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ReactorDesignProject Memo3

In this memo you will add energy balances for your reaction system. Instead of assuming

isothermal conditions, you will now model the adiabatic reactor and heat exchange in your

reactor.

A.EnergyBalanceAdiabatic

1. Calculate the heat of reaction (Hrxn298K) to produce your product from all competing

feedstocksandcommentonthedifferencesbetweenfeedstocks.

2. Calculate an overall energy balance using inlet and outlet enthalpies for a generic reactor

assumingtypicalreactionconversionsdeterminedinmemo2

2.1.First,calculateanoverall isothermalenergybalanceand determineheatdutyrequired to

operateatatypicaltemperature.

2.2.Second,calculateanoveralladiabaticenergybalanceanddeterminethefinaltemperature.

Usetypical inlettemperaturesgiven ineithertheKirkOthmerorMcKettareferencescited inthe

overviewof theproject.Performcalculationsusingphysicalpropertydataobtained fromeither

the

DIPPR

database

or

the

NIST

database

(NIST

WebBook

http://webbook.nist.gov/),

if

possible.

Remembertouseheatcapacitiesasafunctionoftemperature.

Giveasamplecalculationofheatcapacities,enthalpiesandtheenergybalanceandshowunitsin

thiscalculation.Remembertocitereferencesforyourthermochemicalpropertydata.

B.EnergyBalance Heatexchange

Consideramultitubularreactor.Forexothermicreactionswetypicallyconfigurethemultitubular

reactor with cocurrent cooling, offering the best conditions for reactor dynamic stability by

maintaining a relatively constant driving force for heat transfer. For simplicity in this memo,

however,wewillassumeaconstantcoolanttemperaturesothatyouwontneedtoconsiderthe

energybalanceonthecoolant. Youwillneedtoincludetheenergybalancefortheprocessfluid.

Monitoring the reactor hotspot the hottest axial temperature in the reactor provides an

indicationof theviabilityofyour reactordesign from theperspectiveof reactionselectivityand

reactordynamicstability. Whiledynamicmodelingofreactorbehaviorisoutsidethescopeofthis

course,we canuseawellestablished steadystatemeasureasan indicatorof reactor stability.

Oneruleofthumbtestfordynamicstability istoconsiderthereactorgain. Thismetricasks

thequestionwhenyou increase the reactorcoolant temperatureby1C,howmuchdoes the

hotspottemperatureincrease?

Foracooledreactor,wegenerallywantthisquantitytobelessthan2toassuredynamicstability.

(Asimilar

measure

is

evaluated

for

an

adiabatic

reactor

where

we

look

at

how

the

exit

temperaturevarieswithavariationininlettemperature).

Polymathmodel

1. Add theenergybalance toyourPolymathmodeldeveloped inmemo2. Derive theenergy

balance expression in termsof catalystweight ( dT/dW )using your catalyst bulkdensity, and

includethesecalculationsinthismemo. AsasimplificationforyourPolymathmodel,youmay

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wish touse componentCp valuesat your reactor feed temperature so that you can calculate

thesevaluesapriori inaspreadsheet. Youwillalsoneedtoresearchavalueforanoverallheat

transfercoefficientforapackedbedreactor,andselectacoolanttemperature. Beclearabout

whatyouareassumingandstateyourassumptionsinyourmemo.

a.Examineyourreactortemperatureprofilesandselectivitytoyourdesiredproductasafunction

ofgasinlettemperatureandcoolanttemperature.Isthereahotspot? Howwouldyoumoderate

thishotspot

temperature

with

process

variables?

Explore

these

ideas

and

submit

your

results

withyourmodelinthismemo.

b.Whatisthegainforyourselectedreactoroperatingconditions? Doyouexpectyourreactor

to be stabledynamically, and if not, how can youmodify yourprocess conditions to specify a

stablereactor?

c.Howdoesisothermaloperationcomparewithoperatingarealreactorwithheattransfer?

d.Howdoesthecoolanttemperatureimpacttheresults?