Electrodesalination, injection of demulsifiers.

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The main process parameters for electrical desalination of crude oil can be divided into two categories. One category consists of the adjustable parameters during the desalination process, including temperature, water injection volume, type and dosage of demulsifiers, intensity of oil-water mixing, and the electric desalination interface level. All these parameters have a significant impact on the effectiveness of electrical desalination of crude oil. The other category consists of design parameters, which are related to the properties of crude oil, the volume of crude oil processed, and the equipment selected for the desalination unit. In practical operation, since the properties of the crude oil remain constant and the processing volume is fixed, parameters such as desalination pressure, the residence time of the crude oil in the electric field, and electric field strength are also fixed and cannot be adjusted arbitrarily. a. Electrodesalination temperature: This is an important control parameter in the electrodesalination process. In design and control, heat exchange between crude oil and other heat transfer media is typically used; the temperature level has a significant impact on the demulsification of emulsions and the sedimentation of water droplets. According to Stokes’ formula: reducing the viscosity of the oil phase, increasing the density difference between oil and water, and enlarging the diameter of the water droplets can accelerate the settling speed of water. To ensure the maximum settlement rate, the selection principle for the desalination temperature is: the density difference between oil and water should be as large as possible, and the viscosity of the crude oil should be as low as possible. As the desalination temperature increases, the viscosity of the crude oil decreases, reducing the resistance to the movement of water droplets and thus facilitating their movement. Rising temperature also lowers the interfacial tension between oil and water; the water droplets expand due to heat, which weakens the emulsion membrane and promotes demulsification and coalescence. Additionally, higher temperatures increase the speed of Brownian motion, thereby increasing the likelihood of collisions between water droplets. An appropriate increase in temperature is beneficial for demulsification and the sedimentation of water droplets. However, there is a limit to increasing the desalination temperature of crude oil; once the temperature rises to a certain level, the increase in the settling speed of water droplets also begins to decline ； Furthermore, an increase in the temperature of crude oil can also have some adverse effects: as the temperature rises, the electrical conductivity of the crude oil increases, leading to higher power consumption ； In electric insulation rods and insulating hangers (polytetrafluoroethylene), deterioration of the working environment can easily lead to breakdown of these insulation elements, which is not conducive to the stable operation of electrodesalination over long periods of time ； An increase in the circulating water consumption and a rise in the desalination temperature lead to an elevation in the temperature at which the water is separated from the desalinated fluid. To achieve the specified discharge temperature for this separated water, it is necessary to increase the amount of circulating water accordingly. At the same time, the increase in desalination temperature is also limited by the operating pressure of the desalination tank. At a certain operating pressure for electrodialysis, the saturated vapor pressure of water remains constant; therefore, the desalination temperature must be lower than the boiling point of water, otherwise sudden boiling of the water will occur, disrupting the normal operation of desalination. Hence, it is necessary to maintain an appropriate desalination temperature. Currently, the designed temperature for crude oil entering the desalting tank in atmospheric and vacuum distillation units is generally 120–140°C (the optimal temperature). During the actual design process, this temperature can be increased appropriately depending on production conditions and the density of the crude oil, which facilitates adjustment of the desalting temperature during the production process. When the density is 0.87–0.89 g/cm3, temperature control at the lower limit can be used; when the density is 0.90–0.96 g/cm3, the temperature can be controlled at the upper limit. During operation, the desalination temperature must be kept stable, and any adjustments should be made slowly. If the adjustment is made too quickly, the feed temperature will rise suddenly, resulting in a decrease in the density of the crude oil entering the lower part of the desalination tank. This leads to the hot oil in the lower section displacing the cold oil in the upper section, thereby creating \"thermal mixing\". b Washing water: The function of injecting water is to dissolve inorganic salts and some organic substances in the crude oil, thereby removing these salts as the washing water is drained. Appropriately increasing the water injection volume can enhance the cohesion between water droplets, thereby facilitating their aggregation ； It can also increase the salt removal rate. Excessive water injection can affect the stability of the electric field, which is not conducive to electrodialysis operations. The amount of water injected should be adjusted according to the properties of the crude oil; currently, it is generally kept at 5% to 8% of the volume of the crude oil. Requirements for the quality of water used for injection: ammonia nitrogen level ≤ 40 mg/l, chloride level ≤ 300 mg/l, and pH value ≤ 8. A high pH value of the injection water can lead to the formation of emulsions, which not only affects the desalination efficiency but also causes the desalinated wastewater to contain oil. The water injection location can be determined based on the salt and water content of the crude oil; for crude oils that are prone to emulsification, the water injection point can be placed before the mixing valve ； For crude oil with a high salt content, it can be injected after the crude oil pump, with the water injection volume increased appropriately. In earlier designs, the water injection point was located at the inlet of the crude oil pump, primarily to make demulsification more difficult. Currently, in most plants, the water injection point has been moved to behind the heat exchange system and before the desalination tank. However, there are also some devices that place the water injection point before the heat exchange system, allowing for thorough mixing of oil and water; this eliminates the need for static mixers and avoids excessive mixing caused by centrifugal pumps. Moreover, the injection of wash water at a higher temperature (around 80°C) helps to reduce the formation of scale due to inorganic salts and suspended impurities in the heat exchanger, lowers the viscosity of the crude oil, and improves the heat transfer coefficient of the crude oil within the heat exchange system. c. Demulsifier: A demulsifier is injected during the desalination process to break down the oil-water emulsion film and prevent the formation of such a film on the surface of the wash water. The dosage of the demulsifier varies depending on the properties of the crude oil, the requirements for desalination, and the process conditions used for desalination. Generally, the dosage of water-soluble demulsifiers ranges from 20 to 40 mg/l (per unit volume of crude oil), while that of oil-soluble demulsifiers ranges from 3 to 25 mg/l. Demulsifiers are also a type of surfactant; if used in excessive amounts, they can degrade the quality of saline wastewater, making its purification more difficult. Characteristics for selecting demulsifiers: 1. Strong surface activity. 2. It has good wetting properties. 3. It has sufficient flocculation capacity. 4. It has excellent setting capacity. When using demulsifiers, water-soluble demulsifiers should first be prepared into a 1%–2% aqueous solution in a dedicated container, while oil-soluble demulsifiers are used in their original commercial form and added to the crude oil using a specialized small-capacity metering pump. d. Mixing intensity of oil and water: Mixing intensity is an important parameter in the electrodesalination process. The efficiency of removing salts from crude oil depends to a large extent on the degree of mixing between the crude oil, the washing water, and the demulsifier. Proper mixing ensures good contact between the washing water, the demulsifier, and the salt-containing droplets in the crude oil, thereby enabling effective dilution of these droplets. A low mixing intensity makes it difficult to achieve satisfactory desalination results, while a high mixing intensity results in an overly stable emulsion layer that is hard to break apart. The electrodialysis desalination system can select the optimal mixing intensity based on the type of crude oil being processed and the actual conditions of the internal structure of the desalination tanks, thereby improving the desalination efficiency. Based on actual production data, when processing crude oil with a lower density (°API 15–24), the pressure difference △p of the mixing valve should be set at 30–80 kPa ； When processing crude oil with a higher density (°API 15–24), the pressure difference △p of the mixing valve is set at 50–130 kPa. e. Electric field strength: A high electric field strength can enhance the coalescing force between small water droplets, which is beneficial for electrodesalination. However, beyond a certain level, increasing the electric field strength further has little effect on improving the desalination efficiency. The fully impedance-adjustable transformers now being used in domestic electrodialysis units allow the output voltage to be adjusted in 4–5 levels; during operation, the electric field strength can be regulated by changing the output voltage as needed. In general, the electric field strength in areas with weak electric fields is 0.3–0.4 kv/cm, while in areas with strong electric fields it is 0.7–1.0 kv/cm. f. Residence time of crude oil in the electric field: The residence time of crude oil and water in the tank determines the production efficiency of the electrodesalination tank, and it affects the effectiveness of crude oil desalination and dehydration as well as the oil content in the wastewater. The residence time of crude oil in the electric field is an important parameter affecting droplet coalescence. It is generally considered that a residence time of about 2 minutes for crude oil in an electric field is appropriate. g. Electrodesalting interface position: The residence time of crude oil in the tank determines not only its rising speed within the tank but also the position of the oil-water interface; therefore, controlling this interface is also an important process parameter. The oil-water interface in the desalination tank is the transition zone where oil and water come into contact; it must be maintained at a level below the lower plate of the desalination tank. If this interface is too low, oil may end up in the water stream, while a higher interface level can increase the weak electric field strength, thereby improving the desalination effect. However, if the interface level is too high, water may be carried in the crude oil, leading to fluctuations in the desalination current and causing the desalination transformer to trip. Therefore, it is necessary to maintain an appropriate interface level during actual operation. The measuring instruments for the desalination level include resistive, capacitive, internal float type, micro-differential pressure type, radar type, and others. Currently, the internal floating ball type and the micro-pressure difference type are more suitable. By measuring the level, the drainage volume is automatically controlled in order to regulate the level height.

1. The demulsifier should be calculated based on the volume of material to be processed; it can be injected directly using a pump, without the need for dilution； 2. The dosage of water-soluble demulsifiers is generally 20–40 mg/l, while the dosage of oil-soluble demulsifiers is 5–20 mg/l ； 3. There is a critical value for the concentration of demulsifiers used (which varies depending on factors such as the properties of the oil). Below this critical value, increasing the amount used leads to better demulsification and dehydration; above this critical value, however, higher amounts instead cause emulsification.

Oil-water interfacial tension meter: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=1862831&extra=