Developing fuels

Why burn it?

Come up with a definition and description, uses AND equation for COMBUSTION of octane.

Why burn it? Combustion is a process that everyone has

experienced. It is used to heat homes, to power automobiles, and even to produce electricity. Combustion is an exothermic reaction between oxygen and a hydrocarbon. Combustion requires a certain amount of heat to begin, but once started the reaction proceeds spontaneously generating water vapour and carbon dioxide along with large amounts of energy in the form of heat and light.

Equation – complete combustionGeneral formula

ACXHY + ((2AX +AY/2)/2)O2 ---> (AX)CO2 + (AY/2)H2O

Combustion of Methane:

CH4 + 2O2 ---> CO2 + 2H2O

Combustion of Octane:

2C8H18 + 25O2 ---> 16CO2 + 18H2O

Combustion of Benzene:

2C6H6 + 15O2 ---> 12CO2 + 6H2O

Incomplete combustion – what Chemguide says Incomplete combustion (where there isn't enough oxygen

present) can lead to the formation of carbon or carbon monoxide.

As a simple way of thinking about it, the hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets whatever is left over!

The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke. Carbon monoxide is produced as a colourless poisonous gas.

Why carbon monoxide is poisonous Oxygen is carried around the blood by

haemoglobin (US: hemoglobin). Unfortunately carbon monoxide binds to exactly the same site on the haemoglobin that oxygen does.

The difference is that carbon monoxide binds irreversibly - making that particular molecule of haemoglobin useless for carrying oxygen. If you breath in enough carbon monoxide you will die from a sort of internal suffocation.

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

Catalytic Reforming the process re-arranges or re-structures the hydrocarbon

molecules (as well as breaking some of the molecules into smaller molecules.)

The overall effect is that the product reformate contains

hydrocarbons with more complex molecular shapes having higher octane values

The process separates hydrogen atoms from the

hydrocarbon molecules and produces very significant amounts of by-product hydrogen gas for use in a number of the other processes involved in a modern petroleum refinery

Examples – 500oC c.20 atmospheres

Molecular Sieves

A molecular sieve is a material containing tiny pores of a precise and uniform size that is used as an adsorbent for gases and liquids.

Molecules small enough to pass through the pores are adsorbed while larger molecules are not. It is different from a common filter in that it operates on a molecular level.

This is used for separating the straight chains from the branched.