




已阅读5页,还剩8页未读, 继续免费阅读
版权说明:本文档由用户提供并上传,收益归属内容提供方,若内容存在侵权,请进行举报或认领
文档简介
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
温馨提示
- 1. 本站所有资源如无特殊说明,都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
- 2. 本站的文档不包含任何第三方提供的附件图纸等,如果需要附件,请联系上传者。文件的所有权益归上传用户所有。
- 3. 本站RAR压缩包中若带图纸,网页内容里面会有图纸预览,若没有图纸预览就没有图纸。
- 4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
- 5. 人人文库网仅提供信息存储空间,仅对用户上传内容的表现方式做保护处理,对用户上传分享的文档内容本身不做任何修改或编辑,并不能对任何下载内容负责。
- 6. 下载文件中如有侵权或不适当内容,请与我们联系,我们立即纠正。
- 7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。
最新文档
- 电器分销合同协议书范本
- 策划赞助合作协议书范本
- 破坏房屋赔偿协议书范本
- 电梯轿厢清洁协议合同书
- 监控质保与售后合同范本
- 驾校学员培训合同协议书
- 项目工程挂靠协议书范本
- 环保投资股东协议书模板
- 煤矿合同续签协议书模板
- 权利质押反担保合同范本
- 外研版八年级上册英语期末复习:阅读理解 刷题练习题30篇(含答案解析)
- 退休延期留用岗位协议书
- 生物安全记录表
- DB34T 1708-2020 电站堵阀检验规程
- 《幼儿园保育教育质量评估指南》引领下的园本教研转向与新生态
- 四年级数学(小数加减运算)计算题专项练习与答案汇编
- 《老年护理学》考试复习题库(含答案)
- 第1章有理数单元同步练习题 2024-2025学年华东师大版数学七年级上册
- GB/T 29239-2024移动通信设备节能参数和测试方法基站
- 职业技术学校《服装工艺基础》课程标准
- 《Python编程案例教程》全套教学课件
评论
0/150
提交评论