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1、化学反应工程Chapter13TheDispersionModelChoiceofModelsModelsareusefulforrepresentingflowinrealvessels,forscaleup,andfordiagnosingpoorflow.Wehavedifferentkindsofmodelsdependingonwhetherflowisclosetoplug,mixed,orsomewhereinbetween.Chapter13and14dealprimarilywithsmalldeviationfromplugflow.Therearetwomo
2、delsforthis:thedispersionmodelandthetanks-in-seriesmodel.Usetheonethatiscomfortableforyou.化学反应工程Theyareroughlyequivalent.Thesemodelsapplytoturbulentflowinpipes,laminarflowinverylongtubes,flowinpackedbeds,shaftkilns,longchannels,screwconveyers,etc.Forlaminarflowinshorttubesorlaminarflowofvisco
3、usmaterialsthesemodelsmaynotapply,anditmaybethattheparabolicvelocityprofileisthemaincauseofdeviationfromplugflow.Wetreatthissituation,calledthepureconvectionmodel,inChapter15.IfyouareunsurewhichmodeltousegotothechartatthebeginningofChapter15.Itwilltellyouwhichmodelshouldbeusedtorepresentyours
4、etup.化学反应工程13.1AXIALDISPERSIONSupposeanidealpulseoftracerisintroducedintothefluidenteringavessel.Thepulsespreadsasitpassesthroughthevessel,andtocharacterizethespreadingaccordingtothismodel(seeFig.13.1),weassumeadiffusion-likeprocesssuperimposed(添加)onplugflow.Wecallthisdispersionorlongi‘tudina
5、l(纵向的,轴向的)dispersiontodistinguishitfrommoleculardiffusion.ThedispersioncoefficientD(m2/s)representsthisspreadingprocess.Thus●largeDmeansrapidspreadingofthetracercurve●smallDmeansslowspreading●D=0meansnospreading,henceplugflow化学反应工程Figure13.1Thespreadingoftraceraccordingtothedispersionmodel.小结
6、轴向分（扩散）模型基本假定：对于偏离了理想平推流的管式反应器，假设在主流体的流动方向上叠加了一个反向的轴向分（扩）散效应，因而带来了流体粒子的返混。特点：径向上，浓度分布均一；轴向上，流体的流速u和分散系数D均为恒定值化学反应工程Alsoisthedimensionlessgroupcharacterizingthespreadinthewholevessel.WeevaluateDorD/uLbyrecordingtheshapeofthetracercurveasitpassestheexitofthevessel.Inparticular,wemeasure化学反应工程Th
7、evariance(方差)isdefinedas(2)Thesemeasures,andand,aredirectlylinkedbytheorytoDandD/uL.Themeantime,forcontinuousordiscrete(离散的)data,isdefinedas(1)化学反应工程orindiscreteform(3)Thevariancerepresentsthesquareofthespreadofthedistributionasitpassestheves
