Experimental Study of Bubble-Droplet Interactions in Improved Primary Oil Separation

The interactions between dispersed oil droplets and gas bubbles was experimentally studied in this work. An experimental set-up was built in the Multiphase Flow Research Center (NUEM) in the Federal University of Technology – Paraná (UTFPR) to conduct a fundamental evaluation of the interactions between sessile gas bubbles and oil droplets employing side-view flow visualization. Tap water was used as the continuous phase, whereas pure nitrogen and colored vegetable oil were employed as the dispersed phases. The bubble-droplet attachment consisted in the encapsulation of the bubble by the droplet, presenting phenomenological similarities to droplet-droplet coalescence. The contact between the dispersed phases induces the formation of a connecting bridge, which grows rapidly with time, with the height of the bridge being comparable to the size of the droplet after 57.0 ms. The inherent asymmetry of the phenomenon induced a significant horizontal displacement of the bubble towards the droplet, whose position remained unaltered. The evaluation of the bridge meniscus corroborated to this observation, since the contact angle on the droplet side decayed faster with time in comparison to the contact angle on the bubble side. The hydrodynamics of the rising aggregate is also evaluated, by the obtainment of its size, three-dimensional trajectory and terminal velocity. The stable aggregates formed presented an increase factor of 150 to 180%, based on the terminal velocity of the individual droplet. The radius of the bubble was found to be the major influence on the hydrodynamics of the aggregate, allowing the definition of a critical bubble radius based on trajectory instabilities.