Numerical Assessment of Separator Performance for Three-Phase Mixture with High-Viscosity Oil

Horizontal gravity separators are widely used to separate gas, oil, and water phases based on their density differences. While conventional design methods primarily focus on geometric sizing using droplet settling theory, actual separation performance is significantly influenced by inlet flow conditions and internal configurations. This study aims to numerically investigate the effects of inlet flow rate and geometry on the phase behavior and separation efficiency of a three-phase horizontal gravity separator handling a high-viscosity oil mixture. A three-dimensional CFD simulation was performed using an Eulerian multiphase model and an interfacial area concentration for a horizontal separator equipped with a reversed-pipe inlet diverter and a weir. Five cases were analyzed by varying the inlet flow rate, vessel length, and the presence of a perforated baffle. Volume fraction distributions, flow streamlines, and pressure fields were evaluated to understand the underlying separation mechanisms. The results showed that increased inlet flow rates lead to stronger momentum, delaying oil pad formation and allowing water to pass over the weir into the oil outlet, thereby reducing separation efficiency. The addition of a perforated baffle helped suppress flow disturbances and improved water drainage, resulting in a thicker oil pad and higher separation efficiency. In contrast, shortening the vessel slightly improved performance, but may not satisfy slenderness ratio guidelines or handle transient flow effectively. This study highlights the importance of considering operational and internal structural factors to improve the performance of horizontal gravity separators. The findings provide valuable insights for optimizing separator design in compact or constrained field applications.