The purpose of oil dehydration is to remove water from the oil stream to levels that meet pipeline specifications before refining into upgraded petroleum products. Water can present as free water that is not emulsified and can be settled out from oil if given time, or emulsion that requires special treatment for separation.
Emulsion is a temporarily stable mixture of immiscible fluids when one phase is dispersed into the other and does not separate easily. For an emulsion to exist, there must be two immiscible liquids such as oil and water, an emulsifying agent, and sufficient agitation to disperse one phase into the other. In the oil and gas industry there are three kinds of emulsions.
- water is the continuous phase
- oil is the continuous phase
- Multiple Emulsion
- both oil-in-water and water-in-oil emulsions exist simultaneously
Figure 1 below shows a microscopic picture of a typical water-in-oil emulsion in which the oil is the continuous phase and water presents as dispersed droplets. Common emulsifying agents in oil fields are paraffin, resins, organic acids, metallic salts, colloidal silts and clay, and asphaltenes. The emulsifying agents tend to be insoluble in one of the liquid phases, thus concentrate at the interface preventing droplet coalescence. The emulsifying agent can decrease the interfacial tension of the water droplet causing smaller droplets to form, and/or form a viscous coating on the droplets that keeps them from coalescing. The polar molecules in an emulsifier can also prevent coalescing by the electrical charges at the droplet surface.
Emulsion can be broken thermally and/or chemically. Demulsifier is surface-active agent that neutralizes the effect of emulsifying agents and promotes separation. Demulsifier has to be able to migrate rapidly to the droplet interface, produce attraction forces between droplets, and promote droplet interface film rupture that enhances droplets coalescence. A blend of multiple chemical components might be needed to achieve these actions.
Free-water knockouts (FWKO)
FWKO is a vertical or horizontal pressure vessel used to separate gas, oil and/or emulsion, and free water. It is normally located at the last separation stage, where turbulence has been minimized. Its internals are normally coated or protected from corrosion by sacrificial anodes because of its frequent contact with saltwater. Gunbarrel tank is one type of vertical free-water knockouts that are used in heavy crude applications and old low flow rate onshore applications. The water will be transferred to the water processing units, while the emulsion from FWKO will be delivered to downstream processing facility for further treatment.
Heater-treater is a three-phase separator that utilizes heat and mechanical separation devices to accelerate oil and water emulsions separation before transporting the dry oil to pipelines. Heat can reduce the oil viscosity that facilitates water to settle, increase the density difference between oil and water by flashing gas out of liquid and increase the molecular movement that promotes droplet coalescence. Heater-treater can be vertical or horizontal. Figure 2 shows a schematic of a typical vertical heater-treater. Vertical heater-treater consists of four major sections: gas separation, free-water knockout, heating and water-wash, and coalescing-settling sections.
The gas separation section (or inlet degassing section) has an inlet diverter that provides primary gas and liquid separation. A mist extractor further removes unseparated liquid mist in the gas stream. The liquid flows downwards through a down-comer into the free-water knockout section at the base of the treater. The end of the down-comer should be slightly below the oil-water interface, such that it facilitates the oil and gas separation. This process is called “water-wash.” The oil and emulsion rise through the heating and water-wash section, where the fluid is heated by a fire tube. The coalescing-settling section (or oil section) should provide enough retention time to allow water droplets to coalesce and settle to the bottom.
The oil level in a heater-treater is controlled by pneumatic or mechanical dump valves. At the same time, the oil-water interface is maintained by an interface level controller, or adjustable external water let. The gas pressure is controlled by a gas back pressure valve. A burner management system (BMS) is required to manage the heating process in a heater-treater, in order to keep the system operate at optimum conditions.
Horizontal heater-treaters can handle higher volumes of fluids and heavier crudes that require longer retention periods for separation. Besides, the more surface area of the burner tube in contact with emulsion promotes the oil and water separation. On the other side, vertical heater treaters are more cost-effective, do not require as much pressure to operate, and require less footprint.
Some horizontal heater-treater includes an electrostatic grid in their coalescing section to enhance oil and water separation (Figure 3). When emulsion passes through the electrical field, the water droplets become electrically charged, which accelerates their movement, therefore promoting droplet coalescence and improves their settling.
The salts in crude oil can be dissolved in the water phase, which comes together with oil or crystallized and suspended in the oil phase. They are mainly in the form of calcium, magnesium, and sodium chloride. If not removed, the salts will crystallize and deposit as scale within heat exchange equipment, resulting in fouling. They can also cause corrosion by hydrochloric acid formation. In addition, the entrained salt crystals can deactivate catalyst beds and plug downstream processing equipment. Therefore, crude oil must be desalted to levels that satisfy the refinery specifications.
Since the salt is mainly dissolved in the water phase, most of them can be removed by a heater treater or electrostatic treater. However, if the content is still beyond requirement, an oil desalter is needed to lower it down to a specified amount.
The desalting process involves two steps. The first step is to mix the fluid with an appropriate amount of washing water (dilution water) using a mixing valve. The salts are dissolved in the aqueous phase, and the salinity in the produced water is reduced. In the second step, dehydration is performed to remove the water from the oil. For an electrostatic desalter, electrostatic grids are installed to break oil and water emulsion. The desalting process can be conducted in one-stage or two-stage depending on the salt content and specifications. Figure 4 shows a flow chart of a typical two-stage desalting process. For very stable emulsion, demulsifiers are needed to facilitate oil and water separation, which are injected into the stream in the first step.