This presentation was prepared for my physics PhD dissertation defense. I studied the pyrolysis of polyethylene using Molecular Dynamics, GC, and NMR.

1. Konstantin Popov

2. Pyrolysis: thermal decomposition in the
absence of oxygen
Combustion of solids is gas phase
process supplied by gaseous fuel
produced during pyrolysis of the solid

3. ~40% of all plastic
production

4. why
needed?
28.8 mln tons of
plastic landfilled
yearly (USA)
Only 4.77 mln
tons recycled
and combusted
(USA)
6.4 mln tons of
plastic circles the
globe in oceans
recycling

5. why
needed?
Majority of fire
victims are killed
by hazardous
products of
combustion
fire safety

6. why
needed?
Polyethylene
and other longchain alkanes: a
solid fuel for
hybrid rockets
solid rocket fuel

7. initial stages of
pyrolysis of
polyethylene
Reactive
Molecular
Dynamics
study
experimental
study using
GC and NMR
kinetic modeling

8. initial stages of
pyrolysis of
polyethylene
Reactive
Molecular
Dynamics
study
experimental
study using
GC and NMR
kinetic modeling

10. 0
0.5
1
1.5
2

11. 1100
ENERGY / KJ MOL-1
900
700
500
300
100
-100 0
1
2
3
DISTANCE BETWEEN C ATOMS / A
4
5

12. 5
ENERGY / KJ MOL-1
4
3
2
1
0
-1
0
2
4
6
DISTANCE / A
8
10

13. temperature range: 2300 – 3100K
density range: 0 – 0.75 g cm-3
length of polymer chain: 500 monomer units (1000 C atoms)
periodic boundary conditions:

16. Millions
3100K, normal density
350
300
Keff / s-1
250
200
150
50-100 simulations
per point
100
50
0
0.3
0.4
0.5
0.6
RC
0.7
0.8
0.9

18. Two-step mechanism:
k1
(1) R-R ⇄ [R…R]
k-1
kD
(2) [R…R] → R + R

20. 30
kD/keff*10-3
25
20
15
3100K,
normal
density
10
5
0
0
1
2
3
kD*10-12
4
5

21. 30
short
separations
kD/keff*10-3
25
20
15
3100K,
normal
density
10
long
5 separations
0
0
1
2
3
kD*10-12
4
5

22. The effect of density:
9.5
vacuum
reduced density
normal density
Log(k1 / s-1)
9
8.5
8
7.5
7
6.5
6
0.3
0.35
0.4
1000 K/ T
0.45

23. The effect of polymer chain length:
8.5
1
log(k1 / s )
8.0
fixed p
fixed d
7.5
7.0
6.5
2
4
50
200
number of C atoms in a single molecule
1000

24. Although effects of condensed phase were
observed, their magnitude is not dramatic
Gas phase C-C scission rate constant can be
used in modeling condensed phase processes
This result of the RMD part will be used in
kinetic modeling of experimental observations

25. initial stages of
pyrolysis of
polyethylene
Reactive
Molecular
Dynamics
study
experimental
study using
GC and NMR
kinetic modeling

26. experimental apparatus

27. experimental apparatus
thermostat

28. experimental apparatus
stirrer

29. experimental apparatus
thermocouple

30. experimental apparatus
reactor

31. reactor
polyethylene
sample

32. temperature range: 400 – 440oC
time: 0 – 20 min
initial stages: abundances of vinyl groups
kept below 1%

33. Gas Chromatography
C5H12
C2H4
1-C6H12
2-C4H8
C2H6
1-C7H14
C3H8
C6H14
CH4
C7H16
1-C4H8
C3H6
C4H10
C8H18

34. 1H
NMR
R-CH=CH-R
RR1C=CH2
-CH2-
R-CH=CH2
-CH3

35. 13C
NMR
-CH2-
-CH3
-CH3
CHY
CHY

36. examples of gaseous products:
3.00E-05
440 C
Molar fraction
2.50E-05
2.00E-05
1.50E-05
methane
propane
hexane
1.00E-05
ethylene
5.00E-06
0.00E+00
0
5
10
15
Time / min
20
25

37. condensed phase products:
0.0035
440 C
0.003
Molar fraction
0.0025
R-CH=CH2
0.002
vinyl
0.0015
vinylene
vinylidene
0.001
RRC=CH2
0.0005
0
-5
0
-0.0005
R-CH=CH-R
5
10
Time / min
15
20
25

38. initial stages of
pyrolysis of
polyethylene
Reactive
Molecular
Dynamics
study
experimental
study using
GC and NMR
kinetic modeling

39. initiation of the process:
R-R → 2RP
k1

40. propagation reactions:
kβP
RP → C2H4 + RP
kHT
RP + -CH2- → -CH3 + RS
kβS
RS → Q + RP
long-chain olefins
containing vinyl
group

41. radical addition to double bonds
(propagation reactions):
kAPT
R P + Q → RS
kAPN
R P + Q → RP *
kAST
RS + Q → RS*
kASN
RS + Q → RP**

42. termination reactions:
kRPP
RP + RP → R-R
kRPS
RP + RS → CHY
kRSS
RS + RS → CHH

43. reduced mechanism:
initiation
k1
R-R → 2RP
kβP
RP → C2H4 + RP
kHT
propagation
RP + -CH2- → -CH3 + RS
kβS
RS → Q + R P
kAST
RS + Q → RS* (+CHY)
termination
kRSS
RS + RS → CHH

44. assumptions used:
(i) gas phase k1 (supported by RMD)
(ii) gas phase kβP, kβS
(iii) steady-state approximation for
[RP] and [RS]:

45. kinetic equation for [C2H4]:
440oC
1.8E-05
1.6E-05
C2H4 molar fraction
1.4E-05
1.2E-05
1.0E-05
8.0E-06
steady-state
[RP] determined
6.0E-06
4.0E-06
2.0E-06
0.0E+00
0
5
10
15
Time / min
20
25

46. kinetic equation for [-CH3]:
440oC
2.0E-02
1.8E-02
-CH3 molar fraction
1.6E-02
1.4E-02
1.2E-02
1.0E-02
8.0E-03
kHT and steadystate [RS]
determined
6.0E-03
4.0E-03
2.0E-03
0.0E+00
-5
0
5
10
Time / min
15
20
25

47. kinetic equation for [RS]:
kinetic equation for [RP]:
solution:
kRSS
determined

48. kinetics of [Q]:
solution:

49. kinetics of [Q]:
0.0020
440oC
molar fraction
0.0015
kAST determined
0.0010
0.0005
0
200
400
600
time / sec
800
1000
1200

50. possible pathway to produce vinylene
groups:
(i) radical addition to vinyl group
(ii) β-scission

51. possible pathway to produce vinylidene
groups:
(i) H abstraction from a tertiary C atom
(ii) β-scission

52. Rate constant of hydrogen abstraction:
1.34
1.36
1.38
1.4
-14.1
-14.3
Tsang, 1986
Log(kHT)
-14.5
-14.7
-14.9
-15.1
-15.3
-15.5
1000 K / T
1.42
1.44

53. Rate constant of radical recombination:
1.34
-10.5
Log(kHT)
-11
1.36
1.38
1.4
Tsang, 1986
-11.5
-12
-12.5
-13
1000 K / T
1.42
1.44

54. Rate constant of radical addition to double
bonds:
1.34
-14.7
1.36
1.38
1.4
-14.8
Tsang, 1991
Log(kHT)
-14.9
-15
-15.1
-15.2
-15.3
1000 K / T
1.42
1.44

55. Reactive Molecular Dynamics:
Kinetics of C-C bond scission was studied under the
conditions of liquid phase
Two-step kinetic mechanism enabled decoupling the
influence of the cage effect from the kinetics of C-C scission
in the liquid phase
The influence of the polymer chain length on the kinetics
of C-C scission was evaluated for fixed density and fixed
pressure
Important conclusion: usage of gas phase value of k1 is
justified for modeling liquid phase processes

56. experimental study and kinetic modeling:
The kinetics of both liquid phase and gas phase products
of initial stages of HDPE pyrolysis were studied
The kinetics of major products was successfully described
using proposed kinetic mechanism
The values of rate constants of critical reactions
(kHT, kAST, kRSS) were estimated under the conditions of the
liquid phase
Possible pathways for creation of vinylene (R-CH=CH-R)
and vinylidene (RR1C=CH2) were suggested