Increasing solid holdup and de-clustering in downer reactors by counter-current operation

In this work, we investigate the fluid dynamics of a counter-current downer using particle image velocimetry and optical fiber probes. The objective is to characterize the hydrodynamic behavior, specifically holdup and clustering, to establish a balance between risers, which exhibit high holdup and low clustering, and co-current downers, characterized by low holdup and high clustering. Flow patterns and clustering phenomena are quantified for Geldart A and B particles under various operating conditions at two axial positions. The results show that particle flow evolves from an acceleration zone into a more developed, stable region, with notable differences in solid holdup and cluster size distributions between the two measurement locations. By optimizing operating conditions, we achieved a 38% increase in solid holdup for commercial FCC particles and a 45% increase for sand particles, along with a more uniform particle distribution in the case of FCC. Under optimized counter-current operation, cluster size was reduced by 43% for FCC particles and by 33% for sand particles.