The process of coalescence in water treating systems is more time dependent than the process of dispersion. In dispersions of two immiscible liquids, immediate coalescence seldom occurs when two droplets collide. If the droplet pair is exposed to turbulent pressure fluctuations, and the kinetic energy of the oscillations induced in the droplet pair is larger than the energy of adhesion between them, the contact will be broken before coalescence is completed.
It has been shown in the previous chapter that the time to “grow” a droplet size due to coalescence in a gravity settler is proportional to the diameter of the droplet to some power greater than three and inversely proportional to the concentration of the oil phase. From this it can be concluded that after an initial period of coalescence in a settler, additional retention time has a rapidly diminishing ability to cause coalescence and to capture oil droplets.