(584k) CFD Modeling of Immiscible Liquid-Liquid Flow in a Large Scale Crude Oil Storage Tank Equipped with Side-Entering Mixers

The periodic internal cleaning of crude oil storage tanks at terminals and refineries, needed when the rate of Basic Sediment and Water (BSW) accumulation is at a concentration above 0.5 wt%, consists in a demand of great impact for the maintenance team. This results in low overall storage efficiency due to the equipment unavailability and high environmental impact, such as the discharge of the sludge inside the tank and replacement of the bottom plates due to corrosion as a result of the presence of brine. Mineral solid present in petroleum; high molecular weight organic sediments, aqueous brine drops and corrosion residues participate in an undesirable process of formation of complex solid compounds with high hardness and density. Compacted sediment, associated with the degradation of organic compounds inside storage tanks leads to the formation of a thick sludge layer, which by definition is a multi-phase water-oil-solid system of great stability, difficult to remove and / or process and with low chemical treatability. The mixing process is an important physical operation to avoid sludge formation, achieving better homogenization of the fluid (blending). Settling is common when dispersed and continuous phases are of different density and when agitation provides only minimal circulation throughout the vessel. It is known that the mechanical mixer is the more efficient mixing device in terms of the average tank speed per power unit. Furthermore, the side-entry impellers are considered to be a low capital investment and of simple installation. This computational study has been carried out using the Computational Fluid Dynamics package OpenFOAM, which gives an estimate of the liquid-liquid flow pattern, considering the characterization of the mixture using side-entering impellers in large scale circular tanks. The power number and simulated flow of a single impeller in a cylindrical storage tank were validated using empirical correlations. Sensitivity analysis of the mixture by multiple impellers in cylindrical storage tank has been also performed to study the effect of the spacing between the adjacent impellers and the angle of inclination for each of the impellers in the performance of the mixture. It also important to determine the minimum speed for drop suspension in other to keep drops suspended, avoiding this way sludge formation. This of course depends on phase density differences, agitation speed, impeller size, and its location within the vessel.

Keywords: Mixing; crude oil; side-entering mixers; Computational fluid dynamics; immiscible liquid-liquid; suspension of drop