In the petroleum industry, economical separation of oil and water has been a major challenge. Typically produced water accounts for an average of 75% of extracted fluid; oil water separation totally costs $40B/yr. worldwide to the industry. For separation of oil and water, industry traditionally relies on conventional gravity based vessels that are bulky, heavy and expensive. More importantly, separation by only gravity suffers from low efficiency for oil water emulsion as extracted fluid.
In order to improve the split ratio and the separation efficiency of traditional API gravity separation, a dynamic separator with a helical impeller and draft tube (KDS) has been proposed by the Dow Chemical Company. In such device, the centrally-located impeller produces up-pumping flow inside cylindrical draft tube and downward flow between draft tube and container. The flow circulation inside container facilitates phase separation by increasing probability of oil droplet collision. In addition, with slow rotational speed, the shear created by helical impeller inside draft tube may further enhance droplet coalesce by deformation of oil droplet. The novel dynamic separator was found promising with about 8 times faster than gravity separation in preliminary experiments. There is still huge space for improvement with better designs.
Herein, Computational Fluid Dynamics analyses have been used to understand working principles as well as provide optimization for the preliminary design of the dynamic oil-water coalescer. Firstly, the flow pattern of fluid inside has been studied with single phase CFD simulations with SST transition turbulent model. Since a higher shear will lead to droplet breakup rather than coalesce, CFD investigations have been adopted in this paper to identify the critical shear rate and different zones for droplet breakup or coalesce respectively. With the objectives of enhancing droplet coalesce while suppressing breakup, an optimized design has been found by parametric study with different parameters, e.g., impeller twisting angle, draft tube diameter, impeller rotational speed. In addition, multi-phase simulation with Eulerian Model and population Balance Model has also been adopted to studies oil-water separation process and oil droplet diameter evolution. CFD results showed increasing twisting angle of impeller is helpful to develop a more homogeneous flow field inside draft tube and thus reduce high shear region for breakup. The optimized draft tube diameter has also been discussed for highest global circulate flow rate.