油气储运毕业设计文献翻译.docx
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油气储运毕业设计文献翻译
PetroleumRefiningProcesses
Desulphurisation
Sulphuroccursincrudeoilscombinedinavarietyofways,fromthesimplestcompoundH2Stocomplexringstructures.H2SisproducedduringdistillationofthecrudeoilbydecompositionofhigherboilingsulphurcompoundsandappearsintheLPGfromwhichitmustberemovedbecauseofitspoisonousandcorrosivenature.Thisisdonebycountercurrentwashingwithanamine(e.g.diethanolamine),theH2Sbeingremovedforsulphurrecoverybyheatingtheaminesolutioninaseparatevesselthusregeneratingtheamineforrecycletothewashingstage.MercaptanscanbeconsideredderivativesofH2S,inwhichonehydrogenatomisreplacebyacarbon/hydrogengroup,andsharesomeofitsunpleasantpropertiesofbadsmellandcorrosivity.Thosemercaptansboilingbelowabout80℃arereadilydissolvedinalkalinesolutionsbutthesolubilitydecreasesrapidlyabovethattemperature.ForLPGandlightgasolinesthereforethemercaptanscanberemovedbycountercurrentwashingwithcausticsodasolution.
TheUOPMeroxprocessusescausticsodatoextractthemercaptanswhicharethenoxidisedwithairtodisulphidesandthecausticsodaregeneratedforfurtheruse.Theoxidationstepisassistedbyametalcomplexcatalystdissolvedinthecausticsoda.
Theprocesscanberepresentedasfollow:
4C2H5SH+4NaOH=4C2H5SNa+2H2O+2H2O
+O2
2C2H5S—SC2H5+4NaOH
Thedisulphidesarenotsolubleincausticsodaandformanoillayerwhichcanberemoved.
Mercaptansinfractionboilingbetween80℃and250℃cannotbeoxidisedtodisulphidesintheMeroxsolutionwithair.Thedisulphides,whicharenon-corrosiveandhavelittlesmell,remaindissolvedintheoilsothatnoactualdesulphurisationhasbeenachivedbuttheproductshavebeen“sweetened”.Anotherprocessfortheoxidationofmercaptansusescopperchlorideasacatalyst.Bothprocessescanbeusedintheproductionofaviationjetfuels.
Asthecutstakenfromcrudeoilincreaseinboilingpointitisfoundthatthesulphurincreases.Inthe250-350℃rangewhichisusedforbothdieselfuelanddomesticcentral-heatingfuelthesulphurcontentisabout1percentweightfrommostMiddleEastcrudes.WhenthismaterialisburntthesulphurisoxidisedtoSO2which,beingeasilyoxidisedtosulphuricacid,causesatmosphericpollutionandcorrosionofmetals.Thesulphurcannotbetreatedbythemethodspreviouslyoutlinedasitismainlycombinedwithcarbonandhydrogeninformsmuchmorecomplicatedthanthesimplemercaptans.Thesecomplexcompoundshavetobebrokendowntogetatthesulphurwhichisdonebypassingtheoiltogetherwithhydrogenathightemperature(320-420℃)andhighpressure(25-70bar),overacatalystcontainingcobaltandmolybdenumoxidesonanaluminabase,madeintheformofsmallpelletsorextrudates.Thereactioniseasierandthecatalystlifebetterwhentheratioofhydrogentofeedisseveraltimeshigherthanthatnecessarytocompletethereactionchemically.UndertheseconditionsthesulphurcompoundsdecomposeandthesulphurcombineswiththehydrogentogiveH2S.Almostallofthesulphurcompoundscanbedecomposedinthiswaywithoutsignificantlyaffectingtheremaininghydrocarbons.
Thisprocessofdesulphurisation,alsocalled“hydrofining”,iseffectiveinattackingallformsofsulphurcompounds,andcanbeusedtotreatanypartofcrudeoil.
Inprincipletheequipmentusedforallfeedsisbasicallysimlarandwillcontainmeansforcarryingoutthefollowingsteps:
1Supplythefeedandhydrogentothereactoratthecorrecttemperatureandpressure.
2Coolthereactorproducttocondensetheoilandallowtheseparationoftheexcesshydrogensothatitcanberecycledtothereactor.
3RemovetheH2Sandsmallquantity(2-3percent)oflow-boilinghydrocarbonsproductedinthereaction.
Apumptakesthefeedandraisesittotherepuiredpressureandpassesitthroughtubesinafurnacewhereitisheatedtotherequiredtemperaturebeforebeingmixedwiththehydrogenandpassingintothereactor.Thereactorproductiscooled,partiallybythefreshfeedinaheatexchangertosavefuel,andpartiallybywaterinanotherheatexchanger.Excesshydrogenisseparatedfromthecondensedoilinadrumandrecirculatedbacktothereactorbyacompressortogetherwithfreshhydrogentoreplacetheamountconsumedinthereaction.TheliquidfromthedrumispassedintoadistillationcolumnwheretheH2Sandlow-boilingbreakdownproductsareremovedandthedesulphurisedoiltakenfromthebottomofthecolumn.
Muchofthecrudeoilboilingabove350℃isusedtomakeheavyfueloilforpower-stations,shipsandlargeindustrialplantsandcanhaveasulphurcontentof2.5-4percentweightfrommostMiddleEastcrudes.BuringthismaterialreleasesSO2andveryhighchimneyshavetobeused(anumberinthe500-600footrangehavebeenbuilt,oneof800feetintheUSA)sothattheSO2canbedisperedwidelyintheatmospherethusavoidinglocalisedpollution.Theidealsolutionwouldbetodesulphuriseallpartsofthecrudeoil.Unfortunately,althoughthedesulphurisationofdistillatesboilinguptoabout550℃cabberelativelyeasilyaccomplished,thetreatmentofheavycrude-oilresiduesposesmanydifficultproblems.Withincreasingboiling-pointthedifficultyofdesulphurisationincreasesandalsotheproportionofmoleculescontainingsulphurbecomeshigh(possiblyupto50percent)whichmeansthatahighproportionofthemoleculespresentmustbedecomposed.Tracemetalsintheoiltendtodeactivatethemosteffectivedesulphurisationcatalystsandhighpressures(upto170bar)mustbsused.Allthesefactorsresultinhighcostsforfuel-oildesulphurisation.Alsorecentdevelopmentsinthecrude-oilsopplysituationworldwidehaveplacedastongemphasisonenergyconservation.Consequently,fuelintensiveprocesseswouldbeemployedonlyasalastresortwhenalternativemeansofminimingpollutionarenotviable.
InarefinerywheredesulphurisationisusedextensivelytheproductionofH2Scaneasilyreach100tonnesperday.AlthoughtheH2ScouldbeburnttoSO2andventedfromallstacks,itisveryundesirablebecauseoftheatmosphericpollutioncausedandadditionalplantisinstslledtorecoverthesulphur.TheH2SisburnttoSO2withtheoxygensupplylimitedsothataboutone-thirtoftheH2Sburns.Thisgivesamixtureoftwo-thirdsH2Sandone-thirdSO2whichwillcombinetoformsulphurandwater:
2H2S+SO2=3S+2H2O
Thesulphuriscollectedandisusuallysoldtochemicalcompaniesmainlyforthemanufactureofsulphuricacid.
ThermalCracking
Whenhydrocarbonsareheatedtotemperaturesexceedingabout450℃theybegintodecompose.Thelargemoleculesbreakingor“cracking”intosmallerones.Paraffinsarethemosteasilycrackedfollowedbynaphthenes,aromaticsbeingextremelyrefractory.Atonetimethermal-crackingprocesseswerewidelyusedtoimprovetheoctanenumberofnaphthasortoproducegasolineandgasoilfromheavyfractions.Thequalityofthegasolinefromthethermalcrackingofnaphthaisnothighenoughforpresentmotorgasolinesandtheprocesshasfallenoutofuse.Theproductsfromheavyoilcrackingrequirementsandwhileatpresenttheprocessislittleuseditcouldbeofinterestshouldconversionofheavydistillates(upto550℃)togasoilsberequired.Onethermalcrackingprocesspresentlyincommonuseisvisbreakingwhichisthethermalcrackingofviscouscrude-oilresiduestoreducetheirviscousitybybreakingdownthelargecomplexmoleculestosmallerones.Asatisfactoryfueloilcanthembemadewithoutthenecessityofusinggasoilsorkerosinetoblendwiththeviscousresidue.
Anotherthermal-crackingprocesspresentlyemployedisDelayedCoking,whichisnormallyappliedtoatmosphericorvacuumresiduesfromlowsulphurcrudesfortheproductionofelectrodegradecoke(usedmostlyinaluminiumproduction).Theresidueisheatedtoabout500℃andpassedtothebottomofalargedrumwherethecrackingreactionproceedswhichbreaksdownthehigh-boilingmaterials,Thelowerboilingmaterialsformedvaporiseatthehightemperatureinthedrumandpassoutofthetoptothefractionationsystemwheretheyareseparatedintogas,gasolineandgasoilandleavebehindinthedrumaporousmassofcoke.Whenthedrumisfullofcokethefeedisswitchedtoanotherdrumwhichisfilledwhilethefulloneissteamedoutandthecokeremoved.Asinthethermal-crackingprocesstheliquidproductsrequirehydrogenationforuseasnaphthaorgasoil.
CatalyticCracking
Thermalcrackingofheavydistillatesforgasolineproductionisnotselectiveandproducessubstantialquantitiesofgasandfueloiltogetherwiththegasoline,whichisalsonotofverygoodquality.Aboutthirtyyearsagoitwasfoundthatfuller’searthandsimillarmaterialscouldactascrackingcatalystsandgiveagoodyieldofhighoctanenumbergasline(catalyticcracking).Unfortunately,thefuller’searthbecamequicklycoveredincarbonandnolongeractedasacatalystwasreturnedtoitspreviousactivityandthustooperatetheprocesscontinuouslyitwasnecessarytodevisemethodsofalternativelyusingandregeneratingthecatalystcontinuouslyonalargescale.
Oneofthemostsuccessfulmethodsofachievingthisdependsontheuseoffluidisation.Whenagasispassedupthroughabedoffinepowerthebehaviourofthepowerdependsonthevelocityofthegas.Ifitishigh(about1m/sec.)theparticlesaremovedaboutbythegasandthebedofpoweractslikeafluidandcanbetransported,finditsownlevel,ect.justlikealiquid.Byusingthisproperty,thecatalyst,inpowerform,canbecirculatedcontinuouslybetweenareactionstageandaregenerat