Three-phase Eulerian computational fluid dynamics of air-water-oil separator under off-shore operation

A three-phase transient Eulerian computational fluid dynamics (CFD) model was developed to investigate the effect of three angular motions (rolling, yawing, and pitching) on the separation efficiency of an air-water-oil separator. The horizontal cylindrical separator included a feed inlet, a gravity separation zone with a coalescer for water and oil separation, and a mist elimination zone. The CFD model consisted of the continuity, momentum, and standard k-epsilon turbulence equations. The CFD equations were solved in a moving reference frame to take into account the angular motions. The exit pressures at the water and oil outlets were controlled using a user-defined function since the CFD model without pressure control resulted in backflow from the water outlet, which was not realistic. CFD cases, such as no motion, rolling, yawing, 2 degrees pitching, and 4 degrees pitching with a period of 8 s were simulated with exit pressure control. The angular motions with pressure control showed a stable cyclic behavior with a high oil separation efficiency. However, the 4 degrees pitching motion decreased the oil recovery to 93% with 77% water purity at the water outlet.