结晶器原理及设计CHAPTER-8-CRYSTALLIZATION

MISS.RAHIMAHBINTIOTHMAN(Email:rahimah@.my)Chapter6:cRYSTALLIZATIONCOURSEOUTCOMESCODESCRIBEthebasicprinciplesandapplicationsofcrystallizationprocess.CALCULATEtheyields,materialandenergybalanceincrystallization.Basicprinciplesandapplicationsofcrystallizationprocess.CALCULATIONofyields,materialandenergybalanceincrystallization.OUTLINESProcessofproducingcrystalsfromahomogeneousphasewhichisobtainedfromasolution.CapableofproducingbioproductsatveryhighpurityandconsideredtobeapolishingstepandapurificationstepTwodifferentapplicationofcrystallization:CrystallizationforpolishingandpurificationCrystallizationforcrystallographyCRYSTALLIZATIONCOMPARISONOFCRYSTALLIZATIONANDPRECIPITATIONDescriptionCrystallizationPrecipitationSolubilityWiderange,usuallymediumtohighSparinglysolubleRelativesupersaturationLowHighProductmorphologyWell-definedIll-definedProductcrystalsizeLargeSmallNucleationmechanismSecondaryPrimaryNucleationrateLowHighGrowthRateWideRangeLowControllabilityControllableDifficulttocontrolCRYSTALLIZATIONFROMSUPERSATURATEDSOLUTIONSOFSODIUMACETATEDescription:Asupersaturatedsolutionofsodiumacetateiscrystallizedbypouringitontoaseedcrystal,formingastalagmite-likesolid.Heatisradiatedfromthesolid.Source:Shakhashiri,B.Z.ChemicalDemonstrations:AHandbookforTeachersofChemistryCRYSTALLIZATIONPRINCIPLESCrystalsCanappearaspolyhedrons,orsolidsformedbyplanefaceswhenallowedtoformfreely.Anglesmadebythecorrespondingfacesofthesamematerialdonotvary–canbeclassifiedbythischaracteristics.Relativesizesofthefacesofacrystalinaparticularsystemcanvaryconsiderably–resultinginavarietyofcrystalshapes.Thisvariationiscalledahabit.Crystalhabitisinfluencebytheconditionsofcrystallization,particularlybytheimpuritiespresentandbytheparticularsolventorsolventsused.Impuritiescanstuntthegrowthofacrystalincertaindirections.POLYHEDRONSSHAPETABULARBook-like(tablets)thatarethickerthanplaty

butnotaslongatedasbladed.

Wulfenite

formscrystalsthatareagoodexampleoftabularcrystals.

PRISMATICOneofthemostcommonofcrystalhabits.Prismaticcrystalsare"pencil-like",elongatedcrystalsthatarethickerthanneedles(seeacicular).Indicolite(avarietyofelbaite)formsgoodexamplesofprismaticcrystals.

ACICULARLongandneedle-like,thinnerthanprismaticbutthickerthanfibrous.

Natrolite

crystalscanbegoodexamplesofacicularcrystals.

CRYSTALSGROWIN2STEPS:Nucleation-firstaggregation.Growth.ThermodynamicallydistinctWantafewnucleitogrowbigUsethermodynamicstounderstandtherequiredconditionsNucleationThegenerationofultramicroscopicparticlesintheprocessofnucleationisthesumofcontributionsbyprimarynucleationandsecondnucleation.Primarynucleation:occursintheabsenceofcrystals,secondarynucleation:attributedtotheinfluenceofexistingcrystalsPrimarynucleationcanbeeitherhomogeneous(noforeignparticlesarepresent)orheterogeneous(foreignparticlespresentduringheterogeneousnucleation)Rateofprimarynucleationhasbeenmodeledbythefollowingpowerlawexpression:

CrystallizationPrinciples(1)B:numberofnucleiformedperunitvolumeperunittime;N:numberofnucleiperunitvolume;kn:rateconstant;c:instantaneoussoluteconcentration;c*:soluteconcentrationatsaturation.(c-c*)term:supersaturation,theexponentofncanrangeupto10buttypicallyisintherangeof3to4.CRYSTALLIZATIONPRINCIPLESTwotypesofsecondarynucleation:shearnucleation(occursasaresultoffluidshearongrowingcrystalfaces),contactnucleation(happensbecauseofcrystalscollidingwitheachotherandwiththeimpellerandothervesselinternalsurfaces.Rateofsecondarynucleationincrystallizationisthefollowing:(2)k1:rateconstant;MT:suspensiondensity,b:canrangeupto5buthasamostprobablevalueof2;j:rangesupto1.5with1beingthemostprobablevalueCrystallizationPrinciplesFigure1:

Typicalphasediagram.Thecomponentsinsolutionconsistoftheproduct(ordinate)andtheprecipitatingreagent(abscissa).Thelineswitharrowsoutlineonepossiblewayofperformingthecrystallization.ThesupersaturationmustbeabovetheacertainvaluebeforenucleationwillbeginMetastableregion:thesupersaturationislowthatnucleationwillnotstartOncethesupersaturationhasbeenraisedenoughtobeinthelabileregion,nucleationcanbegin.Atthispoint,crystalsbegintogrow,andthesupersaturationdecreasesIfthesupersaturationbecomestoohigh,thenucleationratewillbetoogreat,andamorphousprecipitatewillresult.CrystallizationPrinciplesNucleationSupersaturationPHASEDIAGRAMSPrecipitatantconcentration(salt,PEGetc.)ProteinconcentrationUnder-saturation

(proteinremainssoluble;crystalsdissolve)NucleationzonePrecipitationzoneSolubilitycurveMetastablezone

Crystalsgrow,but

NucleiformonlyinfinitelyslowlyCOURSEOFCRYSTALLIZATIONEXPERIMENT[Precipitatant]ProteinconcentrationNucleationPrecipitationMetastableStartw/solubleprotein(undersaturatedormetastable)NucleateshereIncrease[protein],[precipitant]Crystalgrows

Sequestersprotein

[protein]dropsCrystalstopsgrowing@solubilitycurveExptincr.[protein],[precipitant]

Xtlgrowsagain,untilhitscurve

RepeatsasfollowssolubilitycurveCrystalGrowthPostnucleationprocessinwhichmoleculesinsolutionareaddedtothesurfaceofexistingcrystalsTherateofmassdepositionRduringcrystalgrowthis:Overalllineargrowthratecanalsobeexpressedas:L:characteristicssingledimensionofthecrystal,suchaslengthCrystallizationPrinciples(3)(4)W:massofcrystalspervolumeofsolvent;A:thesurfaceareaofcrystalspervolumeofsolvent;kG

:overallmasstransfercoefficient(dependsontemperature,crystalsize,hydrodynamicconditions,thepresenceofimpurities);g:usually0and2.5CRYSTALLIZATIONPRINCIPLESCrystalgrowthisaprocessthatconsistsoftwostepsinseries–diffusionandsurfaceintegration

Whentheexponentsareunity,combiningEquation3,5,6gives(5)ci:concentrationattheinterfacebetweentheliquidandsolidphase;kd

andkr:masstransfercoefficients(6)(7)Thus,ifsurfaceintegrationisveryfastcomparedwithbulkdiffusion,thenkr>>kd,andkG,kd.YIELDSANDHEATANDMATERIALBALANCESINCRYSTALLIZATIONYieldsandmaterialbalanceincrystallizationThesolution(motherliquor)andthesolidcrystalsareincontactforenoughtimetoreachequilibrium.Hence,themotherliquorissaturatedatthefinaltemperatureatthefinaltemperatureoftheprocess,andthefinalprocess,andthefinalconcentrationofthesoluteinthesolutioncanbeobtainedfromthesolubilitycurve.Theyieldcanbecalculatedknowingtheinitialconcentrationofsolute,thefinaltemperature,andthesolubilityatthistemperature.Inmakingthematerialbalances,thecalculationsarestraightforwardwhenthesolutecrystalsareanhydrous.Simplewaterandsolutematerialbalancesaremade.Whenthecrystallizationsarehydrated,someofthewaterinsolutionisremovedwiththecrystalsasahydrate.EXAMPLE1YieldofaCrystallizationProcessAsaltsolutionweighing10000kgwith30wt%Na2CO3iscooledto293K(20°C).Thesaltcrystallizesasthedecahydrate.WhatwillbetheyieldofNa2CO3•10H2Ocrystalsifthesolubilityis21.5kganhydrousNa2CO3/100kgoftotalwater?Dothisforthefollowingcases:(a)Assumethatnowaterisevaporated.(b)Assumethat3%ofthetotalweightofthesolutionislostbyevaporationofwaterincooling.FIGURE3.ProcessflowforcrystallizationEXAMPLE1SolutionThemolecularweightsare106.0forNa2CO3,180.2for10H20,and286.2forNa2CO3•10H2O.TheprocessflowdiagramisshowninFig.3,withWbeingkgH2Oevaporated,Skgsolution(motherliquor),andCkgcrystalsofNa2CO3•10H2O.Makingamaterialbalancearoundthedashedlineboxforwaterforpart(a),whereW=0.(13)where(180.2)/(286.2)iswtfractionofwaterinthecrystals.Example1MakingabalanceforNa2CO3,Solvingthetwoequationssimultaneously,C=6370kgofNa2CO3

•10H2OcrystalsandS=3630kgsolution.Forpart(b),W=0.03(10000)=300kgH2O.Equation(13)becomesEquation(14)doesnotchange,sincenosaltisintheWstream.SolvingEqs.(14)and(15)simultaneously,C=6630kgofNa2CO3

•10H2OcrystalsandS=3070kgsolution.(14)(15)HeateffectsandheatbalancesincrystallizationWhenacompoundwhosesolubilityincreasesastemperatureincreasesdissolves,thereisanabsorptionofheat,calledtheheatofsolution–occurswhenthesolubilitydecreasesasthetemperatureincreasesAtequilibriumtheheatofcrystallizationisequaltothenegativeoftheheatofsolutionatthesameconcentrationinsolution.TheenthalpyH1oftheenteringsolutionattheinitialtemperatureisreadoffthechart,whereH1iskJforthetotalfeed.TheenthalpyH2ofthefinalmixtureofcrystalsandmotherliquoratthefinaltemperatureisalsoreadoff.Ifsomeevaporationoccurs,theenthalpyHvofthewatervaporisobtainedfromthesteamtables.ThenthetotalheatabsorbedqinkJisIfqispositive,heatmustbeaddedtothesystem.Ifitisnegative,heatisevolvedorgivenoff.YieldsandHeatandMaterialBalancesinCrystallization(16)EXAMPLE2HeatBalanceinCrystallizationAfeedsolutionof2268kgat327.6K(54.4°C)containing48.2kgMgSO4/100kgtotalwateriscooledto293.2K(20°C),whereMgSO4•7H2Ocrystalsareremoved.Thesolubilityofthesaltis35.5kgMgSO4/100kgtotalwater.Theaverageheatcapacityofthefeedsolutioncanbeassumedas2.93kJ/kg•K.Theheatofsolutionat291.2K(18°C)is-13.31x103

kJ/kgmolMgSO4•7H2O.Calculatetheyieldofcrystalsandmakeaheatbalancetodeterminethetotalheatabsorbed,q,assumingthatnowaterisvaporized.EXAMPLE2SolutionMakingawaterbalanceandabalanceforMgSO4usingequationssimilarto(13)and(14)inExample1,C=616.9kgMgSO4•7H2OcrystalsandS=1651.1kgsolution.Tomakeaheatbalance,adatumof293.2K(20°C)willbeused.ThemolecularweightofMgSO4•7H20is246.49.TheenthalpyofthefeedisH1:Example2Theheatofsolutionis-(13.31x103)/246.49=-54.0kJ/kgcrystals.Thentheheatofcrystallizationis-(-54.0)=+54.0kJ/kgcrystals,or54.0(616.9)=33312kJ.Thisassumesthatthevalueat291.2Kisthesameasat293.2K.Thetotalheatabsorbed,q,isSinceqisnegative,heatisgivenoffandmustberemoved.EQUIPMENTFORCRYSTALLIZATIONTankCrystallizationHotsaturatedsolutionsareallowedtocoolinopentanksAfteraperiodoftime,themotherliquorisdrainedandthecrystalsremovedNucleationandthesizeofcrystalsaredifficulttocontrolLaborcostareveryhighHaslimitedapplication;usedtoproducecertainfinechemicalandpharmaceuticalproducts.ScrapedsurfacecrystallizersOnetypeofscrapedsurfacecrystallizeristheSwenson-Walkercrystallizer,whichconsistsofanopentrough0.6mwidewithasemicircularbottomhavingacoolingjacketinside.Slow-speedspiralagitatorrotates

andsuspendsthegrowingcrystalsonturning.Bladespassclosetothewallandbreakoffanydepositsofcrystalsonthecooledwall.Usedincrystallizingicecreamandplasticizingmargarine.EquipmentforCrystallizationCirculating-liquidevaporated-c