tell meall about crude oil refining?

Answer 1

Crude oil is a complicated mixture of hydrocarbons, with a varying composition depending on its source. The hydrocarbons in crude oil have different boiling points, according to the number of carbon atoms their molecules contain and how they are arranged. Fractional distillation uses the difference in boiling point to separate the hydrocarbons in crude oil. The fractionating column is cooler at the top than the bottom, so the vapours cool as they rise.
Vapours condense onto a tray when they reach the part of the column which is cooler than their boiling point. The bubble caps help to slow down the vapours has they rise through the column. As the last gases from the bottom of the column pass through the holes in a tray, any lighter hydrocarbons still in the condensed liquid are boiled off, and rise through the column.

Lowering the pressure over a liquid will lower its boiling point. Heavy residues from the fractionating column are distilled again under a vacuum. This means that the heavier fractions can be further separated without high temperatures which would break them down. These are then passed on to the cracking unit or used to obtain types of lubricating oils or blended into industrial fuels.

Answer 2

http://science.howstuffworks.com/oil-refining.htm