Numerical and Experimental Evaluation of Skimmer Tank Technologies for Gravity Separation of Oil in Produced Water

Abstract

Computational Fluid Dynamics (CFD)and experimental tests were used to carry out a comparative study of gravity separation using skimmer tank technologies for removing low oil concentrations remaining in produced water. In this work, two technologies were evaluated; one without internals (called descendent flow technology) and the other with internals (called baffle technology). For experimental tests, a pilot tank of 2000 liters was built. The flow, residence time and canalization effects were evaluated by performing conductivity measurements, injecting a sodium chloride pulse at the inlet and measuring conductivity profile at the outlet. For the first technology (descendent flow) canalization was strong, and that was evidenced by several conductivity peaks a few minutes after salt injection (the mean theoretical residence time was 4 hours). On the other hand, for baffle technology only one conductivity peak was detected after 140 minutes, showing the much more volume efficiency of the tank.
At first, CFD validation of experimental data was performed by solving the advection-diffusion transport equation for salt concentration(eulerian strategy) over a steady state flow previously calculated for water. Although this methodology catches the first peak for the descendent flow technology, the subsequent peaks are not evidenced. For this reason, an alternative strategy (particle tracking) was used to represent salt flow, allowing to reproduce the full conductivity data profile and to estimate the mean residence time for both skimmer tanks.
Computational time was considerably reduced by the use of lagrangian strategy. Results from descendent flow technology were in good agreement with experimental data, leading to the conclusion that the implemented CFD strategies is suitable for evaluation and design of skimmer tanks with long residence time.