Introduction to the process of crude oil electric desalting
1. Electric desalting process
The crude oil enters the device from the irrigation area, reaches the specified temperature through the heat exchanger, mixes with demulsifier and water and enters the dedesalting tank through the distribution tube from the bottom of the dedesalting tank, dehydrates through the weak electric field first, and then enters the strong electric field. At this time, the remaining water droplets in the crude oil further coalexes, and finally forms large water droplets, which continue to sink by the difference of oil-water density. It enters the clean water layer at the bottom of the desalting tank and is discharged out of the tank. The desalted crude oil flows out from the irrigation roof, and the desalting process of crude oil is completed.
The electrochemical desalination process of crude oil generally goes through three steps: First, water is injected into the crude oil to dilute and wash the salt in the crude oil. The second is that the crude oil is fully mixed with the injected water, and the brine, sediment and crystalline salt of the crude oil in the crude oil are fully diluted and dissolved. The third is to separate as much salt water as possible from the crude oil to achieve the effect of crude oil desalination.
In order to reduce sewage discharge and fresh water consumption, most crude oil waterflooding refineries in China use purified water after wastewater stripping. After heat exchange between the purified water and the cut water of primary desalination, it is generally used as the water injection of secondary electric desalination, and the cut water of secondary electric desalination is used as the water injection of primary electric desalination, and the cut water of primary electric desalination is cooled by the heat exchanger and sent to the crude water injection of the sewage treatment plant.
2. Main process parameters of crude oil electric desalting
The main process parameters of crude oil electric desalting can be divided into two categories. One refers to the adjustable parameters during the desalting operation, mainly including temperature, water injection, demulsifier model and injection amount, oil-water mixing strength, and electric desalting boundary, etc. These parameters have a significant impact on the electric desalting effect of crude oil. The other is the design parameters, which are related to the nature of the crude oil, the amount of crude oil processed and the equipment selected for the desalination plant. In actual operation, due to the certain properties of crude oil, the processing amount is certain, the desalting pressure, the residence time of crude oil in the electric field, and the electric field strength are also certain parameters, which can not be adjusted at will.
a Electric desalting temperature
The desalting temperature is an important control parameter in the electric desalting operation. The design control generally adopts the heat exchange between crude oil and other heat flow media. The temperature has a significant influence on the emulsion demulsification and water drop settling. According to Stokes formula: reducing the viscosity of oil phase, increasing the difference of oil-water density and increasing the diameter of water droplets can accelerate the settling speed of water.
In order to ensure the maximum sedimentation rate, the principle of desalting temperature selection is: the difference between oil and water density is as large as possible, and the viscosity of crude oil is as small as possible. When the desalting temperature increases, the viscosity of crude oil decreases and the movement resistance of water droplets decreases, which is conducive to the movement of water droplets. The increase in temperature also reduces the interfacial tension of oil and water droplets, and the expansion of water droplets under heat weakens the emulsion film, which is conducive to demulsification and agglomeration. In addition, the increase in temperature increases the Browns motion velocity and enhances the collision probability of water droplets. However, the increase of crude oil desalting temperature has a certain limit. When the temperature rises to a certain extent, the growth of water drop settling speed also begins to decline. In addition, the increase of crude oil temperature will also cause some adverse effects: the increase of temperature, the increase of crude oil conductivity, power consumption; The deterioration of the working environment of the electric insulation rod and the insulation hanging (polytetrafluoroethylene) is easy to cause the breakdown of the electric shock rod, which is not conducive to the stable operation of the long period of electric desalting; The increase of circulating water consumption increases the desalting temperature, resulting in the increase of desalting cut water temperature. In order to make the cut water reach the specified discharge temperature, the circulating water quantity must be increased accordingly. At the same time, the increase of the desalting temperature is also limited by the operating pressure of the desalting tank, under a certain operating pressure of electric desalting, the saturated steam pressure of water is certain, the desalting operating temperature must be lower than the vaporization temperature of water, otherwise it will cause a sudden boiling of water, affecting the normal operation of desalting, therefore, it is necessary to control the appropriate desalting temperature.
At present, the temperature of crude oil into the desalting tank designed by the atmospheric and vacuum pressure device is generally 120 ~ 140℃ (the best temperature), and the design temperature can be appropriately increased according to the production situation and crude oil density, which is conducive to the adjustment of the desalting temperature in the production process. When the density is 0.87 ~ 0.89g/cm3, the temperature can be controlled to the lower limit, when the density is 0.90 ~ 0.96g/cm3, the temperature can be controlled to the upper limit. In operation, the desalting temperature control should not only be stable, but also slow when adjusting, if the adjustment amplitude is faster, the feed temperature suddenly rises, resulting in the density of the crude oil entering the lower part of the desalting tank becomes smaller, causing the hot oil in the lower part to replace the cold oil in the upper part to form a "hot agitation".
3D stereoscopic section of an electric desalting tank
3D stereoscopic section of an electric desalting tank
b Washing water
The role of water injection is to dissolve inorganic salt and some organic matter in crude oil, and remove salt with the removal of washing water. Increasing the amount of water injected can improve the cohesiveness of water droplets, which is conducive to condensation of water droplets. At the same time, it can also improve the rate of salt removal.
Excessive water injection will affect the stability of the electric field, which is not conducive to the operation of electric desalination. The size of the water injection should be adjusted according to the nature of the crude oil, and the current water injection is generally controlled at 5% to 8% of the crude oil volume. Water quality requirements: ammonia nitrogen content ≯40mg/l, chloride content ≯300mg/l, PH value ≯8, the PH value of water injection is high, easy to produce emulsion, not only affect the desalting rate, but also cause desalting drainage with oil.
The location of water injection can be selected according to the salt content and water content of crude oil. For easily emulsified crude oil, the location of water injection point can be placed in front of the mixing valve. For crude oil with high salt content, it can be injected after the original oil pump, and the injection water is appropriately increased. The water injection point is located in the earlier design, and the primary water injection is generally at the entrance of the original oil pump, mainly to increase the difficulty of breaking milk. At present, most devices move the water injection point to the front of the desalination tank after the heat exchange system. However, there are also some devices that put the water injection point into the heat exchange system, can make the oil and water mix fully, can cancel the static mixer, and avoid the excessive mixing of the centrifugal pump, and the injection of higher temperature washing water (about 80 ° C) can reduce the scale of inorganic salts and suspended impurities in the heat exchanger, reduce the viscosity of crude oil, and improve the heat transfer coefficient of crude oil in the heat exchange system.
3. Demulsifier
Demulsifier is injected in the desalting process to destroy the oil-water emulsion film and prevent the formation of a demulsifier film outside the wash water droplets. The injection amount of demulsifier varies with the nature of crude oil, the requirements of desalting, and the process conditions of desalting. The injection amount of water-soluble demulsifier is generally 20 ~ 40 mg/l (the amount of crude oil), and the injection amount of oil-soluble demulsifier is 3 ~ 25 mg/l. Demulsifier is also a surfactant, if the injection amount is too large, it will also cause poor water quality of salt-containing sewage, and bring difficulties to the purification of sewage.
Selection characteristics of demulsifier: 1. It has strong surface activity. 2. It has good wetting performance. 3. Sufficient flocculation capacity. 4. It has good coagulation ability.
When the demulsifier is used, the water-soluble demulsifier should first be prepared into an aqueous solution of 1% to 2% in a special container, and the oil-soluble demulsifier should be used in the original commercial state and added to the crude oil with a special small-capacity metering pump.
4. Oil-water mixing strength
Mixing strength is an important parameter of electric desalting operation. The salt removal rate of crude oil depends to a large extent on the mixing degree of crude oil with washing water and demulsifier. Sufficient mixing can ensure the mixing degree of washing water and demulsifier. The salt water droplets in the crude oil can be diluted effectively, and the low mixing strength is difficult to ensure the desalting effect, and the mixing strength makes the emulsion layer too stable and difficult to break milk.
The electric desalting system can choose the optimal mixing strength according to the crude oil and the actual condition of the internal structure of the desalting tank, and improve the desalting effect. According to the actual production data, when processing low density crude oil (°API 15 ~ 24), the mixing valve pressure difference △P is 30 ~ 80kpa; When processing high density crude oil (°API 15 ~ 24), the mixing valve pressure difference △P is 50 ~ 130kpa.
5. Electric field strength
High electric field intensity can improve the condensation force between small water droplets, which is conducive to electric desalination, but beyond a certain range, increasing the electric field intensity has little effect on improving the desalination rate.
At present, the full impedance adjustable transformer used by the domestic electric desalting device has 4 to 5 gear adjustable output voltage, and the electric field strength can be adjusted by changing the output voltage according to the needs of the operation. Generally, the electric field strength of the weak electric field area is 0.3 ~ 0.4KV/cm, and the electric field strength of the strong electric field area is 0.7 ~ 1.0KV/cm.
6. Residence time of crude oil in electric field
The residence time of crude oil and water in the tank determines the production efficiency of the electric desalting tank, and affects the effect of crude oil desalting and dehydration and the amount of oil contained in the drainage. The residence time of crude oil in the electric field is an important parameter affecting the coalescence of water droplets. It is generally considered that the residence time of crude oil in the electric field is about 2min.
7. Electric desalting threshold
The residence time of crude oil in the tank not only determines the rising speed of crude oil in the tank, but also determines the position of the oil-water interface, so the control of the oil-water interface is also a very important process operation parameter. The oil-water interface of the desalting tank is the transition area of oil-water contact, which must be kept below a certain position of the plate under the desalting tank. The boundary level is too low, which is easy to cut the water, and the higher boundary level can improve the strength of the weak electric field, thereby improving the desalting effect, but the boundary level is too high, which is easy to bring water to the crude oil, causing the desalting current fluctuation resulting in the desalting transformer tripping, so in the actual operation process, The proper desalting boundary should be controlled.
The measuring instruments of desalting boundary are resistance type, capacitance type, inner buoy type, micro differential pressure type, radar type and so on. At present, internal float type and micro-differential pressure type are more suitable. Through the measurement of the threshold, the displacement is automatically controlled to control the level of the threshold.
