What is the basic principle of electric desalination?
What is the basic principle of electric desalination?
Crude oil electric desalting at the appropriate temperature, inject the right amount of fresh water and demulsifier, some production units also inject the right amount of decalcifier, for full mixing, adding fresh water is to dissolve the salts remaining in the crude oil, while the original brine is further diluted, the purpose of adding demulsifier is to destroy its emulsification state. Under the action of high pressure electric field and demulsifier, the tiny water droplets are gradually converged into large water droplets, and most of the salts from the crude oil are dissolved in water by gravity, so the purpose of desalting is achieved at the same time of dehydration.
1. Oil-water emulsification. The conditions for the formation of stable emulsion are as follows: one is that the two liquids do not dissolve each other; Second, there is emulsifying stabilizer in the system. The third is the appropriate mixing intensity. In the process of crude oil transportation and processing, the three conditions for the formation of stable emulsion are fully met. The crude oil transported to the refinery contains water, which has two phases of oil and water that are not mutually soluble. There are natural emulsifying stabilizers, such as gum, asphaltene, naphthenic acid, crystalline paraffin, clay, etc. In the process of crude oil mining and transportation, the conditions are fully stirred. The water is dispersed in the crude oil in the droplet state, and the emulsifier in the crude oil is concentrated on the oil-water interface by adsorption, forming a firm molecular film and forming a stable emulsion. Oil-water emulsions generally have the following three types: one is oil-in-water type, the other is oil-in-water type, and the third is multiple emulsions or multistage emulsions. The emulsion in crude oil is mainly oil-in-water. There are many factors affecting the stability of oil-water emulsion, such as the nature of crude oil, the degree of water dispersion and the formation time of the emulsion. The larger the viscosity of crude oil, the smaller the water droplets in crude oil and the more stable the emulsion, because the larger the viscosity, the greater the resistance of water droplets to coalescence and sinking. According to Stokes' law, the sedimentation rate of water droplets in crude oil is proportional to the difference in oil-water density, the smaller the difference in oil-water density, the more stable the emulsion, the longer the emulsion exists, the more difficult to break the emulsion, which is also called "aging" of the emulsion; When the PH value of the water phase is higher than 8, it is easy to produce surface active substances such as soap, and the more stable the emulsion, so the PH value of the general washing water is maintained at about 6-7 to facilitate crude oil demulsification and dehydration. In addition, the degree of stability of the emulsion is also related to the type of emulsifier, for example, when the wax emulsifier is heated above its melting point temperature, the emulsification can be greatly reduced, and the asphaltene emulsifier is generally more difficult to demulsify.
2. Emulsion demulsification
Because the emulsion is a dispersion system formed by strong mixing, the emulsion is a temporary stable system. By proper process method, the stability of the emulsifier on the oil-water interface can be weakened, and the rate of demulsification can be accelerated. Such as controlling the process conditions to decompose the emulsifier; Add chemicals to react with emulsifiers to weaken or lose their emulsifying ability; Increasing the temperature increases the solubility of emulsifier in crude oil and weakens the strength of oil-water interface. The directional arrangement of the emulsifier on the oil-water interface can also be weakened. According to the above principles, the main methods of emulsion demulsification are:
a Chemical method: This method has been widely used, the commonly used method is to add demulsifier to the crude oil, demulsifier has higher activity than natural emulsifier, it spreads in the crude oil, gradually approaches the oil-water interface, is adsorbed by the interface film, replaces the natural emulsifier in the emulsified film, and forms a new film that is not strong, so that the interface film breaks, occurs agglomeration, and realizes oil-water separation. Another function of demulsifier is to wet the solid particles in the crude oil, remove its external oil film, and enter the water phase to discharge.
b electric field method: The small water droplets in the emulsion in the electric field, due to electrostatic induction of the induced dipole, along the direction of the electric field with different symbols of charge on both ends, contact electrode droplets will also carry a net charge. Induced dipole causes the electrostatic attraction between water droplets to attract each other, that is, water droplets coalesce force, water droplets are affected by the coalesce force, the motion speed increases, the kinetic energy increases, on the one hand, it can overcome the resistance of the emulsion film, on the other hand, it increases the chance of collision between water droplets, so that small water droplets coalesce into large water droplets. Therefore, static electricity is generated between adjacent droplets and between droplets and electrodes, and the movement speed of droplets is accelerated by electrostatic action, the kinetic energy is increased, and the probability of collision with each other is increased. Due to the action of kinetic energy and electrostatic potential energy, the droplets in the emulsion can overcome the barrier of emulsifying film and achieve coalescence.
c Other methods: First, mechanical centrifugal separation, the use of dispersed phase and dispersed media relative density is different, under the action of centrifugal force to achieve separation; The second is the filtration method, the pressure makes the emulsion through the adsorption layer (such as activated carbon, silica gel, clay, etc.), because the adsorption layer adsorbed the emulsifier, the emulsion film is destroyed, and the oil-water separation is realized. The third is the foam decomposition method, which uses the foaming method to adsorb the dispersed phase oil droplets to the foam and float to the water surface for oil-water separation.
At present, when crude oil is demulsified, not only a single method is used, but usually a concentrated method is used to improve the demulsification effect. For example, the electric desalting of crude oil currently uses the electric-chemical method, and the deemulsification process is divided into two stages. The first stage is flocculation, in which small water droplets gather together and move closer to each other. In the second stage, two or more water droplets close to each other coalesce and grow, coagulate, become large water droplets, and separate and settle.
3. Crude oil decalcification
Calcium is an alkaline earth metal. Studies have shown that only a small part of metallic calcium exists in the form of inorganic salts in crude oil, such as calcium chloride, calcium carbonate, etc., and most of it exists in the form of organic compounds, such as naphthenates and phenolates. The inorganic calcium in crude oil, in the process of crude oil electric desalting dehydration, calcium chloride can be dissolved in water and removed, calcium carbonate, calcium sulfate is not easy to water, but in the process of electric desalting dehydration by washing, precipitation can be partially removed. However, organic calcium salts with calcium phenol and calcium naphthenate are easily soluble in oil and difficult to dissolve in water, and are difficult to remove through the current demulsification and desalting dehydration process. Even if deep desalting is achieved (the salt content of crude oil after removal is less than or equal to 3mg/l, the water content is less than or equal to 0.3%, and the oil content of drainage is less than or equal to 200mg/l), only a small part of calcium can be removed.
During the distillation of crude oil, almost all calcium salts remain in the atmospheric residual oil due to high boiling point, and the calcium content in crude oil increases greatly, which brings great risks to the secondary processing. In the hydrocracking process, calcium will deactivate and scale the catalyst quickly, and it is easy to deposit between the catalyst particles, resulting in increased reactor pressure. Calcium will also poison the catalytic cracking catalyst and reduce its activity. The calcium sulfate produced will make the catalyst stick together and affect the catalyst fluidization. For delayed coking process, calcium will become ash in petroleum coke and affect the quality of petroleum coke. Both laboratory and field applications show that the oil with high calcium content has higher conductivity, higher desalting current and higher desalting power consumption, which is not conducive to the stable operation of the electric desalting plant. Therefore, the removal of organic calcium from crude oil is very important for normal production.
The best way of crude oil decalcification is to add chemical agents to convert oil-soluble calcium salt into water-soluble calcium salt during the pretreatment process of crude oil electric desalting, which is carried out simultaneously with dehydration and desalting. Calcium in crude oil mainly exists in the form of naphthenate, fatty acid salt, phenol salt, etc. After mixing with water in crude oil, oil-water mixture with a large number of oil-water interface is formed, and these organic calcium salts have a certain ionization balance at the oil-water interface. When chemical agents are added, the added chemical agent molecules have reacted with calcium ions by complexation, chelation or replacement. The formation of water-soluble compounds, so that calcium ions from the oil phase to the water phase, under the action of high pressure electric field and demulsifier with desalting sewage discharge, so as to achieve the purpose of decalcification.
