英文原文MixingeffectsofconstitutingelementsofmixingscrewsinsingleandtwinscrewextrudersD.J.vanZuilichem,E.Kuiper,W.Stolp,T.JagerDepartmentofFoodTechnologyandNutritionalSciences,WageningenAgriculturalUniersity,FoodandBioprocessEngineeringGroup,P.O.Box8129,Bomenweg2,6700EVWageningen,NetherlandsReceived21April1998；accepted29March1999AbstractInextrusion,mixingofsolidsandmeltshasalwaysbeenproblematic,leadingtodiversemodelsdescribingthemeltingprocess.Itisfoundthatforfoodsbasedoncereals,onlyafewarevalid,duetothesimultaneouspresenceofwaterandhighviscousnon-Newtonianmaterial.Mixingtrialsaresummarisedforsingleandtwinscrewextruders,withparticulatesolidsofdifferentparticlesizeslike,maizegrits,wheatflour,sucrosecrystalsandglucosesyrup.Specialmixing-headsforsinglescrewextruderslike,pineapple-heads,andslottedflightsectionsforcounter-rotatingtwinscrewextrudersareinvestigated.Theeffectsofscrewgeometryonmixinghasbeenmeasuredusingco-rotatingtwinscrewextrudermodularelementslike:singleleadelements,mixingpaddlesandreversedpitchelementsinatranslucentmodelextruderbymeansofastudyofflowphenomenaandresidencetimedistributionRTDmeasurements.Mixingeffectsarereportedandtheirinfluencesonviscousdissipation,residencetime,curvespreadandstagnancyareexplained.1999ElsevierScienceS.A.Allrightsreserved.Keywords:Singlescrewextruders；Twinscrewextruders；Mixing1.IntroductionPartofthedefinitionoffoodextrusioncookingisthatthefoodmaterialonitswaythroughtheopenchannelofasinglescrewextruders.s.e.,issheared,mixedandcom-pressed.Inthecaseofatwinscrewextrudert.s.e.Withcloselyintermeshingscrews,thefoodistransportedinso-calledC-shapedchambers,whichareenclosedbytheneighbouringintermeshingscrewandthebarrelwallseeFig.1.ThefoodpowderistakenupatthefeedportintheC-shapedchamberofonescrew,isconveyed,internallymixed,forcedthroughthegapsandclearancesofthisscreworeventuallytransferredtotheneighbouringscrewandfinallythisC-shapedchamberdeliversitscontentatwxthedie13,23.Thedifferencesinthewayofoperationbetweens.s.e.sandt.s.e.sareobviousandeachofthemwillaskforitsowndesignofmodularscrewpartsthatwillenhanceandimprovethemixingeffectsofthescrewswx14,17.Mixingoperationsincookingextruderscanbedistin-guishedinthreedifferenttypes:thefirstoneiscoarsemixingorlongitudinalraxialmixing.Thesecondoneisdispersivemixingorcomminutionofthedispersephase.Thethirdoneisdistributivemixingorradialmixing.Theoperativemechanismtorealisecoarsemixingistherota-tionalmovementofthescrews,causingresidencetimedistributionRTD.Incaseofplugflowhowever,therewillbenocoarsemixing.Thesedifferentmechanismsareresponsibleforthemixingeffects.Fordispersivemixingthefunctionofthecomminutionistobreakagglomeratesofthedispersephaseintosmallerones.Theoperativemechanismistheexistenceofelongationalforcesandrorshearstresses.Distributivemixingiscausedbytheactionofascrewelement,distributingtheconcentrationofthedispersephaseoverthetotalvolumeofthechamberorchanneloftheextrudercross-sectioninsuchawaythattheconcentrationofthedispersephaseineachvolumetricelementequalstheaveragedconcentration.Thisrequire-mentisstricterwhentheelementaryvolumetobeconsid-eredissmaller.Althoughtotallydifferent,thethreemech-anismsdescribedarenotindependentofeachother.Theanismsdescribedarenotindependentofeachother.Thethreetypesofmixingshouldthereforebecarefullydistinguished.Itispossibletodesignscrewswithmixingelementsondifferentlocations,provokingacertaincombinationofthethreemixingtypesatacertainaxiallocation.Thepossibilitiesofthesecombinationsarestronglydependentofextrudergeometry,propertiesoffoodmaterialsandpropertiesunderextrusionconditions.Althoughalotofknowledgeisgatheredconcerningtheuseofwell-knownmixingelementsfromtheplasticatingindustry,incorporationofthisknowledgeintodesignsofs.s.e.sandt.s.e.sarenotalwayssuccessfulforfoodproductsandotherbiopolymers.Thereareseveralreasonsforthisphenomenon.Thefirstdisturbingfactorwhencomparingplasticpolymerswithbiopolymersisthedifferenceinfeedtotheextruder,e.g.,thepresenceofmultiplesolids,waterandeventuallyvegetableoilinthecaseofbiopolymers.Secondly,biopolymersdonothaveawell-definedmelt-indexlikemanychemicalpolymersandingeneraltheirviscositybehaviourdiffersfromthatofchemicalpolymerswx1.Thirdly,thereareanumberofdegradationreactionsofirreversiblenatureoccurringsimultaneouslywhenabiopolymerissubjectedtoatemperatureandstressfieldinanextruder,changingthephysicalandchemicalpropertieswxdrasticallyfromlocationtolocation15.Inthisfieldnumerouspapershavebeenpublishedbywell-knownau-thorsfromresearchinstitutionslikeCAFTrRutgersuniversity,IFRNorwich,INRA,CEMEFFrance.Theaimofthisarticleissummarisingthepossibilitiesandsolutionsformixingproblemsforsinglescrewandcounter-rotatingtwinscrewextrudersandgivingbetterinsightinthefunctionsofscrewelementsinco-rotatingtwinscrewextruders.2.Mixingmechanisms2.1.LongitudinaloraxialmixingWhenfoodparticlesduringtheirlifetimeinanextruderaresubjectedtoaxialmixing,thiscanbeseenandmeasuredbyachangingRTD.Extremesinthiscaseareplugflow,whereallparticleshavethesameresidencetime,orawideRTDwhichiscausedbyaxialmixing.Someparticleswillstayintheextrudercookerforaprocesstimeshorterthantheaverage,whilstotherswilldwellformuchlongerthantheaveragewhendifferentflowsystemsarecompiledseeFig.2.Fig.3showsthetheoreticalRTDforamaterialwithacoefficientns1inthepower-lawequationforthesheardependencyoftheviscosityinans.s.e.ThisfigurealsoshowstheRTDdata,measuredwhenprocessingcorngritsinaBattenfelds.s.e.attwomoisturey1.contentsvs0.142,0.20kgkgandscrewrevolu-wy1wxtionsrangingfromNs1.33,1.66,2.0revs22.TheRTDdataarerepresentedasancumulativeexitageorF-function,whichshowsthefractionFoftheinflow,wxwhichhaslefttheextruderaftertseconds3,4,16.TheE-function,thederivativeoftheF-function,showstheso-calledexitagedistributionoffoodparticles,fedintotheextruderataninfinitesmalltimeandisrelatedtotheF-functionwith:tFtsEtdt1H0Theaverageresidencetimetofthefoodmaterialinthecookingextruderiscalculatedwith:tstEtdt.2HWhentheF-functioninFig.3isobserved,itfollowsthatmostofthecorngritshaveanexitagebetweentandt,0whereast(3r4t.Thesemeasurementsshowthatfora0singlescrewextruderthereisastronginfluenceofthemoisturecontentandthenumberofrevolutionsofthescrewontheshapeoftheF-functionandthebreakthroughwxpoint3.TheoperatingprincipleoftwinscrewextruderscanbedescribedwiththeexistenceoftwoparallelseriesofContinuouslyStirredTankReactorsCSTR,whereeachwxC-shapedchamberrepresentssuchaCSTR13.InFig.4,aschematicisgivenofthisprincipleandtheleakage-gapsand-flowsbetweenthechambersandbetweentheseriesCSTRsareindicated.Theleakageflowsthroughthecalendar-,side-,flight-andtetrahedron-gapsQ,Q,Q,csf.Q,res.andforcedconveyingofthechambersresultsinatwxnettpositivemassflowfromfeedporttothedie13,17.ThisapproachwasthebasisofaseriesofpublicationsoftheWageningenresearchgroupontheRTDofextrusionwxcooking,whichisdescribedbelow812,2426,28.TheRTDsoft.s.e.saremoreasymmetriccomparedtoacascadeofCSTRs.ThedifferentparallelseriesofC-shapedchambersinat.s.e.,canbesummeduptoanRTDmodelofCSTRsinseries.TheseCSTRstravelfromfeedporttodie,duringwhichthechambervolumechangesandthesechambersloosematerialintheoppositedirectionwxseeFig.4.Jageretal.11presentedawaybywhichtheRTDcouldbecalculated.TheaverageresidencetimeandthecurvewidthsarecalculatedfromtheuncorrectedE-curveusingToddsmethodofrecordingthepassing-byof16%,84%and92%oftracermaterial.TheyarecorrectedwithasystematicalerrorascalculatedbyaMonteCarlowxprocedure18.Theuncorrectedaverageresidencetimeiscalculatedusingequationsforthezero,firstandsecond.timemomentM,M,Mofthemeasuredextinction012.Ctas:MstiCtdt.3HitsMrM410AndM21UNs52MMyM021NUgivesthenumberofmixersofaseriesofCSTRs.AhighernumberofNUstandsforaflowthatresemblesamoreplug-flownature.RT