CFD Modelling of a Horizontal Three-Phase Separator: A Population Balance Approach

The performance and internal mult iphase flow behavior in a three-phase separator was investigated. The separator considered represents an existing surface facility belonging to Abu Dhabi Co mpany for Onshore Oil Operations ADCO. A first approach, using the Eulerian-Eu lerian mult iphase model imp lemented in the code ANSYS FLUENT, assumed mono-dispersed oil and water secondary phases excluding the coalescence and breakup phenomena. Interesting results were obtained but noticeable discrepancies were caused by the simp lifying assumption. Therefore, it was decided to use the Population Balance Model PBM to account for the size distribution, coalescence, and breakup of the secondary phases which were the key limitat ions of the Eulerian -Eulerian model. The separator configuration, with upgraded internals, was represented with the maximu m of geometrical details, contrary to the simp lifying approach adopted in most of the previous numerical studies, to minimize the sources of discrepancies. In the absence of field information about the droplet size distribution at the inlet of the separator, three different Rosin-Rammler distributions, referred to as fine, mediu m, and coarse distributions were assumed based on the design values reported in the oil industry. The simulat ion results are compared with the scares laboratory, field tests, and/or semi-empirical data existing in the literature. The coarser size distributions, at the inlet, enhanced the separator performance. It was found that the inlet device, called Schoepentoeter, generates a quasi-mono-dispersed distribution under the effect of coalescence which persists throughout the whole volume of the separator. The mean residence time obtained fro m the simulat ions agreed well with some of the existing approaches in the literature. Finer d istributions generate higher mean residence times . The classical sizing approach, based on representative values of droplet diameter and settling velocity remains limited although useful for design guidelines. In contrast, CFD presents the advantage of calculating the flow variab les locally wh ich yields a mo re co mplete and detailed picture of the entire flo w field. This is very useful for understanding the impact of the internal multiphase flow behaviour on the overall performance of the separator.