Hydrocarbons As Fuels.

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Written by: Mohammed Ali Zaini

Year: 12 AS

Teacher: Miss Perry

Introduction

Hydrocarbons as Fuels

Crude oil/Petroleum

Petroleum is another name for crude oil. Its name refers to the fact that it occurs naturally Pockets within rocks; the word petroleum means 'rock oil'. Crude oil in its natural form is a thick, tarry substance that is difficult to ignite. In its raw state it isn't very useful; it needs to be processed to separate out its most valuable constituents such petrol, lubricating oils, heating oil and power station fuel. Crude oil is also the source of the raw materials used to make detergents, plastics, paints, antifreeze, synthetic rubber and medicines. Seventy per cent of organic chemicals are produced from crude oil and a massive 3000 million tonnes of crude oil products are used worldwide every year.

Where does crude oil come from?

Over 400 million years ago, much of the Earth was covered in sea. Life had envolved but it consisted of primitive cells such as bacteria algae and single-celled animals and plants. There were no larger organisms; this was 200 million years before the appearance of the dinosaurs. Although the individual organisms were small, there were vast numbers of them and they grew and reproduced rapidly in the warm oceans. As they died, they sank to the bottom and formed thick layers of decomposing organic material. Over many years, layers of sediment built up on top of this organic layer and it eventually formed sedimentary rock. During the early stages, bacteria that were able to survive without oxygen continued to break don the organic material in the layer. Over millions of years, the high pressures created by the weight of the overlying layers and the high temperatures generated by decomposition converted this layer of biomass into crude oil and natural gas.

The liquid crude oil seeped into porous rock deposits above. If only porous rock was present, the oil could get up to the surface, forming shallow lakes. In other places, the porous rock were overlaid by a layer of impervious rock which prevented oil travelling any further and an oil reservoir was created. Natural gas often forms a pocket at the surface of oil reservoirs.

What does crude oil contain?

Crude oil is a mixture of about 150 different hydrocarbons. Hydrocarbons are compounds that contain only carbon and hydrogen. The majority of hydrocarbons in crude oil are straight chain alkanes, but with the mixture also contains cycloalkanes and arenas.

Fig. summarises the structure of these important hydrocarbons.

The exact composition of crude oil depends on the condition in which it formed. Samples from different parts of the world have slightly different amounts of each type of hydrocarbon, called its 'fingerprint'.

Different crude oils

Percentage composition

Type of crude oil

Petrol

Kerosene

Gas oil

Fuel oil

North sea oil

23

5

24

38

Arabian light

8

1.5

8

52.5

Arabian heavy

21

5

21

43

Iranian heavy

21

3

20

46

This fingerprint makes it possible to identify the source of an unknown sample by analysing the proportional of hydrocarbons it contains.

What properties should a good fuel have and what is the chemical reaction when a fuel burns?

The essential reaction for any chemical fuel is:

Fuel + oxygen ( or other oxidiser ) oxidation products + energy transfer

Though different fuels are needed for different purposes the ideal characteristics include the following:

- A fuel should react with an oxidiser to release larger amounts of energy.

It is interesting to compare fuels on the basis of energy per unit amount of material (mole) and energy per unit mass (kilogram).

Fuel

Formula

Relative molecular mass

Energy released per mole (KJ mol-1

Energy released per kilogram (KJ Kg-1)

Carbon

C(s)

2

-393

-32750

Methane

CH4(g)

6

-890

-55625

Octane

C8H18(l)

14

-5512

-48350

Methanol

CH3OH(l)

32

-715

-22343

Hydrogen

H2(g)

2

-286

-143000

- A fuel must be oxidised fairly easily, ignite quickly and sustain burning without further intervention.
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Gaseous or easily vaporised fuels usually perform well, as they mix easily and continuously with air/oxygen, which helps the reaction. Solid fuels (coal) are sometimes powered for use in large industrial furnaces.

- A fuel should be readily available, in large quantities and at a reasonable price.

The availability and price of oil, for example, affect national economies so much that governments can fall and countries go to war when these change. The price of any fuel includes many factors:

- the costs of finding it

- extraction, refining and transportation

- ...

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