What to do if salt cannot be removed through electrodesalination?
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Normally, 150 cubic meters of crude oil are fed into the unit for processing. The electrodialysis tanks have two stages, with each stage having a capacity of 106 cubic meters. In our current process, only the first stage of electrodialysis is used, with a dosage of around 80 ppm. The demulsifier is diluted before being injected at a rate of 2500 kg/h. The temperature in the desalination tanks is around 115 degrees, and the pressure is 1550 kPa; the safety valve set point for the electrodialysis tanks is 1800 kPa, while the water level is 30%. The water content decreases by 0.2-0.4% before and after desalination, but the desalination rate is less than 45%; the salt content remains high at 38-46% even after desalination. This often causes the water in the top tank to turn black, and over time the accumulation of salt inside affects operations, forcing the need to clean the tower. Hey, is there any way to reduce this desalination rate?
Let’s check the mixed strength; it seems like the salt hasn’t dissolved in the water at all. The amount of water used and the type of demulsifier are also very important.
This post was last edited by songanasd on 2010-4-14 at 12:47. I’m not very familiar with the operations and process parameters of electrodesalination; it seems like there shouldn’t be any major issues. But nothing was said about injecting water or anything like that. It is recommended to screen some demulsifiers, using a dosage of 80 ppm. Are you using water-soluble or oil-soluble ones? I rarely see such a large amount being used. In normal refineries, it is generally around 30 ppm for water-soluble substances and about 15 ppm for oil-soluble substances.
I remember an expert in electrodialysis from Haichuan saying that normal water removal but abnormal desalination is a typical sign of insufficient water injection volume. I tried to find that post just now, but unfortunately couldn’t locate it; there are quite a few posts related to electrodialysis on Haichuan’s platform, so you can give them a search. Why are you still using only stage 1? I wonder what would happen if you increased the water injection volume and used stage 2.
After drying, it has a low water content but a high salt content; typical mixed strength is insufficient! Is there a significant discrepancy between the designed flow rate and the actual flow rate? Perhaps you should move the water injection point before the last heat exchanger in order to increase the mixing intensity
What is the water content of your crude oil entering the plant? The person upstairs mentioned whether the demulsifier is oil-soluble or water-soluble (the data indicates it is water-soluble) It is recommended to increase the mixing intensity, screen the demulsifiers, and adjust the water volume and quality (special attention should be paid to recycled water, as this has occurred in our company).
For processed heavy oil, it is recommended to increase the temperature before dehydration; around 140 degrees is ideal
What are the density of your crude oil, its viscosity at 20°, and the salt content before dehydration?
The main reason is that the salt has not dissolved in the water. Choosing the right solvent should not be a problem, and the mixing efficiency should also be fairly good. It might be worth considering whether the crude oil used differs significantly from that specified in the design of the desalination tank; trying to increase the water flow rate and raise the temperature could be an option.
It is also possible to consider heating the water being injected
Depending on the properties of different crude oils, their salt content, and the type of salts present, appropriate electrodialysis process parameters should be selected. (1) Temperature: An increase in temperature can reduce the viscosity and density of crude oil, as well as the stability of emulsions, and increase the settling speed of water. If the temperature is too high (>140°C), the density difference between oil and water decreases, which is also unfavorable for dehydration. At the same time, the electrical conductivity of crude oil increases as the temperature rises; therefore, excessively high temperatures not only fail to improve the efficiency of dehydration and desalination, but may also cause tripping due to excessive current in the desalination tanks, thereby disrupting normal power supply. Therefore, the desalination temperature for crude oil is generally set between 105 and 140°C. (II) Pressure: The desalination tank must operate under a certain pressure to prevent the light components in crude oil from vaporizing, which could cause disturbance in the oil layer and affect the sedimentation and separation of water. The operating pressure depends on the content of light fractions in the crude oil and the heating temperature, and is generally between 0.8 and 2 MPa. (III) Water injection volume and water quality: During the desalination process, injecting a certain amount of water into the crude oil mixes with it, increasing the density of the water droplets and thus making it easier for them to coalesce. Additionally, water injection can disrupt the stability of the crude oil emulsion, which is beneficial for desalination. At the same time, the secondary water injection volume has a significant impact on the salt content of the crude oil after desalting. This is because the primary electrodialysis tank primarily removes the salts in the crude oil that are suspended in it as well as those present in the water-in-oil emulsions, while the secondary electrodialysis tank mainly removes the salts contained within the emulsions. The water injection rate is generally 5% to 7%. (IV) Demulsifiers and demetalizing agents: Demulsifiers are one of the most critical factors affecting the desalination rate. In recent years, with the development of new oil wells, the impurities in crude oil have changed significantly, while the petroleum refining industry has increasingly higher requirements for the quality of distillates. In response to this situation, many new types of broad-spectrum, multi-functional demulsifiers have been developed, which are generally composite demulsifiers composed of two or more components. The amount of demulsifier used is generally 10–30 μg/g. To expand the electrodesalination capabilities of crude oil, a new type of demetallizing agent has been developed in recent years. Once it enters the crude oil, it can chelate certain metal ions, causing them to transfer from the oil phase to the water phase where they can then be removed. This demetallizing agent can achieve removal rates of 85.9%, 87.5%, and 74.1% for Ca2+, Mg2+, and Fe2+ in crude oil, respectively. The calcium content in the crude oil after treatment can drop to below 3 μg/g, meeting the requirements for the calcium level in feedstock oil for heavy oil hydrocracking. By reducing the conductive ions in crude oil, its electrical conductivity is lowered, which also reduces the power consumption required for desalination. (5) Electric field gradient: The greater the electric field gradient E, the greater f is. But there is a limit to increasing E. When E is greater than or equal to the critical electric field dispersion gradient, the water droplets are subjected to electric dispersion, causing the larger droplets that have already aggregated to disperse again, resulting in a decrease in the efficiency of dehydration and desalination. The actual strong electric field gradients used in current refineries in our country range from 500 to 1,000 V/cm, while the weak electric field gradients range from 150 to 300 V/cm.