Gas Absorption.docx

Gas Absorption.docx

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GasAbsorption

Chapter18GasAbsorption

Thischapterdealswiththemass-transferoperationsknownasgasabsorptionandstripping,ordesorption.Ingasabsorptionasolublevaporisabsorbedfromitsmixturewithaninertgasbymeansofaliquidinwhichthesolutegasismoreorlesssoluble.AmajorapplicationofabsorptiontechnologyistheremovalofCO2andH2Sfromnaturalgasorsynthesisgasbyabsorptioninsolutionofaminesoralkalinesalts.Anotherexampleisthewashingofammoniafromamixtureofammoniaandairbymeansofliquidwater.Thesoluteissubsequentlyrecoveredfromtheliquidbydistillation,andtheabsorbingliquidcanbeeitherdiscardedorreused.Sometimesasoluteisremovedfromaliquidbybringingtheliquidintocontactwithaninertgas;suchanoperation,thereverseofgasabsorption,isdesorptionorgasstripping.

PRINCIPLESOFABSORPTION

Asshownintheprevioussection,thediameterofapackedabsorptiontowerdependsonthequantitiesofgasandliquidhandled,theirproperties,andtheratioofonestreamtotheother.Theheightofthetower,andhencethetotalvolumeofpacking,dependsonthemagnitudeofthedesiredconcentrationchangesandontherateofmasstransferperunitofpackedvolume.Calculationsofthetowerheight,therefore,restonmaterialbalances,enthalpybalances,andestimatesofdrivingforceandmass-transfercoefficients.

Materialbalances

Inadifferential-contact（微分接触）plantsuchasthepackedabsorptiontowerillustratedinFig.18.9,variationsincompositionarecontinuousfromoneendoftheequipmenttotheother.Materialbalancesfortheportionofthecolumnaboveanarbitrarysection,asshownbythedashedlineinFig.18.9,areasfollows.

Totalmaterial:

（18.2）

ComponentA:

（18.3）

whereGisthemolalflowrateofthegasphaseandLthatoftheliquidphaseatthesamepointinthetower.TheL-phaseandG-phaseconcentrationsxandyapplytothissamelocation.

Theoverallmaterial-balanceequations,basedontheterminalstreams,are

Totalmaterial:

（18.4）

ComponentA:

（18.5）

Therelationshipbetweenxandyatanypointinthecolumn,obtainedbyrearrangingEq.（18.3）,iscalledtheoperating-lineequation.

（18.6）

Theoperatingline（操作线）canbeplottedonanarithmeticgraphalongwiththeequilibriumcurve,asshowninFig.18.10.Theoperatinglinemustlieabovetheequilibriumlineinorderforabsorptiontotakeplace,sincethisgivesapositivedrivingforcey-y*forabsorption.

InEq.（18.6）,xandyrepresentthebulkcompositionsoftheliquidandgas,respectively,incontactwitheachotheratanygivensectionthroughthecolumn.Itisassumedthatthecompositionsatagivenelevationareindependentofpositioninthepacking.TheabsorptionofasolublecomponentfromagasmixturemakesthetotalgasrateGdecreaseasthegaspassesthroughthecolumn,andtheflowofliquidLincreases.Thesechangesmaketheoperatinglineslightlycurved,asshowninFig.8.3.Fordilutemixtures,containinglessthan10%ofsolublegas,theeffectofchangesintotalflowisusuallyignoredandthedesignisbasedontheaverageflowrates.

Gb

G

Ga

Fig.18.9Material-balancediagramforpackedcolumn

Limitinggas-liquidratio（最小液气比）

Fig.18.10Limitinggas-liquidratio

Equation（18.6）showsthattheaverageslopeoftheoperatinglineisL/G,theratioofthemolalflowsofliquidandgas.Thus,foragivengasflow,areductioninliquidflowdecreasestheslopeoftheoperatingline.ConsidertheoperatinglineabinFig.18.10.Assumethatthegasrateandtheterminalconcentrationsxa,ya,andybareheldconstantandtheliquidflowLdecreased.Theupperendoftheoperatinglinethenshiftsinthedirectionoftheequilibriumline,andxb,theconcentrationofthestrongliquor,increases.Themaximumpossibleliquorconcentrationandtheminimumpossibleliquidrateareobtainedwhentheoperatinglinejusttouchestheequilibriumline,asshownbylineab'inFig.18.10.Atthiscondition,aninfinitelydeeppackedsectionisnecessary,astheconcentrationdifferenceformasstransferbecomeszeroatthebottomofthetower.Inanyactualtowertheliquidratemustbegreaterthanthisminimumtoachievethespecifiedchangeingascomposition.

TheL/Gratioisimportantintheeconomicsofabsorptioninacountercurrentcolumn.Thedrivingforceformasstransferisy-y*,whichisproportionaltotheverticaldistancebetweentheoperatinglineandtheequilibriumlineonadiagramsuchasFig.18.10.IncreasingL/Gincreasesthedrivingforceeverywhereinthecolumnexceptattheverytop,andtheabsorptioncolumndoesnotneedtobeastall.However,usingalargeramountofliquidgivesamorediluteliquidproduct,whichmakesitmoredifficulttorecoverthesolutebydesorptionorstripping.Theenergycostforstrippingisoftenamajorpartofthetotalcostofanabsorption-strippingoperation.Theoptimumliquidrateforabsorptionisfoundbybalancingtheoperatingcostsforbothunitsagainstthefixedcostsoftheequipment.Ingeneral,theliquidratefortheabsorbershouldbebetweenl.land1.5timestheminimumrate,unlesstheliquidistobediscardedandnotregenerated.

Theconditionsatthetopoftheabsorberareoftendesignvariablesthatalsohavetobeset,consideringthebalancebetweenequipmentandoperatingcosts.Forexample,iftentative（试验的）specificationscallfor98percentrecoveryofaproductfromagasstream,thedesignermightcalculatehowmuchtallerthecolumnwouldhavetobetoget99percentrecovery.Ifthevalueoftheextraproductrecoveredexceedstheextracosts,theoptimumrecoveryisatleast99percent,andthecalculationshouldberepeatedforevenhigherrecovery.Iftheunremovedsoluteisapollutant,itsconcentrationintheventgasmaybesetbyemissionstandards,andtherequiredpercentrecoverymayexceedtheoptimumvaluebasedonproductvalueandoperatingcosts.

ThediagraminFig.18.10showsasignificantconcentrationofsoluteintheliquidfedtothecolumn,and99percentremovalfromthegaswouldnotbepossibleforthiscase.However,alowervalueofxacouldbeobtainedbybetterstrippingormorecompleteregenerationoftheabsorbingliquid.Thevalueofxacouldbeoptimized,consideringtheextraequipmentandoperatingcostsformorecompleteregenerationandthesavingsfrombetteroperationoftheabsorber.

Rateofabsorption

Therateofabsorptioncanbeexpressedinfourdifferentwaysusingindividualcoefficientsoroverallcoefficientsbasedonthegasorliquidphases.Volumetriccoefficientsareusedformostcalculations,becauseitismoredifficulttodeterminethecoefficientsperunitareaandbecausethepurposeofthedesigncalculationisgenerallytodeterminethetotalabsorbervolume.Inthefollowingtreatmentthecorrectionfactorsforone-waydiffusionareomittedforsimplicity,andthechangesingasandliquidflowratesareneglected.Theequationsarestrictlyvalidonlyforleangases（低浓气体）butcanbeusedwithlittleerrorformixtureswithupto10%solute.Absorptionfromrichgases（高浓气体）istreatedlaterasaspecialcase.

Therateofabsorptionperunitvolumeofpackedcolumnisgivenbyanyofthefollowingequations,whereyandxrefertothemolefractionofthecomponentbeingabsorbed:

（18.7）

（18.8）

（18.9）

（18.10）

Theindividualcoefficientskyaandkxaarebasedonaunitvolume,asaretheoverallcoefficientsKyaandKxa.Theainallthesecoefficientsistheinterfacialareaperunitvolumeofthepackedcolumnorotherdevice.Itishardtomeasureortopredicta,butinmostcasesitisnotnecessarytoknowitsactualvaluesincedesigncalculationscanbebasedonthevolumetriccoefficients.

Theinterfacecomposition（yi,xi）canbeobtainedfromtheoperating-linediagramusingEqs.（18.7）and（18.8）:

（18.11）

Fig.18.11Locationofinterfacecompositions

Thusalinedrawnfromtheoperatinglinewithaslope-kxa/kyawillintersecttheequilibriumlineat（yi,xi）,asshowninFig.8.8.Usuallyitisnotnecessarytoknowtheinterfacecompositions,butthesevaluesareusedforcalculationsinvolvingrichgasesorwhentheequilibriumlineisstronglycurved.

Theoveralldrivingforcesareeasilydeterminedasverticalorhorizontallinesonthey-xdiagram.Theoverallcoefficientsareobtainedfromkyaandkxa,usingthelocalslopeoftheequilibriumcurvem,aswasshowninChap.17[Eq.（17.57）]:

（18.12）

（18.13）

InEq.（18.12）,theterms1/（mky,a）and1/（kxa）aretheresistancestomasstransferinthegasfilmandliquidfilm,respectively.Whenthecoefficientskyaandkxaareofthesameorderofmagnitude,thevaluesoftheslopesmisverymuchgreaterthan1.0,theliquidfilmresistanceissaidtobecontrolling.Thismenasthatanychangeinkxahasanearlyproportionaleffectonbothkyaandkxaandontherateofabsorption,whereasachangeinkyahaslittleeffect.Forewample,Henry’slawcoefficientforCO2inwaterat20℃is1,430atm/molfraction,whichcorrespondstom=1,430forabsorptionat1atmandm=143forabsorptionat10atm.UndertheseconditionstheabsorptionofCO2inwaterisclearlyliquid-filmcontrolled.Increasingthegasvelocitywouldincreasekyabuthaveanegligibleeffectonkxa.Increasingtheliquidvelocitywouldincreasetheinterfacialareaaandprobablyalsoincreasekx,bothleadingtoanincreaseinkxaandkya.

Whenthesolubilityofthegasisveryhigh,suchaswithHCLinwater,misquitesmallandthegas-filmresistancecontrolstherateofabsorption.Withgasesofintermediatesolubilit