![Two-step mechanism:
k1
(1) R-R ⇄ [R…R]
k-1
kD
(2) [R…R] → R + R](https://image.slidesharecdn.com/initialstagesofpyrolysisofpolyethylene-140130164430-phpapp01/85/Initial-stages-of-pyrolysis-of-polyethylene-18-320.jpg)
![assumptions used:
(i) gas phase k1 (supported by RMD)
(ii) gas phase kβP, kβS
(iii) steady-state approximation for
[RP] and [RS]:](https://image.slidesharecdn.com/initialstagesofpyrolysisofpolyethylene-140130164430-phpapp01/85/Initial-stages-of-pyrolysis-of-polyethylene-44-320.jpg)
![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](https://image.slidesharecdn.com/initialstagesofpyrolysisofpolyethylene-140130164430-phpapp01/85/Initial-stages-of-pyrolysis-of-polyethylene-45-320.jpg)
![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](https://image.slidesharecdn.com/initialstagesofpyrolysisofpolyethylene-140130164430-phpapp01/85/Initial-stages-of-pyrolysis-of-polyethylene-46-320.jpg)
![kinetic equation for [RS]:
kinetic equation for [RP]:
solution:
kRSS
determined](https://image.slidesharecdn.com/initialstagesofpyrolysisofpolyethylene-140130164430-phpapp01/85/Initial-stages-of-pyrolysis-of-polyethylene-47-320.jpg)
![kinetics of [Q]:
solution:](https://image.slidesharecdn.com/initialstagesofpyrolysisofpolyethylene-140130164430-phpapp01/85/Initial-stages-of-pyrolysis-of-polyethylene-48-320.jpg)
![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](https://image.slidesharecdn.com/initialstagesofpyrolysisofpolyethylene-140130164430-phpapp01/85/Initial-stages-of-pyrolysis-of-polyethylene-49-320.jpg)
Initial stages of pyrolysis of polyethylene
- 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
- 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
- 32. temperature range: 400 – 440oC time: 0 – 20 min initial stages: abundances of vinyl groups kept below 1%
- 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
- 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