Fluid catalytic cracking (FCC) riser reactors have complex hydrodynamics, which depend not only on operating conditions, feedstock quality, and catalyst particles characteristics, but also on the geometric configurations of the reactor. This paper presents a numerical study of the influence of different riser outlet designs on the dynamic of the flow and reactor efficiency. A three-dimensional, three-phase flow model and a four-lump kinetic scheme were used to predict the performance of the reactor. The phenomenon of vaporization of the liquid oil droplets was also analyzed. Results showed that small changes in the outlet configuration had a significant effect on the flow patterns and consequently, on the reaction yields. 1. Introduction Although commercially established for over half a century, the fluid catalytic cracking (FCC) process is still widely studied nowadays. Since it is a very profitable？？ operation, any improvement in it can result in large savings for the refinery. In the FCC process, preheated high-boiling liquid oil is injected into the riser reactor, where it is vaporized and cracked into smaller molecules by contact and mixing with the very hot catalyst particles coming from the regenerator. These phenomena cause a gas expansion, which drags the catalyst to the top of the reactor. Since catalytic cracking reactions can only occur after the vaporization of liquid feedstock, mixing of hydrocarbon droplets with catalyst must take place in the riser as soon as possible. It is known that riser reactors have complex hydrodynamics. They present a high solids concentration near the walls and are also axially divided into dense and dilute regions. In addition, different riser configurations such as the inlet and outlet structures can have a profound effect on the flow patterns mentioned above. The influence of riser exit geometry on the hydrodynamics of gas-solid circulating fluidized beds (CFB) has been investigated in many studies [1–6]. Although the results reported in these studies apparently conflict quantitatively concerning the influence of riser exit, some common aspects can be observed: the design of the exit has a large effect upon the reflux of solids; abrupt exits cause an increase in the solids holdup and a large backmixing at the top of the riser; increasing the refluxing effect of the exit has proved to increase the mean particle residence time; larger and denser clusters are formed at the walls in the risers with abrupt exits. In an experimental work, Lim et al. [7] investigated a cold model of a circulating fluidized bed, in a
References
[1]
Y. Cheng, F. Wei, G. Yang, and J. Yong, “Inlet and outlet effects on flow patterns in gas-solid risers,” Powder Technology, vol. 98, no. 2, pp. 151–156, 1998.
[2]
S. K. Gupta and F. Berruti, “Evaluation of the gas-solid suspension density in CFB risers with exit effects,” Powder Technology, vol. 108, no. 1, pp. 21–31, 2000.
[3]
E. H. Van Der Meer, R. B. Thorpe, and J. F. Davidson, “Flow patterns in the square cross-section riser of a circulating fluidised bed and the effect of riser exit design,” Chemical Engineering Science, vol. 55, no. 19, pp. 4079–4099, 2000.
[4]
A. T. Harris, J. F. Davidson, and R. B. Thorpe, “The influence of the riser exit on the particle residence time distribution in a circulating fluidised bed riser,” Chemical Engineering Science, vol. 58, no. 16, pp. 3669–3680, 2003.
[5]
C. W. Chan, A. Brems, S. Mahmoudi et al., “PEPT study of particle motion for different riser exit geometries,” Particuology, vol. 8, no. 6, pp. 623–630, 2010.
[6]
X. Wang, L. Liao, B. Fan et al., “Experimental validation of the gas-solid flow in the CFB riser,” Fuel Processing Technology, vol. 91, no. 8, pp. 927–933, 2010.
[7]
M. T. Lim, S. Pang, and J. Nijdam, “Investigation of solids circulation in a cold model of a circulating fluidized bed,” Powder Technology, vol. 226, pp. 57–67, 2012.
[8]
J. W. Chew, R. Hays, J. G. Findlay et al., “Reverse core-annular flow of Geldart Group B particles in risers,” Powder Technology, vol. 221, pp. 1–12, 2012.
[9]
T. Pugsley, D. Lapointe, B. Hirschberg, and J. Werther, “Exit effects in circulating fluidized bed risers,” Canadian Journal of Chemical Engineering, vol. 75, no. 6, pp. 1001–1010, 1997.
[10]
G. Van engelandt, G. J. Heynderickx, J. De Wilde, and G. B. Marin, “Experimental and computational study of T- and L-outlet effects in dilute riser flow,” Chemical Engineering Science, vol. 66, no. 21, pp. 5024–5044, 2011.
[11]
J. De Wilde, G. B. Marin, and G. J. Heynderickx, “The effects of abrupt T-outlets in a riser: 3D simulation using the kinetic theory of granular flow,” Chemical Engineering Science, vol. 58, no. 3–6, pp. 877–885, 2003.
[12]
B. Chalermsinsuwan, P. Kuchonthara, and P. Piumsomboon, “Effect of circulating fluidized bed reactor riser geometries on chemical reaction rates by using CFD simulations,” Chemical Engineering and Processing, vol. 48, no. 1, pp. 165–177, 2009.
[13]
A. K. Das, J. De Wilde, G. J. Heynderickx, and G. B. Marin, “CFD simulation of dilute phase gas-solid riser reactors—part II: simultaneous adsorption of SO2- from flue gases,” Chemical Engineering Science, vol. 59, no. 1, pp. 187–200, 2004.
[14]
H. Ali, S. Rohani, and J. P. Corriou, “Modelling and control of a riser type fluid catalytic cracking (FCC) unit,” Chemical Engineering Research and Design, vol. 75, no. 4, pp. 401–412, 1997.
[15]
I. S. Han and C. B. Chung, “Dynamic modeling and simulation of a fluidized catalytic cracking process—part I: process modeling,” Chemical Engineering Science, vol. 56, no. 5, pp. 1951–1971, 2001.
[16]
I. S. Han and C. B. Chung, “Dynamic modeling and simulation of a fluidized catalytic cracking process—part II: property estimation and simulation,” Chemical Engineering Science, vol. 56, no. 5, pp. 1973–1990, 2001.
[17]
S. V. Nayak, S. L. Joshi, and V. V. Ranade, “Modeling of vaporization and cracking of liquid oil injected in a gas-solid riser,” Chemical Engineering Science, vol. 60, no. 22, pp. 6049–6066, 2005.
[18]
J. S. Ahari, A. Farshi, and K. Forsat, “A mathematical modeling of the riser reactor in industrial FCC unit,” Petroleum and Coal, vol. 50, no. 2, pp. 15–24, 2008.
[19]
F. Van Landeghem, D. Nevicato, I. Pitault et al., “Fluid catalytic cracking: modelling of an industrial riser,” Applied Catalysis A, vol. 138, no. 2, pp. 381–405, 1996.
[20]
C. Derouin, D. Nevicato, M. Forissier, G. Wild, and J. R. Bernard, “Hydrodynamics of riser units and their impact on FCC operation,” Industrial and Engineering Chemistry Research, vol. 36, no. 11, pp. 4504–4515, 1997.
[21]
R. Deng, F. Wei, T. Liu, and Y. Jin, “Radial behavior in riser and downer during the FCC process,” Chemical Engineering and Processing, vol. 41, no. 3, pp. 259–266, 2002.
[22]
G. C. Lopes, L. M. Rosa, M. Mori, J. R. Nunhez, and W. P. Martignoni, “Three-dimensional modeling of fluid catalytic cracking industrial riser flow and reactions,” Computers and Chemical Engineering, 2011.
[23]
K. N. Theologos and N. C. Markatos, “Advanced modeling of fluid catalytic cracking riser-type reactors,” AIChE Journal, vol. 39, no. 6, pp. 1007–1017, 1993.
[24]
K. N. Theologos, A. I. Lygeros, and N. C. Markatos, “Feedstock atomization effects on FCC riser reactors selectivity,” Chemical Engineering Science, vol. 54, no. 22, pp. 5617–5625, 1999.
[25]
A. Gupta and D. S. Rao, “Model for the performance of a fluid catalytic cracking (FCC) riser reactor: effect of feed atomization,” Chemical Engineering Science, vol. 56, no. 15, pp. 4489–4503, 2001.
[26]
G. C. Lopes, L. M. Da Rosa, M. Mori, J. R. Nunhez, and W. P. Martignoni, “The importance of using three-phase 3-D model in the simulation of industrial FCC risers,” Chemical Engineering Transactions, vol. 24, pp. 1417–1422, 2011.
[27]
H. Farag, A. Blasetti, and H. De Lasa, “Catalytic cracking with FCCT loaded with tin metal traps: adsorption constants for gas oil, gasoline, and light gases,” Industrial and Engineering Chemistry Research, vol. 33, no. 12, pp. 3131–3140, 1994.
[28]
S. B. Schut, E. H. Van Der Meer, J. F. Davidson, and R. B. Thorpe, “Gas-solids flow in the diffuser of a circulating fluidised bed riser,” Powder Technology, vol. 111, no. 1-2, pp. 94–103, 2000.
[29]
L. S. Lee, Y. W. Chen, and T. N. Huang, “Four-lump kinetic model for fluid catalytic cracking process,” Canadian Journal of Chemical Engineering, vol. 67, no. 4, pp. 615–619, 1989.
[30]
J. A. Juárez, F. L. Isunza, E. A. Rodrìguez, and J. C. M. Mayorga, “A strategy for kinetic parameter estimation in the fluid catalytic cracking process,” Industrial & Engineering Chemistry Research, vol. 36, pp. 5170–5174, 1997.
[31]
S. A. Morsi and A. J. Alexander, “An investigation of particle trajectories in two-phase flow systems,” The Journal of Fluid Mechanics, vol. 55, no. 2, pp. 193–208, 1972.
[32]
Ansys Inc. (US), ANSYS FLUENT 12. 0—Theory Guide, Ansys, 2009.
[33]
W. Zhang, Y. Tung, and F. Johnsson, “Radial voidage profiles in fast fluidized beds of different diameters,” Chemical Engineering Science, vol. 46, no. 12, pp. 3045–3052, 1991.
[34]
J. H. P？rssinen and J. X. Zhu, “Particle velocity and flow development in a long and high-flux circulating fluidized bed riser,” Chemical Engineering Science, vol. 56, no. 18, pp. 5295–5303, 2001.
[35]
J. C. S. C. Bastos, L. M. Rosa, M. Mori, F. Marini, and W. P. Martignoni, “Modelling and simulation of a gas-solids dispersion flow in a high-flux circulating fluidized bed (HFCFB) riser,” Catalysis Today, vol. 130, no. 2–4, pp. 462–470, 2008.
[36]
K. N. Theologos, I. D. Nikou, A. I. Lygeros, and N. C. Markatos, “Simulation and design of fluid-catalytic cracking riser-type reactors,” Computers and Chemical Engineering, vol. 20, no. 1, pp. S757–S762, 1996.
[37]
J. Gao, C. Xu, S. Lin, G. Yang, and Y. Guo, “Simulations of gas-liquid-solid 3-phase flow and reaction in FCC riser reactors,” AIChE Journal, vol. 47, no. 3, pp. 677–692, 2001.
[38]
W. Martignoni and H. I. De Lasa, “Heterogeneous reaction model for FCC riser units,” Chemical Engineering Science, vol. 56, no. 2, pp. 605–612, 2001.
[39]
D. L. Liu and J. M. Han, “Evaluation on commercial application of LPC type nozzle for FCC feed,” Process Engineering Resources, vol. 22, article 49, 1992.
[40]
A. T. Harris, J. F. Davidson, and R. B. Thorpe, “Influence of exit geometry in circulating fluidized-bed risers,” AIChE Journal, vol. 49, no. 1, pp. 52–64, 2003.
[41]
M. P. Martin, P. Turlier, J. R. Bernard, and G. Wild, “Gas and solid behavior in cracking circulating fluidized beds,” Powder Technology, vol. 70, no. 3, pp. 249–258, 1992.
[42]
X. Wu, F. Jiang, X. Xu, and Y. Xiao, “CFD simulation of smooth and T-abrupt exits in circulating fluidized bed risers,” Particuology, vol. 8, no. 4, pp. 343–350, 2010.
[43]
M. Liu and J. N. Tilton, “Spatial distributions of mean age and higher moments in steady continuous flows,” AIChE Journal, vol. 56, no. 10, pp. 2561–2572, 2010.
[44]
F. Wang, Q. Marashdeh, A. Wang, and L. Fan, “Electrical capacitance volume tomography imaging of three-dimensional flow structures and solids concentration distributions in a riser and a bend of a gas-solid circulating fluidized bed,” Industrial & Engineering Chemistry Research, vol. 51, pp. 10968–10976, 2012.
