Published: March 08, 2011 r2011 American Chemical Society 4264 dx.doi.org/10.1021/ie1014074 | Ind. Eng. Chem. Res. 2011, 50, 4264–4279 ARTICLE pubs.acs.org/IECR New Approach for Kinetic Modeling of Catalytic Cracking of Paraffinic Naphtha Jae Ho Lee, †,‡ Sookil Kang, ‡ Young Kim, § and Sunwon Park* ,† † Department of Chemical & Biomolecular Engineering, KAIST, 335 Gwahak-ro, Yuseong-gu, Daejeon 305-701, South Korea ‡ SK Energy Institute of Technology, 140-1 Wonchon-dong, Yuseong-gu, Daejeon 305-712, South Korea § Korea Institute of Machinery and Materials, 104 Sinseongno, Yuseong-gu, Daejeon 305-343, South Korea ABSTRACT: Catalytic cracking of paraffinic naphtha requires higher temperature than that for cracking of olefinic naphtha. In the high temperature catalytic cracking, thermal cracking also occurs simultaneously. The degree of thermal cracking and its contribution to ethylene and propylene yield in the catalytic cracking of paraffinic naphtha are experimentally investigated. As the degree of thermal cracking is high at over 650 °C, both thermal and catalytic cracking mechanisms have to be simultaneously considered in the kinetic model. An approximate approach based on transition state theory is proposed for kinetic modeling of the catalytic cracking of paraffinic naphtha, which has a complex chemical reaction network. The pertinent parameters of the developed kinetic model are estimated by a genetic algorithm. Additionally, an integrated modeling software package is developed with a graphical user interface. The efficacy of the proposed approach is shown with its application to industrial catalytic cracking of paraffinic naphtha in the circulating fluidized bed reactor system. This approach will be particularly effective for modeling complex chemical reaction network systems. 1. INTRODUCTION The global production levels of ethylene and propylene in 2007 were 114.6 and 73.5 million metric tons per year, respectively. 1 While thermal cracking of naphtha remains as a major technology to produce the olefin products, catalytic cracking is increasingly drawing attention for its lower energy consumption. Thermal cracking operates at 850-950 °C, consuming a huge amount of energy. Even for the most up-to-date thermal crackers, specific energy consumption is about 4500-5000 kcal/kg of ethylene. In previous studies on catalytic cracking, C 5 to C 7 alkanes are reacted below 550 °C. Greensfelder and Voge 2 report the catalytic to thermal cracking rate ratios at 500 °C are 5 and 55 for nC 4 and nC 24 , respectively. Liguras and Allen 3 propose that, at 500 °C, the rates of catalytic cracking of paraffins and olefins are 10 to 1000 times faster than those of thermal cracking, which implies that catalytic cracking is dominant at this temperature. However, for higher temperature operation above 600 °C, the thermal cracking rate should be increased. At higher temperature, the possibility of thermal cracking increases as the adsorption rates of hydrocarbons on to the catalysts decrease. Moreover, it should be noted that thermal cracking of paraffinic light naphtha starts at nearly 600 °C. Therefore, kinetic modeling for catalytic cracking of C 5 to C 7 alkanes above 600 °C should be preceded by identifying the degree of thermal cracking occurring in the catalytic cracker. The catalytic cracking of hydrocarbons involves complex reaction networks with a large number of species, which makes its kinetic modeling difficult. Kinetic modeling of the complex reaction networks has been studied since the 1980s in several works. The single event concept 4-8 adopts transition state theory for calculating frequency factors. The structural entropy change of a reaction is calculated by the ratio between the number of configurations, i.e., the number of single events, of the reactant and the activated complex in the transition state. Although it is theoretically reasonable, calculating all the config- urations of the reactant and activated complex for complex reaction network systems is rather cumbersome. Klein et al. 9-11 suggest a practical approach using linear free energy relationships (LFERs) for various cracking reactions with an integrated modeling toolbox. This approach is practical if a LFER is known for specific reactions. Structure oriented lumping (SOL) 12,13 is an additivity approach for heavy feedstock, with a huge analytical database focus- ing the estimation of the properties of products. Some approaches based on basic elementary steps 14-16 are also practical, but their theoretical backgrounds are relatively weak and their models con- sider catalytic cracking reactions only. However, a more practical approach with less computational load needs to be developed for the high temperature catalytic cracking reactions. In this work, the mechanisms for the high temperature catalytic cracking reactions for paraffinic naphtha are elucidated by experiments. Subsequently, a kinetic model of the reactions is developed on the basis of a newly proposed approximate approach based on the transition state theory. This approach defines the approximate factors, which mean the approximate values of entropy changes in the thermal and catalytic reactions. Only two types of parameters, the correction factors and activa- tion energies, are estimated by a global optimization method. The range of the correction factors is within 0.1-1.5. Activation energies are bounded within literature values. The narrow range of parameters can reduce the stiffness problems in solving a set of ordinary differential equations (ODEs) during the parameter estimation. Table 1 compares various approaches of kinetic modeling for complex chemical reaction networks. Received: July 1, 2010 Accepted: December 23, 2010 Revised: December 9, 2010