By: Shafraz Zafrullah

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Petrol Its Additives

• By Shafraz Zafrullah

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Petrol - Why do we need it?

• Demise of the widespread use of the steam engine
• Limitations - Power, size, cost, etc.
• Pollution - Environmental, noise

• Diversification of modes of transport
• Automobiles
• Aircraft
• Ships

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Petrol - Where does it come from?

• Crude oil is refined through fractional
  distillation.
• Constituents are separated and collected by
  boiling point molecular weight.
• Lower b.p. compounds are the most suitable for
  use as petrol.
• Petrol directly obtained from distillation is
  referred to as straight-run gasoline.

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Typical composition of a petroleum oil fraction
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Straight-run gasoline - Limitations

• Crude oils are not all of the same composition.
• Some are more suitable for petrol than others.
• Results in problems of quality and quantity of
  petrol.

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Main parts of a typical cylinder
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The four stroke cycle
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The octane rating

• Very commonly used and misused term used to rate
  petrol.

• Octane rating of any hydrocarbon is defined as
  being equal to the proportion of iso-octane in a
  mixture of iso-octane and n-heptane.

• Eg. Methylcyclohexane knocks under the same
  conditions as a mixture of 75 iso-octane and
  25 n-heptane, and hence has an octane rating of
  75.

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What is knocking

• Is where the fuel/air mixture ignites ahead of
  the spark plug.
• Characterised by a metallic clattering pinging
  noise.
• Caused by
• low octane petrol
• hot spots in the cylinder

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Why is knocking bad?
Knock damage on piston edge furthest from the
spark plug.
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Overcoming the problems of low quality and
quantity

• Further refinement and processing of the suitable
  fractions of the crude oil through
• Cracking (thermal, catalytic hydro-)
• Alkylation
• Isomerisation
• Re-forming
• Quality of the petrol is also enhanced through
  the use of additives.

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Cracking (thermal, catalytic hydro)

• Heating the higher boiling fractions (C10 and
  above) with a catalyst to 400-500oC.
• This causes the larger molecules to
• Crack into smaller hydrocarbons.
• Cracked hydrocarbons rearrange to branch
  chained hydrocarbons.

• Increases the amount of petrol produced.
• Increases octane rating into the 85-95 range.

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Alkylation

• Involves the heating of isobutane with low
  boiling alkenes (C3-C6) under acidic conditions.
• Causes addition of isobutane to the alkene

• Converts some lower boiling gas fractions into
  high octane fractions - typically 90-95.

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Isomerisation

• Heating at relatively low temperatures, with
  special catalysts, causes isomerisation of
  straight chain hydrocarbons to their branched
  chain isomers

• Improves the octane rating into the mid-80s.
• Is used primarily on the straight-run fraction.

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Re-formation

• Is by far the most important change process in
  refining.
• Involves the heating of the selected fraction
  with catalysts (similar to those used for
  cracking and isomerisation).
• Through careful control of the the conditions,
  hydrocarbons (C6 and above) are converted to
  aromatic compounds.

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Re-formation (cont)

• Re-formed to aromatic compounds like
• benzene
• toluene
• xylenes
• Provides a marked improvement to the octane
  rating - into the mid-to-high 90s.

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After all the refining

• With all of this refining of the crude oil, the
  average composition of the petrol produced today
  is close to
• butane 10
• paraffins 60-65
• aromatics 25-40
• alkenes small
• In addition to this the octane rating has been
  boosted from about 70 to the high 80s.

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Additives

• With the progression of technology, improvement
  in the development of engines in terms of higher
  performance characteristics and efficiency than
  earlier forms that existed earlier (ie. increased
  power, speed. compression ratio, torque etc.).
  Hence it was a natural progression that improved
  engine design went hand in hand with the
  improvement in the quality of the petrol used.
• However a thorough solution of the problem would
  be inconceivable with out the use additives, as
  they constitute the most progressive and
  economically advantageous method of producing
  high quality petrol.

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Additives (cont)

• However a thorough solution of the problem would
  be inconceivable with out the use additives, as
  they constitute the most progressive and
  economically advantageous method of producing
  high quality petrol. As a result the most common
  and expedient way used by petroleum companies to
  improve the properties of petrol, is through the
  addition of certain chemical compounds to the
  petrol prior to its use in internal combustion
  engines. Numerous organic and organometallic
  compounds with a wide variety of functional
  groups have been used over the years as additives
  to improve petrol quality.

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Anti-knock additives

• Attempts to raise the engine compression ratio,
  to increase its performance and fuel efficiency
  caused an increase in the knocking tendency.
• As a result compounds that increased the octane
  rating of the petrol is added to ameliorate the
  increase in knocking tendency.
• Generally two classes of compounds have been
  employed
• Organometallic compounds (used little today)
• Organic ethereal compounds (more common)

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Organometallic Anti-knock additives

• The most commonly used compounds have been lead
  alkyls
• tetraethyl lead (TEL)
• tetramethyl lead (TME)
• As well as physical mixtures and chemically
  reacted blends of these two lead alkyls
• Usually added in concentrations to afford a Pb
  concentration between 0.15 to 0.80g/L. Depending
  on the original octane rating of the petrol to
  begin with.

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Organometallic Anti-knock additives (cont)

• Significant points about the composition of TEL
  and TML are that
• They both contain a fixed concentration of
  elemental Pb.
• Are added with in conjunction with dibromo or
  dichloro-ethane, which scavenge the Pb from the
  cylinder as volatile lead halides.

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Organometallic Anti-knock additives (cont)

• In addition to lead the use of numerous other
  organometallic compounds incorporating Mn, Cu,
  Fe, Ce, Ni, Sn have been developed and used to
  some extent, but the economics and overall
  improvement in performance have not warranted
  their widespread use.

• However the most significant issue that has
  lead to the demise of the use of compounds like
  TEL is to do with the associated health effects
  of releasing heavy metals into the environment.

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Organic Anti-knock additives

• With the trend away from the use of compounds
  that contain heavy metals, the most favoured
  group of compounds that have been used are ethers
  like
• Methyl tert-butyl Ether (MTBE) - most commonly
  used
• Ethyl tert-butyl Ether (ETBE)
• Tert-amyl Methyl Ether (TAME)
• Di-isopropyl Ether (DIPE)
• Typically added to petrol at a level between
  2-15.

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Antioxidant additives

• As is common with most organic materials, petrol
  is subject to deterioration due to oxidation.
• Oxidation gives rise to the formation of gums and
  lowers the octane rating, which seriously
  influence the performance of the engine.
• Additives known as antioxidants retard oxidation
  through scavenging any free radicals present
• Are a cost effective alternative or supplement to
  processing modifications.

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Antioxidant additives (cont.)

• Main advantage is that the antioxidant increases
  the period that the petrol can be stored for,
  without the formation of gums.
• Examples of antioxidants used include
• aromatic diamines
• alkylphenols
• a combination of both aromatic diamines and
  alkylphenols are used

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Aromatic diamines

• Most effective antioxidant for petrol with a high
  alkene content is p-phenyldiamines of the general
  formula R-NH-C6H4-R. Where R R groups can be
  the same or different functionality. Common
  groups used include
• s-butyl
• isopropyl
• 1,4-dimethylpentyl
• 1-methylpentyl
• Added in concentrations of 5-20 ppm

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Alkylphenols

• Most effective antioxidant for petrol with a low
  alkene content, (lt10 by vol).
• Added in concentration of about 5-100 ppm.
• Most commonly used
• 4-methyl-2,6-di-tertbutylphenol
• 2,4-methyl-6-tertbutylphenol
• 2,6-di-tertbutylphenol
• Have good antioxidant stability due to sterically
  hindered hydroxyl groups due to alkyl groups in
  the 2,6 positions.

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Blends of antioxidant

• However mixtures of both aromatic diamines and
  alkyl phenols have proven to be the most
  effective and outperform equivalent
  concentrations of either constituent alone.
• The ratio of diamine to phenolic inhibitor used
  depends on the alkene content of the petrol.

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Corrosion problems

• An unwanted characteristic of most petrol is its
  corrosive aggressiveness, which is manifested
  during shipment and storage as well as under
  operating conditions (ie. within the fuel tank
  and engine).
• As a result corrosion inhibitors are added,
  typically below 20 ppm, to reduce the attack on
  the metallic surfaces that the petrol comes in
  contact with.

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Corrosion inhibitors

• These additives are usually surfactant type
  materials that attach themselves as monomolecular
  layer onto the internal steel surface, resulting
  in the formation of a barrier against attack.

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Corrosion inhibitors

• The most effective polar groups employed as
  corrosion inhibitors are acidic
• carboxylic
• phosphoric
• sulfonic
• Also the amine slats of the aforementioned acids
• In recent years the use of alcohols as octane
  boosters has increased the corrosivity of
  gasoline and has raised the need for the use of
  corrosion inhibitors.

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How do we know what we are getting?

• Quickest and easiest method is by visual
  inspection - ie. COLOUR.
• For this reason as well as to meet legal
  requirements dyes are added (2-10ppm) to colour
  the petrol. Eg
• Regular Unleaded - purpleish blue
• Premium Unleaded - Yellow
• AVGAS 100/130 - Aqua Green
• AVTUR - very pale yellow/almost colourless

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Dyes used

• Generally the dyes used are azo compounds.
• Is quite an important requirement especially in
  the aviation industry.
• Recall the eastern sates aviation fuel crisis
  of 2000.
• Some operators may add their own dyes to pass of
  their petrol as a superior product.
• So there needs to be some way to verify the
  origin of the petrol. Ie. trace it to the
  refinery/batch.

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Markers

• As a result marker chemicals are added by the
  petroleum company after refinement.
• Quite important for security purposes.
• Are detected either by a colouration reaction or
  by chromatography (usually GC).
• Marker compounds used include
• Furfural
• Diphenylamine
• Can be detected in concentrations as low as 5 in
  other fuels.

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Conclusion

• Straight-run gasoline is of too lower quality and
  quantity to be used without further processing
  refinement.
• Cracking (thermal, catalytic, hydro-)
• Alkylation
• Isomerisation
• Re-forming
• These increase both the quality and quantity of
  the petrol.

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Conclusion (cont)

• However, additives are required to further
  improve the quality of the petrol to a suitable
  standard.
• Anti-knock additives
• Antioxidant additives
• Corrosion inhibitors
• Dyes markers - for security and Quality
  assurance purposes

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Conclusion (cont)

• Importantly is a compromise in the overall
  performance and efficiency of the engine as well
  as the cost of production.

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QUESTIONS?