Flow behavior and regime transition in a high-density circulating fluidized bed riser

Flow behavior and flow regime transitions were determined in a circulating fluidized bed riser (0.203 m i.d. x 5.9 m high) of FCC particles (d(p) = 70 pm, rho(p) = 1700 kg/m(3)). A momentum probe was used to measure radial profiles of solids momentum flux at several heights and to distinguish between local net upward and downward flow. In the experimental range covered (U-g = 5-8 m/s; G(s) = 22-345 kg/m(2) s), the fast fluidization flow regime was observed to coexist with dense suspension upflow (DSU). At a constant gas velocity, net downflow of solids near the wall disappeared towards the bottom of the riser with increasing solids mass flux, with dense suspension upflow achieved where there was no refluxing of solids near the riser wall on a time-average basis. The transition to DSU conditions could be distinguished by means of variations of net solids flow direction at the wall, annulus thickness approaching zero and flattening of the solids holdup versus G(s) trend. A new flow regime map is proposed distinguishing the fast fluidization, DSU and dilute pneumatic transport flow regimes. (C) 2004 Elsevier Ltd. All rights reserved.