(133b) Study of FCC Riser Hydrodynamics Using CFD Tools

Past experiences of commercial FCC units indicate that hydrodynamics have a pronounced effect on the riser reactor performance. 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.

Results of the cold flow studies were used to select and validate the best CFD modeling approach. Predictions of several models developed within the Eulerian-Eulerian and Eulerian-Lagrangian frameworks were compared with the experimental solids flux profiles in the cold riser operating at commercially-relevant velocities. Both model classes showed good agreement with experiment in the “normal” riser operation. The Eulerian-Lagrangian approach showed better predictions of experiment when the riser was “pushed” in terms of velocity and solids flux.

The general trends of riser hydrodynamics, per CFD predictions, were identified as follows. Lower vapor velocity, larger riser diameter and higher solids flux through the riser all contribute to hydrodynamic non-idealities, such as catalyst downflow at the wall and vapor channeling through the core of the riser. In practice, these “non-idealities” lead to undesirable shifts in conversion (under-conversion in the core) and selectivity (overcracking in the annulus) of the riser.