Reactor DesignS,S&L Chapter 7Terry A. RingChE

Reactor TypesIdealPFRCSTRRealUnique design geometries and therefore RTDMultiphaseVarious regimes of momentum, mass and heat transfer

Reactor CostReactor isPRFPressure vesselCSTRStorage tank with mixerPressure vesselHydrostatic head gives the pressure to design for

Reactor CostPFRReactor Volume (various L and D) from reactor kineticshoop-stress formula for wall thickness:

t= vessel wall thickness, in.P= design pressure difference between inside and outside of vessel, psigR= inside radius of steel vessel, in.S= maximum allowable stress for the steel. E= joint efficiency (0.9)tc=corrosion allowance = 0.125 in.

Reactor CostPressure Vessel Material of Construction gives metalMass of vessel = metal (VC+2VHead)Vc = DLVHead from tables that are based upon D

Cp= FMCv(W)

Reactors in Process SimulatorsStoichiometric ModelSpecify reactant conversion and extents of reaction for one or more reactionsTwo Models for multiple phases in chemical equilibriumKinetic model for a CSTRKinetic model for a PFRCustom-made models (UDF)

Used in early stages of design

Kinetic Reactors - CSTR & PFRUsed to Size the ReactorUsed to determine the reactor dynamicsReaction Kinetics

PFR no backmixingUsed to Size the Reactor

Space Time = Vol./QOutlet Conversion is used for flow sheet mass and heat balances

CSTR complete backmixingUsed to Size the Reactor

Outlet Conversion is used for flow sheet mass and heat balances

Review : Catalytic Reactors Brief IntroductionMajor StepsA BBulk FluidExternal Surfaceof Catalyst PelletCatalyst SurfaceInternal Surfaceof Catalyst PelletCAbCAs4. Surface Reaction

Catalytic ReactorsVarious Mechanisms depending on rate limiting stepSurface Reaction LimitingSurface Adsorption LimitingSurface Desorption LimitingCombinationsLangmuir-Hinschelwood Mechanism (SR Limiting)H2 + C7H8 (T) CH4 + C6H6(B)

Catalytic Reactors Implications on designWhat effects do the particle diameter and the fluid velocity above the catalyst surface play?What is the effect of particle diameter on pore diffusion ?How the surface adsorption and surface desorption influence the rate law?Whether the surface reaction occurs by a single-site/dual site / reaction between adsorbed molecule and molecular gas?How does the reaction heat generated get dissipated by reactor design?

Enzyme CatalysisEnzyme Kinetics

S= substrate (reactant)E= Enzyme (catalyst)

ProblemsManaging Heat effectsOptimizationMake the most product from the least reactant

Optimization of Desired ProductReaction NetworksMaximize yield, moles of product formed per mole of reactant consumedMaximize SelectivityNumber of moles of desired product formed per mole of undesirable product formedMaximum Attainable Region see discussion in Chapt. 7.Reactors (pfrs &cstrs in series) and bypass Reactor sequencesWhich come first

Managing Heat EffectsReaction Run AwayExothermicReaction DiesEndothermic

Preventing ExplosionsPreventing Stalling

Temperature EffectsOn EquilibriumOn Kinetics

Equilibrium Reactor-Temperature EffectsSingle EquilibriumaA +bB rR + sS

ai activity of component IGas Phase, ai = iyiP, i== fugacity coefficient of iLiquid Phase, ai= i xi exp[Vi (P-Pis) /RT]i = activity coefficient of i Vi =Partial Molar Volume of iVant Hoff eq.

Overview of CRE Aspects related to Process DesignLevenspiel , O. (1999), Chemical Reaction Engineering, John Wiley and Sons , 3rd ed.

Unfavorable EquilibriumIncreasing Temperature Increases the RateEquilibrium Limits Conversion

Overview of CRE Aspects related to Process DesignLevenspiel , O. (1999), Chemical Reaction Engineering, John Wiley and Sons , 3rd ed.

Feed Temperature, HrxnHeat Balance over ReactorCoolingAdiabaticAdiabaticQ = UA Tlm

Reactor with Heating or CoolingQ = UA T

Kinetic Reactors - CSTR & PFR Temperature EffectsUsed to Size the ReactorUsed to determine the reactor dynamicsReaction Kinetics

PFR no backmixingUsed to Size the Reactor

Space Time = Vol./QOutlet Conversion is used for flow sheet mass and heat balances

CSTR complete backmixingUsed to Size the Reactor

Outlet Conversion is used for flow sheet mass and heat balances

Unfavorable EquilibriumIncreasing Temperature Increases the RateEquilibrium Limits Conversion

Various Reactors, Various Reactions

Reactor with Heating or CoolingQ = UA T

Temperature Profiles in a ReactorExothermic ReactionRecycle

Best Temperature Path

Optimum Inlet TemperatureExothermic Rxn

Managing Heat EffectsReaction Run AwayExothermicReaction DiesEndothermic

Preventing ExplosionsPreventing Stalling

Inter-stage CoolerExothermic EquilibriaLowers Temp.

Inter-stage Cold FeedExothermic EquilibriaLowers TempLowers Conversion

Optimization of Desired ProductReaction NetworksMaximize yield, moles of product formed per mole of reactant consumedMaximize SelectivityNumber of moles of desired product formed per mole of undesirable product formedMaximum Attainable Region see discussion in Chapt. 6.Reactors and bypass Reactor sequences

Reactor Design for Selective Product DistributionS,S&L Chapt. 7

OverviewParallel ReactionsA+BR (desired)ASSeries ReactionsABC(desired)DIndependent ReactionsAB (desired)CD+ESeries Parallel ReactionsA+BC+DA+CE(desired)Mixing, Temperature and Pressure Effects

ExamplesEthylene Oxide SynthesisCH2=CH2 + 3O22CO2 + 2H2O

CH2=CH2 + O2CH2-CH2(desired)O

ExamplesDiethanolamine Synthesis

ExamplesButadiene Synthesis, C4H6, from Ethanol

Rate SelectivityParallel ReactionsA+BR (desired)A+BSRate Selectivity

(D- U) >1 make CA as large as possible(D U)>1 make CB as large as possible

(kD/kU)= (koD/koU)exp[-(EA-D-EA-U)/(RT)]EA-D > EA-U TEA-D < EA-U T

Reactor Design to Maximize Desired Product for Parallel Rxns.

Maximize Desired ProductSeries ReactionsAB(desired)CDPlug Flow ReactorOptimum Time in Reactor

Fractional Yield(k2/k1)=f(T)

Real Reaction SystemsMore complicated than either Series ReactionsParallel ReactionsEffects of equilibrium must be consideredConfounding heat effectsAll have Reactor Design Implications

Engineering TricksReactor typesMultiple ReactorsMixtures of ReactorsBypassRecycle after SeparationSplit Feed Points/ Multiple Feed PointsDiluentsTemperature Management with interstage Cooling/Heating

A few words about simulatorsAspenKinetics Must put in with Aspen UnitsEquilibrium constantsMust put in in the formlnK=A+B/T+CT+DT2ProMaxReactor type and Kinetics must match!!KineticsSelectable unitsEquilibrium constants

**parallel*Series parallel*Series parallel , CH3CHO acetaldehyde*