The crude oil electric desalination unit of Luoyang Petrochemical Branch is designed as a 2-channel 3-level AC desalination system, with 3 full impedance transformers installed on each tank. The tank is equipped with 3 layers of horizontal plates, with the middle plate connected to electricity and the upper and lower plates connected to ground. After the sewage from the electric desalination tank is collected, it undergoes heat exchange in the heat exchanger and enters the downstream unit. The properties of crude oil processed by Unit 1: The crude oil processed by Luoyang Petrochemical Branch is a mixture of multiple components, which poses certain difficulties to the production process. In January and February 2016, there were significant changes in the types of crude oil, as shown in Table 1. As shown in Table 1, with the change of crude oil types, there is a significant fluctuation in processing volume, which poses difficulties for refining and processing, resulting in the oil content of electric desalination wastewater exceeding the standard. On February 9, 2016, it reached 357 mg/L (the target requirement is that the oil content should not exceed 100 mg/L). For this purpose, process adjustments were made to ensure that the oil content of the electric desalination wastewater meets the qualified requirements. Due to the stable properties of Tahe crude oil processed by the flash evaporation system in February, with no changes in type and processing volume, the main focus of the month was on optimizing the main flow system. The most significant changes in the types of crude oil processed in January and February were Changqing crude oil, Abu Dhabi Kumu crude oil, and Hango crude oil. The main properties of these three types of crude oil are shown in Table 2. According to Table 2, the characteristics of Changqing crude oil are low density, low viscosity, and low heavy metal content, making it a light, low sulfur paraffin based crude oil; Hango crude oil belongs to sulfur-containing intermediate base crude oil; The density of Abu Dhabi Kumu crude oil is relatively low, with high nickel and vanadium content, and it belongs to intermediate base crude oil. 2. Electric desalination mechanism
95% of crude oil belongs to stable water in oil emulsions. Natural emulsifiers such as cycloalkanoic acid, asphaltene, and gum in crude oil move towards the oil-water interface and cause a decrease in surface tension of the oil phase, thereby stabilizing the system. With the extension of time and the influence of transportation conditions, the emulsion film at the oil-water interface thickens, increasing the stability of the emulsion, which increases the difficulty of electric desalination. In order to disrupt this stable emulsion, various factors such as chemicals, electric fields, and gravity are usually relied upon to ultimately cause water droplets to coalesce and settle, achieving the goal of oil-water separation.
The density difference between crude oil and water is the driving force for sedimentation separation, while the viscosity of the dispersing medium is the resistance. The settling separation of immiscible liquids of oil and water conforms to Stokes' law of free settling of spherical particles in stationary fluids. Gravity settling is the basic method for separating oil and water. The density of saltwater droplets in crude oil is different from that of oil, and they can be settled and separated by heating and standing.
Analysis of the reasons for exceeding the standard of 3 electric desalination processes
3.1 Impact of Density: The density of Hango crude oil is relatively high, while the density of Changqing crude oil is comparable to that of Abu Dhabi crude oil. The density of water at 20 ℃ is 998.2 kg · m-3. Comparing the three types of crude oil, it can be seen that the density difference between oil and water decreased in February 2016. The viscosity of Changqing crude oil and Hango crude oil is higher than that of Abu Dhabi Kumu crude oil. The density difference between crude oil and water is the driving force for sedimentation separation, while the viscosity of the dispersing medium is the resistance. The settling velocity of water droplets in oil follows Stokes' law, as shown in equation (1): u=(d2 × △ r × g) (/18 × r × ρ oil). In equation (1), u represents the settling velocity, m/s; D - droplet diameter, m; △ r - oil-water specific gravity difference; R - crude oil viscosity, m2/s; G - local gravitational acceleration, m/s2, ρ oil - crude oil density, kg˙m-3。
From equation (1), it can be seen that the density difference between oil and water decreases, the viscosity of crude oil increases, and the settling speed of water droplets in crude oil is reduced, which is unfavorable for the electric desalination operation and affects the desalination effect.
3.2 Effects of Salt Composition in Crude Oil: According to Table 2, the chloride and metal contents in crude oil indicate that the nickel and vanadium contents in Hango crude oil are relatively high. An increase in metal content can easily damage the performance of catalysts and demulsifiers, increase the degree of emulsification in electric desalination, and lead to a deterioration in the operation of electric desalination transformers, thereby reducing the effectiveness of electric desalination. And as the degree of emulsification increases, it is necessary to lower the oil-water interface operation, which increases the oil content of the electric desalination wastewater. 3.3 The influence of process parameters in electric desalination operation 3.3.1 Selection of demulsifiers and additives: The function of demulsifiers is to destroy the emulsion film at the oil-water interface, reduce the resistance of water droplet coalescence, and accelerate the oil-water separation speed. In order to adapt to different characteristics of crude oil, Luoyang Petrochemical Company screens demulsifiers in advance when there are significant changes in crude oil characteristics. In February 2016, the main process emulsifier used was FC9300 emulsifier manufactured by Luoyang Huagong Industrial Co., Ltd. However, the current excessive oil content in electric desalination wastewater indicates that the demulsification performance of the demulsifiers currently used may be poor. Therefore, it is necessary to consider screening for more efficient new oil soluble demulsifiers. In response to the high metal content in crude oil, non-metallic additives can be added as an auxiliary to improve the electric desalination effect. 3.3.2 Electric desalination temperature: Temperature is an important factor affecting the electric desalination of crude oil. As the temperature increases, the viscosity of crude oil decreases, the density difference between oil and water increases, the interfacial tension between oil and water weakens, thermal motion accelerates, and the chance of emulsified water droplets colliding increases, promoting water droplet coalescence and sedimentation. However, an increase in desalination temperature can also bring negative effects, as the oil-water interface decreases, leading to increased electrical dispersion and power consumption. For water-soluble emulsifiers, the demulsification effect sharply decreases when reaching the cloud point temperature. To solve the problem of excessive oil content in electric desalination wastewater, the voltage should be appropriately increased to raise the temperature inside the tank by about 5 ℃; After the oil content in the sewage is qualified, in order to ensure the smooth operation of the device and lower the temperature, the average temperature of the main flow electric desalination tank is 133 ℃ during the adjustment process. 3.3.3 Water injection volume and oil-water interface: The purpose of water injection is to wash and dilute the saline water droplets in the crude oil, and to separate the saline water in the desalination tank. The oil-water interface is controlled smoothly, which can ensure stable desalination operation. Improving the oil-water interface can prolong the residence time of saltwater in the tank and reduce the amount of oil carried by wastewater. But it also reduces the residence time of crude oil. The water injection volume should not be too large, otherwise it will cause the water retention time in the desalination tank to be too short, affecting oil-water separation. An excessively high boundary position can cause a short circuit in the electrode rod of the electric desalination tank, resulting in an increase in temperature inside the tank. An excessively low boundary position can affect the efficiency of desalination and oil removal. (1) Water injection volume. On February 6, 2016, after the desalination wastewater contained too much oil, the water injection volume gradually increased, as shown in Table 3.
In order to ensure that the sewage is qualified, we will focus on increasing the water injection volume of the third level electric desalination tank. After March, the properties of crude oil remained stable, and the oil content in the electric desalination wastewater was qualified. In order to ensure economic benefits, the injection volume was slightly reduced after the electric desalination operation was stable.
(2) Oil water interface. To reduce the emulsification degree of electric desalination and ensure the normal operation of electric desalination transformers, the oil-water interface of electric desalination should be appropriately lowered after the oil content in the sewage exceeds the standard, as shown in Table 4. 3.3.4 Mixing strength: Mixing strength reflects the degree of oil-water mixing. Overall, the higher the mixing strength, the better the mixing effect. But when the mixing strength is too high, it will cause the diameter of water droplets dispersed in the oil to decrease, resulting in emulsification and affecting the oil-water separation effect. At present, the control index value for the mixing strength of electric desalination in atmospheric and vacuum distillation units should be between 20 and 150 kPa. Experience has shown that the best electric desalination effect is achieved when the pressure difference is controlled between 80 and 100 kPa. When the electric desalination wastewater is unqualified, the mixed pressure difference of the three-stage electric desalination tank is reduced. Considering that the crude oil properties of the first stage electric desalting tank are more complex than those of the second and third stage electric desalting tanks, the mixing strength of the second stage is slightly higher than that of the first stage, as shown in Table 5. The implementation effect has achieved good results through a series of adjustment measures, and the changes in oil content in sewage are shown in Figure 1. From Figure 1, it can be seen that on February 6, 2016, the oil content in desalinated wastewater began to exceed the standard. It reached its peak on February 9th and then began to decline, indicating that the adjustment has achieved good results. The oil content in the sewage fluctuated in late February, and by March, it had reached the qualified standard and fluctuated at lower levels. To reduce the oil content in the wastewater of the electric desalination unit, comprehensive adjustments need to be made from multiple aspects. In February 2016, when the oil content in the electric desalination wastewater exceeded the standard, adjustments were mainly made to the injection volume, oil-water interface, and mixing strength. After half a month of adjustment, the oil content in the wastewater from the electric desalination unit has reached the qualified standard. Reducing environmental pollution, while also reducing the load on downstream devices and lowering their production costs.
