Doctor of Philosophy
Chemical and Biochemical Engineering
Briens, Cedric L.
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%.
Summary for Lay Audience
Oil sands, or more specifically the bitumen they contain, require a conversion from heavy oil to valuable lighter hydrocarbons. This thesis aims to improve the performance of Fluid Cokers, a type of fluidized thermal cracking unit used for this type of oil conversion.
A fluidized unit consists of gas injected into a bed of solid particles to impart a liquid-like behaviour to the gas-solid mixture. In a Fluid Coker, heavy oils are mixed with steam and sprayed into a fluidized bed of hot coke (a coal derivative) to be converted into lighter products. In Fluid Cokers, one major challenge is wet‑agglomerates formed when coke and bitumen clump together. When these wet-agglomerates reach the unit bottom, the valuable liquid they carry is destroyed, increasing how much raw materials are required per barrel of refined oil. Therefore, optimizing the process would minimize its environmental impact. Wet‑agglomerates also stick and accumulate on the inner walls of the unit bottom, significantly limiting the unit operational lifecycle.
A model is developed to predict how much liquid is carried by wet‑agglomerates to the bottom of commercial Fluid Cokers. This model bundles several key components of an agglomerate lifecycle: formation, drying due to liquid vaporization, possible rewetting, and possible destruction from interactions with the bed. Specific focus is given to possible industrially suitable solutions, such as redistributing the heavy oil injection profile or adding a specific insert (“ring‑baffle”) inside the bed. A radioactive tracer mimicking an agglomerate is used in a cold gas‑solid pilot unit to acquire data for the model. A new mathematical method converting the radiation measurements into tracer coordinates is proposed to improve the accuracy of calculated wet‑agglomerate trajectories.
The model shows that redistributing how bitumen is injected into the Fluid Coker, by shifting the injection toward the upper part of the bed, can reduce the amount of liquid reaching the unit bottom. This improvement is explained by a change in the bed hydrodynamics which slows wet‑agglomerates travelling down. Adding a “ring-baffle” is also helpful: in addition to slowing down agglomerates travel, it also locally promotes their destruction. Combining both methods is beneficial.
Cochet, Yohann Alain François, "Mitigation of fouling and liquid losses in a Fluid Coker™: Influence of operating conditions and internals on wet-agglomerates contribution to liquid carry-under" (2021). Electronic Thesis and Dissertation Repository. 7890.
Available for download on Thursday, July 13, 2023