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CH EN 5253 – Process Design II Effects of Impurities on Reactions and Reactor Design

CH EN 5253 – Process Design II Effects of Impurities on Reactions and Reactor Design. February 11, 2019. Books. There is no chapter in the book on this subject. Impurity Effects. Reactors Heat Exchange Separation Systems Recycle Loops. Location of Separation Units. Impurities in Reactors.

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CH EN 5253 – Process Design II Effects of Impurities on Reactions and Reactor Design

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  1. CH EN 5253 – Process Design IIEffects of Impurities on Reactions and Reactor Design February 11, 2019

  2. Books • There is no chapter in the book on this subject

  3. Impurity Effects • Reactors • Heat Exchange • Separation Systems • Recycle Loops

  4. Location of Separation Units

  5. Impurities in Reactors Point 1: Poisons for Catalysts • Kill Catalyst with time Point 2: Impurities can cause side reactions altering • Reactor conversion • Generating additional undesirable products Point 3: Impurities Impact Equilibrium Conversion Point 4: Impurities Impact Reaction Rates • Lower concentrations Point 5: Impurities have Reaction Heat Effects • Increase Cp

  6. Point 1: Poisons for Catalysts • Kill Catalyst with time • Lead, Sulfur, Manganese in Gasoline kill Catalytic Converter Platinum, Palladium, Rhodium Note: Vehicles equipped with catalytic converters can run only on unleaded fuel

  7. Review: Mechanism ofHeterogeneous Catalysis

  8. Catalytic Reactors • Various Mechanisms depending on rate limiting step • Surface Reaction Limiting • Surface Adsorption Limiting • Surface Desorption Limiting • Combinations

  9. Catalytic Reactors • Toluene Hydrodealkylation (HDA)Process H2 + C7Hi (T) CH4 + C6H6(B) • Catalyst: Chromium or molybdenum or platinum oxides • Langmuir-Hinshelwood Mechanism (Surface Reaction Limiting) • Impurities are adsorbed on catalytic sites • Decrease the Cv concentration of active sites

  10. Point 2: Side reactions altering • Reactor conversion • Generating additional undesirable products (undesired) (undesired)

  11. Point 3: Impurities Impact Equilibrium Conversion • Temperature Effects • Single Equilibrium aA +bBrR + sS Van’t Hoff eq.

  12. Unfavorable Equilibrium • Increasing Temperature Increases the Rate • Equilibrium Limits Conversion Equilibrium line is repositioned and rate curves are repositioned due to impurities

  13. Point 4: Impurities Impact Reaction Rates

  14. Point 5: Reaction Heat Effects Effect of Inert Addition Similar to Impurity Effects Adiabatic Adiabatic ∆T=T2-T1 Q=external heat added With increased inerts Cp rises, and these curves become more closely vertical.

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

  16. Impact on Reactor Design

  17. PFR – no backmixing • Used to Size the Reactor • Space Time = Vol./Q • Outlet Conversion is used for flow sheet mass and heat balances rK is smaller and V is larger due to impurities.

  18. CSTR – complete backmixing • Used to Size the Reactor • Outlet Conversion is used for flow sheet mass and heat balances rK is smaller and V is larger due to impurities.

  19. Temperature Profiles in a Reactor Exothermic Reaction Impurities effect these curves and areas under these curves=size of reactor

  20. Costs • Higher capital cost for • Bigger reactors • Cooling and heating systems • Installation • Higher operating cost for • Utility – hot and cold water • Pumping • Separation

  21. Consideration of Impuritiesin Catalytic Reactor Design • How the surface adsorption and surface desorption influence the rate law? • Whether the surface reaction occurs by a single-site or dual-site reaction between adsorbed molecule and molecular gas? • How does the reaction heat generated get dissipated by reactor design?

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