An experimental study was carried out on the downflow of fine catalysts (d50=44 μm) in a straight vertical cyclone dipleg (diameter 80 mm, height 4.6 m) to clarify various types of powder flow which can exist in the dipleg and to measure the amount of gas entrainment down the dipleg under a wide range of operating conditions. In addition, the radial profiles of solids velocity and solids volume fraction were measured at various heights along the dipleg. The axial pressure profiles show that the flow in the dipleg can be divided in regions: inlet, dilute and dense. A flow diagram shows the occurrence of various flow types depending on operating conditions. The flow in the dense region can be fluidized (preferred) or packed. In the dilute region, the solids velocity is higher than the terminal falling velocity of a single particle. This indicates the existence of clusters, the size of which has been estimated. Gas carry-under is generally not significant when the dense region has developed. Once there is a dilute region only, the gas carry-under can become as large as 20 times the volumetric solids flow. The radial profiles of solids velocity and solids volume fraction show that going from dipleg top to dipleg bottom, the rotating annular flow at the dipleg top (cyclone outlet) transforms into a more or less uniform plug flow, both with respect to velocity and to local concentration.
Wang, J., Bouma, J. H., & Dries, H. W. A. (2000). An experimental study of cyclone dipleg flow in fluidized catalytic cracking. Powder technology, 112(3), 221-228. https://doi.org/10.1016/S0032-5910(00)00296-5