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Summary In this work, the performance of two pilot–scale separators was investigated using computational–fluid–dynamics (CFD) simulation with one operating at low gas volumetric quality comprising a bucket–and–weir configuration, and the other operated at high gas volumetric quality with a weir configuration. The pilot–scale separators were selected for this work because of their availability and the lack of data on industrial separators. The effects of the liquid (oil and water) flow rate and weir height on separation performance have been investigated for the separator operating at low gas volumetric quality. For this separator, the design of experiments (DOE) and a preliminary run of the separator were used to select the number of experiments and simulations to conduct and the levels (values) of the three variables investigated. For the second separator, the effects of the inlet flow rate on separation performance have been investigated. Eulerian and volume–of–fluid (VOF) multiphase–flow models in ANSYS® Fluent (Fluent 2019), combined with a k–ε turbulence model, were used to simulate the fluid–flow pattern and phase behavior inside each of the separators. The numerical solutions were initialized with a water level set at 50% of the weir height using a patching tool. A mesh–independence test was carried out to ensure that the results are not dependent on the mesh quality. The separation efficiencies from both models were compared with that from the experimental data. The results indicated that the two multiphase models, namely, Eulerian and VOF, predict the experimental results within 30% error. However, different separation performances were obtained for the same flow conditions. For the separator operating at low gas volumetric quality, the results from the Eulerian multiphase model produced a maximum deviation of 8%, while results from the VOF multiphase model produced a maximum deviation of 23% of the experimental data. For this separator, the oil flow rate was found to have the greatest effect on the separation efficiency. This is followed by the water flow rate and weir height. For the separator operating at high gas volumetric quality, a maximum percentage error of 30% for the Eulerian model and 21% for the VOF was obtained.

The separation of produced fluids is essential once it reaches the surface. This separation is achieved in gravity separators. The design and sizing of separators can be challenging due to the number of factors involved. Improper separator design can bottleneck and reduce the production of the entire facility. This paper describes the development of a capital cost optimisation model for sizing three phase separators. The developed model uses GRG Non-linear algorithms to determine the minimum cost associated with the construction of horizontal separators subject to four sets of constraints. A numerical sizing example was solved to provide the details associated with the model and the ease with which parameters can be varied to suit the user's needs. Finally, a spreadsheet comparison between results obtained from the developed model and four other extant models is carried out. Results indicated that the developed model predicted results within an absolute error of AE5m 3 in most cases and a maximum of AE12.5m 3 for very high gas flows in comparison to conventional models developed based on retention time theory.

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Summary In the first part of this work, the development of a capital cost optimization model for sizing three-phase separators was described. The developed model uses generalized reduced gradient nonlinear algorithms to determine the minimum cost associated with the construction of horizontal separators subject to four sets of constraints. In the second part, an experimental test rig was designed and used to investigate the effect of gas flow rate, liquid flow rate, and slenderness ratio (L/D) on the separation performance of horizontal three-phase separators. The results indicated an inverse relationship between an increase in gas and liquid flow rate and the separator outlet quality. It also indicated a direct relationship between an increase in slenderness ratio and separator outlet quality. The results also showed that the gradient change of the percentage of water in the oil outlet with respect to slenderness ratio decreased to ratios of 6:1. Hence, the separation rate increased. At ratios greater than 6:1, the separation still increases, but the gradient change in separation drops off, implying that the benefit in terms of separation is diminishing beyond this point. Therefore, the optimal slenderness ratio for technical reasons is 6:1.