Abstract: The effects of different operating conditions on the solid circulation and solid holdup distribution in the axial and radial directions of a novel diameter-changing riser were investigated in a cold-mold test using FCC equilibrium catalyst and room temperature air medium. The flow structure within the diameter expanded segment was described. Experimental results showed that the amount of solid circulation increases with the increase of the superficial gas velocity, and the bed height. When changing the ratio of intake air, the amount of circulation increases with the increase of the proportion of pre-lifting gas. Compared with the traditional riser, there are many kinds of flow patterns in the new structure riser, and the solid holdup in the expanded section is significantly increased. A dense phase turbulent flow pattern exists in the bottom region (εs is about 0.3—0.4) and a dilute phase pneumatic conveying form is observed in the upper region (εs is about 0.05—0.1) where the solid holdup has no obvious change. The novel variable diameter riser is able to redistribute the solid particles coming from the inclined pipe, resulting in inhibition of the traditional “ring-core” flow pattern, and making the radial distribution of solid holdup more uniform.

Key words: novel variable diameter riser, flow structure, solid circulation, solid content