Mitigation of fouling and liquid losses in a Fluid Coker™: Influence of operating conditions and internals on wet-agglomerates contribution to liquid carry-under
Abstract
In Fluid Cokers, heavy oil is sprayed into a fluidized bed of hot particles through feed ring nozzles located at different heights. Unreacted or partially reacted feedstock, trapped in wet‑agglomerates, reaches the reactor bottom where it causes fouling or is lost to the system burner. The thesis objective is to determine whether modifying the feed distribution between rings or adding baffles can reduce the amount of liquid reaching the reactor bottom.
A model was developed to predict the amount of liquid reaching the reactor bottom. It integrates models for agglomerate formation and rewetting in spray jets, agglomerate drying through heat transfer from the hot bed particles, and agglomerate breakage from shear in the turbulent bed. The model requires accurate agglomerate trajectories through the reactor to predict rewetting, drying time, and shear.
Radioactive Particle Tracking (RPT) uses radioactive tracers to provide agglomerate trajectories in a 0.25 m diameter cold model with solid circulation. This thesis shows how RPT accuracy can be enhanced by correcting systematic errors from radiation absorption and random signal fluctuations due to the stochastic nature of radioactive emissions.
The model shows that agglomerates between 10 and 13 mm in diameter are most likely to bring liquid to the reactor bottom. Smaller agglomerates dry quickly. Larger agglomerates are more likely to be broken through shear.
Redistributing feed between rings can reduce the amount of liquid reaching the reactor bottom. Wet-agglomerates formed in the upper regions have more time to dry and break. For example, with five feed rings, redistributing the feed from the lower ring to the upper four rings reduces how much liquid reaches the reactor bottom by about 70-80%.
Adding a ring-baffle in the bed can also reduce the amount of liquid reaching the reactor bottom by about 40-50%. The baffle prevents wet-agglomerates from reaching the reactor bottom quickly, providing more time for drying. The baffle also creates a zone of high shear between the baffle tips, enhancing agglomerate breakage. Combining baffle addition and feed redistribution from the lower ring to the upper feed rings reduces the amount of liquid reaching the reactor bottom by about 80-90%.