FCC Riser Hydrodynamics Improvement Using Simulation Tools

Past experiences of commercial FCC units indicate that hydrodynamics have a pronounced effect on the riser performance. For example, an increase of lift steam velocity has been shown to result in increased gasoline selectivity in many units. For a large FCC unit in Asia the installation of dual-radii feed distributors improved feed-catalyst contacting in the lower riser and also led to better gasoline yields.

A study of FCC riser hydrodynamics was undertaken to understand the phenomena observed in the field. It encompassed computational fluid dynamics (CFD), cold flow modeling, radioactive tracer studies, kinetic modeling and yield estimation. The ultimate goal of the study was to isolate the effects of riser diameter, lift steam velocity, and solids flux on the hydrodynamic behavior and yield performance of FCC risers.

Our CFD approach was successfully validated by cold flow results and applied to several commercial FCC units. The general trends of riser hydrodynamics were identified as follows. Lower vapor velocity, larger riser diameter and higher solids flux through the riser all contribute to hydrodynamic “pathologies”, such as catalyst downflow at the wall and vapor channeling through the core of the riser. In practice, these “non-idealities” lead to “under-conversion” of feed in the core of the riser and overcracking of products in the annulus of the riser.