Numerical gas-solid flow analysis of ring-baffled risers Vivien Rossbach a , Jonathan Utzig a,b , Rodrigo Koerich Decker a , Dirceu Noriler a , Henry França Meier a, ⁎ a Chemical Engineering Department, University of Blumenau (FURB), Rua São Paulo, 3250 I-302, 89030-000 Blumenau, Santa Catarina, Brazil b School of Mechanical Engineering, Federal University of Uberlandia (UFU), Av. João Naves de Ávila, 2121 Bloco 5P, 38400-902 Uberlandia, Minas Gerais, Brazil abstract article info Article history: Received 21 December 2015 Received in revised form 21 March 2016 Accepted 25 April 2016 Available online 27 April 2016 In order to improve the gas-solid flow in a lab-scale circulating fluidized bed (CFB) riser, airfoil-shaped ring-type internals were used in a CFD-based design of experiments. The best geometrical properties and arrangement of the ring baffles in the riser were defined with a design of experiments. Four variables were studied: ring thick- ness, number of rings, spacing between rings and the insertion of a bottom ring. A catalyst-to-gas ratio of 0.181 kg/kg was used, with a gas velocity of 5.6 m/s. Numerical simulations were performed with the k- epsilon turbulence model and the Gidaspow drag model. KTGF was used to describe the solids properties. The solids distribution was evaluated through dispersion coefficient analysis. Four structures for the particle concen- tration were identified: U-shaped, A-shaped, O-shaped and I-shaped. The best result, with a 45% decrease in the solids dispersion coefficient in comparison with the case without rings, was found in the case with 10 mm ring thickness and four rings. Analysis of the best case showed that the rings promote winding flow, observed in the direction of gas and solid velocity vectors. © 2016 Elsevier B.V. All rights reserved. Keywords: CFB riser Inlet region Gas-solid flow Ring baffles 1. Introduction Fluid catalytic cracking (FCC) is an important process in the petro- leum refining industry. Through this process, heavy vacuum gas oil is converted into lighter fractions such as gasoline. An FCC unit is basically composed of a riser reactor, where chemical reactions take place, and a regenerator, responsible for the removal of coke deposited on the cata- lyst particles. The gas oil feed is vaporized and mixed with the catalyst particles in the bottom riser region. The contact between the gas and solid phases occurs in the same way as in a circulating fluidized bed (CFB). Because of the core-annulus profile formation [1], catalyst particles accumulate near the riser wall. Thus, a dense region is formed in contrast to a dilute region in the center, a phenomenon known as turbophoresis [2]. Consequently, overcracking occurs in the dense re- gion and undercracking occurs in the dilute region [3]. Many authors have demonstrated that solid particles are concentrated in the wall re- gion of the riser, while the center region is diluted [4,5]. The complex and turbulent flow in the riser inlet region induces back-mixing and short circuits [3,6,7]. To reduce the solids concentration in the dense re- gion near the wall, Peng et al. [8] studied the use of air jets from the riser circumference numerically. Under appropriate jet velocities, the solids concentration near the wall can be significantly reduced and the flow structure becomes more uniform. The use of internal ring baffles in the riser inlet region improves the solids distribution and helps to increase the mass, heat and momentum transfer [7]. Thus, the conversion of the gas oil into products improves due to better contact between the phases [3,7]. Jiang et al. [9] analyzed ozone decomposition experimentally, using FCC particles and conclud- ed that internal ring baffles improve the chemical reactions involved in the conversion at average and high velocities, by increasing the solids fraction distribution in the radial direction. Zhu et al. [10] studied the influence of the ring opening area on the solids fraction distribution ex- perimentally, using three ring baffles with 70%, 90% and 95% of opening area, installed in different axial positions. For rings with 70% of opening area, a denser region is formed in the riser bottom. Also, the insertion of ring baffles induces the formation of an S-shaped velocity profile in the axial direction [10]. Samruamphianskun et al. [11] studied the gas-solid flow in a ring baffled riser numerically, varying four geometric proper- ties of the square-shaped rings: ring thickness, number of rings, opening area, and the spacing between rings and its uniformity. The ring opening area and the spacing between rings had the most important effect on the solids distribution. Bu and Zhu [12] investigated the influence of ring-type internals on the axial pressure distribution in a circulating fluidized bed experimen- tally, in order to determine its influence on the radial and axial solids distribution. Four ring opening areas between 70% and 95% were stud- ied and their effect under different operational conditions was tested. The opening area showed a strong influence on the flow behavior. The optimum opening area is related to the operational conditions applied and, in this case, a 90% opening produced the most homogeneous gas- solid flow. Guío-Perez et al. [13] investigated the increase in the total pressure drop as a result of ring-type internals installed in a circulating fluidized bed experimentally. Previously designed wedge-shaped rings were Powder Technology 297 (2016) 320–329 ⁎ Corresponding author. E-mail address: meier@furb.br (H.F. Meier). http://dx.doi.org/10.1016/j.powtec.2016.04.044 0032-5910/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Powder Technology journal homepage: www.elsevier.com/locate/powtec