A DETAILED STUDY ABOUT DESALTERS BY ASAD RAZA AND HASAN BAQAR
SIDE VIEW OF DESALTER
WHAT IS DESALTING? Desalting removes salts from the liquids such as crude oil. Produced crude oil is a mixture of hydrocarbon liquids, water, natural gas and salts. Natural gas and free water are easily separated by gravity. The salts are dissolved in water, called brine water except for a small amount of oil-coated salts, therefore removing water will rid the crude of salts. Three types of water are found in crude oil: FREE, EMULSIFIED, and SOLUBLE WATER. Free Water is not intimately mixed with the crude and can be easily removed via
separator vessels. Conversely Soluble Water cannot be physically separated. It is intimately mixed with the crude ad tiny droplets of the emulsified water are scattered throughout the crude. Emulsified Water cannot be separated in sample gravity separator vessels. To remove this type of water we use DESALTERS, to understand how desalter works, one must understand the properties of the emulsified droplets. A brine-water droplet is wrapped in a tough elastic film that repulses other droplets. Voltage applied to droplet elongate this skin. Positive charged molecules are attracted to the negative pole and vise versa for negative charged molecules, AC current alternates the dipole 120 times a second. The droplet’s film elongate and contracts many times and eventually ruptures.
WHAT IS DESALTER? A typical desalter is comprised of a vessel, electric transformer, oil outlet header, electrodes, inlet header, water effluent header, mudwash header and mixing valve (above Fig). The vessel is a horizontal gravity settling vessel in which brine water is separated from the crude oil. The transformers, mounted on top of the desalter vessel, convert primary (plant supply) voltage to secondary (across the electrodes) voltage that enable electric coalescing of the brine. The inlet header introduces the crude into the desalter and evenly distributed the crude (oil/water mixture) within the vessel. The inlet header is located just above the oil/water interface level. The oil outlet collection header is located at the top of the vessel along the entire length of the vessel. It collects the separated oil. The effluent water header located just above the bottom of the vessel and drums the separated water from the crude. The mudwash system, at the bottom of the vessel, injects process water and agitates accumulated solids to be drained through the effluent water header. The mix valve located upstream of the desalter intimately mixes fresh water with the incoming crude to wash the crude thoroughly.
HOW DOES THE DESALTER WORK? A desalter enhances gravity settling of heavier droplets by increasing the falling velocity as shown by Stokes Law,
Where: V d
= Relative velocity of the water droplet falling through oil = Diameter of the water droplet = Density of the water = Density of the oil = Viscosity of the oil
The desalter increases the diameter of the water droplet which is the most dominating parameter of the strokes Law.
The crude oil is heated upstream of desalter to lower its viscosity, which increases the falling speed of water droplet through crude. The heated crude at 100 F to 300 F enters the desalter and is distributed below a high electrical field. Larger water droplet will settle by gravity; the crude, containing emulsified water, will flow upward through the electrical field generated by the electrodes. The electrical field coalesces the emulsified water droplets. Coalesced water droplets will fall by gravity and collect in the bottom of the desalter. The bottom settle water is continuously discharged through the effluent water header. The crude oil must mixed with injected washwater, which should be cleaner then the brine water in crude ( low salt content). The washwater dilutes the salt concentration of the brine water emulsified in the crude and increases the Size of droplet
HELPFUL TIPS TO OPERATE DESALTER: • • •
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Use sufficient washwater: Increase the washwater flowrate always improves desalting efficiency. A washwater flowrate ranges from 4% to 10% and 5% is typical. Washwater pH should be maintain 5 and 8 to prevent corrosion or emulsification problems. The mixing valve pressure drop: A normal range is 5 to 20 psi. The maximum pressure drop is usually limited to 20 psi because excessive pressure drop can break water droplets and causes emulsification in desalter. Desalter temperature: Desalter temperature should be kept b/w 150 to 300 f, low temperature lowers crude viscosity, thereby slowing water droplet falling velocity. Pressure of desalter: At least 20 psi above the vapor pressure of the crude is a good rule. Water level: Water level should be maintain to maximize the water residence time. The highest water level should still be lower than highest trycock valve to prevent excessive power consumption and even short-circuiting problem. Demulsifying chemicals: Demulsifier should be injected with a constant flow rate.
OVERVIEW: This report presents general Electro-static desalting and working of Desalters, however always refer to the manufacturer’s operating and maintenance manuals for operation and maintenance of specific type of desalter.
