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外文翻译--单螺杆和双螺杆挤压机中混合元件的混合性能.doc

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外文翻译--单螺杆和双螺杆挤压机中混合元件的混合性能.doc

英文原文MixingeffectsofconstitutingelementsofmixingscrewsinsingleandtwinscrewextrudersD.J.vanZuilichem,E.Kuiper,W.Stolp,T.JagerDepartmentofFoodTechnologyandNutritionalSciences,WageningenAgriculturalUniÍersity,FoodandBioprocessEngineeringGroup,P.O.Box8129,Bomenweg2,6700EVWageningen,NetherlandsReceived21April1998accepted29March1999AbstractInextrusion,mixingofsolidsandmeltshasalwaysbeenproblematic,leadingtodiversemodelsdescribingthemeltingprocess.Itisfoundthatforfoodsbasedoncereals,onlyafewarevalid,duetothesimultaneouspresenceofwaterandhighviscousnonNewtonianmaterial.Mixingtrialsaresummarisedforsingleandtwinscrewextruders,withparticulatesolidsofdifferentparticlesizeslike,maizegrits,wheatflour,sucrosecrystalsandglucosesyrup.Specialmixingheadsforsinglescrewextruderslike,pineappleheads,andslottedflightsectionsforcounterrotatingtwinscrewextrudersareinvestigated.Theeffectsofscrewgeometryonmixinghasbeenmeasuredusingcorotatingtwinscrewextrudermodularelementslikesingleleadelements,mixingpaddlesandreversedpitchelementsinatranslucentmodelextruderbymeansofastudyofflowphenomenaandresidencetimedistributionRTDmeasurements.Mixingeffectsarereportedandtheirinfluencesonviscousdissipation,residencetime,curvespreadandstagnancyareexplained.1999ElsevierScienceS.A.Allrightsreserved.KeywordsSinglescrewextrudersTwinscrewextrudersMixing1.IntroductionPartofthedefinitionoffoodextrusioncookingisthatthefoodmaterialonitswaythroughtheopenchannelofasinglescrewextruders.s.e.,issheared,mixedandcompressed.Inthecaseofatwinscrewextrudert.s.e.Withcloselyintermeshingscrews,thefoodistransportedinsocalledCshapedchambers,whichareenclosedbytheneighbouringintermeshingscrewandthebarrelwallseeFig.1.ThefoodpowderistakenupatthefeedportintheCshapedchamberofonescrew,isconveyed,internallymixed,forcedthroughthegapsandclearancesofthisscreworeventuallytransferredtotheneighbouringscrewandfinallythisCshapedchamberdeliversitscontentatwxthedie13,23.Thedifferencesinthewayofoperationbetweens.s.e.sandt.s.e.sareobviousandeachofthemwillaskforitsowndesignofmodularscrewpartsthatwillenhanceandimprovethemixingeffectsofthescrewswx14,17.Mixingoperationsincookingextruderscanbedistinguishedinthreedifferenttypesthefirstoneiscoarsemixingorlongitudinalraxialmixing.Thesecondoneisdispersivemixingorcomminutionofthedispersephase.Thethirdoneisdistributivemixingorradialmixing.Theoperativemechanismtorealisecoarsemixingistherotationalmovementofthescrews,causingresidencetimedistributionRTD.Incaseofplugflowhowever,therewillbenocoarsemixing.Thesedifferentmechanismsareresponsibleforthemixingeffects.Fordispersivemixingthefunctionofthecomminutionistobreakagglomeratesofthedispersephaseintosmallerones.Theoperativemechanismistheexistenceofelongationalforcesandrorshearstresses.Distributivemixingiscausedbytheactionofascrewelement,distributingtheconcentrationofthedispersephaseoverthetotalvolumeofthechamberorchanneloftheextrudercrosssectioninsuchawaythattheconcentrationofthedispersephaseineachvolumetricelementequalstheaveragedconcentration.Thisrequirementisstricterwhentheelementaryvolumetobeconsideredissmaller.Althoughtotallydifferent,thethreemechanismsdescribedarenotindependentofeachother.Theanismsdescribedarenotindependentofeachother.Thethreetypesofmixingshouldthereforebecarefullydistinguished.Itispossibletodesignscrewswithmixingelementsondifferentlocations,provokingacertaincombinationofthethreemixingtypesatacertainaxiallocation.Thepossibilitiesofthesecombinationsarestronglydependentofextrudergeometry,propertiesoffoodmaterialsandpropertiesunderextrusionconditions.Althoughalotofknowledgeisgatheredconcerningtheuseofwellknownmixingelementsfromtheplasticatingindustry,incorporationofthisknowledgeintodesignsofs.s.e.sandt.s.e.sarenotalwayssuccessfulforfoodproductsandotherbiopolymers.Thereareseveralreasonsforthisphenomenon.Thefirstdisturbingfactorwhencomparingplasticpolymerswithbiopolymersisthedifferenceinfeedtotheextruder,e.g.,thepresenceofmultiplesolids,waterandeventuallyvegetableoilinthecaseofbiopolymers.Secondly,biopolymersdonothaveawelldefinedmeltindexlikemanychemicalpolymersandingeneraltheirviscositybehaviourdiffersfromthatofchemicalpolymerswx1.Thirdly,thereareanumberofdegradationreactionsofirreversiblenatureoccurringsimultaneouslywhenabiopolymerissubjectedtoatemperatureandstressfieldinanextruder,changingthephysicalandchemicalpropertieswxdrasticallyfromlocationtolocation15.InthisfieldnumerouspapershavebeenpublishedbywellknownauthorsfromresearchinstitutionslikeCAFTrRutgersuniversity,IFRNorwich,INRA,CEMEFFrance.Theaimofthisarticleissummarisingthepossibilitiesandsolutionsformixingproblemsforsinglescrewandcounterrotatingtwinscrewextrudersandgivingbetterinsightinthefunctionsofscrewelementsincorotatingtwinscrewextruders.2.Mixingmechanisms2.1.LongitudinaloraxialmixingWhenfoodparticlesduringtheirlifetimeinanextruderaresubjectedtoaxialmixing,thiscanbeseenandmeasuredbyachangingRTD.Extremesinthiscaseareplugflow,whereallparticleshavethesameresidencetime,orawideRTDwhichiscausedbyaxialmixing.Someparticleswillstayintheextrudercookerforaprocesstimeshorterthantheaverage,whilstotherswilldwellformuchlongerthantheaveragewhendifferentflowsystemsarecompiledseeFig.2.Fig.3showsthetheoreticalRTDforamaterialwithacoefficientns1inthepowerlawequationforthesheardependencyoftheviscosityinans.s.e.ThisfigurealsoshowstheRTDdata,measuredwhenprocessingcorngritsinaBattenfelds.s.e.attwomoisturey1.contentsvs0.142,0.20kgkgandscrewrevoluwy1wxtionsrangingfromNs1.33,1.66,2.0revs22.TheRTDdataarerepresentedasancumulativeexitageorFfunction,whichshowsthefractionFoftheinflow,wxwhichhaslefttheextruderaftertseconds3,4,16.TheEfunction,thederivativeoftheFfunction,showsthesocalledexitagedistributionoffoodparticles,fedintotheextruderataninfinitesmalltimeandisrelatedtotheFfunctionwithtFtsEtdt1H0TheaverageresidencetimetofthefoodmaterialinthecookingextruderiscalculatedwithtstEtdt.2HWhentheFfunctioninFig.3isobserved,itfollowsthatmostofthecorngritshaveanexitagebetweentandt,0whereast3r4t.Thesemeasurementsshowthatfora0singlescrewextruderthereisastronginfluenceofthemoisturecontentandthenumberofrevolutionsofthescrewontheshapeoftheFfunctionandthebreakthroughwxpoint3.TheoperatingprincipleoftwinscrewextruderscanbedescribedwiththeexistenceoftwoparallelseriesofContinuouslyStirredTankReactorsCSTR,whereeachwxCshapedchamberrepresentssuchaCSTR13.InFig.4,aschematicisgivenofthisprincipleandtheleakagegapsandflowsbetweenthechambersandbetweentheseriesCSTRsareindicated.Theleakageflowsthroughthecalendar,side,flightandtetrahedrongapsQ,Q,Q,csf.Q,res.andforcedconveyingofthechambersresultsinatwxnettpositivemassflowfromfeedporttothedie13,17.ThisapproachwasthebasisofaseriesofpublicationsoftheWageningenresearchgroupontheRTDofextrusionwxcooking,whichisdescribedbelow8–12,24–26,28.TheRTDsoft.s.e.saremoreasymmetriccomparedtoacascadeofCSTRs.ThedifferentparallelseriesofCshapedchambersinat.s.e.,canbesummeduptoanRTDmodelofCSTRsinseries.TheseCSTRstravelfromfeedporttodie,duringwhichthechambervolumechangesandthesechambersloosematerialintheoppositedirectionwxseeFig.4.Jageretal.11presentedawaybywhichtheRTDcouldbecalculated.TheaverageresidencetimeandthecurvewidthsarecalculatedfromtheuncorrectedEcurveusingToddsmethodofrecordingthepassingbyof16,84and92oftracermaterial.TheyarecorrectedwithasystematicalerrorascalculatedbyaMonteCarlowxprocedure18.Theuncorrectedaverageresidencetimeiscalculatedusingequationsforthezero,firstandsecond.timemomentM,M,Mofthemeasuredextinction012.CtasMstiCtdt.3HitsMrM410AndM21UNs52MMyM021NUgivesthenumberofmixersofaseriesofCSTRs.AhighernumberofNUstandsforaflowthatresemblesamoreplugflownature.RTDmodelscanbedividedincurvefitmodels,whichdescribethecurveshapeandinmodelsthatdescribetheaxialmixingintheextruderreactorinapredictiveandrordescriptiveway.Anexampleofadescriptivemodelistheplugflowmodelwithwxaxialdispersion16.TheaxialdispersionisdescribedwithaconstanteffectivelongitudinaldispersioncoefficientIDe2y1.wx.ms21.ThePecletnumberPe,theratiooftheaverageaxialconvectivetransportandthetransportbydispersionisequaltoÍLPes6IDeinwhichListheextruderlengthandÍistheaverageaxialvelocity.TheaxialdispersionmodelandamodelofawxcascadeofCSTRsdogivecomparableRTDs16.Jagerwxwxhasshownin199111and199212thatbothmodelsforextruderscanbecombinedandthatPeandNarerelatedasPe2UNs7UyPe2Pey1q10ItispossibletousethisRTDmodelfordifferenttwinscrewextruderdesigns,toevaluatetheirworkingbehaviour.Thiscanberealisedbyintroducingasocalledchambermixingcoefficient,

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