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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