The process of coalescence in oil treating systems is time dependent. 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 coalescing droplet pair is larger than the energy of adhesion between them, the contact will be broken before coalescence is completed. Experiments with deep layer gravity settlers indicate that the time to “grow” a droplet size due to coalescence can be estimated by the following equation:
When the energy of oscillations is very low so that “bouncing” of droplets approaches zero, j approaches 3. Assuming a value of 4, the minimum time required to obtain a desired particle diameter can be expressed:
Assuming d0 is small relative to the droplet size we wish to “grow” by coalescence in our gravity settler, Equation 6-3 can be approximated:
The following qualitative conclusions for coalescence in a gravity settler can be drawn from this relationship:
1. A doubling of residence time increases the maximum size drop grown in a gravity settler less than 19 percent. If the value of “j” is greater than 4 the growth in droplet diameter will be even slower.
2. The more dilute the dispersed phase, the greater the residence time needed to “grow” a given particle size. That is, coalescence occurs more rapidly in concentrated dispersions. This is the reason that oil
is “water washed” by entering the treating vessel below the oil water interface in most gunbarrels and treaters. Flocculation and coalescence therefore occur most effectively at the interface zone between oil and water.