Octane Numbers
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3/17/2015 Octane numbers http://www.4college.co.uk/as/df/octane.php 1/4 Octane numbers The octane number of a fuel is based on how fuels auto ignite, under compression. It is based on the scale where isooctane is 100 (minimal ignition under pressure) and heptane is 0 (easily ignites under pressure). For example, a petrol with an octane number of 92 has the same knock as a mixture of 92% isooctane and 8% heptane. Octane rating decreases with an increase in the carbon chain length. Octane ratings increase with carbon chain branching. Octane ratings increase in aromatics with same number of carbons. It is important to know octane numbers for petrol because the auto ignition of fuels causes a “knocking” effect in petrol engines. This is where it ignites twice; once due to the high pressure and again when the spark ignites the petrol. This causes the car engine to be less efficient and it can also damage the engine. However, diesel engines rely on this knocking effect, as they have no spark plugs and rely on the effect of compression to make their fuel/air mixtures ignite. How octane rating of a fuel can be increased There are two ways to increase the octane number of a fuel. One is to put special additives into the fuel which discourage auto ignition, and the other is to blend highoctane fuels in with the ordinary petrol. Antiknocking additives Antiknocking additives are substances which reduce the tendency of a fuel to autoignite, and so increase the octane number. Since the 1920’s, small amounts of lead compounds have been used as economical and effective antiknock additives. Concern over the environmental effects of leaded petrol has lead to a gradual phasing out of leaded petrol. Petrol companies have focused their attention on refining and blending to get high octane ratings.
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octane rating
Transcript of Octane Numbers
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3/17/2015 Octane numbers
http://www.4college.co.uk/as/df/octane.php 1/4
Octanenumbers
Theoctanenumberofafuelisbasedonhowfuelsautoignite,undercompression.
Itisbasedonthescalewhereisooctaneis100(minimalignitionunderpressure)andheptaneis0(easilyignitesunder
pressure).
Forexample,apetrolwithanoctanenumberof92hasthesameknockasamixtureof92%isooctaneand8%heptane.
Octaneratingdecreaseswithanincreaseinthecarbonchainlength.
Octaneratingsincreasewithcarbonchainbranching.
Octaneratingsincreaseinaromaticswithsamenumberofcarbons.
Itisimportanttoknowoctanenumbersforpetrolbecausetheautoignitionoffuelscausesaknockingeffectinpetrolengines.
Thisiswhereitignitestwiceonceduetothehighpressureandagainwhenthesparkignitesthepetrol.Thiscausesthecar
enginetobelessefficientanditcanalsodamagetheengine.
However,dieselenginesrelyonthisknockingeffect,astheyhavenosparkplugsandrelyontheeffectofcompressiontomake
theirfuel/airmixturesignite.
Howoctaneratingofafuelcanbeincreased
Therearetwowaystoincreasetheoctanenumberofafuel.Oneistoputspecialadditivesintothefuelwhichdiscourageauto
ignition,andtheotheristoblendhighoctanefuelsinwiththeordinarypetrol.
Antiknockingadditives
Antiknockingadditivesaresubstanceswhichreducethetendencyofafueltoautoignite,andsoincreasetheoctanenumber.
Sincethe1920s,smallamountsofleadcompoundshavebeenusedaseconomicalandeffectiveantiknockadditives.
Concernovertheenvironmentaleffectsofleadedpetrolhasleadtoagradualphasingoutofleadedpetrol.
Petrolcompanieshavefocusedtheirattentiononrefiningandblendingtogethighoctaneratings.
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3/17/2015 Octane numbers
http://www.4college.co.uk/as/df/octane.php 2/4
Therightkindofalkane
Thebestkindofalkaneforfuelsisnotthemostplentifulincrudeoil.
Therefinerydoctorsthefuelforourneeds.
Theshorterthealkanechain,thehighertheoctanenumber.However,thevolatilityofthehydrocarbonincreaseswitha
decreaseinthechainlength.
Theshortalkanechainshavetobemixedinpetroltolowertheirvolatilityandhighertheoctanenumberofthepetrol.
Theotherfactorthataffectstheoctanenumberisthedegreeofbranchinginthealkanechain.Themorebranchedthealkane
is,thehighertheoctanenumber.
Crudeoilcontainsbothstraightchainandbranchedalkanes,butitdoesnotcontainenoughbranchedalkanestogiveitahigh
octanenumber.
Chemistshavesolvedthisproblemincreasingtheoctanenumbersbyisomerism,reformingandcracking.
Isomerism
Isomerizationinvolvesheatingupstraightchainedalkanesinthepresenceofsuitablecatalysts,sothatthechainsbreak.
Whenthechainsjointogetheragain,theyaremorelikelytobebranched.
OilrefineriesdothisonalargescalewithC6H14andC5H12.
Theisomerizationreactionisreversibleandhighertemperaturesfavourtheproductionofstraightchained
isomers.Howeverthetemperaturecannotbetooloworthereactionwouldtaketoolong,soacompromiseismade
betweenyieldandrateofreaction.Thetemperatureusedis250oC.
Thecatalystinvolvedintheisomerizationisplatinumtheproductsarethenpassedoveraformofzeolite,whichactsasa
molecularsieve,separatingthestraightchainsfromthebranchedchains.
Reforming
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3/17/2015 Octane numbers
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Reformingiswherealkanesareconvertedtocycloalkanesandcycloalkanestoaromatichydrocarbons.
Substancessuchashexane(whicharecommonincrudeoil)areconvertedtocyclohexane.Hydrogenisalsoproduced,whichis
valuable.
Cracking
Crackingisoneofthemostimportantreactionsinthepetroleumindustry.
Crackinginvolvesbreakinguplargealkaneswhichhavetoomanycarbonsforpetrolintoshorterchainsthatcanbeusedin
petrol.
Theseshorterchainsareoftenbranched,whichgivesthemahigheroctanenumber.
Crackingalsohelpstosolvethesupplyanddemandproblem.Somefractionsareabundantinthecrudeoil,butarenotneeded.
However,otherfuelsareingreatdemandbutarenotcommoninthecrudeoil.
Mostofthecrackingisdonebyheatingupthefuelinthepresenceofacatalyst.Thisprocessisreferredtoascatcracking.The
fuelthatisheatedcanbekerosene,dieseloil,oranyotherheavierfuelsfromtheresidue.
Zeolitesareonceagainusedinthisprocess,astheyactasexcellentcatalysts.ZeoliteYisusedtocrackthefuel,asit
particularlyeffectiveatproducingfuelswithahighoctanerating.
Aswellastheshorterchainedalkanes,alkenesarealsoformed.Thesearemoreoftenbranchedandareimportantinthe
petrochemicalsindustry.
Oneproblemwithcrackingisthatcoke(carbon)tendstoformonthecatalystsurface,andsothecatalysteventuallybecomes
inactive.Toovercomethis,thecatalystiscirculatedwiththeproductsandregeneratedaftereachcycle.
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3/17/2015 Octane numbers
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Usefulbooksforrevision:
ReviseASChemistryforSalters(WrittenbyexperiencedexaminersandteachersofSalter'schemistry) ReviseASChemistryforSalters(OCR)(SaltersAdvancedChemistry)
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