Difference between atmospheric distillation and vacuum distillation pdf

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difference between atmospheric distillation and vacuum distillation pdf

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For sake of simplicity, Figure 4. Crude oil enters a desalter and then a heater. It then undergoes an atmospheric distillation where it refluxes. The atmospheric residue goes to a separate heater and then into a vacuum distillation right where light vacuum gas oil and heavy vacuum gas oil are separated out along with vacuum residue. A temperature gradient is established in the column by removing heat from the overhead vapor.

The Open Chemical Engineering Journal

Vacuum distillation is distillation performed under reduced pressure, which allows the purification of compounds not readily distilled at ambient pressures or simply to save time or energy. This technique separates compounds based on differences in boiling points. This technique is used when the boiling point of the desired compound is difficult to achieve or will cause the compound to decompose. The reduction in boiling point can be calculated using a temperature-pressure nomograph using the Clausius—Clapeyron relation. For samples with high boiling points, short-path distillation apparatus is commonly employed. Rotary evaporation [7] is a common technique used in laboratories to concentrate or isolate a compound from solution.

Atmospheric and Vacuum Distillation Units

Register now or log in to join your professional community. The atmospheric distillation column is used to handle the straight crude oil and it operates at a pressure of roughly1 atm. The vacuum distillation column is used to handle the residue being heavy from the distillation column and that at a reduced pressure in order to decrease the boiling point of the species so that the recovery of heavier components can be done easier. Heavy products being evaporated easily. Atmospheric distillation column is used to separate the crude oil components into its fractions which are valuable products like gasoline, LPG, kerosene, Diesel fuel, naphtha and heavy gas oil. It operates at approx 1 atm. The basic principle used in ADU is fractional distillation distillation on boiling ranges.

The crude oil is heated to - oC and the vapour and liquid are piped into the distilling column. The liquid falls to the bottom and the vapour rises, passing through a series of perforated trays sieve trays. It operates at atmospheric pressure which is mm of Hg. Above this temperature, the oil will thermally crack, or break apart, which impedes the distillation process. The purpose of atmospheric distillation is primary separation of various 'cuts' of hydrocarbons namely, fuel gases, LPG, naptha , kerosene, diesel and fuel oil. The heavy hydrocarbon residue left at the bottom of the atmospheric distillation column is sent to vacuum distillation

Crude Distillation Unit (CDU)

Generally, Petroleum refineries are put in place to convert or refine unprocessed crude oil into more useful products using both physical separation and chemical conversion processes. Albeit, different refining unit are subsets of the physical separation category. The atmospheric and vacuum distillation unit seems to be more prominent. Conventionally, the crude atmospheric residue cannot be further heated in an atmospheric condition due to: coke formation, pipes plugging, thermal cracking and straining of the furnace. A vacuum distillation column is therefore required.

Vacuum distillation

The key difference between atmospheric distillation and vacuum distillation is that the atmospheric distillation is used to separate low boiling fraction of a mixture whereas vacuum distillation allows the components to be separated easily by lowering the boiling point of a high boiling fraction. Distillation is the action of purifying a liquid by a process of heating and cooling.


The next step in the process of crude oil separation is the vacuum distillation of the atmospheric residue that was drawn off from the bottom of the atmospheric column. Why is it called vacuum fractioning? What is the main advantage of it? And what is the difference between the atmospheric and the vaccum fractionation? In practice, vacuum is referred to when the air pressure in an area goes below atmospheric pressure. It can be created by removing air from a space using a vaccum pump or by reducing the pressure using a fast flow of fluid.

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