(583cb) A Reduced Mechanism for the Prediction of Olefin Production and Coke Precursors During the Steam-Cracking of Ethane and Propane

A reduced mechanism
for the prediction of olefin production and coke precursors during the
steam-cracking of ethane and propane

L.C. López ¹,
A.Y. Ramírez¹, M.Kraft², J.C. Vivas³, A.Molina^*,1

¹ Universidad Nacional de
Colombia ? Sede Medellín, Facultad de Minas, Bioprocesos y Flujos reactivos,
Medellín, Colombia

² University of Cambridge,
Department of Chemical Engineering and Biotechnology, Computational Modelling
Group, Cambridge, England

³Instituto Colombiano del
Petróleo, Piedecuesta, Colombia

^*Corresponding author:
amolinao@unal.edu.co

A
reduced mechanism for the steam-cracking of ethane and propane was proposed.
Thermal cracking of hydrocarbons is the main route for the production of valuable
products such as ethylene and propylene from ethane, propane and naphtha.
Recently Ramírez [1] proposed a model that represents coke formation during the
steam-cracking of ethane and ethane-propane mixtures. The success of Ramirez's
model depends on the ability of a gas-phase mechanism to predict the
concentration of coke precursors such as benzene, hydrogen and acetylene. Among
the numerous kinetic mechanisms developed to describe the combustion and
pyrolysis of hydrocarbons, Ramirez recommended those developed by Frenklach et
al. [2, 3] and Wang et al. [4, 5] that can represent the pyrolysis of light
hydrocarbons.

The
kinetic mechanism of Frenklach et al. (ABF mechanism) consists of 99 chemical
species and 544 reactions [2,
3]. It includes the pyrolysis and oxidation of C₁
and C₂ species and considers pyrene as the heaviest aromatic
compound. The kinetic mechanism described by Wang
et al. (USC mechanism), with 352 species and 2083 reactions in its last
published version, takes into account the combustion of H₂/CO/C₁-C₄
and the description of pyrolysis and combustion at high temperature of normal
alkenes up to n-dodecane. It has as the most complex aromatic compound benzene [4,
5]. These mechanisms
involve a large number of chemical species and reactions and, therefore, imply
a high computational cost. As the detail required in the simulation of cracking
processes increases (e.g. in CFD analyses or in optimization processes), large
mechanisms become impractical. A reduced mechanism that decreases the
computational cost without losing information about the most important species
and coke-precursors is, therefore, desirable.

In
the referred literature several authors have worked on the simplified
representation of reaction mechanisms [6-10]. Most authors agree that a
strategy for reducing a kinetic mechanism could combine sensitivity and
principal component analyses (PCA). This study
proposes reduced versions of the ABF and USC mechanisms, keeping in mind its
application to the simulation of coke deposition in a steam-cracking reactor
operating with ethane and ethane/propane mixtures.
The new versions were obtained after a PCA following the methodology described
by Tomlin et al. [6]. A value of 10^-6 was used as threshold for the
eigenvalues and eigenvectors analysis to determine the redundant reactions and
generate the reduced mechanism.  To validate the reduced mechanism,
simulations were carried out for a batch and a PFR reactors with the same
temperature profile (approximating the time as space time for the batch reactor
assuming a residence time of 0.5 s) of an industrial steam-cracking furnace
operating at the conditions described by Ramirez [1]. Fig.1 shows the
concentration profile of important species in a simulation carried out in the
batch reactor. As
the ethane concentration decreases, the concentration of ethylene and propylene
increases.

Fig
1.
Concentration profiles of important species. Ethane, ethylene  and propylene
for the simulation using the reduced USC mechanism

After
applying the PCA analysis the number of reactions for the ABF and USC mechanisms
was reduced to 28% and 16% of the original value, respectively. Fig. 2 and Fig.
3 show, through a parity plot, that the predictions for benzene and acetylene
perfectly agree between the original and reduced mechanisms for the batch
simulations. Similar results were obtained for other species. Fig. 4 shows that
the reduced ABF mechanism correctly represents the conversion of ethane along
the industrial steam-cracking furnace. Similar profiles, obtained for other
coke precursors, indicate that the reduced ABF mechanism, with only 153
reactions, guarantees a good representation of the process of steam-cracking of
ethane.

Fig 2. Comparison of
predictions for benzene concentration during the steam-cracking of ethane in a
batch reactor as obtained with the original and reduced USC mechanisms

Fig 3. Comparison of
predictions for acetylene concentration during the steam-cracking of ethane in
a batch reactor as obtained with the original and reduced ABF mechanisms

Fig.
4. Comparison
of the predicted ethane conversion by the ABF original and reduced mechanisms
along an industrial ethane steam-cracker.

REFERENCES

[1] A. Ramírez. ?A model for the
prediction of olefin production and coke deposition during thermal cracking of light
hydrocarbons?. M.Sc. thesis, Universidad Nacional
de Colombia, 2012. Available on http://www.bdigital.unal.edu.co/7360/1/1017125365.2012.pdf

[2] M. Frenklach, J. Appel, and H.
Bockhorn, ?ABF mechanism.? [Online]. Available:
http://www.me.berkeley.edu/soot/mechanisms/abf.html.

[3] J. Appel, H. Bockhorn, and M.
Frenklach, ?Kinetic modeling of soot formation with detailed chemistry and
physics: laminar premixed flames of C2 hydrocarbons,? Combustion and Flame,
vol. 121, no. 1?2, pp. 122-136, 2000.

[4] H. Wang, X. You, A. V. Joshi, S. G.
Davis, A. Laskin, and F. Egolfopoulos, ?USC Mech Version II. High-Temperature
Combustion Reaction Model of H2/CO/C1-C4 Compounds.? [Online]. Available:
http://ignis.usc.edu/USC_Mech_II.htm.

[5] H. Wang and F. Egolfopoulos,
?JetSurf-A Jet Surrogate Fuel Model.? [Online]. Available:
http://melchior.usc.edu/JetSurF/.

[6] A. S. Tomlin, M. J. Pilling, J. H.
Merkin, J. Brindley, N. Burgess, and A. Gough, ?Reduced Mechanisms for Propane
Pyrolysis?, Industrial & Engineering Chemistry Research, vol. 34, no. 11,
pp. 3749-3760, 1995.

[7] T. Kovács, I. G. Zsély, A.
Kramarics, and T. Turányi, ?Kinetic analysis of mechanisms of complex pyrolytic
reactions" Journal of Analytical and Applied Pyrolysis, vol. 79, no. 1-2,
pp. 252-258, 2007.

[8]  T. Turányi, ?Sensitivity analysis
of complex kinetic systems. Tools and applications," Journal of
Mathematical Chemistry, vol. 5, no. 3, pp. 203-248, 1990.

[9] T. Turányi. ?Reduction of large
mechanisms?. New Journal of Chemistry. Vol. 14 no. 11. pp. 795-803. 1990.

[10] M. S. Okino and M. L.
Mavrovouniotis, ?Simplication of Mathematical Models of Chemical Reaction
Systems" Chemical Reviews, vol. 98, no. 2, pp. 391-408, 1998.