It is the first process of crude oil processing and a pretreatment process.
It is an essential crude oil pretreatment process for the supply of high quality raw materials for equipment plants.
It is an important process for refineries to reduce energy consumption, reduce equipment scaling and corrosion, and prevent catalyst poisoning.
(1) Purpose of pretreatment
The oil extracted from the underground oil layer is accompanied by water. These waters are dissolved with inorganic salts such as NaCl, MgCl2, CaCl2, etc. In the oil field, the crude oil is dehydrated and stabilized, and most of the water and salt in the water can be removed. However, some water cannot be removed because the water is present in the emulsified state in the crude oil. The salt content of the crude oil will bring harm to the transportation, storage, processing and product quality of the crude oil.
Excessive water content of crude oil will cause unstable operation of the distillation column, and even cause accidents in the tower when it is severe. The water content increases the heat energy consumption and increases the load of the cooler and the consumption of cooling water.
(2) The salts in crude oil are generally dissolved in water, and the presence of these salts is very harmful to the processing. Mainly manifested in:
- In the heat exchanger and heating furnace, as the water evaporates, the salt deposits on the pipe wall to form salt scale, which reduces the heat transfer efficiency, increases the flow pressure drop, and even blocks the pipeline and causes shutdown.
2, causing equipment corrosion. Hydrolysis of CaCl2 and MgCl2 to form highly corrosive HCl: MgCl2 + 2H2O Mg(OH)2 + 2HCl If the system has sulfides, the corrosion will be more serious. Fe + H2S FeS + H2 FeS + 2HCl FeCl2 + H2S
- When the salt in crude oil is distilled, most of it remains in the residue and heavy fraction, which will affect the quality of petroleum products. According to the above reasons, at present, domestic and foreign refineries require that the crude oil has a water content of 0.1% to 0.2% and a salt content of <5 mg/L to 10 mg/L before processing.
(3) Basic principles
Most of the salt in the crude oil is dissolved in the water, so desalting and dehydration are carried out simultaneously. In order to remove the salt particles suspended in the crude oil, a certain amount of fresh water is injected into the crude oil (the injection amount is generally 5%), and the mixture is thoroughly mixed, and then the micro water droplets are gradually aggregated under the action of the demulsifier and the high-voltage electric field. Large water droplets are separated from the oil by gravity to achieve the purpose of desalination and dehydration. This is usually called electrochemical desalination and dehydration process.
When the crude oil emulsion passes through the high-voltage electric field, an induced charge is formed on the water droplets of the dispersed phase, and the water droplets with positive and negative charges collide with each other to form large water droplets and accelerate the sedimentation. The larger the diameter of the water droplet, the greater the difference in relative density between crude oil and water. The higher the temperature, the smaller the viscosity of the crude oil and the faster the sedimentation rate. Among these factors, the difference between the diameter of the water droplets and the relative density of the oil and water is the key. When the diameter of the water droplets is so small that the falling speed is lower than the rising speed of the crude oil, the water droplets cannot sink, and as the oil floats, the purpose of sedimentation and separation is not achieved.
(4) Process
Most refineries in China use a two-stage desalination and dehydration process. After the crude oil is pumped out from the oil tank, it is mixed with fresh water and demulsifier in proportion, heated to the specified temperature, and sent to the first-stage desalting tank. The desalination rate of the first-stage electric desalting is between 90% and 95%. Before desalination, fresh water is still injected. The first water injection is to dissolve the suspended salt particles. The secondary water injection is to increase the amount of water in the crude oil to increase the dipole coalescence force of the water droplets.
(5) Type of electric desalination
- a. AC desalination technology
Generally it is a horizontal electrode plate. The electric desalting tank consists of 2 or 3 electrode plates. The upper or middle layer transmits electricity to form a strong electric field region, and the middle and lower layers constitute a weak electric field region.
Typical equipment: Maoming 1#, 3# distillation unit. Changling’s 1# distillation unit.
- b. AC/DC desalination technology
Generally it is a vertical electrode plate. In its transformer plus rectification equipment, the electric field region consists of an AC weak electric field and a DC weak electric field DC strong electric field.
Typical device: Maoming 2# distillation unit. Takahashi 1# distillation unit.
- c. Squirrel cage high efficiency electric desalination technology
Generally, it is an AC type. The electrode plate adopts an axial squirrel cage structure, which consists of a weak electric field, a filtered electric field, and a strong electric field. The crude oil passes through three electric fields in sequence, and is desalted and dehydrated.
Typical equipment: Tahe Branch 4# distillation unit.
- d. High-speed electric desalination technology
Through the specially designed oil inlet distributor, the crude oil directly enters the electric field from the nozzle, and the oil inlet mode is the oil phase oil. The water discharged from the high-speed electric desalting tank settles downward, the oil flow moves upward, and the residence time in the tank is greatly shortened. Three horizontal electrode plates are arranged in the tank to form two strong electric fields. Compared with conventional electric desalting, the treatment capacity can be increased by 75-100%, and the salt rejection rate can reach over 95%.
Typical equipment: Zhenhai 3#, Shanghai Petrochemical distillation unit.
(6) Process parameters and operation control indicators for electric desalination control
- Demulsifier selection and injection volume
- Crude oil inlet temperature 120—140°C
- Crude oil injection volume 5—8%
- Oil-water mixing intensity 30—80 kPa
- 50—130 kPa
- Electric field strength 500-1000 K V/CM2
- 150-300 K V/CM2
Electrical desalination operation control indicators:
- Salt after removal: 3 mg ∕ l
- Sewage oil 200ppm=200g/t
