(328b) Generation of a Kinetic Mechanism for Thermal Cracking of Naphtha Using a 10-Species Surrogate

Generation
of a kinetic mechanism for thermal cracking of naphtha using a 10-species
surrogate
 

A. Y. Arévalo¹,
J.J Arias, A.Y. Ramírez¹, A.Molina
¹_, J.C. Vivas²

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

²Instituto
Colombiano del Petróleo, Piedecuesta, Colombia

Naphtha,
which represents around 10-15% of the petroleum compounds, normally compromises
more than 200 components grouped as n-paraffin, iso-paraffin, olefins,
naphthenes and aromatics. The specific composition of naphtha depends on the
place and conditions where the petroleum is located, therefore, a general
composition cannot be assigned ^[1].

Development
of a kinetic mechanism for naphtha cracking demands a detailed knowledge of its
composition (i.e. molecular composition and molar fraction). As dealing with
more than 200 compounds is difficult, naphthas are normally represented as
surrogates that, with around 10-20 components, can simulate the general
behavior of the naphtha. This paper proposes a naphtha surrogate that was used
as inlet conditions for the development of a naphtha mechanism in the RMG
software (Reaction mechanism generator) ^[2,3]. This research, then,
had two main components: (1) Generation of naphtha surrogate using the Shannon
entropy criterion and (2) Generation of a naphtha mechanism for the proposed
surrogate.

Some research groups have developed methods for naphtha
reconstruction. One of them is CRACKER, a cracking furnace simulator which has
a module for the characterization of naphtha based on a neuronal network model
and with commercial indices such as PINA analysis and the specific density ^[4].
Other groups use the constricted minimization of Shanon's entropy using Lagrange's
multipliers as optimization tool ^[5]. For simplicity, this second
approach was adopted in this research with boiling
temperature, specific density, molecular weight and H/C ratio as commercial
indices. The resulting 10-species surrogate of a Colombian naphtha predicts the
proportion of parrafins, iso-paraffins, olefins, naphthenes and aromatics
components present in the naphtha as well as the selected commercial indices.

Based on this
surrogate, a reaction mechanism for naphtha cracking was constructed using RMG,
an automatic chemical reaction mechanism generator, developed at MIT ^[2], which is able to construct kinetic
mechanisms composed of elementary chemical reaction using a general
understanding of how molecules react. In addition to the surrogate, the
conditions that were supplied to RMG for the generation of the mechanism were
experimental information about the naphtha cracking process, a termination goal
(e.g. n-pentane conversion) and an error tolerance.

The results
obtained with the generated reaction network were compared with results
obtained from a simulation with the kinetic mechanism in a 1D model for the
naphtha cracking pilot plant to verify the models ability to represent the
steam cracking behavior. A good agreement between simulation results and
experimental data for the main reaction products was obtained.

REFERENCES

[1] M. Dente, E.
Ranzi, G. Bozzano, and S. Pierucci, Pyrolysis of Naphtha Feedstocks : Automatic
Generation of Detailed Kinetics and Lumping Procedures, vol. 28. Elsevier B.V.,
2010.

[2] RMG
Documentation, 4.0.0.  W. H. Green, 2013. http://rmg.mit.edu/,
consulted: January 2013.

[3] M. R.
Harper, K. M. Van Geem, S. P. Pyl, ?Comprehensive reaction mechanism for
n-butanol pyrolysis and combustion?, Combustion and Flame, vol.158, pp.16-41,
2011

[4] E. Joo, K.
Lee, M. Lee, and S. Park, Computers & Chemical Engineering a PC based
simulator for industrial cracking furnaces," vol. 24, pp. 1523-1528, 2000.

[5] K. M. Van
Geem, D. Hudebine, M. F. Reyniers, F. Wahl, J. J. Verstraete, and G. B. Marin,
?Molecular reconstruction of naphtha steam cracking feedstocks based on
commercial indices," Computers & Chemical Engineering, vol. 31, pp.
1020-1034, Sept. 2007.