Modeling of oil–water separation efficiency in three-phase separators: Effect of emulsion rheology and droplet size distribution

Abstract

The multi-fluid Eulerian multiphase model coupled with the inhomogeneous population balance model is applied to predict the gas, oil, and water separation behavior and emulsion destabilization in a high-pressure horizontal three-phase separator. The population balance model is applied to predict the evolving droplet size distribution due to droplet coalescence in the crude oil emulsion. The effect of water fraction on the emulsion viscosity was incorporated to predict the existence of the dense packed layer or zone (DPZ) – a high water-fraction emulsion layer formed at the interface between the oil and water phases. The novel emulsion viscosity model incorporates a dependence on the local droplet size. 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. The pilot separator experimental measurements include inlet and outlet flowrates and density, and electrical capacitance measurement of the vertical water concentration profile. The CFD model determines the flow patterns, phase distributions and the dispersed water droplet size distribution in the emulsion in the separator. The model results are in good agreement with the data for the separation efficiency from the water flow rate in and out of the separator, the phase distributions and the DPZ at different flow rates and water fractions. This study demonstrates the importance in multiphase separator design and troubleshooting of including the influence of water fraction on emulsion viscosity and the droplet size distribution with droplet coalescence phenomena. The methodology presented has significant advantages over previously published methods and improves on the value and potential impact toward research and industry practice.