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
- 2023年卧式自动翻洗过滤机项目资金需求报告
- 2024届浙江省宁波市高三上学期期末数学试题(解析版)
- 光泵磁力计(OPM)全球前8强生产商排名及市场份额调查数据
- 经济转型方案
- 装修工程分部分项工程主要施工方案
- 髋关节置换产品市场前景规模分析
- 心理辅导教师辅导学生记录表
- 避免教师对学科本体知识丢失的十种方法
- 柴油发电机突然停车故障特征及原因及排除方法
- 分级诊疗制度(同名1905)
- 员工对赌协议书
- 国际反洗钱师cams考试真题中文版题库汇总(含答案)
- 二年级道德与法治下册全册知识要点梳理
- 急性肾衰竭诊疗规范内科学诊疗规范诊疗指南2023版
- 长沙银行2023年春季招聘工作人员笔试历年难、易错考点试题含答案附详解
- 中医医院业务数据分析报告
- 网络安全责任制
- 在XX医院药品、器械、耗材供应商廉政谈话会上讲话稿范文
- 2023年05月浙江温州医科大学附属第一医院医疗保健中心工作人员招考聘用笔试题库含答案解析
- 某铁路工程路基及附属工程施工组织设计
- 《混凝土结构加固设计规范》宣贯培训教材
评论
0/150
提交评论