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

Abstract

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 (

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