Impregnated-activated-carbon-for-environmental-pro

ImpregnatedactivatedcarbonforenvironmentalprotectionK.-D.HenningandS.SchiiferCarboTechAktivkohlenGmbH,Franz-Fischer-Weg61,D-45207Essen,GermanyImpregnatedactivatedcarbonsarecarbonaceousadsorbentswhichhavechemicalsfinelydistributedontheirinternalsurface.Theimpregnationoptimizestheexistingpropertiesoftheactivatedcarbongivingasynergismbetweenthechemicalsandthecarbon.Thisfacilitatesthecost-effectiveremovalofcertainimpuritiesfromgasstreamswhichwouldbeimpossibleotherwise.Forenvironmentalprotection,variousqualitiesofimpregnatedactivatedcarbonareavailableandhavebeenusedformanyyearsinthefieldsofgaspurification,civilandmilitarygasprotectionandcatalysis.Keywords:activatedcarbon;impregnatedactivatedcarbon;mercury;hydrogensulfide;environmentalprotectionIntroductionActivatedcarbonisthetradenameforacarbonaceousadsorbentwhichisdefinedasfollows’:Activatedcarbonsarenon-h~rdous,processed,carbon-aceousproducts,havingaporousstructureandalargeinternalsurfacearea.Thesematerialscanadsorbawidevarietyofsubstances,i.e.theyareabletoattractmoleculestotheirinternalsurface,andarethereforecalledadsorbents.Thevolumeofporesoftheactivatedcarbonsisgenerallygreaterthan0.2mlg-‘.Theinternalsurfaceareaisgenerallygreaterthan400m*g-‘.Thewidthoftheporesrangesfrom0.3toseveralthousandnm.Allactivatedcarbonsarecharacterizedbytheirramifiedporesystemwithinwhichvariousmesopores(r=l-25nm),micropores(r=0.4-1.0nm)andsub-micropores(r0.4nm)branchofffromwhatwecallExternalsurfaceinternalsurfaceI-internalsurface/--ExternalsurfaceSubmicroporestr0.4nm)Micropores(0.4nmr1nm)Mesopores(1nmr25nmlMacropores(r25nmlPorousstructure(schematically)Figure1Schematicactivatedcarbonmodel0960-4214/93/040235-06@1993Butterworth-HeinemannLtdGasSeparation&Purification1993Vol7No4235fm~regnatadactivatedcarbonforenvironmentalprotection:K.-D.~en~i~~andS.Schlifermacropores(r25nm)(Figure1).Activatedcarbonshavebeenusedformanyyearsquitesuccessfullyforadsorptiveremovalofimpuritiesfromexhaustgasandwastewaterstreams.However,forcost-effectiveremovalofcertainimpuritiescontainedingases(suchashydrogensulfide,mercuryandammo~a),theadsorptioncapacitiesandthefeasibleremovalratesmustbesubstantiallyboostedbyimpregnationoftheactivatedcarbonbysuitablechemicals.Whenthesechemicalsaredepositedontheinternalsurfaceoftheactivatedcarbon,theremovalmechanismalsochanges.Theimpuritiesarenolongerremovedbyadsorptionbutbychemisorption.Threereasonsforimpregnatingactivatedcarbonmaybedefined,andrelevantexamplesaregivenbelow.1OptimizationofexistingpropertiesofactivatedcarbonActivatedcarbonsarecapableofcatalyticoxidationoforganicandinorganiccompounds.Thepropertyasoxidationcatalystcanbeboostedby,forexample,impregnationwithpotassiumiodideactingaspromoter.Potassiumiodide-impregnatedactivatedcarbonsare,infact,alreadyusedforcatalytichydrogensulfideoxidationtoelementalsulfur,asdescribedlater.2Synergismbetweenactivatedcarbonandimpreg-natingagentMercuryandsulfurarenotnormallyconvertedtomercurysulfideatambienttemperature.However,ifthesulfurisdist~butedontotheinternalsurfaceofactivatedcarbon,thisreactioncanberunatlowtemperaturesformercuryremovalfromgases(seedescriptionlaterinthispaper).3UseofactivatedcarbonasaninertporouscarriermaterialActivatedcarbonisusedasaninertporouscarriermaterialfordist~butingchemicalsonthelargeinternalsurface,thusmakingthemaccessibletoreactants.Forexample,activatedcarbonimpregnatedwithphosphoricacidisusedfora~oniaremoval:HXPO1+3NH3+(NH,),PO,Aswellastheporeradiidistributionoftheactivatedcarbontobeimprecated,thechemicalcompositionandthequantityoftheimpregnationagentsusedandtheirdistributionintheporesystemareveryimportant.manufactureForthemanufactureofimpregnatedactivatedcarbon,anactivatedcarbonofsuitablequalityfortheparticularapplicationis~pre~at~withsolutionsofsaltsorotherchemicalswhich,afterdryingorotherafter-treatmentsteps,remainontheinternalsurfaceoftheactivatedcarbon.Aswellassoakingimpregnation,sprayimpregnationcanbeused.Inthatcasetheactivatedcarbonissprayedinarotarykilnorinafluidizedbedunderdefinedcon-ditions.Theimpregnatedwetactivatedcarbonneedstobedriedinanappropriateinstallation(e.g.arotarykilnorfluidi~d-beddrier).Afterthedryingstep,mostofthe236GasSeparation&Purification1993Vol7No40.6AW=microporevolumeL=microporewidth012L(nm)Figure2MicroporedistributionofbasicactivatedcarbonD47/4(a)andofthesamequalityimpr~natedwith15%sulfur(D47/4+S)(0)impregnatedactivatedcarbonscanbeusedindustrially.Insomeapplicationstheimpregnationagentsarepresentintheformofhydroxides,carbonates,chromatesornitratesandmustbesubjectedtothermalafter-treatmentathighertemperatures(150to200°C)todecomposetheanions.Accordingtotheapplication,variousactivatedcarbons(pellets,granularandpowderedqualities)areimpregnatedwithsuitableorganicorinorganicchemicals.Homogeneousdist~butionoftheimpregnatingagentsontheinternalsurfaceofanactivatedcarbonisim-portant.Furthermore,blockingofthemicroporesandmacroporesshouldbeavoidedinordertokeeptheimpregnationagentaccessibleforthereactants.Inform-ationontheimprecatingagent’sdist~butionandaccess-ibilitytotherea