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Reactor Design

Reactor Design. S,S&L Chapter 7 Terry A. Ring ChE. Reactor Types. Ideal PFR CSTR Real Unique design geometries and therefore RTD Multiphase Various regimes of momentum, mass and heat transfer. Reactor Cost. Reactor is PRF Pressure vessel CSTR Storage tank with mixer

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Reactor Design

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

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

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

  4. 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.

  5. 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)

  6. 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

  7. Kinetic Reactors - CSTR & PFR • Used to Size the Reactor • Used to determine the reactor dynamics • Reaction 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 Reactor • Outlet Conversion is used for flow sheet mass and heat balances

  10. Review : Catalytic Reactors – Brief Introduction Major Steps Bulk Fluid CAb B A • External Diffusion • Rate = kC(CAb – CAS) 7 . Diffusion of products from pore mouth to bulk 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 5. Surface Desorption B. S <-> B + S 3. Surface Adsorption A + S <-> A.S A  B 4. Surface Reaction Catalyst Surface

  11. Catalytic Reactors • 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)

  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?

  13. Enzyme Catalysis • Enzyme Kinetics • S= substrate (reactant) • E= Enzyme (catalyst)

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

  15. 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

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

  17. Temperature Effects • On Equilibrium • On Kinetics

  18. 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.

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

  20. Unfavorable Equilibrium • Increasing Temperature Increases the Rate • Equilibrium Limits Conversion

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

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

  23. Reactor with Heating or Cooling Q = UA ΔT

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

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

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

  27. Unfavorable Equilibrium • Increasing Temperature Increases the Rate • Equilibrium Limits Conversion

  28. Various Reactors, Various Reactions

  29. Reactor with Heating or Cooling Q = UA ΔT

  30. Temperature Profiles in a Reactor Exothermic Reaction Recycle

  31. Best Temperature Path

  32. Optimum Inlet TemperatureExothermic Rxn CSTR PFR

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

  34. Inter-stage Cooler Lowers Temp. Exothermic Equilibria

  35. Inter-stage Cold Feed Lowers Temp Lowers Conversion Exothermic Equilibria

  36. 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

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

  38. Overview • Parallel 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

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

  40. Examples • Diethanolamine Synthesis

  41. Examples • Butadiene Synthesis, C4H6,from Ethanol

  42. Rate Selectivity • Parallel Reactions • 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

  43. Reactor Design to Maximize Desired Product for Parallel Rxns.

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

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

  46. 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

  47. 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

  48. 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 A few words about simulators

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