Characterization of the flow patterns in a pressurized circulating fluidized bed

Abstract

To investigate the influence of the solid/gas density ratio ρ S/ ρ F on the flow pattern in a riser reactor, a pressurized circulating fluidized bed with an inner diameter of 0.194 m and 9 m height was used. The experiments have been carried out from ambient conditions to a static pressure p abs of 5 MPa in the regime of fast fluidization and/or pneumatic conveying. With the use of glass beads ( ρ S = 2500 kg/m 3, d P = 85 μm), density ratios ρ S/ ρ F from 2094 to 41 and solids net fluxes G S,Net from 65 to 200 kg/m 2s ( U G = 0.8 to 4.5 m/s) were found. Local information about solids concentrations, solids velocities, and solids mass flows has been obtained by a capacitance probe system. The results show a more uniform axial and radial solids distribution with increasing pressure p abs or decreasing density ratios ρ S/ ρ F, indicating flow conditions which are different to a well known core-annulus flow structure. This flow structure is defined by a dilute upflow in the core region and a dense downflow in the annulus region which is situated immediately near the wall. In agreement, the radial profiles of axial solids mass flux show flow patterns which are untypical for fast fluidization operating conditions. The results depict a distinct wall region that covers almost half of the riser cross-sectional area. Therefore, significant solids downflow occur, pointing out massive solids backmixing even at low solid/gas density ratios ρ S/ ρ F. Finally, the obtained information is used to develop an easy correlation for the prediction of internal solids reflux in a riser reactor as a function of solids/gas density ratios ρ S/ ρ F and the dimensionless superficial gas velocity Fr P.