-
general formula: CnH2n-2 (n is a whole number, usually from 1 to 20)
- linear or branched chain molecules containing two carbon-carbon double-bonds
- can be liquid or gas
- examples: acetylene, butadienes
However, before we get products such as synthetic rubber the crude oil must be extracted from its reserves and then processed.
Today the leading producers of crude oil include, Texas, California, Alaska, Iran, Kuwait, the middle-east etc. As you can see oil can be found all over the world and therefore different extraction methods are put into progress. Before an oil-rig/well is dug, scientific methods are put into place to determine where to find the oil. Gravimeters and magnetometers and seismographs are used to identify the subsurface rock formations which could hold crude oil. Drilling for the crude oil can be extremely difficult due to these conditions and is often a risky process e.g. some wells must be dug 7 miles deep before some oil stores are found. Today much of the oil extraction is located off shore on platforms standing on the ocean bed. In order for the oil to come up to the oil rig it has to be pumped up by using water, gas or air to force it out. Once the oil has been collected it is often transported by tanker or pipeline to the refinery.
Crude oil in its raw form is a complex mixture of hydrocarbons where the hydrocarbons are of varied mass, have differing boiling temperatures and differing lengths. Now as I have mentioned previously these hydrocarbons can be very useful but before we can use them they must be made into smaller more useful hydrocarbons. This state can be achieved through a process called fractional distillation. The technique for this process uses the differing boiling temperatures of the molecules in the crude oil so that all differing molecules can be separated. The long chained hydrocarbons are broken down into narrower fractions where the boiling point amongst that fraction does not vary immensely. This process takes place in a specially designed fractional distillation column.
Process
- Crude oil is vaporized by passing through pipes in a furnace where the oil is heated to 600 degrees Celsius.
- The oil will then boil forming vapor (gas)
- The oil is then fed into the distillation chamber towards the bottom.
- The distillation chamber may be 100m high and consist of vertically stacking rows of steel trays. The trays have many holes (bubble caps) in them to allow the vapor and the liquids to pass through them. The trays will help to collect the liquids that form at various heights in the column. The liquids will flow down the tray over a wier.
- The reason why the oil is separated into narrower fractions can be explained by looking at the temperature gradient in the column. At the bottom of the tank it is extremely hot due to the freshly fed oil vapor and at the top it is cool. As the vapor passes through the tray it will come into contact with a slightly cooler liquid. This causes some of the hydrogen molecules to condense in that tray causing more violate hydrocarbons in the liquid to evaporate. This process will take place at each tray and in each tray a unique narrow range of hydrocarbons with similar properties will form. After approx 45 condensations and evaporations have taken place the crude oil has been separated into fractions.
- The collected liquid fractions may either pass to condensers, which cool them further, and then go to storage tanks. Or go to other areas for further chemical processing.
The more volatile hydrocarbons with the low boiling point form at the top of the tank and the least volatile hydrocarbons with high boiling point at the bottom.
Once operating the column may be kept in an equilibrium state by maintaining the input of the crude oil at a flow rate which balances the total of the flow rate at which the fractions are removed. When a steady state exists the compositions of the liquid and vapor at any one tray do not vary. This enables the fractions at each tray to be drawn individually when required. Each tray will contain a narrow range of fractions with a narrow range of boiling points.
The fractional distillation column will separate the crude oil into the following fractions: Refinery gasses, gasoline and naphtha, Kerosene, gas (diesel), oil and residue.
Refinery gasses – Consist of simple alkanes containing up to four carbon atoms. They are used as fuels or as a source for building other molecules.
Gasoline – Contains Alkanes with 5 – 10 carbons in the chain and is used in petrol.
Naphtha – Most important source of chemicals for the chemical process industry.
Kerosene - is used for jet fuel and domestic heating.
Gas oil - is used as diesel fuel and as a feedstock for catalyst cracking.
Residue – used as a source of lubricating oils and wax and bitumen.
Bitumen – when mixed with crushed stone is the tarmac compound used for road surfaces.
Although the crude oil has been separated into useful fraction, some of the separated ‘trays’ can be further processed to form products that are even more useful.
Cracking – To obtain more useful alkanes and alkenes
Heating the oil fractions with a catalyst. Under these conditions it can brake-down high molecular mass alkanes into low molecular mass alkanes as well as alkenes. The cracking is a random process by which both C-H and C-C bonds can be broken. Therefore it is possible for by products to be produced like: Hydrogen and branched chain alkane isomers.
For example Decane can be broken into:
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3
(Decane)
H
│
→CH3CH2CH=CH2 + H3C─ C─CH3CH2CH3
(But-1-ene) │
CH3
(2-methylpentane)
Decane is broken into these two isomers because there is a larger requirement for small chain isomers than larger ones.
After the cracking the hot vaporized oil fraction and the catalyst behave as a fluid. This is called the fluidized bed. Some of the hydrocarbon fraction can be broke down into carbon which can block the pores of the catalyst. We can recycle the catalyst by pumping it into the regeneration chamber where the carbon coke is burnt off in air at high temperatures.
Isomerisation
Is the process which we use to obtain branched alkanes. The process involves heating the straight chain alkanes with platinum catalyst to form Branched chain alkanes:
CH3
│
CH3CH2CH2CH2CH2CH3 → H3C─C─CH2CH3
│
CH3
(Hexane) (2,2-dimethylbutane)
However these newly formed branched chain alkanes have to be separated from the straight chain alkanes and this is done by a molecular sieve. The sieve is a type of zeolite that has pores through which the straight chain alkanes can pass through but the branched chain alkanes cannot due to there bulky shape and thus they are separated off. The straight chain alkanes may then be recycled to the reactor.
Reforming
This involves the conversion of alkanes to cycloalkanes or cycloalkanes to arenes using a bimetallic catalyst. For example a cluster of platinum and rhenium atoms is very effective in removing hydrogen atoms from methylcyclohexane to form methylbenzene.
(Methylcyclohexane) (Methylbenzene)
A catalyst containing Clusters of iridium atoms and platinum enables conversions of straight chain alkanes to arenes:
CH3CH2CH2CH2CH2CH3 + 4H2
(Hexane)
The metal clusters have to be between 1 and 5nm thick and are deposited on an inert support such as aluminum oxide. The Rhenium and the iridium help prevent the build up of carbon deposits which reduce the activity of the catalyst.
Why are Alkanes fuels?
The reason for this is because of their reaction with oxygen.
Alkane (fuel) + Oxygen (or other oxidizer) → Oxidation products + Energy transfer
This basically means that a fuel must react with oxygen to release large amounts of energy and Oxidation products that aren’t extremely harmful to mankind. Although different chains of alkanes can produce different energy amounts and byproducts and are used for different purposes, they all comply with this equation. Below I will list the ideal characteristics which all fuels must have.
- A fuel must react with an oxidizer to release large amounts of energy.
- A Fuel must be oxidized fairly easily, ignite quickly and sustain burning without further intervention.
- A fuel should be readily available, in large quantities and at a reasonable price.
- A fuel should not burn to give products that are difficult to dispose of, or are unpleasant and harmful.
- A fuel should be convenient to store and transport safely without loss.
So obviously different fuels are used in different environments. In Industry For example larger fuels that produce a lot of energy that may give off lots of harmful gasses can be used. This is because in industry it will have the relevant equipment to extract the dangerous fumes and dispose of them safely. In homes however where there won’t be the relevant equipment to handle toxic fuels, more environmental friendly fuels can be used like gas. And for transport we have to consider the transporting of fuel issue so petrol which is a liquid can be used rather than more environmental friendly hydrogen as there is a possibility of leakage as it is a gas. However the world is subject to change so in 10 years time other fuels maybe used in place of these.
Problems with these fuels
There are various problems with these fuels. One of the major ones is that we as a nation rely on them too much (Coal, oil and gas). As they are fossil fuels they are in effect none renewable resources. At our rate of consumption it is predicted that these resource will be depleted within 100 years.
Also the fossil fuels are the raw materials which supply the feed stock for our chemical industry. They can be processed to produce useful products such as; Polymers, medicines, solvents, adhesives etc. So how long can we afford to burn our chemical feedstock?
There is also the issue of carbon dioxide emissions of these fuels. This is the major contributor to the greenhouse effect which causes the temperature of our environment to increase dramatically. Due to this, precautions are being made to reduce these emissions. Britain has been set a target to reduce its emissions by 35%. This could be achieved by the outright ban of coal and oil but nations are reluctant to do this as they have become so reliant on these resources.
There is also the possibility of a spillage which can pollute rivers and streams and the environment. This can cause death to animals and plant life and there is also the enormous cost of cleaning it up after the disaster.
If the carbon based fuel does not completely combust in the furnaces, carbon monoxide is produced and this can cause death by interfering with the blood stream.
2C + O2 → 2CO (impartial oxidation to give carbon monoxide)
Also sulphur and nitrogen dioxide can be produced which contribute to harmful acid rain.
Development of renewable resources
As our reserves of fossil fuels are limited we must find alternative sources of energy. In the search for the alternatives, chemists and other scientists are now working to develop renewable resources such as:
Biofuels
This is when plants are grown to be used directly as fuels e.g. wood, animal waste and plant waste to produce alcohol and using waste products to produce biogas.
Advantages – Renewable, help to reduce waste, used with simple technology.
Disadvantages – Not large enough supply to replace fossil fuels at present rate of use.
Nuclear Fuels
Chain reaction involving the nuclei of isotopes of uranium 235 splitting to produce vast amounts of energy.
Advantages – No carbon, nitrogen or sulphur as polluting byproducts.
Disadvantages – Radioactive waste products are difficult to store and treat; which is a very expensive process
Moving air: wind
Energy of moving air is transferred into the motion of windmills and wind turbines
Advantages – Renewable pollution and waste free; can be used in locality where energy is needed
Disadvantages – Expensive, not reliable, noisy and ugly to the environment
Hydroelectricity
Water stored behind dams and waterfalls can be released and generate electricity by turning a generator.
Advantages – Renewable, predictable, waste free and can be used on large scale.
Disadvantages – Expensive to install, environmental impact of dams etc.
Solar panels
Panels of solar heat collectors, used to heat water in parts of world were sun is plentiful.
Advantages – Renewable, no pollution
Disadvantages – Require a lot of sunshine, Expensive initial cost, Very large scale to be suitable.
Hydrogen
Hydrogen is extracted quite cheaply from water by electrolysis and is used as a fuel.
Advantages – No pollution as water is the only waste product.
Disadvantages – Too dangerous and explosive, difficult to store and to use for transport or in domestic situations.
Bibliography
I would like to thank the following books, websites and teachers which I have used information from:
Mrs. Chapman (teacher)
(website)
Microsoft Encarta 2002 (CD ROM)
Cambridge chemistry 1 by Brian Ratcliff (Book)
Chemists in context Second edition by GC Hill and JS Holman (Book)
Please note that I have used the information in no particular order however the information taken exactly from the resources has been written in italic.
