Most oil-treating equipment relies on gravity to separate water droplets from the oil continuous phase, because water droplets are heavier than the volume of oil they displace. However, gravity is resisted by a drag force caused by the droplets’ downward movement through the oil. When the two forces are equal, a constant velocity is reached, which can be computed from Stokes’ Law as:
Several conclusions can be drawn from Stokes’ Law:
1. The larger the size of a water droplet, the larger the square of its diameter, and thus, the greater its downward velocity. That is, the bigger the droplet size, the less time it takes for the droplet to settle
to the bottom of the vessel and thus the easier it is to treat the oil.
2. The greater the difference in density between the water droplet and the oil phase, the greater the downward velocity. That is, the lighter the crude, the easier it is to treat the oil. If the crude gravity is 10°API and the water fresh, the settling velocity is zero, as there is no gravity difference.
3. The higher the temperature, the lower the viscosity of the oil, and thus the greater the downward velocity. That is, it is easier to treat the oil at high temperatures than at low temperatures (assuming a
small effect on gravity difference due to increased temperature).