Chemical reactors_by Dr Umesh Barache.ppt

Industrial Chemistry

1. Dr. S. N. Shringare
Assistant Professor
School of Chemical Sciences
Punyashlok Ahilyadevi Holkar Solapur
University Solapur
Chemical Reactors

2. Chemicalreactors
Thereactors,inwhichchemicalsaremadeinindustry,varyinsizefromafewcm3tothe
vaststructures.Forexample,kilnsthatproducelimefromlimestonemaybeover25metreshigh
andhold,atanyonetime,wellover400tonnesofmaterials.Thedesignofthereactorisdetermined
bymanyfactorsbutofparticularimportancearethethermodynamicsandkineticsofthechemical
reactionsbeingcarriedout.
Thetwomaintypesofreactoraretermedbatchandcontinuous.

3. Batchreactors
Batchreactorsareusedformostofthereactionscarriedoutina
laboratory.Thereactantsareplacedinatest-tube,flaskorbeaker.They
aremixedtogether,oftenheatedforthereactiontotakeplaceandare
thencooled.Theproductsarepouredoutand,ifnecessary,purified.
Thisprocedureisalsocarriedoutinindustry,thekeydifferencebeing
oneofsizeofreactorandthequantitiesofreactants.
Followingreaction,thereactoriscleanedreadyforanotherbatch
ofreactantstobeadded.
Batchreactorsareusuallyusedwhenacompanywantsto
producearangeofproductsinvolvingdifferentreactantsand
reactorconditions.Theycanthenusethesameequipmentfor
thesereactions.

4. Continuous reactors
Analternativetoabatchprocessistofeedthereactantscontinuouslyinto
thereactoratonepoint,allowthereactiontotakeplaceandwithdrawtheproductsat
anotherpoint.Theremustbeanequalflowrateofreactantsandproducts.While
continuousreactorsarerarelyusedinthelaboratory,awater-softenercanbe
regardedasanexampleofacontinuousprocess.Hardwaterfromthemainsis
passedthroughatubecontaininganion-exchangeresin.Reactionoccursdownthe
tubeandsoftwaterpoursoutattheexit.
Themainadvantage,theproducttendstobeofamoreconsistentquality
fromacontinuousreactorbecausethereactionparameters(e.g.residencetime,
temperatureandpressure)arebettercontrolledthaninbatchoperations.
Theyalsoproducelesswasteandrequiremuchlowerstorageofbothraw
materialsandproductsresultinginamoreefficientoperation.Capitalcostsper
tonneofproductproducedareconsequentlylower.
Themaindisadvantageistheirlackofflexibilityasoncethereactorhas
beenbuiltitisonlyinrarecasesthatitcanbeusedtoperformadifferentchemical
reaction.

5. Types of continuous reactors
Industry uses several types of continuous reactors.
Tubular reactors
Inatubularreactor,fluids(gasesand/orliquids)flowthroughitathighvelocities.
Asthereactantsflow,forexamplealongaheatedpipe,theyareconvertedto
products(Figure4).Atthesehighvelocities,theproductsareunabletodiffuse
backandthereislittleornobackmixing.Theconditionsarereferredtoasplug
flow.Thisreducestheoccurrenceofsidereactionsandincreasestheyieldofthe
desiredproduct.
Withaconstantflowrate,theconditionsatanyonepointremainconstantwithtime
andchangesintimeofthereactionaremeasuredintermsofthepositionalong
thelengthofthetube.
Thereactionrateisfasteratthepipeinletbecausetheconcentrationofreactants
isatitshighestandthereactionratereducesasthereactantsflowthroughthe
pipeduetothedecreaseinconcentrationofthereactant.
Tubularreactorsareused,for
example,inthesteam
crackingofethane,propane
andbutaneandnaphthato
producealkenes.

6. Fixed bed reactors
Aheterogeneouscatalystisusedfrequentlyinindustrywheregases
flowthroughasolidcatalyst(whichisoftenintheformofsmallpellets
toincreasethesurfacearea).Itisoftendescribedasafixedbedof
catalyst.
Amongtheexamplesoftheirusearethemanufactureofsulfuricacid
(theContactProcess,withvanadium(V)oxideascatalyst),the
manufactureofnitricacidandthemanufactureofammonia(theHaber
Process,withironasthecatalyst).

7. Fluidbedreactors
Afluidbedreactorissometimesusedwherebythecatalystparticles,which
areveryfine,sitonadistributorplate.Whenthegaseousreactantspassthroughthe
distributorplate,theparticlesarecarriedwiththegasesformingafluid.Thisensures
verygoodmixingofthereactantswiththecatalyst,withveryhighcontactbetweenthe
gaseousmoleculesandthecatalystandagoodheattransfer.Thisresultsinarapid
reactionandauniformmixture,reducingthevariabilityoftheprocessconditions.
Oneexampleoftheuseoffluidbedreactorsisintheoxychlorinationof
ethenetochloroethene(vinylchloride),thefeedstockforthepolymer
poly(chloroethene)(PVC).Thecatalystiscopper(II)chlorideandpotassiumchloride
depositedonthesurfaceofalumina.Thissupportissofine,itactsasafluidwhen
gasespassthroughit.

8. Continuousstirredtankreactors,CSTR
InaCSTR,oneormorereactants,forexampleinsolutionorasaslurry,are
introducedintoareactorequippedwithanimpeller(stirrer)andtheproductsare
removedcontinuously.Theimpellerstirsthereagentsvigorouslytoensuregood
mixingsothatthereisauniformcompositionthroughout.Thecompositionatthe
outletisthesameasinthebulkinthereactor.Theseareexactlytheopposite
conditionstothoseinatubularflowreactorwherethereisvirtuallynomixingofthe
reactantsandtheproducts.
ACSTRreactorisused,forexampleintheproductionoftheamide
intermediateformedintheprocesstoproducemethyl2-
methylpropenoate.