Separation of Oil Droplets from Oil-in-Water Emulsion Using a Microbubble Generator

Abstract

The induced air flotation of oil droplets was performed using a microbubble generator to develop a simple and energy saving method for oil-in-water emulsion separation. The microbubble generator, which consisted of a liquid pump and a special line mixer, was set at the bottom of a bubble column. The effects of operation conditions on the oil separation performance were investigated. A sintered glass gas sparger was also used to disperse millibubbles. Oil-in-water emulsions were prepared from soybean oil and water. Compared with millibubbles, the dispersion of microbubbles effectively separated oil. This was because microbubbles had high efficiency of collision with droplets and it was difficult to detach droplets because of the low rising velocity of microbubbles. Millibubbles were difficult to attach to oil droplets and easy to detach droplets because of the high velocity movement of bubbles. The oil separation ratio was a maximum at the gas velocity of 0.2 mm/s because the amount of millibubbles increased as the gas velocity became higher above 0.4 mm/s. The oil separation was largely enhanced at the solution pH below 4.5. The zeta potential of droplets was negative in the pH range from 3 to 11. However, as the solution pH became lower, the zeta potential of microbubbles changed from negative to positive at the solution pH of 4.5. In addition to the hydrophobic attraction, the electrostatic attraction between oil droplets and microbubbles was generated since the zeta potentials of droplets and microbubbles had negative and positive values, respectively.