Presentation on theme: "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%"— Presentation transcript:
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 C 5 to C 7 boiling point 25C -75C) Gasoline and gas oil fractions are sources of petrol components Naptha used for high grade petrol and chemical feedstocks
'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 piston down 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 CH 3 CH 3 l l CH 3 -C-CH 2 -CH-CH 3 l CH 3 2,2,4 – trimethylpentane Low tendency to auto ignite scores 100 CH 3 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 3 Heptane High 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