C.2 Fossil fuels
what is the octane number and why is it used?
2,2,4-trimethylpentane (isooctane) does not knock. It is used as a basis for judging other fuels. The performance of a fuel is given by its octane number, which is based on a scale on which the 2,2,4-trimetylpentane has a value of 100 and the straight-chain molecule heptane, which auto-ignites relatively easily, has an octane number of 0. A fuel with an octane rating of 96 burns as efficiently as a mixture of 96% 2,2,4-trimethylpentane and 4% heptane. Fuels with high octane numbers can be more highly compressed, which results in more power per piston stroke.
Why is crude oil not a lasting solution to energy and feedstock?
90% of refined oil is used as fuel. As crude oil supplies decrease, this proportion may fall. Burning fuels derived from crude oil is the most convenient and economical option, but alternative energy sources and feedstocks need to be developed for when the supply of crude oil is depleted.
Describe the compression stroke of an internal combustion engine.
A cylinder-full of fuel and air is squashed into a small volume and ignited with a span plug. Some fuels auto-ignite without he need of a spark plug as the fuel-ai mixture is compressed by the piston. This pemratue ignition is called knocking and gives use to a knocking sound in the engine. Knocking reduces the efficiency fo the engine as ate energy of the exploding and expanding gas is not applied fully to the piston at the optimum time and can damage the engine.
What is the carbon footprint?
A measure of the impact of human activity on the environment. It is a measurement of all the greenhouse gases individuals produce. It has units of mass of carbon dioxide equivalent and depends on the amount of greenhouse gases people produce in their day-to-day activities through the use of fossil fuels.
What is hydrocracking?
A method used to break large hydrocarbon into smaller useful ones. It produces compounds which can be used in high-quality gasoline. In this process, a heavy hydrocarbon fractions is mixed with hydrogen at pressure about 80 atm and cracked over palladium on a zeolite surface. A high yield of branched-chain alkanes and cycloalkanes and som aromatic compounds is produced.
What is the relative percentage of carbon in anthracite?
Anthracite is formed under conditions of very high heat and pressure and is almost pure carbon, but goal generally contains between 80% - 90% carbon by mass. Coal occurs in many areas, though the majority of the world's supplies are in the northern hemisphere.
What are the advantages and disadvantages of coal, crude oil, and natural gas?
Coal Advantages: cheap and plentiful, can be converted into synthetic liquid fuels and gases, safer than nuclear power, ash produced can be used in making roads Disadvantages: produces CO2, SO2, particulates (electrostatic preceptors can remove most of these), difficult to transport, waste can lead to visual and chemical pollution, mining is dangerous Crude Oil Advantages: easily transported in pipelines or by tankers, convenient fuel for use in cars as volatile and burns easily, sulfur impurities can be easily removed Disadvantages: limited lifespan and invent distribution in the world, contributes to acid rain and global warming, transport can lead to pollution, carbon monoxide is local pollutant produced by incomplete combustion of gasoline in internal combustion engines, photochemical smog produced as secondary pollutant due to reactions of primary pollutants (nitrogen oxides and hydrocarbons) released form internal combustion engines Natural Gas Advantages: produces fewer pollutants per unit energy, easily transported in pipelines and pressurised containers, does not contribute to acid rain, higher specific energy Disadvantages: limited supplies, contributes to global warming, risk of explosion due to leaks
How can supplies of methane by increased by cracking or coal gasification?
Coal can be converted to the more environmentally clean methane by the process of coal gasification. The rushed coal is mixed with superheated steam, and a mixture of carbon monoxide and hydrogen known as synthesis gas is produced. C (s) + H2O (l) -> CO (g) + H2 (g) Synthesis gas can be used directly as fuel. It can be processed further to make methane. It i mixed with additional hydrogen in the presence of a heated catalyst. CO (g) + 3H2 (g) -> CH4 (g) + H2O (g) Synthetic natural gas can slo be made by heating cursed coal in the presence of steam with a potassium hydroxide catalyst to produce methane and carbon dioxide. 2C (s) + 2H2O (g) -> CH4 (g) + CO2 (g)
Describe the pattern of octane number for molecules with the same carbon number, but different isomers.
Cyclic compounds have high octane numbers than linear structures, alkenes have a higher octane number than the isometric cycloalkane, and aromatic compounds with the benzene ring have even higher octane numbers.
How can octane number be increased by catalytic reforming?
In catalytic reforming, the atoms in a molecule can rearrange to form a more branched chain or a cyclic or aromatic compound. The process is often referred to as plat forming if Pt is used as the catalyst, but other rare and expensive metals such as Pd, Ir, or Rh can also be used. For example: 1. hexane can be converted into benzene and hydrogen in a dehydrogenation reaction C6H14 -> C6H6 +4H2 2. heptane can be converted into methylbenzene and hydrogen C7H16 -> C6H5CH3 + 4H2
Describe crude oil.
It is a complex mixture of straight-chain and branched-chain saturated alkanes, cycloalkanes, aromatic compounds, and (in smaller quantities) compounds of nitrogen, oxygen, and sulfur.
What is the Fischer-Tropsch process?
It is a method of coal liquefaction. It involves using a mixture of carbon monoxide and hydrogen as the feedstock and produces a variety of alkanes along with water. (2n+1)H2 + n(CO -> CnH(2n+2) + nH2O
What is vacuum distillation?
It is a method used to vaporise hydrocarbon residue on fractionating columns and separate them at relatively low temperatures. Higher temperatures would lead to the molecules breaking up before separation takes place.
How is crude oil formed?
It is formed over millions of years from marine animal and plant remains, which are trapped under layers of rock. Under high temperature and pressure, organic matter decays in the presence of bacteria and the absence of oxygen.
Describe coal.
It is the most abundant fossil fuel. It is a combustable sedimentary rock formed from the remains of plant life which have been subject to geological heat and pressure. Most coal was formed during the carboniferous period.
What is alkylation?
It is used to increase the quality of crude oil. It is a reverse of cracking in that an alkene and alkane of a short carbon chain combine together to form a high mass alkane with a branched chain. The alkene is effectively adding across the double bon din the alkene - with a hydrogen atom bonding to once of the carbons in the double bond. The rest of the molecule adding to the other carbon of the double bond. The hydrogen atom bonds to the carbon in the double bond which has the least number of hydrogens. For example: methylpropane and methylpropene can convert to 2,2,4-trimethylpentane in presence of an acidic catalyst.
What are uses of crude oil?
It supplies the fuel needed for transport and electricity generation. It is also an important chemical feedstock for the production of importance organic compounds such as polymers, pharmaceuticals, dyes, and solvents.
Describe, in brief, catalytic cracking.
Large hydrocarbons are reacted with catalysts to form carbocations, which are produced and then rearranged on the catalyst surface. Large and intermediate-sized alkanes are passed over an alumina (Al2O3) and silica (SiO2) catalyst, which is in powdered form to increase its surface area. The lower temperature requires less energy and so reduces the cost of the process. Some carbon is formed during the process and this can coat the catalyst and stop it working. The catalyst is cleaned ore generated by separating it from the reaction mixture using steam jets. The carbon coat is removed by heating. The heat produced from the combustion can be used to sustain the cracking reaction. Catalytic cracking tends to produce branched-chain alkenes and compounds which contain the benzene ring (aromatics) - which burn move envy in a car engine than their straight-chain isomers and are used in high-quality gasoline.
What is coal liquefaction?
Liquid hydrocarbon fuels can be produced by reacting coal with hydrogen under high pressure in the presence of a catalyst. The resulting mixture of hydrocarbons can be separate day fractional distillation. As the cost of crude oil increases as a resource, this will be an increasingly economic option. The direct conversion of coal to liquids by hydrogenation process can be summarise by: nC + (n+1)H2 -> CnH(2n+2) where n > 4
What is the difference between MON and RON?
MON is motor octane number and it indicates the octane number measured at 900 rpm. RON is research octane number and it indicates the octane number measured at 600 rpm.
What is natural gas and how as it formed?
Methane is the primary constituent fo natural gas, which also contains nitrogen and sulfur compounds (hydrogen sulphide) as impurities. Natural gas was formed millions of years ago by the action of heat, pressure, and bacteria on buried organic matter. The gas is trapped in geological formations capped by impermeable rock. It is also formed from the decomposition of crude oil and coal deposits. I can occur almost entirely on its won, dissolved under pressure in oil; or in a layer above the oil in a reservoir. Natural gas can also be found associated with coal, when its a major hazard as it forms an explosive mixture with air.
How can branching in a molecule be increased by isomerisation?
Molecules are heated in the presence of a catalyst such as AlCl3. For example: octane -> 2,2,4-trimethylpentane
Describe natural gas in terms of purity.
Natural gas is the cleanest fossil fuel to burn due to its high H : C ratio. Impurities can easily be removed, so the combustion of natural gas produces minimal amounts of carbon monoxide, hydrocarbons, and particulates. Where it is available as mains gas it flows through pipes to wherever it is needed, so little energy is used to get the gas from the sound to the consumer. Setting up such a distraction network involves massive capital investment and in some countries, liquefied gas (butane or propane) is an option for domestic heating and cooking.
What are zeolites?
Naturally occurring minerals of aluminium, silicon, and oxygen. They are good catalysts for catalytic cracking because they crystal structures contain an extensive network which offers the hydrocarbons a large surface area for reaction.
Why has the use of oil overtaken that of coal?
Oil and gas are easier to extract than coal since they are fluids and can be pumped up from underground reserves. It is also possible to extract oil from beneath the sea.
Give an example of the risk of using oil as an energy source.
Petrol or gasoline is a highly concentrated and convenient energy source for transport. However, burning hydrocarbons results in environmental side-effects such as smog and global warming.
What are the four strokes of a typical gasoline engine?
Piston moves to increase volume of chamber. Air and fuel intakes. Piston moves to decrease volume of the chamber. Compression stroke and spark ignition near end of compression. Piston moves to increase volume of chamber. Expansion of gases caused by production of gases and increase temperature. Piston moves to decrease the volume of chamber. Exhaust gases expelled.
Describe an alternative to crude oil for chemical feedstock.
Polymers can be made from coil and from renewable biological materials such as wood, starch, or cotton.
What are the different fractions of crude oil, the number of carbon atoms in them, and their uses?
Refinery Gas Carbon atoms: 1 - 4 Use: fuel and as feedstock for petrochemicals Gasoline/Naphtha Carbon atoms: 5 - 10 Use: gasoline fuel for cars and naphtha for chemical feedstock Kerosene Carbon atoms: 10 - 16 Use: fuel for jets, paraffin for heating Gas Oil Carbon atoms: 13 - 25 Use: fuel for diesel engines, power plants, and heating Residue Carbon atoms: > 25 Use: oil-fired power stations, phishing waxes, lubricating oils, bitumen used to surface roads
Compare straight-chained and branched-chain isomers in terms of knocking.
Straight-Chain molecules have a greater tendency to knock. The branched-chain isomer of octane, 2,2,4-trimethylpentane (isooctane) does not suffer from premature ignition and is considered to be the standard by which other fuels are judged.
Why does crude oil need to be refined before use?
Sulfur impurities, mainly in the form of hydrogen sulphide, must be removed as they would block the active sites of the catalysts used in later chemical processing.
How is crude oil refined?
The acidic hydrogen sulphide is removed by dissolving it in basic potassium carbonate solution: H2S (g) + CO3 2- (aq) <-> HS- (aq) + HCO3 - (aq) The hydrogen sulfide can be recovered from solution by later reversing the reaction. It is burned in the air to form sulfur dioxide: 2H2S (g) + 3O2 (g) -> 2SO2 (g) + H2O (l) The sulfur dioxide produced can then react with more hydrogen sulfide to produce elemental sulfur: 2H2S (g) + SO2 (g) -> 3S (s) + 2H2O (l) The desulfurisation step also reduces acid rain pollution which would otherwise result in the sulfur was burned with the oil. The crude oil is separated into different fractions on the basis of their boiling points. In this fractional distillation process, the crude oil is heated to a temperature of about 400 ˚C. At this temperature, all the different components of the mixture are vaporised and allowed to pass up a distillation column. The level at which molecules condense depends on their size. The smaller molecules containing between one and four carbon atoms collect at the top as the refinery gas fraction. Molecules of successively larger molecular mass condense at lower levels corresponding to their higher boiling points. The residue from a fractionating column consists of hydrogen with high boiling points under normal atmospheric conditions.
How is coal formed?
The action of pressure and heat change plant material in stages from peat to lignite to bituminous soft coal and finally to anthracite. At each stage, the percentage of carbon increases.
What are the trends in fossil fuel advancement?
The carbon content of fuel has decreased over time. Coal was used first because it was abundant and easy to obtain. Coal has been replaced by glassine and natural gas because they heave higher specific energies and energy densities and are easier to transport. Gas and oil are cleaner and produce less CO2 per unit energy.
What is cracking and why is it needed?
The demand for the different fractions does not always match the amounts present int he crude oil supplied. Thus, the hydrocarbon molecules from the crude oil need to be chemically changed. Hydrocarbons with up to 12 carbon atoms are in the most demand as they are more easily vaporised and make the best fuels. The supply of these molecules can be increased by breaking down or cracking larger molecules.
What fraction is in most demand and why?
The fraction with between 5 and 10 carbon atoms as it is used in cars. It is convenient to handle and deliver and has a relatively low boiling point as a liquid. It is easy to vaporise and assists in combustion.
What are the patterns of octane numbers in straight-chain alkanes?
The octane number decreases with an increase in chain length.
How can octane numbers be increased by addition of alcohols are other compounds?
The octane number of a fuel could be increased with the addition of tetraethyl lead (Pb(C2H5)4) and this led to the widespread use of leaded gasoline. Its sue has, however, been phased out in most countries as it both inhibits the action of catalytic converters and is a source of lead air pollution. Lead is a heavy metal poison with cumulative neurological effects that are particularly damaging to young children. Methanol (octane number 107) and ethanol (108) are both added to gasoline instead fo tetraethyl lead.
How are fossil fuels produced?
The slow and partial decomposition of plant and animal matter that is trapped int eh absence of air. Oxygen is lost from biological molecules containing carbon, hydrogen, nitrogen, sulfur, and oxygen at a faster rate than other elements, which results in reduced biological compounds which are often hydrocarbons.
What is thermal cracking and catalytic cracking?
When cracking occurs by heating the starting material. Catalytic cracking is the use of a catalyst that allows the reaction to occur at a lower temperature of 500 ˚C. It helps to give the required product by controlling the mechanism.
