化学反应工程英文课件Chapter-9

化学反应工程Chapter9TemperatureandPressureEffectsInoursearchforfavorableconditionsforreactionwehaveconsideredhowreactortypeandsizeinfluencetheextentofconversionanddistributionofproducts.Thereactiontemperatureandpressurealsoinfluencetheprogressofreactions,anditistheroleofthesevariablethatwenowconsider.化学反应工程Wefollowathree-stepprocedure:First,wemustfindhowequilibriumcomposition,rateofreaction,andproductdistributionareaffectedbychangeinoperatingtemperaturesandpressures.Thiswillallowustodeterminetheoptimumtemperatureprogression,anditisthisthatwestrive(努力,奋斗)toapproximatewitharealdesign.化学反应工程Second,chemicalreactionsareusuallyaccompaniedbyheateffects,andwemustknowhowthesewillchangethetemperatureofthereactingmixture.Withthisinformationweareabletoproposeanumberoffavorablereactorandheatexchangesystems—thosewhichcloselyapproachtheoptimumconditionsandthenseeinghoweconomicconsiderationswillselectoneofthesefavorablesystemsasthebest.化学反应工程So,withtheemphasisonfindingtheoptimumconditionsandthenseeinghowbesttoapproachtheminactualdesignratherthandeterminingwhatspecificreactorswilldo,letusstartwithdiscussionofsinglereactionsandfollowthiswiththespecialconsiderationsofmultiplereactions.化学反应工程9.1SINGLEREACTIONSWithsinglereactionsweareconcernedwithconversionlevelandreactorstability.Questionsofproductdistributiondonotoccur.Thermodynamicsgivestwoimportantpiecesofinformation,thefirstbeingtheheatliberated(释放)orabsorbedforagivenextentofreaction,thesecondbeingthemaximumpossibleconversion.Letusbrieflysummarizethesefindings.化学反应工程HeatsofReactionfromThermodynamicsTheheatliberatedorabsorbedreactionattemperatureTdependsonthenatureofthereactingsystem,theamountofmaterialreacting,andthetemperatureandpressureofthereactingsystem,andiscalculatedfromtheheatofreaction△Hr(反应热),forthereactioninquestion.Whenthisisnotknown,itcaninmostcasesbecalculatedfromknownandtabulatedthermochemicaldataonheatsofformation△Hf(生成热)orheatsofcombustion△Hc(燃烧热)ofthereactingmaterials.化学反应工程aArR+sSThesearetabulatedatsomestandardtemperature,T1,usually25℃.Asabriefreminder,considerthereactionByconvention(习俗,惯例)wedefinetheheatofreactionattemperatureTastheheattransferredtothereactingsystemfromthesurroundingswhenamolesofAdisappeartoproducermolesofRandsmolesofSwiththesystemmeasuredatthesametemperatureandpressurebeforeandafterthechange.ThusaArR+sS△HrTpositive,endothermicnegative,exothermic(1)化学反应工程HeatofReactionandTemperature.ThefirstproblemistoevaluatetheheatofreactionattemperatureT2knowingtheheatofreactionattemperatureT1.Thisisfoundbythelawofconservation(守恒,保存)ofenergyasfollows:化学反应工程2Ttureattemperareactionduringabsorbedheat12TtoTfrometemperaturtheirchangetoreactantstoaddedheat1Ttureattemperareactionduringabsorbedheat12TfromTbacktothembringtoproductstoaddedheat=++(2)化学反应工程Intermofen‘thalpies(焓)ofreactantsandproductsthisbecomes△Hr2=-(H2–H1)reactants+△Hr1+(H2–H1)productsWheresubscripts1and2refertoquantitiesmeasuredattemperatureT1andT2,respectively.Intermofspecificheats(比热，CP)pApSpRpaC-sCrCCWhere21Tr2r1TH=HPCdT(4)(3)(5)化学反应工程CpA=αA+βAT+γAT2CpR=αR+βRT+γRT2CpS=αS+βST+γST2Whenthemolarspecificheatsarefunctionsoftemperaturesasfollows,(6)化学反应工程Weobtain△Hr2=△Hr1+α+β+γT2)dT=△Hr1+α(T2-T1)++21TT()(22122TT)(33132TTWhereα=rαR+sαS-aαAβ=rβR+sβS-aβAγ=rγR+sγS-aγA(7)(8)化学反应工程EXAMPLE9.1△HrATVARIOUSTEMPERATURESFromthe△Hcand△Hftables,I’vecalculatedthatthestandardheatofmygas-phasereactionat25℃isasfollows:A+B2R△Hr,298K=-50000JAt25℃thereactionisstronglyexothermic.Butthisdoesn’tinterestmebecauseIplantorunthereactionat1025℃.Whatisthe△Hratthattemperature,andisthereactionstillexothermicatthattemperature?Data.Between25℃and1025℃theaverageCpvaluesforthevariousreactioncomponentsare=35J/molK=45J/molK=70J/molKpACpBCpRC化学反应工程SOLUTIONFirst,prepareareactionmapasshowninFig.E9.1.Thenanenthalpybalancefor1molA,1molB,2molRgives△H1=△H2+△H3+△H4=(n△T)reactants+△Hr,25℃+(n△T)productspCpC=1(35)(25-1025)[phone])+(-50000)[phone])=10000J△Hr.1025℃=10000Jor化学反应工程FigureE9.1Thereactionisexothermicat25Cendothermicat1025C化学反应工程EquilibriumConstantsfromThermodynamicsFromthesecondlawofthermodynamicsequilibriumconstants,henceequilibriumcompositionsofreactingsystems,maybecalculated.Wemustremember,however,thatrealsystemsdonotnecessarilyachievethisconversion;therefore,theconversionscalculatedfromthermodynamicsareonlysuggestedattainablevalues.Asabriefreminder,thestandardfreeenergy△G0forthereactionofEq.1attemperatureTisdefinedas化学反应工程0000RSAG=rGsGaG=RTlnK00R0lnrsSaAffffRTff(9)wherefisthefu‘gacity(逸度)ofthecomponentatequilibriumconditions;f0isthefugacityofthecomponentatthearbitrarilyselectedstandardstateattemperatureT,thesameoneusedincalculating△G0;化学反应工程G0isthestandardfreeenergyofareactingcomponent,tabulatedformanycompounds;andKisthethermodynamicequilibriumconstantforthereaction.Standardstatesatgiventemperaturearecommonlychosenasfollow:Gas—purecomponentatoneatmosphere,atwhichpressureidealgasbehavioriscloselyapproximated；Soli