Accelerated oil droplet separation from produced water using magnetic nanoparticles

The treatment of highly stable small oil droplets in produced water is challenging for offshore production, where platform space is constrained, because their long residence time requires large equipment volumes. The use of magnetic nanoparticles (MNPs) to remove dispersed droplets is a promising alternative due to their quick response to external magnetic field, allowing easy separation of oil droplets on which MNPs were attached from water. The goal of this study is to prove the concept of magnetically separating oil droplets from produced water using surface-coated MNPs. Batch-scale experiments were performed and they showed that droplets in 5 wt. % of decane-in-water emulsions, which have negative surface charges, were successfully separated from water using cationic surfactant-coated MNPs, with decane removal efficiency of 85 to 99.99%, depending on the experiment conditions. Anionic surfactant-coated MNPs did not remove oil droplets, indicating that the electrostatic attraction between emulsions and MNPs control the attachment of the MNPs to the droplet surface. The settling velocity of a droplet coated with MNPs was derived by extending classical theory to account for the magnetic force as well as buoyancy and drag forces. Under the applied experimental conditions and considering the geometry of the oil-water-MNP system, velocity calculations show that the droplet settles spontaneously when a magnetic field is applied. Otherwise the MNP-coated oil droplets (MNP-droplets) would naturally migrate upwards due to buoyancy. The velocity of a single MNP-droplet is strongly dependent on the intensity of the magnetic field and it changes up to three orders of magnitude within the height of tested sample of approximately 3 cm.