(139e) A Model for Olefin Predictions during Steam Cracking of Naphtha: Naphtha Surrogate, Naphtha Complete and Reduced Mechanism

A mechanism for steam cracking of naphtha was proposed. Naphtha is the most used raw material in steam cracking plant to produce important hydrocarbons for petrochemical industry, such as ethylene, propylene and aromatics. The current and most used technology for olefin production involves injection of a mixture of hydrocarbons, into a long tubular coil located in a furnace. Steam is added to the hydrocarbon mixture in order to control the reactive flow temperature and reduce the partial pressure of hydrocarbons.

Development of a model 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, naphtha’s are normally represented as surrogates that, with around 10-20 components, can simulate the general behavior of the naphtha. The first step follow in this job for modeling naphtha steam cracking was to use a methodology to have the naphtha composition as a surrogate. The methodology selected was the Shannon entropy maximization because the good results present by other authors while using the same methodology [1]. The Shannon entropy maximization is subjected to constraints related with the naphtha properties: MW, density, H/C ratio, TBP and PINA analysis. A 10 species surrogate was obtained with represents in an accurately way the naphtha behavior.

Two mechanisms available in the open-literature were used to evaluate the naphtha steam cracking: the Ranzi´s [2] mechanism and the USC mechanism [3]. In naphtha pilot plant was conduct experiments to validate both mechanisms. The results suggest that the USC is the one that will be the more suitable to predict the species concentration (ethylene, propylene, methane, benzene, and toluene) during steam cracking of naphthas.

This mechanism, however, demand a high computational cost because of its large number of species and reactions (352 and 2083 respectively), for that as a final step for model naphtha steam cracking, a reduced mechanism was proposed. In the referred literature several authors have worked on the simplified representation of reaction mechanisms [4-6]. 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 mechanism, keeping in mind its application to predict the main product from the naphtha steam cracking. The mechanism considers less than 700 reactions y 120 species and was validated with the same set of experiments which ones were validated the original one.

[1] 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.

[2] M. Dente, E. Ranzi, and A. G. Goossens, “Detailed prediction of olefin yields from hydrocarbon pyrolysis through a fundamental simulation model (SPYRO),” Computers & Chemical Engineering, vol. 3, no. 1–4, pp. 61-75, 1979.

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

[4] 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.

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

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