CH 204: Chemical Reaction Engineering - lecture notes

1 CH204:ChemicalReactionEngineering- lecturenotesJanuary-April2010 Departmentof Chemical EngineeringIndianInstituteof ScienceBangalore 560 012 Contents1 Introductionpage12 Reviewof backgroundmaterial8 References49iii1 IntroductionLevenspiel(2004,p. iii)hasgivena conciseandaptdescriptionof chemicalreactionengineering(CRE):Chemica lreactionengineeringis thatengineeringactivity concernedwiththeex-ploitationofchemical thesuccessfuldesignandoperationof chemicalreactors,andprobablymorethanany otherac-tivity, it setschemicalengineeringapartasa distinctbranch of of CREare(i)thermodynamics,(ii)kinetics,(ii i)tranportprocesses,(iv)types of reactors,(v)modeof operationandcon-tacting,(vi)modellingand optimization,and(vii) thereactionThestandardfreeenergyof formation G0of gaseousNOat a temperatureT= 298K anda referencepressurep0= 1 atmis mixtureof N2andO2, andis main-tainedat a constant temperatureTanda pressurep=p0.

2 Considerthereaction12N2(g)+12O2(g) NO(g)( )Herethenotation\(g)"impliesthatthespeci esis present in , \(l)"and\(s)"willbe usedto denotespeciespresent in SomeNOwillbe formedby ( ),buttheequilibriummolefractionof NO(yNO;e) is 1 as G0=(R T) 1. HereR12is thegasconstant, andyNO;eis themolefractionattainedat say thatthereactionis ,butis lessthan1 % evenatT= 1780K andp= 1 reactions ,such as theoxidationof NH3mustbe usedto produceNO(ChatterjeeandJoshi,2008). of reactionThesignof theheatof Reaction Hdetermineswhetherthereactorshouldbe heatedor ( H >0),andthelatterforexothermicreactions( H<0).Themagnitudeof Hdeterminestheamount of heatingor forthermodynamicnon-idealitiesFor gaseousreactionsat highpressureor low temperature,theequilibriumconstantKp, which is basedonpartialpressures,mustbe replacedby theequilibriumconstantKf, which is basedon fugacities(Denbigh,1971,p.)

3 152, ).For example,considertheammoniasynthesisreact ionN2(g) + 3 H2(g) 2 NH3(g)( )AtT= 450 C,thevalueofKpis 10 3atp= 10 3atp= 300atm(Denbigh,1971,p. 152).ThusKpvarieswiththepressure,whereas thevalueofKfis approximatelyconstant in thispressurerange- it is 10 3atp= 10 3atp= variationofKfis causedby theuseof approximateexpressionsforthefugacities,b asedon thevalueof G0, wecancalculateKf, andusingthethermodynamicrelationsbetween fugacitiesandpartialpressures,theequilib riumcompositioncanbe , fora liquidphasereactioninvolvingthesynthesis of methyltert-amyl ether(anadditive forhighoctanegasoline)frommethanoland2-m ethyl-2-butene,thecalculatedactivity coe cient formethanolis in 298K (Heintz et al.

4 , 2007). cationof reactionsReactionsmay be classi edby (a)thenumber of phasesinvolved,(b)thepresenceor absenceof a catalyst,and(c)thenatureof all thereactants andproducts,andcatalysts,if any, arein asinglephase,thereactionis saidto behomogeneous. Anexampleis providedbythethermalcrackingof ethanetoethylene(Froment andBischo ,1990,p. 29)C2H6(g) C2H4(g) + H2(g)( )Ontheotherhand,if morethanonephaseis involved,thereactionis saidtobeheterogeneous. Anexampleis providedby thechemicalvapourdeposition(CVD)of Si ona substrate(Fogler,1999,p. 675)SiH4(g)!Si(s)+ 2 H2(g)( )(silane)Equation( )represents anon-catalyticreaction,whereasammoniasyn thesisinvolves a somecases,a homogeneouscatalystmay be example,an enzymecalledglucoseisomerasecatalyzesthe isomerizationof glucoseto fructosein theliquidphase( ).

5 Glucoseto (2005,p. 24).Schmidt(2005,p. 24)notesthatthisis thelargestbioprocessin thechemicalindustry. Asfructoseis ve timessweeterthanglucose,theprocessis usedto make ,as written, may represent eitheranelementary4reactionor theformeris givenby thegas-phasereaction(Laidler,2007,p. 138)NO2(g) + CO(g) NO(g) + CO2(g)( )HereNOis formedby thecollisionbetweenmoleculesof NO2andCO, ,( )represents a non-elementaryreaction,as it actuallyproceedsbythesequenceof reactionsshownbelow (Fogler,1999,p. 666).SiH4(g) SiH2(g)+ H2(g)SiH2(g)+ !SiH2 SiH2 !Si(s)+ H2(g)( )where* represents anactive which a reactantis usuallyexpressedper unitvolumeof the uidfor uid-phasereactions,andper unitarea(orunitmass)of example,therateof formationof Siby themechanism( )is givenby_rSi=k pSiH4pH2+K pSiH4( )wherepSiH4andpH2arethepartialpressureso f SiH4andH2, ( )canbe derived from( )by alternativeroutesTheconventionalprocessf orthemanufactureof 5-cyanovaleramide(anin-termediatefora herbicide)by thehydrolysisof adiponitrile( )usedMgOas a catalyst(Pereira,1999).

6 Thecatalystwas di cultto recover andreactivate,andtheconversionhadtobe limitedto20% to avoida lowselectivity. Analternative processbasedona supportedenzymecatalystgave a highconversionanda adiponitrileto equationsFor uid-phasereactions, there-actionsinvolve two phasesthatarestrati ed,as in thecaseof a gas-liquidreactionin a falling- lmreactor,separateequationscanbe writtenforeachof onephaseis dispersedin theother,as in thecaseof stirredliquid-liquiddispersionsor uidizedbeds,we caneitherwriteseparateequa-tionsforeach phase,or usesomeformof explicitor implicitaveragingtowritecontinuumequatio nsforeach phase( ,Jackson,2000,Yu et al.)

7 , 2007). a uidphaseconsistingof simple uidssuch as airor water,theNavier-Stokes equationsarecommonlyusedto describe momentumtransfer,withFourier'slaw usionis described eitherby Fick'slawforbinarymixtures,or by astationarysolidphasesuch as a bed of catalystpellets,themomentumbalanceis ,Fourier'slaw andFick'slaw6have to be modi edby replacingthethermalconductivity anddi usivity bythe\e ective" thermalconductivity and\e ective" di usivity, dispersedmultiphasesystems,constitutive equationsaremorecomplicated,andnotas rmlyestablishedas of reactorsTheadjective idealrefersto thestateofmixingin is assumedto be perfectin thecaseof idealbatch,semi-batch, owreactorcorrespondsto theassumptionofperfectmixingin theradialdirection,nomixingin theaxialdirection,anda ataxialvelocity pro Levenspiel(2004, ,321-334)andFogler(1999, ,893-904)a sequenceof idealreactorscansometimesbe usedto theconventionalstirredvessels,\empty"tub ularreactors,andpackedbeds,therearemany othertypes of reactorssuch as uidizedbeds(LeeandLi,2009),tricklebeds(W u et al.

8 , 2009), uidized catalyticcrackers(Yanget al., 2009),bubblecolumns(Tokumuraet al.,2009),mem-branereactors(Rahimpourand Ghader,2004),microchannelreactors(Wanget al., 2009),andmultifunctionalreactors(Fan et al., 2009;Agar,1999).Thereferencesin bracketsrepresent recent articlesdiscussingsuch operationandcontactingReactorscanbe operatedeitherinbatch, rsttwo modescausetheconcentrationsof thespeciesto varywithtime,whereasthelattercanbe operatedin , startup,shutdown,anddisturbancesin feed ow rate, somesystems,it may be advantageoustodeliberatelyoperatein anunsteadymannerto achieve higherselectivity example,Sotowa et al.

9 (2008)examinedthee ectof forcedtemperaturecyclingof a catalystlayer on propylene(C3H6) a highertime-averagedconversionthansteadys tateoperation,forthesamerateof consumptionof energy. TheIntroduction7bombardierbeetleprovides anexampleof a naturalsystemthatreliesonforced periodicoperation(Aneshansleyet al., 1969).For multiphasereactors,severalmodesof contacting,such ascocur-rent,countercurrent, andcross- al.(2008)ex-aminedthee ectof introducingH2at variouspoints alongthelengthofa microchannelreactorontheconversionof COtohydrocarbonsby theFischer-Tropsch theintroductionof H2alongwithCOat theinletof thereactor,anincreasein selectivity was ,control, andoptimizationA mathematicalmodelof thereactorpermitspredictionof theconversion,selectivity (forsystemswithmultiplereactions)

10 , ow patternsandhotspotsor regionsof ectof changesin operatingconditionscanalsobe of a modelpermitsthedevelopmentof suitablecontrolschemesto ensureproductquality, andalsoprovidesavaluableaidfortheoptimiz ationof parametersor operatingpoliciesto sat-isfyspeci cobjective example,Altintenet al. (2008)modelleda batch reactorusedfortheproductionof suitablecon-trolscheme,thereactortempera turewas variedwithtimeso as to follow an\optimum"pro polymerof thedesiredmolecularweight was obtainedin reactionsThefollowingnotationwillbe usedto represent irreversibleandreversiblereactions:A+B!C irreversibleA+B Creversible( )LetAi; i= 1; NrepresentNspeciesparticipatingin a singlereactiona1A1+a2A2+::: amAm+:::aNANwhereairepresents thenumber of molesof speciesAi.