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Unit 24 Demand for different fractions of petroleum.

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1 Unit 24 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 500 o C and 30 atmospheres.  Cracking cracking – vapours of large hydrocarbons are passed over red hot catalyst (aluminium oxide and silicon(IV) oxide) at 500 o 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  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 C 13 to C 25 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 700 o C.  C 6 H H 2 O → 5CO + CH H 2  C 6 H H 2 O → 2CO 2 + 4CO + 15H 2  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%)

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