Gas segregation in a pilot-scale ebullated bed system: Experimental investigation and model validation

Gas hold-up reduction inside hydroprocessors is critical to improving the efficiency of bitumen conversion. The use of internal gas/liquid segregation devices, such as recycle cups, has been the focus of much past research and development over the previous five decades. In this study, the effects of different recycle cup designs and working fluids were evaluated using a pilot-scale cold-flow ebullated bed reactor and compared to multiphase flow simulation predictions. Two recycle cup designs from the literature were fabricated using 3D printing technology and two working fluids were used: air-water and air-5 wt% ethanol aqueous solutions. The addition of ethanol helped create high gas hold-up and foaming conditions frequently observed in commercial hydroprocessors. It was observed that in foaming systems, the performance of the recycle cups deteriorated. Additionally, increasing the liquid flow rate, either through the inlet or increased recycle, resulted in diminished gas/liquid segregation efficiency. The accuracy of simulations using the two-fluid model was evaluated to predict gas hold-up in the pilot scale system as well as inside the recycle line. In the homogeneous flow regime, given the proper bubble size, the simulations predicted the gas hold-up in the column with less than 5% error. However, unlike the experimental observations, simulations did not predict any gas entrained in the recycle line, which is attributed to variation in the average bubble size and limitations of the existing momentum exchange closures.