Three phase horizontal separators of oil, gas and water are considered one of the main surface equipments in the upstream petroleum industry that have a significant impact on the treatment of the produced oil. The flow inside separators is a complex flow, three dimensional with complex internal geometries, turbulent and multiphase, thus presents a challenge for both experimental and numerical studies. Due to this complexity of flow nature besides the time and computational resources required, very few studies exist on the flow inside separators.;In this study, the mixture multiphase model, embedded in Fluent version 14, was used to simulate the multiphase flow and separator performance using mono-dispersed secondary phases (oil and water). The study is extended to include, the Discrete Phase Model (DPM) to investigate the oil carryover for various droplets distributions (poly-dispersed oil), with/without the effect of breakup and coalescence. This was done by modeling the gas compartment of the separator. In both models, the standard k-epsilon turbulence model was used to account for turbulence effects. Due to the absence of knowledge of field information about the droplet size distribution at the inlet of the separator, four different representative distributions (injected with 10, 30, 50 and 80 microns mean diameters) according to Rosin-Rammler size function were considered based on the design values of industrial separators. The simulation results are assessed in terms of overall separation efficiency, internals' effectiveness, local size distribution and residence time, and then compared against results from field tests of ADCO (Abu Dhabi Company for Onshore Oil Operations) and previously published simulations using the Eulerian-Eulerian and Population Balance models (PBM).;The separation efficiencies obtained using the mixture model were in acceptable agreement with previously published values, and ADCO test field results, except for the oil in water contents. In this case, the effect of the coarser mesh used by the mixture model had led to overestimating the diffusion term and hence generating less sharp interfaces and huge amount of oil in water.;The DPM simulation results showed that droplet breakup rarely occurred, especially for fine size distributions, while droplet coalescence was a more common phenomenon, especially in the Schoepentoeter. This behavior played a significant role in increasing the separation efficiency up to nearly 100% when the coalescence and breakup phenomena were introduced to the simulations of all size distributions cases. In general, the model provided acceptable separation efficiencies that had a good agreement with the PBM results at relatively large size distributions (> 80 micron). While for finer size distributions the model overestimated the coalescence rate in the Schoepentoeter region, leading to higher separation efficiencies compared to PBM. In addition, the effectiveness of Schoepentoeter was found to be essential for all size distributions, especially when the coalescence model was present. The other internals were found to contribute moderately to the overall separation efficiency. Moreover, the mean size distribution was found to gradually decrease across the separators' internals when there is no effect of coalescence. However, when coalescence is considered the mean size diameter was found to increase severely downstream the Schoepentoeter, leading to enhanced separation in the settling compartment.;Finally, the mean residence time (MRT) obtained by the DPM (35 sec - 40 sec) showed a good agreement with some of the existing approaches in the literature. However, a discrepancy was found against the published MRT calculated by PBM.
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