A novel settling tank for produced water treatment: CFD simulations and PIV experiments

Removal of solids is a critical step in the treatment of produced water generated by offshore hydrocarbon exploration. Present work evaluates a new design of a settling tank that can be used for continuous separation of fine particles from produced water. The novelty of the design lies in its ability generate swirling flow that promotes the settling of fine particles. The tank was investigated by conducting CFD simulations and experiments. The CFD model was initially validated using the experimental data measured in a lab-scale replica of the tank by using particle image velocimetry (PIV) technique. The validated model was then used to investigate flow patterns and settling behavior of different sized particles at varying operating flow rates and with different inletoutlet configurations. Simulations revealed that the angled entry of water in to the bulk of the tank caused rotational flow that transported the suspended particles towards wall, where downward axial velocity resulted in the settling of particles. The flow patterns showed inward flow at the bottom, which caused the accumulation of settled particles near the center hatch. The flow patterns also indicated an upward flow and the lifting of the settled particles near the hatch at the bottom. The impact of the lifting of particles was mitigated by adjusting the size of inlet openings and operating flowrate. For inlet openings of 40 x 20 cm(2), simulations predicted capture of more than 80% of particles having a size range of 30-1000 mu m at 30000 barrels per day operating flow rate. The settling tank has no moving element, rotating part or filtering medium; and its settling ability is solely governed by the induced hydrodynamics. Due to its simplicity and high efficiency, the settling tank could be used in continuous operation with high operating flowrate, and for both onshore and offshore operations.