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.