Discrete Phase Modeling of Oil Droplets in the Gas Compartment of a Production Separator

Computational Fluid Dynamics (CFD) is a powerful engineering tool that has different applications in the Petroleum Industry. In recent years, CFD has been used to analyze the complex 3D multiphase flow inside production separators. Due to changing reservoir conditions oil companies replace old internals with upgraded ones. In this study, a numerical simulation of the turbulent multiphase flow using the Discrete Phase Model (DPM) is used to assess the effects of the oil droplet size distribution on the oil carry-over in a production separator. Liquid droplet size distributions, meant to represent fine and coarse populations of oil droplets, were generated at the inlet of the separator within the range of sizes recommended in the literature for design purposes. The DPM model accounts for the key phenomena of droplets coalescence and breakup. Although the real case includes three phases, the present DPM simulations do not account for the water phase due to its negligible volume fraction and its prevailing gravitational settling compared to the carry-over effect. The new internals included; an inlet device known as Schoepentoeter, agglomerator, parallel-plates coalescer, and cyclonic mist extractor. Unlike many of the CFD studies reported in the literature, usually representing the internals by numerical models for simplicity, the internals of the separator were replicated with the maximum of geometrical details in this study. The present work was compared with field tests and previous numerical simulations using the Population Balance Model PBM. The PBM simulations considered the whole separator volume and the presence of three phases (gas, oil, water). The mean residence time obtained from the simulations agreed reasonably with some of the results published in the literature using semi-empirical formulas and experiments. The new internals were seen to promote droplet coalescence with minimal breakup. The new inlet device (Schoepentoeter), in particular, was found to contribute considerably to the coalescence of droplets and, hence, to separation.