CFD Modelling of Pilot-Scale Three-Phase Separators

This work uses experimental analysis and Computational Fluid Dynamics (CFD) modelling to investigate the separation performance of three-phase pilot-scale separators operating at different flow conditions. The first separator (Pilot-Sep-A) is a three-phase separator equipped with flat plate inlet diverter and an overflow weir with a length to diameter ratio of 3. The effects of gas, oil and water flowrates were determined on the separation efficiency for this separator. The second separator (PilotSep-B) is equipped with an inlet diverter, mist extractor and a bucket and weir configuration with a length to diameter ratio of 2. For Pilot-Sep-B, the effect of oil flowrate, water flowrate and weir height were determined on the separation efficiency. The two pilot-scale separators are used for demonstration purposes and were selected for this work due to their availability.
The experiments were designed using Minitab Factorial design of experiments (DOE). Eight experiments were conducted at random for each separator at different levels (low and high) of the factors investigated. Randomisation was carried out to provide protection against extraneous factors that can affect the results. For each run, oil samples were obtained and centrifuged to determine quantity of water in the oil outlet. This was then used to determine the separation efficiency. For the CFD modelling, numerical solutions were initialised with predetermined oil and water levels using the patching tool. A sensitivity analysis on multiphase and turbulence models indicated that Eulerian multiphase model with standard k-ε turbulence models predicted the best results. A mesh independence test was also carried out to ensure the results are independent of the mesh size.
The experimental and CFD predicted results in form of cube plot, pareto chart and main effect chart for the separation efficiency were compared for both separators. The factors investigated showed significant effects on separation performance. The oil flowrate was found to have the greatest effect on the separation efficiency for both separators. This is followed by the water flowrate and finally the gas flowrate and weir height for Pilot-Sep-A and Pilot-Sep-B respectively. A maximum deviation between the experimental and CFD predicted results of 18% was obtained when Pilot-Sep-B was set at high oil and water flowrate and low weir height. A white rag placed at the gas outlet indicated no liquid carry over at the gas outlet for both separators. This is consistent with the CFD predicted results. Finally, this work highlights the ability of CFD modelling to predict the separation performance of pilot-scale separators which can then be utilised to improve and optimise gravity separators.