光催化氧化技术

光催化氧化技术 92 GRANULAR ACTIVATED CARBON 12.Gullick,R.W.,Grayman,W.M.,Deininger,R.A.,andMales,Amongthemostcommonlyusedrawmaterials,precursors fortheproductionofcommerciallyactivatedcarbonsare R.M.(2003).Designofearlywarningmonitoringsystemsforwood(130,000tons/year),coal(100,000tons/year),lignite (50,000tons/year),coconutshell(35,000tons/year),and sourcewaters.J.Am.WaterWorksAssoc.95:58–72. peat(35,000tons/year)(4,5). 13.Ropp,S.L.,Jin,Q.,Knight,J.C.,Massung,R.F.,andEspos- ito,J.J.(1995).PCRstrategyforidenti,cationanddiffer-TYPES OF ACTIVATED CARBONS entiationofsmallpoxandotherorthopoxviruses.J.Clin.Basedonthephysicalproperties,activatedcarbonscanbe classi,edintothefollowingbroadcategories: 33:2069–2076. Microbiol. 14.Peruski,A.H.andPeruski,L.F.,Jr.(2003).ImmunologicalPowdered Activated Carbon (PAC) Theseactivatedcarbonsaremadeinthepowdersor methodsfordetectionandidenti,cationofinfectiousdisease,negranulesaccordingtotherequirement.Powdered activatedcarbonshaveadiameterbetween15and25 µm. andbiologicalwarfareagents.Clin.Diagn.Lab.Immunol.Thesecarbonsprovidealargeinternalsurfacewitha smalldiffusiondistance. 10:506–513. 15.Tims,T.B.andLim,D.V.(2004).RapiddetectionofBacillus Granular Activated Carbon (GAC) sporesdirectlyfrompowderswithanevanes-anthracisGranularactivatedcarbon(GAC)iscommonlyusedfor thepuri,cationofliquidsandgases.Granularactivated centwave,ber-opticbiosensor.J.Microbiol.Methods59:carbonadsorbsavastvarietyofdissolvedorganicmate- rials,includingmanythatarenonbiodegradable.GAC 127–130. removesorganiccontaminantsfromwater/wastewater 16.Kijek,T.M.,Rossi,C.A.,Moss,D.,Parke,R.W.,andHen- bytheadsorptionprocessoftheattractionandaccu- chal,E.A.(2000).Rapidandsensitiveimmunomagnetic-mulationofonesubstanceonthesurfaceofanother. Granularactivatedcarbonstypicallyhavesurfaceareasof electrochemiluminescentdetectionofstaphylococcalentero-500–2000m2/g,withsomereportedashighas3000m2/g. toxinB.236:9–17. Muchofthesurfaceareaavailablefortheadsorption J.Immunol.Methods 17.Firmani,M.A.andBroussard,L.A.(2003).Moleculardiag-ingranularactivatedcarbonparticlesisfoundinthe poreswithinthegranularcarbonparticlescreatedduring nostictechniquesforuseinresponsetobioterrorism.Expert Table1.SourceMaterialsUsedforthePreparationof 3:605–616. Rev.Mol.Diagn.ActivatedCarbons defmw1702mw166 GRANULAR ACTIVATED CARBON DINESHMOHAN KUNWARP.SINGH GomtiNagar,Lucknow,Uttar Pradesh,India Activatedcarbonisthegenerictermusedtodescribe afamilyofcarbonaceousadsorbentswithahighly crystallineformandextensivelydevelopedinternal porestructure.Activatedcarbonisdistinguishedfrom elementalcarbonbytheremovalofallnoncarbon rangeofporesizes,usedcarbonizedwoodaround1fromvisiblecracksimpuritiesandtheoxidationofthecarbonsurface(1).B.C.asanadsorbent500andcrevices,tocra cksActivatedcarbonhasthehighestvolumeofadsorbingformedicinalpurposesaswella sapurifyingagent.The porosityofanysubstanceknowntohumans(5gramsof andcrevicesofmol ancientHindusinIndiausedcheculardimensions.activatedcarboncanhavethesurfaceareaofafootballarcoalfor,ltrationof ,eld).Itcanbede,nedas(2): drinkingwater.However,theb asisforindustrialpro-Theuseofactivated Activatedcarbonisacrudeformofgraphite,witharandomcarbonisnotnew.T heEgyptiansductionofactivecarbonswasesoramorphousstructure,whichishighlyporous,overabroadtablishedin1900–1901in ordertoreplacebonecharinthesugarre,ningprocess(3).Bagasse Lampblack Bark Leatherwaste ActivecarbonscanbepreparedfromawiderangeBeat-sugarsludgeMolasses Municipalwaste ofcarbonaceousmaterials,whichincludecoconutshells,Blood Newspaper Bluedust Nutshells woodchar,lignin,petroleumcoke,bonechar,peat,sawBones Oilshale CarbohydratesOlivestones Palmtreecobs dust,carbonblack,ricehulls,sugar,peachpits,,sh, Cereals peat fertilizerwaste,andwasterubbertire.(Table1).TheCoal Petroleumacidsludge Coconutcoir Petroleumcoke rangeofrawmaterialsisdiverseandwidespreadandCoconutshellPotassiumferrocynideresidue greatlyin,uencedbytheneedtoproducelow-costcarbon.Coffeebeans Pulp-millwaste CornCobsandcornstalksRef,nationearth Re,nerywaste CottonseedhullsRicehulls Rubberwaste DistillerywasteSawdust Scraptires FertilizerwasteslurrySpentFuller’searth Sun,owerseeds Fish Sugar-beetsludge Fruitpits Tealeaves Fuller’searthRubberTires Wheatstraw Graphite WoodHumanhairs Jutestick Kelpandseaweed Leatherwaste Lignin Lignite GRANULAR ACTIVATED CARBON93 carbonaceouscompoundsandnonorganizedcarbon. theactivationprocess.GAChaverelativelylargerparti- Theprecursorsusedfortheproductionofactivatedclesixesthanpowderedactivatedcarbonandtherefore carbonshavealargeeffectontheporesizedistribution,provideasmallerexternalsurface.Thesecarbonsarepre- surfacearea,andotherphysicalandchemicalproperties.ferredforallthesorptionofgasesandvapors.GACarealso Table2sumsupsomeofthebasicdifferencesbetweenusedinwater/wastewatertreatment,deodorization,decol- therawmaterialsusedfortheproductionofsomeorization,andseparationofcomponentsin,owsystems. importantactivatedcarbons,whereasthepropertiesof Spherical Activated Carbon (SAC) Thesecarbonsarepreparedfromsmallsphericalballs whereinpitchismeltedinthepresenceofnaphthalene ortetorlinandconvertedintospheres.Thesespheres arethencontactedwithnapthasolution,whichextracts naphthaleneintroducedintotheporousstructure.These porousspheresarethenheatedbetween100and400 ?Cin thepresenceofanoxidizingagent.Theoxidizedspheres arethenheatedbetween150and700 ?Cinthepresence ofammoniatointroducenitrogenintospheresfollowedby activationinsteamorCO2. Impregnated Activated Carbon (IAC) Inchemicalactivation,acatalystmaybeimpregnatedinto thefeedstock.Themostcommonlyusedchemicalactivants includeZnCl2,H3PO4,H2SO4,KOH,K2S,andKCNS. Inthisprocess,anear-saturatedsolutionofcatalyst- impregnatedfeedstockisdriedtoin,uencepyrolysisin suchawaythattarformationandvolatilizationcan bekeptataminimum.Theresultingproductisthen carbonized.Silverimpregnatedactivatedcarbonsareused forpuri,cationofdomesticwater. Polymer Coated Activated Carbons (POAC) Inthisprocess,porouscarboncanbecoatedwith biocompatiblepolymersresultinginasmoothand permeablecoatwithoutblockingthepores.Itiswell documentedinliteraturethatactivatedcarbonspossessa highlydevelopedporoussystem.Theseporesareproduced duringtheactivationprocessofcarbonizedresiduewhen spacesbetweenelementarycrystallineareclearedof importantaspect.Thefollowingpointsmustbeconsidered beforeselectinganyrawmaterialfortheproductionof somecommerciallyavailablecarbonswiththeirsources activatedcarbons: ascollectedfromliteraturearepresentedinTable3. 1.Industriallyinexpensivematerialswithhighcar- METHODS FOR ACTIVATED CARBON DEVELOPMENT bonandlowinorganiccontentshouldalways bepreferred. Themethodsforthedevelopmentofactivatedcarbons 2.Theimpuritiesinrawmaterialsshouldbekeptata arenearlyaswidespreadastheirpotentialusesand minimumbecauseaftertheactivationprocess,many sourcematerials(6).However,thebasicstepsmost ofthesemaybepresentinthecarbonathigher commonlyusedinthepreparationofactivatedcarbons concentrationsthantheprecursorsmaterials. areprecursormaterialpreparation,palletizing,low- 3.Importanceshouldbegiventotheprecursorshaving temperaturecarbonization,followedbychemicalor physicalactivation(Fig.1).Anumberofmethodswere highdensityandsuf,cientvolatilecontent.The volatileresultsinporouschar,whereashighdensity usedforthepreparationofactivatedcarbonsfromwaste materialsfromtimetotimeusingdifferentactivation favorstheenhancementofstructuralstrengthof thecarbonneededtowithstandexcessiveparticle parameters.Differentsteps/activationparametersused forthepreparationofsomeoftheactivatedcarbonsare crumbleduringuse. presentedinTable4.Althoughthelistisnotcomplete,it 4.Therawmaterialshouldbeavailableinabun- willprovideageneralideaofthedifferentmethodsused dancelocally. fortheproductionofactivatedcarbons. Therawmaterialsusedforthepreparationofactivated Raw Materials carbonsvarywiththeirapplications.Acomparisonofsome oftherawmaterialsispresentedinTable2. Theselectionofanappropriaterawmaterial(Table1)for thedevelopmentofgranularactivatedcarbonisthemost Table2.BasicDifferencesinPrecursorMaterialsUsedfortheProductionofActivatedCarbons Percent RawPercentVolatileAshDensityActivated Materials CarbonMatterPercent(kg/m3)CarbonTextureApplications Liquidphaseadsorption Softwithlargeporevolume Hardwood40–4255–600.25–1.20.50–0.8 LiquidphaseadsorptionSoftwithlargeporevolume Vaporphaseadsorption Hardwithlargemultipore Softwood40–4555–600.25–1.00.40–0.50 Nutshells40–4555–600.40–0.601.4 volume Lignite50–7025–405–61.0–1.40HardwithsmallporevolumeLiquidphaseadsorption MediumhardwithmediumLiquidandvaporphaseSoftcoal60–8025–302–121.25–1.50 microporevolume adsorption Semihardcoal70–751–155–151.45HardwithlargeporevolumeVaporphaseadsorption HardwithlargeporevolumeVapourphaseadsorptionHardcoal85–955–102–151.50–2.0 94GRANULAR ACTIVATED CARBON Table3.PropertiesofSomeSelectedActivatedCarbonsGatheredfromtheLiterature Total Moisture Surface as PoreIodine TypesofRawArea,BETVolumeNumberAshUniformityPackedEffectiveApparent Carbons2/g)(%)Material(m(ml/g)(mg/g)(%)Size,mmDensityzpcCoef,cientpH F-100 Bituminouscoal850–900—8502.19.020.8–1.0— 0.55–0.75— F-200850 Bituminouscoal714 —1.98.22 F-300 Bituminouscoal950–1500.8590092.1 1.9 F-400 — 1.4 Bituminouscoal1050–1200 1.5 F-816 1000 — 0.945.4— F-820Bituminouscoal—— — CentaurHSV 9001.44 Bituminouscoal— — NucharSN—9— NucharSABituminouscoal—900 NucharW Wood1400–1800— — HD-4000800 DracoKBWood1400–1800 7NoritGAC840R —900 Wood1400–1600 Lignitecoal625 9003–6— Hardwood1500 9003–6 Reactivated—— 647— — —23 0.932 800 1.8— 9.8 20.8–1.00.5g/cc10.40 — 2 — 0.55–0.7527— 24.0 4.0 21.3–1.5—— — 4 — 1.0–1.2— — 10 10—0.56g/cc 10 8—337–369(k g/m3) 2 —337–369(k g/m3) —240–305(k g/m3) 0.740.40g/ml —0.45g/cc —0.48g/cc Carbon NoritPAC20BCoal——800———3— 0.51g/cc HD-CLignitecoal 556 500 4——— NoritGAC1240coal11000.9510201.8—2—0.50g/cc Barnebey&Coconutshell1100–1200—10502–3————— SutcliffeSE Barnebey&Coconutshell1100–1200—10502–3———— SutcliffePE Barnebey&Coconutshell1150——5———0.48g/cc SutcliffeKE Barnebey&Coconutshell1150——5——5—0.4–0.8g/cc SutcliffeUU PicaPOU/POE 1150 5 — Coconutshell700–2200— — 5—0.45– 0.54g/ cc GX203 carbon PicaPOU/POENCCoconutshell700–2200—11004——3—0.41–0.45g/cc 506 carbon CameronPACarbResinouswood——5008——3—0.45g/cc charcoal SelectoABA4000 300–5000.35–0.55—————— LC Witco517Petroleum1050—10000.51.4—10.890.52g/cc lessorganizedlooselyboundcarbonaceousmaterial.The Carbonization resultingchannelsthroughthegraphiticregions,the Carbonization,sometimescalledcharring,convertsthespacesbetweentheelementarycrystallites,together with,ssureswithinandparalleltothegraphiteplanes constitutetheporousstructure,withlargeinternalsurface organicmaterialintoprimarycarbon,whichisamixturearea(24).Therearetwotypesofactivation,whichareused toimpartaporousstructurewithinastartingmaterial ofash,tars,amorphouscarbon,andcrystallinecarbon.ofrelativelylowsurfacearea,namelythermal/physicalor chemicalactivation. Incarbonization,thematerialisheatedslowlyin Physical or Thermal Activation.Physicalorthermal theabsenceofair.Inthisprocess,mostofthe activationoccursafterinitialtreatmentandpalletizing; noncarbonelements,hydrogen,andoxygenare,rstitinvolvescarbonizationat500–600 ?Ctoeliminatethe bulkofthevolatilematterfollowedbypartialgasi,cation removedingaseousformbypyrolyticdecompositionof usingmildoxidizinggassuchasCO2,steam,orfuelgasat 800–1000 ?Ctodeveloptheporosityandsurfacearea(25). thestartingmaterials.Theimportantparameters,whichAnexampleincludesthegasi,cationofthecarbonized materialwithsteam,andcarbondioxideoccursbythe determinethequalityandquantityofthecarbonizedfollowingendothermicreactions: product,are(a)rateofheating,(b),naltemperature,andC +H2O ????CO +H2 (29kcal) (c)soakingtime. C +CO2 ????2CO (39kcal) Activation Theactivationiscarriedoutbasicallytoenlargethe diametersofthepores,whicharecreatedduringthe carbonizationprocess,andtocreatesomenewporosity, whichresultsintheformationofawell-de,nedand readilyaccessibleporestructurewithlargeinternal surfacearea.Duringtheactivationprocess,thespaces betweentheelementarycrystallitesbecomeclearedor GRANULAR ACTIVATED CARBON95 Precursor materials Grinding/Classifying/ Reconstitution Sizing Pretreatment Carbonization PhysicalActivation Chemical SieveGrinding analysis PACGAC Figure1.Variousstepsusedintheactivated carbonproduction. Chemical Activation.Thesecondtypeofactivation CO +H2O ????CO2 +H2 (10kcal) involvestheincorporationofinorganicadditivesor TheH2OmoleculeissmallerthantheCO2molecule metallicchlorides,suchaszincchlorideorphosphoricandthusdiffusesfasterintotheporesofthecarbon. Consequently,areactionwithsteamisfasterthanthatofacid,intotheprecursorbeforethecarbonization(6).Ithas CO2.Ithasbeenreportedinliteraturethatadecreasein thereactionratewithCO2activationoncarbon-containing wastesisnearlytwotimeslessthanthatofsteam.When airoroxygenisusedasanactivatingagent,problems developbecauseoftheexothermicnatureofthereactions ofcarbonwithair(oxygen),andthusitisdif,cultto control.Despitetheseproblems,severalresearchershave usedthesamefortheactivationoftheirproducts. beenreportedthatcarbonswithwell-developedporous structure,mainlymesoandmicroporous,canbeproduced byZnCl2incorporation.KOHactivationhasalsobeen showntosuccessfullyincreasethesurfaceareaandpore volumeofactivecarbons(20,26) Manyotherchemicals,suchasammoniumsalts, borates,calciumoxide,ferricandferrouscompounds, manganesedioxide,nickelsalts,hydrochloricacid,nitric acid,andsulfuricacid,havealsobeenusedforthe activationpurpose. Thebasicdifferencebetweenphysicalandchemical activationisthenumberofstagesrequiredforactivation andthetemperatureatwhichtheactivationtakes place.Chemicalactivationisaone-stepprocess,whereas physicalactivationisatwo-stepprocess,including carbonizationandactivation.Thetemperaturerequired inphysicalactivation(800–1000 ?C)ishigherthanthatof chemicalactivation(200–800 ?C). 96GRANULAR ACTIVATED CARBON Table4.SomeChemicalActivant-FeedstockCouples[ExtendedFormof(4)] Feedstock Conditions ActivantReferenceCoconutshell 7 1.5partsbyweightH2SO4for 24hrsat140–160 ?C:steamCon.H2SO4 activationat1Kg/m2pressure for30min. Agriculturalbyproductssuchas —82at1123KHeatinginCO almondshell,olivestonesandpeach stones Coconutshell 9 Con.H2SO41.0partsbyweightH2SO4for Fertilizerslurry 10 24hat150 ?C Palmtreecobs 11 450 ?C,1h Coconutshell 12 730 ?C,6h H2O2/H2O,N2Petroleumcoke 3 450 ?C Raf,nationearth 13 700–850 ?C,4h H3PO4/H2SO4Algeriancoal 14 10%v/v,350 ?C Pinesawdust 15 930 ?C Almondandpecanshells16 850 ?C,1h;825 ?C,6h H3PO4 Chemicalactivationwith KOH/H2O H2SO4 KOH/NaOH Fe(NO3)3/CO2 H3PO4 H3PO4/PhysicalCO2 Eucalytuswoodchars —17 Bituminouscoal 18 ZnCl2CO2activation,400–800 ?C N2/400–700 ?C Coalorcoconutshell Phosgeneorchlorinegasat19 180 ?C Petroleumcoke KOH20 Dehydrationat400 ?Cfollowedby Lignite 21 Na2MoO4/activationin500–900 ?C Inertatmosphere/600–800 ?C NaWO4/ NH4VO3/ (NH4)2MoO4/ FeCl3/ Fe(NO3)3 150 ?C,SodiumbicarbonatePeanuthulls Coconutshellandcocnutshell,bers H2SO4600 ?C H2SO4 22 23 Thus,theactivatedcarbonscanbepreparedbyeitherHigherdensityGACispreferredbecauseofthe followingreasons: physical/chemicalactivationoracombinationofbothwith welldistributedporosityandhighsurfacearea. 1.HighdensityGAChasmorecarbonstructure. 2.ThehighdensityGAC,guredthat,foreachcubicCharacterization of GAC meterofvolume,moreGACcouldbeinstalled. Theeffectivenessofactivatedcarbonasaunitprocess3.Bituminouscoal-basedGACprovidesamuchdenser materialthanligniteorsubbituminouscoal. inthetreatmentofwater/wastewater/industrialef,uents Moisture.Itisimportantonlyforshippingand requiresthebestselectionofanappropriateactivatedmanufacturingpurposes.Itisde,nedasthepercentby massofwateradsorbedonactivatedcarbon. carbon.Followingaresomeoftheimportantproperties Hardness and Attrition.Itisanimportantfactorin systemdesign,,lterlife,andproducthandling.Large usedfortheselectionofsuitableactivatedcarbonfordifferencesexistinthehardnessofactivatedcarbons, dependingontherawmaterialsusedandactivitylevel speci,ctreatment: (Tables2and3).Itisde,nedastheresistanceofa granularactivatedcarbontothedegradationactionof Density.ThedensityofcarboncanbeexpressedinsteelballsinaRo–Tapmachine.Itiscalculatedbyusing themassofgranularcarbonretainedonaparticularsieve severaldifferentways.Thetwomostimportantamong afterthecarbonhasbeenwithsteelballs. theseareapparentandparticledensities. Apparent Density.Apparentdensityhaslittlerolein initialevaluationofanactivatedcarbon,butithasan importantroleintheregenerationprocess.Itisde,ned asthemassofcarbonperunitvolumethatcanbepacked intoanemptycolumn.Itisexpressedingramspercubic centimeterorpoundspercubicfoot. Sieve Analysis.Itisveryusefulincarbonproduction Particle Density.Particledensityisde,nedastheratio evaluation.Itisalsoimportantintheevaluationof purchasedcarbontothespeci,cations.Thedistributionof particlesizesinagivensampleisobtainedbymechanicallyofthemassofdryandunloadedcarbonparticlestothe totalvolumeoftheparticles,includingporevolume. GRANULAR ACTIVATED CARBON97 d =w ×n shakingaweighedamountofactivatedcarbonthrougha wherewisthepercentofthetotalweightretainedbyseriesoftestsievesanddeterminingthequantityretained aparticularsieveandnistheaverageofthemesh byorpassinggivensieves. openingofthesievethatcontainedwandthesieveused Abrasion Number.Theabrasionnumberisimportantimmediatelyabove. inevaluatingtheabilityofacarbontowithstandattrition.Byusingthisd,themeanparticlediametercanbe Itisde,nedasameasureoftheresistanceoftheparticlescalculatedas: todegradeonbeingmechanicallyabraded,whichis Meanparticlediameter =sumofalldiametersmeasuredbyputtingacarbonsamplewithsteelballs dividedby100inapanonaRo–Tapmachine.Inotherwords,itis de,nedastheratioofthe,nalaverage(mean)particle diametertotheoriginalaverage(mean)particlediameter times100. Ash Percent.Theashpercentisveryimportantinthe evaluationoftherawmaterialsandthemanufacturing process.Itistheresiduethatremainswhenthe carbonaceousportionisburnedoff.Theashcontained mainlyhassilica,alumina,iron,magnesium,andcalcium. Ashinactivatedcarbonisnotdesirableandisconsidered tobeanimpurity.Itcanbemeasuredbythechangein weightbyburningthecarbonsampletoconstantweight at800 ?C. pH.TheeffectofthecarbononpHofavolumeof waterisverymuchdependentontherelativequantities ofbothactivatedcarbonaswellaswater.ThepHeffect canbestudiedbyputting1.0-gramcarbonwith50mlof de-ionizedwaterandheatingto90 ?Cfollowedbycooling to20 ?CandmeasuringthepHofthesupernatant. Effective Size, Mean Particle Diameter, and Uniformity Coef,cient.Itisusedtoestablishthehydraulicconditions ofanadsorbercolumn.Measurementofthegradationof carbonparticlesplaysanimportantroleintheevaluation ofheadlossinthe,owthroughgranularbeds.Themean particlediametercanbecalculatedasfollows: Firsttheaverageparticlediameter(d)ofeachsieve usedfortheparticledistributioniscalculatedbyusingthe followingequation: atrimoleculardistributionofporesizes(Fig.2),as discussedbelow. coef,cient.The50%passingsievesizeisapproximately themeanparticlediameter.Theuniformitycoef,cient isbasicallyadimensionlessfactor,whichindicatesthe Macropores.Thesearetheporeshavingdiameter degreeofuniformityofGAC.Avalueofoneindicates greaterthan50nm(Fig.2).Theirporevolumevaries from0.2to0.5cm3/gandsurfacearealiesbetween thatallparametersareidenticalinsize,whereasgreater 0.5and2m2/g.Suchlowvalueofsurfacearearenders valuesrelatetoahigherdegreeofvariation. themoflittleuseinadsorptionexceptforlarge adsorbatemolecules. Surface Area.Totalsurfaceareaisveryimportant tocharacterizeporoussolids.Largesurfaceareais generallyarequirementforagoodadsorbent.However, Mesopores.Theseporeshavediameterrangingbetween thetotalsurfaceareahastopossessadequateporesize 2nmand50nm(Fig.2).Theporevolumeofthesepores distributionandsurfacechemistrytoadsorbthetargeted variedfrom0.02to0.01cm3/g.Thesurfaceareaconsti- species.Surfaceareadeterminationreliesontheaccurate tutedbytheseporesliesbetween10and100m2/g.The knowledgeoftheaveragearea.Itisdeterminedbythe capillarycondensationwiththeformationofameniscus sorptionofnitrogengasintothecarbonandisexpressed ofadsorbatemainlytakesplaceinthesepores. insquareareapergramofcarbon.Themostwidelyaccept methodistheBETnitrogenadsorptionmethod. Micropores.Theseporeshavediameterlessthan2nm (Fig.2).Theirsizegenerallycorrespondstothatof Pore Size Distribution.Activatedcarbonisacomplex molecules.Thevalueoftheseporesrangesfrom0.15to networkofporesofdifferentsizesandshapes.Theshapes 0.5cm3/gandsurfacearealiesbetween100and1000m2/g. includecylinders,rectangularcrosssections,andother Theyconstituteabout95%ofthetotalsurfacearea.These irregularshapesandconstrictions.Theidenti,cationof poresareofgreatsigni,canceasfarasadsorptionstudies differentsizesiscalledtheporesizedistribution. areconcerned. Theporesizedistributionisverymuchdependenton Besidesthese,twodifferenttypesofporesareexistin thesourcematerialsandmethodofactivation.According toIUPACacceptedcriteria(27),activatedcarbonshave someofthecarbons. Itisveryimportanttonoteherethatthesmallestsieveis notconsideredincalculatingthemeanparticlediameter.Ultramicropores.Theseporeshavediameter <0.7nm. TheeffectivesizecanthusbecalculatedbyusingtheSupermicropores.Theseporeshavediameterbetween 0.7and2nm. cumulativepercentageofcarbonpassingeachsizeandInpractice,totalporevolumeiscalculatedby measuringthreedistinctvolumesassociatedwithone plottingthesieveopeninginmillimeters(ordinate)versusgramofthematerialataconstanttemperature.These threedistinctvolumesare: thecumulativepercentage(abscissa)onasemilogarithmic scale.Thus,theeffectivesizeistheopeninginmillimeters, whichpasses10%ofthetotalmaterial.The60%passing sievesizedividedbytheeffectivesizegivestheuniformity 98GRANULAR ACTIVATED CARBON actualwaterused.Itisverywelldocumentedinliterature thatatlowpH,phenoladsorptionismuchgreaterthanat neutraloralkalipH. SURFACE CHEMISTRY OF ACTIVATED CARBON Thetypeofstartingmaterialandthemethodofactivation usedinproducingactivatedcarbondeterminesthe natureofvarioussurfacefunctionalgroups.Thesurface chemistryofcarbonhasbeenstudiedextensivelyby severalinvestigatorsfromtimetotime.Thenatureof External Pores Internal Poresthecarbonsurfacebasicallydependsontheconditions andtemperaturesemployedduringtheactivationprocess. Activatedcarboncanbedividedintotwomajortypes, namelyacidic(L)andbasic(H),accordingtoSteenberg’s classi,cation(1)andcanbede,nedasfollows: Micropores L Carbons (d , 2 nm)Thesecarbonsarepreparedbyheatingtherawmaterials Macropores atabout200–400 ?Cinthepresenceofair.Thesecarbons (> 50 nm) Mesoporesassumeanegativecharge(ionized)uponhydrationand, (2 , d , 50 nm) ?Where d represents diameter BoththeiodinenumberandthemolassesnumberFigure2.Schematicactivatedcarbonstructure. decreasewithtimeasadsorptionoccurs. (a)thevolumeofitspurelysolidstructure Methylene Blue Number.Itisde,nedasthenumber(b)theexternalgeometricalvolumeofthesolid (c)thevolumeofagiven,uidthatitcandisplace ofmilligramsofmethyleneblueadsorbedbyonegram Thus,thetotalporevolumecanbegivenbythedifferenceofactivatedcarboninequilibriumwithasolutionof between(b)and(a). methylenebluedyehavingconcentration1.0mg/L. Iodine Number.Itisde,nedasthenumberof milligramsofiodineadsorbedbyonegramofactivatedPhenol Number.Phenolnumberhasbeenfoundtobea carbonwhentheiodineconcentrationoftheresiduallesssensitivetestasitisverymucheffectedbypHofthe ,ltrateis0.02N. Theiodinenumberprovidesanindicationoftheamount ofsmallporesincarbon.Itiscorrelatedwiththesurface ? asmallmolecule,itprovidesanindicationofaparticular carbon’scapacitytoadsorbsmallermolecules. Molasses Number.Themolassesnumberrepresentsthe amountoflargeporesincarbon.Itiscalculatedfromthe ratiooftheopticaldensitiesofthe,ltrateofamolasses solutiontreatedwithastandardactivatedcarbonandone inquestion.Themolassesnumbercanbecorrelatedwith thesurfaceareainporeswithdiametersgreaterthan thus,yieldingacidicpH,arehydrophilicinnatureand canneutralizeastrongbase.Thesecarbonsgenerally developacidicsurfaceoxidesandlowerthepHvalue ofneutralorbasicsolutions.Theyprimarilysorb basesfromthesolutionsandexhibitanegativezeta potential.Wood-basedactivatedcarbonsareusuallyL typeinnature.Thepredominantsurfacefunctional groupsarecarboxyl,phenolichydroxyl,carbonyl(quinone type),carboxylicacid,anhydrides,lactone,andcyclic peroxide(28,29).Ithasbeenreportedthatcarboxylicand lactonegroupstendtodiscouragetheadsorpionofmany aromaticcompounds(29).Carbonylgroupsintheformof quinonestructures,andhydroquinonegroups,however, canenhancetheadsorptionofaromaticcompoundsbythe formationofanelectrondonor-acceptorcomplexbetween thearomaticringandthesurfacecarbonylgroups(30,31). H Carbons Theactivatedcarbonsarepreparedbyheatingtheraw materialsatabout800–1000 ?Cintheabsenceofairorin thepresenceofCO2followedbyexposuretoairatroom temperature.Thesecarbonsassumeapositivecharge (protonated)uponintroductionintowater,thusyielding alkalinepH.Theyarehydrophobicinnatureandcan neutralizestrongacids.Thesecarbonsgenerallydevelop basicsurfaceoxidesandincreasethepHvalueofneutralor acidicsolutions.Theyprimarilysorbstrongacidsfromthe solutionsandexhibitapositivezetapotential.Coconut shell-basedanddustcoalactivatedcarbonsareusually Htypeinnature.Thepredominantsurfacefunctional groupsonthesurfaceofthecarbonsarelactones,quinones, phenols,andcarboxylates.Morereportsofthechromene (benzpyran)groupsonthesurfaceofthecarbonexist(32). BoehmandVoll(33)suggestedthatbasicsurfaceoxide mayberepresentedbypyrone-likestructures. ItisinterestingtonotethatH-typecarbonscanbe convertedintoL-typecarbonswhentheyareoxidesby chemicaloxidantsoragedintheatmosphere. areainporeswithdiameterslessthan10A.Asiodineis 10A. GRANULAR ACTIVATED CARBON99 adsorbentsurface.Ingaseousseparations,activatedTheacidicgroupsonactivatedcarbonsarebelieved tobeoneofthemostimportantpropertiesofactivatedcarbonisoftenusedtoremoveodorsandimpurities carbonsformetalionsadsorption(34).TheL-typecarbonfromindustrialgases,torecovervaluablesolventvapor, isastrongersolidacidthantheH-typecarbon,thereforeandtodehumidifyairandothergases.Inliquid L-typecarbonismoreef,cientfortheadsorptionof separations,activatedcarboncanbeappliedforremovingheavymetalions.Surfaceareaisnotaprimaryfactor thetasteandodorfromwater,decolorizing,andtreatingforadsorptiononactivatedcarbon.Highsurfacearea industrialwastewatercontainingorganics,dyes,anddoesnotmeanhighadsorptioncapacity,asreportedby heavymetalions. Perrich,(35),becauseofthefollowingfactors: (a)Inadsorption,onlythewettedsurfaceareais effective,whichisneverequaltothetotal surfacearea. (b)Insomeadsorptionprocesses,thematerialtobe adsorbedistoolargethatitcannotenterthesmall poreswherethebulkofthesurfaceareaexists. (c)Dataonthesurfacearea,porevolume,andsurface natureusuallyhavenotbeencorrelatedwithdata onthematerialtobeadsorbed. Thephysicalandchemicalpropertiesofthecarbonsare verymuchdependentonthepropertiesofrawmaterials, methodofcarbonization,andactivation.Theproperties ofsomeselectedcarbonsdevelopedfromdifferentsources aregiveninTable3.Thesepropertiesarecollectedfrom varioussourcesincludingbooks,literature,andcompany brochures. ADSORPTION EQUILIBRIUM Todeterminetheultimateadsorptioncapacity,thesolutes arebroughtincontactwithagivenamountofactivated carbonoranyotheradsorbentinaclosedsystem.If adsorptionisthedominatingremovalmechanism,then theresidualconcentrationwillbereachedthatwill remainunchangedwithtime,whichisalsoknowas equilibriumconcentration.Thisprocessisknownas adsorptionequilibrium. Theadsorptionprocessisessentiallyanattraction ofgaseousorliquidadsorbatemoleculesontoaporous Polarity:Lesspolar(orweaklyionized)organics numberoflayersbeingproportionaltothecontaminant areadsorbedmoreeasilythanpolar(orstrongly concentration. ionized)organics. :Lesssolublecompoundsareadsorbedmore Solubility Chemisorption easilythanmoresolublecompounds. Itistheresultofchemicalinteractionbetweenthecarbon surfaceandtheadsorbedsubstance.Itusuallyinvolves ADSORPTION ISOTHERMS strongbonds,andisthereforeirreversible. Anumberoffactorsexistthataffectadsorption, Anadsorptionisothermistherelationshipbetweenthe includingchemicalpropertiesofadsorbateandactivated amountofasubstanceadsorbedontheactivatedcarbon carbon,pH,andtemperatureoftheadsorbate.Chemical surfaceandtheequilibriumconcentrationofdissolved adsorptionisusuallydominantathightemperatures adsorbateataconstanttemperatureandotherconditions. becausechemicalreactionsproceedmorerapidlyat Anadsorptionisothermisanexpressionoftheprinciple elevatedtemperaturesthanatlowtemperatures. ofmicroscopicreversibility,althoughadsorptioncanbe Thefollowingfactorsaffectthesorptionoforganicson irreversible.Themostcommonmethodforgathering activatedcarbons: isothermdataisthebottlepointexperiment.These equilibriumdataareformulatedintoanadsorption HydrocarbonSaturation:Double-ortriple-carbonbond isothermmodel.Brunaueretal.(36)pointedoutthat (unsaturation)organicsareadsorbedmoreeasily althoughtheisothermsaredifferentforallsorbentsand thansingle-carbonbond(saturated)organics. sorbates,moreorlesscommonshapesofisothermsare MolecularStructure:Branch-chainorganicsareadsor- observed.Sixdifferenttypesofisotherms(37)exist,which bedmoreeasilythanstraight-chainorganics. arenamedastype1,typeII,typeIII,typeIV,typeV,and typeVI.Thepictorialrepresentationoftheisothermsare MolecularWeight:Largermoleculesaregenerally giveninFig.3. adsorbedmoreeasilythansmallermolecules. Therearetwotypesofadsorptionprocesses. Type 1 Physical Adsorption Thistypeofisothermisobtainedfromcarbonshaving Physicaladsorptionisareversiblephenomenon.Itresultsmicroporesonly,whichcorrespondstomonolayeradsorp- tionaspostulatedbyLangmuir.Thevolumeofthegas adsorbedapproachesalimitingvalue,justenoughtocom- fromtheactionofvanderWaalsforces,comprisedof pleteamonomolecularlayerevenwhenthegaspressureis Londondispersionforcesandclassicelectrostaticforcesratherlow.Furtherincreasesinpressurehardlyproduces anyfurtherincreaseintheamountofadsorption.These ofattractionbetweenmoleculesofthecarbonandthetypesofisothermsaretypicalofamicroporoussolidwhere onlymonolayeradsorptionoccurs.Examplesincludethesubstanceadsorbed(adsorbate).Physicaladsorptionis usuallydominantatlowtemperatures.Thistypeof adsorptionisusuallymultilayered;i.e.,eachmolecular layerformsontopofthepreviouslayer,withthe 100GRANULAR ACTIVATED CARBON Type IIType I Type III Type IV Hysteresis Loop Type V Type VI Hysteresis Loop Second layer Monolayer coverage bothmicroporesandmesopores.ThesetypesofisothermsFigure3.Typeofisotherms. showlargedeviationsfromtheLangmuirmodel.The adsorptionofnitrogenorhydrogenonmicroporouscarbon amountofadsorptionkeepsincreasingineachcasewithanattemperaturescloseto ?180 ?C. increaseinpressure,whichisattributedtotheformation Type II ofadditionallayersofphysicallyadsorbedgasmolecules.Thistypeofisothermdescribesthephysicaladsorption Itisconsideredthatthegasmoleculesadsorbedinthe ofgasesbynonporoussolids,andmonolayercoverageis,rstlayermayholdbyvanderWaalsforcesasecondlayer succeededbymultilayeradsorptionathigherpressure.ofgasmolecules,which,inturn,mayholdathirdlayer Examplesincludetheadsorptionofnitrogenonironandsoon.TheexamplesoftypeIIIisothermsincludethe catalystat ?195 ?C. adsorptionofbromineonsilicaoraluminagelat80 ?C. Type III Thistypeofisothermisobtainedfromcarbonhaving Relative pressure Type IV Thistypeofisothermisobtainedfrombothnonporousand mesoporoussolids. Theseareobservedinthecaseswhereapossibilityof condensationofgasesexistswithinthenarrowcapillary poresoftheadsorbent.Thisphenomenonisalsoknown ascapillarycondensation.ExamplesoftypeIVinclude adsorptionofbenzeneonsilicagelat50 ?Candthatof watervaporonactivatedcarbonat100 ?C. Type V Thistypeofisothermoriginatesfrommicroporousand mesoporoussolids.TypeVisothermsarebasicallyrelated totypeIIIisothermsandareveryuncommon. Type VI Thistypeofisothermisobtainedfromuniformnonporous surfacesandrepresentsstepwisemultilayeradsorption. Thesharpnessofthestepsdependsonthesystemandthe temperature.Thestepheightrepresentsthemonolayer capacityforeachadsorbedlayer,andinthesimplestcase, itremainsnearlyconstantfortwoorthreeadsorbedlay- ers.Examplesincludetheisothermsobtainedwithargon Amount adsorbed GRANULAR ACTIVATED CARBON101 orkryptonongraphitizedcarbonblackatliquidnitrogenwhereqeistheamountofsoluteadsorbedperunit weightofactivatedcarbon(mg/g),Ceistheequilibrium temperature. concentrationofsoluteinthebulksolution(mg/L),KFis theconstantindicativeoftherelativeadsorptioncapacity oftheadsorbent(mg/g),and1/nistheconstantindicative APPLICATIONS OF ACTIVATED CARBONS oftheintensityoftheadsorption. Activatedcarbonsareproducedingranular,powdered, Langmuir Isotherm.TheLangmuiradsorptionisothermandpalletizedformsandhaveawiderangeofapplications. describesthesurfacetobehomogeneous.TheLangmuir adsorptionisothermassumedthatalltheadsorption Theapplicationsofactivatedcarbonusescanbroadlybe siteshaveequalaf,nityforthemoleculesandthatthe adsorptionatonesitedoesnotaffectadsorptionatan dividedintotwocategories. adjacentsite(39,40). TheLangmuirequationmaybewrittenas Liquid-Phase Applications Liquid-phasegranularactivatedcarbonadsorption (GACA)isanef,cient,easy,andreliabletreatmenttech- nology.Itisverymuchdifferentfromgas-phasecarbon asliquid-phasecarbonshavesigni,cantlymoreporevol- Q0bCeqe = (Nonlinearform) umeinthemacroporerange,whichpermitsadsorbatesto diffusemorerapidlyintothemicroporesandmesopores. 1 Itisconsideredtobeabestavailablecontroltechnol- +(Linearform) Q0 ogy(BACT)bytheU.S.EnvironmentalProtectionAgency 1 +bCe Ce1 qe Q0b (USEPA)andisabenchmarkforotherremediationtech-isnormally0.3–3.0mm,andthesearemostlyusedin nologies.Inorderforcarbonadsorptiontoworkwell,itcontinuous,owsystems(,xedandmovingbed).Inthis isimportantthatthe,naldesignincorporateboththearticle,mostofthediscussionhasbeenrestrictedtothe physicalandadsorptionprocess.Inliquid-phasegranularliquidadsorptiononly. activatedadsorption,activatedcarboncanbeusedeither inpowder,granular,orpalletizedforms.Theaveragesize ofpowderactivatedcarbonrangesbetween15and25 µm andaremostfrequentlyusedinbatchapplications.On theotherhand,thegranularactivatedcarbonparticlesize isreached. qeistheamountofsoluteadsorbedperunitweight where ofadsorbent(mg/g),Ceistheequilibriumconcentration BET Isotherm.TheBrunauer,Emmett,Teller(BET) ofsoluteinthebulksolution(mg/L),Q0isthemonolayer isothermassumesthepertainingofacompoundbetween adsorptioncapacity(mg/g),andistheconstantrelatedto bliquidandsolidcompartmentsorphases.Thisisotherm thefreeenergyofadsorption.Itisthevaluereciprocalof assumesthemultilayersadsorptionofsoluteonactivated concentrationatwhichhalfthesaturationoftheadsorbent carbon(36,40) Batch Systems.Generally,powderedactivatedcarbons BCQ0 NonlinearFormqe = areusedinbatchsystems.Batchsystemconsistsof (Cs ?C)[1 + (B ?1)(C/Cs)]contactingawholevolumeoffeedsolutionwithade,nite quantityofactivatedcarboninbatchstirredvessels. CB ?1C1 Themixtureisstirredoragitatedtofacilitatemass qe ==+Linerform0 transfer.Theimportantprocessdesignparameterscan 0CsBQ (Cs ?C)qe becalculatedfromlaboratorybatchadsorptionisotherms,whereqeistheamountofadsorbateadsorbedperunit weightofactivatedcarbon,Bistheconstantrelatedtothe whichpreciselymodelthefull-scalebatchprocess.Theenergyofinteractionwiththesurface,Cistheequilibrium concentrationofadsorbateinsolution(mg/Lormol/L), batchadsorptionprocessesareseldomusedexceptinQ0isthenumberofmolesofadsorbateperunitweight ofcarbontoformacompletemonolayer,andCsisthe laboratoriesbecausetheyarehighlyinef,cientcomparedsaturationconcentrationoftheadsorbate. Thelimitationsoftheadsorptionisothermsareas withcolumnadsorptionprocessesandarethereforecapitalfollows: intensiveandexpensivetooperate. 1.Isothermsareequilibriumtests,andtherefore,the Varioustheoreticalandempiricalmodelshavebeentimerestrictionsarenotconsidered. 2.Isothermsarebasedoncarbonexhaustion—gra- proposedtodescribethedifferenttypesofadsorp- nularsystemsdonottotallyexhaust—andtheentire tionisothermsinbatchsystems.Themostcommonlybedcontents. usedmodelsincludeFreundlich(38),Langmuir(39),and3.Long-termchemicalandbiologicaleffectsare BETisotherms. Freundlich Isotherm.TheFreundlichadsorptioniso- thermmodelwasgivenbyFreundlich(38).Thisisotherm describestheequilibriumonheterogeneoussurfacesand, hence,doesnotassumemonolayercapacity TheFreundlichequationmaybewrittenas notevident. qe =KFCe1/n(Nonlinearform) logqe =logKF + Thus,batchequilibriumadsorp1 (Linearform)tionisothermtestscannotnlogCe simulateorpredictdynamicper formancedirectly. = ?Ce BQ 102GRANULAR ACTIVATED CARBON ofthezoneiscontrolledbymanyfcatorsdependingon Continuous Systems.Incontinuoussystems,mostly thesoluteconcentrationbeingadsorbed,characteristicsof granularactivatedcarbons(GAC)areused.The,xedactivatedcarbon,andhydraulicfactors. Aplotbetweentheconcentrationoftheadsorbate bedadsorbersystemsaremostwidelyusedforconductingexhibitsanS-shapedcurveintheadsorptionzonewith endsasymptoticallyapproachingzeroandthein,uent adsorptionoperationswheretheadsorbatetobetreatedconcentrationC0.Thiscurveisknownasabreak .Anidealbreakthroughplotobtainedfor throughcurveispassedthrougha,xedbed.Inthe,xedbedadsorbera,xedbedadsorberisdepictedinFig.4.Thesoluteor impurityisadsorbedveryrapidlybythefewinitialupper operation,adegreeofseparationandremovalisachievedlayersofthefreshgranularcarbonduringtheinitial stagesofoperation,asshowninFig.4.Theseupper thatwouldrequiremanystepsinabatchsystem.Thelayersareincontactwiththesorbate/impurityatits highestconcentrationlevel,C0.Thesmallamountsof parameters,whicharerequiredtoestablisha,xed solute/impuritythatescapeadsorptionininitialstages bedreactor,includethetypeofcarbon,physicaland areadsorbedinthelowerstages,andnosoluteescapes chemicalcharacteristicsofthecarbon,columndiameter,fromthe,xedbedadsorberinitially (C1 =0). water/wastewater/ef,uents,owrate,pHoftheef,uent, FIXED BED ADSORBER carbonbeddepth,weightofcarbon,contacttime, concentrationofthein,uent,concentrationoftheef,uent,Anadsorptionprocessinwhichliquidbeingtreated isallowedtopassthroughacarboncolumninthat anddesiredef,uentconcentration carbonbecomesexhaustedandtheunitisremovedfrom Whencontaminatedsolute(warer/wastewater/serviceandcompletelyrechargedwithfreshcarbon.The carbonremains,xedinthepositionduringthewhole ef,uents)ispassedthroughabedofgranularactivatedadsorptionprocess. carbon,awavefrontoramasstransferzone(MTZ)is formedbycontinuousadsorptionofsoluteinthecar- bonbed.Figure4showsthechangeinconcentrationof adsorbedspeciesonthesurfaceofactivatedcarbonwith time.Thesoluteisrapidlyadsorbedonthetoplayersof thebeduntiltheamountadsorbedisinequilibriumwith in,uentsoluteconcentration.Atthisparticulartime,that portionofthebedisexhausted.Belowthiszoneisasecond zonewheredynamicadsorptionistakingplace.Thesolute isbeingtransferredfromtheliquidtotheadsorbedphase. Thiszoneisknownasmasstransferzone,andthedepth C0C0C0C0C0C0 Adsorption zone C0 C1C2CbC4C5Ce Exhaust point Break Point Vb Figure4.Typicaladsorptionzonemove- Effluent volumementina,xedbedadsorber. Effluent concentration GRANULAR ACTIVATED CARBON103 samevelocity.Itisequaltothevolumeoftheemptybed Toconstructa,xedbedadsorber,thefollowingdesign dividedbythe,owrate.Conventionalwatertreatment parametersareused: plantadsorbershaveEBCTsintherangeof7–10min. Itcanbecalculatedbyusingtheequation Volumetric Flow Rate (Q) Itisthequantityofsolutefedperunittime.Flowrate Lt VbAbLin,uencestheadsorptioncapacityofcarboninadynamic EBCT ====α system.Ifa1-inchcolumnistobeused,20.6ml/min,ow FAbV Q Fv rate,whichisequalto1g/min/ft2,istobeused. VbisthevolumeofGACincontactor(m3);Qisthe whereThefollowingvaluesmayfurtherbeconsideredforvolumetric,owrate(m3/h);Abisthecross-sectionalarea ofGAC,m2;ListhelengthoftheGACincontactor(m); designing,xedbedreactors: VFisthelinearvelocityor,ltervelocity(m/h);tisthe retentiontime(h);and αistheVoidvolume(m3). VolumetricFlow ColumnDiameter(cm) Rate(cc/min) F ) Filter Operation Time (t2.56 41.6ItisthetimeperiodthataGACbedhasbeeninoperation. 5.08 7.62 Throughput Volume (VL) 10.16 165.2Itisthevolumeofsolutethathaspassedthroughthe,lter 374.0 661.6 atatime.Carbon Bed Volume (Vb) Itiscalculatedbyusingtheequation ThisisthetotalvolumeofGACpackedbed,whichaccounts VL =tF ×Q forboththeactivatedcarbongrainsandthevoidvolume. Filter Density (ρGAC) Itiscalculatedusingthefollowingequationing/L: Cross-Sectional Area (Ab) Itissimplythecross-sectionalarea. Void Volume (α) mGAC ρGAC =Thevolumebetweenthecarbonparticlesinapacked Vb bedorcolumnexpressedasapercentageofthetotalbedvolumethatisnotoccupiedbyactivatedcarbonparticles volume,whichcorrespondstothepartofthe,xedbed mGACisthemassofGAC(g)andVbisthevolume where (L). Filter or Linear Velocity (vF ) Itisalsoknowassuper,ciallinearvelocityandsurfaceSpeci,c Throughput Volume (Vsp) Asthethroughputvolumedependsonthe,ltersize,it loadingrate,whichisthevelocityinanemptybedwithdoesnotallowadirectcomparisonofdifferentsizeof plants.Ifthroughputvolumeisdividedbythemassof the,ltercross-sectionalarea. activatedcarboninthebed,thespeci,cthroughputvolume Itcanbecalculatedfromthefollowingequation: isobtained.ThemassofGACisdeterminedbymultiplying Q ,lterdensityandbedvolume. vF =Itiscalculatedbyusingtheequation b A Q ×tVb ×t Effective Contact Time, Resident Time, or Retention Time (t)VLVsp ===EBCT ×mGAC Itisde,nedasthetimewithintheGACbedthatis Vb × ρGAC GACm availableforthemasstransferoftheorganicsubstances t Vb ×t==frombulksolutiontotheGACparticle.Itisalsode,nedas EBCT × ρGAC thetheoreticallengthoftimeforaliquidtopassthrough EBCT × (ρGAC ×Vb) acolumnassumingalltheliquidmovesthroughwiththe Throughput Bed Volume (BV) Itisananotherparameterusedforthecomparisonofthe sameuniformvelocity.Itisequaltothevolumeofliquidremovalef,cienciesregardlessofthebedsize.Itisthe ratioofthroughputvolumeandbedvolume inthecolumndividedbytherateof,ow.Thevolumeof liquidinacarboncolumnissimplythetotalvolumeof thecolumntimesthevoidfraction.Itcanbecalculatedby usingthefollowingequation: BV = VFα Q Carbon Usage Rate (CUR) VL t= Vb F EB CT Itisexpressedbythefollowingexpression: Empty Bed Contact Time (EBCT) Thetimerequiredfortheliquidtopassthroughacarbon mGAC CUR,g/m3 =columnassumingthatalltheliquidpassesthroughatthe Q ×t t = 104GRANULAR ACTIVATED CARBON PULSED BED Bed Life The volume of the water treated for a given EBCT,In this type of bed,carbon is removed at intervals From the bottom of the column and replaced at top byExpressed in liters ,and can be calculated as: Fresh adsorbent. mass ?of ?GAC ?for ?given ?EBCTVolume = GAC ?usage ?rate MOVING BED SYSTEM And therefore ,the bed life can be calculated as The most recent development of granular activated carbonvolume ?of ?water ?treated Bed ?Life =Has been the use of moving bed.In this system,the ?for ?given ?EBCT Direction of the liquid ,ow is upward,whereas the carbonQ Moves in a downward direction.The basic principle behind In carbon bed. The typical values of the various important parameters asFor designing of ,xed bed reactors,a number of theories Discussed above with their units presented in Table 5.Have been proposed by various researchers.These theories include: Carbon Dose 1.Length of unused bed (LUB) approachThe amount of GAC required to ,ll each column is 2.Bed-depth-service time (BDST) approachCalculated by the expression: 3.Empty bed residence time (EMRT) approachWeight of carbon =volume of column ×A.D ×0.85 Where A.D is the apparent density and 0.85 is the factorTable5 To allow back washed density. This amount should be degassed and wetted prior to Installation in the column. The degassing can be done by Boiling the carbon in organic-free water for 2 h or soaking At room temperature for 24 h.The degassed carbon should Be charged into the column in small increments as a Slurry keeping a layer of organic-free water above the GAC During charging ,which is best accomplished by ,lling the Column one-third of organic-free water prior to charging The degassed-free water into the column. It is also noted That all connecting tube and other void space must be ,lled With liquid in order to avoid the formation of gas pockets This technique is to have one column (or multiple columnsin literature that only 20% of produced activated carbons Running in parallel) packed completely with a carbon bedare used for gas-phase applications.The carbons applied Of suf,cient height to have the adsorption–wave front andfor gas-phase applications are mostly granular in shape. To provide some operating time with the ef,uent ,ow being REGENERATION Within speci,cation.As the adsorption wave front moves Once the granular activated carbons become saturated,Up the column,it is periodically displaced downward by the It is necessary to change the carbon or to regenerateRemoval of a quantity of saturated carbon from the base The ,xed bed adsorber.Regeneration is the processOf the column and the replacement of the same quantity Of removing adsorbed compounds from the granularWith fresh or regenerated carbon at the top of the column. Activated carbon surface.Here,the carbon surface includesAlthough the principle of the moving bed can be applied External macropores and micropores. In smaller units,it is most frequently used in larger unitsCarbon regeneration frequently is a majorpart of total Where the lower capital investment is important. Operating cost associated with granular activated carbon (GAC) adsorption systems.Following are the importantGAS-PHASE APPLICATIONS And potential regeneration methods: Gas-phase applications of activated carbons include sep- aration,gas storage,and catalysis.It is well documented ParameterTypicalValuesSymbolUnits1.Thermal regeneration Biological regeneration Infrared regeneration Volumetric,owrate50–400 Supercritical ,uids 3Q Bedvolume10–50 Cross-sectionalarea5–30 2. bVm3Length1.8–4.0 Voidfraction0.38–0.42Ab m2 L m α m3/m3 3. 4. GACdensity ivecontacttime Emptybedcontacttime FiltervelocityEffectOperationaltime Spent GAC used in water/wastewater,and other liquid-Throughputvolumeρkg/m3350–550 Phase applications,is generally reactivated using a high-Speci,cthroughput 5–15 m/h vFBedvolume2–10Temperature thermal process where the GAC is heated min5–30tTo about 815 ?C(1500 ?F),allowing drying,baking,and 100–600Gasi,cation to occur.Various types of furnaces used forEBCTmin 104–105The regeneration purpose include multiple hearth furnace days50–200tF (MHF) or rotary kilns (RK),electric belt furnaces (EBF), 2,000–20,000 m3 And ,uidized bed regenerators (FBR).Details about theseVL m3/kgVsp m3/m3BV m/h GRANULAR ACTIVATED CARBON105 Furnaces and their operation are well documented in Pharmaceuticals. Activated carbon is used to purify aliter- ature(41). The two major criteria involved in the Wide range of pharmaceuticals and intermediates.regeneration of the granular activated carbon are (a) characteristicGranular activated carbon is installed to purify the of the spent carbon and (b) choice of the furnace for Recirculating amine to remove degradation products and that particular application.The spent carbon characteristicsDissolved hydrocarbons. include the potential for char formation, corrosion, andMilitary Use Most of the world’s armed forces are using activatedslagging ,whereas the furnace characteristics are assessed by determining the mass transfer ef,ciency, particle resi-Carbons to protect against attack by toxic gases such as dence time,and temperature control. After reviewing theMustard gas.This is also used in military suiting where properties of furnaces and activated carbons,one can selectCombat uniforms are coated by a layer of impregnated the best possible option. Carbon under the outer cover. In thermal regeneration ,5–10% of granular activated Nuclear Reactors Carbon is lost as a result of oxidation and attrition ,and Most of the nuclear reactors,especially in the western By the cost of energy in heating ,the carbon around World ,have activated carbon ventilators to protect800–850 ?C(42). An alternative technique is that of Against radioactive iodine leaks from the core or heatChemical regeneration in which chemical reagents are Exchanger systems, if any .Special activated carbonsApplied to the exhausted granular activated carbon. The Impregnated with potassium iodide or potassium tri-iodideChemical regeneration of exhausted GAC can be achieved Are commonly used for this purpose.Another application inBy two main categories of substances :inorganic chemical Nuclear technology is as a ,lter in emergency ventilationRegenerates with oxidizing powers and organic chemical System for the reactor building ,which is switched onRegenerates with solubilizing powers.The ef,ciency of Automatically in case of breakdowns. Any regenerate is judged on the extent that it effects the Land,ll Leachate Treatment Recovery of the adsorptive powers of the granular activated Granular activated carbon in combination with biologicalcarbons.The regeneration ef,ciency can be calculated as Pretreatment is the leading technology for the treatment Regeneration ef,ciency (RE%) = (Ar/A0) ×100Of land,ll leachate for the removal of Chemical Oxygen Demand (COD), Adsorbable Organic Halogens(AOX), andWhere A0 is the original capacity of GAC for a particular Adsorbate and Ar is the capacity of regenerated carbon.Other toxic substances .Granular activated carbon is now APPLICATIONS AND COMMON USES (BOTH LIQUID PHASE AND GAS PHASE) Used at over 50 sites in Europe for this application. Activated carbon has wide applications in both liquid and gas/solvent phase systems(43). Granular activated carbon is the most common technology Groundwater Remediation Employed to pump and treat groundwater remediationCatalysis Many chemical reactions require a catalyst to improvesystems. It is highly suited to this application and is often used as a single treatment step to remove compounds ef,ciency ,accordingly; in many cases, activated carbonssuch as chlorinated hydrocarbons and aromatic compounds,provide large surface area ,thus ,further improving including benzene, toluene, ethylbenzene ,and xylenethe ef,ciency. (BTEX) .For more highly contaminated groundwater, twoMedicinal Activated Carbon The activated carbon has been applied in medicine forOr more carbon units may be placed in series or carbon may Be used in combination with other treatment technologiesA long time. The carbons originally used were prepared Such as air stripping or advanced oxidation processes.From waste materials of animal origin, especially blood (animal charcoal-carbo animalis).In catarrhal infectionsSoil Vapor Extraction and Air Sparring Activated carbon can also be used for the removal ofOf the digestive system,the use of activated carbon serves Primarily to remove bacterial toxins,which ,being high-Volatile Organic Compounds (VOCs) from air streams molecular-weight substances, are easily adsorbed on activeResulting from in situ removal techniques such as soil carbon .Activated carbons are also prescribed in large doses in all the cases of acute gastritis and enteritis.It is also a very effective antidote in all the cases of poisoning. Vapor extraction. Domestic Use Chemical and Pharmaceutical Industries Activated carbons are used in various home appliances,Chemicals. Activated carbon is suitable for the decol- Including fridge deodorizers,air puri,ers, and cooker Orization and puri,cation of a wide range of or ganic and hoods. The activated carbons are used in the removalInorganic compounds ,including amines ,hydrochloric and of caffeine from coffee.Cigarettes are made that, inOther mineral acids,amino acids,glycols,and hydrocar- addition to an antismoke ,lter,contain ,nely granulatedbons. activated carbon.