Discussion on electrodesalination units

Effect of temperature: At higher temperatures, the viscosity of crude oil decreases, which reduces the resistance to the movement and settlement of water droplets, thereby increasing their speed of movement； At the same time, the surface tension at the oil-water interface decreases, weakening the strength of the emulsion film ； At high temperatures, the stability of certain emulsifiers decreases, which can accelerate the demulsification of crude oil. When the temperature exceeds 120 degrees, the conductivity increases sharply, leading to higher power consumption ； At the same time, magnesium chloride and calcium chloride begin to hydrolyze, which is unfavorable for desalination. The electrodesalination temperature is generally maintained between 105 and 145, with the optimal operating temperature being determined based on the properties of the crude oil.

The water injection volume for electrodesalination is generally controlled at 4~6% of the crude oil volume. Beyond this range, the desalination rate decreases, as an excessive amount of water lowers the desalination temperature and increases the water content in the crude oil after desalination. However, if the crude oil being processed is very heavy and has a high salt content, it becomes necessary to increase the amount of water used. The quality of the water used for injection also has requirements; it is generally soft water. Additionally, the pH value of the injected water affects the desalination efficiency, while the emulsifier operates in a weakly alkaline environment. The water injection point is generally located in front of the electrodialysis mixing valve after the initial heat exchange of the crude oil.

What is the optimal level of water content and mixing strength before crude oil dehydration? What are the effects of having too high or too low levels?

Effect of pressure: At too low a pressure, the water injected into crude oil tends to vaporize at high temperatures (at 120°C–140°C, the saturated vapor pressure of water is 198.48–361.19×103 Pa), which is unfavorable for desalination； The entry of moisture into the electrode causes conductivity, which in turn leads to an increase in current ； Thirdly, the lighter water vapor will be discharged along with the crude oil from the top, resulting in excessive water content in the crude oil after treatment. Given the high pressure, it is necessary to consider the capacity of the electrodialysis unit as well as its impact on subsequent devices; provided that the unit can handle the pressure without affecting those subsequent devices, a higher value is preferable. Generally, the pressure for electrodesalination is about 2-3 Kgf higher than the vapor pressure of water at that temperature.

1# zhaoyuejiang: Let me share my personal suggestion. In the past, our desalination results were not good; attempts to improve them by adjusting parameters such as water injection volume, voltage, and demulsifiers proved ineffective. Later, we increased the electrodesorption temperature to around 140 degrees, and the results were very satisfactory. I suggest that the person who posted this question give it a try.

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1. The mixed pressure difference should be between 0.5 and 1 kilogram. 2. The temperature should not exceed 130 degrees. 3. The pressure should be above 13 kilograms and below the maximum design pressure. 4. The water injection volume should be 5% of the total amount of crude oil per hour

Effect of mixed strength ΔP: A certain amount of washing water must be added to the crude oil before electrodialysis in order to mix it with the oil, thereby increasing the density of the water droplets and making it easier for them to coalesce. Water injection can also dissolve the salts suspended in the crude oil, allowing them to be removed along with the water. It can also disrupt the stability of the emulsion in the crude oil, which facilitates desalting. When crude oil is mixed with water, a certain mixing pressure difference ΔP is required; the larger ΔP is, the finer the dispersion of the injected water, the more thorough its contact with the salts in the crude oil, and the higher the desalination rate. However, if ΔP is too high, emulsification may occur, and there are mainly three reasons for this: 1. The settling speed of water droplets in crude oil follows Stokes’ law. However, if ΔP is too high, the smaller the diameter of the water droplets, the greater the sedimentation speed of these droplets in the crude oil. This sedimentation speed is proportional to the square of the droplet diameter. When the sedimentation speed of the small droplets is less than the upward flow rate of the crude oil, they cannot settle and may be carried into the electric field, thereby causing an increase in current. This also leads to darkening of the drained liquid as well as a high water content in the crude oil after purification, which in turn results in a high salt content in the same fluid. 2. In an electric field, the coalescing force between water droplets is proportional to the sixth power of the droplet radius; the smaller the droplets, the weaker this coalescing force, making it difficult for large droplets to form, which in turn leads to difficulties in dehydration. 3. Demulsifiers are surfactants that generally reside at the oil-water interface, reducing the surface tension between oil and water, which facilitates the aggregation of water droplets enclosed in oil. But if water of the same volume forms countless small droplets due to excessive mixing, my calculations show that when water of a certain volume is dispersed into 8 small droplets of the same volume, its surface area increases by 4 times. As a result, the distribution of the demulsifier on the oil-water interface is reduced to one-fourth of its original level. To effectively solve this problem, it is necessary to increase the amount of demulsifier used. Of course, this is also related to the compatibility between the demulsifier and the crude oil. In electrodesalination, ΔP is generally 30–70 KPa, and the water content in the feed oil should be less than 0.5%; if it exceeds 1%, dehydration becomes difficult and the desalination efficiency drops.

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There are many influencing factors; in addition to the demulsifier and the demineralization unit itself, various operational parameters in actual operation, such as temperature, pressure, the amount of demulsifier used, and the mixing pressure difference, also play a role