化学化工专业英语试题_化学工程与工艺专业英语阅读教程_化学工程与工艺考教师(45)

Infact,therearesomecasesinwhichthedesignengineermightusethemethodsandequationsoftransportphenomenadirectlyinthedesignofequipment.Anexamplewouldbeatubularreactor,whichmightbeillustratedasapipe,e.g.,theheatexchangerdescribedearlier,withahomogeneouschemicalreactionoccurringinthefluidwithin.Thefluidenterswithacertainconcentrationofreactantandleavesthetubewithadecreasedconcentrationofreactantandanincreasedconcentrationofproduct.实际上，在设备设计中有一些情况下设计工程师可能直接使用传递现象的方法和方程。一种情况就是设计可以被称为管道的管式反应器，如，前面所提过的热交换器，在它里面的液相中发生着一个均相化学反应。流体以一定浓度的反应物流进并以浓度降低的反应物和浓度

增加的产物流出反应管。

Ifthereactioniothermic,thereactorwallwillusuallybemaintainedatalowtemperatureinordertoremovetheheatgeneratedbythechemicalreaction.Thereforethetemperaturewilldecreasewithradialposition,i.e.,withthedistancefromthecenterlineofthepipe.Then,sincethereactionrateincreaseswithtemperature,itwillbehigheratthecenter,wherethetemperatureishigh,thanatthewall,wherethetemperatureislow.Accordingly,theproductsofthereactionwilltendtoaccumulateatthecenterlinewhilethereactantsaccumulatenearthewallofthereactor.Hence,concentrationaswellastemperaturewillvarybothwithradialpositionandwithlength.Todesignthereactorwewouldneedtoknow,atanygivenlength,themeanconcentrationofproduct.Sincethismeanconcentrationisobtainedfromthepointvaluesaveragedoverthecrosssection,weactuallyneedtoobtaintheconcentrationateverypointinthereactor,i.e.,ateveryradialpositionandateverylength.Buttocalculatetheconcentrationateverypointweneedtoknowthereactionrateateverypoint,andtocalculatetherateateverypointweneedtoknowboththetemperatureandtheconcentrationateverypoint!Furthermore,tocalculatethetemperaturewealsoneedtoknowtherateandthevelocityofthefluidateverypoint.Wewillnotgointotheequationsinvolved,butobviouslywehaveacomplicatedsetofpartialdifferentialequationsthatmustbesolvedbysophisticatedprocedures,usuallyonacomputer.Itshouldbeapparentthatwecouldnothandlesuchaproblembytheempiricaldesignproceduresusedinunitoperationscoursesforaheatexchanger.Insteadthetheoryandmathematicalproceduresoftransportphenomenaareessential,unlessonewishestogogotheexpenseandtakethetimetobuildpilotplantsofincreasingsizeandmeasuretheconersionineach.Eventhenthefinalscale-upisprecariousanduncertain.如果反应是放热的，为了移除化学反应生成的热量反应器壁通常维持在一个低的温度。