Alkanes and Petroleum

What is the process of fractional distillation?

1. Crude oil is vaporised at about 350°C

2. The vaporised crude oil goes through the bottom of the fractionating column and rises up through the trays

3. The largest hydrocarbons don’t vaporise at all, because their boiling points are too high- they just run to the bottom and form a gooey residue

4. As crude oil vapour rises up the column, it gets cooler

5. Because boiling points of alkanes increase as molecules get bigger, each fraction condenses at a different temp.

6. The hydrocarbons with the lowest boiling points don’t condense. They’re drawn off as gases at the top of the column.

What is cracking?

Cracking is breaking long-chain alkanes into smaller hydrocarbons (which can include alkenes). It involves breaking the C-C bonds

Why do we use cracking?

Most people want loads of light fractions, like petrol and naphtha. They don’t want so much of the heavier stuff like bitumen though. Stuff that’s in high demand is much more valuable than the stuff that isn’t.

What is thermal cracking?

• Takes place at high temperatures (around 1000°C) and high pressure (up to 70atm)

• Produces a lot of alkenes which is used to make a lot of valuable products

What is catalytic cracking?

• Uses a zeolite catalyst at a slight pressure and high temperature (around 500°C)

• Mostly produces aromatic hydrocarbons and the alkanes needed to produce motor fuels

• Using a catalyst cuts costs, because the reaction can be done at a lower temp and pressure and speeds up rate