crude oil and cracking

What is petroleum commonly called

Crude oil

what is petroleum a mixture of

Alkane hydrocarbons

What is a petroleum fraction

Mixture of hydrocarbons with similar chain length and boiling point range

How is petroleum separated into its components

Fractional distillation

Process of fractional distillation

• Oil is pre-heated

• then passed into column.

• The fractions condense at different heights

• The temperature of column decreases upwards

• The separation depends on boiling point.

• Boiling point depends on size of molecules.

• The larger the molecule the larger the van der waals forces

• Similar molecules (size, bp, mass) condense together

• Small molecules condense at the top at lower temperatures

• and big molecules condense at the bottom at higher temperat

Is cracking a physical process

This is a physical process involving the splitting of weak van der waals forces between molecule

What is a vacuum distillation unit

• Heavy residues from the fractionating column are distilled again under a vacuum

• Lowering the pressure over a liquid will lower its boiling point

allows heavier fractions to be further separated without high temperatures which could break them down

What is cracking

Conversion of large hydrocarbons to smaller hydrocarbon molecules by breakage of C-C bonds

Is cracking a chemical process

Yes as involves splitting of strong covalent bonds so requires high temperatures

Economic reasons for cracking

• petroleum fractions with shorter chain lengths more in demand than larger fractions

• Make use of excess larger hydrocarbons and supply demand for shorter ones the long ones are cracked

• Products of cracking more valuable than the starting materials

2 types of cracking

Thermal

Catalytic

Thermal cracking

• high pressure (7000 kPa)

• High temperature (400-900)

• Produces mostly alkenes, sometimes hydrogen

• Bonds can be broken anywhere in molecule by C-C bond fission and CxH bond fission

Catalytic cracking

• Slight or moderate pressure

• High temperature (450)

• Zeolite catalyst

• produces branched and cyclic alkanes and aromatic hydrocarbons

• Used for making motor fuels

Why is catalytic cracking cheaper than thermal cracking

Saves energy as lower temperatures and pressures used

Do branched and cyclic hydrocarbons burn more cleanly

Yes

Used to give fuels higher octane number

Examples of alkanes as fuels

• Methane- natural gas (cookers)

• Propane- LPG (cars,stoves,barbecues)

• Butane- LPG

• Octane- chemicals and petrols