The influence of dissolved naphthenic acids and divalent calcium ions on the performance of a polyanionic and a polycationic flocculant on oil removal by induced gas flotation

Flocculants are commonly added to improve the oil removal in compact gas flotation units during produced water treatment. Polymeric flocculants are used industrially, but there is limited knowledge of how the produced water composition can influence their performance. This study aimed at understanding how dissolved naphthenic acids and the ionic composition of the water influenced the performance of a polycationic and a polyanionic flocculant during oil removal. Oil removal experiments were conducted in a batch induced gas flotation unit at 25˚C and 2 bar, while selected systems were also investigated at 70˚C and 40 bar. Small amounts of both flocculants enhanced the oil removal, while increased temperature and pressure increased it further. Optimal oil removal was reached at 20–30 ppm for the polyanionic flocculant. It was not significantly influenced by calcium ions, while it was lowered in the presence of naphthenic acids. For the polycationic flocculant, optimal oil removal was reached at 15 ppm flocculant and decreased at higher dosages. At this dosage, it increased in the presence of calcium ions and reduced in the presence of naphthenic acids. The study demonstrated that the ionic composition and naphthenic acid content of the water phase are important factors to consider when robust evaluation of flocculant performance in produced water treatment is required.