Production of Llysine from Llysine monohydrochloride by bipolar membrane electrodialysis0516.docx
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Production of Llysine from Llysine monohydrochloride by bipolar membrane electrodialysis0516.docx
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ProductionofLlysinefromLlysinemonohydrochloridebybipolarmembraneelectrodialysis0516
ProductionofL-lysinefromL-lysinemonohydrochloride
bybipolarmembraneelectrodialysis
YapingZhang1,YanChen1,2,MingzhuYue1,LeiWang1
1.Engineering Research Center of Biomass Materials,Ministry of Education,
Southwest University of Science and Technology, 59QinglongRoad,Mianyang 621010, P.R.China
2.SichuanAnxianYinheConstructianal&ChemicalGroupCo.,Ltd,Mianyang 622656, P.R.China
Tel./fax:
+868166089372;E-mailaddress:
zhangyaping@(Y.Zhang)
Abstract
Bipolarmembraneelectrodialysisconsistingofonebipolarmembrane,oneanion-exchangemembraneandonecation-exchangemembranewasperformedtoachievetheproductionofL-lysinefromL-lysinemonohydrochloride.Severalexperimentalparametersincludingtheoperationvoltage,theinitialL-Lys·HClconcentrationandoperationtemperaturewerecomparedanddiscussed.TheCl-removalratio,theelectricconductivity,thepHinvariouscompartments,thecurrentefficiencyandtheenergyconsumptionwerepresentedandanalyzedrespectively.WhentheinitialconcentrationofL-Lys·HClis0.6molL-1andtheconstantvoltageis40V,theremovalratioofCl-reached86.6%,thecurrentefficiency24%andtheenergyconsumption28.2kW·h·kg-1.Elevatingtheoperationtemperaturecanpromotetheelectrodialysisprocessslightly.
Keywords:
bipolarmembrane;electrodialysis;L-lysine;L-lysinemonohydrochloride
1Introduction
L-lysine(L-Lys)isanessentialaminoacidgenerallyconsideredasafundamentalsubstanceofhumanbody,whichhasawiderangeofapplicationinpharmaceuticals,foodandfeedindustry[1].L-Lysisnecessarytophysicalgrowthandskeletaldevelopmentespeciallyforchildren[2].Concretely,itisbeneficialtoabsorbcalcium,keepnitrogenbalance,andensureleanbodymass[3].CommercialL-LysisusuallyproducedasL-lysinemonohydrochloride(L-Lys·HCl),sinceL-Lysisdifficulttopreserveandeasytoabsorbmoistureasanaturalprotein.However,L-Lys·HClisnotcommonlysuitableforeatingdirectly,andCl-needstoberemovedinordertogaintheL-Lysofhighmedicalandediblevalue[4].
Inourearliestwork,Cl-wasremovedbytraditionalionexchangemembraneelectrodialysis[5],wherethefinalCl-removalratioreachedabout73.1%—alittlelowerforpracticalapplication.ThenionsubstitutionelectrodialysiswastriedforproductionofL-Lys,andresultsshownamuchhigherCl-removalratio,95.6%[6].However,inordertoachievethehighestCl-removalratio,alittleamountofbuffersolutionwasaddedtothefeedliquid,whichmayleadtoaslightimpurityofas-producedL-Lys.Therefore,anothernovelelectrodialysismethod,namely,bipolarmembraneelectrodialysis,isexpectedtobedevelopedtoachievepurerL-LysfromL-Lys·HCl.
Bipolarmembraneisaspecialtypeofion-exchangemembraneanditswell-knownfunctionistosplitwaterintoOH−andH+inadirectcurrentfield[7-9].Bipolarmembraneelectrodialysishasbeenusedtoseveralkindsoffieldsandmanyliteraturepapershavebeendevotedtoit[10,11],suchastreatingwastewatercontainingammoniumnitrate[12-14],desalinationoffruitjuice[15,16],productionofcriticacid[17,18]、lacticacid[19]、salicylicacid[20],andsoon.Asforitsapplicationinaminoacidfield,LixinYuetal.[21]havereportedabipolarmembraneelectrodialysisprocesstoregeneratesodiump-toluenesulfonate(orsodium4-methylbenzenesulfonate)intheproductionofD-(-)-p-hydroxyphenylglycine(orR-α-amino-4-hydroxy-benzeneaceticacid).Theconversionofpuresodiump-toluenesulfonatesolutionandactualfeedsolutionthatcontainssmallamountofR-α-amino-4-hydroxy-benzeneaceticacidbesidessodiump-toluenesulfonatewascomparedandanewprocessingroutewasproposed.TheremovalratioofNa+wastypicallyaround80%withanaveragecurrentefficiencyof20~50%.Inaddition,XiaoyanZhangetal.[22]alsoreportedtherecoveryofglutamicacidbytwoelectrodialysisprocesses:
two-compartmentbipolarmembraneelectrodialysisandmodifiedtraditionalelectrodialysis.However,tillnow,fewliteraturepapershavereportedtheproductionofL-LysfromL-Lys·HClbybipolarmembraneelectrodialysis.
Consequently,theproductionofL-LysfromL-Lys·HClbybipolarmembraneelectrodialysiswasevaluatedinthiswork.Effectsofseveralparametersincludingtheoperationvoltage,initialL-Lys·HClconcentrationandtheoperationtemperaturearepresentedonebyone.Theprocessperformanceparametersofthewholeoperationsystem,suchastheelectricconductivity,pHinvariouscompartments,removalratioofCl-,thecurrentefficiencyandtheenergyconsumptionarealsostudied.
2.Experimental
2.1Materials
Abipolarmembrane(BPM,NeoseptaBP-1,TokuyamaSodaInc.,Japan),aheterogeneouscation-exchangemembrane(CEM)andananion-exchangemembrane(AEM)(ShanghaiShanghuaWaterTreatmentMaterialCo.,Ltd.,China)wereusedduringallexperiments,andtheirmaincharacteristicsarelistedinTable1~2.Inordertoavoidtheperturbationoftheimpuritiesinmembranes,thepurchasedmembranes(BPM,CEMandAEM)werepretreatedwith1mol·L-1HClandNaOHfor4hoursalternatelyandthenwashedwithdeionizedwaterbeforeelectrodialysis.
L-lysinemonohydrochloride,sodiumhydroxide,sodiumnitrateandsodiumsulphate,etc.ofARgradewerecommerciallyobtainedandusedwithoutfurtherpurification.Deionizedwaterwasusedthroughly.
2.2Methods
Inbipolarmembraneelectrodialysis,thelaboratory-scaleelectrodialyzerwasconsistedofoneanode,onecathode,onepieceofbipolarmembrane(BPM),onepieceofanion-exchangemembranes(AEM),aswellasonepieceofcation-exchangemembrane(CEM)insertedbetweenthemwiththeinter-membranedistanceof8mm.Therewerefourcompartmentswiththevolumeof25mLfromlefttoright:
compartment1containing0.5mol·L-1ofNa2SO4solution,compartment2containing0.3mol·L-1ofNaNO3solution,compartment3composedofL-Lys·HClsolutionofcertainconcentration,andcompartment4composedof0.5mol·L-1NaOHsolution.Allabovecompartmentswereconnectedtoaseparateexternal500mLbeaker,allowingforcontinuousrecirculationbyfoursubmergedpumps(AT-301,ATMAN)withtheflowrateof15L·h-1.Thesizeofion-exchangemembranewas5cm×5cmwithaneffectiveareaof7.06cm2.ADCpower(DF1731SLL3A,ZhongceElectronicsCo.,Ltd.,China)wasusedtoapplyconstantpotentialacrosstheelectrodes.ThepHvalueofthesolutionwasregularlymonitoredbyanaciditymeter(PHS-2C,ShanghaiHongyiInstrumentationCo.,Ltd.,China).TheschematicdiagramoftheexperimentalapparatuswasshowninFig.1,amongwhichthebipolarmembraneelectrodialysisstackwasillustratedinFig.2.
Thewholeelectrodialysisprocessmainlyinvolvesthefollowingfourstages:
(
)WaterweresplittedbybipolarmembraneanddissociatedintoOH-andH+,andthentheytransferacrosstheanionexchangelayerandcationexchangelayeroftheBPM,gettingintocompartment3and4respectively;(
)substitutionofCl-byOH-inthecompartment3;(
)migrationofCl-fromcompartment3to2throughAEM;(
)generationofH+bywatersplittingatanodeanditsmigrationthroughCEMandsubsequentformationofHClincompartment2.
Allexperimentswererepeatedforthreetimesandtheirmeanvaluewastakenasthefinalresult.Theestimatederrorisabout±5%.
3.Resultsanddiscussion
3.1Effectofoperationvoltage
ExperimentsforproductionofL-Lysbybipolarelectrodialysiswerecarriedoutatdifferentappliedpotentialsrangingbetween20and50VwithinitialL-Lys·HClconcentrationof0.2mol·L-1.Theresultsofoperationprocessincludingconductivity,removalratioofCl-,pHinvariouscompartments,currentefficiencyandenergyconsumptionwereshowninFig.3-7.Bytheway,inordertomakethefiguresinthispaperseenmoreclearly,errorbarsweregiventoshowthelevelofuncertaintyandthesignificanceofthesebehaviors.
3.1.1Currentdensity,conductivityandremovalratioofCl-
Asmentionedabove,whenanelectricfieldisapplied,OH-iondissociatedbybipolarmembranesubstitutesCl-incompartment3andCl-ionmigratesfromcompartment3to2throughtheAEM,thusthefreeL-Lysformsincompartment3.Variationsofelectricconductivityincompartment3withtimeduringsuchanionsubstitutionprocessatvariousoperationvoltagesweredepictedinFig.3.
Atthebeginningofbipolarelectrodialysis,waterdissociationoccursattheinterfaceofthebipolarmembraneandacurrentissustainedbythemigrationofH+andOH-ionstotherespectivecompartmentunderthedriveofelectricfieldforce.AsshowninFig.3,initialelectricconductivityisslightlylowandthenitincreasestosomeextent.Afterattainingthemaximumvalue,itdecreaseswithtimegradually.Duringtheelectrochemicalprocess,appliedvoltageisresponsiblefortheformationortransportationofOH-,H+andCl-.TheH+andOH-producedbywaterdissociationraisetheionicamountincompartment3and4,andthetransportationofCl-tocompartment2alsoincreasestheionconcentration.Therefore,theincreaseinionicconcentrationincompartment2,3and4leadstotherisingelectricconductivityinevitably.However,afterreachingthemaximumconductivity,electrolyte(L-Lys·HCl)concentrationinthecompartment3islowered,whichcausesanincreaseintheoverallelectricalresistance,andthustheconductivitydecreasedgradually.Infact,suchanexperimentaltrendcanbeobservedduringallelectrodialysisprocedures,whichisalsoinaccordancewiththeresultsreportedbyM.Kumaretal.[23].
RemovalratioofCl-isoneofthemostimportantparameters
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