【2026 Vertical Equipment】Vacuum distillation must be employed for atmospheric and vacuum distillation; heavy oils cannot be distilled directly under atmospheric pressure
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In all atmospheric and vacuum distillation units, the process flow features an atmospheric tower first and a vacuum tower afterward; this has been the standard configuration in the oil refining industry for decades. Many people only know that vacuum distillation towers are used to produce wax oil and residue, but they never truly understand a key question: why must heavy oils be distilled under vacuum, rather than directly at atmospheric pressure?
I. The inherent limit of atmospheric distillation: the temperature cannot be increased, and heavier fractions cannot be separated. Crude oil is a complex mixture of hydrocarbons with a very wide boiling range. For the vast majority of crude oils in the country, heavy fractions in the range of 350°C to 500°C account for a very high proportion, approaching 50% of the total distillates. These components are core high-quality resources for producing lubricant base oils, catalytic cracking feedstocks, and hydrogenation feedstocks, and they possess extremely high value. But under normal pressure conditions, it is simply impossible for us to vaporize this fraction, and there is only one reason for that: once the temperature rises, the oil will crack and deteriorate.
1.1 Red line for the actual operating temperature of the unit: Under conditions of producing gasoline and diesel, the outlet temperature of the atmospheric pressure furnace shall not exceed 370°C. Under conditions of jet fuel production, the outlet temperature of the atmospheric pressure furnace does not exceed 365°C.
1.2 Severe consequences of overheating Once the specified temperature is exceeded, heavy oil products undergo thermal cracking and thermal condensation reactions. The color of the oil changes rapidly to a darker, dirtier shade. Gum and asphaltenes are produced rapidly in large quantities. The light components emerge in a disordered manner, and the separation of components is chaotic. Furnace tubes, tray plates, and heat exchangers suffer from accelerated coking, thereby shortening the operational life of the plant.
In other words, the temperature of the atmospheric pressure furnace cannot be increased any further; the stripping capacity of the atmospheric pressure column is restricted by temperature, allowing only the light and medium components at temperatures up to 350–370°C to be stripped out. A large amount of heavy, effective components at temperatures above 370°C are all trapped in the heavy oil at the bottom of the atmospheric pressure tower, making effective separation impossible.
II. The core principle of vacuum distillation: reducing pressure to lower the boiling point and avoid decomposition temperatures. In the basic principles of chemistry, hydrocarbon oils have a key characteristic: their boiling point drops significantly as the system pressure decreases; the lower the pressure, the lower the temperature at which vaporization occurs.
2.1 Process conditions under reduced-pressure operation: Taking advantage of this characteristic, the vacuum tower system is deliberately made to operate under negative pressure, with the pressure inside the tower maintained at a high vacuum level of 2.67–8.0 kPa. Under high-vacuum conditions, heavy components that normally have a boiling point of around 500°C at atmospheric pressure can be easily boiled, vaporized, and separated at only 380–400°C.
2.2 Key advantages of vacuum distillation: It enables the safe and complete extraction of heavy oils, which cannot be distilled at normal pressure and high temperatures, within a safe range that does not exceed the cracking temperature of the oil. It prevents high-temperature pyrolysis and degradation as well as coking and carbon deposition, while maximizing the overall recovery rate of crude oil.
III. The three core production objectives of vacuum distillation 3.1 Preventing thermal cracking of oils to ensure product quality and the long-term operation of the facility. High temperatures at atmospheric pressure can damage the oils and cause coking; high vacuum lowers the vaporization temperature, allowing heavier fractions to vaporize at lower temperatures without cracking or deteriorating. It ensures that the vacuum distillate wax oil has a normal color and stable composition, providing qualified raw materials for subsequent catalytic, hydrogenation, and lubricant production processes.
3.2 Maximizing the value of crude oil and improving the overall recovery rate of the processing unit: Without a vacuum system, the large quantities of high-quality fractions at 350–500°C obtained from the bottom of the atmospheric distillation column can only be treated as low-value residue, resulting in extremely low utilization of crude oil and significant waste of economic benefits. Vacuum distillation is used to extract the high-quality intermediate wax oil components to the greatest extent possible, and it represents the key step in maximizing the efficiency of the entire atmospheric and vacuum distillation unit.
3.3 Reasonably divide the fractions to supply feed to the secondary processing units; the vacuum system precisely separates wax oils with different boiling points into first-stage reduced oil, second-stage reduced oil, and third-stage reduced oil. Light wax oil is suitable for hydrogenation units, while heavy wax oil is suitable for catalytic cracking units. The vacuum residue at the bottom of the tower can be used as raw material for coking and asphalt production. Truly achieve the complete utilization of crude oil, its staged use, and hierarchical processing.