- Atmospheric distillation takes place in a distilling column at or near atmospheric pressure. The crude oil is heated to 350 - 400oC 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).
- Heavier hydrocarbons condense more quickly and settle on lower trays and lighter hydrocarbons remain as a vapour longer and condense on higher trays.
How It Works
- Distillation is based on the fact that the vapour of a boiling mixture will be richer in the components that have lower boiling points.
- Thus, when this vapour is cooled and condensed, the condensate will contain the more volatile components. At the same time, the original mixture will contain more of the less volatile components.
- A fractionating distillation unit, by running the process in parallel, at a number of different temperatures, can generate a number of different hydrocarbon streams, each of a different volatility.
- After desalting the crude oil is pumped through a series of heat exchangers and its temperature is raised to about 300 ° C by heat exchange with product and reflux streams.
- It is then further heated to about 400 ° C in a furnace and charged to the flash zone of the atmospheric fractionators.
- The main components of a distillation column are as follows
- A vertical shell where separation of liquid components is done.
- Column internals e.g.trays/plates and/or packings which are used to enhance component separations.
- A reboiler to provide the necessary vaporization for the distillation process.
- A condenser to cool and condense the vapour leaving the top of the column.
- A reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column.
- Liquid fractions are drawn from the trays and removed. In this way the light gases, methane, ethane, propane and butane pass out the top of the column, petrol is formed in the top trays, kerosene and gas oils in the middle, and fuel oils at the bottom.
- The atmospheric fractionator normally contains 30 to 50 fractionation trays. Separation of the complex mixtures of crude is relatively easy and generally 5 to 8 trays are needed for each side stream product plus the same number above and below the feed plate.
- Thus crude oil atmospheric fractionation tower with four liquid sidestream drawoffs will require some 30 to 42 trays.
- Residue drawn of the bottom may be burned as fuel, processed into lubricating oils, waxes and bitumen or used as feedstock for cracking units.
- To recover additional heavy distillates from this residue, it may be piped to a second distillation column where the process is repeated under vacuum, called vacuum distillation.
- This allows heavy hydrocarbons with boiling points of 450oC and higher to be separated without them partly cracking into unwanted products such as coke and gas.