Fundamental

Atmospheric distillation and vacuum distillation are both physical processes. The desalinated and dehydrated mixed feedstock oil is heated in the distillation tower. According to its boiling point, it is divided into oil products with different boiling points from the top to the bottom of the tower, that is, the distillate. Some of these distillate oils are blended and added with additives and then shipped as products. Most of them are used as raw materials for secondary processing devices. Therefore, atmospheric and vacuum distillation is also called primary processing of crude oil.　

Process overview

Refining atmospheric and vacuum equipment is the first process of crude oil processing. Crude oil is separated into a variety of oil products and raw materials for downstream processing equipment through distillation. The control system and operation level of the atmospheric and vacuum unit have a great influence on the product quality and yield of the refinery and the effective utilization of crude oil.

1. Electric desalination: The salts contained in crude oil will deposit on the process pipes, heating furnace tubes and heat exchanger tube walls during processing to form salt scale, which makes heat transfer difficult and increases fuel consumption. The presence of salt can also cause corrosion, which can lead to corrosion and perforation, oil leakage and cause fires. It can also contaminate the catalyst in the secondary processing and shorten the life of the catalyst.

2. Crude oil distillation:

A. The crude oil distillation plant is generally equipped with an initial distillation tower or a flash distillation tower before the atmospheric fractionation tower. It is to steam out the light oil that has been vaporised during the heat exchange and heating process of the crude oil in time, so that it no longer enters the atmospheric heating furnace. In order to reduce the heat exchange load of the heating furnace and the operating pressure drop of the crude oil heat exchange system. Thereby saving device energy consumption and operating costs. The light gasoline fraction of the top product of the initial distillation tower is used as the raw material for catalytic reforming.

B. Atmospheric towers are set up with 3 to 4 side lines to produce gasoline, solvent oil, kerosene, aviation kerosene, light diesel, heavy diesel and other products or blending components.

C. Catalytic cracking or hydrocracking raw materials are sent from the side of the vacuum tower, the product is relatively simple, and the fractionation accuracy is not high.

D. Decompression towers are generally operated as “wet” or “dry” operations (that is, the decompression tower section and the decompression furnace tube do not or less steam).

3. Fractionation tower: Fractionation tower is the core equipment in the crude oil distillation process. The process conditions mainly include the temperature and pressure of the fractionation tower, that is, the reflux ratio. The flash pressure of the tower is determined by the pressure at the top of the tower and the total pressure drop above the flash section. The pressure of the atmospheric tower is determined by the pressure of the top of the tower and the condensing system. The pressure at the top of the decompression tower is mainly determined by the capacity of the evacuator. Regardless of the atmospheric pressure tower or the vacuum tower, the reduction of its flash pressure means that the temperature of the furnace outlet can be lowered under the same gasification rate, thereby reducing fuel consumption. The pressure above the flashing section is reduced, and the relative volatility between the side cuts increases, which is beneficial to the separation of the side cuts. General optimization control is written around atmospheric towers.

4. Heating furnace: For the heating furnace for crude oil distillation, it is required that the maximum oil temperature in the furnace tube should be lower than the temperature at which the oil starts to crack under the specified gasification rate. (Especially for the decompression heating furnace) Generally, the expansion of the gasification section furnace tube is taken from the equipment point of view, so that the maximum oil temperature in the furnace tube does not exceed 400°C. The control difficulty is the balanced control of the branch outlet temperature.

Crude oil distillation process is basically a physical process of gas-liquid conversion, so all control mechanisms should follow the basic principles of material, heat balance, phase balance, mass and heat transfer.

1. Balanced control of the outlet temperature of the heating furnace branch Atmospheric furnace feed is generally divided into several branches. After adopting DCS, we can conveniently use FDB to implement balanced control of the outlet temperature of the heating furnace branch, which can reach no more than 1°C, and less than 0.5°C at the best.
2. Heating furnace combustion control When heating furnace outlet temperature, furnace negative pressure, flue gas content and other variables are independently adopted, after DCS is adopted, the control method of feed forward and feedback is used; the feedback adjustment object selects the thermal efficiency of the heating furnace, and the execution means adopts air volume adjustment. The feed forward system uses single parameter feedback, and the interference source selects the fuel pressure.
3. The closed-loop quality control of the fractionation tower The closed-loop quality control of the fractionation tower is based on the known real boiling point curve of crude oil and the boiling point curve of each sideline product of the tower, the temperature and pressure of each part of the tower, and the real-time parameters of the flow of each incoming and outgoing tower. Through material balance and thermal balance, the reflux and composition of the gas and liquid phases of each section of the tower are calculated, and then the dry point and initial boiling point of the extractable line product are calculated, and the temperature value is used as the temperature control setting value.
4. Atmospheric tower multi-variable intelligent control. This control function is operating smoothly, overcoming the disturbance caused by the change of crude oil, overcoming the disturbance of the furnace outlet temperature and improving the product quality. The controlled variables: constant top temperature, constant first-line gas phase temperature, and constant third-line gas phase temperature control variables: Constant top cold reflux flow rate, constant first line extraction volume, and normal third line extraction volume can be measured disturbance variables: heater outlet temperature, tower top pressure 5` over gasification rate is controlled in crude oil distillation, in order to ensure the yield and quality of distillate oil, There must be a certain over-gasification rate, but if the over-gasification rate is too large, it will increase the energy consumption of the device. Usually, the over-gasification rate is 2% to 3%. The control method can be closed-loop control through DCS, or open Ring guidance. Using DCS to implement automatic control of atmospheric and vacuum equipment has brought good economic and social benefits to the device. From two aspects: on the one hand, it uses intelligent control and complex control, improves product quality, and improves the extraction rate of light oil. On the other hand, because it replaces conventional instruments, it reduces the intermediate failure rate and improves the safe operation cycle of the equipment. It reduces the number of breakdowns and increases the production time, which also constitutes another aspect of creating benefits.