What is Atmospheric Distillation?
Distillation under atmospheric pressure is a crucial test method to characterize these refined products. It delivers important information about the tested sample in order to e.g. maximize the yield of different petroleum products obtained from crude oil, based on their boiling range characteristics.
In this process, pre-heated crude oil (heated to about 250-260°C) is further heated to a temperature of around 350°C. This heated crude oil is then passed into a distillation column in which the pressure at the top is maintained around 1.2-1.5 atm (nearly the atmospheric pressure).
Feed to the atmospheric distillation column is desalted and pre-heated. The components separated by this distillation method are hydrocarbons such as fuel gases, naphtha, kerosene, diesel, and fuel oil. The residue left at the bottom of the atmospheric distillation column is known as the heavy hydrocarbon fraction. This fraction is sent to the vacuum distillation.
What is Vacuum Distillation?
Vacuum distillation is one of the industrial and laboratory methods of liquid solutions. In this method, by placing the solution under vacuum, the evaporation temperature of the solution decreases. And as a result, materials can be separated from each other by spending less heat energy. and also preserved the properties of the material.
The most important application of vacuum distillation is in refining crude oil, crude oil contains heavy and light hydrocarbons with different boiling points, which requires two distillation processes: an atmospheric distillation without the use of a vacuum, a second distillation is carried out in a vacuum to separate heavy hydrocarbons at low temperatures and minimize thermal cracking and the creation of unwanted by-products.
Atmospheric Distillation vs Vacuum Distillation
Distillation of crude oil is carried out in two units, first in an Atmospheric Distillation Unit (also known as Crude Distillation Unit, CDU), with further processing of the residue from atmospheric distillation in the Vacuum Distillation Unit (VDU). The key difference between atmospheric distillation and vacuum distillation is that atmospheric distillation is used to separate the lighter fractions of a mixture whereas vacuum distillation allows the components to be separated easily by lowering the boiling point of heavier fractions.
| Atmospheric Distillation vs Vacuum Distillation | |
| Atmospheric distillation is a technique used to separate components in crude oil that is performed under atmospheric pressure. | Vacuum distillation is a technique used to separate components in a mixture at a reduced pressure. |
| Pressure | |
| Atmospheric distillation uses pressure nearly similar to the atmospheric pressure (around 1.2-1.5 atm). | Vacuum distillation uses very lower pressure conditions. |
| Theory | |
| Atmospheric distillation is used to separate low boiling fraction of the mixture. | Vacuum distillation allows the components to be separated easily by lowering the boiling point of a high boiling fraction. |
| Fractionation | |
| Atmospheric distillation separates the light fractions of the mixture. | Vacuum distillation separates the heavy fractions of the mixture. |
| Degradation of the Key Components | |
| The atmospheric distillation does not concern about the degradation of the component. | The vacuum distillation allows the components to be separated without thermal decomposition (because some components degrade at high-temperature conditions). |
distillation column
Distillation column is an essential item used to separate a mixture into its component parts, or fractions, based on the differences in volatilities. In general, the distillation column is separated into 3 sections: rectifying (or enriching) section, stripping (or exhausting) section, and feed (or flashing) section.
Process description
Atmospheric Distillation (ADU)
is generally the first process and also has the largest capacity in a refinery complex. Different fractions are produced depending on the difference in their boiling temperature and are further processed in the downstream plants for desired products.
Vacuum distillation (VDU)
When you have a solution that you need to separate using distillation, it must be heated above the boiling point of one component so that it can be converted into vapor form and then condensed to cover again in liquid form. But there are some components with high boiling points. In these cases, we use vacuum distillation which by the creation of a vacuum decreases the boiling points of components. Using this way we can not only save energy but also prevent the cracking of components at high temperatures.
