1. Demand for different fractions of petroleum
Unit 24
Demand for different fractions of petroleum

2. Demand for different fractions of petroleum

3. Demand for different fractions
The lighter, low boiling fractions – made up of small hydrocarbon molecules.
Can be sold in a higher price (of higher economic value).
More demanded by us.

4. Demand for different fractions
Heavier, high boiling fractions – made up of large hydrocarbon molecules.
Supply is more than demand.
Cracking is a process in which large hydrocarbon molecules are broken down into small molecules.
A way to meet our demand for petrol.

5. Cracking of petroleum
When hydrocarbon molecules are heated, they move and vibrate faster. Their more frequent collisions lead to the breaking up of large molecules into smaller ones.

6. Methods of cracking
Thermal cracking – heating hydrocarbons up to 500oC and 30 atmospheres.
Cracking cracking – vapours of large hydrocarbons are passed over red hot catalyst (aluminium oxide and silicon(IV) oxide) at 500o C and 1 atmosphere.

7. Catalyst
A substance that can speed up the rate of reaction, but itself is not used up at the end of reaction (remains unchanged at the end of the reaction).
Compare with enzyme!
Enzymes would be denatured.

8. Cracking in the laboratory

9. Cracking in the laboratory
Broken porous pot (unglazed porcelain pieces) or alumina (aluminium oxide) – used as catalyst.
Products of cracking (differing from case to case). Why?
Carbon soot, hydrogen, small alkanes and small alkenes.

10. Importance of cracking
Producing extra petrol
High boiling fractions of C13 to C25 are cracked into small molecules which are fuels for motor-cars.
As a source of alkenes (raw materials for plastics)
Alkenes – by products of cracking and raw materials for making plastics, polymers and detergents.

11. Town gas Made from the reforming of naphtha.
Naphtha vapour and steam were passed over red-hot nickel catalyst at 700oC.
C6H H2O → 5CO + CH H2
C6H H2O → 2CO2 + 4CO + 15H2
An odorant is added to alert people if there is a leakage of town gas.

12. Composition of town gas
Hydrogen (49%)
Methane (28.5%)
Carbon dioxide (19.5%)
Carbon monoxide (3%)