organic chem:fractional distillation

jadelamlam

Hi,
During fractional distillation,crude oil is heated to a certain temperature,then it is pumped into the fractional tower.
As you know,petroleum is a mixture of alkanes.When it is heated in the furnace(to about 400°C),it does not burn.But lower alkanes(e.g.CH4) are extremely flammable,why it doesn't burn at 400°C?

DrDu

Probably because you carefully avoid contact with air or oxygen.

jadelamlam

Oh,is that means,petroleum is heated with the absence of air in the furnace?

DrDu

Yes. I suppose you flood the columns with nitrogen when you begin operation. Later on, there will be only the vapours of the various alkanes etc. in the column.

jadelamlam

Where does the nitrogen go?

DrDu

I suppose it would leave the column together with the lightes alkanes (methane, ethane) which don't condense.