S,S&L Chapter 7 Terry A. Ring ChE Reactor Design S,S&L Chapter 7 Terry A. Ring ChE

Reactor Types Ideal Real PFR CSTR Unique design geometries and therefore RTD Multiphase Various regimes of momentum, mass and heat transfer

Reactor Cost Reactor is PRF CSTR Pressure vessel Storage tank with mixer Hydrostatic head gives the pressure to design for

Reactor Cost PFR Reactor Volume (various L and D) from reactor kinetics hoop-stress formula for wall thickness: t= vessel wall thickness, in. P= design pressure difference between inside and outside of vessel, psig R= 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 Cost Pressure Vessel – Material of Construction gives ρmetal Mass of vessel = ρmetal (VC+2VHead) Vc = πDL VHead – from tables that are based upon D Cp= FMCv(W)

Reactors in Process Simulators Stoichiometric Model Specify reactant conversion and extents of reaction for one or more reactions Two Models for multiple phases in chemical equilibrium Kinetic model for a CSTR Kinetic model for a PFR Custom-made models (UDF) Used in early stages of design

Kinetic Reactors - CSTR & PFR Used to Size the Reactor Used to determine the reactor dynamics Reaction Kinetics

PFR – no backmixing Used to Size the Reactor Space Time = Vol./Q Outlet Conversion is used for flow sheet mass and heat balances

CSTR – complete backmixing Used to Size the Reactor Outlet Conversion is used for flow sheet mass and heat balances

Review : Catalytic Reactors – Brief Introduction Major Steps A B 7 . Diffusion of products from pore mouth to bulk Bulk Fluid CAb External Diffusion Rate = kC(CAb – CAS) External Surface of Catalyst Pellet CAs 2. Defined by an Effectiveness Factor 6 . Diffusion of products from interior to pore mouth Internal Surface of Catalyst Pellet 3. Surface Adsorption A + S <-> A.S 5. Surface Desorption B. S <-> B + S A  B Catalyst Surface 4. Surface Reaction

Catalytic Reactors Langmuir-Hinschelwood Mechanism (SR Limiting) Various Mechanisms depending on rate limiting step Surface Reaction Limiting Surface Adsorption Limiting Surface Desorption Limiting Combinations Langmuir-Hinschelwood Mechanism (SR Limiting) H2 + C7H8 (T) CH4 + C6H6(B)

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?

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

Problems Managing Heat effects Optimization Make the most product from the least reactant

Optimization of Desired Product Reaction Networks Maximize yield, moles of product formed per mole of reactant consumed Maximize Selectivity Number of moles of desired product formed per mole of undesirable product formed Maximum Attainable Region – see discussion in Chap’t. 7. Reactors (pfrs &cstrs in series) and bypass Reactor sequences Which come first

Managing Heat Effects Reaction Run Away Reaction Dies Exothermic Reaction Dies Endothermic Preventing Explosions Preventing Stalling

Temperature Effects On Equilibrium On Kinetics

Equilibrium Reactor- Temperature Effects Single Equilibrium aA +bB  rR + sS ai activity of component I Gas Phase, ai = φiyiP, φi== fugacity coefficient of i Liquid Phase, ai= γi xi exp[Vi (P-Pis) /RT] γi = activity coefficient of i Vi =Partial Molar Volume of i Van’t Hoff eq.

Overview of CRE – Aspects related to Process Design Le Chatelier’s Principle Levenspiel , O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons , 3rd ed.

Unfavorable Equilibrium Increasing Temperature Increases the Rate Equilibrium Limits Conversion

Overview of CRE – Aspects related to Process Design Levenspiel , O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons , 3rd ed. 21

Feed Temperature, ΔHrxn Adiabatic Adiabatic Cooling Heat Balance over Reactor Q = UA ΔTlm

Reactor with Heating or Cooling Q = UA ΔT

Kinetic Reactors - CSTR & PFR – Temperature Effects Used to Size the Reactor Used to determine the reactor dynamics Reaction Kinetics

PFR – no backmixing Used to Size the Reactor Space Time = Vol./Q Outlet Conversion is used for flow sheet mass and heat balances

CSTR – complete backmixing Used to Size the Reactor Outlet Conversion is used for flow sheet mass and heat balances

Unfavorable Equilibrium Increasing Temperature Increases the Rate Equilibrium Limits Conversion

Various Reactors, Various Reactions

Reactor with Heating or Cooling Q = UA ΔT

Temperature Profiles in a Reactor Exothermic Reaction Recycle

Best Temperature Path

Optimum Inlet Temperature Exothermic Rxn CSTR PFR

Managing Heat Effects Reaction Run Away Reaction Dies Exothermic Reaction Dies Endothermic Preventing Explosions Preventing Stalling

Inter-stage Cooler Lowers Temp. Exothermic Equilibria

Inter-stage Cold Feed Lowers Temp Lowers Conversion Exothermic Equilibria

Optimization of Desired Product Reaction Networks Maximize yield, moles of product formed per mole of reactant consumed Maximize Selectivity Number of moles of desired product formed per mole of undesirable product formed Maximum Attainable Region – see discussion in Chap’t. 6. Reactors and bypass Reactor sequences

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

Overview Parallel Reactions Series Reactions Independent Reactions A+BR (desired) AS Series Reactions ABC(desired)D Independent Reactions AB (desired) CD+E Series Parallel Reactions A+BC+D A+CE(desired) Mixing, Temperature and Pressure Effects

Examples Ethylene Oxide Synthesis CH2=CH2 + 3O22CO2 + 2H2O CH2=CH2 + O2CH2-CH2(desired) O parallel

Examples Diethanolamine Synthesis Series parallel

Examples Butadiene Synthesis, C4H6, from Ethanol Series parallel , CH3CHO acetaldehyde

Rate Selectivity Parallel Reactions Rate Selectivity A+BR (desired) A+BS Rate 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 T EA-D < EA-U T

Reactor Design to Maximize Desired Product for Parallel Rxns.

Maximize Desired Product Series Reactions AB(desired)CD Plug Flow Reactor Optimum Time in Reactor

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

Real Reaction Systems More complicated than either Series Reactions Parallel Reactions Effects of equilibrium must be considered Confounding heat effects All have Reactor Design Implications

Engineering Tricks Reactor types Multiple Reactors Mixtures of Reactors Bypass Recycle after Separation Split Feed Points/ Multiple Feed Points Diluents Temperature Management with interstage Cooling/Heating

A few words about simulators Aspen Kinetics Must put in with “Aspen Units” Equilibrium constants Must put in in the form lnK=A+B/T+CT+DT2 ProMax Reactor type and Kinetics must match!! Kinetics Selectable units Equilibrium constants