Developing Fuels – Storylines notes.

DF1 – Petrol is Popular.

·         Read pages 19-20 DF1 and outline the advantages and disadvantages of electric and petrol fueled vehicles.

ELECTRIC –

PETROL –

DF2 – Getting energy from fuels.

The role of oxygen

·         Fig 3 page 20 gives ∆H_comb^q for 5 fuels – why are they different?

·         To release energy fuels must burn i.e. react with O₂. Energy is released as bonds with oxygen are formed. Therefore ∆H_comb^q of a fuel depends on;

v      The number of bonds to be broken and made (larger molecules have more bonds to break and \make, so will have larger ∆H_comb^q values).

v      The type of bonds involved. Compare equations for combustion of methane and methanol on page 21, the products are the same but CH₃OH already has one O-H bond. This bond does not have to be remade in forming products so less energy is given out when methanol burns. We say methanol is already partially oxidised.

The more oxygen a fuel has in its molecule, the less energy it gives out when it burns.

·         Alcohols are \less energy rich than hydrocarbons – however they are also less polluting.

Important news for slimmers

·         Excess energy from energy rich foods is stored in the body as fat.

·         Compare;

Glucose (carbohydrate)            C₆H₁₂O₆

Glycerol trioleate (olive oil)     C₅₇H₁₀₄O₆

·         Why are fats more fattening?

·         Alcohols are energy rich (can burn as an alternative to petrol!) so are also fattening.

Carrying fuels around.

·         For fuels ∆H_comb^qis important, but this tells us energy released per mole of fuel.

·         For transport purposes energy density can be more important i.e. energy per kilogram. (this can be calculated using ∆H_comb^q and Rmm – see table 1 pg. 22)

DF3 – Focus on Petrol.

What is petrol?

·         A mixture of many compounds, mostly hydrocarbons.

·         Obtained from the gasoline and gas oil fractions of crude oil, separated by fractional distillation, (see fig 8 page 23 – you must understand this process.)

·         Gasoline fraction is made up mainly of alkanes C₅-C₁₀ (Bpt range 20-180°C)

·         Naphtha fraction is also very useful – being used for high grade petrol and for the production of many organic chemicals. Table 2 and fig 9 (both on page 24) give the uses and also supply/demand information on all the fractions.

Distillation

Straight run

What is it?

Modifactions

              Why?

Isomerisation                           Reforming                        Cracking

Products?                                           Products?                                           Products?

Ø      Some useful products can also be obtained from the residue by vacuum distillation (see green box on page 24) e.g. fuel oils for power stations or ships and lubricating oils and waxes.

Winter and summer petrol.

·         Petrol must be blended to give correct properties – e.g. volatility.

·         In a car engine a mixture of petrol vapour and air is produced in the carburettor and ignited in the cylinder ( see fig 10 pg 25)

·         If it is cold the petrol is difficult to vaporise so the car is difficult to start.

·         Petrol companies make different blends for different times of the year – in winter it must be more volatile to vaporise more easily so compounds with lower Bpt’s are added i.e. small molecules with low RMM’s e.g. C₄-C₅.

·         In summer the blend must be less volatile WHY?

·         Hence petrol is blended to suit different countries and climates.

The problem of Knocking.

·         Another important property of petrol is its octane rating – a measure of the tendency of the petrol to cause ‘knock’ by autoignition. (read green box on page 26)

·         In a petrol engine the air/petrol mix must ignite at exactly the right time – just before the piston reaches the top of the cylinder.

·         As the mixture is compressed in the cylinder it gets hot (many modern cars use high compression ratios to achieve greater efficiency).

·         Under these conditions many hydrocarbons autoignite i.e. catch fire as they are compressed.

·         If this happens you get two explosions

·         One due to compression

·         One as spark from spark plug occurs.

·         This causes knocking or pinking.

·         The thrust of the piston no longer occurs at the correct time so performance is affected, and the cylinder may become damaged.

DF4_- Making petrol – getting the right octane number.

·         High performance cars need high octane fuel to prevent knocking.

·         Knocking is controlled by;

·         Additives used to discourage autoignition

·         Blending high octane compounds with normal petrol.

Look no lead

·         Anti-knock additives reduce tendency to autoignite (­ octane rating)

·         Lead compounds used since 1920’s (economical and effective)

·         Now phased out WHY?

·         Blending now seen as best alternative.

Refining and blending

·         The best performing hydrocarbons are not plentiful in crude oil so refinery must modify what they have.

·         The structure of an alkane influences its octane number.

·         Shorter chain alkanes ;

·         Have high octane no. so good to improve cold start (even CH₄  - C₄H₁₀ can be dissolved in petrol)

·         But are more volatile so can’t add too many (WHY?)

·         Branched chained alkanes also have higher octane nos than straight chains but are not plentiful in crude oil.

·         3 processes are used to produce compounds with the desired properties – isomerisation, reforming  and cracking.

Isomerisation.

·         Involves taking straight chain alkanes, heating in the presence of a catalyst – chains break and rejoin in a different arrangement.

·         When they reform they are more likely to be branched.

·        
Refineries do this with pentane and hexane to increase the octane quality of petrol.

·         Note that reactions do not go to completion – they are equilibrium reactions. The result is a mixture of straight and branched chain products.

·         Zeolites (molecular sieves) are used to separate straight and branched chains so straight chains can be recycled and go through the process again. (read purple box on zeolites page 28)

·         In modern plants the catalyst is aluminium oxide coated in finely dispersed platinum.

Reforming.

·         Another way of increasing octane quality of petrol components.

·         Uses naphtha fraction (C₆ – C₁₀) heated to 500°C and passed over a platinum catalyst.

·         See fig. 16 for reactions.

·         Process is called platforming.

·         Sometimes carbon is produced which reduces the efficiency of the catalyst so H₂ sometimes added to the naphtha to suppress this.

·         £5 million worth of catalyst in one reformer – so important to keep it in good working order!

Cracking.

·         One of the most important reactions in the petroleum industry!

·         Involves breaking large alkanes into smaller, more useful molecules.

·         Products often branched – so good for octane rating.

·         Solves supply/demand of fractions problem.

How is it done?

·         Mostly done by heating heavy oils in presence of a catalst – catalytic cracking.

·        Feedstock – heavy fractions C_25-100  (Mostly C_30-40)

·         Cracking produces many products.

·         They are separated using fractionating columns.

·         Zeolites used this time as catalysts.

A Modern Catalytic Cracker

v      Catalytic crackers have been used since late 1940’s;

v    They are flexible.

v      Can be adapted for different feedstocks.

v      Conditions/catalyst easily varied.

v      Hence can give max amount of desired products e.g. branched alkanes for petrol blending.

See Chemical Ideas chapter 12.1 for conditions, feedstocks, products etc. of these three important processes.

Adding oxygenates.

·         Oxygenates are fuels with oxygen in their molecules, e.g. alcohols (contain –OH group) or ethers (contain –C-O-C- group) (read green box on page 31).

·         Table 3 on page 31 shows the most common oxygenates for petrol blending.

·         One is MBTE (methyltertiarybutylether – modern name 2-methoxy-2-methly propane)

CH₃                                         Fig 9 shows the % MBTE

                                                Added (by volume) to

CH₃       C         O         CH₃                 to different petrol blends.

            CH₃                   MBTE

·         Why add oxygenates?

The perfect blend.

·         Fig 20 page 32 gives the octane nos of petrol ingredients.

·         Blender must produce petrol from these ingredients that meets specification required for volatility, density, octane no. etc. but keep cost as low as possible.

·         Price and availability of ingredients must be considered.

·         Computer models used to help make decisions.

·         Blending is a batch process;

·         Mixing is very important to give a homogeneous mixture. (read green box ‘Why do hydrocarbons Mix’)

DF5 – Trouble with emissions.

Burning fuels causes worldwide concerns because of emissions (see fig21)

·         CO₂, CO

·         H₂O

·         C_xH_y (even from parked cars!)

·         NO_x (NO, NO₂)

·         SO_x (SO₂, SO₃)

Photochemical smog.

            It is not just above emissions which cause problems. Ozone (O₃) is a secondary pollutant formed by the action of UV radiation on C_xH_y, NO_x, O₂ and H₂O. (read green box ‘Ozone – a molecule of many parts’)

·         Stratospheric O₃ is essential to block harmful UV radiation from the sun.

·         Tropospheric O₃ is an irritant, toxic to humans, and contributes to global warming.

Photochemical smog;

·         Contains a mixture of 1° and 2° pollutants (see fig 23 page 34)

·         Composition depends on many factors e.g. geography, climate, time of day, nature of pollutants etc.

·         Worse in summer during high pressure conditions when still air traps pollutants near the ground.

·        
Produced by the reactions shown in Fig 24 page 34(above) – read and learn! (even in unpolluted air there is a small amount of O₃ – This starts the reaction which forms the smog. As smog is produced then concⁿ of O₃ ­.

·         The reactions are slow, so the highest O₃ readings are often in the countryside away from the source of pollution.

Effects of smog.

·         Haziness and reduced visibility at ground level.

·         Eye and nose irritation from O₃.

·         Problems worse for asthmatics, the very young or very old.

·         Other chemicals in smog also cause problems.

·         O₃ very reactive – attacks organic matter – so plants and animals affected.

·         C=C attacked by O₃ so some plastics, rubbers, textiles, paints etc. damaged.

·         Exact links between ozone and these problems difficult to establish – research is still going on!

How can chemists help?

·         Monitoring pollutants – looking for patterns during day/year to help explain what is going on – many sites in UK now (see figs 25 and 26 page 35.

·         Laboratory studies – study reactions occuring between various species present in the air under controlled conditions.

·         Computer modelling studies – info from experiments fed in and used to reproduce and predict behaviour of pollutants in the atmosphere.

·         Smog chamber simulations – lab experiments on a grand scale! See fig 27.

DF6 – Tackling the emissions problem.

            Governments are enforcing ever stricter emission limits for new vehicles (see fig 28 page 36). There are 2 ways to tackle problem;

1)      Change car engine.

2)     Change the fuel (covered in DF7)

Changing engine technology.

            Assignment 9 will help you calculate how much air a petrol engine requires.

·         The ratio of air:fuel calculated is the volume of air needed to burn the petrol completely.

·         It is called the stoichiometric ratio i.e. involving the exact amounts shown in the chemical equation.

·        
For an average car the ratio is  15:1  air:petrol   (by mass)

·         Less air – mixture too rich, i.e. too much petrol – use this when running on choke to start the car.

·         More air – mixture is lean – produces less CO and NO_x but level of C_xH_y can increase. If mixture too lean causes misfire of engine therefore ­ emissions. fig 29

·         Lean burn engines use air:fuel of about 18:1 – requires specially designed combustion chambers to prevent misfiring. Get better fuel economy – use less fuel each time engine fires.

Catalysts can help.

·         Catalysts speed up chemical reactions without getting used up themselves.

·         Look at these reactions ;

2CO(g)  +  O₂(g)  ®  2CO₂(g)

C₇H₁₆(g)  +  11O₂(g) ®  7CO₂(g)  +  8H₂O(g)

2NO(g)  +  2CO(g)  ®  N₂(g)  +  2CO₂(g)

·         These occur between exhaust pollutants as they are formed (note products!)

·         They are slow reactions – too slow to effectively reduce pollutants.

·         Having a precious metal catalyst (e.g. Pt or Rh) in the exhaust system speeds up these reactions – these are contained in a catalytic converter.

(Assignment 10)

·         Lean burn engine use an oxidation catalyst to remove CO and C_xH_y (exhaust gases are rich in O₂) This catalyst would not remove NO – but lean burn engines don’t produce much!

·         An ordinary engine produces more NO so a simple oxidation catalyst is not enough – a three way catalyst system is required.

·         3 way catalyst oxidises CO and C_xH_y  but also reduces NO.

·         This system only works if the air:petrol mix is carefully controlled to be exactly the stoichiometric mixture.

·         If too rich not enough O₂ in exaust to remove CO and C_xH_y

·         If too lean problems with misfiring and increased emissions.

·         The car must have an oxygen sensor in the exhaust linked back to the electronically controlled fuel injection system. (see fig 30 page 37)

·         Fig 31 page 38 shows the effect of catalyst on exhaust emissions – 3 way system is most efficient.

Catalytic converters;

·         Only work when hot (Pt at 240°C but alloyed Pt/Rh at 150°C)

·         Are poisoned by lead so only work with lead free fuel.

·         Used as a powder spread on a ceramic support with a network of tiny holes to give a large surface area (2-3 football pitches per catalytic converter!!)

DF7 – Changing the fuel.

            The second way of tackling emissions problem is to change the fuel.

Aromatic hydrocarbons.

Ø      Make up about 40% of lead free petrol.

Ø      Cause high CO, C_xH_y and NO₂ emissions.

Ø      Some cause cancer (especially benzene).

Ø      May need to be controlled in future.

Butane.

Ø      Volatile so responsible for evaporative emissions.

Ø      Hence leads to ozone formation and photochemical smog.

Ø      May also need to be reduced.

• Can’t just take out these high octane components and expect the same performance.
• Therefore need to produce reformulated gasoline with lower volatility, lower benzene content and more oxygenates (to reduce pollutants) BUT without reducing the efficiency of the fuel!!

·         Two possible solutions are methanol and ethanol.

Methanol

            One possible key to future fuels.

Ethanol

            Ethanol can also be added to petrol. It has a high octane number and is less polluting. It can be made from the ethene produced by cracking naphtha, but can also be made by fermenting cane sugar juice – a renewable resource.

            Gasahol (a mixture of ethanol and petrol) is used in Mexico where cane sugar is grown.

DF8- Hydrogen – A Fuel for the Future.

          If hydrogen can be produced without consuming precious fossil fuels it could not only reduce our dependency on these, but also provide an alternative fuel which does not release CO₂ on burning.

          Hydrogen can be obtained from water. It could be distributed as we do natural gas. It can be burned as a heating fuel, used in internal combustion engines or converted into electricity in a fuel cell.

Making Hydrogen.

• Hydrogen is produced by the large scale electrolysis of water.
• Energy would need to be obtained from a renewable resource to carry out the electrolysis.
• Why bother producing electricity to then use it to produce H₂?
• Hydrogen can be stored
• It can be used in an internal combustion engine.

• Fig 37 shows a possible scheme for producing hydrogen all the necessary technology already exists.

The Hydrogen Economy.

• Uses H₂ as a way of storing and distributing energy.
• Can use existing technology and equipment e.g. gas pipelines.
• Distributing H₂ by pipeline may be cheaper than transmitting electricity.
• Fuel cells can be used to generate electricity on a small scale in cars. (see fig 38)
• The main problem for cars is the storage of hydrogen. A large volume of hydrogen gas is needed to get the equivalent mileage to petrol.
• One solution is to store it in a fuel tank under pressure as a liquid.
• Developing a hydrogen economy would require long term, large scale investments, therefore success will depend on political and economical factors.