Oil refining and its Products

Petrol and Crude Oil Crude oil is separated by

fractional distillation

works because the molecules have different boiling/condensation points

many of these hydrocarbons are alkanes, and are sorted into fractions

each fraction has a range of boiling points in the distillation

narrow boiling ranges of limited carbon number (eg light gasoline is C5 to C7 boiling point 25C -75C)

Gasoline and gas oil fractions are sources of petrol components

Naptha used for high grade petrol and chemical feedstocks

Fractionation of Crude OilFractions and their uses

Refinery Gas (LPG) Light Gasoline (Petrol) Naphtha (Petrol) Kerosene (Jet Fuel) Gas Oil (Diesel Fuel) Residue Fractions (Bitumen)

Natural Gas

Natural gas is an extremely fuel both for domestic and industrial use.

It is a mixture consisting mostly of methane, CH4, (at least 85%), ethane, C2H6, (up to 10%) and small amounts of propane, C3H8, and butane, C4H10.

Liquid Petroleum Gas (LPG)

The gases in the refinery gas fraction are bottled and sold for domestic use.

Propane and butane from this fraction can be readily liquefied under pressure and are referred to as liquid petroleum gas (LPG).

Mercaptans

Very smelly, organic sulfur compounds called mercaptans are added to natural gas and LPG so that leaks can be detected

Petrol Composition

Complex mixture of compounds

Mainly Hydrocarbons

Branched – chain alkanes

Aromatic Compounds

Petrol in the InternalCombustion Engine

Vaporised Mixed with air Compressed Ignited and burned Gases produced expand Kinetic Energy

Premature Ignition

Problem: Auto-ignition (i.e. knocking or pinking) Effects: a) Loss of power

b) Engine damage Prevention: a) Additives

b) Use suitable mixtures of high-octane compounds

Octane Rating

Measure of tendency to resist auto-igniteor

Measure of tendency to cause knocking

Low octane rating makes auto-ignition more likely

2,2,4-tri-methylpentane

Octane Number =100

Heptane

Octane Number = 0

Octane Rating

Additives

(i) Lead compounds e.g. tetra ethyl lead Prevents reactions Harmful environmental effects Phased out in 2000(ii) Oxygenates e.g. ROR orROR1 MTBE Raise octane number Cause less pollution

Mixture of compounds with high octane numbers

Molecular features: Degree of branching – the more the better

Chain length – the shorter the better

Presence of rings – highly desirable

High octane numbers can be obtained from low by:

1. Isomerisation

2. Dehydrocyclisation

3. Catalytic cracking

All three methods involve the use of catalysts

Isomerisation

Take a straight chain alkane e.g. pentane (O.N.62)C ─ C ─ C ─ C ─ C

Heat in the presence of a catalyst Chain breaks Bits rejoin to form a branched compound e.g.2-methylbutane

(O.N.93)

C ─ C ─ C ─ C

│ C

Dehydrocyclisation

Take a straight chain alkane e.g. hexane (O.N. 25)

Catalyst causes change to a cycloalkane (O.N. 83)

C6H14 → (CH2)6 + H2

Catalyst causes the cycloalkane to change to an aromatic compound e.g. benzene (O.N. >100)

(CH2)6 → 3H2 + C6H6

Benzene

Catalytic Cracking

Heavy oil e.g. kerosine or diesel High temperature and catalyst Molecule breaks into several smaller molecules Unsaturated products are used as feedstock

for the polymer industry Saturated products are usually high octane

branched chain alkanes suitable for making petrol

CH3 ─ (CH2)10 ─ CH3

↓ CH3 CH3

│ │ CH3 ─ CH ─ CH2 ─ CH ─ CH3

+ CH3 │ CH2 = C ─ CH2 ─ CH3