The results show a more uniform axial and radial solids distribution with increasing pressure pabs or decreasing density ratios ρS/ρF, 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 ρS/ρF.
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 ρS/ρF and the dimensionless superficial gas velocity FrP.