Emulsion Treating Theory
Removing water from crude oil often requires additional processing beyond the normal oil–water separation process, which relies on gravity separation. Crude oil treating equipment is designed to break emulsions by coalescing the water droplets and then using gravity separation to separate the oil and water. In addition, the water droplets must have sufficient time to contact each other and coalesce. The negative buoyant forces acting on the coalesced droplets must be sufficient to enable these droplets to settle to the bottom of the treating vessel. Therefore, it’s important when designing a crude oil treating system to take into account temperature, time, viscosity of the oil, which may inhibit settling, and the physical dimensions of the treating vessel, which determines the velocity at which settling must occur.
When selecting a treating system, several factors should be considered to determine the most desirable method of treating the crude oil to contract requirements. Some of these factors are
- Stability (tightness) of the emulsion,
- Viscosity of crude oil and emulsion at different temperature,
- Specific gravity of the oil and produced water,
- Corrosiveness of the crude oil, produced water, and associated gas,
- Scaling tendencies of the produced water,
- Quantity of fluid to be treated and percent water in the fluid,
- Paraffin-forming tendencies of the crude oil, and pour point of crude oil.
- Desirable operating pressures for equipment,
- Availability of a sales outlet and value of the associated gas produced.
A common method for separating this “water-in-oil” emulsion is to heat the stream. Increasing the temperature of the two immiscible liquids deactivates the emulsifying agent, allowing the dispersed water droplets to collide. As the droplets collide they grow in size and begin to settle.
If designed properly, the water will settle to the bottom of the treating vessel due to differences in specific gravity.
Laboratory analysis, in conjunction with field experience, should be the basis for specifying the configuration of treating vessels. The purpose of this chapter is to present a rational alternative for those instances when laboratory data do not exist or, if it is desirable, to extrapolate field experience.