Oil and gas reservoirs not only contain hydrocarbon liquids and gases but many other impurities as well, such as water, sediment and inorganic salts. These impurities can cause corrosion and fouling in downstream equipment and must be removed at various stages of production, transport and refining. In fact, it is advantageous to remove these impurities as far upstream as possible, as explained by Longtin and Hoffman here. While oil and gas separators and wash tanks can remove some of the contaminants, electrostatic dehydrators and desalters are specifically designed to remove the water-soluble impurities from the produced oil.
An electrostatic dehydrator – also referred to as an electrostatic treater, electrostatic separator, or electrostatic coalescer – uses electricity to remove dispersed water droplets from oil. Crude oil flowing from upstream production separators is partially degassed before entering a dehydrator. Onshore surface production facilities often use mechanical or thermal-mechanical treaters (dehydrators) to remove emulsified water from crude oil as explained in this article.
Offshore floating facilities and onshore oil gathering centers typically use an external heat source downstream of the production separators to heat the oil before entering an electrostatic dehydrator. As the warm oil-water mixture flows through the inlet header and is injected into the lower-half of the dehydrator, it is subjected to an electric field which facilitates better oil-water separation by causing the dispersed water droplets to coalesce. The larger water droplets settle by gravity to the bottom of the vessel and are removed on level control while the partially-dehydrated oil traverses upward and out of the top of the vessel under back-pressure control.
The oil leaving the dehydrator usually meets the outlet BS&W specification (basic sediment and water) but typically exceeds the outlet salt specification for the treatment facility. Thus, the separation facility requires the addition of an electrostatic desalter (electrical desalter).
The Desalting Process
An electrostatic desalter operates in a manner similar to the electrostatic dehydrator. The difference is that a desalter adds dilution water to the feed mixture and then passes the combined stream through a mixing device before entering the desalter vessel. The desalted oil exits the desalter and is cooled before traveling to downstream tankage, pipeline or refinery equipment.
Heat aids gravity separation by reducing the viscosity of the bulk oil phase, thereby allowing the coalesced water droplets to settle to the bottom of the vessel. If the operating temperature of the desalter or dehydrator is too low, the required water settling time is greater, thus increasing the vessel size. If the operating temperature is too high, then the increased electrical conductivity results in larger electrical transformers and higher power consumption.
Stable emulsions can form in the electrostatic coalescer at the oil-water interface no matter what kind of electrostatic technology is used. Therefore, operating facilities inject demulsifier chemicals into the oil prior to the desalter or production separator. In some plants, a water-soluble demulsifier may also be used to clear up dirty or oily effluent water.
Some applications may require a two-stage desalter. These include applications containing high salinity formation water and applications with very low outlet specifications. The 2-stage desalting process is similar to the desalting process previously described, except that the dilution water is injected upstream of the second stage desalter and the effluent water from the second stage is recycled upstream of the first-stage desalter mixing valve. The use of recycle water minimizes the total amount of dilution water required.
The high voltage bushing system is believed by many to be the most important part of an electrostatic dehydrator/desalter unit. Failures or faulty design can cause unexpected downtime and costly repairs. This system must transfer the high voltage from the transformer/reactor to the electrodes under stresses due to high temperatures, high pressures and high voltage.
High Voltage System
The high voltage system must be a rugged and reliable design for use in both refinery and oil field applications. A dual isolation bushing assembly should be used to protect the transformer. Pressure-tested bushings must be used for both the vessel nozzle and for the transformer’s high-voltage connection. Both bushings need to be enclosed in an oil-filled housing which further protects the transformer. Some older designs use a single conduit to connect the transformer to the vessel.
If the bushing leaks, crude oil passes through the conduit into the transformer resulting in a grounded or a burned-out transformer. Both the transformer bushing and entrance bushing can be removed and replaced by the removal of the service inspection blinds. Bushing replacements can be made quickly without requiring the services of a technical representative and without draining the vessel if operating procedures allow.
Captive GridTM electrode assembly consisting of high-voltage conductors and framework made from high alloy steel and high voltage electrical insulators. Like the entrance bushings, the electrical insulators are machined and constructed from high-density virgin Teflon® and polished to a smooth finish.
The entrance bushing insulating material is made of high-density virgin Teflon® that has been thoroughly inspected to assure there are no flaws. The surface is finely machined to a polished finish to minimize surface tracking. Entrance Bushings carry an ATEX certification at no additional cost.
The high voltage electrode assembly consists of two or more grids with adjustable spacing. Depending on the design, the electrode assembly can use either horizontal or vertical grids. Conventional designs typically utilize a three-grid system incorporating two hot grids and one ground grid. This design provides maximum operating flexibility with minimal power consumption and reduced electrical demand. Other multi-grid designs with dual or triple phase wiring are available as required by the application or as requested by the client.