Flow Profiles and Gas/Liquid Separation in First and Second Generation Designs for Ebullated Reactors

Ebullated bed reactor technology is found in the oil and gas industry as part of the hydrocracking process, within which heavy oils are cracked under elevated temperatures and pressures to produce increased fractions of refinable petroleum products. A unique feature of these types of reactors is the presence of an internal gas/liquid separation and liquid recycle line, through which 60 to 90% of the net liquid flow through the column is recycled to maintain fluidized conditions within the internal catalyst bed. The separation efficiency within these systems has a significant impact on overall unit profitability, whereby high levels of gas recirculation results in lower liquid throughput and increased potential of over-cracking of product gases and production of light ends [1]. These units typically operate at gas holdups above 30%, with even small reductions in gas entrainment potentially leading to significant increases in profitability. Due to the severe conditions present within operating units (several MPa pressures, >300°C), pilot-scale experimental systems exploring fluid flow phenomena have typically employed nitrogen and kerosene as analogous fluids[2]. Even within these systems, the ability to visualize flow patterns and parametrically evaluate the effects of separator modifications on gas recirculation has been limited. In an effort to provide strategic focus for future process improvements, Dalhousie University has been collaborating with Ottawa University and Syncrude Canada Ltd. to develop 3D CFD-based simulations of older generation designs to explore fundamental flow characteristics and sensitivity of gas-liquid separation efficiency to changes in geometry and process conditions. This work explores the sensitivity of gas separation efficiency to operational parameters (bubble size, processing rate, gas holdup), geometric design (two generations of separator designs), and computational model choices (drag correlations and packing limiters). Of particular note is the sensitivity of the predicted performance to drag models, for which there is limited empirical validation under the high gas fraction conditions present in this industrial unit, and the sensitivity to packing limiters, which reflect foam formation (an issue observed within operating units). The trends predicted within this work show significant similarities to current operational trends observed in commercial ebullated bed reactors, and provide a basis for predicting the effects of operational changes on the overall performance of these units.