Developing fuels

1
Developing fuels

• DF 3 Petrol is popular

2
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)

3
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
4
Petrol and Crude Oil

5
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)

6
Whats 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

7
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?
8
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 dont 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 ?
9
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 petrolair mixture, then a spark
makes the mixture explode, pushing the
piston down and turning the crankshaft.
10
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
l CH3-C-CH2-CH-CH3 l
CH3 2,2,4 trimethylpentane Low tendency to
auto ignite scores 100

CH3-CH2-CH2-CH2-CH2-CH2-CH3 Heptane High
tendency to auto ignite scores 0
11
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