Verification of a CFD-Population Balance Model for Crude Oil Separation Efficiency in a Three-Phase Separator – Effect of Emulsion Rheology and Droplet Size Distribution

摘要

In this study, a multidimensional computational fluid dynamics (CFD), combined with the Population Balance Model, is used to predict the gas, oil, and water separation behavior in a horizontal three-phase high- pressure, high-temperature pilot separator at different flow rates and water cuts, and predict the thickness of the dense packed emulsion layer above the water interface. The multi-fluid Eulerian multiphase model, coupled with a multivariate population balance model, is applied to predict emulsion destabilization and separation, and validated with experimental data. The population balance model is applied to predict the changing droplet size distribution in the crude oil emulsion due to droplet coalescence. The effect of water cut on the emulsion rheology was incorporated in the CFD model to predict the existence of the dense packed layer or zone (DPZ) – a high water-cut emulsion layer formed at the interface between the oil and water phases. A novel emulsion viscosity model incorporates a dependence on the local droplet size. The CFD modeling predicts the flow patterns and phase distributions throughout the separator, and the dispersed water droplet size distribution in the emulsion/liquid phase. Parameter estimation for the droplet coalescence kernel in the population balance equation is determined from experimental measurements of emulsion separation kinetics. A Proportional Integral Derivative (PID) controller logic is used to maintain the gas/ oil and oil/water interface levels by automatically adjusting the oil and water outlet pressure, respectively. Experimental data from a high-pressure pilot scale separator is compared to the predicted phase flow rates into and out of the vessel, and vertical phase profiles to CFD results. The CFD model reproduced separation efficiency, three-phase distribution and the DPZ influence on oil-water separation efficiency and demonstrates an effective methodology to evaluating full-scale separator performance.