PEPT study of particle motion for different riser exit geometries

CFD study of exit effect of high-density CFB risers with EMMS-based two-fluid model

“…Note that (i) the axial solids concentration profiles reported in figure 6 are limited to the height of 9.9m, which is slightly below the exit; (ii) the center of exit tube is at the height of 10m; (iii) the volume-average solids concentration of bed is the solids inside the whole riser, including those in the cavity or round elbow. It can be seen that the shift from smooth exit to abrupt exit can significantly increase the solids concentration at the upper part of the riser, which is in a qualitative agreement with many previous experiments (Jin et al, 1988;Bai et al, 1992;Brereton and Grace, 1994;Zheng and Zhang, 1994;Pugsley et al, 1997;Harris et al, 2003;Kim et al, 2008;Chan et al, 2010) and CFD studies (Wu et al, 2010;Lopes et al, 2012). Note that the low solids concentration near the bottom gas inlet is due to the neglect of the effect of gas distributor in the simulations and therefore the unrealistic gas inlet boundary condition.…”

“…As shown in pervious works (Jin et al, 1988;Bai et al, 1992;Brereton and Grace, 1994;Zheng and Zhang, 1994;Pugsley et al, 1997;Harris et al, 2003;Kim et al, 2008;Chan et al, 2010), the solid concentration profile is a good indicator of whether the exit effects are important or not, so figure 6 shows the effect of the types of exit on the axial solids concentration profiles and the volume-average solids concentration in the risers. Note that (i) the axial solids concentration profiles reported in figure 6 are limited to the height of 9.9m, which is slightly below the exit; (ii) the center of exit tube is at the height of 10m; (iii) the volume-average solids concentration of bed is the solids inside the whole riser, including those in the cavity or round elbow.…”

“…By contrast, some studies concluded that exit geometry nearly has no effects on gas-solid flow in CFB risers: the abrupt exit appeared not to affect the solids residence time distribution and the voidage profiles along the studied riser (Lackermeier and Werther, 2002). The reason for those conflicting views was due to the fact that the bed hydrodynamics depended not only on the types of exit but also the operating condition and the size of risers (Chan et al, 2010;Harris et al, 2003;Zheng and Zhang, 1994;Gupta and Berruti, 2000;Lackermeier and Werther, 2002). Moreover, pronounced asymmetry can be found in radial distribution of solids concentration and of solids flux near the top of riser with abrupt exit.…”

“…On the other hand, Johnsson et al (1999) concluded that the influence of the cavity height of abrupt exit on the exit pressure drop was minimal. Again, the reason for those conflicting conclusion was possibly due to different operating conditions (Chan et al, 2010;Harris et al, 2003;Zheng and Zhang, 1994;Gupta and Berruti, 2000). In the study of Van der Meer et al (2000), different curvature radii of smooth exit (C) made a notable distinct in reflux ratio (which can measure solids reflux quantitatively), and the reflux ratio increased with the curvature radius of smooth exit.…”

“…In general, two types of exit can be distinguished (Grace, 1996): the "smooth" one, if there is a smooth and gentle transition via a moderate size radius bend from the riser to the cyclone entrance (Grace, 1996) and the "abrupt" one, where the exit is at a right angle to vertical direction of the flow in the riser (Lackermeier and Werther, 2002). The selection of the type of exit significantly affected the bed hydrodynamics: almost all particles were carried out by air with smooth exit, so time-averaged axial solids concentration decreased with increasing height (Chan et al, 2010;Harris et al, 2003;Pugsley et al, 1997;Reddy and Nag, 2001;Zheng and Zhang, 1994;Zheng et al, 1995). However, significant back-mixing can be observed in the riser with abrupt exit due to the collisions of solid particles with the projected roof and therefore their count-flow, this movement resulted in the fact that axial solids concentration profiles were presented as C-shaped, in other words, solids densification not only appeared in the bottom but also in the upper of bed.…”