PetroleumRefiningProcessesDesulphurisationSulphuroccursincrudeoilscombinedinavarietyofways,fromthesimplestcompoundH2Stocomplexringstructures.H2SisproducedduringdistillationofthecrudeoilbydecompositionofhigherboilingsulphurcompoundsandappearsintheLPGfromwhichitmustberemovedbecauseofitspoisonousandcorrosivenature.Thisisdonebycountercurrentwashingwithanamine(e.g.diethanolamine),theH2Sbeingremovedforsulphurrecoverybyheatingtheaminesolutioninaseparatevesselthusregeneratingtheamineforrecycletothewashingstage.MercaptanscanbeconsideredderivativesofH2S,inwhichonehydrogenatomisreplacebyacarbon/hydrogengroup,andsharesomeofitsunpleasantpropertiesofbadsmellandcorrosivity.Thosemercaptansboilingbelowabout80℃arereadilydissolvedinalkalinesolutionsbutthesolubilitydecreasesrapidlyabovethattemperature.ForLPGandlightgasolinesthereforethemercaptanscanberemovedbycountercurrentwashingwithcausticsodasolution.TheUOPMeroxprocessusescausticsodatoextractthemercaptanswhicharethenoxidisedwithairtodisulphidesandthecausticsodaregeneratedforfurtheruse.Theoxidationstepisassistedbyametalcomplexcatalystdissolvedinthecausticsoda.Theprocesscanberepresentedasfollow:4C2H5SH+4NaOH=4C2H5SNa+2H2O+2H2O+O22C2H5S—SC2H5+4NaOHThedisulphidesarenotsolubleincausticsodaandformanoillayerwhichcanberemoved.Mercaptansinfractionboilingbetween80℃and250℃cannotbeoxidisedtodisulphidesintheMeroxsolutionwithair.Thedisulphides,whicharenon-corrosiveandhavelittlesmell,remaindissolvedintheoilsothatnoactualdesulphurisationhasbeenachivedbuttheproductshavebeen“sweetened”.Anotherprocessfortheoxidationofmercaptansusescopperchlorideasacatalyst.Bothprocessescanbeusedintheproductionofaviationjetfuels.Asthecutstakenfromcrudeoilincreaseinboilingpointitisfoundthatthesulphurincreases.Inthe250-350℃rangewhichisusedforbothdieselfuelanddomesticcentral-heatingfuelthesulphurcontentisabout1percentweightfrommostMiddleEastcrudes.WhenthismaterialisburntthesulphurisoxidisedtoSO2which,beingeasilyoxidisedtosulphuricacid,causesatmospher