Presentation on theme: "S,S&L Chapter 7 Terry A. Ring ChE"— Presentation transcript:
1 S,S&L Chapter 7 Terry A. Ring ChE Reactor DesignS,S&L Chapter 7Terry A. RingChE
2 Reactor Types Ideal Real PFR CSTR Unique design geometries and therefore RTDMultiphaseVarious regimes of momentum, mass and heat transfer
3 Reactor Cost Reactor is PRF CSTR Pressure vessel Storage tank with mixerHydrostatic head gives the pressure to design for
4 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 = in.
5 Reactor Cost Pressure Vessel – Material of Construction gives ρmetal Mass of vessel = ρmetal (VC+2VHead)Vc = πDLVHead – from tables that are based upon DCp= FMCv(W)
6 Reactors in Process Simulators Stoichiometric 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
7 Kinetic Reactors - CSTR & PFR Used to Size the ReactorUsed to determine the reactor dynamicsReaction Kinetics
8 PFR – no backmixing Used to Size the Reactor Space Time = Vol./Q Outlet Conversion is used for flow sheet mass and heat balances
9 CSTR – complete backmixing Used to Size the ReactorOutlet Conversion is used for flow sheet mass and heat balances
10 Review : Catalytic Reactors – Brief Introduction Major StepsAB7 . Diffusion of productsfrom pore mouth tobulkBulk FluidCAbExternal DiffusionRate = kC(CAb – CAS)External Surfaceof Catalyst PelletCAs2. Defined by an Effectiveness Factor6 . Diffusion of productsfrom interior to poremouthInternal Surfaceof Catalyst Pellet3. Surface AdsorptionA + S <-> A.S5. Surface DesorptionB. S <-> B + SA BCatalystSurface4. Surface Reaction
12 Catalytic Reactors – Implications on design What 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?
14 Problems Managing Heat effects Optimization Make the most product from the least reactant
15 Optimization of Desired Product Reaction 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 Chap’t. 7.Reactors (pfrs &cstrs in series) and bypassReactor sequencesWhich come first
16 Managing Heat Effects Reaction Run Away Reaction Dies ExothermicReaction DiesEndothermicPreventing ExplosionsPreventing Stalling
18 Equilibrium Reactor- Temperature Effects Single EquilibriumaA +bB rR + sSai activity of component IGas Phase, ai = φiyiP,φi== fugacity coefficient of iLiquid Phase, ai= γi xi exp[Vi (P-Pis) /RT]γi = activity coefficient of iVi =Partial Molar Volume of iVan’t Hoff eq.
19 Overview of CRE – Aspects related to Process Design Le Chatelier’s PrincipleLevenspiel , O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons , 3rd ed.
20 Unfavorable Equilibrium Increasing Temperature Increases the RateEquilibrium Limits Conversion
21 Overview of CRE – Aspects related to Process Design Levenspiel , O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons , 3rd ed.21
22 Feed Temperature, ΔHrxn AdiabaticAdiabaticCoolingHeat Balance over ReactorQ = UA ΔTlm
36 Optimization of Desired Product Reaction 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 Chap’t. 6.Reactors and bypassReactor sequences
37 Reactor Design for Selective Product Distribution S,S&L Chapt. 7
38 Overview Parallel Reactions Series Reactions Independent Reactions A+BR (desired)ASSeries ReactionsABC(desired)DIndependent ReactionsAB (desired)CD+ESeries Parallel ReactionsA+BC+DA+CE(desired)Mixing, Temperature and Pressure Effects
41 Examples Butadiene Synthesis, C4H6, from Ethanol Series parallel , CH3CHO acetaldehyde
42 Rate Selectivity Parallel Reactions Rate Selectivity A+BR (desired)A+BSRate Selectivity(αD- αU) >1 make CA as large as possible(βD –βU)>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
43 Reactor Design to Maximize Desired Product for Parallel Rxns.
44 Maximize Desired Product Series ReactionsAB(desired)CDPlug Flow ReactorOptimum Time in Reactor
46 Real Reaction Systems More complicated than either Series ReactionsParallel ReactionsEffects of equilibrium must be consideredConfounding heat effectsAll have Reactor Design Implications
47 Engineering Tricks Reactor types Multiple ReactorsMixtures of ReactorsBypassRecycle after SeparationSplit Feed Points/ Multiple Feed PointsDiluentsTemperature Management with interstage Cooling/Heating
48 A few words about simulators AspenKineticsMust put in with “Aspen Units”Equilibrium constantsMust put in in the formlnK=A+B/T+CT+DT2ProMaxReactor type and Kinetics must match!!KineticsSelectable unitsEquilibrium constants