Modeling of acetylene pyrolysis under steel vacuum carburizing conditions in a tubular flow reactor

Article abstract of DOI:10.3390/12030290

In the present work, the pyrolysis of acetylene was studied under steel vacuum carburizing conditions in a tubular flow reactor. The pyrolysis temperature ranged from 650 °C to 1050 °C. The partial pressure of acetylene in the feed mixture was 10 and 20 mbar, respectively, while the rest of the mixture consisted of nitrogen. The total pressure of the mixture was 1.6 bar. A kinetic mechanism which consists of seven species and nine reactions has been used in the commercial computational fluid dynamics (CFD) software Fluent. The species transport and reaction model of Fluent was used in the simulations. A comparison of simulated and experimental results is presented in this paper.

Full text of DOI:10.3390/12030290

Molecules 2007, 12, 290-296
molecules
ISSN 1420-3049
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Full Paper
Modeling of Acetylene Pyrolysis under Steel Vacuum
†
Carburizing Conditions in a Tubular Flow Reactor
*
Rafi Ullah Khan , Siegfried Bajohr, Frank Graf and Rainer Reimert
Engler-Bunte-Institut, Bereich Gas, Erdöl und Kohle, Engler Bunte Ring 1, Universität Karlsruhe,
D-76131 Karlsruhe, Germany; E-mails: siegfried.bajohr@ciw.uni-karlsruhe.de,
frank.graf@ciw.uni-karlsruhe.de, rainer.reimert@ciw.uni-karlsruhe.de
*
Author to whom correspondence should be addressed. E-mail: rafi_ullah.khan@ciw.uni-karlsruhe.de
Paper presented at ECSOC-10, 1-30 November 2006
†
Received: 20 February 2007; in revised form: 1 March 2007 / Accepted: 1 March 2007 /
Published: 2 March 2007
Abstract: In the present work, the pyrolysis of acetylene was studied under steel vacuum
carburizing conditions in a tubular flow reactor. The pyrolysis temperature ranged from
6
50 °C to 1050 °C. The partial pressure of acetylene in the feed mixture was 10 and 20
mbar, respectively, while the rest of the mixture consisted of nitrogen. The total pressure
of the mixture was 1.6 bar. A kinetic mechanism which consists of seven species and
nine reactions has been used in the commercial computational fluid dynamics (CFD)
software Fluent. The species transport and reaction model of Fluent was used in the
simulations. A comparison of simulated and experimental results is presented in this
paper
Keywords: Acetylene; Pyrolysis; Modeling; Simulation; Carburizing
Introduction
Amongst the many applications of acetylene, low pressure carburizing of steel is an up and coming
one. The pyrolysis of acetylene has been studied by many researchers under various conditions,
depending upon the application being considered. Detailed kinetic mechanisms proposed in literature

Molecules 2007, 12
291
may consist of hundreds of species and reactions. Typically, these investigations include shock tube
pyrolysis of acetylene [1–12] and pyrolysis in flow systems [13–15]. The deposition of pyrolytic
carbon from various hydrocarbons has also been described by some researchers [16–20]. Although the
detailed kinetic mechanisms are considered to be more accurate and reliable, their use is mostly
limited to ideal flow models. Use of such detailed kinetic mechanisms is not easy with transport
models in CFD softwares with current computational hardware and algorithms. Nevertheless, CFD
softwares normally implement Navier-Stokes equations and are used for modeling the flow field, even
when the geometry is complex. Operational kinetic mechanisms or reduced versions of detailed
mechanisms can be used with CFD softwares for modeling the reacting flow. Vacuum carburizing with
acetylene involves the pyrolysis of acetylene under low pressure, which produces solid pyrolytic
carbon and soot along with other hydrocarbon gases, making the flow field more complex. Diffusion of
carbon into the surface of steel takes place, which is continued until the desired carbon content and
case depth are achieved. In the present work, the pyrolysis of acetylene was studied under such
conditions suitable for vacuum carburizing of steel.
Results and Discussion
The measured pyrolysis products include CH , C H C H , C H , C H . The amount of carbon C
(s)
4
2
2,
2
4
4
4
6
6 .
in the form of soot was calculated from a carbon mass balance of the measured gas phase products.
The product yields have been plotted as a function of temperature. The experimental results are
compared with the simulation results of Fluent version 6.2. The amount of hydrogen was also
calculated by a material balance and is also compared with the simulation results. The temperature
varies along the reactor length. Figure 1 shows the temperature profiles in the reactor at three different
controller temperatures values.
Figure 1. Temperature profiles at selected controller temperatures Tc.

Molecules 2007, 12
292
Figure 2. Mole fraction contours of C H at 900 °C and 20 mbar partial pressure of acetylene.
2
2
Figure 3. Mole fractions contours of C(s) at 900 °C and 20 mbar partial pressure of acetylene.
Figure 4. Comparison of simulated vs experimental results at 10 mbar acetylene partial pressure.
C2H2 sim
C2H2 exp
soot sim
soot exp
100
14
95
90
85
80
75
70
65
60
1
2
0
8
6
4
2
0
1
600
700
800
900
1000
1100
600
700
800
900
1000
1100
Temperature Tc in °C
Temperature Tc in °C
CH4 sim
H2 sim
H2 exp
CH4 exp
1
2
0
8
6
4
2
0
4
3
2
1
0
.5
C2H4 sim
C2H4 exp
C4H4 sim
C4H4-exp
C6H6 sim
C6H6 exp
4
.5
3
1
.5
2
.5
1
.5
0
600
700
800
900
1000
1100
6
00
700
800
900
1000
1100
Temperature Tc in °C
Temperature Tc in °C

Molecules 2007, 12
293
Figures 2 and 3 show two of the typical mole fraction contours obtained from simulations for the
two important species C H and C(s) at 900 °C and 20 mbar partial pressure of acetylene in the reactor.
2
2
Figure 4 shows the comparison of experimental and simulation results for acetylene at 10 mbar partial
pressure for a controller temperature variation of 650 °C to 1050 °C. The carbon content carried by
unconverted acetylene in the mixture decreases from 99 % at 650 °C to 75 % at 1050 °C for 20 mbar,
representing a 25 % conversion of acetylene to other products at the outlet, as shown in Figure 5. The
second major and important component carrying carbon among the pyrolysis products is the soot, for
which results are shown for 10 mbar as well as for 20 mbar partial pressure of acetylene. The
percentage of carbon C in the form of soot increases with an increase in temperature. The formation
(s)
4 4 6 6
of C H and C H increases up to a temperature of 900 °C and then gradually decreases at higher
temperatures. CH and C H are also formed, but the carbon content in these compounds is less than 1
4
2
4
%
under these experimental conditions.
Figure 5. Comparison of simulated vs experimental results at 20 mbar acetylene partial pressure.
C2H2 sim
C2H2 exp
soot sim
soot exp
100
2
1
1
1
0
8
6
4
9
8
7
6
5
4
0
0
0
0
0
0
12
1
0
8
6
4
2
0
6
00
700
800
900
1000
1100
Temperature Tc in °C
600
700
800
900
1000
1100
Temperature Tc in °C
CH4 sim
sim H2
exp H2
6
5
4
3
2
1
0
CH4 exp
C2H4 sim
C2H4 exp
C4H4 sim
C4H4 exp
C6H6 sim
C6H6-exp
1
1
1
6
4
2
10
8
6
4
2
0
600
700
800
900
1000
1100
600
700
800
900
1000
1100
Temperature Tc in °C
Temperature Tc in °C
Reaction Mechanism and CFD Model
The reaction mechanism shown in Table 1 consists of seven species which are the major products
of acetylene pyrolysis under vacuum steel carburizing conditions. These include C(s), which represents
the solid carbon in the form of soot, and hydrocarbons, consisting of CH , C H , C H , C H , and
4
2
2
2
4
4
4
6 6 2
C H , along with H . The overall mechanism consists of nine reactions. The estimated Arrhenius

Molecules 2007, 12
294
parameters, activation energies and proposed reaction rates are also shown in Table 1. A 2-D grid
consisting of 300 x 20 cells (6000 cells in total) has been used to represent a reactor length of 500 mm
with a diameter of 20 mm. The species transport and reaction model in Fluent was used for modeling
the chemistry. The mechanism discussed above was implemented through a user defined function
(
UDF) in Fluent. The operating pressure was set equal to 1.6 bar while inlet temperature and velocity
boundary conditions were used according to the experimental measurements.
Table 1. Proposed reaction mechanism and kinetic parameters of acetylene pyrolysis.
⎛
−EA,i ⎞
⎜
⎜
⎟
⎟
⎝
RT
⎠
Rate constant k = A ⋅e
i
i
3
No. Reaction
Rate Expression
r = k ⋅ c ⋅ cH 2
A
i
(mol, m , sec)
E
A
,
i
(kJ/mol)
0
.36
3
C H + H → C H
1
2
3
4
2
2
2
2
4
2
1
1
C H 2
4.4⋅10
103.0
2
0.5
7
C H → C H + H
r = k ⋅c
2
4
2
4
2
2
C H
3.8⋅10
200.0
150.0
195.0
2
4
0.35
0.22
5
C H + 3H → 2CH
r = k ⋅c
⋅cH2
2
2
2
4
2
3
3
C H
1.4⋅10
2
2
0.21
6
2
CH → C H + 3H
r = k ⋅c
4
2
2
4
4
CH4
8.6⋅10
1.9
c
C H
2
2
6
C H → 2C + H
r = k ⋅
5
2
2
(s)
2
5
5
5.5⋅10
165.0
1
+ 18cH2
1
.6
5
C H + C H → C H
r = k ⋅c
6
7
8
2
2
2
2
4
4
2
6
6
6
C H
1.2⋅10
120.7
335.2
64.5
2
2
4
0
.75
1
5
C H → C H + C H
r = k ⋅c
4
4
2
2
2
7
7
C H
1.0⋅10
4
1.3
0.6
3
C H + C H → C H
r = k ⋅c
⋅c
4
4
2
2
6
8
8
C H
C H
1.8⋅10
2
2
4
4
0
.75
c
C H
6
6
3
C H → 6C + 3 H2 r = k ⋅
9
6
6
(s)
9
9
1.0⋅10
75.0
1
+ 22cH2
As the reactor is not operated under isothermal conditions, a temperature profile was necessary to
model the temperature field. A mathematical fit in the form of a polynomial shown in equation (1)
below was used to describe the temperature profile in the simulations:
2
2
T(T
c
, x) = (a ⋅ x + b ⋅ x + c) ⋅Tc + d ⋅ x + e⋅ x + f
(1)
where T(T ,x) represents the temperature as a function of the controller temperature T and the position
c
c
x along the reactor length, whereas a, b, c, d, e and f are the polynomial coefficients obtained by least
squares fit to the experimental results. This temperature profile was also implemented through a user
defined function (UDF) and compiled before loading into Fluent using the default procedures in the
software. A typical temperature profile implemented in Fluent is shown in Figure 1. The solution was
-6
converged to species residuals of 10 or less so that there was no further variation of these residuals.
The convergence was fast and achieved in less than 500 iterations.
Conclusions
Acetylene pyrolysis has been modeled by using simplified chemistry with a transport model under
the particular conditions of the industrial process of vacuum carburizing of steel. Comparison of
simulated and experimental results shows satisfactory agreement under technical operating conditions.

Molecules 2007, 12
295
Further investigations for complex geometries are necessary to prove the validity of the model for the
application as a prediction tool to control the carburizing process in commercial plants.
Experimental
A ceramic reactor with an inner diameter of 20 mm, outer diameter of 25 mm and a length of 600
mm was used in the present investigation. There is a ceramic filter at the outlet of the reactor which
can separate entrained solid carbon from the gas stream. The pyrolysis products in the gas phase were
measured by gas chromatography. The temperature profile was measured at the center of the reactor in
an interior ceramic pipe with an outer diameter of 6 mm.
Acknowledgements
The financial assistance of the Higher Education Commission of Pakistan (HEC) and the German
Academic Exchange Service (DAAD) are gratefullly acknowledged.
References
1.
2.
3.
4.
Wu, C. H.; Singh, H. J.; Kern, R. D. Pyrolysis of acetylene behind reflected shock waves.
International. Int. J. Chem. Kin. 1987, 19, 975-996.
Frenklach, M.; Durgaprasad, M. B.; Taki, S.; Matula. R. Soot formation in shock-tube pyrolysis
of acetylene, allene, and 1, 3-butadiene. Combust. Flame 1983, 54, 81-101.
Frenklach, M.; Clary, D. W.; Gardiner, W. C.; Stein. S. E. Detailed kinetic modeling of soot
formation in shock-tube pyrolysis of acetylene. Proc. Combust. Inst. 1984, 20, 887-901.
Frenklach, M. Reaction mechanism of soot formation in flames. Phys. Chem. Chem. Phys. 2002,
4, 2028-2037.
5
.
.
Back, M. H. Mechanism of the Pyrolysis of Acetylene. Can. J. Chem. 1971, 49, 2199–2204.
Kiefer J. H.; Von Drasek, W. A. The mechanism of the homogeneous pyrolysis of acetylene. Int.
J. Chem. Kinet. 1990, 22, 747-786.
6
7
8
9
1
1
.
.
.
Gay, I. D.; Kistiakowsky, G. B.; Michael, J. V.; Niki, H. Thermal decomposition of acetylene in
shock waves. J. Chem. Phys. 1965, 43, 1720-1726.
Kruse, T.; Roth, P. Kinetics of C2 Reactions during High-Temperature Pyrolysis of Acetylene. J.
Phys. Chem. A. 1997, 101, 2138–2146.
Tanzawa, T.; Gardiner, W. C., Jr. Reaction mechanism of the homogeneous thermal
decomposition of acetylene. J. Phys. Chem.. 1980, 84, 236-239.
0. Laskin, A.; Wang, H. On initiation reactions of acetylene oxidation in shock tubes- A quantum
mechanical and kinetic modeling study. J. Chem. Phys. Lett. 1999, 303, 43-49.
1. Kiefer, J. H.; Sidhu, S. S.; Kern, R. D.; Xie, K.; Chen, H.; Harding, L. B. The homogeneous
pyrolysis of acetylene II: the high temperature radical chain mechanism. J. Combust. Sci. Tech..
1
992, 82, 101-130.
2. Böhm H.; Jander, H. PAH formation in acetylene-benzene pyrolysis. Phys. Chem. Chem. Phys.
999, 1, 3775-3781.
1
1

Molecules 2007, 12
296
1
1
1
1
1
1
1
2
3. Norinaga, K.; Deutschmann, O.; Hüttinger, K. J. Analysis of gas phase compounds in chemical
vapor deposition of carbon from light hydrocarbons. Carbon 2006, 44, 1790-1800.
4. Dimitrijevic, S. T.; Paterson, S.; Pacey. P. D. Pyrolysis of acetylene during viscous flow at low
conversions; influence of acetone. J. Anal. App. Pyrol. 2000, 53, 107-122.
5. Cullis, C. F.; Franklin, N. H. The pyrolysis of mixtures of acetylene with other hydrocarbons. J.
Combust. Flame 1964, 8, 246-248.
6. Krestinin, A. V. On the Kinetics of Heterogeneous Acetylene Pyrolysis. Kinet. Catal. 2000, 41,
7
29-736.
7. Hu, Z. J.; Hüttinger, K. J. Mechanisms of carbon deposition-A kinetic approach. Carbon 2002,
0, 624-628.
4
8. Hu, Z. J.; Hüttinger, K. J. Influence of the surface area/volume ratio on the chemistry of carbon
deposition from methane. Carbon 2003, 41, 501-1508.
9. Norinaga, K.; Hüttinger, K. J. Kinetics of surface reactions in carbon deposition from light
hydrocarbons. Carbon 2003, 41, 1509-1514.
0. Zhang, W. G.; Hu, Z. J.; Hüttinger, K. J. Chemical vapor infiltration of carbon fiber felt:
optimization of densification and carbon microstructure. Carbon 2002, 40, 2529-2545.
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