In a refinery, crude oil containing high levels of salt will go through a Desalter/Dehydrator before being fed to the atmospheric distillation tower.Removal of salts is important for reducing corrosion in the distillation tower and downstream processing units. If not removed, the salt will form acids when heated that will result in corrosion. Also, the salt can form deposits on heat exchanger surfaces over time, resulting in fouling.Desalting system also removes suspended solids such as sand, dirt, and rust particles picked up in transport.
Desalters and Dehydrators work on the principal of electrostatic coalescence. Water droplets in crude assume spherical shapes. When exposed to an electrical grid, the droplets distort into an elliptical shape, or dipole. The attractive forces between the dipoles result in rapid coalescence into larger droplets. The larger droplets can then be separated by gravity. As Stoke’s law for gravity settling predicts that the settling rate increases as the square of the droplet size, typical brine droplets of 1 micron found in oilfield emulsions increase to 100 microns, thereby increasing their settling rate by a factor of 10000. This greatly reduces the size of the desalter or dehydrator compared to conventional gravity settling equipment.
In the Desalter, the crude oil is heated and then mixed with 5-15% volume of fresh water so that the water can dilute the dissolved salts. The oil-water mix is put into a settling tank to allow the salt-containing water to separate and be drawn off. Frequently, an electric field is used to encourage water separation. Demulsifying chemicals are also used. For high levels of salt and/or to achieve very low final concentrations, two- or three-stage desalting may be used.Typical desalted crude will have concentration of 1 pound/thousand barrels (PTB).
Desalting can be performed in a single stage or in two stages, depending on the requirements of the refinery. Dehydration efficiency of a desalter is usually 95% in a single stage and up to 99% in two stages.
We offer a wide range of electrostatic dehydrators and desalters to effectively and economically treat the range of crude oils produced today.
By providing different vessel sizes, different numbers and types of power units together with several different electrical arrangements, our custom designs meet the most stringent requirements.
Free Water Knockout is a three-phase separator which is used to remove free water held in the vessel and separate brine from crude oil. It is essential to use Free Water Knockout as free water results into the formation of hydrates, corrosion and tight emulsions that are difficult to break in later stages. It is referred to as a three-phase separator because it is capable of segregating oil, gas, and free water. The vessel is also known as oil-water separator.
The Free Water Knockout is designed in such a way that it provides low-velocity flow when combined with a large water/oil interface area. It allows maximum amount of water to settle down. With increasing demand, diverse manufacturers are presenting the vessel in a number of shapes and sizes to meet specific conditions and requirements. These vessels are either horizontal or vertical shaped and separate liquids of different densities. The degree of separation depends on the retention time, the density of differential fluid and the temperature of flowing fluids.
A heater treater is a pressure vessel that uses heat and residence time to separate clean, dry oil from incoming fluids. It usually is located immediately downstream of the Free Water Knockout during the oil dehydration process. The heat allows for easier separation of the oil and water and also allows solid particles such as sand and corrosive products to settle out. The quality of the effluent oil is measured with a bulk sediment and water (BS&W) probe.
There are two separate sections in a heater treater, the heating section and the treating section. The heating section is where fluid is received and heat is applied. Heat can be applied with a burner or steam coils. The heat causes the viscosity of the oil to drop and flow through the water phase easier. At the boundary of the heating section, there is an internal weir which directs flow to the treating section. In the treating section, a lowered oil viscosity and residence time cause the oil to separate from the water.
Gas from the process stream collects in the top portion of the vessel and exits the vessel through a nozzle designed to hold back pressure on the vessel.
Water from the stream exits through the bottom of the vessel. The water is then sent to further processing to remove any additional bulk sediment or trace oil particles. Following this, the water may be filtered and softened for use in steam generation or water injection.
Oil is collected from a weir box near the top of the vessel. Oil discharge may be flow controlled by a control valve or might dump with assistance of an on/ off control valve. At this point, the oil should of sufficient quality for sale and it either transferred directly to a LACT Unit or a holding tank.
Heater treaters can be either direct fired or indirect fired. The orientation of the pressure vessel can be horizontal or vertical. The main difference between a horizontal and vertical orientation is residence time. A horizontal heater treater has longer residence time than a vertical one.
A direct fired heater treater is designed such that the transfer of heat is accomplished by direct contact of the fluid with the firebox/ fire-tube. Direct fired heater treaters are more efficient than indirect fired heater treaters. With a direct fired heater treater, care must be taken to ensure that the fluid level never drops below any portion of the fire tube. If the level drops below the top of the firebox/ fire-tube the heat generated by the burner cannot be dissipated as quickly by the fluids and could cause self-ignition of the gas blanked. For this reason, the heated section of a heater treater is usually fluid packed.
An indirect fired heater treater is one where the heating element heats the water or another heating medium and the heating medium is then used to transfer heat to the process stream. An indirect fired heater treater might be used if the production stream is sandy. Sand falling out of the emulsion can collect on the fire-tube and cause overheating (hot spots) of the tube. In general, an indirect fired heater is less efficient than a direct fired heater because heat from the burner needs to be transferred to the heating medium which is then transferred to the process stream.