Chemical Engineering Science 64 (2009) 3847--3858 Contents lists available at ScienceDirect Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces CFD modeling of gas–solid flow and cracking reaction in two-stage riser FCC reactors Xingying Lan, Chunming Xu, Gang Wang, Li Wu, Jinsen Gao ∗ State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, PR China ARTICLE INFO ABSTRACT Article history: Received 27 August 2008 Received in revised form 25 April 2009 Accepted 14 May 2009 Available online 20 May 2009 Keywords: FCC Two-stage riser Gas–particle turbulent flows Cracking reactions CFD Optimization The Eulerian–Eulerian approach was applied to simulate the flow behavior and catalytic cracking reactions in the riser reactors of two-stage riser fluid catalytic cracking (TSRFCC) technology. A k -  - k p -  p -  gas–solid turbulent flow model was used, which took account of the particle turbulence and the interaction of turbulence between gas and particle phases. A 14-lump kinetics model was used for simulating cracking reactions. The approach and model were validated with both experimental results and commercial data. The distributions of particle fraction volume and velocity, as well as product yields in the TSRFCC riser reactors were first analyzed. The simulations were then carried out for optimization studies to understand the influence of the operating conditions on the performance of commercial TSRFCC riser reactors. The model and results presented here are valuable for the design and optimization of TSRFCC technology. © 2009 Published by Elsevier Ltd. 1. Introduction Fluid catalytic cracking (FCC) is a key and widely used refinery process for converting heavy oils into valuable light products such as gasoline and diesel. About 45% of worldwide gasoline production comes from the FCC process and its ancillary units. Especially for China, due to the lack of hydro-cracking and hydro-conversion units, FCC remains the most important and profitable heavy oil conversion process in the petroleum refining industry. Although the FCC process has been commercially established for over 60 years, the technology continues to evolve to meet new challenges (Chen, 2006). Modern FCC units need to use a wide variety of feedstocks and to adjust op- erating conditions to maximize production of gasoline, middle dis- tillate, or light olefins to meet different market demands. Nowadays, many improvements have been made on the FCC process, and these new processes have been run in commercial scale, such as two- stage riser fluid catalytic cracking (TSRFCC) technology (Shan et al., 2001), a catalytic cracking process for the production of clean gaso- line (MIP-CGP) (Han et al., 2006), flexible dual-riser fluid catalytic cracking (FDFCC) technology (Wang et al., 2003), double-riser tech- nology (Henry et al., 2002), millisecond catalytic cracking (MSCC) technology (Schnaith et al., 1998), and deep catalytic cracking (DCC) (Xie, 1997). These technologies obtain better product distribution than conventional FCC. For conventional FCC, the catalyst activity decreases sharply in the riser entry zone so that cracking reactions are carried out with ∗ Corresponding author. Tel.: +86 10 89733993. E-mail address: jsgao@cup.edu.cn (J. Gao). 0009-2509/$ - see front matter © 2009 Published by Elsevier Ltd. doi:10.1016/j.ces.2009.05.019 quite low catalyst activity in the second half of the riser. More- over, the catalyst deactivation results in poor product selectivity. The conventional single riser FCC unit has less than optimum product distribution. Thus, Shan et al. (2001) developed TSRFCC technology to improve product distribution. Until now, TSRFCC technology has been applied successfully in eight commercial FCC units (Shan et al., 2006). TSRFCC technology has two risers, whose diameter and length is different from the conventional single riser. The fresh feedstock is introduced into the first-stage riser and subjected to a certain de- gree of cracking reactions. The coked catalysts with low activity and selectivity are separated from the oil products. Then, the oil prod- ucts continue cracking reactions over the regenerated catalysts with good activity and selectivity up to the final conversion in the second- stage riser. The two risers share a common disengager and regenera- tor. A series of TSRFCC derivative technologies have been developed to achieve various product yields. The TSRFCC-I scheme is suitable for enhancing the production of light oil, especially higher ratios of diesel to gasoline (Shan et al., 2006). In the TSRFCC-I scheme, the cracking products from both first- and second-stage risers enter a fractionator and are separated. The products of gas, gasoline, and diesel leave the reaction system, while the heavy cycle oil (HCO) enters the second-stage riser and proceeds cracking reactions over regenerated catalysts. The commercial application of TSRFCC-I tech- nology showed that the light oil yield increased by 4 wt%, while the dry gas yield decreased by more than 1% (Shan et al., 2006). Although TSRFCC-I technology has been applied in eight commer- cial FCC units, the procedure of the industrial design primarily de- pending on experience is not able to bring its function into full play. The operation optimization is absolutely necessary in commercial ap- plications of TSRFCC-I technology. The performance of an FCC unit is