High-Temperature and High-Pressure Pyrolysis of Hexadecane

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High-temperature and High-pressure Pyrolysis of Hexadecane: A Molecular Dynamic Simulation Based on Reactive Force Field (ReaxFF) Zhuojun Chen, Weizhen Sun, and Ling Zhao J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b12367 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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The Journal of Physical Chemistry

High-temperature and High-pressure Pyrolysis of Hexadecane: Molecular Dynamic Simulation Based on Reactive Force Field (ReaxFF) Zhuojun Chen, Weizhen Sun* and Ling Zhao State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China ABSTRACT: As important products of heavy oil pyrolysis, heavier components such as gasoline and diesel supply the vast majority of energy demand through combustion, and lighter components such as ethylene and propylene are main sources of industrial chemicals and plastics products. In this work, pyrolysis of hexadecane, as the model compound, was studied by reactive force field (ReaxFF) molecular simulation at high temperatures and high pressures. It was confirmed by unimolecular simulations that there exist eight different initial mechanisms all starting with C-C bond dissociation. The biradical mechanism was verified, through which the pyrolysis process can be accomplished within shorter time. The enthalpy of reaction was calculated by QM method, which was well consistent with ReaxFF calculation results. Multimolecular simulations showed that there is a strong dependency relationship between products distribution and temperature, as well as that between reaction rates and temperature. The optimal condition for ethylene formation in our work is 11.6 MPa and 2000 K, while it is best for hydrogen formation at conditions of 11.6 MPa and 3500 K. Kinetic analysis was performed with the activation energy of 113.03 kJ/mol and pre-exponential factor of 4.55×1012, and it is in good agreement with previous work.

1. INTRODUCTION Pyrolysis of heavy oil plays a vital role in the modern industries since the products, i.e. various straight-chain and branched-chain chemicals are all widely used in energy and chemical industry.

1 ACS Paragon Plus Environment


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Combustion of organic fuels including gasoline, kerosene, and diesel meets the 85% energy demand,1 which is still increasing year by year.2 Additionally, as the primary products and fundamental chemical raw materials, ethylene and propylene consumption are steadily rising.3-6 According to reports, the majority of ethylene and propylene used in industries are produced by pyrolysis of heavy oil.7-8 To release the pressure of energy shortage and improve the utilization efficiency of energy, production of low-carbon fuel9-10 and even no-carbon fuel11 at less cost has been becoming a new task for the research of heavy oil pyrolysis.12 Heavy oil is a complex mixture that involves various kinds of components with larger relative molecular weight, including not only hydrocarbons and non-hydrocarbons but also the colloidal substances and asphalt, therefore it brings many difficulties to relevant researches, especially to kinetic studies. An alternative solution is to use one model compound to represent heavy oil. For example, hexadecane has been chosen as the model compound of heavy oil by many researchers1, 1316

over several decades due to its wide existence in heavy oil and low cost. Besides, its relative

molecular weight is equivalent to average molecular weight of diesel. Fabuss et al.17 studied the rapid thermal cracking of n-hexadecane experimentally at elevated temperatures from 593 °C to 649 °C and with the pressure range of 13-69 bar, resulting in the hexadecane conversions between 1% and 97%. They concluded that polymerization reactions were dominant at the beginning, and the activation energy of pyrolysis reaction (first-order) declined with temperature. Ford et al.18 performed experiments for liquid-phase thermal decomposition of hexadecane with temperature range of 330-420 °C, which focused on the studies of reaction mechanisms. The intermediates and products from C2 to C15 were detected including straight-chain and branched-chain alkenes as well as alkanes, and the activation energy was determined to be 238 kJ/mol. Jackson et al.14, 19 measured the pressure effects on n-hexadecane cracking rates with temperature range of 300-370 °C and pressure range of 15-60 MPa. It was found that lighter compounds came from the single first-order reaction


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kinetics, while heavier compounds were originated from a series of chain extension reactions. And then heavier compounds followed subsequent destruction reaction with the activation energy of 251 kJ/mol. Pressure might have a retarding effect on cracking rate of n-hexadecane.14, 19 Experiments usually provide necessary information to reactions, however, reactions such as pyrolysis, combustion and explosion can be extremely fast at higher temperatures,20 and would produce considerable intermediates including molecules and radicals through various complex reaction mechanisms and pathways. In this sense, experiments are far from enough for deep comprehensions of pyrolysis reactions. Reactive force field (ReaxFF) is a quite useful tool for predicting performance and developing new materials under extreme conditions for its excellent capability on dealing with bond dissociation and bond formation. Using ReaxFF, larger system (millions of atoms) can be computed within shorter simulation time at relative high accuracy,21-22 which could reach the level of quantum chemistry. In recent years, pyrolysis and combustion researches using ReaxFF method have been performed, such as thermal decomposition of poly (dimethylsiloxane) by Chenoweth et al.,23 combustion of 1,5-dinitrobiuret (DNB) by Russo et al.,24 thermal and catalytic cracking of 1-heptene by Castro-Marcano et al.25 and pyrolysis and combustion of JP-10 hydrocarbon jet fuel by Chenoweth et al.26 ReaxFF also showed good applicability in other fields.27-35 In this work, the ReaxFF simulation was firstly utilized to investigate the pyrolysis of hexadecane. Here, we carried out the pyrolysis simulation of the model compound hexadecane at high temperatures to analyse the pyrolysis process at atomistic level and provide more detailed information concerning reaction mechanism and reaction pathway.

2. COMPUTATIONAL METHODOLOGY



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MD-NPT simulations 21.37 Å

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30.4 Å

Heating stage

Figure 1. The construction procedure (from a hexadecane molecule to the simulation box containing 20 hexadecane molecules) and pre-heating procedure (the volume of simulation box changed during NPT ensemble). MD means molecular dynamics, NPT means simulation with constant number of atoms, constant pressure, and constant temperature. 2.1. Reactive force field. Reactive force field (ReaxFF) is a newly emerged force field proposed by van Duin et al.,22 which can describe the chemical reactivity during the molecular simulations. It is driven by the total energy of the system that can be calculated by adding up all the terms of partial energy, including bond energy (Ebond), over-coordination energy (Eover), under-coordination energy (Eunder), valence angle energy (Eval), penalty energy (Epen), torsion energy (Etors), conjugation energy (Econj), and non-bond interactions van der Waals (EvdWaals) and Coulomb (ECoulomb), shown in Eq.(1). All these terms of energy is calculated on the basis of bond orders, which originate from the distance between two atoms. Reactive force field provides the position of each atoms rather than the connectivity information between them, and a 0.3 bond order cut-off36 was utilized in this work. Moreover, as the kernel of ReaxFF, the force field containing numerous parameters is trained against highly accurate DFT calculations and experiments, and the effects of polarization are managed through the EEM (electronegativity equalization method).37-38 Therefore, the reactive force field can provide highly accurate simulation results for reactive system. In this work, we used the Chenoweth


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CHO-2008 force field that has been successfully developed and utilized in previous work21, 39-40 and the “reax” package of LAMMPS41 to simulate the thermal pyrolysis of hexadecane. The Chenoweth CHO-2008 force field data was provided in Supporting Information file. Esystem = Ebond + Eover + Eunder + Eval + Epen + Etors + Econj + EvdWaals + ECoulomb 2.2. Molecular dynamic simulations.

(1)

To study the pyrolysis mechanisms and pathways of

hexadecane, we firstly conducted 20 separate unimolecular simulations. One hexadecane molecule was randomly put in a periodic box. The hexadecane molecule firstly ran a short equilibrium simulation at low temperature of 100 K, then the system was heated up to 3000 K within 5 ps with the time step of 0.05 fs and temperature damping constant of 100 fs. After that, the system was maintained at 3000 K for 150 ps. Short time step of 0.05 fs was applied for not leaving out each possible reaction. Species including radicals and intermediates as well as products with corresponding time steps were listed at output files. By analyzing these data step by step, pyrolysis pathways can be deduced. To further validate the accuracy of reactive force field, the enthalpy of formation of species observed during the unimolecular dynamic simulations were calculated by QM method at the B3LYP/6-311G++(d, p) level of theory26, 42-44 of Gaussian 09, and then the energies of reaction pathways calculated by QM and ReaxFF are compared and discussed. To predict the non-catalytic thermal cracking performance at high temperatures and high pressures, the multimolecular simulations were conducted. The construction and simulation processes are as follows: 1) Construction of boxes and preliminary adjustment of pressure: Cubic simulation boxes with the length of 21.37 Å containing 20 random hexadecane molecules were built at room temperature and atmospheric pressure, resulting a liquid-phase system with the density of 0.77 g/ml containing 1000 atoms. Simulation boxes were constructed with periodic boundary conditions. Then the system was simulated for 50 ps under NPT conditions (constant number of atoms and pressure, and controlled temperature) using Berendsen thermostat and barostat to pre-



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adjust the pressure, resulting in a larger simulation box, for example, the box with the length of 30.4 Å at 1500 K was displayed in Figure 1. All these steps were done by Material Studio software. 2) Minimization and low-temperature equilibration: The systems were energy-minimized and then undergone a low-temperature equilibration simulation at 100 K. 3) Heating stage: NPT ensemble was employed to heat up the systems (50 ps). For temperature-dependence simulations, the pressure was adjusted to 10 MPa at the end of heating stage, and for pressure-dependence simulations, the pressures were altered to 10 MPa, 30 MPa, 50 MPa, 70 MPa, and 90 MPa respectively, since these pressure conditions can give a relatively complete pressure scales for realistic and virtual conditions. 4) High temperature NVT simulations: NVT ensemble (constant number of atoms and volume, and controlled temperature) was performed for a simulation time of 500 ps. The time step was 0.1 fs suggested by Chenoweth et al.21 and temperature damping constant was 100 fs. Temperatures setting included 1500 K, 2000 K, 2500 K, 3000 K, and 3500 K. All the conditions of the simulation are also listed in Table 1. The step 2) ~ 4) were conducted by LAMMPS software. It is noteworthy that in realistic thermal pyrolysis of hydrocarbons, it usually need microsecond to millisecond, but for

Table 1 Parameters of unimolecular and multimolecular simulations.

No.*

Number of Hexadecane Molecule

Initial Pressure (MPa)

Temp (K)

1 2 3 4 5 6 7 8 9 10

1 20 20 20 20 20 20 20 20 20

10 10 10 10 10 30 50 70 90

3000 1500 2000 2500 3000 3500 3000 3000 3000 3000

*No. 1 is for the unimolecular simulation; No. 2 to 6 is for temperature-dependent simulations; No. 5 and No. 7 to 10 are for pressure-dependent simulations.


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reactive simulations, the time scale is normally limited to several dozens of nanoseconds due to the computational cost. Hence, we enhance the temperatures to accelerate the simulation process.

3. RESULTS AND DISCUSSION 3.1. Unimolecular pyrolysis simulations. To

study

the

initial

pyrolysis

pathways

of

hexadecane and validate the availability of reactive force field, unimolecular simulations were carried out in a periodic box of 35.0 Å×35.0 Å×35.0 Å with a single hexadecane molecule at 3000 K for about 150 ps using NVT ensemble. Quantum chemistry method was utilized to calculate the enthalpy of reactions at the B3LYP/6-311G++ (d, p) level of theory. The 20 independent simulations were performed to collect proper statistical results and analyse possible mechanisms. According to previously published work26, 45-46, 20 times of simulations used in this work are adequate. From these unimolecular simulations, we obtained eight different kinds of initial reactions, which were all initiated by C-C bond

Table 2 Initial pyrolysis mechanisms of hexadecane observed within 20 independent unimolecular simulations Name

Reactions

∆EQM/ （kcal/mol）

∆EReaxFF/ （kcal/mol）

Observed Times

Initial A

C16H34 → •CH3 + •C15H31

81.99

76.70

1

Initial B

C16H34 → •C2H5 + •C14H29

78.88

74.69

1

Initial C

C16H34 → •C3H7 + •C13H27

79.41

75.16

4

Initial D

C16H34 → •C4H9 + •C12H25

79.20

71.22

3

Initial E

C16H34 → •C5H11 + •C11H23

78.63

62.64

2

Initial F

C16H34 → •C6H13 + •C10H21

79.21

62.81

3

Initial G

C16H34 → •C7H15 + •C9H19

79.23

63.48

5

Initial H

C16H34 → •C8H17 + •C8H17

79.21

68.53

1 13

dissociation. And some of these reactions were reported in previous work .



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Moreover, no reactions occurred before C-C bond-breaking except for chain bending and stretching. Details are listed in Table 2. According to Initial A of Table 2, hexadecane molecule decomposed into methyl and pentadecyl through C-C bond dissociation. This initial mechanism has the highest enthalpy of reaction of 81.99 kcal/mol by Quantum chemistry and 76.70 kcal/mol by ReaxFF, and was observed once. In the process of Initial B, hexadecane molecule decomposed into ethyl and myristyl with the enthalpy of reaction of 78.88 kcal/mol by Quantum chemistry and 74.69 kcal/mol by ReaxFF, and was observed once. In Initial C, hexadecane molecule decomposed into propyl and tridecyl with the enthalpy of reaction of 79.41 kcal/mol by Quantum chemistry and 75.16 kcal/mol by ReaxFF, and was observed for four times. As seen from Initial D, the hexadecane molecule decomposed into butyl and dodecyl with the enthalpy of reaction of 79.20 kcal/mol by Quantum chemistry and 71.22 kcal/mol by ReaxFF. This reaction was observed for three times. According to Initial E, hexadecane molecule decomposed into pentyl and undecyl with enthalpy of reaction of 78.63 kcal/mol by Quantum chemistry and 62.64 kcal/mol by ReaxFF, and was observed twice. In Initial F, hexadecane molecule decomposed into hexyl and decyl with the enthalpy of reaction of 79.21 kcal/mol by Quantum chemistry and 62.81 kcal/mol by ReaxFF. This mechanism was observed for three times. In Initial G, hexadecane molecule decomposed into heptyl and nonyl with the enthalpy of reaction of 79.23 kcal/mol by Quantum chemistry and 63.48 kcal/mol by ReaxFF, and was observed for five times. As seen from Initial H, hexadecane molecule decomposed into two octyl radicals with the enthalpy of reaction of 79.21 kcal/mol by Quantum chemistry and 68.53 kcal/mol by ReaxFF, and was observed once. Above eight initial reactions contained all the possible position of C-C bond cleavage because of the symmetrical structure of hexadecane molecule. These eight initial reactions


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were all observed in previous experimental research of Ristori et al.13 The small energy difference between Quantum chemistry and ReaxFF could ascribe to the underestimation of radicals in ReaxFF.46 The general conformity in enthalpy of reactions validates the accuracy of ReaxFF. Table 3 Selected Subsequent Reactions Following Different Initial Reactions Observed in Unimolecular Simulations Initial No. Reactions Products Reactions 1

CH3-CH2-CH2-•CH2 → CH3-CH2-CH2=CH2 + H•

Initial B

2

•CH2-[(CH2)7]-CH2-•CH2 → •CH2-[(CH2)7]-•CH-CH3

Initial C

3

CH3-[(CH2)10]-CH2-•CH2 → CH3-[(CH2)10]-CH=CH2 + H•

Initial C

4

CH3-[(CH2)10]-CH=CH2 → CH3-CH2-CH2-•CH2 + •CH2-[(CH2)6]-CH=CH2

Initial C

5

CH3-CH2-•CH2 → CH3-CH=CH2 + H•

Initial C

6

CH3-CH2-•CH2 → CH3-•CH-CH3

Initial C

7

•CH2-CH2-CH2-•CH2 →CH3-CH2-CH=CH2

Initial C

CH3-[(CH2)8]-CH2-•CH-CH3 → CH3-[(CH2)8]-CH-CH3 8

•CH2

Initial D

Among the trajectories of 20 independent unimolecular pyrolysis simulations, eight significant subsequent reactions are listed in Table 3. It should be noted that the reactions



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listed in Table 3 are selected from many subsequent reactions following various Initial reaction mechanisms such as Initial A, Initial B and so on. They are specifically chose since these reactions produce valuable intermediates and products. As a result of this, the first reaction listed in Table 3 is not just the secondary reaction of the primary reaction initial B. The same rule applies to other reactions listed in Table 3. Accordingly, eight specific subsequent reactions are selected, and one reaction is from Initial B mechanism, six reactions are from Initial C mechanism, and one reaction is from Initial D mechanism. The involved reactions include hydrogen dissociation, hydrogen transfer, biradical self-consistent and chain branching. Reaction 1 shows β-dissociation of hydrogen atom from butyl radical and formation of 1-butene. This secondary reaction may be a possible pathway of 1-butene formation. Similar β-dissociation of hydrogen atom is shown in reactions 3 and 5, in which 1tridecene and 1-propene were generated individually, and tridecene further decomposed into smaller species while propene did not. Hydrogen transfer also occurred in secondary reactions shown in reaction 5. End-group propyl radical transformed into middle-group radical. Such hydrogen transfer also found in biradical like decyl biradical shown in reaction 4. One hydrogen on β-carbon transferred to α-carbon resulting another biradical. Reaction 7 shows a self-consistent process of butyl biradical turning into 1-butene, and reaction 8 shows the process of chain branching of dodecyl. The snapshots of species during the unimolecular pyrolysis simulations were also attached in Table 3. It is worth mentioning that some biradicals such as •C11H22• and •C9H18• are generated in the following reaction of Initial C (in Table 4). Here •C11H22• means undecyl biradical containing unpaired electron at each end of the group, similarly, •C9H18• is nonyl biradical containing unpaired electron at each end of the group. Biradicals from pyrolysis processes were difficult to detect with existing analysis methods, because were extremely unstable, which quickly dissociated into other


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smaller species within several femtoseconds, and analysis methods are limited, hence there is almost no relative experiment reports. Such biradicals have been reported before by O’Neal et al.,47 Milov et al.,48 Nagata et al.49 and so on. ReaxFF researches mentioned biradicals were also reported before. Chenoweth et al.26 conducted ReaxFF pyrolysis simulation of JP-10, and they proposed that biradicals were generated from dissociation of C2H4. Liu et al.50 simulated the pyrolysis of highdensity polyethylene and found biradicals in long chains. Zhang et al.51 simulated the thermal cracking of triglyceride at high temperatures and found that cyclopentane could be generated by intramolecular cyclization of a C5 biradical. Together with our results, biradicals tend to transform into smaller species quickly within several femtoseconds before self-consistent phenomenon took place, revealing that possible existence condition of biradicals could be high energy supplied from Table 4 Biradical Pathways Observed in Initial C of Unimolecular Simulations

Name Initial C1+

∆EQM/

∆EReaxFF/

(kcal/mol)

(kcal/mol)

79.41

75.16

78.88

69.53

37.95

39.06

17.90

8.54

31.32

23.32

Reactions C16H34→ CH3-CH2-•CH2+ CH3-[(CH2)10]-CH2-•CH2 CH3-[(CH2)10]-CH2-•CH2→ •CH2-[(CH2)9]-•CH2+

Initial C2+ CH3-•CH2 Initial C3+

CH3-•CH2→ CH2=CH2+ H• •CH2-[(CH2)9]-•CH2→ •CH2-[(CH2)7]-•CH2+

Initial C4+ CH2=CH2 •CH2-[(CH2)7]-•CH2→ •CH2-CH2-CH2-•CH2 Initial C5+ + CH3-CH=CH2+ CH2=CH2 Initial C6+

•CH2-CH2-CH2-•CH2→ CH2=CH2+ CH2=CH2

-42.71

-37.18

Initial C7+

CH3-CH2-•CH2→ CH2=CH2+ •CH3

24.83

27.38

Initial C8•CH3+ H•→ CH4 -103.01 -106.58 (Symbol + represents the number of species in the reaction is increasing, symbol - represents the number of species in the reaction is declining. The number simply represents the rank ordering in this Table.)



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environment. According to observed reaction pathway following Initial C shown in Table 4, the hexadecane molecule firstly dissociated into •C13H27 (tridecyl) and •C3H7 (propyl, non-end group radical) with the enthalpy of reaction of 79.41 kcal/mol calculated by QM method and 75.16 kcal/mol obtained by ReaxFF method. The tridecyl radical then released an ethyl radical remaining an undecyl biradical (•C11H22•), and the ethyl radical released one hydrogen radical leaving an ethylene molecule. Subsequently, chain-breaking reactions proceeded and lasted for dozens of femtoseconds, producing

a) 0 ps

b) 15.3 ps

c) 15.8 ps

d) 16.5 ps

e) 16.6 ps

f) 21.7 ps

g) 22.3 ps

h) 22.8 ps

Figure 2. Graphical unimolecular reaction mechanism of Initial A at 3000 K. a) Unreacted hexadecane molecule; b) A •CH3 radical (methyl) dissociated from the long chain at 15.3 ps; c) A •C3H7 radical (propyl) dropped from the long chain and produced a •C12H24• biradical at 15.8 ps; d) An ethylene molecule dropped from the •C12H24• biradical and •C10H20• biradical was generated at 16.5 ps; e) The second ethylene molecule generated at 16.6 ps leaving a •C8H16• biradical; f) The third ethylene dissociated from •C8H16• biradical at 21.7 ps, resulting a •C6H12• biradical; g) The •C6H12• biradical further released an ethylene molecule at 22.3 ps, leaving a •C4H8• biradical; h) The •C4H8• biradical cracked into two ethylene molecules at 22.8 ps.


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nonyl biradical (•C9H18•), butyl biradical (•C4H8•), and finally resulted in the formation of four ethylene molecules, one methane molecule and one propylene molecule. The formation of methane is a strongly exothermic reaction, which occurred in this unimolecular pathway. This may be attributed to higher temperature used in our work and it is reasonable according to previous work.24, 46 Figure 2 depicts another biradical mechanism following Initial A. The biradical was first generated at 15.8 ps by dissociation of species from different side of the long chain, and cracked into smaller species through ethylene abstraction reactions. Finally, four ethylene molecules were produced at 22.8 ps. Compared with other mechanisms, biradical mechanism proceeded faster which could accomplish within dozens of picoseconds, for biradicals are extremely unstable with high chemical activity and can only exist for a relative short time. Such biradical mechanism has not yet been reported in experimental studies of hexadecane before and can helps to understand the processes of pyrolysis under high temperatures. 3.2. Multimolecular molecular simulations 20 hexadecane molecules were randomly put and equilibrated for a short time of 5 ps in a simulation box with the length of 21.37 Å at room temperature, and then an NPT dynamic simulation was carried out at 1500 K, 2000 K, 2500 K, 3000 K, 3500 K respectively to adjust the temperature and density of the system. Finally, NVT simulations were conducted on every system for 500 ps with the time step of 0.1 fs and the temperature damping constant of 100 fs.

a) 90

H2

80

C2H3

CH

CH2

CH3

b) 180

C2H2

C16 C11 C6

160

C2H4

70

140 Number of species

Number of species

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 50 40 30 20

C15 C10 C5

C14 C9 C4

C13 C8 C3

C12 C7 C2

C1

120 100 80 60 40 20

10

0

0 0

100

200

300

400

500

0

100

200

300

400

500

Time (ps)

Time (ps)

Figure 3. Time evolution of a) number of typical species, b) number of Cn in multimolecular simulations of 3500 K, initial13pressure 11.6 MPa system ACS Paragon Plus Environment


3.2.1 Temperature-dependent simulations

a) 80

1500 K 3500 K

2000 K

2500 K

3000 K

400

1500 K 3500 K

350

60 Number of C16

b) Total number of species

70

50 40 30 20

2000 K

0

300 250 200 150 100

100

200

300

400

0

500

100

200

80

1500 K 3500 K

70

2000 K

300

400

500

Time (ps)

Time (ps)

2500 K

3000 K

d)

80

1500 K 3500 K

70

60

2000 K

2500 K

3000 K

60 Number of C2H2

Number of C2H4

3000 K

0

0

c)

2500 K

50

10

50 40 30

50 40 30

20

20

10

10

0

0

0

100

200

300

400

500

0

100

200

e) 160

1500 K 3500 K

140

2000 K

300

400

500

Time (ps)

Time (ps) 2500 K

3000 K

f)

100

1500 K 3500 K

90

2000 K

2500 K

3000 K

80 Number of H2

120 Number of C2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100 80 60 40

70 60 50 40 30 20

20

10 0

0 0

100

200

300

400

0

500

100

200

300

400

500

Time (ps)

Time (ps)

Figure 4. Time evolution of the number of a) hexadecane molecules, b) kinds of species, c) ethylene molecules, d) ethyne molecules, e) C2 species, f) hydrogen molecules at 1500 K, 2000 K, 2500 K, 3000 K, 3500 K, respectively during NVT simulations. Same as unimolecular simulations, chain bending and stretching took place frequently at the beginning and gradually replaced by bond dissociation reactions. Figure 3 a) shows the evolution of


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number of typical species at 3500 K. It can be seen that ethylene molecule were the firstly generated species which is in good agreement of our unimolecular simulation results. The number of ethylene molecule sharply increased and peaked at 100-150 ps with the number of about 50, and then gradually decreased to 30 at the end of the simulation. Meanwhile, the number of hydrogen gradually rose to about 75 together with the number of ethyne increasing to about 45. Dehydrogenation of ethylene could be the reasonable explanation of this phenomenon. Besides, some radicals like •C2H3 and •CH3 et al. occupied some proportion because of severe vibration under such a high temperature. The number of •C2H3 increased to about 30 within 100 ps and fluctuated around 25, and the number of other species were undulate within 30. Additionally, the yield of ethylene reached about 21.5 % that could match the yield of ethylene from industrial catalytic pyrolysis of heavy oil. This demonstrates that the simulation time of 500 ps is reasonable. In order to obtain an overview of pyrolysis of hexadecane, all the species/products were classified into 16 categories as C1, C2, C3 …, C16, where Cn represent the number of C atom in the species/products. The number of Cn is showed in Figure 3 b). It is clear that C2 is most popular species whose number sharply rose up to 100 at 100 ps and then slightly increased to 110 during the following 450 ps. According to unimolecular simulations discussed before, ethylene abstraction a major pathway from large molecules to small ones, and it will further cracked into ethylene radical and ethyne that were all counted into C2. Another popular category is C1 whose number increased to about 60 within 200 ps and slightly decreased during the rest simulation time. The number of C3, mainly including propylene and allyl, firstly grew to about 20 and then declined slightly. The number of other species is much less. Consequently, the main products of pyrolysis of hexadecane under high temperature were C2, C1, and C3. Compared with previous experimental studies14, 18-19, our main products were of lower molecular mass and it indicates that 500 ps is reasonable for high temperature pyrolysis simulation of hexadecane.



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The effect of temperature on consumption of hexadecane molecules is shown in Figure 4 a). A faster reaction rate can be found in the system of higher temperature. All the reactant molecules were exhausted in the system of 2500 K, 3000 K and 3500 K within the respective time of about 295 ps, 80 ps, and 25 ps, that is to say that the consumption rate of hexadecane is positively correlated with temperature. This phenomenon demonstrates that temperature has a substantial effect on pyrolysis reaction rate. Figure 4 b) shows the time evolution of the total number of species in each system. More intermediates were generated at higher temperatures with the maximum of 300 and the minimum of 70. However, the increasing rate is gradually reducing with temperatures rising. Combining Figure 4 c) and d) it can be concluded that the number of ethylene (C2H4) was increasing at the beginning as temperature increases, but decreasing after about 150 ps at 3000 K and 3500 K. The number of ethyne (C2H2) gradually rose especially in the system of 3500 K in which the number of ethylene has the sharpest drop. This could be ascribed to the dehydrogenation of ethylene, which has been discussed in unimolecular simulations. By controlling the temperature and reaction time, either ethylene or ethyne can be selectively harvested. Figure 4 e) shows the time evolution of the number of C2 at different temperatures. The number of C2 in system of 3500 K reached the equilibrium value of about 120, followed by that of 3000 K and 2500 K. The time evolution of the number of hydrogen molecules is shown in Figure 4 f). A high value is reached for the system of 3500 K while it approaches 0 for the system of 1500 K, which indicates temperature has a huge influence on hydrogen formation. In sum, the production of low-carbon fuel and even no-carbon fuel was closely related to temperature and reaction time. Moreover, the number of larger molecular weight olefins and paraffins are extremely little due to higher simulation temperatures. 3.2.2 Pressure-dependent simulations In pressure dependent simulations, the pressure of systems gradually rose during the last NVT process, because large molecules and radicals decomposed into smaller species. The pressure at the


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a)

H2 (temperature-dependent simulations) H2 (pressure-dependent simulations)

b)

4

C2H4 (temperature-dependent simulations) C2H4 (pressure-dependent simulations) 90 80 70

Number of C2H

60 50 40

00 20

10 0

60

Pre s

20

0

su re

40

) (K

35 00

20

0

80

re tu ra

3 0 00

40

Pre ssu re (

2 5 0 0

60

pe m Te

MP a)

80

00 20

12 0

0 00 15

10 0

(M Pa )

12 0

0 00

15

30 20 10

35 00

30 20 10

3 00 0

60 50 40

2 5 0 0

Number of H2

90 80 70

re tu ra pe m Te

) (K C2H2 (temperature-dependent simulations) C2H2 (pressure-dependent simulations)

d)

H2

C2H4

C2H2

90 80 70

00 20

60

2 5 0

40

35 00

20

0

10 0

su re

15

80

3 00 0

35 00

Pre ssu

30 00

20

0

12 0

0 00

re tu ra pe m Te

2 5 0 0

60

40

MP a)

00 20

10 0

re (

80

Pre s

12 0

0 00

15

30 20 10

0

30 20 10

60 50 40

(M Pa )

60 50 40

typical Number of

products

90 80 70

Number of C2H

2

c)

re tu ra pe m Te

)

) (K

(K

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Distribution of typical products under the effects of temperature and pressure at the end of simulation time, a) H2, b) C2H4, c) C2H2, d) H2, C2H4, and C2H2 together. (Temperature-dependent simulations were carried out at 1500 K, 2000 K, 2500 K, 3000 K, and 3500 K respectively; pressuredependent simulations were carried out with the final pressures of 11.6 MPa, 34.5 MPa, 57.7 MPa, 80.7 MPa, and 103.6 MPa respectively, 3000 K.) end of simulation of 10 MPa, 30 MPa, 50 MPa, 70 MPa, and 90 MPa were 11.6 MPa, 34.5 MPa, 57.7 MPa, 80.7 MPa, and 103.6 MPa respectively. Time evolution of reactants and products in pressure-dependent simulations did not give a regular relationship between pressure and reactants/products except for the positive correlation between the number of H2 and the pressure, and



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the curves overlapped each other (Provided in Supporting Information file). Besides, effects of pressure are not as strong as temperature, so the pressure-dependent simulations and temperaturedependent simulations were combined together to study the distribution of main products. Figure 5 a) shows the formation of H2 under the condition of temperature-dependent simulations (red symbol) and pressure-dependent simulations (blue symbol), in which the number of H2 molecules increases sharply with temperatures, resulting in a quantitative range from 0 to 79, while that slowly increases with the rising pressure. The formation of H2 can be attributed to the following two sources: 1) frequent collision of H• at high temperatures; 2) dehydrogenation of C2H4 or other hydrocarbons. Temperature could accelerate both frequency of collision and rate of dehydrogenation, however, pressure could only facilitate the collision. The formation of C2H4 is shown in Figure 5 b). The number of C2H4 grows with increasing temperature from 1500 K to 2000 K, but declines with temperature from 2000 K to 3500 K. Similarly, increasing pressure has a retarding effect on ethylene which is in accordance with previous researches.14, 19 The reason of reduction may lie in that as for dehydrogenation of C2H4, as temperature rises, more H• and H2 drop from C2H4, resulting in formation of more C2H2, whose number grows with increasing temperature as shown in Figure 5 c). Table 5 Comparison of the activation energy obtained by previous experimental researches and ReaxFF dynamic simulation. Sources

Temperature ranges/K

Ea/(kJ/mol)

Ref.16

573-643

309.8

Ref.

52

790-862

253.7

Ref.

17

866-922

162.4

Ref.

17

922-977

115.5

1500-3500

113.03

This work

Pressure influenced little on the production of C2H2. Figure 5 d) presents the temperature evolution and pressure evolution of the numbers of all the typical products, in which the condition of 11.6 MPa, 2000 K is the optimized combination for C2H4 because of the high ethylene proportion.


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For production of H2, the condition of 11.6 MPa and 3500 K is the best choice. Clearly, high pressure does not contribute to the yield of fuels, but increases the cost of production. 3.2.3. Kinetic analysis of pyrolysis The multimolecular pyrolysis simulation was conducted on a series of temperatures, which range from 1500 K to 3500 K with the temperature interval of 500 K to investigate the kinetic properties of the first step of hexadecane pyrolysis shown in Reaction (a), where parameter m and n are both positive integer and their numerical sum is 16. These simulations were carried out with the time step of 0.1 fs and temperature damping constant of 100 fs. The logarithm of the reaction rate constant (ln k) is plotted versus the reciprocal of temperature (1/T) in Figure 6, where a First-order kinetics is displayed. By linear fit of these splashes, the activation energy (Ea) and pre-exponential factor (A) were calculated using the adapted Arrhenius equation Eq. (2) and Eq. (3), in which the concentration of reactants was simply replaced by the number of reactant molecules.30, 45 C16→ Cm+ Cn

(a)

ln Nt – ln N0 = - kt

(2)

ln k = ln A- Ea /RT

(3)

The activation energy of hexadecane pyrolysis is 113.03 kJ/mol, calculated from slope value of the linear fitting of ln k versus 1/T, which is expressed in Eq. (3). The pre-exponential factor of hexadecane pyrolysis is calculated to be 4.55×1012. The activation energy Ea obtained in this work was compared with previous experimental results. The reported activation energy of pyrolysis of hexadecane varies from 115.5 kJ/mol to 309.8 kJ/mol,16-17,

52

as shown in Table 5. It can be seen that the activation energy declines with the

increasing temperatures. This trend is in good agreement with the conclusion proposed by other researchers, that is, the value of activation energy is variable within a wide range of conditions,53-54



27 26

ln k= - Ea/RT+ lnA Ea=113.03 kJ/mol

25

A= 4.55x10

12

24 -1

ln k (s )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

23 22 21 20 19 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 -1

1000/T (K )

Figure 6. Kinetic analysis of pyrolysis simulation at 1500 K-3500 K and the same trend was proven by Kenneth J. Jackson’s experimental results.14 Additionally, the preexponential factor is consistent with previous work on magnitude.13 Overall, calculated results by ReaxFF pyrolysis simulation of hexadecane are in good agreement with the experimental values.

4. CONCLUSIONS In this work, reactive force field (ReaxFF) molecular simulation was performed to investigate the thermal cracking process of hexadecane system. For unimolecular pyrolysis simulation, the box containing a single hexadecane molecule was simulated separately under 3000 K for 20 times, based on which eight different initial mechanisms have been developed. According to these mechanisms, the hexadecane molecule cracking started with C-C bond dissociation and some biradical species that have been confirmed by previous researchers were detected. Cracking through biradical mechanism could be finished within shorter time compared to normal (non-biradical) dissociation mechanisms. By comparing the enthalpy of reactions calculated by two methods—QM method and ReaxFF method, similar values of enthalpy of reactions were obtained, which verified that ReaxFF is accurate to study the pyrolysis process of hydrocarbons.


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Multimolecular simulations were conducted on boxes containing 20 hexadecane molecules with temperature range of 1500-3000 K and initial pressure range of 10-90 MPa. Temperature-dependent simulations and pressure-dependent simulations were carried out separately, indicating that the conditions of 11.6 MPa and 2000 K was the optimized combination for C2H4 formation, and conditions of 11.6 MPa and 3500 K are the best choice for H2 formation. High pressures were found not suitable for the yield of low-carbon and H2 fuels to some degree. In addition, kinetic analysis was performed through linear fitting of adapted Arrhenius equation with the activation energy of 113.03 kJ/mol and pre-exponential factor of 4.55×1012 s-1, which is consistent with previous results.

AUTHOR INFORMATION

Corresponding Author

*(W.S.) E-mail: [email protected]. Telephone: +86 21 64253027

Notes

The authors declare no competing financial interest.

SUPPORTING INFORMATION Force field data and figures of time evolution of products and reactant molecules in pressure dependent simulations are provided in the supporting information file.

ACKNOWLEDGEMENTS The financial support by the National Natural Science Foundation of China (91434108) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars is gratefully



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acknowledged. The authors also would like to acknowledge Professor Adri C.T. van Duin from Penn State University for providing force field parameters and Professor Li-Chiang Lin from Ohio State University for fruitful discussions and valuable comments.

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TOC Image:


High-Temperature and High-Pressure Pyrolysis of Hexadecane

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