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This post was last edited by LQ198619 on 2017-5-22 at 17:26. Chapter 1: Basic Knowledge of Atmospheric and Vacuum Distillation Units. I. Overview of the Unit: The atmospheric and vacuum distillation unit is the first step in crude oil processing, that is, the initial processing of crude oil. Generally, after being processed through such units, crude oil can be converted into products such as naphtha, kerosene, light diesel, heavy diesel, and wax oil. Certain crude oils rich in gums and asphaltenes can also be directly used to produce road asphalt after vacuum deep distillation. Another main function of the atmospheric and vacuum distillation unit is to provide high-quality feedstock for downstream secondary processing units. Due to the large volume of crude oil processed by atmospheric and vacuum distillation units, these units are large in scale and consume a significant amount of energy; their energy consumption typically accounts for around 20% to 30% of the total energy consumption of a refinery. Electrodesalination of crude oil, as a pretreatment process for crude oil, removes inorganic salts, water, and mechanical impurities from it. It plays a crucial role not only in ensuring the stable operation of the facility, reducing energy consumption, and preventing equipment corrosion, but also in determining the properties of the feedstock for downstream units, the quality and distribution of the products, as well as in preventing equipment corrosion and blockages. A atmospheric and vacuum distillation unit typically consists of six components: the electrodesalination section, the initial distillation section, the atmospheric distillation section, the vacuum distillation section, the crude oil heat exchange network, and the light hydrocarbons recovery section. II. Process principle: Atmospheric distillation refers to the process of \"separating\" crude oil into different fractions under normal pressure, based on the varying boiling points of its various components. Vacuum distillation is a process installed in oil refining units after atmospheric distillation. Crude oil is a complex mixture with a wide boiling range; for most crude oils, the fraction with a boiling point of 350–500°C accounts for about 50% of the total distillates. Oils are prone to thermal decomposition when heated, which causes their color to darken and the amount of gum to increase. When producing gasoline and diesel, the temperature at the outlet of the furnace is generally not higher than 370 °C; the amount of product obtained under normal pressure corresponds to the total amount distilled at an actual boiling point of 350–370 °C. The fractions at 350–500 ℃ are difficult to evaporate under atmospheric pressure, yet these fraction oils are the main raw materials for producing catalytic cracking feedstock. Since the vapor pressure of oils decreases as temperature drops, or in other words, as the pressure in the boiling system decreases, these heavy fractions can be distilled under conditions of 2.67–8.0 kPa and 380–400 °C. Reduced-pressure wax oil is the product under these operating conditions. III. Principles of distillation and the conditions for it: 1. Distillation: The process of heating a mixture to cause the lighter components with lower boiling points to vaporize and condense, thereby achieving a rough separation, is called distillation. 2. Distillation: The process of precisely separating various components by making use of both partial vaporization and partial condensation, simultaneously and multiple times, is called distillation. Distillation is based on the significant differences in the volatility of various components within a liquid mixture [volatility refers to the tendency of each component in a liquid mixture to vaporize], that is, the different boiling points of these components. The essence of distillation is the multiple condensation of the vapor phase and the multiple vaporization of the liquid phase to facilitate mass and heat transfer. Conditions: 1. There must be a location where the vapor and liquid phases can come into full contact, namely trays and packing. 2. There must be vapor reflux and liquid reflux provided to the distillation tower. 3. There must be a temperature difference and a concentration difference between the vapor and liquid phases in contact. 4. On each tray, both vapor and liquid phases must be present simultaneously and in full contact. IV. Typical flow diagram for atmospheric and vacuum distillation V. Raw materials and products Raw materials: Crude oil refers to unprocessed petroleum extracted from underground; it is usually a viscous liquid that ranges in color from light yellow to black, is fluid or semi-fluid in nature, and has a strong odor. Its density is generally less than 1000 kg/m3, typically between 0.85 and 0.95 kg/m3, although the properties of crude oil vary to varying degrees depending on the region where it is produced. The composition of crude oil is not only a mixture of various hydrocarbon and non-hydrocarbon compounds, but also a mixture of components with different boiling points. Those with a relatively low molecular weight, lower boiling points, and ease of vaporization are called light fractions ； Those with a higher molecular weight, higher boiling points, and lower volatility are referred to as heavy fractions. Through heating distillation, the composition can be divided into fractions with different boiling point ranges. Boiling point 500°C; > C36 is the residue fraction. The elemental composition of crude oil consists mainly of carbon and hydrogen; it also contains certain amounts of elements such as nitrogen, oxygen, and sulfur. The carbon and hydrogen content in crude oil generally accounts for over 94%, with carbon making up 83% to 87% of the crude oil ； H accounts for 10%–14% ； The levels of other elements are below 6%; generally, sulfur accounts for 0.05% to 6%, nitrogen for 0.02% to 2%, and oxygen for 0.05% to 2%. Metal elements are usually below 1%. Although these elements are present in small amounts, they, together with hydrocarbons, have a tremendous impact on petroleum processing in all non-hydrocarbon compounds that are formed. The distillation composition of crude oil: Crude oil is a complex mixture composed of various types of hydrocarbons and non-hydrocarbon compounds, with molecular weights ranging from several dozen to several thousand; as a result, its boiling point range is also wide, from room temperature up to over 500°C. Before studying and processing crude oil, it must be separated into several fractions based on their boiling points using distillation; the boiling point range of each fraction is referred to as its boiling range. Distillates do not represent petroleum products per se; rather, based on their boiling range, they may serve as raw materials for producing gasoline, kerosene, diesel, and lubricants. These distillates usually require appropriate processing in order to produce products that meet the relevant quality specifications. l The hydrocarbon composition of crude oil: The main component of crude oil is hydrocarbons. Based on their structure, they can be divided into alkanes, cycloalkanes, aromatics, and hydrocarbons formed by combinations of these three types. Light fractions are rich in alkanes and cycloalkanes, while the content of aromatic hydrocarbons is low. The heavy distillates contain a high proportion of solid long-chain hydrocarbons, and the content of cyclic hydrocarbons, particularly polycyclic naphthenes and naphthoaromatics, increases. Product: The atmospheric and vacuum distillation unit can separate various products with different boiling points from crude oil, as well as raw materials for further processing. When a primary distillation tower is used, a narrow-range reforming feed or gasoline components can be obtained from the tower top. The products that can be produced by an atmospheric tower include: gasoline components at the top of the tower, reforming feedstock, and naphtha ； Jet fuel (aviation kerosene), lamp kerosene, solvent oil, and raw materials for ethylene cracking are commonly produced on this line ； Light diesel and cracking feedstock are often produced in the secondary line ； Heavy diesel or lubricant base oil is often produced in three streams ； Normal residue is obtained from the bottom of the atmospheric pressure tower. The products that can be produced by the vacuum distillation tower include: heavy diesel oil that can be obtained from the first distillation stage ； The properties of the crude oil and the requirements for its use in the other side streams allow it to be used as a feedstock for catalytic cracking, hydrocracking, lubricant base oils, and paraffin production ； Vacuum residue can be used as a feedstock for delayed coking, solvent deasphalting, oxidative asphaltification, and visbreaking, or it can be directly used to produce asphalt or components for fuel oil blending. VI. Functions and Principles of the Main Equipment in the Atmospheric and Vacuum Distillation Unit. The main equipment of an atmospheric and vacuum distillation unit includes: electrical desalination units, heating furnaces, fractionation towers (flash towers, atmospheric towers, stripping towers, vacuum towers), pumps, and heat exchange equipment. 1. Electrodesalting tank: Crude oil contains water, in which salts are dissolved; it also contains natural emulsifiers such as gums and asphaltenes. Due to the intense disturbances that occur during extraction and transportation, water becomes dispersed in the crude oil in the form of tiny droplets. The emulsifiers present in the crude oil accumulate at the oil-water interface through adsorption, forming a stable emulsion. It is therefore necessary to carry out separation and removal processes in an electrodesalting tank. Crude oil electrodialysis involves adding a demulsifier and a certain amount of water to dissolve the salts present in the oil. The demulsifier breaks down the stability of the emulsion in the crude oil. The crude oil, which has been heated to a specified temperature, is then fed into an electrodialysis tank. Under the influence of a high-voltage electric field, the tiny water droplets in the crude oil aggregate into larger droplets. Thanks to the difference in density between oil and water, these water droplets settle and separate from the oil, allowing the salts and water to be removed from the crude oil. 2. Cooling equipment: The products coming out of the atmospheric and vacuum distillation columns are at high temperatures, and they must be cooled to a safe temperature before they can be discharged from the plant. The crude oil coming from the tank farm has a lower temperature, and it also needs to be heated before it can enter the towers and furnaces for further heating and processing. Hot products need to be cooled, while cold crude oil needs to be heated; by exchanging their respective hot and cold temperatures, that is the function of heat exchange equipment. 3. Heating furnace: Normal and reduced pressure distillation takes place during the vaporization and condensation of oil products, and the function of the heating furnace is to provide heat for the vaporization of these oil products. Provides a stable amount of vaporization and heat for the distillation process. 4. Flash tower (some systems use a primary distillation tower): The difference between the two is that in the flash process, the liquid mixture is heated to partial vaporization; after passing through a pressure reducing valve, it enters a vessel (flash tank or evaporation tower), where, under specific temperature and pressure conditions, the vapor and liquid phases are rapidly separated to yield the corresponding vapor and liquid products. Therefore, the flash tower has no trays; the vapor from its top enters the upper part of the atmospheric tower, where there are no condensation or reflux facilities. The distillation column has trays, with the product being obtained from its top; there is reflux, and some of these columns even have side streams (primary side stream). The design basis for both is that fractional distillation towers are used for reformation feedstocks derived from crude oils with high arsenic content. In distillation units for producing chemical products, a flash tower is generally installed. 5. Atmospheric tower: The atmospheric tower can be considered the core of the entire distillation process. Crude oil is fractionated in a distillation tower through heat and mass transfer, ultimately resulting in the separation of crude oil into various products. The function of an atmospheric tower is to separate certain components from crude oil under conditions close to atmospheric pressure, in order to obtain products such as gasoline, kerosene, and diesel. 6. Vacuum distillation tower: A vacuum distillation tower is a device that utilizes pressure reduction to carry out the distillation process, based on the relationship between the boiling point of oils and pressure. 7. Stripping Tower: The purpose of the stripping tower is to use direct steam stripping or indirect heating on the side products distilled from the atmospheric tower, in order to remove the low-boiling-point components from these side products and ensure that their flash points and boiling ranges meet the specified requirements. The commonly used stripping method is direct stripping with water vapor at a higher temperature than the side-stream extraction temperature. VII. Basic operating conditions: Three types of equilibrium that need to be understood in the operation of a distillation tower. Material balance: This refers to the fact that the amount of material entering the tower per unit time should be equal to the sum of the amounts of material leaving the tower. Material balance reflects the production capacity of the tower, which is primarily adjusted based on the feed rate and the outlet rates at the top and bottom of the tower. l Vapour-liquid equilibrium: It primarily reflects the quality of the product as well as any losses that occur. This is achieved by adjusting the operation of the tower [temperature, pressure] and the vapor-liquid contact conditions on the tray. l Heat balance refers to the equilibrium between the heat entering the tower and the heat leaving it. This is reflected specifically in the tower top temperature; heat balance is the basis for achieving material balance as well as vapor-liquid phase equilibrium. The three balances influence and restrict each other; in operation, it is necessary to focus on controlling the material balance and adjust the heat balance accordingly, with the ultimate goal of achieving vapor-liquid phase equilibrium.