Modeling the Fluid Dynamics of a Commercial Ebullated Bed Hydroprocessor
Due to the nature of bitumen extracted from Canada’s Athabascan oil sands, upgrading of this heavy oil to lighter, more valuable, products is required. Ebullated bed hydroprocessors are used for the conversion of bitumen residue from atmospheric, and vacuum, distillation tower bottoms to lighter petroleum products. Studies in support of these industrial hydroprocessors have demonstrated that gas holdup within the reactor is higher than predicted. High gas holdup is unfavorable because it displaces the unconverted liquid hydrocarbons, thereby reducing the liquid phase residence time and overall pitch conversion. Due to the complex interactions between the gas-liquid-solid phases, estimation of liquid residence time, and catalyst bed fluidization behavior are dependent on a thorough understanding of the multiphase fluid dynamics. High gas holdup has been attributed to the entrainment of gas bubbles by the internally recycled liquid. However, previous studied have not addressed the impact of the gas-liquid separation in the freeboard region of the hydroprocessor. This thesis presents a novel fluid dynamic model of an industrial ebullated bed hydroprocessor that accurately accounts for the gas-liquid separation dynamics in the freeboard. Analysis of the freeboard region multiphase flow was conducted by computational fluid dynamics (CFD) to characterize gas entrainment by the recycled liquid phase. A correlation of gas separation to liquid residence time was observed and validated with commercial pilot plant data. Outcomes of this numerical study demonstrated a sensitivity of the gas separation to the bubble drag coefficient. Using an analog fluid system, a bubble swarm drag coefficient correlation was experimentally investigated at atmospheric and elevated pressures. As pressures increased, swarm effects attenuated for bubbles within the optical probe detection limits. Based on these results, comparisons of previous generation separators were completed using CFD. Enhanced separation was primarily due to improved removal of large bubbles, providing insights for future design improvements. These results were incorporated into an integrated fluid dynamic model of an ebullated bed hydroprocessor, and a parametric analysis was conducted to determine the sensitivity of reactor gas holdup to separation efficiency.