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Formation and characteristics of carbonaceous deposits from heavy hydrocarbon vapours Fan, Zhiming


Coking is an important step in the upgrading of bitumen or other heavy hydrocarbons into lighter products. Deposition of carbonaceous material in the cyclone exit line is a chronic problem for all fluid cokers and is a key process limitation to achieving longer run length. It has been a subject of detailed process studies at UBC. The overall aims of this work were to elucidate the causes of deposit formation via coke characterization with a range of techniques, to examine the evolution of deposit composition and structure over time, to compare the laboratory deposits with industrial samples, and to propose a model of deposit structural evolution during the aging process based on kinetics of the deposit aging process and above characterization results. Fresh deposits recovered from a laboratory bench-scale cyclone fouling unit and aged laboratory deposits have been compared with samples from the snout region and exit tube of an industrial fluid coker. Extensive characterization studies were conducted using modern analytical techniques, e.g., elemental analysis, X-ray Fluorescence (XRF), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), Diffusive Reflection Infrared Spectroscopy (DRIFT), and Solid-state ¹³C Nuclear Magnetic Resonance (¹³C NMR). Simulated distillation was also applied to solvent extracts of deposits. Results substantiate that physical condensation rather than chemical reaction is the primary reason for fluid coker cyclone exit line fouling. Entrained liquid droplets also contribute to the deposit formation to a lesser extent. Although the fresh laboratory deposits are much different from the industrial deposits, after days to weeks of aging at elevated temperature, the H/C ratio, TGA characteristics, ¹³C NMR, and DRIFTS spectra of the lab deposits become very similar to those of the graphitic industrial deposits. The differences in morphology which remain after aging, are attributed to the difference in hydrodynamic conditions during the deposit laydown. The various techniques studied yielded a consistent picture of the evolution from the heavy fluid phase components which initially deposit from the vapour to the final massive graphitic coke-like deposit formed in the industrial coker cyclone exit tube. During aging of fresh lab deposits, a considerable amount of volatile components is released, especially in the initial period. Kinetic models were developed to describe reactions in different aging periods based on thermal behavior of deposits. During the heating period from ambient to final aging temperatures, a first order non-isothermal kinetic model was used to describe the de-volatilization reaction properties of cyclone fouling deposits. A two-stage model is used for the kinetics of the laboratory deposits collected at low temperatures (<500°C), but a single-stage kinetic model can well describe the thermal behavior of the industrial deposits and the laboratory deposits collected at high temperatures. The values of apparent activation energies in the heating period suggest that thermal cracking reactions occur. Isothermal kinetic models were used to characterize the weight loss phenomena with time at different final aging temperatures. A 2nd order exponential decay equation can describe the volatiles decline in the initial stage, and another zero order reaction model can be used to describe the subsequent slow aging reaction period. The aged laboratory deposits and industrial deposit samples have similar but not identical kinetic characteristics. The structural evolution of aging cyclone fouling deposits is proposed following the Marsh-Griffiths model. Depending on temperature, pressure, and composition, cyclone exit vapours can contain some liquid droplets. Since the cyclone exit tube wall is at a lower temperature than the bulk fluid, physical condensation may occur on the surface, giving rise to the deposits. Alternately adherence of droplets transferred from the bulk fluid may be the cause. A liquid droplet mass transfer model has been developed to describe the deposition in both laboratory and industrial conditions. In the laminar flow range, calculations show that deposition of fine droplets is very slow; physical condensation on the tube should be the primary reason for the exit tube line fouling under laboratory conditions. In the turbulent flow range, as the droplet size is increased, the deposition mechanism shifts from diffusion to impaction, which would accelerate the droplet deposition rate dramatically and cause much more deposits in the tube entrance region. The droplet mass transfer could cause serious deposition and blockage of tube line in industrial operation conditions, especially for droplets above one micron. Small droplet size and large diameter exit tube will mitigate the deposition problem in industrial fluid coker cyclone operation.

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