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.
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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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
- all the company overheads, such as buildings, salaries and advertising
- fuel taxes levied by governments
- and the capital costs of the equipment needed to burn it
- A fuel should not burn to give products that are difficult to dispose of, or are unpleasant or harmful.
This is a considerable problem for most fuels, as hydrogen is the only fuel with a safe, non-polluting product from its oxidation reaction to water.
- A fuel should be convenient to store and transport safely and without loss.
Over the ages, people have tackled many problems of fuel storage, from how to keep wood dry to how to keep liquid oxygen extremely cold and safe for space flight. If gases such as methane and hydrogen are to be used as alternatives to petrol in vehicles, the problems of storage of large amounts of gas must be solved.
What is fractional distillation?
The process of fractional distillation is used to separate the components of a mixture of liquids by means of the differences in their boiling temperatures. An example apparatus to explain fractional distillation is shown down:
The fractionating column has a large surface area on which ascending vapour and descending liquid come into contact. A mixture rich in the most volatile component distils over at the top of the column, where the thermometer registers its boiling temperature. As distillation continues, the temperature rises towards the boiling temperature of the next most volatile component. The receiver is changed to collect the second component. In this way, the components are distilled over at their boiling temperatures.
The principles underlying fractional distillation are illustrated her down. Imagine that a liquid of composition L1 is heated until it begins to boil at a temperature B1. Then the vapour in equilibrium with the liquid has composition has V1. If the vapour is condensed by meeting the cold surface of a distillation column, it condenses to form a liquid of composition L2. This liquid starts to trickle down the column towards the distillation flask. If it is heated, it begins to boil at B2, to form a vapour of composition V2.If this vapour is condensed, it forms a liquid of composition L3.By repeated vaporisation and condensation, the composition of the vapour is made to follow the curve V1V2V3, becoming richer and richer in A, the more volatile component. The liquid is becoming richer in less volatile component, B, and its composition follows the curve from L1 towards B. The longer the column, the more vaporisation followed by condensation steps will be achieved, and the closer to pure A and pure B will distillate and residue become.
The oil industry uses fractional distillation. Crude petroleum oil is vaporised and fed into a massive fractionation column, which may be 30 to 60 meters high and 3 to 6 meters in diameter. Different fractions, such as gasoline and kerosene, are drawn off continuously from the column at different levels. The next figure shows how liquid and vapour attain equilibrium at each level, so that low boiling temperature fractions are drawn off from the bottom of the column.
Further treatment after distillation
After distillation, the different hydrocarbon fractions are treated in a variety of different ways. These include processes such as vacuum distillation ( to separate out less volatile components such as lubricating oils and waxes from the residue), desulphurisation ( to remove sulphur ) and cracking ( to produce more gasoline and alkanes ). There is insufficient gasoline and naphtha fractions from the primary distillation to satisfy the demand for petrol. So higher boiling fractions are cracked to produce more gasoline and naphtha. Modern petrol engines require higher proportions of branched-chain alkanes, cycloalkanes and arenes to promote efficient combustion. These are produced by reforming and isomerisation.
Cracking
Cracking involves heating the oil fraction with a catalyst. Under these conditions, high-molecular-mass alkanes are broken down into low-molecular-mass alkanes as well as alkanes.
Both C-C and C-h bonds are broken in the process. As the bondbreaking is a random process, a variety of products, including hydrogen, are possible and some of them intermediates can react to produce branched-chain alkane isomers. For example a possible reaction for decane is:
The chemical industry uses alkenes such as ethane for a variety of products ( for example, poly9ethene0 from ethene0. 2,2,4-trimetylpentane is an important component of petrol).
In the catalytic cracker the hot, vaporised oil fraction and the catalyst behave as a fluid. The seething mixture is called a fluidised bed. Some of the hydrocarbon mixture is broken down to carbon, which blocks the pores of the catalyst. The fluidised bed of the catalyst is pumped into a regeneration chamber, where the carbon coke is burnt off in air at a high temperature, allowing the catalyst to be recycled.Reforming
Reforming involves the conversion of alkanes to cycloalkanes, or of cycloalkanes to arenes.
Reforming reactions are catalysed by bimetallic catalysts. For example, a cluster of platinum
And rhenium atoms is very effective at removing hydrogen from methylcyclohexane to form methylbenzene.
A catalyst containing clusters of platinum and iridium atoms enables conversion of straight-chain alkanes to arenes.
These metal clusters are between 1 and 5 nm in diameter and are deposited on an inert support such as aluminium oxide. The rhenium and iridium help prevent the build-up of carbon deposits, which reduce the activity of the catalysts.
Isomerisation
Isomerisation involves heating the straight-chain isomers in the presence of a platinum catalyst.
The resulting mixture of straight- and branched-chain isomers then has to be separated. This is done by using a molecular sieve, which is another type of zeolite that has pores through which the straight-chain isomers are too bulky and thus are separated off; the straight-chain molecules are recycled to the reactor.
Physical properties of Alkanes
Since the electronegativities of carbon and hydrogen are 2.5 and 2.1 respectively have only weak dipole moments.
C----H bonds
Weak attractive forces exist between dipoles in neighbouring molecules, and van der Waals forces also come into the play. The attractive forces are so weak that the lower alkanes, from methane to butane, are gases at room temperature and pressure. Linear molecules of higher homologues can align themselves in a parallel arrangement so that dipole-dipole interactions and van der Waals forces can operate along the whole length of the molecule. The alkanes from C5 to C17 are liquids, while those with larger molecules are solids. Since branched-chain molecules are more spherical in shape than unbranched-chain hydrocarbons, the attractive forces between molecules are more restricted. The boiling temperatures of unbranched-chain alkanes are plotted against molar mass in the figure down.
The difference in boiling temperatures between C1 and C2 alkanes is 73°C, while the C9 and C10 alkanes differ by only 25°C. It is therefore more difficult to separate the higher members by fractional distillation.
The liquids alkanes are less dence than water; oil floats on water. The higher members are viscous liquids, the viscosity increasing with increasing molecular mass as the attractive forces between molecules increase.
Alkanes are only slightly soluble in water. Water molecules interact because of the strong dipoles in the
O---H bonds
The hydrogen bonds formed are stronger than any interaction, which can occur between water molecules and the non-polar alkanes molecules. Dissolution is therefore not favoured by energy considerations.
Problems with chemical fuels
Fossil fuels are non-renewable resources, yet we are consuming them extremely quickly. It is predicted that most of the earth's oil reserves will be depleted over the next hundred years.
Oxidation of the carbon-based compounds in fuels produces vast amounts of carbon dioxide. At one time carbon dioxide was considered to be a relatively harmless gas. Now it is known to be a major contributor to the 'greenhouse effect', which causes an increase in atmospheric temperature. Some governments are so concerned about this effect, which could bring about disastrous climatic change, that many means of reducing carbon dioxide levels in the atmosphere are being considered. Britain has set a target of reducing CO2 emission by 35% of the 1992 level by the year 2000. The simplest solution would be an outright ban on the use of coal, oil and methane. Governments are understandably reluctant to take such drastic action, as national economies have become so dependent on these fuels.
Spillage of fuel often causes great damage to local environments. This damage ranges from streams and ponds polluted by leaks fuel tanks to major disaster when oil tankers break open. There can be immense lose of animal and plant life and enormous costs of cleaning up.
Inefficient burning of carbon-based fuels in defective furnaces and domestic gas fires and in poorly tuned engines produces the very poisonous gas, carbon monoxide. Instead of:
C(s) + O2(g) CO2
Partial oxidation gives:
2C(s) + O2 2CO(g)
Inhalation of carbon monoxide may cause death, as it interferes with the transport of oxygen in the bloodstream. Other dangerous gases produced by the burning of fuels include nitrogen oxides and sulphur oxides, which form strongly acidic solutions in water ( acid rain ). A large variety of compounds, including carcinogens, appear in the smoke from burning coal and wood.
Alternatives to fossil fuels
Biofuels
Plants can be grown to be used directly as fuels, e.g. wood. Plants can also be grown for conversion into fuels, e.g. sugar cane is easily fermented into ethanol. This can be used directly as an alternative to petrol or mixed with petrol. There is increasing use of natural oils, such as rapeseed or sunflower oil, as part of diesel fuels. Plants convert atmospheric carbon dioxide by photosynthesis to cellulose and other plant material. If crops are used either directly as, or for conversion into, a fuel, the carbon dioxide released to the athmosphere simply replaces that removed during plant growth. Scientists working for shell are exploring the potential for growing forests of fast cropping trees and using the biomass as a renewable energy source. The biomass is dried and chipped before being converted to gas and bio-oil by heating in the absence of air. The gas or bio-oil is the used to fuel a gas turbine to generate electricity. Greater overall efficiency results when the biomass is first converted to gas and bio-oil. Waste products large municipal landfill sites produce significant quantities of biogas by anaerobic decay of biological materials. In the past, this gas often seeped into the athmospere where it can form an explosive mixture with air. Now, it is collected in pipes and often flared for safe disposal. Biogas is mainly composed of methane which has a much greater greenhouse effect than its combustion product carbon dioxide. In a few cases, the collection and combustion of biogas from landfill sites is being used to generate electricity.
-Advantages: renewable; helps to reduce waste; used with simple technology.
-Disadvantages: not large enough supply to replace fossil fuels at present rates of use.
Methanol
This simple alcohol can be made quite cheaply from methane. It is often used in racing cars.
-Advantages: methanol burns cleanly and completely; little carbon monoxide is produced.
-Disadvantages: methanol is more toxic than ethanol; it provides much less energy per litre than petrol; mixtures of methanol and petrol absorb water and car engines may corrode; methanol and petrol tend to separate into layers; combustion of methanol produces the carcinogenic aldehyde methanal when there is insufficient air.
Nuclear Fuels
Fission: Energy is released when the nuclei of atoms of isotopes of uranium U-235 undergoes fission (splitting) in a chain reaction. Very large amounts of energy are available from this process . The energy is normally used in power stations to heat water to drive electricity-generating steam turbines.
-Advantages: no carbon, nitrogen or sulphur oxides as polluting by-products.
-Disadvantages: radioactive waste products are difficult to store and treat; safety systems to contain radioactivity are very costly.
Fusion
Energy is released when deuterium and tritium 'fuse' to form helium.
H + H He + n
-Advantages: potentially almost limitless as an energy supply as the 'fuels' come from water.
-Disadvantages; no fusion reactors are yet producing energy at economic rates; they are extremely costly.
Moving air: Wind energy
The energy of moving air is transferred into the motion of windmills and wind turbines. Much science and technology is being devoted to improving the efficiency of the wind machines, and they soon may provide over 10% of the UK energy needs.
-Advantages: renewable; pollution- and waste free; can be used in locality where energy is needed.
-Disadvantages; high initial expense for large-scale generation of electricity; not a reliable source in calm weather.
Moving water
Hydroelectricity: water stored behind dams or from waterfalls can be released through turbines and generate electricity or be used directly to turn wheels in mill. Hydroelectricity is a major source of power in many countries.
Waves: the motion of waves is used to cause oscillation motion in various devices and to generate electricity.
Tides: Incoming in rivers estuaries fill up large water stores behind barrages across the river.
The water can be released through turbines to generate electricity.
-Advantages; renewable; quite predictable; pollution-and waste-free; can be used on large scale.
-Disadvantage: costly to install; environmental impact of dams and barrages.
Sunlight: solar heating and photovoltaics
Solar panels, which are panels of solar heat collectors, are used to heat water in parts of the world where sunshine plentiful.
Photovoltaic cells convert light into electricity. In future, large satellites may generate electricity and beam energy by microwave to Earth:
-Advantages: renewable; pollution-free with no waste products.
-Disadvantages: low sunlight levels in UK; none at night; photovoltaics have high initial costs; very large arrays needed for large scale production of electricity.
Geothermal: hot rocks
Some distance belowthe surface of the Earth, the temperature is high (about 85°C at 2 km below). Water pumped into wells in the hot rock zone is heated; the extracted hot water can be used to heat buildings.
-Advantages: almost unlimited source.
-Disadvantages: not widely available; expensive initially; technological problems.
Hydrogen
Many scientists believe that we should run a 'hydrogen economy'. Hydrogen can be extracted quite cheaply from water by electrolysis. Much scientific and technological effort is being spent on effective storage and transport systems.
-Advantages: no pollution; as water is the only waste product from burning hydrogen in air; available in large quantities.
-Disadvantages: regarded as too dangerously explosive by many people; difficult to store and use for transport or in domestic situations.