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Indian Journal of Science and Technology Vol.2 No 4 (Apr. 2009) ISSN: 0974- 6846

Research article pH-based CO2estimation Asha Masohan et alIndian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Technol

59

A simple pH-based method for estimation of CO

2

absorbed in alkanolamines

AshaMasohan1,MamshadAhmed

1,SushilKumarNirmal

1,AjayKumar

2andMadhukarOnkarnathGarg

1

1IndianInstituteofPetroleum,Mohkampur,Dehradun,Uttarakhand,248005,India.

2ChemicalMetrologySection,

NationalPhysicalLaboratory,NewDelhi,110012,India.

[email protected],[email protected]

Abstract: CO2 is recognized as a major contributor ofglobal warming. We report a facile, sensitive andaccurate method for estimating CO2 content in thealkanolamine solvents. It can be applied to a host ofalkanolamine solvents as well as solvent mixtures forscreeningforprospective absorbent forCO2 capture.Inthismethod,pHof theCO2-loadedabsorbentsample ismeasured and referred to a reference pH graph ofcorresponding amine with sulphuric acid that isconstructedunderspecificconditions.Theauthenticityofthemethodwas validated from knownamounts ofCO2added in the form of CO2-saturated water to threedifferent types of alkanolamines i.e. the primary(monoethanolamine, MEA), secondary (diethanolamine,DEA)andtertiary(triethanolamine,TEA)alkanolamines.The method was compared with known methods andfoundtobebetterintermsofaccuracy.Keywords:CO2estimation,pH,pKa,Alkanolamines,Gasabsorption.Introduction

Carbon dioxide (CO2) has been proven to beresponsiblefor80%ofgreenhousegases,contributingtothe increase of earths temperature. Half of the CO2emission is produced by industries and power plantsusing fossil fuel (Desideri et al., 1999). Consequently,modern research geared towards developing new CO2capturetechnologiesworldwide.

Traditionally, absorption/stripping via circulatedaqueous alkanolamines have been the most favoredtechnologyforremovingCO2fromprocessandwastegasstreams. The existingcommercialprocesses basedonalkanolaminesarehighlyenergy-andcost-intensive.Asmuch as 80% of the total energy consumption in analkanolamine absorption process occurs during solventregeneration(White.etal.,2003).Therefore,searchisonfor new improved solvents with higher rates andcapacities for absorption of CO2, high degradationresistance, low corrosiveness and low energy use forregeneration. Such a solvent will decreaseboth capitalandoperatingcostsoftheprocesstherebyreducingthecost of electricity at thermal power plant when CO2removalisintegratedintheplant.ManynewsolventsarebeingresearchedforpotentialcommercialusetoremoveCO2fromfluegasstreams. Three classes of alkanolamines, primary (MEA),

secondary (DEA) and tertiary (TEA and methyldiethanolamine,MDEA) or their mixtures, are generallyused as absorbing liquids. The different categories ofaminesdifferinthetypeofmechanismswithwhichtheyreactwithCO2aswellasthereactionproductsandtheheatsofreaction.

Typicallyprimaryand secondaryamines formacidicanionspecieslikebicarbonate,carbonateandcarbamatewithCO2(equations1to5).

R2NH+H2OR2NH2++OH

- (1)

CO2+2H2OHCO3-+H3O

+ (2)

HCO3-+H2OCO3

--+H3O

+ (3)

CO2+R2NHR2NH+COO

-(zwitterions) (4)

R2NH+COO

-+R2NH

R2NCOO-+R2NH2

+ (5)

(carbamate)(protonatedamine) Tertiary amines cannot form carbamate speciesbecause they do not have hydrogen attached to thenitrogenatom.Typicallytertiaryaminesreactaccording

toequation6.CO2+R3N+H2O HCO3

-+R3NH

+ (6)

One of the main criteria for selection of suitablesolvent requiresestimation ofCO2 in both thegasandliquid phases for ascertaining the mechanism andmaterialbalance for rateandcapacity estimationofthesolvent.AccurateamountsofCO2 inliquidsolventsarealso required to remove discrepancies in parameteestimation in simulation and modeling bymathematicamodelforplantdesignandfeasibilitystudies.

There aremanymethodsavailablefor estimationocarbon dioxide in absorbing liquids specifically thealkanolamines.TheUOPmethod826-81 (UOPMethod1981)usesphosphoricacidtoliberatethedissolvedCO 2inaminesolutions.Themethodinvolvesquiteafewstepslike use of mercury for determining the volume ocalibrated tube, introducing sample of CO2-containingamine, adding phosphoric acid to liberate CO2 fromsample and use of mercury leveling bulb to force theliberatedgasintothemeasuringburetterepeatedlytillnofurtherCO2remainsdissolvedintheamineandwashingof carbonimeter. Regarding precision of the method noestimated standarddeviation is reporteddue to lack osufficient data to permit this calculation with at least 4degrees of freedom. Also the elapsed time for oneanalysisis1.0hour.

Currently the most popular analytical method fo

determiningtheacidgasconcentrationinaqueousaminesolutions employs a wet chemistry titration. In themethod for CO2, the amine sample is mixed with anexcessofstandardbaseandheatedtoboiling.Sincetheamine-acid gas complex is thermally unstable, the acidgasisconvertedintoanionicspeciesandisprecipitatedby an appropriate metal salt like BaCl2. The filtrate istitrated with a standard acid to determine theconcentrationofuncarbonatedamineinthesample(hereonwards termed as BaCl2 method). Indicators likebromocresol green, cresol red andphenolphthalein etc

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Fig. 1. pH graph of 2.6M AMP vs H2SO 4

0

1

2

3

45

6

7

8

9

10

11

12

13

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

moles of H2SO4mole-1

amine

pH

Fig. 2. pH graph of MEA, DEA, TEA and MDEA

vs H2SO 4

2

3

4

5

6

7

8

9

10

11

12

13

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

moles of acid mole-1of amine

pH

MEA- H2SO4

DEA- H2SO4

TEA- H2SO4MDEA- H2SO4

Fig. 3. pH graph of 2.6M AMP vs acetic acid

4

5

6

7

8

9

10

11

12

13

0 0.1 0.2 0.3 0.4 0.5 0.6 0.

moles of acetic acid mole-1

amine

pH

arecommonlyusedtoindicatetheendpoints(Coldrey&Harris,1976;Ellisetal.,1963;Hikitaetal.,1977;Jensenetal.,1954;GasConditioningFactBook,1962;Jouetal.,1982; Weiland &Trass,1969). The CO2 content of thesolutioniscalculatedasthedifferencebetweenthetotal

amine and uncarbonated amine. However, thecalculations require assumption of 1:1 stoicheometrybetweenamineandCO2(Weilandetal.,1969)whichisnot true. Another titrimetricmethod for determinationofCO2 in ethanolamine is theUOP method 829-82 [hereonwards termed as MeONamethod] (UOP Method,1982). The method involvesdissolution of CO2-absorbedsample in anhydrousmethanol. Theamine/methanol solution istitrated with standard

methanolic sodium hydroxidesolution usingthymolphthalein as theindicator and calculating theCO2content.

The wet chemistrymethods involvingprecipitations and titrationshave serious disadvantages.Thetotaltimerequiredforanexperienced technician toanalyze one carbonatedamine solution could be as

long as 1-2 hours. Duringpart of this time, the CO2-aminesolutionisopentotheatmosphere so both flashingofCO2anddegradationoftheamine can occur. Alsostoicheometry for differenttypes of alkanolamines(primary, secondary andtertiary) for CO2 is usuallyassumedasonewhichisnottrue and leads to erraticresultsforCO2content.Thishasseriousbearingonresults

for the weight of CO2estimated because thespeciesformeduponreactionofalkanolaminewithCO2mayvary from monoionic HCO3

-

and carbamate to di-ionicCO3

--ions(equations.2,3and

5) in different proportions.Freshlyprepared solutionsofstandardized acid and basemust also be available and

verifieddailybeforeuse.Blankrunsshouldalsobemadetoproperlyexecutethetitration.Thesolutionanalysisbytitrationisoftencomplicatedbythepresenceofimpuritiesanddegradationproducts(Arnold&Pearce,1960).Mostchemicalmethods of ethanolamine analysis depend on

amine functionality and are nonspecific and inaccurate(Brydia&Pearce,1967).

A method based on GC has been reported byRobbinsandBullin(Robbins&Bullin,1984)whichisfast

accurate and analyses the aminesolutionforacidgaseslikeH 2SandCO2, hydrocarbons, water andamine content. However, theexample cited is with MDEA onlyandthestructuralcharacteristicsoother alkanolamines are bound toaffect the retention time of theconstituent components and lot owork needs to be done on thei

separation using the prospectivesolventsusedinCO2separationfothismethod tobecome a standardmethod. Moreover the effects oamine salts, that result when thealkanolamine reacts with CO2, onretentiontimeisnotknown.

Thus, the analytical methodsreported above can provide onlythe directional values for totaamountofCO2intheleanandrichamines but are not suited fostudies involving small changes in

CO2 content as is required instudieslikekineticsofabsorptioninthe stirred contactor/wetted-walcolumn.Thenumberofdatapointsin these studies is large and theCO2buildupthatstartsfromjustafew mg is very gradual. Presenstudies were, therefore, aimed todevelop a sensitive and fasmethod for estimation of CO2 foanalyzing sequential build up oCO2 in the alkanolamine solventsranging fromppm topercent levefor absorption-basedstudies. The

methodwasappliedtoCO2-loadedalkanolamineswhichwereobtainedafter absorption of CO2 in thealkanolaminesofdifferentchemicalstructures(MEA,DEA,TEAetc.).Experimental

1. Reagents and equipments: Althe chemicals used in the presenstudy were 97-99% pure. H2SO4(98%),DEA(98%),TEA(97%)andNaOH (97%)werepurchasedfrom

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Fig. 4. pH graph of MEA, DEA and TEA vs

oxalic acids

2

3

4

5

6

7

8

9

10

11

12

13

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

moles of oxalic acid mole-1

of amine

pH

MEA- Oxalic acid

DEA- Oxalic acid

TEA- Oxalic acid

SDFineChem.Ltd.,India.HCl,MEA(98%),oxalicacidampules(volumetricsolutionN/10)andBaCl2(99%)werepurchased from Qualigens, India while MDEA (99+%)waspurchased fromAldrichOrganics,NewJersy,USA.Methanol(>99%) fromMerckLimited, India;Acetic acid

(99.8%) from Rankem FineChemicals Ltd., India andNa2CO3 from BDH, E. Merck,India and 2-amino-2-methyl-1-propanol (AMP) from MerckSchuchardt OHG, Germanywere purchased. All solutionswerepreparedindoubledistilledwater.Thymolphthaleinindicatorsolution taken was from LobaChemiePvt.Ltd.,India. pH meter used was ofThermo Orion, Model 420,Beverly, MA 01915, USA. All

titrations were carried out at25

oC.

2.Preparationofsolutions 100 ml of desired strengths(1Mor2.6M)ofMEA,DEA,TEA,MDEA,andAMPabsorbentswereprepared. Methanolic NaOH(0.05M) was prepared bydissolving 2.1 gNaOH in100mlwaterandmakingup thesolutionto1Lbyaddingmethanolasgiveninreference(UOPMethod,1982).BaCl2 used was 0.68 M while

H2SO4, acetic acid and oxalicacids used for pH titration were0.1N. 250mleachof 0.05NNaOH,

0.06MBaCl2and0.02NHClwerepreparedforuseforestimationofCO2inCO2-saturatedwater.3. Titration for plotting ofreferencegraphs 10 Drops of thymolphthaleinindicatorwereaddedto20mlofmethanol in a 100 ml beaker.Thissolutionwastitratedagainstmethanolic NaOH (termed as

MeONa) till light blue colorappeared.3mlof(2.6M)solutionof AMP was added to thismixture. Moles of amine takenwere calculated by dividingweight ofamine in3 mlsolutionbyitsmolecularweight.pHofthissolutionwasnoted.Theresultingmethanolicsolutionofaminewasthen titrated against H2SO4 withconstant stirring bymagnetic stirrer. After every small

additionofacid (0.5ml) the pHwas noted. Additionoacid increased the temperature of mixture and wasallowed to cool to 25

oC before taking the pH reading

InitialpHofaminewasnearly12.Readingsweretakentill the pHfellto nearly 3 (pH rangeof acid).Weight o

acid added was calculated fromits normality and the volumeadded.Theweightthusobtainedwas converted to moles of acidby dividing with 98 andsubsequentlytomolesacidmole1aminebydividingthemolesoacidbymolesofamine.Agraphwasplottedformolesacidmole

-

amine vs. corresponding pH fothewholepHrange.ThiswasthereferencepHgraph(Fig.1). Similarly reference pHgraphs were obtained for MEA

DEA, TEA and MDEA withH2SO4 (Fig. 2). Graphs werealso plotted for 2.6M AMP (Fig3) with acetic acid and MEADEA and TEA with oxalic acid(Fig. 4) to study the effect ostructureofacidonpHcurveandstoicheometry.4. Preparation of CO2-loadedalkanolaminesamples4.1. Absorption of CO 2 inalkanolamine: 50 ml of 1Msolution of MEAwas taken in a

100ml three-neck round bottomflask. CO2 absorption in aminewasstartedby passingCO2gasataflowrateof5.5secfor10mof CO2 gas. Absorption wascontinuedfor1h.ThisresultedintheCO2-loadedaminesample.4.2. pH Measurement of CO2loaded amine samples: Alkalinesolution of 20 ml methanol wasprepared in the same way asunder3 bytitratingwithMeONatill blue to thymolphthaleinindicator.1mloftheCO2-loaded

MEAsamplepreparedunder4.1was immediately added to thissolution. The blue solutionturned colorless because oacidic CO2 in amine. pH of thissolutionwas noted and referredtothecorrespondingpHvalueonthe reference pH graph plottedunder 3. Corresponding molesacid mole

-1 amine was noted

fromthegraph.

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4.3.CalculationsforweightofCO2inloadedamine:Theweight ofCO2 is calculated from themolesacidmole

-1

amineobtainedfromreferencepHgraphin4.2asgivenbelow(equations7to10):

MolesCO2mole-1amine=

amineofMoles

COofMoles 2 (7)

=amineofMoles

/44COofWeight 2 (8)

Weight of CO2 = Moles CO2 mole-1 amine Moles of

amine44 (9)

=MolesCO2mole-1amine

amineofWt.Mol.

*amineofWeight44..(10)

(*Weightof amineis theweight inthevolume takenforpHestimation)5.Validationofresults5.1. Reparation of saturated solution of CO2 in water:

100ml of distilledwater was taken and degasifiedoverultrasonicbathfor1h.CO2wasthenpassedthroughthiswater(maintainedat25

oC)atthesameflowrateas

under4.1for1h.EstimationofCO2wasdonebytaking5ml of this CO2-saturated water in a mixture of 10 mlNaOHand10mlBaCl2 and titrating excessNaOHwithHCl.AmountofCO2wascalculatedbythedifference.5.2. Adding known volume of CO2-saturated water toamineandestimatingbythemethoddeveloped:3mlofalcoholic amine solutionof2.6MMEAwas prepared insimilarwayasmentionedunder2.A5mlsampleofCO2-saturated water prepared as above was added to thisaminesolution.pHofthismixturewasnoted.Itistobe

noted that addition of 5 ml water (as CO2-saturatedsolution) results in dilution effect on concentration oaminefrom3mlto8mli.e.byafactorof2.66.Sothestrengthofamine reduces to1MafteradditionofCO2saturatedwater.Therefore,thereferencepHgraphto

be used for reference of pH for this sample was thaconstructedwith1MMEAsolution.FromthepHvalueofthe samplemolesof acidmole

-1 aminewas notedfrom

pH graph and amount of CO2 calculated by usingequations7-10. Similarestimationswerealsomadebyadding5mloftheabovementionedCO 2-saturatedwatein2.6M solutions ofDEA and TEA and referring to thereferencepH graphsof1Msolutionsofcorrespondingamines. Theresultsobtained(Table1)werecomparedwiththeamountofCO2actuallypresentin5mlofCO2saturatedwater(estimatedearlierasunder5.1). The estimations for mixtures containing same

volumesofCO2-saturatedwaterinthethreeamineswerealso done by two of the reported, theMeONa and the

BaCl2,methods(Table1).6.EstimationofCO2inloadedamines

Basedontheaccuracyoftheresults,thepresentpH-basedmethodwasthenusedinestimatingCO 2contenintheCO2-loadedaminesamplespreparedasdescribedunder4.1.Completelyloadedsamplesofalkanolamines:3mloftheCO2-absorbed amine sample was added to 20 ml oalkalinemethanolasunder3.pHofthesampleisnotedand molesacidmole

-1 amine obtained from theMEA

H2SO4. Weight of CO2 absorbed in amine is thencalculated from the value of moles acid mole

-1 amine

Results were compared with those obtained by the

MeONaandBaCl2methods.Similaestimations were made for CO2absorbed in DEA and TEA. TheresultsareshowninTable2.Partially loaded samples oalkanolamines containing variablamountsofCO2:Thatthemethodissuitable for any content of CO2 inalkanolamines was tested byconducting kinetic experiments instirred cell.CO2 was passed at theinterphaseof the alkanolaminewithstiiring of the liquid phase andcollecting CO2-absorbedsamples a

an interval of few seconds (30seconds) to minutes till the end oexperiment.TheresultsareshowninFig. 5. Similar estimations weremadeforCO2inDEAandTEAalsoIncaseofsecondaryaminelikeDEAit was observed that the resultsneedetobemultipliedbyafactorof1.9 for exact amounts (perhapsbecause of stoicheometricrequirements).

Table1.EstimationofsaturatedamountofCO2inwaterassuchandafteradditionindifferentamines

Wt.ofCO2in5mLofCO2-saturatedwater(mg)

CO2estimated

Inwater InamineAmine

Strengthofaminesolution,(M)

AlkalineBaCl2Method

BaCl2Method(Literature)

MethanolicNaOHMethod(UOPmethod,1982)

frompHreferencegraphofaminewithH2SO4

MEA 2.67M 5.701 5.5 6.1813 5.984

DEA 2.67M - 8.11297 2.816

1.9=5.35)

TEA 2.67M 11.44 8.11297 5.104MEA 2.67M 5.632 6.41 4.699 5.55

DEA 2.67M - 8.223 3.168

1.9=6.02)

TEA 2.67M 7.32 9.3985 5.346

MEA 2.67M 5.700 6.64 5.9

DEA 2.67M 6.18 2.99

1.9=5.68)

TEA 2.67M 7.78 5.34

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Results and discussions

Becauseofthelargenumber of new solventsthatarebeingscreened today fortranslating into a cost-

effectivetechnologyand theinherent disadvantagesassociatedwith the wet chemistrymethodsasisrealizedbythescientific community, a newmethod developed in thislaboratoryisbeingreported.A.StructuresandpKasofthematerialsused:Structuresofthe alkanolamines used inpresent study are shown inTable 3. These structuresincludeallthethreetypesi.e.

the primary, secondary andtertiaryalkanolmonoamines.The pKa values for theamine-acid systems of thepresent studies areprovidedin Table 4. The reportedvalues are for aqueoussolutions while theexperimentally determinedvaluesareforthemethanolicsolutions of alkanolamines.Thelatterarebeingreportedfor the first time. As

expected pKa of MEA, DEAand triethanolamine (TEA)goes on decreasing as thesubstitutiononnitrogenatomincreases. However, thesevalues are lower thancorrespondingvaluesofaqueoussolutions.B. Basic principle behind the method developed: Alkanolaminebeingthebasicmoleculereactswithacidiccomponents like H2SO4 or CO2 stoicheometrically(equations 1-6). When an acid is added to a knownamountofaminetheresultingpHisthemeasureofmolesacid mole

-1amine reacted and indirectly the moles of

amineleftun-reacted.Thestoicheometryisobtainedafter

completeneutralizationofaminewiththeacidbyplottingpHagainstmolesacidmole

-1amineadded.

The stoicheometry of alkanol monoamines forsulphuricacidis0.5suggestingthattwomolesofaminearereactingwithonemoleofH2SO4(Fig.1,2).Incaseofneutralization with weak monobasic organic acid, likeaceticacid,astoicheometryofnearly0.31wasobtainedfromgraphwhilefordibasicorganicacidlikeoxalicacidthestoicheometryisnearlydoubleascomparedtothatofmonobasic organic acid i.e. >0.6. It was finalized thatH2SO4 being strong acid dissociated completely.

Therefore, itsreferencepHgraph forthe threeaminescouldbeusedforestimationofacidiccomponentlikeCO2inamine.

C. Validation: Based on theabove reasoning the

validation was done for theresults obtained for addingknown volumesof saturatedsolution of CO2 containingknown amount of CO2Repeat values of 0.025molesCO2L

-1wereobtained

for CO2-saturated water asdeterminedbyalkalineBaCl2precipitation.Samevolumes(5mL) of this CO2-saturatedwater, with known CO2content,wereaddedtoeachof the MEA, DEA and TEA

samples simultaneously sothat there was no time gapfor escape of CO2 fromwater.EstimationofthisCO2in each aminewas donebyall the three (MeONa,BaCl2and the present pHmethods.AsseeninTable1values of CO2 estimated inamine by the pH graphmethod are of very highaccuracy as itmatches withthe alkaline BaCl2 results in

water(errorofonly4.4,0.88and -5.5% for MEA, DEAand TEA respectively fosuch low amount as 5.6mgCO2). For secondary aminelikeDEAitwasobservedtha

the values obtained from pH graph needed to bemultipliedbyafactorof1.9forcorrectresults.IncaseofBaCl2method the errorrangewas17.5;9.4and37.7%andincaseofMeONamethodtheerrorwas-16.5,45.8and 66.7%. These values oferrors are rather high fodependableuse.

Itwas also finalized that the reference pHgraphoalkanolaminewithH2SO4 gavemostaccurateresultsas

comparedtothepHgraphswithorganicacids.D.ApplicationofmethodtoaminesloadedwithCO2afteabsorption: Tabulated results of the amount of CO2estimatedin loadedalkanolaminesafterCO2 absorption(Table 2; Fig. 6) show a lotof variation in the repeaexperiments for primary and secondary alkanolamines(MEAandDEA)byMeONaandBaCl2methods.Resultsby the present pHmethod showgood repeatability andhigh accuracy. Also the trendobserved incase of thepresent method shows that capacity for absorption fosecondary amine DEA is higher than primary amine

Table2.EstimationofCO2 intheloadedaminesamples(mg)

Amines BaCl2method MeONamethod ReferencepHgraphmethod

MEA 66.17 76.75 46.28

MEA 71.1 98.7 46.8

MEA 72.5 94 47.7

DEA 64.22 52.8733.16 1.9 = 62.89

DEA 62.4 65.431.1 1.9 = 61.88

DEA 62.8 71.334 1.9 = 64.6

TEA 34.18 20.36 12.67

Table3Structuresofalkanolamineabsorbentsusedforpresentstudies

Monoethanolamine(MEA)

Diethanolamine(DEA)

Triethanolamine,TEA

2-amino-2-methyl-1-propanol(AMP)

N-methyldiethanolaomine(MDEA)

2-(2-aminoethylamino)ethanol(AEAE)

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whichistheobservedtrendforCO2absorption.TEAshows lot of variation in CO2 content by the threemethods. Theseresultscan throwan insight into theexperimental results of absorption/desorption withdifferent solvents when applied to the simulation

modelingof thedatabyeNRTLwhere thepredictionsbased on liquid analysis are still lacking because ofdependableexperimentaldata.

Itisenvisagedthatthedevelopedmethodcanalsobeextendedtomixturesofalkanolamines.Useofsuchmixtures is the latest stateof theart in CO2 capture.Estimation of absorbedCO2 in themixturewill againrequire construction of the reference pH-graph byplottingpHvs.molesofacidmole

-1aminemixtureas

under3followedbyvalidationandthenestimatingtheCO2content(4.2and4.3)inthemixture.Conclusion

A novel, versatile and facile method has beendeveloped for estimation of CO2 loaded into the

alkanolaminewithverygoodaccuracy for use indataanalysis. A pH graph of the methanolic solution ofspecificconcentrationoftheamine(thatistobetestedfor CO2 absorption) with sulphuric acid is plotted asreferencepHgraph.TheCO2contentisthenobtainedbynotingpHoftheCO2-loadedamineandreferringtothe pH graph for moles of CO2 mole

-1 amine and

subsequently the weight of CO2 absorbed can becalculated using the molecular weight of amine. Thismethod has the potential of application to otheralkanolaminesandtomixturesofalkanolaminesaswell.The method is applicable to full range of CO2 loading(fromverylowtohigh)inthesolvent.

Acknowledgement The authors thank Dr. S. Chandra, Jt. Advisor,Ministry ofChemicals and Fertilizers, New Delhi, India,for his constant encouragement and the Ministry ofChemicals and Fertilizer, Govt. of India, for financialassistancetocarryoutthisstudy.References

1. Arnold JL and Pearce RL (1960) Analysis of glycoland amine solutions and the interpretation thereof.GasconditioningInst.,Liberal,Kansas.

2. Brydia LE and Pearce RL (1967) Quantitative gaschromatographic determination of ethanolamines astriflouroacetylderivatives.Anal.Chem.39,1318-1320.

3. Coldrey PW and Harris IJ (1976) Kinetics of liquid

phase reaction between carbon dioxide andDiethanolamine.Can.J.Chem.Eng.54,566-571.

4. Desideri U and Paolucci A (1999) Performancemodeling of a carbon dioxide removal system forpowerplants.EnergyConvrs.Manag.40,1899-1915.

5. EllisGC,LeachmanGS,FormainiRE,HazeltonRFandSmithWS(1963) Rate of desorption of carbondioxide fromMonoethanolamine solution, Proc. 13

th

Ann.GasConditioningConf.Univ.Oklahoma,p:B3.6. Gas Conditioning Fact Book (1962) Dow Chemical

Company,Midland,Michigan(c.f.Ref.-4).

7. HikitaH,AsaiS,IshikawaHandHondaM(1977)Thekinetics of reactions of carbon dioxide withmonoethanolamine, diethanolamine andtriethanolamine by a rapid mixing method. ChemEng.J.13,7-12.

8. Jensen MB, Jorgensen E and Faurholt C (1954Reactions between carbon dioxide and aminoalcohols.ActaChem.Scand.8,1137-1140.

9. JouFY,MatherAEandOttoFD(1982)Solubility osolution and carbon dioxide in aqueousMethyldiethanolamine solution. Ind. Eng. ChemProcessDes.Dev.21,539-544.

10.Perrin RG (1965, Supplement 1972) Dissociationconstants of organic bases in aqueous solutionsButterworths:London,

11.RobbinsGDandBullinJA(1984)AnalysisofaminesolutionsbyGasChromatography.EnergyProgress4,229-232.

12.UOPMethod826-81(1981)Carbondioxideinamine

solution.ASTMIntl.Publ..13.UOPMethod829-82.(1982)Titrimetricdetermination

ofCO2inethanolamines.ASTMIntl.Publ.14.Weiland RH and Trass O (1969) Titrimetric

determinationofacidgases inalkali hydroxidesandamine,Anal.Chem.41,1709-1710.

15.WhiteCM,StrazisarBR,GraniteEJ,HoffmanJSandPennlineHW(2003)Separation andcaptureofCO2from large stationary sources and sequestration ingeological formation-coalbeds and deep salineaquifers,J.AirWasteMangt.Assn.53,645-715.

Table4 ExperimentalandliteraturevaluesofpKasofthealkanolaminesandacids

Amines

Amineacid

system

pKainmethanolic

solution(Exp.)

pKainaqueous

solution(Perrinetal.,1965)

Rangeofmolesofacid

usedmole

-1

amine(max.)

MEA-H2SO4 9.4 9.51 0.50

MEA-Oxalicacid 9.29 0.60

DEA-Oxalicacid 8.78 0.60

DEA-H2SO4 8.72 8.95 0.5

DEA-HCl 8.66 0.5

TEA-Oxalicacid 7.71 0.60

TEA-H2SO4 7.5 7.78

Acids

LiteraturevalueofpKa

pKa1 pKa2

AceticAcid(CH3COOH)

4.74

CarbonicAcid(H2CO3)

6.37 10.25

OxalicAcid[(COOH)2]

1.23 4.19

SulfuricAcid(H2SO4)

none 1.92

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