A Case Study of the Exxon Valdez Oil Spill of 1989

ACaseStudyoftheExxonValdezOilSpillof1989PhyllisA.LeberDepartmentofChemistryFranklinandMarshallCollegeLancaster,PA17604IntroductionPetroleumisamixtureofhydrocarbonsandtheirderivativesthatoccurnaturallyintheearth.Althoughliquidpetroleum,morecommonlyknownascrudeoil,containslightorvolatilehydrocarbonssuchasoctane(C8H18),italsoconsistsofgases,waxes(solidhydrocarbons),andbituminousmaterialssuchasasphaltandtar.Petroleum,whichisdistributedliberallythroughtheearth'scrust,hasbeenutilizedasaheatingandconstructionmaterialsinceantiquity.Forexample,anasphaltringdatingfromtheSumeriansin3500B.C.hasbeenexcavatedatthesiteoftheprehistoriccityofUrinsouthernBabylonia.1Forthenext5450yearstheearth'spopulationexploitedlittleofitsoilreserves.However,inthelast50yearsmajorworldoildependencyhasbeenpromptedbytherapidriseofmasstransportationandtheemergingpetrochemicalindustryafterWorldWarII.Aflourishingworldoiltradehasevolvedasaconsequenceoftheunevendistributionoftheworld'soilreserves.Whileoilispumpedthroughextensivepipelinesonland,severalthousandoiltankerstraversetheworld'sseasascomponentsofanextensivepetroleumtransportationsystem.Althoughoilconsumptionrelativetootherenergysourceshasdecreasedmodestlyduringthelastdecade,Figure1clearlyshowsthatoilisstilladominantenergysource.Figure1.PrimaryEnergySources(1988)Source:ScientificAmerican,Sept.1990,pp.26-8.OriginofPetroleumBecauseitisderivedfromdecayingplantandanimaldebris,primarilyphyto-andzooplankton,apetroleumisreferredtoasafossilfuel.Whilethewater-insolubleorganiccomponentsofthesedepositsinitiallycontainedoxygen,oxygenwaslargelyremovedbytwoprocesses.Theeliminationofwater,ordehydration,wasaccomplishedunderconditionsofhighpressurefromaccumulatingsediment,whiledecarboxylation,theevolutionofcarbondioxideoccuredduetotheactionofanaerobicbacterialdecomposition.Definitiveevidencefavoringtheoriesthatconnectpetroleumtoorganicratherthaninorganicsourcesstemsfromtheobservationofopticalactivityamongcertainpetroleumconstituents.Plausiblesourcematerialsfortheporphyrinbconstituentsofcrudeoilarechlorophyllcandheme.dAdditionally,fattyacidseareprimecandidatesaspetroleumsourcematerialsbecausetheycanbereadilyconvertedtohydrocarbonsbydecarboxylation(equation(1)).RCO2H→RH+CO2(g)(1)fattyacidhydrocarbonforexample:CH3(CH2)15CH2CO2H→CH3(CH2)15CH3+CO2(g)stearicacidheptadecaneAlthoughoiloriginatesinasedimentarymarinesourcebed,itultimatelymigratesthroughfaultsandfracturescreatedbytectonicfplatemovementintoreservoirrock.Reservoirrocksareusuallysedimentaryrockssuchassandstoneorlimestonewhereastypicalcaprocks,whichpreventtheescapeofthevolatilecomponentsofoil,areclaysandshalesthatarefarlesspermeable.Stratificationgintodiscretegas(i.e.,methane),oil,andwaterlayersoftenoccursinthereservoirrock.CompositionofPetroleumTheproportionofoilyconstituentshasbeenusedasthekeycriterionindifferentiatingpetroleumandasphalt.Petroleumisusuallycharacterizedascontaining50%oilyconstituents.Acomplexmixtureofsubstances,petroleumconsistspredominantlyofhydrocarbonswithmetalsandcompoundscontainingnitrogen,sulfur,oroxygenpresentinminorquantities(Table1).Thehydrocarboncomponentsofpetroleumfallintothreeclasses:paraffinsorsaturatedhydrocarbons,eitherbranchedorunbranched(CnH2n+2),cycloparaffinsornaphthenes(CnH2nwhereonlyoneringispresent),andaromaticssuchasbenzene(C6H6).Thenon-hydrocarboncomponentsconsistofeithermetalsorsulfur-,oxygen-,ornitrogen-containingcompounds.Interconversionamongthevarioushydrocarbonconstituentsofpetroleumoccursviahydrogenadditionorloss.Itisprobablethatthesechemicaltransformationsoccurduringtheformationanddevelopmentofoilreserves,possiblyduetothecatalyticeffectofcavitiesinporousclay.Table1.HydrocarbonConstituentsofPetroleumHydrocarbonsExamplesParaffinNaphthenesAromaticsNon-HydrocarbonsExamplesSulfurCompoundsOxygenCompoundsNitrogenCompoundsMetalsFe,Ni,V,CuPropertiesofPetroleumDuetotheirhydrophobicornonpolarnature,hydrocarbonsarenotsolubleinwater.Becausetheyarelessdensethanwater,hydrocarbonsfloat.Crudeoiltypicallyhasadensityof0.85g/mL.Volatilityisoneofthefundamentalcharacteristicsofliquidfuels.Assuch,itformsthebasisforthecharacterizationofliquidpetroleumfuelssuchasliquefiedpetroleum,gasoline,aviationfuel,naphthas,kerosene,gasoils,dieselfuels,andfueloils.PetroleumRefiningSeparationofcommerciallyimportantcomponentsofpetroleumisachievedusingaprocessknownasfractionaldistillation.Infractionaldistillation,averticalfractionatingcolumnwithalargesurfaceareaisutilizedtoimprovedistillationefficiency.Commercialfractionatingcolumns(Figure2)consistofmanyhorizontalplatesthatallowforrepetitivevaporization-condensationcyclesofthedistillatesothatthevaporreachingthetopofthecolumnishighlyenrichedinthemorevolatilecomponentsofthedistilland.Thisfractionationisachievedbytheestablishmentofatemperaturegradientwithinthefractionatingcolumn;thus,thetemperatureatthetopofthecolumnislowerthanisthetemperatureatthebottomofthecolumn.Ideally,thetemperatureatthetopofthecolumnwillcorrespondtotheboilingpointofthemostvolatilecomponentoftheoriginalmixture.Becausehydrocarbo