Production of Llysine from Llysine monohydrochloride by bipolar membrane electrodialysis0516.docx

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