Flocculation of emulsified oil-in-water with dodecylbenzene sulfonate and polyacrylamide and floc separation by dissolved air flotation

This paper presents bench studies on the treatment of saline oil-in-water emulsions (synthetic and real produced water) by a novel flocculation technique combining sodium dodecylbenzene sulfonate and a high molecular weight cationic polyacrylamide, followed by floc separation by gravity and/or with microbubbles (dissolved air flotation - DAF). The effects of temperature, reagents concentration, medium pH, salinity (sodium chloride concentration), and water hardness on the separation efficiency were investigated. The best results for total oil and grease (TOG) removal were around 90% and residual TOG content between 7 and 13 mg L−1 in the treated water. These results were obtained with 20–40 mg L−1 of sodium dodecylbenzene sulfonate followed by 7 mg L−1 of polyacrylamide, at 25°C and at pH between 6 and 7. The microbubbles were generated by hydrodynamic cavitation in a needle valve after dissolving air in water at a saturation pressure of 4 bar with a 20% recycling ratio. The mechanisms involved were discussed in terms of interfacial phenomena occurring at the oil/water interface with the formation of polymer-surfactant complexes (PSCs). Notably, this instantaneous aggregation process occurs without the need for slow mixing, as in conventional flocculation. The flocculation of the emulsified oil droplets appears to involve, simultaneously, the formation of a polymer-surfactant complex, whereby the hydrophobic forces play an important role, and interactions between the oil droplets (coated with sodium dodecylbenzene sulfonate and the polyacrylamide). Here, large and light (cluster-like) hydrophobic flocs, with enmeshed/entrained oil (de-emulsified), immediately rise (as a phase separation) towards the water/air interface due to the low density, together with the small flocs rising with the microbubbles. Due to the high process efficiency and the high quality of the treated water, this flocculation technique appears to have a very good potential for oil-bearing wastewater treatment at a high rate, especially in refineries and offshore platforms of crude oil extraction.