Why is it necessary to desalt crude oil?

The salt and water content in crude oil poses significant hazards to its storage, transportation, processing, product quality, and equipment. These hazards include: first, an increase in the load on equipment as well as the consumption of power, heat energy, and cooling water; second, disruptions to the normal operation of distillation towers. When crude oil contains too much water, the water vaporizes, increasing the volume of the gas phase. This leads to a rise in pressure within the distillation tower, as well as higher gas flow rates, which can cause operational failures such as tower flooding. III. Impact on heat transfer and pipeline blockage: The salts present in crude oil, as water evaporates, form scale on the walls of heat exchangers and heating furnace tubes. This reduces heat transfer efficiency and increases flow resistance; in severe cases, it can lead to pipeline blockage, tube wall breakdown, and accidents. IV. Corrosion of equipment: CaCl2 and MgCl2 can hydrolyze to produce highly corrosive HCl; especially when there is moisture in the parts of the equipment operating at low temperatures, hydrochloric acid is formed, leading to more severe corrosion. When processing sulfur-containing crude oil, sulfur compounds decompose to produce H2S, which is corrosive to equipment; however, the corrosion product FeS, when deposited on the metal surface, can provide some protection for the metal. However, in the presence of HCl as well, HCl can react with FeS and destroy the protective layer, while the released H2S will further react with iron, exacerbating corrosion. V. Impact on product quality: Metals present in salts enter the heavy distillates or residue, poisoning catalysts and affecting the quality of raw materials used in further processing as well as the quality of the final products. Therefore, after crude oil enters the refinery, it must first be desalted and dehydrated to reduce the water content to 0.1%–0.2% and the salt content to <5 mg/l. For refineries with residue hydrogenation or heavy oil catalytic cracking processes, the salt content in crude oil is required to be <3 mg/l.

The salt and water content in crude oil poses significant hazards to its storage, transportation, processing, product quality, and equipment. These hazards include: first, an increase in the load on equipment as well as the consumption of power, heat energy, and cooling water; second, disruptions to the normal operation of distillation towers. When crude oil contains too much water, the water vaporizes, increasing the volume of the gas phase. This leads to a rise in pressure within the distillation tower, as well as higher gas flow rates, which can cause operational failures such as tower flooding. III. Impact on heat transfer and pipeline blockage: The salts present in crude oil, as water evaporates, form scale on the walls of heat exchangers and heating furnace tubes. This reduces heat transfer efficiency and increases flow resistance; in severe cases, it can lead to pipeline blockage, tube wall breakdown, and accidents. IV. Corrosion of equipment: CaCl2 and MgCl2 can hydrolyze to produce highly corrosive HCl; especially when there is moisture in the parts of the equipment operating at low temperatures, hydrochloric acid is formed, leading to more severe corrosion. When processing sulfur-containing crude oil, sulfur compounds decompose to produce H2S, which is corrosive to equipment; however, the corrosion product FeS, when deposited on the metal surface, can provide some protection for the metal. However, in the presence of HCl as well, HCl can react with FeS and destroy the protective layer, while the released H2S will further react with iron, exacerbating corrosion. V. Impact on product quality: Metals present in salts enter the heavy distillates or residue, poisoning catalysts and affecting the quality of raw materials used in further processing as well as the quality of the final products. Therefore, after crude oil enters the refinery, it must first be desalted and dehydrated to reduce the water content to 0.1%–0.2% and the salt content to <5 mg/l. For refineries with residue hydrogenation or heavy oil catalytic cracking processes, the salt content in crude oil is required to be <3 mg/l.

Salts in crude oil are generally dissolved in the water contained within it; sometimes, a portion of them also remains suspended in the crude oil as fine particles. The composition of salts in various crude oils varies; they mainly consist of chlorides of sodium, calcium, and magnesium, with NaCl being the most abundant, typically accounting for about 75%. The presence of these salts poses significant hazards during the processing process, mainly in the following ways: 1. It increases the transportation load and costs; the presence of water raises the weight and volume of the crude oil. This places an additional burden on transportation and storage, increases energy consumption, and raises the costs of storage and delivery. 2. It affects the smooth operation of the processing process. The average molecular weight of crude oil is around 200, while the molecular weight of water is 18. If the water content in crude oil is 1% (w), after vaporization, water accounts for 11% of the total volume. Therefore, during the processing, it invisibly increases the space inside the pipes and equipment. In other words, with the same equipment, the processing capacity is reduced by 11%. Furthermore, the sudden vaporization of water causes an increase in system pressure, raising the pressure at the pump outlet and increasing power consumption. At the same time, it will also increase the steam velocity inside the distillation tower, leading to overpressure and tower flooding accidents. 3. Increased power consumption during the processing process. The vaporization heat of crude oil is approximately 350 kJ/kg ； The latent heat of vaporization of water is 2600 kJ/kg, which is 6 to 8 times that of crude oil. If crude oil contains a high amount of water, the vaporization and condensation processes in crude oil processing will inevitably increase the consumption of fuel and cooling water. 4. Causing scaling and blockage of equipment and pipelines: During crude oil processing, in furnace tubes and heat exchange equipment, the evaporation of water leads to the deposition of inorganic salts contained in the crude oil on the walls of these devices, resulting in scaling. Scaling on the heat exchange tubes affects heat transfer and deteriorates the operation of the heat exchanger. Fouling of the furnace tubes reduces their lifespan and increases pressure drop. In severe cases, it can cause blockages in the furnace tubes or heat exchangers, leading to plant accidents and unplanned shutdowns. 5. Impact on the properties of the raw materials used in secondary processing units: Most of the salts present in crude oil remain in residue and heavy fractions, which directly affects the quality of certain products; for example, it increases the ash content of petroleum coke and reduces the elongation capacity of asphalt. It also increases the metal content in the materials used for secondary processing, thereby exacerbating catalyst contamination and poisoning. 6. Corrosion of pipelines and equipment: The most serious issue is corrosion. During the processing of crude oil, CaCl2 and MgCl2 can hydrolyze to produce highly corrosive HCl; this problem is particularly severe in low-temperature equipment where the presence of water leads to the formation of hydrochloric acid. For example: MgCl2 + 2H2O ——→ Mg(OH)2 + 2HCl. Hydrogen chloride, in the presence of water, can cause severe corrosion to equipment. Fe + 2HCl ——→ FeCl2 + H2. When processing sulfur-containing crude oil, sulfur compounds break down to produce H2S, which is corrosive to equipment; however, the corrosion product FeS, when it adheres to the metal surface, can provide some protection for the metal. However, in the presence of HCl as well, HCl can react with FeS and destroy the protective layer, while the released H2S will further react with iron, thereby accelerating corrosion. Fe + H2S ——→ FeS + H2. FeS + 2HCl ——→ FeCl2 + H2S. The resulting ferrous chloride dissolves in water, causing iron to corrode and the metal surface to be stripped away; meanwhile, the hydrogen sulfide produced continues to react with iron, leading to more severe corrosion. Therefore, in order to eliminate the various aforementioned hazards caused by the water and salt content in crude oil on the processing process, thorough desalination and dewatering must be carried out before the crude oil is processed. Currently, domestic and international refineries generally stipulate a salt content limit for crude oil desalting and dewatering

Poor operation of electrodialysis can cause severe corrosion to subsequent equipment and pipelines. At the same time, the heavy metals in the product will affect the activity of the catalytic cracking catalyst

The metals in salts end up in the heavy distillates or residue, poisoning the catalysts. In salts, substances like sodium are cations; so how do they turn into metallic sodium? Do sodium ions cause damage to catalysts?

The core equipment of the electrodialysis unit is the electrodialysis tank; depending on the scale of the installation, there are generally 4 to 5 such tanks. The figure below shows the electrical desalination tank of a 10 million tons per year atmospheric and vacuum distillation unit.

Crude oil needs to be mixed with water in order to dissolve the inorganic salts contained within it; thereafter, a demulsifier is added and the mixture is passed through an electrodesalination tank. The electrodesalination tank is essentially used for removing water from the crude oil, and the water discharged as a result is saline wastewater. Why is desalination and dehydration necessary? One is to prevent excessive water from entering the initial distillation tower and causing a tower flooding accident. Secondly, inorganic salts contained in crude oil decompose at high temperatures, forming deposits that cause scaling inside heat exchangers and pipes; this reduces heat transfer efficiency and, in severe cases, can lead to explosions. In crude oil, when inorganic salts are decomposed at high temperatures, chloride ions produce hydrochloric acid, which corrodes the pipes. At the same time, hydrochloric acid reacts with ferrous sulfide, causing severe corrosion of the pipes. Fourthly, the high salt content in heavy oil can cause catalyst poisoning in downstream units, such as catalytic cracking. A dechlorinating agent needs to be added to the crude oil before it enters the plant. The above is my understanding; I hope those who are more knowledgeable will point out any mistakes, and everyone is welcome to add further details. Thank you!

Article 2 is very important! I learned it! Thank you!

What material is this device made of?

The supplementary summary is excellent! Welcome more people to join the discussions!