Oil/Water Separation by Induced-Air Flotation

Abstract

Abstract The separation of oil, stabilized with an oil-soluble petroleum sulfonate, from brine solutions by induced-air flotation was studied in a continuous-flow pilot unit. The effects of inlet oil concentration, vessel residence time, air flow rate, bubble diameter, oil drop diameter, temperature, NaCl concentration, and cationic polyelectrolyte concentration were investigated. Oil drop and air bubble diameters, liquid residence time, and concentration of cationic polyelectrolyte were the most significant variables affecting overall flotation performance. Only drops larger than 2 m showed significant removal, while smaller drops were generated by the air-inducing rotor. The cationic polyelectrolyte improved flotation performance by increasing the number of large oil drops.The removal rate for each oil drop size was first order with respect to oil drop concentration, and an experimental procedure permitting determination of the first-order rate constants for removal only due to bubble/drop interactions was developed. The oil drop and air bubble diameters were the only variables which affected these rate constants. Increasing oil drop diameter and decreasing bubble diameter increased the rate constants. Comparison of the experimental and theoretically predicted rate constants showed that the mechanism of oil-droplet removal by bubbles from 0.2- to 0.7-mm is one of hydrodynamic capture in the wake behind the rising bubbles. Introduction Oily wastewaters are generated during the production, processing, transportation, storage, and use of petroleum and its products. Removal of dispersed oil from water is usually accomplished by either dissolved- or dispersed-gas flotation. The processes are similar: gas bubbles are introduced into the oil-containing liquid and the oil drops are captured by the gas bubbles which quickly rise to the surface where the oil is removed. The significant differences between the two flotation processes are the bubble size and mixing conditions. In dissolved-gas flotation, the bubbles are about 50 to 60 m in diameter, whereas induced-gas bubbles are an order of magnitude larger. Dissolved-gas flotation units operate under fairly quiescent conditions and the liquid phase approximates plug flow. For induced-gas flotation, the submerged rotor imparts enough energy to the liquid that the tank contents are mixed nearly perfectly.This research focuses on the induced-air flotation process for the removal of dispersed oil droplets. The industrial use of induced-air flotation devices for oil wastewater separation began in 1969. Basset provides the process development history, equipment description, and operating experience for an induced-air unit similar to the design used in the experiments described here. Although induced-air flotation equipment is simple, the fluid mechanics of the process are not; and the arrangement of the turbine, sleeve, and perforations have been determined necessarily by trail-and-error experimentation with small-scale units.The interaction between gas bubbles and oil drops has been described as follows (1) absorption of an oil drop to a gas bubble due to precipitation of a bubble on the oil surface and collision between the drop and bubble; (2) entrapment of a gas bubble in a flocculated structure of oil drops as it rises; and (3) absorption of bubbles into a flocculated structure as it forms.For dissolved-gas flotation, all these mechanisms probably influence oil removal interdependently. SPEJ P. 579＾