Hydrodynamics and heat transfer characteristics in a circulating fluidized bed

Abstract

Hydrodynamics and heat transfer characteristics have been determined in a circulating fluidized bed (0.1m I.D. × 5.3m high) of FCC particles $(d_p=65㎛, ρ_s=1720 kg/㎥)$. The effects of gas velocity (1.5- 3.5 m/s), solid circulation rate (5 - 50 kg/㎡s), secondary air ratio (0 ~ 50 %), height of injection port (1.5 - 2.5 m) and injection types (radial and tangential) on the axial solid holdup distribution have been determined The transport velocity of FCC particles in the fast fluidization regime has been determined from the pressure drop across the bed and the time required for all the solids to leave the bed. A flow regime map has been proposed to demarcate the flow regimes quantitatively from turbulent fluidization to pneumatic transport regime. The axial solid holdup distributions have been determined from the measured axial pressure profiles. The solid phase forms a dense phase at the bottom section and a dilute phase at the top section of the riser. Total solid holdups in the riser increase with increasing solid circulation rate but decreases with an increase in superficial gas velocity. The solid holdup in the dilute phase has a constant value $(ε_{s∞}$ along the riser height which can be related to the ratio of solid to gas velocities $(G_s/ρ_sU_g)$ by introducing slip factor (ψ) as: $(ε_{s∞}=ψ/[ψ+1/(G_s/ρ_sU_g)]$ At the top region of the riser, the solid holdup increases due to the end effect of the exit structure of the column which increases with increasing gas velocity. The solid holdup in the dense phase decreases slightly with gas velocity. The distributions of void ratio have been correlated with the pertinent dimensionless groups based on the data of the present and previous studies as: ◁수식 삽입▷(원문을 참조하세요) The effect of secondary air injection by the radial and tangential injection nozzles (0.016 m ID) on the axial solid holdup distributions has been determined. The injection of secondary air is found to affect the axial solid holdup distribu...