5.5 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 Oil
Fractions 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
 Mainly

mixture of compounds

Hydrocarbons

 Branched
 Aromatic

– chain alkanes

Compounds

Petrol in the Internal
Combustion 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-ignite
or
Measure of tendency to cause knocking

 Low

octane rating makes auto-ignition more
likely

Octane Rating
 2,2,4-tri-methylpentane

Octane Number =100
Heptane
Octane Number = 0

Additives
(i)



(ii)



Lead compounds e.g. tetra ethyl lead
Prevents reactions
Harmful environmental effects
Phased out in 2000
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.2methylbutane (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

5.5 oil refining and its products

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