents on the six-membered ring. Decalin is taken as reference component for the polycyclic alkanes.
It is the goal of this work to study the reactivity of the previous components during pyrolysis. This is done through a literature survey elaborating on the important reactions that govern the pyrolysis chemistry, as well as on the currently available experimental and kinetic modeling work. Microkinetic models for these reference components are constructed automatically using Genesys and the model performance of the thus generated mechanisms is ascertained through comparison with literature reported experimental data.
In a first chapter, the method of automatically constructing a microkinetic model with Genesys is explained. The experimental set-up on which cyclohexane pyrolysis experiments were performed is also discussed in this chapter.
Chapter 3 discusses the pyrolysis of monocyclic alkanes, and more in particular of the reference components cyclohexane and methyl cyclohexane. For cyclohexane a mechanism was developed in-house (unpublished work). This mechanism is validated against experimental data obtained at the Bench Scale Steam Cracker (BSSC) set-up of the Laboratory for Chemical Technology (LCT) at Ghent University, as well as against literature reported experimental data44 obtained at low pressure (40 mbar) in order to test its applicability under these conditions. Secondly, a model for methyl cyclohexane pyrolysis is constructed with Genesys. The performance of this mechanism is assessed through comparison with experimental data from literature.45
Chapter 1. Meeting Growing Demand 11
In chapter 4, the construction of a microkinetic model for the pyrolysis of decalin is discussed. A literature survey summarizes the experimental and kinetic modeling studies performed to date, and elaborates upon how important decomposition reactions of decalin are considered in these studies. Experimental data46 from literature are then compared with simulations using the developed kinetic model.
Final conclusions and potential future work are presented in the final chapter of this work. 12 Chapter 1. Meeting Growing Demand
1.5 References
1. Gnanou Y, Fontanille M. Organic and Physical Chemistry of Polymers: Wiley; 2008.
2. ExxonMobil. The Outlook for Energy: A View to 2040. 2015.
3. U.S. Energy Information Administration. Internation Energy Outlook 2016. May 2016.
4. Perkins DH. China's Emerging Middle Class: Beyond Economic Transformation – Edited by Cheng Li. The Developing Economies. 2012;50(1):68-70.
5. BP. BP Energy Outlook. 2016.
6. U.S. Energy Information Administration. Secondary sources: Use of Electricity. 2016.
7. U.S. Energy Information Administration. U.S. Electricity Generation by Energy Source. 2015.
8. International Energy Agency. China Electricity Generation by Source and CO₂ Intensity in the New Policies Scenario. 2015.
9. World Nuclear Association. The Nuclear Renaissance. 2015.
10. ExxonMobil. Summary Annual Report. 2014.
11. Huber GW, Iborra S, Corma A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. Chemical Reviews. 2006/09/01 2006;106(9):4044-4098.
12. Liao Y, Liu Q, Wang T, Long J, Ma L, Zhang Q. Zirconium phosphate combined with Ru/C as a highly efficient catalyst for the direct transformation of cellulose to C6 alditols. Green Chemistry. 2014;16(6):3305-3312.
13. Ruppert AM, Weinberg K, Palkovits R. Hydrogenolysis goes bio: from carbohydrates and sugar alcohols to platform chemicals. Angewandte Chemie (International ed. in English). Mar 12 2012;51(11):2564-2601.
14. Van de Vyver S, Geboers J, Jacobs PA, Sels BF. Recent advances in the catalytic conversion of cellulose. ChemCatChem. 2011;3(1):82-94.
15. Op de Beeck B, Dusselier M, Geboers J, et al. Direct catalytic conversion of cellulose to liquid straight-chain alkanes. Energy & Environmental Science. 2015;8(1):230- 240.
16. Zhang Q, Chang J, Wang T, Xu Y. Review of biomass pyrolysis oil properties and upgrading research. Energy Conversion and Management. 1// 2007;48(1):87-92.
17. Lange JP. Lignocellulose conversion: an introduction to chemistry, process and economics. Biofuels, bioproducts and biorefining. 2007;1(1):39-48. Chapter 1. Meeting Growing Demand 13
18. Isikgor FH, Becer CR. Lignocellulosic biomass: a sustainable platform for the production of bio-based chemicals and polymers. Polymer Chemistry. 2015;6(25):4497-4559.
19. Saha P, Baishnab AC, Alam F, Khan MR, Islam A. Production of Bio-fuel (Bio- ethanol) from Biomass (Pteris) by Fermentation Process with Yeast. Procedia Engineering. 2014/01/01 2014;90:504-509.
20. Wayman M, Tallevi A, Winsborrow B. Hydrolysis of biomass by sulphur dioxide. Biomass. 1984/01/01 1984;6(1):183-191.
21. Dijkmans T, Pyl SP, Reyniers M-F, Abhari R, Van Geem KM, Marin GB. Production of bio-ethene and propene: alternatives for bulk chemicals and polymers. Green Chemistry. 2013;15(11):3064-3076.
22. Huber GW, Corma A. Synergies between bio- and oil refineries for the production of fuels from biomass. Angewandte Chemie (International ed. in English). 2007;46(38):7184-7201.
23. Huber GW, O’Connor P, Corma A. Processing biomass in conventional oil refineries: Production of high quality diesel by hydrotreating vegetable oils in heavy vacuum oil mixtures. Applied Catalysis A: General. 10/1/ 2007;329:120-129.
24. Jęczmionek Ł, Porzycka-Semczuk K. Hydrodeoxygenation, decarboxylation and decarbonylation reactions while co-processing vegetable oils over a NiMo hydrotreatment catalyst. Part I: Thermal effects – Theoretical considerations. Fuel. 9/1/ 2014;131:1-5.
25. Toba M, Abe Y, Kuramochi H, Osako M, Mochizuki T, Yoshimura Y. Hydrodeoxygenation of waste vegetable oil over sulfide catalysts. Catalysis Today. 4/30/ 2011;164(1):533-537.
26. Soimakallio S, Antikainen R, Thun R. Assessing the sustainability of liquid biofuels from evolving technologies. A Finnish Approach. Technical Research Centre of Finland (VTT) Research Notes. 2009;2482.
27. Rantanen L, Linnaila R, Aakko P, Harju T. NExBTL-Biodiesel fuel of the second generation: SAE Technical Paper;2005. 0148-7191.
28. Van Geem KM, Abhari R, Pyl S, Reyniers M-F, Marin GB. From Biomass to Ethylene: Steam Cracking of Bio-Synfined Naphtha. Paper presented at: AIChE Ethyl. Prod. Conf. Proc. New York: American Institute of Chemical Engineers2010.
29. Ghanta M, Fahey D, Subramaniam B. Environmental impacts of ethylene production from diverse feedstocks and energy sources. Applied Petrochemical Research. 2014;4(2):167-179.
30. Klass DL. Biomass for renewable energy, fuels, and chemicals: Academic press; 1998.
31. Mohan D, Pittman CU, Steele PH. Pyrolysis of wood/biomass for bio-oil: a critical review. Energy & fuels. 2006;20(3):848-889. 14 Chapter 1. Meeting Growing Demand
32. Moulijn JA, Makkee M, Van Diepen AE. Chemical process technology: John Wiley & Sons; 2013.
33. Karimi K, Taherzadeh MJ. A critical review of analytical methods in pretreatment of lignocelluloses: Composition, imaging, and crystallinity. Bioresource Technology. 1// 2016;200:1008-1018.
34. Bhaskar T, Bhavya B, Singh R, Naik DV, Kumar A, Goyal HB. Chapter 3 - Thermochemical Conversion of Biomass to Biofuels A2 - Pandey, Ashok. In: Larroche C, Ricke SC, Dussap C-G, Gnansounou E, eds. Biofuels. Amsterdam: Academic Press; 2011:51-77.
35. Tran LS, Sirjean B, Glaude P-A, Fournet R, Battin-Leclerc F. Progress in detailed kinetic modeling of the combustion of oxygenated components of biofuels. Energy. 7// 2012;43(1):4-18.
36. Román-Leshkov Y, Barrett CJ, Liu ZY, Dumesic JA. Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates. Nature. 2007;447(7147):982- 985.
37. Renders T, Schutyser W, Van den Bosch S, Koelewijn S, Sels B. Catalytic Reductive Fractionation of Lignocellulose-A Promising Bio-refinery Strategy. 2016.
38. Van den Bosch S, Schutyser W, Koelewijn S-F, Renders T, Courtin C, Sels B. Tuning the lignin oil OH-content with Ru and Pd catalysts during lignin hydrogenolysis on birch wood. Chemical Communications. 2015;51(67):13158-13161.
39. Van den Bosch S, Schutyser W, Vanholme R, et al. Reductive lignocellulose fractionation into soluble lignin-derived phenolic monomers and dimers and processable carbohydrate pulps. Energy & Environmental Science. 2015;8(6):1748-1763.
40. de Beeck BO, Dusselier M, Geboers J, et al. Direct catalytic conversion of cellulose to liquid straight-chain alkanes. Energy & Environmental Science. 2015;8(1):230- 240.
41. Dagaut P, Reuillon M, Cathonnet M. High pressure oxidation of liquid fuels from low to high temperature. 1. n-Heptane and iso-Octane. Combustion Science and Technology. 1993;95(1-6):233-260.
42. Curran HJ, Gaffuri P, Pitz W, Westbrook C. A comprehensive modeling study of iso- octane oxidation. Combustion and flame. 2002;129(3):253-280.
43. Ranzi E, Faravelli T, Gaffuri P, Garavaglia E, Goldaniga A. Primary Pyrolysis and Oxidation Reactions of Linear and Branched Alkanes. Industrial & Engineering Chemistry Research. 1997/08/01 1997;36(8):3336-3344.
44. Wang Z, Cheng Z, Yuan W, et al. An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure. Combustion and Flame. 7// 2012;159(7):2243-2253. Chapter 1. Meeting Growing Demand 15
45. Wang Z, Ye L, Yuan W, et al. Experimental and kinetic modeling study on methylcyclohexane pyrolysis and combustion. Combustion and Flame. 2014;161(1):84-100.
46. Zeng M, Li Y, Yuan W, et al. Experimental and kinetic modeling investigation on decalin pyrolysis at low to atmospheric pressures. Combustion and Flame. 5// 2016;167:228-237.
2. Chapter 2. Kinetic Model Generation and Experimental Set-up
Chapter 2 Kinetic Model Generation and Experimental Set-up
2.1 Introduction
The presence of cyclic components like cyclohexane, cyclopentane, methyl cyclohexane, etc. in renewable steam cracker feedstock has already been proven in previous work.1 Knowledge about the behavior of these components during steam cracking is therefore indispensable. Pyrolysis chemistry allows for example to obtain insights in the stability of the molecule and to identify formation pathways of (poly) aromatic hydrocarbons. An efficient way to gathering this information is by developing a detailed kinetic model, which can predict product yields for a variety of reactor conditions (e.g. pressure, temperature, reactor length). These detailed kinetic models are however characterized by a complexity that makes the manual construction of them sometimes impossible to achieve and certainly tedious and error-prone. Therefore, use will be made of an automatic kinetic model generator named Genesys2 to construct pyrolysis models for certain model components.
18 Chapter 2. Kinetic Model Generation and Experimental Set-up
These models solely consist of elementary reactions, i.e. reactions characterized by a single transition state. The tool allows for kinetic models to be automatically generated fully based on the user’s expertise while separating all chemical information from the network generation code itself. It aggregates the relevant chemistry that molecules can undergo in reaction families which is the direct extension of the idea that sub-molecular patterns and more specifically functional groups inside a chemical species govern the reactivity of a molecule rather than the molecule in its entirety.3
After its construction, a mechanism needs to be validated against experimental data. In this work, experimental data for cyclohexane pyrolysis in a plug flow reactor (PFR) were obtained from literature4, as well as gathered at the Bench Scale Steam Cracking (BSSC) set-up of the Laboratory for Chemical Technology (LCT) of Ghent University (unpublished work). Experimental methyl cyclohexane5 and decalin6 pyrolysis data were taken from literature. For decalin, in-house experiments performed in a continuously stirred tank reactor (CSTR) are simulated as well.
In this chapter, the use of Genesys as an automatic chemical kinetic model generator is discussed in a first part. Next, the plug flow experimental set-up, the BSSC, where the cyclohexane experiments have been performed, and its designated analysis section are elaborated upon. Details on the literature reported experiments are accorded in the next chapter.
Chapter 2. Kinetic Model Generation & Experimental Set-up 19
1.2 Genesys: a tool for automatic kinetic model generation
The Genesys architecture is divided into three sequential parts: input, processing and output (Figure 2-1). An initial set of species is the starting point for the creation of new products and the gradual expansion of the reaction network by the application of the user-defined reaction families. These species can represent the feedstock of the chemical process that is modeled or species that are likely not formed during the network generation but should still be included because they play an important role. Both InChI7 and SMILES8 line identifiers can be used to specify these species in a unique way. They can be imported in most molecule editors for conversion back into two- dimensional drawings or three-dimensional models of the molecules.
Processing CHEMKIN Initial species pool Thermochemistry Network generation Microkinetic reaction Reactor simulations Reaction families mechanism - Mole/mass fraction Thermo and kinetic ….profiles parameter assignment - Rate of production Input Output ….analysis
Figure 2-1 Architecture of Genesys divided into three sequential parts.
A CHEMKIN readable reaction network, which is composed of a set of unique species and unique elementary reactions between those species, is generated in the processing section of Genesys. The microkinetic model follows after assignment of the thermochemical and kinetic parameters to reactions and species. In the next paragraphs, the different steps in the generation of microkinetic model with Genesys are discussed in more detail.
1.2.1 Reaction families
As mentioned above, Genesys requires the definition of elementary reaction families in order to generate a kinetic model. This definition includes: (i) a description of the reactive moiety inside reactant molecules, and of the atoms constituting the reactive moiety, (ii) a user defined recipe- like scheme in .xml format in which keywords specify the type of transformation and symbols represent the atoms of the reactive moiety, and (iii) information on the calculation method of the kinetic parameters of the reaction in this reaction family. In order to indicate which reactive moiety is needed for a particular reaction family without hard coding these rules, a language that expresses atom environment patterns is needed. For this, SMILES identifiers are suitable as well.
20 Chapter 2. Kinetic Model Generation and Experimental Set-up
Several types of reaction families can be distinguished during pyrolysis of cyclic hydrocarbons: intermolecular hydrogen abstractions, intramolecular hydrogen abstractions, C-C !-scission, C-H !-scission, C-C scission, carbon and hydrogen centered radical addition, intramolecular carbon addition to form cyclic species and ring-opening isomerization. All reactions are defined to be equilibrium reactions in order to have thermodynamic consistency of the final kinetic model.
Some of these reactions are defined as reverse reactions. This type of kinetics is created to circumvent the issues with reaction families that are the reverse of each other. For example, the β-scission reaction family is the reverse reaction family of the radical addition reaction family. When both reaction families are added as a reaction family template in Genesys, elementary reactions belonging to both reaction families can both appear in the final kinetic model. Since elementary reactions are defined to be reversible, this leads to unwanted duplicate reactions in the final kinetic model. To avoid creating duplicate reactions, kinetics of a reaction family can be defined as “reverse”. This implies that reactions of this reaction family will not appear in the final kinetic model. Nevertheless, the generated products of these reactions will be used as reactants for further enlargement of the model.
2.2.1.1 Intermolecular hydrogen abstractions
During the pyrolysis of hydrocarbon species, a hydrogen atom can be abstracted by either a carbon-centered radical or by a hydrogen atom. Group additive values (GAV) databases for linear molecules, as constructed by for example Sabbe et al.9, can be used to derive the kinetic parameters of the reactions belonging to these reaction families. Due to transfer of the light hydrogen atom between two other molecules, the contribution of tunneling to the rate coefficient can be significant. Including this tunneling in the ΔGAV°" directly would cause them to be valid in a limited temperature interval only and hence Sabbe et al.9 chose to model the tunneling contributions separately via the Eckart tunneling potential. The same approach is adopted in the definition for the reaction families of inter- and intramolecular H-abstractions.
2.2.1.2 Intramolecular hydrogen abstraction reactions
Intramolecular hydrogen abstractions proceed through a cyclic transition state structure, and their activation energy can, via a Benson-type10 model, be considered to consist of two parts: the activation energy of the corresponding bimolecular reaction plus the ring strain energy
#$ = #&'()* + #,-.$'/ . The former can be determined via the group additive method, whilst the latter solely depends on the ring size. The strain energy for various ring sizes is presented in Table 2-1.
Chapter 2. Kinetic Model Generation & Experimental Set-up 21
Table 2-1 Ring strain energy (298 K).11
-1 Ring size 1234567 [kJ mole ] 3 107.9
4 100.4 5 31.8 6 2.5 7 -1.7
The pre-exponential factors decrease if the size of the ring increases. This decrease is due to the number of rotors lost in the transition states for the alkyl H-shift reactions. The pre-exponential factors of intramolecular hydrogen abstraction reactions can be estimated by the rate rules proposed by Wang et al.11, based on the entropy difference between the reactant and transition state, which correlates with the loss of internal rotors in the cyclic transition state structure. In this work however, rate rules used for intramolecular hydrogen abstraction reactions are taken from Van de Vijver et al.12 who computed these pathways in a similar way.
2.2.1.3 C-H 0-scission/hydrogen addition
Reaction rate coefficients for C-H β-scission and hydrogen addition on C=C double bonds are estimated using the methodology proposed by Sabbe et al.13, who applied the group additivity method for Arrhenius parameters to hydrogen additions to alkenes and alkynes and, to the reverse β-scission reactions. The group additive values were determined from CBS-QB3 ab-initio- calculated rate coefficients. The reference reaction was chosen to be the hydrogen addition reaction to ethylene. In total 33 groups were derived, allowing the calculation of the activation energy and pre-exponential factors for a broad range of hydrogen addition reactions. Tunneling is accounted for by the Eckart correlation. The temperature dependence of the calculated ΔGAV°" is sufficiently low to allow a set of ΔGAV°" at a single temperature to describe the kinetics, even for wide temperature ranges.
2.2.1.4 C-C 0-scission/carbon addition
For the C-C β-scissions, Sabbe et al.14 followed the same approach as for the C-H β-scissions. A consistent set of group additive values ΔGAV°" for 46 groups was derived. As a training set, a database of 51 rate coefficients based on CBS-QB3 calculations with corrections for hindered internal motion was used. The results of the group additive network agreed well with the experimental observed rate coefficients with a mean factor of deviation of three. Again, low temperature dependence was observed.
Intramolecular carbon radical additions to a double bond proceed through a cyclic transition state and result in the formation of cyclic species. The main reactions in this category are the radical 22 Chapter 2. Kinetic Model Generation and Experimental Set-up additions forming 5- and 6-membered rings. Two types can be discerned: exo- and endo- intramolecular addition. The difference between the two is that via the former, substituted rings are formed, as is illustrated in Figure 2-2 for hex-1-en-6-yl radical. GAV’s for these reactions are taken from Wang et al.11
Figure 2-2 Endo- and exo-intramolecular carbon radical addition to a double bond in hex-1-en-6-yl radical.
2.2.1.5 Diels-Alder cyclization
Diels-Alder refers to a cycloaddition reaction between a conjugated diene and an alkene, commonly termed the dienophile, to form a (substituted) cyclohexene ring. As an example, the Diels-Alder reaction of 1,3-butadiene and ethylene to form cyclohexene is depicted in Figure 2-3. Arrhenius parameters for this reaction are taken from the ethane steam cracking model of Sabbe et al.15
Figure 2-3 Diels-Alder cycloaddition of 1,3-butadiene and ethylene to form cyclohexene.
2.2.1.6 Ring-opening
Ring-opening of cyclopentane and cyclohexane has been experimentally studied by Tsang et al.16, who showed that the experimentally obtained global rate parameters were consistent with a diradical mechanism for ring-opening. The proposed reaction scheme for the ring-opening isomerization of cyclohexene towards 1-hexene is presented in Figure 2-4.
Figure 2-4 Diradical mechanism for cyclohexane ring-opening.
Using analogies with reactions of linear alkanes, Benson et al.17 have estimated the equilibrium
18 coefficient 89: = ;)<9/ ;=*),9 via the group additivity method. Sirjean et al. found a transition state for the ring-opening of cyclohexane and determined rate coefficients at the CBS-QB3 level of Chapter 2. Kinetic Model Generation & Experimental Set-up 23 theory. Zhang et al.19 studied the ring-opening of methyl cyclohexane with high-level quantum chemical calculations and Rice−Ramsperger−Kassel−Marcus (RRKM)/master equation simulations.
2.2.2 Network generation
The algorithm that generates and systematically enlarges the reaction network starts with a pool of initial species. Next, a species from this pool of molecules that has not yet been subjected to the set of defined reaction families (‘‘unreacted species’’) is iteratively tested whether it is eligible to undergo a reaction family. Molecular constraints are verified before the substructure algorithm scans the species for a particular reactive moiety. When product species emerge after the execution of the reaction family they are added to the list of unreacted species under the condition that they did not appear in the list of unreacted species or in the list of reacted species.
Genesys adopts termination criteria to prevent endless generation of new reactions. For construction of the models in this work, a rule-based criterion is applied. These rules are user- defined and prevent the reaction family description from generating chemical reactions that are kinetically insignificant or containing chemical species with insignificant concentrations. Two types of constraints are envisioned for each reaction family: molecular and atomic constraints. Molecular constraints concern global molecular features of a candidate species. For example, species that abstract hydrogen atoms are limited to contain a maximum of three carbon atoms, i.e. only methyl, C2 and C3 radicals can abstract hydrogen atoms form other species. Atomic constraints feature restrictions on a particular atom, e.g. the electronic state, the hybridization. For example, in pyrolysis conditions carbon atoms that are part of an aromatic system are relatively unreactive compared to aliphatic carbons. Hence, it can be specified that they do not undergo the reaction families the atom would undergo if the atom possessed an identical electronic configuration. Therefore, the addition of a radical atom to a double bonded carbon, would be restricted to non-aromatic carbon atoms.
An alternative is to use a rate-based criterion, in which the rate and species concentration information is used to select only kinetically significant reactions. This type of criterion is not used in this work because the thermodynamic and kinetic parameters are still too unreliable for cyclic species.
2.2.3 Assignment of kinetic and thermodynamic parameters
Assigning kinetic and thermodynamic parameters to elementary reactions and species is the final step before a kinetic model is obtained that can be used in reactor simulations and produce quantitative results. 24 Chapter 2. Kinetic Model Generation and Experimental Set-up
The temperature dependency of a reaction rate coefficient is expressed with an Arrhenius equation, giving rise to the need for a pre-exponential factor A, and an activation energy Ea. For large kinetic schemes, this proves problematic as experimental data on elementary reactions is often missing and it still is not feasible to perform quantum chemical methods on a large scale in regard of computational time. Application of Benson’s group additivity method for transition states allows for calculation of the Arrhenius parameters of each elementary reaction of a target reaction by adding perturbations to the Arrhenius parameters of a reference reaction.9 These perturbations, or ∆GAV’s, are linked to the difference in structure of the transition state of the considered reaction compared to a reference reaction.
To circumvent the lack of kinetic data for the majority of the reactions in the reaction network, approximations have been developed based on a limited number of high-quality data and applied to a larger set of related reactions. The reaction enthalpy, which is an estimation for the reaction barrier, can for example be assessed using the Bell-Evans-Polanyi principle22 or the Blowers and Masel correlation23.
Saeys et al.24 have developed a methodology in which the group additivity method of Benson is applied for the estimation of kinetic parameters. The rate coefficients in this methodology are expressed as in Equation 2.1.
G Q H (2.1) ; @ = ? @ A9 B P RS
in which ? @ is the Eckhart tunneling coefficient, A9 accounts for degeneracy, B is the pre- exponential factor and #$ is the activation energy. The Arrhenius parameters B and #$ for a given I I reaction can be written as perturbations, ΔEBFGH and ΔEBFJ , to the single event Arrhenius parameters #$,.9L and B.9L. These perturbations are related to the structural differences between the transition state of the considered reaction and the reference reaction. The main advantage of introducing a reference reaction is that the temperature dependence of the Arrhenius parameters is mainly incorporated in the corresponding parameters of the reference reaction, leaving the ∆GAVI almost temperature independent. As an example, the schematic representation of the transition state of a carbon radical !-scission is provided in Figure 2-5.
Chapter 2. Kinetic Model Generation & Experimental Set-up 25
Figure 2-5 Transition state of a carbon radical 0-scission.
The contributions to the Arrhenius parameters can be categorized into primary, secondary and tertiary contributions. The former refers to ligands related to the central atoms involved in the studied reaction (U' , V = 1,2,3), while secondary contributions relate to groups one stage further away from these central atoms ([', \', ]' , V = 1,2,3). Tertiary contributions are a consequence of non-nearest neighbor interactions (^', _', `' , V = 1, … , 9).
Saeys et al.24 have previously shown that the non-nearest neighbor interactions typically only influence the kinetics for reactions with strong sterical hindrance and can often be neglected. Hence, via the group additivity method, following expressions can be obtained for the Arrhenius parameters, truncated after the secondary contributions:
c c f c I I I I #$ @ = #$,.9L @ + ΔEBFGH(U') + ΔEBFGH([') + ΔEBFGH(\') + ΔEBFGH(]') 'de 'de 'de 'de
c c f c I I I I B @ = B.9L @ + ΔEBFJ U' + ΔEBFJ [' + ΔEBFJ (\') + ΔEBFJ (]') 'de 'de 'de 'de
In the current method of determining the thermochemistry of a molecule, it is first verified whether thermodynamic properties of the molecule are present in one of the user-defined databases by InChI comparison of the candidate molecule and the database entries. If this is not the case, estimation techniques are applied to obtain thermochemical properties for that species. Resonance structures are generated for a candidate molecule, and thermochemical properties for every resonance structure is generated. The resonance structure with the lowest standard enthalpy of formation at the designated temperature will be taken as the thermochemistry of the molecule.
26 Chapter 2. Kinetic Model Generation and Experimental Set-up
A Benson group additivity scheme for ideal gas phase thermochemical properties25 is implemented. This predictive method calculates these properties of the molecule by adding contributions of sub-molecular groups present in the molecule and is generally applicable regardless of the involved chemical elements. These contributions are derived by regression of experimentally obtained data or ab initio calculations of a training set of species.
2.3 Chemkin
Kinetic modeling simulations have been performed using the plug flow reactor model of the CHEMKIN PRO software.26 This software allows to incorporate complex chemical kinetics into reactor simulations. Furthermore, with CHEMKIN PRO, it is possible to visualize the chemical reactions networks and to analyze reaction paths. Besides thermodynamic and kinetic parameters of species and reactions respectively, the software requires the specifications of the reactor configuration, which for the experiments in this work is a plug flow reactor. The PFR model requires a temperature and pressure profile, inlet conditions and reactor dimensions.
Chapter 2. Kinetic Model Generation & Experimental Set-up 27
2.4 Experimental set-up
The bench scale pyrolysis set-up is shown schematically in Figure 2-6, and can be divided into three sections: a feed section, reactor section and analysis section. The latter contains three different analyzers: a light oxygenates analyzer (LOA), a refinery gas analyzer (RGA) and a two- dimensional gas chromatograph (GCxGC).
Figure 2-6 Schematic overview of the experimental pyrolysis set-up. Position of temperature and pressure measurements are indicated on the figure. The numbered items are: (1) electronic balance, (2) liquid feed reservoir, (3) peristaltic pump, (4) gaseous diluent / internal standard (N2), (5) coriolis mass flow controller, (6) evaporator / heater, (7) mixer, (8) tubular reactor with electric furnace, (9) heated sampling oven, (10) heated transfer lines, (11) GCxGC-FID/(TOF-MS), (12) light oxygenates analyzer (LOA), (13) pressure control valve, (14) water-cooled heat exchanger, (15) dehydrator, (16) refinery gas analyzer (RGA), (17) condensate drum.
2.4.1 Feed section
The liquid feedstock is pumped to a vaporizer by a peristaltic pump at a predefined mass flow rate, controlled by a Coriolis mass flow controller. The diluent, i.e. N2 (Air Liquide, 99.999%), is heated separately to the same temperature. Both the evaporators/heaters and the mixer are electrically heated units filled with quartz beads, to enable a smooth evaporation of the feed and uniform mixing of feed and diluent. Cyclohexane (>99% purity) was purchased from Sigma- Aldrich. 28 Chapter 2. Kinetic Model Generation and Experimental Set-up
2.4.2 Reactor section
The reactor is a 1.475 m long Incoloy 800HT (30-35 wt% Ni, 19-23 wt% Cr and >39.5 wt% Fe) tube with an internal diameter of 8 mm. The reactor is fixed vertically in an electrically heated furnace. The temperature of the gases at different points along the reactor is measured by eight thermocouples. Two manometers, situated at the inlet and outlet of the reactor, allow to measure the coil inlet pressure (CIP) and coil outlet pressure (COP) respectively. The pressure in the reactor is controlled by a back pressure regulator downstream of the outlet, which is manipulated manually. The pressure drop over the reactor (< 0.002 MPa) was found to be negligible in all experiments performed.
2.4.3 Analysis section
The analysis section of the pyrolysis set-up enables on-line qualification and quantification of the entire product stream. The latter consists of permanent gasses (H2, CO, CO2, etc.), and hydrocarbons ranging from methane to polyaromatic hydrocarbons (PAH). The reactor effluent is sampled on-line, by two high-temperature 6-port 2-way valves, kept at 573 K to prevent condensation of high molecular weight components. The effluent contains significant amounts of light hydrocarbons and is diluted with N2, which reduces component partial pressures, and therefore probabilities of condensation.
Using the valve-based sampling manifold and uniformly heated transfer lines, a gaseous sample of the reactor effluent is injected onto the GCxGC or onto the LOA. The former is equipped with both a Flame Ionization Detector (FID) and Time-of-Flight Mass Spectrometer (TOF-MS), enabling both quantitative and qualitative analyses of the entire product stream, from methane to PAHs.
The LOA, equipped with a Thermal Conductivity Detector (TCD), allows analysis of H2O formaldehyde and other small oxygenates.
Further downstream, a fraction of the reactor effluent is cooled to approximately 425 K using a water cooled heat exchanger, while the remainder of the effluent is sent directly to the vent. Water and condensed products are removed in a liquid separator, while the gaseous components are injected into the RGA using built-in gas sampling valves (355 K). This analysis allows separation and detection of all permanent gasses, such as N2, CO, CO2, and H2, present in the effluent and additional analysis of the lighter hydrocarbons, i.e. C1-C4. Calibration on the BSSC is done as follows: nitrogen is carried along the reactor as an internal standard. This flow is measured by the RGA and is used to quantify the RGA-TCD. The methane flow measured next, is used to quantify the RGA-FID, after which in most cases ethylene is utilized for quantification of the GCxGC.
Chapter 2. Kinetic Model Generation & Experimental Set-up 29
2.5 Literature reported reactor configurations
Most of the available experimental data are carried out in a shock-tube or tubular flow reactor. Some literature reported studies validated their kinetic models using premixed laminar flames as well. Given the importance of the selected experimental apparatus on uncertainty regarding speciation, the main features for each type of device are briefly summarized.
A shock-tube is a device in which a shock wave is normally formed by the rupture of a diaphragm, which divides a gas at high pressure from a test section containing the species of interest at a lower pressure. The shock wave brings the test gas virtually instantaneously to a high temperature and pressure, maintains that condition for some time and then is supplanted by an expansion wave that cools the sample rapidly. During this time, the test gas can be studied by continuous sampling (e.g. time-of-flight mass spectrometer) or, alternatively, sampled at the end of the process by gas chromatography or other appropriate analytical techniques. This results in two distinct types of shock-tubes; the single pulse shock tube (i.e. produces speciation data at the end of the shock heating) and a time-dependent shock-tube (i.e. produces time-dependent speciation data by coupling to a mass spectrometer).27
Tubular flow reactors for studying pyrolysis and combustion chemistry are extensively used due to their operational flexibility. In these reactors, conditions of temperature, pressure, and space time can be carefully controlled. Tubular flow reactors can be operated in such a way that they attain close to ideal behavior, e.g. plug flow, diminishing the mathematical difficulties in the simulation of such environments. The validity of assumptions such as plug flow should be tested for each individual case. 28
Laboratory flames, particularly premixed laminar flames, have also been used to gather experimental data on cyclic compounds. In general applications, from domestic appliances to engines and gas turbines, flames are often non-premixed and/or turbulent. In these cases, the complicated fluid dynamics hamper investigations of the underlying chemistry. While the modeling of 3D reactive flows is still computationally expensive and challenging, contemporary computer codes are capable of modeling one-dimensional flames with complex detailed kinetic schemes. In this respect, premixed laminar flames are used to develop and validate chemical kinetics schemes.28,29 Note that there is significant temperature uncertainty in such flames as a thermocouple is usually installed in the flame. This disturbs the ideal temperature profile and introduces considerable error. Significant efforts are being made to mitigate this error.29
30 Chapter 2. Kinetic Model Generation and Experimental Set-up
2.6 References
1. Op de Beeck B, Dusselier M, Geboers J, et al. Direct catalytic conversion of cellulose to liquid straight-chain alkanes. Energy & Environmental Science. 2015;8(1):230- 240. 2. Vandewiele NM, Van Geem KM, Reyniers M-F, Marin GB. Genesys: Kinetic model construction using chemo-informatics. Chemical Engineering Journal. 10/1/ 2012;207–208:526-538. 3. Blurock ES. Reaction: system for modeling chemical reactions. Journal of chemical information and computer sciences. 1995;35(3):607-616. 4. Wang Z, Cheng Z, Yuan W, et al. An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure. Combustion and Flame. 7// 2012;159(7):2243-2253. 5. Wang Z, Ye L, Yuan W, et al. Experimental and kinetic modeling study on methylcyclohexane pyrolysis and combustion. Combustion and Flame. 2014;161(1):84-100. 6. Zeng M, Li Y, Yuan W, et al. Experimental and kinetic modeling investigation on decalin pyrolysis at low to atmospheric pressures. Combustion and Flame. 5// 2016;167:228-237. 7. McNaught A. The iupac international chemical identifier. Chemistry International. 2006:12-14. 8. DAYLIGHT CIS, Inc. SMILES - A Simplified Chemical Language. 2008. 9. Sabbe MK, Vandeputte AG, Reyniers MF, Waroquier M, Marin GB. Modeling the influence of resonance stabilization on the kinetics of hydrogen abstractions. Physical Chemistry Chemical Physics. 2010;12(6):1278-1298. 10. Gardiner WC, Troe J. Rate Coefficients of Thermal Dissociation, Isomerization, and Recombination Reactions. In: Gardiner WC, ed. Combustion Chemistry. New York, NY: Springer US; 1984:173-196. 11. Wang K, Villano SM, Dean AM. Reactivity–Structure-Based Rate Estimation Rules for Alkyl Radical H Atom Shift and Alkenyl Radical Cycloaddition Reactions. The Journal of Physical Chemistry A. 2015/07/16 2015;119(28):7205-7221. 12. Vijver RVD. Automatic Ab Initio Calculations for Kinetic Model Generation of Gas- Phase Processes. 2017. 13. Sabbe MK, Reyniers MF, Waroquier M, Marin GB. Hydrogen Radical Additions to Unsaturated Hydrocarbons and the Reverse Scission Reactions: Modeling of Activation Energies and Pre Exponential Factors. ChemPhysChem. 2010;11(1):195-210. 14. Sabbe MK, Reyniers MF, Van Speybroeck V, Waroquier M, Marin GB. Carbon Centered Radical Addition and Scission Reactions: Modeling of Activation Energies and Pre exponential Factors. ChemPhysChem. 2008;9(1):124-140. Chapter 2. Kinetic Model Generation & Experimental Set-up 31
15. Sabbe MK, Van Geem KM, Reyniers MF, Marin GB. First principle based simulation of ethane steam cracking. AIChE journal. 2011;57(2):482-496. 16. Tsang W. Thermal decomposition of cyclopentane and related compounds. International Journal of Chemical Kinetics. 1978;10(6):599-617. 17. Benson SW. Thermochemical Kinetics. Wiley, Interscience. 1976;2nd edition. 18. Sirjean B, Glaude P-A, Ruiz-Lopez M, Fournet R. Detailed kinetic study of the ring opening of cycloalkanes by CBS-QB3 calculations. The Journal of Physical Chemistry A. 2006;110(46):12693-12704. 19. Zhang F, Wang Z, Wang Z, Zhang L, Li Y, Qi F. Kinetics of decomposition and isomerization of methylcyclohexane: Starting point for studying monoalkylated cyclohexanes combustion. Energy & Fuels. 2013;27(3):1679-1687. 20. Saeys M, Reyniers M-F, Marin GB, Van Speybroeck V, Waroquier M. Ab initio calculations for hydrocarbons: enthalpy of formation, transition state geometry, and activation energy for radical reactions. The Journal of Physical Chemistry A. 2003;107(43):9147-9159. 21. Sumathi R, Green WH. Missing thermochemical groups for large unsaturated hydrocarbons: Contrasting predictions of G2 and CBS-Q. The Journal of Physical Chemistry A. 2002;106(46):11141-11149. 22. Evans M, Polanyi M. Inertia and driving force of chemical reactions. Transactions of the Faraday Society. 1938;34:11-24. 23. Blowers P, Masel R. Engineering approximations for activation energies in hydrogen transfer reactions. AIChE journal. 2000;46(10):2041-2052. 24. Saeys M, Reyniers MF, Marin GB, Van Speybroeck V, Waroquier M. Ab initio group contribution method for activation energies for radical additions. AIChE journal. 2004;50(2):426-444. 25. Lay TH, Bozzelli JW, Dean AM, Ritter ER. Hydrogen atom bond increments for calculation of thermodynamic properties of hydrocarbon radical species. The Journal of Physical Chemistry. 1995;99(39):14514-14527. 26. 15131 C-P. Reaction Design: San Diego. 2013. 27. Hanson RK, Davidson DF. Recent advances in laser absorption and shock tube methods for studies of combustion chemistry. Progress in Energy and Combustion Science. 2014;44:103-114. 28. Battin-Leclerc F, Simmie JM, Blurock E. Cleaner combustion: Springer; 2013. 29. Ghoddoussi R. An Investigation on Thermal Characteristics of Premixed Counterflow Flames Using Micro-thermocouples2005.
3. Chapter 3. Cyclohexane as Reference Component for Naphthenes
Chapter 3 Cyclohexane as Reference Component for Naphthenes
3.1 Introduction
In the past decades, intensive research has been devoted to the pyrolysis and combustion of normal and branched alkanes, such as pentanes, hexanes and iso-octane, as they are the major components in respectively naphtha1,2 and fuels3. Naphtha steam cracking takes place in tubular reactors inside the firebox of the furnace, at high temperature, low pressure and very short residence time, producing light olefins which are the main building blocks for the petrochemical industry. Branched alkanes with high octane ratings are used in high performance gasoline engines because they can withstand significant compression before reaching their ignition point. Moreover, decomposition of aromatic hydrocarbons like benzene4,5, toluene6,7 and alkylbenzenes8 is a relative mature research area as well.
In contrast, and although they are present in significant amounts in conventional fuels (3% in gasoline and up to 35% in diesel fuel)9,10, relatively little attention has been payed to the decomposition of cycloalkanes. Research on these components is gaining importance with the 34 Chapter 3. Cyclohexane as Reference Component for Naphthenes
increasing interest in the use of renewable feedstock derived from hydrodeoxygenated biomass, containing a higher amount of cycloalkanes than a similar naphtha feedstock.11,12
Knowledge about the behavior of these components during pyrolysis can be provided by a kinetic model. Such a model is constructed based on the occurring reactions and can be obtained automatically through the definition of reaction families, together with kinetic and thermodynamic parameters. The resulting chemistry allows to obtain insight in the stability of the molecules, evaluate soot or deposit formation and identify important intermediates. Since it is impossible to study the behavior of all the compounds contained in, for example diesel fuel, through the use of a kinetic model, it is more typical to use reference components to represent a certain mixture. As an example, iso-octane13-15 and n-dodecane16-18 are typically used to reproduce the reaction behavior of diesel fuels.
In this chapter, the pyrolysis of cyclohexane is studied as a reference for cycloalkanes. A first part describes its potential decomposition pathways through means of a literature survey and an overview of the currently available kinetic and experimental data is given. The model performance of the in-house developed model for cyclohexane pyrolysis is tested through simulation of two sets of experimental data, one at atmospheric pressure (in-house) and one at 40 mbar (Wang et al.19).
Subsequently, methyl cyclohexane decomposition is discussed in order to better understand the effect of substitutions on the C6-ring. For this species, a pyrolysis model is constructed automatically with Genesys. The simulated results of this model are compared with experimental data from literature20 at 1.0 bar. Through a rate of production and sensitivity analysis, differences with cyclohexane pyrolysis are identified.
Chapter 3. Cyclohexane as Reference Component for Naphthenes 35
3.2 Literature survey on the pyrolysis of cyclohexane
Due to the high symmetry of the cyclohexane molecule, the study of its pyrolysis should be rather straightforward. This is confirmed by the relative high amount of research that has been conducted around this molecule compared to other naphthenes like cyclopentane and methyl cyclohexane. The experimental data obtained in these studies increases the understanding of the complexities involved in the pyrolysis and combustion of the considered components and allows to validate the kinetic models.
3.2.1 Literature reported experimental and kinetic modeling studies
In the early years of research on cyclohexane pyrolysis, Tsang21 and Brown et al.22 proposed that the dominant initial pathway is the isomerization to form 1-hexene. However, the experimental results in an annular reactor obtained by Aribike et al.23 did not confirm Tsang’s predictions, since 1-hexene was not observed. Experiments performed by Voisin et al.24 in a jet stirred reactor (JSR) also did not show the presence of 1-hexene. Due to the absence of 1-hexene, Voisin et al.24 and Aribike et al.23 were among the first to propose dissociation channels other than the isomerization reaction of cyclohexane.
Klai et al.25 developed a low-temperature mechanism for cyclohexane oxidation to reproduce experimental results obtained in a batch reactor at 635 K and 0.06 bar (Φ = 9.0). Voisin et al.24 proposed a high temperature mechanism to reproduce experimental results obtained in a jet- stirred reactor. This mechanism was extended to a lower pressure range by El Bakali et al.26, who found the best agreement between modelling and experimental results at 1 bar and at 10 atm. The modelling indicated that, at 10 bar, cyclohexane is mainly consumed by H-atom abstractions and to a far lesser extent by thermal decomposition. At 1 bar, experiments confirmed the occurrence of ring-opening isomerization as 1-hexene was detected.
Granata et al.27 developed a lumped mechanism in order to reduce the complexity of the overall scheme in terms of species and reactions. Particular attention was devoted to the role of the isomerization or internal abstraction of hydrogen atoms, in competition with decomposition reactions. A more detailed kinetic mechanism has been developed by Silke et al.28 to study the oxidation of cyclohexane at both low and high temperature. This mechanism was developed by addition of all species and reactions relating to cyclohexane to the previously developed mechanism for C1-C6 of Curran et al.13
Wang et al.19 considered the pyrolysis of cyclohexane at low pressure (40 mbar) in a plug flow reactor (PFR) from 950 K to 1520 K. Among the products, 1-hexene was formed at the lowest 36 Chapter 3. Cyclohexane as Reference Component for Naphthenes
temperature, indicating that cyclohexane isomerization is the dominant initial decomposition channel under these conditions. They build a kinetic model that could adequately predict product yields, also known as the JetSurf 2.0 mechanism.29 El Bakali et al.26 later extended this study to lower pressures and shorter residence times, and they also revised the kinetic mechanism and extended its domain of validation.
A complete overview of the currently performed experiments is given in Table 3-1. The different reactor types are discussed in Chapter 2.
Table 3-1 Overview of available research concerning cyclohexane pyrolysis and oxidation.
Component Reactor Experiment Temperature Pressure Equivalence Mole % Ref. Type [K] [bar] Ratio CH BR Oxidation 533-633 0.1-0.3 4.5-18.0 - Zeelenberg et al.30 BR Oxidation 473-623 0.03-0.3 9.0 - Bonner et al.31 BR Pyrolysis 753 0.02-0.7 H2/O2 - Gulati et al.32 BR Oxidation 635 0.06 9.0 - Klai et al.25 JSR Oxidation 750-1200 1.0-10.0 0.5-1.5 - Voisin et al.24 RCM Oxidation 600-900 7.0-14.0 1.0 - Lemaire et al.33 RCM Oxidation 824 41.6 0.4 - Tanaka et al.34 ST Oxidation 1230-1840 7.3-9.5 0.5-2.0 0.01 Sirjean et al.10 ST Oxidation 847-1379 11-61 0.25-1.0 0.01 Daley et al.35 ST Oxidation 1280-1480 1.5-3.0 0.5-1.0 0.04 Hong et al.36 ST Pyrolysis 1300-2000 0.03-0.26 ∞ 0.02-0.2 Kiefer et al.37 ST Pyrolysis 1320-1550 1.8-2.2 ∞ 0.0002 Peukert et al.38 PFR Pyrolysis 950-1520 0.04 ∞ 0.02 Wang et al.19 Motor Oxidation 1000-1050 - 9.0 - Bennett et al.39 Motor Oxidation 750-860 10.0-25.0 0.25 - Yang et al.40 Flame a Oxidation - 0.04 1.0 - Yang et al.40 Flame a Oxidation - 1.0 2.33 - Ciajolo et al.41 Flame b Oxidation 353 1.0-20.0 0.7-1.5 - Ji et al.42 BR (batch reactor), JSR (jet stirred reactor), RCM (rapid cooling machine), ST (shock tube reactor), PFR (plug flow reactor) a Species profiles in a premixed laminar flame. b Laminar flame speed (temperature is unburned gases temperature).
In this section, cyclohexane pyrolysis is studied through a literature survey, kinetic modeling and experimental work. The former is performed to better understand the important reactions in the decomposition of cyclohexane and to obtain suitable and accurate rate coefficients for crucial reactions. Secondly, the performance of the in-house microkinetic model for cyclohexane pyrolysis, developed by Muralikrishna Khandavilli, is validated with experimental data, gathered at the Bench Scale Steam Cracker Set-up (BSSC) set-up (cfr. Chapter 2), and compared with that of different mechanisms from literature10,28,29,43. Next, the initial decomposition chemistry and the formation of aromatics in cyclohexane pyrolysis are elaborated upon through a detailed reaction path and sensitivity analysis. The in-house model is also used to reproduce low-pressure experiments (40 mbar) performed by Wang et al.19 Chapter 3. Cyclohexane as Reference Component for Naphthenes 37
3.2.2 Decomposition reactions of cyclohexane
3.2.2.1 Unimolecular reactions of cyclohexane
The two main kinds of unimolecular reactions, decomposition and isomerization, produce only a limited amount of different intermediates. A C-H bond scission yields a cyclohexyl radical and hydrogen atom. This reaction was first proposed and incorporated in a kinetic mechanism by Voisin et al.24 in 1998. The breaking of a C-C bond results in a ring-opening isomerization, which proceeds through a diradical species to yield 1-hexene (Figure 3-1).
Figure 3-1 Unimolecular reactions of cyclohexane. C-H #-scission (top), C-C #-scission (bottom).
The ring-opening reactions of several cycloalkanes were studied by Sirjean et al.44 from the Département de Chimie Physique des Réactions, Nancy. The quantum chemical calculations in this work, performed at the CBS-QB3 level of theory, take the two possible conformations of cyclohexane, the boat and chair conformation, into account. The free enthalpy difference between the boat and chair conformations at 1 atm and 298 K, amounts to 23.8 kJ mole-1.44 The chair conformation is most stable at room temperature, but at high temperatures, the boat conformation cannot simply be neglected. The equilibrium coefficient for the reaction (chair ⇌ boat), fitted in the 600-2000 K temperature range is given by Equation 2.1.44
−3265 % = )*+ + 1.271 (2.1) &' 1
At 1000 K, %&' is equal to 0.136, corresponding to 88.0 wt.% of cyclohexane molecules in the chair conformation.
Sirjean and his team44 identified two different transition states for the ring-opening of cyclohexane, depending on its initial boat or chair conformation. They found that the transition state corresponding to the former is 10.0 kJ mole-1 lower in free energy. The two different pathways and their corresponding transition states to form a diradical are represented in a free energy diagram (Figure 3-2). Since the activation energy for ring-opening is much higher than the energy involved in a boat/chair conformational change, only the lowest transition state should be taken into account. This means that only the biradical depicted on the right hand side of Figure 38 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3-2 will be formed through ring-opening of either of the cyclohexane conformers in the kinetic scheme.
450
10.0
400
350 349.4
300 Energy [kJ/mol]
23.5
250 0.0
200 Reation Coordinate [-] Figure 3-2 Free energy diagram for the ring-opening isomerization of cyclohexane, taking into account the different conformers. (p=1 atm, T=298 K).44
The Gibbs free energy of activation for the ring opening of cyclohexane can be compared to that of other naphthenes like cyclobutane and cyclopentane. The activation Gibbs free energy at 298 K is highest for cyclohexane ( ∆G‡ = 374.5 kJ moleAB ), followed by cyclopentane ( ∆G‡ = 344.8 kJ moleAB) and cyclobutane (∆G‡ = 258.2 kJ moleAB).44 This difference is due to the ring strain energy that is released when going from the cycloalkane to the linear diradical, which is much higher for cyclobutane due to its distorted angels.
In Figure 3-1, the effect of conformers of the diradical is ignored. However, the enthalpy of activation involved in the formation of 1-hexene from the diradical is low (∆H‡ = 4.2 kJ molAB at 298 K)44 and it might be in competition with rotational barriers. In accordance with this assumption, a more extended scheme for the formation of 1-hexene, as shown in Figure 3-3, should be considered.
Figure 3-3 Ring opening isomerization pathways for cyclohexane. Chapter 3. Cyclohexane as Reference Component for Naphthenes 39
Table 3-2 summarizes the modified Arrhenius parameters for the elementary reactions in Figure 3-3.
Table 3-2 Modified Arrhenius parameters for the ring opening of cyclohexane (p=1 atm, 600 K < T < 2000 K) determined by Sirjean et al.44 Note: E = F GH IJK −L MG , with units: mole, s, kJ.
A n L k (1000K)
k13 4.79E+20 -0.80 386.77 1.20E-02
k31 1.20E+11 0.12 2.93 1.90E+11
k23 1.45E+20 -0.81 359.78 8.63E-02
k32 1.70E+11 0.07 3.39 1.87E+11
k34 1.48E+02 2.92 3.22 5.90E+10
k43 1.29E+10 0.72 10.46 5.29E+11
k36 2.51E+11 0.36 12.47 6.69E+11
k46 1.48E+04 2.30 0.84 1.03E+11
As mentioned before, the main consumption pathway for the linear hexyl diradical, is through an isomerization forming 1-hexene. Other routes however are also possible, but owing to their high activation energy compared to that for the formation of 1-hexene, these pathways are often neglected without introducing any major error.19,45 For completeness, all possible reactions and their corresponding modified Arrhenius parameters44 are depicted in Figure 3-4.
5.9 P(1) = 2.88 10T 1T.UVW)*+ X− [ Z1
72.1 P(1) = 4.68 10AT 1\.] )*+ X− [ Z1
185.1 P(1) = 2.51 10B^ 1T.B]U )*+ X− [ Z1
107.7 P(1) = 1.70 10U 1^.WW_ )*+ X− [ Z1
Figure 3-4 Reactions of hex-1,6-diyl and the corresponding rate coefficients as determined by Sirjean et al.44 Unit are NAO, kJ, mole.
According to experiments performed by Granata et al.27, the direct dehydrogenation of cyclohexane yielding cyclohexene and hydrogen, is a reaction of minor importance, as it is responsible for only 3% of the formation of cyclohexene at 1050 K (P=1-10 bar, Φ=0.5-1.5). 40 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3.2.2.2 Bimolecular reaction of cyclohexane
Kiefer et al.37 compared various H-abstraction channels (by H, CH3, C2H3, C2H5, etc.) and found that hydrogen atoms are mainly abstracted from cyclohexane by hydrogen atoms and methyl radicals. They calculated the rate coefficient for the reaction in Figure 3-5 via the transition state theory at the B3LYP/6-311++G** level of theory.
Figure 3-5 H-abstraction reaction of cyclohexane.
Another way to determine rate expressions is through structure related rate rules, eliminating the need for sometimes computationally heavy calculations. The rate coefficient for the reaction in Figure 3-5 can, for example, be taken as that for a H-abstraction from the secondary carbon atom in propane, multiplied by six due to the different number of single events.37
Both the unimolecular C-H bond dissociation and the bimolecular H-abstraction of cyclohexane produce the same, unique cyclohexyl radical. As depicted in Figure 3-6, there are two possible elementary reactions through which cyclohexyl radical is converted. A `-scission of a C-H bond results in the formation of cyclohexene, while a `-scission of a C-C bond will break the cyclic structure, yielding hex-1-en-6-yl radical.
Figure 3-6 Reactions of cyclohexyl radical. C-H #-scission (top), C-C #-scission (bottom).
Granata et al.27 proposed to use the same kinetic parameters for these reactions as those evaluated on the basis of the primary pyrolysis and oxidation of linear and branched alkanes.15,46 For example, the rate coefficient for the C-H `-scission to cyclohexene is the same as the one used for the conversion of 3-pentyl radical to form 2-pentene. Sirjean et al.10 have determined Arrhenius parameters for the reactions listed in Figure 3-6 via transition state theory at the CBS-QB3 level of theory. The evolution of the reaction rate coefficients as a function of temperature for both pathways is depicted in Figure 3-7, showing a preference for the C-C ` -scission at lower temperature ( < 800 K). At relevant steam cracking conditions (900 K – 1100 K), the C-C `-scission is around a factor two larger than that of C-H `-scission. Chapter 3. Cyclohexane as Reference Component for Naphthenes 41
1.0E+11 142.3 P (1) = 2.5 10B_ )*+ X− [ aa Z1 1] 1.0E+08 -
1.0E+05
1.0E+02 120.1 P (1) = 4.00 10B\ )*+ X− [ ab Z1 1.0E-01 Rate Coefficient [s
1.0E-04 500 700 900 1100 1300 1500 Temperature [K] Figure 3-7 Evolution of the rate coefficients for the two reaction pathways of cyclohexyl radical as a function of temperature (P=1.0 bar). C-C #-scission (—), C-H #-scission (– – –). Unit are NAO, kJ, mole.10
Successive ` -scission of the hex-1-en-6-yl radical (Figure 3-8) is in competition with isomerization reactions that form resonantly stabilized hex-1-en-3-yl radicals. A direct C-C `- scission will result in the formation of a 3-butenyl radical and ethylene, whilst an intramolecular H-shift results in the formation of hex-1-en-3-yl radical. The latter subsequently decomposes into butadiene and an ethyl radical. In their kinetic model for cyclohexane pyrolysis at 40 mbar and high dilution, Wang et al.19 concluded that the main butadiene production pathway progresses however via a C-H `-scission of 3-butenyl radical (angled arrow in Figure 3-8).
Figure 3-8 Reactions of hex-1-en-6-yl radical.
In a simplified kinetic model, Granata et al.27 proposed that the kinetic parameters of these reactions can be evaluated without taking into account the double bond, based on the reference kinetic parameters of the oxidation and pyrolysis of branched and normal alkanes, as determined by Ranzi et al.46,47
However, the previous assumption will result in a relatively large error because it does not take into account the resonance stability of the allylic radical (hex-1-en-3-yl radical). Hence, it is better to use rate-rules that do not neglect the double bond in hex-1-en-3-yl radical. Values for the rate coefficients can for example be taken from Wang et al.48, who performed quantum mechanical calculations for these reactions. 42 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3.3 Kinetic modeling of cyclohexane pyrolysis
3.3.1 Experimental method
The thermal decomposition of pure, undiluted cyclohexane (Sigma-Aldrich, >99% purity) has been studied at a constant pressure of 1.7 bar and in a temperature range of 913 – 1073 K in a plug flow reactor. This temperature interval spans the complete conversion range of cyclohexane, i.e. from 3% at 913 K to 94% at 1073 K. Further information on the experimental set-up is provided in Chapter 2. The flow rate of cyclohexane was varied between 9.51 x 10A_ and 1.00 x 10A_ mole/s. The measured temperature profile of the process gas at set point temperatures of 933 K, 953 K, 1013 K and 1073 K are presented in Figure 3-9. Usually, a small overshoot of the set temperature, ±15 K, is measured at around 0.4 m in the reactor, followed by an isothermal section where temperature variations remain within 5 K.
1200
1000
800
600
400 Temperature [K] 200
0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Axial Reactor Coordinate [m]
Figure 3-9 Gas temperature profiles for the cyclohexane pyrolysis experiments along the axial reactor coordinate, corresponding to set temperatures of 913 K (—), 953 K (—), 1013 K (—), 1073 K (—).
The major decomposition products of cyclohexane pyrolysis include ethylene, propylene and 1,3- butadiene. Smaller fractions of ethane, cyclopentadiene and the permanent gas H2 have been detected as well. Significant amounts of aromatics like benzene, toluene and styrene are also formed, especially at high temperatures. An example of a GCxGC-FID chromatogram of the effluent at 1053 K is depicted in Figure 3-10. Chart Title Chapter 3. Cyclohexane as Reference Component for Naphthenes 43
5 C1, C2, C3
C4 4 1,3-C4H6
3
2
1 2nd dimension retention time [s]
0 0 25 50 75 1st dimension retention time [min] Figure 3-10 GCxGC-FID chromatogram of the on-line sampled pure cyclohexane pyrolysis effluent. T=1053 K, Ak p=1.7 bar, HdedfghIJiHI=9.51 x Oj mole/s.
In total, 72 species were detected and identified online by the comprehensive analysis section equipped with a so-called refinery gas analyzer (RGA) and a GC×GC-FID/TOF-MS. The former enables analysis of permanent gases and light hydrocarbons (C1 – C4), while the latter analyzes the whole product spectrum. Every experiment for a given temperature set point was repeated three to five times to establish repeatability. This yielded a average error of less than 10% on the mass fractions of the product species. Carbon balances closed within 1 % error. The detected species are listed in Appendix A, together with the experimental results at different conditions. Eventhough the total number of detected products amounts to 72, many of them are lumped aromatic species such as CB^H] (napthalene, azulene, fulvalene), for which modeling would be difficult. Out of the 72 species, a total of 15 can be qualified as major species with a mass yield higher than 1 wt. %.
3.3.2 Kinetic model development
The LCT model, developed automatically with use of Genesys by Muralikrishna Khandavilli (in- house, unpublished), contains 779 reversible reactions between 235 species. The procedure of automatically constructing such a model using Genesys is described in Chapter 2.
3.3.3 LCT experiments: results and discussion
This section discusses the performance of the LCT model and compares it to mechanisms found in literature. Due to their importance, special attention is paid to the initial decomposition pathways of cyclohexane and to the various production routes toward aromatics. 44 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3.3.3.1 Model performance and comparison to literature models
Reactor simulations have been performed using the LCT model and other literature models: Sirjean et al.10, Silke et al.28, POLIMI27, CSM43, JetSurF 2.029. For this, the PFR code in the CHEMKIN PRO software package was used. The experimental (symbols) and simulated (solid lines) mass percentage of cyclohexane during cyclohexane pyrolysis are shown in Figure 3-11. In general, the LCT model reproduces the initial decomposition temperatures, peak temperature and concentration of cyclohexane very well. Next to the LCT model, the CSM and POLIMI models also perform reasonable. The CSM model, however forms aromatics via lumped reactions and the POLIMI model in general is a lumped model. The LCT model is thus the only well-performing reaction mechanism that consists of elementary reactions.
The prediction of the cyclohexane yield with JetSurf 2.0 and the models of Sirjean et al. and Silke et al. is quite far off; hence these models will no longer be considered in the following discussion. In the JetSurf2.0 mechanism, only H-abstactions from cyclohexane by methyl radicals and hydrogen atoms are considered, which can potentially explain the late onset of cyclohexane consumption (at 1000 K, Figure 3-11). Another explantion can be that the rate coefficients of the ring-opening isomerization and subsequent C-C sciccions of 1-hexene to form smaller radicals, are taken from Kiefer et al.37, who calculated these pathways with the RRKM method in the pressure range 33.3 mbar – 267.6 mbar. The model of Sirjean et al. was developed for cyclohexane oxidation at high temperatures (1230 K – 1800 K) and therefore less applicable for the conditions considered in this work. The model of Silke et al. was also developed specifically for cyclohexane combustion and most of the reactions of cyclohexyl proceed through interaction with molecular oxygen in this mechanism and not through C-C `–scissions, which for pyrolysis are far more improtant.
100
80
60
40
20 Cyclohexane yield [wt. %] 0 900 950 1000 1050 1100 Temperature [K] Figure 3-11 Experimental (symbols) and simulated (lines) mass fraction of cyclohexane as a function of temperature. Simulations are done with the PFR code in CHEMKIN PRO and the models used are: LCT (—), Sirjean et al.10 (— —), Silke et al.28 (····), POLIMI27 (– · –), CSM43 (– – –), JetSurF 2.029 (–– · ––). P=1.7 bar, Ak HdedfghIJiHI=9.51 x Oj mole/s. The error bars represent 95% confidence intervals. Chapter 3. Cyclohexane as Reference Component for Naphthenes 45
The yields of the light olefin pyrolysis products of cyclohexane, ethylene being the most abundant at 25 wt. %, are shown in Figure 3-12. The mass fractions of hydrogen, methane, ethylene and acetylene exhibit a monotonous increasing trend as a function of both conversion and reactor temperature. The model is in qualitative agreement with most of the experimental product profiles, i.e. it is able to capture the effect of conversion on mole fractions, the main exception being acetylene (Figure 3-12) which is over predicted by a factor 3. On the contrary, the POLIMI and JetSurf2.0 mechanisms under predict the acetylene yield.
Propylene and butadiene reach a maximum yield at 90% and 75% conversion respectively. Contrary to the other models, the LCT model succeeds in reproducing both these maxima accordingly. The simulated propylene yield in the JetSurf2.0 model is too high, and the POLIMI model is not able to replicate the overall trend, i.e. the mole fraction of propylene increases in a monotonous way in the considered conversion range. The 1,3-butadiene yield is too high in the POLIMI model and slightly underestimated near the maximum in the JetSurf2.0 mechanism.
1.6 12 H2 CH4 1.2 9
0.8 6 Yield [wt. %] Yield [wt. %] 0.4 3
0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] 0.8 30 C2H2 C2H4 24 0.6
18 0.4 12 Yield [wt. %] 0.2 Yield [wt. %] 6
0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] 16 20 C3H6 1,3-C4H6
12 15
8 10 Yield [wt. %] Yield [wt. %] 4 5
0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] Figure 3-12 Experimental (symbols) and simulated (lines) mass fractions of major cyclohexane light olefin pyrolysis products as function of conversion. Simulations are done with the PFR code in CHEMKIN PRO and the Ak models used are LCT (—), CSM (····), POLIMI ( ̵ ̵ ̵ ). P=1.7 bar, HdedfghIJiHI=9.51 x Oj mole/s. The error bars represent 92.5% confidence intervals. 46 Chapter 3. Cyclohexane as Reference Component for Naphthenes
Figure 3-13 shows the experimental and simulated yields of the main aromatic species. The mass fraction profile of benzene, the main aromatic species, displays a typical exponential increase as a function of temperature (not shown in Figure 3-13), reaching a value of 15 wt. % at the highest temperature of 1073 K, corresponding to 94% conversion. This is in accordance with the experimental data. The POLIMI model does not simulate the benzene mass fraction profile very well due to its lumped nature, as already explained before. The second most abundant aromatic species is toluene. The experimental mass fraction of toluene reaches a value of 3.5 wt. % at 94% conversion, but is under predicted by the model. This is because some pathways to this species are missing in the LCT model. A more detailed analysis is given in the next paragraph. Styrene is present in smaller amounts than either toluene or benzene and its predicted mass fraction is in accordance with the experimental one. Polyaromatic hydrocarbons (PAH’s) are also formed during cyclohexane pyrolysis. The simplest PAH, naphthalene, has a mass fraction of around 1 wt. % at 94% conversion.
20 5 Benzene Toluene 16 4
12 3
8 2 Yield [wt. %] Yield [wt. %] 4 1
0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] 5 5 Styrene Naphthalene 4 4
3 3
2 2 Yield [wt. %] Yield [wt. %] 1 1
0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] Figure 3-13 Experimental (symbols) and simulated (lines) mass fractions of major cyclohexane aromatic pyrolysis products as function of conversion. Simulation are done with PFR in CHEMKIN PRO and models used Ak are: LCT (—), CSM (····), POLIMI ( ̵ ̵ ̵ ). P=1.7 bar, HdedfghIJiHI=9.51 x Oj mole/s. The error bars represent 92.5% confidence intervals.
3.3.3.2 Rate of production analysis of cyclohexane consumption
Figure 3-14 displays the rate of production (ROP) analysis of the initial decomposition pathways of cyclohexane at 933 K and 1033 K near the entrance of the reactor, as well as in the center of the reactor. The entrance is shown because this is where the initial decomposition chemistry of cyclohexane is most distinct. At these temperatures, the conversion of cyclohexane at the end of Chapter 3. Cyclohexane as Reference Component for Naphthenes 47
the reactor is 4 % and 74 % respectively. The ROP analysis shows that cyclohexane is consumed via two channels, i.e. the unimolecular ring-opening isomerization to form 1-hexene, and H- abstraction by radical attack to yield cyclohexyl radical. In Figure 3-14, a distinction is made between the various abstracting radicals as well: hydrogen atoms and methyl, ethyl and allyl radicals are all considered. Other, less occurring, reactions include hydrogen abstractions by vinyl or propyl radicals. At all temperatures and all positions, H-abstractions by hydrogen atoms are dominant, except at very low temperature and near the entrance of the reactor.
24
23 22 1033 K, 75 cm 933 K, 75 cm 21
20
19 1033 K, 15 cm 933 K, 15 cm
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0 10 20 30 40 50 60 70 80 90 100 Rate of consumption [%] Figure 3-14 Rate of production analysis of the initial decomposition pathways of cyclohexane at different temperatures and positions in the reactor with the LCT mechanism. Simulations are done with the PFR code in CHEMKIN PRO. 933 K, 15 cm (full black); 933 K, 75 cm (dotted); 1033 K, 15 cm (oblique stripped); 1033 K, 75 cm (horizontal striped). P=1.7 bar. The percentage shows the fraction of cyclohexane consumed by the corresponding reactions.
The importance of each channel clearly depends on the operating conditions and on the position in the reactor. The ring-opening isomerization reaction is favored especially near the entrance of the reactor. This is logical because the ring-opening isomerization is in competition with radical attack to consume cyclohexane, and near the entrance of the reactor no radicals are present yet. The first radicals in the system are formed through C-C scissions of 1-hexene. These radicals subsequently start a hydrogen abstraction sequence from cyclohexane, in turn resulting in a rapidly increasing amount of radicals. As a consequence, hydrogen abstraction reactions become dominant further along in the reactor. Hence, the importance of the ring-opening consumption pathway of cycloalkanes as an initial source of radicals is evident and should always be considered when studying the pyrolysis of naphthenes. However, modeling of this reaction, proves to be challenging, since it is very hard to find a corresponding transition state. It is therefore difficult to 48 Chapter 3. Cyclohexane as Reference Component for Naphthenes
find adequate, high-level ab-initio calculated rate coefficients. To date, Sirjean et al.44 are the only ones who succeeded in determining a transition state.
Another trend that can be deduced from Figure 3-14 is that the fraction of hydrogen abstractions by hydrogen atoms and methyl radicals increases with temperature, while that of hydrogen abstraction by ethyl radicals does the opposite. This is because much of the ethyl radicals have been converted to the more stable ethylene through a C-H `-scission at higher temperature.
A full rate of production analysis of cyclohexane pyrolysis at 1033 K is presented in Figure 3-15. Cyclohexyl radical and 1-hexene react further through C-C ` -scission and C-C scission respectively. Through the latter, hex-1-en-5-yl is formed which is eligible for further C-C ` - scissions or intramolecular H-abstraction toward more stable, i.e. secondary or allylic, radicals, as can be seen in Figure 3-15. The latter pathway is the dominant one and results in the formation of 1,3-butadiene. As is clear from the experimental and simulated results (Figure 3-12), 1,3- butadiene is not a stable end product. At the governing conditions, this species is converted further to propylene through subsequent hydrogen atom attack. Propylene as well exhibits a maximum in Figure 3-12 and this because it is consumed through radical attack by hydrogen atoms, yielding propyl radicals. These subsequently undergo C-C `-scission to form ethylene and methyl radicals.
The main routes to aromatic species are also shown in Figure 3-15. Hexadienyl radical, formed by recombination of vinyl radical and 1,3-butadiene, undergoes endo-intramolecular carbon radical addition to form cyclohex-1-en-4-yl. A series of subsequent C-C ` -scissions and hydrogen abstractions on this molecule leads to benzene. Styrene and toluene are formed through reaction of benzene with respectively methyl and vinyl radical and subsequent C-H `-scission. Benzyl radical can add to 1,3-butadiene to form C4-substituted benzene. An intramolecular addition results in the formation of two fused rings, which on C-H `-scission and H2 elimination gives naphthalene. Chapter 3. Cyclohexane as Reference Component for Naphthenes 49
Figure 3-15 Rate of production analysis of the major pathways in cyclohexane pyrolysis. T=1033 K, 75 cm along the axial reactor coordinate. Cyclohexane and the experimentally detected species are highlighted. The dominant routes are indicated with bold arrows.
50 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3.3.4 Wang experiments: results and discussion
In order to test the general applicability of the LCT model, experiments performed by Wang et al.19 have also been simulated. These experiments investigate the pyrolysis of cyclohexane at low pressure (40 mbar) in a plug flow reactor at temperatures between 950 K and 1520 K. The reactor has a length of 229 mm and an inner diameter of 0.68 mm. Argon was used as a carrier gas and the cyclohexane flow was set to 1.36 x 10AU mole/s, corresponding to a mole fraction 0.02. The product species have been analyzed by use of synchrotron VUV photoionization mass spectrometry at the National Synchrotron Radiation Laboratory in Hefei, China. The authors report an average error of ±10 % on the mole fractions of all major product species, and ±5 % on the cyclohexane mole fraction. The errors of the total carbon and hydrogen balances for measured species in the whole temperature range are within 10%.
3.3.4.1 Model performance
The low-pressure experiments have been simulated using the Plug Flow Code in the Chemkin- PRO software package. The experiments were simulated first with the original LCT model for cyclohexane pyrolysis, as used in the previous section, secondly with an adapted version of this model to improve the prediction of aromatic compounds at low pressure and thirdly with the Wang model19. The alterations in the adapted LCT model are discussed later in this section. The experimental (symbols) and simulated (solid lines) mole percentage of cyclohexane during pyrolysis are shown in Figure 3-16 as a function of temperature. The original LCT model is not able to reproduce the experimental mole fraction profile of cyclohexane. The onset of cyclohexane consumption is predicted to occur at a much lower temperature than the experimentally observed value. This already gives an indication that the original LCT model is not suitable to be used at low pressures. 2.5
2.0
1.5
1.0 Yield [mole%] 0.5
0.0 1000 1100 1200 1300 1400 1500 1600 Temperature [K] Figure 3-16 Experimental (symbols) and simulated (lines) mass fraction of cyclohexane as a function of temperature. Simulations are done with the PFR code in CHEMKIN PRO and the models used are: LCT adapted 19 Am (—), LCT original ( ̵ ̵ ̵ ), Wang et al. (····). P=40 mbar, HdedfghIJiHI=1.36 x Oj mole/s. The error bars represent the error as determined by Wang et al.19 Chapter 3. Cyclohexane as Reference Component for Naphthenes 51
The mole fraction profiles of the major light olefin pyrolysis products are presented in Figure 3-17. The yields are presented as a function of either the measured or predicted conversion. Thus, the predicted and measured conversions are reconciled, allowing for comparison of the yields at a given conversion. The hydrogen yield is under predicted by all models, and more by the adapted LCT model compared to the original. The simulated methane yield follows the right trend but overall overestimates the experimental one. Ethylene is the major pyrolysis product and is simulated very well by all models. The slope in the acetylene mole fraction is not well reproduced by both the original and new LCT model. The trend in the propylene and 1,3-butadiene mole fractions remains the same in both LCT models, but underestimates the experimental value.
3.0 2.0 H2 CH4 2.5 1.6
2.0 1.2 1.5 0.8 1.0 Yield [mole%] Yield [mole%] 0.4 0.5
0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] 2.5 1.0 C2H4 C2H2 2.0 0.8
1.5 0.6
1.0 0.4 Yield [mole%] Yield [mole%] 0.5 0.2
0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] 0.4 0.8 C3H6 1,3-C4H6
0.3 0.6
0.2 0.4 Yield [mole%] 0.1 Yield [mole%] 0.2
0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-]
Figure 3-17 Experimental (symbols) and simulated (lines) mass fraction of cyclohexane light olefin pyrolysis products as function of reconciled conversion. Simulations are done with the PFR code in CHEMKIN PRO and 19 the models used are: LCT adapted (—), LCT original ( ̵ ̵ ̵.), Wang et al. (····). P=40 mbar, HdedfghIJiHI=1.36 x OjAm mole/s. The error bars represent the error as determined by Wang et al.19
The experimental and simulated mole fractions of the aromatics are displayed in Figure 3-18. Here, the yields are also presented as a function of either the measured or predicted conversion. The simulated results of the original LCT model poorly predict the yield of the aromatic species, 52 Chapter 3. Cyclohexane as Reference Component for Naphthenes
producing virtually no benzene, toluene or styrene. Therefore, missing pathways are searched for in the current model, focused on aromatics.
0.10 0.010 Benzene Styrene 0.08 0.008
0.06 0.006
0.04 0.004 Yield [mole%] Yield [mole%]
0.02 0.002
0.00 0.000 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] 0.010 Toluene 0.008
0.006
0.004 Yield [mole%]
0.002
0.000 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Figure 3-18 Experimental (symbols) and simulated (lines) mass fractions of major cyclohexane aromatic pyrolysis products as function of cyclohexane conversion. Simulations are done with the PFR code in CHEMKIN PRO and the models used are: : LCT adapted (—), LCT original ( ̵ ̵ ̵.), Wang et al.19 (····). P=40 mbar, Am 19 HdedfghIJiHI=1.36 x Oj mole/s. The error bars represent the error as determined by Wang et al.
Formation of aromatic rings such as benzene and its precursor, phenyl radical, has been thought to be the rate controlling step of PAH and soot formation. Therefore, it is crucial to well understand the pathways that lead to benzene production. In the original LCT model, benzene is formed through only one channel: vinyl and 1,3-butadiene combine to form hexandienyl radical. This radical undergoes an endo intramolecular cyclization resulting in cyclohex-1-en-4-yl, which upon C-H `-scission yields 1,4-cyclohexadiene. Hydrogen abstraction from this molecule leads to the di-allylic cyclohexadienyl radical. After another C-H `-scission, benzene is formed.
An experimental study by Hansen et al.49-51 revealed that benzene formation is affected by the fuel structure in premixed flames of isomeric C6H12 fuels. In this work, the behavior of 1-hexene, cyclohexane, methyl cyclopentane and 3,3-dimethyl-1-butene were investigated in laminar flames at low pressure. For a 1-hexene flame, it was suggested that benzene is formed dominantly through C3+C3 routes, be it directly or indirectly, since 1-hexene decomposes dominantly to C3 products. Dominant reactions in these C3 routes include propargyl (C3H3) radical recombination to benzene, phenyl radical and fulvene (Figure 3-19a). The latter is also extensively formed through reactions of propargyl and allyl radicals; and presents a facile route to benzene through subsequent H-assisted isomerization (Figure 3-19b). Chapter 3. Cyclohexane as Reference Component for Naphthenes 53
Figure 3-19 (a) Propargyl (C3H3) recombinations to benzene (left), phenyl (center) and fulvene (right). (b) H- assisted fulvene isomerization.
Hansen et al. also argument that, out of the four isomers, the largest amounts of benzene are formed in the cyclohexane flame due to the additional possibility of successive dehydrogenation. This was later confirmed by simulation results. During the pyrolysis of cyclohexane, 1-hexene, cyclohexyl radical and C3 species are extensively formed. It can hence be expected that more than just the one pathway towards benzene in the original LCT model contributes to its formation. Based on the conclusions of Hansen et al., additional reactions have been added to the original LCT model for cyclohexane pyrolysis. These reactions and their rate coefficients are listed in Table 3-3.
54 Chapter 3. Cyclohexane as Reference Component for Naphthenes
Table 3-3 Summary of the reactions in the submechanism for aromatics formation. Note: E = F GH IJK −L MG , with units mole, nop , s, kJ. The majority of the Arrhenius parameters are only valid at low pressure. An overview of the species in this table is given in the List of Species.
No. Reaction A n E Ref. Comment
Benzene formation 1. C3H3+C3H3=FULVENE 7.25E+65 -16.0 104.75 52,53[52],[53]..… RRKM 4.19E+39 -9.0 25.51 52,53[52],[53]..… RRKM 3. C3H3+C3H3=BENZENE 1.64E+66 -15.9 115.18 52,53[52],[53]..… RRKM 1.20E+35 -7.4 21.16 52,53[52],[53]..… RRKM 5. FULVENE=BENZENE 5.62E+81 -19.4 508.36 54[54].. .. RRKM 6. FULVENE+H=BENZENE+H 3.00E+12 0.5 8.37 54[54]. .. RRKM 7. FULVENE=BENZENE+H 2.57E+97 -23.2 642.12 54[54]. .. RRKM 8. C3H3+C3H5=FULVENE+2H 3.26E+29 -5.4 14.18 54[54]. .. RRKM 9. C3H4+C3H5=BENZENE+H 2.20E+11 0.0 8.37 54[54]. .. RRKM 10. C3H5+C3H6=BENZENE+H 2.20E+11 0.0 8.37 54[54]. .. RRKM 11. C4H5-1+C2H2=BENZENE+H 2.94E+16 -1.09 38.73 54[54]. .. RRKM 12. C4H5-1=C2H2=FULVENE+H 1.52E+15 -0.76 36.66 54[54]. .. RRKM 13. C4H5-2+C2H2=BENZENE+H 1.47E+23 -3.28 104.21 54[54]. .. RRKM 14. C4H5-2=C2H2=FULVENE+H 1.01E+34 -5.94 120.44 54[54]. .. RRKM 15. C3H3+C3H3=C6H5+H 2.10E+11 0.0 18.59 54[54]. .. RRKM 16. C6H5+C2H6=BENZENE+C2H5 5.71E+04 2.43 26.25 54[54]. .. RRKM 17. C6H5+H=BENZENE 8.02E+19 -2.0 8.23 54[54]. .. RRKM 18. C6H5+CH4=BENZENE+CH3 3.89E-03 4.6 21.99 54[54]. .. RRKM
Benzyl formation 19. C7H7+H=C6H5+CH3 4.50E+58 -11.9 216.98 55[55]… - 20. C7H7=CPD-3+C2H2 3.00E+12 0.0 292.88 55[55]... - 21. C7H6+H=CPD-3+C2H2 7.00E+12 0.0 0.0 56[56]… -
Styrene formation 22. C6H5+C2H4=STYRENE+H 5.10E+12 0.0 25.90 19[19]… RRKM C6H5+C2H3=STYRENE 1.90E+48 -10.52 73.17 [19] RRKM
Toluene formation 23. TOLUENE=C6H5+CH3 2.66E+17 0 409.53 57[57]… - 24. C7H7+H=TOLUENE 3.02E+137 -33.35 232.46 57[57]… - 25. TOLUENE+H=C7H7+H2 6.47E+00 3.98 14.20 57[57]… - 26. TOLUENE+CH3=CH4+C7H7 3.16E+11 0 39.75 57[57]… - 27. TOLUENE+C6H5=C7H7+BENZENE 2.10E+12 0 18.41 57[57]… -
The mole fraction profiles of the light olefins and aromatic species, predicted with the new LCT model, i.e. the original model supplemented with the submechanism for aromatics, are presented in Figure 3-17 and Figure 3-18 respectively. The figure shows that the major aromatics, benzene, Chapter 3. Cyclohexane as Reference Component for Naphthenes 55
toluene and styrene, are still all under predicted, but far less than with the original LCT model. A similar performance as the Wang model is observed.
3.3.4.2 Rate of production analysis for the formation of aromatics
A complete rate of production analysis of cyclohexane pyrolysis was already presented in Figure 3-15. Therefore, and because the focus of this section is more on improvements made towards the formation of aromatics, only ROP analysis of benzene (Figure 3-20), toluene and styrene is performed in this paragraph. In this figure, the percentages reflect the molar flux of the corresponding pathway divided by the total molar flux of cyclohexane consumption. The ROP is carried out at 1359 K in the center of the reactor, to elucidate the benzene formation channels. At these conditions, the respective mole fractions of benzene, toluene and styrene are 0.01 mol%, 0.0006 mole% and 0.0003 mole%. As seen from the figure, benzene is formed by multiple channels: 33.2% of benzene is formed from dehydrogenation of cyclohexadienyl, 4.3% benzene is formed from the isomerization of fulvene and the remaining share is directly produced by combination of resonantly stabilized radicals like propargyl radical recombination, recombination of propargyl radical with allene and propyn. For the latter recombination channels, the self-recombination of propargyl radical is dominant (53.9%). Considering that production of fulvene is mainly C3-based (e.g. recombination of propargyl radicals to form fulvene, 87%) as well,
C3+C3 reactions totally contribute about 66.8% to benzene formation.
Figure 3-20 Rate of production analysis for benzene at T=1310 K (=56 % conversion), P=40 mbar, center of the reactor. Simulation are done with PFR in CHEMKIN PRO. The percentages reflect the molar flux of the corresponding pathway divided by the total molar flux of cyclohexane consumption.
Figure 3-21 gives the sensitivity analysis of benzene performed at 56% conversion of cyclohexane at 1 bar and 40 mbar. A positive sensitivity coefficient means that increasing the pre-exponential factor of a particular reaction results in more benzene being produced. It can be seen that the ring- opening isomerization of cyclohexane to 1-hexene has a significant positive coefficient at low pressures. Since most C3 products are formed from 1-hexene, this confirms the observation from Figure 3-20 that reactions involving species with three carbon atoms play an important for benzene formation. For the same reason, the C-C scission of 1-hexene to yield propyl and allyl radical has a positive sensitivity. The largest positive sensitivity coefficient in the atmospheric pressure case is found for the reaction of but-1-en-3-yl radical to ethylene and vinyl radical. This 56 Chapter 3. Cyclohexane as Reference Component for Naphthenes
is in accordance with Figure 3-15, where it is shown that the addition of vinyl radical to 1,3- butadiene contributes to the major pathway towards benzene. At 40 mbar, this reaction has a much lower sensitivity.
8 1 bar, 56% cyclohexane conversion
7 40 mbar, 56% cyclohexane conversion
6
5
4
3
2
1
0.00 0.10 0.20 0.30 0.40 0.50 0.60 Normalized sensitivity coefficient [-]
Figure 3-21 Normalized sensitivity coefficients for main reactions in benzene formation. New LCT model at 1310 K (=56% conversion), P=40 mbar, center of the reactor (full black); New LCT model at 1013 K (=56% conversion), P=1.0 bar, center of the reactor (dotted). Simulations are done with the PFR code in CHEMKIN PRO.
Besides benzene, other aromatics like toluene and styrene are also formed as shown in Figure 3-15. Figure 3-22 displays a rate of production analysis performed at 1310 K to reveal the production channels of these products. In this figure, the percentages reflect the fractions of styrene and toluene formed through the respective pathways, and not relative to the molar flux of cyclohexane consumption because these are below 10Aq %. It can be seen from the figure that these aromatics are mainly formed by addition reactions to phenyl and benzyl. The recombination of benzyl radical with hydrogen atoms and the addition of benzyl radical to ethylene are some examples. Benzyl radical itself is dominantly formed through reaction of phenyl radical with methyl radical, and about 90% of the phenyl radical originates from propargyl radical recombinations. Therefore, it can be concluded that reactions involving species with three carbon atoms play an important role in the formation of benzene and larger aromatics.
Figure 3-22 Rate of production analysis for toluene and styrene at T=1310 K (=56 % conversion), P=40 mbar, center of the reactor. The percentages reflect the fractions of styrene and toluene formed through the respective pathways. Simulations are done with the PFR code in CHEMKIN PRO. Chapter 3. Cyclohexane as Reference Component for Naphthenes 57
3.4 Literature survey on substituted cycloalkanes Substituted cycloalkanes have not been studied as extensively as cyclohexane due to their often complex structure, which gives rise to extensive reaction networks. In this section, the reported kinetic modeling and experimental work concerning methyl cyclohexane, ethyl cyclohexane and dimethyl cyclohexane to date is discussed in order to better understand the important pathways in its decomposition.
3.4.1 Methyl cyclohexane
3.4.1.1 Literature reported experimental and kinetic modeling studies
Yang and Boehman40 made a comparative study of cyclohexane and methyl cyclohexane oxidation under motored engine conditions. Their work shows that the presence of a methyl side chain increases the reactivity of the fuel.
Granata et al.27 reported a semi detailed kinetic model for methyl cyclohexane pyrolysis, which was tested against turbulent flow reactor data from Zeppieri et al.58 This model was developed further by Bieleveld et al.59, to model the critical conditions of extinction and auto ignition of methyl cyclohexane. Orme et al.60 presented a more detailed kinetic model, which was used for simulating ignition delay times in shock tubes. Another model was developed by Pitz et al.61 who compared the model’s predictions with experimentally measured ignition delay times from a rapid compression machine. This model predicted well the shape of the measured ignition times with temperature, but theoretical times were longer than the measured ones.
The most recently developed chemical kinetic model is the one from Wang et al.20 In this work, methyl cyclohexane pyrolysis at various pressures and premixed flames with equivalence ratio of 1.75 were investigated. The kinetic model with 249 species and 1570 reactions was developed based on their existing cyclohexane pyrolysis model by adding a new sub-mechanism of MCH. The decomposition was discussed with special attention to the formation of toluene and benzene.
An overview of the currently available research concerning methyl cyclohexane oxidation and pyrolysis is presented in Table 3-4.
58 Chapter 3. Cyclohexane as Reference Component for Naphthenes
Table 3-4 Overview of available research concerning methyl cyclohexane pyrolysis and oxidation
Reactor Temperature Pressure Equivalence Mole Component Experiment Ref. Type [K] [bar] Ratio % MCH ST Oxidation 1200-2200 0.6-1.7 0.5-2.0 0.01 Orme et al.62 ST Pyrolysis 795-1560 1.0-50.0 0.5-2.0 0.01 Vasu et al.63 Vanderover ST Oxidation 880-1319 10.8-69.5 0.25-1.0 0.01 et al.64 ST Oxidation 1280-1480 1.5-3.0 0.5-1.0 - Hong et al.36 RCM Oxidation 700-1050 10-20 1.0 - Pitz et al.61 Tanaka et RCM Oxidation 827 41.0 0.4 - al.34 Zeppieri et PFR Pyrolysis 1050-1200 1.0 1.25 - al.58 PFR Pyrolysis 900-1250 0.04-1.0 1.75 - ∞ 0.02 Wang et al.20 Flamea Oxidation - 0.02-0.04 1.0-1.9 - Skeen et al.65 Flamea Oxidation 353 1.0-20.0 0.7-1.5 - Wu et al.66 Motorb Oxidation 750-860 10.0-25.0 0.25 - Yang et al.40 BR (batch reactor), JSR (jet stirred reactor), RCM (rapid cooling machine), ST (shock tube reactor), PFR (plug flow reactor) a Species profiles in a premixed laminar flame. b Laminar flame speed (temperature is unburned gases temperature).
3.4.1.2 Initial pyrolysis chemistry: unimolecular reactions
In contrast to cyclohexane, the initial reaction pathways of methyl cyclohexane are more numerous, making its kinetic model more extensive. The two main kinds of unimolecular reactions are decomposition, i.e. C-C and C-H bond scission and ring-opening isomerization as a result of a C-C scission in the six-membered ring. Skeen et al.65, Kiefer et al.37 and Zhang et al.67 showed that the activation energy for C-H dissociation is considerably higher compared to the C- C dissociation and isomerization pathways. Hence, these reactions are omitted in this study.
Only one C-C bond is susceptible to a scission reaction that does not result in opening of the ring. This reaction, forming a cyclohexyl radical and a methyl radical, is represented in Figure 3-23. A possible way to determine its rate coefficient is to use the reverse of a methyl addition to isopropyl radical as calculated by Tsang et al.68 Dissociation of a C-C bond in the six-membered ring results in the formation of C7H14 diradical species. While for cyclohexane only one unique diradical could be formed, methyl cyclohexane ring-opening produces three different diradicals (Figure 3-23). Chapter 3. Cyclohexane as Reference Component for Naphthenes 59
Figure 3-23 C-C bond dissociation reactions of methyl cyclohexane.
Note that not all pathways in Figure 3-23 are equally likely to proceed since C-C bonds next to the methyl substitution are weaker and thus have a higher tendency to break. Recently, the unimolecular reactions of methyl cyclohexane were investigated on the CASPT2/cc-pVDZ level of theory by Skeen et al.65 Their results are summarized in the potential energy diagram depicted in Figure 3-24.
361.5 359.4 357.7 355.6
352.7
348.5 Energy [kJ/mol]
0.0
Reaction Coordinate [-] Figure 3-24 Zero point CASPT2/cc-pVDZ potential energy diagram for C-C scissions in methyl cyclohexane. 65
From Figure 3-24, it is clear that both authors came to the same conclusion that a lower barrier exists for breaking of the C-C bonds adjacent to the side-chain. However, the difference in activation enthalpies for the formation of the three diradicals is modest, suggesting that ring- opening is possible via any of the pathways and that all diradicals on the right side of Figure 3-24 will be formed in similar amounts.
60 Chapter 3. Cyclohexane as Reference Component for Naphthenes
Furthermore, it was found that ring-opening isomerization will compete with the dissociation channel forming cyclohexyl and methyl radicals. This statement is quite different from previous predictions which suggest that formation of methyl and cyclohexyl radicals might be negligible and hence, that heptenes are the more dominant isomerization products. The conclusions of Skeen et al.65 were confirmed by Zhang et al.67, who estimated the rate coefficient for the dissociation channel forming cyclohexyl and methyl radicals by the canonical variational transition state theory at MRCI/6-31+g(d,p)//CAS(2e,2o)/6-31+g(d,p).
The diradicals are only intermediate species and will immediately react further, either through isomerization reactions, forming heptenes and methyl-hexenes, or via a C-C `-scission to form an alkene and another diradical (Figure 3-23). Although the energetic thresholds for the latter reactions are much higher67, these can become significant at high temperatures and should not be omitted. Rate coefficients for the isomerization and dissociation reactions have been calculated at different pressures by Zhang et al.67 The need for pressure dependent rate coefficients is evident from Figure 3-25. Especially at low pressure, the variation is high. At the same time, it is clear that the dissociation of methyl cyclohexane to methyl and cyclohexyl radical plays a more important role than the isomerization pathways.
0.035 0.0037
0.034 ] ] 1 - 1 0.0034 - [s [s 0.033 0.0031 0.032 dissociation isomerization
k 0.0028 0.031 k
0.03 0.0025 0.01 0.1 1 10 100 Pressure [bar] Figure 3-25 Pressure dependence of rate constants of methyl cyclohexane dissociation (—) and overall isomerization ( ̵ ̵ ̵.) at 1000 K. The vertical coordinate on the right hand side corresponds to the dissociation channel, while the one on the left side corresponds to the isomerization channel.67
Some additional conclusions can be made from the calculations of Zhang et al.67 Firstly, when comparing the formation of 1- and 2-heptene, the latter is clearly favored due to the lower energy of its transition state. Secondly, the rate coefficient of reaction 2 and 3 in Figure 3-23 are distinguishable from those of the four other isomerization pathways, which may be caused by the lower barrier of the transition state corresponding to break the C-C bonds adjacent to the methyl group, as can be seen in Figure 3-24; Therefore, these rate coefficient calculations indicate that methyl cyclohexane pyrolysis will yield few methyl-substituted C7-species. Thirdly, the –CH3 loss pathway dominates the unimolecular reactions of methyl cyclohexane at low temperatures, but Chapter 3. Cyclohexane as Reference Component for Naphthenes 61
quickly loses significance with increasing temperature. These conclusions were also confirmed by pyrolysis experiments in a plug flow reactor at very low pressures (40 mbar) performed by the authors.67
3.4.1.3 Initial pyrolysis chemistry: bimolecular reactions
In contrast to cyclohexane, hydrogen atom abstraction from methyl cyclohexane can take place at primary, secondary and tertiary carbon atoms (Figure 3-26).
Figure 3-26 H-abstraction reaction of methyl cyclohexane.
Orme et al.62 and Curran et al.13,62 have reported modified Arrhenius parameters for these site- specific H-abstractions by various radicals. The most important ones are listed in Table 3-5. Electron poor atoms are stabilized by neighboring atoms that can donate electron density. Hence, the stability of the resulting radicals increases in the sequence primary – secondary – tertiary. This is translated to a faster formation rate, as is evidenced in the last column of Table 3-5. The increasing number of electron donating alkyl groups on the carbon atom bearing the free radical explain the latter effect.
Table 3-5 Modified Arrhenius parameters for H-abstraction reactions of methyl cyclohexane.62 E = F GH IJK −L MG , with units: mole, cm3, s, kJ.
Abstraction by C-site A n E k (1000 K) H• Primary 2.22E+05 2.54 28.27 3.09E+11 Secondary 6.50E+05 2.40 18.71 1.09E+12 Tertiary 6.02E+05 2.40 10.81 2.60E+12
• CH\ Primary 1.51E-01 3.65 29.93 3.68E+08 Secondary 7.55E-01 3.46 22.93 1.15E+09 Tertiary 6.01E-10 6.36 3.74 4.61E+09
• The cyclic CqHB_ radicals are mainly consumed by ring-opening reactions. The resulting alkenyl • CqHB_ radicals undergo subsequent reactions: direct `-scission of a C-C bond and H-migration forming allylic radicals, followed by a C-C `-scission. These reactions are shown in Figure 3-27 for methyl cyclohex-1-yl radical. It should be mentioned that other intramolecular H-abstractions, 62 Chapter 3. Cyclohexane as Reference Component for Naphthenes
not forming an allylic radical, are also possible, but are much slower compared to the formation • of the latter. Some of the alkenyl CqHB\ radicals can also undergo intramolecular addition forming five- or six-membered-ring products. This contribution is however negligible and is often omitted in literature.62,65
• Figure 3-27 Formation of alkenyl stuOp radicals through ring-opening of methyl cyclohex-1-yl radical, and subsequent reactions.
Rate coefficients for the reverse ring-closure reactions were originally proposed by Orme et al.62 and Pitz et al.61 More recently, Sirjean et al.69 have performed theoretical calculations at the CBS- QB3 level of theory to determine the rate coefficients for the ring-opening of cyclohexyl radicals. Narayanaswamy et al.70 of Cornell University proposed to use these quantum chemical calculations for the ring-opening and –closing of the methyl cyclohexyl radicals, independent of the nature of the C-C bond that is broken. Although this introduces a considerable error, they found sufficient agreement with their experiments.
3.4.2 Ethyl cyclohexane
3.4.2.1 Literature reported experimental and kinetic modeling studies
Husson et al.71 have studied the oxidation of ethyl cyclohexane in a jet-stirred reactor (JSR) under atmospheric pressure in a temperature range of 500-1100 K. They identified a total of 47 intermediates and showed that the proposed detailed kinetic model (JetSurf1.1 mechanism72) could well describe the mole fractions of the important species.
Ignition delay times of an ethyl cyclohexane/air mixture were measured by Vanderover and Oehlschlaeger64 in a shock tube reactor. They did experiments covering 881-1319 K, 10.8-69.5 bar, Φ = 0.25-1.0 but no modeling work was done. Li et al.42 measured laminar flame speeds of an ethyl cyclohexane/air mixtures at atmospheric pressure and a temperature of 353 K (Φ = 0.7-1.5). Their results were simulated appropriately using the JetSurf 1.1 mechanism.72 Tian et al.45 measured ignition delay times for cyclohexane, ethyl cyclohexane and n-propyl cyclohexane at atmospheric pressure, temperature between 1110-1650 K and equivalence ratios of 0.5-2.0. McEnally et al.73 investigated the unimolecular dissociation of various cycloalkanes (cyclohexane, methyl cyclohexane, ethyl cyclohexane, vinyl cyclohexane, ethynyl cyclohexane) and found that Chapter 3. Cyclohexane as Reference Component for Naphthenes 63
cycloalkanes with saturated side-chains decompose faster than cycloalkanes with unsaturated side-chains.
An overview of the currently available research is presented in Table 3-6.
Table 3-6 Overview of available research concerning ethyl cyclohexane pyrolysis and oxidation. Component Reactor Experiment Temperature Pressure Equivalence Mole % Ref. Type [K] [bar] Ratio ECH JSR Oxidation 500-1100 1.0 0.25-2.0 0.005 Husson et al.71 ST Oxidation 881-1319 10.8- 0.25-1.0 0.02 Vanderover 69.5 et al.64 Flame Oxidation 353 1.0-10.0 0.7-1.5 0.02 Ji et al.42 speed ST Oxidation 1110-1650 1.1 0.5-2.0 0.005 Tian et al.45 BR (batch reactor), JSR (jet stirred reactor), RCM (rapid cooling machine), ST (shock tube reactor), PFR (plug flow reactor)
3.4.2.2 Initial pyrolysis chemistry: unimolecular reactions
In contrast to methyl cyclohexane, ethyl cyclohexane can undergo two C-C bond dissociations which are not followed by opening of the ring. They result in the formation of cyclohexyl, methyl cyclohexyl and small alkyl radicals, as illustrated in Figure 3-28.
Figure 3-28 Decomposition reactions of ethyl cyclohexane.
Rate coefficients for these reactions can be obtained from analogies with other reactions. The rate coefficient for the reverse of the first reaction is similar to the one measured for iso-propyl and ethyl radical recombination, whereas that of the second is similar to the one for propane C-C dissociation.74 An experimental rate expressions for the first reaction was presented by Tsang et al.68,75 More recently, Klippenstein et al.76 determined the rate coefficient for the reverse of this reaction on the CASPT2/cc-pvdz level of theory. Various expression for the rate coefficient of this reaction were adopted and compared in a study performed by Wang et al.74 They found that the best agreement was found when using the rate expression for methyl cyclohexane decomposition to a cyclohexyl radical and a methyl radical, calculated by Zhang et al.67 64 Chapter 3. Cyclohexane as Reference Component for Naphthenes
Oehlschlaeger et al.77 investigated ethane and propane decomposition using RRKM/master equation analysis with a restricted (hindered) Gorin78 model for the transition state and fit these calculations to the current high-temperature dissociation data. Their rate coefficient for propane dissociation can be used for the second reaction in Figure 3-28 after reduction by a factor two, considering degeneracy. The evolution of the rate coefficients as a function of temperature for these competing reactions is given in Figure 3-31, showing only a modest difference, especially at high temperatures. 1.0E+05 453.9 P (1) = 5.93 10V_ 1AB_.BU )*+ X− [ {y{z|x&}yz ~ÄÅ{z Z1 ] 1 - 1.0E+02
1.0E-01
1.0E-04
Rate Coefficient [s 407.5 P (1) = 6.5 10\V 1AU.]_ )*+ X− [ v&wxyz {y{z|x&}yz ~ÄÅ{z Z1 1.0E-07 700 900 1100 1300 1500 1700 Temperature [K]
Figure 3-29 Evolution of the rate coefficients for the unimolecular reaction pathways of ethyl cylcohexane as a function of temperature (P=1.0 bar). Formation of cyclohexyl radical: kcyclohexyl radical 67 ( ̵ ̵ ̵.), formation of methyl cyclohexyl radical: kmethyl cyclohexyl radical 77 (—). Units are: mole, cm3, s, kJ.
Through ring-opening isomerization pathways, several C]HBV alkenes can be formed (Figure 3-30), but not all are equally likely. Similar to the case of methyl cyclohexane, the C-C bonds adjacent to the side-chain have a higher tendency to break than any other of the C-C bonds.
Figure 3-30 Ring-opening isomerization reactions of ethyl cyclohexane. Chapter 3. Cyclohexane as Reference Component for Naphthenes 65
The isomerization of the diradicals of ethyl cyclohexane is very similar to that of methyl cyclohexane. Hence, the same rate expressions can be used, without introducing too large errors. A rate of production analysis, performed by, Wang et al.75, revealed that formation of 3-octene and 2-ethyl-1-heptene are the most important ethyl cyclohexane consumption pathways at 1100 K and 1 atm. Their importance however quickly declines as the temperature and/or pressure increase.75
3.4.2.3 Initial pyrolysis chemistry: bimolecular reactions
Aside from unimolecular reactions, H-abstraction reactions by small radicals are just as important in the consumption of ethyl cyclohexane. Whereas cyclohexane produces only one radical due to its high symmetry number, radical attacks on ethyl cyclohexane can form up to six different • radicals (Figure 3-31). Subsequent reactions of these cyclic C]HBU radicals include C-C and C-H `- scission, and internal H-migration. These subsequent reactions are very similar to those of methyl cyclohexane and will not be discussed here.
Figure 3-31 H-abstraction reactions of ECH.
Figure 3-32 shows a study of the potential energy surface (PES) at the CBS-QB3 level of theory, performed by Wang et al.74,75 for these cyclic radicals. They found that the activation enthalpy for C-C ` -scissions that open the ring (± 124.3 kJ mole-1) is slightly lower than that for C-H ` -
-1 • scissions (± 141.0 kJ mole ), for the various cyclic C]HBU radicals. The ring-opening `-scissions produce linear or branched alkenyl radicals which can react further through another direct C-C `-scission or through intramolecular H-abstraction. 66 Chapter 3. Cyclohexane as Reference Component for Naphthenes
180.7
141.0 135.1 137.2 ]
1 124.3 -
101.3 91.6 Energy [kJ mole
0.0
Reaction Coordinate [-]
Figure 3-32 Potential energy surface of 1-ethylcyclohexyl radical and its decomposition products (T=0K).74,75
3.4.3 Dimethyl cyclohexane
There have been some studies regarding the catalyzed ring opening of dimethyl cyclohexane from authors like for example Dokjampa et al.79, Do et al.80 and Gonzales-Cortés et al.81 However, the thermal ring opening has been neglected to a large extent. This paragraph discusses the scare experimental and kinetic modeling studies for this molecule that have been performed to date. Differences and analogies with ethyl or methyl cyclohexane are considered as well. In this respect, 1,3-dimethyl cyclohexane is taken as a model component.
3.4.3.1 Literature reported experimental and kinetic modeling studies
In very recent work, Kang and coworkers82 made a comparative study of the ignition process of di-substituted cyclohexane and ethyl cyclohexane. They found that ethyl cyclohexane is more reactive than both 1,3-dimethyl cyclohexane and 1,2-dimethyl cyclohexane. Sun et al.83 have performed an experimental and modeling study of the pyrolysis of substituted cycloalkanes. They found that the gaseous product yield for the mono-substituted cyclohexanes with short substituents increased steadily, but that longer alkyl chains depressed the gaseous product yield. Eldeeb et al.84 presented an extension of the JetSurf 2.029 mechanism for dimethyl cyclohexane to verify their experimentally measured ignition delay times. Without too much adaptions of the JetSurf 2.0 mechanism, good agreement was found between predicted and measured data.
An overview of the currently available research concerning dimethyl cyclohexane oxidation and pyrolysis is presented in Table 3-7.
Chapter 3. Cyclohexane as Reference Component for Naphthenes 67
Table 3-7 Overview of available research concerning dimethyl cyclohexane pyrolysis and oxidation.
Component Reactor Experiment Temperature Pressure Equivalence Mole Ref. Type [K] [atm] Ratio % DMCH Motor Oxidation 428 1.0 0.5 - Kang et al.82 ST Pyrolysis 1100-1200 2.5 ∞ 0.005 Rosado- Reyes et al.85 ST Oxidation 1049-1544 3.0-12.0 0.5-2.0 0.018 Eldeeb et al.84
ST Pyrolysis 1238-1406 4.0 ∞ 0.018 Eldeeb et al.84
Flame a Oxidation - 1.0 - - McEnally et al.73 Bench- Pyrolysis 973 40 ∞ - Sun et al.83 scale BR (batch reactor), JSR (jet stirred reactor), RCM (rapid cooling machine), ST (shock tube reactor), PFR (plug flow reactor) a non-premixed laminar flame speeds
3.4.3.2 Initial pyrolysis chemistry: unimolecular reactions
Unimolecular dissociation of a methyl substitution yields a 2-methylcyclohexyl radical and a methyl radical. Ring-opening isomerization leads to the formation of branched C8H16 alkenes. These reactions are presented in Figure 3-33. Due to the weak mutual interaction between the two methyl substitutions, it is feasible to treat the reactions of 1,3-dimethylcyclohexane by analogy to those of methyl cyclohexane, using only a few adaptations.84 For example, the pre- exponential factor of the top reaction in Figure 3-33 should be multiplied by two, due to the degeneracy.
Figure 3-33 Unimolecular reactions of dimethyl cyclohexane.
68 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3.4.3.3 Initial pyrolysis chemistry: bimolecular reactions
H-abstraction of 1,3-dimethylcyclohexane by hydrogen atoms or methyl radicals produces five different 1,3-dimethyl cyclohexane radicals, which can undergo further reactions. A C-C `-scission resulting in ring-opening followed by isomerization, produces linear C8 alkenyl radicals. ` - scission of a C-C bond between the ring and side chain forms methyl cyclohexenes and methyl radicals; whilst `-scission of a C-H bond results in the formation of cyclic C8H14 alkenes. Further reactions are very similar to those of ethyl and methyl cyclohexane pyrolysis and are hence omitted here.
Chapter 3. Cyclohexane as Reference Component for Naphthenes 69
3.5 Kinetic modeling of methyl cyclohexane pyrolysis
Methyl cyclohexane is chosen as a reference component for substituted monocyclic alkanes. In this section, the development of a microkinetic model for methyl cyclohexane pyrolysis is discusses. The simulated results of the new model are compared to experimental data obtained by Wang et al.20 in a plug flow reactor at 1 bar. Special attention is paid to the initial decomposition pathways of methyl cyclohexane and to the ring-opening of methyl cyclohexyl radicals through a rate of production and sensitivity analysis. The formation of aromatics as well is discussed in more detail.
3.5.1 Experimental method
The thermal decomposition of diluted methyl cyclohexane has been studied at a constant pressure of 1 bar and in a temperature range of 900 – 1250 K in a plug flow reactor by Wang et al.20 The complete conversion range of methyl cyclohexane, i.e. from 2% at 942 K to 99.9% at 1263 K, is included in this temperature interval. The methyl cyclohexane (>99% purity) was vaporized in an argon stream before flowing into the alumina plug flow reactor (inner diameter 6.8mm and length 229 mm). The total inlet flow, composed of argon and 2 mole % methyl cyclohexane, was kept constant at 6.82 x 10A_ mole/s. The temperature profiles for the experiments at 945 K, 1050 K, 1155 K, 1230 K are presented in Figure 3-34.
1600
1200
800 Temperature [K] 400
0 0 0.05 0.1 0.15 0.2 Axial reactor coordinate [m]
Figure 3-34 Gas temperature profiles for the methyl cyclohexane pyrolysis experiments along the axial reactor coordinate, corresponding to set temperatures of 945 K (—), 1050 K (—), 1155 K (—), 1230 K (—).
In these experiments, the uncertainties of the measured mole fractions are estimated to be ±10% for hydrogen and methane, and 20% for the other species. The experimental uncertainties of the total carbon and hydrogen balances are around ±10% compared to inlet fluxes of carbon and hydrogen elements.
70 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3.5.2 Kinetic model development
The model for methyl cyclohexane pyrolysis is constructed in a hierarchical way. In a first step, a sub mechanism for the initial reactions of methyl cyclohexane towards (substituted) C7 species is constructed manually. Radical attack on methyl cyclohexane can abstract hydrogen atoms from five different carbon atoms. Rate coefficients for hydrogen centered H-abstractions from methyl cyclohexane are taken from Wang et al.20, who computed these pathways with the CBS-QB3 method. No quantum chemical rate coefficients are available for the carbon centered H- abstractions, hence, these are determined via the group additivity scheme from Sabbe et al.86 Subsequent C-C `-scissions of the resulting methyl cyclohexyl radicals, which open the ring, are also referred to CBS-QB3 calculations of Wang et al.20
The temperature- and pressure-dependent kinetics of unimolecular reactions of methyl cyclohexane, including its dissociation and ring-opening isomerization were computed by Zhang et al.67 with high-level quantum chemical calculations and Rice−Ramsperger− Kassel−Marcus/master equation simulations. An overview of the reactions for the unimolecular and bimolecular initial decomposition channels of methyl cyclohexane and their rate coefficients are listed in Table 3-8.
Chapter 3. Cyclohexane as Reference Component for Naphthenes 71
Table 3-8 Summary of the initial unimolecular and bimolecular reactions in the pyrolysis of methyl cyclohexane. Note: E = F GH IJK −L MG , with units mole, nop, s, kJ. An overview of the species in this table is given in the List of Species.
No. Reaction A n E Ref. Comment
H-abstractions 1. MCH+H=MCHR1+H2 4.53E+06 2.11 17.15 20[20].. CBS-QB3 2. MCH+H=MCHR2+H2 2.08E+07 2.10 27.09 20[20].. CBS-QB3 3. MCH+H=MCHR3+H2 2.07E+07 2.12 26.95 20[20].. CBS-QB3 4. MCH+H=MCHR4+H2 1.16E+07 2.11 26.52 20[20].. CBS-QB3 5. MCH+H=CYCHEXCH2+H2 6.66E+06 2.13 37.22 20[20].. CBS-QB3 6. MCH+CH3=MCHR1+CH4 5.66E+12 0.0 52.40 86[86].. GAV 7. MCH+CH3=MCHR2+CH4 3.09E+13 0.0 62.50 86[86].. GAV 8. MCH+CH3=MCHR3+CH4 3.07E+13 0.0 62.50 86[86].. CAV 9. MCH+CH3=MCHR4+CH4 1.54E+13 0.0 62.50 86[86].. GAV 10. MCH+CH3=CYCHEXCH2+CH4 2.83E+13 0.0 72.20 86[86].. GAV
Ring-opening isomerization 11. MCH = C7H14-1 5.57E+69 -15.49 507.02 67[67].. RRKM 12. MCH = C7H14-2 9.66E+68 -15.21 496.81 67[67].. RRKM 13. MCH = CH3-2-1C6H11 3.27E+74 -16.83 527.02 67[67].. RRKM 14. MCH = CH3-5-1C6H11 2.89E+74 -16.83 529.99 67[67].. RRKM 15. MCH = CH3-3-1C6H11 3.71E+73 -16.58 527.85 67[67].. RRKM
C-C scission 16. MCH = CYC6H11+CH3 5.93E+64 -14.15 453.92 67[67].. RRKM
H-shift 17. CYCHEXCH2=MCHR3 2.30E+07 1.02 120.03 20[20].. CBS-QB3 18. CYCHEXCH2=MCHR4 6.00E+07 0.92 94.98 20[20].. CBS-QB3
#-scission 19. CYCHEXCH2=1-C7H13-7 1.56E+29 -5.39 98.37 20[20].. CBS-QB3 20. MCHR1=2-CH3-C6H10 3.46E+31 -6.06 131.16 20[20].. CBS-QB3 21. MCHR2=3-CH3-C6H10 1.01E+28 -5.23 118.29 20[20].. CBS-QB3 22. MCHR2=2-C7H13-7 1.44E+28 -5.21 116.68 20[20].. CBS-QB3 23. MCHR2=CYC6H10+CH3 2.27E+28 -5.30 123.73 20[20].. CBS-QB3 24. MCHR3=4-CH3-C6H10 5.52E+18 -5.31 121.2 20[20].. CBS-QB3 25. MCHR3=1-C7H13-6 3.29E+28 -5.27 118.30 20[20].. CBS-QB3 26. MCHR4=4-CH3-C6H10 2.59E+32 -6.32 133.97 20[20].. CBS-QB3
These initial decomposition reactions of methyl cyclohexane all yield linear or substituted C7 products. Generation of a pyrolysis mechanism for these products is done with the use of Genesys. The procedure for constructing such a mechanism with Genesys has been explained in Chapter 2.
The adopted reaction families and the source of the kinetic data are shown in Table 3-9. This C7 pyrolysis model contains 7687 reversible reactions between 3842 species. 72 Chapter 3. Cyclohexane as Reference Component for Naphthenes
Table 3-9 Reaction families used for the construction of the C7 pyrolysis model.
Kinetic Reaction family Example reaction Source Hydrogen transfer
Intermolecular H-abstraction 86[86]..
Intramolecular H-abstraction 87[87]..
#-scission/Radical addition Intermolecular carbon radical 88[88].. addition Intermolecular hydrogen 89[89].. radical addition Intramolecular carbon radical 43,90[43],[90]…. addition
Bond dissociation/Radical recombination
C-C recombination 91[86],[91].
C-H recombination 91[91]...
Diels-Alder
Diels-Alder Cyclization 90[90]..
The sub mechanism for the initial decomposition of methyl cyclohexane and the mechanism for the pyrolysis of the resulting C7-species are both merged with the AramcoMech2.092,93 mechanism. This mechanism was developed at the National University of Ireland, Galway, and characterizes the kinetic and thermochemical properties of a large number of C1-C4 based hydrocarbon and oxygenated fuels over a wide range of experimental conditions. The Sharma-Green mechanism94 is used to model the reactions of aromatic species. The final model contains 8333 reversible reactions between 4113 species.
3.5.3 Wang experiments: results and discussion
This section discusses the performance of the developed methyl cyclohexane pyrolysis model and compares it to mechanisms found in literature, more specifically by Wang et al.20 and Orme et al.62 The initial decomposition pathways of cyclohexane and the formation routes of aromatics are analyzed by means of a rate of production and sensitivity analysis.
Chapter 3. Cyclohexane as Reference Component for Naphthenes 73
3.5.3.1 Model performance and comparison to literature models
The developed kinetic model, as described in the previous section, is used to simulate experimental data obtained by Wang et al.20 at a pressure of 1 bar and in a temperature range of 900 K – 1250 K. The same is done for models developed by Orme et al.62 and Wang et al.20 Simulations are performed with the Plug Flow Code in the Chemkin-PRO software package.
The model simulated mole fraction of methyl cyclohexane as a function of temperature is compared with the experimental data in Figure 3-35. The LCT model (full red line in Figure 3-35) reproduces the experimental mole fractions adequately, only slightly under predicting the methyl cyclohexane yield at higher temperatures. The prediction of the methyl cyclohexane mole fraction by the models from Wang et al. and Orme et al. are shown in black dotted and striped lines. Both models predict a too fast consumption of methyl cyclohexane with respect to the temperature.
2.0
1.5
1.0
Yield [mole %] 0.5
0.0 900 1000 1100 1200 1300 Temperature [K] Figure 3-35 Experimental (symbols) and simulated (lines) mole fraction of methyl cyclohexane as function of temperature. Simulations are done with the PFR code in CHEMKIN PRO and the models used are: LCT (—), 20 62 Ak Wang et al. (····), Orme et al. ( ̵ ̵ ̵.). P=1.0 bar, HdedfghIJiHI=6.82 x Oj mole/s. The error bars represent the confidence intervals as determined by Wang et al.20
The mole fraction profiles of the major pyrolysis products are shown as function of conversion in Figure 3-36. All of the initial unimolecular decomposition products, 1-heptene, 2-heptene, and methyl and cyclohexyl radical, were below the limit of detectability at 1 bar in the work of Wang et al.19 and their mole fraction profiles are therefore not shown here. The permanent gases, hydrogen and methane, exhibit a monotonous increasing mole fraction upon rising conversion. The latter species is simulated accurately but the slope in the simulated hydrogen mole fraction profile is lower than the experimentally observed one in all models, thereby under predicting the hydrogen yield at higher temperatures. The maximum ethylene and acetylene yields are significantly over predicted with all mechanisms. In the investigated temperature range, the yields of propylene and 1,3-butadiene reach a maximum at around a conversion of 80%. The mole fraction of propylene agrees quite well with the experimental data, but the 1,3-butadiene yield is largely underestimated. 74 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3.0 1.5 H2 2.5 CH4 1.2 2.0 0.9 1.5 0.6 1.0 Yield [mole %] Yield [mole %] 0.5 0.3
0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-]
3.0 1 C2H4 C2H2 2.5 0.8 2.0 0.6 1.5 0.4 1.0 Yield [mole %] Yield [mole %] 0.2 0.5
0.0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] 0.4 0.8 C3H6 C4H6 0.3 0.6
0.2 0.4 Yield [mole %] Yield [mole %] 0.1 0.2
0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] Figure 3-36 Experimental (symbols) and simulated (lines) mole fractions of methyl cyclohexane and its major pyrolysis products as function of conversion. Simulations are done with the PFR code in CHEMKIN PRO and the 20 62 Ak models used are: LCT (—), Wang et al. (····), Orme et al. ( ̵ ̵ ̵.). P=1.0 bar, HdedfghIJiHI=6.82 x Oj mole/s. The error bars represent the confidence intervals as determined by Wang et al.20
Figure 3-37 shows the measured and simulated mole fractions of benzene, toluene and cyclopentadiene. No experimental data was reported for styrene. The benzene yield is very accurately predicted, but the toluene and cyclopentadiene yields are considerably under and overestimated respectively. It is noticeable that, although methyl cyclohexane seems an ideal precursor for toluene formation, the yield of toluene is still an order of magnitude smaller than that of benzene. This already gives an indication of the reaction paths involved in aromatics formation. Chapter 3. Cyclohexane as Reference Component for Naphthenes 75
0.24 0.06 Benzene Toluene 0.2 0.05
0.16 0.04
0.12 0.03
Yield [mole %] 0.08 0.02 Yield [mole %] 0.04 0.01
0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] 0.2 CPD 0.16
0.12
0.08 Yield [mole %] 0.04
0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Figure 3-37 Experimental (symbols) and simulated (lines) mole fractions of main aromatics and cyclopentadiene in methyl cyclohexane pyrolysis as function of conversion. Simulations are done with PFR in CHEMKIN PRO and the model used are: LCT (—), Wang et al.20 (····), Orme et al.62 ( ̵ ̵ ̵.). P=1.0 bar, Ak HdedfghIJiHI=6.82 x Oj mole/s. The error bars represent the confidence intervals as determined by Wang et al.20
3.5.3.2 Rate of production analysis for methyl cyclohexane consumption
As shown in Figure 3-38, three major consumption pathways exist for methyl cyclohexane at these conditions: C-C scission, H-abstraction and ring-opening isomerization. The percentages represent the fraction of methyl cyclohexane that is consumed by the respective paths, both near the entrance and center of the reactor at 1050 K and 1185 K. The entrance is considered in view of comparing the initial chemistry of cyclohexane and methyl cyclohexane pyrolysis. In Figure 3-38, a distinction is made between the formed methyl cyclohexyl radicals as well. There is a clear preference for H-abstraction reactions at higher temperatures and at longer distances in the reactor. This is expected because these two conditions give rise to an increased radical concentration. The main abstracting species are hydrogen atoms and methyl radicals. 76 Chapter 3. Cyclohexane as Reference Component for Naphthenes
39 1185 K, 20 cm 1050 K, 20 cm 38 37 36 35 1185 K, 5 cm 1050 K, 5 cm 34 33 32 9 31 8 30 29 7 28 6 27 26 5 25 4 24 23 3 22 2 21 1 20 19 18 0 10 20 30 40 50 17 16 Rate of consumption [%] 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 20 40 60 80 100 Rate of consumption [%] Figure 3-38 Rate of production analysis of the initial decomposition pathways of methyl cyclohexane at different temperatures and positions in the reactor. Simulations are done with the PFR code in CHEMKIN PRO. 1050 K, 5 cm (full black); 1050 K, 20 cm (dotted); 1185 K, 5 cm (sloped, stripped); 1185 K, 20 cm (horizontal striped). P=1.0 bar. The percentage shows the fraction of cyclohexane consumed by the corresponding reactions.
According to the quantum chemical calculations performed by Zhang et al.67, 2-heptene is the dominant isomerization product of methyl cyclohexane, which is also confirmed in the ROP analysis. Note that the importance of the ring-opening isomerization of methyl cyclohexane is clearly reduced compared to that of cyclohexane (Figure 3-14). When no radicals are present in the system, i.e. very close to the entrance of the reactor, cyclohexane can only undergo a C-C scission between carbon atoms located in the ring, resulting in a diradical species that is quickly converted through an isomerization reaction to 1-hexene. In contrast, methyl cyclohexane disposes of a C-C bond that can break without resulting in a diradical species, but resulting in cyclohexyl and methyl radical. Hence, this C-C scission is in competition with the ring-opening isomerization of methyl cyclohexane. The rate coefficients at 1000 K, 1100 K and 1200 K of the C- C scission in cyclohexane and of the C-C scission between the methyl substituent and the ring in methyl cyclohexane are presented in Table 3-10. The number of single events has been taken into account. At all temperatures, the rate coefficient for the C-C scission that results in methyl and cyclohexyl radical is higher than that of the C-C scission in cyclohexane. A possible explanation is that through the latter reaction two primary radicals are formed, while via the former reaction one secondary and one primary radical are formed.
Chapter 3. Cyclohexane as Reference Component for Naphthenes 77
Table 3-10 Rate coefficients for C-C scissions in cyclohexane and methyl cyclohexane at 1000 K – 1200 K. P=1.0 bar.
-1 Reaction k [s ] 800 K 900 K 1000 K
Cyclohexane
5.60E-03 1.36E-01 3.34E+00 Methyl cyclohexane
4.09E-02 1.52E+00 2.78E+01
Figure 3-38 displays that all radical species resulting from H-abstraction of methyl cyclohexane are formed in considerable amounts, 1-methyl-2-cyclohexyl and 1-methyl-3-cyclohexyl radical being the dominant radical products. Despite being a tertiary radical, 1-methyl-1-cyclohexyl radical is formed to a lesser extent because of lower number of single events. Cyclohexyl as well is an important species as it is formed through the major consumption pathway of methyl cyclohexane near the entrance of the reactor. A reaction path analysis at a temperature of 1185 K (center of the reactor) starting from these main radical species is shown in Figure 3-39.
The ring opening of cyclohexyl radical (Figure 3-39a) forms hex-5-en-1-yl radical, which undergoes an internal 1,4 H-shift, forming the more stable allylic hex-1-en-3-yl radical, or direct C-C `-scission. Through further `-scissions of these respective radical species, smaller products, such as 1,3-butadiene and ethylene are formed.
1-methyl-2-cyclohexyl radical is mainly consumed through a C-C `-scission that results in linear hept-2-en-7-yl radical, which subsequently undergoes an intramolecular H-shift towards the formation of conjugated pentadiene, or reacts to form ethylene and pent-2-en-5-yl radical. The latter undergoes an intramolecular H-abstraction yielding a more stable allylic radical that is converted to 1,3-butadiene through another C-C `-scission. 3-methyl-1-hexen-6-yl is another `- scission product from 1-methyl-2-cyclohexyl radical. This species reacts further through another direct C-C `-scission or an intramolecular H-shift. Subsequent `-scissions result in the formation of smaller olefinic species such as ethylene and 1,3-butadiene. A minor fraction of the 1-methyl- 2-cyclohexyl radical reacts to form cyclohexene and methyl radical. The reactions of 1-methyl-3- cyclohexyl radical are very similar to those of 1-methyl-2-cyclohexyl radical. They mainly consist out of consecutive C-C `-scissions potentially preceded by an intramolecular H-shift to yield a more stable radical. The main light olefin species formed from this radical are again ethylene and 1,3-butadiene, but propylene is formed as well.
78 Chapter 3. Cyclohexane as Reference Component for Naphthenes
Figure 3-39 Rate of production analysis of the ring-opening of cyclohexyl radical (a), 1-methyl-2-cyclohexyl radical (b) and 1-methyl-3-cyclohexyl radical (c). Simulations are done with the PFR code in CHEMKIN PRO. T=1185 K, P=1.0 bar, center of the reactor. The percentages reflect the molar flux of the corresponding pathway divided by the total molar flux of methyl cyclohexane consumption. The starting species and major experimentally detected product species are highlighted. The dominant routes are indicated with bold arrows.
Chapter 3. Cyclohexane as Reference Component for Naphthenes 79
Note from Figure 3-39 that in some cases larger radicals, such as hexyl or 4-methyl-1-hexen-1-yl radical, prefer to undergo C-C `-scission, yielding ethylene, rather than undergo intramolecular hydrogen shift towards more stable, i.e. allylic or secondary, radicals. This too high tendency for a direct C-C `-scission can be one possible explanation for the overestimation of the ethylene yield.
Additionally, a sensitivity analysis of methyl cyclohexane, performed under the same conditions as the ROP analysis (i.e. P=1.0 bar, T=1185 K, center of the reactor) highlights the large negative sensitivity of the conversion of methyl cyclohexane to cyclohexyl and methyl radical (Figure 3-40). That is, increasing the pre-exponential factor of this reaction causes methyl cyclohexane to be consumed significantly faster. Since the conversion is also predicted adequately (Figure 3-35), it is evident that this is a very important reaction in the decomposition chemistry of methyl cyclohexane. The large negative sensitivity of this reaction is not only due to its direct consumption of methyl cyclohexane, but also because it produces methyl radicals, which are one of the major species that partake in H-abstraction reactions. These H-abstractions in turn also have negative sensitivities because they directly consume methyl cyclohexane. Reactions that result in the formation of hydrogen atoms (not shown in Figure 3-40 due to too low sensitivity) also hold a positive sensitivity. For the opposite reason, reactions that consume important H- abstracting radicals, such as the consumption of methyl radicals through H-abstractions from cyclopentadiene, possess negative sensitivities. The recombination of methyl radicals to form ethane has an adverse effect as well, because it withdraws radicals from the system.
-0.80 -0.70 -0.60 -0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 Normalized sensitivity coefficient [-] Figure 3-40 Sensitivity analysis with respect to the methyl cyclohexane mole fraction. Simulations are done with the PFR code in CHEMKIN PRO. T=1185 K, P=1.0 bar, center of the reactor. 80 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3.5.3.3 Rate of production analysis for the formation of aromatics
Although the benzene yield is predicted adequately, the mole fraction profile of toluene is still heavily underestimated. The goal of this section is to analyze the important reactions that lead to benzene formation and to identify potentially missing pathways in the formation of toluene through a rate of production analysis.
The ROP of benzene formation is shown in Figure 3-41. Only a very small amount of the cyclic • CqHB\ radicals undergo successive H-abstraction and C-H `–scission resulting in the formation of benzene and toluene (<0.2%, not shown in Figure 3-41). The main pathways however, include Diels-Alder cyclization, propargyl recombination and H-assisted fulvene isomerization, similar to the cyclohexane pyrolysis case. The reactions of the resonantly stabilized propargyl radical contribute to about 9% of the total benzene formation, taking into account that fulvene is also the product of propargyl recombination. These smaller radicals are not abundantly present from the start. This is reflected in an increase of the total rate of production of benzene from 5.2 x 10A] mole cmA\ sAB, 12 cm in the reactor, to a maximum of 4.5 x 10Aq mole cmA\ sAB at 16 cm along the axial reactor coordinate. Multiple authors20,62,65 report a larger importance of the dehydrogenation of cyclohexadienyl radical, than observed in this work, be it formed through successive dehydrogenation of through reactions of smaller molecules. For further use, it is advised to revise these pathways.
Figure 3-41 Rate of production analysis of benzene formation. T=1185 K, P=1.0 bar, center of the reactor. The percentages reflect the fraction of benzene formed through the respective pathways.
Figure 3-42 presents a rate of production analysis for toluene formation at 1185 K, 1 bar and in the center of the reactor. In this figure, the percentages reflect the fraction of toluene formed through the respective pathways and not relative to the total flux of methyl cyclohexane consumption because these values are very low. At 1185 K and in the center of the reactor, only 1.24 x 10AV % of methyl cyclohexane is converted to toluene on a molar basis. The main pathways to toluene include the H-abstraction of phenyl radical from cyclopentadiene and the recombination reaction of phenyl radical and hydrogen atoms. The reaction of benzyl radical with methyl radical is also a large contributor to the total toluene formation. Smaller amounts of Chapter 3. Cyclohexane as Reference Component for Naphthenes 81
toluene are formed from the reaction of benzene with methyl radical or the H-abstraction of phenyl radical from methane. Phenyl radical itself is almost for 100% the result of intramolecular carbon radical addition reactions of linear C7 alkenyl species (not shown in Figure 3-42). A possible explanation for the under predicted mole fraction of toluene can be that too few of these
C7 species are formed and it is therefore advised to investigate this route further.
Figure 3-42 Rate of production analysis of toluene formation. T=1185 K, P=1.0 bar, center of the reactor. The percentages reflect the fraction of toluene formed through the respective pathways.
82 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3.6 Conclusion
In this chapter, the pyrolysis of monocyclic hydrocarbons, specifically cyclohexane and methyl cyclohexane, has been discussed. The necessity of insight into the pyrolysis behavior of these components is evident because they are present in significant amounts in lignocellulose-derived steam cracker feedstock. In order to understand their reactivity, i.e. initial decomposition, aromatics formation, etc., a literature survey comprising a summary of the existing experimental and kinetic modeling work is reported.
The in-house developed kinetic model for cyclohexane pyrolysis is used to simulate experiments performed at the LCT. The model calculated mole fraction profiles of cyclohexane and its pyrolysis products are in very good agreement with the LCT experiments. These experiments were performed with non-diluted cyclohexane in a plug flow reactor at 1.7 bar and in a temperature range of 913 K – 1073 K. A sensitivity analysis at the beginning of the reactor and at different temperatures shows the importance of the ring-opening isomerization of cyclohexane to 1- hexene, especially at lower temperatures.
The applicability of the in-house developed model towards other conditions is also tested through reproduction of experimental results obtained by Wang et al. at 40 mbar, 950 K - 1520 K and high dilution. Here, the simulation of light pyrolysis products like methane, ethylene and 1,3-butadiene is acceptable, but the yield of aromatics is highly underestimated. It is found that several important pathways, such as reactions involving species with three carbon atoms (e.g. propargyl recombination), to benzene, toluene and other aromatics are missing in the LCT model to perform well at low pressures. After altering the model with these reactions, the prediction of the mole fractions of the main aromatic species is satisfactory. A rate of production analysis with the adapted model confirms the importance of the C3+C3 reactions. Since most of the C3 species originate from 1-hexene, this again points to the relevance of the ring-opening isomerization, as was also emphasized through a sensitivity analysis for benzene.
Additionally, a microkinetic model for methyl cyclohexane pyrolysis is constructed in a hierarchical way. The initial reactions of methyl cyclohexane are added manually and a kinetic model for the resulting C7 species is developed using Genesys. The model is validated against experimental data obtained by Wang et al.20 for methyl cyclohexane in high dilution, a temperature range of 842 K – 1250 K and at low pressure (40 mbar). The model performance of the LCT model and the models by Orme et al.62 and Wang et al.20 is ascertained for this set of experimental data. Chapter 3. Cyclohexane as Reference Component for Naphthenes 83
The LCT model very accurately, and better than the other mechanisms, predicts the consumption of methyl cyclohexane. The yield of one of the main species, 1,3-butadiene is underestimated, as is that of toluene. All models considerably over predict the ethylene mole fraction. The yield of the main aromatic species, benzene, matches well the experimental data. A rate of production analysis is used to identify the important, and also missing, reaction paths. Although several adaptations have already been made to the model, it should still be further improved, especially with respect to toluene and 1,3-butadiene.
These simulation results clearly show the limitations in the presently available kinetic models and indicate that accurate experimental data as well as accurately calculated rate coefficients are crucial to improve the kinetic models developed for the pyrolysis of cyclic compounds. Ring- opening isomerizations have proven to be important, but since a transition state for these reactions is very hard to find, accurate ab initio rate coefficients are scarce. The formation of aromatics also proves to be challenging and different routes are important at different pressures.
84 Chapter 3. Cyclohexane as Reference Component for Naphthenes
3.7 References
1. Pavone A, Cesar M. Advances in naphtha steam cracking. Process Economics Program Report 248A 2005.
2. Ren T, Patel M, Blok K. Olefins from conventional and heavy feedstocks: Energy use in steam cracking and alternative processes. Energy. 3// 2006;31(4):425-451.
3. Taylor AMKP. Science review of internal combustion engines. Energy Policy. 12// 2008;36(12):4657-4667.
4. Costa ID, Fournet R, Billaud F, Battin-Leclerc F. Experimental and modeling study of the oxidation of benzene. International Journal of Chemical Kinetics. 2003;35(10):503-524.
5. Richter H, Howard JB. Formation and consumption of single-ring aromatic hydrocarbons and their precursors in premixed acetylene, ethylene and benzene flames. Physical Chemistry Chemical Physics. 2002;4(11):2038-2055.
6. Dagaut P, Pengloan G, Ristori A. Oxidation, ignition and combustion of toluene: Experimental and detailed chemical kinetic modeling. Physical Chemistry Chemical Physics. 2002;4(10):1846-1854.
7. Sivaramakrishnan R, Tranter RS, Brezinsky K. A high pressure model for the oxidation of toluene. Proceedings of the Combustion Institute. 1// 2005;30(1):1165-1173.
8. Dagaut P, Ristori A, El Bakali A, Cathonnet M. Experimental and kinetic modeling study of the oxidation of n-propylbenzene. Fuel. 1// 2002;81(2):173-184.
9. Simmie JM. Detailed chemical kinetic models for the combustion of hydrocarbon fuels. Progress in energy and combustion science. 2003;29(6):599-634.
10. Sirjean B, Buda F, Hakka H, et al. The autoignition of cyclopentane and cyclohexane in a shock tube. Proceedings of the Combustion Institute. 1// 2007;31(1):277-284.
11. Renders T, Schutyser W, Van den Bosch S, et al. Influence of acidic (H3PO4) and alkaline (NaOH) additives on the catalytic reductive fractionation of lignocellulose. ACS Catalysis. 2016;6(3):2055-2066.
12. Van den Bosch S, Schutyser W, Vanholme R, et al. Reductive lignocellulose fractionation into soluble lignin-derived phenolic monomers and dimers and processable carbohydrate pulps. Energy & Environmental Science. 2015;8(6):1748-1763.
13. Curran HJ, Gaffuri P, Pitz W, Westbrook C. A comprehensive modeling study of iso- octane oxidation. Combustion and flame. 2002;129(3):253-280.
14. Dagaut P, Reuillon M, Cathonnet M. High pressure oxidation of liquid fuels from low to high temperature. 1. n-Heptane and iso-Octane. Combustion Science and Technology. 1993;95(1-6):233-260. Chapter 3. Cyclohexane as Reference Component for Naphthenes 85
15. Ranzi E, Faravelli T, Gaffuri P, Sogaro A, D'Anna A, Ciajolo A. A wide-range modeling study of iso-octane oxidation. Combustion and Flame. 1997;108(1):24-42.
16. Agosta A, Cernansky N, Miller D, Faravelli T, Ranzi E. Reference components of jet fuels: kinetic modeling and experimental results. Experimental Thermal and Fluid Science. 2004;28(7):701-708.
17. Malewicki T, Brezinsky K. Experimental and modeling study on the pyrolysis and oxidation of n-decane and n-dodecane. Proceedings of the Combustion Institute. 2013;34(1):361-368.
18. You X, Egolfopoulos FN, Wang H. Detailed and simplified kinetic models of n- dodecane oxidation: The role of fuel cracking in aliphatic hydrocarbon combustion. Proceedings of the Combustion Institute. 2009;32(1):403-410.
19. Wang Z, Cheng Z, Yuan W, et al. An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure. Combustion and Flame. 7// 2012;159(7):2243-2253.
20. Wang Z, Ye L, Yuan W, et al. Experimental and kinetic modeling study on methylcyclohexane pyrolysis and combustion. Combustion and Flame. 2014;161(1):84-100.
21. Tsang W. Thermal stability of cyclohexane and 1 hexene. International Journal of Chemical Kinetics. 1978;10(11):1119-1138.
22. Brown TC, King KD, Nguyen TT. Kinetics of primary processes in the pyrolysis of cyclopentanes and cyclohexanes. The Journal of Physical Chemistry. 1986;90(3):419-424.
23. Aribike D, Susu A, Ogunye A. Mechanistic and mathematical modeling of the thermal decompostion of cyclohexane. Thermochimica Acta. 1981;51(2):113-127.
24. Voisin D, Marchal A, Reuillon M, Boettner JC, Cathonnet M. Experimental and Kinetic Modeling Study of Cyclohexane Oxidation in a JSR at High Pressure. Combustion Science and Technology. 1998/09/01 1998;138(1-6):137-158.
25. Klai S, Baronnet F. Study of the Homogeneous Gas-Phas Oxidation of Cyclohexane. Part 1. Experimental Study. Journal de chimie physique et de physico-chimie biologique 1993;90(10):1929-1950.
26. El Bakali A, Braun-Unkhoff M, Dagaut P, Frank P, Cathonnet M. Detailed kinetic reaction mechanism for cyclohexane oxidation at pressure up to ten atmospheres. Proceedings of the Combustion Institute. 2000/01/01 2000;28(2):1631-1638.
27. Granata S, Faravelli T, Ranzi E. A wide range kinetic modeling study of the pyrolysis and combustion of naphthenes. Combustion and Flame. 2// 2003;132(3):533-544.
28. Silke EJ, Pitz WJ, Westbrook CK, Ribaucour M. Detailed Chemical Kinetic Modeling of Cyclohexane Oxidation. The Journal of Physical Chemistry A. 2007/05/01 2007;111(19):3761-3775. 86 Chapter 3. Cyclohexane as Reference Component for Naphthenes
29. Wang H, Dames E, Sirjean B, et al. JetSurF version 2.0. A High-Temperature Chemical Kinetic Model of n-alkane (up to n-dodecane), cyclohexane, and methyl-, ethyl-, n-propyl, and n-butyl-cyclohexane Oxidation at High Temperatures. 2010.
30. Zeelenberg AP, de Bruijn HW. Kinetics, mechanism and products of the gaseous oxidation of cyclohexane. Combustion and Flame. 1965/09/01 1965;9(3):281-295.
31. Bonner BH, Tipper CFH. Cool-flame combustion of hydrocarbons. Symposium (International) on Combustion. 1965/01/01 1965;10(1):145-150.
32. Gulati SK, Walker RW. Addition of cyclohexane to slowly reacting H2-O2 mixtures at 480 [degree]C. Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics. 1989;85(11):1799-1812.
33. Lemaire O, Ribaucour M, Carlier M, Minetti R. The production of benzene in the low-temperature oxidation of cyclohexane, cyclohexene, and cyclohexa-1,3-diene. Combustion and Flame. 10// 2001;127(1–2):1971-1980.
34. Tanaka S, Ayala F, Keck JC, Heywood JB. Two-stage ignition in HCCI combustion and HCCI control by fuels and additives. Combustion and Flame. 1// 2003;132(1– 2):219-239.
35. Daley SM, Berkowitz AM, Oehlschlaeger MA. A shock tube study of cyclopentane and cyclohexane ignition at elevated pressures. International Journal of Chemical Kinetics. 2008;40(10):624-634.
36. Hong Z, Lam K-Y, Davidson DF, Hanson RK. A comparative study of the oxidation characteristics of cyclohexane, methylcyclohexane, and n-butylcyclohexane at high temperatures. Combustion and Flame. 2011;158(8):1456-1468.
37. Kiefer JH, Gupte KS, Harding LB, Klippenstein SJ. Shock Tube and Theory Investigation of Cyclohexane and 1-Hexene Decomposition. The Journal of Physical Chemistry A. 2009/12/03 2009;113(48):13570-13583.
38. Peukert S, Naumann C, Braun-Unkhoff M, Riedel U. Formation of H-atoms in the pyrolysis of cyclohexane and 1-hexene: A shock tube and modeling study. International Journal of Chemical Kinetics. 2011;43(3):107-119.
39. Bennett PJ, Gregory D, Jackson RA. Mechanistic Studies on the Combustion of Isotopically Labelled Cyclohexanes within a Single Cylinder Internal Combustion Engine. Combustion Science and Technology. 1996/06/01 1996;115(1-3):83-103.
40. Yang Y, Boehman AL. Experimental study of cyclohexane and methylcyclohexane oxidation at low to intermediate temperature in a motored engine. Proceedings of the Combustion Institute. // 2009;32(1):419-426.
41. Ciajolo A, Tregrossi A, Mallardo M, Faravelli T, Ranzi E. Experimental and kinetic modeling study of sooting atmospheric-pressure cyclohexane flame. Proceedings of the Combustion Institute. // 2009;32(1):585-591. Chapter 3. Cyclohexane as Reference Component for Naphthenes 87
42. Ji C, Dames E, Sirjean B, Wang H, Egolfopoulos FN. An experimental and modeling study of the propagation of cyclohexane and mono-alkylated cyclohexane flames. Proceedings of the Combustion Institute. // 2011;33(1):971-978.
43. Wang K, Villano SM, Dean AM. Fundamentally-based kinetic model for propene pyrolysis. Combustion and Flame. 2015;162(12):4456-4470.
44. Sirjean B, Glaude PA, Ruiz-Lopez MF, Fournet R. Detailed Kinetic Study of the Ring Opening of Cycloalkanes by CBS-QB3 Calculations. The Journal of Physical Chemistry A. 2006/11/01 2006;110(46):12693-12704.
45. Tian Z, Zhang Y, Yang F, Pan L, Jiang X, Huang Z. Comparative Study of Experimental and Modeling Autoignition of Cyclohexane, Ethylcyclohexane, and n- Propylcyclohexane. Energy & Fuels. 2014/11/20 2014;28(11):7159-7167.
46. Ranzi E, Faravelli T, Gaffuri P, Garavaglia E, Goldaniga A. Primary Pyrolysis and Oxidation Reactions of Linear and Branched Alkanes. Industrial & Engineering Chemistry Research. 1997/08/01 1997;36(8):3336-3344.
47. Ranzi E, Dente M, Goldaniga A, Bozzano G, Faravelli T. Lumping procedures in detailed kinetic modeling of gasification, pyrolysis, partial oxidation and combustion of hydrocarbon mixtures. Progress in Energy and Combustion Science. 2001;27(1):99-139.
48. Wang H, Dames E, Sirjean B, et al. A high-temperature chemical kinetic model of n- alkane (up to n-dodecane), cyclohexane, and methyl-, ethyl-, n-propyl and n-butyl- cyclohexane oxidation at high temperatures. JetSurF version. 2010;2:19.
49. Hansen N, Miller JA, Klippenstein SJ, Westmoreland PR, Kohse-Höinghaus K. Exploring formation pathways of aromatic compounds in laboratory-based model flames of aliphatic fuels. Combustion, Explosion, and Shock Waves. 2012;48(5):508- 515.
50. Law ME, Westmoreland PR, Cool TA, et al. Benzene precursors and formation routes in a stoichiometric cyclohexane flame. Proceedings of the Combustion Institute. 2007;31(1):565-573.
51. Li W, Law ME, Westmoreland PR, Kasper T, Hansen N, Kohse-Höinghaus K. Multiple benzene-formation paths in a fuel-rich cyclohexane flame. Combustion and Flame. 2011;158(11):2077-2089.
52. Hansen N, Miller JA, Kasper T, et al. Benzene formation in premixed fuel-rich 1, 3- butadiene flames. Proceedings of the Combustion Institute. 2009;32(1):623-630.
53. Miller JA, Melius CF. Kinetic and thermodynamic issues in the formation of aromatic compounds in flames of aliphatic fuels. Combustion and flame. 1992;91(1):21-39.
54. Miller JA, Klippenstein SJ. The Recombination of Propargyl Radicals and Other Reactions on a C6H6 Potential. The Journal of Physical Chemistry A. 2003/10/01 2003;107(39):7783-7799. 88 Chapter 3. Cyclohexane as Reference Component for Naphthenes
55. Li Y, Zhang L, Tian Z, et al. Experimental study of a fuel-rich premixed toluene flame at low pressure. Energy & Fuels. 2009;23(3):1473-1485.
56. Cavallotti C, Derudi M, Rota R. On the mechanism of decomposition of the benzyl radical. Proceedings of the Combustion Institute. 2009;32(1):115-121.
57. Tian Z, Pitz WJ, Fournet R, Glaude P-A, Battin-Leclerc F. A detailed kinetic modeling study of toluene oxidation in a premixed laminar flame. Proceedings of the Combustion Institute. 2011;33(1):233-241.
58. Zeppieri S, Brezinsky K, Glassman I. Pyrolysis studies of methylcyclohexane and oxidation studies of methylcyclohexane and methylcyclohexane/toluene blends. Combustion and Flame. 1997/02/01 1997;108(3):266-286.
59. Bieleveld T, Frassoldati A, Cuoci A, et al. Experimental and kinetic modeling study of combustion of gasoline, its surrogates and components in laminar non- premixed flows. Proceedings of the Combustion Institute. 2009;32(1):493-500.
60. Orme JP, Curran HJ, Simmie JM. Experimental and modeling study of methyl cyclohexane pyrolysis and oxidation. The journal of physical chemistry. A. Jan 12 2006;110(1):114-131.
61. Pitz WJ, Naik CV, Mhaoldúin TN, et al. Modeling and experimental investigation of methylcyclohexane ignition in a rapid compression machine. Proceedings of the Combustion Institute. 1// 2007;31(1):267-275.
62. Orme JP, Curran HJ, Simmie JM. Experimental and modeling study of methyl cyclohexane pyrolysis and oxidation. J. Phys. Chem. A. Jan 2006;110(1):114-131.
63. Vasu SS, Davidson DF, Hong Z, Hanson RK. Shock Tube Study of Methylcyclohexane Ignition over a Wide Range of Pressure and Temperature. Energy & Fuels. 2009/01/22 2009;23(1):175-185.
64. Vanderover J, Oehlschlaeger MA. Ignition time measurements for methylcylcohexane- and ethylcyclohexane-air mixtures at elevated pressures. International Journal of Chemical Kinetics. 2009;41(2):82-91.
65. Skeen SA, Yang B, Jasper AW, Pitz WJ, Hansen N. Chemical Structures of Low- Pressure Premixed Methylcyclohexane Flames as Benchmarks for the Development of a Predictive Combustion Chemistry Model. Energy & Fuels. 2011/12/15 2011;25(12):5611-5625.
66. Wu F, Kelley AP, Law CK. Laminar flame speeds of cyclohexane and mono-alkylated cyclohexanes at elevated pressures. Combustion and Flame. 2012;159(4):1417- 1425.
67. Zhang F, Wang Z, Wang Z, Zhang L, Li Y, Qi F. Kinetics of Decomposition and Isomerization of Methylcyclohexane: Starting Point for Studying Monoalkylated Cyclohexanes Combustion. Energy & Fuels. 2013/03/21 2013;27(3):1679-1687. Chapter 3. Cyclohexane as Reference Component for Naphthenes 89
68. Tsang W. Chemical Kinetic Data Base for Combustion Chemistry. Part 3: Propane. Journal of Physical and Chemical Reference Data. 1988;17(2):887-951.
69. Sirjean B, Glaude PA, Ruiz-Lopèz MF, Fournet R. Theoretical Kinetic Study of Thermal Unimolecular Decomposition of Cyclic Alkyl Radicals. The Journal of Physical Chemistry A. 2008/11/20 2008;112(46):11598-11610.
70. Narayanaswamy K, Pitsch H, Pepiot P. A chemical mechanism for low to high temperature oxidation of methylcyclohexane as a component of transportation fuel surrogates. Combustion and Flame. 2015;162(4):1193-1213.
71. Husson B, Herbinet O, Glaude PA, Ahmed SS, Battin-Leclerc F. Detailed product analysis during low- and intermediate-temperature oxidation of ethylcyclohexane. The journal of physical chemistry. A. May 31 2012;116(21):5100-5111.
72. Wang H, Dames E, Sirjean B, et al. A high-temperature chemical kinetic model of n- alkane (up to n-dodecane), cyclohexane, and methyl-, ethyl-, n-propyl and n-butyl- cyclohexane oxidation. . JetSurF. 2010;Version 2.0.
73. McEnally CS, Pfefferle LD. Fuel decomposition and hydrocarbon growth processes for substituted cyclohexanes and for alkenes in nonpremixed flames. Proceedings of the Combustion Institute. 1// 2005;30(1):1425-1432.
74. Wang Z, Bian H, Wang Y, et al. Investigation on primary decomposition of ethylcyclohexane at atmospheric pressure. Proceedings of the Combustion Institute. 2015;35(1):367-375.
75. Wang Z, Zhao L, Wang Y, et al. Kinetics of ethylcyclohexane pyrolysis and oxidation: An experimental and detailed kinetic modeling study. Combustion and Flame. 2015-07 2014.
76. Klippenstein SJ, Georgievskii Y, Harding LB. Predictive theory for the combination kinetics of two alkyl radicals. Physical Chemistry Chemical Physics. 2006;8(10):1133-1147.
77. Oehlschlaeger MA, Davidson DF, Hanson RK. High-temperature ethane and propane decomposition. Proceedings of the Combustion Institute. 2005;30(1):1119-1127.
78. Stewart PH, Smith GP, Golden DM. The pressure and temperature dependence of methane decomposition. International Journal of Chemical Kinetics. 1989;21(10):923-945.
79. Dokjampa S, Rirksomboon T, Phuong DTM, Resasco DE. Ring opening of 1,3- dimethylcyclohexane on Ir catalysts: Modification of product distribution by addition of Ni and K to improve fuel properties. Journal of Molecular Catalysis A: Chemical. 9/3/ 2007;274(1–2):231-240.
80. Do PT, Alvarez WE, Resasco DE. Ring opening of 1,2- and 1,3-dimethylcyclohexane on iridium catalysts. Journal of Catalysis. 3/10/ 2006;238(2):477-488. 90 Chapter 3. Cyclohexane as Reference Component for Naphthenes
81. González-Cortés SL, Dorkjampa S, Do PT, Li Z, Ramallo-López JM, Requejo FG. Tuning the ring-opening reaction of 1, 3-dimethylcyclohexane with the addition of potassium over Ir-containing catalysts. Chemical Engineering Journal. 2008;139(1):147-156.
82. Kang D, Lilik G, Dillstrom V, et al. Impact of branched structures on cycloalkane ignition in a motored engine: Detailed product and conformational analyses. Combustion and Flame. 4// 2015;162(4):877-892.
83. Sun D, Du Y, Li C, et al. Theoretic heat sink simulation and experimental investigation of the pyrolysis of substituted cyclohexanes. Journal of Energy Chemistry. 1// 2015;24(1):119-125.
84. Eldeeb MA, Jouzdani S, Wang Z, Sarathy SM, Akih-Kumgeh B. Experimental and Chemical Kinetic Modeling Study of Dimethylcyclohexane Oxidation and Pyrolysis. Energy & Fuels. 2016/08/30 2016.
85. Rosado-Reyes CM, Tsang W. Isomerization of cis-1,2-dimethylcyclohexane in single-pulse shock tube experiments. The journal of physical chemistry. A. Sep 11 2014;118(36):7707-7714.
86. Sabbe MK, Vandeputte AG, Reyniers M-F, Waroquier M, Marin GB. Modeling the influence of resonance stabilization on the kinetics of hydrogen abstractions. Physical Chemistry Chemical Physics. 2010;12(6):1278-1298.
87. Sabbe MK, Van Geem KM, Reyniers MF, Marin GB. First principle based simulation of ethane steam cracking. AIChE journal. 2011;57(2):482-496.
88. Sabbe MK, Reyniers MF, Van Speybroeck V, Waroquier M, Marin GB. Carbon Centered Radical Addition and Scission Reactions: Modeling of Activation Energies and Pre exponential Factors. ChemPhysChem. 2008;9(1):124-140.
89. Sabbe MK, Reyniers MF, Waroquier M, Marin GB. Hydrogen Radical Additions to Unsaturated Hydrocarbons and the Reverse Scission Reactions: Modeling of Activation Energies and Pre Exponential Factors. ChemPhysChem. 2010;11(1):195-210.
90. Merchant SS. Molecules to engines: combustion chemistry of alcohols and their application to advanced engines, Massachusetts Institute of Technology; 2015.
91. Mehl M, Vanhove G, Pitz WJ, Ranzi E. Oxidation and combustion of the n-hexene isomers: A wide range kinetic modeling study. Combustion and Flame. 2008;155(4):756-772.
92. Metcalfe WK, Burke SM, Ahmed SS, Curran HJ. A Hierarchical and Comparative Kinetic Modeling Study of C1 − C2 Hydrocarbon and Oxygenated Fuels. International Journal of Chemical Kinetics. 2013;45(10):638-675. Chapter 3. Cyclohexane as Reference Component for Naphthenes 91
93. Kéromnès A, Metcalfe WK, Heufer KA, et al. An experimental and detailed chemical kinetic modeling study of hydrogen and syngas mixture oxidation at elevated pressures. Combustion and Flame. 6// 2013;160(6):995-1011.
94. Sharma S, Harper MR, Green WH. Modeling of 1, 3-hexadiene, 2, 4-hexadiene and 1, 4-hexadiene-doped methane flames: Flame modeling, benzene and styrene formation. Combustion and Flame. 2010;157(7):1331-1345.
4. Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
Chapter 4 Decalin as Reference Component for Polycyclic Alkanes
4.1 Introduction
As was discussed in the previous chapter, the oxidation and pyrolysis kinetics of certain hydrocarbon classes have been investigated to varying degrees. Paraffinic compounds, C2-C8 species in particular, and aromatics have been studied the most extensively and less attention has been paid to cyclic alkanes. Within this class, the naphthenes, or monocyclic alkanes, have been considered to a relatively large extent and were the topic of Chapter 3. In particular, there have been very few kinetic studies on polycyclic alkanes. Because it is the simplest polycyclic alkane, decalin is taken as a reference component for this category.
The chemical structure of decalin is presented in Figure 4-1. Decalin is a molecule composed of two fused six-membered rings and exists in both cis and trans isomers, of which the latter is energetically more stable and has a lower boiling point. It is often taken as a representative polycyclic alkane in the development of surrogate fuels and their kinetic models.1 Surrogate mixtures are typically designed to mimic the physical and/or chemical properties of commercial 94 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
crude-based fuels. Chemical surrogates may be designed to capture the laminar burning velocity, high-temperature auto-ignition, low-temperature oxidation, and pollutant and soot formation tendencies of the complex distillate fuel at a range of conditions. The development of surrogate mixtures enables the simulation of combustion processes using chemistry describing only the oxidation of the surrogate mixture and not the thousands of compounds contained in the complex distillate fuel. Moreover, because of its high thermal stability, decalin is an attractive compound to use as an endothermic fuel or propulsion fuel additive. It is also used as a fuel component for liquid fuel based ramjet propulsion systems due to its high energy density.2,3
Figure 4-1 Chemical structure of decalin.
In this chapter, the pyrolysis chemistry of decalin is studied as a reference component for polycyclic alkanes. A first part describes the various decomposition pathways through means of a literature survey and gives an overview of the currently available kinetic and experimental data. The performance of the automatically constructed microkinetic model for decalin pyrolysis is ascertained through comparison with literature reported experimental data.4 Through a rate of production and sensitivity analysis, differences with cyclohexane and methyl cyclohexane pyrolysis are identified.
Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 95
4.2 Literature survey on decalin pyrolysis
This section gives an overview of the experimental and kinetic modeling studies that have been performed to date. The important reactions that occur during decalin pyrolysis, and how these were modelled in literature are discussed as well.
4.2.1 Reported kinetic modeling and experimental studies
The formation of high concentrations of aromatics like toluene, benzene and styrene during decalin pyrolysis was first observed by Billaud et al.5 Some experimental studies on decalin pyrolysis have been performed in past years. Bredael et al.6 investigated the pyrolysis of decalin in a quartz flow reactor at 0.5 bar and temperatures of 973 – 1223 K. Plug flow experiments in a quartz reactor at atmospheric pressure have been executed by Taylor et al.7 Ondruschka et al.8 have investigated the low pressure (10-3 mbar) pyrolysis of trans-decalin at temperatures below 1450 K. Their results confirmed the existence of several unimolecular reaction pathways like C-C bond dissociation and isomerizations.
Recently, a comparative study between cyclohexane and decalin pyrolysis was performed by Comandini et al.3 They accurately simulated the ignition delay time and flame speed experimental results using kinetic models available in literature. Measurements conducted for cyclohexane and decalin at similar conditions provided evidence that the autoignition of the two-ring compound is slower than that of cyclohexane.
Due to the large molecular weight and complex molecular structure, there are only few theoretical and kinetic modeling studies of decalin decomposition. Chae et al.2 performed density functional theory calculations (B3LYP and BH&HLYP functionals) of the potential energy surface to investigate the mechanism of decalin breakdown. They used RRKM- and transition state theory methods to compute high-pressure limit rate constants for the various reaction pathways. Their calculations predict the primary products of decalin decomposition to be monoaromatic species and C2-C3 alkenes. Benzene and toluene are produced through multiple pathways and their product concentrations are significant (64 – 94% benzene and 6 – 31% toluene, depending on the temperature). Xylene, styrene and ethylbenzene are produced only in small quantities.
Ranzi and Dagaut1,9 have proposed a lumped kinetic model for decalin oxidation and validated it against ignition delay times and jet stirred reactor oxidation data.
96 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
A new experimental and kinetic modeling study of decalin pyrolysis has recently been reported by Zeng et al.4 The experimental work in this study has been performed in a plug flow reactor at pressures of 0.04, 0.2 and 1 bar, in a temperature range of 920 – 1500 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for comprehensive detection of pyrolysis species, such as free radicals, isomers, and mono- and polycyclic aromatic hydrocarbons.
A complete overview of the currently available experimental data is given in Table 4-1.
Table 4-1 Overview of available research concerning decalin pyrolysis and oxidation.
Component Reactor Experiment Temperature Pressure Equivalence Mole Ref. Type [K] [bar] Ratio % Decalin PFR Pyrolysis 973-1223 0.5 - - Bredael et al.6 PFR Pyrolysis 973-1223 0.5 - - Hillebrand et al.10 PFR Pyrolysis - 1.0 - - Taylor et al.7 PFR Pyrolysis 770-1020 0.5 - 0.13 Ondruschka et al.8 PFR Pyrolysis 700-750 23-75 - - Yu et al.11 Micro Pyrolysis 1083 4.0 - - Zamostny et al.12 JSR Oxidation 750-1350 1.0-10 0.5-1.5 0.1 Dagaut et al.9 PFR Pyrolysis 920-1500 0.04-1.0 - 1.0 Zeng et al.4 JSR (jet stirred reactor), ST (shock tube reactor), PFR (plug flow reactor), Micro (Micro plug flow reactor)
4.2.2 Decomposition reactions of decalin
4.2.2.1 Unimolecular reactions of decalin
There are two kinds of pathways to open the fused six-membered rings of decalin through a C-C bond dissociation. The first pathway is the dissociation of the bridgehead bond (C1-C1’,Figure 4-2) producing cyclodecene through a diradical intermediate. This bridgehead bond is the weakest C- C bond in the decalin molecule, but one should realize that dissociation of this bond converts the two fused cyclohexane molecules into a ten-membered ring with a ring-strain of around 50.21 kJ !"#$%& . It is therefore suggested by Chae et al.2 and Oehlschlaeger et al.13 that besides the isomerization to cyclodecene, the diradical species can also undergo direct C-C '-scissions. These reactions result in the formation of small, linear hydrocarbons, as shown in Figure 4-2. Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 97
Figure 4-2 Dissociation of the bridgehead bond in decalin and subsequent reaction paths.
The second pathway consists in cleavage of the other C-C bonds, resulting in substituted cyclohexane or cyclohexene molecules through a diradical intermediate. For example, the dissociation of the C1-C2 bond (green in Figure 4-3) can produce 1-propyl-2- methylenecyclohexane or 1-allyl-2-methylcyclohexane. This strain-free pathway is expected to be more prevalent than the former pathway that results in a 10-membered ring.
Figure 4-3 Unimolecular C-C bond dissociation reaction of decalin.
Overall, when not accounting for stereo isomers, the dissociation of a C-C bond in decalin contributes to the formation of five C&)H&+ products, whose chemical structures are shown in Figure 4-3. Similar to decalin itself, these species can undergo further C-C bond dissociations or suffer H-atom abstractions by hydrogen atoms or methyl radicals. For these C&)H&+ species with a double bond, the allylic C-H bond is much weaker compared to the other aliphatic C-H bonds.
4.2.2.2 Bimolecular reactions of decalin
Due to the symmetry of decalin’s chemical structure, H-abstractions yield to the formation of only three different decalyl radicals. The potential energy surface for these reactions has been calculated using density functional theory by Chae et al.2 The energy barriers to produce DECALYL-1, -2 and -3 are presented in Figure 4-4. 98 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
80 TS for DECALYL-2 TS for DECALYL-3 60 TS for DECALYL-1
40
20
0
-20 Energy [kJ/mole]
-40
-60 DECALYL-2 DECALYL-3
-80 DECALYL-1 Figure 4-4 Energy barriers for H-abstractions of decalin by methyl radical.2
It is proposed by Zeng et al.4 to refer the rate constants of these H-abstractions to the theoretically calculated rate constant of similar pathways of methyl cyclohexane by Zhang et al.14 The thus formed decalyl radicals can undergo C-C '-scission reactions to produce seven monocyclic C&)H&, radicals, including six radicals with a six-membered ring and one with a ten-membered ring. In addition to the C-C '-scissions, the three bicyclic decalyl radicals can also suffer C-H '-scissions to produce bicyclic olefins.
The resulting monocyclic C&)H&, radicals can undergo C-C and C-H '-scissions, and isomerization reactions. If the former occurs at the six-membered ring, the ring will be opened, producing linear alkenyl radicals. A ' -scission occurring between the other carbon atoms will result in the formation of a smaller monocyclic species and a small linear species. Ultimately this will lead to the formation of aromatic species combined with smaller alkenes and alkanes.
Starting from the decalyl radicals, Chae et al.2 have identified several pathways towards monocyclic aromatics like benzene, toluene and styrene. In their model, benzene is the major reaction product at low-temperature conditions (700 K). As an example, the reaction path that contributes for 99% - 76% to the total benzene production in the temperature range of 700-1500 K is depicted in Figure 4-5.
Figure 4-5 Main reaction path of decalin producing benzene according to Chae et al.2 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 99
4.3 Kinetic modeling of decalin pyrolysis
This section discusses the development of a microkinetic model for decalin pyrolysis. This model is validated against literature reported experimental data.4 Furthermore, the initial decomposition pathways of decalin, the main routes towards small olefins and the formation routes of aromatics are analyzed by means of a rate of production and sensitivity analysis.
4.3.1 Experimental method
Zeng et al.4 have studied the pyrolysis of decalin in a plug flow reactor at constant pressure (1 bar) in the temperature range 920 K – 1230 K. The conversion of decalin in this temperature interval is not complete and reaches a maximum value of 72%. During these experiments, the gas mixture of argon and decalin (1 mole%) with a total flow rate of 4.84 x 10%/mole/s was fed into an α- alumina plug flow reactor with 7.0mm inner diameter and a length of 22.7 mm. The temperature profiles for the experiments performed at 975 K, 1053 K, 1130 K and 1235 K are shown in Figure 4-6.
1600
1200
800
Temperature [K] 400
0 0.00 0.05 0.10 0.15 0.20 Axial reactor cooridinate [m]
Figure 4-6 Gas temperature profiles for the decalin pyrolysis experiments along the axial reactor coordinate, corresponding to set temperatures of 975 K (—), 1053 K (—), 1130 K (—) and 1235 K(—).
The authors estimate the uncertainties of the evaluated mole fractions in these experiments to be within ±5% for decalin and within ±15% for pyrolysis products.
4.3.2 Kinetic model development
A first and crucial step in the automatic generation of a suitable pyrolysis model for decalin with the use of Genesys, is the identification of relevant reaction families. The initial decomposition reactions of decalin, i.e. ring-opening isomerization and H-abstraction followed by C-C '-scission, are referred to similar reactions of methyl cyclohexane as determined through high-level ab initio calculations performed by Zhang et al.14 and Wang et al.15 100 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
Considering the reaction families that are user-defined in Genesys, the H-abstraction reactions are the most dominant, but bond dissociation/radical recombination, '-scission/radical addition and intramolecular H-abstractions are present as well. Based on literature, additional reactions like the ring-opening isomerization starting from decalin, Diels-Alder cyclization and intramolecular carbon radical addition to a double bond are incorporated in the present mechanism. A complete overview of the considered reaction families and the source of the kinetic data is presented in Table 4-2.
Table 4-2 Used reaction families for the construction of the decalin pyrolysis model.
Reaction family Description Reference Hydrogen transfer Intermolecular H-abstraction 16[16]..
Intramolecular H-abstraction 17[17]..
0-scission/Radical addition Intermolecular carbon radical 18[18].. addition Intermolecular hydrogen 19[19].. radical addition Intramolecular carbon radical 20,21[20],[21]…. addition
Bond scission/Radical recombination
C-C recombination 22[22]..
C-H recombination 22[22]..
Diels-Alder
Diels-Alder Cyclization 21[21]..
Ring-opening
Ring-opening Isomerization 14,23[14],[23]….
AramcoMech2.0 mechanism24,25 is used as a base mechanism. This mechanism was developed at the National University of Ireland, Galway, and characterizes the kinetic and thermochemical properties of a large number of C1-C4 based hydrocarbons over a wide range of experimental conditions. Formation pathways towards aromatics have been incorporated manually based on the work of Sharma and Green et al.26 The final model contains 19,493 reversible reactions between 3,370 species.
Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 101
For a large portion of the rate coefficients, GAV databases are used, because performing ab initio calculations for all possible reactions of decalin, decalyl radicals, etc. would be too time consuming for this study. GAV’s for the reaction families, as discussed in Chapter 2, have however specifically been derived for linear molecules and adopting them for cyclic molecules without any further notice could potentially lead to deviations from reality in the eventual model. Therefore, it is first assessed through the use of some case studies whether or not these GAV’s can be used without introducing too large an error. The considered reactions are: carbon centered H-abstraction from a secondary, tertiary and allylic carbon atom, and C-C-C β-scission. Figure 4-7 shows a schematic representation of the methodology used for justifying the use of GAV-based rate coefficients for linear molecules in the decalin model.
Reaction in decalin pyrolysis model
Comparable reaction in methyl cyclohexane model
Considered reaction in group additivity scheme
Figure 4-7 Methodology for justifying the use of GAV-based rate coefficients for linear molecules in the decalin pyrolysis model.
In case of only small deviation between 1232452 and 1678 , these rate coefficients can be safely adopted. A deviation of a factor five is generally accepted. Table 4-3 gives for each type of reaction the rate coefficient as calculated ab initio via transition state theory for cyclic molecules, divided by the rate coefficients as calculated via group additive values for comparable linear molecules. More details concerning these calculations are given in Appendix B. It can be concluded that H- abstractions from secondary and tertiary carbon atoms can be treated the same for the decomposition products of decalin as for linear molecules. A large deviation however exists for H- abstraction from allylic carbon atoms and for β-scission that open the ring, which is due to the extra ring-strain that is lost compared to linear molecules.
102 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
Table 4-3 9:;:<=:/9>?@ for representative reactions in the decalin model.
Example of reaction in decalin model 9:;:<=:/9>?@ 700 K 900 K 1100 K H-abstraction secondary 2.48 2.20 2.04 C-atom H-abstraction tertiary 0.49 0.37 0.40 C-atom
H-abstraction allylic 35.72 13.04 6.87 C-atom
C-C-C β-scission 358.21 14.45 1.87
Based on the conclusions from Table 4-3, special attention is paid to the rate coefficients of H- abstractions from allylic carbon atoms in cyclic molecules and of β-scissions that open the ring. The Arrhenius parameters for these reactions are based on the Arrhenius parameters for similar reactions from the in-house cyclohexane pyrolysis model, i.e. the C-C '-scission from cyclohexyl radical to hex-5-enyl radical and H-abstraction from cyclohexene to yield cyclohex-2-enyl respectively.
Automatic generation of a microkinetic model for decalin pyrolysis proves to be challenging with the current capabilities of Genesys. Often, a run is aborted because of memory issues as the network already contains more than 50.000 reversible reaction and is still expanding. Therefore, no stereochemistry is considered and stringent constraints are imposed on the model generation. These constraint are shown in Table 4-4.
Table 4-4 Summary of the constraints imposed on the species and reactions in the decalin model.
Type Constraint Species Maximum ten carbon atoms Maximum two double bonds Maximum two tertiary carbon atoms No quaternary carbon atoms No consecutive double bonds
Reactions H-abstractions only by hydrogen atoms or C3- species Intramolecular hydrogen abstractions are limited to 1-5 and 1-6 shifts Only methyl radicals can add to a double bond Only exo- and no endo-intramolecular carbon radical additions can occur
Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 103
An alternative solution would be to use a rate-based termination criterion instead of the rule- based termination criterion. However the kinetic model, generated by former criterion, heavily depends on the applied kinetic and thermodynamic data. Because of the lack of data in databases for cyclic compounds and hence the high uncertainty on this used kinetic and thermodynamic data during kinetic model development, the usage of the rule-based criterion is advised.
4.3.3 Results and discussion
In this section, the performance of the developed model for decalin pyrolysis is ascertained and compared to that of a recently developed mechanism by Zeng et al.4 To this end, experiments executed by the same authors in a plug flow reactor at 1 bar are simulated. The emphasis is on the initial consumption pathways of decalin, as well as on the formation routes towards aromatics.
4.3.3.1 Model performance and comparison to literature model
Plug flow reactor simulations have been performed using both the LCT and Zeng model. For this, the Plug Flow Code in the Chemkin-PRO software package is used. Figure 4-8 shows the experimental (symbols) and simulated (solid lines) mole fraction profile of decalin as a function of the reactor temperature. In the considered temperature range, the conversion of decalin varies from 0.01% to 73%. The LCT model overestimates the onset temperature of decalin consumption by about 50 K. Also, after this point, the slope in the LCT-simulated mole fraction profile is steeper than that of the experimental one, which means that decalin is consumed too fast in the LCT model. Although the Zeng model yields a better prediction of the onset temperature, it is also not able to accurately capture the slope of the experimental data.
1.2
1.0
0.8
0.6
0.4 Yield [mole %]
0.2
0.0 900 1000 1100 1200 1300 Temperature [K]
Figure 4-8 Experimental (symbols) and simulated (lines) mole fraction of decalin as function of temperature.
Simulations are done with the PFR code in CHEMKIN PRO and the models used are: LCT (—), Zeng et al.4 (– – –). %G P=1.0 bar, ABC:D<=A=4.84 x EF mole/s. The error bars represent the average error as determined by Zeng et al.4 104 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
The experimental and simulated yields of the light olefin products of decalin as a function of conversion are presented in Figure 4-9. The most abundant species are hydrogen and ethylene, with 1.4 mole% and 0.9 mole% respectively. Both these molecules exhibit a monotonous increase in their mole fraction upon rising conversion. The hydrogen yield is simulated reasonably well, and that of ethylene is only slightly overestimated at higher conversion. Methane and acetylene are present to a lesser extent. These respective species are slightly under and overestimated by the two models in the entire conversion range. The LCT-simulated mole fraction of propylene is significantly lower than the experimental value and also doesn’t show a maximum. An explanation for this observation is sought for in the next paragraph. The decline in the 1,3-butadiene yield occurs at the same temperature as in the experiments but is overall underestimated.
2.0 0.3 H2 CH4 1.5 0.2
1.0
0.1 Yield [mole %] 0.5 Yield [mole %]
0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-]
1.5 0.3 C2H4 C2H2
1.0 0.2
0.5 0.1 Yield [mole %] Yield [mole %]
0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-]
0.3 0.3 C3H6 C4H6
0.2 0.2
0.1 0.1 Yield [mole %] Yield [mole %]
0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] Figure 4-9 Experimental (symbols) and simulated (lines) mole fractions of major decalin light olefin pyrolysis products as function of conversion. Simulations are done with the PFR code in CHEMKIN PRO and the models
4 %G used are: LCT (—), Zeng et al. (– – –). P=1.0 bar, ABC:D<=A=4.84 x EF mole/s. The error bars represent the average error as determined by Zeng et al.4
Figure 4-10 displays the experimental and simulated yields of cyclopentadiene and of the main aromatic product species, i.e. benzene, toluene, styrene and naphthalene. Benzene is the most Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 105
abundantly produced aromatic species in both the experiments and simulations. As seen from Figure 4-10, the maximum mole fraction of benzene reaches more than 0.25 mole%, corresponding to more than 15% of the total carbon flux. This percentage is very high compared with those in the pyrolysis of cyclohexane and alkyl cyclohexanes.4,15,27,28 The trend in the benzene mole fraction is predicted better than in the model of Zeng et al. but still underestimates the experimentally observed value. Toluene is the second most abundant aromatic product at around 0.08 mole%. In the low- to mid-conversion range, the simulated toluene yield is lower than the experimental one, but seems to recover at high conversion. Small amounts of styrene are formed as well, but its yield is underestimated in the entire considered conversion range. The aromatic counterpart of decalin, naphthalene, is the major polyaromatic hydrocarbon formed during pyrolysis. Its mole fraction is underestimated by a factor two. The cyclopentadiene yield, which goes to a maximum at 85% decalin conversion is in accordance with the experimental values.
0.32 0.12 Benzene Toluene 0.24 0.08
0.16
0.04 Yield [mole %] Yield [mole %] 0.08
0.00 0.00 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] 0.04 0.03 Styrene Naphthalene 0.03 0.02
0.02
0.01 Yield [mole %] 0.01 Yield [mole %]
0.00 0.00 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Conversion [-] 0.12 CPD
0.08
0.04 Yield [mole %]
0.00 0.0 0.2 0.4 0.6 0.8 1.0 Conversion [-] Figure 4-10 Experimental (symbols) and simulated (lines) mole fractions of major aromatic species and cyclopentadiene as function of conversion. Simulations are done with the PFR code in CHEMKIN PRO and the
4 %G models used are: LCT (—), Zeng et al. (– – –). P=1.0 bar, ABC:D<=A=4.84 x EF mole/s. The error bars represent the average error as determined by Zeng et al.4 106 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
4.3.3.2 Rate of production analysis of decalin consumption
It is expected that different reactions govern the consumption of decalin at different conditions. Therefore, a rate of production analysis of the consumption of decalin is executed at a high (1076 K) and low (1230 K) reactor temperature, both near the entrance and in the center of the reactor. At these temperatures, the conversion of decalin is 3.5% and 72% respectively. The result of the ROP is presented in Figure 4-11 and shows that decalin is consumed through two major channels: ring-opening isomerization and H-abstraction by radical attack to yield three different decalyl radicals. The principal consumption pathways of decalin near the entrance of the reactor at low as well as high temperature are the two ring-opening isomerization reactions, forming 3-butenyl cyclohexane and 1-butyl cyclohexene, of which the former species is the dominant product. Further in the reactor, H-abstraction take over, yielding three different decalyl radicals. The major abstracting radicals are hydrogen atoms and methyl radicals (shown in the smaller graph on the right hand side of Figure 4-11). When comparing the share of H-abstraction by methyl radicals during decalin pyrolysis on the one side and during methyl cyclohexane pyrolysis on the other, the heavily reduced importance in the former case stands out. Much more decalyl radicals are formed through reaction between decalin and hydrogen atoms than through reaction between decalin and methyl radicals. This again highlights the importance of the C-C scission during methyl cyclohexane pyrolysis, which acts as a methyl radical source.
24 1230 K, 20 cm 1076 K, 20 cm
23
22 1230 K, 5 cm 1076 K, 5 cm
21
20 9 19 8 18 7 17 6 16 5
15 4
14 3
13 2
12 1
11 0 10 20 30 40 50 10 Rate of consumption [%] 9
8
7
6
5
4
3
2
1
0 10 20 30 40 50 60 70 80 90 100 Rate of consumption [%] Figure 4-11 Rate of production analysis of the initial decomposition pathways of decalin at different temperatures and positions in the reactor. Simulations are done with the PFR code in CHEMKIN PRO. 1076 K, 5 cm (full black); 1076 K, 20 cm (dotted); 1230 K, 5 cm (oblique stripped); 1230 K, 20 cm (horizontal striped). P=1.0 bar. The percentage shows the fraction of cyclohexane consumed by the corresponding reactions. Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 107
Note that, although they are not dominant, ring-opening isomerizations are still significant at higher temperatures. This is opposite to as was observed during cyclohexane and methyl cyclohexane pyrolysis. The contribution of the ring-opening of cyclohexane to 1-hexene is heavily diminished at high temperature at the expense of H-abstractions. Methyl cyclohexane is even hardly ever consumed through ring-opening due to the lower activation barrier of the C-C scission. Moreover, decalin consumption still transpires through ring-opening isomerization to a notable extent around the center of the reactor. This as well is in contrast to the pyrolysis of cyclohexane and methyl cyclohexane. These observations lead to conclude that radical formation is of increased difficulty during decalin pyrolysis. A possible explanation for this is that a lot of the radical species involved at the start of the reaction contain many carbon atoms, i.e. ten, nine or eight, and that these are much more readily consumed through C-C '-scission toward smaller species than to undergo C-H ' -scission producing hydrogen atoms. Furthermore, the high ‘branching’ degree in decalin compared to methyl cyclohexane facilitates ring-opening to a higher extent than with cyclohexane or methyl cyclohexane. This is because there are more ring-opening isomerizations that proceed through relatively more stable secondary diradical species.
Figure 4-11 shows the main primary consumption products of decalin, i.e. the three decalyl radicals, DECALYL-1, DECALYL-2 and DECALYL-3, as well as the main ring-opening isomerization product 1-butyl cyclohexene. A reaction path analysis detailing the consumption routes of these species towards the main aromatic and small olefin products is performed at 1180 K in the center of the reactor. The result is displayed Figure 4-12 for 1-butyl cyclohexene and in Figure 4-13 for the three radical species. The dashed lines in these figures represent indirect routes which are omitted here for simplicity reasons.
Near the entrance of the reactor, when no radical species are present yet, the main consumption routes of the ring-opening isomerization products consist of C-C scission yielding methyl, ethyl, propyl and allyl radicals. These radical species initiate subsequent H-abstraction reactions from decalin, which is the onset of the radical controlled pyrolysis. As soon as radicals are present, 1- butyl cyclohexene as well is primarily converted through radical abstractions, as shown in Figure 4-12. Two of these H-abstracted molecules undergo a C-C '-scission which opens the ring and yields branched alkenyl radicals. Through a subsequent C-C '-scission and intramolecular carbon radical addition, vinyl cyclohexyl radicals are formed, which are finally converted to styrene. 108 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
Figure 4-12 Rate of production analysis of the consumption pathways of 1-butyl cyclohexene. Simulations are done with the PFR code in CHEMKIN-PRO. T=1180 K, P=1.0 bar, center of the reactor. The percentage reflect the molar flux of the corresponding pathway divided by the total molar flux of decalin consumption. The starting molecule and the detected product species are highlighted. Dashed lines represent indirect pathways.
DECALYL-3 radical (Figure 4-13a) undergoes C-C ' -scission yielding 1-butenyl-2-cyclohexyl radical and 1-ethenyl-2-methanidylcyclohexane radical, of which the former is more readily formed due to its secondary radical, in contrast to the primary radical in the latter. A subsequent C-C '-scission of 1-butenyl-2-cyclohexyl radical can open the ring, yielding branched alkenyl species, or can cut of a butenyl chain to form cyclohexene. Cyclohexene is converted further to benzene by a sequence of radical attacks and C-H ' -scissions. The butenyl radical is mainly converted to 1,3-butadiene but also to ethylene. 1-ethenyl-2-methanidylcyclohexane radical primarily undergoes an intramolecular 1-2 H-shift towards a more stable tertiary radical species. This species decomposes to 1-methyl cyclohexene, a precursor to toluene, and allyl radical which reacts with a hydrogen atom to form propylene. Similar to 1-butenyl-2-cyclohexyl radical, a minor percentage of 1-ethenyl-2-methanidylcyclohexane radical suffers from a C-C ' -scission that opens the ring, yielding branched C10 alkenyl radicals. Through a sequence of C-C '-scissions, these branched alkenyl radical are converted to smaller species such as ethylene and 1,3- butadiene. The other decalyl radicals (Figure 4-13b and Figure 4-13c) suffer from the same reactions as DECALYL-3 radical. These reactions mainly produce ethylene, some 1,3-butadiene and aromatic species. Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 109
Figure 4-13 Rate of production analysis of the consumption pathways of DECALYL-1 (a), DECALYL-2 (b) and DECALYL-3 (c). Simulations are done with the PFR code in CHEMKIN-PRO. T=1180 K, P=1.0 bar, center of the reactor. The percentage reflect the molar flux of the corresponding pathway divided by the total molar flux of decalin consumption. The starting molecule and the detected product species are highlighted. Dashed lines represent indirect pathways. 110 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
Note that ethylene has plenty of formation pathways in decalin pyrolysis. Most of the reactions of branched alkenyl radicals produce ethylene, as do the reactions of cyclohexane species with alkyl substitutions. From Figure 4-13, a dissimilarity with cyclohexane and methyl cyclohexane pyrolysis can be noticed with respect to the formation of aromatics. During decalin pyrolysis there are a lot of direct pathways to benzene, styrene and toluene in which recombination or addition reactions of smaller species, like for example propargyl radicals and acetylene, do not partake. At comparable reactor conditions, i.e. high temperature, center of the reactor, the latter reactions contribute to about 98% of the total amount of benzene, toluene and styrene produced during the pyrolysis of methyl cyclohexane. For decalin, this percentage is much lower, as is discussed further in paragraph 4.3.3.3. The ROP from Figure 4-13 also shows the high number of routes toward benzene, compared to toluene and styrene. This explains the higher concentration of benzene that was observed in the experiments performed by Zeng et al.4
Additionally, a sensitivity analysis on decalin consumption is performed at the same conditions as the rate of production analysis, i.e. 1180 K, 1.0 bar, center of the reactor. This sensitivity analysis is executed with CHEMKIN PRO and the result is depicted in Figure 4-14. A negative sensitivity coefficient means that increasing the pre-exponential factor of the corresponding reaction will result in more decalin being consumed, and vice versa. The larger the sensitivity, the larger this effect. As for cyclohexane and methyl cyclohexane consumption, the large negative sensitivity of the ring-opening isomerization producing 1-butyl cyclohexene stands out. This is because it is the dominant isomerization product and the primary source of radicals at the start of the pyrolysis, i.e. near the entrance of the reactor. Through dissociation reactions, the isomerization products of decalin produce methyl, ethyl, propyl and allyl radicals which subsequently initiate the main consumer sequence of decalin: H-abstractions. For the same reason, the ring-opening towards butenyl cyclohexane as well has a negative sensitivity, be it smaller because it is a less dominant ring-opening isomerization product. Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 111
-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 Normalized sensitivity coefficient [-]
Figure 4-14 Sensitivity analysis of decalin consumption. Simulations are done with the PFR code in CHEMKIN PRO. T=1180 K, P=1.0 bar, center of the reactor.
Hydrogen abstractions from decalin are its main consumer pathways and hence have negative sensitivities. Abstractions by hydrogen atoms being the most dominant, they also have the largest sensitivity of all H-abstraction reactions. As a consequence, reactions that remove radicals from the system have a positive sensitivity because they hinder decalin consumption in this way. The recombination reaction of methyl radicals to ethylene has the largest positive sensitivity because this reaction removes two main H-abstractors. In a similar logic, the reaction of allyl radical with a hydrogen atom and the H-abstraction by a hydrogen atom from cyclopentadiene also have positive sensitivities.
4.3.3.3 Rate of production analysis for the production of aromatics
Because of a large difference in the formation of aromatics between decalin and cyclohexane/methyl cyclohexane pyrolysis, rate of production analyses on the main aromatic species are performed. These include benzene, styrene, toluene and naphthalene. A number of main pathways towards the first three aromatic species were already shown and discussed in Figure 4-13. Important intermediates in the formation of benzene are cyclohexene and cyclohexenyl radical, which originate directly from decalin. A rate of production analysis of benzene is presented in Figure 4-15. This ROP is performed at 1180 K, 1.0 bar and in the center of the reactor. Almost 83% of the total benzene is formed by H2-elimination from 1,3- cyclohexadiene, which is a species formed almost exclusively through subsequent '-scissions of the three decalyl radicals. Reactions involving smaller species also contribute to benzene formation. The Diels-Alder cyclization of acetylene and vinyl acetylene is the main reaction in this 112 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
category, but propargyl radical recombination, the reaction of vinyl radical with 1,3-butadiene and the recombination of propargyl and allyl radicals also yield benzene.
Figure 4-15 Rate of production analysis of benzene formation. T=1180 K, P=1.0 bar, center of the reactor. The percentages reflect the molar flux of the corresponding pathway divided by the total molar flux of decalin consumption.
A rate of production analysis of the formation of styrene and toluene is displayed in Figure 4-16.
Both species are dominantly formed through H2-elimination of respectively vinyl cyclohexadiene species and methyl cyclohexadiene. Similar to benzene, these precursors stem directly form the decalyl radicals, i.e. styrene mainly from DECALYL-1 radical and toluene primarily form DECALYL- 3 radical. These pathways are shown in Figure 4-13. Next to these routes, addition of methyl radical and vinyl radical to benzene also contribute to the formation of styrene, respectively toluene, be it only limited for the latter.
Figure 4-16 Rate of production analysis of styrene and toluene formation. T=1180 K, P=1.0 bar, center of the reactor. The percentages reflect the molar flux of the corresponding pathway divided by the total molar flux of decalin consumption.
Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 113
Many polyaromatic hydrocarbons were detected in the study by Zeng et al.4, among which naphthalene has the highest concentration. The main formation pathways of naphthalene are summarized in Figure 4-17. Compared to the monocyclic aromatic hydrocarbons, naphthalene is formed only in very small amounts. The major formation route proceeds via a recombination reaction of cyclopentadienyl radicals. The latter species is also involved in most of the other reactions toward naphthalene as can be seen in the ROP. The cyclopentadienyl radical is formed through a H-abstraction from cyclopentadiene, which in turn is mainly produced via the reaction of but-2-yn-1-yl radical with ethylene or through ring-closure through intramolecular carbon radical addition (not shown in Figure 4-17).
Figure 4-17 Rate of production analysis of naphthalene formation. T=1180 K, P=1.0 bar, center of the reactor. The percentages reflect the molar flux of the corresponding pathway divided by the total molar flux of decalin consumption.
114 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
4.4 Conclusion
Polycyclic alkanes have to a large extent been omitted in recent literature. In this chapter, decalin is chosen as a reference component for these polycyclic alkanes and its pyrolysis chemistry in a plug flow reactor is studied. To this end, a literature survey comprising a summary of the existing experimental and kinetic modeling work was performed. The latter proves to be rather limited; only Zeng et al.4 and Dagaut et al.9 have reported mechanisms, of which the latter one is a lumped model.
A mechanism for decalin pyrolysis is constructed automatically with Genesys based on user defined reaction families. Rate coefficients for the critical initial decomposition reactions, i.e. ring- opening and H-abstractions followed by C-C '-scissions, are adopted from similar reactions of methyl cyclohexane pyrolysis, that are determined with high level quantum chemical calculations.14,15 Automatic generation of a microkinetic model for decalin pyrolysis proves to be challenging with the current capabilities of Genesys, because often the networks become too large to handle. For Genesys to be able to generate a complete model, stringent constraints are imposed.
The model has been validated against experimental results obtained by Zeng et al.4 in a plug flow reactor at 1.0 bar and in a temperature range of 920 K – 1230 K. The overall performance of the model is satisfactory, the major discrepancy between simulated and experimental results being the propylene yield. A rate of production and sensitivity analysis on decalin consumption have shown that the decomposition of decalin is initiated by C-C bond scissions followed by an isomerization reaction. These ring-opening isomerizations produce two main products, 3-butenyl cyclohexane and 1-butyl cyclohexene, which, near the entrance of the reactor, undergo further C- C scissions yielding the first radicals in the system. These radicals subsequently initiate H- abstractions from decalin, which are its main consuming pathway further along in the reactor. The hydrogen abstractions result in three different decalyl radicals, which undergo a sequence of C-C '-scissions towards small olefinic and aromatic species.
An aspect that is different compared to cyclohexane or methyl cyclohexane pyrolysis is that the ring-opening isomerization of decalin is still quite important in the center of the reactor. This is due to the ‘branched’ nature of decalin which facilitates ring-opening because there are an increased number of pathways that proceed through a more stable secondary diradical species than is the case for cyclohexane and methyl cyclohexane. Both the ring-opening isomerization and the hydrogen atom abstractions show large sensitivities to the consumption of decalin. Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 115
The major olefinic product species of decalin pyrolysis at 1.0 mole% is ethylene, but also considerable amounts of methane, 1,3-butadiene and propylene are present. Monocyclic aromatics are mainly produced in the decalin consumption process, instead of a mass growth process from small molecules. This reveals that the two fused six-membered ring structure acts as a solid precursor for aromatics. This is reflected in the high concentration of benzene that is formed, which amounts around 25% of the total molar carbon flux. In contrast to the monoaromatic species, polyaromatic hydrocarbons, of which naphthalene is the most abundant, are primarily formed indirectly from the three decalyl radicals. This means via a mass growth process starting form suitable precursors like cyclopentadiene, cyclopentadienyl radical and benzene.
116 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
4.5 References
1. Ranzi E. A Wide-Range Kinetic Modeling Study of Oxidation and Combustion of Transportation Fuels and Surrogate Mixtures. Energy & Fuels. 2006/05/01 2006;20(3):1024-1032.
2. Chae K, Violi A. Thermal decomposition of decalin: an ab initio study. The Journal of organic chemistry. Apr 27 2007;72(9):3179-3185.
3. Comandini A, Dubois T, Abid S, Chaumeix N. Comparative Study on Cyclohexane and Decalin Oxidation. Energy & Fuels. 2014/01/16 2014;28(1):714-724.
4. Zeng M, Li Y, Yuan W, et al. Experimental and kinetic modeling investigation on decalin pyrolysis at low to atmospheric pressures. Combustion and Flame. 5// 2016;167:228-237.
5. Billaud F, Chaverot P, Freund E. Cracking of decalin and tetralin in the presence of mixtures of n-decane and steam at about 810°C. Journal of Analytical and Applied Pyrolysis. 1987/10/01 1987;11:39-53.
6. Bredael P, Rietvelde D. Pyrolysis of hydronapthalenes. 2. Pyrolysis of cis-decalin. Fuel. 1979/03/01 1979;58(3):215-218.
7. Taylor PH, Rubey WA. Evaluation of the gas-phase thermal decomposition behavior of future jet fuels. Energy & Fuels. 1988/11/01 1988;2(6):723-728.
8. Ondruschka B, Zimmermann G, Remmler M, Sedlackova M, Pola J. Thermal reactions of decalin. I. A comparative study of conventional and laser-driven pyrolysis. Journal of Analytical and Applied Pyrolysis. 1990/08/01 1990;18(1):19- 32.
9. Dagaut P, Ristori A, Frassoldati A, Faravelli T, Dayma G, Ranzi E. Experimental and semi-detailed kinetic modeling study of decalin oxidation and pyrolysis over a wide range of conditions. Proceedings of the Combustion Institute. // 2013;34(1):289-296.
10. Hillebrand W, Hodek W, Kölling G. Steam cracking of coal-derived oils and model compounds. Fuel. 1984/06/01 1984;63(6):756-761.
11. Yu J, Eser S. Thermal Decomposition of Jet Fuel Model Compounds under Near- Critical and Supercritical Conditions. 2. Decalin and Tetralin. Industrial & Engineering Chemistry Research. 1998/12/01 1998;37(12):4601-4608.
12. Zámostný P, Bělohlav Z, Starkbaumová L, Patera J. Experimental study of hydrocarbon structure effects on the composition of its pyrolysis products. Journal of Analytical and Applied Pyrolysis. 3// 2010;87(2):207-216.
13. Oehlschlaeger MA, Shen H-PS, Frassoldati A, Pierucci S, Ranzi E. Experimental and Kinetic Modeling Study of the Pyrolysis and Oxidation of Decalin. Energy & Fuels. 2009/03/19 2009;23(3):1464-1472. Chapter 4. Decalin as Reference Component for Polycyclic Alkanes 117
14. Zhang F, Wang Z, Wang Z, Zhang L, Li Y, Qi F. Kinetics of Decomposition and Isomerization of Methylcyclohexane: Starting Point for Studying Monoalkylated Cyclohexanes Combustion. Energy & Fuels. 2013/03/21 2013;27(3):1679-1687.
15. Wang Z, Ye L, Yuan W, et al. Experimental and kinetic modeling study on methylcyclohexane pyrolysis and combustion. Combustion and Flame. 2014;161(1):84-100.
16. Sabbe MK, Vandeputte AG, Reyniers MF, Waroquier M, Marin GB. Modeling the influence of resonance stabilization on the kinetics of hydrogen abstractions. Physical Chemistry Chemical Physics. 2010;12(6):1278-1298.
17. Sabbe MK, Van Geem KM, Reyniers MF, Marin GB. First principle based simulation of ethane steam cracking. AIChE journal. 2011;57(2):482-496.
18. Sabbe MK, Reyniers MF, Van Speybroeck V, Waroquier M, Marin GB. Carbon Centered Radical Addition and Scission Reactions: Modeling of Activation Energies and Pre exponential Factors. ChemPhysChem. 2008;9(1):124-140.
19. Sabbe MK, Reyniers MF, Waroquier M, Marin GB. Hydrogen Radical Additions to Unsaturated Hydrocarbons and the Reverse Scission Reactions: Modeling of Activation Energies and Pre Exponential Factors. ChemPhysChem. 2010;11(1):195-210.
20. Wang K, Villano SM, Dean AM. Fundamentally-based kinetic model for propene pyrolysis. Combustion and Flame. 2015;162(12):4456-4470.
21. Merchant SS. Molecules to engines: combustion chemistry of alcohols and their application to advanced engines, Massachusetts Institute of Technology; 2015.
22. Mehl M, Vanhove G, Pitz WJ, Ranzi E. Oxidation and combustion of the n-hexene isomers: A wide range kinetic modeling study. Combustion and Flame. 12// 2008;155(4):756-772.
23. Sirjean B, Glaude P-A, Ruiz-Lopez M, Fournet R. Detailed kinetic study of the ring opening of cycloalkanes by CBS-QB3 calculations. The Journal of Physical Chemistry A. 2006;110(46):12693-12704.
24. Kéromnès A, Metcalfe WK, Heufer KA, et al. An experimental and detailed chemical kinetic modeling study of hydrogen and syngas mixture oxidation at elevated pressures. Combustion and Flame. 6// 2013;160(6):995-1011.
25. Metcalfe WK, Burke SM, Ahmed SS, Curran HJ. A Hierarchical and Comparative Kinetic Modeling Study of C1 − C2 Hydrocarbon and Oxygenated Fuels. International Journal of Chemical Kinetics. 2013;45(10):638-675.
26. Sharma S, Harper MR, Green WH. Modeling of 1, 3-hexadiene, 2, 4-hexadiene and 1, 4-hexadiene-doped methane flames: Flame modeling, benzene and styrene formation. Combustion and Flame. 2010;157(7):1331-1345. 118 Chapter 4. Decalin as Reference Component for Polycyclic Alkanes
27. Wang Z, Cheng Z, Yuan W, et al. An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure. Combustion and Flame. 7// 2012;159(7):2243-2253.
28. Wang Z, Bian H, Wang Y, et al. Investigation on primary decomposition of ethylcyclohexane at atmospheric pressure. Proceedings of the Combustion Institute. 2015;35(1):367-375.
5. Chapter 5. Conclusion and Future Work Chapter 5 Conclusion and Future Work
5.1 Conclusions
The thermal decomposition of cyclic species is studied in this work as they are present to significant amounts in lignocellulose-based steam cracker feedstock. More specifically, microkinetic models are generated automatically with Genesys for reference components of these cyclic species. For the monocylic alkanes, cyclohexane and methyl cyclohexane pyrolysis are considered, while decalin is chosen as a reference component to study the pyrolysis of polycyclic alkanes. Simulations with the kinetic models are compared with literature-reported experimental data and reaction path analyses are performed to get a better insight in the decomposition chemistry.
For cyclohexane pyrolysis, reactor simulations are performed with the LCT model and other literature models (Sirjean et al., Silke et al., POLIMI, CSM, JetSurF 2.0) to reproduce experimental data obtained at the BSSC set-up. These experiments were performed with non-diluted cyclohexane in a plug flow reactor at 1.7 bar and in a temperature range of 913 K – 1073 K. In 120 Chapter 5. Conclusion and Future Work general, the LCT model reproduces the initial decomposition temperatures, peak temperature and mass fraction of cyclohexane very well. Next to the LCT model, the CSM and POLIMI models also perform reasonable. The prediction of the yields with JetSurf 2.0 and the models of Sirjean et al. and Silke et al. is quite far off. A sensitivity analysis at the beginning of the reactor and at different temperatures shows the importance of the ring-opening isomerization of cyclohexane to 1- hexene, especially at lower temperatures.
The applicability of the in-house developed model for cyclohexane pyrolysis towards other conditions is also tested through reproduction of experimental results obtained by Wang et al. at 40 mbar, 950 K - 1520 K and high dilution. Here, the simulation of light pyrolysis products like methane, ethylene and butadiene is adequate, but the yield of aromatics is highly underestimated. It is found that several important pathways, like reactions involving three carbon atoms (e.g. propargyl radical recombination and addition of propargyl radical to allene and propyn), to benzene, toluene and other aromatics are missing in the LCT model. After altering the model with these reactions, the prediction of the mole fractions of the main aromatic species is satisfactory.
A rate of production analysis with the adapted model confirms the importance of the C3+C3 reactions, propargyl radical recombinations being the most important. Since most of the C3- species originate from 1-hexene, this again points to the relevance of the ring-opening isomerization, as is also emphasized through a sensitivity analysis for benzene.
Additionally, a model for methyl cyclohexane pyrolysis is constructed in order to ascertain the effect of the substituent. The construction is done in two stages: first, a sub mechanism describing the initial decomposition reactions of methyl cyclohexane, i.e. ring-opening isomerization and hydrogen abstraction followed by C-C !-scission, is constructed manually. Rate coefficients for these reactions are taken from ab initio calculations on the CBS-QB3 level of theory performed by Wang et al. The ring-opening isomerization reactions were computed by Zhang et al. with high- level quantum chemical calculations and RRKM master equation simulations. Next, a kinetic model for the resulting linear or branched C7 species is developed using Genesys. Rate coefficients for the majority of the reactions in this mechanism, such as hydrogen abstractions and C-C / C-H !-scissions are calculated by the group additive framework developed by Sabbe et al.
The model is validated against experimental data obtained by Wang et al. for methyl cyclohexane in high dilution, a temperature range of 842 K – 1250 K and at atmospheric pressure. The model performance of the LCT model and the models by Orme et al. and Wang et al. are assessed for this set of experimental data. The LCT model very accurately, and better than the other mechanisms, predicts the consumption of methyl cyclohexane. The mole fraction profiles of the majority of Chapter 5. Conclusion and Future Work 121 product species are accurately reproduced, but the yield of 1,3-butadiene is underestimated, as is that of toluene. All models considerably over predict the ethylene mole fraction. The yield of the main aromatic species, benzene, matches well the experimental data. A rate of production analysis on the initial consumption pathways of methyl cyclohexane shows that the C-C scission resulting in cyclohexyl and methyl radical dominates the ring-opening isomerization reactions towards 1- and 2-heptene. Near the center of the reactor, methyl cyclohexane is primarily consumed through hydrogen abstractions by methyl radicals and hydrogen atoms. Compared to cyclohexane and decalin, a larger importance of hydrogen abstractions by methyl radicals stands out. This highlights the importance of the C-C scission during methyl cyclohexane pyrolysis, which acts as an additional methyl radical source. Like for cyclohexane, the pyrolysis reactions of smaller species, such as propargyl radicals, dominate the formation routes towards aromatics.
For decalin pyrolysis as well, a microkinetic model is constructed automatically with Genesys based on user defined reaction families. Rate coefficients for the critical initial decomposition reactions, i.e. ring-opening and H-abstractions followed by C-C !-scissions, are adopted from similar reactions of methyl cyclohexane pyrolysis, that have been calculated with high level quantum mechanical methods. Automatic generation of a microkinetic model for decalin pyrolysis proves to be challenging with the current capabilities of Genesys, because often the network becomes too large to handle. For Genesys to be able to generate a complete model, stringent constraints are imposed. Stereochemistry is for example not considered, as well as hydrogen abstraction by radicals containing more than three carbon atoms.
The model is validated against experimental results obtained by Zeng et al. in a plug flow reactor at 1.0 bar and in a temperature range of 920 K – 1230 K. The overall performance of the model is satisfactory, the major discrepancy between simulated and experimental results being the propylene yield. A rate of production and sensitivity analysis on decalin consumption show that the decomposition of decalin is initiated by C-C bond scissions followed by an isomerization reaction. These ring-opening isomerizations produce two main products, 3-butenyl cyclohexane and 1-butyl cyclohexene, which, near the entrance of the reactor, undergo further C-C scissions yielding the first radicals in the system. These radicals subsequently initiate H-abstractions from decalin which are its main consuming pathway further along in the reactor. They result in three different decalyl radicals, which undergo a sequence of C-C !-scissions towards small olefinic and aromatic species.
122 Chapter 5. Conclusion and Future Work
An aspect that is different compared to cyclohexane or methyl cyclohexane pyrolysis, is that the ring-opening isomerization of decalin is still quite important in the center of the reactor. This is due to the ‘branched’ nature of decalin which facilitates ring-opening because there are an increased number of pathways that proceed through a more stable secondary diradical species than is the case for cyclohexane and methyl cyclohexane. Both the ring-opening isomerization and the hydrogen atom abstractions show large sensitivities to the consumption of decalin.
The major olefinic product species of decalin pyrolysis at 1.0 mole% is ethylene, but also considerable amounts of methane, 1,3-butadiene and propylene are present. Monocyclic aromatics are mainly produced in the decalin consumption process, instead of a mass growth process starting from small molecules. This reveals that the two fused six-membered ring structure acts as a solid precursor for aromatics. This is reflected in the high concentration of benzene that is formed, which amounts around 25% of the total molar carbon flux. In contrast to the monoaromatic species, polyaromatic hydrocarbons, of which naphthalene is the most abundant, are primarily formed indirectly from the three decalyl radicals. This means via a mass growth process starting form suitable precursors like cyclopentadiene, cyclopentadienyl radical and benzene.
5.2 Future work
This study has shown that the simulated mole fractions for cyclohexane, methyl cyclohexane and decalin pyrolysis are in good agreement with literature experimental data. These kinetic models are constructed solely based on literature ab initio data and rate-rules that are implemented in Genesys. Moreover, by only using similarities with methyl cyclohexane an adequate model has been developed that accurately describes the pyrolysis chemistry of decalin. Potential further improvements on the kinetic model performance can be obtained through the use of accurate kinetic parameters, calculated by high-level theoretical calculations. A more detailed microkinetic model can be constructed by mitigating the constraints that have been imposed in order to limit the number of reactions that constitute the mechanism. To this end, use can be made of rate-based termination criteria, in which the rate and species concentration information is used to select only kinetically significant reactions. This type of criterion has not been used in this work because the thermodynamic and kinetic parameters are still to unreliable for cyclic species. Further attention should hence be paid to improving these parameters.
Chapter 5. Conclusion and Future Work 123
To further improve the quality of this work, new experimental datasets should be acquired on the BSSC set-up located at the LCT for the pyrolysis of methyl cyclohexane and decalin. The dedicated analysis section of this set-up enables to detect many product species very accurately. Moreover, the experiments for the pyrolysis of cyclohexane can be repeated under diluted conditions, which are more appropriate for kinetic studies.
Furthermore, the microkinetic model that was developed for the pyrolysis of cyclohexane and methyl cyclohexane can be further extended towards other substituted cyclohexanes, like dimethyl cyclohexane and ethyl cyclohexane. Experimental studies on the pyrolysis of these compounds are however scarce, requiring new experimental datasets on the LCT BSSC set-up for validation of these kinetic models.
Appendix A. Experimental Cyclohexane Pyrolysis Data
Appendix A. Experimental Cyclohexane Pyrolysis Data
CYCLOHEXANE EXPERIMENTS: 913 K – 993 K Run nr. 1 2 3 4 5 Feed Cyclohexane [g/h] 288 288 288 304 304 N2 Dilution [g/h] 0 0 0 0 0 N2 Internal Standard [g/h] 20 20 20 20 30 Reactor temperature [K] 913 933 953 973 993 Temperature profile [K] CIT 499.5 511.9 523.1 536.6 550.1 T at 190 mm 642.2 661.3 680.9 702.9 720.0 T at 380 mm 690.0 706.4 722.7 740.4 756.3 T at 480 mm 642.2 659.7 678.6 699.3 720.0 T at 670 mm 608.5 628.7 654.3 680.4 708.0 T at 860 mm 640.0 660.4 680.6 700.4 720.0 T at 1050 mm 649.0 669.5 688.1 706.4 724.3 T at 1240 mm 642.0 662.2 681.8 701.8 721.8 T at 1430 mm 443.5 459.1 475.2 489.8 504.4 COT 477.9 480.4 485.8 443.5 450.3 Pressure profile [bar] CIP 1.7 1.7 1.7 1.7 1.7 COP 1.7 1.7 1.7 1.7 1.7
126 Appendix A. Experimental Cyclohexane Pyrolysis Data
Product yields [wt. %] H2 0.056 0.095 0.182 0.317 0.509 C2H4 0.538 0.930 2.459 4.789 8.600 C2H6 0.412 0.588 1.336 2.433 3.631 C2H2 0.002 0.000 0.000 0.027 0.011 CH4 0.070 0.140 0.419 0.990 2.029 C3H8 0.003 0.007 0.024 0.045 0.080 C3H6 0.326 0.522 1.393 2.678 4.437 PD 0.003 0.001 0.001 0.009 0.018 i-C4H10 0.000 0.002 0.006 0.006 0.009 n-C4H10 0.005 0.003 0.003 0.007 0.014 t-2-C4H8 0.062 0.050 0.221 0.418 0.742 1-C4H8 0.000 0.093 0.333 0.599 0.996 i-C4H8 0.002 0.004 0.015 0.033 0.061 c-2-C4H8 0.024 0.035 0.163 0.313 0.567 MeAc 0.000 0.000 0.002 0.009 0.012 cyc-C3 0.000 0.000 0.000 0.000 0.000 1,3-C4H6 0.413 0.578 2.066 3.969 6.863 2-methyl-2-butene 0.003 0.008 0.023 0.036 0.073 2-methyl-1,3-butadiene 0.002 0.004 0.015 0.000 0.000 trans-2-pentene 0.001 0.003 0.010 0.018 0.051 trans-1,3-pentadiene 0.077 0.121 0.301 0.481 0.848 cyclopentane 0.012 0.021 0.043 0.137 0.242 1,3-cyclopentadiene 0.019 0.037 0.187 0.501 1.140 cyclopentene 0.047 0.072 0.168 0.318 0.488 C6H10 0.084 0.140 0.207 0.312 0.351 1,5-hexadiyne 0.111 0.171 0.199 0.309 0.215 C6H8 0.042 0.073 0.132 0.236 0.199 cyclohexane 96.813 96.056 87.716 82.172 61.995 1,3,5-hexatriene 0.008 0.017 0.060 0.131 0.271 4-methylenecyclopentene 0.009 0.021 0.070 0.172 0.316 cyclohexene 0.769 0.018 1.738 2.087 1.949 benzene 0.079 0.178 0.349 0.791 2.072 toluene 0.006 0.011 0.063 0.218 0.596 C7H10 0.000 0.000 0.009 0.029 0.061 methylcyclohexadiene 0.000 0.000 0.035 0.050 0.094 xylene (m, p) 0.000 0.000 0.005 0.017 0.057 ethylbenzene 0.000 0.000 0.004 0.015 0.040 o-xylene 0.000 0.000 0.003 0.016 0.029 styrene 0.000 0.000 0.003 0.017 0.075 1-pentene 0.000 0.000 0.039 0.071 0.101 indene 0.000 0.000 0.000 0.015 0.065 C9H10 0.000 0.000 0.000 0.000 0.037 1-methyl-1H-Indene 0.000 0.000 0.000 0.000 0.015 3-methyl-1H-Indene 0.000 0.000 0.000 0.000 0.014 1,2-dihydro-Naphthalene 0.000 0.000 0.000 0.000 0.008 Appendix A. Experimental Cyclohexane Pyrolysis Data 127
naphthalene 0.000 0.000 0.000 0.000 0.020 3-methyl-2-butene 0.000 0.000 0.000 0.000 0.000 1-methylene-1H-Indene 0.000 0.000 0.000 0.000 0.000 fulvene 0.000 0.000 0.000 0.000 0.000 1-ethenyl-2-methyl-benzene 0.000 0.000 0.000 0.000 0.000 indane 0.000 0.000 0.000 0.000 0.000 C11H10 0.000 0.000 0.000 0.000 0.000 C2 naphthalene 0.000 0.000 0.000 0.000 0.000 2-methyl-Naphthalene 0.000 0.000 0.000 0.000 0.000 C13H12 diaromatics 0.000 0.000 0.000 0.000 0.000 biphenyl 0.000 0.000 0.000 0.000 0.000 C10H8 0.000 0.000 0.000 0.000 0.000 1-isopropenylnaphthalene 0.000 0.000 0.000 0.000 0.000 acenaphthene 0.000 0.000 0.000 0.000 0.000 acenaphthylene 0.000 0.000 0.000 0.000 0.000 C12H10 0.000 0.000 0.000 0.000 0.000 C13H10 naphthenodiaromatics 0.000 0.000 0.000 0.000 0.000 anthracene 0.000 0.000 0.000 0.000 0.000 phenanthrene 0.000 0.000 0.000 0.000 0.000 C16H10 naphthenotriaromatics 0.000 0.000 0.000 0.000 0.000 pyrene 0.000 0.000 0.000 0.000 0.000 1-methyl-Naphthalene 0.000 0.000 0.000 0.000 0.000 C14H12 naphthenodiaromatics 0.000 0.000 0.000 0.000 0.000 C15H12 triaromatics 0.000 0.000 0.000 0.000 0.000 C15H10 triaromatics 0.000 0.000 0.000 0.000 0.000 C17H14 0.000 0.000 0.000 0.000 0.000 Fluoranthene 0.000 0.000 0.000 0.000 0.000 C17H12 naphthenotriaromatics 0.000 0.000 0.000 0.000 0.000
128 Appendix A. Experimental Cyclohexane Pyrolysis Data
CYCLOHEXANE EXPERIMENTS: 1013 K – 1073 K Run nr. 6 7 8 9 Feed Cyclohexane [g/h] 288 288 288 298 N2 Dilution [g/h] 0 0 0 0 N2 Internal Standard [g/h] 30 30 40 50 Reactor temperature [K] 1013 1033 1053 1073 Temperature profile [K] CIT 565.8 580.4 595.2 609.6 T at 190 mm 741.3 763.2 783.0 801.1 T at 380 mm 769.5 790.0 810.8 834.1 T at 480 mm 739.0 760.2 779.7 798.8 T at 670 mm 733.7 759.0 780.4 797.4 T at 860 mm 740.0 760.4 779.5 799.7 T at 1050 mm 743.2 762.3 780.0 799.3 T at 1240 mm 742.5 763.0 781.3 801.6 T at 1430 mm 516.6 531.9 544.2 551.0 COT 475.0 456.4 475.9 461.7 Pressure profile [bar] CIP 1.7 1.7 1.7 1.7 COP 1.7 1.7 1.7 1.7 Product yield [wt. %] H2 0.720 0.935 1.159 1.409 C2H4 13.477 18.041 22.278 26.029 C2H6 4.679 5.220 5.613 5.402 C2H2 0.018 0.035 0.148 0.299 CH4 3.608 5.461 7.605 10.598 C3H8 0.151 0.202 0.243 0.246 C3H6 6.781 8.256 8.821 8.093 PD 0.023 0.034 0.044 0.210 i-C4H10 0.012 0.043 0.206 0.067 n-C4H10 0.032 0.040 0.041 0.034 t-2-C4H8 1.355 1.317 1.211 0.645 1-C4H8 1.447 1.359 1.155 0.592 i-C4H8 0.131 0.164 0.180 0.145 c-2-C4H8 1.086 1.058 0.988 0.530 MeAc 0.020 0.058 0.130 0.136 cyc-C3 0.000 0.000 0.000 0.000 1,3-C4H6 11.524 12.016 11.751 7.690 2-methyl-2-butene 0.152 0.193 0.133 0.016 2-methyl-1,3-butadiene 0.026 0.170 0.193 0.235 trans-2-pentene 0.097 0.118 0.062 0.049 trans-1,3-pentadiene 1.121 1.207 1.031 0.722 cyclopentane 0.313 0.310 0.258 0.226 1,3-cyclopentadiene 2.018 2.881 3.453 3.345 cyclopentene 0.529 0.653 0.536 0.423 C6H10 0.243 0.273 0.302 0.122 1,5-hexadiyne 0.234 0.169 0.000 0.000 Appendix A. Experimental Cyclohexane Pyrolysis Data 129
C6H8 0.305 0.030 0.017 0.000 cyclohexane 41.219 26.290 12.486 5.856 1,3,5-hexatriene 0.361 0.393 0.398 0.334 4-methylenecyclopentene 0.442 0.504 0.462 0.383 cyclohexene 1.456 0.840 0.471 0.224 benzene 4.196 7.150 10.842 14.573 toluene 1.287 2.061 2.821 3.486 C7H10 0.109 0.104 0.077 0.050 methylcyclohexadiene 0.157 0.016 0.021 0.034 xylene (m, p) 0.196 0.000 0.202 0.226 ethylbenzene 0.000 0.000 0.172 0.189 o-xylene 0.000 0.012 0.084 0.091 styrene 0.119 0.779 0.932 1.492 1-pentene 0.157 0.172 0.131 0.107 indene 0.042 0.376 0.723 0.942 C9H10 0.000 0.447 0.456 0.499 1-methyl-1H-Indene 0.042 0.092 0.011 0.163 3-methyl-1H-Indene 0.033 0.072 0.140 0.151 1,2-dihydro-Naphthalene 0.020 0.052 0.047 0.088 naphthalene 0.063 0.216 0.681 1.216 3-methyl-2-butene 0.000 0.155 0.112 0.096 1-methylene-1H-Indene 0.000 0.027 0.047 0.081 fulvene 0.000 0.000 0.091 0.083 1-ethenyl-2-methyl-benzene 0.000 0.000 0.259 0.259 indane 0.000 0.000 0.026 0.032 C11H10 0.000 0.000 0.096 0.068 C2 naphthalene 0.000 0.000 0.058 0.087 2-methyl-Naphthalene 0.000 0.000 0.085 0.138 C13H12 diaromatics 0.000 0.000 0.044 0.035 biphenyl 0.000 0.000 0.051 0.089 C10H8 0.000 0.000 0.074 0.000 1-isopropenylnaphthalene 0.000 0.000 0.034 0.065 acenaphthene 0.000 0.000 0.024 0.043 acenaphthylene 0.000 0.000 0.087 0.160 C12H10 0.000 0.000 0.016 0.025 C13H10 naphthenodiaromatics 0.000 0.000 0.116 0.233 anthracene 0.000 0.000 0.022 0.066 phenanthrene 0.000 0.000 0.042 0.135 C16H10 naphthenotriaromatics 0.000 0.000 0.000 0.011 pyrene 0.000 0.000 0.000 0.053 1-methyl-Naphthalene 0.000 0.000 0.000 0.144 C14H12 naphthenodiaromatics 0.000 0.000 0.000 0.155 C15H12 triaromatics 0.000 0.000 0.000 0.051 C15H10 triaromatics 0.000 0.000 0.000 0.186 C17H14 0.000 0.000 0.000 0.128 Fluoranthene 0.000 0.000 0.000 0.087 C17H12 naphthenotriaromatics 0.000 0.000 0.000 0.121
Appendix B. Use of Rate Rules for Linear Species in the Decalin Mechanism
Appendix B. Use of Rate Rules for Linear Species in the Decalin Mechanism
B.1 GAV-based rate coefficients in the decalin mechanism
For a large portion of the rate coefficients, GAV databases are used, because performing ab initio calculations for all possible reactions of decalin, decalyl radicals, etc. would be too time consuming for this study. GAV’s for the reaction families, as discussed in Chapter 2, have however specifically been derived for linear molecules and adopting them for cyclic molecules without any further notice could potentially lead to deviations from reality in the eventual model. Therefore, it is first assessed through the use of some case studies whether or not these GAV’s can be used without introducing too large an error. The considered reactions are: carbon centered H-abstraction from a secondary, tertiary and allylic carbon atom, and C-C-C β-scission. Figure B-1 shows a schematic representation of the methodology used for justifying the use of GAV-based rate coefficients for linear molecules in the decalin model. In case of only small deviation between ������� and ����, these rate coefficients can be safely adopted. A deviation of a factor five is generally accepted.
132 Appendix B. Use of Rate Rules for Linear Species in the Decalin Mechanism
Reaction in decalin pyrolysis model
Comparable reaction in methyl cyclohexane model
Considered reaction in group additivity scheme
Figure B-1 Methodology for justifying the use of GAV-based rate coefficients for linear molecules in the decalin pyrolysis model.
The reactions that are compared are summarized in Table B-1. For each reaction class, rate coefficients for the reactions of the cyclic species (������� ) are taken from high-level ab initio calculations by various authors, while rate coefficients for the considered reaction in the group additivity scheme (����) are taken from a n-hexane pyrolysis model which was automatically generated with Genesys in the context of this work. The results of this comparison are summarized in Table B-1. The values of ������� /���� in red are not within a factor of five and hence experience a too large discrepancy between ������� and ����. These reaction should be treated with extra care in the decalin model and cannot simply be adopted from rate rules for linear species.
Appendix B. Use of Rate Rules for Linear Species in the Decalin Mechanism 133
Table B-1 Comparison ������� and ����.
Reaction k(T) Ref. 900 K 1000 K 1100 K 1200 K 1[1], in- 1.71E+10 3.8E+10 7.29E+10 1.25E+11 house
7.79E+09 1.8E+10 3.57E+10 6.32E+10 GAV
�������/���� 2.32 2.20 2.11 2.04
3.11E+09 8.37E+09 1.88E+10 3.7E+10 1[1]
4.06E+10 7.68E+10 1.29E+11 2E+11 GAV
�������/���� 0.05 0.08 0.11 0.15
2.94E+09 6.07E+09 1.16E+10 2.1E+10 2[2]
8.02E+09 1.64E+10 2.95E+10 4.8E+10 GAV
�������/���� 0.40 0.37 0.37 0.40
5.41E+07 8.64E+07 1.27E+08 1.74E+08 2[2]
3.74E+06 1.84E+07 6.76E+07 2.00E+08 GAV
�������/���� 58.87 14.45 4.70 1.87
B.2 References
1. Wang Z, Cheng Z, Yuan W, et al. An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure. Combustion and Flame. 7// 2012;159(7):2243-2253.
2. Wang Z, Ye L, Yuan W, et al. Experimental and kinetic modeling study on methylcyclohexane pyrolysis and combustion. Combustion and Flame. 2014;161(1):84-100.