Developing fuels
DF 3 Petrol is popular
Petrol and Crude Oil Petrol is a mixture of many different
compounds blended to give the right properties
30% - 40% of each barrel of crude oil goes to make petrol
Thick black liquid with gases and solids dissolved inside
Crude oil is a mixture of hydrocarbons (molecules made of a chemical combination of carbon and hydrogen atoms)
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 petrol 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
Petrol and Crude Oil
Petrol and Crude Oil 'straight run' gasoline from primary
distillation doesn't make good petrol, most needs further treatment
supply and demand - surplus of high boiling hydrocarbons which need to be cracked to make more volatile hydrocarbons suitable for petrol etc. (AND in the process making valuable alkenes - important secondary chemical feedstock for a huge number of other chemicals including plastics)
the 'refinery's job' is to convert the crude oil fractions into useful products - wide range of hydrocarbons
Alkanes converted into other types of hydrocarbon cycloalkanes (containing carbon rings) arenes (containing benzene rings)
What’s left over? After distillation there is a residue left over
Can be used to make useful products
First is distilled again under reduced pressure
Vacuum distillation avoids high temps that would be needed at atmospheric pressure (which would crack the hydrocarbons)
More volatile oils distils
Oils are used as fuel oils in power stations or ships
Others are used as base for lubricating oils
Winter and summer petrol Not as simple as sending straight run
gasoline to the pump. Petrol has to be blended to get the right properties – Volatility
In a car engine mixture of petrol vapour and air is ignited
When weather is cold petrol is difficult to vaporise – car difficult to start
How do petrol blenders solve the problem?
Winter and summer petrol Petrol companies make different blends for
different times of year
Winter – more volatile compounds vaporise more easily, more small molecules such as butane and pentane
Summer – in hot weather you don’t want too many volatile compounds – petrol would vaporise to easily and you would lose petrol from the tank – costly and polluting
Different blend for different countries
How would petrol blends differ in Russia and Egypt ?
The problem of knocking Octane rating of petrol is important characteristic
petrol blenders must take into consideration This is a measure of the tendency of the petrol to cause a problem known as ‘Knock’
Figure 12 How a four-stroke petrol engine works. The compression stroke is shown here. The piston compresses the petrol–air mixture, then a spark makes the mixture explode, pushing the pistondown and turning the crankshaft.
Octane number The tendency of a fuel to auto ignite is measured by its
octane number. 2,2,4 – trimethylpentane is a branched alkane with a low
tendency to auto-ignite – given an octane number of 100 Heptane- straight chain alkane - auto-ignites easily and
is given an octane number of 0 The octane number of any fuel is the percentage of 2,2,4
– trimethylpentane in a mixture of 2,2,4 – trimethylpentane and heptane which knocks at the same compression ratio as a given fuel
For example 4-star petrol has an octane number of 97 and knocks at the same compression ratio as a mixture of 97% 2,2,4 – trimethylpentane and 3% heptane
CH3 CH 3
l lCH3-C-CH2-CH-CH3
l CH3 2,2,4 – trimethylpentaneLow tendency to auto ignite scores 100
CH3-CH2-CH2-CH2-CH2-CH2-CH3
HeptaneHigh tendency to auto ignite scores 0
The problem of knocking In a petrol engine, the petrol-air
mixture has to until at the right time
As the fuel air mixture is compressed it heats up
Many hydrocarbons auto-ignite in these conditions
The fuel air mixture catches fire as it is compressed
When this two explosions happen one due to compression and one with the spark
This causes a knocking sound
Engine performance is lowered and cylinder can be damaged